Quantitative targeted absolute proteomics of human blood-brain barrier transporters and receptors

Modeling the blood-brain barrier using stem cell sources

The blood–brain barrier (BBB) is a selective endothelial interface that controls trafficking between the bloodstream and brain interstitial space. During development, the BBB arises as a result of complex multicellular interactions between immature endothelial cells and neural progenitors, neurons, radial glia, and pericytes. As the brain develops, astrocytes and pericytes further contribute to BBB induction and maintenance of the BBB phenotype. Because BBB development, maintenance, and disease states are difficult and time-consuming to study in vivo, researchers often utilize in vitro models for simplified analyses and higher throughput. The in vitro format also provides a platform for screening brain-penetrating therapeutics. However, BBB models derived from adult tissue, especially human sources, have been hampered by limited cell availability and model fidelity. Furthermore, BBB endothelium is very difficult if not impossible to isolate from embryonic animal or human brain, restricting capabilities to model BBB development in vitro. In an effort to address some of these shortcomings, advances in stem cell research have recently been leveraged for improving our understanding of BBB development and function. Stem cells, which are defined by their capacity to expand by self-renewal, can be coaxed to form various somatic cell types and could in principle be very attractive for BBB modeling applications. In this review, we will describe how neural progenitor cells (NPCs), the in vitro precursors to neurons, astrocytes, and oligodendrocytes, can be used to study BBB induction. Next, we will detail how these same NPCs can be differentiated to more mature populations of neurons and astrocytes and profile their use in co-culture modeling of the adult BBB. Finally, we will describe our recent efforts in differentiating human pluripotent stem cells (hPSCs) to endothelial cells with robust BBB characteristics and detail how these cells could ultimately be used to study BBB development and maintenance, to model neurological disease, and to screen neuropharmaceuticals.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

Potent and selective tariquidar bioisosters as potential PET radiotracers for imaging P-gp

Compounds 8a-d have been designed as bioisosters of tariquidar for imaging P-gp expression and density by PET. The results displayed that compounds 8b and 8d could be considered potential P-gp/BCRP ligands suitable as (11)C and (18)F radiotracers, respectively.

Blood-brain barrier integrity and glial support: mechanisms that can be targeted for novel therapeutic approaches in stroke

The blood-brain barrier (BBB) is a critical regulator of brain homeostasis. Additionally, the BBB is the most significant obstacle to effective CNS drug delivery. It possesses specific charcteristics (i.e., tight junction protein complexes, influx and efflux transporters) that control permeation of circulating solutes including therapeutic agents. In order to form this "barrier," brain microvascular endothelial cells require support of adjacent astrocytes and microglia. This intricate relationship also occurs between endothelial cells and other cell types and structures of the CNS (i.e., pericytes, neurons, extracellular matrix), which implies existence of a "neurovascular unit." Ischemic stroke can disrupt the neurovascular unit at both the structural and functional level, which leads to an increase in leak across the BBB. Recent studies have identified several pathophysiological mechanisms (i.e., oxidative stress, activation of cytokine-mediated intracellular signaling systems) that mediate changes in the neurovascular unit during ischemic stroke. This review summarizes current knowledge in this area and emphasizes pathways (i.e., oxidative stress, cytokine-mediated intracellular signaling, glial-expressed receptors/targets) that can be manipulated pharmacologically for i) preservation of BBB and glial integrity during ischemic stroke and ii) control of drug permeation and/or transport across the BBB. Targeting these pathways present a novel opportunity for optimization of CNS delivery of therapeutics in the setting of ischemic stroke.

Quantitative targeted absolute proteomic analysis of transporters, receptors and junction proteins for validation of human cerebral microvascular endothelial cell line hCMEC/D3 as a human blood-brain barrier model

