Osteopontin facilitates angiogenesis, accumulation of osteoclasts, and resorption in ectopic bone

Osteoclastic bone resorption requires a number of complex steps that are un der the control of local regulatory molecules. Osteopontin is expressed in osteoclasts and is also present in bone matrix; however, its biological fun ction has not been fully understood. To elucidate the role of osteopontin i n the process of osteoclastic bone resorption, we conducted ectopic bone im plantation experiments using wild-type and osteopontin knockout mouse. In t he wild-type group, bone discs from calvariae implanted ectopically in musc le were resorbed, and their mass was reduced by 25% within 4 weeks. In cont rast, the mass of the bone discs from calvariae of osteopontin knockout mic e was reduced by only 5% when implanted in osteopontin knockout mice. Histo logical analyses indicated that the number of osteoclasts associated with t he implanted bones was reduced in the osteopontin knockout mice. As osteopo ntin deficiency does not suppress osteoclastogenesis per se, we further exa mined vascularization immunohistologically and found that the number of ves sels containing CD31-positive endothelial cells around the bone discs impla nted in muscle was reduced in the osteopontin knockout mice. Furthermore, s c implantation assays indicated that the length and branching points of the newly formed vasculatures associated with the bone discs were also reduced in the absence of osteopontin. In this assay, tartrate-resistant acid phos phatase-positive area of the bone discs was also reduced in the osteopontin knockout mice, indicating further the link between the osteopontin-depende nt vascularization and osteoclast accumulation. The bone resorption defect could be rescued by topical administration of recombinant osteopontin to th e bones implanted in muscle. These observations indicate that osteopontin i s required for efficient vascularization by the hemangiogenic endothelial c ells and subsequent osteoclastic resorption of bones.