Gliovascular and cytokine interactions modulate brain endothelial barrier in vitro

Validation and characterization of a novel blood-brain barrier platform for investigating traumatic brain injury

The Blood-Brain Barrier (BBB) is a highly-selective physiologic barrier responsible for maintaining cerebral homeostasis. Innovative in vitro models of the BBB are needed to provide useful insights into BBB function with CNS disorders like traumatic brain injury (TBI). TBI is a multidimensional and highly complex pathophysiological condition that requires intrinsic models to elucidate its mechanisms. Current models either lack fluidic shear stress, or neglect hemodynamic parameters important in recapitulating the human in vivo BBB phenotype. To address these limitations in the field, we developed a fluid dynamic novel platform which closely mimics these parameters. To validate our platform, Matrigel-coated Transwells were seeded with brain microvascular endothelial cells, both with and without co-cultured primary human astrocytes and bone-marrow mesenchymal stem cells. In this article we characterized BBB functional properties such as TEER and paracellular permeability. Our platform demonstrated physiologic relevant decreases in TEER in response to an ischemic environment, while directly measuring barrier fluid fluctuation. These recordings were followed with recovery, implying stability of the model. We also demonstrate that our dynamic platform is responsive to inflammatory and metabolic cues with resultant permeability coefficients. These results indicate that this novel dynamic platform will be a valuable tool for evaluating the recapitulating BBB function in vitro, screening potential novel therapeutics, and establishing a relevant paradigm to evaluate the pathophysiology of TBI.

PI3KC2β inactivation stabilizes VE-cadherin junctions and preserves vascular integrity

Endothelium protection is critical, because of the impact of vascular leakage and edema on pathological conditions such as brain ischemia. Whereas deficiency of class II phosphoinositide 3-kinase alpha (PI3KC2α) results in an increase in vascular permeability, we uncover a crucial role of the beta isoform (PI3KC2β) in the loss of endothelial barrier integrity following injury. Here, we studied the role of PI3KC2β in endothelial permeability and endosomal trafficking in vitro and in vivo in ischemic stroke. Mice with inactive PI3KC2β showed protection against vascular permeability, edema, cerebral infarction, and deleterious inflammatory response. Loss of PI3KC2β in human cerebral microvascular endothelial cells stabilized homotypic cell-cell junctions by increasing Rab11-dependent VE-cadherin recycling. These results identify PI3KC2β as a potential new therapeutic target to prevent aggravating lesions following ischemic stroke.

Islet Amyloid Polypeptide: A Partner in Crime With Aβ in the Pathology of Alzheimer's Disease

Diabetes affects hundreds of millions of patients worldwide. Despite the advances in understanding the disease and therapeutic options, it remains a leading cause of death and of comorbidities globally. Islet amyloid polypeptide (IAPP), or amylin, is a hormone produced by pancreatic β-cells. It contributes to the maintenance of glucose physiological levels namely by inhibiting insulin and glucagon secretion as well as controlling adiposity and satiation. IAPP is a highly amyloidogenic polypeptide forming intracellular aggregates and amyloid structures that are associated with β-cell death. Data also suggest the relevance of unprocessed IAPP forms as seeding for amyloid buildup. Besides the known consequences of hyperamylinemia in the pancreas, evidence has also pointed out that IAPP has a pathological role in cognitive function. More specifically, IAPP was shown to impair the blood–brain barrier; it was also seen to interact and co-deposit with amyloid beta peptide (Aß), and possibly with Tau, within the brain of Alzheimer's disease (AD) patients, thereby contributing to diabetes-associated dementia. In fact, it has been suggested that AD results from a metabolic dysfunction in the brain, leading to its proposed designation as type 3 diabetes. Here, we have first provided a brief perspective on the IAPP amyloidogenic process and its role in diabetes and AD. We have then discussed the potential interventions for modulating IAPP proteotoxicity that can be explored for therapeutics. Finally, we have proposed the concept of a “diabetes brain phenotype” hypothesis in AD, which may help design future IAPP-centered drug developmentstrategies against AD.

