Immunocytochemical localization of a putative inhibitory amino acid receptor subunit in the parasitic nematodes Haemonchus contortus and Ascaris suum

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.

A novel BR-SMAD is required for larval development in barber's pole worm Haemonchus contortus

SMAD proteins mediate TGF-β signaling and thereby regulate the metazoan development; however, they are poorly defined in Haemonchus contortus–a common blood-sucking parasitic nematode of small ruminants. Here, we characterized an R-SMAD family protein in H. contortus termed HcSMA2, which is closely related to Caenorhabditis elegans SMA2 (CeSMA2) involved in the bone morphogenetic protein (BMP) signaling. Hcsma2 is transcribed in all developmental stages of H. contortus but highly induced in the adult male worms. The RNA interference with Hcsma2 retarded the transition of infective L3 into L4 larvae. Besides, the bimolecular fluorescence complementation revealed the interaction of HcSMA2 with a TGF-β-activated-R-SMAD (HcDAF8). Together these results show a BMP-like receptor-regulated SMAD in H. contortus that is required for larval differentiation and underscore an adaptive functional repurposing of BMP-signaling in parasitic worms.

Identification and characterization of an R-Smad homologue (Hco-DAF-8) from Haemonchus contortus

Background

Smad proteins are essential cellular mediators within the transforming growth factor-β (TGF-β) superfamily. They directly transmit incoming signals from the cell surface receptors to the nucleus. In spite of their functional importance, almost nothing is known about Smad proteins in parasitic nematodes including Haemonchus contortus, an important blood-sucking nematode of small ruminants.

Methods

Based on genomic and transcriptome data for H. contortus and using bioinformatics methods, a Smad homologue (called Hco-daf-8) was inferred from H. contortus and the structural characteristics of this gene and its encoded protein Hco-DAF-8 established. Using real-time PCR and immunofluorescence assays, temporal transcriptional and spatial expression profiles of Hco-daf-8 were studied. Gene rescue in Caenorhabditis elegans was then applied to assess the function of Hco-daf-8 and a specific inhibitor of human Smad3 (called SIS3) was employed to evaluate the roles of Hco-DAF-8 in H. contortus development.

Results

The features of Hco-DAF-8 (502 amino acids), including conserved R-Smad domains and residues of the L3-loop that determine pathway specificity, are consistent with a TGF-β type I receptor-activated R-Smad. The Hco-daf-8 gene was transcribed in all developmental stages of H. contortus studied, with a higher level of transcription in the fourth-stage larval (L4) females and the highest level in adult males. Hco-DAF-8 was expressed in the platymyarian muscular cells, intestine and reproductive system of adult stages. Gene rescue experiments showed that Hco-daf-8 was able to partially rescue gene function in a daf-8 deficient mutant strain of C. elegans, leading to a resumption of normal development. In H. contortus, SIS3 was shown to affect H. contortus development from the exsheathed third-stage larvae (L3s) to L4s in vitro.

Conclusions

These findings suggest that Hco-DAF-8, encoded by the gene Hco-daf-8, is an important cellular mediator of H. contortus development via the TGF-β signalling pathway. They provide a basis for future explorations of Hco-DAF-8 and associated pathways in H. contortus and other important parasitic nematodes.

Ivermectin resistance and overview of the Consortium for Anthelmintic Resistance SNPs

