Increasing global chromatin accessibility alone is insufficient to enhance global transcription during mouse zygotic...

In mice, zygotic gene activation (ZGA) occurs in two waves: a minor wave at the 1-cell stage and a major wave at the 2-cell stage. At the 1-cell stage, chromatin is highly decondensed and transcriptionally permissive; however, global transcription is low. At the 2-cell stage, global transcription increases with chromatin compaction. Thus, the early embryo departs from the usual association between open chromatin and active transcription. Open chromatin in the 1-cell embryo is regarded as a hallmark of totipotency; however, its contribution to transcription remains unclear. To address this issue, whether increasing accessibility is sufficient to enhance global transcription and whether the response depends on the mechanism of chromatin opening were investigated. Therefore, global chromatin accessibility was increased at each developmental stage using two mechanistically distinct perturbations, KDM4D-mediated removal of H3K9me3 and trichostatin A (TSA)-mediated histone hyperacetylation, and the resulting accessibility and transcription were evaluated by DNase-TUNEL and 5-ethynyl uridine incorporation, respectively. At the 1-cell stage, both perturbations increased accessibility but not global transcription. KDM4D decreased transcription in the maternal pronucleus, whereas TSA reduced transcription in both pronuclei. At the 2-cell stage, KDM4D increased accessibility without altering global transcription. The simultaneous removal of H3K9me3 and H3K27me3 did not increase global transcription. In contrast, TSA increased global accessibility and global transcription. Thus, increasing global chromatin accessibility alone was insufficient to enhance global transcription and could even reduce it. The transcriptional response depended on the mechanism of chromatin perturbation. Furthermore, maternally enriched H3K9me3 did not account for the lower transcriptional activity in the maternal pronucleus.