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, suggesting alternative targets. Moreover, different OPs can cause different adverse outcomes, suggesting that different OPs act through different mechanisms, emphasizing 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 general neurotoxicity. We leverage this feature to investigate potential differential effects of these OPs on the adult and developing brain by performing a comparative high-throughput 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.

Methods
Adult (A) and Regenerating (R) planarians of the species Dugesia japonica were screened for a multitude of morphological and behavioral endpoints on days 7(d7) and 12 (d12) of exposure. Experiments were performed in at least triplicate with n=8 per replicate (total n>=24). For binary endpoints (lethality, body shape, stickiness, scrunching and eye regeneration), the data are shown as the number of planarians affected (n_in) and the total number of planarians analyzed (N). For stickiness and scrunching, the incidence number (n_in) by normalized by the incidence number of the in-plate vehicle controls. For continuous endpoints, the data are provided as the normalized response ("resp"), which has been normalized by the in-plate vehicle controls. These datasets were used as input for the Rcurvep package to calculate the benchmark response and benchmark concentration. For more details, please see the corresponding publication.