Ivermectin (IVM) has transformed nematode parasite control in veterinary medicine and the control of some nematode infections in humans, such as onchocerciasais, lymphatic filariasis in Africa and strongyloidiasis. Unfortunately, IVM resistance is now a serious problem for parasite control in livestock and there is a concern about resistance development and spread in nematode parasites of humans. IVM is believed to act by opening glutamate-gated chloride channels and GABA-gated channels in invertebrate neurons or muscle cells, leading to hyperpolarisation of the cells and to an inhibitory paralysis. However, in the filarial nematodes, it is not altogether clear that the effect of IVM is confined to these actions or even whether these are the most important. Alterations in some ligand-gated ion channel (LGIC) receptor subunits may play a role in the mechanisms of IVM resistance in some nematodes, but the evidence that changes in LGICs are the most important cause of IVM resistance in nematodes is far from clear. What is evident is that IVM is an excellent substrate for some ATP-binding cassette transporters, IVM selects for changes in expression levels of ABC transporters, such as P-glycoproteins, and that altered levels of some ABC transporters contribute to IVM resistance. In addition, there is growing evidence that IVM selects on β-tubulin, at least in some nematodes. Based on these various mechanisms, which contribute to IVM resistance, it may become possible to develop panels of molecular markers for IVM resistance in different nematode parasites. In order to stimulate the development of such markers, an international Consortium for Anthelmintic Resistance SNPs (CARS) has been developed to help coordinate marker development, advance our knowledge of helminth biology and possibly assist with the development of new anthelmintic molecules.

Hco-LGC-38 is novel nematode cys-loop GABA receptor subunit

We have identified and characterized a novel cys-loop GABA receptor subunit (Hco-LGC-38) from the parasitic nematode Haemonchus contortus. This subunit is present in parasitic and free-living nematodes and shares similarity to both the UNC-49 group of GABA receptor subunits from nematodes and the resistant to dieldrin (RDL) receptors of insects. Expression of the Hco-lgc-38 gene in Xenopus oocytes and subsequent electrophysiological analysis has revealed that the gene encodes a homomeric channel sensitive to GABA (EC(50) 19 μM) and the GABA analogue muscimol. The sensitivity of the Hco-LGC-38 channel to GABA is similar to reported values for the Drosophila RDL receptor whereas its lower sensitivity to muscimol is similar to nematode GABA receptors. Hco-LGC-38 is also highly sensitive to the channel blocker picrotoxin and moderately sensitive to fipronil and dieldrin. Homology modeling of Hco-LGC-38 and subsequent docking of GABA and muscimol into the binding site has uncovered several types of potential interactions with binding-site residues and overall appears to share similarity with models of other invertebrate GABA receptors.

Nematode cys-loop GABA receptors: biological function, pharmacology and sites of action for anthelmintics

Parasitic nematode infection of humans and livestock is a major problem globally. Attempts to control nematode populations have led to the development of several classes of anthelmintic, which target cys-loop ligand-gated ion channels. Unlike the vertebrate nervous system, the nematode nervous system possesses a large and diversified array of ligand-gated chloride channels that comprise key components of the inhibitory neurotransmission system. In particular, cys-loop GABA receptors have evolved to play many fundamental roles in nematode behaviour such as locomotion. Analysis of the genomes of several free-living and parasitic nematodes suggests that there are several groups of cys-loop GABA receptor subunits that, for the most part, are conserved among nematodes. Despite many similarities with vertebrate cys-loop GABA receptors, those in nematodes are quite distinct in sequence similarity, subunit composition and biological function. With rising anthelmintic resistance in many nematode populations worldwide, GABA receptors should become an area of increased scientific investigation in the development of the next generation of anthelmintics.

An UNC-49 GABA receptor subunit from the parasitic nematode Haemonchus contortus is associated with enhanced GABA sensitivity in nematode heteromeric channels

We have identified two genes from the parasitic nematode Haemonchus contortus, Hco-unc-49B and Hco-unc-49C that encode two GABA-gated chloride channel subunits. Electrophysiological analysis revealed that this channel has properties similar to those of the UNC-49 channel from the free-living nematode Caenorhabditis elegans. For example, the Hco-UNC-49B subunit forms a functional homomeric channel that responds to GABA and is highly sensitive to picrotoxin. Hco-UNC-49C alone does not respond to GABA but can assemble with Hco-UNC-49B to form a heteromeric channel with a lower sensitivity to picrotoxin. However, we did find that the Hco-UNC-49B/C heteromeric channel is significantly more responsive to agonists compared to the Hco-UNC-49B homomeric channel, which is the opposite trend to what has been found previously for the C. elegans channel. To investigate the subunit requirements for high agonist sensitivity, we generated cross-assembled channels by co-expressing the H. contortus subunits with UNC-49 subunits from C. elegans (Cel-UNC-49). Co-expressing Cel-UNC-49B with Hco-UNC-49C produced a heteromeric channel with a reduced sensitivity to GABA compared to that of the Cel-UNC-49B homomeric channel. In contrast, co-expressing Hco-UNC-49B with Cel-UNC-49C produced a heteromeric channel that, like the Hco-UNC-49B/C heteromeric channel, exhibits an increased sensitivity to GABA. These results suggest that the Hco-UNC-49B subunit is the key determinant for the high agonist sensitivity of heteromeric channels.

