High-content screening assay for identification of chemicals impacting spontaneous activity in zebrafish embryos

Indoor and Personal PM2.5 Samples Differ in Chemical Composition and Alter Zebrafish Behavior Based on Primary Fuel Source

Fine particulate matter (PM2.5) exposure has been linked to diverse human health impacts. Little is known about the potential heterogeneous impacts of PM2.5 generated from different indoor fuel sources and how exposure differs between personal and indoor environments. Therefore, we used PM2.5 collected by one stationary sampler in a kitchen and personal samplers (female and male participants), in homes (n = 24) in Kheri, India, that used either biomass or liquified petroleum gas (LPG) as primary fuel sources. PM2.5 samples (pooled by fuel type and monitor placement) were analyzed for oxidative potential and chemical composition, including elements and 125 organic compounds. Zebrafish (Danio rerio) embryos were acutely exposed to varying concentrations of PM2.5 and behavioral analyses were conducted. We found relatively high PM2.5 concentrations (5-15 times above World Health Organization daily exposure guidelines) and varied human health-related chemical composition based on fuel type and monitor placement (up to 15% carcinogenic polycyclic aromatic hydrocarbon composition). Altered biological responses, including changes to mortality, morphology, and behavior, were elicited by exposure to all sample types. These findings reveal that although LPG is generally ranked the least harmful compared to biomass fuels, chemical characteristics and biological impacts were still present, highlighting the need for further research in determining the safety of indoor fuel sources.

Neurotoxicity of the Cu(OH)2 Nanopesticide through Perturbing Multiple Neurotransmitter Pathways in Developing Zebrafish

The copper hydroxide [Cu(OH)2] nanopesticide is an emerging agricultural chemical that can negatively impact aquatic organisms. This study evaluated the behavioral changes of zebrafish larvae exposed to the Cu(OH)2 nanopesticide and assessed its potential to induce neurotoxicity. Metabolomic and transcriptomic profiling was also conducted to uncover the molecular mechanisms related to potential neurotoxicity. The Cu(OH)2 nanopesticide at 100 μg/L induced zebrafish hypoactivity, dark avoidance, and response to the light stimulus, suggestive of neurotoxic effects. Altered neurotransmitter-related pathways (serotoninergic, dopaminergic, glutamatergic, GABAergic) and reduction of serotonin (5-HT), dopamine (DA), glutamate (GLU), γ-aminobutyric acid (GABA), and several of their precursors and metabolites were noted following metabolomic and transcriptomic analyses. Differentially expressed genes (DEGs) were associated with the synthesis, transport, receptor binding, and metabolism of 5-HT, DA, GLU, and GABA. Transcripts (or protein levels) related to neurotransmitter receptors for 5-HT, DA, GLU, and GABA and enzymes for the synthesis of GLU and GABA were downregulated. Effects on both the glutamatergic and GABAergic pathways in zebrafish were specific to the nanopesticide and differed from those in fish exposed to copper ions. Taken together, the Cu(OH)2 nanopesticide induced developmental neurotoxicity in zebrafish by inhibiting several neurotransmitter-related pathways. This study presented a model for Cu(OH)2 nanopesticide-induced neurotoxicity in developing zebrafish that can inform ecological risk assessments.

Disruption of the intestinal barrier by avermectin in carp involves oxidative stress and apoptosis and leads to intestinal inflammation

Avermectin (AVM) is a widely used insecticide. Due to its sensitive toxicity to aquatic organisms, the toxicology of AVM on fish intestines remains unclear. Here, we established a 96 h AVM acute toxicity model to explore the effects of AVM on the intestinal tract of carp. The 96 h LC50 of carps exposed to AVM was 24.04 μg/L, 12.02 μg/L was selected as the high-dose group and 3.005 μg/L was selected as the low-dose group. After 96 h of exposure, intestinal tissues were collected and subsequently analyzed for histopathology, the activities of antioxidant oxidases (CAT, SOD, GSH-Px), and the expression of mRNA associated with oxidative stress, inflammation, and apoptosis. Our study showed that AVM exposure caused intestinal damage in carp, decreased the expression of the tight junction protein gene, activated oxidative stress, induced apoptosis, and induced intestinal inflammation in carp. Therefore, we demonstrated that AVM exposure compromised the integrity of the intestinal barrier in carp, activated oxidative stress, induced endogenous apoptosis, and induced intestinal inflammatory responses. These results indicate that AVM, as a drug-sensitive to aquatic organisms, has a much more complex toxic effect on the fish intestinal tract, which provides a new perspective for studying the toxicology of AVM on the fish intestinal tract.

Potential toxicity of landfill leachate to zebrafish and mung beans

Landfill leachate has become a major public health concern due to its adverse health effects. However, its toxicological effects have not been thoroughly determined because of its complex composition. To address this issue, two model organisms were used in this study, including mung beans and zebrafish. Bean seedlings were exposed to different concentrations of landfill leachate (1%, 5%, 10%, 15%, and 20%, v/v, leachate/deionized water) for 7 days. Low concentrations (1%) of landfill leachate increased the growth of mung beans, whereas high concentrations (15% and 20%) of landfill leachate inhibited the growth and development of seedlings. Furthermore, landfill leachate reduced chlorophyll levels but increased malondialdehyde levels, leading to an increased rate of root-tip micronuclei. Zebrafish embryos were exposed to different concentrations of landfill leachate (0.5%, 1.0%, 1.2%, and 1.5%, v/v, leachate/E3 medium) for 120 h. The results showed that landfill leachate significantly decreased lower levels of hatching rate and heart rate but increased the mortality and malformation rates of embryos. Moreover, 1.0% landfill leachate reduced the frequency of spontaneous movement and the light stimulation reaction of embryos. Embryos exposed to leachate showed less exploratory behavior and fewer mirror attacks in the black and white areas. Our results suggest that exposure to landfill leachate could cause developmental toxicity and genotoxicity in plants and fish. The findings can improve our understanding of the environmental toxicity of landfill leachate and provide additional evidence for its risk assessment and management.

Avermectin induces cardiac toxicity in early embryonic stage of zebrafish

Avermectin is a widely used insecticide, and it is mainly effective against animal parasites and insects. Given its extensive use in agriculture, a large amount of avermectin is accumulated in natural waters. Avermectin is a neurotoxin that affects the autonomous behavior of zebrafish and inhibits neurological responses in invertebrates via GABA-chloride channels. In this study, we used zebrafish as a model organism to explore the lethal teratogenic effects of different avermectin concentrations. We found that 50-μg/L avermectin could cause significant malformation abnormalities during the development of zebrafish heart, changes in heart rate, and significant reduction in hatching rate and body length. Transcriptome data revealed that 499 genes were upregulated and 877 genes were downregulated at 72 h post-fertilization (hpf), whereas 1805 genes were upregulated and 836 genes were downregulated at 120 hpf. According to gene ontology (GO) enrichment analysis, avermectin affected cardiac circulation and myocardial fiber development. KEGG analysis revealed that avermectin treatment significantly altered the activity of signal pathways associated with cardiac rhythm and vascular smooth muscle contraction. The main target of avermectin was identified as the heart, as it affected heart development and function by altering cardiac-related gene expression that led to a heart defect phenotype. Our findings indicate that developing zebrafish are sensitive to avermectin, which targets the heart.

Subcellular-Level Mitochondrial Energy Metabolism Response in the Fat Body of the German Cockroach Fed Abamectin

Mitochondria are the leading organelle for energy metabolism. The toxic effects of environmental toxicants on mitochondrial morphology, energy metabolism, and their determination of cell fate have already been broadly studied. However, minimal research exists on effects of environmental toxicants such as pesticides on mitochondrial energy metabolism at in vitro subcellular level, particularly from an omics perspectives (e.g., metabolomics). Here, German cockroach (Blattella germanica) was fed diets with (0.01 and 0.001 mg/mL) and without abamectin, and highly purified fat body mitochondria were isolated. Swelling measurement confirmed abnormal mitochondrial swelling caused by abamectin stress. The activity of two key mitochondrial energy metabolism-related enzymes, namely succinic dehydrogenase and isocitrate dehydrogenase, was significantly affected. The metabolomic responses of the isolated mitochondria to abamectin were analyzed via untargeted liquid chromatography/mass spectrometry metabolomics technology. Fifty-two differential metabolites (DMs) were identified in the mitochondria between the 0.001 mg/mL abamectin-fed and the control groups. Many of these DMs were significantly enriched in pathways involved in ATP production and energy consumption (e.g., oxidative phosphorylation, TCA cycle, and pentose phosphate pathway). Nineteen of the DMs were typically related to energy metabolism. This study is valuable for further understanding mitochondrial toxicology under environmental toxicants, particularly its subcellular level.

