Brd4 and P300 regulate zygotic genome activation through histone acetylation

Abstract

The awakening of the zygote genome, signaling the transition from maternal transcriptional control to zygotic control, is a watershed in embryonic development, but the factors and mechanisms controlling this transition are still poorly understood. By combining CRISPR-Cas9-mediated live imaging of the first transcribed genes (miR-430), chromatin and transcription analysis during zebrafish embryogenesis, we observed that genome activation is gradual and stochastic, and the active state is inherited in daughter cells. We discovered that genome activation is regulated through both translation of maternal mRNAs and the effects of these factors on the chromatin. We show that chemical inhibition of H3K27Ac writer (P300) and reader (Brd4) block genome activation, while induction of a histone acetylation prematurely activates transcription, and restore genome activation in embryos where translation of maternal mRNAs is impaired, demonstrating that they are limiting factors for the activation of the genome. In contrast to current models, we do not observe triggering of genome activation by a reduction of the nuclear-cytoplasmic (N/C) ratio or slower cell division. We conclude that genome activation is controlled by a time-dependent mechanism involving the translation of maternal mRNAs and the regulation of histone acetylation through P300 and Brd4. This mechanism is critical to initiating zygotic development and developmental reprogramming.

The Spatio-Temporal Control of Zygotic Genome Activation

“…Similar conclusions were drawn from profiling the zygotic transcriptome of haploid Drosophila (Lu et al, 2009) and cell cycle-arrested zebrafish (Chan et al, 2018). However, the highest number of newly engaged gene bodies is indeed reached at the MBT, confirming it as the developmental stage at which bulk ZGA occurs (Langley et al, 2014).…”

Chromatin accessibility established by Pou5f3, Sox19b and Nanog primes genes for activity during zebrafish genome activation

“…One potential mechanism is that these transcription factors mediate the establishment of H3K27ac at regulatory elements [56,59]. PSN as well as H3K27ac are essential for zygotic genome activation [15,19,59], and the highest H3K27ac enrichment occurs at PSNprimed loci (this study). The interdependence of these factors in the context of priming, however, remains unclear.…”

“…We find that the mir-430 cluster, which is transcribed two orders of magnitude higher than any other region, undergoes a dramatic loss in accessibility when elongation of RNA polymerase II is blocked. The miR-430 cluster is one of the first to be transcribed and initially, all transcribing RNA polymerase II localizes to the two alleles of the miR-430 cluster [19][20][21]56], which might facilitate the formation of RNA-pol II 'trains' capable of evicting nucleosomes, as shown in vitro [33].…”

“…This revealed that regulatory elements that show both the largest increase in accessibility between 256-cell and oblong stage, and the largest reduction in accessibility upon the individual loss of either Pou5f3, Sox19b or Nanog, have the highest expression level at sphere stage ( Figure 5c). Because of the observed correlation between zygotic transcription and H3K27ac levels on regulatory elements during zebrafish ZGA [19,48], we repeated our analysis using H3K27Ac as a proxy for transcriptional activity. This independent analysis confirmed the important role of Pou5f3, Sox19b and Nanog in priming gene expression ( Figure 5d).…”

Global analyses of the dynamics of mammalian microRNA metabolism

“…Metabolic incorporation of 5-ethynyluridine (5EU) into nascent RNA, followed by attachment of a biotin or fluorescent label through click chemistry acting at the ethynyl group of 5EU, provides a powerful method for isolating or detecting newly synthesized RNA (Jao and Salic 2008;Chan et al 2018;Kwasnieski et al 2019). The adaptation of this approach to efficiently isolate ~30-nt fragments of newly synthesized mRNAs (S Eichhorn, T Eisen, D Bartel, unpublished results) suggested that when examining the production and decay dynamics of RNAs as short as miRNAs, 5EU metabolic labeling might offer a favorable alternative to 4-thiouridine labeling.…”