Schistosoma mansoni: analysis of cercarial transformation methods

Suppression of Schistosoma japonicum Acetylcholinesterase Affects Parasite Growth and Development

To further investigate the importance of Schistosoma japonicum acetylcholinesterase (SjAChE) in cholinergic signaling for parasite growth and development, we used RNA interference (RNAi) to knock-down its expression in adults and eggs in vitro. This resulted in its reduced transcription but also expression of other important genes involved both in cholinergic signaling and glucose uptake were impacted substantially. Significant decreases in AChE protein expression, AChE enzymatic activity, and glucose uptake were observed in the SjAChE-knockdown parasites compared with luciferase controls. In vaccine/challenge experiments, we found that immunization of mice with recombinant SjAChE (rSjAChE) expressed in Escherichia coli elicited reductions in male worm numbers (33%), liver granuloma density (41%), and reduced numbers of mature intestinal eggs (73%) in the vaccinated group compared with the control group. These results indicate AChE plays an important role in the metabolism of male worms, and impacts indirectly on female fecundity leading to increased numbers of immature eggs being released and reduced sizes of liver granulomas. Furthermore, cytokine analysis showed that immunization of mice with rSjAChE elicited a predominantly Th1-type immune response characterized by increased production of IFNγ in splenic CD4⁺ T cells of vaccinated mice. The study confirms the potential of SjAChE as a vaccine/drug candidate against zoonotic schistosomiasis japonica.

Acetylcholinesterase and Nicotinic Acetylcholine Receptors in Schistosomes and Other Parasitic Helminths

Schistosomiasis, which is caused by helminth trematode blood flukes of the genus Schistosoma, is a serious health and economic problem in tropical areas, and the second most prevalent parasitic disease after malaria. Currently, there is no effective vaccine available and treatment is entirely dependent on a single drug, praziquantel (PZQ), raising a significant potential public health threat due to the emergence of PZQ drug resistance. It is thus urgent and necessary to explore novel therapeutic targets for the treatment of schistosomiasis. Previous studies demonstrated that acetylcholinesterase (AChE) and nicotinic acetylcholine receptors (nAChRs) play important roles in the schistosome nervous system and ion channels, both of which are targeted by a number of currently approved and marketed anthelminthic drugs. To improve understanding of the functions of the cholinergic system in schistosomes, this article reviews previous studies on AChE and nAChRs in schistosomes and other helminths and discusses their potential as suitable targets for vaccine development and drug design against schistosomiasis.

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.

Hsp70 May Be a Molecular Regulator of Schistosome Host Invasion

Schistosomiasis is a debilitating disease that affects over 240 million people worldwide and is considered the most important neglected tropical disease following malaria. Free-swimming freshwater cercariae, one of the six morphologically distinct schistosome life stages, infect humans by directly penetrating through the skin. Cercariae identify and seek the host by sensing chemicals released from human skin. When they reach the host, they burrow into the skin with the help of proteases and other contents released from their acetabular glands and transform into schistosomula, the subsequent larval worm stage upon skin infection. Relative to host invasion, studies have primarily focused on the nature of the acetabular gland secretions, immune response of the host upon exposure to cercariae, and cercaria-schistosomulum transformation methods. However, the molecular signaling pathways involved from host-seeking through the decision to penetrate skin are not well understood. We recently observed that heat shock factor 1 (Hsf1) is localized to the acetabular glands of infectious schistosome cercariae, prompting us to investigate a potential role for heat shock proteins (HSPs) in cercarial invasion. In this study, we report that cercarial invasion behavior, similar to the behavior of cercariae exposed to human skin lipid, is regulated through an Hsp70-dependent process, which we show by using chemical agents that target Hsp70. The observation that biologically active protein activity modulators can elicit a direct and clear behavioral change in parasitic schistosome larvae is itself interesting and has not been previously observed. This finding suggests a novel role for Hsp70 to act as a switch in the cercaria-schistosomulum transformation, and it allows us to begin elucidating the pathways associated with cercarial host invasion. In addition, because the Hsp70 protein and its structure/function is highly conserved, the model that Hsp70 acts as a behavior transitional switch could be relevant to other parasites that also undergo an invasion process and can apply more broadly to other organisms during morphological transitions. Finally, it points to a new function for HSPs in parasite/host interactions.

