Glutamate-gated chloride channels and the mode of action of the avermectin/milbemycin anthelmintics

Abstract

The macrocyclic lactones are the biggest selling and arguably most effective anthelmintics currently available. They are good substrates for the P-glycoproteins, which might explain their selective toxicity for parasites over their vertebrate hosts. Changes in the expression of these pumps have been implicated in resistance to the macrocyclic lactones, but it is clear that they exert their anthelmintic effects by binding to glutamate-gated chloride channels expressed on nematode neurones and pharyngeal muscle cells. This effect is quite distinct from the channel opening induced by glutamate, the endogenous transmitter acting at these receptors, which produces rapidly opening and desensitising channels. Ivermectin-activated channels open very slowly but essentially irreversibly, leading to a very long-lasting hyperpolarisation or depolarisation of the neurone or muscle cell and therefore blocking further function. Molecular and genetic studies have shown that there are multiple GluCl isoforms in both free-living and parasitic nematodes: the exact genetic make-up and functions of the GluCl may vary between species. The known expression patterns of the GluCl explain most of the observed biological effects of treatment with the macrocyclic lactones, though the reason for the long-lasting inhibition of larval production in filarial species is still poorly understood.

Ivermectin disrupts the function of the excretory-secretory apparatus in microfilariae of Brugia malayi

“…Phylogenetic analysis and naming according to Beech et al (19) allowed us to infer that these subunits are orthologs of the GLC-2, GLC-4, AVR-14A, and AVR-14B subunits in C. elegans and other nematodes. Among them, only AVR-14A and AVR-14B are reported to form IVMsensitive channels (14). Because assembly and annotation of these subunits in the B. malayi genome database was ambiguous and incomplete, the full ORFs for these subunits were assessed (SI Materials and Methods, Fig.…”

“…This remarkably broad spectrum of action and record of mammalian safety is partly a result of the absence of GluCls from vertebrates and the essential role of these channels in neurotransmission in some types of invertebrates (14). Beyond this, the cascade of events following GluCl opening, which leads to the observed effects of IVM treatment in filariases, is still unclear.…”

“…GluCls are members of the "Cys-loop" family of ligand-gated ion channels, which form pentameric structures composed of one or more subunit types (14). GluCl gene diversity across the phylum Nematoda is significant, but the biological processes regulated by the activity of these channels have been investigated in few species.…”

“…Neither their localization nor physiological roles can be simply extrapolated between phylogenetically distant nematode species. In C. elegans, for example, six GluCl genes have been identified as encoding eight possible protein subunits formed by alternative splicing (14). In contrast, homology searching for gene models that represent probable GluCl subunits in the genome of B. malayi This article is a PNAS Direct Submission.…”

Molecular Determinants of Ivermectin Sensitivity at the Glycine Receptor Chloride Channel

“…The biological target for ivermectin and related macrocyclic lactones is a glutamate-gated chloride channel receptor (GluClR) 5 that is expressed in the neurons and muscle cells of nematodes and some arthropods but is absent in vertebrates (2). Ivermectin irreversibly activates these GluClRs at low nanomolar concentrations, thereby inhibiting neuronal activity and muscle contractility and thus inducing death by flaccid paralysis (3). Unfortunately, ivermectin resistance is emerging as a serious problem in nematodes and arthropods (4 -6).…”