Chlorpyrifos and chlorpyrifos-oxon inhibit axonal growth by interfering with the morphogenic activity of acetylcholinesterase

Assessing the Risks of Pesticide Exposure: Implications for Endocrine Disruption and Male Fertility

Pesticides serve as essential tools in agriculture and public health, aiding in pest control and disease management. However, their widespread use has prompted concerns regarding their adverse effects on humans and animals. This review offers a comprehensive examination of the toxicity profile of pesticides, focusing on their detrimental impacts on the nervous, hepatic, cardiac, and pulmonary systems, and their impact on reproductive functions. Additionally, it discusses how pesticides mimic hormones, thereby inducing dysfunction in the endocrine system. Pesticides disrupt the endocrine system, leading to neurological impairments, hepatocellular abnormalities, cardiac dysfunction, and respiratory issues. Furthermore, they also exert adverse effects on reproductive organs, disrupting hormone levels and causing reproductive dysfunction. Mechanistically, pesticides interfere with neurotransmitter function, enzyme activity, and hormone regulation. This review highlights the effects of pesticides on male reproduction, particularly sperm capacitation, the process wherein ejaculated sperm undergo physiological changes within the female reproductive tract, acquiring the ability to fertilize an oocyte. Pesticides have been reported to inhibit the morphological changes crucial for sperm capacitation, resulting in poor sperm capacitation and eventual male infertility. Understanding the toxic effects of pesticides is crucial for mitigating their impact on human and animal health, and in guiding future research endeavors.

Molecular docking analysis of chlorpyrifos at the human α7-nAChR and its potential relationship with neurocytoxicity in SH-SY5Y cells

The organophosphate insecticide chlorpyrifos (CPF), an acetylcholinesterase inhibitor, has raised serious concerns about human safety. Apart from inducing synaptic acetylcholine accumulation, CPF could also act at nicotinic acetylcholine receptors, like the α7-isoform (α7-nAChR), which could potentially be harmful to developing brains. Our aims were to use molecular docking to assess the binding interactions between CPF and α7-nAChR through, to test the neurocytotoxic and oxidative effects of very low concentrations of CPF on SH-SY5Y cells, and to hypothesize about the potential mediation of α7-nAChR. Docking analysis showed a significant binding affinity of CPH for the E fragment of the α7-nAChR (ΔGibbs: -5.63 to -6.85 Kcal/mol). According to the MTT- and Trypan Blue-based viability assays, commercial CPF showed concentration- and time-dependent neurotoxic effects at a concentration range (2.5-20 µM), ten-folds lower than those reported to have crucial effects for sheer CPF. A rise of the production of radical oxygen species (ROS) was seen at even lower concentrations (1-2.5 µM) of CPF after 24h. Notably, our docking analysis supports the antagonistic actions of CPF on α7-nAChR that were recently published. In conclusion, while α7-nAChR is responsible for neuronal survival and neurodevelopmental processes, its activity may also mediate the neurotoxicity of CPF.

"Comparative evaluation of different chemical agents induced Autism Spectrum Disorder in experimental Wistar rats"

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental condition with uncertain etiology and pathophysiology. Several studies revealed that the commonly used animal models like Valproic Acid (VPA) and Propionic Acid (PPA) do not precisely represent the disease as the human patient does. The current study was conducted on different chemically (VPA, PPA, Poly I:C, Dioxin (2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)) & Chlorpyrifos (CPF)) induced ASD-like animal models and validated the best suitable experimental animal model, which would closely resemble with clinical features of the ASD. This validated model might help to explore the pathophysiology of ASD. This study included rat pups prenatally exposed to VPA, PPA, Poly I:C, Dioxin & CPF within GD9 to GD15 doses. The model groups were validated through developmental and behavioral parameters, Gene Expressions, Oxidative Stress, and Pro-inflammatory and Anti-inflammatory cytokines levels. Developmental and neurobehavioral parameters showed significant changes in model groups compared to the control. In oxidative stress parameters and neuro-inflammatory cytokines levels, model groups exhibited high oxidative stress and neuro-inflammation compared to control groups. Gene expression profile of ASD-related genes showed significant downregulation in model groups compared to the control group. Moreover, the Poly I:C group showed more significant results than other model groups. The comparison of available ASD-like experimental animal models showed that the Poly I:C induced model represented the exact pathophysiology of ASD as the human patient does. Poly I:C was reported in the maternal immune system activation via the inflammatory cytokines pathway, altering embryonic development and causing ASD in neonates.

Prediction of in vivo prenatal chlorpyrifos exposure leading to developmental neurotoxicity in humans based on in vitro toxicity data by quantitative in vitro-in vivo extrapolation

Introduction: Epidemiological studies in children suggested that in utero exposure to chlorpyrifos (CPF), an organophosphate insecticide, may cause developmental neurotoxicity (DNT). We applied quantitative in vitro-in vivo extrapolation (QIVIVE) based on in vitro concentration and non-choline esterase-dependent effects data combined with Benchmark dose (BMD) modelling to predict oral maternal CPF exposure during pregnancy leading to fetal brain effect concentration. By comparing the results with data from epidemiological studies, we evaluated the contribution of the in vitro endpoints to the mode of action (MoA) for CPF-induced DNT. Methods: A maternal-fetal PBK model built in PK-Sim^® was used to perform QIVIVE predicting CPF concentrations in a pregnant women population at 15 weeks of gestation from cell lysate concentrations obtained in human induced pluripotent stem cell-derived neural stem cells undergoing differentiation towards neurons and glia exposed to CPF for 14 days. The in vitro concentration and effect data were used to perform BMD modelling. Results: The upper BMD was converted into maternal doses which ranged from 3.21 to 271 mg/kg bw/day. Maternal CPF blood levels from epidemiological studies reporting DNT findings in their children were used to estimate oral CPF exposure during pregnancy using the PBK model. It ranged from 0.11 to 140 μg/kg bw/day. Discussion: The effective daily intake doses predicted from the in vitro model were several orders of magnitude higher than exposures estimated from epidemiological studies to induce developmental non-cholinergic neurotoxic responses, which were captured by the analyzed in vitro test battery. These were also higher than the in vivo LOEC for cholinergic effects. Therefore, the quantitative predictive value of the investigated non-choline esterase-dependent effects, although possibly relevant for other chemicals, may not adequately represent potential key events in the MoA for CPF-associated DNT.

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.

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.

DE-71 affected the cholinergic system and locomotor activity via disrupting calcium homeostasis in zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) can induce neurotoxicity, but the mechanism of their toxicity on the cholinergic system and locomotion behavior remains unclear. In this paper, zebrafish embryos were exposed to DE-71 (0, 1, 3, 10, 30, and 100 µg/L) until 120 h post fertilization, and its effects on the behavior and cholinergic system of zebrafish larvae and its possible mechanism were investigated. Results indicated a general locomotor activity impairment in the light-dark transition stimulation without affecting the secondary motoneurons. However, with the extension of test time in the dark or light, the decreased locomotor activity was diminished, a significant decrease only observed in the 100 µg/L DE-71 exposure groups in the last 10 min. Furthermore, whole-body acetylcholine (ACh) contents decreased after DE-71 exposure, whereas no changes in NO contents and inducible nitric oxide synthase activity were found. The expression of certain genes encoding calcium homeostasis proteins (e.g., grin1a, camk2a, and crebbpb) and the concentrations of calcium in zebrafish larvae were significantly decreased after DE-71 exposure. After co-exposure with calcium channel agonist (±)-BAY K8644, calcium concentrations, ACh contents, and locomotor activity in the light-dark transition stimulation was significantly increased compared with the same concentrations of DE-71 exposure alone, whereas no significant difference was observed compared with the control, indicating that calcium homeostasis is involved in the impairment of cholinergic neurotransmission and locomotor activity. Overall, our results suggested that DE-71 can impair the cholinergic system and locomotor activity by impairing calcium homeostasis. Our paper provides a better understanding of the neurotoxicity of PBDEs.

Green synthesis of rGO-AgNP composite using Curcubita maxima extract for enhanced photocatalytic degradation of the organophosphate pesticide chlorpyrifos

In this study, Curcubita maxima leaves are used as a novel source for green synthesis of reduced graphene oxide - silver nanoparticle composite in a single pot. Characterization of the novel phyto source-driven composite was performed by UV-visible spectroscopy, Fourier transform infrared analysis, X-ray diffraction analysis, and field emission scanning electron microscopic methods. The assessment of degradation effect of chlorpyrifos by the synthesized nanocomposite was performed. The photocatalytic activity of the composite was demonstrated through two different processes as adsorption under room temperature and photocatalysis in the presence of sunlight. Different parameters such as pH, time, photocatalyst dose and pesticide concentration were optimized. The adsorption isotherms governing the photocatalytic adsorption process were investigated to predict the adsorption capacity of the synthesized nanocomposite. In addition, the results of antimicrobial activity of the nanocomposite against gram-positive, gram-negative bacteria and antifungal activity were also been found to be highly promising to utilize this composite for the removal of microbial contaminations in wastewater treatment.

Modulation of CREB and its associated upstream signaling pathways in pesticide-induced neurotoxicity

Human beings are exposed to various environmental xenobiotics throughout their life consisting of a broad range of physical and chemical agents that impart bodily harm. Among these, pesticide exposure that destroys insects mainly by damaging their central nervous system also exerts neurotoxic effects on humans and is implicated in the etiology of several degenerative disorders. The connectivity between CREB (cAMP Response Element Binding Protein) signaling activation and neuronal activity is of broad interest and has been thoroughly studied in various diseased states. Several genes, as well as protein kinases, are involved in the phosphorylation of CREB, including BDNF (Brain-derived neurotrophic factor), Pi3K (phosphoinositide 3-kinase), AKT (Protein kinase B), RAS (Rat Sarcoma), MEK (Mitogen-activated protein kinase), PLC (Phospholipase C), and PKC (Protein kinase C) that play an essential role in neuronal plasticity, long-term potentiation, neuronal survival, learning, and memory formation, cognitive function, synaptic transmission, and suppressing apoptosis. These elements, either singularly or in a cascade, can result in the modulation of CREB, making it a vulnerable target for various neurotoxic agents, including pesticides. This review provides insight into how these various intracellular signaling pathways converge to bring about CREB activation and how the activated or deactivated CREB levels can affect the gene expression of the upstream molecules. We also discuss the various target genes within the cascade vulnerable to different types of pesticides. Thus, this review will facilitate future investigations associated with pesticide neurotoxicity and identify valuable therapeutic targets.

Molecular characterization of HEPCIDIN-1 (HAMP1) gene in red-bellied pacu (Piaractus brachypomus)

Hepcidins are cysteine-rich peptides, which participate in iron metabolism regulation, the inflammatory and antimicrobial response. This study characterizes the hepcidin-1 (HAMP1) gene, its transcript expression in different tissues, as well as its regulation in a model of brain injury in Piaractus brachypomus. Bioinformatic analysis was carried out to determine conserved domains, glycosylation sites and protein structure of HAMP1, and probability that HAMP1 corresponds to an antimicrobial peptide (AMP). Relative gene expression of the P. brachypomus HAMP1 gene was determined by qPCR from cDNA of several tissues, a brain injury model, an organophosphate sublethal toxicity model and anesthetic experiment using the 2^-ΔΔCt method. HAMP1 ORF encodes for a 91 aa pre-prohepcidin conformed for a prodomain with 42 aa and mature peptide of 25 aa. Mature domain was determined as an AMP. HAMP1 transcript is expressed in all the tissues, being higher in the spleen and liver. HAMP1 mRNA level was upregulated in the brain injury group, as well as in the olfactory bulb, optic chiasm and telencephalon of red-bellied pacu brain exposed to an organophosphate. In anesthetic experiment, HAMP1 mRNA level was upregulated in the liver and gills. HAMP1 gene of P. brachypomus may be involved in the inflammatory, antimicrobial, hypoxia and stress oxidative response.

Prenatal exposure to organophosphate pesticides and risk-taking behaviors in early adulthood

INTRODUCTION

Previous studies show evidence for associations of prenatal exposure to organophosphate (OP) pesticides with poorer childhood neurodevelopment. As children grow older, poorer cognition, executive function, and school performance can give rise to risk-taking behaviors, including substance abuse, delinquency, and violent acts. We investigated whether prenatal OP exposure was associated with these risk-taking behaviors in adolescence and young adulthood in a Mexican American cohort.

METHODS

We measured urinary dialkyl phosphates (DAPs), non-specific metabolites of OPs, twice (13 and 26 weeks gestation) in pregnant women recruited in 1999-2000 in the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) study, a birth cohort set in a primarily Latino agricultural community in the Salinas Valley, California. We followed up children throughout their childhood and adolescence; at the 18-year visit, adolescent youth (n = 315) completed a computer-based questionnaire which included questions about substance use, risky sexual activity, risky driving, and delinquency and police encounters. We used multivariable models to estimate associations of prenatal total DAPs with these risk-taking behaviors.

RESULTS

The prevalence of risk-taking behaviors in CHAMACOS youth ranged from 8.9% for smoking or vaping nicotine to 70.2% for committing a delinquent act. Associations of total prenatal DAPs (geometric mean = 132.4 nmol/L) with risk-taking behavior were generally null and imprecise. Isolated findings included a higher risk for smoking or vaping nicotine within the past 30 days (relative risk [RR] per 10-fold increase in prenatal DAPs = 1.89, 95% CI: 1.00, 3.56) and driving without a license (RR = 1.74, 95% CI: 1.25, 2.42). There were no consistent differences by sex or childhood adversity.

DISCUSSION

We did not find clear or consistent evidence for associations of prenatal OP exposure with risk-taking behaviors in adolescence/early adulthood in the CHAMACOS population. Our small sample size may have prevented us from detecting potentially subtle associations of early life OP exposure with these risk-taking behaviors.

