Glio-vascular changes during ageing in wild-type and Alzheimer's disease-like APP/PS1 mice

Transient Changes in Cerebral Tissue Oxygen, Glucose, and Temperature by Microstrokes: A Computational Study

OBJECTIVE

This study focuses on evaluating the disruptions in key physiological parameters during microstroke events to assess their severity.

METHODS

A mathematical model was developed to simulate the changes in cerebral tissue pO2, glucose concentration, and temperature due to blood flow interruptions. The model considers variations in baseline cerebral blood flow (CBF), capillary density, and blood oxygen/glucose levels, as well as ambient temperature changes.

RESULTS

Simulations indicate that complete blood flow obstruction still allows for limited glucose availability, supporting nonoxidative metabolism and potentially exacerbating lactate buildup and acidosis. Partial obstructions decrease tissue pO2, with minimal impact on glucose level, which can remain almost unchanged or even slightly increase. Reduced CBF, capillary density, or blood oxygen due to aging or disease enhances hypoxia risk at lower obstruction levels, with capillary density having a significant effect on stroke severity by influencing both pO2 and glucose levels. Conditions could lead to co-occurrence of hypoxia/hypoglycemia or hypoxia/hyperglycemia, each worsening outcomes. Temperature effects were minimal in deep brain regions but varied near the skull by 0.2-0.8°C depending on ambient temperature.

CONCLUSIONS

The model provides insights into the conditions driving severe stroke outcomes based on estimated levels of hypoxia, hypoglycemia, hyperglycemia, and temperature changes.

Cerebrovascular Dysfunction in Alzheimer's Disease and Transgenic Rodent Models

Alzheimer's Disease (AD) and Alzheimer's Disease-Related Dementia (ADRD) are the primary causes of dementia that has a devastating effect on the quality of life and is a tremendous economic burden on the healthcare system. The accumulation of extracellular beta-amyloid (Aβ) plaques and intracellular hyperphosphorylated tau-containing neurofibrillary tangles (NFTs) in the brain are the hallmarks of AD. They are also thought to be the underlying cause of inflammation, neurodegeneration, brain atrophy, and cognitive impairments that accompany AD. The discovery of APP, PS1, and PS2 mutations that increase Aβ production in families with early onset familial AD led to the development of numerous transgenic rodent models of AD. These models have provided new insight into the role of Aβ in AD; however, they do not fully replicate AD pathology in patients. Familial AD patients with mutations that elevate the production of Aβ represent only a small fraction of dementia patients. In contrast, those with late-onset sporadic AD constitute the majority of cases. This observation, along with the failure of previous clinical trials targeting Aβ or Tau and the modest success of recent trials using Aβ monoclonal antibodies, has led to a reappraisal of the view that Aβ accumulation is the sole factor in the pathogenesis of AD. More recent studies have established that cerebral vascular dysfunction is one of the earliest changes seen in AD, and 67% of the candidate genes linked to AD are expressed in the cerebral vasculature. Thus, there is an increasing appreciation of the vascular contribution to AD, and the National Institute on Aging (NIA) and the Alzheimer's Disease Foundation recently prioritized it as a focused research area. This review summarizes the strengths and limitations of the most commonly used transgenic AD animal models and current views about the contribution of Aβ accumulation versus cerebrovascular dysfunction in the pathogenesis of AD.

Vascular senescence and leak are features of the early breakdown of the blood-brain barrier in Alzheimer's disease models

Alzheimer's disease (AD) is an age-related disease, with loss of integrity of the blood-brain barrier (BBB) being an early feature. Cellular senescence is one of the reported nine hallmarks of aging. Here, we show for the first time the presence of senescent cells in the vasculature in AD patients and mouse models of AD. Senescent endothelial cells and pericytes are present in APP/PS1 transgenic mice but not in wild-type littermates at the time of amyloid deposition. In vitro, senescent endothelial cells display altered VE-cadherin expression and loss of cell junction formation and increased permeability. Consistent with this, senescent endothelial cells in APP/PS1 mice are present at areas of vascular leak that have decreased claudin-5 and VE-cadherin expression confirming BBB breakdown. Furthermore, single cell sequencing of endothelial cells from APP/PS1 transgenic mice confirms that adhesion molecule pathways are among the most highly altered pathways in these cells. At the pre-plaque stage, the vasculature shows significant signs of breakdown, with a general loss of VE-cadherin, leakage within the microcirculation, and obvious pericyte perturbation. Although senescent vascular cells were not directly observed at sites of vascular leak, senescent cells were close to the leak area. Thus, we would suggest in AD that there is a progressive induction of senescence in constituents of the neurovascular unit contributing to an increasing loss of vascular integrity. Targeting the vasculature early in AD, either with senolytics or with drugs that improve the integrity of the BBB may be valid therapeutic strategies.

Microfluidic models of the neurovascular unit: a translational view

The vasculature of the brain consists of specialized endothelial cells that form a blood-brain barrier (BBB). This barrier, in conjunction with supporting cell types, forms the neurovascular unit (NVU). The NVU restricts the passage of certain substances from the bloodstream while selectively permitting essential nutrients and molecules to enter the brain. This protective role is crucial for optimal brain function, but presents a significant obstacle in treating neurological conditions, necessitating chemical modifications or advanced drug delivery methods for most drugs to cross the NVU. A deeper understanding of NVU in health and disease will aid in the identification of new therapeutic targets and drug delivery strategies for improved treatment of neurological disorders.To achieve this goal, we need models that reflect the human BBB and NVU in health and disease. Although animal models of the brain's vasculature have proven valuable, they are often of limited translational relevance due to interspecies differences or inability to faithfully mimic human disease conditions. For this reason, human in vitro models are essential to improve our understanding of the brain's vasculature under healthy and diseased conditions. This review delves into the advancements in in vitro modeling of the BBB and NVU, with a particular focus on microfluidic models. After providing a historical overview of the field, we shift our focus to recent developments, offering insights into the latest achievements and their associated constraints. We briefly examine the importance of chip materials and methods to facilitate fluid flow, emphasizing their critical roles in achieving the necessary throughput for the integration of microfluidic models into routine experimentation. Subsequently, we highlight the recent strides made in enhancing the biological complexity of microfluidic NVU models and propose recommendations for elevating the biological relevance of future iterations.Importantly, the NVU is an intricate structure and it is improbable that any model will fully encompass all its aspects. Fit-for-purpose models offer a valuable compromise between physiological relevance and ease-of-use and hold the future of NVU modeling: as simple as possible, as complex as needed.

