Monepantel irreversibly binds to and opens Haemonchus contortus MPTL-1 and Caenorhabditis elegans ACR-20 receptors

A phenotypic screen of the Global Health Priority Box identifies an insecticide with anthelmintic activity

BACKGROUND

Infection with parasitic nematodes (helminths), particularly those of the order Strongylida (such as Haemonchus contortus), can cause significant and burdensome diseases in humans and animals. Widespread drug (anthelmintic) resistance in livestock parasites, the absence of vaccines against most of these nematodes, and a lack of new and effective chemical entities on the commercial market demands the discovery of new anthelmintics. In the present study, we searched the Global Health Priority Box (Medicines for Malaria Venture) for new candidates for anthelmintic development.

METHODS

We employed a whole-organism, motility-based phenotypic screening assay to identify compounds from the Global Health Priority Box with activity against larvae of the model parasite H. contortus, and the free-living comparator nematode Caenorhabditis elegans. Hit compounds were further validated via dose-response assays, with lead candidates then assessed for nematocidal activity against H. contortus adult worms, and additionally, for cytotoxic and mitotoxic effects on human hepatoma (HepG2) cells.

RESULTS

The primary screen against H. contortus and C. elegans revealed or reidentified 16 hit compounds; further validation established MMV1794206, otherwise known as 'flufenerim', as a significant inhibitor of H. contortus larval motility (half-maximal inhibitory concentration [IC50] = 18 μM) and development (IC50 = 1.2 μM), H. contortus adult female motility (100% after 12 h of incubation) and C. elegans larval motility (IC50 = 0.22 μM). Further testing on a mammalian cell line (human hepatoma HepG2 cells), however, identified flufenerim to be both cytotoxic (half-maximal cytotoxic concentration [CC50] < 0.7 μM) and mitotoxic (half-maximal mitotoxic concentration [MC50] < 0.7 μM).

CONCLUSIONS

The in vitro efficacy of MMV1794206 against the most pathogenic stages of H. contortus, as well as the free-living C. elegans, suggests the potential for development as a broad-spectrum anthelmintic compound; however, the high toxicity towards mammalian cells presents a significant hindrance. Further work should seek to establish the protein-drug interactions of MMV1794206 in a nematode model, to unravel the mechanism of action, in addition to an advanced structure-activity relationship investigation to optimise anthelmintic activity and eliminate mammalian cell toxicity.

The diverse family of Cys-loop receptors in Caenorhabditis elegans: insights from electrophysiological studies

Cys-loop receptors integrate a large family of pentameric ligand-gated ion channels that mediate fast ionotropic responses in vertebrates and invertebrates. Their vital role in converting neurotransmitter recognition into an electrical impulse makes these receptors essential for a great variety of physiological processes. In vertebrates, the Cys-loop receptor family includes the cation-selective channels, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors, and the anion-selective channels, GABAA and glycine receptors, whereas in invertebrates, the repertoire is significantly larger. The free-living nematode Caenorhabditis elegans has the largest known Cys-loop receptor family as well as unique receptors that are absent in vertebrates and constitute attractive targets for anthelmintic drugs. Given the large number and variety of Cys-loop receptor subunits and the multiple possible ways of subunit assembly, C. elegans offers a large diversity of receptors although only a limited number of them have been characterized to date. C. elegans has emerged as a powerful model for the study of the nervous system and human diseases as well as a model for antiparasitic drug discovery. This nematode has also shown promise in the pharmaceutical industry search for new therapeutic compounds. C. elegans is therefore a powerful model organism to explore the biology and pharmacology of Cys-loop receptors and their potential as targets for novel therapeutic interventions. In this review, we provide a comprehensive overview of what is known about the function of C. elegans Cys-loop receptors from an electrophysiological perspective.

