A cardioprotective role for the small heat shock protein, alpha B-crystallin, in ischemia-reperfusion injury

The small heat shock protein, alpha B-crystallin (αBC), is a stress inducible chaperone found in selected tissues, and notably comprising up to 3% of total protein in the heart. Considerable interest in this small heat shock protein (sHSP) derives from emerging evidence of its participation in the cardiac stress response. To address the role for αBC in the myocardium, the recently generated αBC/HSPB2 knockout (KO) mouse model was utilized. Cardiac morphology and function were assessed and the response of KO mouse hearts to ischemia/reperfusion (1/R)-induced stress was analyzed. While initial characterization revealed little morphological and functional differences between KO and wild-type (WT) mouse hearts, following I/R, KO mouse hearts exhibited a two-fold reduction in contractile recovery that correlated with increased necrosis and apoptosis. Thus, while neither αBC nor HSPB2 is essential for myocardial development and function under non-stressful conditions, one or both are required for maximal functional recovery and for protection from the necrotic and apoptotic damage caused by I/R stress. One mechanism thought to mediate protection in cardiac myocytes is the phosphorylation of αBC. In response to various stresses, αBC is phosphorylated on serines −19, −45, and −59. However, it is not known whether αBC phosphorylation directly affects its ability to foster cell survival. A neonatal rat ventricular cardiac myocyte model system was used and cells were transfected with DNA constructs that encoded mutated forms of αBC mimicking or blocking phosphorylation at serines −19, −45, or −59, and their effects on apoptosis was assessed after hypoxic stress. It was revealed that cells expressing pseudophosphorylated forms of αBC were maximally protected from hypoxia-reoxygenation (H/R)-induced apoptosis. In contrast, cells expressing forms of αBC that could not be phosphorylated were more susceptible to apoptosis than control cells. Additionally, the protection afforded by phosphorylation of αBC was mediated, in part, by the attenuation of caspase-3 activity. These studies demonstrate that αBC is required for protection of the myocardium against oxidative stress, and its phosphorylation is a key feature that maximally protects cardiac myocytes against stress-induced cell death.