Pericyte Loss in Diseases

Pericytes are specialized cells located in close proximity to endothelial cells within the microvasculature. They play a crucial role in regulating blood flow, stabilizing vessel walls, and maintaining the integrity of the blood-brain barrier. The loss of pericytes has been associated with the development and progression of various diseases, such as diabetes, Alzheimer's disease, sepsis, stroke, and traumatic brain injury. This review examines the detection of pericyte loss in different diseases, explores the methods employed to assess pericyte coverage, and elucidates the potential mechanisms contributing to pericyte loss in these pathological conditions. Additionally, current therapeutic strategies targeting pericytes are discussed, along with potential future interventions aimed at preserving pericyte function and promoting disease mitigation.

The platelet transcriptome and proteome in Alzheimer's disease and aging: an exploratory cross-sectional study

Introduction: Alzheimer's disease (AD) and aging are associated with platelet hyperactivity. However, the mechanisms underlying abnormal platelet function in AD and aging are yet poorly understood. Methods: To explore the molecular profile of AD and aged platelets, we investigated platelet activation (i.e., CD62P expression), proteome and transcriptome in AD patients, non-demented elderly, and young individuals as controls. Results: AD, aged and young individuals showed similar levels of platelet activation based on CD62P expression. However, AD and aged individuals had a proteomic signature suggestive of increased platelet activation compared with young controls. Transcriptomic profiling suggested the dysregulation of proteolytic machinery involved in regulating platelet function, particularly the ubiquitin-proteasome system in AD and autophagy in aging. The functional implication of these transcriptomic alterations remains unclear and requires further investigation. Discussion: Our data strengthen the evidence of enhanced platelet activation in aging and provide a first glimpse of the platelet transcriptomic changes occurring in AD.

The association of astrogliosis and microglial activation with aging and Alzheimer's disease pathology in the chimpanzee brain

Aging and neurodegenerative disorders, such as Alzheimer's disease (AD), trigger an immune response known as glial activation in the brain. Recent evidence indicates species differences in inflammatory responses to AD pathology, highlighting the need for additional comparative studies to further understand human-specific neuropathologies. In the present study, we report on the occurrence of astrogliosis, microglial activation, and their relationship with age and AD-like pathology in a cohort of male and female chimpanzees (Pan troglodytes). Chimpanzees with severe astrogliosis exhibited widespread upregulation of hypertrophic astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) throughout all layers of the dorsolateral prefrontal cortex and a loss of the interlaminar palisade. In addition, extreme astrogliosis was associated with increased astrocyte density in the absence of significant microglial activation and AD lesions. A shift from decreased resting to increased phagocytotic microglia occurred with aging, although proliferation was absent and no changes in astrogliosis was observed. Vascular amyloid correlated with decreased astrocyte and microglia densities, while tau lesions were associated with morphological changes in microglia and greater total glia density and glia: neuron ratio. These results further our understanding of inflammatory processes within the chimpanzee brain and provide comparative data to improve our understanding of human aging and neuropathological processes.

Immune-mediated platelet depletion augments Alzheimer's disease neuropathological hallmarks in APP-PS1 mice

In Alzheimer's disease (AD), platelets become dysfunctional and might contribute to amyloid beta deposition. Here, we depleted platelets in one-year-old APP Swedish PS1 dE9 (APP-PS1) transgenic mice for five days, using intraperitoneal injections of an anti-CD42b antibody, and assessed changes in cerebral amyloidosis, plaque-associated neuritic dystrophy and gliosis. In APP-PS1 female mice, platelet depletion shifted amyloid plaque size distribution towards bigger plaques and increased neuritic dystrophy in the hippocampus. In platelet-depleted females, plaque-associated Iba1+ microglia had lower amounts of fibrillar amyloid beta cargo and GFAP+ astrocytic processes showed a higher overlap with thioflavin S+ amyloid plaques. In contrast to the popular hypothesis that platelets foster plaque pathology, our data suggest that platelets might limit plaque growth and attenuate plaque-related neuritic dystrophy at advanced stages of amyloid plaque pathology in APP-PS1 female mice. Whether the changes in amyloid plaque pathology are due to a direct effect on amyloid beta deposition or are a consequence of altered glial function needs to be further elucidated.

Dimethyl Fumarate Alleviates Adult Neurogenesis Disruption in Hippocampus and Olfactory Bulb and Spatial Cognitive Deficits Induced by Intracerebroventricular Streptozotocin Injection in Young and Aged Rats

The disorder of adult neurogenesis is considered an important mechanism underlying the learning and memory impairment observed in Alzheimer's disease (AD). The sporadic nonhereditary form of AD (sAD) affects over 95% of AD patients and is related to interactions between genetic and environmental factors. An intracerebroventricular injection of streptozotocin (STZ-ICV) is a representative and well-established method to induce sAD-like pathology. Dimethyl fumarate (DMF) has antioxidant and anti-inflammatory properties and is used for multiple sclerosis treatment. The present study determines whether a 26-day DMF therapy ameliorates the disruption of adult neurogenesis and BDNF-related neuroprotection in the hippocampus and olfactory bulb (OB) in an STZ-ICV rat model of sAD. Considering age as an important risk factor for developing AD, this study was performed using 3-month-old (the young group) and 22-month-old (the aged group) male Wistar rats. Spatial cognitive functions were evaluated with the Morris water maze task. Immunofluorescent labelling was used to assess the parameters of adult neurogenesis and BDNF-related neuroprotection in the hippocampus and OB. Our results showed that the STZ-ICV evoked spatial learning and memory impairment and disturbances in adult neurogenesis and BDNF expression in both examined brain structures. In the aged animals, the deficits were more severe. We found that the DMF treatment significantly alleviated STZ-ICV-induced behavioural and neuronal disorders in both age groups of the rats. Our findings suggest that DMF, due to its beneficial effect on the formation of new neurons and BDNF-related neuroprotection, may be considered as a promising new therapeutic agent in human sAD.

Ultrastructural characterization of dark microglia during aging in a mouse model of Alzheimer's disease pathology and in human post-mortem brain samples

A diverse heterogeneity of microglial cells was previously described in Alzheimer's disease (AD) pathology, including dark microglia, a state characterized by ultrastructural markers of cellular stress. To provide novel insights into the roles of dark microglia during aging in the context of AD pathology, we performed a quantitative density and ultrastructural analysis of these cells using high-throughput scanning electron microscopy in the ventral hippocampus CA1 stratum lacunosum-moleculare of 20-month-old APP-PS1 vs C57BL/6J male mice. The density of dark microglia was significantly higher in APP-PS1 vs C57BL/6J mice, with these cells accounting for nearly half of all microglia observed near amyloid-beta (Aβ) plaques. This dark microglial state interacted more with dystrophic neurites compared to other APP-PS1 microglia and possessed glycogen granules, associated with a metabolic shift toward glycolysis, which provides the first ultrastructural evidence of their presence in microglia. Dark microglia were further observed in aging human post-mortem brain samples showing similar ultrastructural features as in mouse. Overall, our results provide a quantitative ultrastructural characterization of a microglial state associated with cellular stress (i.e., dark microglia) that is primarily restricted near Aβ plaques and dystrophic neurites. The presence of this microglial state in the aging human post-mortem brain is further revealed.

