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.

Awakening of zygotic transcription in animal embryos relies on maternal pioneer transcription factors. The interplay of global and specific functions of these proteins remains poorly understood. Here, we analyze chromatin accessibility and time-resolved transcription in single and double mutant zebrafish embryos lacking pluripotency factors Pou5f3 and Sox19b. We show that two factors modify chromatin in a largely independent manner. We distinguish four types of direct enhancers by differential requirements for Pou5f3 or Sox19b. We demonstrate that changes in chromatin accessibility of enhancers underlie the changes in zygotic expression repertoire in the double mutants. Pou5f3 or Sox19b promote chromatin accessibility of enhancers linked to the genes involved in gastrulation and ventral fate specification. The genes regulating mesendodermal and dorsal fates are primed for activation independently of Pou5f3 and Sox19b. Strikingly, simultaneous loss of Pou5f3 and Sox19b leads to premature expression of genes, involved in regulation of organogenesis and differentiation.

Nanog has been implicated in establishment of pluripotency in mammals and in zygotic genome activation in zebrafish. In this study, we characterize the development of MZ (maternal and zygotic null) mutant zebrafish embryos Without functional Nanog, epiboly is severely affected, embryo axes do not form and massive cell death starts at the end of gastrulation. We show that three independent defects in MZ mutants contribute to epiboly failure: yolk microtubule organization required for epiboly is abnormal, maternal mRNA fails to degrade owing to the absence of miR-430, and actin structure of the yolk syncytial layer does not form properly. We further demonstrate that the cell death in MZ embryos is cell-autonomous. Nanog is necessary for correct spatial expression of the ventral-specifying genes , and , and the neural transcription factor It is also required for the correctly timed activation of endoderm genes and for the degradation of maternal mRNA via miR-430 Our findings suggest that maternal Nanog coordinates several gene regulatory networks that shape the embryo during gastrulation.

Tspan8 exhibits a functional role in many cancer types including pancreatic, colorectal, oesophagus carcinoma, and melanoma. We present a first study on the expression and function of Tspan8 in breast cancer. Tspan8 protein was present in the majority of human primary breast cancer lesions and metastases in the brain, bone, lung, and liver. In a syngeneic rat breast cancer model, Tspan8+ tumours formed multiple liver and spleen metastases, while Tspan8− tumours exhibited a significantly diminished ability to metastasise, indicating a role of Tspan8 in metastases. Addressing the underlying molecular mechanisms, we discovered that Tspan8 can mediate up‐regulation of E‐cadherin and down‐regulation of Twist, p120‐catenin, and β‐catenin target genes accompanied by the change of cell phenotype, resembling the mesenchymal–epithelial transition. Furthermore, Tspan8+ cells exhibited enhanced cell–cell adhesion, diminished motility, and decreased sensitivity to irradiation. As a regulator of the content and function of extracellular vesicles (EVs), Tspan8 mediated a several‐fold increase in EV number in cell culture and the circulation of tumour‐bearing animals. We observed increased protein levels of E‐cadherin and p120‐catenin in these EVs; furthermore, Tspan8 and p120‐catenin were co‐immunoprecipitated, indicating that they may interact with each other. Altogether, our findings show the presence of Tspan8 in breast cancer primary lesion and metastases and indicate its role as a regulator of cell behaviour and EV release in breast cancer. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Abstract:The zebrafish embryo remains transcriptionally quiescent during the first 10 cell cycles. Only then Zygotic Genome Activation (ZGA) occurs and is accompanied by fast chromatin remodeling. At ZGA, homologs of mammalian stem cell transcription factors (TFs) Pou5f3/Oct4, Nanog and Sox19b bind to thousands of developmental enhancers to initiate 5 transcription. So far, how these three ZGA TFs influence chromatin dynamics at ZGA has remained unresolved. To address this question, we analyzed before and after ZGA nucleosome positions in wild-type and Maternal-Zygotic (MZ) mutants for pou5f3 and nanog. 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 10 and high propeller twist DNA shape value. Just prior to ZGA, Pou5f3 and Nanog nonspecifically compete with histones and reduce nucleosome occupancy on strong nucleosome positioning sequences genome-wide. After ZGA, specific binding of Nanog and Pou5f3/SoxB1 complex is necessary to maintain open chromatin state on more than 6,000 HNARs. Nanog binds to the strongest nucleosome positioning sequence within HNAR, while the Pou5f3/SoxB1 15 complex binds to the flanks of it. We suggest a model, where the same intrinsic DNA properties of HNAR promote both high nucleosome occupancy and differential binding of TFs. We hope that our model will provide a useful framework for genome regulatory studies in the variety of biological systems.

SummaryZygotic 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 pioneers on synergistic enhancers, promote histone acetylation and activate transcription. Antagonistic enhancers are activated by pioneer binding of one of these factors. The other TF binds as non-pioneer, 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.Pou5f3 and Nanog compete as pioneer-like and non-pioneer factors on common genomic sites.Pioneer-like binding activates transcription, non-pioneer binding blocks transcription.Competition between Pou5f3 and Nanog establishes the order of gene expression.

PouV and SoxB1 family transcription factors (TFs) have emerged as master regulators of cell fate transitions. To investigate the genetic interactions between Pou5f3 and Sox19b in zebrafish embryos passing through Zygotic Genome Activation (ZGA), we combined time-resolved mutant transcription analysis using the novel tool RNA-sense, chromatin state and phenotypic assays. We distinguish four types of embryonic enhancers, differentially regulated by the two TFs. Pou5f3 is critical for activation of enhancer types 1 and 2, which are responsible for transcription of genes involved in gastrulation and ventral genes. Pou5f3 or Sox19b prevent premature activation of type 3 and 4 enhancers, which are responsible for transcription of organogenesis regulators, differentiation factors and dorsal genes. We also show that the balance between Sox19b and Pou5f3 is important for bulk ZGA timing. Our results uncover how independent activities of maternal Pou5f3 and Sox19b add up or antagonize to determine the early gene expression repertoire. Bullet points:• Pou5f3 and Sox19b bind independently to DNA • Disbalance between maternal Pou5f3 and Sox19b delays ZGA • Pou5f3 suppresses premature transcription of neural patterning genes, activated by SoxB1 factors • Pou5f3 and Sox19b synergistically suppress premature transcription of a wide range of differentiation genes • Pou5f3 and Sox19b restrict the dorsal organizer

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 pioneers on synergistic enhancers, promote histone acetylation and activate transcription. Antagonistic enhancers are activated by pioneer binding of one of these factors. The other TF binds as non-pioneer, 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