Histones package DNA into nucleosomes and lysines within their tail are common targets for acetylation. Acetylation neutralizes the negative charge on lysine allowing DNA to be less tightly wrapped around the nucleosome. This facilitates access by the transcriptional machinery. Acetylation is also a docking site for bromodomain proteins that serve as assembly sites for transcriptional complexes. The minor round of transcription that occurs in the 1-cell embryo requires the onset of global histone 3 lysine 9 acetylation (H3K9ace) and this was perturbed by culture of zygotes in vitro 1. Here we report on the association of H3K9ace with the epigenetic reprogramming that occurs in the 2-cell embryo and the effects of embryo culture on this process.
Hybrid mouse embryos were collected at the 1-cell (18 h post hCG) or 2-cell (42 h post hCG) (fresh) stage. 1-cells were then cultured under standard conditions to an equivalent time of 2-cell development (cultured). H3K9ace was quantified in the resulting 2-cell embryos by indirect fluorescence immuno-staining intensity using Image Pro Plus software.
Mean nuclear H3K9ace staining per pixel and the total staining across the nuclei were significantly lower in cultured embryos compared to fresh (p<0.0001; 0.02), respectively. Average nuclear cross-sectional area was significantly larger in cultured embryos (p=0.001) as was the number of nucleoli precursor bodies per nuclei (p=0.0004). The distribution of heterochromatic staining and its association with H3K9ace was also disturbed in cultured embryos.
Contrary to the hyperacetylation observed in cultured zygotes 1, culture to the 2-cell stage was associated with a relative global hypoacetylation of the genome. This was accompanied by changes in nuclear organisation, including nucleoli maturation. Our studies point to a complex pattern of alterations in the epigenetic landscape in the early embryo in response to the various stresses imposed upon it by culture in vitro.
1.Reproduction 2017:154:375-385