Schistosoma mansoni: the occurrence of microvilli on the surface of the tegument during transformation from cercaria to schistosomulum

Mechanically produced schistosomula as a higher-throughput tools for phenotypic pre-screening in drug sensitivity assays: current research and future trends

It is crucial to develop new antischistosomal drugs since there is no vaccine and the whole world is relying on only a single drug for the treatment of schistosomiasis. One of the obstacles to the development of drugs is the absence of the high throughput objective screening methods to assess drug compounds efficacy. Thus for identification of new drug compounds candidates, fast and accurate in vitro assays are unavoidable and more research efforts in the field of drug discovery can target schistosomula. This review presents a substantial overview of the present state of in vitro drug sensitivity assays developed so far for the determination of anti-schistosomula activity of drug compounds, natural products and derivatives using newly transformed schistosomula (NTS). It highlights some of the challenges involved in in vitro compound screening using NTS and the way forward.

Integument of the tapeworm scolex. 1. Freeze-fracture of the syncytial layer, microvilli and discoid bodies

The tegument of cestodes is the most important and structurally complex metabolic interface between these parasites and the hostile environment in which they reside. In spite of the complex metabolic, regulatory and immunological properties of this layer of syncytial cytoplasm, which are relatively well known, the detailed fine structural anatomy of the cestode tegument remains equivocal. The present study therefore reports the freeze-fracture morphology of the tapeworm (Hymenolepis diminuta) tegument. The most important features revealed by analysis of platinum replicas of freeze-fractured tapeworm scolex-neck tegument include: (a) presence of highly ordered linear and/or circumferentially-orientated rows of intramembrane particles situated on the PF fracture face of microvillar plasma membrane, which may participate in movements of the microvilli, (b) presence of apparent 'pores' (11 nm in diameter) at the tips of the tegumentary microvilli, which could serve as regulated gates through which extramicrovillar surface coating materials can be extruded, and (c) the alignment of cytoplasmic discoid bodies into positions at the bases of the surface microvilli such that they could move into the core of each microvillus and thereby release their contents for extrusion (via the pores) onto the outer surface of the microvilli. Concomitantly, the limiting membrane of the discoid bodies could be added to the tegument plasma membrane and thereby contribute to the rapid turnover of the tegumentary surface. This study provides the first detailed account of the ultrastructural anatomy of the tapeworm tegument and is intended to serve as a point of reference for future investigations of tapeworm tegumentary functions.

Developmental and functional ultrastructure of Ornithodiplostomum ptychocheilus diplostomula (Trematoda: Strigeoidea) during invasion of the brain of the fish intermediate host, Pimephales promelas

We examined tegumental development of the diplostomulum of Ornithodiplostomum ptychocheilus, with respect to structural transformations that have functional relevance to the invasion, migration, and site establishment processes in the brain of the fish second-intermediate host, Pimephales promelas. Using a combination of brightfield, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal microscopy (CM), we demonstrated that the diplostomula become established in the outer region of the optic lobes within 24-48 hr of penetration and continue to grow and transform over a period of 4-14 days. During this period, the J-shaped body consists of 2 distinct regions: (1) a highly motile prosoma with distinctive tegumental spines and (2) an opisthosoma, the tegument of which is elaborated into a dense uniform layer of long, thin microvilli. The prosoma is alternately invaginated into and everted from the opisthosoma, thus constituting a protrusible proboscis. By day 14 postinfection (PI), the body has lost this bipartite structure and has taken on the uniformly flattened form characteristic of metacercariae. The transitory complex structure of the diplostomula appears to be well suited to burrowing through host tissues (primarily by action of the prosoma), followed by rapid dissociation of host tissue and nutrient accumulation (primarily by action of the opisthosoma) in preparation for metacercaria encystment.

