Circadian regulators PER1 and PER2 regulate osteoclastogenesis by balancing competing activities of innate immunity genes

Bone remodeling is influenced by circadian rhythms as demonstrated by global gene expression patterns and the phenotypes of knockout mice of circadian regulators. However, the direct connections between circadian regulators and specific bone genes remain unclear. We previously found that a conditional knockout of Per1, a central circadian regulator, in osteoclasts increased osteoclastogenesis and decreased bone mass, whereas Per2 knockout did not cause these phenotypes. Here, we extended the research to Per1;Per2 conditional double knockout mice and observed different phenotypes and underlying mechanisms from individual knockouts. In contrast to Per1 knockout, the double knockout decreased osteoclastogenesis and increased bone mass. This was accompanied by downregulation of genes involved in innate immunity, including several known promoters and inhibitors of osteoclastogenesis. Chromatin immunoprecipitation and reporter assay suggested direct regulation of some of them by PER proteins. These results indicate that PER1 and PER2 are critical regulators of osteoclastogenesis through balancing multiple competing activities of osteoclastogenesis, rather than acting as simple promoters or inhibitors of osteoclastogenesis. Regulation of innate immunity genes by circadian regulators is widely observed across other monocyte/macrophage lineages. Our results extend this common mechanism to osteoclasts with therapeutic potential to treat inflammatory bone diseases. Lay summaryAlthough circadian rhythms regulate bone remodeling, direct links between circadian regulators and bone genes remain unclear. We previously demonstrated that depletion of Per1, a main circadian regulator, downregulates immunological genes, increases the number of osteoclasts, the main bone resorbing cells, and decreases bone mass in mice. Here, we showed the opposite effects with double depletion of Per1 and Per2 and identified a new set of downregulated immunological genes that promote or inhibit osteoclastogenesis. This study connects circadian rhythms to bone resorption through immunological genes with opposing activities in osteoclastogenesis, supporting therapeutic interventions targeting circadian regulators to treat inflammatory bone diseases.