Osteoblast-Derived Osteomodulin Suppresses Bone Loss in Osteoporosis by Inhibiting Osteoclastogenesis

Osteoporosis is a prevalent systemic skeletal disorder that leads to bone fragility and an elevated risk of fractures. Osteomodulin (OMD) is recognized for its essential role in bone morphogenetic protein 2 (BMP2)-driven osteogenic differentiation. However, its role in osteoporosis, has not been systematically investigated. Here, we carried out omics studies on osteoporotic data obtained from the NCBI database and showed that OMD is mainly expressed in osteoprogenitor cells and that OMD expression levels are down-regulated under bone loss conditions. OMD osteoblast-specific conditional knockout mice (OSTEOCALCIN-Cre (OC-Cre); Omd^flox/flox) were generated by CRISPR and an ovariectomy (OVX)-induced model of osteoporosis was constructed in mice. Micro-CT of OVX mice indicated that the trabecular bone volume fraction (BV/TV) was significantly decreased in OC-Cre; Omd^flox/flox mice. Tartrate resistant acid phosphatase (TRAP) staining showed an increase in the number of osteoclasts in the femurs of OC-Cre; Omd^flox/flox mice. TRAP and immunofluorescence staining indicated that OMD treatment inhibited osteoclast differentiation of bone marrow-derived macrophages (BMDMs) in vitro. RNA sequencing showed that OMD treatment significantly affected osteoclast differentiation signaling pathways, while western blot analysis revealed that OMD treatment decreased the phosphorylation of JNK, ERK, p38, AKT, and IκBα in BMDMs. Molecular docking analysis predicted that OMD binds to receptor activator of nuclear factor kappa beta ligand (RANKL), which was verified by co-immunoprecipitation. Further molecular docking simulations indicated that OMD shares overlapping binding regions on RANKL with osteoprotegerin and RANK. Together, our findings indicate that OMD deficiency exacerbates bone loss in osteoporosis. Furthermore, OMD may impede osteoclastic differentiation in BMDMs by binding to RANKL.