Blood-brain Barrier Transport Systems Research Articles

Silica Nanoparticles Decrease Glutamate Uptake in Blood–Brain Barrier Components

Glutamate is the major excitatory amino acid in the vertebrate brain, playing an important role in most brain functions. It exerts its activity through plasma membrane receptors and transporters, expressed both in neurons and glia cells. Overstimulation of neuronal glutamate receptors is linked to cell death in a process known as excitotoxicity, that is prevented by the efficient removal of the neurotransmitter through glutamate transporters enriched in the glia plasma membrane and in the components of the blood–brain barrier (BBB). Silica nanoparticles (SiO2-NPs) have been widely used in biomedical applications and directed to enter the circulatory system; however, little is known about the potential adverse effects of SiO2-NPs exposure on the BBB transport systems that support the critical isolation function between the central nervous system (CNS) and the peripheral circulation. In this contribution, we investigated the plausible SiO2-NPs-mediated disruption of the glutamate transport system expressed by BBB cell components. First, we evaluated the cytotoxic effect of SiO2-NPs on human brain endothelial (HBEC) and Uppsala 87 Malignant glioma (U-87MG) cell lines. Transport kinetics were evaluated, and the exposure effect of SiO2-NPs on glutamate transport activity was determined in both cell lines. Exposure of the cells to different SiO2-NP concentrations (0.4, 4.8, 10, and 20 µg/ml) and time periods (3 and 6 h) did not affect cell viability. We found that the radio-labeled D-aspartate ([3H]-D-Asp) uptake is mostly sodium-dependent, and downregulated by its own substrate (glutamate). Furthermore, SiO2-NPs exposure on endothelial and astrocytes decreases [3H]-D-Asp uptake in a dose-dependent manner. Interestingly, a decrease in the transporter catalytic efficiency, probably linked to a diminution in the affinity of the transporter, was detected upon SiO2-NPs. These results favor the notion that exposure to SiO2-NPs could disrupt BBB function and by these means shed some light into our understanding of the deleterious effects of air pollution on the CNS.

Characterization of a Primate Blood-Brain Barrier Co-Culture Model Prepared from Primary Brain Endothelial Cells, Pericytes and Astrocytes.

Culture models of the blood-brain barrier (BBB) are important research tools. Their role in the preclinical phase of drug development to estimate the permeability for potential neuropharmaceuticals is especially relevant. Since species differences in BBB transport systems exist, primate models are considered as predictive for drug transport to brain in humans. Based on our previous expertise we have developed and characterized a non-human primate co-culture BBB model using primary cultures of monkey brain endothelial cells, rat brain pericytes, and rat astrocytes. Monkey brain endothelial cells in the presence of both pericytes and astrocytes (EPA model) expressed enhanced barrier properties and increased levels of tight junction proteins occludin, claudin-5, and ZO-1. Co-culture conditions also elevated the expression of key BBB influx and efflux transporters, including glucose transporter-1, MFSD2A, ABCB1, and ABCG2. The correlation between the endothelial permeability coefficients of 10 well known drugs was higher (R2 = 0.8788) when the monkey and rat BBB culture models were compared than when the monkey culture model was compared to mouse in vivo data (R2 = 0.6619), hinting at transporter differences. The applicability of the new non-human primate model in drug discovery has been proven in several studies.

Blood-brain barrier from the point of view of anesthesiologist. Review. Part 1

The blood-brain barrier prevents the penetration of neurotoxic components of plasma, blood cells and pathogens into the brain. At the same time, it regulates the transport of molecules in the central nervous system, maintaining a strictly controlled chemical composition of the neuronal environment. This review discusses the molecular and cellular mechanisms that underlie the functioning of the blood-brain barrier, the physiology of the blood-brain barrier transport systems, the structural components of the blood-cerebrospinal fluid barrier and the intrathecal administration of drugs. Based on the physicochemical properties of antibacterial drugs, we estimated which compounds are currently the most promising for the treatment of infections of the central nervous system.