Schistosome vaccines: a critical appraisal

An effective schistosome vaccine is a desirable control tool but progress towards that goal has been slow. Protective immunity has been difficult to demonstrate in humans, particularly children, so no routes to a vaccine have emerged from that source. The concept of concomitant immunity appeared to offer a paradigm for a vaccine operating against incoming larvae in the skin but did not yield the expected dividends. The mining of crude parasite extracts, the use of monoclonal antibodies and protein selection based on immunogenicity produced a panel of vaccine candidates, mostly of cytoplasmic origin. However, none of these performed well in independent rodent trials, but glutathione-S-transferase from Schistosoma haematobium is currently undergoing clinical trials as an anti-fecundity vaccine. The sequencing of the S. mansoni transcriptome and genome and the development of proteomic and microarray technologies has dramatically improved the possibilities for identifying novel vaccine candidates, particularly proteins secreted from or exposed at the surface of schistosomula and adult worms. These discoveries are leading to a new round of protein expression and protection experiments that will enable us to evaluate systematically all the major targets available for immune intervention. Only then will we know if schistosomes have an Achilles' heel.

The tegument surface membranes of the human blood parasite Schistosoma mansoni: a proteomic analysis after differential extraction

The blood fluke Schistosoma mansoni can live for years in the hepatic portal system of its human host and so must possess very effective mechanisms of immune evasion. The key to understanding how these operate lies in defining the molecular organisation of the exposed parasite surface. The adult worm is covered by a syncytial tegument, bounded externally by a plasma membrane and overlain by a laminate secretion, the membranocalyx. In order to determine the protein composition of this surface, the membranes were detached using a freeze/thaw technique and enriched by sucrose density gradient centrifugation. The resulting preparation was sequentially extracted with three reagents of increasing solubilising power. The extracts were separated by 2-DE and their protein constituents were identified by MS/MS, yielding predominantly cytosolic, cytoskeletal and membrane-associated proteins, respectively. After extraction, the final pellet containing membrane-spanning proteins was processed by liquid chromatographic techniques before MS. Transporters for sugars, amino acids, ions and other solutes were found together with membrane enzymes and proteins concerned with membrane structure. The proteins identified were categorised by their function and putative location on the basis of their homology with annotated proteins in other organisms.

Transgene expression in Schistosoma mansoni: introduction of RNA into schistosomula by electroporation

Despite their significance in human and veterinary medicine, and the ability to maintain the parasites in the mouse, relatively little functional detail is available regarding the biology of schistosomes. This deficit is due largely to the lack of well-developed molecular tools for manipulating gene expression in these parasites. Here, we describe an electroporation protocol that provides a routine approach for efficiently introducing nucleic acids into schistosomes. Using luciferase-encoding RNA for electroporation, and luciferase activity as a read-out, we established 400 microg/ml of RNA, and a 20 ms pulse at 125 V using a square wave electroporation generator to be optimal for electroporating schistosomes. Under these conditions schistosomula from 1 hr to 18 hr old could be successfully electroporated, the majority of parasites within a population expressed the introduced RNA, and acute mortality was negligible. Electroporation, as described here, makes possible experimental studies using transiently expressed constitutively active and/or dominant negative mutant proteins, etc. In addition, the finding that electroporation can be used to introduce RNA into schistosomula raises the possibility of using this approach to introduce either DNA constructs or dsRNA sequences, both of which might be expected to have longer-term, ideally inheritable, effects.

Schistosoma mansoni: ultrastructural demonstration of a miracidial glycocalyx that cross-reacts with antibodies raised against the cercarial glycocalyx

