Differential expression of the multidrug resistance-related proteins ABCb1 and ABCc1 between blood-brain interfaces

Antiretroviral drug dolutegravir induces inflammation at the mouse brain barriers

Integrase strand transfer inhibitors (INSTIs) based antiretroviral therapy (ART) is currently used as first-line regimen to treat HIV infection. Despite its high efficacy and barrier to resistance, ART-associated neuropsychiatric adverse effects remain a major concern. Recent studies have identified a potential interaction between the INSTI, dolutegravir (DTG), and folate transport pathways at the placental barrier. We hypothesized that such interactions could also occur at the two major blood-brain interfaces: blood-cerebrospinal fluid barrier (BCSFB) and blood-brain barrier (BBB). To address this question, we evaluated the effect of two INSTIs, DTG and bictegravir (BTG), on folate transporters and receptor expression at the mouse BCSFB and the BBB in vitro, ex vivo and in vivo. We demonstrated that DTG but not BTG significantly downregulated the mRNA and/or protein expression of folate transporters (RFC/SLC19A1, PCFT/SLC46A1) in human and mouse BBB models in vitro, and mouse brain capillaries ex vivo. Our in vivo study further revealed a significant downregulation in Slc19a1 and Slc46a1 mRNA expression at the BCSFB and the BBB following a 14-day DTG oral treatment in C57BL/6 mice. However, despite the observed downregulatory effect of DTG in folate transporters/receptor at both brain barriers, a 14-day oral treatment of DTG-based ART did not significantly alter the brain folate level in animals. Interestingly, DTG treatment robustly elevated the mRNA and/or protein expression of pro-inflammatory cytokines and chemokines (Cxcl1, Cxcl2, Cxcl3, Il6, Il23, Il12) in primary cultures of mouse brain microvascular endothelial cells (BBB). DTG oral treatment also significantly upregulated proinflammatory cytokines and chemokine (Il6, Il1β, Tnfα, Ccl2) at the BCSFB in mice. We additionally observed a downregulated mRNA expression of drug efflux transporters (Abcc1, Abcc4, and Abcb1a) and tight junction protein (Cldn3) at the CP isolated from mice treated with DTG. Despite the structural similarities, BTG only elicited minor effects on the markers of interest at both the BBB and BCSFB. In summary, our current data demonstrates that DTG but not BTG strongly induced inflammatory responses in a rodent BBB and BCSFB model. Together, these data provide valuable insights into the mechanism of DTG-induced brain toxicity, which may contribute to the pathogenesis of DTG-associated neuropsychiatric adverse effect.

Metabolite transport across central nervous system barriers

Metabolomic analysis of cerebrospinal fluid (CSF) is used to improve diagnostics and pathophysiological understanding of neurological diseases. Alterations in CSF metabolite levels can partly be attributed to changes in brain metabolism, but relevant transport processes influencing CSF metabolite concentrations should be considered. The entry of molecules including metabolites into the central nervous system (CNS), is tightly controlled by the blood-brain, blood-CSF, and blood-spinal cord barriers, where aquaporins and membrane-bound carrier proteins regulate influx and efflux via passive and active transport processes. This report therefore provides reference for future CSF metabolomic work, by providing a detailed summary of the current knowledge on the location and function of the involved transporters and routing of metabolites from blood to CSF and from CSF to blood.

Mechanisms of cerebrospinal fluid secretion by the choroid plexus epithelium: Application to various intracranial pathologies

The choroid plexus (CP) is a small yet highly active epithelial tissue located in the ventricles of the brain. It secretes most of the CSF that envelops the brain and spinal cord. The epithelial cells of the CP have a high fluid secretion rate and differ from many other secretory epithelia in the organization of several key ion transporters. One striking difference is the luminal location of, for example, the vital Na+-K+-ATPase. In recent years, there has been a renewed focus on the role of ion transporters in CP secretion. Several studies have indicated that increased membrane transport activity is implicated in disorders such as hydrocephalus, idiopathic intracranial hypertension, and posthemorrhagic sequelae. The importance of the CP membrane transporters in regulating the composition of the CSF has also been a focus in research in recent years, particularly as a regulator of breathing and hemodynamic parameters such as blood pressure. This review focuses on the role of the fundamental ion transporters involved in CSF secretion and its ion composition. It gives a brief overview of the established factors and controversies concerning ion transporters, and finally discusses future perspectives related to the role of these transporters in the CP epithelium.

The impact of ATP-binding cassette transporters in the diseased brain: Context matters

ATP-binding cassette (ABC) transporters facilitate the movement of diverse molecules across cellular membranes, including those within the CNS. While most extensively studied in microvascular endothelial cells forming the blood-brain barrier (BBB), other CNS cell types also express these transporters. Importantly, disruptions in the CNS microenvironment during disease can alter transporter expression and function. Through this comprehensive review, we explore the modulation of ABC transporters in various brain pathologies and the context-dependent consequences of these changes. For instance, downregulation of ABCB1 may exacerbate amyloid beta plaque deposition in Alzheimer's disease and facilitate neurotoxic compound entry in Parkinson's disease. Upregulation may worsen neuroinflammation by aiding chemokine-mediated CD8 T cell influx into multiple sclerosis lesions. Overall, ABC transporters at the BBB hinder drug entry, presenting challenges for effective pharmacotherapy. Understanding the context-dependent changes in ABC transporter expression and function is crucial for elucidating the etiology and developing treatments for brain diseases.

Characterization of an iPSC-based barrier model for blood-brain barrier investigations using the SBAD0201 stem cell line

BACKGROUND

Blood-brain barrier (BBB) models based on primary murine, bovine, and porcine brain capillary endothelial cell cultures have long been regarded as robust models with appropriate properties to examine the functional transport of small molecules. However, species differences sometimes complicate translating results from these models to human settings. During the last decade, brain capillary endothelial-like cells (BCECs) have been generated from stem cell sources to model the human BBB in vitro. The aim of the present study was to establish and characterize a human BBB model using human induced pluripotent stem cell (hiPSC)-derived BCECs from the hIPSC line SBAD0201.

METHODS

The model was evaluated using transcriptomics, proteomics, immunocytochemistry, transendothelial electrical resistance (TEER) measurements, and, finally, transport assays to assess the functionality of selected transporters and receptor (GLUT-1, LAT-1, P-gp and LRP-1).

RESULTS

The resulting BBB model displayed an average TEER of 5474 ± 167 Ω·cm2 and cell monolayer formation with claudin-5, ZO-1, and occludin expression in the tight junction zones. The cell monolayers expressed the typical BBB markers VE-cadherin, VWF, and PECAM-1. Transcriptomics and quantitative targeted absolute proteomics analyses revealed that solute carrier (SLC) transporters were found in high abundance, while the expression of efflux transporters was relatively low. Transport assays using GLUT-1, LAT-1, and LRP-1 substrates and inhibitors confirmed the functional activities of these transporters and receptors in the model. A transport assay suggested that P-gp was not functionally expressed in the model, albeit antibody staining revealed that P-gp was localized at the luminal membrane.

CONCLUSIONS

In conclusion, the novel SBAD0201-derived BBB model formed tight monolayers and was proven useful for studies investigating GLUT-1, LAT-1, and LRP-1 mediated transport across the BBB. However, the model did not express functional P-gp and thus is not suitable for the performance of drug efflux P-gp reletated studies.

