BAP31 Drives Cartilage Calcification through Disruption of Autophagosome-Lysosome Fusion in Osteoarthritis

Pathological cartilage calcification represents a pivotal pathological alteration in osteoarthritis (OA), yet its underlying mechanisms remain poorly understood. Exosomes are involved in facilitating calcification, and their biogenesis may be linked to autophagy disruption. However, whether autophagy disruption regulates exosome-mediated calcification in OA remains to be elucidated. Here, B-cell receptor-associated protein 31 (BAP31) was identified as a key mechanosensitive regulator bridging this gap. Mechanical stimulation activated BAP31 in chondrocytes. Protein docking, coimmunoprecipitation assays, and competitive pull-down assays demonstrated that activated BAP31 competes with autophagy-related 14 (ATG14) for binding to syntaxin 17 (STX17), thereby disrupting the STX17-ATG14 complex, which is essential for autophagosome‒lysosome fusion. This disruption led to the accumulation of autophagosomes, which in turn drove the release of exosomes that facilitated pathological cartilage calcification. Critically, intra-articular delivery of adeno-associated virus-mediated BAP31 shRNA restored autophagosome‒lysosome fusion, suppressed exosome release, and attenuated cartilage calcification and degeneration in a rat OA model. Collectively, these findings identify BAP31 as a mechanical stress-responsive switch that couples impaired autophagy to exosome-mediated calcification, highlighting its potential as a therapeutic target for OA.