Modulation of P-glycoprotein activity by glial factors and retinoic acid in an immortalized rat brain microvessel endothelial cell line

ABCB1 and ABCG2 Regulation at the Blood-Brain Barrier: Potential New Targets to Improve Brain Drug Delivery

The drug efflux transporters ABCB1 and ABCG2 at the blood-brain barrier limit the delivery of drugs into the brain. Strategies to overcome ABCB1/ABCG2 have been largely unsuccessful, which poses a tremendous clinical problem to successfully treat central nervous system (CNS) diseases. Understanding basic transporter biology, including intracellular regulation mechanisms that control these transporters, is critical to solving this clinical problem.In this comprehensive review, we summarize current knowledge on signaling pathways that regulate ABCB1/ABCG2 at the blood-brain barrier. In Section I, we give a historical overview on blood-brain barrier research and introduce the role that ABCB1 and ABCG2 play in this context. In Section II, we summarize the most important strategies that have been tested to overcome the ABCB1/ABCG2 efflux system at the blood-brain barrier. In Section III, the main component of this review, we provide detailed information on the signaling pathways that have been identified to control ABCB1/ABCG2 at the blood-brain barrier and their potential clinical relevance. This is followed by Section IV, where we explain the clinical implications of ABCB1/ABCG2 regulation in the context of CNS disease. Lastly, in Section V, we conclude by highlighting examples of how transporter regulation could be targeted for therapeutic purposes in the clinic. SIGNIFICANCE STATEMENT: The ABCB1/ABCG2 drug efflux system at the blood-brain barrier poses a significant problem to successful drug delivery to the brain. The article reviews signaling pathways that regulate blood-brain barrier ABCB1/ABCG2 and could potentially be targeted for therapeutic purposes.

P450 oxidoreductase regulates barrier maturation by mediating retinoic acid metabolism in a model of the human BBB

The blood-brain barrier (BBB) selectively regulates the entry of molecules into the central nervous system (CNS). A crosstalk between brain microvascular endothelial cells (BMECs) and resident CNS cells promotes the acquisition of functional tight junctions (TJs). Retinoic acid (RA), a key signaling molecule during embryonic development, is used to enhance in vitro BBB models’ functional barrier properties. However, its physiological relevance and affected pathways are not fully understood. P450 oxidoreductase (POR) regulates the enzymatic activity of microsomal cytochromes. POR-deficient (PORD) patients display impaired steroid homeostasis and cognitive disabilities. Here, we used both patient-specific POR-deficient and CRISPR-Cas9-mediated POR-depleted induced pluripotent stem cell (iPSC)-derived BMECs (iBMECs) to study the role of POR in the acquisition of functional barrier properties. We demonstrate that POR regulates cellular RA homeostasis and that POR deficiency leads to the accumulation of RA within iBMECs, resulting in the impaired acquisition of TJs and, consequently, to dysfunctional development of barrier properties.

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Retinoic acid (RA) promotes functional barrier properties

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POR-deficient iPS-brain endothelial-like cells display impaired barrier development

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POR mediates CYP26-dependent cellular RA catabolism

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RA accumulation induces a pro-inflammatory response

Similarities and differences in the localization, trafficking, and function of P-glycoprotein in MDR1-EGFP-transduced rat versus human brain capillary endothelial cell lines

Background

In vitro models based on brain capillary endothelial cells (BCECs) are among the most versatile tools in blood–brain barrier research for testing drug penetration into the brain and how this is affected by efflux transporters such as P-glycoprotein (Pgp). However, compared to freshly isolated brain capillaries or primary BCECs, the expression of Pgp in immortalized BCEC lines is markedly lower, which prompted us previously to transduce the widely used human BCEC line hCMEC/D3 with a doxycycline-inducible MDR1-EGFP fusion plasmid. The EGFP-labeled Pgp in these cells allows studying the localization and trafficking of the transporter and how these processes are affected by drug exposure. Here we used this strategy for the rat BCEC line RBE4 and performed a face-to-face comparison of RBE4 and hCMEC/D3 wild-type (WT) and MDR1-EGFP transduced cells.

Methods

MDR1-EGFP-transduced variants were derived from WT cells by lentiviral transduction, using an MDR1-linker-EGFP vector. Localization, trafficking, and function of Pgp were compared in WT and MDR1-EGFP transduced cell lines. Primary cultures of rat BCECs and freshly isolated rat brain capillaries were used for comparison.

Results

All cells exhibited typical BCEC morphology. However, significant differences were observed in the localization of Pgp in that RBE4-MDR1-EGFP cells expressed Pgp primarily at the plasma membrane, whereas in hCMEC/D3 cells, the Pgp-EGFP fusion protein was visible both at the plasma membrane and in endolysosomal vesicles. Exposure to doxorubicin increased the number of Pgp-EGFP-positive endolysosomes, indicating a lysosomotropic effect. Furthermore, lysosomal trapping of doxorubicin was observed, likely contributing to the protection of the cell nucleus from damage. In cocultures of WT and MDR1-EGFP transduced cells, intercellular Pgp-EGFP trafficking was observed in RBE4 cells as previously reported for hCMEC/D3 cells. Compared to WT cells, the MDR1-EGFP transduced cells exhibited a significantly higher expression and function of Pgp. However, the junctional tightness of WT and MDR1-EGFP transduced RBE4 and hCMEC/D3 cells was markedly lower than that of primary BCECs, excluding the use of the cell lines for studying vectorial drug transport.

Conclusions

The present data indicate that MDR1-EGFP transduced RBE4 cells are an interesting tool to study the biogenesis of lysosomes and Pgp-mediated lysosomal drug trapping in response to chemotherapeutic agents and other compounds at the level of the blood–brain barrier.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00266-z.

