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Dysfunction of the blood–brain barrier in Alzheimer's disease: Evidence from human studies

The pathological processes leading to synapse loss, neuronal loss, brain atrophy and gliosis in Alzheimer's disease (AD) and their relation to vascular disease and immunological changes are yet to be fully explored. Amyloid‐β (Aβ) aggregation, vascular damage and altered immune response interact at the blood–brain barrier (BBB), affecting the brain endothelium and fuelling neurodegeneration. The aim of the present systematic literature review was to critically appraise and to summarise the published evidence on the clinical correlations and pathophysiological concepts of BBB damage in AD, focusing on human data. The PubMed, Cochrane, Medline and Embase databases were searched for original research articles, systematic reviews and meta‐analyses, published in English language from 01/2000 to 07/2021, using the keywords Alzheimer*, amyloid‐β or β‐amyloid or abeta and BBB. This review shows that specific changes of intercellular structures, reduced expression of transendothelial carriers, induction of vasoactive mediators and activation of both astroglia and monocytes/macrophages characterise BBB damage in human AD and AD models. BBB dysfunction on magnetic resonance imaging takes place early in the disease course in AD‐specific brain regions. The toxic effects of Aβ and apolipoprotein E (ApoE) are likely to induce a non‐cerebral‐amyloid‐angiopathy‐related degeneration of endothelial cells, independently of cerebrovascular disease; however, some of the observed structural changes may just arise with age. Small vessel disease, ApoE, loss of pericytes, proinflammatory signalling and cerebral amyloid angiopathy enhance BBB damage. Novel therapeutic approaches for AD, including magnetic resonance‐guided focused ultrasound, aim to open the BBB, potentially leading to an improved drainage of Aβ along perivascular channels and increased elimination from the brain. In vitro treatments with ApoE‐modifying agents yielded promising effects on modulating BBB function. Reducing cardiovascular risk factors represents one of the most promising interventions for dementia prevention at present. However, further research is needed to elucidate the connection of BBB damage and tau pathology, the role of proinflammatory mediators in draining macromolecules and cells from the cerebral parenchyma, including their contribution to cerebral amyloid angiopathy. Improved insight into these pathomechanisms may allow to shed light on the role of Aβ deposition as a primary versus a secondary event in the complex pathogenesis of AD.