Cercariae are covered by a glycocalyx that is highly antigenic. Here, we have examined the surface of miracidia for a similar structure. The miracidia are covered by epithelial plates and syncytial ridges. By transmission electron microscopy, the plates and ridges were covered by a 0.5-micron-thick glycocalyx composed of a mesh of 9- to 10-nm fibrils that were stained by ruthenium red delivered in the aldehydes or ferrocyanide-reduced osmium tetroxide. Rabbit antibodies prepared against phenol extracted and chromatographed cercarial glycocalyx were detected by immunoelectron microscopy with secondary antibodies conjugated to horseradish peroxidase. Reaction product bound to both the miracidial and cercarial glycocalyx. In addition, the outer leaflets of the cercarial tegumental membrane and membranes of the miracidial surface structures, including plates, ridges, terebratorium, and sensory papillae, had reaction product. Controls incubated with nonspecific rabbit serum had no reaction product. By indirect immunofluorescence, antibodies against the cercarial glycocalyx stained both plates and ridges. As the miracidia transformed to sporocysts, the glycocalyx remained associated with the plates as they were sloughed. These studies demonstrate that miracidia possess a glycocalyx similar in structure and antigenicity to the cercarial glycocalyx.

Schistosoma mansoni: development of the cercarial glycocalyx

The development of the cercarial glycocalyx of Schistosoma mansoni was studied by transmission electron microscopy and immunofluorescence light microscopy employing antibodies raised against extracted and chromatographed glycocalyx. By electron microscopy, cercariae present in the brood chamber of daughter sporocysts were surrounded by an electron-dense granular and fibrillar matrix. This material appeared structurally distinct from the glycocalyx which was coarsely fibrillar and located only on the surface of organisms that had developed a final tegument. The thickness of the glycocalyx apparently increased with the maturation of the tegument, since teguments that had many spines also had the thickest glycocalyx. Immunofluorescent staining of frozen sections of infected snail hepatopancreas showed that glycocalyx antigens were present on the surface of the cercariae and not in the matrix within the brood chamber or in snail tissues. Immunofluorescent staining of isolated larval cercariae showed staining of some but not all parasites with partially elongated tails. These studies suggest that the glycocalyx develops late in cercarial development (late in Stage 6 or in Stage 7 of Cheng and Bier), is made by the cercariae themselves, and is not a product of either the sporocyst wall cells or snail hepatopancreas.

Analysis of total and surface membrane lipids of Schistosoma mansoni

Lipids extracted from whole worm homogenates and tegumental outer membranes of guinea pig-derived 5-day, 2-, 3- and 6-week old schistosomes have been analysed by thin layer chromatography. Six-week hamster-derived parasites have been studied for comparative purposes. All homogenates contained neutral lipids, cholesterol and several phospholipids; phosphatidyl choline and phosphatidyl ethanolamine were major components. Phospholipid (P3) was absent from homogenates of 5-day worms but was present in older parasites. A single phospholipid (P4) which co-chromatographed with phosphatidyl glycerol was unique to hamster-derived parasites. One glycolipid (G1) was ubiquitous to all homogenates and co-chromatographed with the monogalactosyl ceramide marker. A second sugar-containing lipid (G2) was unique to 3-week worm homogenates, and was highly polar. It was resolved beneath the trigalactosyl ceramide marker. Tegumental membranes isolated from 6-week adults contained at least five glycolipids, four of which were also highly polar. Cholesterol and two dominant phospholipids occurred in the membranes of 2-, 3-, and 6-week worms. One phospholipid co-chromatographed with phosphatidyl choline; the other had an Rf value (relative band speed) equivalent to phosphatidyl ethanolamine. Membranes from liver stage parasites contained a further phospholipid which cochromatographed with sphingomyelin, and three additional, phosphate-staining lipids (P1, P3 and P6). Five sugar-containing lipids occurred in adult membranes only; four were highly polar, being resolved near the origin. Similar components were identified in extracts of host erythrocytes. The remaining membrane lipid appeared homologous to G1 identified in the whole worm homogenates. Important changes in lipid composition thus occur during schistosome growth and maturation in guinea pigs; moreover, worms derived from different rodents express different lipids.

