Patent Application
20140073520
The invention disclosed herein generally relates to methods and systems that analyze biological systems in microscopic and macroscopic scales at the same time. Specifically, the invention disclosed herein relates to methods and systems that combine the power and advantages of genomics and single cells analyses. More specifically, the invention disclosed herein relates to methods and systems that encode cellular constituents with indicia based on genomics and then employ powerful tools such as super resolution microscopes that can resolve such indicia.
Advances in genomic research have revolutionized the fields of biology, genetics, and biochemistry. In particular, microarray technologies have enabled the study of molecular interactions at a large scale. As the collective understanding of biological systems progress, it is increasingly important that molecular interactions are understood both macroscopically and microscopically in a systematic fashion.
As a field of study, system biology is the study of the interactions between the components of biological systems, and how these interactions give rise to the function and behavior of that system (for example, the enzymes and metabolites in a metabolic pathway).
Current tools for macroscopic and systematic analyses of biological systems require tremendous input in both resource and manpower. There is a need in the art for methods and systems that can carry out such analyses with more efficiency and economy.
In various embodiments, the invention teaches a method, including: (i) creating a molecular barcode for one or more DNA sequences of interest in a chromosome of a cell by a method including: (a) providing a plurality of probe-pairs that are each specific to a DNA sequence of interest, wherein each probe-pair includes an activator fluorophore and a reporter fluorophore; (b) hybridizing, within said cell, a quantity of said one or more DNA sequences of interest with a plurality of said probe-pairs, wherein when the probe-pairs hybridize with said DNA sequences of interest, the reporter fluorophore and activator fluorophore of each probe-pair are in sufficiently close proximity to form a functional dye pair; and wherein each of the DNA sequences of interest that is hybridized with said plurality of probe-pairs emits two or more distinct signals, so as to create the molecular barcode; and (ii) resolving said molecular barcode by resolving said signals emitted from said plurality of probe-pairs associated with each of said DNA sequences of interest, wherein each emitted signal is a component of the barcode associated with each of said DNA sequences of interest and wherein each DNA sequence of interest is associated with a distinct barcode, so as to detect one or more DNA sequences of interest in a chromosome of a single cell. In some embodiments, the barcode is resolved using super-resolution technology. In certain embodiments, the cell is essentially intact or undisrupted. In certain embodiments, the cell is selected from the group consisting of a fungus cell, an animal cell, a mammalian cell, a mouse cell, a human cell, a cancer cell, a lymphocyte, an erythrocyte, a white blood cell, an epithelial cell, a pituitary cell, a gut cell, a respiratory tract cell, a gland cell, a thyroid gland cell, a parathyroid gland cell, an adrenal gland cell, a muscle cell, a ciliated cell, an embryonic cell, a sensory transducer cell, a neuron, a glial cell, a lens cell, a kidney cell, a pigment cell, and a pancreatic cell. In some embodiments, the plurality of probes includes oligonucleotides. In some embodiments, the fluorophores are selected from the group consisting of fluorescein, rhodamine, Alexa Fluors, DyLight fluors, ATTO Dyes, and analogs or derivatives thereof. In some embodiments, the molecular barcode is a linear, two-dimensional or three-dimensional pattern of signals emitted from said plurality of probes. In certain embodiments, the super resolution technology is selected from the group consisting of: Stimulated Emission Depletion microscopy (STEDM), Ground State Depletion microscopy (GSDM), Spatially Structured Illumination microscopy (SSIM), Photo-Activated Localization Microscopy (PALM), Fluorescence-PALM (FPALM), Stochastical Optical Reconstruction Microscopy (STORM), Fluorescence Imaging with One-Nanometer Accuracy (FIONA), and combinations thereof. In some embodiments, the method also includes determining the location of one or more DNA sequences of interest in the chromosome, based upon the signals detected. In some embodiments, the cell is acquired from a biopsy of a subject. In some embodiments, the method also includes diagnosing the subject with the presence or absence of cancer based upon one or more characteristics of one or more DNA sequences of interest determined by resolving one or more molecular bar code, wherein the one or more characteristics are selected from the group consisting of: presence of the sequence in the chromosome, absence of the sequence in the chromosome, location of the sequence in the chromosome relative to another DNA sequence, and abundance. In various embodiments, the invention teaches a method, including: (i) creating a molecular barcode for one or more DNA sequences of interest in a chromosome of a cell by a method including: (a) providing a plurality of probe-pairs that are each specific to a DNA sequence of interest, wherein each probe-pair includes an activator fluorophore and a reporter fluorophore; (b) hybridizing, within said cell, a quantity of said one or more DNA sequences of interest with a plurality of said probe-pairs, wherein when the probe-pairs hybridize with said one or more DNA sequences of interest, the reporter fluorophore and activator fluorophore of each probe-pair are in sufficiently close proximity to form a functional dye pair; and wherein each of the DNA sequences of interest that is hybridized with said plurality of probe-pairs emits two or more distinct signals, so as to create the molecular barcode; (ii) creating a molecular barcode for one or more mRNA sequences of interest in the cell, by a method including: (a) providing a plurality of probe-pairs that are specific to each of said mRNA sequences of interest, wherein each probe-pair comprises an activator fluorophore and a reporter fluorophore; (b) hybridizing, within said cell, a quantity of said mRNA sequences of interest with a plurality of said probe-pairs, wherein when the probe-pairs hybridize with said mRNA sequences of interest, the reporter and activator fluorophore of each probe-pair are in sufficiently close proximity to form a functional dye pair; and wherein each of the mRNA sequences of interest that is hybridized with said probe-pairs emits two or more distinct signals, so as to create the molecular barcode; (iii) resolving said molecular barcodes by resolving said signals emitted from said plurality of probe-pairs associated with each of said DNA and mRNA sequences of interest, wherein each emitted signal is a component of the barcode associated with each of said DNA or mRNA sequences of interest and wherein each DNA and mRNA sequence of interest is associated with a distinct barcode, so as to detect one or more DNA and mRNA sequences of interest in a single cell. In certain embodiments, the barcode is resolved using super resolution technology. In certain embodiments, the cell is essentially intact or undisrupted. In some embodiments, the cell is selected from the group consisting of a fungus cell, an animal cell, a mammalian cell, a mouse cell, a human cell, a cancer cell, a lymphocyte, an erythrocyte, a white blood cell, an epithelial cell, a pituitary cell, a gut cell, a respiratory tract cell, a gland cell, a thyroid gland cell, a parathyroid gland cell, an adrenal gland cell, a muscle cell, a ciliated cell, an embryonic cell, a sensory transducer cell, a neuron, a glial cell, a lens cell, a kidney cell, a pigment cell, and a pancreatic cell. In certain embodiments, the plurality of probes includes oligonucleotides. In some embodiments, the fluorophores are selected from the group consisting of fluorescein, rhodamine, Alexa Fluors, DyLight fluors, ATTO Dyes, and analogs or derivatives thereof. In some embodiments, the resolvable molecular barcode is a linear, two-dimensional or three-dimensional pattern of signals emitted from said plurality of probes. In certain embodiments, one or more barcodes are resolved using super resolution technology. In certain embodiments, the super resolution technology is selected from the group consisting of Stimulated Emission Depletion microscopy (STEDM), Ground State Depletion microscopy (GSDM), Spatially Structured Illumination microscopy (SSIM), Photo-Activated Localization Microscopy (PALM), Fluorescence-PALM (FPALM), Stochastical Optical Reconstruction Microscopy (STORM), Fluorescence Imaging with One-Nanometer Accuracy (FIONA), and combinations thereof. In various embodiments, the cell is acquired from a biopsy of a subject. In some embodiments, the method also includes diagnosing the subject with the presence or absence of cancer based upon (1) one or more characteristics of said one or more DNA sequences of interest determined by resolving one or more molecular barcode, wherein the one or more characteristics are selected from the group consisting of: presence of the sequence on the chromosome, absence of the sequence on the chromosome, location of the sequence on the chromosome relative to another DNA sequence, and abundance of the sequence; and/or (2) the presence or absence and/or abundance of one or more mRNA sequence of interest.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. Those of skill in the art will understand that the drawings, described below, are for illustrative purposes only. The drawings are not intended to limit the scope of the present teachings in any way.
All references cited herein are incorporated by reference in their entirety as though fully set forth. Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Singleton et al., Dictionary of Microbiology and Molecular Biology 3rd ed., J. Wiley & Sons (New York, N.Y. 2001); March, Advanced Organic Chemistry Reactions, Mechanisms and Structure 5th ed., J. Wiley & Sons (New York, N.Y. 2001); and Sambrook and Russel, Molecular Cloning: A Laboratory Manual 3rd ed., Cold Spring Harbor Laboratory Press (Cold Spring Harbor, N.Y. 2001), provide one skilled in the art with a general guide to many of the terms used in the present application.
One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present invention. Indeed, the present invention is in no way limited to the methods and materials described. For purposes of the present invention, certain terms are defined below.
As used herein, the term “an essentially intact or undisrupted cell” refers to a cell that is completely intact or largely conserved with respect to its macromolecular cellular content. For example, a cell within the meaning of this term can include a cell that is made at least partially permeable such that external buffers and/or reagents can be introduced into the cell. Such external reagents can include but are not limited to probes, labels, labeled probes, and/or combinations thereof.
As used herein, the term “cellular constituent” refers to any measurable biological variables that can be used in accordance with the present invention. Exemplary cellular constituents include but are not limited to any large biomolecules such as a DNA molecule or a fragment thereof, an RNA molecule or a fragment thereof, an mRNA molecule or a fragment thereof, a protein molecule or a fragment thereof, an mRNA complex or a section thereof, a protein complex or a section thereof, an organelle or a section thereof, or combinations thereof. Exemplary properties of cellular constituents include but are not limited to abundance level, location within a cell, abundance level or location with respect to other cellular constituents, relation to other cellular constituents, etc.
As used herein, the term “indicia” or “indicium” refers to any method, composition or system that can be associated with one or more cellular constituents to characterize at least one property of a cellular constituent at issue. As used herein, the term “molecular barcode” or “barcode” is used interchangeably with the term “indicium” or “indicia.” The process of creating the indicia or barcode can be referred to as a barcoding process.
As used herein, the term “probe” refers to any composition that can be specifically associated with a target cellular constituent within a cell. A probe can be a small molecular or a large molecule. Exemplary probes include but are not limited to nucleic acids such as oligos, peptides, proteins (such as antibodies), or hybrids thereof.
As used herein, the term “label” refers to any composition that can be used to generate the signals that constitute an indicium. The signals generated by a label can be of any form that can be resolved subsequently to constitute the indicium. Preferably, the signal is a light within the visible range. In some embodiments, the signal is a light not in the visible range. In some embodiments, the signal is a radio signal, an X-ray signal, or an electro-magnetic signal. However, it will be understood by one of skill in the art that equipment and devices are available for recording and monitoring light of virtually any wavelength.
As used herein, the term “biological state” is broadly defined to refer to a state, a characteristic, or a property that is associated with a cellular constituent. For example, it can be the number of copies of mRNA transcripts associated with a particular gene and the locations of these transcripts within a cell. It can also be the identity and location of other cellular constituents that interact or bind to the target cellular constituents. Exemplary biological states include but are not limited to sequence, size, abundance level, activity level, two-dimensional structure, three-dimensional structure, charged state, surface accessibility, location within the cellular context, binding affinity and specificity to another cellular constituent, or a combination thereof.
Two sets of orthogonal systems biology approaches, large scale and small scale techniques, have traditionally been undertaken to elucidate the cellular interactions and biochemical networks. The large scale systems approaches, exemplified by microarrays and sequencing techniques, can profile comprehensively the transcriptional and the genomic state of a population of cells. On the other hand, the small scale systems approaches, for example, those using fluorescence microscopy techniques, can examine key genetic and regulatory interactions in individual cells while preserving the spatial context of the interactions.
The present invention combines the advantages of both approaches to bring the power of genomics into single cells. In one aspect, the methods and systems described herein provide unprecedented resolution in molecular interactions occurring in biological networks. In one aspect, the methods and systems described herein provide important clinical tools in identifying molecular signatures of disease. In another aspect, the methods and systems described herein can revolutionize the day-to-day experimental routines in the field of biological sciences, including but not limited to cell biology, molecular biology, biochemistry, biophysics and chemistry.
One of skill in the art would understand that most of the methods and systems described herein are applicable to all types of cells, including but not limited to bacteria, archaea, protists, fungi, plant, and animal cells, especially mammalian cells, mouse cells, human cells. Exemplary human cells include but are not limited to cancer cells, blood cells, lymphocytes, erythrocytes, white blood cells, epithelial cells, pituitary cells, gut and respiratory tract cells, various gland cells (e.g., thyroid, parathyroid, or adrenal glands), muscle cells, ciliated cells, embryonic cells, sensory transducer cells, various neuron cells, glial cells, lens cells, kidney cells, pigment cells, pancreatic cells, combinations thereof, etc.
In one aspect, the power of genomics is brought into single cells via the creation of molecular barcodes or indicia, where barcodes or indicia are associated with specific cellular constituents.
Taking the high-throughput approach into a single cell may enable powerful exploration in many biological systems. However, multiplex detection of molecular species in single cells faces several fundamental challenges. First, individual cells need to be isolated. Then, molecules within those cells need to be separated, identified and quantified. Optical microscopy circumvents the need to isolate individual cells, but limits molecular discrimination, as molecules cannot be resolved beyond the diffraction limit (−300 nm). Referring to
The present methods and systems of molecular barcoding are used to illustratively detect multiple mRNA species in single Saccharomyces cerevisiae cells (Table 1).
As described above, a molecular barcode is broadly defined as a form of indicia that can be used to determine the identity or any other characteristics and/or properties of a cellular constituent in a cell. The methods and systems described herein can be used for research, diagnostic, prognostic or any other purposes. The cell can be a prokaryotic cell or a eukaryotic cell. For example, barcoding can be done in simple model organisms such as E. coli or yeast to monitor and study processes that include but are not limited to transcription, translation, protein folding, and protein-trafficking. Alternatively, methods and systems of barcoding can be used in more advanced organisms such as animal and human cells, for example, to determine a complicated structure of molecular complexes, to dissect a signaling pathway, to characterize alternative splicing events, or to monitor and quantify changes within the cells.
In some embodiments, a molecular barcode or indicium comprises a visual component, for example, in the form of a combination of different visible colors affixed to labels that are in turn affiliated with probes bound to one or more target cellular constituents in a cell. In some embodiments, the color type and frequency of these labeled probes are used to create a molecular barcode. In some embodiments, the color type, frequency and position of these labeled probes are used to create a molecular barcode.
Referring to
Exemplary barcodes associated with transcripts mRNA-1, mRNA-2 and mRNA-3 are described further to illustrate the concept of molecular barcoding. In particular, barcode 1, which corresponds to mRNA-1 transcript, comprises five oligo nucleotide probes that each bind to a specific segment of the mRNA-1 transcript, including three probes with label A and two probes with label B. In this example, label A is red and label B is blue. The combined effect of three blue labels and two copies of red label is depicted as purple-like color and revealed by super resolution imaging to correspond to an arrangement of Red-Blue-Red-Red-Blue. In a different example, one probe with red label A, three probes with blue label B and two probes with green label C constitute barcode 2, which is used to represent mRNA-2. Barcode-2 is read under super resolution imaging to correspond to an arrangement of Blue-Blue-Green-Blue-Green-Red. In still another example, Barcode-3 is read under super resolution imaging to correspond to an arrangement of Red-Blue-Green-Green-Blue-Red.
As illustrated above, the types of signals (e.g., color) associated with each probe, the frequency and arrangement of these labeled probes can all be used to define a molecular barcode. Although no obvious gaps are present in the exemplary barcodes in
In some embodiments, in addition to the more or less linear order arrangements described above, more complex arrangements of the colored probes can also be used to define a molecular barcode associated with a particular cellular constituent. Exemplary non-linear arrangements include, for example, two-dimensional grids, maps, or three-dimensional lattices.
