The blood-brain barrier/neurovascular unit in health and disease

Caveolin-1 regulates expression of junction-associated proteins in brain microvascular endothelial cells

Recent evidence from this laboratory indicated that reduced expression of caveolin-1 accompanied the diminished expression of tight junction (TJ)-associated proteins occludin and zonula occludens-1 (ZO-1) following stimulation of brain microvascular endothelial cells (BMECs) with the chemokine CCL2 (formerly called MCP-1). Because attenuated caveolin-1 levels have also been correlated with heightened permeability of other endothelia, the objective of this study was to test the hypothesis that reduced caveolin-1 expression is causally linked to the action of CCL2 on BMEC junctional protein expression and barrier integrity. This was achieved using adenovirus to nondestructively deliver caveolin-1 siRNA (Ad-siCav-1) to BMEC monolayers, which model the blood-brain barrier (BBB). Treatment with siRNA reduced the caveolin-1 protein level as well as occludin and ZO-1. Additionally, occludin exhibited dissociation from the cytoskeletal framework. These changes were attended by comparable alterations in adherens junction (AJ)-associated proteins, VE-cadherin and beta-catenin, increased BMEC paracellular permeability, and facilitated the ability of CCL2 to stimulate monocytic transendothelial migration. Furthermore, treating BMECs with cavtratin, a synthetic cell-permeable peptide encoding the caveolin-1 scaffolding domain, antagonized effects of both Ad-siCav-1 and CCL2. These results collectively highlight caveolin-1 loss as a critical step in CCL2-induced modulation of BMEC junctional protein expression and integrity, and possibly serve a crucial role in regulating inflammation at the BBB.

Estradiol reduces activity of the blood-brain barrier Na-K-Cl cotransporter and decreases edema formation in permanent middle cerebral artery occlusion

Estrogen has been shown to protect against stroke-induced brain damage, yet the mechanism is unknown. During the early hours of stroke, cerebral edema forms as increased transport of Na and Cl from blood into brain occurs across an intact blood-brain barrier (BBB). We showed previously that a luminal BBB Na-K-Cl cotransporter is stimulated by hypoxia and arginine vasopressin (AVP), factors present during cerebral ischemia, and that inhibition of the cotransporter by intravenous bumetanide greatly reduces edema in rats subjected to permanent middle cerebral artery occlusion (MCAO). The present study was conducted to determine whether estrogen protects in stroke at least in part by reducing activity of the BBB cotransporter, thereby decreasing edema formation. Ovariectomized rats were subjected to 210 mins of permanent MCAO after 7-day or 30-min pretreatment with 17beta-estradiol and then brain swelling and 2,3,5-triphenyltetrazolium chloride staining were assessed as measures of brain edema and lesion volume, respectively. Diffusion-weighed imaging was used to monitor permanent MCAO-induced decreases in apparent diffusion coefficient (ADC) values, an index of changes in brain water distribution and mobility. Na-K-Cl cotransporter activity of cerebral microvascular endothelial cells (CMECs) was assessed as bumetanide-sensitive K influx and cotransporter abundance by Western blot analysis after estradiol treatment. Estradiol significantly decreased brain swelling and lesion volume and attenuated the decrease in ADC values during permanent MCAO. Estradiol also abolished CMEC cotransporter stimulation by chemical hypoxia or AVP and decreased cotransporter abundance. These findings support the hypothesis that estrogen attenuates stimulation of BBB Na-K-Cl cotransporter activity, reducing edema formation during stroke.

