A new model of the blood--brain barrier: co-culture of neuronal, endothelial and glial cells under dynamic conditions

On the quest of reliable 3D dynamic in vitro blood-brain barrier models using polymer hollow fiber membranes: Pitfalls, progress, and future perspectives

With the increasing concern of neurodegenerative diseases, the development of new therapies and effective pharmaceuticals targeted to central nervous system (CNS) illnesses is crucial for ensuring social and economic sustainability in an ageing world. Unfortunately, many promising treatments at the initial stages of the pharmaceutical development process, that is at the in vitro screening stages, do not finally show the expected results at the clinical level due to their inability to cross the human blood-brain barrier (BBB), highlighting the inefficiency of in vitro BBB models to recapitulate the real functionality of the human BBB. In the last decades research has focused on the development of in vitro BBB models from basic 2D monolayer cultures to 3D cell co-cultures employing different system configurations. Particularly, the use of polymeric hollow fiber membranes (HFs) as scaffolds plays a key role in perfusing 3D dynamic in vitro BBB (DIV-BBB) models. Their incorporation into a perfusion bioreactor system may potentially enhance the vascularization and oxygenation of 3D cell cultures improving cell communication and the exchange of nutrients and metabolites through the microporous membranes. The quest for developing a benchmark 3D dynamic in vitro blood brain barrier model requires the critical assessment of the different aspects that limits the technology. This article will focus on identifying the advantages and main limitations of the HFs in terms of polymer materials, microscopic porous morphology, and other practical issues that play an important role to adequately mimic the physiological environment and recapitulate BBB architecture. Based on this study, we consider that future strategic advances of this technology to become fully implemented as a gold standard DIV-BBB model will require the exploration of novel polymers and/or composite materials, and the optimization of the morphology of the membranes towards thinner HFs (<50 μm) with higher porosities and surface pore sizes of 1–2 µm to facilitate the intercommunication via regulatory factors between the cell co-culture models of the BBB.

Targetability of the neurovascular unit in inflammatory diseases of the central nervous system

The blood-brain barrier (BBB) is a selectively permeable barrier separating the periphery from the central nervous system (CNS). The BBB restricts the flow of most material into and out of the CNS, including many drugs that could be used as potent therapies. BBB permeability is modulated by several cells that are collectively called the neurovascular unit (NVU). The NVU consists of specialized CNS endothelial cells (ECs), pericytes, astrocytes, microglia, and neurons. CNS ECs maintain a complex "seal" via tight junctions, forming the BBB; breakdown of these tight junctions leads to BBB disruption. Pericytes control the vascular flow within capillaries and help maintain the basal lamina. Astrocytes control much of the flow of material that has moved beyond the CNS EC layer and can form a secondary barrier under inflammatory conditions. Microglia survey the border of the NVU for noxious material. Neuronal activity also plays a role in the maintenance of the BBB. Since astrocytes, pericytes, microglia, and neurons are all able to modulate the permeability of the BBB, understating the complex contributions of each member of the NVU will potentially uncover novel and effective methods for delivery of neurotherapies to the CNS.

A novel tri-culture model for neuroinflammation

Neuroinflammation is believed to play a primary role in the pathogenesis of most neurodegenerative diseases including Alzheimer's disease, Parkinson's disease and schizophrenia. Currently, suitable in vitro neuroinflammation models for studying cellular interactions and inflammatory mechanisms at the neurovascular unit are still scarce. In this study, we established an experimentally flexible tri-culture neuroinflammation model combining murine microglial cells (N11), mouse neuroblastoma Nuro2A cell lines and brain microvascular endothelial MVEC(B3) cells in a transwell co-culture system stimulated with lipopolysaccharides. Neuroinflammation was induced in this tri-culture model as manifested by activated N11 cells via toll-like receptor 4, resulting in increased release of proinflammatory mediators (nitric oxide, interleukin-6 and tumour necrosis factor-α) through the activation of nuclear factor-κB signalling pathway. The released inflammatory cytokines from N11 in turn, damaged the tight junction in microvascular endothelial MVEC(B3) cells, increased permeability of endothelial barrier, and induced tau phosphorylation and up-regulated caspase-3 expression in mouse neuroblastoma Nuro2A cell lines, leading to neuroinflammation injury. In summary, this tri-culture inflammation model mimics the microenvironment, the cellular crosstalk and the molecular events that take place during neuroinflammation. It provides a robust in vitro model for studying neuroinflammation mechanisms and screening for potential therapeutics to treat various neurodegenerative diseases.

A cross-platform approach to characterize and screen potential neurovascular unit toxicants

Development of the neurovascular unit (NVU) is a complex, multistage process that requires orchestrated cell signaling mechanisms across several cell types and ultimately results in formation of the blood-brain barrier. Typical high-throughput screening (HTS) assays investigate single biochemical or single cell responses following chemical insult. As the NVU comprises multiple cell types interacting at various stages of development, a methodology combining high-throughput results across pertinent cell-based assays is needed to investigate potential chemical-induced disruption to the development of this complex cell system. To this end, we implemented a novel method for screening putative NVU disruptors across diverse assay platforms to predict chemical perturbation of the developing NVU. HTS assay results measuring chemical-induced perturbations to cellular key events across angiogenic and neurogenic outcomes in vitro were combined to create a cell-based prioritization of NVU hazard. Chemicals were grouped according to similar modes of action to train a logistic regression literature model on a training set of 38 chemicals. This model utilizes the chemical-specific pairwise mutual information score for PubMed MeSH annotations to represent a quantitative measure of previously published results. Taken together, this study presents a methodology to investigate NVU developmental hazard using cell-based HTS assays and literature evidence to prioritize screening of putative NVU disruptors towards a knowledge-driven characterization of neurovascular developmental toxicity. The results from these screening efforts demonstrate that chemicals representing a range of putative vascular disrupting compound (pVDC) scores can also produce effects on neurogenic outcomes and characterizes possible modes of action for disrupting the developing NVU.

