Changes in brain β-amyloid deposition and aquaporin 4 levels in response to altered agrin expression in mice

Laminins and the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic structure that maintains brain homeostasis. BBB breakdown is a key pathological hallmark of almost all neurological diseases. Although the regulation of BBB integrity by different cells has been extensively studied, the function of its non-cellular component-the basal lamina in BBB regulation remains largely unknown. Laminin, a trimeric protein with multiple isoforms, is one of the most important constituents of the basal lamina. In the CNS, different cells synthesize distinct laminin isoforms, which differentially regulate BBB integrity in both physiological and pathological conditions. A thorough understanding of laminin expression and function in BBB integrity could lead to the identification of novel therapeutic targets and potentially result in effective treatments for neurological disorders involving BBB disruption. Here in this review, we first briefly introduce the BBB and basal lamina with a focus on laminin. Next, we elucidate laminin expression and its function in BBB maintenance/repair in a cell-specific manner. Potential functional compensation among laminin isoforms is also discussed. Last, current challenges in the field and future directions are summarized. Our goal is to provide a synthetic review to encourage novel ideas and stimulate new research in the field.

Structural changes in cerebral microvasculature induced by ferroptosis contribute to blood-brain barrier disruption in Alzheimer's disease: an autopsy study

INTRODUCTION

Cerebrovascular lesions are associated with cognitive impairment. However, the impact of AD neuropathological changes (ADNC) on cerebral microvasculature is not completely understood.

METHODS

Twelve decedents with ADNC and 15 matched controls were selected from the Brain Bank. The changes in the median tunica and basement membrane-related extracellular matrix (ECM) contents of the microvasculature were quantified and compared. Additionally, we explored the related mechanisms of agrin in pericytes.

RESULTS

Venular collagenosis was significantly more severe in AD patients (p < 0.001), and ECM remolding was significantly correlated with ADNC. In the AD group, blood-brain barrier (BBB) disruption and decreased pericytes were observed. Finally, we confirmed that agrin induced ferroptosis in pericytes and BBB disruption in vitro.

DISCUSSION

Our data indicate that venular collagenosis and significant ECM remolding are important contributors to ADNC. The mechanism by which agrin's role in disrupting the BBB by inducing ferroptosis presents a potential new target.

HIGHLIGHTS

Changes in the median tunica and basement membrane-related ECM contents of the microvasculature were quantified in human brains. Venular collagenosis was significantly more severe in AD patients. In the AD group, BBB disruption and ECM remodeling were important contributors to AD neuropathological changes. Agrin disrupted the BBB by inducing ferroptosis in pericytes, which presents a potential new target.

Update on the intriguing roles of AQP4 expression and redistribution in the progression and treatment of glioma

Aquaporin 4 (AQP4) is abundant in the human brain and has an important role in brain homeostasis and diseases. AQP4 expression has been found to be associated with glioma malignancies. However, the complete understanding of the biological processes and curative importance of AQP4 in glioma remains unclear. The impact of AQP4 subcellular mislocalization on glioma progression and the precise mechanisms regarding AQP4 translocation in glioma need further investigation. In this review, we update recent findings about disturbed AQP4 expression in glioma and explore targeting AQP4 to modulate the glioma progression. Thereafter we discuss some possible mechanisms of action of AQP4 translocations in glioma. The present article offers an appropriate introduction to the potential involvement of AQP4 in the emergence and progression of glioma. Both comprehensive research into the mechanisms and systematically intervention studies focusing on AQP4 are essential. By embracing this strategy, we can obtain a new and insightful outlook on managing cancerous glioma. Although the observations summarized in this review should be confirmed with more studies, we believe that they could provide critical information for the design of more focused research that will allow for systematic and definitive evaluation of the role of AQP4 in glioma treatments.

Biomimetic strategies for the deputization of proteoglycan functions

Proteoglycans (PGs), which have glycosaminoglycan chains attached to their protein cores, are essential for maintaining the morphology and function of healthy body tissues. Extracellular PGs perform various functions, classified into the following four categories: i) the modulation of tissue mechanical properties; ii) the regulation and protection of the extracellular matrix; iii) protein sequestration; and iv) the regulation of cell signaling. The depletion of PGs may significantly impair tissue function, encompassing compromised mechanical characteristics and unregulated inflammatory responses. Since PGs play critical roles in the function of healthy tissues and their synthesis is complex, the development of PG mimetic molecules that recapitulate PG functions for tissue engineering and therapeutic applications has attracted the interest of researchers for more than 20 years. These approaches have ranged from semisynthetic graft copolymers to recombinant PG domains produced by cells that have undergone genetic modifications. This review discusses some essential extracellular PG functions and approaches to mimicking these functions.

Single-cell transcriptome atlas in C57BL/6 mice encodes morphological phenotypes in the aging kidneys

C57BL/6 mice are frequently utilized as murine models with the desired genetic background for altertion in multiple research contexts. So far, there is still a lack of comprehensive kidney morphology and single-cell transcriptome atlas at all stages of growth of C57BL/6 mice. To provide an interactive set of reference standards for the scientific community, we performed the current study to investigate the kidney's development throughout the capillary-loop stage until senescence. Eight groups, with five to six mice each, represented embryonic stage (embryos 18.5 days), suckling period (1 day after birth), juvenile stage (1 month old), adulthood (containing 3 months old, 6 months old and 10 months old), reproductive senescence stage (20 months old), and post-senescence stage (30 months old), respectively. With age, the thickness of the glomerular basement membrane (GBM) was increased. Notably, GBM knobs appeared at three months and became frequent with age. Using single-cell transcriptome data, we evaluated how various biological process appear in particular cell types and investigated the potential mechanism of formation of GBM konbs. In conclusion, having access to detailed kidney morphology and single-cell transcriptome maps from C57BL/6 mice at various developmental stages of C57BL/6 mice would be a novel and major resource for biological research and testing of prospective therapeutic approaches.

Heparan sulfate proteoglycan in Alzheimer's disease: aberrant expression and functions in molecular pathways related to amyloid-β metabolism

Alzheimer's disease (AD) is the most common form of dementia. Currently, there is no effective treatment for AD, as its etiology remains poorly understood. Mounting evidence suggests that the accumulation and aggregation of amyloid-β peptides (Aβ), which constitute amyloid plaques in the brain, is critical for initiating and accelerating AD pathogenesis. Considerable efforts have been dedicated to shedding light on the molecular basis and fundamental origins of the impaired Aβ metabolism in AD. Heparan sulfate (HS), a linear polysaccharide of the glycosaminoglycan family, co-deposits with Aβ in plaques in the AD brain, directly binds and accelerates Aβ aggregation, and mediates Aβ internalization and cytotoxicity. Mouse model studies demonstrate that HS regulates Aβ clearance and neuroinflammation in vivo. Previous reviews have extensively explored these discoveries. Here, this review focuses on the recent advancements in understanding abnormal HS expression in the AD brain, the structural aspects of HS-Aβ interaction, and the molecules involved in modulating Aβ metabolism through HS interaction. Furthermore, this review presents a perspective on the potential effects of abnormal HS expression on Aβ metabolism and AD pathogenesis. In addition, the review highlights the importance of conducting further research to differentiate the spatiotemporal components of HS structure and function in the brain and AD pathogenesis.

