An electrically tight in vitro blood-brain barrier model displays net brain-to-blood efflux of substrates for the ABC transporters, P-gp, Bcrp and Mrp-1

Investigative analysis of blood-brain barrier penetrating potential of electronic nicotine delivery systems (e-cigarettes) chemicals using predictive computational models

INTRODUCTION

Seizures are known potential side effects of nicotine toxicity and have been reported in electronic nicotine delivery systems (ENDS, e-cigarettes) users, with the majority involving youth or young adults.

AREAS COVERED

Using chemoinformatic computational models, chemicals (including flavors) documented to be present in ENDS were compared to known neuroactive compounds to predict the blood-brain barrier (BBB) penetration potential, central nervous system (CNS) activity, and their structural similarities. The literature search used PubMed/Google Scholar, through September 2023, to identify individual chemicals in ENDS and neuroactive compounds.The results show that ENDS chemicals in this study contain >60% structural similarity to neuroactive compounds based on chemical fingerprint similarity analyses. The majority of ENDS chemicals we studied were predicted to cross the BBB, with approximately 60% confidence, and were also predicted to have CNS activity; those not predicted to passively diffuse through the BBB may be actively transported through the BBB to elicit CNS impacts, although it is currently unknown.

EXPERT OPINION

In lieu of in vitro and in vivo testing, this study screens ENDS chemicals for potential CNS activity and predicts BBB penetration potential using computer-based models, allowing for prioritization for further study and potential early identification of CNS toxicity.

From cells to organoids: The evolution of blood-brain barrier technology for modelling drug delivery in brain cancer

Brain cancer remains the deadliest cancer. The blood-brain barrier (BBB) is impenetrable to most drugs and is a complex 3D network of multiple cell types including endothelial cells, astrocytes, and pericytes. In brain cancers, the BBB becomes disrupted during tumor progression and forms the blood-brain tumor barrier (BBTB). To advance therapeutic development, there is a critical need for physiologically relevant BBB in vitro models. 3D cell systems are emerging as valuable preclinical models to accelerate discoveries for diseases. Given the versatility and capability of 3D cell models, their potential for modelling the BBB and BBTB is reviewed. Technological advances of BBB models and challenges of in vitro modelling the BBTB, and application of these models as tools for assessing therapeutics and nano drug delivery, are discussed. Quantitative, in vitro BBB models that are predictive of effective brain cancer therapies will be invaluable for accelerating advancing new treatments to the clinic.

Selective protection of normal cells from chemotherapy, while killing drug-resistant cancer cells

Cancer therapy is limited by toxicity in normal cells and drug-resistance in cancer cells. Paradoxically, cancer resistance to certain therapies can be exploited for protection of normal cells, simultaneously enabling the selective killing of resistant cancer cells by using antagonistic drug combinations, which include cytotoxic and protective drugs. Depending on the mechanisms of drug-resistance in cancer cells, the protection of normal cells can be achieved with inhibitors of CDK4/6, caspases, Mdm2, mTOR, and mitogenic kinases. When normal cells are protected, the selectivity and potency of multi-drug combinations can be further enhanced by adding synergistic drugs, in theory, eliminating the deadliest cancer clones with minimal side effects. I also discuss how the recent success of Trilaciclib may foster similar approaches into clinical practice, how to mitigate systemic side effects of chemotherapy in patients with brain tumors and how to ensure that protective drugs would only protect normal cells (not cancer cells) in a particular patient.

Role of interleukin-6 and interleukin-10 in morphological and functional changes of the blood-brain barrier in hypertriglyceridemia

BACKGROUND

Hypertriglyceridemia is closely linked to atherosclerosis related inflammatory processes and blood-brain barrier (BBB) dysfunction. Using apolipoprotein B-100 (APOB-100) transgenic mice, an animal model of chronic hypertriglyceridemia, we analyzed BBB function and morphology in vitro and ex vivo. Our objective was to determine which BBB characteristics are produced mainly by interleukin (IL)-6, an atherosclerosis promoting cytokine, and whether these actions can be antagonized by IL-10, an anti-inflammatory cytokine.

METHODS

Brain endothelial and glial cell cultures and brain microvessels were isolated from wild type (WT) and APOB-100 transgenic mice and were treated with IL-6, IL-10 and their combination. First, IL-6 and IL-10 production was measured in WT and APOB-100 microvessels using qPCR. Then functional parameters of endothelial cell cultures were analyzed and immunocytochemistry for key BBB proteins was performed.

