Cerebral microvessels and derived cells in tissue culture: isolation and preliminary characterization

Uptake Transporters at the Blood-Brain Barrier and Their Role in Brain Drug Disposition

Uptake drug transporters play a significant role in the pharmacokinetic of drugs within the brain, facilitating their entry into the central nervous system (CNS). Understanding brain drug disposition is always challenging, especially with respect to preclinical to clinical translation. These transporters are members of the solute carrier (SLC) superfamily, which includes organic anion transporter polypeptides (OATPs), organic anion transporters (OATs), organic cation transporters (OCTs), and amino acid transporters. In this systematic review, we provide an overview of the current knowledge of uptake drug transporters in the brain and their contribution to drug disposition. Here, we also assemble currently available proteomics-based expression levels of uptake transporters in the human brain and their application in translational drug development. Proteomics data suggest that in association with efflux transporters, uptake drug transporters present at the BBB play a significant role in brain drug disposition. It is noteworthy that a significant level of species differences in uptake drug transporters activity exists, and this may contribute toward a disconnect in inter-species scaling. Taken together, uptake drug transporters at the BBB could play a significant role in pharmacokinetics (PK) and pharmacodynamics (PD). Continuous research is crucial for advancing our understanding of active uptake across the BBB.

Modeling the blood-brain barrier for treatment of central nervous system (CNS) diseases

The blood-brain barrier (BBB) is the most specialized biological barrier in the body. This configuration of specialized cells protects the brain from invasion of molecules and particles through formation of tight junctions. To learn more about transport to the brain, in vitro modeling of the BBB is continuously advanced. The types of models and cells selected vary with the goal of each individual study, but the same validation methods, quantification of tight junctions, and permeability assays are often used. With Transwells and microfluidic devices, more information regarding formation of the BBB has been observed. Disease models have been developed to examine the effects on BBB integrity. The goal of modeling is not only to understand normal BBB physiology, but also to create treatments for diseases. This review will highlight several recent studies to show the diversity in model selection and the many applications of BBB models in in vitro research.

Blood-brain barrier models: Rationale for selection

Brain delivery is a broad research area, the outcomes of which are far hindered by the limited permeability of the blood-brain barrier (BBB). Over the last century, research has been revealing the BBB complexity and the crosstalk between its cellular and molecular components. Pathologically, BBB alterations may precede as well as be concomitant or lead to brain diseases. To simulate the BBB and investigate options for drug delivery, several in vitro, in vivo, ex vivo, in situ and in silico models are used. Hundreds of drug delivery vehicles successfully pass preclinical trials but fail in clinical settings. Inadequate selection of BBB models is believed to remarkably impact the data reliability leading to unsatisfactory results in clinical trials. In this review, we suggest a rationale for BBB model selection with respect to the addressed research question and downstream applications. The essential considerations of an optimal BBB model are discussed.

Advancing brain barriers RNA sequencing: guidelines from experimental design to publication

BACKGROUND

RNA sequencing (RNA-Seq) in its varied forms has become an indispensable tool for analyzing differential gene expression and thus characterization of specific tissues. Aiming to understand the brain barriers genetic signature, RNA seq has also been introduced in brain barriers research. This has led to availability of both, bulk and single-cell RNA-Seq datasets over the last few years. If appropriately performed, the RNA-Seq studies provide powerful datasets that allow for significant deepening of knowledge on the molecular mechanisms that establish the brain barriers. However, RNA-Seq studies comprise complex workflows that require to consider many options and variables before, during and after the proper sequencing process.

MAIN BODY

In the current manuscript, we build on the interdisciplinary experience of the European PhD Training Network BtRAIN ( https://www.btrain-2020.eu/ ) where bioinformaticians and brain barriers researchers collaborated to analyze and establish RNA-Seq datasets on vertebrate brain barriers. The obstacles BtRAIN has identified in this process have been integrated into the present manuscript. It provides guidelines along the entire workflow of brain barriers RNA-Seq studies starting from the overall experimental design to interpretation of results. Focusing on the vertebrate endothelial blood-brain barrier (BBB) and epithelial blood-cerebrospinal-fluid barrier (BCSFB) of the choroid plexus, we provide a step-by-step description of the workflow, highlighting the decisions to be made at each step of the workflow and explaining the strengths and weaknesses of individual choices made. Finally, we propose recommendations for accurate data interpretation and on the information to be included into a publication to ensure appropriate accessibility of the data and reproducibility of the observations by the scientific community.

CONCLUSION

Next generation transcriptomic profiling of the brain barriers provides a novel resource for understanding the development, function and pathology of these barrier cells, which is essential for understanding CNS homeostasis and disease. Continuous advancement and sophistication of RNA-Seq will require interdisciplinary approaches between brain barrier researchers and bioinformaticians as successfully performed in BtRAIN. The present guidelines are built on the BtRAIN interdisciplinary experience and aim to facilitate collaboration of brain barriers researchers with bioinformaticians to advance RNA-Seq study design in the brain barriers community.

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

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

Improved Method for the Establishment of an In Vitro Blood-Brain Barrier Model Based on Porcine Brain Endothelial Cells

The aim of this protocol presents an optimized procedure for the purification and cultivation of pBECs and to establish in vitro blood-brain barrier (BBB) models based on pBECs in mono-culture (MC), MC with astrocyte-conditioned medium (ACM), and non-contact co-culture (NCC) with astrocytes of porcine or rat origin. pBECs were isolated and cultured from fragments of capillaries from the brain cortices of domestic pigs 5-6 months old. These fragments were purified by careful removal of meninges, isolation and homogenization of grey matter, filtration, enzymatic digestion, and centrifugation. To further eliminate contaminating cells, the capillary fragments were cultured with puromycin-containing medium. When 60-95% confluent, pBECs growing from the capillary fragments were passaged to permeable membrane filter inserts and established in the models. To increase barrier tightness and BBB characteristic phenotype of pBECs, the cells were treated with the following differentiation factors: membrane permeant 8-CPT-cAMP (here abbreviated cAMP), hydrocortisone, and a phosphodiesterase inhibitor, RO-20-1724 (RO). The procedure was carried out over a period of 9-11 days, and when establishing the NCC model, the astrocytes were cultured 2-8 weeks in advance. Adherence to the described procedures in the protocol has allowed the establishment of endothelial layers with highly restricted paracellular permeability, with the NCC model showing an average transendothelial electrical resistance (TEER) of 1249 ± 80 Ω cm², and paracellular permeability (Papp) for Lucifer Yellow of 0.90 10^-6 ± 0.13 10^-6 cm sec^-1 (mean ± SEM, n=55). Further evaluation of this pBEC phenotype showed good expression of the tight junctional proteins claudin 5, ZO-1, occludin and adherens junction protein p120 catenin. The model presented can be used for a range of studies of the BBB in health and disease and, with the highly restrictive paracellular permeability, this model is suitable for studies of transport and intracellular trafficking.

