Mediation of Trypanosoma cruzi invasion by heparan sulfate receptors on host cells and penetrin counter-receptors on the trypanosomes

The First Contact of Human Dendritic Cells With Trypanosoma cruzi Reveals Response to Virus as an Unexplored Central Pathway

Chagas disease is a debilitating and neglected disease caused by the protozoan Trypanosoma cruzi. Soon after infection, interactions among T. cruzi and host innate immunity cells can drive/contribute to disease outcome. Dendritic cells (DCs), present in all tissues, are one of the first immune cells to interact with Trypanosoma cruzi metacyclic trypomastigotes. Elucidating the immunological events triggered immediately after parasite-human DCs encounter may aid in understanding the role of DCs in the establishment of infection and in the course of the disease. Therefore, we performed a transcriptomic analysis of a 12 h interaction between T. cruzi and MoDCs (monocyte-derived DCs) from three human donors. Enrichment analyses of the 468 differentially expressed genes (DEGs) revealed viral infection response as the most regulated pathway. Additionally, exogenous antigen processing and presentation through MHC-I, chemokine signaling, lymphocyte co-stimulation, metallothioneins, and inflammasome activation were found up-regulated. Notable, we were able to identify the increased gene expression of alternative inflammasome sensors such as AIM2, IFI16, and RIG-I for the first time in a T. cruzi infection. Both transcript and protein expression levels suggest proinflammatory cytokine production during early T. cruzi-DCs contact. Our transcriptome data unveil antiviral pathways as an unexplored process during T. cruzi-DC initial interaction, disclosing a new panorama for the study of Chagas disease outcomes.

Interaction and assembly of two novel proteins in the spore wall of the microsporidian species Nosema bombycis and their roles in adherence to and infection of host cells

Microsporidia are obligate intracellular parasites with rigid spore walls that protect against various environmental pressures. Despite an extensive description of the spore wall, little is known regarding the mechanism by which it is deposited or the role it plays in cell adhesion and infection. In this study, we report the identification and characterization of two novel spore wall proteins, SWP7 and SWP9, in the microsporidian species Nosema bombycis. SWP7 and SWP9 are mainly localized to the exospore and endospore of mature spores and the cytoplasm of sporonts, respectively. In addition, a portion of SWP9 is targeted to the spore wall of sporoblasts earlier than SWP7 is. Both SWP7 and SWP9 are specifically colocalized to the spore wall in mature spores. Furthermore, immunoprecipitation, far-Western blotting, unreduced SDS-PAGE, and yeast two-hybrid data demonstrated that SWP7 interacted with SWP9. The chitin binding assay showed that, within the total spore protein, SWP9 and SWP7 can bind to the deproteinated chitin spore coats (DCSCs) of N. bombycis. However, binding of the recombinant protein rSWP7-His to the DCSCs is dependent on the combination of rSWP9-glutathione S-transferase (GST) with the DCSCs. Finally, rSWP9-GST, anti-SWP9, and anti-SWP7 antibodies decreased spore adhesion and infection of the host cell. In conclusion, SWP7 and SWP9 may have important structural capacities and play significant roles in modulating host cell adherence and infection in vitro. A possible major function of SWP9 is as a scaffolding protein that supports other proteins (such as SWP7) that form the integrated spore wall of N. bombycis.

Effects of chlorate on the sulfation process of Trypanosoma cruzi glycoconjugates. Implication of parasite sulfates in cellular invasion

Sulfation, a post-translational modification which plays a key role in various biological processes, is inhibited by competition with chlorate. In Trypanosoma cruzi, the agent of Chagas' disease, sulfated structures have been described as part of glycolipids and we have reported sulfated high-mannose type oligosaccharides in the C-T domain of the cruzipain (Cz) glycoprotein. However, sulfation pathways have not been described yet in this parasite. Herein, we studied the effect of chlorate treatment on T. cruzi with the aim to gain some knowledge about sulfation metabolism and the role of sulfated molecules in this parasite. In chlorate-treated epimastigotes, immunoblotting with anti-sulfates enriched Cz IgGs (AS-enriched IgGs) showed Cz undersulfation. Accordingly, a Cz mobility shift toward higher isoelectric points was observed in 2D-PAGE probed with anti-Cz antibodies. Ultrastructural membrane abnormalities and a significant decrease of dark lipid reservosomes were shown by electron microscopy and a significant decrease in sulfatide levels was confirmed by TLC/UV-MALDI-TOF-MS analysis. Altogether, these results suggest T. cruzi sulfation occurs via PAPS. Sulfated epitopes in trypomastigote and amastigote forms were evidenced using AS-enriched IgGs by immunoblotting. Their presence on trypomastigotes surface was demonstrated by flow cytometry and IF with Cz/dCz specific antibodies. Interestingly, the percentage of infected cardiac HL-1 cells decreased 40% when using chlorate-treated trypomastigotes, suggesting sulfates are involved in the invasion process. The same effect was observed when cells were pre-incubated with dCz, dC-T or an anti-high mannose receptor (HMR) antibody, suggesting Cz sulfates and HMR are also involved in the infection process by T. cruzi.

NbHSWP11, a microsporidia Nosema bombycis protein, localizing in the spore wall and membranes, reduces spore adherence to host cell BME

Microsporidia are obligate intracellular parasites, and a derivative of fungi, which harbor a rigid spore wall to resist adverse environmental pressures. The spore wall protein, which is thought to be the first and direct protein interacting with the host cell, may play a key role in the process of microsporidia infection. In this study, we report a protein, NbHSWP11, with a dnaJ domain. The protein also has 6 heparin-binding motifs which are known to interact with extracellular glycosaminoglycans. Syntenic analysis indicated that gene loci of Nbhswp11 are conserved and syntenic between Nosema bombycis and Nosema ceranae. Phylogenetic tree analysis showed that Nbhswp11 clusters with fungal dnaJ proteins and has 98% identity with an N. bombycis dnaJ protein. Nbhswp11 was transcribed throughout the entire life stages, and gradually increased during 1-7 days, in a silkworm that was infected by N. bombycis, as determined by reverse-transcription PCR (RT-PCR). The recombinant protein NbHSWP11 (rSWP11-HIS) was obtained and purified using gene cloning and prokaryotic expression. Western blotting analysis displayed NbHSWP11 expressed in the total mature spore proteins and spore coat proteins. Indirect immunofluorescence assay revealed NbHSWP11 located at the spore wall of mature spores and the spore coats. Furthermore, immune electron microscopy showed that NbHSWP11 localized in the cytoplasm of the sporont. Within the developmental process of N. bombycis, a portion of NbHSWP11 is targeted to the spore wall of sporoblasts and mature spores. However, most of NbHSWP11 distributes on the membraneous structures of the sporoblast and mature spore. In addition, using a host cell binding assay, native protein NbHSWP11 in the supernatant of total soluble mature spore proteins is shown to bind to the host cell BmE surface. Finally, an antibody blocking assay showed that purified rabbit antibody of NbHSWP11 inhibits spore adherence and decreases the adherence rate of spores by 20% compared to untreated spores. Collectively, the present results suggest that NbHSWP11 is involved in host cell adherence in vitro. Therefore NbHSWP11, which has a dnaJ domain, may modulate protein assembly, disassembly, and translocation in N. bombycis.

