Purification of Mouse Brain Vessels

Brain endothelial GSDMD activation mediates inflammatory BBB breakdown

The blood-brain barrier (BBB) protects the central nervous system from infections or harmful substances1; its impairment can lead to or exacerbate various diseases of the central nervous system2-4. However, the mechanisms of BBB disruption during infection and inflammatory conditions5,6 remain poorly defined. Here we find that activation of the pore-forming protein GSDMD by the cytosolic lipopolysaccharide (LPS) sensor caspase-11 (refs. 7-9), but not by TLR4-induced cytokines, mediates BBB breakdown in response to circulating LPS or during LPS-induced sepsis. Mice deficient in the LBP-CD14 LPS transfer and internalization pathway10-12 resist BBB disruption. Single-cell RNA-sequencing analysis reveals that brain endothelial cells (bECs), which express high levels of GSDMD, have a prominent response to circulating LPS. LPS acting on bECs primes Casp11 and Cd14 expression and induces GSDMD-mediated plasma membrane permeabilization and pyroptosis in vitro and in mice. Electron microscopy shows that this features ultrastructural changes in the disrupted BBB, including pyroptotic endothelia, abnormal appearance of tight junctions and vasculature detachment from the basement membrane. Comprehensive mouse genetic analyses, combined with a bEC-targeting adeno-associated virus system, establish that GSDMD activation in bECs underlies BBB disruption by LPS. Delivery of active GSDMD into bECs bypasses LPS stimulation and opens the BBB. In CASP4-humanized mice, Gram-negative Klebsiella pneumoniae infection disrupts the BBB; this is blocked by expression of a GSDMD-neutralizing nanobody in bECs. Our findings outline a mechanism for inflammatory BBB breakdown, and suggest potential therapies for diseases of the central nervous system associated with BBB impairment.

VC-resist glioblastoma cell state: vessel co-option as a key driver of chemoradiation resistance

Glioblastoma (GBM) is a highly lethal type of cancer. GBM recurrence following chemoradiation is typically attributed to the regrowth of invasive and resistant cells. Therefore, there is a pressing need to gain a deeper understanding of the mechanisms underlying GBM resistance to chemoradiation and its ability to infiltrate. Using a combination of transcriptomic, proteomic, and phosphoproteomic analyses, longitudinal imaging, organotypic cultures, functional assays, animal studies, and clinical data analyses, we demonstrate that chemoradiation and brain vasculature induce cell transition to a functional state named VC-Resist (vessel co-opting and resistant cell state). This cell state is midway along the transcriptomic axis between proneural and mesenchymal GBM cells and is closer to the AC/MES1-like state. VC-Resist GBM cells are highly vessel co-opting, allowing significant infiltration into the surrounding brain tissue and homing to the perivascular niche, which in turn induces even more VC-Resist transition. The molecular and functional characteristics of this FGFR1-YAP1-dependent GBM cell state, including resistance to DNA damage, enrichment in the G2M phase, and induction of senescence/stemness pathways, contribute to its enhanced resistance to chemoradiation. These findings demonstrate how vessel co-option, perivascular niche, and GBM cell plasticity jointly drive resistance to therapy during GBM recurrence.

Dynamic local mRNA localization and translation occurs during the postnatal molecular maturation of perivascular astrocytic processes

Astrocytes are highly ramified and send out perivascular processes (PvAPs) that entirely sheathe the brain's blood vessels. PvAPs are equipped with an enriched molecular repertoire that sustains astrocytic regulatory functions at the vascular interface. In the mouse, PvAP development starts after birth and is essentially complete by postnatal day (P) 15. Progressive molecular maturation also occurs over this period, with the acquisition of proteins enriched in PvAPs. The mechanisms controlling the development and molecular maturation of PvAPs have not been extensively characterized. We reported previously that mRNAs are distributed unequally in mature PvAPs and are locally translated. Since dynamic mRNA localization and local translation influence the cell's polarity, we hypothesized that they might sustain the postnatal maturation of PvAPs. Here, we used a combination of molecular biology and imaging approaches to demonstrate that the development of PvAPs is accompanied by the transport of mRNA and polysomal mRNA into PvAPs, the development of a rough endoplasmic reticulum (RER) network and Golgi cisternae, and local translation. By focusing on genes and proteins that are selectively or specifically expressed in astrocytes, we characterized the developmental profile of mRNAs, polysomal mRNAs and proteins in PvAPs from P5 to P60. We found that some polysomal mRNAs polarized progressively towards the PvAPs. Lastly, we found that expression and localization of mRNAs in developing PvAPs is perturbed in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts. Our results indicate that dynamic mRNA localization and local translation influence the postnatal maturation of PvAPs.

Effects of plateau hypoxia on pharmacokinetic parameters and cerebral-blood distribution of levetiracetam in rats

OBJECTIVES

Plateau hypoxia exposure causes changes in pharmacokinetic parameters and cerebral-blood distribution of drugs, including many substrates of P-glycoprotein (P-gp). Levetiracetam, a kind of antiepileptic drugs, is a substrate of P-gp. Whether plateau hypoxia exposure changes its pharmacokinetic characteristics and cerebral-blood distribution remains unclear. This study aims to investigate the effects of plateau hypoxia on the pharmacokinetics and cerebra-blood distribution of levetiracetam.

METHODS

Wistar rats were divided into a low-altitude control group, a high-altitude group, a solvent group, and a P-gp induction group. After 24 h of exposure at altitude of 4 010 m, rats in the high-altitude group were given levetiracetam orally or intravenously. The plasma was respectively collected at 0.083, 0.25, 0.5, 0.83, 1.25, 2, 4, 6, 8, 10, 12, and 24 h after oral administration of the drug, while both plasma and brain were respectively collected at 5, 45, 60,120 and 240 min after intravenous injection. After 3 days administration of dexamethasone, plasma and brain of rats in the P-gp induction group were collected at 120 min after intravenously giving levetiracetam. Plasma and brain concentrations of the drug were determined by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The expression of P-gp in blood-brain barrier was detected by Western blotting.

RESULTS

Compared with the low-altitude control group, the area under the curve (AUC) and mean residence time (MRT) of levetiracetam were respectively decreased by 14.69% (P<0.01) and 15.42% (P<0.01), while the clearance (CL) was increased by 16.67% (P<0.01) in the high-altitude group. The ratio of brain/blood plasma drug concentration was decreased by 22.82% (P<0.05), 12.42% (P<0.05), 17.40% (P<0.01), and 13.22% (P<0.01) at 5, 45, 120, and 240 min after injection, respectively. The expression of P-gp on the blood-brain barrier was increased by 86.3% (P<0.05). Compared with the solvent control group, the expression of P-gp on the blood-brain barrier in the P-gp induction group was increased by 56.3% (P<0.05), the ratio of brain/blood plasma drug concentration was decreased by 19.3% (P<0.05).

CONCLUSIONS

After acute plateau hypoxia exposure, the pharmacokinetic of levetiracetam in rats are altered, and the cerebral-blood distribution of the drug in rats is decreased, which may be related to the up-regulation of P-gp expression on the blood-brain barrier.

Hydrogen alleviated cognitive impairment and blood‒brain barrier damage in sepsis-associated encephalopathy by regulating ABC efflux transporters in a PPARα-dependent manner

Hydrogen (H₂) can protect against blood‒brain barrier (BBB) damage in sepsis-associated encephalopathy (SAE), but the mechanism is still unclear. We examined whether it is related to PPARα and its regulatory targets, ABC efflux transporters. After injection with DMSO/GW6471 (a PPARα inhibitor), the mice subjected to sham/caecal ligation and puncture (CLP) surgery were treated with H₂ for 60 min postoperation. Additionally, bEnd.3 cells were grown in DMSO/GW6471-containing or saline medium with LPS. In addition to the survival rates, cognitive function was assessed using the Y-maze and fear conditioning tests. Brain tissues were stained with TUNEL and Nissl staining. Additionally, inflammatory mediators (TNF-α, IL-6, HMGB1, and IL-1β) were evaluated with ELISA, and PPARα, ZO-1, occludin, VE-cadherin, P-gp, BCRP and MRP2 were detected using Western blotting. BBB destruction was assessed by brain water content and Evans blue (EB) extravasation. Finally, we found that H₂ improved survival rates and brain dysfunction and decreased inflammatory cytokines. Furthermore, H₂ decreased water content in the brain and EB extravasation and increased ZO-1, occludin, VE-cadherin and ABC efflux transporters regulated by PPARα. Thus, we concluded that H₂ decreases BBB permeability to protect against brain dysfunction in sepsis; this effect is mediated by PPARα and its regulation of ABC efflux transporters.

