Schistosoma mansoni miracidia transformed by particle bombardment infect Biomphalaria glabrata snails and develop into transgenic sporocysts

Schistosome Transgenesis: The Long Road to Success

As research on parasitic helminths has entered the post-genomic era, research efforts have turned to deciphering the function of genes in the public databases of genome sequences. It is hoped that, by understanding the role of parasite genes in maintaining their parasitic lifestyle, critical insights can be gained to develop new intervention and control strategies. Methods to manipulate and transform parasitic worms are now developed to a point where it has become possible to gain a comprehensive understanding of the molecular mechanisms underlying host-parasite interplay, and here, we summarise and discuss the advances that have been made in schistosome transgenesis over the past 25 years. The ability to genetically manipulate schistosomes holds promise in finding new ways to control schistosomiasis, which ultimately may lead to the eradication of this debilitating disease.

Transfection of Platyhelminthes

Flatworms are one of the most diverse groups within Lophotrochozoa with more than 20,000 known species, distributed worldwide in different ecosystems, from the free-living organisms in the seas and lakes to highly specialized parasites living in a variety of hosts, including humans. Several infections caused by flatworms are considered major neglected diseases affecting countries in the Americas, Asia, and Africa. For several decades, a particular interest on free-living flatworms was due to their ability to regenerate considerable portions of the body, implying the presence of germ cells that could be important for medicine. The relevance of reverse genetics for this group is clear; understanding the phenotypic characteristics of specific genes will shed light on developmental traits of free-living and parasite worms. The genetic manipulation of flatworms will allow learning more about the mechanisms for tissue regeneration, designing new and more effective anthelmintic drugs, and explaining the host-parasite molecular crosstalk so far partially inaccessible for experimentation. In this review, availability of transfection techniques is analyzed across flatworms, from the initial transient achievements to the stable manipulations now developed for free-living and parasite species.

Knocking down schistosomes - promise for lentiviral transduction in parasites

Underpinned by major advances in our understanding of the genomes of schistosomes, progress in the development of functional genomic tools is providing unique prospects to gain insights into the intricacies of the biology of these blood flukes, their host relationships, and the diseases that they cause. This article reviews some key applications of double-stranded RNA interference (RNAi) in Schistosoma mansoni, appraises delivery systems for transgenesis and stable gene silencing, considers ways of increasing efficiency and specificity of gene silencing, and discusses the prospects of using a lentivirus delivery system for future functional genomic-phenomic explorations of schistosomes and other parasites. The ability to achieve effective and stable gene perturbation in parasites has major biological implications and could facilitate the development of new interventions.

Evaluation of schistosome promoter expression for transgenesis and genetic analysis

Schistosome worms of the genus Schistosoma are the causative agents of schistosomiasis, a devastating parasitic disease affecting more than 240 million people worldwide. Schistosomes have complex life cycles, and have been challenging to manipulate genetically due to the dearth of molecular tools. Although the use of gene overexpression, gene knockouts or knockdowns are straight-forward genetic tools applied in many model systems, gene misexpression and genetic manipulation of schistosome genes in vivo has been exceptionally challenging, and plasmid based transfection inducing gene expression is limited. We recently reported the use of polyethyleneimine (PEI) as a simple and effective method for schistosome transfection and gene expression. Here, we use PEI-mediated schistosome plasmid transgenesis to define and compare gene expression profiles from endogenous and nonendogenous promoters in the schistosomula stage of schistosomes that are potentially useful to misexpress (underexpress or overexpress) gene product levels. In addition, we overexpress schistosome genes in vivo using a strong promoter and show plasmid-based misregulation of genes in schistosomes, producing a clear and distinct phenotype--death. These data focus on the schistosomula stage, but they foreshadow strong potential for genetic characterization of schistosome molecular pathways, and potential for use in overexpression screens and drug resistance studies in schistosomes using plasmid-based gene expression.

