Bone remodeling involves the interplay of bone resorption and formation and is accurately controlled to maintain bone mass. Both processes require transcellular Ca 2؉ transport, but the molecular mechanisms engaged remain largely elusive. The epithelial Ca 2؉ channel TRPV5 is one of the most Ca 2؉ -selective transient receptor potential (TRP) channels. In this study, the functional role of TRPV5 in bone was investigated. TRPV5 mRNA was expressed in human and murine bone samples and in osteoclasts along with other genes involved in transcellular Ca 2؉ transport, including calbindin-D9K and calbindin-D28K, Na ؉ ͞Ca 2؉ exchanger 1, and plasma membrane Ca 2؉ -ATPase 1b. TRPV5 expression in murine osteoclasts was confirmed by immunostaining and showed predominant localization to the ruffled border membrane. However, TRPV5 was absent in osteoblasts. Analyses of femoral bone sections from TRPV5 knockout (TRPV5 ؊͞؊ ) mice revealed increased osteoclast numbers and osteoclast area, whereas the urinary bone resorption marker deoxypyridinoline was reduced compared with WT (TRPV5 ؉͞؉ ) mice. In an in vitro bone marrow culture system, the amount of osteoclasts and number of nuclei per osteoclast were significantly elevated in TRPV5 ؊/؊ compared with TRPV5 ؉/؉ mice. However, using a functional resorption pit assay, we found that bone resorption was nearly absent in osteoclast cultures from TRPV5 ؊/؊ mice, supporting the impaired resorption observed in vivo. In conclusion, TRPV5 deficiency leads to an increase in osteoclast size and number, in which Ca 2؉ resorption is nonfunctional. This report identifies TRPV5 as an epithelial Ca 2؉ channel that is essential for osteoclastic bone resorption and demonstrates the significance of transcellular Ca 2؉ transport in osteoclastic function.tartrate-resistant acid phosphatase ͉ 1,25(OH)2D3 ͉ osteoblast ͉ Coomassie blue ͉ laser scanning confocal microscopy M aintenance of body Ca 2ϩ is of crucial importance for many physiological functions, including neuronal excitability, muscle contraction, and bone formation. Bone is the major Ca 2ϩ storage of the body and regulates in concerted action with kidney and intestine the whole-body Ca 2ϩ balance. Transcellular Ca 2ϩ transport is an important process in maintaining Ca 2ϩ balance by these tissues (1, 2). In bone, it is crucial for bone formation͞ mineralization to achieve adequate bone quality and strength, but also for osteoclastic bone resorption, which contributes to Ca 2ϩ balance in the blood. However, the proteins involved in transcellular Ca 2ϩ transport in bone cells are largely elusive. Recently, TRPV5 and TRPV6, members of the superfamily of transient receptor potential (TRP) cation channels, have been identified as the gatekeepers of transepithelial Ca 2ϩ transport in kidney and intestine, respectively (3-5). These highly selective epithelial Ca 2ϩ channels are part of a three-step process, facilitating transcellular Ca 2ϩ transport (3, 4, 6). After entry of Ca 2ϩ into the cell through TRPV5 and TRPV6, Ca 2ϩ bound to calbindin-D 28...

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancerbinding protein b (C/EBPb) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP*, LAP) and a short repressive (LIP) isoforms from a single C/EBPb transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBPb null (c/ebpb À/À ) or in LIP knock-in (L/L) osteoclast precursors. C/EBPb mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBPb isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression.