Patent Grant
8703483
1. Field of the Invention
This invention provides methods for toxicological screening of pharmaceuticals and other chemical compounds. The invention specifically provides reagents that are human embryonic stem cells (hESC) or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, as well as methods for using these cells to detect developmental toxicity or teratogenic effects of pharmaceutical compounds and other chemicals. More particularly, the invention provides an in vitro means for analyzing toxicity of compounds predictive of their toxicity during human development. Candidate predictive biomarkers for toxic or teratogenic effects are also identified and provided herein.
2. Background of Invention
Birth defects are a major cause of infant morbidity in the United States, affecting 1 in every 33 infants born (Brent & Beckman, 1990, Bull NY Acad Med 66: 123-63; Rosano et al., 2000, J Epidemiology Community Health 54:660-66), or approximately 125,000 newborns per year. It is understood that developmental toxicity can cause birth defects, and can generate embryonic lethality, intrauterine growth restriction (IUGR), dysmorphogenesis (such as skeletal malformations), and functional toxicity, which can lead to cognitive disorders such as autism. There is an increasing concern about the role that chemical exposure can play in the onset of these disorders. Indeed, it is estimated that 5% to 10% of all birth defects are caused by in utero exposure to known teratogenic agents (Beckman & Brent, 1984, Annu Rev Pharmacol 24: 483-500).
Concern exists that chemical exposure may be playing a significant and preventable role in producing birth defects (Claudio et al., 2001, Environm Health Perspect 109: A254-A261). This concern has been difficult to evaluate, however, since the art has lacked a robust and efficient model for testing developmental toxicity for the more than 80,000 chemicals in the market, plus the new 2,000 compounds introduced annually (General Accounting Office (GAO), 1994, Toxic Substances Control Act: Preliminary Observations on Legislative Changes to Make TSCA More Effective, Testimony, Jul. 13, 1994, GAO/T-RCED-94-263). Fewer than 5% of these compounds have been tested for reproductive outcomes and even fewer for developmental toxicity (Environmental Protective Agency (EPA), 1998, Chemical Hazard Data Availability Study, Office of Pollution Prevention and Toxins). Although some attempts have been made to use animal model systems to assess toxicity (Piersma, 2004, Toxicology Letters 149:147-53), inherent differences in the sensitivity of humans in utero have limited the predictive usefulness of such models. Development of a human-based cell model system would have an enormous impact in drug development and risk assessment of chemicals.
Toxicity, particularly developmental toxicity, is also a major obstacle in the progression of compounds through the drug development process. Currently, toxicity testing is conducted on animal models as a means to predict adverse effects of compound exposure, particularly on development and organogenesis in human embryos and fetuses. The most prevalent models that contribute to FDA approval of investigational new drugs are whole animal studies in rabbits and rats (Piersma, 2004, Toxicology Letters 149: 147-53). In vivo studies rely on administration of compounds to pregnant animals at different stages of pregnancy and embryonic/fetal development (first week of gestation, organogenesis stage and full gestation length). However, these in vivo animal models are limited by a lack of robustness between animal and human responses to chemical compounds during development. Species differences are often manifested in trends such as dose sensitivity and pharmacokinetic processing of compounds. At present, animal models are only 50% efficient in predicting human developmental response to compounds (Greaves et al., 2004, Nat Rev Drug Discov 3:226-36). Thus, human-directed predictive in vitro models present an opportunity to reduce the costs of new drug development and enable safer drugs.
In vitro models have been employed in the drug industry for over 20 years (Huuskonen, 2005, Toxicology & Applied Pharm 207:S495-S500). Many of the current in vitro assays involve differentiation models using primary cell cultures or immortalized cells lines (Huuskonen, 2005, Toxicology & Applied Pharm 207:S495-S500). Unfortunately, these models differ significantly from their in vivo counterparts in their ability to accurately assess development toxicity. In particular, the ECVAM initiative (European Center for Validation of Alternative Methods) has used mouse embryonic stem cells as a screening system for predictive developmental toxicology. The embryonic stem cell test (EST) has shown very promising results, with a 78% statistically significant correlation to in vivo studies, and the test was able to differentiate strong teratogens from moderate/weak or non-embryotoxic compounds (Spielmann et al., 1997, In Vitro Toxicology 10:119-27). This model is limited in part because toxicological endpoints are defined only for compounds that impair cardiac differentiation. This model also fails to account for interspecies developmental differences between mice and humans, and so does not fully address the need in the art for human-specific model systems.
Thus there remains a need in this art for a human-specific in vitro method for reliably determining developmental toxicity in pharmaceutical agents and other chemical compounds. There also is a need in the art to better understand human development and its perturbation by toxins and other developmental disrupting agents, to assist clinical management of acquired congenital disorders and the many diseases that share these biochemical pathways, such as cancer.
The present invention provides for the assessment of a plurality of small molecules, preferably secreted or excreted by hES cells or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, and is determined and correlated with health and disease or insult state. Similar analyses have been applied to other biological systems in the art (Want et al., 2005 Chem Bio Chem 6: 1941-51), providing biomarkers of disease or toxic responses that can be detected in biological fluids (Sabatine et al., 2005 Circulation 112:3868-875).
The present invention provides reagents and methods for in vitro screening of toxicity and teratogenicity of pharmaceutical and non-pharmaceutical chemicals using undifferentiated human embryonic stem cells (hESC) or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells. The invention provides human-specific in vitro methods for reliably determining toxicity, particularly developmental toxicity and teratogenicity, of pharmaceuticals and other chemical compounds using human embryonic stem cells (hESCs) or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells. As provided herein, hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, are useful for assessing toxic effects of chemical compounds, particularly said toxic and teratogenic effects on human development, thus overcoming the limitations associated with interspecies animal models. In particular, the invention demonstrates that metabolite profiles of hES cells or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells are altered in response to known disruptors of human development.
The invention shows that the hESC metabolome is a source of human biomarkers for disease and toxic response. In particular embodiments, exposure of hESC to valproate induced significant changes in different metabolic pathways, consistent with its known activity as a human teratogen. In other embodiments, hESC exposure to varying levels of ethanol induced significant alterations in metabolic pathways consistent with alcohol's known effects on fetal development.
In one aspect, the invention provides methods for using undifferentiated pluripotent human embryonic stem cells (hESC) or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, for in vitro evaluation. In the inventive methods, undifferentiated hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells are exposed to test compounds, preferably at concentrations reflective of in vivo levels or at levels found in maternal circulation. Further embodiments of this aspect of the invention provide for determination of the capacity of the test compound to induce differentiation of pluripotent hESC into particular cell types. In other embodiments, the inventive methods are provided using pluripotent, non-lineage restricted cells. The benefit of utilizing pluripotent stem cells is they permit analysis of global toxic response(s) and are isolated from the physiological target of developmental toxicity, i.e. the human embryo. In addition, because these cells have not differentiated into a specific lineage, the potential for false negatives is reduced. In yet further embodiments are provided methods using hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, for assessing toxicity and particularly developmental toxicity and teratogenicity.
