The contribution of β-amyloid, Tau and α-synuclein to blood–brain barrier damage in neurodegenerative disorders

Since in the brain of patients affected by NDDs Aβ, Tau and α-syn are continuously generated, failure to remove them results in chronic neuroinflammation that, in addition to contributing to neuronal damage, further increases BBB permeability and vascular dysfunction [54, 102, 148]. Amyloidogenic proteins can initiate a sterile immune response by acting as DAMPs that activate pattern recognition receptors, such as toll-like receptors (TLRs), on cerebral myeloid cells [54, 148].

Soluble parenchymal Aβ (and other amyloids) can drain into the perivascular space [63], where it encounters PvMΦ. Macrophages, as the resident immune cells within the vasculature, are professional phagocytes and can efficiently engulf amyloid aggregates. High amyloid load and cellular stress can impair phagocytosis function, leading to macrophage cell death and exacerbating CAA pathology [143]. Moreover, a recent study has shown that PvMΦ regulates the flow rate of CSF, which decreases with aging, further reducing the rate of amyloid clearance [35].

The degree of the pro-inflammatory factor release depends on the receptors involved. Receptors like scavenger receptor (SR)-A, SR-BI and triggering receptor expressed on myeloid cells 2 promote the uptake of extracellular amyloid deposits without eliciting a significant inflammatory response, whereas TLR2/4 and CD36 stimulation induces the production of neurotoxic cytokines and ROS via nuclear factor κB (NFκB) and NLRP3-ASC-inflammasome pathways, thus promoting neuronal and vascular degeneration [54, 64, 131]. Reduction in SR-A and SR-BI impairs PvMΦ function and enhances CAA [83, 131], while CD36 deficiency in PvMΦ is protective in transgenic mouse models [135].

In addition, post-mortem and experimental studies have shown that neurons with Tau pathology are surrounded by reactive microglia/macrophages [8, 9], suggesting that intraneuronal pathological Tau also activates brain macrophages. Indeed, neurons with Tau filaments expose abnormally high levels of phosphatidylserine triggering opsonin milk-fat-globule EGF-factor-8-mediated phagocytosis by microglia, that in turn become hypofunctional [15, 16]. Phagocytosed Tau has also been reported to trigger inflammatory activation via polyglutamine binding protein 1 cyclic GMP-AMP synthase-Stimulator of interferon genes-NFκB pathway [66].

Other evidence supports that the NLRP3 inflammasome pathway can be induced by fibrillary α-syn or by α-syn-pathology-dependent dopaminergic failure, which results in the reduction of microglia dopamine D1 and D2 receptor stimulation and consequent microglia activation [105, 126].

By compromising BBB permeability and increasing the expression and binding affinity of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 on ECs, pathological protein-related neuroinflammatory cascades also contribute to the recruitment, activation and extravasation into the parenchyma of peripheral immune cells, including neutrophils and T cells, into the brain [104, 144, 145]. In turn, the accumulation of peripheral leukocytes in the blood vessels and perivascular space can decrease blood circulation and increase BBB leakage, thus further contributing to brain damage [10, 24].

In support of the key role of neuroinflammation in T cell recruitment there is evidence indicating that the brain areas exhibiting marked neuroinflammation and microglial activation in post-mortem PD brains frequently show the presence of leukocytes next to MHCII-positive astrocytes in proximity to blood vessels [61, 111]. Another study described that CAMs are localized in close proximity to T cells in post-mortem PD brains and showed that these cells act as the main antigen-presenting cells necessary to initiate a CD4 T cell recruitment and neuroinflammation in response to α-syn accumulation [118]. Similar findings have been described in post-mortem brains of AD and LB dementia patients, where significant T lymphocyte recruitment to both grey and white matter has been reported [3, 134].

Collectively, these observations support that Aβ, Tau and α-syn pathology act as main DAMPs for brain-macrophage activation-related BBB disruption.

The exact origin of Aβ depositions in the vasculature is not fully understood yet, but evidence suggests that neurons and astrocytes are the main sources, with subsequent spreading or movement towards the vasculature for clearance [39]. However, recent findings also support that ECs may contribute to the production of Aβ and CAA [128].

Aβ exerts both direct or indirect effects on ECs within brain microvasculature by altering the distribution of TJPs, promoting ECs death, elevating oxidative stress and inducing proinflammatory cytokine production in glial cells [130, 145]. Aβ accumulation adversely affects brain vessel walls, leading to BBB damage and potential hemorrhage in CAA mouse models and human patients [42, 55]. In vitro studies have shown that Aβ exposure disrupts actin organization and induces apoptosis in ECs [129]. Furthermore, CAA patients and amyloid precursor protein (APP)-overexpressing mice exhibit decreased TJPs expression and increased levels of matrix metalloproteinases (MMPs) [55]. The presence of Aβ also hampers endothelial nitric oxide synthase/heat shock protein 90 interaction [80] and promotes the formation of von Willebrand factor (VWF) fibers, which contribute to inflammatory and thrombogenic responses in brain vessels [121]. These findings highlight the detrimental effects of Aβ deposition on vessel walls, including ECs impairment, compromised BBB and alterations in inflammatory and thrombogenic responses. All these mechanisms can contribute to the impairment of neurovascular control.

