Patent Application
20150141633
Pursuant to 37 C.F.R. §1.821(c) or (e), a file containing a PDF version of the Sequence Listing has been submitted concomitant with this application, the contents of which are hereby incorporated by reference. The transmittal documents of this application include a Request to Transfer CRF from the parent application.
The disclosure relates to the fields of plant secondary metabolites with pharmacological or other industrial properties and metabolic engineering of these phytochemicals. More specifically, the disclosure relates to a method for enhancing the biosynthesis and/or secretion of sapogenin intermediates in the culture medium of plant and microbial cell cultures. Further, the disclosure also relates to the identification of novel genes involved in the biosynthesis of sapogenin intermediates, as well as to novel sapogenin compounds.
Plants synthesize an overwhelming variety of triterpene saponins with an enormous range of biological activities relevant for the pharmaceutical and chemical industry (e.g., additives to foods and cosmetics). Interest in triterpenoid saponins and its precursors has increased recently because of data showing their diverse biological activities and beneficial properties, which include antifungal, antibacterial, antiviral, antitumor, molluscicidal, insecticidal, and antifeedant activities (Suzuki et al. 2002; Sparg et al. 2004; Huhman et al. 2005). Saponins are synthesized by multiple glycosylations of sapogenin building blocks, which, in turn, are produced by multiple modifications (e.g., hydroxylations) of basic sapogenin backbones such as β-amyrin, lupeol, and dammarenediol. These diverse backbones are generated by specific cyclizations of 2,3-oxidosqualene, which is also an intermediate in the synthesis of membrane sterols. As an illustration, more than seventy saponins have been identified in the model legume, M. truncatula (Huhman and Sumner 2002; Pollier et al. 2011), the core of this diversity being centralized in a few aglycones (sapogenins). Also, these precursor sapogenins are very valuable compounds and are important starter molecules for further synthetic modifications. For example, the naturally occurring triterpenoid sapogenin oleanolic acid and its derivatives possess several promising pharmacological activities, such as hepato-protective effects, and anti-inflammatory, antioxidant, or anticancer activities (Pollier and Goossens 2012).
The first committed step in triterpenoid saponin biosynthesis is the cyclization of 2,3-oxidosqualene (
Present availability of saponins and sapogenins depends on their extractability from plants and is often uneconomical and inefficient. Often, laborious extraction schemes have to be developed for each specific metabolite of interest and a steady supply of sufficient amounts of specific sapo(ge)nins from plants that accumulate mixtures of structurally related compounds is not feasible. Synthetic chemistry mainly attempts to address these issues by chemically linking desired side chains to extracted sapogenins, as was done for oleanolic acid. However, the structural complexity of the sapogenins hampers chemical synthesis and the availability of corresponding sapogenins forms a major bottleneck.
The culture of plant cells has been explored since the 1960s as a viable alternative for the production of complex phytochemicals of industrial interest. For example, the use of large-scale plant cell cultures in bioreactors for the production of alkaloids has been extensively studied (Verpoorte et al. 1999). Despite the promising features and developments, the production of plant-derived pharmaceuticals by plant cell cultures has not been fully commercially exploited. The main reasons for this reluctance shown by industry to produce phytochemicals by means of cell cultures, compared to the conventional extraction of whole plant material, are economical ones based on the slow growth and the low production levels of phytochemicals by such plant cell cultures. Important causes are the toxicity of such compounds to the plant cell, and the role of catabolism of these compounds. Another important problem is that many phytochemicals, such as the triterpene saponins and its precursors, are mostly retained intracellularly, complicating the downstream processing and purification. Another important problem is that for many phytochemicals, the precursors or intermediates in the pathway do not accumulate or only in trace amounts, because they are readily converted by the downstream enzymes.
Biotechnological production of either complete saponins, or of sapogenin pathway intermediates that are not readily accessible, may circumvent the limitation of natural sapo(ge)nin availability. However, circumvention of laborious and uneconomical extraction procedures for industrial production from plants is also hampered by lack of knowledge and availability of genes in saponin biosynthesis. As a consequence, although triterpene synthases have been expressed in microbial hosts such as Saccharomyces cerevisiae, there has been little effort made so far to engineer the metabolism of a microbial host for enhanced production of triterpenes. By contrast, there have been many considerable efforts to engineer microbes for higher production of mono-, sesqui- and diterpenes. Notably, triterpene production may not be as amenable to engineering efforts as the volatile sesquiterpenes and monoterpenes that readily diffuse out of the cell.
Therefore, a need exists for the cost-effective biotechnological production of high value sapo(ge)nins or other triterpene building blocks in a convenient host cell.
Evidence is available that sapogenins, when produced in their natural hosts (plants), are often only found in trace amounts intracellularly in plant cells, as also demonstrated in Example 1. Moreover, and as shown in Examples 3 and 4, although sapogenins can be heterologously produced in genetically engineered yeast cells, they are only detected when extracted from the cells, and they are not found in the growth medium. In order to overcome these problems, it has been found that by incubating a eukaryotic cell culture that is capable of intracellularly synthesizing sapogenins in a culture medium with cyclodextrins, significant amounts of sapogenins can be extracted from the culture medium (Examples 5, 6 and 7). Cyclodextrins, which are cyclic oligosaccharides consisting of five or more α-D-glucopyranose residues linked by a(1→4) glucosidic bonds, are known to act as sequestering agents of phytosterols from membranes (Raffaele et al. 2009) and have been used as elicitors to increase the production and extraction of phytosterols from cultures of plant cells (EP2351846; Sabater-Jara et al. 2010). In the disclosure, it is shown that intracellularly synthesized sapogenins can be released in the growth medium of eukaryotic cell cultures in the presence of cyclodextrins and can accumulate in significant amounts. In addition, it was shown that substantially higher amounts can be obtained by repeatedly adding cyclodextrins to the culture medium.
Thus, in a first aspect, the disclosure relates to a method for producing triterpenoid sapogenins in the extracellular medium of a eukaryotic cell culture comprising:
In certain embodiments, the eukaryotic cells naturally produce triterpenoid sapogenins, such as plants cells. Alternatively, the eukaryotic cells are genetically engineered to produce triterpenoid sapogenins. Such genetically engineered eukaryotic cells can be microbial cells, such as yeast cells, or plant cells, or algal cells.
In certain embodiments, the cyclodextrins are selected from the group comprising randomly methylated cyclodextrins or hydroxypropylated cyclodextrins. Preferably, the cyclodextrin is a β-cyclodextrin. According to particular embodiments, the cyclodextrins may be added to the culture medium once or at different consecutive time points.
In other aspects, the disclosure also envisages eukaryotic cells genetically engineered to synthesize sapogenins.
Further, a sapogenin obtained by any of the above-described methods is also encompassed.
According to yet another aspect, the disclosure relates to an isolated polypeptide selected from the group consisting of:
In one embodiment, the disclosure relates to any of the above-described polypeptides wherein the polypeptide sequence is consisting of an amino acid sequence as set forth in SEQ ID NO:3 or 4 and polypeptide sequences having at least 75% identity to SEQ ID NO:3 or 4.
According to yet another aspect, the disclosure relates to an isolated polynucleotide selected from the group consisting of:
In a specific embodiment, the disclosure relates to a chimeric gene comprising the following operably linked sequences: a) a promoter region capable of directing expression in a eukaryotic cell (as defined hereinbefore); b) a DNA region encoding a polypeptide as defined above; and c) a 3′ polyadenylation and transcript termination region.
A vector comprising a polynucleotide sequence or a chimeric gene as defined above also forms part of the disclosure, as well as a host cell comprising a polynucleotide sequence or a chimeric gene or a vector as defined above.
According to a particular aspect, the disclosure provides a transgenic plant or a cell derived thereof that is transformed with the above-described vector.
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.
The disclosure will be described with respect to particular embodiments and with reference to certain drawings; the disclosure is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term “comprising” is used in this description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g., “a” or “an,” “the,” this includes a plural of that noun unless something else is specifically stated. Furthermore, the terms “first,” “second,” “third,” and the like, in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in sequences other than described or illustrated herein.
Unless otherwise defined herein, scientific and technical terms and phrases used in connection with the disclosure shall have the meanings that are commonly understood by those of ordinary skill in the art. Generally, nomenclatures used in connection with, and techniques of molecular and cellular biology, genetics and protein and nucleic acid chemistry and hybridization described herein are those well-known and commonly used in the art. The methods and techniques of the disclosure are generally performed according to conventional methods well known in the art and as described in various general and more specific references that are cited and discussed throughout this specification unless otherwise indicated. See, for example, Sambrook et al., Molecular Cloning: A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989); Ausubel et al., Current Protocols in Molecular Biology, Greene Publishing Associates (1992, and Supplements to 2002); and Leach, Molecular Modelling: Principles and Applications, 2d ed., Prentice Hall, New Jersey (2001).
According to a first aspect, the disclosure relates to a method for producing triterpenoid sapogenins in the extracellular medium of a eukaryotic cell culture comprising:
Saponins are a group of natural bio-active compounds that consist of an isoprenoidal-derived aglycone, designated “genin” or “sapogenin,” covalently linked to one or more sugar moieties. This combination of polar and non-polar structural elements in their molecules explains their soap-like behavior in aqueous solutions. Most known saponins are plant-derived secondary metabolites, though several saponins are also found in marine animals such as sea cucumbers and starfish. In plants, saponins are generally considered to be part of defense systems due to anti-microbial, fungicidal, allelopathic, insecticidal and molluscicidal, etc., activities. Typically, saponins reside inside the vacuoles of plant cells. Extensive reviews on molecular activities, biosynthesis, evolution, classification, and occurrence of saponins are given by, e.g., Augustin et al. (2011) and Vincken et al. (2007). Thus, the term “sapogenin,” as used herein, refers to an aglycone, or non-saccharide, moiety of the family of natural products known as saponins.
The commonly used nomenclature for saponins distinguishes between triterpenoid saponins (also: triterpene saponins) and steroidal saponins, which is based on the structure and biochemical background of their aglycones. Both sapogenin types are thought to derive from 2,3-oxidosqualene, a central metabolite in sterol biosynthesis. In phytosterol anabolism, 2,3-oxidosqualene is mainly cyclized into cycloartenol. “Triterpenoid sapogenins” branch off the phytosterol pathway by alternative cyclization of 2,3-oxidosqualene, while “steroidal sapogenins” are thought to derive from intermediates in the phytosterol pathway downstream of cycloartenol formation (see also
According to the above definitions, and as used herein, the “triterpenoid sapogenins” may be selected from the group comprising dammarane-type sapogenins, tirucallane-type sapogenins, lupane-type sapogenins, oleanane-type sapogenins, ursane-type sapogenins, and hopane-type sapogenins. Thus, according to specific embodiments, the triterpenoid sapogenins as produced by the method of the disclosure are dammarane-type sapogenins, or tirucallane-type sapogenins, or lupane-type sapogenins, or oleanane-type sapogenins, or ursane-type sapogenins, or hopane-type sapogenins.
