Schistosoma mansoni: cloning and characterization of the Ras homologue

Protein tyrosine kinases in Schistosoma mansoni

The identification and description of signal transduction molecules and mechanisms are essential to elucidate Schistosoma mansoni host-parasite interactions and parasite biology. This mini review focuses on recent advancements in the study of signalling molecules and transduction mechanisms in S. mansoni, drawing special attention to the recently identified and characterised protein tyrosine kinases of S. mansoni.

Tyrosine kinase and cooperative TGFβ signaling in the reproductive organs of Schistosoma mansoni

Drug-induced suppression of female schistosome sexual maturation is an auspicious strategy to combat schistosomiasis since the eggs are the causative agent. The establishment of drug targets requires knowledge about the molecular mechanisms that regulate the development of the female reproductive organs, which include vitellarium and ovary. This review summarizes recent studies suggesting tyrosine kinases as important factors for the regulation of female gonad development. In this context, especially cytoplasmatic tyrosine kinases of the Src class seem to play dominant roles. Moreover, experimental data and theoretical concepts are provided supporting a crosstalk between tyrosine kinase and TGFbeta signaling in the production of vitellocytes. Finally, we take advantage from the schistosome genome project to propose a model for the regulation of vitelline-cell production and differentiation.

Signal transduction in larval trematodes: putative systems associated with regulating larval motility and behaviour

The multi-host lifestyle of parasitic trematodes necessitates their ability to communicate with their external environment in order to invade and navigate within their hosts' internal environment. Through recent EST and genome sequencing efforts, it has become clear that members of the Trematoda possess many of the elaborate signal transduction systems that have been delineated in other invertebrate model systems like Drosophila melanogaster and Caenorhabditis elegans. Gene homologues representing several well-described signal receptor families including receptor tyrosine kinases, receptor serine tyrosine kinases, G protein-coupled receptors and elements of their downstream signalling systems have been identified in larval trematodes. A majority of this work has focused on the blood flukes, Schistosoma spp. and therefore represents a narrow sampling of the diverse digenean helminth taxon. Despite this fact and given the substantial evidence supporting the existence of such signalling systems, the question then becomes, how are these systems employed by larval trematodes to aid them in interpreting signals received from their immediate environment to initiate appropriate responses in cells and tissues comprising the developing parasite stages? High-throughput, genome-wide analysis tools now allow us to begin to functionally characterize genes differentially expressed throughout the development of trematode larvae. Investigation of the systems used by these parasites to receive and transduce external signals may facilitate the creation of technologies for achieving control of intramolluscan schistosome infections and also continue to yield valuable insights into the basic mechanisms regulating motility and behaviour in this important group of helminths.

Growth factor receptors in helminth parasites: Signalling and host-parasite relationships

Parasitic helminths remain major pathogens of both humans and animals throughout the world. The success of helminth infections depends on the capacity of the parasite to counteract host immune responses but also to exploit host-derived signal molecules for its development. Recent progress has been made in the characterization of growth factor receptors of various nematode and flatworm parasites with the demonstration that transforming growth factor beta (TGF-beta), epidermal growth factor (EGF) and insulin receptor signalling pathways are conserved in helminth parasites and potentially implicated in the host-parasite molecular dialogue and parasite development.

An historical and genomic view of schistosome conjugal biology with emphasis on sex-specific gene expression

The genetic programmes associated with the sexual biology of dioecious schistosomes remain a critically important but significantly understudied area of parasitology. Throughout the last four decades, progress has been slow in describing the gross antigenic and proteomic differences linked to sexually mature schistosomes and in characterizing some of the sex-associated transcripts and regulatory mechanisms induced during developmental maturation. These investigations have been severely hindered by the lack of complete EST/genomic information, as well as corresponding post- and functional-genomic tools for studying these pathogenic parasites. As near complete transcriptomes forSchistosoma japonicumandS. mansonihave recently been reported, and both DNA microarrays and post-transcriptional gene silencing have been applied to schistosomes, the tools and techniques for the high-throughput identification and characterization of transcripts involved in conjugal biology are now readily available. Here, an historical review is presented that summarizes some of the most significant findings associated with schistosome sex and sexual maturation during the last several decades. Following this discussion is a current overview of some modern day genomic approaches used to study schistosomes, which illustrates how major advances in the field of conjugal biology will be achieved.

Molecular cloning and characterization of Ras- and Raf-homologues from the fox-tapeworm Echinococcus multilocularis

