Chlorpyrifos-oxon disrupts zebrafish axonal growth and motor behavior

Identification of a Novel Inhibitor of Cimex lectularius Acetylcholinesterase

Acetylcholinesterase (AChE) stands as a primary target of commercial insecticides, notably organophosphates and carbamates. Despite their widespread use in agricultural and indoor pest control, concerns over their high toxicity and the emergence of resistance have restricted their efficacy. In this study, we conducted high-throughput virtual screening against both wild-type (WT) and resistant Cimex lectularius AChE utilizing a library encompassing 1 270 000 compounds. From this screening, we identified 100 candidate compounds and subsequently assessed their inhibitory effects on purified AChE enzymes. Among these candidates, AE027 emerged as a potent inhibitor against both WT and resistant AChE, exhibiting IC50 values of 10 and 43 μM, respectively. Moreover, the binding of AE027 significantly stabilized AChE, elevating its melting temperature by approximately 7 °C. Through molecular docking and molecular dynamics simulation, we delineated the binding mode of AE027, revealing its interaction with a site adjacent to the catalytic center, which is distinct from known inhibitors, with differing poses observed between WT and resistant AChE. Notably, the resistance mutation F348Y, positioned at a site directly interfacing with AE027, impedes ligand binding through steric hindrance. Furthermore, we evaluated the toxicity and pharmacokinetic properties of AE027 utilizing bioinformatics tools. These findings lay a crucial foundation for the development of a novel generation of insecticides that can combat both WT and resistant pest populations effectively and safely.

Effect of short-term exposure to the strobilurin fungicide dimoxystrobin: Morphofunctional, behavioural and mitochondrial alterations in Danio rerio embryos and larvae

Strobilurins, among the most used fungicides worldwide, are considered non-toxic to mammals and birds, but there is growing evidence that these compounds are highly toxic to aquatic species. Dimoxystrobin has been included in the 3rd Watch List of the European Commission, and it has been classified as very toxic to aquatic life. However, previous studies focused on acute toxicity and only two reports are available on its impact on fish, and none on its effects during the early life stages. Here, we evaluated for the first time the effects induced on zebrafish embryos and larvae by two dimoxystrobin sublethal concentrations (6.56 and 13.13 μg/L) falling in the range of predicted environmental concentrations. We demonstrated that short-term exposure to dimoxystrobin may exert adverse effects on multiple targets, inducing severe morphological alterations. Moreover, we showed enhanced mRNA levels of genes related to the mitochondrial respiratory chain and ATP production. Impairment of the swim bladder inflation has also been recorded, which may be related to the observed swimming performance alterations.

Network Toxicology and Molecular Docking to Investigate the Non-AChE Mechanisms of Organophosphate-Induced Neurodevelopmental Toxicity

Organophosphate pesticides (OPs) are toxic substances that contaminate aquatic environments, interfere with the development of the nervous system, and induce Neurodevelopmental Toxicity (NDT) in animals and humans. The canonical mechanism of OP neurotoxicity involves the inhibition of acetylcholinesterase (AChE), but other mechanisms non-AChE are also involved and not fully understood. We used network toxicology and molecular docking to identify molecular targets and toxicity mechanisms common to OPs. Targets related to diazinon-oxon, chlorpyrifos oxon, and paraoxon OPs were predicted using the Swiss Target Prediction and PharmMapper databases. Targets related to NDT were compiled from GeneCards and OMIM databases. In order to construct the protein-protein interaction (PPI) network, the common targets between OPs and NDT were imported into the STRING. Network topological analyses identified EGFR, MET, HSP90AA1, and SRC as hub nodes common to the three OPs. Using the Reactome pathway and gene ontology, we found that signal transduction, axon guidance, cellular responses to stress, and glutamatergic signaling activation play key roles in OP-induced NDT.

Different developmental insecticide exposure windows trigger distinct locomotor phenotypes in the early life stages of zebrafish

Due to their extensive use and high biological activity, insecticides largely contribute to loss of biodiversity and environmental pollution. The regulation of insecticides by authorities is mainly focused on lethal concentrations. However, sub-lethal effects such as alterations in behavior and neurodevelopment can significantly affect the fitness of individual fish and their population dynamics and therefore deserve consideration. Moreover, it is important to understand the impact of exposure timing during development, about which there is currently a lack of relevant knowledge. Here, we investigated whether there are periods during neurodevelopment of fish, which are particularly vulnerable to insecticide exposure. Therefore, we exposed zebrafish embryos to six different insecticides with cholinergic mode of action for 24 h during different periods of neurodevelopment and measured locomotor output using an age-matched behavior assay. We used the organophosphates diazinon and dimethoate, the carbamates pirimicarb and methomyl as well as the neonicotinoids thiacloprid and imidacloprid because they are abundant in the environment and cholinergic signaling plays a major role during key processes of neurodevelopment. We found that early embryonic motor behaviors, as measured by spontaneous tail coiling, increased upon exposure to most insecticides, while later movements, measured through touch-evoked response and a light-dark transition assay, rather decreased for the same insecticides and exposure duration. Moreover, the observed effects were more pronounced when exposure windows were temporally closer to the performing of the respective behavioral assay. However, the measured behavioral effects recovered after a short period, indicating that none of the exposure windows chosen here are particularly critical, but rather that insecticides acutely interfere with neuronal function at all stages as long as they are present. Overall, our results contribute to a better understanding of risks posed by cholinergic insecticides to fish and provide an important basis for the development of safe regulations to improve environmental health.

Environmentally relevant concentrations of organic (benzophenone-3) and inorganic (titanium dioxide nanoparticles) UV filters co-exposure induced neurodevelopmental toxicity in zebrafish

UV filters, widely used in personal care products, are ubiquitous environmental pollutants detected and pose a significant public health concern. Benzophenone-3 (BP3) and titanium dioxide nanoparticles (nano-TiO₂) are the predominant organic and inorganic UV filters in environmental media. However, few studies have explored the combined developmental neurotoxic (DNT) effects and the underlying mechanisms when co-exposed to BP3 and nano-TiO₂. In the present study, zebrafish (Danio rerio) embryos were exposed to environmentally relevant concentrations of BP3 (10 μg/L), nano-TiO₂ (100 μg/L), and mixtures starting from 6 h post fertilization (hpf), respectively. Developmental indicators and motor behaviors were investigated at various developmental stages. Our results showed that BP3 alone or co-exposed with nano-TiO₂ increased spontaneous movement at 24 hpf, co-exposure decreased touch response at 30 hpf and hatching rate at 60 hpf. Consistent with these motor deficits, co-exposure to BP3 and nano-TiO₂ inhibited relative axon length of primary motor neuron during the early developmental stages, disturbed the expression of axonal growth-related genes at 30 and 48 hpf, increased cell apoptosis on the head region and mRNA levels of apoptosis-related genes, and also increased reactive oxygen species (ROS) levels in zebrafish, suggesting the functional relevance of structural changes. Taken together, our findings demonstrated that BP3 alone or in combination with nano-TiO₂ at environmentally relevant concentrations induced evident neurotoxic effects on the developing embryos in zebrafish.

Bioactivation and detoxification of organophosphorus pesticides in freshwater planarians shares similarities with humans

Organophosphorus pesticides (OPs) are a chemically diverse class of insecticides that inhibit acetylcholinesterase (AChE). Many OPs require bioactivation to their active oxon form via cytochrome P450 to effectively inhibit AChE. OP toxicity can be mitigated by detoxification reactions performed by carboxylesterase and paraoxonase. The relative extent of bioactivation to detoxification varies among individuals and between species, leading to differential susceptibility to OP toxicity. Because of these species differences, it is imperative to characterize OP metabolism in model systems used to assess OP toxicity. We have shown that the asexual freshwater planarian Dugesia japonica is a suitable model to assess OP neurotoxicity and developmental neurotoxicity via rapid, automated testing of adult and developing organisms in parallel using morphological and behavioral endpoints. D. japonica has two cholinesterase enzymes with intermediate properties between AChE and butyrylcholinesterase that are sensitive to OP inhibition. Here, we demonstrate that D. japonica contains the major OP metabolic machinery to be a relevant model for OP neurotoxicity studies. Adult and regenerating D. japonica can bioactivate chlorpyrifos and diazinon into their respective oxons. Significant AChE inhibition was only observed after in vivo metabolic activation but not when the parent OPs were directly added to planarian homogenate using the same concentrations and timing. Using biochemical assays, we found that D. japonica has both carboxylesterase (24 nmol/(min*mg protein)) and paraoxonase (60 pmol/(min*mg protein)) activity. We show that planarian carboxylesterase activity is distinct from cholinesterase activity using benzil and tacrine. These results further support the use of D. japonica for OP toxicity studies.

