WO2001000794A1 - Method for producing cloned tigers by employing inter-species nuclear transplantation technique - Google Patents

Abstract

The present invention provides a method for producing cloned tigers by employing inter-species nuclear transplantation technique. The method for producing cloned tigers of the invention comprises the steps of: preparing donor somatic cell lines collected from tiger; maturing oocytes collected from ovary of cow or cat in vitro; removing the cumulus cells surrounding the oocytes; cutting a portion of zona pellucida of the matured oocytes to make a slit, and squeezing out a portion of cytoplasm including the first polar body through the slit to give enucleated recipient oocytes; transferring a nucleus to the recipient oocyte by injection of the donor cells to the enucleated recipient oocytes, followed by the subsequent electrofusion and activation of the electrofused cells to give embryos; postactivating and culturing the embryos in vitro; and, transferring the cultured embryos into surrogate mothers to produce cloned tigers. The present invention can be widely applied in preservation of genetic characters of rare or endangered species permanently in the form of nuclear transferred embryos.

Description

Method For Producing Cloned Tigers By Employing Inter-species Nuclear Transplantation Technique

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method for producing cloned tigers by employing inter-species nuclear transplantation technique, more specifically, a method for producing cloned tigers by employing inter-species nuclear transplantation technique by which nuclei of somatic cells derived from the tissues of tiger are transferred into oocytes originated from cow or cat. It also relates to cloned tiger embryos and cloned tigers produced by the method described above.

Bac gound of the Invention

Animals have long been considered to be produced by fertilization involving male and female gametes. However, tremendous efforts have been made on generating cloned animals with the identical appearance and genetic characteristics.

Cloning of zygotes has been known to be possible only in amphibians for 30 years until the success in producing a cloned offspring by substituting a pronucleus of one-cell zygote in mice (.s_e_e_: McGrath and Solter, Science, 220:1300-1302, 1983). Despite this first success in cloning animals, the same success in industrial animals (s_£e_: akayama et al., Nature, 394:369-374, 1998) has been reported much later since production of cloned mice employing mature oocytes and zygote blastomeres after 2-cell stage have several problems such as decrease in reprogramming .

With regard to the production of cloned industrial animals by nuclear transfer, it was the first report that an offspring was produced in sheep by employing blastomeres of 8- to lβ-cell zygote as donor cells (.s_e_e.: Wiladsen, Nature, 320:63-65, 1986). Since then, only the blastomeres of zygote with totipotency by which a cell can be differentiated into every single cell have been considered to be cloned by nuclear transfer. However, the first cloned sheep was produced by introducing nuclei from somatic cells (s_e_e.: Wilmut et al., Nature, 385:810-813, 1997), thus making correction in the prior developmental theory and enabling many successful examples to be reported in production of cloned cows (.s_e_e_: Wells et al, Reprod. Fertil. and Develop., 10:369-378, 1998) and pigs.

Embryos produced by exploiting somatic cells of existent wild animals and the technique for producing the embryos have been appreciated very valuable for preserving genetic characters of rare or endangered species permanently. However, there has been no report yet on successful cloning of existent wild animals. When it comes to cloning these wild animals, it has difficulties in obtaining the recipient oocytes in case of cloning animals which are rare or under protection. Therefore, their cloned animals should be produced by employing inter- species nuclear transplantation and the oocytes derived from the closely related species. This inter-species nuclear transplantation technique was published previously (s_e_e.: Dominko et al., Biol . Reprod., 60(6):1496- 1502(1999) . This was, however, directed to employing the oocytes and somatic cells of industrial animals on which much research work had already been done, thus still making the application of this technique in production of cloned wild animals difficult.

Under the circumstances, there are strong reasons for exploring and developing an improved method for producing cloned wild animals by employing inter-species nuclear transplantation and somatic cells of wild animals. Summary of the Invention

In accordance with the present invention, it has been discovered that: cloned tiger embryos and cloned tigers developed from the said embryos can be produced by inter-species nuclear transplantation technique involving fusion of the oocytes from cow or cat with somatic cells of tiger .

