Toxicokinetics of Polar Chemicals in Zebrafish Embryo (Danio rerio): Influence of Physicochemical Properties and of Biological Processes

Xenometabolomics in Ecotoxicology: Concepts and Applications

Nontargeted high-resolution mass spectrometry (HRMS) allows for the characterization of a large fraction of the exposome, i.e., the entirety of chemicals an organism is exposed to, and helps detect important exogenous chemical compounds that could be key drivers of toxicological impact. Along with these chemical compounds occur endogenous metabolites that are essential for the health of the host organism. Chemical compounds derived from the biotransformation of xenobiotics present in the exposome are referred to as the xenometabolome, while endogenous metabolites derived from the host organism are referred to as the endometabolome. Recent advancements in HRMS technology allow for the detection of chemical features of biological and ecological importance in the context of chemical safety assessments with unprecedented sensitivity and resolution. In this perspective, we highlight the application of HRMS-based metabolomics of organisms in the context of ecotoxicology, the complexity of comprehensively characterizing the endometabolome, and distinguishing chemical compounds of the xenometabolome.

Effects of cyanobacterium Phormidium nigroviride K3 on zebrafish embryos and genomic insights into its toxic potential

Cyanobacterial blooms in freshwater systems pose significant environmental and public health risks, largely due to their production of toxic secondary metabolites. This study investigated the effects of Phormidium nigroviride K3 extracts on early zebrafish development, focusing on organismal toxicity and gene expression changes. Acute toxicity was assessed by monitoring developmental deformities and evaluating relative changes in the expression of six genes associated with xenobiotic metabolism, apoptosis, endoplasmic reticulum stress, and endocrine processes. Whole-genome sequencing, in silico genome annotation and mining were conducted to identify biosynthetic gene clusters involved in toxin production. Zebrafish embryos exposed to P. nigroviride K3 extracts developed spinal deformities, pericardial edema, yolk sac edema and reduced eye size, with spinal deformities being the most prevalent malformation (EC50 of 215.6 µg d.w. mL⁻¹). Significant alterations in the expression of genes involved in xenobiotic metabolism were detected, including a dose-dependent 4.6-fold induction of cyp1a1 and a 4.47-fold increase in abcb4 expression at the highest extract concentration. Genome analysis of P. nigroviride K3 revealed 15 biosynthetic gene clusters for secondary metabolites, including a match with the Cylindrospermopsis raciborskii AWT205 cluster, responsible for the cyanotoxin cylindrospermopsin biosynthesis. The LC-MS/MS analysis confirmed the production of cylindrospermopsin in P. nigroviride K3, providing new insights into cyanotoxin biosynthesis in Phormidium species, a genus that has been underexplored in the context of toxin production. These findings expand cyanobacterial genomic databases, enhancing our understanding of cyanobacterial toxic potential. Such knowledge is crucial for predicting and mitigating the risks associated with cyanotoxins in aquatic ecosystems.

New Insights into Melatonin's Function on Thiacloprid-Induced Pyroptosis and Inflammatory Response in Head Kidney Lymphocytes of Cyprinus carpio: Implicating Mitochondrial Metabolic Imbalance and mtROS/cGAS-STING/NF-κB Axis

Thiacloprid (THI) is a synthetic insecticide, the misuse is targeted chiefly to control aphid pest species in orchards and vegetables. Melatonin (MET) is a hormone that plays crucial physiological roles in anti-inflammatory capacities of fish. We explored the function of MET (100 μM) to mitigate the toxicity induced by THI (20 μM) in lymphocytes. Our results indicate that THI led to a notable rise in lymphocyte mortality. Lymphocytes exposed to THI exhibited a heightened incidence of pyroptosis, accompanied by upregulation in expression associated with pyroptosis (NLRP3, GSDMEA, and IL-18). Meanwhile, THI exposure led to a decrease in lymphocyte mitochondrial membrane potential, an increase in mtROS levels, and a reduction in intracellular ATP, DNA, and NADPH/NADP+ levels, indicating an imbalance in the mitochondrial metabolism within the lymphocytes. Additionally, these effects were reversed by MET treatment, where MitoQ treatment showed that the suppression of mtROS reduced the lymphocyte pyroptosis caused by THI via the mtROS/cGAS-STING/NF-κB axis. Importantly, MET provided defense against the immunotoxic impacts of THI by ameliorating pyroptosis and enhancing anti-inflammatory capability via the mtROS/cGAS-STING/NF-κB axis. Our research potentiates the safeguarding of cultured fish from biological hazards caused by THI and highlights the valuable application of MET in common carp.

Dynamics in zebrafish development define transcriptomic specificity after angiogenesis inhibitor exposure

Testing for developmental toxicity is an integral part of chemical regulations. The applied tests are laborious and costly and require a large number of vertebrate test animals. To reduce animal numbers and associated costs, the zebrafish embryo was proposed as an alternative model. In this study, we investigated the potential of transcriptome analysis in the zebrafish embryo model to support the identification of potential biomarkers for key events in developmental toxicity, using the inhibition of angiogenesis as a proof of principle. Therefore, the effects on the zebrafish transcriptome after exposure to the tyrosine kinase inhibitors, sorafenib (1.3 µM and 2.4 µM) and SU4312 (1 µM, 2 µM, and 5 µM), and the putative vascular disruptor compound rotenone (25 nM and 50 nM) were analyzed. An early (2 hpf-hours post fertilization) and a late (24 hpf) exposure start with a time resolved transcriptome analysis was performed to compare the specificity and sensitivity of the responses with respect to anti-angiogenesis. We also showed that toxicodynamic responses were related to the course of the internal concentrations. To identify differentially expressed genes (DEGs) the time series data were compared by applying generalized additive models (GAMs). We observed mainly unspecific developmental toxicity in the early exposure scenario, while a specific repression of vascular related genes was only partially observed. In contrast, differential expression of vascular-related genes could be identified clearly in the late exposure scenario. Rotenone did not show angiogenesis-specific response on a transcriptomic level, indicating that the observed mild phenotype of angiogenesis inhibition may represent a secondary effect.

Rapid determination of chemical losses in a microplate bioassay using fluorescence spectroscopy

The increase in production and innovation of chemicals that humans interface with has enhanced the need for rapid toxicity testing of new and existing chemicals. This need, along with efforts to reduce animal testing, has led to the development of high-throughput bioassays typically conducted in microplates. These bioassays offer time and resource advantages over traditional animal models; however, significant chemical losses can occur in microplates. Current methods for measuring chemical losses in microplates require extensive sample preparation and highly sensitive instruments. We propose the use of fluorescence spectroscopy to measure chemical losses in high-throughput bioassays as a low resource alternative to the existing methods. A fluorescent plate reader was used to develop methods for quantifying the aqueous concentrations of two chemicals, 2-hydroxynaphthalene and acridine, in microwells of a 96-well microplate. A high-throughput, 5 day embryonic zebrafish bioassay was used as the model bioassay for method development. Chemical losses were attributed to a combination of photodegradation, sorption, and uptake by the zebrafish embryos, kinetics of which were derived from a pseudo-first order model. Chemical uptake amount was calculated to be approximately 50% and 21% of the total chemical amount added for 2-hydroxynaphthalene and acridine, respectively. Unexpected cranial deformities were observed in the embryonic zebrafish, suggesting further investigation of potential additive toxicity of the ultraviolet radiation exposure from fluorescence measurements and chemical exposure is needed. Nonetheless, this novel method provides a rapid, low resource approach to measuring chemical losses in microplates that can be extended to a variety of autofluorescent chemicals and microplate-based bioassays.

Integrating Time-Resolved nrf2 Gene-Expression Data into a Full GUTS Model as a Proxy for Toxicodynamic Damage in Zebrafish Embryo

The immense production of the chemical industry requires an improved predictive risk assessment that can handle constantly evolving challenges while reducing the dependency of risk assessment on animal testing. Integrating omics data into mechanistic models offers a promising solution by linking cellular processes triggered after chemical exposure with observed effects in the organism. With the emerging availability of time-resolved RNA data, the goal of integrating gene expression data into mechanistic models can be approached. We propose a biologically anchored TKTD model, which describes key processes that link the gene expression level of the stress regulator nrf2 to detoxification and lethality by associating toxicodynamic damage with nrf2 expression. Fitting such a model to complex data sets consisting of multiple endpoints required the combination of methods from molecular biology, mechanistic dynamic systems modeling, and Bayesian inference. In this study, we successfully integrate time-resolved gene expression data into TKTD models and thus provide a method for assessing the influence of molecular markers on survival. This novel method was used to test whether nrf2 can be applied to predict lethality in zebrafish embryos. With the presented approach, we outline a method to successfully approach the goal of a predictive risk assessment based on molecular data.