Human cerebral microvascular endothelial cell line hCMEC/D3 is an established model of the human blood-brain barrier (BBB). The purpose of the present study was to determine, by means of quantitative targeted absolute proteomics, the protein expression levels in hCMEC/D3 cells of multiple transporters, receptors and junction proteins for comparison with our previously reported findings in isolated human brain microvessels. Among 91 target molecules, 12 transporters, 2 receptors, 1 junction protein and 1 membrane marker were present at quantifiable levels in plasma membrane fraction of hCMEC/D3 cells. ABCA2, MDR1, MRP4, BCRP, GLUT1, 4F2hc, MCT1, ENT1, transferrin and insulin receptors and claudin-5 were detected in both hCMEC/D3 cells and human brain microvessels. After normalization based on Na(+)/K(+) ATPase expression, the differences in protein expression levels between hCMEC/D3 cells and human brain microvessels were within 4-fold for these proteins, with the exceptions of ENT1, transferrin receptor and claudin-5. ABCA8, LAT1, LRP1 and γ-GTP were below the limit of quantification in the cells, but were found in human brain microvessels. ABCA3, ABCA6, MRP1 and ATA1 were found only in hCMEC/D3 cells. Furthermore, compared with human umbilical vein endothelial cells (HUVECs) as reference nonbrain endothelial cells, MDR1 was found only in hCMEC/D3 cells, and GLUT1 expression was 15-fold higher in hCMEC/D3 cells than in HUVECs. In conclusion, this is the first study to examine the suitability and limitations of the hCMEC/D3 cell line as a BBB functional model in terms of quantitative expression levels of transporters, receptors and tight junction proteins.

Identification of transporters associated with Etoposide sensitivity of stomach cancer cell lines and methotrexate sensitivity of breast cancer cell lines by quantitative targeted absolute proteomics

Membrane transporter proteins may influence the sensitivity of cancer cells to anticancer drugs that can be recognized as substrates. The purpose of this study was to identify proteins that play a key role in the drug sensitivity of stomach and breast cancer cell lines by measuring the absolute protein expression levels of multiple transporters and other membrane proteins and examining their correlation to drug sensitivity. Absolute protein expression levels of 90 membrane proteins were examined by quantitative targeted absolute proteomics using liquid chromatography-linked tandem mass spectrometry. Among them, 11 and 14 membrane proteins, including transporters, were present in quantifiable amounts in membrane fraction of stomach cancer and breast cancer cell lines, respectively. In stomach cancer cell lines, the protein expression level of multidrug resistance-associated protein 1 (MRP1) was inversely correlated with etoposide sensitivity. MK571, an MRP inhibitor, increased both the cell-to-medium ratio of etoposide and the etoposide sensitivity of MRP1-expressing stomach cancer cell lines. In breast cancer cell lines, the protein expression level of reduced folate carrier 1 (RFC1) was directly correlated with methotrexate (MTX) sensitivity. Initial uptake rate and steady-state cell-to-medium ratio of [(3)H]MTX were correlated with both RFC1 expression level and MTX sensitivity. These results suggest that MRP1 modulates the etoposide sensitivity of stomach cancer cell lines and RFC1 modulates the MTX sensitivity of breast cancer cell lines. Our results indicate that absolute quantification of multiple membrane proteins could be a useful strategy for identification of candidate proteins involved in drug sensitivity.

Tight junctions at the blood brain barrier: physiological architecture and disease-associated dysregulation

The Blood–brain barrier (BBB), present at the level of the endothelium of cerebral blood vessels, selectively restricts the blood-to-brain paracellular diffusion of compounds; it is mandatory for cerebral homeostasis and proper neuronal function. The barrier properties of these specialized endothelial cells notably depend on tight junctions (TJs) between adjacent cells: TJs are dynamic structures consisting of a number of transmembrane and membrane-associated cytoplasmic proteins, which are assembled in a multimolecular complex and acting as a platform for intracellular signaling. Although the structural composition of these complexes has been well described in the recent years, our knowledge about their functional regulation still remains fragmentary. Importantly, pericytes, embedded in the vascular basement membrane, and perivascular microglial cells, astrocytes and neurons contribute to the regulation of endothelial TJs and BBB function, altogether constituting the so-called neurovascular unit.

The present review summarizes our current understanding of the structure and functional regulation of endothelial TJs at the BBB. Accumulating evidence points to a correlation between BBB dysfunction, alteration of TJ complexes and progression of a variety of CNS diseases, such as stroke, multiple sclerosis and brain tumors, as well as neurodegenerative diseases like Parkinson’s and Alzheimer’s diseases. Understanding how TJ integrity is controlled may thus help improve drug delivery across the BBB and the design of therapeutic strategies for neurological disorders.