A primary object of the present invention is, therefore, to provide a method for producing cloned tigers by inter-species nuclear transplantation technique.

The other object of the invention is to provide cloned tiger embryos produced by the said method. Another object is to provide somatic cell-derived cloned tigers produced by the said method.

BRTEF DESCRIPTION OF THE DRAWINGS

The above and the other objects and features of the present invention will become apparent from the following description given in the conjunction with the accompanying drawings, in which:

Figure 1 is a photograph of donor somatic cells.

Figure 2 is a photograph showing the process of cutting zona pellucida of a recipient oocyte with a holding pipette and cutting pipette. Figure 3 is a photograph showing the process of enucleation by removing the first polar body and nucleus from a recipient oocyte . Figure 4 is a photograph showing the process of transferring a somatic cell into an enucleated oocyte with a holding pipette and injection pipette. DETAILED DESCRIPTION OF THE INVENTION

The method for producing cloned tigers of present invention comprises the steps of: preparing donor somatic cell lines collected from tiger; maturing oocytes collected from ovary of cow or cat in vitro; removing cumulus cells surrounding the oocytes, cutting a portion of zona pellucida of the matured oocytes and squeezing out a portion of cytoplasm including the first polar body to give enucleated recipient oocytes; transferring a nucleus to the recipient oocyte by injection of the donor cells to the enucleated oocytes, followed by the subsequent electrofusion and activation of the electrofused cells to give embryos; postactivating and culturing the embryos in vitro; and, transferring the cultured embryos into surrogate mothers to produce cloned tigers.

The method for producing cloned tigers of the invention is further illustrated as follows.

Step 1 : Preparation of donor cells

Somatic cell lines collected from tiger are prepared as donor cells: although cells collected from a tiger are not limited for donor cells, preferable cell lines include cells collected from uterine flushing fluid, endometrium, oviduct, ear or muscle, cumulus cells or fetal fibroblasts, which are prepared by employing the conventionally known method (.s_e_e.: Mather & Barnes, Methods in Cell Biology, Vol.57, Animal Cell Culture Methods, Academic Press, 1998) with some modifications.

For example, cells are collected by the addition of

PBS (phosphate buffered saline) containing 1% penicillin- streptomycin (Gibco; lOOOOU/ml penicillin, lOmg/ml streptomycin) to uterine flushing fluid and subsequent centrifugation. The cells collected from uterine flushing fluid are cultured in DMEM (Dulbecco' s modified Eagle's medium) supplemented with non-essential amino acids, 10% FBS (fetal bovine serum) and 1% penicillin-streptomycin under an environment of 39 ^°C, 5% C0₂. Uterine epithelial cells collected from endometrium or oviduct are washed with the said PBS, trypsinized, and cultured under the same conditions as described above.

For cumulus cells, cumulus-oocyte complexes are treated with hyaluronidase solution to isolate cumulus cells surrounding oocytes. The cumulus cells are trypsinized for 30 to 60 minutes under an environment of

39^°C, 5% CO_: before they are cultured in a similar manner as described above.

For ear fibroblasts and fetal fibroblasts, they are obtained from the inner side of skin lined along with cartilage tissue and from tissue collected from trunk and limbs of fetus, respectively, by washing and mincing the tissues aseptically, followed by treatment of trypsin and collagenase type II under an environment of 39^°C, 5% C0₂. These cells are also cultured analogously as in the somatic donor cell lines described above.

The somatic cell lines are stored by subculture, serum starvation culture or freezing. The subculture of donor cell lines is carried out at regular intervals by changing the old medium to new one after trypsinization. The serum starvation culture is performed by employing DMEM supplemented with 0.5% FBS and the method of Wilmut et al.(≤_e_: Wilmut et al . , Nature, 385:810-813, 1997). The cell lines thus stored are used for later step as donor cells.

