Mechanism of Src kinase induction of cortical cataract following exposure to stress: destabilization of cell-cell junctions.

PURPOSE Activation of stress pathways is a primary cause of age-related cataracts. Our laboratory previously developed a lens cataract model in which activation of the p38 kinase/Src family kinase (SFK) stress-signaling pathway is responsible for the induction of cortical opacities. Here, we use this model to investigate further the mechanism of stress-induced cataract. METHODS Cortical cataracts were induced by mechanical stress and prevented by exposure to the SFK inhibitor PP1. Organization of the actin cytoskeleton, cadherin junctions and membrane structure was determined by fluorescence imaging. Apoptosis was examined by both terminal dUTP nick-end labeling (TUNEL) and DEVDase assays. N-cadherin cleavage was examined by western blot analysis. RESULTS In this cortical cataract model, activation of SFKs in the lens epithelium caused increased cleavage of N-cadherin, the loss of cadherin cell-cell junctions, and the disorganization of the actin cytoskeleton. As the function of cadherin junctions and the cytoskeleton are required for assembling a polarized epithelium and protecting it against apoptotic cell death, the SFK-dependent disassembly of these structures was likely responsible for the observed loss of integrity and apoptosis of the lens epithelium. Apoptotic death of lens epithelial cells preceded the appearance of cortical opacities, suggesting that the lens epithelium provided an important line of defense for the lens fiber cells. Opacification was related to defects in the membrane structure of cortical fiber cells. Linearity of fiber cell membranes was lost. Extensive undulations and interdigitations of these membranes occurred as well as large separations between fiber cells as is common to many types of cataract. CONCLUSIONS Src kinase activation-induced loss of cadherin junctions and apoptosis of the lens epithelium leads to altered fiber cell organization and lens opacification.