Comparative analysis of midgut bacterial communities of Aedes aegypti mosquito strains varying in vector competence to dengue virus

Differences in midgut bacterial communities of Aedes aegypti, the primary mosquito vector of dengue viruses (DENV), might influence the susceptibility of these mosquitoes to infection by DENV. As a first step toward addressing this hypothesis, comparative analysis of bacterial communities from midguts of mosquito strains with differential genetic susceptibility to DENV was performed. 16S rRNA gene libraries and real-time PCR approaches were used to characterize midgut bacterial community composition and abundance in three Aedes aegypti strains: MOYO, MOYO-R, and MOYO-S. Although Pseudomonas spp.-related clones were predominant across all libraries, some interesting and potentially significant differences were found in midgut bacterial communities among the three strains. Pedobacter sp.- and Janthinobacterium sp.-related phylotypes were identified only in the MOYO-R strain libraries, while Bacillus sp. was detected only in the MOYO-S strain. Rahnella sp. was found in MOYO-R and MOYO strains libraries but was absent in MOYO-S libraries. Both 16S rRNA gene library and real-time PCR approaches confirmed the presence of Pedobacter sp. only in the MOYO-R strain. Further, real-time PCR-based quantification of 16S rRNA gene copies showed bacterial abundance in midguts of the MOYO-R strain mosquitoes to be at least 10-100-folds higher than in the MOYO-S and MOYO strain mosquitoes. Our study identified some putative bacteria with characteristic physiological properties that could affect the infectivity of dengue virus. This analysis represents the first report of comparisons of midgut bacterial communities with respect to refractoriness and susceptibility of Aedes aegypti mosquitoes to DENV and will guide future efforts to address the potential interactive role of midgut bacteria of Aedes aegypti mosquitoes in determining vectorial capacity for DENV.

Adhesion of Trypanosoma cruzi trypomastigotes to fibronectin or laminin modifies tubulin and paraflagellar rod protein phosphorylation

Background

The unicellular parasite Trypanosoma cruzi is the causative agent of Chagaś disease in humans. Adherence of the infective stage to elements of the extracellular matrix (ECM), as laminin and fibronectin, is an essential step in host cell invasion. Although members of the gp85/TS, as Tc85, were identified as laminin and fibronectin ligands, the signaling events triggered on the parasite upon binding to these molecules are largely unexplored.

Methodology/Principal Findings

Viable infective parasites were incubated with laminin, fibronectin or bovine serum albumin for different periods of time and the proteins were separated by bidimensional gels. The phosphoproteins were envisaged by specific staining and the spots showing phosphorylation levels significantly different from the control were excised and identified by MS/MS. The results of interest were confirmed by immunoblotting or immunoprecipitation and the localization of proteins in the parasite was determined by immunofluorescence. Using a host cell-free system, our data indicate that the phosphorylation contents of T. cruzi proteins encompassing different cellular functions are modified upon incubation of the parasite with fibronectin or laminin.

Conclusions/Significance

Herein it is shown, for the first time, that paraflagellar rod proteins and α-tubulin, major structural elements of the parasite cytoskeleton, are predominantly dephosphorylated during the process, probably involving the ERK1/2 pathway. It is well established that T. cruzi binds to ECM elements during the cell infection process. The fact that laminin and fibronectin induce predominantly dephosphorylation of the main cytoskeletal proteins of the parasite suggests a possible correlation between cytoskeletal modifications and the ability of the parasite to internalize into host cells.

Trypanosoma cruzi heparin-binding proteins present a flagellar membrane localization and serine proteinase activity

Heparin-binding proteins (HBPs) play a key role in Trypanosoma cruzi-host cell interactions. HBPs recognize heparan sulfate (HS) at the host cell surface and are able to induce the cytoadherence and invasion of this parasite. Herein, we analysed the biochemical properties of the HBPs and also evaluated the expression and subcellular localization of HBPs in T. cruzi trypomastigotes. A flow cytometry analysis revealed that HBPs are highly expressed at the surface of trypomastigotes, and their peculiar localization mainly at the flagellar membrane, which is known as an important signalling domain, may enhance their binding to HS and elicit the parasite invasion. The plasmon surface resonance results demonstrated the stability of HBPs and their affinity to HS and heparin. Additionally, gelatinolytic activities of 70 kDa, 65·8 kDa and 59 kDa HBPs over a broad pH range (5·5–8·0) were revealed using a zymography assay. These proteolytic activities were sensitive to serine proteinase inhibitors, such as aprotinin and phenylmethylsulfonyl fluoride, suggesting that HBPs have the properties of trypsin-like proteinases.

Trypanosoma cruzi heparin-binding proteins mediate the adherence of epimastigotes to the midgut epithelial cells of Rhodnius prolixus

Heparin-binding proteins (HBPs) have been demonstrated in both infective forms of Trypanosoma cruzi and are involved in the recognition and invasion of mammalian cells. In this study, we evaluated the potential biological function of these proteins during the parasite-vector interaction. HBPs, with molecular masses of 65·8 kDa and 59 kDa, were isolated from epimastigotes by heparin affinity chromatography and identified by biotin-conjugated sulfated glycosaminoglycans (GAGs). Surface plasmon resonance biosensor analysis demonstrated stable receptor-ligand binding based on the association and dissociation values. Pre-incubation of epimastigotes with GAGs led to an inhibition of parasite binding to immobilized heparin. Competition assays were performed to evaluate the role of the HBP-GAG interaction in the recognition and adhesion of epimastigotes to midgut epithelial cells of Rhodnius prolixus. Epithelial cells pre-incubated with HBPs yielded a 3·8-fold inhibition in the adhesion of epimastigotes. The pre-treatment of epimastigotes with heparin, heparan sulfate and chondroitin sulfate significantly inhibited parasite adhesion to midgut epithelial cells, which was confirmed by scanning electron microscopy. We provide evidence that heparin-binding proteins are found on the surface of T. cruzi epimastigotes and demonstrate their key role in the recognition of sulfated GAGs on the surface of midgut epithelial cells of the insect vector.

Parasite-specific aptamers as biosynthetic reagents and potential pharmaceuticals

Aptamers are short, synthetic nucleic acid molecules. They are generated by a Darwinian-type in vitro evolution method known as 'systematic evolution of ligands by exponential enrichment' (SELEX). SELEX represents an experimental platform to identify rare ligands with predetermined functionality from combinatorial nucleic acid libraries. Since its discovery about 20 years ago the method has been instrumental in identifying a large number of aptamers that recognize targets of very different chemistry and molecular complexity. Although aptamers have been converted into sophisticated biomolecular tools for a diverse set of technologies, only a limited number of aptamers have been selected as binding reagents for parasites or parasite-derived molecules. Here the published examples of aptamers that target Leishmania-, Trypanosoma- and Plasmodia-specific molecules are reviewed.