High-throughput screening paradigms in ecotoxicity testing: Emerging prospects and ongoing challenges

The rapidly increasing number of new production chemicals coupled with stringent implementation of global chemical management programs necessities a paradigm shift towards boarder uses of low-cost and high-throughput ecotoxicity testing strategies as well as deeper understanding of cellular and sub-cellular mechanisms of ecotoxicity that can be used in effective risk assessment. The latter will require automated acquisition of biological data, new capabilities for big data analysis as well as computational simulations capable of translating new data into in vivo relevance. However, very few efforts have been so far devoted into the development of automated bioanalytical systems in ecotoxicology. This is in stark contrast to standardized and high-throughput chemical screening and prioritization routines found in modern drug discovery pipelines. As a result, the high-throughput and high-content data acquisition in ecotoxicology is still in its infancy with limited examples focused on cell-free and cell-based assays. In this work we outline recent developments and emerging prospects of high-throughput bioanalytical approaches in ecotoxicology that reach beyond in vitro biotests. We discuss future importance of automated quantitative data acquisition for cell-free, cell-based as well as developments in phytotoxicity and in vivo biotests utilizing small aquatic model organisms. We also discuss recent innovations such as organs-on-a-chip technologies and existing challenges for emerging high-throughput ecotoxicity testing strategies. Lastly, we provide seminal examples of the small number of successful high-throughput implementations that have been employed in prioritization of chemicals and accelerated environmental risk assessment.

Potency matters: Impacts of embryonic exposure to nAChR agonists thiamethoxam and nicotine on hatching success, growth, and neurobehavior in larval zebrafish

Thiamethoxam (TM) is a neonicotinoid insecticide that acts as a nicotinic acetylcholine receptor (nAChR) agonist. While designed to specifically target invertebrate nAChRs, recent studies have reported adverse effects of neonicotinoid exposure in early life-stage fish. This study examined the health and neurobehavioral impacts of chronic exposure to various concentrations of TM or nicotine (NIC) in early life zebrafish (Danio rerio) in conjunction with in-silico molecular docking to compare their ligand-receptor interactions with vertebrate nAChR. Chronic exposure to both reduced survival by approximately 20% (163 µg TM/l) and 25-100% (≥0.49 µg NIC/l). Hatching and growth were impaired following exposure to ≥0.21 µg TM/l or 4.9 µg NIC/l. Both TM and NIC produced morphological and behavioral indicators of neurotoxicity, with more potent effects following NIC exposure. NIC impaired embryonic motor activity by 40% (49 µg NIC/l), while both TM and NIC significantly altered predator escape response in larvae, specifically the latency and the initial burst movement of the response were impacted. Molecular docking predicted variations in the type and strength of interactions that occur between NIC or TM and vertebrate nAChR. These findings demonstrate that chronic exposure to TM might impact general health and neurobehavior of early-stage zebrafish. Our data support hypotheses that TM presents low affinity for vertebrate nAChR but may still pose an adverse risk to larval fish growth and neurobehavior.

Neurotoxic effects of synthetic phenolic antioxidants on dopaminergic, serotoninergic, and GABAergic signaling in larval zebrafish (Danio rerio)

Synthetic phenolic antioxidants (SPAs) are an environmental concern because they are widely detected in aquatic ecosystems and can pose potential threats to organisms. Studies have reported developmental deficits and behavioral changes in response to SPAs, indicating possible neurotoxic effects. However, their neuroactive potency as well as their mode of action (MoA) remain unclear. As such, this study evaluated the potential neurotoxicity of three SPAs [butylated hydroxytoluene (BHT), 2,4-di-tert-butylphenol (2,4-DTBP), and 4-tert-octylphenol (4-t-OP)] at three concentrations (0.01, 0.1 and 1 μM) to zebrafish larvae. Both 2,4-DTBP and BHT decreased spontaneous tail coiling (STC) at 28 hpf (hours post fertilization) whereas 4-t-OP increased STC. Locomotor activity, based on the velocity and distance of larvae (144 hpf) travelled, was promoted by 2,4-DTBP while it decreased in larvae with exposure to 4-t-OP and BHT. In the light-dark preference assay, exposure to either 2,4-DTBP or BHT resulted in variability in the visiting frequency to the dark zone, and larvae (144 hpf) spent less time in the dark, suggesting anxiety-like behavior. Conversely, zebrafish exposed to 4-t-OP, especially at 1 μM concentration, were hypoactive and spent more time in dark, suggestive of anxiolytic-like responses. RNA-seq was conducted to discern mechanisms underlying behavioral responses. Transcriptomic analysis revealed that gene networks related to neuroactive ligand-receptor interaction as well as neurotransmitter-related pathways were altered by all three SPAs based on gene set and subnetwork enrichment analysis. Modulation of dopaminergic, serotoninergic, and/or GABAergic signaling at the transcript level was noted for each of the three SPAs, but different expression patterns were observed, indicating SPA- and dose-specific responses of the transcriptome. The present study provides novel insight into potential mechanisms associated with neurotoxicity of SPAs congeners.

A high-throughput screening assay for identification of chemicals with liver tumor promoting potential using a transgenic zebrafish line

Traditional high-throughput methods for identification of chemicals with liver tumor promotion potentials are based on established cancer cell lines, and rapid and cost-effective high-throughput screening assays in whole organisms are presently lacking. In this study, a transgenic zebrafish liver cancer model was employed to develop a method that could be used to identify chemicals with liver tumor promotion effect quickly and accurately. The method consisted of three parts, including exposure preparation, exposure process and image acquisition. In brief, after chemical exposure for 7 days, 96-well plate exposure system for zebrafish larvae was assessed by microplate reader. Then, the liver cancer promoting potential chemicals were evaluated by field area and field average intensity of fluorescence. The results were further validated by conducting histopathological examination. Our data demonstrated that the high-throughput screening assay developed in this study was reproducible and could be used to rapidly screen chemicals with liver tumor promoting potentials by using tris-(2-chloropropyl)-phosphate (TDCIPP) as a positive control. Furthermore, some other positive chemicals found in previous studies and environmental compounds were assessed using the established method. Results indicated that 86.7% of the positive chemicals and five environmental compounds out of seventeen compounds could enhance liver tumor progression.

New evidence for neurobehavioral toxicity of deltamethrin at environmentally relevant levels in zebrafish

Deltamethrin, a widely used type II pyrethroid insecticide, was reported with neurotoxicity to aquatic organisms, such as fish. However, the effects and potential mechanisms on the central nervous system remain largely unknown, especially under environmental concentrations. Therefore, we exposed adult female zebrafish to environmentally relevant levels of deltamethrin (30, 100, and 333 ng/L) for 21 days to assess neurobehavioral changes related to the central nervous system and explore the modes of action. Behavioral assays revealed significant increases in the swimming speeds, residence time near other fish and the shoaling cohesion in exposed fish. Transcriptomic results enriched the disrupted neural functions involving the glutamatergic and dopaminergic synapses in the brain. The qRT-PCR confirmed the upregulation of the factors for promoting the glutamate release. The measurement of neurotransmitters showed significantly increased content of the excitatory neurotransmitter glutamate in the brain. Taken together, deltamethrin exposure increased the glutamate level and promoted the release of such an excitatory neurotransmitter between the glutamatergic synapses in the brain, which eventually led to hyperactivity of social behaviors in adult zebrafish.

Micronized Curcumin Causes Hyperlocomotion in Zebrafish Larvae

Zebrafish larvae have been widely used in neuroscience and drug research and development. In the larval stage, zebrafish present a broad behavioral repertoire and physiological responses similar to adults. Curcumin (CUR), a major component of Curcuma longa L. (Zingiberaceae), has demonstrated the ability to modulate several neurobiological processes relevant to mental disorders in animal models. However, the low bioavailability of this compound can compromise its in vivo biological potential. Interestingly, it has been shown that micronization can increase the biological effects of several compounds. Thus, in this study, we compared the effects of acute exposure for 30 min to the following solutions: water (control), 0.1% DMSO (vehicle), 1 μM CUR, or 1 μM micronized curcumin (MC) in zebrafish larvae 7 days post-fertilization (dpf). We analyzed locomotor activity (open tank test), anxiety (light/dark test), and avoidance behavior (aversive stimulus test). Moreover, we evaluated parameters of oxidative status (thiobarbituric acid reactive substances and non-protein thiols levels). MC increased the total distance traveled and absolute turn angle in the open tank test. There were no significant differences in the other behavioral or neurochemical outcomes. The increase in locomotion induced by MC may be associated with a stimulant effect on the central nervous system, which was evidenced by the micronization process.

