Adhesion of Helicobacter pylori strains to alpha-2,3-linked sialic acids

Helicobacter pylori and Its Role in Gastric Cancer

Gastric cancer is a challenging public health concern worldwide and remains a leading cause of cancer-related mortality. The primary risk factor implicated in gastric cancer development is infection with Helicobacter pylori. H. pylori induces chronic inflammation affecting the gastric epithelium, which can lead to DNA damage and the promotion of precancerous lesions. Disease manifestations associated with H. pylori are attributed to virulence factors with multiple activities, and its capacity to subvert host immunity. One of the most significant H. pylori virulence determinants is the cagPAI gene cluster, which encodes a type IV secretion system and the CagA toxin. This secretion system allows H. pylori to inject the CagA oncoprotein into host cells, causing multiple cellular perturbations. Despite the high prevalence of H. pylori infection, only a small percentage of affected individuals develop significant clinical outcomes, while most remain asymptomatic. Therefore, understanding how H. pylori triggers carcinogenesis and its immune evasion mechanisms is critical in preventing gastric cancer and mitigating the burden of this life-threatening disease. This review aims to provide an overview of our current understanding of H. pylori infection, its association with gastric cancer and other gastric diseases, and how it subverts the host immune system to establish persistent infection.

Carbohydrate-Dependent and Antimicrobial Peptide Defence Mechanisms Against Helicobacter pylori Infections

The human stomach is a harsh and fluctuating environment for bacteria with hazards such as gastric acid and flow through of gastric contents into the intestine. H. pylori gains admission to a stable niche with nutrient access from exudates when attached to the epithelial cells under the mucus layer, whereof adherence to glycolipids and other factors provides stable and intimate attachment. To reach this niche, H. pylori must overcome mucosal defence mechanisms including the continuously secreted mucus layer, which provides several layers of defence: (1) mucins in the mucus layer can bind H. pylori and transport it away from the gastric niche with the gastric emptying, (2) mucins can inhibit H. pylori growth, both via glycans that can have antibiotic like function and via an aggregation-dependent mechanism, (3) antimicrobial peptides (AMPs) have antimicrobial activity and are retained in a strategic position in the mucus layer and (4) underneath the mucus layer, the membrane-bound mucins provide a second barrier, and can function as releasable decoys. Many of these functions are dependent on H. pylori interactions with host glycan structures, and both the host glycosylation and concentration of antimicrobial peptides change with infection and inflammation, making these interactions dynamic. Here, we review our current understanding of mucin glycan and antimicrobial peptide-dependent host defence mechanisms against H. pylori infection.

Tropism for Spasmolytic Polypeptide-Expressing Metaplasia Allows Helicobacter pylori to Expand Its Intragastric Niche

BACKGROUND & AIMS

In patients with chronic Helicobacter pylori (H pylori) infection, parietal and chief cell atrophy in the gastric corpus, a process known as spasmolytic polypeptide-expressing metaplasia (SPEM), increases the risk for progression to cancer. The relation between H pylori and these metaplastic changes is unclear. We investigated whether H pylori localizes to regions of SPEM.

METHODS

We developed an in situ adherence assay in which we incubated H pylori with free-floating tissue sections from the gastric corpora of mice; we assessed H pylori distribution along the gastric unit by immunofluorescence. We analyzed the interactions of H pylori with tissue collected from mice with acute SPEM, induced by high-dose tamoxifen. We also evaluated how adhesin-deficient H pylori strains, chemical competition assays, and epithelial glycosylation affected H pylori adhesion to SPEM glands. Mice colonized with the mouse-adapted PMSS1 strain were analyzed for H pylori colonization in vivo during tamoxifen-induced SPEM or after decrease of stomach acid with omeprazole.

RESULTS

Compared with uninjured glands, H pylori penetrated deep within SPEM glands, in situ, through interaction of its adhesin, SabA, with sialyl-Lewis X, which expanded in SPEM. H pylori markedly increased gastric corpus colonization when SPEM was induced, but this proximal spread reversed in mice allowed to recover from SPEM. Decreasing corpus acidity also promoted proximal spread. However, H pylori penetrated deep within corpus glands in vivo only when sialyl-Lewis X expanded during SPEM.

CONCLUSIONS

Helicobacter pylori differentially binds SPEM glands in situ and in mice, in large part by interacting with sialyl-Lewis X. Our findings indicate that H pylori expands its niche into the gastric corpus by promoting and exploiting epithelial metaplastic changes that can lead to tumorigenesis.

Saccharomyces boulardii expresses neuraminidase activity selective for α2,3-linked sialic acid that decreases Helicobacter pylori adhesion to host cells

Helicobacter pylori is a major causative agent of gastritis and peptic ulcer disease and is an established risk factor for gastric malignancy. Antibiotic combination therapy can eradicate H. pylori. As these same regimens can evoke adverse effects and resistance, new alternative therapies or adjunctive treatments are needed. A probiotic approach may provide a novel strategy for H. pylori treatment. In the current study, two probiotic bacteria, Lactobacillus acidophilus and Lactobacillus reuteri, and a probiotic yeast, Saccharomyces boulardii, were evaluated for their ability to influence H. pylori viability, adherence to gastric and duodenal cells, as well as the effect of S. boulardii on cell surface expression of sialic acid. Our results indicate that S. boulardii contains neuraminidase activity selective for α(2-3)-linked sialic acid. This neuraminidase activity removes surface α(2-3)-linked sialic acid, the ligand for the sialic acid-binding H. pylori adhesin, which in turn, inhibits H. pylori adherence to duodenal epithelial cells.

Gangliosides and sialic acid effects upon newborn pathogenic bacteria adhesion: an in vitro study

The effect of the main gangliosides (GM(1), GM(3), GD(3)) and free sialic acid (Neu5Ac) upon the adhesion of pathogenic bacteria implicated in infant diarrhoea is assessed in vitro using the Caco-2 cell line. Concentrations of the bioactive compounds found in the bioaccessible (soluble) fraction of infant formula and human milk are employed. Bacterial adhesion behaviour included enterotoxigenic Escherichia coli (ETEC), enteropathogenic E.coli (EPEC), Listeria monocytogenes, Salmonella entericaserovartyphi, Shigella sonnei, Campylobacter jejuni and Helicobacter pylori. Three different approaches were assayed: pre-incubation of bacteria and compounds before addition to cells (competition); pre-incubation of the cells with compounds (exclusion); and pre-incubation of cells with bacteria (displacement). Furthermore, the spatial localization of the most abundant gangliosides, GM(3) and GD(3), in Caco-2 cells has been determined using confocal microscopy. Results show that GM(3), GD(3), GM(1) and Neu5Ac at the assayed concentrations are able to interfere with the adhesion of several pathogenic bacteria involved in neonatal diseases-the greatest effect corresponding to Neu5Ac, followed by GD(3), GM(1) and GM(3). Gangliosides GM(3) and GD(3) are located in the apical and basolateral membranes of the Caco-2 cells.