RESULTS

IL-6 mRNA levels were higher in brain microvessels than in brain parenchyma of APOB-100 transgenic mice. Transendothelial electric resistance and P-glycoprotein activity were lower, and paracellular permeability was higher in cultured APOB-100 brain endothelial cells. These features were sensitive to both IL-6 and IL-10 treatments. A decreased P-glycoprotein immunostaining was measured in transgenic endothelial cells under control conditions and in WT cells after treating them with IL-6. This effect was antagonized by IL-10. Changes in immunostaining for tight junction proteins were observed after IL-6 exposure, which were in part antagonized by IL-10. In glial cell cultures an increase in aquaporin-4 immunolabeling in the transgenic group and an increase in microglia cell density in WT glia cultures was detected after IL-6 treatment, which was antagonized by IL-10. In isolated brain microvessels a decrease in P-glycoprotein immunolabeled area fraction was measured in APOB-100 microvessels under control conditions and in WT microvessels after every cytokine treatment. ZO-1 immunolabeling showed characteristics similar to that of P-glycoprotein. No change was seen in claudin-5 and occludin immunoreactive area fractions in microvessels. A decrease in aquaporin-4 immunoreactivity was measured in WT microvessels treated by IL-6, which was antagonized by IL-10.

CONCLUSION

IL-6 produced in microvessels contributes to BBB impairment observed in the APOB-100 mice. We showed that IL-10 partly antagonizes the effects of IL-6 at the BBB.

Transport Mechanisms at the Blood-Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood-brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a "gateway" that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

Drug Penetration into the Central Nervous System: Pharmacokinetic Concepts and In Vitro Model Systems

Delivery of most drugs into the central nervous system (CNS) is restricted by the blood-brain barrier (BBB), which remains a significant bottleneck for development of novel CNS-targeted therapeutics or molecular tracers for neuroimaging. Consistent failure to reliably predict drug efficiency based on single measures for the rate or extent of brain penetration has led to the emergence of a more holistic framework that integrates data from various in vivo, in situ and in vitro assays to obtain a comprehensive description of drug delivery to and distribution within the brain. Coupled with ongoing development of suitable in vitro BBB models, this integrated approach promises to reduce the incidence of costly late-stage failures in CNS drug development, and could help to overcome some of the technical, economic and ethical issues associated with in vivo studies in animal models. Here, we provide an overview of BBB structure and function in vivo, and a summary of the pharmacokinetic parameters that can be used to determine and predict the rate and extent of drug penetration into the brain. We also review different in vitro models with regard to their inherent shortcomings and potential usefulness for development of fast-acting drugs or neurotracers labeled with short-lived radionuclides. In this regard, a special focus has been set on those systems that are sufficiently well established to be used in laboratories without significant bioengineering expertise.

Human Induced Pluripotent Stem Cell-Derived Brain Endothelial Cells: Current Controversies

Brain microvascular endothelial cells (BMECs) possess unique properties that are crucial for many functions of the blood-brain-barrier (BBB) including maintenance of brain homeostasis and regulation of interactions between the brain and immune system. The generation of a pure population of putative brain microvascular endothelial cells from human pluripotent stem cell sources (iBMECs) has been described to meet the need for reliable and reproducible brain endothelial cells in vitro. Human pluripotent stem cells (hPSCs), embryonic or induced, can be differentiated into large quantities of specialized cells in order to study development and model disease. These hPSC-derived iBMECs display endothelial-like properties, such as tube formation and low-density lipoprotein uptake, high transendothelial electrical resistance (TEER), and barrier-like efflux transporter activities. Over time, the de novo generation of an organotypic endothelial cell from hPSCs has aroused controversies. This perspective article highlights the developments made in the field of hPSC derived brain endothelial cells as well as where experimental data are lacking, and what concerns have emerged since their initial description.

The blood-brain barrier studied in vitro across species

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (BECs) supported by pericytes and astrocytes. The BBB maintains homeostasis and protects the brain against toxic substances circulating in the blood, meaning that only a few drugs can pass the BBB. Thus, for drug screening, understanding cell interactions, and pathology, in vitro BBB models have been developed using BECs from various animal sources. When comparing models of different species, differences exist especially in regards to the transendothelial electrical resistance (TEER). Thus, we compared primary mice, rat, and porcine BECs (mBECs, rBECs, and pBECs) cultured in mono- and co-culture with astrocytes, to identify species-dependent differences that could explain the variations in TEER and aid to the selection of models for future BBB studies. The BBB models based on primary mBECs, rBECs, and pBECs were evaluated and compared in regards to major BBB characteristics. The barrier integrity was evaluated by the expression of tight junction proteins and measurements of TEER and apparent permeability (Papp). Additionally, the cell size, the functionality of the P-glycoprotein (P-gp) efflux transporter, and the expression of the transferrin receptor were evaluated and compared. Expression and organization of tight junction proteins were in all three species influenced by co-culturing, supporting the findings, that TEER increases after co-culturing with astrocytes. All models had functional polarised P-gp efflux transporters and expressed the transferrin receptor. The most interesting discovery was that even though the pBECs had higher TEER than rBECs and mBECs, the Papp did not show the same variation between species, which could be explained by a significantly larger cell size of pBECs. In conclusion, our results imply that the choice of species for a given BBB study should be defined from its purpose, instead of aiming to reach the highest TEER, as the models studied here revealed similar BBB properties.

