High affinity binding of the Entamoeba histolytica lectin to polyvalent N-acetylgalactosaminides

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Characterization of the carbohydrate binding specificity and kinetic parameters of lectins by using surface plasmon resonance

An accurate, rapid, and sensitive method for characterizing the carbohydrate binding properties of lectins using a BIAcore apparatus and the detection method of surface plasmon resonance is described. As a model study, the sialic acid binding lectins from Sambucus nigra and Maackia amurensis, which are specific for the epitopes Neu5Ac(alpha2-6)Gal and Neu5Ac(alpha2-3)Gal, respectively, were chosen as suitable candidates. Two systems, one for the analysis of oligosaccharides and the other for glycoproteins, were developed after a rigorous analysis and evaluation of such parameters as binding conditions, buffers, and regeneration conditions. The systems take into account nonspecific binding, using the respective denatured lectin as negative blank, and avoid loss of activity: regeneration of the surface using either 10 mM NaOAc (pH 4.3) buffer (oligosaccharide system) or 20 mM HCl (glycoprotein system). The specificity of the lectins is well illustrated, while the kinetics parameters are shown to be sensitive to subtle changes in the recognized epitopes, and to be affected by steric hindrance. Surface plasmon resonance is a suitable technique for the analysis and characterization of lectins.

The cysteine-rich region of the Entamoeba histolytica adherence lectin (170-kilodalton subunit) is sufficient for high-affinity Gal/GalNAc-specific binding in vitro

Adherence of Entamoeba histolytica trophozoites to colonic mucin, epithelium, and other target cells is mediated by the amebic Gal/GalNAc lectin. We constructed in vitro expression vectors containing full-length (residues 1 to 1280), cysteine-poor (1 to 353 and 1 to 480), and cysteine-rich (356 to 1143 and 480 to 900) fragments of the gene encoding the heavy subunit of the adherence lectin, hgl2. In vitro transcription followed by translation using a nuclease-treated rabbit reticulocyte lysate system was carried out. Immunoreactivity of in vitro-translated Hgl2 was confirmed by immunoprecipitation with lectin-specific monoclonal antibodies (MAbs) 1G7 and 8A3, which recognize linear epitopes. Protein disulfide isomerase (PDI) refolding of Hgl2 enhanced immunoreactivity (P < 0.05) with the conformationally dependent MAb 3F4. Binding of PDI-refolded full-length (P < 0.001) and cysteine-rich (P = 0.005) Hgl2 to CHO cells was galactose dependent and competitively inhibited by native hololectin (50% inhibitory concentration of 39.6 ng/ml). The cysteine-poor region (1 to 353) did not bind CHO cells. Both full-length (1 to 1280) and cysteine-rich (356 to 1143) Hgl2 bound the glyconeoconjugate GalNAc(19)BSA in a GalNAc-specific manner. The smaller cysteine-rich fragment (480 to 900) also exhibited GalNAc-specific binding but to a lesser extent (P < 0.05) than residues 1 to 1280 and 356 to 1143. Neither the cysteine-poor fragment (1 to 480), luciferase (protein control), nor control translation reactions (without hgl2 lectin mRNA) bound GalNAc(19)BSA. Binding to GalNAc(19)BSA was shown to be dependent on the concentration of GalNAc(19)BSA coated in each well or (35)S-lectin added (K(D) = 0.85 +/- 0.37 pM). Binding was competitively inhibited by the terminal GalNAc-containing glycoprotein asialofetuin (P < 0.005). Taken together, these data provide direct evidence that the cysteine-rich region of the Gal/GalNAc lectin heavy subunit contains one or more carbohydrate-binding domains.