APOE-ε4 synergizes with sleep disruption to accelerate Aβ deposition and Aβ-associated tau seeding and spreading

Alzheimer's disease (AD) is the most common cause of dementia. The APOE-ε4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset AD. The APOE genotype modulates the effect of sleep disruption on AD risk, suggesting a possible link between apoE and sleep in AD pathogenesis, which is relatively unexplored. We hypothesized that apoE modifies Aβ deposition and Aβ plaque-associated tau seeding and spreading in the form of neuritic plaque-tau (NP-tau) pathology in response to chronic sleep deprivation (SD) in an apoE isoform-dependent fashion. To test this hypothesis, we used APPPS1 mice expressing human APOE-ε3 or -ε4 with or without AD-tau injection. We found that SD in APPPS1 mice significantly increased Aβ deposition and peri-plaque NP-tau pathology in the presence of APOE4 but not APOE3. SD in APPPS1 mice significantly decreased microglial clustering around plaques and aquaporin-4 (AQP4) polarization around blood vessels in the presence of APOE4 but not APOE3. We also found that sleep-deprived APPPS1:E4 mice injected with AD-tau had significantly altered sleep behaviors compared with APPPS1:E3 mice. These findings suggest that the APOE-ε4 genotype is a critical modifier in the development of AD pathology in response to SD.

In mice and humans, brain microvascular contractility matures postnatally

Although great efforts to characterize the embryonic phase of brain microvascular system development have been made, its postnatal maturation has barely been described. Here, we compared the molecular and functional properties of brain vascular cells on postnatal day (P)5 vs. P15, via a transcriptomic analysis of purified mouse cortical microvessels (MVs) and the identification of vascular-cell-type-specific or -preferentially expressed transcripts. We found that endothelial cells (EC), vascular smooth muscle cells (VSMC) and fibroblasts (FB) follow specific molecular maturation programs over this time period. Focusing on VSMCs, we showed that the arteriolar VSMC network expands and becomes contractile resulting in a greater cerebral blood flow (CBF), with heterogenous developmental trajectories within cortical regions. Samples of the human brain cortex showed the same postnatal maturation process. Thus, the postnatal phase is a critical period during which arteriolar VSMC contractility required for vessel tone and brain perfusion is acquired and mature.

Brain blood vessel autoantibodies in patients with NMDA and GABAA receptor encephalitis: identification of unconventional Myosin-X as target antigen

Introduction: The antibody repertoire from CSF-derived antibody-secreting cells and memory B-cells in patients with encephalitis contains a considerable number of antibodies that do not target the disease-defining autoantigen such as the GABA or NMDA receptors. This study focuses on the functional relevance of autoantibodies to brain blood vessels in patients with GABA_A and NMDA receptor encephalitis. Methods: We tested 149 human monoclonal IgG antibodies from the cerebrospinal fluid of six patients with different forms of autoimmune encephalitis on murine brain sections for reactivity to blood vessels using immunohistochemistry. Positive candidates were tested for reactivity with purified brain blood vessels, effects on transendothelial electrical resistance (TEER), and expression of tight junction proteins as well as gene regulation using human brain microvascular endothelial hCMEC/D3 cells as in vitro blood-brain barrier model. One blood-vessel reactive antibody was infused intrathecally by pump injection in mice to study in vivo binding and effects on tight junction proteins such as Occludin. Target protein identification was addressed using transfected HEK293 cells. Results: Six antibodies reacted with brain blood vessels, three were from the same patient with GABA_AR encephalitis, and the other three were from different patients with NMDAR encephalitis. One antibody from an NMDAR encephalitis patient, mAb 011-138, also reacted with cerebellar Purkinje cells. In this case, treatment of hCMEC/D3 cells resulted in decreased TEER, reduced Occludin expression, and mRNA levels. Functional relevance in vivo was confirmed as Occludin downregulation was observed in mAb 011-138-infused animals. Unconventional Myosin-X was identified as a novel autoimmune target for this antibody. Discussion: We conclude that autoantibodies to blood vessels occur in autoimmune encephalitis patients and might contribute to a disruption of the blood-brain barrier thereby suggesting a potential pathophysiological relevance of these antibodies.

The Prevention and Reversal of a Phenytoin-Resistant Model by N-acetylcysteine Therapy Involves the Nrf2/P-Glycoprotein Pathway at the Blood-Brain Barrier

The transporter hypothesis is one of the most popular hypotheses of drug-resistant epilepsy (DRE). P-glycoprotein (P-gp), a channel protein at the blood-brain barrier (BBB), plays an important role in the transport of some anti-seizure drugs from brain tissue into vessels, which reduces drug concentrations and diminishes the effects of drug treatment. We performed this study to test whether P-gp is overexpressed in DRE and identify ways to prevent and reverse DRE. In this study, we established a phenytoin (PHT)-resistant mouse model and revealed that P-gp was overexpressed at the BBB in PHT-resistant mice. The P-gp inhibitor nimodipine decreased the resistance of phenytoin. Antioxidative preventive treatment with N-acetylcysteine (NAC) prevented the mice from entering a PHT-resistant state, and NAC therapy tended to reverse PHT resistance into sensitivity. We were also able to induce PHT resistance by activating the Nrf2/P-gp pathway, which indicates that oxidative stress plays an important role in drug resistance. Taken together, these findings suggest that antioxidative therapy may be a promising strategy for overcoming DRE.

Methyl-Beta-Cyclodextrin Restores KIR Channel Function in Brain Endothelium of Female Alzheimer's Disease Mice

Background:

As the sixth-leading cause of death in the United States, Alzheimer’s disease (AD) entails deteriorating endothelial control of blood flow throughout the brain. In particular, reduced inward-rectifying K⁺ (K_IR) channel function in animal models of aging and AD compromises endothelial function and optimal perfusion of brain parenchyma. Deficient endothelial K_IR channels may result from aberrant interaction with plasma membrane cholesterol as a primary regulator of membrane fluidity and ion channels.

Objective:

We tested the hypothesis that mild methyl-β-cyclodextrin (MβCD) treatment to reduce membrane cholesterol may restore endothelial K_IR channel function in brain endothelium of old AD mice.

Methods:

Membrane potential was continuously measured in isolated endothelial tubes from posterior cerebral arteries of young (1 to 3 months) and old (16 to 19 months) female 3xTg-AD mice before and after mild treatment with the cholesterol-removing agent MβCD (1 mmol/L). Elevated extracellular potassium ([K⁺]_E; 15 mmol/L) and NS309 (1μmol/L) activated K_IR and Ca²⁺-activated K⁺ (SK_Ca/IK_Ca) channels respectively before and after MβCD treatment.

Results:

SK_Ca/IK_Ca channel function for producing hyperpolarization remained stable regardless of age group and MβCD treatment (ΔV_m: ∼–33 mV). However, as deficient during AD, K_IR channel function was restored (ΔV_m: –9±1 mV) versus pre-MβCD conditions (–5±1 mV); a progressive effect that reached –14±1 mV hyperpolarization at 60 min following MβCD washout.

Conclusion:

In female animals, MβCD treatment of brain endothelium selectively restores K_IR versus SK_Ca/IK_Ca channel function during AD. Thus, the endothelial cholesterol-K_IR channel interface is a novel target for ameliorating perfusion of the AD brain.

Megalencephalic leukoencephalopathy with subcortical cysts is a developmental disorder of the gliovascular unit

Absence of the astrocyte-specific membrane protein MLC1 is responsible for megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare type of leukodystrophy characterized by early-onset macrocephaly and progressive white matter vacuolation that lead to ataxia, spasticity, and cognitive decline. During postnatal development (from P5 to P15 in the mouse), MLC1 forms a membrane complex with GlialCAM (another astrocytic transmembrane protein) at the junctions between perivascular astrocytic processes. Perivascular astrocytic processes along with blood vessels form the gliovascular unit. It was not previously known how MLC1 influences the physiology of the gliovascular unit. Here, using the Mlc1 knock-out mouse model of MLC, we demonstrated that MLC1 controls the postnatal development and organization of perivascular astrocytic processes, vascular smooth muscle cell contractility, neurovascular coupling, and intraparenchymal interstitial fluid clearance. Our data suggest that MLC is a developmental disorder of the gliovascular unit, and perivascular astrocytic processes and vascular smooth muscle cell maturation defects are primary events in the pathogenesis of MLC and therapeutic targets for this disease.