Distribution of the branched chain aminotransferase proteins in the human brain and their role in glutamate regulation

The branched chain aminotransferase enzymes (BCAT) serve as nitrogen donors for the production of 30% of de novo glutamate synthesis in rat brain. Despite the importance of this major metabolite and excitatory neurotransmitter, the distribution of BCAT proteins in the human brain (hBCAT) remains unreported. We have studied this and report, for the first time, that the mitochondrial isoform, hBCATm is largely confined to vascular endothelial cells, whereas the cytosolic hBCATc is restricted to neurons. The majority of hBCATc-labelled neurons were either GABA-ergic or glutamatergic showing both cell body and axonal staining indicating a role for hBCATc in both glutamate production and glutamate release during excitation. Strong staining in hormone secreting cells suggests a further role for the transaminases in hormone regulation potentially similar to that proposed for insulin secretion. Expression of hBCATm in the endothelial cells of the vasculature demonstrates for the first time that glutamate could be metabolized by aminotranferases in these cells. This has important implications given that the dysregulation of glutamate metabolism, leading to glutamate excitotoxicity, is an important contributor to the pathogenesis of several neurodegenerative conditions, where the role of hBCATm in metabolizing excess glutamate may factor more prominently.

Drug transport across the blood-brain barrier

The blood-brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. This property arises from the epithelial-like tight junctions within the brain capillary endothelium. The BBB is anatomically and functionally distinct from the blood-cerebrospinal fluid barrier at the choroid plexus. Certain small molecule drugs may cross the BBB via lipid-mediated free diffusion, providing the drug has a molecular weight <400 Da and forms <8 hydrogen bonds. These chemical properties are lacking in the majority of small molecule drugs, and all large molecule drugs. Nevertheless, drugs can be reengineered for BBB transport, based on the knowledge of the endogenous transport systems within the BBB. Small molecule drugs can be synthesized that access carrier-mediated transport (CMT) systems within the BBB. Large molecule drugs can be reengineered with molecular Trojan horse delivery systems to access receptor-mediated transport (RMT) systems within the BBB. Peptide and antisense radiopharmaceuticals are made brain-penetrating with the combined use of RMT-based delivery systems and avidin-biotin technology. Knowledge on the endogenous CMT and RMT systems expressed at the BBB enable new solutions to the problem of BBB drug transport.

A novel PET protocol for visualization of breast cancer resistance protein function at the blood-brain barrier

Breast cancer resistance protein (BCRP) is the most abundant multidrug efflux transporter at the human blood-brain barrier (BBB), restricting brain distribution of various drugs. In this study, we developed a positron emission tomography (PET) protocol to visualize Bcrp function at the murine BBB, based on the dual P-glycoprotein (P-gp)/Bcrp substrate radiotracer [(11)C]tariquidar in combination with the Bcrp inhibitor Ko143. To eliminate the contribution of P-gp efflux to [(11)C]tariquidar brain distribution, we studied mice in which P-gp was genetically knocked out (Mdr1a/b((-/-)) mice) or chemically knocked out by pretreatment with cold tariquidar. We found that [(11)C]tariquidar brain uptake increased dose dependently after administration of escalating doses of Ko143, both in Mdr1a/b((-/-)) mice and in tariquidar pretreated wild-type mice. After 15 mg/kg Ko143, the maximum increase in [(11)C]tariquidar brain uptake relative to baseline scans was 6.3-fold in Mdr1a/b((-/-)) mice with a half-maximum effect dose of 4.98 mg/kg and 3.6-fold in tariquidar (8 mg/kg) pretreated wild-type mice, suggesting that the presented protocol is sensitive to visualize a range of different functional Bcrp activities at the murine BBB. We expect that this protocol can be translated to the clinic, because tariquidar can be safely administered to humans at doses that completely inhibit cerebral P-gp.

TNAP and EHD1 are over-expressed in bovine brain capillary endothelial cells after the re-induction of blood-brain barrier properties

Although the physiological properties of the blood-brain barrier (BBB) are relatively well known, the phenotype of the component brain capillary endothelial cells (BCECs) has yet to be described in detail. Likewise, the molecular mechanisms that govern the establishment and maintenance of the BBB are largely unknown. Proteomics can be used to assess quantitative changes in protein levels and identify proteins involved in the molecular pathways responsible for cellular differentiation. Using the well-established in vitro BBB model developed in our laboratory, we performed a differential nano-LC MALDI-TOF/TOF-MS study of Triton X-100-soluble protein species from bovine BCECs displaying either limited BBB functions or BBB functions re-induced by glial cells. Due to the heterogeneity of the crude extract, we increased identification yields by applying a repeatable, reproducible fractionation process based on the proteins' relative hydrophobicity. We present proteomic and biochemical evidence to show that tissue non-specific alkaline phosphatase (TNAP) and Eps15 homology domain-containing protein 1(EDH1) are over-expressed by bovine BCECs after the re-induction of BBB properties. We discuss the impact of these findings on current knowledge of endothelial and BBB permeability.

