Effect of epigenetic regulation during swine embryogenesis and on cloning by nuclear transfer

Somatic Cell Nuclear Transfer in Pigs

Somatic cell nuclear transfer (SCNT) has been successfully applied to clone animals of several species. Pigs are one of the main livestock species for food production and are also important for biomedical research due to their physiopathological similarities with humans. In the past 20 years, clones of several swine breeds have been produced for a variety of purposes, including biomedical and agricultural applications. In this chapter, we describe a protocol to produce cloned pigs by SCNT.

Transplantation of human cells into Interleukin-2 receptor gamma gene knockout pigs under several conditions

Introduction

Previously, we performed gene knockout (KO) of interleukin-2 receptor gamma (IL2RG) in porcine fetal fibroblasts using zinc finger nuclease-encoding mRNAs, subsequently generating IL2RG KO pigs using these cells through somatic cell nuclear transfer. The IL2RG KO pigs lacked a thymus and were deficient in T lymphocytes and natural killer cells, similar to human X-linked severe combined immunodeficiency (SCID) patients. The present study aimed to evaluate whether pigs can support the growth of xenografted human cells and have the potential to be an effective animal model.

Methods

The IL2RG X^KOY pigs used in this study were obtained by mating IL2RG X^KOX females with wild-type boars. This permitted the routine production of IL2RG KO pigs via natural breeding without complicated somatic cell cloning procedures; therefore, a sufficient number of pigs could be prepared. We transplanted human HeLa S3 cells expressing the tandem dimer tomato into the ears and pancreas of IL2RG KO pigs. Additionally, a newly developed method for the aseptic rearing of SCID pigs was used in case of necessity.

Results

Tumors from the transplanted cells quickly developed in all pigs and were verified by histology and immunohistochemistry. We also transplanted these cells into the pancreas of designated pathogen-free pigs housed in novel biocontainment facilities, and large tumors were confirmed.

Conclusions

IL2RG KO pigs have the potential to become useful animal models in a variety of translational biology fields.

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The present study aimed to evaluate whether IL2RG KO SCID-like pigs can host and support the growth of xenografted human cells under several conditions.

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Tumors from transplanted cells quickly developed in all pigs, as verified by histology and immunohistochemistry.

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IL2RG KO pigs have the potential to become extremely useful animal models in a variety of translational biology fields.

Epigenetic manipulation to improve mouse SCNT embryonic development

Cloned mammals can be achieved through somatic cell nuclear transfer (SCNT), which involves reprogramming of differentiated somatic cells into a totipotent state. However, low cloning efficiency hampers its application severely. Cloned embryos have the same DNA as donor somatic cells. Therefore, incomplete epigenetic reprogramming accounts for low development of cloned embryos. In this review, we describe recent epigenetic barriers in SCNT embryos and strategies to correct these epigenetic defects and avoid the occurrence of abnormalities in cloned animals.

A Dystrophin Exon-52 Deleted Miniature Pig Model of Duchenne Muscular Dystrophy and Evaluation of Exon Skipping

Duchenne muscular dystrophy (DMD) is a lethal X-linked recessive disorder caused by mutations in the DMD gene and the subsequent lack of dystrophin protein. Recently, phosphorodiamidate morpholino oligomer (PMO)-antisense oligonucleotides (ASOs) targeting exon 51 or 53 to reestablish the DMD reading frame have received regulatory approval as commercially available drugs. However, their applicability and efficacy remain limited to particular patients. Large animal models and exon skipping evaluation are essential to facilitate ASO development together with a deeper understanding of dystrophinopathies. Using recombinant adeno-associated virus-mediated gene targeting and somatic cell nuclear transfer, we generated a Yucatan miniature pig model of DMD with an exon 52 deletion mutation equivalent to one of the most common mutations seen in patients. Exon 52-deleted mRNA expression and dystrophin deficiency were confirmed in the skeletal and cardiac muscles of DMD pigs. Accordingly, dystrophin-associated proteins failed to be recruited to the sarcolemma. The DMD pigs manifested early disease onset with severe bodywide skeletal muscle degeneration and with poor growth accompanied by a physical abnormality, but with no obvious cardiac phenotype. We also demonstrated that in primary DMD pig skeletal muscle cells, the genetically engineered exon-52 deleted pig DMD gene enables the evaluation of exon 51 or 53 skipping with PMO and its advanced technology, peptide-conjugated PMO. The results show that the DMD pigs developed here can be an appropriate large animal model for evaluating in vivo exon skipping efficacy.

TDG is a pig-specific epigenetic regulator with insensitivity to H3K9 and H3K27 demethylation in nuclear transfer embryos

Pig cloning by somatic cell nuclear transfer (SCNT) frequently undergoes incomplete epigenetic remodeling during the maternal-to-zygotic transition, which leads to a significant embryonic loss before implantation. Here, we generated the first genome-wide landscapes of histone methylation in pig SCNT embryos. Excessive H3K9me3 and H3K27me3, but not H3K4me3, were observed in the genomic regions with unfaithful embryonic genome activation and donor-cell-specific gene silencing. A combination of H3K9 demethylase KDM4A and GSK126, an inhibitor of H3K27me3 writer, were able to remove these epigenetic barriers and restore the global transcriptome in SCNT embryos. More importantly, thymine DNA glycosylase (TDG) was defined as a pig-specific epigenetic regulator for nuclear reprogramming, which was not reactivated by H3K9me3 and H3K27me3 removal. Both combined treatment and transient TDG overexpression promoted DNA demethylation and enhanced the blastocyst-forming rates of SCNT embryos, thus offering valuable methods to increase the cloning efficiency of genome-edited pigs for agricultural and biomedical purposes.

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Identification of reprogramming-resistant genes and regions in porcine SCNT embryos

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H3K9me3 and H3K27me3 are enriched in reprogramming-resistant genes and regions

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Removing H3K9me3 and H3K27me3 by KDM4A and GSK126 facilitates nuclear reprogramming

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Transient TDG overexpression promotes DNA demethylation and improves reprogramming

Challenges and Considerations during In Vitro Production of Porcine Embryos

Genetically modified pigs have become valuable tools for generating advances in animal agriculture and human medicine. Importantly, in vitro production and manipulation of embryos is an essential step in the process of creating porcine models. As the in vitro environment is still suboptimal, it is imperative to examine the porcine embryo culture system from several angles to identify methods for improvement. Understanding metabolic characteristics of porcine embryos and considering comparisons with other mammalian species is useful for optimizing culture media formulations. Furthermore, stressors arising from the environment and maternal or paternal factors must be taken into consideration to produce healthy embryos in vitro. In this review, we progress stepwise through in vitro oocyte maturation, fertilization, and embryo culture in pigs to assess the status of current culture systems and address points where improvements can be made.