Organophosphate Insecticide Toxicity in Neural Development, Cognition, Behaviour and Degeneration: Insights from Zebrafish

Organophosphate (OP) insecticides are used to eliminate agricultural threats posed by insects, through inhibition of the neurotransmitter acetylcholinesterase (AChE). These potent neurotoxins are extremely efficacious in insect elimination, and as such, are the preferred agricultural insecticides worldwide. Despite their efficacy, however, estimates indicate that only 0.1% of organophosphates reach their desired target. Moreover, multiple studies have shown that OP exposure in both humans and animals can lead to aberrations in embryonic development, defects in childhood neurocognition, and substantial contribution to neurodegenerative diseases such as Alzheimer's and Motor Neurone Disease. Here, we review the current state of knowledge pertaining to organophosphate exposure on both embryonic development and/or subsequent neurological consequences on behaviour, paying particular attention to data gleaned using an excellent animal model, the zebrafish (Danio rerio).

Differences in neurotoxic outcomes of organophosphorus pesticides revealed via multi-dimensional screening in adult and regenerating planarians

Organophosphorus pesticides (OPs) are a chemically diverse class of commonly used insecticides. Epidemiological studies suggest that low dose chronic prenatal and infant exposures can lead to life-long neurological damage and behavioral disorders. While inhibition of acetylcholinesterase (AChE) is the shared mechanism of acute OP neurotoxicity, OP-induced developmental neurotoxicity (DNT) can occur independently and/or in the absence of significant AChE inhibition, implying that OPs affect alternative targets. Moreover, different OPs can cause different adverse outcomes, suggesting that different OPs act through different mechanisms. These findings emphasize the importance of comparative studies of OP toxicity. Freshwater planarians are an invertebrate system that uniquely allows for automated, rapid and inexpensive testing of adult and developing organisms in parallel to differentiate neurotoxicity from DNT. Effects found only in regenerating planarians would be indicative of DNT, whereas shared effects may represent neurotoxicity. We leverage this unique feature of planarians to investigate potential differential effects of OPs on the adult and developing brain by performing a comparative screen to test 7 OPs (acephate, chlorpyrifos, dichlorvos, diazinon, malathion, parathion and profenofos) across 10 concentrations in quarter-log steps. Neurotoxicity was evaluated using a wide range of quantitative morphological and behavioral readouts. AChE activity was measured using an Ellman assay. The toxicological profiles of the 7 OPs differed across the OPs and between adult and regenerating planarians. Toxicological profiles were not correlated with levels of AChE inhibition. Twenty-two "mechanistic control compounds" known to target pathways suggested in the literature to be affected by OPs (cholinergic neurotransmission, serotonin neurotransmission, endocannabinoid system, cytoskeleton, adenyl cyclase and oxidative stress) and 2 negative controls were also screened. When compared with the mechanistic control compounds, the phenotypic profiles of the different OPs separated into distinct clusters. The phenotypic profiles of adult vs. regenerating planarians exposed to the OPs clustered differently, suggesting some developmental-specific mechanisms. These results further support findings in other systems that OPs cause different adverse outcomes in the (developing) brain and build the foundation for future comparative studies focused on delineating the mechanisms of OP neurotoxicity in planarians.

Decabromodiphenyl ethane induced hyperactivity in developing zebrafish at environmentally relevant concentrations

Decabromodiphenyl ethane (DBDPE), a widely used novel brominated flame retardant, is gaining concerns due to rapidly increased contents in various environmental and biota samples. In the present study, zebrafish (Danio rerio) embryos were exposed to 2.91, 9.71, 29.14 and 97.12 μg/L of DBDPE until 120 h post-fertilization (hpf) to investigate the potential developmental neurotoxicity and underlying mechanisms. Chemical analysis revealed concentration-dependently increased body burdens of DBDPE in zebrafish larvae, with bioaccumulation factors (BCFs) ranging from 414 to 726. Embryonic exposure to DBDPE caused hyperactivity without affecting the development of secondary motoneuron axons and muscle fibers. However, further results implicated that DBDPE may affect the locomotor regulatory network via different mechanisms at lower and higher concentrations. On the one hand, embryonic exposure to 2.91 μg/L DBDPE transiently promoted spontaneous coiling contractions, but showed no effects on touch-response and swimming activity in zebrafish larvae. The whole-body contents of neurotransmitters were significantly decreased. Significant decreased protein abundances of α1-TUBULIN and SYN2a and molecular docking results pointed out possible interactions of DBDPE with these two proteins. However, these changes may be unconcerned with the transient hyperactivity, and the exact molecular mechanisms need further investigation. On the other hand, 29.14 and 97.12 μg/L DBDPE exposure caused longer-lasting effects in promoting spontaneous coiling contractions, and also touch-response and swimming activity. At the same time, increased ACh contents (without changes of other neurotransmitters) and ChAT activity and inhibited transcription of nAChRs were observed at higher concentrations. Molecular docking indicated direct interaction of DBDPE with ChAT. The results suggested that DBDPE induced hyperactivity at higher concentrations was probably involved with disrupted cholinergic system, with ChAT as a potential target. Given that the body burden of DBDPE in lower concentration group was comparable with those detected in wild fish, the current results may provide useful information for ecological risk assessment.

Long-Term Exposure to SSRI Citalopram Induces Neurotoxic Effects in Zebrafish

Residual antidepressants are of increasing concern worldwide, yet critical information on their long-term neurotoxic impacts on nontarget aquatic animals is lacking. Here, we investigated the long-term effects (from 0 to 150 days postfertilization) of the selective serotonin reuptake inhibitor citalopram (0.1-100 μg/L) on motor function, learning, and memory in zebrafish over two generations and explored the reversibility of the effect in F1 larvae. Unlike F0⁺ larvae, we found that F1⁺ larvae displayed decreased sensorimotor performance when continuously exposed to citalopram at 100 μg/L. No adverse effects were found in F1^- larvae after they were transferred to a clean medium. Whole-mount immunofluorescence assays suggested that the motor impairments were related to axonal projections of the spinal motor neurons (MNs). For F0⁺ adults, long-term citalopram exposure mainly caused male-specific declines in motor, learning, and memory performance. Analysis of serotonergic and cholinergic MNs revealed no significant changes in the male zebrafish spinal cord. In contrast, the number of glutamatergic spinal MNs decreased, likely associated with the impairment of motor function. Additionally, treatment with 100 μg/L citalopram significantly reduced the number of dopaminergic neurons, but no significant neuronal apoptosis was observed in the adult telencephalon. Overall, this study provides neurobehavioral evidence and novel insights into the neurotoxic mechanisms of long-term citalopram exposure and may facilitate the assessment of the environmental and health risks posed by citalopram-containing antidepressant drugs.

Transcriptomic profiling and differential analysis revealed the neurodevelopmental toxicity mechanisms of zebrafish (Danio rerio) larvae in response to tetrabromobisphenol A bis(2-hydroxyethyl) ether (TBBPA-DHEE) exposure

Tetrabromobisphenol A bis(2-hydroxyetyl) ether (TBBPA-DHEE) is among the main derivatives of Tetrabromobisphenol A (TBBPA). Result from previous study showed that TBBPA-DHEE can cause neurotoxicity in rat. In this study, zebrafish larvae were used for evaluation of TBBPA-DHEE-induced developmental toxicity, apoptosis, oxidative stress and the potential molecular mechanisms of action. Our result showed that TBBPA-DHEE exposure caused a significant concentration-dependent developmental toxicity endpoints like death rate, malformation rate, growth rate. TBBPA-DHEE altered locomotor and enzymes activities of larvae and caused apoptosis within the brain indicating the potential TBBPA-DHEE-induced cardiac, brain impairment in the zebrafish larvae. Our transcriptomic analysis shows that 691 genes were differentially expressed (DEGs) (539 upregulated, 152 downregulated). The KEGG and GO enrichment pathway analysis shows that the DEGs were involved in development, immunity, enzyme activity. Our study provides novel evidence on the neurodevelopmental toxicity and toxicity mechanism of TBBPA-DHEE which are vital for assessment of the environmental toxicity and risk assessment of the chemical.