Gestational Exposure to Organophosphate Pesticides and Longitudinally Assessed Behaviors Related to Attention-Deficit/Hyperactivity Disorder and Executive Function

The brain’s prefrontal cortex directs higher-order cognitive and behavioral processes that are important for attention, working memory, and inhibitory control. We investigated whether gestational exposure to organophosphate (OP) pesticides was associated with these abilities in childhood and early adolescence. Between 1999 and 2000, we enrolled pregnant women in a birth cohort drawn from an agricultural region of California. We measured dialkyl phosphate (DAP) metabolites of OP pesticides in maternal pregnancy urine samples (13 and 26 weeks) and estimated associations with behaviors related to attention-deficit/hyperactivity disorder and executive function, assessed longitudinally; 351 families provided neurodevelopmental outcome data at any point when the child was aged 7–12 years. We assessed function across multiple dimensions (e.g., working memory, attention), methods (e.g., behavior reports, child assessment), and reporters (e.g., mothers, teachers, child self-reports). Higher gestational DAP concentrations were consistently associated with behaviors related to attention-deficit/hyperactivity disorder and executive function. For example, a 10-fold increase in gestational DAP concentration was associated with poorer longitudinally assessed Behavior Rating Inventory of Executive Function scores, as reported by mothers (β = 4.0 (95% confidence interval: 2.1, 5.8); a higher score indicates more problems), and Weschler Intelligence Scale for Children—Fourth Edition Working Memory scores (a 3.8-point reduction; β = −3.8 (95% confidence interval: −6.2, −1.3)). Reducing gestational exposure to OP pesticides through public health policy is an important goal.

Isolation, characterization and identification of pesticide degrading bacteria from contaminated soil for bioremediation

In this study, malathion and chlorpyrifos degrading bacteria were isolated from agricultural soil samples taken from the Himachal region in India. A total of 52 organisms were isolated which were further screened for their efficiency for chlorpyrifos and malathion degradation. Screening was done by checking the growth on Nutrient Agar, Mineral Salt Medium and MacConkey agar plates containing chlorpyrifos and malathion; 37 isolates showed growth in these. Biomass assay and minimum inhibitory concentration (MIC) determination were carried out for the selection of most efficient bacterial isolates. Out of the seven isolates which showed good biomass assay and MIC, only three isolates (PDM-2, PDM-15 and PDM-20) were selected for further studies. These were characterized by various biochemical tests, Gram staining, indole test, methyl red test, Voges-Proskauer test, citrate utilization test and carbohydrate fermentation test. Out of three isolates, PDM-15 showed good resistance against the antibiotics such as erythromycin, chloramphenicol, ampicillin and penicillin and identified as Kocuria assamensis. Degradation of 71.3% of chlorpyrifos and 85% of malathion was observed by the gas chromatography. Therefore, the Kocuria assamensis can be used in the bioremediation of pesticide-contaminated soil.

Corticosterone and chlorpyrifos oxon exposure elicits spatiotemporal MAPK phosphoprotein signaling in a mouse brain

Chlorpyrifos (CPF) is one of the most widely-used pesticides globally for agricultural purposes. Certain occupations (e.g., farmers, military) are at an increased risk for high-dose exposure to CPF, which can lead to seizures and irreversible brain injury. Workers with the highest risk of exposure typically experience increased circulating cortisol levels, which is related to physiological stress. To better represent this exposure scenario, a mouse model utilized exogenous administration of corticosterone (CORT; high physiologic stress mimic) in combination with chlorpyrifos oxon (CPO; oxon metabolite of CPF); this combination increases neuroinflammation post-exposure. In the present study adult male C57BL/6J mice were given CORT (200 μg/mL) in drinking water for seven days followed by a single intraperitoneal injection of CPO (8.0 mg/kg) on day eight, and euthanized 0.5, 2, and 24 h post-injection. Ten post-translationally modified proteins were measured in the frontal cortex and striatum to evaluate brain region-specific effects. The spatiotemporal response to CORT + CPO sequentially activated phosphoproteins (p-ERK1/2, p-MEK1/2, p-JNK) involved in mitogen-activated protein kinase (MAPK) signaling. Observed p-ZAP70 responses further integrated MAPK signaling and provided a spatiotemporal connection between protein phosphorylation and neuroinflammation. This study provides insight into the spatiotemporal cellular signaling cascade following CORT + CPO exposure that represent these vulnerable populations.

Gene-Environment Interactions in Developmental Neurotoxicity: a Case Study of Synergy between Chlorpyrifos and CHD8 Knockout in Human BrainSpheres

BACKGROUND

Autism spectrum disorder (ASD) is a major public health concern caused by complex genetic and environmental components. Mechanisms of gene-environment (G × E ) interactions and reliable biomarkers associated with ASD are mostly unknown or controversial. Induced pluripotent stem cells (iPSCs) from patients or with clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9 (CRISPR/Cas9)-introduced mutations in candidate ASD genes provide an opportunity to study (G × E ) interactions.

OBJECTIVES

In this study, we aimed to identify a potential synergy between mutation in the high-risk autism gene encoding chromodomain helicase DNA binding protein 8 (CHD8) and environmental exposure to an organophosphate pesticide (chlorpyrifos; CPF) in an iPSC-derived human three-dimensional (3D) brain model.

METHODS

This study employed human iPSC-derived 3D brain organoids (BrainSpheres) carrying a heterozygote CRISPR/Cas9-introduced inactivating mutation in CHD8 and exposed to CPF or its oxon-metabolite (CPO). Neural differentiation, viability, oxidative stress, and neurite outgrowth were assessed, and levels of main neurotransmitters and selected metabolites were validated against human data on ASD metabolic derangements.

RESULTS

Expression of CHD8 protein was significantly lower in CHD8 heterozygous knockout (C H D 8 + / - ) BrainSpheres compared with C H D 8 + / + ones. Exposure to CPF/CPO treatment further reduced CHD8 protein levels, showing the potential (G × E ) interaction synergy. A novel approach for validation of the model was chosen: from the literature, we identified a panel of metabolic biomarkers in patients and assessed them by targeted metabolomics in vitro. A synergistic effect was observed on the cholinergic system, S-adenosylmethionine, S-adenosylhomocysteine, lactic acid, tryptophan, kynurenic acid, and α -hydroxyglutaric acid levels. Neurite outgrowth was perturbed by CPF/CPO exposure. Heterozygous knockout of CHD8 in BrainSpheres led to an imbalance of excitatory/inhibitory neurotransmitters and lower levels of dopamine.

DISCUSSION

This study pioneered (G × E ) interaction in iPSC-derived organoids. The experimental strategy enables biomonitoring and environmental risk assessment for ASD. Our findings reflected some metabolic perturbations and disruption of neurotransmitter systems involved in ASD. The increased susceptibility of CHD 8 + / - BrainSpheres to chemical insult establishes a possibly broader role of (G × E ) interaction in ASD. https://doi.org/10.1289/EHP8580.

Associations between pesticide mixtures applied near home during pregnancy and early childhood with adolescent behavioral and emotional problems in the CHAMACOS study

Supplemental Digital Content is available in the text.

Studies suggest that exposure to pesticides during pregnancy and early childhood is associated with adverse child neurodevelopment. Research to date has focused primarily on exposure to single pesticides or pesticide classes in isolation; there are little data on the effect of exposure to pesticide mixtures on child and adolescent neurodevelopment.

