Schistosoma mansoni: histochemical analysis of the postacetabular gland secretion of cercariae

Search, find, and penetrate: ultrastructural data of furcocercariae of Cardiocephaloides longicollis (Digenea, Strigeidae) explain their transmission and infection strategy into fish hosts

The present study provides an overview of the structures linked to fish host finding, recognition, and invasion of one of the most commonly occurring morphotypes among trematodes, furcocercariae. For this, we use free-swimming cercariae of the strigeid Cardiocephaloides longicollis (Rudolphi 1819) Dubois, 1982. Their elongated cercarial body and bifurcated tail are covered by a tegument with an irregular surface, showing numerous folds arranged in different directions and a typical syncytial organization. Both the body and the bifurcated tail are covered with short spines, rose-thorn shaped, as well as four types of sensory papillae, distinguished by the presence or absence of a cilium, its length, and their position on the cercarial body. These papillae are especially important for free-living stages that rely on external stimuli to locate and adhere to the host. A specialized anterior organ is located at the anterior part of the cercariae and is encircled by a triangle-shaped group of enlarged pre-oral spines followed by a transverse row of enlarged post-oral spines that, together with the sensory papillae, allow active finding, recognition, and penetration into fish. The ventral sucker, covered with inner-oriented spines, sensory papillae, and cilia, helps during this process. The cercariae of C. longicollis possess three types of gland cells (a head gland and two types of penetration glands), each containing different types of secretory granules that play a role in host invasion. The protonephridial excretory system consists of an excretory bladder, a system of collecting tubules, flame cells, and two excretory pores in the middle of each furcae, which serve to control osmoregulation in their marine environment, as well as to eliminate metabolic waste. Together with the four types of sensory endings, the central ganglion forms the nervous system. Our results add novel information on the ultrastructure of strigeid furcocercariae, being essential to interpret these data in relation of their functional role to better understand the transmission and penetration strategies that cercariae display to infect their fish hosts.

Schistosoma mansoni Larval Extracellular Vesicle protein 1 (SmLEV1) is an immunogenic antigen found in EVs released from pre-acetabular glands of invading cercariae

Extracellular Vesicles (EVs) are an integral component of cellular/organismal communication and have been found in the excreted/secreted (ES) products of both protozoan and metazoan parasites. Within the blood fluke schistosomes, EVs have been isolated from egg, schistosomula, and adult lifecycle stages. However, the role(s) that EVs have in shaping aspects of parasite biology and/or manipulating host interactions is poorly defined. Herein, we characterise the most abundant EV-enriched protein in Schistosoma mansoni tissue-migrating schistosomula (Schistosoma mansoni Larval Extracellular Vesicle protein 1 (SmLEV1)). Comparative sequence analysis demonstrates that lev1 orthologs are found in all published Schistosoma genomes, yet homologs are not found outside of the Schistosomatidae. Lifecycle expression analyses collectively reveal that smlev1 transcription peaks in cercariae, is male biased in adults, and is processed by alternative splicing in intra-mammalian lifecycle stages. Immunohistochemistry of cercariae using a polyclonal anti-recombinant SmLEV1 antiserum localises this protein to the pre-acetabular gland, with some disperse localisation to the surface of the parasite. S. mansoni-infected Ugandan fishermen exhibit a strong IgG1 response against SmLEV1 (dropping significantly after praziquantel treatment), with 11% of the cohort exhibiting an IgE response and minimal levels of detectable antigen-specific IgG4. Furthermore, mice vaccinated with rSmLEV1 show a slightly reduced parasite burden upon challenge infection and significantly reduced granuloma volumes, compared with control animals. Collectively, these results describe SmLEV1 as a Schistosomatidae-specific, EV-enriched immunogen. Further investigations are now necessary to uncover the full extent of SmLEV1's role in shaping schistosome EV function and definitive host relationships.