Human α-Defensin-6 Neutralizes Clostridioides difficile Toxins TcdA and TcdB by Direct Binding

Rising incidences and mortalities have drawn attention to Clostridioides difficile infections (CDIs) in recent years. The main virulence factors of this bacterium are the exotoxins TcdA and TcdB, which glucosylate Rho-GTPases and thereby inhibit Rho/actin-mediated processes in cells. This results in cell rounding, gut barrier disruption and characteristic clinical symptoms. So far, treatment of CDIs is limited and mainly restricted to some antibiotics, often leading to a vicious circle of antibiotic-induced disease recurrence. Here, we demonstrate the protective effect of the human antimicrobial peptide α-defensin-6 against TcdA, TcdB and the combination of both toxins in vitro and in vivo and unravel the underlying molecular mechanism. The defensin prevented toxin-mediated glucosylation of Rho-GTPases in cells and protected human cells, model epithelial barriers as well as zebrafish embryos from toxic effects. In vitro analyses revealed direct binding to TcdB in an SPR approach and the rapid formation of TcdB/α-defensin-6 complexes, as analyzed with fluorescent TcdB by time-lapse microscopy. In conclusion, the results imply that α-defensin-6 rapidly sequesters the toxin into complexes, which prevents its cytotoxic activity. These findings extend the understanding of how human peptides neutralize bacterial protein toxins and might be a starting point for the development of novel therapeutic options against CDIs.

CO2 induced seawater acidification impacts survival and development of European eel embryos

Fish embryos may be vulnerable to seawater acidification resulting from anthropogenic carbon dioxide (CO₂) emissions or from excessive biological CO₂ production in aquaculture systems. This study investigated CO₂ effects on embryos of the European eel (Anguilla anguilla), a catadromous fish that is considered at risk from climate change and that is targeted for hatchery production to sustain aquaculture of the species. Eel embryos were reared in three independent recirculation systems with different pH/CO₂ levels representing “control” (pH 8.1, 300 μatm CO₂), end-of-century climate change (“intermediate”, pH 7.6, 900 μatm CO₂) and “extreme” aquaculture conditions (pH 7.1, 3000 μatm CO₂). Sensitivity analyses were conducted at 4, 24, and 48 hours post-fertilization (hpf) by focusing on development, survival, and expression of genes related to acute stress response (crhr1, crfr2), stress/repair response (hsp70, hsp90), water and solute transport (aqp1, aqp3), acid-base regulation (nkcc1a, ncc, car15), and inhibitory neurotransmission (GABAAα6b, Gabra1). Results revealed that embryos developing at intermediate pH showed similar survival rates to the control, but egg swelling was impaired, resulting in a reduction in egg size with decreasing pH. Embryos exposed to extreme pH had 0.6-fold decrease in survival at 24 hpf and a 0.3-fold change at 48 compared to the control. These observed effects of acidification were not reflected by changes in expression of any of the here studied genes. On the contrary, differential expression was observed along embryonic development independent of treatment, indicating that the underlying regulating systems are under development and that embryos are limited in their ability to regulate molecular responses to acidification. In conclusion, exposure to predicted end-of-century ocean pCO₂ conditions may affect normal development of this species in nature during sensitive early life history stages with limited physiological response capacities, while extreme acidification will negatively influence embryonic survival and development under hatchery conditions.

Sediment toxicity assessment using zebrafish (Danio rerio) as a model system: Historical review, research gaps and trends

Sediment is an important compartment in aquatic environments and acts as a sink for environmental pollutants. Sediment toxicity tests have been suggested as critical components in environmental risk assessment. Since the zebrafish (Danio rerio) has been indicated as an emerging model system in ecotoxicological tests, a scientometric and systematic review was performed to evaluate the use of zebrafish as an experimental model system in sediment toxicity assessment. A total of 97 papers were systematically analyzed and summarized. The historical and geographical distributions were evaluated and the data concerning the experimental design, type of sediment toxicity tests and approach (predictive or retrospective), pollutants and stressors, zebrafish developmental stages and biomarkers responses were summarized and discussed. The use of zebrafish to assess the sediment toxicity started in 1996, using mainly a retrospective approach. After this, research showed an increasing trend, especially after 2014-2015. Zebrafish exposed to pollutant-bound sediments showed bioaccumulation and several toxic effects, such as molecular, biochemical, morphological, physiological and behavioral changes. Zebrafish is a suitable model system to assess the toxicity of freshwater, estuarine and marine sediments, and sediment spiked in the laboratory. The pollutant-bound sediment toxicity in zebrafish seems to be overall dependent on physical and chemical properties of pollutants, experimental design, environmental factor, developmental stages and presence of organic natural matter. Overall, results showed that the zebrafish embryos and larvae are suitable model systems to assess the sediment-associated pollutant toxicity.

Toxicity of gabapentin-lactam on the early developmental stage of zebrafish (Danio rerio)

Gabapentin-lactam (GBP-L) is a transformation product (TP) of gabapentin (GBP), a widely used anti-epileptic pharmaceutical. Due to its high persistence, GBP-L has been frequently detected in the surface water. However, the effects of GBP-L on aquatic organisms have not been thoroughly investigated. In the present study, zebrafish (Danio rerio) embryos as a model organism were used to study the impacts of GBP-L in terms of embryos LC₅₀, spontaneous movement at 24 hpf (hours post fertilization), heartbeat rates at 48 hpf, and body length at 72 hpf, with the concentrations of GBP-L down to 0.01 μg/L, covering its environmental concentrations. Various biomarkers from nervous, antioxidant and immune systems of zebrafish larvae were analyzed, including acetylcholinesterase, acetylcholine, dopamine, gamma-aminobutyric acid, superoxide dismutase, catalase, glutathione S-transferase, C reactive protein, and lysozyme, to assess its toxicity on these systems. RT-qPCR was then used to further verify the results and explain the toxicological mechanism at the gene level. The results demonstrated that GBP-L is much more toxic than its parent compound, and could lead to adverse impacts on the aquatic organisms even at every low concentrations.

Computational modeling and experimental validation of the EPI-X4/CXCR4 complex allows rational design of small peptide antagonists

EPI-X4, a 16-mer fragment of albumin, is a specific endogenous antagonist and inverse agonist of the CXC-motif-chemokine receptor 4 (CXCR4) and thus a key regulator of CXCR4 function. Accordingly, activity-optimized synthetic derivatives of EPI-X4 are promising leads for the therapy of CXCR4-linked disorders such as cancer or inflammatory diseases. We investigated the binding of EPI-X4 to CXCR4, which so far remained unclear, by means of biomolecular simulations combined with experimental mutagenesis and activity studies. We found that EPI-X4 interacts through its N-terminal residues with CXCR4 and identified its key interaction motifs, explaining receptor antagonization. Using this model, we developed shortened EPI-X4 derivatives (7-mers) with optimized receptor antagonizing properties as new leads for the development of CXCR4 inhibitors. Our work reveals the molecular details and mechanism by which the first endogenous peptide antagonist of CXCR4 interacts with its receptor and provides a foundation for the rational design of improved EPI-X4 derivatives.

