INTRODUCTION: Osteoarthritis (OA) is a prevalent degenerative joint disease accompanied by increased number of senescent chondrocytes. Mitochondrial dysfunction is a well-established hallmark of chondrocyte senescence in OA pathogenesis. Sterile α and Toll/Interleukin-1 Receptor motif-containing 1 (SARM1), known to drive mitochondrial impairment in various cell types, has not been thoroughly investigated in the context of chondrocyte aging or OA. METHODS: We established a doxorubicin (DOX)-induced senescence model in primary mouse chondrocytes. Gain- and loss-of-function approaches were employed using siRNA-mediated knockdown and lentiviral overexpression of SARM1, followed by assessment of senescence markers, mitochondrial function, and morphology. To investigate the mechanistic pathway, exogenous cyclic ADP-ribose (cADPR) and its specific inhibitor 8-Br-cADPR were applied, with subsequent evaluation of intracellular calcium dynamics and Drp1 translocation to mitochondria. We next tested the efficacy of blocking this SARM1/cADPR axis both ex vivo on human femoral head tissue as well as in an experimental OA mouse model. RESULTS: Chondrocytes isolated from human OA cartilage and aged murine cartilage showed increased expression of SARM1. Knockdown of SARM1 reduced DOX-induced chondrocyte senescence and mitochondrial dysfunction, while overexpression of wild-type but not catalytic-inactive SARM1-TIR domain mutant (TIR-E642A) induced intrinsic apoptosis and mitochondrial fragmentation. Exogenous cADPR recapitulated senescence and mitochondrial fragmentation, whereas treatment with 8-Br-cADPR abolished SARM1-dependent effects. Mechanistically, SARM1-generated cADPR increased intracellular calcium levels, triggering Drp1 phosphorylation at Ser616 and dephosphorylation at Ser637, thereby resulting in Drp1-FIS1 interaction and mitochondrial fission. Interestingly, pharmacological or genetic inhibition of the SARM1/cADPR pathway ameliorated cartilage degradation in the experimental OA model. CONCLUSION: We show that SARM1 mediates mitochondrial fragmentation by activating cADPR-dependent calcium signaling, which in turn promotes Drp1 binding to FIS1. This suggests an unappreciated role for the SARM1-cADPR pathway in OA etiology and presents this pathway as an attractive candidate to be targeted therapeutically.
Gu et al. (Tue,) studied this question.