In some embodiments, for a less linear cellular constituent such as a protein, more complex spatial arrangements are needed to create the corresponding molecular barcode. For example, labeled antibodies targeting surface epitopes can be used to create one or more maps that uniquely identify the protein at issue. One of skill in the art would understand that a molecular barcode for a protein relies on knowledge of the structure of the protein at issues, the distribution of surface epitopes, as well as the availability of antibodies targeting such surface epitopes.
In some embodiments, small synthetic antibodies are used as probes when targeting one or more proteins. Synthetic antibody libraries have proven immensely useful for the de novo isolation of antibodies without the need for animal immunization. Focused libraries designed to recognize particular classes of ligands, such as haptens or proteins, have been employed to facilitate the selection of high affinity antibodies. Focused libraries are built using V regions encoding combinations of canonical structures that resemble the structural features of antibodies that bind the desired class of ligands and sequence diversity is introduced at residues typically involved in recognition. Synthetic antibodies are generated and experimentally validated with different scFv libraries that efficiently generate binders to peptides, a class of molecules that has proven to be a difficult target for antibody generation. Diversity was introduced in the VH using the profile of amino acids found at positions that frequently contact peptide antigens. Both libraries yielded binders to two model peptides, angiotensin and neuropeptide Y, following screening by solution phage panning. In particular, mouse libraries yielded antibodies with high affinities (e.g., below 20 nM) to both targets even though only the VH had been subjected to diversification.
In some embodiments, nucleic acids capable of binding to specific labels are attached to the natural or synthetic antibodies to generate the signals that ultimately create the molecular barcodes and/or indicia. In some embodiments, one or more secondary antibodies are used to generate the signals.
In some embodiments, synthetically evolved small peptides are used as “synthetic antibodies.” The peptides have nanomolar affinity to target proteins and can be around 10 amino acids or longer; around 12 amino acids or longer; around 15 amino acids or longer; around 18 amino acids or longer; around 20 amino acids or longer; around 22 amino acids or longer; around 25 amino acids or longer; around 30 amino acids or longer; around 35 amino acids or longer; around 40 amino acids or longer; around 50 amino acids or longer; around 60 amino acids or longer; around 80 amino acids or longer; around 100 amino acids or longer; around 120 amino acids or longer; around 150 amino acids or longer; around 180 amino acids or longer; around 200 amino acids or longer; around 250 amino acids or longer; around 300 amino acids or longer around 400 amino acids or longer; or around 500 amino acids or longer.
More details on synthetic antibodies applicable to the present methods and systems can be found, for example, in Cobaugh et al., 2008, “Synthetic Antibody Libraries Focused Towards Peptide Ligands,” J Mol. Biol. 378(3): 622-633; Benhar I. 2007, “Design of synthetic antibody libraries,” Expert Opin Biol Ther. 7(5):763-779; Nahary and Benhar, 2009, “Design of a human synthetic combinatorial library of single-chain antibodies,” Methods Mol. Biol. 525:61-80; Bostrom and Fuh, 2009, “Design and construction of synthetic phage-displayed Fab libraries,” Methods Mol. Biol. 562:17-35; Fellouse et al., 2004, “Synthetic antibodies from a four-amino-acid code: A dominant role for tyrosine in antigen recognition,” Proc. Natl. Acad. Sci. U.S.A. 101(34):12467-12472; Agnew et al., 2009, “Iterative in situ click chemistry creates antibody-like protein-capture agents,” Angew Chem Int Ed Engl. 48(27):4944-4948; Rohde et al., 2006, “A non-oxidative approach toward chemically and electrochemically functionalizing Si(111),” J Am Chem. Soc. 128(29):9518-9525; each of which (including Supplemental Material) is hereby incorporated by reference herein in its entirety.
In some embodiments, aptamers can be used as probes to bind to cellular constituents, especially proteins. Aptamers are oligonucleic acid or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. More specifically, aptamers can be classified as: DNA or RNA aptamers, which comprise (usually short) strands of oligonucleotides and peptide aptamers, which comprise a short variable peptide domain, attached at both ends to a protein scaffold.
Nucleic acid aptamers are nucleic acid species that have been engineered through repeated rounds of in vitro selection or equivalently, SELEX (systematic evolution of ligands by exponential enrichment) to bind to various molecular targets such as small molecules, proteins, nucleic acids, and even cells, tissues and organisms. Aptamers are useful in biotechnological and therapeutic applications as they offer molecular recognition properties that rival that of the commonly used biomolecule, antibodies. In addition to their discriminate recognition, aptamers offer advantages over antibodies as they can be engineered completely in a test tube, are readily produced by chemical synthesis, possess desirable storage properties, and elicit little or no immunogenicity in therapeutic applications.
Peptide aptamers are proteins that are designed to interfere with other protein interactions inside cells. They consist of a variable peptide loop attached at both ends to a protein scaffold. This double structural constraint greatly increases the binding affinity of the peptide aptamer to levels comparable to an antibody's (nanomolar range).
The variable loop length is typically composed of ten to twenty amino acids, and the scaffold may be any protein which has good solubility and compacity properties. Currently, the bacterial protein Thioredoxin-A is the most used scaffold protein, the variable loop being inserted within the reducing active site, which is a -Cys-Gly-Pro-Cys- loop in the wild protein, the two Cysteines lateral chains being able to form a disulfide bridge. Peptide aptamer selection can be made using different systems, but the most used is currently the yeast two-hybrid system. Selection of Ligand Regulated Peptide Aptamers (LiRPAs) has been demonstrated. By displaying 7 amino acid peptides from a novel scaffold protein based on the trimeric FKBP-rapamycin-FRB structure, interaction between the randomized peptide and target molecule can be controlled by the small molecule Rapamycin or non-immunosuppressive analogs.
In some embodiments, AptaBiD or Aptamer-Facilitated Biomarker Discovery is adopted for probe design. AptaBiD is based on multi-round generation of an aptamer or a pool of aptamers for differential molecular targets on the cells which facilitates exponential detection of biomarkers. It involves three major stages: (i) differential multi-round selection of aptamers for biomarker of target cells; (ii) aptamer-based isolation of biomarkers from target cells; and (iii) mass spectrometry identification of biomarkers. The important feature of the AptaBiD technology is that it produces synthetic affinity probes (aptamers) simultaneously with biomarker discovery. In AptaBiD, aptamers are developed for cell surface biomarkers in their native state and conformation. In addition to facilitating biomarker identification, such aptamers can be directly used for cell isolation, cell visualization, and tracking cells in vivo. They can also be used to modulate activities of cell receptors and deliver different agents (e.g., siRNA and drugs) into the cells.
In some embodiments, the aptamer probes themselves comprise labels that can generate signals that create the molecular barcodes or indicia. In some embodiments, secondary or even tertiary labels are used to generate signals that create the molecular barcodes or indicia.
Additional information on aptamer can be found, for example, in Ellington, et al., 1990, “In vitro selection of RNA molecules that bind specific ligands,” Nature 346 (6287): 818-822; Bock, et al., 1992, “Selection of single-stranded DNA molecules that bind and inhibit human thrombin,” Nature 355 (6360): 564-566; Hoppe-Seyler, et al., 2000, “Peptide aptamers: powerful new tools for molecular medicine,” J Mol. Med. 78 (8): 426-430; Carothers, et al., 2004, “Informational complexity and functional activity of RNA structures,” J Am Chem. Soc. 126 (16): 5130-5137; Cohen et al., 1998, “An artificial cell-cycle inhibitor isolated from a combinatorial library,” Proc Natl Acad Sci USA. 95 (24): 14272-14277; Binkowski et al., 2005, “Ligand-regulated peptides: a general approach for modulating protein-peptide interactions with small molecules,” Chem. Biol. 12(7):847-855; Sullenger et al., 2002, “Emerging clinical applications of RNA,” Nature 418 (6894): 252-258; Ng E W et al., 2006, “Pegaptanib, a targeted anti-VEGF aptamer for ocular vascular disease,” Nat Rev Drug Discov 5 (2): 123-132; Drabovich et al., 2006, “Selection of smart aptamers by methods of kinetic capillary electrophoresis,” Anal Chem. 78 (9): 3171-3178; Cho et al., 2009, “Applications of Aptamers as Sensors,” Annual Review of Analytical Chemistry 2(1): 241-264; each of which (including Supplemental Material) is hereby incorporated by reference herein in its entirety.
The complexity of a particular barcode or a particular set of barcodes is determined by the ultimate purpose for which the barcodes are intended. For example, if the barcodes are used for profiling of large number of cellular constituents, the barcodes will be more sophisticated. For example, there will be more colors within each respective barcodes. The barcodes will comprise larger number of labeled probes. In addition, there will be more variations in linear frequencies and/or spatial arrangement of the labeled probes with respect to each barcode for large scale profiling type analysis. Alternatively, if the barcodes are intended for quantifying one or more particular cellular constituents or analyzing the interactions between specific cellular constituents, care will be taken to ensure accuracy by using redundant probes and multiplicity in barcoding. For example, probes bound to the same or overlapping region of a particular mRNA transcript can be tagged with different types of labels to provide redundancy data to improve accuracy and precision.
In some embodiments, the same cellular constituent can be represented by multiple barcodes. In some embodiments, only selected regions of a cellular constituent is used in creating a barcode. In some embodiments, the entire cellular constituent is used in creating a barcode.
In some embodiments, multiple probes carrying the same type of label are used in creating a barcode/indicium. This design results in differences in intensity of signals observed for different types of signals.
In some embodiments, probes are designed to stabilize the target cellular constituents, thus increasing the persistence length and stiffening the probe-target cellular constituent complex. The strategy of “spatial barcoding” is depicted in
One of skill in the art would understand that, although described in connection with mRNA transcripts, the strategies of intensity barcoding and spatial barcoding are applicable to other cellular constituents.
Additional details on probe design and labeling probes can be found herein below.
As described above, an important aspect of creating a molecular barcode is the selection and creation of the probes that specifically recognize a target cellular constituent. As illustrated above, when the target cellular constituent is an mRNA transcript, the probes that are used to recognize and bind to the mRNA transcript are oligonucleotides, or “oligos.” In some embodiments, the oligo probes are 10-mers or longer. In some embodiments, the oligo probes are 15-mers or longer. In some embodiments, the oligos are 20-mers or longer; 25-mers or longer; 30-mers or longer; 40-mers or longer; 50-mers or longer; 70-mers or longer; 100-mers or longer; 150-mers or longer; 200-mers or longer; 250-mers or longer; 300-mers or longer; 500-mers or longer; or 1,000-mers or longer.
In some embodiments, the oligo probes are designed by using complementary sequences to randomly selected sequences or segments of sequences in a target cellular constituent. In some embodiments, the target cellular constituent is an mRNA transcript. In some embodiments, the target cellular constituent is an mRNA isoform.
In some embodiments, the oligo probes are designed by deliberately selecting sequences or segments of sequences that bind to a target cellular constituent with known or predicted binding affinity. This is called “intelligent probe design,” where structure, sequence and biochemical data are all considered to create probes that will likely have better binding properties to a target cellular constituent. In particular, the preferred regions in a target cellular constituent are either identified experimentally or predicted by algorithms based on experimental data or computation data. For example, computed binding energy and/or theoretical melting temperature can be used as selection criteria in intelligent probe design. In certain embodiments, the cellular constituent is an mRNA transcript. In some embodiments, the cellular constituent is an mRNA isoform. In some embodiments, the mRNA is associated with a gene involved in neural patterning or tumorigenesis.
Tools are available for automated designs of probes that will have either actual or predicted optimal binding properties to the target cellular constituents. For example, the Designer program is routinely used for designing probes that bind to a particular target RNA sequence as part of the established single molecule RNA Fluorescent in-situ hybridization technology (FISH), which was developed at the University of Medicine and Dentistry of New Jersey (UMDNJ). For the Designer program, the open reading frame (ORF) of the gene of interest is typically used as input. This approach is used to exclude the more repetitive regions and low complexity sequence contained in Un-translated Regions (UTRs). Probes are designed to minimize deviations from the specified target GC percentage. The program will output the maximum number of probes possible up to the number specified. Sequence input is stripped of all non-sequence characters. A user can specify parameters such as the number of probes, target GC content, length of oligonucleotide and spacing length. Most success has been achieved with target GC contents of 45%. Typically, oligos are designed as 20 nucleotides in length and are spaced a minimum of two nucleotides apart.
To ensure accuracy, three major design considerations to target coding sequences of gene transcripts are used 1) mRNA length, 2) repeat sequences and 3) sequences of low complexity (such as GC content). As for length, the number of probes that can be accommodated for robust detection is primarily sequence dependent. Naturally longer RNA transcripts would require more probes or longer probes to ensure accuracy. Repetitive elements and low complexity sequences must be screened in advance and manually removed, further restricting the sequence space available for design. For this reason, certain genes such as the keratins may present unusual challenges. The optimal target has an overall GC content of 40-50%, although mRNAs with higher GC content may yield good results by using more stringent washing conditions.
The Stellaris RNA FISH method is applicable to a variety of biological specimens, including but not limited to bacteria, yeast, mammalian cells, C. elegans embryos and L1-L2 larvae, Drosophila melanogaster wing imaginal discs, and primary rat hippocampal neurons.
Additional description of single molecule FISH can be found in, for example, Raj A., et al., 2008, “Imaging individual mRNA molecules using multiple singly labeled probes,” Nature Methods 5(10): 877-879; Femino A., et al., 1998, “Visualization of single RNA transcripts in situ,” Science 280: 585-590; Vargas D., et al., 2005, “Mechanism of mRNA transport in the nucleus,” Proc. Natl. Acad. Sci. of USA 102: 17008-17013; Raj A., et al., 2006, “Stochastic mRNA synthesis in mammalian cells,” PLoS Biology 4(10):e309; Maamar H., et al., 2007, “Noise in gene expression determines cell fate in B. subtilis,” Science, 317: 526-529; and Raj A., et al., 2010 “Variability in gene expression underlies incomplete penetrance,” Nature 463:913; each of which (including any Supplemental Material) is hereby incorporated by reference herein in its entirety.
The rationale of intelligent probe design also applies to probes that are not nucleic acids, such as proteins. Given the three-dimensional nature of protein molecules, intelligent designs of antibody probes that would bind to a target cellular constituent can be more challenging. For example, protein structures and known epitope data or prediction algorithms will be considered to identify accessible surface epitopes that will likely combine to create an indicium or molecular barcode that is associated with the target cellular constituent and can be used to identify it within a cellular environment or to determine properties associated with the target cellular constituent, such as location within the cell and possible binding partners.
In some embodiments, protein indicia can be identified by in vitro analysis. For example, purified protein samples can be conjugated with one or more labeled antibodies. The locations of these antibodies can then be determined by electron microscopy, X-ray diffraction, or combined methods. Electron microscopes (EM) have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than visible light (photons), and can achieve better than 0.2 nm resolution and magnifications of up to 2,000,000×. The pre-determined indicia can then be used as standards (or positive controls) to assist the identification of molecular indicia or barcodes of proteins in a cell using the super resolution technologies of the present invention.
In some embodiments, the present methods and systems can be used to carry out whole cell labeling of nucleic acids and proteins. Previously, over 1,000 genes were each tagged individually in different cells with fluorescent labels, and then protein and mRNA copies in individual cells were counted using a high-throughput system. In another example, over 7,000 genes were classified by applying FISH to one gene one at a time in the Berkeley fly genome project. See, for example, insitu<dot>fruitfly<dot>org; Taniguchi Y., et al., 2010, “Quantifying E. coli proteome and transcriptome with single-molecule sensitivity in single cells,” Science 329:533-538; Tomancak et al., 2002, “Systematic determination of patterns of gene expression during Drosophila embryogenesis,” Genome Biol. 3(12):1-14; Tomancak et al., 2007, “Global analysis of patterns of gene expression during Drosophila embryogenesis,” Genome Biol. 8(7):R145; each of which (including any Supplemental Material) is hereby incorporated by reference herein in its entirety. By using different molecular barcodes, the present systems and methods all allow one to label multiple cellular constituents in the same cell.
