Biogenic amines and the control of neuromuscular signaling in schistosomes

Rhodopsin orphan GPCR20 interacts with neuropeptides and directs growth, sexual differentiation, and egg production in female Schistosoma mansoni

IMPORTANCE

Schistosomes cause schistosomiasis, one of the neglected tropical diseases as defined by the WHO. For decades, the treatment of schistosomiasis relies on a single drug, praziquantel. Due to its wide use, there is justified fear of resistance against this drug, and a vaccine is not available. Besides its biological relevance in signal transduction processes, the class of G protein-coupled receptors (GPCRs) is also well suited for drug design. Against this background, we characterized one GPCR of Schistosoma mansoni, SmGPCR20, at the molecular and functional level. We identified two potential neuropeptides (NPPs) as ligands, SmNPP26 and SmNPP40, and unraveled their roles, in combination with SmGPCR20, in neuronal processes controlling egg production, oogenesis, and growth of S. mansoni females. Since eggs are closely associated with the pathogenesis of schistosomiasis, our results contribute to the understanding of processes leading to egg production in schistosomes, which is under the control of pairing in this exceptional parasite.

Schistosoma japonicum Tyrosine Hydroxylase is promising targets for immunodiagnosis and immunoprotection of Schistosomiasis japonica

Identification of promising schistosome antigen targets is crucial for the development of anti-schistosomal strategies. Schistosomes rely on their neuromuscular systems to coordinate important locomotory behaviors. Tyrosine hydroxylase (TH) is critical in the initial rate-limiting step in biosynthesis of catecholamine, the important neuroactive agents, which promote the lengthening of the worm through muscular relaxation and are therefore of great importance to the movement of the organism both within and between its hosts. THs from both Schistosoma mansoni and Schistosoma japonicum and their enzyme activities have been discovered; however, the role of these proteins during infection have not been explored. Herein, a recombinant protein of the nonconserved fragment of S. japonicum TH (SjTH) was produced and the corresponding polyclonal antibody was generated. The expression and antigenicity of SjTH were detected by qRT-PCR, western blotting, immunofluorescence assays, and ELISA. Mice immunized with the recombinant SjTH were challenged with cercariae to evaluate the immunoprotective value of this protein. Our results showed SjTH not only distributed in the head associated with the central nervous system, but also expressed along the tegument and the intestinal intima, which are involved in the movement, coupling and digestion of the parasites and associated with the peripheral nervous system. This protein can effectively stimulate humoral immune responses in mammalian hosts and has high potential as a biomarker for schistosomiasis immunodiagnosis. Furthermore, immunization with recombinant SjTH showed to reduce the worm and egg burden of challenged mice, and to contribute to the systemic balance of the Th1/Th2 responses. Taken together, these results suggest that SjTH is an important pathogenic molecule in S. japonicum and may be a possible target for anti-schistosomal approaches.

Molecular characterization of Smtdc-1 and Smddc-1 discloses roles as male-competence factors for the sexual maturation of Schistosoma mansoni females

Introduction

Schistosomes are the only mammalian flatworms that have evolved separate sexes. A key question of schistosome research is the male-dependent sexual maturation of the female since a constant pairing contact with a male is required for the onset of gonad development in the female. Although this phenomenon is long known, only recently a first peptide-based pheromone of males was identified that contributes to the control of female sexual development. Beyond this, our understanding of the molecular principles inducing the substantial developmental changes in a paired female is still rudimentary.

Objectives

Previous transcriptomic studies have consistently pointed to neuronal genes being differentially expressed and upregulated in paired males. These genes included Smp_135230 and Smp_171580, both annotated as aromatic-L-amino-acid decarboxylases (DOPA decarboxylases). Here, we characterized both genes and investigated their roles in male-female interaction of S. mansoni.

