Vascular amyloidosis impairs the gliovascular unit in a mouse model of Alzheimer's disease

Vascular Function and Ion Channels in Alzheimer's Disease

This review paper explores the critical role of vascular ion channels in the regulation of cerebral artery function and examines the impact of Alzheimer's disease (AD) on these processes. Vascular ion channels are fundamental in controlling vascular tone, blood flow, and endothelial function in cerebral arteries. Dysfunction of these channels can lead to impaired cerebral autoregulation, contributing to cerebrovascular pathologies. AD, characterized by the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles, has been increasingly linked to vascular abnormalities, including altered vascular ion channel activity. Here, we briefly review the role of vascular ion channels in cerebral blood flow control and neurovascular coupling. We then examine the vascular defects in AD, the current understanding of how AD pathology affects vascular ion channel function, and how these changes may lead to compromised cerebral blood flow and neurodegenerative processes. Finally, we provide future perspectives and conclusions. Understanding this topic is important as ion channels may be potential therapeutic targets for improving cerebrovascular health and mitigating AD progression.

Challenges and Future Perspectives in Modeling Neurodegenerative Diseases Using Organ-on-a-Chip Technology

Neurodegenerative diseases (NDDs) affect more than 50 million people worldwide, posing a significant global health challenge as well as a high socioeconomic burden. With aging constituting one of the main risk factors for some NDDs such as Alzheimer's disease (AD) and Parkinson's disease (PD), this societal toll is expected to rise considering the predicted increase in the aging population as well as the limited progress in the development of effective therapeutics. To address the high failure rates in clinical trials, legislative changes permitting the use of alternatives to traditional pre-clinical in vivo models are implemented. In this regard, microphysiological systems (MPS) such as organ-on-a-chip (OoC) platforms constitute a promising tool, due to their ability to mimic complex and human-specific tissue niches in vitro. This review summarizes the current progress in modeling NDDs using OoC technology and discusses five critical aspects still insufficiently addressed in OoC models to date. Taking these aspects into consideration in the future MPS will advance the modeling of NDDs in vitro and increase their translational value in the clinical setting.

Loss of Direct Vascular Contact to Astrocytes in the Hippocampus as an Initial Event in Alzheimer's Disease. Evidence from Patients, In Vivo and In Vitro Experimental Models

Alzheimer's disease (AD) is characterized by the accumulation of aggregated amyloid peptides in the brain parenchyma and within the walls of cerebral vessels. The hippocampus-a complex brain structure with a pivotal role in learning and memory-is implicated in this disease. However, there is limited data on vascular changes during AD pathological degeneration in this susceptible structure, which has distinctive vascular traits. Our aim was to evaluate vascular alterations in the hippocampus of AD patients and PDAPP-J20 mice-a model of AD-and to determine the impact of Aβ40 and Aβ42 on endothelial cell activation. We found a loss of physical astrocyte-endothelium interaction in the hippocampus of individuals with AD as compared to non-AD donors, along with reduced vascular density. Astrocyte-endothelial interactions and levels of the tight junction protein occludin were altered early in PDAPP-J20 mice, preceding any signs of morphological changes or disruption of the blood-brain barrier in these mice. At later stages, PDAPP-J20 mice exhibited decreased vascular density in the hippocampus and leakage of fluorescent tracers, indicating dysfunction of the vasculature and the BBB. In vitro studies showed that soluble Aβ40 exposure in human brain microvascular endothelial cells (HBMEC) was sufficient to induce NFκB translocation to the nucleus, which may be linked with an observed reduction in occludin levels. The inhibition of the membrane receptor for advanced glycation end products (RAGE) prevented these changes in HBMEC. Additional results suggest that Aβ42 indirectly affects the endothelium by inducing astrocytic factors. Furthermore, our results from human and mouse brain samples provide evidence for the crucial involvement of the hippocampal vasculature in Alzheimer's disease.

Alzheimer's disease pathophysiology in the Retina

The retina is an emerging CNS target for potential noninvasive diagnosis and tracking of Alzheimer's disease (AD). Studies have identified the pathological hallmarks of AD, including amyloid β-protein (Aβ) deposits and abnormal tau protein isoforms, in the retinas of AD patients and animal models. Moreover, structural and functional vascular abnormalities such as reduced blood flow, vascular Aβ deposition, and blood-retinal barrier damage, along with inflammation and neurodegeneration, have been described in retinas of patients with mild cognitive impairment and AD dementia. Histological, biochemical, and clinical studies have demonstrated that the nature and severity of AD pathologies in the retina and brain correspond. Proteomics analysis revealed a similar pattern of dysregulated proteins and biological pathways in the retina and brain of AD patients, with enhanced inflammatory and neurodegenerative processes, impaired oxidative-phosphorylation, and mitochondrial dysfunction. Notably, investigational imaging technologies can now detect AD-specific amyloid deposits, as well as vasculopathy and neurodegeneration in the retina of living AD patients, suggesting alterations at different disease stages and links to brain pathology. Current and exploratory ophthalmic imaging modalities, such as optical coherence tomography (OCT), OCT-angiography, confocal scanning laser ophthalmoscopy, and hyperspectral imaging, may offer promise in the clinical assessment of AD. However, further research is needed to deepen our understanding of AD's impact on the retina and its progression. To advance this field, future studies require replication in larger and diverse cohorts with confirmed AD biomarkers and standardized retinal imaging techniques. This will validate potential retinal biomarkers for AD, aiding in early screening and monitoring.

Arteriolar degeneration and stiffness in cerebral amyloid angiopathy are linked to β-amyloid deposition and lysyl oxidase

Cerebral amyloid angiopathy (CAA) is a vasculopathy characterized by vascular β-amyloid (Aβ) deposition on cerebral blood vessels. CAA is closely linked to Alzheimer's disease (AD) and intracerebral hemorrhage. CAA is associated with the loss of autoregulation in the brain, vascular rupture, and cognitive decline. To assess morphological and molecular changes associated with the degeneration of penetrating arterioles in CAA, we analyzed post-mortem human brain tissue from 26 patients with mild, moderate, and severe CAA end neurological controls. The tissue was optically cleared for three-dimensional light sheet microscopy, and morphological features were quantified using surface volume rendering. We stained Aβ, vascular smooth muscle (VSM), lysyl oxidase (LOX), and vascular markers to visualize the relationship between degenerative morphological features, including vascular dilation, dolichoectasia (variability in lumenal diameter) and tortuosity, and the volumes of VSM, Aβ, and LOX in arterioles. Atomic force microscopy (AFM) was used to assess arteriolar wall stiffness, and we identified a pattern of morphological features associated with degenerating arterioles in the cortex. The volume of VSM associated with the arteriole was reduced by around 80% in arterioles with severe CAA and around 60% in cases with mild/moderate CAA. This loss of VSM correlated with increased arteriolar diameter and variability of diameter, suggesting VSM loss contributes to arteriolar laxity. These vascular morphological features correlated strongly with Aβ deposits. At sites of microhemorrhage, Aβ was consistently present, although the morphology of the deposits changed from the typical organized ring shape to sharply contoured shards with marked dilation of the vessel. AFM showed that arteriolar walls with CAA were more than 400% stiffer than those without CAA. Finally, we characterized the association of vascular degeneration with LOX, finding strong associations with VSM loss and vascular degeneration. These results show an association between vascular Aβ deposition, microvascular degeneration, and increased vascular stiffness, likely due to the combined effects of replacement of VSM by β-amyloid, cross-linking of extracellular matrices (ECM) by LOX, and possibly fibrosis. This advanced microscopic imaging study clarifies the association between Aβ deposition and vascular fragility. Restoration of physiologic ECM properties in penetrating arteries may yield a novel therapeutic strategy for CAA.

