Characterization and tissue localization of zebrafish homologs of the human ABCB1 multidrug transporter

The roles of ABCB1/P-glycoprotein drug transporters in regulating gut microbes and inflammation: insights from animal models, old and new

Commensal enteric bacteria have evolved systems that enable growth in the ecologic niche of the host gastrointestinal tract. Animals evolved parallel mechanisms to survive the constant exposure to bacteria and their metabolic by-products. We propose that drug transporters encompass a crucial system to managing the gut microbiome. Drug transporters are present in the apical surface of gut epithelia. They detoxify cells from small molecules and toxins (xenobiotics) in the lumen. Here, we review what is known about commensal structure in the absence of the transporter ABCB1/P-glycoprotein in mammalian models. Knockout or low-activity alleles of ABCB1 lead to dysbiosis, Crohn's disease and ulcerative colitis in mammals. However, the exact function of ABCB1 in these contexts remain unclear. We highlight emerging models-the zebrafish Danio rerio and sea urchin Lytechinus pictus-that are poised to help dissect the fundamental mechanisms of ATP-binding cassette (ABC) transporters in the tolerance of commensal and pathogenic communities in the gut. We and others hypothesize that ABCB1 plays a direct role in exporting inflammatory bacterial products from host epithelia. Interdisciplinary work in this research area will lend novel insight to the transporter-mediated pathways that impact microbiome community structure and accelerate the pathogenesis of inflammatory bowel disease when perturbed. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

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.

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.

Ivermectin Toxicokinetics in Rainbow Trout (Oncorhynchus mykiss) following P-glycoprotein Induction

Alterations in ivermectin (IVM, 22,23-dihydro avermectin B1a+22,23-dihydro avermectin B1b) toxicokinetics following P-glycoprotein (P-gp) induction by clotrimazole (CTZ) were examined in rainbow trout (Oncorhynchus mykiss) to assess the potential importance of P-gp activity levels in xenobiotic distribution and kinetics in fish. Control and fish pretreated with CTZ (30 µmol/kg) were administered 175 µg/kg 3H-IVM into the caudal vasculature. At various time points (0.25, 0.5, 1, 3, 24, 48, 96, and 168 h) following injection, tissues (blood, liver, kidney, gill, intestines, brain [5 regions], eye, gonad and fat) were removed analyzed for IVM-derived radioactivity. IVM concentration declined in blood, liver, kidney and gill, and concentrations in other tissues remained constant over the sampling period. The highest measured concentrations were found in kidney, followed by liver, with the lowest values found in brain, eye and gonad. The highest % of the administered dose was found in the liver and kidney in the immediate hours post-administration, and in the intestines and fat at 24 h post-administration. P-gp induction by CTZ did not alter IVM distribution or any calculated toxicokinetic parameter (AUC, mean residence time, T1/2, clearance rate, volume of distribution), suggesting that P-gp induction may be limited or that P-gp plays a lesser role in xenobiotic kinetics in fish compared to mammals.

Effects of the chemosensitizer verapamil on P-glycoprotein substrate efflux in rainbow trout hepatocytes