Monepantel-based anthelmintic combinations to optimize parasite control in cattle

Improvement in the use of existing anthelmintics is a high priority need for the pharmaco-parasitology research field, considering the magnitude and severity of anthelmintic resistance as an important issue in livestock production. In the work described here, monepantel (MNP) was given alone or co-administered with either macrocyclic lactone (ML) or benzimidazole (BZ) anthelmintics to calves naturally infected with ML- and BZ-resistant gastrointestinal (GI) nematodes on two different commercial cattle farms. Both pharmacokinetic (PK) and efficacy assessments were performed. On Farm A, male calves (n = 15 per group) were treated with either MNP orally (2.5 mg/kg), IVM s.c. (0.2 mg/kg), ricobendazole (RBZ) s.c. (3.75 mg/kg) or remained untreated. On Farm B, eight groups (n = 15) of male calves received treatment with either: MNP, abamectin (ABM, oral, 0.2 mg/kg), RBZ (s.c., 3.75 mg/kg), albendazole (ABZ, oral, 5 mg/kg), MNP+ABM, MNP+RBZ, MNP+ABZ (all at the above-mentioned routes and doses) or remained untreated. Seven animals from each treated group (Farm B) were randomly selected to perform the PK study. MNP and its metabolite monepantel sulphone (MNPSO2) were the main analytes recovered in plasma after HPLC analysis. The combined treatments resulted in decreased systemic exposures to MNP parent drug compared with that observed after treatment with MNP alone (P < 0.05). However, the systemic availability of the main MNP metabolite (MNPSO2) was unaffected by co-administration with either ABM, RBZ or ABZ. Efficacies of 98% (Farm A) and 99% (Farm B) demonstrated the high efficacy of MNP given alone (P < 0.05) against GI nematodes resistant to ML and BZ in cattle. While the ML (IVM, ABM) failed to control Haemonchus spp., Cooperia spp. and Ostertagia spp., MNP achieved 99% to 100% efficacy against those nematode species on both commercial farms. However, MNP alone failed to control Oesophagostomum spp. (60% efficacy) on Farm A. The co-administered treatments MNP+ABZ and MNP+RBZ reached a 100% reduction against all GI nematode genera. In conclusion, the oral treatment with MNP should be considered to deal with resistant nematode parasites in cattle. The use of MNP in combination with BZ compounds could be a valid strategy to extend its lifespan for use in cattle as well as to reverse its poor activity against Oesophagostomum spp.

Functional validation of the truncated UNC-63 acetylcholine receptor subunit in levamisole resistance

Levamisole is a broad-spectrum anthelmintic which permanently activates cholinergic receptors from nematodes, inducing a spastic paralysis of the worms. Whereas this molecule is widely used to control parasitic nematodes impacting livestock, its efficacy is compromised by the emergence of drug-resistant parasites. In that respect, there is an urgent need to identify and validate molecular markers associated with resistance. Previous transcriptomic analyses revealed truncated cholinergic receptor subunits as potential levamisole resistance markers in the trichostrongylid nematodes Haemonchus contortus, Telodorsagia circumcincta and Trichostrongylus colubriformis. In the present study we used the Xenopus oocyte, as well as the free-living model nematode Caenorhabditis elegans, as heterologous expression systems to functionally investigate truncated isoforms of the levamisole-sensitive acetylcholine receptor (L-AChR) UNC-63 subunit. In the Xenopus oocyte, we report that truncated UNC-63 from C. elegans has a strong dominant negative effect on the expression of the recombinant C. elegans L-AChRs. In addition, we show that when expressed in C. elegans muscle cells, truncated UNC-63 induces a drastic reduction in levamisole susceptibility in transgenic worms, thus providing the first known functional validation for this molecular marker in vivo.