A large portion of the astrocyte proteome is dedicated to perivascular endfeet, including critical components of the electron transport chain

The perivascular astrocyte endfoot is a specialized and diffusion-limited subcellular compartment that fully ensheathes the cerebral vasculature. Despite their ubiquitous presence, a detailed understanding of endfoot physiology remains elusive, in part due to a limited understanding of the proteins that distinguish the endfoot from the greater astrocyte body. Here, we developed a technique to isolate astrocyte endfeet from brain tissue, which was used to study the endfoot proteome in comparison to the astrocyte somata. In our approach, brain microvessels, which retain their endfoot processes, were isolated from mouse brain and dissociated, whereupon endfeet were recovered using an antibody-based column astrocyte isolation kit. Our findings expand the known set of proteins enriched at the endfoot from 10 to 516, which comprised more than 1/5th of the entire detected astrocyte proteome. Numerous critical electron transport chain proteins were expressed only at the endfeet, while enzymes involved in glycogen storage were distributed to the somata, indicating subcellular metabolic compartmentalization. The endfoot proteome also included numerous proteins that, while known to have important contributions to blood-brain barrier function, were not previously known to localize to the endfoot. Our findings highlight the importance of the endfoot and suggest new routes of investigation into endfoot function.

CC17 group B Streptococcus exploits integrins for neonatal meningitis development

Group B Streptococcus (GBS) is the major cause of human neonatal infections. A single clone, designated CC17-GBS, accounts for more than 80% of meningitis cases, the most severe form of the infection. However, the events allowing blood-borne GBS to penetrate the brain remain largely elusive. In this study, we identified the host transmembrane receptors α5β1 and αvβ3 integrins as the ligands of Srr2, a major CC17-GBS-specific adhesin. Two motifs located in the binding region of Srr2 were responsible for the interaction between CC17-GBS and these integrins. We demonstrated in a blood-brain-barrier cellular model that both integrins contributed to the adhesion and internalization of CC17-GBS. Strikingly, both integrins were overexpressed during the postnatal period in the brain vessels of the blood-brain barrier and blood-cerebrospinal fluid barrier and contributed to juvenile susceptibility to CC17 meningitis. Finally, blocking these integrins decreased the ability of CC17-GBS to cross into the CNS of juvenile mice in an in vivo model of meningitis. Our study demonstrated that CC17-GBS exploits integrins in order to cross the brain vessels, leading to meningitis. Importantly, it provides host molecular insights into neonate's susceptibility to CC17-GBS meningitis, thereby opening new perspectives for therapeutic and prevention strategies of GBS-elicited meningitis.

Endothelial C3a receptor mediates vascular inflammation and blood-brain barrier permeability during aging

Dysfunction of immune and vascular systems has been implicated in aging and Alzheimer disease; however, their interrelatedness remains poorly understood. The complement pathway is a well-established regulator of innate immunity in the brain. Here, we report robust age-dependent increases in vascular inflammation, peripheral lymphocyte infiltration, and blood-brain barrier (BBB) permeability. These phenotypes were subdued by global inactivation and by endothelial cell-specific ablation of C3ar1. Using an in vitro model of the BBB, we identified intracellular Ca2+ as a downstream effector of C3a/C3aR signaling and a functional mediator of vascular endothelial cadherin junction and barrier integrity. Endothelial C3ar1 inactivation also dampened microglia reactivity and improved hippocampal and cortical volumes in the aging brain, demonstrating a crosstalk between brain vasculature dysfunction and immune cell activation and neurodegeneration. Further, prominent C3aR-dependent vascular inflammation was also observed in a tau-transgenic mouse model. Our studies suggest that heightened C3a/C3aR signaling through endothelial cells promotes vascular inflammation and BBB dysfunction and contributes to overall neuroinflammation in aging and neurodegenerative disease.

Identification of the GlialCAM interactome: the G protein-coupled receptors GPRC5B and GPR37L1 modulate megalencephalic leukoencephalopathy proteins

Megalencephalic Leukoencephalopathy with subcortical Cysts (MLC) is a type of vacuolating leukodystrophy, which is mainly caused by mutations in MLC1 or GLIALCAM. The two MLC-causing genes encode for membrane proteins of yet unknown function that have been linked to the regulation of different chloride channels such as the ClC-2 and VRAC. To gain insight into the role of MLC proteins, we have determined the brain GlialCAM interacting proteome. The proteome includes different transporters and ion channels known to be involved in the regulation of brain homeostasis, proteins related to adhesion or signaling as several G protein-coupled receptors (GPCRs), including the orphan GPRC5B and the proposed prosaposin receptor GPR37L1. Focusing on these two GPCRs, we could validate that they interact directly with MLC proteins. The inactivation of Gpr37l1 in mice upregulated MLC proteins without altering their localization. Conversely, a reduction of GPRC5B levels in primary astrocytes downregulated MLC proteins, leading to an impaired activation of ClC-2 and VRAC. The interaction between the GPCRs and MLC1 was dynamically regulated upon changes in the osmolarity or potassium concentration. We propose that GlialCAM and MLC1 associate with different integral membrane proteins modulating their functions and acting as a recruitment site for various signaling components as the GPCRs identified here. We hypothesized that the GlialCAM/MLC1 complex is working as an adhesion molecule coupled to a tetraspanin-like molecule performing regulatory effects through direct binding or influencing signal transduction events.

Enrichment of Vascular Fragments from Mouse Embryonic Brains for Endothelial Cell Analysis

Endothelial cells in the brain interact with other cell types, forming the blood-brain barrier. This barrier controls the movement of solutes into and out of the brain, regulating pathophysiological processes and drug delivery to the brain. Common isolation methods used to study these cells during embryonic development involve enzymatic treatment and cell sorting using specific markers. This process modifies the cell state and produces minute amounts of sample. Here, we describe a protocol for the enrichment of vascular cells from embryonic brains based on dextran separation. In this method, the brain is lightly disrupted with a pestle and then resuspended in a dextran solution. Low-speed centrifugation permits the separation of the parenchymal and vascular fractions. Further centrifugation steps improve fractionation. This method is simple and fast and produces enough sample for biochemical assays. Graphic abstract: Purification of vascular fragments from an embryonic brain.

Aging Alters Cerebrovascular Endothelial GPCR and K+ Channel Function: Divergent Role of Biological Sex

Age-related dementia entails impaired blood flow to and throughout the brain due, in part, to reduced endothelial nitric oxide signaling. However, it is unknown whether sex affects cerebrovascular Gq-protein-coupled receptors (GPCRs) and K+ channels underlying endothelium-derived hyperpolarization (EDH) during progressive aging. Thus, we simultaneously evaluated intracellular Ca2+ ([Ca2+]i) and membrane potential (Vm) of intact endothelial tubes freshly isolated from posterior cerebral arteries of young (4-6 mo), middle-aged (12-16 mo), and old (24-28 mo) male and female C57BL/6 mice. Purinergic receptor function (vs. muscarinic) was dominant and enhanced for [Ca2+]i increases in old females versus old males. However, Ca2+-sensitive K+ channel function as defined by NS309-evoked Vm hyperpolarization was mildly impaired in females versus males during old age. This sex-based contrast in declined function of GPCRs and K+ channels to produce EDH may support a greater ability for physiological endothelial GPCR function to maintain optimal cerebral blood flow in females versus males during old age. As reflective of the pattern of cerebral blood flow decline in human subjects, inward-rectifying K+ (KIR) channel function decreased with progressive age regardless of sex. Combined age-related analyses masked male versus female aging and, contrary to expectation, hydrogen peroxide played a minimal role. Altogether, we conclude a sex-based divergence in cerebrovascular endothelial GPCR and K+ channel function while highlighting a previously unidentified form of age-related endothelial dysfunction as reduced KIR channel function.

