Trapping of ivermectin by a pentameric ligand-gated ion channel upon open-to-closed isomerization

Ivermectin (IVM) is a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. By activating invertebrate pentameric glutamate-gated chloride channels (GluCl receptors; GluClRs), IVM induces sustained chloride influx and long-lasting membrane hyperpolarization that inhibit neural excitation in nematodes. Although IVM activates the C. elegans heteromeric GluCl alpha/beta receptor, it cannot activate a homomeric receptor composed of the C. elegans GluCl beta subunits. To understand this incapability, we generated a homopentameric alpha 7-GluCl beta chimeric receptor that consists of an extracellular ligand-binding domain of an alpha 7 nicotinic acetylcholine receptor known to be potentiated by IVM, and a chloride-selective channel domain assembled from GluCl beta subunits. Application of IVM prior to acetylcholine inhibited the responses of the chimeric alpha 7-GluCl beta R. Adding IVM to activated alpha 7-GluCl beta Rs, considerably accelerated the decline of ACh-elicited currents and stabilized the receptors in a non-conducting state. Determination of IVM association and dissociation rate constants and recovery experiments suggest that, following initial IVM binding to open alpha 7-GluCl beta Rs, the drug induces a conformational change and locks the ion channel in a closed state for a long duration. We further found that IVM also inhibits the activation by glutamate of a homomeric receptor assembled from the C. elegans full-length GluCl beta subunits.