Ion channels and drug transporters as targets for anthelmintics

Neuronal gene expression in two generations of the marine parasitic worm, Cryptocotyle lingua

Trematodes, or flukes, undergo intricate anatomical and behavioral transformations during their life cycle, yet the functional changes in their nervous system remain poorly understood. We investigated the molecular basis of nervous system function in Cryptocotyle lingua, a species of relevance for fisheries. Transcriptomic analysis revealed a streamlined molecular toolkit with the absence of key signaling pathways and ion channels. Notably, we observed the loss of nitric oxide synthase across the Platyhelminthes. Furthermore, we identified upregulated neuronal genes in dispersal larvae, including those involved in aminergic pathways, synaptic vesicle trafficking, TRPA channels, and surprisingly nitric oxide receptors. Using neuronal markers and in situ hybridization, we hypothesized their functional relevance to larval adaptations and host-finding strategies. Additionally, employing a behavior quantification toolkit, we assessed cercaria motility, facilitating further investigations into the behavior and physiology of parasitic flatworms. This study enhances our understanding of trematode neurobiology and provides insights for targeted antiparasitic strategies.

Praziquantel activates a native cation current in Schistosoma mansoni

Introduction

Praziquantel (PZQ), an anthelmintic drug discovered in the 1970s, is still used to treat schistosomiasis and various other infections caused by parasitic flatworms. PZQ causes a triad of phenotypic effects on schistosome worms - rapid depolarization, muscle contraction, and damage throughout the worm tegument. The molecular target mediating these effects has been intimated as a Ca2+-permeable ion channel, but native currents evoked by PZQ have not been reported in any schistosome cell type. The properties of the endogenous PZQ activated conductance therefore remain unknown.

Methods

Here, invasive electrophysiology was used to probe for responses to PZQ from different locales in a living schistosome worm.

Results and discussion

No direct response was seen in tegument-derived vesicles, or from the sub-tegumental muscle layer despite the presence of voltage-operated currents. However, PZQ rapidly triggered a sustained, non-selective cation current in recordings from neuronal tissue, targeting both the anterior ganglion and the main longitudinal nerve cord. The biophysical signature of this PZQ-evoked current resolved at single channel resolution matched that of a transient receptor potential ion channel named TRPMPZQ, recently proposed as the molecular target of PZQ. The endogenous PZQ-evoked current was also inhibited by a validated TRPMPZQ antagonist. PZQ therefore is a neuroactive anthelmintic, causing a sustained depolarization through ion channels with the characteristics of TRPMPZQ.

Effect of Temperature and Ionic Substitutions on the Tegumental Potentials of Protoscoleces of Echinococcus granulosus

The protoscolex (PSC) is generated by asexual reproduction at the larval stage of taeniid Echinococcus granulosus that causes cystic echinococcosis or hydatidosis, a worldwide zoonosis. The PSC is enveloped by a complex cellular syncytial tegument responsible for ionic movements and the hydroelectrolytic balance of the parasite. We recently reported on two electrical potentials in bovine lung protoscoleces (PSCs) that reflect differences in ionic movements between the parasite's invaginated and evaginated developmental stages. Here, we explored the effect of temperature and ionic substitutions on the tegumental potentials of bovine lung PSCs of Echinococcus granulosus by microelectrode impalements. We observed that the transient peak potential was temperature-dependent, consistent with an active transport component in the invaginated state only. Further changes in the electrical potentials by high K⁺ depolarization, low external Ca²⁺, and addition of the diuretic amiloride are in agreement with the presence of a Ca²⁺-sensitive cation-selective electrodiffusional pathway in the outer surface of the parasite. Variations in electrical potential differences through the tegument provide an accessible and valuable parameter for studying ionic transport mechanisms and, therefore, potential targets for developing novel antiparasitic drugs.

Multivariate chemogenomic screening prioritizes new macrofilaricidal leads

Development of direct acting macrofilaricides for the treatment of human filariases is hampered by limitations in screening throughput imposed by the parasite life cycle. In vitro adult screens typically assess single phenotypes without prior enrichment for chemicals with antifilarial potential. We developed a multivariate screen that identified dozens of compounds with submicromolar macrofilaricidal activity, achieving a hit rate of >50% by leveraging abundantly accessible microfilariae. Adult assays were multiplexed to thoroughly characterize compound activity across relevant parasite fitness traits, including neuromuscular control, fecundity, metabolism, and viability. Seventeen compounds from a diverse chemogenomic library elicited strong effects on at least one adult trait, with differential potency against microfilariae and adults. Our screen identified five compounds with high potency against adults but low potency or slow-acting microfilaricidal effects, at least one of which acts through a novel mechanism. We show that the use of microfilariae in a primary screen outperforms model nematode developmental assays and virtual screening of protein structures inferred with deep learning. These data provide new leads for drug development, and the high-content and multiplex assays set a new foundation for antifilarial discovery.

