Possible roles of cAMP and Ca2+ in the regulation of miracidial transformation in Schistosoma mansoni

Serotonin stimulates Echinococcus multilocularis larval development

Background

Serotonin is a phylogenetically ancient molecule that is widely distributed in most metazoans, including flatworms. In addition to its role as a neurotransmitter, serotonin acts as a morphogen and regulates developmental processes. Although several studies have focused on the serotonergic nervous system in parasitic flatworms, little is known on the role of serotonin in flatworm development.

Methods

To study the effects of serotonin on proliferation and development of the cestode Echinococcus multilocularis, we cloned the genes encoding the E. multilocularis serotonin transporter (SERT) and tryptophan hydroxylase (TPH), analyzed gene expression by transcriptome analysis and whole mount in situ hybridization (WMISH) and performed cell culture experiments.

Results

We first characterized orthologues encoding the SERT and TPH, the rate-limiting enzyme in serotonin biosynthesis. WMISH and transcriptomic analyses indicated that the genes for both SERT and TPH are expressed in the parasite nervous system. Long-term treatment of parasite stem cell cultures with serotonin stimulated development towards the parasite metacestode stage. Mature metacestode vesicles treated with serotonin showed increased rates of incorporation of the thymidine analogue 5-ethynyl-2′-deoxyuridine (EdU), indicating stimulated cell proliferation. In contrast, treatment with the selective serotonin reuptake inhibitor paroxetine strongly affected the viability of parasite cells. Paroxetine also caused structural damage in metacestode vesicles, suggesting that serotonin transport is crucial for the integrity of parasite vesicles.

Conclusions

Our results indicate that serotonin plays an important role in E. multilocularis development and proliferation, providing evidence that the E. multilocularis SERT and TPH are expressed in the nervous system of the protoscolex. Our results further suggest that the E. multilocularis SERT has a secondary role outside the nervous system that is essential for parasite integrity and survival. Since serotonin stimulated E. multilocularis metacestode development and proliferation, serotonin might also contribute to the formation and growth of the parasite in the liver.

In-depth proteomic characterization of Schistosoma haematobium: Towards the development of new tools for elimination

BACKGROUND

Schistosomiasis is a neglected disease affecting hundreds of millions worldwide. Of the three main species affecting humans, Schistosoma haematobium is the most common, and is the leading cause of urogenital schistosomiasis. S. haematobium infection can cause different urogenital clinical complications, particularly in the bladder, and furthermore, this parasite has been strongly linked with squamous cell carcinoma. A comprehensive analysis of the molecular composition of its different proteomes will contribute to developing new tools against this devastating disease.

METHODS AND FINDINGS

By combining a comprehensive protein fractionation approach consisting of OFFGEL electrophoresis with high-throughput mass spectrometry, we have performed the first in-depth characterisation of the different discrete proteomes of S. haematobium that are predicted to interact with human host tissues, including the secreted and tegumental proteomes of adult flukes and secreted and soluble egg proteomes. A total of 662, 239, 210 and 138 proteins were found in the adult tegument, adult secreted, soluble egg and secreted egg proteomes, respectively. In addition, we probed these distinct proteomes with urine to assess urinary antibody responses from naturally infected human subjects with different infection intensities, and identified adult fluke secreted and tegument extracts as being the best predictors of infection.

CONCLUSION

We provide a comprehensive dataset of proteins from the adult and egg stages of S. haematobium and highlight their utility as diagnostic markers of infection intensity. Protein composition was markedly different between the different extracts, highlighting the distinct subsets of proteins that different development stages present in their different niches. Furthermore, we have identified adult fluke ES and tegument extracts as best predictors of infection using urine antibodies of naturally infected people. This study provides the first steps towards the development of novel tools to control this important neglected tropical disease.