Polyethyleneimine mediated DNA transfection in schistosome parasites and regulation of the WNT signaling pathway by a dominant-negative SmMef2

Schistosomiasis is a serious global problem and the second most devastating parasitic disease following malaria. Parasitic worms of the genus Schistosoma are the causative agents of schistosomiasis and infect more than 240 million people worldwide. The paucity of molecular tools to manipulate schistosome gene expression has made an understanding of genetic pathways in these parasites difficult, increasing the challenge of identifying new potential drugs for treatment. Here, we describe the use of a formulation of polyethyleneimine (PEI) as an alternative to electroporation for the efficacious transfection of genetic material into schistosome parasites. We show efficient expression of genes from a heterologous CMV promoter and from the schistosome Sm23 promoter. Using the schistosome myocyte enhancer factor 2 (SmMef2), a transcriptional activator critical for myogenesis and other developmental pathways, we describe the development of a dominant-negative form of the schistosome Mef2. Using this mutant, we provide evidence that SmMef2 may regulate genes in the WNT pathway. We also show that SmMef2 regulates its own expression levels. These data demonstrate the use of PEI to facilitate effective transfection of nucleic acids into schistosomes, aiding in the study of schistosome gene expression and regulation, and development of genetic tools for the characterization of molecular pathways in these parasites.

Progress with schistosome transgenesis

Genome sequences for Schistosoma japonicum and Schistosoma mansoni are now available. The schistosome genome encodes ~13,000 protein encoding genes for which the function of only a minority is understood. There is a valuable role for transgenesis in functional genomic investigations of these new schistosome gene sequences. In gain-of-function approaches, transgenesis can lead to integration of transgenes into the schistosome genome which can facilitate insertional mutagenesis screens. By contrast, transgene driven, vector-based RNA interference (RNAi) offers powerful loss-of-function manipulations. Our laboratory has focused on development of tools to facilitate schistosome transgenesis. We have investigated the utility of retroviruses and transposons to transduce schistosomes. Vesicular stomatitis virus glycoprotein (VSVG) pseudotyped murine leukemia virus (MLV) can transduce developmental stages of S. mansoni including eggs. We have also observed that the piggyBac transposon is transpositionally active in schistosomes. Approaches with both VSVG-MLV and piggyBac have resulted in somatic transgenesis and have lead to integration of active reporter transgenes into schistosome chromosomes. These findings provided the first reports of integration of reporter transgenes into schistosome chromosomes. Experience with these systems is reviewed herewith, along with findings with transgene mediated RNAi and germ line transgenesis, in addition to pioneering and earlier reports of gene manipulation for schistosomes.

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.

Paving the way for transgenic schistosomes

In parasitological research, significant progress has been made with respect to genomics and transcriptomics but transgenic systems for functional gene analyses are mainly restricted to the protozoan field. Gene insertion and knockout strategies can be applied to parasitic protozoa as well as gene silencing by RNA interference (RNAi). By contrast, research on parasitic helminthes still lags behind. Along with the major advances in genome and transcriptome analyses e.g. for schistosomes, methods for the functional characterization of genes of interest are still in their initial phase and have to be elaborated now, at the beginning of the post-genomic era. In this review we will summarize attempts made in the last decade regarding the establishment of protocols to transiently and stably transform or transfect schistosomes. Besides approaches using particle bombardment, electroporation or virus-based infection strateies to introduce DNA constructs into adult and larval schistosome stages to express reporter genes, first approaches have also been made in establishing protocols based on soaking, lipofection, and/or electroporation for RNA interference to silence gene activity. Although in these cases remarkable progress can be seen, the schistosome community eagerly awaits major breakthroughs especially with respect to stable transformation, but also for silencing or knock-down strategies for every schistosome gene of interest.

Genetic manipulation of schistosomes--progress with integration competent vectors

Draft genome sequences for Schistosoma japonicum and S. mansoni are now available. The schistosome genome encodes ∼13,000 protein-encoding genes for which the functions of few are well understood. Nonetheless, the new genes represent potential intervention targets, and molecular tools are being developed to determine their importance. Over the past 15 years, noteworthy progress has been achieved towards development of tools for gene manipulation and transgenesis of schistosomes. A brief history of genetic manipulation is presented, along with a review of the field with emphasis on reports of integration of transgenes into schistosome chromosomes.

Advances in mRNA silencing and transgene expression: a gateway to functional genomics in schistosomes

The completion of the WHO Schistosoma Genome Project in 2008, although not fully annotated, provides a golden opportunity to actively pursue fundamental research on the parasites genome. This analysis will aid identification of targets for drugs, vaccines and markers for diagnostic tools as well as for studying the biological basis of drug resistance, infectivity and pathology. For the validation of drug and vaccine targets, the genomic sequence data is only of use if functional analyses can be conducted (in the parasite itself). Until recently, gene manipulation approaches had not been seriously addressed. This situation is now changing and rapid advances have been made in gene silencing and transgenesis of schistosomes.