Parasitic schistosome worms cause morbid disease in over 240 million individuals worldwide. Acute infections with these worms can lead to Katayama fever, while chronic infections can lead to portal hypertension, enlarged abdomen, and liver damage. The infective larval stage, called cercariae, are free-swimming and can detect, seek, and penetrate human skin to enter the human host circulatory system, eventually developing into egg-laying adult worms that cause schistosomiasis. Molecular pathways associated with the initial cercarial invasion of the host, however, are largely unknown, especially with respect to the parasite-specific signals involved in host detection and subsequent decision to invade. Here, we describe a role for Hsp70 in cercarial invasion behavior. To date, only generic stimulation with skin lipid, linoleic acid or L-arginine are known to induce cercarial invasion behavior; thus, we can begin an initial investigation of molecular requirements for host invasion and environment transition for schistosomes and possibly other parasitic organisms.

Functional characterisation of Schistosoma japonicum acetylcholinesterase

BACKGROUND

Acetylcholinesterase (AChE) is an important metabolic enzyme of schistosomes present in the musculature and on the surface of the blood stage where it has been implicated in the modulation of glucose scavenging from mammalian host blood. As both a target for the antischistosomal drug metrifonate and as a potential vaccine candidate, AChE has been characterised in the schistosome species Schistosoma mansoni, S. haematobium and S. bovis, but not in S. japonicum. Recently, using a schistosome protein microarray, a predicted S. japonicum acetylcholinesterase precursor was significantly targeted by protective IgG1 immune responses in S. haematobium-exposed individuals that had acquired drug-induced resistance to schistosomiasis after praziquantel treatment.

RESULTS

We report the full-length cDNA sequence and describe phylogenetic and molecular structural analysis to facilitate understanding of the biological function of AChE (SjAChE) in S. japonicum. The protein has high sequence identity (88 %) with the AChEs in S. mansoni, S. haematobium and S. bovis and has 25 % sequence similarity with human AChE, suggestive of a highly specialised role for the enzyme in both parasite and host. We immunolocalized SjAChE and demonstrated its presence on the surface of adult worms and schistosomula, as well as its lower expression in parenchymal regions. The relatively abundance of AChE activity (90 %) present on the surface of adult S. japonicum when compared with that reported in other schistosomes suggests SjAChE may be a more effective drug or immunological target against this species. We also demonstrate that the classical inhibitor of AChE, BW285c51, inhibited AChE activity in tegumental extracts of paired worms, single males and single females by 59, 22 and 50 %, respectively, after 24 h incubation with 200 μM BW284c51.

CONCLUSIONS

These results build on previous studies in other schistosome species indicating major differences in the enzyme between parasite and mammalian host, and provide further support for the design of an anti-schistosome intervention targeting AChE.

Functional expression of a novel Kunitz type protease inhibitor from the human blood fluke Schistosoma mansoni

BACKGROUND

Schistosomes are able to survive for prolonged periods in the blood system, despite continuous contact with coagulatory factors and mediators of the host immune system. Protease inhibitors likely play a critical role in host immune modulation thereby promoting parasite survival in this extremely hostile environment. Even though Kunitz type serine protease inhibitors have been shown to play important physiological functions in a range of organisms these proteins are less well characterised in parasitic helminths.

METHODS

We have cloned one gene sequence from S. mansoni, Smp_147730 (SmKI-1) which is coded for single domain Kunitz type protease inhibitor, E. coli-expressed and purified. Immunolocalisation and western blotting was carried out using affinity purified polyclonal anti-SmKI-1 murine antibodies to determine SmKI-1 expression in the parasite. Protease inhibitor assays and coagulation assays were performed to evaluate the functional roles of SmKI-1.

RESULTS

SmKI-1 is localised in the tegument of adult worms and the sub-shell region of eggs. Furthermore, this Kunitz protein is secreted into the host in the ES products of the adult worm. Recombinant SmKI-1 inhibited mammalian trypsin, chymotrypsin, neutrophil elastase, FXa and plasma kallikrein with IC50 values of 35 nM, 61 nM, 56 nM, 142 nM and 112 nM, respectively. However, no inhibition was detected for pancreatic elastase or cathepsin G. SmKI-1 (4 μM) delayed blood clot formation, reflected in an approximately three fold increase in activated partial thromboplastin time and prothrombin time.

CONCLUSIONS

We have functionally characterised the first Kunitz type protease inhibitor (SmKI-1) from S. mansoni and show that it has anti-inflammatory and anti-coagulant properties. SmKI-1 is one of a number of putative Kunitz proteins in schistosomes that have presumably evolved as an adaptation to protect these parasites from the defence mechanisms of their mammalian hosts. As such they may represent novel vaccine candidates and/or drug targets for schistosomiasis control.