Methodologies/findings

Sequence analyses indicated that Smp_135230 represents an L-tyrosine decarboxylase (Smtdc-1), whereas Smp_171580 represents a DOPA decarboxylase (Smddc-1). By qRT-PCR, we confirmed the male-specific and pairing-dependent expression of both genes with a significant bias toward paired males. RNA-interference experiments showed a strong influence of each gene on gonad differentiation in paired females, which was enhanced by double knockdown. Accordingly, egg production was significantly reduced. By confocal laser scanning microscopy, a failure of oocyte maturation was found in paired knockdown females. Whole-mount in situ hybridization patterns exhibited the tissue-specific occurrence of both genes in particular cells at the ventral surface of the male, the gynecophoral canal, which represents the physical interface of both genders. These cells probably belong to the predicted neuronal cluster 2 of S. mansoni.

Conclusion

Our results suggest that Smtdc-1 and Smddc-2 are male-competence factors that are expressed in neuronal cells at the contact zone between the genders as a response of pairing to subsequently control processes of female sexual maturation.

Pharmacological Profiling of a Brugia malayi Muscarinic Acetylcholine Receptor as a Putative Antiparasitic Target

The diversification of anthelmintic targets and mechanisms of action will help ensure the sustainable control of nematode infections in response to the growing threat of drug resistance. G protein-coupled receptors (GPCRs) are established drug targets in human medicine but remain unexploited as anthelmintic substrates despite their important roles in nematode neuromuscular and physiological processes. Bottlenecks in exploring the druggability of parasitic nematode GPCRs include a limited helminth genetic toolkit and difficulties establishing functional heterologous expression. In an effort to address some of these challenges, we profile the function and pharmacology of muscarinic acetylcholine receptors in the human parasite Brugia malayi, an etiological agent of human lymphatic filariasis. While acetylcholine-gated ion channels are intensely studied as targets of existing anthelmintics, comparatively little is known about metabotropic receptor contributions to parasite cholinergic signaling. Using multivariate phenotypic assays in microfilariae and adults, we show that nicotinic and muscarinic compounds disparately affect parasite fitness traits. We identify a putative G protein-linked acetylcholine receptor of B. malayi (Bma-GAR-3) that is highly expressed across intramammalian life stages and adapt spatial RNA in situ hybridization to map receptor transcripts to critical parasite tissues. Tissue-specific expression of Bma-gar-3 in Caenorhabditis elegans (body wall muscle, sensory neurons, and pharynx) enabled receptor deorphanization and pharmacological profiling in a nematode physiological context. Finally, we developed an image-based feeding assay as a reporter of pharyngeal activity to facilitate GPCR screening in parasitized strains. We expect that these receptor characterization approaches and improved knowledge of GARs as putative drug targets will further advance the study of GPCR biology across medically important nematodes.

Annotation of G-Protein Coupled Receptors in the Genomes of Parasitic Blood Flukes

Infection with Schistosoma parasitic flatworms ( Schistosoma haematobium, Schistosoma mansoni and Schistosoma japonicum ) causes the neglected tropical disease schistosomiasis. There is a need to identify new chemotherapies to treat these parasites, and G-protein coupled receptors (GPCRs) are a logical druggable targets to explore given they control key aspects of schistosome biology such as neuromuscular function and reproduction. Updated chromosome level genome assemblies for each of the three major species have recently been released. However, studies on these GPCRs require accurate, updated genome annotations. Here, we have re-annotated the GPCRs present in each of the three major schistosome species.

Neuromuscular system of the causative agent of dicrocoeliosis, Dicrocoelium lanceatum. I. 5-Hydroxytryptamine in the nervous system