A Biomphalaria glabrata peptide that stimulates significant behaviour modifications in aquatic free-living Schistosoma mansoni miracidia

The human disease schistosomiasis (or bilharzia) is caused by the helminth blood fluke parasite Schistosoma mansoni, which requires an intermediate host, the freshwater gastropod snail Biomphalaria glabrata (the most common intermediate host). The free-swimming parasite miracidia utilise an excellent chemosensory sense to detect and locate an appropriate host. This study investigated the biomolecules released by the snail that stimulate changes in the behaviour of the aquatic S. mansoni miracidia. To achieve this, we have performed an integrated analysis of the snail-conditioned water, through chromatography and bioassay-guided behaviour observations, followed by mass spectrometry. A single fraction containing multiple putative peptides could stimulate extreme swimming behaviour modifications (e.g. velocity, angular variation) similar to those observed in response to crude snail mucus. One peptide (P12;—R-DITSGLDPEVADD-KR—) could replicate the stimulation of miracidia behaviour changes. P12 is derived from a larger precursor protein with a signal peptide and multiple dibasic cleavage sites, which is synthesised in various tissues of the snail, including the central nervous system and foot. P12 consists of an alpha helix secondary structure as indicated by circular dichroism spectroscopy. This information will be helpful for the development of approaches to manipulate this parasites life cycle, and opens up new avenues for exploring other parasitic diseases which have an aquatic phase using methods detailed in this investigation.

In aquatic environments, where the vast majority of animals live in darkness, key relationships are often formed and maintained by chemical communication (including smell and taste). Parasites with an aquatic life phase rely on an exquisite sense of chemosensation to detect host biomolecules (kairomones), allowing them to locate and infect their host. Our study identifies the first kairomone released by the freshwater gastropod snail Biomphalaria glabrata, an intermediate host for the helminth blood fluke parasite Schistosoma mansoni. This is a key aspect of the S. mansoni life-cycle that ultimately leads to human infection, causing the disease schistosomiasis (or bilharzia), which is considered the most devastating human helminth infection in terms of global morbidity and mortality. The kairomone we identify is a peptide that does not appear to share any similarity with any other known animal peptide. This information will be helpful as we explore methods to interrupt parasite infection, and therefore break the cycle of infection that causes a major human disease.

Kinases: Molecular Stage Directors for Schistosome Development and Differentiation

Understanding schistosome biology is still a challenging mission. The reproductive biology of this parasitic trematode is closely associated with the pathologic consequences of schistosomiasis, the devastating infectious disease caused by members of the family Schistosomatidae worldwide. Recent studies of signaling mechanisms confirmed the prominent roles of protein kinases (PKs) in directing schistosome biology, and first evidence was obtained for an additional contribution of kinases with substrates different from proteins (non-PKs). This review provides an overview of the Schistosoma mansoni kinome in the context of male-female interaction and summarizes recent studies of kinases controlling development and differentiation. Due to their importance for schistosome biology, kinases represent Achilles' heels and are therefore of high value also for translational research.

Phenotypic, chemical and functional characterization of cyclic nucleotide phosphodiesterase 4 (PDE4) as a potential anthelmintic drug target

BACKGROUND

Reliance on just one drug to treat the prevalent tropical disease, schistosomiasis, spurs the search for new drugs and drug targets. Inhibitors of human cyclic nucleotide phosphodiesterases (huPDEs), including PDE4, are under development as novel drugs to treat a range of chronic indications including asthma, chronic obstructive pulmonary disease and Alzheimer's disease. One class of huPDE4 inhibitors that has yielded marketed drugs is the benzoxaboroles (Anacor Pharmaceuticals).

METHODOLOGY/PRINCIPAL FINDINGS

A phenotypic screen involving Schistosoma mansoni and 1,085 benzoxaboroles identified a subset of huPDE4 inhibitors that induced parasite hypermotility and degeneration. To uncover the putative schistosome PDE4 target, we characterized four PDE4 sequences (SmPDE4A-D) in the parasite's genome and transcriptome, and cloned and recombinantly expressed the catalytic domain of SmPDE4A. Among a set of benzoxaboroles and catechol inhibitors that differentially inhibit huPDE4, a relationship between the inhibition of SmPDE4A, and parasite hypermotility and degeneration, was measured. To validate SmPDE4A as the benzoxaborole molecular target, we first generated Caenorhabditis elegans lines that express a cDNA for smpde4a on a pde4(ce268) mutant (hypermotile) background: the smpde4a transgene restored mutant worm motility to that of the wild type. We then showed that benzoxaborole inhibitors of SmPDE4A that induce hypermotility in the schistosome also elicit a hypermotile response in the C. elegans lines that express the smpde4a transgene, thereby confirming SmPDE4A as the relevant target.