P-glycoprotein induction and its energetic costs in rainbow trout (Oncorhynchus mykiss)

Biological organisms are constantly challenged by xenobiotics and have evolved mechanisms to reduce, neutralize, or repair toxic outcomes. The various chemical defenses all utilize energy, but their specific costs and impacts on energy budgets are currently unknown. In this study, the energetic costs associated with the induction and substrate transport of the efflux transporter P-glycoprotein (P-gp [ABCB1, MDR1]) were examined in rainbow trout. An intraperitoneal injection of the P-gp inducer clotrimazole (0, 0.1, 1.0, and 10 mg/kg) increased P-gp activity (as measured by a competitive rhodamine 123 transport assay in hepatocytes) in a dose-dependent manner reaching a maximum induction of 2.8-fold. Maximum P-gp induction occurred at 50 h post-administration with the highest dose; significant induction of P-gp activity remained elevated over constitutive values until the last sampling time point (168 h). In vitro measurements of hepatocyte respiration indicated that basal P-gp activity transporting R123 as a substrate did not significantly increase respiration rates (range 18.0 to 23.2 ng O₂/min/10⁶ cells); however, following the induction of P-gp by clotrimazole and exposure to the P-gp substrate R123, respiration rates increased significantly (3.52-fold) over baseline values. Using whole animal respirometry, it was shown that respiration rates in fish exposed to R123 only or induced with clotrimazole were not different from controls (range 1.2 to 2.1 mg O₂/kg/min); however, respiration rates were significantly increased in fish with induced P-gp levels and also exposed to R123. This work indicates that basal and induced levels of P-gp activity do not incur significant energetic costs to fish; however, upon induction of P-gp and concomitant substrate exposures, energetic costs can increase and could pose challenges to organisms facing limited energy resources.

Human induced pluripotent stem cells (BIONi010-C) generate tight cell monolayers with blood-brain barrier traits and functional expression of large neutral amino acid transporter 1 (SLC7A5)

The barrier properties of the brain capillary endothelium, the blood-brain barrier (BBB) restricts uptake of most small and all large molecule drug compounds to the CNS. There is a need for predictive human in vitro models of the BBB to enable studies of brain drug delivery. Here, we investigated whether human induced pluripotent stem cell (hiPSC) line (BIONi010-C) could be differentiated to brain capillary endothelial- like cells (BCEC) and evaluated their potential use in drug delivery studies. BIONi010-C hIPSCs were differentiated according to established protocols. BCEC monolayers displayed transendothelial electrical resistance (TEER) values of 5,829±354 Ω∙cm², a P_app,_mannitol of 1.09±0.15 ∙ 10^-6 cm∙s^-1 and a P_app,diazepam of 85.7 ± 5.9 ∙ 10^-6 cm ∙s^-1. The P_diazepam/P_mannitol ratio of ~80, indicated a large dynamic passive permeability range. Monolayers maintained their integrity after medium exchange. Claudin-5, Occludin, Zonulae Occludens 1 and VE-Cadherin were expressed at the cell-cell contact zones. Efflux transporters were present at the mRNA level, but functional efflux of substrates was not detected. Transferrin-receptor (TFR), Low density lipoprotein receptor-related protein 1 (LRP1) and Basigin receptors were expressed at the mRNA-level. The presence and localization of TFR and LRP1 were verified at the protein level. A wide range of BBB-expressed solute carriers (SLC's) were detected at the mRNA level. The presence and localization of SLC transporters GLUT1 and LAT1 was verified at the protein level. Functional studies revealed transport of the LAT1 substrate [³H]-L-Leucine and the LRP1 substrate angiopep-2. In conclusion, we have demonstrated that BIONi010-C-derived BCEC monolayers exhibited, BBB properties including barrier tightness and integrity, a high dynamic range, expression of some of the BBB receptor and transporter expression, as well as functional transport of LAT1 and LRP1 substrates. This suggests that BIONi010-C-derived BCEC monolayers may be useful for studying the roles of LAT-1 and LRP1 in brain drug delivery.

Activation of RARα, RARγ, or RXRα Increases Barrier Tightness in Human Induced Pluripotent Stem Cell-Derived Brain Endothelial Cells

The blood-brain barrier (BBB) is critical to central nervous system (CNS) health. Brain microvascular endothelial cells (BMECs) are often used as in vitro BBB models for studying BBB dysfunction and therapeutic screening applications. Human pluripotent stem cells (hPSCs) can be differentiated to cells having key BMEC barrier and transporter properties, offering a renewable, scalable source of human BMECs. hPSC-derived BMECs have previously been shown to respond to all-trans retinoic acid (RA), and the goal of this study was to identify the stages at which differentiating human induced pluripotent stem cells (iPSCs) respond to activation of RA receptors (RARs) to impart BBB phenotypes. Here the authors identified that RA application to iPSC-derived BMECs at days 6-8 of differentiation led to a substantial elevation in transendothelial electrical resistance and induction of VE-cadherin expression. Specific RAR agonists identified RARα, RARγ, and RXRα as receptors capable of inducing barrier phenotypes. Moreover, RAR/RXRα costimulation elevated VE-cadherin expression and improved barrier fidelity to levels that recapitulated the effects of RA. This study elucidates the roles of RA signaling in iPSC-derived BMEC differentiation, and identifies directed agonist approaches that can improve BMEC fidelity for drug screening studies while also distinguishing potential nuclear receptor targets to explore in BBB dysfunction and therapy.

An isogenic blood-brain barrier model comprising brain endothelial cells, astrocytes, and neurons derived from human induced pluripotent stem cells

The blood-brain barrier (BBB) is critical in maintaining a physical and metabolic barrier between the blood and the brain. The BBB consists of brain microvascular endothelial cells (BMECs) that line the brain vasculature and combine with astrocytes, neurons and pericytes to form the neurovascular unit. We hypothesized that astrocytes and neurons generated from human-induced pluripotent stem cells (iPSCs) could induce BBB phenotypes in iPSC-derived BMECs, creating a robust multicellular human BBB model. To this end, iPSCs were used to form neural progenitor-like EZ-spheres, which were in turn differentiated to neurons and astrocytes, enabling facile neural cell generation. The iPSC-derived astrocytes and neurons induced barrier tightening in primary rat BMECs indicating their BBB inductive capacity. When co-cultured with human iPSC-derived BMECs, the iPSC-derived neurons and astrocytes significantly elevated trans-endothelial electrical resistance, reduced passive permeability, and improved tight junction continuity in the BMEC cell population, while p-glycoprotein efflux transporter activity was unchanged. A physiologically relevant neural cell mixture of one neuron: three astrocytes yielded optimal BMEC induction properties. Finally, an isogenic multicellular BBB model was successfully demonstrated employing BMECs, astrocytes, and neurons from the same donor iPSC source. It is anticipated that such an isogenic facsimile of the human BBB could have applications in furthering understanding the cellular interplay of the neurovascular unit in both healthy and diseased humans. Read the Editorial Highlight for this article on page 843.

