An essential role for the association of CD47 to SHPS-1 in skeletal remodeling

Integrin-associated protein (IAP/CD47) has been implicated in macrophage-macrophage fusion. To understand the actions of CD47 on skeletal remodeling, we compared Cd47 ^-/- mice with Cd47 ^+/+ controls. Cd47 ^-/- mice weighed less and had decreased areal bone mineral density compared with controls. Cd47 ^-/- femurs were shorter in length with thinner cortices and exhibited lower trabecular bone volume owing to decreased trabecular number and thickness. Histomorphometry revealed reduced bone-formation and mineral apposition rates, accompanied by decreased osteoblast numbers. No differences in osteoclast number were observed despite a nonsignificant but 40% decrease in eroded surface/bone surface in Cd47 ^-/- mice. In vitro, the number of functional osteoclasts formed by differentiating Cd47 ^-/- bone marrow cells was significantly decreased compared with wild-type cultures and was associated with a decrease in bone-resorption capacity. Furthermore, by disrupting the CD47-SHPS-1 association, we found that osteoclastogenesis was markedly impaired. Assays for markers of osteoclast maturation suggested that the defect was at the point of fusion and not differentiation and was associated with a lack of SHPS-1 phosphorylation, SHP-1 phosphatase recruitment, and subsequent dephosphorylation of non-muscle cell myosin IIA. We also demonstrated a significant decrease in osteoblastogenesis in bone marrow stromal cells derived from Cd47 ^-/- mice. Our finding of cell-autonomous defects in Cd47 ^-/- osteoblast and osteoclast differentiation coupled with the pronounced skeletal phenotype of Cd47 ^-/- mice support the conclusion that CD47 plays an important role in regulating skeletal acquisition and maintenance through its actions on both bone formation and bone resorption.