A Critical Role of DMP-1 in Mineralization

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DESCRIPTION (provided by applicant): The mechanisms by which bone mineralizes are currently unclear. It has been proposed that mineralization is a passive physicochemical process that is controlled mainly at the level of inhibition by molecules such as Matrix Gla Protein. Conversely, it has been proposed that mineralization is an active process that is regulated and initiated or triggered by extracellular matrix (ECM) proteins. Our data supports the latter hypothesis, as we have shown that mice lacking the bone ECM protein, Dentin matrix protein-1 (DMP-1) exhibit a dramatic osteomalacia/rickets phenotype with impaired mineralization. In these mice, a large percentage of the bone remains unmineralized, leading to bone fragility, deformation, and impaired fracture healing. Recent biochemical studies suggest that DMP-1 must be proteolytically processed to yield fragments of 37kDa and 57kDa that are responsible for its bioactivity. Preliminary data suggest that the 57kDa fragment is a nucleator of hydroxyapatite formation. Therefore, our central hypothesis is that DMP-1, an ECM protein specific for mineralizing tissues, plays a key role in controlling mineralization and consequently bone remodeling through its different, proteolytically processed forms. To test this hypothesis, molecular and transgenic approaches will be used to determine the effects of overexpression of bioactive fragments of DMP-1 as well as to determine the ability of these fragments to rescue the phenotype of Dmp-1 null mice. In aim 1, the mineralization defects in mice lacking DMP-1 will be characterized in detail and in relation to skeletal maturity. In aim 2 the function of intact DMP-1, the 37kDa and 57kDa DMP-1 fragments as well as Dmp-1 that is mutated to prevent cleavage will be determined in osteoblast differentiation and mineralization using in vitro overexpression and rescue approaches. In aim 3 the in vivo function of intact, mutated, and cleaved fragments of DMP-1 will be determined by transgenic overexpression and rescue approaches using the Drop-1 null mice. These studies will identify the important functional domains of DMP-1 essential for its role as a regulator of mineralization and may highlight its potential role as a common mediator for mineralization defects observed in other models of osteomalacia/rickets. Completion of these studies will enhance our understanding of the molecular mechanisms of bone mineralization and will identify novel targets for therapeutic intervention in diseases of abnormal bone mineralization such as osteomalacia, rickets, and metabolic bone disease such as osteoporosis.