Electrical potentials of protoscoleces of the cestode Echinococcus granulosus from bovine origin

Larval stages of taeniid Echinococcus granulosus are the infective forms of cystic echinococcosis or hydatidosis, a worldwide zoonosis. The protoscolex that develops into the adult form in the definitive host is enveloped by a complex cellular syncytial tegument, where all metabolic interchange takes place. Little information is available as to the electrical activity of the parasite in this developmental stage. To gain insight into the electrical activity of the parasite at the larval stage, we conducted microelectrode impalements of bovine lung protoscoleces (PSCs) of Echinococcus granulosus in standard saline solution. We observed two distinct intra-parasitic potentials, a transient peak potential, and a stable second potential, most likely representing tegumental and intra-parasitic extracellular space electrical potential differences. These values changed on the developmental status of the parasite, its anatomical regions, or time course after harvesting. Changes in electrical potential differences of the parasite provide an accessible and valuable parameter for the study of transport mechanisms and potential targets for developing novel antiparasitic therapeutics.

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.

Drug Discovery and Target Identification against Schistosomiasis: a Reality Check on Progress and Future Prospects

Schistosomiasis ranks among the most important infectious diseases, with over 200 million people currently being infected and > 280,000 deaths reported annually. Chemotherapeutic treatment has relied on one drug, praziquantel, for four decades, while other drugs, such as oxamniquine and metrifonate, are no longer preferred for clinical use due to their narrow spectrum of activity - these are only active against S. mansoni and S. haematobium, respectively. Despite being cheap, safe, and effective against all schistosome species, praziquantel is ineffective against immature worms, which may lead to reinfections and treatment failure in endemic areas; a situation that necessitates repeated administration besides other limitations. Therefore, novel drugs are urgently needed to overcome this situation. In this paper, an up to date review of drug targets identified and validated against schistosomiasis while also encompassing promising clinical and preclinical candidate drugs is presented. While there are considerable efforts aimed at identifying and validating drug targets, the pipeline for new antischistosomals is dry. Moreover, the majority of compounds evaluated preclinically are not really advanced because most of them were evaluated in very small preclinical species such as mice alone. Overall, it appears that although a lot of research is going on at discovery phases, unfortunately, it does not translate to advanced preclinical and clinical evaluation.

Antiparasitic Properties of Cardiovascular Agents against Human Intravascular Parasite Schistosoma mansoni

The intravascular parasitic worm Schistosoma mansoni is a causative agent of schistosomiasis, a disease of great global public health significance. Praziquantel is the only drug available to treat schistosomiasis and there is an urgent demand for new anthelmintic agents. Adopting a phenotypic drug screening strategy, here, we evaluated the antiparasitic properties of 46 commercially available cardiovascular drugs against S. mansoni. From these screenings, we found that amiodarone, telmisartan, propafenone, methyldopa, and doxazosin affected the viability of schistosomes in vitro, with effective concentrations of 50% (EC₅₀) and 90% (EC₉₀) values ranging from 8 to 50 µM. These results were further supported by scanning electron microscopy analysis. Subsequently, the most effective drug (amiodarone) was further tested in a murine model of schistosomiasis for both early and chronic S. mansoni infections using a single oral dose of 400 mg/kg or 100 mg/kg daily for five consecutive days. Amiodarone had a low efficacy in chronic infection, with the worm and egg burden reduction ranging from 10 to 30%. In contrast, amiodarone caused a significant reduction in worm and egg burden in early infection (>50%). Comparatively, treatment with amiodarone is more effective in early infection than praziquantel, demonstrating the potential role of this cardiovascular drug as an antischistosomal agent.

Phytochemical analysis and in vitro anthelmintic activity of Imperata cylindrica underground parts

Background

Imperata cylindrica is considered as an agricultural weed, but it is a valuable plant in the traditional medicines of Southeast Asia. In the Mizo traditional medicine of India and Myanmar, the rhizomes and roots are used as a remedy for bacterial, fungal and intestinal helminth infections.

Methods

An extract of the whole underground parts was prepared in Soxhlet apparatus using chloroform as a solvent. After concentrating in a vacuum rotary evaporator, the extract was analysed using gas chromatography-mass spectrometry. Anthelmintic activity was tested in vitro against the tapeworm Raillietina tetragona and the roundworm Ascaridia galli. Scanning electron microscopy was used to examine the structural changes on the helminths after treatment with the plant extract.

Results

Twenty-two compounds were identified from the plant extract out of which fatty acids were the predominant compounds. Palmitic acid was the most abundant. Bioactive phytosterols such as campesterol and stigmasterol were also detected. The plant extract was significantly effective on both the helminths and showed dose-dependent anthelmintic activity as that of albendazole. The tapeworm treated with the plant extract showed deformities on the suckers, clumping of the spines, tegumental folds and erosion of microtriches. Extensive damage was also seen on the roundworm including cuticular shrinkage, collapse of the lips, and formation of warty surface throughout the body.

Conclusion

I. cylindrica extract effectively killed and caused detrimental effects on parasitic tapeworm and roundworm. The study therefore validates the traditional usage among the Mizo people, and guarantees further investigation on the exact compound(s) and mechanism of action.