A Microtus fortis protein, serum albumin, is a novel inhibitor of Schistosoma japonicum schistosomula

Schistosomiasis is an endemic parasite disease and praziquantel is the only drug currently in use to control this disease. Experimental and epidemiological evidence strongly suggests that Microtus fortis ( Mf ) is a naturally resistant vertebrate host of Schistosoma japonicum . In the present study, we found that Mf serum albumin ( Mf -albumin) and the conditioned medium of pcDNA3.1- Mf -albumin caused 46.2% and 38.7% schistosomula death rates in 96 h, respectively, which were significantly higher than that of the negative control (p < 0.05). We also found that mice injected with Mf -albumin had a 43.5% reduction in worm burden and a 48.1% reduction in liver eggs per gram (p < 0.05) in comparison to the control animals. To characterise the mechanisms involved in clearance, schistosomula were incubated with fluorescein isothiocyanate-labelled Mf -albumin and fluorescent enrichment effects were found in the gut lumen of schistosomula after 48 h of incubation. Next, digestive tract excretions from schistosomula were collected and the sensitivity of Mf -albumin to digestive tract excretions was evaluated. The results indicated that schistosomula digestive tract excretions showed indigestibility of Mf -albumin. The death of schistosomula could be partially attributed to the lack of digestion of Mf -albumin by digestive tract excretions during the development of the schistosomula stage. Therefore, these data indicate the potential of Mf -albumin as one of the major selective forces for schistosomiasis.

Comparative study of transcriptome profiles of mechanical- and skin-transformed Schistosoma mansoni schistosomula

Schistosome infection begins with the penetration of cercariae through healthy unbroken host skin. This process leads to the transformation of the free-living larvae into obligate parasites called schistosomula. This irreversible transformation, which occurs in as little as two hours, involves casting the cercaria tail and complete remodelling of the surface membrane. At this stage, parasites are vulnerable to host immune attack and oxidative stress. Consequently, the mechanisms by which the parasite recognises and swiftly adapts to the human host are still the subject of many studies, especially in the context of development of intervention strategies against schistosomiasis infection. Because obtaining enough material from in vivo infections is not always feasible for such studies, the transformation process is often mimicked in the laboratory by application of shear pressure to a cercarial sample resulting in mechanically transformed (MT) schistosomula. These parasites share remarkable morphological and biochemical similarity to the naturally transformed counterparts and have been considered a good proxy for parasites undergoing natural infection. Relying on this equivalency, MT schistosomula have been used almost exclusively in high-throughput studies of gene expression, identification of drug targets and identification of effective drugs against schistosomes. However, the transcriptional equivalency between skin-transformed (ST) and MT schistosomula has never been proven. In our approach to compare these two types of schistosomula preparations and to explore differences in gene expression triggered by the presence of a skin barrier, we performed RNA-seq transcriptome profiling of ST and MT schistosomula at 24 hours post transformation. We report that these two very distinct schistosomula preparations differ only in the expression of 38 genes (out of ∼11,000), providing convincing evidence to resolve the skin vs. mechanical long-lasting controversy.

Schistosomiasis is an endemic parasitic disease affecting ∼200 million people in the most socioeconomically deprived regions of the world. Human infection occurs during water contact where free-living larvae called cercariae penetrate host skin and become parasitic organisms called schistosomula. This stage represents the first encounter of the parasites with the host and is also regarded as one of the most vulnerable stages of the parasite's life cycle. Therefore, schistosomula are the focus of many studies, many of which look at changes in the expression of genes as a way of understanding the process of infection, identifying potential drug targets and vaccine candidates. Because collecting enough parasitic material from natural infections is not possible for certain types of studies (for example, gene expression studies), a mechanical transformation of the cercariae into schistosomula is often used instead and assumed as a good proxy for the natural transformation process. However, the equivalency of gene expression profiles between naturally transformed parasites and the mechanically transformed counterparts has never been studied. In this report, we analyse differences in gene expression patterns between these two different parasite preparations and provide enough data to resolve a long-lasting controversy.

Development of an in vitro drug screening assay using Schistosoma haematobium schistosomula

BACKGROUND

The development of novel antischistosomal drugs is crucial, as currently no vaccine and only a single drug is available for the treatment of schistosomiasis. Fast and accurate in vitro assays are urgently needed to identify new drug candidates and research efforts should include Schistosoma haematobium. The aim of the present study was to develop a S. haematobium drug sensitivity assay based on newly transformed schistosomula (NTS).

