Bone And Tooth Matrix Biochemistry And Metabolism

Dental & Craniofacial Research

Investigators

Linked publications & trials

Abstract

The Matrix Biochemistry Section focuses its research on the functions of five major noncollagenous proteins found associated with the mineralized matrix of bones and teeth. These include: bone sialoprotein (BSP); osteopontin (OPN); dentin matrix protein 1 (DMP1); dentin sialophosphoprotein (DSPP); and matrix extracellular phosphoglycoprotein (MEPE). We have made a strong case for the genetic relatedness of these seemingly different proteins and there is increasing acceptance of the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family concept. The genes encoding these proteins are all clustered in a tandem fashion within a 400 kb region of human chromosome 4 and they all have similar exon properties separated by type-0 introns. Because the same genes are clustered together in all mammals studied, they are likely to be the result of gene duplications and subsequent divergences many millions of years ago. One of the conserved motifs among the different SIBLING members (and among animal species for any single protein) is the integrin-binding tripeptide, arginine-glycine-aspartate (RGD), that can link otherwise soluble binding partners to cell surfaces. In recent years for example, we have shown that at least three members of the SIBLING family bind and activate three different members of the secreted matrix metalloproteinase (MMP) family as well as complement Factor H and may modify their activities by bridging these otherwise soluble proteins to cell surfaces.[unreadable] [unreadable] Although studied for many years in bones and teeth, the specific functions of these proteins in the mineralized tissues remains surprisingly unclear. One observation made many years ago was that the acidic SIBLINGs, particularly OPN and BSP, appeared to accumulate to a large extent in layers within the mineralized matrices of bone called cement or reversal lines. These lines are where one round of highly organized collagen matrix synthesis was separated in time and space from subsequent collagen matrix assembly/mineralization events. These narrow regions of low collagen matrix were expected to confer specific properties on the biomatrix. One hypothesis is that the cement line, lacking collagen fibrils, would be a region where strain cracks would propagate under heavy loads if left unchecked. In collaboration with experts in atomic force microscopy at UC Santa Barbara, our recombinant OPN, BSP, and DMP1 were tested for the ability to absorb energy much as classic soft glues do. All three SIBLINGs (individually) were found to absorb relatively large amounts of energy in their model system. Indeed, the SIBLINGs continued to absorb energy over distances much larger than the lengths of the individual proteins suggesting that three-dimensional complexes were responsible for these interesting properties. This entropy-driven process was found to be calcium dependent and reversible suggesting that no covalent bonds were broken. Therefore it is reasonable to hypothesize that the SIBLINGs form calcium-dependent complexes that absorb energy as cracks try and propagate along the cement lines. As the forces are dissipated and the matrix returns to its original state, the complexes reform and are ready for the next strain event.[unreadable] [unreadable] Although originally thought to be specific to bones and teeth, we have also shown in the last 2-3 years that all five members of the SIBLING family are also expressed in normal adult salivary gland and kidney. (Others had previously shown that OPN is expressed outside of bones and teeth including the kidney were it is commonly called uropontin.) We have hypothesized that the SIBLINGs and their partner MMPs may be cooperatively expressed in all metabolically active ductal cells that recover salts from the initial fluids made by different organs. There the SIBLINGs and MMPs would be used to replace pericellular matrix proteins damaged by free radical byproducts generated by the abundant mitochondria needed to make the ATP used in the rapid salt/small molecule recovery processes. We proposed that sweat glands (which also recover salts against a chemical gradient) but not tear gland ducts (tears are salty because the salts are not removed from the fluid as it is secreted) would express the SIBLINGs and MMPs. Our results did show that the relatively inert lacrymal gland ducts do lack all SIBLINGs and MMP expression while the sweat glands express BSP, OPN, DMP1, DSPP, and their cognate MMPs (when known). MEPE was an interesting exception as it was not expressed in the sweat gland. Unlike kidney and salivary gland, sweat glands do not actively transport free phosphate groups into or out of the newly formed fluid. This led us to a secondary hypothesis that MEPE may be specifically related to active phosphate transport functions.[unreadable] [unreadable] We and our collaborators have shown over the last decade that BSP, OPN and DMP1 are frequently highly expressed in a number of cancers. This year we have added oral squamous cell carcinoma (the most common cancer of the oral cavity) to the growing list of cancers that strongly up-regulate one or more SIBLINGs during tumorigenesis. Indeed, over 90% of the oral squamous cell carcinomas studied expressed BSP, OPN and/or DSPP. No DMP1 or MEPE was observed by immunohistochemistry in any of these cancers. MMP-2 and MMP-3 (partners for BSP and OPN respectively) were also expressed thereby showing for the first time that the two families of secreted proteins may function together in cancerous tissues in vivo. The SIBLINGs seemed to turn on early in the oral cancer process in that there was no correlation of expression with tumor size. We are currently checking to see if these three SIBLINGs and/or their MMP partners may serve as markers for precancerous oral lesions that have a high propensity to progress to frank oral cancer.[unreadable] [unreadable] The Matrix Biochemistry Unit freely gives probes (antisera, cDNA, proteins etc.) to any laboratory in the world that makes a reasonable request. In FY07 we sent 250 probes to 90 laboratories (20% of which were in the dental field) around the world. Others in the Branch have sent these same probes during this time as well.