An emerging role of astrocytes in vascular contributions to cognitive impairment and dementia

Cerebral Small Vessel Disease: a Review of the Pathophysiological Mechanisms

Cerebral small vessel disease (cSVD) refers to the age-dependent pathological processes involving the brain small vessels and leading to vascular cognitive impairment, intracerebral hemorrhage, and acute lacunar ischemic stroke. Despite the significant public health burden of cSVD, disease-specific therapeutics remain unavailable due to the incomplete understanding of the underlying pathophysiological mechanisms. Recent advances in neuroimaging acquisition and processing capabilities as well as findings from cSVD animal models have revealed critical roles of several age-dependent processes in cSVD pathogenesis including arterial stiffness, vascular oxidative stress, low-grade systemic inflammation, gut dysbiosis, and increased salt intake. These factors interact to cause a state of endothelial cell dysfunction impairing cerebral blood flow regulation and breaking the blood brain barrier. Neuroinflammation follows resulting in neuronal injury and cSVD clinical manifestations. Impairment of the cerebral waste clearance through the glymphatic system is another potential process that has been recently highlighted contributing to the cognitive decline. This review details these mechanisms and attempts to explain their complex interactions. In addition, the relevant knowledge gaps in cSVD mechanistic understanding are identified and a systematic approach to future translational and early phase clinical research is proposed in order to reveal new cSVD mechanisms and develop disease-specific therapeutics.

GWAS of multiple neuropathology endophenotypes identifies new risk loci and provides insights into the genetic risk of dementia

Genome-wide association studies (GWAS) have identified >80 Alzheimer's disease and related dementias (ADRD)-associated genetic loci. However, the clinical outcomes used in most previous studies belie the complex nature of underlying neuropathologies. Here we performed GWAS on 11 ADRD-related neuropathology endophenotypes with participants drawn from the following three sources: the National Alzheimer's Coordinating Center, the Religious Orders Study and Rush Memory and Aging Project, and the Adult Changes in Thought study (n = 7,804 total autopsied participants). We identified eight independent significantly associated loci, of which four were new (COL4A1, PIK3R5, LZTS1 and APOC2). Separately testing known ADRD loci, 19 loci were significantly associated with at least one neuropathology after false-discovery rate adjustment. Genetic colocalization analyses identified pleiotropic effects and quantitative trait loci. Methylation in the cerebral cortex at two sites near APOC2 was associated with cerebral amyloid angiopathy. Studies that include neuropathology endophenotypes are an important step in understanding the mechanisms underlying genetic ADRD risk.

Olig2+/NG2+/BLBP+ astrocyte progenitors: a novel component of the neurovascular unit in the developing mouse hippocampus

Astrocytes are key components of the neurovascular unit. While we have recently identified Olig2+ astrocyte progenitors (ASPs) in the developing mouse dentate gyrus (DG), their molecular signature remains incompletely characterized. Here we demonstrate that Olig2+ ASPs predominantly express brain lipid-binding protein (BLBP), while only a small population of them expresses gfap-GFP. These Olig2+/BLBP+ ASPs co-express the transcription factors Sox3, Sox9 and the proteoglycan NG2 but not Sox10, a marker for oligodendrocyte progenitors (OLPs). Olig2+ ASPs appear from embryonic day 18 (E18) onwards and decline at postnatal day 14 (P14). Consistent with the proliferation of both Olig2+ and NG2+ glial cells after brain injury, intrauterine intermittent hypoxia (IH) led to an increase in Olig2+/NG2+/BLBP+ ASPs in the postnatal DG. IH also promoted both angiogenesis and vascular coupling of Olig2+/NG2+ ASPs. Our data suggest that IH-induced expression of HIF1a increases Olig2+/NG2+/BLBP+ ASPs in a cell non-autonomous manner. Our data also revealed increased vascular coupling of GFAP+ astrocytes following IH, while the number of GFAP+ astrocytes remains unchanged. Given that BLBP, Olig2 and NG2 are expressed in reactive astrocytes, our findings suggest that Olig2+/NG2+/BLBP+ ASPs represent a subtype of reactive astrocyte progenitors. Furthermore, the enhanced vascular coupling of Olig2+/NG2+/BLBP+ ASPs appears to be an adaptive response to hypoxic brain injury. This study provides new insights into the molecular characteristics of Olig2+/NG2+/BLBP+ ASPs and their potential role in the brain's response to hypoxic injury, contributing to our understanding of neurovascular unit dynamics in both development and pathological conditions.

Parallel electrophysiological abnormalities due to COVID-19 infection and to Alzheimer's disease and related dementia

Many coronavirus disease 2019 (COVID-19) positive individuals exhibit abnormal electroencephalographic (EEG) activity reflecting "brain fog" and mild cognitive impairments even months after the acute phase of infection. Resting-state EEG abnormalities include EEG slowing (reduced alpha rhythm; increased slow waves) and epileptiform activity. An expert panel conducted a systematic review to present compelling evidence that cognitive deficits due to COVID-19 and to Alzheimer's disease and related dementia (ADRD) are driven by overlapping pathologies and neurophysiological abnormalities. EEG abnormalities seen in COVID-19 patients resemble those observed in early stages of neurodegenerative diseases, particularly ADRD. It is proposed that similar EEG abnormalities in Long COVID and ADRD are due to parallel neuroinflammation, astrocyte reactivity, hypoxia, and neurovascular injury. These neurophysiological abnormalities underpinning cognitive decline in COVID-19 can be detected by routine EEG exams. Future research will explore the value of EEG monitoring of COVID-19 patients for predicting long-term outcomes and monitoring efficacy of therapeutic interventions. HIGHLIGHTS: Abnormal intrinsic electrophysiological brain activity, such as slowing of EEG, reduced alpha wave, and epileptiform are characteristic findings in COVID-19 patients. EEG abnormalities have the potential as neural biomarkers to identify neurological complications at the early stage of the disease, to assist clinical assessment, and to assess cognitive decline risk in Long COVID patients. Similar slowing of intrinsic brain activity to that of COVID-19 patients is typically seen in patients with mild cognitive impairments, ADRD. Evidence presented supports the idea that cognitive deficits in Long COVID and ADRD are driven by overlapping neurophysiological abnormalities resulting, at least in part, from neuroinflammatory mechanisms and astrocyte reactivity. Identifying common biological mechanisms in Long COVID-19 and ADRD can highlight critical pathologies underlying brain disorders and cognitive decline. It elucidates research questions regarding cognitive EEG and mild cognitive impairment in Long COVID that have not yet been adequately investigated.

PNA5, A Novel Mas Receptor Agonist, Improves Neurovascular and Blood-Brain-Barrier Function in a Mouse Model of Vascular Cognitive Impairment and Dementia

It is well established that decreased brain blood flow, increased reactive oxygen species production (ROS), and pro-inflammatory mechanisms accelerate neurodegenerative disease progressions, including vascular cognitive impairment and dementia (VCID). Previous studies in our laboratory have shown that our novel glycosylated Angiotensin-(1-7) Mas receptor agonist PNA5 reverses cognitive deficits, decreases ROS production, and inhibits inflammatory cytokine production in our preclinical mouse model of VCID that is induced by chronic heart failure (VCID-HF). In the present study, the effects of VCID-HF and treatment with PNA5 on microglia activation, blood-brain-barrier (BBB) integrity, and neurovascular coupling were assessed in our mouse model of VCID-HF. Three-month-old male C57BL/6J mice were subjected to myocardial infarction (MI) to induce heart failure for four weeks and then treated with subcutaneous injections of extended-release PNA5. Microglia activation, BBB permeability, cerebral perfusion, and neurovascular coupling were assessed. Results show that in our VCID-HF model, there was an increase in microglial activation and recruitment within the CA1 and CA3 regions of the hippocampus, a disruption in BBB integrity, and a decrease in neurovascular coupling. Treatment with PNA5 reversed these neuropathological effects of VCID-HF, suggesting that PNA5 may be an effective disease-modifying therapy to treat and prevent VCID. This study identifies potential mechanisms by which heart failure may induce VCID and highlights the possible mechanisms by which treatment with our novel glycosylated Angiotensin-(1-7) Mas receptor agonist, PNA5, may protect cognitive function in our model of VCID.