It will be understood by one of skill in the art that indicia corresponding to longer, larger or more complex cellular constituents require more sophisticated combination of probes. For example, probes of longer oligonucleotides or more probes are needed to recognize and distinguish mRNA transcripts bearing similar sequences. Similarly, distinguishing proteins bearing similar structural or functional domains will also require more complex indicia.
It will be understood by one of skill in the art that the current methods and systems can be applied to a combination of cellular constituents. For example, DNA, RNA and protein can be labeled and analyzed in one single experiment.
One of skill in the art would also understand that length or size of probes will vary, depending on the target cellular constituents and purposes of the analysis.
Labels are associated with the specific probes to allow them to emit signals that will be used in subsequence super resolution analysis. Any labels suitable for generating such signals can be used in the present invention. In some embodiments, the signals are generated by fluorophores. Fluorescent labeling, e.g., the process of covalently attaching a fluorophore to a probe that binds to a cellular constituent (such as a protein or nucleic acid) is generally accomplished using a reactive derivative of the fluorophore that selectively binds to a functional group contained in the target molecule. In some embodiments, exemplary probes to which the labels are attached include but are not limited to antibodies, proteins, amino acids and peptides. Common reactive groups include amine reactive isothiocyanate derivatives such as FITC and TRITC (derivatives of fluorescein and rhodamine), amine reactive succinimidyl esters such as NHS-fluorescein, and sulfhydryl reactive maleimide activated fluors such as fluorescein-5-maleimide.
Following a fluorescent labeling reaction, it is often necessary to remove any non-reacted fluorophore from the labeled target molecule. This is often accomplished by size exclusion chromatography, taking advantage of the size difference between fluorophore and labeled protein, nucleic acid, etc. Fluorophores may interact with the separation matrix and reduce the efficiency of separation. For this reason, specialized dye removal columns that account for the hydrophobic properties of fluorescent dyes are sometimes used. Reactive fluorescent dyes are available from many sources. They can be obtained with different reactive groups for attachment to various functional groups within the target molecule. They are also available in labeling kits that contain all the components to carry out a labeling reaction.
In some embodiments, labels of the present invention comprise one or more fluorescent dyes, including but not limited to fluorescein, rhodamine, Alexa Fluors, DyLight fluors, ATTO Dyes, or any analogs or derivatives thereof.
In some embodiments, labels of the present invention include but are not limited to fluorescein and chemical derivatives of fluorescein; Eosin; Carboxyfluorescein; Fluorescein isothiocyanate (FITC); Fluorescein amidite (FAM); Erythrosine; Rose Bengal; fluorescein secreted from the bacterium Pseudomonas aeruginosa; Methylene blue; Laser dyes; Rhodamine dyes (e.g., Rhodamine, Rhodamine 6G, Rhodamine B, Rhodamine 123, Auramine O, Sulforhodamine 101, Sulforhodamine B, and Texas Red).
In some embodiments, labels of the present invention include but are not limited to ATTO dyes; Acridine dyes (e.g., Acridine orange, Acridine yellow); Alexa Fluor; 7-Amino actinomycin D; 8-Anilinonaphthalene-1-sulfonate; Auramine-rhodamine stain; Benzanthrone; 5,12-Bis(phenylethynyl)naphthacene; 9,10-Bis(phenylethynyl)anthracene; Blacklight paint; Brainbow; Calcein; Carboxyfluorescein; Carboxyfluorescein diacetate succinimidyl ester; Carboxyfluorescein succinimidyl ester; 1-Chloro-9,10-bis(phenylethynyl)anthracene; 2-Chloro-9,10-bis(phenylethynyl)anthracene; 2-Chloro-9,10-diphenylanthracene; Coumarin; Cyanine dyes (e.g., Cyanine such as Cy3 and Cy5, DiOC6, SYBR Green I); DAPI, Dark quencher, DyLight Fluor, Fluo-4, FluoProbes; Fluorone dyes (e.g., Calcein, Carboxyfluorescein, Carboxyfluorescein diacetate succinimidyl ester, Carboxyfluorescein succinimidyl ester, Eosin, Eosin B, Eosin Y, Erythrosine, Fluorescein, Fluorescein isothiocyanate, Fluorescein amidite, Indian yellow, Merbromin); Fluoro-Jade stain; Fura-2; Fura-2-acetoxymethyl ester; Green fluorescent protein, Hoechst stain, Indian yellow, Indo-1, Lucifer yellow, Luciferin, Merocyanine, Optical brightener, Oxazin dyes (e.g., Cresyl violet, Nile blue, Nile red); Perylene; Phenanthridine dyes (Ethidium bromide and Propidium iodide); Phloxine, Phycobilin, Phycoerythrin, Phycoerythrobilin, Pyranine, Rhodamine, Rhodamine 123, Rhodamine 6G, RiboGreen, RoGFP, Rubrene, SYBR Green I, (E)-Stilbene, (Z)-Stilbene, Sulforhodamine 101, Sulforhodamine B, Synapto-pHluorin, Tetraphenyl butadiene, Tetrasodium tris(bathophenanthroline disulfonate)ruthenium(II), Texas Red, TSQ, Umbelliferone, or Yellow fluorescent protein.
In some embodiments, labels of the present invention include but are not limited to the Alexa Fluor family of fluorescent dyes (Molecular Probes, Oregon). Alexa Fluor dyes are typically used as cell and tissue labels in fluorescence microscopy and cell biology. The excitation and emission spectra of the Alexa Fluor series cover the visible spectrum and extends into the infrared. The individual members of the family are numbered according roughly to their excitation maxima (in nm). Alexa Fluor dyes are synthesized through sulfonation of coumarin, rhodamine, xanthene (such as fluorescein), and cyanine dyes. Sulfonation makes Alexa Fluor dyes negatively charged and hydrophilic. Alexa Fluor dyes are generally more stable, brighter, and less pH-sensitive than common dyes (e.g. fluorescein, rhodamine) of comparable excitation and emission, and to some extent the newer cyanine series. However, they are also more expensive. Exemplary Alexa Fluor dyes include but are not limited to Alexa-350, Alexa-405, Alexa-430, Alexa-488, Alexa-500, Alexa-514, Alexa-532, Alexa-546, Alexa-555, Alexa-568, Alexa-594, Alexa-610, Alexa-633, Alexa-647, Alexa-660, Alexa-680, Alexa-700, or Alexa-750.
In some embodiments, labels of the present invention comprise one or more members of the DyLight Fluor family of fluorescent dyes (Dyomics and Thermo Fisher Scientific). Exemplary DyLight Fluor family dyes include but are not limited to DyLight-350, DyLight-405, DyLight-488, DyLight-549, DyLight-594, DyLight-633, DyLight-649, DyLight-680, DyLight-750, or DyLight-800.
In some embodiments, when pairs of dyes are used (as described in greater detail herein below) the activator choices include Alexa405, 488, 532 and 568, and the emitter choices include Cy5, Cy5.5, Cy7, and 7.5. Using these particular choices, because they can be mixed and matched to give functional dye pairs, there are 16 possible pairs (4×4) in all. In some embodiments, for RNA FISH, emitters used are Alexa 647 or Dynomics 632, Cy5.5, Cy7, and IR800CW. In some embodiments, for DNA FISH, they are Alexa647, Cy5.5, Alexa 750 and Alexa 790.
In some embodiments, the same type of labels can be attached to different probes for different types of cellular constituents, including nucleic acids and proteins.
For example, in some embodiments, DNA or RNA probes are labeled with either Cy3 or Cy5 that has been synthesized to carry an N-hydroxysuccinimidyl ester (NHS-ester) reactive group. Since, NHS-esters react readily only with aliphatic amine groups, which nucleic acids lack, nucleotides have to be modified with aminoallyl groups. This can be done through incorporating aminoallyl-modified nucleotides during synthesis reactions. In some embodiments, a label is used in every 60 bases to avoid quenching effects.
For example, in some embodiments, protein probes (e.g., antibodies) are also labeled with either Cy3 or Cy5. For protein labeling, Cy3 and Cy5 dyes sometimes bear maleimide reactive groups instead. The maleimide functionality allows conjugation of the fluorescent dye to the sulfhydryl group of cysteine residues. Cysteines can be added and removed from the protein domain of interest via PCR mutagenesis. Cy5 is sensitive to the electronic environment in which it resides. Changes in the conformation of the protein to which the label is attached can produce an enhancement or quenching of the emission. The rate of this change can be measured to determine enzyme kinetic parameters. Cy3 and Cy5 are used in proteomics experiments so that samples from two sources can be mixed and run together thorough the separation process. This eliminates variations due to differing experimental conditions that are inevitable if the samples were run separately. These variations make it extremely difficult, if not impossible, to use computers to automate the acquisition of the data after the separation is complete. Importantly, using these dyes makes the automation trivial.
One of skill in the art would readily appreciate that choices for a label are determined based on a variety of factors, including, for example, size, types of signals generated, methods of attachment to or incorporation into a probe, properties of the cellular constituents, including their locations within the cell, properties of the cells, types of interactions being analyzed, and so forth.
Incorporating Labels into Probes
In some embodiments, labels such as fluorophores are attached to the probes as a secondary addition. In these embodiments, the probes are synthesized or formed prior to the addition of the labels. In some embodiments, labels such as fluorophores are attached to specific locale of the probes. For example, pre-synthesized probes (e.g., oligonucleotides or peptides) are mixed with fluorophores under predefined reaction conditions such that attachment of the fluorophores to the probes results.
In some embodiments, labels are embedded within the probes themselves. In these embodiments, one or more labels are incorporated into probes while they are being synthesized or formed. For example, a fluorophore can be embedded in an oligonucleotide probe during synthesis. In some embodiments, one or more labels (e.g., fluorophores) are attached to multiple identical probes (e.g., oligos with identical sequences).
In some embodiments, different labels (e.g., fluorophores) are attached to multiple identical probes (e.g., oligos with identical sequences). For example, multiple indicia can be created with minor variations in signal arrangement for the same cellular constituent. Such near-redundancy or near-duplicity is used to ensure accuracy of barcoding. In some embodiments, the same label (e.g., fluorophores emitting red light at the same wavelength) is attached to multiple identical probes (e.g., oligos with identical sequences).
In some embodiments, for example, when aptamers are used as probes, signal-emitting labels are added in a secondary or tertiary step. For example, aptamers that specifically bind to a protein are first synthesized. Complementary DNA oligos, which already have labels attached or embedded, are added later to allow binding to the DNA element in the aptamers. In such embodiments, signal-emitting labels are not directly associated with the probes, but through an intermediary composition—the DNA molecule in an aptamer. Similarly, intermediary binding partners of a peptide element or an RNA element of an aptamer can also be used to affixing labels that will emit the signals for the molecular barcodes or indicia.
As used herein, an intermediary composition is any molecule or structure to which a label can be attached or embedded to form the final molecular barcodes or indicia. Exemplary intermediary compositions include but are not limited to a DNA or derivatives thereof, an RNAs or derivatives thereof, DNA-RNA hybrids or derivatives thereof, a peptide or analogs thereof,
In some embodiments, multiple intermediary compositions can be used to permit final attachment of labels that emit the signals for the molecular barcodes or indicia. For example, the number of intermediary composition used can be one, two, three, four, five, six, seven or more, or ten or more.
In some embodiments, when fluorophores are used as labels, a spare, optically resolvable subset of fluorophores is selectively activated by using photo-switchable fluorophores. For example, multicolor super resolution imaging can be done with a family of photo-switchable fluorescent probes, using Stochastic Optical Reconstruction Microscopy (STORM), as described and demonstrated in the experiments set forth in the “Examples” section herein. Some of the commonly used fluorophores, such as Cyanine dyes can undergo reversible photoswitching, where the fluorophore can be switched between a fluorescent state and a dark state upon exposure to light of different wavelengths. The rate of switching to the dark state depends on the concentration of the primary thiol in the solution and the solution pH in a manner quantitatively consistent with the formation of an encounter complex between the cyanine dye and ionized thiol prior to their conjugation. Mass spectrometry suggests that the photo-conversion product is a thiol-cyanine adduct in which covalent attachment of the thiol to the polymethine bridge disrupts the original conjugated 7r-electron system of the dye. In particular, Cy5 has demonstrated such “optical switching” properties: its fluorescence emission can be switched on and off using pulses of light. During each excitation, Cy5 emits thousands of photons before going dark. A brief pulse of ultraviolet light will then efficiently reactivate the molecule to its fluorescent state, and this process can be repeated for hundreds of cycles. The switchable fluorescence exhibited by Cy5 is a strongly nonlinear process, and this nonlinearity can be used to overcome the diffraction limit of resolution.
In some embodiments, a probe is associated with a photo-switchable “reporter” fluorophore (such as those described above) that can be cycled between fluorescent and dark states, and an “activator” that facilitates photo-activation of the reporter. In some embodiments, pairs of reporter-activator fluorophores are used as labels, as demonstrated in the “Examples” section. Combinatorial pairing of reporters and activators allows the creation of probes with many distinct colors. Iterative, color-specific activation of sparse subsets of these probes allows their localization with nanometer accuracy, enabling the construction of a super-resolution STORM image. Using this approach, multicolor imaging of DNA model samples and mammalian cells can be done with 20- to 30-nanometer resolution.
In some embodiments, pools of probes are use against multiple target cellular constituents using a tiered strategy (Table 1 and
Additional details concerning photo-switchable fluorophores used as labels can be found herein in the “Examples” section. More details on switchable fluorophores can be found, for example, in Bates et al., 2007, “Multicolor Super-Resolution Imaging with Photo-Switchable Fluorescent Probes,” Science 317(5845): 1749-1753 and Supplemental Online Material, and Dempsey et al., 2009, “Photoswitching Mechanism of Cyanine Dyes,” J. Am. Chem. Soc., 131(51): 18192-18193, each of which (including any Supplemental Material) is hereby incorporated by reference herein in its entirety.
Labeled probes are subsequently introduced into the cell and hybridized to target cellular constituents. In general, in situ hybridization of yeast cells is almost identical to mammalian cells, except that the cell wall has to be removed by spheroplasting the yeast cells prior to hybridization. Additional details can be found, for example, in Long R M, et al., 1995, RNA (10): 1787-1794 and at singlemoleculefish<dot>com/protocols<dot>html, each of which (including any Supplemental Material) is hereby incorporated by reference herein in its entirety.
In some embodiments, a hybridization process comprises the steps of probe preparation, fixation, hybridization, washing, and mounting. In some embodiments, RNAse treatment and antibody detection are also included.
One of skill in the art would understand that hybridization conditions of the probes to target cellular constituents changes with respect to the specific purposes for which a barcoding method/system is used.
In some embodiments, signals from cellular constituents bearing multi-signal molecular barcodes are recorded at the same time. In some embodiments, signals from cellular constituents bearing multi-signal molecular barcodes are recorded at different times, one signal at a time. Data collected from multiple channels can be combined to produce one or more composite images. One of skill in the art would readily appreciate that cross-talking among labels can be reduced by modifying the probe design, types of label attached, use of equipment with better resolution, or by improving the methods or algorithms by which the data are processed.
In some embodiments, drifts in multichannel experiments are corrected by using fluorescent beads as fiducial markers. These beads can be localized to 1-2 nm using Gaussian fitting, and can be used accurately to correct for small changes in stage positions. In some embodiments, where multi-imaging channels are used, gold nanoparticles can be used to correct for chromatic aberrations. In some embodiments, some magnetic beads are used to correct drifting in multichannel experiments. Additional details on such technologies can be found, for example, in Shroff et al., 2007, “Dual-color super-resolution imaging of genetically expressed probes within individual adhesion complexes,” Proc. Natl. Acad. Sci. USA 104(51):20308-20313 and Bates et al., 2007, “Multicolor Super-Resolution Imaging with Photo-Switchable Fluorescent Probes,” Science 317(5845): 1749-1753, which (including any Supplemental Material) is incorporated by reference herein in its entirety.
