Resetting H3K4me3, H3K27ac, H3K9me3 and H3K27me3 during the maternal-to-zygotic transition and blastocyst lineage specification in bovine embryos
It remains poorly understood how histone modifications regulate changes in gene expression during preimplantation development. Using a bovine model, we profiled changes in two activating (H3K4me3 and H3K27ac) and two repressive (H3K9me3 and H3K27me3) marks in oocytes, 2-, 4- and 8-cell embryos (that developed in the presence or absence of the transcription inhibitor a-amanitin), morula, blastocysts, inner cell mass cells and trophectoderm. In oocytes, we find that broad bivalent domains of H3K4me3 and H3K27me3 mark developmental genes, and that prior to genome activation, H3K9me3 and H3K27me3 co-occupy gene bodies. During genome activation, chromatin accessibility is established before canonical H3K4me3 and H3K27ac, and although embryonic transcription is required for this active remodeling, it is dispensable for maintenance of pre-established histone marks. Finally, blastocyst lineages are defined by differential Polycomb repression and transcription factor activity. Overall, these results further support the use of bovine as a more appropriate model system than the mouse to study genome activation and cell lineage specification during human preimplantation development.