The trematode Dicrocoelium lanceatum known as lancet fluke, is a causative agent of dicrocoeliosis, a widespread parasitic disease of the grazing ruminants. The investigation of the major neurotransmitters and their functions are an important step in the development of a new pharmacological strategy of the struggle against the dicrocoeliosis affecting the neuronal signal substances and the functions of its nervous system. The aim of this work was to study the presence and localization of the neurotransmitter serotonin (5-HT, 5-Hydroxytryptamine) in the nervous system of D. lanceatum using immunocytochemical technique and confocal laser scanning microscopy. For the first time the data on the presence and distribution of serotonin-immunopositive components in the central and peripheral compartments of the nervous system of D. lanceatum has been obtained. Serotonin-immunopositive neurons and neurites were identified in paired brain ganglia, in the brain commissure, longitudinal nerve cords and connective nerve commissures. The innervation of the oral and ventral suckers by serotonergic nerve structures was revealed. The distal part of the reproductive system and the region of the reproductive pore were intensively innervated by serotonergic neurites. Serotonin-immunopositive neurons and neurites were also revealed in the proximal region of the reproductive system. The data obtained suggest that the serotonergic nervous system is involved in the regulation of the attachment organs and the reproductive system functions in D. lanceatum. The new results on the morphological and functional organization of the D. lanceatum nervous system increase our knowledge of the structure and function of nervous system of trematodes of various taxonomic groups and support the possibility of the exploitation of the serotonergic system of the parasite as a target for anthelmintic drugs.

Serotonin Signalling in Flatworms: An Immunocytochemical Localisation of 5-HT7 Type of Serotonin Receptors in Opisthorchis felineus and Hymenolepis diminuta

The study is dedicated to the investigation of serotonin (5-hydroxytryptamine, 5-HT) and 5-HT₇ type serotonin receptor of localisation in larvae of two parasitic flatworms Opisthorchis felineus (Rivolta, 1884) Blanchard, 1895 and Hymenolepis diminuta Rudolphi, 1819, performed using the immunocytochemical method and confocal laser scanning microscopy (CLSM). Using whole mount preparations and specific antibodies, a microscopic analysis of the spatial distribution of 5-HT₇-immunoreactivity(-IR) was revealed in worm tissue. In metacercariae of O. felineus 5-HT₇-IR was observed in the main nerve cords and in the head commissure connecting the head ganglia. The presence of 5-HT₇-IR was also found in several structures located on the oral sucker. 5-HT₇-IR was evident in the round glandular cells scattered throughout the larva body. In cysticercoids of H. diminuta immunostaining to 5-HT₇ was found in flame cells of the excretory system. Weak staining to 5-HT₇ was observed along the longitudinal and transverse muscle fibres comprising the body wall and musculature of suckers, in thin longitudinal nerve cords and a connective commissure of the central nervous system. Available publications on serotonin action in flatworms and serotonin receptors identification were reviewed. Own results and the published data indicate that the muscular structures of flatworms are deeply supplied by 5-HT₇-IR elements. It suggests that the 5-HT₇ type receptor can mediate the serotonin action in the investigated species and is an important component of the flatworm motor control system. The study of the neurochemical basis of parasitic flatworms can play an important role in the solution of fundamental problems in early development of the nervous system and the evolution of neuronal signalling components.

Identification of annotated bioactive molecules that impair motility of the blood fluke Schistosoma mansoni

Neglected tropical diseases are of growing worldwide concern and schistosomiasis, caused by parasitic flatworms, continues to be a major threat with more than 200 million people requiring preventive treatment. As praziquantel (PZQ) remains the treatment of choice, an urgent need for alternative treatments motivates research to identify new lead compounds that would complement PZQ by filling the therapeutic gaps associated with this treatment. Because impairing parasite neurotransmission remains a core strategy for control of parasitic helminths, we screened a library of 708 compounds with validated biological activity in humans on the blood fluke Schistosoma mansoni, measuring their effect on the motility on schistosomulae and adult worms. The primary phenotypic screen performed on schistosomulae identified 70 compounds that induced changes in viability and/or motility. Screening different concentrations and incubation times identified molecules with fast onset of activity on both life stages at low concentration (1 μM). To complement this study, similar assays were performed with chemical analogs of the cholinomimetic drug arecoline and the calcilytic molecule NPS-2143, two compounds that rapidly inhibited schistosome motility; 17 arecoline and 302 NPS-2143 analogs were tested to enlarge the pool of schistosomicidal molecules. Finally, validated hit compounds were tested on three functionally-validated neuroregulatory S. mansoni G-protein coupled receptors (GPCRs): Sm5HTR (serotonin-sensitive), SmGPR2 (histamine) and SmD2 (dopamine), revealing NPS-2143 and analogs as potent inhibitors of dopamine/epinine responses on both human and S. mansoni GPCRs. This study highlights the potential for repurposing known human therapeutic agents for potential schistosomicidal effects and expands the list of hits for further progression.