Genome-wide DNA methylation and transcription analysis in tongue and biceps femoris muscles of cloned pigs with macroglossia

Cloned animals are prone to abnormal phenotypes such as enlarged tongue, fetal oversize, and progeria. In the present study, whole-genome bisulfite sequencing and mRNA sequencing were performed on tongue and biceps femoris muscles of cloned piglets with and without macroglossia, in an attempt to elucidate the epigenetic causes of the macroglossia phenotype. We identified 14 958 and 18 752 differentially methylated regions in the tongue and biceps femoris muscles, respectively, of macroglossia piglets and these correspond to 4574 and 4772 differentially methylated genes compared with the control group (piglets without macroglossia). Larger methylation difference was found in tongue muscle than in biceps femoris muscle. In total, 114 genes in tongue and 72 genes in biceps femoris muscles were found to be differentially expressed between the two groups. Of these differentially expressed genes in tongue muscle, 31 were also differentially methylated genes, among which DIO3 and ZIC1 were imprinting or predicted imprinting genes. These two and another six overlapping genes (ALDH1A2, MKX, MAB21L2, CA3, RANBP3L, and MYL10) are crucial factors involved in embryonic development or tissue and organ development. GO enrichment analysis suggested possible alteration of these processes. Our study provides novel molecular insights into the formation of macroglossia in cloned pigs.

Development of a porcine model of phenylketonuria with a humanized R408W mutation for gene editing

Phenylketonuria (PKU) is a metabolic disorder whereby phenylalanine metabolism is deficient due to allelic variations in the gene for phenylalanine hydroxylase (PAH). There is no cure for PKU other than orthotopic liver transplantation, and the standard of care for patients is limited to dietary restrictions and key amino acid supplementation. Therefore, Pah was edited in pig fibroblasts for the generation of PKU clone piglets that harbor a common and severe human mutation, R408W. Additionally, the proximal region to the mutation was further humanized by introducing 5 single nucleotide polymorphisms (SNPs) to allow for development of gene editing machinery that could be translated directly from the pig model to human PKU patients that harbor at least one classic R408W allele. Resulting piglets were hypopigmented (a single Ossabaw piglet) and had low birthweight (all piglets). The piglets had similar levels of PAH expression, but no detectable enzymatic activity, consistent with the human phenotype. The piglets were fragile and required extensive neonatal care to prevent failure to thrive and early demise. Phenylalanine levels rose sharply when dietary Phe was unrestricted but could be rapidly reduced with a low Phe diet. Fibroblasts isolated from R408W piglets show susceptibility to correction using CRISPR or TALEN, with subsequent homology-directed recombination to correct Pah. This pig model of PKU provides a powerful new tool for development of all classes of therapeutic candidates to treat or cure PKU, as well as unique value for proof-of-concept studies for in vivo human gene editing platforms in the context of this humanized PKU allele.

Assessment of the Growth and Reproductive Performance of Cloned Pietrain Boars

How to maximize the use of the genetic merits of the high-ranking boars (also called superior ones) is a considerable question in the pig breeding industry, considering the money and time spent on selection. Somatic cell nuclear transfer (SCNT) is one of the potential ways to answer the question, which can be applied to produce clones with genetic resources of superior boar for the production of commercial pigs. For practical application, it is essential to investigate whether the clones and their progeny keep behaving better than the "normal boars", considering that in vitro culture and transfer manipulation would cause a series of harmful effects to the development of clones. In this study, 59,061 cloned embryos were transferred into 250 recipient sows to produce the clones of superior Pietrain boars. The growth performance of 12 clones and 36 non-clones and the semen quality of 19 clones and 28 non-clones were compared. The reproductive performance of 21 clones and 25 non-clones were also tested. Furthermore, we made a comparison in the growth performance between 466 progeny of the clones and 822 progeny of the non-clones. Our results showed that no significant difference in semen quality and reproductive performance was observed between the clones and the non-clones, although the clones grew slower and exhibited smaller body size than the non-clones. The F1 progeny of the clones showed a greater growth rate than the non-clones. Our results demonstrated through the large animal population showed that SCNT manipulation resulted in a low growth rate and small body size, but the clones could normally produce F1 progeny with excellent growth traits to bring more economic benefits. Therefore, SCNT could be effective in enlarging the merit genetics of the superior boars and increasing the economic benefits in pig reproduction and breeding.

Locus-specific analysis of DNA methylation patterns in cloned and in vitro fertilized porcine embryos

Porcine somatic cell nuclear transfer (SCNT) is currently inefficient, as 1–3.95% of reconstructed embryos survive to term; inadequate or erroneous epigenetic reprogramming of the specialized donor somatic nucleus could be a primary reason. Therefore, a locus-specific analysis of DNA methylation dynamics in embryogenesis and the DNA methylation status of gametes and donor cells used for SCNT were conducted in the following developmentally important gene loci: POU5F1, NANOG, SOX2, H19, IGF2, IGF2R, XIST; and the retrotransposon LINE-1. There were significant epigenetic differences between the gametes and the somatic donor cells. Three gamete-specific differentially methylated regions (DMRs) in POU5F1, XIST, and LINE-1 were identified. A delayed demethylation process at POU5F1 and LINE-1 loci occurred after three successive cleavages, compared to the in vitro fertilized (IVF) embryos. Although cloned embryos could undergo de-methylation and re-methylation dynamics at the DMRs of imprinted genes (H19,IGF2R, and XIST), the re-methylation process was compromised, unlike in fertilized embryos. LINE-1 loci are widely dispersed across the whole genome, and LINE-1 DMR might be a potential porcine nuclear reprogramming epi-marker. Data from observations in our present and previous studies, and two published articles were pooled to produce a schematic diagram of locus-specific, DNA methylation dynamics of cloned and IVF embryos during porcine early embryogenesis. This also indicated aberrant DNA methylation reprogramming events, including inadequate DNA demethylation and insufficient re-methylation in cloned embryos. Further research should focus on mechanisms underlying demethylation during the early cleavage of embryos and de novo DNA methylation at the blastocyst stage.

Application of genome-editing systems to enhance available pig resources for agriculture and biomedicine

Traditionally, genetic engineering in the pig was a challenging task. Genetic engineering of somatic cells followed by somatic cell nuclear transfer (SCNT) could produce genetically engineered (GE) pigs carrying site-specific modifications. However, due to difficulties in engineering the genome of somatic cells and developmental defects associated with SCNT, a limited number of GE pig models were reported. Recent developments in genome-editing tools, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) 9 system, have markedly changed the effort and time required to produce GE pig models. The frequency of genetic engineering in somatic cells is now practical. In addition, SCNT is no longer essential in producing GE pigs carrying site-specific modifications, because direct injection of genome-editing systems into developing embryos introduces targeted modifications. To date, the CRISPR/Cas9 system is the most convenient, cost-effective, timely and commonly used genome-editing technology. Several applicable biomedical and agricultural pig models have been generated using the CRISPR/Cas9 system. Although the efficiency of genetic engineering has been markedly enhanced with the use of genome-editing systems, improvements are still needed to optimally use the emerging technology. Current and future advances in genome-editing strategies will have a monumental effect on pig models used in agriculture and biomedicine.

Sperm-borne small RNAs regulate α-tubulin acetylation and epigenetic modification of early bovine somatic cell nuclear transfer embryos

Accumulated evidence indicates that sperm-borne small RNA plays a crucial role in embryonic development, especially the absence of the sperm-borne small RNA might be a major cause of the abnormal development of cloned embryos. In this study, we found that sperm-borne small RNA can affect abnormal pronuclear-like structures, postpone the timing of first embryo cleavage and enhance developmental competence of bovine somatic cell nuclear transfer (SCNT) embryos. In addition, the supplementation of sperm-borne small RNA can significantly increase live birth rates and decrease the birth weights of cloned offspring. To investigate the underlying mechanisms, the levels of α-tubulin K40 acetylation (Ac α-tubulin K40) and histone H3 lysine 9 trimethylation (H3K9me3) during early embryo development were investigated in SCNT embryos with sperm-borne small RNA supplementation (termed as T-NT), compared to those normal SCNT embryos and embryos obtained from standard IVF. The results showed that sperm-borne small RNA can significantly decrease the H3K9me3 levels at the pronuclear and two-cell stages, while significantly increase Ac α-tubulin K40 levels at anaphase and telophase of bovine SCNT embryos during the first cleavage. Collectively, our study for the first time demonstrates that sperm-borne small RNA plays a crucial role in the developmental competence of SCNT embryos by regulating H3K9me3 and Ac α-tubulin K40. Further studies will be required to determine how sperm small RNA regulate the H3K9me3 and Acα-tubulin K40. Our study suggests that the supplementation of sperm-borne small RNA is a potential application to improve the cloning efficiency.

