The action of Trypanosoma cruzi trans-sialidase on glycolipids and glycoproteins

Recent progress on health effects and biosynthesis of two key sialylated human milk oligosaccharides, 3'-sialyllactose and 6'-sialyllactose

Human milk oligosaccharides (HMOs), the third major solid component in breast milk, are recognized as the first prebiotics for health benefits in infants. Sialylated HMOs are an important type of HMOs, accounting for approximately 13% of total HMOs. 3'-Sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL) are two simplest sialylated HMOs. Both SLs display promising prebiotic effects, especially in promoting the proliferation of bifidobacteria and shaping the gut microbiota. SLs exhibit several health effects, including antiadhesive antimicrobial ability, antiviral activity, prevention of necrotizing enterocolitis, immunomodulatory activity, regulation of intestinal epithelial cell response, promotion of brain development, and cognition improvement. Both SLs have been approved as "Generally Recognized as Safe" by the American Food and Drug Administration and are commercially added to infant formula. The biosynthesis of SLs using enzymatic or microbial approaches has been widely studied. The enzymatic synthesis of SLs can be realized by two types of enzymes, sialidases with trans-sialidase activity and sialyltransferases. Microbial synthesis can be achieved by the multiple recombinant bacteria in one-pot reaction, which express the enzymes involved in SL synthesis pathways separately or in combination, or by metabolically engineered strains in a fermentation process. In this article, the physiological properties of 3'-SL and 6'-SL are summarized in detail and the biosynthesis of these SLs via enzymatic and microbial synthesis is comprehensively reviewed.

trans-Sialylation: a strategy used to incorporate sialic acid into oligosaccharides

Sialic acid, as a component of cell surface glycoconjugates, plays a crucial role in recognition events. Efficient synthetic methods are necessary for the supply of sialosides in enough quantities for biochemical and immunological studies. Enzymatic glycosylations obviate the steps of protection and deprotection of the constituent monosaccharides required in a chemical synthesis. Sialyl transferases with CMP-Neu5Ac as an activated donor were used for the construction of α2-3 or α2-6 linkages to terminal galactose or N-acetylgalactosamine units. trans-Sialidases may transfer sialic acid from a sialyl glycoside to a suitable acceptor and specifically construct a Siaα2-3Galp linkage. The trans-sialidase of Trypanosoma cruzi (TcTS), which fulfills an important role in the pathogenicity of the parasite, is the most studied one. The recombinant enzyme was used for the sialylation of β-galactosyl oligosaccharides. One of the main advantages of trans-sialylation is that it circumvents the use of the high energy nucleotide. Easily available glycoproteins with a high content of sialic acid such as fetuin and bovine κ-casein-derived glycomacropeptide (GMP) have been used as donor substrates. Here we review the trans-sialidase from various microorganisms and describe their application for the synthesis of sialooligosaccharides.

Synthesis of the hexasaccharide from Trypanosoma cruzi mucins with the Galp(1 → 2)Galf unit constructed with a superarmed thiogalactopyranosyl donor

Hexasaccharide β-D-Galp-(1→ 2)-[β-D-Galp-(1 → 3)]-β-D-Galp-(1 → 6)-[β-D-Galp-(1 → 2)-β-D-Galf-(1 → 4)]-D-GlcNAc (1) was found O-linked in mucins of Trypanosoma cruzi epimastigotes and metacyclic trypomatigotes. Studies on the biological pathways and functionalities of the mucin oligosaccharides are prompted in order to understand the interactions of these molecules with the insect host. Trisaccharide constituent β-D-Galp-(1 → 2)-β-D-Galf-(1 → 4)-D-GlcNAc was constructed from the reducing to the non-reducing end. We discuss the difficulties to introduce a Galp unit at the O-2 position of a partially protected galactofuranosyl unit which were overcome using an anchimerically superarmed donor. By this route and employing a [3 + 3] nitrilium convergent approach hexasaccharide 1 was synthesized in moderate yield.

Trypanosoma cruzi trans-sialidase. A tool for the synthesis of sialylated oligosaccharides

Cells are covered by a complex array of carbohydrates. Among them, sialosides are of key importance in intracellular adhesion, recognition and signaling. The need for structurally diverse sialosides impelled the search for efficient synthetic methods since their isolation from natural sources is a difficult task. The enzymatic approach obviates the need of a chemical synthesis for protecting or participating groups in the substrates. The trans-sialidase of Trypanosoma cruzi (TcTS) is highly stereospecific for the transfer of sialic acid from an α-sialylglycoside donor to a terminal β-galactopyranosyl unit in the acceptor substrate to form the α-Neu5Ac-(2 → 3)-β-D-Galp motif. The enzyme was cloned and easily available glycoproteins, e.g. fetuin, may be used as donors of sialic acid, constituting strong points for the scalability of TcTS-catalyzed reactions. This review outlines the preparative use of TcTS for the sialylation of oligosaccharides. A detailed description of the substrates used as sialic acid donors, the acceptor substrates and the methods employed to monitor the reaction is included.

Genomic comparison of Trypanosoma conorhini and Trypanosoma rangeli to Trypanosoma cruzi strains of high and low virulence

Background

Trypanosoma conorhini and Trypanosoma rangeli, like Trypanosoma cruzi, are kinetoplastid protist parasites of mammals displaying divergent hosts, geographic ranges and lifestyles. Largely nonpathogenic T. rangeli and T. conorhini represent clades that are phylogenetically closely related to the T. cruzi and T. cruzi-like taxa and provide insights into the evolution of pathogenicity in those parasites. T. rangeli, like T. cruzi is endemic in many Latin American countries, whereas T. conorhini is tropicopolitan. T. rangeli and T. conorhini are exclusively extracellular, while T. cruzi has an intracellular stage in the mammalian host.

Results

Here we provide the first comprehensive sequence analysis of T. rangeli AM80 and T. conorhini 025E, and provide a comparison of their genomes to those of T. cruzi G and T. cruzi CL, respectively members of T. cruzi lineages TcI and TcVI. We report de novo assembled genome sequences of the low-virulent T. cruzi G, T. rangeli AM80, and T. conorhini 025E ranging from ~ 21–25 Mbp, with ~ 10,000 to 13,000 genes, and for the highly virulent and hybrid T. cruzi CL we present a ~ 65 Mbp in-house assembled haplotyped genome with ~ 12,500 genes per haplotype. Single copy orthologs of the two T. cruzi strains exhibited ~ 97% amino acid identity, and ~ 78% identity to proteins of T. rangeli or T. conorhini. Proteins of the latter two organisms exhibited ~ 84% identity. T. cruzi CL exhibited the highest heterozygosity. T. rangeli and T. conorhini displayed greater metabolic capabilities for utilization of complex carbohydrates, and contained fewer retrotransposons and multigene family copies, i.e. trans-sialidases, mucins, DGF-1, and MASP, compared to T. cruzi.