Schistosoma japonicum cathepsin B2 (SjCB2) facilitates parasite invasion through the skin

Cercariae invasion of the human skin is the first step in schistosome infection. Proteases play key roles in this process. However, little is known about the related hydrolytic enzymes in Schistosoma japonicum. Here, we investigated the biochemical features, tissue distribution and biological roles of a cathepsin B cysteine protease, SjCB2, in the invasion process of S. japonicum cercariae. Enzyme activity analysis revealed that recombinant SjCB2 is a typical cysteine protease with optimum temperature and pH for activity at 37°C and 4.0, respectively, and can be totally inhibited by the cysteine protease inhibitor E-64. Immunoblotting showed that both the zymogen (50 kDa) and mature enzyme (30.5 kDa) forms of SjCB2 are expressed in the cercariae. It was observed that SjCB2 localized predominantly in the acetabular glands and their ducts of cercariae, suggesting that the protease could be released during the invasion process. The protease degraded collagen, elastin, keratin, fibronectin, immunoglobulin (A, G and M) and complement C3, protein components of the dermis and immune system. In addition, proteomic analysis demonstrated that SjCB2 can degrade the human epidermis. Furthermore, it was showed that anti-rSjCB2 IgG significantly reduced (22.94%) the ability of the cercariae to invade the skin. The cysteine protease, SjCB2, located in the acetabular glands and their ducts of S. japonicum cercariae. We propose that SjCB2 facilitates skin invasion by degrading the major proteins of the epidermis and dermis. However, this cysteine protease may play additional roles in host-parasite interaction by degrading immunoglobins and complement protein.

Schistosomiasis is one of the most prevalent parasitic diseases in the world, with about 200 million humans infected in 74 tropical countries. The infection of schistosome is initiated when the larvae, cercariae, penetrate the human skin. Proteolytic enzymes are likely involved in the invasion process, but these have yet to be characterized for S. japonicum. Here, we have functionally expressed a recombinant form of the cathepsin B cysteine protease SjCB2 in the yeast Pichia pastoris. Our study showed that SjCB2 degraded a number of proteins associated with the skin and immune systems, and disrupted the structure of the human epidermis. The enzyme was located in the acetabular glands and their ducts in the cercariae, where it would be stored before released into the skin. Antibody-blocking studies revealed that SjCB2 had a 22.94% contribution during the cercariae invasion process. Taken together, our findings suggest that SjCB2 helped cercariae penetrating the skin barrier and evading the immune attack to allow successful infection in the mammalian host.

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.

Immunolocalization of anti-hsf1 to the acetabular glands of infectious schistosomes suggests a non-transcriptional function for this transcriptional activator

Schistosomiasis is a chronically debilitating disease caused by parasitic worms of the genus Schistosoma, and it is a global problem affecting over 240 million people. Little is known about the regulatory proteins and mechanisms that control schistosome host invasion, gene expression, and development. Schistosome larvae, cercariae, are transiently free-swimming organisms and infectious to man. Cercariae penetrate human host skin directly using proteases that degrade skin connective tissue. These proteases are secreted from anucleate acetabular glands that contain many proteins, including heat shock proteins. Heat shock transcription factors are strongly conserved activators that play crucial roles in the maintenance of cell homeostasis by transcriptionally regulating heat shock protein expression. In this study, we clone and characterize the schistosome Heat shock factor 1 gene (SmHSF1). We verify its ability to activate transcription using a modified yeast one-hybrid system, and we show that it can bind to the heat shock binding element (HSE) consensus DNA sequence. Our quantitative RT-PCR analysis shows that SmHSF1 is expressed throughout several life-cycle stages from sporocyst to adult worm. Interestingly, using immunohistochemistry, a polyclonal antibody raised against an Hsf1-peptide demonstrates a novel localization for this conserved, stress-modulating activator. Our analysis suggests that schistosome Heat shock factor 1 may be localized to the acetabular glands of infective cercariae.

Schistosome parasites are the cause of human schistosomiasis and infect more than 240 million people worldwide. Schistosome larvae, termed cercariae, are a free-swimming mobile developmental stage responsible for host infection. These larvae produce enzymes that degrade human skin, allowing them to pass into the human host. After invasion, they continue to evade the immune system and develop into adult worms. The transition from free-swimming larvae in freshwater to invasion into a warm-blooded saline environment requires that the parasite regulate genes to adapt to these changes. Heat shock factor 1 is a well-characterized activator of stress and heat response that functions in cellular nuclei. Using immunohistochemistry, we observed non-nuclear localization for anti-Heat shock factor 1 signal in the secretory glands necessary for the invasive function of schistosome larvae. This observation expands the potential mechanistic roles for Heat shock factor 1 and may aid in our understanding of schistosome host invasion and early development.

