Development in vitro and mitochondrial fate of interspecies cloned embryos

Quantitative analysis of mitochondrial DNA in porcine-mouse cloned embryos

The aim of the research is to identify that porcine oocytes can function as recipients for interspecies cloning and have the ability to develop to blastocysts. Furthermore each mitochondrial DNA (mtDNA) in interspecises cloned embryos was analyzed. For the study, mouse-porcine and porcine-porcine cloned embryos were produced with mouse fetal fibroblasts (MFF) and porcine fetal fibroblasts (PFF), respectively, introduced as donor cells into enucleated porcine oocytes. The developmental rate and cell numbers of blastocysts between intraspecies porcine-porcine and interspecies mouse-porcine cloned embryos were compared and real-time polymerase chain reaction (PCR) was performed for the estimate of mouse and porcine mtDNA copy number in mouse-porcine cloned embryos at different stages.There was no significant difference in the developmental rate or total blastocyst number between mouse-porcine cloned embryos and porcine-porcine cloned embryos (11.1 ± 0.9%, 25 ± 3.5 vs. 10.1 ± 1.2%, 24 ± 6.3). In mouse-porcine reconstructed embryos, the copy numbers of mouse somatic cell-derived mtDNA decreased between the 1-cell and blastocyst stages, whereas the copy number of porcine oocyte-derived mtDNA significantly increased during this period, as assessed by real-time PCR analysis. In our real-time PCR analysis, we improved the standard curve construction-based method to analyze the level of mtDNA between mouse donor cells and porcine oocytes using the copy number of mouse beta-actin DNA as a standard. Our findings suggest that mouse-porcine cloned embryos have the ability to develop to blastocysts in vitro and exhibit mitochondrial heteroplasmy from the 1-cell to blastocyst stages and the mouse-derived mitochondria can be gradually replaced with those of the porcine oocyte in the early developmental stages of mouse-porcine cloned embryos.

Extranuclear Inheritance of Mitochondrial Genome and Epigenetic Reprogrammability of Chromosomal Telomeres in Somatic Cell Cloning of Mammals

The effectiveness of somatic cell nuclear transfer (SCNT) in mammals seems to be still characterized by the disappointingly low rates of cloned embryos, fetuses, and progeny generated. These rates are measured in relation to the numbers of nuclear-transferred oocytes and can vary depending on the technique applied to the reconstruction of enucleated oocytes. The SCNT efficiency is also largely affected by the capability of donor nuclei to be epigenetically reprogrammed in a cytoplasm of reconstructed oocytes. The epigenetic reprogrammability of donor nuclei in SCNT-derived embryos appears to be biased, to a great extent, by the extranuclear (cytoplasmic) inheritance of mitochondrial DNA (mtDNA) fractions originating from donor cells. A high frequency of mtDNA heteroplasmy occurrence can lead to disturbances in the intergenomic crosstalk between mitochondrial and nuclear compartments during the early embryogenesis of SCNT-derived embryos. These disturbances can give rise to incorrect and incomplete epigenetic reprogramming of donor nuclei in mammalian cloned embryos. The dwindling reprogrammability of donor nuclei in the blastomeres of SCNT-derived embryos can also be impacted by impaired epigenetic rearrangements within terminal ends of donor cell-descended chromosomes (i.e., telomeres). Therefore, dysfunctions in epigenetic reprogramming of donor nuclei can contribute to the enhanced attrition of telomeres. This accelerates the processes of epigenomic aging and replicative senescence in the cells forming various tissues and organs of cloned fetuses and progeny. For all the above-mentioned reasons, the current paper aims to overview the state of the art in not only molecular mechanisms underlying intergenomic communication between nuclear and mtDNA molecules in cloned embryos but also intrinsic determinants affecting unfaithful epigenetic reprogrammability of telomeres. The latter is related to their abrasion within somatic cell-inherited chromosomes.

Dynamic characteristics of the mitochondrial genome in SCNT pigs

Most animals generated by somatic cell nuclear transfer (SCNT) are heteroplasmic; inheriting mitochondrial genetics from both donor cells and recipient oocytes. However, the mitochondrial genome and functional mitochondrial gene expression in SCNT animals are rarely studied. Here, we report the production of SCNT pigs to study introduction, segregation, persistence and heritability of mitochondrial DNA transfer during the SCNT process. Porcine embryonic fibroblast cells from male and female Xiang pigs were transferred into enucleated oocytes from Yorkshire or Landrace pigs. Ear biopsies and blood samples from SCNT-derived pigs were analyzed to characterize the mitochondrial genome haplotypes and the degree of mtDNA heteroplasmy. Presence of nuclear donor mtDNA was less than 5% or undetectable in ear biopsies and blood samples in the majority of SCNT-derived pigs. Yet, nuclear donor mtDNA abundance in 14 tissues in F0 boars was as high as 95%. Additionally, mtDNA haplotypes influenced mitochondrial respiration capacity in F0 fibroblast cells. Our results indicate that the haplotypes of recipient oocyte mtDNA can influence mitochondrial function. This leads us to hypothesize that subtle developmental influences from SCNT-derived heteroplasmy can be targeted when using donor and recipient mitochondrial populations from breeds of swine with limited evolutionary divergence.