Step 2 : Preparation of recipient oocytes

Immature oocytes collected from ovary of cow or cat are matured in vitro: immature oocytes are selected from ovary in TCM199 washing medium containing 10 mM HEPES (N-

[hydroxyethyl] piperazine-N' - [2-ethanesulfonic acid]), and matured in different culture media, depending on the kind of animals from which the oocytes were obtained. To mature the oocytes derived from cow, TCM199-1 culture medium (see : Table 1) is employed. For maturation of the oocytes collected from cat, the cells are cultured in TCM199-2 culture medium (s_e_e_: Table 2) supplemented with human Chorionic Gonatropin (hCG) . In both cases, the culture was performed for 16 to 22hr under an environment of 39^°C, 5% C0₂.

Table 1: TCM199-1 culture medium

Table 2: TCM199-2 culture medium

Step 3: Enucleation of recipient oocytes

After removing cumulus cells surrounding the mature recipient oocytes and cutting a portion of zona pellucida of the oocytes, a portion of cytoplasm including the first polar body is removed from the oocytes to give enucleated oocytes: first, cumulus cells surrounding the mature oocytes are removed physically with a denuding pipette in TCM199 washing medium containing hyaluronidase . Then, denuded oocytes are washed with TCM199 washing medium and transferred into cytochalasin B solution. For enucleation of the denuded oocytes, a portion of zona pellucida of the denuded oocytes is penetrated by a cutting pipette to give a slit through which 10 to 15 % of cytoplasm including the first polar body can be squeezed out of the oocytes. The enucleated oocytes are washed and incubated in TCM199 culture medium. The said cytochalasin B solution is prepared by diluting cytochalasin B dissolved in DMSO (dimethylsulfoxide) with the TCM199 culture medium.

Step 4 : Electrofusion of donor cells with recipient oocytes and activation of the electrofused cells

The donor cells are transferred to the recipient oocytes, followed by subsequent electrofusion and activation of the electrofused cells: before the injection of donor cells into recipient oocytes, the enucleated oocytes are washed with TCM199 culture medium and transferred to PHA-P (phytohemagglutinin) solution. Then, the donor cells are transferred to the enucleated oocytes by injecting donor cells to the slit made on zona pellucida of the oocytes in PHA-P solution.

The electrofusion is carried out by employing an Electro Cell Manipulator (BTX ECM2001). The reconstructed embryos in mannitol solution supplemented with TCM199 washing solution are placed in a chamber with two electrodes, one on either side. Before placing the embryos with their donor cells facing the cathode in the chamber, the chamber was filled with mannitol solution. After the embryos are electrofused by applying DC pulse of 0.75 to 2.00 kV/cm twice with one second's interval for 15μs each time, the electrofused embryos are washed with mannitol solution and TCM199 washing medium, incubated in cytochalasin B solution, and activated. The electrofusion and activation occur in a simultaneous manner provided that the electrofusion is carried out in a mannitol medium containing Ca²⁺. Otherwise, the activation is performed after electrofusion. When the electrofusion is carried out in a Ca^2*-free mannitol medium, the activation step is performed by incubating the embryos in ionomycin solution in the dark. Then, ionomycin is removed from the embryos by washing them with TCM199 washing medium containing FBS or BSA. The said ionomycin solution is prepared by diluting ionomycin dissolved in DMSO with TCM199 washing medium containing BSA.