VEGFR-2 expression in brain injury: its distribution related to brain-blood barrier markers

VEGF is a major regulator of angiogenesis and vascular permeability in development and injury. The involvement of one of its receptors, Flk-1 in angiogenesis has been widely demonstrated, but few studies elucidate its role as a mediator of the BBB permeability and none displays its distribution following a cortical micronecrosis. A microvascular marker (LEA lectin), two BBB markers (EBA, GluT-1) and the VEGFR2 receptor were studied in adult rats after a minimal brain injury. Immunohistochemistry shows an increase of positive vessels, somata and processes around the micronecrosis from 6 to 72 hours after injury. Flk-1 was overexpressed mainly in endothelial cells, but also in astrocytes, neuronal somata and processes adjacent to the damage. This increase correlates to the lose of positivity for EBA. After injury, VEGFR-2 expression increases and its distribution corresponds to VEGF one. The whole system seems to play a role in the disruption of the BBB.

Side by side comparison between dynamic versus static models of blood–brain barrier in vitro: A permeability study

Endothelial cells in vivo are continuously exposed to shear stress, a tangential force generated by the flow of blood across their apical surfaces that affects endothelial cell structure and function. By contrast, the Transwell apparatus cannot reproduce the presence of intraluminal blood flow that is essential for the formation and differentiation of the BBB. In contrast, the dynamic in vitro model of the BBB (DIV-BBB) mimics both functionally and anatomically the brain microvasculature, creating quasi-physiological conditions for co-culturing human and non-human endothelial cells and astrocytes in a capillary-like structure. We used intraluminal bovine aortic endothelial cells (BAEC) co-cultured with extraluminal glial cells (C6) to obtain elevated trans-endothelial electrical resistance (TEER) and selective permeability to sucrose and phenytoin. The experiments were performed in parallel using Transwell systems DIV-BBB models and data were then cross compared. By contrast with Transwell, C6 and BAEC co-cultured in the DIV-BBB demonstrated predominantly aerobic metabolism evidenced by a robust increase in glucose consumption that was paralleled by a similar change in lactate production. BAEC exposed to glia under dynamic conditions grow in a monolayer fashion and developed a more stringent barrier as demonstrated by high TEER values and a selective permeability to [14C] phenytoin and the well-known paracellular marker [3H] sucrose. In conclusion, these data demonstrate that the exposure to intraluminal flow plays an essential role in promoting endothelial cell differentiation and increasing BBB tightness, thus making the use of the DIV-BBB well suited for pharmacological studies.

Advancing the study of stroke in women: summary and recommendations for future research from an NINDS-Sponsored Multidisciplinary Working Group

BACKGROUND AND PURPOSE

Women have poorer outcomes from stroke than men. Women also have risk factors that are unique, including pregnancy and hormone therapy. Hormone therapy for postmenopausal replacement increased the risk of ischemic stroke according to results of the Women's Health Initiative clinical trials. Based on the current understanding of the mechanisms of action of estrogen, the reasons for this increased risk are uncertain. One method to better understand the reasons for this increased risk is to re-evaluate estrogen's role in the neurovascular unit, simplistically comprised of the neurons, glia, and endothelial cells, as well as the processes of inflammation, and hemostasis/thrombosis. Besides the role of estrogen there are many gaps of knowledge about issues specific to women and stroke.

SUMMARY OF REVIEW

A multidisciplinary workshop was held in August 2005 to summarize the current evidence for estrogen and, more generally, stroke in women, and to provide recommendations for future basic, preclinical, and clinical research studies.

CONCLUSIONS

These studies may ultimately change the approach to stroke prevention and treatment in women.

A potential role for CD8+ T-cells as regulators of CNS vascular permeability

The role of immune cells in promoting central nervous system (CNS) vascular permeability is poorly understood. In recent years, there is a growing body of literature that suggests CD8+ T-cells are potent mediators of vascular permeability in peripheral viral infections as well as in immune mediated neurological diseases. This review outlines the recent advances in tissue culture and animal models used to study vascular permeability. In addition, we put forth our hypothesis that CD8+ T-cells promote the opening of tight junctions between cerebral endothelial cells, enabling the infiltration of white blood cells and in certain models even leading to microhemorrhages in the CNS. Determining the mechanism by which CD8+ T-cells and other immune cells promote CNS vascular permeability in animal models could define new targets for immune mediated neurological conditions characterized by vascular permeability.