[Modelling of the blood-brain barrier]

The blood-brain barrier (BBB) is located at the brain microvessel level and isolates the brain from the whole body, thus restricting molecule and cell exchanges between cerebral and peripheral compartments. In order to better decipher and understand the BBB physiology and development, and to investigate transport mechanism and toxicity of neuropharmaceuticals, several in vitro BBB models have been developed using animal or human cells, primary or immortalized cells. The aim of this review is to explain to the reader the major criteria required for a pertinent in vitro BBB model and to briefly expose the different models currently available with their characteristics with a special focus on the static models.

In vitro models of the blood-brain barrier: An overview of commonly used brain endothelial cell culture models and guidelines for their use

The endothelial cells lining the brain capillaries separate the blood from the brain parenchyma. The endothelial monolayer of the brain capillaries serves both as a crucial interface for exchange of nutrients, gases, and metabolites between blood and brain, and as a barrier for neurotoxic components of plasma and xenobiotics. This "blood-brain barrier" function is a major hindrance for drug uptake into the brain parenchyma. Cell culture models, based on either primary cells or immortalized brain endothelial cell lines, have been developed, in order to facilitate in vitro studies of drug transport to the brain and studies of endothelial cell biology and pathophysiology. In this review, we aim to give an overview of established in vitro blood-brain barrier models with a focus on their validation regarding a set of well-established blood-brain barrier characteristics. As an ideal cell culture model of the blood-brain barrier is yet to be developed, we also aim to give an overview of the advantages and drawbacks of the different models described.

Differentiation and characterization of human pluripotent stem cell-derived brain microvascular endothelial cells

The blood-brain barrier (BBB) is a critical component of the central nervous system (CNS) that regulates the flux of material between the blood and the brain. Because of its barrier properties, the BBB creates a bottleneck to CNS drug delivery. Human in vitro BBB models offer a potential tool to screen pharmaceutical libraries for CNS penetration as well as for BBB modulators in development and disease, yet primary and immortalized models respectively lack scalability and robust phenotypes. Recently, in vitro BBB models derived from human pluripotent stem cells (hPSCs) have helped overcome these challenges by providing a scalable and renewable source of human brain microvascular endothelial cells (BMECs). We have demonstrated that hPSC-derived BMECs exhibit robust structural and functional characteristics reminiscent of the in vivo BBB. Here, we provide a detailed description of the methods required to differentiate and functionally characterize hPSC-derived BMECs to facilitate their widespread use in downstream applications.

Characterization of an in vitro blood-brain barrier model system for studying drug transport and metabolism

Bovine brain micro vessel endothelial cells have been isolated and grown in culture to monolayers. These endothelial cell monolayers have been characterized morphologically with electron microscopy, histochemically for brain endothelium enzyme markers, alkaline phosphatase and γ-glutamyl trans-peptidase, and by immunofluorescence to detect Factor VIII antigen, an exclusive endothelial antigen. Results of these studies indicate that the cells forming the monolayers are of endothelial origin and possess many features of the in vivo brain endothelium responsible for formation of the blood-brain barrier. This in vitro blood-brain barrier model system was used in experiments to determine the permeability of the cultured monolayer to sucrose, leucine, and propranolol. Leucine rapidly moved across the monolayers of this in vitro system and tended to plateau after approximately 10 min. In contrast, the rates of sucrose and propranolol movement were linear during a 1-hr observation period, with the rate of propranolol movement across the monolayer greater than that of sucrose. The ability to detect differences in the permeability of the monolayers to leucine, propranolol, and sucrose with radioactive tracers suggests that this in vitro model system will be an important tool for the investigation of the role of the blood-brain barrier in the delivery of centrally acting drugs and nutrients.

Homocysteine induces cerebral endothelial cell death by activating the acid sphingomyelinase ceramide pathway

Homocysteine (Hcy) levels may rise after a stroke, but the mechanism of Hcy-induced cerebral endothelial cell (CEC) dysfunction has not been explored. In this study we examined the role of the acid sphingomyelinase (Asm)-ceramide pathway in the molecular mechanism of Hcy-induced CEC dysfunction. Murine CECs were prepared from fresh mouse brains. CECs were treated with 50-500 μM Hcy and 30-100 μM C2-ceramide for 48 h. Sphingomyelinase assays were performed to determine Asm activity. Quantitative assessments of cell survival and death by the MTT reduction and LDH release were conducted. Treatment of murine CECs with Hcy and ceramide caused cell death in a dose-dependent manner as determined by LDH and MTT assays. 250 μM Hcy and 50 μM C2-ceramide caused 50% cell death. Hcy induced murine CEC death also occurred in a time-dependant manner with substantial cell death noted as early as 24h after Hcy exposure. C2-ceramide-induced murine CEC death occurred earlier than Hcy-induced cell death by about 18h. Hcy treatment increased Asm activity and intracellular ceramide accumulation. This study demonstrated that Hcy and C2-ceramide can cause murine CEC death. Hcy induces CEC death possibly by activating the Asm-ceramide pathway.

Characterization of microvascular endothelial cells isolated from the dermis of adult mouse tails

Dermal microvascular endothelial cells (DMECs) play an important role in physiological and pathophysiological processes such as wound healing, cell differentiation, antigen-presentation, inflammation, tumor metastasis, and diabetes. The study of these processes requires a suitable and accessible in vitro model, such as murine DMECs (mDMECs). However, since these cells are difficult to isolate and propagate, some of their properties are not fully characterized. We isolated these cells from C57BL/6J adult mouse tail skin and purified them using magnetic sorting. Then, we tested several culture conditions and oxygen concentrations for mDMEC growth and propagation. After obtaining optimal culture conditions, we characterized the expression of EC markers and compared such expression with an established murine microvascular EC line (EOMA). Our results indicate that mDMECs isolated from mouse tails expressed most of the characteristic EC markers such as von Willebrand Factor (vWF), CD31, Tie1, Tie2, ANGPT1, ANGPT2, FLK-1, FLT-1, and VEGF-A. Further characterization demonstrated that these cells also expressed proteins involved in organogenesis such as bone morphogenetic proteins-2, -4 (BMP-2/-4), and their receptor (BMPR1A). Surprisingly, higher expression of vWF, ANGPT1, and BMP-2 was observed in mDMECs compared to EOMA cells. For mDMEC in vitro propagation, we found a twofold increase in cell proliferation in cells that grew at 1% O(2) compared to those cells that grew at standard 20% O(2.) Therefore, the method described herein for mDMECs isolation and propagation allowed us to analyze in more detail their biological properties that can be relevant for the study of pathological processes using mouse models.