Induction of ICAM1 in Brain Vessels is Implicated in an Early AD Pathogenesis by Modulating Neprilysin

Intercellular adhesion molecule 1 (ICAM1) is a vessel adhesion protein induced during brain vascular inflammation, which could be closely linked with the development of Alzheimer's disease (AD). This study investigated the effect of ICAM1 on amyloid-degrading enzymes (ADEs) in endothelial cells and their potential involvement in inflammation and AD progression. TNF-α treatment increased ICAM1 in human brain microvascular endothelial cells (HBMVECs) but decreased the neprilysin (NEP) protein level. Knock-down of ICAM1 using siRNA enhanced NEP, which increased the degradation of amyloid-β. In the brains of 4-month-old AD transgenic mice (APPswe/PSEN1dE9), there were significantly higher levels of ICAM1 expression and amyloid deposits but lower levels of NEP and insulin-degrading enzymes (IDE), demonstrating an inverse correlation of ICAM1 with NEP and IDE expression. Further studies demonstrated significantly increased GFAP protein levels in the brain, specifically localized near blood vessels, of both TNF-α-injected and 4-month-old AD transgenic mice. Taken together, the induction of ICAM1 in endothelial cells suppresses NEP expression, accelerating the accumulation of amyloid-β in blood vessels. It also enhances leukocyte adhesion to blood vessels stimulating the migration of leukocytes into the brain, subsequently triggering brain inflammation.

Transcranial Direct Current Stimulation Alleviates Neurovascular Unit Dysfunction in Mice With Preclinical Alzheimer’s Disease

Neurons, glial cells and blood vessels are collectively referred to as the neurovascular unit (NVU). In the Alzheimer’s disease (AD) brain, the main components of the NVU undergo pathological changes. Transcranial direct current stimulation (tDCS) can protect neurons, induce changes in glial cells, regulate cerebral blood flow, and exert long-term neuroprotection. However, the mechanism by which tDCS improves NVU function is unclear. In this study, we explored the effect of tDCS on the NVU in mice with preclinical AD and the related mechanisms. 10 sessions of tDCS were given to six-month-old male APP/PS1 mice in the preclinical stage. The model group, sham stimulation group, and control group were made up of APP/PS1 mice and C57 mice of the same age. All mice were histologically evaluated two months after receiving tDCS. Protein content was measured using Western blotting and an enzyme-linked immunosorbent assay (ELISA). The link between glial cells and blood vessels was studied using immunofluorescence staining and lectin staining. The results showed that tDCS affected the metabolism of Aβ; the levels of Aβ, amyloid precursor protein (APP) and BACE1 were significantly reduced, and the levels of ADAM10 were significantly increased in the frontal cortex and hippocampus in the stimulation group. In the stimulation group, tDCS reduced the protein levels of Iba1 and GFAP and increased the protein levels of NeuN, LRP1 and PDGRFβ. This suggests that tDCS can improve NVU function in APP/PS1 mice in the preclinical stage. Increased blood vessel density and blood vessel length, decreased IgG extravasation, and increased the protein levels of occludin and coverage of astrocyte foot processes with blood vessels suggested that tDCS had a protective effect on the blood-brain barrier. Furthermore, the increased numbers of Vimentin, S100 expression and blood vessels (lectin-positive) around Aβ indicated that the effect of tDCS was mediated by astrocytes and blood vessels. There was no significant difference in these parameters between the model group and the sham stimulation group. In conclusion, our results show that tDCS can improve NVU function in APP/PS1 mice in the preclinical stage, providing further support for the use of tDCS as a treatment for AD.

Neurovascular unit dysfunction as a mechanism of seizures and epilepsy during aging

The term neurovascular unit (NVU) describes the structural and functional liaison between specialized brain endothelium, glial and mural cells, and neurons. Within the NVU, the blood‐brain barrier (BBB) is the microvascular structure regulating neuronal physiology and immune cross‐talk, and its properties adapt to brain aging. Here, we analyze a research framework where NVU dysfunction, caused by acute insults or disease progression in the aging brain, represents a converging mechanism underlying late‐onset seizures or epilepsy and neurological or neurodegenerative sequelae. Furthermore, seizure activity may accelerate brain aging by sustaining regional NVU dysfunction, and a cerebrovascular pathology may link seizures to comorbidities. Next, we focus on NVU diagnostic approaches that could be tailored to seizure conditions in the elderly. We also examine the impending disease‐modifying strategies based on the restoration of the NVU and, more in general, the homeostatic control of anti‐ and pro‐inflammatory players. We conclude with an outlook on current pre‐clinical knowledge gaps and clinical challenges pertinent to seizure onset and conditions in an aging population.

Dimethyl fumarate does not mitigate cognitive decline and β-amyloidosis in female APPPS1 mice

INTRODUCTION

Alzheimer's disease (AD) is the leading cause of dementia and a major global health issue. Currently, only limited treatment options are available to patients. One possibility to expand the treatment repertoire is repurposing of existing drugs such as dimethyl fumarate (DMF). DMF is approved for treatment of multiple sclerosis and previous animal studies have suggested that DMF may also have a beneficial effect for the treatment of AD.

METHODS

We used an APPPS1 transgenic model of senile β-amyloidosis and treated female mice orally with DMF in two treatment paradigms (pre and post onset). We quantified learning and memory parameters, β-amyloidosis, and neuroinflammation to determine the potential of DMF as AD therapeutics.

RESULTS

Treatment with DMF had no influence on water maze performance, β-amyloid accumulation, plaque formation, microglia activation, and recruitment of immune cells to the brain. Compared to vehicle-treated animals, oral DMF treatment could not halt or retard disease progression in the mice.

DISCUSSION

Our results do not favour the use of DMF as treatment for AD. While our results stand in contrast to previous findings in other models, they emphasize the importance of animal model selection and suggest further studies to elucidate the mechanisms leading to conflicting results.

