Osteoporosis is a disease characterized by abnormal skeletal fragility due to an imbalance in bone homeostasis. Finding therapeutic targets that promote bone formation is valuable for treating osteoporosis. Discoidin domain receptor 2 (DDR2) is a receptor tyrosine kinase that promotes the differentiation of bone-depositing cells, osteoblasts, and bone formation. We investigated the role of DDR2 in osteoclastogenesis, the formation of cells responsible for bone resorption. We found that, although differentiated osteoclasts had catalytically active DDR2, its abundance was decreased. Overexpression of DDR2 inhibited the expression of osteoclastic markers, osteoclast maturation, and osteoclast-mediated bone resorption in a culture model of bone matrix. Conversely, knocking down Ddr2 through RNA interference accelerated osteoclast differentiation and bone resorption in mice. Further, we identified the co-receptor Neuropilin-1 (Nrp1) as a DDR2-interacting protein. DDR2 facilitated the binding of Nrp1 and the co-receptor PlexinA1 by forming a DDR2-Nrp1-PlexinA1 complex, which blocked PlexinA1-mediated stimulation of osteoclastogenesis. DDR2 prevented PlexinA1 from interacting with the receptor TREM2 and the adaptor DAP12. Moreover, knocking down Nrp1 rescued the expression of osteoclastic markers observed in DDR2-overexpressing cells, whereas ectopic expression of Nrp1 in DDR2-silenced cells inhibited the induction of osteoclastogenesis. Nrp1 enhanced the function of DDR2 in promoting osteoblast differentiation and bone formation in mice. In an ovariectomized mouse model of osteoporosis, adenovirus-mediated delivery of DDR2 to the femur bone alleviated osteopenic phenotypes. Our results revealed that DDR2 functions as an inhibitor of osteoclastogenesis, as well as a promoter of osteoblastogenesis, suggesting that enhancing DDR2 may be therapeutic for patients with osteoporosis.
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