Polymorphisms in exon 11 of the mptl-1 gene and monepantel resistance in Haemonchus contortus

Chemical treatments are the main strategy to control gastrointestinal nematodes in sheep, and the emergence of anthelmintic resistance, as consequence, results in control failures and leads to economic losses. Thus, molecular tests may constitute an excellent tool for the early detection of anthelmintic resistance-related mutations. Thus, a polymerase chain reaction (PCR)-based genotyping assay followed by polyacrylamide gel electrophoresis (PAGE) was developed to detect polymorphisms in exon 11 of the acetylcholine receptor monepantel-1 gene (mptl-1) that were previously associated with monepantel resistance through a genome-wide study in Haemonchus contortus. DNA samples recovered from individual and pooled third-stage larvae from two susceptible field-derived isolates and five (three in vivo-derived and two field-derived) resistant populations were used. New polymorphisms, including a 6-bp deletion and a 3-bp insertion, were detected in resistant individuals. These indels, confirmed using sequencing of cloned PCR products, are predicted to result in amino acid changes in transmembrane domain 2 (TMD2) of the MPTL-1 protein. The two susceptible isolates showed only the presence of the wild-type allele (100%), whereas lower frequencies of the wild-type allele were detected in monepantel-resistant populations (11.1 to 66.7%). These findings report new polymorphisms in the mptl-1 gene, validate the results obtained through genomic mapping for monepantel resistance, and provide a PCR-based assay to genotype indels located in exon 11 of mptl-1 in H. contortus.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

The Caenorhabditis elegans DEG-3/DES-2 Channel Is a Betaine-Gated Receptor Insensitive to Monepantel

Natural plant compounds, such as betaine, are described to have nematocidal properties. Betaine also acts as a neurotransmitter in the free-living model nematode Caenorhabditis elegans, where it is required for normal motility. Worm motility is mediated by nicotinic acetylcholine receptors (nAChRs), including subunits from the nematode-specific DEG-3 group. Not all types of nAChRs in this group are associated with motility, and one of these is the DEG-3/DES-2 channel from C. elegans, which is involved in nociception and possibly chemotaxis. Interestingly, the activity of DEG-3/DES-2 channel from the parasitic nematode of ruminants, Haemonchus contortus, is modulated by monepantel and its sulfone metabolite, which belong to the amino-acetonitrile derivative anthelmintic drug class. Here, our aim was to advance the pharmacological knowledge of the DEG-3/DES-2 channel from C. elegans by functionally expressing the DEG-3/DES-2 channel in Xenopus laevis oocytes and using two-electrode voltage-clamp electrophysiology. We found that the DEG-3/DES-2 channel was more sensitive to betaine than ACh and choline, but insensitive to monepantel and monepantel sulfone when used as direct agonists and as allosteric modulators in co-application with betaine. These findings provide important insight into the pharmacology of DEG-3/DES-2 from C. elegans and highlight the pharmacological differences between non-parasitic and parasitic nematode species.

Drug discovery: Insights from the invertebrate Caenorhabditis elegans

Therapeutic drug development is a long, expensive, and complex process that usually takes 12-15 years. In the early phases of drug discovery, in particular, there is a growing need for animal models that ensure the reduction in both cost and time. Caenorhabditis elegans has been traditionally used to address fundamental aspects of key biological processes, such as apoptosis, aging, and gene expression regulation. During the last decade, with the advent of large-scale platforms for screenings, this invertebrate has also emerged as an essential tool in the pharmaceutical research industry to identify novel drugs and drug targets. In this review, we discuss the reasons why C. elegans has been positioned as an outstanding cost-effective option for drug discovery, highlighting both the advantages and drawbacks of this model. Particular attention is paid to the suitability of this nematode in large-scale genetic and pharmacological screenings. High-throughput screenings in C. elegans have indeed contributed to the breakthrough of a wide variety of candidate compounds involved in extensive fields including neurodegeneration, pathogen infections and metabolic disorders. The versatility of this nematode, which enables its instrumentation as a model of human diseases, is another attribute also herein underscored. As illustrative examples, we discuss the utility of C. elegans models of both human neurodegenerative diseases and parasitic nematodes in the drug discovery industry. Summing up, this review aims to demonstrate the impact of C. elegans models on the drug discovery pipeline.