Transduction of Schistosoma japonicum schistosomules with vesicular stomatitis virus glycoprotein pseudotyped murine leukemia retrovirus and expression of reporter human telomerase reverse transcriptase in the transgenic schistosomes

Although draft genome sequences of two of the major human schistosomes, Schistosoma japonicum and Schistosoma mansoni are available, the structures and characteristics of most genes and the influence of exogenous genes on the metabolism of schistosomes remain uncharacterized. Furthermore, which functional genomics approaches will be tractable for schistosomes are not yet apparent. Here, the vesicular stomatitis virus glycoprotein (VSVG)-pseudotyped pantropic retroviral vector pBABE-puro was modified to incorporate the human telomerase reverse transcriptase gene (hTERT) as a reporter, under the control of the retroviral long terminal repeat (LTR). Pseudotyped virions were employed to transduce S. japonicum to investigate the utility of retrovirus-mediated transgenesis of S. japonicum and the activity of human telomerase reverse transcriptase as a reporter transgene in schistosomes. Schistosomules perfused from experimentally infected rabbits were cultured for 6 days after exposure to the virions after which genomic DNAs from virus exposed and control worms were extracted. Analysis of RNA from transduced parasites and immunohistochemistry of thin parasite sections revealed expression of hTERT in the transduced worms. Expression of hTERT was also confirmed by immunoblot analysis. These findings indicated that S. japonicum could be effectively transduced by VSVG-pseudotyped retrovirus carrying the hTERT gene. Given the potential of hTERT to aid in derivation of immortalized cells, these findings suggest that this pantropic retroviral approach can be employed to transduce cells from specific tissues and organs of schistosomes to investigate the influence of transgene hTERT on growth and proliferation of schistosome cells.

Vector-based RNA interference of cathepsin B1 in Schistosoma mansoni

In helminth parasites, proteolytic enzymes have been implicated in facilitating host invasion, moulting, feeding, and evasion of the host immune response. These key functions render them potential targets for anti-parasite chemotherapy and immunotherapy. Schistosomes feed on host blood and the digested haemoglobin is their major source of amino acids. Haemoglobin digestion is essential for parasite development, growth, and reproduction. We recently reported the use of pseudotyped Moloney murine leukaemia virus to accomplish transformation of Schistosoma mansoni. Here, we report the design of a viral vector expressing a dsRNA hairpin to silence expression of the schistosome cathepsin B1 (SmCB1) gene. We observed 80% reduction in transcript level 72 h after virus exposure and complete silencing of enzyme activity in transduced worms. This is the first report using this technology in any helminth parasite. It will facilitate the evaluation of potential drug targets and biochemical pathways for novel interventions in schistosomes.

Electroporation facilitates introduction of reporter transgenes and virions into schistosome eggs

Background

The schistosome egg represents an attractive developmental stage at which to target transgenes because of the high ratio of germ to somatic cells, because the transgene might be propagated and amplified by infecting snails with the miracidia hatched from treated eggs, and because eggs can be readily obtained from experimentally infected rodents.

Methods/Findings

We investigated the utility of square wave electroporation to deliver transgenes and other macromolecules including fluorescent (Cy3) short interference (si) RNA molecules, messenger RNAs, and virions into eggs of Schistosoma mansoni. First, eggs were incubated in Cy3-labeled siRNA with and without square wave electroporation. Cy3-signals were detected by fluorescence microscopy in eggs and miracidia hatched from treated eggs. Second, electroporation was employed to introduce mRNA encoding firefly luciferase into eggs. Luciferase activity was detected three hours later, whereas luciferase was not evident in eggs soaked in the mRNA. Third, schistosome eggs were exposed to Moloney murine leukemia virus virions (MLV) pseudotyped with vesicular stomatitis virus glycoprotein (VSVG). Proviral transgenes were detected by PCR in genomic DNA from miracidia hatched from virion-exposed eggs, indicating the presence of transgenes in larval schistosomes that had been either soaked or electroporated. However, quantitative PCR (qPCR) analysis determined that electroporation of virions resulted in 2–3 times as many copies of provirus in these schistosomes compared to soaking alone. In addition, relative qPCR indicated a copy number for the proviral luciferase transgene of ∼20 copies for 100 copies of a representative single copy endogenous gene (encoding cathepsin D).

Conclusions

Square wave electroporation facilitates introduction of transgenes into the schistosome egg. Electroporation was more effective for the transduction of eggs with pseudotyped MLV than simply soaking the eggs in virions. These findings underscore the potential of targeting the schistosome egg for germ line transgenesis.