In another aspect the invention provides methods for identifying predictive biomarkers of toxic responses to chemical compounds, particularly pharmaceutical and non-pharmaceutical chemicals, and particularly to known teratogens. In embodiments of this aspect, a dynamic set representative of a plurality of cellular metabolites, preferably secreted or excreted by hES cells or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, is determined and correlated with health and disease or toxic insult state. Cellular metabolites according to this aspect of the invention generally range from about 10 to about 1500 Daltons, more particularly from about 100 to about 1000 Daltons, and include but are not limited to compounds such as sugars, organic acids, amino acids, fatty acids and signaling low-molecular weight compounds. Said biomarker profiles are diagnostic for toxicity of chemical compounds, particularly pharmaceutical and non-pharmaceutical chemicals, that participate in and reveal functional mechanisms of cellular response to pathological or toxic chemical insult, thus serving as biomarkers of disease or toxic response that can be detected in biological fluids. In particularly preferred embodiments of this aspect of the invention, these biomarkers are useful for identifying active (or activated) metabolic pathways following molecular changes predicted, inter alia, by other methods (such as transcriptomics and proteomics).
The invention thus also provides biomarker and pluralities of biomarkers, in some instances associated with metabolites from particular metabolic pathways, that are indicative of toxic or teratogenic insult. Said markers as provided by the invention are used to identify toxic and teratogenic insult, and in particular embodiments are used to characterize the amount or extent of said insult by being correlated with the amount or extent of the particular biomarker or plurality of biomarkers detected in cell culture media. In particular embodiments, said plurality of biomarkers provide a diagnostic pattern of toxic or teratogenic insult, more particularly identifying one or a multiplicity of specific metabolic pathways comprising metabolites detected after toxic or teratogenic insult.
The present invention is advantageous compared with inter alia the ECVAM mouse model because toxicity testing and biomarker identification are performed with human cells, specifically human embryonic stem cells (hESC). Human embryonic stem cells are able to recapitulate mammalian organogenesis in vitro (Reubinoff et al., 2000, Nature Biotechnology 18:399-404; He et al., 2003, Circ Res 93:32-9; Zeng et al., 2004, Stem Cells 22:925-40; Lee et al., 2000, Mol Genet Metab 86:257-68; Yan et al., 2005, Stem Cells 22:781-90) because they are pluripotent and self-renewing cells. Thus, hESCs can reveal mechanisms of toxicity, particularly developmental toxicity, and identify developmental pathways that are particularly sensitive to chemicals during early human development. The “human for human” embryonic model provided by the inventive methods disclosed herein permits a better understanding of the pathways associated with developmental toxicity, as this is a system developed directly from the target organism, as well as being a more accurate and sensitive assay for toxic or teratogenic insult in human development.
The methods of the invention provide further advantages in identifying important biomarkers for toxicity and teratogenicity by functional screening of hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells. These biomarkers advantageously identify metabolic and cellular pathways and mechanisms of toxicity, particularly developmental toxicity. Importantly, these biomarkers may also assist in the evaluation of toxic effects of chemicals on the developing human embryo.
In yet another aspect of the invention, differentially-detected secreted or excreted cellular products identified by methods of the invention include those associated with neurodevelopmental disorders and alterations in associated metabolic pathways, and include but are not limited to kynurenine, glutamate, pyroglutamic acid, 8-methoxykynurenate, N′-formylkynurenine 5-hydroxytryptophan, N-acetyl-D-tryptophan and other metabolites in the tryptophan and glutamate metabolic pathways.
Functional toxicity in post-natal life can be predicted using hESC since differentiated cells with critical in vivo properties can be generated in vitro. hESCs can be used to produce lineage-specific cells, including lineage-specific stem cells, precursor cells and terminally-differentiated cells, providing therein enriched populations of cells typically present in vivo in mixtures of different cell types comprising tissues. The invention thus provides methods for using hESCs to produce said enriched and developmental stage-specific populations of cells for toxicity screening of chemical compounds, particularly drugs, drug lead compounds and candidate compounds in drug development, to identify human-specific toxicities of said chemical compounds. These aspects of the methods of the invention are advantageous over art-recognized in vitro and in vivo animal model systems.
Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawing wherein:
The invention provides reagents, including human embryonic stem cells (hESC) or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells produced therefrom, for assessing developmental toxicity using the human embryonic stem cell metabolome. Human embryonic stem cells are pluripotent, self-renewing cells isolated directly from preimplantation human embryos that recapitulate organogenesis in vitro. Lineage-specific precursor cells are derived from hES cells and have entered a specific cellular lineage, but yet remain multipotent with regard to cell type within that specific lineage. For example, neural precursors have committed to neural differentiation but yet remain unrestricted as to its neural cell type. Also within the scope of the inventive methods are terminally-differentiated cell types, such as neurons. Biochemical pathways of human development and disease are active in hESCs and or hESC-derived lineage-specific cells, because they recapitulate differentiation into functional somatic cells. Disruption of these pathways during development contributes to disorders such as neural tube defects (NTDs) and cognitive impairment. Environmental agents, namely chemicals or drugs, participate in the ontogenesis of certain acquired congenital disorders. The question of which pathways during early human development are particularly susceptible to the effects of the environment remains unsolved.
The metabolome, defined as the total dynamic set of cellular metabolites present in cells, is a product of health or disease/insult states. Metabolomics is particularly sensitive to environmental effects in comparison to other “omic” areas of study, such as genomics and proteomics. Cellular metabolites include but are not limited to sugars, organic acids, amino acids and fatty acids, particularly those species secreted or excreted from cells, that participate in functional mechanisms of cellular response to pathological or chemical insult. These cellular metabolites serve as biomarkers of disease or toxic response and can be detected in biological fluids (Soga et al., 2006, J Biol Chem 281:16768-78; Zhao et al., 2006, Birth Defects Res A Clin Mol Teratol 76:230-6), including hESC culture media. Importantly, metabolomic profiling may confirm functional changes that are often predicted by transcriptomics and proteomics.
However, because it was known that hESCs are highly sensitive to the culture microenvironment (Levenstein et al., 2005, Stem Cells 24: 568-574; Li et al., 2005, Biotechnol Bioeng 91:688-698.), their application as a source of predictive biomarkers in response to chemical compounds, including toxins, teratogens and particularly pharmaceutical agents, drug lead compounds and candidate compounds in drug development, and their usefulness in establishing in vitro models of disease and development was uncertain, inter alia because those of skill in the art could anticipate that exposure to an exogenous chemicals could be highly detrimental to survival of hES cells and preclude obtaining useful information from them. This concern has turned out not to be justified.
