The narrow-spectrum anthelmintic oxantel is a potent agonist of a novel acetylcholine receptor subtype in whipworms

Abstract

In the absence of efficient alternative strategies, the control of parasitic nematodes, impacting human and animal health, mainly relies on the use of broad-spectrum anthelmintic compounds. Unfortunately, most of these drugs have a limited single-dose efficacy against infections caused by the whipworm, Trichuris. These infections are of both human and veterinarian importance. However, in contrast to a wide range of parasitic nematode species, the narrow-spectrum anthelmintic oxantel has a high efficacy on Trichuris spp. Despite this knowledge, the molecular target(s) of oxantel within Trichuris is still unknown. In the distantly related pig roundworm, Ascaris suum, oxantel has a small, but significant effect on the recombinant homomeric Nicotine-sensitive ionotropic acetylcholine receptor (N-AChR) made up of five ACR-16 subunits. Therefore, we hypothesized that in whipworms, a putative homolog of an ACR-16 subunit, can form a functional oxantel-sensitive receptor. Using the pig whipworm T. suis as a model, we identified and cloned a novel ACR-16-like receptor subunit and successfully expressed the corresponding homomeric channel in Xenopus laevis oocytes. Electrophysiological experiments revealed this receptor to have distinctive pharmacological properties with oxantel acting as a full agonist, hence we refer to the receptor as an O-AChR subtype. Pyrantel activated this novel O-AChR subtype moderately, whereas classic nicotinic agonists surprisingly resulted in only minor responses. We demonstrated that the novel Tsu-ACR-16-like receptor is indeed a target for oxantel and is more responsive to oxantel than the ACR-16 receptor from A. suum. These finding most likely explain the high sensitivity of whipworms to oxantel, and highlights the importance of the discovery of additional distinct receptor subunit types within Trichuris that can be used as valuable screening tools to evaluate the effect of new synthetic or natural anthelmintic compounds. Author SummaryThe human whipworm, Trichuris trichiura, is an intestinal parasitic nematode infecting approximately 289.6 million people globally, primarily children living in developing countries. Chronic T. trichiura infection may cause dysentery, growth stunting and decreased cognitive performance. Whipworm infections are notoriously difficult to control with most available anthelmintics, including those commonly used in mass drug administration programs. Recently performed randomised controlled trials with whipworm-infected humans, have reported superior efficacies of oxantel, a classic, narrow-spectrum anthelmintic, developed for the treatment of Trichuris infections. Despite this knowledge, the molecular target(s) of oxantel within the whipworm has not been identified. In this study, we used the whipworm from pigs as a model and identified a receptor, which was explored using the Xenopus oocyte expression system. We demonstrated that this receptor is highly responsive to oxantel, and therefore a major target of oxantel within Trichuris. In addition, we discovered that this receptor-type is distinctive and only present in the ancient group of parasitic nematodes, Clade I, which also includes the important zoonotic parasite Trichinella. Our findings, explain the specific mode of action of oxantel and open the way for additional characterization of similar receptor subtypes in other medically or veterinary important parasitic nematodes of Clade I.