The ATP-dependent membrane transporter P-glycoprotein (P-gp) is associated with resistance to a wide variety of chemical substrates, as well as the multi-drug resistance (MDR) phenotype in mammals. Less is known regarding P-gp's function and relevance in teleosts; this study expanded the range of known substrates and the inhibitory effects of a model chemosensitizer verapamil. The P-gp-mediated uptake and efflux dynamics of 5 known mammalian substrates (berberine, cortisol, doxorubicin, rhodamine 123 [R123], and vinorelbine) were examined in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes with and without co-exposure to varying doses of verapamil. Initial substrate uptake rates (pmol/106 cells/min) varied widely and were in order: berberine (482 ± 94) > R123 (364 ± 67) > doxorubicin (158 ± 41) > cortisol (20.3 ± 5.9) > vinorelbine (15.3 ± 3.5). Initial efflux rates (pmol/106 cells/min) were highest in berberine (464 ± 110) > doxorubicin (341 ± 57) > R123 (106 ± 33) > cortisol (26.6 ± 6.1) > vinorelbine (9.0 ± 2.4). Transport of vinorelbine and R123 is verapamil sensitive, but verapamil had no effect on transport of berberine, cortisol, or doxorubicin. Cortisol and doxorubicin showed evidence of high P-gp affinity, thus displacing verapamil from their shared P-gp binding site. Cortisol, doxorubicin, R123, and vinorelbine transport by rainbow trout P-gp was confirmed, while berberine could not be confirmed or excluded as a substrate. Binding sites and affinities were similar between mammalian and trout P-gp for doxorubicin, R123, and vinorelbine, while fish P-gp had a higher affinity for cortisol than mammalian P-gp. This study demonstrated that the range of substrates, as well as binding sites and affinities, of fish P-gp are well-aligned with those in mammals.

The effects of P-glycoprotein induction on ivermectin-induced behavioural alterations in zebrafish (Danio rerio) under varying diets

The neuroprotective effects of inducing the blood-brain barrier ATP-binding cassette protein transporter P-glycoprotein (P-gp) with clotrimazole (CTZ) in both fed and fasted zebrafish (Danio rerio) against the CNS-toxicant ivermectin (IVM, 22,23-dihydro avermectin B1a + 22,23-dihydro avermectin B1b) were examined. Zebrafish were administered 2 μmol/kg IVM intraperitoneally, and various behavioural assays (swimming performance, exploratory behaviour, olfactory responses, motor coordination, and escape responses) were used to measure neurological dysfunction. IVM administration alone caused a decrease in mean swim speed (91 % of controls), maximal speed (71 %), passage rate (81 %), 90° turns (81 %), and response to food stimulus (39 %). IVM exposure also increased the percent time that fish spent immobile (45 % increase over controls) and the percent of lethargic fish (40 % increase). Fish administered 30 μmol/kg of the P-gp inducer CTZ intraperitoneally 3 d prior to IVM exposure exhibited a change in only the % time spent immobile. These data indicate that P-gp induction may be limited in protecting the zebrafish CNS from IVM over baseline. Fasted fish did not differ from fed fish in the effects of IVM on behaviour, and no differences were seen following P-gp induction with CTZ. These results suggest that this chemical defence system is not downregulated when fish are challenged with limited energy availability.

Identification of NanoLuciferase Substrates Transported by Human ABCB1 and ABCG2 and their Zebrafish Homologs at the Blood-Brain Barrier

ATP-binding cassette (ABC) transporters expressed at the blood-brain barrier (BBB) impede delivery of therapeutic agents to the brain, including agents to treat neurodegenerative diseases and primary and metastatic brain cancers. Two transporters, P-glycoprotein (P-gp, ABCB1) and ABCG2, are highly expressed at the BBB and are responsible for the efflux of numerous clinically useful chemotherapeutic agents, including irinotecan, paclitaxel, and doxorubicin. Based on a previous mouse model, we have generated transgenic zebrafish in which expression of NanoLuciferase (NanoLuc) is controlled by the promoter of glial fibrillary acidic protein, leading to expression in zebrafish glia. To identify agents that disrupt the BBB, including inhibitors of ABCB1 and ABCG2, we identified NanoLuc substrates that are also transported by P-gp, ABCG2, and their zebrafish homologs. These substrates will elevate the amount of bioluminescent light produced in the transgenic zebrafish with BBB disruption. We transfected HEK293 cells with NanoLuc and either human ABCB1, ABCG2, or their zebrafish homologs Abcb4 or Abcg2a, respectively, and expressed at the zebrafish BBB. We evaluated the luminescence of ten NanoLuc substrates, then screened the eight brightest to determine which are most efficiently effluxed by the ABC transporters. We identified one substrate efficiently pumped out by ABCB1, two by Abcb4, six by ABCG2, and four by Abcg2a. These data will aid in the development of a transgenic zebrafish model of the BBB to identify novel BBB disruptors and should prove useful in the development of other animal models that use NanoLuc as a reporter.