The genome sequences of two of the three major species of schistosomes are now available. Molecular tools are needed to determine the importance of these new genes. With this in mind, we investigated introduction of reporter transgenes into schistosome eggs, with the longer-term aim of manipulation of schistosome genes and gene functions. The egg is a desirable developmental stage for genome manipulation, not least because it contains apparently accessible germ cells. Introduction of transgenes into the germ cells of schistosome eggs might result in transgenic schistosomes. However, the egg is surrounded by a thick shell which might block access to entry of transgenes. We cultured eggs in the presence of three types of reporter transgenes of increasing molecular size, and in addition we tried to produce transient holes in the eggs by electroporation to investigate whether the transgenes would more easily enter the eggs. Electroporation of eggs appeared to allow entry of two larger types of transgenes into cultured schistosome eggs, messenger RNA encoding firefly luciferase, and retroviral virions. We anticipate that this approach, electroporation of transgenes into schistosome eggs, will facilitate genetic manipulation of schistosomes for investigating the importance of schistosome genes and gene products as new intervention targets.

Neuronal signaling in schistosomes: current status and prospects for postgenomicsThe present review is one of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

Parasitic platyhelminths of the genus Schistosoma Weinland, 1858 (Trematoda, Digenea) are the etiological agents of human schistosomiasis, one of the most prevalent and debilitating parasitic diseases worldwide. Praziquantel is the only drug treatment available in most parts of the world and the effectiveness of the drug is threatened by the prospect of drug resistance. There is a pressing need to learn more about the basic biology of this organism and to identify molecular targets for new therapeutic drugs. The nervous system of schistosomes coordinates many activities that are essential for parasite survival, and as such is an attractive target for chemotherapeutic intervention. Until recently, very little was known about the molecular mechanisms of neuronal signaling in these organisms, but this is rapidly changing following the completion of the genome sequence and several recent developments in schistosome transgenesis and gene silencing. Here we review the current status of schistosome neurobiology and discuss prospects for future research as the field moves into a postgenomics era. One of the themes that will emerge from this discussion is that schistosomes have a rich diversity of neurotransmitters and receptors, indicating a more sophisticated system of neuronal communication than might be expected of a parasitic flatworm. Moreover, many of these transmitter receptors share little sequence homology with those of the human host, making them ideally suited for selective drug targeting. Strategies for characterization of these important parasite proteins will be discussed.

In vitro manipulation of gene expression in larval Schistosoma: a model for postgenomic approaches in Trematoda

With rapid developments in DNA and protein sequencing technologies, combined with powerful bioinformatics tools, a continued acceleration of gene identification in parasitic helminths is predicted, potentially leading to discovery of new drug and vaccine targets, enhanced diagnostics and insights into the complex biology underlying host-parasite interactions. For the schistosome blood flukes, with the recent completion of genome sequencing and comprehensive transcriptomic datasets, there has accumulated massive amounts of gene sequence data, for which, in the vast majority of cases, little is known about actual functions within the intact organism. In this review we attempt to bring together traditional in vitro cultivation approaches and recent emergent technologies of molecular genomics, transcriptomics and genetic manipulation to illustrate the considerable progress made in our understanding of trematode gene expression and function during development of the intramolluscan larval stages. Using several prominent trematode families (Schistosomatidae, Fasciolidae, Echinostomatidae), we have focused on the current status of in vitro larval isolation/cultivation as a source of valuable raw material supporting gene discovery efforts in model digeneans that include whole genome sequencing, transcript and protein expression profiling during larval development, and progress made in the in vitro manipulation of genes and their expression in larval trematodes using transgenic and RNA interference (RNAi) approaches.

Imaging schistosomes in vivo

Schistosomes are intravascular, parasitic helminths that cause a chronic, often debilitating disease afflicting over 200 million people in over 70 countries. Here we describe novel imaging methods that, for the first time, permit visualization of live schistosomes within their living hosts. The technology centers on fluorescent agent uptake and activation in the parasite's gut, and subsequent detection and signal quantitation using fluorescence molecular tomography (FMT). There is a strong positive correlation between the signal detected and parasite number. Schistosoma mansoni parasites of both sexes recovered from infected experimental animals exhibit vivid fluorescence throughout their intestines. Likewise, the remaining important human schistosome parasites, S. japonicum and S. hematobium, also exhibit gut fluorescence when recovered from infected animals. Imaging has been used to efficiently document the decline in parasite numbers in infected mice treated with the antischistosome drug praziquantel. This technology will provide a unique opportunity both to help rapidly identify much-needed, novel antischistosome therapies and to gain direct visual insight into the intravascular lives of the major schistosome parasites of humans.