METHODS

We first undertook comparative studies on the cercarial emergence rhythms of the intermediate host snails Biomphalaria glabrata (S. mansoni) and Bulinus truncatus (S. haematobium). Two transformation methods as well as three purification methods were studied on S. haematobium cercariae in order to produce a large number of viable and clean NTS. Known antischistosomal drugs were tested in the established NTS assay in vitro. Drug effects were evaluated either microscopically or fluorometrically, using a resazurin based viability marker. Microscopically obtained IC₅₀ values were compared with results obtained for S. mansoni.

RESULTS

A circadian rhythm existed in both snail species. Infected B. truncatus snails shed less cercariae than B. glabrata during the testing period. The highest transformation rate (69%) of S. haematobium cercariae into NTS was obtained with the vortex transformation (mechanical input) and the highest purification factor was observed using Percoll®. The fluorimetric readout based on resazurin was very precise in detecting dead or/and severely damaged schistosomula.

CONCLUSIONS

With the use of viability markers such as resazurin, drug screening assays using S. haematobium NTS can be efficiently performed. However, drugs acting on the morphology and motility of S. haematobium NTS, such as metrifonate are missed. Drug sensitivity assays with NTS of both species, S. haematobium and S. mansoni, showed very similar results using known antischistosomal drugs. The S. mansoni NTS assay might be more suitable as primary screen in drug discovery efforts, which ultimately aim for a broad-spectrum antischistosomal drug as a larger number of S. mansoni NTS can be generated.

In vitro cercariae transformation: comparison of mechanical and nonmechanical methods and observation of morphological changes of detached cercariae tails

Schistosomula, the larval stage of schistosomes in vertebrate hosts, are highly vulnerable and considered an ideal target for vaccine and drug development. Although the schistosomule stage is essential for biological studies, collecting sufficient numbers of schistosomula from their definitive hosts in vivo is difficult to accomplish. However, in vitro collection via cercariae transformation can effectively yield high numbers of schistosomula. We compared a current and widely used double-ended-needle mechanical transformation method to a culture medium based on a nonmechanical method. We found the rates of transformed cercariae, i.e., separated cercariae heads from tails, differed by only 2-7% at 0.5, 1, and 2 days in culture and that there was no significant difference in the number of transformed cercariae between the transformation methods at 3 and 4 days in culture. Notably, the mechanical and nonmechanical cercariae transformation methods both yielded significantly large and similar quantities of viable schistosomula. Given that the nonmechanical method is simpler and less damaging to the parasites, we recommend the use of it as an alternative way for in vitro cercariae transformation. In addition, we also observed morphological changes of the detached cercariae tails in culture medium. Interestingly, the tails are able to regenerate head-like organs/tissues and survive for at least 4 days. This intriguing change suggests unique biological features of the cells in the tails.

Karyopherin alpha 2 (KPNA2) is associated with the natural resistance to Schistosoma japanicum infection in Microtus fortis

Microtus fortis is a naturally vertebrate host resistant to Schistosoma japonicum infection. In order to understand the molecular mechanism and identify the molecules related to the natural resistance to S. japanicum infection of M. fortis, we screened a gene pool named gE76 by expression cloning and proved it to have high anti-schistosomula effects in our previous work. In this study we identified a clone named gE76.44. We found that the conditioned medium of pcDNA1.1-gE76.44 caused 14.0% schistosomula death rate in 96 h, which was significantly higher than that of negative control (P<0.05). The gE76.44 was sequenced and the full-length cDNA was 2008 bp with ORF of 1590bp encoding a polypeptide of 529 amino acid residues. Bioinformatics analysis indicated it was the homologue of karyopherin alpha 2 (KPNA2). To further confirm its anti-schistosome activity, we inserted full length of Mf-KPNA2 (KPNA2 of M. fortis) gene into a retroviral expression vector pLXSN and packaged the recombinant virus with PA317 cells. Mice infected with S. japanicum cercariae were administrated by intravenous injection through tail vein and treated with pLXSN-KPNA2. Adult worms and egg reduction were counted after heart perfusion of mice 42 d after infection. We found that compared with the control, mice injected with Mf-KPNA2 had 39.42% worm burden reduction and 76.50% reduction in LEPG (liver eggs per gram) (P<0.01), indicating its anti-schistosome effect of Mf-KPNA2 in vivo. Taken together, the results suggested Mf-KPNA2 as a novel anti-schistosome molecule in vitro and in vivo.

