Oxidative and Nitrosative Stress

Oxidative stress plays a critical role in cerebral ischemic injury and occurs when there is an overproduction of free radicals and reactive oxygen species (ROS) beyond the ability of a biologic system to neutralize their adverse effects. ROS include oxygen ions, free radicals, and peroxides, and are products of cellular metabolism. Iron and its metabolites are crucial in the formation as well as destruction of ROS . These molecules are involved in normal physiologic processes, such as synaptic transmission, cell signaling, induction of mitogenic responses, and immune defense. Under physiologic conditions the cerebral vascular tone is controlled in part by endothelium-derived nitric oxide (NO) and other ROS. However, under periods of ischemic stress leading to mitochondrial uncoupling, ROS are generated at higher rates than the system’s ability to clear them, leading to oxidative damage.

ROS are also produced as part of the natural immune response. A prime example is the inflammatory burst produced by neutrophils that protects the host against pathogens. However, when inflammation is unchecked, ROS are produced in pathological levels leading to damage not just to the invading organism but also to the host. Although stroke is a sterile injury, cellular damage leads to immune activation via the activation of danger associated molecular patterns both in the periphery and within the brain itself. The induction of poststroke inflammation induces expression of adhesion molecules and proinflammatory cytokines post stroke . Furthermore, leukocyte adhesion and aggregation lead to the release of additional free radicals, further propagating secondary injury. Leukocyte adhesion and aggregation also lead to vascular stasis and occlusion, which further limits tissue perfusion and destabilizes the blood–brain barrier.

Inflammation, reperfusion injury, excitotoxicity, calcium imbalance, and respiratory inhibition all contribute to the generation of free radicals including superoxide ions, hydroxyl radicals, and NO-like reactive nitrogen species (RNS), such as the potent oxidant peroxynitrite (ONOO ⁻ ). These molecules react with molecular targets including proteins, lipids, and/or nucleic acids contributing to cellular dysfunction or death.

Neurons have high oxygen consumption, relatively low antioxidants (i.e. catalase and glutathione), and are exquisitely vulnerable to oxidative stress. In addition, neuronal cell membranes are rich in polyunsaturated fatty acids, and hence prone to ROS damage. Brain cells also have higher levels of iron, which acts as a prooxidant under pathologic conditions. ROS lead to lipid peroxidation, further producing aldehydes, alkanes, and dienals, which are toxic to neurons and white matter, and induce apoptosis. Studies have revealed variable susceptibility of specific brain regions to oxidative injury. Dopaminergic areas seem to be specifically sensitive to ROS/RNS-induced injury.

Cell death can be seen as a spectrum comprising apoptosis, necrosis, necroptosis, and parthanatos. Necroptosis (a form of regulated necrosis) involves proteins such as receptor-interacting serine/threonine-protein kinase 3 and Mixed Lineage Kinase domain like ( protein), which are induced by death receptors, interferons, toll-like receptors, and other mediators. Enhanced mitochondrial ROS production was found to be the terminal event in experimental mouse cells undergoing necroptosis. Ferroptosis is a type of regulated necrosis that is characterized by an iron-dependent production of ROS . Cell death involving poly-ADP ribose (PAR) polymerase formation is termed as parthanatos.

Oxidative Stress in Ischemic Stroke

Arterial blockade by a thrombus or an embolus results in impaired cerebral perfusion. A complex biochemical cascade is triggered after an ischemic injury to the brain. Oxidative stress plays a major role in the series of events. Depletion of ATP inhibits the Na ⁺ /K ⁺ -ATPase pump, subsequently leading to electrolyte imbalance, which further results in anoxic depolarization of the membrane and promotes membrane instability. This usually takes place at the core of the ischemic stroke, and further expansion of the core depends on this biochemical cascade. The deprivation of oxygen and glucose and increased intracellular Ca ²⁺ , Na ⁺ , and ADP leads to increased mitochondrial production of ROS. Normal cell homeostasis depends on effective mitochondrial calcium buffering. Increased calcium levels induce opening of the mitochondrial permeability transition pore, which results in diffusion of molecules including ROS from the mitochondria to the cytoplasm, further exacerbating mitochondrial dysfunction. Along with leakage from the mitochondrial electron transport chain, free radicals are also generated via action of NADPH oxide synthases, xanthine oxidase, and cyclooxygenase ( Fig. 49.1 ).