Donor-host mitochondrial compatibility improves efficiency of bovine somatic cell nuclear transfer

Background  

The interaction between the karyoplast and cytoplast plays an important role in the efficiency of somatic cell nuclear transfer (SCNT), but the underlying mechanism remains unclear. It is generally accepted that in nuclear transfer embryos, the reprogramming of gene expression is induced by epigenetic mechanisms and does not involve modifications of DNA sequences. In cattle, oocytes with various mitochondrial DNA (mtDNA) haplotypes usually have different ATP content and can further affect the efficiency of in vitro production of embryos. As mtDNA comes from the recipient oocyte during SCNT and is regulated by genes in the donor nucleus, it is a perfect model to investigate the interaction between donor nuclei and host oocytes in SCNT.     

Segregation of donor cell mitochondrial DNA in gaur-bovine interspecies somatic cell nuclear transfer embryos, fetuses and an offspring

The fate of foreign mitochondrial DNA (mtDNA) following somatic cell nuclear transfer (SCNT) is still controversial. In this study, we examined the transmission of the heteroplasmic mtDNA of gaur donor cells and recipient bovine oocytes to an offspring and aborted and mummified fetuses at various levels during the development of gaur-bovine interspecies SCNT (iSCNT) embryos. High levels of the donor cell mtDNA were found in various tissue samples but they did not have any beneficial effect to the survival of iSCNT offspring. However, the factors on mtDNA inheritance are unique for each iSCNT experiment and depend on the recipient oocyte and donor cell used, which might play an important role in the efficiency of iSCNT.     

Association between mitochondrial DNA haplotype compatibility and increased efficiency of bovine intersubspecies cloning

Reconstructed embryos derived from intersubspecies somatic cell nuclear transfer(SCNT) have poorer developmental potential than those from intrasubspecies SCNT.Based on our previous study that Holstein dairy bovine(HD) mitochondrial DNA(mtDNA) haplotype compatibility between donor karyoplast and recipient cytoplast is crucial for SCNT embryo development,we performed intersubspecies SCNT using HD as donor karyoplast and Luxi yellow heifer(LY) as recipient cytoplast according to mtDNA haplotypes determined...     

Aberrant heteroplasmic transmission of mtDNA in cloned pigs arising from double nuclear transfer

Double nuclear transfer begins with the transfer of nuclear DNA from a donor cell into an enucleated recipient oocyte. This reconstructed oocyte is allowed to develop to the pronuclear stage, where the pronuclei are transferred into an enucleated zygote. This reconstructed zygote is then transferred to a surrogate sow. The genetic integrity of cloned offspring can be compromised by the transmission of mitochondrial DNA from the donor cell, the recipient oocyte and the recipient zygote. We have verified through the use of sequence analysis, restriction fragment length polymorphism analysis, allele specific PCR and primer extension polymorphism analysis that following double nuclear transfer the donor cell mtDNA is eliminated. However, it is likely that the recipient oocyte and zygote mitochondrial DNA are transmitted to the offspring, indicating bimaternal mitochondrial DNA transmission. This pattern of mtDNA inheritance is similar to that observed following cytoplasmic transfer and violates the strict unimaternal inheritance of mitochondrial DNA to offspring. This form of transmission raises concerns regarding the genetic integrity of cloned offspring and their uses in studies that require metabolic analysis or a stable genetic environment where only one variable is under analysis, such as in knockout technology.     

Association between mitochondrial DNA haplotype compatibility and increased efficiency of bovine intersubspecies cloning