Identification of the resistance of a novel molecule heat shock protein 90alpha (HSP90alpha) in Microtus fortis to Schistosoma japonicum infection

Microtus fortis is a naturally resistant vertebrate host of Schistosoma japonicum by preventing completion of parasite's life cycle. Sera of M. fortis were found to have anti-schistosome effect in vitro and in vivo. In order to identify genes associated with the anti-schistosome effect of M. fortis, we screened a M. fortis marrow cDNA expression library by expression cloning and identified a 331-bp clone gC14.75. It was the homologue of heat shock protein 90alpha (HSP90alpha). Full-length of M. fortis HSP90alpha gene, Mf-HSP90alpha, was amplified according to gC14.75 and Cricetulus griseus HSP90alpha. To test the potential anti-schistosome function of Mf-HSP90alpha, we prepared conditioned medium of Mf-HSP90alpha and added it to schistosomula cultured in vitro. It caused 27.0% schistosomula death rate in 96h, which was considerably higher than that of negative control. We transferred Mf-HSP90alpha by retroviral expression vector pLXSN into mice to investigate its anti-schistosome effect in vivo. Compared with those of DMEM injection control, mice injected with Mf-HSP90alpha recombinant retrovirus had 40.8% worm burden reduction and 57.9% reduction in liver eggs per gram (LEPG) indicating its anti-schistosome effect in vivo. Taken together, our results suggested Mf-HSP90alpha as a novel anti-schistosome molecule in vitro and in vivo.

Suppression of mRNAs encoding tegument tetraspanins from Schistosoma mansoni results in impaired tegument turnover

Schistosomes express a family of integral membrane proteins, called tetraspanins (TSPs), in the outer surface membranes of the tegument. Two of these tetraspanins, Sm-TSP-1 and Sm-TSP-2, confer protection as vaccines in mice, and individuals who are naturally resistant to S. mansoni infection mount a strong IgG response to Sm-TSP-2. To determine their functions in the tegument of S. mansoni we used RNA interference to silence expression of Sm-tsp-1 and Sm-tsp-2 mRNAs. Soaking of parasites in Sm-tsp dsRNAs resulted in 61% (p = 0.009) and 74% (p = 0.009) reductions in Sm-tsp-1 and Sm-tsp-2 transcription levels, respectively, in adult worms, and 67%–75% (p = 0.011) and 69%–89% (p = 0.004) reductions in Sm-tsp-1 and Sm-tsp-2 transcription levels, respectively, in schistosomula compared to worms treated with irrelevant control (luciferase) dsRNA. Ultrastructural morphology of adult worms treated in vitro with Sm-tsp-2 dsRNA displayed a distinctly vacuolated and thinner tegument compared with controls. Schistosomula exposed in vitro to Sm-tsp-2 dsRNA had a significantly thinner and more vacuolated tegument, and morphology consistent with a failure of tegumentary invaginations to close. Injection of mice with schistosomula that had been electroporated with Sm-tsp-1 and Sm-tsp-2 dsRNAs resulted in 61% (p = 0.005) and 83% (p = 0.002) reductions in the numbers of parasites recovered from the mesenteries four weeks later when compared to dsRNA-treated controls. These results imply that tetraspanins play important structural roles impacting tegument development, maturation or stability.

Schistosomes, or blood flukes, reside in the blood vessels surrounding the liver and bowel of their human hosts. They infect 200 million people and kill many thousands each year in developing countries. The parasites cover themselves in a unique series of cell membranes called the tegument. Molecules in the tegument membranes are a major target for the development of new drugs and vaccines against the parasite. Here we show that at least one member of a family of tegument membrane proteins called tetraspanins, Sm-TSP-2, is integral to the proper formation of the tegument and subsequent survival of the parasite in its human host, providing a potential mechanism by which a vaccine based on Sm-TSP-2 protects immunized hosts.

The 26S proteasome in Schistosoma mansoni: Bioinformatics analysis, developmental expression, and RNA interference (RNAi) studies

The 26S proteasome is a proteolytic complex responsible for the degradation of the vast majority of eukaryotic proteins. Regulated proteolysis by the proteasome is thought to influence cell cycle progression, transcriptional control, and other critical cellular processes. Here, we used a bioinformatics approach to identify the proteasomal constituents of the parasitic trematode Schistosoma mansoni. A detailed search of the S. mansoni genome database identified a total of 31 putative proteasomal subunits, including 17 subunits of the regulatory (19S) complex and 14 predicted catalytic (20S) subunits. A quantitative real-time RT-PCR analysis of subunit expression levels revealed that the S. mansoni proteasome components are differentially expressed among cercaria, schistosomula, and adult worms. In particular, the data suggest that the proteasome may be downregulated during the early stages of schistosomula development and is subsequently upregulated as the parasite matures to the adult stage. To test for biological relevance, we developed a transfection-based RNA interference method to knockdown the expression of the proteasome subunit, SmRPN11/POH1. Transfection of in vitro transformed S. mansoni schistosomula with specific short-interfering RNAs (siRNAs) diminished SmRPN11/POH1 expression nearly 80%, as determined by quantitative RT-PCR analysis, and also decreased parasite viability 78%, whereas no significant effect could be seen after treatment with the same amount of an irrelevant siRNA. These results indicate that the subunit SmRPN11/POH1 is an essential gene in schistosomes and further suggest an important role for the proteasome in parasite development and survival.