The barrier and interface mechanisms of the brain barrier, and brain drug delivery

Three different barriers are formed between the cerebrovascular and the brain parenchyma: the blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the cerebrospinal fluid-brain barrier (CBB). The BBB is the main regulator of blood and central nervous system (CNS) material exchange. The semipermeable nature of the BBB limits the passage of larger molecules and hydrophilic small molecules, Food and Drug Administration (FDA)-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Although the complexity of the BBB affects CNS drug delivery, understanding the composition and function of the BBB can provide a platform for the development of new methods for CNS drug delivery. This review summarizes the classification of the brain barrier, the composition and role of the basic structures of the BBB, and the transport, barrier, and destruction mechanisms of the BBB; discusses the advantages and disadvantages of different drug delivery methods and prospects for future drug delivery strategies.

ATP-binding cassette (ABC) drug transporters in the developing blood-brain barrier: role in fetal brain protection

The blood-brain barrier (BBB) provides essential neuroprotection from environmental toxins and xenobiotics, through high expression of drug efflux transporters in endothelial cells of the cerebral capillaries. However, xenobiotic exposure, stress, and inflammatory stimuli have the potential to disrupt BBB permeability in fetal and post-natal life. Understanding the role and ability of the BBB in protecting the developing brain, particularly with respect to drug/toxin transport, is key to promoting long-term brain health. Drug transporters, particularly P-gp and BCRP are expressed in early gestation at the developing BBB and have a crucial role in developmental homeostasis and fetal brain protection. We have highlighted several factors that modulate drug transporters at the developing BBB, including synthetic glucocorticoid (sGC), cytokines, maternal infection, and growth factors. Some factors have the potential to increase expression and function of drug transporters and increase brain protection (e.g., sGC, transforming growth factor [TGF]-β). However, others inhibit drug transporters expression and function at the BBB, increasing brain exposure to xenobiotics (e.g., tumor necrosis factor [TNF], interleukin [IL]-6), negatively impacting brain development. This has implications for pregnant women and neonates, who represent a vulnerable population and may be exposed to drugs and environmental toxins, many of which are P-gp and BCRP substrates. Thus, alterations in regulated transport across the developing BBB may induce long-term changes in brain health and compromise pregnancy outcome. Furthermore, a large portion of neonatal adverse drug reactions are attributed to agents that target or access the nervous system, such as stimulants (e.g., caffeine), anesthetics (e.g., midazolam), analgesics (e.g., morphine) and antiretrovirals (e.g., Zidovudine); thus, understanding brain protection is key for the development of strategies to protect the fetal and neonatal brain.

The Daily Expression of ABCC4 at the BCSFB Affects the Transport of Its Substrate Methotrexate

The choroid plexuses (CPs), located in the brain ventricles, form an interface between the blood and the cerebrospinal fluid named the blood-cerebrospinal barrier, which, by the presence of tight junctions, detoxification enzymes, and membrane transporters, limits the traffic of molecules into the central nervous system. It has already been shown that sex hormones regulate several CP functions, including the oscillations of its clock genes. However, it is less explored how the circadian rhythm regulates CP functions. This study aimed to evaluate the impact of sex hormones and circadian rhythms on the function of CP membrane transporters. The 24 h transcription profiles of the membrane transporters rAbca1, rAbcb1, rAbcc1, rAbcc4, rAbcg2, rAbcg4, and rOat3 were characterized in the CPs of intact male, intact female, sham-operated female, and gonadectomized rats. We found that rAbcc1 is expressed in a circadian way in the CPs of intact male rats, rAbcg2 in the CPs of intact female rats, and both rAbcc4 and rOat3 mRNA levels were expressed in a circadian way in the CPs of intact male and female rats. Next, using an in vitro model of the human blood–cerebrospinal fluid barrier, we also found that methotrexate (MTX) is transported in a circadian way across this barrier. The circadian pattern of Abcc4 found in the human CP epithelial papilloma cells might be partially responsible for MTX circadian transport across the basal membrane of CP epithelial cells.

Choroid Plexus and Drug Removal Mechanisms

Timely and efficient removal of xenobiotics and metabolites from the brain is crucial in maintaining the homeostasis and normal function of the brain. The choroid plexus (CP) forms the blood-cerebrospinal fluid barrier and vitally removes drugs and wastes from the brain through several co-existing clearance mechanisms. The CP epithelial (CPE) cells synthesize and secrete the cerebrospinal fluid (CSF). As the CSF passes through the ventricular and subarachnoid spaces and eventually drains into the general circulation, it collects and removes drugs, toxins, and metabolic wastes from the brain. This bulk flow of the CSF serves as a default and non-selective pathway for the removal of solutes and macromolecules from the brain interstitium. Besides clearance by CSF bulk flow, the CPE cells express several multispecific membrane transporters to actively transport substrates from the CSF side into the blood side. In addition, several phase I and II drug-metabolizing enzymes are expressed in the CPE cells, which enzymatically inactivate a broad spectrum of reactive or toxic substances. This review summarizes our current knowledge of the functional characteristics and key contributors to the various clearance pathways in the CP-CSF system, overviewing recent developments in our understanding of CSF flow dynamics and the functional roles of CP uptake and efflux transporters in influencing CSF drug concentrations.

Transfer of rhodamine-123 into the brain and cerebrospinal fluid of fetal, neonatal and adult rats

Background

Adenosine triphosphate binding cassette transporters such as P-glycoprotein (PGP) play an important role in drug pharmacokinetics by actively effluxing their substrates at barrier interfaces, including the blood-brain, blood-cerebrospinal fluid (CSF) and placental barriers. For a molecule to access the brain during fetal stages it must bypass efflux transporters at both the placental barrier and brain barriers themselves. Following birth, placental protection is no longer present and brain barriers remain the major line of defense. Understanding developmental differences that exist in the transfer of PGP substrates into the brain is important for ensuring that medication regimes are safe and appropriate for all patients.

Methods

In the present study PGP substrate rhodamine-123 (R123) was injected intraperitoneally into E19 dams, postnatal (P4, P14) and adult rats. Naturally fluorescent properties of R123 were utilized to measure its concentration in blood-plasma, CSF and brain by spectrofluorimetry (Clariostar). Statistical differences in R123 transfer (concentration ratios between tissue and plasma ratios) were determined using Kruskal-Wallis tests with Dunn’s corrections.

Results

Following maternal injection the transfer of R123 across the E19 placenta from maternal blood to fetal blood was around 20 %. Of the R123 that reached fetal circulation 43 % transferred into brain and 38 % into CSF. The transfer of R123 from blood to brain and CSF was lower in postnatal pups and decreased with age (brain: 43 % at P4, 22 % at P14 and 9 % in adults; CSF: 8 % at P4, 8 % at P14 and 1 % in adults). Transfer from maternal blood across placental and brain barriers into fetal brain was approximately 9 %, similar to the transfer across adult blood-brain barriers (also 9 %). Following birth when placental protection was no longer present, transfer of R123 from blood into the newborn brain was significantly higher than into adult brain (3 fold, p < 0.05).

Conclusions

Administration of a PGP substrate to infant rats resulted in a higher transfer into the brain than equivalent doses at later stages of life or equivalent maternal doses during gestation. Toxicological testing of PGP substrate drugs should consider the possibility of these patient specific differences in safety analysis.