21-Day dermal exposure to aircraft engine oils: effects on esterase activities in brain and liver tissues, blood, plasma, and clinical chemistry parameters for Sprague Dawley rats

This dermal study tested the potential toxicity of grade 3 (G3) and 4 (G4) organophosphate-containing aircraft engine oils in both new (G3-N, G4-N) and used states (G3-U, G4-U) to alter esterase activities in blood, brain and liver tissues, clinical chemistry parameters, and electrophysiology of hippocampal neurons. A 300 µl volume of undiluted oil was applied in Hill Top Chamber Systems®, then attached to fur-free test sites on backs of male and female Sprague Dawley rats for 6 hr/day, 5 days/week for 21 days. Recovery rats received similar treatments and kept for 14 days post-exposure to screen for reversibility, persistence, or delayed occurrence of toxicity. In brain, both versions of G3 and G4 significantly decreased (32-41%) female acetylcholinesterase (AChE) activity while in males only G3-N and G4-N reduced (33%) AChE activity. Oils did not markedly affect AChE in liver, regardless of gender. In whole blood, G3-U decreased female AChE (29%) which persisted during recovery (32%). G4-N significantly lowered (29%) butyrylcholinesterase (BChE) in male plasma, but this effect was resolved during recovery. For clinical chemistry indices, only globulin levels in female plasma significantly increased following G3-N or G4-N exposure. Preliminary electrophysiology data suggested that effects of both versions of G3 and G4 on hippocampal function may be gender dependent. Aircraft maintenance workers may be at risk if precautions are not taken to minimize long-term aircraft oil exposure.

Polychlorinated Biphenyls (PCBs): Risk Factors for Autism Spectrum Disorder?

Autism spectrum disorder (ASD) includes a group of multifactorial neurodevelopmental disorders defined clinically by core deficits in social reciprocity and communication, restrictive interests and repetitive behaviors. ASD affects one in 54 children in the United States, one in 89 children in Europe, and one in 277 children in Asia, with an estimated worldwide prevalence of 1-2%. While there is increasing consensus that ASD results from complex gene x environment interactions, the identity of specific environmental risk factors and the mechanisms by which environmental and genetic factors interact to determine individual risk remain critical gaps in our understanding of ASD etiology. Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that have been linked to altered neurodevelopment in humans. Preclinical studies demonstrate that PCBs modulate signaling pathways implicated in ASD and phenocopy the effects of ASD risk genes on critical morphometric determinants of neuronal connectivity, such as dendritic arborization. Here, we review human and experimental evidence identifying PCBs as potential risk factors for ASD and discuss the potential for PCBs to influence not only core symptoms of ASD, but also comorbidities commonly associated with ASD, via effects on the central and peripheral nervous systems, and/or peripheral target tissues, using bladder dysfunction as an example. We also discuss critical data gaps in the literature implicating PCBs as ASD risk factors. Unlike genetic factors, which are currently irreversible, environmental factors are modifiable risks. Therefore, data confirming PCBs as risk factors for ASD may suggest rational approaches for the primary prevention of ASD in genetically susceptible individuals.

Gestational exposures to organophosphorus insecticides: From acute poisoning to developmental neurotoxicity

For over three-quarters of a century, organophosphorus (OP) insecticides have been ubiquitously used in agricultural, residential, and commercial settings and in public health programs to mitigate insect-borne diseases. Their broad-spectrum insecticidal effectiveness is accounted for by the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that catalyzes acetylcholine (ACh) hydrolysis, in the nervous system of insects. However, because AChE is evolutionarily conserved, OP insecticides are also toxic to mammals, including humans, and acute OP intoxication remains a major public health concern in countries where OP insecticide usage is poorly regulated. Environmental exposures to OP levels that are generally too low to cause marked inhibition of AChE and to trigger acute signs of intoxication, on the other hand, represent an insidious public health issue worldwide. Gestational exposures to OP insecticides are particularly concerning because of the exquisite sensitivity of the developing brain to these insecticides. The present article overviews and discusses: (i) the health effects and therapeutic management of acute OP poisoning during pregnancy, (ii) epidemiological studies examining associations between environmental OP exposures during gestation and health outcomes of offspring, (iii) preclinical evidence that OP insecticides are developmental neurotoxicants, and (iv) potential mechanisms underlying the developmental neurotoxicity of OP insecticides. Understanding how gestational exposures to different levels of OP insecticides affect pregnancy and childhood development is critical to guiding implementation of preventive measures and direct research aimed at identifying effective therapeutic interventions that can limit the negative impact of these exposures on public health.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

Effects of chlorpyrifos on cell death and cellular phenotypic specification of human neural stem cells

Chlorpyrifos (CPF) is an organophosphate pesticide widely used in agriculture, whose traditional and well-known mechanism of action is the inhibition of the enzyme Acetylcholinesterase (AChE). Subacute exposures to CPF have been associated with alterations different from the inhibition of AChE. Because of the vulnerability of the developing nervous system, prenatal and early postnatal exposures are of special concern. Human neural stem cells (hNSC) provide the opportunity to study early stages of neural development and may be a valuable tool for developmental neurotoxicology (DNT). In the current work, the cell line hNS1 was used as a model system with the aim of validating this cell line as a reliable testing method. To evaluate the effects of CPF on early developmental stages, hNS1 cells were exposed to different concentrations of the pesticide and cell death, proliferation and cell fate specification were analyzed under differentiation conditions. Since hNS1 cells responded to CPF in a similar way to other human cell lines, we consider it may be a valid model for DNT chemical assessment. CPF induced apoptotic cell death only at the highest doses tested, suggesting that it is not toxic for the specific developmental stage here addressed under short term exposure. In addition, the higher doses of CPF promoted the generation of astroglial cells, without affecting neurogenesis.

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.

Health Effects of Pesticides on Pregnant Women and Children

Pesticides, along with hybrid seeds and fertilizers, are an integral part of the green revolution and are used to control and eradicate disease vectors for the improvement of agricultural production. Pesticides is an umbrella term for insecticides, nematocides, fungicides, herbicides, fumigants, repellents, and attractants. Pesticides are used against unwanted plants and animals to control diseases and losses. Efforts at different levels may help to reduce the impact of pesticides on newborn babies and on pregnant women. Different efforts can be considered at clinical, educational, and policymaking institutes. Use of risk assessment tools, encouragement of organic diets, educating parents working in agricultural fields from hazards of pesticides particularly in pregnancy and breast feeding, implementation of integrated pest management (IPM) programs, and encouraging policies supporting IPM can help in tackling the menace of pesticide hazards.

A neuroprotective role of kaempferol against chlorpyrifos-induced oxidative stress and memory deficits in rats via GSK3β-Nrf2 signaling pathway

Chlorpyrifos (CPF) is an agricultural pesticide and a potential food contaminant, which causes neurotoxicity. Here, we aimed at exploring the link between the repeated exposure to CPF and memory dysfunction in rats and the possible protective effect of kaempferol, a flavonoid with appreciable antioxidant and anti-inflammatory activities. Rats were divided into: Control group (received drug vehicles for 14 days); CPF-treated group (received subcutaneous 18 mg/kg BW of CPF daily for 14 days and CPF + Kaempferol treated group (received the same CPF dose +21 mg/kg BW of Kaempferol intraperitoneally for 14 days. On the 14^th day, Y-maze and novel target recognition behavioral tests were employed to evaluate memory deficits. 24 h after the last dose of CPF, animals were sacrificed, and brain tissues were used for the determination of oxidative stress biomarkers and gene expression levels of GSK3β and Nrf2. The results revealed that CPF-treated rats suffered from severe deterioration of spatial and non-spatial memory functions with low activities of antioxidant enzymes and acetylcholinesterase (AChE). The administration of kaempferol significantly protected against CPF-induced neuronal damage, increased the activities of antioxidant enzymes and AChE and induced a better performance in the behavioral tests. The protective effect of kaempferol was mediated through the inhibition of GSK3β gene expression and the induction of Nrf2 expression in the brain tissues. In conclusion, the repeated exposure to CPF is associated with oxidative stress and memory deficits in rats. However, kaempferol administration effectively alleviated CPF- induced brain toxicity, possibly through the modulation of GSK3β-Nrf2 signaling pathway.