Aberrant activation of hippocampal astrocytes causes neuroinflammation and cognitive decline in mice

Reactive astrocytes are associated with neuroinflammation and cognitive decline in diverse neuropathologies; however, the underlying mechanisms are unclear. We used optogenetic and chemogenetic tools to identify the crucial roles of the hippocampal CA1 astrocytes in cognitive decline. Our results showed that repeated optogenetic stimulation of the hippocampal CA1 astrocytes induced cognitive impairment in mice and decreased synaptic long-term potentiation (LTP), which was accompanied by the appearance of inflammatory astrocytes. Mechanistic studies conducted using knockout animal models and hippocampal neuronal cultures showed that lipocalin-2 (LCN2), derived from reactive astrocytes, mediated neuroinflammation and induced cognitive impairment by decreasing the LTP through the reduction of neuronal NMDA receptors. Sustained chemogenetic stimulation of hippocampal astrocytes provided similar results. Conversely, these phenomena were attenuated by a metabolic inhibitor of astrocytes. Fiber photometry using GCaMP revealed a high level of hippocampal astrocyte activation in the neuroinflammation model. Our findings suggest that reactive astrocytes in the hippocampus are sufficient and required to induce cognitive decline through LCN2 release and synaptic modulation. This abnormal glial-neuron interaction may contribute to the pathogenesis of cognitive disturbances in neuroinflammation-associated brain conditions.

SPAK inhibitor ZT-1a attenuates reactive astrogliosis and oligodendrocyte degeneration in a mouse model of vascular dementia

BACKGROUND

Astrogliosis and white matter lesions (WML) are key characteristics of vascular contributions to cognitive impairment and dementia (VCID). However, the molecular mechanisms underlying VCID remain poorly understood. Stimulation of Na-K-Cl cotransport 1 (NKCC1) and its upstream kinases WNK (with no lysine) and SPAK (the STE20/SPS1-related proline/alanine-rich kinase) play a role in astrocytic intracellular Na+ overload, hypertrophy, and swelling. Therefore, in this study, we assessed the effect of SPAK inhibitor ZT-1a on pathogenesis and cognitive function in a mouse model of VCID induced by bilateral carotid artery stenosis (BCAS).

METHODS

Following sham or BCAS surgery, mice were randomly assigned to receive either vehicle (DMSO) or SPAK inhibitor ZT-1a treatment regimen (days 14-35 post-surgery). Mice were then evaluated for cognitive functions by Morris water maze, WML by ex vivo MRI-DTI analysis, and astrogliosis/demyelination by immunofluorescence and immunoblotting.

RESULTS

Compared to sham control mice, BCAS-Veh mice exhibited chronic cerebral hypoperfusion and memory impairments, accompanied by significant MRI DTI-detected WML and oligodendrocyte (OL) death. Increased activation of WNK-SPAK-NKCC1-signaling proteins was detected in white matter tissues and in C3d+ GFAP+ cytotoxic astrocytes but not in S100A10+ GFAP+ homeostatic astrocytes in BCAS-Veh mice. In contrast, ZT-1a-treated BCAS mice displayed reduced expression and phosphorylation of NKCC1, decreased astrogliosis, OL death, and WML, along with improved memory functions.

CONCLUSION

BCAS-induced upregulation of WNK-SPAK-NKCC1 signaling contributes to white matter-reactive astrogliosis, OL death, and memory impairment. Pharmacological inhibition of the SPAK activity has therapeutic potential for alleviating pathogenesis and memory impairment in VCID.

Development of a monoclonal antibody specific for a calpain-generated ∆48 kDa calcineurin fragment, a marker of distressed astrocytes

BACKGROUND

Calcineurin (CN) is a Ca2+/calmodulin-dependent protein phosphatase. In healthy tissue, CN exists mainly as a full-length (∼60 kDa) highly-regulated protein phosphatase involved in essential cellular functions. However, in diseased or injured tissue, CN is proteolytically converted to a constitutively active fragment that has been causatively-linked to numerous pathophysiologic processes. These calpain-cleaved CN fragments (∆CN) appear at high levels in human brain at early stages of cognitive decline associated with Alzheimer's disease (AD).

NEW METHOD

We developed a monoclonal antibody to ∆CN, using an immunizing peptide corresponding to the C-terminal end of the ∆CN fragment.

RESULTS

We obtained a mouse monoclonal antibody, designated 26A6, that selectively detects ∆CN in Western analysis of calpain-cleaved recombinant human CN. Using this antibody, we screened both pathological and normal human brain sections provided by the University of Kentucky's Alzheimer's Disease Research Center. 26A6 showed low reactivity towards normal brain tissue, but detected astrocytes both surrounding AD amyloid plaques and throughout AD brain tissue. In brain tissue with infarcts, there was considerable concentration of 26A6-positive astrocytes within/around infarcts, suggesting a link with anoxic/ischemia pathways.

COMPARISON WITH EXISTING METHOD

The results obtained with the new monoclonal are similar to those obtained with a polyclonal we had previously developed. However, the monoclonal is an abundant tool available to the dementia research community.

CONCLUSIONS

The new monoclonal 26A6 antibody is highly selective for the ∆CN proteolytic fragment and labels a subset of astrocytes, and could be a useful tool for marking insidious brain pathology and identifying novel astrocyte phenotypes.

In severe ADNC, hippocampi with comorbid LATE-NC and hippocampal sclerosis have substantially more astrocytosis than those with LATE-NC or hippocampal sclerosis alone

Limbic-predominant age-related TDP-43 encephalopathy neuropathologic change (LATE-NC) and hippocampal sclerosis of aging (HS-A) pathologies are found together at autopsy in ∼20% of elderly demented persons. Although astrocytosis is known to occur in neurodegenerative diseases, it is currently unknown how the severity of astrocytosis is correlated with the common combinations of pathologies in aging brains. To address this knowledge gap, we analyzed a convenience sample of autopsied subjects from the University of Kentucky Alzheimer's Disease Research Center community-based autopsy cohort. The subjects were stratified into 5 groups (n = 51 total): pure ADNC, ADNC + LATE-NC, ADNC + HS-A, ADNC + LATE-NC + HS-A, and low-pathology controls. Following GFAP immunostaining and digital slide scanning with a ScanScope, we measured GFAP-immunoreactive astrocytosis. The severities of GFAP-immunoreactive astrocytosis in hippocampal subfield CA1 and subiculum were compared between groups. The group with ADNC + LATE-NC + HS-A had the most astrocytosis as operationalized by either any GFAP+ or strong GFAP+ immunoreactivity in both CA1 and subiculum. In comparison to that pathologic combination, ADNC + HS or ADNC + LATE-NC alone showed lower astrocytosis. Pure ADNC had only marginally increased astrocytosis in CA1 and subiculum, in comparison to low-pathology controls. We conclude that there appeared to be pathogenetic synergy such that ADNC + LATE-NC + HS-A cases had relatively high levels of astrocytosis in the hippocampal formation.

Hippocampal glial inflammatory markers are differentially altered in a novel mouse model of perimenopausal cerebral amyloid angiopathy

Dementia is often characterized by age-dependent cerebrovascular pathology, neuroinflammation, and cognitive deficits with notable sex differences in risk, disease onset, progression and severity. Women bear a disproportionate burden of dementia, and the onset of menopause (i.e., perimenopause) may be a critical period conferring increased susceptibility. However, the contribution of early ovarian decline to the neuroinflammatory processes associated with cerebrovascular dementia risks, particularly at the initial stages of pathology that may be more amenable to proactive intervention, is unknown. To better understand the influence of early ovarian failure on dementia-associated neuroinflammation we developed a model of perimenopausal cerebral amyloid angiopathy (CAA), an important contributor to dementia. For this, accelerated ovarian failure (AOF) was induced by 4-vinylcyclohexene diepoxide (VCD) treatment to isolate early-stage ovarian failure comparable to human perimenopause (termed "peri-AOF") in transgenic SWDI mice expressing human vasculotropic mutant amyloid beta (Aβ) precursor protein, that were also tested at an early stage of amyloidosis. We found that peri-AOF SWDI mice showed increased astrocyte activation accompanied by elevated Aβ in select regions of the hippocampus, a brain system involved in learning and memory that is severely impacted during dementia. However, although SWDI mice showed signs of increased hippocampal microglial activation and impaired cognitive function, this was not further affected by peri-AOF. In sum, these results suggest that elevated dysfunction of key elements of the neurovascular unit in select hippocampal regions characterizes the brain pathology of mice at early stages of both CAA and AOF. However, neurovascular unit pathology may not yet have passed a threshold that leads to further behavioral compromise at these early periods of cerebral amyloidosis and ovarian failure. These results are consistent with the hypothesis that the hormonal dysregulation associated with perimenopause onset represents a stage of emerging vulnerability to dementia-associated neuropathology, thus providing a selective window of opportunity for therapeutic intervention prior to the development of advanced pathology that has proven difficult to repair or reverse.

