Deficiency of Nrf2 exacerbates white matter damage and microglia/macrophage levels in a mouse model of vascular cognitive impairment

Emma Sigfridsson,Jill H Fowler,Giles E Hardingham,Karen Horsburgh,Martina Marangoni

doi:10.1186/s12974-020-02038-2

Abstract

BackgroundChronic cerebral hypoperfusion causes damage to the brain’s white matter underpinning vascular cognitive impairment. Inflammation and oxidative stress have been proposed as key pathophysiological mechanisms of which the transcription factor Nrf2 is a master regulator. We hypothesised that white matter pathology, microgliosis, blood-brain barrier breakdown and behavioural deficits induced by chronic hypoperfusion would be exacerbated in mice deficient in the transcription factor Nrf2.MethodsMice deficient in Nrf2 (male heterozygote or homozygous for Nrf2 knockout) or wild-type littermates on a C57Bl6/J background underwent bilateral carotid artery stenosis (BCAS) to induce chronic cerebral hypoperfusion or sham surgery and survived for a further 6 weeks. White matter pathology was assessed with MAG immunohistochemistry as a marker of altered axon-glial integrity; alterations to astrocytes and microglia/macrophages were assessed with GFAP and Iba1 immunohistochemistry, and blood-brain barrier breakdown was assessed with IgG immunohistochemistry. Behavioural alterations were assessed using 8-arm radial arm maze, and alterations to Nrf2-related and inflammatory-related genes were assessed with qRT-PCR.ResultsChronic cerebral hypoperfusion induced white matter pathology, elevated microglial/macrophage levels and blood-brain barrier breakdown in white matter tracts that were increased in Nrf2+/− mice and further exacerbated by the complete absence of Nrf2. Chronic hypoperfusion induced white matter astrogliosis and induced an impairment in behaviour assessed with radial arm maze; however, these measures were not affected by Nrf2 deficiency. Although Nrf2-related antioxidant gene expression was not altered by chronic cerebral hypoperfusion, there was evidence for elevated pro-inflammatory related gene expression following chronic hypoperfusion that was not affected by Nrf2 deficiency.ConclusionsThe results demonstrate that the absence of Nrf2 exacerbates white matter pathology and microgliosis following cerebral hypoperfusion but does not affect behavioural impairment.

Highlights

Chronic cerebral hypoperfusion causes damage to the brain’s white matter underpinning vascular cognitive impairment
Deficient mice compared to wild-type mice post-bilateral carotid artery stenosis (BCAS) We previously demonstrated alterations in the intensity of myelin-associated glycoprotein (MAG) immunostaining in response to chronic cerebral hypoperfusion that is indicative of altered axon-glial integrity [11] and associated with alterations in white matter integrity assessed with diffusion tensor imaging [10]
These data show that cerebral hypoperfusion induced by BCAS surgery causes white matter disruption in two major white matter tracts and that this disruption is exacerbated in the absence of nuclear factor erythroid 2related factor (Nrf2)

Summary

Introduction

Chronic cerebral hypoperfusion causes damage to the brain’s white matter underpinning vascular cognitive impairment. We hypothesised that white matter pathology, microgliosis, blood-brain barrier breakdown and behavioural deficits induced by chronic hypoperfusion would be exacerbated in mice deficient in the transcription factor Nrf. Sustained cerebral hypoperfusion is one of the pathophysiological mechanisms contributing to cognitive decline in ageing, vascular cognitive impairment (VCI) and Alzheimer’s disease (AD) by causing damage to the brain’s white matter [1,2,3]. We demonstrated that bilateral carotid artery stenosis (BCAS) in mice causes chronic cerebral hypoperfusion [9], white matter alterations detected using diffusion-tensor MRI [10] and impaired axon-glial integrity [11, 12]. We showed that the white matter disruption caused by BCAS induces a selective deficit in spatial working memory [11], reminiscent of the disrupted frontal cortical circuitry found in VCI, that progresses to encompass spatial reference memory deficits in the longer term response [13]

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