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.

Developmental neurotoxicity of triphenyl phosphate in zebrafish larvae

Triphenyl phosphate (TPhP), a typical organophosphate ester, is frequently detected in the environment and biota samples. It has been implicated as a neurotoxin as its structure is similar to neurotoxic organophosphate pesticides. The purpose of the present study was to investigate its potential developmental neurotoxicity in fish by using zebrafish larvae as a model. Zebrafish (Danio rerio) embryos were exposed to 0.8, 4, 20 and 100 μg/L of TPhP from 2 until 144 h post-fertilization. TPhP was found to have high bioconcentrations in zebrafish larvae after exposure. Further, it significantly reduced locomotor activity as well as the heart rate at the 100 μg/L concentration. TPhP exposure significantly altered the content of the neurotransmitters γ-aminobutyric and histamine. Downregulation of the genes related to central nervous system development (e.g., α1-tubulin, mbp, syn2a, shha, and elavl3) as well as the corresponding proteins (e.g., α1-tubulin, mbp, and syn2a) was observed, but the gap-43 protein was found to upregulated. Finally, marked inhibition of total acetylcholinesterase activity, which is considered as a biomarker of neurotoxicant exposure, was also observed in the larvae. Our results indicate that exposure to environmentally relevant concentrations of TPhP can affect different parameters related to center nervous system development, and thus contribute to developmental neurotoxicity in early developing zebrafish larvae.

Knockdown of Butyrylcholinesterase but Not Inhibition by Chlorpyrifos Alters Early Differentiation Mechanisms in Human Neural Stem Cells

Butyrylcholinesterase (BChE) is the evolutionary counterpart to acetylcholinesterase (AChE). Both are expressed early in nervous system development prior to cholinergic synapse formation. The organophosphate pesticide chlorpyrifos (CPF) primarily exerts toxicity through the inhibition of AChE, which results in excess cholinergic stimulation at the synapse. We hypothesized that the inhibition of AChE and BChE by CPF may impair early neurogenesis in neural stem cells (NSCs). To model neurodevelopment in vitro, we used human NSCs derived from induced pluripotent stem cells (iPSCs) with a focus on the initial differentiation mechanisms. Over the six days of NSC differentiation, the BChE activity and mRNA expression significantly increased, while the AChE activity and expression remained unchanged. The CPF treatment (10 μM) caused 82% and 92% inhibition of AChE and BChE, respectively. The CPF exposure had no effect on the cell viability or the expression of the differentiation markers HES5, DCX, or MAP2. However, the shRNA-knockdown of the BChE expression resulted in the decreased or delayed expression of the transcription factors HES5 and HES3. BChE may have a role in the differentiation of NSCs independent of, or in addition to, its enzymatic activity.

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.

Cholinesterases and the fine line between poison and remedy

Acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC 3.1.1.8) are related enzymes found across the animal kingdom. The critical role of acetylcholinesterase in neurotransmission has been known for almost a century, but a physiological role for butyrylcholinesterase is just now emerging. The cholinesterases have been deliberately targeted for both therapy and toxicity, with cholinesterase inhibitors being used in the clinic for a variety of disorders and conversely for their toxic potential as pesticides and chemical weapons. Non-catalytic functions of the cholinesterases (ChEs) participate in both neurodevelopment and disease. Manipulating either the catalytic activities or the structure of these enzymes can potentially shift the balance between beneficial and adverse effect in a wide number of physiological processes.

Redox imbalance caused by pesticides: a review of OPENTOX-related research

Pesticides are a highly diverse group of compounds and the most important chemical stressors in the environment. Mechanisms that could explain pesticide toxicity are constantly being studied and their interactions at the cellular level are often observed in well-controlled in vitro studies. Several pesticide groups have been found to impair the redox balance in the cell, but the mechanisms leading to oxidative stress for certain pesticides are only partly understood. As our scientific project "Organic pollutants in environment - markers and biomarkers of toxicity (OPENTOX)" is dedicated to studying toxic effects of selected insecticides and herbicides, this review is focused on reporting the knowledge regarding oxidative stress-related phenomena at the cellular level. We wanted to single out the most important facts relevant to the evaluation of our own findings from studies conducted on in vitro cell models.

A Novel Method for the Development of Environmental Public Health Indicators and Benchmark Dose Estimation Using a Health-Based End Point for Chlorpyrifos

BACKGROUND

Organophosphorus (OP) compounds are the most widely used group of insecticides in the world. Risk assessments for these chemicals have focused primarily on 10% inhibition of acetylcholinesterase in the brain as the critical metric of effect. Aside from cholinergic effects resulting from acute exposure, many studies suggest a linkage between cognitive deficits and long-term OP exposure.

OBJECTIVE

In this proof-of-concept study, we focused on one of the most widely used OP insecticides in the world, chlorpyrifos (CPF), and utilized an existing physiologically based pharmacokinetic (PBPK) model and a novel pharmacodynamic (PD) dose-response model to develop a point of departure benchmark dose estimate for cognitive deficits following long-term, low-dose exposure to this chemical in rodents.

METHODS

Utilizing a validated PBPK/PD model for CPF, we generated a database of predicted biomarkers of exposure and internal dose metrics in both rat and human. Using simulated peak brain CPF concentrations, we developed a dose-response model to predict CPF-induced spatial memory deficits and correlated these changes to relevant biomarkers of exposure to derive a benchmark dose specific to neurobehavioral changes. We extended these cognitive deficit predictions to humans and simulated corresponding exposures using a model parameterized for humans.

RESULTS

Results from this study indicate that the human-equivalent benchmark dose (BMD) based on a 15% cognitive deficit as an end point is lower than that using the present threshold for 10% brain AChE inhibition. This predicted human-equivalent subchronic BMD threshold compares to occupational exposure levels determined from biomarkers of exposure and corresponds to similar exposure conditions where deficits in cognition are observed.

CONCLUSIONS

Quantitative PD models based on neurobehavioral testing in animals offer an important addition to the methodologies used for establishing useful environmental public health indicators and BMDs, and predictions from such models could help inform the human health risk assessment for chlorpyrifos. https://doi.org/10.1289/EHP1743.

