Effects of Enucleation and Caffeine on Maturation-Promoting Factor (MPF) and Mitogen-Activated Protein Kinase (MAPK) Activities in Ovine Oocytes Used as Recipient Cytoplasts for Nuclear Transfer1

Beneficial effect of young oocytes for rabbit somatic cell nuclear transfer

This study was designed to examine the effect of the age of rabbit oocytes on the developmental potential of cloned embryos. The metaphase II oocytes used for nuclear transfer (NT) were collected at 10, 12, 14, and 16 h post-hCG injection (hpi). The total number of oocytes collected per donor (21.4-23.7) at 12 to 16 hpi was similar, but significantly higher than that collected at 10 hpi (16.2). Additionally, a significant improvement in blastocyst development was achieved with embryos generated by electrically mediated cell fusion (56.0%), compared to those from nuclear injection (13.1 %) (Experiment 1). Markedly higher blastocyst development (45.8-54.5%) was also achieved with oocytes collected at 10-12 hpi than from those collected 14-16 hpi (8.3-14.3%) (Experiment 2). In Experiment 3, the blastocyst rates of NT embryos derived from oocytes harvested 12 hpi (39.2-42.8 %) were significantly higher than from those collected at 16 hpi (6.8-8.4 %) (p < 0.05), regardless of the donor cell age. Kinase activity assays showed variable changes of activity in rabbit oocytes over the period of 10-16 hpi; however, there was no correlation with preimplantational development (blastocyst rate vs. MPF, R = 0.326; blastocyst rate vs. MAPK, R = -0.131). Embryo transfer of NT embryos utilizing 12 hpi oocytes resulted in one full-term but stillborn, and one live cloned rabbit; thus, an efficiency of 1.7 % (n = 117) (Experiment 4). These results demonstrated that NT utilizing relatively young rabbit oocytes, harvested at 10-12 h after hCG injection, was beneficial for the development of NT embryos.

Treatment with proteasome inhibitor MG132 during cloning improves survival and pronuclear number of reconstructed rat embryos

In several mammalian species including rats, successfully cloned animals have been generated using somatic cell nuclear transfer (SCNT). However, in the case of rats, additional treatment with MG132, a proteasome inhibitor, before enucleation of oocytes seems to be required for successful cloning because ovulated rat oocytes are spontaneously activated, and hence, their suppression is the key to successful cloning. A previous study on rats demonstrated that matured oocytes potentially possess lower cytostatic factor (CSF) activity compared to mouse oocytes, resulting in a low incidence of premature chromosome condensation in the reconstructed embryos after SCNT. It is known that mice having more than two pronuclei are generally observed in nuclear-transferred oocytes after induction of premature chromosome condensation, which implies successful reprogramming. This leads us to the hypothesis that MG132 treatment affects not only the inhibition of spontaneous activation but also the reprogramming and developmental ability of reconstructed rat embryos. If so, prolonged MG132 treatment during and/or after SCNT may further improve the survivability. However, the effect of MG132 treatment on reconstructed embryos after SCNT has been very limited in rats and other species. We show here that prolonged MG132 treatment during and after SCNT improves survival and the number of pronuclei in reconstructed rat embryos after activation. These reconstructed embryos treated before, during, and after SCNT showed significantly higher p34(cdc2) kinase activity involving CSF activity compared to that of the control embryos. On the other hand, p34(cdc2) kinase activity was not recovered in nuclear-transferred oocytes without MG132, which suggested that the enucleation had detrimental effects on the development of reconstructed oocytes. Taken together, MG132 treatment during SCNT increases survival and pronuclear numbers in reconstructed rat embryos via maintenance of high CSF activity. The data suggest that MG132 treatment is indispensable for at least rat SCNT.