In Vitro Models of the Blood-Brain Barrier

Knowledge about the transport of active compounds across the blood-brain barrier is of essential importance for drug development. Systemically applied drugs for the central nervous system (CNS) must be able to cross the blood-brain barrier in order to reach their target sites, whereas drugs that are supposed to act in the periphery should not permeate the blood-brain barrier so that they do not trigger any adverse central adverse effects. A number of approaches have been pursued, and manifold in silico, in vitro, and in vivo animal models were developed in order to be able to make a better prediction for humans about the possible penetration of active substances into the CNS. In this particular case, however, in vitro models play a special role, since the data basis for in silico models is usually in need of improvement, and the predictive power of in vivo animal models has to be checked for possible species differences. The blood-brain barrier is a dynamic, highly selective barrier formed by brain capillary endothelial cells. One of its main tasks is the maintenance of homeostasis in the CNS. The function of the barrier is regulated by cells of the microenvironment and the shear stress mediated by the blood flow, which makes the model development most complex. In general, one could follow the credo "as easy as possible, as complex as necessary" for the usage of in vitro BBB models for drug development. In addition to the description of the classical cell culture models (transwell, hollow fiber) and guidance how to apply them, the latest developments (spheroids, microfluidic models) will be introduced in this chapter, as it is attempted to get more in vivo-like and to be applicable for high-throughput usage with these models. Moreover, details about the development of models based on stem cells derived from different sources with a special focus on human induced pluripotent stem cells are presented.

Comparison of Blood-Brain Barrier Models for in vitro Biological Analysis: One Cell Type vs Three Cell Types

Different types of in vitro blood-brain barrier (BBB) models have been constructed and applied for drug transport to evaluate the efficacy of nano-carrier based drug delivery. However, the effectiveness of different types of BBB models has not been reported. In this paper, we developed two types of in vitro models: one-cell type BBB model developed using only endothelial cells and three-cell type BBB model obtained by co-culturing endothelial, pericyte and astrocyte cells. The nanoparticle transport mechanisms through BBB and transport efficiencies of the Lactoferrin attached silica nanoparticles were studied using both types of the in vitro BBB models. Compared with one-cell type model, the PSi-Lf NPs exhibit relatively lower transport efficiency across three-cell type BBB system. Moreover, the effects of the nanoparticle size on the transport efficacies are consistent for both models. For both types of BBB models, the transport efficacies of the NPs are size dependent, and the highest efficacies are achieved for NPs with 25 nm in diameter. Our experimental results indicate that the one-cell type and three-cell type BBB models are equivalent for evaluating and optimizing nanoparticle transport across BBB.

Isolation of Cerebral Capillaries from Fresh Human Brain Tissue

Understanding blood-brain barrier function under physiological and pathophysiological conditions is critical for the development of new therapeutic strategies that hold the promise to enhance brain drug delivery, improve brain protection, and treat brain disorders. However, studying the human blood-brain barrier function is challenging. Thus, there is a critical need for appropriate models. In this regard, brain capillaries isolated from human brain tissue represent a unique tool to study barrier function as close to the human in vivo situation as possible. Here, we describe an optimized protocol to isolate capillaries from human brain tissue at a high yield and with consistent quality and purity. Capillaries are isolated from fresh human brain tissue using mechanical homogenization, density-gradient centrifugation, and filtration. After the isolation, the human brain capillaries can be used for various applications including leakage assays, live cell imaging, and immune-based assays to study protein expression and function, enzyme activity, or intracellular signaling. Isolated human brain capillaries are a unique model to elucidate the regulation of the human blood-brain barrier function. This model can provide insights into central nervous system (CNS) pathogenesis, which will help the development of therapeutic strategies for treating CNS disorders.

Neurovascular dysfunction in dementia - human cellular models and molecular mechanisms

From the earliest stages of development, when cerebral angiogenesis and neurogenesis are entwined, to the end of life, the interplay between vascular and neural systems of the brain is critical in health and disease. Cerebral microvascular endothelial cells constitute the blood-brain barrier and in concert with pericytes or smooth muscle cells, glia and neurons, integrate into a functional neurovascular unit (NVU). This multicellular NVU maintains homoeostasis of the brain's microenvironment by restricting the entry of systemic pathogens and neurotoxins as well as meeting the metabolic demands of neural activity. Recent evidence of cerebral microvascular pathologies in vascular diseases and dementia, including Alzheimer's disease, has challenged the notion that vascular events are merely the consequence of neuronal pathology. This review focuses on molecular mechanisms of neurovascular dysfunction in dementia and outlines currently employed in vitro models to decode such mechanisms. Deciphering neurovascular crosstalk is likely to be more important in understanding the molecular mechanisms of disease than previously anticipated and may offer novel therapeutic opportunities for dementia and related conditions.

In vitro models and systems for evaluating the dynamics of drug delivery to the healthy and diseased brain

The blood-brain barrier (BBB) plays a crucial role in maintaining brain homeostasis and transport of drugs to the brain. The conventional animal and Transwell BBB models along with emerging microfluidic-based BBB-on-chip systems have provided fundamental functionalities of the BBB and facilitated the testing of drug delivery to the brain tissue. However, developing biomimetic and predictive BBB models capable of reasonably mimicking essential characteristics of the BBB functions is still a challenge. In addition, detailed analysis of the dynamics of drug delivery to the healthy or diseased brain requires not only biomimetic BBB tissue models but also new systems capable of monitoring the BBB microenvironment and dynamics of barrier function and delivery mechanisms. This review provides a comprehensive overview of recent advances in microengineering of BBB models with different functional complexity and mimicking capability of healthy and diseased states. It also discusses new technologies that can make the next generation of biomimetic human BBBs containing integrated biosensors for real-time monitoring the tissue microenvironment and barrier function and correlating it with the dynamics of drug delivery. Such integrated system addresses important brain drug delivery questions related to the treatment of brain diseases. We further discuss how the combination of in vitro BBB systems, computational models and nanotechnology supports for characterization of the dynamics of drug delivery to the brain.

Hollow fiber bioreactor technology for tissue engineering applications

Hollow fiber bioreactors are the focus of scientific research aiming to mimic physiological vascular networks and engineer organs and tissues in vitro. The reason for this lies in the interesting features of this bioreactor type, including excellent mass transport properties. Indeed, hollow fiber bioreactors allow limitations to be overcome in nutrient transport by diffusion, which is often an obstacle to engineer sizable constructs in vitro. This work reviews the existing literature relevant to hollow fiber bioreactors in organ and tissue engineering applications. To this purpose, we first classify the hollow fiber bioreactors into 2 categories: cylindrical and rectangular. For each category, we summarize their main applications both at the tissue and at the organ level, focusing on experimental models and computational studies as predictive tools for designing innovative, dynamic culture systems. Finally, we discuss future perspectives on hollow fiber bioreactors as in vitro models for tissue and organ engineering applications.