Disease-specific biomarker discovery by aptamers

RNA and DNA aptamers developed by an in vitro selection process, Systematic Evolution of Ligands by EXponential enrichment (SELEX), comprise a novel class of high-affinity and specific capture agents, which can be easily modified for cytometry and in vivo applications. A novel application of this technique (Cell SELEX) explores the expression of cell surface epitopes that differ between two given cell types or between healthy and diseased cells. Using whole cells as targets, aptamer libraries can be identified that bind to biomarkers expressed by target cells and not by any other cells. Aptamers have been developed that specifically interact with cell surface epitopes of trypanosomes or distinguish between the differences in molecular signature of somatic and cancer cells. Aside from its use for target cell identification by image and flow cytometry and laser-scanning microscopy, aptamers can be used for ligand-mediated purification and identification of their binding proteins in target cell membranes. In this review, we discuss an approach for the development of aptamers targeting parasite-derived surface proteins of Trypanosoma and Plasmodium.

cDNA cloning and partial characterization of amastigote specific surface protein from Trypanosoma cruzi

Trypanosoma cruzi amastigote surface proteins are the target of both humoral and cell-mediated immune responses; however, few such molecules have been thoroughly studied. In order to study a T. cruzi amastigote-specific protein (SSP4), we used antibodies against the deglycosylated form of this molecule to clone cDNA. The selected cDNA clone (2070 bp) encodes for a 64 kDa protein product whose sequence analysis revealed no N-glycosylation signal. The DNA sequence showed high homology with a member of a previously reported dispersed repetitive gene family of T. cruzi. Antibodies against the recombinant protein reacted strongly with a 66 kDa protein and weakly with an 84 kDa protein in amastigote extracts. Immunoelectron microscopy studies showed that intracellular amastigotes express the native protein on their surfaces and flagellar pockets. The antibody label was also associated with an amorphous material present in the parasitic cavity and in direct contact with the parasite surface, which suggest that amastigotes are releasing this material. On cell-free amastigotes, the antibody showed strong decoration of the cell surface and labeling of intracellular vesicles. Immunofluorescence analysis showed that the superficial protein is expressed shortly after trypomastigotes begin to transform into amastigotes. Anti-recombinant protein antibodies recognized proteins of 100 kDa and 50-60 kDa in protein extracts of rat heart and skeletal muscle, respectively.

Role of the gp85/trans-sialidase superfamily of glycoproteins in the interaction of Trypanosoma cruzi with host structures

Invasion of mammalian cells by T. cruzi trypomastigotes is a multi-step and complex process involving several adhesion molecules, signaling events and proteolytic activities. From the blood to the cell target in different tissues the parasite has to interact with different cells and the extracellular matrix (ECM). The review focus on the role of the gp85/ trans-sialidase superfamily members in the interaction of the parasite with the host cell, particularly with ECM components, with emphasis on the significant variability among the ligands and receptors involved. Use of the SELEX technique to evolve nuclease-resistant RNA aptamers for receptor identification is briefly discussed.

Trypanosoma cruzi heparin-binding proteins and the nature of the host cell heparan sulfate-binding domain

Trypanosoma cruzi invasion is mediated by receptor-ligand recognition between the surfaces of both parasite and target cell. We have previously demonstrated the role of heparan sulfate proteoglycan in the attachment and invasion of T. cruzi in cardiomyocytes. Herein, we have isolated the T. cruzi heparin-binding proteins (HBP-Tc) and investigated the nature of cardiomyocyte heparan sulfate (HS)-binding site to the parasite surface ligand. Two major heparin-binding proteins with molecular masses of 65.8 and 59 kDa were observed in total extract of amastigote and trypomastigote forms of T. cruzi. Hydrophobic [S(35)]methionine labeled proteins eluted from heparin-sepharose affinity chromatography also revealed both proteins in trypomastigotes but only the 59 kDa is strongly recognized by biotin-conjugated glycosaminoglycans. Competition assays were performed to analyze the role of sulfated proteoglycans, including heparin, keratan sulfate and both acetylated and highly sulfated domains of heparan sulfate, in the recognition and invasion process of T. cruzi. Significant inhibitions of 84% and 35% in the percentage of infection were revealed after treatment of the parasites with heparin and the N-acetylated/ N-sulfated heparan sulfate domain, respectively, suggesting the important role of the glycuronic acid and NS glucosamine domain of the HS chain in the recognition of the HBP-Tc during the T. cruzi-cardiomyocyte interaction.

Implications of genetic variability of Trypanosoma cruzi for the pathogenesis of Chagas disease

Trypanosoma cruzi, the etiological agent of Chagas disease, presents a high degree of intraspecific genetic variability, with possible implications for the clinical forms of the disease, like the development of cardiopathy, megaesophagus, and megacolon, alone or in combination. This tissue tropism involved in the pathogenesis of Chagas disease has still not been totally elucidated. Thus, the current review approaches key aspects of T. cruzi genetic diversity, the clinical forms of Chagas disease, and the infection of the host cell by the parasite and the immune response. Other aspects discussed here include the release of immunosuppressive factors by the parasite, acting in the host's immune response pathways; host cell apoptosis inhibition; the pathogenesis of chagasic megaesophagus, which can be related to host-parasite interaction; and finally the association between megaesophagus and increased risk for the development of squamous-cell esophageal carcinoma. However, despite great advances in the understanding of this disease, it is still not possible to establish the true relationship between the parasite's genetic variability and the clinical form of Chagas disease.

Involvement of Ssp-4-related carbohydrate epitopes in mammalian cell invasion by Trypanosoma cruzi amastigotes

We examined whether the expression of Ssp-4-related carbohydrate epitopes defined by monoclonal antibodies 1D9 and 2B7 was related to cell invasion by Trypanosoma cruzi amastigotes from different isolates and whether the highest expression of the epitope defined by MAb 1D9 would confer greater infectivity. Confocal microscopy showed that both epitopes localize to the membrane of amastigotes from 569, 588, 573, 587 and SC2005 isolates, similar to the G isolate, whereas the CL isolate showed a punctate and diffuse staining. Flow cytometry revealed inter- and intra-isolate variable expression of these epitopes. Apart from the lower expression of MAb 2B7 epitope by intracellular amastigotes of the SC2005 isolate, amastigotes from chagasic patient isolates expressed both epitopes similar to the G isolate, in contrast to CL isolate, that showed lower expression of both epitopes. MAb 1D9 did not react with CL isolate on immunoblots and reacted poorly with 588 and 587 parasites. MAb 2B7 preferentially reacted with an epitope on an 84 kDa component in G and 573 isolates. Invasion assays revealed that despite the fact that amastigotes from chagasic patient isolates displayed high levels of the epitope defined by MAb 1D9, only isolate 588 invaded host cells in levels comparable to that of isolate G. Both MAbs specifically inhibited cell invasion by G and 588, but not CL. These results suggested that the highest expression of MAb 1D9 epitope was not sufficient to confer higher infectivity on the isolate, and besides the two epitopes, other factors may modulate the invasiveness of extracellular amastigotes from the different isolates.