Triterpenoid sapogenins typically have a tetracyclic or pentacyclic skeleton. As described in the Background section, the sapogenin building blocks themselves may have multiple modifications, e.g., small functional groups, including hydroxyl, keto, aldehyde, and carboxyl moieties, of precursor sapogenin backbones such as β-amyrin, lupeol, and dammarenediol.
The terms “triterpene” and “triterpenoid” are used interchangeably herein.
It is to be understood that the triterpenoid sapogenins, as used herein, also encompass new-to-nature triterpenoid compounds, which are structurally related to the naturally occurring triterpenoid sapogenins. These new-to-nature triterpenoid sapogenins may be currently unextractable compounds by making use of existing extraction procedures or may be novel compounds that can be obtained after genetic engineering of the synthesizing eukaryotic host cell (see further herein).
For the sake of clarity, the term “triterpenoid sapogenins,” as used herein, is not meant to cover phytosterols or phytosterol pathway intermediates. Phytosterols, which encompass plant sterols and stanols, are triterpenes that are important structural components of plant membranes and free phytosterols serve to stabilize phospholipid bilayers in plant cell membranes just as cholesterol does in animal cell membranes. Stanols are a fully saturated subgroup of phytosterols (contain no double bonds). Non-limiting examples of phytosterols, which form important structural components of plant membranes, are stigmasterol, β-sitosterol, fucosterol, and campesterol.
Medicago
Panax
Bupleurum
Maesa
Saponaria
Betula
Oleander
The method of the disclosure makes use of cyclodextrins for the production of triterpenoid sapogenins in the culture medium of eukaryotic cells that are capable of synthesizing triterpenoid sapogenins. With “cyclodextrins” (CDs) (sometimes also called cycloamyloses) are meant cyclic oligosaccharides composed of five or more (α-1,4)-linked α-D-glucopyranose subunits, which are well-known in the art. As used herein, “cyclodextrins” encompass both naturally occurring cyclodextrins as well as chemical derivatives thereof, as described further herein. Cyclodextrins possess a cage-like supramolecular structure, and are capable of forming inclusion complexes with a variety of guest molecules: CDs incorporate compounds in their hydrophobic cavities depending on the cavity size. The most typical cyclodextrins contain a set of six to eight glucopyranoside units in a ring (the cyclodextrin core), creating a cone shape. Within this family, α-cyclodextrins (αCD) have six glucopyranoside units, β-cyclodextrins (βCD) have seven glucopyranoside units, and γ-cyclodextrins (γCD) have eight glucopyranoside units in a ring. Each glucopyranoside unit has, according to the standard atom numbering system, one primary alcohol group at carbon 6 and two secondary alcohol groups at carbons 2 and 3. These natural cyclodextrins, in particular βCD, are of limited aqueous solubility. Therefore, several derivatives of cyclodextrins have been developed. Numerous chemical modifications of cyclodextrins are known in the art, as summarized, for instance, by A. Croft and R. Bartsch in Tetrahedron Report No. 147, Tetrahedron (1983) 39(9):1417-1474, and which is incorporated herein by reference. These derivatives usually are produced by aminations, esterifications or etherifications of primary and secondary hydroxyl groups of the cyclodextrins. Depending on the substituent, the solubility of the cyclodextrin derivatives is usually different from that of their parent cyclodextrins. Virtually all derivatives have a changed hydrophobic cavity volume and also these modifications can improve solubility, stability against light or oxygen and help control the chemical activity of guest molecules. For example, and without the purpose of being limitative, water-soluble cyclodextrin derivatives of commercial interest include the hydroxypropyl derivatives of βCD and γCD, the randomly methylated β-cyclodextrin (RMβCD), and sulfobutylether β-cyclodextrin sodium salt (SBEβCD).
Thus, according to a preferred embodiment, the cyclodextrin that is used in the method of the disclosure is chosen from the group comprising randomly methylated cyclodextrin or hydroxypropylated cyclodextrin. Preferably, the degree of substitution by methyls per glucose unit of the randomly methylated cyclodextrin is between one and three, and more preferably, the degree of substitution by methyls per glucose unit of the randomly methylated cyclodextrin is two. Preferably, the degree of substitution by hydroxypropyls per glucose unit of the hydroxypropylated cyclodextrin is between 0.6 and 0.9. According to another preferred embodiment, the cyclodextrin that is used in the method of the disclosure is a β-cyclodextrin. According to still another embodiment, the cyclodextrin that is used in the method of the disclosure is a methylated β-cyclodextrin.
In another embodiment, the concentration of cyclodextrins in the culture medium is less than 25 mM, preferably less than 10 mM, more preferably between 2 and 7 mM. According to more specific embodiments, the concentration of cyclodextrins in the culture medium is 5 mM or 2.5 mM, or 1 mM. In one other embodiment, cyclodextrins are added to the culture medium at one point in time, for example, immediately before or after inoculation with eukaryotic cells. Preferably, cyclodextrins are added to the culture medium at different consecutive time points, for example, immediately before or after inoculation with eukaryotic cells, and then on a daily basis after the first addition of cyclodextrins, or every other day after the first addition of cyclodextrins. This is further illustrated, without the purpose of being limitative, in Example 5.
With “production” of triterpenoid sapogenins is meant both intracellular production as well as secretion into the medium. According to a preferred embodiment, the triterpenoid sapogenins as produced by the method of the disclosure are secreted into the extracellular medium. The production of triterpenoid sapogenins typically is enhanced or induced by using the method of the disclosure. An “enhanced production” of a triterpenoid sapogenin means that there exists already a detectable amount of this metabolite in the eukaryotic cell in the absence of cyclodextrins but that detection only becomes possible in the extracellular medium upon adding cyclodextrins according to the disclosure. An “induced production” of a triterpenoid sapogenin means that there is no detectable production of this metabolite in the eukaryotic cell in the absence of cyclodextrins but that detection becomes possible in the cell and the extracellular medium upon addition of cyclodextrins according to the disclosure. With an increase in the production of one or more sapogenins according to the method of the disclosure, it is understood that production may be enhanced or induced with a factor 2, 3, 4, 5, 10, 20, 50, 100 or more, relative to the production in the absence of cyclodextrins.
Eukaryotic cells provided in the above-described method can be of any unicellular or multicellular eukaryotic organism, but, in particular embodiments, microbial, plant, and algal cells are envisaged. The nature of the cells used will typically depend on the desired sapogenins and/or the ease and cost of producing the sapogenins. According to particular embodiments, the plant cell as used is derived from a plant of the genus selected from the group comprising Medicago, Nova, Bupleurum, Maesa, Saponaria, Betula, Quillaja, Aesculus, Chenopodium, Hedera, Acacia, Centella, Oleander, Avena, Arabidopsis, or Nicotiana. The term “plant” as used herein refers to vascular plants (e.g., gymnosperms and angiosperms). According to further particular embodiments, the “microbial cell” as used herein is a yeast cell, in particular, a yeast cell of a Saccharomyces species (e.g., Saccharomyces cerevisiae), a Hansenula species (e.g., Hansenula polymorpha), a Yarrowia species (e.g., Yarrowia lipolytica), a Kluyveromyces species (e.g., Kluyveromyces lactis), a Pichia species (e.g., Pichia pastoris) or a Candida species (e.g., Candida utilis). According to a specific embodiment, the eukaryotic cells as used are Saccharomyces cells. According to further particular embodiments, the algal cells are derived from algae of the genus selected from the group comprising Dunaliella, Chlorella, or Chlamydomonas. According to a very particular embodiment, the eukaryotic cells as used are not plant cells.
In a particular embodiment, the eukaryotic cells may naturally have the capability of synthesizing triterpene saponins and triterpene sapogenin building blocks, such as plant cells. A “plant cell” is understood, according to the disclosure, as being any cell that is derived from or found in a plant and that is able to form or is part of undifferentiated tissues, such as calli or cell cultures, differentiated tissues such as embryos, parts of plants, plants or seeds.
In an alternative embodiment, the “eukaryotic cells,” as used herein, themselves do not naturally produce triterpenoid sapo(ge)nins, but may do so after genetic engineering. Thus, preferably, eukaryotic cells artificially producing sapogenins refers to cells that, while not naturally having the ability to synthesize sapogenins, have acquired such ability by means of genetic manipulation processes including transgenesis. This particularly applies to yeast cells or algal cells, which naturally do not synthesize sapogenins. In yet another embodiment, the eukaryotic cells as used herein may be genetically engineered to produce another spectrum of sapogenins, as compared to the natural spectrum that is produced by the wild-type strain, which particularly may apply to plant cells.
Thus, according to a preferred embodiment, the “plant cell,” as used herein, may be a genetically engineered plant cell, which is a plant cell derived from a transgenic plant. A “transgenic plant,” as used herein, refers to a plant comprising a recombinant polynucleotide and/or a recombinant polypeptide resulting in the expression of a regulatory or biosynthetic enzyme of the sapogenin biosynthesis pathway. A transgenic plant refers to a whole plant as well as to a plant part, such as seed, fruit, leaf, or root, plant tissue, plant cells or any other plant material, and progeny thereof. A transgenic plant can be obtained by transforming a plant cell with an expression cassette and regenerating such plant cell into a transgenic plant. Such plants can be propagated vegetatively or reproductively. The transforming step may be carried out by any suitable means, including by Agrobacterium-mediated transformation and non-Agrobacterium-mediated transformation, as discussed further below. Plants can be regenerated from the transformed cell (or cells) by techniques known to those skilled in the art. Where chimeric plants are produced by the process, plants in which all cells are transformed may be regenerated from chimeric plants having transformed germ cells, as is known in the art. Methods that can be used to transform plant cells or tissue with expression vectors include both Agrobacterium and non Agrobacterium vectors. Agrobacterium-mediated gene transfer exploits the natural ability of Agrobacterium tumefaciens to transfer DNA into plant chromosomes and is described in detail in G. Gheysen, G. Angenon, and M. Van Montagu, 1998, Agrobacterium-mediated plant transformation: a scientifically intriguing story with significant applications in K. Lindsey (Ed.), Transgenic Plant Research, Harwood Academic Publishers, Amsterdam, pp. 1-33; and in H. A. Stafford (2000), Botanical Review 66:99-118. A second group of transformation methods is the non-Agrobacterium-mediated transformation and these methods are known as direct gene transfer methods. An overview is brought by P. Barcelo and P. A. Lazzeri (1998), Direct gene transfer: chemical, electrical and physical methods in K. Lindsey (Ed.), Transgenic Plant Research, Harwood Academic Publishers, Amsterdam, pp. 35-55. Methods include particle gun delivery, microinjection, electroporation of intact cells, polyethyleneglycol-mediated protoplast transformation, electroporation of protoplasts, liposome-mediated transformation, silicon-whiskers-mediated transformation, etc. A suitable control plant would include a genetically unaltered or non-transgenic plant of the parental line (wild-type) used to generate a transgenic plant herein.
