The first differentiation event in the mammalian life-cycle occurs during the transition from the 8-cell to the blastocyst. This results in the formation of the trophectoderm and inner cell mass lineages (ICM). Differentiation requires global reprogramming of the cell’s epigenetic landscape and a range of covalent modifications to histone proteins and DNA bases are the dominant mechanisms. The combinatorial arrangement of these modifications determined chromatin structure and transcriptional capacity. In this study we examined global changes in the major histone modifications H3K9 acetylation (H3K9Ace), H3K9 trimethylation (H3K9me3), H3K4me3, and H3K27me3 across the 8-cell to blastocyst transition. We also undertake preliminary analysis of the interaction between the histone modification and the levels of DNA methylation.
Immunolocalization of these modifications showed that H3K9ace, H3K9me3, H3K4me3 and H3K27me3 were uniformly present in each cell of the 8-cell but by the blastocyst stage each modification became restricted to the trophectoderm while the ICM showed a marked loss of these modifications. These differential levels of one activating (H3K9ace) and three repressive modifications mirrored the differential levels of DNA methylation present between these two lineages. Exposure of embryos to the broad-spectrum histone deacetylase inhibitor (trichostatin A, TSA, 30nM) demonstrated an interaction between acetylation and DNA methylation, by reducing the global nuclear level of methylated cytosine in the resulting blastocysts.
These results demonstrate extensive global epigenetic remodeling of multiple epigenetic modifications accompany the first differentiation event in the embryo. The similar profile of global changes for both activating and repressive modifications indicate a complexity of the process. This complexity is exacerbated by interactions between modifications, with changes in acetylation levels influencing DNA methylation profiles. The study provides a foundation for detailed analysis of the control of this remodeling with differentiation.