Impact of Biotransformation on Internal Concentrations and Specificity Classification of Organic Chemicals in the Zebrafish Embryo (Danio rerio)

Internal concentrations (ICs) are crucial for linking exposure to effects in the development of New Approach Methodologies. ICs of chemicals in aquatic organisms are primarily driven by hydrophobicity and modulated by biotransformation and efflux. Comparing the predicted baseline to observed toxicity enables the estimation of effect specificity, but biological processes can lead to overestimating ICs and bias the specificity assessment. To evaluate the prediction of a mass balance model (MBM) and the impact of biotransformation on ICs, experimental ICs of 63 chemicals in zebrafish embryos were compared to predictions with physicochemical properties as input parameters. Experimental ICs of 79% (50 of 63) of the chemicals deviated less than 10-fold from predictions, and the remaining 13 deviated up to a factor of 90. Using experimental ICs changed the classification for 19 chemicals, with ICs 5 to 90 times lower than predicted, showing the bias of specificity classification. Uptake kinetics of pirinixic acid, genistein, dexamethasone, ethoprophos, atorvastatin, and niflumic acid were studied over a 96 h exposure period, and transformation products (TPs) were elucidated using suspect- and nontarget screening with UPLC-HRMS. 35 TPs (5 to 8 TPs per compound) were tentatively identified and semiquantified based on peak areas, suggesting that biotransformation may partly account for the overpredictions of ICs.

TnP and AHR-CYP1A1 Signaling Crosstalk in an Injury-Induced Zebrafish Inflammation Model

Aryl Hydrocarbon Receptor (AHR) signaling is crucial for regulating the biotransformation of xenobiotics and physiological processes like inflammation and immunity. Meanwhile, Thalassophryne nattereri Peptide (TnP), a promising anti-inflammatory candidate from toadfish venom, demonstrates therapeutic effects through immunomodulation. However, its influence on AHR signaling remains unexplored. This study aimed to elucidate TnP's molecular mechanisms on the AHR-cytochrome P450, family 1 (CYP1) pathway upon injury-induced inflammation in wild-type (WT) and Ahr2-knockdown (KD) zebrafish larvae through transcriptomic analysis and Cyp1a reporters. TnP, while unable to directly activate AHR, potentiated AHR activation by the high-affinity ligand 6-Formylindolo [3,2-b]carbazole (FICZ), implying a role as a CYP1A inhibitor, confirmed by in vitro studies. This interplay suggests TnP's ability to modulate the AHR-CYP1 complex, prompting investigations into its influence on biotransformation pathways and injury-induced inflammation. Here, the inflammation model alone resulted in a significant response on the transcriptome, with most differentially expressed genes (DEGs) being upregulated across the groups. Ahr2-KD resulted in an overall greater number of DEGs, as did treatment with the higher dose of TnP in both WT and KD embryos. Genes related to oxidative stress and inflammatory response were the most apparent under inflamed conditions for both WT and KD groups, e.g., Tnfrsf1a, Irf1b, and Mmp9. TnP, specifically, induces the expression of Hspa5, Hsp90aa1.2, Cxcr3.3, and Mpeg1.2. Overall, this study suggests an interplay between TnP and the AHR-CYP1 pathway, stressing the inflammatory modulation through AHR-dependent mechanisms. Altogether, these results may offer new avenues in novel therapeutic strategies, such as based on natural bioactive molecules, harnessing AHR modulation for targeted and sustained drug effects in inflammatory conditions.

Diving into drug-screening: zebrafish embryos as an in vivo platform for antimicrobial drug discovery and assessment

The rise of multidrug-resistant bacteria underlines the need for innovative treatments, yet the introduction of new drugs has stagnated despite numerous antimicrobial discoveries. A major hurdle is a poor correlation between promising in vitro data and in vivo efficacy in animal models, which is essential for clinical development. Early in vivo testing is hindered by the expense and complexity of existing animal models. Therefore, there is a pressing need for cost-effective, rapid preclinical models with high translational value. To overcome these challenges, zebrafish embryos have emerged as an attractive model for infectious disease studies, offering advantages such as ethical alignment, rapid development, ease of maintenance, and genetic manipulability. The zebrafish embryo infection model, involving microinjection or immersion of pathogens and potential antibiotic hit compounds, provides a promising solution for early-stage drug screening. It offers a cost-effective and rapid means of assessing the efficacy, toxicity and mechanism of action of compounds in a whole-organism context. This review discusses the experimental design of this model, but also its benefits and challenges. Additionally, it highlights recently identified compounds in the zebrafish embryo infection model and discusses the relevance of the model in predicting the compound's clinical potential.

Zebrafish and nematodes as whole organism models to measure developmental neurotoxicity

Despite the growing epidemiological evidence of an association between toxin exposure and developmental neurotoxicity (DNT), systematic testing of DNT is not mandatory in international regulations for admission of pharmaceuticals or industrial chemicals. However, to date around 200 compounds, ranging from pesticides, pharmaceuticals and industrial chemicals, have been tested for DNT in the current OECD test guidelines (TG-443 or TG-426). There are calls for the development of new approach methodologies (NAMs) for DNT, which has resulted in a DNT testing battery using in vitro human cell-based assays. These assays provide a means to elucidate the molecular mechanisms of toxicity in humans which is lacking in animal-based toxicity tests. However, cell-based assays do not represent all steps of the complex process leading to DNT. Validated models with a multi-organ network of pathways that interact at the molecular, cellular and tissue level at very specific timepoints in a life cycle are currently missing. Consequently, whole model organisms are being developed to screen for, and causally link, new molecular targets of DNT compounds and how they affect whole brain development and neurobehavioral endpoints. Given the practical and ethical restraints associated with vertebrate testing, lower animal models that qualify as 3 R (reduce, refine and replace) models, including the nematode (Caenorhabditis elegans) and the zebrafish (Danio rerio) will prove particularly valuable for unravelling toxicity pathways leading to DNT. Although not as complex as the human brain, these 3 R-models develop a complete functioning brain with numerous neurodevelopmental processes overlapping with human brain development. Importantly, the main signalling pathways relating to (neuro)development, metabolism and growth are highly conserved in these models. We propose the use of whole model organisms specifically zebrafish and C. elegans for DNT relevant endpoints.

Physiology-informed toxicokinetic model for the zebrafish embryo test developed for bisphenols

Zebrafish embryos (ZFE) is a widely used model organism, employed in various research fields including toxicology to assess e.g., developmental toxicity and endocrine disruption. Variation in effects between chemicals are difficult to compare using nominal dose as toxicokinetic properties may vary. Toxicokinetic (TK) modeling is a means to estimate internal exposure concentration or dose at target and to enable extrapolation between experimental conditions and species, thereby improving hazard assessment of potential pollutants. In this study we advance currently existing TK models for ZFE with physiological ZFE parameters and novel experimental bisphenol data, a class of chemicals with suspected endocrine activity. We developed a five-compartment model consisting of water, plastic, chorion, yolk sack and embryo in which surface area and volume changes as well as the processes of biotransformation and blood circulation influence mass fluxes. For model training and validation, we measured internal concentrations in ZFE exposed individually to BPA, bisphenol AF (BPAF) and Z (BPZ). Bayesian inference was applied for parameter calibration based on the training data set of BPZ. The calibrated TK model predicted internal ZFE concentrations of the majority of external test data within a 5-fold error and half of the data within a 2-fold error for bisphenols A, AF, F, and tetrabromo bisphenol A (TBBPA). We used the developed model to rank the hazard of seven bisphenols based on predicted internal concentrations and measured in vitro estrogenicity. This ranking indicated a higher hazard for BPAF, BPZ, bisphenol B and C (BPB, BPC) than for BPA.

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a widely used antioxidant in tire rubber known to enter the aquatic environment via road runoff. The associated transformation product (TP) 6-PPD quinone (6-PPDQ) causes extreme acute toxicity in some fish species (e.g., coho salmon). To interpret the species-specific toxicity, information about biotransformation products of 6-PPDQ would be relevant. This study investigated toxicokinetics of 6-PPD and 6-PPDQ in the zebrafish embryo (ZFE) model. Over 96 h of exposure, 6-PPD and 6-PPDQ accumulated in the ZFE with concentration factors ranging from 140 to 2500 for 6-PPD and 70 to 220 for 6-PPDQ. A total of 22 TPs of 6-PPD and 12 TPs of 6-PPDQ were tentatively identified using liquid chromatography coupled to high-resolution mass spectrometry. After 96 h of exposure to 6-PPD, the TPs of 6-PPD comprised 47% of the total peak area (TPA), with 4-hydroxydiphenylamine being the most prominent in the ZFE. Upon 6-PPDQ exposure, >95% of 6-PPDQ taken up in the ZFE was biotransformed, with 6-PPDQ + O + glucuronide dominating (>80% of the TPA). Among other TPs of 6-PPD, a reactive N-phenyl-p-benzoquinone imine was found. The knowledge of TPs of 6-PPD and 6-PPDQ from this study may support biotransformation studies in other organisms.