Step 5: Postactivation and in vitro culture of embryos

The embryos are postactivated and cultured in vitro: the activated embryos incubated in TCM199 washing medium containing FBS or BSA are postactivated by incubating in cycloheximide solution or DAMP (4-dimethylaminopurine) solution, and cultured in vitro under an environment of 5% C0₂, or a mixture of 5% C0₂, 7% 0₂ and 88% N₂. The said cycloheximide solution or DAMP solution is prepared by adding cycloheximide dissolved in ethanol or DAMP to media for in vitro culture, respectively. The media for in vitro culture include mTALP (.s_ae_: Table 3), mSOF (.≥e_e_: Table 4) and mCR2aa (.s_e_e_: Table 5) medium, all of which comprise NaCl, KC1, NaHC0₃, NaH₂P0₄, CaCl₂, Na-lactate, glucose, phenol red, BSA, kanamycin, essential amino acids, non-essential amino acids and L-glutamine. Optionally, the embryos cultured in vitro are stored by freezing for later use, and subjected to thawing when they are intended to be used. To freeze the embryos, they are washed with PBS containing FBS, put in a freezing medium containing penicillin-streptomycin, CaCl₂, glucose, MgCl₂, Na-pyruvate and PBS. Then, the embryos in the freezing medium are subjected to slow freezing, followed by rapid freezing in liquid N₂. When the frozen embryos are taken from liquid N₂ and thawed, they are put in the air for about 5 seconds and then thawed in warm water. To remove the freezing medium from the thawed embryos, they are put serially in media containing glycerol from its high concentration to low concentration. Table 3: mTALP medium

Table 4 : mSOF medium

Table 5: mCR2aa medium

Step 6: Production of cloned tigers

The embryos cultured in vitro are transferred into surrogate mothers to produce tigers: the embryos in PBS containing FBS are implanted to uterus of surrogate mothers.

Based on the method described above, the present inventors produced an embryo, SNU5 (Korean Tiger NT Embryo), by using ear cells of tiger and oocytes of Korean cow as nucleus donors and recipient oocytes, respectively. The embryo was deposited with an international depositary authority, KCTC (Korean Collection for Type Cultures; KRIBB #52, Oun-dong, Yusong-ku, Taejon, 305-333, Republic of Korea) on Mar. 10, 2000, under an accession number of KCTC 0752BP.

The present invention is further illustrated in the following examples, which should not be taken to limit the scope of the invention.

Example 1: Preparation of donor cells and recipient oocytes

To prepare donor cells, disinfection was carried out with ethanol and betadine around the apical ear tissue of a male adult tiger after shaving the hairs around the tissue. Skin tissue ( l-2cm² in area) was collected with sterilized instruments, transferred into a 50ml test-tube containing phosphate buffered saline (PBS, Gibco BRL Life Technologies, USA) with 0.5% penicillin (lOOOOU/ml) -streptomycin (lOmg/ml) . Then, the cartilage tissue and hair-containing skin were separated from the said collected skin tissue with sterilized scissors and surgical blades to give the tissue of inner side skin lined along with the cartilage tissue for donor cells. This tissue was washed with PBS and minced into 100 mesh size. Then, the tissue was incubated in PBS containing 0.25% trypsin, ImM EDTA and lmg/ml collagenase type II for lhr under an environment of 39^°C, 5% C0₂. After the tissue was digested with the enzymes, it was centrifuged at 1,500 rpm for 2 minutes, and suspended in DMEM (Dulbecco' s modified Eagle's medium, Gibco BRL, Life Technologies, USA) supplemented with 10% FBS, 1% NEAA (non- essential amino acids) and 1% penicillin-streptomycin. The suspension was transferred to dishes for cell culture and incubated under an environment of 39^°C, 5% C0₂ to give a somatic cell line. After that, the cells were trypsinized in solution containing 0.25% trypsin and ImM EDTA, and the cell number was adjusted to be 2 X 10⁴ cells/ml to aliquot the cells in effendorf-tubes .

Figure 1 depicts the somatic cells isolated as single cells for nucleus donor.

On the other hand, for recipient oocytes, follicles of which size was about 2 to 6 mm in diameter were aspirated from ovaries of Korean cows with a 10ml syringe having an 18G needle. Then, the follicular fluid were transferred into a 100mm dish with a grid (the length between lines was 1 cm) drawn on its bottom, and oocytes with homogeneous cytoplasm and sufficient number of cumulus cell layers around them were screened. The selected oocytes were washed three times with 2 ml of TCM199 washing medium (s_e_e_: Table 6) in 35mm dishes, and subsequently, once with TCM199-1 culture medium (s_e_£: Table 1) . Finally, the oocytes were cultured in TCM199 culture medium containing 0.1% estradiol solution (s_e_e_: Table 7), 2.5% follicle stimulating hormone solution (s_e_e_: Table 8) and 10% FBS to give recipient oocytes.

