Remodeling of the skeleton is a physiological process crucial for the maintenance of skeletal integrity and systemic calcium homeostasis. Local extracellular calcium concentration ([Ca2+]e) fluctuates dramatically within bone multicellular units, from ~0.5 mM during bone formation, when the extracellular matrix is being mineralized, to ≥2 mM during bone resorption and matrix degradation. Osteoblasts sense and respond to fluctuations in [Ca2+]e independently of systemic factors: high [Ca2+]e promote osteoblast chemotaxis, proliferation, maturation, gene expression, and matrix mineralization. Osteoblasts express the calcium receptor (CaR) that detects and responds to changes in [Ca2+]e. Important physiological functions of the CaR have been shown in the parathyroid and kidney cells.

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Because of the aging of the general population in all developed countries, the prevalence of osteoporosis is increasing worldwide and the fight against this disease has important consequences in terms of public health. This has prompted the International Osteoporosis Foundation (IOF) and Servier to build a partnership to encourage young scientists to engage in cutting-edge research in bone disease, and increase awareness and understanding of osteoporosis. With this aim, they created a grant designed to support investigators under the age of 40 in original research projects on osteoporosis of high scientific value and international relevance. The winning project will be supported by an unrestricted ¿40 000 grant from Servier.

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The active form of vitamin D, calcitriol, is essential to bone turnover and remodeling and to calcium-phosphate homeostasis. Alterations in the vitamin D endocrine system may contribute to bone loss with aging. One hypothesis proposes that age-related bone loss results from decreased intestinal absorption of calcium, leading to increased PTH, which, in turn, increases the rates of bone remodeling and bone loss. Decreases in the production of 1,25(OH)2 vitamin D, intestinal responsiveness to 1,25(OH)2 vitamin D, and levels of 25(OH) vitamin D may all contribute to the decreased intestinal absorption of calcium and increased bone loss seen with aging.

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Various molecules coordinate osteoclast function with that of osteoblasts, such as the RANK/RANKL pathway. However, molecules that mediate osteoclast-osteoblast interactions by simultaneous signal transduction in both cell types have not yet been identified.

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