Characterization of the Schistosoma mansoni gene encoding the glycolytic enzyme, triosephosphate isomerase

Pathogen-host interaction mediated by vesicle-based secretion in schistosomes

As part of the parasite's excretory/secretory system, extracellular vesicles (EVs) represent a potent communication tool of schistosomes with their human host to strike the balance between their own survival in a hostile immunological environment and a minimal damage to the host tissue. Their cargo consists of functional proteins, lipids, and nucleic acids that facilitate biological processes like migration, nutrient acquisition, or reproduction. The most important impact of the vesicle-mediated communication, however, is the promotion of the parasite survival via mimicking host protein function and directly or indirectly modulating the immune response of the host. Overcoming this shield of immunological adaption in the schistosome-host relation is the aim of current research activities in this field and crucial for the development of a reliable anti-schistosomal therapy. Not least because of their prospective use in clinical applications, research on EVs is now a rapidly expanding field. We herein focus on the current state of knowledge of vesicle-based communication of schistosomes and discussing the role of EVs in facilitating biological processes and immune modulatory properties of EVs considering the different life stages of the parasite.

Culture of cells from two life stages of Schistosoma mansoni

The ability to culture continuously proliferating cell lines of various organisms in vitro has provided numerous advantages in experimental approaches toward the understanding of basic biology and disease. Although in vitro approaches are common in many disciplines, this methodology has proven difficult to exploit in the study of helminthic parasites. A major cause of parasitic disease, particularly in tropical countries, is the trematode Schistosoma mansoni. We have developed in vitro techniques that allow the long term maintenance of cell cultures from two stages of the life cycle of this organism, associated with its mammalian and the molluscan hosts. We have developed quantitative assays of cell survival and proliferation in our culture systems, and obtained evidence for limited proliferation in vitro. Although the cultures we have achieved thus far are useful for many kinds of experiments in vitro, development of continuously proliferating cell lines remains our goal.

Excretory-secretory proteome of larval Schistosoma mansoni and Echinostoma caproni, two parasites of Biomphalaria glabrata

Schistosoma mansoni and Echinostoma caproni are two trematode species that use different strategies (mimicry and immunosuppression, respectively) to interfere with the snail innate immune system. Parasites excretory-secretory (ES) products have been shown to play a key role in these host-parasite immune interactions. However, they remain largely uncharacterized in larval trematodes. We developed a global proteomic approach to characterize the ES proteome of S. mansoni and E. caproni primary sporocysts. In ES products of both parasites, we found proteins involved in reactive oxygen species scavenging, glycolysis, signalling or calcium binding (superoxide dismutase Cu/Zn; glutathione S-transferase; aldo-keto-reductase; triose-phosphate isomerase; glyceraldehyde-3-phosphate dehydrogenase; aldolase, enolase, MICAL-like, calreticulin). According to their predicted functions, we propose a model in which these proteins (i) are involved in antioxidant activity, (ii) prevent hemocyte encapsulation process or (iii) favor invasion and migration of sporocysts in host tissues. These results suggest that S. mansoni and E. caproni sporocysts develope a strong immune protection during the first hours of infection giving them enough time to build up a long lasting immune evasion strategy relying on molecular mimicry or immunosuppression, respectively.

Cloning, expression and partial characterization of a gene encoding the S15a ribosomal protein of Taenia solium

Ribosomes, ribosomal proteins (r-proteins), and messenger and transfer RNAs catalyze the synthesis of proteins in organisms. To understand and define the components involved in this event in Taenia solium, we isolated and characterized a T. solium cDNA encoding the basic ribosomal protein S15a (TsS15a). The TsS15a cDNA produces a protein with M(r) (relative molecular mass) 14,988, which contains 22.3% of basic amino acids. Analysis comparing TsS15a protein with other S15a r-proteins indicates that this protein is highly conserved. A recombinant TsS15a protein with similar M(r) was produced in bacteria. Antibodies against recombinant TsS15a react with a 15-kDa protein in extracts from all life stages of T. solium and from all helminths tested. Hybridization studies showed the presence of two genes encoding a mRNA of 0.5 kb. Moreover, the gene presents an intron of 30 bp. Our phylogenetic analysis using S15a r-proteins reproduced the topologies reported for 16/18S rRNA.

