Environmental contaminants modulate transport activity of zebrafish (Danio rerio) multidrug and toxin extrusion protein 3 (Mate3/Slc47a2.1)

Zebrafish Mate3 is one of six co-orthologs of human multidrug and toxin extrusion proteins. It is highly expressed in the kidneys, intestine, testes, and brain of males. Initial interaction studies showed its interaction with xenobiotic compounds, suggesting a role in the efflux of toxic compounds. In this study, we aimed to test various environmental contaminants for their interaction with zebrafish Mate3. We developed a stable zebrafish Mate3 cell line and optimized a high-throughput screening assay using DAPI and ASP+ as fluorescent model substrates. To gain insight into the structure and function of the Mate3 protein and relate these to the results of the DAPI and ASP+ transport measurements, we predicted its 3D structure using the AlphaFold2 algorithm. A 3D structure with high per residue confidence scores with 13 transmembrane segments (TMs) was obtained, with topology and mutual positioning characteristic of the Mate protein family in a shape open to the extracellular part. Molecular docking methods were used to identify DAPI and ASP+ binding sites on the surface and in the center of the protein cavity. Because our kinetics experiments combined with molecular docking indicated that there may be additional active sites in zebrafish Mate3, additional cytotoxicity experiments were performed and highly potent Mate3 interactors were identified from a set of 55 different environmental contaminants. Our results suggest that some of the identified interactors may be of environmental concern, as their interaction with Mate3 could lead to an impairment of its normal efflux function, making fish more sensitive to harmful substances commonly released into the aquatic environment. Finally, the quality of zebrafish Mate3 structures predicted by the AlphaFold2 algorithm opens up the possibility of successfully using this tool for in silico research on transport preferences of other Mate proteins.

Stage-dependent localization of F-actin and Na+ /K+ -ATPase in zebrafish embryos detected using optimized cryosectioning immunostaining protocol

The increasing use of the zebrafish model in biomedical and (eco)toxicological studies aimed at understanding the function of various proteins highlight the importance of optimizing existing methods to study gene and protein expression and localization in this model. In this context, zebrafish cryosections are still underutilized compared with whole-mount preparations. In this study, we used zebrafish embryos (24-120 hpf) to determine key factors for the preparation of high-quality zebrafish cryosections and to determine the optimal protocol for (immuno)fluorescence analyses of Na⁺ /K⁺ -ATPase and F-actin, across developmental stages from 1 to 5 dpf. The results showed that the highest quality zebrafish cryosections were obtained after the samples were fixed in 4% paraformaldehyde (PFA) for 1 h, incubated in 2.5% bovine gelatin/25% sucrose mixture, embedded in OCT, and then sectioned to 8 μm thickness at -20°C. Fluorescence microscopy analysis of phalloidin-labeled zebrafish skeletal muscle revealed that 1-h-4% PFA-fixed samples allowed optimal binding of phalloidin to F-actin. Further immunofluorescence analyses revealed detailed localization of F-actin and Na⁺ /K⁺ -ATPase in various tissues of the zebrafish and a stage-dependent increase in their respective expression in the somitic muscles and pronephros. Finally, staining of zebrafish cryosections and whole-mount samples revealed organ-specific and zone-dependent localizations of the Na⁺ /K⁺ -ATPase α1-subunit. RESEARCH HIGHLIGHTS: This study brings optimization of existing protocols for preparation and use of zebrafish embryos cryosections in (immuno)histological analyses. It reveals stage-dependent localization/expression of F-actin and Na⁺ /K⁺ -ATPase in zebrafish embryos.

