OSCAR

OSCAR (Osteoclast-associated immunoglobulin-like receptor), also known as PIgR-3 (polymeric immunoglobulin-like receptor), is a type I transmembrane protein of the leukocyte receptor complex (LRC) family. OSCAR is specifically expressed by pre-osteoclasts and signals via FcR gamma. OSCAR co-stimulates one of the major FcR gamma -associated pathways required for osteoclastogenesis in vivo. Collagen type I and collagen type II serve as ligands for OSCAR. OSCAR binds to specific motifs within fibrillary collagens in the extracellular matrix (ECM). OSCAR promoted osteoclastogenesis in vivo, and its binding to the collagen motif led to signaling that increased numbers of osteoclasts in culture. OSCAR contains two immunoglobulin-like domains, D1 and D2. Direct binding assays on OSCAR molecules showed that D2, but not D1, is critical for collagen binding. OSCAR may contribute to the pathogenesis and severity of a number of diseases including osteoporosis, atherosclerosis, chronic obstructive pulmonary disease and rheumatoid arthritis.