Pou5f3, SoxB1, and Nanog Remodel Chromatin on High Nucleosome Affinity Regions at Zygotic Genome Activation

The zebrafish embryo is transcriptionally mostly quiescent during the first 10 cell cycles, until the main wave of zygotic genome activation (ZGA) occurs, accompanied by fast chromatin remodeling. At ZGA, homologs of the mammalian stem cell transcription factors (TFs) Pou5f3, Nanog, and Sox19b bind to thousands of developmental enhancers to initiate transcription. So far, how these TFs influence chromatin dynamics at ZGA has remained unresolved. To address this question, we analyzed nucleosome positions in wild-type and maternal-zygotic (MZ) mutants for pou5f3 and nanog by MNase-seq. We show that Nanog, Sox19b, and Pou5f3 bind to the high nucleosome affinity regions (HNARs). HNARs are spanning over 600 bp, featuring high in vivo and predicted in vitro nucleosome occupancy and high predicted propeller twist DNA shape value. We suggest a two-step nucleosome destabilization-depletion model, in which the same intrinsic DNA properties of HNAR promote both high nucleosome occupancy and differential binding of TFs. In the first step, already before ZGA, Pou5f3 and Nanog destabilize nucleosomes at HNAR centers genome-wide. In the second step, post-ZGA, Nanog, Pou5f3, and SoxB1 maintain open chromatin state on the subset of HNARs, acting synergistically. Nanog binds to the HNAR center, whereas the Pou5f3 stabilizes the flanks. The HNAR model will provide a useful tool for genome regulatory studies in a variety of biological systems

Zebrafish Pou5f1-dependent transcriptional networks in temporal control of early development

Time-resolved transcriptome analysis of early pou5f1 mutant zebrafish embryos identified groups of developmental regulators, including SoxB1 genes, that depend on Pou5f1 activity, and a large cluster of differentiation genes which are prematurely expressed.Pou5f1 represses differentiation genes indirectly via activation of germlayer-specific transcriptional repressor genes, including her3, which may mediate in part Pou5f1-dependent repression of neural genes.A dynamic mathematical model is established for Pou5f1 and SoxB1 activity-dependent temporal behaviour of downstream transcriptional regulatory networks. The model predicts that Pou5f1-dependent increase in SoxB1 activity significantly contributes to developmental timing in the early gastrula.Comparison to mouse Pou5f1/Oct4 reveals evolutionary conserved targets. We show that Pou5f1 developmental function is also conserved by demonstrating rescue of Pou5f1 mutant zebrafish embryos by mouse POU5F1/OCT4

Nucleolus: A Central Hub for Nuclear Functions

International audienceThe nucleus contains distinct nuclear bodies (NBs); nucleolus is the largest and the most studied NB, but its role in the functioning of the nucleus is far from being fully understood. The nucleolus is not surrounded by a membrane, yet it contains DNA, RNA and a set of proteins that can either be retained in the nucleolus or rapidly shuttle between the nucleoplasm, the nucleolus and the cytoplasm in response to various stimuli. The emerging evidence points to the central function of the nucleolus in organizing many nuclear functions besides RNA polymerase I transcription and ribosome biogenesis. Here we discuss the functions of the nucleolus related to the shuttling of proteins and nucleic acids between nucleolus and nucleoplasm. The functional processes affected by shuttling of nucleolar components include 3D organization of the genome, stress response, DNA repair and recombination, transcription regulation, telomere maintenance and other essential cellular functions

Initiation of cyp26a1 Expression in the Zebrafish Anterior Neural Plate by a Novel Cis-Acting Element

International audienceEarly patterning of the vertebrate neural plate involves a complex hierarchy of inductive interactions orchestrated by signalling molecules and their antagonists. The morphogen ret-inoic acid, together with the Cyp26 enzymes which degrade it, play a central role in this process. The cyp26a1 gene expressed in the anterior neural plate thus contributes to the fine modulation of the rostrocaudal retinoic acid gradient. Despite this important role of cyp26a1 in early brain formation, the mechanisms that control its expression in the anterior neural plate are totally unknown. Here, we present the isolation of a 310-base-pair DNA element adjacent to cyp26a1 promoter, displaying enhancer activity restricted to the anterior neural plate of the zebrafish gastrula. We show that unlike that of cyp26a1, expression driven by this cyp26a1 anterior neural plate element (cANE) is independent of retinoic acid. Through deletion analysis, we identify a 12-nucleotide motif essential for cANE activity. A consensus bipartite binding site for SoxB:Oct transcription factors overlaps with this motif. Mutational analysis suggests that SoxB binding is essential for its activity. We discuss the contribution of this study to the elucidation of the regulatory hierarchy involved in early neural plate patterning

Activator-blocker model of transcriptional regulation by pioneer-like factors

Abstract Zygotic genome activation (ZGA) in the development of flies, fish, frogs and mammals depends on pioneer-like transcription factors (TFs). Those TFs create open chromatin regions, promote histone acetylation on enhancers, and activate transcription. Here, we use the panel of single, double and triple mutants for zebrafish genome activators Pou5f3, Sox19b and Nanog, multi-omics and mathematical modeling to investigate the combinatorial mechanisms of genome activation. We show that Pou5f3 and Nanog act differently on synergistic and antagonistic enhancer types. Pou5f3 and Nanog both bind as pioneer-like TFs on synergistic enhancers, promote histone acetylation and activate transcription. Antagonistic enhancers are activated by binding of one of these factors. The other TF binds as non-pioneer-like TF, competes with the activator and blocks all its effects, partially or completely. This activator-blocker mechanism mutually restricts widespread transcriptional activation by Pou5f3 and Nanog and prevents premature expression of late developmental regulators in the early embryo