An Improved Method for Physical Separation of Cerebral Vasculature and Parenchyma Enables Detection of Blood-Brain-Barrier Dysfunction

The neurovascular niche is crucial for constant blood supply and blood-brain barrier (BBB) function and is altered in a number of different neurological conditions, making this an intensely active field of research. Brain vasculature is unique for its tight association of endothelial cells with astrocytic endfeet processes. Separation of the vascular compartment by centrifugation-based methods confirmed enrichment of astrocytic endfeet processes, making it possible to study the entire vascular niche with such methods. Several centrifugation-based separation protocols are found in the literature; however, with some constraints which limit their applicability and the scope of the studies. Here, we describe and validate a protocol for physically separating the neurovascular niche from the parenchyma, which is optimized for smaller tissue quantities. Using endothelial, neuronal, and astrocyte markers, we show that quantitative Western blot-based target detection can be performed of both the vessel-enriched and parenchymal fractions using as little as a single mouse brain hemisphere. Validation of our protocol in rodent stroke models by detecting changes in tight junction protein expression, serum albumin signals and astrocyte activation, i.e., increased glial fibrillary acidic protein expression, between the ipsilateral and the lesion-free contralateral hemisphere demonstrates this protocol as a new way of detecting BBB breakdown and astrogliosis, respectively.

Local translation in perisynaptic and perivascular astrocytic processes - a means to ensure astrocyte molecular and functional polarity?

Together with the compartmentalization of mRNAs in distal regions of the cytoplasm, local translation constitutes a prominent and evolutionarily conserved mechanism mediating cellular polarization and the regulation of protein delivery in space and time. The translational regulation of gene expression enables a rapid response to stimuli or to a change in the environment, since the use of pre-existing mRNAs can bypass time-consuming nuclear control mechanisms. In the brain, the translation of distally localized mRNAs has been mainly studied in neurons, whose cytoplasmic protrusions may be more than 1000 times longer than the diameter of the cell body. Importantly, alterations in local translation in neurons have been implicated in several neurological diseases. Astrocytes, the most abundant glial cells in the brain, are voluminous, highly ramified cells that project long processes to neurons and brain vessels, and dynamically regulate distal synaptic and vascular functions. Recent research has demonstrated the presence of local translation at these astrocytic interfaces that might regulate the functional compartmentalization of astrocytes. In this Review, we summarize our current knowledge about the localization and local translation of mRNAs in the distal perisynaptic and perivascular processes of astrocytes, and discuss their possible contribution to the molecular and functional polarity of astrocytes.

Circulating tPA contributes to neurovascular coupling by a mechanism involving the endothelial NMDA receptors

The increase of cerebral blood flow evoked by neuronal activity is essential to ensure enough energy supply to the brain. In the neurovascular unit, endothelial cells are ideally placed to regulate key neurovascular functions of the brain. Nevertheless, some outstanding questions remain about their exact role neurovascular coupling (NVC). Here, we postulated that the tissue-type plasminogen activator (tPA) present in the circulation might contribute to NVC by a mechanism dependent of its interaction with endothelial N-Methyl-D-Aspartate Receptor (NMDAR). To address this question, we used pharmacological and genetic approaches to interfere with vascular tPA-dependent NMDAR signaling, combined with laser speckle flowmetry, intravital microscopy and ultrafast functional ultrasound in vivo imaging. We found that the tPA present in the blood circulation is capable of potentiating the cerebral blood flow increase induced by the activation of the mouse somatosensorial cortex, and that this effect is mediated by a tPA-dependent activation of NMDAR expressed at the luminal part of endothelial cells of arteries. Although blood molecules, such as acetylcholine, bradykinin or ATP are known to regulate vascular tone and induce vessel dilation, our present data provide the first evidence that circulating tPA is capable of influencing neurovascular coupling (NVC).

Cerebrovascular Senescence Is Associated With Tau Pathology in Alzheimer's Disease

Alzheimer's Disease (AD) is associated with neuropathological changes, including aggregation of tau neurofibrillary tangles (NFTs) and amyloid-beta plaques. Mounting evidence indicates that vascular dysfunction also plays a key role in the pathogenesis and progression of AD, in part through endothelial dysfunction. Based on findings in animal models that tau pathology induces vascular abnormalities and cellular senescence, we hypothesized that tau pathology in the human AD brain leads to vascular senescence. To explore this hypothesis, we isolated intact microvessels from the dorsolateral prefrontal cortex (PFC, BA9) from 16 subjects with advanced Braak stages (Braak V/VI, B3) and 12 control subjects (Braak 0/I/II, B1), and quantified expression of 42 genes associated with senescence, cell adhesion, and various endothelial cell functions. Genes associated with endothelial senescence and leukocyte adhesion, including SERPINE1 (PAI-1), CXCL8 (IL8), CXCL1, CXCL2, ICAM-2, and TIE1, were significantly upregulated in B3 microvessels after adjusting for sex and cerebrovascular pathology. In particular, the senescence-associated secretory phenotype genes SERPINE1 and CXCL8 were upregulated by more than 2-fold in B3 microvessels after adjusting for sex, cerebrovascular pathology, and age at death. Protein quantification data from longitudinal plasma samples for a subset of 13 (n = 9 B3, n = 4 B1) subjects showed no significant differences in plasma senescence or adhesion-associated protein levels, suggesting that these changes were not associated with systemic vascular alterations. Future investigations of senescence biomarkers in both the peripheral and cortical vasculature could further elucidate links between tau pathology and vascular changes in human AD.

Multidimensional Proteome Profiling of Blood-Brain Barrier Perturbation by Group B Streptococcus

Group B Streptococcus (GBS) remains the leading cause of neonatal meningitis, a disease associated with high rates of adverse neurological sequelae. The in vivo relationship between GBS and brain tissues remains poorly characterized, partly because past studies had focused on microbial rather than host processes. Additionally, the field has not capitalized on systems-level technologies to probe the host-pathogen relationship. Here, we use multiplexed quantitative proteomics to investigate the effect of GBS infection in the murine brain at various levels of tissue complexity, beginning with the whole organ and moving to brain vascular substructures. Infected whole brains showed classical signatures associated with the acute-phase response. In isolated brain microvessels, classical blood-brain barrier proteins were unaltered, but interferon signaling and leukocyte recruitment proteins were upregulated. The choroid plexus showed increases in peripheral immune cell proteins. Proteins that increased in abundance in the vasculature during GBS invasion were associated with major histocompatibility complex (MHC) class I antigen processing and endoplasmic reticulum dysfunction, a finding which correlated with altered host protein glycosylation profiles. Globally, there was low concordance between the infection proteome of whole brains and isolated vascular tissues. This report underscores the utility of unbiased, systems-scale analyses of functional tissue substructures for understanding disease.IMPORTANCE Group B Streptococcus (GBS) meningitis remains a major cause of poor health outcomes very early in life. Both the host-pathogen relationship leading to disease and the massive host response to infection contributing to these poor outcomes are orchestrated at the tissue and cell type levels. GBS meningitis is thought to result when bacteria present in the blood circumvent the selectively permeable vascular barriers that feed the brain. Additionally, tissue damage subsequent to bacterial invasion is mediated by inflammation and by immune cells from the periphery crossing the blood-brain barrier. Indeed, the vasculature plays a central role in disease processes occurring during GBS infection of the brain. Here, we employed quantitative proteomic analysis of brain vascular substructures during invasive GBS disease. We used the generated data to map molecular alterations associated with tissue perturbation, finding widespread intracellular dysfunction and punctuating the importance of investigations relegated to tissue type over the whole organ.

NLRP3 Is Involved in the Maintenance of Cerebral Pericytes

Pericytes play a central role in regulating the structure and function of capillaries in the brain. However, molecular mechanisms that drive pericyte proliferation and differentiation are unclear. In our study, we immunostained NACHT, LRR and PYD domains-containing protein 3 (NLRP3)-deficient and wild-type littermate mice and observed that NLRP3 deficiency reduced platelet-derived growth factor receptor β (PDGFRβ)-positive pericytes and collagen type IV immunoreactive vasculature in the brain. In Western blot analysis, PDGFRβ and CD13 proteins in isolated cerebral microvessels from the NLRP3-deficient mouse brain were decreased. We further treated cultured pericytes with NLRP3 inhibitor, MCC950, and demonstrated that NLRP3 inhibition attenuated cell proliferation but did not induce apoptosis. NLRP3 inhibition also decreased protein levels of PDGFRβ and CD13 in cultured pericytes. On the contrary, treatments with IL-1β, the major product of NLRP3-contained inflammasome, increased protein levels of PDGFRβ, and CD13 in cultured cells. The alteration of PDGFRβ and CD13 protein levels were correlated with the phosphorylation of AKT. Inhibition of AKT reduced both protein markers and abolished the effect of IL-1β activation in cultured pericytes. Thus, NLRP3 activation might be essential to maintain pericytes in the healthy brain through phosphorylating AKT. The potential adverse effects on the cerebral vascular pericytes should be considered in clinical therapies with NLRP3 inhibitors.

