Praziquantel efficacy and long-term appraisal of schistosomiasis control in Pemba Island.

OBJECTIVE: To determine whether praziquantel (PZQ) has retained its efficacy against Schistosoma haematobium on Pemba Island after 20 years of mass administration--albeit discontinuous--and to analyse retrospectively the performance of schistosomiasis control programmes. METHODS: A sample of Pemba schoolchildren was examined before and after PZQ treatment by urine filtration, macro- and micro-haematuria and viability of excreted eggs. RESULTS: Although 5% of treated children continued to pass some eggs in the urine up to the seventh week after PZQ administration, none of these eggs was viable, indicating an effective schistosomicidal activity followed by a slow release of dead eggs from host tissues. CONCLUSION: No signs of PZQ resistance could be detected in the population under study. An overall retrospective analysis of schistosomiasis control activities in Pemba Island revealed that mass drug administration is clearly effective in reducing infection prevalence, but soon after interruption of drug distribution prevalence returns rapidly to pre-intervention levels.

Praziquantel: its use in control of schistosomiasis in sub-Saharan Africa and current research needs.

Treatment with praziquantel (PZQ) has become virtually the sole basis of schistosomiasis control in sub-Saharan Africa and elsewhere, and the drug is reviewed here in the context of the increasing rate that it is being used for this purpose. Attention is drawn to our relative lack of knowledge about the mechanisms of action of PZQ at the molecular level, the need for more work to be done on schistosome isolates that have been collected recently from endemic areas rather than those maintained in laboratory conditions for long periods, and our reliance for experimental work mainly on Schistosoma mansoni, little work having been done on S. haematobium. There is no evidence that resistance to PZQ has been induced in African schistosomes as a result of its large-scale use on that continent to date, but there is also no assurance that PZQ and/or schistosomes are in any way unique and that resistant organisms will not be selected as a result of widespread drug usage. The failure of PZQ to produce complete cures in populations given a routine treatment should therefore solicit considerable concern. With few alternatives to PZQ currently available and/or on the horizon, methods to monitor drug-susceptibility in African schistosomes need to be devised and used to help ensure that this drug remains effective for as long a time as possible.

Praziquantel and the benzodiazepine Ro 11-3128 do not compete for the same binding sites in schistosomes.

The benzodiazepine Ro 11-3128 (methyl-clonazepam) presents several similarities with praziquantel with regard to its anti-schistosomal mode of action, since both drugs cause spastic paralysis, calcium influx and tegumental disruption in the parasites. In order to know whether the two compounds share the same binding sites in the schistosomes, we performed in vivo and in vitro competition experiments. We took advantage of the fact that Ro 11-3128 is active against immature Schistosoma mansoni (whereas praziquantel is inactive), and praziquantel is active against S. japonicum (which is insensitive to Ro 11-3128). An excess of praziquantel did not inhibit the activity of Ro 11-3128 against immature S. mansoni and an excess of Ro 11-3128 did not inhibit the activity of praziquantel against S. japonicum, suggesting that the schistosome binding sites of the two drugs are different. On the other hand, cytochalasin D, an agent known to perturb--among other things--calcium channel function, was capable of inhibiting the schistosomicidal activity of both praziquantel and Ro 11-3128, thus adding another element of similarity between the two anti-schistosomal agents. A similar, albeit partial, inhibition of the schistosomicidal activity of the two drugs was exerted by some of the classical calcium channel blockers. Taken together, these results suggest that praziquantel and Ro 11-3128, although binding to different schistosome receptor sites, may use the same basic anti-schistosomal effector mechanisms.

The distribution of motor proteins in the muscles and flame cells of the Schistosoma mansoni miracidium and primary sporocyst.

Schistosoma mansoni eggs, miracidia and primary sporocysts were labelled with phalloidin-rhodamine to visualize filamentous actin structures. Analysis of these forms by confocal fluorescence microscopy revealed the presence of previously well-defined circular and longitudinal muscle layers. Besides these muscular layers that sustain and provide motility to these parasite forms, we found in these 3 consecutive developmental stages of the parasite previously unidentified actin-rich tubular structures. In the 3 forms, 4 actin-rich tubules could be observed by optical sectioning underneath the well-developed muscle layers. The tubules appear in pairs, transversal to the length of the parasite, and located towards the extremities. By using an anti-flame cell specific antibody we confirmed that the tubules co-localize with flame cells and also determined that the tubule core is filled with microtubules. The additional presence of myosin in these tubules strongly suggests that they are contractile structures.

Molecular cloning of Schistosoma mansoni calcineurin subunits and immunolocalization to the excretory system.