Sheep and Goat Genome Engineering: From Random Transgenesis to the CRISPR Era

Sheep and goats are valuable livestock species that have been raised for their production of meat, milk, fiber, and other by-products. Due to their suitable size, short gestation period, and abundant secretion of milk, sheep and goats have become important model animals in agricultural, pharmaceutical, and biomedical research. Genome engineering has been widely applied to sheep and goat research. Pronuclear injection and somatic cell nuclear transfer represent the two primary procedures for the generation of genetically modified sheep and goats. Further assisted tools have emerged to enhance the efficiency of genetic modification and to simplify the generation of genetically modified founders. These tools include sperm-mediated gene transfer, viral vectors, RNA interference, recombinases, transposons, and endonucleases. Of these tools, the four classes of site-specific endonucleases (meganucleases, ZFNs, TALENs, and CRISPRs) have attracted wide attention due to their DNA double-strand break-inducing role, which enable desired DNA modifications based on the stimulation of native cellular DNA repair mechanisms. Currently, CRISPR systems dominate the field of genome editing. Gene-edited sheep and goats, generated using these tools, provide valuable models for investigations on gene functions, improving animal breeding, producing pharmaceuticals in milk, improving animal disease resistance, recapitulating human diseases, and providing hosts for the growth of human organs. In addition, more promising derivative tools of CRISPR systems have emerged such as base editors which enable the induction of single-base alterations without any requirements for homology-directed repair or DNA donor. These precise editors are helpful for revealing desirable phenotypes and correcting genetic diseases controlled by single bases. This review highlights the advances of genome engineering in sheep and goats over the past four decades with particular emphasis on the application of CRISPR/Cas9 systems.

The development and expression of pluripotency genes in embryos derived from nuclear transfer and in vitro fertilization

Somatic cell nuclear transfer can be used to produce embryonic stem (ES) cells, cloned animals, and can even increase the population size of endangered animals. However, the application of this technique is limited by the low developmental rate of cloned embryos, a situation that may result from abnormal expression of some zygotic genes. In this study, sheep–sheep intra-species cloned embryos, goat–sheep inter-species cloned embryos, or sheep in vitro fertilized embryos were constructed and cultured in vitro and the developmental ability and expression of three pluripotency genes, SSEA-1, Nanog and Oct4, were examined. The results showed firstly that the developmental ability of in vitro fertilized embryos was significantly higher than that of cloned embryos. In addition, the percentage of intra-species cloned embryos that developed to morula or blastocyst stages was also significantly higher than that of the inter-species cloned embryos. Secondly, all three types of embryos expressed SSEA-1 at the 8-cell and morula stages. At the 8-cell stage, a higher percentage of in vitro fertilized embryos expressed SSEA-1 than occurred for cloned embryos. However, at the morula stage, all detected embryos could express SSEA-1. Thirdly, the three types of embryos expressed Oct4 mRNA at the morula and blastocyst stages, and embryos at the blastocyst stage expressed Nanog mRNA. The rate of expression of Oct4 and Nanog mRNA at these developmental stages was higher in in vitro fertilized embryos than in cloned embryos. These results indicated that, during early development, the failure to reactivate some pluripotency genes maybe is a reason for the low cloning efficiency found with cloned embryos.

Mitochondrial DNA transmission and confounding mitochondrial influences in cloned cattle and pigs

Although somatic cell nuclear transfer (SCNT) is a powerful tool for production of cloned animals, SCNT embryos generally have low developmental competency and many abnormalities. The interaction between the donor nucleus and the enucleated ooplasm plays an important role in early embryonic development, but the underlying mechanisms that negatively impact developmental competency remain unclear. Mitochondria have a broad range of critical functions in cellular energy supply, cell signaling, and programmed cell death; thus, affect embryonic and fetal development. This review focuses on mitochondrial considerations influencing SCNT techniques in farm animals. Donor somatic cell mitochondrial DNA (mtDNA) can be transmitted through what has been considered a "bottleneck" in mitochondrial genetics via the SCNT maternal lineage. This indicates that donor somatic cell mitochondria have a role in the reconstructed cytoplasm. However, foreign somatic cell mitochondria may affect the early development of SCNT embryos. Nuclear-mitochondrial interactions in interspecies/intergeneric SCNT (iSCNT) result in severe problems. A major biological selective pressure exists against survival of exogenous mtDNA in iSCNT. Yet, mtDNA differences in SCNT animals did not reflect transfer of proteomic components following proteomic analysis. Further study of nuclear-cytoplasmic interactions is needed to illuminate key developmental characteristics of SCNT animals associated with mitochondrial biology.