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

In Vitro Blood-Brain Barrier Models-An Overview of Established Models and New Microfluidic Approaches

The societal need for new central nervous system (CNS) medicines is substantial, because of the global increase in life expectancy and the accompanying increase in age-related CNS diseases. Low blood-brain barrier (BBB) permeability has been one of the major causes of failure for new CNS drug candidates. There has therefore been a great interest in cell models, which mimic BBB permeation properties. In this review, we present an overview of the performance of monocultured, cocultured, and triple-cultured primary cells and immortalized cell lines, including key parameters such as transendothelial electrical resistance values, permeabilities of paracellular flux markers, and expression of BBB-specific marker proteins. Microfluidic systems are gaining ground as a new automated technical platform for cell culture and systematic analysis. The performance of these systems was compared with current state-of-the-art models and it was noted that, although they show great promise, these systems have not yet reached beyond the proof-of-concept stage. In general, it was found that there were large variations in experimental protocols, BBB phenotype markers, and paracellular flux markers used. It is the author's opinion that the field may benefit greatly from developing standardized methodologies and initiating collaborative efforts on optimizing culture protocols.

Setting-up an in vitro model of rat blood-brain barrier (BBB): a focus on BBB impermeability and receptor-mediated transport

The blood brain barrier (BBB) specifically regulates molecular and cellular flux between the blood and the nervous tissue. Our aim was to develop and characterize a highly reproducible rat syngeneic in vitro model of the BBB using co-cultures of primary rat brain endothelial cells (RBEC) and astrocytes to study receptors involved in transcytosis across the endothelial cell monolayer. Astrocytes were isolated by mechanical dissection following trypsin digestion and were frozen for later co-culture. RBEC were isolated from 5-week-old rat cortices. The brains were cleaned of meninges and white matter, and mechanically dissociated following enzymatic digestion. Thereafter, the tissue homogenate was centrifuged in bovine serum albumin to separate vessel fragments from nervous tissue. The vessel fragments underwent a second enzymatic digestion to free endothelial cells from their extracellular matrix. The remaining contaminating cells such as pericytes were further eliminated by plating the microvessel fragments in puromycin-containing medium. They were then passaged onto filters for co-culture with astrocytes grown on the bottom of the wells. RBEC expressed high levels of tight junction (TJ) proteins such as occludin, claudin-5 and ZO-1 with a typical localization at the cell borders. The transendothelial electrical resistance (TEER) of brain endothelial monolayers, indicating the tightness of TJs reached 300 ohm·cm² on average. The endothelial permeability coefficients (Pe) for lucifer yellow (LY) was highly reproducible with an average of 0.26 ± 0.11 x 10^-3 cm/min. Brain endothelial cells organized in monolayers expressed the efflux transporter P-glycoprotein (P-gp), showed a polarized transport of rhodamine 123, a ligand for P-gp, and showed specific transport of transferrin-Cy3 and DiILDL across the endothelial cell monolayer. In conclusion, we provide a protocol for setting up an in vitro BBB model that is highly reproducible due to the quality assurance methods, and that is suitable for research on BBB transporters and receptors.

Development of a physiologically-based pharmacokinetic model of the rat central nervous system

Central nervous system (CNS) drug disposition is dictated by a drug's physicochemical properties and its ability to permeate physiological barriers. The blood-brain barrier (BBB), blood-cerebrospinal fluid barrier and centrally located drug transporter proteins influence drug disposition within the central nervous system. Attainment of adequate brain-to-plasma and cerebrospinal fluid-to-plasma partitioning is important in determining the efficacy of centrally acting therapeutics. We have developed a physiologically-based pharmacokinetic model of the rat CNS which incorporates brain interstitial fluid (ISF), choroidal epithelial and total cerebrospinal fluid (CSF) compartments and accurately predicts CNS pharmacokinetics. The model yielded reasonable predictions of unbound brain-to-plasma partition ratio (Kpuu,brain) and CSF:plasma ratio (CSF:Plasmau) using a series of in vitro permeability and unbound fraction parameters. When using in vitro permeability data obtained from L-mdr1a cells to estimate rat in vivo permeability, the model successfully predicted, to within 4-fold, Kpuu,brain and CSF:Plasmau for 81.5% of compounds simulated. The model presented allows for simultaneous simulation and analysis of both brain biophase and CSF to accurately predict CNS pharmacokinetics from preclinical drug parameters routinely available during discovery and development pathways.

Theranostic quantum dots for crossing blood-brain barrier in vitro and providing therapy of HIV-associated encephalopathy

The blood–brain barrier (BBB) is a complex physiological checkpoint that restricts the free diffusion of circulating molecules from the blood into the central nervous system. Delivering of drugs and other active agents across the BBB is one of the major technical challenges faced by scientists and medical practitioners. Therefore, development of novel methodologies to address this challenge holds the key for both the diagnosis and treatment of brain diseases, such as HIV-associated encephalopathy. Bioconjugated quantum dots (QDs) are excellent fluorescent probes and nano-vectors, being designed to transverse across the BBB and visualize drug delivery inside the brain. This paper discusses the use of functionalized QDs for crossing the blood–brain barrier and treating brain disease. We highlight the guidelines for using in vitro BBB models for brain disease studies. The theranostic QDs offers a strategy to significantly improve the effective dosages of drugs to transverse across the BBB and orientate to the targets inside the brain.