Multidimensional autophagy nano-regulator boosts Alzheimer's disease treatment by improving both extra/intraneuronal homeostasis

Intraneuronal dysproteostasis and extraneuronal microenvironmental abnormalities in Alzheimer's disease (AD) collectively culminate in neuronal deterioration. In the context of AD, autophagy dysfunction, a multi-link obstacle involving autophagy downregulation and lysosome defects in neurons/microglia is highly implicated in intra/extraneuronal pathological processes. Therefore, multidimensional autophagy regulation strategies co-manipulating "autophagy induction" and "lysosome degradation" in dual targets (neuron and microglia) are more reliable for AD treatment. Accordingly, we designed an RP-1 peptide-modified reactive oxygen species (ROS)-responsive micelles (RT-NM) loading rapamycin or gypenoside XVII. Guided by RP-1 peptide, the ligand of receptor for advanced glycation end products (RAGE), RT-NM efficiently targeted neurons and microglia in AD-affected region. This nano-combination therapy activated the whole autophagy-lysosome pathway by autophagy induction (rapamycin) and lysosome improvement (gypenoside XVII), thus enhancing autophagic degradation of neurotoxic aggregates and inflammasomes, and promoting Aβ phagocytosis. Resultantly, it decreased aberrant protein burden, alleviated neuroinflammation, and eventually ameliorated memory defects in 3 × Tg-AD transgenic mice. Our research developed a multidimensional autophagy nano-regulator to boost the efficacy of autophagy-centered AD therapy.

Cerebrovascular Reserve Imaging: Problems and Solutions

The current standard of practice for assessing patients with cerebrovascular steno-occlusive disease is based on measuring resting blood flow metrics using MR imaging and CT perfusion imaging. However, the reliability of these methods decreases as the degree and number of stenoses increase. The reason for this is that measures of adequate baseline blood flow in highly collateralized circulations do not account for possible shortfalls in recruitable blood flow or increased metabolic demand. The following offers a clinically tested solution for this purpose using cerebrovascular reactivity methodology that applies a quantifiable vasodilatory stimulus improving reproducibility and repeatability essential for optimizing patient management.

Effect and Mechanism of Rapamycin on Cognitive Deficits in Animal Models of Alzheimer's Disease: A Systematic Review and Meta-analysis of Preclinical Studies

Background

Alzheimer's disease (AD), the most common form of dementia, remains long-term and challenging to diagnose. Furthermore, there is currently no medication to completely cure AD patients. Rapamycin has been clinically demonstrated to postpone the aging process in mice and improve learning and memory abilities in animal models of AD. Therefore, rapamycin has the potential to be significant in the discovery and development of drugs for AD patients.

Objective

The main objective of this systematic review and meta-analysis was to investigate the effects and mechanisms of rapamycin on animal models of AD by examining behavioral indicators and pathological features.

Methods

Six databases were searched and 4,277 articles were retrieved. In conclusion, 13 studies were included according to predefined criteria. Three authors independently judged the selected literature and methodological quality. Use of subgroup analyses to explore potential mechanistic effects of rapamycin interventions: animal models of AD, specific types of transgenic animal models, dosage, and periodicity of administration.

Results

The results of Morris Water Maze (MWM) behavioral test showed that escape latency was shortened by 15.60 seconds with rapamycin therapy, indicating that learning ability was enhanced in AD mice; and the number of traversed platforms was increased by 1.53 times, indicating that the improved memory ability significantly corrected the memory deficits.

CONCLUSIONS

Rapamycin therapy reduced age-related plaque deposition by decreasing AβPP production and down-regulating β-secretase and γ-secretase activities, furthermore increased amyloid-β clearance by promoting autophagy, as well as reduced tau hyperphosphorylation by up-regulating insulin-degrading enzyme levels.

Reactive astrocytes associated with prion disease impair the blood brain barrier

BACKGROUND

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

RESULTS

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

CONCLUSIONS

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Astrocyte Endfeet in Brain Function and Pathology: Open Questions

Astrocyte endfeet enwrap the entire vascular tree within the central nervous system, where they perform important functions in regulating the blood-brain barrier (BBB), cerebral blood flow, nutrient uptake, and waste clearance. Accordingly, astrocyte endfeet contain specialized organelles and proteins, including local protein translation machinery and highly organized scaffold proteins, which anchor channels, transporters, receptors, and enzymes critical for astrocyte-vascular interactions. Many neurological diseases are characterized by the loss of polarization of specific endfoot proteins, vascular dysregulation, BBB disruption, altered waste clearance, or, in extreme cases, loss of endfoot coverage. A role for astrocyte endfeet has been demonstrated or postulated in many of these conditions. This review provides an overview of the development, composition, function, and pathological changes of astrocyte endfeet and highlights the gaps in our knowledge that future research should address.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ_1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

Preserved blood-brain barrier and neurovascular coupling in female 5xFAD model of Alzheimer's disease

Introduction

Dysfunction of the cerebral vasculature is considered one of the key components of Alzheimer's disease (AD), but the mechanisms affecting individual brain vessels are poorly understood.

Methods

Here, using in vivo two-photon microscopy in superficial cortical layers and ex vivo imaging across brain regions, we characterized blood-brain barrier (BBB) function and neurovascular coupling (NVC) at the level of individual brain vessels in adult female 5xFAD mice, an aggressive amyloid-β (Aβ) model of AD.

Results

We report a lack of abnormal increase in adsorptive-mediated transcytosis of albumin and preserved paracellular barrier for fibrinogen and small molecules despite an extensive load of Aβ. Likewise, the NVC responses to somatosensory stimulation were preserved at all regulatory segments of the microvasculature: penetrating arterioles, precapillary sphincters, and capillaries. Lastly, the Aβ plaques did not affect the density of capillary pericytes.

Conclusion

Our findings provide direct evidence of preserved microvascular function in the 5xFAD mice and highlight the critical dependence of the experimental outcomes on the choice of preclinical models of AD. We propose that the presence of parenchymal Aβ does not warrant BBB and NVC dysfunction and that the generalized view that microvascular impairment is inherent to Aβ aggregation may need to be revised.

Two decades of astrocytes in neurovascular coupling

The brain is a highly energy demanding organ, which accounts in humans for the 20% of total energy consumption at resting state although comprising only 2% of the body mass. The necessary delivery of nutrients to brain parenchyma is ensured by the cerebral circulatory system, through the exchange of glucose and oxygen (O2) at the capillary level. Notably, a tight spatial and temporal correlation exists between local increases in neuronal activity and the subsequent changes in regional cerebral blood flow. The recognized concept of neurovascular coupling (NVC), also named functional hyperemia, expresses this close relationship and stands at the basis of the modern functional brain imaging techniques. Different cellular and molecular mechanisms have been proposed to mediate this tight coupling. In this context, astrocytes are ideally positioned to act as relay elements that sense neuronal activity through their perisynaptic processes and release vasodilator agents at their endfeet in contact with brain parenchymal vessels. Two decades after the astrocyte involvement in neurovascular coupling has been proposed, we here review the experimental evidence that contributed to unraveling the molecular and cellular mechanisms underlying cerebral blood flow regulation. While traveling through the different controversies that moved the research in this field, we keep a peculiar focus on those exploring the role of astrocytes in neurovascular coupling and conclude with two sections related to methodological aspects in neurovascular research and to some pathological conditions resulting in altered neurovascular coupling.