Table 6: TCM199 washing medium

Table 7 : Estradiol solution

Table 8: Follicle stimulating hormone solution

Example 2 : Nuclear transfer of somatic cells

The recipient oocytes prepared in Example 1 were washed once with TCM199 washing medium and transferred in 0.1% hyaluronidase (Sigma Chemical Co., U.S.A.) solution prepared by mixing 1ml of TCM199 washing medium with lllμl of hyaluronidase stock solution (lOmg/ml in TCM199 washing medium) . After cumulus cells were removed from the oocytes in the presence of 0.1% hyaluronidase, the denuded oocytes were washed three times and incubated in TCM199 washing medium. Then, the oocytes were transferred to cytochalasin B (Sigma Chemical Co., U.S.A.) solution prepared by mixing lml of TCM199 washing medium containing 10% FBS with lμl of cytochalasin stock solution (7.5mg/ml in DMSO), and zona pellucida of each oocyte was cut by employing micromanipulator to make a slit through which 10 to 15% of cytoplasm can be squeezed out of the oocyte to give an enucleated oocyte. The enucleation step is more specifically illustrated as following: a working dish was put on the micromanipulator plate, and the micromanipulator was equipped with a holding pipette on its left arm and a cutting pipette on its right arm. Then, the holding pipette and cutting pipette were placed in the direction of 9 o'clock and 3 o'clock, respectively, and adjusted to move freely in all directions by placing a pipette controller in the middle. These two pipettes were further adjusted to let them not touch the working dish and their tips placed to the middle of a microdroplet by moving them up and down over the microdroplet. Then, the oocytes were transferred from TCM199 washing medium to cytochalasin B solution by employing washing mouth pipettes (>200μm inner diameter). The micromanipulator was first focused on the oocyte by using its coarse adjustment knob and fine adjustment knob, and the focus was further adjusted by moving the two pipettes up and down. The oocyte was placed with its first polar body oriented toward the direction of 12 o'clock, and the holding pipette was placed close to the oocyte in the direction of 9 o'clock of the oocyte to fix the oocyte by applying hydraulic pressure. Figure 2 shows the process of cutting zona pellucida of the oocyte with the holding pipette and cutting pipette. As shown in Figure 2, the oocyte was penetrated by the cutting pipette (2) from the direction of 1 o'clock to the direction of 11 o'clock with special care not to damage the cytoplasm of the oocyte. After that, hydraulic pressure was applied to the holding pipette (1) to separate the oocyte (3), and the holding pipette was contacted with the cutting pipette penetrating the zona pellucida bordering on the upper part of the first polar body to cut the portion of zona pellucida by rubbing the two pipettes. The slit on the oocyte made above was used for both enucleation and donor cell injection. Figure 3 depicts the process of enucleation removing the first polar body and nucleus from the oocyte. As shown in Figure 3, the oocyte (3) was placed with its slit oriented vertically, held with the holding pipette (1) on its lower part to prevent it from moving, and squeezed mildly on its upper part with the cutting pipette (2) to give an enucleated oocyte. The enucleated oocyte was washed three times with TCM199 washing medium and incubated in TCM199 culture medium.

After that, donor cells prepared in advance were transferred to enucleated oocytes by employing micromanipulator. First, 4μl of injection microdroplet was made on the middle of the working dish by using PHA-P solution prepared by mixing 400μl of TCM199 washing solution and lOOμl of PHA-P (phytohemagglutinin) stock solution (0.5mg/ml in TCM199 washing solution). And then, two microdroplets for donor cells were made with one above and the other below the injection microdroplet on the same working dish by using PBS containing 1% FBS. After these microdroplets were spread over with mineral oil, the working dish was placed on the micromaniulator plate.

