Changes in histone modification upon activation of dormant mouse blastocysts

Abstract

ABSTRACTGene expression in the implanting blastocyst is altered by stimulation with estrogen secreted from maternal ovaries. In the present study, to understand the mechanism regulating the changes in gene expression, diverse histone modifications in blastocysts were studied using a delayed implantation model, in which embryos were kept in a dormant state in the uterus by maternal ovariectomy and progesterone treatment, and then activated by injection with estrogen. Total transcriptional activity increased markedly in activated embryos, and immunocytochemistry with antibodies recognizing specific histone modifications revealed differential modification of several histones in the trophectoderm (TE) and inner cell mass (ICM) of dormant and activated embryos. High levels of histone H3 lysine 9 (H3K9) dimethylation, which suppresses gene expression, were observed in the ICM, but not in the TE, of dormant embryos, and the levels decreased when the embryos were activated, consistent with changes in transcriptional activity. Substitution of histone H3.3, a variant of H3, for dominant H3.1 increased in activated embryos, suggesting that histone substitution is involved in inducing gene expression associated with activation. In the nucleus, H3.3 was mainly localized in the nucleoli of activated embryos but not in those of dormant ones. In contrast, there were no obvious differences in the trimethylation of histone H3K9 or the acetylation of histones H3K9, H3K18 and H4K12 between dormant and activated embryos. These results suggest that a decrease in H3K9 dimethylation contributes to the acquisition of implantation competence by releasing genes from suppression. In addition, histone H3.3 substitution seems to be involved in global gene activation and facilitates the prompt recovery of dormant blastocysts to the active state by inducing rRNA synthesis, resulting in an increase in translational activity.