Blood-brain barrier: structural components and function under physiologic and pathologic conditions

The blood-brain barrier (BBB) is the specialized system of brain microvascular endothelial cells (BMVEC) that shields the brain from toxic substances in the blood, supplies brain tissues with nutrients, and filters harmful compounds from the brain back to the bloodstream. The close interaction between BMVEC and other components of the neurovascular unit (astrocytes, pericytes, neurons, and basement membrane) ensures proper function of the central nervous system (CNS). Transport across the BBB is strictly limited through both physical (tight junctions) and metabolic barriers (enzymes, diverse transport systems). A functional polarity exists between the luminal and abluminal membrane surfaces of the BMVEC. As a result of restricted permeability, the BBB is a limiting factor for the delivery of therapeutic agents into the CNS. BBB breakdown or alterations in transport systems play an important role in the pathogenesis of many CNS diseases (HIV-1 encephalitis, Alzheimer's disease, ischemia, tumors, multiple sclerosis, and Parkinson's disease). Proinflammatory substances and specific disease-associated proteins often mediate such BBB dysfunction. Despite seemingly diverse underlying causes of BBB dysfunction, common intracellular pathways emerge for the regulation of the BBB structural and functional integrity. Better understanding of tight junction regulation and factors affecting transport systems will allow the development of therapeutics to improve the BBB function in health and disease.

Possible involvement of gap junctions in the barrier function of tight junctions of brain and lung endothelial cells

Gap-junction plaques are often observed with tight-junction strands of vascular endothelial cells but the molecular interaction and functional relationships between these two junctions remain obscure. We herein show that gap-junction proteins connexin40 (Cx40) and Cx43 are colocalized and coprecipitated with tight-junction molecules occludin, claudin-5, and ZO-1 in porcine blood-brain barrier (BBB) endothelial cells. Gap junction blockers 18beta-glycyrrhetinic acid (18beta-GA) and oleamide (OA) did not influence expression of Cx40, Cx43, occludin, claudin-5, junctional adhesion molecule (JAM)-A, JAM-B, JAM-C, or ZO-1, or their subcellular localization in the porcine BBB endothelial cells. In contrast, these gap-junction blocking agents inhibited the barrier function of tight junctions in cells, determined by measurement of transendothelial electrical resistance and paracellular flux of mannitol and inulin. 18beta-GA also significantly reduced the barrier property in rat lung endothelial (RLE) cells expressing doxycycline-induced claudin-1, but did not change the interaction between Cx43 and either claudin-1 or ZO-1, nor their expression levels or subcellular distribution. These findings suggest that Cx40- and/or Cx43-based gap junctions might be required to maintain the endothelial barrier function without altering the expression and localization of the tight-junction components analyzed.

Causes and consequences of methamphetamine and MDMA toxicity

Methamphetamine (METH) and its derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) are 2 substituted amphetamines with very high abuse liability in the United States. These amphetamine-like stimulants have been associated with loss of multiple markers for dopaminergic and serotonergic terminals in the brain. Among other causes, oxidative stress, excitotoxicity and mitochondrial dysfunction appear to play a major role in the neurotoxicity produced by the substituted amphetamines. The present review will focus on these events and how they interact and converge to produce the monoaminergic depletions that are typically observed after METH or MDMA administration. In addition, more recently identified consequences of METH or MDMA-induced oxidative stress, excitotoxicity, and mitochondrial dysfunction are described in relation to the classical markers of METH-induced damage to dopamine terminals.

Cognate interaction between endothelial cells and T cells

Endothelial cells lining the blood vessels form a barrier between circulating immune cells and parenchymal tissue. While the molecular mechanisms involved in antigen-independent recruitment of leukocytes into infected tissue have been extensively studied, the mechanisms involving antigen-specific recruitment of T cells into tissue have remained largely elusive. Here I shall review the experimental evidence that endothelial cells function as antigen-presenting cells and in this function contribute first to regulation of immune responses and second, to antigen-specific recruitment of T cells.