CONCLUSIONS/SIGNIFICANCE

The orthogonal chemical, biological and genetic strategies employed identify SmPDE4A's contribution to parasite motility and degeneration, and its potential as a drug target. Transgenic C. elegans is highlighted as a potential screening tool to optimize small molecule chemistries to flatworm molecular drug targets.

Calmodulin disruption impacts growth and motility in juvenile liver fluke

Background

Deficiencies in effective flukicide options and growing issues with drug resistance make current strategies for liver fluke control unsustainable, thereby promoting the need to identify and validate new control targets in Fasciola spp. parasites. Calmodulins (CaMs) are small calcium-sensing proteins with ubiquitous expression in all eukaryotic organisms and generally use fluctuations in intracellular calcium levels to modulate cell signalling events. CaMs are essential for fundamental processes including the phosphorylation of protein kinases, gene transcription, calcium transport and smooth muscle contraction. In the blood fluke Schistosoma mansoni, calmodulins have been implicated in egg hatching, miracidial transformation and larval development. Previously, CaMs have been identified amongst liver fluke excretory-secretory products and three CaM-like proteins have been characterised biochemically from adult Fasciola hepatica, although their functions remain unknown.

Methods

In this study, we set out to investigate the biological function and control target potential of F. hepatica CaMs (FhCaMs) using RNAi methodology alongside novel in vitro bioassays.

Results

Our results reveal that: (i) FhCaMs are widely expressed in parenchymal cells throughout the forebody region of juvenile fluke; (ii) significant transcriptional knockdown of FhCaM1-3 was inducible by exposure to either long (~200 nt) double stranded (ds) RNAs or 27 nt short interfering (si) RNAs, although siRNAs were less effective than long dsRNAs; (iii) transient long dsRNA exposure-induced RNA interference (RNAi) of FhCaMs triggered transcript knockdown that persisted for ≥ 21 days, and led to detectable suppression of FhCaM proteins; (iv) FhCaM RNAi significantly reduced the growth of juvenile flukes maintained in vitro; (v) FhCaM RNAi juveniles also displayed hyperactivity encompassing significantly increased migration; (vi) both the reduced growth and increased motility phenotypes were recapitulated in juvenile fluke using the CaM inhibitor trifluoperazine hydrochloride, supporting phenotype specificity.

Conclusions

These data indicate that the Ca²⁺-modulating functions of FhCaMs are important for juvenile fluke growth and movement and provide the first functional genomics-based example of a growth-defect resulting from gene silencing in liver fluke. Whilst the phenotypic impacts of FhCaM silencing on fluke behaviour do not strongly support their candidature as new flukicide targets, the growth impacts encourage further consideration, especially in light of the speed of juvenile fluke growth in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1324-9) contains supplementary material, which is available to authorized users.

Application of RNAi to Genomic Drug Target Validation in Schistosomes

Concerns over the possibility of resistance developing to praziquantel (PZQ), has stimulated efforts to develop new drugs for schistosomiasis. In addition to the development of improved whole organism screens, the success of RNA interference (RNAi) in schistosomes offers great promise for the identification of potential drug targets to initiate drug discovery. In this study we set out to contribute to RNAi based validation of putative drug targets. Initially a list of 24 target candidates was compiled based on the identification of putative essential genes in schistosomes orthologous of C. elegans essential genes. Knockdown of Calmodulin (Smp_026560.2) (Sm-Calm), that topped this list, produced a phenotype characterised by waves of contraction in adult worms but no phenotype in schistosomula. Knockdown of the atypical Protein Kinase C (Smp_096310) (Sm-aPKC) resulted in loss of viability in both schistosomula and adults and led us to focus our attention on other kinase genes that were identified in the above list and through whole organism screening of known kinase inhibitor sets followed by chemogenomic evaluation. RNAi knockdown of these kinase genes failed to affect adult worm viability but, like Sm-aPKC, knockdown of Polo-like kinase 1, Sm-PLK1 (Smp_009600) and p38-MAPK, Sm-MAPK p38 (Smp_133020) resulted in an increased mortality of schistosomula after 2-3 weeks, an effect more marked in the presence of human red blood cells (hRBC). For Sm-PLK-1 the same effects were seen with the specific inhibitor, BI2536, which also affected viable egg production in adult worms. For Sm-PLK-1 and Sm-aPKC the in vitro effects were reflected in lower recoveries in vivo. We conclude that the use of RNAi combined with culture with hRBC is a reliable method for evaluating genes important for larval development. However, in view of the slow manifestation of the effects of Sm-aPKC knockdown in adults and the lack of effects of Sm-PLK-1 and Sm-MAPK p38 on adult viability, these kinases may not represent suitable drug targets.