As used herein, the term “human embryonic stem cells (hESCs)” is intended to include undifferentiated stem cells originally derived from the inner cell mass of developing blastocysts, and specifically pluripotent, undifferentiated human stem cells and partially-differentiated cell types thereof (e.g., downstream progenitors of differentiating hESC). As provided herein, in vitro cultures of hESC are pluripotent and not immortalized, and can be induced to produce lineage-specific cells and differentiated cell types using methods well-established in the art. In preferred embodiments, hESCs useful in the practice of the methods of this invention are derived from preimplantation blastocysts as described by Thomson et al., in co-owned U.S. Pat. No. 6,200,806. Multiple hESC cell lines are currently available in US and UK stem cell banks.
The terms “stem cell progenitor,” “lineage-specific cell,” “hESC derived cell” and “differentiated cell” as used herein are intended to encompass lineage-specific cells that are differentiated from hES cells such that the cells have committed to a specific lineage of diminished potency. In some embodiments, these lineage-specific precursor cells remain undifferentiated with regard to final cell type. For example, neuronal stem cells are derived from hESCs and have differentiated enough to commit to neuronal lineage. However, the neuronal precursor retains ‘stemness’ in that it retains the potential to develop into any type of neuronal cell. Additional cell types include terminally-differentiated cells derived from hESCs or lineage-specific precursor cells, for example neural cells.
The term “cellular metabolite” as used herein refers to any small molecule secreted and/or excreted by a hESC or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, produced therefrom. In preferred embodiments, cellular metabolites include but are not limited to sugars, organic acids, amino acids, fatty acids, hormones, vitamins, oligopeptides (less than about 100 amino acids in length), as well as ionic fragments thereof. Cells may also be lysed in order to measure cellular products present within the cell. In particular, said cellular metabolites are from about 10 to about 3600 Daltons in molecular weight, more particularly about 10 to about 1500 Daltons, and yet more particularly from about 100 to about 1000 Daltons.
hESCs are cultured according to the methods of the invention using standard methods of cell culture well-known in the art, including, for example those methods disclosed in Ludwig et al. (2006, Feeder-independent culture of human embryonic stem cells, Nat Methods 3: 637-46.). In preferred embodiments, hESCs are cultured in the absence of a feeder cell layer during the practice of the inventive methods; however, hESCs may be cultured on feeder cell layer prior to the practice of the methods of this invention.
The term “administering” as used herein refers to contacting in vitro cultures of hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells produced therefrom with a toxic, teratogenic, or test chemical compound. In a preferred embodiment the dosage of the compound is administered in an amount equivalent to levels achieved or achievable in vivo, for example, in maternal circulation.
The phrases “identifying cellular metabolites that are differentially produced” or “detecting alterations in the cells or alternations in cell activity” as used herein include but are not limited to comparisons of treated hES cells or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, to untreated (control) cells (i.e., cells cultured in the presence (treated) or absence (untreated) of a toxic, teratogenic, or test chemical compound). Detection or measurement of variations in cellular metabolites, excreted or secreted therefrom, between treated and untreated cells is included in this definition. In a preferred embodiment, alterations in cells or cell activity are measured by determining a profile of changes in cellular metabolites having a molecular weight of less than 3000 Daltons, more particularly between 10 and 1500 Daltons, and even more particularly between 100 and 1000 Daltons, in a treated versus untreated cell as illustrated in
The term “correlating” as used herein refers to the positive correlation or matching of alterations in cellular metabolites including but not limited to sugars, organic acids, amino acids, fatty acids, and low molecular weight compounds excreted or secreted from hES cells or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, to an in vivo toxic response. The screened cellular metabolites can be involved in a wide range of biochemical pathways in the cells and related to a variety of biological activities including, but not limited to inflammation, anti-inflammatory response, vasodilation, neuroprotection, oxidative stress, antioxidant activity, DNA replication and cell cycle control, methylation, and biosynthesis of, inter alia, nucleotides, carbohydrates, amino acids and lipids, among others. Alterations in specific subsets of cellular metabolites can correspond to a particular metabolic or developmental pathway and thus reveal effects of a test compound on in vivo development.
The term “physical separation method” as used herein refers to any method known to those with skill in the art sufficient to produce a profile of changes and differences in small molecules produced in hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, contacted with a toxic, teratogenic or test chemical compound according to the methods of this invention. In a preferred embodiment, physical separation methods permit detection of cellular metabolites including but not limited to sugars, organic acids, amino acids, fatty acids, hormones, vitamins, and oligopeptides, as well as ionic fragments thereof and low molecular weight compounds (preferably with a molecular weight less than 3000 Daltons, more particularly between 10 and 1500 Daltons, and even more particularly between 100 and 1000 Daltons). In particular embodiments, this analysis is performed by liquid chromatography/electrospray ionization time of flight mass spectrometry (LC/ESI-TOF-MS), however it will be understood that cellular metabolites as set forth herein can be detected using alternative spectrometry methods or other methods known in the art for analyzing these types of cellular compounds in this size range.
Data for statistical analysis were extracted from chromatograms (spectra of mass signals) using the Agilent Mass Hunter software (Product No. G3297AA, Agilent Technologies, Inc., Santa Clara, Calif.); it will be understood that alternative statistical analysis methods can be used. Masses were binned together if they were within 10 ppm and eluted within a 2 minutes retention time window. A binned mass was considered to be the same molecule across different LC/ESI-TOF-MS analyses (referred to herein as an “exact mass,” which will be understood to be ±10 ppm). Binning of the data is required for statistical analysis and comparison of masses across the entire experiment. If multiple peaks with the same mass at the same retention time within a single sample were detected by Mass Hunter, they were averaged to assist data analysis. Masses lacking a natural isotopic distribution or with a signal-to-noise ratio of less than 3 were removed from the data prior to analysis. One of skill in the art will appreciate that the results from this assay provide relative values that are assessed according to annotated values within 10 ppm to provide an identity for the molecular weight detected. Thus, a mass shift within 10 ppm is considered consistent with determining the identity of a specific cellular metabolite annotated known in the art due to differences in ionization source and instrumentation, e.g. between different experiments or using different instruments.
As used herein, a mass was considered to be the same across LC/ESI-TOF-MS runs using a simple algorithm that first sorts the data by mass and retention time. After sorting, a compound was considered unique if it had a retention time difference of less than or equal to three minutes and a mass difference less than or equal the weighted formula (0.000011× mass). If a series of measurements fit this definition it was considered to be from the same compound. If either the mass or the retention time varied by more than the limits listed above it was considered to be a different compound and given a new unique designation.
Significance tests were determined by performing ANOVAs on the log base 2 transformed abundance values of unique compounds present in treated and untreated media at each time point. A randomized complete block design was used with the ANOVA model including the effects of treatment, experiments, and a residual term, with the following formula: Log2(abundancetb)=treatmentt+experimentb+errortb.
