Abstract 26: Role of Cytochrome c Oxidase Subunit 4- Isoform 2 in Neuronal Susceptibility to Lethal Ischemia/Reperfusion-Induced Injury

Abstract

Brain ischemia-reperfusion injury (I/R) caused by stroke or cardiac arrest-resuscitation is associated with a high incidence of mortality and, in survivors, often results in profound neurological deficits. Specific regions of the brain (in particular, neurons in the CA1 hippocampus) are especially vulnerable to I/R injury, possibly due to exacerbated production of reactive oxygen species (ROS) and the resultant oxidative damage. However, the molecular mechanisms underlying excessive ROS generation in vulnerable neurons is unknown. Hypothesis: We propose that the hyper-susceptibility of the CA1 hippocampus to I/R injury is due in part to the localized expression of mitochondrial cytochrome c oxidase subunit 4-isoform 2 ( COX4-2 ) - i.e., a specific hyperactive isoform of COX. COX is the rate-limiting enzyme of oxidative phosphorylation and its activity indirectly controls mitochondrial ROS production by regulating mitochondrial membrane potential. Therefore expression of a hyperactive COX isoform (COX4-2) would result in excessive ROS production and promote cell death. Methods and Results: We performed COX activity measurements of isolated COX complexes containing COX4-2 compared to COX without COX4-2 and found COX4-2 expression causes a two-fold increase in activity. Next, we generated COX4-2 knock-out mice to determine the role of COX4-2 in selective vulnerability of neurons. Wild-type mice (WT: with documented COX4-2 expression in the CA1) and COX4-2 knock-out mice (KO) underwent 12 min of global brain ischemia (bilateral carotid occlusion with hypotension) or no intervention (time-matched shams). At 3 days post-reperfusion, the number of viable neurons in the CA1 hippocampus was quantified by cresyl violet staining. As expected: (i) shams displayed 100% neuronal viability; while (ii), in wild-type mice, only 14±2.3% of neurons in the CA1 remained viable following I/R. In contrast, COX4-2 KO mice were characterized by a significant, ∼5-fold increase in the proportion of surviving neurons (68±1.1% viability; p<0.05 vs WT I/R, n = 3). Conclusion: These data provide novel evidence that the absence of COX4-2 confers protection to the CA1 and suggest that expression of the hyperactive COX4-2 isoform in the CA1 hippocampus contributes to the vulnerability of these neurons to lethal I/R injury.