Functional genomic characterization of neoblast-like stem cells in larval Schistosoma mansoni

Schistosomes infect hundreds of millions of people in the developing world. Transmission of these parasites relies on a stem cell-driven, clonal expansion of larvae inside a molluscan intermediate host. How this novel asexual reproductive strategy relates to current models of stem cell maintenance and germline specification is unclear. Here, we demonstrate that this proliferative larval cell population (germinal cells) shares some molecular signatures with stem cells from diverse organisms, in particular neoblasts of planarians (free-living relatives of schistosomes). We identify two distinct germinal cell lineages that differ in their proliferation kinetics and expression of a nanos ortholog. We show that a vasa/PL10 homolog is required for proliferation and maintenance of both populations, whereas argonaute2 and a fibroblast growth factor receptor-encoding gene are required only for nanos-negative cells. Our results suggest that an ancient stem cell-based developmental program may have enabled the evolution of the complex life cycle of parasitic flatworms.

DOI:http://dx.doi.org/10.7554/eLife.00768.001

Schistosomiasis—a disease caused by parasitic flatworms known as schistosomes—affects more than 200 million people worldwide, mainly in tropical regions, and in public health importance is second only to malaria (according to the World Health Organization). Chronic infection leads to damage to internal organs, and the disease is responsible for roughly 250,000 deaths each year.

The schistosome parasite has a complex life cycle, and the worms are capable of infecting mammals during just one stage of this cycle. Infection occurs through contact with contaminated freshwater, with the infectious form of the parasite burrowing through skin. Once inside the body, the parasites mature into adults, before reproducing sexually and laying eggs that are excreted by their host back into the water supply.

However, to generate the form of the parasite that can infect mammals, schistosomes must first infect an intermediate host, namely a freshwater snail. When the larval form of the parasite—which cannot infect mammals—enters the snail, the larvae undergo an unusual type of asexual embryogenesis. This results in thousands of parasites that are capable of infecting mammals. Studies suggest that a population of cells known as germinal cells are responsible for this transformation and replication process, but little is known about these cells at the molecular level.

Here, Wang et al. report the gene expression profile of these cells in a species of schistosome, and use RNA-mediated silencing techniques to explore the functions of the genes. This analysis revealed that the germinal cells have a molecular signature similar to that of neoblasts—adult pluripotent stem cells found in free-living flatworms such as planarians. Neoblasts can develop into any cell type in the body, enabling planarians to repair or even replace damaged body parts.

The similarity between neoblasts and germinal cells led Wang et al. to suggest that schistosomes may have evolved their parasitic life cycle partly by adapting a program of development based on stem cells in non-parasitic worms.

DOI:http://dx.doi.org/10.7554/eLife.00768.002

Inferences on the biochemical and environmental regulation of universal stress proteins from Schistosomiasis parasites

BACKGROUND

Human schistosomiasis is a freshwater snail-transmitted disease caused by parasitic flatworms of the Schistosoma genus. Schistosoma haematobium, Schistosoma mansoni, and Schistosoma japonicum are the three major species infecting humans. These parasites undergo a complex developmental life cycle, in which they encounter a plethora of environmental signals. The presence of genes encoding the universal stress protein (USP) domain in the genomes of Schistosoma spp. suggests these flatworms are equipped to respond to unfavorable conditions. Though data on gene expression is available for USP genes, their biochemical and environmental regulation are incompletely understood. The identification of additional regulatory molecules for Schistosoma. USPs, which may be present in the human, snail, or water environments, could also be useful for schistosomiasis interventions.

METHODS

We developed a protocol that includes a visual analytics stage to facilitate integration, visualization, and decision making, from the results of sequence analyses and data collection on a set of 13 USPs from S. mansoni and S. japonicum.

RESULTS

Multiple sequence alignment identified conserved sites that could be key residues regulating the function of USPs of the Schistosoma spp. Based on the consistency and completeness of sequence annotation, we prioritized for further research the gene for a 184-amino-acid-long USP that is present in the genomes of the three human-infecting Schistosoma spp. Calcium, zinc, and magnesium ions were predicted to interact with the protein product of the gene.

