- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Article Processing Charges ·
- Articles in Press ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents
Journal of Parasitology Research
Volume 2012 (2012), Article ID 480824, 7 pages
Transcriptional Profile and Structural Conservation of SUMO-Specific Proteases in Schistosoma mansoni
1Departamento de Ciências Biológicas/Núcleo de Pesquisas em Ciências Biológicas, Instituto de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto, Campus Morro do Cruzeiro, 35400-000 Ouro Preto, MG, Brazil
2Departamento de Parasitologia, Instituto de Ciências Biomédicas, Universidade de São Paulo, Av. Lineu Prestes, 1374-Butantan, 05508-900 São Paulo, SP, Brazil
3Instituto de Genética e Bioquímica, Universidade Federal de Uberlândia, Av. Getúlio Vargas, Palácio dos Cristais-Centro, 38700-126 Patos de Minas, MG, Brazil
4Fundação Oswaldo Cruz, Centro de Pesquisas René Rachou, Laboratório de Malacologia, Av. Augusto de Lima, 1715-Barro Preto, 30190-002 Belo Horizonte, MG, Brazil
Received 24 April 2012; Revised 9 August 2012; Accepted 23 August 2012
Academic Editor: Cristina Toscano Fonseca
Copyright © 2012 Roberta Verciano Pereira et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Small ubiquitin-related modifier (SUMO) is involved in numerous cellular processes including protein localization, transcription, and cell cycle control. SUMOylation is a dynamic process, catalyzed by three SUMO-specific enzymes and reversed by Sentrin/SUMO-specific proteases (SENPs). Here we report the characterization of these proteases in Schistosoma mansoni. Using in silico analysis, we identified two SENPs sequences, orthologs of mammalian SENP1 and SENP7, confirming their identities and conservation through phylogenetic analysis. In addition, the transcript levels of Smsenp1/7 in cercariae, adult worms, and in vitro cultivated schistosomula were measured by qRT-PCR. Our data revealed upregulation of the Smsenp1/7 transcripts in cercariae and early schistosomula, followed by a marked differential gene expression in the other analyzed stages. However, no significant difference in expression profile between the paralogs was observed for the analyzed stages. Furthermore, in order to detect deSUMOylating capabilities in crude parasite extracts, SmSENP1 enzymatic activity was evaluated using SUMO-1-AMC substrate. The endopeptidase activity related to SUMO-1 precursor processing did not differ significantly between cercariae and adult worms. Taken together, these results support the developmentally regulated expression of SUMO-specific proteases in S. mansoni.
Reversible posttranslational modification by ubiquitin and ubiquitin-like proteins (Ubls) plays a crucial role in dynamic regulation of protein function, fate, binding partners, activity, and localization. Small ubiquitin-related modifier (SUMO) is a member of Ubls family that is covalently attached to lysine residues of their protein targets in cells, altering function and subcellular localization . SUMOylation has been implicated in the regulation of numerous biological processes including transcription  and cell cycle control . SUMO conjugation mechanism is a highly dynamic and reversible process closely related to ubiquitylation. The sequential actions of E1, E2, and E3 enzymes catalyze the attachment of SUMO to target proteins, while deconjugation is promoted by SUMO-specific proteases. In addition, like other Ubls, SUMO polypeptide is synthesized as an inactive precursor, which requires a C-terminal cleavage to expose the glycine residue for substrate conjugation. This process is also catalyzed by SUMO-specific protease through its hydrolase activity. The catalytic activity is maintained within a highly conserved 200 amino acid region at the C-terminus of the proteases [4–6].
Recently, the importance of reversible SUMOylation for normal cell physiology has been demonstrated. Excessive SUMO conjugation using knockouts for either SENP1 or SENP2 induced an embryonic lethal phenotype [7, 8] and deregulation of either SUMO conjugation or deconjugation can contribute to cancer progression as reviewed by . In humans there are six SENP enzymes (SENP1, -2, -3, -5, -6, and -7) that deconjugate mono-SUMOylated proteins or disassemble polymeric SUMO side chains, while in Saccharomyces cerevisiae there are only two deSUMOylating enzymes, Ulp1 and Ulp2. Based on these activities and affinities for SUMO isoforms, SENPs can be categorized into three independent subfamilies: SENP1 and SENP2; SENP3 and SENP5; SENP6 and SENP7 .
