Investigation into pathogenic mechanisms leading to neuro-glial-vascular damage and cognitive decline in a mouse model of vascular cognitive impairment
Vascular cognitive impairment (VCI) refers to the contribution of cerebrovascular disease to a spectrum of cognitive impairments, ranging from subjective cognitive decline to dementia. Compromised cerebral blood flow (CBF) has been heavily implicated in the pathogenesis of cerebrovascular disease, however, the key underlying mechanisms remain to be fully elucidated. Bilateral common carotid artery stenosis (BCAS) is a surgical method in which micro-coils are applied permanently to both carotid arteries to reduce CBF. The BCAS mouse model recapitulates many of the pathological and functional hallmarks of VCI, making it a valuable experimental model. A prominent feature of the BCAS model is a robust increase in white matter microglial numbers, which are significantly associated with cognitive impairments. The first aim of this thesis was to test the hypothesis that microglial proliferation directly leads to white matter damage and cognitive impairment following BCAS. BCAS surgery was found to elicit a significant and persistent reduction in CBF, alongside increased microglial proliferation. Pharmacological inhibition of microglial proliferation, through GW2580 treatment, prevented microglial proliferation, reduced microglial lysosomal expression, preserved white matter integrity, and restored spatial learning deficits. The second aim was to investigate, using the Cx3Cr1 eGFP microglial reporter line and intravital multiphoton imaging, structural and functional changes within microglial cells following BCAS. The additional pathogenic effects of amyloidosis as a co-morbidity using the transgenic App23 mouse model were also assessed. Microglial structure and process motility were found to be unaltered, at 1-week following BCAS, within both Cx3Cr1 eGFP/+ and Cx3Cr1 eGFP/+App23 mice. Following 3-months of BCAS, microglial density was found to be unaltered, alongside intact neurovascular coupling responses and spatial learning, although, spatial memory was impaired within Cx3Cr1 eGFP/+ mice. Microglial density was also found to be unchanged within Cx3Cr1 eGFP/+App23 mice following 3-months of BCAS. Neurovascular coupling, however, was significantly impaired within Cx3Cr1 eGFP/+App23 mice following BCAS surgery. Spatial learning and memory deficits were found within Cx3Cr1 eGFP/+App23 mice alone, with no additional BCAS mediated deficit. As a means of explaining the lack of microglial response within the Cx3Cr1 eGFP/+ mice, qPCR analysis was carried out and confirmed a ≈5-fold reduction in Cx3Cr1 receptor expression. Considerable evidence has implicated cerebrovascular dysfunction as a pivotal mechanism in the pathophysiology of VCI and dementia. The studies in chapter 3 aimed to test the hypothesis that BCAS causes vascular dysfunction leading to the onset of neuro-glial-vascular damage. Multiphoton imaging of C57BL/6J wild-type mice found significantly reduced RBC velocity alongside impaired arterial pulsation, and increased leukocyte trafficking, 1-month following BCAS surgery. In conclusion, the work described within this thesis demonstrates that microglial proliferation plays a causative role in white matter damage and cognitive decline following BCAS, and that this can be successfully targeted to reverse pathological damage and functional deficits. Furthermore, Cx3Cr1 receptor signalling was found to play a significant role in the regulation of microglial responses following BCAS. Finally, BCAS was found to not simply be a model of reduced CBF, with impairments in arterial pulsation and increased endothelial activation providing a new framework to contextualise BCAS mediated effects in future studies.