MXPA98001646A - Cellular populations inactive for transfer celu - Google Patents

Abstract

The present invention relates to a method for reconstructing an animal embryo involving transferring the nucleus of an inactive donor cell to a suitable recipient cell. The donor cell is inactive, since it leaves the growth and division cycle in G1 and stops in the G0 state. Nuclear transfer can occur through the fusion of the cell. The reconstituted embryo then gives rise to one or more animals. The invention is useful in the production of transgenic animals, as well as non-transgenic animals, of high genetic merit.

Description

INACTIVELY CELLULAR POPULATIONS FOR CELLULAR TRANSFER DESCRIPTION OF THE INVENTION This invention relates to the generation of animals, including, but not limited to, genetically selected and / or modified animals. The reconstruction of mammalian embryos through the transfer of a nucleus from a donor embryo to an enucleated oocyte or a cell zygote allows the production of genetically identical individuals (ie, as biological controls) and also in commercial applications (ie , multiplication of genetically valuable livestock, uniformity of meat products, animal handling). One problem with the use of early embryos as nuclear donors is that the number of progeny, which can be produced from an individual embryo, is limited by both the number of cells (embryos in cell stage 32-64 are the most widely used in farm animal species) and the efficiency of the nuclear transfer protocol. In contrast to the use of embryos as nuclear donors, the ability to produce living progeny through nuclear transfer from cells that can be kept in culture is an object that has been sought for some time by animal breeders. The ability to produce the progeny cloned from a cultured cell line could offer a large number of advantages over the use of early embryos. These include: the production of large numbers of identical offspring over a long period (cultured cells can be frozen and stored) and the ability to modify and / or genetically select cell populations of the required genotype (eg, sex) before of the reconstruction of the embryo. One type of potential cell to be used in these procedures is the embryonic stem (ES) cell. ES cells have been isolated in the mouse, however, although there are no reports of development after use in nuclear transfer. Currently, there is only one report of ES as cells in pigs, which have contributed to the development after injected into the blastocoel cavity of blastocysts produced in vivo (Wheeler, Reprod. Fertile, Dev. 6 563-568 (1994) ), but there are no reports of chimerism in other farm livestock species and there are no reports of development to term after nuclear transfer in any mammalian species of any established cell line. There are several alternatives for using the ES cell lines; one of these is to look for other cell populations, which are capable of promoting development when they are used for nuclear transfer. Several reports have been suggested that Primordial Germ Cells offer an adequate candidate; however, no development to thermal has yet been reported. Established cell lines of early embryos have been suggested; although development has been reported from early passage cells in sheep (Campbell et al., Therio 43 181 (1995)) in prolonged culture, no development was obtained using conventional nuclear transfer protocols (Campbell et al., J. Abstract Series (5) 31 (1995)). In order to obtain full-term development after the nuclear transfer, the development clock of the transferred core must be readjusted. For this to happen, the transcription must be stopped and then restarted in a highly regulated pattern. Previous reports have shown that the development of the blastocyst stage can be obtained from a wide variety of cell types in the cow, sheep, pig, rabbit and mouse. However, in all these reports, no term development has been reported. The birth of live lambs after nuclear transfer from primary cell lines (up to and including passage 3) has been previously reported, which were established from embryonic disk (ED) of 9 sheep embryos (Campbell et al. al., Therio 43 181 (1995)). However, in subsequent cultures, no forward development has been obtained, using conventional nuclear transfer protocols (in passage 6 and 11) (Cam pbell et al., J. Reprod. Fertil., Abstract Series (5 ) 31 (1995)). These results can be interpreted in a number of ways; First, it can be postulated that all cells derived from ED obtained during early periods of culture are capable of promoting development. However, in a culture prolonged during the establishment of a cultured cell line, these cells change and thus are not able to control the development when they are used as nuclear donors for nuclear transfer in the "Universal Receptor" denominated in the documents shown Alternatively, it can be postulated that during the early culture period, a sub-population of cells retains the ability to promote development and that this could explain the production of live offspring after nuclear transfer during these early stages. Previous studies have emphasized the role of cell cycle coordination of the donor nucleus and recipient cytoplasm in the development of reconstructed embryos through nuclear transfer (Campbell et al., Biol. Reprod. 49 933-942 (1993) and Biol. Reprod. 50 1385-1393 (1994)). Two possible alternative strategies for that that depend on the isolation of a cell line, which is totipotent for nuclear transfer using published nuclear transfer protocols, are: (1) modify existing transfer procedures; or (2) modify the chromatin of the donor cell prior to nuclear transfer. A totipotent cell can direct the development of a whole animal (when embryos are constructed through nuclear transfer from a donor cell to a recipient cell, such as an enucleated oocyte, it is the nucleus of the donor cell which is totipotent) . This includes directing the development of extra-embryonic lineages, that is, the placenta. In this definition, a fertilized zygote and in some individual blastomere species are also totipotent. In contradiction, a pluripotent or multipotent cell type (ie, an embryonic stem cell) has been defined as one that can form all tissues in the conceptus / progeny after injection into the blastocoel cavity. In both strategies (1) and (2) of nuclear transfer observed above, a method is required, which will allow the reprogramming of the expression of the transferred core gene: this method could then allow the use of differentiated or partially differentiated cells as nuclear donors and could "emit" their inherent totipotency. It has now been found that inactive cells, i.e. cells that are not actively proliferating through the cell cycle, can advantageously be used as nuclear donors in the reconstitution of an animal embryo. These embryos are then allowed to develop at term. It seems that changes in the donor nucleus, which are observed after reconstitution of the embryo and which are required for efficient nuclear transfer, can be induced in cell nuclei before being used as nuclear donors, causing them to enter to the inactive state. This reality has been exploited in the present application. According to a first aspect of the present invention, a method for reconstituting an animal embryo is provided, the method comprising transferring the nucleus of an inactive donor cell to a suitable recipient cell. In principle, the invention is applicable to all animals, including birds, such as poultry, amphibian species and aquatic species (fish). In practice, however, they will be non-human animals, especially mammals (not humans), particularly placental mammals, where the greatest commercially useful applicability will be observed. This is with ungulates, particularly economically important ungulates such as cattle, sheep, goats, buffalo from India, camels and pigs, where the invention is probably very useful, both as means to clone animals and as means to generate transgenic animals or genetically modified. It should also be noted that the invention is probably also applicable to other economically important animal species such as, for example, horses, llamas or rodents, for example, rats or mice, or rabbits. The invention is equally applicable in the production of transgenic animals, as well as non-transgenic animals. Transgenic animals can be produced from genetically altered donor cells. The whole procedure has a number of advantages over conventional procedures for the production of transgenic (ie, genetically modified) animals, which can be summarized as follows: (1) very few receptors will be required; (2) multiple syngeneic founders can be generated using clonal donor cells; (3) Subtle genetic alteration through the target gene is allowed; (4) all animals produced from embryos prepared by means of the invention must transmit the relevant genetic modification through the germline since each animal is derived from an individual nucleus; in contrast, the production of transgenic animals through pronuclear injection or chimerism after the inclusion of stem cell populations modified by blastocyst injection, or other procedures, produces a proportion of mosaic animals, in which all cells do not contain the modification and the resulting animal can not transmit the modification through the germ line; and (5) the cells can be selected for the site of genetic modification (eg, integration) prior to generation of the whole animal. It should be noted that the term "transgenic", in relation to animals, should not be taken as limiting with reference to animals that contain in their germline one or more genes of another species, although many transgenic animals will contain said gene or genes. Rather, the term refers more broadly to any animal whose germline has been the subject of technical intervention by recombinant DNA technology. Thus, for example, an animal in whose germline an endogenous gene has been deleted, duplicated, activated or modified, is a transgenic animal for the purposes of this invention, as well as an animal for whose germline a DNA sequence has been added. exogenous In the embodiments of the invention, where the animal is transgenic, the donor nucleus is genetically modified. The donor nucleus may contain one or more transgenes and the genetic modification may occur before nuclear transfer and reconstitution of the embryo. Although microinjection, analogous to injection to the pronucleus of the male or female of a zygote, can be used as a method of genetic modification, the invention is not limited to that methodology; Transformation and transfection techniques can also be used, for example, electroporation, transfection or viral lipofection. In the method of the invention described above, a core is transferred from an inactive donor cell to a recipient cell. The use of this method is not restricted to a particular type of donor cell. All normal karyotype cells, including embryonic, fetal, and adult somatic cells, which can be induced to enter inactivity or exit into an inactive state in vivo, can prove to be totipotent using this technology. The invention, therefore, contemplates the use of at least one partially differentiated cell, including a fully differentiated cell. The donor cells may be, but do not have to be, in culture. Below, cultured bovine primary fibroblasts, an embryo derived ovine cell line (TNT4), a sheep mammary epithelial cell (OME) -derived cell line from a 6-year-old adult sheep, a cell line from sheep, are illustrated. fibroblast derived from fetal sheep tissue (BLWF1) and an epithelial-type cell line derived from a 9-day-old adult sheep embryo (SEC1). A class of embryo derived cell lines useful in the invention, which include the cell line TNT4 are also the subject of co-pending PCT Patent Application No. PCT / GB95 / 02095, published as WO96 / 07732. To be useful in the invention, the donor cells are inactive, ie they are not actively proliferating through the mitotic cell cycle. The mitotic cell cycle has four different phases, G1, S, G2 and M. The beginning in the cell cycle, called start, occurs in the G1 phase and has a unique function. The decision or presentation to experience another cell cycle is made at the beginning. Once a cell has passed through start, it goes through the rest of the G1 phase, which is the pre-DNA synthesis phase. The second stage, the S phase, is when DNA synthesis is presented. This is followed by the G2 phase, which is the period between DNA synthesis and mitosis. The same mitosis occurs in the M phase. Generally, inactive cells (which include cells in which inactivity has been induced, as well as those cells which are naturally inactive, such as certain fully differentiated cells) are considered not to belong to none of these four phases of the cycle; they are usually described as being in the GO state, to indicate that they normally would not be able to progress through the cycle. The nuclei of inactive GO cells have a diploid DNA content.

