Two related questions are investigated in this proposal. First, what is the biological function of the vitamin K-dependent, bone specific protein, bone Gla protein (BGP, Osteocalcin)? BGP is an abundant, noncollagenous protein which comprises approximately 1-2% of the total proteins present. A number of key experiments point to the function and control of biosynthesis of BGP. BGP is found in minute quantities in the circulation, and the levels are altered in metabolic bone diseases. Bone Gla protein requires its full complement of 3 delta-carboxyglutamic acid residues in order to bind to the bone mineral phase, as treatment with the vitamin K antagonist warfarin abolishes the delta-carboxylation of BGP and its mineral binding capacity. Thus, rats treated with warfarin do not accumulate BGP in bone, although they continue to grow normally. The post translational delta-carboxylation of BGP is essential for its normal intracellular processing and secretion from the cell. Bone culture experiments have shown that this protein is made in bone by osteoblasts, and is synthesized at a rate comparable to type I collagen. When vitamin D metabolites are added to cultures, collagen synthesis and bone Gla protein synthesis are regulated in opposite directions. Bone Gla protein levels in bone and serum decline with age, as does the capacity to form new bone. The second, related question is, why does bone formation rate and osteoblastic function decline in aged rats. Either the bone formation rate or the bone resorption rat or both are abnormal in osteoporosis. We propose to determine if stem cell depletion or a loss of systemic factors are the cause of the decreased bone formation in the old rat. A specific radioimmunoassay for BGP will be combined with in vivo and in vitro experiments to explore the biosynthesis and function of this protein. Two model systems, one in vivo, and one in vitro will be used to test for stem cell activity from rats of different ages.
