Transcriptome analysis of the acoelomate human parasite Schistosoma mansoni

Schistosomiasis control: keep taking the tablets

Despite the limited reports of praziquantel resistance, the relative success of chemotherapy-based control programmes for schistosomiasis has prompted overdue efforts to expand the use of cheap, generic, praziquantel in sub-Saharan Africa. The likely impact of such programmes on the development and spread of praziquantel resistance is uncertain, but this possibility reinforces the need for monitoring the spectrum of praziquantel sensitivity of schistosome populations and for an improved knowledge of the precise targets for the action of the drug. The search for alternatives to praziquantel and other tools for control of schistosomiasis must continue.

Schistosomiasis vaccines

While there are many challenges in vaccine development, none is greater than that of developing vaccines against large metazoan parasites such as schistosomes, the parasitic worms that are responsible for schistosomiasis. Initial optimism stemming from the identification of the first vaccine candidate antigens that gave protection in animals has been dashed by the failure, as yet, of any of the vaccine candidate antigens to enter Phase III clinical trials. Now, despite an improved understanding of the biology of the parasites and of the immune responses they stimulate in naturally exposed populations, the vaccine effort is stalled. The control effort has switched heavily in favour of the wider use of conventional chemotherapy with praziquantel, which is now affordable by all but the poorest countries. Disagreements among researchers in the schistosome field as to whether or not a vaccine is needed have not helped convince funding agencies that schistosomiasis vaccines, rather than drugs, should be a priority. With the schistosome genome projects at an advanced stage plus the power of the proteomics, perhaps it is still too early to call time on schistosome vaccine development.

Linked thioredoxin-glutathione systems in platyhelminths

The thioredoxin and glutathione systems play a central role in thiol-disulfide redox homeostasis in many organisms by providing electrons to essential enzymes, and defence against oxidative stress. These systems have recently been characterized in platyhelminth parasites, and the emerging biochemical scenario is the existence of linked processes with the enzyme thioredoxin glutathione reductase supplying reducing equivalents to both pathways. In contrast to their hosts, conventional thioredoxin reductase and glutathione reductase enzymes appear to be absent. Analysis of published data and expressed-sequence tag databases indicates the presence of linked thioredoxin-glutathione systems in the cytosolic and mitochondrial compartments of these parasites.

Merlin, a new superfamily of DNA transposons identified in diverse animal genomes and related to bacterial IS1016 insertion sequences

Several new families of DNA transposons were identified by computer-assisted searches in a wide range of animal species that includes nematodes, flat worms, mosquitoes, sea squirt, zebrafish, and humans. Many of these elements have coding capacity for transposases, which are related to each other and to those encoded by the IS1016 group of bacterial insertion sequences. Although these transposases display a motif similar to the DDE motif found in many transposases and integrases, they cannot be directly allied to any of the previously described eukaryotic transposases. Other common features of the new eukaryotic and bacterial transposons include similarities in their terminal inverted repeats and 8-bp or 9-bp target-site duplications. Together, these data indicate that these elements belong to a new superfamily of DNA transposons, called Merlin/IS1016, which is common in many eubacterial and animal genomes. We also present evidence that these transposons have been recently active in several animal species. This evidence is particularly strong in the parasitic blood fluke Schistosoma mansoni, in which Merlin is also the first described DNA transposon family.

A vaccine against Asian schistosomiasis

There is continued transmission of schistosomiasis japonica in China and Philippines despite highly effective control programs that focus on the application of the highly effective drug praziquantel (PZQ). The massive Three Gorges Dam across the Yangtze River in Southern China, soon to be completed, is expected to significantly increase schistosomiasis transmission and introduce the disease into areas currently unaffected. After long-term experience it is generally accepted that PZQ chemotherapy, although the cornerstone of current control programs, does have significant limitations. Furthermore, efficient drug delivery requires a substantial infrastructure to regularly cover all parts of an endemic area. Although there is not yet clear-cut evidence for the existence of PZQ-resistant schistosome strains, decreased susceptibility to the drug has been observed in several countries. As a result, a protective vaccine represents an essential component for the long-term control of schistosomiasis. This article briefly reviews aspects of anti-schistosome protective immunity that are important in the context of vaccine development. The current status in the development of vaccines against Schistosoma japonicum will then be discussed as will new approaches that may improve on the efficacy of available vaccines, and aid in the identification of new targets for immune attack. With new and extensive data becoming available from the S. japonicum genome project, the prospects for developing an effective vaccine are encouraging. The challenges that remain are many but it is crucial that the momentum towards developing effective anti-schistosome vaccines is maintained.