Tau reduction in aged mice does not impact Microangiopathy

Microangiopathy, including proliferation of small diameter capillaries, increasing vessel tortuosity, and increased capillary blockage by leukocytes, was previously observed in the aged rTg4510 mouse model. Similar gene expression changes related to angiogenesis were observed in both rTg4510 and Alzheimer's disease (AD). It is uncertain if tau is directly responsible for these vascular changes by interacting directly with microvessels, and/or if it contributes indirectly via neurodegeneration and concurrent neuronal loss and inflammation. To better understand the nature of tau-related microangiopathy in human AD and in tau mice, we isolated capillaries and observed that bioactive soluble tau protein could be readily detected in association with vasculature. To examine whether this soluble tau is directly responsible for the microangiopathic changes, we made use of the tetracycline-repressible gene expression cassette in the rTg4510 mouse model and measured vascular pathology following tau reduction. These data suggest that reduction of tau is insufficient to alter established microvascular complications including morphological alterations, enhanced expression of inflammatory genes involved in leukocyte adherence, and blood brain barrier compromise. These data imply that 1) soluble bioactive tau surprisingly accumulates at the blood brain barrier in human brain and in mouse models, and 2) the morphological and molecular phenotype of microvascular disturbance does not resolve with reduction of whole brain soluble tau. Additional consideration of vascular-directed therapies and strategies that target tau in the vascular space may be required to restore normal function in neurodegenerative disease.

Physiological blood-brain transport is impaired with age by a shift in transcytosis

The vascular interface of the brain, known as the blood-brain barrier (BBB), is understood to maintain brain function in part via its low transcellular permeability^1-3. Yet, recent studies have demonstrated that brain ageing is sensitive to circulatory proteins^4,5. Thus, it is unclear whether permeability to individually injected exogenous tracers-as is standard in BBB studies-fully represents blood-to-brain transport. Here we label hundreds of proteins constituting the mouse blood plasma proteome, and upon their systemic administration, study the BBB with its physiological ligand. We find that plasma proteins readily permeate the healthy brain parenchyma, with transport maintained by BBB-specific transcriptional programmes. Unlike IgG antibody, plasma protein uptake diminishes in the aged brain, driven by an age-related shift in transport from ligand-specific receptor-mediated to non-specific caveolar transcytosis. This age-related shift occurs alongside a specific loss of pericyte coverage. Pharmacological inhibition of the age-upregulated phosphatase ALPL, a predicted negative regulator of transport, enhances brain uptake of therapeutically relevant transferrin, transferrin receptor antibody and plasma. These findings reveal the extent of physiological protein transcytosis to the healthy brain, a mechanism of widespread BBB dysfunction with age and a strategy for enhanced drug delivery.

FGF21 Protects against Aggravated Blood-Brain Barrier Disruption after Ischemic Focal Stroke in Diabetic db/db Male Mice via Cerebrovascular PPARγ Activation

Recombinant fibroblast growth factor 21 (rFGF21) has been shown to be potently beneficial for improving long-term neurological outcomes in type 2 diabetes mellitus (T2DM) stroke mice. Here, we tested the hypothesis that rFGF21 protects against poststroke blood–brain barrier (BBB) damage in T2DM mice via peroxisome proliferator-activated receptor gamma (PPARγ) activation in cerebral microvascular endothelium. We used the distal middle cerebral occlusion (dMCAO) model in T2DM mice as well as cultured human brain microvascular endothelial cells (HBMECs) subjected to hyperglycemic and inflammatory injury in the current study. We detected a significant reduction in PPARγ DNA-binding activity in the brain tissue and mRNA levels of BBB junctional proteins and PPARγ-targeting gene CD36 and FABP4 in cerebral microvasculature at 24 h after stroke. Ischemic stroke induced a massive BBB leakage two days after stroke in T2DM mice compared to in their lean controls. Importantly, all abnormal changes were significantly prevented by rFGF21 administration initiated at 6 h after stroke. Our in vitro experimental results also demonstrated that rFGF21 protects against hyperglycemia plus interleukin (IL)-1β-induced transendothelial permeability through upregulation of junction protein expression in an FGFR1 activation and PPARγ activity elevation-dependent manner. Our data suggested that rFGF21 has strong protective effects on acute BBB leakage after diabetic stroke, which is partially mediated by increasing PPARγ DNA-binding activity and mRNA expression of BBB junctional complex proteins. Together with our previous investigations, rFGF21 might be a promising candidate for treating diabetic stroke.

ABC Transporters at the Blood-Brain Interfaces, Their Study Models, and Drug Delivery Implications in Gliomas

Drug delivery into the brain is regulated by the blood-brain interfaces. The blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the blood-arachnoid barrier (BAB) regulate the exchange of substances between the blood and brain parenchyma. These selective barriers present a high impermeability to most substances, with the selective transport of nutrients and transporters preventing the entry and accumulation of possibly toxic molecules, comprising many therapeutic drugs. Transporters of the ATP-binding cassette (ABC) superfamily have an important role in drug delivery, because they extrude a broad molecular diversity of xenobiotics, including several anticancer drugs, preventing their entry into the brain. Gliomas are the most common primary tumors diagnosed in adults, which are often characterized by a poor prognosis, notably in the case of high-grade gliomas. Therapeutic treatments frequently fail due to the difficulty of delivering drugs through the brain barriers, adding to diverse mechanisms developed by the cancer, including the overexpression or expression de novo of ABC transporters in tumoral cells and/or in the endothelial cells forming the blood-brain tumor barrier (BBTB). Many models have been developed to study the phenotype, molecular characteristics, and function of the blood-brain interfaces as well as to evaluate drug permeability into the brain. These include in vitro, in vivo, and in silico models, which together can help us to better understand their implication in drug resistance and to develop new therapeutics or delivery strategies to improve the treatment of pathologies of the central nervous system (CNS). In this review, we present the principal characteristics of the blood-brain interfaces; then, we focus on the ABC transporters present on them and their implication in drug delivery; next, we present some of the most important models used for the study of drug transport; finally, we summarize the implication of ABC transporters in glioma and the BBTB in drug resistance and the strategies to improve the delivery of CNS anticancer drugs.

The amyloid-β degradation intermediate Aβ34 is pericyte-associated and reduced in brain capillaries of patients with Alzheimer's disease

An impairment of amyloid β-peptide (Aβ) clearance is suggested to play a key role in the pathogenesis of sporadic Alzheimer’s disease (AD). Amyloid degradation is mediated by various mechanisms including fragmentation by enzymes like neprilysin, matrix metalloproteinases (MMPs) and a recently identified amyloidolytic activity of β-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 cleavage of Aβ40 and Aβ42 results in the formation of a common Aβ34 intermediate which was found elevated in cerebrospinal fluid levels of patients at the earliest disease stages. To further investigate the role of Aβ34 as a marker for amyloid clearance in AD, we performed a systematic and comprehensive analysis of Aβ34 immunoreactivity in hippocampal and cortical post-mortem brain tissue from AD patients and non-demented elderly individuals. In early Braak stages, Aβ34 was predominantly detectable in a subset of brain capillaries associated with pericytes, while in later disease stages, in clinically diagnosed AD, this pericyte-associated Aβ34 immunoreactivity was largely lost. Aβ34 was also detected in isolated human cortical microvessels associated with brain pericytes and its levels correlated with Aβ40, but not with Aβ42 levels. Moreover, a significantly decreased Aβ34/Aβ40 ratio was observed in microvessels from AD patients in comparison to non-demented controls suggesting a reduced proteolytic degradation of Aβ40 to Aβ34 in AD. In line with the hypothesis that pericytes at the neurovascular unit are major producers of Aβ34, biochemical studies in cultured human primary pericytes revealed a time and dose dependent increase of Aβ34 levels upon treatment with recombinant Aβ40 peptides while Aβ34 production was impaired when Aβ40 uptake was reduced or BACE1 activity was inhibited. Collectively, our findings indicate that Aβ34 is generated by a novel BACE1-mediated Aβ clearance pathway in pericytes of brain capillaries. As amyloid clearance is significantly reduced in AD, impairment of this pathway might be a major driver of the pathogenesis in sporadic AD.

