Circadian rhythms regulate osteoclast recycling through gut microbiota-dependent Th17 cell expansion

The circadian clock coordinates diverse biological processes to maintain physiological function and homeostasis in mammals under the day-night light cycle. Disruption of circadian rhythms impairs immune and metabolic functions and increases susceptibility to various diseases. Here, we demonstrate that long-term rest-phase time-restricted feeding (TRF), which disrupts circadian rhythmicity, induces bone loss and gut microbiota dysbiosis in male mice. Fecal microbiota transplantation (FMT) from circadian-misaligned feeding donors to germ-free recipients increased Th17 cell populations, thereby promoting the fusion of osteomorphs-a recently identified osteoclast precursor-into mature osteoclasts through the RANKL-RANK-OPG signaling pathway. Collectively, our findings identify a gut microbiota-Th17-osteomorph axis as a critical mediator of circadian disruption-induced bone loss, uncovering a previously unrecognized mechanism by which circadian rhythms regulate skeletal homeostasis.