Trypanosoma cruzi proline racemases are involved in parasite differentiation and infectivity

Polyclonal lymphocyte activation is one of the major immunological disturbances observed after microbial infections and among the primary strategies used by the parasite Trypanosoma cruzi to avoid specific immune responses and ensure survival. T. cruzi is the insect-transmitted protozoan responsible for Chagas' disease, the third public health problem in Latin America. During infection of its mammalian host, the parasite secretes a proline racemase that contributes to parasite immune evasion by acting as a B-cell mitogen. This enzyme is the first described eukaryotic amino acid racemase and is encoded by two paralogous genes per parasite haploid genome, TcPRACA and TcPRACB that give rise, respectively, to secreted and intracellular protein isoforms. While TcPRACB encodes an intracellular enzyme, analysis of TcPRACA paralogue revealed putative signals allowing the generation of an additional, non-secreted isoform of proline racemase by an alternative trans-splicing mechanism. Here, we demonstrate that overexpression of TcPRAC leads to an increase in parasite differentiation into infective forms and in its subsequent penetration into host cells. Furthermore, a critical impairment of parasite viability was observed in functional knock-down parasites. These results strongly emphasize that TcPRAC is a potential target for drug design as well as for immunomodulation of parasite-induced B-cell polyclonal activation.

Molecular basis of mammalian cell invasion by Trypanosoma cruzi

Establishment of infection by Trypanosoma cruzi, the agent of Chagas' disease, depends on a series of events involving interactions of diverse parasite molecules with host components. Here we focus on the mechanisms of target cell invasion by metacyclic trypomastigotes (MT) and mammalian tissue culture trypomastigotes (TCT). During MT or TCT internalization, signal transduction pathways are activated both in the parasite and the target cell, leading to Ca2+ mobilization. For cell adhesion, MT engage surface glycoproteins, such as gp82 and gp35/50, which are Ca2+ signal-inducing molecules. In T. cruzi isolates that enter host cells in gp82-mediated manner, parasite protein tyrosine kinase as well as phospholipase C are activated, and Ca2+ is released from I P3-sensitive stores, whereas in T. cruzi isolates that attach to target cells mainly through gp35/50, the signaling pathway involving adenylate cyclase appears to be stimulated, with Ca2+ release from acidocalciosomes. In addition, T. cruzi isolate-dependent inhibitory signals, mediated by MT-specific gp90, may be triggered both in the host cell and the parasite. The repertoire of TCT molecules implicated in cell invasion includes surface glycoproteins of gp85 family, with members containing binding sites for laminin and cytokeratin 18, enzymes such as cruzipain, trans-sialidase, and an oligopeptidase B that generates a Ca2+-agonist from a precursor molecule.

Chondroitin sulphate A as an adherence receptor for Plasmodium falciparum-infected erythrocytes

Until recently, the sequestration of erythrocytes infected with Plasmodium falciparum has been thought to be due to one of a number of protein-protein interactions. In this article, Stephen Rogerson and Graham Brown summarize the emerging evidence that, in vitro, infected erythrocytes can also adhere to the glycosaminoglycan chondroitin sulphate A (CSA) expressed on the surface of cells and immobilized on plastic. In vivo, binding of infected erythrocytes to CSA could be crucial to the development of malarial infection of the placenta, and possibly to sequestration in the lung and brain. The consequences of this may include maternal morbidity and mortality, low birth weight in the infant, pulmonary oedema and cerebral malaria. They discuss the need to characterize the molecular basis of this interaction, and to investigate the possible therapeutic role of CSA in malaria. Chondroitin sulphates are nontoxic compounds already in use for other diseases in humans. Vaccines based on inhibiting this receptor-ligand interaction could also be appropriate.

Role of sulfated glycans in adherence of the microsporidian Encephalitozoon intestinalis to host cells in vitro

Microsporidia are obligate intracellular opportunistic protists that infect a wide variety of animals, including humans, via environmentally resistant spores. Infection requires that spores be in close proximity to host cells so that the hollow polar tube can pierce the cell membrane and inject the spore contents into the cell cytoplasm. Like other eukaryotic microbes, microsporidia may use specific mechanisms for adherence in order to achieve target cell proximity and increase the likelihood of successful infection. Our data show that Encephalitozoon intestinalis exploits sulfated glycans such as the cell surface glycosaminoglycans (GAGs) in selection of and attachment to host cells. When exogenous sulfated glycans are used as inhibitors in spore adherence assays, E. intestinalis spore adherence is reduced by as much as 88%. However, there is no inhibition when nonsulfated glycans are used, suggesting that E. intestinalis spores utilize sulfated host cell glycans in adherence. These studies were confirmed by exposure of host cells to xylopyranoside, which limits host cell surface GAGs, and sodium chlorate, which decreases surface sulfation. Spore adherence studies with CHO mutant cell lines that are deficient in either surface GAGs or surface heparan sulfate also confirmed the necessity of sulfated glycans. Furthermore, when spore adherence is inhibited, host cell infection is reduced, indicating a direct association between spore adherence and infectivity. These data show that E. intestinalis specifically adheres to target cells by way of sulfated host cell surface GAGs and that this mechanism serves to enhance infectivity.

Sulfated glycosaminoglycans in two hematophagous arthropod vectors of Chagas disease, Triatoma brasiliensis and Rhodnius prolixus (Hemiptera: Reduviidae)

The characterization of sulfated glycosaminoglycans (GAGs) in hematophagous arthropod vectors in general has been limited, with the exception of the studies in the triatomine Rhodnius prolixus. Heparan sulfate (HS) and chondroitin sulfate (CS) were previously identified and structurally characterized in extracts of whole bodies of fourth instar larvae of R. prolixus. Recently, we showed the expression of these two sulfated GAGs in specific body tissues of adult males and females and in embryos of R. prolixus. In the present work, we identified and compared the sulfated GAG composition in specific tissues of adult insects and in embryos of another triatomine species, Triatoma brasiliensis. Sulfated GAGs were isolated from the fat body, intestinal tract, and the reproductive tracts of adult insects and from embryos. Only HS and CS were found in the tissues analyzed. The present results extend the initial observations on the sulfated GAG composition in R. prolixus by showing that these molecules are widely distributed among internal organs of triatomines. These observations may be useful for future investigations aiming to evaluate the possible implication of these compounds in physiological events that take place in a specific organ(s) in these insects.

Parasite-derived trans-sialidase binds to heart tissue in Trypanosoma cruzi-infected animals

Trypanosoma cruzi is an obligate intracellular protozoan parasite that actively penetrates into non-phagocytic mammalian cells. To accomplish this, the parasite relies on the binding of cell surface ligands. It is reported herein that the T. cruzi trans-sialidase (TS), which is exposed on the parasite surface, binds to mouse heart cells, and should therefore be further studied as a possible cell penetration-related ligand. In addition, as has been proposed elsewhere, the binding of T. cruzi to tissues may turn them into targets for parasite-specific immune reactions. Washed heart sections from T. cruzi-infected mice were subjected to immunoenzymatic staining with antisera against whole T. cruzi and with polyclonal or monoclonal antibodies against TS. The anti-TS antibodies stained both parasites and uninfected heart cells in the vicinity of T. cruzi nest remains/trypomastigotes. On the other hand, an anti-T. cruzi serum, which did not recognize TS, only stained the parasites. In addition, normal heart sections from uninfected nude mice were shown to react with both enzymatically active and inactive recombinant TS molecules, probably through their amino-terminal region, since a recombinant TS lacking this region failed to bind.