ABC transporter-mediated MXR mechanism in fish embryos and its potential role in the efflux of nanoparticles

ATP-binding cassette (ABC) transporters are believed to protect aquatic organisms by pumping xenobiotics out, and recent investigation has suggested their involvement in the detoxification and efflux of nanoparticles (NPs), but their roles in fish embryos are poorly understood. In this regard, this paper summarizes the recent advances in research pertaining to the development of ABC transporter-mediated multi-xenobiotic resistance (MXR) mechanism in fish embryos and the potential interaction between ABC transporters and NPs. The paper focuses on: (1) Expression, function, and modulation mechanism of ABC proteins in fish embryos; (2) Potential interaction between ABC transporters and NPs in cell models and fish embryos. ABC transporters could be maternally transferred to fish embryos and thus play an important role in the detoxification of various chemical pollutants and NPs. There is a need to understand the specific mechanism to benefit the protection of aquatic resources.

Chemical biology fluorescent tools for in vitro investigation of the multidrug resistant P-glycoprotein (P-gp) expression in tumor cells

Selective P-glycoprotein (P-gp)-targeted fluorescent conjugates are desirable tools to investigate the role of P-gp, a protein strongly implicated in mediating multidrug resistance and a major cause of chemotherapy failure. Herein, we report the development of 25 novel fluorescent small-molecule conjugates with varying physicochemical and optical properties, and their biological evaluation in a cell model as P-gp targeted constructs. This investigation revealed relationships between molecular structure and cell behavior and uncovered the capacity of conjugates with varying fluorophores to selectively target P-gp. Sulfocyanine 3 labeled conjugates (5, 10, 24, 29, 34) showed a particular intracellular staining pattern. Other conjugates bearing a boron dipyrromethene (BODIPY) core (3, 8, 13, 22, 27 (BODIPY FL), 12 (BODIPY 564/570) and 4, 9 (BODIPY 650/665)) or a 7-nitrobenz-2-oxa-1,3-diazole (NBD) core (11, 30) showed potential for global P-gp direct detection and quantification. These fluorescent conjugates holds key advantages over existing methods for drug resistance evaluation with regards to P-gp expression and could be used as innovative tools in preclinical assays and clinical diagnosis.

Generation and characterization of a zebrafish knockout model of abcb4, a homolog of the human multidrug efflux transporter P-glycoprotein

The ATP-binding cassette subfamily B member 1 (ABCB1), encoding a multidrug transporter referred to as P-glycoprotein (Pgp), plays a critical role in the efflux of xenobiotics in humans and is implicated in cancer resistance to chemotherapy. Therefore, developing high-throughput animal models to screen for Pgp function and bioavailability of substrates and inhibitors is paramount. Here, we generated and validated a zebrafish knockout line of abcb4, a human Pgp transporter homolog. CRISPR/Cas9 genome editing technology was deployed to generate a frameshift mutation in exon 4 of zebrafish abcb4. The zebrafish abcb4 homozygous mutant exhibited elevated accumulation of fluorescent rhodamine 123, a substrate of human Pgp, in the intestine and brain area of embryos. Moreover, abcb4 knockout embryos were sensitized toward toxic compounds such as doxorubicin and vinblastine compared to the WT zebrafish. Immunostaining for zebrafish Abcb4 colocalized in the endothelial brain cells of adult zebrafish. Transcriptome profiling using Gene Set Enrichment Analysis uncovered that the 'cell cycle process,' 'mitotic cell cycles,' and 'microtubule-based process' were significantly downregulated in the abcb4 knockout brain with age. This study establishes and validates the abcb4 knockout zebrafish as an animal model to study Pgp function in vivo. Unexpectedly it reveals a potentially novel role for zebrafish abcb4 in age-related changes in the brain. The zebrafish lines generated here will provide a platform to aid in the discovery of modulators of Pgp function as well as the characterization of human mutants thereof.