Сentral and peripheral mechanisms of mu-opioid analgesia and tolerance

Objective – An analysis of the basic science and clinical publications found in PubMed, Medline, and Web of Science. The search covered modern laboratory and clinical mechanisms of peripheral mu opioid analgesia, the role of peripheral mu receptors in systemic analgesia and the development of tolerance to the analgesic effect of opioids. The review discusses the regulatory mechanisms of synthesis and transport of mu-opioid receptors in the primary afferent neurons and the molecular mechanisms responsible for modulating the conduction of nociceptive information from the periphery to the spinal cord. According to some authors, the peripheral component can account for 50-90% of the total analgesic effect after the systemic administration of morphine and methadone. The review reports on the important role of glycoprotein-P and the blood-brain barrier transport system in modulating the peripheral component of the analgesic effect of morphine as well as the synergistic interaction between central and peripheral mu receptors. The results of the reviewed studies convincingly show the key role of peripheral mu receptors in the development of tolerance to the analgesic effect of morphine after its systemic administration. The mechanisms of opioid tolerance also involve peripheral anti-opioid, pronociceptive systems such as NMDA receptors. It is well known that the same mechanisms are involved in maintaining peripheral hyperalgesia and allodynia. The development of analgesic drugs that act on peripheral antinociceptive systems offers a promising perspective on the possible treatment of acute and chronic pain.

Intravenous infusion of mesenchymal stem cells improves impaired cognitive function in a cerebral small vessel disease model

Cerebral small vessel disease (CSVD) is not only a cause of vascular dementia (VD) but also a contributing factor to Alzheimer's disease (AD). The essential pathological feature of CSVD is the disruption of blood–brain barrier (BBB). Dysfunction of BBB due to degeneration of both endothelial cells and pericytes in capillaries leads to neuronal damage and progressive brain atrophy. Moreover, deterioration of amyloid-β (Aβ) clearance due to the failure of the transvascular BBB transport system results in accumulation of Aβ in the brain. Intravenous infusion of mesenchymal stem cells (MSCs) elicits functional recovery in experimental models including stroke and spinal cord injury. One effect of MSCs is to restore disrupted BBB through remodeling of microvasculature. Using spontaneously hypertensive rats (stroke-prone) with impaired cognitive function as a CSVD model, we have shown that infused MSCs has a therapeutic effect for cognitive function. Restoration of BBB function via remodeling of microvasculature and inhibition of Aβ accumulation could inhibit progressive brain atrophy and lead to restore cognitive dysfunction. Gene expression analysis indicated that infused MSCs activates both transforming growth factor-β and angiopoietin 1 signaling pathways and promotes the remodeling of microvasculature. Thus, infused MSCs may represent a novel therapy for both VD and AD.

The effect of nanoparticle size on the ability to cross the blood-brain barrier: an in vivo study.

Our goal was to develop an efficient nanoparticle-based system that can overcome the restrictive mechanism of the blood-brain barrier (BBB) by targeting insulin receptors and would thus enable drug delivery to the brain. Insulin-coated gold nanoparticles (INS-GNPs) were synthesized to serve as a BBB transport system. The effect of nanoparticle size (20, 50 and 70 nm) on their ability to cross the BBB was quantitatively investigated in Balb/C mice. The most widespread biodistribution and highest accumulation within the brain were observed using 20 nm INS-GNPs, 2 h post injection. In vivo CT imaging revealed that particles migrated to specific brain regions, which are involved in neurodegenerative and neuropsychiatric disorders. These findings promote the optimization of nanovehicles for transport of drugs through the BBB. The insulin coating of the particles enabled targeting of specific brain regions, suggesting the potential use of INS-GNPs for delivery of various treatments for brain-related disorders.

Sialic acid (SA)-modified selenium nanoparticles coated with B6 peptide for potential use in Alzheimer's disease