Treating brain diseases using systemic parenterally-administered protein therapeutics: Dysfunction of the brain barriers and potential strategies

The parenteral administration of protein therapeutics is increasingly gaining importance for the treatment of human diseases. However, the presence of practically impermeable blood-brain barriers greatly restricts access of such pharmaceutics to the brain. Treating brain disorders with proteins thus remains a great challenge, and the slow clinical translation of these therapeutics may be largely ascribed to the lack of appropriate brain delivery system. Exploring new approaches to deliver proteins to the brain by circumventing physiological barriers is thus of great interest. Moreover, parallel advances in the molecular neurosciences are important for better characterizing blood-brain interfaces, particularly under different pathological conditions (e.g., stroke, multiple sclerosis, Parkinson's disease, and Alzheimer's disease). This review presents the current state of knowledge of the structure and the function of the main physiological barriers of the brain, the mechanisms of transport across these interfaces, as well as alterations to these concomitant with brain disorders. Further, the different strategies to promote protein delivery into the brain are presented, including the use of molecular Trojan horses, the formulation of nanosystems conjugated/loaded with proteins, protein-engineering technologies, the conjugation of proteins to polymers, and the modulation of intercellular junctions. Additionally, therapeutic approaches for brain diseases that do not involve targeting to the brain are presented (i.e., sink and scavenging mechanisms).

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.

Purified IgG from aquaporin-4 neuromyelitis optica spectrum disorder patients alters blood-brain barrier permeability

BACKGROUND

Neuromyelitis optica spectrum disorders (NMOSD) is a primary astrocytopathy driven by antibodies directed against the aquaporin-4 water channel located at the end-feet of the astrocyte. Although blood-brain barrier (BBB) breakdown is considered one of the key steps for the development and lesion formation, little is known about the molecular mechanisms involved. The aim of the study was to evaluate the effect of human immunoglobulins from NMOSD patients (NMO-IgG) on BBB properties.

METHODS

Freshly isolated brain microvessels (IBMs) from rat brains were used as a study model. At first, analysis of the secretome profile from IBMs exposed to purified NMO-IgG, to healthy donor IgG (Control-IgG), or non-treated, was performed. Second, tight junction (TJ) proteins expression in fresh IBMs and primary cultures of brain microvascular endothelial cells (BMEC) was analysed by Western blotting (Wb) after exposition to NMO-IgG and Control-IgG. Finally, functional BBB properties were investigated evaluating the presence of rat-IgG in tissue lysate from brain using Wb in the rat-model, and the passage of NMO-IgG and sucrose in a bicameral model.

RESULTS

We found that NMO-IgG induces functional and morphological BBB changes, including: 1) increase of pro-inflammatory cytokines production (CXCL-10 [IP-10], IL-6, IL-1RA, IL-1β and CXCL-3) in IBMs when exposed to NMO-IgG; 2) decrease of Claudin-5 levels by 25.6% after treatment of fresh IBMs by NMO-IgG compared to Control-IgG (p = 0.002), and similarly, decrease of Claudin-5 by at least 20% when BMEC were cultured with NMO-IgG from five different patients; 3) a higher level of rat-IgG accumulated in periventricular regions of NMO-rats compared to Control-rats and an increase in the permeability of BBB after NMO-IgG treatment in the bicameral model.

CONCLUSION

Human NMO-IgG induces both structural and functional alterations of BBB properties, suggesting a direct role of NMO-IgG on modulation of BBB permeability in NMOSD.

The molecular anatomy and functions of the choroid plexus in healthy and diseased brain

The choroid plexus (CP) is located in the ventricular system of the brain (one in each ventricle), and the CP epithelial cells form an important barrier between the blood and the cerebrospinal fluid (CSF). Their main function comprises CSF secretion, maintenance of brain homeostasis, signalling, and forming a neuroprotective barrier against harmful external and internal compounds. The CPs mature early and demonstrate expressional changes of barrier-specific genes and proteins related to location and developmental stage of the CP. Important proteins for the barrier function include tight junction proteins, numerous transporters and enzymes. Natural senescence leads to structural changes in the CP cells and reduced or loss of function, while further loss of CP function and changes in immune status may be relevant in neurodegenerative diseases such as Alzheimer's disease and Multiple Sclerosis. Neuroprotective genes expressed at CPs may be unexplored targets for new therapies for neurodegenerative diseases.

A Practical Perspective on the Evaluation of Small Molecule CNS Penetration in Drug Discovery

The separation of the brain from blood by the blood-brain barrier and the bloodcerebrospinal fluid (CSF) barrier poses unique challenges for the discovery and development of drugs targeting the central nervous system (CNS). This review will describe the role of transporters in CNS penetration and examine the relationship between unbound brain (Cu-brain) and unbound plasma (Cu-plasma) or CSF (CCSF) concentration. Published data demonstrate that the relationship between Cu-brain and Cu-plasma or CCSF can be affected by transporter status and passive permeability of a drug and CCSF may not be a reliable surrogate for CNS penetration. Indeed, CCSF usually over-estimates Cu-brain for efflux substrates and it provides no additional value over Cu-plasma as the surrogate of Cu-brain for highly permeable non-efflux substrates. A strategy described here for the evaluation of CNS penetration is to use in vitro permeability, P-glycoprotein (Pgp) and breast cancer resistance protein efflux assays and Cu-brain/Cu-plasma in preclinical species. Cu-plasma should be used as the surrogate of Cu-brain for highly permeable non-efflux substrates with no evidence of impaired distribution into the brain. When drug penetration into the brain is impaired, we recommend using (total brain concentration * unbound fraction in the brain) as Cu-brain in preclinical species or Cu-plasma/in vitro Pgp efflux ratio if Pgp is the major limiting mechanism for brain penetration.

Medications for pregnant women: A balancing act between the interests of the mother and of the fetus

Drug entry into the adult brain is controlled by efflux mechanisms situated in various brain barrier interfaces. The effectiveness of these protective mechanisms in the embryo, fetus and newborn brain is less clear. The longstanding belief that "the" blood-brain barrier is absent or immature in the fetus and newborn has led to many misleading statements with potential clinical implications. Here we review the properties of brain barrier mechanisms in the context of drug entry into the developing brain and discuss the limited number of studies published on the subject. We noticed that most of available literature suffers from some experimental limitations, notably that drug levels in fetal blood and cerebrospinal fluid have not been measured. This means that the relative contribution to the overall brain protection provided by individual barriers such as the placenta (which contains similar efflux mechanisms) and the brain barriers cannot be separately ascertained. Finally, we propose that systematic studies in appropriate animal models of drug entry into the brain at different stages of development would provide a rational basis for use of medications in pregnancy and in newborns, especially prematurely born, where protection usually provided by the placenta is no longer present.

The bitter taste receptor TAS2R14 regulates resveratrol transport across the human blood-cerebrospinal fluid barrier

The regulation of transport mechanisms at brain barriers must be thoroughly understood, so that novel strategies for improving drug delivery to the brain can be designed. The blood-cerebrospinal fluid barrier (BCSFB) established by the choroid plexus (CP) epithelial cells has been poorly studied in this regard despite its relevance for the protection of the central nervous system (CNS). This study assessed the role of bitter taste receptors (TAS2Rs), TAS2R14 and TAS2R39, in the transport of resveratrol across CP epithelial cells using an in vitro model of the human BCSFB. Both receptors are expressed in human CP cells and known to bind resveratrol. First, Ca²⁺ imaging assays demonstrated that resveratrol specifically activates the TAS2R14 receptor, but not TAS2R39, in these human CP epithelial cells. Then, we proceeded with permeation studies that showed resveratrol can cross the human BCSFB, from the blood to the CSF side and that TAS2R14 knockdown decreased the transport of resveratrol across these cells. Conversely, inhibition of efflux transporters ABCC1, ABCC4 or ABCG2 also restrained the transport of resveratrol across these cells. Interestingly, resveratrol upregulated the expression of ABCG2 located at the apical membrane of the cells via TAS2R14, whereas ABCC1 and ABCC4 at the basolateral membrane of the cells were not affected. Altogether, our study demonstrates that the BCSFB is a gateway for resveratrol entrance into the CNS and that the receptor TAS2R14 regulates its transport by regulating the action of efflux transporters at CP epithelial cells.

