Helicobacter pylori Sialic Acid-Specific Surface Lectin

Abstract: Helicobacter pylori is a gastric pathogen colonising the gastric mucus layer and epithelium of gastric tissue and is associated with chronic type B gastritis and peptic ulcer disease. The attachment of H. pylori to gastric epithelial cells involves several structures recognised by specific bacterial surface proteins. Since colonisation is an initial and critical step in the development of a chronic disease, the identification of H. pylori adhesins is important for defining specific interactions between host and pathogen with the aim to develop adhesin-receptor analogues, which can inhibit gastric tissue colonisation. H. pylori sialic acid (Sia)-binding surface proteins represent a group of lectin-like adhesins, which are most likely crucial for the colonisation of the human stomach mucosa. Haemagglutination experiments with enzymatically derivatised erythrocytes carrying Sias only on defined glycans demonstrated that one third of those H. pylori strains investigated express Sia-binding activity. All of these were specific for a-2,3-linked Sia. This specificity was confirmed by microscopy studies using fluorescent neoglycoconjugates as targets. Further examination by electron microscopy revealed the surface localisation of this Sia-specific lectin (SAL) condensed into complexes. The lectin was extracted from the surface of the bacteria by water extraction and affinity purified on glutardialdehyde-fixed erythrocytes. The analysis of proteins in the isolated H. pylori SAL complex by proteomics approach revealed the presence of two H. pylori outer membrane proteins AlpA/B, which had been proposed to function in H. pylori gastric tissue adherence processes. Since the binding specificity of these proteins is unknown, the Sia-binding activity may reside in a minor component of the complex associated with AlpA/B. H. pylori Sia-binding lectins seem to be involved in interactions with gastrointestinal mucins facilitating the spread of the pathogen also to the gastric epithelium. Mucin-like sialylated components from bovine milk show high inhibitory potencies for H. pylori binding to sialylglycoconjugates and could probably be developed as potent anti-infectious components against this pathogen. A cuvette-based optical biosensor, IAsys? was efficiently used for analysis of the structural requirements for H. pylori interactions with such glycoconjugates. In contrast to a previous report, no evidence for sialidase activity of H. pylori was obtained. Therefore, it can be assumed that H. pylori may regulate the expression of its Sia-binding lectins to be released from either mucin or cell surfaces for further spreading.