Cloning and sequence analysis of the Schistosoma mansoni membrane glycoprotein antigen gene GP22

Altered patterns of gene expression underlying the enhanced immunogenicity of radiation-attenuated schistosomes

BACKGROUND

Schistosome cercariae only elicit high levels of protective immunity against a challenge infection if they are optimally attenuated by exposure to ionising radiation that truncates their migration in the lungs. However, the underlying molecular mechanisms responsible for the altered phenotype of the irradiated parasite that primes for protection have yet to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

We have used a custom microarray comprising probes derived from lung-stage parasites to compare patterns of gene expression in schistosomula derived from normal and irradiated cercariae. These were transformed in vitro and cultured for four, seven, and ten days to correspond in development to the priming parasites, before RNA extraction. At these late times after the radiation insult, transcript suppression was the principal feature of the irradiated larvae. Individual gene analysis revealed that only seven were significantly down-regulated in the irradiated versus normal larvae at the three time-points; notably, four of the protein products are present in the tegument or associated with its membranes, perhaps indicating a perturbed function. Grouping of transcripts using Gene Ontology (GO) and subsequent Gene Set Enrichment Analysis (GSEA) proved more informative in teasing out subtle differences. Deficiencies in signalling pathways involving G-protein-coupled receptors suggest the parasite is less able to sense its environment. Reduction of cytoskeleton transcripts could indicate compromised structure which, coupled with a paucity of neuroreceptor transcripts, may mean the parasite is also unable to respond correctly to external stimuli.

CONCLUSIONS/SIGNIFICANCE

The transcriptional differences observed are concordant with the known extended transit of attenuated parasites through skin-draining lymph nodes and the lungs: prolonged priming of the immune system by the parasite, rather than over-expression of novel antigens, could thus explain the efficacy of the irradiated vaccine.

Patterns of gene expression in schistosomes: localization by whole mount in situ hybridization

As a consequence of comprehensive transcriptome analysis followed by sequencing and draft assembly of the genome, the emphasis of schistosome research is shifting from the identification of genes to the characterization of their functions and interactions. Developmental biologists have long used whole mount in situ hybridization (WISH) to determine gene expression patterns, as a vital tool for formulating and testing hypotheses about function. This paper describes the application of WISH to the study of gene expression in larval and adult schistosomes. Fixed worms were permeablized by proteinase K treatment for hybridization with digoxygenin-labelled RNA probes, with binding being detected by alkaline phosphatase-coupled anti-digoxygenin antibodies, and BM Purple substrate. Discrete staining patterns for the transcripts of the molecules Sm29, cathepsin L, antigen 10.3 and chorion were observed in the tegument cell bodies, gut epithelium, oesophageal gland and vitelline lobules, respectively, of adult worms. Transcripts of the molecules SGTP4, GP18-22 and cathepsin L were localized to tegument cell bodies and embryonic gut, respectively, of lung schistosomula. We also showed that Fast Red TR fluorescent substrate can refine the pattern of localization permitting use of confocal microscopy. We believe that method of WISH will find broad application, in synergy with other emerging post-genomic techniques, such as RNA interference, to studies focused at increasing our molecular understanding of schistosomes.

Cloning and characterization of SmZF1, a gene encoding a Schistosoma mansoni zinc finger protein

The zinc finger motifs (Cys2His2) are found in several proteins playing a role in the regulation of transcripton. SmZF1, a Schistosoma mansoni gene encoding a zinc finger protein was initially isolated from an adult worm cDNA library, as a partial cDNA. The full sequence of the gene was obtained by subcloning and sequencing cDNA and genomic fragments. The collated gene sequence is 2181 nt and the complete cDNA sequence is 705 bp containing the full open reading frame of the gene. Analysis of the genome sequence revealed the presence of three introns interrupting the coding region. The open reading frame theoretically encodes a protein of 164 amino acids, with a calculated molecular mass of 18,667Da. The predicted protein contains three zinc finger motifs, usually present in transcription regulatory proteins. PCR amplification with specific primers for the gene allowed for the detection of the target in egg, cercariae, schistosomulum and adult worm cDNA libraries indicating the expression of the mRNA in these life cycle stages of S. mansoni. This pattern of expression suggests the gene plays a role in vital functions of different life cycle stages of the parasite. Future research will be directed to elucidate the functional role of SmZF1.