Astrocyte-Derived Paracrine Signals: Relevance for Neurogenic Niche Regulation and Blood-Brain Barrier Integrity

Astrocytes are essential for proper regulation of the central nervous system (CNS). Importantly, these cells are highly secretory in nature. Indeed they can release hundreds of molecules which play pivotal physiological roles in nervous tissues and whose abnormal regulation has been associated with several CNS disorders. In agreement with these findings, recent studies have provided exciting insights into the key contribution of astrocyte-derived signals in the pleiotropic functions of these cells in brain health and diseases. In the future, deeper analysis of the astrocyte secretome is likely to further increase our current knowledge on the full potential of these cells and their secreted molecules not only as active participants in pathophysiological events, but as pharmacological targets or even as therapeutics for neurological and psychiatric diseases. Herein we will highlight recent findings in our and other laboratories on selected molecules that are actively secreted by astrocytes and contribute in two distinct functions with pathophysiological relevance for the astroglial population: i) regulation of neural stem cells (NSCs) and their progeny within adult neurogenic niches; ii) modulation of the blood–brain barrier (BBB) integrity and function.

High-Fat Diet Increases Amylin Accumulation in the Hippocampus and Accelerates Brain Aging in hIAPP Transgenic Mice

The accumulation of human islet amyloid polypeptide (hIAPP) in pancreatic islets under induction by a high-fat diet plays a critical role in the development of type-2 diabetes mellitus (T2DM). T2DM is a risk factor of late-onset Alzheimer’s disease (AD). Nevertheless, whether hIAPP in combination with hyperlipidemia may lead to AD-like pathological changes in the brain remains unclear. hIAPP transgenic mice were fed with a high-fat diet for 6 or 12 months to establish the T2DM model. The accumulation of amylin, the numbers of Fluoro-Jade C (FJC)-positive and β-gal positive cells, and the deposition level of Aβ42 in the hippocampi of the transgenic mice were detected by using brain sections. Cytoplasmic and membrane proteins were extracted from the hippocampi of the transgenic mice, and the ratio of membrane GLUT4 expression to cytoplasmic GLUT4 expression was measured through Western blot analysis. Changes in the cognitive functions of hIAPP transgenic mice after 12 months of feeding with a high-fat diet were evaluated. hIAPP transgenic mice fed with a high-fat diet for 6 or 12 months showed elevated blood glucose levels and insulin resistance; increased amylin accumulation, number of FJC-positive and β-gal positive cells, and Aβ42 deposition in the hippocampi; and reduced membrane GLUT4 expression levels. hIAPP transgenic mice fed with a high-fat diet for 12 months showed reductions in social cognitive ability and passive learning ability. A high-fat diet increased amylin accumulation in the hippocampi of hIAPP transgenic mice, which presented AD-like pathology and behavior characterized by neural degeneration, brain aging, Aβ42 deposition, and impaired glucose utilization and cognition.

Astrocytic thrombin-evoked VEGF release is dependent on p44/42 MAPKs and PAR1

Intracerebral haemorrhage (ICH) often causes severe neurological deficits in survived patients, although its underlying mechanisms remain elusive. A common feature of ICH is the accumulation of thrombin around the lesion site. Previous studies showed that thrombin promotes VEGF release and angiogenesis at a late stage post ICH [1]. In current study, we explored the source for thrombin-induced VEGF release by adding thrombin or its receptor agonist peptide to the neuronal or astrocytic primarily cultures. We identified that astrocytes specifically respond to thrombin by up-regulating and releasing VEGF. Furthermore, such release is dependent on p44/42 MAPKs and PAR1, a thrombin specific receptor. Our study therefore helps clarifying the underlying mechanisms of thrombin-induced VEGF release in ICH, which will further provide novel insights into the designing principles for treating ICH and traumatic brain injuries.

Triphenyl Phosphate (TPHP)-Induced Neurotoxicity in Adult Male Chinese Rare Minnows ( Gobiocypris rarus)

The neurotoxicity of triphenyl phosphate (TPHP) in exposed humans and laboratory animals is under debate. The rapid crossing of the blood-brain barrier (BBB) and high distribution of TPHP in fish brains have raised widespread concerns about potential neurotoxicity. Adult male Chinese rare minnows ( Gobiocypris rarus) were used as a model and exposed to 0, 20, or 100 μg/L TPHP for 28 days. We evaluated the BBB permeability, neuroinflammatory response, cell proliferation and apoptosis, synaptic plasticity and synapse loss in fish brains via the learning/memory performance of fish following 28 days of TPHP exposure. TPHP significantly increased the BBB permeability, activated the neuroinflammatory response, and decreased the tight junction-related mRNA levels of claudin-5α and occludin in the fish brain. In addition, cell proliferation was inhibited by treatment with 100 μg/L TPHP, but no significant apoptosis was observed in the brain. Fish exposed to 100 μg/L TPHP exhibited significantly decreased dendritic arborization in pyramidal neurons in the cerebellum (Ce), and the maze test indicated impaired learning/memory performance. Taken together, these findings provide scientific evidence that TPHP is neurotoxic to fish and further suggest that TPHP may not be a safe alternative for aquatic organisms.

