PARENT SESSION

(T287) CHANGES IN HISTONE MODIFICATION UPON THE ACTIVATION OF DORMANT MOUSE EMBRYOS.

Matsuhashi, Tamako¹, Aoki, Fugaku², Sakai, Senkiti¹, ¹ University of Tokyo, Tokyo, Japan² University of Tokyo, Kashiwa, Japan

ABSTRACT- In mice ovariectomized early on day 4 of pregnancy and then given progesterone daily, the embryos arrested in the blastocyst stage without implanting in the uteri for several days. These dormant embryos were activated and started implanting on the administration of estradiol. It has been reported that gene expression patterns differ markedly in day-4, dormant, and activated embryos. To clarify the mechanisms that regulate these changes in gene expression, the total transcriptional activity and modifications of histones were examined in these embryos. The total transcriptional activity, which was measured by the incorporation of BrUTP using an in vitro run-on assay, was low in the dormant embryos and increased significantly when the embryos were activated by the administration of estradiol. Global patterns of histone acetylation and methylation were examined on various lysine residues using fluorescence immunocytochemistry. Dimethylation of histone H3 lysine 9 (H3K9) was detected at a low level in day-4 embryos both in the trophectoderm (TE) and inner cell mass (ICM). In the ICM, this increased in dormant embryos and then decreased in activated ones, while it did not change in the TE. The level of trimethylated H3K9 was higher in the TE than in the ICM in day-4 embryos. Although this difference was maintained in dormant embryos, the level in the ICM decreased to a marginal level in the activated embryos, but not in the TE. No appreciable change was detected in the acetylation level of H3K9, which was always higher in the TE than the ICM in any type of embryo. The acetylation level of H3K18 was constantly high in the ICM. By contrast, in the TE, it was high only in day-4 embryos: it decreased in dormant embryos and remained at a low level in activated ones. The level of acetylated H4K12 was higher in the TE than in the ICM in day-4 embryos. In the dormant embryos, it decreased in the TE, but not in the ICM, which resulted in similar levels of acetylated H4K12 in the ICM and TE. It increased again only in the TE of the activated embryos. These results suggest that changes in histone modification are responsible for the changes in the gene expression patterns during the process of embryo dormancy and activation.

KEY WORDS: histone, methylation, acetylation, dormant embryo