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.

Osteoclasts (OCLs) are key players in controlling bone remodeling. Modifications in their differentiation or bone resorbing activity are associated with a number of pathologies ranging from osteopetrosis to osteoporosis, chronic inflammation and cancer, that are all characterized by immunological alterations. Therefore, the 2000s were marked by the emergence of osteoimmunology and by a growing number of studies focused on the control of OCL differentiation and function by the immune system. At the same time, it was discovered that OCLs are much more than bone resorbing cells. As monocytic lineage-derived cells, they belong to a family of cells that displays a wide heterogeneity and plasticity and that is involved in phagocytosis and innate immune responses. However, while OCLs have been extensively studied for their bone resorption capacity, their implication as immune cells was neglected for a long time. In recent years, new evidence pointed out that OCLs play important roles in the modulation of immune responses toward immune suppression or inflammation. They unlocked their capacity to modulate T cell activation, to efficiently process and present antigens as well as their ability to activate T cell responses in an antigen-dependent manner. Moreover, similar to other monocytic lineage cells such as macrophages, monocytes and dendritic cells, OCLs display a phenotypic and functional plasticity participating to their anti-inflammatory or pro-inflammatory effect depending on their cell origin and environment. This review will address this novel vision of the OCL, not only as a phagocyte specialized in bone resorption, but also as innate immune cell participating in the control of immune responses.

Various studies indicate that periodontal ligament fibroblasts (PLF) have some similarities to osteoblasts, for example they have the capacity to induce the formation of osteoclast-like cells. Here, we investigated whether a second population of tooth-associated fibroblasts, gingival fibroblasts (GF), has similar osteoclastogenesis properties. PLF and GF were co-cultured with peripheral blood mononuclear cells (PBMC) in the presence and absence of dexamethasone and 1alpha,25dihydroxycholecalciferol (dex + vit D(3)) on plastic and on cortical bone slices. Tartrate resistant acid phosphatase (TRACP) positive multinucleated cells (MNCs) were more abundant in co-cultures with PLF than in GF-PBMC co-cultures, more abundant on plastic compared to bone and more abundant in the presence of dex + vit D(3). In line with these findings was an inhibition of MNC formation and not inhibition of existing osteoclasts by medium conditioned by GF. We next investigated whether expression of molecules important for osteoclastogenesis differed between the two types of fibroblasts and whether these molecules were regulated by dex + vit D(3). OPG was detected at high levels in both fibroblast cultures, whereas RANKL could not be detected. Resorption of bone did not occur by the MNCs formed in the presence of either fibroblast subpopulation, suggesting that the fibroblasts secrete inhibitors of bone resorption or that the osteoclast-like cells were not functional. The incapacity of the MNCs to resorb was abolished by culturing the fibroblast-PBMC cultures with M-CSF and RANKL. Our results suggest that tooth-associated fibroblasts may trigger the formation of osteoclast-like cells, but more importantly, they play a role in preventing bone resorption, since additional stimuli are required for the formation of active osteoclasts.

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Considerable heterogeneity in responsiveness to P. gingivalis exists both between GF and PDLF and between individuals, which may be crucial determinants for the susceptibility to develop periodontitis.

Blood monocytes are precursors of dendritic cells, macrophages, and osteoclasts. They are a heterogeneous cell population with differences in size, phenotype, and function. Although monocytes maintain several tissue-specific populations of immune cells in homeostasis, their contribution to populations of dendritic cells, macrophages, and osteoclasts is significantly increased in inflammation. Identification of a growing number of functionally different subsets of cells within populations of monocyte-derived immune cells has recently put monocyte heterogeneity into sharp focus. Here, we summarize recent findings in monocyte heterogeneity and their differentiation into dendritic cells, macrophages, and osteoclasts. We also discuss these advances in the context of the formation of functionally different monocyte-derived subsets of dendritic cells, macrophages, and osteoclasts.

Fibre metal laminates were developed at Delft University during the last two decades as a family of new hybrid materials consisting of bonded thin metal sheets and fibre/adhesive layers. This laminated structure provides the material with excellent fatigue, impact and damage tolerance characteristics and a low density. While the 20 per cent weight reduction was the prime driver behind the development of this new family of materials, it turns out that additional benefits like cost reduction and an improved safety level have become more and more important. The combination of these aspects in one material makes fibre metal laminates a strong candidate material for fuselage skin structures of the new generation of high capacity aircraft. The focus on this application currently leads to industrialization and qualification that makes this material available to the aircraft designer.