Trypanosoma rangeli Transcriptome Project: Generation and analysis of expressed sequence tags

Trypanosoma rangeli is an important hemoflagellate parasite of several mammalian species in Central and South America, sharing geographical areas, vectors and reservoirs with T. cruzi, the causative agent of Chagas disease. Thus, the occurrence of single and/or mixed infections, including in humans, must be expected and are of great importance for specific diagnosis and epidemiology. In comparison to several Trypanosomatidae species, the T. rangeli biology and genome are little known, reinforcing the needs of a gene discovery initiative. The T. rangeli transcriptome initiative aims to promote gene discovery through the generation of expressed sequence tags (ESTs) and Orestes (ORF ESTs) from both epimastigote and trypomastigote forms of the parasite, allowing further studies of the parasite biology, taxonomy and phylogeny.

Schistosome transcriptomes: new insights into the parasite and schistosomiasis

Schistosomiasis is one of the most serious parasitic diseases. More than 250 million people are infected with schistosomes in the tropics or subtropics. The parasitic flukes have some unique biological features: dioecism, complex life cycles, mechanisms to avoid host immune responses, and an apparent reliance on host endocrine and immune signals to complete their development, maturation and egg production. Recently, a large dataset of expressed sequence tags (ESTs) were generated from Schistosoma japonicum and Schistosoma mansoni, from which numerous novel genes were identified. The transcriptome analyses provide the basis for a comprehensive understanding of the molecular mechanisms involved in schistosome nutrition and metabolism, host-dependent development and maturation, immune evasion and invertebrate evolution. In addition, new potential vaccine candidates and drug targets have been predicted.

Leucine aminopeptidase of the human blood flukes, Schistosoma mansoni and Schistosoma japonicum

An array of schistosome endoproteases involved in the digestion of host hemoglobin to absorbable peptides has been described, but the exoprotease responsible for catabolising these peptides to amino acids has yet to be identified. By searching the public databases we found that Schistosoma mansoni and Schistosoma japonicum express a gene encoding a member of the M17 family of leucine aminopeptidases (LAPs). A functional recombinant S. mansoni LAP produced in insect cells shared biochemical properties, including pH optimum for activity, substrate specificity and reliance on metal cations for activity, with the major aminopeptidase activity in soluble extracts of adult worms. The pH range in which the enzyme functions and the lack of a signal peptide indicate that the enzyme functions intracellularly. Immunolocalisation studies showed that the S. mansoni LAP is synthesised in the gastrodermal cells surrounding the gut lumen. Accordingly, we propose that peptides generated in the lumen of the schistosome gut are absorbed into the gastrodermal cells and are cleaved by LAP to free amino acids before being distributed to the internal tissues of the parasite. Since LAP was also localised to the surface tegument it may play an additional role in surface membrane re-modelling.

Genomics of parasitic flatworms

Although we live in what is often touted as the 'post-genomic era', this term is hardly appropriate when we consider the paucity of knowledge of the genomic biology of parasitic flatworms. The situation is, however, changing-at least for two species of Schistosoma. Recent transcriptome analysis of Schistosoma mansoni and of Schistosoma japonicum has identified novel genes and genes not previously reported for schistosomes, as well as the identification of the molecular mechanisms for host-dependent maturation, immune evasion, development, signalling and sexual dimorphism. The analyses also identify potential vaccine candidates and drug targets. Here, the current state of knowledge is reviewed for mitochondrial and nuclear genomes of parasitic flatworms. We highlight the remarkable recent progress in gene discovery for schistosomes and the goal of sequencing complete schistosome genomes.

Saci-1, -2, and -3 and Perere, four novel retrotransposons with high transcriptional activities from the human parasite Schistosoma mansoni

Using the data set of 180,000 expressed sequence tags (ESTs) of the blood fluke Schistosoma mansoni generated recently by our group, we identified three novel long-terminal-repeat (LTR)- and one novel non-LTR-expressed retrotransposon, named Saci-1, -2, and -3 and Perere, respectively. Full-length sequences were reconstructed from ESTs and have deduced open reading frames (ORFs) with several uncorrupted features, characterizing them as possible active retrotransposons of different known transposon families. Alignment of reconstructed sequences to available preliminary genome sequence data confirmed the overall structure of the transposons. The frequency of sequenced transposon transcripts in cercariae was 14% of all transcripts from that stage, twofold higher than that in schistosomula and three- to fourfold higher than that in adults, eggs, miracidia, and germ balls. We show by Southern blot analysis, by EST annotation and tallying, and by counting transposon tags from a Serial Analysis of Gene Expression library, that the four novel retrotransposons exhibit a 10- to 30-fold lower copy number in the genome and a 4- to 200-fold-higher transcriptional rate per copy than the four previously described S. mansoni retrotransposons [corrected]. Such differences lead us to hypothesize that there are two different populations of retrotransposons in S. mansoni genome, occupying different niches in its ecology. Examples of retrotransposon fragment inserts were found into the 5' and 3' untranslated regions of four different S. mansoni target gene transcripts. The data presented here suggest a role for these elements in the dynamics of this complex human parasite genome.