Development of functional genomic tools in trematodes: RNA interference and luciferase reporter gene activity in Fasciola hepatica

The growing availability of sequence information from diverse parasites through genomic and transcriptomic projects offer new opportunities for the identification of key mediators in the parasite–host interaction. Functional genomics approaches and methods for the manipulation of genes are essential tools for deciphering the roles of genes and to identify new intervention targets in parasites. Exciting advances in functional genomics for parasitic helminths are starting to occur, with transgene expression and RNA interference (RNAi) reported in several species of nematodes, but the area is still in its infancy in flatworms, with reports in just three species. While advancing in model organisms, there is a need to rapidly extend these technologies to other parasites responsible for several chronic diseases of humans and cattle. In order to extend these approaches to less well studied parasitic worms, we developed a test method for the presence of a viable RNAi pathway by silencing the exogenous reporter gene, firefly luciferase (fLUC). We established the method in the human blood fluke Schistosoma mansoni and then confirmed its utility in the liver fluke Fasciola hepatica. We transformed newly excysted juveniles of F. hepatica by electroporation with mRNA of fLUC and three hours later were able to detect luciferase enzyme activity, concentrated mainly in the digestive ceca. Subsequently, we tested the presence of an active RNAi pathway in F. hepatica by knocking down the exogenous luciferase activity by introduction into the transformed parasites of double-stranded RNA (dsRNA) specific for fLUC. In addition, we tested the RNAi pathway targeting an endogenous F. hepatica gene encoding leucine aminopeptidase (FhLAP), and observed a significant reduction in specific mRNA levels. In summary, these studies demonstrated the utility of RNAi targeting reporter fLUC as a reporter gene assay to establish the presence of an intact RNAi pathway in helminth parasites. These could facilitate the study of gene function and the identification of relevant targets for intervention in organisms that are by other means intractable. More specifically, these results open new perspectives for functional genomics of F. hepatica, which hopefully can lead to the development of new interventions for fascioliasis.

Reverse genetics tools allow assessing the function of unknown genes. Their application for the study of neglected infectious diseases could lead eventually to the identification of relevant gene products to be used in diagnosis, or as drug targets or immunization candidates. Being technically more simple and less demanding than other reverse genetics tools such as transgenesis or knockouts, the suppression of gene activity mediated by double-stranded RNA has emerged as a powerful tool for the analysis of gene function. RNAi appeared as an obvious alternative to apply in complex biological systems where information is still scarce, a situation common to several infectious and parasitic diseases. However, several technical or practical difficulties have hampered the development of this technique in parasites to the expectations originally generated. We developed a simple method to test the presence of a viable RNAi pathway by silencing an exogenous reporter gene. The method was tested in F. hepatica, describing the conditions for transfection and confirming the existence of a viable RNAi pathway in this parasite. The experimental design created can be useful as a first approach in organisms where genetic analysis is still unavailable, providing a tool to unravel gene function and probably advancing new candidates relevant in pathobiology, prevention or treatment.

Expression of helminth genes in Leishmania: an experimental transfection system to test immunological function

Functional analysis of genes from parasitic helminths requires, at the present time, heterologous expression. We have adapted the well-characterized system of transfection in Leishmania protozoal parasites, as a means of analysing the effect of single filarial genes on the mammalian immune system. For example, testing the function of the Brugia malayi abundant larval transcript (ALT) gene-transfected Leishmania mexicana were found to be significantly more virulent in macrophages in vitro. The course of infection in vivo is also aggravated by expression of the ALT gene. Examples are also given of transgenes which reduced in vitro growth within macrophages, as well as others which exert no effect on the protozoal parasitism. Thus, Leishmania transfection provides a tractable system to analyse helminth gene function within the context of the host immune system.

Transgenesis of schistosomes: approaches employing mobile genetic elements

Draft genome sequences for Schistosoma mansoni and Schistosoma japonicum are now available. However, the identity and importance of most schistosome genes have yet to be determined. Recently, progress has been made towards the genetic manipulation and transgenesis of schistosomes. Both loss-of-function and gain-of-function approaches appear to be feasible in schistosomes based on findings described in the past 5 years. This review focuses on reports of schistosome transgenesis, specifically those dealing with the transformation of schistosomes with exogenous mobile genetic elements and/or their endogenous relatives for the genetic manipulation of schistosomes. Transgenesis mediated by mobile genetic elements offers a potentially tractable route to introduce foreign genes to schistosomes, a means to determine the importance of schistosome genes, including those that could be targeted in novel interventions and the potential to undertake large-scale forward genetics by insertional mutagenesis.