Minimum Transendothelial Electrical Resistance Thresholds for the Study of Small and Large Molecule Drug Transport in a Human in Vitro Blood-Brain Barrier Model

A human cell-based in vitro model that can accurately predict drug penetration into the brain as well as metrics to assess these in vitro models are valuable for the development of new therapeutics. Here, human induced pluripotent stem cells (hPSCs) are differentiated into a polarized monolayer that express blood-brain barrier (BBB)-specific proteins and have transendothelial electrical resistance (TEER) values greater than 2500 Ω·cm². By assessing the permeabilities of several known drugs, a benchmarking system to evaluate brain permeability of drugs was established. Furthermore, relationships between TEER and permeability to both small and large molecules were established, demonstrating that different minimum TEER thresholds must be achieved to study the brain transport of these two classes of drugs. This work demonstrates that this hPSC-derived BBB model exhibits an in vivo-like phenotype, and the benchmarks established here are useful for assessing functionality of other in vitro BBB models.

Resistance to cancer chemotherapy: failure in drug response from ADME to P-gp

Cancer chemotherapy resistance (MDR) is the innate and/or acquired ability of cancer cells to evade the effects of chemotherapeutics and is one of the most pressing major dilemmas in cancer therapy. Chemotherapy resistance can arise due to several host or tumor-related factors. However, most current research is focused on tumor-specific factors and specifically genes that handle expression of pumps that efflux accumulated drugs inside malignantly transformed types of cells. In this work, we suggest a wider and alternative perspective that sets the stage for a future platform in modifying drug resistance with respect to the treatment of cancer.

Brain pharmacokinetics of ganciclovir in rats with orthotopic BT4C glioma

Ganciclovir (GCV) is an essential part of the Herpes simplex virus thymidine kinase (HSV-tk) gene therapy of malignant gliomas. The purpose of this study was to investigate the brain pharmacokinetics and tumor uptake of GCV in the BT4C rat glioma model. GCV's brain and tumor uptakes were investigated by in vivo microdialysis in rats with orthotopic BT4C glioma. In addition, the ability of GCV to cross the blood-brain barrier and tumor vasculature was assessed with in situ rat brain perfusion. Finally, the extent to which GCV could permeate across the BT4C glioma cell membrane was assessed in vitro. The areas under the concentration curve of unbound GCV in blood, brain extracellular fluid (ECF), and tumor ECF were 6157, 1658, and 4834 μM⋅min, respectively. The apparent maximum unbound concentrations achieved within 60 minutes were 46.9, 11.8, and 25.8 μM in blood, brain, and tumor, respectively. The unbound GCV concentrations in brain and tumor after in situ rat brain perfusion were 0.41 and 1.39 nmol/g, respectively. The highly polar GCV likely crosses the fenestrated tumor vasculature by paracellular diffusion. Thus, GCV is able to reach the extracellular space around the tumor at higher concentrations than that in healthy brain. However, GCV uptake into BT4C cells at 100 μM was only 2.1 pmol/mg of protein, and no active transporter-mediated disposition of GCV could be detected in vitro. In conclusion, the limited efficacy of HSV-tk/GCV gene therapy may be due to the poor cellular uptake and rapid elimination of GCV.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

The pivotal role of astrocytes in an in vitro stroke model of the blood-brain barrier

Stabilization of the blood-brain barrier during and after stroke can lead to less adverse outcome. For elucidation of underlying mechanisms and development of novel therapeutic strategies validated in vitro disease models of the blood-brain barrier could be very helpful. To mimic in vitro stroke conditions we have established a blood-brain barrier in vitro model based on mouse cell line cerebEND and applied oxygen/glucose deprivation (OGD). The role of astrocytes in this disease model was investigated by using cell line C6. Transwell studies pointed out that addition of astrocytes during OGD increased the barrier damage significantly in comparison to the endothelial monoculture shown by changes of transendothelial electrical resistance as well as fluorescein permeability data. Analysis on mRNA and protein levels by qPCR, western blotting and immunofluorescence microscopy of tight junction molecules claudin-3,-5,-12, occludin and ZO-1 revealed that their regulation and localisation is associated with the functional barrier breakdown. Furthermore, soluble factors of astrocytes, OGD and their combination were able to induce changes of functionality and expression of ABC-transporters Abcb1a (P-gp), Abcg2 (bcrp), and Abcc4 (mrp4). Moreover, the expression of proteases (matrixmetalloproteinases MMP-2, MMP-3, MMP-9, and t-PA) as well as of their endogenous inhibitors (TIMP-1, TIMP-3, PAI-1) was altered by astrocyte factors and OGD which resulted in significant changes of total MMP and t-PA activity. Morphological rearrangements induced by OGD and treatment with astrocyte factors were confirmed at a nanometer scale using atomic force microscopy. In conclusion, astrocytes play a major role in blood-brain barrier breakdown during OGD in vitro.

A retinoic acid-enhanced, multicellular human blood-brain barrier model derived from stem cell sources

Blood-brain barrier (BBB) models are often used to investigate BBB function and screen brain-penetrating therapeutics, but it has been difficult to construct a human model that possesses an optimal BBB phenotype and is readily scalable. To address this challenge, we developed a human in vitro BBB model comprising brain microvascular endothelial cells (BMECs), pericytes, astrocytes and neurons derived from renewable cell sources. First, retinoic acid (RA) was used to substantially enhance BBB phenotypes in human pluripotent stem cell (hPSC)-derived BMECs, particularly through adherens junction, tight junction, and multidrug resistance protein regulation. RA-treated hPSC-derived BMECs were subsequently co-cultured with primary human brain pericytes and human astrocytes and neurons derived from human neural progenitor cells (NPCs) to yield a fully human BBB model that possessed significant tightness as measured by transendothelial electrical resistance (~5,000 Ωxcm(2)). Overall, this scalable human BBB model may enable a wide range of neuroscience studies.