Inhibition of Complement Drives Increase in Early Growth Response Proteins and Neuroprotection Mediated by Salidroside After Cerebral Ischemia

Salidroside is neuroprotective across a wide therapeutic time-window after cerebral ischemia-reperfusion injury (IRI). Here, we investigated the role of complement in mediating effects of salidroside after cerebral IRI in rats. Rats were administrated with vehicle or salidroside 50 mg/kg, given daily for either 24 or 48 h, after middle cerebral artery occlusion (MCAO) for 2 h and reperfusion for 1 h. Levels of proteins in ischemic brain were measured by immunofluorescence and western blotting. We observed early increases in the deposition of immunoglobulin M, mannose-binding lectin 2, and annexin IV on cerebral endothelial cells, induction of the complement components C3 and C3a, by 24 h after IRI, and a later significant increase in the complement component C1q by 48 h. Salidroside prevented these changes. The neuroplasticity-related early growth response proteins Egr1, Egr2, and Egr4 and activity-regulated cytoskeleton-associated protein increased transiently in the first 6 h after IRI but then decreased below baseline by 48 h after IRI. Neither salidroside nor a C3a receptor antagonist (C3aRA) affected these proteins 24 h after IRI, but both reversed their later decreases to similar and non-additive extents. Salidroside and C3aRA increased NeuN in a non-additive manner after IRI. Our results suggest that salidroside exerts neuroprotection by reducing early activation of the lectin pathway on the cerebral endothelium and inhibiting the gradual activation of the classical pathway after cerebral IRI. This prolonged neuroprotection may depend, at least in part, on increased expression of neuroplasticity-related genes driven by reduced complement activation.

"Immune Gate" of Psychopathology-The Role of Gut Derived Immune Activation in Major Psychiatric Disorders

Interaction between the gastrointestinal tract (GI) and brain functions has recently become a topic of growing interest in psychiatric research. These multidirectional interactions take place in the so-called gut-brain axis or more precisely, the microbiota-gut-brain axis. The GI tract is the largest immune organ in the human body and is also the largest surface of contact with the external environment. Its functions and permeability are highly influenced by psychological stress, which are often a precipitating factor in the first episode, reoccurrence and/or deterioration of symptoms of psychiatric disorders. In recent literature there is growing evidence that increased intestinal permeability with subsequent immune activation has a major role in the pathophysiology of various psychiatric disorders. Numerous parameters measured in this context seem to be aftermaths of those mechanisms, yet at the same time they may be contributing factors for immune mediated psychopathology. For example, immune activation related to gut-derived bacterial lipopolysaccharides (LPS) or various food antigens and exorphins were reported in major depression, schizophrenia, bipolar disorder, alcoholism and autism. In this review the authors will summarize the evidence and roles of such parameters and their assessment in major psychiatric disorders.

Polymerase delta-interacting protein 2 deficiency protects against blood-brain barrier permeability in the ischemic brain

BACKGROUND

Polymerase δ-interacting protein 2 (Poldip2) is a multifunctional protein that regulates vascular extracellular matrix composition and matrix metalloproteinase (MMP) activity. The blood-brain barrier (BBB) is a dynamic system assembled by endothelial cells, basal lamina, and perivascular astrocytes, raising the possibility that Poldip2 may be involved in maintaining its structure. We investigated the role of Poldip2 in the late BBB permeability induced by cerebral ischemia.

METHODS

Transient middle cerebral artery occlusion (tMCAO) was induced in Poldip2+/+ and Poldip2+/- mice. The volume of the ischemic lesion was measured in triphenyltetrazolium chloride-stained sections. BBB breakdown was evaluated by Evans blue dye extravasation. Poldip2 protein expression was evaluated by western blotting. RT-PCR, zymography, and ELISAs were used to measure mRNA levels, activity, and protein levels of cytokines and MMPs. Cultured astrocytes were transfected with Poldip2 siRNA, and mRNA levels of cytokines were evaluated as well as IκBα protein degradation.