In some embodiments, Cy5.5 or Cy7 based dyes are used to increase the available colors to at least 6. Incorporating Spiroamides can add additional colors to the palette, allowing drastically enhanced multiplex capabilities. Introduction of these dyes would require chromatic corrections on the STORM reconstructed images. In addition, two color repeat barcodes can be used, depending mainly on the photophysical properties of the Cy5 based STORM dyes. The relatively poor contrast ratio (1:200) of the Cy5 based dyes means that stronger than ideal activation power is needed to overcome the non-specific blinking rate of Cy5. This increases the frequency of multiple activation of fluorophores within the same diffraction limited volume, and generates spatial blurring in between the repeat coding position. Thus for the repeat barcode experiments, the activation power is tuned to be above the non-specific activation rate, but lower than the rate that would generate multiple activations within the same diffraction limited spot.
In some embodiments, only simple three-color barcodes are implemented where higher activation intensity and multiple switching events in one color do not distort the barcode image to avoid complications caused by the nonspecific blinking of dyes. In some embodiments, the axial dimensions of the fluorophores are resolved. The approach of using astigmatic or dumbbell shaped point-spread-function can improve the axial resolution to 50 nm, which can be helpful to resolve 2 barcoded mRNAs if they overlap in the xy but not z dimension. In some embodiments, interferometric PALM can be used to further resolve axial resolution, e.g., to 5 nm. The ultimate limiting factor in the multiplexing is the contrast ratio of Cy5 dyes. It limits the labeling density as no more than 100 Cy5 molecules can be in the same diffraction limited volume for super-resolution reconstruction and prevents the effective usage of the repeat barcodes.
In some embodiments, techniques are used to achieve z-resolution on super resolution scale, for example, on the nanometer scale. The z-resolution is generally defined as the optical thickness of the optical z-plane. Methods for improving z-resolution are known in the art and can be applied to the present methods and systems.
In some embodiments, as described hereinabove, a Huffman coding type of strategy is used to facilitate signal resolution based on known additional expression data.
In some embodiments, a cell sample is subject to 3D-sectioning to collection data that will be used to reconstruct the three-dimensional structure of the cell. The laser light section method is a 3D-procedure to measure object profiles in one sectional plane. The principle of the laser triangulation requires an orthogonal to the objects surface positioned detector area (e.g., CCD- or CMOS-matrix) to measure the lateral displacement or the deformation of a laser line projected in an angle (between 0 and 90 degrees) onto the objects surface. Laser light sectioning is the two-dimensional extension of the laser triangulation. With projecting the expanded laser line, an elevation profile of the object under test is obtained.
Methods for data processing, especially those for digital imaging data processing can be used in the present invention to improve or optimize the process for resolving barcode/indicium. Digital image processing is the only practical technology for classification, feature extraction, pattern recognition, projection, and multi-scale signal analysis, each of which aspect is applicable to dissolving molecular barcodes/indicia. Exemplary techniques or algorithms that are used in digital image processing include but are not limited to pixelization, linear filtering, principal components analysis, independent component analysis, hidden Markov models, anisotropic diffusion, partial differential equations, self-organizing maps, neural networks, and wavelets.
Indicia created by molecular barcoding are resolved or discerned by super resolution technologies. In some embodiments, super resolution technologies of the present invention include super resolution microscopy. In some embodiments, the super resolution technology has a resolution of about 100 nm or higher; about 80 nm or higher; about 60 nm or higher; about 50 nm or higher; about 40 nm or higher; about 30 nm or higher; about 25 nm or higher; about 20 nm or higher; about 15 nm or higher; about 10 nm or higher; about 8 nm or higher; about 6 nm or higher; about 5 nm or higher; about 4 nm or higher; about 3 nm or higher; about 2 nm or higher; about 1 nm or higher; about 0.5 nm or higher; about 0.2 nm or higher; about 0.1 nm or higher; about 0.05 nm or higher; or about 0.01 nm or higher.
One of skill in the art would understand that the specific characteristics (e.g., size) of the cellular constituents will determine the resolution at which a particular indicium will be resolved.
Super resolution techniques allow the capture of images with a higher resolution than the diffraction limit. They fall into two broad categories, “true” super resolution techniques, which capture information contained in evanescent waves, and “functional” super resolution techniques, which uses clever experimental techniques and known limitations on the matter being imaged to reconstruct a super resolution image. True sub-wavelength imaging techniques include those that utilize the Pendry Superlens and near-field scanning optical microscopy. Most techniques of importance in biological imaging fall into the functional category.
Exemplary super resolution technologies include but are not limited to I5M microscopy, 4Pi-microscopy, Stimulated Emission Depletion microscopy (STEDM), Ground State Depletion microscopy (GSDM), Spatially Structured Illumination microscopy (SSIM), Photo-Activated Localization Microscopy (PALM), Reversible Saturable Optically Linear Fluorescent Transition (RESOLFT), Total Internal Reflection Fluorescence Microscope (TIRFM), Fluorescence-PALM (FPALM), Stochastical Optical Reconstruction Microscopy (STORM), Fluorescence Imaging with One-Nanometer Accuracy (FIONA), and combinations thereof. Descriptions of relevant techniques can be found in Chi, 2009 “Super-resolution microscopy: breaking the limits, Nature Methods 6(1):15-18; Blow 2008, “New ways to see a smaller world,” Nature 456:825-828; Hell, et al., 2007, “Far-Field Optical Nanoscopy,” Science 316: 1153; R. Heintzmann and G. Ficz, 2006, “Breaking the resolution limit in light microscopy,” Briefings in Functional Genomics and Proteomics 5(4):289-301; Garini et al., 2005, “From micro to nano: recent advances in high-resolution microscopy,” Current Opinion in Biotechnology 16:3-12; Bewersdorf et al., 2006, “Comparison of I5M and 4Pi-microscopy,” 222(2):105-117; and Wells, 2004, “Man the Nanoscopes,” JCB 164(3):337-340; each of which (including Supplemental Material) is hereby incorporated by reference herein in its entirety.
In some embodiments, electron microscopes (EM) are used to resolve an indicium. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than visible light (photons), and can achieve better than 0.2 nm resolution and magnifications of up to 2,000,000 times.
In some embodiments, a plurality of cellular constituents is barcoded by methods and systems of the present invention. For example, molecular barcoding can be applied to one cellular constituent; two or more cellular constituents; three or more cellular constituents; four or more cellular constituents; five or more cellular constituents; six or more cellular constituents; eight or more cellular constituents; ten or more cellular constituents; 15 or more cellular constituents; 20 or more cellular constituents; 30 or more cellular constituents; 50 or more cellular constituents; 80 or more cellular constituents; 100 or more cellular constituents; 150 or more cellular constituents; 200 or more cellular constituents; 300 or more cellular constituents; 500 or more cellular constituents; 1,000 or more cellular constituents; 1,500 or more cellular constituents; or 2,000 or more cellular constituents.
In one aspect, the present invention is used to measure a biological state of a cell, for example, its transcriptional state. The transcriptional state of a cell includes the identities and abundances of the constituent RNA species, especially mRNAs, in the cell under a given set of conditions. For example, a fraction of the constituent RNA species in the cell can be measured for genetic profiling.
In some embodiments, only one type of cellular constituent is analyzed, for example, mRNA transcript. In some embodiments, one or more mRNA isoforms are analyzed in order to characterize alternative splicing events. One of skill in the art would readily appreciate that one or more mRNA isoforms can be analyzed using any of the labeling schemes described herein. In some embodiments, the labeling scheme involves labeling one or more introns and one or more exons, as depicted in Example 10, and described in greater detail herein below.
In some embodiments, two or more types of cellular constituents are analyzed. For example, interactions between RNAs and proteins can be analyzed.
In some embodiments, time resolved analysis can be carried out. For example, sample cells can be synchronized by chemical arrest or starvation. Thereafter, cells will be taken at different time points and are sequentially subjected to analysis by molecular barcoding followed by super resolution de-coding. By doing so, a time course of the desired reaction or process can be constructed.
In some embodiments, the present methods and systems can be used to conduct biochemical assay in a single cell setting. For example, the present methods and systems can be used to study interactions between any cellular constituents, including protein-protein interactions, protein-nucleic acid interaction, and others. In some embodiments, time-resolved biochemical assays can be performed using the present methods and systems.
In some embodiments, a transcriptional state of a cell is imaged by detecting and distinguishing individual mRNAs. Florescence In-Situ Hybridization (FISH) allows single mRNAs molecule in fixed cells to be labeled and imaged. This is accomplished by hybridizing the mRNA with a set of short oligonucleotide probes complementary to the mRNA sequence. Conventionally, these probes are labeled all with the same fluorophore to increase the contrast compared to non-specific bound probes in the cell. This allows individual mRNA to be visualized as a bright fluorescent dot in the cell. In such embodiments, the number of species that can be labeled simultaneously is determined by the availability of spectrally distinct fluorophores. Further, if the expression level of the targeted mRNA is high, then individual hybridized transcripts cannot be optically resolved from each other, preventing accurate quantitation of the copy number.
In some embodiments, different super-resolution fluorophores are attached within a set of oligo probes, such that as they hybridize against their target mRNA, a molecular barcode or indicium can be readout by super-resolution microscopy. The barcode can then serve to uniquely identify that particular transcript. Thus, by using different barcodes for different mRNA species, many mRNAs can be labeled simultaneously as illustrated in
The systems and methods described herein have enormous capacity and can effectively barcode the entire human transcriptome with at least 6 distinct super-resolution fluorophores. The entire human transcriptome has about 20,000 genes and 6 distinct super-resolution fluorophores at 6 barcode positions render over 45,000 combinations (66=46656). In addition, with the giga-pixel volume of a single cell under super-resolution microscopy, individual mRNAs can be imaged and their barcode read out without concerns of overcrowding in the optical space. The expression level for each species of mRNAs can then be tallied by counting the abundance of the corresponding barcodes. This technique not only detects transcripts with single molecule sensitivity, therefore highly accurate in quantitation, but also preserves the intracellular and intercellular spatial context in which transcription occurs. These advantages make it especially applicable for investigations in heterogeneous cell populations, such as cell cultures, tissue sections, and embryos.
Mapping Chromosome Structures.
Chromosomal rearrangements have been implicated in many forms of cancer, and recent investigations revealed that chromosomes in eukaryotes are packed in a non-linear and complex fashion. Super-resolution barcoding can be applied to image the structure of chromosomes and determine their conformation in single cells. Conventional DNA-FISH can label only 4-5 distinct chromosomal locations limited by the number of distinct fluorophores. However, with the super-resolution barcoding technique, a large set of genomic loci can be labeled each with a distinct barcode and resolved by microscopy. Thus, the physical location of many genes can be mapped and serve as landmarks on the chromosomes, thereby allowing for detection of chromosomal translocations and other rearrangement events in cells. A physical image of the chromosomes in cells with the addresses of individual genes will allow for an unprecedented look at how the genome is compacted, compare organization in transcriptionally active versus repressed regions, and detect subtle changes in genomic structure in tumor cells.
Imaging Transcription Factor Binding in Single Cells.
In some embodiments, the methods and systems described herein are used to analyze transcription factor binding in single cells. Transcription factors (TFs) control genes in transcriptional networks through binding sites on the DNA and interactions with regulatory proteins. The distribution of positions and binding states of a particular TF on the chromosome determines the transcriptional program it is accessing in the cell. By fusing TFs with photo-switchable fluorescent protein or labeling with antibodies, the physical location of individual TFs can be determined with 10 nm resolution. The genomic location of the TF can then be assigned by overlaying those positions on top of the high resolution chromosome map developed from technique 2. Results from ChIP-seq experiments can be compared to determine the occupancy of each binding site and higher order structures at the promoters in single cells. In some embodiments, analysis by the present methods and systems focus specifically on Crz1, a TF in budding yeast that has been shown to pulse in its activity. In some embodiments, the fraction of Crz1 binding sites that are occupied during a pulse can be determined. In some embodiments, it can be determined whether occupancy of different binding sites is correlated depending on their physical proximity in the nucleus.
Having described the invention in detail, it will be apparent that modifications, variations, and equivalent embodiments are possible without departing the scope of the invention defined in the appended claims. Furthermore, it should be appreciated that all examples in the present disclosure are provided as non-limiting examples.
The following non-limiting examples are provided to further illustrate embodiments of the invention disclosed herein. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent approaches that have been found to function well in the practice of the invention, and thus can be considered to constitute examples of modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.
Probes (20mers) were designed to match melting temperature whenever possible, with the exception of STORM probes which were designed to have 2 base pair spacing between the probes to allow efficient reactivation of the STORM pair dyes (
Imaging of the hybridized cells is carried out on automated fluorescence microscopes. For FIONA, images were acquired on an Olympus IX81 with a 100× sapo objective with laser illumination at 532 nm, 594 nm, and 640 nm. Images were taken with Andor IQ software and an Andor Ikon CCD. FIONA images were acquired in 3 different fluorescence channels (Semrock zero line filters). The centroids of the FISH dots were calculated in each color or wavelength channel and a center of mass of all the dots were calculated and aligned between the channels by a simple translation. This was sufficient for most alignments without additional corrections from rotation and dilations.
STORM imaging was performed on a Nikon Tleclipse microscope with PFS autofocus lock. 640 nm laser (Crystallasers) was used as the main imaging laser and brought to the sample past the critical angle by a TIRFM objective. Lasers at 405 nm, 473 nm and 532 nm were used as activation lasers and automation is controlled by u-manager software. The microscope stages (Prior and ASI) were automated and also controlled by the acquisition software to enable multi-position imaging. Images were then analyzed in a custom written Mathematica script. Buffers used in STORM imaging follows the protocol similar to those in Bates et al., 2007, “Multicolor Super-Resolution Imaging with Photo-Switchable Fluorescent Probes,” Science 317(5845): 1749-1753, which (including any Supplemental Material) is incorporated by reference herein in its entirety. Glucose Oxidase was used as the oxygen scavenger and BME was used as the reducing agent.
To multiplex mRNA detection in single cells, 9 target genes of the yeast transcription regulator Crz1 were chosen. A combination of barcodes was used to ensure the accuracy in determining the abundances of each of the genes. To do so, the simplest barcode was designed for the most highly abundant and shortest mRNAs. YLR414c and YLR194c are each 700 bps long and expressed strongly based on microarray experiments. Thus, they were assigned single color barcodes. Cmk2, another highly expressed gene, was also assigned a single color barcode with 12 paired-probes. The next tier expression levels are PMC1, NPT1 and YPS1. These genes were expressed at a lower level and were assigned a 2-position barcode. Lastly, Sok2, GYP7 and PUTT are lowly expressed and were assigned the 3 different 3 color barcodes. As 3 color barcodes were resolved correctly 70% of the time, care was taken to avoid having the incorrectly resolved barcode leak into other barcodes at a significant level. Thus assigning the lowest expressed genes with the most complex barcode mitigates the crosstalk problems.
The barcoding capacity could be drastically improved by using repeat barcodes and more color than the three cy5-pair dyes (
The copy number of all of the genes was measured in single cells simultaneously by counting the number of super-resolution reconstructed barcodes (Table 1). Using this data, the distribution of copy numbers for each gene was determined (
Another question was whether transcription factor availability is limiting in creating stochastic transcription bursting. In order to investigate this issue, the same target genes were profiled in cells with Crz1 strongly expressed from a plasmid (PLE66). The affinity of the promoters to Crz1 can be inferred from the fold change in the expression level of that gene as Crz1 is over-expressed. High affinity promoters such as NPT1 and Pmc1 were expressed at the same levels in wt and over-expressed cells, suggesting the promoter is already saturated at wt Crz1 levels; while lower affinity promoters such as Cmk2 and YLR194c showed a 2 fold increase when Crz1 was over-expressed. There was a stronger correlation between the high affinity promoters, likely due to the tight coupling of transcriptional bursting with the Crz1 pulse. Meanwhile, the weaker promoters showed weaker correlation as they burst stochastically and independently following every Crz1 pulse. As Crz1 was over-expressed, the correlation among weak promoters increased, suggesting that promoter occupancy was enhanced. A smaller but non-zero population of cells exhibited bursting in only one gene in over-expressed cells, suggesting that promoter affinity does not explain all of the variations in correlations among genes. In addition, while not wishing to be bound by any one particular theory, no connection was found between chromosome positions and the gene correlations.