Recent progress in translational engineered in vitro models of the central nervous system

The complexity of the human brain poses a substantial challenge for the development of models of the CNS. Current animal models lack many essential human characteristics (in addition to raising operational challenges and ethical concerns), and conventional in vitro models, in turn, are limited in their capacity to provide information regarding many functional and systemic responses. Indeed, these challenges may underlie the notoriously low success rates of CNS drug development efforts. During the past 5 years, there has been a leap in the complexity and functionality of in vitro systems of the CNS, which have the potential to overcome many of the limitations of traditional model systems. The availability of human-derived induced pluripotent stem cell technology has further increased the translational potential of these systems. Yet, the adoption of state-of-the-art in vitro platforms within the CNS research community is limited. This may be attributable to the high costs or the immaturity of the systems. Nevertheless, the costs of fabrication have decreased, and there are tremendous ongoing efforts to improve the quality of cell differentiation. Herein, we aim to raise awareness of the capabilities and accessibility of advanced in vitro CNS technologies. We provide an overview of some of the main recent developments (since 2015) in in vitro CNS models. In particular, we focus on engineered in vitro models based on cell culture systems combined with microfluidic platforms (e.g. 'organ-on-a-chip' systems). We delve into the fundamental principles underlying these systems and review several applications of these platforms for the study of the CNS in health and disease. Our discussion further addresses the challenges that hinder the implementation of advanced in vitro platforms in personalized medicine or in large-scale industrial settings, and outlines the existing differentiation protocols and industrial cell sources. We conclude by providing practical guidelines for laboratories that are considering adopting organ-on-a-chip technologies.

Drug Development for Central Nervous System Diseases Using In vitro Blood-brain Barrier Models and Drug Repositioning

An important goal of biomedical research is to translate basic research findings into practical clinical implementation. Despite the advances in the technology used in drug discovery, the development of drugs for central nervous system diseases remains challenging. The failure rate for new drugs targeting important central nervous system diseases is high compared to most other areas of drug discovery. The main reason for the failure is the poor penetration efficacy across the blood-brain barrier. The blood-brain barrier represents the bottleneck in central nervous system drug development and is the most important factor limiting the future growth of neurotherapeutics. Meanwhile, drug repositioning has been becoming increasingly popular and it seems a promising field in central nervous system drug development. In vitro blood-brain barrier models with high predictability are expected for drug development and drug repositioning. In this review, the recent progress of in vitro BBB models and the drug repositioning for central nervous system diseases will be discussed.

Evaluation of the Influence of Astrocytes on In Vitro Blood-Brain Barrier Models

In vitro blood-brain barrier (BBB) models are a useful tool to screen the permeability and toxicity of new drugs. Currently, many different in vitro BBB models coexist, but none stands out as being notably better than the rest. Therefore, there is still a need to evaluate the quality of BBB models under various conditions and assess their ability to mimic the in vivo situation. In this study, two brain endothelial cell lines (bEnd.3 and hCMEC/D3) and two epithelial-like cell lines (MDCKII and Caco-2) were selected for BBB modelling purposes. They were grown as monolayers of a single cell type, under the following conditions: in coculture with either primary or immortalised astrocytes; or in the presence of primary or immortalised astrocyte-derived conditioned media. A total of 20 different BBB models were established in this manner, in order to assess the effects of the astroglial components on the BBB phenotype in each case. To this end, six parameters were studied: the expression of selected tight junction proteins; the enzyme activities of alkaline phosphatase and of gamma glutamyl transpeptidase; the transendothelial/transepithelial electrical resistance (TEER); restriction in paracellular transport; and efflux transporter inhibition were each evaluated and correlated. The results showed that coculturing with either primary or immortalised astrocytes led to a general improvement in all parameters studied, evidencing the contribution of this cell type to effective BBB formation. Furthermore, the permeability coefficient (P _e) of the tracer molecule, Lucifer Yellow, correlated with three of the six parameters studied. In addition, this study highlights the potential for the use of the Lucifer Yellow P _e value as an indicator of barrier integrity in in vitro BBB models, which could be useful for screening the permeability of new drugs.

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.