RESULTS

Cerebral ischemia induced the expression of Poldip2. Compared to Poldip2+/+ mice, Poldip2+/- animals exhibited decreased Evans blue dye extravasation and improved survival 24 h following stroke. Poldip2 expression was upregulated in astrocytes exposed to oxygen and glucose deprivation (OGD) and siRNA-mediated downregulation of Poldip2 abrogated OGD-induced IL-6 and TNF-α expression. In addition, siRNA against Poldip2 inhibited TNF-α-induced IκBα degradation. TNF-α, IL-6, MCP-1, VEGF, and MMP expression induced by cerebral ischemia was abrogated in Poldip2+/- mice. The protective effect of Poldip2 depletion on the increased permeability of the BBB was partially reversed by systemic administration of TNF-α.

CONCLUSIONS

Poldip2 is upregulated following ischemic stroke and mediates the breakdown of the BBB by increasing cerebral cytokine production and MMP activation. Therefore, Poldip2 appears to be a promising novel target for the development of therapeutic strategies to prevent the development of cerebral edema in the ischemic brain.

Mechanisms of macular edema: Beyond the surface

Macular edema consists of intra- or subretinal fluid accumulation in the macular region. It occurs during the course of numerous retinal disorders and can cause severe impairment of central vision. Major causes of macular edema include diabetes, branch and central retinal vein occlusion, choroidal neovascularization, posterior uveitis, postoperative inflammation and central serous chorioretinopathy. The healthy retina is maintained in a relatively dehydrated, transparent state compatible with optimal light transmission by multiple active and passive systems. Fluid accumulation results from an imbalance between processes governing fluid entry and exit, and is driven by Starling equation when inner or outer blood-retinal barriers are disrupted. The multiple and intricate mechanisms involved in retinal hydro-ionic homeostasis, their molecular and cellular basis, and how their deregulation lead to retinal edema, are addressed in this review. Analyzing the distribution of junction proteins and water channels in the human macula, several hypotheses are raised to explain why edema forms specifically in the macular region. "Pure" clinical phenotypes of macular edema, that result presumably from a single causative mechanism, are detailed. Finally, diabetic macular edema is investigated, as a complex multifactorial pathogenic example. This comprehensive review on the current understanding of macular edema and its mechanisms opens perspectives to identify new preventive and therapeutic strategies for this sight-threatening condition.

Annexin A2 Plus Low-Dose Tissue Plasminogen Activator Combination Attenuates Cerebrovascular Dysfunction After Focal Embolic Stroke of Rats

Previous studies showed recombinant annexin A2 (rA2) in combination with low-dose tissue-type plasminogen activator (tPA) improved thrombolytic efficacy and long-term neurological outcomes after embolic focal ischemia in rats. The objective of this study was to investigate the effects and mechanisms of the combination in early BBB integrity and cerebrovascular patency in the rat focal embolic stroke model. Ischemic brain infarct volume and hemorrhagic transformation were quantified at 24 h after stroke. At an earlier time point, 16 h after stroke, BBB integrity was evaluated by IgG extravasation, and the involved mechanisms were assessed for tight junction ZO-1 and adhesion junction ve-cadherin protein expression, matrix metalloproteinase activation, extracellular matrix collagen IV and endothelial barrier antigen expression, and activation of microglia/macrophages and astrocytes. While at the same time point, cerebrovascular patency was assessed by intravascular fibrin and platelet depositions. At 24 h after stroke, the combination showed significant reduction in brain infarction and intracerebral hemorrhage. At 16 h after stroke onset, the combination therapy significantly reduced BBB disruption, and improved preservation of the junction proteins ZO-1 and ve-cadherin, decreased activation of matrix metalloproteinase, inhibited degradation of extracellular matrix collagen IV and endothelial barrier antigen, and reduced microglia/macrophage and astrocytes activations. Meanwhile, the combination also significantly improved cerebrovascular patency by reducing intravascular fibrin and platelet depositions in the peri-infarct brain tissues. These results suggest the beneficial effects of the rA2 plus low-dose tPA combination may be mediated in part by the amelioration of BBB disruption and improvement of cerebrovascular patency.

Islet amyloid polypeptide: Another key molecule in Alzheimer's pathogenesis?