Causes and consequences of baseline cerebral blood flow reductions in Alzheimer's disease

Reductions of baseline cerebral blood flow (CBF) of ∼10-20% are a common symptom of Alzheimer's disease (AD) that appear early in disease progression and correlate with the severity of cognitive impairment. These CBF deficits are replicated in mouse models of AD and recent work shows that increasing baseline CBF can rapidly improve the performance of AD mice on short term memory tasks. Despite the potential role these data suggest for CBF reductions in causing cognitive symptoms and contributing to brain pathology in AD, there remains a poor understanding of the molecular and cellular mechanisms causing them. This review compiles data on CBF reductions and on the correlation of AD-related CBF deficits with disease comorbidities (e.g. cardiovascular and genetic risk factors) and outcomes (e.g. cognitive performance and brain pathology) from studies in both patients and mouse models, and discusses several potential mechanisms proposed to contribute to CBF reductions, based primarily on work in AD mouse models. Future research aimed at improving our understanding of the importance of and interplay between different mechanisms for CBF reduction, as well as at determining the role these mechanisms play in AD patients could guide the development of future therapies that target CBF reductions in AD.

Picturing Breast Cancer Brain Metastasis Development to Unravel Molecular Players and Cellular Crosstalk

Simple Summary

Breast cancer is a devastating disorder affecting millions of women worldwide. With improved therapeutics for the primary tumor, the appearance of metastasis has been increasing. Breast cancer frequently metastasizes to the brain, constituting a major hurdle without cure and with a poor survival. It is imperative to better understand the mechanisms involved in malignant cell transposition of the brain microvasculature and parenchymal colonization by deciphering the alterations occurring in the tumor and microvascular cells, as well as the occurrence of intercellular communication during the process. We aimed to profile the process of the formation of breast cancer brain metastasis and the timeline of events governing it. We used a specific mouse model of the disease to perform extensive microscopic analyses. We identified phenotypic changes and the activation of relevant molecular players in tumorigenesis, together with vascular alterations, and the occurrence of crosstalk. Our findings unravel putative therapeutic targets to tackle breast cancer brain metastasis.

Abstract

With breast cancer (BC) therapy improvements, the appearance of brain metastases has been increasing, representing a life-threatening condition. Brain metastasis formation involves BC cell (BCC) extravasation across the blood–brain barrier (BBB) and brain colonization by unclear mechanisms. We aimed to disclose the actors involved in BC brain metastasis formation, focusing on BCCs’ phenotype, growth factor expression, and signaling pathway activation, correlating with BBB alterations and intercellular communication. Hippocampi of female mice inoculated with 4T1 BCCs were examined over time by hematoxylin-eosin, immunohistochemistry and immunofluorescence. Well-established metastases were observed at seven days, increasing thereafter. BCCs entering brain parenchyma presented mesenchymal, migratory, and proliferative features; however, with time, they increasingly expressed epithelial markers, reflecting a mesenchymal–epithelial transition. BCCs also expressed platelet-derived growth factor-B, β₄ integrin, and focal adhesion kinase, suggesting autocrine and/or paracrine regulation with adhesion signaling activation, while balance between Rac1 and RhoA was associated with the motility status. Intercellular communication via gap junctions was clear among BCCs, and between BCCs and endothelial cells. Thrombin accumulation, junctional protein impairment, and vesicular proteins increase reflect BBB alterations related with extravasation. Expression of plasmalemma vesicle-associated protein was increased in BCCs, along with augmented vascularization, whereas pericyte contraction indicated mural cells’ activation. Our results provide further understanding of BC brain metastasis formation, disclosing potential therapeutic targets.

Impact of aging and comorbidities on ischemic stroke outcomes in preclinical animal models: A translational perspective

Ischemic stroke is a highly complex and devastating neurological disease. The sudden loss of blood flow to a brain region due to an ischemic insult leads to severe damage to that area resulting in the formation of an infarcted tissue, also known as the ischemic core. This is surrounded by the peri-infarct region or penumbra that denotes the functionally impaired but potentially salvageable tissue. Thus, the penumbral tissue is the main target for the development of neuroprotective strategies to minimize the extent of ischemic brain damage by timely therapeutic intervention. Given the limitations of reperfusion therapies with recombinant tissue plasminogen activator or mechanical thrombectomy, there is high enthusiasm to combine reperfusion therapy with neuroprotective strategies to further reduce the progression of ischemic brain injury. Till date, a large number of candidate neuroprotective drugs have been identified as potential therapies based on highly promising results from studies in rodent ischemic stroke models. However, none of these interventions have shown therapeutic benefits in stroke patients in clinical trials. In this review article, we discussed the urgent need to utilize preclinical models of ischemic stroke that more accurately mimic the clinical conditions in stroke patients by incorporating aged animals and animal stroke models with comorbidities. We also outlined the recent findings that highlight the significant differences in stroke outcome between young and aged animals, and how major comorbid conditions such as hypertension, diabetes, obesity and hyperlipidemia dramatically increase the vulnerability of the brain to ischemic damage that eventually results in worse functional outcomes. It is evident from these earlier studies that including animal models of aging and comorbidities during the early stages of drug development could facilitate the identification of neuroprotective strategies with high likelihood of success in stroke clinical trials.

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.

Blood-brain barrier dysfunction in aging induces hyperactivation of TGFβ signaling and chronic yet reversible neural dysfunction

Aging involves a decline in neural function that contributes to cognitive impairment and disease. However, the mechanisms underlying the transition from a young-and-healthy to aged-and-dysfunctional brain are not well understood. Here, we report breakdown of the vascular blood-brain barrier (BBB) in aging humans and rodents, which begins as early as middle age and progresses to the end of the life span. Gain-of-function and loss-of-function manipulations show that this BBB dysfunction triggers hyperactivation of transforming growth factor-β (TGFβ) signaling in astrocytes, which is necessary and sufficient to cause neural dysfunction and age-related pathology in rodents. Specifically, infusion of the serum protein albumin into the young rodent brain (mimicking BBB leakiness) induced astrocytic TGFβ signaling and an aged brain phenotype including aberrant electrocorticographic activity, vulnerability to seizures, and cognitive impairment. Furthermore, conditional genetic knockdown of astrocytic TGFβ receptors or pharmacological inhibition of TGFβ signaling reversed these symptomatic outcomes in aged mice. Last, we found that this same signaling pathway is activated in aging human subjects with BBB dysfunction. Our study identifies dysfunction in the neurovascular unit as one of the earliest triggers of neurological aging and demonstrates that the aging brain may retain considerable latent capacity, which can be revitalized by therapeutic inhibition of TGFβ signaling.