In vitro and in vivo study of dolichyl phosphate on the efflux activity of P-glycoprotein at the blood-brain barrier

It has been commonly recognized that accumulated amyloid-β (Aβ) in the brain plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Since the deficiency of the P-glycoprotein (P-gp) at the blood-brain barrier (BBB) in AD may aggravate Aβ deposition and the P-gp reversal agents display lower selectivity of the action, to selectively restore activity of the efflux pump is eagerly required. This study was designed to investigate the influence of dolichyl-phosphate (dolichyl-P) on the P-gp at the BBB. The results revealed that treatment with dolichyl-P increased transendothelial transfer of Rhodamine123 (Rh123) and Aβ42 from the apical compartment to the basolateral compartment but reduced that from the basolateral compartment to the apical compartment in the co-culture of rat brain microvessel endothelial cells (rBMECs) and astrocytes, down regulated P-gp expression in rBMECs and significantly elevated content of Rh123 in rat cortex and hippocampus tissues. The present results implied that accumulated dolichyl-P in the brain may exert an important role in the depression of the P-gp at the BBB, which may suggest valuable clues to promote function of the pump at the BBB in AD.

Blood-brain barrier dysfunction as a cause and consequence of Alzheimer's disease

The blood-brain barrier (BBB) plays critical roles in the maintenance of central nervous system (CNS) homeostasis. Dysfunction of the BBB occurs in a number of CNS diseases, including Alzheimer's disease (AD). A prevailing hypothesis in the AD field is the amyloid cascade hypothesis that states that amyloid-β (Aβ) deposition in the CNS initiates a cascade of molecular events that cause neurodegeneration, leading to AD onset and progression. In this review, the participation of the BBB in the amyloid cascade and in other mechanisms of AD neurodegeneration will be discussed. We will specifically focus on three aspects of BBB dysfunction: disruption, perturbation of transporters, and secretion of neurotoxic substances by the BBB. We will also discuss the interaction of the BBB with components of the neurovascular unit in relation to AD and the potential contribution of AD risk factors to aspects of BBB dysfunction. From the results discussed herein, we conclude that BBB dysfunction contributes to AD through a number of mechanisms that could be initiated in the presence or absence of Aβ pathology.

Modulation of P-glycoprotein in rat brain microvessel endothelial cells under oxygen glucose deprivation

Objectives

To investigate modulation of P-glycoprotein (P-gp) in rat brain microvessel endothelial cells (rBMECs) under oxygen glucose deprivation (OGD).

Methods

The coculture of rBMECs and astrocytes was established to investigate the time course of P-gp, tumour necrosis factor-α (TNF-α), endothelin-1 (ET-1), nitric oxide synthase (NOS) and protein kinase C (PKC) expression in the rBMECs as well as rhodamine 123 (Rh123) transendothelial transfer under OGD using Western blot and HPLC, respectively. The influence of pharmacological tools including H398, JKC-301, RES-701-1, L-NMMA, BIM and SN50 on the P-gp expression as well as Rh123 transendothelial transfer was evaluated at 3 h time point of OGD.

Key findings

Elevated P-gp, TNF-α, ET-1, NOS and PKC expression in the rBMECs, as well as increased P-gp efflux activity were observed after 2 h or more time of OGD. Incubation of H398 and other pharmacological tools downregulated P-gp expression and functional activity in the rBMECs at 3 h time point of OGD.

Conclusions

This report suggested that TNF-α, ET-1, NOS and PKC may mediate upregulation of P-gp in the rBMECs under OGD, which may be worthy of being referenced for the investigation of P-gp at the blood–brain barrier in the early period of stroke.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Retinoic acid induces blood-brain barrier development

The blood-brain barrier (BBB) is crucial in the maintenance of a controlled environment within the brain to safeguard optimal neuronal function. The endothelial cells (ECs) of the BBB possess specific properties that restrict the entry of cells and metabolites into the CNS. The specialized BBB endothelial phenotype is induced during neurovascular development by surrounding cells of the CNS. However, the molecular differentiation of the BBB endothelium remains poorly understood. Retinoic acid (RA) plays a crucial role in the brain during embryogenesis. Because radial glial cells supply the brain with RA during the developmental cascade and associate closely with the developing vasculature, we hypothesize that RA is important for the induction of BBB properties in brain ECs. Analysis of human postmortem fetal brain tissue shows that the enzyme mainly responsible for RA synthesis, retinaldehyde dehydrogenase, is expressed by radial glial cells. In addition, the most important receptor for RA-driven signaling in the CNS, RA-receptor β (RARβ), is markedly expressed by the developing brain vasculature. Our findings have been further corroborated by in vitro experiments showing RA- and RARβ-dependent induction of different aspects of the brain EC barrier. Finally, pharmacologic inhibition of RAR activation during the differentiation of the murine BBB resulted in the leakage of a fluorescent tracer as well as serum proteins into the developing brain and reduced the expression levels of important BBB determinants. Together, our results point to an important role for RA in the induction of the BBB during human and mouse development.

In vitro models of the blood-brain barrier

The chapter provides an introduction and brief overview of currently available in vitro blood-brain barrier models, pointing out the major advantages and disadvantages of the respective models and potential applications. Bovine brain microvessel endothelial cell isolation, culture, and transendothelial permeability measurement procedures are discussed in detail as a model system for a laboratory to begin brain vascular investigations.

The mouse blood-brain barrier transcriptome: a new resource for understanding the development and function of brain endothelial cells

The blood-brain barrier (BBB) maintains brain homeostasis and limits the entry of toxins and pathogens into the brain. Despite its importance, little is known about the molecular mechanisms regulating the development and function of this crucial barrier. In this study we have developed methods to highly purify and gene profile endothelial cells from different tissues, and by comparing the transcriptional profile of brain endothelial cells with those purified from the liver and lung, we have generated a comprehensive resource of transcripts that are enriched in the BBB forming endothelial cells of the brain. Through this comparison we have identified novel tight junction proteins, transporters, metabolic enzymes, signaling components, and unknown transcripts whose expression is enriched in central nervous system (CNS) endothelial cells. This analysis has identified that RXRalpha signaling cascade is specifically enriched at the BBB, implicating this pathway in regulating this vital barrier. This dataset provides a resource for understanding CNS endothelial cells and their interaction with neural and hematogenous cells.