Comparative serum metabolomics between SCID mice and BALB/c mice with or without Schistosoma japonicum infection: Clues to the abnormal growth and development of schistosome in SCID mice

The small blood flukes of genus Schistosoma, which cause one of the most prevalent and serious parasitic zoonosis schistosomiasis, are dependent on immune-related factors of their mammalian host to facilitate their growth and development, and the formation of granulomatous pathology caused by eggs deposited in host's liver and intestinal wall. Schistosome development is hampered in the mice lacking just T cells, and is even more heavily retarded in the severe combined immunodeficient (SCID) mice lacking both T and B lymphocytes. Nevertheless, it's still not clear about the underlying regulatory molecular mechanisms of schistosome growth and development by host's immune system. This study, therefore, detected and compared the serum metabolic profiles between the immunodeficient mice and immunocompetent mice (SCID mice vs. BALB/c mice) before and after S. japonicum infection (on the thirty-fifth day post infection using liquid chromatography-mass spectrometry (LC-MS). Totally, 705 ion features in electrospray ionization in positive-ion mode (ESI+) and 242 ion features in ESI- mode were identified, respectively. First, distinct serum metabolic profiles were identified between SCID mice and BALB/c mice without S. japonicum worms infection. Second, uniquely perturbed serum metabolites and their enriched pathways were also obtained between SCID mice and BALB/c mice after S. japonicum infection, which included differential metabolites due to both species differences and differential responses to S. japonicum infection. The metabolic pathways analysis revealed that arachidonic acid metabolism, biosynthesis of unsaturated fatty acids, linoleic acid metabolism, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, alpha-linolenic acid metabolism, glycerophospholipid metabolism, sphingolipid metabolism and purine metabolism were enriched based on the differential serum metabolites between SCID mice and BALB/c mice after S. japonicum infection, which was addressed to be related to the retarded growth and development of S. japonicum in SCID mice. These findings provide new clues to the underlying molecular events of host's systemic metabolic changes on the growth and development of S. japonicum worms, and also provide quite promising candidates for exploitation of drugs or vaccines against schistosome and schistosomiasis.

A rotifer-derived paralytic compound prevents transmission of schistosomiasis to a mammalian host

Schistosomes are parasitic flatworms that infect over 200 million people, causing the neglected tropical disease, schistosomiasis. A single drug, praziquantel, is used to treat schistosome infection. Limitations in mass drug administration programs and the emergence of schistosomiasis in nontropical areas indicate the need for new strategies to prevent infection. It has been known for several decades that rotifers colonizing the schistosome's snail intermediate host produce a water-soluble factor that paralyzes cercariae, the life cycle stage infecting humans. In spite of its potential for preventing infection, the nature of this factor has remained obscure. Here, we report the purification and chemical characterization of Schistosome Paralysis Factor (SPF), a novel tetracyclic alkaloid produced by the rotifer Rotaria rotatoria. We show that this compound paralyzes schistosome cercariae and prevents infection and does so more effectively than analogous compounds. This molecule provides new directions for understanding cercariae motility and new strategies for preventing schistosome infection.

Identification of a new panel of reference genes to study pairing-dependent gene expression in Schistosoma mansoni