Genetically transformed hairy root cultures can be obtained by transformation with virulent strains of Agrobacterium rhizogenes, and they can produce high contents of secondary metabolites, including triterpenoid sapo(ge)nins, characteristic to the mother plant. Protocols used for establishing of hairy root cultures vary, as well as the susceptibility of plant species to infection by Agrobacterium (Toivounen et al. 1993; Vanhala et al. 1995). It is known that the Agrobacterium strain used for transformation has a great influence on root morphology and the degree of secondary metabolite accumulation in hairy root cultures. It is possible by systematic clone selection, e.g., via protoplasts, to find high yielding, stable, and from single-cell-derived hairy root clones. This is possible because the hairy root cultures possess a great somaclonal variation. Another possibility of transformation is the use of viral vectors (Turpen 1999).
Any plant tissue or plant cells capable of subsequent clonal propagation, whether by organogenesis or embryogenesis, may be transformed with an expression vector of interest. The team “organogenesis” means a process by which shoots and roots are developed sequentially from meristematic centers; the term “embryogenesis” means a process by which shoots and roots develop together in a concerted fashion (not sequentially), whether from somatic cells or gametes. The particular tissue chosen will vary depending on the clonal propagation systems available for, and best suited to, the particular species being transformed. Exemplary tissue targets include protoplasts, leaf disks, pollen, embryos, cotyledons, hypocotyls, megagametophytes, callus tissue, existing meristematic tissue (e.g., apical meristems, axillary buds, and root meristems), and induced meristem tissue (e.g., cotyledon meristem and hypocotyls meristem).
According to a particularly preferred embodiment, the “yeast cell,” as used herein in any of the above-described methods, is a genetically engineered yeast cell, such as a yeast cell expressing an exogenous regulatory or biosynthetic enzyme of the sapogenin biosynthesis pathway (see also Tables 2 and 3). Preferably, the genetically engineered yeast cell is overexpressing an oxidosqualene cyclase (EC 5.4.99.-) and/or a cytochrome P450 (EC 1.14.-). According to a particular embodiment, the “genetically engineered yeast cell,” as used herein in any of the above-described methods, is overexpressing a cytochrome P450, which is capable of hydroxylating a carbon at position 21 of an oleanane-type backbone as pictured in
According to a further preferred embodiment, the genetically engineered yeast cell as used is deficient in expression and/or activity of an enzyme involved in endogenous sterol synthesis. For example, and without the purpose of being limitative, key enzymes in the yeast ergosterol biosynthetic pathway may be down-regulated, in particular, the lanosterol synthase gene (EC 5.4.99.7) to enable maximal and controllable flux toward the heterologous production of the desired triterpenoid sapogenin compounds.
The term “endogenous” as used herein, refers to substances (e.g., genes) originating from within an organism, tissue, or cell. Analogously, “exogenous” as used herein, is any material originated outside of an organism, tissue, or cell, but that is present (and typically can become active) in that organism, tissue, or cell.
Table 2. Overview of oxidosqualene cyclase (OSCs) reported in the literature as involved in sapogenin biosynthesis. 2,3-Oxidosqualene cyclization products identified to emerge from the activity of the corresponding OSC are indicated in squared brackets: aa—α-amyrin, ba—β-amyrin, bau—baurenol, da—damyrin, dam—dammarenediol, fri—friedelin, ge—germanicol, glu—glutinol, isotir—isotirucallol, lu—lupeol, lud—lupane-3β,20-diol, tir—tirucalla-7,24-dien-3β-ol, minor—additional byproducts either reported to be of minor appearance or to represent <10% of the observed products (Table 2 derived from Augustin et al. 2011).
A.
annua
A.
sedifolius
A.
strigosa
B.
gymnorhiza
B.
platyphylla
E.
tirucalli
G.
glabra
G.
straminea
L.
japonicus
M.
truncatula
N.
sativa
P.
ginseng
P.
ginseng
P.
sativum
S.
lycopersicum
S.
vaccaria
B.
gymnorhiza
B.
platyphylla
G.
glabra
L.
japonicus
O.
europaea
R.
communis
T.
officinale
C.
asiatica
P.
ginseng
P.
ginseng
A.
thaliana
A.
thaliana
A.
thaliana
C.
speciosus
L.
japonicus
K.
candel
K.
daigremontiana
O.
europaea
P.
sativum
R.
stylosa
S.
lycopersicum
A.
strigosa
M.
truncatula
V.
vinifera
V.
vinifera
C.
roseus
P.
ginseng
P.
ginseng
M.
truncatula
M.
truncatula
M.
truncatula
G.
uralensis
G.
uralensis
G.
max
M.
truncatula
G.
uralensis
Suitable cell culture media for eukaryotic cells, in particular, plant cells and microbial cells, are known in the art. For plant cells, exemplary media include standard growth media, many of which are commercially available (e.g., Sigma Chemical Co., St. Louis, Mo.). Examples include Schenk-Hildebrandt (SH) medium, Linsmaier-Skoog (LS) medium, Murashige and Skoog (MS) medium, Gamborg's B5 medium, Nitsch & Nitsch medium, White's medium, and other variations and supplements well known to those of skill in the art (see, e.g., Plant Cell Culture, Dixon, ed. IRL Press, Ltd. Oxford (1985); and George et al., Plant Culture Media, Vol. 1, Formulations and Uses Exegetics Ltd. Wilts, UK, (1987)). (See, e.g., Plant Cell Culture, Dixon, ed. IRL Press, Ltd. Oxford (1985); and George et al., Plant Culture Media, Vol. 1, Formulations and Uses Exegetics Ltd. Wilts, UK, (1987)).
For yeast cells, exemplary media include standard growth media, many of which are commercially available (e.g., Clontech, Sigma Chemical Co., St. Louis, Mo.). Examples include Yeast Extract Peptone Dextrose (YPD or YPED) medium, Yeast Extract Peptone Glycerol (YPG or YPEG) medium, Hartwell's complete (HC) medium, Synthetic complete (SC) medium, Yeast Nitrogen Base (YNB), and other variations and supplements well known to those of skill in the art (see, Yeast Protocol Handbook, Clontech).
The incubation conditions (temperature, photoperiod, shaking, auxin/cytokine hormone ratio, promoter-inducing conditions, promoter-repressing conditions, etc.) will depend, among other factors, on the cells to be incubated and are standard techniques in the art. In a particular embodiment, the current disclosure can be combined with other known methods to enhance the production and/or the secretion of triterpenoid sapogenin production in eukaryotic cell cultures, for example (1) by improvement of the cell culture conditions, (2) by metabolic engineering, (3) by the addition of specific elicitors to the cell culture.
Preferably, the eukaryotic cell is induced before it produces secondary metabolites such as triterpenoid sapogenins, meaning that the cell culture is stimulated by the addition of an external factor. External factors include the application of heat, the application of cold, the addition of acids, bases, metal ions, fungal membrane proteins, sugars and the like. In the case of plants, it is demonstrated that better production of plant secondary metabolites occurs via elicitation. Elicitors are compounds capable of inducing defense responses in plants. These are usually not found in intact plants but their biosynthesis is induced after wounding or stress conditions. Commonly used elicitors are jasmonates, mainly jasmonic acid and its methyl ester, methyl jasmonate. Jasmonates are linoleic acid derivatives of the plasma membrane and display a wide distribution in the plant kingdom. They were originally classified as growth inhibitors or promoters of senescence but now it has become apparent that they have pleiotropic effects on plant growth and development. Jasmonates appear to regulate cell division, cell elongation and cell expansion and thereby stimulate organ or tissue formation. They are also involved in the signal transduction cascades that are activated by stress situations such as wounding, osmotic stress, desiccation and pathogen attack. Methyl jasmonate (MeJA) is known to induce the accumulation of numerous defense-related secondary metabolites through the induction of genes coding for the enzymes involved in the biosynthesis of these compounds in plants. Jasmonates can modulate gene expression from the (post)transcriptional to the (post)translational level, both in a positive as well as in a negative way. Genes that are up-regulated are, e.g., defense- and stress-related genes (PR proteins and enzymes involved with the synthesis of phytoalexins and other secondary metabolites), whereas the activity of housekeeping proteins and genes involved with photosynthetic carbon assimilation are down-regulated. For example: the biosynthesis of phytoalexins and other secondary products in plants can also be boosted up by signal molecules derived from microorganisms or plants (such as peptides, oligosaccharides, glycopeptides, salicylic acid and lipophilic substances), as well as by various abiotic elicitors like UV-light, heavy metals (Cu, VOSO4, Cd) and ethylene. The effect of any elicitor is dependent on a number of factors, such as the specificity of an elicitor, elicitor concentration, the duration of the treatment and growth stage of the culture.
A number of suitable culture media for callus induction and subsequent growth on aqueous or solidified media are known. Exemplary media include standard growth media, many of which are commercially available (e.g., Sigma Chemical Co., St. Louis, Mo.). Examples include Schenk-Hildebrandt (SH) medium, Linsmaier-Skoog (LS) medium, Murashige and Skoog (MS) medium, Gamborg's B5 medium, Nitsch & Nitsch medium, White's medium, and other variations and supplements well known to those of skill in the art
During or after the sapogenin production in the growth medium of a eukaryotic cell culture, according to any of the above described methods, the sapogenins can be extracted from the cells. According to a preferred embodiment, the sapogenins are extracted from the culture medium wherein the sapogenins are secreted. Accordingly, the methods of producing sapogenins may optionally also comprise the step of extracting the sapogenins from the culture medium. Eventually, the sapogenins may also be further purified. Means that may be employed to this end are known to the skilled person. Generally, triterpenoid sapogenins can be measured intracellularly or in the extracellular space by methods known in the art. Such methods comprise analysis by thin-layer chromatography, high-pressure liquid chromatography, capillary electrophoresis, gas chromatography combined with mass spectrometric detection, radioimmuno-assay (RIA) and enzyme immuno-assay (ELISA). For example, Medicago triterpene sapo(ge)nin content can be analyzed by reverse-phase UPLC/ICR/FT-MS, and is also further illustrated in the Examples section.