Inhibition of glycosphingolipid biosynthesis reverts multidrug resistance by differentially modulating ABC transporters in chronic myeloid leukemias

Multidrug resistance (MDR) in cancer arises from cross-resistance to structurally- and functionally-divergent chemotherapeutic drugs. In particular, MDR is characterized by increased expression and activity of ATP-binding cassette (ABC) superfamily transporters. Sphingolipids are substrates of ABC proteins in cell signaling, membrane biosynthesis, and inflammation, for example, and their products can favor cancer progression. Glucosylceramide (GlcCer) is a ubiquitous glycosphingolipid (GSL) generated by glucosylceramide synthase, a key regulatory enzyme encoded by the UDP-glucose ceramide glucosyltransferase (UGCG) gene. Stressed cells increase de novo biosynthesis of ceramides, which return to sub-toxic levels after UGCG mediates incorporation into GlcCer. Given that cancer cells seem to mobilize UGCG and have increased GSL content for ceramide clearance, which ultimately contributes to chemotherapy failure, here we investigated how inhibition of GSL biosynthesis affects the MDR phenotype of chronic myeloid leukemias. We found that MDR is associated with higher UGCG expression and with a complex GSL profile. UGCG inhibition with the ceramide analog d-threo-1-(3,4,-ethylenedioxy)phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (EtDO-P4) greatly reduced GSL and monosialotetrahexosylganglioside levels, and co-treatment with standard chemotherapeutics sensitized cells to mitochondrial membrane potential loss and apoptosis. ABC subfamily B member 1 (ABCB1) expression was reduced, and ABCC-mediated efflux activity was modulated by competition with nonglycosylated ceramides. Consistently, inhibition of ABCC-mediated transport reduced the efflux of exogenous C₆-ceramide. Overall, UGCG inhibition impaired the malignant glycophenotype of MDR leukemias, which typically overcomes drug resistance through distinct mechanisms. This work sheds light on the involvement of GSL in chemotherapy failure, and its findings suggest that targeted GSL modulation could help manage MDR leukemias.

Alteration in the Function and Expression of SLC and ABC Transporters in the Neurovascular Unit in Alzheimer's Disease and the Clinical Significance

The neurovascular unit (NVU) plays an important role in maintaining the function of the central nervous system (CNS). Emerging evidence has indicated that the NVU changes function and molecules at the early stage of Alzheimer’s disease (AD), which initiates multiple pathways of neurodegeneration. Cell types in the NVU have become attractive targets in the interventional treatment of AD. The NVU transportation system contains a variety of proteins involved in compound transport and neurotransmission. Brain transporters can be classified as members of the solute carrier (SLC) and ATP-binding cassette (ABC) families in the NVU. Moreover, the transporters can regulate both endogenous toxins, including amyloid-beta (Aβ) and xenobiotic homeostasis, in the brains of AD patients. Genome-wide association studies (GWAS) have identified some transporter gene variants as susceptibility loci for late-onset AD. Therefore, the present study summarizes changes in blood-brain barrier (BBB) permeability in AD, identifies the location of SLC and ABC transporters in the brain and focuses on major SLC and ABC transporters that contribute to AD pathology.

Brain Distribution of Drugs: Pharmacokinetic Considerations

It is crucial to understand the basic principles of drug transport, from the site of delivery to the site of action within the CNS, in order to evaluate the possible utility of a new drug candidate for CNS action, or possible CNS side effects of non-CNS targeting drugs. This includes pharmacokinetic aspects of drug concentration-time profiles in plasma and brain, blood-brain barrier transport and drug distribution within the brain parenchyma as well as elimination processes from the brain. Knowledge of anatomical and physiological aspects connected with drug delivery is crucial in this context. The chapter is intended for professionals working in the field of CNS drug development and summarizes key pharmacokinetic principles and state-of-the-art experimental methodologies to assess brain drug disposition. Key parameters, describing the extent of unbound (free) drug across brain barriers, in particular blood-brain and blood-cerebrospinal fluid barriers, are presented along with their application in drug development. Special emphasis is given to brain intracellular pharmacokinetics and its role in evaluating target engagement. Fundamental neuropharmacokinetic differences between small molecular drugs and biologicals are discussed and critical knowledge gaps are outlined.

ABC efflux transporters at blood-central nervous system barriers and their implications for treating spinal cord disorders

The barriers present in the interfaces between the blood and the central nervous system form a major hurdle for the pharmacological treatment of central nervous system injuries and diseases. The family of ATP-binding cassette (ABC) transporters has been widely studied regarding efflux of medications at blood-central nervous system barriers. These efflux transporters include P-glycoprotein (abcb1), 'breast cancer resistance protein' (abcg2) and the various 'multidrug resistance-associated proteins' (abccs). Understanding which efflux transporters are present at the blood-spinal cord, blood-cerebrospinal fluid and cerebrospinal fluid-spinal cord barriers is necessary to determine their involvement in limiting drug transfer from blood to the spinal cord tissue. Recent developments in the blood-brain barrier field have shown that barrier systems are dynamic and the profile of barrier defenses can alter due to conditions such as age, disease and environmental challenge. This means that a true understanding of ABC efflux transporter expression and localization should not be one static value but instead a range that represents the complex patient subpopulations that exist. In the present review, the blood-central nervous system barrier literature is discussed with a focus on the impact of ABC efflux transporters on: (i) protecting the spinal cord from adverse effects of systemically directed drugs, and (ii) limiting centrally directed drugs from accessing their active sites within the spinal cord.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Mind the Gaps: Ontogeny of Human Brain P-gp and Its Impact on Drug Toxicity

Available data on human brain P-glycoprotein ontogeny during infancy and childhood are limited. This review discusses the current body of data relating to maturation of human brain P-glycoprotein including transporter expression levels in post-mortem human brain samples, in vivo transporter activity using probe substrates, surrogate marker endpoints, and extrapolations from animal models. Overall, the data tend to confirm that human brain P-glycoprotein activity keeps developing after birth, although with a developmental time frame that remains unclear. This knowledge gap is a concern given the critical role of brain P-glycoprotein in drug safety and efficacy, and the vulnerable nature of the pediatric population. Future research could include the measurement of brain P-glycoprotein activity across age groups using positron emission tomography or central pharmacodynamic responses. For now, caution is advised when extrapolating adult data to children aged younger than 2 years for drugs with P-glycoprotein-dependent central nervous system activity.

Clinical Imaging of Choroid Plexus in Health and in Brain Disorders: A Mini-Review

The choroid plexuses (ChPs) perform indispensable functions for the development, maintenance and functioning of the brain. Although they have gained considerable interest in the last years, their involvement in brain disorders is still largely unknown, notably because their deep location inside the brain hampers non-invasive investigations. Imaging tools have become instrumental to the diagnosis and pathophysiological study of neurological and neuropsychiatric diseases. This review summarizes the knowledge that has been gathered from the clinical imaging of ChPs in health and brain disorders not related to ChP pathologies. Results are discussed in the light of pre-clinical imaging studies. As seen in this review, to date, most clinical imaging studies of ChPs have used disease-free human subjects to demonstrate the value of different imaging biomarkers (ChP size, perfusion/permeability, glucose metabolism, inflammation), sometimes combined with the study of normal aging. Although very few studies have actually tested the value of ChP imaging biomarkers in patients with brain disorders, these pioneer studies identified ChP changes that are promising data for a better understanding and follow-up of diseases such as schizophrenia, epilepsy and Alzheimer’s disease. Imaging of immune cell trafficking at the ChPs has remained limited to pre-clinical studies so far but has the potential to be translated in patients for example using MRI coupled with the injection of iron oxide nanoparticles. Future investigations should aim at confirming and extending these findings and at developing translational molecular imaging tools for bridging the gap between basic molecular and cellular neuroscience and clinical research.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Recent Developments in Understanding Barrier Mechanisms in the Developing Brain: Drugs and Drug Transporters in Pregnancy, Susceptibility or Protection in the Fetal Brain?

