Bovine Ooplasm Partially Remodels Primate Somatic Nuclei following Somatic Cell Nuclear Transfer

Interspecific Nuclear Transfer Blastocysts Reconstructed from Arabian Oryx Somatic Cells and Domestic Cow Ooplasm

Cloning, commonly referred to as somatic cell nuclear transfer (SCNT), is the technique of enucleating an oocyte and injecting a somatic cell into it. This study was carried out with interspecific SCNT technology to clone the Arabian Oryx utilizing the oryx's fibroblast cells and transfer it to the enucleated oocytes of a domestic cow. The recipient oocytes were extracted from the cows that had been butchered. Oryx somatic nuclei were introduced into cow oocytes to produce embryonic cells. The study was conducted on three groups, Oryx interspecific somatic cell nuclear transfer into enucleated oocytes of domestic cows, cow SCNT "the same bovine family species", used as a control group, and in vitro fertilized (IVF) cows to verify all media used in this work. The rates of different embryo developmental stages varied slightly (from 1- cell to morula stage). Additionally, the oryx interspecies Somatic cell nuclear transfer blastocyst developmental rate (9.23%) was comparable to that of cow SCNT (8.33%). While the blastula stage rate of the (IVF) cow embryos exhibited a higher cleavage rate (42%) in the embryo development stage. The results of this study enhanced domestic cow oocytes' ability to support interspecific SCNT cloned oryx, and generate a viable embryo that can advance to the blastula stage.

Cell division- and DNA replication-free reprogramming of somatic nuclei for embryonic transcription

Nuclear transfer systems represent the efficient means to reprogram a cell and in theory provide a basis for investigating the development of endangered species. However, conventional nuclear transfer using oocytes of laboratory animals does not allow reprogramming of cross-species nuclei owing to defects in cell divisions and activation of embryonic genes. Here, we show that somatic nuclei transferred into mouse four-cell embryos arrested at the G2/M phase undergo reprogramming toward the embryonic state. Remarkably, genome-wide transcriptional reprogramming is induced within a day, and ZFP281 is important for this replication-free reprogramming. This system further enables transcriptional reprogramming of cells from Oryx dammah, now extinct in the wild. Thus, our findings indicate that arrested mouse embryos are competent to induce intra- and cross-species reprogramming. The direct induction of embryonic transcripts from diverse genomes paves a unique approach for identifying mechanisms of transcriptional reprogramming and genome activation from a diverse range of species.

Relative abundance of pluripotency-associated candidate genes in immature oocytes and in vitro-produced buffalo embryos (Bubalus bubalis)

The present study was undertaken to analyze the relative abundance (RA) of pluripotency-associated genes (NANOG, OCT4, SOX2, c-MYC, and FOXD3) in different grades of immature oocytes and various stages of in vitro-produced buffalo embryos using RT-qPCR. Results showed that the RA of NANOG, OCT4, and FOXD3 transcripts was significantly higher (P < 0.05) in A grade oocytes compared with the other grades of oocytes. The RA of the c-MYC transcript was significantly higher (P < 0.05) in A grade compared with the C and D grades of oocytes, but the values did not differ significantly from the B grade of oocytes. The RA of the SOX2 transcript was almost similar in all grades of the oocytes. The expression levels of NANOG (P > 0.05), OCT4 (P > 0.05), c-MYC (P > 0.05) and SOX2 (P < 0.05) were higher in the blastocysts compared with the other stages of the embryos. Markedly, FOXD3 expression was significantly higher (P < 0.05) in 8-16-cell embryos compared with the 2-cell and 4-cell embryos and blastocyst, but did not differ significantly from the morula stage of the embryos. In the study, the majority of pluripotency-associated genes showed higher expression in A grade immature oocytes. Therefore, it is concluded that the A grade oocytes appeared to be more developmental competent and are suitable candidates for nuclear cloning research in buffalo. In buffalo, NANOG, OCT4, SOX2, and c-MYC are highly expressed in blastocysts compared with the other stages of embryos.

The perspective of the incompatible of nucleus and mitochondria in interspecies somatic cell nuclear transfer for endangered species

Taking into account the latest Red List of the International Union for Conservation of Nature in which 25% of all mammals are threatened with extinction, somatic cell nuclear transfer (SCNT) could be a beneficial tool and holds a lot of potential for aiding the conservation of endangered, exotic or even extinct animal species if somatic cells of such animals are available. In the case of shortage and sparse amount of wild animal oocytes, interspecies somatic cell nuclear transfer (iSCNT), where the recipient ooplasm and donor nucleus are derived from different species, is the alternative SCNT technique. The successful application of iSCNT, resulting in the production of live offspring, was confirmed in several combination of closely related species. When nucleus donor cells and recipient oocytes have been used in many other combinations, very often with a very distant taxonomical relation iSCNT resulted only in the very early stages of cloned embryo development. Problems encountered during iSCNT related to mitochondrial DNA (mtDNA)/genomic DNA incompatibility, mtDNA heteroplasmy, embryonic genome activation of the donor nucleus by the recipient oocyte and availability of suitable foster mothers for iSCNT embryos. Implementing assisted reproductive technologies, including iSCNT, to conservation programmes also raises concerns that the production of genetically identical populations might cause problems with inbreeding. The article aims at presenting achievements, limitations and perspectives of iSCNT in maintaining animal biodiversity.