The expression of ceruloplasmin in astrocytes is essential for postnatal myelination and myelin maintenance in the adult brain

Ceruloplasmin (Cp) is a ferroxidase enzyme that is essential for cell iron efflux. The absence of this protein in humans and rodents produces progressive neurodegeneration with brain iron accumulation. Astrocytes express high levels of Cp and iron efflux from these cells has been shown to be central for oligodendrocyte maturation and myelination. To explore the role of astrocytic Cp in brain development and aging we generated a specific conditional KO mouse for Cp in astrocytes (Cp cKO). Deletion of Cp in astrocytes during the first postnatal week induced hypomyelination and a significant delay in oligodendrocyte maturation. This abnormal myelin synthesis was exacerbated throughout the first two postnatal months and accompanied by a reduction in oligodendrocyte iron content, as well as an increase in brain oxidative stress. In contrast to young animals, deletion of astrocytic Cp at 8 months of age engendered iron accumulation in several brain areas and neurodegeneration in cortical regions. Aged Cp cKO mice also showed myelin loss and oxidative stress in oligodendrocytes and neurons, and at 18 months of age, developed abnormal behavioral profiles, including deficits in locomotion and short-term memory. In summary, our results demonstrate that iron efflux-mediated by astrocytic Cp-is essential for both early oligodendrocyte maturation and myelin integrity in the mature brain. Additionally, our data suggest that astrocytic Cp activity is central to prevent iron accumulation and iron-induced oxidative stress in the aging CNS.

Neurovascular glial unit: A target of phytotherapy for cognitive impairments

BACKGROUND

Neurovascular glial unit (NVGU) dysfunction has been reported to be an early and critical event in the pathophysiology of Alzheimer's disease (AD) and vascular dementia (VD). Although herbal medicines, with their favorable safety profiles and low adverse effects, have been suggested to be useful for the treatment of cognitive impairment, the potential role of the NVGU as the target of the effects of herbal medicines is still unclear.

PURPOSE

This review aimed to retrieve evidence from experimental studies of phytopharmaceuticals targeting the NVGU for the treatment of cognitive impairment in AD and VD, and discussed the potential of phytopharmaceuticals to improve cognitive impairment from the perspective of the NVGU.

STUDY DESIGN AND METHODS

We systematically searched PubMed, Google Scholar, Web of Science, and CNKI. The keywords used for searching information on the NVGU in the treatment of cognitive impairments included "Alzheimer's disease," "Vascular dementia," "Herbal medicines," "Natural products," "Neurovascular," "Adverse reaction," and "Toxicity, etc." We selected studies on the basis of predefined eligibility criteria.

RESULTS

NVGU mainly consists of endothelial cells, pericytes, astrocytes, microglia, oligodendrocytes, and neurons, and damage to these cells can induce cognitive impairment by impairing the blood-brain barrier (BBB) and cerebral blood flow (CBF) as well as neuronal function. The active components of herbal medicines, including Ginkgo biloba L., Ginseng Radix et Rhizoma, Epimedium Folium, Chuanxiong Rhizoma, Carthami flos, and Acorus tatarinowii Schott, as well as traditional Chinese medicine prescriptions have shown the potential to improve BBB function and increase CBF to prevent cognitive impairment by inhibiting astrocyte and microglia activation, protecting oligodendrocyte myelin function, reducing neuronal apoptosis, and promoting angiogenesis.

CONCLUSIONS

Herbal medicines demonstrate great potential to prevent cognitive impairment. Multiple components from herbal medicines may function through different signaling pathways to target the NVGU. Future studies using novel drug-carrier or delivery systems targeting the NVGU will certainly facilitate the development of phytopharmaceuticals for AD and VD.

Proteomics identifies lipocalin-2 in neonatal inflammation associated with cerebrovascular alteration in mice and preterm infants

Staphylococcus (S.) epidermidis is the most common nosocomial coagulase-negative staphylococci infection in preterm infants. Clinical signs of infection are often unspecific and novel markers to complement diagnosis are needed. We investigated proteomic alterations in mouse brain after S. epidermidis infection and in preterm infant blood. We identified lipocalin-2 (LCN2) as a crucial protein associated with cerebrovascular changes and astrocyte reactivity in mice. We further proved that LCN2 protein expression was associated with endothelial cells but not astrocyte reactivity. By combining network analysis and differential expression approaches, we identified LCN2 linked to blood C-reactive protein levels in preterm infants born <28 weeks of gestation. Blood LCN2 levels were associated with similar alterations of cytokines and chemokines in both infected mice and human preterm infants with increased levels of C-reactive protein. This experimental and clinical study suggests that LCN2 may be a marker of preterm infection/inflammation associated with cerebrovascular changes and neuroinflammation.

Relationship between insulin-like growth factor-1 and cerebral small vessel disease and its mechanisms: advances in the field

Insulin-like growth factor-1 (IGF-1) is an active polypeptide protein that closely resembles the structural sequence of insulin and is involved in a variety of metabolic processes in the body. Decreased IGF-1 circulation levels are associated with an increased risk of stroke and a poorer prognosis, but the relationship with cerebral small vessel disease (cSVD) is unclear. Some studies found that the level of IGF-1 in patients with cSVD was significantly reduced, but the clinical significance and underlying mechanisms are unknown. This article reviews the correlation between IGF-1 and cerebrovascular disease and explores the potential relationship and mechanism between IGF-1 and cSVD.

A single mild juvenile TBI in male mice leads to regional brain tissue abnormalities at 12 months of age that correlate with cognitive impairment at the middle age

Traumatic brain injury (TBI) has the highest incidence amongst the pediatric population and its mild severity represents the most frequent cases. Moderate and severe injuries as well as repetitive mild TBI result in lasting morbidity. However, whether a single mild TBI sustained during childhood can produce long-lasting modifications within the brain is still debated. We aimed to assess the consequences of a single juvenile mild TBI (jmTBI) at 12 months post-injury in a mouse model. Non-invasive diffusion tensor imaging (DTI) revealed significant microstructural alterations in the hippocampus and the in the substantia innominata/nucleus basalis (SI/NB), structures known to be involved in spatial learning and memory. DTI changes paralled neuronal loss, increased astrocytic AQP4 and microglial activation in the hippocampus. In contrast, decreased astrocytic AQP4 expression and microglia activation were observed in SI/NB. Spatial learning and memory were impaired and correlated with alterations in DTI-derived derived fractional ansiotropy (FA) and axial diffusivity (AD). This study found that a single juvenile mild TBI leads to significant region-specific DTI microstructural alterations, distant from the site of impact, that correlated with cognitive discriminative novel object testing and spatial memory impairments at 12 months after a single concussive injury. Our findings suggest that exposure to jmTBI leads to a chronic abnormality, which confirms the need for continued monitoring of symptoms and the development of long-term treatment strategies to intervene in children with concussions.

Updated Understanding of the Glial-Vascular Unit in Central Nervous System Disorders

The concept of the glial-vascular unit (GVU) was raised recently to emphasize the close associations between brain cells and cerebral vessels, and their coordinated reactions to diverse neurological insults from a "glio-centric" view. GVU is a multicellular structure composed of glial cells, perivascular cells, and perivascular space. Each component is closely linked, collectively forming the GVU. The central roles of glial and perivascular cells and their multi-level interconnections in the GVU under normal conditions and in central nervous system (CNS) disorders have not been elucidated in detail. Here, we comprehensively review the intensive interactions between glial cells and perivascular cells in the niche of perivascular space, which take part in the modulation of cerebral blood flow and angiogenesis, formation of the blood-brain barrier, and clearance of neurotoxic wastes. Next, we discuss dysfunctions of the GVU in various neurological diseases, including ischemic stroke, spinal cord injury, Alzheimer's disease, and major depression disorder. In addition, we highlight the possible therapies targeting the GVU, which may have potential clinical applications.

