Nuclear mechanotransduction has been implicated in the control of chromatin organization and gene expression. Wang et al. show that, in Drosophila myofibers, the LINC complex is required for the regulation of DNA replication and synchronized cell-cycle progression in myonuclei.

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex transduces nuclear mechanical inputs suggested to control chromatin organization and gene expression; however, the underlying mechanism is currently unclear. We show here that the LINC complex is needed to minimize chromatin repression in muscle tissue, where the nuclei are exposed to significant mechanical inputs during muscle contraction. To this end, the genomic binding profiles of Polycomb, Heterochromatin Protein1 (HP1a) repressors, and of RNA-Pol II were studied in Drosophila larval muscles lacking functional LINC complex. A significant increase in the binding of Polycomb and parallel reduction of RNA-Pol-II binding to a set of muscle genes was observed. Consistently, enhanced tri-methylated H3K9 and H3K27 repressive modifications and reduced chromatin activation by H3K9 acetylation were found. Furthermore, larger tri-methylated H3K27me3 repressive clusters, and chromatin redistribution from the nuclear periphery towards nuclear center, were detected in live LINC mutant larval muscles. Computer simulation indicated that the observed dissociation of the chromatin from the nuclear envelope promotes growth of tri-methylated H3K27 repressive clusters. Thus, we suggest that by promoting chromatin–nuclear envelope binding, the LINC complex restricts the size of repressive H3K27 tri-methylated clusters, thereby limiting the binding of Polycomb transcription repressor, directing robust transcription in muscle fibers.

Unlike recent progress in cellular reprogramming, the mechanisms and requirements for misspecification of entire organs are largely unknown. A canonic model for organ 15 "reprogramming" was provided by the induction of haltere-to-wing transformations in response to early exposure of fly embryos to ether. Using this model, we identify a mechanistic chain of events explaining why and how stage-specific exposure leads to organ transformation at a later stage. We show that ether interferes with protein integrity and compromises Trithorax-mediated establishment of H3K4 tri-methylations. The altered pattern of H3K4me3 pre-disposes early-20 methylated Ubx targets and wing genes for later up-regulation in the larval haltere disc, hence the wing-like outcome. Consistent with protein destabilization by ether, this transformation is enhanced by reduced function of Hsp90 and emerges spontaneously by joint deficiency in Hsp90 and Trithorax. The morphogenetic impact of chaperone response at the onset of epigenetic patterning may comprise a general scheme for organ reprogramming by environmental cues. 25 Main TextCell identities and patterns of expression in flies are established during embryonic development and are maintained by epigenetic means, particularly by the Polycomb and Trithorax systems 1,2 . Early embryonic exposure to environmental stimuli (e.g. ether vapor and heat) can alter these 30 patterns and induce homeotic transformations, such as haltere-to-wing (bithorax) phenocopies 3-8 . The induced bithorax phenocopies can be further stabilized (assimilated) by repeated exposures over several generations 4,9,10 . Consistent with the similarity to the phenotypes of Ultrabithorax mutations, the penetrance of the induced phenocopy is enhanced by loss-of-function mutations in Ubx, and its upstream regulators, trx 10,11 . However, the molecular mechanisms that mediate the 35 45

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex transduces nuclear mechanical inputs suggested to control chromatin epigenetics. We analyzed the epigenetic landscape and genomic binding profile of HP1 and Polycomb transcription repressors, and of RNA-Pol II in Drosophila fully differentiated larval muscles lacking functional LINC complex. Our findings suggest a significant increase in chromatin repression promoted by enhanced binding of Polycomb and concomitant reduction of RNA-Pol II binding in the LINC mutant muscles. Consistently these mutants exhibited elevated levels of epigenetic repressive marks, tri-methylated H3K9 and H3K27, and reduced chromatin activation by H3K9 acetylation. These changes correlated with enhanced condensation of the DNA observed in the LINC mutant myonuclei. Importantly, we find larger repressive chromatin clusters marked by H3K27me3-GFP in live LINC mutant larval muscles. We suggest that the LINC complex is required for the stabilization of chromatin epigenetic landscape in non-dividing muscle fibers, possibly by inhibiting DNA condensation and restricting repressive cluster formation.