Schistosome TRP channels: An appraisal

Ion channels underlie electrical excitability in cells and are essential for a variety of functions, most notably neuromuscular and sensory activity. They are also validated targets for a preponderance of approved anthelmintic compounds. Transient receptor potential (TRP) channels constitute an ion channel superfamily whose members play important roles in sensory signaling, regulation of ion homeostasis, organellar trafficking, and other key cellular and organismal activities. Unlike most other ion channels, TRP channels are often polymodal, gated by a variety of mechanisms. Furthermore, TRP channels fall into several classes or subtypes based on sequence and structure. Until recently, there had been very little investigation of the properties and functions of TRP channels from parasitic helminths, including schistosomes, but that situation has changed in the past few years. Indeed, it is now clear that at least some schistosome TRP channels exhibit unusual pharmacological properties, and, intriguingly, both a mammalian and a schistosome TRP channel are activated by praziquantel, the current antischistosomal drug of choice. With the latest release of the Schistosoma mansoni genome database, several changes in predicted TRP channel sequences appeared, some of which were significant. This review updates and reassesses the TRP channel repertoire in S. mansoni, examines recent findings regarding these potential therapeutic targets, and provides guideposts for some of the physiological functions that may be mediated by these channels in schistosomes.

EAT-18 is an essential auxiliary protein interacting with the non-alpha nAChR subunit EAT-2 to form a functional receptor

Nematode parasites infect approximately 1.5 billion people globally and are a significant public health concern. There is an accepted need for new, more effective anthelmintic drugs. Nicotinic acetylcholine receptors on parasite nerve and somatic muscle are targets of the cholinomimetic anthelmintics, while glutamate-gated chloride channels in the pharynx of the nematode are affected by the avermectins. Here we describe a novel nicotinic acetylcholine receptor on the nematode pharynx that is a potential new drug target. This homomeric receptor is comprised of five non-α EAT-2 subunits and is not sensitive to existing cholinomimetic anthelmintics. We found that EAT-18, a novel auxiliary subunit protein, is essential for functional expression of the receptor. EAT-18 directly interacts with the mature receptor, and different homologs alter the pharmacological properties. Thus we have described not only a novel potential drug target but also a new type of obligate auxiliary protein for nAChRs.

Soil-transmitted helminths affect about a quarter of the worlds’ population. Chemical anthelmintics not only alleviate the threat to human and animal health but also improve agricultural economics and food security. Here we have identified a “druggable” nicotinic acetylcholine receptor (nAChR) subunit, EAT-2, that constitutes the pharyngeal cholinergic receptor in nematodes. The receptor is required for feeding and possibly for reproductive behavior in worms. A selective therapeutic compound targeting this nAChR should either starve the worms or make them sluggish, helping with faster expulsion from the host. The EAT-2 pharyngeal nAChR is a unique receptor formed by five non-α subunits that lack vicinal cysteines in the ligand binding loop-C. To date, all cation selective nAChRs contain at least two α subunits. It is possible that EAT-2 subunits have retained functionality without the vicinal cysteines due to evolutionary modifications and expresses as a new nAChR subtype which doesn’t fit the established dogma based on the study of vertebrate receptors. Our findings also identified a new type of auxiliary protein subunit, which is essential for functional expression of the pharyngeal nAChR and also modulates its pharmacology. To the best of our knowledge, this is the first report of an auxiliary protein that is essential for functional expression in any cys-loop ligand-gated ion channel.

Revisited - Failure of tetrodotoxin to protect red-spotted newts, Notophthalmus viridescens, from endoparasites

Red-spotted newts, Notophthalmus viridescens, contain tetrodotoxin (TTX) and its analogue 6-epiTTX in variable concentrations. In a follow-up study, newts were sampled from a pond in Pennsylvania, USA, in 2010, 2014, and 2018. Their toxin levels were assayed by liquid-chromatography-fluorescence detection (LC-FLD), and assessment of their infection with endoparasites such as nematodes and helminths was performed by histological examination of internal organs. In the 2010 and 2014 samples, average prevalence of parasite infection was 53 and 60%, respectively, but reached 100% in the 2018 sample, where metacercaria stages of the digenean trematode genus Australapatemon/Apatemon (family: Strigeidae) were predominant causing severe tissue damage in liver and kidney. Mean values of TTX and 6-epiTTX were not significantly different in parasitized or parasite-free newts over the study period, confirming previous findings that host toxicity and parasite load are not negatively correlated. Whereas the role of TTX in defence against predators is undisputed, its efficacy to prevent parasitic infections is less obvious. Toxin-resistance of various metazoan parasites may promote their widespread occurrence in poisonous newts.

In Vitro and In Vivo Studies of Spironolactone as an Antischistosomal Drug Capable of Clinical Repurposing

Schistosomiasis is a parasitic flatworm disease that infects over 200 million people worldwide, especially in poor communities. Treatment and control of the disease rely on just one drug, praziquantel. Since funding for drug development for poverty-associated diseases is very limited, drug repurposing is a promising strategy. In this study, from a screening of 13 marketed diuretics, we identified that spironolactone, a potassium-sparing diuretic, had potent antischistosomal effects on Schistosoma mansoni in vitro and in vivo in a murine model of schistosomiasis. In vitro, spironolactone at low concentrations (<10 µM) is able to alter worm motor activity and the morphology of adult schistosomes, leading to parasitic death. In vivo, oral treatment with spironolactone at a single dose (400 mg/kg) or daily for five consecutive days (100 mg/kg/day) in mice harboring either patent or prepatent infections significantly reduced worm burden, egg production, and hepato- and splenomegaly (P < 0.05 to P < 0.001). Taken together, with the safety profile of spironolactone, supported by its potential to affect schistosomes, these results indicate that spironolactone could be a potential treatment for schistosomiasis and make it promising for repurposing.