Conclusions

Our analyses of the T. rangeli and T. conorhini genomes closely reflected their phylogenetic proximity to the T. cruzi clade, and were largely consistent with their divergent life cycles. Our results provide a greater context for understanding the life cycles, host range expansion, immunity evasion, and pathogenesis of these trypanosomatids.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5112-0) contains supplementary material, which is available to authorized users.

The Glycoscience of Immunity

Carbohydrates, or glycans, are as integral to biology as nucleic acids and proteins. In immunology, glycans are well known to drive diverse functions ranging from glycosaminoglycan-mediated chemokine presentation and selectin-dependent leukocyte trafficking to the discrimination of self and non-self through the recognition of sialic acids by Siglec (sialic acid-binding Ig-like lectin) receptors. In recent years, a number of key immunological discoveries are driving a renewed and burgeoning appreciation for the importance of glycans. In this review, we highlight these findings which collectively help to define and refine our knowledge of the function and impact of glycans within the immune response.

Synthesis of a model trisaccharide for studying the interplay between the anti α-Gal antibody and the trans-sialidase reactions in Trypanosoma cruzi

Trypanosoma cruzi, the etiologic agent of Chagas disease, is covered by a dense glycocalix mainly composed by glycoproteins called mucins which are also the acceptors of sialic acid in a reaction catalyzed by a trans-sialidase (TcTS). Sialylation of trypomastigote mucins protects the parasite from lysis by the anti α-Galp antibodies from serum. The TcTS is essential for the infection process since T. cruzi is unable to biosynthesize sialic acid. The enzyme specifically transfers it from a terminal β-d-Galp unit in the host glycoconjugate to terminal β-d-Galp units in the parasite mucins to construct the d-NeuNAc(α2→3)β-d-Galp motif. On the other hand, although galactose is the most abundant sugar in mucins of both, the infective trypomastigotes and the insect stage epimastigotes, α-d-Galp is only present in the infective stage whereas β-d-Galf is characteristic of the epimastigote stage of the less virulent strains. Neither α-d-Galp nor d-Galf is acceptor of sialic acid. In the mucins, some of the oligosaccharides are branched with terminal β-d-Galp units to be able to accept sialic acid in the TcTS reaction. Based on previous reports showing that anti α-Galp antibodies only partially colocalize with sialic acid, we have undertaken the synthesis of the trisaccharide α-d-Galp(1→3)-[β-d-Galp(1→6)]-d-Galp, the smallest structure containing both, the antigenic d-Galp(α1→3)-d-Galp unit and the sialic acid-acceptor β-d-Galp unit. The trisaccharide was obtained as the 6-aminohexyl glycoside to facilitate further conjugation for biochemical studies. The synthetic approach involved the α-galactosylation at O-4 of a suitable precursor of the reducing end, followed by β-galactosylation at O-6 of the same precursor and introduction of the 6-aminohexyl aglycone. The fully deprotected trisaccharide was successfully sialylated by TcTS using either 3'-sialyllactose or fetuin as donors. The product, 6-aminohexyl α-d-NeuNAc(2→3)-β-d-Galp(1→6)-[α-d-Galp(1→3)]-β-d-Galp, was purified and characterized.

Transcriptomic analysis reveals metabolic switches and surface remodeling as key processes for stage transition in Trypanosoma cruzi

American trypanosomiasis is a chronic and endemic disease which affects millions of people. Trypanosoma cruzi, its causative agent, has a life cycle that involves complex morphological and functional transitions, as well as a variety of environmental conditions. This requires a tight regulation of gene expression, which is achieved mainly by post-transcriptional regulation. In this work we conducted an RNAseq analysis of the three major life cycle stages of T. cruzi: amastigotes, epimastigotes and trypomastigotes. This analysis allowed us to delineate specific transcriptomic profiling for each stage, and also to identify those biological processes of major relevance in each state. Stage specific expression profiling evidenced the plasticity of T. cruzi to adapt quickly to different conditions, with particular focus on membrane remodeling and metabolic shifts along the life cycle. Epimastigotes, which replicate in the gut of insect vectors, showed higher expression of genes related to energy metabolism, mainly Krebs cycle, respiratory chain and oxidative phosphorylation related genes, and anabolism related genes associated to nucleotide and steroid biosynthesis; also, a general down-regulation of surface glycoprotein coding genes was seen at this stage. Trypomastigotes, living extracellularly in the bloodstream of mammals, express a plethora of surface proteins and signaling genes involved in invasion and evasion of immune response. Amastigotes mostly express membrane transporters and genes involved in regulation of cell cycle, and also express a specific subset of surface glycoprotein coding genes. In addition, these results allowed us to improve the annotation of the Dm28c genome, identifying new ORFs and set the stage for construction of networks of co-expression, which can give clues about coded proteins of unknown functions.

Synthesis of the O-linked hexasaccharide containing β-D-Galp-(1→2)-D-Galf in Trypanosoma cruzi mucins. Differences on sialylation by trans-sialidase of the two constituent hexasaccharides

The hexasaccharide β-D-Galp-(1→2)-[β-D-Galp-(1→3)]-β-D-Galp-(1→6)-[β-D-Galp(1→2)-β-D-Galf(1→4)]-D-GlcNAc (10) and its β-D-Galf-(1→2)-β-D-Galf containing isomer (7) are the largest carbohydrates in mucins of some strains of Trypanosoma cruzi. The terminal β-D-Galp units are sites of sialylation by the parasite trans-sialidase. Hexasaccharide 10 was chemically synthesized for the first time by a [3+3] nitrilium based convergent approach, using the trichloroacetimidate method of glycosylation. The (1)H NMR spectrum of its alditol was identical to the spectrum of the product released by β-elimination from the parasite mucin. The trans-sialylation reaction studied on the benzyl glycoside of 10 showed two monosialylated products whose relative abundance changed with time. On the other hand, only one product was produced by sialylation of the benzyl glycoside of 7. A preparative synthesis of the latter and spectroscopic analysis of the product unequivocally established the sialylation site at the less hindered (1→3)-linked galactopyranose.