Nematode ligand-gated chloride channels: an appraisal of their involvement in macrocyclic lactone resistance and prospects for developing molecular markers

Ligand-gated chloride channels, including the glutamate-(GluCl) and GABA-gated channels, are the targets of the macrocyclic lactone (ML) family of anthelmintics. Changes in the sequence and expression of these channels can cause resistance to the ML in laboratory models, such as Caenorhabditis elegans and Drosophila melanogaster. Mutations in multiple GluCl subunit genes are required for high-level ML resistance in C. elegans, and this can be influenced by additional mutations in gap junction and amphid genes. Parasitic nematodes have a different complement of channel subunit genes from C. elegans, but a few genes, including avr-14, are widely present. A polymorphism in an avr-14 orthologue, which makes the subunit less sensitive to ivermectin and glutamate, has been identified in Cooperia oncophora, and polymorphisms in several subunits have been reported from resistant isolates of Haemonchus contortus. This has led to suggestions that ML resistance may be polygenic. Possible reasons for this, and its consequences for the development of molecular tests for resistance, are explored.

Selection at a γ-aminobutyric acid receptor gene in Haemonchus contortus resistant to avermectins/milbemycins

Gamma-aminobutyric acid (GABA) Type A receptors are inhibitory chloride channels in membranes of vertebrate and invertebrate neuromuscular cells. Gating of the channels by GABA leads to an influx of chloride ions into, and hyperpolarisation of, the cell. GABA receptors are believed to form channels by the association of five protein molecules of varying subunit types, with the second transmembrane (M2) domain of each protein molecule forming a central pore through which chloride ions can pass. We have analysed by single-strand conformation polymorphism the genetic variation of a GABA-receptor gene, HG1, from two sets of unselected and anthelmintic-selected strains of the parasitic nematode Haemonchus contortus. Significant differences in allele frequencies were detected between one unselected strain and its derived ivermectin-selected strain and between the other unselected strain and its derived ivermectin- and moxidectin-selected strains. In each set of strains, one allele increased substantially in frequency in the drug-selected strains relative to their respective unselected strains. The selected allele, however, differed between the two sets of strains. Similar analyses were performed on a phosphoenolpyruvate carboxykinase gene and a nicotinic acetylcholine receptor subunit gene. No significant differences were found in allele frequencies between the unselected and their derived anthelmintic-selected strains. These results indicate the GABA receptor as a possible site of action for avermectins and milbemycins, and suggest its involvement in resistance to these anthelmintics.

Distribution of glutamate-gated chloride channel subunits in the parasitic nematode Haemonchus contortus