Efflux mechanisms situated in various brain barrier interfaces control drug entry into the adult brain; this review considers the effectiveness of these protective mechanisms in the embryo, fetus, and newborn brain. The longstanding belief that the blood-brain barrier is absent or immature in the fetus and newborn has led to many misleading statements with potential clinical implications. The immature brain is undoubtedly more vulnerable to damage by drugs and toxins; as is reviewed here, some developmentally regulated normal brain barrier mechanisms probably contribute to this vulnerability. We propose that the functional status of brain barrier efflux mechanisms should be investigated at different stages of brain development to provide a rational basis for the use of drugs in pregnancy and in newborns, especially in those prematurely born, where protection usually provided by the placenta is no longer present.

Blood-brain barrier development: Systems modeling and predictive toxicology

The blood-brain barrier (BBB) serves as a gateway for passage of drugs, chemicals, nutrients, metabolites, and hormones between vascular and neural compartments in the brain. Here, we review BBB development with regard to the microphysiology of the neurovascular unit (NVU) and the impact of BBB disruption on brain development. Our focus is on modeling these complex systems. Extant in silico models are available as tools to predict the probability of drug/chemical passage across the BBB; in vitro platforms for high-throughput screening and high-content imaging provide novel data streams for profiling chemical-biological interactions; and engineered human cell-based microphysiological systems provide empirical models with which to investigate the dynamics of NVU function. Computational models are needed that bring together kinetic and dynamic aspects of NVU function across gestation and under various physiological and toxicological scenarios. This integration will inform adverse outcome pathways to reduce uncertainty in translating in vitro data and in silico models for use in risk assessments that aim to protect neurodevelopmental health.

The myth of the immature barrier systems in the developing brain: role in perinatal brain injury

Central nervous system homeostasis is maintained by cellular barriers that protect the brain from external environmental changes and protect the CNS from harmful molecules and pathogens in the blood. Historically, for many years these barriers were thought of as immature, with limited functions, during brain development. In this review, we will present advances in the understanding of the barrier systems during development and evidence to show that in fact the barriers serve many important neurodevelopmental functions and that fetal and newborn brains are well protected. We will also discuss how ischaemic injury or systemic inflammation may breach the integrity of the barriers in the developing brain.

Neurosteroid Transport in the Brain: Role of ABC and SLC Transporters

Neurosteroids, comprising pregnane, androstane, and sulfated steroids can alter neuronal excitability through interaction with ligand-gated ion channels and other receptors and have therefore a therapeutic potential in several brain disorders. They can be formed in brain cells or are synthesized by an endocrine gland and reach the brain by penetrating the blood–brain barrier (BBB). Especially sulfated steroids such as pregnenolone sulfate (PregS) and dehydroepiandrosterone sulfate (DHEAS) depend on transporter proteins to cross membranes. In this review, we discuss the involvement of ATP-binding cassette (ABC)- and solute carrier (SLC)-type membrane proteins in the transport of these compounds at the BBB and in the choroid plexus (CP), but also in the secretion from neurons and glial cells. Among the ABC transporters, especially BCRP (ABCG2) and several MRP/ABCC subfamily members (MRP1, MRP4, MRP8) are expressed in the brain and known to efflux conjugated steroids. Furthermore, several SLC transporters have been shown to mediate cellular uptake of steroid sulfates. These include members of the OATP/SLCO subfamily, namely OATP1A2 and OATP2B1, as well as OAT3 (SLC22A3), which have been reported to be expressed at the BBB, in the CP and in part in neurons. Furthermore, a role of the organic solute transporter OSTα-OSTβ (SLC51A/B) in brain DHEAS/PregS homeostasis has been proposed. This transporter was reported to be localized especially in steroidogenic cells of the cerebellum and hippocampus. To date, the impact of transporters on neurosteroid homeostasis is still poorly understood. Further insights are desirable also with regard to the therapeutic potential of these compounds.

Neuroimmune Axes of the Blood-Brain Barriers and Blood-Brain Interfaces: Bases for Physiological Regulation, Disease States, and Pharmacological Interventions

Central nervous system (CNS) barriers predominantly mediate the immune-privileged status of the brain, and are also important regulators of neuroimmune communication. It is increasingly appreciated that communication between the brain and immune system contributes to physiologic processes, adaptive responses, and disease states. In this review, we discuss the highly specialized features of brain barriers that regulate neuroimmune communication in health and disease. In section I, we discuss the concept of immune privilege, provide working definitions of brain barriers, and outline the historical work that contributed to the understanding of CNS barrier functions. In section II, we discuss the unique anatomic, cellular, and molecular characteristics of the vascular blood-brain barrier (BBB), blood-cerebrospinal fluid barrier, and tanycytic barriers that confer their functions as neuroimmune interfaces. In section III, we consider BBB-mediated neuroimmune functions and interactions categorized as five neuroimmune axes: disruption, responses to immune stimuli, uptake and transport of immunoactive substances, immune cell trafficking, and secretions of immunoactive substances. In section IV, we discuss neuroimmune functions of CNS barriers in physiologic and disease states, as well as pharmacological interventions for CNS diseases. Throughout this review, we highlight many recent advances that have contributed to the modern understanding of CNS barriers and their interface functions.

Barriers to Drug Distribution into the Perinatal and Postnatal Brain

Drug bioavailability to the developing brain is a major concern in the treatment of neonates and infants as well as pregnant and breast-feeding women. Central adverse drug reactions can have dramatic consequences for brain development, leading to major neurological impairment. Factors setting the cerebral bioavailability of drugs include protein-unbound drug concentration in plasma, local cerebral blood flow, permeability across blood-brain interfaces, binding to neural cells, volume of cerebral fluid compartments, and cerebrospinal fluid secretion rate. Most of these factors change during development, which will affect cerebral drug concentrations. Regarding the impact of blood-brain interfaces, the blood-brain barrier located at the cerebral endothelium and the blood-cerebrospinal fluid barrier located at the choroid plexus epithelium both display a tight phenotype early on in embryos. However, the developmental regulation of some multispecific efflux transporters that also limit the entry of numerous drugs into the brain through barrier cells is expected to favor drug penetration in the neonatal brain. Finally, drug cerebral bioavailability is likely to be affected following perinatal injuries that alter blood-brain interface properties. A thorough investigation of these mechanisms is mandatory for a better risk assessment of drug treatments in pregnant or breast-feeding women, and in neonate and pediatric patients.

