Ooplasm transfer and interspecies somatic cell nuclear transfer: heteroplasmy, pattern of mitochondrial migration and effect on embryo development

Although interspecies somatic cell nuclear transfer (iSCNT) has potential applications in the conservation of exotic species, an in vitro developmental block has been observed in embryos produced by this approach. It has been suggested that mitochondrial mismatch between donor cell and recipient oocyte could cause embryonic developmental arrest. A series of experiments was conducted to investigate the effect of mixed mitochondrial populations (heteroplasmy) on early development of iSCNT-derived cloned embryos. The effect of combining the techniques of ooplasm transfer (OT) and somatic cell nuclear transfer (SCNT) was examined by monitoring in vitro embryonic development; the presence and pattern of migration of foreign mitochondria after OT was analysed by MitoTracker staining. In addition, the effect of transferring caprine ooplasm (iOT) into the bovine enucleated oocytes used in iSCNT was analysed. There was no significant effect of the sequence of events (OT-SCNT or SCNT-OT) on the number of fused, cleaved, blastocyst or hatched blastocyst stage embryos. MitoTracker Green staining of donor oocytes used for OT confirmed the introduction of foreign mitochondria. The distribution pattern of transferred mitochondria most commonly remained in a distinct cluster after 12, 74 and 144 h of in vitro culture. When goat ooplasm was injected into bovine enucleated oocytes (iSCNT), there was a reduction (p < 0.05) in fusion (52 vs. 82%) and subsequent cleavage rates (55 vs. 78%). The procedure of iOT prior to iSCNT had no effect in overcoming the 8- to 16-cell in vitro developmental block, and only parthenogenetic cow and goat controls reached the blastocyst (36 and 32%) and hatched blastocyst (25 and 12%) stages, respectively. This study indicates that when foreign mitochondria are introduced at the time of OT, these organelles tend to remain as distinct clusters without relocation after a few mitotic divisions. Although the bovine cytoplast appears capable of supporting mitotic divisions after iOT-iSCNT, heteroplasmy or mitochondrial incompatibilities may affect nuclear-ooplasmic events occurring at the time of genomic activation.

Nuclear transfer in cats and its application

Nuclear transfer (NT) technology is typically used for generating identical individuals, but it is also a powerful resource for understanding the cellular and molecular aspects of nuclear reprogramming. Most recently, the procedure has been used in humans for producing patient-specific embryonic stem cells. The successful application of NT in cats was demonstrated by the birth of domestic and non-domestic cloned kittens at a similar level of efficiency to that reported for other mammalian species. In cats, it has been demonstrated that either in vivo or in vitro matured oocytes can be used as donor cytoplasts. The length of in vitro oocyte maturation affects in vitro development of reconstructed embryos, and oocytes matured in vitro for shorter periods of time are the preferred source of donor cytoplasts. For NT, cat somatic cells can be synchronized into the G0/G1 phase of the cell cycle by using different methods of cell synchronization without affecting the frequency of in vitro development of cloned embryos. Also, embryo development to the blastocyst stage in vitro is not influenced by cell type, but the effect of cell type on the percentage of normal offspring produced requires evaluation. Inter-species NT has potential application for preserving endangered felids, as live offspring of male and female African wildcats (AWC, Felis silvestris lybica) have been born and pregnancies have been produced after transferring black-footed cat (Felis nigripes) cloned embryos into domestic cat (Felis silvestris catus) recipients. Also, successful in vitro embryo development to the blastocyst stage has been achieved after inter-generic NT of somatic cells of non-domestic felids into domestic cat oocytes, but no viable progeny have been obtained. Thus, while cat cytoplasm induces early nuclear remodeling of cell nuclei from a different genus, the high incidence of early embryo developmental arrest may be caused by abnormal nuclear reprogramming. Fetal resorption and abortions were frequently observed at various stages of pregnancy after transfer of AWC cloned embryos into domestic cat recipients. Abnormalities, such as abdominal organ exteriorization and respiratory failure and septicemia were the main causes of death in neonatal cloned kittens. Nonetheless, several live domestic and AWC cloned kittens have been born that are seemingly normal and healthy. It is important to continue evaluating these animals throughout their lives and to examine their capability for natural reproduction.