To better understand growth regulation in the human parasitic cestode Echinococcus multilocularis, we have cloned and characterized the parasite's orthologues of the key regulatory factors Ras and Raf. Using a degenerative PCR approach a gene, emras, was identified whose gene product, EmRas, showed high homology (79% identical residues) to human Ras and contained all amino acid residues which are characteristic for this subfamily of small GTPases at the corresponding positions. Recombinantly expressed EmRas bound GTP and was farnesylated, but not geranyl-geranylated, by Echinococcus lysate in an in vitro prenylation assay. Furthermore, upon expression in yeast, emras was able to functionally complement the Saccharomyces orthologue RAS2 in an invasive growth assay. In Western blot analyses using an anti-EmRas antibody, the Echinococcus factor could be detected in lysates of the larval stages metacestode and protoscolex. By immune-histochemistry, EmRas was shown to localize to the germinal layer of the metacestode and to tegumental structures of the protoscolex, particularly around the rostellum and the sucker regions. In addition, we fully characterized the gene emraf whose product, EmRaf, displayed considerable homology to mammalian Raf-kinases and orthologous factors from Drosophila and Caenorhabditis elegans. emraf was co-expressed with emras in the larval stages metacestode and protoscolex during in vitro cultivation and during an infection of the intermediate host as assessed by RT-PCR experiments. The emraf gene was composed of nine exons and eight introns and shared four highly conserved exon-intron boundaries with the human gene encoding Raf-1, suggesting that both genes derived from a common evolutionary ancestor. Southern blot hybridizations demonstrated that emraf is a single copy gene. Using the yeast two-hybrid system, EmRaf was shown to interact with EmRas, but not with EmRal, a previously characterized orthologue of mammalian Ral GTPases. This is the first characterization of a Ras orthologue from a cestode and the first report on a Raf-like kinase from a platyhelminth. The data presented herein will form a solid basis for further investigations on Echinococcus signaling systems that are involved in growth control and development of the parasite.

Echinococcus multilocularis: identification and molecular characterization of a Ral-like small GTP-binding protein

In mammals, Ral (Ras-like) GTPases have been implicated in the regulation of several cellular key processes such as oncogenic transformation, endocytosis, and actin-cytoskeleton dynamics. Here we provide, for the first time, molecular data on a Ral homologue from a parasitic helminth. We have cloned and characterized the complete cDNA molecule and the chromosomal locus encoding a novel GTP binding protein, EmRal, of the human parasite Echinococcus multilocularis. The encoded protein contained all highly conserved amino acid residues of the protein family at corresponding positions and shared significant sequence homologies with human RalA (53% identity) and RalB (54%). Upon heterologous expression of EmRal in Escherichia coli, the recombinant protein was able to bind GTP, thus indicating functionality of the Echinococcus factor. Using an in vitro prenylation assay, the purified protein was shown to be geranylgernylated, but not farnesylated, in both rabbit reticulocyte and Echinococcus cell extracts. The EmRal mRNA was found to be processed via trans-splicing and, using RT-PCR and virtual Northern blot experiments, expression of the factor could be demonstrated for the larval stages metacestode and protoscolex during an infection of the intermediate host. The data presented herein provide a solid basis for further investigations on Ras-Ral signaling mechanisms in Echinococcus.

Characterisation of a Rho homologue of Schistosoma mansoni

The development and survival of the helminth parasite, Schistosoma mansoni, is dependent on its ability to interpret signals from its environment. Currently, little is known about signal transduction in schistosomes. Rho is a member of a super-family of small GTP-binding proteins. Rho is involved in a number of cell signalling pathways with effects on actin cytoskeleton organisation, gene transcription, cell cycle progression, and membrane trafficking. We have cloned an S. mansoni protein (Rho1) that has 71-75% identity and approximately 85% similarity with human Rho A, B, and C proteins. We have optimised expression of recombinant S. mansoni Rho1 protein in Escherichia coli by co-expression with rare tRNAs. Western blot analysis results showed expression of Rho1 protein in adult worm stages especially female worms. In vitro prenylation of recombinant S. mansoni Rho1 determined that, similar to Rho from other organisms, Rho1 is geranynlgeranylated but not farnesylated. A search of the gene database indicates that Rho GTPases exist as a small family in S. mansoni including orthologues of Rho, Cdc42, and Rac. These data suggest that S. mansoni Rho1 plays a role in signalling in adult worms, especially females.

Using RNA interference to manipulate endogenous gene expression in Schistosoma mansoni sporocysts

Direct assessments of gene function in parasitic flatworms have been hampered by the lack of effective tools to alter gene expression. The aim of the present study was to use RNA-interference (RNAi) to achieve targeted gene knockdown in larval stages of the human blood fluke, Schistosoma mansoni. We selected two S. mansoni genes for RNAi experiments: SGTP1, a facilitated diffusion glucose transporter and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). When S. mansoni larvae were treated in vitro for 6 days with dsRNA specific to one of these two genes, targeted transcript levels were reduced by 70-80% as determined by quantitative PCR (qPCR), while non-targeted transcripts were unaffected. Parasite exposure to SGTP1 dsRNA, but not GAPDH dsRNA, reduced larval glucose-uptake capacity by 40%, demonstrating that SGTP1 transcript knockdown results in the functional phenotype of reduced glucose transport activity. The effect of dsRNA treatment on transcript level was evident for up to 28 days after an initial dsRNA treatment. Interestingly, dsRNA treatment was effective only when miracidia were allowed to undergo the transition to sporocysts in its presence, while treatment of fully transformed sporocysts was ineffective. Fluorescence patterns in larvae exposed to rhodamine-labeled dsRNA as miracidia and sporocysts were similar, suggesting that the difference in susceptibility to dsRNA treatment between the two life stages may not be due to differences in dsRNA entry. Overall, this technology will enable direct assessment of the roles of individual genes in physiological processes of larval stages of S. mansoni, a crucial step in the identification of novel intervention targets for this important human pathogen.

