Molecular mechanisms of ischemia-reperfusion injury in brain: pivotal role of the mitochondrial membrane potential in reactive oxygen species generation

Abstract

Stroke and circulatory arrest cause interferences in blood flow to the brain that result in considerable tissue damage. The primary method to reduce or prevent neurologic damage to patients suffering from brain ischemia is prompt restoration of blood flow to the ischemic tissue. However, paradoxically, restoration of blood flow causes additional damage and exacerbates neurocognitive deficits among patients who suffer a brain ischemic event. Mitochondria play a critical role in reperfusion injury by producing excessive reactive oxygen species (ROS) thereby damaging cellular components, and initiating cell death. In this review, we summarize our current understanding of the mechanisms of mitochondrial ROS generation during reperfusion, and specifically, the role the mitochondrial membrane potential plays in the pathology of cerebral ischemia/reperfusion. Additionally, we propose a temporal model of ROS generation in which posttranslational modifications of key oxidative phosphorylation (OxPhos) proteins caused by ischemia induce a hyperactive state upon reintroduction of oxygen. Hyperactive OxPhos generates high mitochondrial membrane potentials, a condition known to generate excessive ROS. Such a state would lead to a "burst" of ROS upon reperfusion, thereby causing structural and functional damage to the mitochondria and inducing cell death signaling that eventually culminate in tissue damage. Finally, we propose that strategies aimed at modulating this maladaptive hyperpolarization of the mitochondrial membrane potential may be a novel therapeutic intervention and present specific studies demonstrating the cytoprotective effect of this treatment modality.

Figure 1. Progression of Ischemia/reperfusion Injury

(A) Under normal, non-stressed conditions, OxPhos components are phosphorylated (as illustrated for Cytc and CcO), promoting controlled electron transfer and maintaining the ΔΨ_m in the 80-140mV range. This respiratory state is conducive to maximal ATP production and minimal ROS generation. (B) Ischemia induces a starvation state were the ETC cannot function due to lack of O₂. Dephosphorylation of OxPhos during ischemia renders these enzymes ‘primed’ for hyperactivity, however they cannot operate due to lack of the terminal substrate for respiration, O₂. (C) Reintroduction of O₂ with reperfusion initiates electron transfer, proton pumping and ATP synthesis. However, in this hyperactive (dephosphorylated) state, OxPhos hyperpolarizes the ΔΨ_m, causing an exponential increase in ROS generation at complexes I and/or III. (D) This burst in ROS can act as a signal for triggering apoptosis. In addition, damage caused by ROS induces a mitochondrial dysfunction state, with reduced electron transfer kinetics and reduced ΔΨ_m levels, which results in energetic failure.

Figure 2. Mechanism of ROS generation during reperfusion

During extended brain ischemia, increased intramitochondrial Ca²⁺ activates phosphatases that dephosphorylate OxPhos components, as shown for Cytc and CcO in B. This promotes a state of OxPhos hyperactivity; however, because O₂ is absent electron transport cannot proceed. (C) Upon induction of reperfusion, OxPhos is allowed to operate at maximal activity, generating high ΔΨ_m levels, which in turn promotes ROS generation.