Comparative rates of sialylation by recombinant trans-sialidase and inhibitor properties of synthetic oligosaccharides from Trypanosoma cruzi mucins-containing galactofuranose and galactopyranose

Synthesis of Oligosaccharides Containing the S-Galp(α1 → 3)Galp Unit, Glycomimetic of the Epitope Recognized by Lytic Antibodies

Two important activities take place in the surface of Trypanosoma cruzi, the agent of Chagas disease: the trans-sialidase (TcTS) catalyzes the transfer of sialic acid from the host glycoconjugates to the mucin-like glycoproteins from the parasite and the presence of lytic antibodies recognize the epitope α-Galp(1 → 3)-β-Galp(1 → 4)-α-GlcNAcp. This antigenic structure is known to be present in the parasite mucins; however, in order to be substrates of trans-sialidase, some of the galactose residues should be in the β-Galp configuration. To study the interaction between both activities, it is important to count the synthetic structures as well as the structural-related glycomimetics. With this purpose, we addressed the synthesis of a trisaccharide and two isomeric tetrasaccharides containing the 1-S-α-Galp(1 → 3)-β-Galp motif, the thio analog of the epitope recognized by lytic antibodies. Starting with a common lactose precursor, the sulfur function was incorporated by double inversion of the configuration of the galactose residue that was further glycosylated using different activated donors. Both tetrasaccharides were good acceptors of sialic acid in the reaction catalyzed by TcTS, as determined by high-performance anion exchange chromatography.

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.

The Glycan Structure of T. cruzi mucins Depends on the Host. Insights on the Chameleonic Galactose

Trypanosoma cruzi, the protozoa that causes Chagas disease in humans, is transmitted by insects from the Reduviidae family. The parasite has developed the ability to change the structure of the surface molecules, depending on the host. Among them, the mucins are the most abundant glycoproteins. Structural studies have focused on the epimastigotes and metacyclic trypomastigotes that colonize the insect, and on the mammal trypomastigotes. The carbohydrate in the mucins fulfills crucial functions, the most important of which being the accepting of sialic acid from the host, a process catalyzed by the unique parasite trans-sialidase. The sialylation of the parasite influences the immune response on infection. The O-linked sugars have characteristics that differentiate them from human mucins. One of them is the linkage to the polypeptide chain by the hexosamine, GlcNAc, instead of GalNAc. The main monosaccharide in the mucins oligosaccharides is galactose, and this may be present in three configurations. Whereas β-d-galactopyranose (β-Galp) was found in the insect and the human stages of Trypanosoma cruzi, β-d-galactofuranose (β-Galf) is present only in the mucins of some strains of epimastigotes and α-d-galactopyranose (α-Galp) characterizes the mucins of the bloodstream trypomastigotes. The two last configurations confer high antigenic properties. In this review we discuss the different structures found and we pose the questions that still need investigation on the exchange of the configurations of galactose.

Trypanosoma cruzi trans-sialidase alternative substrates: Study of the effect of substitution in C-6 in benzyl β-lactoside

Trypanosoma cruzi trans-sialidase (TcTS) is a cell surface protein that participates in the adhesion and invasion mechanisms of the parasite into the host cells, making it an attractive target for inhibitors design. In order to contribute to the knowledge of the interaction between TcTS and their acceptor substrates, we designed and synthesized a library of 20 benzyl lactosides substituted in C-6 of the glucose residue with a series of 1,2,3-triazole derivatives containing different aromatic substituents in the C-4 position. The library was prepared by alkyne-azide cycloaddition reaction catalyzed by Cu(I) ("click chemistry") between a benzyl β-lactoside functionalized with an azide group in the C-6 position and a series of 2-propargyl phenyl ethers. Herein we analyzed the chromatographic behavior on high performance anion exchange chromatography (HPAEC) of the triazoyl-lactose derivatives and their activity as acceptors of TcTS and inhibitors of the sialylation of N-acetyllactosamine. The triazoyl derivatives were obtained with excellent yields and all of them behaved as moderate alternative substrates. The presence of bulky hydrophobic substituents dramatically increased the retention times in HPAEC but did not affect significantly their acceptor properties toward TcTS.

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.

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.

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”.

Addition of α-O-GlcNAc to threonine residues define the post-translational modification of mucin-like molecules in Trypanosoma cruzi