The cutting pipette installed on the micromanipulator was substituted with an injection pipette. The enucleated oocytes were washed three times with TCM199 washing medium and transferred into the injection microdroplet. The donor cells were drawn up into the injection pipette and transferred into the injection microdroplet. Figure 4 shows the process of transferring a somatic cell into an enucleated oocyte. As shown in Figure 4, the enucleated oocyte was placed with its slit oriented toward the direction of 1 o'clock, fixed by using the holding pipette, ana injected with the donor cell through the slit by employing the injection pipette and hydraulic pressure to give a reconstructed embryo. The embryo was washed three times with and incubated in TCM199 washing medium.

Example 3: Electrofusion and activation

The reconstructed embryos were subjected to electrofusion employing an Electrocell Manipulator (ECM 2001, BTX, USA), followed by activation. 15μl of mannitol solution containing 0.28M mannitol, 0.5mM HEPES (pH 7.2), O.lmM MgS0₄ and 0.05% BSA was added to TCM199 culture medium containing the reconstructed embryos by employing a mouth pipette for washing. After 1 minute's incubation in the said medium, the embryos were incubated for 1 minute in mannitol solution supplemented with TCM199 washing solution, and finally transferred into mannitol solution by employing the mouth pipette for washing. The chamber (3.2mm chamber No. 453) of the Electrocell Manipulator was filled with mannitol solution supplemented with TCM199 washing medium, and then the embryos were placed in the chamber with their donor cell part facing the cathode. After the embryos were electrofused by applying DC pulse of 0.75 to 2.00kV/cm twice with one second's interval for 15μs each time, they were transferred into and washed three times with TCM199 washing medium by way of mannitol solution. To activate the electrofused embryos, they were incubated in the dark for 4 minutes in ionomycin (Sigma Chemical Co., USA) solution which was TCM199 washing medium containing 5μM ionomycin and 1% BSA. The ionomycin stock solution was prepared by dissolving lmg of ionomycin m 1.34 ml of DMSO. The activated embryos were incubated for 5 minutes in a 35mm dish containing TCM199 washing medium supplemented with 10% FBS to remove ionomycin from the embryos .

Example : Postactivation and in vitro culture of the electrofused embryos

The activated embryos were postactivated for 4hrs in 25μl of cycloheximide (Sigma Chemical Co., USA) solution prepared by adding cycloheximide stock solution (lOmg/ml in ethanol) to an in vitro culture medium, mTALP (.s_e_e_: Table 3) in a final concentration of 10 μg/ml. Then, the embryos were screened, and the selected embryos were incubated for 7 days under an environment of 39^°C, 5% C0₂.

Based on the method described above, the present inventors produced an embryo, SNU5 (Korean Tiger NT Embryo), by using ear cells of tiger and oocytes of Korean cow as nuclear donors and recipient oocytes, respectively. The embryo was deposited with an international depositary authority, KCTC (Korean Collection for Type Cultures; KRIBB #52, Oun-dong, Yusong-ku, Taejon, 305-333, Republic of Korea) on Mar. 10, 2000, under an accession number of KCTC 0752BP.

Example 5 : Production of embryos employing the oocytes obtained from cat

The present inventors produced and cultured the embryos by employing the same method described in Examples 1 to 4 except using the oocytes obtained from cat as recipient oocytes.

Example 6: Freeze and thaw of embryos and transplantation

The embryos produced in Examples 4 and 5 were frozen for long-term storage. First, a freezing medium (.s_e_e.:

Tables 9 and 10) was distributed into 35mm dishes, and a freezer was turned on to be maintained at -5^°C. The embryos selected for freezing were washed with PBS containing 10% FBS, and incubated m the freezing medium for 20 minutes. Then, the embryos were drawn up into a 0.25ml French straw to let the straw have the freezing medium containing the embryos m the middle and two layers of air. After the straw was heat-sealed at both ends by using a heated forcep, it was placed into the freezer, held at -5^°C for 5 minutes, and seeded with a forcep prechilled by liquid N-, . After seeding, the straw was cooled down at C a rate of -0.3^°C/mιn to -30^°C, held for 10 minutes when the temperature reached -30^°C. Finally, the embryos were stored in a liquid N_ tank.