Bioconcentration of carbamazepine, enalapril, and sildenafil in neotropical fish species

Sewage effluents are the main source of entry of Human Pharmaceutical Active Ingredients (HPAIs) to surface water bodies. Carbamazepine (CBZ), psychiatric drug, enalapril (ENA) antihypertensive, and sildenafil (SIL), to treat erectile dysfunction, have been frequently detected in receiving wastewater and in wild fish species from Argentina. This study aimed to assess the bioconcentration of selected HPAIs in native fish species of the Del Plata Basin. In a first trial, the bioconcentration factors of CBZ, ENA, and SIL were obtained by exposing Cnesterodon decemmaculatus, respectively, to 135, 309, and 70 μg/L during 96 h. Then the bioconcentration kinetic of SIL was comparatively assessed in C. decemmaculatus and Piaractus mesopotamicus exposed, respectively, to 44.1 and 16.2 μg/L during a one-week, followed by a four-day depuration phase. HPAIs concentrations in water and tissue were measured by HPLC-MS after 0.22 μm filtration and direct injection or solid-liquid extraction, respectively. Bioconcentration factors obtained empirically (BCFe) for C. decemmaculatus were CBZ = 1.5, SIL = 1.4, and ENA = 0.007. Parameters estimated by the SIL bioconcentration kinetic model for C. decemmaculatus were: uptake rate constant (k1) = 5.5 L/kg d, elimination rate constant during uptake phase (k2u) = 0.00175 d-1, maximum predicted tissue concentration (Ct(max)) = 138588 μg/kg, estimated bioconcentration factor (BCFm) = 3143, lag time between the exposure and the first detection in tissue (tlag) = 0 d, elimination rate constant in the depuration phase (k2d) = 0.49 d-1 and half-life in the tissue (t1/2) = 1.4 d. The model parameters for P. mesopotamicus were k1: 7.3 L/kg d, k2u: 0.0836 d-1, Ct(max): 1423 μg/kg, BCFm: 88, tlag: 3.8 d in the uptake phase and k2d: 0.31 d-1 and t1/2: 2.3 d in the depuration phase. The reached conclusions were: 1) the bioconcentration capacity of CBZ and SIL are similar but around 200 times higher than ENA, 2) the time to reach the bioconcentration equilibrium for SIL is longer than 1 week, then estimated BCFm are between 1 and 3 orders of magnitude higher than BCFe obtained after 96 h exposure, but actual values need to be verified, 3) substantial differences (≈30 fold) were observed in the estimated BCF of SIL among species, indicating the need for further studies toward understanding such diversity to improve HPAIs ecological risk assessment worldwide.

Study of IsCT analogue peptide against Candida albicans and toxicity/teratogenicity in zebrafish embryos (Danio rerio)

Aim: An IsCT analogue peptide (PepM3) was designed based on structural studies of wasp mastoparans and tested against Candida albicans. Its effects on fungal cell membranes and toxicity were evaluated. Materials & methods: Antifungal activity was analyzed using a microdilution susceptibility test. Toxicity was assessed using human skin keratinocytes (HaCaT) and zebrafish embryos. Results: PepM3 demonstrated activity against C. albicans and a synergistic effect with amphotericin B. The peptide presented fungicidal action with damage to the fungal cell membrane, low toxicity in HaCat cells and was nonteratogenic in zebrafish embryos. Conclusion: Evaluating structural modifications is essential for the development of new agents with potential activity against fungal pathogens and for the reduction of toxic and teratogenic effects.

Impact of Drug Administration Routes on the In Vivo Efficacy of the Natural Product Sorangicin a Using a Staphylococcus aureus Infection Model in Zebrafish Embryos

Staphylococcus aureus causes a wide range of infections, and it is one of the leading pathogens responsible for deaths associated with antimicrobial resistance, the rapid spread of which among S. aureus urges the discovery of new antibiotics. The evaluation of in vivo efficacy of novel drug candidates is usually performed using animal models. Recently, zebrafish (Danio rerio) embryos have become increasingly attractive in early drug discovery. Herein, we established a zebrafish embryo model of S. aureus infection for evaluation of in vivo efficacy of novel potential antimicrobials. A local infection was induced by microinjecting mCherry-expressing S. aureus Newman followed by treatment with reference antibiotics via microinjection into different injection sites as well as via waterborne exposure to study the impact of the administration route on efficacy. We successfully used the developed model to evaluate the in vivo activity of the natural product sorangicin A, for which common mouse models were not successful due to fast degradation in plasma. In conclusion, we present a novel screening platform for assessing in vivo activity at the antibiotic discovery stage. Furthermore, this work provides consideration for the choice of an appropriate administration route based on the physicochemical properties of tested drugs.

Atlantic Salmon Gill Epithelial Cell Line (ASG-10) as a Suitable Model for Xenobiotic Biotransformation

Fish are exposed to xenobiotics in the water. Uptake occurs mainly through the gills, which function as an exchange point with the environment. The gills' ability to detoxify harmful compounds by biotransformation is an essential protection mechanism. The enormous numbers of waterborne xenobiotics requiring ecotoxicological assessment makes it necessary to replace in vivo fish studies with predictive in vitro models. Here, we have characterized the metabolic capacity of the ASG-10 gill epithelial cell line from Atlantic salmon. Inducible CYP1A expression was confirmed by enzymatic assays and immunoblotting. The activities of important cytochrome P450 (CYP) and uridine 5'-diphospho-glucuronosyltransferase (UGT) enzymes were established using specific substrates and metabolite analysis by liquid chromatography (LC) triple quadrupole mass spectrometry (TQMS). Metabolism of the fish anesthetic benzocaine (BZ) in ASG-10 confirmed esterase and acetyl transferase activities through the production of N-acetylbenzocaine (AcBZ), p-aminobenzoic acid (PABA) and p-acetaminobenzoic acid (AcPABA). Moreover, we were able to determine hydroxylamine benzocaine (BZOH), benzocaine glucuronide (BZGlcA) and hydroxylamine benzocaine glucuronide (BZ(O)GlcA) by LC high-resolution tandem mass spectrometry (HRMS/MS) fragment pattern analysis for the first time. Comparison to metabolite profiles in hepatic fractions, and in plasma of BZ-euthanized salmon, confirmed the suitability of the ASG-10 cell line for investigating biotransformation in gills.

Biotransformation, metabolic response, and toxicity of UV-234 and UV-326 in larval zebrafish (Danio rerio)

Benzotriazole ultraviolet stabilizers (BUVSs) are emerging pollutants that are widely detected in aquatic ecosystems. While structure-dependent effects of BUVSs are reported, the relationship between biotransformation and toxicity outcomes remains unclear. In this study, zebrafish embryos were exposed to two common BUVSs (UV-234 and UV-326) at 1, 10, and 100 µg/L for up to 7 days. Comparison of their uptake and biotransformation revealed that the bioaccumulation capacity of UV-234 was higher than that of UV-326, while UV-326 was more extensively biotransformed with additional conjugation reactions. However, UV-326 showed low metabolism due to inhibited phase II enzymes, which may result in the comparable internal concentrations of both BUVSs in larval zebrafish. Both BUVSs induced oxidative stress while decreased MDA, suggesting the disturbance of lipid metabolism. The subsequent metabolomic profiling revealed that UV-234 and UV-326 exerted different effects on arachidonic acid, lipid, and energy metabolism. However, both BUVSs negatively impacted the cyclic guanosine monophosphate / protein kinase G pathway. This converged metabolic change resulted in comparable toxicity of UV-234 and UV-326, which was confirmed by the induction of downstream apoptosis, neuroinflammation, and abnormal locomotion behavior. These data have important implications for understanding the metabolism, disposition, and toxicology of BUVSs in aquatic organisms.

Influence of habitats and physicochemical factors on trophic transfer processes of antibiotics in a freshwater ecosystem: Application of stable isotopes and human health risks

Habitats of species and physicochemical factors are of great importance in determining the trophic transfer of contaminants in freshwater ecosystems. There is little information on how multiple physicochemical factors and habitats influence the trophic transfer of antibiotics in freshwater food webs. This study investigated the concentrations of 7 sulfonamides (SAs), 4 fluoroquinolones (FQs), 4 tetracyclines (TCs) and 2 macrolides (MLs) in the Lake Dianshan food web. Stable isotope analysis (SIA), and mathematical models were used to assign trophic levels and distinguish between the benthic food web (BFW) and pelagic food web (PFW). Values of stable nitrogen isotope (δ¹⁵N‰) and stable carbon isotope (δ¹³C‰) ranged from 10.2 ± 0.11 to 19.72 ± 0.05 and -33.67 ± 0.18 to -20.79 ± 0.50, respectively. Total concentrations of antibiotics ranged from 36.63 ± 12.73 ng/g dry weight (dw) to 105.85 ± 12.95 ng/g dw for all species. The relative abundance of antibiotics was in the following order: ∑FQs (36.49 %) > ∑SAs (26.70 %), >∑MLs (12.63 %) for all biotas. Trophic magnification factor (TMFs) values for individual antibiotics ranged from 0.10 to 1.20 and 0.31 to 1.82 for PFW and BFW, respectively. Three classes of antibiotics ∑FQs (p < 0.05), ∑TCs (p < 0.05), and ∑MLs (p < 0.05) showed significant trophic dilution in PFW, opposite to non-significant trophic dilution in BFW. The influence of various physicochemical factors was not strong over trophic transfer (e.g., octanol-water partition coefficient-LogK_ow (r = -0.05 in PFW, r = -0.14 in BFW) and distribution coefficient-LogD (r = 0.06 in PWF, r = -0.28 in BFW)) except for aqueous solubility (LogS). Results indicated a significantly higher trophic dilution of antibiotics in the PFW than in the BFW. Among the studied six physicochemical factors, only LogS significantly influences (p < 0.05) the trophic transfer of antibiotics in the freshwater food web. Health risk assessments indicated that currently, there were no serious risks present for urban and rural populations.