Selective interaction of microcystin congeners with zebrafish (Danio rerio) Oatp1d1 transporter

Microcystins (MCs) are the most studied cyanotoxins. The uptake of MCs in cells and tissues of mammals and fish species is mostly mediated by organic anion-transporting polypeptides (OATPs in humans and rodents; Oatps in other species), and the Oatp1d1 appears to be a major transporter for MCs in fish. In this study, six MC congeners of varying physicochemical properties (MC-LR, -RR, -YR, -LW, -LF, -LA) were tested by measuring their effect on the uptake of model Oatp1d1 fluorescent substrate Lucifer yellow (LY) in HEK293T cells transiently or stably overexpressing zebrafish Oatp1d1. MC-LW and -LF showed the strongest interaction resulting in an almost complete inhibition of LY transport with IC₅₀ values of 0.21 and 0.26 μM, while congeners -LR, -YR and -LA showed lower inhibitory effects. To discern between Oatp1d1 substrates and inhibitors, results were complemented by Michaelis-Menten kinetics and chemical analytical determinations of MCs uptake, along with molecular docking studies performed using the developed zebrafish Oatp1d1 homology model. Our study showed that Oatp1d1-mediated transport of MCs could be largely dependent on their basic physicochemical properties, with log P_OW being the most obvious determinant. Finally, apart from determination of the chemical composition of cynobacterial blooms, a reliable risk assessment should take into account the interaction of identified MC congeners with Oatp1d1 as their primary transporter, and herewith we demonstrated that such a comprehensive approach could be based on the use of highly specific in vitro models, accompanied by chemical assessment and in silico molecular docking studies.

Environmental contaminants modulate transport activity of zebrafish organic anion transporters Oat1 and Oat3

Organic anion transporters (OATs) are transmembrane proteins which belong to SLC22 subfamily. They are responsible for the uptake of various endo- and xenobiotics into the cells of different organs and tissues. Following our previous work on characterization of zebrafish Oat1 and Oat3, in this study we analyzed interaction of various classes of environmental contaminants with these membrane transporters using the transport activity assay with HEK293 Flp-In cell line stably overexpressing zebrafish Oat1 and Oat3, respectively. Based on the initial screening of a series of 36 environmental contaminants on their ability to interact with zebrafish Oat1 and Oat3, the most potent interactors were selected, their IC50 values calculated and type of interaction determined. Finally, to further confirm the type of interaction and initially evaluate their toxic potential, the cytotoxicity assays were performed. Broad ligand selectivity and similarity of zebrafish Oat1 and Oat3 with mammalian orthologs was confirmed and potent interactors among environmental contaminants identified.

Interaction of environmental contaminants with zebrafish (Danio rerio) multidrug and toxin extrusion protein 7 (Mate7/Slc47a7)

Zebrafish Mate7 belongs to solute carrier protein superfamily and specifically to subfamily of multidrug and toxin extruders. It is co-orthologous to mammalian Mates, and is ubiquitously expressed in zebrafish tissues with the highest expression in kidney. It has been shown to interact with both endogenous (steroid hormones) and xenobiotic compounds (pharmaceuticals), implying a role in efflux of toxic compounds. The objective of our study was to analyse interaction of environmental contaminants with zebrafish Mate7 using a newly developed high throughput screening (HTS) Mate7 assay. A full-length zebrafish mate7 sequence was obtained from zebrafish cDNA originating from male kidney, and a stable expression of Mate7 in genetically engineered HEK293 Flp-In cells was achieved. Stable Mate7 transfectants were then used for development and optimization of a new HTS cellular uptake protocol, with DAPI and ASP + as model fluorescent substrates. The developed assay was used for identifying zebrafish Mate 7 interactors and discerning the type of interaction. A series of 89 diverse environmental contaminants, including industrial chemicals, pesticides, and pharmaceuticals, was tested and highly effective Mate7 interactors were identified in all of the aforementioned groups. Some of the inhibitors identified could be of environmental concern because they may potentially impair Mate7 efflux function, lowering the fish defence capacity against environmental contaminants, or interfering with transport of yet unidentified physiological substrates. In addition, we found significant differences between zebrafish Mate7 and mammalian Mates' substrate preferences, a finding that should be taken into consideration when using zebrafish as a model organism in toxicokinetic studies.

