Regulatory mechanism of bone metabolism by multimers of RANKL

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We identified three RANKL isoforms. RANKL1 was identical to the originally reported RANKL. RANKL2 had a shorter intracellular domain. RANKL3 did not have the intracellular or transmembrane domains. The three RANKL isoforms was cloned into expression vectors, respectively, and transfected into NIH3T3 cells. In addition, multiple RANKL isoforms were cotransfected into NIH3T3 cells. NIH3T3 cells expressing RANKL isoform(s) were cocultured with bone marrow macrophages, and osteoclastogeneisis was analyzed. NIH3T3 cells expressing RANKL1 or RANKL2 formed tartrate-resistant acid phosphatase-positive cells. Coexpression of RANKL1 and RANKL2 induced fusion of perosteoclasts. NIH3T3 cells expressing RANKL3 had no effect on the formation of tartrate-resistant acid phosphatase-positive cells, but significantly inhibited fusion of preosteoclasts when coexpressed with RANKL1 and RANKL2. These findings suggest that multiple multimeric RANKL structures regulate bone metabolism.We also developed new sandwich ELISA system to detect RANKL protein, and analyzed the concentration of soluble RANKL using serum taken from 50 postmenopausal women. Serum concentration of RANKL was positively correlated with age and concentration of urinary deoxypiridinoline. These findings suggest that serum concentration of RANKL reflects bone metabolism.We also analyzed the expression of RANKL in human peripheral T lymphocytes. The expression of RANKL was very low in the T lymphocytes, but higher expression was detected in the stimulated T lymphocytes and T lymphocytes derived from inflammatory lesions. These findings suggest that RANKL is important to the function of T lymphocytes in addition to osteoclastogenesis.