A novel Syk-family tyrosine kinase from Schistosoma mansoni which is preferentially transcribed in reproductive organs

The complete coding deoxyribonucleic acid for a novel tyrosine kinase (TK) of the human parasite Schistosoma mansoni has been cloned and characterized. The molecule was designated TK4. The sequence predicts a translation product of about 140 kDa containing two Src homology 2 domains and a tyrosine kinase domain. Data base analyses indicate that TK4 belongs to the Syk family of TKs which has not been identified in schistosomes or other Acoelomata yet. The presence of a member of the Syk family in this phylum supports previous findings demonstrating that TK subclasses were established early in evolution. Although Northern blot and reverse transcription polymerase chain reaction analyses show transcription of TK4 in larval stages and adult schistosomes of both genders, TK4 is more abundantly transcribed in males. In situ hybridization data demonstrate the gender-independent occurrence of TK4 transcripts in parenchymatic cells. Significant signals were detected in the oocytes of the female and in the spermatocytes of the male suggesting that TK4, among other functions, may play a role in germ cell development. This is an unexpected finding considering that Syk-family TKs of invertebrates and vertebrates described so far are not involved in the differentiation of the gonads.

Identification of Schistosoma mansoni gender-associated gene transcripts by cDNA microarray profiling

BACKGROUND

Parasitic helminths of the genus Schistosoma mate, achieve sexual maturity and produce eggs in the bloodstream of their definitive hosts, and the most important pathological consequences of the infection are associated with this process. We have used cDNA microarray technology to initiate genome-wide gene-expression studies of sex and sexual development in mature Schistosoma mansoni parasites.

RESULTS

An S. mansoni-specific cDNA microarray was fabricated using 576 expressed sequence tags selected from three cDNA libraries and originating from two different parasite developmental stages. Five independent cDNA microarray hybridizations were analyzed using stringent filtering criteria and careful quality control, leading to the identification of 12 new female-associated and 4 new male-associated gene transcripts in the mature adult schistosome. Statistical analysis of variation demonstrated high levels of agreement within a cDNA microarray (correlation coefficient 0.91; median coefficient of variation 11.1%) and between cDNA microarrays (correlation coefficient 0.90; median coefficient of variation 14.4%). RT-PCR analysis confirmed the cDNA microarray results, thereby supporting the reliability of the system.

CONCLUSIONS

Our study expands the list of S. mansoni gender-associated gene transcripts from all previous studies by a factor of two. Among the new associations identified, a tyrosinase ortholog was preferentially expressed in the adult female, and a dynein light-chain ortholog was highly induced in the adult male. cDNA microarrays offer the potential for exponential leaps in the understanding of parasite biology and this study shows how molecules involved in sexual biology can be rapidly identified.

The guanine protein coupled receptor rhodopsin is developmentally regulated in the free-living stages of Schistosoma mansoni

Schistosoma mansoni parasites inhabit three distinct environments including water, intermediate molluscan hosts, and definitive vertebrate hosts. Determining how schistosomes interact with these environments may be one mechanism by which suitable vaccines or novel chemotherapeutic targets will be identified. Towards this end, we describe the identification of a 36-kDa S. mansoni protein that shares extensive sequence similarity to light absorbing rhodopsin guanine protein coupled receptors (GPCRs). This protein, S. mansoni rhodopsin (SmRHO), is the first molecularly characterized GPCR described in schistosomes. Sequence analysis reveals that SmRHO shares extensive phylogenetic conservation among rhodopsins/opsins expressed in water-dwelling invertebrates, possibly indicative of orthology. We demonstrate here that SmRHO is expressed in the free-living, light responsive miracidia and cercaria stages and is down-regulated in the adult, vertebrate residing forms. Moreover, we show that SmRHO is localized to sub-tegumental structures found towards the anterior end of cercariae. As SmRHO may be implicated in schistosome photoreception processes, we have begun a search for additional parasite encoded GPCR super-family members, which may be associated with chemoreception, chemotaxis, and olfaction. Identifying and characterizing new GPCRs may uncover hidden aspects of parasite biology useful towards the development of novel intervention strategies.

The Schistosoma gene discovery program: state of the art

Schistosoma are dioecious digenetic trematodes carrying a large (270 Mb) genome. Gaining knowledge about the genome of these parasites is of importance for the understanding of their biology, mechanisms of drug resistance and antigenic variation that determine escape from the host's immune system. This review will provide an update on the Schistosoma Gene Discovery Program, which is part of the Schistosoma Genome Project created in 1992. One of the main objectives of this program is the discovery and characterisation of new genes of Schistosoma mansoni and Schistosoma japonicum in an attempt to search for new targets for drugs and vaccine development. The success of the Schistosoma Gene Discovery Program is demonstrated by the number of catalogued genes, that now reaches 15 to 20% of the full gene complement of its genome.