Short- and Long-Term Effects of Chlorpyrifos on Thyroid Hormone Axis and Brain Development in Xenopus laevis

INTRODUCTION

The extensive use of the insecticide chlorpyrifos (CPF) throughout the world has brought increased scrutiny on its environmental and health impact. CPF is a cholinergic neurotoxicant; however, exposure to low noncholinergic doses is associated with numerous neurodevelopmental effects in animal models. In this study, we aimed to assess CPF for its potential to disrupt thyroid hormone signalling and investigate the short- and long-term effects on neurodevelopment by using Xenopus laevis.

METHODS

The thyroid hormone (TH) disrupting potential of CPF was assessed using TH-sensitive transgenic Tg(thibz:eGFP) tadpoles. The consequences of early embryonic exposure were examined by exposing fertilized eggs for 72 h to environmentally relevant CPF concentrations (10-10 M and 10-8 M). Three endpoints were evaluated: (1) gene expression in whole embryonic brains immediately after exposure, (2) mobility and brain morphology 1 week after exposure, and (3) brain morphology and axon diameters at the end of metamorphosis (2 months after the exposure).

RESULTS

CPF disrupted TH signalling in Tg(thibz:eGFP) tadpoles. The expression of genes klf9, cntn4, oatp1c1, and tubb2b was downregulated in response to CPF. Tadpoles exposed to CPF exhibited increased mobility and altered brain morphology compared to control tadpoles. Early embryonic exposure of CPF affected myelinated axon diameter, with exposed animals exhibiting shifted frequency distributions of myelinated axons diameters towards smaller diameters in the hindbrain of froglets.

DISCUSSION/CONCLUSION

This study provides more evidence of the endocrine and neurodevelopment disrupting activity of CPF. Further experimental and epidemiological studies are warranted to determine the long-term consequences of early CPF exposure on brain development.

A bioinformatics framework for targeted gene expression assay design: Application to in vitro developmental neurotoxicity screening in a rat model

Brain development involves a series of intricately choreographed neuronal differentiation and maturation steps that are acutely vulnerable to interferences from chemical exposures. Many genes involved in neurodevelopmental processes show evolutionarily conserved expression patterns in mammals and may constitute useful indicators/biomarkers for the evaluation of potential developmental neurotoxicity. Based on these premises, this study developed a bioinformatics framework to guide the design of a gene expression-based in vitro developmental neurotoxicity assay targeting evolutionary conserved genes associated with neuronal differentiation and maturation in rat cerebellar granule cells (CGCs). Rat, mouse and human genes involved in neurodevelopment and presenting one-to-one orthology were selected and orthologous exons within these genes were identified. PCR primer sets were designed within these orthologous exons and their specificity was evaluated in silico. The performance and specificity of rat, mouse and human PCR primer sets were then confirmed experimentally. Finally, RT-qPCR analyses in CGCs exposed in vitro to well-known neurotoxicants (Chlorpyrifos and Chlorpyrifos oxon) uncovered perturbations of expression levels for most of the selected genes. This bioinformatics framework for gene and target sequence selection may facilitate the identification of transcriptional biomarkers for developmental neurotoxicity assays and the comparison of gene expression data across experimental models from different mammalian species.

Antagonistic effects and mechanisms of carbendazim and chlorpyrifos on the neurobehavior of larval zebrafish

Residues from multiple pesticides are frequently detected on vegetables, which may produce combined toxicity not predicted by individual toxicity data. As these combined effects present additional dangers to food safety, we have compared individual to combined effects for a variety of pesticides. Carbendazim and chlorpyrifos are the two most commonly detected pesticides in vegetables, and previous studies reported that combined exposure results in synergistic developmental toxicity to zebrafish embryos. In this study, individual and combined effects on zebrafish motor activity were examined following individual and combined exposure to assess nervous system toxicity. Further, transcriptomics methods were used to identify potential molecular mechanisms for individual and combined toxicity. Carbendazim alone induced a disorganized swim pattern characterized by increased angular velocity, turn angle, meander, and acceleration during light-dark transition, while chlorpyrifos alone reduced average swim speed and light-dark acceleration. Combined treatment significantly reduced average swim velocity and total distance traveled. Combination indices indicated strong antagonism between compounds for average speed and light-dark acceleration. Transcriptomics (RNA-seq) showed that carbendazim significantly altered the expression of genes involved in antigen processing and presentation, apoptosis, autophagy, and metabolism, including ctslb, cyp7a1, hsp70l, and ugt1a1. Alternatively, chlorpyrifos significantly altered genes involved in various nervous system-related pathways, including glutamatergic, GABAergic, dopaminergic, and calcium signaling. Protein-protein interaction (PPI) network analysis suggested that chlorpyrifos significantly downregulated genes related to light transduction, resulting in decreased sensitivity to light-dark transitions, while antagonism mainly reflected divergent effects on phototransduction and retinol metabolism. Carbendazim had no significant effects on vision-related genes such as gnat1 and gngt1, while chlorpyrifos downregulated expression, an effect reversed by the combination. Comprehensive toxicity analyses must include joint effects of co-applied pesticides for enhanced food safety.

Nano-TiO2 aggravates bioaccumulation and developmental neurotoxicity of triphenyl phosphate in zebrafish larvae

This study explored the combined effects of titanium dioxide nanoparticles (nano-TiO₂) and triphenyl phosphate (TPhP) on the neurodevelopment of zebrafish larvae as well as the underlying mechanisms. With this regard, zebrafish embryos were exposed to nano-TiO₂ of 100 μg·L^-1, TPhP of 0, 8, 24, 72, and 144 μg·L^-1, or their combinations until 120 h post-fertilization (hpf). Results indicated 100 μg·L^-1 nano-TiO₂ alone to be nontoxic to zebrafish larvae. However, obvious developmental toxicity manifested as inhibition of surviving rate, heart rate and body length as well as increased malformation was observed in the higher concentrations of TPhP (72 and 144 μg·L^-1) alone and the co-exposure groups. Additionally, results suggested that nano-TiO₂ significantly enhanced the bioaccumulation of TPhP in zebtafish larvae, and thus aggravated the abnormities of spontaneous movement and swimming behavior in zebrafish larvae induced by TPhP. Nano-TiO₂ also exacerbated the TPhP-induced inhibition of the axonal growth on the secondary motor neuron, and aggravated the TPhP-induced decrease on expressions of neuron-specific green fluorescent protein (GFP) and neuronal marker genes (ngn1 and elavl3). Further, the content of neurotransmitter serotonin was not altered by TPhP alone exposure, but was decreased significantly in the co-exposure group of 144 μg·L^-1 TPhP and nano-TiO₂. Our data indicated that nano-TiO₂ might aggravate the neuron abnormities and serotonin system dysfunction by enhancing the TPhP accumulation, leading to exacerbated abnormal locomotors in zebrafish larvae.

Pesticides applied to crops and amyotrophic lateral sclerosis risk in the U.S

BACKGROUND

Environmental exposures are implicated in the etiology of amyotrophic lateral sclerosis (ALS). Application of insecticides, herbicides, and fungicides with neurotoxic properties to crops is permitted in the U.S., however reporting of the quantities is government mandated.

OBJECTIVE

To identify pesticides that may be associated with ALS etiology for future study.

METHODS

We geospatially estimated exposure to crop-applied pesticides as risk factors for ALS in a large de-identified medical claims database, the SYMPHONY Integrated Dataverse®. We extracted residence at diagnosis of ∼26,000 nationally distributed ALS patients, and matched non-ALS controls. We mapped county-level U.S. Geological Survey data on applications of 423 pesticides to estimate local residential exposure. We randomly broke the SYMPHONY dataset into two groups to form independent discovery and validation cohorts, then confirmed top hits using residential history information from a study of NH, VT, and OH.