Automated measurement of the spontaneous tail coiling of zebrafish embryos as a sensitive behavior endpoint using a workflow in KNIME

Neuroactive substances are the largest group of chemicals detected in European surface waters. Mixtures of neuroactive substances occurring at low concentrations can induce adverse neurological effects in humans and organisms in the environment. Therefore, there is a need to develop new screening tools to detect these chemicals. Measurement of behavior or motor effects in rodents and fish are usually performed to assess potential neurotoxicity for risk assessment. However, due to pain and stress inflicted on these animals, the scientific community is advocating for new alternative methods based on the 3R principle (reduce, replace and refine). As a result, the behavior measurement of early stages of zebrafish embryos such as locomotor response, photomotor response and spontaneous tail coiling are considered as a valid alternative to adult animal testing. In this study, we developed a workflow to investigate the spontaneous tail coiling (STC) of zebrafish embryos and to accurately measure the STC effect in the KNIME software. We validated the STC protocol with 3 substances (abamectin, chlorpyrifos-oxon and pyracostrobin) which have different mechanisms of action. The KNIME workflow combined with easy and cost-effective method of video acquisition makes this STC protocol a valuable method for neurotoxicity testing.•

Video acquisition duration of 60 s at 25 ± 1 hpf was used

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20 embryos exposed per dish and acclimatized for 30 min before video acquisition

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Capability to inspect and correct errors for high accuracy

Gran1: A Granulysin-Derived Peptide with Potent Activity against Intracellular Mycobacterium tuberculosis

Granulysin is an antimicrobial peptide (AMP) expressed by human T-lymphocytes and natural killer cells. Despite a remarkably broad antimicrobial spectrum, its implementation into clinical practice has been hampered by its large size and off-target effects. To circumvent these limitations, we synthesized a 29 amino acid fragment within the putative cytolytic site of Granulysin (termed “Gran1”). We evaluated the antimicrobial activity of Gran1 against the major human pathogen Mycobacterium tuberculosis (Mtb) and a panel of clinically relevant non-tuberculous mycobacteria which are notoriously difficult to treat. Gran1 efficiently inhibited the mycobacterial proliferation in the low micro molar range. Super-resolution fluorescence microscopy and scanning electron microscopy indicated that Gran1 interacts with the surface of Mtb, causing lethal distortions of the cell wall. Importantly, Gran1 showed no off-target effects (cytokine release, chemotaxis, cell death) in primary human cells or zebrafish embryos (cytotoxicity, developmental toxicity, neurotoxicity, cardiotoxicity). Gran1 was selectively internalized by macrophages, the major host cell of Mtb, and restricted the proliferation of the pathogen. Our results demonstrate that the hypothesis-driven design of AMPs is a powerful approach for the identification of small bioactive compounds with specific antimicrobial activity. Gran1 is a promising component for the design of AMP-containing nanoparticles with selective activity and favorable pharmacokinetics to be pushed forward into experimental in vivo models of infectious diseases, most notably tuberculosis.

Delivery by Dendritic Mesoporous Silica Nanoparticles Enhances the Antimicrobial Activity of a Napsin-Derived Peptide Against Intracellular Mycobacterium tuberculosis

Tuberculosis remains a serious global health problem causing 1.3 million deaths annually. The causative pathogen Mycobacterium tuberculosis (Mtb) has developed several mechanisms to evade the immune system and resistances to many conventional antibiotics, so that alternative treatment strategies are urgently needed. By isolation from bronchoalveolar lavage and peptide optimization, a new antimicrobial peptide named NapFab is discovered. While showing robust activity against extracellular Mtb, the activity of NapFab against intracellular bacteria is limited due to low intracellular availability. By loading NapFab onto dendritic mesoporous silica nanoparticles (DMSN) as a carrier system, cellular uptake, and consequently antimycobacterial activity against intracellular Mtb is significantly enhanced. Furthermore, using lattice light-sheet fluorescence microscopy, it can be shown that the peptide is gradually released from the DMSN inside living macrophages over time. By electron microscopy and tomography, it is demonstrated that peptide loaded DMSN are stored in vesicular structures in proximity to mycobacterial phagosomes inside the cells, but the nanoparticles are typically not in direct contact with the bacteria. Based on the combination of functional and live-cell imaging analyses, it is hypothesized that after being released from the DMSN NapFab is able to enter the bacterial phagosome and gain access to the bacilli.

Avermectin induced DNA damage to the apoptosis and autophagy in human lung epithelial A549 cells

Avermectin (AVM), as a biological insecticide, is widely used in agriculture and forestry production globally. However, inhalation of AVM may pose a risk, and the lung is the direct target, but the cytotoxicity of AVM on human lung cells is still unclear. Here, we attempted to elucidate the cytotoxic effect and molecular mechanism of AVM on human lung A549 cells. The results indicated that AVM inhibits cell proliferation, and enhances programmed cell death (apoptosis and autophagy). In addition, we found the AVM-treated cells showed an obvious drop in mitochondrial membrane potential and LC3-I/II, increased ROS production, DNA double-strand breaks, caspase-3/9 activated, PARP cleaved, cytochrome c and Bax/Bcl-2 content rise. The results showed that AVM induced mitochondria-related apoptosis and autophagy in lung A549 cells. These results indicate that AVM can pose a potential threat to human health by inducing DNA damage and programmed cell death.

Toward High-Throughput Fish Embryo Toxicity Tests in Aquatic Toxicology

Addressing the shift from classical animal testing to high-throughput in vitro and/or simplified in vivo proxy models has been defined as one of the upcoming challenges in aquatic toxicology. In this regard, the fish embryo toxicity test (FET) has gained significant popularity and wide standardization as one of the sensitive alternative approaches to acute fish toxicity tests in chemical risk assessment and water quality evaluation. Nevertheless, despite the growing regulatory acceptance, the actual manipulation, dispensing, and analysis of living fish embryos remains very labor intensive. Moreover, the FET is commonly performed in plastic multiwell plates under static or semistatic conditions, potentially inadequate for toxicity assessment of some organic, easily degradable or highly adsorptive toxicants. Recent technological advances in the field of mechatronics, fluidics and digital vision systems demonstrate promising future opportunities for automation of many analytical stages in embryo toxicity testing. In this review, we highlight emerging advances in fluidic and laboratory automation systems that can prospectively enable high-throughput FET testing (HT-FET) akin to pipelines commonly found in in vitro drug discovery pipelines. We also outline the existing challenges, barriers to future development and provide an outlook of ground-breaking fluidic technologies in embryo toxicity testing.

Rapid Zebrafish Behavioral Profiling Assay Accelerates the Identification of Environmental Neurodevelopmental Toxicants

Rapid and cost-effective in vivo assays to screen potential environmental neurodevelopmental toxicants are necessary to address the limitations of in vitro platforms, such as the inability to fully recapitulate the developmental and physiological processes of whole organisms. In the present study, a rapid zebrafish behavioral profiling assay was developed to characterize the neurodevelopmental effects of environmental substances by quantitatively evaluating multiple spontaneous movement features of zebrafish embryos. This video analysis-based assay automatically segmented every embryo and thus was able to accurately quantify spontaneous movement features, including frequency, duration, intensity, interval, and the number of continuous movements. When tested with eight environmental substances known to be neurodevelopmental toxicants, such as chlorpyrifos and bisphenol A, the assay successfully captured frequency alterations that were well-documented in previous studies while also providing additional information. Using an optimized procedure, we further assessed 132 potential neurotoxins that spanned a wide range of molecular targets, many of which were previously detected in environmental waterbodies. The distinct altered behavioral barcodes indicated that the spontaneous movement was impacted by diverse neuroactive substances, and the effects could be effectively evaluated with the developed assay. The web-based tool, named EMAnalysis, is further provided at http://www.envh.sjtu.edu.cn/zebrafish_contraction.jsp. Thus, this assay provides an efficient platform to accelerate the pace of neurotoxic environmental contaminant discoveries.

Rare Earth Elements Lanthanum and Praseodymium Adversely Affect Neural and Cardiovascular Development in Zebrafish (Danio rerio)

Increasing rare earth element (REE) mining and refining activities have led to a considerable release of these substances into aquatic environment, yet the knowledge of their impacts on aquatic organisms is still limited. Here, we explored the developmental effects of 16 REEs (concentration ranged from 0.46 to 1000 mg/L) to zebrafish embryos and highlighted the adverse effects of lanthanum (La) and praseodymium (Pr). Among the multiple developmental parameters measured, the significant effects on swimming behavior and cardiac physiology were the most prominent. Transcriptomic analysis of La and Pr at concentrations of 1.1 to 10 mg/L revealed their rather uniform effects at molecular levels. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis revealed that among others, notch, glutamate, and serotonin signaling, as well as cardiac hypertrophy and cardiac muscle contraction, were significantly affected. These changes of neural signaling were consistent with behavior effects observed and supported by neurotransmitter changes and thus provide a reasonable molecular mechanistic explanation. Furthermore, increased DNA damage and apoptotic activity at high concentrations were observed, especially in the heart. They may contribute to explain the observed adverse morphological and physiological outcomes, such as pericardial edema. The effect concentrations observed in the present study were comparable to the concentrations of REE residues at highly contaminated sites (several mg/L), indicating ecotoxicological effects at environmentally relevant concentrations. Overall, the present data help to clarify the potential developmental toxicity of REEs that was not yet fully recognized and thus contribute to their environmental risk assessment.