Synaptic Loss, ER Stress and Neuro-Inflammation Emerge Late in the Lateral Temporal Cortex and Associate with Progressive Tau Pathology in Alzheimer's Disease

The complex multifactorial nature of AD pathogenesis has been highlighted by evidence implicating additional neurodegenerative mechanisms, beyond that of amyloid-β (Aβ) and tau. To provide insight into cause and effect, we here investigated the temporal profile and associations of pathological changes in synaptic, endoplasmic reticulum (ER) stress and neuro-inflammatory markers. Quantifications were established via immunoblot and immunohistochemistry protocols in post-mortem lateral temporal cortex (n = 46). All measures were assessed according to diagnosis (non-AD vs. AD), neuropathological severity (low (Braak ≤ 2) vs. moderate (3–4) vs. severe (≥ 5)) and individual Braak stage, and were correlated with Aβ and tau pathology and cognitive scores. Postsynaptic PSD-95, but not presynaptic synaptophysin, was decreased in AD cases and demonstrated a progressive decline across disease severity and Braak stage, yet not with cognitive scores. Of all investigated ER stress markers, only phospho-protein kinase RNA-like ER kinase (p-PERK) correlated with Braak stage and was increased in diagnosed AD cases. A similar relationship was observed for the astrocytic glial fibrillary acidic protein (GFAP); however, the associated aquaporin 4 and microglial Iba1 remained unchanged. Pathological alterations in these markers preferentially correlated with measures of tau over those related to Aβ. Notably, GFAP also correlated strongly with Aβ markers and with all assessments of cognition. Lateral temporal cortex-associated synaptic, ER stress and neuro-inflammatory pathologies are here determined as late occurrences in AD progression, largely associated with tau pathology. Moreover, GFAP emerged as the most robust indicator of disease progression, tau/Aβ pathology, and cognitive impairment.

Inhibition of formyl peptide receptors improves the outcome in a mouse model of Alzheimer disease

Background

An important hallmark of Alzheimer’s disease (AD) is the increase of Aβ1-42 burden and its accumulation to senile plaques, leading the reactive gliosis and neurodegeneration. The modulation of glia cell function represents an attractive therapeutic strategy, but is currently limited by an incomplete understanding of its relevance for AD. The chemotactic G-protein coupled formyl peptide receptor (FPR), which is known to modulate Aβ1-42 uptake and signal transduction, might be one candidate molecule regulating glia function in AD. Here, we investigate whether the modulation of FPR exerts beneficial effects in an AD preclinical model.

Methods

To address this question, APP/PS1 double-transgenic AD mice were treated for 20 weeks with either the pro-inflammatory FPR agonist fMLF, the FPR1/2 antagonist Boc2 or the anti-inflammatory FPR2 agonist Ac2-26. Spatial learning and memory were evaluated using a Morris water maze test. Immunohistological staining, gene expression studies, and flow cytometry analyses were performed to study neuronal loss, gliosis, and Aß-load in the hippocampus and cortex, respectively.

Results

FPR antagonism by Boc2-treatment significantly improved spatial memory performance, reduced neuronal pathology, induced the expression of homeostatic growth factors, and ameliorated microglia, but not astrocyte, reactivity. Furthermore, the elevated levels of amyloid plaques in the hippocampus were reduced by Boc2-treatment, presumably by an induction of amyloid degradation.

Conclusions

We suggest that the modulation of FPR signaling cascades might be considered as a promising therapeutic approach for alleviating the cognitive deficits associated with early AD. Additional studies are now needed to address the downstream effectors as well as the safety profile of Boc2.

Perivascular Unit: This Must Be the Place. The Anatomical Crossroad Between the Immune, Vascular and Nervous System

Most neurological disorders seemingly have heterogenous pathogenesis, with overlapping contribution of neuronal, immune and vascular mechanisms of brain injury. The perivascular space in the brain represents a crossroad where those mechanisms interact, as well as a key anatomical component of the recently discovered glymphatic pathway, which is considered to play a crucial role in the clearance of brain waste linked to neurodegenerative diseases. The pathological interplay between neuronal, immune and vascular factors can create an environment that promotes self-perpetration of mechanisms of brain injury across different neurological diseases, including those that are primarily thought of as neurodegenerative, neuroinflammatory or cerebrovascular. Changes of the perivascular space can be monitored in humans in vivo using magnetic resonance imaging (MRI). In the context of glymphatic clearance, MRI-visible enlarged perivascular spaces (EPVS) are considered to reflect glymphatic stasis secondary to the perivascular accumulation of brain debris, although they may also represent an adaptive mechanism of the glymphatic system to clear them. EPVS are also established correlates of dementia and cerebral small vessel disease (SVD) and are considered to reflect brain inflammatory activity. In this review, we describe the “perivascular unit” as a key anatomical and functional substrate for the interaction between neuronal, immune and vascular mechanisms of brain injury, which are shared across different neurological diseases. We will describe the main anatomical, physiological and pathological features of the perivascular unit, highlight potential substrates for the interplay between different noxae and summarize MRI studies of EPVS in cerebrovascular, neuroinflammatory and neurodegenerative disorders.

Platelets in Amyloidogenic Mice Are Activated and Invade the Brain

Alzheimer’s disease (AD) is a neurodegenerative disease with a complex and not fully understood pathogenesis. Besides brain-intrinsic hallmarks such as abnormal deposition of harmful proteins, i.e., amyloid beta in plaques and hyperphosphorylated Tau in neurofibrillary tangles, blood-derived elements, in particular, platelets have been discussed to be involved in AD pathogenesis. The underlying mechanisms, however, are rather unexplored. Here, we investigate a potential role of platelets in an AD transgenic animal model with severe amyloid plaque formation, the APP-PS1 transgenic mice, and analyzed the presence, spatial location and activation status of platelets within the brain. In APP-PS1 mice, a higher number of platelets were located within the brain parenchyma, i.e., outside the cerebral blood vessels compared to WT controls. Such platelets were activated according to the expression of the platelet activation marker CD62P and to morphological hallmarks such as membrane protrusions. In the brain, platelets were in close contact exclusively with astrocytes suggesting an interaction between these two cell types. In the bloodstream, although the percentage of activated platelets did not differ between transgenic and age-matched control animals, APP-PS1 blood-derived platelets showed remarkable ultrastructural peculiarities in platelet-specific organelles such as the open canalicular system (OCS). This work urges for further investigations on platelets and their yet unknown functional roles in the brain, which might go beyond AD pathogenesis and be relevant for various age-related neurodegenerative diseases.

Neuroprotective Effect of ent-Kaur-15-en-17-al-18-oic Acid on Amyloid Beta Peptide-Induced Oxidative Apoptosis in Alzheimer's Disease

ent-Kaur-15-en-17-al-18-oic acid, extracted from the Chinese well known folk herb Leontopodium longifolium, performed a significantly neuroprotective effect on amyloid beta peptide 25-35 (Aβ_25-35)-induced SH-SY5Y cells neurotoxicity in Alzheimer's disease. The results demonstrated that this compound maintained oxidative stress balance, reduced levels of reactive oxygen species (ROS), malondialdehyde (MDA), and improved contents of glutathione (GSH) and superoxide dismutase (SOD) without obvious cytotoxicity. This compound also obviously relieved oxidative stress-induced apoptosis associated with p53 and nuclear factor κB (NF-κB) pathways accompanied by upregulating B-cell lymphoma-2 (bcl-2) and downregulating p53, nuclear factor κB (NF-κB), Bax, Cleaved-caspase 3, and Cytochrome C protein expressions further. Briefly, ent-kaur-15-en-17-al-18-oic acid protected cells from oxidative apoptosis associated with p53 and NF-κB pathways.