Biochemical characterization of trans-sialidase TS1 variants from Trypanosoma congolense

Background

Animal African trypanosomiasis, sleeping sickness in humans and Nagana in cattle, is a resurgent disease in Africa caused by Trypanosoma parasites. Trans-sialidases expressed by trypanosomes play an important role in the infection cycle of insects and mammals. Whereas trans-sialidases of other trypanosomes like the American T. cruzi are well investigated, relatively little research has been done on these enzymes of T. congolense.

Results

Based on a partial sequence and an open reading frame in the WTSI database, DNA sequences encoding for eleven T. congolense trans-sialidase 1 variants with 96.3% overall amino acid identity were amplified. Trans-sialidase 1 variants were expressed as recombinant proteins, isolated and assayed for trans-sialylation activity. The purified proteins produced α2,3-sialyllactose from lactose by desialylating fetuin, clearly demonstrating their trans-sialidase activity. Using an HPLC-based assay, substrate specificities and kinetic parameters of two variants were characterized in detail indicating differences in substrate specificities for lactose, fetuin and synthetic substrates. Both enzymes were able to sialylate asialofetuin to an extent, which was sufficient to reconstitute binding sites for Siglec-4. A mass spectrometric analysis of the sialylation pattern of glycopeptides from fetuin revealed clear but generally similar changes in the sialylation pattern of the N-glycans on fetuin catalyzed by the trans-sialidases investigated.

Conclusions

The identification and characterization of a trans-sialidase gene family of the African parasite T. congolense has opened new perspectives for investigating the biological role of these enzymes in Nagana and sleeping sickness. Based on this study it will be interesting to address the expression pattern of these genes and their activities in the different stages of the parasite in its infection cycle. Furthermore, these trans-sialidases have the biotechnological potential to be used for enzymatic modification of sialylated glycoconjugates.

Binding of the periodontitis associated bacterium Porphyromonas gingivalis to glycoproteins from human epithelial cells

INTRODUCTION

In the present study we examined the ability of the periodontal pathogen Porphyromonas gingivalis to adhere to glycoconjugates on intact cells and to protein preparations of epithelial cells (KB cells).

METHODS

The KB cell protein preparation was separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose membranes by Western blotting. The membranes were used in overlay assays with labeled P. gingivalis. Flow cytometry was used to analyze attachment of bacteria to intact KB cells.

RESULTS

Glycoconjugate expression on the KB cells and in the protein preparation was confirmed. Binding was detected to several bands on the Western blots. Flow cytometry showed a distinct increase in fluorescence for strain FDC 381. Preincubation of the bacteria with mannose, fucose, N-acetylglucosamine and N-acetylgalactosamine inhibited the binding to KB cells by approximately 30% whereas preincubation with N-acetylneuraminic acid reduced the binding by 60%.

CONCLUSION

These results indicate that carbohydrate structures are involved in the binding process of P. gingivalis to oral epithelial cells and that neuraminic acid plays a significant role in the adhesion process.

Helicobacter pylori binding to new glycans based on N-acetyllactosamine

Previously we reported binding of Helicobacter pylori to various nonacid and sialylated neolacto carbohydrate structures using a wide range of natural and chemically modified sequences. A novel nonsialylated neolacto-based binding epitope, GlcNAc beta 3Gal beta 4GlcNAc, and analogous structures carrying terminal GalNAc beta 3, GalNAc alpha 3, or Gal alpha 3 showed the binding activity (Miller-Podraza H, Lanne B, Angström J, Teneberg S, Abul-Milh M, Jovall P-A, Karlsson H, Karlsson K-A. 2005. Novel binding epitope for Helicobacter pylori found in neolacto carbohydrate chains. J Biol Chem. 280:19695-19703). The present work reports two other H. pylori-binding nonsialylated neolacto-based structures, GlcA beta 3Gal beta 4GlcNAc beta 3-R and Glc beta 3Gal beta 4GlcNAc beta 3-R, and two amide derivatives (N-methyl and N-ethyl) of GlcA beta 3Gal beta 4GlcNAc beta 3-R which were bound by H. pylori. The latter structures turned out to be more effective as H. pylori binders than the parent saccharide. New reducing-end variants of the neolacto epitope including species containing N-acetyllactosamine linked beta 6 to GlcNAc or Gal with similarity to branched polylactosamines and mucins were prepared and tested. The results extend our previous findings on binding specificities of H. pylori and show that this pathogen is able to interact with an array of N-acetyllactosamine/neolacto structures, which may be of importance for the in vivo interaction of the bacterium with human cells. The information gained in this work may also be of value for rational design of anti-H. pylori drugs.

The Multiple Carbohydrate Binding Specificities of Helicobacter pylori

Persistent colonization of the human stomach by Helicobacter pylori is a risk factor for the development of peptic ulcer disease and gastric cancer. Adhesion of microbes to the target tissue is an important determinant for successful initiation, establishment and maintenance of infection, and a variety of different candidate carbohydrate receptors for H. pylori have been identified. Here the different the binding specifities, and their potential role in adhesion to human gastric epithelium are described. Finally, recent findings on the roles of sialic acid binding SabA adhesin in interactions with human neutrophils and erythrocytes are discussed.