Traumatic brain injury increases TGFβRII expression on endothelial cells

Transforming growth factor beta (TGFbeta) modulates a variety of growth related functions following traumatic injury. The cellular response to TGFbeta is predominantly mediated through TGFbeta receptor I (TGFbetaRI) and receptor II (TGFbetaRII) on the cell surface and SMAD proteins intracellularly. We investigated the expression of TGFbeta receptors in the acute and chronic phases of a traumatic cerebral injury (TCI) by immunohistochemistry and in cultures of murine brain microvascular endothelial (EN) cells using cytofluorimetry. Here, we report that TGFbetaRII expression significantly increases on brain endothelial cells in the chronic phase of TCI. SMAD3 and SMAD4 protein expression were also upregulated suggesting the activation of TGFbeta receptor intracellular signaling. When TGFbetaRI and TGFbetaRII expression was studied in in vitro cultures of murine brain microvessel EN cells, TGFbetaRII showed increased expression on proliferating cells that are incorporating BrdU. These data show a differential expression of TGFbetaRI and TGFbetaRII on brain microvessel EN cells in the acute and chronic phases of TCI that might be associated with EN proliferation following injury.

New approaches to in vitro models of blood-brain barrier drug transport

The pharmaceutical industry has been searching for an in vitro blood-brain barrier (BBB) model that preserves in vivo transporter functions in CNS drug discovery and development. The application of conditionally immortalized cell lines derived from transgenic animals harboring temperature-sensitive SV40 large T-antigen gene, is a rational and promising approach to such a workable in vitro BBB model. The established brain capillary endothelial cell lines retain the in vivo transport rate of several compounds and various forms of gene expression. Furthermore, this new approach has enabled the development of stable and reproducible co-culture models with a pericyte cell line and/or an astrocyte cell line.

Models for Angiogenesis in Gliomas

During the last decades a lot of attention has been focussed on mechanisms of glioma vascularization, particularly in terms of investigating vascular growth factors and receptors. Recently, these efforts resulted in various approaches for antiangiogenic treatment strategies using in vitro cell culture systems as well as experimental orthotopic and non-orthotopic brain tumors. These basic science and preclinical trials need an assortment of models, which should allow investigating a variety of questions. Several objectives concerning basic endothelial cell (EC) characteristics can adequately be studied in vitro using EC monolayer assays. Three-dimensional spheroid techniques respect the more complex cell-cell and cell-environment interplay within a 3-dimensional culture. Recent advances in molecular genetic techniques offer a wide access to the genome of EC. Using these micro array or chip methods differences between micro- and macromolecular EC as well as variations within the gene pool of different organ specific EC can be assessed. To optimize the imitation of the crucial interaction of human gliomas with host endothelial cells, immunological cells and extracellular matrix, animal models are mandatory. An essential rule is to utilize an orthotopic model, since tumor-host-interaction is organ specific. To avoid alloimmunogenic responses, it is desirable to use weak or non-immunogenic glioma grafts, which is best accomplished in a syngeneic model. However, since rat gliomas poorly resemble human glioma growth patterns, human glioma xenografting into immunocompromized animals should be considered. In vivo-monitoring techniques like videoscopy via a cranial window or magnetic resonance imaging (MRI) allow for functional studies and improve the validity of the model employed. Finally, it is essentially to recognize the limitations of each model considered and to select that model which seems to be most appropriate for the objectives to be investigated.

A simple method for isolation and characterization of mouse brain microvascular endothelial cells

Brain endothelial cells, a site of the blood-brain barrier in vivo, regulate a number of physiological and pathophysiological processes in the brain including transport of nutrients, export of critical toxins, transmigration of circulating leukocytes and formation of new blood vessels. In this report, we describe a simple and reproducible method to isolate pure (>99%), functionally active endothelial cells from small quantities of adult mouse brain tissue. In vitro, these cells express typical phenotypic markers of differentiated brain endothelium such as von Willebrand factor, multiple drug resistant protein and glucose transporter-1, demonstrate uptake of acetylated low-density lipoprotein, and possess morphological and ultrastructural characteristics of microvascular endothelium. They form tight junctions and capillary-like tubes when stimulated by growth factors in an in vitro angiogenesis assay. In response to tumor necrosis factor-alpha, isolated mouse brain endothelial cells (MBEC) express vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). The protocol described here provides an effective and reliable method to isolate pure cerebral endothelium from adult mouse brain that should offer a useful tool for studying the role of altered vascular biology in mice with genetically manipulated brain disorders.

Antiangiogenic therapy against experimental glioblastoma using genetically engineered cells producing interferon-alpha, angiostatin, or endostatin

Inhibition of angiogenesis has been considered among the most promising approaches to treat highly vascularized solid tumors such as glioblastoma. In this study, we designed and validated a new in vitro assay system based on the implantation of tumor cells into organotypic brain slice cultures. We evaluated the effects of local production of three endogenous inhibitors of angiogenesis, angiostatin, endostatin, and interferon (IFN)-alpha(1), using stably transfected rat (9L) and human (GL15) glioblastoma cells on tumor vascularization and growth. Despite similar effectiveness of the three proteins in a classic in vitro endothelial cell migration assay, IFN-alpha(1) demonstrated the most potent antiangiogenic effect in organotypic brain slice cultures. In vivo, after intracerebral implantation of such genetically modified glioblastoma cells, IFN-alpha(1) caused a dramatic decrease in tumor volume revealed by magnetic resonance imaging and by postmortem histology. The mechanisms of this antitumor effect were most likely caused by the major antiangiogenic action of the cytokine, because IFN-alpha(1) expression provoked a pronounced decrease in blood vessel density, which was accompanied by extensive necrosis in the body mass of the tumors. The median survival time of rats implanted intracerebrally with IFN-alpha-expressing 9L cells tripled, and was still significantly increased when these constituted only 1% of transplanted tumor cells. A similar effect was seen when 50% of the transplanted cells were replaced by IFN-alpha-expressing bone marrow stromal cells. These data point to the local delivery of IFN-alpha(1) using cell vectors as a potent tool for the inhibition of tumor-induced angiogenesis.

LC-MS/MS detection of peroxynitrite-derived 3-nitrotyrosine in rat microvessels

3-Nitrotyrosine (3NT) is used as a biomarker of nitrative pathology caused by peroxynitrite (PN), myeloperoxidase (MPO)-, and/or eosinophil peroxidase (EPO)-dependent nitrite oxidation. 3NT measurements in biological materials are usually based on either antibody staining, HPLC detection, or GC detection methodologies. In this report, a procedure is described for the measurement of 3NT and tyrosine (TYR) by LC-MS/MS that is simple, direct, and sensitive. Though highly specialized in its use as an assay, LC-MS/MS technology is available in many research centers in academia and industry. The critical assay for 3NT was linear below 100 ng/ml and the limit of detection was below 100 pg/ml. Regarding protein digested samples, we found that MRM was most selective with 133.1 m/z as the daughter ion. In comparison, LC-ECD was 100 times less sensitive. Basal levels of 3NT in extracted digests of rat brain homogenate were easily detected by LC-MS/MS, but were below detection by LC-ECD. The LC-MS/MS assay was used to detect 3NT in rat brain homogenate that was filtered through a 180 micron nylon mesh. Three fractions were collected and examined by phase contrast microscopy. The mass ratio (3NT/TYR) of 3NT in fractions of large vessel enrichment, microvessel enrichment, and vessel depletion was 0.6 ng/mg, 1.2 ng/mg, and 0.2 ng/mg, respectively. Ultimately, we found that the basal 3NT/TYR mass ratio as determined by LC-MS/MS was six times greater in microvessel-enriched brain tissue vs. tissue devoid of microvessels.