IPEC-J2 rMdr1a, a New Cell Line with Functional Expression of Rat P-glycoprotein Encoded by Rat Mdr1a for Drug Screening Purposes

The efflux pump P-glycoprotein (P-gp) affects drug distribution after absorption in humans and animals. P-gp is encoded by the multidrug resistance gene (MDR1) gene in humans, while rodents (the most common preclinical animal model) express the two isoforms Mdr1a and Mdr1b. Differences in substrate selectivity has also been reported. Our aim was to generate an in vitro cell model with tight barrier properties, expressing functional rat Mdr1a P-gp, as an in vitro tool for investigating species differences. The IPEC-J2 cell line forms extremely tight monolayers and was transfected with a plasmid carrying the rat Mdr1a gene sequence. Expression and P-gp localization at the apical membrane was demonstrated with Western blots and immunocytochemistry. Function of P-gp was shown through digoxin transport experiments in the presence and absence of the P-gp inhibitor zosuquidar. Bidirectional transport experiments across monolayers of the IPEC-J2 rMDR1a cell line and the IPEC-J2 MDR1 cell line, expressing human P-gp, showed comparable magnitude of transport in both the absorptive and efflux direction. We conclude that the newly established IPEC-J2 rMdr1a cell line, in combination with our previously established cell line IPEC-J2 MDR1, has the potential to be a strong in vitro tool to compare P-gp substrate profiles of rat and human P-gp.

Modeling blood-brain barrier pathology in cerebrovascular disease in vitro: current and future paradigms

The complexity of the blood-brain barrier (BBB) and neurovascular unit (NVU) was and still is a challenge to bridge. A highly selective, restrictive and dynamic barrier, formed at the interface of blood and brain, the BBB is a "gatekeeper" and guardian of brain homeostasis and it also acts as a "sensor" of pathological events in blood and brain. The majority of brain and cerebrovascular pathologies are associated with BBB dysfunction, where changes at the BBB can lead to or support disease development. Thus, an ultimate goal of BBB research is to develop competent and highly translational models to understand mechanisms of BBB/NVU pathology and enable discovery and development of therapeutic strategies to improve vascular health and for the efficient delivery of drugs. This review article focuses on the progress being made to model BBB injury in cerebrovascular diseases in vitro.

Culture-induced changes in mRNA expression levels of efflux and SLC-transporters in brain endothelial cells

Background

The complexity of the neurovascular unit (NVU) poses a challenge in the investigations of drug transport across the blood–brain barrier (BBB) and the function of the brain capillary endothelium. Several in vitro models of the brain capillary endothelium have been developed. In vitro culture of primary endothelial cells has, however, been reported to alter the expression levels of various brain endothelial proteins. Only a limited number of studies have addressed this in detail. The aim of the present study was to investigate mRNA levels of selected BBB transporters and markers in in vitro models of the BBB based on bovine primary endothelial cells and compare these to the levels estimated in freshly isolated bovine brain capillaries.

Methods

Brain capillaries were isolated from bovine cerebral cortex grey matter. Capillaries were seeded in culture flasks and endothelial cells were obtained using a brief trypsinization. They were seeded onto permeable supports and cultured in mono-, non-contact- or contact co-culture with/without primary rat astrocytes. mRNA-expression levels of the selected BBB markers and transporters were evaluated using qPCR and monolayer integrity of resulting monolayers was evaluated by measuring the transendothelial electrical resistance (TEER).

Results

The capillary mRNA transcript profile indicated low expression of ABCC1 and CLDN1. The mRNA expression levels of TPA, OCLN, ABCB1, SLC2A1, SLC16A1 and SLC7A5 were significantly decreased in all culture configurations compared to freshly isolated bovine brain capillaries. ALP, VWF, ABCC1 and ABCC4 were upregulated during culture, while the mRNA expression levels of F11R, TJP1, CLDN5, CLDN1 and ABCG2 were found to be unaltered. The mRNA expression levels of VWF, ALP, ABCB1 and ABCC1 were affected by the presence of rat astrocytes.

Conclusion

The endothelial mRNA transcript profile in bovine capillaries obtained in this study correlated nicely with profiles reported in mice and humans. Cultured endothelial cells drastically downregulated the mRNA expression of the investigated SLC transporters but maintained expression of efflux transporter and junctional protein mRNA, implying that the bovine in vitro BBB models may serve well to investigate basic barrier biology and in vivo permeation of passively permeating compounds and efflux transporter substrates but may be less well suited for investigations of SLC-mediated transport.