SabA is the H. pylori hemagglutinin and is polymorphic in binding to sialylated glycans

Adherence of Helicobacter pylori to inflamed gastric mucosa is dependent on the sialic acid–binding adhesin (SabA) and cognate sialylated/fucosylated glycans on the host cell surface. By in situ hybridization, H. pylori bacteria were observed in close association with erythrocytes in capillaries and post-capillary venules of the lamina propria of gastric mucosa in both infected humans and Rhesus monkeys. In vivo adherence of H. pylori to erythrocytes may require molecular mechanisms similar to the sialic acid–dependent in vitro agglutination of erythrocytes (i.e., sialic acid–dependent hemagglutination). In this context, the SabA adhesin was identified as the sialic acid–dependent hemagglutinin based on sialidase-sensitive hemagglutination, binding assays with sialylated glycoconjugates, and analysis of a series of isogenic sabA deletion mutants. The topographic presentation of binding sites for SabA on the erythrocyte membrane was mapped to gangliosides with extended core chains. However, receptor mapping revealed that the NeuAcα2–3Gal-disaccharide constitutes the minimal sialylated binding epitope required for SabA binding. Furthermore, clinical isolates demonstrated polymorphism in sialyl binding and complementation analysis of sabA mutants demonstrated that polymorphism in sialyl binding is an inherent property of the SabA protein itself. Gastric inflammation is associated with periodic changes in the composition of mucosal sialylation patterns. We suggest that dynamic adaptation in sialyl-binding properties during persistent infection specializes H. pylori both for individual variation in mucosal glycosylation and tropism for local areas of inflamed and/or dysplastic tissue.

Helicobacter pylori infections are very common worldwide and cause chronic inflammation in the stomach (gastritis), which may progress to peptic ulcer disease and stomach cancer. In the gastric epithelium, H. pylori infections induce expression of inflammation-associated “sialylated” carbohydrates. The ability to bind to the glycosylated epithelial cells is considered to be essential for H. pylori to cause persistent infection and disease. Here the authors show that during established infection, H. pylori also binds to red blood cells in gastric mucosal blood vessels in both infected humans and Rhesus monkeys. The authors found that “sialic acid–binding adhesin” (SabA), is the bacterial surface protein that mediates binding of H. pylori to red blood cells. Furthermore, they show that clinical H. pylori isolates demonstrate “polymorphism” in their abilities to bind various sialylated carbohydrates, and that the variation in binding properties depends on the sialic acid–binding adhesin protein itself. This variability may adapt the binding properties of H. pylori both to individual hosts and the changing epithelial glycosylation patterns during chronic inflammation. Continuous adaptation to inflamed tissue during persistent infections is probably a general feature of microbial pathogens, although their binding properties have not yet been explored in detail.

Helicobacter pylori SabA adhesin evokes a strong inflammatory response in human neutrophils which is down-regulated by the neutrophil-activating protein

The human pathogen Helicobacter pylori expresses two dominant adhesins; the Lewis b blood group antigen binding adhesin, BabA, and the sialic acid-binding adhesin, SabA. These adhesins recognize specific carbohydrate moieties of the gastric epithelium, i.e. the Lewis b antigen, Le(b), and the sialyl-Lewis x antigen, sLe(x), respectively, which promote infection and inflammatory processes in the gastroduodenal tract. To assess the contribution of each of BabA, SabA and the neutrophil activating protein (HP-NAP) in a local inflammation, we investigated the traits of H. pylori mutants in their capacity to interact with and stimulate human neutrophils. We thence found that the SabA adhesin was not only the key inducer of oxidative metabolism (Unemo et al. J Biol Chem 280:15390-15397, 2005), but also essential in phagocytosis induction, as evaluated by flow cytometry, fluorescence microscopy and luminol-enhanced chemiluminescence. The napA deletion resulted in enhanced generation of reactive oxygen species and impaired adherence to the host cells. In conclusion, the SabA adhesin stimulates human neutrophils through selectin-mimicry. Interestingly, HP-NAP modulates the oxidative burst, which could tune the impact of the H. pylori infection for establishment of balanced and chronic inflammation of the gastric mucosa.

Erythrocyte invasion by Babesia parasites: Current advances in the elucidation of the molecular interactions between the protozoan ligands and host receptors in the invasion stage

During an asexual growth cycle of Babesia parasites in a natural host, the extracellular merozoites invade (i.e., attach to, penetrate, and internalize) the host erythrocytes (RBC) via multiple adhesive interactions of several protozoan ligands with the target receptors on the host cell surface. After internalizing the host RBC, they asexually multiply, egress from the RBC by rupturing the host cells, and then invade the new RBC again. In the invasion stage, several surface-coating molecules of merozoites might be involved in the initial attachment to the RBC, while proteins secreted from apical organelles (rhoptry, microneme, and spherical body) are proposed to play roles mainly in erythrocyte penetration or internalization. On the other hand, several components located on the surface of the RBC, such as sialic acid residues, protease-sensitive proteins, or sulphated glycosaminoglycans, are identified or suspected as the host receptors of erythrocyte invasion by Babesia parasites. The detailed molecular interactions between Babesia merozoites and the host RBC are incompletely understood. In this review, these identified or suspected molecules (protozoan ligands/erythrocyte receptors) are described by especially focusing on Babesia bovis.

Identification of a new sialic acid-binding protein in Helicobacter pylori

A novel sialic acid-specific lectin has been isolated from Helicobacter pylori lysate using fetuin-agarose affinity chromatography followed by cleavage of the alpha(2,3) and alpha(2,6) linkages of sialic acids using neuraminidase. The protein had a molecular weight of 17.5 kDa on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) and was identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry to be protein of unknown function with gene number HP0721. Recombinant HP0721 was shown to bind to fetuin-agarose and sialic acid-containing glycosphingolipids on thin-layer plates suggesting this protein may represent another sialic acid-specific adhesin of H. pylori. A H. pylori mutant defective for HP0721 was generated and its ability to bind to human AGS cells assayed.

Identification and characterization of binding properties of Helicobacter pylori by glycoconjugate arrays

The microaerophilic bacterium Helicobacter pylori is well established for its role in development of different gastric diseases. Bacterial adhesins and corresponding binding sites on the epithelial surface allow H. pylori to colonize the gastric tissue. In this investigation, the adhesion of H. pylori to dot blot arrays of natural glycoproteins and neoglycoproteins was studied. Adhesion was detected by overlay with fluorescence-labeled bacteria on immobilized (neo)glycoproteins. The results confirmed the interaction between the adhesin BabA and the H-1-, Lewis b-, and related fucose-containing antigens. In addition, H. pylori bound to terminal alpha2-3-linked sialic acids as previously described. The use of a sabA mutant and sialidase treatment of glycoconjugate arrays showed that the adherence of H. pylori to laminin is mediated by the sialic acid-binding adhesin, SabA. The adhesion to salivary mucin MUC5B is mainly associated with the BabA adhesin and to a lesser extent with the SabA adhesin. This agrees with reports, that MUC5B carries both fucosylated blood group antigens and alpha2-3-linked sialic acids. The adhesion of H. pylori to fibronectin and lactoferrin persisted in the babA/sabA double mutant. Because binding to these molecules was abolished by denaturation rather than by deglycosylation, it was suggested to depend on the recognition of unknown receptor moieties by an additional unknown bacterial surface component. The results demonstrate that the bacterial overlay method on glycoconjugate arrays is a useful tool for exploration and the characterization of unknown adhesin specificities of H. pylori and other bacteria.