The nature and action of host signals

It is clear that excystations in vitro of the coccidia so far examined involves two steps, in the first of which CO2 is important, and the second, in which an external source of chymotrypsin and surface-active agents are required. However, the details of the mechanism of excystment are not clear. We do not know how the presence of CO2 changes the permeability of the oocyst wall. We do not know whether CO2 does anything to the sporozoite or sporocyst; the circumstance that mechanically-released sporocysts readily excyst under appropriate conditions without the necessity for high concentrations of, or perhaps any, CO2 suggests it does not. Circumstantial evidence suggests that the substrate in which chymotrypsin acts is the Stieda body, but whether the enzyme has other roles we do not know. Similarly, the role of bile is ill-defined, although it does seem that the induction of activity is important--but how is this brought about? The techniques available to excyst oocysts are, for many species, very efficient. If CO2 is, as it seems to be, a fundamental stimulus, then efficiency might be enhanced if more attention was given, not so much to increasing the time of exposure and amount of CO2 in the gas phase, but rather to the pH of the medium, which is rarely stated or apparently, controlled. The pH determines the proportion of the different carbonate species in solution, which may be of greater significance than the partial pressure of CO2 in the gas phase (see also Section V A). Although high numbers of excysted sporocysts can be obtained with a particular technique, this does not necessarily mean that all the signals supplied by the host are reproduced in vitro. Jackson (1962) found it necessary to wash oocysts in water or dilute buffers between the primary phase and the secondary phase, a step which implies a deficiency in the methods he used. Commonly, oocysts are exposed to a strong solution of L-cysteine. Does this reflect a general deficiency in the technique, or a counterpart of strongly reducing conditions in ruminant and non-ruminant alike? It seems that we have only a very general outline of excystment, and that we do not understand the details. Yet the problem seems to have been put aside; the most recent relevant reference we have found is dated 1983.

Schistosoma mansoni: phospholipid methylation and the escape of schistosomula from in vitro cytotoxic reaction

The ability of Schistosoma mansoni schistosomula to evade in vitro cytotoxic activity of antibodies plus complement is shown to be increased by incubation with Concanavalin A (Con A) or with non-immune inactivated human serum. This effect was not observed if S-adenosyl-homocysteine (SAH) a methyltransferase inhibitor was added to the incubation medium. Methyl group incorporation occurs in schistosomulum phospholipids if parasites are incubated in Earle's balanced salt solution. This incorporation is increased by Con A addition and this increase is inhibited by SAH. Supernatants of schistosomula incubated in culture media containing Con A were able to promote phospholipid methylation, showing that methyltransferases were liberated into the culture media. The possible roles played by these phenomena in host-parasite interactions are discussed.

Inhibition of human peripheral blood mononuclear cell proliferative responses by released materials from Schistosoma mansoni cercariae

During in vitro transformation of Schistosoma mansoni cercariae into schistosomula, surface and glandular materials are released into the culture medium. Extracts of these materials, termed cercarial released extracts 1 and 2 (CRE-1 and CRE-2), were analysed and found to consist primarily of protein and carbohydrate at ratios of 5:1 (CRE-1) and 7:1 (CRE-2). It was observed that inclusion of either CRE-1 or CRE-2 in cultures of human peripheral blood mononuclear cells (PBMN) led to decreased cell proliferation. This was true whether the cells were resting, control cultures or were stimulated with either phytohaemagglutinin or an antigenic preparation from adult S. mansoni worms. The inhibition was equally effective with PBMN of patients with active schistosomal infection or PBMN from uninfected individuals. Since these materials are released spontaneously during cercarial-to-schistosomular transformation they may have a putative immunosuppressive effect in decreasing antischistosomular activities early after cercarial penetration.