Assaying the probabilities of obtaining maternally inherited heteroplasmy as the basis for modeling OXPHOS diseases in animals

Gross alterations in cell energy metabolism underlie manifestations of hereditary OXPHOS (oxidative phosphorylation) diseases, many of which depend on proportion of mutant mitochondrial DNA (mtDNA) in tissues. An animal model of OXPHOS disease with maternal inheritance of mitochondrial heteroplasmy might help understanding the peculiarities of abnormal mtDNA distribution and its effect on pre- and postnatal development. Previously we obtained mice that carry human mtDNA in some tissues. It co-existed with murine mtDNA (heteroplasmy) and was transmitted maternally to the progeny of animals developed from zygotes injected with human mitochondria. To analyze the probability of obtaining heteroplasmic mice we increased the number of experiments with early embryos and obtained more specimens from F1. About 33% of zygotes injected with human mtDNA developed into post-implantation embryos (7th-13th days). Lower amount of such developed into neonate mice (ca. 21%). Among post-implantation embryos and in generations F0 and F1 percentages of human mtDNA-carriers were ca. 14-16%. Such percentages are sufficient for modeling maternally inherited heteroplasmy in small animal groups. More data are needed to understand the regularities of anomalous mtDNA distribution among cells and tissues and whether heart and muscles frequently carrying human mtDNA in our experiments are particularly susceptible to heteroplasmy.

Obtaining mice that carry human mitochondrial DNA transmitted to the progeny

To study human diseases associated with mutations in mitochondrial DNA one needs an animal model in which the distribution of abnormal mtDNA and its impact on the phenotype might be followed. We isolated human mitochondria from HepG2 cell culture and microinjected them into murine zygotes, upon which those were transplanted to the pseudopregnant mice. PCR with species-specific primers allowed detecting human mtDNA in the tissues of 7-13-day embryos. No serious alterations in the development of transmitochondrial embryos were noticed. Among various organs/tissues of the 13-day embryos, human mtDNA was detected only in the heart, skeletal muscles, and stomach, which is in line with its uneven distribution among the blastomeres of an early mouse embryo that we described previously. In four recipient females, the microinjected zygotes were allowed to develop to term, the four neonate males of their joint litter were sacrificed, and in three of them human mtDNA was detected in the heart, skeletal muscles, stomach, brain, testes, and bladder. Six females of that joint litter were grown and mated to intact males. In the progeny (F1) of one of the females two mice were carrying human mtDNA in the heart, skeletal muscles, stomach, brain, lungs, uterus, ovaries, and kidneys. The study confirms the possibility to obtain transmitochondrial mice carrying human mtDNA that is transmitted to the animals of the next generation. Our results also indicate that among the organs to which human mtDNA is distributed some are more likely to receive it than others.

Proliferation of donor mitochondrial DNA in nuclear transfer calves (Bos taurus) derived from cumulus cells

In embryos derived by nuclear-transfer (NT), fusion of donor cell and recipient oocyte caused mitochondrial heteroplasmy. Previous studies from other laboratories have reported either elimination or maintenance of donor-derived mitochondrial DNA (mtDNA) from somatic cells in cloned animals. Here we examined the distribution of donor mtDNA in NT embryos and calves derived from somatic cells. Donor mitochondria were clearly observed by fluorescence labeling in the cytoplasm of NT embryos immediately after fusion; however, fluorescence diminished to undetectable levels at 24 hr after nuclear transfer. By PCR-mediated single-strand conformation polymorphism (PCR-SSCP) analysis, donor mtDNAs were not detected in the NT embryos immediately after fusion (less than 3-4%). In contrast, three of nine NT calves exhibited heteroplasmy with donor cell mtDNA populations ranging from 6 to 40%. These results provide the first evidence of a significant replicative advantage of donor mtDNAs to recipient mtDNAs during the course of embryogenesis in NT calves from somatic cells.