Synthesis of divalent ligands of β-thio- and β-N-galactopyranosides and related lactosides and their evaluation as substrates and inhibitors of Trypanosoma cruzi trans-sialidase

In this work we describe the synthesis of mono- and divalent β-N- and β-S-galactopyranosides and related lactosides built on sugar scaffolds and their evaluation as substrates and inhibitors of the Trypanosoma cruzi trans-sialidase (TcTS). This enzyme catalyzes the transfer of sialic acid from an oligosaccharidic donor in the host, to parasite βGalp terminal units and it has been demonstrated that it plays an important role in the infection. Herein, the enzyme was also tested as a tool for the chemoenzymatic synthesis of sialic acid containing glycoclusters. The transfer reaction of sialic acid was performed using a recombinant TcTS and 3’-sialyllactose as sialic acid donor, in the presence of the acceptor having βGalp non reducing ends. The products were analyzed by high performance anion exchange chromatography with pulse amperometric detection (HPAEC-PAD). The ability of the different S-linked and N-linked glycosides to inhibit the sialic acid transfer reaction from 3’-sialyllactose to the natural substrate N-acetyllactosamine, was also studied. Most of the substrates behaved as good acceptors and moderate competitive inhibitors. A di-N-lactoside showed to be the strongest competitive inhibitor among the compounds tested (70% inhibition at equimolar concentration). The usefulness of the enzymatic trans-sialylation for the preparation of sialylated ligands was assessed by performing a preparative sialylation of a divalent substrate, which afforded the monosialylated compound as main product, together with the disialylated glycocluster.

Unraveling the differences of the hydrolytic activity of Trypanosoma cruzi trans-sialidase and Trypanosoma rangeli sialidase: a quantum mechanics-molecular mechanics modeling study

Chagas' disease, also known as American trypanosomiasis, is a lethal, chronic disease that currently affects more than 10 million people in Central and South America. The trans-sialidase from Trypanosoma cruzi (T. cruzi, TcTS) is a crucial enzyme for the survival of this parasite: sialic acids from the host are transferred to the cell surface glycoproteins of the trypanosome, thereby evading the host's immune system. On the other hand, the sialidase of T. rangeli (TrSA), which shares 70% sequence identity with TcTS, is a strict hydrolase and shows no trans-sialidase activity. Therefore, TcTS and TrSA represent an excellent framework to understand how different catalytic activities can be achieved with extremely similar structures. By means of combined quantum mechanics-molecular mechanics (QM/MM, SCC-DFTB/Amberff99SB) calculations and umbrella sampling simulations, we investigated the hydrolysis mechanisms of TcTS and TrSA and computed the free energy profiles of these reactions. The results, together with our previous computational investigations, are able to explain the catalytic mechanism of sialidases and describe how subtle differences in the active site make TrSA a strict hydrolase and TcTS a more efficient trans-sialidase.

Sialylation of lactosyl lipids in membrane microdomains byT. cruzi trans-sialidase

SolubleT. cruzi trans-sialidase transformed a synthetic lactosyl glycolipid in microdomains more slowly than the same substrate dispersed across the bilayer surface, producing phospholipid vesicles with a Neu5Ac(α2-3)Gal(β1-4)Glc “glycocalyx”.

Insights into the activity and specificity of Trypanosoma cruzi trans-sialidase from molecular dynamics simulations

Trypanosoma cruzitrans-sialidase (TcTS), which catalyzes the transfer or hydrolysis of terminal sialic acid residues, is crucial to the development and proliferation of the T. cruzi parasite and thus has emerged as a potential drug target for the treatment of Chagas disease. We here probe the origin of the observed preference for the transfer reaction over hydrolysis where the substrate for TcTS is the natural sialyl donor (represented in this work by sialyllactose). Thus, acceptor lactose preferentially attacks the sialyl-enyzme intermediate rather than water. We compare this with the weaker preference for such transfer shown by a synthetic donor substrate, 4-methylumbelliferyl α-d-acetylneuraminide. For this reason, we conducted molecular dynamics simulations of TcTS following its sialylation by the substrate to examine the behavior of the asialyl leaving group by the protein. These simulations indicate that, where lactose is released, this leaving group samples well-defined interactions in the acceptor site, some of which are mediated by localized water molecules; also, the extent of the opening of the acceptor site to solvent is reduced as compared with those of unliganded forms of TcTS. However, where there is release of 4-methylumbelliferone, this leaving group explores a range of transient poses; surrounding active site water is also more disordered. The acceptor site explores more open conformations, similar to the case in which the 4-methylumbelliferone is absent. Thus, the predicted solvent accessibility of sialylated TcTS is increased when 4-methylumbelliferyl α-d-acetylneuraminide is the substrate compared to sialyllactose; this in turn is likely to contribute to a greater propensity for hydrolysis of the covalent intermediate. These computational simulations, which suggest that protein flexibility has a role in the transferase/sialidase activity of TcTS, have the potential to aid in the design of anti-Chagas inhibitors effective against this neglected tropical disease.

Infection, inflammation and host carbohydrates: a Glyco-Evasion Hypothesis

Microbial immune evasion can be achieved through the expression, or mimicry, of host-like carbohydrates on the microbial cell surface to hide from detection. However, disparate reports collectively suggest that evasion could also be accomplished through the modulation of the host glycosylation pathways, a mechanism that we call the "Glyco-Evasion Hypothesis". Here, we will summarize the evidence in support of this paradigm by reviewing three separate bodies of work present in the literature. We review how infection and inflammation can lead to host glycosylation changes, how host glycosylation changes can increase susceptibility to infection and inflammation and how glycosylation impacts molecular and cellular function. Then, using these data as a foundation, we propose a unifying hypothesis in which microbial products can hijack host glycosylation to manipulate the immune response to the advantage of the pathogen. This model reveals areas of research that we believe could significantly improve our fight against infectious disease.

Trans-sialidase and mucins of Trypanosoma cruzi: an important interplay for the parasite

A dense glycocalix covers the surface of Trypanosoma cruzi, the agent of Chagas disease. Sialic acid in the surface of the parasite plays an important role in the infectious process, however, T. cruzi is unable to synthesize sialic acid or the usual donor CMP-sialic acid. Instead, T. cruzi expresses a unique enzyme, the trans-sialidase (TcTS) involved in the transfer of sialic acid from host glycoconjugates to mucins of the parasite. The mucins are the major glycoproteins in the insect stage epimastigotes and in the infective trypomastigotes. Both, the mucins and the TcTS are anchored to the plasma membrane by a glycosylphosphatidylinositol anchor. Thus, TcTS may be shed into the bloodstream of the mammal host by the action of a parasite phosphatidylinositol-phospholipase C, affecting the immune system. The composition and structure of the sugars in the parasite mucins is characteristic of each differentiation stage, also, interstrain variations were described for epimastigote mucins. This review focus on the characteristics of the interplay between the trans-sialidase and the mucins of T. cruzi and summarizes the known carbohydrate structures of the mucins.