Conserved nicotine-activated neuroprotective pathways involve mitochondrial stress

Tobacco smoking is a risk factor for several human diseases. Conversely, smoking also reduces the prevalence of Parkinson's disease, whose hallmark is degeneration of substantia nigra dopaminergic neurons (DNs). We use C. elegans as a model to investigate whether tobacco-derived nicotine activates nicotinic acetylcholine receptors (nAChRs) to selectively protect DNs. Using this model, we demonstrate conserved functions of DN-expressed nAChRs. We find that DOP-2, a D3-receptor homolog; MCU-1, a mitochondrial calcium uniporter; PINK-1 (PTEN-induced kinase 1); and PDR-1 (Parkin) are required for nicotine-mediated protection of DNs. Together, our results support involvement of a calcium-modulated, mitochondrial stress-activated PINK1/Parkin-dependent pathway in nicotine-induced neuroprotection. This suggests that nicotine-selective protection of substantia nigra DNs is due to the confluence of two factors: first, their unique vulnerability to mitochondrial stress, which is mitigated by increased mitochondrial quality control due to PINK1 activation, and second, their specific expression of D3-receptors.

Complementary Approaches with Free-living and Parasitic Nematodes to Understanding Anthelmintic Resistance

Anthelmintic drugs are the major line of defense against parasitic nematode infections, but the arsenal is limited and resistance threatens sustained efficacy of the available drugs. Discoveries of the modes of action of these drugs and mechanisms of resistance have predominantly come from studies of a related nonparasitic nematode species, Caenorhabditis elegans, and the parasitic nematode Haemonchus contortus. Here, we discuss how our understanding of anthelmintic resistance and modes of action came from the interplay of results from each of these species. We argue that this 'cycle of discovery', where results from one species inform the design of experiments in the other, can use the complementary strengths of both to understand anthelmintic modes of action and mechanisms of resistance.

Caenorhabditis elegans in anthelmintic research - Old model, new perspectives

For more than four decades, the free-living nematode Caenorhabditis elegans has been extensively used in anthelmintic research. Classic genetic screens and heterologous expression in the C. elegans model enormously contributed to the identification and characterization of molecular targets of all major anthelmintic drug classes. Although these findings provided substantial insights into common anthelmintic mechanisms, a breakthrough in the treatment and control of parasitic nematodes is still not in sight. Instead, we are facing increasing evidence that the enormous diversity within the phylum Nematoda cannot be recapitulated by any single free-living or parasitic species and the development of novel broad-spectrum anthelmintics is not be a simple goal. In the present review, we summarize certain milestones and challenges of the C. elegans model with focus on drug target identification, anthelmintic drug discovery and identification of resistance mechanisms. Furthermore, we present new perspectives and strategies on how current progress in C. elegans research will support future anthelmintic research.

Cholinergic transmission in C. elegans: Functions, diversity, and maturation of ACh-activated ion channels

Acetylcholine is an abundant neurotransmitter in all animals. Effects of acetylcholine are excitatory, inhibitory, or modulatory depending on the receptor and cell type. Research using the nematode C. elegans has made ground-breaking contributions to the mechanistic understanding of cholinergic transmission. Powerful genetic screens for behavioral mutants or for responses to pharmacological reagents identified the core cellular machinery for synaptic transmission. Pharmacological reagents that perturb acetylcholine-mediated processes led to the discovery and also uncovered the composition and regulators of acetylcholine-activated channels and receptors. From a combination of electrophysiological and molecular cellular studies, we have gained a profound understanding of cholinergic signaling at the levels of synapses, neural circuits, and animal behaviors. This review will begin with a historical overview, then cover in-depth current knowledge on acetylcholine-activated ionotropic receptors, mechanisms regulating their functional expression and their functions in regulating locomotion.

