Effect of telophase enucleation on bovine somatic nuclear transfer

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Procedure used for denuding pig oocytes influences oocyte damage, and development of in vitro and nuclear transfer embryos

The effects of different denuding procedures used during the in vitro culture of porcine embryos on oocyte damage and aspects of porcine embryo development were investigated in a series of studies. Oocytes were denuded by vortexing or pipetting after 44h in vitro maturation (IVM) or pre-denuded after 22h IVM. The total oocyte death rate was significantly (P<0.05) higher for pre-denuded (27.3±1.4%) than for vortexed (20.3±1.2%) or pipetted (16.2±2.2%) oocytes. There was no significant difference between the treatments in the percentage of oocytes that extruded the first polar body. The type I cortical granule distribution (reflecting complete maturity) and normal spindle formation rates were significantly lower in the pre-denuding than in the vortexing and pipetting treatments. Blastocyst formation rates were significantly lower for the pre-denuding treatment in PA (25.7±4.5%) and IVF (6.1±1.5%) culture than in the vortexing (PA 42.0±4.5%; IVF 11.2±0.5%) and pipetting (PA 43.4±3.1%; IVF 9.4±1.6%) treatments. The proportion of oocytes developing to blastocysts in SCNT culture was not significantly different between treatments ranging from 9.9±1.8% for pre-denuding to 12.3±2.7% for vortexing. No significant differences in apoptosis or embryonic fragmentation were observed. This study shows that the denuding procedure used for porcine oocytes during the in vitro production of embryos can significantly affect oocyte damage, spindle patterns, oocyte maturation, embryo development but not embryonic apoptosis or the frequency of fragmentation.

Effects of demecolcine and sucrose on the incidence of cytoplasmic protrusions containing chromosomes in pig oocytes matured in vitro

The present study was carried out to examine whether demecolcine and sucrose affect the formation of a cytoplasmic protrusion containing chromosomes in pig oocytes independently or in combination. In the presence of 20 mM sucrose, the rates of oocytes with a cytoplasmic protrusion after culture for 60 min with 0.2-1.0 microg/ml demecolcine were significantly higher than those with 0.01-0.05 microg/ml demecolcine. When oocytes were cultured for 15 min in the presence of 0.2 microg/ml demecolcine and 20 mM sucrose, 35.1% of them extruded a cytoplasmic protrusion; this rate was significantly lower than those of oocytes cultured for 30-90 min. In the presence of 0.2 microg/ml demecolcine, significantly fewer oocytes extruded a cytoplasmic protrusion after culture for 30 min with 160 mM sucrose than with 0-80 mM sucrose. Significantly more oocytes extruded a cytoplasmic protrusion after culture for 30 min with 0.2 microg/ml demecolcine than without it, regardless of the presence or absence of 20 mM sucrose. In 88.9-100% of the oocytes, the cytoplasmic protrusions contained chromosomes with no significant differences among the different concentrations of demecolcine and sucrose and among the different treatment times. The results of the present study show that the cytoplasmic protrusion containing chromosomes in the pig oocyte is attributable to demecolcine, but sucrose does not affect its formation.

Somatic cell nuclear transfer: Past, present and future perspectives

It is now over a decade since the birth, in 1996, of Dolly the first animal to be produced by nuclear transfer using an adult derived somatic cell as nuclear donor. Since this time similar techniques have been successfully applied to a range of species producing live offspring and allowing the development of transgenic technologies for agricultural, biotechnological and medical uses. However, though applicable to a range of species, overall, the efficiencies of development of healthy offspring remain low. The low frequency of successful development has been attributed to incomplete or inappropriate reprogramming of the transferred nuclear genome. Many studies have demonstrated that such reprogramming occurs by epigenetic mechanisms not involving alterations in DNA sequence, however, at present the molecular mechanisms underlying reprogramming are poorly defined. Since the birth of Dolly many studies have attempted to improve the frequency of development, this review will discuss the process of animal production by nuclear transfer and in particular changes in the methodology which have increased development and survival, simplified or increased robustness of the technique. Although much of the discussion is applicable across species, for simplicity we will concentrate primarily on published data for cattle, sheep, pigs and mice.

