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

Tg-SwDI transgenic mice: A suitable model for Alzheimer's disease and cerebral amyloid angiopathy basic research and preclinical studies

Alzheimer's disease (AD) is the most common neurodegenerative disease and the most frequent cause of dementia. Characteristic features observed in the brain of AD patients are the accumulation of amyloid beta peptide (Aβ) aggregates, neurofibrillary tangles (NFT) composed of hyperphosphorylated Tau protein, neuronal and synaptic loss, and elevated levels of oxidative stress and inflammatory markers. Cerebral amyloid angiopathy (CAA) is another common cause of cognitive decline characterized by the accumulation of Aβ in the cerebral vasculature. The precise overlapping pathogenic mechanisms underlying the co-occurrence of AD and CAA are not very well understood. However, vascular dysfunction observed at early stages is considered a key phenomenon. Tg-SwDI transgenic mice expressing human Aβ precursor protein (AβPP) harboring the Swedish K670N/M671L and vasculotropic Dutch/Iowa E693Q/D694N mutations in the brain have been extensively used to study many pathological features observed in AD/CAA patients and to design biomarkers and therapeutic strategies. The present review summarizes studies addressing different features mimicking human disease in Tg-SwDI mice: parenchymal and cerebral vascular amyloid accumulation, neuroinflammation, complement overactivation, cerebrovascular, mitochondrial and GABAergic system dysfunction, altered NO synthesis, circadian rhythm disruptions, lead exposure effect, among others. Also, reports that evaluated anti-Aβ and anti-inflammatory strategies and compounds capable of delaying or reversing vascular dysfunction and the impairment of GABAergic transmission in Tg-SwDI mice are analyzed. This review may help researchers determine this model's appropriateness for future studies of a particular mechanism or a novel treatment protocol.

Imaging the Activity of Efflux Transporters at the Blood-Brain Barrier in Neurologic Diseases: Radiotracer Selection Criteria

Efflux transporters of the adenosine triphosphate-binding cassette (ABC) superfamily, such as P-glycoprotein (P-gp/ABCB1) and breast cancer resistance protein (BCRP/ABCG2), are highly expressed at the blood-brain barrier (BBB), where they contribute to maintaining brain homeostasis. P-gp may serve as an imaging biomarker to assess the contribution of BBB functionality rather than integrity to the onset or progression of various neurologic diseases. Considerable efforts have been made to develop radiolabeled P-gp substrates to assess cerebral P-gp activity with PET. However, initially developed radiotracers have limited clinical utility as they lack sensitivity to detect moderate, physiologically relevant changes in cerebral P-gp activity. Learning from this molecular imaging area has called for specific criteria, different from those classically used for other central nervous system targets, for developing and selecting suitable PET tracers to study ABC transporter activity at the BBB in different neurologic diseases.

The impact of the ATP-binding cassette (ABC) transporter family on multidrug resistance in head and neck tumors

The ATP-binding cassette (ABC) transporter family is among the largest protein superfamilies, consisting of seven subfamilies, and plays an important role in various physiological processes and in the clinical manifestations of many diseases. The early clinical signs of head and neck cancer (HNC) are often subtle, resulting in most patients being diagnosed at more advanced stages. This late diagnosis adversely affects tumor treatment, and the resistance of certain tumors to chemotherapy further poses significant challenges for clinical management. Several previous studies have indicated a correlation between the ABC protein family and multidrug resistance (MDR) in tumors. This article offers a thorough review of the subfamilies, structures, functions, and roles of ABC transporters in MDR related to head and neck tumors, with the aim of providing insights and recommendations for overcoming MDR in this context.