A perspective on the discovery of selected compounds with anthelmintic activity against the barber's pole worm-Where to from here?

Parasitic roundworms (nematodes) cause substantial morbidity and mortality in animals worldwide. Anthelmintic treatment is central to controlling these worms, but widespread resistance to most of the commercially available anthelmintics for veterinary and agricultural use is compromising control, such that there is an urgency to discover new and effective drugs. The purpose of this article is to review information on parasitic nematodes, the treatment and control of parasitic nematode infections and aspects of discovering new anthelmintics in the context of anthelmintic resistance problems, and then to discuss some progress that our group has made in identifying selected compounds with activity against nematodes. The focus of our recent work has been on discovering new chemical entities and known drugs with anthelmintic activities against Haemonchus contortus as well as other socioeconomically important parasitic nematodes for subsequent development. Using whole worm-based phenotypic assays, we have been screening compound collections obtained via product-development-partnerships and/or collaborators, and active compounds have been assessed for their potential as anthelmintic candidates. Following the screening of 15,333 chemicals from five distinct compound collections against H. contortus, we have discovered one new chemical entity (designated SN00797439), two human kinase inhibitors (SNS-032 and AG-1295), 14 tetrahydroquinoxaline analogues, one insecticide (tolfenpyrad) and two tolfenpyrad (pyrazole-5-carboxamide) derivatives (a-15 and a-17) with anthelmintic activity in vitro. Some of these 20 'hit' compounds have selectivity against H. contortus in vitro when compared to particular human cell lines. In our opinion, some of these compounds could represent starting points for 'lead' development. Accordingly, the next research steps to be pursued include: (i) chemical optimisation of representative chemicals via structure-activity relationship (SAR) evaluations; (ii) assessment of the breadth of spectrum of anthelmintic activity on a range of other parasitic nematodes, such as strongyloids, ascaridoids, enoplids and filarioids; (iii) detailed investigations of the absorption, distribution, metabolism, excretion and toxicity (ADMET) of optimised chemicals with broad nematocidal or nematostatic activity; and (iv) establishment of the modes of action of lead candidates.

Genotypic characterisation of monepantel resistance in historical and newly derived field strains of Teladorsagia circumcincta

Recent reports of monepantel (MPTL) resistance in UK field isolates of Teladorsagia circumcincta has highlighted the need for a better understanding of the mechanism of MPTL-resistance in order to preserve its anthelmintic efficacy in this economically important species. Nine discrete populations of T. circumcincta were genotypically characterised; three MPTL-susceptible isolates, three experimentally selected MPTL-resistant strains and three field derived populations. Full-length Tci-mptl-1 gene sequences were generated and comparisons between the MPTL-susceptible isolates, MPTL-resistant strains and one field isolate, showed that different putative MPTL-resistance conferring mutations were present in different resistant isolates. Truncated forms of the Tci-mptl-1 gene were also observed. The genetic variability of individual larvae, within and between populations, was examined using microsatellite analyses at 10 'neutral' loci (presumed to be unaffected by MPTL). Results confirmed that there was little background genetic variation between the populations, global FST <0.038. Polymorphisms present in exons 7 and 8 of Tci-mptl-1 enabled genotyping of individual larvae. A reduction in the number of genotypes was observed in all MPTL-resistant strains compared to the MPTL-susceptible strains that they were derived from, suggesting there was purifying selection at Tci-mptl-1 as a result of MPTL-treatment. The potential link between benzimidazole (BZ)-resistance and MPTL-resistance was examined by screening individual larvae for the presence of three SNPs associated with BZ-resistance in the β-tubulin isotype-1 gene. The majority of larvae were BZ-susceptible homozygotes at positions 167 and 198. Increased heterozygosity at position 200 was observed in the MPTL-resistant strains compared to their respective MPTL-susceptible population. There was no decrease in the occurrence of BZ-resistant genotypes in larvae from each population. These differences, in light of the purifying selection at this locus in all MPTL-resistant isolates, suggests that Tci-mptl-1 confers MPTL-resistance in T. circumcincta, as in Haemonchus contortus, but that different mutations in Tci-mptl-1 can confer resistance in different populations.