GM1 Oligosaccharide Crosses the Human Blood-Brain Barrier In Vitro by a Paracellular Route

Ganglioside GM1 (GM1) has been reported to functionally recover degenerated nervous system in vitro and in vivo, but the possibility to translate GM1′s potential in clinical settings is counteracted by its low ability to overcome the blood–brain barrier (BBB) due to its amphiphilic nature. Interestingly, the soluble and hydrophilic GM1-oligosaccharide (OligoGM1) is able to punctually replace GM1 neurotrophic functions alone, both in vitro and in vivo. In order to take advantage of OligoGM1 properties, which overcome GM1′s pharmacological limitations, here we characterize the OligoGM1 brain transport by using a human in vitro BBB model. OligoGM1 showed a 20-fold higher crossing rate than GM1 and time–concentration-dependent transport. Additionally, OligoGM1 crossed the barrier at 4 °C and in inverse transport experiments, allowing consideration of the passive paracellular route. This was confirmed by the exclusion of a direct interaction with the active ATP-binding cassette (ABC) transporters using the “pump out” system. Finally, after barrier crossing, OligoGM1 remained intact and able to induce Neuro2a cell neuritogenesis by activating the TrkA pathway. Importantly, these in vitro data demonstrated that OligoGM1, lacking the hydrophobic ceramide, can advantageously cross the BBB in comparison with GM1, while maintaining its neuroproperties. This study has improved the knowledge about OligoGM1′s pharmacological potential, offering a tangible therapeutic strategy.

ABC Transporters at the Blood-Brain Interfaces, Their Study Models, and Drug Delivery Implications in Gliomas

Drug delivery into the brain is regulated by the blood-brain interfaces. The blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the blood-arachnoid barrier (BAB) regulate the exchange of substances between the blood and brain parenchyma. These selective barriers present a high impermeability to most substances, with the selective transport of nutrients and transporters preventing the entry and accumulation of possibly toxic molecules, comprising many therapeutic drugs. Transporters of the ATP-binding cassette (ABC) superfamily have an important role in drug delivery, because they extrude a broad molecular diversity of xenobiotics, including several anticancer drugs, preventing their entry into the brain. Gliomas are the most common primary tumors diagnosed in adults, which are often characterized by a poor prognosis, notably in the case of high-grade gliomas. Therapeutic treatments frequently fail due to the difficulty of delivering drugs through the brain barriers, adding to diverse mechanisms developed by the cancer, including the overexpression or expression de novo of ABC transporters in tumoral cells and/or in the endothelial cells forming the blood-brain tumor barrier (BBTB). Many models have been developed to study the phenotype, molecular characteristics, and function of the blood-brain interfaces as well as to evaluate drug permeability into the brain. These include in vitro, in vivo, and in silico models, which together can help us to better understand their implication in drug resistance and to develop new therapeutics or delivery strategies to improve the treatment of pathologies of the central nervous system (CNS). In this review, we present the principal characteristics of the blood-brain interfaces; then, we focus on the ABC transporters present on them and their implication in drug delivery; next, we present some of the most important models used for the study of drug transport; finally, we summarize the implication of ABC transporters in glioma and the BBTB in drug resistance and the strategies to improve the delivery of CNS anticancer drugs.

Involvement of human and canine MRP1 and MRP4 in benzylpenicillin transport

The blood-brain barrier (BBB) is a dynamic and complex interface between blood and the central nervous system (CNS). It protects the brain by preventing toxic substances from entering the brain but also limits the entry of therapeutic agents. ATP-binding cassette (ABC) efflux transporters are critical for the functional barrier and present a formidable impediment to brain delivery of therapeutic agents including antibiotics. The aim of this study was to investigate the possible involvement of multidrug resistance-associated protein 1 and 4 (MRP1 and MRP4), two ABC transporters, in benzylpenicillin efflux transport using wild-type (WT) MDCKII cells and cells overexpressing those human transporters, as well as non-selective and selective inhibitors. We found that inhibiting MRP1 or MRP4 significantly increased [3H]benzylpenicillin uptake in MDCKII-WT, -MRP1 or -MRP4 cells. Similar results were also found in HepG2 cells, which highly express MRP1 and MRP4, and hCMEC/D3 cells which express MRP1. The results indicate that human and canine MRP1 and MRP4 are involved in benzylpenicillin efflux transport. They could be potential therapeutic targets for improving the efficacy of benzylpenicillin for treating CNS infections since both MRP1 and MRP4 express at human blood-brain barrier.

Effects of oxygen-glucose deprivation (OGD) on barrier properties and mRNA transcript levels of selected marker proteins in brain endothelial cells/astrocyte co-cultures

