Induced DNA hypomethylation by Folic Acid Deprivation in Bovine Fibroblast Donor Cells Improves Reprogramming of Somatic Cell Nuclear Transfer Embryos

Oct-4 activating compound 1 (OAC1) could improve the quality of somatic cell nuclear transfer embryos in the bovine

Previous studies described aberrant nuclear reprogramming in somatic cell nuclear transfer (SCNT) embryos that is distinctly different from fertilized embryos. This abnormal nuclear reprogramming hampers the proper pre- and/or post-implantation development. It has been demonstrated that SCNT blastocysts aberrantly expressed POU5F1 and POU5F1-related genes. With regard to this, it has been postulated that promoting the expression of POU5F1 in SCNT embryos may enhance reprogramming in SCNT embryos. In this study, we treated either fibroblast donor cells or SCNT embryos with OAC1 as a novel small molecule that has been reported to induce POU5F1 expression. Quantitative results from the MTS assay revealed that lower concentrations of OAC1 (1, 1.5, and 3 μM) are non-toxic after 2, 4, and 6 days, but higher concentrations (6, 8, 10, and 12 μM) are toxic and reduced the proliferation of cells after 6 days. No enhancement in the expression of endogenous POU5F1 was observed when both mouse and bovine fibroblast cells were treated with 1.5 and 3 μM OAC1 for up to 6 consecutive days. Subsequently, we treated either fibroblast as donor cells in the SCNT procedure (BFF-OAC1 group) or SCNT embryos [for 4 days (IVC-OAC1: D4-D7 group) or 7 days (IVC-OAC1: D0-D7 group)] with 1.5 μM OAC1. We observed that neither treatment of fibroblast donor cells nor SCNT embryos improved the cleavage and blastocyst rates. Interestingly, we observed that treatment of SCNT embryos all throughout the in vitro culture (IVC) (IVC-OAC1: D0-D7) with 1.5 μM OAC1 improves the quality of derived blastocyst which was indexed by morphological grading, blastomere allocation, epigenetic marks and mRNA expression of target genes. In conclusion, our results showed that supplementation of IVC medium with 1.5 μM OAC1 (D0-D7) accelerates SCNT reprogramming in bovine species.

Strategies to Improve the Efficiency of Somatic Cell Nuclear Transfer

Mammalian oocytes can reprogram differentiated somatic cells into a totipotent state through somatic cell nuclear transfer (SCNT), which is known as cloning. Although many mammalian species have been successfully cloned, the majority of cloned embryos failed to develop to term, resulting in the overall cloning efficiency being still low. There are many factors contributing to the cloning success. Aberrant epigenetic reprogramming is a major cause for the developmental failure of cloned embryos and abnormalities in the cloned offspring. Numerous research groups attempted multiple strategies to technically improve each step of the SCNT procedure and rescue abnormal epigenetic reprogramming by modulating DNA methylation and histone modifications, overexpression or repression of embryonic-related genes, etc. Here, we review the recent approaches for technical SCNT improvement and ameliorating epigenetic modifications in donor cells, oocytes, and cloned embryos in order to enhance cloning efficiency.

The regulation mechanisms and the Lamarckian inheritance property of DNA methylation in animals

DNA methylation is a stable and heritable epigenetic mechanism, of which the main functions are stabilizing the transcription of genes and promoting genetic conservation. In animals, the direct molecular inducers of DNA methylation mainly include histone covalent modification and non-coding RNA, whereas the fundamental regulators of DNA methylation are genetic and environmental factors. As is well known, competition is present everywhere in life systems, and will finally strike a balance that is optimal for the animal's survival and reproduction. The same goes for the regulation of DNA methylation. Genetic and environmental factors, respectively, are responsible for the programmed and plasticity changes of DNA methylation, and keen competition exists between genetically influenced procedural remodeling and environmentally influenced plastic alteration. In this process, genetic and environmental factors collaboratively decide the methylation patterns of corresponding loci. DNA methylation alterations induced by environmental factors can be transgenerationally inherited, and exhibit the characteristic of Lamarckian inheritance. Further research on regulatory mechanisms and the environmental plasticity of DNA methylation will provide strong support for understanding the biological function and evolutionary effects of DNA methylation.

Manipulating the Epigenome in Nuclear Transfer Cloning: Where, When and How

The nucleus of a differentiated cell can be reprogrammed to a totipotent state by exposure to the cytoplasm of an enucleated oocyte, and the reconstructed nuclear transfer embryo can give rise to an entire organism. Somatic cell nuclear transfer (SCNT) has important implications in animal biotechnology and provides a unique model for studying epigenetic barriers to successful nuclear reprogramming and for testing novel concepts to overcome them. While initial strategies aimed at modulating the global DNA methylation level and states of various histone protein modifications, recent studies use evidence-based approaches to influence specific epigenetic mechanisms in a targeted manner. In this review, we describe-based on the growing number of reports published during recent decades-in detail where, when, and how manipulations of the epigenome of donor cells and reconstructed SCNT embryos can be performed to optimize the process of molecular reprogramming and the outcome of nuclear transfer cloning.