Lupin (Lupinus spp.) seeds exert anthelmintic activity associated with their alkaloid content

The growing range of drug resistant parasitic nematode populations threatens the sustainability of ruminant farming worldwide. In this context, nutraceuticals, animal feed that provides necessary dietary requirements while ensuring parasite control, could contribute to increase farming sustainability in developed and low resource settings. In this study, we evaluated the anthelmintic potential of lupin seed extracts against the major ruminant trichostrongylids, Haemonchus contortus and Teladorsagia circumcincta. In vitro observations showed that seed extracts from commercially available lupin varieties could significantly but moderately inhibit larval migration. This anthelmintic effect was mediated by the seed alkaloid content and was potent against both fully susceptible and multidrug resistant H. contortus isolates as well as a susceptible T. circumcincta isolate. Analytical chemistry revealed a set of four lupanine and sparteine-derivatives with anthelmintic activity, and electrophysiology assays on recombinant nematode acetylcholine receptors suggested an antagonistic mode of action for lupin alkaloids. An in vivo trial in H. contortus infected lupin-fed ewes and goats failed to demonstrate any direct anthelmintic effect of crude lupin seeds but infected lupin-fed goats suffered significantly less parasite-mediated blood losses. Altogether, our findings suggest that the anthelmintic potential of lupin remains limited. However, the potent alkaloids identified could lead to the development of novel drugs or may be used in combination with current anthelmintics to improve their efficacy.

The interactions of anthelmintic drugs with nicotinic receptors in parasitic nematodes

Parasitic nematodes express a large number of distinct nicotinic acetylcholine receptors and these in turn are the targets of many classes of anthelmintic drug. This complexity poses many challenges to the field, including sorting the exact subunit composition of each of the receptor subtypes and how much they vary between species. It is clear that the model organism Caenorhabditis elegans does not recapitulate the complexity of nicotinic pharmacology of many parasite species and data using this system may be misleading when applied to them. The number of different receptors may allow nematodes some plasticity which they can exploit to evolve resistance to a specific cholinergic drug; however, this may mean that combinations of cholinergic agents may be effective at sustainably controlling them. Resistance may involve the expression of truncated receptor subunits that affect the expression levels of the receptors via mechanisms that remain to be deciphered.

Monepantel is a non-competitive antagonist of nicotinic acetylcholine receptors from Ascaris suum and Oesophagostomum dentatum

Zolvix^® is a recently introduced anthelmintic drench containing monepantel as the active ingredient. Monepantel is a positive allosteric modulator of DEG-3/DES-2 type nicotinic acetylcholine receptors (nAChRs) in several nematode species. The drug has been reported to produce hypercontraction of Caenorhabditis elegans and Haemonchus contortus somatic muscle. We investigated the effects of monepantel on nAChRs from Ascaris suum and Oesophagostomum dentatum heterologously expressed in Xenopus laevis oocytes. Using two-electrode voltage-clamp electrophysiology, we studied the effects of monepantel on a nicotine preferring homomeric nAChR subtype from A. suum comprising of ACR-16; a pyrantel/tribendimidine preferring heteromeric subtype from O. dentatum comprising UNC-29, UNC-38 and UNC-63 subunits; and a levamisole preferring subtype (O. dentatum) comprising UNC-29, UNC-38, UNC-63 and ACR-8 subunits. For each subtype tested, monepantel applied in isolation produced no measurable currents thereby ruling out an agonist action. When monepantel was continuously applied, it reduced the amplitude of acetylcholine induced currents in a concentration-dependent manner. In all three subtypes, monepantel acted as a non-competitive antagonist on the expressed receptors. ACR-16 from A. suum was particularly sensitive to monepantel inhibition (IC₅₀ values: 1.6 ± 3.1 nM and 0.2 ± 2.3 μM). We also investigated the effects of monepantel on muscle flaps isolated from adult A. suum. The drug did not significantly increase baseline tension when applied on its own. As with acetylcholine induced currents in the heterologously expressed receptors, contractions induced by acetylcholine were antagonized by monepantel. Further investigation revealed that the inhibition was a mixture of competitive and non-competitive antagonism. Our findings suggest that monepantel is active on multiple nAChR subtypes.