Stromal Cell-Derived Factor 1 Protects Brain Vascular Endothelial Cells from Radiation-Induced Brain Damage

Stromal cell-derived factor 1 (SDF-1) and its main receptor, CXC chemokine receptor 4 (CXCR4), play a critical role in endothelial cell function regulation during cardiogenesis, angiogenesis, and reendothelialization after injury. The expression of CXCR4 and SDF-1 in brain endothelial cells decreases due to ionizing radiation treatment and aging. SDF-1 protein treatment in the senescent and radiation-damaged cells reduced several senescence phenotypes, such as decreased cell proliferation, upregulated p53 and p21 expression, and increased senescence-associated beta-galactosidase (SA-β-gal) activity, through CXCR4-dependent signaling. By inhibiting extracellular signal-regulated kinase (ERK) and signal transducer and activator of transcription protein 3 (STAT3), we confirmed that activation of both is important in recovery by SDF-1-related mechanisms. A CXCR4 agonist, ATI2341, protected brain endothelial cells from radiation-induced damage. In irradiation-damaged tissue, ATI2341 treatment inhibited cell death in the villi of the small intestine and decreased SA-β-gal activity in arterial tissue. An ischemic injury experiment revealed no decrease in blood flow by irradiation in ATI2341-administrated mice. ATI2341 treatment specifically affected CXCR4 action in mouse brain vessels and partially restored normal cognitive ability in irradiated mice. These results demonstrate that SDF-1 and ATI2341 may offer potential therapeutic approaches to recover tissues damaged during chemotherapy or radiotherapy, particularly by protecting vascular endothelial cells.

Magnesium Regulates Endothelial Barrier Functions through TRPM7, MagT1, and S1P1

Mg2+-deficiency is linked to hypertension, Alzheimer's disease, stroke, migraine headaches, cardiovascular diseases, and diabetes, etc., but its exact role in these pathophysiological conditions remains elusive. Mg2+ can regulate vascular functions, yet the mechanistic insight remains ill-defined. Data show that extracellular Mg2+ enters endothelium mainly through the TRPM7 channel and MagT1 transporter. Mg2+ can act as an antagonist to reduce Ca2+ signaling in endothelium. Mg2+ also reduces the intracellular reactive oxygen species (ROS) level and inflammation. In addition, Mg2+-signaling increases endothelial survival and growth, adhesion, and migration. Endothelial barrier integrity is significantly enhanced with Mg2+-treatment through S1P1-Rac1 pathways and barrier-stabilizing mediators including cAMP, FGF1/2, and eNOS. Mg2+ also promotes cytoskeletal reorganization and junction proteins to tighten up the barrier. Moreover, Mg2+-deficiency enhances endothelial barrier permeability in mice, and Mg2+-treatment rescues histamine-induced transient vessel hyper-permeability in vivo. In summary, Mg2+-deficiency can cause deleterious effects in endothelium integrity, and Mg2+-treatment may be effective in the prevention or treatment of vascular dysfunction.

Memantine, a Noncompetitive N-Methyl-D-Aspartate Receptor Antagonist, Attenuates Cerebral Amyloid Angiopathy by Increasing Insulin-Degrading Enzyme Expression

Sporadic cerebral amyloid angiopathy (CAA) is characterized by cerebrovascular amyloid beta (Aβ) deposits and causes cerebral hemorrhages and dementia in elderly people. Memantine is used in Alzheimer's disease to inhibit the glutamatergic system by blocking N-methyl-D-aspartate receptors. Its therapeutic effects in CAA are unclear, however. Here, we used APP23 transgenic mice (CAA model) to investigate whether memantine has direct therapeutic effects on cerebrovascular Aβ deposits. We treated APP23 mice and age-matched wild-type littermates with memantine at ages 6-18 months. We counted the numbers of vessels with Aβ and hemosiderin deposits. We measured soluble and insoluble Aβ40 and Aβ42 levels and levels of amyloid precursor protein (APP), APP-processing enzymes (α-, β-, γ-secretase), and Aβ-degrading enzymes (insulin-degrading enzyme [IDE], neprilysin). Memantine reduced cerebrovascular Aβ and hemosiderin deposits in APP23 mice. Compared with controls, memantine-treated APP23 mice had reduced Aβ40 levels and increased levels of hippocampal and vascular IDE. Our results suggest that memantine reduces cerebrovascular Aβ deposits by enhancing Aβ-cleaving IDE expression. The clinical availability of memantine may allow its use as a novel therapeutic agent in CAA.

Beta-amyloid pathology in human brain microvessel extracts from the parietal cortex: relation with cerebral amyloid angiopathy and Alzheimer's disease

Several pieces of evidence suggest that blood-brain barrier (BBB) dysfunction is implicated in the pathophysiology of Alzheimer's disease (AD), exemplified by the frequent occurrence of cerebral amyloid angiopathy (CAA) and the defective clearance of Aβ peptides. However, the specific role of brain microvascular cells in these anomalies remains elusive. In this study, we validated by Western, ELISA and immunofluorescence analyses a procedure to generate microvasculature-enriched fractions from frozen samples of human cerebral cortex. We then investigated Aβ and proteins involved in its clearance or production in microvessel extracts generated from the parietal cortex of 60 volunteers in the Religious Orders Study. Volunteers were categorized as AD (n = 38) or controls (n = 22) based on the ABC scoring method presented in the revised guidelines for the neuropathological diagnosis of AD. Higher ELISA-determined concentrations of vascular Aβ40 and Aβ42 were found in persons with a neuropathological diagnosis of AD, in apoE4 carriers and in participants with advanced parenchymal CAA, compared to respective age-matched controls. Vascular levels of two proteins involved in Aβ clearance, ABCB1 and neprilysin, were lower in persons with AD and positively correlated with cognitive function, while being inversely correlated to vascular Aβ40. In contrast, BACE1, a protein necessary for Aβ production, was increased in individuals with AD and in apoE4 carriers, negatively correlated to cognitive function and positively correlated to Aβ40 in microvessel extracts. The present report indicates that concentrating microvessels from frozen human brain samples facilitates the quantitative biochemical analysis of cerebrovascular dysfunction in CNS disorders. Data generated overall show that microvessels extracted from individuals with parenchymal CAA-AD contained more Aβ and BACE1 and less ABCB1 and neprilysin, evidencing a pattern of dysfunction in brain microvascular cells contributing to CAA and AD pathology and symptoms.

PDGF-B Is Required for Development of the Glymphatic System

The glymphatic system is a highly polarized cerebrospinal fluid (CSF) transport system that facilitates the clearance of neurotoxic molecules through a brain-wide network of perivascular pathways. Herein we have mapped the development of the glymphatic system in mice. Perivascular CSF transport first emerges in hippocampus in newborn mice, and a mature glymphatic system is established in the cortex at 2 weeks of age. Formation of astrocytic endfeet and polarized expression of aquaporin 4 (AQP4) consistently coincided with the appearance of perivascular CSF transport. Deficiency of platelet-derived growth factor B (PDGF-B) function in the PDGF retention motif knockout mouse line Pdgfbret/ret suppressed the development of the glymphatic system, whose functions remained suppressed in adulthood compared with wild-type mice. These experiments map the natural development of the glymphatic system in mice and define a critical role of PDGF-B in the development of perivascular CSF transport.

Postnatal development of the astrocyte perivascular MLC1/GlialCAM complex defines a temporal window for the gliovascular unit maturation

Astrocytes, the most abundant glial cells of the central nervous system are morphologically complex. They display numerous processes interacting with synapses and blood vessels. At the vascular interface, astrocyte endfeet-terminated processes almost entirely cover the blood vessel surface and participate to the gliovascular unit where important vascular properties of the brain are set such as the blood-brain barrier (BBB) integrity. How specific morphological and functional interactions between astrocytes and the vascular compartment develop has not been fully investigated. Here, we elaborated an original experimental strategy to study the postnatal development of astrocyte perivascular endfeet. Using purified gliovascular units, we focused on the postnatal expression of MLC1 and GlialCAM, two transmembrane proteins forming a complex enriched at the junction between mature astrocyte perivascular endfeet. We showed that MLC1 and GlialCAM were enriched and assembled into mature complexes in astrocyte perivascular endfeet between postnatal days 10 and 15, after the formation of astrocyte perivascular Aquaporin 4 water channels. These events correlated with the increased expression of Claudin-5 and P-gP, two endothelial-specific BBB components. These results illustrate for the first time that astrocyte perivascular endfeet differentiation is a complex and progressive process which correlates with BBB maturation. Moreover, our results suggest that maturation of the astrocyte endfeet MLC1/GlialCAM complex between postnatal days 10 and 15 might be a key event in the gliovascular unit maturation.