The blood–brain barrier (BBB) is a formidable gatekeeper toward exogenous substances, playing an important role in brain homeostasis and maintaining a healthy microenvironment for complex neuronal activities. However, it also greatly hinders drug permeability into the brain and limits the management of brain diseases. The development of new drugs that show improved transport across the BBB represents a promising strategy for Alzheimer's disease (AD) intervention. Whereas a previous study of receptor-mediated endogenous BBB transport systems has focused on a strategy of using transferrin to facilitate brain drug delivery system, a system that still suffers from limitations including synthesis procedure, stability and immunological response, in the present study, we synthetized sialic acid (SA)-modified selenium (Se) nanoparticles conjugated with an alternative peptide-B6 peptide (B6-SA-SeNPs, a synthetic selenoprotein analogue), which shows high permeability across the BBB and has the potential to serve as a novel nanomedicine for disease modification in AD. Laser-scanning confocal microscopy, flow cytometry analysis and inductively coupled plasma-atomic emission spectroscopy ICP-AES revealed high cellular uptake of B6-SA-SeNPs by cerebral endothelial cells (bEnd.3). The transport efficiency of B6-SA-SeNPs was evaluated in a Transwell experiment based on in vitro BBB model. It provided direct evidence for B6SA-SeNPs crossing the BBB and being absorbed by PC12 cells. Moreover, inhibitory effects of B6-SA-SeNPs on amyloid-β peptide (Aβ) fibrillation could be demonstrated in PC12 cells and bEnd3 cells. B6-SA-SeNPs could not only effectively inhibit Aβ aggregation but could disaggregate preformed Aβ fibrils into non-toxic amorphous oligomers. These results suggested that B6SA-SeNPs might provide a promising platform, particularly for the application of nanoparticles in the treatment of brain diseases.

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice.

BackgroundWe recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.MethodsLPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.ResultsIn both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.ConclusionsThe results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Sialic acid (SA)-modified selenium nanoparticles coated with a high blood–brain barrier permeability peptide-B6 peptide for potential use in Alzheimer’s disease

The blood–brain barrier (BBB) is a formidable gatekeeper toward exogenous substances, playing an important role in brain homeostasis and maintaining a healthy microenvironment for complex neuronal activities. However, it also greatly hinders drug permeability into the brain and limits the management of brain diseases. The development of new drugs that show improved transport across the BBB represents a promising strategy for Alzheimer’s disease (AD) intervention. Whereas, previous study of receptor-mediated endogenous BBB transport systems has focused on a strategy of using transferrin to facilitate brain drug delivery system, a system that still suffers from limitations including synthesis procedure, stability and immunological response. In the present study, we synthetised sialic acid (SA)-modified selenium (Se) nanoparticles conjugated with an alternative peptide-B6 peptide (B6-SA-SeNPs, a synthetic selenoprotein analogue), which shows high permeability across the BBB and has the potential to serve as a novel nanomedicine for disease modification in AD. Laser-scanning confocal microscopy, flow cytometry analysis and inductively coupled plasma-atomic emission spectroscopy ICP-AES revealed high cellular uptake of B6-SA-SeNPs by cerebral endothelial cells (bEnd.3). The transport efficiency of B6-SA-SeNPs was evaluated in a Transwell experiment based on in vitro BBB model. It provided direct evidence for B6-SA-SeNPs crossing the BBB and being absorbed by PC12 cells. Moreover, inhibitory effects of B6-SA-SeNPs on amyloid-β peptide (Aβ) fibrillation could be demonstrated in PC12 cells and bEnd3 cells. B6-SA-SeNPs could not only effectively inhibit Aβ aggregation but could disaggregate preformed Aβ fibrils into non-toxic amorphous oligomers. These results suggested that B6-SA-SeNPs may provide a promising platform, particularly for the application of nanoparticles in the treatment of brain diseases. Statement of SignificanceAlzheimer’s disease (AD) is the world’s most common form of dementia characterized by intracellular neurofibrillary tangles in the brain. Over the past decades, the blood–brain barrier (BBB) limits access of therapeutic or diagnostic agents into the brain, which greatly hinders the development of new drugs for treating AD. In this work, we evaluated the efficiency of B6-SA-SeNPs across BBB and investigated the interactions between B6-SA-SeNPs and amyloid-β peptide (Aβ). We confirm that B6-SA-SeNPs could provide a promising platform because of its high brain delivery efficiency, anti-amyloid properties and anti-oxidant properties, which may serve as a novel nanomedicine for the application in the treatment of brain diseases.

