Background: Eukaryotic circadian clocks require chromatin modifications and remodeling.Results: SET1 is required for proper expression of the Neurospora clock gene frequency (frq). SET1 modifies chromatin at frq with the peak in H3K4me3 occurring after the peak in activation.Conclusion: H3K4 methylation appears to mitigate White Collar complex (WCC)-mediated expression.Significance: Chromatin is a key component underlying circadian oscillations in gene expression.

During embryonic morphogenesis, cells and tissues undergo dramatic movements under the control of F-actin regulators. Our studies of epidermal cell migrations in developing Caenorhabditis elegans embryos have identified multiple plasma membrane signals that regulate the Rac GTPase, thus regulating WAVE and Arp2/3 complexes, to promote branched F-actin formation and polarized enrichment. Here, we describe a pathway that acts in parallel to Rac to transduce membrane signals to control epidermal F-actin through the GTPase RHO-1/RhoA. RHO-1 contributes to epidermal migration through effects on underlying neuroblasts. We identify signals to regulate RHO-1-dependent events in the epidermis. HUM-7, the C. elegans homolog of human MYO9A and MYO9B, regulates F-actin dynamics during epidermal migration. Genetics and biochemistry support that HUM-7 behaves as a GTPase-activating protein (GAP) for the RHO-1/RhoA and CDC-42 GTPases. Loss of HUM-7 enhances RHO-1-dependent epidermal cell behaviors. We identify SAX-3/ROBO as an upstream signal that contributes to attenuated RHO-1 activation through its regulation of HUM-7/Myo9. These studies identify a new role for RHO-1 during epidermal cell migration, and suggest that RHO-1 activity is regulated by SAX-3/ROBO acting on the RhoGAP HUM-7.

CDC-42 regulation of non-muscle myosin/NMY-2 is required for polarity maintenance in the one-cell embryo of Caenorhabditis elegans. CDC-42 and NMY-2 regulate polarity throughout embryogenesis, but their contribution to later events of morphogenesis are less understood. We have shown that epidermal enclosure requires the GTPase CED-10/Rac1 and WAVE/Scar complex, its effector, to promote protrusions that drive enclosure through the branch actin regulator Arp2/3. Our analysis here of RGA-8, a homolog of SH3BP1/Rich1/ARHGAP17/Nadrin, with BAR and RhoGAP motifs, suggests it regulates CDC-42, so that actin and myosin/NMY-2 promote ventral enclosure during embryonic morphogenesis. Genetic and molecular data suggest RGA-8 regulates CDC-42, and phenocopies the CDC-42 pathway regulators WASP-1/WSP-1 and the F-BAR proteins TOCA-1 and TOCA-2. Live imaging shows RGA-8 and WSP-1 enrich myosin and regulate F-actin in migrating epidermal cells during ventral enclosure. Loss of RGA-8 alters membrane recruitment of active CDC-42. We propose TOCA proteins and RGA-8 use BAR domains to localize and regenerate CDC-42 activity, thus regulating F-actin levels, through the branched actin regulator WSP-1, and myosin enrichment. RhoGAP RGA-8 thus polarizes epithelia, to promote cell migrations and cell shape changes of embryonic morphogenesis.

The circadian clock and aging are intertwined. Disruption to the normal diurnal rhythm accelerates aging and corresponds with telomere shortening. Telomere attrition also correlates with increase cellular senescence and incidence of chronic disease. In this report, we examined diurnal association of White Collar 2 (WC-2) in Neurospora and BMAL1 in zebrafish and mice and found that these circadian transcription factors associate with telomere DNA in a rhythmic fashion. We also identified a circadian rhythm in Telomeric Repeat-containing RNA (TERRA), a lncRNA transcribed from the telomere. The diurnal rhythm in TERRA was lost in the liver of Bmal1-/- mice indicating it is a circadian regulated transcript. There was also a BMAL1-dependent rhythm in H3K9me3 at the telomere in zebrafish brain and mouse liver, and this rhythm was lost with increasing age. Taken together, these results provide evidence that BMAL1 plays a direct role in telomere homeostasis by regulating rhythms in TERRA and heterochromatin. Loss of these rhythms may contribute to telomere erosion during aging.