Regulation of Entamoeba invadens encystation and gene expression with galactose and N-acetylglucosamine

Encystation of Entamoeba invadens parasites is prevented by the presence of free galactose or N-acetylglucosamine in the encystation medium. Galactose prevents the formation of amoeba cellular aggregates which develop during the early phase of encystation, suggesting the presence of functional cell surface galactose-binding molecules, whereas N-acetylglucosamine allows aggregation to occur and prevents cyst formation at a later point. While studying sugar inhibition of amoeba encystation, it was found that high efficiency encystation required the inclusion in encystation medium of precise amounts of polyvalent galactose-terminated molecules, and these molecules could be supplied by serum or by defined glycoconjugates, including mucin. Addition of free galactose to encystation medium prevented the accumulation of three transcripts which are normally upregulated during encystation, and N-acetylglucosamine prevented accumulation of one of the transcripts. These results suggest the presence of distinct sugar-sensitive pathways that regulate differentiation of the amoeba trophozoite into infectious cysts.

Interaction of human macrophage C-type lectin with O-linked N-acetylgalactosamine residues on mucin glycopeptides

A fluorescein-labeled synthetic peptide, PTTTPITTTTK, was converted into O-glycosylated glycopeptides with various numbers of attached N-acetyl-D-galactosamines (GalNAcs) by in vitro glycosylation with UDP-GalNAc and a microsomal fraction of LS174T human colon carcinoma cells. Glycopeptides with 1, 3, 5, and 6 GalNAc residues (G1, G3, G5, and G6) were obtained, and their sizes were confirmed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Their sequences were determined by a peptide sequencer to be PTTTGalNAcPITTTTK for G1, PTGalNAcTTPITGalNAcTGalNAcTTK for G3, PTTGalNAcTGalNAcPITGalNAcTGalNAcTGalNAcTK for G5, and PTGalNAcTGalNAcTGalNAcPITGalNAcTGalNAcTGalNAcTK for G6. A calcium-type human macrophage lectin (HML) was prepared in a recombinant form, and its interaction with these glycopeptides was investigated by surface plasmon resonance (SPR) spectroscopy and fluorescence polarization. The affinity of recombinant HML (rHML) for immobilized glycopeptides increased, as revealed by SPR, in parallel with the number of GalNAc. The highest affinity was obtained when the G6-peptide was immobilized at high density. Fluorescence polarization equilibrium-binding assays also revealed that the affinity of rHML for soluble gly-copeptides increased, depending on the number of attached GalNAcs. Carbohydrate recognition domain (CRD) fragments of HML were prepared, and their affinity for these four glycopeptides was also determined, this affinity was apparently lower than that of rHML. Affinity constants of rHML for the G3- and G5-peptides were 11- and 38-fold higher, respectively, than for the G1-peptide, whereas those of CRD fragments were only 2- and 6-fold higher, respectively. A chemical cross-linking study revealed that rHML but not recombinant CRD forms trimers in an aqueous solution. Thus, preferential binding of densely glycosylated O-linked glycopeptides should be due to the trimer formation of rHML.

Substructural specificity and polyvalent carbohydrate recognition by the Entamoeba histolytica and rat hepatic N-acetylgalactosamine/galactose lectins

Both the Entamoeba histolytica lectin, a virulence factor for the causative agent of amebiasis, and the mammalian hepatic lectin bind to N-acetylgalactosamine (GalNAc) and galactose (Gal) nonreducing termini on oligosaccharides, with preference for GalNAc. Polyvalent GalNAc-derivatized neoglycoproteins have >1000-fold enhanced binding affinity for both lectins (Adler,P., Wood,S.J., Lee,Y.C., Lee,R.T., Petri,W.A.,Jr. and Schnaar,R.L.,1995, J. Biol. Chem ., 270, 5164-5171). Substructural specificity studies revealed that the 3-OH and 4-OH groups of GalNAc were required for binding to both lectins, whereas only the E.histolytica lectin required the 6-OH group. Whereas GalNAc binds with 4-fold lower affinity to the E.histolytica lectin than to the mammalian hepatic lectin, galactosamine and N-benzoyl galactosamine bind with higher affinity to the E. histolytica lectin. Therefore, a synthetic scheme for converting polyamine carriers to poly-N-acyl galactosamine derivatives (linked through the galactosamine primary amino group) was developed to test whether such ligands would bind the E.histolytica lectin with high specificity and high affinity. Contrary to expectations, polyvalent derivatives including GalN6lys5, GalN4desmosine, GalN4StarburstTMdendrimer, and GalN8StarburstTMdendrimer demonstrated highly enhanced binding to the mammalian hepatic lectin but little or no enhancement of binding to the E.histolytica lectin. We propose that the mammalian hepatic lectin binds with greatest affinity to GalNAc "miniclusters," which mimic branched termini of N-linked oligosaccharides, whereas the E.histolytica lectin binds most effectively to "maxiclusters," which may mimic more widely spaced GalNAc residues on intestinal mucins.