RESULTS

Pesticides with the largest positive statistically significant associations in both the discovery and the validation studies and evidence of neurotoxicity in the literature were the herbicides 2,4-D (OR 1.25 95 % CI 1.17-1.34) and glyphosate (OR 1.29 95 %CI 1.19-1.39), and the insecticides carbaryl (OR 1.32 95 %CI 1.23-1.42) and chlorpyrifos (OR 1.25 95 %CI 1.17-1.33).

SIGNIFICANCE

Our geospatial analysis results support potential neurotoxic pesticide exposures as risk factors for sporadic ALS. Focused studies to assess these identified potential relationships are warranted.

Multi Species Analyses Reveal Testicular T3 Metabolism and Signalling as a Target of Environmental Pesticides

Thyroid hormones (THs) regulate many biological processes in vertebrates, including reproduction. Testicular somatic and germ cells are equipped with the arrays of enzymes (deiodinases), transporters, and receptors necessary to locally maintain the optimal level of THs and their signalling, needed for their functions and spermatogenesis. Pesticides, as chlorpyrifos (CPF) and ethylene thiourea (ETU), impair the function of thyroid and testis, affecting male fertility. However, their ability to disarrange testicular T3 (t-T3) metabolism and signalling is poorly considered. Here, a multi-species analysis involving zebrafish and mouse suggests the damage of t-T3 metabolism and signalling as a mechanism of gonadic toxicity of low-doses CPF and ETU. Indeed, the developmental exposure to both compounds reduces Dio2 transcript in both models, as well as in ex-vivo cultures of murine seminiferous tubules, and it is linked to alteration of steroidogenesis and germ cell differentiation. A major impact on spermatogonia was confirmed molecularly by the expression of their markers and morphologically evidenced in zebrafish. The results reveal that in the adopted models, exposure to both pesticides alters the t-T3 metabolism and signalling, affecting the reproductive capability. Our data, together with previous reports suggest zebrafish as an evaluable model in assessing the action of compounds impairing locally T3 signalling.

Androgenic activation, impairment of the monoaminergic system and altered behavior in zebrafish larvae exposed to environmental concentrations of fenitrothion

Fenitrothion is an organophosphorus insecticide usually found in aquatic ecosystems at concentrations in the range of low ng/L. In this manuscript we show that 24 h exposure to environmental concentrations of fenitrothion, from ng/L to low μg/L, altered basal locomotor activity, visual-motor response and acoustic/vibrational escape response of zebrafish larvae. Furthermore, fenitrothion and expression of gap43a, gfap, atp2b1a, and mbp exhibited a significant non-monotonic concentration-response relationship. Once determined that environmental concentrations of fenitrothion were neurotoxic for zebrafish larvae, a computational analysis identified potential protein targets of this compound. Some of the predictions, including interactions with acetylcholinesterase, monoamine-oxidases and androgen receptor (AR), were experimentally validated. Binding to AR was the most suitable candidate for molecular initiating event, as indicated by both the up-regulation of cyp19a1b and sult2st3 and the non-monotonic relationship found between fenitrothion and the observed responses. Finally, when the integrity of the monoaminergic system was evaluated, altered levels of L-DOPA, DOPAC, HVA and 5-HIAA were found, as well as a significant up-regulation of slc18a2 expression at the lowest concentrations of fenitrothion. These data strongly suggest that concentrations of fenitrothion commonly found in aquatic ecosystems present a significant environmental risk for fish communities.

In vitro biolayer interferometry analysis of acetylcholinesterase as a potential target of aryl-organophosphorus flame-retardants

Organophosphorus flame retardants (OPFRs) have been implicated as neurotoxicants, but their potential neurotoxicity and mechanisms remain poorly understood. Herein, we investigated the neurotoxicity of selected OPFRs using zebrafish as a model organism. Environmentally relevant concentrations (3-1500 nM) of three classes of OPFRs (aryl-OPFRs, chlorinated-OPFRs, and alkyl-OPFRs) were tested in zebrafish larvae (2-144 h post-fertilisation) alongside the neurotoxic chemical chlorpyrifos (CPF) that inhibits acetylcholinesterase (AChE). Exposure to aryl-OPFRs and CPF inhibited AChE activities, while chlorinated- and alkyl-OPFRs did not inhibit these enzymes. Biolayer interferometry (BLI) was used to probe interactions between OPFRs and AChE. The association and dissociation response curves showed that, like CPF, all three selected aryl-OPFRs, triphenyl phosphate (TPHP), tricresyl phosphate (TCP) and cresyl diphenyl phosphate (CDP), bound directly to AChE. The affinity constant (K_D) for TPHP, TCP, CDP and CPF was 2.18 × 10^-4, 5.47 × 10^-5, 1.05 × 10^-4 and 1.70 × 10^-5 M, respectively. In addition, molecular docking revealed that TPHP, TCP, CDP and CPF bound to AChE with glide scores of - 7.8, - 8.3, - 8.1 and - 7.3, respectively. Furthermore, the calculated binding affinity between OPFRs and AChE correlated well with the K_D values measured by BLI. The present study revealed that aryl-OPFRs can act as potent AChE inhibitors, and may therefore present a significant ecological risk to aquatic organisms.

In Vivo Effects of Neonicotinoid-Sulfoximine Insecticide Sulfoxaflor on Acetylcholinesterase Activity in the Tissues of Zebrafish (Danio rerio)

Sulfoxaflor is the first member of the neonicotinoid-sulfoximine insecticides that acts as an agonist of nicotinic acetylcholine receptors (nAChRs). This study investigated the acute effects of sulfoxaflor on acetylcholinesterase (AChE; EC 3.1.1.7) enzyme activity in the brain and muscle tissues of zebrafish (Danio rerio) as a model organism. The zebrafish were exposed to 0.87 mg/L (2.5% of 96 h 50% lethal concentration (LC₅₀), 1.75 mg/L (5% of 96 h LC₅₀) and 3.51 mg/L (10% of 96 h LC₅₀) of sulfoxaflor for 24 h-48 h and 96 h periods. AChE enzyme activities were analysed by a spectrophotometric method in the brain and muscle tissues. The results of this study showed that in vivo acute sulfoxaflor exposure significantly increased AChE enzyme activity in the brain and muscle tissues of zebrafish. The induction percentages of AChE were between 10 and 83%, and 19 and 79% for brain and muscle tissues, respectively. As a result, it was found that sulfoxaflor had an effect on AChE enzyme activity in the two main tissues containing this enzyme, and it can be considered as a potential neuroactive compound for zebrafish.

Approaches to Test the Neurotoxicity of Environmental Contaminants in the Zebrafish Model: From Behavior to Molecular Mechanisms

The occurrence of neuroactive chemicals in the aquatic environment is on the rise and poses a potential threat to aquatic biota of currently unpredictable outcome. In particular, subtle changes caused by these chemicals to an organism's sensation or behavior are difficult to tackle with current test systems that focus on rodents or with in vitro test systems that omit whole-animal responses. In recent years, the zebrafish (Danio rerio) has become a popular model organism for toxicological studies and testing strategies, such as the standardized use of zebrafish early life stages in the Organisation for Economic Co-operation and Development's guideline 236. In terms of neurotoxicity, the zebrafish provides a powerful model to investigate changes to the nervous system from several different angles, offering the ability to tackle the mechanisms of action of chemicals in detail. The mechanistic understanding gained through the analysis of this model species provides a good basic knowledge of how neuroactive chemicals might interact with a teleost nervous system. Such information can help infer potential effects occurring to other species exposed to neuroactive chemicals in their aquatic environment and predicting potential risks of a chemical for the aquatic ecosystem. In the present article, we highlight approaches ranging from behavioral to structural, functional, and molecular analysis of the larval zebrafish nervous system, providing a holistic view of potential neurotoxic outcomes. Environ Toxicol Chem 2021;40:989-1006. © 2020 SETAC.