Transferrin Receptor-Mediated Uptake at the Blood-Brain Barrier Is Not Impaired by Alzheimer's Disease Neuropathology

The transferrin receptor (TfR) is highly expressed by brain capillary endothelial cells (BCECs) forming the blood-brain barrier (BBB) and is therefore considered as a potential target for brain drug delivery. Monoclonal antibodies binding to the TfR, such as clone Ri7, have been shown to internalize into BCECs in vivo. However, since Alzheimer's disease (AD) is accompanied by a BBB dysfunction, it raises concerns about whether TfR-mediated transport becomes inefficient during the progression of the disease. Measurements of TfR levels using Western blot analysis in whole homogenates from human post-mortem parietal cortex and hippocampus did not reveal any significant difference between individuals with or without a neuropathological diagnosis of AD (respectively, n = 19 and 22 for the parietal cortex and n = 12 and 14 for hippocampus). Similarly, TfR concentrations in isolated human brain microvessels from parietal cortex were similar between controls and AD cases. TfR levels in isolated murine brain microvessels were not significantly different between groups of 12- and 18-month-old NonTg and 3xTg-AD mice, the latter modeling Aβ and τ neuropathologies. In situ brain perfusion assays were then conducted to measure the brain uptake and internalization of fluorolabeled Ri7 in BCECs upon binding. Consistently, TfR-mediated uptake in BCECs was similar between 3xTg-AD mice and nontransgenic controls (∼0.3 μL·g^-1·s^-1) at 12, 18, and 22 months of age. Fluorescence microscopy analysis following intravenous administration of fluorolabeled Ri7 highlighted that the signal from the antibody was widely distributed throughout the cerebral vasculature but not in neurons or astrocytes. Overall, our data suggest that both TfR protein levels and TfR-dependent internalization mechanisms are preserved in the presence of Aβ and τ neuropathologies, supporting the potential of TfR as a vector target for drug delivery into BCECs in AD.

Low-level blast exposure disrupts gliovascular and neurovascular connections and induces a chronic vascular pathology in rat brain

Much concern exists over the role of blast-induced traumatic brain injury (TBI) in the chronic cognitive and mental health problems that develop in veterans and active duty military personnel. The brain vasculature is particularly sensitive to blast injury. The aim of this study was to characterize the evolving molecular and histologic alterations in the neurovascular unit induced by three repetitive low-energy blast exposures (3 × 74.5 kPa) in a rat model mimicking human mild TBI or subclinical blast exposure. High-resolution two-dimensional differential gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of purified brain vascular fractions from blast-exposed animals 6 weeks post-exposure showed decreased levels of vascular-associated glial fibrillary acidic protein (GFAP) and several neuronal intermediate filament proteins (α-internexin and the low, middle, and high molecular weight neurofilament subunits). Loss of these proteins suggested that blast exposure disrupts gliovascular and neurovascular interactions. Electron microscopy confirmed blast-induced effects on perivascular astrocytes including swelling and degeneration of astrocytic endfeet in the brain cortical vasculature. Because the astrocyte is a major sensor of neuronal activity and regulator of cerebral blood flow, structural disruption of gliovascular integrity within the neurovascular unit should impair cerebral autoregulation. Disrupted neurovascular connections to pial and parenchymal blood vessels might also affect brain circulation. Blast exposures also induced structural and functional alterations in the arterial smooth muscle layer. Interestingly, by 8 months after blast exposure, GFAP and neuronal intermediate filament expression had recovered to control levels in isolated brain vascular fractions. However, despite this recovery, a widespread vascular pathology was still apparent at 10 months after blast exposure histologically and on micro-computed tomography scanning. Thus, low-level blast exposure disrupts gliovascular and neurovascular connections while inducing a chronic vascular pathology.

A Volumetric Method for Quantification of Cerebral Vasospasm in a Murine Model of Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a subtype of hemorrhagic stroke. Cerebral vasospasm that occurs in the aftermath of the bleeding is an important factor determining patient outcome and is therefore frequently taken as a study endpoint. However, in small animal studies on SAH, quantification of cerebral vasospasm is a major challenge. Here, an ex vivo method is presented that allows quantification of volumes of entire vessel segments, which can be used as an objective measure to quantify cerebral vasospasm. In a first step, endovascular casting of the cerebral vasculature is performed using a radiopaque casting agent. Then, cross-sectional imaging data are acquired by micro computed tomography. The final step involves 3-dimensional reconstruction of the virtual vascular tree, followed by an algorithm to calculate center lines and volumes of the selected vessel segments. The method resulted in a highly accurate virtual reconstruction of the cerebrovascular tree shown by a diameter-based comparison of anatomical samples with their virtual reconstructions. Compared with vessel diameters alone, the vessel volumes highlight the differences between vasospastic and non-vasospastic vessels shown in a series of SAH and sham-operated mice.

SUR1-TRPM4 channel activation and phasic secretion of MMP-9 induced by tPA in brain endothelial cells

Background

Hemorrhagic transformation is a major complication of ischemic stroke, is linked to matrix metalloproteinase-9 (MMP-9), and is exacerbated by tissue plasminogen activator (tPA). Cerebral ischemia/reperfusion is characterized by SUR1-TRPM4 (sulfonylurea receptor 1—transient receptor potential melastatin 4) channel upregulation in microvascular endothelium. In humans and rodents with cerebral ischemia/reperfusion (I/R), the SUR1 antagonist, glibenclamide, reduces hemorrhagic transformation and plasma MMP-9, but the mechanism is unknown. We hypothesized that tPA induces protease activated receptor 1 (PAR1)-mediated, Ca²⁺-dependent phasic secretion of MMP-9 from activated brain endothelium, and that SUR1-TRPM4 is required for this process.

Methods

Cerebral I/R, of 2 and 4 hours duration, respectively, was obtained using conventional middle cerebral artery occlusion. Immunolabeling was used to quantify p65 nuclear translocation. Murine and human brain endothelial cells (BEC) were studied in vitro, without and with NF-κB activation, using immunoblot, zymography and ELISA, patch clamp electrophysiology, and calcium imaging. Genetic and pharmacological manipulations were used to identify signaling pathways.

Results

Cerebral I/R caused prominent nuclear translocation of p65 in microvascular endothelium. NF-κB-activation of BEC caused de novo expression of SUR1-TRPM4 channels. In NF-κB-activated BEC: (i) tPA caused opening of SUR1-TRPM4 channels in a plasmin-, PAR1-, TRPC3- and Ca²⁺-dependent manner; (ii) tPA caused PAR1-dependent secretion of MMP-9; (iii) tonic secretion of MMP-9 by activated BEC was not influenced by SUR1 inhibition; (iv) phasic secretion of MMP-9 induced by tPA or the PAR1-agonist, TFLLR, required functional SUR1-TRPM4 channels, with inhibition of SUR1 decreasing tPA-induced MMP-9 secretion.

Conclusions

tPA induces PAR1-mediated, SUR1-TRPM4-dependent, phasic secretion of MMP-9 from activated brain endothelium.

The Microvascular Pericyte: Approaches to Isolation, Characterization, and Cultivation

The microvascular pericyte was identified in 1873 by the French scientist Charles Benjamin Rouget and originally called the Rouget cell (Rouget.Sciences 88:916-8, 1879). However, it was not until the early 1900s that Rouget's work was confirmed, and the Rouget cell renamed the pericyte by virtue of its peri-endothelial location (Dore. Brit J Dermatol 35:398-404, 1923; Zimmermann. Z Anat Entwicklungsgesch 68:3-109, 1923). Over the years a large number of publications have emerged, but the pericyte has remained a truly enigmatic cell. This is due, in part, by the paucity of easy and reliable methods to isolate and characterize the cell as well as its heterogeneity and pluripotent characteristics. However, more recent advances in molecular genetics and development of novel cell isolation and imaging techniques have enable scientists to more readily define pericyte function. This chapter will discuss general approaches to the isolation, characterization, and propagation of primary pericytes in the establishment of cell lines. We will attempt to dispel misinterpretations about the pericyte that cloud the literature.