CONCLUSION

Given that the initial effects of praziquantel on schistosomes include the influx of calcium ions, additional investigations are required to (1) functionally characterize the interactions of calcium ions with the amino acid residues of Schistosoma USPs; and (2) determine the transcriptional response of Schistosoma. USP genes to praziquantel. The data sets produced, and the visual analytics views that were developed, can be easily reused to develop new hypotheses.

Characterization of a flatworm inositol (1,4,5) trisphosphate receptor (IP₃R) reveals a role in reproductive physiology

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that elevate cytoplasmic Ca²⁺ in response to the second messenger IP3. Here, we describe the identification and in vivo functional characterization of the planarian IP₃R, the first intracellular Ca²⁺ channel to be defined in flatworms. A single IP₃R gene in Dugesia japonica encoded a 2666 amino acid protein (Dj.IP₃R) that shared well conserved structural features with vertebrate IP₃R counterparts. Expression of an NH₂-terminal Dj.IP₃R region (amino acid residues 223-585) recovered high affinity ³H-IP₃ binding (0.9±0.1 nM) which was abolished by a single point mutation of an arginine residue (R495L) important for IP₃ coordination. In situ hybridization revealed that Dj.IP₃R mRNA was most strongly expressed in the pharynx and optical nerve system as well as the reproductive system in sexualized planarians. Consistent with this observed tissue distribution, in vivo RNAi of Dj.IP₃R resulted in a decreased egg-laying behavior suggesting Dj.IP₃R plays an upstream role in planarian reproductive physiology.

Ca²⁺ channels and praziquantel: a view from the free world

Targeting the cellular Ca(2+) channels and pumps that underpin parasite Ca(2+) homeostasis may realize novel antihelmintic agents. Indeed, the antischistosomal drug praziquantel (PZQ) is a key clinical agent that has been proposed to work in this manner. Heterologous expression data has implicated an action of PZQ on voltage-operated Ca(2+) channels, although the relevant in vivo target of this drug has remained undefined over three decades of clinical use. The purpose of this review is to bring new perspective to this issue by discussing the potential utility of free-living planarian flatworms for providing new insight into the mechanism of PZQ action. First, we discuss in vivo functional genetic data from the planarian system that broadly supports the molecular data collected in heterologous systems and the 'Ca(2+) hypothesis' of PZQ action. On the basis of these similarities we highlight our current knowledge of platyhelminth voltage operated Ca(2+) channels, their unique molecular pharmacology and the downstream functional PZQ interactome engaged by dysregulation of Ca(2+) influx that has potential to yield novel antischistosomal targets. Overall the broad dataset underscores a common theme of PZQ-evoked disruptions of Ca(2+) homeostasis in trematodes, cestodes and turbellarians, and showcases the utility of the planarian model for deriving insight into drug action and targets in parasitic flatworms.

Functional genomics approaches in parasitic helminths

As research on parasitic helminths is moving into the post-genomic era, an enormous effort is directed towards deciphering gene function and to achieve gene annotation. The sequences that are available in public databases undoubtedly hold information that can be utilized for new interventions and control but the exploitation of these resources has until recently remained difficult. Only now, with the emergence of methods to genetically manipulate and transform parasitic worms will it be possible to gain a comprehensive understanding of the molecular mechanisms involved in nutrition, metabolism, developmental switches/maturation and interaction with the host immune system. This review focuses on functional genomics approaches in parasitic helminths that are currently used, to highlight potential applications of these technologies in the areas of cell biology, systems biology and immunobiology of parasitic helminths.

Cloning and functional characterization of two calmodulin genes during larval development in the parasitic flatworm Schistosoma mansoni