Coexpression analysis identifies nuclear reprogramming barriers of somatic cell nuclear transfer embryos

The success of cloned animal "Dolly Sheep" demonstrated the somatic cell nuclear transfer (SCNT) technique holds huge potentials for mammalian asexual reproduction. However, the extremely poor development of SCNT embryos indicates their molecular mechanism remain largely unexplored. Deciphering the spatiotemporal patterns of gene expression in SCNT embryos is a crucial step toward understanding the mechanisms associated with nuclear reprogramming. In this study, a valuable transcriptome recourse of SCNT embryos was firstly established, which derived from different inter-/intra donor cells. The gene co-expression analysis identified 26 cell-specific modules, and a series of regulatory pathways related to reprogramming barriers were further enriched. Compared to the intra-SCNT embryos, the inter-SCNT embryos underwent only complete partially reprogramming. As master genome trigger genes, the transcripts related to TFIID subunit, RNA polymerase and mediators were incomplete activated in inter-SCNT embryos. The inter-SCNT embryos only wasted the stored maternal mRNA of master regulators, but failed to activate their self-sustained pathway of RNA polymerases. The KDM family of epigenetic regulator also seriously delayed in inter-SCNT embryo reprogramming process. Our study provided new insight into understanding of the mechanisms of nuclear reprogramming.

Proper reprogramming of imprinted and non-imprinted genes in cloned cattle gametogenesis

Epigenetic abnormalities in cloned animals are caused by incomplete reprogramming of the donor nucleus during the nuclear transfer step (first reprogramming). However, during the second reprogramming step that occurs only in the germline cells, epigenetic errors not corrected during the first step are repaired. Consequently, epigenetic abnormalities in the somatic cells of cloned animals should be erased in their spermatozoa or oocytes. This is supported by the fact that offspring from cloned animals do not exhibit defects at birth or during postnatal development. To test this hypothesis in cloned cattle, we compared the DNA methylation level of two imprinted genes (H19 and PEG3) and three non-imprinted genes (XIST, OCT4 and NANOG) and two repetitive elements (Satellite I and Satellite II) in blood and sperm DNAs from cloned and non-cloned bulls. We found no differences between cloned and non-cloned bulls. We also analyzed the DNA methylation levels of four repetitive elements (Satellite I, Satellite II, Alpha-satellite and Art2) in oocytes recovered from cloned and non-cloned cows. Again, no significant differences were observed between clones and non-clones. These results suggested that imprinted and non-imprinted genes and repetitive elements were properly reprogramed during gametogenesis in cloned cattle; therefore, they contributed to the soundness of cloned cattle offspring.

Lack of effects of ooplasm transfer on early development of interspecies somatic cell nuclear transfer bison embryos

Background

Successful development of iSCNT (interspecies somatic cell nuclear transfer) embryos depends on complex interactions between ooplasmic and nuclear components, which can be compromised by genetic divergence. Transfer of ooplasm matching the genetic background of the somatic cell in iSCNT embryos is a valuable tool to study the degree of incompatibilities between nuclear and ooplasmic components. This study investigated the effects of ooplasm transfer (OT) on cattle (Bos taurus) and plains bison (Bison bison bison) embryos produced by iSCNT and supplemented with or without ooplasm from cattle or plains bison oocytes.

Results

Embryos in all groups were analysed for developmental competence that included cleavage rates, ATP content, and expression of nuclear- and mitochondrial- encoded genes at 8–16 cell stage. Interestingly, no significant differences were observed in embryo development, ATP content, and expression of nuclear respiratory factor 2 (NRF2), mitochondrial transcription factor A (TFAM) and mitochondrial subunit 2 of cytochrome c oxidase (mt-COX2) among groups. Thus, although OT did not result in any detrimental effects on the reconstructed embryos due to invasive manipulation, significant benefits of OT were not observed up to the 8–16 cell stage.

Conclusions

This study showed that a viable technique for OT + SCNT is possible, however, further understanding of the effects of OT on blastocyst development is necessary.