A PBPK model to evaluate zebrafish eleutheroembryos' actual exposure: bisphenol A and analogs' (AF, F, and S) case studies

The zebrafish eleutheroembryo model is increasingly used to assess the toxicity and developmental adverse effects of xenobiotics. However, the actual exposure is seldom measured (poorly accessible), while a predictive model could estimate these concentrations. The predictions with a new eleutheroembryo physiologically based pharmacokinetic (PBPK) model have been evaluated using datasets obtained from literature data for several bisphenols. The model simulated the toxicokinetics of bisphenols A (BPA), AF, F, and S through the eleutheroembryo tissues while considering the body and organ growth. We further improved the predictions by adding dynamic flows through the embryo and/or its chorion, impact of experimental temperature, metabolic clearance, and saturation of the absorption by Bayesian calibration. The model structure was determined using the BPA dataset and generalized to the other bisphenols. This model revealed the central role of the chorion in the compound uptake in the first 48 h post-fertilization. The predictions for the BPA substitutes estimated by our PBPK model were compared to available toxicokinetics data for zebrafish embryos, and 63% and 88% of them were within a twofold and fivefold error intervals of the corresponding experimental values, respectively. This model provides a tool to design new eleutheroembryo assays and evaluate the actual exposure.

Kinetics and toxicity of an environmentally relevant mixture of halogenated organic compounds in zebrafish embryo

Persistent and semi-persistent halogenated compounds cause health problems for the animals occupying the upper level of the food web in the Baltic Sea. Atlantic salmon (Salmo salar), being a top piscivore in the Baltic Sea, has been observed to carry a large body burden of halogenated toxins. Here, a mixture of nine halogenated compounds belonging to different groups was created, based on the observed composition of halogenated toxins in salmon serum. The toxicokinetic properties of the compounds were studied in zebrafish (Danio rerio) embryos to achieve the same proportions between the internal doses of the compounds in the zebrafish as in the salmon. Toxicity was evaluated for the compounds dosed individually as well as in a mixture. Perfluorooctanesulfonic acid (PFOS) was the dominant compound in the salmon and was observed to be the driving force for effects on swimbladder inflation caused by the mixture with a 50% effect concentration of 4.8 µM nominal dose, or 1300 µMD based on the area under the internal concentration-time curve (AUC). The driving compound for other severe effects caused by the mixture, including lethality, spinal deformity, and edemas, was the hydroxylated polybrominated diphenyl ether 6-OH-BDE47, which was observed to have a 50% lethality concentration of 93 nM, corresponding to 94 µMD based on internal dose (AUC). The individual compounds were observed to act additively on most of the documented outcomes when dosed as a mixture.

Toxicokinetic Modeling of Per- and Polyfluoroalkyl Substance Concentrations within Developing Zebrafish (Danio rerio) Populations

Per- and polyfluoroalkyl substances (PFAS) are pervasive environmental contaminants, and their relative stability and high bioaccumulation potential create a challenging risk assessment problem. Zebrafish (Danio rerio) data, in principle, can be synthesized within a quantitative adverse outcome pathway (qAOP) framework to link molecular activity with individual or population level hazards. However, even as qAOP models are still in their infancy, there is a need to link internal dose and toxicity endpoints in a more rigorous way to further not only qAOP models but adverse outcome pathway frameworks in general. We address this problem by suggesting refinements to the current state of toxicokinetic modeling for the early development zebrafish exposed to PFAS up to 120 h post-fertilization. Our approach describes two key physiological transformation phenomena of the developing zebrafish: dynamic volume of an individual and dynamic hatching of a population. We then explore two different modeling strategies to describe the mass transfer, with one strategy relying on classical kinetic rates and the other incorporating mechanisms of membrane transport and adsorption/binding potential. Moving forward, we discuss the challenges of extending this model in both timeframe and chemical class, in conjunction with providing a conceptual framework for its integration with ongoing qAOP modeling efforts.

Human Apparent Volume of Distribution Predicts Bioaccumulation of Ionizable Organic Chemicals in Zebrafish Embryos

Chemicals with elevated bioaccumulation profiles present potential hazards to public health and the environment. Ionizable organic compounds (IOCs) increasingly represent a large proportion of commercial chemicals; however, historical approaches for bioaccumulation determinations are mainly developed for neutral chemicals, which were not appropriate for IOCs. Herein, we employed the zebrafish embryo, a common vertebrate model in environmental and biomedical studies, to elucidate toxicokinetics and bioconcentration of eight IOCs with diverse physicochemical properties and pharmacokinetic parameters. At an environmentally relevant pH (7.5), most IOCs exhibited rapid uptake and depuration in zebrafish, suggesting the ionized forms of IOCs are readily bioavailable. Bioconcentration factors (BCF) of these IOCs ranged from 0.0530 to 250 L·kg^-1 wet weight. The human pharmacokinetic proportionality factor, apparent volume of distribution (V_D), better predicted the BCF of selected IOCs than more commonly used hydrophobicity-based parameters (e.g., pH-dependent octanol-water distribution ratio, D_ow). Predictive bioaccumulation models for IOCs were constructed and validated using V_D alone or with D_ow. Significant relationships between fish BCF and human V_D, which is readily available for pharmaceuticals, highlighted the utility of biologically based "read-across" approaches for predicting bioaccumulative potential of IOCs. Our novel findings thus provided an understanding of the partitioning behavior and improved predictive bioconcentration modeling for IOCs.

A critical review of effect modeling for ecological risk assessment of plant protection products

A wide diversity of plant protection products (PPP) is used for crop protection leading to the contamination of soil, water, and air, which can have ecotoxicological impacts on living organisms. It is inconceivable to study the effects of each compound on each species from each compartment, experimental studies being time consuming and cost prohibitive, and animal testing having to be avoided. Therefore, numerous models are developed to assess PPP ecotoxicological effects. Our objective was to provide an overview of the modeling approaches enabling the assessment of PPP effects (including biopesticides) on the biota. Six categories of models were inventoried: (Q)SAR, DR and TKTD, population, multi-species, landscape, and mixture models. They were developed for various species (terrestrial and aquatic vertebrates and invertebrates, primary producers, micro-organisms) belonging to diverse environmental compartments, to address different goals (e.g., species sensitivity or PPP bioaccumulation assessment, ecosystem services protection). Among them, mechanistic models are increasingly recognized by EFSA for PPP regulatory risk assessment but, to date, remain not considered in notified guidance documents. The strengths and limits of the reviewed models are discussed together with improvement avenues (multigenerational effects, multiple biotic and abiotic stressors). This review also underlines a lack of model testing by means of field data and of sensitivity and uncertainty analyses. Accurate and robust modeling of PPP effects and other stressors on living organisms, from their application in the field to their functional consequences on the ecosystems at different scales of time and space, would help going toward a more sustainable management of the environment. Graphical Abstract Combination of the keyword lists composing the first bibliographic query. Columns were joined together with the logical operator AND. All keyword lists are available in Supplementary Information at https://doi.org/10.5281/zenodo.5775038 (Larras et al. 2021).

Grouping of chemicals into mode of action classes by automated effect pattern analysis using the zebrafish embryo toxicity test

A central element of high throughput screens for chemical effect assessment using zebrafish is the assessment and quantification of phenotypic changes. By application of an automated and more unbiased analysis of these changes using image analysis, patterns of phenotypes may be associated with the mode of action (MoA) of the exposure chemical. The aim of our study was to explore to what extent compounds can be grouped according to their anticipated toxicological or pharmacological mode of action using an automated quantitative multi-endpoint zebrafish test. Chemical-response signatures for 30 endpoints, covering phenotypic and functional features, were generated for 25 chemicals assigned to 8 broad MoA classes. Unsupervised clustering of the profiling data demonstrated that chemicals were partially grouped by their main MoA. Analysis with a supervised clustering technique such as a partial least squares discriminant analysis (PLS-DA) allowed to identify markers with a strong potential to discriminate between MoAs such as mandibular arch malformation observed for compounds interfering with retinoic acid signaling. The capacity for discriminating MoAs was also benchmarked to an available battery of in vitro toxicity data obtained from ToxCast library indicating a partially similar performance. Further, we discussed to which extent the collected dataset indicated indeed differences for compounds with presumably similar MoA or whether other factors such as toxicokinetic differences could have an important impact on the determined response patterns.

Specificity of time- and dose-dependent morphological endpoints in the fish embryo acute toxicity (FET) test for substances with diverse modes of action: the search for a "fingerprint"

The fish embryo acute toxicity (FET) test with the zebrafish (Danio rerio) embryo according to OECD TG 236 was originally developed as an alternative test method for acute fish toxicity testing according to, e.g., OECD TG 203. Given the versatility of the protocol, however, the FET test has found application beyond acute toxicity testing as a common tool in environmental hazard and risk assessment. Whereas the standard OECD guideline is restricted to four core endpoints (coagulation as well as lack of somite formation, heartbeat, and tail detachment) for simple, rapid assessment of acute toxicity, further endpoints can easily be integrated into the FET test protocol. This has led to the hypothesis that an extended FET test might allow for the identification of different classes of toxicants via a "fingerprint" of morphological observations. To test this hypothesis, the present study investigated a set of 18 compounds with highly diverse modes of action with respect to acute and sublethal endpoints. Especially at higher concentrations, most observations proved toxicant-unspecific. With decreasing concentrations, however, observations declined in number, but gained in specificity. Specific observations may at best be made at test concentrations ≤ EC10. The existence of a "fingerprint" based on morphological observations in the FET is, therefore, highly unlikely in the range of acute toxicity, but cannot be excluded for experiments at sublethal concentrations.