Glutamate-gated chloride channels (GluCl) are related to gamma-aminobutyric acid-A (GABA(A)) receptors and are the target sites for the avermectin/milbemycin (A/M) anthelmintics, drugs that cause paralysis of the somatic and pharyngeal muscles in nematodes. We have previously identified four GluCl subunits, HcGluClalpha, HcGluClbeta, HcGluClalpha3A, and HcGluClalpha3B from the sheep parasite Haemonchus contortus. We raised specific antisera against all of these subunits and used them in immunofluorescence experiments on adult parasites. All of the subunits were expressed in the motor nervous system, especially motor neuron commissures. Double-immunostaining experiments suggested that HcGluClalpha and HcGluClbeta were expressed on the same commissures; these were also stained with an anti-GABA antibody, suggesting that they may be inhibitory motor neurons. The HcGluClbeta subunit was also detected in lateral and sublateral nerve cords. The HcGluClalpha3A and -B subunits, products of an alternatively spliced gene, were expressed in different neurons. We found HcGluClalphaA in a pair of sensory, possibly amphid, neurons in the head, in addition to the motor neuron commissures. HcGluClalpha3B was detected in three cell bodies, probably of pharyngeal neurons, and to ventral and lateral cords. These results indicate that the GluCl are widely distributed in the H. contortus nervous system and suggest that they have critical roles controlling locomotion, pharyngeal function, and possibly sensory processing in parasitic nematodes. They also provide an explanation for the observed effects of the A/M anthelmintics.

Genetic variability following selection of Haemonchus contortus with anthelmintics

Genetic diversity in nematodes leads to variation in response to anthelmintics. Haemonchus contortus shows enormous genetic diversity, allowing anthelmintic resistance alleles to be rapidly selected. Anthelmintic resistance is now a widespread problem, especially in H. contortus. Here, I compare the genes involved in anthelmintic resistance in H. contortus with those that confer susceptibility or resistance on the free living nematode Caenorhabditis elegans. I also discuss the latest knowledge of genes associated with resistance to benzimidazoles, levamisole and the macrocyclic lactones and the need for DNA markers for anthelmintic resistance.

Ligand-gated chloride channel subunits encoded by the Haemonchus contortus and Ascaris suum orthologues of the Caenorhabditis elegans gbr-2 (avr-14) gene

The alternatively-spliced Caenorhabditis elegans gbr-2/avr-14 gene encodes two subunits of the nematode ligand-gated chloride channel family which forms an important molecular target for the avermectin and related anthelminthics. We used reverse transcriptase-polymerase chain reaction (RT-PCR) techniques to isolate cDNAs encoding the products of the gbr-2/avr-14 orthologues from the parasitic nematodes Haemonchus contortus and Ascaris suum. The predicted polypeptides possess all the characteristics of subunits of the ligand-gated chloride channels, sharing greater than 80% amino-acid identity with their counterparts in C. elegans and with partial sequences from the filarial species Onchocerca volvulus and Dirofilaria immitis. The pattern of alternative splicing of the gbr-2/avr-14 gene observed in C. elegans is conserved in H. contortus but may not be in A. suum. Affinity-purified anti-GBR-2 antibodies were used to study the expression of these subunits in adult worms and they reacted specifically with the nerve ring, the ventral and dorsal nerve cords, the anterior portion of the dorsal sub-lateral cord and motor-neuron commissures in H. contortus. Specific immunofluorescence of the nerve cords was confirmed in A. suum; isolated muscle cells did not react with the antibody.

Cloning and localisation of an avermectin receptor-related subunit from Haemonchus contortus

Ivermectin is believed to exert its anthelminthic effects by binding to glutamate-gated chloride channels (Glu-Cl) and several cDNAs encoding subunits of Glu-Cl have been cloned from Caenorhabditis elegans. We report the cloning of cDNAs encoding a putative Glu-Cl subunit (HG4) from the parasite Haemonchus contortus. The HG4 cDNAs were isolated using RT-PCR and the sequence of the predicted polypeptide has 82% amino-acid identity with the C. elegans Glu-Cl beta subunit. Individual HG4 cDNAs showed up to 4% sequence variation at the nucleotide level, but the vast majority of these polymorphisms were translationally silent. A synthetic peptide corresponding to sequence near the N-terminus of the mature polypeptide was used to raise an antiserum that recognised the N-terminal domain of HG4 expressed in E. coli. Affinity purified antibodies reacted with motor neuron commissures in immuno-localisation studies: these commissures were limited to the anterior portion of the worms, from a region level with the nerve ring to just anterior of the vulva. Some possible nerve cord staining was also observed, but no expression of HG4 on pharyngeal muscle could be detected.