TRP channels as potential targets for antischistosomals

Ion channels are membrane protein complexes that underlie electrical excitability in cells, allowing ions to diffuse through cell membranes in a regulated fashion. They are essential for normal functioning of the neuromusculature and other tissues. Ion channels are also validated targets for many current anthelmintics, yet the properties of only a small subset of ion channels in parasitic helminths have been explored in any detail. Transient receptor potential (TRP) channels comprise a widely diverse superfamily of ion channels with important roles in sensory signaling, regulation of ion homeostasis, organellar trafficking, and other functions. There are several subtypes of TRP channels, including TRPA1 and TRPV1 channels, both of which are involved in, among other functions, sensory, nociceptive, and inflammatory signaling in mammals. Several lines of evidence indicate that TRPA1-like channels in schistosomes exhibit pharmacological sensitivities that differ from their mammalian counterparts and that may signify unique physiological properties as well. Thus, in addition to responding to TRPA1 modulators, schistosome TRPA1-like channels also respond to compounds that in other organisms modulate TRPV1 channels. Notably, TRPV channel genes are not found in schistosome genomes. Here, we review the evidence leading to these conclusions and examine potential implications. We also discuss recent results showing that praziquantel, the current drug of choice against schistosomiasis, selectively targets host TRP channels in addition to its likely primary targets in the parasite. The results we discuss add weight to the notion that schistosome TRP channels are worthy of investigation as candidate therapeutic targets.

Cyclic Octamer Peptoids: Simplified Isosters of Bioactive Fungal Cyclodepsipeptides

Cyclic peptoids have recently emerged as an important class of bioactive scaffolds with unique conformational properties and excellent metabolic stabilities. In this paper, we describe the design and synthesis of novel cyclic octamer peptoids as simplified isosters of mycotoxin depsipeptides bassianolide, verticilide A1, PF1022A and PF1022B. We also examine their complexing abilities in the presence of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB) salt and explore their general insecticidal activity. Finally, we discuss the possible relationship between structural features of free and Na⁺-complexed cyclic octamer peptoids and bioactivities in light of conformational isomerism, a crucial factor affecting cyclic peptoids' biomimetic potentials.

Bioassay-guided isolation of three anthelmintic compounds from Warburgia ugandensis Sprague subspecies ugandensis, and the mechanism of action of polygodial

Parasitic helminths continue to pose problems in human and veterinary medicine, as well as in agriculture. Resistance to current anthelmintics has prompted the search for new drugs. Anthelmintic metabolites from medicinal plants could be good anthelmintic drug candidates. However, the compounds active against nematodes have not been identified in most medicinal plants with anthelmintic activity. In this study, we aimed to identify the active compounds against helminths in Warburgia ugandensis Sprague subspecies ugandensis (Canellaceae) and study the underlying mechanism of action. A bioassay-guided isolation of anthelmintic compounds from the plant was performed using a Caenorhabditis elegans (C. elegans) test model with a WMicrotracker instrument to monitor motility. Three active compounds were purified and identified by nuclear magnetic resonance and high resolution MS: warburganal (IC₅₀: 28.2 ± 8.6 μM), polygodial (IC₅₀: 13.1 ± 5.3 μM) and alpha-linolenic acid (ALA, IC₅₀: 70.1 ± 17.5 μM). A checkerboard assay for warburganal and ALA as well as polygodial and ALA showed a fractional inhibitory concentration index of 0.41 and 0.37, respectively, suggesting that polygodial and ALA, as well as warburganal and ALA, have a synergistic effect against nematodes. A preliminary structure-activity relationship study for polygodial showed that the α,β-unsaturated 1,4-dialdehyde structural motif is essential for the potent activity. None of a panel of C. elegans mutant strains, resistant against major anthelmintic drug classes, showed significant resistance to polygodial, implying that polygodial may block C. elegans motility through a mechanism which differs from that of currently marketed drugs. Further measurements showed that polygodial inhibits mitochondrial ATP synthesis of C. elegans in a dose-dependent manner (IC₅₀: 1.8 ± 1.0 μM). Therefore, we believe that the underlying mechanism of action of polygodial is probably inhibition of mitochondrial ATP synthesis. In conclusion, polygodial could be a promising anthelmintic drug candidate worth considering for further development.