Ischemic stroke has been shown to induce breakdown of the blood-brain barrier, although these changes are not fully characterized. Oxygen-glucose deprivation (OGD) has been used to investigate the effects of ischemia in cultured brain capillary endothelial cells, however this involves a change of medium which in itself may affect the cells. The aim of the present study was to investigate the effect of OGD and simple medium exchange followed by 48 h of reperfusion on barrier properties of primary bovine endothelial cells co-cultured with rat astrocytes. Barrier properties were evaluated by transendothelial electrical resistance measurements, passive permeability of flux markers, RT-qPCR and immunocytochemistry. Both OGD and simple medium exchange caused an increase in endothelial monolayer permeability. This correlated with reduced transcript levels of a number of tight junction and tight junction-associated proteins (claudin-1, claudin-5, occludin, ZO-1, tricellulin, marveld3 and PECAM-1), as well as with altered transcript level of several transporters and receptors (GLUT-1, HB-EGF, InsR, TfR, two members of the low density lipoprotein receptor family, LDLR and LRP-1, and the efflux transporter BCRP). In contrast, effects induced specifically by OGD were transient de-localization of claudin-5 from the junction zone, increased InsR localization at the plasma membrane and transient downregulation of MRP-1 and P-gp transcript levels. In conclusion, OGD caused changes in claudin-5 and InsR localization, as well as in MRP-1 and P-gp transcript levels. Our results however also indicated that medium exchange alone caused changes in functional barrier properties and expression levels of wide range of proteins.

Investigating Maternal Brain Alterations in Preeclampsia: the Need for a Multidisciplinary Effort

PURPOSE OF REVIEW

To provide insight into the mechanisms underlying cerebral pathophysiology and to highlight possible methods for evaluation, screening, and surveillance of cerebral complications in preeclampsia.

RECENT FINDINGS

The pathophysiology of eclampsia remains enigmatic. Animal studies show that the cerebral circulation in pregnancy and preeclampsia might be affected with increased permeability over the blood-brain barrier and altered cerebral blood flow due to impaired cerebral autoregulation. The increased blood pressure cannot be the only underlying cause of eclampsia and cerebral edema, since some cases of eclampsia arise without simultaneous hypertension. Findings from animal studies need to be confirmed in human tissues. Evaluation of brain alterations in preeclampsia and eclampsia is challenging and demands a multidisciplinary collaboration, since no single method can accurately and fully describe how preeclampsia affects the brain. Cerebral complications of preeclampsia are significant factors in maternal morbidity and mortality worldwide. No single method can accurately describe the full picture of how preeclampsia affects the brain vasculature and parenchyma. We recommend an international and multidisciplinary effort not only to overcome the issue of limited sample availability but also to optimize the quality of research.

[Consideration for future in vitro BBB models - technical development to investigate the drug delivery to the CNS]

Blood vessels in the central nervous system (CNS) limit the material exchange between blood and parenchyma by blood brain barrier (BBB). At present, no appropriate in vitro BBB models are available for the investigation whether or not the candidate compounds for new drugs could be delivered to the CNS. This causes huge difficulties of the development of CNS drugs and prediction of CNS adverse effects. In this review, I first outline the structures and functions of BBB, together with the parameters used for the quantification of BBB functions. I also introduce the history of in vitro BBB models used in the drug development so far, i.e., the transition from non-cell models to the models using primary culture of rodent cells, porcine, bovine, cell lines, etc. More recently, the application of human cells differentiated from human induced pluripotent stem cells and microfluidic engineering have already started. BBB is essential for the maintenance of brain homeostasis and the mechanisms of the BBB development will be clarified by reproducing functional BBB on the dish. The new in vitro models and the data may provide accurate prediction of drug delivery to the CNS and the improvement of the evaluation system for toxicity and safety, thereby leading to successful launch of new drugs on the market.

Evaluating the involvement of cerebral microvascular endothelial Na+/K+-ATPase and Na+-K+-2Cl- co-transporter in electrolyte fluxes in an in vitro blood-brain barrier model of dehydration

The blood-brain barrier (BBB) is involved in brain water and salt homeostasis. Blood osmolarity increases during dehydration and water is osmotically extracted from the brain. The loss of water is less than expected from pure osmotic forces, due to brain electrolyte accumulation. Although the underlying molecular mechanisms are unresolved, the current model suggests the luminally expressed Na⁺-K⁺-2Cl^- co-transporter 1 (NKCC1) as a key component, while the role of the Na⁺/K⁺-ATPase remains uninvestigated. To test the involvement of these proteins in brain electrolyte flux under mimicked dehydration, we employed a tight in vitro co-culture BBB model with primary cultures of brain endothelial cells and astrocytes. The Na⁺/K⁺-ATPase and the NKCC1 were both functionally dominant in the abluminal membrane. Exposure of the in vitro BBB model to conditions mimicking systemic dehydration, i.e. hyperosmotic conditions, vasopressin, or increased [K⁺]_o illustrated that NKCC1 activity was unaffected by exposure to vasopressin and to hyperosmotic conditions. Hyperosmotic conditions and increased K⁺ concentrations enhanced the Na⁺/K⁺-ATPase activity, here determined to consist of the α1 β1 and α1 β3 isozymes. Abluminally expressed endothelial Na⁺/K⁺-ATPase, and not NKCC1, may therefore counteract osmotic brain water loss during systemic dehydration by promoting brain Na⁺ accumulation.