Pharmacokinetic modeling of P-glycoprotein function at the rat and human blood-brain barriers studied with (R)-[11C]verapamil positron emission tomography

Background

This study investigated the influence of P-glycoprotein (P-gp) inhibitor tariquidar on the pharmacokinetics of P-gp substrate radiotracer (R)-[¹¹C]verapamil in plasma and brain of rats and humans by means of positron emission tomography (PET).

Methods

Data obtained from a preclinical and clinical study, in which paired (R)-[¹¹C]verapamil PET scans were performed before, during, and after tariquidar administration, were analyzed using nonlinear mixed effects (NLME) modeling. Administration of tariquidar was included as a covariate on the influx and efflux parameters (Q_in and Q_out) in order to investigate if tariquidar increased influx or decreased outflux of radiotracer across the blood–brain barrier (BBB). Additionally, the influence of pilocarpine-induced status epilepticus (SE) was tested on all model parameters, and the brain-to-plasma partition coefficient (V_T-NLME) was calculated.

Results

Our model indicated that tariquidar enhances brain uptake of (R)-[¹¹C]verapamil by decreasing Q_out. The reduction in Q_out in rats during and immediately after tariquidar administration (sevenfold) was more pronounced than in the second PET scan acquired 2 h after tariquidar administration (fivefold). The effect of tariquidar on Q_out in humans was apparent during and immediately after tariquidar administration (twofold reduction in Q_out) but was negligible in the second PET scan. SE was found to influence the pharmacological volume of distribution of the central brain compartment V_br1. Tariquidar treatment lead to an increase in V_T-NLME, and pilocarpine-induced SE lead to increased (R)-[¹¹C]verapamil distribution to the peripheral brain compartment.

Conclusions

Using NLME modeling, we were able to provide mechanistic insight into the effects of tariquidar and SE on (R)-[¹¹C]verapamil transport across the BBB in control and 48 h post SE rats as well as in humans.

The role of glucose transporters in brain disease: diabetes and Alzheimer’s Disease

The occurrence of altered brain glucose metabolism has long been suggested in both diabetes and Alzheimer’s diseases. However, the preceding mechanism to altered glucose metabolism has not been well understood. Glucose enters the brain via glucose transporters primarily present at the blood-brain barrier. Any changes in glucose transporter function and expression dramatically affects brain glucose homeostasis and function. In the brains of both diabetic and Alzheimer’s disease patients, changes in glucose transporter function and expression have been observed, but a possible link between the altered glucose transporter function and disease progress is missing. Future recognition of the role of new glucose transporter isoforms in the brain may provide a better understanding of brain glucose metabolism in normal and disease states. Elucidation of clinical pathological mechanisms related to glucose transport and metabolism may provide common links to the etiology of these two diseases. Considering these facts, in this review we provide a current understanding of the vital roles of a variety of glucose transporters in the normal, diabetic and Alzheimer’s disease brain.

Post-marketing assessment of neuropsychiatric adverse events in influenza patients treated with oseltamivir: an updated review

A 2008 review by our group concluded that the risk of neuropsychiatric adverse events (NPAEs) in influenza patients was not increased by oseltamivir exposure, and did not identify any mechanism by which oseltamivir or its metabolites could cause or worsen such events. The current article reviews new information on this topic. Between September 16, 2007 and May 15, 2010, 1,805 spontaneously-reported NPAEs were identified in 1,330 patients receiving oseltamivir: 767 (42.5%) from Japan, 296 (16.4%) from the USA, and 742 (41.1%) from other countries. NPAEs were more common in children: 1,072 (59.4%) events were in those aged ≤16 years. NPAEs often occurred within 48 h of treatment initiation (953 events; 52.8%). Nearly half of the events were serious in nature (838; 46.4%). The three largest categories of events were abnormal behavior (457 events, 25.3%), miscellaneous psychiatric events (370; 20.5%), and delusions/perceptual disturbances (316 events, 17.5%). A total of 1,545 events (85.6%) in eight different categories were considered to be delirium or delirium-like. Twenty-eight suicide-related events were reported. A US healthcare claims database analysis showed that the risk of NPAEs in 7,798 oseltamivir-treated patients was no higher than that in 10,411 patients not on antivirals, but a study on oseltamivir and abnormal behavior in Japan was less conclusive. NPAE frequency in oseltamivir-exposed Japanese and Taiwanese children with influenza was the same as in unexposed children. New analysis of the UK General Practice Research Database showed that the relative adjusted risk of NPAEs in influenza patients was 2.18-times higher than in the general population. Other epidemiology studies report frequent occurrence of encephalitis and similar disorders in influenza patients independently of oseltamivir exposure. The new data support the findings of the original assessment. Evidence suggests that influenza-related encephalopathies are caused by influenza-induced inflammatory responses, but more work is needed to confirm the underlying mechanisms.