Development of interspecies cloned embryos reconstructed with rabbit (Oryctolagus cuniculus) oocytes and cynomolgus monkey (Macaca fascicularis) fibroblast cell nuclei

Interspecies somatic cell nuclear transfer (ISCNT) has been proposed as a technique to produce cloned offspring of endangered species as well as to investigate nucleus-cytoplasm interactions in mammalian embryo. However, it is still not known which embryo culture medium is optimal for ISCNT embryos for the nuclear donor or the oocyte recipient. We assessed the effects of the culture medium on the developmental competence of the ISCNT embryos by introducing cynomolgus monkey (Macaca fascicularis) fibroblast nuclei into enucleated rabbit (Oryctolagus cuniculus) oocytes (monkey-rabbit embryo). The monkey-rabbit ISCNT embryos that were cultured in mCMRL-1066 developed to the blastocyst stage, although all monkey-rabbit ISCNT embryos cultured in M199 were arrested by the 4-cell stage. When monkey-rabbit ISCNT and rabbit-rabbit somatic cell nuclear transfer (SCNT) embryos were cultured in mCMRL-1066, the blastocyst cell numbers of the monkey-rabbit ISCNT embryos corresponded to the cell numbers of the control rabbit-rabbit SCNT embryos, which were produced from a rabbit fibroblast nucleus and an enucleated rabbit oocyte. In addition, the presence of mitochondria, which were introduced with monkey fibroblasts into rabbit recipient cytoplasm, was confirmed up to the blastocyst stage by polymerase chain reaction (PCR). This study demonstrated that: (1) rabbit oocytes can reprogramme cynomolgus monkey somatic cell nuclei, and support preimplantation development; (2) monkey-rabbit ISCNT embryos developed well in monkey culture medium at early embryonic developmental stages; (3) the cell number of monkey-rabbit ISCNT embryos is similar to that of rabbit-rabbit SCNT embryos; and (4) the mitochondrial fate of monkey-rabbit ISCNT embryos is heteroplasmic from the time just after injection to the blastocyst stage that has roots in both rabbit oocytes and monkey fibroblasts.

Constant transmission of mitochondrial DNA in intergeneric cloned embryos reconstructed from swamp buffalo fibroblasts and bovine ooplasm

Although interspecies/intergeneric somatic cell nuclear transfer (iSCNT) has been proposed as a tool to produce offspring of endangered species, conflict between donor nucleus and recipient cytoplasm in iSCNT embryos has been identified as an impediment to implementation for agricultural production. To investigate the nuclear-mitochondrial interactions on the developmental potential of iSCNT embryos, we analyzed the mtDNA copy numbers in iSCNT embryos reconstructed with water buffalo (swamp type) fibroblasts and bovine enucleated oocytes (buffalo iSCNT). As controls, SCNT embryos were derived from bovine fibroblasts (bovine SCNT). Buffalo iSCNT and bovine SCNT embryos showed similar rates of cleavage and development to the 8-cell stage (P>0.05). However, buffalo iSCNT embryos did not develop beyond the 16-cell stage. Both bovine and buffalo mtDNA content in buffalo iSCNT embryos was stable throughout the nuclear transfer process, and arrested at the 8- to 16-cell stage (P>0.05). In bovine SCNT embryos that developed to the blastocyst stage, mtDNA copy number was increased (P<0.05). In conclusion, both the donor cell and recipient cytoplast mtDNAs of buffalo iSCNT embryos were identified and maintained through the iSCNT process until the 8-16-cell stage. In addition, the copy number of mtDNA per embryo was a useful monitor to investigate nuclear-mitochondrial interactions.

Two-staged nuclear transfer can enhance the developmental ability of goat-sheep interspecies nuclear transfer embryos in vitro

The technique of interspecies somatic cell nuclear transfer, in which interspecies cloned embryos can be reconstructed by using domestic animal oocytes as nuclear recipients and endangered animal or human somatic cells as nuclear donors, can afford more opportunities in endangered animal rescue and human tissue transplantation, but the application of this technique is limited by extremely low efficiency which may be attributed to donor nucleus not fully reprogrammed by xenogenic cytoplasm. In this study, goat fetal fibroblasts (GFFs) were used as nuclear donors, in vitro-matured sheep oocytes were used as nuclear recipients, and a two-stage nuclear transfer procedure was performed to improve the developmental ability of goat-sheep interspecies clone embryos. In the first stage nuclear transfer (FSNT), GFFs were injected into the ooplasm of enucleated sheep metaphase-II oocytes, then non-activated reconstructed embryos were cultured in vitro, so that the donor nucleus could be exposed to the ooplasm for a period of time. Subsequently, in the second stage nuclear transfer, FSNT-derived non-activated reconstructed embryo was centrifuged, and the donor nucleus was then transferred into another freshly enucleated sheep oocyte. Compared with the one-stage nuclear transfer, two-stage nuclear transfer could significantly enhance the blastocyst rate of goat-sheep interspecies clone embryos, and this result indicated that longtime exposure to xenogenic ooplasm benefits the donor nucleus to be reprogrammed. The two-stage nuclear transfer procedure has two advantages, one is that the donor nucleus can be exposed to the ooplasm for a long time, the other is that the problem of oocyte aging can be solved.