Reactive astrocytes associated with prion disease impair the blood brain barrier

Background

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

Results

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

Conclusions

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Endothelial Dysfunction in Neurodegenerative Diseases

The cerebral vascular system stringently regulates cerebral blood flow (CBF). The components of the blood-brain barrier (BBB) protect the brain from pathogenic infections and harmful substances, efflux waste, and exchange substances; however, diseases develop in cases of blood vessel injuries and BBB dysregulation. Vascular pathology is concurrent with the mechanisms underlying aging, Alzheimer's disease (AD), and vascular dementia (VaD), which suggests its involvement in these mechanisms. Therefore, in the present study, we reviewed the role of vascular dysfunction in aging and neurodegenerative diseases, particularly AD and VaD. During the development of the aforementioned diseases, changes occur in the cerebral blood vessel morphology and local cells, which, in turn, alter CBF, fluid dynamics, and vascular integrity. Chronic vascular inflammation and blood vessel dysregulation further exacerbate vascular dysfunction. Multitudinous pathogenic processes affect the cerebrovascular system, whose dysfunction causes cognitive impairment. Knowledge regarding the pathophysiology of vascular dysfunction in neurodegenerative diseases and the underlying molecular mechanisms may lead to the discovery of clinically relevant vascular biomarkers, which may facilitate vascular imaging for disease prevention and treatment.

Altered dynamics of glymphatic flow in a mature-onset Tet-off APP mouse model of amyloidosis

BACKGROUND

Alzheimer's disease (AD) is an incurable neurodegenerative disorder characterised by the progressive buildup of toxic amyloid-beta (Aβ) and tau protein aggregates eventually leading to cognitive decline. Recent lines of evidence suggest that an impairment of the glymphatic system (GS), a brain waste clearance pathway, plays a key role in the pathology of AD. Moreover, a relationship between GS function and neuronal network integrity has been strongly implicated. Here, we sought to assess the efficacy of the GS in a transgenic Tet-Off APP mouse model of amyloidosis, in which the expression of mutant APP was delayed until maturity, mimicking features of late-onset AD-the most common form of dementia in humans.

METHODS

To evaluate GS function, we used dynamic contrast-enhanced MRI (DCE-MRI) in 14-month-old Tet-Off APP (AD) mice and aged-matched littermate controls. Brain-wide transport of the Gd-DOTA contrast agent was monitored over time after cisterna magna injection. Region-of-interest analysis and computational modelling were used to assess GS dynamics while characterisation of brain tissue abnormalities at the microscale was performed ex vivo by immunohistochemistry.

RESULTS

We observed reduced rostral glymphatic flow and higher accumulation of the contrast agent in areas proximal to the injection side in the AD group. Clustering and subsequent computational modelling of voxel time courses revealed significantly lower influx time constants in AD relative to the controls. Ex vivo evaluation showed abundant amyloid plaque burden in the AD group coinciding with extensive astrogliosis and microgliosis. The neuroinflammatory responses were also found in plaque-devoid regions, potentially impacting brain-fluid circulation.

CONCLUSIONS

In a context resembling late-onset AD in humans, we demonstrate the disruption of glymphatic function and particularly a reduction in brain-fluid influx in the AD group. We conjecture that the hindered circulation of cerebrospinal fluid is potentially caused by wide-spread astrogliosis and amyloid-related obstruction of the normal routes of glymphatic flow resulting in redirection towards caudal regions. In sum, our study highlights the translational potential of alternative approaches, such as targeting brain-fluid circulation as potential therapeutic strategies for AD.

Microglia-Mediated Neurovascular Unit Dysfunction in Alzheimer's Disease

The neurovascular unit (NVU) is involved in the pathological changes in Alzheimer's disease (AD). The NVU is a structural and functional complex that maintains microenvironmental homeostasis and metabolic balance in the central nervous system. As one of the most important components of the NVU, microglia not only induce blood-brain barrier breakdown by promoting neuroinflammation, the infiltration of peripheral white blood cells and oxidative stress but also mediate neurovascular uncoupling by inducing mitochondrial dysfunction in neurons, abnormal contraction of cerebral vessels, and pericyte loss in AD. In addition, microglia-mediated dysfunction of cellular components in the NVU, such as astrocytes and pericytes, can destroy the integrity of the NVU and lead to NVU impairment. Therefore, we review the mechanisms of microglia-mediated NVU dysfunction in AD. Furthermore, existing therapeutic advancements aimed at restoring the function of microglia and the NVU in AD are discussed. Finally, we predict the role of pericytes in microglia-mediated NVU dysfunction in AD is the hotspot in the future.

Extracellular vesicles and Alzheimer's disease in the novel era of Precision Medicine: implications for disease progression, diagnosis and treatment

Extracellular vesicles (EVs), secreted membranous nano-sized particles, are critical intercellular messengers participating in nervous system homeostasis, while recent evidence implicates EVs in Alzheimer's disease (AD) pathogenesis. Specifically, small EVs have been shown to spread toxic proteins, induce neuronal loss, and contribute to neuroinflammation and AD progression. On the other hand, EVs can reduce amyloid-beta deposition and transfer neuroprotective substances between cells, mitigating disease mechanisms. In addition to their roles in AD pathogenesis, EVs also exhibit great potential for the diagnosis and treatment of other brain disorders, representing an advantageous tool for Precision Medicine. Herein, we summarize the contribution of small EVs to AD-related mechanisms and disease progression, as well as their potential as diagnostic and therapeutic agents for AD.

Molecular and histological correlates of cognitive decline across age in male C57BL/6J mice

INTRODUCTION

Increasing age is the number one risk factor for developing cognitive decline and neurodegenerative disease. Aged humans and mice exhibit numerous molecular changes that contribute to a decline in cognitive function and increased risk of developing age-associated diseases. Here, we characterize multiple age-associated changes in male C57BL/6J mice to understand the translational utility of mouse aging.

METHODS

Male C57BL/6J mice from various ages between 2 and 24 months of age were used to assess behavioral, as well as, histological and molecular changes across three modalities: neuronal, microgliosis/neuroinflammation, and the neurovascular unit (NVU). Additionally, a cohort of 4- and 22-month-old mice was used to assess blood-brain barrier (BBB) breakdown. Mice in this cohort were treated with a high, acute dose of lipopolysaccharide (LPS, 10 mg/kg) or saline control 6 h prior to sacrifice followed by tail vein injection of 0.4 kDa sodium fluorescein (100 mg/kg) 2 h later.

RESULTS

Aged mice showed a decline in cognitive and motor abilities alongside decreased neurogenesis, proliferation, and synapse density. Further, neuroinflammation and circulating proinflammatory cytokines were increased in aged mice. Additionally, we found changes at the BBB, including increased T cell infiltration in multiple brain regions and an exacerbation in BBB leakiness following chemical insult with age. There were also a number of readouts that were unchanged with age and have limited utility as markers of aging in male C57BL/6J mice.