Atypical pharmacology of schistosome TRPA1-like ion channels

Parasitic flatworms of the genus Schistosoma cause schistosomiasis, a neglected tropical disease estimated to affect over 200 million people worldwide. Praziquantel is the only antischistosomal currently available for treatment, and there is an urgent need for new therapeutics. Ion channels play key roles in physiology and are targets for many anthelmintics, yet only a few representatives have been characterized in any detail in schistosomes and other parasitic helminths. The transient receptor potential (TRP) channel superfamily comprises a diverse family of non-selective cation channels that play key roles in sensory transduction and a wide range of other functions. TRP channels fall into several subfamilies. Members of both the TRPA and TRPV subfamilies transduce nociceptive and inflammatory signals in mammals, and often also respond to chemical and thermal signals. We previously showed that although schistosomes contain no genes predicted to encode TRPV channels, TRPV1-selective activators such as capsaicin and resiniferatoxin elicit dramatic hyperactivity in adult worms and schistosomula. Surprisingly, this response requires expression of a S. mansoni TRPA1-like orthologue (SmTRPA). Here, we show that capsaicin induces a rise in intracellular Ca²⁺ in mammalian cells expressing either SmTRPA or a S. haematobium TRPA1 orthologue (ShTRPA). We also test SmTRPA and ShTRPA responses to various TRPV1 and TRPA1 modulators. Interestingly, in contrast to SmTRPA, ShTRPA is not activated by the TRPA1 activator AITC (allyl isothiocyanate), nor do S. haematobium adult worms respond to this compound, a potentially intriguing species difference. Notably, 4-hydroxynonenal (4-HNE), a host-derived, inflammatory product that directly activates mammalian TRPA1, also activates both SmTRPA and ShTRPA. Our results point to parasite TRPA1-like channels which exhibit atypical, mixed TRPA1/TRPV1-like pharmacology, and which may also function to transduce endogenous host signals.

Schistosomes are parasitic flatworms that infect hundreds of millions of people worldwide. They cause schistosomiasis, a disease with major consequences for human health and economic development. There is only a single drug available for treatment and control of this highly prevalent disease, and there is an urgent need for development of new treatments. TRP ion channels play key roles in sensory (and other) functions. One type of TRP channel, TRPV1, is activated by capsaicin, the active ingredient in hot peppers. However, schistosomes do not have any TRPV-like channels. Nonetheless, we previously showed that capsaicin and similar compounds induce dramatic hyperactivity in schistosomes, and that this response is abolished by suppressing expression of SmTRPA, a schistosome TRPA1-like channel. Mammalian TRPA1 channels are not sensitive to capsaicin. Here, we show that the SmTRPA channel itself responds to capsaicin, resulting in an influx of Ca²⁺ into cells. ShTRPA, a TRPA1-like channel from another schistosome, S. haematobium, is also sensitive to capsaicin. Thus, the pharmacology of schistosome TRPA1 channels apparently differs from that of host mammalian channels, a characteristic that could indicate mixed TRPA/TRPV functionality and might be exploitable for development of new antischistosomal drugs. Furthermore, we show that schistosome TRPA1-like channels are activated by host-derived compounds, perhaps indicating a mechanism by which the parasite can respond to host signals.

G-protein-coupled receptor genes of Dirofilaria immitis

The diversity and uniqueness of nematode heterotrimeric G-protein-coupled receptors (GPCRs) provides impetus for identifying ligands that can be used as therapeutics for treating diseases caused by parasitic nematode infections. In human medicine, GPCRs have represented the largest group of 'drugable' targets exploited in the market today. In the filarial nematode Dirofilaria immitis, which causes heartworm disease, the macrocyclic lactones (ML) have been used as the sole preventatives for more than 25 years and now there is confirmed ML resistance in this parasite. A novel anthelmintic emodepside, with antifilarial activity, can act on a GPCR. In view of the ML resistance, there is an urgent need to identify new drug targets and GPCRs of D. immitis may be promising receptors. Knowledge of polymorphism within the GPCR superfamily is of interest. A total of 127 GPCR genes have been identified, so far, in the genome of D. immitis. Whole genome sequencing data from four ML susceptible and four ML loss of efficacy populations was used to identify 393 polymorphic loci in 35 D. immitis GPCR genes. Out of 57 SNPs in exonic regions, 36 of them caused a change in an amino acid, out of which 2 changed the predicted secondary structure of the protein. Knowledge about GPCR genes and their polymorphism is valuable information for drug design processes. Further studies need to be carried out to more fully understand the implications of each of the SNPs identified by this study.

Diversity within diversity: Parasite species richness in poison frogs assessed by transcriptomics

Symbionts (e.g., endoparasites and commensals) play an integral role in their host's ecology, yet in many cases their diversity is likely underestimated. Although endoparasites are traditionally characterized using morphology, sequences of conserved genes, and shotgun metagenomics, host transcriptomes constitute an underused resource to identify these organisms' diversity. By isolating non-host transcripts from host transcriptomes, individual host tissues can now simultaneously reveal their endoparasite species richness (i.e., number of different taxa) and provide insights into parasite gene expression. These approaches can be used in host taxa whose endoparasites are mostly unknown, such as those of tropical amphibians. Here, we focus on the poison frogs (Dendrobatidae) as hosts, which are a Neotropical clade known for their bright coloration and defensive alkaloids. These toxins are an effective protection against vertebrate predators (e.g., snakes and birds), bacteria, and skin-biting ectoparasites (e.g., mosquitoes); however, little is known about their deterrence against eukaryotic endoparasites. With de novo transcriptomes of dendrobatids, we developed a bioinformatics pipeline for endoparasite identification that uses host annotated RNA-seq data and set of a priori parasite taxonomic terms, which are used to mine for specific endoparasites. We found a large community of helminths and protozoans that were mostly restricted to the digestive tract and a few systemic parasites (e.g., Trypanosoma). Contrary to our expectations, all dendrobatid frogs regardless of the presence of alkaloid defenses have endoparasites, with their highest species richness located in the frog digestive tract. Some of these organisms (e.g., roundworms) might prove to be generalists, as they were not found to be co-diversifying with their frog hosts. We propose that endoparasites may escape poison frogs' chemical defenses by colonizing tissues with fewer alkaloids than the frog's skin, where most toxins are stored.