Trypanosoma cruzi, an intracellular protozoan etiologic agent of Chagas disease is covered by a dense coat of mucin-type glycoproteins, which is important to promote the parasite entry and persistence in the mammalian host cells. The O-glycosylation of T. cruzi mucins (Tc-mucins) is initiated by enzymatic addition of α-O-N-acetylglucosamine (GlcNAc) to threonine (Thr) by the UDP-GlcNAc:polypeptide α-N-acetylglucosaminyltransferase (pp-α-GlcNAcT) in the Golgi. The Tc-mucin is characterized by the presence of a high structural diversity of O-linked oligosaccharides found among different parasite strains, comprising two O-glycan Cores. In the Core 1, from strains principally associated with the domestic transmission cycle of Chagas disease, the GlcNAc O-4 is substituted with a β-galactopyranose (βGalp) unit, and in the most complex oligosaccharides the GlcNAc O-6 is further processed by the addition of β1 → 2-linked Galp residues creating a short linear Galp-containing chain. In the Core 2 structures, expressed by strains isolated from T. cruzi sylvatic hosts, the GlcNAc O-4 carries a β-galactofuranose (βGalf) unit and the GlcNAc O-6 can carry a branched Galpβ1 → 3[Galpβ1 → 2]Galpβ1 → 6 motif. The O-glycans carrying nonreducing terminal βGalp are available for sialylation by a surface T. cruzi trans-sialidase activity. Based on structural results, this review summarizes available data on the highly conserved process, which adds the GlcNAc unit in α-linkage to Thr residues the basis of the post-translational modification system in T. cruzi mucins. In addition, a mechanism unique employed by the parasite to transfer exogenous sialic acid residues to Tc-mucins is presented.

Sialic acid: a sweet swing between mammalian host and Trypanosoma cruzi

Commonly found at the outermost ends of complex carbohydrates in extracellular medium or on outer cell membranes, sialic acids play important roles in a myriad of biological processes. Mammals synthesize sialic acid through a complex pathway, but Trypanosoma cruzi, the agent of Chagas’ disease, evolved to obtain sialic acid from its host through a trans-sialidase (TcTS) reaction. Studies of the parasite cell surface architecture and biochemistry indicate that a unique system comprising sialoglycoproteins and sialyl-binding proteins assists the parasite in several functions including parasite survival, infectivity, and host–cell recognition. Additionally, TcTS activity is capable of extensively remodeling host cell glycomolecules, playing a role as virulence factor. This review presents the state of the art of parasite sialobiology, highlighting how the interplay between host and parasite sialic acid helps the pathogen to evade host defense mechanisms and ensure lifetime host parasitism.

Synthesis of the O-linked hexasaccharide containing β-D-Galf-(1→2)-β-D-Galf in Trypanosoma cruzi mucins

The hexasaccharide β-D-Galp-(1→2)-[β-D-Galp-(1→3)]-β-D-Galp-(1→6)-[β-D-Galf(1→2)-β-D-Galf(1→4)]-D-GlcNAc (1) is the largest carbohydrate structure released as alditol by reductive β-elimination from mucins of some strains of T. cruzi. The terminal β-D-Galp units are sites of sialylation by trans-sialidase which transfers sialic acid from the host to the parasite. Hexasaccharide 1 was synthesized by a [3 + 3]-convergent strategy based on a nitrile assisted glycosylation, using the trichloroacetimidate method. The β-D-Galf-(1→2)-β-D-Galf-D-GlcNAc synthon was sequentially constructed from the reducing end to the non-reducing end employing benzyl α-D-galactofuranoside as starting material for the internal Galf unit. The choice of this novel precursor, obtained in one-reaction step from galactose, allowed the introduction of an orthogonal and participating levulinoyl group at O-2. Thus, the diastereoselective construction of the Galf-β(1→4)-GlcNAc linkage by the trichloroacetimidate method of glycosylation was achieved. The (1)H NMR spectrum of alditol 2 was identical to the product released by β-elimination from the parasite mucin.

Improved bioavailability of inhibitors of Trypanosoma cruzi trans-sialidase: PEGylation of lactose analogs with multiarm polyethyleneglycol

The trans-sialidase of Trypanosoma cruzi (TcTS) catalyzes the transfer of sialic acid from host glycoconjugates to terminal β-galactopyranosides in the mucins of the parasite. During infection, the enzyme is actively shed by the parasite to the bloodstream inducing hematological alterations. Lactitol prevents cell apoptosis caused by the TcTS, although it is rapidly eliminated from the circulatory system. Linear polyethyleneglycol (PEG) conjugates of lactose analogs were prepared but their clearance from blood was still quite fast. With the aim of improving their circulating half-lives in vivo, we now synthesized covalent conjugates of eight-arm PEG. The star-shape of these conjugates allows an increase in the molecular weight together with the loading of the active sugar. Two approaches were used for PEGylation of disaccharide derivatives containing β-D-Galp as the non-reducing unit. (1) Amide formation between benzyl β-D-galactopyranosyl-(1→6)-2-amino-2-deoxy-α-D-glucopyranoside and a succinimide-activated PEG. (2) Conjugation of lactobionolactone with amino end-functionalized PEG. Two 8-arm PEG derivatives (20 and 40 kDa) were used for each sugar. Substitution of all arms was proved by (1)H nuclear magnetic resonance (NMR) spectroscopy. The bioavailability of the conjugates in mice plasma was considerably improved with respect to the 5 kDa linear PEG conjugates retaining their inhibitory properties.

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.