Missing data were omitted from the test changing the degrees of freedom rather than assuming the missing data were absent. This assumption was made because the extensive filtering performed by the Mass Hunter software may miss or filter certain peaks because they are below a certain abundance threshold and not zero. The ANOVA F-test was considered significant if its p-value was less than 0.05. Fold changes were calculated using the least squared means for a given time and treatment.
The term “biomarker” as used herein refers to cellular metabolites that exhibit significant alterations between treated and untreated controls. In preferred embodiments, biomarkers are identified as set forth above, by methods including LC/ESI-TOF-MS. Metabolomic biomarkers are identified by their unique molecular mass and consistency with which the marker is detected in response to a particular toxic, teratogenic or test chemical compound; thus the actual identity of the underlying compound that corresponds to the biomarker is not required for the practice of this invention. Alternatively, certain biomarkers can be identified by, for example, gene expression analysis, including real-time PCR, RT-PCR, Northern analysis, and in situ hybridization, but these will not generally fall within the definition of the term “cellular metabolites” as set forth herein.
The basal metabolome of undifferentiated hESCs served as a collection of biochemical signatures of functional pathways that are relevant for sternness and self-renewal. Metabolite profiling was conducted on excreted or secreted cellular metabolites as opposed to intracellular compounds. Ultimately, biomarkers discovered in vitro are expected to be useful for analyzing in vivo biofluids such as serum, amniotic fluid and urine, complex mixtures of extracellular biomolecules. This is advantageous over invasive procedures such as tissue biopsies because small molecules in biofluids can be detected non-invasively (in contrast to intracellular compounds). In addition, processing cellular supernatant for mass spectrometry is more robust and less laborious than cellular extracts. However, cellular extracts (from, for example, lysed cells) can be utilized in the methods of the invention.
The term “biomarker profile” as used herein refers to a plurality of biomarkers identified by the inventive methods. Biomarker profiles according to the invention can provide a molecular “fingerprint of the toxic and teratogenic effects of a test compound and convey what cellular metabolites, specifically excreted and secreted cellular metabolites, were significantly altered following test compound administration to hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells. In these embodiments, each of the plurality of biomarkers is characterized and identified by its unique molecular mass and consistency with which the biomarker is detected in response to a particular toxic, teratogenic or test chemical compound; thus the actual identity of the underlying compound that corresponds to the biomarker is not required for the practice of this invention.
The term “biomarker portfolio” as used herein refers to a collection of individual biomarker profiles. The biomarker portfolios may be used as references to compare biomarker profiles from novel or unknown compounds. Biomarker portfolios can be used for identifying common pathways, particularly metabolic or developmental pathways, of toxic or teratogenic response.
These results set forth herein demonstrated that human embryonic stem cell metabolomics, and metabolomics from hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, can be used in biomarker discovery and pathway identification. Metabolomics detected small molecules secreted by hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, produced therefrom and the identified biomarkers can be used for at least two purposes: first, to determine specific metabolic or developmental pathways that respond to or are affected by toxin or teratogen exposure, particularly said pathways utilized or affected during early development that are sensitive to toxic, teratogenic or test chemical compounds that are developmental disruptors and participate in the ontogenesis of birth defects; and second, to provide cellular metabolites that can be measured in biofluids to assist management and diagnosis of toxic exposure, birth defects or other disease.
A biomarker portfolio from hESCs or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, produced therefrom can also serve as a high throughput screening tool in preclinical phases of drug discovery. In addition, this approach can be used to detect detrimental effects of environmental (heavy metals, industrial waste products) and nutritional chemicals (such as alcohol) on human development. Ultimately, the methods of this invention utilizing the hESC metabolome or the metabolome of or hESC-derived lineage-specific cells, such as neural stem cells, neural precursor cells and neural cells, can assist pharmaceutical, biotechnology and environmental agencies on decision-making towards development of compounds and critical doses for human exposure. The integration of chemical biology to embryonic stem cell technology also offers unique opportunities to strengthen understanding of human development and disease. Metabolomics of cells differentiated from hESC should serve similar roles and be useful for elucidating mechanisms of toxicity and disease with greater sensitivity for particular cell or tissue types, and in a human-specific manner. For example, key metabolic pathways, including as set forth herein folate, glutamate and tryptophan synthesis and degradation, may be differentially disrupted in earlier versus later stages of human development. In addition, metabolite profiles of neural precursor cells or neuronal cell populations can reveal biomarkers of neurodevelopmental disorders in target cell types. The association of metabolomics to stem cell biology can inform the mechanisms of action of folic acid and neural tube defects in the early human embryo.
Biomarker portfolios produced using the hESC-dependent and hESC-derived lineage-specific cell-dependent methods of this invention can also be used in high throughput screening methods for preclinical assessment of drug candidates and lead compounds in drug discovery. This aspect of the inventive methods produces minimal impact on industry resources in comparison to current developmental toxicology models, since implementation of this technology does not require experimental animals. The resulting positive impact on productivity enables research teams in the pharmaceutical industry to select and advance compounds into exploratory development with greater confidence and decreased risk of encountering adverse developmental effects.
The term “developmental pathway” as used herein refers to developmental or metabolic pathways in embryonic and fetal development.
“Supernatant” as used herein may include but is not limited to extracellular media, co-cultured media, cells, or a solution of fractionated or lysed cells.
Cellular metabolite profiles obtained from analysis of toxins, teratogens, alcohol, and test chemical compounds can be used to compose a library of biomarker portfolios. These portfolios can then be used as a reference for toxicological analysis of unknown chemical compounds. A similar strategy has been validated as a means to determine cellular changes that arise in response to chemicals in non-hESC systems (Daston & Nacliff, 2005, Reprod Toxicolog 19:381-94; Fella et al., 2005, Proteomics 5:1914-21). Metabolic profiles of novel compounds can be compared to known biomarker portfolios to identify common mechanisms of toxic response. This approach can reveal functional markers of toxic response, which serve as screening molecules that are shared at least in part as a consequence of exposure to various different toxic and teratogenic compounds. Such hESC-derived small molecules can be used as measurable mediators of toxic response that refine or replace costly and complex screening systems (such as in vivo animal models) and have the additional advantage of being specific for human cells and human metabolic and developmental pathways.
The Examples which follow are illustrative of specific embodiments of the invention, and various uses thereof. They are set forth for explanatory purposes only, and are not to be taken as limiting the invention.