Biology of the schistosome lung-stage schistosomulum

Past and more recent research has examined the ultrastructure, metabolism, cell biology, genomics and post-genomics of schistosome schistosomula. These areas are considered and discussed in this review with particular emphasis on (1) the early migration phases through the host, (2) interaction of the host immune response with the parasite surface, (3) glucose uptake mechanisms, and (4) defining the transcriptional profiles of lung-stage schistosomula compared with other developmental stages using microarrays. The microarray profiling studies suggest caution is required when considering the use of schistosomes obtained by in vitro means for molecular or biochemical studies.

Schistosoma japonicum and S. mansoni cercariae: different effects of protein in medium, of mechanical stress, and of an intact complement system on in vitro transformation to schistosomula

The cercariae of Schistosoma japonicum were subjected in vitro to treatments known for Schistosoma mansoni to generate schistosomula-like organisms. As a technical prerequisite to pipette or to otherwise handle the sticky cercariae of S. japonicum, the addition of protein to water or medium was found to abolish the stickiness of cercariae of this species. Shearing forces exerted in vitro by syringe (22 G) passage are known since long to fully transform S. mansoni cercariae, but this treatment was found to be much less efficient with S. japonicum. Thus, even with very narrow needles (27 G), complete transformation of cercariae was not obtained with S. japonicum. Complement, provided by fresh human serum, is also well known to induce rapid transformation of S. mansoni cercariae with subsequent killing of the schistosomula. This treatment of S. japonicum cercariae induced degeneration of the tails and strongly promoted the transformation to schistosomula-like organisms, but at a much slower pace. These effects were absent from sera either heat-inactivated or depleted of factor B or of complement component C8, but were restored after adding the purified respective complement components. The schistosomula-like organisms of S. japonicum were not susceptible to lysis after 1 day of in vitro culture in the presence of 50% fresh human serum, although both cercariae and schistosomula of S. mansoni were killed under these conditions. In conclusion, the dynamics of in vitro transformation of S. japonicum cercariae differ significantly from those of S. mansoni, and complement has a major transformation-promoting activity.

A role for parasite-induced PGE2 in IL-10-mediated host immunoregulation by skin stage schistosomula of Schistosoma mansoni

Significant quantities of PGE(2) were produced by cercariae of Schistosoma mansoni following incubation with linoleic acid, a free fatty acid found on the surface of the skin. Cyclooxygenase (COX) 2 inhibitors failed to block this PGE(2) production, suggesting that a different biochemical pathway may be involved in the production of PGE(2) by the parasite. In addition, the parasites were also able to induce PGE(2) and IL-10 from human and mouse keratinocytes. Analysis of mouse skin during skin migratory phases of infection confirmed these in vitro observations. COX2 inhibitors blocked the parasite-induced PGE(2) and IL-10 from keratinocytes. Further analysis of the parasite secretions showed that the PGE(2)/IL-10-inducing effect was associated with a fraction <30 kDa molecular size. Addition of this fraction or parasite-stimulated keratinocyte culture supernatant to Con A-stimulated spleen cells resulted in the suppression of cell proliferation. This effect could be blocked by anti-IL-10 treatment. In sharp contrast, attenuation of the parasites with gamma-irradiation significantly abrogated their ability to induce PGE(2) or IL-10 from skin cells. Significance of IL-10 in host immunoregulation by skin stage schistosomula of S. mansoni was further confirmed by using IL-10-deficient mice. In these mice the normal subdued cutaneous reaction to the parasite was absent. Instead, a prominent cellular reaction occurred around the parasite, and there was considerable delay in parasitic migration through the skin. Thus these results suggest a key role for parasite-induced PGE(2) in IL-10-dependent down-regulation of host immune responses in the skin.