Translating Neurobehavioral Toxicity Across Species From Zebrafish to Rats to Humans: Implications for Risk Assessment

There is a spectrum of approaches to neurotoxicological science from high-throughput in vitro cell-based assays, through a variety of experimental animal models to human epidemiological and clinical studies. Each level of analysis has its own advantages and limitations. Experimental animal models give essential information for neurobehavioral toxicology, providing cause-and-effect information regarding risks of neurobehavioral dysfunction caused by toxicant exposure. Human epidemiological and clinical studies give the closest information to characterizing human risk, but without randomized treatment of subjects to different toxicant doses can only give information about association between toxicant exposure and neurobehavioral impairment. In vitro methods give much needed high throughput for many chemicals and mixtures but cannot provide information about toxicant impacts on behavioral function. Crucial to the utility of experimental animal model studies is cross-species translation. This is vital for both risk assessment and mechanistic determination. Interspecies extrapolation is important to characterize from experimental animal models to humans and between different experimental animal models. This article reviews the literature concerning extrapolation of neurobehavioral toxicology from established rat models to humans and from zebrafish a newer experimental model to rats. The functions covered include locomotor activity, emotion, and cognition and the neurotoxicants covered include pesticides, metals, drugs of abuse, flame retardants and polycyclic aromatic hydrocarbons. With more complete understanding of the strengths and limitations of interspecies translation, we can better use animal models to protect humans from neurobehavioral toxicity.

Titanium dioxide nanoparticle affects motor behavior, neurodevelopment and axonal growth in zebrafish (Danio rerio) larvae

More attention has been recently paid to the ecotoxicity of titanium dioxide nanoparticles (nano-TiO₂) owing to its common use in many fields. Although previous studies have shown that nano-TiO₂ is neurotoxic, the mechanism is still largely unknown. In the present study, zebrafish embryos were exposed to 0.01, 0.1, and 1.0 mg/L nano-TiO₂ and 1.0 mg/L micro-TiO₂ for up to 6 days post-fertilization (dpf). Exposure to 1.0 mg/L nano-TiO₂ significantly decreased the body length and weight of zebrafish larvae; however, the hatching and mortality rate of zebrafish embryos did not change. Behavioral tests showed that nano-TiO₂ exposure significantly reduced the swimming speed and clockwise rotation times of the larvae. The results revealed that nano-TiO₂ treatment adversely affected motor neuron axon length in Tg (hb9-GFP) zebrafish and decreased central nervous system (CNS) neurogenesis in Tg (HuC-GFP) zebrafish. Additionally, real-time polymerase chain reaction analysis demonstrated that genes associated with neurogenesis (nrd and elavl3) and axonal growth (α1-tubulin, mbp, and gap43) were significantly affected by nano-TiO₂ exposure. In conclusion, our study demonstrated that early-life stage exposure of zebrafish to nano-TiO₂ causes adverse neural outcomes through the inhibition of neurodevelopment and motor neuron axonal growth.

Systematic Assessment of Exposure Variations on Observed Bioactivity in Zebrafish Chemical Screening

The embryonic zebrafish is a powerful tool for high-throughput screening of chemicals. While this model has significant potential for use in safety assessments and chemical prioritization, a lack of exposure protocol harmonized across laboratories has limited full model adoption. To assess the potential that exposure protocols alter chemical bioactivity, we screened a set of eight chemicals and one 2D nanomaterial across four different regimens: (1) the current Tanguay laboratory's standard protocol of dechorionated embryos and static exposure in darkness; (2) exposure with chorion intact; (3) exposure under a 14 h light: 10 h dark cycle; and (4) exposure with daily chemical renewal. The latter three regimens altered the concentrations, resulting in bioactivity of the test agents compared to that observed with the Tanguay laboratory's standard regimen, though not directionally the same for each chemical. The results of this study indicate that with the exception for the 2D nanomaterial, the screening design did not change the conclusion regarding chemical bioactivity, just the nominal concentrations producing the observed activity. Since the goal of tier one chemical screening often is to differentiate active from non-active chemicals, researchers could consider the trade-offs regarding cost, labor, and sensitivity in their study design without altering hit rates. Taken further, these results suggest that it is reasonably feasible to reach agreement on a standardized exposure regiment, which will promote data sharing without sacrificing data content.

Melatonin attenuates 17β-trenbolone induced insomnia-like phenotype and movement deficiency in zebrafish

17β-trenbolone (17β-TBOH) is one of the dominant metabolites of trenbolone acetate, which is widely applied in beef cattle operations around the globe. The effects of environmental concentrations of 17β-trenbolone on the early development of zebrafish embryos have received very little attention. Melatonin could regulate sleep-wake cycle and plays a protective role in various adverse conditions. Here, environmentally realistic concentrations of 17β-trenbolone (1 ng/L, 10 ng/L, 50 ng/L) has been exposure to zebrafish embryos at 2 h postfertilization (hpf). The results showed that 10 ng/L and 50 ng/L 17β-trenbolone disturbed the distribution of caudal primary motoneurons and downregulated expression of motoneuron development related genes along with locomotion decreasing. While melatonin could recover the detrimental effects caused by 17β-trenbolone. Interestingly, 17β-trenbolone exposure increased waking activity and decreased rest even in a low dose (1 ng/L). Moreover, it upregulated hypocretin/orexin (Hcrt) signaling which promotes wakefulness. Melatonin restored the insomnia-like alternation induced by 17β-trenbolone exposure. Collectively, we conclude that 17β-trenbolone disturbed motoneuron development and altered sleep/wake behavior, while melatonin could alleviate the deleterious influence on motoneuron development and recover the circadian rhythm.

Neurodevelopmental toxicity assessments of alkyl phenanthrene and Dechlorane Plus co-exposure in zebrafish

Alkyl phenanthrene (A-Phen) and Dechlorane Plus (DP) are ubiquitous environmental pollutants that widely co-exist in the environment. It has been established that both A-Phen and DP elicit neurotoxicity, but the potential interactive toxicity of these contaminants is not well-known. To determine whether a mixture of A-Phen and DP would exhibit interactive effects on neurodevelopment, we co-exposed 3-methylphenanthrene (3-MP), a representative of A-Phen, with DP. Our results illustrated that exposure to 5 or 20 μg/L 3-MP alone or in combination with 60 μg/L DP caused neurobehavioral anomalies in zebrafish. In accordance with the behavioral deficits, 3-MP alone or co-exposed with DP significantly decreased axonal growth of secondary motoneurons, altered intracellular Ca²⁺ homeostasis and induced cell apoptosis in the muscle of zebrafish. Additionally, 3-MP alone or co-exposed with DP significantly increased reactive oxygen species (ROS) and the mRNA levels of apoptosis-related genes. These findings indicate that 3-MP alone or co-exposed with DP induces neurobehavioral deficits through the combined effects on neuronal connectivity and muscle function. Chemical analysis revealed significant increases in 3-MP and DP bioaccumulation in zebrafish co-exposed with 3-MP and DP. Elevated bioaccumulation resulting from mixture exposure may represent a significant contribution of the synergistic effects observed in combined chemical exposure.

Adverse outcome pathway of developmental neurotoxicity resulting from prenatal exposures to cannabis contaminated with organophosphate pesticide residues

There is growing concern that increased use of medical and recreational cannabis may result in increased exposure to contaminants on the cannabis, such as pesticides. Several states are moving towards implementing robust regulation of the sales, cultivation, and manufacture of cannabis products. However, there are challenges with creating health-protective regulations in an industry that, to date, has been largely unregulated. The focus of this publication is a theoretical examination of what may happen when women are exposed pre-conceptually or during pregnancy to cannabis contaminated with pesticides. We propose an adverse outcome pathway of concomitant prenatal exposure to cannabinoids and the organophosphate pesticide chlorpyrifos by curating what we consider to be the key events at the molecular, cellular, and tissue levels that result in developmental neurotoxicity. The implications of this adverse outcome pathway underscore the need to elucidate the potential developmental neurotoxicity that may result from prenatal exposure to pesticide-contaminated cannabis.