To elucidate a more global picture of how expression is correlated, higher order correlations are instructive. Each gene was classified as either highly or lowly expressed in a cell, thresholding on the average expression level of that gene in the population, for a total of 29 binary states. The dataset can be mapped to vertices of a 9 dimensional hypercube, which can be collapsed onto a Petrie projection graph (
Extension of the mRNA to allow spatial resolution of the barcode is important to the high multiplex potential of the technique. Several approaches were tried to generate spatial extension. First, a DNA origami type of strategy was used to fold the mRNA into a stiff rod like configuration (
It has been suggested from previous FISH experiments that the mRNA is covalently attached to the protein matrix by formaldehyde. The inventors experimented with fixation methods where only proteins are crosslinked or precipitated to observe the effect on the mRNA. The inventors used the methanol fixation and DSS a NHS ester based protein crosslinker in separate experiments. As methanol fixation is supposed to only precipitate the proteins and does not crosslink nucleic acids, the inventors wondered whether they could move the mRNAs out of the cell with an applied E field. Again, the same copy number of transcripts was detected by FISH and the FIONA reconstruction showed no further extension of the mRNA compared to non-electrophoresed cells. While not wishing to be bound by any one particular theory his experiment strongly suggests that mRNA is fixed in the cells by interactions of the ribosomes on the mRNA with the protein background during fixation, rather than through direct nucleic acid to protein interactions. This is consistent with the lack of ability to use DNA origami methods to fold the mRNA. In a previous experiment, it was suggested short RNAs rapidly escape the cell during the hybridization process and these miRNAs are recovered in the solution. The inventors' observation of longer transcripts shows that physical escape of the transcript from the cells is improbable and likely hindered by ribosomes, whereas most of the shorter miRNAs likely do not associate with proteins and thus are not permanently fixed in the cell. Experiments in which Puromycin and Harringtonine were used to dissociate or stall the ribosomes were inconclusive, probably due to poor permeability and poor kinetics of the compounds at 25-30° C.
Thus, while not wishing to be bound by any one particular theory, one of the best mechanisms for the extension of mRNA is by physical compression of the cells as they are sandwiched between coverslips during imaging. As the mRNAs are held in place by ribosomes fixed to the cellular matrix, a flattening of the cells creates a shear flow within the cells that moves the ribosomes and stretches the mRNA out in the lateral direction (
To demonstrate the feasibility of this approach, multiple mRNAs species in single Saccharomyces cerevisiae cells were detected using the methods and system described herein. The current methods and systems differ from the single molecule FISH (smFISH) techniques. Instead of detecting each mRNA with 40 20mer oligonucleotide probes labeled with the same fluorophore as in smFISH, a nanoscopic barcode was imparted on each transcript by hybridizing probes labeled with different fluorophores in a spatially ordered fashion. The current SRM resolution of 15 nm allows a code region of 50 bp long to be resolved. Different species of mRNA can be uniquely barcoded and quantitated by tabulating barcodes in individual cells (
It was first demonstrated that barcodes on mRNAs can be resolved spatially, using Fluorescence Imaging with One-Nanometer Accuracy (FIONA). Twelve oligo probes targeting GFP mRNA were grouped in sets of 4 and labeled by 3 different fluorophores. In hybridized yeast cells, target mRNA appeared as co-localized, near-diffraction limited, spots in the fluorescence channels. 99±2% of spots co-localized in all three channels and each spot typically consists of 2.7±1 probes, as determined by photobleaching (SOM). These results suggested that hybridization is 70±10% efficient and single labeled probes can be readily detected. The centroid positions of the probes can be determined accurately by Gaussian fitting, with a localization accuracy of ˜4 nm (with ˜104 photons), and with an error of ˜5 nm due to chromatic aberration in our imaging setup (SOM). Following center-of-mass image alignment, the correct spatial order was observed in 80±10% of the labeled mRNAs, with the average spatial separation of 25±10 nm between centroids. The spatial distances is shorter than the 40 nm expected for a fully extended 120 bp region of hybridized mRNA, but significantly more than the potentially folded native structure of the mRNA. It was hypothesized that a partially stretched mRNAs is observed, because as the cells are sandwiched between 2 glass coverslips for imaging, the compression of the cells generates a shear flow acting on the ribosomes attached to the mRNA, resulting in extension of the mRNA in the planar direction. This extension of the mRNAs from their native conformation allows the 80% fidelity in reading out the barcodes.
To demonstrate the robustness of the FIONA barcode technique, the order the fluorophore labeling was switched and the distances between 2 barcode positions was increased. The corresponding switch was observed in the ordering of the centroids with 70±10% correct ordering as well as the lengthening of the distances between the barcode positions (from 25 nm to 40±10 nm). In addition, mRNAs of different lengths from 700 bp to 14 kb were labeled using this method, without observing significant differences in the detection of the correct ordering of the barcode. The FIONA approach is easy to implement with conventional fluorophores and has high localization accuracy. However, the approach is limited in labeling density by diffraction and prevents repeat usage of colors for coding. For example, a RGR coding scheme cannot be resolved by FIONA, as the two red positions are within the same diffraction limited volume. To circumvent both problems, photoswitchable fluorophores were used to label the oligonucleotide probes.
A barcode from photoswitchable dyes labeled oligos based upon the Cy5 dye-pairs were used. In the STORM experiment, an activator dye (Alexa405, Alexa488, or Cy3) were placed in close proximity (˜1 nm) to the switchable dye (Cy5, Cy5.5 or Cy7). As Cy5 is imaged and switched off by a 640 nm laser, it can be stochastically re-activated by weak illumination in the activator dye wavelength. Three activator dyes can be paired with three emitter dyes to provide 9 colors in STORM. The probes were designed such that two probes, one labeled in its 3′ position with the activator and the other in 5′ position with the Cy5 emitter dye (
Since the images were all acquired in the Cy5 channel, there is no need for chromatic aberration corrections. Three color barcodes (
Practically, the accurate readout of barcode is constrained by the hybridization efficiency and non-specific blinking of the Cy5 dyes which distort super-resolution reconstruction. As each probe has 70% chance of hybridizing, the pairs are generated successfully 50% of the time. With 4 redundant probe pairs per barcode position, the chance of having all three positions present with at least one pair of probes is 81%, consistent with the inventors' observation that 77% 3 color codes are complete. In the directly labeled probes used in the FIONA experiments, a 4 probe redundancy was sufficient to ensure that 98% of time at least one probe is bound in each channel. Thus, it was anticipated that with the development of fluorophores with improved non-specific activation rates. Thus, barcodes could be reconstructed more accurately, and oligo probes could be labeled directly to increase the chance that all the code positions are present.
The unique capabilities of the barcoding FISH approach in studying genetic networks in single cells was demonstrated by profiling a set of co-regulated genes controlled by a transcription factor Crz1. It was previously shown that Crz1 pulses in its nuclear localization in frequency-modulated fashion. However, not every Crz1 localization pulse gives rise to a transcriptional burst in a particular target gene. The super-resolution barcoding approach allowed the inventors to determine whether these transcriptional bursts were uncorrelated across different target genes, suggesting an intrinsic source of stochasticity at the promoter, or correlated, suggesting extrinsic contributions. By fixing cells in conditions in which the interval between Crz1 pulses is longer than the typical mRNA lifetime, the distribution of transcriptional responses among genes to each Crz1 pulse in single cells can be captured.
Nine Crz1 target genes were chosen based on previous microarray and flow-cytometry experiments to represent a range of expression levels and loci on different chromosomes (Table 1). The transcripts were encoded in a manner analogous to Huffman coding, with the highest expressed genes assigned the least complex barcode. As the inventors' 3 color barcodes have an error rate of 20% in crosstalk to the 2 color barcodes due to hybridization efficiency, assigning the lowest expressed genes with 3 color codes diminishes the error in over-estimating the abundances of 2 color coded genes. From tabulating barcodes, the distribution of copy numbers for each gene was determined, and the pairwise correlation between target genes was calculated. Significant stochasticity was found in the expression levels for many of the genes (CV=0.9±0.1), suggesting presence of transcription bursts. In addition, large variability was observed in the pairwise correlation between different genes, from R=0.19±0.14 to R=0.8±0.07. In genes pairs that were weakly correlated, a stark disparity in expressions was observed in >50% of cells in which some genes were clearly highly expressed with a copy number of 20-30 per cell, while others genes in the same cells were not expressed at all (
Nine genes were barcoded with each circle representing 4 pair of probes, and their abundances in individual cells were tabulated. Note the heterogeneity in expression levels within a single cell.
The multidimensional transcriptional dataset captured by barcoded FISH cannot be fully represented through pairwise correlations. To present in an intuitive manner the higher order correlations in the Crz1 targets, each gene was classified as either highly or lowly expressed in a cell, thresholding on the average expression level of that gene. The states were then enumerated with a given subset of genes highly expressed and represent the probability of finding cells in that state by the size of the circle in
Super-resolution barcode FISH (SURF) of the present invention highlights an alternative path to genome-wide transcriptional profiling in single cells. Nine genes were multiplexed by utilizing only 3 super-resolution colors, without the use of repeated barcodes (
In experiments related to those previously described herein, the inventors targeted the YLR414c mRNA with 3 sets of oligo probes labeled with different fluorophores (
Alternative splicing is ubiquitous in eukaryotes and significantly increases the complexity of the proteome. Recent deep sequencing experiments reveal that 30% of the human genome is spliced with an average exon size of 50-100 bps. Alternative splicing occurs stochastic in individual cells and has been shown to be crucial in cellular differentiation and patterning events. Thus, it is important to determine the distribution of splice variants that are present in individual cells to understand how they are regulated. The inventors performed a proof of principle experiment of single cell alternative splicing using barcoded FISH on the S. cerevisiae gene PTC7. The PTC7 gene is composed of an intron flanked by two exons (
The inventors observed that on average 17.3% of the transcripts were unspliced (N=98), consistent with the 13.3% determined from qPCR. However, at the single cell level, the efficiency of splicing is heterogeneously distributed across cells (
Conventional fluororescence microscopy is only useful in cases where transcript levels are low. When the density of transcripts is high, diffraction-limited fluorescent spots will overlap and make barcode readout by centroid fitting impossible. Super-resolution imaging is essential for single-cell microscopy to achieve high-density multiplex barcoding. The inventors turned to the Cyanine dye based photoswitchable dye pairs developed by Bates et al (as cited above). In the PALM, STORM, and FPALM implementations of SRM, subsets of fluorophores are photo-activated, imaged as single molecules, localized by centroid fitting and switched off. After many iterations of this cycle, a high-resolution image can be reconstructed from all of the centroid fits in the individual images. The inventors exploited the pairs of fluorophores used by STORM microscopy to dramatically reduce the background from non-specifically bound probes. The STORM scheme places an activator dye in close proximity to a Cy5 emitter, which can be switched off by imaging and re-activated by exciting the activator, as described in greater detail above. The inventors designed the oligos such that two probes were used: one labeled at its 3′ position with the activator and the other at the 5′ position with the emitter (
As demonstrated by the previously described experiments, the spectral barcoding scheme is immediately applicable and robust with current technology. In spectral barcoding (
The disadvantage of the spectral coding scheme is its low multiplex capacity. However, using just 9 dye pairs, a 4 position spectral coding scheme allows 9C4=126 genes to be multiplexed at the single cell level. This scheme scales exponentially with the number of fluorophores available. As indicated in Dempsey et al., “Evaluation of fluorophores for optimal performance in localization-based super-resolution imaging” Nat. Methods. 2011 Nov. 6; 8(12):1027-36, which is incorporated herein by reference in its entirety as though fully set forth, at least one additional emitter is available to pair with the activators, giving at least 3 additional SRM fluorophores. With just one additional emitter, close to a thousand genes (12C5=792) can be multiplexed in single cells.
In fact, recent studies indicate that using the above-described scheme, for activators there are at least 4 choices, including Alexa405, 488, 532 and 568, and for emitters, there are also at least 4 choices, including Cy5, Cy5.5, Cy7, and 7.5. With these choices, because they can be mixed and matched to give functional dye pairs, there are 16 possible pairs (4×4) in all. Recent studies also indicate that the particular characteristics of these molecules can influence their suitability for specific applications. For example, for RNA FISH particularly good emitters are Alexa 647 or Dynomics 632, Cy5.5, Cy7, and IR800CW. For DNA FISH, they are Alexa647, Cy5.5, Alexa 750 and Alexa 790.
When the activators and emitters described directly above are considered, a much easier 2 color barcode can get to the same multiplex capacity (16C2=120) described above. In fact, the inventors have used this method for the analysis of almost 100 genes.
It has been recently observed that many transcription factors (TFs), particularly those involved in stress responses, activate gene expression in discrete pulses. These TF pulses occur in a wide range of organisms, such as SOS in bacteria, Msn2 and Crz1 in budding yeast, and p53 and NF-kB in mammalian cells. Since many of the pulses occur stochastically in single cells and are averaged out in population experiments, the inventors set out to determine if pulsing in many of the stress response genes is correlated and in doing so to ascertain their modes of combinatorial regulation by upstream TFs. The inventors hypothesized that co-regulated genes are more likely to bursts together. By sampling many stress response genes using super-resolution barcode imaging, the correlations amongst the single cell gene expression patterns can be used to infer regulatory architectures.
The calcium signaling pathway in budding yeast is an ideal model to address these questions. The inventors have previously shown that in response to external signals, the master transcription factor Crz1 translocates in and out of the nucleus in short (2-3 minute) well-defined pulses. These pulses occur stochastically in time and involve most of the Crz1 molecules in the cell. In addition to Crz1, calcium stress also triggers the Msn2 pathway, a general stress response regulator that also pulses in its localization. To ensure that they were observing the products of individual TF pulses, the inventors fixed cells under conditions where the average interval between pulses was longer than the typical mRNA lifetime. This experiment necessitated the application of the inventive barcoding technique as it is difficult to multiplex more than 10 genes with existing single cell techniques, and population measurement would have been averaged over the unsynchronized Crz1 pulses in different cells. The inventors' technique provides a unique snapshot of expression of a battery of stress response genes.
The inventors selected 14 genes that are regulated by Crz1, 5 general stress response genes, as well as 13 other aging and stress markers, for a total of 32 genes. To label these genes, the inventors used spectral coding of combinations of 3 out of 7 super-resolution dye pairs (3C7=35). Barcode assignments are shown below in Table 2. Codes 135, 235, and 895 are left empty. In the barcode scramble experiments, the activators are permuted. 1->2, 2->3, 3->1.
Cells were imaged by super-resolution and transcript levels in individual cells quantified based on the abundances of the corresponding barcodes. All genes were assigned barcodes containing combinations of 3 fluorophore pairs. The main concern with multiplexing a large probe set is the crosstalk among the barcodes. This can arise from several sources: fluorescent background from the cell, blinking of nonspecifically bound probes, crosstalk in the fluorophores, and errors in computation analysis. The inventors controlled for each of these sources of errors (
With the single cell data (
The inventors next asked whether there are subgroups of the Crz1 genes that tend to burst together and whether they correspond to different regulatory architecture. By agglomerative hierarchal clustering with Ward's criterion of the correlation distances between genes, the inventors resolved two distinct clusters for the Crz1 responsive genes with high p values (0.09, 0.08) (
The inventors last examined whether combinatorial regulation affects the level of coordination within the combinatorial subgroup of Crz1 genes. The inventors found that when both groups of genes were on, the combinatorial target genes were as well coordinated as the pure Crz1 targets (
Under a super-resolution microscope, cells become virtual microarrays with giga-pixel information density. Because individual molecules can be spatially resolved, their identities can be assigned a unique combinatorial code. The inventors demonstrated two such combinatorial labeling schemes, one based on the resolving the spatial sequence of labels on mRNAs and the other dependent on discriminating the combination of fluorophores in the label. On the one hand, the spatial coding scheme can be efficiently scaled up, but is error prone and requires molecules to be stretched out. On the other hand, the spectral coding scheme is more robust and can apply to other molecules besides mRNAs, but requires additional fluorophores to increase throughput. The inventors showed that by using 7 super-resolution fluorophores, 32 genes can be detected simultaneously in single cells. This multiplexing capacity is increased to 100 genes if 9 of the currently available fluorophores are used (9C4=126). As multiplex capacity increases exponentially with the number of fluorophores available, even the spectral coding scheme has the potential to achieve transcriptome level multiplexing if additional emitters are used (18C6=18,532).
