Suppression of bone formation by osteoclastic expression of semaphorin 4D

The holy grail in the bone field is the identification of a pharmacological compound that promotes bone growth during osteoporosis. Hiroshi Takayanagi and his colleagues take a step forward in that direction by showing that osteoclast-mediated expression of semaphorin D inhibits osteoblast differentiation and that, by inhibiting this pathway with a blocking antibody, they could prevent bone loss in a mouse model of osteoporosis. Most of the currently available drugs for osteoporosis inhibit osteoclastic bone resorption; only a few drugs promote osteoblastic bone formation. It is thus becoming increasingly necessary to identify the factors that regulate bone formation. We found that osteoclasts express semaphorin 4D (Sema4D), previously shown to be an axon guidance molecule, which potently inhibits bone formation. The binding of Sema4D to its receptor Plexin-B1 on osteoblasts resulted in the activation of the small GTPase RhoA, which inhibits bone formation by suppressing insulin-like growth factor-1 (IGF-1) signaling and by modulating osteoblast motility. Sema4d −/− mice, Plxnb1 −/− mice and mice expressing a dominant-negative RhoA specifically in osteoblasts showed an osteosclerotic phenotype due to augmented bone formation. Notably, Sema4D-specific antibody treatment markedly prevented bone loss in a model of postmenopausal osteoporosis. Thus, Sema4D has emerged as a new therapeutic target for the discovery and development of bone-increasing drugs.