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.

Systemic Inflammation Causes Microglial Dysfunction With a Vascular AD phenotype

Background

Studies in rodents and humans have indicated that inflammation outside CNS (systemic inflammation) affects brain homeostasis contributing to neurodevelopmental disorders. Itis becoming increasingly evident that such early insults may also belinked to neurodegenerative diseases like late-onset Alzheimer's disease (AD). Importantly, lifestyle and stress, such as viral or bacterial infection causing chronic inflammation, may contribute to neurodegenerative dementia. Systemic inflammatory response triggers a cascade of neuroinflammatory responses, altering brain transcriptome, cell death characteristic of AD, and vascular dementia. Our study aimed to assess the temporal evolution of the pathological impact of systemic inflammation evoked by prenatal and early postnatal peripheral exposure of viral mimetic Polyinosinic:polycytidylic acid (PolyI:C) and compare the hippocampal transcriptomic changes with the profiles of human post-mortem AD and vascular dementia brain specimens.

Methods

We have engineered the PolyI:C sterile infection model in wildtype C57BL6 mice to achieve chronic low-grade systemic inflammation. We have conducted a cross-sectional analysis of aging PolyI:C and Saline control mice (3 months, 6 months, 9 months, and 16 months), taking the hippocampus as a reference brain region, and compared the brain aging phenotype to AD progression in humans with mild AD, severe AD, and Controls (CTL), in parallel to Vascular dementia (VaD) patients' specimens.

Results

We found that PolyI:C mice display both peripheral and central inflammation with a peak at 6 months, associated with memory deficits. The hippocampus is characterized by a pronounced and progressive tauopathy. In PolyI:C brains, microglia undergo aging-dependent morphological shifts progressively adopting a phagocytic phenotype. Transcriptomic analysis reveals a profound change in gene expression throughout aging, with a peak in differential expression at 9 months. We show that the proinflammatory marker Lcn2 is one of the genes with the strongest upregulation in PolyI:C mice upon aging. Validation in brains from patients with increasing severity of AD and VaD shows the reproducibility of some gene targets in vascular dementia specimens as compared to AD ones.

Conclusions

The PolyI:C model of sterile infection demonstrates that peripheral chronic inflammation causes progressive tau hyperphosphorylation, changes in microglia morphology, astrogliosis and gene reprogramming reflecting increased neuroinflammation, vascular remodeling, and the loss of neuronal functionality seen to some extent in human AD and Vascular dementia suggesting early immune insults could be crucial in neurodegenerative diseases.

A Microfluidic Model of AQP4 Polarization Dynamics and Fluid Transport in the Healthy and Inflamed Human Brain: The First Step Towards Glymphatics-on-a-Chip

Dysfunction of the aquaporin-4 (AQP4)-dependent glymphatic waste clearance pathway has recently been implicated in the pathogenesis of several neurodegenerative diseases. However, it is difficult to unravel the causative relationship between glymphatic dysfunction, AQP4 depolarization, protein aggregation, and inflammation in neurodegeneration using animal models alone. There is currently a clear, unmet need for in vitro models of the brain's waterscape, and the first steps towards a bona fide "glymphatics-on-a-chip" are taken in the present study. It is demonstrated that chronic exposure to lipopolysaccharide (LPS), amyloid-β(1-42) oligomers, and an AQP4 inhibitor impairs the drainage of fluid and amyloid-β(1-40) tracer in a gliovascular unit (GVU)-on-a-chip model containing human astrocytes and brain microvascular endothelial cells. The LPS-induced drainage impairment is partially retained following cell lysis, indicating that neuroinflammation induces parallel changes in cell-dependent and matrisome-dependent fluid transport pathways in GVU-on-a-chip. Additionally, AQP4 depolarization is observed following LPS treatment, suggesting that LPS-induced drainage impairments on-chip may be driven in part by changes in AQP4-dependent fluid dynamics.

Pharmaceutical and pharmacological studies of Shen Ma Yi Zhi granule for prevention of vascular dementia: A review

Background: With dementia significantly increasing hospitalization and disability rates, worldwide aging of the population presents major challenges to public health. The majority of cases of cognitive dysfunction among the elderly, however, are characterized by an identifiable, preventable and treatable vascular component. As such, increased study of preventative methods in the context of dementia is warranted. Traditional Chinese medicine compounds have been reported to be neuroprotective and improve cognitive function via a variety of mechanisms. Shen Ma Yi Zhi granule (SMYZG) is one such collection of compounds that has been proven clinically effective. Pharmacological mechanisms of action, pharmacokinetics and clinical applications of SMYZG have been previously studied using a variety of vascular dementia animal models. SMYZG activates and regulates four main signaling pathways relevant to vascular dementia including the AMPK/PPARα/PGC-1α/UCP2, Nrf2/HO-1, HIF-1/VEGF/Notch, and VEGF/Flk-1/p8 MAPK pathways. Furthermore, SMYZG influences anti-inflammatory and anti-oxidant stress responses, reverses demyelination of brain white matter and vascular endothelium, regulates pericyte function and normalizes mitochondrial metabolism. Neuroprotective effects of SMYZG, as well as those promoting regeneration of vascular endothelium, have also been reported in studies of rat models of vascular dementia. Future research concerning SMYG is warranted for development of vascular dementia preventative management strategies.

cGMP Signaling in the Neurovascular Unit-Implications for Retinal Ganglion Cell Survival in Glaucoma

Glaucoma is a progressive age-related disease of the visual system and the leading cause of irreversible blindness worldwide. Currently, intraocular pressure (IOP) is the only modifiable risk factor for the disease, but even as IOP is lowered, the pathology of the disease often progresses. Hence, effective clinical targets for the treatment of glaucoma remain elusive. Glaucoma shares comorbidities with a multitude of vascular diseases, and evidence in humans and animal models demonstrates an association between vascular dysfunction of the retina and glaucoma pathology. Integral to the survival of retinal ganglion cells (RGCs) is functional neurovascular coupling (NVC), providing RGCs with metabolic support in response to neuronal activity. NVC is mediated by cells of the neurovascular unit (NVU), which include vascular cells, glial cells, and neurons. Nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling is a prime mediator of NVC between endothelial cells and neurons, but emerging evidence suggests that cGMP signaling is also important in the physiology of other cells of the NVU. NO-cGMP signaling has been implicated in glaucomatous neurodegeneration in humans and mice. In this review, we explore the role of cGMP signaling in the different cell types of the NVU and investigate the potential links between cGMP signaling, breakdown of neurovascular function, and glaucoma pathology.

Modulation of Small RNA Signatures by Astrocytes on Early Neurodegeneration Stages; Implications for Biomarker Discovery

Diagnosis of neurodegenerative disease (NDD) is complex, therefore simpler, less invasive, more accurate biomarkers are needed. small non-coding RNA (sncRNA) dysregulates in NDDs and sncRNA signatures have been explored for the diagnosis of NDDs, however, the performance of previous biomarkers is still better. Astrocyte dysfunction promotes neurodegeneration and thus derived scnRNA signatures could provide a more precise way to identify of changes related to NDD course and pathogenesis, and it could be useful for the dissection of mechanistic insights operating in NDD. Often sncRNA are transported outside the cell by the action of secreted particles such as extracellular vesicles (EV), which protect sncRNA from degradation. Furthermore, EV associated sncRNA can cross the BBB to be found in easier to obtain peripheral samples, EVs also inherit cell-specific surface markers that can be used for the identification of Astrocyte Derived Extracellular Vesicles (ADEVs) in a peripheral sample. By the study of the sncRNA transported in ADEVs it is possible to identify astrocyte specific sncRNA signatures that could show astrocyte dysfunction in a more simpler manner than previous methods. However, sncRNA signatures in ADEV are not a copy of intracellular transcriptome and methodological aspects such as the yield of sncRNA produced in ADEV or the variable amount of ADEV captured after separation protocols must be considered. Here we review the role as signaling molecules of ADEV derived sncRNA dysregulated in conditions associated with risk of neurodegeneration, providing an explanation of why to choose ADEV for the identification of astrocyte-specific transcriptome. Finally, we discuss possible limitations of this approach and the need to improve the detection limits of sncRNA for the use of ADEV derived sncRNA signatures.