An atlas for Schistosoma mansoni organs and life-cycle stages using cell type-specific markers and confocal microscopy

Schistosomiasis (bilharzia) is a tropical disease caused by trematode parasites (Schistosoma) that affects hundreds of millions of people in the developing world. Currently only a single drug (praziquantel) is available to treat this disease, highlighting the importance of developing new techniques to study Schistosoma. While molecular advances, including RNA interference and the availability of complete genome sequences for two Schistosoma species, will help to revolutionize studies of these animals, an array of tools for visualizing the consequences of experimental perturbations on tissue integrity and development needs to be made widely available. To this end, we screened a battery of commercially available stains, antibodies and fluorescently labeled lectins, many of which have not been described previously for analyzing schistosomes, for their ability to label various cell and tissue types in the cercarial stage of S. mansoni. This analysis uncovered more than 20 new markers that label most cercarial tissues, including the tegument, the musculature, the protonephridia, the secretory system and the nervous system. Using these markers we present a high-resolution visual depiction of cercarial anatomy. Examining the effectiveness of a subset of these markers in S. mansoni adults and miracidia, we demonstrate the value of these tools for labeling tissues in a variety of life-cycle stages. The methodologies described here will facilitate functional analyses aimed at understanding fundamental biological processes in these parasites.

Schistosomes are parasitic flatworms that infect hundreds of millions of people worldwide. The development of genomic resources and recent application of functional genomic tools (e.g., global gene expression studies, inhibition of gene expression by RNA interference, and transgenesis) hold the promise of revolutionizing the study of schistosome biology. These advances necessitate the introduction of molecular markers for examining the consequences of manipulating schistosome genes. In this manuscript we report the use of several cell type-specific markers and confocal microscopy for visualizing various schistosome tissues in a variety of life-cycle stages. Our analysis provides an atlas of the major organ systems in three different life-cycle stages in these important parasites. The tools and methodologies reported here are widely available and can be readily adopted by researchers interested in more detailed studies of these organisms. We anticipate that these resources will be particularly useful for detailed phenotypic characterization following gene inhibition or over-expression studies.

Penetration of human skin by the cercariae of Schistosoma mansoni: an investigation of the effect of multiple cercarial applications

It has previously been postulated that L-arginine emitted by penetrating Schistosoma mansoni cercariae serves as an intraspecific signal guiding other cercariae to the penetration site. It was suggested that penetrating in groups offers a selective advantage. If this hypothesis is correct and group penetration at one site on the host offers an advantage, it would follow that at such a site, successive groups of cercariae would be able to penetrate skin in either greater numbers or at a faster rate. This prediction was tested by the use of an in vitro model of cercarial penetration based on the Franz cell and using human skin. It was demonstrated that there was no increase in the percentage of cercariae able to penetrate the skin with subsequent exposures. Consequently, it seems unlikely that the release of L-arginine by cercariae during penetration could have evolved as a specific orientation system based on a selective advantage offered by group penetration.

Cercarial elastase is encoded by a functionally conserved gene family across multiple species of schistosomes

Water borne cercaria(ae) of the trematode genus Schistosoma rapidly penetrate host skin. A single serine protease activity, cercarial elastase, is deposited in advance of the invading parasite by holocytosis of vesicles from ten large acetabular gland cells. Cercarial elastase activity is a composite of multiple isoforms. Genes coding for the isoforms can be divided into two classes by amino acid and promoter sequence homology. Two of the five genes identified in Schistosoma mansoni account for over 90% of the activity and protein released. The remaining genes produce little protein or are silent. Positional scanning synthetic combinatorial substrate libraries demonstrate that the two major isoforms have similar substrate specificities and are, therefore, isoenzymes. The closely related Schistosoma hematobium and the distantly related Schistosomatium douthitti also contain multiple orthologous cercarial elastase genes suggesting that gene duplication may have occurred after speciation in Schistosoma evolution and that this duplication has been conserved.

