Schistosoma mansoni: purification and characterization of a membrane-associated leucine aminopeptidase

Membrane-associated leucine aminopeptidase (EC 3.4.11.1, LAP) has been purified to homogeneity from Schistosoma mansoni egg homogenates by a combination of ultracentrifugation, chromatofocusing, and molecular sieve chromatography. A 260-fold increase in specific activity was observed after purification. This is a metalloenzyme, containing carbohydrate moieties. Optimal enzyme activity was found at neutral pH. Enzyme activity was measured using L-leucine-7-amino-4-trifluoromethylcoumarin (L-Leu-AFC); in addition, schistosome egg LAP hydrolyzed a variety of other aminopeptidase substrates. Hydrolysis of L-Leu-AFC was inhibited by a number of aminopeptidase inhibitors, including 1,10-phenanthroline, bestatin, and amastatin.     

Leucine aminopeptidase and hatching of Schistosoma mansoni eggs

Leucine aminopeptidase (LAP) activity has been measured in extracts of eggs, miracidia, cercariae and adult worms of Schistosoma mansoni. Activity measured at pH 7.2 using L-leu-7-amino-4-trifluoro-methylcoumarin as substrate is 6- to 17-fold greater in eggs than in other life stages. LAP activity is also high in soluble egg antigen preparations and in hatching fluid. The release of LAP from eggs parallels hatching, and inhibitors of LAP also inhibit hatching. The possible role of LAP in the hatching process of S. mansoni eggs is discussed.     

Biochemical characterization of 2-Cys peroxiredoxins from Schistosoma mansoni

Peroxiredoxins are a large family of peroxidases that have important antioxidant and cell signaling functions. Genes encoding two novel 2-cysteine peroxiredoxin proteins were identified in the expressed sequence tag data base of the helminth parasite Schistosoma mansoni, a causative agent of schistosomiasis. The recombinant proteins showed peroxidase activity in vitro with a variety of hydroperoxides and used both the thioredoxin and the glutathione systems as electron donors. Steady-state kinetic analysis indicated that the new peroxiredoxins had saturable kinetics, whereas a previously identified schistosome peroxiredoxin was found to function with more typical unsaturable (ping-pong) kinetics. The catalytic efficiencies S. mansoni peroxiredoxins were similar to those for other peroxiredoxins studied (10(4)-10(5) m(-1) s(-1)). Mutagenesis of S. mansoni peroxiredoxins indicated that glutathione dependence and kinetic differences were conferred by the C-terminal alpha-helix forming 22 amino acids. This is the first report of 2-cysteine peroxiredoxins efficiently utilizing reducing equivalents from both the thioredoxin and glutathione systems. Studies to determine the resistance to oxidative inactivation, important in regulating cell signaling pathways, showed that S. mansoni possess both bacterial-like resistant and mammalian-like sensitive peroxiredoxins. The susceptibility to oxidative inactivation was conferred by the C-terminal tail containing a tyrosine-phenylalanine motif. S. mansoni is the first organism shown to possess both robust and sensitive peroxiredoxins. The ability of schistosome peroxiredoxins to use alternative electron donors, and their variable resistance to overoxidation may reflect their presence in different cellular sites and emphasizes the significant differences in overall redox balance mechanisms between the parasite and its mammalian host.     

Methionine uptake of larval and adult Schistosoma mansoni;

The uptake of methyl-¹⁴C-methionine by schistosomula, 21 day-old and adult Schistosoma mansoni in vitro has been investigated. Ligation of the adult pharynx and comparison between the kinetics of uptake of schistosomula and adults suggests that methionine is absorbed, in the system described, primarily via the tegument. All stages of the parasite examined absorb methionine by, at least, two kinetically distinguishable systems, one which is saturable and a second which appears to be simple diffusion. The saturable system has relatively low specificity for amino acids but shows no affinity for other classes of compounds. The effects of pH, temperature, metabolic inhibitors, and sodium ion concentration have been examined. The results are discussed with reference to schistosome gut function and also to the differential response to chemotherapy according to age of infection.     

Schistosoma mansoni: purification and characterization of the major acidic proteinase from adult worms

We report purification of the major digestive proteinase from adult worms of Schistosoma mansoni. This enzyme is a thiol proteinase with a pH optimum of 5 and is activated by thiol reagents. It was purified 300-fold using a combination of gel chromatography and chromatofocusing. It readily hydrolyzed hemoglobin with an apparent Km of 0.29 microM and a specific activity of 27 micrograms degraded/min/mg enzyme at 37 C. Peptides with positively charged amino acids were preferentially cleaved. The enzyme degraded Boc-Arg-Arg-7-amino-4-methyl coumarin with a kcat/Km of 9083 M-1 sec-1. Lengthening the peptide chain to 3 amino acids or substituting glycine for the amino terminal arginine resulted in decreased activity. The enzyme was inhibited by chloromethylketone-derivatized peptides of similar sequence and by leupeptin. The purified proteinase exhibits microheterogeneity in different preparations with forms ranging in molecular weight from 30,000 to 35,000, and pI 5.7-6.0.     