The astrocyte odyssey

Neurons have long held the spotlight as the central players of the nervous system, but we must remember that we have equal numbers of astrocytes and neurons in the brain. Are these cells only filling up the space and passively nurturing the neurons, or do they also contribute to information transfer and processing? After several years of intense research since the pioneer discovery of astrocytic calcium waves and glutamate release onto neurons in vitro, the neuronal-glial studies have answered many questions thanks to technological advances. However, the definitive in vivo role of astrocytes remains to be addressed. In addition, it is becoming clear that diverse populations of astrocytes coexist with different molecular identities and specialized functions adjusted to their microenvironment, but do they all belong to the umbrella family of astrocytes? One population of astrocytes takes on a new function by displaying both support cell and stem cell characteristics in the neurogenic niches. Here, we define characteristics that classify a cell as an astrocyte under physiological conditions. We will also discuss the well-established and emerging functions of astrocytes with an emphasis on their roles on neuronal activity and as neural stem cells in adult neurogenic zones.

Microvascular P-glycoprotein expression at the blood-brain barrier following focal astrocyte loss and at the fenestrated vasculature of the area postrema

The multidrug transporter, P-glycoprotein, expressed at the blood-brain barrier is thought to be important for limiting access of toxic agents to the brain, but its relationship to astrocyte expression is unclear. We have studied P-glycoprotein expression in the inferior colliculus after a temporary loss of blood-brain barrier integrity following chemically induced astrocyte loss and at the fenestrated vascular endothelium of the area postrema. Male Fisher F344 rats given 3-chloropropanediol showed astrocyte loss from 12 to 24 h until the lesion was repopulated 8-28 days later. In non-dosed tissue, P-glycoprotein expression was seen the entire length of platelet endothelial cell adhesion molecule immunoreactive vessels. Within 6 h of dosing, a significant (p<0.05) reduction in the total length of P-glycoprotein immunoreactive vasculature was evident. By 48 h, P-glycoprotein immunoreactivity was heavily fragmented. The total length of P-glycoprotein immunoreactive vessels became minimal at 4 days (p<0.001) but was still present in many vessels. From 6 to 28 days, P-glycoprotein immunoreactivity returned across the inferior colliculus, in parallel with astrocytic repopulation of the lesion, and by 28 days resembled that seen in control tissue. The area postrema showed GFAP immunoreactive astrocytes but which made limited contact with the vasculature, while the platelet endothelial cell adhesion molecule immunoreactive vasculature showed no expression of P-glycoprotein. These findings provide evidence supporting a link between GFAP-astrocyte and P-glycoprotein expression in the mature brain vasculature in vivo.

Dynamics of CNS barriers: evolution, differentiation, and modulation

(1) Three main barrier layers at the interface between blood and tissue protect the central nervous system (CNS): the endothelium of brain capillaries, and the epithelia of the choroid plexus (CP) and the arachnoid. The classical work on these barriers in situ until the 1970s laid the foundations for modern understanding. Techniques for brain endothelial cell isolation and culture pioneered by Ferenc Joó in the 1970s opened up new fields of examination, enabling study of mechanisms at the cellular and molecular level. (2) Astrocytic glial cells are closely associated with the brain endothelial barrier. During evolution the barrier appears to have shifted from the glial to the endothelial layer, in parallel with the increasing importance of the microvasculature and its regulation. Vestiges of the barrier potential of glia remain in the modern mammalian CNS. (3) Evolutionary evidence suggests that the advantage derived from ionic homeostasis around central synapses was the major selective pressure leading to refinement of CNS barrier systems. This is one element of the modern 'multitasking' barrier function. (4) While epithelia are constitutively able to form barriers at appropriate interfaces, the 'default' condition for endothelia is more leaky; inductive influences from associated cells especially astrocytes are important in generating the full blood-brain barrier (BBB) phenotype in brain capillaries. The underlying mechanisms are being elucidated at the molecular and genomics level. (5) The barrier layers of the nervous system can be modulated by a number of receptor-mediated processes, involving several signal transduction pathways, both calcium dependent and independent. Some agents acting as 'inducers' in the long term can act as 'modulators' in the short-term, with some overlap of signaling pathways. Modulating agents may be derived both from the blood and from cells associated with cerebral vessels. Less is known about the modulation of the CP. (6) The challenge for the next era of CNS barrier studies will be to apply new knowledge from proteomics and genomics to understanding the in vivo condition in physiology and pathology.

Relationship between expression of multiple drug resistance proteins and p53 tumor suppressor gene proteins in human brain astrocytes

Multiple drug resistance occurs when cells fail to respond to chemotherapy. Although it has been established that the drug efflux protein P-glycoprotein protects the brain from xenobiotics, the mechanisms involved in the regulation of expression of multiple drug resistance genes and proteins are not fully understood. Re-entry into the cell cycle and integrity of the p53 signaling pathway have been proposed as triggers of multiple drug resistance expression in tumor cells. Whether this regulation occurs in non-tumor CNS tissue is not known. Since multiple drug resistance overexpression has been reported in glia and blood vessels from epileptic brain, we investigated the level of expression of multidrug resistance protein, multidrug resistance-associated proteins and lung resistance protein in endothelial cells and astrocytes isolated from epileptic patients or studied in situ in surgical tissue samples by double label immunocytochemistry. Reverse transcriptase-polymerase chain reaction and Western blot analyses revealed that multiple drug resistance, multidrug resistance protein, and lung resistance protein are expressed in these cells. Given that lung resistance proteins have been reported to be preferentially expressed by tumors, we investigated expression of tumor suppressor genes in epileptic cortices. The pro-apoptotic proteins p53 and p21 could not be detected in "epileptic" astrocytes, while endothelial cells from the same samples readily expressed these proteins, as did normal brain astroglia and normal endothelial cells. Other apoptotic markers were also absent in epileptic glia. Our results suggest a possible link between loss of p53 function and expression of multiple drug resistance in non-tumor CNS cells.