Probing the acceptor substrate binding site of Trypanosoma cruzi trans-sialidase with systematically modified substrates and glycoside libraries

Systematically modified octyl galactosides and octyl N-acetyllactosamines were assessed as inhibitors of, and substrates for, T. cruzi trans-sialidase (TcTS) in the context of exploring its acceptor substrate binding site. These studies show that TcTS, which catalyses the α-(2→3)-sialylation of non-reducing terminal β-galactose residues, is largely intolerant of substitution of the galactose 2 and 4 positions whereas substitution of the galactose 6 position is well tolerated. Further studies show that even the addition of a bulky sugar residue (glucose, galactose) does not impact negatively on TcTS binding and turnover, which highlights the potential of 'internal' 6-substituted galactose residues to serve as TcTS acceptor substrates. Results from screening a 93-membered thiogalactoside library highlight a number of structural features (notably imidazoles and indoles) that are worthy of further investigation in the context of TcTS inhibitor development.

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.

Synthesis of trisaccharides containing internal galactofuranose O-linked in Trypanosoma cruzi mucins

The trisaccharides beta-D-Galf-(1-->2)-beta-D-Galf-(1-->4)-D-GlcNAc (5) and beta-D-Galp-(1-->2)-beta-D-Galf-(1-->4)-D-GlcNAc (6) constitute novel structures isolated as alditols when released by reductive beta-elimination from mucins of Trypanosoma cruzi (Tulahuen strain). Trisaccharides 5 and 6 were synthesized employing the aldonolactone approach. Thus, a convenient D-galactono-1,4-lactone derivative was used for the introduction of the internal galactofuranose and the trichloroacetimidate method was employed for glycosylation reactions. Due to the lack of anchimeric assistance on O-2 of the galactofuranosyl precursor, glycosylation studies were performed under different conditions. The nature of the solvent strongly determined the stereochemical course of the glycosylation reactions when the galactofuranosyl donor was substituted either by 2-O-Galp or 2-O-Galf.

Chemistry and biology of galactofuranose-containing polysaccharides

The thermodynamically less stable form of galactose-galactofuranose (Galf)-is essential for the viability of several pathogenic species of bacteria and protozoa but absent in this form in mammals, so the biochemical pathways by which Galf-containing glycans are assembled and catabolysed are attractive sites for drug action. This potential has led to increasing interest in the synthesis of molecules containing Galf residues, their subsequent use in studies directed towards understanding the enzymes that process these residues and the identification of potential inhibitors of these pathways. Major achievements of the past several years have included an in-depth understanding of the mechanism of UDP-galactopyranose mutase (UGM), the enzyme that produces UDP-Galf, which is the donor species for galactofuranosyltransferases. A number of methods for the synthesis of galactofuranosides have also been developed, and practitioners in the field now have many options for the initiation of a synthesis of glycoconjugates containing either alpha- or beta-Galf residues. UDP-Galf has also been prepared by a number of approaches, and it appears that a chemoenzymatic approach is currently the most viable method for producing multi-milligram amounts of this important intermediate. Recent advances both in the understanding of the mechanism of UGM and in the synthesis of galactofuranose and its derivatives are highlighted in this review.

The trans-sialidase from Trypanosoma cruzi efficiently transfers alpha-(2-->3)-linked N-glycolylneuraminic acid to terminal beta-galactosyl units

The trans-sialidase from Trypanosoma cruzi (TcTS), the agent of Chagas' disease, is a unique enzyme involved in mammalian host-cell invasion. Since T. cruzi is unable to synthesize sialic acids de novo, TcTS catalyzes the transfer of alpha-(2-->3)-sialyl residues from the glycoconjugates of the host to terminal beta-galactopyranosyl units present on the surface of the parasite. TcTS also plays a key role in the immunomodulation of the infected host. Chronic Chagas' disease patients elicit TcTS-neutralizing antibodies that are able to inhibit the enzyme. N-Glycolylneuraminic acid has been detected in T. cruzi, and the trans-sialidase was pointed out as the enzyme involved in its incorporation from host glycoconjugates. However, N-glycolylneuraminic acid alpha-(2-->3)-linked-containing oligosaccharides have not been analyzed as donors in the T. cruzi trans-sialidase reaction. In this paper we studied the ability of TcTS to transfer N-glycolylneuraminic acid from Neu5Gc(alpha2-->3)Gal(beta1-->4)GlcbetaOCH(2)CH(2)N(3) (1) and Neu5Gc(alpha2-->3)Gal(beta1-->3)GlcNAcbetaOCH(2)CH(2)N(3) (2) to lactitol, N-acetyllactosamine and lactose as acceptor substrates. Transfer from 1 was more efficient (50-65%) than from 2 (20-30%) for the three acceptors. The reactions were inhibited when the enzyme was preincubated with a neutralizing antibody. K(m) values were calculated for 1 and 2 and compared with 3'-sialyllactose using lactitol as acceptor substrate. Analysis was performed by high-performance anion-exchange (HPAEC) chromatography. A competitive transfer reaction of compound 1 in the presence of 3'-sialyllactose and N-acetyllactosamine showed a better transfer of Neu5Gc than of Neu5Ac.