Investigation of the selective toxicity of neonicotinoids using the nematode worm Caenorhabditis elegans

Abstract

An ability of insecticides to selectively target pests without affecting non-target species is a key determinant of success of compounds used in agriculture. Neonicotinoids which encompass seven different types of chemical representing three distinct chemical classes, namely the cyanoamidines, nitroguanidines and nitromethylenes, are a major class of insecticides. They effectively control a wide range of insect pests and have low toxicity against mammals, however they can also negatively impact on non-target species of bees, threatening food safety. Neonicotinoids act by targeting insect nicotinic acetylcholine receptors (nAChRs), which are major excitatory receptors in the insect central nervous system. Difficulties in heterologous expression of these proteins hinders their pharmacological characterisation and identification of the molecular determinants of neonicotinoid-toxicity. This thesis describes efforts into developing Caenorhabditis elegans (C. elegans) as a platform in which the mode of action and selective toxicity of neonicotinoid-insecticides can be studied.

We determined the effects of neonicotinoids on C. elegans behaviours governed by the cholinergic neurotransmission. The cyanoamidine represented by clothianidin, the nitroguanidine represented by thiacloprid and the nitromethylene represented by nitenpyram showed low efficacy on locomotion, pharyngeal pumping, egg-laying and egg-hatching of young adult wild-type C. elegans. Exposure of mutant worm with enhanced cuticular permeability showed increased susceptibility of worms to all three neonicotinoids tested, suggesting an adult cuticle limits drug access. The role of the cuticle in neonicotinoids susceptibility was investigated in C. elegans cut-head preparation, in which the cuticle is removed and the effects of compounds on pharyngeal pumping are scored. Out of the three neonicotinoids applied, clothianidin showed the greatest efficacy. It stimulated pharyngeal pumping at ≥ 75 µM (18.75 ppm). Generally, the concentrations effective against the function of the pharynx are an order or magnitude lower than the residual, average concentration of neonicotinoids in the soil, suggesting C. elegans is not impacted in the field, and at least several fold lower than lethal doses in insect-pests. The difference in neonicotinoid-susceptibility between adult C. elegans and insects precludes the use of C. elegans pharynx as a platform for the mode of action studies, but highlights its potential as a suitable background for the heterologous expression of insect nAChRs.

Further experiments showed that C. elegans eat-2 nAChR mutant is a suitable genetic background, in which the expression of heterologous nAChRs can be Expression of the exogenous receptor, human α7 in the pharynx of eat-2 mutant led to a cell-surface expression, as shown by staining with labeled α-bungarotoxin (α-bgtx). However the feeding and pharmacological phenotypes of the mutant were not rescued. C. elegans strain in which human α7 is expressed in the wild-type genetic background was also generated to determine whether the pharmacology of the human receptor can be imposed on the C. elegans pharynx. No difference in the pharyngeal response to nAChR agonists cytisine, nicotine or acetylcholine were noted. The lack of apparent functionality of α7 receptor could be due to the incorrect cellular localisation of this protein. α-bgtx staining showed that α7 receptor is expressed in the specific cells of the pharyngeal muscle, however this localisation does not overlap with the localisation of native EAT-2 receptors. A transgenic strain in which exogenous proteins are expressed using EAT-2 native promoter should be made. scored. Expression of C. elegans nAChR EAT-2 in the pharyngeal muscle rescued the blunted feeding phenotype and 5-HT insensitivity of the eat-2 mutant.

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Identifiers

ORCID for Lindy Holden-Dye: orcid.org/0000-0002-9704-1217

ORCID for Vincent O'connor: orcid.org/0000-0003-3185-5709

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