acr-23 Encodes a Monepantel-Sensitive Channel in Caenorhabditis elegans

Abstract

Monepantel is a member of the recently identified class of anthelmintics known as the amino-acetonitrile derivatives (AADs). Monepantel controls all major gastro-intestinal nematodes in sheep including those that are resistant to the classical anthelmintics. Previous studies have shown that the Caenorhabditis elegans acr-23 and the Haemonchus contortus Hco-mptl-1 genes may be prominent targets of monepantel. With this discovery it became possible to investigate the mode of action of monepantel in nematodes at the molecular level. In the present study, we show that a C. elegans mutant acr-23 strain is fully rescued by expressing the wild-type acr-23 gene. Moreover, we present a new mutant allele, and characterize acr-23 alleles genetically. We also show that acr-23 is expressed in body wall muscle cells, and provide therefore a possible explanation for the paralysis caused by monepantel. Furthermore, genetic evidence suggests that the chaperone RIC-3 is required for expression of full monepantel resistance. Finally, we present reconstitution of the C. elegans ACR-23 receptor in Xenopus laevis oocytes and provide direct evidence of its modulation by monepantel. Conversely, co-injection of the chaperone RIC-3 had no impact for channel reconstitution in X. laevis oocytes. These results reinforce the involvement of the ACR-23 family in the mode of action of monepantel and advance our understanding of this new class of anthelmintics.

Acquired resistance to monepantel in C. elegans – what about parasitic nematodes?

“…We found that choline and monepantel act as agonists on ACR-23 and therefore proposed that ACR-23 forms a monepantel-sensitive channel that is permeable to monovalent cations. 3 We also suggested that the anthelmintic blocks the channel in its open configuration and found ACR-23 to be mainly expressed in body wall muscle cells. We therefore proposed that monepantel blocks muscle function by continuous depolarization of the membrane of muscle cells.…”

“…3 A more recent report showed that a reporter construct driven by the acr-23 promoter was mainly expressed in the mechanosensory neurons PLM, PVM, AVM and ALM and in body wall muscle cells. 6 In our transgenic strain, we can observe two cells expressing acr-23 in the tail, but their position and shape neither correspond to the two PLM, nor to cells of the body wall muscles.…”

“…3 However monepantel has little or no efficacy against certain parasitic nematodes like Taenia ovis, Esophagostomum venulosum, or the free-living nematode Pristionchus pacificus, which lacks the monepantel receptor. 4,5 C. elegans is not a parasite, but a nematode that can easily and conveniently be maintained in culture.…”

“…We therefore proposed that monepantel blocks muscle function by continuous depolarization of the membrane of muscle cells. 3 More recently, glycine betaine was identified as the natural ligand of ACR-23. 6 Moreover, in this study monepantel was shown to boost betaine action, resulting in spastic paralysis.…”

“…This study, however, proposes that ACR-23 acts in neurons rather than in muscles where ACR-23 is predominantly expressed. 3,6 In any case, the nervous system is tightly coupled to the muscular system via the neuromuscular junctions, so they can possibly function in tandem with regards to ACR-23, therefore accounting for the discrepancies observed in the two studies.…”