Osteoporosis is a public health problem that affects approximately 25 million people in the United States. Regardless of the etiology, osteoporosis is characterized by an imbalance in bone remodeling, such that osteoclastic bone resorption exceeds osteoblastic bone formation, which leads to low bone mass or osteopenia. New information about mechanisms of bone resorption and osteoclastogenesis may facilitate the development of novel strategies to decrease bone resorption and may lead to new treatments for osteoporosis. We have used the model of cherubism, a rare disorder characterized by giant-cell bone resorptive tumors of the mandible and maxilla, to explore novel pathways that regulate osteoclast development and function. Cherubism is caused by heterozygous missense mutations within a six-amino-acid region (415-420) of the Src homology 3 binding protein 2 (SH3BP2) gene, which encodes a widely expressed adaptor protein. The specific timing and localized expression of cherubism indicates that it is a unique and novel model of human osteoclastogenesis, bone resorption and osteopenia. Moreover the discrete location of these lesions may have important implications for understanding the pathophysiology of osteonecrosis of the jaw. We have assessed 17 cherubism probands and have found that the SH3BP2 mutations activate nuclear factor of activated T cells (NFAT). NFATc1 (also termed NFAT2) is the master switch of osteoclastogenesis. All of these SH3BP2 mutations also induce expression of the osteoclast marker tartrate resistant acid phosphatase (TRAP) in the RAW 264.7 pre-osteoclastic cell line. Our recent studies suggest that SH3BP2 stimulates a form of phosphatidylinositol-specific phospholipase C (PI-PLC), and the production of inositol phosphate 3 (IP3) leads to release of calcium (Ca2+) from the endoplasmic reticulum. The increase in intracellular calcium can activate calcineurin, which dephosphorylates NFAT and promotes its translocation to the nucleus. Hypothesis: SH3BP2 has a role in osteoclastogenesis and SH3BP2 mutations in cherubism patients alter the physical properties (post-translational modifications and/or binding to other proteins) of SH3BP2, with at least one result being enhanced NFATc1 activity and consequent osteoclastogenesis. Aim 1. To examine post-translational modifications in wild-type and mutant SH3BP2. Aim 2. To characterize SH3BP2 Protein-Protein Interactions. Aim 3. To distinguish the osteoclastogenic mechanism for wild-type and mutant SH3BP2. PUBLIC HEALTH RELEVANCE Osteoporosis is a public health problem that affects approximately 25 million people in the United States. Regardless of the etiology, osteoporosis is characterized by an imbalance in bone remodeling, such that osteoclastic bone resorption exceeds osteoblastic bone formation, which leads to low bone mass or osteopenia. New information about mechanisms of bone resorption may facilitate the development of novel strategies to decrease bone resorption and may lead to new treatments for osteoporosis.
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