Single‐Cell Atlas of Subchondral Bone Marrow Lesions Reveals Proteostasis Dysfunction as a Druggable Mechanism for Early Osteoarthritis

ABSTRACT Subchondral bone marrow lesions (BMLs) constitute a pathognomonic imaging feature of both incipient and progressive osteoarthritis (OA). However, the pathological features and molecular mechanisms underlying BMLs remain poorly characterized. Here, we innovatively established and dynamically characterized a standardized mouse model, simulating human BMLs on magnetic resonance imaging (MRI) that correlates significantly with cartilage degeneration, while revealing substantial aberrant bone matrix accumulation within BML regions. By establishing an osteochondral single‐cell atlas of mouse knee joints during BML development, we found that proteasome dysfunction and abnormal secretion of misfolded collagen, driven by dysregulated heat shock protein 70 (HSP70) and deubiquitinase 19 (USP19) expression in osteoarthritic subchondral osteoblasts, constitute a key mechanism of BML formation. Furthermore, cell‐cell communication and Col10a1‐Cre; R26 ^tdt+ fate‐mapping analyses uncovered that osteoblast‐secreted WNT5A mediates crosstalk with hypertrophic chondrocytes (HTCs), accelerating their hypertrophy and cell death. Critically, pharmacological HSP70 targeting by TRC051384 inhibited collagen misfolding/secretion, preventing BML formation. Notably, results of Mendelian randomization demonstrated a significant correlation between proteasome gene expression and OA risk in humans, further supporting a potential role of proteasome dysfunction in BML pathogenesis. Collectively, these data reveal mechanisms underlying BML formation and therapeutic targets for early OA intervention.