Reasons to Be Nervous about Flukicide Discovery

The majority of anthelmintics dysregulate neuromuscular function, a fact most prominent for drugs against nematode parasites. In contrast to the strong knowledge base for nematode neurobiology, resource and tool deficits have prevented similar advances in flatworm parasites since those driven by bioimaging, immunocytochemistry, and neuropeptide biochemistry 20-30 years ago. However, recent developments are encouraging a renaissance in liver fluke neurobiology that can now support flukicide discovery. Emerging data promote neuromuscular signalling components, and especially G protein-coupled receptors (GPCRs), as next-generation targets. Here, we summarise these data and expose some of the new opportunities to accelerate progress towards GPCR-targeted flukicides for Fasciola hepatica.

Effects of proteasome inhibitor MG-132 on the parasite Schistosoma mansoni

Proteasome is a proteolytic complex responsible for intracellular protein turnover in eukaryotes, archaea and in some actinobacteria species. Previous work has demonstrated that in Schistosoma mansoni parasites, the proteasome inhibitor MG-132 affects parasite development. However, the molecular targets affected by MG-132 in S. mansoni are not entirely known. Here, we used expression microarrays to measure the genome-wide changes in gene expression of S. mansoni adult worms exposed in vitro to MG-132, followed by in silico functional analyses of the affected genes using Ingenuity Pathway Analysis (IPA). Scanning electron microscopy was used to document changes in the parasites’ tegument. We identified 1,919 genes with a statistically significant (q-value ≤ 0.025) differential expression in parasites treated for 24 h with MG-132, when compared with control. Of these, a total of 1,130 genes were up-regulated and 790 genes were down-regulated. A functional gene interaction network comprised of MG-132 and its target genes, known from the literature to be affected by the compound in humans, was identified here as affected by MG-132. While MG-132 activated the expression of the 26S proteasome genes, it also decreased the expression of 19S chaperones assembly, 20S proteasome maturation, ubiquitin-like NEDD8 and its partner cullin-3 ubiquitin ligase genes. Interestingly, genes that encode proteins related to potassium ion binding, integral membrane component, ATPase and potassium channel activities were significantly down-regulated, whereas genes encoding proteins related to actin binding and microtubule motor activity were significantly up-regulated. MG-132 caused important changes in the worm tegument; peeling, outbreaks and swelling in the tegument tubercles could be observed, which is consistent with interference on the ionic homeostasis in S. mansoni. Finally, we showed the down-regulation of Bax pro-apoptotic gene, as well as up-regulation of two apoptosis inhibitor genes, IAP1 and BRE1, and in contrast, down-regulation of Apaf-1 apoptotic activator, thus suggesting that apoptosis is deregulated in S. mansoni exposed to MG-132. A considerable insight has been gained concerning the potential of MG-132 as a gene expression modulator, and overall the data suggest that the proteasome might be an important molecular target for the design of new drugs against schistosomiasis.

Functional genomics in Brugia malayi reveal diverse muscle nAChRs and differences between cholinergic anthelmintics

Many techniques for studying functional genomics of important target sites of anthelmintics have been restricted to Caenorhabditis elegans because they have failed when applied to animal parasites. To overcome these limitations, we have focused our research on the human nematode parasite Brugia malayi, which causes elephantiasis. Here, we combine single-cell PCR, whole muscle cell patch clamp, motility phenotyping (Worminator), and dsRNA for RNAi for functional genomic studies that have revealed, in vivo, four different muscle nAChRs (M-, L-, P-, and N-). The cholinergic anthelmintics had different selectivities for these receptors. We show that motility and patch-clamp responses to levamisole and pyrantel, but not morantel or nicotine, require the unc-38 and/or unc-29 genes. Derquantel behaved as a competitive antagonist and distinguished M-nAChRs activated by morantel (Kb 13.9 nM), P-nAChRs activated by pyrantel (Kb 126 nM), and L-nAChRs activated by levamisole (Kb 0.96 µM) and bephenium. Derquantel was a noncompetitive antagonist of nicotine, revealing N-type nAChRs. The presence of four diverse nAChRs on muscle is perhaps surprising and not predicted from the C. elegans model. The diverse nAChRs represent distinguishable drug targets with different functions: Knockdown of unc-38+unc-29 (L- and/or P-receptors) inhibited motility but knockdown of acr-16+acr-26 (M- and/or N-receptors) did not.