To demonstrate the efficacy of hESCs as a model system for developmental toxicity testing, hESCs were treated with a known teratogen, valproate (VPA). Valproate is a common mood stabilizer and anti-convulsant drug with clinical indications in epilepsy and bipolar disorder (Williams et al., 2001, Dev Med Child Neuro 43:202-6) that has been associated with developmental abnormalities (Meador et al., 2006, Neurology 67: 407-412). The mechanism by which valproate produces developmental defects, however, is not fully understood, despite the increased susceptibility of the nervous system (Bjerkedal et al., 1982, Lance 2:109: Wyszynski et al., 2005, Neurology 64:961-5; Rasalam et al., 2005, Dev Med Child Neuro 47:551-555). Exposure to valproate results in a pronounced increase in spina bifida and neural tube defects (NTDs; Bjerkedal et al., 1982, Lancet 2:109) at ten-to-twenty times that of the general population, as well as cognitive disorders such as autism (Adab et al., 2004, J Neurol Neurosurg Psychiatry 75:1575-83). However, since VPA is an anti-convulsant drug with clinical indications in epilepsy and bipolar disorder (Williams et al., 2001, Dev Med Child Neurol 43:202-06), treatment generally must be sustained throughout pregnancy.
Folic acid supplementation prior to pregnancy reduces the incidence of spina bifida by 70% (Shaw et al., 1995, Epidemiolog 6:219-226) although its precise mechanism of action is unknown. In addition, homocysteine and glutathione have also been implicated in NTDs (Zhao et al., 2006, Birth Defects Res A Clin Mol Teratol 76:230-6). Thus, metabolite profiles of folate and related pathways were candidates for changes in response to valproate. In the results set forth herein, folic acid was significantly increased (by 16%) in the extracellular media of hES cells treated with valproate (p=0.022 at eight days, Table 3 and
Exposure of hESCs was performed as follows. H1 hESC (passage 41) were cultured on Matrigel (BD Scientific, San Jose, Calif.) in the absence of a feeder layer. hESCs were maintained in conditioned medium (CM) collected from mouse embryonic fibroblasts (MEFs) (80% DMEM/F12, Invitrogen, Carlsbad, Calif.) and 20% KNOCKOUT serum replacement (Invitrogen) supplemented with 1 mM L-glutamine (Invitrogen), 1% MEM non-essential amino acids (Invitrogen), and 0.1 mM 2-mercaptoethanol (Sigma, Chemical Co., St. Louis, Mo.). Prior to feeding hESCs, the culture medium was supplemented with 4 ng/mL human recombinant basic fibroblast growth factor (Invitrogen). hESCs were passaged when the wells were ˜80% confluent. To passage, hESCs were incubated in a 1 mg/mL dispase (Invitrogen)/DMEM/F12 solution for 7-10 minutes at 37° C. After this treatment hESCs were washed and seeded on fresh Matrigel coated plates. In parallel studies disclosed herein, H1 and H9 cells were cultured in defined medium known as TeSR (Ludwig et al., 2006, Id.).
H1 and H9 (equivalent to NIH code WA01/WA09) hESC were treated with valproate (VPA) (22 μM and 1 mM) (Sigma # P4543) according to the procedure outlined below; each experiment involved three separate VPA treatments, and each treatment group had a parallel control group with a total of six 6-well culture dishes (Nunc, Naperville, Ill.) per experiment (two 6-well culture dishes per treatment). Treatment 1 (labeled 24 H) exposed hESC cells to 1 mM VPA (Sigma) for 24 hours followed by collection of supernatant and cell pellets. In a second treatment group (labeled 4 D), hESC cells were exposed to 22 μM or 1 mM VPA for 4 days and harvested on day 4. In a third treatment group (labeled extended culture, EC), hESC cells received 22 μM or 1 mM VPA for 4 days followed by culture in standard hESC media for an additional four days. For this group, cells and supernatant were harvested on day eight.
To assess the effects of teratogenic VPA treatment on hESCs, the treated cells were analyzed as set forth below to determine changes in a total dynamic set of small molecules present in cells according to health and disease or insult states. Small molecules including but not limited to sugars, organic acids, amino acids, fatty acids, hormones, vitamins, oligopeptides (less than about 100 amino acids in length), as well as ionic fragments thereof and signaling low molecular weight compounds were known to participate in and reveal functional mechanisms of cellular response to pathological or chemical insult. These analyses were also used to identify active pathways following molecular changes predicted by other analyses including for example transcriptomics and proteomics.
Supernatant from VPA-treated and control hESCs were subjected to liquid chromatography and electrospray ionization time of flight mass spectrometry (LC/ESI-TOF-MS) to assess changes and differences in small molecules (as defined herein) produced by the cells in the presence and absence of VPA treatment. Supernatant was collected from control and treated plates of hESCs at 24 H, 4 D, and 8 D, and CM was collected as a “no treatment” control. The supernatant and media were stored at −80° C. until preparation for mass spectrometry analysis. For analysis, samples were prepared in a 20% Acetonitrile (Fisher Scientific Co., Pittsburgh, Pa.) solution (comprising 500 μL of supernatant, 400 μL acetonitrile and 1.1 mL distilled water) and centrifuged through a Millipore 3 kDa Centricon column (Millipore, Billerica, Mass.) for 3 hours at 4575×g to remove proteins. The flow-through was retained for analysis, as it contains small molecules free of high molecular weight compounds such as proteins. In each analysis, three replicates for each sample were injected into a 2.1×200 mm C18 column using a 90 minute gradient from 5% Acetonitrile, 95% Water, 0.1% Formic Acid to 100% Acetonitrile, 0.1% Formic Acid at a flow rate of 40 μL/min. ESI-TOF-MS (TOF) was performed on the flow-through using an Agilent ESI-TOF mass spectrometer. Data was collected from 100-3600 m/z, and particularly in the 0-1500 m/z range. The raw data was analyzed to identify the separated small molecules using a computer compilation and analysis program (Mass Hunter) provided by the manufacturer and according to manufacturer's instructions (Agilent; statistical analyses were performed as described above in the Detailed Description and Preferred Embodiments. This analysis generated lists of retention time/accurate mass pair feature. Another program (Mass Profiler, Agilent) was used to compare multiple run sets to find ion intensity changes of features that changed between the sample conditions. Significance tests were determined by performing ANOVAs on the log base 2 transformed abundance values of unique compounds present in treated and untreated media at each time point.
The plurality of small molecules identified using these methods were then compared with exact mass and retention time from ESI-TOF-MS using public databases (for example, at http://metlin.scripps.edu., www.nist.gov/srd/chemistry.htm; http://www.metabolomics.ca/). Mass spectrometry analysis also included predicted chemical structures of small molecules based upon exact mass, although currently-available public databases do not in every instance include matching small molecules due to database limitations. In addition, more comprehensive private databases are available for comparative analysis, such as the NIST/EPA/NIH Mass Spectral Library: 05. NIST ASCII Version.
