Osteocalcin promotes mineralization in bone microenvironment via regulating hydroxyapatite formation and integration

Within the bone microenvironment, the intricate interplay and regulation among matrix components form a complex network. Disentangling this network is crucial for uncovering potential therapeutic targets in bone pathology. Osteocalcin (OCN), the most abundant non-collagenous bone protein, is an essential node within this network. However, the function of OCN in bone mineralization remains controversial. By combining in vitro and in vivo approaches, we evaluated the function and mechanism of OCN and its derived peptides on bone formation and mineralization. CRISPR/Cas9-mediated OCN knockout mice exhibited reduced bone mineral density, compromised biomechanical strength, and impaired osteogenic differentiation in BMSCs. Raman spectroscopy revealed diminished crystal size and atomic order in Hydroxyapatite (HAp) of OCN^-/- bone. Further studies using atomic force microscopy and nano-isothermal titration calorimetry identified OCN and its derived peptide OC22 as enhancers of collagen stiffness via affinity for HAp. These findings establish OCN as a positive regulator of bone mineralization, and provide mechanistic insights for targeting bone-related pathologies.