Predicting fish acute toxicity using a fish gill cell line-based toxicity assay

The fish embryo test (FET): origin, applications, and future

Originally designed as an alternative for the acute fish toxicity test according to, e.g., OECD TG 203, the fish embryo test (FET) with the zebrafish (Danio rerio) has been optimized, standardized, and validated during an OECD validation study and adopted as OECD TG 236 as a test to assess toxicity of embryonic forms of fish. Given its excellent correlation with the acute fish toxicity test and the fact that non-feeding developmental stages of fish are not categorized as protected stages according to the new European Directive 2010/63/EU on the protection of animals used for scientific purposes, the FET is ready for use not only for range-finding but also as a true alternative for the acute fish toxicity test, as required for a multitude of national and international regulations. If-for ethical reasons-not accepted as a full alternative, the FET represents at least a refinement in the sense of the 3Rs principle. Objections to the use of the FET have mainly been based on the putative lack of biotransformation capacity and the assumption that highly lipophilic and/or high molecular weight substances might not have access to the embryo due to the protective role of the chorion. With respect to bioactivation, the only substance identified so far as not being activated in the zebrafish embryo is allyl alcohol; all other biotransformation processes that have been studied in more detail so far were found to be present, albeit, in some cases, at lower levels than in adult fish. With respect to larger molecules, the extension of the test duration to 96 h (i.e., beyond hatch) has-at least for the substances tested so far-compensated for the reduced access to the embryo; however, more research is necessary to fully explore the applicability of the FET to substances with a molecular weight >3 kDa as well as substances with a neurotoxic mode of action. An extension of the endpoints to also cover sublethal endpoints makes the FET a powerful tool for the detection of teratogenicity, dioxin-like activity, genotoxicity and mutagenicity, neurotoxicity, as well as various forms of endocrine disruption.

Immunomodulatory Potency of Microcystin, an Important Water-Polluting Cyanobacterial Toxin

Microcystins (MCs) are primarily hepatotoxins produced by cyanobacteria and are responsible for intoxication in humans and animals. There are many incidents of chronic exposure to MCs, which have been attributed to the inappropriate treatment of water supplies or contaminated food. Using RAW 264.7 macrophages, we showed the potency of microcystin-LR (MC-LR) to stimulate production of pro-inflammatory cytokines (tumor necrosis factor α and interleukin-6) as a consequence of fast nuclear factor κB and nitrogen-activated protein kinase activation. In contrast to other studies, the observed effects were not attributed to the intracellular inhibition of protein phosphatases 1/2A due to lack of specific transmembrane transporters for MCs. However, the MC-LR-induced activation of macrophages was effectively inhibited by a specific peptide that blocks signaling of receptors, which play a pivotal role in the innate immune responses. Taken together, we showed for the first time that MC-LR could interfere with macrophage receptors that are responsible for triggering the above-mentioned signaling pathways. These findings provide an interesting mechanistic explanation of some adverse health outcomes associated with toxic cyanobacteria and MCs.

Environmental monitoring of urban streams using a primary fish gill cell culture system (FIGCS)

The primary fish gill cell culture system (FIGCS) is an in vitro technique which has the potential to replace animals in whole effluent toxicity tests. In the current study FIGCS were transported into the field and exposed to filtered (0.2μm) river water for 24h from 4 sites, on 2 different sampling dates. Sites 1 and 2 are situated in an urban catchment (River Wandle, London, UK) with site 1 downstream of a sewage treatment work; site 3 is located in a suburban park (River Cray, Kent, UK), and site 4 is more rural (River Darent, Kent, UK). The change in transepithelial electrical resistance (TER), the expression of the metal responsive genes metallothionein A (mta) and B (mtb), cytochrome P450 1A1 (cyp1a1) and 3A27 (cyp3a27), involved in phase 1 metabolism, were assessed following exposure to sample water for 24h. TER was comparable between FIGCS exposed to 0.2μm filtered river water and those exposed to synthetic moderately soft water for 24h. During the first sampling time, there was an increase in mta, cyp1a1 and cyp3a27 gene expression in epithelium exposed to water from sites 1 and 2, and during the second sampling period an increase in cyp3a27 gene expression at sites 1 and 4. Urban river water is a complex mixture of contaminants (e.g., metals, pesticides, pharmaceuticals and polyaromatic hydrocarbons) and the increase in the expression of genes encoding mta, cyp1a1 and cyp3a27 in FIGCS is indicative of the presence of biologically active pollutants.