NVP-BEZ235 (Dactolisib) Has Protective Effects in a Transgenic Mouse Model of Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease and the main cause of dementia. Its major symptom is memory loss, which is a result of neuronal cell death, which is accompanied by neuroinflammation. Some studies indicate the overactivation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mechanistic target of rapamycin (mTOR) pathway in this disease, being, thus, a potential target for pharmacological treatment. Here, we used a transgenic mouse model of AD that expresses a mutant amyloid-β precursor protein (T41 mice) to investigate the effects of dactolisib (alternative name: NVP-BEZ235, abbreviation BEZ), a dual PI3K/mTOR inhibitor. Ten-months-old T41 animals were treated for 14 days with BEZ or vehicle via oral gavage and then submitted to social memory, open field and contextual conditioned fear tests. Hippocampal slices were prepared and Aβ_1-42 content, NeuN, Iba-1, CD68 and GFAP were evaluated. Tissues were further processed to evaluate cytokines levels through cytometric bead array. The treatment with BEZ (5 mg/kg) reduced social memory impairment in T41 mice. However, BEZ did not have any effect on altered Aβ levels, NeuN, or GFAP staining. The drug reduced the CD68/Iba-1 ratio in CA3 region of hippocampus. Finally, BEZ diminished IL-10 levels in T41 mice. Thus, although its mechanisms are not clear, BEZ protects against memory impairment, reduces microglial activation and reestablishes IL-10 levels, revealing beneficial effects, which should be further investigated for the treatment of AD.

A positive allosteric modulator of mGluR5 promotes neuroprotective effects in mouse models of Alzheimer's disease

Alzheimer's Disease (AD) is the most prevalent neurodegenerative disorder. Despite advances in the understanding of its pathophysiology, none of the available therapies prevents disease progression. Excess glutamate plays an important role in excitotoxicity by activating ionotropic receptors. However, the mechanisms modulating neuronal cell survival/death via metabotropic glutamate receptors (mGluRs) are not completely understood. Recent data indicates that CDPPB, a positive allosteric modulator of mGluR5, has neuroprotective effects. Thus, this work aimed to investigate CDPPB treatment effects on amyloid-β (Aβ) induced pathological alterations in vitro and in vivo and in a transgenic mouse model of AD (T41 mice). Aβ induced cell death in primary cultures of hippocampal neurons, which was prevented by CDPPB. Male C57BL/6 mice underwent stereotaxic surgery for unilateral intra-hippocampal Aβ injection, which induced memory deficits, neurodegeneration, neuronal viability reduction and decrease of doublecortin-positive cells, a marker of immature neurons and neuronal proliferation. Treatment with CDPPB for 8 days reversed neurodegeneration and doublecortin-positive cells loss and recovered memory function. Fourteen months old T41 mice presented cognitive deficits, neuronal viability reduction, gliosis and Aβ accumulation. Treatment with CDPPB for 28 days increased neuronal viability (32.2% increase in NeuN⁺ cells) and reduced gliosis in CA1 region (Iba-1⁺ area by 31.3% and GFAP⁺ area by 37.5%) in transgenic animals, without inducing hepatotoxicity. However, it did not reverse cognitive deficit. Despite a four-week treatment did not prevent memory loss in aged transgenic mice, CDPPB is protective against Aβ stimulus. Therefore, this drug represents a potential candidate for further investigations as AD treatment.

The effects of endothelium-specific CYP2J2 overexpression on the attenuation of retinal ganglion cell apoptosis in a glaucoma rat model

Glaucoma is a leading cause of irreversible blindness worldwide. Vascular factors play a substantial role in the pathogenesis of glaucoma. Expressed in the vascular endothelium, cytochrome P450 (CYP) 2J2 is one of the CYP epoxygenases that metabolize arachidonic acid to produce epoxyeicosatrienoic acids and exert pleiotropic protective effects on the vasculature. In the present study, we investigated whether endothelium-specific overexpression of CYP2J2 (tie2-CYP2J2-Tr) protects against retinal ganglion cell (RGC) loss induced by glaucoma and in what way retinal vessels are involved in this process. We used a glaucoma model of retinal ischemia-reperfusion (I/R) injury in rats and found that endothelium-specific overexpression of CYP2J2 attenuated RGC loss induced by retinal I/R. Moreover, retinal I/R triggered retinal vascular senescence, indicated by up-regulated senescence-related proteins p53, p16, and β-galactosidase activity. The senescent endothelial cells resulted in pericyte loss and increased endothelial secretion of matrix metallopeptidase 9, which further contributed to RGC loss. CYP2J2 overexpression alleviated vascular senescence, pericyte loss, and matrix metallopeptidase 9 secretion. CYP2J2 suppressed endothelial senescence by down-regulating senescence-associated proteins p53 and p16. These 2 proteins were positively regulated by microRNA-128-3p, which was inhibited by CYP2J2. These results suggest that CYP2J2 protects against endothelial senescence and RGC loss in glaucoma, a discovery that may lead to the development of a potential treatment strategy for glaucoma.-Huang, J., Zhao, Q., Li, M., Duan, Q., Zhao, Y., Zhang, H. The effects of endothelium-specific CYP2J2 overexpression on the attenuation of retinal ganglion cell apoptosis in a glaucoma rat model.

Astrocyte Function Is Affected by Aging and Not Alzheimer's Disease: A Preliminary Investigation in Hippocampi of 3xTg-AD Mice

Old age is a risk factor for Alzheimer's disease (AD), which is characterized by hippocampal impairment together with substantial changes in glial cell functions. Are these alterations due to the disease progression or are they a consequence of aging? To start addressing this issue, we studied the expression of specific astrocytic and microglial structural and functional proteins in a validated transgenic model of AD (3×Tg-AD). These mice develop both amyloid plaques and neurofibrillary tangles, and initial signs of the AD-like pathology have been documented as early as three months of age. We compared male 3×Tg-AD mice at 6 and 12 months of age with their wild-type age-matched counterparts. We also investigated neurons by examining the expression of both the microtubule-associated protein 2 (MAP2), a neuronal structural protein, and the brain-derived neurotrophic factor (BDNF). The latter is indeed a crucial indicator for synaptic plasticity and neurogenesis/neurodegeneration. Our results show that astrocytes are more susceptible to aging than microglia, regardless of mouse genotype. Moreover, we discovered significant age-dependent alterations in the expression of proteins responsible for astrocyte-astrocyte and astrocyte-neuron communication, as well as a significant age-dependent decline in BDNF expression. Our data promote further research on the unexplored role of astroglia in both physiological and pathological aging.

