H3K27me3 may be associated with Oct4 and Sox2 in mouse preimplantation embryos.

As a core member of polycomb repressive complex 2, the transcription and enzyme activity of enhancer of zeste homolog 2 (Ezh2) is directly involved in the trimethylation of lysine 27 on histone H3. In this study, the fluorescence intensity of H3K27me3 in mouse in vivo morulae and blastocysts was compared by indirect immunofluorescence staining. We found that demethylation of H3K27me3 occurred during the blastocyst stage. Real-time polymerase chain reaction was performed to investigate Ezh2 expression in oocytes and in preimplantation embryos. Ezh2 expression peaked during the zygote stage and gradually decreased from the 2-cell stage, exhibiting an inverse pattern when compared with Oct4 and Sox2 mRNA in mouse preimplantation embryos. To understand the role of development-related genes on the transcription of mouse Ezh2, a promoter assay was performed in NIH/3T3 cells. Ezh2 expression was markedly suppressed by Oct4 and Sox2 alone in a dose-dependent manner, while Ezh2 promoter activity in co-transfection with Nanog, Klf-4, and c-Myc groups showed no significant change as compared with the control. Our data suggest that the demethylation of H3K27me3 is caused by the degressive expression and activity of Ezh2 in blastocysts, leading to increased expression of developmentally important transcription factors. We also observed negative effects of Oct4 and Sox2 on the transcription of Ezh2 and identified Oct4 and Sox2 as novel negative regulators of Ezh2 at the post-translation level in a mouse preimplantation embryo.

Differences in H3K4 trimethylation in in vivo and in vitro fertilization mouse preimplantation embryos.

Trimethylation of lysine 4 at histone 3 (H3K4me3) is considered a marker of active transcription; it plays an important role in transcription reprogramming efficiency. We compared the levels of H3K4me3 in mouse preimplantation embryos from MII stage oocytes produced by in vivo and in vitro fertilization (IVF) using immunofluorescence histochemistry. IVF embryos were further treated with trichostatin A (a histione deacetylase inhibitor) to investigate the effect of histone acetylation on H3K4me3. We found higher levels of H3K4me3 in MII stage oocytes in metaphase chromosomes. The pattern of H3K4 trimethylation of in vivo embryos from zygote to blastocyst stages was similar to that of IVF embryos; however, the concentration of H3K4me3 was significantly higher in the in vivo fertilization embryos. The levels of H3K4me3 in the trichostatin A-treated groups were also significantly increased. We conclude that culture condition and environmental changes can cause histone modification and that the effect of these environmental conditions on epigenetic changes should be taken into consideration.