Biokinetics in repeated-dosing in vitro drug toxicity studies

The aim of the EU FP7 Predict-IV project was to improve the predictivity of in vitro assays for unwanted effects of drugs after repeated dosing. The project assessed the added benefit of integrating long-lived in vitro organotypic cell systems with 'omics' technologies and in silico modelling, including systems biology and pharmacokinetic assessments. RPTEC/TERT1 kidney cells, primary rat and human hepatocytes, HepaRG liver cells and 2D and 3D primary brain cultures were dosed daily or every other day for 14 days to a selection of drugs varying in their mechanism of pharmacological action. Since concentration-effect relationships not only depend on the activity of the drug or the sensitivity of the target, but also on the distribution of compounds in the in vitro system, the concentration of a selection of drugs in cells, microtitre plate plastic and medium was measured over time. Results, reviewed in this paper, indicate that lipophilic drugs bind significantly to plastic labware. A few drugs, including less lipophilic drugs, bind to cell-attachment matrices. Chemicals that reach high concentrations in cells, including cyclosporin A and amiodarone, significantly accumulate over time after repeated dosing, partly explaining their increased toxicity after repeated dosing, compared to a single dose.

Toxicology across scales: Cell population growth in vitro predicts reduced fish growth

Environmental risk assessment of chemicals is essential but often relies on ethically controversial and expensive methods. We show that tests using cell cultures, combined with modeling of toxicological effects, can replace tests with juvenile fish. Hundreds of thousands of fish at this developmental stage are annually used to assess the influence of chemicals on growth. Juveniles are more sensitive than adult fish, and their growth can affect their chances to survive and reproduce. Thus, to reduce the number of fish used for such tests, we propose a method that can quantitatively predict chemical impact on fish growth based on in vitro data. Our model predicts reduced fish growth in two fish species in excellent agreement with measured in vivo data of two pesticides. This promising step toward alternatives to fish toxicity testing is simple, inexpensive, and fast and only requires in vitro data for model calibration.

A primary FIsh Gill Cell System (FIGCS) for environmental monitoring of river waters

Studies were conducted to assess the feasibility of a primary FIsh Gill Cell culture system (FIGCS) for both laboratory and field based environmental monitoring of rivers known to be affected by metal contamination. FIGCS were exposed in the laboratory and in the field to water from the River Hayle, a metal-contaminated system in Cornwall, United Kingdom. Water chemistry, including transition metal concentrations, changes in transepithelial electrical resistance (TEER), cell viability and the expression of metal responsive genes, metallothionein A and B were measured. FIGCS tolerated river water in the laboratory showing no loss in TEER or cell viability following 24h exposure. The cells also tolerated transport to the field (∼1000 km and 30 h) and exposure to unfiltered and filtered river water. Metallothionein A and B, a measure of intracellular biologically active metals, expression was induced in the laboratory and field on exposure to water from sites with elevated metal concentrations compared to those sites where metal levels were below water metal Environmental Quality Standards. This demonstrates that FIGCS detects bioreactive metals in river waters on exposure in the laboratory or field and can be used for on-site environmental monitoring as well as investigations into bioavailability and toxicity of contaminant mixtures in natural waters.