Nuclear-mitochondrial incompatibility in interorder rhesus monkey-cow embryos derived from somatic cell nuclear transfer

Monkey interorder somatic cell nuclear transfer (iSCNT) using enucleated cow oocytes yielded poor blastocysts development and contradictory results among research groups. Determining the reason for this low blastocyst development is a prerequisite for optimizing iSCNT in rhesus monkeys. The aim of this study was to elucidate nuclear-mitochondrial incompatibility of rhesus monkey-cow iSCNT embryos and its relationship to low blastocyst development. Cytochrome b is a protein of complex III of the electron transport chain (ETC). According to meta-analysis of amino acid sequences, the homology of cytochrome b is 75 % between rhesus monkeys and cattle. To maintain the function of ETC after iSCNT, 4n iSCNT embryos were produced by fusion of non-enucleated cow oocytes and rhesus monkey somatic cells. The blastocyst development rate of 4n iSCNT embryos was higher than that of 2n embryos (P < 0.01). Formation of reactive oxygen species (ROS) is an indirect indicator of ETC activity of cells. The ROS levels of 4n iSCNT embryos was higher than that of 2n embryos (P < 0.01). Collectively, rhesus monkey iSCNT embryos reconstructed with cow oocytes have nuclear-mitochondrial incompatibility due to fundamental species differences between rhesus monkeys and cattle. Nuclear-mitochondrial incompatibility seems to correlate with low ETC activity and extremely low blastocyst development of rhesus monkey-cow iSCNT embryos.

Developmental competence of 8?16-cell stage bison embryos produced by interspecies somatic cell nuclear transfer

Altered communication between nuclear and cytoplasmic components has been linked to impaired development in interspecies somatic cell nuclear transfer (iSCNT) embryos as a result of genetic divergence between the two species. This study investigated the developmental potential and mitochondrial function of cattle (Bos taurus), plains bison (Bison bison bison) and wood bison (Bison bison athabascae) embryos produced by iSCNT using domestic cattle oocytes as cytoplasts. Embryos in all groups were analysed for development, accumulation of ATP, apoptosis and gene expression of nuclear- and mitochondrial-encoded genes at the 8-16-cell stage. The results of this study showed no significant differences in the proportion of developed embryos at the 2-, 4- and 8-16-cell stages between groups. However, significantly higher ATP levels were observed in cattle SCNT embryos compared with bison iSCNT embryos. Significantly more condensed and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL)-positive nuclei were found in plains bison iSCNT embryos. No significant differences in the expression levels of nuclear respiratory factor 2 (NRF2) or mitochondrial subunit 2 of cytochrome c oxidase (mt-COX2) were found in any of the groups. However, mitochondrial transcription factor A (TFAM) expression significantly differed between groups. The results of this study provide insights into the potential causes that might lead to embryonic arrest in bison iSCNT embryos, including mitochondrial dysfunction, increased apoptosis and abnormal gene expression.

Irregular transcriptome reprogramming probably causes thec developmental failure of embryos produced by interspecies somatic cell nuclear transfer between the Przewalski's gazelle and the bovine

Background

Interspecies somatic cell nuclear transfer (iSCNT) has been regarded as a potential alternative for rescuing highly endangered species and can be used as a model for studying nuclear–cytoplasmic interactions. However, iSCNT embryos often fail to produce viable offspring. The alterations in normal molecular mechanisms contributing to extremely poor development are for the most part unknown.

Results

Przewalski’s gazelle–bovine iSCNT embryos (PBNT) were produced by transferring Przewalski’s gazelle fibroblast nuclei into enucleated bovine oocytes. The percentages of PBNT embryos that developed to morula/blastocyst stages were extremely low even with the use of various treatments that included different SCNT protocols and treatment of embryos with small molecules. Transcriptional microarray analyses of the cloned embryos showed that the upregulation of reprogramming-associated genes in bovine–bovine SCNT (BBNT) embryos was significantly higher than those observed in PBNT embryos (1527:643). In all, 139 transcripts related to various transcription regulation factors (TFs) were unsuccessfully activated in the iSCNT embryos. Maternal degradation profiles showed that 1515 genes were uniquely downregulated in the BBNT embryos, while 343 genes were downregulated in the PBNT embryos. Incompatibilities between mitochondrial DNA (mtDNA) and nuclear DNA revealed that the TOMM (translocase of outer mitochondrial membrane)/TIMM (translocase of inner mitochondrial membrane) complex-associated genes in BBNT embryos had the highest expression levels, while the PBNT embryos exhibited much lower expression rates.

Conclusions

Improper degradation of maternal transcripts, incomplete activation of TFs and abnormal expression of genes associated with mitochondrial function in PBNT embryos likely contributed to incomplete reprogramming of the donor cell nuclei and therefore led to the developmental failure of these cloned embryos.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1113) contains supplementary material, which is available to authorized users.

Role of aberrant protein modification, assembly, and localization in cloned embryo phenotypes

Aberrant post-translational modifications of proteins contribute markedly to the abnormal characteristics of cloned embryos. This review summarizes aberrant aspects of protein modifications and protein interactions, taking an inside-outside view to the cell. These aberrant aspects affect a range of processes including the control of chromatin structure, expression of pluripotency genes, propagation of epigenetic inheritance, protein trafficking, localization and signaling, cytoskeletal structure, mitosis, and correct localization of membrane proteins. By observing these aberrant features of cloned embryos, how they arise, and their impacts on development, it is possible to gain insight into normal development and identify novel strategies for enhancing cloning outcomes.

