Ctsk+ Osteoclasts Orchestrate Condylar Morphogenesis Via Hypoxic Lysosome

This study aims to evaluate the response mechanisms of Cathepsin K (ctsk)-positive osteoclasts to hypoxia and their distinct roles in condylar tissue structures. Functional disappear of ctsk+ cells in Diphtheria Toxin Receptor (DTR) mice and Hypoxic inducible factor-1α (HIF-1α) conditional knockout (CKO) mice were hybridized (n=6). DTR mice displayed large accumulation of calcified cartilage in mandibular condylar cartilage (mcc) and insufficient subchondral bone formation, associated with osteoclasts deficiency. Ctsk+ cells inhibited proliferation and premature hypertrophy of mcc and suppressed osteogenesis of subchondral bone. Furthermore, CKO mice caused cartilage accumulation, where the morphology and arrangement of fibrocartilage layers before 5 weeks of age were basically consistent with DTR group. However, after 6 weeks of age, CKO mice showed decreased cartilage and accelerated mineralization. Consistent with continuous reduction of osteoclasts, extensive chondrocyte apoptosis and high expression of MMP13 were major factors in the gradual thinning of CKO mcc. Meanwhile, a persistent lack of osteoclasts blocked the replacement of cartilage tissue by newly formed bone, leading to a delayed but accelerated mineralization in subchondral bone. Mechanistically, osteoclasts in CKO mice displayed the disorder of ruffled border and the formation of lysosomes. Furthermore, we demonstrated a molecular role for osteoclastic TSC2-mTORC1-TFEB signaling that presumably regulated hypoxic lysosomal formation. Our findings revealed an unique role of ctsk+ cell-mediated bone absorption signaling in suppressing chondrocyte proliferation, premature hypertrophy and osteogenesis, and the potential role of HIF-1α in osteoclastogenesis and lysosomes modulation in the degradation of condylar calcified cartilage.