Pou5f1/Oct4 Promotes Cell Survival via Direct Activation of <i>mych</i> Expression during Zebrafish Gastrulation

<div><p>Myc proteins control cell proliferation, cell cycle progression, and apoptosis, and play important roles in cancer as well in establishment of pluripotency. Here we investigated the control of <i>myc</i> gene expression by the Pou5f1/Oct4 pluripotency factor in the early zebrafish embryo. We analyzed the expression of all known zebrafish Myc family members, <i>myca</i>, <i>mycb</i>, <i>mych</i>, <i>mycl1a, mycl1b</i>, and <i>mycn</i>, by whole mount <i>in situ</i> hybridization during blastula and gastrula stages in wildtype and maternal plus zygotic <i>pou5f1</i> mutant (MZ<i>spg</i>) embryos, as well as by quantitative PCR and in time series microarray data. We found that the broad blastula and gastrula stage <i>mych</i> expression, as well as late gastrula stage <i>mycl1b</i> expression, both depend on Pou5f1 activity. We analyzed ChIP-Seq data and found that both Pou5f1 and Sox2 bind to <i>mych</i> and <i>mycl1b</i> control regions. The regulation of <i>mych</i> by Pou5f1 appears to be direct transcriptional activation, as overexpression of a Pou5f1 activator fusion protein in MZ<i>spg</i> embryos induced strong <i>mych</i> expression even when translation of zygotically expressed mRNAs was suppressed. We further showed that MZ<i>spg</i> embryos develop enhanced apoptosis already during early gastrula stages, when apoptosis was not be detected in wildtype embryos. However, Mych knockdown alone did not induce early apoptosis, suggesting potentially redundant action of several early expressed <i>myc</i> genes, or combination of several pathways affected in MZ<i>spg.</i> Experimental <i>mych</i> overexpression in MZ<i>spg</i> embryos did significantly, but not completely suppress the apoptosis phenotype. Similarly, p53 knockdown only partially suppressed apoptosis in MZ<i>spg</i> gastrula embryos. However, combined knockdown of p53 and overexpression of Mych completely rescued the MZ<i>spg</i> apoptosis phenotype. These results reveal that Mych has anti-apoptotic activity in the early zebrafish embryo, and that p53-dependent and Myc pathways are likely to act in parallel to control apoptosis at these stages.</p></div

Role of SoxB binding sites in the regulation of <i>cyp26a1</i> expression in the anterior neural plate (ANP).

<p>(A) Representation of the cANE region in the zebrafish genome; 3 predicted SoxB binding sites are indicated; the green channel represents the intensity of the anti-Sox2 ChipSeq signal in this region according to [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150639#pone.0150639.ref034" target="_blank">34</a>]. (B) A logo representing the composite consensus binding site for SoxB/Oct factors [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150639#pone.0150639.ref033" target="_blank">33</a>] is aligned with the predicted Sox binding site (Sox_BS1) overlapping Motif1 (cANE 45–59). The mutSox TT->CC mutation destroying the Sox-binding half-site is indicated. (C-F) egfp in situ hybridization representing enhancer activity of cANE ΔSoxBS2-3, where both Sox_BS2 and Sox_BS3 have been deleted (C), compared with intact cANE (D), at the 90% epiboly stage, and enhancer activity of Motif1 mutation mutSox (F) in 1–222 context compared with wild type 1–222 (E), at the 75% epiboly stage. Dorsal views; anterior is to the left. (G) RT-PCR relative quantification of total <i>egfp</i> expression stable transgenic embryos for constructs cANE (1–310), 81–310, 1–222 and 1–222 mutSox, as well as non transgenic embryos (Ctl); error bars represent SEM; units are arbitrary.</p

Characterization of cANE activity as an early neural plate specific enhancer.

<p>(A-I,A’-I’) Compared expression patterns of <i>cyp26a1</i> (A-I) and <i>egfp</i> driven by cANE (A’-I’) during early embryonic development. (J) Double in situ hybridization showing <i>barhl2</i> expression domain (blue) exactly filling the gap in the <i>cyp26a1</i> expression domain (red). (A,B,C,E,G,A’,B’,C’,E’,G’) are lateral views. (d,f,h,I,D’,f’,h’,I’) are animal pole views. White arrowheads: anterior neural plate. Black arrowheads: blastoderm marginal zone. Arrows in (H-J): gap in the anterior neural plate domain of <i>cyp26a1</i> expression. All stages are indicated in the pictures. (k,k’,l,l’) The effect of 100 nM retinoic acid (RA) treatment between 2,5 hpf and 8,5 hpf on on stable transgenic cANE_endo:::<i>egfp</i> (K-K’) and cANE::<i>egfp</i> (L-L’) expression. (M) EGFP fluorescence in a 12 hpf stable transgenic cANE:::<i>egfp</i> embryo. Lateral view with dorsal to the left. (N) Schematic representation of cANE and all three reported retinoic acid responsive elements (R1, R2, R3) identified previously. cANE is located from -504 bp to -195 bp relative to <i>cyp26a1</i> ATG codon. An: animal, Vg: vegetal; V: ventral; D: dorsal, A: anterior, P: posterior, L: left, R: right, CTRL: control embryo, RA: retinoic-acid treated embryo.</p