Pollution characteristics, exposure assessment and potential cardiotoxicities of PM2.5-bound benzotriazole and its derivatives in typical Chinese cities

Benzotriazole and its derivatives (BTRs), classified as high-volume production chemicals, have been widely detected in various environmental media, including the atmosphere, water, soil and dust, as well as organisms. However, studies on the pollution characteristics and health impact of PM_2.5 related BTRs are so far limited. This study is the first to demonstrate the regional scale distribution of PM_2.5-bound BTRs and their potential cardiotoxicities. Optimized methods of extraction, purification and GC-EI-MS/MS were applied to characterize and analyze PM_2.5-bound BTRs from three cities in China during the winter of 2018. The concentration of ∑BTRs in Taiyuan (6.28 ng·m^-3) was more than three times that in Shanghai (1.53 ng·m^-3) and Guangzhou (1.99 ng·m^-3). Benzotriazole (BTR) and 5-methyl-1H-benzotriazole (5TTR) contributed more than 80% of ∑BTRs concentration as the major pollutants among three cities. The correlation analysis indicated that there was a positive correlation between temperature and concentration of BTR and a negative correlation between temperature and concentration of 5TTR. In addition, the risk of BTRs exposure to toddlers should be paid more attention in Taiyuan by the human exposure assessment. Furthermore, toxicity screening by experimental methods indicated that 4-methyl-1H-benzotriazole (4TTR) was the most harmful to cardiomyocytes. The western blot assay showed a ROS-mediated mitochondrial apoptosis signaling pathway was activated after exposure to 4TTR in neonatal rat cardiomyocytes (NRCMs). On the other hand, metabolomics revealed that exposure of 4TTR to NRCMs disturbed mitochondrial energy metabolism by disturbing pantothenate and coenzyme A synthesis pathway. Our study not only clarifies the contamination profiles of PM_2.5-bound BTRs in typical Chinese cities but also reveals their cardiotoxicities associated with mitochondrial dysfunction.

Zebrafish Oatp1d1 Acts as a Cellular Efflux Transporter of the Anionic Herbicide Bromoxynil

Toxicokinetics (TK) of ionic compounds in the toxico-/pharmacological model zebrafish embryo (Danio rerio) depend on absorption, distribution, metabolism, and elimination (ADME) processes. Previous research indicated involvement of transport proteins in the TK of the anionic pesticide bromoxynil in zebrafish embryos. We here explored the interaction of bromoxynil with the organic anion-transporting polypeptide zebrafish Oatp1d1. Mass spectrometry imaging revealed accumulation of bromoxynil in the gastrointestinal tract of zebrafish embryos, a tissue known to express Oatp1d1. In contrast to the Oatp1d1 reference substrate bromosulfophthalein (BSP), which is actively taken up by transfected HEK293 cells overexpressing zebrafish Oatp1d1, those cells accumulated less bromoxynil than empty vector-transfected control cells. This indicates cellular efflux of bromoxynil by Oatp1d1. This was also seen for diclofenac but not for carbamazepine, examined for comparison. Correspondingly, internal concentrations of bromoxynil and diclofenac in the zebrafish embryo were increased when coexposed with BSP, inhibiting the activities of various transporter proteins, including Oatp1d1. The effect of BSP on accumulation of bromoxynil and diclofenac was enhanced in further advanced embryo stages, indicating increased efflux activity in those stages. An action of Oatp1d1 as an efflux transporter of ionic environmental compounds in zebrafish embryos should be considered in future TK assessments.

Exploring Neurobehaviour in Zebrafish Embryos as a Screening Model for Addictiveness of Substances

Tobacco use is the leading cause of preventable death worldwide and is highly addictive. Nicotine is the main addictive compound in tobacco, but less is known about other components and additives that may contribute to tobacco addiction. The zebrafish embryo (ZFE) has been shown to be a good model to study the toxic effects of chemicals on the neurological system and thus may be a promising model to study behavioral markers of nicotine effects, which may be predictive for addictiveness. We aimed to develop a testing protocol to study nicotine tolerance in ZFE using a locomotion test with light-dark transitions as behavioral trigger. Behavioral experiments were conducted using three exposure paradigms: (1) Acute exposure to determine nicotine’s effect and potency. (2) Pre-treatment with nicotine dose range followed by a single dose of nicotine, to determine which pre-treatment dose is sufficient to affect the potency of acute nicotine. (3) Pre-treatment with a single dose combined with acute exposure to a dose range to confirm the hypothesized decreased potency of the acute nicotine exposure. These exposure paradigms showed that (1) acute nicotine exposure decreased ZFE activity in response to dark conditions in a dose-dependent fashion; (2) pre-treatment with increasing concentrations dose-dependently reversed the effect of acute nicotine exposure; and (3) a fixed pre-treatment dose of nicotine induced a decreased potency of the acute nicotine exposure. This effect supported the induction of tolerance to nicotine by the pre-treatment, likely through neuroadaptation. The interpretation of these effects, particularly in view of prediction of dependence and addictiveness, and suitability of the ZFE model to test for such effects of other compounds than nicotine, are discussed.

Cytochrome P450-dependent biotransformation capacities in embryonic, juvenile and adult stages of zebrafish (Danio rerio)-a state-of-the-art review

Given the strong trend to implement zebrafish (Danio rerio) embryos as translational model not only in ecotoxicological, but also toxicological testing strategies, there is an increasing need for a better understanding of their capacity for xenobiotic biotransformation. With respect to the extrapolation of toxicological data from zebrafish embryos to other life stages or even other organisms, qualitative and quantitative differences in biotransformation pathways, above all in cytochrome P450-dependent (CYP) phase I biotransformation, may lead to over- or underestimation of the hazard and risk certain xenobiotic compounds may pose to later developmental stages or other species. This review provides a comprehensive state-of-the-art overview of the scientific knowledge on the development of the CYP1-4 families and corresponding phase I biotransformation and bioactivation capacities in zebrafish. A total of 68 publications dealing with spatiotemporal CYP mRNA expression patterns, activities towards mammalian CYP-probe substrates, bioactivation and detoxification activities, as well as metabolite profiling were analyzed and included in this review. The main results allow for the following conclusions: (1) Extensive work has been done to document mRNA expression of CYP isoforms from earliest embryonic stages of zebrafish, but juvenile and adult zebrafish have been largely neglected so far. (2) There is insufficient understanding of how sex- and developmental stage-related differences in expression levels of certain CYP isoforms may impact biotransformation and bioactivation capacities in the respective sexes and in different developmental stages of zebrafish. (3) Albeit qualitatively often identical, many studies revealed quantitative differences in metabolic activities of zebrafish embryos and later developmental stages. However, the actual relevance of age-related differences on the outcome of toxicological studies still needs to be clarified. (4) With respect to current remaining gaps, there is still an urgent need for further studies systematically assessing metabolic profiles and capacities of CYP isoforms in zebrafish. Given the increasing importance of Adverse Outcome Pathway (AOP) concepts, an improved understanding of CYP capacities appears essential for the interpretation and outcome of (eco)toxicological studies.

Microtubules and motor proteins support zebrafish neuronal migration by directing cargo

Neuronal migration during development is necessary to form an ordered and functional brain. Postmitotic neurons require microtubules and dynein to move, but the mechanisms by which they contribute to migration are not fully characterized. Using tegmental hindbrain nuclei neurons in zebrafish embryos together with subcellular imaging, optogenetics, and photopharmacology, we show that, in vivo, the centrosome's position relative to the nucleus is not linked to greatest motility in this cell type. Nevertheless, microtubules, dynein, and kinesin-1 are essential for migration, and we find that interference with endosome formation or the Golgi apparatus impairs migration to a similar extent as disrupting microtubules. In addition, an imbalance in the traffic of the model cargo Cadherin-2 also reduces neuronal migration. These results lead us to propose that microtubules act as cargo carriers to control spatiotemporal protein distribution, which in turn controls motility. This adds crucial insights into the variety of ways that microtubules can support successful neuronal migration in vivo.

Zebrafish early life stages as alternative model to study 'designer drugs': Concordance with mammals in response to opioids

The number of new psychoactive substances (NPS) on the illicit drug market increases fast, posing a need to urgently understand their toxicity and behavioural effects. However, with currently available rodent models, NPS assessment is limited to a few substances per year. Therefore, zebrafish (Danio rerio) embryos and larvae have been suggested as an alternative model that would require less time and resources to perform an initial assessment and could help to prioritize substances for subsequent evaluation in rodents. To validate this model, more information on the concordance of zebrafish larvae and mammalian responses to specific classes of NPS is needed. Here, we studied toxicity and behavioural effects of opioids in zebrafish early life stages. Synthetic opioids are a class of NPS that are often used in pain medication but also frequently abused, having caused multiple intoxications and fatalities recently. Our data shows that fentanyl derivatives were the most toxic among the tested opioids, with toxicity in the zebrafish embryo toxicity test decreasing in the following order: butyrfentanyl>3-methylfentanyl>fentanyl>tramadol> O-desmethyltramadol>morphine. Similar to rodents, tramadol as well as fentanyl and its derivatives led to hypoactive behaviour in zebrafish larvae, with 3-methylfentanyl being the most potent. Physico-chemical properties-based predictions of chemicals' uptake into zebrafish embryos and larvae correlated well with the effects observed. Further, the biotransformation pattern of butyrfentanyl in zebrafish larvae was reminiscent of that in humans. Comparison of toxicity and behavioural responses to opioids in zebrafish and rodents supports zebrafish as a suitable alternative model for rapidly testing synthetic opioids.