Unraveling the interplay between meningitis and mitochondria: Etiology, pathogenesis, and therapeutic insights

Meningitis, characterized by an inflammatory response affecting the membranes surrounding the brain and spinal cord, poses a formidable challenge to global public health. Its etiology spans a spectrum of infectious agents, ranging from bacteria, to viruses, fungi, and parasites. Concurrently, mitochondria-traditionally known as 'cellular powerhouses'-have emerged as critical players in various essential biological functions, including but not limited to, energy production, metabolic regulation, and cell fate determination. Emerging evidence suggests that mitochondria may play vital roles in the pathogenesis of meningitis. In this review, we delineated the definition, classification, etiology, pathogenesis, and clinical manifestations of meningitis, and elucidated the structure, dynamics and functions of mitochondria. We subsequently delved into the intricate interplay between meningitis and mitochondria, identifying potential therapeutic interventions targeting mitochondria for the first time. With clinical trials on the horizon, our review lays the foundation for a transformative era in meningitis therapeutics, where unraveling the intricate interplay between meningitis and mitochondria offers promise for mitigating neuroinflammation and improving patient outcomes.

Unraveling the transcriptomic landscape of brain vascular cells in dementia: A systematic review

INTRODUCTION

Cerebrovascular dysfunction plays a critical role in the pathogenesis of dementia and related neurodegenerative disorders. Recent omics-driven research has revealed associations between vascular abnormalities and transcriptomic alterations in brain vascular cells, particularly endothelial cells (ECs) and pericytes (PCs). However, the impact of these molecular changes on dementia remains unclear.

METHODS

We conducted a comparative analysis of gene expression in ECs and PCs across neurodegenerative conditions, including Alzheimer's disease (AD), Huntington's disease, and arteriovenous malformation, utilizing transcriptomic data from published postmortem human tissue studies.

RESULTS

We identified differentially expressed genes (DEGs) consistently dysregulated in ECs and PCs across these pathologies. Notably, several DEGs are linked to vascular cell zonation and genetic risks for AD and cerebral small vessel disease.

DISCUSSION

Our findings provide insights into the cellular and molecular mechanisms underlying vascular dysfunction in dementia, highlight the knowledge gaps, and suggest potential novel vascular therapeutic targets, including genes not previously investigated in this context.

HIGHLIGHTS

Systematic review of differentially expressed genes (DEGs) in vascular cells from neurodegenerative single-nuclear RNA-sequencing (snRNA-seq) studies. Identify overlapping DEGs in multiple vascular cell types across studies. Examine functional relevance and associations with genetic risk for common DEGs. Outline future directions for the vascular omics field.

Device-assisted strategies for drug delivery across the blood-brain barrier to treat glioblastoma

The blood-brain barrier, essential for protecting the central nervous system, also restricts drug delivery to this region. Thus, delivering drugs across the blood-brain barrier is an active research area in immunology, oncology, and neurology; moreover, novel methods are urgently needed to expand therapeutic options for central nervous system pathologies. While previous strategies have focused on small molecules that modulate blood-brain barrier permeability or penetrate the barrier, there is an increased focus on biomedical devices-external or implanted-for improving drug delivery. Here, we review device-assisted drug delivery across the blood-brain barrier, emphasizing its application in glioblastoma, an aggressively malignant primary brain cancer in which the blood-brain barrier plays a central role. We examine the blood-brain barrier and its features in glioblastoma, emerging models for studying the blood-brain barrier, and device-assisted methods for crossing the blood-brain barrier. We conclude by presenting methods to monitor the blood-brain barrier and paradigms for combined cross-BBB drug delivery.

The role of ABCG2 in health and disease: Linking cancer therapy resistance and other disorders