Cultured cells can be induced to enter the inactive state through various methods, including chemical treatments, nutrient deprivation, growth inhibition or manipulation of gene expression. Currently, the reduction of serum levels in the culture medium has been successfully used to induce inactivity in cell lines of both sheep and cattle. In this situation, the cells exit the growth cycle during the G1 phase and stop, as explained above, in the so-called GO stage. These cells can remain in this state for several days (possibly longer depending on the cell) until they are re-stimulated when they re-enter the growth cycle. The inactive cells arrested in the GO being are diploid. The GO state is the point in the cell cycle from which the cells are able to differentiate. In inactivity, a number of metabolic changes have been reported and these include: monophosphorylated histones, ciliated centrioles, complete reduction or cessation throughout protein synthesis, increased proteolysis, reduced transcription and increased RNA turnover resulting in a reduction in the total RNA of the cell, disaggregation of polyribosomes, accumulation of inactive 80S ribosomes and chromatin condensation (reviewed by Whitfield et al., Conrol of Animal Cell Proliferation, 1 331-365 (1985)). Many of those aspects are those, which are required to occur after the transfer of a nucleus to an enucleated oocyte. The fact that the GO state is associated with cell differentiation suggests that this may provide a nuclear / chromatin structure, which is more susceptible to either remodeling and / or reprogramming by the cytoplasm of the recipient cell. In this way, by virtue of the nuclear donor cells being in the inactive state, the chromatin of the donor nuclei can be modified before the reconstitution or reconstruction of the embryo, so that the nuclei are capable of directing the development. This differs from all previously reported methods of nuclear transfer, in that the donor cell chromatin is modified before using the cells as nuclear donors. The recipient cell to which the donor cell nucleus is transferred may be an oocyte or other suitable cell. You can use recipient cells in a variety of different stages of development, from oocytes in metaphase I to metaphase I I, to zygotes and two-cell embryos. Each has its advantages and its disadvantages. The use of fertilized eggs ensures efficient activation, while parthenogenetic activation is required with oocytes (see below). Another mechanism that may favor the use of embryos of cleavage stage in some species is the degree to which the reprogramming of gene expression is required. Transcription is initiated during the second cell cycle in the mouse and no major changes in the nature of the proteins that are being synthesized were revealed, through bi-dimensional electrophoresis until the blastocyst stage (Howlett and Bolton, J. Embryol. Morphol 87, 175-206 (1985)). Although in many cases, the receptor cells will be oocytes. It is preferred that the receiver be enucleated. Since it has generally been assumed that enucleation of receptor oocytes in nuclear transfer procedures is essential, there is no published experimental confirmation of this judgment. The original procedure described for ungulates involved the division of the cell into two halves, one of which was probably enucleated (Willadsen Nature 320 (6) 63-65 (1986)). This procedure has the disadvantage that the unknown half will continue to have the metaphase apparatus and that the reduction in cytoplasm volume is thought to accelerate the differentiation pattern of the new embryo (Eviskov et al., Development 1 09 322-328 (1990)) . More recently, different procedures have been used with the attempt to remove the chromosomes with a minimum of cytoplasm. It was found that aspiration of the first polar body and surrounding cytoplasm removes the metaphase I I apparatus in 67% of sheep oocytes (Smith &; Wilmut Bio. Reprod. 40 1027-1035 (1989)). Only with the use of specific fluorochrome for DNA (Hoechst 33342) was a method provided by which enucleation could be guaranteed with the minimum reduction in cytoplasmic volume (Tsunoda et al., J. Reprod. Fertil. 82 173 (1988 )). In livestock species, this is probably the routine method of use at present (Prather &First J. Reprod. Fertile, Sppl 41 125 (1990), Westhusin et al., Biol. Reprod. (Suppl.) 42 176 (1990)). There have been very few reports of non-aggressive aspects for enucleation in mammals, whereas in amphibians, irradiation with ultraviolet light is used as a routine procedure (Gurdon Q. J. Microsc. Soc. 101 299-31 1 (1960)). There are no detailed reports regarding the use of this aspect in mammals, although during the use of DNA-specific fluorescein, it was observed that exposure of mouse oocytes to ultraviolet light for more than 30 seconds reduced the development potential of the cell (Tsunoda et al., J. Reprod. Fertile, 82 173 (1988)). It is preferred that recipient host cells, to which the donor cell nucleus is transferred, are an enucleated metaphase II oocyte, an inactivated oocyte enucleated by an enucleated preactivated oocyte. At least where the receptor is an enucleated metaphase II oocyte, activation may occur at the time of transfer. Alternatively, at least where the receptor is an enucleated inactivated metaphase I I oocyte, activation may occur subsequently. As described above, enucleation can be achieved physically, through actual removal of the nucleus, pro-nucleus or metaphase plate (depending on the recipient cell), or functionally, such as through the application of ultraviolet radiation or another influence of enucleation. Three suitable receptor cytoplasts (enucleated oocyte) are: 1 . The "MAG IC Receiver" (Metaphase Arrested G1 / G0 Acceptl ng Cytoplast) described in our co-pending PCT patent application No. PCT / GB96 / 02098, filed today (priority claimed from GB 9517779.6). 2. The "GOAT" (G0 / G 1 Activation and Transfer) - a Mi l oocyte (metaphase II) at the moment of activation (Campbell et al., Biol Reprod 49 933-942 (1993). Universal receptor "(Campbell et al., Biol. Reprod. 649 933- 942 (1993), Biol. Reprod. 50 1385-1393 (1994). All these three receptors could result in normal ploid, when nuclei are used. donors in GO in the reconstructed embryo, however, recent reports have suggested that a proportion of inactive cell nuclei are not able to enter the synthetic phase of DNA when they are placed in a S phase cytoplasm without undergoing separation from the nucleus. nuclear envelope (log &Munshi, J. Cell Biol. 127 (1) 5-14 (1994)). Therefore, although a proportion of embryos will be developed when the "Universal Receptor" is used, it is postulated that the use of of Mil oocytes containing high levels of MPF (promoter factor of the M phase or maturation promoter factor) as cytoplast receptors or either through method 1 or 2, will result in a higher frequency of development. Once the appropriate donor and recipient cells have been detected, it is necessary that the nucleus of the previous ones be transferred to the latter. More conveniently, the nuclear transfer is effected by fusion. Three established methods, which have been used to induce fusion are: (1) exposing the cells to chemical fusion promoting compounds, such as polyethylene glycol; (2) use inactivated viruses, such as Sendai virus; and (3) use electrical stimulation. Exposure of cells to fusion-promoting chemical compounds, such as polyethylene glycol or other glycols, is a routine procedure for somatic cell fusion, but has not been widely used with embryos. Since polyethylene glycol is toxic, it is necessary to expose the cells for a minimum period and the need to be able to remove the chemical rapidly, may require removal of the zona pellucida (Kanka et al., Mol. Reprod. Dev. 29 1 10-1 16 (1991)). In experiments with mouse embryos, the inactivated Sendai virus provides efficient means for the fusion of cells from cleavage stage embryos (Graham Wistar Inst. Symp. Monogr. 19 19 (1969)), with the additional experimental advantage of that the activation is not induced. In ungulates, fusion is commonly achieved through the same electrical stimulation that is used to induce partogenetic activation (Willadsen Nature 320 (6) 63-65 (1986), Prather et al., Biol. Reprod. 37 859-866 (1987)). In these species, the Sendai virus induces fusion in a proportion of cases, but is not reliable enough for routine application (Willadsen Nature 320 (6) 63-65 (1986)). Since cell-cell fusion is a preferred method of effecting nuclear transfer, it is not only the method that can be used. Other suitable techniques include microinjection (Ritchie and Campbell, J. Reproduction and Fertility, Abstract Series No. 15, p60). Before or (preferably) after nuclear transfer (or, in some cases at least, concomitantly with it), it is generally necessary to stimulate the recipient cell to develop through parthenogenetic activation, at least if the cell is an oocyte. Recent experiments have shown that the requirements for partogenetic activation are more complicated than one would have imagined. It has been assumed that the activation is a complete phenomenon or without anything and that a large number of treatments capable of inducing the formation of a pronucleus all cause "activation". However, exposure of rabbit oocytes to repeated electrical pulses revealed that only the selection of an appropriate series of pulses and control of Ca2 + was able to promote the development of diploidized oocytes to middle gestation (Ozil Development 109 117-127 (1990 )). During fertilization, there are repeated, transient increases in intracellular calcium concentration (Cutbertson & amp;; Cobbold Nature 316 541-542 (1985)) and it is believed that electrical pulses cause analogous increases in calcium concentration. There is evidence that the calcium passenger pattern varies with the species, and that it can be anticipated that the optimal pattern of electrical pulses will vary in a similar way. The interval between the pulses in the rabbit is approximately 4 minutes (Ozil Development 109 117-127 (1990)), and in the mouse 10 to 20 minutes (Cutbertson &Cobbold Nature 316 541-542 (1985)), while that there are preliminary observations in the cow that the interval is approximately 20 to 30 minutes (Robl et al., in Symposium on Cloning Mammals by Nuclear Transplantation (Seidel ed.), Colorado State University, 24-27 (1992)). In most published experiments, activation was induced with an individual electrical pulse, but new observations suggest that the proportion of reconstituted embryos that develop increases through exposure to several pulses (Collas &Robl Biol. Reprod. 43 877-884 (1990)). In any case, routine adjustments can be made to optimize the number of pulses, the field resistance and the duration of the pulses and the calcium concentration of the medium. According to a second aspect of the present invention, a reconstituted animal embryo prepared by a method as previously described is provided. According to a third aspect of the present invention, there is provided a method for preparing an animal, comprising: (a) reconstituting an animal embryo as described above; and (b) having an animal develop from the embryo at term; and (c) optionally, spawning of the animal thus formed. Step (a) has been described in depth previously. The second step, step (b) in the method of this aspect of the invention is to make an animal develop at term from the embryo. This can be done directly or indirectly. In direct development, the reconstituted embryo from step (a) is simply allowed to develop without further intervention beyond whatever is necessary to allow development to occur. In indirect development, however, the embryo can be further manipulated before development is fully present. For example, the embryo can be divided and the cells clonaintly expanded, for the purpose of improving performance.

