The effect of mechanical compression on chondrocyte gene expression

Chondrocytes are the cells responsible for the growth, maintenance, and repair of articular cartilage. Recent research has shown that chondrocytes are very sensitive to mechanical forces in their environment, and they can alter their synthesis of extracellular matrix (ECM) molecules to meet the functional demands of loading in vivo. Presently, the mechanisms by which mechanical forces cause changes in gene expression or other intracellular pathways that regulate chondrocyte synthesis of matrix molecules are not known. The goal of this thesis is to investigate the influences of mechanical compression chondrocyte gene expression (through messenger RNA analyses) and the resulting down-stream synthesis of matrix macromolecules.

In previous studies, static mechanical compression has been shown to decrease chondrocyte synthesis of aggrecan and type II collagen. However, there have only been limited studies examining the effects of static loading on chondrocyte gene expression and aggrecan and type II collagen. We have shown that static compression can cause a down-regulation in expression of both aggrecan and type II collagen that is dependent on the duration and magnitude of the compression. In addition, our studies suggest that static compression does not act to accelerate the decay of existing mRNA molecules, rather it acts to inhibit new transcription of these mRNA molecules. Finally, our studies have shown that inhibition of macromolecular synthesis occurs earlier after the application of static compression then corresponding changes in expression. This suggests that the total decrease in chondrocyte production of aggrecan and type II collagen is only in part due to down-regulation in the expression levels of these molecules.

Motivated by examining mechanical loads more closely resembling forces experienced in vivo, we also examined the affects of dynamic compression on chondrocyte gene expression of aggrecan and type II collagen. Previous reports have shown that dynamic stimulation can cause significant increases in production of aggrecan and type II collagen molecules. Our series of experiments have shown that dynamic loading can result in increased expression of aggrecan and type II collagen. Interestingly in some cases, we have seen an increase in macromolecular synthesis of some ECM molecules without a corresponding upregulation of mRNA expression. This phenomena appears to be related to the bulk levels of the surrounding extracellular matrix.

Together our data show that chondrocytes are influenced by mechanical compression at the gene-expression level. In trends similar to those previously reported for macromolecular protein synthesis, static compression appears to down-regulate matrix molecule mRNA expression and dynamic loading can upregulate ECM-specific messages. However, it still remains to be determined what intracellular mechanisms are responsible for these changes and what initiates the cascade that ultimately leads to alterations in extracellular matrix production. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)