Modelling organophosphate intoxication in C. elegans: a framework for new treatments vistas

Abstract

Organophosphates are potent neurotoxins that bind to acetylcholinesterase and inhibit its function. The inhibited enzyme fails to hydrolyse acetylcholine at the synaptic cleft and neuromuscular junctions which leads to accumulate. The excess of neurotransmitter reflects prolonged activation of both nicotinic and muscarinic types of receptors triggering varying levels of toxicity in humans. Asphyxia is the main cause of death and arises due to the paralysis of the breathing musculature aggravated by the disruption of the respiratory centres in the brain.

The evolutionary conservation of acetylcholinesterase has led to the wide use of organophosphates as pesticides. Despite their known toxicity, a large volume of organophosphates is still in use for this purpose. In addition, organophosphates are used as nerve agents for chemical warfare and terrorism. The limited efficiency of the current therapy urges the necessity of developing alternative strategies to treat organophosphate poisoning. These strategies frequently emerge from investigations using mammalian model organisms.

Here, I exploit the free-living nematode C. elegans as suitable model to achieve this goal. This nematode exhibits a set of well-characterized behaviours that rely on the proper signalling of acetylcholine at its neuromuscular junctions. Behavioural and biochemical studies highlighted the pharyngeal pumping on food as the most suitable phenotype to research organophosphate intoxication, recovery and antidotes. Further investigations indicated that the profound organophosphate-induced inhibition of pumping is actually triggered by overstimulation of the body wall muscle nerve transmission. This raises the idea of an inter-tissue communication happened in this stress condition.

Systematic investigation of pharyngeal and body wall cholinergic transmission revealed an unexpected organophosphate-induced plasticity in which prolonged incubation in presence of organophosphates sees a mitigation against toxicity. A methodical screening of strains deficient in cholinergic transmission identified that the integrity of the levamisole-sensitive acetylcholine receptor at the body wall muscle is central to the organophosphate-induced plasticity.

Importantly, two auxiliary proteins these receptors involved in the synaptic homeostasis during poisoning (OIG-4 and RSU-1) have been identified to modulate the severity of the drug-induced plasticity. In addition, key receptor subunit alleles (UNC-29 P284S and LEV-1 G461E) and the positive allosteric modulator MOLO-1 were identified as determinants underpinning the level of mitigating plasticity. I take these results to provoke the idea of using nicotinic receptors and/or the auxiliary proteins involved in their function as drug targets to develop new antidotes against organophosphate poisoning. Overall, the outcome of this thesis highlights a novel whole organism quantitative approach capable of revealing new insights into the mechanism and mitigation of this important class of neurotoxins.

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Identifiers

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

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

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