Ultrastructure of the Schistosoma mansoni cercaria

The cercaria of the schistosome parasite is a short-lived, free-swimming larval stage that is infective for the mammalian, definitive host. This atlas describes the ultrastructure of the cells that comprise the cercaria of Schistosoma mansoni, a leading causative agent of human schistosomiasis. In addition to the cells which make up the various organ systems, such as the nervous, tegumental, osmoregulatory, muscular and primordial digestive systems, also we show the ultrastructure of those cells whose organization or location are not as well defined structurally but are essential nevertheless for the success of the parasite. These latter include the various support cells, and those cells that, upon differentiation into the adult worm, serve reproductive functions. A description is also given of the cells whose sole functions are realized only at the cercarial stage, chiefly involved in the vigorous act of skin penetration. Although we include a detailed review of the ultrastructure of S. mansoni cercariae, much of the information reported has not been previously published. In summary, this paper brings together an ultrastructural description of all the cell types presently known that make up the much studied larval stage of this medically important trematode.

Strongyloides venezuelensis: adhesion of adult worms to culture vessels by orally secreted mucosubstances

Adults worms of Strongyloides venezuelensis were cultured in vitro. After overnight incubation, about 60% of the worms adhered firmly to the bottom of culture vessels by secreting adhesive substances from the mouth. A single worm produced 24.5 +/- 10.1 of the adhesion spots overnight. When they were transferred to new culture vessels, they still produced new spots comparable to those produced for first 24 hr. The adhesion spots were positively stained with Coomassie brilliant blue and also with mucicarmine, periodic acid-Schiff, and alcian blue, pH 2.5, but not with alcian blue, pH 0.3, indicating their glycoprotein nature. The substances were amorphous and did not contain cells or nuclei. Histologic staining with a panel of lectins showed that the adhesive substances were rich in mannose, N-acetyl galactosamine, and N-acetyl glucosamine, but devoid of sialic acid. These characteristics were distinct from those of jejunal goblet cell mucins of rats. Adhesive substances contained antigenic components recognized by sera from infected rats. Thus, the adhesive substances secreted from the mouth of S. venezuelensis were clearly of parasite origin. We consider the production/secretion of the adhesive substances by S. venezuelensis adult worms a key step for the parasites to invade and establish the host epithelial layer.

Paramyosin isoforms of Schistosoma mansoni are phosphorylated and localized in a large variety of muscle types

Paramyosin, although a widely distributed muscle component among invertebrates, has hitherto not clearly been shown to occur in the muscles of schistosomes. Instead, it has been reported to occur in the tegument. In the present study, a specific antibody reacting with each of 10 isoforms of paramyosin was used for light microscopical immunolocalization in sections of Schistosoma mansoni. Specimens were fixed by a new method to immobilize antigens with uranyl acetate-trehalose-methanol. In cercariae, schistosomula, and adults, the circular and longitudinal muscles of the body wall, the dorsoventral muscles and those surrounding the gut and the pharynx as well as the fast moving cross-striated muscles of the tail of cercariae intensely reacted with the antibody. However, neither immunohistologically nor on Western blots of isolated tegument, were indications found for the presence of paramyosin in the tegument. In vivo phosphorylation and binding of anti-phospho-tyrosine and anti-phospho-serine antibodies show phosphorylation of paramyosin which probably is responsible for the generation of the isoforms.

Physiological analyses of host-finding behaviour in trematode cercariae: adaptations for transmission success

Physiological analyses of the behaviour of several cercarial species which actively find and invade their hosts have revealed very complex sequences of behaviour patterns and responses to very different stimuli from the environment and the host. A result of these physiological studies is that the behaviour patterns of each of the species investigated are surprisingly individual. The behavioural patterns of host-finding of those species analysed in some detail reveal profound adaptations to maximize transmission success. This can be demonstrated for movement patterns during swimming, for responses to environmental conditions such as gravity, light and temperature, for responses to stimuli emanating from the host such as shadows, water turbulence and chemical compounds and especially for the responses after contact with the host. The behaviour patterns can be interpreted as adaptations to: (1) dispersal by leaving the habitat of the snail intermediate host and distribution within the area; (2) long survival by energy saving swimming behaviour, by avoiding responses to inappropriate stimuli, by selecting favourable microhabitats and probably by avoiding predation; (3) finding and invading particular host types by selecting microhabitats frequented by the hosts and responding to sequences of specific stimuli emanating from the hosts.