Modulation of transendothelial permeability and expression of ATP-binding cassette transporters in cultured brain capillary endothelial cells by astrocytic factors and cell-culture conditions

Confluent cell monolayers of brain capillary endothelial cells (BCEC) are used widely as an in vitro cell culture model of the blood-brain barrier. The present study describes the influence of cell-culture conditions on tight junctions, filamentous-actin cytoskeleton, and expression of ATP-binding cassette (ABC) transporters in primary cell cultures of porcine BCEC. Astrocyte as well as C6 glioma-conditioned cell culture medium was used in combination with retinoic acid, dexamethasone, cyclic adenosine monophosphate (cAMP) analogs, or 1,25-dihydroxyvitamin D3. It was shown that C6-conditioned medium led to a reorganization of filamentous actin and to an improved staining of zonula occludens-associated protein-1 (ZO-1). Further optimization of these culture conditions was achieved with cAMP analogs and dexamethasone. Retinoic acid, as well as 1,25-dihydroxyvitamin D3, did not improve cellular tight junctions as judged by filamentous actin, ZO-1 rearrangement, and transcellular electrical resistance (TER) measurements. However, these morphological changes did not influence the paracellular permeability of the extracellular marker sucrose. Expression of ABC transporters such as P-glycoprotein, multidrug resistance-associated protein-1(MRP1), and MRP2 were compared by measuring messenger RNA (mRNA) levels in whole-brain tissue, isolated brain capillaries, and cultured cells. In freshly isolated BCEC, mRNA levels of MRP2 and P-glycoprotein dropped by two- to sevenfold, respectively, whereas MRP1 mRNA levels were slightly increased. During cell culture, mRNA levels of MRP1 and MRP2 decreased by up to fivefold, while P-glycoprotein levels remained constant. These results were unaltered by different cell-culture conditions. In conclusion, the present study suggests that paracellular permeability, as well as mRNA expression of the studied ABC transporters in primary cultures, of porcine BCEC are insensitive toward changes in cell-culture conditions.

Protection against hypoxia-induced increase in blood-brain barrier permeability: role of tight junction proteins and NFkappaB

Co-culture with glial cells and glia-conditioned media can induce blood-brain barrier properties in microvessel endothelial cells and protect against hypoxia-induced blood-brain barrier breakdown. We examined the effect of two types of glia-conditioned media on brain microvessel endothelial cell permeability and tight junction protein expression, and studied potential mechanisms of action. We found that C6-glioma-conditioned media, but not rat astrocyte-conditioned media, protected against an increase in permeability induced by exposure to 1% oxygen for 24 hours. This hypoxic stress caused an increase in the expression of tight junction proteins claudin-1 and actin, particularly in cells treated with C6-conditioned media. We found that C6-conditioned media has a significantly higher level of both basic fibroblast growth factor and vascular endothelial growth factor. Treatment with C6-conditioned media for 1 or 3 days protects against hypoxia-induced permeability increases, and this protective effect may be mediated by signal transduction pathways terminating at the transcription factor NFkappaB.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Astrocyte-endothelial interactions and blood-brain barrier permeability

The blood-brain barrier (BBB) is formed by brain endothelial cells lining the cerebral microvasculature, and is an important mechanism for protecting the brain from fluctuations in plasma composition, and from circulating agents such as neurotransmitters and xenobiotics capable of disturbing neural function. The barrier also plays an important role in the homeostatic regulation of the brain microenvironment necessary for the stable and co-ordinated activity of neurones. The BBB phenotype develops under the influence of associated brain cells, especially astrocytic glia, and consists of more complex tight junctions than in other capillary endothelia, and a number of specific transport and enzyme systems which regulate molecular traffic across the endothelial cells. Transporters characteristic of the BBB phenotype include both uptake mechanisms (e.g. GLUT-1 glucose carrier, L1 amino acid transporter) and efflux transporters (e.g. P-glycoprotein). In addition to a role in long-term barrier induction and maintenance, astrocytes and other cells can release chemical factors that modulate endothelial permeability over a time-scale of seconds to minutes. Cell culture models, both primary and cell lines, have been used to investigate aspects of barrier induction and modulation. Conditioned medium taken from growing glial cells can reproduce some of the inductive effects, evidence for involvement of diffusible factors. However, for some features of endothelial differentiation and induction, the extracellular matrix plays an important role. Several candidate molecules have been identified, capable of mimicking aspects of glial-mediated barrier induction of brain endothelium; these include TGFbeta, GDNF, bFGF, IL-6 and steroids. In addition, factors secreted by brain endothelial cells including leukaemia inhibitory factor (LIF) have been shown to induce astrocytic differentiation. Thus endothelium and astrocytes are involved in two-way induction. Short-term modulation of brain endothelial permeability has been shown for a number of small chemical mediators produced by astrocytes and other nearby cell types. It is clear that endothelial cells are involved in both long- and short-term chemical communication with neighbouring cells, with the perivascular end feet of astrocytes being of particular importance. The role of barrier induction and modulation in normal physiology and in pathology is discussed.

The influence of P-glycoprotein on morphine transport in Caco-2 cells. Comparison with paclitaxel

In vitro monolayer studies using Caco-2 cells were employed here to explore P-glycoprotein mediated transport of morphine. Bi-directional transport studies of 10-75 microM morphine showed efflux to be twofold higher than influx (4 x 10(-6) compared to 2 x 10(-6) cm/s) and cellular accumulation in the efflux direction was eightfold higher. The cyclosporin analogue (PSC-833) equilibrated morphine transport in both directions. Depletion of intracellular glutathione had a greater effect on increasing cellular morphine accumulation than P-glycoprotein inhibitors, suggesting a role for glutathione in morphine transport. P-glycoprotein had a substantially greater effect on paclitaxel accumulation, efflux and bi-directional transport than for morphine. Paclitaxel transport was below detection (<0.1 x 10(-6) cm/s) in the influx direction, yet efflux was very high (18.4 x 10(-6) cm/s) and P-glycoprotein inhibition increased accumulation >100-fold. These results reinforce the substantial role P-glycoprotein has in paclitaxel transport. Conversely, P-glycoprotein regulated morphine transport is weak. Nevertheless, morphine transport rates could be doubled when administered with P-glycoprotein substrates. Therefore, increased analgesia through P-glycoprotein inhibition should be possible.