From the Cover: BDE-47 and BDE-49 Inhibit Axonal Growth in Primary Rat Hippocampal Neuron-Glia Co-Cultures via Ryanodine Receptor-Dependent Mechanisms

Polybrominated diphenyl ethers (PBDEs) are widespread environmental contaminants associated with adverse neurodevelopmental outcomes in children and preclinical models; however, the mechanisms by which PBDEs cause developmental neurotoxicity remain speculative. The structural similarity between PBDEs and nondioxin-like (NDL) polychlorinated biphenyls (PCBs) suggests shared toxicological properties. Consistent with this, both NDL PCBs and PBDEs have been shown to stabilize ryanodine receptors (RyRs) in the open configuration. NDL PCB effects on RyR activity are causally linked to increased dendritic arborization, but whether PBDEs similarly enhance dendritic growth is not known. In this study, we quantified the effects of individual PBDE congeners on not only dendritic but also axonal growth since both are regulated by RyR-dependent mechanisms, and both are critical determinants of neuronal connectivity. Neuronal-glial co-cultures dissociated from the neonatal rat hippocampus were exposed to BDE-47 or BDE-49 in the culture medium. At concentrations ranging from 20 pM to 2 µM, neither PBDE congener altered dendritic arborization. In contrast, at concentrations ≥ 200 pM, both congeners delayed neuronal polarization resulting in significant inhibition of axonal outgrowth during the first few days in vitro. The axon inhibitory effects of these PBDE congeners occurred independent of cytotoxicity, and were blocked by pharmacological antagonism of RyR or siRNA knockdown of RyR2. These results demonstrate that the molecular and cellular mechanisms by which PBDEs interfere with neurodevelopment overlap with but are distinct from those of NDL PCBs, and suggest that altered patterns of neuronal connectivity may contribute to the developmental neurotoxicity of PBDEs.

Effects of Chlorpyrifos or Methyl Parathion on Regional Cholinesterase Activity and Muscarinic Receptor Subtype Binding in Juvenile Rat Brain

The effects of developmental exposure to two organophosphorus (OP) insecticides, chlorpyrifos (CPF) and methyl parathion (MPS), on cholinesterase (ChE) activity and muscarinic acetylcholine receptor (mAChR) binding were investigated in preweanling rat brain. Animals were orally gavaged daily with low, medium, and high dosages of the insecticides using an incremental dosing regimen from postnatal day 1 (PND1) to PND20. On PND12, PND17 and PND20, the cerebral cortex, corpus striatum, hippocampus, and medulla-pons were collected for determination of ChE activity, total mAChR density, and the density of the individual mAChR subtypes. ChE activity was inhibited by the medium and high dosages of CPF and MPS at equal levels in all four brain regions at all three ages examined. Exposure to both compounds decreased the levels of the M1, M2/M4, and M3 subtypes and the total mAChR level in all brain regions, but the effects varied by dosage group and brain region. On PND12, only the high dosages induced receptor changes while on PND17 and PND20, greater effects became evident. In general, the effects on the M1 subtype and total receptor levels appeared to be greater in the cerebral cortex and hippocampus than in the corpus striatum and medulla-pons. This did not appear to be the case for the M2/M4 and M3 subtypes effects. The differences between CPF and MPS were minimal even though in some cases, CPF exerted statistically greater effects than MPS did. In general, repeated exposure to organophosphorus insecticides can alter the levels of the various mAChR subtypes in various brain regions which could induce perturbation in cholinergic neurochemistry during the maturation of the brain regions.

Planarian cholinesterase: molecular and functional characterization of an evolutionarily ancient enzyme to study organophosphorus pesticide toxicity

The asexual freshwater planarian Dugesia japonica has emerged as a medium-throughput alternative animal model for neurotoxicology. We have previously shown that D. japonica are sensitive to organophosphorus pesticides (OPs) and characterized the in vitro inhibition profile of planarian cholinesterase (DjChE) activity using irreversible and reversible inhibitors. We found that DjChE has intermediate features of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Here, we identify two candidate genes (Djche1 and Djche2) responsible for DjChE activity. Sequence alignment and structural homology modeling with representative vertebrate AChE and BChE sequences confirmed our structural predictions, and show that both DjChE enzymes have intermediate sized catalytic gorges and disrupted peripheral binding sites. Djche1 and Djche2 were both expressed in the planarian nervous system, as anticipated from previous activity staining, but with distinct expression profiles. To dissect how DjChE inhibition affects planarian behavior, we acutely inhibited DjChE activity by exposing animals to either an OP (diazinon) or carbamate (physostigmine) at 1 µM for 4 days. Both inhibitors delayed the reaction of planarians to heat stress. Simultaneous knockdown of both Djche genes by RNAi similarly resulted in a delayed heat stress response. Furthermore, chemical inhibition of DjChE activity increased the worms' ability to adhere to a substrate. However, increased substrate adhesion was not observed in Djche1/Djche2 (RNAi) animals or in inhibitor-treated day 11 regenerates, suggesting this phenotype may be modulated by other mechanisms besides ChE inhibition. Together, our study characterizes DjChE expression and function, providing the basis for future studies in this system to dissect alternative mechanisms of OP toxicity.

Developmental neurotoxicity of the organophosphorus insecticide chlorpyrifos: from clinical findings to preclinical models and potential mechanisms

Organophosphorus (OP) insecticides are pest-control agents heavily used worldwide. Unfortunately, they are also well known for the toxic effects that they can trigger in humans. Clinical manifestations of an acute exposure of humans to OP insecticides include a well-defined cholinergic crisis that develops as a result of the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter acetylcholine (ACh). Prolonged exposures to levels of OP insecticides that are insufficient to trigger signs of acute intoxication, which are hereafter referred to as subacute exposures, have also been associated with neurological deficits. In particular, epidemiological studies have reported statistically significant correlations between prenatal subacute exposures to OP insecticides, including chlorpyrifos, and neurological deficits that range from cognitive impairments to tremors in childhood. The primary objectives of this article are: (i) to address the short- and long-term neurological issues that have been associated with acute and subacute exposures of humans to OP insecticides, especially early in life (ii) to discuss the translational relevance of animal models of developmental exposure to OP insecticides, and (iii) to review mechanisms that are likely to contribute to the developmental neurotoxicity of OP insecticides. Most of the discussion will be focused on chlorpyrifos, the top-selling OP insecticide in the United States and throughout the world. These points are critical for the identification and development of safe and effective interventions to counter and/or prevent the neurotoxic effects of these chemicals in the developing brain. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Prenatal chlorpyrifos leads to autism-like deficits in C57Bl6/J mice

BACKGROUND

Children are at daily risk for exposure to organophosphate insecticides, of which the most common is chlorpyrifos (CPF). Exposure of pregnant women to CPF was linked to decreased birth weight, abnormal reflexes, reduction in IQ, as well as increased maternal reports of signs of pervasive developmental disorder. The aim of current study was to examine the long term effects of prenatal exposure to CPF in C57BL/6 J (B6) mice with specific focus on social and repetitive behavior.

METHODS

B6 female mice were treated with vehicle, 2.5 mg/kg CPF or 5 mg/kg of CPF on gestational days 12-15 by oral gavage. On postnatal days (PND's) 6-12 early development and neuromotor ability were assessed by measuring 3 neonatal reflexes in the offspring. In adulthood, PND 90, social behavior was investigated using the social preference, social novelty and social conditioned place preference tasks. Object recognition and restricted interest, measured by the repetitive novel object contact task (RNOC), were also assessed on PN D 90. In order to rule out the possibility that CPF administration induced alterations in maternal care, the dams' behavior was evaluated via the maternal retrieval task.

RESULTS

CPF treatment resulted in delayed development of neonatal reflexes on PND's 6-12. On PND 90, mice treated prenatally with the 5.0 mg/kg dose exhibited reduced preference towards an unfamiliar conspecific in the social preference test and reduced social conditioned place preference. In the RNOC task, mice exposed prenatally to 2.5 mg/kg dose of CPF showed enhanced restricted interest. CPF administration did not impair dams' behavior and did not cause memory or recognition deficit as was observed in the object recognition task.