Identification and tissue-specific distribution of sulfated glycosaminoglycans in the blood-sucking bug Rhodnius prolixus (Linnaeus)

We have previously characterized heparan sulfate (HS) as the major ovarian sulfated glycosaminoglycan (GAG) in females of Rhodnius prolixus, while chondroitin sulfate (CS) was the minor component. Using histochemical procedures we found that GAGs were concentrated in the ovarian tissue but not found inside the oocytes. Here, we extend our initial observations of GAG expression in R. prolixus by characterizing these molecules in other organs: the fat body, intestinal tract, and the reproductive tracts. Only HS and CS were found in the three organs analyzed, however CS was the major GAG species in these tissues. We also determined the compartmental distribution of GAGs in these organs by histochemical analysis using 1,9-dimethylmethylene blue, and evaluated the specific distribution of CS within both male and female reproductive tracts by immunohistochemistry using an anti-CS antibody. We also determined the GAG composition in eggs at days 0 and 6 of embryonic development. Only HS and CS were found in eggs at day 6, while no sulfated GAGs were detected at day 0. Our results demonstrate that HS and CS are the only sulfated GAG species expressed in the fat body and in the intestinal and reproductive tracts of Rhodnius male and female adults. Both sulfated GAGs were also identified in Rhodnius embryos. Altogether, these results show no qualitative differences in the sulfated GAG composition regarding tissue-specific or development-specific distribution.

Lysosome recruitment during host cell invasion by Trypanosoma cruzi

The protozoan parasite Trypanosoma cruzi uses an unusual mechanism to enter cells. Recent observations revealed that instead of trypanosomes being brought in to fuse with lysosomes, it is the lysosomes that migrate to the trypanosomes and actually participate in their internalization. Signalling events involving intracellular free Ca2+ occur upon contact of the parasites with host cells and may contribute to the regulation of this unusual process.

In vitro selection of high-affinity nucleic acid ligands to parasite target molecules

The logic of using nucleic acids as pharmaceutical reagents is in part based on their capacity to interact with high affinity and specificity with other biological components. Considerable progress has been made over the past 10 years in the development of nucleic acid-based drug molecules using a variety of different technologies. One approach is a combinatorial technology that involves an iterative Darwinian-type in vitro evolution process, which has been termed SELEX for 'systematic evolution of ligands by exponential enrichment'. The procedure is a highly efficient method of identifying rare ligands from combinatorial nucleic acid libraries of very high complexity. It allows the selection of nucleic acid molecules with desired functions and it has been instrumental in the identification of a number of synthetic DNA and RNA molecules, so-called aptamers that recognise ligands of different chemical origin. The method is fast, it does not require special equipment and the selected aptamers typically bind their target with high affinity and high specificity. Here we summarise the recent examples of the SELEX technique within the context of identifying high-affinity ligands against parasite target molecules.

Heparan sulfate proteoglycans mediate the invasion of cardiomyocytes by Trypanosoma cruzi

Cytoadherence is an important step for the invasion of a mammalian host cell by Trypanosoma cruzi. Cell surface macromolecules are implicated in the T. cruzi-cardiomyocyte recognition process. Therefore, we investigated the role of cell surface proteoglycans during this invasion process and analyzed their expression after the parasite infected the target cells. Treatment of trypomastigote forms of T. cruzi with soluble heparan sulfate resulted in a significant inhibition in successful invasion, while chondroitin sulfate had no effect. Removal of sulfated glycoconjugates from the cardiomyocyte surface using glycosaminoglycan (GAG) lyases demonstrated the specific binding of the parasites to heparan sulfate proteoglycans. Infection levels were reduced by 42% whenthe host cells were previously treated with heparitinase II. No changes were detected in the expression of GAGs infected cardiomyocytes even after 96 h of infection. Our data demonstrate that heparan sulfate proteoglycans, but not chondroitin sulfate, mediate both attachment and invasion of cardiomyocytes by T. cruzi.

GapA and CrmA coexpression is essential for Mycoplasma gallisepticum cytadherence and virulence

It was previously demonstrated that avirulent Mycoplasma gallisepticum strain R(high) (passage 164) is lacking three proteins that are expressed in its virulent progenitor, strain R(low) (passage 15). These proteins were identified as the cytadhesin molecule GapA, the putative cytadhesin-related molecule CrmA, and a component of a high-affinity transporter system, HatA. Complementation of R(high) with wild-type gapA restored expression in the transformant (GT5) but did not restore the cytadherence phenotype and maintained avirulence in chickens. These results suggested that CrmA might play an essential role in the M. gallisepticum cytadherence process. CrmA is encoded by the second gene in the gapA operon and shares significant sequence homology to the ORF6 gene of Mycoplasma pneumoniae, which has been shown to play an accessory role in the cytadherence process. Complementation of R(high) with wild-type crmA resulted in the transformant (SDCA) that lacked the cytadherence and virulence phenotype comparable to that found in R(high) and GT5. In contrast, complementation of R(high) with the entire wild-type gapA operon resulted in the transformant (GCA1) that restored cytadherence to the level found in wild-type R(low). In vivo pathogenesis trials revealed that GCA1 had regained virulence, causing airsacculitis in chickens. These results demonstrate that both GapA and CrmA are required for M. gallisepticum cytadherence and pathogenesis.

Cell signalling and Trypanosoma cruzi invasion

Mammalian cell invasion by the protozoan pathogen Trypanosoma cruzi is critical to its survival in the host. To promote its entry into a wide variety of non-professional phagocytic cells, infective trypomastigotes exploit an arsenal of heterogenous surface glycoproteins, secreted proteases and signalling agonists to actively manipulate multiple host cell signalling pathways. Signals initiated in the parasite upon contact with mammalian cells also function as critical regulators of the invasion process. Whereas the full spectrum of cellular responses modulated by T. cruzi is not yet known, mounting evidence suggests that these pathways impinge on a number of cellular processes, in particular the ubiquitous wound-repair mechanism exploited for lysosome-mediated parasite entry. Furthermore, differential engagement of host cell signalling pathways in a cell type-specific manner and modulation of host cell gene expression by T. cruzi are becoming recognized as essential determinants of infectivity and intracellular survival by this pathogen.

In vitro selection of RNA aptamers that bind to cell adhesion receptors of Trypanosoma cruzi and inhibit cell invasion

Trypanosoma cruzi causing Chagas' disease needs to invade host cells to complete its life cycle. Macromolecules on host cell surfaces such as laminin, thrombospondin, heparan sulfate, and fibronectin are believed to be important in mediating parasite-host cell adhesions and in the invasion process of the host cell by the parasite. The SELEX technique (systematic evolution of ligands by exponential enrichment) was used to evolve nuclease-resistant RNA ligands (aptamer = to fit) that bind with affinities of 40-400 nm to parasite receptors for the host cell matrix molecules laminin, fibronectin, thrombospondin, and heparan sulfate. After eight consecutive rounds of in vitro selection four classes of RNA aptamers based on structural similarities were isolated and sequenced. All members of each class shared a common sequence motif and competed with the respective host cell matrix molecule that was used for displacement during the selection procedure. RNA pools following seven and eight selection rounds as well as individual aptamers sharing consensus motifs were active in inhibiting invasion of LLC-MK(2) monkey kidney cells by T. cruzi in vitro.