Glutathione Conjugation at the Blood-CSF Barrier Efficiently Prevents Exposure of the Developing Brain Fluid Environment to Blood-Borne Reactive Electrophilic Substances

Exposure of the developing brain to toxins, drugs, or deleterious endogenous compounds during the perinatal period can trigger alterations in cell division, migration, differentiation, and synaptogenesis, leading to lifelong neurological impairment. The brain is protected by cellular barriers acting through multiple mechanisms, some of which are still poorly explored. We used a combination of enzymatic assays, live tissue fluorescence microscopy, and an in vitro cellular model of the blood-CSF barrier to investigate an enzymatic detoxification pathway in the developing male and female rat brain. We show that during the early postnatal period the choroid plexus epithelium forming the blood-CSF barrier and the ependymal cell layer bordering the ventricles harbor a high detoxifying capacity that involves glutathione S-transferases. Using a functional knock-down rat model for choroidal glutathione conjugation, we demonstrate that already in neonates, this metabolic pathway efficiently prevents the penetration of blood-borne reactive compounds into CSF. The versatility of the protective mechanism results from the multiplicity of the glutathione S-transferase isoenzymes, which are differently expressed between the choroidal epithelium and the ependyma. The various isoenzymes display differential substrate specificities, which greatly widen the spectrum of molecules that can be inactivated by this pathway. In conclusion, the blood-CSF barrier and the ependyma are identified as key cellular structures in the CNS to protect the brain fluid environment from different chemical classes of potentially toxic compounds during the postnatal period. This metabolic neuroprotective function of brain interfaces ought to compensate for the liver postnatal immaturity.SIGNIFICANCE STATEMENT Brain homeostasis requires a stable and controlled internal environment. Defective brain protection during the perinatal period can lead to lifelong neurological impairment. We demonstrate that the choroid plexus forming the blood-CSF barrier is a key player in the protection of the developing brain. Glutathione-dependent enzymatic metabolism in the choroidal epithelium inactivates a broad spectrum of noxious compounds, efficiently preventing their penetration into the CSF. A second line of detoxification is located in the ependyma separating the CSF from brain tissue. Our study reveals a novel facet of the mechanisms by which the brain is protected at a period of high vulnerability, at a time when the astrocytic network is still immature and liver xenobiotic metabolism is limited.

Molecular anatomy and functions of the choroidal blood-cerebrospinal fluid barrier in health and disease

The barrier between the blood and the ventricular cerebrospinal fluid (CSF) is located at the choroid plexuses. At the interface between two circulating fluids, these richly vascularized veil-like structures display a peculiar morphology explained by their developmental origin, and fulfill several functions essential for CNS homeostasis. They form a neuroprotective barrier preventing the accumulation of noxious compounds into the CSF and brain, and secrete CSF, which participates in the maintenance of a stable CNS internal environment. The CSF circulation plays an important role in volume transmission within the developing and adult brain, and CSF compartments are key to the immune surveillance of the CNS. In these contexts, the choroid plexuses are an important source of biologically active molecules involved in brain development, stem cell proliferation and differentiation, and brain repair. By sensing both physiological changes in brain homeostasis and peripheral or central insults such as inflammation, they also act as sentinels for the CNS. Finally, their role in the control of immune cell traffic between the blood and the CSF confers on the choroid plexuses a function in neuroimmune regulation and implicates them in neuroinflammation. The choroid plexuses, therefore, deserve more attention while investigating the pathophysiology of CNS diseases and related comorbidities.

Drug Clearance from Cerebrospinal Fluid Mediated by Organic Anion Transporters 1 (Slc22a6) and 3 (Slc22a8) at Arachnoid Membrane of Rats

Although arachnoid mater epithelial cells form the blood-arachnoid barrier (BAB), acting as a blood-CSF interface, it has been generally considered that the BAB is impermeable to water-soluble substances and plays a largely passive role. Here, we aimed to clarify the function of transporters at the BAB in regulating CSF clearance of water-soluble organic anion drugs based on quantitative targeted absolute proteomics (QTAP) and in vivo analyses. Protein expression levels of 61 molecules, including 19 ATP-binding-cassette (ABC) transporters and 32 solute-carrier (SLC) transporters, were measured in plasma membrane fraction of rat leptomeninges using QTAP. Thirty-three proteins were detected; others were under the quantification limits. Expression levels of multidrug resistance protein 1 (Mdr1a/P-gp/Abcb1a) and breast cancer resistance protein (Bcrp/Abcg2) were 16.6 and 3.27 fmol/μg protein (51.9- and 9.82-fold greater than in choroid plexus, respectively). Among those organic anion transporters detected only at leptomeninges, not choroid plexus, organic anion transporter 1 (oat1/Slc22a6) showed the greatest expression (2.73 fmol/μg protein). On the other hand, the protein expression level of oat3 at leptomeninges was 6.65 fmol/μg protein, and the difference from choroid plexus was within two-fold. To investigate oat1's role, we injected para-aminohippuric acid (PAH) with or without oat1 inhibitors into cisterna magna (to minimize the contribution of choroid plexus function) of rats. A bulk flow marker, FITC-inulin, was not taken up from CSF up to 15 min, whereas uptake clearance of PAH was 26.5 μL/min. PAH uptake was completely blocked by 3 mM cephalothin (inhibits both oat1 and oat3), while 17% of PAH uptake was inhibited by 0.2 mM cephalothin (selectively inhibits oat3). These results indicate that oat1 and oat3 at the BAB provide a distinct clearance pathway of organic anion drugs from CSF independently of choroid plexus.

Potential Pathways for CNS Drug Delivery Across the Blood-Cerebrospinal Fluid Barrier

The blood-brain interfaces restrict the cerebral bioavailability of pharmacological compounds. Various drug delivery strategies have been developed to improve drug penetration into the brain. Most strategies target the microvascular endothelium forming the blood-brain barrier proper. Targeting the blood-cerebrospinal fluid (CSF) barrier formed by the epithelium of the choroid plexuses in addition to the blood-brain barrier may offer added-value for the treatment of central nervous system diseases. For instance, targeting the CSF spaces, adjacent tissue, or the choroid plexuses themselves is of interest for the treatment of neuroinflammatory and infectious diseases, cerebral amyloid angiopathy, selected brain tumors, hydrocephalus or neurohumoral dysregulation. Selected CSF-borne materials seem to reach deep cerebral structures by mechanisms that need to be understood in the context of chronic CSF delivery. Drug delivery through both barriers can reduce CSF sink action towards parenchymal drugs. Finally, targeting the choroid plexus-CSF system can be especially relevant in the context of neonatal and pediatric diseases of the central nervous system. Transcytosis appears the most promising mechanism to target in order to improve drug delivery through brain barriers. The choroid plexus epithelium displays strong vesicular trafficking and secretory activities that deserve to be explored in the context of cerebral drug delivery. Folate transport and exosome release into the CSF, plasma protein transport, and various receptor-mediated endocytosis pathways may prove useful mechanisms to exploit for efficient drug delivery into the CSF. This calls for a clear evaluation of transcytosis mechanisms at the blood-CSF barrier, and a thorough evaluation of CSF drug delivery rates.

Obstacles to Brain Tumor Therapy: Key ABC Transporters

The delivery of cancer chemotherapy to treat brain tumors remains a challenge, in part, because of the inherent biological barrier, the blood-brain barrier. While its presence and role as a protector of the normal brain parenchyma has been acknowledged for decades, it is only recently that the important transporter components, expressed in the tightly knit capillary endothelial cells, have been deciphered. These transporters are ATP-binding cassette (ABC) transporters and, so far, the major clinically important ones that functionally contribute to the blood-brain barrier are ABCG2 and ABCB1. A further limitation to cancer therapy of brain tumors or brain metastases is the blood-tumor barrier, where tumors erect a barrier of transporters that further impede drug entry. The expression and regulation of these two transporters at these barriers, as well as tumor derived alteration in expression and/or mutation, are likely obstacles to effective therapy.

P-glycoprotein (ABCB1) and Oxidative Stress: Focus on Alzheimer's Disease

ATP-binding cassette (ABC) transporters, in particular P-glycoprotein (encoded by ABCB1), are important and selective elements of the blood-brain barrier (BBB), and they actively contribute to brain homeostasis. Changes in ABCB1 expression and/or function at the BBB may not only alter the expression and function of other molecules at the BBB but also affect brain environment. Over the last decade, a number of reports have shown that ABCB1 actively mediates the transport of beta amyloid (Aβ) peptide. This finding has opened up an entirely new line of research in the field of Alzheimer's disease (AD). Indeed, despite intense research efforts, AD remains an unsolved pathology and effective therapies are still unavailable. Here, we review the crucial role of ABCB1 in the Aβ transport and how oxidative stress may interfere with this process. A detailed understanding of ABCB1 regulation can provide the basis for improved neuroprotection in AD and also enhanced therapeutic drug delivery to the brain.