All biological systems have adenosine triphosphate (ATP) binding cassette (ABC) transporters, one of the significant protein superfamilies involved in transport across membranes. ABC transporters have been implicated in the etiology of diseases like metabolic disorders, cancer, and Alzheimer's disease. ATP-binding cassette superfamily G member 2 (ABCG2), one of the ABC transporters, is necessary for the ATP-dependent efflux of several endogenous and exogenous substances. Consequently, it maintained cellular homeostasis and shielded tissue from xenobiotic substances. ABCG2 was initially identified in an Adriamycin-selected breast cancer cell line (MCF-7/AdrVp) and was linked to the emergence of multidrug resistance (MDR) in cancerous cells. Under many pathophysiological conditions, including inflammation, disease pathology, tissue injury, infection, and in response to xenobiotics and endogenous substances, the expression of ABCG2 undergoes alterations that result in modifications in its function and activity. Genetic variants in the ABCG2 transporter can potentially impact its expression and function, contributing to the development of many disorders. This review aimed to illustrate the impact of ABCG2 expression and its variants on oral drug bioavailability, MDR in specific cancer cells, explore the relationship between ABCG2 expression and other disorders such as gout, Alzheimer's disease, epilepsy, and erythropoietic protoporphyria, and demonstrate the influence of various synthetic and natural compounds in regulating ABCG2 expression.

MiRNAs function in the development of resistance against doxorubicin in cancer cells: targeting ABC transporters

Resistance to chemotherapeutic agents poses a significant challenge in cancer treatment, particularly with doxorubicin, a widely used drug for various cancers, including breast cancer, leukaemia, osteosarcoma, and gastrointestinal cancers. This review aims to elucidate the critical role of microRNAs (miRNAs) in the development of doxorubicin resistance, focusing on their interactions with ATP-binding cassette (ABC) transporters. Despite extensive research, the molecular mechanisms governing doxorubicin resistance still need to be completed, particularly regarding the regulatory influence of miRNAs on ABC transporter expression. By analyzing current literature, this review identifies a notable gap: the lack of comprehensive insight into how specific miRNAs modulate the expression and activity of ABC transporters in cancer cells, contributing to doxorubicin resistance. We systematically examine recent findings on the interplay between miRNAs and ABC transporters, providing a detailed assessment of potential therapeutic strategies that leverage miRNA modulation to overcome drug resistance. Ultimately, this review underscores the significance of integrating miRNA research into existing therapeutic frameworks to enhance the efficacy of doxorubicin in cancer treatment.

Unveiling the role of phytochemicals in autism spectrum disorder by employing network pharmacology and molecular dynamics simulation

Autism Spectrum Disorder (ASD) comprises a myriad of disorders with vast pathologies, aetiologies, and involvement of genetic and environmental risk factors. Given the polygenic aspect of ASD, targeting several genes/proteins responsible for pathogenesis at once might prove advantageous in its remediation. Various phytochemicals have been proven to possess neuroprotective, anti-inflammatory, and antioxidant properties by alleviating symptoms and targeting a complex network of genes/proteins related to disease pathology. However, the effects of many of these phytochemicals on ASD are enigmatic, and their molecular targets and molecular mechanisms are still elusive. Here, we provide a comprehensive comparative study on the therapeutic potential of 6 phytochemicals viz. Cannabidiol, Crocetin, Epigallocatechin-3-gallate, Fisetin, Quercetin, and Resveratrol based on their neuroprotective properties in managing ASD. We aimed to identify and target a network of core proteins in the pathology of ASD via phytochemicals using network pharmacology, molecular docking, and simulation studies. The methodology includes screening genes/proteins implicated in ASD as targets of each phytochemical, followed by network construction using Protein-Protein Interactions, Gene Ontology, and enrichment analysis. The constructed network was further narrowed down to the hub genes in the network, followed by their spatio-temporal analysis, molecular docking, and molecular dynamics simulation. 6 core genes were obtained for ASD, 3 of which are directly involved in disease pathogenesis. The study provides a set of novel genes that phytochemicals can target to ameliorate and regulate ASD pathogenesis. Cannabidiol can inhibit ABCG2, MAOB, and PDE4B, Resveratrol can target ABCB1, and Quercetin can regulate AKR1C4 and XDH. This study demonstrated the potential of phytochemicals to target and regulate ABCG2, ABCB1, AKR1C4, MAOB, PDE4B, and XDH, which in turn modulate the dysfunctional network present in the ASD pathology and provide therapeutic potential in the management of ASD.