Is the blood-brain barrier really disrupted in all glioblastomas? A critical assessment of existing clinical data

The blood-brain barrier (BBB) excludes the vast majority of cancer therapeutics from normal brain. However, the importance of the BBB in limiting drug delivery and efficacy is controversial in high-grade brain tumors, such as glioblastoma (GBM). The accumulation of normally brain impenetrant radiographic contrast material in essentially all GBM has popularized a belief that the BBB is uniformly disrupted in all GBM patients so that consideration of drug distribution across the BBB is not relevant in designing therapies for GBM. However, contrary to this view, overwhelming clinical evidence demonstrates that there is also a clinically significant tumor burden with an intact BBB in all GBM, and there is little doubt that drugs with poor BBB permeability do not provide therapeutically effective drug exposures to this fraction of tumor cells. This review provides an overview of the clinical literature to support a central hypothesis: that all GBM patients have tumor regions with an intact BBB, and cure for GBM will only be possible if these regions of tumor are adequately treated.

How to make P-glycoprotein (ABCB1, MDR1) harbor mutations and measure its expression and activity in cell cultures?

Several polymorphisms have been identified in ABCB1, the gene encoding for the P-glycoprotein. This transporter alters the pharmacokinetics or effectiveness of drugs by excreting them from cells where it is expressed (e.g., blood-brain barrier, intestine or tumors). No consensus has been reached regarding the functional consequences of these polymorphisms in the transporter's function. The aim of this review was to describe a methodology that allows the assessment of P-gp function when harboring polymorphisms. We describe how to obtain cell lines with high expression levels of the transporter with polymorphisms and several tactics to measure its expression and activity. This methodology may help elucidate the contribution of polymorphisms in ABCB1 to drug pharmacokinetics, effectiveness and safety or to cancer chemotherapy failure.

The insulin receptor is expressed and functional in cultured blood-brain barrier endothelial cells but does not mediate insulin entry from blood to brain

Insulin and its receptor are known to be present and functional in the brain. Insulin cerebrospinal fluid concentrations have been shown to correlate with plasma levels of insulin in a nonlinear fashion, indicative of a saturable transport pathway from the blood to the brain interstitial fluid. The aim of the present study was to investigate whether insulin was transported across brain endothelial cells in vitro via an insulin receptor-dependent pathway. The study showed that the insulin receptor was expressed at both the mRNA and protein levels in bovine brain endothelial cells. Luminally applied radiolabeled insulin showed insulin receptor-mediated binding to the endothelial cells. This caused a dose-dependent increase in Akt-phosphorylation, which was inhibited by coapplication of an insulin receptor inhibitor, s961, demonstrating activation of insulin receptor signaling pathways. Transport of insulin across the blood-brain barrier in vitro was low and comparable to that of a similarly sized paracellular marker. Furthermore, insulin transport was not inhibited by coapplication of an excess of unlabeled insulin or an insulin receptor inhibitor. The insulin transport and uptake studies were repeated in mouse brain endothelial cells demonstrating similar results. Although it cannot be ruled out that culture-induced changes in the cell model could have impaired a potential insulin transport mechanism, these in vitro data indicate that peripheral insulin must reach the brain parenchyma through alternative pathways rather than crossing the blood-brain barrier via receptor mediated transcytosis.

Characterization of the L-glutamate clearance pathways across the blood-brain barrier and the effect of astrocytes in an in vitro blood-brain barrier model

The aim was to characterize the clearance pathways for L-glutamate from the brain interstitial fluid across the blood-brain barrier using a primary in vitro bovine endothelial/rat astrocyte co-culture. Transporter profiling was performed using uptake studies of radiolabeled L-glutamate with co-application of transporter inhibitors and competing amino acids. Endothelial abluminal L-glutamate uptake was almost abolished by co-application of an EAAT-1 specific inhibitor, whereas luminal uptake was inhibited by L-glutamate and L-aspartate (1 mM). L-glutamate uptake followed Michaelis-Menten-like kinetics with high and low affinity at the abluminal and luminal membrane, respectively. This indicated that L-glutamate is taken up via EAAT-1 at the abluminal membrane and exits at the luminal membrane via a low affinity glutamate/aspartate transporter. Metabolism of L-glutamate and transport of metabolites was examined using [U-¹³C] L-glutamate. Intact L-glutamate and metabolites derived from oxidative metabolism were transported through the endothelial cells. High amounts of L-glutamate-derived lactate in the luminal medium indicated cataplerosis via malic enzyme. Thus, L-glutamate can be transported intact from brain to blood via the concerted action of abluminal and luminal transport proteins, but the total brain clearance is highly dependent on metabolism in astrocytes and endothelial cells followed by transport of metabolites.