Reconstructed embryos derived from intersubspecies somatic cell nuclear transfer (SCNT) have poorer developmental potential than those from intrasubspecies SCNT. Based on our previous study that Holstein dairy bovine (HD) mitochondrial DNA (mtDNA) haplotype compatibility between donor karyoplast and recipient cytoplast is crucial for SCNT embryo development, we performed intersubspecies SCNT using HD as donor karyoplast and Luxi yellow heifer (LY) as recipient cytoplast according to mtDNA haplotypes determined by polymerase chain reactionrestriction fragment length polymorphism (PCR-RFLP) analysis. The results demonstrated that intersubspecies mtDNA homotype SCNT embryos had higher pre- and post-implantation developmental competence than intrasubspecies mtDNA heterotype embryos as well as improved blastocyst reprogramming status, including normal H3K9 dimethylation pattern and promoter hypomethylation of pluripotent genes such as Oct4 and Sox2, suggesting that intersubspecies SCNT using LY oocytes maintains HD cloning efficiency and may reprogram HD nuclei to develop into a normal cloned animal ultimately. Our results indicated that karyoplast-cytoplast interactions and mtDNA haplotype compatibility may affect bovine intersubspecies SCNT efficiency. This study on bovine intersubspecies SCNT is valuable for understanding the mechanisms of mtDNA haplotype compatibility between karyoplast and cytoplast impacting the bovine SCNT efficiency, and provides an alternative and economic resource for HD cloning.     

Mitochondrial DNA heteroplasmy in ovine fetuses and sheep cloned by somatic cell nuclear transfer

Background  

The mitochondrial DNA (mtDNA) of the cloned sheep "Dolly" and nine other ovine clones produced by somatic cell nuclear transfer (SCNT) was reported to consist only of recipient oocyte mtDNA without any detectable mtDNA contribution from the nucleus donor cell. In cattle, mouse and pig several or most of the clones showed transmission of nuclear donor mtDNA resulting in mitochondrial heteroplasmy. To clarify the discrepant transmission pattern of donor mtDNA in sheep clones we analysed the mtDNA composition of seven fetuses and five lambs cloned from fetal fibroblasts.     

Risk assessment of porcine reproductive and respiratory syndrome virus (PRRSV) transmission via somatic cell nuclear transfer (SCNT) embryo production using oocytes from commercial abattoirs

Somatic cell nuclear transfer (SCNT) technology has become a powerful tool for reproductive biology to preserve and propagate valuable genetics for livestock. Embryo production through SCNT involves enucleation of the oocyte and insertion of a somatic donor cell into the oocyte. These procedures lead to a few small openings on the zona pellucida that may elevate risk of viral infection for the produced SCNT embryos. The oocytes used for SCNT are mainly obtained from abattoirs where viral contamination is almost inevitable. Therefore, a systematic evaluation of risk of disease transmission through SCNT embryo production is necessary prior large scale implementation of this technology in the livestock industry. The objective of the current study was to evaluate the risk of disease transmission via SCNT embryo production and transfer by testing for the presence of porcine reproductive and respiratory syndrome virus (PRRSV) throughout the process of SCNT embryo production. The presence of PRRSV in each step of SCNT embryo production, from donor cells to pre-implantation SCNT embryo culture, was carefully examined using a real-time PCR assay with a sensitivity of five copies per-reaction. All 114 donor cell lines derived from pig skin tissue over a period of 7 years in our facility tested negative for PRRSV. Out of the 68 pooled follicular fluid samples collected from 736 ovaries, only four (5.9%) were positive indicating a small amount of viral molecule present in the oocyte donor population. All 801 Day 7 SCNT embryos produced in four separate trials and over 11,571 washed oocytes obtained in 67 batches over 10 months tested negative. These oocytes were collected from multiple abattoirs processing animals from areas with high density of pig population and correspond to a donor population of over 5828 individuals. These results indicate that the oocytes from abattoirs were free of PRRSV infection and therefore could be safely used for in vitro embryo production. Additionally, the established SCNT embryo production system, including donor cell testing, oocytes decontamination, and pathogen free embryo reconstruction and culturing, bears no risk of PRRSV transmission.     