Incompatibility of nucleus and mitochondria causes xenomitochondrial cybrid unviable across human, mouse, and pig cells

The nucleus and mitochondria are on correlative dependence; they interact in the process of protein transportation and energy metabolism. The compatibility of nucleus and mitochondria is essential for interspecies somatic cell nuclear transfer (iSCNT) and xenomitochondrial cybrid. In order to test the compatibility of nucleus and mitochondria among human, mouse, and pig cells, we compared the performances of cybrids that fused inter- and intra-species. The ρ0 cells from human and pig cell lines were created as nucleus donors which were transfected with GFP-neo for cell selective system in advance, and mitochondria donor cells were labeled by Mitochondria-RFP. Human and mouse platelets were also used as a mitochondrial donor. Results indicated that all interspecies cybrids declined to die in 2-4 d after the cell fusion in the selection medium, while intraspecies cybrid cells survived and formed stable clones. As a conclusion, the incompatibility between nucleus and mitochondria is the critical factor for the formation of interspecies cybrids.

Somatic cells, stem cells, and induced pluripotent stem cells: how do they now contribute to conservation?

More than a decade has now passed since the birth of the first endangered species produced from an adult somatic cell reprogrammed by somatic cell nuclear transfer. At that time, advances made in domestic and laboratory animal species provided the necessary foundation for attempting cutting-edge technologies on threatened and endangered species. In addition to nuclear transfer, spermatogonial stem cell transplantation and induction of pluripotent stem cells have also been explored. Although many basic scientific questions have been answered and more than 30 wild species have been investigated, very few successes have been reported. The majority of studies document numerous obstacles that still need to be overcome to produce viable gametes or embryos for healthy offspring production. This chapter provides an overview of somatic cell and stem cell technologies in different taxa (mammals, fishes, birds, reptiles and amphibians) and evaluates the potential and impact of these approaches for animal species conservation.

Cryopreservation and long-term maintenance of bovine embryo-derived cell lines

The aim of this study was to develop methods for cryopreservation and long-term maintenance of putative bovine embryonic stem cells (ESCs). Putative bovine ESC (bESC) lines (n=3) isolated in conventional medium were used to compare slow-freezing and vitrification. After warming, vitrified cells (96.9%) demonstrated significantly (P<0.05) better survival than frozen-thawed cells (81.5%) and formed significantly more colonies with good morphology (vitrification: 93/93, 100.0%; slow-freezing: 74/106, 69.81%; P<0.05). The effect of inhibitors of differentiation (PD184352, SU5402, CHIR99021) on ESC maintenance was assessed on putative bESC lines established in N2B27-3i medium (n=8) or conventional medium (n=1) after culture over 30 passages (>240 days). All cell lines expressed ALP, SSEA1, SSEA4, OCT4, REX1 and SSEA1. OCT4 expression was confirmed by relative real-time PCR and was upregulated in early passages of putative bESCs cultured in N2B27-3i (2.9±0.89-fold higher at Passage (P) 2-4), whereas the converse was observed later (P22-26; 2.2±0.1-fold increase in conventional medium). Putative bESC lines isolated in N2B27-3i medium (n=3) or conventional medium (n=1) were vitrified at P18 and, after warming, were cultured for a further 12 passages. These cells survived vitrification and expressed OCT4, REX1, SSEA1, ALP, SSEA1 and SSEA4. These results demonstrate that putative bESC lines that express pluripotent markers can be cultured long term and retain expression of pluripotent markers after vitrification.

Interspecies somatic cell nuclear transfer: advancements and problems

Embryologists working with livestock species were the pioneers in the field of reprogramming by somatic cell nuclear transfer (SCNT). Without the "Dolly experiment," the field of cellular reprogramming would have been slow and induced plutipotent cells (iPSCs) would not have been conceived. The major drive of the work in mammalian cloning was the interest of the breeding industry to propagate superior genotypes. Soon it was realized that the properties of oocytes could be used also to clone endangered mammalian species or to reprogram the genomes of unrelated species through what is known as interspecies (i) SCNT, using easily available oocytes of livestock species. iSCNT for cloning animals works only for species that can interbreed, and experiments with taxonomically distant species have not been successful in obtaining live births or deriving embryonic stem cell (ESC) lines to be used for regenerative medicine. There are controversial reports in the literature, but in most cases these experiments have underlined some of the cellular and molecular mechanisms that are incomplete during cell nucleus reprogramming, including the failure to organize nucleoli, silence somatic cell genes, activate the embryonic genome, and resume mitochondrial replication and function, thus indicating nucleus-cytoplasmic incompatibility.

