Pericyte, vascular and microglia changes in Alzheimer's disease and neuroinflammation

Alzheimer's disease (AD) is the most common form of dementia and is characterised by the accumulation of amyloid-beta (A˜í‚â§) and neurofibrillary tangles (NFTs) in the brain. However, vascular-related changes including reduced cerebral blood flow (CBF), neuroinflammation and blood-brain barrier (BBB) breakdown, have all been found to occur in AD, suggesting vascular dysfunction may be contributing to AD development. A possible mediator of these neurovascular changes could be pericytes, a contractile cell located on capillaries, which have shown to be important for controlling many vascular functions. However, the role of pericytes in AD has only just begun to be investigated. Therefore, the overall aim of this thesis was to determine whether changes to pericytes and the vasculature contributes to AD development, and whether pericytes contribute or respond to neuroinflammation, prevalent in AD, through interactions with microglia, the immune cell of the brain. The first aim of this thesis was to establish whether pericytes and the vasculature are lost as AD pathology severity increased, in an AD resilient brain region of human post-mortem AD tissue. Subsequently, the second aim was to determine whether pericyte and vascular changes precede or follow A˜í‚⧠plaque formation and deposition in APP/PS1 mice. Next, this thesis aimed to determine the effects of neuroinflammation on pericytes and the vasculature using the lipopolysaccharide (LPS) model of systemically induced inflammation. Lastly, this thesis aimed to investigate the association between pericytes and microglia in the healthy and inflamed brain, to provide insight into whether microglia may be a mediator of pericyte loss/dysfunction in AD. For chapter 2, post-mortem primary visual cortex brain tissue (resilient to AD development), diagnosed as moderate AD, severe AD or non-neurological controls, was obtained and stained for pericytes and the vasculature, to determine changes based on AD pathology of A˜í‚⧠plaques and NFTs. Quantification of pericytes, microglia, the vasculature and other pathological/physiological measures was conducted. For chapter 3, APP/PS1 and wildtype mouse brain tissue, at 3, 6, 9, 12 and 18-months of age was used to quantify pericytes, vasculature and insoluble levels of A˜í‚â§. For chapter 4, NG2-DsRed mice were injected with 3 or 4.5mg/kg of LPS, with brain tissue collected 4 days later to determine changes to pericytes, the vasculature and microglia in the context of neuroinflammation. For chapter 5, NG2-DsRed x CX3CR1-GFP mice underwent cranial window surgery and in vivo two-photon microscopy in the somatosensory cortex for 28 days, to detect potential associations between pericytes, the vasculature and microglia over time. Subsequently, NG2-DsRed x CX3CR1-GFP mice were administered 3mg/kg of LPS and imaged in vivo at 24 hours, to detect real-time changes in pericytes, vasculature and microglia in response to microglia-induced inflammation. Post-mortem AD brains showed no overt pericyte or vascular changes with increasing AD severity, with pericyte and vascular density in both grey and white matter of the primary visual cortex remaining constant across control, moderate AD, and severe AD cases. In APP/PS1 mice, no pericyte loss or vascular regression was present across any brain regions assessed as A˜í‚⧠accumulated across the lifespan and compared to wildtype mice. However, pericyte and vascular density was highest prior to the development of A˜í‚⧠plaques, at 3-months of age, with pericyte and vascular density returning to wildtype levels at 6-months of age, when A˜í‚⧠plaques were present. The LPS model produced systemic neuroinflammation in NG2-DsRed mice, shown through increased reactivity of microglia 4 days post-LPS. However, no changes in pericytes or the vasculature were found. In healthy NG2-DsRed x CX3CR1-GFP mice a subpopulation of microglia, termed pericyte-associated microglia (PEM), were found associating with pericytes (within 10˜í¬¿m). These associations were dynamic over a 28-day period. Administration of LPS to NG2-DsRed x CX3CR1-GFP mice showed rapid migration of microglia to pericytes within 24 hours, with an increase in the number of PEM post-LPS. The findings from this thesis suggest that pericyte and vascular loss may not be a pathological feature of AD, at least not in AD resilient regions, as AD pathology severity increased. However, results from the APP/PS1 model of amyloidosis suggest that hypervascularisation, potentially mediated by pericytes and prior to A˜í‚⧠plaque deposition, may be an early response occurring in AD, potentially to enhance the clearance of soluble A˜í‚â§. These findings contribute to the growing body of knowledge of AD, by highlighting that although pericyte and vascular loss may not be a consistent pathology of AD, early alterations to pericytes and the underlying vasculature may prevent or delay the development of AD. Results in this thesis also suggest that microglia-induced neuroinflammation, following LPS administration, does not cause pericyte and/or vascular loss in the brain, indicating neuroinflammation may not be a mediator of pericyte and vascular loss in AD. This thesis also highlighted a novel pericyte-microglia association, with the number of these associations increased following LPS administration. These pericyte-microglia associations may provide a link between the brain‚ÄövÑv¥s immune system and the cerebrovasculature, which may potentially be altered in AD. Thus, this thesis provides a potential pericyte-mediated mechanism that may be preventing the development of AD pathology, and a possible cellular link between the brain‚ÄövÑv¥s immune system and the cerebrovasculature, which could give rise to novel therapeutic targets for AD.