Proton transfer facilitated by ligand binding. An energetic analysis of the catalytic mechanism of Trypanosoma cruzi trans-sialidase

Trans-sialidase is a crucial enzyme for the infection of Trypanosoma cruzi, the protozoa responsible for Chagas' disease in humans. This enzyme catalyzes the transfer of sialic acids from mammalian host cells to parasitic cell surfaces in order to mask the infection from the host's immune system. It represents a promising target for the development of therapeutics to treat the disease and has been subject of extensive structural studies. Elaborate experiments suggested formation of a long-lived covalent intermediate in the catalytic mechanism and identified a Tyr/Glu pair as an unusual catalytic couple. This requires that the tyrosine hydroxyl proton is transferred to the carboxylate group of glutamate before the nucleophilic attack. Since the solution pK(a)s of tyrosine and glutamate are very different, this transfer can only be accomplished if the reaction environment selectively stabilizes the product state. We compute the free energy profile for the proton transfer in different environments, and our results indicate that it can take place in the active site of trans-sialidase, but only after substrate binding. By means of the energy decomposition method, we explain the influence that the active site residues exert on the reaction and how the pattern is changed when the substrate is present. This study represents an initial step that can shed light on our understanding of the catalytic mechanism of this reaction.

Continuous nonradioactive method for screening trypanosomal trans-sialidase activity and its inhibitors

Trypanosoma cruzi, the agent of American trypanosomiasis is unable to synthesize sialic acid (SA). Instead of using the corresponding nucleotide sugar as donor of the monosaccharide, the transfer occurs from alpha-2,3-linked SA in the host sialoglycoconjugates to terminal beta-galactopyranosyl units of the parasite mucins. For that purpose, T. cruzi expresses a glycosylphosphatidylinositol-anchored trans-sialidase (TcTS) that is shed into the milieu, being detected in the blood during the acute phase of the infection. The essential role of TcTS in infection and the absence of a similar activity in mammals make this enzyme an attractive target for the development of alternative chemotherapies. However, there is no effective inhibitor toward this enzyme. In vitro, 3'-sialyllactose (SL) as donor and radioactive lactose as acceptor substrate are widely used to measure TcTS activity. The radioactive sialylated product is then isolated by anion exchange chromatography and measured. Here we describe a new nonradioactive assay using SL or fetuin as donor and benzyl beta-d-Fuc-(1-->6)-alpha-d-GlcNAc (1) as acceptor. Disaccharide 1 was easily synthesized by regioselective glycosylation of benzyl alpha-d-GlcNAc with tetra-O-benzoyl-d-fucose followed by debenzoylation. Compound 1 lacks the hydroxyl group at C-6 of the acceptor galactose and therefore is not a substrate for galactose oxidase. Our method relies on the specific quantification of terminal galactose produced by trans-sialylation from the donor to the 6-deoxy-galactose (D-Fuc) unit of 1 by a spectrophotometric galactose oxidase assay. This method may also discriminate sialidase and trans-sialylation activities by running the assay in the absence of acceptor 1.

Identification of glycoproteins targeted by Trypanosoma cruzi trans-sialidase, a virulence factor that disturbs lymphocyte glycosylation

Trypanosoma cruzi, the agent of the American trypanosomiasis or Chagas disease, bypasses its lack of de novo synthesis of sialic acids by expressing a surface-anchored trans-sialidase. This enzyme transfers sialic acid residues from the host's sialylglycoconjugates to the parasite's galactosylglycoconjugates. In addition to carrying out a pivotal role in parasite persistence/replication within the infected mammal, the trans-sialidase is shed into the bloodstream and induces alterations in the host immune system by modifying the sialylation of the immune cells. A major obstacle to understand these events is the difficulty to identify the transferred sialic acid among all those naturally occurring on the cell surface. Here, we report the use of azido-modified unnatural sialic acid to identify those molecules that act as cell surface acceptors of the sialyl residue in the trans-sialidase-catalyzed reaction, which might then be involved in the immune alterations induced. In living parasites, we readily observed the transfer of azido-sialic acid to surface mucins. When evaluating mouse thymocytes and splenocytes as acceptors of the azido-sugar, a complex pattern of efficiently tagged glycoproteins was revealed. In both leukocyte populations, the main proteins labeled were identified as different CD45 isoforms. Disruption of the cell architecture increased the number and the molecular weight distribution of azido-sialic acid tagged proteins. Nevertheless, CD45 remained to be the main acceptor. Mass spectrometry assays allowed us to identify other acceptors, mainly integrins. The findings reported here provide a molecular basis to understand the abnormalities induced in the immune system by the trans-sialidase during T. cruzi infection.

The role of parasite persistence in pathogenesis of Chagas heart disease

Chagas disease (CD) is caused by the infection with the protozoan haemoflagellate Trypanosoma cruzi. This disease is still a great menace to public health, and is largely neglected as it affects mostly the poorest populations of Latin America. Nonetheless, there are neither effective diagnostic markers nor therapeutic options to accurately detect and efficiently cure this chronic infection. In spite of the great advances in the knowledge of the biology of natural transmission, as well as the immunobiology of the host-parasite interaction, the understanding of the pathogenesis of CD remains largely elusive. In the recent decades, a controversy in the research community has developed about the relevance of parasite persistence or autoimmune phenomena in the development of chronic cardiac pathology. One of the most notable aspects of chronic CD is the progressive deterioration of cardiac function, derived mostly from structural derangement, as a consequence of the intense inflammatory process. Here we review the evidence supporting the multifactorial nature of Chagas heart disease comprising pathogen persistence and altered host immunoregulatory mechanisms.

Modulation of catalytic function by differential plasticity of the active site: case study of Trypanosoma cruzi trans-sialidase and Trypanosoma rangeli sialidase

trans-Sialidase is an essential enzyme for Trypanosoma cruzi, the causative agent of Chagas' disease, to escape from the host immune system and to invade the host cells. Therefore, T. cruzi trans-sialidase (TcTS) presents a potential and appealing therapeutic target for this lethal disease. The availability of a structurally very similar enzyme with strict hydrolase activity (Trypanosoma rangeli sialidase, TrSA) provides us a unique opportunity to understand the determinants of their structure and catalytic mechanism. In this study, we compare the catalytic cleft plasticity of free (apo) and ligand-bound (holo) forms of the two enzymes using molecular dynamics simulations. We focus on the mouth of the catalytic cleft that is defined by two residues: W312 and Y119 in TcTS and W312 and S119 in TrSA. Our results indicate that TcTS has a very flexible, widely open catalytic cleft, mostly due to W312 loop motion, in apo form. However, when the catalytic cleft is occupied by a ligand, the flexibility and solvent exposure of TcTS is significantly reduced. On the other hand, TrSA maintains a more open catalytic cleft compared to its crystal structures in both apo and holo forms (and compared to TcTS in holo forms). The reduced solvent exposure of TcTS catalytic cleft might be partially or fully responsible for TcTS to be a less efficient hydrolase than TrSA.