Failed central nervous system regeneration: a downside of immune privilege?

Immunity is required to eliminate dangerous or degenerated material and to support regeneration, but also causes significant parenchymal damage. In the eye and the brain, in which cornea and lens poorly regenerate and neurons are hardly replaceable, early transplantation experiments demonstrated remarkable tolerance to various grafts. This "immunologically privileged status" (Billingham and Boswell, 1953) may reflect evolutionary pressure to downmodulate certain actions of immune cells within particularly vulnerable tissues. As an example, tolerating certain "neurotrophic" viruses may often be a more successful strategy for survival than the elimination of all infected neurons. While several constitutive and inducible signals maintaining or re-establishing immune tolerance within the brain have been identified, it has also become evident that the resulting anti-inflammatory environment limits certain beneficial effects of neuroinflammation such as neurotrophin secretion or glutamate buffering by T-cells and the clearance of growth-inhibiting myelin or amyloid. Following spinal cord injury, the costs and benefits of neuroinflammation seem to come close because enhancing as well as suppressing innate or adaptive immunity caused amelioration and aggravation of functional regeneration in similar experiments. Evaluating such balances has also begun in (animal models of) Alzheimer's disease, central nervous system trauma, and stroke, and the appreciation of the beneficial side of neuroinflammation has caused a rethinking of the ill-defined use of immune suppressants. As dual roles for individual molecules have been recognized (Merrill and Benveniste, 1996), we are uncovering an already fine-tuned system, but the challenge remains to further support beneficial immune cascades without causing additional damage, and vice versa.

HIV tat and neurotoxicity

HIV tat is the transactivator of HIV-1, supporting efficient viral replication by stabilizing the transcription of viral genes. Tat can be released from HIV-infected cells and alter several functions in uninfected cells. In the brain, tat induces neuronal dysfunction/toxicity, even though neurons cannot be directly infected with HIV, resulting in CNS pathology, such as the dementia and encephalitis associated with NeuroAIDS. This review discusses the most recent data addressing tat-induced neurotoxicity and integrates these new findings in the context of NeuroAIDS.

Astrocyte-endothelial interactions at the blood-brain barrier

The blood-brain barrier, which is formed by the endothelial cells that line cerebral microvessels, has an important role in maintaining a precisely regulated microenvironment for reliable neuronal signalling. At present, there is great interest in the association of brain microvessels, astrocytes and neurons to form functional 'neurovascular units', and recent studies have highlighted the importance of brain endothelial cells in this modular organization. Here, we explore specific interactions between the brain endothelium, astrocytes and neurons that may regulate blood-brain barrier function. An understanding of how these interactions are disturbed in pathological conditions could lead to the development of new protective and restorative therapies.

Human cerebral malaria and the blood-brain barrier

Malaria represents a continuing and major global health challenge and our understanding of how the Plasmodium parasite causes severe disease and death remains poor. One serious complication of the infection is cerebral malaria, a clinically complex syndrome of coma and potentially reversible encephalopathy, associated with a high mortality rate and increasingly recognised long-term sequelae in survivors. Research into the pathophysiology of cerebral malaria, using a combination of clinical and pathological studies, animal models and in vitro cell culture work, has focussed attention on the blood-brain barrier (BBB). This represents the key interface between the brain parenchyma and the parasite, which develops within an infected red cell but remains inside the vascular space. Studies of BBB function in cerebral malaria have provided some evidence for parasite-induced changes secondary to sequestration of parasitised red blood cells and host leukocytes within the cerebral microvasculature, such as redistribution of endothelial cell intercellular junction proteins and intracellular signaling. However, the evidence for a generalised increase in BBB permeability, leading to cerebral oedema, is conflicting. As well as direct cell adhesion-dependent effects, local adhesion-independent effects may activate and damage cerebral endothelial cells and perivascular cells, such as decreased blood flow, hypoxia or the effects of parasite toxins such as pigment. Finally, a number of systemic mechanisms could influence the BBB during malaria, such as the metabolic and inflammatory complications of severe disease acting 'at a distance'. This review will summarise evidence for these mechanisms from human studies of cerebral malaria and discuss the possible role for BBB dysfunction in this complex and challenging disease.

