Corneal fibroblasts respond rapidly to changes in local mechanical stress

Purpose. To investigate the response of corneal fibroblasts to local changes in extracellular matrix (ECM) tension. Methods. Rabbit and human corneal fibroblasts were plated inside fibrillar collagen matrices. After 18 to 72 hours, a glass microneedle was inserted into the ECM and either pushed toward a cell to reduce local tension, or pulled away to increase tension. Time-lapse differential interference contrast (DIC) imaging was performed both before and after needle micromanipulation. ECM displacements were quantified, and strain maps were generated by finite element modeling. In some experiments, cells were treated with the Rho-kinase inhibitor Y-27632 either 30 minutes before, or 1 hour after they were pushed with the microneedle. Changes in focal adhesion organization were also evaluated in a subset of cells expressing green fluorescent protein (GFP)-zyxin, by simultaneous fluorescent and DIC imaging. Results. Pulling on the ECM resulted in initial cell elongation, followed by disengagement and retraction of pseudopodia. In contrast, pushing the ECM toward a cell induced rapid shortening (contraction), presumably since existing cellular forces were no longer counterbalanced by ECM tension. Pseudopodial extension (spreading) was then observed at both ends of the cell. The ECM was pulled inward during this secondary spreading, and rapid turnover of focal adhesions was observed along extending pseudopodia. Preincubation with Y-27632 or cytochalasin D blocked both the initial contractile and secondary spreading responses. Conclusions. Overall, the data suggest that corneal fibroblasts actively respond to increases or decreases in local matrix stress in an attempt to maintain tensional homeostasis(constant tension), and that this response may be mediated by Rho and/or Rac.