CONCLUSIONS

Here we propose that these changes may be used as molecular and histological readouts that correspond to aging-related behavioral decline. These comprehensive findings, in the context of the published literature, are an important resource toward deepening our understanding of normal aging and provide an important tool for studying aging in mice.

γ-Glutamylcysteine Exerts Neuroprotection Effects against Cerebral Ischemia/Reperfusion Injury through Inhibiting Lipid Peroxidation and Ferroptosis

Ferroptosis is a non-apoptotic form of cell death driven by iron-dependent lipid peroxidation. Recent evidence indicates that inhibiting ferroptosis could alleviate cerebral ischemia/reperfusion (CIR) injury. γ-glutamylcysteine (γ-GC), an intermediate of glutathione (GSH) synthesis, can upregulate GSH in brains. GSH is the co-factor of glutathione peroxidase 4 (GPX4), which is the negative regulator of ferroptosis. In this study, we explored the effect of γ-GC on CIR-induced neuronal ferroptosis and brain injury. We found that γ-GC significantly reduced the volume of cerebral infarction, decreased the loss of neurons and alleviated neurological dysfunction induced by CIR in rats. Further observation showed that γ-GC inhibited the CIR-caused rupture of the neuronal mitochondrial outer membrane and the disappearance of cristae, and decreased Fe2+ deposition and lipid peroxidation in rat cerebral cortices. Meanwhile, γ-GC altered the expression of some ferroptosis-related proteins in rat brains. Mechanistically, γ-GC increased the expression of GSH synthetase (GSS) for GSH synthesis via protein kinase C (PKC)ε-mediated activation of nuclear factor erythroid 2-related factor (Nrf2). Our findings suggest that γ-GC not only serves as a raw material but also increases the GSS expression for GSH synthesis against CIR-induced lipid peroxidation and ferroptosis. Our study strongly suggests that γ-GC has potential for treating CIR injury.

FMNL2 regulates gliovascular interactions and is associated with vascular risk factors and cerebrovascular pathology in Alzheimer's disease

Alzheimer's disease (AD) has been associated with cardiovascular and cerebrovascular risk factors (CVRFs) during middle age and later and is frequently accompanied by cerebrovascular pathology at death. An interaction between CVRFs and genetic variants might explain the pathogenesis. Genome-wide, gene by CVRF interaction analyses for AD, in 6568 patients and 8101 controls identified FMNL2 (p = 6.6 × 10-7). A significant increase in FMNL2 expression was observed in the brains of patients with brain infarcts and AD pathology and was associated with amyloid and phosphorylated tau deposition. FMNL2 was also prominent in astroglia in AD among those with cerebrovascular pathology. Amyloid toxicity in zebrafish increased fmnl2a expression in astroglia with detachment of astroglial end feet from blood vessels. Knockdown of fmnl2a prevented gliovascular remodeling, reduced microglial activity and enhanced amyloidosis. APP/PS1dE9 AD mice also displayed increased Fmnl2 expression and reduced the gliovascular contacts independent of the gliotic response. Based on this work, we propose that FMNL2 regulates pathology-dependent plasticity of the blood-brain-barrier by controlling gliovascular interactions and stimulating the clearance of extracellular aggregates. Therefore, in AD cerebrovascular risk factors promote cerebrovascular pathology which in turn, interacts with FMNL2 altering the normal astroglial-vascular mechanisms underlying the clearance of amyloid and tau increasing their deposition in brain.

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

Neurovascular coupling mechanisms in health and neurovascular uncoupling in Alzheimer's disease

To match the metabolic demands of the brain, mechanisms have evolved to couple neuronal activity to vasodilation, thus increasing local cerebral blood flow and delivery of oxygen and glucose to active neurons. Rather than relying on metabolic feedback signals such as the consumption of oxygen or glucose, the main signalling pathways rely on the release of vasoactive molecules by neurons and astrocytes, which act on contractile cells. Vascular smooth muscle cells and pericytes are the contractile cells associated with arterioles and capillaries, respectively, which relax and induce vasodilation.

Much progress has been made in understanding the complex signalling pathways of neurovascular coupling, but issues such as the contributions of capillary pericytes and astrocyte calcium signal remain contentious. Study of neurovascular coupling mechanisms is especially important as cerebral blood flow dysregulation is a prominent feature of Alzheimer’s disease. In this article we will discuss developments and controversies in the understanding of neurovascular coupling and finish by discussing current knowledge concerning neurovascular uncoupling in Alzheimer’s disease.

Improving mouse models for the study of Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disease whose risk is influenced by genetic and environmental factors. Although a number of pathological hallmarks have been extensively studied over the last several decades, a complete picture of disease initiation and progression remains unclear. We now understand that numerous cell types and systems are involved in AD pathogenesis, and that this cellular profile may present differently for each individual, making the creation of relevant mouse models challenging. However, with increasingly diverse data made available by genome-wide association studies, we can identify and examine new genes and pathways involved in genetic risk for AD, many of which involve vascular health and inflammation. When developing mouse models, it is critical to assess (1) an aging timeline that represents onset and progression in humans, (2) genetic variants and context, (3) environmental factors present in human populations that result in both neuropathological and functional changes-themes that we address in this chapter.

High-Resolution Digital Panorama of Multiple Structures in Whole Brain of Alzheimer's Disease Mice

Simultaneously visualizing Amyloid-β (Aβ) plaque with its surrounding brain structures at the subcellular level in the intact brain is essential for understanding the complex pathology of Alzheimer's disease, but is still rarely achieved due to the technical limitations. Combining the micro-optical sectioning tomography (MOST) system, whole-brain Nissl staining, and customized image processing workflow, we generated a whole-brain panorama of Alzheimer's disease mice without specific labeling. The workflow employed the steps that include virtual channel splitting, feature enhancement, iso-surface rendering, direct volume rendering, and feature fusion to extract and reconstruct the different signals with distinct gray values and morphologies. Taking advantage of this workflow, we found that the denser-distribution areas of Aβ plaques appeared with relatively more somata and smaller vessels, but show a dissimilar distributing pattern with nerve tracts. In addition, the entorhinal cortex and adjacent subiculum regions present the highest density and biggest diameter of plaques. The neuronal processes in the vicinity of these Aβ plaques showed significant structural alternation such as bending or abrupt branch ending. The capillaries inside or adjacent to the plaques were observed with abundant distorted micro-vessels and abrupt ending. Depicting Aβ plaques, somata, nerve processes and tracts, and blood vessels simultaneously, this panorama enables us for the first time, to analyze how the Aβ plaques interact with capillaries, somata, and processes at a submicron resolution of 3D whole-brain scale, which reveals potential pathological effects of Aβ plaques from a new cross-scale view. Our approach opens a door to routine systematic studies of complex interactions among brain components in mouse models of Alzheimer's disease.