Developmental Arrest and Cytoskeletal Anomalies of Rat Embryos Reconstructed by Somatic Cell Nuclear Transfer

Many factors influence success rates in animal cloning by somatic cell nuclear transfer (SCNT), including cell cycle stage of donor cells and recipient oocytes, the procedure of micromanipulation, and the activation protocol. This study was conducted to determine the effects of cell cycle coordination for cloning rats from fetal fibroblasts (FFs). Moreover, enucleated zygotic and parthenogenetic ooplasts were used for serial cloning with pronuclear and two-cell stage blastomeres derived from SCNT. Metaphase donor cells had a significantly higher cleavage rate than G0/G1-phase FFs with MII oocytes and G2-phase FFs with TII oocytes. However, reconstructed embryos were unable to develop beyond the two-cell stage, neither in vitro nor in vivo. Moreover, the developmental arrest at the two-cell stage was not overcome, even when using serial cloning with zygotic and parthenogenetic recipients. To assess the cytoskeleton after SCNT, reconstructed two-cell stage embryos were harvested at different times after cleavage for immunostaining (anti-alpha-tubulin) and mRNA abundance (beta-actin, alpha-tubulin, alpha-actinin). Reconstructed two-cell embryos showed abnormal microtubule distribution and down-regulated expression of several cytoskeletal transcripts. Therefore, it seems that the developmental arrest of rat SCNT embryos is associated with improper transcription of cytoskeleton genes, presumably resulting in abnormal microtubule distribution.

Telophase-stage host ooplasts support complete reprogramming of roscovitine-treated somatic cell nuclei in cattle

Nuclear-cytoplasmic incompatibilities are known to play a significant role in the developmental outcome of embryos produced by nuclear transfer, particularly when metaphase arrested oocytes are used as hosts for interphase donor nuclei. To further our understanding of how cell cycle coordination affects somatic cell cloning, somatic cells at different stages of the cell cycle were fused to host oocytes either before (metaphase II, M-II) or after (telophase II, T-II) activation. To obtain cells at different stages of the cell cycle, fetal fibroblast (FF) and granulosa cells (GC) were treated with roscovitine, an inhibitor of cyclin-dependent kinases (CDKs) resulting in a large percentage of cells in S/G(2)-phase. In contrast to the M-II group, which did better with confluent cells, embryos reconstructed with T-II cytoplasts resulted in higher rates of blastocyst formation when fused to cells recovered at 16-24 h after passage. Embryos reconstructed with FF treated with roscovitine and T-II cytoplasts (Rosc/T-II) resulted in similar blastocyst rate compared to those produced with confluent cells and M-II cytoplasts (Conf/M-II). Transfer of blastocysts to surrogate heifers resulted pregnancies and birth of healthy calves from Rosc/T-II and Conf/M-II reconstructed embryos. These results indicate that, when combined with nuclear donor cells at specific cell cycle stages, M-II and T-II bovine oocytes are similarly effective in supporting the reprogramming of somatic cell nuclei.

Effect of some factors on the fusion rate of bovine–rabbit interspecies reconstructed eggs

Tests were conducted to highlight the effects of donor cell treatment (bovine G0- and non-G0 phase somatic cells refrigerated for 1–5 or 12–14 days at 4°C), fusion solution (with or without cytochalasin B) and age of the rabbit (young, 3–5-months old; or multiparous, 10–12 months old) as oocyte donor on the fusion and mortality rates of bovine–rabbit interspecies reconstructed eggs. Results showed that there were no significant differences (P>0.05) in fusion and mortality rates of eggs reconstructed with G0 cells or non-G0 cells as nuclear donor somatic cells. The same was also true when nuclear donor somatic cells (G0 cells or non-G0 cells) were treated at low temperature (4°C) for 1–5 or 12–14 days. The fusion rate of reconstructed eggs did not change significantly (P>0.05) when adding cytochalasin B into the fusion solution, but the mortality rate increased significantly (P<0.05). No significant effect (P>0.05) was found in fusion rate of the eggs reconstructed with young rabbits' and multiparous rabbits' oocytes as recipient, but the mortality rate decreased significantly (P<0.01) for young rabbit oocytes compared with multiparous rabbit ones.

Development of water buffalo (Bubalus bubalis) embryos from in vitro matured oocytes reconstructed with fetal skin fibroblast cells as donor nuclei