In a further aspect, the disclosure also provides a eukaryotic cell genetically engineered to synthesize sapogenins and/or pathway intermediates. In particular, the genetically engineered cell is a yeast cell, for example a Saccharomyces, Schizosaccharomyces, Pichia, Yarrowia, Candida or Hansenula cell. According to a particularly preferred embodiment, the yeast cell is genetically engineered to express an exogenous regulatory or biosynthetic enzyme of the sapogenin biosynthesis pathway (see also Tables 2 and 3). Preferably, the yeast cell is genetically engineered to overexpress an oxidosqualene cyclase (EC 5.4.99.-) and/or a cytochrome P450 (EC 1.14.-). According to a particular embodiment, the yeast cell is genetically engineered to overexpress a cytochrome P450, which is capable of hydroxylating a carbon at position 21 of an oleanane-type backbone as pictured in
In still another aspect, the disclosure also encompasses existing or novel sapogenins obtained by any of the above-described methods. The sapogenins that are extracellularly accumulating in the growth medium of a eukaryotic cell culture in the presence of cyclodextrins are readily accessible and can be exploited by industry for a variety of purposes, either directly, or after further synthetic chemistry. For example, sapogenins and its derivatives (including saponins) can be used as additives to foods and cosmetics, preservatives, flavor modifiers, agents for removal of cholesterol from dietary products, and may also be very valuable for their pharmacological properties. For example, several saponins and sapogenins are considered to possess activities such as anti-inflammatory, anti-carcinogenic, anti-bacterial, anti-fungal and anti-viral effects. Saponins are also of interest as valuable adjuvants and the first saponin-based vaccines are introduced commercially (reviewed in Sun et al. 2009).
In a specific embodiment, sapogenins that are hydroxylated on a carbon at position 21 of an oleanane-type backbone as pictured in
According to another aspect, the disclosure relates to an isolated polypeptide selected from the group consisting of:
In one embodiment, the disclosure relates to any of the above-described polypeptides wherein the polypeptide sequence is consisting of an amino acid sequence as set forth in SEQ ID NO:3 or 4 and polypeptide sequences having at least 75% identity to SEQ ID NO:3 or 4.
According to another aspect, the disclosure relates to an isolated polynucleotide selected from the group consisting of:
As used herein, the terms “polypeptide,” “protein,” and “peptide” are used interchangeably and refer to a polymeric form of amino acids of any length, which can include coded and non-coded amino acids, chemically or biochemically modified or derivatized amino acids, and polypeptides having modified peptide backbones.
As used herein, the terms “nucleic acid,” “polynucleotide,” and “polynucleic acid” are used interchangeably and refer to a polymeric form of nucleotides of any length, either deoxyribonucleotides or ribonucleotides, or analogs thereof. Polynucleotides may have any three-dimensional structure, and may perform any function, known or unknown. Non-limiting examples of polynucleotides include a gene, a gene fragment, exons, introns, messenger RNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA, recombinant polynucleotides, branched polynucleotides, plasmids, vectors, isolated DNA of any sequence, control regions, isolated RNA of any sequence, nucleic acid probes, and primers. The polynucleotide molecule may be linear or circular. The polynucleotide may comprise a promoter, an intron, an enhancer region, a polyadenylation site, a translation initiation site, 5′ or 3′ untranslated regions, a reporter gene, a selectable marker or the like. The polynucleotide may comprise single-stranded or double-stranded DNA or RNA. The polynucleotide may comprise modified bases or a modified backbone. A nucleic acid that is up to about 100 nucleotides in length, is often also referred to as an oligonucleotide.
An “isolated polypeptide” or an “isolated polynucleotide,” as used herein, refers to, respectively, an amino acid sequence or a polynucleotide sequence that is not naturally occurring or no longer occurring in the natural environment wherein it was originally present.
As used herein, the terms “identical” or “percent identity,” in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same (e.g., 75% identity over a specified region), when compared and aligned for maximum correspondence over a comparison window or designated region as measured using sequence comparison algorithms or by manual alignment and visual inspection. Preferably, the identity exists over a region that is at least about 25 amino acids or nucleotides in length, or, more preferably, over a region that is 50-100 amino acids or nucleotides or even more in length. According to preferred embodiments, the disclosure relates to an isolated polypeptide comprising a polypeptide sequence having at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to the polypeptide encoded by a polynucleotide sequence having SEQ ID NO:1 or 2, or having at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to a polypeptide comprising an amino acid sequence as set forth in SEQ ID NO:3 or 4. In other preferred embodiments, the disclosure relates to an isolated polynucleotide comprising a polynucleotide sequence having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identity to the sequence having SEQ ID NO:1 or 2.
In a particular embodiment, fragments and variants of any of the above polynucleotides or polypeptides also form part of this disclosure.
In reference to a nucleotide sequence, “a fragment” refers to any sequence of at least 15 consecutive nucleotides, preferably at least 30 consecutive nucleotides, more preferably at least 50, 60, 70, 80, 90, 100, 150, 200 consecutive nucleotides or more, of any of the sequences provided herein. If desired, the fragment may be fused at either terminus to additional base pairs, which may number from 1 to 20, typically 50 to 100, but up to 250 to 500 or more.
A “fragment,” as referred to polypeptides, refers to a subsequence of the polypeptide. Fragments may vary in size from as few as five amino acids to the length of the intact polypeptide, but are preferably at least 10, 15, 20, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 55, 60, 65, 70, 75 amino acids in length. If desired, the fragment may be fused at either terminus to additional amino acids, which may number from 1 to 20, typically 50 to 100, but up to 250 to 500 or more. A “functional fragment” means a polypeptide fragment possessing the ability to hydroxylate a carbon at position 21 of an oleanane-type backbone as pictured in
A “variant” as used herein refers to homologs, orthologs and paralogs and include, but are not limited to, homologs, orthologs and paralogs of SEQ ID NOS:1-4. Homologs of a protein encompass peptides, oligopeptides and polypeptides having amino acid substitutions, deletions and/or insertions, preferably by a conservative change, relative to the unmodified protein in question and having similar biological and functional activity as the unmodified protein from which they are derived; or in other words, without significant loss of function or activity. Orthologs and paralogs, which are well-known terms by the skilled person, define subcategories of homologs and encompass evolutionary concepts used to describe the ancestral relationships of genes. Paralogs are genes within the same species that have originated through duplication of an ancestral gene; orthologs are genes from different organisms that have originated through speciation, and are also derived from a common ancestral gene. Several different methods are known by those of skill in the art for identifying and defining these functionally homologous sequences. General methods for identifying orthologues and paralogues include phylogenetic methods, sequence similarity and hybridization methods. Percentage similarity and identity can be determined electronically. Examples of useful algorithms are PILEUP (Higgins & Sharp, CABIOS 5:151 (1989)), BLAST and BLAST 2.0 (Altschul et al., J. Mol. Biol. 215:403 (1990); software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/)). Preferably, the homologue, orthologue or paralogue has a sequence identity at a protein level of at least 50%, preferably 60%, more preferably 70%, even more preferably 80%, most preferably 90% as measured in a BLASTp.
Further, it will be appreciated by those of skill in the art, that any of a variety of polynucleotide sequences are capable of encoding the polypeptides of the disclosure. Due to the degeneracy of the genetic code, many different polynucleotides can encode identical and/or substantially similar polypeptides. Sequence alterations that do not change the amino acid sequence encoded by the polynucleotide are termed “silent” variations. With the exception of the codons ATG and TGG, encoding methionine and tryptophan, respectively, any of the possible codons for the same amino acid can be substituted by a variety of techniques, e.g., site-directed mutagenesis, available in the art. Accordingly, any and all such variations of a sequence are a feature of the disclosure. In addition to silent variations, other conservative variations that alter one, or a few amino acids in the encoded polypeptide, can be made without altering the function of the polypeptide (i.e., enhanced secondary metabolite production, in the context of the disclosure), these conservative variants are, likewise, a feature of the disclosure.
Conservative substitutions or variations, as used herein, are those in which at least one residue in the amino acid sequence has been removed and a different residue inserted in its place. Such substitutions generally are made in accordance with the table depicted below. This table shows amino acids that can be substituted for an amino acid in a protein and that are typically regarded as conservative substitutions.
Substitutions that are less conservative than those in the above table can be selected by picking residues that differ more significantly in their effect on maintaining (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a sheet or helical conformation, (b) the charge or hydrophobicity of the molecule at the target site, or (c) the bulk of the side chain. The substitutions that, in general, are expected to produce the greatest changes in protein properties will be those in which (a) a hydrophilic residue, e.g., seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl or alanyl; (b) a cysteine or proline is substituted for (or by) any other residue; (c) a residue having an electropositive side chain, e.g., lysyl, arginyl, or histidyl, is substituted for (or by) an electronegative residue, e.g., glutamyl or aspartyl; or (d) a residue having a bulky side chain, e.g., phenylalanine, is substituted for (or by) one not having a side chain, e.g., glycine.
Substitutions, deletions and insertions introduced into the sequences are also envisioned by the disclosure. Such sequence modifications can be engineered into a sequence by site-directed mutagenesis or the other methods known in the art. Amino acid substitutions are typically of single residues; insertions usually will be on the order of from about one to ten amino acid residues; and deletions will range from about one to thirty residues. In preferred embodiments, deletions or insertions are made in adjacent pairs, e.g., a deletion of two residues or insertion of two residues. Substitutions, deletions, insertions or any combination thereof can be combined to arrive at a sequence. The mutations that are made in the polynucleotides of the disclosure should not create complementary regions that could produce secondary mRNA structure. Preferably, the polypeptide encoded by the DNA performs the desired function (i.e., hydroxylating a carbon at position 21 of an oleanane-type backbone as pictured in
In a specific embodiment, the disclosure relates to a chimeric gene comprising the following operably linked sequences: a) a promoter region capable of directing expression in a eukaryotic cell (as defined hereinbefore); b) a DNA region encoding a polypeptide as defined above; and c) a 3′ polyadenylation and transcript termination region.
The term “operably linked” as used herein refers to a linkage in which the regulatory sequence is contiguous with the gene of interest to control the gene of interest, as well as regulatory sequences that act in trans or at a distance to control the gene of interest. For example, a DNA sequence is operably linked to a promoter when it is ligated to the promoter downstream with respect to the transcription initiation site of the promoter and allows transcription elongation to proceed through the DNA sequence. A DNA for a signal sequence is operably linked to DNA coding for a polypeptide if it is expressed as a pre-protein that participates in the transport of the polypeptide. Linkage of DNA sequences to regulatory sequences is typically accomplished by ligation at suitable restriction sites or adapters or linkers inserted in lieu thereof using restriction endonucleases known to one of skill in the art.