Models for assessment of angiogenesis in gliomas

In the last two decades, much attention has been focussed on mechanisms of glioma vascularization including the investigation of growth factors and receptors involved. Recently, these efforts resulted in various approaches for antiangiogenic treatment of experimental brain tumors. These basic science and preclinical trials need an assortment of models, which should allow investigating a variety of questions. Several objectives concerning basic endothelial cell (EC) characteristics can adequately be studied in vitro using EC monolayer assays. Three-dimensional spheroid techniques respect the more complex cell-cell and cell-environment interplay within a three-dimensional culture. To optimize the imitation of the crucial interaction of human gliomas with host endothelial cells, immunological cells and extracellular matrix, animal models are mandatory. An essential rule is to utilize an orthotopic model, since tumor-host interaction is organ specific. To avoid alloimmunogenic responses, it is desirable to use weakly or not immunogenic glioma grafts, what is best accomplished in a syngeneic model. However, since rat gliomas poorly resemble human glioma growth patterns, human glioma xenografting into immunocompromized animals should be considered. In vivo monitoring techniques like videoscopy via a cranial window or magnetic resonance imaging (MRI) allow for functional studies and improve the validity of the model employed. Finally, it is essentially to recognize the limitations of each model considered and to select that model, which seems to be most appropriate for the objectives to be investigated.

Reoxygenation injury of human brain capillary endothelial cells

1. Many studies have demonstrated that endothelial cells from several species can generate oxygen free radicals when subjected to anoxia and reoxygenation. However, due to the heterogeneity of the endothelium within different organs and species, the effects of superoxide dismutase (SOD), catalase, and allopurinol on reoxygenated cultured cells remain quite controversial. 2. This review outlines the possible sources of oxygen free radicals within brain endothelial cells. 3. We examine the aspects of the effects of SOD catalase and allopurinol on cultured human brain capillary endothelial cells upon reoxygenation. 4. Also, we introduce briefly a method of culturing human brain capillary endothelial cells and present our experimental results on the effects of SOD, catalase, and allopurinol in these cultured cells following anoxia and reoxygenation.

[Transport of drugs across the blood-brain barrier]

The blood-brain barrier prevents an indifferent medicine existing in the blood to enter also in the brain. This barrier has got an anatomical base: it is first consisting in a cerebrovascular layer of endothelial capillary vessels of the peripheral tissue. It is moreover covered by outgrowths of the flial cells, which are called astrocytes. There are, for that reason, important limits to a size of molecules which can reach the cerebral tissue through a paracellular way (through what is called in English "tight-junctions"). Most medicines must use the transcellular way. Lipophily is necessary to follow that way. Year after year, it appeared, thanks to a comparative study of the substances, that there exists--grosso modo--a positive correlation between the lipophilic level and the permeation-level of a substance in the cerebral tissue. There are, however, several exceptions: it is so that hydrophilic substances, possessing an important nourishing function (such as glucosis, amino-acids) seem to penetrate much more easily than we could expect when we consider their physicochemical characteristics. This is the result of the fact that there exist specifical transport-mechanisms for these substances at the level of the endothelial cell-membranes, allowing the penetration of such substances. There exist, on the contrary, lipophilic components that penetrate the cerebral tissue much less strongly than we should expect. This happens because there also exist pumping-mechanisms at the level of the hemato-encephalic barrier. The concerning substance, which was recently discovered is the "glycoprotein P", which is also responsible for the "multi-drug-resistance" and for the resistance of tumors to cytostatics. This phenomenon relies on a very efficient pumping of substances which have penetrated cells in which this protein expressed itself in the membranous structure. In order to obtain a better understanding of the function of the hemato-encephalic barrier, comprising the transport of medicines, it is most important to have reliable experimental models. It is to that aim that, during former years, the technique of cultivating endothelial cerebrovascular cells was developed. These cells are isolated from brains of calves or rats and, subsequently, cultivated on a laboratory medium; about a week later, they have grown a single and confluent layer. This layer represents a kint of "hemato-encephalic barrier" in vitro, which allows us to study the transfer of substances through the layer and thus also the details concerning the transport mechanisms, as well as the factors influencing the permeability of the cells-layer (for instance the inflammatory stimuli). Concerning the "in vivo" research, the technique of intracerebral microdialysis in lab-animals proved to be very promising. In order to effect this microdialysis, a semipermeable microcannula is introduced in the brain tissue, across which an iso-osmotic liquid is being injected continuously. The substances staying in the interstitial liquid of the cerebral tissue will diffuse under the influence of a concentration gradient, into the dialysing liquid and they will also be ready to be analysed. Thanks to this technique, it is possible to follow, in the same animal, the evolution of the concentration in the brain of a substance which has, for instance been injected in a peripheral region. In this way, we obtain, indirectly and in vivo, informations about the functioning-process of the "hemato-encephalic barrier". We can, moreover, effect measures on a specific spot, for instance in tumoral brain tissue: this allows us to study the influence of specific transport-mechanisms. These rather recent techniques, as well in vitro as in vivo, will allow us, in consequence, to increase, during the next years, our understanding of the way the hemato-encephalic barrier functions as to the transfer of medicines towards the central nervous system. This understanding may lead us to new strategies allowing

A general strategy for isolation of endothelial cells from murine tissues. Characterization of two endothelial cell lines from the murine lung and subcutaneous sponge implants

A rapid, reproducible method for the isolation of murine endothelial cells (ECs) has been developed. Murine ECs were highly enriched by collagenase digestion of mechanically minced lung and subcutaneous sponge implants followed by specific selection with rat anti-mouse CD31 (i.e., PECAM-1) monoclonal antibody-coated magnetic beads (Dynabeads). Pure EC populations were isolated from primary cultures by a second cycle of immunomagnetic selection. The cells from the lung were then cloned by a limiting-dilution method to exclude the possibility of nonendothelial cell contamination. Of the 300 cells plated, 29 clones (approximately 10%) were obtained. The clones were positive for CD31 as measured by flow cytometry, and one clone from the lungs (1G11) and the cells from sponge implants (designated as SIECs) were then subjected to subsequent culture in vitro for 40 and 30 passages (up to 5 months), respectively. Characterization was performed on cells between passage 3 and 10. Both cell types formed contact-inhibited monolayers on gelatin and capillary-like "tubes" on Matrigel. However, 1G11 cells exhibited a "cobblestone" morphology, whereas SIECs had a fibroblast-like appearance at confluence. By flow cytometry and enzyme-linked immunosorbent assay, these cells constitutively expressed CD31, VE-cadherin (cadherin-5), CD34, ICAM-1, VCAM-1, and P-selectin. After stimulation with 30 ng/mL of tumor necrosis factor-alpha, the cells became positive for E-selectin (at 4 hours poststimulation) and the expression of ICAM-1, VCAM-1, and P-selectin was upregulated (after 24 hours of stimulation). The presence of VE-cadherin in 1G11 cells and SIECs was confirmed by fluorescence microscopy and Northern blot analysis. The phenotype and morphology of both cell types were stable during 5 months of culture, and there was no evidence of overgrowth by contaminating cells. Taken together, the approach outlined herein may provide a general strategy for the isolation and culture of ECs from a variety of murine tissues. The general strategy outlined here is simple, effective, and flexible, allowing the inclusion of further positive or negative selection steps.