Neuroinflammation in Dementia—Therapeutic Directions in a COVID-19 Pandemic Setting

Although dementia is a heterogenous group of diseases, inflammation has been shown to play a central role in all of them and provides a common link in their pathology. This review aims to highlight the importance of immune response in the most common types of dementia. We describe molecular aspects of pro-inflammatory signaling and sources of inflammatory activation in the human organism, including a novel infectious agent, SARS-CoV-2. The role of glial cells in neuroinflammation, as well as potential therapeutic approaches, are then discussed. Peripheral immune response and increased cytokine production, including an early surge in TNF and IL-1β concentrations activate glia, leading to aggravation of neuroinflammation and dysfunction of neurons during COVID-19. Lifestyle factors, such as diet, have a large impact on future cognitive outcomes and should be included as a crucial intervention in dementia prevention. While the use of NSAIDs is not recommended due to inconclusive results on their efficacy and risk of side effects, the studies focused on the use of TNF antagonists as the more specific target in neuroinflammation are still very limited. It is still unknown, to what degree neuroinflammation resulting from COVID-19 may affect neurodegenerative process and cognitive functioning in the long term with ongoing reports of chronic post-COVID complications.

Mineralocorticoid receptor antagonism improves transient receptor potential vanilloid 4-dependent dilation of cerebral parenchymal arterioles and cognition in a genetic model of hypertension

OBJECTIVE

In a model of secondary hypertension, mineralocorticoid receptor (MR) antagonism during the development of hypertension prevents the impairment of transient receptor potential vanilloid 4 (TRPV4) activation in parenchymal arterioles (PAs) and cognitive impairment. However, it is unknown whether MR antagonism can improve these impairments when treatment begins after the onset of essential hypertension. We tested the hypothesis that MR activation in stroke-prone spontaneously hypertensive rats (SHRSP) leads to impaired TRPV4-mediated dilation in PAs that is associated with cognitive dysfunction and neuroinflammation.

METHODS

20-22-week-old male SHRSP ± eplerenone (EPL; 100 mg/kg daily for 4 weeks) were compared to normotensive Sprague-Dawley (SD) rats. Pressure myography was used to assess PA function. Cognition was tested using Y-maze. Neuroinflammation was assessed using immunofluorescence and qRT-PCR.

RESULTS

Carbachol-mediated endothelium-dependent dilation was impaired in SHRSP, and MR antagonism improved this without affecting myogenic tone. Dilation to TRPV4 agonist GSK1016790A was impaired in SHRSP, and ELP treatment restored this. Intermediate conductance potassium channel (IKCa)/small conductance potassium channel (SKCa)-mediated dilation was impaired by hypertension and unaffected by EPL treatment. TRPV4 and IKCa/SKCa channel mRNA expression were reduced in PAs from hypertensive rats, and EPL did not improve this. Impairments in PA dilation in SHRSP were associated with cognitive decline, microglial activation, reactive astrogliosis, and neuroinflammation; cognitive and inflammatory changes were improved with MR blockade.

CONCLUSIONS

These data advance our understanding of the effects of hypertension on cerebral arterioles using a clinically relevant model and treatment paradigm. Our studies suggest TRPV4 and the MR are potential therapeutic targets to improve cerebrovascular function and cognition during hypertension.

Neuroinflammation in Vascular Cognitive Impairment and Dementia: Current Evidence, Advances, and Prospects

Vascular cognitive impairment and dementia (VCID) is a major heterogeneous brain disease caused by multiple factors, and it is the second most common type of dementia in the world. It is caused by long-term chronic low perfusion in the whole brain or local brain area, and it eventually develops into severe cognitive dysfunction syndrome. Because of the disease’s ambiguous classification and diagnostic criteria, there is no clear treatment strategy for VCID, and the association between cerebrovascular pathology and cognitive impairment is controversial. Neuroinflammation is an immunological cascade reaction mediated by glial cells in the central nervous system where innate immunity resides. Inflammatory reactions could be triggered by various damaging events, including hypoxia, ischemia, and infection. Long-term chronic hypoperfusion-induced ischemia and hypoxia can overactivate neuroinflammation, causing apoptosis, blood–brain barrier damage and other pathological changes, triggering or aggravating the occurrence and development of VCID. In this review, we will explore the mechanisms of neuroinflammation induced by ischemia and hypoxia caused by chronic hypoperfusion and emphasize the important role of neuroinflammation in the development of VCID from the perspective of immune cells, immune mediators and immune signaling pathways, so as to provide valuable ideas for the prevention and treatment of the disease.

Alpha lipoic acid reverses scopolamine-induced spatial memory loss and pyramidal cell neurodegeneration in the prefrontal cortex of Wistar Rats

Neurodegenerative disorders are linked to oxidative tissue damage characterized by gradual loss of cognitive functions and neuronal cells. Alpha-lipoic acid (AHA) has a strong antioxidant property. Scopolamine is an anti-muscarinic agent used to study the mechanism of memory loss in an animal model. This study is aimed at evaluating the antioxidant role of alpha lipoic acid in reversing scopolamine induced memory loss and neurodegenerative process in the prefrontal cortex of Wistar rats. Twenty adult male Wistar rats used were divided into four groups (n = 5): Group 1 received vehicle (Control), Group 2 had scopolamine (1 mg/kg, i.p) for 4 days, Group 3 received AHA (200 mg/kg, p.o) for 10 days while Group 4 were pretreated with scopolamine (1 mg/kg, i.p) for 4 days followed by oral administration of 200 mg/kg of AHA for 10 days. The rats were subjected to Y-maze test to assess their spatial memory. The rats were euthanized, the prefrontal area was excised and fixed in 10% formol-calcium and processed for Haematoxylin and Eosin, Cresyl fast violet for Nissl Bodies (Ribosome), and Glial Fibrillary Acidic Protein (GFAP) stains. Scopolamine caused a significant decline in spatial working memory, prefrontal neuron cell loss, and increased proliferation of reactive astrocytes (astrogliosis) when compared with the control and AHA treated group. AHA process of reversing scopolamine-induced memory deficit, prefrontal neuron cell loss, and generation of reactive astrocytes (astrogliosis) is mediated by its antioxidant mediated positive modulation of astrocyte-neuronal interaction during neuroinflammation in response to oxidative tissue damage.

Solving neurodegeneration: common mechanisms and strategies for new treatments

Across neurodegenerative diseases, common mechanisms may reveal novel therapeutic targets based on neuronal protection, repair, or regeneration, independent of etiology or site of disease pathology. To address these mechanisms and discuss emerging treatments, in April, 2021, Glaucoma Research Foundation, BrightFocus Foundation, and the Melza M. and Frank Theodore Barr Foundation collaborated to bring together key opinion leaders and experts in the field of neurodegenerative disease for a virtual meeting titled "Solving Neurodegeneration". This "think-tank" style meeting focused on uncovering common mechanistic roots of neurodegenerative disease and promising targets for new treatments, catalyzed by the goal of finding new treatments for glaucoma, the world's leading cause of irreversible blindness and the common interest of the three hosting foundations. Glaucoma, which causes vision loss through degeneration of the optic nerve, likely shares early cellular and molecular events with other neurodegenerative diseases of the central nervous system. Here we discuss major areas of mechanistic overlap between neurodegenerative diseases of the central nervous system: neuroinflammation, bioenergetics and metabolism, genetic contributions, and neurovascular interactions. We summarize important discussion points with emphasis on the research areas that are most innovative and promising in the treatment of neurodegeneration yet require further development. The research that is highlighted provides unique opportunities for collaboration that will lead to efforts in preventing neurodegeneration and ultimately vision loss.