Extracellular matrix proteins in construction and function of in vitro blood-brain barrier models

The blood-brain barrier (BBB) is a highly impermeable barrier separating circulating blood and brain tissue. A functional BBB is critical for brain health, and BBB dysfunction has been linked to the pathophysiology of diseases such as stroke and Alzheimer’s disease. A variety of models have been developed to study the formation and maintenance of the BBB, ranging from in vivo animal models to in vitro models consisting of primary cells or cells differentiated from human pluripotent stem cells (hPSCs). These models must consider the composition and source of the cellular components of the neurovascular unit (NVU), including brain microvascular endothelial cells (BMECs), brain pericytes, astrocytes, and neurons, and how these cell types interact. In addition, the non-cellular components of the BBB microenvironment, such as the brain vascular basement membrane (BM) that is in direct contact with the NVU, also play key roles in BBB function. Here, we review how extracellular matrix (ECM) proteins in the brain vascular BM affect the BBB, with a particular focus on studies using hPSC-derived in vitro BBB models, and discuss how future studies are needed to advance our understanding of how the ECM affects BBB models to improve model performance and expand our knowledge on the formation and maintenance of the BBB.

Urinary protein biomarkers based on LC-MS/MS analysis to discriminate vascular dementia from Alzheimer's disease in Han Chinese population

Objective

This study aimed to identify the potential urine biomarkers of vascular dementia (VD) and unravel the disease-associated mechanisms by applying Liquid chromatography tandem-mass spectrometry (LC-MS/MS).

Methods

LC-MS/MS proteomic analysis was applied to urine samples from 3 groups, including 14 patients with VD, 9 patients with AD, and 21 normal controls (NC). By searching the MS data by Proteome Discoverer software, analyzing the protein abundances qualitatively and quantitatively, comparing between groups, combining bioinformatics analysis using Gene Ontology (GO) and pathway crosstalk analysis using Kyoto Encyclopedia of Genes and Genomes (KEGG), and literature searching, the differentially expressed proteins (DEPs) of VD can be comprehensively determined at last and were further quantified by receiver operating characteristic (ROC) curve methods.

Results

The proteomic findings showed quantitative changes in patients with VD compared to patients with NC and AD groups; among 4,699 identified urine proteins, 939 and 1,147 proteins displayed quantitative changes unique to VD vs. NC and AD, respectively, including 484 overlapped common DEPs. Then, 10 unique proteins named in KEGG database (including PLOD3, SDCBP, SRC, GPRC5B, TSG101/STP22/VPS23, THY1/CD90, PLCD, CDH16, NARS/asnS, AGRN) were confirmed by a ROC curve method.

Conclusion

Our results suggested that urine proteins enable detection of VD from AD and VC, which may provide an opportunity for intervention.

Analysis of Cerebral Small Vessel Changes in AD Model Mice

Amyloid β (Aβ) peptide is deposited in the brains of sporadic Alzheimer's disease (AD) due to impaired vessel-dependent clearance. To understand the mechanisms, we investigated time-dependent cerebrovascular changes in AD model mice. Cerebrovascular and other pathological changes were analyzed in AD model mice (J20 strain) aging from 2 to 9 months by immunostaining. At 2 months, Aβ was only intraneuronal, whereas vessels were positive from 3 months in J20 mice. Compared to wild-type (WT), vessel density was increased at 2 months but decreased at 9 months in J20 mice, claudin-5 levels were decreased, and vascular endothelial growth factor (VEGF) levels were increased in the cortex and hippocampus of J20 mice brain at all time points. Albumin extravasation was evident from 3 months in J20 brains. Collagen 4 was increased at 2 and 3 months. Aquaporin 4 was spread beyond the vessels starting from 3 months in J20, which was restricted around the vessel in wild-type mice. In conclusion, the study showed that an early decrease in claudin-5 was associated with VEGF expression, indicating dysfunction of the blood-brain barrier. Decreased claudin-5 might cause the leakage of blood constituents into the parenchyma that alters astrocyte polarity and its functions.

The mechanisms underlying the actions of Xuefu Zhuyu decoction pretreatment against neurological deficits after ischemic stroke in mice: The mediation of glymphatic function by aquaporin-4 and its anchoring proteins

Ischemic stroke (IS) has been associated with an impairment in glymphatic function. Xuefu Zhuyu Decoction (XFZYD) is widely used in the prevention and treatment of ischemic stroke. We hypothesized that Xuefu Zhuyu decoction pretreatment could attenuate early neurological deficits after ischemic stroke by enhancing the function of the glymphatic system. To prove our hypothesis, we carried out temporary middle cerebral artery occlusion and reperfusion surgery on C57BL/6 mice and then measured neurological score, infarct size and performed hematoxylin-eosin staining to assess stroke outcomes after 24 h of reperfusion. Subsequently, we injected fluorescent tracers in to the cisterna magna and evaluated tracer distribution in coronal brain sections. The polarization of aquaporin-4 (AQP4), colocalization of aquaporin-4, α-dystroglycan, β-dystroglycan and agrin were determined by immunofluorescence. Our research showed that pretreatment with Xuefu Zhuyu decoction significantly alleviated neurological scores, neurological deficits and pathological abnormalities in a mouse model of ischemic stroke. Importantly, Xuefu Zhuyu decoction pretreatment enhanced cerebrospinal fluid influx, protected aquaporin-4 depolarization and promoted the colocalization of aquaporin-4 with its anchoring proteins in the brain. Our findings highlight novel mechanisms underlying the neuroprotective effect of Xuefu Zhuyu decoction pretreatment on ischemic stroke-induced brain damage through the glymphatic system. Xuefu Zhuyu decoction pretreatment may offer a promising approach to slow the onset and progression of ischemic stroke.

MRI-Visible Perivascular Spaces and Risk of Incident Dementia: The Framingham Heart Study

BACKGROUND AND OBJECTIVES

Perivascular spaces (PVS) visible on MRI scans may represent key aspects in the pathophysiology of stroke and dementia, including cerebral small vessel disease and glymphatic dysfunction. This study aimed to determine the association between MRI-visible PVS burden and the risk of incident dementia.

METHODS

This study included community-dwelling Framingham Heart Study Original and Offspring cohort participants with available brain MRI-PVS ratings, free of stroke and dementia. Multivariable Cox proportional hazard regression was used to obtain hazard ratios (HRs) and 95% CIs of the association between MRI-visible PVS and incident dementia. PVS were rated using validated methods in the basal ganglia (BG) and centrum semiovale (CSO). The outcomes included all-cause dementia, Alzheimer dementia (AD), and vascular dementia (VaD).