The term “regulatory sequence” as used herein refers to polynucleotide sequences that are necessary to affect the expression of coding sequences to which they are operably linked. Expression control sequences are sequences that control the transcription, post-transcriptional events and translation of nucleic acid sequences. Expression control sequences include appropriate transcription initiation, termination, promoter and enhancer sequences; efficient RNA processing signals such as splicing and polyadenylation signals; sequences that stabilize cytoplasmic mRNA; sequences that enhance translation efficiency (e.g., ribosome binding sites); sequences that enhance protein stability; and when desired, sequences that enhance protein secretion. The nature of such control sequences differs depending upon the host organism. The term “control sequences” is intended to include, at a minimum, all components whose presence is essential for expression, and can also include additional components whose presence is advantageous, for example, leader sequences and fusion partner sequences.
A vector comprising a polynucleotide sequence or a chimeric gene as defined above also forms part of the disclosure, as well as a host cell comprising a polynucleotide sequence or a chimeric gene or a vector as defined above.
The term “vector” as used herein is intended to refer to a nucleic acid molecule capable of transporting another nucleic acid molecule to which it has been linked. The vector may be of any suitable type including, but not limited to, a phage, virus, plasmid, phagemid, cosmid, bacmid or even an artificial chromosome. Certain vectors are capable of autonomous replication in a host cell into which they are introduced (e.g., vectors having an origin of replication that functions in the host cell). Other vectors can be integrated into the genome of a host cell upon introduction into the host cell, and are thereby replicated along with the host genome. Moreover, certain preferred vectors are capable of directing the expression of certain genes of interest. Such vectors are referred to herein as “recombinant expression vectors” (or simply, “expression vectors”). Suitable vectors have regulatory sequences, such as promoters, enhancers, terminator sequences, and the like as desired and according to a particular host organism (e.g., plant cell). Typically, a recombinant vector according to the disclosure comprises at least one “chimeric gene” or “expression cassette.” Expression cassettes are generally DNA constructs preferably including (5′ to 3′ in the direction of transcription): a promoter region, a polynucleotide sequence, homologue, variant or fragment thereof of the disclosure operably linked with the transcription initiation region, and a termination sequence including a stop signal for RNA polymerase and a polyadenylation signal. It is understood that all of these regions should be capable of operating in biological cells, such as plant cells, to be transformed. The promoter region comprising the transcription initiation region, which preferably includes the RNA polymerase binding site, and the polyadenylation signal may be native to the biological cell to be transformed or may be derived from an alternative source, where the region is functional in the biological cell.
The term “recombinant host cell” (“expression host cell,” “expression host system,” “expression system” or simply “host cell”), as used herein, is intended to refer to a cell into which a recombinant vector has been introduced. It should be understood that such terms are intended to refer not only to the particular subject cell but to the progeny of such a cell. Because certain modifications may occur in succeeding generations due to either mutation or environmental influences, such progeny may not, in fact, be identical to the parent cell, but are still included within the scope of the term “host cell” as used herein. A recombinant host cell may be an isolated cell or cell line grown in culture or may be a cell that resides in a living tissue or organism. Host cells can be of bacterial, fungal, plant or mammalian origin.
According to yet another aspect, the disclosure provides a transgenic plant or a cell derived thereof that is transformed with the above-described vector.
The term “plant” as used herein refers to vascular plants (e.g., gymnosperms and angiosperms). A “transgenic plant” refers to a plant comprising a recombinant polynucleotide and/or a recombinant polypeptide according to the disclosure. A transgenic plant refers to a whole plant as well as to a plant part, such as seed, fruit, leaf, or root, plant tissue, plant cells or any other plant material, and progeny thereof. A transgenic plant can be obtained by transforming a plant cell with an expression cassette of the disclosure and regenerating such plant cell into a transgenic plant. Such plants can be propagated vegetatively or reproductively. The transforming step may be carried out by any suitable means, including by Agrobacterium-mediated transformation and non-Agrobacterium-mediated transformation, as discussed in detail below. Plants can be regenerated from the transformed cell (or cells) by techniques known to those skilled in the art. Where chimeric plants are produced by the process, plants in which all cells are transformed may be regenerated from chimeric plants having transformed germ cells, as is known in the art. Methods that can be used to transform plant cells or tissue with expression vectors of the disclosure include both Agrobacterium and non-Agrobacterium vectors. Agrobacterium-mediated gene transfer exploits the natural ability of Agrobacterium tumefaciens to transfer DNA into plant chromosomes and is described in detail in G. Gheysen, G. Angenon, and M. Van Montagu, 1998, Agrobacterium-mediated plant transformation: a scientifically intriguing story with significant applications in K. Lindsey (Ed.), Transgenic Plant Research, Harwood Academic Publishers, Amsterdam, pp. 1-33; and in H. A. Stafford (2000), Botanical Review 66:99-118. A second group of transformation methods is the non-Agrobacterium-mediated transformation and these methods are known as direct gene transfer methods. An overview is brought by P. Barcelo and P. A. Lazzeri (1998), Direct gene transfer: chemical, electrical and physical methods in K. Lindsey (Ed.), Transgenic Plant Research, Harwood Academic Publishers, Amsterdam, pp. 35-55.
Methods include particle gun delivery, microinjection, electroporation of intact cells, polyethyleneglycol-mediated protoplast transformation, electroporation of protoplasts, liposome-mediated transformation, silicon-whiskers-mediated transformation, etc.
Hairy root cultures can be obtained by transformation with virulent strains of Agrobacterium rhizogenes, and they can produce high content of secondary metabolites characteristic to the mother plant. Protocols used for establishing of hairy root cultures vary, as well as the susceptibility of plant species to infection by Agrobacterium (Toivounen et al. 1993; Vanhala et al. 1995). It is known that the Agrobacterium strain used for transformation has a great influence on root morphology and the degree of secondary metabolite accumulation in hairy root cultures. It is possible by systematic clone selection, e.g., via protoplasts, to find high yielding, stable, and from single-cell-derived hairy root clones. This is possible because the hairy root cultures possess a great somaclonal variation. Another possibility of transformation is the use of viral vectors (Turpen 1999).
Any plant tissue or plant cells capable of subsequent clonal propagation, whether by organogenesis or embryogenesis, may be transformed with an expression vector of the disclosure. The term “organogenesis” means a process by which shoots and roots are developed sequentially from meristematic centers; the term “embryogenesis” means a process by which shoots and roots develop together in a concerted fashion (not sequentially), whether from somatic cells or gametes. The particular tissue chosen will vary depending on the clonal propagation systems available for, and best suited to, the particular species being transformed. Exemplary tissue targets include protoplasts, leaf disks, pollen, embryos, cotyledons, hypocotyls, megagametophytes, callus tissue, existing meristematic tissue (e.g., apical meristems, axillary buds, and root meristems), and induced meristem tissue (e.g., cotyledon meristem and hypocotyls meristem).
A “control plant” as used in the disclosure refers to a plant cell, seed, plant component, plant tissue, plant organ or whole plant used to compare against transgenic or genetically modified plant for the purpose of identifying a difference in production of sapogenins in the transgenic or genetically modified plant. A control plant may in some cases be a transgenic plant line that comprises an empty vector or marker gene, but does not contain the recombinant polynucleotide of the disclosure that is expressed in the transgenic or genetically modified plant being evaluated. In general, a control plant is a plant of the same line or variety as the transgenic or genetically modified plant being tested. A suitable control plant would include a genetically unaltered or non-transgenic plant of the parental line (wild-type) used to generate a transgenic plant herein.
Plants of the disclosure may include, but are not limited to, plants or plant cells of agronomically important crops that are or are not intended for animal or human nutrition, such as maize or corn, wheat, barley, oat, Brassica spp. plants such as Brassica napus or Brassica juncea, soybean, bean, alfalfa, pea, rice, sugarcane, beetroot, tobacco, sunflower, cotton, Arabidopsis, vegetable plants such as cucumber, leek, carrot, tomato, lettuce, peppers, melon, watermelon, diverse herbs such as oregano, basilicum and mint. It may also be applied to plants that produce valuable compounds, e.g., useful as, for instance, pharmaceuticals, as ajmalicine, vinblastine, vincristine, ajmaline, reserpine, rescinnamine, camptothecine, ellipticine, quinine, and quinidine, taxol, morphine, scopolamine, atropine, cocaine, sanguinarine, codeine, genistein, daidzein, digoxinu, calystegins or as food additives such as anthocyanins, and vanillin, including, but not limited to, the classes of compounds mentioned above. Examples of such plants include, but not limited to, Papaver spp., Rauwolfia spp., Taxus spp., Cinchona spp., Eschscholtzia californica, Camptotheca acuminata, Hyoscyamus spp., Berberis spp., Coptis spp., Datura spp., Atropa spp., Thalictrum spp., Peganum spp. Preferred members of the genus Taxus comprise Taxus brevifolia, Taxus baccata, Taxus cuspidata, Taxus canadensis and Taxus floridana.
The polynucleotide sequence, homologue, variant or fragment thereof of the disclosure may be expressed in, for example, a plant cell under the control of a promoter that directs constitutive expression or regulated expression. Regulated expression comprises temporally or spatially regulated expression and any other form of inducible or repressible expression. Temporally means that the expression is induced at a certain time point, for instance, when a certain growth rate of the plant cell culture is obtained (e.g., the promoter is induced only in the stationary phase or at a certain stage of development). Spatially means that the promoter is only active in specific organs, tissues, or cells (e.g., only in roots, leaves, epidermis, guard cells or the like). Other examples of regulated expression comprise promoters whose activity is induced or repressed by adding chemical or physical stimuli to the plant cell. In a preferred embodiment, the expression is under control of environmental, hormonal, chemical, and/or developmental signals. Such promoters for plant cells include promoters that are regulated by (1) heat, (2) light, (3) hormones, such as abscisic acid, and methyl jasmonate (4) wounding or (5) chemicals such as salicylic acid, chitosans or metals. Indeed, it is well known that the expression of secondary metabolites can be boosted by the addition of, for example, specific chemicals, jasmonate and elicitors. In a particular embodiment, the co-expression of several (more than one) polynucleotide sequences or homologues or variants or fragments thereof, in combination with the induction of secondary metabolite synthesis, is beneficial for an optimal and enhanced production of secondary metabolites. Alternatively, the at least one polynucleotide sequence, homologue, variant or fragment thereof is placed under the control of a constitutive promoter. A constitutive promoter directs expression in a wide range of cells under a wide range of conditions. Examples of constitutive plant promoters useful for expressing heterologous polypeptides in plant cells include, but are not limited to, the cauliflower mosaic virus (CaMV) 35S promoter, which confers constitutive, high-level expression in most plant tissues including monocots, the nopaline synthase promoter and the octopine synthase promoter. The expression cassette is usually provided in a DNA or RNA construct that is typically called an “expression vector,” which is any genetic element, e.g., a plasmid, a chromosome, a virus, behaving either as an autonomous unit of polynucleotide replication within a cell (i.e., capable of replication under its own control) or being rendered capable of replication by insertion into a host cell chromosome, having attached to it another polynucleotide segment, so as to bring about the replication and/or expression of the attached segment. Suitable vectors include, but are not limited to, plasmids, bacteriophages, cosmids, plant viruses and artificial chromosomes. The expression cassette may be provided in a DNA construct, which also has at least one replication system. In addition to the replication system, there will frequently be at least one marker present, which may be useful in one or more hosts, or different markers for individual hosts. The markers may a) code for protection against a biocide, such as antibiotics, toxins, heavy metals, certain sugars or the like; b) provide complementation, by imparting prototrophy to an auxotrophic host; or c) provide a visible phenotype through the production of a novel compound in the plant. Exemplary genes that may be employed include neomycin phosphotransferase (NPTII), hygromycin phosphotransferase (HPT), chloramphenicol acetyltransferase (CAT), nitrilase, and the gentamicin resistance gene. For plant host selection, non-limiting examples of suitable markers are β-glucuronidase, providing indigo production, luciferase, providing visible light production, Green Fluorescent Protein and variants thereof, NPTII, providing kanamycin resistance or G418 resistance, HPT, providing hygromycin resistance, and the mutated aroA gene, providing glyphosate resistance.