B6 Peptide-Modified PEG-PLA Nanoparticles for Enhanced Brain Delivery of Neuroprotective Peptide

The blood-brain barrier (BBB), which is formed by the brain capillary wall, greatly hinders the development of new drugs for the brain. Over the past decades, among the various receptor-mediated endogenous BBB transport systems, the strategy of using transferrin or anti-transferrin receptor antibodies to facilitate brain drug delivery system is of particular interest. However, the application of large proteins still suffers from the drawbacks including synthesis procedure, stability, and immunological response. Here, we explored a B6 peptide discovered by phase display as a substitute for transferrin, and conjugated it to PEG-PLA nanoparticles (NP) with the aim of enhancing the delivery of neuroprotective drug across the BBB for the treatment of Alzheimer's disease. B6-modified NP (B6-NP) exhibited significantly higher accumulation in brain capillary endothelial cells via lipid raft-mediated and clathrin-mediated endocytosis. In vivo, fluorescently labeled B6-NP exhibited much higher brain accumulation when compared with NP. Administration of B6-NP encapsulated neuroprotective peptide-NAPVSIPQ (NAP)-to Alzheimer's disease mouse models showed excellent amelioration in learning impairments, cholinergic disruption, and loss of hippocampal neurons even at lower dose. These findings together suggested that B6-NP might serve as a promising DDS for facilitating the brain delivery of neuropeptides.

AGT-181: Expression in CHO cells and pharmacokinetics, safety, and plasma iduronidase enzyme activity in Rhesus monkeys

Enzyme replacement therapy is not effective for the brain, owing to the lack of transport of the enzyme across the blood–brain barrier (BBB). Recombinant proteins such as the lysosomal enzyme, iduronidase, can penetrate the human BBB, following the re-engineering of the protein as an IgG fusion protein, where the IgG moiety targets an endogenous BBB transport system. The IgG acts as a molecular Trojan horse to ferry the fused protein into brain. AGT-181 is a genetically engineered fusion protein of human iduronidase and a chimeric monoclonal antibody against the human insulin receptor. Adult Rhesus monkeys were administered repeat intravenous doses of AGT-181 ranging from 0.2 to 20 mg/kg. Chronic AGT-181 dosing resulted in no toxicity at any dose, no changes in organ histology, no change in plasma or cerebrospinal fluid glucose, and no significant immune response. AGT-181 was rapidly removed from plasma, based on measurements of either plasma immunoreactive AGT-181 or plasma iduronidase enzyme activity. Plasma pharmacokinetics analysis showed a high systemic volume of distribution, and a clearance rate comparable to a small molecule. The safety pharmacology studies provide the basis for future drug development of AGT-181 as a new therapeutic approach to treatment of the brain in Hurler's syndrome.

Endothelial Protein C Receptor-Assisted Transport of Activated Protein C across the Mouse Blood—Brain Barrier

Activated protein C (APC), a serine-protease with anticoagulant, anti-inflammatory, and cytoprotective activities, is neuroprotective and holds potential to treat different neurologic disorders. It is unknown whether APC crosses the blood-brain barrier (BBB) to reach its therapeutic targets in the brain. By using a brain vascular perfusion technique, we show that (125)I-labeled plasma-derived mouse APC enters the brain from cerebrovascular circulation by a concentration-dependent mechanism. The permeability surface area product of (125)I-APC (0.1 nmol/L) in different forebrain regions ranged from 3.11 to 4.13 microL/min/g brain. This was approximately 80- to 110-fold greater than for (14)C-inulin, a simultaneously infused reference tracer. The K(m) value for APC BBB cortical transport was 1.6+/-0.2 nmol/L. Recombinant APC variants with reduced anticoagulant activity, 5A-APC and 3K3A-APC, but not protein C, exhibited high affinity for the APC BBB transport system. Blockade of APC-binding site on endothelial protein C receptor (EPCR), but not blockade of its protease-activated receptor-1 (PAR1) catalytic site, inhibited by >85% APC entry into the brain. APC brain uptake was reduced by 64% in severely deficient EPCR mice, but not in PAR1 null mice. These data suggest that APC and its variants with reduced anticoagulant activity cross the BBB via EPCR-mediated saturable transport.