Induction cues for tegument formation during the transformation of Schistosoma mansoni cercariae

Adult schistosomes are parasitic blood flukes that have a continuous double lipid bilayered membrane surrounding the entire worm. This tegumental membrane is synthesised during invasion of the vertebrate host by free-swimming infectious forms called cercariae. As cercariae invade their final hosts they lose their tails and encounter a changing environment that includes altered temperature, sugar concentration and osmolarity. We have identified a glucose transporter protein designated SGTP4 that is found exclusively in the outer adult tegument and on membranous vesicles within the tegumental cytoplasm. By using immunofluorescence analysis to monitor the appearance and distribution of SGTP4 we can track the process of new tegumental membrane formation and examine the cues that trigger this developmental pathway. Cercariae in water do not transform their tegument while those incubated in rich medium do so rapidly. We have examined which of the many constituents of rich medium are responsible for triggering this transformation. Incubation in a solution of moderate osmolarity (120 mOsM PBS) is sufficient by itself to trigger tegument transformation, albeit at a slower rate relative to incubation in rich medium. Adding either glucose (to 100 mM) to the solution or increasing the temperature of incubation (from 22 degrees C to 37 degrees C) further increased the rate of tegument biogenesis. The introduction of glucose together with an increase in the incubation temperature further accelerated the process, suggesting that these factors act synergistically to promote transformation rates. The critical nature of osmolarity in inducing the process is highlighted by the fact that transformation proceeds as efficiently in 360 mOsM alone as it does in rich medium. While the fatty acids linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic acid at 1 mM) and capric acid (Decanoic acid, at 0.1 mM) have both been proposed to stimulate tegumental transformation, we show that neither promotes the morphogenesis of a normal schistosomulum tegument. The schistosomicide praziquantel (to 1 mM) has no detectable effect on new tegument formation.

ICAM-1 and iNOS expression increased in the skin of mice after vaccination with gamma-irradiated cercariae of Schistosoma mansoni

Host responses to migrating schistosomula of Schistosoma mansoni were compared in the skin of naive, multiply infected, or vaccinated (with gamma-irradiated cercariae) mice during the first 72 hr after cercarial penetration. Cellular response to the migrating parasite was minimal in the skin of naive mice for up to 72 hr after infection. In sharp contrast, the multiply infected or vaccinated animals exhibited a marked inflammatory response in the skin as early as 8 hr after cutaneous penetration of the challenge cercariae. This early inflammatory response in the skin of sensitized animals was characterized by a significant increase in the number of infiltrating cells, predominantly mononuclear cells and neutrophils. Increased exudation of serum proteins was also present in the skin of sensitized animals in areas of cercarial challenge. A time course of analyses revealed that mononuclear cell numbers increased significantly in the skin of vaccinated animals as early as 60 min after a challenge infection and continued to be present at a significantly higher level up to 72 hr after challenge. Peak neutrophil responses occurred in the skin at 24 hr (in multiply infected animals) and at 48 hr (in vaccinated animals) after a challenge infection. Along with the massive cellular infiltration there was an increased tissue expression of ICAM-1 and mRNA for iNOS in the skin of sensitized animals. Further analysis showed that in sensitized animals increased ICAM-1 expression was predominantly found on endothelial cells lining dermal capillaries, especially in areas around schistosomular migration and on cells that surrounded schistosomula in the dermis. In naive animals, however, a similar infection did not induce any ICAM-1 expression or iNOS production in the skin. Thus, an ICAM-1 mediated early accumulation of mononuclear cells in the skin and local production of nitric oxide may be important for the initial cutaneous inflammatory immune responses to migrating schistosomula of S. mansoni in vaccinated animals. On the contrary, in naive animals a potential parasite-induced suppression of ICAM-1 may play an important role in reducing cellular reaction in the skin and consequently help the parasite evade immune responses in the skin.

Cysteine protease inhibitors block schistosome hemoglobin degradation in vitro and decrease worm burden and egg production in vivo

Schistosome parasites utilize hemoglobin as a major protein source for their metabolism. Degradation of hemoglobin has been hypothesized to be mediated by both cysteine and aspartyl proteases secreted into the lumen of the parasite intestine. We now show that two distinct types of irreversible cysteine protease-specific inhibitors both arrest schistosome hemoglobin degradation in vitro. Arrest of hemoglobin degradation is followed by death of developing schistosomula 1 week later. Schistosome infected mice treated by a dose of 2 mg inhibitor per day for 1 week early in infection, and 2 weeks at the time of egg production, showed a significant reduction in worm burden, hepatomegaly, and the number of eggs produced per female worm. Histopathology showed a minimal immune response to those eggs which were produced, consistent with a delay in egg production relative to untreated infections. By tagging the inhibitor with biotin, specific cysteine protease targets were identified in extracts of schistosome worms.