Generation of Transgenic Cloned Buffalo Embryos Harboring the EGFP Gene in the Y Chromosome Using CRISPR/Cas9-Mediated Targeted Integration

Sex control technology is of great significance in the production of domestic animals, especially for rapidly breeding water buffalo (bubalus bubalis), which served as a research model in the present study. We have confirmed that a fluorescence protein integrated into the Y chromosome is fit for sexing pre-implantation embryos in the mouse. Firstly, we optimized the efficiency of targeted integration of exogenous gene encoding enhanced green fluorescent protein (eGFP) and mCherry in Neuro-2a cells, mouse embryonic stem cells, mouse embryonic cells (NIH3T3), buffalo fetal fibroblast (BFF) cells. The results showed that a homology arm length of 800 bp on both sides of the target is more efficient that 300 bp or 300 bp/800 bp. Homology-directed repair (HDR)-mediated knock-in in BFF cells was also significantly improved when cells were supplemented with pifithrin-μ, which is a small molecule that inhibits the binding of p53 to mitochondria. Three pulses at 250 V resulted in the most efficient electroporation in BFF cells and 1.5 μg/mL puromycin was found to be the optimal concentration for screening. Moreover, Y-Chr-eGFP transgenic BFF cells and cloned buffalo embryos were successfully generated using CRISPR/Cas9-mediated gene editing combined with the somatic cell nuclear transfer (SCNT) technique. At passage numbers 6–8, the growth rate and cell proliferation rate were significantly lower in Y-Chr-eGFP transgenic than in non-transgenic BFF cells; the expression levels of the methylation-related genes DNMT1 and DNMT3a were similar; however, the expression levels of the acetylation-related genes HDAC1, HDAC2, and HDAC3 were significantly higher (p < 0.05) in Y-Chr-eGFP transgenic BFF cells compared with non-transgenic cells. Y-Chr-eGFP transgenic BFFs were used as donors for SCNT, the results showed that eGFP reporter is suitable for the visualization of the sex of embryos. The blastocyst rates of cloned buffalo embryos were similar; however, the cleavage rates of transgenic cloned embryos were significantly lower compared with control. In summary, we optimized the protocol for generating transgenic BFF cells and successfully generated Y-Chr-eGFP transgenic embryos using these cells as donors.

Lessons Learned from Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) has been an area of interest in the field of stem cell research and regenerative medicine for the past 20 years. The main biological goal of SCNT is to reverse the differentiated state of a somatic cell, for the purpose of creating blastocysts from which embryonic stem cells (ESCs) can be derived for therapeutic cloning, or for the purpose of reproductive cloning. However, the consensus is that the low efficiency in creating normal viable offspring in animals by SCNT (1–5%) and the high number of abnormalities seen in these cloned animals is due to epigenetic reprogramming failure. In this review we provide an overview of the current literature on SCNT, focusing on protocol development, which includes early SCNT protocol deficiencies and optimizations along with donor cell type and cell cycle synchrony; epigenetic reprogramming in SCNT; current protocol optimizations such as nuclear reprogramming strategies that can be applied to improve epigenetic reprogramming by SCNT; applications of SCNT; the ethical and legal implications of SCNT in humans; and specific lessons learned for establishing an optimized SCNT protocol using a mouse model.

Generation of viable PDX1 gene-edited founder pigs as providers of nonmosaics

Pancreatic duodenal homeobox 1 (PDX1) is a crucial gene for pancreas development during the fetal period. PDX1-modified pigs have the potential to be used as a model of diabetes mellitus. However, the severe health problems caused by the PDX1 mutation limit phenotypic studies of PDX1-modified pigs as diabetes models. In this study, we generated PDX1-modified pigs by the CRISPR/Cas9 system introduced into zygotes via electroporation and investigated the mosaicism, phenotypes, and inheritance of the resulting pigs. After the embryo transfer of PDX1-modified zygotes, nine mutant piglets were delivered. Two piglets were apancreatic biallelic mutants. For the other seven piglets, the ratio of mutant alleles to total alleles was 17.5-79.7%. Two mutant piglets with high mutation rates (67.7% and 79.7%) exhibited hypoplasia of the pancreas, whereas the other five piglets were healthy. One of the male mutant piglets was further analyzed. The ejaculated semen from the pig contained PDX1-mutant spermatozoa and the pig showed normal reproductive ability. In conclusion, the frequency of the PDX1 mutation is presumed to relate to pancreas formation, and PDX1 mutant founder pigs generated from zygotes introduced to the CRISPR/Cas9 system can serve as providers of nonmosaics to contribute to medical research on diabetes mellitus.

Overexpression of MBD3 Improves Reprogramming of Cloned Pig Embryos

Methyl-CpG-binding domain protein 3 (MBD3) is a core component of the nucleosome remodeling and deacetylase (NuRD) complex, which is crucial for pluripotent stem cell differentiation and embryonic development. MBD3 was shown to play important roles in transcription factor-induced somatic cell reprogramming. Expression level of MBD3 was demonstrated to be higher in somatic cell nuclear transfer-generated cloned pig embryos than in fertilization-derived porcine embryos. However, the functions of MBD3 in nuclear transfer-mediated somatic cell reprogramming are unknown. In this study, MBD3 was overexpressed in cloned pig embryos, and the effects of MBD3 overexpression on gene transcription, DNA methylation, and in vitro developmental competence of cloned pig embryos were analyzed. Results indicated that overexpression of MBD3 in cloned pig embryos not only increased blastocyst rate and number of cells per blastocyst but also upregulated mRNA expression levels and decreased the DNA methylation of NANOG, OCT4, and LINE1 genes to the levels close to those in in vivo fertilization-produced pig embryos. These findings suggest that overexpression of MBD3 improves reprogramming of cloned pig embryos.

Generation of transgenic-cloned Huanjiang Xiang pigs systemically expressing enhanced green fluorescent protein

Huanjiang Xiang pig is a unique native minipig breed originating in Guangxi, China, and has great utility value in agriculture and biomedicine. Reproductive biotechnologies such as somatic cell nuclear transfer (SCNT) and SCNT-mediated genetic modification show great potential value in genetic preservation and utilization of Huanjiang Xiang pigs. Our previous work has successfully produced cloned and transgenic-cloned embryos using somatic cells from a Huanjiang Xiang pig. In this study, we firstly report the generation of transgenic-cloned Huanjiang Xiang pigs carrying an enhanced green fluorescent protein (eGFP) gene. A total of 504 SCNT-derived embryos were transferred to two surrogate recipients, one of which became pregnant and gave birth to three live piglets. Exogenous eGFP transgene had integrated in all of the three Huanjiang Xiang piglets identified by genotyping. Furthermore, expression of eGFP was also detected from in vitro cultured skin fibroblast cells and various organs or tissues from positive transgenic-cloned Huanjiang Xiang pigs. The present work provides a practical method to preserve this unique genetic resource and also lays a foundation for genetic modification of Huanjiang Xiang pigs with improved values in agriculture and biomedicine.