The term “promoter activity” refers to the extent of transcription of a polynucleotide sequence, homologue, variant or fragment thereof that is operably linked to the promoter whose promoter activity is being measured. The promoter activity may be measured directly by measuring the amount of RNA transcript produced, for example, by Northern blot or indirectly by measuring the product coded for by the RNA transcript, such as when a reporter gene is linked to the promoter.
According to a further aspect of the disclosure, the above-described polynucleotide sequences (and encoded proteins) can be used for the biosynthesis of (novel) triterpenoid sapogenin compounds. To illustrate this further, without the purpose of being limitative, one is referred to the Examples section below.
The following examples are intended to promote a further understanding of the disclosure. While this disclosure is described herein with reference to illustrated embodiments, it should be understood that the disclosure is not limited hereto. Those having ordinary skill in the art and access to the teachings herein will recognize additional modifications and embodiments within the scope thereof. Therefore, the disclosure is limited only by the claims attached herein.
In an attempt to detect triterpenoid sapogenins from plant cultures, hairy roots of the model legume Medicago truncatula transformed with a GATEWAY™ plasmid pK7WG2D-GUS, containing a non-functional β-glucuronidase gene (GUS) expressed from a 35S promoter (Pollier et al., 2011) were analyzed. The hairy roots were grown for three weeks in 30 ml Murashige and Skoog basal salt mixture including vitamins (Duchefa) prior to an organic extraction and gas chromatography-mass spectrometry (GC-MS) analysis. The roots were harvested from the culture medium, frozen in liquid nitrogen and ground to a fine powder. A total metabolite extraction was performed on this ground material using 1 ml of methanol. The methanol phase was evaporated to dryness and the subsequent pellet was extracted with 1 ml hexane, since the triterpenoid sapogenins are extremely hydrophobic in nature. The hexane phase was separated from the undissolved residue, evaporated to dryness and trimethylsilylated as described (Radosevich et al. 1985). This derivatized material was subjected to GC-MS (GC model 6890, MS model 5973, Agilent) analysis where, a 1 μl aliquot was injected in splitless mode into a VF-5 ms capillary column (VARIAN® CP9013, Agilent) and operated at a constant helium flow of 1 ml/minute. The injector temperature was set to 280° C. and the oven temperature was held at 80° C. for 1 minute post-injection, ramped to 280° C. at 20° C./minute, held at 280° C. for 45 minutes, ramped to 320° C. at 20° C./minute, held at 320° C. for 1 minute, and finally cooled down to 80° C. at 50° C./minute at the end of the run. The MS transfer line was set to 250° C., the MS ion source to 230° C., and the quadrupole to 150° C., throughout.
Owing to the hydrophobicity of sterols, multiple sterols and sterol intermediates were detected in the GC chromatogram of this extract (
Triterpene saponin and sterol biosynthesis depend on the same precursor, i.e., oxidosqualene (
The ergosterol biosynthetic pathway of the Saccharomyces cerevisiae strain S288c BY4742 was modified as described (Kirby et al., 2008) with adaptations. The lanosterol synthase gene (ERG7) (GenBank accession number NM—001179202) was made conditionally down-regulatable by replacing the native ERG7 promoter with a methionine-repressible MET3 promoter as described for ERG9 in S. cerevisiae CEN.PK 113-7D (Asadollahi et al., 2008), to generate strain TM1. The amount of ergosterol produced by TM1 in the presence of different concentrations of methionine was quantified using the sterol quantification method as described (Arthington-Skaggs et al., 1999). A 60% reduction in ergosterol accumulation was observed in TM1 with 1.5 mM methionine when compared to wild-type cells. Further, a truncated, feedback-uncoupled, copy of isoform 1 of the rate-limiting enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (tHMG1) (GenBank accession number NM—001182434) was generated as described (Polakowski et al., 1998). The tHMG1 was cloned into the multiple cloning site (MCS) 1 of the high copy number plasmid pESC-URA (Agilent Technologies) behind the galactose inducible GAL10 promoter to generate pESC-URA[GAL10/tHMG1], which was transformed into TM1 to generate strain TM5.
The Glycyrrhiza glabra β-amyrin synthase (GgbAS) (GenBank accession number AB037203; (Hayashi et al., 2001)) was cloned into the MCS 2 of plasmid pESC-URA[GAL10/tHMG1] to generate pESC-URA[GAL10/tHMG1; GAL1/bAS]. Strain TM1 was transformed with this plasmid using the lithium acetate-mediated transformation method to generate strain TM3. To validate the production of β-amyrin in TM3, strains TM3 and TM5 were first precultured in minimal synthetic defined (SD) base-containing drop out (DO) supplement—Ura (SD-Ura) (Clontech) medium for 18-20 hours at 30° C. and 250 rpm. The precultures were washed prior to inoculating Minimal SD Base Gal/Raf containing DO supplement—Ura (SD Gal/Raf-Ura) (Clontech) medium to a starting optical density of 0.25. The cultures were incubated as before for 24 hours, prior to addition of 10 mM methionine to a final concentration of 1.5 mM, following which they were incubated further for 48 hours. Yeast cells from a 1 ml culture were used for extraction and gas chromatography-mass spectrometry (GC-MS) analysis as described (Kirby et al., 2008) with modifications. The cell pellet was resuspended in an equal volume of 40% potassium hydroxide and 50% ethanol prior to lysis by boiling at 95° C. for 10 minutes. An organic extraction was performed on the lysate using an equivalent volume of hexane and vortexing at high speed for 1 minute. The hexane extraction was repeated thrice before the phases were pooled and evaporated to dryness. A trimethylsilyl derivatization was performed on the dried material, and used for GC-MS analysis. The GC chromatograms showed the presence of a single peak at 27.2 minutes corresponding to 36.2 mg/L of β-amyrin in TM3 but not in TM5 (
The Arabidopsis thaliana lupeol synthase (AtLUS1) (GenBank accession number U49919; (Herrera et al., 1998)) was cloned into the MCS 2 of plasmid pESC-URA[GAL10/tHMG1] to generate pESC-URA[GAL10/tHMG1; GAL1/AtLUS1]. The plasmid was transformed into strain TM1 to generate a lupeol-producing strain TM6. The production of lupeol by strain TM6 was verified by culturing as described in Example 3 and analyzing the trimethylsilylated fraction by GC-MS. A single peak at 28.9 minutes corresponding to 46.3 mg/L of lupeol with an E1 pattern identical to a standard of lupeol (Extrasynthese) (data not shown), was observed in the GC chromatograms of TM6 but not TM5. Again, no lupeol was detected in the spent medium of strains TM6 and TM5 (
Strains TM3, TM6 and TM5 were cultured as in Example 2 with modifications. The precultures were prepared and inoculated into SD Gal/Raf-Ura medium and incubated for 24 hours as described. Along with methionine, 250 mM methyl-β-cyclodextrin (CAVASOL©, Wacker Quimica Ibérica S.A.) was also added to a final concentration of 5 mM and the cultures were incubated further under the same conditions. Post 24 hours incubation, 250 mM methyl-β-cyclodextrin was added once again to a concentration of 5 mM and the cultures were incubated further for 24 hours. The cells and spent medium were harvested from 1 ml of the culture and processed separately for extraction and GC-MS analysis as described in Example 3. GC chromatograms confirmed the presence of β-amyrin and lupeol in the cell pellet as well as in spent medium of strains TM3 and TM6, respectively, but not TM5, with higher concentrations of both β-amyrin and lupeol quantified from the extracts of spent medium (37.3 mg/L and 164.2 mg/L, respectively) when compared to cell pellet (20.4 mg/L and 41.7 mg/L, respectively) (
Further, it was determined whether cyclodextrin facilitates the secretion of triterpene sapogenin backbones in a dose-dependent manner. For this, the β-amyrin-producing strain TM3 was employed and applied cyclodextrin at different times during culturing. A total of seven culturing conditions were set up, which included an untreated control (C) and six treated samples, with cyclodextrin added to a concentration of 5 mM each time. To samples 11, 12 and 13, cyclodextrin was added on Day 1 immediately after inoculation into SD Gal/Raf-Ura medium. To samples I2 and I3, an additional dose of cyclodextrin was added on Day 2, together with addition of methionine. Additionally, to sample I3, a third dose of cyclodextrin was added on Day 3. Further, to samples R1, R2, and AR1, cyclodextrin was added on; Day 2 only, Day 2 and Day 3, and Day 3 only, respectively. Extractions were performed on the spent medium of all the samples on Day 4 and quantified for β-amyrin using GC-MS as described. Surprisingly, a direct correlation was observed between the amount of β-amyrin quantified from the spent medium and the number of times cyclodextrin was added to the sample, thereby suggesting the dose-dependent nature of this secretion (
To produce triterpene sapogenins in the yeast strain, β-amyrin and lupeol were modified with three characterized cytochrome P450 monooxygenases (CYPs), CYP716A12 (GenBank accession number FN995113; (Carelli et al., 2011)), CYP88D6 (GenBank accession number AB433179; (Seki et al., 2008)) and CYP93E2 (GenBank accession number DQ335790; (Li et al., 2007)). The CYPs need a CYP reductase (CPR) as a redox partner for their activity. Therefore, A. thaliana CPR, ATR1 (At4g24520) along with the CYPs were simultaneously cloned. Both the CYPs and CPR were PCR amplified and cloned into the entry vector pDONR221 by GATEWAY™ recombination (Invitrogen Life Technologies). Further, the CYPs were GATEWAY™ recombined into the high copy number expression vector, pAG423GAL-ccdB (Addgene plasmid 14149) containing the GAL1 promoter and HIS3 auxotrophic marker to generate the plasmids pAG423[GAL1/CYP716A12], pAG423[GAL1/CYP88D6] and pAG423[GAL1/CYP93E2]. The CPR was GATEWAY™ recombined into the high copy number expression vector pAG425GAL-ccdB (Addgene plasmid 14153) having the GAL1 promoter and LEU2 auxotrophic marker to generate plasmid pAG425[GAL1/AtATR1].