Control of nuclear remodelling and subsequent in vitro development and methylation status of porcine nuclear transfer embryos

We attempted to control the nuclear remodelling of somatic cell nuclear transfer embryos (NTs) and examined their subsequent development and DNA methylation patterns in pigs. Porcine foetal fibroblasts were fused to enucleated oocytes treated with either 5 mM caffeine for 2.5 h or 0.5 mM vanadate for 0.5 h. After activation, NTs were cultured in vitro for 6 days to examine their development. The nuclear remodelling type of the reconstituted embryos was evaluated 1 h after fusion. Methylated DNA of in vitro-fertilised (IVF) embryos and NTs at various developmental stages and of donor cells was detected using a 5-methylcytosine (5-MeC) antibody. Caffeine-treated NTs induced premature chromosome condensation at a high rate (P<0.05), whereas most vanadate-treated NTs formed a pronucleus-like structure. Although cleavage rates to the two-cell stage did not differ among groups, delayed cleavage was observed in the vanadate-treated group. The blastocyst formation rate was significantly reduced by vanadate treatment compared with caffeine-treated and non-treated (control) NT groups (P<0.05). The apoptotic cell index of NT blastocysts was lower in the caffeine-treated group than in other groups (P<0.05). The methylation patterns were similar among NTs, but more hypermethylated DNA was observed at the four-cell stage of control and vanadate-treated NTs when compared with that in IVF embryos (P<0.05). Thus, the nuclear remodelling type controlled by caffeine or vanadate treatment can affect in vitro development and the methylation status of NTs in relation to nuclear reprogramming.

Unexpected nuclear localization of Cdc25C in bovine oocytes, early embryos, and nuclear-transferred embryos

It is clear from a wide range of studies that the nuclear/cytoplasmic distribution of Cdc25C has important functional consequences for cell cycle control. It is now admitted that in somatic cells, the localization of Cdc25C in the cytoplasm is required to maintain the cell in an interphasic state and that Cdc25C has to translocate to the nucleus just before M-phase to induce mitotic events. We characterized the expression and localization of Cdc25C during oocyte maturation, the first embryo mitosis, and the first steps of somatic cell nuclear transfer (SCNT) in cattle. We demonstrated that Cdc25C was expressed throughout the maturation process and the early development. We clearly showed that Cdc25C was localized in the nucleus at the germinal vesicle stage and during the early development until the blastocyst stage. However, the signal change in blastocyst and Cdc25C became cytoplasmic as is the case in somatic cells. Thus, oocytes and early embryonic cells presented a specific nuclear Cdc25C localization different from the one observed in somatic cells, suggesting that Cdc25C could have a particular localization/regulation in undifferentiated cells. Following SCNT, Cdc25C became nuclear as soon as the nucleus swelled, and this localization persisted until the blastocyst stage, as is the case in in vitro fertilized embryos. The Cdc25C nuclear localization appeared to constitute a major change, which could be associated with the reorganization of the somatic nucleus upon nuclear transfer.

Precision animal breeding

We accept that we are responsible for the quality of life of animals in our care. We accept that the activities of man affect all the living things with which we share this planet. But we are slow to realize that as a result we have a duty of care for all living things. That duty extends to the breeding of animals for which we are responsible. When animals are bred by man for a purpose, the aim should be to meet certain goals: to improve the precision with which breeding outcomes can be predicted; to avoid the introduction and advance of characteristics deleterious to well-being; and to manage genetic resources and diversity between and within populations as set out in the Convention on Biological Diversity. These goals are summed up in the phrase precision animal breeding. They should apply whether animals are bred as sources of usable products or services for medical or scientific research, for aesthetic or cultural considerations, or as pets. Modern molecular and quantitative genetics and advances in reproductive physiology provide the tools with which these goals can be met.

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.

Buffalos (Bubalus bubalis) Cloned by Nuclear Transfer of Somatic Cells1

Cloning of buffalos (Bubalus bubalis) through nuclear transfer is a potential alternative approach in genetic improvement of buffalos. However, to our knowledge, cloned offspring of buffalos derived from embryonic, fetal, or somatic cells have not yet been reported. Thus, factors affecting the nuclear transfer of buffalo somatic cells were examined, and the possibility of cloning buffalos was explored in the present study. Treatment of buffalo fibroblasts and granulosa cells with aphidicolin plus serum starvation resulted in more cells being arrested at the G0/G1 phase, the proportion of cells with DNA fragmentation being less, and the number of embryos derived from these cells that developed to blastocysts being greater. In addition, a difference was found in the development of embryos reconstructed with fetal fibroblasts from different individuals (P < 0.001). Forty-two blastocysts derived from granulosa cells and fetal fibroblasts were transferred into 21 recipient swamp buffalos, and 4 recipients were confirmed to be pregnant by rectal palpation on Day 60 of gestation. One recipient received two embryos from fetal fibroblasts aborted on Day 300 of gestation and delivered two female premature calves. Three recipients maintained pregnancy to term and delivered three female cloned calves after Days 338-349 of gestation. These results indicate that buffalo embryos derived from either fetal fibroblasts or granulosa cells can develop to the term of gestation and result in newborn calves.

