Osteoblasts Provide a Suitable Microenvironment for the Action of Receptor Activator of Nuclear Factor-κB Ligand

Deficiency of osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of nuclear factor-κB ligand (RANKL), in mice induces osteoporosis caused by enhanced bone resorption. Serum concentrations of RANKL are extremely high in OPG-deficient (OPG−/−) mice, suggesting that circulating RANKL is involved in osteoclastogenesis. RANKL−/− mice exhibit osteopetrosis, with the absence of osteoclasts. We examined the requirements for osteoclastogenesis using OPG−/− mice, RANKL−/− mice, and a system involving bone morphogenetic protein 2 (BMP-2)-induced ectopic bone formation. When collagen disks containing BMP-2 (BMP-2-disks) or vehicle were implanted into OPG−/− mice, osteoclast-like cells (OCLs) and alkaline phosphatase-positive OCLs appeared in BMP-2-disks but not the control disks. F4/80-positive osteoclast precursors were similarly distributed in both BMP-2- and control disks. Cells expressing RANKL were detected in the BMP-2-disks, and the addition of OPG to the disk inhibited OCL formation. Muscle cells in culture differentiated into alkaline phosphatase-positive cells in the presence of BMP-2 and accordingly expressed RANKL mRNA in response to PTH. This suggests that RANKL expressed by osteoblasts is a requirement for osteoclastogenesis. We then examined how osteoblasts are involved in osteoclastogenesis other than RANKL expression, using RANKL−/− mice. BMP-2- and control disks were implanted into RANKL−/− mice, which were injected with RANKL for 7 d. Many OCLs were observed in the BMP-2-disks and bone tissues but not the control disks. These results suggest that osteoblasts also play important roles in osteoclastogenesis through offering the critical microenvironment for the action of RANKL.