title: Comparative pharmacology of flatworm and roundworm glutamate-gated chloride channels: Implications for potential anthelmintics.
abstract: Pharmacological targeting of glutamate-gated chloride channels (GluCls) is a potent anthelmintic strategy, evidenced by macrocyclic lactones that eliminate numerous roundworm infections by activating roundworm GluCls. Given the recent identification of flatworm GluCls and the urgent need for drugs against schistosomiasis, flatworm GluCls should be evaluated as potential anthelmintic targets. This study sought to identify agonists or modulators of one such GluCl, SmGluCl-2 from the parasitic flatworm Schistosoma mansoni. The effects of nine glutamate-like compounds and three monoterpenoid ion channel modulators were measured by electrophysiology at SmGluCl-2 recombinantly expressed in Xenopus laevis oocytes. For comparison with an established anthelmintic target, experiments were also performed on the AVR-14B GluCl from the parasitic roundworm Haemonchus contortus. l-Glutamate was the most potent agonist at both GluCls, but l-2-aminoadipate, d-glutamate and d-2-aminoadipate activated SmGluCl-2 (EC50 1.0 ± 0.1 mM, 2.4 ± 0.4 mM, 3.6 ± 0.7 mM, respectively) more potently than AVR-14B. Quisqualate activated only SmGluCl-2 whereas l-aspartate activated only AVR-14B GluCls. Regarding the monoterpenoids, both GluCls were inhibited by propofol, thymol and menthol, SmGluCl-2 most potently by thymol (IC50 484 ± 85 μM) and least potently by menthol (IC50 > 3 mM). Computational docking suggested that agonist and inhibitor potency is attributable to particular interactions with extracellular or membrane-spanning amino acid residues. These results reveal that flatworm GluCls are pharmacologically susceptible to numerous agonists and modulators and indicate that changes to the glutamate γ-carboxyl or to the propofol 6-isopropyl group can alter the differential pharmacology at flatworm and roundworm GluCls. This should inform the development of more potent compounds and in turn lead to novel anthelmintics.
authors.affiliation: Neurophysiology and Neurosensory Systems, Technical University of Darmstadt, 64287 Darmstadt, Germany.
authors.forename: Timothy
authors.initials: T
authors.lastname: Lynagh
authors.affiliation: Health Innovations Research Institute and School of Medical Sciences, RMIT University, Bundoora, Victoria 3083, Australia.
authors.forename: Brett A
authors.initials: BA
authors.lastname: Cromer
authors.affiliation: Centre for Host-Parasite Interactions, Institute of Parasitology, McGill University - MacDonald Campus, Sainte-Anne-de-Bellevue, Québec H9X 3V9, Canada.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Dufour
authors.affiliation: Neurophysiology and Neurosensory Systems, Technical University of Darmstadt, 64287 Darmstadt, Germany.
authors.forename: Bodo
authors.initials: B
authors.lastname: Laube
keywords: Anthelmintic; Binding site; ECD, extracellular domain; GABA, γ-aminobutyric acid; GABAAR, type A γ-aminobutyric acid receptor; GluCl; GluCl, glutamate-gated chloride channel; GlyR, glycine receptor; Propofol; Schistosomiasis; TMD, transmembrane domain; Thymol; cis-ACBD, cis-1-aminocyclobutane-1,3-dicarboxylate; iGluR, (tetrameric) ionotropic glutamate receptor; pLGIC, pentameric ligand-gated ion channel (or Cys-loop receptor)