Junctional adhesion molecule 3 controls bone homeostasis by orchestrating the differentiation of osteoblasts and osteoclasts

Junctional adhesion molecule 3 (JAM3) has been increasingly recognized as a critical regulator of cell migration and adhesion, and JAM3 mutation in humans results in severe skeletal dysplasia and death. However, the underlying mechanism by which JAM3 regulates bone formation is unclear. Here, we identified increased JAM3 levels in both osteoblast and adipocyte differentiation. Functional experiments revealed that JAM3 in bone marrow mesenchymal stromal cells (BMSCs) promoted osteoblast differentiation while inhibiting adipogenesis. In vivo , both the bone marrow cavity-delivered Jam3 siRNA mouse model and the osteoblast progenitor-specific Jam3 conditional knockout mouse model (Osx-Cre; Jam3 fl/fl ) consistently exhibited a marked reduction in osteoblasts, an increase in adipocytes and osteoclasts, and a significant decrease in tibial bone mass and bone density. It has been demonstrated that JAM3 can activate LRP5 protein. Structural modeling using AlphaFold3, combined with molecular dynamics simulations, confirmed that JAM3 could form a stable complex with LRP5. Consistently, cellular assays demonstrated that JAM3 suppressed the degradation of LRP5, supporting its role in stabilizing the receptor. Mechanistic studies confirmed that inhibiting β-catenin activation significantly reversed the effect of JAM3 on regulating osteogenesis/adipogenesis differentiation in BMSCs. Regarding osteoclast differentiation, overexpression of JAM3 in BMSCs led to an increased Rankl/Opg ratio, indicating that JAM3 promotes osteoclastogenesis and bone resorption via a RANKL-dependent pathway. This study provides evidence that JAM3 in BMSCs reciprocally regulates bone homeostasis by modulating LRP5/β-catenin and down-regulating the expression of RANKL.