siRNA inhibition and not chemical inhibition of Suv39h1/2 enhances pre-implantation embryonic development of bovine somatic cell nuclear transfer embryos

H3K9me3 Levels Affect the Proliferation of Bovine Spermatogonial Stem Cells

Spermatogonial stem cells (SSCs) possess the characteristics of self-renewal and differentiation, as well as the ability to generate functional sperm. Their unique stemness has broad applications in male infertility treatment and species preservation. In rodents, research on SSCs has been widely reported, but progress is slow in large livestock such as cattle and pigs due to long growth cycles, difficult proliferation in vitro, and significant species differences. Previously, we showed that histone 3 (H3) lysine 9 (K9) trimethylation (H3K9me3) is associated with the proliferation of bovine SSCs. Here, we isolated and purified SSCs from calf testicular tissues and investigated the impact of different H3K9me3 levels on the in vitro proliferation of bovine SSCs. The enriched SSCs eventually formed classical stem cell clones in vitro in our feeder-free culture system. These clones expressed glial cell-derived neurotrophic factor family receptor alpha-1 (GFRα1, specific marker for SSCs), NANOG (pluripotency protein), C-KIT (germ cell marker), and strong alkaline phosphatase (AKP) positivity. qRT-PCR analysis further showed that these clones expressed the pluripotency genes NANOG and SOX2, and the SSC-specific marker gene GFRα1. To investigate the dynamic relationship between H3K9me3 levels and SSC proliferation, H3K9me3 levels in bovine SSCs were first downregulated using the methyltransferase inhibitor, chaetocin, or transfection with the siRNA of H3K9 methyltransferase suppressor of variegation 3-9 homologue 1 (SUV39H1). The EDU (5-Ethynyl-2'-deoxyuridine) assay revealed that SSC proliferation was inhibited. Conversely, when H3K9me3 levels in bovine SSCs were upregulated by transfecting lysine demethylase 4D (KDM4D) siRNA, the EDU assay showed a promotion of cell proliferation. In summary, this study established a feeder-free culture system to obtain bovine SSCs and explored its effects on the proliferation of bovine SSCs by regulating H3K9me3 levels, laying the foundation for elucidating the regulatory mechanism underlying histone methylation modification in the proliferation of bovine SSCs.

Reduction of H3K9 methylation by G9a inhibitors improves the development of mouse SCNT embryos

Removal of somatic histone H3 lysine 9 trimethylation (H3K9me3) from the embryonic genome can improve the efficiency of mammalian cloning using somatic cell nuclear transfer (SCNT). However, this strategy involves the injection of histone demethylase mRNA into embryos, which is limiting because of its invasive and labor-consuming nature. Here, we report that treatment with an inhibitor of G9a (G9ai), the major histone methyltransferase that introduces H3K9me1/2 in mammals, greatly improved the development of mouse SCNT embryos. Intriguingly, G9ai caused an immediate reduction of H3K9me1/2, a secondary loss of H3K9me3 in SCNT embryos, and increased the birth rate of cloned pups about 5-fold (up to 3.9%). G9ai combined with the histone deacetylase inhibitor trichostatin A further improved this rate to 14.5%. Mechanistically, G9ai and TSA synergistically enhanced H3K9me3 demethylation and boosted zygotic genome activation. Thus, we established an easy, highly effective SCNT protocol that would enhance future cloning research and applications.

High Expression of ZFP42 Improves Early Development of Pig Embryos Produced by Handmade Cloning

Handmade Cloning (HMC) is a pivotal technique for cloning pig embryos. Despite its significance, the low efficiency of this method hampers its widespread application. Although numerous factors and signaling pathways influencing embryo development have been studied, the mechanisms underlying low developmental capacity and insufficient reprogramming of cloned embryos remain elusive. In the present study, we sought to elucidate key regulatory factors involved in the development of pig HMC embryos by comparing and analyzing the gene expression profiles of HMC embryos with those of naturally fertilized (NF) embryos at the 4-cell, 8-cell, and 16-cell stages. The results showed that ZFP42 expression is markedly higher in NF embryos than in cloned counterparts. Subsequent experiments involving the injection of ZFP42 messenger RNA (mRNA) into HMC embryos showed that ZFP42 could enhance the blastocyst formation rate, upregulate pluripotent genes and metabolic pathways. This highlights the potential of ZFP42 as a critical factor in improving the development of pig HMC embryos.