The TM3 strain was transformed with the plasmid pAG425[GAL1/AtATR1], in combination with either pAG423[GAL1/CYP716A12], pAG423[GAL1/CYP88D6], pAG423[GAL1/CYP93E2] or pAG423GAL-ccdB to generate strains TM10, TM11, TM12 and TM26, respectively. The spent medium of strains TM10, TM11, TM12 and TM26, cultured in SD Gal/Raf-Ura/-His/-Leu medium with cyclodextrin treatment as described in Example 5, was analyzed by GC-MS. Peaks corresponding to erythrodiol, oleanolic aldehyde and oleanolic acid at high concentrations were detected in the GC chromatograms of extract from TM10, indicating the C-28 hydroxylation of β-amyrin mediated by CYP716A12. Similarly, 11-hydroxy-β-amyrin and 11-oxo-β-amyrin were detected in the chromatogram of strain TM11 and 24-hydroxy-β-amyrin in the extract of strain TM12. The identity of all the hydroxylated β-amyrin peaks was confirmed by comparing their E1 patterns against available standards (erythrodiol, oleanolic acid (Extrasynthese)), or previous reports when a commercial standard was unavailable. No hydroxylated β-amyrin was detected in the chromatogram of TM26 (
Next, it was determined whether cyclodextrin could also facilitate the secretion of lupane-type triterpene sapogenins from the cells to the culture medium. For this, strains TM22 and TM28 were generated by transforming strain TM6 with the plasmids pAG425[GAL1/AtATR1] and pAG423[GAL1/CYP716A12] or pAG423GAL-ccdB, respectively. Strains TM22 and TM28 were cultured as described using SD Gal/Raf-Ura/-His/-Leu medium with cyclodextrin treatment and the spent medium was used for extraction and GC-MS analysis. 7.2 mg/L betulin and 2.4 mg/L betulinic acid were detected in the chromatograms of TM22 but not TM28, indicating the C-28 hydroxylation of lupeol by CYP716A12 (
It was then determined whether cyclodextrins could also facilitate the production and/or secretion of triterpene sapogenins from the model legume M. truncatula by analyzing the spent medium of M. truncatula hairy roots transformed with pK7WG2D-GUS. The hairy roots were grown for two weeks in 20 ml Murashige and Skoog basal salt mixture including vitamins (Duchefa) prior to addition of 250 mM cyclodextrin to a final concentration of 25 mM, in combination with or without 100 μM methyl jasmonate, and compared to untreated and 100 μM methyl jasmonate only treated controls. The roots were harvested 48 hours after treatment and the spent medium was extracted and analyzed by GC-MS. Surprisingly, the triterpene sapogenins erythrodiol, oleanolic acid and oleanolic aldehyde, corresponding to the building blocks of the most abundant saponins produced by the hairy roots (Pollier et al., 2011), could now be detected in the GC chromatograms of medium of roots treated with cyclodextrin, but not in control roots (
The genus Bupleurum consists of perennial herbs and forms an integral part of Asian traditional medicine in which it is used, either alone or in combination with other ingredients, for the treatment of common colds, fever and inflammatory disorders in the form of over-the-counter herbal teas. Saikosaponins constitute the largest class of secondary metabolites in Bupleurum and account for ˜7% of the total dry weight of roots. More than 120 closely related glycosylated oleanane- and ursane-type saikosaponins have been identified from this genus that can be distinguished only by the positions and numbers of double bonds in rings C and D and oxygenation patterns on C-16, C-23, C-28 and C-30 (
To identify new saponin biosynthesis genes, a genome-wide cDNA-AFLP®-based transcript profiling was performed on the roots of hydroponically grown B. falcatum plants. B. falcatum seeds, obtained from a commercial source (on the World Wide Web at SandMountainHerbs.com), were sown in soil, and 2 weeks after germination, seedlings were transferred to aerated hydroponics medium containing 1 g/L 10-30-20 salts (Scotts, Ohio, USA), pH 6.5. Plants were grown at 16 hours/8 hours light/dark regime, at 21° C. The pH was monitored daily and adjusted to 6.5 by adding KOH to the hydroponics medium. Three weeks after the plants were transferred to the hydroponics medium, they were treated with 50 μM methyl jasmonate (MeJA) (dissolved in ethanol (EtOH)) or an equivalent amount of EtOH as a control, by adding the EtOH or MeJA solution directly to the hydroponics medium. For transcript profiling, roots were harvested 0, 0.5, 1, 2, 4, 8 and 24 hours after treatment, frozen in liquid nitrogen, and stored at −70° C. For each sample, three individual plants were pooled.
A full genome-wide cDNA-AFLP®-based transcript profiling on the roots of hydroponically grown B. falcatum plants was carried out as described in Vuylsteke et al. (2007). Gel images were analyzed with the AFLP-QUANTAR® PRO software (KEYGENE®, Wageningen, The Netherlands), allowing accurate quantification of band intensities. Extraction and analysis of expression data of all individual bands, selection of tags displaying differential expression, cluster analysis, sequencing, and BLAST analysis was performed as described (Rischer et al., 2006).
Using the complete set of 128 BstYI+1/MseI+2 primer combinations, the expression of a total of 18,800 transcript tags was monitored over time. In total, 1,771 MeJA-responsive transcript tags were isolated (hereafter referred to as “BF tags”). Direct sequencing of the reamplified BF tags gave good-quality sequences for 1217 (68.7%) of the fragments. To the remaining 554 tags (31.3%), no unique sequence could be attributed unambiguously, indicating that they might not represent unique gene tags, hence, these tags were not considered for further analysis. A BLAST search with the nucleotide sequences of the 1217 unique cDNA-AFLP® tags led to the annotation of 776 (63.7%) of the BF tags.
Average linkage hierarchical clustering analysis of the expression profiles of the 776 annotated BF tags showed that, upon MeJA treatment, the selected genes are either transcriptionally activated or transcriptionally repressed. The activated gene tags can be divided into different subclusters, based on the timing of the MeJA response. In one subcluster, genes are activated within 2 hours after the MeJA treatment, and their expression remains high thereafter. In this group, tags corresponding to genes encoding enzymes that catalyze early steps in the triterpene saponin biosynthesis, including squalene synthase (SQS) and β-amyrin synthase (bAS), can be found. These tags displayed an almost identical expression pattern, suggesting a tight co-regulation, and reached maximum levels of expression 8-24 hours post-elicitation (
Yeast strain TM7 was generated by super-transforming strain TM3 (see Example 3) with plasmids pAG423[GAL1/CYP716AO21] and pAG425[GAL1/AtATR1], expressing CYP716AO21 and the A. thaliana CytP450 reductase (CPR), AtATR1 (At4g24520), respectively, from the galactose inducible GAL1 promoter. In parallel, a control strain TM26 harboring only pAG425[GAL1/AtATR1] in TM3 was also generated. Cell pellets analyzed by GC-MS showed the presence of a unique new peak eluting at 31.8 minutes in TM7 (
Since CYP716AO21 was tentatively annotated as a homolog of the M. truncatula CYP716A12 (GenBank accession number FN995113; (Carelli et al., 2011)), the GC elution time was compared to the E1 pattern of the new peak in TM7 with a standard of erythrodiol (28-hydroxy-β-amyrin). The strain TM10 was generated by transforming plasmid pAG423[GAL1/CYP716A12] along with pAG425[GAL1/AtATR1] in TM3, and compared its GC-MS profile with TM7, TM26 and an erythrodiol standard. A peak corresponding to the elution time and EI of standard erythrodiol (
The endoplasmic reticulum (ER) localized CPRs are flavoproteins, containing both a redox cofactor flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), that serve as electron donor proteins for several ER oxygenases, including CytP450s. Therefore, optimal interaction between CPR and CytP450 is essential to allow the reducing equivalents from NADPH to pass from the CPR to the CytP450 (Reed and Backes, 2012). In an attempt to increase the efficiency of hydroxylation of β-amyrin by CYP716AO21, the effect of the ratio of CPR to CytP450 was determined by varying the expression level of AtATR1 while keeping the expression of CYP716AO21 constant.
The CPR:CytP450 ratios between 1:5 and 1:30 have been reported to be ideal for the efficient functioning of yeast and mammalian CytP450s (Reed and Backes, 2012). Therefore, the AtATR1 was expressed from either an integrated (pAG305, 1 copy per cell), low-copy number (pAG415, 3-5 copies per cell), or high-copy number (pAG425, 10-40 copies per cell) vector, in combination with CYP716AO21 always expressed from the high-copy number plasmid (pAG423, 10-40 copies per cell). Thus, two strains were generated, TM8 and TM9, overexpressing pAG423[GAL1/CYP716AO21] along with pAG305[GAL1/AtATR1] or pAG415[GAL1/AtATR1], respectively, and compared the amount of hydroxylated β-amyrin produced by these strains with that of TM7. In accordance with this assumption, strain TM9 accumulated higher levels of hydroxylated β-amyrin compared to TM8 and TM7, with the lowest accumulation in the strain expressing the integrated copy of CPR (
For the following experiments with strain TM9, condition R2 was employed to avoid the excessive secretion and, hence, loss of β-amyrin, the precursor for CYP716AO21, from the cells into the medium. Surprisingly, the hydroxylated β-amyrin eluting at 31.8 minutes was only observed in the GC chromatograms of the spent medium and not cell pellets of TM9 upon MβCD treatment (
The specificity of the type of CD used was also determined on the sequestering of hydroxylated β-amyrin from the cells into the spent medium of strain TM9. The most abundant variants of CD are α, β and γCD, which have 6, 7 and 8 glucose units, respectively. Therefore, αCD, βCD, γCD, Random MβCD (RMβCD) or MβCD were applied to a final concentration of 5 mM as in condition R2 and analyzed the spent medium on Day 4, for quantification of the amount of hydroxylated β-amyrin secreted into the medium. Sequestering was only observed with the βCD and its methylated versions and the highest amount of hydroxylated β-amyrin was detected upon RMβCD and MβCD treatment, suggesting a strong specificity of the methylated forms of CD over the unmethylated forms, for sequestering of triterpenoid sapogenins from yeast cells (
Maesa lanceolata, a member of the Myrsinaceae family, is a shrub or small tree indigenous to Africa. African traditional healers use extracts and/or parts of the plant for the treatment of a wide range of diseases including infectious hepatitis, bacillary dysentery, impetigo, ozena, dermatoses and neuropathies. Methanol extracts of M. lanceolata leaves are rich in maesasaponins and have been shown to possess virucidal, molluscicidal, fungistatic and antimutagenic activities (Sindambiwe et al., 1998). The maesasaponins identified so far are derived from an oleanane skeleton via modifications of the β-amyrin backbone, resulting in a characteristic C-13,28 hemiacetal or ester bridge and oxidations on C-16, C-21 and C-22 (
To identify new saponin biosynthetic genes, a transcript profiling was performed on methyl jasmonate (MeJA) treated M. lanceolata axenic shoot cultures. M. lanceolata axenic shoot cultures were generated and maintained as described (Faizal et al., 2011). For elicitation, each pot of shoot culture was sprayed with 2 ml deionized water containing 0.05% (v/v) TWEEN®-20 in combination with 500 μM MeJA (10 μl of 100 mM stock dissolved in ethanol) or an equivalent amount of ethanol as control. For transcript profiling, samples were collected 0, 0.5, 1, 2, 4, 8, 24 and 48 hours after elicitor or mock treatments. For each sample, three different plants were pooled.