The results of these analyses are shown in
Further, the results shown in
These studies demonstrated that the claimed methods for assessing developmental toxicity and the identification of biomarkers using hESCs provided robust information on changes in small molecule content of cells in response to being contacted with a known teratogen, VPA. The results concerning a compound (VPA) that is involved in the etiology of spina bifida and neural tube defects (NTDs) (Bjerkedal et al., 1982, Lancet 2:109) when exposed to a developing human conceptus are particularly striking. The results shown here indicated a marked increase (2 to 8 fold) in key metabolites of the folate pathway (dihydrofolic acid, tetrahydrofolic acid, S-adenosylmethionine) following treatment with VPA (in comparison to untreated cells). These methods were reproducible, having been repeated with consistent results obtained in three independent studies using hESCs and on non-embryonic cells (human fibroblasts) as controls (data not shown), and suggested a heretofore unknown adaptive response of the fetus to the chemical/environmental insult and identified sensitive markers for said insult(s).
The mechanism for VPA developmental defects, however, is not fully understood despite the fact that the nervous system is particularly sensitive to its effects (Bjerkedal et al., 1982, Lancet 2:109; Narita et al., 2000, Pediatric Res 52:576-79; Rasalam et al., 2005, Dev Med Child Neurol 47:551-55). Folic acid supplementation prior to pregnancy prevents the incidence of spina bifida by 70% (Shaw et al., 1995, Epidemiolog 6:219-226), although the exact mechanism of action is also unknown. The information obtained herein can be used to elucidate mechanisms of action of folic acid and neural tube defects in the early human embryo. These methods can also be applied to other known teratogens, such as retinoic acid, warfarin, and thalidomide (Franks et al., 2004, Lancet 363:1802-11) to validate the predictive ability of hESCs using the methods of the invention.
Table 1: Candidate small molecules (biomarkers) of developmental toxicity detected in undifferentiated human embryonic stem cells treated with 1 mM valproate in comparison to untreated controls.
As discussed above, metabolite profiles were determined at 24 hours, four days and eight days after valproate treatment. At four days after treatment, multiple candidate biomarkers were upregulated in treated versus untreated human embryonic stem cells (shown in
hESCs were cultured in conditioned media from mouse embryonic fibroblasts, which generated 1277 of the 3241 measured compounds. Many metabolites in human development and disease are likely present in conditioned media from mouse embryonic fibroblasts due to common metabolic pathways. Rigorous investigation is required to validate candidate biomarkers that are not exclusive to hESCs and are also present in the media.
The efficacy of the analysis shown in Example 1 was confirmed by gene expression studies, wherein changes in gene expression were observed following VPA treatment of hESCs. VPA treatment was not detrimental to hESCs, which remained viable for multiple passages following teratogen exposure, thus enabling gene expression analysis to be performed.
Treated and control H1/NIH code WA01 hES cells (passage 41) were analyzed by real-time PCR, and each treatment group was paired with a corresponding control group that received the standard growth media combination of CM+bFGF without VPA. In these studies, total cellular RNA was extracted from cells harvested at 24 hours (24 H), 4 days (4 D) and 8 days (EC) using the RNA Easy Kit (Qiagen, Valencia, Calif.) according to the manufacturer's instructions.
Expression levels of candidate test genes and a housekeeping gene (Beta-2-microglobulin) were evaluated by quantitative real-time PCR using a DNA Engine-Opticon 2 Detection System (MJ Research, Watertown, Mass.). The housekeeping gene acts as an internal control for normalization of RNA levels. The primers used for real-time PCR reactions were designed using Beacon Designer software (Premier Biosoft International, Palo Alto, Calif.). RNA was reverse transcribed using iScript cDNA Synthesis kit (Bio-Rad, Hercules, Calif.), wherein each cDNA synthesis reaction (20 μL) included 4 μL of 5× iScript reaction mix, 1 μL of iScript reverse transcriptase, and 2 μL of RNA. PCR was performed on cDNA in PCR reaction mixtures (25 μL) each containing 12.5 μL of Supermix (contains dNTPs, Taq DNA polymerase, SYBR Green I, and fluorescein), 250 nM forward primer, 250 nM reverse primer, and 1.6 μL RT-PCR products. Melting curve analysis and agarose gel electrophoresis were performed after real-time PCR reaction to monitor PCR specificity, wherein PCR products were detected with SYBR Green I using the iQ SYBR Green Supermix kit (Bio-Rad).
Quantifying the relative expression of real-time PCR was performed using the 2-ΔΔCt method (Livak & Schmittgen, 2001, Methods 25:402-8), and a general linear model was employed to fit the expression data. The PROC GLM procedure in SAS (version 8.2; SAS Institute, Cary, N.C.) was used to estimate least squares means in expression between treated and untreated hESCs and P<0.05 was considered statistically significant
Real-time PCR was conducted on samples from 24 hours (24 H), 4 days (4 D) and 8 days (EC) after VPA treatment to investigate expression levels of epigenetic regulators (such as DNA methyltransferase-1, DNMT-1, BMI-1, EED) and critical transcription factors responsible for embryonic patterning and neurodevelopment (RUNX2, BMP7, FGF8, CBX2, GLI3, SSH and SP8) in human embryonic stem cells. These experiments showed hESCs treated with VPA were subject to marked changes in their transcriptional activity following teratogen treatment. VPA induced overall marked (2 to 30 fold) downregulation of transcription levels as early as 24 hours after exposure in all genes tested (with the exception of DNMT-1 and Shh). At 4 days after treatment, however, expression of the ubiquitous DNA methyltransferase-1 was almost abolished, and sonic hedgehog, which is absolutely critical for neurogenesis (Ye et al., 1998, Cell 93:755-66), was down-regulated five-fold in comparison to untreated controls. At 8 days after VPA treatment, the majority of the genes were upregulated in comparison to untreated controls.
These results embodied two major implications for developmental toxicology. First, VPA induced persistent changes in key epigenetic modulators that also participate in differentiation of other tissues, such as DNMT-1 and the polycomb family member EED. Second, the effects of teratogens persisted in hESCs during critical stages of neurogenesis and organogenesis. For example, genes whose expression was affected as shown herein (including sonic hedgehog and FGF-8) are known to be master regulators of differentiation of serotonergic neurons in the brain (Ye et al., 1998, Cell 93:755-66). Of particular notice is the fact that DNMT-1 expression is almost abolished at four days after treatment. In vivo, disruption of this enzyme is lethal to embryos, since it is the major maintenance methyltransferase during DNA replication (Li et al., 1992, Cell 69:915-26).
Following teratogen exposure, temporal-specific alterations in developmental gene expression were observed. Developmental genes differ in their susceptibility to teratogens at different times. This indication may be critical to understanding specificity of epigenetic disruptors on certain organs or tissues. RUNX2, for example, is a transcriptional activator of bone development (Napierala et al., 2005, Mol Genet Metab 86:257-68), and is more sensitive to VPA-mediated up-regulation at very early or late stages following exposure. Real-time PCR results from hESCs disclosed herein were correlated to previous findings in vivo in mice (Okada et al., 2004, Birth Defects Res A Clin Mol Teratol 70:870-879) and rats (Miyazaki et al., 2005, Int J Devl Neuroscience 23:287-97). In these animal studies, VPA inhibited the expression of Polycomb genes, Eed, Bmil and Cbx2 and induced downregulation of Shh while FGF8 levels remained unchanged. The results shown here at four days following VPA treatment (Table 2) were consistent with these observations using other developmental model systems.
