Calcium/calmodulin-signaling supports TRPV4 activation in osteoclasts and regulates bone mass

Osteoclast differentiation is critically dependent on calcium (Ca2+) signaling. Transient receptor potential vanilloid 4 (TRPV4), mediates Ca2+ influx in the late stage of osteoclast differentiation and thereby regulates Ca2+ signaling. However, the system-modifying effect of TRPV4 activity remains to be determined. To elucidate the mechanisms underlying TRPV4 activation based on osteoclast differentiation, TRPV4 gain-of-function mutants were generated by the amino acid substitutions R616Q and V620I in TRPV4 and were introduced into osteoclast lineage in Trpv4 null mice to generate Trpv4R616Q/V620I transgenic mice. As expected, TRPV4 activation in osteoclasts increased the number of osteoclasts and their resorption activity, thereby resulting in bone loss. During in vitro analysis, Trpv4R616Q/V620I osteoclasts showed activated Ca2+/calmodulin signaling compared with osteoclasts lacking Trpv4. In addition, studies of Trpv4R616Q/V620I mice that lacked the calmodulin-binding domain indicated that bone loss due to TRPV4 activation was abrogated by loss of interactions between Ca2+/calmodulin signaling and TRPV4. Finally, modulators of TRPV4 interactions with the calmodulin-binding domain were investigated by proteomic analysis. Interestingly, nonmuscle myosin IIa was identified by liquid chromatographytandem mass spectroscopy (LC-MS/MS) analysis, which was confirmed by immunoblotting following coimmunoprecipitation with TRPV4. Furthermore, myosin IIa gene silencing significantly reduced TRPV4 activation concomitant with impaired osteoclast maturation. These results indicate that TRPV4 activation reciprocally regulates Ca2+/calmodulin signaling, which involves an association of TRPV4 with myosin IIa, and promotes sufficient osteoclast function. (C) 2012 American Society for Bone and Mineral Research.