ADAMTS13 maintains cerebrovascular integrity to ameliorate Alzheimer-like pathology

Blood-brain barrier (BBB) defects and cerebrovascular dysfunction contribute to amyloid-β (Aβ) brain accumulation and drive Alzheimer disease (AD) pathology. By regulating vascular functions and inflammation in the microvasculature, a disintegrin and metalloprotease with thrombospondin type I motif, member 13 (ADAMTS13) plays a significant protective effect in atherosclerosis and stroke. However, whether ADAMTS13 influences AD pathogenesis remains unclear. Using in vivo multiphoton microscopy, histological, behavioral, and biological methods, we determined BBB integrity, cerebrovascular dysfunction, amyloid accumulation, and cognitive impairment in APPPS1 mice lacking ADAMTS13. We also tested the impact of viral-mediated expression of ADAMTS13 on cerebrovascular function and AD-like pathology in APPPS1 mice. We show that ADAMTS13 deficiency led to an early and progressive BBB breakdown as well as reductions in vessel density, capillary perfusion, and cerebral blood flow in APPPS1 mice. We found that deficiency of ADAMTS13 increased brain plaque load and Aβ levels and accelerated cerebral amyloid angiopathy (CAA) by impeding BBB-mediated clearance of brain Aβ, resulting in worse cognitive decline in APPPS1 mice. Virus-mediated expression of ADAMTS13 attenuated BBB disruption and increased microvessels, capillary perfusion, and cerebral blood flow in APPPS1 mice already showing BBB damage and plaque deposition. These beneficial vascular effects were reflected by increase in clearance of cerebral Aβ, reductions in Aβ brain accumulation, and improvements in cognitive performance. Our results show that ADAMTS13 deficiency contributes to AD cerebrovascular dysfunction and the resulting pathogenesis and cognitive deficits and suggest that ADAMTS13 may offer novel therapeutic opportunities for AD.

Astrocytic changes with aging and Alzheimer's disease-type pathology in chimpanzees

Astrocytes are the main homeostatic cell of the central nervous system. In addition, astrocytes mediate an inflammatory response when reactive to injury or disease known as astrogliosis. Astrogliosis is marked by an increased expression of glial fibrillary acidic protein (GFAP) and cellular hypertrophy. Some degree of astrogliosis is associated with normal aging and degenerative conditions such as Alzheimer's disease (AD) and other dementing illnesses in humans. The recent observation of pathological markers of AD (amyloid plaques and neurofibrillary tangles) in aged chimpanzee brains provided an opportunity to examine the relationships among aging, AD-type pathology, and astrocyte activation in our closest living relatives. Stereologic methods were used to quantify GFAP-immunoreactive astrocyte density and soma volume in layers I, III, and V of the prefrontal and middle temporal cortex, as well as in hippocampal fields CA1 and CA3. We found that the patterns of astrocyte activation in the aged chimpanzee brain are distinct from humans. GFAP expression does not increase with age in chimpanzees, possibly indicative of lower oxidative stress loads. Similar to humans, chimpanzee layer I astrocytes in the prefrontal cortex are susceptible to AD-like changes. Both prefrontal cortex layer I and hippocampal astrocytes exhibit a high degree of astrogliosis that is positively correlated with accumulation of amyloid beta and tau proteins. However, unlike humans, chimpanzees do not display astrogliosis in other cortical layers. These results demonstrate a unique pattern of cortical aging in chimpanzees and suggest that inflammatory processes may differ between humans and chimpanzees in response to pathology.

Aging Exacerbates Neuroinflammatory Outcomes Induced by Acute Ozone Exposure

The role of environmental stressors, particularly exposure to air pollution, in the development of neurodegenerative disease remains underappreciated. We examined the neurological effects of acute ozone (O3) exposure in aged mice, where increased blood-brain barrier (BBB) permeability may confer vulnerability to neuroinflammatory outcomes. C57BL/6 male mice, aged 8-10 weeks or 12-18 months were exposed to either filtered air or 1.0 ppm O3 for 4 h; animals received a single IP injection of sodium fluorescein (FSCN) 20 h postexposure. One-hour post-FSCN injection, animals were transcardially perfused for immunohistochemical analysis of BBB permeability. β-amyloid protein expression was assessed via ELISA. Flow cytometric characterization of infiltrating immune cells, including neutrophils, macrophages, and microglia populations was performed 20 h post-O3 exposure. Flow cytometry analysis of brains revealed increased microglia "activation" and presentation of CD11b, F4/80, and MHCII in aged animals relative to younger ones; these age-induced differences were potentiated by acute O3 exposure. Cortical and limbic regions in aged brains had increased reactive microgliosis and β-amyloid protein expression after O3 insult. The aged cerebellum was particularly vulnerable to acute O3 exposure with increased populations of infiltrating neutrophils, peripheral macrophages/monocytes, and Ly6C+ inflammatory monocytes after insult, which were not significantly increased in the young cerebellum. O3 exposure increased the penetration of FSCN beyond the BBB, the infiltration of peripheral immune cells, and reactive gliosis of microglia. Thus, the aged BBB is vulnerable to insult and becomes highly penetrable in response to O3 exposure, leading to greater neuroinflammatory outcomes.

Dysregulation of sonic hedgehog pathway and pericytes in the brain after lentiviral infection

BACKGROUND

Impairment of the blood-brain barrier (BBB) has been associated with cognitive decline in many CNS diseases, including HIV-associated neurocognitive disorders (HAND). Recent research suggests an important role for the Sonic hedgehog (Shh) signaling pathway in the maintenance of BBB integrity under both physiological and pathological conditions.

METHODS

In the present study, we sought to examine the expression of Shh and its downstream effectors in relation to brain pericytes and BBB integrity in HIV-infected humans and rhesus macaques infected with simian immunodeficiency virus (SIV), an animal model of HIV infection and CNS disease. Cortical brain tissues from uninfected (n = 4) and SIV-infected macaques with (SIVE, n = 6) or without encephalitis (SIVnoE, n = 4) were examined using multi-label, semi-quantitative immunofluorescence microscopy of Shh, netrin-1, tight junction protein zona occludens 1 (ZO1), glial fibrillary acidic protein, CD163, platelet-derived growth factor receptor b (PDGFRB), glucose transporter 1, fibrinogen, and SIV Gag p28.

