We study the mechanisms by which proteins control normal calcification of bone and prevent abnormal calcification of soft tissues. Bone is primarily a composite of hydroxyapatite mineral and type I collagen fibers. We are presently investigating the effects of type I collagen's inherent size exclusion abilities on its ability to exclude calcification inhibiting proteins from its internal structure, resulting in a calcification that only occurs within the collagen itself.
Another area of focus is on two vitamin K dependent proteins that we have discovered, the 50 residue bone Gla protein (BGP) and the 83 residue matrix Gla protein (MGP). Studies using the vitamin K antagonist warfarin and targeted gene deletion have shown that BGP and MGP both inhibit calcification in vivo. Impaired BGP expression causes the spread of calcification from bone into adjacent cartilaginous structures and impaired MGP expression causes calcification of arteries.
We are investigating the relationship between the impaired synthesis of MGP in aging humans and the calcification of arteries as well as the development of atherosclerotic plaque. Our hypothesis is that calcification of the artery in aging humans is caused in part by dietary vitamin K deficiency, and that artery calcification contributes to the formation of the atherosclerotic plaque. This hypothesis is supported by the observation that the extent of artery calcification is the best predictor of future myocardial infarction found to date. It is also supported by the fact that animals on a cholesterol rich diet will not form atherosclerotic plaques unless arteries have been previously induced to calcify.
We are also investigating the in vitro mechanism by which MGP and BGP bind to the surface of hydroxyapatite crystals and prevent their growth and the relationship between the structure of these proteins and their ability to inhibit calcification. We are particularly interested in the role of the vitamin K-dependent calcium binding amino acid, g-carboxyglutamic acid (Gla), in the activities of these proteins and have developed a variety of methods to detect and modify Gla residues.
We have shown that MGP and BGP are normally expressed in bone and that the expression of both proteins by osteoblasts is strongly induced by the active metabolite of vitamin D, 1,25(OH)2D3. 1,25(OH)2D3 restores serum calcium levels to normal by reducing the rate at which serum calcium is deposited in bone. We believe that the induction of BGP and MGP by 1,25(OH)2D3 is part of the mechanism by which the hormone regulates bone calcification, and we are studying the mechanisms by which the expression of these proteins mediate the action of vitamin D on bone.
We have recently found that the only class of drugs currently approved for the treatment of osteoporosis, the bisphosphonates, have strong and specific effects on the levels of BGP and MGP in serum. There is a similarity in the chemical structure of bisphonates and the g-carboxyglutamyl side chain, and we believe that these effects of bisphonates on MGP and BGP are due in part to the competition of the drug with these proteins for binding to sites in bone mineral. We are presently investigating the relationship between the action of bisphonates on bone and the effect of these drugs on the metabolism of MGP and BGP.