Neuropeptide signalling systems in flatworms

Two distinct families of neuropeptides are known to endow platyhelminth nervous systems - the FMRFamide-like peptides (FLPs) and the neuropeptide Fs (NPFs). Flatworm FLPs are structurally simple, each 4-6 amino acids in length with a carboxy terminal aromatic-hydrophobic-Arg-Phe-amide motif. Thus far, four distinct flatworm FLPs have been characterized, with only one of these from a parasite. They have a widespread distribution within the central and peripheral nervous system of every flatworm examined, including neurones serving the attachment organs, the somatic musculature and the reproductive system. The only physiological role that has been identified for flatworm FLPs is myoexcitation. Flatworm NPFs are believed to be invertebrate homologues of the vertebrate neuropeptide Y (NPY) family of peptides. Flatworm NPFs are 36-39 amino acids in length and are characterized by a caboxy terminal GRPRFamide signature and conserved tyrosine residues at positions 10 and 17 from the carboxy terminal. Like FLPs, NPF occurs throughout flatworm nervous systems, although less is known about its biological role. While there is some evidence for a myoexcitatory action in cestodes and flukes, more compelling physiological data indicate that flatworm NPF inhibits cAMP levels in a manner that is characteristic of NPY action in vertebrates. The widespread expression of these neuropeptides in flatworm parasites highlights the potential of these signalling systems to yield new targets for novel anthelmintics. Although platyhelminth FLP and NPF receptors await identification, other molecules that play pivotal roles in neuropeptide signalling have been uncovered. These enzymes, involved in the biosynthesis and processing of flatworm neuropeptides, have recently been described and offer other distinct and attractive targets for therapeutic interference.

An improved and secreted luciferase reporter for schistosomes

Schistosomes are multicellular parasites of humans exhibiting interesting biological adaptations to their parasitic lifestyle. Concerted and in depth analyses of these adaptations and their cell and molecular biology requires further development of molecular genetic tools in schistosomes. In the current study, we demonstrate that a Gaussia luciferase reporter leads to significantly higher levels of luciferase activity in schistosomes compared to other tested luciferases. In addition, Gaussia luciferase can be secreted into culture media enabling non-invasive analysis of reporter activity. The secretion of Gaussia luciferase should allow a variety of new experimental paradigms for schistosome studies. Comparison of biolistic and electroporation transfection methods using luciferase RNA reporters and the luciferase acitivty produced indicates that electroporation of sporocysts and schistosomula is the most efficient transfection method for the four stages analyzed. These data should facilitate additional studies in schistosomes and provide a framework for further development of DNA transfection and gene expression analysis.

piggyBac transposon mediated transgenesis of the human blood fluke, Schistosoma mansoni

The transposon piggyBac from the genome of the cabbage looper moth Trichoplusia ni has been observed in the laboratory to jump into the genomes of key model and pathogenic eukaryote organisms including mosquitoes, planarians, human and other mammalian cells, and the malaria parasite Plasmodium falciparum. Introduction of exogenous transposons into schistosomes has not been reported but transposon-mediated transgenesis of schistosomes might supersede current methods for functional genomics of this important human pathogen. In the present study we examined whether the piggyBac transposon could deliver reporter transgenes into the genome of Schistosoma mansoni parasites. A piggyBac donor plasmid modified to encode firefly luciferase under control of schistosome gene promoters was introduced along with 7-methylguanosine capped RNAs encoding piggyBac transposase into cultured schistosomula by square wave electroporation. The activity of the helper transposase mRNA was confirmed by Southern hybridization analysis of genomic DNA from the transformed schistosomes, and hybridization signals indicated that the piggyBac transposon had integrated into numerous sites within the parasite chromosomes. piggyBac integrations were recovered by retrotransposon-anchored PCR, revealing characteristic piggyBac TTAA footprints in the vicinity of the endogenous schistosome retrotransposons Boudicca, SR1, and SR2. This is the first report of chromosomal integration of a transgene and somatic transgenesis of this important human pathogen, in this instance accomplished by mobilization of the piggyBac transposon.