Schistosoma mansoni: cell-specific expression and secretion of a serine protease during development of cercariae

Eukaryotic serine proteases are an important family of enzymes whose functions include fertilization, tissue degradation by neutrophils, and host invasion by parasites. To avoid damaging the cells or organisms that produced them, serine proteases must be tightly regulated and sequestered. This study elucidates how the parasitic blood fluke Schistosoma mansoni synthesizes, stores, and releases a serine protease during differentiation of its invasive larvae. In situ hybridization with a cDNA probe localized the protease mRNA to acetabular cells, the first morphologically distinguishable parasite cells that differentiate from the embryonic cell masses present in the intermediate host snail. The acetabular cells contained vimentin but not cytokeratins, consistent with a mesenchymal, not epithelial, origin. Antiprotease antibodies, localized by immunoperoxidase, showed that the protease progressively accumulated in these cells and was packaged in vesicles of three morphologic types. Extension of cytoplasmic processes containing protease vesicles formed "ducts" which reached the anterior end of fully differentiated larvae. During invasion of human skin, groups of intact vesicles were released through the acetabular cytoplasmic processes and ruptured within the host tissue. Ruptured protease vesicles were noted adjacent to degraded epidermal cells and dermal-epidermal basement membrane, as well as along the surface of the penetrating larvae themselves. These observations are consistent with the proposed dual role for the enzyme in facilitating invasion of host skin by larvae and helping to release the larval surface glycocalyx during metamorphosis to the next stage of the parasite.

Purification and characterization of proteases secreted by transforming schistosomula of Schistosoma mansoni

Schistosomula of Schistosoma mansoni which are mechanically transformed at 4 degrees C and are then incubated at 37 degrees C in defined medium spontaneously secrete two proteases, a major one of 28 kDa and a minor one of 60 kDa. These were purified by ion exchange chromatography on DEAE-cellulose and gel filtration on Ultrogel AcA 54 with yields of 33% and 29%, respectively. Both appeared as single bands by silver staining following sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis. The 28 kDa protease is a glycoprotein that has a pI of 11 or higher and an optimal activity around pH 9.0. It cleaves casein, gelatin and human C3 and C3b. It is metal-ion independent and is inhibited by diisopropyl fluorophosphate, phenylmethanesulfonyl fluoride, soy-bean trypsin inhibitor, alpha 1 antitrypsin, Zn2+ ions, sodium dodecyl sulphate and normal human serum. The 60 kDa protease is a glycoprotein with a pI of 9.2. It can also cleave casein and gelatin and its activity is inhibited by phenylmethanesulfonyl fluoride but not by diisopropyl fluorophosphate or sodium dodecyl sulphate. We suggest that these proteases may play a role during cercarial penetration of the skin and in shedding of the cercarial glycocalyx.

Schistosoma mansoni: visualization with fluorescent lectins of secretions and surface carbohydrates of living cercariae

Attachment of Schistosoma mansoni cercariae was studied during their explorative movements along a glass surface using labeled lectins as markers. Fluorochrome-labeled lectins selectively labeled surface material produced at the cercarial attachment sites and part of the cercarial surface. The deposited secretions reacted with most of the lectins used but differences in the staining intensity were noted. Secreted material was visualized at the attachment sites within a few seconds after cercarial attachment. The deposited material appeared as "footprints" located at a constant distance from each other. The footprints were formed by a regular cercarial "looping" movement along the glass surface and led to a site of massive deposition of secretions partly covering the body.

Purification and properties of a proteolytic enzyme from the cercariae of the human trematode parasite Schistosoma mansoni

Skin penetration by the cercarial stage of the human trematode parasite Schistosoma mansoni is mediated by the secretion of proteolytic enzymes able to digest components of mammalian connective tissues. In the present study the purification of these proteinases from cercarial homogenates is reported. The major proteinase species has a mol. wt. of approx. 25 000 and exists in monomeric form as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This proteinase has an isoelectric point of 6.0. Studies presented here, with a variety of substrates and inhibitors, confirm previous claims that these proteinases belong to the serine class, and, in addition, suggest that they resemble the vertebrate chymotrypsins rather than trypsins or elastases. However, the amino acid composition of the cercarial proteinase differs significantly from bovine chymotrypsin and from the human leucocyte chymotrypsin-like cathepsin G. The amino-acid-composition differences between these proteinases are consistent with their differences in isoelectric point. In order to obtain an insight into the role of the proteinase in skin penetration, its activity on cartilage proteoglycan monomers and on the isolated peptide backbone of proteoglycan was studied. The results of the present study indicate that the cercarial enzyme catalyses a limited specific digestion of the peptide core.