RESULTS

While Shh presence in the brain persisted during HIV/SIV infection, both netrin-1 immunoreactivity and the size of PDGFRB+ pericytes, a cellular source of netrin-1, were increased around non-lesion-associated vessels in encephalitis compared to uninfected brain or brain without encephalitis, but were completely absent in encephalitic lesions. Hypertrophied pericytes were strongly localized in areas of fibrinogen extravasation and showed the presence of intracellular SIVp28 and HIVp24 by immunofluorescence in all SIV and HIV encephalitis cases examined, respectively.

CONCLUSIONS

The lack of pericytes and netrin-1 in encephalitic lesions, in line with downregulation of ZO1 on the fenestrated endothelium, suggests that pericyte loss, despite the strong presence of Shh, contributes to HIV/SIV-induced BBB disruption and neuropathogenesis in HAND.

Growth Differentiation Factor 11 treatment leads to neuronal and vascular improvements in the hippocampus of aged mice

Aging is the biggest risk factor for several neurodegenerative diseases. Parabiosis experiments have established that old mouse brains are improved by exposure to young mouse blood. Previously, our lab showed that delivery of Growth Differentiation Factor 11 (GDF11) to the bloodstream increases the number of neural stem cells and positively affects vasculature in the subventricular zone of old mice. Our new study demonstrates that GDF11 enhances hippocampal neurogenesis, improves vasculature and increases markers of neuronal activity and plasticity in the hippocampus and cortex of old mice. Our experiments also demonstrate that systemically delivered GDF11, rather than crossing the blood brain barrier, exerts at least some of its effects by acting on brain endothelial cells. Thus, by targeting the cerebral vasculature, GDF11 has a very different mechanism from that of previously studied circulating factors acting to improve central nervous system (CNS) function without entering the CNS.

Aβ inhibits mesenchymal stem cell-pericyte transition through MAPK pathway

Multiple independent reports have demonstrated pericyte loss in both the hippocampus and cortex in human Alzheimer's disease (AD). The differentiation and recruitment of pericytes are the essential steps in vasculature development. However, the role of amyloid beta (Aβ) in pericyte differentiation has not yet been fully elucidated. In this study, we investigated the interaction between Aβ and differentiation of mesenchymal stem cells (MSCs) toward pericytes in culture. Our results showed that mice overexpressing Aβ-precursor protein (APP/PS1) exhibited the loss of pericytes compared with the control group mice, evidenced by the lack of desmin expression in the cortex of 12-month-old mice. Interestingly, we further found that both Aβ40 and Aβ42 inhibited the expressions of pericyte markers (α-SMA, desmin, and PDGFRβ) in cultured MSCs which can be differentiated into mature pericytes. Mechanistically, the inhibitory effects of Aβs on MSC-pericyte transition is mediated by the activation of the ERK1/2 MAPK signal pathway. These new insights into the roles of Aβ in pericyte differentiation may help to develop more effective strategies for the treatment of AD.

Age-associated changes in the blood-brain barrier: comparative studies in human and mouse

Aims

While vascular pathology is a common feature of a range of neurodegenerative diseases, we hypothesized that vascular changes occur in association with normal ageing. Therefore, we aimed to characterize age‐associated changes in the blood–brain barrier (BBB) in human and mouse cohorts.

Methods

Immunohistochemistry and Evans blue assays were used to characterize BBB dysfunction (tight junction protein expression and serum plasma protein accumulation), vascular pathology (pericyte loss and vascular density) and glial pathology (astrocyte and microglial density) in ageing neurological control human prefrontal cortex (a total of 23 cases from 5 age groups representing the spectrum of young adult to old age: 20–30 years, 31–45 years, 46–60 years, 61–75 years and 75+) and C57BL/6 mice (3 months, 12 months, 18 months and 24 months, n = 5/6 per group).

Results

Quantification of the tight junction protein ZO‐1 within the cortex and cerebellum of the mouse cohort showed a significant trend to both increased number (cortex P < 0.001, cerebellum P < 0.001) and length (cortex P < 0.001, cerebellum P < 0.001) of junctional breaks associated with increasing age. GFAP expression significantly correlated with ageing in the mice (P = 0.037). In the human cohort, assessment of human protein accumulation (albumin, fibrinogen and human IgG) demonstrated cells morphologically resembling clasmatodendritic astrocytes, indicative of BBB dysfunction. Semiquantitative assessment of astrogliosis in the cortex expression revealed an association with age (P = 0.003), while no age‐associated changes in microglial pathology, microvascular density or pericyte coverage were detected.

Conclusions

This study demonstrates BBB dysfunction in normal brain ageing, both in human and mouse cohorts.

Heterogeneity of the blood-brain barrier

The brain microvascular network is comprised of capillaries, arterioles and venules, all of which retain - although to a different extent - blood-brain barrier (BBB) properties. Capillaries constitute the largest and tightest microvasculature. In contrast, venules have a looser junctional arrangement, while arterioles have a lower expression of P-gp. Development and maintenance of the BBB depends on the interaction of cerebral endothelial cells with pericytes and astrocytes, which are all heterogeneous in different regions of the central nervous system. At the level of circumventricular organs microvessels are permeable, containing fenestrations and discontinuous tight junctions. In addition, the blood-spinal cord barrier - where the number of pericytes is lower and expression of junctional proteins is reduced - is also more permeable than the BBB. However, much less is known about the cellular, molecular and functional differences among other regions of the brain. This review summarizes our current knowledge on the heterogeneity of the brain microvasculature.

Dissecting the Contribution of Vascular Alterations and Aging to Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive decline that afflicts as many as 45 % of individuals who survive past the age of 85. AD has been associated with neurovascular dysfunction and brain accumulation of amyloid-β peptide, as well as tau phosphorylation and neurodegeneration, but the pathogenesis of the disease is still somewhat unclear. According to the amyloid cascade hypothesis of AD, accumulation of amyloid-β peptide (Aβ) aggregates initiates a sequence of events leading to neuronal injury and loss, and dementia. Alternatively, the vascular hypothesis of AD incorporates the vascular contribution to the disease, stating that a primary insult to brain microcirculation (e.g., stroke) not only contributes to amyloidopathy but initiates a non-amyloidogenic pathway of vascular-mediated neuronal dysfunction and injury, which involves blood-brain barrier compromise, with increased permeability of blood vessels, leakage of blood-borne components into the brain, and, consequently, neurotoxicity. Vascular dysfunction also includes a diminished brain capillary flow, causing multiple focal ischemic or hypoxic microinjuries, diminished amyloid-β clearance, and formation of neurotoxic oligomers, which lead to neuronal dysfunction. Here we present and discuss relevant findings on the contribution of vascular alterations during aging to AD, with the hope that a better understanding of the players in the "orchestra" of neurodegeneration will be useful in developing therapies to modulate the "symphony".