Recent epidemiological evidence reveals that patients suffering from type 2 diabetes mellitus (T2DM) often experience a significant decline in cognitive function, and approximately 70% of those cases eventually develop Alzheimer's disease (AD). Although several pathological processes are shared by AD and T2DM, the exact molecular mechanisms connecting these two diseases are poorly understood. Aggregation of human islet amyloid polypeptide (hIAPP), the pathological hallmark of T2DM, has also been detected in brain tissue and is associated with cognitive decline and AD development. In addition, hIAPP and amyloid β protein (Aβ) share many biophysical and physiological properties as well as exert similar cytotoxic mechanisms. Therefore, it is important to examine the possible role of hIAPP in the pathogenesis of AD. In this article, we introduce the basics on this amyloidogenic protein. More importantly, we discuss the potential mechanisms of hIAPP-induced AD development, which will be beneficial for proposing novel and feasible strategies to optimize AD prevention and/or treatment in diabetics.

Characterization of passive permeability at the blood-tumor barrier in five preclinical models of brain metastases of breast cancer

The blood-brain barrier (BBB) is compromised in brain metastases, allowing for enhanced drug permeation into brain. The extent and heterogeneity of BBB permeability in metastatic lesions is important when considering the administration of chemotherapeutics. Since permeability characteristics have been described in limited experimental models of brain metastases, we sought to define these changes in five brain-tropic breast cancer cell lines: MDA-MB-231BR (triple negative), MDA-MB-231BR-HER2, JIMT-1-BR3, 4T1-BR5 (murine), and SUM190 (inflammatory HER2 expressing). Permeability was assessed using quantitative autoradiography and fluorescence microscopy by co-administration of the tracers (14)C-aminoisobutyric acid (AIB) and Texas red conjugated dextran prior to euthanasia. Each experimental brain metastases model produced variably increased permeability to both tracers; additionally, the magnitude of heterogeneity was different among each model with the highest ranges observed in the SUM190 (up to 45-fold increase in AIB) and MDA-MB-231BR-HER2 (up to 33-fold in AIB) models while the lowest range was observed in the JIMT-1-BR3 (up to 5.5-fold in AIB) model. There was no strong correlation observed between lesion size and permeability in any of these preclinical models of brain metastases. Interestingly, the experimental models resulting in smaller mean metastases size resulted in shorter median survival while models producing larger lesions had longer median survival. These findings strengthen the evidence of heterogeneity in brain metastases of breast cancer by utilizing five unique experimental models and simultaneously emphasize the challenges of chemotherapeutic approaches to treat brain metastases.

Microfluidic organ-on-chip technology for blood-brain barrier research

Organs-on-chips are a new class of microengineered laboratory models that combine several of the advantages of current in vivo and in vitro models. In this review, we summarize the advances that have been made in the development of organ-on-chip models of the blood-brain barrier (BBBs-on-chips) and the challenges that are still ahead. The BBB is formed by specialized e3ndothelial cells and separates blood from brain tissue. It protects the brain from harmful compounds from the blood and provides homeostasis for optimal neuronal function. Studying BBB function and dysfunction is important for drug development and biomedical research. Microfluidic BBBs-on-chips enable real-time study of (human) cells in an engineered physiological microenvironment, for example incorporating small geometries and fluid flow as well as sensors. Examples of BBBs-on-chips in literature already show the potential of more realistic microenvironments and the study of organ-level functions. A key challenge in the field of BBB-on-chip development is the current lack of standardized quantification of parameters such as barrier permeability and shear stress. This limits the potential for direct comparison of the performance of different BBB-on-chip models to each other and existing models. We give recommendations for further standardization in model characterization and conclude that the rapidly emerging field of BBB-on-chip models holds great promise for further studies in BBB biology and drug development.

