Role of histone acetylation in reprogramming of somatic nuclei following nuclear transfer

Before fertilization, chromatins of both mouse oocytes and spermatozoa contain very few acetylated histones. Soon after fertilization, chromatins of both gametes become highly acetylated. The same deacetylation-reacetylation changes occur with histones of somatic nuclei transferred into enucleated oocytes. The significance of these events in somatic chromatin reprogramming to the totipotent state is not known. To investigate their importance in reprogramming, we injected cumulus cell nuclei into enucleated mouse oocytes and estimated the histone deacetylation dynamics with immunocytochemistry. Other reconstructed oocytes were cultured before and/or after activation in the presence of the highly potent histone deacetylase inhibitor trychostatin A (TSA) for up to 9 h postactivation. The potential of TSA-treated and untreated oocytes to develop to the blastocyst stage and to full term was compared. Global deacetylation of histones in the cumulus nuclei occurred between 1 and 3 h after injection. TSA inhibition of histone deacetylation did not affect the blastocyst rate (37% with and 34% without TSA treatment), whereas extension of the TSA treatment beyond the activation point significantly increased the blastocyst rate (up to 81% versus 40% without TSA treatment) and quality (on average, 59 versus 45 cells in day 4 blastocysts with and without TSA treatment, respectively). TSA treatment also slightly increased full-term development (from 0.8% to 2.8%). Thus, deacetylation of somatic histones is not important for reprogramming, and hyperacetylation might actually improve reprogramming.