RESULTS

One thousand four hundred forty-nine participants 50 years or older (46% male) were included. Over a median follow-up period of 8.3 years, the incidence of all-cause dementia, AD, and VaD was 15.8%, 12.5%, and 2.5%, respectively. In models that adjusted for vascular risk factors and cardiovascular disease, the hazard for dementia increased steadily as PVS burden increased, rising 2-fold for those with grade II PVS (HR 2.44, 95% CI 1.51-3.93) to 5-fold in participants with grade IV (HR 5.05, 95% CI 2.75-9.26) compared with grade I PVS in CSO. In the BG, hazards increased 1.6-fold (HR 1.62, 95% CI 1.15-2.27) for grade II to 2.6-fold (HR 2.67, 95% CI 1.04-6.88) for grade IV compared with grade I PVS. The association remained significant for CSO but not for BG, after adjustment for white matter hyperintensity volume (WMHV), covert infarcts, and total brain volume. Similar findings were observed for AD, but VaD, limited by a small number of events, was not statistically significant.

DISCUSSION

Higher burden of PVS in CSO was associated with increased risk of developing dementia, independent of vascular risk factors, total brain volume, WMHVs, and covert infarcts. This finding supports a role for PVS as a subclinical MRI marker to identify individuals in subclinical stages at high risk of developing dementia who may benefit from early intervention.

Blood-Brain Barrier Alterations and Edema Formation in Different Brain Mass Lesions

Differential diagnosis of brain lesion pathologies is complex, but it is nevertheless crucial for appropriate clinical management. Advanced imaging methods, including diffusion-weighted imaging and apparent diffusion coefficient, can help discriminate between brain mass lesions such as glioblastoma, brain metastasis, brain abscesses as well as brain lymphomas. These pathologies are characterized by blood-brain barrier alterations and have been extensively studied. However, the changes in the blood-brain barrier that are observed around brain pathologies and that contribute to the development of vasogenic brain edema are not well described. Some infiltrative brain pathologies such as glioblastoma are characterized by glioma cell infiltration in the brain tissue around the tumor mass and thus affect the nature of the vasogenic edema. Interestingly, a common feature of primary and secondary brain tumors or tumor-like brain lesions characterized by vasogenic brain edema is the formation of various molecules that lead to alterations of tight junctions and result in blood-brain barrier damage. The resulting vasogenic edema, especially blood-brain barrier disruption, can be visualized using advanced magnetic resonance imaging techniques, such as diffusion-weighted imaging and apparent diffusion coefficient. This review presents a comprehensive overview of blood-brain barrier changes contributing to the development of vasogenic brain edema around glioblastoma, brain metastases, lymphomas, and abscesses.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

The amyloid plaque proteome in early onset Alzheimer's disease and Down syndrome

Amyloid plaques contain many proteins in addition to beta amyloid (Aβ). Previous studies examining plaque-associated proteins have shown these additional proteins are important; they provide insight into the factors that drive amyloid plaque development and are potential biomarkers or therapeutic targets for Alzheimer's disease (AD). The aim of this study was to comprehensively identify proteins that are enriched in amyloid plaques using unbiased proteomics in two subtypes of early onset AD: sporadic early onset AD (EOAD) and Down Syndrome (DS) with AD. We focused our study on early onset AD as the drivers of the more aggressive pathology development in these cases is unknown and it is unclear whether amyloid-plaque enriched proteins differ between subtypes of early onset AD. Amyloid plaques and neighbouring non-plaque tissue were microdissected from human brain sections using laser capture microdissection and label-free LC-MS was used to quantify the proteins present. 48 proteins were consistently enriched in amyloid plaques in EOAD and DS. Many of these proteins were more significantly enriched in amyloid plaques than Aβ. The most enriched proteins in amyloid plaques in both EOAD and DS were: COL25A1, SMOC1, MDK, NTN1, OLFML3 and HTRA1. Endosomal/lysosomal proteins were particularly highly enriched in amyloid plaques. Fluorescent immunohistochemistry was used to validate the enrichment of four proteins in amyloid plaques (moesin, ezrin, ARL8B and SMOC1) and to compare the amount of total Aβ, Aβ40, Aβ42, phosphorylated Aβ, pyroglutamate Aβ species and oligomeric species in EOAD and DS. These studies showed that phosphorylated Aβ, pyroglutamate Aβ species and SMOC1 were significantly higher in DS plaques, while oligomers were significantly higher in EOAD. Overall, we observed that amyloid plaques in EOAD and DS largely contained the same proteins, however the amount of enrichment of some proteins was different in EOAD and DS. Our study highlights the significant enrichment of many proteins in amyloid plaques, many of which may be potential therapeutic targets and/or biomarkers for AD.

Emerging roles for dynamic aquaporin-4 subcellular relocalization in CNS water homeostasis

Aquaporin channels facilitate bidirectional water flow in all cells and tissues. AQP4 is highly expressed in astrocytes. In the CNS, it is enriched in astrocyte endfeet, at synapses, and at the glia limitans, where it mediates water exchange across the blood-spinal cord and blood-brain barriers (BSCB/BBB), and controls cell volume, extracellular space volume, and astrocyte migration. Perivascular enrichment of AQP4 at the BSCB/BBB suggests a role in glymphatic function. Recently, we have demonstrated that AQP4 localization is also dynamically regulated at the subcellular level, affecting membrane water permeability. Ageing, cerebrovascular disease, traumatic CNS injury, and sleep disruption are established and emerging risk factors in developing neurodegeneration, and in animal models of each, impairment of glymphatic function is associated with changes in perivascular AQP4 localization. CNS oedema is caused by passive water influx through AQP4 in response to osmotic imbalances. We have demonstrated that reducing dynamic relocalization of AQP4 to the BSCB/BBB reduces CNS oedema and accelerates functional recovery in rodent models. Given the difficulties in developing pore-blocking AQP4 inhibitors, targeting AQP4 subcellular localization opens up new treatment avenues for CNS oedema, neurovascular and neurodegenerative diseases, and provides a framework to address fundamental questions about water homeostasis in health and disease.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

Integration of System Biology Tools to Investigate Huperzine A as an Anti-Alzheimer Agent

Aim: The present study aimed to investigate huperzine A as an anti-Alzheimer agent based on the principle that a single compound can regulate multiple proteins and associated pathways, using system biology tools. Methodology: The simplified molecular-input line-entry system of huperzine A was retrieved from the PubChem database, and its targets were predicted using SwissTargetPrediction. These targets were matched with the proteins deposited in DisGeNET for Alzheimer disease and enriched in STRING to identify the probably regulated pathways, cellular components, biological processes, and molecular function. Furthermore, huperzine A was docked against acetylcholinesterase using AutoDock Vina, and simulations were performed with the Gromacs package to take into account the dynamics of the system and its effect on the stability and function of the ligands. Results: A total of 100 targets were predicted to be targeted by huperzine A, of which 42 were regulated at a minimum probability of 0.05. Similarly, 101 Kyoto Encyclopedia of Genes and Genomes pathways were triggered, in which neuroactive ligand-receptor interactions scored the least false discovery rate. Also, huperzine A was predicted to modulate 54 cellular components, 120 molecular functions, and 873 biological processes. Furthermore, huperzine A possessed a binding affinity of -8.7 kcal/mol with AChE and interacted within the active site of AChE via H-bonds and hydrophobic interactions.