Regulation of Trypanosoma cruzi invasion of nonphagocytic cells by the endocytically active GTPases dynamin, Rab5, and Rab7

During invasion of nonphagocytic cells by Trypanosoma cruzi (T. cruzi), host cell lysosomes are recruited to the plasma membrane attachment site followed by lysosomal enzyme secretion. The membrane trafficking events involved in invasion have not been delineated. We demonstrate here that T. cruzi invasion of nonphagocytic cells was completely abolished by overexpression of a dominant negative mutant of dynamin. Likewise, overexpression of a dominant negative mutant of Rab5, the rate-limiting GTPase for endocytosis, resulted in reduced infection rates compared with cells expressing Rab5 wild-type. Moreover, cells expressing the activated mutant of Rab5 experienced higher infection rates. A similar pattern was also observed when Rab7-transfected cells were examined. Confocal microscopy experiments showed that parasites colocalized with green fluorescent protein-Rab5-positive early endosomes after 5 min of invasion. These data clearly indicate that newly forming T. cruzi phagosomes first interact with an early endosomal compartment and subsequently with other late component markers before lysosomal interaction occurs.

Heparan sulfate modulates kinin release by Trypanosoma cruzi through the activity of cruzipain

Trypanosoma cruzi activates the kinin pathway through the activity of its major cysteine proteinase, cruzipain. Because kininogen molecules may be displayed on cell surfaces by binding to glycosaminoglycans, we examined whether the ability of cruzipain to release kinins from high molecular weight kininogen (HK) is modulated by heparan sulfate (HS). Kinetic assays show that HS reduces the cysteine proteinase inhibitory activity (K(i app)) of HK about 10-fold. Conversely, the catalytic efficiency of cruzipain on kinin-related synthetic fluorogenic substrates is enhanced up to 6-fold in the presence of HS. Analysis of the HK breakdown products generated by cruzipain indicated that HS changes the pattern of HK cleavage products. Direct measurements of bradykinin demonstrated an up to 35-fold increase in cruzipain-mediated kinin liberation in the presence of HS. Similarly, kinin release by living trypomastigotes increased up to 10-fold in the presence of HS. These studies suggest that the efficiency of T. cruzi to initiate kinin release is potently enhanced by the mutual interactions between cruzipain, HK, and heparan sulfate proteoglycans.

Binding specificity of mannose-specific carbohydrate-binding protein from the cell surface of Trypanosoma cruzi

The sugar binding specificity of the recently described mannose-specific carbohydrate-binding proteins (CBP) isolated to homogeneity from both the epimastigote and trypomastigote stages of the pathogenic protozoa Trypanosoma cruzi has been studied by quantitative hapten inhibition of the biotinylated CBPs to immobilized thyroglobulin using model oligosaccharides. The results clearly show a differential specificity toward high-mannose glycans between the CBPs from the two developmental stages. Thus, the isolated CBP from epimastigotes exhibited stronger affinity for higher mannose oligomers containing the Manalpha1-2Manalpha1-6Manalpha1-6 structure. Its affinity decreased, as did the number of mannose residues on the oligomer or removal of the terminal Manalpha1-2-linked mannose. By contrast the CBP isolated from the trypomastigote stage showed about 400-fold lower avidity than the epimastigote form, and contrary to it, it was slightly more specific toward Man5GlcNAc than Man9GlcNAc. Analysis of the interaction of epimastigote-Man-CBP with its ligands by UV difference spectroscopy indicates the existence of an extended binding site in that protein with a large enthalpic contribution to the binding. The thermodynamic parameters of binding were obtained by isothermal titration calorimetry and been found that the DeltaH values to be in good agreement with the van't Hoff values. The binding reactions are mainly enthalpically driven and exhibit enthalpy-enthropy compensation. In addition, analysis of the high-mannose glycans from different parts of the digestive tract of the reduviid insect vector of T. cruzi suggest a role of the CBP in the retention of the epimastigote stage in the anterior portion of the gut.

Host cell invasion by Trypanosoma cruzi is potentiated by activation of bradykinin B(2) receptors

The parasitic protozoan Trypanosoma cruzi employs multiple molecular strategies to invade a broad range of nonphagocytic cells. Here we demonstrate that the invasion of human primary umbilical vein endothelial cells (HUVECs) or Chinese hamster ovary (CHO) cells overexpressing the B(2) type of bradykinin receptor (CHO-B(2)R) by tissue culture trypomastigotes is subtly modulated by the combined activities of kininogens, kininogenases, and kinin-degrading peptidases. The presence of captopril, an inhibitor of bradykinin degradation by kininase II, drastically potentiated parasitic invasion of HUVECs and CHO-B(2)R, but not of mock-transfected CHO cells, whereas the B(2)R antagonist HOE 140 or monoclonal antibody MBK3 to bradykinin blocked these effects. Invasion competence correlated with the parasites' ability to liberate the short-lived kinins from cell-bound kininogen and to elicit vigorous intracellular free calcium ([Ca(2+)](i)) transients through B(2)R. Invasion was impaired by membrane-permeable cysteine proteinase inhibitors such as Z-(SBz)Cys-Phe-CHN(2) but not by the hydrophilic inhibitor 1-trans-epoxysuccinyl-l-leucyl-amido-(4-guanidino) butane or cystatin C, suggesting that kinin release is confined to secluded spaces formed by juxtaposition of host cell and parasite plasma membranes. Analysis of trypomastigote transfectants expressing various cysteine proteinase isoforms showed that invasion competence is linked to the kinin releasing activity of cruzipain, herein proposed as a factor of virulence in Chagas' disease.

A B-cell mitogen from a pathogenic trypanosome is a eukaryotic proline racemase

Lymphocyte polyclonal activation is a generalized mechanism of immune evasion among pathogens. In a mouse model of Trypanosoma cruzi infection (American trypanosomiasis), reduced levels of polyclonal lymphocyte responses correlate with resistance to infection and cardiopathy. We report here the characterization of a parasite protein with B-cell mitogenic properties in culture supernatants of infective forms, the cloning of the corresponding gene and the analysis of the biological properties of its product. We characterized the protein as a co-factor-independent proline racemase, and show that its expression as a cytoplasmic and/or membrane-associated protein is life-stage specific. Inhibition studies indicate that availability of the racemase active site is necessary for mitogenic activity. This is the first report to our knowledge of a eukaryotic amino acid racemase gene. Our findings have potential consequences for the development of new immune therapies and drug design against pathogens.

Characterization of the heparin-binding site of the mycobacterial heparin-binding hemagglutinin adhesin

The mycobacterial adhesin heparin-binding hemagglutinin (HBHA) contains several lysine-rich repeats at its carboxyl-terminal end. Using truncated recombinant HBHA forms and hybrid proteins containing HBHA repeats grafted onto the Escherichia coli maltose-binding protein (MBP), we found that these repeats are responsible for heparin binding. Immunofluorescence microscopy studies revealed that their deletion abrogates binding of HBHA to human pneumocytes. Conversely, when fused to MBP, the HBHA repeats confer pneumocyte adherence properties to the hybrid protein. Treatment of pneumocytes with glycosaminoglycan-degrading enzymes showed that HBHA binding depends on the presence of heparan sulfate chains on the cell surface. The epitope of a monoclonal antibody that inhibits mycobacterial adherence to epithelial cells was mapped within the lysine-rich repeats, confirming their involvement in mycobacterial adherence to epithelial cells. Surface plasmon resonance analyses showed that recombinant HBHA binds to immobilized heparin with fast association kinetics (k(a) = 5.62 (+/- 0.10) x 10(5) m(-1) s(-1)), whereas the dissociation kinetics were slower (k(d) = 0.015 (+/- 0.002) s(-1)), yielding a K(D) value of 26 nm. Similar analyses with grafted MBP indicated similar kinetic constants, indicating that the carboxyl-terminal repeats contain the entire heparin-binding site of HBHA. The molecular characterization of the interactions of HBHA with epithelial glycosaminoglycans should help to better understand mycobacterial adherence within the lungs and may ultimately lead to new approaches for therapy or immunoprophylaxis.