Interaction of Helicobacter pylori with sialylated carbohydrates: the dependence on different parts of the binding trisaccharide Neu5Acα3Galβ4GlcNAc

We have recently shown that binding of Helicobacter pylori to sialylated carbohydrates is dependent on the presence of the carboxyl group and the glycerol chain of Neu5Ac. In this work we investigated the importance of GlcNAc in the binding trisaccharide Neu5Acalpha3Galbeta4GlcNAc and the role of the N-acetamido groups of both Neu5Ac and GlcNAc. An important part of the project was epitope dissection, that is chemical derivatizations of the active carbohydrate followed by binding studies. In addition we used a panel of various unmodified carbohydrate structures in the form of free oligosaccharides or glycolipids. These were tested for binding by hemagglutination inhibition assay, TLC overlay tests, and a new quantitative approach using radiolabeled neoglycoproteins. The studies showed that the N-acetamido group of Neu5Ac is important for binding by H. pylori, whereas the same group of GlcNAc is not. In addition, Fuc attached to GlcNAc, as tested with sialyl-Lewis x, did not affect the binding. Free Neu5Ac was inactive as inhibitor, and Neu5Acalpha3Gal turned out to be active. The binding preference for neolacto structures was confirmed, although one strain also was inhibited by lacto chains. The combined results revealed that an intact Neu5Ac is crucial for the interactions with H. pylori. Parts of Gal also seem to be necessary, whereas the role of the GlcNAc is secondary. GlcNAc does influence binding, however, primarily serving as a guiding carrier for the binding epitope rather than being a part of the binding structure.

Helicobacter pylori and complex gangliosides

Recognition of sialic acid-containing glycoconjugates by the human gastric pathogen Helicobacter pylori has been repeatedly demonstrated. To investigate the structural requirements for H. pylori binding to complex gangliosides, a large number of gangliosides were isolated and characterized by mass spectrometry and proton nuclear magnetic resonance. Ganglioside binding of sialic acid-recognizing H. pylori strains (strains J99 and CCUG 17874) and knockout mutant strains with the sialic acid binding adhesin SabA or the NeuAcalpha3Galbeta4GlcNAcbeta3Galbeta4GlcNAcbeta-binding neutrophil-activating protein HPNAP deleted was investigated using the thin-layer chromatogram binding assay. The wild-type bacteria bound to N-acetyllactosamine-based gangliosides with terminal alpha3-linked NeuAc, while gangliosides with terminal NeuGcalpha3, NeuAcalpha6, or NeuAcalpha8NeuAcalpha3 were not recognized. The factors affecting binding affinity were identified as (i) the length of the N-acetyllactosamine carbohydrate chain, (ii) the branches of the carbohydrate chain, and (iii) fucose substitution of the N-acetyllactosamine core chain. While the J99/NAP(-) mutant strain displayed a ganglioside binding pattern identical to that of the parent J99 wild-type strain, no ganglioside binding was obtained with the J99/SabA(-) mutant strain, demonstrating that the SabA adhesin is the sole factor responsible for the binding of H. pylori bacterial cells to gangliosides.

Interactions of the gastrotropic bacterium Helicobacter pylori with the leukocyte-endothelium adhesion molecules, the selectins--a preliminary report

The deleterious effects of Helicobacter pylori infection of the stomach are largely the result of a vigorous chronic inflammatory response, and include chronic gastritis, peptic ulceration and gastric cancer. We are exploring the possibility that carbohydrate components on H. pylori contribute to the persistent inflammation through interactions with leukocyte-endothelial adhesion molecules of the host. Lipopolysaccharides of most H. pylori strains contain sequences related to the Lewis (Le(x) or Le(a)) antigens. Carbohydrate sequences of this family encompass ligands for the leukocyte-endothelium adhesion molecules of the host, namely, the E- and P-selectins, which are expressed on inflamed endothelia, and L-selectin, which is constitutively expressed on leukocytes. Here we investigate H. pylori isolates from patients with chronic gastritis, duodenal ulcer and gastric cancer for their interactions with the selectins. Our results provide unequivocal evidence of interactions of isolates from each of the diagnostic groups with E- and L-selectins.

Glycoconjugate binding of gastric and enterohepatic Helicobacter spp

Helicobacter pylori is able to utilize several lectin-like, protein-carbohydrate interactions for binding to mucins, cell surfaces, and extracellular matrix proteins. As determined by hemagglutination assays and binding of radiolabeled bacteria to glycosphingolipids on thin-layer chromatograms, strains of gastric helicobacters and enterohepatic helicobacters, including Helicobacter canis, Helicobacter hepaticus, and Helicobacter bilis, also demonstrated evidence for the presence of lectin-hemagglutinin adhesins. In addition, in H. hepaticus and H. bilis, binding may be sialic acid dependent. The presence or absence and differences in the levels of activity of lectin adhesins may reflect the species' ecological niche.

Purification and partial characterization of a 32-kilodalton sialic acid-specific lectin from Aspergillus fumigatus

Adherence of the opportunistic fungus Aspergillus fumigatus to the extracellular matrix components is considered a crucial step in the establishment of the infection. Given the high carbohydrate content of these glycoproteins and the role of carbohydrate-protein interactions in numerous adherence processes, the presence of a lectin in A. fumigatus was investigated. Different fungal extracts obtained by sonication or grinding in liquid nitrogen from resting or swollen conidia, as well as from germ tubes and mycelium, were tested by hemagglutination assays using rabbit erythrocytes. A lectin activity was recovered in all the extracts tested. However, sonication of resting conidia resulted in the highest specific activity. Purification of the lectin was achieved by gel filtration followed by ion-exchange and hydrophobic-interaction chromatographies. Analysis of the purified lectin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed an apparent molecular mass of 32 kDa, which is similar to that of the alkaline protease already identified from different strains of A. fumigatus. However, as evidenced by the use of an alkaline protease-deficient mutant, the two activities were supported by distinct proteins. In addition, hemagglutination inhibition experiments using different saccharides and glycoproteins demonstrated the specificity of the lectin for sialic acid residues. Together these results suggest that this lectin may contribute to the attachment of conidia to the extracellular matrix components through the recognition of the numerous terminal sialic acid residues of their carbohydrate chains.