Super-resolution barcoding provides a useful single cell follow-up technique to high-throughput sequencing technologies. One can sequence a population of cells from cultures or tissue samples, identify the genes of interest and then profile them in single cells with super-resolution barcoding. The advantage of this approach is three fold. First, direct imaging of the sample preserves the spatial information both within cells and among cells. With the application of light sheet microscopy, the technique can be extended into optically thick samples. This advantage makes it a powerful tool in studying signaling in heterogeneous systems such as microbial ecosystems, tissue and embryos, where interactions among different cellular populations play an essential role in cellular decisions. Second, because of the single-molecule and in situ nature of the technique, the method is quantitative and avoids intrinsic bias in RNA extraction and conversion to cDNA. Lastly, many cells can be imaged simultaneously under a microscope quickly and throughput can be scaled up without significant costs. Compared to the high cost and long waiting time for sequencing single cells, the super-resolution FISH approach is economical to scale up. After the initial cost of the probe set synthesis, the probe set can be hybridized many thousands of times to wild-type and mutant organisms.
The combinatorial labeling scheme can be applied to many types of molecules in the cell. The inventors demonstrated labeling mRNAs to quantify expression levels and splice variants. It is a short leap to consider combinatorially labeling chromosomes and proteins, for single cell proteomics and possibly ChIP experiments. For many types of biochemical techniques, such as microarrays, there is an equivalent single-cells experiment possible by application of super-resolution barcoding. The spatial separation step performed traditionally by gels or dilution on a chip can be replaced conceptually by super-resolution imaging to resolve molecules in situ. Further implementations of the super-resolution combinatorial imaging techniques will allow for direct observations of the interactions of biological components within single cells and aid in gaining an integrative picture of how they give rise to biological function.
25mer oligonucleotide probes were designed to match melting temperature whenever possible. STORM probes were designed with 2 base pair spacing between probes to allow efficient reactivation of the STORM pair dyes, often leading to varying TM between STORM probes. Alexa 405, Fluorescein, and Cy3 were used as the activators and Cy5 as the switchable dye. Labeling and purification of the probes followed the protocol previously described herein. Yeast cells were grown in minimal media with 50 mM calcium and fixed in log growth phase following the Singer lab protocols with the addition of 0.1% NaBH4 treatment before the ethanol permeabilization step. The inventors found the NaBH4 treatment significantly decreased the auto-fluorescent background of fixed yeast cells. Cells were stored at −20C in Eppendorf tubes and aliquoted out for hybridization experiments. Cells were hybridized with the probes overnight at room temperature in 20% Formamide and 10% dextran sulfate. After hybridization, cells were washed in Formamide and 0.2×SSC solution 3 times and imaged.
For conventional epi-fluorescence microscopy, images were acquired on an Olympus IX81 with a 100× sapo objective with laser illumination at 532 nm, 594 nm, and 640 nm. Images were acquired with Micromanager software and an Andor Ikon CCD. FIONA images were acquired in 3 different fluorescence channels (Semrock zero line filters). The centroids of the FISH PSFs were calculated in each channel and the images were aligned by center of mass alignment of co-localized PSFs between channels. This was sufficient for alignment without correcting for rotation and dilation. STORM imaging was performed on a Nikon TI-eclipse microscope with PFS autofocus lock. The imaging lasers, a 640 nm laser along with a 30 mW 691 nm and a 30 mW 730 nm laser (Coherent Lasers) were brought to the sample through a 100×TIRFM objective. 405 nm, 473 and 556 lasers were used as activation lasers and imaging automation was controlled by Micromanager software.
For the spatial coding images, samples were first imaged with only the 640 nm laser for 100 frames to switch off Cy5 and to determine the non-specific blinking rate. Then, 100 frames were acquired in each activation channel by co-illumination of the activation and the imaging lasers. This reduced the cross-talk among the different activation channels.
For the spectral coding images, the samples were first bleached in all imaging channels for 6 frames. Samples were imaged in order of the activators, starting at 556 nm through 473 nm and to 405 nm. For each activator, the microscope cycled through imaging with the 730 nm followed by either the 640 nm or 691 nm laser for 48 cycles. Samples were only illuminated with the activation light in the cy7 channel. Activation powers were selected to maximize the activation rate while avoiding crosstalk among the channels. The 640 nm and 691 nm excitation lasers alternated after every emitter cycle. This scheme reduces the photobleaching due to spectral overlap between the Cy5 and Cy5.5 emitters. Following this imaging cycle, the 640 nm lasers were continuously used to image Cy5 for another 30 frames of specific activation. Throughout the imaging routine, for every 4 cycles of the imaging lasers, 2 cycles without the activator lasers were acquired. This enabled ruling out many of the false positive and nonspecific blinking events in the images. The activation lasers were controlled by an Arudino microcontroller board and a servo motor shutter. Fluorescent beads (Invitrogen F-8810) were used as fiducial markers to correct for stage drifts. The microscope stages (Prior and ASI) were automated and controlled by acquisition software to enable multi-position imaging. Buffers used in STORM imaging followed the protocol in Bates et al. (previously cited), with Glucose Oxidase as the oxygen scavenger and BME as the reducing agent.
Images from FIONA and STORM experiments were analyzed with a Mathematica script. In the STORM analysis, the beads were first aligned to determine the stage drifts. Beads emit on the order of 50,000 photons per image, and could be localized to a few nanometers. Beads close to the cells were eliminated from analysis as the switching of fluorophores in cells can disrupt the bead alignment. Then, fluorophores were selected from each image by intensity thresholding, and their centroid calculated. Because the samples were densely labeled (10-20 Cy5 dyes per mRNA molecule), the probability of multiple fluorophore activation was high. A relatively high activation rate, often leading to simultaneous fluorophore activation, is essential for overcoming the nonspecific activation frequency in densely labeled samples. The inventors did not reject activation events that involved multiple fluorophores. To determine which of the barcode colors are present, instead of nonspecific activation, the inventors compared the number of activation events that occurred in the specific activation channel versus the activations observed in frames with no activation. If the nonspecific activation events exceed specific activation events, then that channel was rejected. As several of the activators and emitter channels can crosstalk into other channels, the inventors quantified the crosstalk ratio and rejected activation events if they fell below certain thresholds, as described herein. Switching events that spatially cluster together were grouped to display the resolved barcode. For spectral barcodes, activations were clustered together on a 184 nm grid. Activation events near grid vertices were assigned to a neighboring region containing localizations of the same fluorophore pair. For 3 color barcodes, the center color was determined by finding the position that was not one of the two localized colors separated by the longest distance. Cell positions were determined by manual segmentation. Barcodes were collected and tabulated for each single cell. Cross-correlation was calculated using the correlation function in Mathematica and the standard errors were calculated from re-sampling the data 100 times.
To determine the hybridization efficiency of the probes, the inventors used photobleaching to measure the number of bleaching steps. 12 27mer probes targeting Cmk2 were coupled to Cy3 and imaged with a 532 nm laser. Discrete photobleaching steps were observed corresponding to bleaching of single fluorophores (
For the 3 color YLR414c centroid reconstruction, the inventors observed that 74±8% (N=28) of codes reconstruct correctly. For the PTC7 reconstructions, the inventors observed that 67% (N=12) of the reconstructions contain the correct order. The lower reconstruction rate for the PTC7 transcript is because the lower copy number of PTC7 unspliced transcripts make it less accurate to correct for the offset between the fluorescence channels without extra fiducial markers. This can be corrected by using exogenous fiducial markers. With the cy3 cy5 pair for super-resolution imaging, this labeling scheme improves background rejection, as both probes are required for the fluorophore to be re-activated. Non-specifically bound Cy5 probes in the cell cannot be reactivated. In comparison, directly labeling oligos with Cy3-Cy5 covalently-linked pairs will have the same non-specific background as standard FISH and have a drastically increased blinking rate due to the complex photophysical properties of the Cy3-Cy5 heterodimer. Indeed, the inventors observed prior to inactivation, cells contain a hazy background of singly bound probes in addition to the hybridized FISH spots. After imaging with the 640 nm laser for 4-5 frames, these non-specifically bound molecules switched off and blinked at the non-specific activation rate of cy5. Upon activation with 405,473 or 532 nm lasers, these background probes did not reactivate. It is highly unlikely that probes with an activator would be non-specifically bound within STORM distance (<1 nm) from a Cy5. The majority of spots that reactivated were specific mRNA targets, although noise was observed from cellular auto-fluorescence and probe complexes. Some of this noise was due to x-talk among specifically bound dyes. The inventors observed x-talk ratios of around 7% for the most egregious Cy5-dye pairs (
The cost of the background rejection of cy5 pair-probes is reduced effective hybridization rate. As both probes are required for a functional dye pair, the effective hybridization efficiency is (67%)2=45%. Thus the probability of having at least 1 pair formed out of a redundant set of 4 probes pairs is 140.45)4=0.9. With a 3 color barcode, the theoretical probability of having all three colors present is 0.93=0.72. The inventors observed a 61±8% probability (50 out of 85 reconstructions) that 3 colors were present on a given mRNA, and a 33±6% probability (28/85) of resolving only 2 colors. A typical reconstruction from an image is shown in
The inventors' use of physical compression allowed for imaging most RNAs in a single focal plane. This simple physical treatment permitted the inventors to forgo axial resolution of barcodes. Currently, 2 approaches are available to improve axial resolution. Interferometric PALM would allow for resolving axial resolution to 5 nm, and astigmatic or dumbbell shaped point-spread-function can improve axial resolution to 50 nm. Implementing the astigmatic approach would be insufficient to resolve the color positions within a barcode, but it would be helpful to discern different barcoded mRNAs if they overlap in the xy but not z dimension.
Spatial and spectral barcoding schemes have different strategies for scaling up the throughput. Spatial barcoding is efficient. In principle, five-position barcodes (shown in
The spectral coding approach is more robust because errors associated with identifying spatial positions can be avoided. However, crosstalk among different fluorophores can impede the identification of the proper barcodes and result in leakage among the barcodes. To control for crosstalk, we performed several control experiments.
First, the inventors imaged individual dye pairs with the full imaging routine, going through all activators and emitters to quantify the amount of leakage from each dye pair into the others. By examining all 7 dye pairs used in in the inventors' study, they found the most leakage occurs from Cy3 activators, which can be activated by the 473 and 405 nm lasers. However, Alexa 488 and Alexa 405 cannot be activated with the 555 nm laser, so the crosstalk only appears in one direction. From the single dye pair experiments the inventors quantified the idealized x-talk with 12 of the target dye pairs, imaged in exactly the same routine as their barcode quantification. There is a small amount (˜1%) of non-specific activation in Alexa 405 and 473 with 556 nm activation, due to non-specific blinking of the dye pair. The probe pairs that exhibit x-talk at a rate higher than previously reported are A488 and Cy3 (6-6.5%). While not wishing to be bound by any one particular theory, this may be due to the close proximity of the dyes to each other in the probe design. This x-talk was still clearly separable from signal. In cy5.5, the inventors only used the cy3-cy5.5 pair, thus no crosstalk between 5.5 dyes could occur. To reject the false activation of cy3 by the 473 laser, the inventors discarded activations in the 473 channel that were less than 30% of the activations observed in the 555 nm laser channel. Similarly, the inventors set the threshold for rejection at 15% and 30% respectively for 405 nm activation of A488 and Cy3. In addition, there is crosstalk between the cy5 and cy5.5 emitter channels. Since this crosstalk only occurs in the cy3 activation channel, the inventors compared the activation intensity in cy3-cy5 vs cy3-cy5.5 channels and found about 12% spectral crosstalk between the 2 emitters. Thus, any activation in cy5.5 that was less than 30% of the activation in cy5 was rejected.
Second, to test the accuracy of the 3 color barcode readout, the inventors used the barcode that is the most prone to crosstalk, which is the cy3 activator paired with all three emitters. Several false-positive barcodes were observed at a 20% crosstalk rate with the proper barcode. However, this represents the worst-case scenario for crosstalk, since Cy3 can be activated by both 405 and 473 nm lasers. In addition, the gene targeted with this probe set has a relatively low copy number, so false barcodes due to cellular background and nonspecific blinking appeared at a relatively higher frequency compared to the correct barcode (
Third, when the inventors analyzed data of the full dataset with 32 genes, they examined the frequency of observing the barcode position that was not coded. With a total of 35 possible coding positions in the scheme used, there were 3 empty code positions that should not show up. This false identification frequency is 0.67±0.84 copies per cell, suggesting the inventors' entire barcode set imaged simultaneously is not significantly affected by false positives. In addition, the inventors performed analysis on the full data set with a single gene barcode dropped out, as a built in control on barcoding. The inventors observed that the empty position which is normally present at 4.9±2.3 copies per cell was present at 0.75±0.84 copies per cell, indicating a relatively small amount of crosstalk into that position from other barcodes.
Fourth, the inventors took a 20 gene probe set containing cy5 and cy7 emitters, and circularly permute the activators (405->488, 488->cy3, cy3->405). This effectively scrambled the barcode assignment since the emitters remained in the same position. The inventors observed a strong correlation between genes measured amongst both probe sets, indicating no significant bias is introduced by a particular assignment of the barcode. One significant outlier existed in the analysis, ylr194c. This outlier was dropped based upon its high Cook's distance of 2.08226 (
Fifth, the inventors performed single molecule FISH experiments measuring the expression of 11 genes, including 8 crz1, 1 msn2 and 2 aging genes. The inventors observed a R2=0.95 correlation between the mean levels measured by smFISH and the barcode approach.
Sixth, The inventors also performed q-PCR experiments measuring the mean copy number of 8 crz1 target genes. The inventors observed 0.95 correlation between the qPCR and the barcode data. The qPCR experiments were performed in triplicates and quantitated using lx, 10× and 100× serial dilutions.
The inventors measured the copy number of probed genes by tabulating the barcode reconstructions in single cells, for 62 cells. As shown in
Because Crz1 pulses occur stochastically in individual cells, promoter responses should be averaged out in population measurements. The mRNA copy numbers are shown in
The inventors found no connection between chromosome positions and gene correlations. Yps1, Put1, Pmc1, YLR194c and YLR414c are on chromosome 7 in that order. Cmk2 and Npt1 are on chromosome 15. Gyp7 is on Chromosome 4 and Sok2 is on chromosome 13. Npt1 and Pmc1 are well correlated, but are on different chromosomes, while YLR414c and yps1 are not as well correlated, but on the same chromosome. Previous experiments revealed little pairwise correlation (˜25%) among mRNAs of constitutive highly expressed genes. Thus the correlations observed were not due to static heterogeneity in the cell. At the same time, there appears to be an intrinsic limit to how well coordinated genes within a regulon can be. The inventors tested 2 extreme conditions where Crz1 concentration should not be the limiting factor, one by over-expressing Crz1, and the other by imaging cells immediately after treating with a high concentration of calcium (10 minutes at 200 mM). The inventors still observed that not every Crz1 target gene switched on, indicating additional barriers to transcriptional activation at individual promoters.
Imaging chromosome structures by super-resolution FISH with single-dye labeled oligonucleotides
Fluorescence In Situ Hybridization (FISH) is used extensively for labeling chromosomes for cytogenetics and cancer diagnostics. This technology is used for detecting aberrations in chromosome structure in tumor samples. However, up until now, technology relied on probes generated from Bacterial Artificial Chromosomes (BACs). The fluorescently labeled BACs probes are made by PCR and then sheared to 200-500 nt fragments. Thus, the region that the probes target spans from hundreds of kilobases to megabases. Unfortunately, the availability of probes depends on the availability of the BACs against particular regions of the chromosome. Also, as the probes cover a large genomic distance, they can have significant non-specific interactions, increasing the background of the detection. Thus, each probe set needs to be rigorously validated in many cell types, increasing the expense of the development process.
