Abundant expression of tetraspanin CD9 in activated osteoclasts in ovariectomy-induced osteoporosis and in bone erosions of collagen-induced arthritis.

Tetraspanin CD9 has been shown to be critically involved in multinucleation and cell fusion during osteoclastogenesis, however, its in vivo pathophysiological role in bone-resorbing disorders such as osteoporosis and rheumatoid arthritis, has not been elucidated. To investigate the involvement of tetraspanin CD9 in bone destruction in such diseases, we examined the expression and distribution of tetraspanin CD9 using murine experimental models of osteoporosis and arthritis. In results, CD9 protein is abundantly expressed on the activated osteoclasts in the bone tissues whose trabeculae are severely reduced in ovariectomy-induced osteoporosis. The expression of CD9 is also detected at the sites of bone erosion in arthritic lesions of collagen-induced arthritis (CIA), where tartate-resistant acid phosphatase (TRAP) staining-positive activated osteoclasts are present. These data suggest that tetraspanin CD9 play important roles in bone destructions in osteoporosis and arthritis, and therefore, functional alterations of tetraspanin CD9 may have therapeutic potential in such bone-resorptive disorders.