Unbiased Identification of Angiogenin as an Endogenous Antimicrobial Protein With Activity Against Virulent Mycobacterium tuberculosis

Tuberculosis is a highly prevalent infectious disease with more than 1.5 million fatalities each year. Antibiotic treatment is available, but intolerable side effects and an increasing rate of drug-resistant strains of Mycobacterium tuberculosis (Mtb) may hamper successful outcomes. Antimicrobial peptides (AMPs) offer an alternative strategy for treatment of infectious diseases in which conventional antibiotic treatment fails. Human serum is a rich resource for endogenous AMPs. Therefore, we screened a library generated from hemofiltrate for activity against Mtb. Taking this unbiased approach, we identified Angiogenin as the single compound in an active fraction. The antimicrobial activity of endogenous Angiogenin against extracellular Mtb could be reproduced by synthetic Angiogenin. Using computational analysis, we identified the hypothetical active site and optimized the lytic activity by amino acid exchanges. The resulting peptide-Angie1-limited the growth of extra- and intracellular Mtb and the fast-growing pathogens Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Toward our long-term goal of evaluating Angie1 for therapeutic efficacy in vivo, we demonstrate that the peptide can be efficiently delivered into human macrophages via liposomes and is not toxic for zebrafish embryos. Taken together, we define Angiogenin as a novel endogenous AMP and derive the small, bioactive fragment Angie1, which is ready to be tested for therapeutic activity in animal models of tuberculosis and infections with fast-growing bacterial pathogens.

Vape flavourants dull sensory perception and cause hyperactivity in developing zebrafish embryos

E-cigarette use (vaping) during pregnancy has been increasing, and the potential exists for the developing brain in utero to be exposed to chemical constituents in the vape. Vapes come in over 7000 unique flavours with and without nicotine, and while nicotine is a known neurotoxicant, the effects of vape flavouring alone, in the absence of nicotine, on brain function are not well understood. Here, we performed a screen of vape aerosol extracts (VAEs) to determine the potential for prenatal neurotoxicity using the zebrafish embryo photomotor response (PMR)-a translational biosensor of neurobehavioural effects. We screened three commonly used aerosolized vape liquids (flavoured and flavourless) either with or without nicotine. No neurobehavioural effects were detected in flavourless, nicotine-free VAEs, while the addition of nicotine to this VAE dulled sensory perception. Flavoured nicotine-free VAEs also dulled sensory perception and caused hyperactivity in zebrafish embryos. The combination of flavour and nicotine produced largely additive effects. Flavoured VAEs without nicotine had similar neuroactive potency to nicotine. Together, using zebrafish PMR as a high throughput translational behavioural model for prenatal exposure, our results demonstrate that e-cigarette flavourants that we screened elicit neurobehavioural effects worthy of further investigation for long-term neurotoxic potential and also have the potential to modulate nicotine impact on the developing brain.

Avermectin inhibits neutrophil extracellular traps release by activating PTEN demethylation to negatively regulate the PI3K-ERK pathway and reducing respiratory burst in carp

Excessive residual avermectin (AVM) in the environment can have toxic effects on non-target organisms. AVM can exert immunotoxicity by inducing genomic demethylation, but its effect on neutrophil extracellular traps (NETs) release in carp is unclear. In this study, carp neutrophils were pretreated with 5 μg/L AVM or 4 μM DNA demethylation inhibitor (aurintricarboxylic acid, ATA), alone or in combination, and then treated with 4 μM phorbol 12-myristate 13-acetate (PMA) to stimulate NETs release. The results showed that exposure of carp neutrophils to AVM significantly suppressed NETs release and MPO expression, increased ROS production, and dramatically reduced PMA-induced cellular respiratory burst. In addition, AVM could bind to the MBD2 molecule, markedly upregulate MBD2 expression to cause demethylation, and clearly activate PTEN expression, thereby inhibiting the expression of PI3K, AKT, Raf, MEK, and ERK. However, these effects were alleviated by ATA. In conclusion, our study showed that AVM could inhibit NETs release in carp by inducing demethylation of PTEN to negatively regulate NETs synthesis pathways and reducing respiratory burst level. Our findings clarify the mechanism of AVM immunotoxicity to fish and are of great significance for efforts to protect the ecological environment and human health.

Toxicity testing of pesticides in zebrafish-a systematic review on chemicals and associated toxicological endpoints

The use of zebrafish (Danio rerio) has arisen as a promising biological platform for toxicity testing of pesticides such as herbicides, insecticides, and fungicides. Therefore, it is relevant to assess the use of zebrafish in models of exposure to investigate the diversity of pesticide-associated toxicity endpoints which have been reported. Thus, this review aimed to assess the recent literature on the use of zebrafish in pesticide toxicity studies to capture data on the types of pesticide used, classes of pesticides, and zebrafish life stages associated with toxicity endpoints and phenotypic observations. A total of 352 articles published between September 2012 and May 2019 were curated. The results show an increased trend in the use of zebrafish for testing the toxicity of pesticides, with a great diversity of pesticides (203) and chemical classes (58) with different applications (41) being used. Furthermore, experimental outcomes could be clustered in 13 toxicity endpoints, mainly developmental toxicity, oxidative stress, and neurotoxicity. Organophosphorus, pyrethroid, azole, and triazine were the most studied classes of pesticides and associated with various toxicity endpoints. Studies frequently opted for early life stages (embryos and larvae). Although there is an evident lack of standardization of nomenclatures and phenotypic alterations, the information gathered here highlights associations between (classes of) pesticides and endpoints, which can be used to relate mechanisms of action specific to certain classes of chemicals.

Duchenne muscular dystrophy animal models for high-throughput drug discovery and precision medicine

Introduction: Duchenne muscular dystrophy (DMD) is an X-linked handicapping disease due to the loss of an essential muscle protein dystrophin. Dystrophin-null animals have been extensively used to study disease mechanisms and to develop experimental therapeutics. Despite decades of research, however, treatment options for DMD remain very limited.Areas covered: High-throughput high-content screening and precision medicine offer exciting new opportunities. Here, the authors review animal models that are suitable for these studies.Expert opinion: Nonmammalian models (worm, fruit fly, and zebrafish) are particularly attractive for cost-effective large-scale drug screening. Several promising lead compounds have been discovered using these models. Precision medicine for DMD aims at developing mutation-specific therapies such as exon-skipping and genome editing. To meet these needs, models with patient-like mutations have been established in different species. Models that harbor hotspot mutations are very attractive because the drugs developed in these models can bring mutation-specific therapies to a large population of patients. Humanized hDMD mice carry the entire human dystrophin gene in the mouse genome. Reagents developed in the hDMD mouse-based models are directly translatable to human patients.

Analysis of tail coiling activity of zebrafish (Danio rerio) embryos allows for the differentiation of neurotoxicants with different modes of action

In (eco)toxicology, there is a critical need for efficient methods to evaluate the neurotoxic potential of environmental chemicals. Recent studies proposed analysis of early coiling activity in zebrafish embryos as a powerful tool for the identification of neurotoxic compounds. In order to demonstrate that the analysis of early tail movements of zebrafish embryos allows for the discrimination of neurotoxicants acting via different mechanisms, the present study investigated the effects of four different neurotoxicants on the embryogenesis (fish embryo toxicity test) and early tail coiling movements of zebrafish embryos. Cadmium predominantly increased the frequency of tail coiling at the late pharyngula stage. Dichlorvos delayed embryonic development and caused convulsive tail movements resulting in prolonged duration of tail coils. Embryos exposed to teratogenic concentrations of fluoxetine and citalopram displayed absence of spontaneous tail movements at 24 h post-fertilization. In contrast, a non-teratogenic test concentration of citalopram decreased coiling frequency at multiple time points. Results demonstrated that the analysis of tail coiling movements of zebrafish embryos has the potential to discriminate neurotoxic compounds with different primary modes of action. In addition, chemical-induced effects on coiling activity were shown to potentially overlap with effects on embryogenesis. Further studies are needed to clarify the interplay of unspecific developmental toxicity of neurotoxic chemicals and effects resulting from specific neurotoxic mechanisms.