Using the complete set of 128 BstYI+1/MseI+2 primer combinations, a genome-wide cDNA-AFLP® transcript profiling analysis (Vuylsteke et al., 2007; see also Example 8) was carried out to monitor the expression of a total of 13,558 transcript tags over time. In total, 733 MeJA-responsive transcript tags were isolated (hereafter referred to as “ML tags”). Direct sequencing of the reamplified ML tags gave good quality sequences for 545 (74.4%) of the fragments. To the remaining 188 tags (25.6%), no unique sequence could be attributed unambiguously, indicating that they might not represent unique gene tags and, hence, these were not considered for further analysis. A BLAST search with the nucleotide sequences of the 545 unique cDNA-AFLP® tags led to the annotation of 312 (57.2%) of the ML tags. Average linkage hierarchical clustering analysis of the expression profiles of the ML tags showed that, upon MeJA treatment, the genes are either transcriptionally activated or transcriptionally repressed. The activated gene tags can be divided into different subclusters, based on their MeJA response time. In one subcluster, a gene tag that reached maximum levels of expression 24-48 hours post-elicitation and corresponding to squalene epoxidase (SQE) can be found (
To characterize the putative CytP450, ML593 from our transcript profiling, strain TM21 was generated from the β-amyrin-producing strain TM3 (Table 4; see Example 3), by supertransforming with the plasmids pAG415[GAL1/AtATR1] and pAG423[GAL1/ML593]. The strains TM21 and TM27 (Table 4) were cultured in the presence of MβCD and the spent medium analyzed by GC-MS. A new peak eluting at 31.8 minutes corresponding to a hydroxylated β-amyrin in strain TM21 was observed (
Combinatorial biosynthesis, also known as combinatorial biochemistry, involves the combination of genes from different organisms in a heterologous host to produce bioactive compounds by establishing novel enzyme-substrate combinations in vivo, which, in turn, could lead to the biosynthesis of novel natural products (Pollier et al., 2011). Although CYP716AO21 is tentatively annotated as a homolog of CYP716A12, the GC elution time and E1 pattern of the β-amyrin hydroxylation product of CYP716AO21 is different from erythrodiol (
The FL-ORF of CYP716A12 was amplified with a 5′-overhang of the partial T2A sequence using the primers P24
such that there was an overlap of 7 bp between the two amplified sequences. Since the primers P23 and P24 contain an Uracil each, the CYP716AO21 and CYP716A12 were PCR amplified using the Pfu Turbo Cx polymerase (Stratagene). The purified gel fragments were used for Uracil-Specific Excision or USER™ Cloning (New England Biolabs) to generate two fragments with complementary sticky ends that were ligated in vitro using the T4 DNA ligase (Invitrogen). The ligated DNA product was once again gel purified and used as template for amplification with the primers P19+P22. This amplicon was GATEWAY™ recombined into pDONR221, sequence verified and further recombined into pAG423GAL-ccdB to generate pAG423[GAL1/CYP716AO21-T2A-CYP716A 12].
The spent medium of strain TM30 cultured in the presence of MβCD was analyzed by GC-MS and compared to the GC chromatograms of spent medium from strains TM9, and TM17 overexpressing pAG423[GAL1/CYP716A12] and pAG415[GAL1/AtATR1] in TM3. A unique peak was observed at 40.5 minutes in strain TM30 (
In conclusion, using a combinatorial approach in S. cerevisiae, it was revealed that CYP716AO21 is not a C-28 oxygenase but a novel cytochrome P450 that encodes a β-amyrin 16β/21β/29α-hydroxylase involved in the synthesis of saikosaponins in Bupleurum.
First, to confirm the identity of ML593 as a functional homolog of CYP716AO21, strain TM31 was generated, similar to strain TM30 (Table 4) (see Example 14). Strain TM31 was created by supertransforming TM3 with the plasmids pAG415[GAL1/AtATR1] and pAG423[GAL1/ML593-T2A-CYP716A12], where ML593 and CYP716A12 form a self-processing polyprotein stitched together by the 2A oligopeptide (de Felipe et al., 2006) and is expressed from a single GAL1 promoter. The strains TM31, TM17 and TM21 (Table 4) were cultured in the presence of MβCD and the spent medium was analyzed using GC-MS. Similar to strain TM30 (
To generate a combinatorial scaffold using β-amyrin, ML593 (see Examples 12 and 13) and CYP88D6 were expressed in strain TM3. The CytP450 CYP88D6 (GenBank accession number AB433179; (Seki et al., 2008)) catalyzes a two-step oxidation of β-amyrin to 11-oxo-β-amyrin through an 11-hydroxy-β-amyrin intermediate. First, the activity of CYP88D6 was confirmed in the β-amyrin-producing strain by generating strain TM18 (Table 4) and culturing it in the presence of MβCD along with the control strain TM27. Therefore, the full-length open reading frame (FL-ORF) of CYP88D6 was PCR amplified using the primers P45 (GGGGACAAGTTTGTACAAAAAAGCAGGCTTAATGGAAGTACATTGGGTTTG (SEQ ID NO:16))+P46 (GGGGACCACTTTGTACAAGAAAGCTGGGTACTAAGCACATG AAACCTTTA (SEQ ID NO:17)) and cloned into pDONR221. The CytP450 CYP88D6 was then GATEWAY™ recombined into the high-copy number expression vector pAG423GAL-ccdB (Addgene plasmid 14149). GC chromatograms of the spent medium of strain TM18 showed the presence of four unique peaks (
Further, the strain TM32 was generated by supertransforming strain TM3 with the plasmids pAG415[GAL1/AtATR1] and pAG423[GAL1/ML593-T2A-CYP88D6], where ML593 and CYP88D6 are stitched together with the 2A oligopeptide, resulting in the generation of a self-processing polypeptide (de Felipe et al., 2006). The self-processing polyprotein of ML593 and CYP88D6 was generated by amplifying the FL-ORF of ML593 without a stop codon and having a 3′-overhang of the partial T2A sequence using the primer pair P39
The FL-ORF of CYP88D6 was amplified with a 5′-overhang of the partial T2A sequence using the primers P48
such that there was an overlap of 7 bp between the two amplified sequences. Since the primers P48 and P47 contain a Uracil each, the ML593 and CYP88D6 were PCR amplified using the Pfu Turbo Cx polymerase (Stratagene). The purified gel fragments were used for USER™ Cloning (New England Biolabs) to generate two fragments with complementary sticky ends, which were ligated in vitro using the T4 DNA ligase (Invitrogen). The ligated DNA product was used as template for amplification with the primers P39+P46. This amplicon was GATEWAY™ recombined into pDONR221, sequence verified and further recombined into pAG423GAL-ccdB to generate pAG423[GAL1/ML593-T2A-CYP88D6]. The spent medium of strain TM32 (
In an attempt to generate a dammarenediol-producing yeast strain, the plasmid pESC-URA[GAL10/tHMG1; GAL1/CaDDS] was expressed, harboring a dammarenediol synthase gene (CaDDS) from Centella asiatica (GenBank accession number AY520818; (Kim et al., 2009)), in the sterol-modified yeast strain TM1, to generate strain TM33. The CaDDS was amplified with XhoI and NheI containing primers P43 (GGGGACAAGTTTGTACAAAAAAGCAGG CTTActcgagATGTGGAAGCTGAAGATAGCA (SEQ ID NO:14))+P44 (GGGGACCACTTTGTACAAGAAAGCTGGGTTgctagcTCAATTGGAGAGCCACAAGCG (SEQ ID NO:15)) to generate pESC-URA[GAL10/tHMG1; GAL1/CaDDS].
Strain TM33 and the control strain TM5 was cultured in medium containing MβCD and then analyzed the GC chromatograms obtained from the spent medium of both the strains. Unexpectedly, three new peaks were found in strain TM33 eluting at 27.2 minutes, 28.6 minutes, and 33.5 minutes (
Strain TM37 was generated from strain TM33 by supertransforming with the plasmids pAG415[GAL1/AtATR1] and pAG423[GAL1/ML593] (see Table 4). The ML593 was characterized as a putative C-21 hydroxylase of β-amyrin (see Examples 12 and 13) and the substrate specificity of this CytP450 was determined by expressing it together with the multifunctional cyclase CaDDS. Strain TM37 and the control strain TM38 were cultured in the presence of MβCD and compared GC chromatograms for the presence of unique peaks. Two peaks were identified eluting at 31.8 minutes and 33.2 minutes in the spent medium of TM37 (
The sequences in lower case represent the restriction recognition site used for restriction enzyme-mediated cloning. The underlined sequence corresponds to T2A partial sequences.
Kushiro T., M. Shibuya, and Y. Ebizuka (1998). Beta-amyrin synthase—cloning of oxidosqualene cyclase that catalyzes the formation of the most popular triterpene among higher plants. Eur. J. Biochem. 1998 Aug. 15; 256(1):238-44.
This application is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/EP2013/059821, filed May 13, 2013, designating the United States of America and published in English as International Patent Publication WO 2013/167751 A1 on Nov. 14, 2013, which claims the benefit under Article 8 of the Patent Cooperation Treaty and under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/732,817, filed Dec. 3, 2012 and to U.S. Provisional Patent Application Ser. No. 61/645,998, filed May 11, 2012.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/059821 | 5/13/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61645998 | May 2012 | US | |
| 61732817 | Dec 2012 | US |