The Differential Binding of Antipsychotic Drugs to the ABC Transporter P-Glycoprotein Predicts Cannabinoid-Antipsychotic Drug Interactions

Cannabis use increases rates of psychotic relapse and treatment failure in schizophrenia patients. Clinical studies suggest that cannabis use reduces the efficacy of antipsychotic drugs, but there has been no direct demonstration of this in a controlled study. The present study demonstrates that exposure to the principal phytocannabinoid, Δ⁹-tetrahydrocannabinol (THC), reverses the neurobehavioral effects of the antipsychotic drug risperidone in mice. THC exposure did not influence D₂ and 5-HT_2A receptor binding, the major targets of antipsychotic action, but it lowered the brain concentrations of risperidone and its active metabolite, 9-hydroxy risperidone. As risperidone and its active metabolite are excellent substrates of the ABC transporter P-glycoprotein (P-gp), we hypothesized that THC might increase P-gp expression at the blood-brain barrier (BBB) and thus enhance efflux of risperidone and its metabolite from brain tissue. We confirmed that the brain disposition of risperidone and 9-hydroxy risperidone is strongly influenced by P-gp, as P-gp knockout mice displayed greater brain concentrations of these drugs than wild-type mice. Furthermore, we demonstrated that THC exposure increased P-gp expression in various brain regions important to risperidone's antipsychotic action. We then showed that THC exposure did not influence the neurobehavioral effects of clozapine. Clozapine shares a very similar antipsychotic mode of action to risperidone, but unlike risperidone is not a P-gp substrate. Our results imply that clozapine or non-P-gp substrate antipsychotic drugs may be better first-line treatments for schizophrenia patients with a history of cannabis use.

Modulation of bilirubin neurotoxicity by the Abcb1 transporter in the Ugt1-/- lethal mouse model of neonatal hyperbilirubinemia

Moderate neonatal jaundice is the most common clinical condition during newborn life. However, a combination of factors may result in acute hyperbilirubinemia, placing infants at risk of developing bilirubin encephalopathy and death by kernicterus. While most risk factors are known, the mechanisms acting to reduce susceptibility to bilirubin neurotoxicity remain unclear. The presence of modifier genes modulating the risk of developing bilirubin-induced brain damage is increasingly being recognised. The Abcb1 and Abcc1 members of the ABC family of transporters have been suggested to have an active role in exporting unconjugated bilirubin from the central nervous system into plasma. However, their role in reducing the risk of developing neurological damage and death during neonatal development is still unknown.To this end, we mated Abcb1a/b-/- and Abcc1-/- strains with Ugt1-/- mice, which develop severe neonatal hyperbilirubinemia. While about 60% of Ugt1-/- mice survived after temporary phototherapy, all Abcb1a/b-/-/Ugt1-/- mice died before postnatal day 21, showing higher cerebellar levels of unconjugated bilirubin. Interestingly, Abcc1 role appeared to be less important.In the cerebellum of Ugt1-/- mice, hyperbilirubinemia induced the expression of Car and Pxr nuclear receptors, known regulators of genes involved in the genotoxic response.We demonstrated a critical role of Abcb1 in protecting the cerebellum from bilirubin toxicity during neonatal development, the most clinically relevant phase for human babies, providing further understanding of the mechanisms regulating bilirubin neurotoxicity in vivo. Pharmacological treatments aimed to increase Abcb1 and Abcc1 expression, could represent a therapeutic option to reduce the risk of bilirubin neurotoxicity.

Quantification of Transporter and Receptor Proteins in Dog Brain Capillaries and Choroid Plexus: Relevance for the Distribution in Brain and CSF of Selected BCRP and P-gp Substrates

Transporters at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a pivotal role as gatekeepers for efflux or uptake of endogenous and exogenous molecules. The protein expression of a number of them has already been determined in the brains of rodents, nonhuman primates, and humans using quantitative targeted absolute proteomics (QTAP). The dog is an important animal model for drug discovery and development, especially for safety evaluations. The purpose of the present study was to clarify the relevance of the transporter protein expression for drug distribution in the dog brain and CSF. We used QTAP to examine the protein expression of 17 selected transporters and receptors at the dog BBB and BCSFB. For the first time, we directly linked the expression of two efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), to regional brain and CSF distribution using specific substrates. Two cocktails, each containing one P-gp substrate (quinidine or apafant) and one BCRP substrate (dantrolene or daidzein) were infused intravenously prior to collection of the brain. Transporter expression varied only slightly between the capillaries of different brain regions and did not result in region-specific distribution of the investigated substrates. There were, however, distinct differences between brain capillaries and choroid plexus. Largest differences were observed for BCRP and P-gp: both were highly expressed in brain capillaries, but no BCRP and only low amounts of P-gp were detected in the choroid plexus. K_p,uu,brain and K_p,uu,CSF of both P-gp substrates were indicative of drug efflux. Also, K_p,uu,brain for the BCRP substrates was low. In contrast, K_p,uu,CSF for both BCRP substrates was close to unity, resulting in K_p,uu,CSF/K_p,uu,brain ratios of 7 and 8, respectively. We conclude that the drug transporter expression profiles differ between the BBB and BCSFB in dogs, that there are species differences in the expression profiles, and that CSF is not a suitable surrogate for unbound brain concentrations of BCRP substrates in dogs.

Brain barriers and functional interfaces with sequential appearance of ABC efflux transporters during human development

Adult brain is protected from entry of drugs and toxins by specific mechanisms such as ABC (ATP-binding Cassette) efflux transporters. Little is known when these appear in human brain during development. Cellular distribution of three main ABC transporters (ABCC1, ABCG2, ABCB1) was determined at blood-brain barriers and interfaces in human embryos and fetuses in first half of gestation. Antibodies against claudin-5 and -11 and antibodies to α-fetoprotein were used to describe morphological and functional aspects of brain barriers. First exchange interfaces to be established, probably at 4–5 weeks post conception, are between brain and embryonic cerebrospinal fluid (eCSF) and between outer surface of brain anlage and primary meninx. They already exclude α-fetoprotein and are immunopositive for both claudins, ABCC1 and ABCG2. ABCB1 is detectable within a week of blood vessels first penetrating into brain parenchyma (6–7 weeks post conception). ABCC1, ABCB1 and ABCG2 are present at blood-CSF barrier in all choroid plexuses from first appearance (7 weeks post conception). Outer CSF-brain interfaces are established between 9–11 weeks post conception exhibiting immunoreactivity for all three transporters. Results provide evidence for sequential establishment of brain exchange interfaces and spatial and temporal timetable for three main ABC transporters in early human brain.

The Role of Efflux Pumps in Tuberculosis Treatment and Their Promise as a Target in Drug Development: Unraveling the Black Box

Insight into drug transport mechanisms is highly relevant to the efficacious treatment of tuberculosis (TB). Major problems in TB treatment are related to the transport of antituberculosis (anti-TB) drugs across human and mycobacterial membranes, affecting the concentrations of these drugs systemically and locally. Firstly, transporters located in the intestines, liver, and kidneys all determine the pharmacokinetics and pharmacodynamics of anti-TB drugs, with a high risk of drug-drug interactions in the setting of concurrent use of antimycobacterial, antiretroviral, and antidiabetic agents. Secondly, human efflux transporters limit the penetration of anti-TB drugs into the brain and cerebrospinal fluid, which is especially important in the treatment of TB meningitis. Finally, efflux transporters located in the macrophage and Mycobacterium tuberculosis cell membranes play a pivotal role in the emergence of phenotypic tolerance and drug resistance, respectively. We review the role of efflux transporters in TB drug disposition and evaluate the promise of efflux pump inhibition from a novel holistic perspective.