Screening for Developmental Neurotoxicity at the National Toxicology Program: The Future Is Here

The National Toxicology Program (NTP) receives requests to evaluate chemicals with potential to cause adverse health effects, including developmental neurotoxicity (DNT). Some recent requests have included classes of chemicals such as flame retardants, polycyclic aromatic compounds, perfluoroalkyl substances, and bisphenol A analogs with approximately 20-50 compounds per class, many of which include commercial mixtures. However, all the compounds within a class cannot be tested using traditional DNT animal testing guideline studies due to resource and time limitations. Hence, a rapid and biologically relevant screening approach is required to prioritize compounds for further in vivo testing. Because neurodevelopment is a complex process involving multiple distinct cellular processes, one assay will unlikely address the complexity. Hence, the NTP sought to characterize a battery of in vitro and alternative animal assays to quantify chemical effects on a variety of neurodevelopmental processes. A culmination of this effort resulted in a NTP-hosted collaborative project with approximately 40 participants spanning across domains of academia, industry, government, and regulatory agencies; collaborators presented data on cell-based assays and alternative animal models that was generated using a targeted set of compounds provided by the NTP. The NTP analyzed the assay results using benchmark concentration (BMC) modeling to be able to compare results across the divergent assays. The results were shared with the contributing researchers on a private web application during the workshop, and are now publicly available. This article highlights the overview and goals of the project, and describes the NTP's approach in creating the chemical library, development of NTPs data analysis strategy, and the structure of the web application. Finally, we discuss key issues with emphasis on the utility of this approach, and knowledge gaps that need to be addressed for its use in regulatory decision making.

Review of high-content screening applications in toxicology

High-content screening (HCS) technology combining automated microscopy and quantitative image analysis can address biological questions in academia and the pharmaceutical industry. Various HCS experimental applications have been utilized in the research field of in vitro toxicology. In this review, we describe several HCS application approaches used for studying the mechanism of compound toxicity, highlight some challenges faced in the toxicological community, and discuss the future directions of HCS in regards to new models, new reagents, data management, and informatics. Many specialized areas of toxicology including developmental toxicity, genotoxicity, developmental neurotoxicity/neurotoxicity, hepatotoxicity, cardiotoxicity, and nephrotoxicity will be examined. In addition, several newly developed cellular assay models including induced pluripotent stem cells (iPSCs), three-dimensional (3D) cell models, and tissues-on-a-chip will be discussed. New genome-editing technologies (e.g., CRISPR/Cas9), data analyzing tools for imaging, and coupling with high-content assays will be reviewed. Finally, the applications of machine learning to image processing will be explored. These new HCS approaches offer a huge step forward in dissecting biological processes, developing drugs, and making toxicology studies easier.

Time-course of coiling activity in zebrafish (Danio rerio) embryos exposed to ethanol as an endpoint for developmental neurotoxicity (DNT) - Hidden potential and underestimated challenges

Detection of developmental neurotoxicity (DNT) has been recognized as a major challenge by regulatory bodies and science. In search of sensitive and specific test methods, spontaneous tail coiling of embryonic zebrafish has been recommended as a promising tool for identification of DNT-inducing chemicals. The present study was designed to develop a protocol for a prolonged test to study neurotoxicity during the entire development of coiling movement in zebrafish embryos. Ambient illumination was found to modulate coiling activity from the very onset of tail movements representing the earliest behavioral response to light possible in zebrafish. In the dark, embryos displayed increased coiling activity in a way known from photokinesis, a stereotypical element of the visual motor response. Elevated coiling activity during dark phases allows for the development of test strategies that integrate later coiling movements under the control of a further developed nervous system. Furthermore, zebrafish embryos were exposed to ethanol, and coiling activity was analyzed according to the new test protocol. Exposure of embryos to non-teratogenic concentrations of ethanol (0.4-1%) resulted in a delay of the onset of coiling activity and heartbeat. Moreover, ethanol produced a dose-dependent increase in coiling frequency at 26 h post-fertilization, indicating the involvement of neurotoxic mechanisms. Analysis of coiling activity during prolonged exposure allowed for (1) attributing effects on coiling activity to different mechanisms and (2) preventing false interpretation of results. Further research is needed to verify the potential of this test protocol to distinguish between different mechanisms of neurotoxicity.

Underlying mechanisms of reproductive toxicity caused by multigenerational exposure of 2, bromo-4, 6-dinitroaniline (BDNA) to Zebrafish (Danio rerio) at environmental relevant levels

2-bromo-4, 6-dinitroaniline (BDNA) is a mutagenic aromatic amine involved in the production and degradation of Disperse blue 79, one of the most extensively used brominated azo dyes. In our previous study, a multigenerational exposure of BDNA (0.5, 5, 50 and 500 μg/L) to zebrafish from F₀ adult to F₂ larvae including a recovery group in F₂ larvae was conducted. The effects on apical points observed in individuals and the long-term effects predicted on population were all related to reproduction. In this study, we performed molecular analysis to elucidate the underlying mechanisms of the reproductive toxicity of BDNA. In F₁ generation, measurement of vitellogenin and transcription levels of genes associated with hypothalamus-pituitary-gland (HPG) axis, estrogen receptor (ER) and androgen receptor (AR) were conducted. There was a decrease in VTG level in the blood of F₁ female fish and transcription of genes related to ER was more affected than that of genes related to AR. These results were consistent with adverse effects that sexual differentiation was biased towards males and fecundity was impaired in a concentration-dependent manner in adults of F₁ generation after 150 days exposure. In F₂ generation, global gene transcriptions of F₂ larvae were investigated. It was uncovered that processes related to apoptosis, development and DNA damage were strongly affected. Alterations to these biological pathways accounted for the irreversible parental influence on a significant decrease in hatchability and increase in abnormality of F₂ larvae. All evidence suggested that the multigenerational exposure of BDNA posed lasting effects transmitted from parents to offspring that persisted after exposure ceased.

Doing the locomotion: Insights and potential pitfalls associated with using locomotor activity as a readout of the circadian rhythm in larval zebrafish

BACKGROUND

Zebrafish have been used as a model to study circadian rhythms (CRs) for over 20 years by analyzing various endpoints including locomotor activity. Such studies often utilize high-throughput analysis monitoring activity of larvae placed in well plates numbering >48 wells per plate. Although the CR can be influenced by numerous factors, it is not clear if such effects are permanent. Here, we investigated the variability of CRs of larvae analyzed in different types of well plates and determined the permanency of experimentally-induced aberrations in CRs.

NEW METHOD

Utilized the tracking software Ethovision XT to investigate how different well plate sizes influence the CR. Re-tested subjects for recovery from long-term CR disruptions and evaluated CR patterns at the individual level.

RESULTS

CR tracking using locomotion as a readout is best in 24 well plates. CR consistency is not maintained in larvae tracked in 48 or 96 well plates. A perturbed CR due to constant light recovered after just 3 days of a normal light/dark cycle.

COMPARISON WITH EXISTING METHODS

Unlike other CR locomotor-based assays, our approach allowed for a medium-throughput analysis of individual CRs, minimized variability and allowed for the re-evaluation of larval CRs 4-5 days later.

CONCLUSIONS

This medium-throughput locomotor CR analysis allows for a standardized, less variable approach whereby larvae can be re-tested to identify potential long-term changes after experimental manipulations. Long-term behavioral experiments in 48 or 96 well plates may impart stress on the larvae due to space constraints which could impact nervous system function and/or behavior.

Additives migrating from 3D-printed plastic induce developmental toxicity and neuro-behavioural alterations in early life zebrafish (Danio rerio)

The environmental impact of exposure to 3D-printed plastics as well as potential migration of toxic chemicals from 3D-printed plastics remains largely unexplored. In this work we applied leachates from plastics fabricated using a stereolithography (SLA) process to early developmental stages of zebrafish (Danio rerio) to investigate developmental toxicity and neurotoxicity. Migration of unpolymerized photoinitiator, 1-hydroxycyclohexyl phenyl ketone (1-HCHPK) from a plastic solid phase to aqueous media at up to 200 mg/L in the first 24 h was detected using gas chromatography-mass spectrometry. Both plastic extracts (LC₅₀ 22.25% v/v) and 1-HCHPK (LC₅₀ 60 mg/L) induced mortality and teratogenicity within 48 h of exposure. Developmental toxicity correlated with in situ generation of reactive oxygen species (ROS), an increase in lipid peroxidation and protein carbonylation markers and enhanced activity of superoxide dismutase (SOD) and glutathione-S-transferase (GST) in embryos exposed to concentrations as low as 20% v/v for plastic extracts and 16 mg/L for 1-HCHPK. ROS-induced cellular damage led to induction of caspase-dependent apoptosis which could be pharmacologically inhibited with both antioxidant ascorbic acid and a pan-caspase inhibitor. Neuro-behavioral analysis showed that exposure to plastic leachates reduced spontaneous embryonic movement in 24-36 hpf embryos. Plastic extracts in concentrations above 20% v/v induced rapid retardation of locomotion, changes in photomotor response and habituation to photic stimuli with progressive paralysis in 120 hpf larvae. Significantly decreased acetylcholinesterase (AChE) activity with lack of any CNS-specific apoptotic phenotypes as well as lack of changes in motor neuron density, axonal growth, muscle segment integrity or presence of myoseptal defects were detected upon exposure to plastic extracts during embryogenesis. Considering implications of the results for environmental risk assessment and the growing usage of 3D-printing technologies, we speculate that some 3D-printed plastic waste may represent a significant and yet very poorly uncharacterized environmental hazard that merits further investigation on a range of aquatic and terrestrial species.