Modulation of insulin transport in rat brain microvessel endothelial cells by an ecto-phosphatase activity

The physiological function of alkaline phosphatase (ALP) remains controversial. It was recently suggested that this membrane-bound enzyme has a role in the modulation of transmembranar transport systems into hepatocytes and Caco-2 cells. ALP activity expressed on the apical surface of blood-brain barrier cells, and its relationship with (125)I-insulin internalization were investigated under physiological conditions using p-nitrophenylphosphate (p-NPP) as substrate. For this, an immortalized cell line of rat capillary cerebral endothelial cells (RBE4 cells) was used. ALP activity and (125)I-insulin internalization were evaluated in these cells. The results showed that RBE4 cells expressed ALP, characterized by an ecto-oriented active site which was functional at physiological pH. Orthovanadate (100 microM), an inhibitor of phosphatase activities, decreased both RBE4-ALP activity and (125)I-insulin internalization. In the presence of L-arginine (1 mM) or adenosine (100 microM) RBE4-ALP activity and (125)I-insulin, internalization were significantly reduced. However, D-arginine (1 mM) had no significant effect. Additionally, RBE4-ALP activity and (125)I-insulin internalization significantly increased in the presence of the bioflavonoid kaempferol (100 microM), of the phorbol ester PMA (80 nM), IBMX (1 mM), progesterone (200 microM and 100 microM), beta-estradiol (100 microM), iron (100 microM) or in the presence of all-trans retinoic acid (RA) (10 microM). The ALP inhibitor levamisole (500 microM) was able to reduce (125)I-insulin internalization to 69.1 +/- 7.1% of control. Our data showed a positive correlation between ecto-ALP activity and (125)I-insulin incorporation (r = 0.82; P < 0.0001) in cultured rat brain endothelial cells, suggesting that insulin entry into the blood-brain barrier may be modulated through ALP.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Effect of probenecid on fluorescein transport in the central nervous system using in vitro and in vivo models

PURPOSE

The purpose of this study was to characterize the function of multidrug resistance-associated proteins (MRPs) (or MRP-like organic anion transport systems) in the blood-brain harrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) using both an in vitro BBB model and an in vivo microdialysis model.

METHODS

In vitro functional studies were performed using bovine brain microvessel endothelial cells (BBMEC). The accumulation of fluorescein, an anionic fluorescent dye, in BBMEC was determined with and without the presence of inhibitors of various efflux transport proteins. In vivo microdialysis simultaneously monitored fluorescein concentrations in cortical extracellular fluid and cerebrospinal fluid. The effect of probenecid on the in vivo distribution of fluorescein was studied using a balanced crossover design in the rat.

RESULTS

In vitro experiments showed that probenecid, indomethacin, LY-329146, and all MRP inhibitors significantly increased (two- to threefold) the accumulation of fluorescein in BBMEC, whereas LY-335979, a P-gp inhibitor, had no effect on the accumulation of fluorescein. Probenecid significantly increased fluorescein plasma concentration and the plasma free fraction in vivo. The distribution of fluorescein across the BBB and BCSFB was enhanced by 2.2- and 1.9-fold, respectively, when probenecid was coadministered, even after correction for increased fluorescein plasma concentrations and free fraction.

CONCLUSIONS

These results demonstrate that MRPs or MRP-like transport system(s) may play an important role in fluorescein distribution across both BBB and BCSFB. This study showed that microdialysis proved to be a powerful in vivo technique for the study of transport systems in the central nervous system, and in vitro/in vivo correlations are possible using these model systems.

Ivermectin excretion by isolated functionally intact brain endothelial capillaries

1. Functionally intact brain endothelial capillaries were isolated from porcine brain. p-Glycoprotein was localized at the lumenal membrane of intact capillaries by immunohistochemistry using a murine monoclonal antibody and a secondary FITC fluorescent labelled anti-mouse IgG. Western blot staining of p-glycoprotein in isolated endothelial cells confirmed the immunohistochemistry. 2. Excretion of the fluorescent labelled anthelmintic drug Ivermectin (BODIPY-Ivermectin) was studied in the isolated brain endothelial capillaries. Drug accumulation in the capillary lumen was visualized by fluorescence confocal laser scanning microscopy and was measured by image analysis. Secretion of BODIPY-Ivermectin into the capillary lumen exhibited characteristics of specific and energy-dependent transport. Steady state lumenal fluorescence intensity averaged 1.6 times cellular fluorescence and was reduced 3--4 times below cellular levels when metabolism was inhibited by NaCN. 3. BODIPY-Ivermectin secretion was inhibited in a concentration-dependent manner by unlabeled Ivermectin. In addition, lumenal but not cellular fluorescence intensity was significantly decreased when capillaries were incubated with PSC-833, Cyclosporin A or Verapamil, all inhibitors of p-glycoprotein. Conversely, unlabelled Ivermectin reduced the p-glycoprotein (Pgp)-mediated secretion of a fluorescent derivative of Verapamil, (BODIPY-Verapamil). 4. BODIPY-Ivermectin secretion was not affected in the presence of Leucotriene C(4) (LTC(4)), a potent inhibitor of multidrug resistance related protein (mrp)-mediated transport processes. In addition, excretion of Fluorescein-Methotrexate, an mrp-substrate, was not inhibited by Ivermectin. 5. Uptake experiments with isolated porcine brain capillary cells showing increased cellular uptake of BODIPY-Ivermectin in the presence of unlabelled drug or PSC-833 supported the findings of a Pgp interaction in intact capillaries. 6. The data are consistent with BODIPY-Ivermectin and Ivermectin being transported across the lumenal membrane of brain capillaries. For the first time Pgp-interaction of Ivermectin at the blood brain barrier is demonstrated on a cellular level in an intact vascular tissue.

Regulatory pathways and uptake of L-DOPA by capillary cerebral endothelial cells, astrocytes, and neuronal cells

We examined the nature and regulation of the inward L-3,4-dihydroxyphenylalanine (L-DOPA) transporter in rat capillary cerebral endothelial (RBE4) cells, type 1 astrocytes (DI TNC1), and Neuro-2a neuroblastoma cells. In all three cell types, the inward transfer of L-DOPA was largely promoted through the 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid-sensitive and sodium-independent L-type amino acid transporter. Only in DI TNC1 cells was the effect of maneuvers that increase intracellular cAMP levels accompanied by increases in L-DOPA uptake. Also, only in DI TNC1 cells was the effect of the guanylyl cyclase inhibitor LY-83583 accompanied by a 65% increase in L-DOPA accumulation, whereas the nitric oxide donor sodium nitroprusside produced a 25% decrease in L-DOPA accumulation. In all three cell types, the Ca2+/calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-DOPA uptake in a noncompetitive manner. Thapsigargin (1 and 3 microM) and A-23187 (1 and 3 microM) failed to alter L-DOPA accumulation in RBE4 and Neuro-2a cells but markedly increased L-DOPA uptake in DI TNC1 cells. We concluded that L-DOPA in RBE4, DI TNC1, and Neuro-2a cells is transported through the L-type amino acid transporter and appears to be under the control of Ca2+/calmodulin-mediated pathways. Astrocytes, however, are endowed with other processes that appear to regulate the accumulation of L-DOPA, responding positively to increases in intracellular Ca2+ and cAMP and to decreases in cGMP.