Rapid in-plate screening of biotransformation products in single zebrafish embryos

A procedure was developed for rapid screening of xenobiotic biotransformation products (bioTPs) in single zebrafish (ZF; Danio rerio) embryos. Exposure was carried out from 0–120 hours post fertilization (hpf) to 6 different concentrations of the model compound propranolol (PPL). Following in-plate extraction and non-target instrumental analysis by high resolution mass spectrometry, suspected bioTPs were identified using custom data filtration scripts and matching to in silico structural predictions. A total of eight PPL bioTPs were identified (five at a level 1 confidence and one at a level 2–3 confidence). These findings supplement previously generated toxicometabolomic models derived from the same dataset, and were obtained without conducting additional exposure experiments. In addition to facilitating assessments of inter-individual variability in bioTP production in ZF embryos, we demonstrate that bioTPs can be elucidated using extremely small quantities of biomass (i.e. ∼200 μg). To the best of our knowledge, this is the first time bioTP elucidation has been carried out in single ZF embryos.

A procedure was developed for rapid screening of xenobiotic biotransformation products (bioTPs) in single zebrafish (ZF; Danio rerio) embryos.

Do environmentally relevant concentrations of fluoxetine and citalopram impair stress-related behavior in zebrafish (Danio rerio) embryos?

Selective serotonin reuptake inhibitors (SSRIs) have been shown to interfere with various physiological functions of aquatic organisms, yet the neuroactive potential of low concentrations of SSRIs in the aquatic environment is unclear. The current study investigated the effects of fluoxetine and citalopram on the visual motor response (VMR) of 107 h old zebrafish (Danio rerio) embryos. Results document a reduction in stress-related swimming activity of zebrafish embryos at environmentally relevant concentration levels, with fluoxetine being more effective than citalopram. Further experiments were designed to elucidate (1) if the lower neuroactive potential of citalopram is due to differences in uptake kinetics, (2) if the metabolite of fluoxetine, norfluoxetine, contributes to the neuroactive potential of fluoxetine, (3) and how SSRIs and their metabolites interact in equimolar mixtures. At the stage of 120 h, zebrafish embryos accumulate citalopram at significantly lower rates (up to 127 times) than fluoxetine. Moreover, it was demonstrated that norfluoxetine reduces the embryonic VMR similarly to fluoxetine resulting in additive effects of these substances on stress-related behavior in zebrafish embryos. In contrast, the interaction of fluoxetine, norfluoxetine and citalopram varied with test concentrations of the equimolar mixtures. Findings provide evidence that environmentally relevant concentrations of fluoxetine reduce stress-related behavior of zebrafish embryos, while these effects may be enhanced by the interaction of multiple SSRIs and their metabolites in environmental exposure scenarios.

Potential of mucoadhesive nanocapsules in drug release and toxicology in zebrafish

Mucoadhesive polymeric nanocapsules have attracted interest of researchers from different fields from natural sciences because of their ability to interact with the mucosa and increase drug permeation. Anesthesia by immersion causes absorption through the skin and gills of fish, so it is important to evaluate the exposure of these organs to drug nanosystems. Benzocaine (BENZ) is one of the most popular anesthetic agents used in fish anesthesia, but it has drawbacks because of its low bioavailability, resulting in weak absorption after immersion. Here we describe method developed for preparing and characterizing chitosan-coated PLGA mucoadhesive nanoparticles containing BENZ (NPMAs) for zebrafish immersion anesthesia. We determined the lowest effective concentration, characterized the interaction of the mucoadhesive system with fish, measured the anesthetic efficacy, and evaluated possible toxic effects in embryos and adults exposed to the nanoformulations. This study opens perspectives for using nanoformulations prepared with BENZ in aquaculture, allowing reduction of dosage as well as promoting more effective anesthesia and improved interaction with the mucoadhesive system of fish.

Stereoselective uptake and metabolism of prothioconazole caused oxidative stress in zebrafish (Danio rerio)

Prothioconazole (PTA) is a novel, broad-spectrum, chiral triazole fungicide that is mainly used to prevent and control the disease of cereal crops. However, the adverse effects of PTA and its major metabolite on nontarget organisms have aroused wide concern. In the present work, the acute toxic of the metabolite prothioconazole-desthio (PTA-desthio), with an LC₅₀ of 1.31 mg L^-1, was 3.5-fold more toxic than the parent compound, indicating that the metabolism of PTA in zebrafish was toxic. The stereoselective uptake and metabolism of PTA and PTA-desthio in zebrafish was firstly investigated using LC-MS/MS. Remarkable enantioselectivity was observed: S-PTA and S-PTA-desthio were preferentially uptake with the uptake rate constants of 8.22 and 8.15 d^-1 at exposure concentration of 0.5 mg L^-1, respectively, and the R-PTA-desthio were preferentially metabolized. PTA-desthio was rapidly formed during the uptake processes. The antioxidant enzyme activities in the zebrafish changed significantly, and these effects were reversible. A metabolic pathway including 13 phase I metabolites and 2 phase II metabolites was firstly proposed. A glucuronic acid conjugate and sulfate conjugate were observed in zebrafish. The results of this work provide information that highlights and can help mitigate the potential toxicity of PTA to the ecological environment and humans health.

Yolk Sac of Zebrafish Embryos as Backpack for Chemicals?

The zebrafish embryo (Danio rerio) has developed into one of the most important nonsentient animal models for the hazard assessments of chemicals, but the processes governing its toxicokinetics (TK) are poorly understood. This study compares the uptake of seven test compounds into the embryonic body and the yolk sac of the zebrafish embryo using TK experiments, a dialysis approach, thermodynamic calculations, and kinetic modeling. Experimental data show that between 95% (4-iodophenol) and 67% (carbamazepine) of the total internal amount in 26 h post fertilization (hpf) embryos and between 80 and 49% in 74 hpf embryos were found in the yolk. Thus, internal concentrations determined for the whole embryo overestimate the internal concentration in the embryonic body: for the compounds of this study, up to a factor of 5. Partition coefficients for the embryonic body and a one-compartment model with diffusive exchange were calculated for the neutral test compounds and agreed reasonably with the experimental data. For prevalently ionic test compounds at exposure pH (bromoxynil, paroxetine), however, the extent and the speed of uptake were low and could not be modeled adequately. A better understanding of the TK of ionizable test compounds is essential to allow assessment of the validity of this organismic test system for ionic test compounds.

Enantiomer-specific bioaccumulation and distribution of chiral pharmaceuticals in a subtropical marine food web

There is a growing concern about the occurrence of chiral pharmaceuticals in the aquatic environment. However, trophic transfer of pharmaceutical enantiomers in marine organisms is still largely unknown. This study assessed the bioaccumulation and spatial distribution of four frequently detected pharmaceuticals - atenolol, metoprolol, venlafaxine, and chloramphenicol, in a subtropical marine food web in Hong Kong waters. Twenty-four species were analyzed, including mollusks, crustaceans, and fishes. Special focus was placed in the chirality of the four analytes comprising ten different stereoisomers. Results showed that mean concentrations of individual pharmaceuticals ranged from <0.03 to 5.88 ng/g wet weight, and invertebrates generally had higher concentrations than fishes. Organisms from Hong Kong western waters were likely more contaminated by the studied pharmaceuticals than those from southern and eastern waters. Trophic dilution was observed for atenolol and chloramphenicol, with trophic magnification factors of 0.164 and 0.517, respectively. R-(+)-atenolol, S-(-)-metoprolol, and R-(-)-venlafaxine were selectively accumulated in fishes, and stereoisomeric impurities of chloramphenicol, i.e., enantiomers apart from R,R-para-form, were widespread in the investigated species. Under the worst-case scenario, atenolol and metoprolol in collected fishes might exceed toxic threshold, while local adults were unlikely to experience health risks from pharmaceutical exposure via seafood consumption.

Development of a capillary electrophoresis-mass spectrometry method for the analysis of metformin and its transformation product guanylurea in biota

A method with capillary electrophoresis coupled to mass spectrometry was optimized to determine the uptake of metformin and its metabolite guanylurea by zebrafish (Danio rerio) embryos and brown trout (Salmo trutta f. fario) exposed under laboratory conditions. Metformin was extracted from fish tissues by sonication in methanol, resulting in an absolute recovery of almost 90%. For the extraction of guanylurea from brown trout, solid-phase extraction was implemented with a recovery of 84%. The use of a mixture of methanol and glacial acetic acid as a non-aqueous background electrolyte was vital to achieve robust analysis using a bare fused-silica capillary with an applied voltage of +30 kV. Problems with adsorption associated with an aqueous background electrolyte were eliminated using a non-aqueous background electrolyte made of methanol/acetic acid (97:3) with 25 mM ammonium acetate (for zebrafish embryos) or 100 mM ammonium acetate (for brown trouts), depending on the sample complexity and matrix influences. High resolution and high separation selectivity from matrix components were achieved by optimization of the ammonium acetate concentration in the background electrolyte. An extensive evaluation of matrix effects was conducted with regard to the complex matrices present in the fish samples. They required adapting the background electrolyte to higher concentrations. Applying this method to extracts of zebrafish embryos and brown trout tissue samples, limits of detection for both metformin and guanylurea in zebrafish embryos (12.2 μg/l and 15 μg/l) and brown trout tissues (15 ng/g and 34 ng/g) were in the low μg/l or ng/g range. Finally, metformin and guanylurea could be both quantified for the first time in biota samples from exposure experiments.