Measured and modeled toxicokinetics in cultured fish cells and application to in vitro-in vivo toxicity extrapolation

Effect concentrations in the toxicity assessment of chemicals with fish and fish cells are generally based on external exposure concentrations. External concentrations as dose metrics, may, however, hamper interpretation and extrapolation of toxicological effects because it is the internal concentration that gives rise to the biological effective dose. Thus, we need to understand the relationship between the external and internal concentrations of chemicals. The objectives of this study were to: (i) elucidate the time-course of the concentration of chemicals with a wide range of physicochemical properties in the compartments of an in vitro test system, (ii) derive a predictive model for toxicokinetics in the in vitro test system, (iii) test the hypothesis that internal effect concentrations in fish (in vivo) and fish cell lines (in vitro) correlate, and (iv) develop a quantitative in vitro to in vivo toxicity extrapolation method for fish acute toxicity. To achieve these goals, time-dependent amounts of organic chemicals were measured in medium, cells (RTgill-W1) and the plastic of exposure wells. Then, the relation between uptake, elimination rate constants, and log KOW was investigated for cells in order to develop a toxicokinetic model. This model was used to predict internal effect concentrations in cells, which were compared with internal effect concentrations in fish gills predicted by a Physiologically Based Toxicokinetic model. Our model could predict concentrations of non-volatile organic chemicals with log KOW between 0.5 and 7 in cells. The correlation of the log ratio of internal effect concentrations in fish gills and the fish gill cell line with the log KOW was significant (r>0.85, p = 0.0008, F-test). This ratio can be predicted from the log KOW of the chemical (77% of variance explained), comprising a promising model to predict lethal effects on fish based on in vitro data.

Toxicity of silver nanoparticles to a fish gill cell line: role of medium composition

In aqueous solutions, silver nanoparticle (AgNP) behavior is affected by a variety of factors which lead to altered AgNP size and toxicity. Our research aims to explore the effect of media composition on citrate-coated AgNP (cit-AgNP) behavior and toxicity to the cell line from rainbow trout (Oncorhynchus mykiss) gill, RTgill-W1. Three different exposure media (L15/ex, L15/ex w/o Cl and d-L15/ex) were used. These were characterized by varying ionic strength and chloride content, both of which had a dominant effect on the behaviour of cit-AgNP. Comparing the behaviour and toxicity of cit-AgNP in the different media, stronger agglomeration of cit-AgNP correlated with higher toxicity. Deposition of cit-AgNP on cells might explain the higher toxicity of agglomerated cit-AgNP compared to that of suspended cit-AgNP. The cit-AgNP concentration-response curves as a function of dissolved silver ions, and the limited prevention of toxicity by silver ligands, indicated that cit-AgNP elicited a particle-specific effect on the cells. Furthermore, the lysosomal membrane integrity was significantly more sensitive to cit-AgNP exposure than cellular metabolic activity or cell membrane integrity and showed the weakest protection by silver ligands. This revealed that cit-AgNP toxicity seems to particularly act on RTgill-W1 cell lysosomes. The newly developed low ionic strength medium, d-L15/ex, which can stabilize cit-AgNP and better mimic the freshwater environment, offers an excellent exposure solution to study cellular and molecular effects of NP to gill cells.

Transient overexpression of adh8a increases allyl alcohol toxicity in zebrafish embryos

Fish embryos are widely used as an alternative model to study toxicity in vertebrates. Due to their complexity, embryos are believed to more resemble an adult organism than in vitro cellular models. However, concerns have been raised with respect to the embryo's metabolic capacity. We recently identified allyl alcohol, an industrial chemical, to be several orders of magnitude less toxic to zebrafish embryo than to adult zebrafish (embryo LC50 = 478 mg/L vs. fish LC50 = 0.28 mg/L). Reports on mammals have indicated that allyl alcohol requires activation by alcohol dehydrogenases (Adh) to form the highly reactive and toxic metabolite acrolein, which shows similar toxicity in zebrafish embryos and adults. To identify if a limited metabolic capacity of embryos indeed can explain the low allyl alcohol sensitivity of zebrafish embryos, we compared the mRNA expression levels of Adh isoenzymes (adh5, adh8a, adh8b and adhfe1) during embryo development to that in adult fish. The greatest difference between embryo and adult fish was found for adh8a and adh8b expression. Therefore, we hypothesized that these genes might be required for allyl alcohol activation. Microinjection of adh8a, but not adh8b mRNA led to a significant increase of allyl alcohol toxicity in embryos similar to levels reported for adults (LC50 = 0.42 mg/L in adh8a mRNA-injected embryos). Furthermore, GC/MS analysis of adh8a-injected embryos indicated a significant decline of internal allyl alcohol concentrations from 0.23-58 ng/embryo to levels below the limit of detection (< 4.6 µg/L). Injection of neither adh8b nor gfp mRNA had an impact on internal allyl alcohol levels supporting that the increased allyl alcohol toxicity was mediated by an increase in its metabolization. These results underline the necessity to critically consider metabolic activation in the zebrafish embryo. As demonstrated here, mRNA injection is one useful approach to study the role of candidate enzymes involved in metabolization.