Increased severity of fragile X spectrum disorders in the agricultural community of Ricaurte, Colombia

Premutation carriers of the FMR1 gene (CGG repeats between 55 and 200) usually have normal intellectual abilities but approximately 20% are diagnosed with developmental problems or autism spectrum disorder. Additionally, close to 50% have psychiatric problems such as anxiety, ADHD and/or depression. The spectrum of fragile X disorders also includes Fragile-X-associated primary ovarian insufficiency (FXPOI) in female carriers and Fragile-X-associated tremor/ataxia syndrome (FXTAS) in older male and female carriers. We evaluated 25 premutation carriers in the rural community of Ricaurte Colombia and documented all behavioral problems, social deficits and clinical signs of FXPOI and FXTAS as well as reviewed the medical and obstetric history. We found an increased frequency and severity of symptoms of fragile X spectrum disorders, which might be related to the vulnerability of FMR1 premutation carriers to higher exposure to neurotoxic pesticides in this rural community.

Can Environmentally Relevant Neuroactive Chemicals Specifically Be Detected with the Locomotor Response Test in Zebrafish Embryos?

Chemicals considered as neuroactive (such as certain pesticides, pharmaceuticals, and industrial chemicals) are among the largest groups of bioactive substances recently detected in European rivers. However, the determination of nervous-system-specific effects has been limited using in vitro tests or conventional end points including lethality. Thus, neurobehavioral tests using in vivo models (e.g., zebrafish embryo) have been proposed as complementary approaches. To investigate the specificity and sensitivity of a light-dark transition locomotor response (LMR) test in 4 to 5 days post fertilization zebrafish with respect to different modes of action (MoAs), we analyzed a set of 18 environmentally relevant compounds with various anticipated MoAs. We found that exposure-induced behavioral alterations were reproducible and dependent on concentration and time. Comparative and quantitative analyses of the obtained locomotor patterns revealed that behavioral effects were not restricted to compounds primarily known to target the nervous system. A clear distinction of MoAs based on locomotor patterns was not possible for most compounds. Furthermore, chemicals with an anticipated same MoA did not necessarily provoke similar behavioral phenotypes. Finally, we determined an increased sensitivity (≥10-fold) compared to observed mortality in the LMR assay for five of eight neuroactive chemicals as opposed to non-neuroactive compounds.

The Utility of Zebrafish as a Model for Screening Developmental Neurotoxicity

The developing central nervous system and the blood brain barrier are especially vulnerable and sensitive to different chemicals, including environmental contaminants and drugs. Developmental exposure to these compounds has been involved in several neurological disorders, such as autism spectrum disorders as well as Alzheimer's and Parkinson's diseases. Zebrafish (Danio Rerio) have emerged as powerful toxicological model systems that can speed up chemical hazard assessment and can be used to extrapolate neurotoxic effects that chemicals have on humans. Zebrafish embryos and larvae are convenient for high-throughput screening of chemicals, due to their small size, low-cost, easy husbandry, and transparency. Additionally, zebrafish are homologous to other higher order vertebrates in terms of molecular signaling processes, genetic compositions, and tissue/organ structures as well as neurodevelopment. This mini review underlines the potential of the zebrafish as complementary models for developmental neurotoxicity screening of chemicals and describes the different endpoints utilized for such screening with some studies illustrating their use.

Endoplasmic reticulum stress mediates 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT)-induced toxicity and liver lipid metabolism changes in Nile tilapia (Oreochromis niloticus)

DCOIT (4,5-dichloro-2-n-octyl-4-isothiazolin-3-one) is the main active ingredient in an emerging water environment antifoulant, the toxicity and environmental impacts of which need to be further investigated. Thus, this study examined the toxicity of DCOIT on Nile tilapia (Oreochromis niloticus), including its effects on behavior, respiration and energy metabolism as well as the role of endoplasmic reticulum stress (ER stress) in mediating its toxicity and metabolic changes. The changes in fish behavior, respiration, neuronal signal transmission, energy metabolism, ER stress, and liver histology were examined via acute (4 days) and chronic (28 days) exposures to 0, 3, 15, 30 μg/L DCOIT in vivo. Additionally, ER stress levels were measured in 24-h periods of hepatocyte exposure to 0, 3, 15, 30 and 300 μg/L DCOIT in vitro. The hyper-locomotor activities decreased, but the respiration rate increased after a 4-day acute exposure period, indicating that DCOIT exposure altered fish energy metabolism. After acute exposure at a low DCOIT concentration, the activation of ER stress induced triglyceride accumulation in the liver. After chronic exposure for 28 days, the prolonged ER stress induced a series of pathological cellular changes. At the cellular level, exposure to a high DCOIT concentration induced ER stress in the hepatocytes. In addition, as a neurotoxin, DCOIT has the potential to disrupt the neurotransmission of the cholinergic system, resulting in motor behavior disruption. This study demonstrates that DCOIT plays a role in time- and concentration-dependent toxicity and that changes in lipid metabolism are directly related to endoplasmic reticulum function after exposure to an antifouling agent. This work advances the understanding of the toxic mechanism of DCOIT, which is necessary for its evaluation.

Mechanisms of organophosphorus pesticide toxicity in the context of airway hyperreactivity and asthma

Numerous epidemiologic studies have identified an association between occupational exposures to organophosphorus pesticides (OPs) and asthma or asthmatic symptoms in adults. Emerging epidemiologic data suggest that environmentally relevant levels of OPs may also be linked to respiratory dysfunction in the general population and that in utero and/or early life exposures to environmental OPs may increase risk for childhood asthma. In support of a causal link between OPs and asthma, experimental evidence demonstrates that occupationally and environmentally relevant OP exposures induce bronchospasm and airway hyperreactivity in preclinical models. Mechanistic studies have identified blockade of autoinhibitory M2 muscarinic receptors on parasympathetic nerves that innervate airway smooth muscle as one mechanism by which OPs induce airway hyperreactivity, but significant questions remain regarding the mechanism(s) by which OPs cause neuronal M2 receptor dysfunction and, more generally, how OPs cause persistent asthma, especially after developmental exposures. The goals of this review are to 1) summarize current understanding of OPs in asthma; 2) discuss mechanisms of OP neurotoxicity and immunotoxicity that warrant consideration in the context of OP-induced airway hyperreactivity and asthma, specifically, inflammatory responses, oxidative stress, neural plasticity, and neurogenic inflammation; and 3) identify critical data gaps that need to be addressed in order to better protect adults and children against the harmful respiratory effects of low-level OP exposures.

Neurotoxicity in acute and repeated organophosphate exposure

The term organophosphate (OP) refers to a diverse group of chemicals that are found in hundreds of products worldwide. As pesticides, their most common use, OPs are clearly beneficial for agricultural productivity and the control of deadly vector-borne illnesses. However, as a consequence of their widespread use, OPs are now among the most common synthetic chemicals detected in the environment as well as in animal and human tissues. This is an increasing environmental concern because many OPs are highly toxic and both accidental and intentional exposures to OPs resulting in deleterious health effects have been documented for decades. Some of these deleterious health effects include a variety of long-term neurological and psychiatric disturbances including impairments in attention, memory, and other domains of cognition. Moreover, some chronic illnesses that manifest these symptoms such as Gulf War Illness and Aerotoxic Syndrome have (at least in part) been attributed to OP exposure. In addition to acute acetylcholinesterase inhibition, OPs may affect a number of additional targets that lead to oxidative stress, axonal transport deficits, neuroinflammation, and autoimmunity. Some of these targets could be exploited for therapeutic purposes. The purpose of this review is thus to: 1) describe the important uses of organophosphate (OP)-based compounds worldwide, 2) provide an overview of the various risks and toxicology associated with OP exposure, particularly long-term neurologic and psychiatric symptoms, 3) discuss mechanisms of OP toxicity beyond cholinesterase inhibition, 4) review potential therapeutic strategies to reverse the acute toxicity and long term deleterious effects of OPs.