Transgenic chicken, mice, cattle, and pig embryos by somatic cell nuclear transfer into pig oocytes

This study explored the possibility of producing transgenic cloned embryos by interspecies somatic cell nuclear transfer (iSCNT) of cattle, mice, and chicken donor cells into enucleated pig oocytes. Enhanced green florescent protein (EGFP)-expressing donor cells were used for the nuclear transfer. Results showed that the occurrence of first cleavage did not differ significantly when pig, cattle, mice, or chicken cells were used as donor nuclei (p>0.05). However, the rate of blastocyst formation was significantly higher in pig (14.9±2.1%; p<0.05) SCNT embryos than in cattle (6.3±2.5%), mice (4.2±1.4%), or chicken (5.1±2.4%) iSCNT embryos. The iSCNT embryos also contained a significantly less number of cells per blastocyst than those of SCNT pig embryos (p<0.05). All (100%) iSCNT embryos expressed the EGFP gene, as evidenced by the green florescence under ultraviolet (UV) illumination. Microinjection of purified mitochondria from cattle somatic cells into pig oocytes did not have any adverse effect on their postfertilization in vitro development and embryo quality (p>0.05). Moreover, NCSU23 medium, which was designed for in vitro culture of pig embryos, was able to support the in vitro development of cattle, mice, and chicken iSCNT embryos up to the blastocyst stage. Taken together, these data suggest that enucleated pig oocytes may be used as a universal cytoplast for production of transgenic cattle, mice, and chicken embryos by iSCNT. Furthermore, xenogenic transfer of mitochondria to the recipient cytoplast may not be the cause for poor embryonic development of cattle-pig iSCNT embryos.

Heterochromatin reprogramming in rabbit embryos after fertilization, intra-, and inter-species SCNT correlates with preimplantation development

To investigate the embryonic genome organization upon fertilization and somatic cell nuclear transfer (SCNT), we tracked HP1β and CENP, two well-characterized protein markers of pericentric and centromeric compartments respectively, in four types of embryos produced by rabbit in vivo fertilization, rabbit parthenogenesis, rabbit-to-rabbit, and bovine-to-rabbit SCNT. In the interphase nuclei of rabbit cultured fibroblasts, centromeres and associated pericentric heterochromatin are usually isolated. Clustering into higher-order chromatin structures, such as the chromocenters seen in mouse and bovine somatic cells, could not be observed in rabbit fibroblasts. After fertilization, centromeres and associated pericentric heterochromatin are quite dispersed in rabbit embryos. The somatic-like organization is progressively established and completed only by the 8/16-cell stage, a stage that corresponds to major embryonic genome activation in this species. In SCNT embryos, pericentric heterochromatin distribution typical for rabbit and bovine somatic cells was incompletely reverted into the 1-cell embryonic form with remnants of heterochromatin clusters in 100% of bovine-to-rabbit embryos. Subsequently, the donor cell nuclear organization was rapidly re-established by the 4-cell stage. Remarkably, the incomplete remodeling of bovine-to-rabbit 1-cell embryos was associated with delayed transcriptional activation compared with rabbit-to-rabbit embryos. Together, the results confirm that pericentric heterochromatin spatio-temporal reorganization is an important step of embryonic genome reprogramming. It also appears that genome reorganization in SCNT embryos is mainly dependent on the nuclear characteristics of the donor cells, not on the recipient cytoplasm.

On the cellular and developmental lethality of a Xenopus nucleocytoplasmic hybrid

Nucleocytoplasmic hybrid (cybrid) embryos result from the combination of the nucleus of one species, and the egg cytoplasm of another species. Cybrid embryos can be obtained either in the haploid state by the cross-fertilization or intra-cytoplasmic injection of an enucleated egg with sperm from another species, or in the diploid state by the technique of interspecies somatic cell nuclear transfer (iSCNT). Cybrids that originate from the combination of the nucleus and the cytoplasm of distantly related species commonly expire during early embryonic development, and the cause of this arrest is currently under investigation. Here we show that cells isolated from a Xenopus cybrid (Xenopus (Silurana) tropicalis haploid nucleus combined with Xenopus laevis egg cytoplasm) embryo are unable to proliferate and expand normally in vitro. We also provide evidence that the lack of nuclear donor species maternal poly(A)⁺ RNA-dependent factors in the recipient species egg may contribute to the developmental dead-end of distantly-related cybrid embryos. Overall, the data are consistent with the view that the development promoted by one species’ nucleus is dependent on the presence of maternally-derived, mRNA encoded, species-specific factors. These results also show that cybrid development can be improved without nuclear species mitochondria supplementation or replacement.

Reprogramming and development in nuclear transfer embryos and in interspecific systems

Nuclear transfer (NT) remains the most effective method to reprogram somatic cells to totipotency. Somatic cell nuclear transfer (SCNT) efficiency however remains low, but recurrent problems occurring in partially reprogrammed cloned embryos have recently been identified and some remedied. In particular, the trophectoderm has been identified as a lineage whose reprogramming success has a large influence on SCNT embryo development. Several interspecific hybrid and cybrid reprogramming systems have been developed as they offer various technical advantages and potential applications, and together with SCNT, they have led to the identification of a series of reprogramming events and responsible reprogramming factors. Interspecific incompatibilities hinder full exploitation of cross-species reprogramming systems, yet recent findings suggest that these may not constitute insurmountable obstacles.