Basal lamina changes in neurodegenerative disorders

BACKGROUND

Neurodegenerative disorders are a group of age-associated diseases characterized by progressive degeneration of the structure and function of the CNS. Two key pathological features of these disorders are blood-brain barrier (BBB) breakdown and protein aggregation.

MAIN BODY

The BBB is composed of various cell types and a non-cellular component---the basal lamina (BL). Although how different cells affect the BBB is well studied, the roles of the BL in BBB maintenance and function remain largely unknown. In addition, located in the perivascular space, the BL is also speculated to regulate protein clearance via the meningeal lymphatic/glymphatic system. Recent studies from our laboratory and others have shown that the BL actively regulates BBB integrity and meningeal lymphatic/glymphatic function in both physiological and pathological conditions, suggesting that it may play an important role in the pathogenesis and/or progression of neurodegenerative disorders. In this review, we focus on changes of the BL and its major components during aging and in neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). First, we introduce the vascular and lymphatic systems in the CNS. Next, we discuss the BL and its major components under homeostatic conditions, and summarize their changes during aging and in AD, PD, and ALS in both rodents and humans. The functional significance of these alterations and potential therapeutic targets are also reviewed. Finally, key challenges in the field and future directions are discussed.

CONCLUSIONS

Understanding BL changes and the functional significance of these changes in neurodegenerative disorders will fill the gap of knowledge in the field. Our goal is to provide a clear and concise review of the complex relationship between the BL and neurodegenerative disorders to stimulate new hypotheses and further research in this field.

Extracellular protein components of amyloid plaques and their roles in Alzheimer's disease pathology

Alzheimer's disease (AD) is pathologically defined by the presence of fibrillar amyloid β (Aβ) peptide in extracellular senile plaques and tau filaments in intracellular neurofibrillary tangles. Extensive research has focused on understanding the assembly mechanisms and neurotoxic effects of Aβ during the last decades but still we only have a brief understanding of the disease associated biological processes. This review highlights the many other constituents that, beside Aβ, are accumulated in the plaques, with the focus on extracellular proteins. All living organisms rely on a delicate network of protein functionality. Deposition of significant amounts of certain proteins in insoluble inclusions will unquestionably lead to disturbances in the network, which may contribute to AD and copathology. This paper provide a comprehensive overview of extracellular proteins that have been shown to interact with Aβ and a discussion of their potential roles in AD pathology. Methods that can expand the knowledge about how the proteins are incorporated in plaques are described. Top-down methods to analyze post-mortem tissue and bottom-up approaches with the potential to provide molecular insights on the organization of plaque-like particles are compared. Finally, a network analysis of Aβ-interacting partners with enriched functional and structural key words is presented.

A Comprehensive Analysis of Unique and Recurrent Copy Number Variations in Alzheimer's Disease and its Related Disorders

BACKGROUND

Copy number variations (CNVs) play an important role in the genetic etiology of various neurological disorders, including Alzheimer's disease (AD). Type 2 diabetes mellitus (T2DM) and major depressive disorder (MDD) were shown to have share mechanisms and signaling pathways with AD.

OBJECTIVE

We aimed to assess CNVs regions that may harbor genes contributing to AD, T2DM, and MDD in 67 Saudi familial and sporadic AD patients, with no alterations in the known genes of AD and genotyped previously for APOE.

METHODS

DNA was analyzed using the CytoScan-HD array. Two layers of filtering criteria were applied. All the identified CNVs were checked in the Database of Genomic Variants (DGV).

RESULTS

A total of 1086 CNVs (565 gains and 521 losses) were identified in our study. We found 73 CNVs harboring genes that may be associated with AD, T2DM or MDD. Nineteen CNVs were novel. Most importantly, 42 CNVs were unique in our studied cohort existing only in one patient. Two large gains on chromosomes 1 and 13 harbored genes implicated in the studied disorders. We identified CNVs in genes that encode proteins involved in the metabolism of amyloid-β peptide (AGRN, APBA2, CR1, CR2, IGF2R, KIAA0125, MBP, RER1, RTN4R, VDR and WISPI) or Tau proteins (CACNAIC, CELF2, DUSP22, HTRA1 and SLC2A14).

CONCLUSION

The present work provided information on the presence of CNVs related to AD, T2DM, and MDD in Saudi Alzheimer's patients.

Transcriptome Changes in the Alzheimer's Disease Middle Temporal Gyrus: Importance of RNA Metabolism and Mitochondria-Associated Membrane Genes

We used Illumina Human HT-12 v4 arrays to compare RNA expression of middle temporal gyrus (MTG; BA21) in Alzheimer's disease (AD = 97) and non-demented controls (ND = 98). A total of 938 transcripts were highly differentially expressed (adj p < 0.01; log2 FC ≥ |0.500|, with 411 overexpressed and 527 underexpressed in AD. Our results correlated with expression profiling in neurons from AD and ND obtained by laser capture microscopy in MTG from an independent dataset (log2 FC correlation: r = 0.504; p = 2.2e-16). Additionally, selected effects were validated by qPCR. ANOVA analysis yielded no difference between genders in response to AD, but some gender specific genes were detected (e.g., IL8 and AGRN in males, and HSPH1 and GRM1 in females). Several transcripts were associated with Braak staging (e.g., AEBP1 and DNALI1), antemortem MMSE (e.g., AEBP1 and GFAP), and tangle density (e.g., RNU1G2, and DNALI1). At the pathway level, we detected enrichment of synaptic vesicle processes and GABAergic transmission genes. Finally, applying the Weighted Correlation Network Analysis, we identified four expression modules enriched for neuronal and synaptic genes, mitochondria-associated membrane, chemical stimulus and olfactory receptor and non-coding RNA metabolism genes. Our results represent an extensive description of MTG mRNA profiling in a large sample of AD and ND. These data provide a list of genes associated with AD, and correlated to neurofibrillary tangles density. In addition, these data emphasize the importance of mitochondrial membranes and transcripts related to olfactory receptors in AD.

