Trends in Neurosciences
Targeting eNOS for stroke protection
Section snippets
NO, eNOS and vascular disease
Endothelium-derived NO, the classic vasodilator discovered by Furchgott in 1980 [7], is produced by the enzyme eNOS through oxidative conversion of l-arginine to l-citrulline. When formed by the vascular endothelium, NO diffuses to the adjacent cells and activates soluble guanylate cyclase, which in turn mediates many of the beneficial effects of NO. In vascular smooth muscle, NO is a potent vasodilator and regulates regional blood flow 3, 4, 5, 6. In addition, NO is antithrombotic,
Augmentation of endothelial NO production protects from cerebral ischaemia
Pharmacological and genetic approaches in animal models of cerebral ischaemia have clearly demonstrated that eNOS and vascular NO plays a prominent role in maintaining cerebral blood flow and preventing neuronal injury 5, 6, 14, 15. Vascular NO production regulates cerebrovascular perfusion and protects against stroke by increasing collateral flow to the ischaemic area. Indeed, eNOS knockout mice show decreased blood flow in the ischaemic border zone and develop larger cerebral infarctions [14]
The Janus effect of NO in the brain
In addition to eNOS, additional NOS isoforms play a role during cerebral ischaemia; similar to eNOS, nNOS is constitutively expressed and Ca2+-dependent. Macrophage, ‘immunologic’ or inducible NOS (iNOS; also called NOS2 or NOS II) is induced by selective immunological stimuli and is Ca2+-independent 5, 6. Experiments using nNOS-, eNOS- or iNOS-knockout mice have helped to clarify the respective roles of the different NOS isoforms in ischaemic injury 6, 8. Although NO generated by eNOS is
Novel modalities to upregulate or activate eNOS serve as powerful protectants from ischaemic brain injury
Only recently, several therapeutic modalities to upregulate and/or activate eNOS have been discovered. By increasing NO bioavailability they might mediate NO-dependent stroke-protective effects (Box 1).
A cautionary note: oxidative stress leads to uncoupling of eNOS
eNOS upregulation might not always be good. Similar to nNOS, eNOS can produce large amounts of reactive oxygen species (ROS) when deprived of its crucial cofactor tetrahydrobiopterin (BH4) or its substrate l-arginine 54, 55, 56. Electron flow through the eNOS enzyme is then diverted to molecular oxygen rather than to l-arginine, leading to a condition called eNOS uncoupling, which causes production of superoxide rather than NO. Activation of NADPH oxidase could initiate a vicious cycle of ROS
Dysfunctional eNOS as a stroke risk factor?
Epidemiological evidence links DNA variants in the eNOS gene (located on chromosome 7q35–36 and comprising 26 exons) to increased vascular risk in humans 58, 59. For example, the T786C eNOS polymorphism located in the 5′ flanking region of the gene encoding eNOS affects cerebral circulation in smokers [60], and is also associated with carotid artery stenosis and coronary spasm [61]. Another polymorphism located on exon 7, the G894T mutation (also referred to as Glu298Asp polymorphism) generates
Could eNOS be targeted to protect against stroke in humans?
Are strategies to increase eNOS expression and activity feasible for acute or prophylactic stroke treatment in humans? In fact, one might argue that most modalities that diminish stroke risk might do so through eNOS induction. For example, statin treatment, regular physical activity and regular moderate consumption of red wine have all been demonstrated to associate with reduced stroke risk in large cohorts 30, 67, 68. Although it is likely that some of the beneficial effects on stroke
Concluding remarks
Compelling data from animals models suggest that eNOS, in contrast to nNOS and iNOS, plays a protective role during brain ischaemia. This review has discussed recent experimental evidence suggesting that interventions that lead to eNOS upregulation or activation can protect from ischaemic stroke. Novel modalities to selectively augment endothelial NO production include statins, physical activity, steroid hormones and nutrients, all of which reduce injury in animal models of cerebral ischaemia.
Acknowledgements
We receive funding from the DFG [to M.E. (Heisenberg program) and U.L.], BMBF (CompetenceNet Stroke Project A5 and Berlin NeuroImaging Centre Project 8 to M.E.), Schilling Foundation (to M.E.) and NIH (to J.K.L. and M.A.M.).
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