Schistosomiasis is endemic in over 70 countries, in which more than 200 million people are infected with the various schistosome species. Understanding the physiological processes underlying key developmental events could be useful in developing novel chemotherapeutic reagents or infection intervention strategies. Calmodulin is a small, calcium-sensing protein found in all eukaryotes and, although the protein has been previously identified in various Schistosoma mansoni stages and implicated in egg hatching and miracidia transformation, few molecular and functional data are available for this essential protein. Herein, we report the molecular cloning, expression, and functional characterization of calmodulin in the miracidia and primary sporocyst stages of S. mansoni. Two transcripts, SmCaM1 and SmCaM2, were cloned and sequenced, and a recombinant SmCaM1 protein was expressed in Escherichia coli and used to generate anti-CaM antibodies. The 2 protein sequences were highly conserved when compared to other model organisms. The alignment of the predicted proteins of both SmCaM1 and SmCaM2 exhibited 99% identity to each other and 97-98% identity with mammalian calmodulins. Analysis of steady-state transcript abundance indicate that the 2 calmodulin transcripts differ in their stage-associated expression patterns, although the CaM protein isotype appears to be constitutively expressed during early larval development. Application of RNAi to larval parasites results in a "stunted growth" phenotype in sporocysts with 30 and 35% reduction in transcript abundance for SmCaM1 and SmCaM2, respectively, and a corresponding 35% reduction in protein level after incubation in double-stranded RNA. Differential expression of CaM transcripts during early larval development and a growth defect-inducing effect associated with partial transcript and protein inhibition as a result of RNAi suggest a potentially important role of calmodulin during early larval development.

Developmental regulation of protein kinase A expression and activity in Schistosoma mansoni

cAMP-dependent protein kinases (PKAs) are the main transducers of cAMP signalling in eukaryotic cells. Recently we reported the identification and characterisation of a PKA catalytic subunit (SmPKA-C) in Schistosoma mansoni that is required for adult schistosome viability in vitro. To gain further insights into the role of SmPKA-C in biological processes during the schistosome life cycle, we undertook a quantitative analysis of SmPKA-C mRNA expression in different life cycle stages. Our data shows that SmPKA-C mRNA expression is developmentally regulated, with the highest levels of expression in cercariae and adult female worms. To evaluate the biological role of SmPKA-C in these developmental stages, cercariae and adult worms were treated with various concentrations of PKA inhibitors. Treatment of cercariae with H-89 or PKI 14-22 amide resulted in loss of viability suggesting that, as in adults, PKA is an essential enzyme activity in this infectious larval stage. In adult worms, in vitro exposure to sub-lethal concentrations of H-89 or PKI 14-22 amide resulted in inhibition of egg production in a dose-dependent manner. Furthermore, using a murine model of schistosome infection where S. mansoni fecundity is impaired, we show that reduced rates of egg production in vivo correlate with significant reductions in SmPKA-C mRNA expression and PKA activity. Finally, restoration of parasite egg production in vivo also resulted in normalisation of SmPKA-C mRNA expression and PKA activity. Taken together, our data suggest that PKA signalling is required for cercarial viability and may play a specific role in the reproductive activity of adult worms.

The identification of inhibitors of Schistosoma mansoni miracidial transformation by incorporating a medium-throughput small-molecule screen

In Schistosoma mansoni, the miracidium-to-primary sporocyst transformation process is associated with many physiological, morphological, transcriptional and biochemical changes. In the present study, we use a medium-throughput small-molecule screen to identify chemical compounds inhibiting or delaying the in vitro transformation of miracidia to the sporocyst stage. The Sigma-Aldrich Library of Pharmacologically Active Compounds (LOPAC) contains 1280 well-characterized chemical compounds with various modes of action including enzyme inhibitors, antibiotics, cell-cycle regulators, apoptosis inducers and GPCR ligands. We identified 47 compounds that greatly reduce or delay this transformation process during a primary screen of live miracidia. The majority of compounds inhibiting larval transformation were from dopaminergic, serotonergic, ion channel and phosphorylation classes. Specifically, we found that dopamine D2-type antagonists, serotonin reuptake inhibitors, voltage-gated calcium channel antagonists and a PKC activator significantly reduced in vitro miracidial transformation rates. Many of the targets of these compounds regulate adenylyl cyclase activity, with the inhibition or activation of these targets resulting in increased cAMP levels in miracidia and concomitant blocking/delaying of larval transformation.

A schistosome cAMP-dependent protein kinase catalytic subunit is essential for parasite viability

Eukaryotes, protozoan, and helminth parasites make extensive use of protein kinases to control cellular functions, suggesting that protein kinases may represent novel targets for the development of anti-parasitic drugs. Because of their central role in intracellular signaling pathways, cyclic nucleotide–dependent kinases such as cAMP-dependent protein kinase (PKA) represent promising new targets for the treatment of parasitic infections and neoplastic disorders. However, the role of these kinases in schistosome biology has not been characterized and the genes encoding schistosome PKAs have not been identified. Here we provide biochemical evidence for the presence of a PKA signaling pathway in adult Schistosoma mansoni and show that PKA activity is required for parasite viability in vitro. We also provide the first full description of a gene that encodes a PKA catalytic subunit in S. mansoni, named SmPKA-C. Finally we demonstrate, through RNA interference, that SmPKA-C contributes to the PKA activity we detected biochemically and that inhibition of SmPKA-C expression in adult schistosomes results in parasite death. Together our data show that SmPKA-C is a critically important gene product and may represent an attractive therapeutic target for the treatment and control of schistosomiasis.