Schistosoma mansoni gene GP22 encodes the tegumental antigen sm25: (1) antibodies to a predicted B-cell epitope of Sm25 cross-react with other candidate vaccine worm antigens; (2) characterization of a recombinant product containing tandem-repeats of this peptide as a vaccine

Monospecific antibodies against two putative epitopes of schistosome protein encoded by gene GP22 (182 codons, no introns) were used to probe worm extracts fractionated by lentil-lectin affinity chromatography or by electrophoresis. Anti-peptide-alpha (codons 70-84) exclusively identifies the N-glycanase-sensitive, 25 kDa tegumental glycoprotein Sm25 in the lectin-bound fraction of detergent-solubilized adult worm extract S3. In contrast, antipeptide-delta (codons 151-162) does not react with Sm25 but cross-reacts with other schistosome proteins, including candidate vaccine antigens paramyosin (Sm97) and glutathione-S-transferases (Sm26, Sm28, Sj26). Recombinant protein r4 x 47, constructed to express multiple copies of codon sequence 117-163 (containing delta), reacts with anti-delta and is uniquely recognized by protective Fischer twice-infected (F-2x) rat antiserum. Immunization with r4 x 47 induces antibodies with cross reactivities similar to anti-delta, but which also recognize Sm25. Despite these cross-reactivities with protective antigens, rodents vaccinated with r4 x 47 were not protected against cercarial infection. On the basis of these data, two hypotheses are proposed: (1) antigenic epitopes other than delta are present within the r4 x 47 sequence which induce antibodies reactive with Sm25 and/or (2) peptide-delta assumes alternative antigenic conformations, dependent upon the context of neighbouring sequences, some of which mimic epitopes of proteins encoded by other schistosome genes. These mimotopes are not targets of protective antibodies.

Cloning of Schistosoma mansoni transcription factor NF-YA subunit: phylogenic conservation of the HAP-2 homology domain

The CCAAT-binding factor NF-Y (CBF/CP1) is a heteromeric transcription factor involved in the regulation of a variety of eukaryotic genes. We identified NF-Y as the CCAAT activity binding to the promoter region of the gene coding for the 28-kDa glutathione S-transferase of the human parasite Schistosoma mansoni (Sm28GST). We isolated the NF-YA cDNA from S. mansoni (SmNF-YA): the complete 268 amino acid sequence harbors a region in its C-terminal part that shows homology with the subunit interaction and DNA-binding domains of the mammalian NF-YA; the N-terminal region has an amino acid composition reminiscent of the mammalian and echinoderm counterparts, rich in glutamine and hydrophobic residues, but shows no sequence similarity at the primary level. In vitro synthesized SMNF-YA is able to associate with mammalian NF-YB/C subunits in the absence of DNA and to bind to the Sm28GST CCAAT box. Surprisingly, a monoclonal antibody directed against the non-conserved Q-rich activation domain of mammalian NF-YA supershifts and immunoprecipitates SMNF-YA, strongly suggesting structure conservation in the activation domain between divergent species.