Acute toxicity of the cationic surfactant C12-benzalkonium in different bioassays: how test design affects bioavailability and effect concentrations

Using an ion-exchange-based solid-phase microextraction (SPME) method, the freely dissolved concentrations of C12-benzalkonium were measured in different toxicity assays, including 1) immobilization of Daphnia magna in the presence or absence of dissolved humic acid; 2) mortality of Lumbriculus variegatus in the presence or absence of a suspension of Organisation for Economic Co-Operation and Development (OECD) sediment; 3) photosystem II inhibition of green algae Chlorella vulgaris; and 4) viability of in vitro rainbow trout gill cell line (RTgill-W1) in the presence or absence of serum proteins. Furthermore, the loss from chemical adsorption to the different test vessels used in these tests was also determined. The C12-benzalkonium sorption isotherms to the different sorbent phases were established as well. Our results show that the freely dissolved concentration is a better indicator of the actual exposure concentration than the nominal or total concentration in most test assays. Daphnia was the most sensitive species to C12-benzalkonium. The acute Daphnia and Lumbriculus tests both showed no enhanced toxicity from possible ingestion of sorbed C12-benzalkonium in comparison with water-only exposure, which is in accordance with the equilibrium partitioning theory. Moreover, the present study demonstrates that commonly used sorbent phases can strongly affect bioavailability and observed effect concentrations for C12-benzalkonium. Even stronger effects of decreased actual exposure concentrations resulting from sorption to test vessels, cells, and sorbent phases can be expected for more hydrophobic cationic surfactants.

Gill cell culture systems as models for aquatic environmental monitoring

A vast number of chemicals require environmental safety assessments for market authorisation. To ensure acceptable water quality, effluents and natural waters are monitored for their potential harmful effects. Tests for market authorisation and environmental monitoring usually involve the use of large numbers of organisms and, for ethical, cost and logistic reasons, there is a drive to develop alternative methods that can predict toxicity to fish without the need to expose any animals. There is therefore a great interest in the potential to use cultured fish cells in chemical toxicity testing. This review summarises the advances made in the area and focuses in particular on a system of cultured fish gill cells grown into an epithelium that permits direct treatment with water samples.

The use of fish-derived cell lines for investigation of environmental contaminants: an update following OECD's fish toxicity testing framework No. 171

Protocols for evaluating chemical toxicity at the cellular level using fish cell lines are described in this unit. Routine methodologies for growing salmonid cell lines, and using them in aquatic toxicology studies that support the mandate of the Organization for Economic Co-operation and Development (OECD) to reduce the use of whole animals in toxicity testing, are presented. Rapid, simple, cost-effective tests evaluating viability of cells with three indicator dyes per sample provides a broad overview of the sensitivity of cells to chemical contaminants. This fluorometric assay involves: (1) alamar blue for metabolic activity, (2) CFDA-AM for membrane integrity, and (3) neutral red for lysosomal function. These protocols are conveniently performed in semi-unison within the same multiwell plates and read at three different wavelengths. Detailed step-by-step descriptions of the assays, parameters to consider, troubleshooting, and guidelines for data interpretation are provided as essential tools for investigating environmental aquatic contaminants at the cellular level.