The initial molecular characterization of SUMOylation pathway in S. mansoni showed the presence of two SUMO paralogs called SMT3C and SMT3B , suggesting a variety of targeted substrates for these modifications. Recently, our group reported the conservation of SUMO conjugating enzyme SmUBC9 by phylogeny, primary and modeled tertiary structures, as well as mRNA transcription and protein levels throughout the S. mansoni life cycle. Our data showed that Smubc9 transcription levels are upregulated in early schistosomula, suggesting the utilization of this posttranslational modification mechanism S. mansoni development, particularly at the initial phase of invasion of the vertebrate host .
To extend the investigation of the SUMOylation pathway in S. mansoni, we firstly retrieved through homology-based searches putative SmSENPs using the publically available S. mansoni databases. Furthermore, the levels of Smsenp1/7 transcripts were evaluated by qRT-PCR using mRNA from cercariae, adult worms and mechanically-transformed schistosomula (MTS) in vitro-cultivated during 3.5 h, 1, 2, 3, 5, and 7 days. We also measured the endopeptidase activities of SmSENPs in cercariae and adult worm crude extracts. The present study reveals differences in the gene expression profile of Smsenp1/7 during schistosomula development. In addition, similar SmSENP1 activity was observed in cercariae and adult worms, suggesting the importance of this proteolytic activity during the parasite’s life cycle.
2. Materials and Methods
2.1. Ethics Statement
All experiments involving animals were authorized by the Ethical Committee for Animal Care of Federal University of Ouro Preto, and were in accordance with the national and international regulation accepted for laboratory animal use and care.
2.2. Parasites. S. mansoni
LE strain was maintained by routine passage through Biomphalaria glabrata snails and BALB/c mice. The infected snails were induced to shed cercariae under light exposure for 2 h and the cercariae were recovered by sedimentation on ice. Adult worm parasites were obtained by liver perfusion of mice after 50 days of infection. Mechanically transformed schistosomula (MTS) were prepared as described by Harrop and Wilson . Briefly, cercariae were recovered, and washed in RPMI 1640 medium (Invitrogen), before vortexing at maximum speed for 90 s and immediately cultured during 3.5 h at 37°C, 5% CO2. Then the recovered schistosomula were washed with RPMI 1640 until no tails were detected. For subsequent incubations, the parasites were maintained in M169 medium supplemented with 10% FBS, penicillin, and streptomycin at 100 μg/mL and 5% Schneider's medium  at 37°C on 5% CO2 during 3.5 h, 1, 2, 3, 5, and 7 days.
2.3. Identification and Computational Analysis of SmSENP1/7
SmSENP1/7 sequences were retrieved from the S. mansoni genome database version 5.0 available at http://www.genedb.org/genedb/smansoni/ through BLAST searches. Amino acid sequences from Drosophila melanogaster, Caenorhabditis elegans and Homo sapiens SENP orthologs were used as queries. The BLASTp algorithm, underpinned by the Pfam (v26.0), allowed detection of conserved protein domains or motifs from S. mansoni sequences. Multiple alignments of SmSENP1/7 were performed by ClustalX 2.0 and phylogenetic analysis was conducted in MEGA 5 . A phylogenetic tree of these sequences was inferred using the Neighbor-Joining method . The bootstrap consensus tree inferred from 1000 replicates was used to represent the evolutionary history of the taxa analyzed. Branches corresponding to partitions reproduced in less than 50% bootstrap replicates are collapsed. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (1000 replicates) is shown next to the branches. The tree was drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. All positions containing gaps and missing data were eliminated from the dataset.