A BRIEF REVIEW ON THE MODE OF ACTION OF ANTINEMATODAL DRUGS

Anthelmintics are some of the most widely used drugs in veterinary medicine. Here we review the mechanism of action of these compounds on nematode parasites. Included are the older classes of compounds; the benzimidazoles, cholinergic agonists and macrocyclic lactones. We also consider newer anthelmintics, including emodepside, derquantel and tribendimidine. In the absence of vaccines for most parasite species, control of nematode parasites will continue to rely on anthelmintic drugs. As a consequence, vigilance in detecting drug resistance in parasite populations is required. Since resistance development appears almost inevitable, there is a continued and pressing need to fully understand the mode of action of these compounds. It is also necessary to identify new drug targets and drugs for the continued effective control of nematode parasites.

Pharmacological profile of Ascaris suum ACR-16, a new homomeric nicotinic acetylcholine receptor widely distributed in Ascaris tissues

Background and Purpose

Control of nematode parasite infections relies largely on anthelmintic drugs, several of which act on nicotinic ACh receptors (nAChRs), and there are concerns about the development of resistance. There is an urgent need for development of new compounds to overcome resistance and novel anthelmintic drug targets. We describe the functional expression and pharmacological characterization of a homomeric nAChR, ACR‐16, from a nematode parasite.

Experimental Approach

Using RT‐PCR, molecular cloning and two‐electrode voltage clamp electrophysiology, we localized acr‐16 mRNA in Ascaris suum (Asu) and then cloned and expressed acr‐16 cRNA in Xenopus oocytes. Sensitivity of these receptors to cholinergic anthelmintics and a range of nicotinic agonists was tested.

Key Results

Amino acid sequence comparison with vertebrate nAChR subunits revealed ACR‐16 to be most closely related to α7 receptors, but with some striking distinctions. acr‐16 mRNA was recovered from Asu somatic muscle, pharynx, ovijector, head and intestine. In electrophysiological experiments, the existing cholinergic anthelmintic agonists (morantel, levamisole, methyridine, thenium, bephenium, tribendimidine and pyrantel) did not activate Asu‐ACR‐16 (except for a small response to oxantel). Other nAChR agonists: nicotine, ACh, cytisine, 3‐bromocytisine and epibatidine, produced robust current responses which desensitized at a rate varying with the agonists. Unlike α7, Asu‐ACR‐16 was insensitive to α‐bungarotoxin and did not respond to genistein or other α7 positive allosteric modulators. Asu‐ACR‐16 had lower calcium permeability than α7 receptors.

Conclusions and Implications

We suggest that ACR‐16 has diverse tissue‐dependent functions in nematode parasites and is a suitable drug target for development of novel anthelmintic compounds.

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

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.

