Brd4 and P300 Confer Transcriptional Competency during Zygotic Genome Activation

Abstract

The awakening of the genome after fertilization is a cornerstone of animal development. However, the mechanisms that activate the silent genome after fertilization are poorly understood. Here, we show that transcriptional competency is regulated by Brd4-and P300-dependent histone acetylation in zebrafish. Live imaging of transcription revealed that genome activation, beginning at the miR-430 locus, is gradual and stochastic. We show that genome activation does not require slowdown of the cell cycle and is regulated through the translation of maternally inherited mRNAs. Among these, the enhancer regulators P300 and Brd4 can prematurely activate transcription and restore transcriptional competency when maternal mRNA translation is blocked, whereas inhibition of histone acetylation blocks genome activation. We conclude that P300 and Brd4 are sufficient to trigger genome-wide transcriptional competency by regulating histone acetylation on the first zygotic genes in zebrafish. This mechanism is critical for initiating zygotic development and developmental reprogramming.

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

“…Here too, analysis of shield stage embryos led to the same conclusion (S13C Fig). Because of the observed correlation between zygotic transcription and H3K27ac levels on regulatory elements during zebrafish ZGA [22,52], 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 ( Fig 5D).…”

“…However, zygotic genome activation is a gradual process with different genes starting to be transcribed at different times [15][16][17][18]. It starts as early as the 64-cell stage, when the miR-430 gene cluster is activated [19][20][21][22], and ends when the cells in the embryo start to adopt different fates during gastrulation. Recently, Pou5f3, SoxB1 and Nanog have been identified as key transcription factors involved in activation of the zygotic genome; they bind thousands of putative regulatory elements, and loss of these TFs results in the reduced expression of many genes [15,[23][24][25].…”

Pluripotency factors determine gene expression repertoire at zygotic genome activation

“…In parallel with the increase of chromatin accessibility during ZGA, zebrafish genome acquires H3K27ac and H3K4me3 histone tail modification marks which correlate with active enhancers and promoters, respectively Creyghton et al, 2010;Heintzman et al, 2007). Deposition of H3K27ac on early enhancers is functionally important for ZGA (Chan et al, 2019). To test if histone mark deposition on genome regulatory elements depends on Pou5f3 and Sox19b, we immunoprecipitated embryonic chromatin originating from late blastula stage (dome, 4.3 hpf) of the WT, MZspg and MZsox19b embryos with H3K27ac and H3K4me3 antibodies, sequenced and compared the mapped data.…”

“…In zebrafish, Xenopus and Drosophila, where development starts with rapid cell cycles, excessive maternal core histones serve as general transcriptional repressors before ZGA occurs (Almouzni and Wolffe, 1995;Amodeo et al, 2015;Joseph et al, 2017;Shindo and Amodeo, 2019;Wilky et al, 2019). Activators that are translated before ZGA and reach critical levels at ZGA include three types of proteins: basal transcription factors (Ferg et al, 2007;Veenstra et al, 1999), the regulators of H3K27ac enhancer mark (Chan et al, 2019;Sato et al, 2019) and maternal transcription factors (TFs). TFs that broadly activate zygotically expressed genes have been identified in Drosophila (Zelda, Liang et al, 2008), zebrafish (Pou5f3, Sox19b and Nanog, Lee et al, 2013;Leichsenring et al, 2013), Xenopus (Pou5f3 and Sox3, Gentsch et al, 2019), human (POU5F1, DUX4, Gao et al, 2018;Hendrickson et al, 2017), and mouse (Nfya, Dux, Dppa2 and Dppa4, De Iaco et al, 2017;Eckersley-Maslin et al, 2019;Lu et al, 2016).…”

The nuclear to cytoplasmic ratio directly regulates zygotic transcription inDrosophilathrough multiple modalities

“…In vitro, at least one transcript is directly sensitive to the N/C ratio in cell cycle arrested Xenopus egg extracts, but it is unclear if this direct relationship is maintained in vivo for any or all genes (Amodeo et al, 2015). In vivo, manipulations in ploidy coupled with RNA-seq, microarrays, or qPCR have found that haploid Drosophila, Xenopus, and zebrafish embryos have reduced gene expression when compared to their wild type counterparts with a spectrum of N/C-dependence across transcribed genes (Lu et al, 2009;Jevtić and Levy, 2017;Chan et al, 2019;). However, these experiments are ill suited to determine if the observed changes in transcription are a direct response to the altered N/C ratio or an indirect response to changes in cell cycle duration because they lack the temporal resolution required to properly account for the cumulative changes in interphase length.…”