The crystal structure and mode of action of trans-sialidase, a key enzyme in Trypanosoma cruzi pathogenesis

Trans-sialidases (TS) are GPI-anchored surface enzymes expressed in specific developmental stages of trypanosome parasites like Trypanosoma cruzi, the etiologic agent of Chagas disease, and T. brucei, the causative agent of sleeping sickness. TS catalyzes the transfer of sialic acid residues from host to parasite glycoconjugates through a transglycosidase reaction that appears to be critical for T. cruzi survival and cell invasion capability. We report here the structure of the T. cruzi trans-sialidase, alone and in complex with sugar ligands. Sialic acid binding is shown to trigger a conformational switch that modulates the affinity for the acceptor substrate and concomitantly creates the conditions for efficient transglycosylation. The structure provides a framework for the structure-based design of novel inhibitors with potential therapeutic applications.

Role of the alpAB proteins and lipopolysaccharide in adhesion of Helicobacter pylori to human gastric tissue

The attachment of the bacterial pathogen Helicobacter pylori (H. pylori) to gastric epithelial cells is commonly believed to be required for an efficient and persistent colonization of the human stomach as well as for host cell trans-membrane signaling. In the past, several putative adhesins were postulated, including the outer membrane proteins AlpAB and the bacterial lipopolysaccharide (LPS) presenting oligomeric Lewis x (Le(x)) sugar components. We investigated the AlpAB-mediated and the Le(x)-dependent binding by knockout mutagenesis in one distinct strain, H. pylori P1. We show here that the mutagenesis of either alpA and/or alpB dramatically reduced the adherence of H. pylori P1 to a given gastric biopsy section. None of these mutations influenced the surface exposure of the Le(x) antigen, suggesting that the assembly and/or presentation of LPS is independent of the AlpAB outer membrane proteins. However, a truncation of the LPS O-side chain by a galE mutation abolished the presentation of the Le(x) epitope. This Le(x)-negative strain did not show any significant reduction in its binding capacity to the gastric tissue, when compared with the corresponding wild-type strain. From these data we conclude that the AlpAB-specific adherence is independent of the composition of the LPS and that the oligomeric Le(x) structure does not confer binding to the gastric biopsy material used in this study. As the adhesion properties of our H. pylori strain P1 vary in dependence on the respective biopsy donor it is assumed that the surface-exposed Le(x) epitope recognizes a different host cell receptor than AlpAB, which was probably not present in the tissue sections used in this study.

Helicobacter pylori-binding gangliosides of human gastric adenocarcinoma

Acidic and neutral glycosphingolipids were isolated from a human gastric adenocarcinoma, and binding of Helicobacter pylori to the isolated glycosphingolipids was assessed using the chromatogram binding assay. The isolated glycosphingolipids were characterized using fast atom bombardment mass spectrometry and by binding of antibodies and lectins. The predominating neutral glycosphingolipids were found to migrate in the di- to tetraglycosylceramide regions as revealed by anisaldehyde staining and detection with lectins. No binding of H. pylori to these compounds was obtained. The most abundant acidic glycosphingolipids, migrating as the GM3 ganglioside and sialyl-neolactotetraosylceramide, were not recognized by the bacteria. Instead, H. pylori selectively interacted with slow-migrating, low abundant gangliosides not detected by anisaldehyde staining. Binding-active gangliosides were isolated and characterized by mass spectrometry, proton nuclear magnetic resonance, and lectin binding as sialyl-neolactohexaosylceramide (NeuAcalpha3Galbeta4GlcNAcbeta3Galbeta4GlcNAcbeta3Galbeta4Glcbeta1Cer) and sialyl-neolactooctaosylceramide (NeuAcalpha3Galbeta4GlcNAcbeta3Galbeta4GlcNAcbeta3Galbeta4GlcNAcbeta3Galbeta4Glcbeta1Cer).

Sialidase treatment exposes the beta1-integrin active ligand binding site on HL60 cells and increases binding to fibronectin

The migration of neutrophils from the circulation to areas of inflammation is the result of the sequential activation of multiple cellular adhesion molecules. beta1-Integrins are cell surface glycoproteins and the class of adhesion molecules responsible for binding to the extracellular matrix. The goal of this study was to determine the contribution of glycosylation, specifically the presence of sialic acid, to beta1-integrin adhesion in a neutrophil model. beta1-Integrins on differentiated HL60 cells were remodeled by treatment with the exoglycosidases, sialidase and beta-galactosidase. beta1-Integrin activity was determined by measuring adherence to the extracellular matrix protein fibronectin. The expression of beta1-integrins, beta2-integrins and activated beta1-integrins was determined by flow cytometry. Remodeling of beta1-integrins by treatment with sialidase increased adhesion by greater than 1,000%. Flow cytometric analysis of remodeled beta1-integrins demonstrated an increased expression of the activated beta1-integrin, but only minor increases in the expression of total beta1- and beta2-integrins. We postulate that glycosidase treatment increases adhesion and expression of activated beta1-integrins by exposure of the normally hidden ligand-binding site. The glycosylation of beta1-integrins on neutrophils may act to hide the ligand-binding site in unstimulated cells thereby contributing to the affinity modulation observed in neutrophil beta1-integrin function.

Helicobacter pylori adhesins: review and perspectives

It is highly unlikely that chronic infection with H. pylori could occur in the absence of adhesin-host cell interactions. Also, there is no evidence that any of the serious outcomes of H. pylori infection such as gastric and duodenal ulcers, gastric cancer or mucosa-associated lymphoid tissue (MALT) lymphoma could occur without prior colonization of the gastric epithelium mediated by H. pylori adhesins. H. pylori is highly adaptable, as evidenced by the fact that it can occupy a single host for decades. An important facet of this adaptability is its ability to physically interact with various types of host cells and also with host mucins and extracellular matrix proteins using a number of different adhesins displaying a variety of unique receptor specificities. Thus it is highly unlikely that any one particular H. pylori adhesin will ever be proven responsible for a particular outcome such as duodenal ulcer, MALT lymphoma, or adenocarcinoma. Also, while the search for additional H. pylori adhesins should and certainly will continue, we suggest that the scope of this effort should be expanded to include investigations into the patterns of expression and interaction between individual outer membrane proteins. Which of the numerous H. pylori outer membrane proteins (OMPs) actually function as adhesins (i.e., have receptor-binding sites) and which OMPs are simply necessary for optimal display of the adhesive OMPs? There are many other important questions about H. pylori adhesins waiting to be answered. For example, which adhesins are responsible for loose adherence to host cells and which adhesins are responsible for intimate, or membrane-to-membrane, adherence, and do these adhesins normally work in concert or in a sequential fashion? Also, is a specific type of adhesin necessary for type IV protein translocation into host cells and, if so, is adhesin expression coregulated with the effector protein export?