The trans-sialidase from Trypanosoma cruzi triggers apoptosis by target cell sialylation

The trans-sialidase, a modified sialidase that transfers sialyl residues among macromolecules, is a unique enzymatic activity expressed by some parasitic trypanosomes being essential for their survival in the mammalian host and/or in the insect vector. The enzyme from Trypanosoma cruzi, the agent of Chagas disease, is found in blood and able to act far from the infection site by inducing apoptosis in cells from the immune system. A central and still unsolved question is whether trans-sialidase-mediated addition or removal of sialic acid to/from host acceptor molecules is the event associated with the apoptosis induced by the enzyme. Here we show that lactitol, a competitive inhibitor that precluded the transference of the sialyl residue to endogenous acceptors but not the hydrolase activity of the enzyme, prevented ex vivo and in vivo the apoptosis caused by the trans-sialidase. By lectin histochemistry, the transference of sialyl residue to the cell surface was demonstrated in vivo and found associated with the apoptosis induction. The sialylation of the CD43 mucin, a key molecule involved in trans-sialidase-apoptotic process, was readily detected and also prevented by lactitol on thymocytes. Therefore, lesions induced by trans-sialidase on the immune system are due to the sialylation of endogenous acceptor molecules.

The synthesis and biological evaluation of lactose-based sialylmimetics as inhibitors of rotaviral infection

Rotaviruses are the most significant cause of gastroenteritis in young children and are responsible for over 600,000 infant deaths annually. The rotaviral haemagglutinin protein (VP8*) of some strains has been implicated in early recognition and binding events of host cell-surface sialoglycoconjugates, and is therefore an attractive target for potential therapeutic intervention. Since N-acetylneuraminic acid alpha(2,3)-linked to galactose is believed to be the minimum binding epitope of rotavirus to host cells, we report here our development of an efficient and flexible synthetic route to a range of lactose-based sialylmimetics of alpha(2,3)-linked thiosialosides. These compounds were biologically evaluated as inhibitors of rotaviral infection using an in vitro neutralisation assay. The results suggest that these lactose-based sialylmimetics are not inhibitors of the rhesus rotavirus strain; however, they do exhibit modest inhibition of the human (Wa) strain, presumably through inhibition of the rotaviral adhesion process.

Development of a S. cerevisiae whole cell biocatalyst for in vitro sialylation of oligosaccharides

Absence of sialylation on recombinant glycoproteins compromises their efficacy as therapeutic agents, as it results in rapid clearance from the human bloodstream. To circumvent this, several strategies are followed, including the implementation of a post-secretion glycosylation step. In this paper we describe the engineering of yeast cells expressing active surface exposed Trypanosoma cruzi trans-sialidase (TS) fused to the yeast Aga2 protein, and the use of this yeast in the sialylation of synthetic oligosaccharides. In an attempt to improve overall protein accessibility on the yeast surface, we abolished hyperglycosylation on the yeast cell wall proteins. This was achieved by disrupting the OCH1 gene of the TS surface expressing strain, which resulted in increased enzymatic activity. Using a fluorescence-based activity assay and DSA-FACE structural analysis, we obtained almost complete conversion to a fully sialylated acceptor, whereas in the wild type situation this conversion was only partial. Increasing protein accessibility on the yeast surface by modifying the glycosylation content thus proved to be a valuable approach in increasing the cell wall associated activity of an immobilised enzyme, hence resulting in a more effective biocatalyst system.

Specificity of human trans-sialidase as probed with gangliosides

It has been shown that human blood contains a soluble 67 kDa enzyme, belonging by its donor-acceptor properties to trans-sialidases. The enzyme is capable of both cleaving and synthesizing alpha2-3 and alpha2-6 sialosides [Atherosclerosis2001, 159, 103]. In this work the study of donor-acceptor specificity of the new enzyme was extended. It has been demonstrated in vitro that trans-sialidase possesses the ability of transferring Neu5Ac residue to acceptor (asialofetuin) both from alpha2-3- (GM1, GM3, GD1a), and alpha2-8-sialylated gangliosides (GD3 and GD1b, but not GT1b and GQ1b). Transfer of radiolabeled Neu5Ac from fetuin to glycosphingolipids demonstrated that Lac-Cer>mono- and disialogangliosides>GT1b>GQ1b were acceptors for this enzyme. Two methods were used to reveal whether alpha2-8 bond can be formed between Neu5Ac residues during trans-sialylation, that is immunochemical detection using monoclonal antibodies specific to alpha2-8 di- and oligosialic acids, and fluorometric C7/C9 analysis. Both methods demonstrated the formation of Neu5Acalpha2-8Neu5Ac termination by trans-sialidase, for example, in case of the use 3'SL as sialic acid donor and Neu5Ac-PAA or LDL as acceptor. Thus, human trans-sialidase in vitro displays wide substrate specificity: the enzyme is capable of digesting as well as synthesizing alpha2-3, alpha2-6, and alpha2-8 sialosides.

Structural Insights into the Catalytic Mechanism of Trypanosoma cruzi trans-Sialidase

Sialidases are a superfamily of sialic-acid-releasing enzymes that are of significant interest due to their implication as virulence factors in the pathogenesis of a number of diseases. However, extensive studies of viral and microbial sialidases have failed to provide a comprehensive picture of their mechanistic properties, in part because the structures of competent enzyme-substrate complexes and reaction intermediates have never been described. Here we report these structures for the Trypanosoma cruzi trans-sialidase (TcTS), showing that catalysis by sialidases occurs via a similar mechanism to that of other retaining glycosidases, but with some intriguing differences that may have evolved in response to the substrate structure.