Astrocyte plasticity in mice ensures continued endfoot coverage of cerebral blood vessels following injury and declines with age

Astrocytes extend endfeet that enwrap the vasculature, and disruptions to this association which may occur in disease coincide with breaches in blood-brain barrier (BBB) integrity. Here we investigate if focal ablation of astrocytes is sufficient to disrupt the BBB in mice. Targeted two-photon chemical apoptotic ablation of astrocytes induced a plasticity response whereby surrounding astrocytes extended processes to cover vascular vacancies. In young animals, replacement processes occur in advance of endfoot retraction, but this is delayed in aged animals. Stimulation of replacement astrocytes results in constriction of pre-capillary arterioles, suggesting that replacement astrocytes are functional. Pharmacological inhibition of pSTAT3, as well as astrocyte specific deletion of pSTAT3, reduces astrocyte replacement post-ablation, without perturbations to BBB integrity. Similar endfoot replacement occurs following astrocyte cell death due to reperfusion in a stroke model. Together, these studies uncover the ability of astrocytes to maintain cerebrovascular coverage via substitution from nearby cells.

The effect of parthenolide on methamphetamine-induced blood-brain barrier and astrocyte alterations

BACKGROUND

Methamphetamine abuse is a worldwide concern with long-term health complications. Its impact on neurons has been extensively investigated, and it is currently known that glial cells, including astrocytes, are involved in drug-induced outcomes. Importantly, METH also causes blood-brain barrier (BBB) disruption and astrocytes are critical for BBB (dys)function. Therefore, we aimed to clarify the involvement of neuroinflammation mediated by astrocytes in BBB permeability and brain oedema induced by METH. Further, we aimed to identify a new approach to counteract METH effects.

METHODS

Mice were administered with a METH binge regimen (4 × 10 mg/kg) alone or in combination with parthenolide (PTL; 4 × 1 mg/kg), and hippocampi were analysed. For in vitro studies, mouse primary cultures of astrocytes were exposed to 250 µM METH, alone or co-treated with 10 µM PTL.

RESULTS

We observed a neuroinflammatory response characterized by astrocytic morphological changes and increased TNF-α, iNOS and ICAM-1 protein levels (213.62%, 205.76% and 191.47% of control, respectively). Additionally, brain oedema and BBB disruption were identified by increased water content (81.30% of tissue weight) and albumin (224.40% of control) in the hippocampal tissue, as well as a significant decrease in vessel coverage by astrocytes after METH exposure. Regarding astrocyte cultures, we further identified TNF-α as a key player in METH-induced cell swelling. Importantly, PTL (present in feverfew plant) prevented both animal and in vitro effects induced by METH.

CONCLUSIONS

We provided important insights on brain dysfunction induced by METH, and we also suggest a new approach to counteract such negative effects.

Addressing Blood–Brain Barrier Impairment in Alzheimer’s Disease

The blood–brain barrier (BBB) plays a vital role in maintaining the specialized microenvironment of the brain tissue. It facilitates communication while separating the peripheral circulation system from the brain parenchyma. However, normal aging and neurodegenerative diseases can alter and damage the physiological properties of the BBB. In this review, we first briefly present the essential pathways maintaining and regulating BBB integrity, and further review the mechanisms of BBB breakdown associated with normal aging and peripheral inflammation-causing neurodegeneration and cognitive impairments. We also discuss how BBB disruption can cause or contribute to Alzheimer’s disease (AD), the most common form of dementia and a devastating neurological disorder. Next, we document overlaps between AD and vascular dementia (VaD) and briefly sum up the techniques for identifying biomarkers linked to BBB deterioration. Finally, we conclude that BBB breakdown could be used as a biomarker to help diagnose cognitive impairment associated with normal aging and neurodegenerative diseases such as AD.

Minocycline improves the functional recovery after traumatic brain injury via inhibition of aquaporin-4

Traumatic brain injury (TBI) is one of the main concerns worldwide as there is still no comprehensive therapeutic intervention. Astrocytic water channel aquaporin-4 (AQP-4) system is closely related to the brain edema, water transport at blood-brain barrier (BBB) and astrocyte function in the central nervous system (CNS). Minocycline, a broad-spectrum semisynthetic tetracycline antibiotic, has shown anti-inflammation, anti-apoptotic, vascular protection and neuroprotective effects on TBI models. Here, we tried to further explore the underlying mechanism of minocycline treatment for TBI, especially the relationship of minocycline and AQP4 during TBI treatment. In present study, we observed that minocycline efficaciously reduces the elevation of AQP4 in TBI mice. Furthermore, minocycline significantly reduced neuronal apoptosis, ameliorated brain edema and BBB disruption after TBI. In addition, the expressions of tight junction protein and astrocyte morphology alteration were optimized by minocycline administration. Similar results were found after treating with TGN-020 (an inhibitor of AQP4) in TBI mice. Moreover, these effects were reversed by cyanamide (CYA) treatment, which notably upregulated AQP4 expression level in vivo. In primary cultured astrocytes, small-interfering RNA (siRNA) AQP4 treatment prevented glutamate-induced astrocyte swelling. To sum up, our study suggests that minocycline improves the functional recovery of TBI through reducing AQP4 level to optimize BBB integrity and astrocyte function, and highlights that the AQP4 may be an important therapeutic target during minocycline treating for TBI.

Neurovascular dysfunction and vascular amyloid accumulation as early events in Alzheimer's disease

Alzheimer's disease (AD) is clinically characterized by a progressive loss of cognitive functions and short-term memory. AD patients present two distinctive neuropathological lesions: neuritic plaques and neurofibrillary tangles (NFTs), constituted of beta-amyloid peptide (Aβ) and phosphorylated and truncated tau proteins. Aβ deposits around cerebral blood vessels (cerebral amyloid angiopathy, CAA) is a major contributor to vascular dysfunction in AD. Vascular amyloid deposits could be early events in AD due to dysfunction in the neurovascular unit (NVU) and the blood-brain barrier (BBB), deterioration of the gliovascular unit, and/or decrease of cerebral blood flow (CBF). These pathological events can lead to decreased Aβ clearance, facilitate a neuroinflammatory environment as well as synaptic dysfunction and, finally, lead to neurodegeneration. Here, we review the histopathological AD hallmarks and discuss the two-hit vascular hypothesis of AD, emphasizing the role of neurovascular dysfunction as an early factor that favors vascular Aβ aggregation and neurodegeneration. Addtionally, we emphasize that pericyte degeneration is a key and early element in AD that can trigger amyloid vascular accumulation and NVU/BBB dysfunction. Further research is required to better understand the early pathophysiological mechanisms associated with NVU alteration and CAA to generate early biomarkers and timely treatments for AD.

Differential pial and penetrating arterial responses examined by optogenetic activation of astrocytes and neurons

A variety of brain cells participates in neurovascular coupling by transmitting and modulating vasoactive signals. The present study aimed to probe cell type-dependent cerebrovascular (i.e., pial and penetrating arterial) responses with optogenetics in the cortex of anesthetized mice. Two lines of the transgenic mice expressing a step function type of light-gated cation channel (channelrhodopsine-2; ChR2) in either cortical neurons (muscarinic acetylcholine receptors) or astrocytes (Mlc1-positive) were used in the experiments. Photo-activation of ChR2-expressing astrocytes resulted in a widespread increase in cerebral blood flow (CBF), extending to the nonstimulated periphery. In contrast, photo-activation of ChR2-expressing neurons led to a relatively localized increase in CBF. The differences in the spatial extent of the CBF responses are potentially explained by differences in the involvement of the vascular compartments. In vivo imaging of the cerebrovascular responses revealed that ChR2-expressing astrocyte activation led to the dilation of both pial and penetrating arteries, whereas ChR2-expressing neuron activation predominantly caused dilation of the penetrating arterioles. Pharmacological studies showed that cell type-specific signaling mechanisms participate in the optogenetically induced cerebrovascular responses. In conclusion, pial and penetrating arterial vasodilation were differentially evoked by ChR2-expressing astrocytes and neurons.

