Functional reconstitution of Haemonchus contortus acetylcholine receptors in Xenopus oocytes provides mechanistic insights into levamisole resistance.

The cholinergic agonist levamisole is widely used to treat parasitic nematode infestations. This anthelmintic drug paralyses worms by activating a class of levamisole-sensitive acetylcholine receptors (L-AChRs) expressed in nematode muscle cells. However, levamisole efficacy has been compromised by the emergence of drug-resistant parasites, especially in gastrointestinal nematodes such as Haemonchus contortus. We report here the first functional reconstitution and pharmacological characterization of H. contortus L-AChRs in a heterologous expression system. In the free-living nematode Caenorhabditis elegans, five AChR subunit and three ancillary protein genes are necessary in vivo and in vitro to synthesize L-AChRs. We have cloned the H. contortus orthologues of these genes and expressed them in Xenopus oocytes. We reconstituted two types of H. contortus L-AChRs with distinct pharmacologies by combining different receptor subunits. The Hco-ACR-8 subunit plays a pivotal role in selective sensitivity to levamisole. As observed with C. elegans L-AChRs, expression of H. contortus receptors requires the ancillary proteins Hco-RIC-3, Hco-UNC-50 and Hco-UNC-74. Using this experimental system, we demonstrated that a truncated Hco-UNC-63 L-AChR subunit, which was specifically detected in a levamisole-resistant H. contortus isolate, but not in levamisole-sensitive strains, hampers the normal function of L-AChRs, when co-expressed with its full-length counterpart. We provide the first functional evidence for a putative molecular mechanism involved in levamisole resistance in any parasitic nematode. This expression system will provide a means to analyse molecular polymorphisms associated with drug resistance at the electrophysiological level.