From immune system to health systems--challenges for health research

There are many similarities between the immune system and health systems. Both are knowledge-based learning systems with an ultimate common objective of maintaining the health of individuals and populations, respectively. The immune system learns from the environment, and knowledge and discovery are key drivers of health systems improvement and improving health status. But discovery goes well beyond immunology, virology and genomics, for example, in the quest to develop new drugs, vaccines and diagnostic tests. New ways must also be discovered to make sure that these products reach the people who need it most. We must discover ways for knowledge to be turned into effective policies, to inform the practice of healthcare and to inform the public and society at large. Only if discovery is seen in its broadest perspective will we be able to reach the goal of health for all.

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.

Sexual Biology of Schistosomes

Schistosomes are unusual, together with some of the didymozoidae, in that they are dioecious instead of being hermaphrodite. This gonochorism is accompanied with morphological, ecological, behavioural and molecular differences between the male and the female parasites all through their life cycle. This review is an overview of the sexual biology of schistosomes and aims to provide the most recent information that may help to build future control strategies against these parasites. It proposes a new view of the life cycle of schistosomes, taking into account the sexual status of each developmental stage. It presents the relevant information available on the genetic and phenotypic sexual dimorphisms of these parasites; it proposes a comparison between the host-male parasite and the host-female parasite interactions in both the molluscan intermediate and the mammalian definitive hosts; it exposes the male-female parasite interactions that exist in both the mollusc and the mammalian hosts at the parasite individual and populational levels. This review highlights the domains of research that are still unexplored but that would be of great interest for a better knowledge of the sexual way of life of the parasites which are still responsible for one of the most important human parasitic diseases in the world.

Characterization of the Schistosoma transcriptome opens up the world of helminth genomics

Among the metazoan parasites that cause debilitating disease in man, schistosomes are the first group for which near-complete transcriptome complements have been described. This new genomic information will have an enormous impact on all future investigations into the biology, pathogenesis and control of schistosomiasis.

Among the metazoan parasites that cause debilitating disease in man, schistosomes are the first group for which near-complete transcriptome complements have been described. This new genomic information will have an enormous impact on all future investigations into the biology, pathogenesis and control of schistosomiasis.

Expression of functional Schistosoma mansoni Smad4: role in Erk-mediated transforming growth factor beta (TGF-beta) down-regulation

Members of the transforming growth factor (TGF)-beta superfamily play pivotal roles in cell migration, differentiation, adhesion, pattern formation, and apoptosis. The family of Smad proteins acts as intracellular signal transducers of TGF-beta and related peptides. Smad4, a common mediator Smad (co-Smad), performs a central role in transmitting signals from TGF-beta, BMP, and activins. Schistosoma mansoni receptor-regulated Smad1 and SmSmad2 were previously identified and shown to act in TGF-beta signaling. Herein, we report the identification and characterization of a Smad4 homologue from S. mansoni and provide details about its role in mediation and down-regulation of TGF-beta signaling in schistosomes. In order to identify the schistosome co-Smad, we designed degenerate primers based on the sequence of the conserved MH1/MH2 domains of Smad4 proteins, which were used in PCR to amplify a 137-bp PCR product. A S. mansoni adult worm pair cDNA library was screened resulting in the isolation of a cDNA clone that encodes a 738 amino acid protein (SmSmad4). SmSmad4 was shown to interact with schistosome R-Smads (SmSmad1 and SmSmad2) in vivo and in vitro. The interaction with SmSmad2 was dependent on the receptor-mediated phosphorylation of SmSmad2. In addition, several potential phosphorylation sites for Erk1/2 kinases were identified in the SmSmad4 linker region and shown to be phosphorylated in vitro by an active mutant of mammalian Erk2. Furthermore, Erk-mediated phosphorylation of SmSmad4 decreased its interaction with the receptor-activated form of SmSmad2, in vitro. SmSmad4 was shown to complement a human Smad4 deficiency through the restoration of TGF-beta-responsiveness in MDA-MB-468 breast cancer cells.