Investigation of inflammatory and oxidative stress mechanisms in the disruption of white matter structure and function following chronic cerebral hypoperfusion

Vascular cognitive impairment (VCI) describes a heterogeneous condition caused by cerebrovascular disease and disturbances in cerebral blood flow delivery. It is the second leading form of dementia and vascular factors such as hypertension, diabetes and obesity are associated with an increased risk of developing VCI. White matter alterations are a prominent pathological feature observed in patients with VCI thought to underlie cognitive impairment. Neuroimaging studies show a positive correlation between the burden of white matter alterations and progressive cognitive impairment. Similarly associated both with white matter alterations and cognitive impairment is chronic cerebral hypoperfusion, sustained subtle reductions in cerebral blood flow. Cerebral hypoperfusion is observed before the onset of cognitive decline in humans and reducing cerebral blood flow in animal models replicates important aspects of VCI, suggesting hypoperfusion is an early driver of white matter disruption and VCI. Human neuropathology and preclinical animal models of chronic cerebral hypoperfusion studies have repeatedly identified increased inflammation and oxidative stress. This led to the hypothesis for this thesis; that inflammation and oxidative stress are key drivers of structural and functional white matter disruption when cerebral blood flow is reduced. The studies reported in this thesis were developed to investigate mechanisms involving inflammation and oxidative stress that can inform future treatments aimed at preventing the disruption of white matter and cognitive impairment in VCI. One such mechanism is the Nrf2 (Nuclear factor erythroid 2-related factor 2) signalling pathway. Nrf2 is a transcription factor that acts to detect and resolve inflammation and oxidative stress via induction of over 200 antioxidant and anti-inflammatory genes. Studies have shown that modulation of Nrf2 alters levels of inflammation and oxidative stress which impact on disease progression in models of Alzheimer’s disease, Parkinson’s disease and multiple sclerosis. To date, no one has investigated the direct role of Nrf2 in cerebral hypoperfusion-induced white matter disruption. While Nrf2 represents a promising network approach, another targeted mechanism of interest is microglial proliferation. Many neurodegenerative diseases including human VCI demonstrate increases in microglia, a sign of chronic neuroinflammation thought to be detrimental to cells, tissues and synapses. Work by our group has found an association between increasing numbers of microglia and the progressive disruption of white matter structure and function when cerebral blood flow is reduced in a mouse model, however, whether this is cause or consequence has yet to be determined. The first study of this thesis aimed to test the hypothesis that deficiency of Nrf2 exacerbates white matter pathology and cognitive decline when cerebral blood flow is reduced. Using wild type and Nrf2 knockout mice the study investigated cortical perfusion, white matter disruption and gliosis, cognitive impairment and white matter gene changes following sham or surgically-induced cerebral hypoperfusion (bilateral carotid artery stenosis). There were no differences in the severity of blood flow reductions between genotypes initially, however, wild type mice displayed improved recovery compared to Nrf2 deficient mice. Hypoperfusion induced white matter disruption and microgliosis in the corpus callosum and the optic tract in both genotypes, exacerbated by the absence of Nrf2. Further, hypoperfusion induced white matter astrogliosis and upregulated pro-inflammatory gene signalling in the optic tract and induced an impairment in spatial working memory. However, these measures were not affected by Nrf2 deficiency. The results demonstrate that the absence of Nrf2 exacerbates white matter pathology and microgliosis following cerebral hypoperfusion but does not impact on functional outcome. The second study aimed to test the hypothesis that enhancing astrocytic Nrf2- signalling preserves white matter structure and cognitive decline when cerebral blood flow is reduced. Astrocytes have larger antioxidant capacity than other cell types in the brain and overexpressing Nrf2 in astrocytes is associated with reduced white matter damage in a model of multiple sclerosis, as well as improved outcome in models of Parkinson’s and Huntington’s disease. Similar to the first study, wild type mice and mice overexpressing Nrf2 in astrocytes (GFAP-Nrf2) were subjected to bilateral carotid artery stenosis and cortical perfusion, white matter disruption and gliosis, cognitive impairment and white matter gene changes were assessed. There were no differences in the severity of blood flow reductions between genotypes. Akin to the first study, hypoperfusion induced white matter disruption, micro- and astrogliosis and pro-inflammatory gene signalling in the optic tract. The majority of these alterations were ameliorated in GFAP-Nrf2 mice. In addition, the impairment in spatial working memory induced by cerebral hypoperfusion was modestly improved in GFAP-Nrf2 mice compared to wild type controls. These findings support the hypothesis that astrocytic Nrf2 preserves white matter structure and function following cerebral hypoperfusion. The first two studies identified structural and functional consequences of altered inflammation mediated via alterations in Nrf2 signalling. To thoroughly investigate the Nrf2 signalling pathway following cerebral hypoperfusion the next step would ideally have been to study microglial Nrf2, however due to the lack of a suitable animal model, the third and final study instead aimed to test the hypothesis that microglial colony-stimulating factor 1 receptor (CSF1R) signalling is a driver of white matter disruption and cognitive decline when cerebral blood flow is reduced. Wild type mice treated with a pharmacological inhibitor of CSF1R (GW2580) or vehicle control, as an oral gavage or in diet, were studied by a similar experimental protocol as the first two studies. There were no differences in the severity of cerebral hypoperfusion between GW2580- or vehicle-treated animals either at one or six weeks following bilateral carotid artery stenosis. One week of GW2580 treatment was shown to modulate microglial proliferation and pro-inflammatory signalling in white matter. Remarkably, treatment with GW2580 for six weeks completely rescued impairments in spatial learning, protected against white matter disruption and prevented increased both white matter micro- and astrogliosis compared to wild type controls. These results suggest that CSF1R signalling in microglia is an important driver of the pathophysiological mechanisms that lead to white matter disruption and cognitive impairment when cerebral blood flow is reduced, and importantly, that targeted inhibition of this improves functional outcome. In conclusion, the work described in this thesis provides evidence of the contribution of inflammation and oxidative stress to the disruption and functional impairment of cerebral white matter. The results indicate that these mechanisms are amenable to alteration, and that direct microglial inflammatory mechanisms play an important role in the pathogenesis of white matter disruption and cognitive decline. The results demonstrate that targeted inhibition of CSF1R signalling in microglia and increased astrocytic Nrf2 expression leads to improved structural and functional outcome and as such represent a basis for potential treatment which warrants further investigation