Function of thyroid hormone transporters in the central nervous system

BACKGROUND

Iodothyronines are charged amino acid derivatives that cannot passively cross a phospholipid bilayer. Transport of thyroid hormones across plasma membranes is mediated by integral membrane proteins belonging to several gene families. These transporters therefore allow or limit access of thyroid hormones into brain. Since thyroid hormones are essential for brain development and cell differentiation, it is expected that genetic deficiency of such transporters would result in neurodevelopmental derangements.

SCOPE OF REVIEW

We introduce concepts of thyroid hormone transport into the brain and into brain cells. Important thyroid hormone transmembrane transporters are presented along with their expression patterns in different brain cell types. A focus is placed on monocarboxylate transporter 8 (MCT8) which has been identified as an essential thyroid hormone transporter in humans. Mutations in MCT8 underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome.

MAJOR CONCLUSIONS

Thyroid hormone transporter molecules are expressed in a developmental and cell type-specific pattern. Any thyroid hormone molecule has to cross consecutively the luminal and abluminal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane to finally cross another plasma membrane on its way towards its target nucleus.

GENERAL SIGNIFICANCE

We can expect more transporters being involved in or contributing to in neurodevelopmental or neuropsychiatric disease. Due to their expression in cellular components regulating the hypothalamus-pituitary-thyroid axis, mutations and polymorphisms are expected to impact on negative feedback regulation and hormonal setpoints. This article is part of a Special Issue entitled Thyroid hormone signalling.

Pathophysiology of the neurovascular unit: disease cause or consequence?

Pathophysiology of the neurovascular unit (NVU) is commonly seen in neurological diseases. The typical features of NVU pathophysiology include tissue hypoxia, inflammatory and angiogenic activation, as well as initiation of complex molecular interactions between cellular (brain endothelial cells, astroctyes, pericytes, inflammatory cells, and neurons) and acellular (basal lamina) components of the NVU, jointly resulting in increased blood-brain barrier permeability, brain edema, neurovascular uncoupling, and neuronal dysfunction and damage. The evidence of important role of the brain vascular compartment in disease pathogenesis has elicited the debate whether the primary vascular events may be a cause of the neurological disease, as opposed to a mere participant recruited by a primary neuronal origin of pathology? Whereas some hereditary and acquired cerebral angiopathies could be considered a primary cause of neurological symptoms of the disease, the epidemiological studies showing a high degree of comorbidity among vascular disease and dementias, including Alzheimer's disease, as well as migraine and epilepsy, suggested that primary vascular pathology may be etiological factor causing neuronal dysfunction or degeneration in these diseases. This review focuses on recent hypotheses and evidence, suggesting that pathophysiology of the NVU may be initiating trigger for neuronal pathology and subsequent neurological manifestations of the disease.

Microdialysis evaluation of clozapine and N-desmethylclozapine pharmacokinetics in rat brain

A significant barrier to realization of the full potential of clozapine as a therapeutic agent in the treatment of schizophrenia is the substantial interpatient variability that exists along the therapeutic continuum of no response-efficacious response-adverse response. Genetic polymorphisms that manifest as highly variable pharmacodynamic and pharmacokinetic measures are its expected causes. To support investigations that seek to understand these causes, the plasma and central nervous system pharmacokinetics of clozapine were determined in rats, the latter using microdialysis sampling. Results obtained with clozapine and N-desmethylclozapine, a pharmacologically active human metabolite that was administered to a separate group of animals, support a conclusion of net carrier-mediated efflux of both compounds across the blood-brain barrier. These results are supported by the replication of published findings regarding the passive transport and net efflux transport of two model compounds, escitalopram and risperidone, respectively. The results obtained with clozapine and N-desmethylclozapine are considered a first step in the development of preclinical pharmacokinetic-pharmacodynamic models that will support deeper mechanistic studies of clozapine in in vivo pharmacology, as well as the development of translational models that augment pharmacogenetic investigations that seek to improve the safety and efficacy of clozapine therapeutic intervention in the treatment of schizophrenia.