In response to the aforementioned problems, the inventors developed a technique to target specific gene loci on the chromosome with short 20-40mer oligonucleotides that are singly labeled with fluorophores for FISH. The inventors have demonstrated this technique in single bacteria and yeast cells, and it could also be used in human and mouse cells The inventors observed high hybridization efficiency for these short probes. When the inventors hybridized a gene on a chromosome with non-overlapping probe sets labeled with different colors, a high rate of colocalization (>95%) was observed, indicating most of the coding region is open to hybridization. If hybridization efficiency were low, then fluorescent spots from different probe sets would not colocalize (
The inventors have demonstrated that the novel DNA FISH technology is compatible with super-resolution fluorophores and super-resolution microscopy, as described herein. The inventors' protocol allows chromosomes to be hybridized efficiently using singly-labeled oligo probes. In addition, the inventors can FISH against RNA and DNA simultaneously to probe the transcript levels and the position of the chromosome in the same cell.
The inventors have applied super-resolution barcoding to label multiple chromosomal positions in bacteria using DNA FISH. This technique can enable the detection of multiple (up to 100) loci in the chromosomes of cells. Advantageously, the inventive technique is highly precise with kilobase resolution compared to almost megabase resolution of BACs FISH technology. This allows inexpensive probes to be made for any loci of interest on the chromosomes, without the limitation of the BACs library.
Importantly, in many forms of cancer there are significant copy number variations in the chromosome copy numbers and translocation events among chromosome arms, with large cell-to-cell variations. Using the inventive methods, copy number variations and translocations, as well as other structural variations known to be associated with diseases (e.g. deletions, insertions, and inversions), can be detected in a cell of a subject, thus facilitating diagnosis. Merely by way of non-limiting examples, the following diseases could be diagnosed using the inventive methods: Burkitt's lymphoma, mantle cell lymphoma, follicular lymphoma, papillary thyroid cancer, follicular thyroid cancer, acute myeloblastic leukemia with maturation, chronic myelogenous leukemia, acute lymphoblastic leukemia, acute promyelocytic leukemia, congenital fibrosarcoma, secretory breast carcinoma, acute myeloid leukemia, anaplastic large cell lymphoma, Ewing's sarcoma, DFSP, Acute myelogenous leukemia, synovial sarcoma, oligodendroglioma, oligoastrocytoma, low-grade fibromyxoid sarcoma, and schizophrenia.
In summary, the inventive methods can accomplish all of the current applications of DNA FISH in cancer diagnostics with the additional benefit of being highly multiplexible.
Ribosome production is an essential process and consumes a significant fraction of cellular resources. It has been estimated that ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) makes up about 95% of the RNAs in cells. In many prokaryotes, there are multiple copies of ribosomal RNA (rRNA) scattered at different loci along the chromosome (seven for E. coli, four for P. aeruginosa, five for S. aureous, two for Caulobacter crecentus, and eleven for B. sutbilis). The transcriptions of ribosomal RNAs (rRNAs) as well as ribosomal proteins are carefully coordinated to avoid potential imbalances in stoichiometry of the components.
The inventors determined that genomically distant ribosomal biogenesis genes are actually physically clustered in many prokaryotic species (
Supporting the inventors' hypothesis that spatial organization of ribosomal clusters may serve to coordinate expression levels among operons, it has been observed that clusters can be regulated by external conditions, as clusters dissociate in stressed cells. The dissociation of the clusters is correlated with the repression of ribosomal biogenesis as well as other essential genes for cellular maintenance. Thus the spatial clustering of functional related genes may serve as a one-stop mechanism for cells to synchronize cell growth rate with essential cellular machinery synthesis.
These preliminary results suggest surprisingly that there is sub-cellular organization even a bacteria cell. Lastly, recent investigations revealed that on a population level chromosomes are packed in a complex fashion in cells. A physical image of the genome in individual cells will provide an unprecedented look at how the genome is compacted, and how that organization coordinates transcriptional activities. The inventive technology described herein can be used to determine whether cells actively regulate chromosome conformation and facilitate switching between different transcriptional programs through spatial re-organization of the genome.
The inventors have made 3 key observations regarding ribosomal clusters in prokaryotes. 1. Using DNA FISH, it was observed that all seven rRNA operons colocalize in a single spot near the cell poles in individual E. coli cells (
As indicated above, the inventors have demonstrated DNA FISH with short oligos work efficiently in E. coli cells and 1 kb regions on the chromosome can be clearly resolved. Applying the techniques developed for single molecule FISH to hybridizing DNA described herein, the inventors used 12 probes 60 nucleotide in length to tile along a chromosome position. When the inventors hybridize a gene on the chromosome with non-overlapping probe sets labeled with different colors, they observed a high rate of colocalization (>95%) indicating most of the coding region is open to hybridization (as mentioned above). If hybridization efficiency were low, then fluorescent spots from different probe sets would not colocalize (
By designing probes that hybridize to the homologous region of the rDNA, the inventors can target all the rRNA operon at the same time. In addition, to resolve each of the rRNA operons, the inventors designed oligo probes that target the non-homologous sequence upstream of the rRNA loci. With both measurements, the inventors observed a distinct focus in the cell that corresponded to all rRNA loci assembled into a single position near the poles of the cell (
Given the seven rRNA operons are distributed across half of the chromosome, their clustering significantly affect the compaction of the chromosome in the cell. In order to image the physical organization of the chromosome, the inventors developed a technique called super-resolution barcoding (described above) to label multiple chromosomal loci in individual cells. The inventors have recently demonstrated used this technique to multiplex 32 mRNAs species in single cells. The underlying idea is that the extraordinary resolution in super-resolution microscopy (SRM) can be harnessed for systems biology: for the typical bacteria cell of 1-21=3, the 10-20 nm resolution of SRM translates into 105-106 independent volume elements (voxels) per cell. Many molecules have abundances far below this number in an E. coli cell: for instance, there are 103 operons and 104 total transcripts per cell. Thus, a large portion of the genome can be simultaneously resolved with SRM. As indicated above, the inventors used Fluorescence In Situ Hybridization (FISH) to label individual mRNAs or chromosomal loci with a distinct barcode and imaged the cells under a super-resolution microscope to readout the barcodes. The inventors can then quantitate the positions and abundances of many chromosomal positions simultaneously in situ by identifying their barcodes. For example, using only 9 of the photoswitchable fluorophore pairs currently available for SRM, over a hundred combinations of quadruplet (9C4=126) barcodes can be used. The abundances of each molecule can be quantitated by counting the number of times the corresponding barcode is observed in the super-resolution image of the cell, within their native cellular and inter-cellular contexts.
In validating their technique, the inventors targeted 11 loci on the bacterial chromosome. Each locus is labeled with 12 oligo probes. Using the super-resolution barcode technique, the inventors resolved the spatial positions of those eleven genes in the cell (
A combined genomics and microscopy approach can be used to uncover the puzzle of how the ribosomal clusters form and how they shape the structure of the chromosome in the cell. Pioneering efforts in resolving these interactions with massively parallel sequencing technology have begun to reveal the complexity of chromosome structure. However, these approaches are expensive and have poor resolution in highly repetitive regions, such as the rRNA operons. The inventors apply a more focused approach using directed chromosomal capture ligation to discover regions of the chromosomal that are physically close to the rRNA operons. Only regions that are in physical proximity would be ligated. The ligated product is then amplified using primers specific for rRNA operons, enriching only interactions between the rDNA and other loci. The enriched PCR products are then sequenced to determine the sequence of the interacting loci.
The inventors' preliminary 3C-seq results confirm that rRNA operons interact strongly with each other. While there are other sites of interactions, the rDNA loci primarily interact with each other. For example, rrnD interacts strongly with rrnG, while rrnH interacts rrnA, B, C, E. This bias in the interaction strength may indicate that there are preferential interactions amongst rRNA operons within the same polarity. One potential reason for this that homologous recombination of rRNA operons with opposite polarity can be high detrimental to the cell. This will be explored further in the mechanism and function section. Bioinformatics analysis of these non-ribosomal interaction partners may provide mechanistic and functional insight on the ribosomal clusters.
Using DNA FISH and RNA FISH to probe the rDNA loci in many bacteria species, the inventors observed that a single locus corresponding to the ribosomal clustering in diverse prokaryotic species such as E. coli, P. Areginosa, P. putida, and B. subtlis (
It is far from clear what the molecular mechanisms responsible for ribosomal clusters are. Extensive works have been performed to investigate the forces involved in shaping the nucleoid in bacteria cells. In previous works, it had been hypothesized that transcription and translation exerted opposing forces on the nucleoid compaction state. In addition, NAPs such as HNS have been shown to cluster chromosomal regions that contain HNS binding sites.
However, since ribosomal clusters had not been observed previously, it remains to be determined whether the forces that shape the nucleoid also affect the ribosomal clusters formation. The inventors' preliminary results suggest that ribosomal clusters states are not determined entirely by the overall nucleoid state in the cell. For example, when cells are treated with tetracycline, the nucleoid compacts while the ribosomal clusters dissociate (
rRNA Sequence May Drive Cluster Formation
The inventors have begun the investigation with ribosome-specific processes. The seven rRNA operons are highly homologous. The promoters regions all consist of P1 and P2 promoters. The ribosomal 23s and 16s RNAs are cotranscribed from the operon, separated by tRNAs and a spacer region. There are two types of tRNAs among the seven operons and are more or less equally distributed on the left and right arm of the chromosome. The inventors will perform systematic perturbations of each of these regions to observe their effects on the cluster formation.
The ribosomal operons are essential for cluster formation. The inventors have observed that in cells with deletion of 1 or more operons, the deleted regions are no longer inside the ribosomal clusters (FIG. 33A,B). The inventors used DNA-FISH to probe the regions upstream of the rRNA operons in WT and deletion cells. In rrnD deletion cells, the inventors observed that the region 500 bp upstream of rrnD operons is no longer in the ribosomal clusters, whereas in WT cells they are colocalized with the ribosomal clusters. In cells with only rrnD remaining, with all six other operons deleted, the inventors observed that none of the regions upstream to the rRNA operons cluster. Thus the operons are necessary for the presence of clusters.
The inventors are planning to construct mutants with part of the operon deleted to zoom in on the component of the operon that determines the cluster formation, In particular, the inventors will construct strains with deletion in only the rRNA region, including 23S, 16S and t-RNAs at the rrnA, D and H operons. These rRNA operons are chosen because rrnA and rrnD contain different t-RNAs and rrnH is on the right arm of the replication fork. By preserving the promoter, the inventors can ask whether it is the proteins bound to the promoters that catalyze the formation of the clusters or it is the rRNA coding sequence that is determinative.
Conversely, the inventors will also generate mutants that express the rRNA genes from an inducible T7 promoter replacing the endogenous rRNA promoter. By switching on and off the rRNA production from this locus, the inventors can determine whether the production of rRNA from these loci can bring it in and out of the ribosomal clusters. These experiments will tell the inventors whether the rRNA sequence itself and the transcription of rRNA are necessary and sufficient conditions for cluster formation.
The inventors will move one of the rRNA operons moved to a new position on the chromosome in order to determine whether the native placement of rRNA operon on the chromosome affect the ribosomal clusters. The global structure of chromosome is organized in macrodomains. It is possible that even with rRNA expressed in a different position the global steric constraints prevent the rRNA operons from clustering. Another possibility is that there are sequences upstream and downstream of the native rRNA operons that facilitate ribosomal clusters formation, without which would not occur efficiently. The inventors will walk the rRNA operons from the replication origin to the terminus on the arm of the chromosome to determine whether global chromosomal factors can impact the ability of rRNA operon to cluster. The inventors will perform this walking with the rrnH, because it is the only rRNA operon present on the right side of the replication fork. Lastly, it will be interesting to examine ribosomal clusters in E. coli strains with large chromosomal rearrangement. For example, in strain W3110, a large chromosomal inversion occurred between rrnD and rrnA operons. The inventors will examine whether ribosomal clusters for all the rRNA operons are affected in these rearrangement mutants.
The previous set of experiments allows for pinpointing the genomic sequences that allow ribosomal clusters formation. To identify the protein complexes that facilitate ribosomal clusters formation, the inventors will use the mutants from the Keio knockout collection and chemical inhibitors to perturb the function of these complexes and measure their effects on the ribosomal clusters.
As the rRNA operons are homologous, the inventors will first examine the role of the homologous recombination pathway in catalyzing cluster formation. The inventors have examined RecA, RecB deletions mutants which are parts of the homologous repair recBCD pathway. The inventors observed that the ribosomal clusters dissociates in both strains (
Consistent with the result that the homologous recombination pathway is involved in ribosomal clustering, the inventors treated cells with mitomycin, which generated DNA damage and recruit recA and recBCD to the damaged regions. The inventors observed a similar dispersion of rDNA in the cell, suggesting that ribosomal clusters may be dissociating because recA, and recBCD have been dispersed throughout the chromosome. It had been previously reported that RecA-GFP in WT cells are localized near the poles, while UV damaged cells has RecA-GFP dispersed throughout the cell.
To further test the hypothesis that the recBCD system binds to rRNA operons, the inventors will use super-resolution microscopy and ChIP-seq to determine the binding sites of recA and recB in WT and UV damaged cells. With the 20-30 nm resolution of super-resolution techniques, the inventors can colocalize recA-mEOS2 (a photoswitchable fluorescent protein) with the ribosomal clusters labeled with RNA-FISH. To address the possibility that fluorescent proteins fusions may disturb protein function, the inventors will use Chromatin Immuno-Precipitation (ChIP-seq) to map out the binding sites of recA and recB. Each protein can be fused with a small HIS tag, which will perturb the proteins minimally. These tagged proteins can be pulled down with antibodies with the bound DNA which can then be released and sequenced. The inventors will determine whether in WT cells recA and recB will bind within close proximity to the ribosomal operons.
Since the inventors observe that many other sites in the chromosome associate with ribosomal clusters besides the rRNA operons in the 3C-seq experiments, the inventors will determine whether recA and recB bind to the non-rDNA sites from the ChIP-seq data. If recAB binds to the sites other than rDNA and overlaps with the 3C-seq data, that would suggest that those sequences are “fragile” and recAB recruit them into the ribosomal clusters. However, if the inventors find the recAB ChIP seq and the rDNA 3C-seq data do not overlap, it strongly suggests that recAB are only partially responsible for ribosomal clusters formation and other factors contribute. Then the inventors need to examine the epistatic interactions between the recAB complexes with other pathways involved in transcriptional regulation of the rRNA operons and nucleoid organization. Lastly, performing bioinformatics analysis on the recAB ChIP seq data set as well as the 3C-seq data will provide insights on the sequences that may attract the homologous recombination and nucleoid associated proteins (NAPs). In particular, the inventors will examine whether CHI sequences are over-represented on near the sites that interacts with rDNA and recAB proteins.
To further elucidate the mechanisms underlying ribosomal clusters formation, the inventors will examine null mutants of key nucleoid proteins, such as F is, IHF, HU and StpA, as well as proteins involved in rRNA transcriptional regulation and termination. In addition, the inventors will investigate how chemical inhibitors of specific process, such as transcription, translation and topoisomerases, affect the ribosomal clusters formation process. Lastly, the inventors will examine the epistatic interactions between the NAPs, the transcriptional processes, and the homologous recombination pathway.