Schistosomes are parasitic flatworms that inhabit the circulatory system and are the cause of a debilitating and insidious disease for millions of people worldwide. Like other complex organisms, schistosomes and other parasitic worms regulate their cell biology through extensive use of enzymes called protein kinases that phosphorylate other proteins to alter their function. One such protein kinase, cAMP-dependent protein kinase (PKA), has been proposed as a therapeutic target for the treatment of parasitic infections and cancer. Here we use biochemical techniques to show that schistosome worms possess a functional PKA pathway that is required for survival of the parasites. We also identify a parasite gene that encodes a functional PKA enzyme and show that silencing this gene results in both significant loss of PKA activity in schistosome worms and parasite death. These findings suggest that the gene we have identified is critically important to schistosomes and that its protein product may represent a target for the development of much-needed new drugs to treat schistosome infections.

Transcriptome analysis of Schistosoma mansoni larval development using serial analysis of gene expression (SAGE)

SUMMARY

Infection of the snail, Biomphalaria glabrata, by the free-swimming miracidial stage of the human blood fluke, Schistosoma mansoni, and its subsequent development to the parasitic sporocyst stage is critical to establishment of viable infections and continued human transmission. We performed a genome-wide expression analysis of the S. mansoni miracidia and developing sporocyst using Long Serial Analysis of Gene Expression (LongSAGE). Five cDNA libraries were constructed from miracidia and in vitro cultured 6- and 20-day-old sporocysts maintained in sporocyst medium (SM) or in SM conditioned by previous cultivation with cells of the B. glabrata embryonic (Bge) cell line. We generated 21 440 SAGE tags and mapped 13 381 to the S. mansoni gene predictions (v4.0e) either by estimating theoretical 3' UTR lengths or using existing 3' EST sequence data. Overall, 432 transcripts were found to be differentially expressed amongst all 5 libraries. In total, 172 tags were differentially expressed between miracidia and 6-day conditioned sporocysts and 152 were differentially expressed between miracidia and 6-day unconditioned sporocysts. In addition, 53 and 45 tags, respectively, were differentially expressed in 6-day and 20-day cultured sporocysts, due to the effects of exposure to Bge cell-conditioned medium.

Regulation of guanylyl cyclase by intracellular Ca2+ in relation to the infectivity of the protozoan parasite, Leishmania donovani

A neuronal type Ca2+ stimulated nitric oxide synthase was earlier reported by us to be present in the protozoan parasite Leishmania donovani. As part of nitric oxide-cyclic GMP transduction signaling operative in higher eukaryotes and involved in the long-term potentiation, a soluble guanylyl cyclase has also been detected in this lower eukaryote. However, detailed biochemical characterization revealed the enzyme to be Ca2+ modulated and unstimulated by nitric oxide donors as opposed to higher eukaryotes. The possible role of intracellular Ca2+ level in the regulation of guanylyl cyclase activity as well as L. donovani infectivity was explored by measuring the intracellular survival of the parasites in mammalian macrophages after treatments, which decrease or elevate the intracellular Ca2+. Parasites loaded with intracellular Ca2+ chelators displayed significantly decreased infectivity and cyclic GMP level. In contrast, pretreatment with Ca2+ ionophores, which elevated Ca2+ levels in L. donovani, significantly enhanced the cyclic GMP level as well as the infectivity of the parasites. Moreover, treatment with selective inhibitors of soluble guanylyl cyclase also reduced infectivity, even in cases of calcium ionophore-treated parasites. The gene encoding the soluble guanylyl cyclase was cloned, sequenced and over expressed in bacterial system. The recombinant protein showed enzyme characteristics similar to that obtained in L. donovani promastigote cytosol. Together these results suggest a possible link between guanylyl cyclase, intracellular Ca2+ content and parasite infectivity.