Alternatively or additionally, it may be possible to obtain increased yields of viable embryos through the present invention by clonal expansion of donors and / or using the serial (nuclear) transfer method. A limitation in the speed of blastocyst formation currently achieved is due to the fact that the vast majority of embryos do not "reprogram" (although an acceptable number does so). If this is the case, then the speed can be improved as follows. Each embryo that develops on its own can be used as a nuclear donor, such as, for example, in the morula or cell stage 32-64; alternatively, cells of internal cell mass can be used in the blastocyst stage. Embryos derived from these subsequent transfers can also be used as potential nuclear donors to further improve efficiency. If these embryos actually reflect those which have reprogrammed gene expression and those nuclei are actually reprogrammed (as seems most likely), then each developing embryo must be multiplied in this way through the efficiency of the transfer procedure nuclear. The degree of improvement likely to be achieved depends on the type of cell. In sheep, it is easily possible to obtain 55% embryos in the blastocyst stage by transferring an individual blastomere from a 16-cell embryo to a preactivated "Universal Receptor" oocyte. So it is reasonable to hypothesize that each embryo developed from an individual cell can give rise to eight in the 16-cell stage. Although these figures are only an approximate guide, it is clear that in the later stages of development, the degree of benefit could depend on the efficiency of the procedure at that stage. It is also contemplated that a new cell line acts as a source of nuclear donor cells, can be produced from embryos formed according to the preceding description or the resulting fetuses or adults. In certain cases, where there may be restrictions on the development of an embryo reconstituted in term, it may be preferable to generate a chimeric animal formed from cells derived from a naturally formed embryo and an embryo reconstructed through nuclear transfer. Said chimera can be formed by taking a proportion of cells from the natural embryo and a proportion of the cells of the reconstructed embryo at any stage up to the blastocyst stage and forming a new embryo by aggregation or injection. The proportion of cells can be in the ratio of 50: 50 or another suitable ratio to achieve the formation of an embryo, which develops at term. The presence of normal cells under these circumstances is believed to help rescue the reconstituted embryo and allow successful development at term and a live birth. In addition to the issuance of conveniences to improve performance, the reconstituted embryo can be cultured, in vivo or in vitro to the blastocyst.