2.4. RNA Preparation and Expression Analysis of Smsenp1/7 by qRT-PCR
Total RNA from adult worms, cercariae, and schistosomula was obtained using a combination of the Trizol reagent (GIBCO-BRL) and chloroform for extraction, and then on column purified using the “SV total RNA Isolation System” (Promega, Belo Horizonte, Brazil). The preparation was treated with RNase-free DNase I in 3 different rounds by decreasing enzyme concentration (RQ1 DNase; Promega). RNA was quantified using a spectrophotometer and an aliquot containing 1 μg of total RNA reverse transcribed using an oligodT primer from the Thermoscript RT-PCR System (Invitrogen São Paulo, Brazil) as described by the manufacturer. The efficiency of DNAse I treatment was evaluated by PCR amplification of the cDNA reaction mix without the addition of the Thermoscript enzyme. S. mansoni specific SENP1/7 primers were designed using the program GeneRunner for SmSENP1 (GeneDB access number Smp_033260.2) (forward 5′-AGGAAACGGAGGCGGGATTC-3′, reverse 5′-ACACTGGAGACACGGGATGAGC-3′) and for SmSENP7 (GeneDB access number Smp_159120) (forward 5′-TCAGTTACACGGCCCTTTATC-3′, reverse 5′-CCTGAGAAGTGGATGCGATC-3′). Reverse-transcribed cDNA samples were used as templates for PCR amplification using SYBR Green Master Mix UDG-ROX (Invitrogen) and 7300 Real Time PCR System (Applied Biosystems, Rio de Janeiro, Brazil). Specific primers for S. mansoniα-tubulin were used as an endogenous control (GenBank access number M80214) (forward 5′-CGTATTCGCAAGTTGGCTGACCA-3′, reverse 5′-CCATCGAAGCGCAGTGATGCA-3′) . The efficiency of each pair of primers was evaluated according to the protocol developed by the Applied Biosystems application (cDNA dilutions were 1 : 10, 1 : 100 and 1 : 1000). For all investigated transcripts three biological replicates were performed and their gene expression normalized against the α-tubulin transcript according to the method  using the Applied Biosystems 7300 software.
2.5. Endopeptidase Activity
To determine the enzymatic activity of SENP proteases present in adult worms and cercariae crude extracts we used the fluorogenic substrate SUMO-1-AMC that is specific for SENP1 (Enzo Life Sciences, São Paulo, Brazil). In these assays 10 μg of total protein were used and 0,45 μM of the fluorogenic substrate in 50 mM Tris-HCl pH 8, 10 mM MgCl2 ± 0,45 μM SUMO-1-aldehyde (Enzo Life Sciences) to control for specific enzyme inhibition. Each enzymatic assay was conducted in a final volume of 100 μL for both extracts, followed by 60 minutes incubation at 37°C. The reaction was stopped by addition of 2 mL of 99.5% ethanol. The fluorometric readings were taken at wavelengths of 380 nm (excitation) and 440 nm (emission) in a spectrofluorimeter (Turner QuantechTM Fluorometer), and the results expressed in fluorescence units per μg of total protein.
2.6. Statistical Analysis
Statistical analysis was performed using GraphPad Prism version 5.0 software package (Irvine, CA, USA). Normality of the data was established using one way analysis of variance (ANOVA). Tukey post tests were used to investigate significant differential expression of SUMO proteases throughout the investigated stages. In all cases, the differences were considered significant when P values were <0.05.
3.1. SUMO Specific-Proteases: The Conservation of SmSENP1/7
In silico analysis of SmSENPs revealed the conserved putative domains of SUMO proteases in S. mansoni. The parasite database has two sequences encoding putative SENPs: Smp_033260.2 (putative SENP1) and Smp_159120 (putative SENP7). SENP1 has an alternative spliced form annotated as Smp_033260.1. We evaluated the conservation of these proteins throughout evolution using a phylogenetic approach. Our results showed that each SENP was grouped in 2 distinct clades, reinforcing their structural conservation among the thirteen analyzed orthologs (Figure 1).
To confirm that these predicted proteases are cysteine-family members, analyses using the Pfam protein domain database were conducted. For both SmSENPs entries a conserved Peptidase_C48 domain was revealed. Our in silico analysis demonstrated SmSENP1 more closely related to HsSENP1 (GenBank access number NP_055369.1), whereas SmSENP7 is related to HsSENP7 (GenBank access number NP_065705.3). Alignment of their catalytic domains with the human SENPs showed the presence of the three essential catalytic residues (Cys-His-Asp), highlighted in Figure 2.
