Evaluation of sublethal effects of dichlorvos uponCaenorhabditis elegansbased on a set of end points of toxicity

C. elegans pharyngeal pumping provides a whole organism bio-assay to investigate anti-cholinesterase intoxication and antidotes

Inhibition of acetylcholinesterase by either organophosphates or carbamates causes anti-cholinesterase poisoning. This arises through a wide range of neurotoxic effects triggered by the overstimulation of the cholinergic receptors at synapses and neuromuscular junctions. Without intervention, this poisoning can lead to profound toxic effects, including death, and the incomplete efficacy of the current treatments, particularly for oxime-insensitive agents, provokes the need to find better antidotes. Here we show how the non-parasitic nematode Caenorhabditis elegans offers an excellent tool for investigating the acetylcholinesterase intoxication. The C. elegans neuromuscular junctions show a high degree of molecular and functional conservation with the cholinergic transmission that operates in the autonomic, central and neuromuscular synapses in mammals. In fact, the anti-cholinesterase intoxication of the worm's body wall neuromuscular junction has been unprecedented in understanding molecular determinants of cholinergic function in nematodes and other organisms. We extend the use of the model organism's feeding behaviour as a tool to investigate carbamate and organophosphate mode of action. We show that inhibition of the cholinergic-dependent rhythmic pumping of the pharyngeal muscle correlates with the inhibition of the acetylcholinesterase activity caused by aldicarb, paraoxons and DFP exposure. Further, this bio-assay allows one to address oxime dependent reversal of cholinesterase inhibition in the context of whole organism recovery. Interestingly, the recovery of the pharyngeal function after such anti-cholinesterase poisoning represents a sensitive and easily quantifiable phenotype that is indicative of the spontaneous recovery or irreversible modification of the worm acetylcholinesterase after inhibition. These observations highlight the pharynx of C. elegans as a new tractable approach to explore anti-cholinesterase intoxication and recovery with the potential to resolve critical genetic determinants of these neurotoxins' mode of action.

Protective manifestation of bacoside A and bromelain in terms of cholinesterases, gamma-amino butyric acid, serotonin level and stress proteins in the brain of dichlorvos-intoxicated mice

The objective of the study was to evaluate the neuroprotective effects of bacoside A and bromelain against dichlorvos-incited toxicity. Healthy 6-8-week old, male Swiss mice were administered subacute doses of dichlorvos (40 mg/kg bw), bacoside A (5 mg/kg bw) and bromelain (70 mg/kg bw). AChE, BChE, GABA, serotonin and total protein content and their expressions were used for determination of toxic action of dichlorvos. Protective effects of bacoside A and bromelain were evaluated on the same parameters. Exposure to dichlorvos leads to significant decline in activities of AChE (p < 0.01, p < 0.001), BChE (p < 0.05) and GABA (p < 0.01) and total protein levels (p < 0.01). Antioxidant treatment significantly increased the activities of AChE (p < 0.01, p < 0.001), BChE (p < 0.05), GABA (p < 0.01) and total protein level (p < 0.05) compared to those in dichlorvos-treated mice. Overexpression of Hsp 70 protein and underexpression of phosphorylase a and b, catalase SOD and GPx were observed after dichlorvos exposure which suggests the oxidative stress. The results indicate that dichlorvos-induced neuronal damage which results in the generation of molecular expression of proteins is in agreement with the biochemical data ameliorated by bacoside A and bromelain.

Sublethal Toxicity Endpoints of Heavy Metals to the Nematode Caenorhabditis elegans

Caenorhabditis elegans, a free-living nematode, is commonly used as a model organism in ecotoxicological studies. The current literatures have provided useful insight into the relative sensitivity of several endpoints, but few direct comparisons of multiple endpoints under a common set of experimental conditions. The objective of this study was to determine appropriate sublethal endpoints to develop an ecotoxicity screening and monitoring system. C. elegans was applied to explore the sublethal toxicity of four heavy metals (copper, zinc, cadmium and chromium). Two physiological endpoints (growth and reproduction), three behavioral endpoints (head thrash frequency, body bend frequency and feeding) and two enzymatic endpoints (acetylcholine esterase [AChE] and superoxide dismutase [SOD]) were selected for the assessment of heavy metal toxicity. The squared correlation coefficients (R²) between the responses observed and fitted by Logit function were higher than 0.90 and the RMSE were lower than 0.10, indicating a good significance statistically. There was no significant difference among the half effect concentration (EC50) endpoints in physiological and behavioral effects of the four heavy metals, indicating similar sensitivity of physiological and behavioral effects. AChE enzyme was more sensitive to copper, zinc, and cadmium than to other physiological and behavioral effects, and SOD enzyme was most sensitive to chromium. The EC50 of copper, zinc, and cadmium, to the AChE enzyme in the nematodes were 0.68 mg/L, 2.76 mg/L, and 0.92 mg/L respectively and the EC50 of chromium to the SOD enzyme in the nematode was 1.58 mg/L. The results of this study showed that there was a good concentration-response relationship between all four heavy metals and the sublethal toxicity effects to C. elegans. Considering these sublethal endpoints in terms of simplicity, accuracy, repeatability and costs of the experiments, feeding is the relatively ideal sublethal toxicity endpoint of heavy metals to C. elegans.