Facilitated by the Schistosoma mansoni genome project, multiple transcriptomic studies were performed over the last decade to elucidate gene expression patterns among different developmental stages of the complex schistosome life cycle. While these analyses enable the identification of candidate genes with key functions in schistosome biology, a diverse molecular tool set is needed that allows comprehensive functional characterization at the single gene level. This includes the availability of reliable reference genes to confirm changes in the transcription of genes of interest over different biological samples and experimental conditions. In particular, the investigation of one key aspect of schistosome biology, the pairing-dependent gene expression in females and males, requires knowledge on reference genes that are expressed independently of both pairing and of in vitro culture effects. Therefore, the present study focused on the identification of quantitative reverse transcription (qRT)-PCR reference genes suitable for the investigation of pairing-dependent gene expression in the S. mansoni male. The "pipeline" we present here is based on qRT-PCR analyses of high biological replication combined with three different statistical analysis tools, BestKeeper, geNorm, and NormFinder. Our approach resulted in a statistically robust ranking of 15 selected reference genes with respect to their transcription stability between pairing-unexperienced and -experienced males. We further tested the top seven candidate genes for their transcription stability during invitro culture of adult S. mansoni. Of these, the two most suitable reference genes were used to investigate the influence of the pairing contact on the transcription of genes of interest, comprising a tyrosine decarboxylase gene Smtdc1, an ebony ortholog Smebony, and the follistatin ortholog Smfst in S. mansoni males. Performing pairing, separation and re-pairing experiments with adult S. mansoni in vitro, our results indicate for the first time that pairing can act as a molecular on/off-switch of specific genes to strictly control their expression in schistosome males.

Tissue-specific transcriptome analyses provide new insights into GPCR signalling in adult Schistosoma mansoni

Schistosomes are blood-dwelling trematodes with global impact on human and animal health. Because medical treatment is currently based on a single drug, praziquantel, there is urgent need for the development of alternative control strategies. The Schistosoma mansoni genome project provides a platform to study and connect the genetic repertoire of schistosomes to specific biological functions essential for successful parasitism. G protein-coupled receptors (GPCRs) form the largest superfamily of transmembrane receptors throughout the Eumetazoan phyla, including platyhelminths. Due to their involvement in diverse biological processes, their pharmacological importance, and proven druggability, GPCRs are promising targets for new anthelmintics. However, to identify candidate receptors, a more detailed understanding of the roles of GPCR signalling in schistosome biology is essential. An updated phylogenetic analysis of the S. mansoni GPCR genome (GPCRome) is presented, facilitated by updated genome data that allowed a more precise annotation of GPCRs. Additionally, we review the current knowledge on GPCR signalling in this parasite and provide new insights into the potential roles of GPCRs in schistosome reproduction based on the findings of a recent tissue-specific transcriptomic study in paired and unpaired S. mansoni. According to the current analysis, GPCRs contribute to gonad-specific functions but also to nongonad, pairing-dependent processes. The latter may regulate gonad-unrelated functions during the multifaceted male-female interaction. Finally, we compare the schistosome GPCRome to that of another parasitic trematode, Fasciola, and discuss the importance of GPCRs to basic and applied research. Phylogenetic analyses display GPCR diversity in free-living and parasitic platyhelminths and suggest diverse functions in schistosomes. Although their roles need to be substantiated by functional studies in the future, the data support the selection of GPCR candidates for basic and applied studies, invigorating the exploitation of this important receptor class for drug discovery against schistosomes but also other trematodes.

Signalling pathways in schistosomes: novel targets for control interventions against schistosomiasis

Over the last decade, there has been accumulating evidence showing that signalling pathways are involved in extensive biological and physiological processes in the human blood fluke schistosomes, playing essential roles in environmental sensing, host penetration, growth, development, maturation, embryogenesis, tissue self-renewal and survival. Owing to the likelihood of resistance developing against praziquantel, the only drug currently available that is effective against all the human schistosome species, there is an urgent requirement for an alternative treatment, arguing for continuing research into novel or repurposed anti-schistosomal drugs. An increasing number of anticancer drugs are being developed which block abnormal signalling pathways, a feature that has stimulated interest in developing novel interventions against human schistosomiasis by targeting key cell signalling components. In this review, we discuss the functional characterization of signal transduction pathways in schistosomes and consider current challenges and future perspectives in this important area of research.