Specificity of the binding site of the sialic acid-binding lectin from ovine placenta, deduced from interactions with synthetic analogues

The specificity of the sialic acid-binding lectin from ovine placenta was examined in detail by haemagglutination inhibition assays applying a panel of 32 synthetic sialic acid analogues. The carboxylic acid group is a prerequisite for the interaction with the lectin, the alpha-anomer of the methyl glycoside is only a little more effective as an inhibitor than the beta-anomer and the most potent inhibitor was 9-deoxy-10-carboxylic acid Neu5Ac, followed by 4-oxo-Neu5Ac. In contrast to the majority of known sialic acid-binding lectins, the N-acetyl group of Neu5Ac is not indispensable for binding, neither is the hydroxyl group at C-9 since substitutions at this carbon atom are well tolerated. Furthermore, all sulfur-containing substituents at C-9 enhanced the affinity of the lectin. This is the first sialic acid-binding lectin found to strongly bind thio derivatives.

Theoretical and experimental approaches for studying factors defining the Helicobacter pylori-host relationship

Mathematical modeling has helped develop hypotheses about the role of microbial and host parameters in the initial and subsequent phases of Helicobacter pylori colonization. Transgenic mice have been used to test the hypothesis that the outcome of colonization is influenced by whether bacteria can adhere to available epithelial cell receptors. Complementary use of modeling and experimental approaches should facilitate studies of H. pylori pathogenesis.

Treatment of Helicobacter pylori infection in rhesus monkeys using a novel antiadhesion compound

BACKGROUND & AIMS

Helicobacter pylori can be eradicated by administration of antimicrobials, but resistant strains have emerged, and there is a need for novel therapeutic approaches against this infection. This study aimed to determine the safety and efficacy of 3'-sialyllactose sodium salt (3'SL), an oligosaccharide that occurs naturally in human and bovine milk and that can inhibit the adhesion of H. pylori to human epithelial cells in vitro.

METHODS

Twelve H. pylori-positive rhesus monkeys were given 3'SL, either alone (regimens 1 and 2; n = 6) or in combination with omeprazole (regimen 3; n = 4), or bismuth subsalicylate (regimen 4; n = 6). Videogastroscopies were performed before, during, and after treatment, and gastric biopsy specimens were obtained for quantitative cultures and histology. The H. pylori strains colonizing the animals were genotyped.

RESULTS

After regimen 1 or 2, 2 of 6 animals were cured permanently, and a third animal was transiently cleared. The 3 other animals remained persistently colonized and did not respond to regimen 3. Regimen 4 resulted in transient decreases in colony counts in 3 of 6 other animals. Gastritis was suppressed only in the 2 animals who became persistently H. pylori negative. There was no apparent relation between 3'SL efficacy and any of the H. pylori tested genotypes. No side effects were observed in any of the animals receiving 3'SL.

CONCLUSIONS

Antiadhesive therapy is safe; it can cure or decrease H. pylori colonization in some rhesus monkeys, but the addition of a proton pump inhibitor or bismuth subsalicylate does not increase cure rate.

Neu5Acalpha3Gal is part of the Helicobacter pylori binding epitope in polyglycosylceramides of human erythrocytes

The sialic acid dependent binding by the human pathogen Helicobacter pylori to polyglycosylceramides of human erythrocytes was investigated. Polyglycosylceramides, complex glycosphingolipids with a branched N-acetyllactosamine core, were isolated from human erythrocytes, blood group O, and subfractionated after peracetylation by anion-exchange chromatography. Three subfractions were deacetylated, analysed by matrix-assisted laser desorption ionization-time of flight MS and 2D 1H NMR spectroscopy. The observed mass ranges were m/z = 3093-7622, 3968-7255 and 3459-7987 in the mass spectra of the first, second and third fractions, respectively. The observed ions agreed with the general formula Hex(x+2)HexNAcxFucyNeu5AczCer. Two-dimensional 1H total correlation spectra of the mixtures showed that the first fraction contained 3-linked sialic acid and the second and third fractions contained both 3-linked and 6-linked sialic acid. Thin-layer chromatogram binding assays using the lectins from Maackia amurensis, specific for Neu5Acalpha3Galbeta4GlcNAc, and Sambucus nigra, specific for Neu5Acalpha6Gal/GalNAc, were used to confirm this distribution. H. pylori recognized all three fractions in the binding assay, indicating that the 3-linked, rather than 6-linked, sialic acid is essential for binding.

Helicobacter pylori and neutrophils: sialic acid-dependent binding to various isolated glycoconjugates

Helicobacter pylori has been shown to agglutinate erythrocytes in a sialic acid-dependent manner. However, very few studies have examined relevant target cells in the human stomach. Neutrophils are required for the onset of gastritis, and the inflammatory reaction may be induced on contact between bacteria and neutrophils. In the present work, glycolipids and glycoproteins were isolated from neutrophils and were studied for binding by overlay with radiolabeled bacteria on thin-layer chromatograms and on membrane blots. There was a complex pattern of binding bands. The only practical binding activity found was sialic acid dependent, since treatment of glycoconjugates with neuraminidase or mild periodate eliminated binding. As shown before for binding to erythrocytes and other glycoconjugates, bacterial cells grown on agar bound to many glycoconjugates, while growth in broth resulted in bacteria that would bind only to polyglycosylceramides, which are highly heterogeneous and branched poly-N-acetyllactosamine-containing glycolipids. Approximately seven positive bands were found for glycoproteins, and the traditional ganglioside fraction showed a complex, slow-moving interval with very strong sialic-acid-dependent binding, probably explained by Fuc substitutions on GlcNAc.

Helicobacter pylori interactions with human gastric mucin studied with a resonant mirror biosensor

Helicobacter pylori, a human pathogen colonizing the gastrointestinal tract, is first interacting with mucus glycoproteins to penetrate the gastric mucus layer and then attach to specific epithelial cell targets. An optical biosensor technique based on the resonant mirror was used to study H. pylori interactions with human gastric mucin. The mucin preparation was immobilized on the sensor surface to set up the experimental model, close to the in vivo situation. Both sialylated and sulphated oligosaccharides interfered with the H. pylori binding to the immobilized mucin by reducing or abolishing the binding of the bacteria. Furthermore, the displacement of the bacteria from immobilized mucin by highly sulphated glycosaminoglycans was observed.