Quantitative proteomics of transporter expression in brain capillary endothelial cells isolated from P-glycoprotein (P-gp), breast cancer resistance protein (Bcrp), and P-gp/Bcrp knockout mice

The objective of this study was to quantitatively examine the protein expression of relevant transporters and other proteins in the brain capillary endothelial cells isolated from wild-type mice and P-glycoprotein (P-gp), breast cancer resistance protein (Bcrp), and P-gp/Bcrp knockout mice. After the isolation of brain capillary endothelial cells, a highly sensitive liquid chromatography-tandem mass spectrometry method with multiple reaction monitoring was used to determine the quantitative expression of membrane transporters at the blood-brain barrier (BBB) of the various mouse genotypes. Quantitative expression of 29 protein molecules, including 12 ATP-binding cassette transporters, 10 solute carrier transporters, five receptors, and two housekeeping proteins, was examined by quantitative proteomics in the four mouse genotypes. There was no significant difference in the expression of P-gp between the wild-type and Bcrp1(-/-) mice. Likewise, Bcrp expression was not significantly different between the wild-type and Mdr1a/b(-/-) mice. There was no significant difference in the expression of any of the measured proteins in the brain capillary endothelial cells across the genotypes, except for the lack of expression of the corresponding protein in the mice that had a genetic deletion of P-gp or Bcrp. In conclusion, using a quantitative proteomic approach, we have shown that there are no changes in the expression of several relevant transporters in brain capillary endothelial cells isolated from single and combination knockout mice. These data suggest that the mechanism behind the functional compensation between P-gp and Bcrp at the BBB is not related to compensatory changes in transporter expression.

Development of a multiplex UPLC-MRM MS method for quantification of human membrane transport proteins OATP1B1, OATP1B3 and OATP2B1 in in vitro systems and tissues

OATP1B1, OATP1B3 and OATP2B1 are important members of the organic anion transporting polypeptides (OATP) family and are implicated in the hepatic disposition of endobiotics and xenobiotics. Quantitating the expression levels of human OATP1B1, OATP1B3 and OATP2B1 in in vitro systems and tissue samples could significantly improve attempts to scale up in vitro data and result in more effective in vitro-in vivo correlation of transporter-mediated effects on drug disposition, such as hepatic clearance. In the present study, a quantification method was developed, characterized, and implemented for simultaneous determination of human OATP1B1, OATP1B3 and OATP2B1 in HEK cells transfected with OATP-expressing plasmid vectors (SLCO1B1, SLCO1B3, and SLCO2B1, respectively), human hepatocytes, human brain capillary endothelial cells, and humanized mouse liver tissue using UPLC-MRM MS. Purified membrane protein standards prepared and characterized as previously reported (Protein Expr. Purif. 2008, 57, 163-71) were first used as standards for absolute quantification of the expression levels of the three human OATP membrane proteins. The specificity of the optimized MRM transitions were characterized by analyzing the tryptic digests of the membrane protein fraction of wild type HEK cells and control mouse liver tissue using the herein reported UPLC-MRM MS method. The linearity of the calibration curve spanned from 0.2 μg mL(-1) (0.040 μg mg(-1)) to 20 μg mL(-1) (4.0 μg mg(-1)), with accuracy (% RE) within 15% at all concentrations examined for all three OATPs of interest in this study. The intra- and inter-day assay accuracy (% RE) and coefficient of variations (% CV) of triplicates are all within 15% for all levels of quality control samples prepared by mixing the membrane fraction of control mouse liver tissue with the required amount of purified human OATP1B1, OATP1B3 and OATP2B1.

Pgp-mediated interaction between (R)-[11C]verapamil and tariquidar at the human blood-brain barrier: a comparison with rat data

Using positron emission tomography (PET) imaging we assessed, in vivo, the interaction between a microdose of (R)-[(11)C]verapamil (a P-glycoprotein (Pgp) substrate) and escalating doses of the Pgp inhibitor tariquidar (3, 4, 6, and 8 mg/kg) at the blood-brain barrier (BBB) in healthy human subjects. We compared the dose-response relationship of tariquidar in humans with data obtained in rats using a similar methodology. Tariquidar was equipotent in humans and rats in its effect of increasing (R)-[(11)C]verapamil brain uptake (expressed as whole-brain volume of distribution (V(T))), with very similar half-maximum-effect concentrations. Both in humans and in rats, brain V(T) approached plateau levels at plasma tariquidar concentrations >1,000 ng/ml. However, Pgp inhibition in humans led to only a 2.7-fold increase in brain V(T) relative to baseline scans (before administration of tariquidar) as compared with 11.0-fold in rats. The results of this translational study add to the accumulating evidence that there are marked species-dependent differences in Pgp expression and functionality at the BBB.