3.2. Smsenp1/7 Are Differentially Expressed in S. mansoni
The gene expression profile of Smsenp1 and Smsenp7 was determined using qRT-PCR. We designed specific primers to amplify Smsenp1 and Smsenp7 transcripts in the cercariae-schistosomula transition and adult worm (Figure 3). We observed that Smsenp1 and Smsenp7 transcripts are expressed in basal levels from MTS-2d to 7 days and adult worms. The levels of the SENPs transcripts were significantly higher in MTS-3.5 h being approximately 10-fold higher when compared to levels in adult worms and during schistosomula development (MTS-2, 3, 5, and 7 days) and 3-fold higher when compared to cercariae and MTS-1d. We also observed that Smsenp1 and Smsenp7 transcription levels were not significantly different () within a given stage, except for MTS-3.5 h where Smsenp7 levels are 3 fold-higher than Smsenp1.
3.3. SmSENP1 Enzymatic Activity in Cercariae and Adult Worms
In vitro endopeptidase assays were performed using crude extract from cercariae and adult worms (Figure 4). The SmSENP1 enzymatic activity was slightly higher in cercariae compared to adult worms, although the levels were not significantly different (). Enzyme activities were also recorded in the presence of a commercially available HsSENP1 inhibitor but significant differences were not observed in the analyzed stages.
Previous reports from our group showed the presence of two SUMO paralogs (SMT3C and SMT3B) in S. mansoni, exhibiting differential expression between larvae and adult worms. The electrophoretic pattern of SUMO-conjugated molecules also differed comparing the analyzed stages . Furthermore, our group demonstrated a differential expression profile for UBC9 throughout the parasite life cycle . The structural SUMO conservation in S. mansoni reinforces SUMO as a candidate for a transcription regulator and a degradation signal by facilitating ubiquitylation of certain target proteins during parasite development.
In order to understand the role of SUMOylation in S. mansoni, we evaluated the conservation of SUMO proteases and their gene expression profile in cercariae, schistosomula, and adult worm. Firstly, we retrieved, through homology-based searches, two sequences containing conserved domains of SUMO proteases. A phylogenetic approach then revealed two putative proteases closely associated to human SENP1 and SENP7. Alignment of parasite sequences with their human counterparts also demonstrated conservation of the catalytic triad for both proteases . The active site residues within the core domain of Ulp1 and Ulp2 have been proven to be necessary for catalytic activity and in vivo function in yeast [19, 20]. Furthermore, Ulp1 lacks obvious sequence similarity to any known deubiquitinating enzyme so it is unlikely that it is able to process ubiquitin-linked substrates .
Considering that the SUMOylation pathway dictates the function of a variety of substrates , and the existence of two SUMO paralogs in S. mansoni , it is plausible that the worm utilizes distinct SUMO proteases for SUMO maturation and deSUMOylation of target substrates. Discrimination between SUMO-1 and SUMO-2/3 paralogs may also account for distinct functions of SENP proteases in S. mansoni. In mammalian cells, proteomic studies have identified proteins that are selectively modified by either SUMO-1 or SUMO-2/3 . It is well established that SENP1 processes SUMO-1 in preference to SUMO-2, and has a very low catalytic constant for SUMO-3 . In contrast, the recent characterization of the catalytic domain of SENP7 in humans demonstrated a greater deconjugating activity for SUMO-2/3 chains . As discussed by Shen  SENP7 acts as a SUMO-2/3 specific protease that is likely to regulate the metabolism of polySUMO-2/3 rather than SUMO-1 conjugation in vivo.
In the present investigation, the transcriptional profile of SUMO proteases was also evaluated by qPCR, using total RNA from distinct stages of the S. mansoni life cycle. The gene expression data revealed similar expression profile for Smsenp1 and Smsenp7 at any given stage, but with a remarkable differential expression for both proteases comparing the larvae and adult stages. The differential gene expression profile observed for Smsenp1/7 throughout the S. mansoni life cycle attests for the distinct patterns of SUMO conjugates observed during parasite development . PolySUMOylation has been reported to increase during heat-shock treatment and stress conditions, with SENP7 being responsible for dismantling SUMO polymers [25, 27]. Given that the levels of Smsenp1/7 transcripts were significantly higher in cercariae and MTS-3.5 h, we suggest a stress-induced expression of the SUMOylation machinery during the parasite’s body adaptation to the new metabolic and morphological alterations it undergoes . Our findings are also in agreement with Pereira  which demonstrated a similar pattern of gene expression profile for Smubc9 in the aforementioned parasite stages.
