TRPV-5 mediates a receptor activator of NF-κB (RANK) ligand-induced increase in cytosolic Ca2+in human osteoclasts and down-regulates bone resorption

Abstract

Most of the signaling effectors located downstream of receptor activator of NF-κB (RANK) activation are calcium-sensitive. However, the early signaling events that lead to the mobilization of intracellular calcium in human osteoclasts are still poorly understood. The Ca2+-sensitive fluorescent probe Fura2 was used to detect changes in the intracellular concentration of Ca2+ ([Ca2+]i) in a model of human osteoclasts. Stimulating these cells with receptor activator of NF-κB ligand (RANKL) induced a rapid and significant increase in [Ca 2+]i. Adding extracellular Ca2+ chelators, depleting intracellular stores, and the use of a phospholipase C inhibitor all indicated that the Ca2+ was of extracellular origin, suggesting the involvement of a Ca2+ channel. We showed that none of the classical Ca2+ channels (L-, T-, or R-type) were involved in the RANKL-induced Ca2+ spike. However, the effect of high doses of Gd3+ did suggest that TRP family channels were present in human osteoclasts. The TRPV-5 channel was expressed in osteoclasts and was mainly located in the cellular area in contact with the bone surface. Furthermore, the RNA inactivation of TRPV-5 channel completely inhibited the RANKL-induced increase in [Ca 2+]i, which was accompanied in the long term by marked activation of bone resorption. Overall, our results show that RANKL induced a significant increase in [Ca2+]i of extracellular origin, probably as a result of the opening of TRPV-5 calcium channels on the surface of human osteoclasts. Our findings suggest that TRPV-5 contributes to maintaining the homeostasis of the human skeleton via a negative feedback loop in RANKL-induced bone resorption. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.