The neurogenic niche in Alzheimer's disease

Adult hippocampal neurogenesis is the process of generation and functional incorporation of new neurons, formed by adult neural stem cells in the dentate gyrus. Adult hippocampal neurogenesis is highly dependent upon the integration of dynamic external stimuli and is instrumental in the formation of new spatial memories. Adult hippocampal neurogenesis is therefore uniquely sensitive to the summation of neuronal circuit and neuroimmune environments that comprise the neurogenic niche, and has powerful implications in diseases of aging and neurological disorders. This sensitivity underlies the neurogenic niche alterations commonly observed in Alzheimer's disease, the most common form of dementia. This review summarizes Alzheimer's disease associated changes in neuronal network activity, neuroinflammatory processes, and adult neural stem cell fate choice that ultimately result in neurogenic niche dysfunction and impaired adult hippocampal neurogenesis. A more comprehensive understanding of the complex changes mediating neurogenic niche disturbances in Alzheimer's disease will aid development of future therapies targeting adult neurogenesis.

Hyperbaric oxygen therapy alleviates vascular dysfunction and amyloid burden in an Alzheimer's disease mouse model and in elderly patients

Vascular dysfunction is entwined with aging and in the pathogenesis of Alzheimer's disease (AD) and contributes to reduced cerebral blood flow (CBF) and consequently, hypoxia. Hyperbaric oxygen therapy (HBOT) is in clinical use for a wide range of medical conditions. In the current study, we exposed 5XFAD mice, a well-studied AD model that presents impaired cognitive abilities, to HBOT and then investigated the therapeutical effects using two-photon live animal imaging, behavioral tasks, and biochemical and histological analysis. HBOT increased arteriolar luminal diameter and elevated CBF, thus contributing to reduced hypoxia. Furthermore, HBOT reduced amyloid burden by reducing the volume of pre-existing plaques and attenuating the formation of new ones. This was associated with changes in amyloid precursor protein processing, elevated degradation and clearance of Aß protein and improved behavior of 5XFAD mice. Hence, our findings are consistent with the effects of HBOT being mediated partially through a persistent structural change in blood vessels that reduces brain hypoxia. Motivated by these findings, we exposed elderly patients with significant memory loss at baseline to HBOT and observed an increase in CBF and improvement in cognitive performances. This study demonstrates HBOT efficacy in hypoxia-related neurological conditions, particularly in AD and aging.

Blood-Brain Barrier Breakdown: An Emerging Biomarker of Cognitive Impairment in Normal Aging and Dementia

The blood–brain barrier (BBB) plays a vital role in maintaining the specialized microenvironment of the neural tissue. It separates the peripheral circulatory system from the brain parenchyma while facilitating communication. Alterations in the distinct physiological properties of the BBB lead to BBB breakdown associated with normal aging and various neurodegenerative diseases. In this review, we first briefly discuss the aging process, then review the phenotypes and mechanisms of BBB breakdown associated with normal aging that further cause neurodegeneration and cognitive impairments. We also summarize dementia such as Alzheimer's disease (AD) and vascular dementia (VaD) and subsequently discuss the phenotypes and mechanisms of BBB disruption in dementia correlated with cognition decline. Overlaps between AD and VaD are also discussed. Techniques that could identify biomarkers associated with BBB breakdown are briefly summarized. Finally, we concluded that BBB breakdown could be used as an emerging biomarker to assist to diagnose cognitive impairment associated with normal aging and dementia.

Aberrant waste disposal in neurodegeneration: why improved sleep could be the solution

Sleep takes up a large percentage of our lives and the full functions of this state are still not understood. However, over the last 10 years a new and important function has emerged as a mediator of brain clearance. Removal of toxic metabolites and proteins from the brain parenchyma generated during waking activity and high levels of synaptic processing is critical to normal brain function and only enabled during deep sleep. Understanding of this process is revealing how impaired sleep contributes an important and likely causative role in the accumulation and aggregation of aberrant proteins such as β-amyloid and phosphorylated tau, as well as inflammation and neuronal damage. We are also beginning to understand how brain slow-wave activity interacts with vascular function allowing the flow of CSF and interstitial fluid to drain into the body's lymphatic system. New methodology is enabling visualization of this process in both animals and humans and is revealing how these processes break down during ageing and disease. With this understanding we can begin to envisage novel therapeutic approaches to the treatment of neurodegeneration, and how reversing sleep impairment in the correct manner may provide a way to slow these processes and improve brain function.

Blood-Brain Barrier and Neurodegenerative Diseases-Modeling with iPSC-Derived Brain Cells

The blood-brain barrier (BBB) regulates the delivery of oxygen and important nutrients to the brain through active and passive transport and prevents neurotoxins from entering the brain. It also has a clearance function and removes carbon dioxide and toxic metabolites from the central nervous system (CNS). Several drugs are unable to cross the BBB and enter the CNS, adding complexity to drug screens targeting brain disorders. A well-functioning BBB is essential for maintaining healthy brain tissue, and a malfunction of the BBB, linked to its permeability, results in toxins and immune cells entering the CNS. This impairment is associated with a variety of neurological diseases, including Alzheimer's disease and Parkinson's disease. Here, we summarize current knowledge about the BBB in neurodegenerative diseases. Furthermore, we focus on recent progress of using human-induced pluripotent stem cell (iPSC)-derived models to study the BBB. We review the potential of novel stem cell-based platforms in modeling the BBB and address advances and key challenges of using stem cell technology in modeling the human BBB. Finally, we highlight future directions in this area.

Neurovascular Coupling in Development and Disease: Focus on Astrocytes

Neurovascular coupling is a crucial mechanism that matches the high energy demand of the brain with a supply of energy substrates from the blood. Signaling within the neurovascular unit is responsible for activity-dependent changes in cerebral blood flow. The strength and reliability of neurovascular coupling form the basis of non-invasive human neuroimaging techniques, including blood oxygen level dependent (BOLD) functional magnetic resonance imaging. Interestingly, BOLD signals are negative in infants, indicating a mismatch between metabolism and blood flow upon neural activation; this response is the opposite of that observed in healthy adults where activity evokes a large oversupply of blood flow. Negative neurovascular coupling has also been observed in rodents at early postnatal stages, further implying that this is a process that matures during development. This rationale is consistent with the morphological maturation of the neurovascular unit, which occurs over a similar time frame. While neurons differentiate before birth, astrocytes differentiate postnatally in rodents and the maturation of their complex morphology during the first few weeks of life links them with synapses and the vasculature. The vascular network is also incomplete in neonates and matures in parallel with astrocytes. Here, we review the timeline of the structural maturation of the neurovascular unit with special emphasis on astrocytes and the vascular tree and what it implies for functional maturation of neurovascular coupling. We also discuss similarities between immature astrocytes during development and reactive astrocytes in disease, which are relevant to neurovascular coupling. Finally, we close by pointing out current gaps in knowledge that must be addressed to fully elucidate the mechanisms underlying neurovascular coupling maturation, with the expectation that this may also clarify astrocyte-dependent mechanisms of cerebrovascular impairment in neurodegenerative conditions in which reduced or negative neurovascular coupling is noted, such as stroke and Alzheimer’s disease.

