Targeting the SARM1-cADPR-Ca²⁺ pathway attenuates mitochondrial fragmentation and osteoarthritis progression

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.

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.

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.

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.