Contribution of Carrier-Mediated Transport Systems to the Blood–Brain Barrier as a Supporting and Protecting Interface for the Brain; Importance for CNS Drug Discovery and Development

The blood-brain barrier (BBB) forms an interface between the circulating blood and the brain and possesses various carrier-mediated transport systems for small molecules to support and protect CNS function. For example, the blood-to-brain influx transport systems supply nutrients, such as glucose and amino acids. Consequently, xenobiotic drugs recognized by influx transporters are expected to have high permeability across the BBB. On the other hand, efflux transporters, including ATP-binding cassette transporters such as P-glycoprotein located at the luminal membrane of endothelial cells, function as clearance systems for metabolites and neurotoxic compounds produced in the brain. Drugs recognized by these transporters are expected to show low BBB permeability and low distribution to the brain. Despite recent progress, the transport mechanisms at the BBB have not been fully clarified yet, especially in humans. However, an understanding of the human BBB transport system is critical, because species differences mean that it can be difficult to extrapolate data obtained in experimental animals during drug development to humans. Recent progress in methodologies is allowing us to address this issue. Positron emission tomography can be used to evaluate the activity of human BBB transport systems in vivo. Proteomic studies may also provide important insights into human BBB function. Construction of a human BBB transporter atlas would be a most important advance from the viewpoint of CNS drug discovery and drug delivery to the brain.

薬物の生体膜輸送特性とＤＤＳ　　脳関門輸送の分子機構と脳へのＤＤＳ

薬物の生体膜輸送特性とＤＤＳ　　脳関門輸送の分子機構と脳へのＤＤＳ

The Physiological Function of the Blood‐Brain Barrier Transporters as the CNS Supporting and Protecting System

AbstractFor Abstract see ChemInform Abstract in Full Text.

Blood-brain Barrier Drug Discovery for Central Nervous System Infections

Central nervous system (CNS) infections are formidable diseases with high rates of morbidity and mortality. Since the majority of antimicrobial agents discovered so far do not cross the blood-brain barrier (BBB), the treatment of CNS infections is a major challenge issue. The development of drugs to treat those diseases requires consideration of achievable brain concentrations by targeting the following question. How can the chemistry and biology of the BBB, and infectomics be exploited for the development of drugs against CNS infections? To date drug targeting approaches, such as chemistry-based, biology-based, and infectomics-based, have been implicated in the development of drugs for treatment of CNS infections. The chemistry-based strategies rely on lipid-mediated BBB drug transport as substances that readily permeate the BBB. These usually include small molecular weight of lipophilic or hydrophobic molecules. The biology-based strategies depend on endogenous BBB transport systems, including carrier-mediated transport (CMT), active efflux transport (AET), and receptor-mediated transport (RMT). These transporters play important roles in the influxes and/or effluxes of drugs including antimicrobial agents in brain capillary endothelial cells that form the BBB. Both microbial and host signatures of infectomes, which can be dissected by infectomics, provide invaluable fountains in the search for novel antimicrobial therapies. Key markers associated with the mechanisms of neuronal injury may be identified, and thus, provide important targets for the prevention and treatment of CNS infections. This review focuses on the major BBB drug targeting strategies in the development of therapeutics for CNS infections. A combination of these strategies will ultimately lead to improved treatments.

The brain-to-blood efflux transport of taurine and changes in the blood–brain barrier transport system by tumor necrosis factor-α

The purpose of this study was to examine whether the efflux transport system for taurine from brain to blood is present at the blood–brain barrier (BBB) by using the brain efflux index (BEI) method and to determine whether the taurine transport system is regulated after central nervous system cell damage by tumor necrosis factor-α (TNF-α) in vivo. [ 3H]Taurine was microinjected into the parietal cortex area 2 of the rat brain, and was eliminated from the brain with an efflux transport rate of 1.22×10 −2 min −1, and the process is saturable with a K m of 39.1 μM. This process was significantly inhibited by taurine transporter (TAUT) inhibitors, such as unlabeled taurine, β-alanine, betaine, nipecotic acid and γ-aminobutyric acid (GABA). In addition, the effect of tumor necrosis factor-α on [ 3H]taurine transport was investigated. [ 3H]Taurine uptake was increased and its efflux was reduced by pretreatment with tumor necrosis factor-α. Also, [ 3H]taurine efflux was reduced by tumor necrosis factor-α in a time- and dose-dependent manner. In conclusion, there is the efflux pump for taurine at the blood–brain barrier to reduce taurine concentration in the brain interstitial fluid, and this process was carrier mediated. In addition, the transport system for taurine through the blood–brain barrier was found to be regulated by tumor necrosis factor-α in vivo.