Slowed Temporal and Parietal Cerebrovascular Response in Patients with Alzheimer's Disease

BACKGROUND

Recent investigations now suggest that cerebrovascular reactivity (CVR) is impaired in Alzheimer's disease (AD) and may underpin part of the disease's neurovascular component. However, our understanding of the relationship between the magnitude of CVR, the speed of cerebrovascular response, and the progression of AD is still limited. This is especially true in patients with mild cognitive impairment (MCI), which is recognized as an intermediate stage between normal aging and dementia. The purpose of this study was to investigate AD and MCI patients by mapping repeatable and accurate measures of cerebrovascular function, namely the magnitude and speed of cerebrovascular response (τ) to a vasoactive stimulus in key predilection sites for vascular dysfunction in AD.

METHODS

Thirty-three subjects (age range: 52-83 years, 20 males) were prospectively recruited. CVR and τ were assessed using blood oxygen level-dependent MRI during a standardized carbon dioxide stimulus. Temporal and parietal cortical regions of interest (ROIs) were generated from anatomical images using the FreeSurfer image analysis suite.

RESULTS

Of 33 subjects recruited, 3 individuals were excluded, leaving 30 subjects for analysis, consisting of 6 individuals with early AD, 11 individuals with MCI, and 13 older healthy controls (HCs). τ was found to be significantly higher in the AD group compared to the HC group in both the temporal (p = 0.03) and parietal cortex (p = 0.01) following a one-way ANCOVA correcting for age and microangiopathy scoring and a Bonferroni post-hoc correction.

CONCLUSION

The study findings suggest that AD is associated with a slowing of the cerebrovascular response in the temporal and parietal cortices.

Foveal Avascular Zone and Choroidal Thickness Are Decreased in Subjects with Hard Drusen and without High Genetic Risk of Developing Alzheimer's Disease

A family history (FH+) of Alzheimer’s disease (AD) and ɛ4 allele of the ApoE gene are the main genetic risk factors for developing AD, whereas ɛ4 allele plays a protective role in age-related macular degeneration. Ocular vascular changes have been reported in both pathologies. We analyzed the choroidal thickness using optical coherence tomography (OCT) and the foveal avascular zone (FAZ) using OCT-angiography and compared the results with ApoE gene expression, AD FH+, and the presence or absence of hard drusen (HD) in 184 cognitively healthy subjects. Choroidal thickness was statistically significantly different in the (FH−, ɛ4−, HD+) group compared with (i) both the (FH−, ɛ4−, HD−) and the (FH+, ɛ4+, HD+) groups in the superior and inferior points at 1500 μm, and (ii) the (FH+, ɛ4−, HD+) group in the superior point at 1500 μm. There were statistically significant differences in the superficial FAZ between the (FH+, ɛ4−, HD+) group and (i) the (FH+, ɛ4−, HD−) group and (ii) the (FH+, ɛ4+, HD−) group. In conclusion, ocular vascular changes are not yet evident in participants with a genetic risk of developing AD.

Vascular Risk Factors and Alzheimer's Disease: Blood-Brain Barrier Disruption, Metabolic Syndromes, and Molecular Links

Alzheimer's disease (AD) is a neurodegenerative disorder, marked by cortical and hippocampal deposition of amyloid-β (Aβ) plaques and neurofibrillary tangles and cognitive impairment. Studies indicate a prominent link between cerebrovascular abnormalities and the onset and progression of AD, where blood-brain barrier (BBB) dysfunction and metabolic disorders play key risk factors. Pericyte degeneration, endothelial cell damage, astrocyte depolarization, diminished tight junction integrity, and basement membrane disarray trigger BBB damage. Subsequently, the altered expression of low-density lipoprotein receptor-related protein 1 and receptor for advanced glycation end products at the microvascular endothelial cells dysregulate Aβ transport across the BBB. White matter lesions and microhemorrhages, dyslipidemia, altered brain insulin signaling, and insulin resistance contribute to tau and Aβ pathogenesis, and oxidative stress, mitochondrial damage, inflammation, and hypoperfusion serve as mechanistic links between pathophysiological features of AD and ischemia. Deregulated calcium homeostasis, voltage gated calcium channel functioning, and protein kinase C signaling are also common mechanisms for both AD pathogenesis and cerebrovascular abnormalities. Additionally, APOE polymorphic alleles that characterize impaired cerebrovascular integrity function as primary genetic determinants of AD. Overall, the current review enlightens key vascular risk factors for AD and underscores pathophysiologic relationship between AD and vascular dysfunction.

Cardiovascular brain impulses in Alzheimer's disease

Accumulation of amyloid-β is a key neuropathological feature in brain of Alzheimer’s disease patients. Alterations in cerebral haemodynamics, such as arterial impulse propagation driving the (peri)vascular CSF flux, predict future Alzheimer’s disease progression. We now present a non-invasive method to quantify the three-dimensional propagation of cardiovascular impulses in human brain using ultrafast 10 Hz magnetic resonance encephalography. This technique revealed spatio-temporal abnormalities in impulse propagation in Alzheimer’s disease. The arrival latency and propagation speed both differed in patients with Alzheimer’s disease. Our mapping of arterial territories revealed Alzheimer’s disease-specific modifications, including reversed impulse propagation around the hippocampi and in parietal cortical areas. The findings imply that pervasive abnormality in (peri)vascular CSF impulse propagation compromises vascular impulse propagation and subsequently glymphatic brain clearance of amyloid-β in Alzheimer’s disease.

Astrocytes respond to a neurotoxic Aβ fragment with state-dependent Ca2+ alteration and multiphasic transmitter release

Excessive amounts of amyloid β (Aβ) peptide have been suggested to dysregulate synaptic transmission in Alzheimer's disease (AD). As a major type of glial cell in the mammalian brain, astrocytes regulate neuronal function and undergo activity alterations upon Aβ exposure. Yet the mechanistic steps underlying astrocytic responses to Aβ peptide remain to be elucidated. Here by fluorescence imaging of signaling pathways, we dissected astrocytic responses to Aβ25-35 peptide, a neurotoxic Aβ fragment present in AD patients. In native health astrocytes, Aβ25-35 evoked Ca²⁺ elevations via purinergic receptors, being also dependent on the opening of connexin (CX) hemichannels. Aβ25-35, however, induced a Ca²⁺ diminution in Aβ-preconditioned astrocytes as a result of the potentiation of the plasma membrane Ca²⁺ ATPase (PMCA). The PMCA and CX protein expression was observed with immunostaining in the brain tissue of hAPPJ20 AD mouse model. We also observed both Ca²⁺-independent and Ca²⁺-dependent glutamate release upon astrocytic Aβ exposure, with the former mediated by CX hemichannel and the latter by both anion channels and lysosome exocytosis. Our results suggest that Aβ peptide causes state-dependent responses in astrocytes, in association with a multiphasic release of signaling molecules. This study therefore helps to understand astrocyte engagement in AD-related amyloidopathy.