Opportunities and challenges for using the zebrafish to study neuronal connectivity as an endpoint of developmental neurotoxicity

Chemical exposures have been implicated as environmental risk factors that interact with genetic susceptibilities to influence individual risk for complex neurodevelopmental disorders, including autism spectrum disorder, schizophrenia, attention deficit hyperactivity disorder and intellectual disabilities. Altered patterns of neuronal connectivity represent a convergent mechanism of pathogenesis for these and other neurodevelopmental disorders, and growing evidence suggests that chemicals can interfere with specific signaling pathways that regulate the development of neuronal connections. There is, therefore, a growing interest in developing screening platforms to identify chemicals that alter neuronal connectivity. Cell-cell, cell-matrix interactions and systemic influences are known to be important in defining neuronal connectivity in the developing brain, thus, a systems-based model offers significant advantages over cell-based models for screening chemicals for effects on neuronal connectivity. The embryonic zebrafish represents a vertebrate model amenable to higher throughput chemical screening that has proven useful in characterizing conserved mechanisms of neurodevelopment. Moreover, the zebrafish is readily amenable to gene editing to integrate genetic susceptibilities. Although use of the zebrafish model in toxicity testing has increased in recent years, the diverse tools available for imaging structural differences in the developing zebrafish brain have not been widely applied to studies of the influence of gene by environment interactions on neuronal connectivity in the developing zebrafish brain. Here, we discuss tools available for imaging of neuronal connectivity in the developing zebrafish, review what has been published in this regard, and suggest a path forward for applying this information to developmental neurotoxicity testing.

Tris (2-butoxyethyl) phosphate affects motor behavior and axonal growth in zebrafish (Danio rerio) larvae

Tris (2-butoxyethyl) phosphate (TBOEP) is an environmental contaminant that poses serious risks to aquatic organisms and their associated ecosystem. Recently, the reproductive and developmental toxicology of TBOEP has been reported. However, fewer studies have assessed the neurotoxic effects in zebrafish (Danio rerio) larvae. In this study, zebrafish embryos were subjected to waterborne exposure of TBOEP at 0, 50, 500, 1500 and 2500 μg/L from 2 to 144-h post-fertilization (hpf). Behavioral measurements showed that TBOEP exposure reduced embryonic spontaneous movement and decreased swimming speed of larvae in response to dark stimulation. In accordance with these motor effects, TBOEP treatment reduced neuron-specific GFP expression in transgenic Tg (HuC-GFP) zebrafish larvae and inhibited the growth of secondary motoneurons, as well as decreased expression of marker genes related to central nervous system development in TBOEP treated group. Furthermore, increased concentrations of reactive oxygen species (ROS) and malondialdehyde (MDA), as well as reduction of SOD activity were detected in TBOEP exposure group. The present results showed that the alteration in motor neuron and oxidative stress could together lead to the motor behavior alterations induced by TBOEP.

Prenatal Organophosphate Pesticide Exposure and Traits Related to Autism Spectrum Disorders in a Population Living in Proximity to Agriculture

BACKGROUND

Prenatal exposure to organophosphate (OP) pesticides has been linked with poorer neurodevelopment and behaviors related to autism spectrum disorders (ASD) in previous studies, including in the Center for Health Assessment of Mothers and Children of Salinas (CHAMACOS) study, a birth cohort living in the agricultural Salinas Valley in California.

OBJECTIVES

To investigate the association of prenatal exposure to OP pesticides with traits related to ASD, in childhood and adolescents in CHAMACOS.

METHODS

We assessed OP exposure during pregnancy with measurements of dialkyl phosphates (DAP) metabolites in urine, and residential proximity to OP use during pregnancy using California's Pesticide Use Reporting (PUR) data and estimated associations with ASD-related traits using linear regression models. We measured traits reported by parents and teachers as well as the child's performance on tests that evaluate the ability to use facial expressions to recognize the mental state of others at 7, 10¹/₂, and 14 years of age.

RESULTS

Prenatal DAPs were associated with poorer parent and teacher reported social behavior [e.g., a 10-fold DAP increase was associated with a 2.7-point increase (95% confidence interval (CI): 0.9, 4.5) in parent-reported Social Responsiveness Scale, Version 2, T-scores at age 14]. We did not find clear evidence of associations between residential proximity to OP use during pregnancy and ASD-related traits.

CONCLUSIONS

These findings contribute mixed evidence linking OP pesticide exposures with traits related to developmental disorders like ASD. Subtle pesticide-related effects on ASD-related traits among a population with ubiquitous exposure could result in a rise in cases of clinically diagnosed disorders like ASD. https://doi.org/10.1289/EHP2580.

Tetrabromobisphenol A caused neurodevelopmental toxicity via disrupting thyroid hormones in zebrafish larvae

Tetrabromobisphenol A (TBBPA), one of the most widely used brominated flame retardants (BFRs), has resulted in worldwide environmental contamination. TBBPA has been reported as a thyroid endocrine disruptor and a potential neurotoxicant. However, the underlying mechanism is still not clear. In this study, zebrafish (Danio rerio) embryos (2 h post-fertilization, hpf) were exposed to different concentrations of TBBPA (50, 100, 200 and 400 μg/L) alone or in combination with 3,3',5-triiodo-l-thyronine (T3, 20 μg/L + TBBPA, 200 μg/L). The results confirmed that TBBPA could evoke thyroid disruption by observations of increased T4 contents and decreased T3 contents, accompanied by up-regulated tshβ, tg mRNA and down-regulated ttr and trβ mRNA levels in zebafish larvae. TBBPA-induced neurodevelopmental toxicity was also indicated by down-regulated transcription of genes related to central nervous system (CNS) development (e.g., α1-tubulin, mbp and shha), and decreased locomotor activity and average swimming speed. Our results further demonstrated that treatment with T3 could reverse or eliminate TBBPA-induced effects on thyroidal and neurodevelopmental parameters. Given the above, we hypothesize that the observed neurodevelopmental toxicity in the present study could be attributed to the thyroid hormone disruptions by TBBPA.

Developmental toxicity induced by PM2.5 through endoplasmic reticulum stress and autophagy pathway in zebrafish embryos

The aims of this study were to investigate the mechanism underlying the developmental toxicity of fine particulate matter (PM_2.5) and provide a more thorough understanding of the toxicity of PM_2.5 in an ecological environment. Zebrafish embryos at 4 h post-fertilization were exposed to PM_2.5 at doses of 200, 300, 400, 500, 600 and 800 μg/mL for 120 h. The mortality, hatching rate, morphology score, body length, locomotor capacity, histological changes, antioxidant defense system, leukocyte migration, inflammation-related gene mRNA expression, endoplasmic reticulum stress (ERS) and autophagy were evaluated to study PM_2.5-induced developmental toxicity and its underlying mechanisms. PM_2.5 exposure significantly increased the mortality and malformations and reduced the hatching rate and body length of the zebrafish. PM_2.5 significantly reduced the locomotor capacity of zebrafish larvae, increased the levels of ROS and disturbed the antioxidant defense system in zebrafish larvae. In addition, a histological examination showed that the heart, liver, intestines and muscle of the PM_2.5-treated zebrafish exhibited abnormal changes and a significant increase in cellular autophagic accumulation. RT-PCR showed that the expression of genes related to inflammation (tgfβ and cox2), ERS (hspa5, chop, ire1, xbp1s, and atf6) and autophagy (lc3, beclin1 and atg3) pathways was significantly increased in the PM_2.5-treated zebrafish, indicating that PM_2.5 induced inflammation and promoted ERS and autophagy responses via the activation of the IRE1-XBP1 and ATF6 pathways. Together, our data indicate that PM_2.5 induced a dose- and time-dependent increase in developmental toxicity to zebrafish embryos. Additionally, ERS and autophagy may play important roles in PM_2.5-induced developmental toxicity.

Fishing for contaminants: identification of three mechanism specific transcriptome signatures using Danio rerio embryos

In ecotoxicology, transcriptomics is an effective way to detect gene expression changes in response to environmental pollutants. Such changes can be used to identify contaminants or contaminant classes and can be applied as early warning signals for pollution. To do so, it is important to distinguish contaminant-specific transcriptomic changes from genetic alterations due to general stress. Here we present a first step in the identification of contaminant class-specific transcriptome signatures. Embryos of zebrafish (Danio rerio) were exposed to three substances (methylmercury, chlorpyrifos and Aroclor 1254, each from 24 to 48 hpf exposed) representing sediment typical contaminant classes. We analyzed the altered transcriptome to detect discriminative genes significantly regulated in reaction to the three applied contaminants. By comparison of the results of the three contaminants, we identified transcriptome signatures and biologically important pathways (using Cytoscape/ClueGO software) that react significantly to the contaminant classes. This approach increases the chance of finding genes that play an important role in contaminant class-specific pathways rather than more general processes.