H-ferritin expression in astrocytes is necessary for proper oligodendrocyte development and myelination

How iron is delivered to the CNS for myelination is poorly understood. Astrocytes are the most abundant glial cells in the brain and are the only cells in close contact with blood vessels. Therefore, they are strategically located to obtain nutrients, such as iron, from circulating blood. To determine the importance of astrocyte iron uptake and storage in myelination and remyelination, we conditionally knocked-out the expression of the divalent metal transporter 1 (DMT1), the transferrin receptor 1 (Tfr1), and the ferritin heavy subunit (Fth) in Glast-1-positive astrocytes. DMT1 or Tfr1 ablation in astrocytes throughout early brain development did not significantly affects oligodendrocyte maturation or iron homeostasis. However, blocking Fth production in astrocytes during the first postnatal week drastically delayed oligodendrocyte development and myelin synthesis. Fth knockout animals presented an important decrease in the number of myelinating oligodendrocytes and a substantial reduction in the percentage of myelinated axons. This postnatal hypomyelination was accompanied by a decline in oligodendrocyte iron uptake and with an increase in brain oxidative stress. We also tested the relevance of astrocytic Fth expression in the cuprizone model of myelin damage and repair. Fth deletion in Glast1-positive astrocytes significantly reduced myelin production and the density of mature myelinating oligodendrocytes throughout the complete remyelination process. These results indicate that Fth iron storage in astrocytes is vital for early oligodendrocyte development as well as for the remyelination of the CNS.

Neurofilament light: a possible prognostic biomarker for treatment of vascular contributions to cognitive impairment and dementia

Background

Decreased cerebral blood flow and systemic inflammation during heart failure (HF) increase the risk for vascular contributions to cognitive impairment and dementia (VCID) and Alzheimer disease-related dementias (ADRD). We previously demonstrated that PNA5, a novel glycosylated angiotensin 1–7 (Ang-(1–7)) Mas receptor (MasR) agonist peptide, is an effective therapy to rescue cognitive impairment in our preclinical model of VCID. Neurofilament light (NfL) protein concentration is correlated with cognitive impairment and elevated in neurodegenerative diseases, hypoxic brain injury, and cardiac disease. The goal of the present study was to determine (1) if treatment with Ang-(1–7)/MasR agonists can rescue cognitive impairment and decrease VCID-induced increases in NfL levels as compared to HF-saline treated mice and, (2) if NfL levels correlate with measures of cognitive function and brain cytokines in our VCID model.

Methods

VCID was induced in C57BL/6 male mice via myocardial infarction (MI). At 5 weeks post-MI, mice were treated with daily subcutaneous injections for 24 days, 5 weeks after MI, with PNA5 or angiotensin 1–7 (500 microg/kg/day or 50 microg/kg/day) or saline (n = 15/group). Following the 24-day treatment protocol, cognitive function was assessed using the Novel Object Recognition (NOR) test. Cardiac function was measured by echocardiography and plasma concentrations of NfL were quantified using a Quanterix Simoa assay. Brain and circulating cytokine levels were determined with a MILLIPLEX MAP Mouse High Sensitivity Multiplex Immunoassay. Treatment groups were compared via ANOVA, significance was set at p < 0.05.

Results

Treatment with Ang-(1–7)/MasR agonists reversed VCID-induced cognitive impairment and significantly decreased NfL levels in our mouse model of VCID as compared to HF-saline treated mice. Further, NfL levels were significantly negatively correlated with cognitive scores and the concentrations of multiple pleiotropic cytokines in the brain.

Conclusions

These data show that treatment with Ang-(1–7)/MasR agonists rescues cognitive impairment and decreases plasma NfL relative to HF-saline-treated animals in our VCID mouse model. Further, levels of NfL are significantly negatively correlated with cognitive function and with several brain cytokine concentrations. Based on these preclinical findings, we propose that circulating NfL might be a candidate for a prognostic biomarker for VCID and may also serve as a pharmacodynamic/response biomarker for therapeutic target engagement.

The "Neuro-Glial-Vascular" Unit: The Role of Glia in Neurovascular Unit Formation and Dysfunction

The neurovascular unit (NVU) is a complex multi-cellular structure consisting of endothelial cells (ECs), neurons, glia, smooth muscle cells (SMCs), and pericytes. Each component is closely linked to each other, establishing a structural and functional unit, regulating central nervous system (CNS) blood flow and energy metabolism as well as forming the blood-brain barrier (BBB) and inner blood-retina barrier (BRB). As the name suggests, the “neuro” and “vascular” components of the NVU are well recognized and neurovascular coupling is the key function of the NVU. However, the NVU consists of multiple cell types and its functionality goes beyond the resulting neurovascular coupling, with cross-component links of signaling, metabolism, and homeostasis. Within the NVU, glia cells have gained increased attention and it is increasingly clear that they fulfill various multi-level functions in the NVU. Glial dysfunctions were shown to precede neuronal and vascular pathologies suggesting central roles for glia in NVU functionality and pathogenesis of disease. In this review, we take a “glio-centric” view on NVU development and function in the retina and brain, how these change in disease, and how advancing experimental techniques will help us address unanswered questions.

Pathologic sequelae of vascular cognitive impairment and dementia sheds light on potential targets for intervention

Vascular contributions to cognitive impairment and dementia (VCID) is one of the leading causes of dementia along with Alzheimer's disease (AD) and, importantly, VCID often manifests as a comorbidity of AD(Vemuri and Knopman 2016; Schneider and Bennett 2010)(Vemuri and Knopman 2016; Schneider and Bennett 2010). Despite its common clinical manifestation, the mechanisms underlying VCID disease progression remains elusive. In this review, existing knowledge is used to propose a novel hypothesis linking well-established risk factors of VCID with the distinct neurodegenerative cascades of neuroinflammation and chronic hypoperfusion. It is hypothesized that these two synergistic signaling cascades coalesce to initiate aberrant angiogenesis and induce blood brain barrier breakdown trough a mechanism mediated by vascular growth factors and matrix metalloproteinases respectively. Finally, this review concludes by highlighting several potential therapeutic interventions along this neurodegenerative sequalae providing diverse opportunities for future translational study.

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.

Association Between Adenosine A2A Receptors and Connexin 43 Regulates Hemichannels Activity and ATP Release in Astrocytes Exposed to Amyloid-β Peptides

Increasing evidence implicates astrocytes and the associated purinergic modulation in Alzheimer's disease (AD), characterized by cognitive deficits involving the extracellular amyloid-β peptides (Aβ) accumulation. Aβ can affect astrocytic gliotransmitters release, namely ATP, which is rapidly metabolized into adenosine by ecto-5'-nucleotidase, CD73, resulting in adenosine A_2A receptors (A_2AR) activation that bolsters neurodegeneration. AD's brains exhibit an upregulation of A_2AR and of connexin 43 (Cx43), which in astrocytes forms hemichannels that can mediate ATP release. However, a coupling between astrocytic A_2AR and Cx43 remains to be established. This was now investigated using astrocytic primary cultures exposed to Aβ_1-42 peptides. Aβ triggered ATP release through Cx43 hemichannels, a process blocked by A_2AR antagonists and mimicked by selective A_2AR activation. A_2AR directly regulated hemichannels activity and prevented Cx43 upregulation and phosphorylation observed in Aβ_1-42-exposed astrocytes. Moreover, a proximity ligand assay revealed a physical association between astrocytic A_2AR and Cx43. Finally, the blockade of CD73-mediated extracellular formation of ATP-derived adenosine prevented the Aβ-induced increase of Cx43 hemichannel activity and of ATP release. Overall, the data identify a feed-forward loop involving astrocytic A_2AR and Cx43 hemichannels, whereby A_2AR increase Cx43 hemichannel activity leading to increased ATP release, which is converted into adenosine by CD73, sustaining the increased astrocytic A_2AR activity in AD-like conditions.

Reactive astrocytes: The nexus of pathological and clinical hallmarks of Alzheimer's disease

Astrocyte reactivity is a hallmark of neuroinflammation that arises with Alzheimer’s disease (AD) and nearly every other neurodegenerative condition. While astrocytes certainly contribute to classic inflammatory processes (e.g. cytokine release, waste clearance, and tissue repair), newly emerging technologies for measuring and targeting cell specific activities in the brain have uncovered essential roles for astrocytes in synapse function, brain metabolism, neurovascular coupling, and sleep/wake patterns. In this review, we use a holistic approach to incorporate, and expand upon, classic neuroinflammatory concepts to consider how astrocyte dysfunction/reactivity modulates multiple pathological and clinical hallmarks of AD. Our ever-evolving understanding of astrocyte signaling in neurodegeneration is not only revealing new drug targets and treatments for dementia but is suggesting we reimagine AD pathophysiological mechanisms.