Identification of Candidate Genes for Reactivity in Guzerat (Bos indicus) Cattle: A Genome-Wide Association Study

Temperament is fundamental to animal production due to its direct influence on the animal-herdsman relationship. When compared to calm animals, the aggressive, anxious or fearful ones exhibit less weight gain, lower reproductive efficiency, decreased milk production and higher herd maintenance costs, all of which contribute to reduced profits. However, temperament is a trait that is complex and difficult to assess. Recently, a new quantitative system, REATEST^®, for assessing reactivity, a phenotype of temperament, was developed. Herein, we describe the results of a Genome-wide association study for reactivity, assessed using REATEST^® with a sample of 754 females from five dual-purpose (milk and meat production) Guzerat (Bos indicus) herds. Genotyping was performed using a 50k SNP chip and a two-step mixed model approach (Grammar-Gamma) with a one-by-one marker regression was used to identify QTLs. QTLs for reactivity were identified on chromosomes BTA1, BTA5, BTA14, and BTA25. Five intronic and two intergenic markers were significantly associated with reactivity. POU1F1, DRD3, VWA3A, ZBTB20, EPHA6, SNRPF and NTN4 were identified as candidate genes. Previous QTL reports for temperament traits, covering areas surrounding the SNPs/genes identified here, further corroborate these associations. The seven genes identified in the present study explain 20.5% of reactivity variance and give a better understanding of temperament biology.

Role and modulation of drug transporters in HIV-1 therapy

Current treatment of human immunodeficiency virus type-1 (HIV-1) infection involves a combination of antiretroviral drugs (ARVs) that target different stages of the HIV-1 life cycle. This strategy is commonly referred to as highly active antiretroviral therapy (HAART) or combined antiretroviral therapy (cART). Membrane-associated drug transporters expressed ubiquitously in mammalian systems play a crucial role in modulating ARV disposition during HIV-1 infection. Members of the ATP-binding cassette (ABC) and solute carrier (SLC) transporter superfamilies have been shown to interact with ARVs, including those that are used as part of first-line treatment regimens. As a result, the functional expression of drug transporters can influence the distribution of ARVs at specific sites of infection. In addition, pathological factors related to HIV-1 infection and/or ARV therapy itself can alter transporter expression and activity, thus further contributing to changes in ARV disposition and the effectiveness of HAART. This review summarizes current knowledge on the role of drug transporters in regulating ARV transport in the context of HIV-1 infection.

Cryopreservation by slow cooling of rat neuronal cells

Although primary neuronal cells are routinely used for neuroscience research, with potential clinical applications such as neuronal transplantation and tissue engineering, a gold standard protocol for preservation has not been yet developed. In the present work, a slow cooling methodology without ice seeding was studied and optimized for cryopreservation of rat cerebellar granular cells. Parameters such as cooling rate, plunge temperature and cryoprotective agent concentration were assessed using a custom built device based on Pye's freezer idea. Cryopreservation outcome was evaluated by post thawing cell viability/viable cell yield and in culture viability over a period of 14 days. The best outcome was achieved when 10% of Me2SO as cryoprotective agent, a cooling rate of 3.1 ± 0.2 °C/min and a plunge temperature of -48.2 ± 1.5 °C were applied. The granular cells cryopreserved under these conditions exhibited a cell viability of 82.7 ± 2.7% and a viable cell yield of 28.6 ± 2.2%. Moreover, cell viability in culture remained above 50%, very similar to not cryopreserved cells (control). Our results also suggest that post-thaw viability (based on membrane integrity assays) not necessarily reflects the quality of the cryopreservation procedure and proper functionality tests must be carried out in order to optimize both post thaw viability/cell yield and in culture performance.

ABC transporters at the blood-brain barrier

INTRODUCTION

The blood-brain barrier (BBB) possesses an outstanding ability to protect the brain against xenobiotics and potentially poisonous metabolites. Owing to this, ATP binding cassette (ABC) export proteins have garnered significant interest in the research community. These transport proteins are predominantly localized to the luminal membrane of brain microvessels, where they recognize a wide range of different substrates and transport them back into the blood circulation.

AREAS COVERED

This review summarizes recent findings on these transport proteins, including their expression in the endothelial cell membrane and their substrate recognition. Signaling cascades underlying the expression and function of these proteins will be discussed as well as their role in diseases such as Alzheimer's disease, epilepsy, amyotrophic lateral sclerosis and brain tumors.

EXPERT OPINION

ABC transporters represent an integral part of the human transportome and are of particular interest at the blood-brain barrier they as they significantly contribute to brain homeostasis. In addition, they appear to be involved in myriad CNS diseases. Therefore studying their mechanisms of action as well as their signaling cascades and responses to internal and external stimuli will help us understand the pathogenesis of these diseases.

The biological significance of brain barrier mechanisms: help or hindrance in drug delivery to the central nervous system?

Barrier mechanisms in the brain are important for its normal functioning and development. Stability of the brain's internal environment, particularly with respect to its ionic composition, is a prerequisite for the fundamental basis of its function, namely transmission of nerve impulses. In addition, the appropriate and controlled supply of a wide range of nutrients such as glucose, amino acids, monocarboxylates, and vitamins is also essential for normal development and function. These are all cellular functions across the interfaces that separate the brain from the rest of the internal environment of the body. An essential morphological component of all but one of the barriers is the presence of specialized intercellular tight junctions between the cells comprising the interface: endothelial cells in the blood-brain barrier itself, cells of the arachnoid membrane, choroid plexus epithelial cells, and tanycytes (specialized glial cells) in the circumventricular organs. In the ependyma lining the cerebral ventricles in the adult brain, the cells are joined by gap junctions, which are not restrictive for intercellular movement of molecules. But in the developing brain, the forerunners of these cells form the neuroepithelium, which restricts exchange of all but the smallest molecules between cerebrospinal fluid and brain interstitial fluid because of the presence of strap junctions between the cells. The intercellular junctions in all these interfaces are the physical basis for their barrier properties. In the blood-brain barrier proper, this is combined with a paucity of vesicular transport that is a characteristic of other vascular beds. Without such a diffusional restrain, the cellular transport mechanisms in the barrier interfaces would be ineffective. Superimposed on these physical structures are physiological mechanisms as the cells of the interfaces contain various metabolic transporters and efflux pumps, often ATP-binding cassette (ABC) transporters, that provide an important component of the barrier functions by either preventing entry of or expelling numerous molecules including toxins, drugs, and other xenobiotics. In this review, we summarize these influx and efflux mechanisms in normal developing and adult brain, as well as indicating their likely involvement in a wide range of neuropathologies. There have been extensive attempts to overcome the barrier mechanisms that prevent the entry of many drugs of therapeutic potential into the brain. We outline those that have been tried and discuss why they may so far have been largely unsuccessful. Currently, a promising approach appears to be focal, reversible disruption of the blood-brain barrier using focused ultrasound, but more work is required to evaluate the method before it can be tried in patients. Overall, our view is that much more fundamental knowledge of barrier mechanisms and development of new experimental methods will be required before drug targeting to the brain is likely to be a successful endeavor. In addition, such studies, if applied to brain pathologies such as stroke, trauma, or multiple sclerosis, will aid in defining the contribution of brain barrier pathology to these conditions, either causative or secondary.