Minocycline or iNOS inhibition block 3-nitrotyrosine increases and blood-brain barrier leakiness in amyloid beta-peptide-injected rat hippocampus

This work has examined levels of 3-nitrotyrosine (3-NT, a marker for peroxynitrite formation) and intactness of blood-brain barrier (BBB) in amyloid beta-peptide (Abeta(1-42))-injected rat hippocampus. Immunohistochemical analysis demonstrated 3-NT immunoreactivity in microglia/macrophages and astrocytes were significantly increased at 7 days post-Abeta(1-42) injection. Administration of the broad spectrum anti-inflammatory agent minocycline or the selective iNOS inhibitor 1400W markedly reduced 3-NT levels. Double immunofluorescence staining showed that 3-NT was prominently expressed in microglia/macrophages and astrocytes located in proximity to blood vessels. Additionally, Abeta(1-42) injection caused a marked increase in permeability of the BBB to immunoglobulin G (IgG); both minocycline and 1400W were highly effective in decreasing the leakiness of the BBB. Our results suggest the involvement of glial-derived reactive nitrogen species in mediating increased BBB permeability in Abeta(1-42) injected rat hippocampus.

Role of Site-Specific Binding to Plasma Albumin in Drug Availability to Brain

Many studies have reported greater drug uptake into brain than that predicted based upon existing models using the free fraction (f(u)) of drug in arterial serum. To explain this difference, circulating plasma proteins have been suggested to interact with capillary membrane in vivo to produce a conformational change that favors net drug dissociation and elevation of f(u). Albumin, the principal binding protein in plasma, has two main drug binding sites, Sudlow I and II. We tested this hypothesis using drugs that bind selectively to either site I (warfarin) or site II (ibuprofen), as well as mixed ligands that have affinity for both sites (tolbutamide and valproate). Brain uptake was determined in the presence and absence of albumin using the in situ rat brain perfusion technique. Unidirectional brain uptake transfer constants (K(in)) were measured and compared with those predicted using the modified Kety-Crone-Renkin model: K(in) = F(1-e(-f(u) x PS(u)/F)), where F is perfusion flow and PS(u) is the permeability-surface area product to free drug of brain capillaries. The results demonstrated good agreement between measured and predicted K(in) over a 100-fold range in perfusion fluid albumin concentration using albumin from three different species (i.e., human, bovine, and rat), as well as whole-rat serum. K(in) decreased in the presence of albumin in direct proportion to perfusion fluid f(u) with constant PS(u). The results show that brain uptake of selected Sudlow site I and II ligands matches that predicted by the modified Kety-Crone-Renkin model with no evidence for enhanced dissociation.

Assembly of a perivascular astrocyte protein scaffold at the mammalian blood–brain barrier is dependent on α-syntrophin