Interaction between L-DOPA and 3-O-methyl-L-DOPA for transport in immortalised rat capillary cerebral endothelial cells

The present study aimed to determine the kinetics of L-3,4-dihydroxyphenylalanine (L-DOPA) uptake in an immortalised cell line of rat capillary cerebral endothelial cells (clones RBE 4 and RBE 4B), to define the type of interaction with 3-O-methyl-L-DOPA (3-OM-L-DOPA), sensitivity to 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BHC), N-(methylamino)-isobutyric acid (MeAIB) and sodium. Non-linear analysis of the saturation curves for L-DOPA and 3-OM-L-DOPA revealed in RBE 4 cells Km values (in microM) of 72 (53, 91) and 40 (25, 57) and in RBE 4B cells Km values (in microM) of 60 (46, 74) and 44 (13, 75), respectively. IC50 values for 3-OM-L-DOPA (RBE 4, 642 [542, 759] microM; RBE 4B, 482 [475, 489] microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were greater than the corresponding Ki values (RBE 4, 143 [121, 170] microM; RBE 4B, 93 [92, 95] microM) obtained in the presence of a nearly saturating (250 microM) concentration of 3-OM-L-DOPA; this is compatible with a competitive type of interaction between L-DOPA and 3-OM-L-DOPA. Uptake of both L-DOPA and 3-OM-L-DOPA in RBE 4 and RBE 4B cells was sensitive to BHC with similar IC50 values. MeAIB (up to 2.5 mM) was found not to interfere with the uptake of both L-DOPA and 3-OM-L-DOPA. Uptake of (250 microM) L-DOPA and 3-OM-L-DOPA in the absence of sodium in the incubation medium was similar to that observed in the presence of increasing concentrations of sodium (20-140 mM). Homogenates of both cell lines were endowed with considerable COMT activity. Incubation of RBE 4 and RBE 4B cells with L-DOPA (25 microM) in the presence of a methyl donor (S-adenosyl-L-methionine) resulted in the formation of 3-OM-L-DOPA; this was abolished by 1 microM tolcapone. The fractional outflow of intracellular L-DOPA through the luminal and abluminal cell side was not affected by the presence of intracellular 3-OM-L-DOPA. The fractional outflow of exogenous 3-OM-L-DOPA applied from the luminal cell border was similar to that observed for 3-OM-L-DOPA with origin in L-DOPA. It is concluded that RBE 4 and RBE 4B cells are endowed with the L-type amino acid transporter through which L-DOPA and 3-OM-L-DOPA can be taken up, and 3-OM-L-DOPA behaves as a competitive inhibitor for the uptake of L-DOPA. This, however, only occurs for luminal cell inward movement but not for abluminal cell outward movement of the substrates.

L-DOPA transport properties in an immortalised cell line of rat capillary cerebral endothelial cells, RBE 4

The present study aimed to determine the kinetics of L-3, 4-dihydroxyphenylalanine (L-DOPA) uptake in an immortalised cell line of rat capillary cerebral endothelial cells (clones RBE 4 and RBE 4B), to define the type of inhibition produced by L-5-hydroxytryptophan (L-5-HTP), 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BHC) and N-(methylamino)-isobutyric acid (MeAlB) and its sodium dependence. Non-linear analysis of the saturation curves for L-DOPA and L-5-HTP revealed in RBE 4 cells Km values (in microM) of 72 and 102 and in RBE 4B cells Km values (in microM) of 60 and 118, respectively. IC50 values for L-5-HTP (RBE 4, 1026 microM; RBE 4B, 831 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were almost 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. IC50 values for BHC obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were also 6- to 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. MeAlB (up to 2.5 mM) was found not to interfere with the uptake of L-DOPA. In RBE 4 cells, Vmax values for L-DOPA uptake were identical in the absence and the presence of 150 microM L-5-HTP or 150 microM BHC, but Km values (microM) were significantly greater (P<0.05) when L-DOPA uptake was studied in the presence of L-5-HTP or BHC. Similar findings were observed when RBE 4B cells were used. Uptake of (250 microM) L-DOPA in the absence of sodium in the incubation medium was similar to that observed in the presence of increasing concentrations of sodium (20 to 140 mM). It is concluded that RBE 4 and RBE 4B cells are endowed with the L-type amino acid transporter through which L-DOPA and L-5-HTP can be taken up, and suggested that this immortalised cell line of rat capillary cerebral endothelium might constitute an interesting in vitro model for the study of BBB mechanisms, namely those concerning solute and nutrient transfer across the brain capillary endothelium.

Teratology of retinoids

Either an excess or a deficiency of vitamin A and related compounds (retinoids) causes abnormal morphological development (teratogenesis). Potential retinoid sources come from dietary intake, nutritional supplements, and some therapeutic drugs. Therefore, understanding the mechanisms of retinoid teratogenesis is important. This review first gives an overview of the principles of teratology as they apply to retinoid-induced malformations. It then describes relevant aspects of the biochemical pathway and signal transduction of retinoids. The teratogenic activity of various retinoid compounds, the role of the retinoid receptors, and important toxicokinetic parameters in teratogenesis are reviewed.

Dynamic modeling of the blood-brain barrier: a novel tool for studies of drug delivery to the brain

Apart from in exceptional and rare circumstances when drugs may be inserted directly into the CNS, it is often difficult to achieve therapeutically relevant dosages in the brain. This is because oral or parenteral administration effectively results in the exclusion of most drugs by the blood-brain barrier (BBB). Therefore, it is important to develop systems that allow for rapid and inexpensive determination of the BBB-permeability properties of novel therapeutic drugs. Recent advances allow for the manipulation of brain transport of drugs by chemical facilitation with lipophylic compounds or viral vectors. Studies performed on viable in vitro models are set to accelerate the design of drugs that selectively and aggressively target the CNS.