The accumulation of Tau oligomers in cerebral microvessels has been reported in human AD, LB dementia and progressive supranuclear palsy patients [22]. Microbleeds have been detected in the brains of patients affected by frontotemporal dementia [28] and cerebrovascular inflammation has been associated with Tau pathology in Guam parkinsonism dementia and chronic bilirubin encephalopathy. In these cases, areas with significant accumulation of neurofibrillary tangles exhibited upregulation of adhesion molecules, disruption of TJPs, morphological alterations in brain microvessels, including thickening of the vessel wall, vessel lumen reduction as well as increased in collagen-type IV content per vessel [84]. Still, Tau pathology is associated with small vessel disease [70] and immune cells trafficking across the BBB also appears to be modulated by neurofibrillary pathology in Tauopathies [85]. In P301S transgenic mice, brain microvascular ECs uptake soluble pathogenic Tau oligomers from the extracellular space, which over time accumulate within ECs further leading to mitochondrial damage and ECs senescence [60]. Aged Tau-overexpressing mice were found to develop cortical blood vessel changes such as abnormal spiralling morphologies and reduced diameters in parallel to increased vessel density [10]. These changes were accompanied by alteration in cortical CBF and increased expression of angiogenesis-related genes such as Vegfa, Serpine1, and Plau in CD31-positive ECs [10], hinting that Tau pathological changes in neurons can impact brain ECs biology, thus altering the integrity of brain microvasculature. This observation is corroborated by the fact that other lines of Tau transgenic mice exhibit progressive BBB leakage with IgG, T cell, and red blood cell infiltration that is prevented by Tau depletion [11].

Numerous studies support that α-syn can differentially impact on ECs homeostasis as previously reviewed [13]. For instance, transgenic expression of the human A53T mutated form of the protein in mouse brains decreases the expression of TJPs resulting in increased vascular permeability and also leads to the accumulation of oligomeric α-syn in activated astrocytes that release vascular endothelial growth factor (VEGF) A and nitric oxide [81], key regulators of ECs homeostasis. Moreover, toxic forms of α-syn can modulate ECs function by limiting the release of inflammatory cytokines and adhesion molecules from their secretory granules or downregulating the expression of TJPs [72, 79]. This has been recently confirmed in an innovative human brain-on-a-chip model of the substantia nigra and containing dopaminergic neurons, astrocytes, microglia, pericytes, and microvascular brain ECs cultured under fluid flow and exposed to synthetic α-syn preformed fibrils [103]. By using this model, authors found that α-syn synthetic fibrils impair BBB permeability by directly interacting with ECs and altering the expression of vascular channels, TJPs and other key genes involved in BBB homeostasis, including extracellular proteases of the Serpin family and collagens.

Pericyte degeneration is associated with Aβ deposition [137], facilitating the progression of neurodegenerative pathology [114]. Consistently, a correlation between Aβ accumulation and pericyte loss in patients with NDDs and transgenic mouse models has been demonstrated [91, 114]. Interestingly, though Aβ42 is commonly considered more cytotoxic than Aβ40 due to its aggregation-prone nature, Aβ40 has been found to exert similar effects on pericytes. In patients with AD, a decreased population of NG2-positive pericytes in the hippocampus was observed, negatively correlating with Aβ40 levels. In vitro experiments revealed that exposure to Aβ40 monomers enhanced pericyte viability and proliferation while reducing caspase 3/7 activity. However, exposure to fibril-enriched Aβ40 resulted in decreased pericyte viability and proliferation, along with increased caspase 3/7 activity [119]. These findings suggest that Aβ40 may serve as a key regulator of the pericyte population in the brain, both in disease and in healthy conditions. The loss of pericytes reduces TJPs expression in ECs, compromising BBB integrity and increasing Aβ load in the brain [107].

Similarly, a study on a human-induced pluripotent stem cell-derived BBB model comprising astrocytes, ECs and pericytes suggested that pericytes play a key role in the intravascular accumulation of Aβ [12].

Moreover, Aβ has been discovered to induce pericyte-mediated constriction of brain capillaries through the upregulation of endothelin-1 release, which is mediated by the overproduction of ROS [98]. In vitro studies further support that exposure to Aβ oligomers leads to the development of a hypercontractile phenotype in pericytes [58]. Increased constriction of brain microvessels by hypercontractile pericytes likely contributes to the reduction in CBF observed in NDDs.

Other evidence indicates that following repeated head trauma in mice both types of mural cells, pericytes and VSMCs, uptake recombinant human tau more efficiently as compared to astrocytes and ECs [100]. Interestingly, other studies support that VSMCs undergo significant phenotypic changes and acquire an inflammatory phenotype under AD-like conditions, coinciding with Tau hyperphosphorylation [2]. Although evidence regarding Tau pathology in pericytes is limited, it can be hypothesised that this pathological protein may similarly impact both mural cell types.

Finally, recent studies have shed light on the significant involvement of pericytes in α-syn pathology. In vitro investigations have demonstrated that pericytes can contribute to the clearance of α-syn aggregates by internalizing and degrading them [32, 127]. However, under conditions of additional cellular stress, this process can lead to increased production of ROS in pericytes, ultimately resulting in their apoptosis [127]. Other cell culture studies showed that monomeric α-syn can lead to BBB dysfunction through activated brain pericytes releasing inflammatory mediators such as interleukin 1β, interleukin 6, tumor necrosis factor-α, monocyte chemotactic protein-1 and MMP-9 [34]. Additionally, studies utilizing primary brain pericytes obtained from individuals with PD have shown the formation of tunneling nanotubes, which are membranous channels composed of F-actin and facilitate the transfer and spread of α-syn between cells [31]. These findings highlight the complex involvement of pericytes in α-syn pathology, which appears to encompass both beneficial and detrimental effects.