A novel brain neurovascular unit model with neurons, astrocytes and microvascular endothelial cells of rat

A novel triple cell neurovascular unit (NVU) model co-culturing with neurons, brain microvascular endothelial cells (BMECs) and astrocytes was established in this study for investigating the cerebral diseases and screening the candidates of therapeutic drug. We have first performed the cell identification and morphological characterization, analyzed the specific protein expression and determined the blood-brain barrier (BBB) function of the co-culture model under normal condition. Then, we further determined the BBB function, inflammation, cell injury and the variation of neuroprotective factor in this model after anoxia-reoxygenation. The results suggest that this model exhibited a better BBB function and significantly increased expression of P-glycoprotein (Pg-P) and ZO-1 compared with BMECs only or co-culture with astrocytes or neurons. After anoxia-reoxygenation, the pathological changes of this model were basically resemblance to the pathological changes of brain cells and BBB in vivo. And nimodipine, an antagonist of calcium, could reverse those changes as well. According to our observations, we deduce that this triple cell co-culture model exhibits the basic structure, function and cell-cell interaction of NVU, which may offer a more proper in vitro system of NVU for the further investigation of cerebral diseases and drug screening.

A new dynamic in vitro modular capillaries-venules modular system: cerebrovascular physiology in a box

Background

The study of the cerebrovascular physiology is crucial to understand the pathogenesis of neurological disease and the pharmacokinetic of drugs. Appropriate models in vitro often fail to represent in vivo physiology. To address these issues we propose the use of a novel artificial vascular system that closely mimics capillary and venous segments of human cerebrovasculature while also allowing for an extensive control of the experimental variables and their manipulation.

Results

Using hollow fiber technology, we modified an existing dynamic artificial model of the blood–brain barrier (BBB) (DIV-capillary) to encompass the distal post-capillary (DIV-venules) segments of the brain circulatory system. This artificial brain vascular system is comprised of a BBB module serially connected to a venule segment. A pump generates a pulsatile flow with arterial pressure feeding the system. The perfusate of the capillary module achieves levels of shear stress, pressure, and flow rate comparable to what observed in situ. Endothelial cell exposure to flow and abluminal astrocytic stimuli allowed for the formation of a highly selective capillary BBB with a trans-endothelial electrical resistance (TEER; >700 ohm cm²) and sucrose permeability (< 1X10^-u cm/sec) comparable to in vivo. The venule module, which attempted to reproduce features of the hemodynamic microenvironment of venules, was perfused by media resulting in shear stress and intraluminal pressure levels lower than those found in capillaries. Because of altered cellular and hemodynamic factors, venule segments present a less stringent vascular bed (TEER <250 Ohm cm²; P_sucrose > 1X10^-4 cm/sec) than that of the BBB. Abluminal human brain vascular smooth muscle cells were used to reproduce the venular abluminal cell composition.

Conclusion

The unique characteristics afforded by the DIV-BBB in combination with a venule segment will realistically expand our ability to dissect and study the physiological and functional behavior of distinct segments of the human cerebrovascular network.

Effects of bacterial toxins on endothelial tight junction in vitro: a mechanism-based investigation

ABSTRACT Lipopolysaccharide (LPS) and lipoteichoic acid (LTA), principal cell wall components of Gram-negative and Gram-positive bacteria, respectively, play a central role in altering the blood-brain barrier and facilitate bacterial infection of the host brain. Despite the significance of bacterial toxins in disease pathogenesis, mechanisms by which toxins impair the barrier are not yet known. This study, using an in vitro cell culture model, showed that LPS and LTA interacted with the endothelial cells and disrupted the tight junction between the cells that increased the barrier's permeability. Both toxins increased inducible nitric oxide synthase (iNOS) mRNA that is indicative of an increase in intracellular NO release, disrupted architecture of the tight junction proteins, suppressed zonula occludens-1 (ZO-1) and occludin (OCL) and junctional adhesive molecules (JAM) mRNA levels, and increased tumor necrosis factor alpha (TNFalpha) and interleukin-1 beta (IL-1beta) mRNA levels. Anti-CD14 antibodies blocked the increase in TNFalpha and IL-1beta mRNA levels but did not affect either changes in the tight junction or iNOS, ZO-1, OCL, and JAM mRNA levels in endothelial cells and astrocytes. Although both toxins did not cross the endothelial barrier, the abluminal neurons exhibited high inflammatory activity characterized by a sequential increase in TNFalpha, IL-1beta, external receptor kinase (ERK), and RelA-p50 that induced inflammation, followed by an increase in anti-inflammatory/apoptotic factors including IL-10 and cysteine-aspartic acid protease-8 (CASPASE-8), which resolve inflammation and induce apoptosis. Anti-CD14 antibodies in luminal buffer blocked the pro- and anti-inflammatory effects of the toxins in neurons. Thus, the CD14-TLR cascade that participates in the inflammatory effects of toxins may not participate in the toxin-induced barrier disruption in vitro. Since the toxins did not cross the endothelial barrier, induction of inflammation in neurons was due to a release of proinflammatory cytokines in the abluminal fluid.

Nanomaterial-mediated CNS delivery of diagnostic and therapeutic agents

Research into the diagnosis and treatment of central nervous system (CNS) diseases has been enhanced by rapid advances in nanotechnology and an expansion in the library of nanostructured carriers. This review discusses the latest applications of nanomaterials in the CNS with an emphasis on brain tumors. Novel administration routes and transport mechanisms for nanomaterial-mediated CNS delivery of diagnostic and therapeutic agents to bypass or cross the blood brain barrier (BBB) are also discussed. These include temporary disruption of the BBB, use of impregnated polymers (polymer wafers), convection-enhanced delivery (CED), and intranasal delivery. Moreover, an in vitro BBB model capable of mimicking geometrical, cellular and rheological features of the human cerebrovasculature has been developed. This is a useful tool that can be used for screening CNS nanoparticles or therapeutics prior to in vivo and clinical investigation. A discussion of this novel model is included.

Experimental models for assaying microvascular endothelial cell pathophysiology in stroke

It is important to understand the molecular mechanisms underlying neuron death following stroke in order to develop effective neuroprotective strategies. Since studies on human stroke are extremely limited due to the difficulty in collecting post-mortem tissue at different time points after the onset of stroke, brain ischaemia research focuses on information derived from in-vitro models of neuronal death through ischaemic injury [1]. This review aims to provide an update on the different in-vitro stroke models with brain microvascular endothelial cells that are currently being used. These models provide a physiologically relevant tool to screen potential neuroprotective drugs in stroke and to study the molecular mechanisms involved in brain ischaemia.

