Chronic oxidative stress and oxidative damage of the cerebral microvasculature and brain cells has become one of the most convincing theories in neurodegenerative pathology. Controlled oxidative metabolism and redox signalling in the central nervous system are crucial for maintaining brain function; however, excessive production of reactive oxygen species and enhanced redox signalling damage neurons. While several enzymes and metabolic processes can generate intracellular reactive oxygen species in the brain, recently an O2−‐generating enzyme, NADPH oxidase 2 (Nox2), has emerged as a major source of oxidative stress in ageing‐related vascular endothelial dysfunction and neurodegenerative diseases. The currently available inhibitors of Nox2 are not specific, and general antioxidant therapy is not effective in the clinic; therefore, insights into the mechanism of Nox2 activation and its signalling pathways are needed for the discovery of novel drug targets to prevent or treat these neurodegenerative diseases. This review summarizes the recent developments in understanding the mechanisms of Nox2 activation and redox‐sensitive signalling pathways and biomarkers involved in the pathophysiology of the most common neurodegenerative diseases, such as ageing‐related mild cognitive impairment, Alzheimer's disease and Parkinson's disease.

Oxidative stress plays an important role in aging-related neurodegeneration. This study used littermates of WT and Nox2-knockout (Nox2KO) mice plus endothelial cell–specific human Nox2 overexpression–transgenic (HuNox2Tg) mice to investigate Nox2-derived ROS in brain aging. Compared with young WT mice (3–4 months), aging WT mice (20–22 months) had obvious metabolic disorders and loss of locomotor activity. Aging WT brains had high levels of angiotensin II (Ang II) and ROS production; activation of ERK1/2, p53, and γH2AX; and losses of capillaries and neurons. However, these abnormalities were markedly reduced in aging Nox2KO brains. HuNox2Tg brains at middle age (11–12 months) already had high levels of ROS production and activation of stress signaling pathways similar to those found in aging WT brains. The mechanism of Ang II–induced endothelial Nox2 activation in capillary damage was examined using primary brain microvascular endothelial cells. The clinical significance of Nox2-derived ROS in aging-related loss of cerebral capillaries and neurons was investigated using postmortem midbrain tissues of young (25–38 years) and elderly (61–85 years) adults. In conclusion, Nox2 activation is an important mechanism in aging-related cerebral capillary rarefaction and reduced brain function, with the possibility of a key role for endothelial cells.

Oxidative stress attributable to the activation of a Nox2-containing NADPH oxidase is involved in the development of vascular diseases and in aging. However, the mechanism of Nox2 activation in normal aging remains unclear. In this study, we used age-matched wild-type (WT) and Nox2 knockout (KO) mice at 3–4 months (young); 11–12 months (middle-aged) and 21–22 months (aging) to investigate age-related metabolic disorders, Nox2 activation and endothelial dysfunction. Compared to young mice, middle-aged and aging WT mice had significant hyperglycaemia, hyperinsulinaemia, increased systemic oxidative stress and higher blood pressure. Endothelium-dependent vessel relaxation to acetylcholine was significantly impaired in WT aging aortas, and this was accompanied by increased Nox2 and ICAM-1 expressions, MAPK activation and decreased insulin receptor expression and signaling. However, these aging-associated disorders were significantly reduced or absent in Nox2KO aging mice. The effect of metabolic disorder on Nox2 activation and endothelial dysfunction was further confirmed using high-fat diet-induced obesity and insulin resistance in middle-aged WT mice treated with apocynin (a Nox2 inhibitor). In vitro experiments showed that in response to high glucose plus high insulin challenge, WT coronary microvascular endothelial cells increased significantly the levels of Nox2 expression, activation of stress signaling pathways and the cells were senescent, e.g. increased p53 and β–galactosidase activity. However, these changes were absent in Nox2KO cells. In conclusion, Nox2 activation in response to aging-associated hyperglycaemia and hyperinsulinaemia plays a key role in the oxidative damage of vascular function. Inhibition or knockout of Nox2 preserves endothelial function and improves global metabolism in old age.

Oxidative stress and oxidative damage of cerebral vasculature and neuronal cells have been found to play an important role in ageing-related neurodegenerative behaviour disorders. Although several enzymatic sources are involved in brain ROS generation, recently NADPH oxidase 2 (Nox2) has been suggested as a major source of ROS generation in ageing-related vascular dysfunction and neurodegenerative diseases. In this study, we used age-matched littermates of wild-type (WT) and Nox2 knockout (Nox2-/-) mice on a C57BL/6 background at young (3–4 month old) and old age (18–21 month old) to investigate the relationship between Nox2-derived ROS production, cerebral vascular damage and the loss of locomotor activity in ageing. There was no significant difference in locomotor activity between WT and Nox2-/- young mice as measured digitally using laser-beam motility chambers over a 42 h period. Compared to young mice, WT ageing mice (but not Nox2-/- ageing mice) had a significant decrease in horizontal activity (p < 0.05) and total activity in the dark period. We then examined the levels of ROS production in brain homogenates by lucigenin (5 µM)-chemiluminescence and in brain sections by DHE fluorescence. Compared to WT young mouse brains, there were significant increases in the levels of ROS production in WT ageing brains and this was accompanied with an average 2-fold reduction in the numbers of CD31-positive vessels (all p < 0.05) as revealed by immunofluorescence microscopy and vessel number counting. Compared to WT ageing mice, knockout of Nox2 completely inhibited the age-related increase in brain ROS production and preserved the cerebral microvasculature density and the locomotor activity. In conclusion, Nox2-derived ROS plays an important role in oxidative damage of cerebral vasculature and the loss of locomotor activity at old age.

Excess endothelial production of reactive oxygen species (ROS) has been found to play a major role in the pathogenesis of many cardiovascular diseases. Ageing is a primary risk factor of cardiovascular diseases, however the role of oxidative stress in the development of vascular dysfunction in the elderly remains unknown. In this study we used wild-type C57BL/6 J mice, at young (3–4 m) and old (20–24 m) age to investigate the potential role of ROS in age-related metabolic disorders and vascular dysfunction. The body weight, heart weight and fasting serum glucose were recorded. There was no significant difference in the heart/body weight ratio, and the fasting glucose between the two groups. However, there was a significant increase in blood pressure in ageing mice (141 mm Hg) compared to young mice (126 mm Hg) measured by volume pressure recording tail plethysmography. Vascular tone was examined using aortic rings in an organ bath. There was no significant difference in vessel relaxation to a NO donor (sodium nitroprusside) between the young and ageing mice. However, there was a significant decrease in endothelium- dependent vessel relaxation to acetylcholine in ageing mice compared to young mice. There was a significant increase in the levels of superoxide production by the ageing vessels compared to the young controls as measured using lucigenin-chemiluminescence. In conclusion, there was a significant increase in the levels of ROS production in the vasculature of ageing mice, which might contribute to the age-related endothelial dysfunction and high blood pressure.