Variable small protein (Vsp)-dependent and Vsp-independent pathways for glycosaminoglycan recognition by relapsing fever spirochaetes

Tick-borne relapsing fever, caused by pathogenic Borrelia such as B. hermsii and B. turicatae, features recurrent episodes of bacteraemia, each of which is caused by a population of spirochaetes that expresses a different variable major protein. Relapsing fever is also associated with the infection of a variety of tissues, such as the central nervous system. In this study, we show that glycosaminoglycans (GAGs) mediate the attachment of relapsing fever spirochaetes to mammalian cells. B. hermsii strain DAH bound to immobilized heparin, and heparin and dermatan sulphate blocked bacterial binding to host cells. Bacterial binding was diminished by inhibition of host cell GAG synthesis or sulphation, or by the enzymatic removal of GAGs. GAGs mediated the attachment of relapsing fever spirochaetes to potentially relevant target cells, such as endothelial and glial cells. B. hermsii was able to attach to GAGs independently of variable major proteins, because strains expressing the variable major proteins Vsp33, Vlp7 or no variable major protein at all each recognized GAGs. Nevertheless, we found that a variable major protein of B. turicatae directly promoted GAG binding by this relapsing fever spirochaete. B. turicatae strain Oz1 serotype B, which expresses the variable major protein VspB, bound to GAGs more efficiently than did B. turicatae Oz1 serotype A, which expresses VspA. Recombinant VspB, but not VspA, bound to heparin and dermatan sulphate. Previous studies have shown that strain Oz1 serotype B grows to higher concentrations in the blood than does Oz1 serotype A. Thus, relapsing fever spirochaetes have the potential to express Vsp-dependent and Vsp-independent GAG-binding activities and, for one pair of highly related B. turicatae strains, differences in GAG binding correlate with differences in tissue tropism.

Interferon- or interleukin-10 production is induced by related Trypanosoma cruzi antigens

The purpose of this study was to evaluate the effects of a crude Trypanosoma cruzi antigen (TCA) and its partially purified subfractions TCF1, TCF2 on peripheral blood mononuclear cells (PBMC) of normal donors and chagasic patients. TCFI and TCF2 stimulated cells from normal donors and chagasic patients in association with a significant production of interleukin (IL)-10. Only PBMC from chagasic patients multiplied after incubation with TCA and released mainly interferon-y but also IL-10. Neither the production of IL-2 and IL-4 nor CD4/CD8 ratios were changed after culture with antigens. These data suggest that some antigens active during the acute phase of T. cruzi infection would stimulate the production of cytokines that promote progression of infection, and the immune system can produce a desired cytokine(s) once the appropriate antigenic stimulus is used.

A Toxoplasma lectin-like activity specific for sulfated polysaccharides is involved in host cell infection

Toxoplasma gondii is one of the most widespread parasites of humans and animals. The parasite has a remarkable ability to invade a broad range of cells within its mammalian hosts by mechanisms that are poorly understood at the molecular level. This broad host cell specificity suggests that adhesion should involve the recognition of ubiquitous surface-exposed host molecules or, alternatively, the presence of various parasite attachment molecules able to recognize different host cell receptors. We have discovered a sugar-binding activity (lectin) in tachyzoites of T. gondii that plays a role in vitro in erythrocyte agglutination and infection of human fibroblasts and epithelial cells. The ability to agglutinate erythrocytes can be reversed by a variety of soluble glycoconjugates, of which heparin, fucoidan, and dextran sulfate were the most effective. Interestingly, infectivity of tachyzoites for human foreskin fibroblasts, cells that are commonly used to grow T. gondii in vitro, was increased by low concentrations of the sulfated glycoconjugates that inhibited hemagglutination activity (i.e. dextran sulfate and fucoidan) whereas high concentrations inhibited parasite infection. Furthermore, inhibition of glycosaminoglycan biosynthesis and sulfation on the host cells reduced Toxoplasma infectivity. Finally, Toxoplasma tachyzoites showed a reduced ability to infect epithelial cell mutants deficient in the biosynthesis of surface proteoglycans. The probable identity of the hemagglutinin(s) was investigated by 1) direct binding of red blood cells to filter blots of Toxoplasma proteins separated by polyacrylamide gel electrophoresis, and 2) binding of metabolically labeled parasite proteins to fixed mammalian cells. Three parasite bands were thus identified as candidate adhesins. These results suggest that attachment of T. gondii to its target cell is mediated by parasite lectins and that sulfated sugars on the surface of host cells may function as a key parasite receptor.

Kininogenase activity by the major cysteinyl proteinase (cruzipain) from Trypanosoma cruzi

The major isoform of Trypanosoma cruzi cysteinyl proteinase (cruzipain) has generated Lys-bradykinin (Lys-BK or kallidin), a proinflammatory peptide, by proteolysis of kininogen. The releasing of this peptide was demonstrated by mass spectrometry, radioimmunoassay, and ileum contractile responses. The kinin-releasing activity was immunoabsorbed selectively by monoclonal antibodies to the characteristic COOH-terminal domain of cruzipain. To determine the hydrolysis steps that account for the kininogenase activity of cruzipain, we synthesized a fluorogenic peptide (o-aminobenzoyl-Leu-Gly-Met-Ile-Ser-Leu-Met-Lys-Arg-Pro-Pro-Gly-Phe-S er-Pro-Phe-Arg389-Ser390-Ser-Arg-Ile-NH2) based on the sequence Leu373 to Ile393 of the human high molecular weight kininogen. The hydrolysis products from this peptide were isolated by high performance liquid chromatography, and Lys-BK was characterized as the major released kinin by mass spectrometry. Intramolecularly quenched fluorogenic peptides spanning the Met379-Lys380 and Arg389-Ser390 bradykinin-flanking sequences were then used to assess the substrate specificity requirements of the parasite-derived protease compared with two COOH-terminal truncated recombinant isoforms (cruzain and cruzipain 2). In contrast to the high catalytic efficiency of parasite-derived cruzipain, the recombinant proteinases cleaved the bradykinin-flanking sites at markedly different rates. In addition, we also demonstrated that cruzipain activates plasmatic prekallikrein, which would be a second and indirect way of the parasite protease to release bradykinin.

A lectin-like molecule is discharged from mucocysts of Tetrahymena pyriformis in the presence of insulin

By use of a monoclonal antibody directed against purified lectin from the sponge Geodia cydonium it was demonstrated that the mucocysts of Tetrahymena pyriformis contain a substance immunologically similar to that found in G. cydonium. In extracts of T. pyriformis the monoclonal antibody recognizes a 36 kDa protein; binding could be abolished by adsorption of the antibody with (i) crude extract, (ii) purified lectin from G. cydonium and (iii) a 29 aa long peptide. In addition the data show that 10(-6) M of insulin causes first the release of mucocyst material, which reacts with the lectin antibody, and second its subsequent redistribution on the surface of the somatic cilia and the oral field.

