White Matter Damage as a Consequence of Vascular Dysfunction in a Spontaneous Mouse Model of Chronic Hypoperfusion with eNOS Deficiency

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Abstract Background: Vascular cognitive impairment and dementia (VCID) is the second most common form of dementia after Alzheimer’s disease (AD). Currently, the mechanistic insights into the evolution and progression of VCID are not fully understood. White matter change represents an invariant feature of both VCID and AD. Compelling clinical neuroimaging and pathological evidence suggest a link between white matter changes and neurodegeneration. Our prior study detected non-perfusion lesions in mice with partial deficiency of endothelial nitric oxide (eNOS) expression at a very young age. These lesions developed in multiple brain regions in an age-dependent manner, precisely matching to those hypoperfused areas identified in preclinical AD patients (i.e., temporoparietal and retrosplenial granular cortexes, and hippocampus). We therefore reasoned that eNOS- deficient mice could serve as a spontaneous model of chronic hypoperfusion.Methods/Results: White matter tracts are particularly susceptible to the vascular damage induced by chronic hypoperfusion. Using immunohistochemistry, we detected massive demyelination in the middle aged eNOS-deficient mice. The demyelinated areas were confined to cortical and subcortical areas including the corpus callosum and hippocampus, but did not involve the striatum. The intensity of demyelination correlated with behavioral gait deficits. By Evans blue angiography, we detected blood-brain barrier (BBB) leakage as another early pathological change affecting frontal and parietal cortex in eNOS-deficient mice, which occurs in as early as 3-4 months of age. Sodium nitrate fortified drinking water provided to young and middle aged eNOS-deficient mice completely prevented non-perfusion, BBB leakage, and white matter pathology, indicating that impaired endothelium-derived NO signaling may have caused these pathological events. Conclusions: Using eNOS-deficient mice, we identified BBB breakdown and non-perfusion as the two earliest pathological events, resulting from insufficient vascular NO signaling. We speculate that the compromised BBB and chronic hypoperfusion trigger vascular damage, along with oxidative stress and astrogliosis, accounting for the white matter pathological changes in the eNOS-deficient mouse model. We conclude that eNOS-deficient mice represent an ideal spontaneous evolving model for studying the earliest events leading to spontaneous white matter changes, which will be instrumental to future therapeutic testing of drug candidates and for targeting novel/specific vascular mechanisms contributing to VCID and AD.

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