The trans-sialidase from the african trypanosome Trypanosoma brucei

Trypanosoma brucei is the cause of the diseases known as sleeping sickness in humans (T. brucei ssp. gambiense and ssp. rhodesiense) and ngana in domestic animals (T. brucei brucei) in Africa. Procyclic trypomastigotes, the tsetse vector stage, express a surface-bound trans-sialidase that transfers sialic acid to the glycosylphosphatidylinositol anchor of procyclin, a surface glycoprotein covering the parasite surface. Trans-sialidase is a unique enzyme expressed by a few trypanosomatids that allows them to scavenge sialic acid from sialylated compounds present in the infected host. The only enzyme extensively characterized is that of the American trypanosome T. cruzi (TcTS). In this work we identified and characterized the gene encoding the trans-sialidase from T. brucei brucei (TbTS). TbTS genes are present at a small copy number, at variance with American trypanosomes where a large gene family is present. The recombinant TbTS protein has both sialidase and trans-sialidase activity, but it is about 10 times more efficient in transferring than in hydrolysing sialic acid. Its N-terminus contains a region of 372 amino acids that is 45% identical to the catalytic domain of TcTS and contains the relevant residues required for catalysis. The enzymatic activity of mutants at key positions involved in the transfer reaction revealed that the catalytic sites of TcTS and TbTS are likely to be similar, but are not identical. As in the case of TcTS and TrSA, the substitution of a conserved tryptophanyl residue changed the substrate specificity rendering a mutant protein capable of hydrolysing both alpha-(2,3) and alpha-(2,6)-linked sialoconjugates.

Thymocyte depletion in Trypanosoma cruzi infection is mediated by trans-sialidase-induced apoptosis on nurse cells complex

Trypanosoma cruzi, the causative agent of Chagas' disease, induces transient thymic aplasia early after infection-a phenomenon that still lacks a molecular explanation. The parasite sheds an enzyme known as trans-sialidase (TS), which is able to direct transfer-sialyl residues among macromolecules. Because cell-surface sialylation is known to play a central role in the immune system, we tested whether the bloodstream-borne TS is responsible for the thymic alterations recorded during infection. We found that recombinant TS administered to naive mice was able to induce cell-count reduction mediated by apoptosis, mimicking cell subsets distribution and histologic findings observed during the acute phase of the infection. Thymocytes taken after TS treatment showed low response to Con A, although full ability to respond to IL-2 or IL-2 plus Con A was conserved, which resembles findings from infected animals. Alterations were found to revert several days after TS treatment. The administration of TS-neutralizing Abs to T. cruzi-infected mice prevented thymus alterations. Results indicate that the primary target for the TS-induced apoptosis is the so-called "nurse cell complex". Therefore, we report here supporting evidence that TS is the virulence factor from T. cruzi responsible for the thymic alterations via apoptosis induction on the nurse cell complex, and that TS-neutralizing Abs elicitation during infection is associated with the reversion to thymic normal parameters.

The nematode Caenorhabditis elegans synthesizes unusual O-linked glycans: identification of glucose-substituted mucin-type O-glycans and short chondroitin-like oligosaccharides

The free-living nematode Caenorhabditis elegans is a relevant model for studies on the role of glycoconjugates during development of multicellular organisms. Several genes coding for glycosyltransferases involved in the synthesis of N- and O-linked glycans have already been isolated, but, apart from repetitive dimers of glycosaminoglycans, no detailed structure of either type of component has been published so far. This study aimed to establish the structures of the major O-glycans synthesized by C. elegans to give an insight into the endogenous glycosyltransferase activities expressed in this organism. By the use of NMR and MS, we have resolved the sequence of seven of these components that present very unusual features. Most of them were characterized by the type-1 core substituted on Gal and/or GalNAc by (beta1-4)Glc and (beta1-6)Glc residues. Another compound exhibited the GalNAc(beta1-4)N-acetylglucosaminitol sequence in the terminal position, to which was attached a tetramer of beta-Gal substituted by both Fuc and 2-O-methyl-fucose residues. Our experimental procedure led also to the isolation of glycosaminoglycan-like components and oligomannosyl-type N-glycans. In particular, the data confirmed that C. elegans synthesizes the ubiquitous linker sequence GlcA(beta1-3)Gal(beta1-3)Gal(beta1-4)Xyl.

Probing molecular function of trypanosomal sialidases: single point mutations can change substrate specificity and increase hydrolytic activity

Sialidases are present on the surface of several trypanosomatid protozoan parasites. They are highly specific for sialic acid linked in alpha-(2,3) to a terminal beta-galactose and include the strictly hydrolytic enzymes and trans-sialidases (sialyl-transferases). Based on the structural comparison of the sialidase from Trypanosoma rangeli and the trans-sialidase from T. cruzi (the agent of Chagas' disease in humans), we have explored the role of specific amino acid residues sought to be important for substrate specificity. The substitution of a conserved tryptophanyl residue in the two enzymes, Trp312/313-Ala, changed substrate specificity, rendering the point mutants capable to hydrolyze both alpha-(2,3)- and alpha-(2,6)-linked sialoconjugates. The same mutation abolished sialyl-transferase activity, indicating that transfer (but not hydrolysis) requires a precise orientation of the bound substrate. The exchange substitution of another residue that modulates oligosaccharide binding, Gln284-Pro, was found to significantly increase the hydrolytic activity of sialidase, and residue Tyr119 was confirmed to be part of a second binding site for the acceptor substrate in trans-sialidase. Together with the structural information, these results provide a consistent framework to account for the unique enzymatic properties of trypanosome trans-sialidases.

Preliminary 1H NMR investigation of sialic acid transfer by the trans-sialidase from Trypanosoma cruzi

1H NMR spectroscopy has been used to investigate the transfer of sialic acid from sialic acid donor molecules to acceptor molecules using the trans-sialidase from Typanosoma cruzi. It is clearly demonstrated that NMR spectroscopy is an efficient and powerful means of monitoring the trans-sialidase promoted transfer of sialic acid from donor to acceptor.

Separation of Galfbeta1-->XGlcNAc and Galpbeta1-->XGlcNAc (X = 3, 4, and 6) as the alditols by high-pH anion-exchange chromatography and thin-layer chromatography: characterization of mucins from Trypanosoma cruzi

The O-linked N-acetylglucosamine oligosaccharides in the mucins of Trypanosoma cruzi may contain galactofuranose or galactopyranose, depending on the strain, one of the components being the disaccharide. Since galactofuranose is a site for antibody recognition, it is desirable to have a sensitive method for the detection of the galactofuranosyl structures. In this paper, we present procedures for the separation of Galfbeta1-->XGlcNAc and Galpbeta1-->XGlcNAc (X = 3, 4, and 6) as the corresponding alditols by high-pH anion-exchange chromatography with pulse amperometric detection. All the isomeric disaccharides could be resolved on a CarboPac PA-10 column, the galactofuranose-containing disaccharides being more retained in the column. GlcNAcol and Galfbeta1-->4(Galpbeta1-->6)GlcNAcol could be analyzed in the same run. The compounds could also be separated by thin-layer chromatography on silica gel 60, a convenient method for analysis of the radiolabeled alditols obtained by reductive beta-elimination in the presence of NaB(3)H(4). Both methods were applied for the analysis of the O-linked sugars in the mucins of T. cruzi CL 14 and revealed that they contained only N-acetylglucosamine and the disaccharide Galpbeta1-->4GlcNAc.