The inventors have already observed that several nucleoid protein null mutants affect distances between particular loci with DNA-FISH. For example, in H-NS null cells, the ribosomal genes which are usually strongly co-localized (
Environmental challenges like heat shock, deprivation of nutrients and oxygen, and antibiotics, have been shown to trigger global transcriptional changes. The inventors observed that these stresses also trigger significant ribosomal clusters and chromosomal changes in cells by FISH and DAPI (
Chemical inhibitors that disrupt other cellular processes besides transcription have distinct ribosomal clusters phenotypes. In particular, the inventors will examine DNA damage (UV and mitomycin C), transcriptional inhibitions (rifampicin), protein synthesis inhibitions (chloramphenicol and tetracycline), topoisomerase inhibitition (Norfloxacin) as well as cell wall synthesis (ampicillin) and DNA synthesis inhibitors (trimethoprim). It had been previously observed that these drugs have direct effects on the shape of the nucleoid. The inventors will examine their effects on the ribosomal clusters, which may not be correlated with the overall nucleoid shape. For example, with Chloramphenicol, the inventors observed that while the overall nucleoid condenses in the cell, the ribosomal clusters dissociate and disperse. In addition, the ribosomal clusters dissociate in a heterogeneous fashion in different cells, providing mechanistic insight that indicating a multi-step stochastic process in ribosomal loci clustering. The inventors expect the epistatic interaction between chemical inhibitors and the null mutants will reveal interactions with the ribosomal clusters interactions that would normally be masked by compensatory processes in WT cells.
Lastly, the inventors will examine the role of transertion in ribosomal clusters formation. As translation is co-transcriptional in prokaryotes, nascent membrane proteins that are still attached to the mRNA can drag the mRNA near the membrane. It has been hypothesized that this process of transertion can affect the overall morphology of the nucleoid. The inventors will examine the role of membrane proteins in formation of ribosomal clusters. The inventors will start by examining the spatial location of the highly expressed membrane proteins loci by DNA FISH. If the membrane proteins operons are co-localized with the ribosomal clusters, that would suggest transertion exerts a force on the nascent ribosomes. There are some preliminary evidences suggesting that a few membrane proteins are colocalized with ribosomal clusters, while others are not. The inventors will pursue this further by putting these membrane proteins on inducible promoter and examine whether their expression affects their spatial positions in the cell. As the membrane insertion process is facilitated by the Signal Recognition Particle (SRP) complex, the inventors will inhibit SRP function by a temperature sensitive mutant. These experiments will allow the inventors to measure the contribution of transertion on ribosomal clusters.
The inventors expect to observe that one or two dominant processes determine the clustering process, with many inputs modulating the strength and duration of the interactions to combinatorially regulate the ribosomal clusters formation processes. The inventors will use DNA-FISH as a first step to observe whether ribosomal clusters formation is perturbed with coding sequence deletion and null mutants. The inventors will then use 3C-seq, ChIP-seq to measure the global conformation changes in the chromosome structure in the presence of perturbations. The inventors will follow up the global studies with super-resolution imaging to examine the heterogeneity of ribosomal clusters phenotypes in a cell population. The detailed global and single cell characterization of how different cellular processes perturb ribosomal clusters will reveal the molecular mechanism of the ribosomal clusters formation.
Given that there are many house-keeping genes clustered around rRNA loci, and that ribosomal cluster is dynamically regulated by stress conditions, it is possible that clustering of rRNA is used by the cell to coordinately regulate many essential biogenesis processes at the same time. The inventors will use microscopy and genomics techniques to address these questions.
As the ribosomal cluster significantly reorganizes the chromosome in the cell, the inventors will investigate how rRNA clusters affect the transcription regulation of genes in the genome. There are at least two possible molecular mechanisms. First, given that rRNA production accounts for the 90% of RNAP activity in the cell, ribosomal clusters leads to a concentration of RNAP at the cell pole where the clusters are. Indeed, it has been observed that RNAP-GFP is localized near the poles in fast growing cells. This locally higher concentration of RNAP may lead to up-regulation of genes near rRNA operons by virtue of physical approximity. As RNAP genes and metabolic genes cluster around the rRNA operons, it is possible that these highly expressed house-keeping genes are taking advantage of the high local concentration of RNAP to facilitate their own transcription.
To measure the effect of local RNA polymerase concentration, the inventors will use qPCR and RNA-seq to quantify the expression of genes near the rDNA. The inventors will compare strains with deleted or shuffled rRNA operons. The inventors have shown that deletion of a particular rRNA operon results in that region dissociating from the ribosomal clusters. With each of the rDNA deleteion strains, the inventors will examine the expression level of several highly expressed genes around the operon by qPCR. If there is preliminary evidence from qPCR suggesting that rDNA deletion impact local gene expression, the inventors will apply RNA-seq for a high density map of the all genes surrounding the operon. The RNA-seq data will allow the inventors to analyze whether gene regulation is distance dependent from the rDNA operon. While rrnD, G, and H are spaced far from other rDNAs on the chromosome, rrnA, B, C, E are clustered within lmegabases of each other near the origin of replication (OriC). It will be interesting to compare genes in that region to other genes close to other rRNA operons to explore whether there are synergistic effects due to the compacted nature of that region.
In a forward experiment, by moving the rrnH operon from its native location to other parts of the chromosome, the inventors can ascertain whether moving a segment of the chromosome into the ribosomal clusters can up-regulate the genes close to that segment. The inventors will examine multiple positions on the chromosome to determine whether there are local topological constraints that affects gene expression. RNA-seq will also be used if the preliminary qPCR yield promising results to provide a high resolution map of the transcriptional activities near the ribosomal clusters.
Local concentration of RNAP may be the only one of the factor regulating transcription at the ribosomal clusters. Topological constraints from ribosomal clusters may directly affect transcriptional activities of gene proximal to the rDNA loci. A consequence of the high rate of rRNAs transcription is the buildup of positive super-coiling in front of the RNAP, and negative super-coiling behind the RNAP. As the ribosomal clusters provide an anchor for the DNA sequence flanked by two adjacent rDNA operons, the super-coiling can accumulate in a directional fashion in those regions. Topo-isomerase and gyrase are responsible for relieving negative and positive super-coiling strains. However, given the high frequency of transcription of rRNA, the topoisomerases and gyrases may not have sufficient time to relax the supercoiling buildups on the operons.
The inventors propose several experiments to explore the role super-coiling plays in ribosomal clusters. First, by controlling the expression level of the rRNA transcripts with an inducible promoter, the inventors can tune the amount of super-coiling accumulated near the operon. For this experiment, the inventors will use the T7 inducible rRNA strain generated for the mechanistic studies. The inventors can then measure the abundances of the adjacent genes as a function of the induction level to measure how super-coiling tunes the transcription regulation of nearby operons. Second, the inventors will use Norfloxacin, an inhibitor of topoisomerase, to perturb the level of supercoiling in native rRNA operons. To control for global effects of the topoll inhibitor, the inventors use rDNA deletion strains and profile the regions around the deleted operon. As those regions are now out of the ribosomal clusters and without active rRNA transcription, they should not be directly affected by the inhibitor. Lastly, the inventors can induce the expression of exogeneous topoll and gyrase to control the net positive or negative super-coiling in the native rDNA loci for the qPCR and RNA-seq experiments.
Regulation of Individual rRNA Operons
Regulation of rRNA operons have been extensively studied and found to be strictly controlled by a number of factors. Since the ribosomal synthesis rate is strongly coupled to the growth rates, many pathways that sense extracellular nutrient and stress conditions, such as NAPs and ppGpp bind to regulatory sequences on the rRNA promoters. However, given that all the rRNA operons are clustered, how does the spatial organization affect regulation of their expressions?
The inventors propose to examine the ribosomal biosynthesis with the spatial organization in mind. The inventors will begin by imaging the rRNA production rate at individual rDNA loci. The inventors will use oligo probes that hybridize against the spacer region in the rRNA transcript in order to target nascent rRNA transcription sites. The nascent transcription sites can then be resolved by super-resolution microscopy and assigned to specific operons by colocalization with the operon specific DNA FISH probes. In this fashion, the inventors can uniquely assign the instantaneous transcription rate at each rRNA operon. As there are two types of t-RNA (i-a vs g) sandwiched between the 23s and 16s rRNA, probes can designed to distinguish between different subsets of the rDNA loci. This allows for further resolving the transcriptional activities on the distinct rDNA loci.
When the inventors FISH against nascent rRNAs, they observe that rRNA synthesis is still occurring on the dissociated loci. The intensity of the nascent transcription signal is highly variable among loci. By using super-resolution multiplex imaging to tag the seven rRNA loci by DNA FISH as well as the nascent transcripts by RNA FISH, the inventors can determine whether there are loci are expressing preferentially and whether there are subsets of operons that tend to express in a correlated fashion. In addition, the inventors can tune the rRNA expression levels by slowing down cell growth by controlling nutrient levels, as well as using drugs that mimic ATP depletion conditions. The inventors can then directly observing the heterogeneity in the expression levels of rRNA as a function of these perturbations.
In fact, rRNAs can be expressed from two promoters, aptly named P1 vs P2. Because the initial sequences for the two promoters are distinct, we can distinguish the abundance of transcript from either promoter. It is known that P1 and P2 promoters are regulated differentially by FIS and αCTD at different loci. The inventors expect to observe differential expression of P1 and P2 rRNAs at distinct loci in individual cells. In addition, at P1 has been shown to be strongly regulated by external stress while P2 is less so. Using the inventive technique, one can look at the loci specific expression of P1 and P2 under stress conditions as well as mutants generated in this study to understand the effect of ribosomal clusters on their regulation.
As the rRNA operons are homologous, there is a significantly high probability for homologous recombination to occur between the different loci, as well as among sister chromosomes during replication. These recombination events will results in significant shuffling of the chromosome. In the worst case scenario, operons with sequences running in opposite directions can generate inversions in the chromosome. These inversions events appear to be selected against, as Salmonella chromosomes are highly homologous with E. coli and are free of inversions. It is possible that ribosomal clustering prevents spurious recombination events among the homologous rRNA operons over an evolution timescale. To examine this hypothesis, the inventors will focus on the placement of rDNA loci during replication. As sister chromosomes are synthesized with multiple copies of rRNA operons, the inventors will investigate whether these sites cluster with each other and how are they separated eventually by cell division. The inventors observed tantalizing hints that some of the rRNA operons dissociate from the main ribosomal clusters late in division and may pull apart to serve as a new nucleating point for ribosomal clusters for the daughter cell. The inventors will use super-resolution imaging with cell-cycle synchronized cells to observe this process, as well as using integrated lacO and tetR arrays to measure the movement of 1 or 2 operons in time. These experiments will establish the time sequence of ribosomal clusters formation and segregation during replication and division.
The feedback regulations between ribosomal proteins and rRNAs have been shown to be crucial for cellular function, as a mis-balanced in rRNA and ribosomal protein levels results in slower cell growth rates. Ribosomal protein, RNAP subunits and many housekeeping genes are located adjacent to the rRNA operons. In anaerobic, energy stress, and H-NS mutants, the inventors observed significant changes in chromosome shape and physical proximity between ribosomal genes (
Besides measuring the transcription events, the inventors can visualize the ribosomes directly by smFISH. The inventors have shown that they can target rRNA in the 16s or 23s subunits of the bacterial ribosome. The inventors observed that under different conditions, ribosomes are differentially localized. In rich media, ribosomes are localized to the periphery of the nucleoid near the cell membrane. In starvation media, ribosomes are enriched in the poles of the cell. This is consistent with previous electron microscopy images in starved cells. Thus, cells can traffic components in their translational machinery and control their activity at a global level. Since ribosomal activity has been conjectured to play a counter-acting role to nucleoid compaction, the inventors will measure the ribosomal distribution under different stress conditions and assess their correlation with the nucleoid conformation in single cells. These results will provide functional insight on how translation is regulated in cells and affect nucleoid organization.
In addition to the ribosomal clusters, there may be other genes that cluster in stress response pathways. TFs have been found to interact with NAPs to bend DNA and assist in forming gene clusters. For example, FNR, the transcriptional regulator in response to anaerobic stress, binds to the nir promoter along with IHF, H-NS and FIS to form a nucleoprotein complex and regulate the transcription of the promoter. Other anaerobic TFs, ArcA and YgiP have similar and overlapping consensus sequence. While FNR has been found to exist as a dimer, ArcA and YgiP are known to multimerize and can potentially form clusters. Thus, the inventors will investigate the effect of FNR, YgiP and ArcA null strains on the nucleoid structure during anaerobic shift. The inventors have imaged the nucleoid structures with DAPI in all three null strains and observed significant changes in aerobic and anaerobic conditions. Imaging with super-resolution barcoding will provide a high resolution map of the chromosomal interactions that may lead to discovery of alternative gene clusters and regulatory dynamics.
In conclusion, clustering of ribosomal genes shows bacterial chromosomes are not ordered in a simple linear order. The clustering of rRNA operons appears to be evolutionarily conserved across many prokaryotes. These results indicate there are common mechanistic and functional roles for ribosomal clusters. With the super-resolution tools described in this application to map chromosomal structures in single cells, significant progress can be made in understanding how bacteria use spatial organization to regulate gene expression over many biological processes in a coordinated fashion.
The various methods and techniques described above provide a number of ways to carry out the invention. Of course, it is to be understood that not necessarily all objectives or advantages described may be achieved in accordance with any particular embodiment described herein. Thus, for example, those skilled in the art will recognize that the methods can be performed in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objectives or advantages as may be taught or suggested herein. A variety of advantageous and disadvantageous alternatives are mentioned herein. It is to be understood that some preferred embodiments specifically include one, another, or several advantageous features, while others specifically exclude one, another, or several disadvantageous features, while still others specifically mitigate a present disadvantageous feature by inclusion of one, another, or several advantageous features.
Furthermore, the skilled artisan will recognize the applicability of various features from different embodiments. Similarly, the various elements, features and steps discussed above, as well as other known equivalents for each such element, feature or step, can be mixed and matched by one of ordinary skill in this art to perform methods in accordance with principles described herein. Among the various elements, features, and steps some will be specifically included and others specifically excluded in diverse embodiments.
Although the invention has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the embodiments of the invention extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and modifications and equivalents thereof.
Many variations and alternative elements have been disclosed in embodiments of the present invention. Still further variations and alternate elements will be apparent to one of skill in the art. Among these variations, without limitation, are the specific number of antigens in a screening panel or targeted by a therapeutic product, the type of antigen, the type of cancer, and the particular antigen(s) specified. Various embodiments of the invention can specifically include or exclude any of these variations or elements.
In some embodiments, the numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
In some embodiments, the terms “a” and “an” and “the” and similar references used in the context of describing a particular embodiment of the invention (especially in the context of certain of the following claims) can be construed to cover both the singular and the plural. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations on those preferred embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. It is contemplated that skilled artisans can employ such variations as appropriate, and the invention can be practiced otherwise than specifically described herein. Accordingly, many embodiments of this invention include all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
Furthermore, numerous references have been made to patents and printed publications throughout this specification. Each of the above cited references and printed publications are herein individually incorporated by reference in their entirety.
In closing, it is to be understood that the embodiments of the invention disclosed herein are illustrative of the principles of the present invention. Other modifications that can be employed can be within the scope of the invention. Thus, by way of example, but not of limitation, alternative configurations of the present invention can be utilized in accordance with the teachings herein. Accordingly, embodiments of the present invention are not limited to that precisely as shown and described.
This application is a continuation-in-part of U.S. patent application Ser. No. 13/043,413, filed on Mar. 8, 2011, now pending, which claims the benefit of U.S. Provisional Patent Application No. 61/311,654, filed on Mar. 8, 2010, now expired. This application also claims the benefit of U.S. patent application Ser. No. 13/725,717 which was filed on Dec. 21, 2012 and claims the benefit of Provisional Patent Application No. 61/579,772, filed on Dec. 23, 2011, now expired. Finally, this application claims the benefit of U.S. Provisional Patent Application No. 61/692,913, filed on Aug. 24, 2012. All of the aforementioned applications are incorporated herein by reference in their entirety.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant Nos.: GM087588 and 1DP2OD008530, awarded by National Institutes of Health.
| Number | Date | Country | |
|---|---|---|---|
| 61579772 | Dec 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13725717 | Dec 2012 | US |
| Child | 14010492 | US |