The Biochemistry of Haemonchus contortus and Other Parasitic Nematodes

Different life cycle stages of Haemonchus contortus adapt to different ecosystems. This adaptation is accompanied by alterations in gene transcription and expression associated with the energy, amino acid, nitrogen, lipid and/or nucleic acid metabolism of the respective stages. For example, the aerobic metabolism of larvae depends on an efficient citric acid cycle, whereas the anaerobic metabolism of adults requires glycolysis, resulting in the production of volatile fatty acids, such as acetic acid and propionic acid. There are only few anthelmintics targeting nematode energy metabolism. In addition, H. contortus has reduced pathways for amino acid metabolism, polyamine metabolism and nitrogen excretion pathways. Moreover, nucleic acid metabolism comprising purine and pyrimidine salvage pathways as well as lipid metabolism are reduced. In addition, nematodes possess a particular composition of their cuticle. Energy production of adult worms is mainly linked to egg production and complex regulation of the neuromuscular system in both females and males. In this context, microtubules consisting of α- and β-tubulin heterodimers play a crucial role in the presynaptic vesicle transport. Due to the significant distinction of its quarternary structure in nematodes in comparison to other organisms, β-tubulin was identified as a major target for benzimidazoles used for anthelmintic treatment. Concerning the function of the neuromuscular system, acetylcholine, a ligand of the nicotinic acetylcholine receptor (nAChR), is the major excitatory neurotransmitter in H. contortus. In contrast, glutamate-gated chloride channels, calcium- and voltage-dependent potassium channels as well as γ-aminobutyric acid (GABA)A and its receptors act as inhibitory neurotransmitters and thus opponents to nAChR. For example, the calcium- and voltage-dependent potassium channel SLO-1 is an important target of emodepside, which is involved in the sensitive regulation of activatory and inhibitory receptors of the nervous system. Most of the modern anthelmintics target these different neuromuscular receptors. The mechanisms of resistance to anthelmintics, either specific or non-specific, are associated with changes in the molecular targets of the drugs, changes in metabolism of the drug (inactivation, removal or prevention of its activation) and/or increased efflux systems. The biochemical and molecular analyses of key developmental, metabolic and structural process of H. contortus still require substantial efforts. The nAChR, glutamate-gated chloride channel and calcium- and voltage-dependent potassium channel SLO-1 have long been known as being essential for nematode survival. Therefore, future research should be intensified to fully resolve the three-dimensional structures of these receptors, as has already been started for glutamate-gated chloride channel. With this knowledge, it should be possible to design new anthelmintics, which possess improved binding capacities to corresponding receptors.

Assessment of the nematocidal activity of metallocenyl analogues of monepantel

Ferrocenyl and ruthenocenyl analogues of the nematocidal drug monepantel show organometallic-dependent activity against Haemonchus contortus and Trichostrongylus colubriformis.

Functional Characterization of a Novel Class of Morantel-Sensitive Acetylcholine Receptors in Nematodes

Acetylcholine receptors are pentameric ligand-gated channels involved in excitatory neuro-transmission in both vertebrates and invertebrates. In nematodes, they represent major targets for cholinergic agonist or antagonist anthelmintic drugs. Despite the large diversity of acetylcholine-receptor subunit genes present in nematodes, only a few receptor subtypes have been characterized so far. Interestingly, parasitic nematodes affecting human or animal health possess two closely related members of this gene family, acr-26 and acr-27 that are essentially absent in free-living or plant parasitic species. Using the pathogenic parasitic nematode of ruminants, Haemonchus contortus, as a model, we found that Hco-ACR-26 and Hco-ACR-27 are co-expressed in body muscle cells. We demonstrated that co-expression of Hco-ACR-26 and Hco-ACR-27 in Xenopus laevis oocytes led to the functional expression of an acetylcholine-receptor highly sensitive to the anthelmintics morantel and pyrantel. Importantly we also reported that ACR-26 and ACR-27, from the distantly related parasitic nematode of horses, Parascaris equorum, also formed a functional acetylcholine-receptor highly sensitive to these two drugs. In Caenorhabditis elegans, a free-living model nematode, we demonstrated that heterologous expression of the H. contortus and P. equorum receptors drastically increased its sensitivity to morantel and pyrantel, mirroring the pharmacological properties observed in Xenopus oocytes. Our results are the first to describe significant molecular determinants of a novel class of nematode body wall muscle AChR.