The Physiological Function of the Blood-Brain Barrier Transporters as the CNS Supporting and Protecting System

The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain and restricts drug permeability into the brain. Our latest studies have revealed that the BBB transporters play important physiological roles in maintaining the brain environment. For an energy-storing system, the creatine transporter localized at the brain capillary endothelial cells (BCECs) mediates the supply of creatine from the blood to the brain. The BBB is involved in the brain-to-blood efflux transport of gamma-aminobutyric acid, and GAT2/BGT-1 mediates this transport process. BCECs also express serotonin and norepinephrine transporters. Organic anion transporter 3 (OAT3) and ASCT2 are localized at the abluminal membrane of the BCECs. OAT3 is involved in the brain-to-blood efflux of a dopamine metabolite, a uremic toxin, and thiopurine nucleobase analogues. ASCT2 plays a role in L-isomer-selective aspartic acid efflux transport at the BBB. Dehydroepiandrosterone sulfate and small neutral amino acids undergo brain-to-blood efflux transport mediated by organic anion transporting polypeptide 2 and ATA2, respectively. The BBB transporters are regulated by various factors: ATA2 by osmolarity, taurine transporter by tumor necrosis factor-alpha, and L-cystine/L-glutamic acid exchange transporter by oxidative stress. Clarifying the physiological roles of BBB transport systems should give important information allowing the development of better central nervous system (CNS) drugs and improving our understanding of the relationship between CNS disorders and BBB function.

New aspects of the blood-brain barrier transporters; its physiological roles in the central nervous system.

The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain, and restricts drug permeability into the brain. Our latest studies have revealed that the BBB transporters play important physiological roles in maintaining the brain milieu. The BBB supplies creatine to the brain for an energy-storing system, and creatine transporter localized at the brain capillary endothelial cells (BCECs) is involved in BBB creatine transport. The BBB is involved in the brain-to-blood efflux transport of the suppressive neurotransmitter, gamma-aminobutyric acid, and GAT2/BGT-1 mediates this transport process. BCECs also express serotonin and norepinephrine transporters. Organic anion transporter 3 (OAT3) and ASCT2 are localized at the abluminal membrane of the BCECs. OAT3 is involved in the brain-to-blood efflux of a dopamine metabolite, a uremic toxin and thiopurine nucleobase analogs. ASCT2 plays a role in L-isomer-selective aspartic acid efflux transport at the BBB. Dehydroepiandrosterone sulfate and small neutral amino acids undergo brain-to-blood efflux transport mediated by organic anion transporting polypeptide 2 and ATA2, respectively. The BBB transporters are regulated by various factors, ATA2 by osmolarity, taurine transporter by TNF-alpha, and L-cystine/L-glutamic acid exchange transporter by oxidative stress. Clarifying the physiological roles of BBB transport systems should give us important information allowing the development of better CNS drugs and improving our understanding of the relationship between CNS disorders and BBB function.

Effect of reduced flow on blood–brain barrier transport systems

Pathological states (i.e. stroke, cardiac arrest) can lead to reduced blood flow to the brain potentially altering blood–brain barrier (BBB) permeability and regulatory transport functions. BBB disruption leads to increased cerebrovascular permeability, an important factor in the development of ischemic brain injury and edema formation. In this study, reduced flow was investigated to determine the effects on cerebral blood flow (CBF), pressure, basal BBB permeability, and transport of insulin and K + across the BBB. Anesthetized adult female Sprague–Dawley rats were measured at normal flow (3.1 ml min −1), half flow (1.5 ml min −1), and quarter flow (0.75 ml min −1), using bilateral in situ brain perfusion for 20 min followed by capillary depletion analysis. Reduction in perfusion flow rates demonstrated a modest reduction in CBF (1.27–1.56 ml min −1 g −1), a decrease in pressure, and no significant effect on basal BBB permeability indicating that autoregulation remained functional. In contrast, there was a concomittant decrease in BBB transport of both insulin and K + with reduced flow. At half and quarter flow, insulin transport was significantly reduced ( R Br%=17.2 and R Br%=16.2, respectively) from control ( R Br%=30.4). Additionally, a significant reduction in [ 86Rb +] was observed at quarter flow ( R Br%=2.5) as compared to control ( R Br%=4.8) suggesting an alteration in ion homeostasis as a result of low flow. This investigation suggests that although autoregulation maintains CBF, BBB transport mechanisms were significantly compromised in states of reduced flow. These flow alterations may have a significant impact on brain homeostasis in pathological states.