Chromatin accessibility memory of donor cells disrupts bovine somatic cell nuclear transfer blastocysts development

The post-transfer developmental capacity of bovine somatic cell nuclear transfer (SCNT) blastocysts is reduced, implying that abnormalities in gene expression regulation are present at blastocyst stage. Chromatin accessibility, as an indicator for transcriptional regulatory elements mediating gene transcription activity, has heretofore been largely unexplored in SCNT embryos, especially at blastocyst stage. In the present study, single-cell sequencing assay for transposase-accessible chromatin (scATAC-seq) of in vivo and SCNT blastocysts were conducted to segregate lineages and demonstrate the aberrant chromatin accessibility of transcription factors (TFs) related to inner cell mass (ICM) development in SCNT blastocysts. Pseudotime analysis of lineage segregation further reflected dysregulated chromatin accessibility dynamics of TFs in the ICM of SCNT blastocysts compared to their in vivo counterparts. ATAC- and ChIP-seq results of SCNT donor cells revealed that the aberrant chromatin accessibility in the ICM of SCNT blastocysts was due to the persistence of chromatin accessibility memory at corresponding loci in the donor cells, with strong enrichment of trimethylation of histone H3 at lysine 4 (H3K4me3) at these loci. Correction of the aberrant chromatin accessibility through demethylation of H3K4me3 by KDM5B diminished the expression of related genes (e.g., BCL11B) and significantly improved the ICM proliferation in SCNT blastocysts. This effect was confirmed by knocking down BCL11B in SCNT embryos to down-regulate p21 and alleviate the inhibition of ICM proliferation. These findings expand our understanding of the chromatin accessibility abnormalities in SCNT blastocysts and BCL11B may be a potential target to improve SCNT efficiency.

High-resolution ribosome profiling reveals translational selectivity for transcripts in bovine preimplantation embryo development

High-resolution ribosome fractionation and low-input ribosome profiling of bovine oocytes and preimplantation embryos has enabled us to define the translational landscapes of early embryo development at an unprecedented level. We analyzed the transcriptome and the polysome- and non-polysome-bound RNA profiles of bovine oocytes (germinal vesicle and metaphase II stages) and early embryos at the two-cell, eight-cell, morula and blastocyst stages, and revealed four modes of translational selectivity: (1) selective translation of non-abundant mRNAs; (2) active, but modest translation of a selection of highly expressed mRNAs; (3) translationally suppressed abundant to moderately abundant mRNAs; and (4) mRNAs associated specifically with monosomes. A strong translational selection of low-abundance transcripts involved in metabolic pathways and lysosomes was found throughout bovine embryonic development. Notably, genes involved in mitochondrial function were prioritized for translation. We found that translation largely reflected transcription in oocytes and two-cell embryos, but observed a marked shift in the translational control in eight-cell embryos that was associated with the main phase of embryonic genome activation. Subsequently, transcription and translation become more synchronized in morulae and blastocysts. Taken together, these data reveal a unique spatiotemporal translational regulation that accompanies bovine preimplantation development.

Pluripotent Core in Bovine Embryos: A Review

Early development in mammals is characterized by the ability of each cell to produce a complete organism plus the extraembryonic, or placental, cells, defined as pluripotency. During subsequent development, pluripotency is lost, and cells begin to differentiate to a particular cell fate. This review summarizes the current knowledge of pluripotency features of bovine embryos cultured in vitro, focusing on the core of pluripotency genes (OCT4, NANOG, SOX2, and CDX2), and main chemical strategies for controlling pluripotent networks during early development. Finally, we discuss the applicability of manipulating pluripotency during the morula to blastocyst transition in cattle 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.