Transient loss of alphaB-crystallin: an early cellular response to mechanical stretch

Human trabecular meshwork (HTM) is distended and stretched with increases in intraocular pressure. During this stretching, there is a rearrangement of actin filaments. The HTM cells express alpha B-crystallin, a small heat shock protein that may have a role in the stabilization and regulation of the cytoskeleton in mammalian cells. The levels of alpha B-crystallin were examined in trabecular meshwork cells after mechanical stretch. Human TM primary cell cultures, plated onto silicone sheets, were subjected to a single 10% linear stretch and samples were prepared at various times after stretch for immunoblotting or Northern blotting. Immunoblots of total protein extracts with antibody specific for alpha B-crystallin detected a 26% decrease of cellular alpha B-crystallin levels within 2 minutes. After 1 hour alpha B-crystallin levels had decreased 90% compared to control cells. The levels of alpha B-crystallin began to recover in cells stretched for 2 hours and returned to initial levels by 24 hours. Northern blots probed with alpha B-crystallin exon III cDNA detected a transcript of 0.65 kb in human TM cells and the levels of the alpha B mRNA remained constant during alpha B-crystallin protein decrease. Later, levels of the 0.65 kb transcript of alpha B-crystallin increased during the cellular recovery. These results suggest that decreased levels of alpha B-crystallin after mechanical stretch were probably not due to transcriptional changes but rather to increased degradation of alpha B-crystallin protein. An increase in mRNA levels may play a role in the recovery of alpha B-crystallin during reorganization of the cytoskeleton and attachment to the substratum. These data raise the possibility of a specific proteolysis of alpha B-crystallin protein in cells after a physiological challenge.