Table 9: Freezing PBS

Table 10: Freezing medium

To thaw the frozen embryos, a thawing medium containing PBS supplemented with 20% FBS was prepared m 35mm dishes, and added with glycerol to give thawing media each having 0%, 3% and 6% glycerol (.s_e_e_: Tables 9 and 11). Then, the frozen straw was taken out from the liquid N₂, held in the air for 5 seconds, and thawed m a container (>20cm in diameter) containing warm water(30^°C). After thawing, the straw was cut on the air layers at both ends, and the medium containing the embryos was collected. The embryos were examined under the microscope. To remove the freezing medium from the embryos, they were consecutively incubated in the thawing media containing 6% glycerol, 3% glycerol and 0% glycerol, each for 5 minutes.

Table 11: Thawing media

The thawed embryo was placed in PBS containing 20% FBS, and drawn up into a straw. And then, it was transferred into the uterus of a surrogate mother.

Example 7 : Comparison of embryos employing various donor cells

To compare the differences among the embryos produced by employing different recipient oocytes, the embryos produced in Examples 4 and 5 were implanted to surrogate mothers, and compared regarding the following terms: number of electrofused oocytes, electrofusion rate(%), division rate(%), number (%) of 8-cell embryos, number (%) of 16-cell embryos, number (%) of 32-cell embryos, number (%) of morulae/blastocysts developed, number of transferred embryos and number of pregnancies following the embryo transfer (s_e_e_: Table 12). Number(%) of morulae/blastocysts represents the ratio of embryos developed by in vitro culture to the stage right before implantation over the total embryos produced by nuclear transfer. Table 12: Comparison of cloned tiger embryos

As shown in Table 12, employing the oocytes of cow indicated higher development rate and possibility of pregnancy than employing the oocytes of cat. This can be explained by the fact that many previous studies on- exploiting the oocytes of cows have helped establish the optimal conditions for nuclear transplantation, whereas there has not been much research work on employing the cat oocytes .

The previous studies have showed that industrial animals on which much research has been done can be applied to inter-species nuclear transplantation. However, none of them has succeeded in production of cloned offspring. In connection with the circumstances described above, the present invention, therefore, achieved great advancement in production of somatic cell-derived cloned animals since it produced somatic cell-derived cloned offspring.

As clearly illustrated and explained above, the present invention provides a method for producing cloned tigers by inter-species nuclear transplantation technique involving the fusion of somatic cells of tiger with the oocytes obtained from cow or cat. It also provides cloned tiger embryos and cloned tigers developed from the said embryos. In accordance with the method of the invention, genetic characters of rare or endangered species can be preserved permanently by employing the inter-species nuclear transplantation to produce their cloned embryos as a way to preserve wild animals. In addition to preservation of wild animals, the method of the present invention is also expected to be employed for development of many related applications involving inter-species nuclear transplantation technique.

Various modifications of the invention in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing descriptions. Such modifications are also intended to fall within the scope of the appended claims.

INDICATIONS RELATING TO DEPOSITED MICROORGANISM OR OTHER BIOLOGICAL MATERIAL

(PCT Rule \3bιs)

A The indications made below relate to the deposited microorganism or other biological material referred to in description On page LC! , lines 10 - 18~

B IDENTIFICATION OF DEPOSIT Further deposits are identified on additional sheet D

Name of depositary institution

Korean Collection for Type CuItures(KCTC)

Address of depositary institution (including postal code and country)

Korean Collection for Type Cultures(KCTC) KRIBB #52, Oun-dong, Yusong-ku Taejon, 305-333, Republic of Korea

Date of deposit Accession Number

Mar. 10, 2000 KCTC 0752B P

C ADDITIONAL INDICATIONS (leave blank if not applicable) This information continues on an additional sheet D

D DESIGNATED STATES FOR WHICH INDICATIONS ARE MADE (if the indications are not for all designated States)

E SEPARATE FURNISHING OF INDICATIONS (leave blank if not applicable)