Polymorphism in ABC transporter genes of Dirofilaria immitis

Dirofilaria immitis, a filarial nematode, causes dirofilariasis in dogs, cats and occasionally in humans. Prevention of the disease has been mainly by monthly use of the macrocyclic lactone (ML) endectocides during the mosquito transmission season. Recently, ML resistance has been confirmed in D. immitis and therefore, there is a need to find new classes of anthelmintics. One of the mechanisms associated with ML resistance in nematodes has been the possible role of ATP binding cassette (ABC) transporters in reducing drug concentrations at receptor sites. ABC transporters, mainly from sub-families B, C and G, may contribute to multidrug resistance (MDR) by active efflux of drugs out of the cell. Gene products of ABC transporters may thus serve as the targets for agents that may modulate susceptibility to drugs, by inhibiting drug transport. ABC transporters are believed to be involved in a variety of physiological functions critical to the parasite, such as sterol transport, and therefore may also serve as the target for drugs that can act as anthelmintics on their own. Knowledge of polymorphism in these ABC transporter genes in nematode parasites could provide useful information for the process of drug design. We have identified 15 ABC transporter genes from sub-families A, B, C and G, in D. immitis, by comparative genomic approaches and analyzed them for polymorphism. Whole genome sequencing data from four ML susceptible (SUS) and four loss of efficacy (LOE) pooled populations were used for single nucleotide polymorphism (SNP) genotyping. Out of 231 SNPs identified in those 15 ABC transporter genes, 89 and 75 of them were specific to the SUS or LOE populations, respectively. A few of the SNPs identified may affect gene expression, protein function, substrate specificity or resistance development and may be useful for transporter inhibitor/anthelmintic drug design, or in order to anticipate resistance development.

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In the D. immitis genome, all ABC-A, -B, -C and -G transporter genes were identified.

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Within 15 ABC transporter genes identified in D. immitis, 231 SNP loci were found.

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Four exonic SNPs caused changes in predicted secondary structure of ABC proteins.

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D. immitis populations have low genetic variability among ABC transporter genes.

Utilizing the planarian voltage-gated ion channel transcriptome to resolve a role for a Ca2+ channel in neuromuscular function and regeneration

The robust regenerative capacity of planarian flatworms depends on the orchestration of signaling events from early wounding responses through the stem cell enacted differentiative outcomes that restore appropriate tissue types. Acute signaling events in excitable cells play an important role in determining regenerative polarity, rationalized by the discovery that sub-epidermal muscle cells express critical patterning genes known to control regenerative outcomes. These data imply a dual conductive (neuromuscular signaling) and instructive (anterior-posterior patterning) role for Ca²⁺ signaling in planarian regeneration. Here, to facilitate study of acute signaling events in the excitable cell niche, we provide a de novo transcriptome assembly from the planarian Dugesia japonica allowing characterization of the diverse ionotropic portfolio of this model organism. We demonstrate the utility of this resource by proceeding to characterize the individual role of each of the planarian voltage-operated Ca²⁺ channels during regeneration, and demonstrate that knockdown of a specific voltage operated Ca²⁺ channel (Ca_v1B) that impairs muscle function uniquely creates an environment permissive for anteriorization. Provision of the full transcriptomic dataset should facilitate further investigations of molecules within the planarian voltage-gated channel portfolio to explore the role of excitable cell physiology on regenerative outcomes. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

TRP channels in schistosomes

Praziquantel (PZQ) is effectively the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions of people worldwide. Many anthelmintics, likely including PZQ, target ion channels, membrane protein complexes essential for normal functioning of the neuromusculature and other tissues. Despite this fact, only a few classes of parasitic helminth ion channels have been assessed for their pharmacological properties or for their roles in parasite physiology. One such overlooked group of ion channels is the transient receptor potential (TRP) channel superfamily. TRP channels share a common core structure, but are widely diverse in their activation mechanisms and ion selectivity. They are critical to transducing sensory signals, responding to a wide range of external stimuli. They are also involved in other functions, such as regulating intracellular calcium and organellar ion homeostasis and trafficking. Here, we review current literature on parasitic helminth TRP channels, focusing on those in schistosomes. We discuss the likely roles of these channels in sensory and locomotor activity, including the possible significance of a class of TRP channels (TRPV) that is absent in schistosomes. We also focus on evidence indicating that at least one schistosome TRP channel (SmTRPA) has atypical, TRPV1-like pharmacological sensitivities that could potentially be exploited for future therapeutic targeting.

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We provide an overview of transient receptor potential (TRP) channels in schistosomes and other parasitic helminths.

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TRP channels are important for sensory signaling, ion homeostasis, organellar trafficking, and a host of other functions.

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Very little work has been done on TRP channels in parasitic helminths.

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TRPV channels, found throughout the Metazoa, appear not to be present in parasitic platyhelminths.

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TRP channels in schistosomes appear to have atypical pharmacology, perhaps an entrée for therapeutic targeting.