PRIMARY FIBROBLAST CELL CYCLE SYNCHRONIZATION AND EFFECTS ON HANDMADE CLONED (HMC) BOVINE EMBRYOS

Abstract Spatial and temporal synchrony and compatibility between the receptor oocyte and the donor cell nucleus are necessary for the process of embryo cloning to allow nuclear reprogramming and early embryonic development. The objective of the present study was to evaluate three cell cycle synchronization methods on a primary bovine fibroblast culture for 24, 48, or 72 h. These fibroblasts were used as nuclear donors to evaluate their in vitro developmental potential and the quality of the embryos produced through handmade cloning (HMC). No differences were found between the methods used for fibroblast synchronization in G0/G1 (p > 0.05). Production of clones from fibroblasts in four groups- no treatment at 0 h and using serum restriction SR, high culture confluence HCC, and SR+HCC at 24 h- resulted in high cleavage rates that were not different. Embryo production rates were 37.9%, 29.5%, and 30.9% in the 0h, SR24h, and SR+HHC24h groups, respectively, and 19.3% in the HCC group, which was significantly different from the other three (p < 0.05). There were no differences in the quality parameter among the clones produced with fibroblasts subjected to the different synchronization. Finally, when overall clone production was compared versus parthenotes and IVF embryos, the only difference was between clones and parthenogenetic embryos with zona pellucida (30.2% vs 38.6%). The number of blastomeres from the blastocytes produced through IVF was significantly greater than those from embryos activated parthenogenetically and from clones (117, 80, 75.9, and 67.1, respectively). The evaluation of three synchronization methods at different time points did not demonstrate an increase in the percentage of fibroblasts in the G0/G1 phases of the cell cycle; however, good quality and high cloning rates were obtained, suggesting that it is not always necessary to subject the cells to any synchronization treatments, as they would yield equally good cloning results.

Maternal dietary supplementation of arginine increases the ratio of total cloned piglets born to total transferred cloned embryos by improving the pregnancy rate of recipient sows

The extremely low full-term developmental efficiency of cloned pig embryos limits the practical application of pig cloning techniques. Maternal dietary supplementation of the nutritionally important amino acid, arginine, can enhance prenatal developmental rate of in vivo fertilization-derived pig embryos. It was hypothesized that maternal dietary addition of arginine can also improve the developmental capacity of cloned pig embryos. To test this hypothesis, there was a comparison of the reproductive performance between recipient sows fed an L-arginine-supplemented diet (L-Arg group) and those fed the control diet (control group). There was a subsequent comparison of the developmental indexes of cloned piglets farrowed in the L-Arg and control groups of surrogate sows. Dietary supplementation of L-arginine during gestation days 14-75 increased the plasma concentrations of arginine and arginine metabolites, including nitric oxide, spermidine, and putrescine in recipient sows of transferred cloned pig embryos. Although maternal arginine addition did not affect the birth weight and placental development indexes of newborn cloned piglets, it significantly increased the ratio of total cloned piglets born to total transferred cloned pig embryos by increasing the pregnancy rate of recipient sows. The results of this study suggest that nutritional management of recipient sows is an effective approach to improve the developmental rate of cloned pig embryos.

Can Reprogramming of Overall Epigenetic Memory and Specific Parental Genomic Imprinting Memory within Donor Cell-Inherited Nuclear Genome be a Major Hindrance for the Somatic Cell Cloning of Mammals? – A Review

Successful cloning of animals by somatic cell nuclear transfer (SCNT) requires epigenetic transcriptional reprogramming of the differentiated state of the donor cell nucleus to a totipotent embryonic ground state. It means that the donor nuclei must cease its own program of gene expression and restore a particular program of the embryonic genome expression regulation that is necessary for normal development. Transcriptional activity of somatic cell-derived nuclear genome during embryo pre- and postimplantation development as well as foetogenesis is correlated with the frequencies for spatial remodeling of chromatin architecture and reprogramming of cellular epigenetic memory. This former and this latter process include such covalent modifications as demethylation/re-methylation of DNA cytosine residues and acetylation/deacetylation as well as demethylation/re-methylation of lysine residues of nucleosomal core-derived histones H3 and H4. The main cause of low SCNT efficiency in mammals turns out to be an incomplete reprogramming of transcriptional activity for donor cell-descended genes. It has been ascertained that somatic cell nuclei should undergo the wide DNA cytosine residue demethylation changes throughout the early development of cloned embryos to reset their own overall epigenetic and parental genomic imprinting memories that have been established by re-methylation of the nuclear donor cell-inherited genome during specific pathways of somatic and germ cell lineage differentiation. A more extensive understanding of the molecular mechanisms and recognition of determinants for epigenetic transcriptional reprogrammability of somatic cell nuclear genome will be helpful to solve the problems resulting from unsatisfactory SCNT effectiveness and open new possibilities for common application of this technology in transgenic research focused on human biomedicine.

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Suberoylanilide hydroxamic acid during in vitro culture improves development of dog-pig interspecies cloned embryos but not dog cloned embryos

This study was conducted to investigate whether the treatment of dog to pig interspecies somatic cell nuclear transfer (iSCNT) embryos with a histone deacetylase inhibitor, to improve nuclear reprogramming, can be applied to dog SCNT embryos. The dog to pig iSCNT embryos were cultured in fresh porcine zygote medium-5 (PZM-5) with 0, 1, or 10 µM suberoylanilide hydroxamic acid (SAHA) for 6 h, then transferred to PZM-5 without SAHA. Although there were no significant differences in cleavage rates, the rates of 5-8-cell stage embryo development were significantly higher in the 10 µM group (19.5 ± 0.8%) compared to the 0 µM groups (13.4 ± 0.8%). Acetylation of H3K9 was also significantly higher in embryos beyond the 4-cell stage in the 10 µM group compared to the 0 or 1 µM groups. Treatment with 10 µM SAHA for 6 h was chosen for application to dog SCNT. Dog cloned embryos with 0 or 10 µM SAHA were transferred to recipients. However, there were no significant differences in pregnancy and delivery rates between the two groups. Therefore, it can be concluded that although porcine oocytes support nuclear reprogramming of dog fibroblasts, treatment with a histone deacetylase inhibitor that supports nuclear reprogramming in dog to pig iSCNT embryos was not sufficient for reprogramming in dog SCNT embryos.

Zygote injection of RNA encoding Cre recombinase results in efficient removal of LoxP flanked neomycin cassettes in pigs

Genetically engineered pigs are often created with a targeting vector that contains a loxP flanked selectable marker like neomycin. The Cre-loxP recombinase system can be used to remove the selectable marker gene from the resulting offspring or cell line. Here is described a new method to remove a loxP flanked neomycin cassette by direct zygote injection of an mRNA encoding Cre recombinase. The optimal concentration of mRNA was determined to be 10 ng/μL when compared to 2 and 100 ng/μL (P < 0.0001). Development to the blastocyst stage was 14.1% after zygote injection with 10 ng/μL. This method successfully removed the neomycin cassette in 81.9% of injected in vitro derived embryos; which was significantly higher than the control (P < 0.0001). Embryo transfer resulted in the birth of one live piglet with a Cre deleted neomycin cassette. The new method described can be used to efficiently remove selectable markers in genetically engineered animals without the need for long term cell culture and subsequent somatic cell nuclear transfer.