Understanding the blood-brain barrier using gene and protein expression profiling technologies

The blood-brain barrier (BBB) contributes to the brain homeostasis by regulating the passage of endogenous and exogenous compounds. This function is in part due to well-known proteins such as tight junction proteins, plasma membrane transporters and metabolic barrier proteins. Over the last decade, genomics and proteomics have emerged as supplementary tools for BBB research. The development of genomic and proteomic technologies has provided several means to extend the BBB knowledge and to investigate additional routes for the bypass of this barrier. These profiling technologies have been used on BBB models to decipher the physiological characteristics and, under stress conditions, to understand the molecular mechanisms of brain diseases. In this review, we will report and discuss the genomic and proteomic studies recently carried out to enhance the understanding of BBB features.

The blood-brain barrier in brain homeostasis and neurological diseases

Brain endothelial cells are unique among endothelial cells in that they express apical junctional complexes, including tight junctions, which quite resemble epithelial tight junctions both structurally and functionally. They form the blood-brain-barrier (BBB) which strictly controls the exchanges between the blood and the brain compartments by limiting passive diffusion of blood-borne solutes while actively transporting nutrients to the brain. Accumulating experimental and clinical evidence indicate that BBB dysfunctions are associated with a number of serious CNS diseases with important social impacts, such as multiple sclerosis, stroke, brain tumors, epilepsy or Alzheimer's disease. This review will focus on the implication of brain endothelial tight junctions in BBB architecture and physiology, will discuss the consequences of BBB dysfunction in these CNS diseases and will present some therapeutic strategies for drug delivery to the brain across the BBB.

Pericytes from brain microvessels strengthen the barrier integrity in primary cultures of rat brain endothelial cells

(1) The blood-brain barrier (BBB) characteristics of cerebral endothelial cells are induced by organ-specific local signals. Brain endothelial cells lose their phenotype in cultures without cross-talk with neighboring cells. (2) In contrast to astrocytes, pericytes, another neighboring cell of endothelial cells in brain capillaries, are rarely used in BBB co-culture systems. (3) Seven different types of BBB models, mono-culture, double and triple co-cultures, were constructed from primary rat brain endothelial cells, astrocytes and pericytes on culture inserts. The barrier integrity of the models were compared by measurement of transendothelial electrical resistance and permeability for the small molecular weight marker fluorescein. (4) We could confirm that brain endothelial monolayers in mono-culture do not form tight barrier. Pericytes induced higher electrical resistance and lower permeability for fluorescein than type I astrocytes in co-culture conditions. In triple co-culture models the tightest barrier was observed when endothelial cells and pericytes were positioned on the two sides of the porous filter membrane of the inserts and astrocytes at the bottom of the culture dish. (5) For the first time a rat primary culture based syngeneic triple co-culture BBB model has been constructed using brain pericytes beside brain endothelial cells and astrocytes. This model, mimicking closely the anatomical position of the cells at the BBB in vivo, was superior to the other BBB models tested. (6) The influence of pericytes on the BBB properties of brain endothelial cells may be as important as that of astrocytes and could be exploited in the construction of better BBB models.

Early excitability changes in a novel acute model of transient focal ischemia and reperfusion in the in vitro isolated guinea pig brain

The study of the early events that characterize cerebral ischemia is limited in available experimental models. The study of neurophysiological network changes that occur in brain tissue during the early minutes that follow focal ischemia induction is restricted in the in vivo condition. Very simplified systems, such as in vitro brain slices and in isolated neurons, have been utilized for this type of studies. We describe here a new model of transient focal ischemia and reperfusion developed in the isolated guinea pig brain, maintained in vitro by arterial perfusion with a complex saline solution without blood cells. In this preparation, that combines the advantage of an in vitro preparation with the functional preservation of both vascular and neuronal compartments, the arteries of the Willis circle are directly accessible by visual control. To induce transitory focal ischemia, one medial cerebral artery (MCA) was transiently tied for 30 min, while brain activity was recorded with multiple electrodes positioned in brain areas within and outside MCA territory. Anoxic depression in ischemic areas propagated to the surrounding tissue and was associated with the abolition of evoked responses due to both functional impairment of afferent olfactory input and tissue depression. Recovery of evoked responses was obtained after MCA reperfusion. The spatial distribution of hypoxic depressions was characterized and was correlated with the extension of brain damage, defined by immunohistochemical analysis with antibodies against microtubule-associated protein (MAP-2). We propose that the present model can be utilized to analyze brain activity changes that occur in early stages of focal brain ischemia and reperfusion.

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.

A novel flow based hollow-fiber blood-brain barrier in vitro model with immortalised cell line PBMEC/C1-2

A flow based hollow-fiber in vitro model of the blood-brain barrier (BBB) was established. The immortalised porcine brain microvascular endothelial cell line PBMEC/C1-2 was cultured in a pulsatile hollow-fiber cartridge system (Cellmax Quad). The usability of PBMEC/C1-2 in the flow based hollow-fiber model was increased from three days in the originally used Transwell model up to four months due to the application of shear stress and co-culturing with glioma cell line C6. It was shown that the tightness of PBMEC/C1-2 layers was enhanced significantly in astrocyte conditioned medium (ACM) and in co-culture. The morphology of PBMEC/C1-2 and C6 was visualised by environmental scanning electron microscopy (ESEM). Permeation studies were accomplished with a set of benzodiazepines. The raw data were processed with three different calculation models and the results were compared with permeability coefficients obtained with an established Transwell model. In summary a flow based hollow-fiber BBB in vitro model was developed, which can be used to perform experiments with physiological (e.g., regulation of BBB permeability), pharmacological (e.g., pharmacokinetics and dynamics) and pathophysiological (e.g., effects of diseases on BBB permeability and vice versa) objectives.

Monocytes form a vascular barrier and participate in vessel repair after brain injury

Subpopulations of bone marrow-derived cells can be induced to assume a number of endothelial properties in vitro. However, their ability to form a functional vascular barrier has not been demonstrated. We report that human CD14+ peripheral blood monocytes cultured under angiogenic conditions develop a number of phenotypic and functional properties similar to brain microvascular endothelial cells. These cells express the tight junction proteins zonula occludens 1 (ZO-1) and occludin and form a barrier with a transcellular electrical resistance (TCER) greater than 100 ohm cm2 and low permeability to 4 kDa and 20 kDa dextrans. The TCER of the cellular barrier is decreased by bradykinin and histamine. We also demonstrate that these cells associate with repairing vasculature in areas of brain and skin injury. Our data suggest that CD14+ peripheral blood monocytes participate in the repair of the vascular barrier after brain injury.