Reactive astrocyte nomenclature, definitions, and future directions

Reactive astrocytes are astrocytes undergoing morphological, molecular, and functional remodeling in response to injury, disease, or infection of the CNS. Although this remodeling was first described over a century ago, uncertainties and controversies remain regarding the contribution of reactive astrocytes to CNS diseases, repair, and aging. It is also unclear whether fixed categories of reactive astrocytes exist and, if so, how to identify them. We point out the shortcomings of binary divisions of reactive astrocytes into good-vs-bad, neurotoxic-vs-neuroprotective or A1-vs-A2. We advocate, instead, that research on reactive astrocytes include assessment of multiple molecular and functional parameters-preferably in vivo-plus multivariate statistics and determination of impact on pathological hallmarks in relevant models. These guidelines may spur the discovery of astrocyte-based biomarkers as well as astrocyte-targeting therapies that abrogate detrimental actions of reactive astrocytes, potentiate their neuro- and glioprotective actions, and restore or augment their homeostatic, modulatory, and defensive functions.

More than just summed neuronal activity: how multiple cell types shape the BOLD response

Functional neuroimaging techniques are widely applied to investigations of human cognition and disease. The most commonly used among these is blood oxygen level-dependent (BOLD) functional magnetic resonance imaging. The BOLD signal occurs because neural activity induces an increase in local blood supply to support the increased metabolism that occurs during activity. This supply usually outmatches demand, resulting in an increase in oxygenated blood in an active brain region, and a corresponding decrease in deoxygenated blood, which generates the BOLD signal. Hence, the BOLD response is shaped by an integration of local oxygen use, through metabolism, and supply, in the blood. To understand what information is carried in BOLD signals, we must understand how several cell types in the brain-local excitatory neurons, inhibitory neurons, astrocytes and vascular cells (pericytes, vascular smooth muscle and endothelial cells), and their modulation by ascending projection neurons-contribute to both metabolism and haemodynamic changes. Here, we review the contributions of each cell type to the regulation of cerebral blood flow and metabolism, and discuss situations where a simplified interpretation of the BOLD response as reporting local excitatory activity may misrepresent important biological phenomena, for example with regards to arousal states, ageing and neurological disease. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Multifunctional Nanostructure RAP-RL Rescues Alzheimer's Cognitive Deficits through Remodeling the Neurovascular Unit

Cerebrovascular dysfunction characterized by the neurovascular unit (NVU) impairment contributes to the pathogenesis of Alzheimer's disease (AD). In this study, a cerebrovascular-targeting multifunctional lipoprotein-biomimetic nanostructure (RAP-RL) constituted with an antagonist peptide (RAP) of receptor for advanced glycation end-products (RAGE), monosialotetrahexosyl ganglioside, and apolipoprotein E3 is developed to recover the functional NVU and normalize the cerebral vasculature. RAP-RL accumulates along the cerebral microvasculature through the specific binding of RAP to RAGE, which is overexpressed on cerebral endothelial cells in AD. It effectively accelerates the clearance of perivascular Aβ, normalizes the morphology and functions of cerebrovasculature, and restores the structural integrity and functions of NVU. RAP-RL markedly rescues the spatial learning and memory in APP/PS1 mice. Collectively, this study demonstrates the potential of the multifunctional nanostructure RAP-RL as a disease-modifying modality for AD treatment and provides the proof of concept that remodeling the functional NVU may represent a promising therapeutic approach toward effective intervention of AD.

Cerebral artery dilation during transient ischemia is impaired by amyloid β deposition around the cerebral artery in Alzheimer's disease model mice

Transient ischemia is an exacerbation factor of Alzheimer's disease (AD). We aimed to examine the influence of amyloid β (Aβ) deposition around the cerebral (pial) artery in terms of diameter changes in the cerebral artery during transient ischemia in AD model mice (APPNL-G-F) under urethane anesthesia. Cerebral vasculature and Aβ deposition were examined using two-photon microscopy. Cerebral ischemia was induced by transient occlusion of the unilateral common carotid artery. The diameter of the pial artery was quantitatively measured. In wild-type mice, the diameter of arteries increased during occlusion and returned to their basal diameter after re-opening. In AD model mice, the artery response during occlusion differed depending on Aβ deposition sites. Arterial diameter changes at non-Aβ deposition site were similar to those in wild-type mice, whereas they were significantly smaller at Aβ deposition site. The results suggest that cerebral artery changes during ischemia are impaired by Aβ deposition.

Imaging and optogenetic modulation of vascular mural cells in the live brain

Mural cells (smooth muscle cells and pericytes) are integral components of brain blood vessels that play important roles in vascular formation, blood-brain barrier maintenance, and regulation of regional cerebral blood flow (rCBF). These cells are implicated in conditions ranging from developmental vascular disorders to age-related neurodegenerative diseases. Here we present complementary tools for cell labeling with transgenic mice and organic dyes that allow high-resolution intravital imaging of the different mural cell subtypes. We also provide detailed methodologies for imaging of spontaneous and neural activity-evoked calcium transients in mural cells. In addition, we describe strategies for single- and two-photon optogenetics that allow manipulation of the activity of individual and small clusters of mural cells. Together with measurements of diameter and flow in individual brain microvessels, calcium imaging and optogenetics allow the investigation of pericyte and smooth muscle cell physiology and their role in regulating rCBF. We also demonstrate the utility of these tools to investigate mural cells in the context of Alzheimer's disease and cerebral ischemia mouse models. Thus, these methods can be used to reveal the functional and structural heterogeneity of mural cells in vivo, and allow detailed cellular studies of the normal function and pathophysiology of mural cells in a variety of disease models. The implementation of this protocol can take from several hours to days depending on the intended applications.

An Overview of Astrocyte Responses in Genetically Induced Alzheimer's Disease Mouse Models

Alzheimer's disease (AD) is the most common form of dementia. Despite many years of intense research, there is currently still no effective treatment. Multiple cell types contribute to disease pathogenesis, with an increasing body of data pointing to the active participation of astrocytes. Astrocytes play a pivotal role in the physiology and metabolic functions of neurons and other cells in the central nervous system. Because of their interactions with other cell types, astrocyte functions must be understood in their biologic context, thus many studies have used mouse models, of which there are over 190 available for AD research. However, none appear able to fully recapitulate the many functional changes in astrocytes reported in human AD brains. Our review summarizes the observations of astrocyte biology noted in mouse models of familial and sporadic AD. The limitations of AD mouse models will be discussed and current attempts to overcome these disadvantages will be described. With increasing understanding of the non-neuronal contributions to disease, the development of new methods and models will provide further insights and address important questions regarding the roles of astrocytes and other non-neuronal cells in AD pathophysiology. The next decade will prove to be full of exciting opportunities to address this devastating disease.

Astrocytes in the regulation of cerebrovascular functions

Astrocytes are the most numerous type of neuroglia in the brain and have a predominant influence on the cerebrovascular system; they control perivascular homeostasis, the integrity of the blood-brain barrier, the dialogue with the peripheral immune system, the transfer of metabolites from the blood, and blood vessel contractility in response to neuronal activity. These regulatory processes occur in a specialized interface composed of perivascular astrocyte extensions that almost completely cover the cerebral blood vessels. Scientists have only recently started to study how this interface is formed and how it influences cerebrovascular functions. Here, we review the literature on the astrocytes' role in the regulation of the cerebrovascular system. We cover the anatomy and development of the gliovascular interface, the known gliovascular functions, and molecular factors, the latter's implication in certain pathophysiological situations, and recent cutting-edge experimental tools developed to examine the astrocytes' role at the vascular interface. Finally, we highlight some open questions in this field of research.