Recent Duplication and Functional Divergence in Parasitic Nematode Levamisole-Sensitive Acetylcholine Receptors

Abstract

Helminth parasites rely on fast-synaptic transmission in their neuromusculature to experience the outside world and respond to it. Acetylcholine plays a pivotal role in this and its receptors are targeted by a wide variety of both natural and synthetic compounds used in human health and for the control of parasitic disease. The model, Caenorhabditis elegans is characterized by a large number of acetylcholine receptor subunit genes, a feature shared across the nematodes. This dynamic family is characterized by both gene duplication and loss between species. The pentameric levamisole-sensitive acetylcholine receptor has been characterized from C. elegans, comprised of five different subunits. More recently, cognate receptors have been reconstituted from multiple parasitic nematodes that are found to vary in subunit composition. In order to understand the implications of receptor composition change and the origins of potentially novel drug targets, we investigated a specific example of subunit duplication based on analysis of genome data for 25 species from the 50 helminth genome initiative. We found multiple independent duplications of the unc-29, acetylcholine receptor subunit, where codon substitution rate analysis identified positive, directional selection acting on amino acid positions associated with subunit assembly. Characterization of four gene copies from a model parasitic nematode, Haemonchus contortus, demonstrated that each copy has acquired unique functional characteristics based on phenotype rescue of transgenic C. elegans and electrophysiology of receptors reconstituted in Xenopus oocytes. We found evidence that a specific incompatibility has evolved for two subunits co-expressed in muscle. We demonstrated that functional divergence of acetylcholine receptors, driven by directional selection, can occur more rapidly than previously thought and may be mediated by alteration of receptor assembly. This phenomenon is common among the clade V parasitic nematodes and this work provides a foundation for understanding the broader context of changing anthelmintic drug targets across the parasitic nematodes.

Deciphering the molecular determinants of cholinergic anthelmintic sensitivity in nematodes: When novel functional validation approaches highlight major differences between the model Caenorhabditis elegans and parasitic species

“…Recently, we showed that C . elegans was a tractable model to express AChR receptors from parasitic nematode species [ 35 , 38 , 39 ]. Therefore, to address the functional relationship between ACR-8 and LEV-8, we conducted an in vivo characterization of the parasitic nematode ACR-8 using C .…”

“…contortus : Hco-L-AChR-1.3 and Hco-L-AChR-1.4. [ 35 ]. Both receptors that differed in their subunit composition by the presence of distinct H .…”

Acetylcholinesterase and Nicotinic Acetylcholine Receptors in Schistosomes and Other Parasitic Helminths

“…An analysis of genomic data from 25 nematode parasite species indicated multiple independent duplications of unc-29 as positive, directional selections acting on amino acid positions associated with subunit assembly. It has been demonstrated that functional divergence in AChRs with novel pharmacology occurs more rapidly and may be mediated by alteration of receptor assembly, providing a foundation for understanding the broader context of changing anthelmintic drug targets across the parasitic nematodes [ 61 ]. Furthermore, identification of four gene copies of AChRs from the sheep nematode, Haemonchus contortus , also demonstrated that each copy has acquired unique functional characteristics [ 61 ].…”

Activation ofCaenorhabditis elegansLevamisole-Sensitive and Mammalian Nicotinic Receptors by the Antiparasitic Bephenium

“…Parasitic nematodes contain nAChR subunit genes that show high similarity with their C. elegans counterparts. Whereas C. elegans muscle contains two main nAChR subtypes (L-and N-) of fixed subunit composition, parasitic nematode muscle has a greater diversity of nAChRs owing to subunit duplication and to different arrangements of the orthologous subunits (Qian et al, 2006;Boulin et al, 2008Boulin et al, , 2011Buxton et al, 2014;Duguet et al, 2016;Verma et al, 2017). In different species, L-, N-, P-(pyrantel), M-(morantel), and B-(bephenium) subtypes have been described (Martin et al, 1997;Robertson et al, 2002;Qian et al, 2006;Verma et al, 2017).…”