alpha-Syntrophin, a member of the dystrophin-associated protein complex, is required for proper localization of the water channel aquaporin-4 at the blood-brain barrier. Mice lacking alpha-syntrophin have reduced levels of aquaporin-4 in perivascular astroglial endfeet. Consequently, they exhibit reduced edema and infarct volume in brain trauma models and reduced K+ clearance from the neuropil, leading to increased seizure susceptibility. We have used the alpha-syntrophin null mice to investigate whether alpha-syntrophin is required for proper localization of other components of the dystrophin complex at the blood-brain barrier. We find that alpha-syntrophin is required for the full recruitment of gamma2-syntrophin and alpha-dystrobrevin-2 to glial endfeet in adult cerebellum. In contrast, the localization of beta1- and beta2-syntrophin and alpha-dystrobrevin-1 at the blood-brain barrier is not dependent on the presence of alpha-syntrophin. The localization patterns of alpha-dystrobrevin-1 and -2 in wild type cerebellum are strikingly different; while alpha-dystrobrevin-1 is present in glial endfeet throughout the cerebellum, alpha-dystrobrevin-2 is restricted to glial endfeet in the granular layer alone. Finally, we show that the enrichment of dystrophin in glial endfeet depends on the presence of alpha-syntrophin. This finding is the first demonstration that dystrophin localization is dependent on syntrophin. Since the localization of gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin is contingent on alpha-syntrophin, we conclude that alpha-syntrophin is a central organizer of the astrocyte dystrophin complex, an important molecular scaffold for localization of aquaporin-4 at the blood-brain barrier.

Inhibition of vascular endothelial growth factor receptor 2 activity in experimental brain contusions aggravates injury outcome and leads to early increased neuronal and glial degeneration

Angiogenesis following traumatic brain injuries (TBIs) may be of importance for post-traumatic reparative processes and the development of secondary injuries. We have previously shown expression of vascular endothelial growth factor (VEGF), a major regulator of endothelial cell proliferation, angiogenesis and vascular permeability, and VEGF receptors (VEGFR1 and 2) after TBI in rat. In the present work we tried to further elucidate the role of VEGF after TBI by performing specific VEGFR2 activity inhibition. In rats subjected to VEGFR2 blockage we report an increased haemorrhagic area (P < 0.05), early increase in serum levels of neural injury marker neuron-specific enolase (P < 0.05) and glial injury marker S100beta (P < 0.05), and increased numbers of terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labelling- (TUNEL-) and FluoroJade B- (P < 0.05) positive cells, all increases preceding the known VEGF/VEGFR vascular response in brain trauma. An increase in lesion area, as measured by decreased microtubuli-associated protein 2 expression (P < 0.05) and increased glial fibrillary acidic protein reactivity (P < 0.05), could also be demonstrated. In addition, vascular density, as measured by von Willebrandt factor-positive cells, was decreased (P < 0.05). No differences in post-traumatic inflammatory response, as measured by stainings for macrophages, granulocytes and intracellular adhesion molecules, were shown between the groups. Taken together, our findings point towards VEGF/VEGFR2 up-regulation after TBI as being an important endogenous cytoprotective mechanism in TBI. The possible importance of VEGF on the vascular, neuronal and glial compartments of the neurovascular unit after TBI is discussed.

Fluorescence imaging of blood-brain barrier disruption

Pathological alterations of the blood-brain barrier (BBB) can be topographically heterogeneous. The goal of this study was to develop a method to assess rapidly the magnitude and spatial distribution of permeability changes. Rats were perfused via the common carotid arteries with Ringer's solution containing sodium fluorescein (NF) and Evans Blue albumin (EB). Global NF uptake was determined by fluorimetry and EB uptake was determined by absorbance spectroscopy. NF uptake was linear in control animals and at a rate comparable to sucrose, whereas uptake of EB was negligible. Infusion of 1.6 M mannitol immediately prior to perfusion significantly increased uptake of NF while EB uptake was unchanged. BBB disruption was confirmed by confocal microscopy of fresh-frozen sections. In control animals, NF and EB staining were limited to the edges of slices and to the circumventricular organs. In mannitol-treated animals, heavy NF staining was observed throughout the brain, and EB staining was localized around some microvessels. In animals given a approximately 500 microl air embolus prior to perfusion, a discrete area of NF and EB staining could be observed near the ventral midline, while the rest of the brain remained unaltered. We find that brain perfusion with NF/EB enables a rapid, reliable, and highly sensitive assessment of global BBB permeability and microscopic visualization of discrete BBB disruptions.