Effects of the removal of cytoplasm on the development of early cloned bovine embryos

Oocyte cytoplasm plays a prominent role in cloned embryonic development. To investigate the influence of oocyte cytoplasmic amount on cloned embryo development, we generated bovine somatic cell nuclear transfer (SCNT) embryos containing high (30-40% of the cytoplasm was removed), medium (15-25% of the cytoplasm was removed) and low (<10% of the cytoplasm was removed) nucleocytoplasmic volume ratios (N/C) using enucleated metaphase II oocyte as recipient, and fibroblast as donor nucleus, and analyzed the expression levels of ND1, Cytb and ATPase6, as well as the embryonic quality. The results indicated: (1) the process of embryonic development was not influenced by <40% of cytoplasm removal; (2) the rate of blastocyst formation, the total number of blastomere and the ratio of ICM to TE were inversely proportional to the N/C; (3) SCNT embryos with reduced volume equal to 75-85% or >90% of an intact oocyte volume showed similar karyotype structure of the donor cells; (4) the number of mtDNA copy was larger in low N/C embryos than that in medium or high N/C embryos, and the expression levels of each gene hardly varied from the 2-cell to 8-cell stage, while the expression levels increased dramatically at the blastocyst stage; (5) from 16-cell to the blastocyst stage, the change of the expression level of each gene was not significant between low N/C embryos and IVF embryos, but it was more significant than those of high or medium N/C embryos. The results suggest that the decrease of mtDNA copy number and mitochondrial gene expression may be related to the impairment in early embryonic development, and removal of <10% adjacent cytoplasm volume may be optimal for bovine SCNT embryo development.     

Effect of individual heifer oocyte donors on cloned embryo development in vitro

The aim of this study was to determine the effect of individual oocyte donors on cloned embryo development in vitro. Five Holstein heifers of varied genetic origins were subject to ovum pick up (OPU) once weekly. In total, 913 oocytes were recovered from 1304 follicles. A mean of 7.7+/-0.4 oocytes was recovered per session per animal. Individual mean oocyte production varied significantly in quantity but not in quality (morphological categories) among heifers. Oocytes from individual heifers were used as recipient cytoplasm for somatic cell nuclear transfer (SCNT). Cumulus cells, collected from a single Holstein cow genetically unrelated to the oocyte donor, were used as donor cells. Although the percentage of reconstructed embryos that started to cleave was nearly constant, the percentage of cleaved embryos that developed into blastocysts showed clear individual heifer variation (61%, 51%, 31%, 28% and 24%, respectively), with a mean of 38% showing blastocyst formation. In vitro fertilization (IVF) was also conducted with oocyte from the same heifers used in SCNT. A variation of blastocyst production among individual heifers was also shown in the IVF experiment, but the rank of oocyte donor based on the blastocyst rate was changed. In conclusion, individual oocyte donor may have an effect on cloned embryo development in vitro, which differed from the effect on IVF embryos.     

Maternal-fetal transplacental leakage of mitochondrial DNA in bovine nuclear transfer pregnancies: potential implications for offspring and recipients

The synepitheliochorial placenta of ruminants is constructed of multiple tissue layers that separate maternal and fetal blood. In nuclear transfer cloned ruminants, however, placental anomalies such as abnormal vascular development and hemorrhagic cotyledons have been reported. We have investigated the possible exchange of genetic material between somatic cell nuclear transfer cloned (SCNT) bovine fetuses and recipients at day 80 of gestation using mitochondrial DNA (mtDNA) as a marker. Twenty-three recovered SCNT-fetuses and their recipients were screened for divergent and thus informative mtDNA combinations. Twenty-one fetuses generated by in vitro fertilization (IVF) or multiple ovulation embryo transfer (MOET) and the corresponding recipients served as controls. A search for recipient mtDNA haplotype in DNA extracts from fetal blood by PCR-RFLP analysis revealed three cases of chimerism (two SCNT, one IVF) among a total of 19 informative fetus-recipient pairs (eight SCNT, seven IVF, four MOET). Placental anomalies have also been observed in some IVF fetuses and the present data therefore suggests transplacental leakage of cell components or cells from the recipient into some fetuses generated by in vitro techniques. Further studies are necessary to determine (i) the nature of leaked material, (ii) whether there is bi-directional leakage, and (iii) whether leaked material is present in recipients and calves after parturition, i.e. whether leakage takes place in vivo. If recipients were chimeric for DNA or cells derived from genetically modified SCNT (or IVF) embryos, their subsequent utilization might be affected.     