The present study was carried out to explore the feasibility of using buffalo fetal skin fibroblasts as donor nuclei and to find out the developmental competence of embryos following transfer of these nuclei to in vitro matured enucleated buffalo oocytes. Skin cells were isolated from 1 to 2-month-old fetuses obtained from slaughterhouse, by enzymatic digestion (0.5% w/v trypsin +0.05% w/v collagenase in Dulbecco's PBS) for 15-20 min. The cells were washed 4 times with Dulbecco's PBS and then once with RPMI-1640+10% FBS by centrifugation at 600 x g. The cells were then cultured in the same medium in a CO2 incubator (5% CO2 in air) at 38.5 degrees C for 2-3 days. Cumulus-oocyte complexes (COCs) collected from slaughterhouse buffalo ovaries were subjected to IVM in the IVM medium (TCM-199 + 5 microg/ml FSH-P + 10 microg/ml LH+10% FBS) for 20-22 h in a CO2 incubator (5% CO2 in air) at 38.5 degrees C. Oocytes were denuded with 0.1% trypsin followed by repeated pipetting and then enucleated by aspirating the first polar body with 10-15% of nearby cytoplasm with a micromanipulator. Two different types of donor cells (growing cells and those arrested with cytochalasin-B) were used for reconstruction of oocytes. The reconstructs were electro fused and incubated in the activation medium (TCM-199 + 8 microg/ml cytochalasin-B+10% FBS) for 4 h. These were then cultured in IVC medium (TCM-199+10% FBS) in a CO2 incubator (5% CO2 in air) at 38.5 degrees C for 48 h. The cleaved embryos were then co-cultured with buffalo oviduct cells in embryo development media (EDM). Out of 119 denuded matured oocytes which were enucleated and reconstructed with growing cells, 78 (65.5%) were electro fused, activated and cultured, out of which 4 (5.1%) reconstructs cleaved and developed to 2-cell stage, 3 (3.8%) reached to 4-cell stage and 3 (3.8%) reached to 8-cell stage. In the synchronized group, out of 62 denuded matured oocytes which were reconstructed with cytochalasin-B blocked cells, 40 (65%) were electrofused, activated and cultured, out of which 4 (10%) developed to 2-cell stage, 3 (7.50%) to 4-cell stage, 2 (5.0%) to early morula stage and 1 (2.50%) to blastocysts stage. These results suggest that buffalo fetal skin fibroblasts could be used as donor nuclei for the production of buffalo embryos after nuclear transfer to enucleated in vitro matured buffalo oocytes.

Cloning of Asian yellow goat (C. hircus) by somatic cell nuclear transfer: Telophase enucleation combined with whole cell intracytoplasmic injection

Our and other previous studies have shown that telophase enucleation is an efficient method for preparing recipient cytoplasts in nuclear transfer. Conventional methods of somatic cell nuclear transfer either by electro-fusion or direct nucleus injection have very low efficiency in animal somatic cell cloning. To simplify the manipulation procedure and increase the efficiency of somatic cell nuclear transfer, this study was designed to study in vitro and in vivo development of Asian yellow goat cloned embryos reconstructed by direct whole cell intracytoplasmic injection (WCICI) into in vitro matured oocytes enucleated at telophase II stage. Our results demonstrated that the rates of cleavage and blastocyst development of embryos reconstructed by WCICI were slightly higher than in conventional subzonal injection (SUZI) group, but no statistic difference (P > 0.05) existed between these two methods. However, the percentage of successful embryonic reconstruction in WCICI group was significantly higher than that in SUZI group (P < 0.05). After embryo transfer at 4-cell stage, the foster in both groups gave birth to offspring. Therefore, the present study suggests that the telophase ooplasm could properly reprogram the genome of somatic cells, produce Asian yellow goat cloned embryos and viable kids, and whole cell intracytoplasmic injection is an efficient protocol for goat somatic cell nuclear transfer.

Cloning: Eight Years After Dolly

It is now 8 years since the birth of Dolly, the first animal produced by nuclear transfer using a donor cell population established from an adult animal. During this time, the technique of nuclear transfer has been successfully applied to a range of mammalian species for the production of offspring using a plethora of donor cell types derived from both foetal and adult tissues. In addition, when coupled with genetic manipulation of the donor cells, transgenic offspring have been produced with a range of genetic modifications including gene knockouts and gene knockings. Despite the apparent successes of the technology, the efficiency of development to live offspring has remained low and developmental abnormalities still occur. The objectives of this paper are to review some of the successes and failures of the nuclear transfer procedure since the production of Dolly. In particular, we will review the major steps in the procedure and discuss studies from our laboratory and others which have modified the procedure in ways which may impact on development.