Development and validation of an LC-MS/MS method for simultaneous quantification of eight drugs in plasma and brain: Application in a pharmacokinetic study in mice

A selective and sensitive liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneous quantitation of a cassette of 8 drugs, including docetaxel, erlotinib, loperamide, riluzole, vemurafenib, verapamil, elacridar and tariquidar. Stable isotopically labeled compounds were available for use as internal standards for all compounds, except for tariquidar for which we used elacridar-d4. Sample pre-treatment involved liquid-liquid extraction using tert-butyl-methyl ether as this resulted in good recovery and low ion suppression. Chromatographic separation was achieved using a Zorbax Extend C18 analytical column and a linear gradient from 20 % to 95 % methanol in 0.1 % (v/v) formic acid in water. MS/MS detection using multiple reaction monitoring was done in positive ionization mode. We validated this assay for human and mouse plasma and mouse brain homogenates. The calibration curves were linear over a range 1-200 nM for each drug in the mix, except for tariquidar probably due to the lack of a stable isotope labeled analog. The intra-day and inter-day accuracies were within the 85-115 % range for all compounds at low, medium and high concentrations in the three different matrices. Similarly, the precision for all compounds at three different concentration levels ranged below 15 %, with the exception of tariquidar in mouse plasma and brain homogenate and riluzole in brain homogenate. Pilot studies have confirmed that the method is suitable for the analysis of mouse plasma samples and brain homogenates following cassette dosing of this mixture in mice.

Omega-3 Polyunsaturated Fatty Acids and Blood-Brain Barrier Integrity in Major Depressive Disorder: Restoring Balance for Neuroinflammation and Neuroprotection

Major depressive disorder (MDD), affecting over 264 million individuals globally, is associated with immune system dysregulation and chronic neuroinflammation, potentially linked to neurodegenerative processes. This review examines blood-brain barrier (BBB) dysfunction in MDD, focusing on key regulators like matrix metalloproteinase 9 (MMP9), aquaporin-4 (AQP4), and ATP-binding cassette subfamily B member 1 (ABCB1). We explore potential mechanisms by which compromised BBB integrity in MDD may contribute to neuroinflammation and discuss the therapeutic potential of omega-3 polyunsaturated fatty acids (n-3 PUFAs). n-3 PUFAs have demonstrated anti-inflammatory and neuroprotective effects, and potential ability to modulate MMP9, AQP4, and ABCB1, thereby restoring BBB integrity in MDD. This review aims to elucidate these potential mechanisms and evaluate the evidence for n-3 PUFAs as a strategy to mitigate BBB dysfunction and neuroinflammation in MDD.

Histologic and ultrastructural study of intracranial Gaucheroma causing deafness in a patient with Gaucher disease type 3: Effects of substrate reduction therapy

Hearing loss is frequently associated with Gaucher disease (GD). Gaucher cells are enlarged reticuloendothelial cells containing glucocerebroside in the lysosomes due to deficiency of the glucocerebrosidase. Gaucheromas consist of accumulated Gaucher cells. Gaucher cells accumulate in variable tissues including the liver, spleen, bone marrow, and the middle ear and the mastoid causing conductive hearing loss. Neurons and astrocytes in the central nervous system are affected in neuronopathic GD leading to sensorineural hearing loss. Gaucheromas can develop even in patients treated with enzyme replacement therapy (ERT). We report a 19-year-old female patient with GD type 3 who developed profound bilateral hearing loss associated with intracranial Gaucheroma. Combination therapy of ERT with imiglucerase and substrate reduction therapy (SRT) with eliglustat significantly decreased the size of Gaucher cells and cleared the characteristic microtubular structures in the lysosomes in Gaucher cells. Early implementation of SRT may prevent at least conductive hearing impairment in GD although it may not prevent sensorineural hearing loss due to inner hair cell dysfunction which is also known to be associated with neuronopathic GD.