The impact of chromosomal alteration on embryo development

Chromosome alterations, such as those affecting telomere erosion, predictably occur with each cell division, others, which involve changes to the expression and replication of the X-chromosome occur at particular stages of development, while those that involve loss or gain of chromosomes occur in a random and so far unpredictable manner. The production of embryos in vitro and by somatic cell nuclear transfer (SCNT) has been associated with altered expression of marker genes on the X-chromosome and an increased incidence of chromosomally abnormal cells during early development. In the case of SCNT embryos chromosome abnormalities may be associated with the nuclear donor cell. Telomere rebuilding subsequent to SCNT appears to vary according to species and type of donor cell used. It is speculated that the rate of telomere erosion and incidence of chromosome abnormalities affects developmental potential of early embryos and may be potential predictors of developmental outcome.     

Interspecies somatic cell nuclear transfer is dependent on compatible mitochondrial DNA and reprogramming factors

Interspecies somatic cell nuclear transfer (iSCNT) involves the transfer of a nucleus or cell from one species into the cytoplasm of an enucleated oocyte from another. Once activated, reconstructed oocytes can be cultured in vitro to blastocyst, the final stage of preimplantation development. However, they often arrest during the early stages of preimplantation development; fail to reprogramme the somatic nucleus; and eliminate the accompanying donor cell's mitochondrial DNA (mtDNA) in favour of the recipient oocyte's genetically more divergent population. This last point has consequences for the production of ATP by the electron transfer chain, which is encoded by nuclear and mtDNA. Using a murine-porcine interspecies model, we investigated the importance of nuclear-cytoplasmic compatibility on successful development. Initially, we transferred murine fetal fibroblasts into enucleated porcine oocytes, which resulted in extremely low blastocyst rates (0.48%); and failure to replicate nuclear DNA and express Oct-4, the key marker of reprogramming. Using allele specific-PCR, we detected peak levels of murine mtDNA at 0.14±0.055% of total mtDNA at the 2-cell embryo stage and then at ever-decreasing levels to the blastocyst stage (<0.001%). Furthermore, these embryos had an overall mtDNA profile similar to porcine embryos. We then depleted porcine oocytes of their mtDNA using 10 μM 2',3'-dideoxycytidine and transferred murine somatic cells along with murine embryonic stem cell extract, which expressed key pluripotent genes associated with reprogramming and contained mitochondria, into these oocytes. Blastocyst rates increased significantly (3.38%) compared to embryos generated from non-supplemented oocytes (P<0.01). They also had significantly more murine mtDNA at the 2-cell stage than the non-supplemented embryos, which was maintained throughout early preimplantation development. At later stages, these embryos possessed 49.99±2.97% murine mtDNA. They also exhibited an mtDNA profile similar to murine preimplantation embryos. Overall, these data demonstrate that the addition of species compatible mtDNA and reprogramming factors improves developmental outcomes for iSCNT embryos.     

Large scale in vivo risk assessment of bovine viral diarrhea virus (BVDV) transmission through transfer of bovine embryos produced via somatic cell nuclear transfer (SCNT)

The objective was to use the bovine viral diarrhea virus (BVDV) as a model to assess the risk of infectious disease transmission in the system of in vitro embryo production and transfer via somatic cell nuclear transfer (SCNT) technology. The risks of BVDV transmission in the SCNT embryo production were previously evaluated [1]. In that in vitro study, following standard operating procedures (SOP), including pre-nuclear transfer donor cell testing, oocyte decontamination and virus-free cell and embryo culture conditions, SCNT embryos produced were free of detectable viral RNA. The current study focused on the evaluation of the potential risk of disease transmission from SCNT embryos to recipients, and the risk of producing persistently infected animals via SCNT embryo transfer. Blood samples were collected from 553 recipients of SCNT embryos and 438 cloned calves and tested for the presence of BVDV viral RNA via a sensitive real time PCR method. All samples tested were negative. These results, in conjunction with the previous in vitro study, confirmed that the established SCNT embryo production and transfer system is safe and presents no detectable risk of disease transmission.     

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.