Fish models for ecotoxicology

Development of 3D cultures of zebrafish liver and embryo cell lines: a comparison of different spheroid formation methods

Fish cell spheroids are promising 3D culture models for vertebrate replacement in ecotoxicology. However, new alternative ecotoxicological methods must be adapted for applications in industry and for regulatory purposes; such methods must be cost-effective, simple to manipulate and provide rapid results. Therefore, we compared the effectiveness of the traditional hanging drop (HD), orbital shaking (OS), and HD combined with OS (HD+OS) methods on the formation of zebrafish cell line spheroids (ZFL and ZEM2S). Time in HD (3-5 days) and different 96-well plates [flat-bottom or ultra-low attachment of round-bottom (ULA-plates)] in OS were evaluated. Easy handling, rapid spheroid formation, uniform-sized spheroids, and circularity were assessed to identify the best spheroid protocol. Traditional HD alone did not result in ZFL spheroid formation, whereas HD (5 days)+OS did. When using the OS, spheroids only formed on the ULA-plate. Both HD+OS and OS were reproducible in size (177.50 ± 2.81 µm and 225.62 ± 19.20 µm, respectively) and circularity (0.83 ± 0.02 and 0.80 ± 0.01, respectively) of ZFL spheroids. Nevertheless, HD+OS required a considerable time to completely form spheroids (10 days) and intensive handling, whereas the OS was fast (5 days of incubation) and simple. OS also yielded reproducible ZEM2S spheroids in 1 day (226.23 ± 0.57 µm diameter and 0.80 ± 0.01 circularity). In conclusion, OS in ULA-plate is an effective and simple spheroid protocol for high-throughput ecotoxicity testing. This study contributes to identify a fast, reproducible, and simple protocol of single piscine spheroid formation in 96-well plates and supports the application of fish 3D model in industry and academia.

Towards improved fish tests in ecotoxicology - Efficient chronic and multi-generational testing with the killifish Nothobranchius furzeri

As many freshwaters are chemically polluted, one of the challenges for policy makers is to determine the potential impact of these pollutants on ecosystems and to define safe concentrations. Common practice is the use of ecotoxicological assays to assess the response of model organisms from different trophic levels such as algae, invertebrates and fish during exposure to dilutions of a specific compound. Ideally, ecotoxicological assessments of (pseudo-)persistent chemicals should be performed across the life-cycle or even multiple generations for an accurate risk assessment. Multigenerational tests with fish are, however, impractical and costly given the long lifespan and generation time of classic model species. Here, we suggest a framework for more relevant, time- and cost-efficient fish-based testing in ecotoxicology and align it with accredited test guidelines. Next, we introduce an upcoming fish model, the turquoise killifish Nothobranchius furzeri, and show how it facilitates such research agendas due to a short lifespan and generation time. Through a review of fish-based exposure studies with a set of reference toxicants, we position N. furzeri as a sensitive species, suitable for screening effects of different pollutant types. Ultimately, we perform a cost-benefit analysis and propose a plan of action for the introduction of N. furzeri into accredited test guidelines.

Supplementation of nano-selenium in fish diet: Impact on selenium assimilation and immune-regulated selenoproteome expression in monosex Nile tilapia (Oreochromis niloticus)

Selenium (Se), a fundamental element of nutrigenomic science in fish nutrition, was used to investigate its impact on selenoproteome expression and Se regulation in tilapia. Different concentrations (T₁ - 0, T₂ - 0.5, T₃ - 1.0 and T₄ - 2.0 mg/kg of feed) of dietary nano-Se were incorporated in the diets of monosex Nile tilapia. A total of 180 tilapia fingerlings with initial weight (15.73 ± 0.05 g) were stocked in 150 L capacity FRP tanks categorized into four diet groups with triplicate each for a feeding trial of 90 days. At the end of first, second and third months of the feeding trial, gill, liver, kidney and muscle tissues were harvested to evaluate the effect on the kinetics of Se bioaccumulation and assimilation as well as immune-regulated selenoprotein transcripts (GPx2, SelJ, SelL, SelK, SelS, SelW and Sepp1a) and their synthesis factors (SPS1 and Scly). The findings depicted that significantly (p < 0.05) higher weight gain was found in the diet supplemented with 1.0 mg/kg of nano-Se. The theory of second-order polynomial regression supported the same. The liver showed significantly (p < 0.05) higher Se accumulation and concentration factor among the harvested tissues in a different timeline. All the selected immune-regulated selenoproteins and synthesis factors in different fish tissues showed significantly (p < 0.05) up-regulation in the diet supplemented with 1.0 mg/kg of nano-Se for the second month. Therefore, the present findings suggested that the supplementation of nano-Se could be more effective for improved growth, better selenium regulation and expression of immune-regulated selenoproteins in the fish model.

