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.