Amphibian interorder nuclear transfer embryos reveal conserved embryonic gene transcription, but deficient DNA replication or chromosome segregation

Early interspecies nuclear transfer (iNT) experiments suggested that a foreign nucleus may become permanently damaged after a few rounds of cell division in the cytoplasm of another species. That is, in some distant species combinations, nucleocytoplasmic hybrid (cybrid) blastula nuclei can no longer support development, even if they are back-transferred into their own kind of egg cytoplasm. We monitored foreign DNA amplification and RNA production by quantitative PCR (qPCR) and RT-qPCR in interorder amphibian hybrids and cybrids formed by the transfer of newt (Pleurodeles waltl) embryonic nuclei into intact and enucleated frog (Xenopus laevis) eggs. We found a dramatic reduction in the expansion of foreign DNA and cell numbers in developing cybrid embryos that correlated with reduced gene transcription. Interestingly, expansion in cell numbers was rescued by the recipient species (Xenopus) maternal genome in iNT hybrids, but it did not improve P. waltl DNA expansion or gene transcription. Also, foreign gene transcripts, normalized to DNA copy numbers, were mostly normal in both iNT hybrids and cybrids. Thus, incomplete foreign DNA replication and/or chromosome segregation during cell division may be the major form of nuclear damage occurring as a result of nuclear replication in a foreign cytoplasmic environment. It also shows that the mechanisms of embryonic gene transcription are highly conserved across amphibians and may not be a major cause of cybrid lethality.

Deficient induction response in a Xenopus nucleocytoplasmic hybrid

Defects in induction signaling and response underlie the nucleocytoplasmic incompatibility between two evolutionarily distant frog species, while specific treatments partially restore this response in explants and whole embryos.

Incompatibilities between the nucleus and the cytoplasm of sufficiently distant species result in developmental arrest of hybrid and nucleocytoplasmic hybrid (cybrid) embryos. Several hypotheses have been proposed to explain their lethality, including problems in embryonic genome activation (EGA) and/or nucleo-mitochondrial interactions. However, conclusive identification of the causes underlying developmental defects of cybrid embryos is still lacking. We show here that while over 80% of both Xenopus laevis and Xenopus (Silurana) tropicalis same-species androgenetic haploids develop to the swimming tadpole stage, the androgenetic cybrids formed by the combination of X. laevis egg cytoplasm and X. tropicalis sperm nucleus invariably fail to gastrulate properly and never reach the swimming tadpole stage. In spite of this arrest, these cybrids show quantitatively normal EGA and energy levels at the stage where their initial gastrulation defects are manifested. The nucleocytoplasmic incompatibility between these two species instead results from a combination of factors, including a reduced emission of induction signal from the vegetal half, a decreased sensitivity of animal cells to induction signals, and differences in a key embryonic protein (Xbra) concentration between the two species, together leading to inefficient induction and defective convergence-extension during gastrulation. Indeed, increased exposure to induction signals and/or Xbra signalling partially rescues the induction response in animal explants and whole cybrid embryos. Altogether, our study demonstrates that the egg cytoplasm of one species may not support the development promoted by the nucleus of another species, even if this nucleus does not interfere with the cytoplasmic/maternal functions of the egg, while the egg cytoplasm is also capable of activating the genome of that nucleus. Instead, our results provide evidence that inefficient signalling and differences in the concentrations of key proteins between species lead to developmental defects in cybrids. Finally, they show that the incompatibilities of cybrids can be corrected by appropriate treatments.

When two species evolve separately for several million years, their respective genomes accumulate many small changes that together are responsible for the differences in their characters. Some of these affect the way eggs are prepared inside the germline, and/or how embryos develop, such that the egg cytoplasm of a given species can only support development promoted by its own genome or nucleus. Thus, developmental incompatibility arises between the cytoplasm and the nucleus of distant species during evolution and we don't know its mechanism. We have studied this phenomenon in an advantageous system using two evolutionarily distant frog species (Xenopus laevis and Xenopus tropicalis). We found that hybrid frog embryos with X. laevis cytoplasm and X. tropicalis nuclei are always defective in an important process that is necessary to generate morphogenetic cell movements during development. Through a series of experiments in which we dissect out and/or recombine parts of such hybrid embryos and observe their behaviour in culture, we show that this phenomenon occurs because of malfunctions in the signalling cascade that is responsible for generating these cell movements. Thus, we postulate that inefficient molecular signalling contributes to the death of such hybrids.

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 reaction-restriction 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.