The proteome of mouse brain microvessel membranes and basal lamina

The blood-brain barrier (BBB) is a multicellular vascular structure separating blood from the brain parenchyma that is composed of endothelial cells with tight intercellular junctions, surrounded by a basal lamina, astrocytes, and pericytes. Previous studies have generated detailed databases of the microvessel transcriptome; however, less information is available on the BBB at the protein level. In this study, we specifically focused on characterization of the membrane fraction of cells within the BBB to generate a more complete understanding of membrane transporters, tight junction proteins, and associated extracellular matrix proteins that are functional hallmarks of the BBB. We used Multidimensional Protein Identification Technology to identify a total of 1,143 proteins in mouse brain microvessels, of which 53% were determined to be membrane associated. Analyses of specific classes of BBB-associated proteins in the context of recent transcriptome reports provide a unique database to assess the relative contribution of genes at the level of both RNA and protein in the maintenance of normal BBB integrity.

Attenuation of prostaglandin E2 elimination across the mouse blood-brain barrier in lipopolysaccharide-induced inflammation and additive inhibitory effect of cefmetazole

Background

Peripheral administration of lipopolysaccharide (LPS) induces inflammation and increases cerebral prostaglandin E₂ (PGE₂) concentration. PGE₂ is eliminated from brain across the blood-brain barrier (BBB) in mice, and this process is inhibited by intracerebral or intravenous pre-administration of anti-inflammatory drugs and antibiotics such as cefmetazole and cefazolin that inhibit multidrug resistance-associated protein 4 (Mrp4/Abcc4)-mediated PGE₂ transport. The purpose of this study was to examine the effect of LPS-induced inflammation on PGE₂ elimination from brain, and whether antibiotics further inhibit PGE₂ elimination in LPS-treated mice.

Methods

[³H]PGE₂ elimination across the BBB of intraperitoneally LPS-treated mice was assessed by the brain efflux index (BEI) method. Transporter protein amounts in brain capillaries were quantified by liquid chromatography-tandem mass spectrometry.

Results

The apparent elimination rate of [³H]PGE₂ from brain was lower by 87%, in LPS-treated mice compared with saline-treated mice. The Mrp4 protein amount was unchanged in brain capillaries of LPS-treated mice compared with saline-treated mice, while the protein amounts of organic anion transporter 3 (Oat3/Slc22a8) and organic anion transporting polypeptide 1a4 (Oatp1a4/Slco1a4) were decreased by 26% and 39%, respectively. Either intracerebral or intravenous pre-administration of cefmetazole further inhibited PGE₂ elimination in LPS-treated mice. However, intracerebral or intravenous pre-administration of cefazolin had little effect on PGE₂ elimination in LPS-treated mice, or in LPS-untreated mice given Oat3 and Oatp1a4 inhibitors. These results indicate that peripheral administration of cefmetazole inhibits PGE₂ elimination across the BBB in LPS-treated mice.

Conclusion

PGE₂ elimination across the BBB is attenuated in an LPS-induced mouse model of inflammation. Peripheral administration of cefmetazole further inhibits PGE₂ elimination in LPS-treated mice.

Simultaneous absolute protein quantification of transporters, cytochromes P450, and UDP-glucuronosyltransferases as a novel approach for the characterization of individual human liver: comparison with mRNA levels and activities