Connexin 30 is expressed in a subtype of mouse brain pericytes

Pericytes are mural cells of blood microvessels which play a crucial role at the neurovascular interface of the central nervous system. They are involved in the regulation of blood-brain barrier integrity, angiogenesis, clearance of toxic metabolites, capillary hemodynamic responses, and neuroinflammation, and they demonstrate stem cell activity. Morphological and molecular studies to characterize brain pericytes recently pointed out some heterogeneity in pericyte population. Nevertheless, a clear definition of pericyte subtypes is still lacking. Here, we demonstrate that a fraction of brain pericytes express Connexin 30 (Cx30), a gap junction protein, which, in the brain parenchyma, was thought to be exclusively found in astrocytes. Cx30 could thus be a candidate protein in the composition of the gap junction channels already described between endothelial cells and pericytes. It could also form hemichannels or acts in a channel-independent manner to regulate pericyte morphology, as already observed in astrocytes. Altogether, our results suggest that Cx30 defines a novel brain pericyte subtype.

Sushi repeat-containing protein 1: a novel disease-associated molecule in cerebral amyloid angiopathy

Sporadic cerebral amyloid angiopathy (CAA) is characterized by cerebrovascular amyloid beta (Aβ) deposits and causes cerebral hemorrhage and dementia. The exact molecules that co-accumulate with cerebrovascular Aβ deposits are still not fully known. In our study here, we performed proteomic analyses with microdissected leptomeningeal arteries and cerebral neocortical arterioles from 8 cases with severe CAA, 12 cases with mild CAA, and 10 control cases without CAA, and we determined the levels of highly expressed proteins in cerebral blood vessels in CAA. We focused on sushi repeat-containing protein 1 (SRPX1), which is specifically expressed in CAA-affected cerebral blood vessels. Because SRPX1, which is known as a tumor suppressor gene, reportedly induced apoptosis in tumor cells, we hypothesized that SRPX1 may play an important role in Aβ-induced apoptosis in CAA. Immunohistochemical studies revealed that SRPX1 co-accumulated with Aβ deposits in cerebral blood vessels of all autopsied cases with severe CAA. In contrast, no SRPX1 co-accumulated with Aβ deposits in senile plaques. Furthermore, we demonstrated that both Aβ40 and Aβ42 bound to SRPX1 in vitro and enhanced SRPX1 expression in primary cultures of cerebrovascular smooth muscle cells. SRPX1 enhanced caspase activity induced by Aβ40. Knockdown of SRPX1, in contrast, reduced the formation of Aβ40 accumulations and the activity of caspase in cultured cerebrovascular smooth muscle cells. SRPX1 may thus be a novel molecule that is up-regulated in cerebrovascular Aβ deposits and that may increase Aβ-induced cerebrovascular degeneration in CAA.

Selective knockout of astrocytic Na+ /H+ exchanger isoform 1 reduces astrogliosis, BBB damage, infarction, and improves neurological function after ischemic stroke

Stimulation of Na⁺ /H⁺ exchanger isoform 1 (NHE1) in astrocytes causes ionic dysregulation under ischemic conditions. In this study, we created a Nhe1^flox/flox (Nhe1^f/f ) mouse line with exon 5 of Nhe1 flanked with two loxP sites and selective ablation of Nhe1 in astrocytes was achieved by crossing Nhe1^f/f mice with Gfap-Cre^ERT2 Cre-recombinase mice. Gfap-Cre^ERT2+/- ;Nhe1^f/f mice at postnatal day 60-90 were treated with either corn oil or tamoxifen (Tam, 75 mg/kg/day, i.p.) for 5 days. After 30 days post-injection, mice underwent transient middle cerebral artery occlusion (tMCAO) to induce ischemic stroke. Compared with the oil-vehicle group (control), Tam-treated Gfap-Cre^ERT2+/- ;Nhe1^f/f (Nhe1 KO) mice developed significantly smaller ischemic infarction, less edema, and less neurological function deficits at 1-5 days after tMCAO. Immunocytochemical analysis revealed less astrocytic proliferation, less cellular hypertrophy, and less peri-lesion gliosis in Nhe1 KO mouse brains. Selective deletion of Nhe1 in astrocytes also reduced cerebral microvessel damage and blood-brain barrier (BBB) injury in ischemic brains. The BBB microvessels of the control brains show swollen endothelial cells, opened tight junctions, increased expression of proinflammatory protease MMP-9, and significant loss of tight junction protein occludin. In contrast, the Nhe1 KO mice exhibited reduced BBB breakdown and normal tight junction structure, with increased expression of occludin and reduced MMP-9. Most importantly, deletion of astrocytic Nhe1 gene significantly increased regional cerebral blood flow in the ischemic hemisphere at 24 hr post-MCAO. Taken together, our study provides the first line of evidence for a causative role of astrocytic NHE1 protein in reactive astrogliosis and ischemic neurovascular damage.

Serum amyloid P component: A novel potential player in vessel degeneration in CADASIL

In cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), granular osmiophilic material (GOM) may play some roles in inducing cerebrovascular events. To elucidate the pathogenesis of CADASIL, we used laser microdissection and liquid chromatography-tandem mass spectrometry to analyze cerebrovascular lesions of patients with CADASIL for GOM. The analyses detected serum amyloid P component (SAP), annexin A2, and periostin as the proteins with the largest increase in the samples, which also demonstrated NOTCH3. For the three proteins, anti-human SAP antibody had the strongest reaction in the lesions where the anti-human NOTCH3 antibody showed positive staining. Moreover, immunofluorescence staining with the two antibodies clearly showed co-localization of SAP and NOTCH3. mRNA analyses indicated no positive SAP expression in the brain materials, which suggested that the source of SAP found in the GOM was only the liver. A solid phase enzyme-linked immunosorbent assay confirmed the binding of SAP with NOTCH3. Serum SAP concentrations were neither up-regulated nor down-regulated in CADASIL patients, when compared with those in control subjects. SAP may play an important role in GOM formation although precise mechanisms remain to be elucidated.

Size-selective opening of the blood-brain barrier by targeting endothelial sphingosine 1-phosphate receptor 1

The vasculature of the central nervous system (CNS) forms a selective barrier termed the blood-brain barrier (BBB). Disruption of the BBB may contribute to various CNS diseases. Conversely, the intact BBB restricts efficient penetration of CNS-targeted drugs. Here, we report the BBB-regulatory role of endothelial sphingosine 1-phosphate (S1P) receptor-1, a G protein-coupled receptor known to promote the barrier function in peripheral vessels. Endothelial-specific S1pr1 knockout mice (S1pr1^iECKO ) showed BBB breach for small-molecular-mass fluorescence tracers (<3 kDa), but not larger tracers (>10 kDa). Chronic BBB leakiness was associated with cognitive impairment, as assessed by the novel object recognition test, but not signs of brain inflammation. Brain microvessels of S1pr1^iECKO mice showed altered subcellular distribution of tight junctional proteins. Pharmacological inhibition of S1P₁ function led to transient BBB breach. These data suggest that brain endothelial S1P₁ maintain the BBB by regulating the proper localization of tight junction proteins and raise the possibility that endothelial S1P₁ inhibition may be a strategy for transient BBB opening and delivery of small molecules into the CNS.

Translation in astrocyte distal processes sets molecular heterogeneity at the gliovascular interface

Astrocytes send out long processes that are terminated by endfeet at the vascular surface and regulate vascular functions as well as homeostasis at the vascular interface. To date, the astroglial mechanisms underlying these functions have been poorly addressed. Here we demonstrate that a subset of messenger RNAs is distributed in astrocyte endfeet. We identified, among this transcriptome, a pool of messenger RNAs bound to ribosomes, the endfeetome, that primarily encodes for secreted and membrane proteins. We detected nascent protein synthesis in astrocyte endfeet. Finally, we determined the presence of smooth and rough endoplasmic reticulum and the Golgi apparatus in astrocyte perivascular processes and endfeet, suggesting for local maturation of membrane and secreted proteins. These results demonstrate for the first time that protein synthesis occurs in astrocyte perivascular distal processes that may sustain their structural and functional polarization at the vascular interface.