Identifying sublethal endpoints for evaluating neurotoxic compounds utilizing the fish embryo toxicity test

Fish embryos are increasingly being utilized in aquatic toxicity testing, as evidenced by the Organisation for Economic Co-operation and Development's approval of the fish embryo acute toxicity (FET) test. However, the FET test only allows for the estimation of acute toxicity, whereas other test methods such as the larval growth and survival (LGS) test allow for the estimation of both acute and chronic toxicity. Additionally, it has been demonstrated that the FET test is less sensitive than other test methods for some neurotoxic compounds. To address these limitations, efforts to identify sublethal endpoints that increase FET test sensitivity and allow for the prediction of sublethal adverse effects have begun. As such, the objectives of the current study were 1) to compare estimated LC50 values from the FET and LGS test for three known neurotoxicants: fluoride (F), nickel (Ni), and cadmium (Cd) and 2) to evaluate the responsiveness of potential sublethal endpoints for the FET test related to growth (i.e., wet weight and snout-vent length), neurological development (i.e., spontaneous contraction frequency and eye size), and cardiovascular function (i.e., heart rate and pericardial area). The calculated LC50 values from the F and Cd FET test were significantly higher than those from the LGS test, demonstrating that the FET test is less sensitive than the LGS test for neurotoxic compounds. Only Cd exposure resulted in alterations in any of the sublethal endpoints investigated. Embryos/eleutheroembryos exposed to Cd displayed alterations in length, eye size, and pericardial area at concentrations five-fold less than the estimated LC50 value, suggesting that for Cd the inclusion of these sublethal endpoints would improve the sensitivity of the FET test. Overall, these results provide evidence that for some neurotoxicants, the inclusion of sublehtal endpoints may improve the utility of the FET test; however, further research utilizing a broader range of neurotoxicants with differing mechanisms of action, is needed to fully establish such endpoints in the context of routine FET test.

Lethal and sublethal toxicity of abamectin and difenoconazole (individually and in mixture) to early life stages of zebrafish

In recent years, the need for the development of alternative test methods for the conventional acute fish toxicity test (AFT) with adult fish has often been discussed. In addition, concerns have been raised on the potential risks related with environmentally realistic pesticide mixtures since risk evaluations have traditionally been based on individual pesticides. The insecticide/acaricide abamectin and the fungicide difenoconazole are the main pesticides that are intensively used in Brazilian strawberry crop and are hence likely to occur simultaneously in edge-of-field waterbodies. The aim of the present study was therefore to evaluate the lethal and sublethal toxicity of single and mixture exposures of these pesticides to zebrafish early life stages (embryos and juveniles). By comparing the derived toxicity data of the individual compounds with that previously determined for zebrafish adults, the order of life stage sensitivity was juvenile > adult > embryo. The pesticide mixture revealed a dose-level dependent deviation of the independent action model, with antagonism at low dose levels and synergism at high dose levels. Sublethal parameters (especially those related with locomotion) were considerably more sensitive than lethality. Subsequently, the inclusion of sublethal parameters may greatly improve the sensitivity of FET tests and hence its suitability as a substitution of adult fish testing in risk assessment evaluations.

Design and validation of an ontology-driven animal-free testing strategy for developmental neurotoxicity testing

Developmental neurotoxicity entails one of the most complex areas in toxicology. Animal studies provide only limited information as to human relevance. A multitude of alternative models have been developed over the years, providing insights into mechanisms of action. We give an overview of fundamental processes in neural tube formation, brain development and neural specification, aiming at illustrating complexity rather than comprehensiveness. We also give a flavor of the wealth of alternative methods in this area. Given the impressive progress in mechanistic knowledge of human biology and toxicology, the time is right for a conceptual approach for designing testing strategies that cover the integral mechanistic landscape of developmental neurotoxicity. The ontology approach provides a framework for defining this landscape, upon which an integral in silico model for predicting toxicity can be built. It subsequently directs the selection of in vitro assays for rate-limiting events in the biological network, to feed parameter tuning in the model, leading to prediction of the toxicological outcome. Validation of such models requires primary attention to coverage of the biological domain, rather than classical predictive value of individual tests. Proofs of concept for such an approach are already available. The challenge is in mining modern biology, toxicology and chemical information to feed intelligent designs, which will define testing strategies for neurodevelopmental toxicity testing.

Assessing the ecotoxicity of potentially neurotoxic substances - Evaluation of a behavioural parameter in the embryogenesis of Danio rerio

The pollution of the aquatic environment is currently characterised by a large number of contaminants, especially by mixtures of micro-pollutants including neurotoxins. The ecotoxicological consequences of this burden are not yet assessable. Within the present study, a new test method was applied which evaluates behavioural changes in zebrafish (Danio rerio) embryos to quantify the neurotoxic effect of selected chemicals. Changes in the frequency of spontaneous tail movements - a parameter of locomotion - occurring during embryogenesis was assessed as parameter for neurotoxic effects. Embryos were exposed in 24-microwell plates to neurotoxic compounds. Behaviour was examined after 24 h of exposure by videotaping and quantifying spontaneous locomotion. Additionally, acute toxicity was determined after 48 h of exposure by utilising the fish embryo toxicity test. Abamectin, emamectin benzoate, chlorpyrifos-oxon and carbamazepine were analysed using both: the new and classic acute test methods. The results showed the neurotoxic effect of the substances. Furthermore, an increased sensitivity compared to acute toxicity data was shown. The aim of the present study, to illustrate the sensitivity and specificity of the established endpoint to reliably assess adverse, neurotoxic effects of compounds on the model organism Danio rerio and to apply a new test method was achieved. Therefore, the present study is a substantial contribution to an effect-based risk assessment of contaminants in aquatic ecosystems.

Behavioral screening of the LOPAC1280 library in zebrafish embryos

Spontaneous activity represents an early, primitive form of motor activity within zebrafish embryos, providing a potential readout for identification of neuroactive compounds. However, despite use as an endpoint in chemical screens around the world, the predictive power and limitations of assays relying on spontaneous activity remain unclear. Using an improved high-content screening assay that increased throughput from 384 to 3072 wells per week, we screened a well-characterized library of 1280 pharmacologically active compounds (LOPAC¹²⁸⁰) - 612 of which target neurotransmission - to identify which targets are detected using spontaneous activity as a readout. Results from this screen revealed that (1) 8% of the LOPAC¹²⁸⁰ library was biologically active; (2) spontaneous activity was affected by compounds spanning a broad array of targets; (3) only 4% of compounds targeting neurotransmission impacted spontaneous activity; and (4) hypoactivity was observed for 100% of hits detected, including those that exhibit opposing mechanisms of action for the same target. Therefore, while this assay was able to rapidly identify potent neuroactive chemicals, these data suggest that spontaneous activity may lack the ability to discriminate modes of action for compounds interfering with neurotransmission, an issue that may be due to systemic uptake following waterborne exposure, persistent control variation, and/or interference with non-neurotransmission-related mechanisms.

The legacy pesticide dieldrin acts as a teratogen and alters the expression of dopamine transporter and dopamine receptor 2a in zebrafish (Danio rerio) embryos

Dieldrin (DLD) is a lipophilic pesticide that shows environmental persistence. The objectives were to determine the effects of DLD on GABAergic and dopaminergic systems in developing zebrafish. Both chorionated and dechorionated embryos (~24h post-hatch) were exposed to a single concentration of DLD (0.347-3470μM) for 48h. Following exposure, a subset of larvae was placed into clean water for 6days (i.e. depuration phase). Chorionated embryos showed <15% mortality while dechorionated embryos showed higher mortality (>30%), suggesting that the chorion protected the embryos. Over a 6day depuration phase, there was a dose dependent effect observed in both the "dechorionated and chorionated embryo" treatments for larval mortality (>60%). At the end of depuration, there was no detectable change in neuro-morphological endpoints that included the ratio of notochord length to body length (%) and the ratio of head area to body area (%). However, DLD did induce cardiac edema, skeletal deformities, and tremors. GABA-related transcripts were not affected in abundance by DLD. Conversely, the relative mRNA levels of dopamine transporter (dat1) and dopamine receptor drd2a mRNA were decreased in dechorionated, but not chorionated, embryos. These data suggest that DLD can alter the expression of transcripts related to dopaminergic signaling. Lastly, GABA_A receptor subunits gabrB1 and gabrB2, as well as dopamine receptors drd1 and drd2a, were inherently higher in abundance in dechorionated embryos compared to chorionated embryos. This is an important consideration when incorporating transcriptomics into embryo testing as expression levels can change with removal of the chorion prior to exposure.