We have not observed a positive correlation between differential transcriptional levels of Smsenp1 and enzymatic activity evaluated using synthetic SUMO-1-AMC substrate. The levels of enzymatic activity for SmSENP1 were slightly higher in extracts of cercariae compared to adult worms. In the presence of a specific inhibitor, no significant change in enzymatic activity was detected. This may account for the limited amino acid sequence identity (28%) shared between SmSENP1 and its human ortholog, from which the synthesis of SUMO-1 aldehyde is based. Additional studies aimed at elucidating substrate selection of SmSENP1/7 in S. mansoni and a detailed kinetic analysis of their enzymatic activities, particularly during the cercarie to schistosomula transition may provide further insights into the biological activities of these enzymes and the role of SUMO conjugation in S. mansoni.
The authors thank the transcriptome initiatives: São Paulo Transcriptome Consortium; Minas Gerais GenomeNetwork; Wellcome Trust Genome Iniatiative (UK). This work was supported by the following Brazilian research agencies: FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais, CBB 0558/09), Núcleo de Bioinformática da Universidade Federal de Ouro Preto (NuBio UFOP) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico).
- J. R. Gareau and C. D. Lima, “The SUMO pathway: emerging mechanisms that shape specificity, conjugation and recognition,” Nature Reviews Molecular Cell Biology, vol. 11, no. 12, pp. 861–871, 2010.
- J. M. P. Desterro, M. S. Rodriguez, and R. T. Hay, “SUMO-1 modification of IκBα inhibits NF-κB activation,” Molecular Cell, vol. 2, no. 2, pp. 233–239, 1998.
- H. D. Ulrich, “Regulating post-translational modifications of the eukaryotic replication clamp PCNA,” DNA Repair, vol. 8, no. 4, pp. 461–469, 2009.
- D. Mukhopadhyay and M. Dasso, “Modification in reverse: the SUMO proteases,” Trends in Biochemical Sciences, vol. 32, no. 6, pp. 286–295, 2007.
- J. H. Kim and S. H. Baek, “Emerging roles of desumoylating enzymes,” Biochimica et Biophysica Acta, vol. 1792, no. 3, pp. 155–162, 2009.
- Z. Xu, H. Y. Chan, W. L. Lam et al., “SUMO proteases: redox regulation and biological consequences,” Antioxidants and Redox Signaling, vol. 11, no. 6, pp. 1453–1484, 2009.
- J. Cheng, X. Kang, S. Zhang, and E. T. H. Yeh, “SUMO-specific protease 1 is essential for stabilization of HIF1α during hypoxia,” Cell, vol. 131, no. 3, pp. 584–595, 2007.
- S. Y. Chiu, N. Asai, F. Costantini, and W. Hsu, “SUMO-specific protease 2 is essential for modulating p53-Mdm2 in development of trophoblast stem cell niches and lineages,” PLOS Biology, vol. 6, no. 12, p. e310, 2008.
- T. Bawa-Khalfe and E. T. H. Yeh, “SUMO losing balance: SUMO proteases disrupt SUMO homeostasis to facilitate cancer development and progression,” Genes and Cancer, vol. 1, no. 7, pp. 748–752, 2010.
- F. J. Cabral, O. S. Pereira Jr., C. S. Silva, R. Guerra-Sá, and V. Rodrigues, “Schistosoma mansoni encodes SMT3B and SMT3C molecules responsible for post-translational modification of cellular proteins,” Parasitology International, vol. 57, no. 2, pp. 172–178, 2008.
- R. V. Pereira, F. J. Cabral, M. S. Gomes et al., “Molecular characterization of SUMO E2 conjugation enzyme: differential expression profile in Schistosoma mansoni,” Parasitology Research, vol. 109, no. 6, pp. 1537–1546, 2011.