Permeability studies on in vitro blood-brain barrier models: physiology, pathology, and pharmacology

(1) The specifically regulated restrictive permeability barrier to cells and molecules is the most important feature of the blood-brain barrier (BBB). The aim of this review was to summarize permeability data obtained on in vitro BBB models by measurement of transendothelial electrical resistance and by calculation of permeability coefficients for paracellular or transendothelial tracers. (2) Results from primary cultures of cerebral microvascular endothelial cells or immortalized cell lines from bovine, human, porcine, and rodent origin are presented. Effects of coculture with astroglia, neurons, mesenchymal cells, blood cells, and conditioned media, as well as physiological influence of serum components, hormones, growth factors, lipids, and lipoproteins on the barrier function are discussed. (3) BBB permeability results gained on in vitro models of pathological conditions including hypoxia and reoxygenation, neurodegenerative diseases, or bacterial and viral infections have been reviewed. Effects of cytokines, vasoactive mediators, and other pathogenic factors on barrier integrity are also detailed. (4) Pharmacological treatments modulating intracellular cyclic nucleotide or calcium levels, and activity of protein kinases, protein tyrosine phosphatases, phospholipases, cyclooxygenases, or lipoxygenases able to change BBB integrity are outlined. Barrier regulation by drugs involved in the metabolism of nitric oxide and reactive oxygen species, as well as influence of miscellaneous treatments are also listed and evaluated. (5) Though recent advances resulted in development of improved in vitro BBB model systems to investigate disease modeling, drug screening, and testing vectors targeting the brain, there is a need for checking validity of permeability models and cautious interpretation of data.

Loss of shear stress induces leukocyte-mediated cytokine release and blood-brain barrier failure in dynamic in vitro blood-brain barrier model

Brain ischemia is associated with an acute release of pro-inflammatory cytokines, notably TNF-alpha and IL-6 and failure of the blood-brain barrier. Shear stress, hypoxia-hypoglycemia, and blood leukocytes play a significant role in blood-brain barrier failure during transient or permanent ischemia. However, these mechanisms have not been studied as independent variables for in vitro ischemia. The present study, using a dynamic in vitro blood-brain barrier model, showed that flow cessation/reperfusion under normoxia-normoglycemia or hypoxia-hypoglycemia without blood leukocytes in the luminal perfusate had a modest, transient effect on cytokine release and blood-brain barrier permeability. By contrast, exposure to normoxic-normoglycemic flow cessation/reperfusion with blood leukocytes in the luminal perfusate led to a significant increase in TNF-alpha and IL-6, accompanied by biphasic blood-brain barrier opening. Enhanced permeability was partially prevented with an anti-TNF-alpha antibody. In leukocyte-free cartridges, the same levels of IL-6 had no effect, while TNF-alpha caused a moderate increase in blood-brain barrier permeability, suggesting that blood leukocytes are the prerequisite for cytokine release and blood-brain barrier failure during reduction or cessation of flow. These cells induce release of TNF-alpha early after ischemia/reperfusion; TNF-alpha triggers release of IL-6, since blockade of TNF-alpha prevents IL-6 release, whereas blockade of IL-6 induces TNF-alpha release. Pre-treatment of blood leukocytes with the cyclooxygenase (COX) inhibitor, ibuprofen, inhibited cytokine release and completely preserved blood-brain barrier permeability during the reperfusion period. In conclusion, loss of flow (flow cessation/reperfusion) independent of hypoxia-hypoglycemia plays a significant role in blood-brain barrier failure by stimulating leukocyte-mediated inflammatory mechanisms.

Use of a three-dimensional in vitro model of the rat blood-brain barrier to assay nucleoside efflux from brain

Extracellular adenosine is produced in brain during physiological and pathophysiological conditions. Once produced, this adenosine can undergo one or more of the following fates: it can interact with its receptors, it can be scavenged by astrocytes and/or neurons for ATP resynthesis, it can be transported across the blood-brain barrier and lost from the central nervous system, or it can be metabolized to inosine and hypoxanthine. The present study used a three-dimensional in vitro cell culture model of the rat blood-brain barrier, in which forebrain astrocytes and microvascular endothelial cells were cultured in cartridges containing multiple parallel polypropylene hollow fibers. Endothelial cells were cultured on the inner surfaces and astrocytes on the outer surfaces of these fibers. Growth medium was constantly perfused through the lumen of the fibers to mimic blood flow across endothelial cells in vivo. This co-culture system was used to examine the permeation of adenosine, and its metabolite inosine, from the astrocyte compartment to the endothelial cell compartment. Dipyridamole was added to the media perfusing the endothelial cell compartment to test whether it could decrease permeation of adenosine and inosine across the in vitro blood-brain barrier. Our results indicate that dipyridamole decreased permeation of total purines, especially inosine, across the barrier. Furthermore, permeation of fluorescein isothiocyanate-labeled albumin and radiolabeled sucrose, markers of the paracellular permeation pathway, were also decreased by dipyridamole. In conclusion, these data indicate that in addition to inhibiting nucleoside efflux across the barrier, dipyridamole can also improve blood-brain barrier function in this model.

Transendothelial electrical resistance of bovine retinal capillary endothelial cells is influenced by cell growth patterns: an ultrastructural study

The purpose of this study was to describe the ultrastructural features of an in vitro capillary endothelial cell model of blood-retinal barrier permeability and to relate morphological features with transendothelial electrical resistance. The electrical resistance of endothelial cell monocultures on small and large pore size polycarbonate Transwell filters was measured and compared with cocultures of endothelial cells and Müller cells. There was a wide variation in electrical resistance measurements with many preparations not achieving a functional barrier. The ultrastructural features associated with barrier function in vitro were studied by comparing cultures that exhibited a 'tight' or 'leaky' barrier when measured immediately prior to processing for electron microscopy. Preparations with low transendothelial electrical resistance were associated with irregular cell growth when studied morphologically. It was concluded that parallel light and electron microscopic studies are important for validation of in vitro models of vascular endothelial permeability.