The indications listed below will be submitted to the International Bureau later (specify the general nature of the indications e g "Accession Number of Deposit ")

For receiving Office use only For International Bureau use only

Ll This sheet was received with the international application 0 This sheet was received by the International Bureau on

Authorized officer Authorized officer

Form PCT/R0/134(July 1998)

Claims

WHAT IS CLAIMED IS:

1. A method for producing cloned tigers which comprises the steps of: (i) preparing donor somatic cell lines collected from tiger;

(ii) maturing oocytes collected from ovary of cow or cat in vitro;

(iii) removing cumulus cells surrounding the oocytes, cutting a portion of zona pellucida of the matured oocytes to make a slit, and squeezing out a portion of cytoplasm including the first polar body through the slit to give enucleated recipient oocytes;

(iv) transferring a nucleus to the recipient oocyte by injection of the donor cells to the enucleated recipient oocytes, followed by the subsequent electrofusion and activation of the electrofused cells to give embryos;

(v) postactivating and culturing the embryos in vitro; and, (vi) transferring the cultured embryos into surrogate mothers to produce cloned tigers.

2. The method for producing cloned tigers of claim 1, wherein the somatic cell lines prepared in Step(i) include cells collected from uterine flushing fluid, endometrium, oviduct, ear or muscle, cumulus cells or fetal fibroblasts.

3. The method for producing cloned tigers of claim 1, wherein the somatic cell lines are stored by subculture, serum starvation culture or freezing.

4. The method for producing cloned tigers of claim 1, wherein the cumulus cells surrounding the oocytes in Step

(iii) are physically removed with a denuding pipette after treatment of hyaluronidase.

5. The method for producing cloned tigers of claim 1, wherein the enucleation of oocytes in Step (iii) is carried out by making a slit on the oocyte by cutting it with micromanipulator; placing the oocyte with its slit oriented vertically and holding a lower part of the oocyte with a holding pipette to prevent the cell from moving; squeezing the upper part of the oocyte with a cutting pipette to let 10 to 15% of cytoplasm containing the first polar body out of the oocyte through the slit.

6. The method for producing cloned tigers of claim 1, wherein the nuclear transfer in Step(ιv) is carried out by injecting a donor cell into a recipient enucleated oocyte through the slit made on zona pellucida of the oocyte.

7. The method for producing cloned tigers of claim 1, wherein the electrofusion m Step(iv) is carried out by applying DC pulse of 0.75 to 2.00kV/cm twice with one second's interval for 15μs each time.

8. The method for producing cloned tigers of claim 1, wherein the activation in Step(iv) occurs in a simultaneous manner with electrofusion provided that the electrofusion is performed in a medium containing Ca²⁺.

9. The method for producing cloned tigers of claim 1, wherein the activation in Step(ιv) is performed in ionomycin solution in the dark provided that the electrofusion is carried out in a Ca²⁺-free medium.

10. The method for producing cloned tigers of claim

1, wherein the postactivation in Step(v) is carried out by cultuπng embryos in cycloheximide solution or DMAP(4- dimethylaminopuπne) solution.

11. The method for producing cloned tigers of claim

1, wherein m vitro culture in Step(v) is carried out by cultuπng the postactivated embryos in mTALP, mSOF or mCR2aa medium.

12. The method for producing cloned tigers of claim 1, further comprising a step of storing embryos cultured in vitro in Step(v) for later use after freezing the embryos in a freezing medium containing penicillin-streptomycin, CaCl_:, glucose, MgCl₂, Na-pyruvate and phosphate buffered saline .

13. An embryo, SNU5 (Korean Tiger NT Embryo, KCTC

0752BP) which is produced by the Steps (i) through (v) of claim 1 employing ear cells of tiger and oocytes of Korean cow (Bos ta urus coreanae) as nucleus donor and recipient oocyte, respectively.

14. A cloned tiger which is produced by the Step(vi) of claim 1 employing SNU5 (Korean Tiger NT Embryo, KCTC 0752BP)of claim 13 as an embryo.