In vivo seeding and cross-seeding of localized amyloidosis: a molecular link between type 2 diabetes and Alzheimer disease

Several proteins have been identified as amyloid forming in humans, and independent of protein origin, the fibrils are morphologically similar. Therefore, there is a potential for structures with amyloid seeding ability to induce both homologous and heterologous fibril growth; thus, molecular interaction can constitute a link between different amyloid forms. Intravenous injection with preformed fibrils from islet amyloid polypeptide (IAPP), proIAPP, or amyloid-beta (Aβ) into human IAPP transgenic mice triggered IAPP amyloid formation in pancreas in 5 of 7 mice in each group, demonstrating that IAPP amyloid could be enhanced through homologous and heterologous seeding with higher efficiency for the former mechanism. Proximity ligation assay was used for colocalization studies of IAPP and Aβ in islet amyloid in type 2 diabetic patients and Aβ deposits in brains of patients with Alzheimer disease. Aβ reactivity was not detected in islet amyloid although islet β cells express AβPP and convertases necessary for Aβ production. By contrast, IAPP and proIAPP were detected in cerebral and vascular Aβ deposits, and presence of proximity ligation signal at both locations showed that the peptides were <40 nm apart. It is not clear whether IAPP present in brain originates from pancreas or is locally produced. Heterologous seeding between IAPP and Aβ shown here may represent a molecular link between type 2 diabetes and Alzheimer disease.

Measuring direct current trans-epithelial electrical resistance in organ-on-a-chip microsystems

Trans-epithelial electrical resistance (TEER) measurements are widely used as real-time, non-destructive, and label-free measurements of epithelial and endothelial barrier function. TEER measurements are ideal for characterizing tissue barrier function in organs-on-chip studies for drug testing and investigation of human disease models; however, published reports using this technique have reported highly conflicting results even with identical cell lines and experimental setups. The differences are even more dramatic when comparing measurements in conventional Transwell systems with those obtained in microfluidic systems. Our goal in this work was therefore to enhance the fidelity of TEER measurements in microfluidic organs-on-chips, specifically using direct current (DC) measurements of TEER, as this is the most widely used method reported in the literature. Here we present a mathematical model that accounts for differences measured in TEER between microfluidic chips and Transwell systems, which arise from differences in device geometry. The model is validated by comparing TEER measurements obtained in a microfluidic gut-on-a-chip device versus in a Transwell culture system. Moreover, we show that even small gaps in cell coverage (e.g., 0.4%) are sufficient to cause a significant (~80%) drop in TEER. Importantly, these findings demonstrate that TEER measurements obtained in microfluidic systems, such as organs-on-chips, require special consideration, specifically when results are to be compared with measurements obtained from Transwell systems.

Neuronal and astrocytic interactions modulate brain endothelial properties during metabolic stresses of in vitro cerebral ischemia

Neurovascular and gliovascular interactions significantly affect endothelial phenotype. Physiologically, brain endothelium attains several of its properties by its intimate association with neurons and astrocytes. However, during cerebrovascular pathologies such as cerebral ischemia, the uncoupling of neurovascular and gliovascular units can result in several phenotypical changes in brain endothelium. The role of neurovascular and gliovascular uncoupling in modulating brain endothelial properties during cerebral ischemia is not clear. Specifically, the roles of metabolic stresses involved in cerebral ischemia, including aglycemia, hypoxia and combined aglycemia and hypoxia (oxygen glucose deprivation and re-oxygenation, OGDR) in modulating neurovascular and gliovascular interactions are not known. The complex intimate interactions in neurovascular and gliovascular units are highly difficult to recapitulate in vitro. However, in the present study, we used a 3D co-culture model of brain endothelium with neurons and astrocytes in vitro reflecting an intimate neurovascular and gliovascular interactions in vivo. While the cellular signaling interactions in neurovascular and gliovascular units in vivo are much more complex than the 3D co-culture models in vitro, we were still able to observe several important phenotypical changes in brain endothelial properties by metabolically stressed neurons and astrocytes including changes in barrier, lymphocyte adhesive properties, endothelial cell adhesion molecule expression and in vitro angiogenic potential.

Ischemic neurons activate astrocytes to disrupt endothelial barrier via increasing VEGF expression

Blood-brain barrier (BBB) disruption occurring within the first few hours of ischemic stroke onset is closely associated with hemorrhagic transformation following thrombolytic therapy. However, the mechanism of this acute BBB disruption remains unclear. In the neurovascular unit, neurons do not have direct contact with the endothelial barrier; however, they are highly sensitive and vulnerable to ischemic injury, and may act as the initiator for disrupting BBB when cerebral ischemia occurs. Herein, we employed oxygen-glucose deprivation (OGD) and an in vitro BBB system consisting of brain microvascular cells and astrocytes to test this hypothesis. Neurons (CATH.a cells) were exposed to OGD for 3-h before co-culturing with endothelial monolayer (bEnd 3 cells), or endothelial cells plus astrocytes (C8-D1A cells). Incubation of OGD-treated neurons with endothelial monolayer alone did not increase endothelial permeability. However, when astrocytes were present, the endothelial permeability was significantly increased, which was accompanied by loss of occludin and claudin-5 proteins as well as increased vascular endothelial growth factor (VEGF) secretion into the conditioned medium. Importantly, all these changes were abolished when VEGF was knocked down in astrocytes by siRNA. Our findings suggest that ischemic neurons activate astrocytes to increase VEGF production, which in turn induces endothelial barrier disruption.