Experience suggests that embryos derived by nuclear transfer are different from normal embryos and sometimes benefit from or even require in vivo culture conditions different from those in which embryos are usually cultured (at least in vivo). The reason for this is not known. In the multiplication routine bovine embryos were cultured reconstituted embryos (many of them once) in oviducts of sheep for 5 to 6 days (as described by Willadsen, In Mammalian Egg Transfer (Adams, EE, ed.) 185 CRC Press, Boca Raton, Florida (1982)). In the practice of the present invention, however, in order to protect the embryo, it should preferably be imbibed in a protective medium such as agar before transfer and then dissected from the agar after recovery from the temporary receptor. The function of protective agar or other media is twofold: first, it acts as a structural auxiliary for the embryo by keeping the zona pellucida together; and secondly, it acts as a barrier to cells of the immune system of the recipient animal. Although this aspect increases the proportion of embryos that form blastocysts, there is a disadvantage that a number of embryos may be lost. If in vitro conditions are used, those routinely employed in the art are fully acceptable. In the blastocyst stage, the embryo can be classified to be suitable for development at term. Typically, this will be done when the embryo is transgenic and the classification and selection of stable members have been made. The classification of non-transgenic genetic markers can also be carried out at this stage. However, since the method of the invention allows the classification of donors at an early stage, which will generally be preferred. After classification, if the classification is performed, the blastocyst embryo is allowed to develop at term. This will generally be in vivo. If the development to blastocyst occurs in vitro, then the transfer to the final recipient animal occurs at this stage. If the blastocyst development has been presented in vivo, although in principle the blastocyst is allowed to develop at term in the pre-blastocyst host, in practice the blastocyst will usually be removed from the pre-blastocyst (temporal) receptor and, after the dissection of the protective medium will be transferred to the pre-blastocyst (permanent) receptor. In the optional step (c) of this aspect of the invention, animals can be bred from the an imal prepared by the preceding steps. In this way, an animal can be used to establish a herd or herd of animals having the desired genetic characteristics. Animals produced through the transfer of nuclei from a source of genetically identical cells share the same nucleus, but they are not strictly identical since they are derived from different oocytes. The importance of this different origin is not clear, but it can affect commercial aspects. Recent analyzes of mitochondrial DNA from dairy cattle, at State University Breeding Hard, revealed association with milk yield and reproduction (Freeman &Beitz, in Symposium on Cloning Mammals by Nuclear Transplantation (Seidel, GE Jr., ed. ) 17-20, Colorado State University, Colorado (1992)). It is confirmed that similar effects are present throughout the livestock population and consider whether it is possible or necessary in specific situations to consider the selection of oocytes. In the area of livestock breeding, the ability to produce large numbers of embryos from high genetic merit donors can have considerable potential value in disseminating genetic improvement through the national herd. The scale of application will depend on the cost of each embryo and the proportion of transferred embryos capable of developing at term. By way of illustration and summary, the following scheme establishes a typical procedure by which transgenic and non-transgenic animals can be prepared. The procedure can be considered to involve seven steps: (1) selection and isolation of suitable donor cells, which may include karyotype determination, induction of inactivity (stop in GO) and / or induction of development; (2) application of molecular biology techniques for the production of genetically modified cell populations. Such techniques may include gene additions, gene detonators, gene introduction, and other gene modifications. Optionally, transgenesis can also be employed by transfection with suitable constructs, with or without selectable markers; (3) optionally classifying and selecting modified cell populations or clones for the required genotype / phenotype (ie, stable integrants); (4) induction of inactivity in the modified cell population; (5) reconstitution of the embryo through nuclear transfer; (6) culture, in vivo or in vitro, to the blastocyst; (6a) optionally classify and select for stable members - omit if it is done in (3) - or other characteristics; (7) transfer, if necessary, to the final recipient. According to a fourth aspect of the invention, an animal prepared as described above is provided. Preferred features for each aspect of the invention are as for each aspect to each other, mutatis mutandis. The present invention will now be described with reference to the appended Examples, which are provided for purposes of illustration and are not construed as limiting the present invention.