Exposure of hES cells to the teratogen valproate induced significant changes in different metabolic pathways, including pathways important during pregnancy and development. An alternative metabolic pathway activated during pregnancy are shown in
Metabolome analysis was performed as described in Example 1 (Wu and McAllister, 2003, J Mass Spectrom 38:1043-53). Complex mixtures were separated by liquid chromatography (LC) prior to electrospray ionization (ESI) time of flight (TOF) mass spectrometry according to the methods described in this Example and Example 1. Mass Hunter (Agilent) software was applied to deconvolute the data and determine the abundance of each mass. Data were extracted from the entire mass spectrum using the m/z range of 0 to 1500 and the top 2 million most abundant mass peaks from each sample were used for data deconvolution. The minimum signal-to-noise ratio was set to 5. The masses with a minimum relative abundance greater than 0.1% were exported from the Mass Hunter software and used for further analysis.
hESCs treated with 22 μM valproate resulted in 3,241 detected mass signals 42 injections. Of the total of 3,241 mass signals detected in these experiments, 1,963 compounds were measured solely in hES cells and 1,278 compounds were also present in conditioned media; 443 of these were only measured in 1 of 42 injections. 110 compounds (3%) had statistically-significant differences in at least one time point in valproate-treated hESCs compared with control. Fold changes as high as seven- to thirteen-fold were measured after valproate treatment, but these mass signals exhibited high variability across experiments. Representative masses identified following treatment of cells with 1 mM and 22 μM VPA are summarized in Tables 3 and 4, respectively. Several peaks (1,963) were detected in hES cells but not in conditioned media. One of these small molecules was kynurenine, a compound produced by an alternative tryptophan metabolic pathway, activated during pregnancy and immune response. The levels of kynurenine increased by 44% (p value=0.004 at four days, Table 5) following valproate treatment. Kynurenine was detected exclusively in hES cells and absent in conditioned media. The chemical identity of this peak was confirmed by comparative mass spectrometry in the presence of the chemical standard (
The results of these experiments suggested that kynurenine is a candidate biomarker for neurodevelopmental disorders, in particular those originated by exposure of the human embryo to anti-epileptic drugs such as VPA (Ornoy et al., 2006, Reproductive Toxicol 21:399-409). Strikingly, recent studies have suggested that kynurenine metabolism may be a novel target for the mechanism of action of anti-epileptic drugs (Kocki et al., 2006, Eur J Pharmacol 542:147-51). Cognitive and behavioral disorders are known adverse effects of antiepileptic exposure during pregnancy. Tryptophan is the precursor of serotonin, a key neurotransmitter in the pathogenesis of these and other diseases, such as depression. In addition, increased plasma levels of kynurenine have been linked to postpartum depression (Kohl et al., 2005, J Affect Disord 86:135-42). The alteration in tryptophan metabolism detected herein is a means for examining novel mechanisms in pathogenesis of serotonin-related behavioral disorders such as autism (Chugani, 2004, Ment Retard Dev Disabil Res Rev 10:112-116). An increase in kynurenine levels during development may reduce the bioavailability of tryptophan and consequently serotonin, leading to cognitive dysfunction.
Glutamate and pyroglutamic acid were also elevated in hESCs treated with valproate. Glutamate and pyroglutamic acid were elevated in response to valproate (20% and 27%, respectively), although only pyroglutamic acid exhibited statistically significant changes (p=0.021 at 4 days,
These results suggested that valproate affects the glutamate synthesis pathway in the developing human embryo. The affinity of anti-epileptic drugs towards glutamate targets has been previously suggested (Rogawski and Loscher, 2004, Nat Rev Neurosci 2004 5:553-64). Abnormal levels of glutamate metabolites were measured in maternal serum and amniotic fluid of pregnant women whose infants were diagnosed spina bifida (Groenen et al., 2004, Eur J Obstet Gynecol Reprod Biol.; 112:16-23) with nuclear magnetic resonance (NMR). The levels of glutamine and hydroxyproline were significantly higher in NTDs, and as a result the hESC methods provided herein provide a robust resource to model in vivo alterations of development.
Table 5. Changes in metabolic profiles of four compounds in hES cells treated with valproate versus untreated controls at 24 hours (24 h), 4 days (4 D), and eight days (8 D) after treatment.
0.021
0.022
0.004
Kynurenine was shown in Example 3 to be detected in valproate-treated hES cells. Kynurenine (along with glutamate and pyroglutamic acid) was differentially produced in valproate-treated human embryonic stem cells (hES) versus controls. Kynurenine is a novel biomarker useful for the identification of neurodevelopmental disorders in infants and in vitro developmental toxicity of chemicals. This example describes the identification of biomarkers for neurodevelopmental disorders, including cellular products differentially produced in teratogen-treated hESCs.
The amino acid tryptophan (TRP) is a precursor of the neurotransmitter serotonin, a key mediator of numerous CNS disorders, such as depression, neurodegeneration and cognitive impairment. Tryptophan catabolism into kynurenic acid is an alternative route for tryptophan metabolism (
In addition, kynurenic acid (KYNA), one of the end products of this tryptophan metabolic pathway, is an antagonist of glutamate neurotransmission and N-methyl-D-aspartate (NMDA) receptors. Recent studies have demonstrated that kynurenic acid is a druggable target via its role in the activation of the previously orphan GPCR receptor GPR35 (Wang et al., 2006, J Biol Chem 281:22021-8). Quinolinic acid (QUIN), another end product of the pathway (
Exposure of hES cells to valproate, a disruptor of human development, induced significant changes in different metabolic pathways, including the production of kynurenine (exact neutral mass 208.08), which was significantly upregulated in response to valproate as detected by liquid chromatography electrospray ionization time of flight mass spectrometry (LC/ESI-TOF-MS) as described in Example 4. Additionally, novel chemical entities, having exact neutral masses of 328.058, 336.163, 343.080, were detected and are not yet catalogued in public databases.
When neural precursors derived from hESCs were exposed to 1 mM valproate, a marked decrease in both serotonin (176.0946) and indoleacetaldehyde (159.0689), a downstream sub-product of serotonin generated by monoaminoxoidase activity (MAO) was observed (Table 6). Glutamate and pyroglutamic acid or hydroxyproline (p=0.021) were also elevated in hES cells treated with valproate. These results suggest that valproate affects the glutamate synthesis pathway in the developing human embryo. This finding emulates in vivo neurophysiology, where compounds from the kynurenine pathway modulate activity at NMDA glutamate receptors and produce epileptic phenotypes, including seizures (Perkins and Stone, 1982, Brain Res 247:184-187.).