Polymorphism in ion channel genes of Dirofilaria immitis: Relevant knowledge for future anthelmintic drug design

Dirofilaria immitis, a filarial parasite, causes cardiopulmonary dirofilariasis in dogs, cats and wild canids. The macrocyclic lactone (ML) class of drugs has been used to prevent heartworm infection. There is confirmed ML resistance in D. immitis and thus there is an urgent need to find new anthelmintics that could prevent and/or control the disease. Targeting ion channels of D. immitis for drug design has obvious advantages. These channels, present in the nematode nervous system, control movement, feeding, mating and respond to environmental cues which are necessary for survival of the parasite. Any new drug that targets these ion channels is likely to have a motility phenotype and should act to clear the worms from the host. Many of the successful anthelmintics in the past have targeted these ion channels and receptors. Knowledge about genetic variability of the ion channel and receptor genes should be useful information for drug design as receptor polymorphism may affect responses to a drug. Such information may also be useful for anticipation of possible resistance development. A total of 224 ion channel genes/subunits have been identified in the genome of D. immitis. Whole genome sequencing data of parasites from eight different geographical locations, four from ML-susceptible populations and the other four from ML-loss of efficacy (LOE) populations, were used for polymorphism analysis. We identified 1762 single nucleotide polymorphic (SNP) sites (1508 intronic and 126 exonic) in these 224 ion channel genes/subunits with an overall polymorphic rate of 0.18%. Of the SNPs found in the exon regions, 129 of them caused a non-synonymous type of polymorphism. Fourteen of the exonic SNPs caused a change in predicted secondary structure. A few of the SNPs identified may have an effect on gene expression, function of the protein and resistance selection processes.

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In the Dirofilaria immitis genome, 126 ion channel genes were identified.

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Within 126 ion channel genes, 1762 polymorphic loci were identified.

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Fourteen exonic SNPs caused a change in predicted secondary structure.

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SNPs may effect gene expression, protein function or resistance selection.

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D. immitis populations have low genetic variability among ion channel genes.

Curiouser and Curiouser: The Macrocyclic Lactone, Abamectin, Is also a Potent Inhibitor of Pyrantel/Tribendimidine Nicotinic Acetylcholine Receptors of Gastro-Intestinal Worms

Nematode parasites may be controlled with drugs, but their regular application has given rise to concerns about the development of resistance. Drug combinations may be more effective than single drugs and delay the onset of resistance. A combination of the nicotinic antagonist, derquantel, and the macrocyclic lactone, abamectin, has been found to have synergistic anthelmintic effects against gastro-intestinal nematode parasites. We have observed in previous contraction and electrophysiological experiments that derquantel is a potent selective antagonist of nematode parasite muscle nicotinic receptors; and that abamectin is an inhibitor of the same nicotinic receptors. To explore these inhibitory effects further, we expressed muscle nicotinic receptors of the nodular worm, Oesophagostomum dentatum (Ode-UNC-29:Ode-UNC-63:Ode-UNC-38), in Xenopus oocytes under voltage-clamp and tested effects of abamectin on pyrantel and acetylcholine responses. The receptors were antagonized by 0.03 μM abamectin in a non-competitive manner (reduced R_max, no change in EC₅₀). This antagonism increased when abamectin was increased to 0.1 μM. However, when we increased the concentration of abamectin further to 0.3 μM, 1 μM or 10 μM, we found that the antagonism decreased and was less than with 0.1 μM abamectin. The bi-phasic effects of abamectin suggest that abamectin acts at two allosteric sites: one high affinity negative allosteric (NAM) site causing antagonism, and another lower affinity positive allosteric (PAM) site causing a reduction in antagonism. We also tested the effects of 0.1 μM derquantel alone and in combination with 0.3 μM abamectin. We found that derquantel on these receptors, like abamectin, acted as a non-competitive antagonist, and that the combination of derquantel and abamectin produced greater inhibition. These observations confirm the antagonistic effects of abamectin on nematode nicotinic receptors in addition to GluCl effects, and illustrate more complex effects of macrocyclic lactones that may be exploited in combinations with other anthelmintics.

Evidence for Novel Pharmacological Sensitivities of Transient Receptor Potential (TRP) Channels in Schistosoma mansoni

Schistosomiasis, caused by parasitic flatworms of the genus Schistosoma, is a neglected tropical disease affecting hundreds of millions globally. Praziquantel (PZQ), the only drug currently available for treatment and control, is largely ineffective against juvenile worms, and reports of PZQ resistance lend added urgency to the need for development of new therapeutics. Ion channels, which underlie electrical excitability in cells, are validated targets for many current anthelmintics. Transient receptor potential (TRP) channels are a large family of non-selective cation channels. TRP channels play key roles in sensory transduction and other critical functions, yet the properties of these channels have remained essentially unexplored in parasitic helminths. TRP channels fall into several (7-8) subfamilies, including TRPA and TRPV. Though schistosomes contain genes predicted to encode representatives of most of the TRP channel subfamilies, they do not appear to have genes for any TRPV channels. Nonetheless, we find that the TRPV1-selective activators capsaicin and resiniferatoxin (RTX) induce dramatic hyperactivity in adult worms; capsaicin also increases motility in schistosomula. SB 366719, a highly-selective TRPV1 antagonist, blocks the capsaicin-induced hyperactivity in adults. Mammalian TRPA1 is not activated by capsaicin, yet knockdown of the single predicted TRPA1-like gene (SmTRPA) in S. mansoni effectively abolishes capsaicin-induced responses in adult worms, suggesting that SmTRPA is required for capsaicin sensitivity in these parasites. Based on these results, we hypothesize that some schistosome TRP channels have novel pharmacological sensitivities that can be targeted to disrupt normal parasite neuromuscular function. These results also have implications for understanding the phylogeny of metazoan TRP channels and may help identify novel targets for new or repurposed therapeutics.