Cloning and characterization of the SmIMP25 integral membrane protein of the parasitic helminth Schistosoma mansoni

The cDNA and genomic clones encoding a 25 kDa integral membrane protein, termed SmIMP25, were isolated from Schistosoma mansoni. The 2.2 kb SmIMP25 mRNA was found in all developmental stages of the parasite tested: miracidium, sporocyst, cercaria and adult worm. The SmIMP25 gene is at least 16 kb long and it is split by four introns ranging in size from 36 bp to > or = 9 kb. Excluding the introns, the gene and the cDNA show 100% sequence identity. The cDNA has an open reading frame encoding a protein 223 amino acids long. The predicted sequence reveals a distinct hydrophobic domain of 20 amino acids located 12 residues from the carboxyl-terminal end. The properties of this domain (marked hydrophobicity, size, flanking by charged residues and C-terminal location) are typical of the transmembrane segments of integral membrane proteins. The presence of three potential N-glycosylation sites is also consistent with membrane proteins that are often glycosylated at the extracellular domain. Accordingly we propose that SmIMP25 is an integral membrane protein in which residues 1-191 are extracellular, residues 192-211 comprise the hydrophobic domain that spans the membrane, and residues 212-223 are intracellular. The SmIMP25 was synthesized as a fusion protein in bacteria and antibodies were elicited in rabbits. Antibodies against SmIMP25 specifically precipitated a 25 kDa protein from cell-free products programmed by schistosome mRNA, in agreement with the size of the protein predicted from the cDNA sequence. Immunofluorescence studies showed SmIMP25 on the surface of the parasite. Surface molecules expressed at the host-parasite interface are likely to provide information on host parasite relationship and may serve as targets for protective immunity.

Structure of the gene encoding a putative Schistosoma mansoni tegumental antigen precursor

In order to obtain the complete gene encoding the putative precursor of a 15-kDa Schistosoma mansoni tegumental antigen (Sm15), two cDNAs (A70 and A184) and two fragments of independent genomic clones were subcloned and sequenced. The collated sequence contains 4700 nucleotides and represents the full length open reading frame of the gene, encoding a protein of 1032 amino acids with a calculated molecular mass of 116,900. Thus, the gene encodes a much longer protein than that identified in the tegumental membranes, suggesting that it encodes a precursor that is subsequently highly processed. A 964-bp region composed of 5 closely related repeats was found to be present within the translated frame. The predicted protein is highly acidic and there is no indication of hydrophobic domains that may represent transmembrane regions or indicate attachment of a GPI anchor. The coding region has no homologies in the currently available data bases. In the 5' non-transcribed area a copy of the SM alpha repeat family is present. The coding region is preceded by putative CCAAT and TATA boxes that may be involved in the control of expression.

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

The complete gene encoding Schistosoma mansoni triosephosphate isomerase (TPI) was isolated from a lambda phage genomic library on 2 overlapping clones. These genomic clones have been characterized by restriction mapping and DNA sequencing of the 5' flanking region, the exons, the intron boundaries and the polyadenylation addition site. S. mansoni TPI is encoded by 6 exons spanning a region of about 12 kb. The 5 introns are located at positions precisely analogous to those of mammalian TPI genes but one of the 6 mammalian TPI introns is missing in S. mansoni. We find no evidence of spliced leader involvement in TPI gene expression. The gene is preceded by at least 4 tandem copies of a 2.5-kb repetitive sequence. While the 12-kb size for the S. mansoni TPI gene is much larger than the 3-4 kb typical of mammalian TPI genes, the 42-bp first intron is unusually short. The transcription initiation site for the S. mansoni TPI gene is heterogeneous. Genomic Southern blot analysis suggests that TPI is expressed from a single copy gene.

Cloning and characterisation of the gene encoding the 28-kDa glutathione S-transferase of Schistosoma mansoni

The 28-kDa glutathione S-transferase (GST) of Schistosoma mansoni is considered a possible vaccine candidate for use against this medically important parasite. The gene encoding this molecule has been isolated from a lambda EMBL4 library by using the corresponding cDNA sequence as a probe. The gene contains four exons and is approximately 5.5 kb in length. Analysis of the 5' flanking region revealed the presence of a consensus AP-1 recognition site, 5'-TGACTCA, between nucleotides -231 and -225. Southern blot analysis suggested the presence of a single gene encoding the 28-kDa GST in the S. mansoni genome.