Functional characterization of rainbow trout (Oncorhynchus mykiss) Abcg2a (Bcrp) transporter

ABCG2 (BCRP - breast cancer resistance protein) belongs to the ATP-binding cassette (ABC) superfamily. It plays an important role in the disposition and elimination of xeno- and endobiotics and/or their metabolites in mammals. Likewise, the protective role of ABC transporters, including Abcg2, has been reported for aquatic organisms. In our previous study we have cloned the full gene sequence of rainbow trout (Oncorhynchus mykiss) Abcg2a and showed its high expression in liver and primary hepatocytes. Based on those insights, the main goal of this study was to perform a detailed functional characterization of trout Abcg2a using insect ovary cells (Spodoptera frugiperda, Sf9) as a heterologous expression system. Membrane vesicles preparations from Sf9 cells were used for the ATPase assay determinations and basic biochemical properties of fish Abcg2a versus human ABCG2 have been compared. A series of 39 physiologically and/or environmentally relevant substances was then tested on interaction with trout Abcg2a and human ABCG2. Correlation analysis reveals highly similar pattern of activation and inhibition. Significant activation of trout Abcg2a ATPase was observed for prazosin, doxorubicine, sildenafil, furosemid, propranolol, fenofibrate and pheophorbide. Pesticides showed either a weak activation (malathione) or strong (endosulfan) to weak (chlorpyrifos, fenoxycarb, DDE) inhibition of trout Abcg2a ATPase while the highest activation was obtained for benzo(a)pyrene, curcumine and testosterone. In conclusion, data from this study offer the first characterization of fish Abcg2a, reveal potent interactors among physiologically or environmentally relevant substances and point to similarities regarding strengths and interactor preferences between human ABCG2 and fish Abcg2a.

The first characterization of multidrug and toxin extrusion (MATE/SLC47) proteins in zebrafish (Danio rerio)

Multidrug and toxin extrusion (MATE) proteins are involved in the extrusion of endogenous compounds and xenobiotics across the plasma membrane. They are conserved from bacteria to mammals, with different numbers of genes within groups. Here, we present the first data on identification and functional characterization of Mate proteins in zebrafish (Danio rerio). Phylogenetic analysis revealed six Mates in teleost fish, annotated as Mate3-8, which form a distinct cluster separated from the tetrapod MATEs/Mates. Synteny analysis showed that zebrafish mate genes are orthologous to human MATEs. Gene expression analysis revealed that all the mate transcripts were constitutively and differentially expressed during embryonic development, followed by pronounced and tissue-specific expression in adults. Functional analyses were performed using transport activity assays with model substrates after heterologous overexpression of five zebrafish Mates in HEK293T cells. The results showed that zebrafish Mates interact with both physiological and xenobiotic substances but also substantially differ with respect to the interacting compounds and interaction strength in comparison to mammalian MATEs/Mates. Taken together, our data clearly indicate a potentially important role for zebrafish Mate transporters in zebrafish embryos and adults and provide a basis for detailed functional characterizations of single zebrafish Mate transporters.

Identification of P-glycoprotein inhibitors in contaminated freshwater sediments

P-glycoprotein (P-gp, ABCB1) is an important part of the multixenobiotic resistance (MXR) defense system in aquatic organisms. The main goal of this study was identification of P-gp inhibitors in contaminated sediments using the effect-directed analysis (EDA) approach. The samples were collected from the Gorjak creek (Zagreb, Croatia), a recipient of wastewater effluents from the pharmaceutical industry. Sediment samples were extracted and fractionated using a two-tiered approach. Resulting nonpolar, medium polar, and polar fractions were tested on the inhibition of P-gp activity using P-gp overexpressing PLHC-1/dox cells and calcein-AM as model substrate. The obtained EC50 values (up to 757 μg/g, expressed in toxicity equivalents of model P-gp inhibitor cyclosporine A) revealed high inhibitory potential of polar fractions of investigated sediments and clearly reflected the impact of pharmaceutical wastewater. P-gp specific ATPase assay and the cytotoxicity modulation experiments with colchicine indicated that most of the observed P-gp inhibition was due to the presence of noncompetitive inhibitors. A detailed chemical analysis by ultrahigh-performance liquid chromatography-quadrupole/time-of-flight mass spectrometry (UPLC-QTOFMS) revealed nonionic surfactants, including alcohol polyethoxylates (LAEOs) and polypropylene glycols (PPGs), as the major components of the most active subfractions. Testing of several LAEO and PPG commercial mixtures confirmed their potential to inhibit the fish P-glycoprotein and modulate toxicity of other xenobiotics present in complex environmental samples.