Generation and characterization of a zebrafish knockout model of abcb4, a homolog of the human multidrug efflux transporter P-glycoprotein

The ATP-binding cassette subfamily B member 1 (ABCB1), encoding a multidrug transporter referred to as P-glycoprotein (Pgp), plays a critical role in the efflux of xenobiotics in humans and is implicated in cancer resistance to chemotherapy. Therefore, developing high throughput animal models to screen for Pgp function and bioavailability of substrates and inhibitors is paramount. Here, we generated and validated a zebrafish knockout line of abcb4 , a human Pgp transporter homolog. CRISPR/Cas9 genome editing technology was deployed to generate a frameshift mutation in exon 4 of zebrafish abcb4 . The zebrafish abcb4 homozygous mutant exhibited elevated accumulation of fluorescent rhodamine 123, a substrate of human Pgp, in the intestine and brain area of embryos. Moreover, abcb4 knockout embryos were sensitized toward toxic compounds such as doxorubicin and vinblastine compared to the WT zebrafish. Immuno-staining for zebrafish Abcb4 colocalized in the endothelial brain cells of adult zebrafish. Transcriptome profiling using Gene Set Enrichment Analysis (GSEA) uncovered that the 'cell cycle process,' 'mitotic cell cycles,' and 'microtubule-based process' were significantly downregulated in the abcb4 knockout brain with age. This study establishes and validates the a bcb4 knockout zebrafish as an animal model to study Pgp function in vivo. Unexpectedly it reveals a potentially novel role for zebrafish abcb4 in age-related changes in the brain. The zebrafish lines generated here will provide a platform to aid in the discovery of modulators of Pgp function as well as the characterization of human mutants thereof.

Expression and Function of ABC Proteins in Fish Intestine

In fish, the intestine is fundamental for digestion, nutrient absorption, and other functions like osmoregulation, acid-base balance, and excretion of some metabolic products. These functions require a large exchange surface area, which, in turn, favors the absorption of natural and anthropogenic foreign substances (xenobiotics) either dissolved in water or contained in the food. According to their chemical nature, nutrients, ions, and water may cross the intestine epithelium cells' apical and basolateral membranes by passive diffusion or through a wide array of transport proteins and also through endocytosis and exocytosis. In the same way, xenobiotics can cross this barrier by passive diffusion or taking advantage of proteins that transport physiological substrates. The entry of toxic substances is counterbalanced by an active efflux transport mediated by diverse membrane proteins, including the ATP binding cassette (ABC) proteins. Recent advances in structure, molecular properties, and functional studies have shed light on the importance of these proteins in cellular and organismal homeostasis. There is abundant literature on mammalian ABC proteins, while the studies on ABC functions in fish have mainly focused on the liver and, to a minor degree, on the kidney and other organs. Despite their critical importance in normal physiology and as a barrier to prevent xenobiotics incorporation, fish intestine's ABC transporters have received much less attention. All the ABC subfamilies are present in the fish intestine, although their functionality is still scarcely studied. For example, there are few studies of ABC-mediated transport made with polarized intestinal preparations. Thus, only a few works discriminate apical from basolateral transport activity. We briefly describe the main functions of each ABC subfamily reported for mammals and other fish organs to help understand their roles in the fish intestine. Our study considers immunohistochemical, histological, biochemical, molecular, physiological, and toxicological aspects of fish intestinal ABC proteins. We focus on the most extensively studied fish ABC proteins (subfamilies ABCB, ABCC, and ABCG), considering their apical or basolateral location and distribution along the intestine. We also discuss the implication of fish intestinal ABC proteins in the transport of physiological substrates and aquatic pollutants, such as pesticides, cyanotoxins, metals, hydrocarbons, and pharmaceutical products.