Human immune responses to Schistosoma mansoni vaccine candidate antigens

To determine the naturally occurring immunological responses to the Schistosoma mansoni antigens paramyosin, IrV-5, Sm-23 (MAP-3), and triose phosphate isomerase (MAP-4), a total of 119 subjects from an area of endemicity for schistosomiasis, including "resistant" subjects (n = 17) were evaluated. Specific immunoglobulin G1 (IgG1), IgG2, IgG3, IgG4, and IgA levels for each of the antigens and the cytokine profile in culture supernatants from antigen-stimulated peripheral blood mononuclear cells (PBMC) were determined. Although all the subjects had a high degree of contaminated water exposure, their infection levels were variable (0 to 1,128 eggs/g of stool). There were direct correlations between infection levels and levels of SWAP- and paramyosin-specific IgG1 and IgG4 (P < 0.05). However, an inverse correlation between infection levels and specific IgG2 to IrV-5 (P < 0.01) was observed. The evaluation of the cytokine profile (interleukin 5 [IL-5], IL-10, gamma interferon [IFN-gamma], and tumor necrosis factor alpha) in response to these antigens showed inverse correlations between the degree of infection and IFN-gamma levels in PBMC supernatants stimulated with paramyosin (P < 0.05) and IrV-5 (P < 0.01). Additionally, inverse correlations between the degree of infection and IL-5 levels in MAP-3- and MAP-4-stimulated PBMC supernatants (P < 0.01) were found. Logistic regression analysis was performed to adjust the results of cytokine profile by age. IL-5 production in MAP-3-stimulated PBMC supernatants was associated with lower infection levels (odds ratio = 11.2 [95% confidence interval, 2.7 to 45.8]).

Schistosoma mansoni: differential expression of cathepsins L1 and L2 suggests discrete biological functions for each enzyme

Schistosoma mansoni cathepsins L1 (SmCL1) and L2 (SmCL2) were expressed as active recombinant proteinases in Saccharomyces cerevisiae. The recombinant enzymes exhibited substrate preferences characteristic of cathepsin-L-like cysteine proteinases. However, the enzymes differed in their substrate specificities; SmCL1 cleaved Boc-Val-Leu-Lys-NHMec with a higher efficiency than it cleaved Z-Phe-Arg-NHMec, whereas the opposite was true for SmCL2. The enzymes also differed in their pH profiles of activity; SmCL1 exhibited a broad pH profile with an optimum of pH 6. 5, while SmCL2 was active only in the acidic pH range with an optimum of 5.35. Immunoblot and RT-PCR analyses revealed that the native forms of both SmCL1 and SmCL2 are expressed in male and female worms, but at higher levels in adult female compared to male schistosomes. Additionally, both enzymes were observed in the excretory/secretory products of adult worms. The RT-PCR analysis indicated that neither enzyme is expressed in S. mansoni eggs or in miracidia, suggesting that the cathepsin-L-like activity that has been previously reported to be expressed in these stages may be the product of another gene(s). Cercariae do not express SmCL2, but appear to express SmCL1 in its inactive precursor form. Together with the findings of previous immunolocalization and phylogenetic analyses, the results reported here demonstrate that SmCL1 and SmCL2 are distinct cathepsin cysteine proteinases and strongly suggest that they play discrete biological roles.