Antioxidant and neuroprotective potential of chitooligomers in Caenorhabditis elegans exposed to Monocrotophos

The objectives of this investigation were to establish the propensity of the chitooligomers (COS) to ameliorate neurodegeneration and oxidative stress in Caenorhabditis elegans induced by an organophosphorus insecticide, Monocrotophos (MCP). COS was prepared from α-chitosan by the enzymatic method using chitosanase and characterized by HPLC and electron spray ionization-TOF-(ESI-TOF)-MS. We exposed age synchronized L4 C. elegans worms (both wild type N2 and transgenic strain BZ555 (Pdat-1:GFP) to sublethal concentration of MCP (0.75mM) for 24h in the presence or absence of COS (0.2mM). The neuroprotective effect of COS was examined in N2 worms in terms of brood size, lifespan, egg laying, dopamine content, acetylcholinesterase and carboxylesterase activity and by direct visualization and quantification of degeneration of dopaminergic neurons in BZ555. Exposure to COS extended lifespan, normalized egg laying, increased brood size, decreased the dopaminergic neurodegeneration, increased the dopamine content and increased AChE and carboxylesterase activity in C. elegans treated with MCP. COS induced a significant decrease in reactive oxygen species and increased the reduced glutathione level as well as increased superoxide dismutase and catalase activity. Our findings demonstrate that COS significantly inhibits the dopaminergic neurodegeneration and associated physiological alterations induced by MCP in C. elegans by attenuating the oxidative stress as well.

Glucose feeding during development aggravates the toxicity of the organophosphorus insecticide Monocrotophos in the nematode, Caenorhabditis elegans

Several studies have demonstrated that high glucose feeding induced oxidative stress and apoptosis thereby affecting growth, fertility, aging and lifespan in Caenorhabditis elegans. Earlier studies from our laboratory had clearly established the propensity of monocrotophos, an OPI to alter the physiological and behavioral responses of C. elegans. The present study was aimed to investigate the effect of monocrotophos (MCP) on physiological/behavioral and biochemical responses in C. elegans that were maintained on high glucose diet. We exposed the worms through development to high glucose diet (2%) and then treated with sublethal concentrations of MCP (0.5, 0.75, 1.5mM). We measured the behavioral responses in terms of locomotion, physiological responses in terms of egg laying, brood size, lifespan; morphological alterations; and biochemical responses including glucose content. The worms exposed from egg stage through development to high glucose diet showed enhanced toxic outcome of MCP in terms of physiological, behavioral and biochemical responses. Our studies showed that C. elegans is a good model to study glucose-OPI interactive neurotoxicity since all the responses could be studied at ease in this organism and the outcome could be well extrapolated to those that one would expect in higher animals.

Toxicity testing of neurotoxic pesticides in Caenorhabditis elegans

The use of pesticides is ubiquitous worldwide, and these chemicals exert adverse effects on both target and nontarget species. Understanding the modes of action of pesticides, as well as quantifying exposure concentration and duration, is an important goal of clinicians and environmental health scientists. Some chemical exposures result in adverse effects on the nervous system. The nematode Caenorhabditis elegans (C. elegans) is a model lab organism well established for studying neurotoxicity, since the components of its nervous system are mapped and known, and most of its neurotransmitters correspond to human homologs. This review encompasses published studies in which C. elegans nematodes were exposed to pesticides with known neurotoxic actions. Endpoints measured include changes in locomotion, feeding behavior, brood size, growth, life span, and cell death. From data presented, evidence indicates that C. elegans can serve a role in assessing the effects of neurotoxic pesticides at the sublethal cellular level, thereby advancing our understanding of the mechanisms underlying toxicity induced by these chemicals. A proposed toxicity testing scheme for water-soluble chemicals is also included.

Biosafety assessment of titanium dioxide nanoparticles in acutely exposed nematode Caenorhabditis elegans with mutations of genes required for oxidative stress or stress response

We used Caenorhabditis elegans to investigate whether acute exposure to TiO2-NPs at the concentration of 20 μg L(-1) reflecting predicted environmental relevant concentration and 25 mg L(-1) reflecting concentration in food can cause toxicity on nematodes with mutations of susceptible genes. Among examined mutants associated with oxidative stress and stress response, we found that genes of sod-2, sod-3, mtl-2, and hsp-16.48 might be susceptible for TiO2-NPs toxicity. Mutations of these genes altered functions of both possible primary and secondary targeted organs in nematodes exposed to 25 mg L(-1) of TiO2-NPs for 24-h. Mutations of these genes caused similar expression patterns of genes required for oxidative stress in TiO2-NPs exposed mutant nematodes, implying their similar mechanisms to form the susceptible property. Nevertheless, acute exposure to 20 μg L(-1) of TiO2-NPs for 24-h and 25 mg L(-1) of TiO2-NPs for 0.48-h or 5.71-h did not influence functions of both possible primary and secondary targeted organs in sod-2, sod-3, mtl-2, and hsp-16.48 mutants. Therefore, our results suggest the relatively safe property of acute exposure to TiO2-NPs with certain durations at predicted environmental relevant concentrations or concentrations comparable to those in food in nematodes with mutations of some susceptible genes.