Helicobacter pylori attaches to NeuAc alpha 2,3Gal beta 1,4 glycoconjugates produced in the stomach of transgenic mice lacking parietal cells

Helicobacter pylori infection of the human stomach is associated with altered acid secretion, loss of acid-producing parietal cells, and, in some hosts, adenocarcinoma. We have used a transgenic mouse model to study the effects of parietal cell ablation on H. pylori pathogenesis. Ablation results in amplification of the presumptive gastric epithelial stem cell and its immediate committed daughters. The amplified cells produce sialylated oncofetal carbohydrate antigens that function as receptors for H. pylori adhesins. Attachment results in enhanced cellular and humoral immune responses. NeuAc alpha 2,3Gal beta 1,4 glycoconjugates may not only facilitate persistent H. pylori infection in a changing gastric ecosystem, but by promoting interactions with lineage progenitors and/or initiated cells contribute to tumorigenesis in patients with chronic atrophic gastritis.

Novel therapies for Helicobacter pylori infection

BACKGROUND

Increasing antibiotic resistance has begun to impair our ability to cure Helicobacter pylori infection.

AIM

To evaluate orally administered novel therapies for the treatment of H. pylori infection.

METHODS

Healthy H. pylori infected volunteers received: (a) hyperimmune bovine colostral immune globulins, (b) an oligosaccharide containing an H. pylori adhesion target, Neu5Aca2-3Galb1-4Glc-(3'-sialyllactose), or (c) recombinant human lactoferrin. Outcome was assessed by urea breath test or histological assessment of the number of H. pylori present.

RESULTS

None of the novel therapies appeared effective and no adverse events occurred.

CONCLUSION

Although in vitro data appeared promising, in vivo results were disappointing. Higher doses, longer duration of therapy, adjunctive acid suppression, or a combination could possibly yield better results.

Analysis of 3- and 6-linked sialic acids in mixtures of gangliosides using blotting to polyvinylidene difluoride membranes, binding assays, and various mass spectrometry techniques with application to recognition by Helicobacter pylori

A convenient approach to analyze 3- and 6-linked sialic acids in mixtures of biologically active gangliosides was developed. The procedure was adapted to work on small amounts of material and included parallel tests, which allowed direct analysis of structure and activity. The initial step in the procedure was separation of a mixture of gangliosides by thin-layer chromatography (TLC) and blotting to a polyvinylidene difluoride membrane. The gangliosides were then analyzed (a) by direct desorption from the membrane and fast atom bombardment mass spectrometry (FAB MS), (b) by membrane-binding assay using the NeuAcalpha3- and NeuAcalpha6-specific lectins from Maackia amurensis and Sambucus nigra, respectively, and (c) by TLC binding assay with radiolabeled bacteria. All experiments were performed on a mixture of gangliosides from human leucocytes, which contained species with affinity for the human gastric pathogen Helicobacter pylori. The procedure was used with good results for gangliosides with up to seven sugars per ceramide. A three-sugar ganglioside was identified as GM3 with ceramides composed of sphingosine (d18:1) and 20:0, h20:0, and 24:0 fatty acids. The sequences of four bands in the five-sugar region were consistent with sialylparagloboside (NeuAcalpha3/6Galbeta4GlcNAcbeta3Galbeta4GlcbetaCer). The ceramides were composed of d18:1 and 24:1 fatty acid in the first and third bands, and d18:1 and 16:0 fatty acid in the second and fourth bands from the top. The sialic acid was shown to be 3-linked in the upper two bands and 6-linked in the lower two bands. The same distribution of sialic acid and ceramides but the sequence elongated with one N-acetyllactosamine unit was observed in the less resolved interval containing seven-sugar glycosphingolipids. The direct comparison of binding of lectins and radiolabeled bacteria showed that H. pylori recognized 3-linked sialic acid only. These results were supported by a novel technique of analysis of the sialic acid linkage position by trifluoroacetolysis and gas chromatography/MS. Direct membrane/FAB MS was ineffective for species migrating below the seven-sugar region on the TLC. In this case, the membranes were instead cut in bands and the gangliosides extracted by methanol before analysis by FAB MS.

Simple synthesis of sialyllactose-carrying polystyrene and its binding with influenza virus

Glycoconjugate polystyrenes bearing sialyllactose moieties were prepared via a simple method from a mixture of alpha2-6 and a2-3 linked sialyllactose isomers of bovine milk origin. The reducing end of sialyllactose was converted to an amino function with ammonium hydrogen carbonate and then coupled with p-vinylbenzoyl chloride. The resulting styrene derivative substituted with sialyllactose via an amide linkage was polymerized with ammonium peroxodisulfate and N,N,N',N-tetramethylethylenediamine in water at 30 degrees C. The interaction of the glycopolymer with influenza A and B viruses was investigated by three different methods. The glycopolymer inhibited the hemagglutination of influenza A virus (PR/8/34) and its activity was 10(3) times higher than that of the oligosaccharide itself. The cytopathic effect of virus-infected MDCK (Madine-Darby canine kidney) cells was inhibited by the glycopolymer. The homopolymer showed 10(2) times higher inhibitory activity than naturally-occurring fetuin. It was also found that various viruses could be trapped by the glycopolymer adsorbed on a polystyrene surface. The inhibitory and trapping activities of the glycopolymers were correlated with the sialyl linkage specificities of the virus strains.

Selective binding by Helicobacter pylori of leucocyte gangliosides with 3-linked sialic acid, as identified by a new approach of linkage analysis

The human gastric pathogen Helicobacter pylori has been shown to bind to glycoconjugates of human leucocytes in a sialic acid-dependent way. In order to improve the identification of the binding epitope, a new technique was developed to analyze the ketosidic linkage position between a terminal sialic acid and the consecutive monosaccharide. Permethylation and reduction with LiAlH4 followed by trifluoroacetolysis in 1000:1 trifluoroacetic anhydride:trifluoroacetic acid (24 h, 100 degrees C) results in the cleavage of glycosidic but not ketosidic bonds. The disaccharide products were analyzed by gas chromatography-mass spectrometry and sialyl-3 or -6 position and NeuAc or NeuGc are identified by their separate retention times and mass spectra. The method was worked out on model saccharides and applied on five-sugar gangliosides (sialylparaglobosides) of human leucocytes. Radiolabeled Helicobacter pylori was shown to bind to the upper part, but not to the lower part, of the five-sugar interval of a mixture of gangliosides separated on a thin-layer chromatogram. Using a membrane blotting procedure the active and inactive bands were isolated and shown to be NeuAcalpha2-3- and NeuAcalpha2-6-paraglobosides, respectively.