The involvement of cyclic adenosine monophosphate in the control of schistosome miracidium cilia

This study examined the possible involvement of cyclic adenosine monophosphate (cAMP) in the control of ciliary action of Schistosoma mansoni miracidia. Miracidia immobilized in hypertonic NaCl solution were treated with 3 compounds that are known to increase intracellular cAMP concentrations. Forskolin, at a concentration of 50 microM, induced 50.1% of the miracidia to swim in hypertonic solution. The corresponding values obtained for 3-isobutyl-1-methylxanthine (IBMX) at 1 mM and 8-bromo-cAMP at 10 mM were 42.2 and 50.4%, respectively. The motility-enhancing effect of these compounds was dose dependent. Nevertheless, the swimming speed of miracidia activated in this way was only 10% of that observed in artificial pond water (APW). Cholera toxin had no apparent effect on miracidia swimming in hypertonic NaCl solution. Likewise, swimming in APW treated with forskolin at 50 microM, IBMX at 1 mM, or 8-bromo-cAMP at 10 mM did not induce any apparent change in motility. Miracidia swimming in APW were then treated with 3 compounds that decrease the intracellular concentration of cAMP. MDL-12,330A, at a concentration of 250 microM, caused a dramatic decrease in swimming over a period of 1 hr. Likewise, SQ22536 and imidazole, at concentrations of 20 and 50 mM, respectively, caused 36.5 and 73.4% decreases in swimming under the same conditions. Finally, inhibitors of cAMP-dependent protein kinase, i.e., PKI(14-22)amide, H89, and H88, completely inhibited miracidia swimming in APW at concentrations of 25, 50, and 100 microM, respectively. These results suggest that cAMP and cAMP-dependent protein kinase are involved in osmosis-controlled ciliary motion of schistosome miracidia.

External stimuli and intracellular signalling in the modification of the nematode surface during transition to the mammalian host environment

Previous work has shown that the surface of infective larvae of parasitic nematodes will not bind the fluorescent lipid analogue 5-N-(octadecanoyl)aminofluorescein (AF18) until after exposure of the parasite to mammalian tissue-culture conditions. In this study, culture media which are permissive or non-permissive for the acquisition of lipophilicity for AF18 were altered in order to examine possible stimuli involved. This showed that external alkaline pH and high sodium ion concentration were highly stimulatory. The internal signalling pathways which may be involved in the surface alteration were then examined using agents which are known to affect intracellular signalling in mammalian cells. The results indicated that elevation of cGMP levels was stimulatory whereas inhibition of a putative Na+/H+ antiporter or calcium mobilization was inhibitory, and it is argued that high intracellular levels of cAMP may be inhibitory. Whilst the precise effects of the agents used on nematode cells remain to be established, these results provide a framework for the examination of the processes involved in the modification of the nematode surface which takes place immediately after the infection event.

Possible roles of Ca2+ and cGMP as mediators of the exflagellation of Plasmodium berghei and Plasmodium falciparum

The roles of Ca2+ and cyclic nucleotides as secondary, intracellular messengers for exflagellation of Plasmodium berghei and Plasmodium falciparum were investigated. Treatment with Ca2+ antagonists such as TMB-8 (an inhibitor of intracellular Ca2+ release) or W-7 (a calmodulin inhibitor) strongly inhibited exflagellation induced by alkaline medium at pH 8.0 whereas EGTA (a Ca2+ chelator) or nicardipine and nifedipine (Ca2+ channel inhibitors) had no effect. These results may indicate that mobilization of parasites' internal resources of Ca2+ is a prerequisite for exflagellation. Agents which increase cAMP levels did not induce exflagellation at the non-permissive pH of 7.3, and had no significant inhibitory effect at the permissive pH of 8.0. IBMX (cAMP/cGMP-phosphodiesterase inhibitor), however, enhanced exflagellation at pH 7.3, indicating the possibility that cGMP, but not cAMP, may be involved in the induction of exflagellation. Furthermore, cGMP or agents which increase cGMP levels such as nitroprusside (a potent activator of guanylate cyclase), enhanced exflagellation at pH 7.3, whereas N-methyl-hydroxylamine (guanylate cyclase inhibitor) inhibited the exflagellation at pH 8.0. From these results, it may be concluded that the induction of exflagellation requires both Ca2+ mobilization and an increase in cGMP levels.