Enhancing of cerebral Abeta clearance by modulation of ABC transporter expression: a review of experimental approaches

Clearance of amyloid-beta (Aβ) from the brain is impaired in both early-onset and late-onset Alzheimer's disease (AD). Mechanisms for clearing cerebral Aβ include proteolytic degradation, antibody-mediated clearance, blood brain barrier and blood cerebrospinal fluid barrier efflux, glymphatic drainage, and perivascular drainage. ATP-binding cassette (ABC) transporters are membrane efflux pumps driven by ATP hydrolysis. Their functions include maintenance of brain homeostasis by removing toxic peptides and compounds, and transport of bioactive molecules including cholesterol. Some ABC transporters contribute to lowering of cerebral Aβ. Mechanisms suggested for ABC transporter-mediated lowering of brain Aβ, in addition to exporting of Aβ across the blood brain and blood cerebrospinal fluid barriers, include apolipoprotein E lipidation, microglial activation, decreased amyloidogenic processing of amyloid precursor protein, and restricting the entrance of Aβ into the brain. The ABC transporter superfamily in humans includes 49 proteins, eight of which have been suggested to reduce cerebral Aβ levels. This review discusses experimental approaches for increasing the expression of these ABC transporters, clinical applications of these approaches, changes in the expression and/or activity of these transporters in AD and transgenic mouse models of AD, and findings in the few clinical trials which have examined the effects of these approaches in patients with AD or mild cognitive impairment. The possibility that therapeutic upregulation of ABC transporters which promote clearance of cerebral Aβ may slow the clinical progression of AD merits further consideration.

Neural Stem Cell-Derived Small Extracellular Vesicles: key Players in Ischemic Stroke Therapy - A Comprehensive Literature Review

Ischemic stroke, being a prominent contributor to global disability and mortality, lacks an efficacious therapeutic approach in current clinical settings. Neural stem cells (NSCs) are a type of stem cell that are only found inside the nervous system. These cells can differentiate into various kinds of cells, potentially regenerating or restoring neural networks within areas of the brain that have been destroyed. This review begins by providing an introduction to the existing therapeutic approaches for ischemic stroke, followed by an examination of the promise and limits associated with the utilization of NSCs for the treatment of ischemic stroke. Subsequently, a comprehensive overview was conducted to synthesize the existing literature on the underlying processes of neural stem cell-derived small extracellular vesicles (NSC-sEVs) transplantation therapy in the context of ischemic stroke. These mechanisms encompass neuroprotection, inflammatory response suppression, and endogenous nerve and vascular regeneration facilitation. Nevertheless, the clinical translation of NSC-sEVs is hindered by challenges such as inadequate targeting efficacy and insufficient content loading. In light of these limitations, we have compiled an overview of the advancements in utilizing modified NSC-sEVs for treating ischemic stroke based on current methods of extracellular vesicle modification. In conclusion, examining NSC-sEVs-based therapeutic approaches is anticipated to be prominent in both fundamental and applied investigations about ischemic stroke.

Circadian ABCG2 Expression Influences the Brain Uptake of Donepezil across the Blood-Cerebrospinal Fluid Barrier

Donepezil (DNPZ) is a cholinesterase inhibitor used for the management of Alzheimer's disease (AD) and is dependent on membrane transporters such as ABCG2 to actively cross brain barriers and reach its target site of action in the brain. Located in the brain ventricles, the choroid plexus (CP) forms an interface between the cerebrospinal fluid (CSF) and the bloodstream, known as the blood-CSF barrier (BCSFB). Historically, the BCSFB has received little attention as a potential pathway for drug delivery to the central nervous system (CNS). Nonetheless, this barrier is presently viewed as a dynamic transport interface that limits the traffic of molecules into and out of the CNS through the presence of membrane transporters, with parallel activity with the BBB. The localization and expression of drug transporters in brain barriers represent a huge obstacle for drug delivery to the brain and a major challenge for the development of therapeutic approaches to CNS disorders. The widespread interest in understanding how circadian clocks modulate many processes that define drug delivery in order to predict the variability in drug safety and efficacy is the next bridge to improve effective treatment. In this context, this study aims at characterizing the circadian expression of ABCG2 and DNPZ circadian transport profile using an in vitro model of the BCSFB. We found that ABCG2 displays a circadian pattern and DNPZ is transported in a circadian way across this barrier. This study will strongly impact on the capacity to modulate the BCSFB in order to control the penetration of DNPZ into the brain and improve therapeutic strategies for the treatment of AD according to the time of the day.

