The genetics of ivermectin resistance in Caenorhabditis elegans

Abstract

The ability of organisms to evolve resistance threatens the effectiveness of every antibiotic drug. We show that in the nematode Caenorhabditis elegans, simultaneous mutation of three genes, avr-14, avr-15, and glc-1, encoding glutamate-gated chloride channel (GluCl) ␣-type subunits confers high-level resistance to the antiparasitic drug ivermectin. In contrast, mutating any two channel genes confers modest or no resistance. We propose a model in which ivermectin sensitivity in C. elegans is mediated by genes affecting parallel genetic pathways defined by the family of GluCl genes. The sensitivity of these pathways is further modulated by unc-7, unc-9, and the Dyf (dye filling defective) genes, which alter the structure of the nervous system. Our results suggest that the evolution of drug resistance can be slowed by targeting antibiotic drugs to several members of a multigene family. Ivermectin is used to treat numerous parasitic infections of humans, pets, and livestock (1). Treatment with ivermectin is the cornerstone of efforts to eradicate river blindness (onchocerciasis). However, reports of resistance to ivermectin in nematodes are increasingly common (2-4). Ivermectin also kills the nematode Caenorhabditis elegans at therapeutic concentrations, making C. elegans a useful model system in which to examine mechanisms of ivermectin toxicity and resistance. Ivermectin activates glutamate-gated chloride channels (GluCls) that contain ␣-type channel subunits (5-7). In C. elegans, ␣-type subunits are encoded by a family of genes including: glc-1 (encoding GLC-1͞GluCl␣1), avr-15 (encoding AVR-15͞GluCl␣2), and possibly other uncharacterized genes found in the genome sequence (5-8). Severe loss-of-function mutations in glc-1 or avr-15 do not make worms resistant to ivermectin (6, 7), either because GluCls are not physiologically important targets of ivermectin, or because multiple GluCl genes contribute independently to ivermectin sensitivity. To clarify the role of the GluCls in the nematocidal effects of ivermectin, we had screened for ivermectin-resistant mutants (6). Here we analyze the effects of these and other, previously characterized mutations on ivermectin sensitivity. We show that simultaneous mutation of three genes encoding GluCl ␣-type subunits confers high-level resistance to ivermectin. Our results suggest that the ability of ivermectin to target several members of a multigene family may decrease the rate at which resistance evolves. MethodsGenetics. Unless otherwise indicated the mutant alleles used were: avr-14(ad1302), avr-15(ad1051), gcl-1(pk54::Tc1), unc-7(e5), unc-9(e101), osm-1(ad1307), osm-5(ad1308), dyf-11(ad1303), and che-3(ad1306). avr-15(ad1051), glc-1(pk54::Tc1), unc-7(e5), and unc-9(e101) appear to be molecular nulls (refs. 6,7,and 18; T. Starich, personal communication). Ivermectin-resistant mutants were isolated in a screen for ivermectin resistance in an avr-15(ad1051) background by using the mutagen ethyl methanesulfonate as described (6). All strains were outcrossed twice with N2 ...

Regulation of Intermuscular Electrical Coupling by theCaenorhabditis elegansInnexininx-6

“…The uncoordinated phenotype could result from the aberrant formation of an UNC-7-dependent channel or may reflect ectopic electrical junctions between motorneurons and interneurons in unc-7 mutants (Starich et al, 1993;Dent et al, 2000). The mutant phenotype of another innexin gene, unc-9, is very similar to that of unc-7, indicating that UNC-9 subunit may partner with UNC-7 to form the functional gap junction (Barnes and Hekimi, 1997).…”

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

“…As no additional orthologs of IVM-sensitive channels were found in the B. malayi genome database, these proteins must represent the main putative target for IVM in this parasite based on data obtained in other species of nematodes (26)(27)(28). Identifying their anatomical localization in mf provides new insights to explain the pharmacological consequences of treatment of filariases with IVM.…”

Comparative pharmacology and computational modelling yield insights into allosteric modulation of human α7 nicotinic acetylcholine receptors

“…In addition to nematode acetylcholine receptors, these targets also include P2X receptors of Schistosoma [32] and of mouse ( [33]) and histamine-gated chloride channels of Drosophila [34], as well as nematode [35][36][37][38] and insect [39,40] glutamate-gated chloride channels, GABA receptors from nematodes to vertebrates [41][42][43][44] and human glycine receptors [45]. There is also evidence for an action of IVM on an intracellular ligand-gated ion channel, the ryanodine receptor [46].…”