Prioritisation of organic contaminants in a river basin using chemical analyses and bioassays

Region-specific contaminant prioritisation is an important prerequisite for sustainable and cost-effective monitoring due to the high number of different contaminants that may be present. Surface water and sediment samples from the Sava River, Croatia, were collected at four locations covering a 150-km-long river section characterised by well-defined pollution gradients. Analysis of contaminant profiles along the pollution gradients was performed by combining toxicity screening using a battery of small-scale or in vitro bioassays, which covered different modes of action, with detailed chemical characterisation based on gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/quadrupole time-of-flight mass spectrometry (LC-QTOF-MS). A large number of contaminants, belonging to different toxicant classes, were identified in both analysed matrices. Analyses of water samples showed that contaminants having polar character occurred in the highest concentrations, while in sediments, contributions from both non-polar and amphiphilic contaminants should be taken into account. Estimated contributions of individual contaminant classes to the overall toxicity indicated that, besides the classical pollutants, a number of emerging contaminants, including surfactants, pharmaceuticals, personal care products and plasticizers, should be taken into consideration in future monitoring activities. This work demonstrates the importance of the integrated chemical and bioanalytical approach for a systematic region-specific pollutant prioritisation. Finally, the results presented in this study confirm that hazard assessment in complex environmental matrices should be directed towards identification of key pollutants, rather than focusing on a priori selected contaminants alone.

First characterization of fish P-glycoprotein (abcb1) substrate specificity using determinations of its ATPase activity and calcein-AM assay with PLHC-1/dox cell line

P-glycoprotein (P-gp; abcb1) is one of the major ABC transport proteins that mediates multixenobiotic resistance (MXR) defense in fish. In order to offer a sound evaluation of its ecotoxicological relevance it is critical to characterize substrate specificity of fish P-gp. Measurement of the ATPase activity is a reliable approach often used to discern type of interaction of various drugs with mammalian P-gp. A similar assay has never been used for characterization of P-gp in aquatic organisms and the main goal of this study was to develop a specific ATPase assay for characterization of fish P-gp. For this purpose we have used P-gp enriched membrane vesicles isolated from fish hepatoma PLHC-1/dox cells characterized by high overexpression of P-gp. As additional demonstration of a P-gp specific phenotype, we have quantified transcript expression of a series of eight ABC efflux transporter genes constitutively expressed in PLHC-1 wild type and PLHC-1/dox cells. Transcript expression analysis confirmed high and specific P-gp transcript overexpression in PLHC-1/dox cells. Provided that the transcript abundance is translated to protein, the development of ATPase assay is enabled. Using this model we determined Km(ATP) of 0.4mM, baseline ATPase activity from 35-50nmol/mg(PROT)/min, and maximal activation of ATPase activity obtained for fish P-gp in our system was 1.8-2.5-fold over baseline. All these values were in good agreement with data previously reported for mammalian P-gp. In order to perform a more detailed characterization of fish P-gp substrate specificity, in the next step of our study we used the developed ATPase assay to test 50 different compounds for their interaction with fish P-gp. The same set of compounds was also tested with calcein-AM (Ca-AM) transport activity assay both using PLHC-1/dox cells and NIH 3T3/MDR1 fibroblast cells overexpressing human P-gp. Our results showed that there is a clear difference for some substances-five compounds specifically interacted only with fish P-gp, while seven compounds exhibited interaction with human P-gp only. Most of the compounds tested in this study showed similar behavior in respect to fish or human P-gp and relatively high correlation in the interaction potency was found between fish and human P-gp. In summary, the described results represent the first in depth insight into substrate specificity of an important xenobiotic efflux transporter in fish. In addition, our study showed that combination of Ca-AM assay and the developed ATPase assay using inside/out vesicles isolated from PLHC-1/dox cells, offers a high-throughput and reliable approach for identification of environmentally relevant pollutants that interact with fish P-gp.