Dynamic transcriptomes identify biogenic amines and insect-like hormonal regulation for mediating reproduction in Schistosoma japonicum

Eggs produced by the mature female parasite are responsible for the pathogenesis and transmission of schistosomiasis. Female schistosomes rely on a unique male-induced strategy to accomplish reproductive development, a process that is incompletely understood. Here we map detailed transcriptomic profiles of male and female Schistosoma japonicum across eight time points throughout the sexual developmental process from pairing to maturation. The dynamic gene expression pattern data reveal clear sex-related characteristics, indicative of an unambiguous functional division between males and females during their interplay. Cluster analysis, in situ hybridization and RNAi assays indicate that males likely use biogenic amine neurotransmitters through the nervous system to control and maintain pairing with females. In addition, the analyses indicate that reproductive development of females involves an insect-like hormonal regulation. These data sets and analyses serve as a foundation for deeper study of sexual development in this pathogen and identification of novel anti-schistosomal interventions.

Protein kinase A signalling in Schistosoma mansoni cercariae and schistosomules

Cyclic AMP (cAMP)-dependent protein kinase/protein kinase A regulates multiple processes in eukaryotes by phosphorylating diverse cellular substrates, including metabolic and signalling enzymes, ion channels and transcription factors. Here we provide insight into protein kinase A signalling in cercariae and 24h in vitro cultured somules of the blood parasite, Schistosoma mansoni, which causes human intestinal schistosomiasis. Functional mapping of activated protein kinase A using anti-phospho protein kinase A antibodies and confocal laser scanning microscopy revealed activated protein kinase A in the central and peripheral nervous system, oral-tip sensory papillae, oesophagus and excretory system of intact cercariae. Cultured 24h somules, which biologically represent the skin-resident stage of the parasite, exhibited similar activation patterns in oesophageal and nerve tissues but also displayed striking activation at the tegument and activation in a region resembling the germinal 'stem' cell cluster. The adenylyl cyclase activator, forskolin, stimulated somule protein kinase A activation and produced a hyperkinesia phenotype. The biogenic amines, serotonin and dopamine known to be present in skin also induced protein kinase A activation in somules, whereas neuropeptide Y or [Leu(31),Pro(34)]-neuropeptide Y attenuated protein kinase A activation. However, neuropeptide Y did not block the forskolin-induced somule hyperkinesia. Bioinformatic investigation of potential protein associations revealed 193 medium confidence and 59 high confidence protein kinase A interacting partners in S. mansoni, many of which possess putative protein kinase A phosphorylation sites. These data provide valuable insight into the intricacies of protein kinase A signalling in S. mansoni and a framework for further physiological investigations into the roles of protein kinase A in schistosomes, particularly in the context of interactions between the parasite and the host.

Ergot Alkaloids (Re)generate New Leads as Antiparasitics

Praziquantel (PZQ) is a key therapy for treatment of parasitic flatworm infections of humans and livestock, but the mechanism of action of this drug is unresolved. Resolving PZQ-engaged targets and effectors is important for identifying new druggable pathways that may yield novel antiparasitic agents. Here we use functional, genetic and pharmacological approaches to reveal that serotonergic signals antagonize PZQ action in vivo. Exogenous 5-hydroxytryptamine (5-HT) rescued PZQ-evoked polarity and mobility defects in free-living planarian flatworms. In contrast, knockdown of a prevalently expressed planarian 5-HT receptor potentiated or phenocopied PZQ action in different functional assays. Subsequent screening of serotonergic ligands revealed that several ergot alkaloids possessed broad efficacy at modulating regenerative outcomes and the mobility of both free living and parasitic flatworms. Ergot alkaloids that phenocopied PZQ in regenerative assays to cause bipolar regeneration exhibited structural modifications consistent with serotonergic blockade. These data suggest that serotonergic activation blocks PZQ action in vivo, while serotonergic antagonists phenocopy PZQ action. Importantly these studies identify the ergot alkaloid scaffold as a promising structural framework for designing potent agents targeting parasitic bioaminergic G protein coupled receptors.