CONCLUSIONS

Our data indicate that gestational exposure to CPF has long-term deleterious effects on social behavior and limits exploration of novel objects.

Developmental neurotoxicity of different pesticides in PC-12 cells in vitro

The detection of developmental neurotoxicity (DNT) of chemicals has high relevance for protection of human health. However, DNT of many pesticides is only little known. Furthermore, validated in vitro systems for assessment of DNT are not well established. Here we employed the rat phaeochromocytoma cell line PC-12 to evaluate DNT of 18 frequently used pesticides of different classes, including neonicotinoids, pyrethroids, organophosphates, organochlorines, as well as quaternary ammonium compounds, the organic compound used in pesticides, piperonyl butoxide, as well as the insect repellent diethyltoluamide (DEET). We determined the outgrowth of neurites in PC-12 cells co-treated with nerve growth factor and different concentrations of biocides for 5days. Furthermore, we determined transcriptional alterations of selected genes that may be associated with DNT, such as camk2α and camk2β, gap-43, neurofilament-h, tubulin-α and tubulin-β. Strong and dose- dependent inhibition of neurite outgrowth was induced by azamethiphos and chlorpyrifos, and dieldrin and heptachlor, which was correlated with up-regulation of gap-43. No or only weak effects on neurite outgrowth and transcriptional alterations occurred for neonicotinoids acetamiprid, clothianidin, imidacloprid and thiamethoxam, the pyrethroids λ-cyhalothrin, cyfluthrin, deltamethrin, and permethrin, the biocidal disinfectants C12-C14-alkyl(ethylbenzyl)dimethylammonium (BAC), benzalkonium chloride and barquat (dimethyl benzyl ammonium chloride), and piperonyl butoxide and DEET. Our study confirms potential developmental neurotoxicity of some pesticides and provides first evidence that azamethiphos has the potential to act as a developmental neurotoxic compound. We also demonstrate that inhibition of neurite outgrowth and transcriptional alterations of gap-43 expression correlate, which suggests the employment of gap-43 expression as a biomarker for detection and initial evaluation of potential DNT of chemicals.

Developmental neurotoxicity of succeeding generations of insecticides

Insecticides are by design toxic. They must be toxic to effectively kill target species of insects. Unfortunately, they also have off-target toxic effects that can harm other species, including humans. Developmental neurotoxicity is one of the most prominent off-target toxic risks of insecticides. Over the past seven decades several classes of insecticides have been developed, each with their own mechanisms of effect and toxic side effects. This review covers the developmental neurotoxicity of the succeeding generations of insecticides including organochlorines, organophosphates, pyrethroids, carbamates and neonicotinoids. The goal of new insecticide development is to more effectively kill target species with fewer toxic side effects on non-target species. From the experience with the developmental neurotoxicity caused by the generations of insecticides developed in the past advice is offered how to proceed with future insecticide development to decrease neurotoxic risk.

Mother gestational exposure to organophosphorus pesticide induces neuron and glia loss in daughter adult brain

Chlorpyrifos (CPF) is a widely used organophosphorus pesticide with developmental neurotoxicity such as morphogenesis toxicity. In the present study, we assessed the effects of prenatal CPF exposure on systemic parameters and cytoarchitecture of medial prefrontal cortex (mPFC) in adulthood. Gestational dams were exposed to 5mg/kg/d of CPF during gestational days 13-17, while body weight, organ coefficient, and neuron and glia counts of offspring were determined on postnatal day 60. Our results showed that CPF treatment induced little or no effects on body weight and organ coefficients. There were also no significant pathological changes in mPFC. However, neuron and glia count analysis showed that CPF treatment reduced neuron and glia counts in anterior cingulate, prelimbic, and infralimbic areas of mPFC. The CPF react pattern was similar in both sexes, and there was no statistical difference in most of the sub-regions. Thus, our results revealed an embryonic origin brain deficit induced by gestational mother pesticide exposure.

Neurotoxicity in Preclinical Models of Occupational Exposure to Organophosphorus Compounds

Organophosphorus (OPs) compounds are widely used as insecticides, plasticizers, and fuel additives. These compounds potently inhibit acetylcholinesterase (AChE), the enzyme that inactivates acetylcholine at neuronal synapses, and acute exposure to high OP levels can cause cholinergic crisis in humans and animals. Evidence further suggests that repeated exposure to lower OP levels insufficient to cause cholinergic crisis, frequently encountered in the occupational setting, also pose serious risks to people. For example, multiple epidemiological studies have identified associations between occupational OP exposure and neurodegenerative disease, psychiatric illness, and sensorimotor deficits. Rigorous scientific investigation of the basic science mechanisms underlying these epidemiological findings requires valid preclinical models in which tightly-regulated exposure paradigms can be correlated with neurotoxicity. Here, we review the experimental models of occupational OP exposure currently used in the field. We found that animal studies simulating occupational OP exposures do indeed show evidence of neurotoxicity, and that utilization of these models is helping illuminate the mechanisms underlying OP-induced neurological sequelae. Still, further work is necessary to evaluate exposure levels, protection methods, and treatment strategies, which taken together could serve to modify guidelines for improving workplace conditions globally.

An in vivo study in mice: mother's gestational exposure to organophosphorus pesticide retards the division and migration process of neural progenitors in the fetal developing brain

Background: Widely utilized pesticides such as chlorpyrifos (CPF) can cause cognitive abnormalities, neurotransmitter disruptions and brain cytoarchitecture deficits in adulthood due to exposure in the prenatal period, but the mechanism underlying the development and maintenance of such neurotoxicity in embryonic neurogenesis remains largely unclear. Using embryonic neocortex slices, we investigated mitosis population constituents and characteristic interkinetic nuclear migration (INM) to evaluate the CPF effects on the proliferation process of neural progenitors. Methods: Gestational days (GD) 14 and GD 7.5-11.5 ICR dams were exposed to 5 mg kg-1 of CPF to investigate immediate toxicity and sustained toxicity. Proliferating nuclei were labeled with 50 mg kg-1 of Brdu at 1, 3, 6 and 9 hours before samples were collected. The mitoses count and Brdu positive nuclei (BPN) location were measured and analyzed in standard sections of the embryonic dorsolateral cortex. Results: CPF reduced the mitoses count in the primary progenitors but not in the secondary progenitors which are time sustained. CPF retarded BPN migration with a 6-9 μm delay of the relative location in the immediate groups and a 3-6 μm delay in the sustained ones. CPF had no or little effects on the global mitoses count and BPN count. Conclusion: Prenatal CPF exposure disrupts the proliferation process of primary progenitors in the embryonic dorsolateral cortex immediately and with sustained effects, which may contribute to explain the toxicity mechanism in early neurogenesis.