A Role of Low-Density Lipoprotein Receptor-Related Protein 4 (LRP4) in Astrocytic Aβ Clearance

Amyloid-β (Aβ) deposition occurs years before cognitive symptoms appear and is considered a cause of Alzheimer's disease (AD). The imbalance of Aβ production and clearance leads to Aβ accumulation and Aβ deposition. Increasing evidence indicates an important role of astrocytes, the most abundant cell type among glial cells in the brain, in Aβ clearance. We explored the role of low-density lipoprotein receptor-related protein 4 (LRP4), a member of the LDLR family, in AD pathology. We show that Lrp4 is specifically expressed in astrocytes and its levels in astrocytes were higher than those of Ldlr and Lrp1, both of which have been implicated in Aβ uptake. LRP4 was reduced in postmortem brain tissues of AD patients. Genetic deletion of the Lrp4 gene augmented Aβ plaques in 5xFAD male mice, an AD mouse model, and exacerbated the deficits in neurotransmission, synchrony between the hippocampus and PFC, and cognition. Mechanistically, LRP4 promotes Aβ uptake by astrocytes likely by interacting with ApoE. Together, our study demonstrates that astrocytic LRP4 plays an important role in Aβ pathology and cognitive function.SIGNIFICANCE STATEMENT This study investigates how astrocytes, a type of non-nerve cells in the brain, may contribute to Alzheimer's disease (AD) development. We demonstrate that the low-density lipoprotein receptor-related protein 4 (LRP4) is reduced in the brain of AD patients. Mimicking the reduced levels in an AD mouse model exacerbates cognitive impairment and increases amyloid aggregates that are known to damage the brain. We show that LRP4 could promote the clearance of amyloid protein by astrocytes. Our results reveal a previously unappreciated role of LRP4 in AD development.

HIV-1 and Amyloid Beta Remodel Proteome of Brain Endothelial Extracellular Vesicles

Amyloid beta (Aβ) depositions are more abundant in HIV-infected brains. The blood-brain barrier, with its backbone created by endothelial cells, is assumed to be a core player in Aβ homeostasis and may contribute to Aβ accumulation in the brain. Exposure to HIV increases shedding of extracellular vesicles (EVs) from human brain endothelial cells and alters EV-Aβ levels. EVs carrying various cargo molecules, including a complex set of proteins, can profoundly affect the biology of surrounding neurovascular unit cells. In the current study, we sought to examine how exposure to HIV, alone or together with Aβ, affects the surface and total proteomic landscape of brain endothelial EVs. By using this unbiased approach, we gained an unprecedented, high-resolution insight into these changes. Our data suggest that HIV and Aβ profoundly remodel the proteome of brain endothelial EVs, altering the pathway networks and functional interactions among proteins. These events may contribute to the EV-mediated amyloid pathology in the HIV-infected brain and may be relevant to HIV-1-associated neurocognitive disorders.

Investigating Causal Relations Between Risk Tolerance, Risky Behaviors, and Alzheimer's Disease: A Bidirectional Two-Sample Mendelian Randomization Study

BACKGROUND

Several studies have shown risky behaviors and risk tolerance are associated with Alzheimer's disease. However, the underlying causality remains unclear. Risky behavior and risk tolerance may induce the onset of Alzheimer's disease, and/or vulnerability to Alzheimer's disease may result in more risky behaviors.

OBJECTIVE

To examine bidirectional relationships between risky behavior, risk tolerance, and Alzheimer's disease using Mendelian randomization method for assessing potential causal inference.

METHODS

This bidirectional two-sample Mendelian randomization study used independent genetic variants associated with risky behaviors and risk tolerance (n = 370, 771- 939, 908), and Alzheimer's disease (n = 71, 880 - 37, 613) as genetic instruments from large meta-analyses of genome-wide association studies.

RESULTS

Our results support a strong protective casual effect of risk-taking tendency on AD (odds ratio [OR] = 0.79; 95% CI, 0.67- 0.94, p = 0.007). There was weak statistically significant relationship between number of sexual partners and AD (OR = 0.50, 95% CI, 0.27- 0.93, p = 0.04), and between family history of AD and automobile speeding propensity (OR = 1.018, 95% CI, 1.005 to 1.031; p = 0.007). Contrary to expectations, there was no statistically significant causal effect of AD on risk-taking tendency (β=  0.015, 95% CI, - 0.005 to 0.04; p = 0.14).

CONCLUSION

Under Mendelian randomization assumptions, our results suggest a protective relationship between risk-taking tendency and the risk of AD. This finding may provide valuable insights into Alzheimer's disease pathogenesis and the development of preventive strategies.

Chronic stress impairs the aquaporin-4-mediated glymphatic transport through glucocorticoid signaling

BACKGROUND

The glymphatic system has recently been proposed to function as a brain-wide macroscopic system for the clearance of potentially harmful molecules, such as amyloid beta (e.g., Aβ), from the brain parenchyma. Previous literatures have established that the glymphatic function is dramatically suppressed by aging, traumatic brain injury, and some diseases. However, the effect of chronic stress on the glymphatic function and its underlying mechanism remains largely unknown.

METHODS

Adult mice were randomly divided into four groups: chronic unpredictable mild stress (CUMS)-treated group, CUMS simultaneously treated with mifepristone (MFP) group, dexamethasone (DEX)-treated group, and control group. Stress response was observed by assessing the change of body weight, plasma corticosterone level, and behavior tests. The level of Aβ42 in cerebral tissue was assessed by ELISA. The glymphatic function was determined by using fluorescence tracer injection. The expression and localization of aquaporin-4 (AQP4) were evaluated by immunohistochemistry and western blot. The transcription level of AQP4 and anchoring molecules was evaluated by real-time PCR.

FINDINGS

Compared with control group, CUMS-treated mice exhibited the impairment of global glymphatic function especially in the anterior brain. This change was accompanied by the decreased expression and polarization of AQP4, reduced transcription of AQP4, agrin, laminin, and dystroglycan in the anterior cortex. Similarly, the glucocorticoid receptor (GR) agonist DEX exposure could reduce the glymphatic function and AQP4 expression. Moreover, the GR antagonist MFP treatment could significantly rescue the glymphatic function and reverse the expression and polarization of AQP4 impaired by CUMS.

CONCLUSION

Chronic stress could impair the AQP4-mediated glymphatic transport in the brain through glucocorticoid signaling. Our results also suggest that GR antagonist could be beneficial to rescue the glymphatic function suppressed by chronic stress.

Blood-Cerebrospinal Fluid Barrier Gradients in Mild Cognitive Impairment and Alzheimer's Disease: Relationship to Inflammatory Cytokines and Chemokines

Background: The pathophysiology underlying altered blood-cerebrospinal fluid barrier (BCSFB) function in Alzheimer's disease (AD) is unknown but may relate to endothelial cell activation and cytokine mediated inflammation. Methods: Cerebrospinal fluid (CSF) and peripheral blood were concurrently collected from cognitively healthy controls (N = 21) and patients with mild cognitive impairment (MCI) (N = 8) or AD (N = 11). The paired serum and CSF samples were assayed for a panel of cytokines, chemokines, and related trophic factors using multiplex ELISAs. Dominance analysis models were conducted to determine the relative importance of the inflammatory factors in relationship to BCSFB permeability, as measured by CSF/serum ratios for urea, creatinine, and albumin. Results: BCSFB disruption to urea, a small molecule distributed by passive diffusion, had a full model coefficient of determination (r²) = 0.35, and large standardized dominance weights (>0.1) for monocyte chemoattractant protein-1, interleukin (IL)-15, IL-1rα, and IL-2 in serum. BCSFB disruption to creatinine, a larger molecule governed by active transport, had a full model r² = 0.78, and large standardized dominance weights for monocyte inhibitor protein-1b in CSF and tumor necrosis factor-α in serum. BCSFB disruption to albumin, a much larger molecule, had a full model r² = 0.62, and large standardized dominance weights for IL-17a, interferon-gamma, IL-2, and VEGF in CSF, as well IL-4 in serum. Conclusions: Inflammatory proteins have been widely documented in the AD brain. The results of the current study suggest that changes in BCSFB function resulting in altered permeability and transport are related to expression of specific inflammatory proteins, and that the shifting distribution of these proteins from serum to CSF in AD and MCI is correlated with more severe perturbations in BCSFB function.