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.

Cerebrovascular development: mechanisms and experimental approaches

The cerebral vasculature plays a central role in human health and disease and possesses several unique anatomic, functional and molecular characteristics. Despite their importance, the mechanisms that determine cerebrovascular development are less well studied than other vascular territories. This is in part due to limitations of existing models and techniques for visualisation and manipulation of the cerebral vasculature. In this review we summarise the experimental approaches used to study the cerebral vessels and the mechanisms that contribute to their development.

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

Cholesterol, Atherosclerosis, and APOE in Vascular Contributions to Cognitive Impairment and Dementia (VCID): Potential Mechanisms and Therapy

Vascular contributions to cognitive impairment and dementia (VCID) are a common cause of cognitive decline, yet limited therapies exist. This cerebrovascular disease results in neurodegeneration via acute, chronic, local, and systemic mechanisms. The etiology of VCID is complex, with a significant impact from atherosclerosis. Risk factors including hypercholesterolemia and hypertension promote intracranial atherosclerotic disease and carotid artery stenosis (CAS), which disrupt cerebral blood flow and trigger ischemic strokes and VCID. Apolipoprotein E (APOE) is a cholesterol and phospholipid carrier present in plasma and various tissues. APOE is implicated in dyslipidemia and Alzheimer disease (AD); however, its connection with VCID is less understood. Few experimental models for VCID exist, so much of the present information has been drawn from clinical studies. Here, we review the literature with a focus on the clinical aspects of atherosclerotic cerebrovascular disease and build a working model for the pathogenesis of VCID. We describe potential intermediate steps in this model, linking cholesterol, atherosclerosis, and APOE with VCID. APOE4 is a minor isoform of APOE that promotes lipid dyshomeostasis in astrocytes and microglia, leading to chronic neuroinflammation. APOE4 disturbs lipid homeostasis in macrophages and smooth muscle cells, thus exacerbating systemic inflammation and promoting atherosclerotic plaque formation. Additionally, APOE4 may contribute to stromal activation of endothelial cells and pericytes that disturb the blood-brain barrier (BBB). These and other risk factors together lead to chronic inflammation, atherosclerosis, VCID, and neurodegeneration. Finally, we discuss potential cholesterol metabolism based approaches for future VCID treatment.

Brain arteriolosclerosis

Brain arteriolosclerosis (B-ASC), characterized by pathologic arteriolar wall thickening, is a common finding at autopsy in aged persons and is associated with cognitive impairment. Hypertension and diabetes are widely recognized as risk factors for B-ASC. Recent research indicates other and more complex risk factors and pathogenetic mechanisms. Here, we describe aspects of the unique architecture of brain arterioles, histomorphologic features of B-ASC, relevant neuroimaging findings, epidemiology and association with aging, established genetic risk factors, and the co-occurrence of B-ASC with other neuropathologic conditions such as Alzheimer's disease and limbic-predominant age-related TDP-43 encephalopathy (LATE). There may also be complex physiologic interactions between metabolic syndrome (e.g., hypertension and inflammation) and brain arteriolar pathology. Although there is no universally applied diagnostic methodology, several classification schemes and neuroimaging techniques are used to diagnose and categorize cerebral small vessel disease pathologies that include B-ASC, microinfarcts, microbleeds, lacunar infarcts, and cerebral amyloid angiopathy (CAA). In clinical-pathologic studies that factored in comorbid diseases, B-ASC was independently associated with impairments of global cognition, episodic memory, working memory, and perceptual speed, and has been linked to autonomic dysfunction and motor symptoms including parkinsonism. We conclude by discussing critical knowledge gaps related to B-ASC and suggest that there are probably subcategories of B-ASC that differ in pathogenesis. Observed in over 80% of autopsied individuals beyond 80 years of age, B-ASC is a complex and under-studied contributor to neurologic disability.

Sleep Deprivation Induces Cognitive Impairment by Increasing Blood-Brain Barrier Permeability via CD44

Sleep deprivation occurs frequently in older adults, which can result in delirium and cognitive impairment. CD44 is a key molecular in blood-brain barrier (BBB) regulation. However, whether CD44 participates in the role of sleep deprivation in cognitive impairment remains unclear. In this study, the effect of sleep deprivation on cognitive ability, tissue inflammation, BBB permeability, and astrocyte activity were evaluated in vivo. The differentially expressed genes (DEGs) were identified by RNA sequencing. A CD44 overexpression in the BBB model was performed in vitro to assess the effect and mechanisms of CD44. Sleep deprivation impaired the learning and memory ability and increased the levels of inflammatory cytokines, along with increased BBB permeability and activated astrocytes in hippocampus tissue. RNA sequencing of the hippocampus tissue revealed that 329 genes were upregulated in sleep deprivation-induced mice compared to control mice, and 147 genes were downregulated. GO and pathways showed that DEGs were mainly involved in BBB permeability and astrocyte activation, including nervous system development, neuron development, and brain development, and neuroactive ligand-receptor interaction. Moreover, the PCR analysis revealed that CD44 was dramatically increased in mice with sleep deprivation induction. The overexpression of CD44 in astrocytes promoted BBB permeability in vitro and induced the expression of the downstream gene NANOG. Our results indicate that sleep deprivation upregulated CD44 expression in hippocampus tissue, and increased BBB permeability, resulting in cognitive impairment.

Iron Metabolism in Oligodendrocytes and Astrocytes, Implications for Myelination and Remyelination

Iron is a key nutrient for normal central nervous system (CNS) development and function; thus, iron deficiency as well as iron excess may result in harmful effects in the CNS. Oligodendrocytes and astrocytes are crucial players in brain iron equilibrium. However, the mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes during CNS development or under pathological situations such as demyelination are not completely understood. In the CNS, iron is directly required for myelin production as a cofactor for enzymes involved in ATP, cholesterol and lipid synthesis, and oligodendrocytes are the cells with the highest iron levels in the brain which is linked to their elevated metabolic needs associated with the process of myelination. Unlike oligodendrocytes, astrocytes do not have a high metabolic requirement for iron. However, these cells are in close contact with blood vessel and have a strong iron transport capacity. In several pathological situations, changes in iron homoeostasis result in altered cellular iron distribution and accumulation and oxidative stress. In inflammatory demyelinating diseases such as multiple sclerosis, reactive astrocytes accumulate iron and upregulate iron efflux and influx molecules, which suggest that they are outfitted to take up and safely recycle iron. In this review, we will discuss the participation of oligodendrocytes and astrocytes in CNS iron homeostasis. Understanding the molecular mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes is necessary for planning effective strategies for iron management during CNS development as well as for the treatment of demyelinating diseases.

TGF-β Signaling: A Therapeutic Target to Reinstate Regenerative Plasticity in Vascular Dementia?

Vascular dementia (VaD) is the second leading form of memory loss after Alzheimer's disease (AD). Currently, there is no cure available. The etiology, pathophysiology and clinical manifestations of VaD are extremely heterogeneous, but the impaired cerebral blood flow (CBF) represents a common denominator of VaD. The latter might be the result of atherosclerosis, amyloid angiopathy, microbleeding and micro-strokes, together causing blood-brain barrier (BBB) dysfunction and vessel leakage, collectively originating from the consequence of hypertension, one of the main risk factors for VaD. At the histopathological level, VaD displays abnormal vascular remodeling, endothelial cell death, string vessel formation, pericyte responses, fibrosis, astrogliosis, sclerosis, microglia activation, neuroinflammation, demyelination, white matter lesions, deprivation of synapses and neuronal loss. The transforming growth factor (TGF) β has been identified as one of the key molecular factors involved in the aforementioned various pathological aspects. Thus, targeting TGF-β signaling in the brain might be a promising therapeutic strategy to mitigate vascular pathology and improve cognitive functions in patients with VaD. This review revisits the recent understanding of the role of TGF-β in VaD and associated pathological hallmarks. It further explores the potential to modulate certain aspects of VaD pathology by targeting TGF-β signaling.