EXAMPLES EXAMPLE 1 Induction of Inactivity in Donor Cells Several methods have been shown to induce inactivity in cultured cell lines, including: contact inhibition or serum starvation (reviewed by Whitfield et al., Control of Animal Cell Proliferation, 1 331-365 (1985) .The method of induction of inactivity. it is not believed to be of importance, the important step is that the cells exit the growth cycle, stop in a GO state with a diploid DNA content and remain viable In Examples 3 and 4, the starvation of fibroblast serum Primary bovine cells, a bovine cell line established from the internal cell mass of day 7 in vivo produced blastocysts, and an ovule-derived embryonic cell line (TNT4), was used to induce inactivity and arrest cells in the GO phase of the cell cycle The starvation of serum was similarly used to induce the inactivity of donor cells described in Example 5.

EXAMPLE 2 Isolation of Oocytes and Nuclear Transfer Oocytes can be obtained through (i) in vitro maturation of slaughterhouse material, or transvaginal follicle puncture; (ii) in vivo maturation and surgical recovery; or (iii) any other suitable procedure. All oocytes matured in vivo should be harvested by washing from the oviduct in saline regulated at their pH with calcium-magnesium-free phosphate (PBS) containing 1.0% fetal calf serum (FCS). In vitro matured oocytes are harvested and transferred to calcium free M2 (Wittingham and Wales Aust. J. Biol. Sci. 22 1065-1068 (1969)) containing 1.0% FCS. The oocytes are stripped of cumulus and enucleated cells as previously described (Campbell et al., Biol. Reprod. 49 933-942 (1993) and Biol. Reprod. 50 1385-1393 (1994)) with the exception that the medium Calcium free is used for all procedures. The fusion procedures are modifications of those previously reported (Campbell et al., 1993, 1994 loe cit) and are as described in the relevant section below, alternatively the nucleus can be introduced by injecting the donor cell into the enucleated oocyte (Ritchie & amp; Campbell, J. Reprod. Fertile, Abstract Series (5) 60 (1995)). The time of these cases depends on the species, the following two protocols emphasize the use of oocytes of ovine matured in vivo and of bovine matured in vitro.

EXAMPLE 3 Sheep Nuclear Transfer 3. 1 Superimulation of donor female sheep and oocyte retrieval Scottish "Blackface" female sheep were synchronized with progestogen sponges for 14 days (Veramix ™, Upjohn, UK) and were induced to superovulate with individual injections of 3.0 mg / day (total 6.0 mg ) of ovine follicle stimulating hormone (FSH) (Ivagen ™, Immuno-chemical Products Ltd. N ew Zealand) on two successive days. Ovulation was induced with an individual dose of 8 mg of a gonadotropin-releasing hormone analogue (GnRH Recepta! ™, Hoechst, U K) 24 hours after the second injection of FSH. Unfertilized metaphase II oocytes were recovered by flushing the oviduct 24-29 hours after the GnRH injection, using pH regulated Saline Dubelcco, containing 1.0% fetal calf serum (FCS) maintained at 37 ° C until use. 3. 2 Oocyte manipulation The recovered oocytes were maintained at 37 ° C, washed in PBS containing 1.0% FCS and transferred to a calcium free M2 medium containing 10% Fetal Calf Serum (FCS) at 37 ° C.