Zebrafish Models in Neural and Behavioral Toxicology across the Life Stages

The industry is increasingly relying on fish for toxicity assessment. However, current guidelines for toxicity assessment focus on teratogenicity and mortality. From an ecotoxicological point of view, however, these endpoints may not reflect the “full picture” of possible deleterious effects that can nonetheless result in decreased fitness and/or inability to adapt to a changing environment, affecting whole populations. Therefore, assessing sublethal effects add relevant data covering different aspects of toxicity at different levels of analysis. The impacts of toxicants on neurobehavioral function have the potential to affect many different life-history traits, and are easier to assess in the laboratory than in the wild. We propose that carefully-controlled laboratory experiments on different behavioral domains—including anxiety, aggression, and exploration—can increase our understanding of the ecotoxicological impacts of contaminants, since these domains are related to traits such as defense, sociality, and reproduction, directly impacting life-history traits. The effects of selected contaminants on these tests are reviewed, focusing on larval and adult zebrafish, showing that these behavioral domains are highly sensitive to small concentrations of these substances. These strategies suggest a way forward on ecotoxicological research using fish.

The genome of the Java medaka (Oryzias javanicus): Potential for its use in marine molecular ecotoxicology

The Java medaka (Oryzias javanicus) is distributed in tropical brackish water and is considered as an ecotoxicological experimental organism for assessing diverse pollutions and global climate change effects in the ocean. In this study, we sequenced and assembled the genome of O. javanicus using the Oxford Nanopore technique and anchored the scaffolds to the 24 genetic linkage map of a sister species Oryzias melastigma. The assembled genome consisted of 773 scaffolds including 24 LG-based scaffolds, and the estimated genome length was 846.3 Mb (N50 = 19.3 Mb), containing 24,498 genes. As detoxification processes are crucial in aquatic organisms, antioxidant-related genes including glutathione S-transferases, superoxide dismutase, catalase, and glutathione peroxidase were identified in this study. In the genome of O. javanicus, a total of 21 GSTs, 4 SODs, 1 CAT, and 7 GPxs were identified and showed high similarities between sister species O. melastigma and Oryzias latipes. In addition, despite having 8 classes of cytosolic GSTs family, medaka showed no presence of GST pi and sigma classes, which are predominantly found in carp and salmon, but not in neoteleostei. This study adds another set to genome-library of Oryzias spp. and is a useful resource for better understanding of the molecular ecotoxicology.

Behavioral toxicity of tannery effluent in zebrafish (Danio rerio) used as model system

The ecotoxicity of untreated tannery effluent (UTE) in several animal models has been reported; however, its effects on fish behavior, and neurotoxicity, remain unknown. Thus, the hypothesis that the chronic exposure to UTE can induce behavioral changes in adult zebrafish (Danio rerio) representatives, even when it is highly diluted in water, was tested. Animals exposed to 0.1% and 0.3% UTE for 30 days showed behavioral changes in visual social preference tests through their co-specific and antipredator defensive responses, which had indicated neurotoxic actions. Zebrafish exposed to UTE appeared to have not co-specific preference when it is paired with Poecilia sphrenops. In addition, only animals in the control group showed aversive behavior in the presence of the herein used predatory stimulus (Oreochromis niloticus). However, Cr, Na and Mg bioaccumulation was higher in zebrafish exposed to 0.1% and 0.3% UTE, although anxiogenic and anxiolytic effects were not observed in the models exposed to UTE in the novel tank diving or aggressiveness-increase-in-the-mirror tests. This outcome allowed associating the exposure to the pollutant and bioaccumulation with the observed behavioral changes. The present study is pioneer in scientifically evidencing the sublethal impact caused by chronic exposure to UTE in experimental environment simulating realistic aquatic pollution conditions. Accordingly, results in the current research should motivate further investigations to broaden the knowledge about the real magnitude of UTE biological impacts on the aquatic biota.

Optimizing fluorescent protein choice for transgenic embryonic medaka models

Early-life-stage transgenic medaka are recognized as a pertinent model by the Organisation for Economic Co-operation and Development and are noncompliant with the European definition of a laboratory animal. However, autofluorescence confounds readout of fluorescent biomarkers. The authors determined the fluorescence emission spectrum of different embryonic stages of medaka submitted to a range of excitation wavelengths. This allows selection of high signal-to-noise ratio fluorescent proteins and combining multiple biomarkers within a single embryo.