Potential of biosensor technology for the characterization of interactions by quantitative affinity chromatography

This review places the characterization of interactions by biosensor technology in the broader context of their study by quantitative affinity chromatography. The general reluctance to consider biosensor-based characterization as a form of quantitative affinity chromatography on the grounds of a difference in aims of the two techniques reflects a mistaken belief that BIAcore and IAsys studies characterize the kinetics of the chemical reaction responsible for biospecific adsorption of a soluble reactant to an immobilized form of its affinity partner. It now transpires that the association and dissociation rate constants thereby determined refer to thermodynamic characterization of biospecific adsorption in terms of a single-phase model in which affinity sites are distributed uniformly throughout the liquid-phase volume accessible to the partitioning reactant--the model used for characterization of biospecific adsorption by quantitative affinity chromatography. In that light the most important attribute of biosensor technology is its potential for thermodynamic characterization of biospecific adsorption by virtue of its ability to monitor complex formation directly; and hence its potential for the characterization of interactions with affinities that are too strong for study by forms of quantitative affinity chromatography that monitor complex formation on the basis of reactant depletion from the liquid phase. Kinetic as well as thermodynamic analyses of biosensor data are described for attainment of that potential.

Polyvalent binding to carbohydrates immobilized on an insoluble resin

Numerous studies have established that polyvalency is a critical feature of cell surface carbohydrate recognition. Nevertheless, carbohydrate-protein interactions are typically evaluated by using assays that focus on the behavior of monovalent carbohydrate ligands in solution. It has generally been assumed that the relative affinities of monovalent carbohydrate ligands in solution correlate with their polyvalent avidities. In this paper we show that carbohydrate ligands synthesized directly on TentaGel beads interact with carbohydrate-binding proteins in a polyvalent manner. The carbohydrate-derivatized beads can, therefore, be used as model systems for cell surfaces to evaluate polyvalent carbohydrate-protein interactions. By using a combinatorial approach to synthesize solid-phase libraries of polyvalent carbohydrates, one can rapidly address key issues in the area of cell surface carbohydrate recognition. For example, studies reported herein demonstrate that there is an unanticipated degree of specificity in recognition processes involving polyvalent carbohydrates. However, the correlation between polyvalent avidities and solution affinities is poor. Apparently, the presentation of carbohydrates on the polymer surface has a profound influence on the interaction of the ligand with the protein receptor. These findings have implications for how carbohydrates function as recognition signals in nature, as well as for how polyvalent carbohydrate-protein interactions should be studied.

Identification of a Gal/GalNAc lectin in the protozoan Hartmannella vermiformis as a potential receptor for attachment and invasion by the Legionnaires' disease bacterium