- R. Harrop and R. A. Wilson, “Protein synthesis and release by cultured schistosomula of Schistosoma mansoni,” Parasitology, vol. 107, no. 3, pp. 265–274, 1993.
- P. F. Basch and J. J. DiConza, “In vitro development of Schistosoma mansoni cercariae,” Journal of Parasitology, vol. 63, no. 2, pp. 245–249, 1977.
- S. Kumar, K. Tamura, and M. Nei, “MEGA3: integrated software for Molecular Evolutionary Genetics Analysis and sequence alignment,” Briefings in bioinformatics, vol. 5, no. 2, pp. 150–163, 2004.
- N. Saitou and M. Nei, “The neighbor-joining method: a new method for reconstructing phylogenetic trees,” Molecular Biology and Evolution, vol. 4, no. 4, pp. 406–425, 1987.
- P. J. Webster, K. A. Seta, S. C. Chung, and T. E. Mansour, “A cDNA encoding an α-tubulin from Schistosoma mansoni,” Molecular and Biochemical Parasitology, vol. 51, no. 1, pp. 169–170, 1992.
- K. J. Livak and T. D. Schmittgen, “Analysis of relative gene expression data using real-time quantitative PCR and the 2-ΔΔCT method,” Methods, vol. 25, no. 4, pp. 402–408, 2001.
- S. J. Li and M. Hochstrasser, “The Ulp1 SUMO isopeptidase: distinct domains required for viability, nuclear envelope localization, and substrate specificity,” Journal of Cell Biology, vol. 160, no. 7, pp. 1069–1081, 2003.
- S. J. Li and M. Hochstrasser, “A new protease required for cell-cycle progression in yeast,” Nature, vol. 398, no. 6724, pp. 246–251, 1999.
- A. V. Strunnikov, L. Aravind, and E. V. Koonin, “Saccharomyces cerevisiae SMT4 encodes an evolutionarily conserved protease with a role in chromosome condensation regulation,” Genetics, vol. 158, no. 1, pp. 95–107, 2001.
- S. J. Li and M. Hochstrasser, “The yeast ULP2 (SMT4) gene encodes a novel protease specific for the ubiquitin-like Smt3 protein,” Molecular and Cellular Biology, vol. 20, no. 7, pp. 2367–2377, 2000.
- G. Rosas-Acosta, W. K. Russell, A. Deyrieux, D. H. Russell, and V. G. Wilson, “A universal stategy for proteomic studies of SUMO and other ubiquitin-like modifiers,” Molecular and Cellular Proteomics, vol. 4, no. 1, pp. 56–72, 2005.
- A. C. O. Vertegaal, J. S. Andersen, S. C. Ogg, R. T. Hay, M. Mann, and A. I. Lamond, “Distinct and overlapping sets of SUMO-1 and SUMO-2 target proteins revealed by quantitative proteomics,” Molecular and Cellular Proteomics, vol. 5, no. 12, pp. 2298–2310, 2006.
- L. N. Shen, C. Dong, H. Liu, J. H. Naismith, and R. T. Hay, “The structure of SENP1-SUMO-2 complex suggests a structural basis for discrimination between SUMO paralogues during processing,” Biochemical Journal, vol. 397, no. 2, pp. 279–288, 2006.
- C. D. Lima and D. Reverter, “Structure of the human SENP7 catalytic domain and poly-SUMO deconjugation activities for SENP6 and SENP7,” Journal of Biological Chemistry, vol. 283, no. 46, pp. 32045–32055, 2008.
- L. N. Shen, M. C. Geoffroy, E. G. Jaffray, and R. T. Hay, “Characterization of SENP7, a SUMO-2/3-specific isopeptidase,” Biochemical Journal, vol. 421, no. 2, pp. 223–230, 2009.
- F. Golebiowski, I. Matic, M. H. Tatham et al., “System-wide changes to sumo modifications in response to heat shock,” Science Signaling, vol. 2, no. 72, p. ra24, 2009.
- J. R. Lawson and R. A. Wilson, “Metabolic changes associated with the migration of the schistosomulum of Schistosoma mansoni in the mammal host,” Parasitology, vol. 81, no. 2, pp. 325–336, 1980.