Schistosoma mansoni: protein phosphorylation during transformation of cercariae to schistosomula

Infectivity of the multicellular pathogen Schistosoma mansoni for the human host is dependent upon the ability of free-living cercariae to transform rapidly into parasitic schistosomula. The biochemical pathways that regulate this transitional period are unknown. The role of protein phosphorylation was investigated by examining the incorporation of [32Pi]phosphate into proteins of S. mansoni. A sevenfold increase in total phosphorylation was found in 3-hr-old schistosomula as compared to cercariae. Analysis of radiolabeled proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography demonstrated that a 14-kDa protein served as a marker for transformation, being phosphorylated in schistosomula but not cercariae. The protein was phosphorylated on a serine residue. Phosphorylation was stimulated by a shift of parasites from water to salt-containing medium at 23 degrees C. Incubation of organisms in water at 37 degrees C did not initiate phosphorylation of this protein. The 14-kDa phosphoprotein was extracted from parasite homogenates with 1 M NaCl but was insoluble in 1% Triton X-100. Protein phosphorylation during the cercarial-schistosomula transformation may represent an important biochemical event that regulates infectivity of the parasite for the human host.

The transition from an aerobic to an anaerobic energy metabolism in transforming Schistosoma mansoni cercariae occurs exclusively in the head

It has been shown that in intact cercariae of Schistosoma mansoni in water, both head and tail had an identical, aerobic energy metabolism. As long as the environment was water, glucose was mainly degraded to carbon dioxide by both head and tail whether or not these two were still connected to each other. Transfer of intact cercariae into a simple salt medium supplemented with glucose resulted in a very rapid transition towards a more anaerobic energy metabolism: the production of lactate and pyruvate increased, whereas the production of carbon dioxide remained more or less constant. A concomitant rise in temperature to 37 degrees C was not essential for this biochemical transition, but made it more pronounced. Experiments on isolated cercarial bodies and tails in a transforming medium demonstrated that the tail oxidized glucose to carbon dioxide, whereas bodies produced mainly pyruvate and lactate. The results showed that the metabolic transition towards a more anaerobic energy metabolism occurred only in the head and not in the tail of the cercariae. Loss of the tail was shown not to be a pre-requisite for this transition, nor did it by itself trigger a metabolic switch in the resulting cercarial body.

Analysis of polyunsaturated fatty acid composition of Strongyloides ratti in relation to development

The effect of linolenic acid (C18:3 omega 3) on the development of Strongyloides ratti first-stage larvae (L1) in culture was studied. The fatty acid composition of S. ratti free-living generations was analyzed by gas chromatography. L1 had abundant linoleic acid (C18:2 omega 6) but its proportion decreased with development. On the contrary, eicosapentaenoic acid (C20:5 omega 3) and C20:4 omega 3 were prominent in the filariform larva (L3). Because C20:5 omega 3 is generally synthesized from C18:3 omega 3 via C20:4 omega 3, the high ratio of C20:5 omega 3/C18:3 omega 3 of L3 in all the free-living generations suggested that polyunsaturated fatty acid metabolism, particularly the omega-3 series, and eicosanoids produced had important roles in the development of S. ratti L1.

Schistosoma mansoni: increasing saline concentration signals cercariae to transform to schistosomula

Cercariae of S. mansoni shed the surface glycocalyx, form a double lipid bilayer on their surface, and transform to schistosomula when tails are removed and parasites are transferred from pond water to 300 mOsm phosphate-buffered saline. To determine whether the absolute concentration of saline or the relative change in saline concentration was the signal for surface transformation, cercariae were isolated from the snail hepatopancreas, sheared to remove the tails, and incubated in defined media for 3 hr at 37 degrees C. Surface transformation was assayed using the binding of the fluorescein-conjugated lectin concanavalin A to the schistosomular double unit membrane but not to the cercarial glycocalyx. An increase in salinity either from 18 mOsm (artificial pond water) to 120 mOsm (the snail osmolarity) or from 120 to 300 mOsm (the mammalian osmolarity) triggered transformation to schistosomula. Organisms constantly exposed to 120 mOsm or shifted from 120 mOsm to pond water did not transform their surfaces. The signal for transformation appeared to be increasing salinity rather than increasing osmolarity because cercarial bodies did not become schistosomula in 300 mOsm mannitol. Surface transformation was inhibited when cercariae were incubated with the acetylcholinesterase inhibitor eserine sulfate during a 10 min time when the osmolarity was raised. We conclude that increasing salinity rather than the absolute saline concentration is the signal for surface transformation and that eserine sulfate may inhibit the receipt of this signal.