Histone H4K20 methylation synchronizes cytoskeletal dynamics with cell cycle phases during epidermal differentiation
Histone tails are subject to various post-translational modifications, which play a fundamental role in altering chromatin accessibility. Although they are thought to regulate progression through development, the impact of the most abundant histone modification in vertebrates, i.e., histone H4 lysine 20 dimethylation (H4K20m2), has remained largely elusive. H4K20m2 arises from sequential methylation of new, unmodified histone H4 proteins, incorporated into chromatin during DNA replication, by the mono-methylating enzyme PR-SET7/KMT5A during G2/M phases, followed by conversion to the dimethylated state by SUV4-20H1 enzymes in the following G1/G0 phase. To address its function, we have blocked the deposition of this mark by depleting Xenopus embryos of SUV4-20H1/H2 methyltransferases, which convert H4K20 monomethylated to di- and tri-methylated states, respectively In the frog larval epidermis this results in a severe loss of cilia in multiciliated cells (MCC), a key component of all mucociliary epithelia. MCC precursor cells are correctly specified and amplify centrioles, but ultimately fail in ciliogenesis due to perturbation of cytoplasmic processes. Genome wide transcriptome profiling reveals that SUV4-20H1/H2 depleted ectodermal Animal Cap explants preferentially down-regulate the expression of several hundred cytoskeleton and cilium related genes as a consequence of persistent H4K20 monomethyl marks on postmitotic chromatin. Further analysis demonstrated that knockdown of SUV4-20H1 alone is sufficient to generate the MCC phenotype and that overexpression of the H4K20m1-specific histone demethylase PHF8 rescues the ciliogenic defect in significant, although partial, manner. Taken together, this indicates that the conversion of H4K20m1 to H4K20m2 by SUV4-20H1 is critical to synchronize cytoskeletal dynamics in concert with the cell cycle.