Schistosoma mansoni: germ-line transformation approaches and actin-promoter analysis

Towards germ-line transformation miracidia were biolistically transformed with GFP reporter gene constructs and successfully reintroduced into the schistosome cycle. By PCR and confocal microscopy the presence and the expression of GFP were confirmed in cercariae or adults of the F(0) and F(1) generations. This indicated the presence of the constructs in the germ-line, although no evidence for genome integration was obtained. About 3kb of 5' upstream sequences of the actin gene SmAct1 were identified by in silico analyses, and different fragments up to 1.5kb subcloned for GFP-vector construction. A 445bp fragment was sufficient for transcription initiation in larvae or adults as confirmed by confocal microscopy. An actin gene characteristic assembly of TATA, CArG, and CAAT boxes has been identified, which seems to be functionally conserved between vertebrates and invertebrates. However, a vertebrate-specific intron containing an additional regulatory CArG box was not found indicating that the regulation of SmAct1 transcription depends exclusively on its promoter region.

Genetic manipulation of schistosomes

In contrast to the situations with model organisms and parasitic protozoa, progress with gene manipulation with schistosomes has been delayed by impediments that include our inability to maintain the life cycle in vitro, absence of immortalized cell lines, large genome sizes, unavailability of drug resistance markers and other difficulties. However, in the past few years, tangible progress has been reported towards development of tools for gene manipulation and transgenesis of schistosomes, and there is reason to believe that the field is on the verge of transformation into an era where genetic manipulation is routine. Recent reports dealing with approaches and tools to manipulate the genome and gene expression in schistosomes are reviewed here.

Manipulating the manipulators: advances in parasitic helminth transgenesis and RNAi

Because tropical medicine and parasitology research has moved into the postgenomic era, an enormous amount of gene sequence information for parasitic helminths is now accumulating. These sequences undoubtedly hold information that can be used for new interventions and control. However, to exploit the new resource, methods for gene manipulation and transformation of parasitic worms are needed. Until recently, gene manipulation approaches had not been seriously addressed. This situation is now changing in response to the availability of genome sequences and other advances. In this article, we review advances in the transgenesis and gene silencing of parasitic worms.

Schistosoma mansoni TGF-beta receptor II: role in host ligand-induced regulation of a schistosome target gene

Members of transforming growth factor-beta (TGF-β) superfamily play pivotal roles in development in multicellular organisms. We report the functional characterization of the Schistosoma mansoni type II receptor (SmTβRII). Mining of the S. mansoni expressed sequence tag (EST) database identified an EST clone that shows homology to the kinase domain of type II receptors from different species. The amplified EST sequence was used as a probe to isolate a cDNA clone spanning the entire coding region of a type II serine/threonine kinase receptor. The interaction of SmTβRII with SmTβRI was elucidated and shown to be dependent on TGF-β ligand binding. Furthermore, in the presence of human TGF-β1, SmTβRII was able to activate SmTβRI, which in turn activated SmSmad2 and promoted its interaction with SmSmad4, proving the transfer of the signal from the receptor complex to the Smad proteins. Gynaecophoral canal protein (GCP), whose expression in male worms is limited to the gynaecophoric canal, was identified as a potential TGF-β target gene in schistosomes. Knocking down the expression of SmTβRII using short interfering RNA molecules (siRNA) resulted in a concomitant reduction in the expression of GCP. These data provide evidence for the direct involvement of SmTβRII in mediating TGF-β–induced activation of the TGF-β target gene, SmGCP, within schistosome parasites. The results also provide additional evidence for a role for the TGF-β signaling pathway in male-induced female reproductive development.

Schistosomes are multicellular parasites that infect 200 million people worldwide. Schistosome development in the human host likely involves host molecules that regulate biological processes of the parasite. Members of transforming growth factor-beta (TGF-β) superfamily play pivotal roles in development in multicellular organisms. TGF-β signaling requires ligand binding to a specific surface receptor, TGF-β type II receptor. The authors isolated the schistosome TGF-β type II receptor (SmTβRII), which was found to be biologically active and responded to stimulation by host TGF-β. The gynaecophoric canal is a ventral groove in the male worm in which the female must reside for sexual maturity. Gynaecophoral canal protein (GCP) is a protein whose expression in male worms is limited to the gynaecophoric canal and is implicated in female reproductive maturation. GCP expression was found to be regulated by human TGF-β. Knocking down the expression of SmTβRII resulted in a concomitant reduction in the expression of GCP, providing evidence for the direct involvement of SmTβRII-mediated, host TGF-β–induced regulation of schistosome gene expression. This study implicates the TGF-β signaling pathway in worm pairing, a prerequisite for female egg production. Because the eggs produced by the worm pairs are responsible for pathogenesis, the authors' research identifies potential targets for intervention.