The purpose of the present study was to determine the absolute protein expression levels of multiple drug-metabolizing enzymes and transporters in 17 human liver biopsies, and to compare them with the mRNA expression levels and functional activities to evaluate the suitability of the three measures as parameters of hepatic metabolism. Absolute protein expression levels of 13 cytochrome P450 (P450) enzymes, NADPH-P450 reductase (P450R) and 6 UDP-glucuronosyltransferase (UGT) enzymes in microsomal fraction, and 22 transporters in plasma membrane fraction were determined using liquid chromatography/tandem mass spectrometry. CYP2C9, CYP2E1, CYP3A4, CYP2A6, UGT1A6, UGT2B7, UGT2B15, and P450R were abundantly expressed (more than 50 pmol/mg protein) in human liver microsomes. The protein expression levels of CYP3A4, CYP2B6, and CYP2C8 were each highly correlated with the corresponding enzyme activity and mRNA expression levels, whereas for other P450s, the protein expression levels were better correlated with the enzyme activities than the mRNA expression levels were. Among transporters, the protein expression level of organic anion-transporting polypeptide 1B1 was relatively highly correlated with the mRNA expression level. However, other transporters showed almost no correlation. These findings indicate that protein expression levels determined by the present simultaneous quantification method are a useful parameter to assess differences of hepatic function between individuals.

Molecular-weight-dependent, anionic-substrate-preferential transport of β-lactam antibiotics via multidrug resistance-associated protein 4

β-Lactam antibiotics have cerebral and peripheral adverse effects. Multidrug resistance-associated protein 4 (MRP4) has been reported to transport several β-lactam antibiotics, and its expression at the blood-brain barrier also serves to limit their distribution to the brain. Therefore, the purpose of this study was to clarify the structure-activity relationship of MRP4-mediated transport of β-lactam antibiotics using MRP4-expressing Sf9 membrane vesicles. The transport activity was evaluated as MRP4-mediated transport per MRP4 protein [nL/(min·fmol MRP4 protein)] based on measurement of MRP4 protein expression by means of liquid chromatography-tandem mass spectrometry. Cefotiam showed the greatest MRP4-mediated transport activity [8.90 nL/(min·fmol MRP4 protein)] among the β-lactam antibiotics examined in this study. Measurements of differential transport activity of MRP4 for various β-lactam antibiotics indicated that (i) cephalosporins were transported via MRP4 at a greater rate than were penams, β-lactamase inhibitors, penems, or monobactams; (ii) MRP4-mediated transport activity of anionic cephalosporins was greater than that of zwitterionic cephalosporins; and (iii) higher-molecular-weight anionic β-lactam antibiotics showed greater MRP4-mediated transport activity than lower-molecular-weight ones, whereas zwitterionic β-lactam antibiotics did not show molecular weight dependency of MRP4-mediated transport. These quantitative data should prove useful for understanding MRP-related adverse effects of β-lactam antibiotics and their derivatives.

Targeting blood-brain barrier changes during inflammatory pain: an opportunity for optimizing CNS drug delivery

The blood-brain barrier (BBB) is the most significant obstacle to effective CNS drug delivery. It possesses structural and biochemical features (i.e., tight-junction protein complexes and, influx and efflux transporters) that restrict xenobiotic permeation. Pathophysiological stressors (i.e., peripheral inflammatory pain) can alter BBB tight junctions and transporters, which leads to drug-permeation changes. This is especially critical for opioids, which require precise CNS concentrations to be safe and effective analgesics. Recent studies have identified molecular targets (i.e., endogenous transporters and intracellular signaling systems) that can be exploited for optimization of CNS drug delivery. This article summarizes current knowledge in this area and emphasizes those targets that present the greatest opportunity for controlling drug permeation and/or drug transport across the BBB in an effort to achieve optimal CNS opioid delivery.

Quantitative targeted absolute proteomics-based ADME research as a new path to drug discovery and development: methodology, advantages, strategy, and prospects

An understanding of the functional roles of proteins, for example, in drug absorption, distribution, metabolism, elimination, toxicity, and efficacy (ADMET/efficacy), is important for drug discovery and development. Equally, detailed information about protein expression is required. Recently, a new protein quantification method, called quantitative targeted absolute proteomics (QTAP), has been developed on the basis of separation and identification of protein digests by liquid chromatography-linked tandem mass spectrometry with multiple reaction monitoring. Target peptides for quantification are selected only from sequence information, so time-consuming procedures such as antibody preparation and protein purification are unnecessary. In this review, we introduce the technical features of QTAP and summarize its advantages with reference to recently reported results. These include the evaluation of species differences of blood-brain barrier protein levels among human, monkey, and mouse. The high selectivity of QTAP and its ability to quantify multiple proteins simultaneously make it possible to determine the absolute expression levels of many proteins in tissues and cells in both physiological and disease states. Knowledge of absolute expression amounts, together with data on intrinsic protein activity, allows us to reconstruct in vivo protein function, and this should be an efficient strategy to predict ADMET/efficacy of drug candidates in humans in various disease states.