The segregation of eukaryotic genomes into euchromatin and heterochromatin represents a fundamental and poorly understood process. Here, we demonstrate that genome-wide establishment of heterochromatin is triggered by the maternal to zygotic transition (MZT) during zebrafish embryogenesis. We find that prior to MZT, zebrafish lack hallmarks of heterochromatin including histone H3 lysine 9 trimethylation (H3K9me3) and condensed chromatin ultrastructure. Global establishment of heterochromatic features occurs following MZT and requires both activation of the zygotic genome and degradation of maternally deposited RNA. Mechanistically, we demonstrate that zygotic transcription of the micro RNA miR-430 promotes degradation of maternal RNA encoding the chromatin remodeling protein Smarca2, and that clearance of Smarca2 is required for global heterochromatin establishment in the early embryo. Our results identify MZT as a key developmental regulator of heterochromatin establishment during vertebrate embryogenesis and uncover functions for Smarca2 in protecting the embryonic genome against heterochromatinization.
Kupffer's vesicle (KV) is the zebrafish organ of laterality, patterning the embryo along its left-right (LR) axis. Regional differences in cell shape within the lumen-lining KV epithelium are essential for its LR patterning function. However, the processes by which KV cells acquire their characteristic shapes are largely unknown. Here, we show that the notochord induces regional differences in cell shape within KV by triggering extracellular matrix (ECM) accumulation adjacent to anterior-dorsal (AD) regions of KV. This localized ECM deposition restricts apical expansion of lumen-lining epithelial cells in AD regions of KV during lumen growth. Our study provides mechanistic insight into the processes by which KV translates global embryonic patterning into regional cell shape differences required for its LR symmetry-breaking function.
Pericentromeric satellite repeats are enriched in 5-methylcytosine (5mC). Loss of 5mC at these sequences is common in cancer and is a hallmark of Immunodeficiency, Centromere and Facial abnormalities (ICF) syndrome. While the general importance of 5mC is well-established, the specific functions of 5mC at pericentromeres are less clear. To address this deficiency, we generated a viable animal model of pericentromeric hypomethylation through mutation of the ICF-gene ZBTB24. Deletion of zebrafish zbtb24 caused a progressive loss of 5mC at pericentromeres and ICF-like phenotypes. Hypomethylation of these repeats triggered derepression of pericentromeric transcripts and activation of an interferon-based innate immune response. Injection of pericentromeric RNA is sufficient to elicit this response in wild-type embryos, and mutation of the MDA5-MAVS dsRNA-sensing machinery blocks the response in mutants. These findings identify activation of the innate immune system as an early consequence of pericentromeric hypomethylation, implicating derepression of pericentromeric transcripts as a trigger of autoimmunity.Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).
Pericentromeric satellite repeats are enriched in 5-methylcytosine (5mC). Loss of 5mC at these sequences is common in cancer and is a hallmark of Immunodeficiency, Centromere and Facial abnormalities (ICF) syndrome. While the general importance of 5mC is well-established, the specific functions of 5mC at pericentromeres are less clear. To address this deficiency, we generated a viable animal model of pericentromeric hypomethylation through mutation of the ICF-gene ZBTB24. Deletion of zebrafish zbtb24 caused a progressive loss of 5mC at pericentromeres and ICF-like phenotypes. Hypomethylation of these repeats triggered derepression of pericentromeric transcripts and activation of an interferon-based innate immune response. Injection of pericentromeric RNA is sufficient to elicit this response in wild-type embryos, and mutation of the MDA5-MAVS dsRNA-sensing machinery blocks the response in mutants. These findings identify activation of the innate immune system as an early consequence of pericentromeric hypomethylation, implicating derepression of pericentromeric transcripts as a trigger of autoimmunity.
Nucleoli are surrounded by Peri-Centromeric Heterochromatin (PCH), reflecting a close spatial association between the two largest biomolecular condensates in eukaryotic nuclei. We have investigated how this highly conserved organization is established de novo during early Drosophila development and whether these distinct condensates influence each other's 3D organization. High-resolution live imaging revealed a highly dynamic process in which the PCH progressively surrounds nucleoli through a series of stage-specific intermediates. To assess interplay between the condensates, nucleolus assembly was eliminated by deleting the ribosomal RNA genes (rDNA), resulting in increased PCH compaction and subsequent reorganization to a hollow shell. In addition, in embryos lacking rDNA, some nucleolar proteins were abnormally redistributed into new bodies or 'neocondensates,' including enrichment in the PCH hollow core. These observations, combined with computational modeling, led to the hypothesis that nucleolar-PCH associations are mediated by a hierarchy of affinities between PCH, nucleoli, and 'amphiphilic' protein(s) that interact with both nucleolar and PCH components. We identified the nucleolar protein Pitchoune as a candidate for such an amphiphilic protein because it also contains a PCH-interaction motif and fills the PCH hollow core in embryos lacking rDNA. Together, these results unveil a dynamic program for establishing nucleolar-PCH associations during animal development, demonstrate that nucleoli are required for normal PCH organization, and identify Pitchoune as a likely molecular link for stabilizing PCH-nucleolar associations. Finally, we propose that disrupting affinity hierarchies could cause cellular disease phenotypes by liberating components that form 'neocondensates' or other abnormal structures through self-association and secondary affinities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.