Dynamic changes of SETD2, a histone H3K36 methyltransferase, in porcine oocytes, IVF and SCNT embryos

SETD2 (SET domain containing protein 2) acts as a histone H3 lysine 36 (H3K36)-specific methyltransferase and may play important roles in active gene transcription in human cells. However, its expression and role in porcine oocytes and preimplantation embryos are not well understood. Here, we used immunofluorescence and laser scanning confocal microscopy to examine SETD2 expression in porcine fetal fibroblasts, oocytes, and preimplantation embryos derived from in vitro fertilization (IVF), parthenogenetic activation (PA), and somatic cell nuclear transfer (SCNT). In porcine fetal fibroblasts, SETD2 expression was detected in interphase cells, but not in M (mitotic)-phase cells. The SETD2 signal was observed in non-surrounded nucleolus (NSN)-stage oocytes, but not in surrounded nucleolus (SN)-, metaphase I (MI)-, or metaphase II (MII)-stage oocytes. The SETD2 signal was detectable in sperm, and undetectable immediately after fertilization, detectable at the 2-cell stage, and peaked at the 4-cell stage of IVF embryos in which porcine embryonic genome is activated. Similar to the pattern found in IVF embryos, the SETD2 signal was absent from PA embryos at the 1-cell stage, but it was detected at the 2-cell stage and thereafter maintained to the blastocyst stage. Interestingly, unlike the IVF and PA embryos, the SETD2 signal was detected throughout the development of SCNT embryos, including at the 1-cell stage. These data suggest that SETD2 may be functional for embryonic gene transcription in porcine preimplantation embryos. It is further speculated that the aberrant expression of SETD2 at the 1-cell stage of porcine SCNT embryos may be a factor in the low efficiency of cloning in pig.

Modeling lethal X-linked genetic disorders in pigs with ensured fertility

The development of therapies for rare and intractable genetic disorders represents a significant unmet medical need. Disease model pigs characterized by physiological, anatomical, and pathogenetic similarities to humans allow translational studies to be performed, yielding valuable data that can be extrapolated to patients. The establishment of an efficient reproduction system is a key element in the practical application of disease model pigs, which often suffer from reproductive inability due to severe symptoms. Here, we showed that the valuable trait of genetically modified disease model pigs can be maximized by generating unique chimeric boars composed of mutant and normal cells.

Genetically engineered pigs play an indispensable role in the study of rare monogenic diseases. Pigs harboring a gene responsible for a specific disease can be efficiently generated via somatic cell cloning. The generation of somatic cell-cloned pigs from male cells with mutation(s) in an X chromosomal gene is a reliable and straightforward method for reproducing X-linked genetic diseases (XLGDs) in pigs. However, the severe symptoms of XLGDs are often accompanied by impaired growth and reproductive disorders, which hinder the reproduction of these valuable model animals. Here, we generated unique chimeric boars composed of mutant cells harboring a lethal XLGD and normal cells. The chimeric boars exhibited the cured phenotype with fertility while carrying and transmitting the genotype of the XLGD. This unique reproduction system permits routine production of XLGD model pigs through the male-based breeding, thereby opening an avenue for translational research using disease model pigs.

Inactivation of porcine endogenous retrovirus in pigs using CRISPR-Cas9

Xenotransplantation is a promising strategy to alleviate the shortage of organs for human transplantation. In addition to the concerns about pig-to-human immunological compatibility, the risk of cross-species transmission of porcine endogenous retroviruses (PERVs) has impeded the clinical application of this approach. We previously demonstrated the feasibility of inactivating PERV activity in an immortalized pig cell line. We now confirm that PERVs infect human cells, and we observe the horizontal transfer of PERVs among human cells. Using CRISPR-Cas9, we inactivated all of the PERVs in a porcine primary cell line and generated PERV-inactivated pigs via somatic cell nuclear transfer. Our study highlights the value of PERV inactivation to prevent cross-species viral transmission and demonstrates the successful production of PERV-inactivated animals to address the safety concern in clinical xenotransplantation.

Effect of Cell Cycle Interactions and Inhibition of Histone Deacetylases on Development of Porcine Embryos Produced by Nuclear Transfer

The aim of this study was to evaluate if the positive effects of inhibiting histone deacetylase enzymes on cell reprogramming and development of somatic cell nuclear transfer (SCNT) embryos is affected by the cell cycle stage of nuclear donor cells and host oocytes at the time of embryo reconstruction. SCNT embryos were produced with metaphase II (MII) or telophase II (TII) cytoplasts and nuclear donor cells that were either at the G1-0 or G2/M stages. Embryos reconstructed with the different cell cycle combinations were treated or not with the histone deacetylase inhibitor (HDACi) Scriptaid for 15 h and then cultured in vitro for 7 days. Embryos reconstructed with MII-G1-0 and TII-G2/M developed to the blastocyst stage with a higher frequency compared to the other groups, confirming the importance of cell cycle interactions on cell reprogramming and SCNT embryo development. Treatment with HDACi improved development of SCNT embryos produced with MII but not TII cytoplasts, independently of the cell cycle stage of nuclear donor cells. These findings provide evidence that the positive effect of HDACi treatment on development of SCNT embryos depends upon cell cycle interactions between the host cytoplast and the nuclear donor cells.

Successful cloning of an adult breeding boar from the novel Chinese Guike No. 1 swine specialized strain

Somatic cloning, also known as somatic cell nuclear transfer (SCNT), is a promising technology which has been expected to rapidly extend the population of elaborately selected breeding boars with superior production performance. Chinese Guike No. 1 pig breed is a novel swine specialized strain incorporated with the pedigree background of Duroc and Chinese Luchuan pig breeds, thus inherits an excellent production performance. The present study was conducted to establish somatic cloning procedures of adult breeding boars from the Chinese Guike No. 1 specialized strain. Ear skin fibroblasts were first isolated from a three-year-old Chinese Guike No. 1 breeding boar, and following that, used as donor cell to produce nuclear transfer embryos. Such cloned embryos showed full in vitro development and with the blastocyst formation rate of 18.4 % (37/201, three independent replicates). Finally, after transferring of 1187 nuclear transfer derived embryos to four surrogate recipients, six live piglets with normal health and development were produced. The overall cloning efficiency was 0.5 % and the clonal provenance of such SCNT derived piglets was confirmed by DNA microsatellite analysis. All of the cloned piglets were clinically healthy and had a normal weight at 1 month of age. Collectively, the first successful cloning of an adult Chinese Guike No. 1 breeding boar may lay the foundation for future improving the pig production industry.