To remove the chromosomes, (enucleation) the oocytes were placed in calcium free M2 containing 10% FCS, 7.5 μg / ml cytochalasin B (Sigma) and 5.0 μg / ml Hoechst 33342 (Sigma) at 37 ° C for 20 days. minutes A small amount of cytoplasm directly behind the first polar body was then aspirated using a 20 μM glass pipette. Enucleation was confirmed by exposing the aspirated portion of the cytoplasm to UV light and verifying the presence of a metaphase plate. 3. 3 Embryo Reconstruction Groups of 10-20 oocytes were enucleated and placed in 20 μl drops of M "medium free of calcium at 37 ° C and 5% CO2 under mineral oil (SIGMA), each of the following three protocols ( a), (b) and (c) were used for the reconstruction of the embryo. (a) "MAGIC" (Acceptance Cytoplasm G1 / G0 Stopped in the Metaphase) The sooner after the enucleation, a single cell was placed in contact with the oocyte using a glass pipette to transfer the cell through the previously made hole in the zona pellucida. The cytoplast / cell pair was then transferred to the fusion chamber in 200 μl of 0.3M mannitol in distilled water and manually aligned between the electrodes. A pulse of AC (alternating current) of 5V was applied for 3 seconds followed by 3 pulses of DC (direct current) of 1.25kV / cm during 80 μsegs. The pairs were then washed in calcium free M2, 10% FCS at 37 ° C and incubated in the same medium under oil at 37 ° C and 5% CO2. Thirty minutes before activation, the pairs were transferred to calcium free M2 medium, 10% FCS containing 5 μM nocodazole. Activation was induced at 32-34 hours after the hCG injection as described above. After activation, the reconstructed zygotes were incubated in the TC199 medium (Gibco), 10% FCS at 37 ° C and 5% CO2 for 3 more hours. They were then washed 3 times for 5 minutes at 37 ° C in the same medium without nocodazole and cultured for 12-15 more hours before being transferred to female temporary recipient sheep. (b) "GOAT" (Activation and Transfer of G0 / G 1) At 32-34 hours after the hCG injection, a single cell was contacted with the enucleated oocyte. The pair was transferred to the fusion chamber (see below) in 200 μl of 0.3M mannitol, 0.1 mM MgSO4, O.OO l mM CaCl2 in distilled water. Fusion and activation were induced by applying a 3V AC pulse for 5 seconds followed by 3 DC pulses of 1.25kV / cm for 80 μsegs. The pairs were then washed in TC199, 10% FCS containing 7.5 μg / ml cytochalasin B and incubated in this medium for 1 hour at 37 ° C and 5% CO2-After the pairs were washed in TC199, 10% FCS and were cultured for 12-15 more hours on TC199, 10% FCS at 37 ° C and 5% CO2. (c) "UNIVERSAL RECEIVER" Enucleated oocytes were activated (as described below) 32-34 hours after the hCG injection and then cultured on TC199, 10% FCS at 37 ° C and 5% CO2 for 4-5 hours. 6 hours. Then an individual cell was contacted with the oocyte and fusion was induced as described below. The pairs were then incubated in TC199, 10% FCS, 7.5 μg cytochalasin B for 1 hour at 37 ° C and 5% CO2. The pairs were then washed and cultured on TC199, 10% FCS at 37 ° C and 5% CO2 for 8-11 hours more. 3. 4 Fusion and Activation For activation, oocytes were placed between two parallel electrodes in 200 μl of 0.3M mannitol, 0.1mM MgSO, O.OOlmM CaCl2 in distilled water (Willadsen, Nature 320 63-65 (9186)). Activation was induced by the application of 1 DC pulse of 1.25kV / cm for 80 μs. For fusion, the manipulated embryos were tested in a similar manner with the addition that the contact surface between the enucleated oocyte and the cell was arranged parallel to the electrodes. The fusion was induced by the application of an AC current of 3V for 5 seconds followed by 3 pulses of DC of 1.25kV / cm for 80 μs. 3. 5 Embryo Determination Culture (all groups) After the culture period, the fused pairs were imbibed in duplicate in 1% and 1.2% agar (DIFCO) in PBS and transferred to the bound oviduct of unsynchronized female ewes. The pair was embedded in agar to prevent or reduce immune rejection of the embryo through the recipient female sheep and to help keep the pair together. After 6 hours, the recipient female sheep were sacrificed and the embryos were recovered by washing the oviduct using 10% FCS in PBS. The embryos were dissected from the agar fragments using 2 needles and the development was analyzed through the microscope. All the embryos, which were developed at the morula / blastocyst stage, were transferred as soon as possible to the uterine tube of the final recipient synchronized female sheep. In vitro techniques are also suitable instead of a temporary female sheep, to achieve the development of the embryo in the blastocyst stage.