Reprogramming following somatic cell nuclear transfer in primates is dependent upon nuclear remodeling

BACKGROUND

Somatic cell nuclear transfer (SCNT) requires cytoplast-mediated reprogramming of the donor nucleus. Cytoplast factors such as maturation promoting factor are implicated based on their involvement in nuclear envelope breakdown (NEBD) and premature chromosome condensation (PCC). Given prior difficulties in SCNT in primates using conventional protocols, we hypothesized that the ability of cytoplasts to induce nuclear remodeling was instrumental in efficient reprogramming.

METHODS

NEBD and PCC in monkey (Macaca mulatta) SCNT embryos were monitored by lamin A/C immunolabeling.

RESULTS

Initially, a persistent lamin A/C signal from donor cell nuclei after fusion with cytoplasts was observed indicative of incomplete NEBD following SCNT and predictive of developmental arrest. We then identified fluorochrome-assisted enucleation and donor cell electrofusion as likely candidates for inducing premature cytoplast activation and a consequent lack of nuclear remodeling. Modified protocols designed to prevent premature cytoplast activation during SCNT showed robust NEBD and PCC. Coincidently, over 20% of SCNT embryos reconstructed with fetal fibroblasts progressed to blastocysts. Similar results were obtained with other somatic cells. Reconstructed blastocysts displayed patterns of Oct-4 expression similar to fertilized embryos reflecting successful reprogramming.

CONCLUSIONS

Our results represent a significant breakthrough in elucidating the role of nuclear remodeling events in reprogramming following SCNT.

Effect of Enucleation on Inactivation of Cytostatic Factor Activity in Matured Rat Oocytes

In mammals, matured oocytes are arrested at the MII stage until fertilization, which is regulated by cytostaticfactor (CSF) activity. Maturation-promoting factor (MPF) and the mitogen-activated protein kinase (MAPK) pathway are known as candidates for CSF. Despite of the results that nuclear and perinuclear materials were dispensable for activation of MPF and MAPK in other species, our previous study in rats demonstrated that MPF activity was rapidly decreased after enucleation. We showed here for the first time that nuclear and perinuclear materials were indispensable for CSF activity in matured rat oocytes. In both cytoplasm-removed and enucleated oocytes, high activity of p34(cdc2) kinase was observed immediately after manipulation, but the activity of enucleated oocytes was dramatically reduced within 1 h. Cyclin B level was also decreased, corresponding with inactivation of p34(cdc2) kinase. In enucleated oocytes, the Mos level was dramatically decreased, and both MEK and MAPK dephosphorylation were also induced. A combined treatment with a proteasome inhibitor, MG132, and a protein phosphatase inhibitor, okadaic acid, dramatically improved both levels of p-MAPK and cyclin B in these enucleated oocytes. These data suggest that nuclear and perinuclear materials of matured rat oocytes suppress proteasome and protein phosphatase activation, which is indispensable for stability of CSF.

Contrasting effects of in vitro fertilization and nuclear transfer on the expression of mtDNA replication factors