The cercarial glycocalyx of Schistosoma mansoni

Cercariae, the freshwater stage of Schistosoma mansoni infectious to man, are covered by a single unit membrane and an immunogenic glycocalyx. When cercariae penetrate the host skin, they transform to schistosomula by shedding tails, secreting mucous and enzymes, and forming microvilli over their surface. Here the loss of the glycocalyx from cercariae transforming in vitro was studied morphologically and biochemically. By scanning electron microscopy, the glycocalyx was a dense mesh composed of 15-30 nm fibrils that obscured spines on the cercarial surface. The glycocalyx was absent on organisms fixed without osmium and was partially lost when parasites aggregated in their own secretions before fixation. By transmission electron microscopy, a 1-2 microns thick mesh of 8-15-nm fibrils was seen on parasites incubated with anti-schistosomal antibodies or fixed in aldehydes containing tannic acid or ruthenium red. Cercariae transformed to schistosomula when tails were removed mechanically and parasites were incubated in saline. Within 5 min of transformation, organisms synchronously formed microvilli which elongated to 3-5 microns by 20 min and then were shed. However, considerable fibrillar material remained adherent to the double unit membrane surface of schistosomula. For biochemical labeling, parasites were treated with eserine sulfate, which blocked cercarial swimming, secretion, infectivity, and transformation to schistosomula. Material labeled by periodate oxidation and NaB3H4 was on the surface as shown by autoradiography and had an apparent molecular weight of greater than 10(6) by chromatography. Periodate-NaB3H4 glycocalyx had an isoelectric point of 5.0 +/- 0.4 and was precipitable with anti-schistosomal antibodies. More than 60% of the radiolabeled glycocalyx was released into the medium by transforming parasites in 3 h and was recovered as high molecular weight material. Parasites labeled with periodate and fluorescein-thiosemicarbazide and then transformed had a corona of fluorescence containing microvilli, much of which was shed onto the slide. Material on cercariae labeled by lodogen-catalyzed iodination was also of high molecular weight and was antigenic. In conclusion, the cercarial glycocalyx appears to be composed of acidic high molecular weight fibrils which are antigenic and incompletely cleared during transformation.

Characterization of chemical stimuli for the penetration of Schistosoma mansoni cercariae. II. Conditions and mode of action

The mode of action of chemical substances which trigger the penetration of S. mansoni cercariae into agar substrata is studied. The effectiveness of these substances is largely independent on their polarity and water solubility. Thus, they do not seem to act by a passive membrane permeation process, but they may interact with specific receptor sites, which are characterized. The receptor sites seem to respond to the following chemical characteristics of the stimulating aliphatic hydrocarbon chain: Carboxylic end group, lipophilic end group, chain length, cis-double bond. The penetration stimulating substances cause, even in cercariae in free water, a transformation of the tegument, manifested as a reduction of the Cercarienhüllen-Reaktion and a loss of osmotic protection.

Scanning electron microscopical observations on antigen-antibody coat formation on mechanically transformed Schistosoma mansoni schistosomula

As part of a study of the antigenic composition of the tegument of mechanically transformed Schistosoma mansoni schistosomula, the phenomenon of coat formation around such schistosomula when incubated in medium containing serum of S. mansoni infected mice was investigated with light and scanning electron microscopy. Coat formation was only observed on freshly transformed schistosomula, while schistosomula that had been incubated overnight at 37 degrees C in medium without serum from infected mice were no longer able to produce a coat. When freshly transformed schistosomula were incubated in medium containing serum from infected mice, it was found that individual spines on the tegument were covered with a coat within 1 h. After 2 h of incubation, the schistosomula were completely covered with a thick coat, starting at the posterior end. This coat was actively shed by the schistosomula after 3 h of incubation. To investigate the mechanism of coat formation, experiments were carried out using serum from uninfected mice, heat-inactivated serum from infected mice, and the IgG fraction of serum from infected mice. From the experiments it was concluded that the coat consisted of antigen-antibody complexes, involving IgG antibodies and complement. A detailed analysis of the easily obtainable coats may lead to a better understanding of the antigenic structure of the cercarial and schistosomular tegumental membrane.