EXAMPLE 4 Bovine Nuclear Transfer 4. 1 Maturation of the Oocyte In vitro Ovaries were obtained from a local cattle slaughterhouse and maintained at 28-32 ° C during transport to the laboratory. The cumulus oocyte complexes (COC) of follicles with a diameter of 3-10 mm were aspirated using a hypodermic needle (1.2 mm internal diameter) and placed in universal, sterile plastic containers. The universal containers were placed in a warm chamber (35 ° C) and the follicular material was allowed to settle for 10-15 minutes before emptying three quarters of the supernatant. The remaining follicular material was diluted with an equal volume of dissection medium (TCM 199 with Eagles salts (Gibco), 75.0 mg / l kanamycin, 30.0 mM Hepes, pH 7.4, osmolarity 280 nOsmoles / Kg H2O) supplemented with 10% bovine serum, they were transferred to an 85 mm Petri dish and the COCs were searched under a dissecting microscope. The complexes were selected with at least 2-3 compact layers of cumulus cells, washed three times in the dissecting medium and transferred to the maturation medium (TC 199 medium with Eagles salts (Gibco), 75 mg / l of kanamycin, 30.0mM Hepes, 7.69mM NaHCOs, pH 7.8, osmolarity 280 nOsmoles / Kg H2O) supplemented with 10% bovine serum and 1x106 granulosa cells / ml and were grown until required on an oscillating table. 39 ° C in an atmosphere of 5% CO2 in air. 4. 2 Oocyte manipulation Mature oocytes were separated from cumulus cells 18 hours after the beginning of maturation. Then the stripped oocytes were washed in the calcium-free M2 medium containing % of Fetal Calf Serum (FCS) and were kept in this medium at 37 ° C. To remove the chromosomes (enucleation), the oocytes were placed in calcium free M2 containing 10% FCS, 7.5 μg / ml Hoechst 3342 (Sigma) at 37 ° C for 20 minutes. Then, a small amount of cytoplasm directly below the first polar body was aspirated using a 20 μM glass pipette. Enucleation was confirmed by exposing the aspirated portion of the cytoplasm to UV light and verifying the presence of a metaphase plate. 4. 3 Reconstruction of the Embryo After the enucleated oocytes were used for each of the three reconstruction methods (a), (b) and (c), as detailed below. (a) "MAGIC" (Accepted Cytoplasm G1 / G0 Stopped at the Metaphase) Enucleated oocytes were maintained on calcium free M2 in 10% FCS at 39 ° C, as soon as possible after enucleation, a single cell was placed in contact with the oocyte using a glass pipette to transfer the cell through the previously made hole in the zona pellucida. The cytoplasts / cell pair was then transferred to the fusion chamber in 200 μl of 0.3M mannitol in distilled water. The pair was manually aligned between the electrodes. A pulse of AC (alternating current) of 3V was applied for 5 seconds followed by 3 pulses of DC (direct current) of 1.25kV / cm during 80 μsegs. The pairs were then washed in calcium free M2, 10% FCS at 37 ° C and incubated in the same medium under oil at 37 ° C and 5% CO2. Thirty minutes before activation, the pairs were transferred to calcium free M2 medium, 10% FCS containing 5 μM nocodazole. Activation was induced as described below, after activation, reconstructed zygotes were incubated in the TC199 medium, 10% FCS at 37 ° C and 5% CO2 for 3 more hours. They were then washed 3 times for 5 minutes at 37 ° C in the same medium without nocodazole and cultured for 12-15 more hours before being transferred to female temporary recipient sheep (female sheep are a less expensive alternative as a temporary recipient for the reconstructed embryo). (b) "GOAT" (Activation and Transfer of G0 / G1 Enucleated oocytes were returned to the maturation medium.) At 30 or 42 hours after the start of maturation, an individual cell was placed in contact with the oocyte hallucinate. The pair was transferred to the melting chamber (see below) in 200 μl of 0.3M mannitol, 0.1 mM MgSO4, 0.OOl mM CaCl2 in distilled water. Fusion and activation were induced by the application of a 3V AC pulse for 5 seconds followed by 3 DC pulses of 1.25kV / cm for 80 μsegs. The pairs were then washed in TC199, 10% FCS containing and incubated at 37 ° C and 5% CO2 for 15-20 hours (group 30 hpm) or 4-8 hours (group 42hpm) [The abbreviation "hpm" it is normal for "post-maturation hours"]. (c) "UNIVERSAL RECEIVER" Enucleated oocytes were activated (as described below) 30 or 42 hours after the start of maturation and then cultured on TC199, 10% FCS at 37 ° C and 5% CO2 for 8 hours. -10 hours (group 30 hpm) p 4-6 hours (group 42 hpm). Then an individual cell was contacted with the oocyte and fusion was induced as described below. The pairs were then incubated in TC199, 10% FCS at 37 ° C and 5% CO2 for 12-16 hours plus group 30 hpm) or 4-6 hours (group 42 hpm). 4. 4 Fusion and Activation For activation, oocytes were placed between two parallel electrodes in 200 μl of 0.3M mannitol, 0.1mM MgSO4lO.OOlmM CaCl2 in distilled water (Willadsen, Nature 320 63-65 (9186)). Activation was induced by the application of 1 DC pulse of 1.25kV / cm for 80 μs. For fusion, the manipulated embryos were tested in a similar manner with the addition that the contact surface between the enucleated oocyte and the cell was arranged parallel to the electrodes. The fusion was induced by the application of an AC current of 3V for 5 seconds followed by 3 pulses of DC of 1.25kV / cm for 80 μs. 4. 5 Embryo Determination Culture (all groups) After the culture period, the fused pairs were imbibed in duplicate at 1% and 1.2% agar (DIFCO) in PBS and transferred to the bound oviduct of unsynchronized female sheep (the female sheep are a less expensive alternative as a temporary recipient for the reconstructed embryo). The pair was embedded in agar to prevent or reduce immune rejection of the embryo through the recipient female sheep and to help keep the pair together. After 6 hours, the recipient female sheep were sacrificed and the embryos were recovered by washing from the oviduct using 10% FCS in PBS. The embryos were dissected from the agar fragments using 2 needles and the development was analyzed through the microscope. In vitro techniques are also suitable instead of a temporary female sheep, to achieve the development of the embryo in the blastocyst stage.

Results of Example 3 (sheep cells) and of Example 4 (bovine cells) The techniques herein have been applied to both the reconstruction of sheep and bovine embryos. Currently, embryos in the blastocyst stage have been obtained in cattle; however, no transfers of these embryos have been made to final recipients. In 7 female recipient sheep that became pregnant, the birth of 5 live lambs was presented (2 of which died just after birth). The results of these experiments are summarized in Tables 1-3. Table 1 shows the results of the development to the blastocyst stage of reconstructed sheep embryos using populations of inactive TNT4 cell and 3 different cytoplasts receptors. The reconstructed embryos were cultured in the bound oviduct of a temporary recipient sheep until day 7 after reconstruction. The results are expressed as the percentage of embryos in the morula / blastocyst stage in relation to the total number of embryos recovered.