Reference compounds for alternative test methods to indicate developmental neurotoxicity (DNT) potential of chemicals: example lists and criteria for their selection and use

There is a paucity of information concerning the developmental neurotoxicity (DNT) hazard posed by industrial and environmental chemicals. New testing approaches will most likely be based on batteries of alternative and complementary (non-animal) tests. As DNT is assumed to result from the modulation of fundamental neurodevelopmental processes (such as neuronal differentiation, precursor cell migration or neuronal network formation) by chemicals, the first generation of alternative DNT tests target these processes. The advantage of such types of assays is that they capture toxicants with multiple targets and modes-of-action. Moreover, the processes modelled by the assays can be linked to toxicity endophenotypes, i.e. alterations in neural connectivity that form the basis for neurofunctional deficits in man. The authors of this review convened in a workshop to define criteria for the selection of positive/negative controls, to prepare recommendations on their use, and to initiate the setup of a directory of reference chemicals. For initial technical optimization of tests, a set of >50 endpoint-specific control compounds was identified. For further test development, an additional “test” set of 33 chemicals considered to act directly as bona fide DNT toxicants is proposed, and each chemical is annotated to the extent it fulfills these criteria. A tabular compilation of the original literature used to select the test set chemicals provides information on statistical procedures, and toxic/non-toxic doses (both for pups and dams). Suggestions are provided on how to use the >100 compounds (including negative controls) compiled here to address specificity, adversity and use of alternative test systems.

Phenobarbital use and neurological problems in FMR1 premutation carriers

Fragile X Syndrome (FXS) is a neurodevelopmental disorder caused by a CGG expansion in the FMR1 gene located at Xq27.3. Patients with the premutation in FMR1 present specific clinical problems associated with the number of CGG repeats (55-200 CGG repeats). Premutation carriers have elevated FMR1 mRNA expression levels, which have been associated with neurotoxicity potentially causing neurodevelopmental problems or neurological problems associated with aging. However, cognitive impairments or neurological problems may also be related to increased vulnerability of premutation carriers to neurotoxicants, including phenobarbital. Here we present a study of three sisters with the premutation who were exposed differentially to phenobarbital therapy throughout their lives, allowing us to compare the neurological effects of this drug in these patients.

Paraoxon and Pyridostigmine Interfere with Neural Stem Cell Differentiation

Acetylcholinesterase (AChE) inhibition has been described as the main mechanism of organophosphate (OP)-evoked toxicity. OPs represent a human health threat, because chronic exposure to low doses can damage the developing brain, and acute exposure can produce long-lasting damage to adult brains, despite post-exposure medical countermeasures. Although the main mechanism of OP toxicity is AChE inhibition, several lines of evidence suggest that OPs also act by other mechanisms. We hypothesized that rat neural progenitor cells extracted on embryonic day 14.5 would be affected by constant inhibition of AChE from chronic exposure to OP or pyridostigmine (a reversible AChE blocker) during differentiation. In this work, the OP paraoxon decreased cell viability in concentrations >50 μM, as measured with the MTT assay; however, this effect was not dose-dependent. Reduced viability could not be attributed to blockade of AChE activity, since treatment with 200 µM pyridostigmine did not affect cell viability, even after 6 days. Although changes in protein expression patterns were noted in both treatments, the distribution of differentiated phenotypes, such as the percentages of neurons and glial cells, was not altered, as determined by flow cytometry. Since paraoxon and pyridostigmine each decreased neurite outgrowth (but did not prevent differentiation), we infer that developmental patterns may have been affected.

Analysis of differentially expressed proteins in zebrafish (Danio rerio) embryos exposed to chlorpyrifos

In this study, the protein expression profiles of zebrafish embryos under chlorpyrifos (CPF) stress were investigated. Zebrafish embryos were exposed to 0.25 mg/L CPF, and embryo samples were collected until 24 h post-fertilization (hpf). To gain a better understanding of the response of zebrafish embryos to CPF exposure, two-dimensional polyacrylamide gel electrophoresis (2D PAGE) coupled with mass spectrometry was employed to carry out a comparative proteomic analysis. Total proteins were extracted from the control and treated samples, separated by 2D PAGE, and visualized by silver staining. A total of 59 protein spots showed reproducible changes compared with the control. Of these 59 spots, 19 were selected and subjected to matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight mass spectrometry (TOF/TOF) analysis; 9 differentially expressed proteins were successfully identified, including 3 up-regulated proteins and 6 down-regulated proteins. The increased expression of 3 proteins associated with detoxification and stress response suggested that the activation of protective proteins was required in zebrafish embryos exposed to CPF. On the other hand, the decreased expression of 6 proteins is mainly involved in cytoskeleton structure, protein translation, signal transduction and lipoprotein metabolism. These data may help us understand the functions and the molecular mechanisms of these proteins in zebrafish embryos' response to CPF exposure.

Chlorpyrifos and cypermethrin induce apoptosis in human neuroblastoma cell line SH-SY5Y

Our previous in vivo studies showed that chlorpyrifos (CPF) and cypermethrin (CM) in a mixture dermally administered, strongly inhibited cholinesterase activity in plasma and the brain and were very toxic to the rat central nervous system. In this work, the mechanisms of neurotoxicity have not been elucidated. We used human undifferentiated SH-SY5Y cells to study mechanisms of pesticide-induced neuronal cell death. It was found that chlorpyrifos (CPF) and its mixture with cypermethrin (CPF+CM) induced cell death of SH-SY5Y cells in a dose- and time-dependent manner, as shown by MTT assays. Pesticide-induced SH-SY5Y cell death was characterized by concentration-dependent down-regulation of Bcl-2 and Bcl-xL as well as an increase in the caspase 3 activation. Pan-caspase inhibitor Q-VD-OPh produced a slight but significant reversal effect of pesticide-induced toxicity indicating that the major caspase pathways are not integral to CPF- and CPF+CM-induced cell death. Furthermore, signal transduction inhibitors PD98059, SL-327, SB202190, SP600125 and mecamylamine failed to attenuate pesticides effect. Atropine exhibited minimal ability to reverse toxicity. Finally, it was shown that inhibition of TNF-α by pomalidomide attenuated CPF-/CPF+CM-induced apoptosis. Overall, our data suggest that FAS/TNF signalling pathways may participate in CPF and CPF+CM toxicity.

Oral intake of hydrogen-rich water ameliorated chlorpyrifos-induced neurotoxicity in rats

Chronic exposure to low-levels of organophosphate (OP) compounds, such as chlorpyrifos (CPF), induces oxidative stress and could be related to neurological disorders. Hydrogen has been identified as a novel antioxidant which could selectively scavenge hydroxyl radicals. We explore whether intake of hydrogen-rich water (HRW) can protect Wistar rats from CPF-induced neurotoxicity. Rats were gavaged daily with 6.75mg/kg body weight (1/20 LD50) of CPF and given HRW by oral intake. Nissl staining and electron microscopy results indicated that HRW intake had protective effects on the CPF-induced damage of hippocampal neurons and neuronal mitochondria. Immunostaining results showed that the increased glial fibrillary acidic protein (GFAP) expression in astrocytes induced by CPF exposure can be ameliorated by HRW intake. Moreover, HRW intake also attenuated CPF-induced oxidative stress as evidenced by enhanced level of MDA, accompanied by an increase in GSH level and SOD and CAT activity. Acetylcholinesterase (AChE) activity tests showed significant decrease in brain AChE activity after CPF exposure, and this effect can be ameliorated by HRW intake. An in vitro study demonstrated that AChE activity was more intense in HRW than in normal water with or without chlorpyrifos-oxon (CPO), the metabolically-activated form of CPF. These observations suggest that HRW intake can protect rats from CPF-induced neurotoxicity, and the protective effects of hydrogen may be mediated by regulating the oxidant and antioxidant status of rats. Furthermore, this work defines a novel mechanism of biological activity of hydrogen by directly increasing the AChE activity.