Reactive Astrocytes as Drug Target in Alzheimer's Disease

Alzheimer's disease is a neurodegenerative disease characterized by deposition of extracellular amyloid-β, intracellular neurofibrillary tangles, and loss of cortical neurons. However, the mechanism underlying neurodegeneration in Alzheimer's disease (AD) remains to be explored. Many of the researches on AD have been primarily focused on neuronal changes. Current research, however, broadens to give emphasis on the importance of nonneuronal cells, such as astrocytes. Astrocytes play fundamental roles in several cerebral functions and their dysfunctions promote neurodegeneration and, eventually, retraction of neuronal synapses, which leads to cognitive deficits found in AD. Astrocytes become reactive as a result of deposition of Aβ, which in turn have detrimental consequences, including decreased glutamate uptake due to reduced expression of uptake transporters, altered energy metabolism, altered ion homeostasis (K⁺ and Ca⁺), increased tonic inhibition, and increased release of cytokines and inflammatory mediators. In this review, recent insights on the involvement of, tonic inhibition, astrocytic glutamate transporters and aquaporin in the pathogenesis of Alzheimer's disease are provided. Compounds which increase expression of GLT1 have showed efficacy for AD in preclinical studies. Tonic inhibition mediated by GABA could also be a promising target and drugs that block the GABA synthesizing enzyme, MAO-B, have shown efficacy. However, there are contradictory evidences on the role of AQP4 in AD.

The potential roles of aquaporin 4 in malignant gliomas

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system and is primarily expressed in astrocytes. Recently, accumulated evidence has pointed to AQP4 as a key molecule that could play a critical role in glioma development. Discoveries of the role of AQP4 in cell migration suggest that AQP4 could be a significant factor regarding glioma malignancies. However, the AQP4 expression levels in glioma have not been fully elucidated; furthermore, the correlation of AQP4 expression with glioma malignancy remains controversial. Here, we review the expression pattern and predictive significance of AQP4 in malignant glioma. The molecular mechanism of AQP4 as it pertains to the migration and invasion of human glioma cells has been summarized. In addition, the important roles of AQP4 in combating drug resistance as well as potential pharmacological blockers of AQP4 have been systematically discussed. More research should be conducted to elucidate the potential roles of AQP4 in malignant glioma for identifying the tumor type, progression stages and optimal treatment strategies. The observed experimental results strongly emphasize the importance of this topic for future investigations.

Pla2g6 Deficiency in Zebrafish Leads to Dopaminergic Cell Death, Axonal Degeneration, Increased β-Synuclein Expression, and Defects in Brain Functions and Pathways

This study aimed to gain insights into the pathophysiology underlying PLA2G6-associated neurodegeneration that is implicated in three different neurological disorders, suggesting that other, unknown genetic or environmental factors might contribute to its wide phenotypic expression. To accomplish this, we downregulated the function of pla2g6 in the zebrafish nervous system, performed parkinsonism-related phenotypic characterization, and determined the effects of gene regulation upon the loss of pla2g6 function by using RNA sequencing and downstream analyses. Pla2g6 deficiency resulted in axonal degeneration, dopaminergic and motor neuron cell loss, and increased β-synuclein expression. We also observed that many of the identified, differentially expressed genes were implicated in other brain disorders, which might explain the variable phenotypic expression of pla2g6-associated disease, and found that top enriched canonical pathways included those already known or suggested to play a major role in the pathogenesis of Parkinson's disease. Our data support that pla2g6 is relevant for cranial motor development with significant implications in the pathophysiology underlying Parkinson's disease.

The vascular basement membrane in the healthy and pathological brain

The vascular basement membrane contributes to the integrity of the blood-brain barrier (BBB), which is formed by brain capillary endothelial cells (BCECs). The BCECs receive support from pericytes embedded in the vascular basement membrane and from astrocyte endfeet. The vascular basement membrane forms a three-dimensional protein network predominantly composed of laminin, collagen IV, nidogen, and heparan sulfate proteoglycans that mutually support interactions between BCECs, pericytes, and astrocytes. Major changes in the molecular composition of the vascular basement membrane are observed in acute and chronic neuropathological settings. In the present review, we cover the significance of the vascular basement membrane in the healthy and pathological brain. In stroke, loss of BBB integrity is accompanied by upregulation of proteolytic enzymes and degradation of vascular basement membrane proteins. There is yet no causal relationship between expression or activity of matrix proteases and the degradation of vascular matrix proteins in vivo. In Alzheimer's disease, changes in the vascular basement membrane include accumulation of Aβ, composite changes, and thickening. The physical properties of the vascular basement membrane carry the potential of obstructing drug delivery to the brain, e.g. thickening of the basement membrane can affect drug delivery to the brain, especially the delivery of nanoparticles.

A Novel Egr-1-Agrin Pathway and Potential Implications for Regulation of Synaptic Physiology and Homeostasis at the Neuromuscular Junction

Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the AGRN gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1−/−) display a marked increase in AGRN mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1−/− mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1−/− vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1−/− mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.

Human and mouse cortical astrocytes differ in aquaporin-4 polarization toward microvessels

Aquaporin‐4 (AQP4), the predominant water channel in the brain, is expressed in astrocytes and ependymal cells. In rodents AQP4 is highly polarized to perivascular astrocytic endfeet and loss of AQP4 polarization is associated with disease. The present study was undertaken to compare the expression pattern of AQP4 in human and mouse cortical astrocytes. Cortical tissue specimens were sampled from 11 individuals undergoing neurosurgery wherein brain tissue was removed as part of the procedure, and compared with cortical tissue from 5 adult wild‐type mice processed similarly. The tissue samples were immersion‐fixed and prepared for AQP4 immunogold electron microscopy, allowing quantitative assessment of AQP4's subcellular distribution. In mouse we found that AQP4 water channels were prominently clustered around vessels, being 5 to 10‐fold more abundant in astrocytic endfoot membranes facing the capillary endothelium than in parenchymal astrocytic membranes. In contrast, AQP4 was markedly less polarized in human astrocytes, being only two to three‐fold enriched in astrocytic endfoot membranes adjacent to capillaries. The lower degree of AQP4 polarization in human subjects (1/3 of that in mice) was mainly due to higher AQP4 expression in parenchymal astrocytic membranes. We conclude that there are hitherto unrecognized species differences in AQP4 polarization toward microvessels in the cerebral cortex.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Neuronal heparan sulfates promote amyloid pathology by modulating brain amyloid-β clearance and aggregation in Alzheimer's disease