The Influence of Interspecies Somatic Cell Nuclear Transfer on Epigenetic Enzymes Transcription in Early Embryos

One of the main reason for the incorrect development of embryos derived from somatic cell nuclear transfer is caused by insufficient demethylation of injected somatic chromatin to a state comparable with an early embryonic nucleus. It is already known that the epigenetic enzymes transcription in oocytes and early embryos of several species including bovine and porcine zygotes is species-dependent process and the incomplete DNA methylation correlates with the nuclear transfer failure rate in mammals. In this study the transcription of DNA methyltransferase 1 and 3a (DNMT1, DNMT3a) genes in early embryonic stages of interspecies (bovine, porcine) nuclear transfer embryos (iSCNT) by RT-PCR were analyzed. Coming out from the diverse timing of embryonic genome activation (EGA) in porcine and bovine preimplantation embryos, the intense effect of ooplasm on transferred somatic cell nucleus was expected. In spite of the detection of ooplasmic DNA methyltransferases, the somatic genes for DNMT1 and DNMT3a enzymes were not expressed and the development of intergeneric embryos stopped at the 4-cell stage. Our results indicate that the epigenetic reprogramming during early mammalian development is strongly influenced by the ooplasmic environment.

Somatic cell reprogramming-free generation of genetically modified pigs

Genetically modified pigs for biomedical applications have been mainly generated using the somatic cell nuclear transfer technique; however, this approach requires complex micromanipulation techniques and sometimes increases the risks of both prenatal and postnatal death by faulty epigenetic reprogramming of a donor somatic cell nucleus. As a result, the production of genetically modified pigs has not been widely applied. We provide a simple method for CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 gene editing in pigs that involves the introduction of Cas9 protein and single-guide RNA into in vitro fertilized zygotes by electroporation. The use of gene editing by electroporation of Cas9 protein (GEEP) resulted in highly efficient targeted gene disruption and was validated by the efficient production of Myostatin mutant pigs. Because GEEP does not require the complex methods associated with micromanipulation for somatic reprogramming, it has the potential for facilitating the genetic modification of pigs.

An Improved System for Generation of Diploid Cloned Porcine Embryos Using Induced Pluripotent Stem Cells Synchronized to Metaphase

Pigs provide outstanding models of human genetic diseases due to their striking similarities with human anatomy, physiology and genetics. Although transgenic pigs have been produced using genetically modified somatic cells and nuclear transfer (SCNT), the cloning efficiency was extremely low. Here, we report an improved method to produce diploid cloned embryos from porcine induced pluripotent stem cells (piPSCs), which were synchronized to the G2/M stage using a double blocking method with aphidicolin and nocodazole. The efficiency of this synchronization method on our piPSC lines was first tested. Then, we modified our traditional SCNT protocol to find a workable protocol. In particular, the removal of a 6DMAP treatment post-activation enhanced the extrusion rate of pseudo-second-polar bodies (p2PB) (81.3% vs. 15.8%, based on peak time, 4hpa). Moreover, an immediate activation method yielded significantly more blastocysts than delayed activation (31.3% vs. 16.0%, based on fused embryos). The immunofluorescent results confirmed the effect of the 6DMAP treatment removal, showing remarkable p2PB extrusion during a series of nuclear transfer procedures. The reconstructed embryos from metaphase piPSCs with our modified protocol demonstrated normal morphology at 2-cell, 4-cell and blastocyst stages and a high rate of normal karyotype. This study demonstrated a new and efficient way to produce viable cloned embryos from piPSCs when synchronized to the G2/M phase of the cell cycle, which may lead to opportunities to produce cloned pigs from piPSCs more efficiently.

Engineering Large Animal Species to Model Human Diseases

Animal models are an important resource for studying human diseases. Genetically engineered mice are the most commonly used species and have made significant contributions to our understanding of basic biology, disease mechanisms, and drug development. However, they often fail to recreate important aspects of human diseases and thus can have limited utility as translational research tools. Developing disease models in species more similar to humans may provide a better setting in which to study disease pathogenesis and test new treatments. This unit provides an overview of the history of genetically engineered large animals and the techniques that have made their development possible. Factors to consider when planning a large animal model, including choice of species, type of modification and methodology, characterization, production methods, and regulatory compliance, are also covered. © 2016 by John Wiley & Sons, Inc.

Embryo Aggregation in Pig Improves Cloning Efficiency and Embryo Quality

In this study, we analyzed the effects of the cloned embryo aggregation on in vitro embryo development and embryo quality by measuring blastocyst diameter and cell number, DNA fragmentation levels and the expression of genes associated with pluripotency, apoptosis, trophoblast and DNA methylation in the porcine. Zona-free reconstructed cloned embryos were cultured in the well of the well system, placing one (1x non aggregated group) or three (3x group) embryos per microwell. Our results showed that aggregation of three embryos increased blastocyst formation rate and blastocyst diameter of cloned pig embryos. DNA fragmentation levels in 3x aggregated cloned blastocysts were significantly decreased compared to 1x blastocysts. Levels of Oct4, Klf4, Igf2, Bax and Dnmt 1 transcripts were significantly higher in aggregated embryos, whereas Nanog levels were not affected. Transcripts of Cdx2 and Bcl-xl were essentially non-detectable. Our study suggests that embryo aggregation in the porcine may be beneficial for cloned embryo development and embryo quality, through a reduction in apoptotic levels and an improvement in cell reprogramming.

Alteration of the DNA methylation status of donor cells impairs the developmental competence of porcine cloned embryos

Nuclear reprogramming induced by somatic cell nuclear transfer is an inefficient process, and donor cell DNA methylation status is thought to be a major factor affecting cloning efficiency. Here, the role of donor cell DNA methylation status regulated by 5-aza-2'-deoxycytidine (5-aza-dC) or 5-methyl-2'-deoxycytidine-5'-triphosphate (5-methyl-dCTP) in the early development of porcine cloned embryos was investigated. Our results showed that 5-aza-dC or 5-methyl-dCTP significantly reduced or increased the global methylation levels and altered the methylation and expression levels of key genes in donor cells. However, the development of cloned embryos derived from these cells was reduced. Furthermore, disrupted pseudo-pronucleus formation and transcripts of early embryo development-related genes were observed in cloned embryos derived from these cells. In conclusion, our results demonstrated that alteration of the DNA methylation status of donor cells by 5-aza-dC or 5-methyl-dCTP disrupted nuclear reprogramming and impaired the developmental competence of porcine cloned embryos.

Altering histone acetylation status in donor cells with suberoylanilide hydroxamic acid does not affect dog cloning efficiency

Although dog cloning technology has been applied to conservation of endangered canids, propagation of elite dogs, and production of transgenic dogs, the efficiency of cloning is still very low. To help overcome this problem, we evaluated the effect of treating donor cells with suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, on dog cloning efficiency. Relative messenger RNA expressions of the bax1/bcl2 ratio and Dnmt1 in fibroblasts treated with different concentrations (0, 1, 10, 50 μM) of SAHA and durations (0, 20, 44 hours) were compared. Treatment with 1 μM for 20 hours showed significantly lower bax1/bcl2 and Dnmt1 transcript abundance. Acetylation of H3K9 was significantly increased after SAHA treatment, but H4K5, H4K8 and H4K16 were not changed. After SCNT using control or donor cells treated with SAHA, a total of 76 and 64 cloned embryos were transferred to seven and five recipients, respectively. Three fetuses were diagnosed in both control and SAHA-treated groups by ultrasonography 29 days after the embryo transfer, but there was no significant difference in the pregnancy rate (4.2% vs. 4.3%). In conclusion, although SAHA treatment as used in this study significantly decreased bax1/bcl2 and Dnmt1 transcripts of donor nuclei, as well as increased H3 acetylation, it was not enough to increase in vivo developmental competence of cloned dog embryos.