Hyaluronic Acid Prevents Fusion of Brain Tumor-Derived Spheroids and Selectively Alters Their Gene Expression Profile

Hyaluronic acid (HA), a major glycosaminoglycan of the brain extracellular matrix, modulates cell behaviors through binding its receptor, Cd44. In this study, we assessed the influence of HA on high-grade brain tumors in vitro. The model comprised cell cultures derived from six rodent carcinogen-induced brain tumors, forming 3D spheroids prone to spontaneous fusion. Supplementation of the standard culture medium with 0.25% HA significantly inhibited the fusion rates, preserving the shape and size uniformity of spheroids. The 3D cultures were assigned to two groups; a Cd44lo group had a tenfold decreased relative expression of Cd44 than another (Cd44hi) group. In addition, these two groups differed by expression levels of Sox2 transcription factor; the correlation analysis revealed a tight negative association for Cd44 and Sox2. Transcriptomic responses of spheroids to HA exposure also depended on Cd44 expression levels, from subtle in Cd44lo to more pronounced and specific in Cd44hi, involving cell cycle progression, PI3K/AKT/mTOR pathway activation, and multidrug resistance genes. The potential HA-induced increase in brain tumor 3D models' resistance to anticancer drug therapy should be taken into account when designing preclinical studies using HA scaffold-based models. The property of HA to prevent the fusion of brain-derived spheroids can be employed in CNS regenerative medicine and experimental oncology to ensure the production of uniform, controllably fusing neurospheres when creating more accurate in vitro brain models.

The role of ATP-binding cassette transporter G1 (ABCG1) in Alzheimer's disease: A review of the mechanisms

The maintenance of cholesterol homeostasis is essential for central nervous system function. Consequently, factors that affect cholesterol homeostasis are linked to neurological disorders and pathologies. Among them, ATP-binding cassette transporter G1 (ABCG1) plays a significant role in atherosclerosis. However, its role in Alzheimer's disease (AD) is unclear. There is inconsistent information regarding ABCG1's role in AD. It can increase or decrease amyloid β (Aβ) levels in animals' brains. Clinical studies show that ABCG1 is involved in AD patients' impairment of cholesterol efflux capacity (CEC) in the cerebrospinal fluid (CSF). Lower Aβ levels in the CSF are correlated with ABCG1-mediated CEC dysfunction. ABCG1 modulates α-, β-, and γ-secretase activities in the plasma membrane and may affect Aβ production in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) cell compartment. Despite contradictory findings regarding ABCG1's role in AD, this review shows that ABCG1 has a role in Aβ generation via modulation of membrane secretases. It is, however, necessary to investigate the underlying mechanism(s). ABCG1 may also contribute to AD pathology through its role in apoptosis and oxidative stress. As a result, ABCG1 plays a role in AD and is a candidate for drug development.

Abcg2a is the functional homolog of human ABCG2 expressed at the zebrafish blood-brain barrier

BACKGROUND

A principal protective component of the mammalian blood-brain barrier (BBB) is the high expression of the multidrug efflux transporters P-glycoprotein (P-gp, encoded by ABCB1) and ABCG2 (encoded by ABCG2) on the lumenal surface of endothelial cells. The zebrafish P-gp homolog Abcb4 is expressed at the BBB and phenocopies human P-gp. Comparatively little is known about the four zebrafish homologs of the human ABCG2 gene: abcg2a, abcg2b, abcg2c, and abcg2d. Here we report the functional characterization and brain tissue distribution of zebrafish ABCG2 homologs.