Role of ooplasm in nuclear and nucleolar remodeling of intergeneric somatic cell nuclear transfer embryos during the first cell cycle

Initially, development of the zygote is under control of the oocyte ooplasm. However, it is presently unknown if and to what extent is the ooplasm able to interact with a transferred somatic cell from another species in the context of interspecies somatic cell nuclear transfer (SCNT). Here, one-cell stage embryos were processed at different points in time post activation (2 hpa, 4 hpa, 8 hpa, and 12 hpa) for detailed nuclear and nucleolar analysis by TEM, and immunofluorescence for visualization of nucleolar proteins related to transcription (UBF) and processing (fibrillarin). Bovine and porcine intergeneric SCNT embryos were compared to their parthenogenetic counterparts to assess the effects of the introduced somatic cell. Despite the absence of morphological remodeling (premature chromatin condensation, nuclear envelope breakdown), reconstructed embryos showed nuclear and nucleolar precursor body (NPB) morphology similar to the host ooplasm, which, together with detected posttranslational activity of somatic cell introduced into the bovine ooplasm, suggests a universal function of ooplasmic factors. However, the lack of distinct UBF localization in intergeneric embryos indicates failures in sequence-specific interactions between the ooplasm and chromatin of another genus. In conclusion, the results demonstrate a possible reason why the intergeneric SCNT embryos never reached the full term.

Reprogrammed transcriptome in rhesus-bovine interspecies somatic cell nuclear transfer embryos

Background

Global activation of the embryonic genome (EGA), one of the most critical steps in early mammalian embryo development, is recognized as the time when interspecies somatic cell nuclear transfer (iSCNT) embryos fail to thrive.

Methodology/Principal Findings

In this study, we analyzed the EGA-related transcriptome of rhesus-bovine iSCNT 8- to 16-cell embryos and dissected the reprogramming process in terms of embryonic gene activation, somatic gene silencing, and maternal RNA degradation. Compared with fibroblast donor cells, two thousand and seven genes were activated in iSCNT embryos, one quarter of them reaching expression levels comparable to those found in in vitro fertilized (IVF) rhesus embryos. This suggested that EGA in iSCNT embryos had partially recapitulated rhesus embryonic development. Eight hundred and sixty somatic genes were not silenced properly and continued to be expressed in iSCNT embryos, which indicated incomplete nuclear reprogramming. We compared maternal RNA degradation in bovine oocytes between bovine-bovine SCNT and iSCNT embryos. While maternal RNA degradation occurred in both SCNT and iSCNT embryos, we saw more limited overall degradation of maternal RNA in iSCNT embryos than in SCNT embryos. Several important maternal RNAs, like GPF9, were not properly processed in SCNT embryos.

Conclusions/Significance

Our data suggested that iSCNT embryos are capable of triggering EGA, while a portion of somatic cell-associated genes maintain their expression. Maternal RNA degradation seems to be impaired in iSCNT embryos. Further understanding of the biological roles of these genes, networks, and pathways revealed by iSCNT may expand our knowledge about cell reprogramming, pluripotency, and differentiation.

Nuclear-cytoplasmic incompatibility and inefficient development of pig-mouse cytoplasmic hybrid embryos

Inter-species somatic cell nuclear transfer (iSCNT) embryos usually fail to develop to the blastocyst stage and beyond due to incomplete reprogramming of donor cell. We evaluated whether using a karyoplast that would require less extensive reprogramming such as an embryonic blastomere or the meiotic spindle from metaphase II oocytes would provide additional insight into the development of iSCNT embryos. Our results showed that karyoplasts of embryonic or oocyte origin are no different from somatic cells; all iSCNT embryos, irrespective of karyoplast origin, were arrested during early development. We hypothesized that nuclear-cytoplasmic incompatibility could be another reason for failure of embryonic development from iSCNT. We used pig-mouse cytoplasmic hybrids as a model to address nuclear-cytoplasmic incompatibility in iSCNT embryos. Fertilized murine zygotes were reconstructed by fusing with porcine cytoplasts of varying cytoplasmic volumes (1/10 (small) and 1/5 (large) total volume of mouse zygote). The presence of pig cytoplasm significantly reduced the development of mouse zygotes to the blastocyst stage compared with control embryos at 120 h post-human chorionic gondotropin (41 vs 6 vs 94%, P<0.05; 1/10, 1/5, control respectively). While mitochondrial DNA copy numbers remained relatively unchanged, expression of several important genes namely Tfam, Polg, Polg2, Mfn2, Slc2a3 (Glut3), Slc2a1 (Glut1), Bcl2, Hspb1, Pou5f1 (Oct4), Nanog, Cdx2, Gata3, Tcfap2c, mt-Cox1 and mt-Cox2 was significantly reduced in cytoplasmic hybrids compared with control embryos. These results demonstrate that the presence of even a small amount of porcine cytoplasm is detrimental to murine embryo development and suggest that a range of factors are likely to contribute to the failure of inter-species nuclear transfer embryos.