Headspace-free setup of in vitro bioassays for the evaluation of volatile disinfection by-products

The conventional setup of in vitro bioassays in microplates does not prevent the loss of volatile compounds, which hampers the toxicological characterization of waterborne volatile disinfection by-products (DBPs). To minimize the loss of volatile test chemicals, we adapted four in vitro bioassays to a headspace-free setup using eight volatile organic compounds (four trihalomethanes, 1,1-dichloroethene, bromoethane, and two haloacetonitriles) that cover a wide range of air-water partition coefficients. The nominal effect concentrations of the test chemicals decreased by up to three orders of magnitude when the conventional setup was changed to a headspace-free setup for the bacterial cytotoxicity assay using bioluminescence inhibition of Vibrio fischeri. The increase of apparent sensitivity correlated significantly with the air-water partition coefficient. Purge and trap GC/MS analysis revealed a reduced loss of dosed volatile compounds in the headspace free setup (78-130% of nominal concentration) compared to a substantial loss in the conventional set up (2-13% of the nominal concentration). The experimental effect concentrations converged with the headspace-free setup to the effect concentrations predicted by a QSAR model, confirming the suitability of the headspace-free approach to minimize the loss of volatile test chemicals. The analogue headspace-free design of the bacterial bioassays for genotoxicity (umuC assay) and mutagenicity (Ames fluctuation assay) increased the number of compounds detected as genotoxic or mutagenic from one to four and zero to two, respectively. In a bioassay with a mammalian cell line applied for detecting the induction of the Nrf-2-mediated oxidative stress response (AREc32 assay), the headspace-free setup improved the apparent sensitivity by less than one order of magnitude, presumably due to the retaining effect of the serum components in the medium, which is also reflected in the reduced aqueous concentrations of compounds. This study highlights the importance of adapting bioanalytical test setups when volatile/semivolatile compounds are present in the sample to avoid the loss of chemicals and thus to avoid underestimating the toxicity of mixtures and complex environmental samples.

Identification of metabolic pathways in Daphnia magna explaining hormetic effects of selective serotonin reuptake inhibitors and 4-nonylphenol using transcriptomic and phenotypic responses

The molecular mechanisms explaining hormetic effects of selective serotonin reuptake inhibitors (SSRIs) and 4-nonylphenol in Daphnia magna reproduction were studied in juveniles and adults. Transcriptome analyses showed changes in mRNA levels for 1796 genes in juveniles and 1214 genes in adults (out of 15000 total probes) exposed to two SSRIs (fluoxetine and fluvoxamine) or to 4-nonylphenol. Functional annotation of affected genes was improved by assuming the annotations of putatively homologous Drosophila genes. Self-organizing map analysis and partial least-square regression coupled with selectivity ratio procedures analyses allowed to define groups of genes with specific responses to the different treatments. Differentially expressed genes were analyzed for functional enrichment using Gene Ontology and Kyoto Encyclopaedia of Genes and Genomes databases. Serotonin metabolism, neuronal developmental processes, and carbohydrates and lipid metabolism functional categories appeared as selectively affected by SSRI treatment, whereas 4-nonylphenol deregulated genes from the carbohydrate metabolism and the ecdysone regulatory pathway. These changes in functional and metabolic pathways are consistent with previously reported SSRIs and 4-nonylphenol hormetic effects in D. magna, including a decrease in reserve carbohydrates and an increase in respiratory metabolism.

Challenges for exposure prediction in ecological risk assessment

Evaluating organism exposure in the ecosystems is a difficult task and can be carried out measuring or predicting concentrations in the environment. Although current regulatory approaches favor a modeling approach, they either use a static representation of the environment and of the chemical discharge or a simplified dynamic approach (e.g., dealing with pesticides). Improving the ecological realism of exposure prediction offers a number of challenges. Some are related to the understanding of basic mechanisms such as bioavailability and the determination of internal exposure or the need to develop new paradigms for polar and ionized chemicals. Other issues are the need to provide monitoring data to understand the environmental fate of chemical mixtures, polar and ionized chemicals and metabolites, to understand the complexity of exposure in spatially and temporally variable environments. Exposure models require the development of suitable approaches to simulate the complexity of exposure in the ecosystems including the development of a variety of temporal and spatial scenarios and the integration of submodels (such as aquatic and terrestrial food webs). Finally, the integration of dynamic exposure and effect models is envisaged to fully carry out a more realistic ecological risk assessment.