Analysis of the Cytoprotective Role of alpha-Crystallins in Cell Survival and Implication of the alpha A-Crystallin C-Terminal Extension Domain in Preventing Bax-Induced Apoptosis

alpha-Crystallins, initially described as the major structural proteins of the lens, belong to the small heat shock protein family. Apart from their function as chaperones, alpha-crystallins are involved in the regulation of intracellular apoptotic signals. alpha A- and alpha B-crystallins have been shown to interfere with the mitochondrial apoptotic pathway triggering Bax pro-apoptotic activity and downstream activation of effector caspases. Differential regulation of alpha-crystallins has been observed in several eye diseases such as age-related macular degeneration and stress-induced and inherited retinal degenerations. Although the function of alpha-crystallins in healthy and diseased retina remains poorly understood, their altered expression in pathological conditions argue in favor of a role in cellular defensive response. In the Rpe65(-/-) mouse model of Leber's congenital amaurosis, we previously observed decreased expression of alpha A- and alpha B-crystallins during disease progression, which was correlated with Bax pro-death activity and photoreceptor apoptosis. In the present study, we demonstrated that alpha-crystallins interacted with pro-apoptotic Bax and displayed cytoprotective action against Bax-triggered apoptosis, as assessed by TUNEL and caspase assays. We further observed in staurosporine-treated photoreceptor-like 661W cells stably overexpressing alpha A- or alpha B-crystallin that Bax-dependent apoptosis and caspase activation were inhibited. Finally, we reported that the C-terminal extension domain of alpha A- crystallin was sufficient to provide protection against Bax-triggered apoptosis. Altogether, these data suggest that alpha-crystallins interfere with Bax-induced apoptosis in several cell types, including the cone-derived 661W cells. They further suggest that alpha A-crystallin-derived peptides might be sufficient to promote cytoprotective action in response to apoptotic cell death.