CHONDROCYTE GENE EXPRESSION DURING CARTILAGE REPAIR

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Regulation of the biosynthesis and turnover of extracellular matrix components in articular cartilage must involve an interplay between the chondrocyte and its immediate pericellular environment. In osteoarthritic cartilage from humans as well as in animal models of the disease, there occurs a protease mediated loss of extracellular matrix molecules. This may in turn lead to a disruption in normal adhesive interactions between the chondrocyte and its immediate pericellular environment. Loss of normal cell-matrix interactions could profoundly influence chondrocyte gene expression, and may explain in part the apparent inability of cartilage to effectively repair protease-mediated matrix damage. Since articular cartilage metabolism cannot easily be studied in vivo, explant cultures have been used as model systems for investigating the response of chondrocytes to injurious stimuli. We propose to test the hypothesis that chondrocyte biosynthetic activity in matrix-depleted cartilage differs from that occurring in normal cartilage. We will define the chondrocyte response to matrix alterations at a level of resolution limited by the availability of extracellular matrix probes. Preliminary efforts have involved isolation and characterization of bovine specific probes for link protein and aggrecan and efforts are in progress to generate probes for other major extracellular matrix components. Explants of cartilage will be used as a model system to study the response of the chondrocyte to depletion of the extracellular matrix by exogenously added proteases, to matrix depletion induced by IL-1 stimulated endogenous protease activity and in matrix-depleted osteoarthritic rabbit cartilage. Levels of mRNA transcripts for extracellular matrix proteins will be determined by Northern blot analysis of chondrocyte RNA. Using a cDNA library subtraction strategy an effort will be made to identify phenotypic differences in gene expression between the normal and the reparative chondrocyte from matrix-depleted cartilage. Library subtraction will permit the identification of sequences for intracellular and extracellular matrix chondrocyte proteins whose expression is modulated in chondrocytes involved in cartilage repair based only upon changes in phenotype, requiring no previous knowledge of protein or DNA sequences.