Previous research has suggested that the
Previous research has suggested that the acetylcholine-gated chloride channels (ACCs) in Caenorhabditis elegans (Putrenko et al., 2005) exhibit the characteristics of promising drug targets. The genes that encode the various subunits of this family are present across the nematode phylum and appear to have fairly broad function in the nematode nervous system (Wever et al., 2015). One of the channels, Cel-ACC-1 exhibits a pharmacology distinct from mammalian cys-loop ddhGTP channels and appears in C. elegans to be localized to ventral cord and extrapharyngeal neurons (Wever et al., 2015). Due to their broad role in the C. elegans nervous system, the ACC family of receptors appear to be promising targets for effective anthelmintic action against parasitic nematodes (Wever et al., 2015). However, directly extrapolating results from C. elegans to parasitic nematodes should be done with caution since the expression patterns of individual LGCCs have been shown in some cases to be different (Portillo et al., 2003).
Here we have isolated a member of the ACC family (Hco-ACC-1) from the parasitic nematode H. contortus. Electrophysiological examination revealed that while Hco-ACC-1 does not form a functional homomeric channel in Xenopus oocytes, co-expression with a previously characterized subunit (Hco-ACC-2) can form a channel highly sensitive to acetylcholine and carbachol. The ACC-1/2 heteromeric channel was 3x more sensitive to acetylcholine compared to the ACC-2 homomeric channel. When expressed in C. elegans, hco-acc-1, localizes to pharyngeal neurons and enhances reversal avoidance to octanol demonstrating that hco-acc-1 can function in vivo. In H. contortus, ACC-1 may to play an essential role in the pharynx as immunolocalization revealed expression in a specific region of the pharyngeal muscle. Overall, this research has provided some novel insight into the possible role of ACC receptors in parasitic nematodes.
Materials and methods
Discussion Here we report the isolation and characterization of an acetylcholine-gated chloride channel from a parasitic nematode. Recently, Wever et al. (2015) provided evidence that the ACC family of receptors were potentially good anthelmintic targets in C. elegans. Here avr-15 (encoding a GluCl subunit) under the control of acc promoters exhibited high level of sensitivity to ivermectin demonstrating that the ACC family of receptors function in what was referred to as “essential” tissues. In this case one of the essential tissues was extrapharyngeal neurons (Wever et al., 2015). Interestingly, while we did not observe the localization of Hco-ACC-1 in any neurons in H. contortus we consistently found this receptor in pharyngeal muscle tissue. It appears therefore that although ACC-1 localizes to different tissues in H. contortus compared to C. elegans they both appear to function in tissues that would be considered essential from the point of view of anthelmintic action. The difference in the localization of ACC receptors between C. elegans and H. contortus resembles the situation observed with the ivermectin target, the GluCl. In C. elegans, GluCl subunits have been localized to pharyngeal muscle and various neurons (Dent et al., 1997, 2000; Laughton et al., 1997), but in H. contortus, the GluCl receptors localized thus far were found to be exclusively neuronal (Portillo et al., 2003). However, both nematodes are still very sensitive to ivermectin with respect to both locomotion and pharyngeal pumping (Dent et al., 1997; Wolstenholme, 2012). Here, we have found that Hco-ACC-1 localizes to pharyngeal muscle tissue in H. contortus but localizes to pharyngeal neurons in C. elegans. It is tempting to speculate that like the GluCls, the ACCs family of receptors show different expression patterns but have some overlapping function between H. contortus and C. elegans. We are in the process of characterizing all additional ACC subunits from H. contortus to provide better insight into how conserved the function is between free-living and parasitic nematodes.