Venous endothelial injury in central nervous system diseases

The role of the venous system in the pathogenesis of inflammatory neurological/neurodegenerative diseases remains largely unknown and underinvestigated. Aside from cerebral venous infarcts, thromboembolic events, and cerebrovascular bleeding, several inflammatory central nervous system (CNS) diseases, such as multiple sclerosis (MS), acute disseminated encephalomyelitis (ADEM), and optic neuritis, appear to be associated with venous vascular dysfunction, and the neuropathologic hallmark of these diseases is a perivenous, rather than arterial, lesion. Such findings raise fundamental questions about the nature of these diseases, such as the reasons why their pathognomonic lesions do not develop around the arteries and what exactly are the roles of cerebral venous inflammation in their pathogenesis. Apart from this inflammatory-based view, a new hypothesis with more focus on the hemodynamic features of the cerebral and extracerebral venous system suggests that MS pathophysiology might be associated with the venous system that drains the CNS. Such a hypothesis, if proven correct, opens new therapeutic windows in MS and other neuroinflammatory diseases. Here, we present a comprehensive review of the pathophysiology of MS, ADEM, pseudotumor cerebri, and optic neuritis, with an emphasis on the roles of venous vascular system programming and dysfunction in their pathogenesis. We consider the fundamental differences between arterial and venous endothelium, their dissimilar responses to inflammation, and the potential theoretical contributions of venous insufficiency in the pathogenesis of neurovascular diseases.

The hCMEC/D3 cell line as a model of the human blood brain barrier

Since the first attempts in the 1970s to isolate cerebral microvessel endothelial cells (CECs) in order to model the blood-brain barrier (BBB) in vitro, the need for a human BBB model that closely mimics the in vivo phenotype and is reproducible and easy to grow, has been widely recognized by cerebrovascular researchers in both academia and industry. While primary human CECs would ideally be the model of choice, the paucity of available fresh human cerebral tissue makes wide-scale studies impractical. The brain microvascular endothelial cell line hCMEC/D3 represents one such model of the human BBB that can be easily grown and is amenable to cellular and molecular studies on pathological and drug transport mechanisms with relevance to the central nervous system (CNS). Indeed, since the development of this cell line in 2005 over 100 studies on different aspects of cerebral endothelial biology and pharmacology have been published. Here we review the suitability of this cell line as a human BBB model for pathogenic and drug transport studies and we critically consider its advantages and limitations.

The hCMEC/D3 cell line as a model of the human blood brain barrier

Since the first attempts in the 1970s to isolate cerebral microvessel endothelial cells (CECs) in order to model the blood-brain barrier (BBB) in vitro, the need for a human BBB model that closely mimics the in vivo phenotype and is reproducible and easy to grow, has been widely recognized by cerebrovascular researchers in both academia and industry. While primary human CECs would ideally be the model of choice, the paucity of available fresh human cerebral tissue makes wide-scale studies impractical. The brain microvascular endothelial cell line hCMEC/D3 represents one such model of the human BBB that can be easily grown and is amenable to cellular and molecular studies on pathological and drug transport mechanisms with relevance to the central nervous system (CNS). Indeed, since the development of this cell line in 2005 over 100 studies on different aspects of cerebral endothelial biology and pharmacology have been published. Here we review the suitability of this cell line as a human BBB model for pathogenic and drug transport studies and we critically consider its advantages and limitations.