Schistosoma mansoni: ingestion of dextrans, serum albumin, and IgG by schistosomula.

Schistosomula of Schistosoma mansoni develop the ability to ingest and digest red blood cells after the fourth day post-transformation. Here, we have used fluorescently-labeled dextrans and two plasma proteins, albumin and IgG, to test whether day-old schistosomula can ingest and process macromolecules prior to the time that they eat red cells. Worms ingested dextrans of molecular weights 4,000, 70,000 and 2 x 10(6) in a time- and concentration-dependent manner. The dextran remained in the cecal lumen for up to 2 days after feeding. Parasites ingested both fluorescein-conjugated bovine serum albumin and rabbit IgG, but neither of these proteins remained confined to the cecum over time. Instead, fluorescence redistributed to the acetabular glands within a few hours. Thin-layer chromatography indicated that schistosomula degraded fluorescein-conjugated albumin to fluorescein-conjugated peptides approximately 10-15 amino acids long. The volume of the cecum was estimated to be 2431 microns 3 and the surface area 299 microns 2. These results demonstrate that larval schistosomes can ingest both proteins and complex carbohydrates shortly after transformation, before they can ingest red cells. Further, the gut apparently releases proteases that cleave plasma proteins, but not saccharidases that cleave dextran.     

Extracellular lipid droplets promote hemozoin crystallization in the gut of the blood fluke Schistosoma mansoni

Hemozoin (Hz) is a heme crystal produced upon hemoglobin digestion as the main mechanism of heme disposal in several hematophagous organisms. Here, we show that, in the helminth Schistosoma mansoni, Hz formation occurs in extracellular lipid droplets (LDs). Transmission electron microscopy of adult worms revealed the presence of numerous electron-lucent round structures similar to LDs in gut lumen, where multicrystalline Hz assemblies were found associated to their surfaces. Female regurgitates promoted Hz formation in vitro in reactions partially inhibited by boiling. Fractionation of regurgitates showed that Hz crystallization activity was essentially concentrated on lower density fractions, which have small amounts of pre-formed Hz crystals, suggesting that hydrophilic-hydrophobic interfaces, and not Hz itself, play a key catalytic role in Hz formation in S. mansoni. Thus, these data demonstrate that LDs present in the gut lumen of S. mansoni support Hz formation possibly by allowing association of heme to the lipid-water interface of these structures.     

Invasion by schistosome cercariae: neglected aspects in Schistosoma japonicum

Skin invasion by schistosome cercariae was recently discussed in Trends in Parasitology. However, only Schistosoma mansoni was considered, possibly because this species predominates in laboratory studies (at least outside China). One may be tempted to extrapolate from the "model" S. mansoni to other schistosomes, but Schistosoma japonicum must not be neglected. This schistosome is distinguishable from others (particularly S. mansoni) by virtue of its remarkable speed and success of migration, as well as by specific biochemical and immunological features. This leads to the hypothesis that S. japonicum is atypical with respect to the enzymes that facilitate skin penetration.     

Structure and bioactivity of neuropeptide F from the human parasites Schistosoma mansoni and Schistosoma japonicum

The blood flukes Schistosoma mansoni and Schistosoma japonicum inflict immense suffering as agents of human schistosomiasis. Previous investigations have found the nervous systems of these worms contain abundant immunoreactivity to antisera targeting invertebrate neuropeptide Fs (NPFs) as well as structurally similar neuropeptides of the mammalian neuropeptide Y (NPY) family. Here, cDNAs encoding NPF in these worms were identified, and the mature neuropeptides from the two species differed by only a single amino acid. Both neuropeptides feature the characteristics common among NPFs; they are 36 amino acids long with a carboxyl-terminal Gly-Arg-X-Arg-Phe-amide and Tyr residues at positions 10 and 17 from the carboxyl terminus. Synthetic S. mansoni NPF potently inhibits the forskolin-stimulated accumulation of cAMP in worm homogenates, with significant effects at 10(-11) m. This is the first demonstration of an endogenous inhibition of cAMP by an NPF, and because this is the predominant pathway associated with vertebrate NPY family peptides, it demonstrates a conservation of downstream signaling pathways used by NPFs and NPY peptides.     