Removal of sialic acid from mucin-like surface molecules of Trypanosoma cruzi metacyclic trypomastigotes enhances parasite-host cell interaction

The 35/50 kDa mucin-like surface glycoprotein (gp35/50) of Trypanosoma cruzi metacyclic trypomastigotes has been implicated in mammalian cell invasion. In this study we investigated whether the sialyl residues of gp35/50 are required for interaction of parasites with target cells. After treatment with bacterial neuraminidase, the metacyclic forms (G strain) remained reactive with the monoclonal antibody (mAb) 10D8 but lost their reactivity with mAb 3C9, that recognizes sialic acid-containing epitopes on gp35/50, and entered HeLa cells in significantly higher numbers as compared to untreated controls. Resialylation of gp35/50, by incubation of parasites with T. cruzi trans-sialidase and sialyl lactose, restored the reactivity with mAb 3C9 as well as the affinity for sialic acid specific lectin. Accordingly, the rate of invasion of resialylated parasites was reduced to levels similar to those observed before desialylation. Purified G strain gp35/50, desialylated by neuraminidase treatment, bound to HeLa cells more than its sialylated counterpart. The Ca2+ signaling activity, which has been associated with cell invasion, was also determined by measuring the cytosolic Ca2+ concentration ([Ca2+]i), in HeLa cells upon interaction with sonicated extracts from untreated or neuraminidase-treated parasites, or with purified gp35/50 in its sialylated or desialylated form. Consistent with the results of cell invasion assay, the desialylated parasite preparations, as well as the sialic acid free gp35/50, induced an average elevation in [Ca2+]i significantly higher than that triggered by untreated controls. None of these effects, namely the increase in infectivity and Ca2+ signaling activity, was observed with neuraminidase-treated CL strain metacyclic trypomastigotes, which express a variant form of sialic acid gp35/50 molecule that is not recognized by mAb 10D8 and apparently is not involved in target cell invasion.

Characterization of the parasite-host cell interactions involved in Theileria parva sporozoite invasion of bovine lymphocytes

Sporozoite invasion of bovine lymphocytes by Theileria parva is a pH-dependent process that occurs without the need for de novo protein synthesis. The process was inhibited by RGD(S) peptides, fibronectin and, in the presence of serum, by antibodies reactive with fibronectin. Invasion was also blocked by a range of sulphated glycoconjugates, but treatment of lymphocytes with heparitinase did not inhibit entry. Enzymic modifications of the lymphocyte surface demonstrated that trypsin-insensitive glycoproteins containing O- and N-linked carbohydrates as well as phospholipase-sensitive molecules on the host cell surface were critical to sporozoite entry. Modification of the lymphocyte surface with NEM and DTT had only marginal effects on sporozoite binding but blocked parasite internalization. Invasion was also blocked by several antibodies which cross-reacted with sporozoite surface molecules. While only a few experimental conditions specifically blocked sporozoite binding, a wider range of reagents and treatments inhibited parasite entry. The reasons for this are discussed in terms of the nature of the zippering process that facilitates sporozoite internalization.

Invasive phenotype of Trypanosoma cruzi restricted to a population expressing trans-sialidase

Trypanosoma cruzi expresses a developmentally regulated trans-sialidase implicated in the pathogenesis of Chagas' disease. On inhabitation of the extracellular milieu of cultured cells by infective trypomastigotes, the enzyme is restricted to a small (20 to 30%) population of parasites. The biological significance of trans-sialidase expression on this subset, termed TS+, and not on the majority (70 to 80%) of morphologically similar trypanosomes, named TS-, is unknown. To determine the roles of the TS+ and TS- subsets in T. cruzi invasion, we prepared pure populations of TS- and TS+ trypanosomes using magnetic beads coated with a monoclonal antibody specific for the tandem repeat unit of the trans-sialidase C terminus. After removal of nonadherent TS- trypomastigotes, the TS+ trypomastigotes were isolated from the beads by specific elusion with a synthetic peptide epitope of the trans-sialidase monoclonal antibody. Confirmation of TS+ and TS- phenotypes was obtained by immunofluorescence, immunoblotting, and sialidase or sialyl transferase activity measurements. The TS+ trypanosomes were highly invasive, as they attached to, penetrated, and thrived in cultured mammalian cells much more efficiently than did unfractionated parasites. The critical role of the trans-sialidase in invasion was underscored by the observation that infection was neutralized by human antibodies to transsialidase. What's more, the TS- parasites, in sharp contrast to their TS+ counterparts, were extremely inefficient in invading epithelial cells and fibroblasts. Further, introduction of small amounts of exogenous trans-sialidase into suspensions of nonpenetrating TS- parasites converted them to a highly invasive phenotype indistinguishable from that of the TS+ population. Rescue of the invasive phenotype was specific for the T. cruzi enzyme, for it didn't happen with bacterial and viral sialidases. The in vitro results were confirmed in the murine model of Chagas' disease, as TS- trypomastigotes were relatively avirulent while TS+ trypomastigotes were more virulent than unfractionated parasites.

Release of membrane-bound trails by Trypanosoma cruzi amastigotes onto modified surfaces and mammalian cells

Upon incubation at 37 degrees C onto glass coverslips coated with Concanavalin A, poly-L-lysine, or a monoclonal antibody (1D9) directed to the parasite major surface glycoprotein Ssp-4, extracellular Trypanosoma cruzi amastigotes release trails of material barely visible by light microscopy. This release is not associated with parasite movements. Immunolabeling studies confirmed that the material is derived from the parasite's membrane since thin section through samples labeled with 1D9 revealed that the trails are membrane-bound structures. Scanning electron microscopy showed that the approximately 0.1-micron(s) thick trails of material emerging from the amastigotes can be uniform or beaded, indicating a tendency to vesiculation. The trails are preferentially released from the flagellar pocket region and/or at the opposite posterior end of the parasite body, and seem to be devoid of microtubules. The release is time and temperature-dependent and fixed parasites do not form trails. All attempts to inhibit trail release using drugs (antimycin A, sodium azide, cytochalasin D, nocodazole, genistein, staurosporine, EGTA) failed. The observation of trails associated with intracellular parasites and amastigotes invading Vero cells suggests that this is probably a physiological process.

Adherence of Plasmodium falciparum to chondroitin sulfate A in the human placenta

Women are particularly susceptible to malaria during first and second pregnancies, even though they may have developed immunity over years of residence in endemic areas. Plasmodium falciparum-infected red blood cells (IRBCs) were obtained from human placentas. These IRBCs bound to purified chondroitin sulfate A (CSA) but not to other extracellular matrix proteins or to other known IRBC receptors. IRBCs from nonpregnant donors did not bind to CSA. Placental IRBCs adhered to sections of fresh-frozen human placenta with an anatomic distribution similar to that of naturally infected placentas, and this adhesion was competitively inhibited by purified CSA. Thus, adhesion to CSA appears to select for a subpopulation of parasites that causes maternal malaria.