Vascular neuroprotection via TrkB- and Akt-dependent cell survival signaling

The cerebral endothelium can be a vital source of signaling factors such as brain-derived neurotrophic factor that defends the neuronal parenchyma against stress and injury. But the underlying mechanisms remain to be fully defined. Here, we use cell models to ask how vascular neuroprotection is sustained. Human brain endothelial cells were grown in culture, and conditioned media were transferred to primary rat cortical neurons. Brain endothelial cell-conditioned media activated neuronal Akt signaling and protected neurons against hypoxia and oxygen-glucose deprivation. Blockade of Akt phosphorylation with the PI3-kinase inhibitor LY294002 negated this vascular neuroprotective effect. Upstream of Akt signaling, the brain-derived neurotrophic factor receptor TrkB (neurotrophic tyrosine kinase receptor, type 2) was involved because depletion with TrkB/Fc eliminated the ability of endothelial-conditioned media to protect neurons against hypoxia. Downstream of Akt signaling, activation of GSK-3β (glycogen synthase kinase 3 beta), caspase 9, caspase 3 and Bad pathways were detected. Taken together, these findings suggest that the molecular basis for vascular neuroprotection involves TrkB-Akt signaling that ameliorates neuronal apoptosis. Further investigation of these mechanisms may reveal new approaches for augmenting endogenous vascular neuroprotection in stroke, brain injury, and neurodegeneration.

Emerging roles of endothelial cells in multiple sclerosis pathophysiology and therapy

Although multiple sclerosis (MS) has traditionally been viewed and researched as an immune-mediated demyelinating and neurodegenerative disease of the human central nervous system (CNS), its highly complex pathogenesis clearly includes a significant vascular inflammatory component and many therapeutic approaches achieve benefit by direct or indirect effects on cerebrovascular endothelial cells. Cerebral endothelial cells create and separate the compartments of the peripheral circulation and CNS creating the blood-brain barrier (BBB), a selectively permeable boundary layer between these spaces. Interactions between activated leukocytes and cerebral endothelium play essential roles in mediating their trans-BBB diapedesis during normal immune surveillance and during pathogenesis of neuroinflammatory diseases like MS. Extravasation of activated and committed leukocytes from the peripheral circulation through the endothelial layer of the BBB into the CNS milieu is the most fundamental step in formation of MS lesions. During MS pathogenesis, once the activated leukocytes enter the CNS environment, they propagate a massive wave of destruction which culminates in the loss of both myelin/oligodendrocyte complex and neurodegeneration. Multiple clinical and basic scientific observations support endothelial cell 'stress' and apoptosis as a hallmark characteristic of MS. The manipulation of the endothelial biology aiming to block trans-endothelial migration of activated immune cells into the CNS is a potent form of treatment for MS achieving significant reductions in disease activity and new lesion formation. In particular, endothelial microparticles are now well-recognized as important biomarkers and mediators of this type of stress. In this review, we discuss recent findings and new advances in our knowledge regarding leukocyte migration through the endothelial frontier of the BBB and how this can be exploited toward treating MS patients.

HIV-1, methamphetamine and astrocyte glutamate regulation: combined excitotoxic implications for neuro-AIDS

Glutamate, the most abundant excitatory transmitter in the brain can lead to neurotoxicity when not properly regulated. Excitotoxicity is a direct result of abnormal regulation of glutamate concentrations in the synapse, and is a common neurotoxic mediator associated with neurodegenerative disorders. It is well accepted that methamphetamine (METH), a potent central nervous stimulant with high abuse potential, and human immunodeficiency virus (HIV)-1 are implicated in the progression of neurocognitive malfunction. Both have been shown to induce common neurodegenerative effects such as astrogliosis, compromised blood brain barrier integrity, and excitotoxicity in the brain. Reduced glutamate uptake from neuronal synapses likely leads to the accumulation of glutamate in the extracellular spaces. Astrocytes express the glutamate transporters responsible for majority of the glutamate uptake from the synapse, as well as for vesicular glutamate release. However, the cellular and molecular mechanisms of astrocyte-mediated excitotoxicity in the context of METH and HIV-1 are undefined. Topics reviewed include dysregulation of the glutamate transporters, specifically excitatory amino acid transporter-2, metabotropic glutamate receptor(s) expression and the release of glutamate by vesicular exocytosis. We also discuss glutamate concentration dysregulation through astrocytic expression of enzymes for glutamate synthesis and metabolism. Lastly, we discuss recent evidence of various astrocyte and neuron crosstalk mechanisms implicated in glutamate regulation. Astrocytes play an essential role in the neuropathologies associated with METH/HIV-1-induced excitotoxicity. We hope to shed light on common cellular and molecular pathways astrocytes share in glutamate regulation during drug abuse and HIV-1 infection.