Enzymically inactive members of the trans-sialidase family from Trypanosoma cruzi display beta-galactose binding activity

trans-sialidase is a unique sialidase in that, instead of hydrolizing sialic acid, it preferentially transfers the monosaccharide to a terminal beta-galactose in glycoproteins and glycolipids. This enzyme, originally identified in Trypanosoma cruzi, belongs to a large family of proteins. Some members of the family lack the enzymatic activity. No function has been yet assigned to them. In this work, the gene copy number and the possible function of inactive members of the trans -sialidase family was studied. It is shown that genes encoding inactive members are not a few, but rather, are present in the same copy number (60-80 per haploid genome) as those encoding active trans -sialidases. Recombinant inactive proteins were purified and assayed for sialic acid and galactose binding activity in agglutination tests. The enzymatically inactive trans -sialidases were found to agglutinate de-sialylated erythrocytes but not untreated red blood cells. Assays made with mouse and rabbit red blood cells suggest that inactive trans -sialidases bind to beta, rather than alpha, terminal galactoses, the same specificity required by active trans -sialidases. A recombinant molecule that was made enzymatically inactive through a mutation in a single amino acid also retained the galactose binding activity. The binding was competed by lactose and was dependent on conservation of the protein native conformation. Therefore, at least some molecules in the trans -sialidase family that have lost their enzymatic function still retain their Gal-binding properties and might have a function as lectins in the parasite-host interaction.

One-pot synthesis of beta-D-Galf(1-->4)[beta-D-Galp(1-->6)]-D-GlcNAc, a 'core' trisaccharide linked O-glycosidically in glycoproteins of Trypanosoma cruzi

Tin(IV) chloride-promoted condensation of benzyl 2-acetamido-3-O-benzoyl-2-deoxy-alpha-D-glucopyranoside (4) with penta-O-benzoyl-beta-D-galactopyranose (6) gave the derivative of beta-D-Galp-(1-->6)-alpha-D-GlcNAc 7 in 80% yield. This was glycosylated with penta-O-benzoyl-alpha, beta-D-galactofuranose (5), employing the same catalyst, to afford the protected benzyl per-O-benzoyl-beta-D-Galf(1-->4)[beta-D-Galp(1-->6)]D-GlcNAc 10 in 41% yield. Alternatively, compound 10 was obtained directly in a one-pot reaction from 4, by sequential addition of 6 and 5 (34% yield). beta-Glycosidic linkages were diastereoselectively formed. De-O-benzoylation of 10, followed by heterogeneous catalytic transfer hydrogenolysis of the benzyl group afforded the free trisaccharide beta-D-Galf(1-->4)[beta-D-Galp(1-->6)]-D-GlcNAc (14) in 98% yield from 10. Sodium borohydride reduction of 14 gave the corresponding alditol, whose spectral data were identical to those reported for the alditol obtained from the 38-43 kDa cell-surface glycoprotein of Trypanosoma cruzi.

The lipid structure of the glycosylphosphatidylinositol-anchored mucin-like sialic acid acceptors of Trypanosoma cruzi changes during parasite differentiation from epimastigotes to infective metacyclic trypomastigote forms

The major acceptors of sialic acid on the surface of metacyclic trypomastigotes, which are the infective forms of Trypanosoma cruzi found in the insect vector, are mucin-like glycoproteins linked to the parasite membrane via glycosylphosphatidylinositol anchors. Here we have compared the lipid and the carbohydrate structure of the glycosylphosphatidylinositol anchors and the O-linked oligosaccharides of the mucins isolated from metacyclic trypomastigotes and noninfective epimastigote forms obtained in culture. The single difference found was in the lipid structure. While the phosphatidylinositol moiety of the epimastigote mucins contains mainly 1-O-hexadecyl-2-O-hexadecanoylphosphatidylinositol, the phosphatidylinositol moiety of the metacyclic trypomastigote mucins contains mostly (approximately 70%) inositol phosphoceramides, consisting of a C18:0 sphinganine long chain base and mainly C24:0 and C16:0 fatty acids. The remaining 30% of the metacyclic phosphatidylinositol moieties are the same alkylacylphosphatidylinositol species found in epimastigotes. In contrast, the glycosylphosphatidylinositol glycan cores of both molecules are very similar, mainly Man alpha 1-2Man alpha 1-2Man alpha 1- 6Man alpha 1-4GlcN. The glycans are substituted at the GlcN residue and at the third alpha Man distal to the GlcN residue by ethanolamine phosphate or 2-aminoethylphosphonate groups. The structures of the desialylated O-linked oligosaccharides of the metacyclic trypomastigote mucin-like molecules, released by beta-elimination with concomitant reduction, are identical to the structures reported for the epimastigote mucins (Previato, J. O., Jones, C., Gonçalves, L. P. B., Wait, R., Travassos, L. R., and Mendoça-Previato, L. (1994) Biochem. J. 301, 151-159). In addition, a significant amount of nonsubstituted N-acetylglucosaminitol was released from the mucins of both forms of the parasite. Taken together, these results indicate that when epimastigotes transform into infective metacyclic trypomastigotes, the phosphatidylinositol moiety of the glycosylphosphatidylinositol anchor of the major acceptor of sialic acid is modified, while the glycosylphosphatidylinositol anchor and O-linked sugar chains remain essentially unchanged.

A single tyrosine differentiates active and inactive Trypanosoma cruzi trans-sialidases

Several genes encode members of the Trypanosoma cruzi (Tc) trans-sialidase (TS) family. These proteins contain an enzymatic domain on the N terminus, the only one required for TS activity, and an antigenic domain (SAPA (shed acute phase antigen) amino acid (aa) repeats) on the C terminus. Only some members of this glycoprotein family are enzymatically active. The complete sequence of two clones encoding the enzymatic domain of active and inactive protein from each of two Tc strains has now been obtained. Comparison of these sequences showed a limited divergence among them: 20 out of the 642 deduced aa in the enzymatic domain were found to differ. From these 20 aa, only one was found to be essential for enzymatic activity. A Tyr342 residue is deduced in both active proteins while a His342 is present in both inactive ones. This naturally occurring Tyr342-->His substitution completely abolished the TS activity. In addition to Tyr342, a second deduced aa, Pro231, was found to be necessary for full enzymatic TS activity; a Pro231-->Ala change rendered the TS protein partially active. Fourteen aa residues, including Tyr342, out of the 16 aa in the active site of a sialidase from Salmonella typhimurium are present at the same or very similar positions in the Tc TS.