Melatonin protects bovine cerebral endothelial cells from hyperoxia-induced DNA damage and death

Hyperoxia leads to excessive formation of reactive oxygen species (ROS). ROS cause damage to many cellular components, including DNA. Exposure of bovine cerebral endothelial cells to 95 or 100% oxygen resulted in an increase in DNA fragmentation, the appearance of DNA ladders, and cell death with morphological features suggestive of apoptosis. Melatonin, an antioxidant, reduced hyperoxia-induced DNA fragmentation and cell death in a dose-dependent manner. Results from the present study support the contention that ROS play a major role in DNA damage and apoptotic death. Melatonin is an effective agent in reducing ROS-mediated DNA fragmentation and death in bovine cerebral endothelial cells.

Regulation of cytokine-induced iNOS expression by a hairpin oligonucleotide in murine cerebral endothelial cells

Inducible nitric oxide synthase (iNOS) is expressed in response to cytokines by a number of cell types participating in CNS inflammation, including brain cerebral endothelial cells. NF-kappaB, a transcription factor, mediates effector actions of pro-inflammatory cytokines. A combination of tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) enhanced the expression of iNOS in murine cerebral endothelial cells (MCECs). In an attempt to modulate TNF-alpha+IFN-gamma induced expression of iNOS in MCECs, we designed a double-strand hairpin (hp) oligonucleotide carrying the NF-kappaB motif. This hp oligonucleotide inhibited NF-kappaB binding activity and decreased both iNOS mRNA and protein expression induced by TNF-alpha+IFN-gamma. As a control, a mutant hp oligonucleotide was without effect. The present study confirms the role of transcription factor NF-kappaB in iNOS expression induced by TNF-alpha+IFN-gamma in MCECs. More importantly, it demonstrates that an appropriately designed hp oligonucleotide is an effective tool to modulate iNOS expression and may be of potential pharmacological use.

Osmotic regulation of Na-myo-inositol cotransporter mRNA level and activity in endothelial and neural cells

Myo-inositol (MI) is an important factor in the synthesis of phosphoinositides, and as an osmolyte, MI contributes to the regulation of cell volume. In cells of renal origin, hypertonicity causes an increase in sodium-dependent MI transporter (SMIT) mRNA levels and MI transport. However, it is unknown whether changes in osmolarity regulate transport of MI in neural or endothelial cells. IN these studies, neural and endothelial cells were exposed to hyperosmotic medium for up to 48 h, and the effect on MI transport was determined. Transport of MI was maximally increased by exposing the cells to hyperosmotic medium for 24 h. Kinetic analysis of high-affinity MI transport demonstrated an increase in the apparent maximal velocity with no significant change in the apparent Km. The hyperosmotic induction of MI transport was blocked by the addition of cycloheximide, indicating a requirement for protein synthesis, and was associated with increased levels of SMIT mRNA. In contrast to the effect of hypertonicity, exposure of neural and endothelial cells to hypotonic conditions caused a decrease in SMIT mRNA levels and MI transport in endothelial cells. These studies demonstrate that, in extrarenal cell types, changes in osmolarity also regulate SMIT activity and mRNA levels.

L-fucose reduces collagen and noncollagen protein production in cultured cerebral microvessel endothelial cells

L-fucose is a monosaccharide which is present in low concentrations in normal serum but is increased in diabetes, cancer, and inflammatory diseases. The contribution that abnormal L-fucose levels make to the progression of these disorders is unknown. In a previous study we showed that increased L-fucose concentration reduced proliferation and proteoglycan production by cultured cerebral microvessel endothelial cells. In the present study we show that exposing cerebral microvessel endothelial cells for 2 weeks to medium containing an increased concentration of L-fucose causes a significant decrease in collagen and to a lesser extent noncollagen protein production. The effect of L-fucose on collagen and noncollagen protein production is concentration-dependent: 1 mM L-fucose causes a significant decrease in collagen production but has no effect on noncollagen protein production; a 5 mM L-fucose concentration causes a maximum decrease in both collagen and noncollagen protein production. This defect is unrelated to the reduction in myo-inositol uptake caused by L-fucose and is not prevented by aminoguanidine. Collagen production can be improved by restoring L-fucose-conditioned cells to normal medium. Culturing cells for 2 weeks in medium containing 10 mM L-fucose resulted in a 50% decrease in collagen production, which was restored to 75% of control after cells were transferred to normal medium for 7 days. In contrast, noncollagen protein production was totally restored after 3 days in normal medium. Increasing levels of L-fucose in serum of rats also resulted in a decrease in collagen production. Collagenase digestible incorporation of L-[2,3,4,5-3H]proline into protein of the articular cartilage from rats fed a diet containing 20% L-fucose for 3 weeks was reduced by about 40% compared to rats fed a normal diet. The decrease in collagen production in L-fucose fed rats was less than the reduction that occurred in streptozotocin-induced diabetic rats. These data suggest that changes in L-fucose concentration itself may be a factor in the regulation of collagen production.

Enzyme histochemical studies of membrane proteases in rat subfornical organ

Localization of membrane proteases glutamyl aminopeptidase (EAP), microsomal alanyl aminopeptidase (mAAP), dipeptidyl peptidase IV (DPP IV) and gamma-glutamyl transpeptidase (gamma-GTP) were studied in vessels of the rat subfornical organ (SFO), ependyma which cover the surface of the SFO, and adjacent brain structures. Results of enzyme histochemical reactions showed strong activity for EAP, mAAP, and gamma-GTP, but absence of DPP IV in microvessels of SFO. The ependyma which cover the SFO was positive for gamma-GTP, but negative for other studied proteases. Our results showed that the spectrum of enzymes in the majority of the vessels of SFO is similar to that of the microvessels of the adjacent brain tissue which were positive for EAP, mAAP, and gamma-GTP, but negative for DPP IV. The relative intensity of the enzyme reactions in vessels varied from central to lateral locations in the SFO and the adjacent brain tissue. There was also a difference in the relative reaction intensity from one enzyme to the other. The presence and heterogeneous distribution of the enzymes are consistent with the hypothesis that membrane proteases of the microvascular endothelium constitute an enzyme-barrier between blood and parenchyma of the SFO and between blood and brain tissue, and may be involved in metabolism or modulation of various peptides when they contact the plasma membrane of the endothelial cells of the vessels.