Ultrastructural alterations in adult Schistosoma mansoni caused by artemether

Progress has been made over the last decade with the development and clinical use of artemether as an agent against major human schistosome parasites. The tegument has been identified as a key target of artemether, implying detailed studies on ultrastructural damage induced by this compound. We performed a temporal examination, employing a transmission electron microscope to assess the pattern and extent of ultrastructural alterations in adult Schistosoma mansoni harboured in mice treated with a single dose of 400 mg/kg artemether. Eight hours post-treatment, damage to the tegument and subtegumental structures was seen. Tegumental alterations reached a peak 3 days after treatment and were characterized by swelling, fusion of distal cytoplasma, focal lysis of the tegumental matrix and vacuolisation. Tubercles and sensory organelles frequently degenerated or collapsed. Typical features of subtegumental alterations, including muscle fibres, syncytium and parenchyma tissues, were focal or extensive lysis, vacuolisation and degeneration of mitochondria. Severe alterations were also observed in gut epithelial cells and vitelline cells of female worms. Our findings of artemether-induced ultrastructural alterations in adult S. mansoni confirm previous results obtained with juvenile S. mansoni and S. japonicum of different ages.     

The in vitro effects of various lysosomotropic agents on the gut of Schistosoma mansoni schistosomula

Cultured Schistosoma mansoni schistosomula of various ages were exposed to several lysosomotropic agents. Weak bases such as chloroquine, ammonium chloride, and acridine orange caused gut swelling upon protonation. The latter compound fluoresced a bright orange indicating the acidic nature of the gut contents. Hydrolysis of ingested L-amino acid methyl esters also resulted in gut swelling, indicating the nonpermeant nature of the resulting L-amino acids. Neither age nor feeding status influenced these swelling effects. Treatment of schistosomula with D-amino acid esters, free L-amino acids, or methanol had no effect. Thin-layer chromatographic analysis of worms treated with radiolabeled L-leucine methyl ester provided evidence that the ester was hydrolyzed. These results support the premise that the schistosome gut is an acidic compartment and is reminiscent of a secondary lysosome in its reaction to lysosomotropic agents.     

Effect of mutagen on cultured Schistosoma mansoni

Although lightly homogenized three-week-old Schistosoma mansoni incubated in Mitsuhashi and Maramorosch insect tissue culture medium or the medium of Weller and Wheeldon produced adherent cell layers, continued growth of these cells did not occur. Non-adherent cells obtained by trypsinization also failed to produce long term cell cultures even after the addition of a range of growth factors. The possibility of producing tumour-like schistosome cells by the use of the mutagen ethyl methane sulphonate was therefore examined. Four-hour exposure of three-week-old schistosomes caused in some worms (a) large fluid filled 'ballooning', which also occurred in adult males, (b) enlargement of the gut, (c) increase in numbers of large round cells within the worms and (d) tissue outgrowths. It is suggested that these effects of mutagen offer new approaches to obtaining permanent schistosome cell cultures.     

Cloning and characterization of a cDNA coding for the alpha-subunit of a stimulatory G protein from Schistosoma mansoni.

Guanine nucleotide-binding proteins (G proteins) mediate signals between serotonin receptors and adenylate cyclase in Schistosoma mansoni. A bovine Gs alpha cDNA probe was used to isolate a cDNA clone, SG12, encoding the entire alpha-subunit of a G protein of S. mansoni. The cDNA is 1897 base pairs long, contains an open reading frame of 1137 base pairs, and codes for a deduced protein of 379 amino acids. The putative protein encoded by the clone has an exact amino acid match with bovine Gs alpha of 65% and a 78% match when conserved amino acid substitutions are considered. In contrast, the exact and conserved matches of the schistosome alpha-subunit with bovine Gi are 41 and 61%, respectively. A comparison of the deduced amino acid sequence of SG12 with a variety of different G alpha proteins indicates that all the major structural features characteristic of a Gs alpha protein are present in the S. mansoni gene. The schistosome clone contains the putative site for ADP-ribosylation by cholera toxin found in Gs alpha but does not contain the ADP-ribosylation site for pertussis toxin present in Gi alpha. The amino acids are completely conserved at the GTP-binding sites. On a Northern blot, the cDNA hybridizes to a major band of 3.1 kilobases in RNA from adult schistosomes. The message appears to be absent in miracidia and cercariae, but a faint 3.1-kilobase band is visible in the early schistosomule stage preceding adulthood. This evidence, when added to previous biochemical data, indicates that the expression of this gene is developmentally controlled.     