METHODS

To determine substrates of the transporters, we stably expressed each in HEK-293 cells and performed cytotoxicity and fluorescent efflux assays with known ABCG2 substrates. To assess the expression of transporter homologs, we used a combination of RNAscope in situ hybridization probes and immunohistochemistry to stain paraffin-embedded sections of adult and larval zebrafish.

RESULTS

We found Abcg2a had the greatest substrate overlap with ABCG2, and Abcg2d appeared to be the least functionally similar. We identified abcg2a as the only homolog expressed at the adult and larval zebrafish BBB, based on its localization to claudin-5 positive brain vasculature.

CONCLUSIONS

These results demonstrate the conserved function of zebrafish Abcg2a and suggest that zebrafish may be an appropriate model organism for studying the role of ABCG2 at the BBB.

Utilizing non-coding RNA-mediated regulation of ATP binding cassette (ABC) transporters to overcome multidrug resistance to cancer chemotherapy

Multidrug resistance (MDR) is one of the primary factors that produces treatment failure in patients receiving cancer chemotherapy. MDR is a complex multifactorial phenomenon, characterized by a decrease or abrogation of the efficacy of a wide spectrum of anticancer drugs that are structurally and mechanistically distinct. The overexpression of the ATP-binding cassette (ABC) transporters, notably ABCG2 and ABCB1, are one of the primary mediators of MDR in cancer cells, which promotes the efflux of certain chemotherapeutic drugs from cancer cells, thereby decreasing or abolishing their therapeutic efficacy. A number of studies have suggested that non-coding RNAs (ncRNAs), particularly microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play a pivotal role in mediating the upregulation of ABC transporters in certain MDR cancer cells. This review will provide updated information about the induction of ABC transporters due to the aberrant regulation of ncRNAs in cancer cells. We will also discuss the measurement and biological profile of circulating ncRNAs in various body fluids as potential biomarkers for predicting the response of cancer patients to chemotherapy. Sequence variations, such as alternative polyadenylation of mRNA and single nucleotide polymorphism (SNPs) at miRNA target sites, which may indicate the interaction of miRNA-mediated gene regulation with genetic variations to modulate the MDR phenotype, will be reviewed. Finally, we will highlight novel strategies that could be used to modulate ncRNAs and circumvent ABC transporter-mediated MDR.

Abcg2a is the functional homolog of human ABCG2 expressed at the zebrafish blood-brain barrier

Background

A principal protective component of the mammalian blood-brain barrier (BBB) is the high expression of the multidrug efflux transporters P-glycoprotein (P-gp, encoded by ABCB1) and ABCG2 (encoded by ABCG2) on the lumenal surface of endothelial cells. The zebrafish P-gp homolog Abcb4 is expressed at the BBB and phenocopies human P-gp. Comparatively little is known about the four zebrafish homologs of the human ABCG2 gene: abcg2a, abcg2b, abcg2c, and abcg2d. Here we report the functional characterization and brain tissue distribution of zebrafish ABCG2 homologs.

Methods

To determine substrates of the transporters, we stably expressed each in HEK-293 cells and performed cytotoxicity and fluorescent efflux assays with known ABCG2 substrates. To assess the expression of transporter homologs, we used a combination of RNAscope in situ hybridization probes and immunohistochemistry to stain paraffin-embedded sections of adult and larval zebrafish.

Results

We found Abcg2a had the greatest substrate overlap with ABCG2, and Abcg2d appeared to be the least functionally similar. We identified abcg2a as the only homolog expressed at the adult and larval zebrafish BBB, based on its localization to claudin-5 positive brain vasculature.

Conclusions

These results demonstrate the conserved function of zebrafish Abcg2a and suggest that zebrafish may be an appropriate model organism for the studying the role of ABCG2 at the BBB.