Semliki Forest virus infects mouse brain endothelial cells and causes blood-brain barrier damage

Induction of experimental allergic encephalomyelitis is facilitated in a genetically resistant BALB/c mouse strain by a nonpathogenic strain of a neurotropic alphavirus, Semliki Forest virus (SFV-A7). One possible explanation for this enhancement is virus infection of endothelial cells (EC), causing increased permeability of the blood-brain barrier. We have now sought evidence for virus infection of EC in vivo by immunocytochemistry and in situ hybridization. SFV-A7 antigens and RNA were detected in vascular EC and perivascular neurons in cerebellar and spinal cord white matter. Expression of viral antigens was followed by fibrinogen leakage from the blood vessels into brain parenchyma. This was shown by immunoperoxidase staining detecting fibrinogen extravascularly in central nervous system sections of infected mice. Simultaneously, expression of ICAM-1 (intercellular adhesion molecule 1) was induced on brain EC. SFV-A7 replicated in mouse brain microvascular EC in vitro and caused lysis of the cells. SFV-A7 did not induce ICAM-1 expression of mouse brain microvascular EC in vitro, while ICAM-1 was readily induced by gamma interferon and interleukin 1 beta. The observed increase of ICAM-1 expression on EC is immune mediated and not a direct effect of the virus infection. We conclude that SFV-A7 infection causes cerebral microvascular damage which contributes to the facilitation of experimental allergic encephalomyelitis in BALB/c mice.

Enhanced expression of the blood-brain barrier GLUT1 glucose transporter gene by brain-derived factors

The blood-brain barrier GLUT1 glucose transporter is localized in brain to the capillary endothelium, which makes up the blood-brain barrier (BBB) in vivo. However, its expression is markedly downregulated in cultured bovine brain capillary endothelium (ECL cells), possibly due to the absence of brain-derived or astrocyte trophic factors in the tissue culture medium. To examine this hypothesis, we studied the effect of a bovine brain homogenate (BBH), and conditioned media and plasma membranes obtained from the rat C6 glioma cell line, on the abundance of the GLUT1 transcript in ECL cells. BBH induced a significant increase in the abundance of both GLUT1 and actin mRNAs, and this effect was dose and time dependent. The increase in the GLUT1 mRNA levels correlated with an increase in the transcriptional rate of this gene measured by nuclear run-on experiments. C6 conditioned media and C6 plasma membranes had no effect on the abundance of either GLUT1 or actin mRNA. To determine whether known growth factors cause BBH-like induction of GLUT1 and actin mRNAs, a series of growth factors was also tested. EGF and PDGF had no effect on the levels of these mRNAs. Basic FGF had a moderate effect and TNF alpha partially mimicked the effect of BBH on both GLUT1 and actin transcripts. The present data suggests that brain-derived trophic factors present in BBH stimulate BBB-GLUT1 glucose transporter gene expression in ECL cells through a transcriptional mechanism. Although this effect was partially mimicked by TNF alpha, C6 cell membranes or C6 conditioned media were unable to induce changes in the abundance of GLUT1 mRNA. Therefore, BBH may be a useful model to study the characterization of soluble brain-derived trophic factors involved in the induction of BBB-GLUT1 gene expression.

Cloned mouse cerebrovascular endothelial cells that maintain their differentiation markers for factor VIII, low density lipoprotein, and angiotensin-converting enzyme

This communication describes a relatively novel cell culture technique for the isolation of cerebrovascular endothelial cells from three strains of inbred mice. Cerebrovascular endothelial cells were identified by their morphology, the presence of Factor VIII-related antigen and angiotensin-converting enzyme, and the uptake of acetylated low-density lipoprotein. Cloned cerebrovascular endothelial cells were found to maintain their differentiated state and diploid genotype through 15 serial passages. The morphology and growth characteristics of these cells were found to be altered when cultured on different extracellular matrices. The isolation and cloning methods described are simple and highly reproducible.

Drug transport across the blood-brain barrier. II. Experimental techniques to study drug transport

This is part II of a review on the transport of drugs across the blood-brain barrier. In this part, the emphasis is on the various experimental techniques that can be used to characterize the blood-brain barrier transport of drugs. Generally speaking, three approaches can be distinguished: in vitro techniques using isolated brain capillaries, cerebrovascular endothelial cells in primary culture or endothelium-derived cell lines; in vivo techniques (both single-passage and multi-passage techniques) and in situ perfusion techniques. Each of these techniques has specific advantages and disadvantages associated with it. Therefore, in many instances, a combination of different approaches is needed to study the fundamental aspects of drug transport across the blood-brain barrier.

Effect of L-fucose on proliferation and myo-inositol metabolism in cultured cerebral microvessel and aortic endothelial cells

Decreased myo-inositol metabolism possibly contributes to the development of diabetic complications including micro and macrovascular disease. Previous studies have shown that hyperglycemia may be partially responsible for this defect. We have found that L-fucose, a monosaccharide present in low concentrations in normal circulation and found to be elevated in diabetes, causes defects in cultured endothelial cells, including alterations in myo-inositol metabolism and proliferation. Murine cerebral microvessel and bovine aortic endothelial cells take up L-fucose from the medium in a time and concentration-dependent manner. Both acute and chronic exposure of these cultured endothelial cells to media containing L-fucose at concentrations that may exist in diabetic sera cause a significant decrease in the accumulation of myo-inositol and its incorporation into inositol phospholipids. There is a concomitant decrease in the intracellular levels of myo-inositol. Kinetic analysis of the effect of L-fucose on myo-inositol uptake suggests that L-fucose competitively inhibits the transport of myo-inositol, exhibiting a Ki in the range of 1.6-4.1 mM for both cell types. Endothelial cells exposed to L-fucose concentrations of 0.5-20 mM exhibit depressed rates of proliferation in a concentration-dependent fashion. Furthermore, L-fucose causes a concentration-dependent decrease in synthesis of proteoglycan by cultured cerebral microvessel endothelial cells as measured by incorporation of 35S; however, this effect is not observed in the aortic endothelia. These data suggest that L-fucose at concentrations that may exist in diabetic sera may impair myo-inositol metabolism and proliferation of the vascular endothelium.

Characterization of a rat retinal endothelial cell culture and the expression of P-glycoprotein in brain and retinal endothelium in vitro

Retinal vascular endothelia form one aspect of the blood-retinal barrier and, like the blood-brain barrier, control the passage of molecules and cells into the parenchyma. To facilitate comparative in vitro studies, rat retinal endothelial cells have been cultured and characterised. Using immunocytochemical techniques, retinal endothelium was positive for von Willebrand's factor, tight junction-associated polypeptide (ZO-1) and the transferrin receptor. The cells also expressed high-affinity uptake of acetylated low-density lipoprotein. Using the monoclonal antibodies JSB-1 and C219, the product of the multidrug resistance gene, P-glycoprotein, was found to be expressed on primary cultures of both brain and retinal endothelium.