Epigenetic Modification Agents Improve Gene-Specific Methylation Reprogramming in Porcine Cloned Embryos

Incomplete DNA methylation reprogramming in cloned embryos leads to poor cloning efficiency. Epigenetic modification agents can improve genomic methylation reprogramming and the development of cloned embryos, however, the effect of epigenetic modification agents on gene-specific methylation reprogramming remains poorly studied. Here, we investigated DNA methylation reprogramming of pluripotency (Oct4) and tissue specific (Thy1) genes during early embryo development in pigs. In this study, we found that compared with in vitro fertilized counterparts, cloned embryos displayed the disrupted patterns of Oct4 demethylation and Thy1 remethylation. When 5-aza-2'-deoxycytidine (5-aza-dC) or trichostatin A (TSA) enhanced the development of cloned embryos, the transcripts of DNA methyltransferases (Dnmt1 and Dnmt3a), histone acetyltransferase 1 (Hat1) and histone deacetylase 1 (Hdac1) and the methylation and expression patterns of Oct4 and Thy1 became similar to those detected in in vitro fertilized counterparts. Further studies showed that Dnmt1 knockdown in cloned embryos enhanced the methylation reprogramming of Oct4 and Thy1 and promoted the activation of Oct4 and the silence of Thy1. In conclusion, our results demonstrated that cloned embryos displayed incomplete gene-specific methylation reprogramming and disrupted expression patterns of pluripotency and tissue specific genes, and epigenetic modification agents improved gene-specific methylation reprogramming and expression pattern by regulating epigenetic modification related genes. This work would have important implications in improving cloning efficiency.

Epigenetic Modification of Cloned Embryos Improves Nanog Reprogramming in Pigs

Incomplete reprogramming of pluripotent genes in cloned embryos is associated with low cloning efficiency. Epigenetic modification agents have been shown to enhance the developmental competence of cloned embryos; however, the effect of the epigenetic modification agents on pluripotent gene reprogramming remains unclear. Here, we investigated Nanog reprogramming and the expression patterns of pluripotent transcription factors during early embryo development in pigs. We found that compared with fertilized embryos, cloned embryos displayed higher methylation in the promoter and 5'-untranslated region and lower methylation in the first exon of Nanog. When 5-aza-2'-deoxycytidine (5-aza-dC) or trichostatin A (TSA) enhanced the development of porcine cloned embryos, Nanog methylation reprogramming was also improved, similar to that detected in fertilized counterparts. Furthermore, our results showed that the epigenetic modification agents improved the expression levels of Oct4 and Sox2 and effectively promoted Nanog transcription in cloned embryos. In conclusion, our results demonstrated that the epigenetic modification agent 5-aza-dC or TSA improved Nanog methylation reprogramming and the expression patterns of pluripotent transcription factors, thereby resulting in the enhanced expression of Nanog and high development of porcine cloned embryos. This work has important implications in the improvement of cloning efficiency.

Long-term effect on in vitro cloning efficiency after treatment of somatic cells with Xenopus egg extract in the pig

In somatic cell nuclear transfer (SCNT), donor cell reprogramming is considered as a biologically important and vulnerable event. Various donor cell pre-treatments with Xenopus egg extracts can promote reprogramming. Here we investigated if the reprogramming effect of one treatment with Xenopus egg extract on donor cells was maintained for several cell passages. The extract treatment resulted in increased cell-colony formation from early passages in treated porcine fibroblasts (ExTES), and increased development of cloned embryos. Partial dedifferentiation was observed in ExTES cells, shown as a tendency towards upregulation of NANOG, c-MYC and KLF-4 and downregulation of DESMIM compared with ExTES at Passage 2. Compared with our routine SCNT, continuously increased development of cloned embryos was observed in the ExTES group, and ExTES cloned blastocysts displayed hypermethylated DNA patterns and hypermethylation of H3K4me3 and H3K27me3 in ICM compared with TE. All seven recipients became pregnant after transferral of ExTES cloned embryos and gave birth to 7-22 piglets per litter (average 12). In conclusion, our results demonstrate that one treatment of porcine fibroblasts with Xenopus egg extract can result in long-term increased ability of the cells to promote their in vitro function in subsequent SCNT. Finally these cells can also result in successful development of cloned embryos to term.

Trichostatin A-mediated epigenetic transformation of adult bone marrow-derived mesenchymal stem cells biases the in vitro developmental capability, quality, and pluripotency extent of porcine cloned embryos

The current research was conducted to explore the in vitro developmental outcome and cytological/molecular quality of porcine nuclear-transferred (NT) embryos reconstituted with adult bone marrow-derived mesenchymal stem cells (ABM-MSCs) that were epigenetically transformed by treatment with nonspecific inhibitor of histone deacetylases, known as trichostatin A (TSA). The cytological quality of cloned blastocysts was assessed by estimation of the total cells number (TCN) and apoptotic index. Their molecular quality was evaluated by real-time PCR-mediated quantification of gene transcripts for pluripotency- and multipotent stemness-related markers (Oct4, Nanog, and Nestin). The morula and blastocyst formation rates of NT embryos derived from ABM-MSCs undergoing TSA treatment were significantly higher than in the TSA-unexposed group. Moreover, the NT blastocysts generated using TSA-treated ABM-MSCs exhibited significantly higher TCN and increased pluripotency extent measured with relative abundance of Oct4 and Nanog mRNAs as compared to the TSA-untreated group. Altogether, the improvements in morula/blastocyst yields and quality of cloned pig embryos seem to arise from enhanced abilities for promotion of correct epigenetic reprogramming of TSA-exposed ABM-MSC nuclei in a cytoplasm of reconstructed oocytes. To our knowledge, we are the first to report the successful production of mammalian high-quality NT blastocysts using TSA-dependent epigenomic modulation of ABM-MSCs.

Influence of embryo handling and transfer method on pig cloning efficiency

The somatic cell nuclear transfer (SCNT) technique could be used to produce genetically superior or genetically engineered cloned pigs that have wide application in agriculture and bioscience research. However, the efficiency of porcine SCNT currently is very low. Embryo transfer (ET) is a key step for the success of SCNT. In this study, the effects of several ET-related factors, including cloned embryo culture time, recipient's ovulation status, co-transferred helper embryos and ET position, on the success rate of pig cloning were investigated. The results indicated that transfer of cloned embryos cultured for a longer time (22-24h vs. 4-6h) into pre-ovulatory sows decreased recipient's pregnancy rate and farrowing rate, and use of pre-ovulatory and post-ovulatory sows as recipients for SCNT embryos cultured for 22-24h resulted in a similar porcine SCNT efficiency. Use of insemination-produced in vivo fertilized, parthenogenetically activated and in vitro fertilized embryos as helper embryos to establish and/or maintain pregnancy of SCNT embryos recipients could not improve the success rate of porcine SCNT. Transfer of cloned embryos into double oviducts of surrogates significantly increased pregnancy rate as well as farrowing rate of recipients, and the developmental rate of transferred cloned embryos, as compared to unilateral oviduct transfer. This study provided useful information for optimization of the embryo handling and transfer protocol, which will help to improve the ability to generate cloned pigs.