Effects of transient loss of shear stress on blood-brain barrier endothelium: role of nitric oxide and IL-6

Loss of blood-brain barrier (BBB) function may contribute to post-ischemic cerebral injury by yet unknown mechanisms. Ischemia is associated with anoxia, aglycemia and loss of flow (i.e. shearing forces). We tested the hypothesis that loss of shear stress alone does not acutely affect BBB function due to a protective cascade of mechanisms involving cytokines and nitric oxide (NO). To determine the relative contribution of shear stress on BBB integrity we used a dynamic in vitro BBB model based on co-culture of rat brain microvascular endothelial cells (RBMEC) and astrocytes. Trans-endothelial electrical resistance (TEER), IL-6 release and NO levels were measured from the lumenal and ablumenal compartments throughout the experiment. Flow-exposed RBMEC were challenged with 1 h of normoxic-normoglycemic flow cessation (NNFC) followed by reperfusion for 2 to 24 h. NNFC caused a progressive drop in nitric oxide production during flow cessation followed by a time-dependent increase in ablumenal IL-6 associated with a prolonged NO increase during reperfusion. The nitric oxide synthetase (NOS) inhibitor L-NAME (10 microM) abrogated all effects of NNFC, including changes in NO and cytokine production. BBB permeability did not increase during or after NNFC/reperfusion, but was increased by treatment with L-NAME or when the effects of IL-6 were blocked. Flow adapted RBMEC and astrocytes respond to NNFC/reperfusion by overproduction of IL-6, possibly secondary to increased production of NO during the reperfusion. Maintenance of BBB function during and following NNFC appears to depend on intact NO signaling and IL-6 release.

The blood-brain barrier and its role in immune privilege in the central nervous system

The blood-brain barrier (BBB) provides both anatomical and physiological protection for the central nervous system (CNS), strictly regulating the entry of many substances and blood borne cells into the nervous tissue. Increased understanding of how the unique microenvironment in the CNS influences the BBB is crucial for developing novel therapeutic approaches to CNS diseases. In this review, we discuss those characteristics of the BBB that play an important role in maintaining immune privilege in the CNS, as well as factors that regulate immune cell invasion through the BBB and thereby modulate immune responses in the nervous tissue. In general, immune cell invasion across the BBB is highly restricted and carefully regulated. A florid invasion of activated white blood cells can create a predominantly proinflammatory local environment in the CNS, leading to immune-mediated diseases of the nervous tissue. Recent developments in cellular and molecular biological methods have allowed closer analysis of BBB function, and led to an improved understanding of the active role of the BBB in immune-mediated diseases of the CNS.

Chemokine-dependent mechanisms of leukocyte trafficking across a model of the blood-brain barrier

Leukocyte transmigration across the blood-brain barrier (BBB) is a multistep process that can be mediated by chemokines. These low-molecular-weight chemoattractant proteins are secreted by cells within the central nervous system (CNS) in response to injury or on activation. Leukocytes transmigrate toward this chemokine gradient, crossing the BBB and gaining access to the CNS parenchyma. Depending on the chemokine, the nature of the insult, and the type of cell that transmigrates, the BBB integrity may be disrupted, leading to its increased permeability. Both the inflammation resulting from leukocyte transmigration and BBB perturbations contribute to CNS pathology. The mechanisms that mediate leukocyte transmigration and BBB disruption, as well as tissue culture models that are used to study leukocyte trafficking, are the focus of this review.

Mechanisms of endothelial survival under shear stress

Endothelial cells (ECs) are exposed to cytotoxic reactive oxygen species and oxidation products of NO, yet they are characterized by low apoptotic rates and have an average life span of many years. EC exposure to flow has been shown to downregulate cell cycle-related genes and cause cytoskeletal rearrangement. We hypothesized that exposure to flow also causes molecular and physiological changes that induce antioxidant properties in ECs. We used cDNA array expression profiling and protein analysis to study the responses of human ECs exposed to flow in a hollow fiber apparatus or the same ECs grown under static conditions. Our results show that shear-induced synchronized expression of processes control oxidant production; these changes included upregulation of NADH-producing enzymes (Krebs cycle dehydrogenases and glyceraldehyde-3-phosphate dehydrogenase [GAPDH]) accompanied by simultaneous decrease in NADH-depleting pathways (e.g., lactate dehydrogenase [LDH]) and diminished production of lactate. Exposure to flow upregulated cytoskeletal genes. Our results suggest that, in addition to inhibition of cell cycle, exposure to flow influences ECs by controlling expression of enzymes involved in the generation of antioxidant intermediates and in adaptive control of cell shape. These changes may explain longevity and antioxidant efficiency of ECs and may provide insight in mechanisms leading to pathological conditions such as arteriosclerosis.

Characterization of an in vitro rhesus macaque blood-brain barrier

The blood-brain barrier (BBB) has been modeled in vitro in a number of species, including rat, cow and human. Coculture of multiple cell types is required for the correct expression of tight junction proteins by microvascular brain endothelial cells (MBEC). Markers of inflammation, especially MHC-II, and cell adhesion molecules, such as VCAM-1, are not expressed on the luminal surface of the barrier under resting conditions. The rhesus macaque model has been used to study early events of HIV-neuropathogenesis in vivo, but a suitable in vitro model has not been available for detailed mechanistic studies. Here we describe an in vitro rhesus macaque blood-brain barrier that utilizes autologous MBEC and astrocytes. We believe that this model is highly relevant for examining immunological events at the blood-brain barrier and demonstrate its potential usefulness for examining early events in AIDS neuropathogenesis.

A new dynamic in vitro model for the multidimensional study of astrocyte–endothelial cell interactions at the blood–brain barrier

Blood-brain barrier endothelial cells are characterized by the presence of tight intercellular junctions, the absence of fenestrations, and a paucity of pinocytotic vesicles. The in vitro study of the BBB has progressed rapidly over the past several years as new cell culture techniques and improved technologies to monitor BBB function became available. Studies carried out on viable in vitro models are set to accelerate the design of drugs that selectively and aggressively can target the CNS. Several systems in vitro attempt to reproduce the physical and biochemical behavior of intact BBB, but most fail to reproduce the three-dimensional nature of the in vivo barrier and do not allow concomitant exposure of endothelial cells to abluminal (glia) and lumenal (flow) influences. For this purpose, we have developed a new dynamic in vitro BBB model (NDIV-BBB) designed to allow for extensive pharmacological, morphological and physiological studies. Bovine aortic endothelial cells (BAEC) developed robust growth and differentiation when co-cultured alone. In the presence of glial cells, BAEC developed elevated Trans-Endothelial Electrical Resistance (TEER). Excision of individual capillaries proportionally decreased TEER; the remaining bundles were populated with healthy cells. Flow played an essential role in EC differentiation by decreasing cell division. In conclusion, this new dynamic model of the BBB allows for longitudinal studies of the effects of flow and co-culture in a controlled and fully recyclable environment that also permits visual inspection of the abluminal compartment and manipulation of individual capillaries.

