BMAL1-TTK-H2Bub1 loop deficiency contributes to impaired BM-MSC-mediated bone formation in senile osteoporosis

During the aging process, the reduced osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) results in decreased bone formation, which contributes to senile osteoporosis. Previous studies have confirmed that interrupted circadian rhythm plays an indispensable role in age-related disease. However, the mechanism underlying the impaired osteogenic differentiation of BM-MSCs during aging and its relationship with interrupted circadian rhythm remains unclear. In this study, we confirmed that the circadian rhythm was interrupted in aging mouse skeletal systems. The level of the core rhythm component BMAL1 but not that of CLOCK in the osteoblast lineage was decreased in senile osteoporotic specimens from both human and mouse. BMAL1 targeted TTK as a circadian-controlled gene to phosphorylate MDM2 and regulate H2Bub1 level, while H2Bub1 in turn regulated the expression of BMAL1. The osteogenic capacity of BM-MSCs was maintained by a positive loop formed by BMAL1-TTK-MDM2-H2Bub1. Furthermore, we demonstrated that using bone-targeting recombinant adeno-associated virus 9 (rAAV9) to enhance Bmal1 or Ttk might have a therapeutic effect on senile osteoporosis and delays bone repair in aging mice. In summary, our study indicated that targeting the BMAL1-TTK-MDM2-H2Bub1 axis via bone-targeting rAAV9 might be a promising strategy for the treatment of senile osteoporosis.