Norfluoxetine Is the Only Metabolite of Fluoxetine in Zebrafish (Danio rerio) Embryos That Accumulates at Environmentally Relevant Exposure Scenarios

Fluoxetine has been recognized as one of the most toxic pharmaceuticals in the aquatic environment. Since there is growing evidence that the toxic potential of fluoxetine in surface waters is markedly influenced by its own metabolism in aquatic species, this study investigated the biotransformation of fluoxetine in the zebrafish embryo - an aquatic model organism of intermediate complexity. Zebrafish embryos were exposed to 0.1, 1.0, 10, 50, and 5000 μg/L of fluoxetine from 48 to 120 h post-fertilization (hpf), and the accumulation of fluoxetine and its metabolites was analyzed over time. Additionally, depuration of fluoxetine and its metabolites from 96 to 120 hpf was investigated, and autoinhibitory effects of fluoxetine on phase I biotransformation were analyzed. Exposure to 5000 μg/L fluoxetine resulted in elevated 7-ethoxyresorufin-O-deethylase (EROD) activity of cytochrome P450 enzymes and continuous accumulation of fluoxetine and 11 fluoxetine metabolites. Embryos exposed to 10 and 50 μg/L fluoxetine were able to reduce fluoxetine accumulation from 94 to 120 hpf. During depuration, accumulation of fluoxetine and most metabolites was clearly reduced, and biotransformation shifted in favor of norfluoxetine, the primary fluoxetine metabolite in humans. Findings demonstrated that norfluoxetine is the only metabolite of fluoxetine that accumulates in zebrafish embryos at environmentally relevant exposure scenarios.

Evaluation of Developmental Toxicity, Developmental Neurotoxicity, and Tissue Dose in Zebrafish Exposed to GenX and Other PFAS

BACKGROUND

Per- and polyfluoroalkyl substances (PFAS) are a diverse class of industrial chemicals with widespread environmental occurrence. Exposure to long-chain PFAS is associated with developmental toxicity, prompting their replacement with short-chain and fluoroether compounds. There is growing public concern over the safety of replacement PFAS.

OBJECTIVE

We aimed to group PFAS based on shared toxicity phenotypes.

METHODS

Zebrafish were developmentally exposed to 4,8-dioxa-3H-perfluorononanoate (ADONA), perfluoro-2-propoxypropanoic acid (GenX Free Acid), perfluoro-3,6-dioxa-4-methyl-7-octene-1-sulfonic acid (PFESA1), perfluorohexanesulfonic acid (PFHxS), perfluorohexanoic acid (PFHxA), perfluoro-n-octanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), or 0.4% dimethyl sulfoxide (DMSO) daily from 0-5 d post fertilization (dpf). At 6 dpf, developmental toxicity and developmental neurotoxicity assays were performed, and targeted analytical chemistry was used to measure media and tissue doses. To test whether aliphatic sulfonic acid PFAS cause the same toxicity phenotypes, perfluorobutanesulfonic acid (PFBS; 4-carbon), perfluoropentanesulfonic acid (PFPeS; 5-carbon), PFHxS (6-carbon), perfluoroheptanesulfonic acid (PFHpS; 7-carbon), and PFOS (8-carbon) were evaluated.

RESULTS

PFHxS or PFOS exposure caused failed swim bladder inflation, abnormal ventroflexion of the tail, and hyperactivity at nonteratogenic concentrations. Exposure to PFHxA resulted in a unique hyperactivity signature. ADONA, PFESA1, or PFOA exposure resulted in detectable levels of parent compound in larval tissue but yielded negative toxicity results. GenX was unstable in DMSO, but stable and negative for toxicity when diluted in deionized water. Exposure to PFPeS, PFHxS, PFHpS, or PFOS resulted in a shared toxicity phenotype characterized by body axis and swim bladder defects and hyperactivity.

CONCLUSIONS

All emerging fluoroether PFAS tested were negative for evaluated outcomes. Two unique toxicity signatures were identified arising from structurally dissimilar PFAS. Among sulfonic acid aliphatic PFAS, chemical potencies were correlated with increasing carbon chain length for developmental neurotoxicity, but not developmental toxicity. This study identified relationships between chemical structures and in vivo phenotypes that may arise from shared mechanisms of PFAS toxicity. These data suggest that developmental neurotoxicity is an important end point to consider for this class of widely occurring environmental chemicals. https://doi.org/10.1289/EHP5843.

In vivo fluorescence-based characterization of cytochrome P450 activity during embryonic development of zebrafish (Danio rerio)

Zebrafish (Danio rerio) early life-stages are increasingly gaining attention as an alternative model in both human and environmental toxicology. Whereas there is amble knowledge about the transcription of various cytochrome P450 isoforms, the level of information about functional implications is still limited. This study investigated the development of CYP2-dependent 7-methoxycoumarin-O-demethylase (MCOD) activity throughout the early zebrafish development from 5 to 118 h post-fertilization (hpf) via confocal laser scanning microscopy. Results demonstrate that zebrafish embryos exhibit constitutive MCOD activity from as early as 5.5 hpf. Characteristic spatiotemporal patterns were documented with MCOD activities localized in several tissues and organs, namely the cardiovascular system, the brain, the digestive system, and the urinary tract. The study thereby contributes to a better understanding of the development and functional role of CYP enzymes in zebrafish early life-stages.

Development of a generic zebrafish embryo PBPK model and application to the developmental toxicity assessment of valproic acid analogs

In order to better explain, predict, or extrapolate to humans the developmental toxicity effects of chemicals to zebrafish (Danio rerio) embryos, we developed a physiologically-based pharmacokinetic (PBPK) model designed to predict organ concentrations of neutral or ionizable chemicals, up to 120 h post-fertilization. Chemicals' distribution is modeled in the cells, lysosomes, and mitochondria of ten organs of the embryo. The model's partition coefficients are calculated with sub-models using physicochemical properties of the chemicals of interest. The model accounts for organ growth and changes in metabolic clearance with time. We compared ab initio model predictions to data obtained on culture medium and embryo concentrations of valproic acid (VPA) and nine analogs during continuous dosing under the OECD test guideline 236. We further improved the predictions by estimating metabolic clearance and partition coefficients from the data by Bayesian calibration. We also assessed the performance of the model at reproducing data published by Brox et al. (2016) on VPA and 16 other chemicals. We finally compared dose-response relationships calculated for mortality and malformations on the basis of predicted whole embryo concentrations versus those based on nominal water concentrations. The use of target organ concentrations substantially shifted the magnitude of dose-response parameters and the relative toxicity ranking of chemicals studied.

Embryo bioassays with aquatic animals for toxicity testing and hazard assessment of emerging pollutants: A review

This review article gathers the available information on the use of embryo-tests as high-throughput tools for toxicity screening, hazard assessment and prioritization of new and existing chemical compounds. The approach is contextualized considering the new legal trends for animal experimentation, fostering the 3R policy, with reduction of experimental animals, addressing the potential of embryo-tests as high-throughput toxicity screening and prioritizing tools. Further, the current test guidelines, such as the ones provided by OECD and EPA, focus mainly in a limited number of animal lineages, particularly vertebrates and arthropods. To extrapolate hazard assessment to the ecosystem scale, a larger diversity of taxa should be tested. The use of new experimental animal models in toxicity testing, from a representative set of taxa, was thoroughly revised and discussed in this review. Here, we critically review current tools and the main advantages and drawbacks of different animal models and set researcher priorities.

