? Inherited and tumor acquired bone-mineral disorders have provided powerful models that have helped understand some of the molecular-mechanisms responsible for maintaining a healthy, dynamic, mineralized-skeleton. X-linked hypophosphatemic rickets (HYP) is one such disease and is associated with severe defects in mineralization, renal phosphate handling, and vitamin D metabolism. MEPE, (a bone-matrix protein) and osteoblastic-proteases are elevated markedly in HYP. Also, PHEX, an osteoblast-expressed, plasma-membrane anchored Zn-metallopeptidase is the primary defect responsible for the disease. We have substantive preliminary data that supports the hypothesis that a protein-protein interaction between MEPE and PHEX regulates mineralization. PHEX protects MEPE from proteolytic cleavage by proteases (notably cathepsin B) in-vitro and osteoblastic-proteases are elevated in Hyp. In the absence of PHEX, MEPE is cleaved, releasing a phosphorylated, acidic, serine-aspartate rich, C-terminal fragment (ASARM-peptide). The ASARM-peptide (2 kDa), is remarkably resistant to a vast array of proteases resulting in increased levels in Hyp. The ASARM-peptide inhibits mineralization and elevated ASARM-peptide is proposed to be wholly or in part responsible for the mineralization defects. This application will focus on the role of MEPE and ASARM-peptides in mineralization.
Our specific aims are: 1. Structural characterization and quantification of MEPE ASARM-peptide(s) in-vitro and in-vivo in normal and Hyp osteoblasts and serum. 2. Determination of MEPE ASARM-peptide effects on mineralization in-vitro and in-vivo. 3. Elucidation of MEPE-PHEX protein-protein interactions. 4. The role of elevated protease(s) in Hyp-osteoblasts in relation to abnormal mineralization and MEPE-PHEX interactions. The ASARM-motif is present in a number of tooth-bone matrix and salivary proteins. These include MEPE, DMP1, osteopontin, statherin, DSPP that all map to chromosome 4q21. Thus, the elucidation of its role in mineralization is of prime importance for understanding bone mineralization mechanisms. Importantly, the part the motif plays will have relevance to the treatment and understanding of bone-fracture risk, osteoporosis, tumor-induced osteomalacia (OHO), HYP, periodontal disease, disorders of mineralization in teeth, bone, renal-stones and ectopic calcifications in arteries and renal-osteodystrophy. ? ?
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