Which NADPH oxidase isoform is relevant for ischemic stroke? The case for nox 2

Abstract

Significance and Recent Advances: Ischemic stroke is the leading cause of disability and third in mortality in industrialized nations. Immediate restoration of cerebral blood flow is crucial to salvage brain tissue, but only few patients are eligible for recanalization therapy. Thus, the need for alternative neuroprotective strategies is huge, and antioxidant interventions have long been studied in this context. Reactive oxygen species (ROS) physiologically serve as signaling molecules, but excessive amounts of ROS, as generated during ischemia/reperfusion (I/R), contribute to tissue injury.

Critical issues: Nevertheless and despite a strong rational of ROS being a pharmacological target, all antioxidant interventions failed to improve functional outcome in human clinical trials. Antioxidants may interfere with physiological functions of ROS or do not reach the crucial target structures of ROS-induced injury effectively.

Future directions: Thus, a potentially more promising approach is the inhibition of the source of disease-promoting ROS. Within recent years, NADPH oxidases (Nox) of the Nox family have been identified as mediators of neuronal pathology. As, however, several Nox homologs are expressed in neuronal tissue, and as many of the pharmacological inhibitors employed are rather unspecific, the concept of Nox as mediators of brain damage is far from being settled. In this review, we will discuss the contribution of Nox homologs to I/R injury at large as well as to neuronal damage in particular. We will illustrate that the current data provide evidence for Nox2 as the most important NADPH oxidase mediating cerebral injury.

FIG. 1.

NADPH oxidases in the brain: Structure, activation, and expression. Only data for Nox1, Nox2, and Nox4 are shown, as the other Nox protein have basically not been studied in nervous tissue. H₂O₂, hydrogen peroxide; I/R, ischemia/reperfusion; Nox, NADPH oxidases; O₂, oxygen; O₂⁻, superoxide; PDI, protein disulfide isomerase; TGF β1, transforming growth factor β1.

FIG. 2.

Nox-mediated blood–brain barrier disruption in cerebral ischemia/reperfusion. Ischemia/reperfusion activates elements upstream of Nox2, including PI3-kinase, PKC, and calcium. The subsequent ROS formation not only activates downstream targets involved blood brain–barrier opening but also promotes further stimulation of the elements upstream of Nox2: ROS are known to increase calcium and to activate PKC, PI3-kinase, and Rac. MLC kinase, myosin light-chain kinase; MMP, matrix metalloproteinase; PKC, protein kinase C; ROS, reactive oxygen species.

FIG. 3.

Nox-mediated neuronal cell death in cerebral ischemia/reperfusion. Ischemia/reperfusion as well as NMDA receptor activation increase intracellular calcium and other elements involved in Nox2 activation, which leads to subsequent ROS formation. As illustrated for the ROS-dependent inactivation of casein kinase 2, several positive feedback loops further increase ROS formation in a vicious circle entertained by an oxidation of target molecules. Apoptosis is eventually initiated by energy breakdown through PARP and activation of apoptosis-inducing factors as well as MTP opening. Necrosis can also occur, for example, as a consequence of the calcium- and ROS-mediated activation of the protease calpain by proteolysis, which subsequently cleaves a large number of cellular proteins with high activity. MTP, mitochondrial transition pore; NMDA, N-methyl d-aspartic acid; PARP, poly-ADP ribose polymerase.