Upregulation of P-Glycoprotein and Breast Cancer Resistance Protein Activity in Newly Developed in Vitro Rat Blood-Brain Barrier Spheroids Using Advanced Glycation End-Products

The blood-brain barrier (BBB) is a dynamic interface controlling the compound translocation between the blood and the brain, thereby maintaining neural homeostasis. There is cumulative evidence that BBB impairment during diabetes mellitus (DM) takes part in the progression of cognitive dementia. As tight junction proteins and ATP-binding cassette (ABC) transporters regulate substance exchange between the circulating blood and brain, the expression and function of these molecules under DM should be fully clarified. To understand the alteration of ABC transporter function in the BBB under DM, in vitro multicellular rat BBB spheroids consisting of conditionally immortalized rat brain capillary endothelial cells, astrocytes, and pericytes were newly developed. Immunostaining and permeability analysis of paracellular transport markers suggested the construction of tight junctions on the surface of the BBB spheroids. Transport analyses using fluorescence substrates of P-glycoprotein (P-gp), the breast cancer resistance protein (BCRP), and multidrug resistance-associated protein 4 (MRP4) indicate the functional expression of these transporters in the spheroids. After treatment with advanced glycation end-products (AGEs), involved in various signals during DM, the mRNA expression of tight junction molecules and ABC transporters in the BBB spheroids was upregulated. Furthermore, the functional changes in P-gp and BCRP in the BBB spheroids exposed to AGEs were canceled by the inhibitors of the receptor for AGEs (RAGE). These results suggest that AGE-RAGE interaction upregulates P-gp and BCRP function in the BBB.

Dissimilar Effect of P-Glycoprotein and Breast Cancer Resistance Protein Inhibition on the Distribution of Erlotinib to the Retina and Brain in Humans and Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two ATP-binding cassette efflux transporters that are coexpressed at the human blood-brain barrier (BBB) and blood-retina barrier (BRB). While pharmacological inhibition of P-gp and/or BCRP results in increased brain distribution of dual P-gp/BCRP substrate drugs, such as the tyrosine kinase inhibitor erlotinib, the effect of P-gp and/or BCRP inhibition on the retinal distribution of such drugs has hardly been investigated. In this study, we used positron emission tomography (PET) imaging to assess the effect of transporter inhibition on the distribution of [11C]erlotinib to the human retina and brain. Twenty two healthy volunteers underwent two PET scans after intravenous (i.v.) injection of a microdose (<5 μg) of [11C]erlotinib, a baseline scan, and a second scan either with concurrent i.v. infusion of tariquidar to inhibit P-gp (n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, 650 mg, or 1000 mg, n = 17) to saturate erlotinib transport. In addition, transport of [3H]erlotinib to the retina and brain was assessed in mice by in situ carotid perfusion under various drug transporter inhibition settings. In comparison to the baseline PET scan, coadministration of tariquidar or erlotinib led to a significant decrease of [11C]erlotinib total volume of distribution (VT) in the human retina by -25 ± 8% (p ≤ 0.05) and -41 ± 16% (p ≤ 0.001), respectively. In contrast, erlotinib intake led to a significant increase in [11C]erlotinib VT in the human brain (+20 ± 16%, p ≤ 0.001), while administration of tariquidar did not result in any significant changes. In situ carotid perfusion experiments showed that both P-gp and BCRP significantly limit the distribution of erlotinib to the mouse retina and brain but revealed a similar discordant effect at the mouse BRB and BBB following co-perfusion with tariquidar and erlotinib as in humans. Co-perfusion with prototypical inhibitors of solute carrier transporters did not reveal a significant contribution of organic cation transporters (e.g., OCTs and OCTNs) and organic anion-transporting polypeptides (e.g., OATP2B1) to the retinal and cerebral distribution of erlotinib. In conclusion, we observed a dissimilar effect after P-gp and/or BCRP inhibition on the retinal and cerebral distribution of [11C]erlotinib. The exact mechanism for this discrepancy remains unclear but may be related to the function of an unidentified erlotinib uptake carrier sensitive to tariquidar inhibition at the BRB. Our study highlights the great potential of PET to study drug distribution to the human retina and to assess the functional impact of membrane transporters on ocular drug distribution.