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

Abstract

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 to adequately evaluate potential human hazard. 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 this planarian 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. Additionally, we found that D. japonica has both carboxylesterase and paraoxonase activity. Using specific chemical inhibitors, we show that carboxylesterase activity is distinct from cholinesterase activity. Taken together, these results further support the use of D. japonica for OP toxicity studies.

Acetylcholinesterase Activity Staining in Freshwater Planarians

“…The worms are now ready for imaging but can be stored at 4°C in the dark for a few days before mounting and imaging. Specificity of the staining can be determined using concentrations of AChE inhibitors known to inhibit AChE in planarians, such as physostigmine or OPs like diazinon (Hagstrom et al, 2018;Ireland et al, 2022aIreland et al, , 2022b.…”

“…Several research groups have shown that AChE activity can be detected in various planarian species (Bullock & Nachmansohn, 1942; De Simone et al., 1994; Villar et al., 1994; Zheng et al., 2011). We have shown that AChE activity can be detected during all stages of neurodevelopment in D. japonica planarians (Ireland et al., 2022a). This species is also sensitive to and can metabolize organophosphorus pesticides (Hagstrom et al., 2017; Ireland et al., 2022a).…”

“…We have shown that AChE activity can be detected during all stages of neurodevelopment in D. japonica planarians (Ireland et al., 2022a). This species is also sensitive to and can metabolize organophosphorus pesticides (Hagstrom et al., 2017; Ireland et al., 2022a). Together, this makes D. japonica planarians an especially suitable invertebrate model for studying the developmental neurotoxicity of these compounds.…”

New Worm on the Block: Planarians in (Neuro)Toxicology

“…It remains to be established what the context of use is for the planarian system, considering its unique strengths and limitations. We need to better understand the toxicokinetics and toxicodynamics of planarians, as only limited studies of planarian xenobiotic metabolism have been undertaken (Ireland, Rabeler, Gong, & Collins, 2022). How does planarian screening complement other models when integrated into a DNT test battery?…”

Dissecting the Enantioselective Neurotoxicity of Isocarbophos: Chiral Insight from Cellular, Molecular, and Computational Investigations

“…Organophosphorus compounds play a major role in chiral pesticide pollutants because this type of agrochemicals is the most produced and applied plant protection agent in the world. , Especially in some developing countries such as Brazil, China, and India, the usage of organophosphorus pesticides even reaches more than 60–70% of the total amount of pesticides. , However, commercial organophosphorus pesticides are sold and used in the form of racemate for a long time, without strict discrimination between single enantiomers, which will entail a serious threat to human health. , Zhang et al found that chiral organophosphorus malaoxon, isomalathion, and methamidophos have very similar enantioselectivity to target acetylcholinesterase (AChE) on the inhibition kinetics and postinhibitory reactions of acid α-naphthyl acetate esterase, which will greatly increase the risk of these organophosphates to human health. The results of wet experiments and computational chemistry reported by Zhang et al clearly unknot that ( R )-trichloronate, ( S )-leptophos, and ( R )-fonofos have significantly stronger inhibitory activity and affinity for human AChE than their antipodes, and they further argued that the toxicity risk of racemic organophosphorus pollutants presented widely in the environment needs to be reassessed.…”