Activated bovine cytoplasts prepared by demecolcine-induced enucleation support development of nuclear transfer embryos in vitro

Demecolcine-induced enucleation (IE) of mouse oocytes has been shown to improve development to term of cloned mice. In this study, we characterized the kinetics and morphological progression of bovine oocytes subjected to IE, and evaluated their ability to support embryo development to the blastocyst stage after nuclear transfer (NT). In vitro matured bovine oocytes were parthenogenetically activated and subsequently exposed to demecolcine at various times post-activation. Onset and duration of demecolcine treatment significantly altered activation and IE frequencies, which varied from 7.1% to 100% and 33.3% to 91.7%, respectively, at 5 hr post-activation. A significant decrease in IE frequencies was observed at 17 hr post-activation (3.4%-46.1%), possibly due to reincorporation of chromosomes into the oocyte after incomplete second polar body (PB) extrusion. Oocytes were reconstructed by NT before (treatment 1) or after (treatment 2) activation and demecolcine treatment, and cultured in vitro. Cleavage (48.1%-54.2%) and blastocyst rates (15.7%-19%) were equivalent for the two treatments, as well as the total cell number in NT blastocysts. Furthermore, most of the blastocysts were completely diploid (treatment 2) or heteroploid but with a majority of diploid nuclei (treatment 1). Our results demonstrate that the IE method can be successfully used to produce enucleated bovine cytoplasts that are competent to support development to the blastocyst stage after NT. This technically simple approach may provide a more efficient method to enhance the success rate of NT procedures. Further studies are needed to improve the in vitro development efficiency and to expand our understanding of the mechanism(s) involved in demecolcine-induced enucleation.

Differential development of rabbit embryos derived from parthenogenesis and nuclear transfer

Parthenogenetic development (PA) is often used as a model to investigate activation protocols for nuclear transfer (NT) embryos. The objective of this study was to compare the development, as well as the dynamics of the nuclear materials and microtubules of PA and NT embryos following similar activation treatment. Our results demonstrate that, during parthenogenesis, activation through either electrical pulses or chemical stimulation alone resulted in low cleavage rates and compromised development. A combination of two sets of electrical pulses and a 2-h-exposure to chemical activation medium (5 microg/ml cycloheximide (CHX) and 2 mM 6-dimethylaminopurine (6-DMAP) in KSOM+0.1% BSA) could effectively activate rabbit oocytes, and resulted in a 99% (n = 73) cleavage rate with greater than 60% (n = 73) developing to blastocysts at day 4. However, the same activation protocol following NT resulted in only 65-72% of oocytes cleaved (depending on donor cell type), with less than 20% developing to the blastocyst stage. The differences observed between NT and PA embryos subjected to the same activation protocol were also evident in terms of the time required for their development to the blastocyst stage, as well as the cell numbers present in blastocysts at day 6. Furthermore, laser confocal microscopy revealed that pronuclear formation in the NT embryos was delayed by comparison to that in the parthenotes. In conclusion, our study suggests that an effective protocol for parthenogenesis cannot promise a comparable outcome for NT embryos.

Review of Enucleation Methods and Procedures Used in Animal Cloning: State of the Art

Enucleation of a recipient oocyte is a crucially important process for nuclear transfer efficiency. Several procedures have been developed and used in the production of nuclear transfer embryos. Although the use of excitable fluorochromes and ultraviolet (UV) light are commonly used for complete enucleation, they also pose the risk of damaging the maternal cytoplast. Telophase and chemically assisted enucleation have also been used for cloning, but the quality and quantity of the recipient cytoplasm varies with the procedure used. This paper reviews various methods used for enucleation, and discusses their benefits and limitations with respect to cloning efficiency.

Effect of Activation Methods for Bovine Oocytes after Intracytoplasmic Injection

A principal nuclear transfer procedure is to inject a donor cell into the perivitelline space in an enucleated oocyte and then electric fusion is performed (cell fusion method). The effects of activation methods in reconstructed oocytes for the serum-starved somatic cell cloning procedure were investigated in this study by means of intracytoplasmic injection (i.c.i.). Bovine oocytes were enucleated at 18-22 h for in vitro maturation, and subsequently the nucleus of cumulus cell collected from Japanese Black Bulls (JBCC) after 5-7 days of starved culture was injected into the recipient cytoplast with a piezo-micromanipulator. At 1 h after i.c.i., reconstructed oocytes were stimulated with ethanol (ET) or calcium ionophore (CaI) as the first activation treatment, followed by cycloheximide (CHX) or 6-dimethylaminopurin (DMAP) treatment as the second activation. In the experiment on the first activation method, the proportion of reconstructed oocytes developing to the blastocyst stage was significantly (p<0.01) higher in the ET activation method than that with CaI (10.5% and 4.7%, respectively). And the experiment on the second activation method after ET treatment showed similar proportions of blastocyst development in both CHX and DMAP treatments (5.9% and 2.8%, respectively). The present results indicated that combined activation treatment with ET and CHX was efficient for reconstructed bovine oocytes by i.c.i.