Formation of nucleoli in interspecies nuclear transfer embryos derived from bovine, porcine, and rabbit oocytes and nuclear donor cells of various species

The most successful development of interspecies somatic cell nuclear transfer (iSCNT) embryos has been achieved in closely related species. The analyses of embryonic gene activity in iSCNT embryos of different species combinations have revealed the existence of significant aberrations in expression of housekeeping genes and genes dependent on the major embryonic genome activation (EGA). However, there are many studies with successful blastocyst (BL) development of iSCNT embryos derived from donor cells and oocytes of animal species with distant taxonomical relations (inter-family/inter-class) that should indicate proper EGA at least in terms of RNA polymerase I activation, nucleoli formation, and activation of genes engaged in morula and BL formation. We investigated the ability of bovine, porcine, and rabbit oocytes to activate embryonic nucleoli formation in the nuclei of somatic cells of different mammalian species. In iSCNT embryos, nucleoli precursor bodies originate from the oocyte, while most proteins engaged in the formation of mature nucleoli should be transcribed from genes de novo in the donor nucleus at the time of EGA. Thus, the success of nucleoli formation depends on species compatibility of many components of this complex process. We demonstrate that the time and cell stage of nucleoli formation are under the control of recipient ooplasm. Oocytes of the studied species possess different abilities to support nucleoli formation. Formation of nucleoli, which is a complex but small part of the whole process of EGA, is essential but not absolutely sufficient for the development of iSCNT embryos to the morula and BL stages.

Production of transgenic canine embryos using interspecies somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) has emerged as an important tool for producing transgenic animals and deriving transgenic embryonic stem cells. The process of SCNT involves fusion of in vitro matured oocytes with somatic cells to make embryos that are transgenic when the nuclear donor somatic cells carry ‘foreign’ DNA and are clones when all the donor cells are genetically identical. However, in canines, it is difficult to obtain enough mature oocytes for successful SCNT due to the very low efficiency of in vitro oocyte maturation in this species that hinders canine transgenic cloning. One solution is to use oocytes from a different species or even a different genus, such as bovine oocytes, that can be matured easily in vitro. Accordingly, the aim of this study was: (1) to establish a canine fetal fibroblast line transfected with the green fluorescent protein (GFP) gene; and (2) to investigate in vitro embryonic development of canine cloned embryos derived from transgenic and non-transgenic cell lines using bovine in vitro matured oocytes. Canine fetal fibroblasts were transfected with constructs containing the GFP and puromycin resistance genes using FuGENE 6®. Viability levels of these cells were determined by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay. Interspecies SCNT (iSCNT) embryos from normal or transfected cells were produced and cultured in vitro. The MTT measurement of GFP-transfected fetal fibroblasts (mean OD = 0.25) was not significantly different from non-transfected fetal fibroblasts (mean OD = 0.35). There was no difference between transgenic iSCNT versus non-transgenic iSCNT embryos in terms of fusion rates (73.1% and 75.7%, respectively), cleavage rates (69.7% vs. 73.8%) and development to the 8–16-cell stage (40.1% vs. 42.7%). Embryos derived from the transfected cells completely expressed GFP at the 2-cell, 4-cell, and 8–16-cell stages without mosaicism. In summary, our results demonstrated that, following successful isolation of canine transgenic cells, iSCNT embryos developed to early pre-implantation stages in vitro, showing stable GFP expression. These canine–bovine iSCNT embryos can be used for further in vitro analysis of canine transgenic cells and will contribute to the production of various transgenic dogs for use as specific human disease models.

Development, embryonic genome activity and mitochondrial characteristics of bovine-pig inter-family nuclear transfer embryos

The best results of inter-species somatic cell nuclear transfer (iSCNT) in mammals were obtained using closely related species that can hybridise naturally. However, in the last years, many reports describing blastocyst development following iSCNT between species with distant taxonomical relations (inter-classes, inter-order and inter-family) have been published. This indicates that embryonic genome activation (EGA) in xeno-cytoplasm is possible, albeit very rarely. Using a bovine-pig (inter-family) iSCNT model, we studied the basic characteristics of EGA: expression and activity of RNA polymerase II (RNA Pol II), formation of nucleoli (as an indicator of RNA polymerase I (RNA Pol I) activity), expression of the key pluripotency gene NANOG and alteration of mitochondrial mass. In control embryos (obtained by IVF or iSCNT), EGA was characterised by RNA Pol II accumulation and massive production of poly-adenylated transcripts (detected with oligo dT probes) in blastomere nuclei, and formation of nucleoli as a result of RNA Pol I activity. Conversely, iSCNT embryos were characterised by the absence of accumulation and low activity of RNA Pol II and inability to form active mature nucleoli. Moreover, in iSCNT embryos, NANOG was not expressed, and mitochondria mass was significantly lower than in intra-species embryos. Finally, the complete developmental block at the 16-25-cell stage for pig-bovine iSCNT embryos and at the four-cell stage for bovine-pig iSCNT embryos strongly suggests that EGA is not taking place in iSCNT embryos. Thus, our experiments clearly demonstrate poor nucleus-cytoplasm compatibility between these animal species.