The parasitic infection schistosomiasis afflicts millions of people worldwide and is clinically treated using a single drug, praziquantel (PZQ). Despite the fact that PZQ has served as a stalwart anthelmintic for decades, the molecular basis of action of this clinical agent is poorly understood. This lack of mechanistic information impedes the rational design of alternative therapies and highlights the need for new approaches for studying the target(s) and effectors engaged by PZQ in vivo. Here, we exploit the predictive phenology between free-living planarian regenerative screens and parasitic neuromuscular physiology to reveal a broad efficacy of ergot alkaloids in phenocopying the action of PZQ. In planarian regenerative screens, data highlight structural features of the ergoline scaffold that yield specific regenerative effects to promote or inhibit head regeneration. Ergot alkaloids with efficacy in regenerative assays were also found to modulate the contractility of schistosomules. Overall, these data highlight a possible therapeutic potential of ergot alkaloids as antischistosomals and the action of PZQ as an ergomimetic.

Exploring the role of biogenic amines in schistosome host-parasite interactions

Biogenic amines (BAs) are important neurotransmitters of the schistosome nervous system, but their role in the host-parasite interaction is poorly understood. Recent findings suggest that BAs may play an important role in the interaction with the snail intermediate host. This new evidence adds an important piece of information to our understanding of this complex system.

A constitutively active G protein-coupled acetylcholine receptor regulates motility of larval Schistosoma mansoni

The neuromuscular system of helminths controls a variety of essential biological processes and therefore represents a good source of novel drug targets. The neuroactive substance, acetylcholine controls movement of Schistosoma mansoni but the mode of action is poorly understood. Here, we present first evidence of a functional G protein-coupled acetylcholine receptor in S. mansoni, which we have named SmGAR. A bioinformatics analysis indicated that SmGAR belongs to a clade of invertebrate GAR-like receptors and is related to vertebrate muscarinic acetylcholine receptors. Functional expression studies in yeast showed that SmGAR is constitutively active but can be further activated by acetylcholine and, to a lesser extent, the cholinergic agonist, carbachol. Anti-cholinergic drugs, atropine and promethazine, were found to have inverse agonist activity towards SmGAR, causing a significant decrease in the receptor's basal activity. An RNAi phenotypic assay revealed that suppression of SmGAR activity in early-stage larval schistosomulae leads to a drastic reduction in larval motility. In sum, our results provide the first molecular evidence that cholinergic GAR-like receptors are present in schistosomes and are required for proper motor control in the larvae. The results further identify SmGAR as a possible candidate for antiparasitic drug targeting.

Identification of G protein-coupled receptors in Schistosoma haematobium and S. mansoni by comparative genomics

BACKGROUND

Schistosomiasis is a parasitic disease affecting ~200 million people worldwide. Schistosoma haematobium and S. mansoni are two relatively closely related schistosomes (blood flukes), and the causative agents of urogenital and hepatointestinal schistosomiasis, respectively. The availability of genomic, transcriptomic and proteomic data sets for these two schistosomes now provides unprecedented opportunities to explore their biology, host interactions and schistosomiasis at the molecular level. A particularly important group of molecules involved in a range of biological and developmental processes in schistosomes and other parasites are the G protein-coupled receptors (GPCRs). Although GPCRs have been studied in schistosomes, there has been no detailed comparison of these receptors between closely related species. Here, using a genomic-bioinformatic approach, we identified and characterised key GPCRs in S. haematobium and S. mansoni (two closely related species of schistosome).

METHODS

Using a Hidden Markov Model (HMM) and Support Vector Machine (SVM)-based pipeline, we classified and sub-classified GPCRs of S. haematobium and S. mansoni, combined with phylogenetic and transcription analyses.

RESULTS

We identified and classified classes A, B, C and F as well as an unclassified group of GPCRs encoded in the genomes of S. haematobium and S. mansoni. In addition, we characterised ligand-specific subclasses (i.e. amine, peptide, opsin and orphan) within class A (rhodopsin-like).

CONCLUSIONS

Most GPCRs shared a high degree of similarity and conservation, except for members of a particular clade (designated SmGPR), which appear to have diverged between S. haematobium and S. mansoni and might explain, to some extent, some of the underlying biological differences between these two schistosomes. The present set of annotated GPCRs provides a basis for future functional genomic studies of cellular GPCR-mediated signal transduction and a resource for future drug discovery efforts in schistosomes.