Conservation of epidermal growth factor receptor function in the human parasitic helminth Schistosoma mansoni

The epidermal growth factor receptor (EGF-R) plays an important role in development and cell differentiation, and homologues of EGF-R have been identified in a broad range of vertebrate and invertebrate organisms. This work concerns the functional characterization of SER, the EGF-R-like molecule previously identified in the helminth parasite Schistosoma mansoni. Transactivation assays performed in epithelial Madin-Darby canine kidney cells co-transfected with SER and a Ras-responsive reporter vector indicated that SER was able to trigger a Ras/ERK pathway in response to human epidermal growth factor (EGF). These results were confirmed in Xenopus oocytes showing that human EGF induced meiosis reinitiation characterized by germinal vesicle breakdown in SER-expressing oocytes. Germinal vesicle breakdown induced by EGF was dependent on receptor kinase activity and shown to be associated with phosphorylation of SER and of downstream ERK proteins. (125)I-EGF binding experiments performed on SER-expressing oocytes revealed high affinity (2.9 x 10(-9) M) of the schistosome receptor for human EGF. Phosphorylation of the native SER protein present in S. mansoni membranes was also shown to occur upon binding of human EGF. These data demonstrate the ability of the SER schistosome receptor to be activated by vertebrate EGF ligands as well as to activate the classical ERK pathway downstream, indicating the conservation of EGF-R function in S. mansoni. Moreover, human EGF was shown to increase protein and DNA synthesis as well as protein phosphorylation in parasites, supporting the hypothesis that host EGF could regulate schistosome development. The possible role of SER as a receptor for host EGF peptides and its implication in host-parasite signaling and parasite development are discussed.     

Functional properties of a rat monoclonal IgE antibody specific for Schistosoma mansoni

A rat monoclonal antibody of IgE isotype (B48-14) raised against Schistosoma mansoni has been generated by the fusion of mesenteric lymph node cells from LOU/M rats immunized with a preparation of adult schistosome worms and IR973F nonsecreting rat myeloma cells. Investigation of the in vitro effector functions of this IgE antibody showed a high level of cytotoxicity against S. mansoni schistosomula in the presence of eosinophils, macrophages, and platelets. A significant level of protection (40 to 60%) against a challenge infection with S. mansoni cercariae was achieved by passive transfer experiment of B48-14 IgE to naive recipient rats. By immunoprecipitation, B48-14 IgE antibodies were shown to react with an antigen of 26 kDa present in excretion-secretion products of schistosomula, previously described as a potential immunogen eliciting a protective IgE response against schistosomiasis.     

A 14-kDa Schistosoma mansoni polypeptide is homologous to a gene family of fatty acid binding proteins

The complete nucleotide sequence encoding a Schistosoma mansoni protein termed Sm14 was determined from cDNA clones propagated in bacteriophage lambda gt11 in Escherichia coli. The 14.8-kDa protein bears significant homologies with a family of related polypeptides which bind hydrophobic ligands. Members of this group of cytosolic proteins were originally identified based on their affinity for long chain fatty acids. The purified recombinant protein exhibited an affinity to fatty acids, in contrast to a mutant lacking 16 N-terminal amino acids. Immunofluorescence experiments show that tubercles, which are structures located on the dorsal surface of adult male schistosome and known to contain lipids, are stained using antibodies raised to the beta-galactosidase fusion protein. A regular staining pattern is also evident in the muscle layers as well as in the body of the parasite. As the schistosome cannot synthesize fatty acids de novo and is dependent on the uptake of lipids from serum, the available data support a role for Sm14 in the transport of fatty acids.     

Analysis of cDNA encoding the hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) of Schistosoma mansoni; a putative target for chemotherapy.

Because of the lack of de novo purine biosynthesis, hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) is a critical enzyme in the purine metabolic pathway of the human parasite, Schistosoma mansoni. Using a cDNA clone encoding mouse HGPRTase and subsequently a synthetic oligonucleotide derived from sequencing a clone of genomic DNA, two clones were isolated from an adult schistosome cDNA library. One clone is 1.374 Kilobases (Kb) long and has an open reading frame of 693 bases. The deduced 231 amino acid sequence has 47.9% identity in a 217 amino acid overlap with human HGPRTase. Northern blot analysis indicates that the full length of mRNA for the S. mansoni HGPRTase is 1.45-1.6 Kb. Analysis of the primary structures of the putative active site for human and parasite enzymes reveal specific differences which may eventually be exploitable in the design of drugs for the treatment of schistosomiasis.