In order to explain the schistosomicidal effect of cyclosporin A, the hypothesis was advanced that the drug, complexed with cyclophilin, inhibits the phosphatase activity of parasite calcineurin (CN), with mechanisms similar to those operating in its immunosuppressive action. As a preparatory step to the testing of this hypothesis, we report the molecular cloning of both CN subunits in Schistosoma mansoni. The catalytic (A) subunit has a predicted sequence of 607 amino acids and shows substantial similarity to other cloned CNs, except for the carboxy-terminal end that is highly divergent. The regulatory (B) subunit consists of 169 amino acids that are 86% identical to those of the human counterpart and, from its anomalous electrophoretic mobility, it appears to be myristoylated. The results of Southern blotting experiments are compatible with the existence of multiple genes for CNA and a single gene for CNB. Western blots showed that both subunits are present at all stages of the parasite life cycle and can be detected both in the soluble and in the membrane fraction. Immunofluorescence confocal microscopy revealed a striking concentration of the anti-CNA reactivity in 6-8 discrete spots in the schistosomula and in distinct spots along the body of the adult parasite, corresponding to the expected localization of flame cells. Both patterns were confirmed by a perfect co-localization of the anti-CNA signal with that of a previously characterized anti-flame cell monoclonal antibody. The preferential confinement of schistosome CN to the protonephridial system suggests that the enzyme in the parasite may fulfil similar functions to those performed in mammalian kidneys.

Changes of mate occur in Schistosoma mansoni.

Male and female schistosomes are generally assumed to form stable monogamous pairs for the whole span of their long existence in the mammalian host. Recent evidence from mixed infections has shown that Schistosoma mansoni males can displace S. intercalatum males from their homologous partners, but no information exists about the existence of similar phenomena within a single schistosome species. Here, we determine whether male S. mansoni can displace males of the same species from pre-formed pairs in vivo. The availability of clear-cut genetic markers of drug resistance in schistosomes was exploited to show that hycanthone sensitive S. mansoni males can displace homospecific hycanthone resistant males from pre-formed pairs and vice versa. The frequency of changes is dependent on the magnitude of the excess single males competing with paired worms. The possible mechanics and the biological significance of mate changing are discussed.

Stage-specific expression of a Schistosoma mansoni polypeptide similar to the vertebrate regulatory protein stathmin.

The ubiquitous vertebrate protein stathmin is expressed and phosphorylated in response to a variety of external and internal signals. Stathmin, in turn, controls cell growth and differentiation through its capacity to regulate microtubule assembly dynamics. This is the first report on the molecular cloning and characterization of a stathmin-like protein (SmSLP) in an invertebrate, the human blood fluke Schistosoma mansoni. SmSLP is first synthesized at high levels in the intermediate molluscan host and completely disappears 48 h after penetration into the mammalian host. The protein is preferentially iodinated in intact immature parasites using the Bolton-Hunter reagent, can be quantitatively extracted in high salt buffers, and remains soluble after boiling. Native SmSLP was partially sequenced, and its complete structure was derived from the cloning and sequencing of its cDNA. The sequence is up to 26% identical to vertebrate stathmin sequences and contains two potential phosphorylation sites. Native SmSLP is indeed phosphorylated because phosphatase digestion shifts its mobility in electrofocusing gels. SmSLP associates with tubulin, as suggested by immune co-precipitation results. In vitro experiments demonstrated that SmSLP inhibits tubulin assembly and causes the depolymerization of preassembled microtubules, thus probably fulfilling regulatory roles in critical steps of schistosome development.

Effect of bile salts on oviposition in vitro by Schistosoma mansoni.

Adult pairs of Schistosoma mansoni were kept in culture in the presence or absence of various bile salts and the numbers of parasite eggs deposited in vitro were monitored for 2 weeks. The hydrophilic bile salt tauroursodeoxycholic acid (TUDCA) was found to produce a highly significant increase in the number of eggs deposited during the 1st week of culture. The hydrophobic bile salt taurochenodeoxycholic acid (TCDCA) and the intermediately hydrophobic salt taurocholate (TCA) produced more moderate increases. These results expand previous data showing that schistosomes kept in the presence of portal blood have higher oviposition rates than schistosomes kept in systemic blood.

Cyclosporins: lack of correlation between antischistosomal properties and inhibition of cyclophilin isomerase activity.

The immunosuppressive fungal products cyclosporin A (CsA) and FK506 bind with high affinity to intracellular receptor proteins: cyclophilin (CYP) is one of the receptors for CsA and FK506-binding protein (FKBP) is one of the receptors for FK506. These proteins catalyze the in vitro isomerization from a cis to a trans conformation of peptidyl-prolyl bonds in oligopeptides. The relative importance of the peptidyl-prolyl cis-trans isomerase (PPI ase) activity of CYP compared to FKBP in schistosomes is not known. Here, we examine the effects of CsA and FK506 and show that the former inhibits PPIase activity in schistosome extracts, whereas the latter does not. Since CsA is specific for the CYP protein, this result is indicative of the fact that the PPIase activity in the parasite is mostly attributable to CYP. The observation that CsA was significantly more effective than FK506 as an antischistosomal agent, both in vivo and in vitro raises the possibility that killing of schistosomes is caused by the inhibition of schistosome CYP PPIase. We compared a number of Cs analogs for their antischistosomal effects and for the inhibition of CYP PPIase, but were unable to find a correlation between the two properties. We therefore conclude that the lethal effect of CsA is not directly linked to the inhibition of the enzymatic activity of schistosome CYPs.