Surprising diversity and distribution of spliced leader RNAs in flatworms

Trans-splicing generates the mature 5' ends of certain mRNAs through the addition of a small spliced leader (SL) exon to pre-mRNAs. To search for novel flatworm spliced leaders, degenerate oligonucleotides and 5' RACE [corrected was used to isolate and characterize the 5' terminal sequences of enolase mRNAs in diverse flatworms. Several new spliced leaders and their SL RNA genes were identified, characterized, and compared. All parasitic trematodes examined trans-splice enolase. A primitive polyclad turbellarian, Stylochus zebra, also contains a trans-spliced enolase mRNA. The S. zebra SL is the longest SL yet identified, 51 nucleotides. Comparison of flatworm SLs indicates that they vary significantly in sequence and length. This suggests that neither spliced leader exon sequence nor size is likely to be essential for trans-splicing in flatworms. Flatworm SL RNAs have unusual Sm binding sites with characteristics distinct from other known flatworm snRNA Sm binding sites. Predicted flatworm SL RNA secondary structures show variation exhibiting 2-4 stem loops. Although limited in sequence similarity, phylogenetically conserved regions within the diverse flatworm SL RNAs suggest that they are likely to be derived from a common ancestor and provide information on potentially important SL RNA elements. The identification of a SL in a primitive flatworm suggests that trans-splicing may have been an ancestral feature in the phylum. Representative species of other early and more recent clades within the phylum, however, do not trans-splice enolase, nor do they or representatives of several other flatworm groups, have an SL RNA with a phylogenetically conserved region identified in the current study.

Cloning the gene encoding Schistosoma mansoni p50, an immunophilin

A 2.2-kb fragment of genomic DNA encoding Schistosoma mansoni immunophilin p50 (Smp50) was identified on a 14-kb genomic clone. The sequence of Smp50 reveals seven exons interrupted by six small introns ranging from 28-35 bp in size. The transcription start point, defined by primer extension analysis of schistosome RNA, begins at 30 bp upstream from the start AUG codon. Smp50 lacks a TATA box and appears to be a single-copy gene.

A rab-related GTP-binding protein in Schistosoma mansoni

Schistosomiasis is a parasitic disease initiated by the deposition of eggs in host tissues by the blood fluke Schistosoma. A gene encoding a low-molecular weight GTP-binding protein (LMWGP) was cloned from Schistosoma mansoni using a polymerase chain reaction (PCR)-based strategy. The gene was termed smrab (Schistosoma mansoni rab-related protein). Northern blot analysis hybridizes smrab cDNA with a 1.5-kb band of mRNA; this mRNA is abundantly expressed in male schistosomes, while only slightly in females. The deduced amino acid sequence of smrab shares ca. 70% homology with that of several rab-related LMWGPs. Smrab terminates in a CCXXX motif, which is one of several signals for post-translational isoprenoid modification by geranylgeranyl protein transferase (GGPT) type II. A GGPT assay with in vitro translation product confirms that smrab is geranylgeranylated. Recombinant expression of smrab in the pET3a expression vector yields insoluble protein which migrates as a 23-kDa band on SDS-PAGE. N-terminal sequence information of the recombinant protein matches the predicted amino acid sequence of smrab. GTP-binding analysis indicates that the recombinant protein binds GTP. Therefore, smrab meets the criteria recently established for acceptance into the ras superfamily of GTP-binding proteins (Kahn, R.A., Der, C.J. and Bokoch, G.M. (1992) The ras superfamily of GTP-binding proteins: guidelines on nomenclature. FASEB J. 6, 2512-2513, Ref. [21]).

Molecular cloning and expression of the gene encoding a cysteine proteinase of Spirometra erinacei

A cDNA library constructed from plerocercoid of Spirometra erinacei (SEP) was immunoscreened using rabbit anti-plerocercoid proteinase polyclonal antibody. A 1.0-kb cDNA clone encoding a cysteine proteinase composed of 336 amino acids was isolated. The amino acid sequence predicted from the cDNA showed significant homology with human and mouse cathepsin L. N-terminal amino acid sequence of the native cysteine proteinase extracted from SEP was the same as that of mature proteinase predicted from the cloned gene. The gene encoding the proteinase was characterized by Southern and Northern blot analysis using the cDNA as a probe. The proteinase with a molecular mass of 34 kDa was demonstrated in in vitro translation products using anti-proteinase polyclonal antibody. A fusion protein derived from the cDNA synthesized by Escherichia coli (TB1) using the expression vector, pMAL-c2 was identified as an immunodominant antigen by epitope-selection method and had no cross-reactivity with other parasite-infected sera. A genomic DNA library derived from SEP was screened by the colony hybridization technique using the cDNA probe. A gene with 4.5 kb encoding the proteinase was obtained, which comprised three exons and two introns.

