Role And Mechanism Of Maternal Ezh1/2 In The Development Of Mouse Embryo

Mammalian embryo development is a complex process regulated by various epigenetic modifications.PcG(Polycomb Group)are key epigenetic regulators and important components of a widely conserved cellular memory system that prevents changes in cellular identity.The proteins composing the Polycomb group(PcG)play central roles during embryogenesis when they regulate a large number of target genes involved in differentiation and development.Ezhl and Ezh2 belong to the core components of PRC2 complex in PcG protein family and is the histone methyltransferase of H3K27.Ezhl and Ezh2 have shown to play important roles in embryonic development and embryonic stem cell differentiation.H3K27me3 catalyzed by PRC2 also contributes to various biological processes,including resetting epigenetic memory in early embryo development,establishment of bivalent epigenetic modifications with H3K4me3 on genes regulating development and X chromosome inactivation.Role of maternal Ezh1/2 in mouse embryo development has not been studied.In this study,Ezh1 and Ezh2 were knocked out in oocytes for exploring effects of maternal Ezh1/2 and H3K27me3 on embryo development.Our observation show that the fertility of female mice was not affected when Ezh2 was deleted alone in oocyte,but the fertility was significantly decreased when Ezhl and Ezh1 were both knocked out in oocyte,suggesting that Ezh2 could be compensated by Ezh1 in oocyte.Immunofluorescence of in vivo pre-implantation embryo showed that H3K27me2 recovered completely at the 4-cell stage in sKO/+embryos,but the recovery of H3K27me3 was delayed until late blastocyst.In dKO/+embryos,H3K27me2 did not recover until the late blastocyst and almost all morulae showed spot signals,which may be related to XCI.However,H3K27me3 has not recovered in the late blastocyst.These results suggest that Ezh1 may compensate for Ezh2 through some interaction between H3K27me2 and H3K27me3.Most dKO/+embryos were arrested and absorbed after implantation,which was the main reason for decreased fertility in double knockout mice.dKO/+embryos show a large placental phenotype and a female-biased sex ratio in the late stage of embryo development.The number of Nanog+cells decreased in ICM and embryo development was abnormal during peri-implantation,which was one of the reasons for retardation of dKO/+embryos after implantation.The reduction of Nanog+ cells may be related to the delayed recovery of H3K27me3.In addition,we also found abnormal XCI in early dKO/+embryos and identified some potential imprinted genes depending on maternal H3K27me3.Our study reveals the function of maternal Ezh1/2 in the development of mouse embryo,and finds that maternal Ezh1/2 plays a critical role in X chromosome inactivation,inner cell mass and placental development.We also explore the mechanism of maternal Ezh1/2 in regulating epiblast fate in late blastocyst.Epiblast fate may be maintained by the function of H3K27me3 in inhibiting differentaion factors.This study reveals the importance of maternal Ezh1/2 and H3K27me3 in mammalian embryo development.