As a consequence of the identification of kynurenine herein, chemical inhibitors of kynurenine synthesis can be used as novel therapeutics in mood disorders; for example, small molecules that antagonize indoleamine 2,3-dioxygenase (IDO) or kynurenine formylase activities, which converts tryptophan (TRP) into kynurenine (KYN). Inhibition of TRP catabolism to KYN can be used to ameliorate disease symptoms in cognitive and neurodegenerative disorders by increasing serotonin levels, via elevated synthesis of this neurotransmitter or reduced depletion through the kynurenine pathway.
Collectively, the metabolite changes detected in hES cells in response to valproate converge functionally towards folate, kynurenine and glutamate pathways.
The efficacy of the analysis in Example 4 was confirmed by gene expression studies, wherein changes in gene expression were observed following VPA treatment of hESC. Valproate treatment of human embryonic stem cells induced a marked upregulation in the small molecule kynurenine, an intermediate metabolite in the catabolism of tryptophan. Tryptophan is the precursor of the neurotransmitter serotonin (5HT). Thus, whether expression of enzymes in the metabolism of tryptophan to kynurenine and its opposite route, serotonin synthesis, was altered in human embryonic stem cells was investigated to examine the mechanistic properties of the kynurenine pathway and its response to valproate.
Human embryonic stem cells treated with 1 mM valproate and untreated controls were harvested at four days after treatment and stored at −80° C. prior to RNA isolation using RNeasy (Qiagen). 5 μg of RNA templates were reverse transcribed and amplified (QIAGEN OneStep RT-PCR) according to the manufacturer's instructions using primers designed for transcribed human sequences of the following genes: INDO, indoleamine 2,3 dioxygenase, TDO or TDO2, tryptophan 2,3-dioxygenase, AFMID, arylformamidase, TPH1, tryptophan hydroxylase the rate-limiting enzyme in serotonin biosynthesis, AADAT, aminoadipate aminotransferase, KYNU, kynunreninase, KMO, kynurenine 3-monooxygenase, GAPDH, glyceraldehyde 3-phosphate dehydrogenase.
The results of this study showed that the majority of enzymes in the kynurenine pathway and serotonin synthesis were expressed in hES cells at four days after treatment of hES cells with 1 mM valproate (
To identify differentially secreted metabolites in response to alcohol, as well as the pathways involved in fetal alcohol syndrome, human embryonic stem cells were treated with 0, 0.1 and 0.3% ethanol for four days followed by LC/ESI-TOF mass spectrometry according to the general methods described above for valproate in Example 1. Extracellular media was collected and processed at 24 hours and four days after treatment, and 49,481 mass signals were detected following three technical replications. Of the 49,481 mass signals, 1,860 compounds were significantly different (p<0.05) in at least one treatment and had a significant time change (<1 or >1). (Table 7). Binned masses were annotated in silico by querying the neutral masses in several different databases. These databases included Metlin, Biological Magnetic Resonance Data Bank (BMRB), NIST Chemistry WebBook, and the Human Metabolome Database. A mass was considered identified when its neutral mass was within 10 ppm of a known compound annotated in one of the databases listed above.
The putative kynurenine compound (measured exact neutral mass 208.0816) was upregulated three-fold at day four, but not 24 hours, in both treatments (0.1%, p=0.001 and 0.3% p=0.002, respectively). Another putative metabolite in the kynurenine pathway, 8-methoxykynurenate (219.0532) was also upregulated at four days in response to both 0.1% and 0.3% alcohol treatment (p<0.05). The analysis also detected a significant downregulation of 5-hydroxy-L-tryptophan (220.0848) at four days following 0.3% alcohol treatment (p<0.05) in comparison to untreated controls. 5-hydroxy-L-tryptophan is the only intermediate metabolite between tryptophan and serotonin and its synthesis is mediated by tryptophan hydroxylase, the rate limiting enzyme in serotonin synthesis. These results suggest that alcohol exposure during human development can affect serotonin bioavailability due to upregulation of tryptophan catabolism into kynurenines. In addition, alcohol exposure induced significant changes in metabolic pathways and small molecules involved in neural development such as glutamate, gabapentin, adrenaline and glutathione.
Metabolomic assessment of teratogens on embryonic development is not limited exclusively to hESCs. The methods of the invention are also useful with other progenitor stem cells, including lineage-restricted stem cells such as neural precursor cells. To illustrate the efficacy of toxicology screening on lineage-specific stem cells, neuronal precursors derived from hESCs were treated with 1 mM valproate according to the methods described in Example 1.
Approximately 135 compounds were differentially secreted in VPA-treated neuronal precursors versus control. (See Table 6). The results of this study illustrated that the methods of the invention reveal alterations in the metabolic profile of lineage-specific stem cells in response to teratogen exposure.
The results disclosed herein are set forth in the following tables
0.020
0.028
0.013
0.040
0.006
0.021
0.034
0.038
0.011
0.034
0.002
0.004
0.014
0.002
0.019
0.049
0.017
0.003
0.004
0.018
0.042
0.034
0.004
0.033
0.012
0.004
0.023
0.033
0.044
0.004
0.018
0.001
0.031
0.044
0.003
0.041
0.022
0.001
0.037
0.041
0.024
0.002
0.004
0.016
0.010
All references cited herein are incorporated by reference. In addition, the invention is not intended to be limited to the disclosed embodiments of the invention. It should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit and scope of the invention as set forth in the appended claims.
This application claims the priority benefit of U.S. provisional patent applications, Ser. Nos. 60/790,647, filed Apr. 10, 2006, and 60/822,163, filed Aug. 11, 2006, the entirety of which are incorporated by reference herein.
| Number | Name | Date | Kind |
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| 6197575 | Griffith et al. | Mar 2001 | B1 |
| 6200806 | Thomson | Mar 2001 | B1 |
| 20020019023 | Dasseux et al. | Feb 2002 | A1 |
| 20030219866 | Kruijer et al. | Nov 2003 | A1 |
| 20040073958 | Katsuki et al. | Apr 2004 | A1 |
| 20040121305 | Wiegand et al. | Jun 2004 | A1 |
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| 2560334 | Oct 2005 | CA |
| 0937779 | Aug 1999 | EP |
| 0194616 | Dec 2001 | WO |
| 03005628 | Jan 2003 | WO |
| 03018760 | Mar 2003 | WO |
| 03089635 | Oct 2003 | WO |
| 2004065616 | Aug 2004 | WO |
| 2005005162 | Jan 2005 | WO |
| 2005005621 | Jan 2005 | WO |
| 2005080551 | Sep 2005 | WO |
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| Number | Date | Country | |
|---|---|---|---|
| 20070248947 A1 | Oct 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60790647 | Apr 2006 | US | |
| 60822163 | Aug 2006 | US |
