Methionine adenosyltransferase 2A regulates mouse zygotic genome activation and morula to blastocyst transition†

Methionine adenosyltransferase II (MAT2A) is essential to the synthesis of S-adenosylmethionine, a major methyl donor, from L-methionine and ATP. Upon fertilization, zygotic genome activation (ZGA) marks the period that transforms the genome from transcriptional quiescence to robust transcriptional activity. During this period, embryonic epigenome undergoes extensive modifications, including histone methylation changes. However, whether MAT2A participates in histone methylation at the ZGA stage is unknown. Herein, we identified that MAT2A is a pivotal factor for ZGA in mouse embryos. Mat2a knockdown exhibited 2-cell embryo arrest and reduced transcriptional activity but did not affect H3K4me2/3 and H3K9me2/3. When the cycloleucine, a selective inhibitor of MAT2A catalytic activity, was added to a culture medium, embryos were arrested at the morula stage in the same manner as the embryos cultured in an L-methionine-deficient medium. Under these two culture conditions, H3K4me3 levels of morula and blastocyst were much lower than those cultured under normal medium. Furthermore, cycloleucine treatment or methionine starvation apparently reduced the developmental potential of blastocysts. Thus, Mat2a is indispensable for ZGA and morula-to-blastocyst transition.

Deubiquitylase ubiquitin-specific protease 7 plays a crucial role in the lineage differentiation of preimplantation blastocysts†

Preimplantation embryos undergo a series of important biological events, including epigenetic reprogramming and lineage differentiation, and the key genes and specific mechanisms that regulate these events are critical to reproductive success. Ubiquitin-specific protease 7 (USP7) is a deubiquitinase involved in the regulation of a variety of cellular functions, yet its precise function and mechanism in preimplantation embryonic development remain unknown. Our results showed that RNAi-mediated silencing of USP7 in mouse embryos or treatment with P5091, a small molecule inhibitor of USP7, significantly reduced blastocyst rate and blastocyst quality, and decreased total and trophectoderm cell numbers per blastocyst, as well as destroyed normal lineage differentiation. The results of single-cell RNA-seq, reverse transcription-quantitative polymerase chain reaction, western blot, and immunofluorescence staining indicated that interference with USP7 caused failure of the morula-to-blastocyst transition and was accompanied by abnormal expression of key genes (Cdx2, Oct4, Nanog, Sox2) for lineage differentiation, decreased transcript levels, increased global DNA methylation, elevated repressive histone marks (H3K27me3), and decreased active histone marks (H3K4me3 and H3K27ac). Notably, USP7 may regulate the transition from the morula to blastocyst by stabilizing the target protein YAP through the ubiquitin-proteasome pathway. In conclusion, our results suggest that USP7 may play a crucial role in preimplantation embryonic development by regulating lineage differentiation and key epigenetic modifications.