Accumulation of amyloid-β (Aβ) peptide in the brain is the first critical step in the pathogenesis of Alzheimer's disease (AD). Studies in humans suggest that Aβ clearance from the brain is frequently impaired in late-onset AD. Aβ accumulation leads to the formation of Aβ aggregates, which injure synapses and contribute to eventual neurodegeneration. Cell surface heparan sulfates (HSs), expressed on all cell types including neurons, have been implicated in several features in the pathogenesis of AD including its colocalization with amyloid plaques and modulatory role in Aβ aggregation. We show that removal of neuronal HS by conditional deletion of the Ext1 gene, which encodes an essential glycosyltransferase for HS biosynthesis, in postnatal neurons of amyloid model APP/PS1 mice led to a reduction in both Aβ oligomerization and the deposition of amyloid plaques. In vivo microdialysis experiments also detected an accelerated rate of Aβ clearance in the brain interstitial fluid, suggesting that neuronal HS either inhibited or represented an inefficient pathway for Aβ clearance. We found that the amounts of various HS proteoglycans (HSPGs) were increased in postmortem human brain tissues from AD patients, suggesting that this pathway may contribute directly to amyloid pathogenesis. Our findings have implications for AD pathogenesis and provide insight into therapeutic interventions targeting Aβ-HSPG interactions.

Synthesis and deposition of basement membrane proteins by primary brain capillary endothelial cells in a murine model of the blood-brain barrier

The brain vascular basement membrane is important for both blood-brain barrier (BBB) development, stability, and barrier integrity and the contribution hereto from brain capillary endothelial cells (BCECs), pericytes, and astrocytes of the BBB is probably significant. The aim of this study was to analyse four different in vitro models of the murine BBB for expression and possible secretion of major basement membrane proteins from murine BCECs (mBCECs). mBCECs, pericytes and glial cells (mainly astrocytes and microglia) were prepared from brains of C57BL/6 mice. The mBCECs were grown as monoculture, in co-culture with pericytes or mixed glial cells, or as a triple-culture with both pericytes and mixed glial cells. The integrity of the BBB models was validated by measures of transendothelial electrical resistance (TEER) and passive permeability to mannitol. The expression of basement membrane proteins was analysed using RT-qPCR, mass spectrometry and immunocytochemistry. Co-culturing mBCECs with pericytes, mixed glial cells, or both significantly increased the TEER compared to the monoculture, and a low passive permeability was correlated with high TEER. The mBCECs expressed all major basement membrane proteins such as laminin-411, laminin-511, collagen [α1(IV)]₂ α2(IV), agrin, perlecan, and nidogen 1 and 2 in vitro. Increased expression of the laminin α5 subunit correlated with the addition of BBB-inducing factors (hydrocortisone, Ro 20-1724, and pCPT-cAMP), whereas increased expression of collagen IV α1 primarily correlated with increased levels of cAMP. In conclusion, BCECs cultured in vitro coherently form a BBB and express basement membrane proteins as a feature of maturation. Cover Image for this issue: doi: 10.1111/jnc.13789.

Factors controlling permeability of the blood-brain barrier

As the primary protective barrier for neurons in the brain, the blood-brain barrier (BBB) exists between the blood microcirculation system and the brain parenchyma. The normal BBB integrity is essential in protecting the brain from systemic toxins and maintaining the necessary level of nutrients and ions for neuronal function. This integrity is mediated by structural BBB components, such as tight junction proteins, integrins, annexins, and agrin, of a multicellular system including endothelial cells, astrocytes, pericytes, etc. BBB dysfunction is a significant contributor to the pathogeneses of a variety of brain disorders. Many signaling factors have been identified to be able to control BBB permeability through regulating the structural components. Among those signaling factors are inflammatory mediators, free radicals, vascular endothelial growth factor, matrix metalloproteinases, microRNAs, etc. In this review, we provide a summary of recent progress regarding these structural components and signaling factors, relating to their roles in various brain disorders. Attention is also devoted to recent research regarding impact of pharmacological agents such as isoflurane on BBB permeability and how iron ion passes across BBB. Hopefully, a better understanding of the factors controlling BBB permeability helps develop novel pharmacological interventions of BBB hyperpermeability under pathological conditions.

Reduced CSF Water Influx in Alzheimer's Disease Supporting the β-Amyloid Clearance Hypothesis

OBJECTIVE

To investigate whether water influx into cerebrospinal fluid (CSF) space is reduced in Alzheimer's patients as previously shown in the transgenic mouse model for Alzheimer's disease.

METHODS

Ten normal young volunteers (young control, 21-30 years old), ten normal senior volunteers (senior control, 60-78 years old, MMSE ≥ 29), and ten Alzheimer's disease (AD) patients (study group, 59-84 years old, MMSE: 13-19) participated in this study. All AD patients were diagnosed by neurologists specializing in dementia based on DSM-IV criteria. CSF dynamics were analyzed using positron emission tomography (PET) following an intravenous injection of 1,000 MBq [15O]H2O synthesized on-line.

RESULTS

Water influx into CSF space in AD patients, expressed as influx ratio, (0.755 ± 0.089) was significantly reduced compared to young controls (1.357 ± 0.185; p < 0.001) and also compared to normal senior controls (0.981 ± 0.253, p < 0.05). Influx ratio in normal senior controls was significantly reduced compared to young controls (p < 0.01).

CONCLUSION

Water influx into the CSF is significantly reduced in AD patients. β-amyloid clearance has been shown to be dependent on interstitial flow and CSF production. The current study indicates that reduction in water influx into the CSF may disturb the clearance rate of β-amyloid, and therefore be linked to the pathogenesis of AD.

TRIAL REGISTRATION

UMIN Clinical Trials Registry UMIN000011939.

Can agrin cerebrospinal fluid concentration be used as an early biomarker for Alzheimer's disease?

The need for effective treatments halting Alzheimer's disease (AD) urges the discovery of the earliest possible biomarkers. Agrin is increased in the early stages of AD and is involved in amyloid-β (Aβ) fibrillation and synaptogenesis. We investigated the potential of agrin as an early AD cerebrospinal fluid (CSF) biomarker. We analyzed the agrin CSF concentration in nondemented controls (n = 20) and those with mild (n = 20) and severe (n = 20) AD. The levels of agrin CSF were not significantly divergent among the different patient groups and did not correlate with the concentration of Aβ42, total tau, phosphorylated tau, or the Mini Mental State Examination scores. However, agrin strongly correlated with age in those with dementia. The results indicate that agrin cannot be used as an early AD CSF biomarker using the current immunoassay. However, our population was relatively young; thus, the correlation between agrin and age suggests that stronger differences in agrin concentrations might be found in older groups with more heterogeneous AD pathologic features.