ZEBRAFISH AS AN IN VIVO MODEL FOR SUSTAINABLE CHEMICAL DESIGN

Heightened public awareness about the many thousands of chemicals in use and present as persistent contaminants in the environment has increased the demand for safer chemicals and more rigorous toxicity testing. There is a growing recognition that the use of traditional test models and empirical approaches is impractical for screening for toxicity the many thousands of chemicals in the environment and the hundreds of new chemistries introduced each year. These realities coupled with the green chemistry movement have prompted efforts to implement more predictive-based approaches to evaluate chemical toxicity early in product development. While used for many years in environmental toxicology and biomedicine, zebrafish use has accelerated more recently in genetic toxicology, high throughput screening (HTS), and behavioral testing. This review describes major advances in these testing methods that have positioned the zebrafish as a highly applicable model in chemical safety evaluations and sustainable chemistry efforts. Many toxic responses have been shown to be shared among fish and mammals owing to their generally well-conserved development, cellular networks, and organ systems. These shared responses have been observed for chemicals that impair endocrine functioning, development, and reproduction, as well as those that elicit cardiotoxicity and carcinogenicity, among other diseases. HTS technologies with zebrafish enable screening large chemical libraries for bioactivity that provide opportunities for testing early in product development. A compelling attribute of the zebrafish centers on being able to characterize toxicity mechanisms across multiple levels of biological organization from the genome to receptor interactions and cellular processes leading to phenotypic changes such as developmental malformations. Finally, there is a growing recognition of the links between human and wildlife health and the need for approaches that allow for assessment of real world multi-chemical exposures. The zebrafish is poised to be an important model in bridging these two conventionally separate areas of toxicology and characterizing the biological effects of chemical mixtures that could augment its role in sustainable chemistry.

Assessment of the developmental and neurotoxicity of the mosquito control larvicide, pyriproxyfen, using embryonic zebrafish

In 2014, as an attempt to address the Zika health crisis by controlling the mosquito population, Brazil took the unprecedented action of applying a chemical larvicide, pyriproxyfen, to drinking water sources. The World Health Organization has established an acceptable daily intake of pyriproxyfen to be 100 μg per kg of body weight per day, but studies have demonstrated that at elevated doses (>5000 mg/kg), there are adverse effects in mice, rats and dogs. To better understand the potential developmental toxicity of pyriproxyfen, we utilized the embryonic zebrafish. Our results demonstrate that the concentration resulting in 50% of animals presenting adverse morphological effects (EC50), including craniofacial defects, was 5.2 μM for daily renewal exposure, and above this concentration, adverse behavioral effects were also observed in animals that followed a static exposure regimen. Thus, zebrafish data suggest that the developmental toxicity of pyriproxyfen may not be limited to insects.

Multilevel assessment of ivermectin effects using different zebrafish life stages

Several studies have shown high toxicity of the veterinary pharmaceutical ivermectin (a semisynthetic avermectin) for aquatic invertebrates however, few data is found for fish species. The present study evaluated the toxicity of ivermectin, to embryos, juveniles, and adults of zebrafish at different levels of biological organization including developmental, behavioural and biochemical. Toxicity tests were performed based on OECD protocols and mortality and behavioural changes were assed for all stages. Biochemical responses were assessed in adults and embryos and included cholinesterases (ChEs), catalase (CAT) (only in embryos), glutathione-S-Transferase (GST), lactate dehydrogenase (LDH) and vitellogenin (VTG) like proteins (only in embryos). Genotoxicity was evaluated in adults. Results showed a higher sensitivity of juvenile and adults of zebrafish (96h-LC10 values of 14.0 and 55.4μg/L, respectively). For embryos a 96h-LC10 of 147.1μg/L was calculated, moreover developmental anomalies and hatching inhibition were observed only at high concentrations (>400μg/L), whereas biochemical and behavioural responses occurred at lower concentrations (<60μg/L). Behavioural responses (lethargy) occurred in all life stages. Biochemical responses were observed including the inhibition of GST in adults and changes in ChE, CAT, LDH activities and VTG levels in embryos. Ivermectin did not show to be genotoxic for adult fish. The species sensitivity distribution analysis, based on fish and invertebrate species, indicated a Hazardous Concentration for 5% of the population (HC5) value of 0.057μg/L; suggesting high sensitivity of both groups to ivermectin and a high risk of this compound to aquatic ecosystems.

Advancing toxicology research using in vivo high throughput toxicology with small fish models

Small freshwater fish models, especially zebrafish, offer advantages over traditional rodent models, including low maintenance and husbandry costs, high fecundity, genetic diversity, physiology similar to that of traditional biomedical models, and reduced animal welfare concerns. The Collaborative Workshop on Aquatic Models and 21st Century Toxicology was held at North Carolina State University on May 5-6, 2014, in Raleigh, North Carolina, USA. Participants discussed the ways in which small fish are being used as models to screen toxicants and understand mechanisms of toxicity. Workshop participants agreed that the lack of standardized protocols is an impediment to broader acceptance of these models, whereas development of standardized protocols, validation, and subsequent regulatory acceptance would facilitate greater usage. Given the advantages and increasing application of small fish models, there was widespread interest in follow-up workshops to review and discuss developments in their use. In this article, we summarize the recommendations formulated by workshop participants to enhance the utility of small fish species in toxicology studies, as well as many of the advances in the field of toxicology that resulted from using small fish species, including advances in developmental toxicology, cardiovascular toxicology, neurotoxicology, and immunotoxicology. We alsoreview many emerging issues that will benefit from using small fish species, especially zebrafish, and new technologies that will enable using these organisms to yield results unprecedented in their information content to better understand how toxicants affect development and health.

Developmental Effects of the ToxCast™ Phase I and Phase II Chemicals in Caenorhabditis elegans and Corresponding Responses in Zebrafish, Rats, and Rabbits

BACKGROUND

Modern toxicology is shifting from an observational to a mechanistic science. As part of this shift, high-throughput toxicity assays are being developed using alternative, nonmammalian species to prioritize chemicals and develop prediction models of human toxicity.

METHODS

The nematode Caenorhabditis elegans (C. elegans) was used to screen the U.S. Environmental Protection Agency's (EPA's) ToxCast™ Phase I and Phase II libraries, which contain 292 and 676 chemicals, respectively, for chemicals leading to decreased larval development and growth. Chemical toxicity was evaluated using three parameters: a biologically defined effect size threshold, half-maximal activity concentration (AC50), and lowest effective concentration (LEC).

RESULTS

Across both the Phase I and Phase II libraries, 62% of the chemicals were classified as active ≤ 200 μM in the C. elegans assay. Chemical activities and potencies in C. elegans were compared with those from two zebrafish embryonic development toxicity studies and developmental toxicity data for rats and rabbits. Concordance of chemical activity was higher between C. elegans and one zebrafish assay across Phase I chemicals (79%) than with a second zebrafish assay (59%). Using C. elegans or zebrafish to predict rat or rabbit developmental toxicity resulted in balanced accuracies (the average value of the sensitivity and specificity for an assay) ranging from 45% to 53%, slightly lower than the concordance between rat and rabbit (58%).

CONCLUSIONS

Here, we present an assay that quantitatively and reliably describes the effects of chemical toxicants on C. elegans growth and development. We found significant overlap in the activity of chemicals in the ToxCast™ libraries between C. elegans and zebrafish developmental screens. Incorporating C. elegans toxicological assays as part of a battery of in vitro and in vivo assays provides additional information for the development of models to predict a chemical's potential toxicity to humans.

CITATION

Boyd WA, Smith MV, Co CA, Pirone JR, Rice JR, Shockley KR, Freedman JH. 2016. Developmental effects of the ToxCast™ Phase I and II chemicals in Caenorhabditis elegans and corresponding responses in zebrafish, rats, and rabbits. Environ Health Perspect 124:586-593; http://dx.doi.org/10.1289/ehp.1409645.