Meaning and therapeutic potential of microbial recognition of host glycoconjugates

The microbe-host interface is currently in focus because of attempts to develop infection therapy in humans based on either natural receptor saccharide (respiratory and gastrointestinal disease) or sophisticated sialic acid analogues designed from crystal structures (influenza). Most of the known host receptors for microbes are glycoconjugates, and the diversity and selectivity of host tissue glycosylation allow for the tropism of infections. However, among the many binding specificities detected so far, the biological role has been proven only in a few infectious model systems. The existence of multiple specificities of a single microbe is both a complicating factor and a challenge, requiring expanded research with a special demand on glycoscience.

Inhibition of Helicobacter pylori sialic acid-specific haemagglutination by human gastrointestinal mucins and milk glycoproteins

Helicobacter pylori, a human gastric pathogen causing chronic gastritis and duodenal ulcer disease, has been found in large amounts in gastric mucous gel layer. Mucin preparations, separated from human gastric juices and isolated from different colon regions, were examined for their ability to inhibit haemagglutination of H. pylori with the emphasis on evaluating the role of sialic acid-dependent haemagglutinins of the bacteria in colonisation of the stomach. The mucins showed high inhibitory activity for H. pylori, which was significantly decreased after the removal of sialic acids from the mucins. The inhibitory potencies using high molecular mass mucin-like components from bovine milk were comparable with those obtained for gastric mucins, suggesting their possible role in the prevention of H. pylori infection.

Characterization of Helicobacter pylori interactions with sialylglycoconjugates using a resonant mirror biosensor

A new optical biosensor technique based on the resonant mirror was used to characterize Helicobacter pylori strains according to their sialic acid binding, demonstrating the suitability of using intact bacteria in real-time measurements and classifying strains based on their binding abilities. Results obtained from both competition and displacement assays using different glycoconjugates confirmed that several, but not all, H.pylori strains express sialic acid-binding adhesin(s), specific for alpha-2,3-sialyllactose. The adhesin, removable from the bacterial surface by water extraction, is not related to other reported H.pylori cell surface proteins with binding ability to sialylated compounds such as sialylglycoceramides.

Helicobacter pylori

Helicobacter pylori is a gram-negative bacterium which causes chronic gastritis and plays important roles in peptic ulcer disease, gastric carcinoma, and gastric lymphoma. H. pylori has been found in the stomachs of humans in all parts of the world. In developing countries, 70 to 90% of the population carries H. pylori. In developed countries, the prevalence of infection is lower. There appears to be no substantial reservoir of H. pylori aside from the human stomach. Transmission can occur by iatrogenic, fecal-oral, and oral-oral routes. H. pylori is able to colonize and persist in a unique biological niche within the gastric lumen. All fresh isolates of H. pylori express significant urease activity, which appears essential to the survival and pathogenesis of the bacterium. A variety of tests to diagnose H. pylori infection are now available. Histological examination of gastric tissue, culture, rapid urease testing, DNA probes, and PCR analysis, when used to test gastric tissue, all require endoscopy. In contrast, breath tests, serology, gastric juice PCR, and urinary excretion of [15N]ammonia are noninvasive tests that do not require endoscopy. In this review, we highlight advances in the detection of the presence of the organism and methods of differentiating among types of H. pylori, and we provide a background for appropriate chemotherapy of the infection.

Helicobacter pylori

Helicobacter pylori is a gram-negative bacterium which causes chronic gastritis and plays important roles in peptic ulcer disease, gastric carcinoma, and gastric lymphoma. H. pylori has been found in the stomachs of humans in all parts of the world. In developing countries, 70 to 90% of the population carries H. pylori. In developed countries, the prevalence of infection is lower. There appears to be no substantial reservoir of H. pylori aside from the human stomach. Transmission can occur by iatrogenic, fecal-oral, and oral-oral routes. H. pylori is able to colonize and persist in a unique biological niche within the gastric lumen. All fresh isolates of H. pylori express significant urease activity, which appears essential to the survival and pathogenesis of the bacterium. A variety of tests to diagnose H. pylori infection are now available. Histological examination of gastric tissue, culture, rapid urease testing, DNA probes, and PCR analysis, when used to test gastric tissue, all require endoscopy. In contrast, breath tests, serology, gastric juice PCR, and urinary excretion of [15N]ammonia are noninvasive tests that do not require endoscopy. In this review, we highlight advances in the detection of the presence of the organism and methods of differentiating among types of H. pylori, and we provide a background for appropriate chemotherapy of the infection.

Binding of Helicobacter pylori to sialic acid-containing glycolipids of various origins separated on thin-layer chromatograms

Two standard strains of Helicobacter pylori, grown on solid or in liquid medium, were studied for their binding to sialic acid-containing glycosphingolipids on thin-layer plates. NCTC 11637, but not strain 11638, bound to mixtures of gangliosides of various human and animal origins with similar binding patterns and also to polyglycosylceramides of human erythrocytes, leukocytes, and placenta. There was an apparent specificity for NeuAc alpha3Gal of the neolacto series of gangliosides, since NeuAc alpha6Gal or ganglio-series gangliosides did not bind.

Lack of evidence for sialidase activity in Helicobacter pylori

The role of sialic acid for the adhesion of Helicobacter pylori to gastric mucosa cells and/or to the mucin layer is still under debate. Several but not all H. pylori strains express a sialic acid-binding adhesin, specific for terminal alpha-2,3-sialic acid residues. Recently, the production of sialidase by H. pylori was reported [Dwarakanath, A.D. et al. (1995) FEMS Immunol. Med. Microbiol. 12,213 216]. We analysed several strains isolated from gastric biopsies cultivated both in liquid media and on agar plates for sialidase. Activity of this enzyme was first assayed using the fluorigenic substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid. Since the fluorimetric assay can give false-positive results caused by non-specific interactions with umbelliferyl-tagged substances, we used also the more sensitive and specific assay with sialyl-[3H]lactitol as a substrate. No evidence for sialidase activity of H. pylori strains, cultivated under both inducible and non-inducible conditions, was obtained.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.