N-acetyl-D-neuraminic acid synthesis in Escherichia coli K1 occurs through condensation of N-acetyl-D-mannosamine and pyruvate

Two enzymes have been found to be involved in bacterial N-acetyl-D-neuraminic acid (NeuAc) synthesis: NeuAc synthase, which condenses N-acetyl-L,D-mannosamine and phosphoenolpyruvate, and NeuAc lyase or NeuAc aldolase, which condenses N-acetyl-D-mannosamine and pyruvate. When we used Escherichia coli K1 crude extracts, we observed the generation of NeuAc in the presence of N-acetylmannosamine and both phosphoenolpyruvate (NeuAc synthase activity) or pyruvate (NeuAc lyase activity). However, when crude extracts were fractionated by Sephacryl S-200 chromatography, NeuAc synthase activity disappeared. A chromatographic peak of NeuAc synthase activity was detected when column fractions were re-tested in the presence of the active NeuAc lyase peak. Furthermore, crude extracts converted phosphoenolpyruvate into pyruvate. Pyruvate depletion, due to the addition of pyruvate decarboxylase to the NeuAc synthase reaction mixture, blocked NeuAc formation. Moreover, after NeuAc lyase immunoprecipitation no NeuAc synthase was detected. These findings suggest that NeuAc synthase is not present in E. coli K1 and therefore that NeuAc lyase is the only enzyme responsible for NeuAc synthesis in this bacterium.

Structural characterization of the major glycosylphosphatidylinositol membrane-anchored glycoprotein from epimastigote forms of Trypanosoma cruzi Y-strain

We have investigated the structure of the glycosylphosphatidylinositol (GPI) anchor and the O-linked glycan chains of the 40/45-kDa glycoprotein from the cell surface of the protozoan parasite Trypanosoma cruzi. This glycoconjugate is the major acceptor for sialic acid transferred by trans-sialidase of T. cruzi Y-strain, epimastigote form. The GPI anchor was liberated by treatment with hot alkali, and the phosphoinositol-oligosaccharide moiety was characterized and shown to have the following structure. [formula: see text] Unusually the glucosamine was 6-O-substituted with 2-aminoethylphosphonate, and 2-aminoethylphosphonate was also present on the third mannose residue distal to glucosamine, partially replacing the ethanolamine phosphate. The beta-eliminated reduced oligosaccharide chains showed that two novel classes of O-linked N-acetylglucosamine oligosaccharide were present. The first series had the structures Galp beta 1-3GlcNAc-ol; Galp beta 1-6(Galp beta 1-3)GlcNAc-ol; and Galp beta 1-2Galp beta 1-6(Galp beta 1-3)GlcNAc-ol, whereas the other series had a 1-4 linkage to N-acetylglucosaminitol and had structures Galp beta 1-4GlcNAc-ol, Galp beta 1-6(Galp beta 1-4)GlcNAc-ol, and Galp beta 1-2Galp beta 1-6(Galp beta 1-4)GlcNAc-ol. We have also investigated the kinetics of in vitro sialylation of these O-linked oligosaccharides by the T. cruzi transsialidase and have shown that incorporation of one molecule of sialic acid hinders entry of a second molecule when two potential acceptor sites are present.

O-glycosidically linked N-acetylglucosamine-bound oligosaccharides from glycoproteins of Trypanosoma cruzi

In this report we describe studies on the structures of the O-linked carbohydrate units in cell-surface glycoproteins of epimastigote forms of the G-strain of Trypanosoma cruzi. Mild alkaline reductive degradation of the 38/43 kDa glycoproteins resulted in beta-elimination of glycosylated threonine and/or serine residues, and the liberation of N-acetylglucosaminitol, galactobiosyl-, galactotriosyl-, galactotetraosyl- and galactopentaosyl-N-acetylglucosaminitol. The structures of these oligosaccharide alditols were established by n.m.r. spectroscopy and methylation analysis as: Galf beta 1-4(Galp beta 1-6)GlcNAc-ol; Galp beta 1-3Galp beta 1-6(Galf beta 1-4)GlcNAc-ol; [(Galp beta 1-3)(Galp beta 1-2)Galp beta 1-6](Galf beta 1-4)GlcNAc-ol; [(Galp beta 1-3)(Galp beta 1-2)Galp beta 1-6](Galp beta 1-2Galf beta 1-4)GlcNAc-ol.

Trans-sialidase, SAPA amino acid repeats and the relationship between Trypanosoma cruzi and the mammalian host

During invasion of multicellular organisms, protozoan parasites expose functional molecules that become targets for the host immune response. Recent research on Trypanosoma cruzi, the agent of Chagas' disease, suggests a new model of how the parasite might deal with this problem. Several antigens of T. cruzi have tandemly repeated amino acid motifs in molecules with as yet unknown functions. In two cases, these repeats are in molecules with a defined structure or function. Both proteins are implicated in the invasion of host-cells by the parasite. One of these is the core protein of a putative mucin-like glycoprotein that has Thr/Pro-rich repeats which, by themselves, might define the structure of a highly O-glycosylated molecule. The other protein is SAPA/trans-sialidase/neuraminidase, a molecule able to transfer sialic acid, that has so far only been described in trypanosomes. The amino acid repeats present in SAPA/transsialidase/neuraminidase are unrelated to the enzymic activity and constitute an immunodominant C-terminal domain. The N-terminal domain of SAPA/trans-sialidase/neuraminidase controls the enzymic activity since a recombinant molecule lacking the repeats conserves trans-sialidase activity. That both domains are functionally independent is also indicated by experiments that show that antibodies directed against the amino acid repeats are unable to inhibit trans-sialidase activity. A large number of proteins having trans-sialidase related sequences but lacking enzymic activity are also present in the surface membrane of the parasite. The immunodominant SAPA/trans-sialidase/neuraminidase repeats, together with the complex network of cross-reacting epitopes present in related but enzymatically inactive proteins might contribute to the delay in mounting an effective antibody response.(ABSTRACT TRUNCATED AT 250 WORDS)