Biochemical and physiological responses in Caenorhabditis elegans exposed to sublethal concentrations of the organophosphorus insecticide, monocrotophos

The objective of this study was to investigate the impact of sublethal concentrations of MCP on definitive markers of toxicity and oxidative balance in the nematode, Caenorhabditis elegans. Exposure of worms to 0.85, 1.7 and 3.4mM (1/40, 1/20 and 1/10 LC50; LC50=34 mM) for 4h at 20°C induced significant perturbations in physiological parameters such as decreased brood size (47-73%), increased paralysis (47-85%) and inhibition of the activities of acetylcholinesterase (75-86%) and carboxylesterase (76-81%). These changes were accompanied by distinct oxidative impairments as evidenced by increased reactive oxygen species, decline in glutathione content and decrease in superoxide dismutase activity. Our results clearly demonstrate that low concentrations of MCP may alter the physiological and biochemical status in the nematode, thereby affecting the organism's fitness. Our findings on C. elegans provide an easy diagnosis for OPI contamination and may become helpful in evaluating the ecotoxicological effects of OPI in the aquatic environment near agricultural fields.

Alterations in gene expression in Caenorhabditis elegans associated with organophosphate pesticide intoxication and recovery

Background

The principal toxicity of acute organophosphate (OP) pesticide poisoning is the disruption of neurotransmission through inhibition of acetylcholinesterase (AChE). However, other mechanisms leading to persistent effects and neurodegeneration remain controversial and difficult to detect. Because Caenorhabditis elegans is relatively resistant to OP lethality—particularly through the inhibition of AChE—studies in this nematode provide an opportunity to observe alterations in global gene expression following OP exposure that cannot be readily observed in less resistant organisms.

Results

We exposed cultures of worms in axenic, defined medium to dichlorvos under three exposure protocols. In the first, worms were exposed continuously throughout the experiment. In the second and third, the worms were exposed for either 2 or 8 h, the dichlorvos was washed out of the culture, and the worms were allowed to recover. We then analyzed gene expression using whole genome microarrays from RNA obtained from worms sampled at multiple time points throughout the exposure. The worms showed a time-dependent increase in the expression of genes involved in stress responses. Early in the exposure, the predominant effect was on metabolic processes, while at later times, an immune-like response and cellular repair mechanisms dominated the expression pattern. Following removal of dichlorvos, the gene expression in the worms appeared to relatively rapidly return to steady-state levels.

Conclusion

The changes in gene expression observed in the worms following exposure to dichlorvos point towards two potential mechanisms of toxicity: inhibition of AChE and mitochondrial disruption.

Evaluation of environmental safety concentrations of DMSA Coated Fe2O3-NPs using different assay systems in nematode Caenorhabditis elegans

Dimercaptosuccinic acid (DMSA) coating improves the uptake efficiency presumably by engendering the Fe(2)O(3)-NPs. In the present study, we investigated the possible environmental safety concentrations of Fe(2)O(3)-NPs using different assay systems in nematode Caenorhabditis elegans with lethality, development, reproduction, locomotion behavior, pharyngeal pumping, defecation, intestinal autofluorescence and reactive oxygen species (ROS) production as the endpoints. After exposure from L4-larvae for 24-hr, DMSA coated Fe(2)O(3)-NPs at concentrations more than 50 mg/L exhibited adverse effects on nematodes. After exposure from L1-larvae to adult, DMSA coated Fe(2)O(3)-NPs at concentrations more than 500 μg/L had adverse effects on nematodes. After exposure from L1-larvae to day-8 adult, DMSA coated Fe(2)O(3)-NPs at concentrations more than 100 μg/L resulted in the adverse effects on nematodes. Accompanied with the alterations of locomotion behaviors, ROS production was pronouncedly induced by exposure to DMSA coated Fe(2)O(3)-NPs in the examined three assay systems, and the close associations of ROS production with lethality, growth, reproduction, locomotion behavior, pharyngeal pumping, defecation, or intestinal autofluorescence in nematodes exposed to DMSA coated Fe(2)O(3)-NPs were confirmed by the linear regression analysis. Moreover, mutations of sod-2 and sod-3 genes, encoding Mn-SODs, showed more susceptible properties than wild-type when they were used for assessing the DMSA coated Fe(2)O(3)-NPs-induced toxicity, and the safety concentrations for DMSA coated Fe(2)O(3)-NPs should be defined as concentrations lower than 10 μg/L in sod-2 and sod-3 mutant nematodes.