Dynamic compression of chondrocytes induces a Rho kinase-dependent reorganization of the actin cytoskeleton.

The signal transduction mechanisms in chondrocytes that recognize applied forces and elicit the appropriate biochemical cellular responses are not well characterized. A current theory is that the actin cytoskeleton provides an intracellular framework onto which mechanosensation mechanisms are assembled. The actin cytoskeleton is linked to the extracellular matrix at multi-protein complexes called focal adhesions, and evidence exists that focal adhesions mediate the conversion of external physical forces into appropriate biochemical signal transduction events. The Rho GTPases affect the arrangement of actin cytoskeletal structures, and enhance the formation of focal adhesions, which link the cytoskeleton to the extracellular matrix. A major effector pathway downstream of Rho is the activation of Rho kinase (ROCK), which phosphorylates and activates Lim kinase, which in turn phosphorylates and inhibits the actin-depolymerizing protein cofilin. The objectives of this study were threefold: first, to quantify the actin reorganization in response to dynamic compression of agarose-embedded chondrocytes. Second, to test whether Rho kinase is required for the actin cytoskeletal reorganization induced by dynamic compression. Third, to test whether dynamic compression alters the intracellular localization of Rho kinase and actin remodeling proteins in chondrocytes. Dynamic compression of agarose-embedded chondrocytes induced actin cytoskeletal remodeling causing a significant increase in punctate F-actin structures. Rho kinase activity was required for these cytoskeletal changes. Dynamic compression increased the amount of phosphorylated Rho kinase. The chemokine CCL20 and inducible nitric oxide synthase (iNOS) were the most highly upregulated genes by dynamic compression and this response was reduced by the Rho kinase inhibitors. In conclusion, we show that dynamic compression induces changes in the actin cytoskeleton of agarose-embedded chondrocytes, and we establish methodology to quantify these changes. Furthermore, we show that Rho kinase activity is required for this actin reorganization and gene expression induced by dynamic compression.