Morphological studies on the killing of schistosomula of Schistosoma mansoni by human eosinophil and neutrophil cationic proteins in vitro

Purified eosinophil and neutrophil cationic proteins isolated from the lysosomal secretion granules of human granulocytes, evoke characteristic, dose-dependent morphological changes in young schistosomula of S. mansoni. The first sign of damage is seen with in 15-30 min of incubation and involves the formation of surface microvilli and blebs. Subsequently, tegumental evaginations of varying size are developed, but these appear to explode with rapidity, so that lengths of expanded tegumental outer membrane are deposited over the severely damaged surface of the parasite. Both types of granulocyte proteins are able to effect comparable damage at equimolar concentration. Other cationic proteins such as protamine and poly-L-arginine also damage the parasite surface but the pathological changes differ from those induced by the granulocyte proteins and they take longer to develop. In contrast, lysozyme-treated parasites are virtually similar to control schistosomula incubated in medium alone. These findings are discussed in relation to published data concerning the interaction of intact granulocytes with young schistosomula both in vitro and in vivo.

A freeze fracture study of the developing tegumental outer membrane of Schistosoma mansoni

The freeze fracture technique has been used to quantify changes in the integral components of the double outer membrane of Schistosoma mansoni during the 6-week period of development within the mouse. The intramembraneous particle (IMP) density on the P1 face begins to rise within 6 h of host penetration, reaches a maximum at day 4 and then falls rapidly after day 9, so that it is at a low level between 3 and 6 weeks. The E1 face IMP density follows the same course as that of the P1 face except that maximum particle density is recorded on day 1 and the counts begin to fall on day 5. The IMP density on the P2 face remains at a consistently low level throughout development. The E2 face IMP density rises gradually to a peak at day 4, when the parasites have migrated to the lungs, and remains thereafter at a similar level, so that by 6 weeks the E2 face has a higher IMP density than the other three fracture faces. The E2 face IMP show a marked increase in size on day 4. Morphological studies indicate that a different type of inclusion body makes a transient appearance in the tegument of the lung worms, and immunocytochemical techniques show the lung worms to be nonimmunogenic. It is suggested, therefore, that the E2 face IMP may represent complexes of parasite antigens and acquired host antigens. The tegumental membranes of cultured specimens have also been examined by freeze fracturing and the IMP densities compared with those obtained from in vivo parasites; the cultured schistosomula have a lower E2 face particle density than the in vivo specimens.

Schistosoma mansoni: a comparative study of artificially transformed schistosomula and schistosomula recovered after cercarial penetration of isolated skin

A comparison was made of the ultrastructure, development and antigenic nature of the surfaces and of the viability of three types of Schistosoma mansoni: schistosomula formed after cercariae had penetrated isolated skin (SS) schistosomula produced after mechanical separation of cercarial tails from bodies (MS), and schistosomula transformed from cercariae after incubation in fresh rat serum (RS). Within 2h of transformation, the surface membranes of all three types of schistosomula had changed from trilaminate to heptalaminate structures and SS and MS had lost their cercarial glycocalyx. Initially a dense amorphous material was demonstrated on the surfaces of RS, which was thought to be the result of an interaction between a factor in rat serum and the glycocalyx; this material was greatly reduced within 2 h of transformation. The pre-acetabular glands of SS were emptied while those of MS and RS retained their contents. Immunofluorescent studies showed that all schistosomula bound serum from mice immune to S. mansoni, but the binding was stronger with MS and RS. The mixed agglutination reaction demonstrated the presence of human A and B blood group-like antigenic determinants on approximately 30% of 3h old SS; these determinants were not detected on MS or RS. In vitro, the development of MS and RS was similar to SS; the first schistosomula reached the "gut-closed" stage by day 10; 50-70% of SS reached this stage by day 12, in contrast to only 25-50% of MS and RS. Between 28 and 45% of all schistosomula developed to maturity when injected intravenously into mice. It was concluded that the two types of artificially prepared schistosomula fulfil the main criteria of transformation from cercaria to schistosomulum. Further, it is suggested that MS are the most appropriate source of material for immunochemical and physiological studies.