Mitochondrial DNA (mtDNA) is normally only inherited through the oocyte. However, nuclear transfer (NT), the fusion of a donor cell with an enucleated oocyte, can transmit both donor cell and recipient oocyte mtDNA. mtDNA replication is under the control of nuclear-encoded replication factors, such as polymerase gamma (POLG) and mitochondrial transcription factor A (TFAM). These are first expressed during late preimplantation embryo development. To account for the persistence of donor cell mtDNA, even when introduced at residual levels (mtDNA(R)), we hypothesized that POLG and TFAM would be upregulated in intra- and interspecific (ovine-ovine) and intergeneric (caprine-ovine) NT embryos when compared to in vitro fertilized (IVF) embryos. For the intra- and interspecific crosses, PolGA (catalytic subunit), PolGB (accessory subunit), and TFAM mRNA were expressed at the 2-cell stage in both nondepleted (mtDNA(+)) and mtDNA(R) embryos with protein being expressed up to the 16-cell stage for POLGA and TFAM. However, at the 16-cell stage, there was significantly more PolGA expression in the mtDNA(R) embryos compared to their mtDNA(+) counterparts. Expression for all three genes first matched IVF embryos at the blastocyst stage. In the intergeneric model, POLG was upregulated during preimplantation development. Although these embryos did not persist further than the 16+-cell stage, significantly more mtDNA(R) embryos reached this stage. However, the vast majority of these embryos were homoplasmic for recipient oocyte mtDNA. The upreglation in mtDNA replication factors was most likely due to the donor cells still expressing these factors prior to NT.

Premature Chromosome Condensation Is Not Essential for Nuclear Reprogramming in Bovine Somatic Cell Nuclear Transfer1

Premature chromosome condensation (PCC) was believed to promote nuclear reprogramming and to facilitate cloning by somatic cell nuclear transfer (NT) in mammalian species. However, it is still uncertain whether PCC is necessary for the successful reprogramming of an introduced donor nucleus in cattle. In the present study, fused NT embryos were subjected to immediate activation (IA, simultaneous fusion and activation), delayed activation (DA, activation applied 4 h postfusion), and IA with aged oocytes (IAA, activation at the same oocyte age as group DA). The morphologic changes, such as nuclear swelling, the occurrence of PCC, and microtubule/aster formation, were analyzed in detail by laser-scanning confocal microscopy. When embryos were subjected to IA in both IA and IAA groups, the introduced nucleus gradually became swollen, and a pronuclear-like structure formed within the oocyte, but PCC was not observed. In contrast, delaying embryo activation resulted in 46.5%-91.2% of NT embryos exhibiting PCC. This PCC was observed beginning at 4 h postcell fusion and was shown as one, two, or multiple chromosomal complexes. Subsequently, a diversity of pronuclear-like structures existed in NT embryos, characterized as single, double, and multiple nuclei. In the oocytes exhibiting PCC, the assembled spindle structure was observed to be an interactive mass, closely associated with condensed chromosomes, but no aster had formed. Regardless of whether they were subjected to IA, IAA, or DA treatments, if the oocytes contained pronuclear-like structures, either one or two asters were observed in proximity to the nuclei. A significantly higher rate of development to blastocysts was achieved in embryos that were immediately activated (IA, 59.1%; IAA, 40.7%) than in those for which activation was delayed (14.2%). The development rate was higher in group IA than in group IAA, but it was not significant (P = 0.089). Following embryo transfer, there was no statistically significant difference in the pregnancy rates (Day 70) between two of the groups (group IA, 11.7%, n = 94 vs. group DA, 12.3%, n = 130; P > 0.05) or live term development (group IA, 4.3% vs. group DA, 4.6%; P > 0.05). Our study has demonstrated that the IA of bovine NT embryos results in embryos with increased competence for preimplantational development. Moreover, PCC was shown to be unnecessary for the reprogramming of a transplanted somatic genome in a cattle oocyte.

Oocyte cryopreservation: oocyte assessment and strategies for improving survival

Despite significant progress in cryopreservation of mammalian oocytes and embryos, many of the molecular and biochemical events that underlie this technology are poorly understood. In recent years, researchers have focused on obtaining viable oocytes that are developmentally competent. Even under the most favourable conditions, experimental approaches have achieved only limited success compared with fresh oocytes used in routine in vitro embryo production. Chilling injuries and toxic effects of the cryoprotectants are the major adverse consequences following cryoprocedures. To overcome these problems, different strategies have been developed for improving cryopreservation results. These strategies include reducing container volumes, increasing the thermal gradient, changing the cell surface/volume ratio, enhancing cryotolerance by supplementation with various additives or modifying the lipid composition of the oocyte membrane. In order to develop new strategies for reducing the various forms of stress associated with oocyte cryopreservation, it is fundamental to gain a better understanding of the major changes responsible for poor post-thaw survival. With this knowledge, we hope that oocyte cryostorage will become a fully reliable reproductive technique in the near future.