Temporally restricted spatial localization of acetylated isoforms of histone H4 and RNA polymerase II in the 2-cell mouse embryo

Using immunofluorescent labeling and laser-scanning confocal microscopy, we show that isoforms of histone H4 acetylated on lysine 5, 8 and/or 12 (H4.Ac5-12), as well as RNA polymerase II, become enriched at the nuclear periphery around the time of zygotic gene activation, i.e., the 2-cell stage, in the preimplantation mouse embryo. In contrast, DNA and H4 acetylated on lysine 16 are uniformly distributed throughout the cytoplasm. Culture of embryos with inhibitors of histone deacetylase trichostatin A and trapoxin results in an increase in the (1) amount of acetylated histone H4 detected by immunoblotting, (2) intensity and sharpness of the peripheral staining for H4.Ac5-12, and (3) relative rate of synthesis of proteins that are markers for zygotic gene activation. The enhanced staining for H4.Ac5-12 at the nuclear periphery seems to require DNA replication, but appears independent of cytokinesis or transcription, since its development is inhibited by aphidicolin but not by either cytochalasin D or alpha-amanitin. Lastly, the restricted localization of H4.Ac 5-12 is not observed in the 4-cell embryo or at later stages of preimplantation development. These results suggest that changes in chromatin structure underlie, at least in part, zygotic gene activation in the mouse.