Receptor-linked hydrolysis of phosphoinositides and production of prostacyclin in cerebral endothelial cells

The receptor agonist-mediated hydrolysis of phosphoinositides and production of prostacyclin were studied in murine cerebral endothelial cells (MCEC). Of 11 neurotransmitters and neuromodulators examined, carbachol, noradrenaline (NE), bradykinin, and thrombin significantly increased 3H-inositol phosphate accumulation in the presence of LiCl (20 mM). The maximal stimulation of [3H]inositol monophosphate ([3H]IP1) reached approximately 11, 11, seven, and four times the basal levels for carbachol, NE, bradykinin, and thrombin, respectively. The EC50 values of IP1 accumulation for carbachol and NE were 34 and 0.16 microM, respectively. The muscarinic antagonists, atropine and pirenzepine, blocked the carbachol-induced IP1 accumulation with Ki values of 0.3 and 30 nM, respectively. The adrenergic antagonist, prazosin, blocked NE-induced IP1 accumulation with a Ki of 0.1 nM. The calcium ionophore A23187, histamine, glutamate, vasopressin, serotonin, platelet activating factor, and substance P did not stimulate IP1 accumulation. A23187, bradykinin, and thrombin stimulated prostacyclin release to approximately four, four, and two times the basal levels, respectively, whereas carbachol and NE had little effect upon prostacyclin release. These results suggest that the activation of phospholipase C and of phospholipase A2 in MCEC are regulated separately.

Chapter 17: Blood-brain barrier: a molecular approach to its structural and functional characterization

Our approach to analyze molecular components of the blood-brain barrier led to the identification of additional transcripts which can be regarded as "BBB markers". Other candidates are presently analyzed in order to find hitherto unknown cell type-specific transcripts. We investigated the expression of these marker-genes in cell culture and found all genes still being transcribed after 10 days in primary cultures, although at a lower level. This is surprising, since other authors report the disappearance of BBB characteristics under such conditions. Moreover, the BBB marker gamma-GT is found to be not only expressed in BMEC, but also in the closely associated pericytes. The hitherto unknown physiological function of the enzyme, especially the abundance in pericytes is still under investigation. Since the method of subtractive cloning has been proven as a fruitful approach, we consider to establish further subtractive cDNA libraries, using different subtraction parameters. The PCR method is applicable for amplification of subtracted cDNA (Timblin et al., 1990) and we expect to find additional clones, mainly of lower abundance which are of functional importance for the BBB phenomenon. The described characterization of cultured BMEC now allows to proceed to study BBB-specific gene expression with special regard to regulatory elements. We will perform these experiments by use of enhancer trap vectors transfected into BMEC. The isolation of the corresponding genomic DNA fragments of the BBB markers is in progress.

The cerebral microvessels in culture, an update

Recent advances in our knowledge of the blood-brain barrier (BBB) have in part been made by studying the properties and function of cerebral endothelial cells in vitro. After an era of working with a fraction, enriched in cerebral microvessels by centrifugation, the next generation of in vitro BBB model systems was introduced, when the conditions for routinely culturing the endothelial cells were established. This review summarizes the results obtained from this rapidly growing field. It can be stated with certainty that, in addition to providing a better insight into the chemical composition of cerebral endothelial cells, much has been learned from these studies about the characteristics of transport processes and cell-to-cell interactions during the last 12 years. With the application of new technologies, the approach offers a new means of investigation, applicable not only to biochemistry and physiology but also to the drug research, and may improve the transport of substances through the BBB. The in vitro approach has been and should remain an excellent model of the BBB to help unravel the complex molecular interactions underlying and regulating the permeability of the cerebral endothelium.

Differential metabolism of hydroxyeicosatetraenoic acid isomers by mouse cerebromicrovascular endothelium

Hydroxyeicosatetraenoic acid (HETE) derivatives of arachidonic acid are produced in the brain and have been implicated as pathologic mediators in various types of brain injury. To understand better their fate in the brain, particularly in cerebral microvessels, several HETEs were incubated with cultured mouse cerebromicrovascular endothelium for 1, 2, and 4 h, followed by HPLC analysis of medium and cellular lipids. 5(S)-, 8(RS)-, and 9(RS)-HETE were not metabolized by the cells, but were extensively incorporated, unmodified, into cell lipids. On the other hand, 11(RS)-, 12(S)-, and 15(S)-HETE were extensively metabolized and only minimally incorporated into cell lipids. Previously, the major 12-HETE metabolite was identified as 8-hydroxyhexadecatrienoic acid. In the present study, we identified the major 11-HETE metabolite as 7-hydroxyhexadecatrienoic acid and the major 15-HETE metabolite as 11-hydroxyhexadecatrienoic acid. omega-3 compounds, 15(S)- and 12(S)-hydroxyeicosapentaenoic acids (HEPE), were also metabolized to more polar compounds, but to a lesser extent than their tetraenoic acid, omega-6 counterparts. Comparison of 5-, 12-, and 15-HETE enantiomers revealed no differences in metabolism or incorporation between the R and S stereoisomers. These data suggest that many isomers of HETE and HEPE can be incorporated into cell lipids or metabolized by pathways that do not distinguish between enantiomers. These pathways, however, are sensitive to the position or number of double bonds and are selective based on the position of the hydroxyl group.

Isolation of arteriolar microvessels and culture of smooth muscle cells from cerebral cortex of guinea pig

Published methods for the isolation of cerebral microvessels primarily yield terminal resistance vessels and capillary networks, not the more proximal, subpial penetrating arterioles desired for certain studies. We report a novel method for isolating microvessels from the cerebral cortex of a single guinea-pig brain that yields large arteriolar complexes that are up to 50% intact. Instead of using homogenization to disperse brain parenchyma, we digested cortical fragments with trypsin, gently dispersed the parenchyma mechanically, and recovered microvascular complexes by sieving. Phase-contrast and electron microscopy showed primary (penetrating) arterioles, secondary arterioles, and capillary networks that frequently were in continuity as intact microvascular units. Culture of microvascular cells was carried out by enzymatic dissociation followed by an overnight incubation in a recovery medium at 4 degrees C before plating onto fibronectin-modified surfaces. Viability of isolated cells was demonstrated by good cell attachment and prompt proliferation that resulted in confluent cultures after 10 days. Confluent secondary cultures demonstrated characteristic features of smooth muscle cells, including a "hill-and-valley" growth pattern and expression of alpha-actin. Less than 1% of cells were endothelial or astrocytic cells by immunocytochemical and morphologic criteria. Ultrastructural studies demonstrated evidence of a synthetic phenotype of smooth muscle cell and absence of a significant number of fibroblasts. This method demonstrates that viable smooth muscle cells from the cerebral parenchymal microvasculature can be isolated in bulk quantities for study in vitro.