Expression cloning and biochemical characterizations of recombinant cyclophilin proteins from Schistosoma mansoni.

Recombinant Schistosoma mansoni cyclophilin proteins of the A and the B subtypes (SmCYP A and B) were expressed in bacterial cells as histidine- and maltose-binding fusion proteins and also as nonfused proteins. In addition, S. mansoni CYPs were produced in Sf9 insect cells in their natural forms. Purified recombinant SmCYP B was found to possess a peptidyl-prolyl cis-trans isomerase (PPIase) activity, with a kcat/Km value of 8.2 x 10(5) M-1 s-1. The SmCYP B isoform is approximately two to three times more active than SmCYP A. SmCYP B-specific RNA appears to be more abundant in adult schistosomes than SmCYP A RNA in Northern blots. These results support the conclusion that SmCYP B represents the major schistosomal CYP. The PPIase-associated activity of both CYPs was inhibitable by the immunosuppressive drug cyclosporin A (CsA). We attempt to explain differences in PPIase activities and in CsA inhibition by examining models of the two CYPs complexed to CsA.

Chemotherapy of schistosomiasis: an update.

No major changes have occurred during the past 20 years regarding the therapeutic tools available to the clinician for the treatment of schistosomiasis. If anything, the two drugs (oxamniquine and metrifonate) that are valuable alternatives to the drug of choice (praziquantel) have become more difficult to procure in some African countries. Here, Donato Cioli summarizes some of the most recent and interesting laboratory studies on potential antischistosomal compounds, and then reviews recent developments related to the mechanism of action of praziquantel and to the possible emergence of praziquantel-resistant schistosomes.

Enzymatic basis for the lack of oxamniquine activity in Schistosoma haematobium infections.

The notion that oxamniquine is active against Schistosoma mansoni but inactive against S. haematobium was confirmed using in vitro cultures of adult worms. Since oxamniquine and hycanthone have been shown to become effective upon activation by a schistosome enzyme, enzymatic tests were carried out to detect possible differences between the enzyme of S. mansoni and that of S. haematobium. It was found that the S. mansoni enzyme could activate hycanthone and, to a lesser extent, oxamniquine. The S. haematobium enzyme, on the other hand, was capable of activating hycanthone but virtually incapable of activating oxamniquine. It is concluded that the different activity of oxamniquine in the two species is due to differences in the drug-activating enzyme.

Characterization of a Schistosoma mansoni cDNA encoding a B-like cyclophilin and its expression in Escherichia coli.

A cDNA encoding a Schistosoma mansoni cyclophilin (SmCyP) has been cloned by polymerase chain reaction amplification using degenerate oligonucleotides based on known conserved cyclophilin (CyP) sequences and by screening an expression cDNA library. The cDNA sequence encodes a 21.5-kDa protein, which shares 59% sequence identity with human CyP B. The SmCyP protein was expressed in Escherichia coli with a hexahistidine affinity tag at its amino terminus and antibodies to the purified (His6)-SmCyP fusion protein were raised in a rabbit. Fractionation of parasite material followed by immunoblot analysis revealed that schistosome CyP is a soluble protein. The N-terminus of the predicted protein contains a hydrophobic region, suggestive of a signal sequence. Accordingly, a recombinant SmCyP protein, lacking the first 23 amino acids was found to share the same gel electrophoretic mobility as the parasite-derived CyP protein, suggesting cleavage of a leader sequence. Hybridization of genomic DNA to a full-length cDNA probe indicates that the SmCyP gene is present as a single copy. Immunohistological experiments in conjunction with confocal scanning laser microscopy and immune electron microscopy show that SmCyP is present in abundance in the adult worm as well as in the schistosomula. The function of CyP in the schistosome is presently unclear, but since its ligand, cyclosporin A, has antischistosomal activity, its function is expected to be a vital one.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Sequence and structure similarities of cathepsin B from the parasite Schistosoma mansoni and human liver.

A three-dimensional structure of Schistosoma mansoni cathepsin B was modelled using the coordinates of the crystal structure of the human liver enzyme. Both enzymes appear to share remarkable structural similarity. However, an examination of the models complexed with two synthetic inhibitors revealed differences in inhibitor binding, as confirmed by differences in the 50% inhibitory concentration of the same inhibitor.