Using evolutionary changes to achieve species-specific inhibition of enzyme action--studies with triosephosphate isomerase

BACKGROUND

Many studies that attempt to design species-specific drugs focus on differences in the three-dimensional structures of homologous enzymes. The structures of homologous enzymes are generally well conserved especially at the active site, but the amino-acid sequences are often very different. We reasoned that if a non-conserved amino acid is fundamental to the function or stability of an enzyme from one particular species, one should be able to inhibit only the enzyme from that species by using an inhibitor targeted to that residue. We set out to test this hypothesis in a model system.

RESULTS

We first identified a non-conserved amino acid (Cys14) whose integrity is important for catalysis in triosephosphate isomerase (TIM) from Trypanosoma brucei. The equivalent residues in rabbit and yeast TIM are Met and Leu, respectively. A Cys14Leu mutant of trypanosomal TIM had a tendency to aggregate, reduced stability and altered kinetics. To model the effects of a molecule targeted to Cys14, we used methyl methanethiosulfonate (MMTS) to derivatize Cys14 to a methyl sulfide. This treatment dramatically inhibited TIMs with a Cys residue at a position equivalent to Cys14, but not rabbit TIM (20% inhibition) or yeast TIM (negligible inhibition), which lack this residue.

CONCLUSIONS

Cys14 of trypanosomal TIM is a non-conserved amino acid whose alteration leads to loss of enzyme structure and function. TIMs that have a cysteine residue at position 14 could be selectively inhibited by MMTS. This approach may offer an alternative route to species-specific enzyme inhibition.

RNA trans-splicing in flatworms. Analysis of trans-spliced mRNAs and genes in the human parasite, Schistosoma mansoni

Characteristics of trans-splicing in Schistosoma mansoni were examined to explore the significance and determinants of spliced leader (SL) addition in flatworms. Only a small subset of mRNAs acquire the SL. Analysis of 30 trans-spliced mRNAs and four genes revealed no discernable patterns or common characteristics in the genes, mRNAs, or their encoded proteins that might explain the functional significance of SL addition. While the mRNA encoding the glycolytic enzyme enolase is trans-spliced, mRNAs encoding four other glycolytic enzymes are not, indicating trans-splicing is not prevalent throughout this metabolic pathway. Although the 3' end of flatworm SLs contribute an AUG to mRNAs, the SL AUG does not typically serve to provide a methionine for translation initiation of reading frames in recipient mRNAs. SL RNA expression exhibits no apparent sex, tissue, or cell specificity. Trans-spliced genes undergo both cis- and trans-splicing, and the sequence contexts for these respective acceptor sites are very similar. These results suggest trans-splicing in flatworms is most likely associated either with some property conferred on recipient mRNAs by SL addition or related to some characteristic of the primary transcripts or transcription of trans-spliced genes.

Evidence against the exon theory of genes derived from the triose-phosphate isomerase gene

The exon theory of genes proposes that the introns of protein-encoding nuclear genes are remnants of the DNA spacers between ancient minigenes. The discovery of an intron at a predicted position in the triose-phosphate isomerase (EC 5.3.1.1) gene of Culex mosquitoes has been hailed as an evidential pillar of the theory. We have found that that intron is also present in Aedes mosquitoes, which are closely related to Culex, but not in the phylogenetically more distant Anopheles, nor in the fly Calliphora vicina, nor in the moth Spodoptera littoralis. The presence of this intron in Culex and Aedes is parsimoniously explained as the result of an insertion in a recent common ancestor of these two species rather than as the remnant of an ancient intron. The absence of the intron in 19 species of very diverse organisms requires at least 10 independent evolutionary losses in order to be consistent with the exon theory.

Dinucleotide biases in the platyhelminth Schistosoma mansoni

The analysis of dinucleotide biases in coding and flanking regions, introns, rDNA and repetitive sequences, in the flatworm Schistosoma mansoni is reported. Except for rDNA, all regions display CpG avoidance and TpG plus CpA excess, which might be evidence of the presence of 5mC. The distribution and hierarchies of dinucleotides differ from the data published for invertebrate and vertebrate coding sequences.