Developmental and behavioral alterations in zebrafish embryonically exposed to valproic acid (VPA): An aquatic model for autism

Autism spectrum disorder (ASD) has complex neurodevelopmental impairments and origins that are linked to both genetic and environmental factors. Hence, there is an urgency to establish animal models with ASD-like characteristics to understand the underlying mechanisms of ASD. Prenatal exposure to valproic acid (VPA) produced ASD-like symptoms in humans, rats, and recently zebrafish. The present study investigated the use of VPA exposure to generate an ASD model in zebrafish. Early life stage exposures produced ASD-like phenotypes in the developing brain development and behavioral changes in embryonic and larval zebrafish. Our findings revealed that treating zebrafish embryos with VPA starting at 8h post fertilization (hpf) resulted in significant: increase in the ASD macrocephalic phenotype; hyperactivity of embryo/larvae movement behaviors; and increases of ASD-like larval social behaviors. Further analysis showed increases in cell proliferation, the proportion of mature newborn neurons, and neural stem cell proliferation in the brain region, which may contribute to the brain overgrowth and macrocephaly observed following VPA exposure. Our study demonstrated that VPA exposure generates ASD-like phenotypes and behaviors, indicating that zebrafish is an alternative model to investigate underlying ASD mechanisms.

Tris(1,3-dichloro-2-propyl) phosphate disrupts axonal growth, cholinergic system and motor behavior in early life zebrafish

Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) could have neurotoxic effects and alter motor behaviors in zebrafish (Danio rerio) larvae, however, the underlying mechanisms are still unknown. In this study, zebrafish embryos were subjected to waterborne exposure of TDCIPP at 100, 300, 600, 900μg/L from 2 to 120-h post-fertilization (hpf). Behavioral measurements indicate that TDCIPP exposure significantly elevated spontaneous movement, and altered swimming behavior response of larvae to both light and dark stimulation. Interestingly, in accordance with these motor effects, TDCIPP significantly decreased expression of the neuron-specific GFP in transgenic (HuC-GFP) zebrafish larvae as well as decreased expression of the neural marker genes elavl3 and ngn1, inhibited the axonal growth of the secondary motoneurons and altered the expressions of axon-related genes (α1-tubulin, shha and netrin2) in zebrafish larvae. Furthermore, TDCIPP exposure at 900μg/L significantly increased the activity of acetylcholinesterase (AChE) enzyme, and decreased the total acetylcholine (ACh) concentration. Our data indicate that the alteration in motor neuron and inhibition of cholinergic system could together lead to the TDCIPP induced motor behavior alterations in zebrafish larvae.

Comparison of social behavior responses of Japanese medaka (Oryzias latipes) to lethal and sublethal chlorpyrifos concentrations at different exposure times

Chlorpyrifos (CPF) is one of the most widely used insecticides and has been found in both urban and rural water bodies. In this study, we studied variations in the social behavioral patterns, swimming behavior, and brain acetylcholinesterase (AChE) activity of Japanese medaka exposed to lethal (0.12mg/L) and sublethal (0.012mg/L) concentrations of CPF after different exposure times. Group behavior performance (schooling, shoaling, and solitary) was determined on day 4 of lethal exposure and on days 4, 8, and 12 of sublethal exposure. Swimming speed and brain AChE activity were measured on days 4, 8, 12, 16, and 20 of sublethal CPF exposure. We observed significant decreases in social behavior and swimming speed (i.e., hypoactivity) in fish exposed to lethal CPF concentrations for 4 days. At the sublethal concentration, there was increased schooling duration and hyperactivity of fish on day 8 but not on day 4. In contrast, 12 days of sublethal CPF exposure resulted in social behavior responses similar to those after 4 days' lethal exposure, i.e., significant decreases in schooling frequency and duration with a notable increase in duration of solitary behavior. Brain AChE activity was inhibited in a time-dependent manner. Altered fish behavior in response to organophosphorus pesticides such as CPF may be mediated by more than AChE inhibition alone.

Fish larval recruitment to reefs is a thyroid hormone-mediated metamorphosis sensitive to the pesticide chlorpyrifos

Larval recruitment, the transition of pelagic larvae into reef-associated juveniles, is a critical step for the resilience of marine fish populations but its molecular control is unknown. Here, we investigate whether thyroid-hormones (TH) and their receptors (TR) coordinate the larval recruitment of the coral-reef-fish Acanthurus triostegus. We demonstrate an increase of TH-levels and TR-expressions in pelagic-larvae, followed by a decrease in recruiting juveniles. We generalize these observations in four other coral reef-fish species. Treatments with TH or TR-antagonist, as well as relocation to the open-ocean, disturb A. triostegus larvae transformation and grazing activity. Likewise, chlorpyrifos, a pesticide often encountered in coral-reefs, impairs A. triostegus TH-levels, transformation, and grazing activity, hence diminishing this herbivore’s ability to control the spread of reef-algae. Larval recruitment therefore corresponds to a TH-controlled metamorphosis, sensitive to endocrine disruption. This provides a framework to understand how larval recruitment, critical to reef-ecosystems maintenance, is altered by anthropogenic stressors.

Many animals go through a larval phase before developing into an adult. This transformation is called metamorphosis, and it is regulated by hormones of the thyroid gland in vertebrates. For example, most fish found on coral reefs actually spend the first part of their life as free-swimming larvae out in the ocean. The larvae usually look very different from the juveniles and adults. When these fish become juveniles, the larvae undergo a range of physical and behavioral changes to prepare for their life on the reef. Yet, until now it was not known what hormones control metamorphosis in these fish.

To address this question, Holzer, Besson et al. studied the convict surgeonfish Acanthurus triostegus. This herbivorous coral-reef fish lives in the Indo-Pacific Ocean, and the results showed that thyroid hormones do indeed regulate the metamorphosis of its larvae. This includes changing how the larvae behave and how their adult features develop. Further, Holzer, Besson et al. found that this was also true for four other coral-reef fish, including the lagoon triggerfish and the raccoon butterflyfish. In A. triostegus, thyroid hormones controlled the changes that enabled the juveniles to efficiently graze on algae growing on the reef such as an elongated gut.

When the fish larvae were then exposed to a pesticide called chlorpyrifos, a well-known reef pollutant, their hormone production was disturbed. This in turn affected their grazing behavior and also their metamorphosis. These fish had shortened, underdeveloped guts and could not graze on algae as effectively.

Herbivorous fish such as A. triostegus play a major role in supporting coral reef ecosystems by reducing algal cover and therefore promoting coral recruitment. These new findings show that pollutants from human activities could disturb the metamorphosis of coral-reef fish and, as a consequence, their ability to maintain the reefs. A next step will be to test what other factors can disrupt the hormones in coral-reef fish and thus pose a threat for fish populations and the coral-reef ecosystem.

Biological impacts of glyphosate on morphology, embryo biomechanics and larval behavior in zebrafish (Danio rerio)

All of these days, residues of herbicides such as glyphosate are widely distributed in the environment. The ubiquitous use of glyphosate has drawn extensive attention to its toxicity as an organic pollutant. In this study, we employed larval zebrafish as an animal model to evaluate the effect of different concentrations of glyphosate on early development via morphological, biomechanics, behavioral and physiological analyses. Morphological results showed that an obvious delay occurred in the epiboly process and body length, eye and head area were reduced at concentrations higher than 10 mg/L. The expression of ntl (no tail) shortened and krox20 (also known as Egr2b, early growth response 2b) changed as the glyphosate concentration increased, but there was no change in the expression of shh (sonic hedgehog). In addition, biomechanical analysis of the elasticity of chorion indicated that treated embryos' surface tension was declined. Furthermore, a 48-h locomotion test revealed that embryonic exposure to glyphosate significantly elevated locomotor activities, which is probably attributed to motoneuronal damage. The decreased surface tension of chorion and the increased locomotive activities may contribute to the hatching rates after glyphosate treatment. Our study enriches the researches of evaluating glyphosate toxicity and probablely plays a warning role in herbicides used in farming.

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.