Transduction of Schistosoma mansoni by vesicular stomatitis virus glycoprotein-pseudotyped Moloney murine leukemia retrovirus

Retroviral transduction of cultured schistosomes offers a potential means to establish transgenic lines of schistosomes and thereby to facilitate the elucidation of schistosome gene function and expression. The Moloney murine leukemia retroviral (MMLV) vector pLNHX was modified to incorporate EGFP or luciferase reporter genes under control of schistosome endogenous gene promoters from the spliced leader RNA and HSP70 genes. These constructs and a plasmid encoding vesicular stomatitis virus glycoprotein (VSVG) were utilized along with GP2-293 cells to produce replication incompetent retrovirus particles pseudotyped with the VSVG envelope. Exposure of several developmental stages, including sporocysts, of Schistosoma mansoni to these virions was facilitated by incubation with polybrene and/or by centrifugation. The early stages of binding and uptake of virus to the parasite tegument were demonstrated by the immunofluorescence colocalization of VSVG envelope and retroviral capsid proteins. Southern hybridization analysis indicated the integration of proviral forms of the MMLV constructs in genomic DNA isolated from the virus exposed schistosomes. Furthermore, analysis of RNA isolated from virus treated parasites demonstrated the presence of transcripts encoding reporter transgenes. Together these results indicated productive transduction by VSVG pseudotyped MMLV of cultured schistosomes, and suggest a tractable route forward towards heritable schistosome transgenesis.

Schistosoma japonicum: A method for transformation by electroporation

Despite of our knowledge of genetic make up of schistosomes, a number of genes have not been characterized largely due to lack of effective transformation protocols. Here we present electroporation as a strategy for effective introduction of plasmids DNA into schistosomula and adults. Using plasmids of pEGFP-C1 as an expression vector, we first verified that the CMV promoter could direct EGFP to express in primary culture cells from Schistosoma japonicum. Subsequently, the plasmids were introduced into schistosomula and adults by electroporation and EGFP expression was demonstrated using molecular and microscopical methods. Our findings indicate that electroporation is an effective method for transformation of S. japonicum.

Organization and functional analysis of the Schistosoma mansoni cathepsin D-like aspartic protease gene promoter

We have cloned a 969-bp fragment of genomic DNA that spans 821 bp of the 5' untranslated region, exon 1, a short intron, and part of exon 2 of the Schistosoma mansoni cathepsin D gene by inverse PCR. Inspection of this sequence revealed the presence of two TATA-box motifs, two inverted CCAAT-box (inverted NF-Y) motifs and sequences with homology to binding sites for the transcription factors, AP-1 and NF-Y. This sequence and deletion variants were cloned into reporter gene constructs, in order to examine the ability of these putative regulatory sequences to drive heterologous reporter gene activity. PCR products were cloned into the luciferase reporter vector pXP2. These reporter gene constructs were used to transform HeLa cells which were cultured and examined for luciferase activity. Additionally, HeLa cells transiently transfected with an EGFP reporter plasmid driven by the putative promoter from the S. mansoni cathepsin D gene were examined for EGFP transcripts and fluorescence. The 5' untranslated region of the S. mansoni cathepsin D gene, from position -772 to +40 (translation start ATG), included functional regulatory sequences capable of driving luciferase and EGFP expression, whereas shorter fragments from position -264 or -185 to +40 were insufficient to drive reporter activities.

Genome and genomics of schistosomes

Schistosomes infect over 200 million people and 600 million are at risk. Genomics and post-genomic studies of schistosomes will contribute greatly to developing new reagents for diagnostic purposes and new vaccines that are of interest to the biotechnology industry. In this review, the most recent advances in these fields as well as new projects and future perspectives will de described. A vast quantity of data is publicly available, including short cDNA and genomic sequences, complete large genomic fragments, and the mitochondrial genomes of three species of the genus Schistosoma. The physical structure of the genome is being studied by physically mapping large genomic fragments and characterizing the highly abundant repetitive DNA elements. Bioinformatic manipulations of the data have already been carried out, mostly dealing with the functional analysis of the genes described. Specific search tools have also been developed. Sequence variability has been used to better understand the phylogeny of the species and for population studies, and new polymorphic genomic markers are currently being developed. The information generated has been used for the development of post-genomic projects. A small microarray detected genes that were differentially expressed between male and female worms. The identification of two-dimensional spots by mass spectrometry has also been demonstrated.