Oxamflatin treatment enhances cloned porcine embryo development and nuclear reprogramming

Faulty epigenetic reprogramming of somatic nuclei is thought to be the main reason for low cloning efficiency by somatic cell nuclear transfer (SCNT). Histone deacetylase inhibitors (HDACi), such as Scriptaid, improve developmental competence of SCNT embryos in several species. Another HDACi, Oxamflatin, is about 100 times more potent than Scriptaid in the ability to inhibit nuclear-specific HDACs. The present study determined the effects of Oxamflatin treatment on embryo development, DNA methylation, and gene expression. Oxamflatin treatment enhanced blastocyst formation of SCNT embryos in vitro. Embryo transfer produced more pigs born and fewer mummies from the Oxamflatin-treated group compared to the Scriptaid-treated positive control. Oxamflatin also decreased DNA methylation of POU5F1 regulatory elements and centromeric repeat elements in day-7 blastocysts. When compared to in vitro-fertilized (IVF) embryos, the methylation status of POU5F1, NANOG, and centromeric repeat was similar in the cloned embryos, indicating these genes were successfully reprogrammed. However, compared to the lack of methylation of XIST in day-7 IVF embryos, a higher methylation level in day-7 cloned embryos was observed, implying that X chromosomes were activated in day-7 IVF blastocysts, but were not fully activated in cloned embryos, i.e., reprogramming of XIST was delayed. A time-course analysis of XIST DNA methylation on day-13, -15, -17, and -19 in vivo embryos revealed that XIST methylation initiated at about day 13 and was not completed by day 19. The methylation of the XIST gene in day-19 control cloned embryos was delayed again when compared to in vivo embryos. However, methylation of XIST in Oxamflatin-treated embryos was comparable with in vivo embryos, which further demonstrated that Oxamflatin could accelerate the delayed reprogramming of XIST gene and thus might improve cloning efficiency.

Generation of recombination activating gene-1-deficient neonatal piglets: a model of T and B cell deficient severe combined immune deficiency

Although severe combined immune deficiency (SCID) is a very important research model for mice and SCID mice are widely used, there are only few reports describing the SCID pig models. Therefore, additional research in this area is needed. In this study, we describe the generation of Recombination activating gene-1 (Rag-1)-deficient neonatal piglets in Duroc breed using somatic cell nuclear transfer (SCNT) with gene targeting and analysis using fluorescence-activated cell sorting (FACS) and histology. We constructed porcine Rag-1 gene targeting vectors for the Exon 2 region and obtained heterozygous/homozygous Rag-1 knockout cell colonies using SCNT. We generated two Rag-1-deficient neonatal piglets and compared them with wild-type neonatal piglets. FACS analysis showed that Rag-1 disruption causes a lack of Immunoglobulin M-positive B cells and CD3-positive T cells in peripheral blood mononuclear cells. Consistent with FACS analysis, histological analysis revealed structural defects and an absence of mature lymphocytes in the spleen, mesenteric lymph node (MLNs), and thymus in Rag-1-deficient piglets. These results confirm that Rag-1 is necessary for the generation of lymphocytes in pigs, and Rag-1-deficient piglets exhibit a T and B cell deficient SCID (T-B-SCID) phenotype similar to that of rodents and humans. The T-B-SCID pigs with Rag-1 deficiency generated in this study could be a suitably versatile model for laboratory, translational, and biomedical research, including the development of a humanized model and assessment of pluripotent stem cells.

Effects of Trichostatin A on In vitro Development of Porcine Embryos Derived from Somatic Cell Nuclear Transfer

Many different approaches have been developed to improve the efficiency of animal cloning by somatic cell nuclear transfer (SCNT), one of which is to modify histone acetylation levels using histone deacetylase inhibitors (HDACi) such as trichostatin A (TSA). In the present study, we examined the effect of TSA on in vitro development of porcine embryos derived from SCNT. We found that TSA treatment (50 nM) for 24 h following oocyte activation improved blastocyst formation rates (to 22.0%) compared with 8.9% in the non-treatment group and total cell number of the blastocysts for determining embryo quality also increased significantly (88.9→114.4). Changes in histone acetylation levels as a result of TSA treatment were examined using indirect immunofluorescence and confocal microscopy scanning. Results showed that the histone acetylation level in TSA-treated embryos was higher than that in controls at both acetylated histone H3 lysine 9 (AcH3K9) and acetylated histone H4 lysine 12 (AcH4K12). Next, we compared the expression patterns of seven genes (OCT4, ID1; the pluripotent genes, H19, NNAT, PEG1; the imprinting genes, cytokeratin 8 and 18; the trophoblast marker genes). The SCNT blastocysts both with and without TSA treatment showed lower levels of OCT4, ID1, cytokeratin 8 and 18 than those of the in vivo blastocysts. In the case of the imprinting genes H19 and NNAT, except PEG1, the SCNT blastocysts both with and without TSA treatment showed higher levels than those of the in vivo blastocysts. Although the gene expression patterns between cloned blastocysts and their in vivo counterparts were different regardless of TSA treatment, it appears that several genes in NT blastocysts after TSA treatment showed a slight tendency toward expression patterns of in vivo blastocysts. Our results suggest that TSA treatment may improve preimplantation porcine embryo development following SCNT.

Epigenetic modification agents improve genomic methylation reprogramming in porcine cloned embryos

Incomplete DNA methylation reprogramming in cloned embryos leads to low cloning efficiency. Our previous studies showed that the epigenetic modification agents 5-aza-2'-deoxycytidine (5-aza-dC) or trichostatin A (TSA) could enhance the developmental competence of porcine cloned embryos. Here, we investigated genomic methylation dynamics and specific gene expression levels during early embryonic development in pigs. In this study, our results showed that there was a typical wave of DNA demethylation and remethylation of centromeric satellite repeat (CenRep) in fertilized embryos, whereas in cloned embryos, delayed demethylation and a lack of remethylation were observed. When cloned embryos were treated with 5-aza-dC or TSA, CenRep methylation reprogramming was improved, and this was similar to that detected in fertilized counterparts. Furthermore, we found that the epigenetic modification agents, especially TSA, effectively promoted silencing of tissue specific genes and transcription of early embryo development-related genes in porcine cloned embryos. In conclusion, our results showed that the epigenetic modification agent 5-aza-dC or TSA could improve genomic methylation reprogramming in porcine cloned embryos and regulate the appropriate expression levels of genes related to early embryonic development, thereby resulting in high developmental competence.

Reshaping the transcriptional frontier: epigenetics and somatic cell nuclear transfer

Somatic-cell nuclear transfer (SCNT) experiments have paved the way to the field of cellular reprogramming. The demonstrated ability to clone over 20 different species to date has proven that the technology is robust but very inefficient, and is prone to developmental anomalies. Yet, the offspring from cloned animals exhibit none of the abnormalities of their parents, suggesting the low efficiency and high developmental mortality are epigenetic in origin. The epigenetic barriers to reprogramming somatic cells into a totipotent embryo capable of developing into a viable offspring are significant and varied. Despite their intimate relationship, chromatin structure and transcription are often not uniformly reprogramed after nuclear transfer, and many cloned embryos develop gene expression profiles that are hybrids between the donor cell and an embryonic blastomere. Recent advances in cellular reprogramming suggest that alteration of donor-cell chromatin structure towards that found in an normal embryo is actually the rate-limiting step in successful development of SCNT embryos. Here we review the literature relevant to the transformation of a somatic-cell nucleus into an embryo capable of full-term development. Interestingly, while resetting somatic transcription and associated epigenetic marks are absolutely required for development of SCNT embryos, life does not demand perfection.