Recommendations for Improving Methods and Models for Aquatic Hazard Assessment of Ionizable Organic Chemicals

Ionizable organic chemicals (IOCs) such as organic acids and bases are an important substance class requiring aquatic hazard evaluation. Although the aquatic toxicity of IOCs is highly dependent on the water pH, many toxicity studies in the literature cannot be interpreted because pH was not reported or not kept constant during the experiment, calling for an adaptation and improvement of testing guidelines. The modulating influence of pH on toxicity is mainly caused by pH-dependent uptake and bioaccumulation of IOCs, which can be described by ion-trapping and toxicokinetic models. The internal effect concentrations of IOCs were found to be independent of the external pH because of organisms' and cells' ability to maintain a stable internal pH milieu. If the external pH is close to the internal pH, existing quantitative structure-activity relationships (QSARs) for neutral organics can be adapted by substituting the octanol-water partition coefficient by the ionization-corrected liposome-water distribution ratio as the hydrophobicity descriptor, demonstrated by modification of the target lipid model. Charged, zwitterionic and neutral species of an IOC can all contribute to observed toxicity, either through concentration-additive mixture effects or by interaction of different species, as is the case for uncoupling of mitochondrial respiration. For specifically acting IOCs, we recommend a 2-step screening procedure with ion-trapping/QSAR models used to predict the baseline toxicity, followed by adjustment using the toxic ratio derived from in vitro systems. Receptor- or plasma-binding models also show promise for elucidating IOC toxicity. The present review is intended to help demystify the ecotoxicity of IOCs and provide recommendations for their hazard and risk assessment. Environ Toxicol Chem 2020;39:269-286. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Responses of seeds of typical Brassica crops to tetracycline stress: Sensitivity difference and source analysis

Antibiotics can induce adverse effects on plants. Brassica crop seeds, for their advantages, are used widely in seed germination test to investigate phytotoxicity of substances. However, their performances on evaluating antibiotics remain to be studied to select sensitive species for control of potential risks. In this work, common species of Chinese cabbage (Brassica rapa L.), edible rape (Brassica napus L.), and cabbage (Brassica oleracea L.) with three cultivars each were selected to compare and analyze the sensitivity difference of their seeds to tetracycline (TC) stress. Results showed that the ratio of axis to cotyledon (RAC) by fresh weight was an alternative endpoint besides radicle length (RL) in the test. The species sensitivity distribution (SSD) based on the effective concentrations causing x% inhibition (EC_x) in RL of seeds exposed to TC was applied to compare the sensitivity of seeds and estimate the hazardous concentration for x% species (HC_x). From the species-dependent sensitivity and the sensitivity difference of cultivars in the same species of seeds to TC, the performance of Chinese cabbage was the best in the study. The sensitivity of seeds to TC could be evaluated by EC₂₀ related to seed physical traits and germination indices, while the extent of seeds affected by TC could be evaluated by EC₅₀ related to the composition of seed storage reserves. We recommended that it was a new idea to analyze responses of different seeds to TC at large scale according to seed innate characteristics.

Combined Ion-Trapping and Mass Balance Models To Describe the pH-Dependent Uptake and Toxicity of Acidic and Basic Pharmaceuticals in Zebrafish Embryos ( Danio rerio)

The aim of the current study was to understand and develop models to predict the pH-dependent toxicity of ionizable pharmaceuticals in embryos of the zebrafish Danio rerio. We found a higher uptake and toxicity with increasing neutral fraction of acids (diclofenac, genistein, naproxen, torasemide, and warfarin) and bases (metoprolol and propranolol). Simple mass balance models accounting for the partitioning to lipids and proteins in the zebrafish embryo were found to be suitable to predict the bioconcentration after 96 h of exposure if pH values did not differ much from the internal pH of 7.55. For other pH values, a kinetic ion-trap model for the zebrafish embryo explained the pH dependence of biouptake and toxicity. The total internal lethal concentrations killing 50% of the zebrafish embryos (ILC₅₀) were calculated from the measured BCF and LC₅₀. The resulting ILC₅₀ were independent of external pH. Critical membrane concentrations were deduced by an internal mass balance model, and apart from diclofenac, whose specific toxicity in fish had already been established, all pharmaceuticals were confirmed to act as baseline toxicants in zebrafish.

Transformation Products of Fluoxetine Formed by Photodegradation in Water and Biodegradation in Zebrafish Embryos ( Danio rerio)

The present study investigates the transformation of the antidepressant fluoxetine (FLX) by photo- and biodegradation and shows similarities and differences in transformation products (TPs). TPs were identified using LC-high-resolution mass spectrometry with positive and negative electrospray ionization. In a sunlight simulator, photodegradation was carried out using ultrapure water (pH 6, 8, and 10) and surface water (pH 8) to study the effect of direct and indirect photolysis, respectively. The well-known metabolite norfluoxetine (NFLX) proved to be a minor TP in photolysis (≤2% of degraded FLX). In addition, 26 TPs were detected, which were formed by cleavage of the phenolether bond ( O-dealkylation) which primarily formed 3-(methylamino)-1-phenyl-1-propanol (TP 166) and 4-(trifluoromethyl)phenol, by hydroxylation of the benzyl moiety, by CF₃ substitution to benzoic aldehyde/acid, and by adduct formation at the amine group ( N-acylation with aldehydes and carboxylic acids). Higher pH favors the neutral species of FLX and the neutral/anionic species of primary TPs and, therefore, photodegradation. In zebrafish embryos, the bioconcentration factor of FLX was found to be 110, and about 1% of FLX taken up by the embryos was transformed to NFLX. Seven metabolites known from photodegradation and formed by hydrolysis, hydroxylation, and N-acylation as well as three new metabolites formed by N-hydroxylation, N-methylation, and attachment of an amine group were identified in zebrafish embryos. The study highlights the importance of considering a broad range of TPs of FLX in fresh water systems and in ecotoxicity tests and to include TP formation in both environmental processes and metabolism in organisms.

Dose-dependent effects of a glyphosate commercial formulation - Roundup® UltraMax - on the early zebrafish embryogenesis

The use of herbicides with glyphosate as an active ingredient, the so-called glyphosate-based herbicides (GBH), has increased dramatically in recent years currently being the most widely used in the world. Therefore, glyphosate residues have been detected in water and soils near the application sites. Recent studies indicate that GBH may cause adverse effects on vertebrates although these have been attributed to the presence of adjuvants in the commercial formulations rather than to the sole compound. Accordingly, the objective of this work was to investigate the lethal and sub-lethal developmental effects, neurotoxic potential and oxidative stress responses of zebrafish embryos to Roundup® Ultramax (RU) exposure. Embryos were exposed during 72 h to 0, 2, 5, 8.5 μg a.i. mL-1 of RU. Increased mortality was observed in embryos exposed to concentrations above 8.5 μg a.i. mL-1 as well as increased number of malformations. Decreased heart rate and hatchability were also observed. By contrast, exposure to concentrations that do not evoke teratogenic outcomes induced a dose-dependent decrease of heart rate although not inducing significant developmental changes. However, histological changes were not observed in the larvae exposed to these concentrations. Moreover, the generation of reactive oxygen species, the antioxidant enzymes activities (SOD and CAT), the GST biotransformation activity, the glutathione levels (GSH and GSSG), the oxidative damage (MDA) and the acetylcholinesterase and lactate dehydrogenase were similar among groups following exposure. Overall, available evidence suggests a dose-dependent toxicological effect of this formulation at concentrations that are not routinely detected in the environment. However, additional studies should be performed to better understand the underlying molecular mechanisms in favor of this formulation.

Toxicokinetics of Perfluorinated Alkyl Acids Influences Their Toxic Potency in the Zebrafish Embryo ( Danio rerio)

Perfluorinated alkyl acids (PFAA) are highly persistent and bioaccumulative and have been associated with several adverse health effects. The chemical structure mainly differs in two ways: the length of the hydrophobic alkyl chain and the type of hydrophilic end group. Little is known how the chemical structure affects the toxicokinetics (TK) in different organisms. We studied the TK of four PFAA (PFOS, PFHxS, PFOA, and PFBA) with different chain lengths (4-8 carbons) and functional groups (sulfonic and carboxylic acid) in zebrafish ( Danio rerio) embryo. The time courses of the external (ambient water) and internal concentrations were determined at three exposure concentrations from 2 up to 120 h postfertilization (hpf). Three of the four PFAA showed a biphasic uptake pattern with slow uptake before hatching (around 48 hpf) and faster uptake thereafter. A two-compartment TK model adequately described the biphasic uptake pattern, suggesting that the chorion functions as an uptake barrier until 48 hpf. The bioconcentration factors (BCF) determined at 120 hpf varied widely between PFAA with averages of approximately 4000 (PFOS), 200 (PFHxS), 50 (PFOA), and 0.8 (PFBA) L kg dry weight^-1, suggesting that both the alkyl chain length and the functional group influence the TK. The differences in toxic potency were reduced by 3 orders of magnitude when comparing internal effect concentrations instead of effective external concentrations.

Advancing the Zebrafish embryo test for endocrine disruptor screening using micro-injection: Ethinyl estradiol as a case study

Fish (embryo) toxicity test guidelines are mostly based on aquatic exposures. However, in some cases, other exposure routes can be more practical and relevant. Micro-injection into the yolk of fish embryos could offer a particular advantage for administering hydrophobic compounds, such as many endocrine disruptors. Single-dose micro-injection was compared with continuous aquatic exposure in terms of compound accumulation and biological responses. 17α-Ethinyl estradiol (EE2) was used as a model compound. First, the optimal solvent and droplet size were optimized, and needle variation was assessed. Next, biological endpoints were evaluated. The accumulated internal dose of EE2 decreased over time in both exposure scenarios. Estrogen receptor activation was concentration/injected dose dependent, increased daily, and was related to esr2b transcription. Transcription of vitellogenin 1 (vtg1) and brain aromatase (cyp19a1b) was induced in both scenarios, but the cyp19a1b transcription pattern differed between routes. Injection caused an increase in cyp19a1b transcripts from 48 hours post fertilization (hpf) onward, whereas after aquatic exposure the main increase occurred between 96 and 120 hpf. Some malformations only occurred after injection, whereas others were present for both scenarios. We conclude that responses can differ between exposure routes and therefore micro-injection is not a direct substitute for, but can be complementary to aquatic exposure. Nevertheless, vtg1and cyp19a1b transcription and estrogen receptor activation are suitable biomarkers for endocrine disruptor screening in both scenarios. Environ Toxicol Chem 2019;38:533-547. © 2018 SETAC.