Tight junctions of the blood-brain barrier: development, composition and regulation

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The main characteristic features of blood-brain barrier endothelial cells are an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. 2. Endothelial blood-brain barrier tight junctions differ from epithelial tight junctions, not only by distinct morphological and molecular properties, but also by the fact that endothelial tight junctions are more sensitive to microenvironmental than epithelial factors. 3. Many ubiquitous molecular tight junction components have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin and 7H6. Signaling pathways involved in tight junction regulation include G-proteins, serine-, threonine- and tyrosine-kinases, extra and intracellular calcium levels, cAMP levels, proteases and cytokines. Common to most of these pathways is the modulation of cytoskeletal elements and the connection of tight junction transmembrane molecules to the cytoskeleton. Additionally, crosstalk between components of the tight junction- and the cadherin-catenin system of the adherens junction suggests a close functional interdependence of the two cell-cell contact systems. 4. Important new molecular aspects of tight junction regulation were recently elucidated. This review provides an integration of these new results.

A co-culture-based model of human blood-brain barrier: application to active transport of indinavir and in vivo-in vitro correlation

The growing array of in vitro models of the blood-brain barrier (BBB) which have been used makes it difficult to draw firm conclusions concerning the BBB penetration of HIV-1 protease inhibitors. What is needed is a combined in vivo and in vitro study on biological models that mimic as closely as possible the normal human BBB, to establish whether and how indinavir crosses the BBB. We developed a new human BBB model using primary endothelial cells and astrocytes. The biological relevance of this model was checked with respect on the one hand, to the close relationship between the log of drug permeability coefficient normalized to molecular weight and the log of the 1-octanol/water partition coefficient, and on the other hand to the functional P-glycoprotein (P-gp) expression. We employed this model to perform transport studies with indinavir and showed that the rate of in vitro indinavir transport from the basal to apical compartment was higher than the rate of apical to basal transport. Pretreatment of the BBB model with the P-gp inhibitor, quinidine, significantly increased apical to basal transport. Intracellular indinavir accumulation was increased in BBB as a result of inhibition of active transport. These data were correlated with the indinavir-mediated P-gp ATPase modulation showing that indinavir specifically interacted with a binding site on P-gp. Moreover, the activation of P-gp ATPase by indinavir was inhibited by quinidine. In addition, the in vivo brain to plasma concentration ratio of indinavir into mice showed that indinavir concentration was up to five times higher in the brain of mdr1a(-/-) mice than in the brain of mdr1a(+/+) mice. All these results confirm the role of P-gp in preventing the passage of indinavir across BBB and thus its entry into the central nervous system (CNS). Our human BBB model represents a useful tool for the evaluation of drug penetration into the CNS.

Blood-brain barrier preservation in the in vitro isolated guinea pig brain preparation

The morphofunctional preservation of the blood-brain barrier (BBB) was evaluated in the isolated guinea pig brain maintained in vitro by arterial perfusion. Electron microscopy evaluation after 5 hr in vitro demonstrated that cerebral capillaries and BBB specializations in this preparation retain features compatible with structural integrity. BBB-impermeable and -permeable atropine derivatives arterially perfused to antagonize carbachol-induced fast oscillatory activity confirmed the functional preservation of the BBB in vitro. To study BBB function further, changes in extracellular K+ concentration during arterial perfusion of a high-K+ solution were measured with K+-sensitive electrodes positioned in the cortex and, as control, at the brain venous outlet, where the solution perfused through the brain arterial system was collected. After 5 hr in vitro, the [K+](o) values measured during high-K+ perfusion in the piriform and entorhinal cortices were 5.02 +/- 0.17 mM (mean +/- SE) and 5.2 +/- 0.21 mM, respectively (n = 6). Coperfusion of the high-K+ solution with the Na+/K+ pump blocker ouabain (10 microM; n = 4) induced consistently spreading depression preceded by a rise in [K+](o). Finally, sporadic, isolated spots of extravasation of the fluorescent marker fluorescein isothiocyanate (FITC)-dextran preferentially circumscribed to deep cortical layers was observed in brains perfused with FITC-dextran after 5 hr in vitro. The study demonstrates that the in vitro isolated guinea pig brain is viable for studying cerebrovascular interactions and BBB permeability of compounds active in the central nervous system.

Mechanisms of glucose transport at the blood-brain barrier: an in vitro study

How the brain meets its continuous high metabolic demand in light of varying plasma glucose levels and a functional blood-brain barrier (BBB) is poorly understood. GLUT-1, found in high density at the BBB appears to maintain the continuous shuttling of glucose across the blood-brain barrier irrespective of the plasma concentration. We examined the process of glucose transport across a quasi-physiological in vitro blood-brain barrier model. Radiolabeled tracer permeability studies revealed a concentration ratio of abluminal to luminal glucose in this blood-brain barrier model of approximately 0.85. Under conditions where [glucose](lumen) was higher than [glucose](ablumen), influx of radiolabeled 2-deoxyglucose from lumen to the abluminal compartment was approximately 35% higher than efflux from the abluminal side to the lumen. However, when compartmental [glucose] were maintained equal, a reversal of this trend was seen (approximately 19% higher efflux towards the lumen), favoring establishment of a luminal to abluminal concentration gradient. Immunocytochemical experiments revealed that in addition to segregation of GLUT-1 (luminal>abluminal), the intracellular enzyme hexokinase was also asymmetrically distributed (abluminal>luminal). We conclude that glucose transport at the CNS/blood interface appears to be dependent on and regulated by a serial chain of membrane-bound and intracellular transporters and enzymes.