The Legionnaire's disease bacterium, Legionella pneumophila, is a facultative intracellular pathogen which invades and replicates within two evolutionarily distant hosts, free-living protozoa and mammalian cells. Invasion and intracellular replication within protozoa are thought to be major factors in the transmission of Legionnaire's disease. Although attachment and invasion of human macrophages by L. pneumophila is mediated in part by the complement receptors CR1 and CR3, the protozoan receptor involved in bacterial attachment and invasion has not been identified. To define the molecular events involved in invasion of protozoa by L. pneumophila, we examined the role of protein tyrosine phosphorylation of the protozoan host Hartmannella vermiformis upon attachment and invasion by L. pneumophila. Bacterial attachment and invasion were associated with a time-dependent tyrosine dephosphorylation of multiple host cell proteins. This host cell response was highly specific for live L. pneumophila, required contact with viable bacteria, and was completely reversible following washing off the bacteria from the host cell surface. Tyrosine dephosphorylation of host proteins was blocked by a tyrosine phosphatase inhibitor but not by tyrosine kinase inhibitors. One of the tyrosine dephosphorylated proteins was identified as the 170-kD galactose/N-acetylgalactosamine-inhibitable lectin (Gal/GalNAc) using immunoprecipitation and immunoblotting by antibodies generated against the Gal/GalNAc lectin of the protozoan Entamoeba histolytica. This Gal/GalNAc-inhibitable lectin has been shown previously to mediate adherence of E. histolytica to mammalian epithelial cells. Uptake of L. pneumophila by H. vermiformis was specifically inhibited by two monovalent sugars, Gal and GalNAc, and by mABs generated against the 170-kD lectin of E. histolytica. Interestingly, inhibition of invasion by Gal and GalNAc was associated with inhibition of bacterial-induced tyrosine dephosphorylation of H. vermiformis proteins. High stringency DNA hybridization confirmed the presence of the 170-kD lectin gene in H. vermiformis. We conclude that attachment of L. pneumophila to the H. vermiformis 170-kD lectin is required for invasion and is associated with tyrosine dephosphorylation of the Gal lectin and other host proteins. This is the first demonstration of a potential receptor used by L. pneumophila to invade protozoa.

Glycobiology in Medicine

Glycoconjugates play important roles in biological reactions (for example sialyl Lewis(x) in 'homing' of leukocytes and mannose-6-phosphate in targeting of lysosomal enzymes) and thus aberration in carbohydrate structures in glycoconjugates can lead to abnormal biological behaviors. In fact, glycoconjugates expressed on the surfaces of tumor and cancer cells are considerably different from those of the normal cells, at least quantitatively. There are many known carbohydrate-deficient glycoprotein syndromes. As recognition of carbohydrate groups is mostly performed by carbohydrate-binding proteins, aberration in these proteins also results in disease status (for example I-cell disease). Many pathogens use carbohydrates as recognition markers for invasion (examples are influenza virus and cholera toxin). The carbohydrate receptors in various organs can be used for targeting drugs, antibodies and even DNAs. Conjugation of polysaccharides derived from pathogenic micro-organisms with appropriate proteins provides effective vaccines against the micro-organisms. Copyright 1996 S. Karger AG, Basel

Analysis of cell-adhesion molecule interactions using surface plasmon resonance

The molecular interactions that mediate cell adhesion are often very weak, making them difficult to study. However, real-time optical biosensors based on surface plasmon resonance (SPR) are greatly facilitating the biochemical analysis of these interactions. Analysis of the T cell surface molecule CD2 has shown that adhesion molecules can interact with very low affinities (Kd approximately 100 microM) and dissociate with half lives of approximately 0.2 seconds or less. SPR has been combined with site-directed mutagenesis to delineate the interacting surfaces of CD2 and its ligand, CD48, quantify the contribution of individual residues to the binding energy, and determine the binding orientation of these surfaces in the CD2-CD48 complex. Furthermore, SPR has been combined with in situ modification of carbohydrates on purified glycoproteins to analyze the binding specificity of lectins such as CD22. Researchers have discovered the potential pitfalls of SPR, which can lead to inaccurate affinity and kinetic measurements.

Microbial recognition of target-cell glycoconjugates

Attachment of microbial protein to host cell-surface carbohydrate is considered essential for successful infection. Information of conceptual interest, including crystal structures of protein-saccharide complexes and convincing models of target membrane penetration, continues to emerge for viruses and bacterial toxins. Other important data relate to therapeutic receptor blockade, through the use of saccharide analogs or vaccines directed against the microbial adhesin.