Astrocyte mitochondria: Central players and potential therapeutic targets for neurodegenerative diseases and injury

Mitochondrial function has long been the focus of many therapeutic strategies for ameliorating age-related neurodegeneration and cognitive decline. Historically, the role of mitochondria in non-neuronal cell types has been overshadowed by neuronal mitochondria, which are responsible for the bulk of oxidative metabolism in the brain. Despite this neuronal bias, mitochondrial function in glial cells, particularly astrocytes, is increasingly recognized to play crucial roles in overall brain metabolism, synaptic transmission, and neuronal protection. Changes in astrocytic mitochondrial function appear to be intimately linked to astrocyte activation/reactivity found in most all age-related neurodegenerative diseases. Here, we address the importance of mitochondrial function to astrocyte signaling and consider how mitochondria could contribute to both the detrimental and protective properties of activated astrocytes. Strategies for protecting astrocytic mitochondrial function, promoting bidirectional transfer of mitochondria between astrocytes and neurons, and transplanting healthy mitochondria to diseased nervous tissue are also discussed.

Pathological Continuum From the Rise in Pulse Pressure to Impaired Neurovascular Coupling and Cognitive Decline

The "biomechanical hypothesis" stipulates that with aging, the cumulative mechanical damages to the cerebral microvasculature, magnified by risk factors for vascular diseases, contribute to a breach in cerebral homeostasis producing neuronal losses. In other words, vascular dysfunction affects brain structure and function, and leads to cognitive failure. This is gathered under the term Vascular Cognitive Impairment and Dementia (VCID). One of the main culprits in the occurrence of cognitive decline could be the inevitable rise in arterial pulse pressure due to the age-dependent stiffening of large conductance arteries like the carotids, which in turn, could accentuate the penetration of the pulse pressure wave deeper into the fragile microvasculature of the brain and damage it. In this review, we will discuss how and why the vascular and brain cells communicate and are interdependent, describe the deleterious impact of a vascular dysfunction on brain function in various neurodegenerative diseases and even of psychiatric disorders, and the potential chronic deleterious effects of the pulsatile blood pressure on the cerebral microcirculation. We will also briefly review data from antihypertensive clinical trial aiming at improving or delaying dementia. Finally, we will debate how the aging process, starting early in life, could determine our sensitivity to risk factors for vascular diseases, including cerebral diseases, and the trajectory to VCID.

Small Vessel Disease-Related Dementia: An Invalid Neurovascular Coupling?

The arteriosclerosis-dependent alteration of brain perfusion is one of the major determinants in small vessel disease, since small vessels have a pivotal role in the brain's autoregulation. Nevertheless, as far as we know, endothelium distress can potentiate the flow dysregulation and lead to subcortical vascular dementia that is related to small vessel disease (SVD), also being defined as subcortical vascular dementia (sVAD), as well as microglia activation, chronic hypoxia and hypoperfusion, vessel-tone dysregulation, altered astrocytes, and pericytes functioning blood-brain barrier disruption. The molecular basis of this pathology remains controversial. The apparent consequence (or a first event, too) is the macroscopic alteration of the neurovascular coupling. Here, we examined the possible mechanisms that lead a healthy aging process towards subcortical dementia. We remarked that SVD and white matter abnormalities related to age could be accelerated and potentiated by different vascular risk factors. Vascular function changes can be heavily influenced by genetic and epigenetic factors, which are, to the best of our knowledge, mostly unknown. Metabolic demands, active neurovascular coupling, correct glymphatic process, and adequate oxidative and inflammatory responses could be bulwarks in defense of the correct aging process; their impairments lead to a potentially catastrophic and non-reversible condition.

Astroglial connexins and cognition: memory formation or deterioration?

Connexins are the membrane proteins that form high-conductance plasma membrane channels and are the important constituents of gap junctions and hemichannels. Among different types of connexins, connexin 43 is the most widely expressed and studied gap junction proteins in astrocytes. Due to the key involvement of astrocytes in memory impairment and abundant expression of connexins in astrocytes, astroglial connexins have been projected as key therapeutic targets for Alzheimer's disease. On the other hand, the role of connexin gap junctions and hemichannels in memory formation and consolidation has also been reported. Moreover, deletion of these proteins and loss of gap junction communication result in loss of short-term spatial memory. Accordingly, both memory formation and memory deteriorating functions of astrocytes-located connexins have been documented. Physiologically expressed connexins may be involved in the memory formation, while pathologically increased expression of connexins with consequent excessive activation of astrocytes may induce neuronal injury and cognitive decline. The present review describes the memory formation as well as memory deteriorating functions of astroglial connexins in memory disorders of different etiology with possible mechanisms.

Norvaline Restores the BBB Integrity in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the leading cause of dementia. The disease progression is associated with the build-up of amyloid plaques and neurofibrillary tangles in the brain. However, besides the well-defined lesions, the AD-related pathology includes neuroinflammation, compromised energy metabolism, and chronic oxidative stress. Likewise, the blood-brain barrier (BBB) dysfunction is suggested to be a cause and AD consequence. Accordingly, therapeutic targeting of the compromised BBB is a promising disease-modifying approach. We utilized a homozygous triple-transgenic mouse model of AD (3×Tg-AD) to assess the effects of L-norvaline on BBB integrity. We scrutinized the perivascular astrocytes and macrophages by measuring the immunopositive profiles in relation to the presence of β-amyloid and compare the results with those found in wild-type animals. Typically, 3×Tg-AD mice display astroglia cytoskeletal atrophy, associated with the deposition of β-amyloid in the endothelia, and declining nitric oxide synthase (NOS) levels. L-norvaline escalated NOS levels, then reduced rates of BBB permeability, amyloid angiopathy, microgliosis, and astrodegeneration, which suggests AD treatment agent efficacy. Moreover, results undergird the roles of astrodegeneration and microgliosis in AD-associated BBB dysfunction and progressive cognitive impairment. L-norvaline self-evidently interferes with AD pathogenesis and presents a potent remedy for angiopathies and neurodegenerative disorders intervention.

The Emerging Roles of Ferroptosis in Vascular Cognitive Impairment

Vascular cognitive impairment (VCI) is a clinical syndrome that encompasses all forms of cognitive deficits caused by cerebrovascular disease, from mild cognitive impairment to dementia. Vascular dementia, the second most common type of dementia after Alzheimer’s disease (AD), accounts for approximately 20% of dementia patients. Ferroptosis is a recently defined iron-dependent form of cell death, which is distinct from apoptosis, necrosis, autophagy, and other forms of cell death. Emerging evidence suggests that ferroptosis has significant implications in neurological diseases such as stroke, traumatic brain injury, and AD. Additionally, ferroptosis inhibition has an obvious neuroprotective effect and ameliorates cognitive impairment in various animal models. Here, we summarize the underlying mechanisms of ferroptosis and review the close relationship between ferroptosis and VCI.

Vascular Cognitive Impairment: Information from Animal Models on the Pathogenic Mechanisms of Cognitive Deficits

Vascular cognitive impairment (VCI) is the second most common cause of cognitive deficit after Alzheimer's disease. Since VCI patients represent an important target population for prevention, an ongoing effort has been made to elucidate the pathogenesis of this disorder. In this review, we summarize the information from animal models on the molecular changes that occur in the brain during a cerebral vascular insult and ultimately lead to cognitive deficits in VCI. Animal models cannot effectively represent the complex clinical picture of VCI in humans. Nonetheless, they allow some understanding of the important molecular mechanisms leading to cognitive deficits. VCI may be caused by various mechanisms and metabolic pathways. The pathological mechanisms, in terms of cognitive deficits, may span from oxidative stress to vascular clearance of toxic waste products (such as amyloid beta) and from neuroinflammation to impaired function of microglia, astrocytes, pericytes, and endothelial cells. Impaired production of elements of the immune response, such as cytokines, and vascular factors, such as insulin-like growth factor 1 (IGF-1), may also affect cognitive functions. No single event could be seen as being the unique cause of cognitive deficits in VCI. These events are interconnected, and may produce cascade effects resulting in cognitive impairment.