EVOLUTIONARY DISTRIBUTION OF DENTIN MATRIX PROTEINS

Northwestern University, Evanston IL

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Abstract

The process of biomineralization has been widespread in both vertebrates for several hundred million years. Although the strategies used are complex and diverse, some aspects of mineralization may have been universally conserved throughout evolution in a very diverse range of organisms. Invertebrates comprise about 95% of animal species, yet a substantial body of work relates to mineralization in vertebrates. In vertebrate bone and dentine, apatite crystals are specifically grown within type I collagen matrices. The basic working hypothesis has been that certain acid, phosphorylated, non-collagenous extracellular matrix proteins (NCP) are localized by an interaction with collagen I fibril surfaces, and then direct the nucleation of crystal deposition and orientation of the crystal axes relative to the fibril axes. Further interactions of the NCPs with the growing crystals are postulated to specifically regulate crystal growth in terms of habit, shape, and size. Although bone and dentin share many NCPs, dentin appears to contain several unique, NCPs (e.g., DMP1, phosphophoryns [PP, DSP) that play a role in mineralization. The mineralized compartments of invertebrates do not contain collagen, but do have polymeric components that play an equivalent role. Moreover, several studies have shown that invertebrate mineralized tissues may have acid proteins which are the counterparts of the vertebrate NCPs. Teeth and exoskeletal denticles are among the most primitive vertebrate mineralized tissues. With the molecular tools now available, probes for specific acidic proteins could be used to explore relationships between various vertebrate and invertebrate species. We have chosen to investigate the phosphoryns of invertebrate (sea urchin) teeth, bichir scales and human dentin in order to enhance our comprehension of evolutionary processes that may reveal a similarity and/or diversity of NCPs throughout time. mRNA extract from human teeth, bichir scales and different portions of the sea urchin tooth will be used to create cDNA libraries. These libraries will be probed for the expression of DMP1, DMP2, and DMP3. The libraries will also be probed for bone NCPs such as ON, OPN, BGP and BSP. Antibodies for phosphophoryn will be used to relate expression of phosphophoryn or related proteins in human dentin, bichir scales and different portions of the sea urchin tooth. In addition, the biochemistry of the proteins found in these tooth related organs will be compared through extraction, purification and acidic protein content and composition.