Angptl7+ chondrocytes and Sparc+ osteoblasts mediated the increased angiogenesis and abnormal remodeling of subchondral bone in the development of knee osteoarthritis

With an increasing number of affected individuals and a trend towards younger age of onset, research on the treatment of knee osteoarthritis (KOA) is facing significant challenges. The pathogenesis of KOA is complex for being a multifactorial disease affecting the entire joint, and pathological remodeling of subchondral bone may be one of the key factors mediating the degeneration of the overlying cartilage. This study constructed a postmenopausal KOA mice model in bipedal mice to better understand how subchondral bone remodeling and cartilage degeneration interact during KOA development. The femoral condyle tissue, excluding the growth plate, was isolated from the distal epiphyseal line in KOA mice for single cell RNA sequencing and analysis. And a single-cell atlas of the osteochondral composite tissue, including chondrocytes, endothelia cells, osteoblasts, progenitor cells, and so on, was successfully constructed. Furthermore, three novel subtypes of chondrocytes, including Smoc2+ angiogenic chondrocytes, Angptl7+ angiogenic chondrocytes, and Col1a1+ osteogenic chondrocytes, were identified. Angptl7+ chondrocytes activated endothelia cells via the Fgf2-Fgfr2 interaction pathway, and promoted angiogenesis and vascular invasion in subchondral bone of KOA mice. The quantity of H-type vessels, which recruit numerous osterix+ osteoprogenitor cells and stimulate osteogenesis, exhibited an increase within the subchondral bone of mice with KOA. While Sparc+ osteoblast negatively regulated the bone mineralization and osteoblastic differentiation, aggravated the pathological remodeling of subchondral bone and KOA progression. These findings suggest that there are new avenues for potential therapeutic interventions in the treatment of KOA.