Nuclear transfer in practice

The technique of nuclear transfer (NT) allows the production of embryos, fetuses, and offspring from a range of embryonic, fetal, and adult derived cell types in a range of species. Successful development is dependent upon numerous factors, including type of recipient cell, source of recipient cell, method of reconstruction, activation, embryo culture, donor cell type, and donor and recipient cell cycle stages. The present review will discuss the uses of NT, the techniques presently available, and the factors affecting subsequent development.

Simplification of bovine somatic cell nuclear transfer by application of a zona-free manipulation technique

Contemporary nuclear transfer techniques often require the involvement of skilled personnel and extended periods of micromanipulation. Here, we present details of the development of a nuclear transfer technique for somatic cells that is both simpler and faster than traditional methods. The technique comprises the bisection of zona-free oocytes and the reconstruction of embryos comprising two half cytoplasts and a somatic cell by adherence using phytohaemagglutinin-P (PHA) followed by an electropulse and subsequent culture in microwells (termed WOWs--well of the well). The development of the system was based on results using parthenogenetic and in vitro fertilized zygotes in order to (a) select the optimal primary activation agent that induced the lowest lysis rate but highest parthenogenetic blastocyst yield, (b) evaluate the quantity and quality of zona-free blastocysts produced in WOWs, and (c) establish any potential embryotoxic effects of PHA-P. The initial data indicated that, of calcium ionophore A23187, ionomycin, and electropulse treatments as primary activation agents, the two former were equally efficient even with reduced exposure times. WOW-culture of zona-free versus zona-intact zygotes were not different in either blastocyst yield (44.6 +/- 2.4% versus 51.8 +/- 13.5% [mean +/- SEM]) or quality (126.3 +/- 48.4 versus 119.9 +/- 32.6 total cells), and exposure of zygotes to PHA-P did not reduce blastocyst yields compared to vehicle control (40.8 +/- 11.6% versus 47.1 +/- 20.8% of cultured oocytes). Subsequent application of the optimized technique for nuclear transfer using nine different granulosa cell primary cultures (cultured in 0.5% serum for 5-12 days) generated 37.6 +/- 3.9% (11 replicates; range, 16.4-58.1%) blastocysts per successfully fused and surviving reconstructed embryo (after activation), and 33.6 +/- 3.7% blastocysts per attempted reconstructed embryo. Mean day 7 total blastocyst cell numbers from 5 clone families was 128.1 +/- 15.3. The ongoing pregnancy rate of recipients each receiving two nuclear transfer blastocysts is 3/13 (23.1%) recipients pregnant at 5 months after transfer. These results suggest that the zona-free nuclear transfer technique generates blastocysts of equivalent quantity and quality compared to conventional micromanipulation methods, requires less technical expertise, is less time consuming and can double the daily output of reconstructed embryos (even after taking into consideration the rejection of the half oocytes containing the metaphase plate).

Hypertonic medium treatment for localization of nuclear material in bovine metaphase II oocytes

Oocytes enucleated at the second metaphase stage (MII) are often used as recipient cytoplasts for nuclear transfer. The oocyte's nuclear material has been traditionally removed blindly by aspirating the first polar body (Pb1) along with a portion of the cytoplasm. However, the Pb1-guided enucleation method is unreliable because the position of the Pb1 is variable. A previous study showed that pretreatment of mouse oocytes with 3% (0.09 M) sucrose allowed visualization of the metaphase spindle and chromosomes under standard light microscopy and led to a 100% enucleation rate. The same sucrose treatment, however, did not produce the same effect in bovine oocytes. In this study, we increased the concentration of sucrose to 0.3-0.9 M in PBS containing 20% fetal bovine serum (SPF) and found that the majority of the treated bovine oocytes (75%-86%) formed a small transparent bud into the perivitelline space, as compared with the 0.1 M sucrose (6%) or the no sucrose (0%) control groups. Staining of DNA with Hoechst 33342 revealed that these projections coincided with the position of the metaphase chromosomes in 100% of sucrose-treated oocytes, whereas only 31% of oocytes showed alignment of the position of Pb1 with their nuclear materials. Furthermore, 95% of oocytes treated in 0.3 M SPF were successfully enucleated by removing a small amount of cytoplasm adjacent to the projection. This is a significantly higher enucleation rate than that obtained by conventional Pb1-guided enucleation, even when a larger amount of cytoplasm was removed. For nuclear transfer, the enucleated oocytes treated with sucrose did not differ from the control oocytes in rates of fusion, cleavage, or development to blastocysts, or in the average cell numbers in blastocysts. This study demonstrated that 0.3 M sucrose treatment of bovine oocytes facilitates the localization of metaphase chromosomes under normal light microscopy and hence increases enucleation efficiency without compromising the in vitro development potential of cloned embryos by nuclear transfer.