TABLE 1 Table 2 shows the results of the induction of pregnancy after transfer of all the reconstructed embryos from the morula / blastocyst stage to the uterine horn of synchronized final recipient female sheep. The Table shows the total number of embryos of each group transferred and the frequency of pregnancy in terms of female sheep and embryos (in most cases, 2 embryos were transferred to each female sheep.) An individual double pregnancy was established using the cytoplasm "MAGIC".

TABLE 2 Table 3 shows the outcome of established pregnancies after the transfer of morula / blastocyst stage embryos to the final recipient female sheep.

TABLE 3 EXAMPLE 5 Sheep Nuclear Transfer and Embryo Reconstruction Using OME Cells. BLWF1 and SEC1 The nuclear transfer has been conducted using three types of new cells, designated as OME, BLFW1 and SEC1. OME cells (mammary epithelial sheep) are an established epithelial cell line from a biopsy removed from the mammary gland of a female sheep, Fin-Dorset, 6 years old, following the procedure of Finch et al. (Biochem.Soc Trans 24 369S (196c96).) BLFW1 cells (Black Welsh Fibroblast) are a fibroblast cell line obtained by dissecting and culturing a 26-day Black Welsh fetus obtained following the Natural Pairing of a Black Welsh sheep to a Black Welsh father ram The isolation method of primary fetal fibroblasts is according to Robertson, EJ, in Teratocarcinomas and embryonic stem cells: A practice! approach, 71-112, IRL Press Oxford (1987). SEC1 (sheep embryonic cell) is a cell line of epithelial type derived from a 9-day embryo, obtained from a female sheep Pol-Dorset super ovulated and paired to a parent ram Pol-Dorset.The SEC1 cells are different from the TNT cells described in co-pending PCT application No. PCT / GB95 / 02095, published as WO 96/07732 for the following reasons: Firstly, the morphology of the cells of the two cell lines are completely different, and second, the methods used to isolate the cell lines were different. The SEC1 cell line was established from an individual embryo, while the TNT cell lines are derived from groups of cells. All cell lines were marked with karyotype and showed a modal chromosome number of 54 (2n). Before being used as nuclear donors for embryo reconstruction, induction of inactivity after reduction of serum levels to 0.5% was verified as previously described (Campbell et al., Nature 380 64-66 (996 )). The preparation of the reconstructed embryos was as described above in the previous examples. Table 4 shows a summary of the development of nuclear transfer embryos reconstructed from different cell types. The table shows the number of reconstructed embryos, the development to the blastocyst stage and the number of pregnancies for each of the cell types. All cell lines were marked with karyotype before being used for embryo reconstruction. These cell lines had a modal number of 54 chromosomes. One to three embryos in blastocyst stage were transferred to each synchronized final recipient female sheep. The reconstructed embryos, which were cultured in vitro, were placed in drops of 10 μl (4 embryos) of SOFM (synthetic oviduct fluid medium) containing 10% human serum and cultured in a humidified atmosphere of 5% CO2 , 5% CO2 and 90% N2 at 39 ° C. Cultured embryos were transferred to a fresh medium every two days. The SOFM medium was prepared according to Gadner et al. , Biology of Reproduction 50 390-400 (1994) and Thompson et al. , Biology of Reproduction 53 1385-1391 (1995). Table 5 shows the identification of the recipient female sheep that became pregnant on June 24, 1996, the type of cell used for the reconstruction of the embryo and the expected date of the lamb. The pregnancies were established through the transfer of 1 to 3 morula / blastocyst stage embryos (day 7 after reconstruction) to the synchronized final recipient female sheep. Table 4 shows details of the reconstructed numbers. The abbreviations are: PD = Pol-Dorset, BW = Blach Welsh, FD = End-Dorset, * = embryo cultured in vitro to the blastocyst stage. TABLE 4 TABLE 5

Claims (21)

1. - A method for reconstituting an animal embryo, the method comprising transferring the nucleus of an inactive donor cell to a suitable recipient cell.

2. A method according to claim 1, wherein the animal is a species of ungulate.

3. A method according to claim 2, wherein the animal is a cow or bull, pig, goat, sheep, camel or buffalo from India.

4. A method according to any of claims 1 to 3, wherein the nucleus of the donor cell is genetically modified.

5. A method according to claim 5, wherein the donor cell is genetically modified before reconstitution of the embryo.

6. A method according to claim 5, wherein the recipient cell is an oocyte.

7. A method according to claim 6, wherein the oocyte is enucleated.

8. A method according to claims 1 to 7, wherein the donor cell is an adult somatic cell.

9. A method according to claims 1 to 7, wherein the donor cell is an embryonic somatic cell.

10. A method according to claims 1 to 7, wherein the donor cell is a fetal somatic ce

ll. 11. A method for preparing an animal, the method comprising: (a) reconstituting an animal embryo as described in the preceding claim; and (b) having an animal develop from the embryo at term; and (c) optionally, spawning of the animal thus formed.

12. A method according to claim 11, wherein the animal embryo is further manipulated prior to the complete development of the embryo.

13. A method according to claim 12, wherein more than one animal is derived from the embryo.

14. A reconstituted animal embryo prepared by transferring the nucleus of an inactive donor cell to a suitable recipient cell.

15. A reconstituted embryo according to claim 14, wherein the donor cell is an adult somatic cell.

16. A reconstituted embryo according to claim 14, wherein the donor cell is an embryonic somatic cell.

17. A reconstituted embryo according to claim 14, wherein the donor cell is a fetal somatic cell.

18. An animal prepared by the method according to any of claims 1 to 13.

19. An animal developed from a reconstituted animal embryo according to any of claims 14 to 17. 20.- A cell line derived from a reconstituted embryo according to any of claims 14-17. 21. A cell derived from a reconstituted embryo according to any of claims 14-17.