'Death and axes': unexpected Ca²⁺ entry phenologs predict new anti-schistosomal agents

Schistosomiasis is a parasitic flatworm disease that infects 200 million people worldwide. The drug praziquantel (PZQ) is the mainstay therapy but the target of this drug remains ambiguous. While PZQ paralyses and kills parasitic schistosomes, in free-living planarians PZQ caused an unusual axis duplication during regeneration to yield two-headed animals. Here, we show that PZQ activation of a neuronal Ca²⁺ channel modulates opposing dopaminergic and serotonergic pathways to regulate ‘head’ structure formation. Surprisingly, compounds with efficacy for either bioaminergic network in planarians also displayed antischistosomal activity, and reciprocally, agents first identified as antischistocidal compounds caused bipolar regeneration in the planarian bioassay. These divergent outcomes (death versus axis duplication) result from the same Ca²⁺ entry mechanism, and comprise unexpected Ca²⁺ phenologs with meaningful predictive value. Surprisingly, basic research into axis patterning mechanisms provides an unexpected route for discovering novel antischistosomal agents.

Schistosomiasis (Bilharzia) is one of the most burdensome parasitic worm infections, encumbering third world economies with an annual loss of several million disability-adjusted life years. The key treatment for schistosome infections is the drug praziquantel but the mechanism of action of this drug remains controversial hampering targeted development of next generation antischistosomal agents. Here we provide fresh insight into the signaling pathways engaged by PZQ, by resolving commonalities in the action of PZQ with the process of regenerative signaling in free-living planarian flatworms. A similar calcium-dependent network is engaged in both model systems, but with divergent phenotypic outcomes. This relationship provides predictive insight such that basic research on signaling pathways involved in tissue regeneration reveals novel drug leads for schistosomiasis, and reciprocally schistosomal drug screens reveal targets involved in regenerative signaling. We believe this phenology will be helpful for uncovering new antischistosomal drug targets by exploiting broader vulnerabilities within the PZQ interactome.

The functional role of a serotonin transporter in Schistosoma mansoni elucidated through immunolocalization and RNA interference (RNAi)

Serotonin is an important neurotransmitter in both vertebrates and invertebrates. In the parasitic flatworm, Schistosoma mansoni, serotonin stimulates worm movement and potentiates muscle contraction. A specific serotonin transporter (SmSERT) was previously cloned from S. mansoni and characterized in vitro. Here we conduct a first investigation of the native protein in the worm so as to elucidate the biological role of SmSERT and to assess its drug targeting potential. Confocal immunofluorescence studies using specific antibodies determined that SmSERT is expressed predominantly in the nervous system both in adult worms and larvae (schistosomula). SmSERT immunoreactivity was detected in the main nerve cords of the central nervous system and the peripheral nerve plexus of the body wall in adult males and females, in apparent nerve endings of the male tubercles and possibly the male tegument. In the larvae, SmSERT localized mainly to the peripheral nerve plexus of the body wall. Co-localization experiments showed that the pattern of SmSERT expression coincides with that of serotonin itself, suggesting that SmSERT is present in serotonergic neurons. To test whether SmSERT is involved in the motor effects of serotonin, we treated S. mansoni schistosomula with SmSERT blockers or SmSERT-specific short-interfering RNAs (siRNAs) and then recorded larval motility, using a quantitative imaging assay. In both cases, the treatment produced a strongly hyperactive phenotype, corresponding to a ~3-fold increase in larval motility, roughly the same effect as treatment with an excess of exogenous serotonin. The siRNA effect correlated with a ≈50% decrease in expression of the SmSERT when tested by real-time qPCR. To test if SmSERT mediates transport of exogenous serotonin across the tegument, uptake assays were also performed in intact schistosomula treated with SmSERT siRNAs or an irrelevant siRNA. We found a significant but modest decrease (~25%) in serotonin uptake in the siRNA-suppressed larvae when compared to the negative controls. These results suggest that the SmSERT's function is primarily neuromuscular and may also play a secondary role in the uptake of exogenous (host-derived) serotonin.