Isolation and characterization of microvessels from normal brain and brain tumors

We describe a new technique for isolating microvessels from both brain and brain tumors. This method is relatively quick and provides a microvessel preparation free of contamination by other brain tissue. Using this method, structurally intact microvessels from normal rat brain and from a malignant rat astrocytoma were isolated and characterized with light microscopy, scanning electron microscopy and transmission electron microscopy. In contrast to microvessels derived from normal rat brain, rat astrocytoma microvessels had endothelial cells with multilayered basement membranes, extensive microvilli on the cell surfaces, and a significant increase in the number of pinocytes in the cytoplasm. Furthermore, astrocytoma microvessel endothelial cells had pleomorphic electron dense nuclei with pale perichromatin, whereas the nuclei of endothelial cells of microvessels derived from normal brain tissue were finely granular and homogeneous with characteristically electron dense perichromatin. The morphologic characteristics of the astrocytoma microvessels are similar to the histologic changes seen in astrocytoma tissue in situ, and correlate well with the known altered functions of brain tumor neovasculature.

Acute and chronic exposure of mouse cerebral microvessel endothelial cells to increased concentrations of glucose and galactose: effect on myo-inositol metabolism, PGE2 synthesis, and Na+/K(+)-ATPase transport activity

Cultured mouse cerebral microvessel endothelial cells have a large intracellular myo-inositol content and rapidly take up extracellular myo-inositol. Myo-inositol uptake occurs by a high- and low-affinity transport system. Both transport systems appear to be Na(+)-dependent. The high- and low-affinity transport systems have a Km of 11 and 198 mumol/L and a Vmax of 47 and 381 pmol/min/mg protein, respectively. Acute exposure of cultured cells to 30 mmol/L D-glucose or D-galactose causes a decrease in myo-inositol uptake. The acute effect of glucose and galactose on myo-inositol uptake is sensitive to the extracellular myo-inositol concentration. The acute effect of glucose is apparently due to a competitive inhibition of high-affinity myo-inositol transport and has a Ki of 21 mmol/L. L-Glucose is more effective than D-glucose in decreasing myo-inositol uptake. In contrast, 2-deoxyglucose or 3-0-methylglucose does not acutely inhibit myo-inositol uptake. This suggests that the hydroxyl groups on carbons 2 and 3 of glucose are necessary for inhibitory activity. Chronic exposure of cells to media containing 136.4 mumol/L myo-inositol and 30 mmol/L glucose has no effect on myo-inositol accumulation from the extracellular fluid, myo-inositol incorporation into inositol phospholipids, or total myo-inositol content. Chronic exposure of the cells to media containing 30 mmol/L glucose causes only a small increase in the intracellular sorbitol content. In contrast, chronic exposure of the cells to media containing 30 mmol/L galactose causes a large increase in galactitol content and a decrease in myo-inositol accumulation, myo-inositol incorporation into inositol phospholipids, and intracellular myo-inositol content. Sorbinil treatment of the galactose-supplemented media protects the cells form changes in myo-inositol metabolism and content. Chronic exposure of the cells to media containing 30 mmol/L glucose or 30 mmol/L galactose causes a decrease in ouabain-sensitive Na+/K(+)-ATPase transport activity, which is corrected by the addition of sorbinil to the media. Chronic exposure of the cells to media containing 45 mmol/L glucose, but not galactose, causes an increase in PGE2 production. These studies suggest that acute or chronic exposure of cultured microvessel endothelial cells to increased concentrations of glucose or galactose causes a decrease in myo-inositol uptake by different mechanisms. Chronic exposure of the cells to increased concentrations of glucose or galactose causes alterations in endothelial cell properties, including Na+/K(+)-ATPase transport activity and eicosanoid synthesis. The data are not clearly supportive of polyol accumulation and myo-inositol depletion as being responsible for the decrease in Na+/K+ pump activity.

Blood-brain barrier: transport studies in isolated brain capillaries and in cultured brain endothelial cells

The development of in vitro BBB models consisting of isolated brain capillaries and cultured brain microvessel endothelial cells has made possible the study of BBB transport phenomena at the cellular level. Basic characteristics of BBB transport of endogenous and exogenous solutes and their biochemical, pharmacological, ontogenic, and pathological regulation mechanisms have been investigated. This information has led not only to a better understanding of BBB transport but also to the construction of strategies for improving drug delivery to the CNS for diagnosis and therapeutics. To elucidate the complexity of BBB transport, in vivo studies are always necessary at some point; however, in vitro systems can be useful complements to the in vivo systems. The tissue culture systems seem to be especially important in the clarification of cellular, biochemical and molecular features of BBB transport. Appropriate systems should be selected or combined, depending on the purpose of the investigation.

Role of the blood-brain barrier in the formation of long-chain omega-3 and omega-6 fatty acids from essential fatty acid precursors

Elongated, more highly polyunsaturated derivatives of linoleic acid (18:2 omega-6) and linolenic acid (18:3 omega-3) accumulate in brain, but their sites of synthesis and mechanism of entry are not well characterized. To investigate the role of the blood-brain barrier in this process, cultured murine cerebromicrovascular endothelia were incubated with [1-14C]18:2 omega-6 or [1-14C]18:3 omega-3 and their elongation/desaturation products determined. The major metabolite of 18:2 omega-6 was 20:4 omega-6, whereas the primary product from 18:3 omega-3 was 20:5 omega-3. Although these products were found primarily in cell lipids, they were also released from the cells and gradually accumulated in the extracellular fluid. Eicosanoid production was observed from the 20:4 omega-6 and 20:5 omega-3 that were formed. No 22:5 omega-6 or 22:6 omega-3 fatty acids were detected, suggesting that these endothelial cells are not the site of the final desaturation step. Although the uptake of 18:3 omega-3 and 18:2 omega-6 was nearly identical, 18:3 omega-3 was more extensively elongated and desaturated. Competition experiments demonstrated a preference for 18:3 omega-3 by the elongation/desaturation pathway. These findings suggest that the blood-brain barrier can play an important role in the elongation and desaturation of omega-3 and omega-6 essential fatty acids during their transfer from the circulation into the brain.

Quantitative detection of blood-brain barrier-associated enzymes in cultured endothelial cells of porcine brain microvessels

The present study deals with a rapid and convenient assay for blood-brain barrier (BBB)-associated enzymes, gamma-glutamyl transpeptidase (gamma-GTP) and alkaline phosphatase (ALP), in cultured endothelial cells and other cells. These enzyme activities in cultured cells could be efficiently measured by direct incubation of each substrate in the culture plates without pretreatment of the cells. This new direct in situ-in plate assay was more rapid and convenient than conventional ex-plate assays, and these assays gave similar values for specific enzyme activities. gamma-GTP and ALP activities could be detected by this in situ method in primary-cultured endothelial cells of porcine brain microvessels, but their levels were lower than those before culture. The degree of loss due to culture differed between gamma-GTP and ALP; a relatively large amount of ALP remained but the gamma-GTP level decreased greatly. In this direct in situ-in plate assay, cultured porcine aortic endothelial cells exhibited negligibly small activities for both enzymes, whereas cultured astroglial cells of neonatal porcine brain showed moderate gamma-GTP activity and a trace of ALP activity. This direct in situ-in plate assay can be used for microculture and automatic measurement and offers a convenient means for studying the possible regulatory mechanisms of the expression of the BBB-associated enzymes.