The heparan sulfate proteoglycan agrin contributes to barrier properties of mouse brain endothelial cells by stabilizing adherens junctions

Barrier characteristics of brain endothelial cells forming the blood–brain barrier (BBB) are tightly regulated by cellular and acellular components of the neurovascular unit. During embryogenesis, the accumulation of the heparan sulfate proteoglycan agrin in the basement membranes ensheathing brain vessels correlates with BBB maturation. In contrast, loss of agrin deposition in the vasculature of brain tumors is accompanied by the loss of endothelial junctional proteins. We therefore wondered whether agrin had a direct effect on the barrier characteristics of brain endothelial cells. Agrin increased junctional localization of vascular endothelial (VE)-cadherin, β-catenin, and zonula occludens-1 (ZO-1) but not of claudin-5 and occludin in the brain endothelioma cell line bEnd5 without affecting the expression levels of these proteins. This was accompanied by an agrin-induced reduction of the paracellular permeability of bEnd5 monolayers. In vivo, the lack of agrin also led to reduced junctional localization of VE-cadherin in brain microvascular endothelial cells. Taken together, our data support the notion that agrin contributes to barrier characteristics of brain endothelium by stabilizing the adherens junction proteins VE-cadherin and β-catenin and the junctional protein ZO-1 to brain endothelial junctions.

In vivo models of brain tumors: roles of genetically engineered mouse models in understanding tumor biology and use in preclinical studies

Although our knowledge of the biology of brain tumors has increased tremendously over the past decade, progress in treatment of these deadly diseases remains modest. Developing in vivo models that faithfully mirror human diseases is essential for the validation of new therapeutic approaches. Genetically engineered mouse models (GEMMs) provide elaborate temporally and genetically controlled systems to investigate the cellular origins of brain tumors and gene function in tumorigenesis. Furthermore, they can prove to be valuable tools for testing targeted therapies. In this review, we discuss GEMMs of brain tumors, focusing on gliomas and medulloblastomas. We describe how they provide critical insights into the molecular and cellular events involved in the initiation and maintenance of brain tumors, and illustrate their use in preclinical drug testing.

Postnatal development of the molecular complex underlying astrocyte polarization

Astrocytes are highly polarised cells with processes that ensheath microvessels, cover the brain surface, and abut synapses. The endfoot membrane domains facing microvessels and pia are enriched with aquaporin-4 water channels (AQP4) and other members of the dystrophin associated protein complex (DAPC). Several lines of evidence show that loss of astrocyte polarization, defined by the loss of proteins that are normally enriched in astrocyte endfeet, is a common denominator of several neurological diseases such as mesial temporal lobe epilepsy, Alzheimer's disease, and stroke. Little is known about the mechanisms responsible for inducing astrocyte polarization in vivo. Here we introduce the term endfoot-basal lamina junctional complex (EBJC) to denote the proteins that consolidate and characterize the gliovascular interface. The present study was initiated in order to resolve the developmental profile of the EBJC in mouse brain. We show that the EBJC is established after the first week postnatally. Through a combination of methodological approaches, including light microscopic and high resolution immunogold cytochemistry, quantitative RT-PCR, and Western blotting, we demonstrate that the different members of this complex exhibit distinct ontogenic profiles—with the extracellular matrix (ECM) proteins laminin and agrin appearing earlier than the other members of the complex. Specifically, while laminin and agrin expression peak at P7, quantitative immunoblot analyses indicate that AQP4, α-syntrophin, and the inwardly rectifying K(+) channel Kir4.1 expression increases towards adulthood. Our findings are consistent with ECM having an instructive role in establishing astrocyte polarization in postnatal development and emphasize the need to explore the involvement of ECM in neurological disease.

Physiological roles of aquaporin-4 in brain

Aquaporin-4 (AQP4) is one of the most abundant molecules in the brain and is particularly prevalent in astrocytic membranes at the blood-brain and brain-liquor interfaces. While AQP4 has been implicated in a number of pathophysiological processes, its role in brain physiology has remained elusive. Only recently has evidence accumulated to suggest that AQP4 is involved in such diverse functions as regulation of extracellular space volume, potassium buffering, cerebrospinal fluid circulation, interstitial fluid resorption, waste clearance, neuroinflammation, osmosensation, cell migration, and Ca(2+) signaling. AQP4 is also required for normal function of the retina, inner ear, and olfactory system. A review will be provided of the physiological roles of AQP4 in brain and of the growing list of data that emphasize the polarized nature of astrocytes.

Blood-brain barrier dysfunction as a cause and consequence of Alzheimer's disease

The blood-brain barrier (BBB) plays critical roles in the maintenance of central nervous system (CNS) homeostasis. Dysfunction of the BBB occurs in a number of CNS diseases, including Alzheimer's disease (AD). A prevailing hypothesis in the AD field is the amyloid cascade hypothesis that states that amyloid-β (Aβ) deposition in the CNS initiates a cascade of molecular events that cause neurodegeneration, leading to AD onset and progression. In this review, the participation of the BBB in the amyloid cascade and in other mechanisms of AD neurodegeneration will be discussed. We will specifically focus on three aspects of BBB dysfunction: disruption, perturbation of transporters, and secretion of neurotoxic substances by the BBB. We will also discuss the interaction of the BBB with components of the neurovascular unit in relation to AD and the potential contribution of AD risk factors to aspects of BBB dysfunction. From the results discussed herein, we conclude that BBB dysfunction contributes to AD through a number of mechanisms that could be initiated in the presence or absence of Aβ pathology.

Evidence that pericytes regulate aquaporin-4 polarization in mouse cortical astrocytes

Aquaporin-4 (AQP4) water channels are concentrated in astrocytic endfoot membranes at the brain–blood and brain–cerebrospinal fluid interfaces. The mechanisms underpinning the polarized distribution of AQP4 are poorly understood. Here we tested the hypothesis that pericytes regulate AQP4 anchoring to perivascular astrocytic endfoot membranes. AQP4 immunofluorescence of brain sections obtained from novel transgenic double reporter mice expressing enhanced green fluorescent protein (eGFP) in astrocytes and Discoma Red (DsRed) in pericytes revealed strong AQP4 signal in astrocytic processes adjacent to pericytes. Quantitative immunogold analysis of C57BL/6 mice showed that the AQP4 expression was higher in endfoot membranes abutting pericytes than in those facing endothelial cells. Similar findings were made for α-syntrophin, a member of the dystrophin-associated protein complex (DAPC). The enrichment of α-syntrophin in membranes ensheathing pericytes persisted after Aqp4 gene deletion. Our data support the concept that pericytes regulate AQP4 polarization.

Amyloid beta accumulation in HIV-1-infected brain: The role of the blood brain barrier

In recent years, we face an increase in the aging of the HIV-1-infected population, which is not only due to effective antiretroviral therapy but also to new infections among older people. Even with the use of the antiretroviral therapy, HIV-associated neurocognitive disorders represent an increasing problem as the HIV-1-infected population ages. Increased amyloid beta (Aβ) deposition is characteristic of HIV-1-infected brains, and it has been hypothesized that brain vascular dysfunction contributes to this phenomenon, with a critical role suggested for the blood-brain barrier in brain Aβ homeostasis. This review will describe the mechanisms by which the blood-brain barrier may contribute to brain Aβ accumulation, and our findings in the context of HIV-1 infection will be discussed.