Current research into brain barriers and the delivery of therapeutics for neurological diseases: a report on CNS barrier congress London, UK, 2017

This is a report on the CNS barrier congress held in London, UK, March 22–23rd 2017 and sponsored by Kisaco Research Ltd. The two 1-day sessions were chaired by John Greenwood and Margareta Hammarlund-Udenaes, respectively, and each session ended with a discussion led by the chair. Speakers consisted of invited academic researchers studying the brain barriers in relation to neurological diseases and industry researchers studying new methods to deliver therapeutics to treat neurological diseases. We include here brief reports from the speakers.

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

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

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

Transferrin receptor expression and role in transendothelial transport of transferrin in cultured brain endothelial monolayers

Receptor-mediated transcytosis of the transferrin receptor has been suggested as a potential transport system to deliver therapeutic molecules into the brain. Recent studies have however shown that therapeutic antibodies, which have been reported to cross the brain endothelium, reach greater brain exposure when the affinity of the antibodies to the transferrin receptor is lowered. The lower affinity of the antibodies to the transferrin receptor facilitates the dissociation from the receptor within the endosomal compartments, which may indicate that the receptor itself does not necessarily move across the endothelial cells by transcytosis. The aim of the present study was to investigate transferrin receptor expression and role in transendothelial transferrin transport in cultured bovine brain endothelial cell monolayers. Transferrin receptor mRNA and protein levels were investigated in endothelial mono-cultures and co-cultures with astrocytes, as well as in freshly isolated brain capillaries using qPCR, immunocytochemistry and Western blotting. Transendothelial transport and luminal association of holo-transferrin was investigated using [¹²⁵I]holo-transferrin or [⁵⁹Fe]-transferrin. Transferrin receptor mRNA expression in all cell culture configurations was lower than in freshly isolated capillaries, but the expression slightly increased during six days of culture. The mRNA expression levels were similar in mono-cultures and co-cultures. Immunostaining demonstrated comparable transferrin receptor localization patterns in mono-cultures and co-cultures. The endothelial cells demonstrated an up-regulation of transferrin receptor mRNA after treatment with the iron chelator deferoxamine. The association of [¹²⁵I]holo-transferrin and [⁵⁹Fe]-transferrin to the endothelial cells was inhibited by an excess of unlabeled holo-transferrin, indicating receptor mediated association. However, over time the cell associated [⁵⁹Fe]-label exceeded that of [¹²⁵I]holo-transferrin, which could indicate release of iron in the endothelial cells and receptor recycling. Luminal-to-abluminal transport of [¹²⁵I]holo-transferrin across endothelial cell monolayers was low and not inhibited by unlabeled holo-transferrin. This indicated that transendothelial transferrin transport was independent of transferrin receptor-mediated transcytosis.

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

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

IPEC-J2 MDR1, a Novel High-Resistance Cell Line with Functional Expression of Human P-glycoprotein (ABCB1) for Drug Screening Studies

The P-glycoprotein (P-gp) efflux pump has been shown to affect drug distribution and absorption in various organs and to cause drug resistance in cancer therapy. The aim of this work was to develop a cell line to serve as a screening system for potential substrates of P-gp. This requires a cell line with high paracellular tightness, low expression of nonhuman ABC transporters, and high expression of functional human P-gp (ABCB1). The porcine intestinal epithelial cell line, IPEC-J2, was selected as a transfection host, due to its ability to form extremely high-resistance monolayers (>10,000 Ω·cm(2)) and its low endogenous expression of ABC-type efflux transporters. The IPEC-J2 cells were transfected with a plasmid that contained the sequence of the human MDR1 gene, which encodes P-gp, followed by a selection of successfully transfected cells with geneticin and puromycin. The resulting cell line, IPEC-J2 MDR1, retained its high transepithelilal resistance (>15,000 Ω·cm(2)), which translated into low permeability of the small hydrophilic tracer, mannitol (P < 10(-7) cm·s(-1)). The lipophilic compound, diazepam, displayed high permeability resulting in a dynamic range of 1500 (PDiazepam/Pmannitol) to separate high and low permeability compounds. Human P-gp was expressed predominantly in the apical membrane, as demonstrated by immunocytochemistry, Western blots, and a high efflux ratios (Pbasolateral-apical/Papical-basolateral) of known P-gp substrates. P-gp was demonstrated to be responsible for the efflux transport by substrate profiling, combined with application of P-gp and BCRP inhibitors. Furthermore, the compounds atenolol, citalopram, and mitoxantrone were identified as P-gp substrates. Functional P-gp expression was shown to be stable through at least 10 cell passages. In conclusion, the IPEC-J2 MDR1 cell line displays high paracellular tightness combined with high expression of human P-gp and low expression of porcine ABC transporters, and it may serve as a useful tool in drug development studies.

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

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