Essential components of antimicrobial gastrointestinal epithelial barrier: specific interaction of mucin with an integral apical membrane protein of gastric mucosa

BACKGROUND: The gastric mucosal protective barrier consists of two essential elements: mucus glycoprotein, mucin, secreted by gastric mucosal cells, and the mucin binding protein (MBP), an integral component of the apical epithelial membrane. The studies described here provide evidence on the structure of MBP, its interaction with mucin, and the susceptibility to phospholipase C (PLC) and Helicobacter pylori protease. MATERIAL AND METHODS: The rat gastric mucosa was used to isolate mucin and the apical epithelial membranes. A buffered saline extract of the mucosal cells was used for the isolation of mucin and the 1% Triton X-100-insoluble gastric apical membranes for the preparation of MBP. RESULTS: The studies on MBP, the mucosal mucin receptor revealed that the protein is anchored in apical membrane through glycosylphosphatidylinositol (GPI). The deamination of MBP with nitrous acid afforded phosphatidylinositols (PIs) and a water soluble, 97 kDa glycosylated protein. The in situ studies with untreated rat gastric mucosa and the mucosa depleted of mucin showed that MBP without mucin was susceptible to the proteolytic degradation with pepsin and H. pylori proteases, but was not released from the apical membrane by the treatment with bacterial PLC. CONCLUSION: The study of carbohydrate ligands for MBP revealed binding of octa- and decasaccharides of gastric mucin. The severe impairment in mucin adhesion to MBP, induced by the diet containing ethanol, supports the conclusion that specific carbohydrate determinants participate in mucin attachment to MBP and epigenetic control of the processes that coordinates its interaction with apical mucosal epithelium in the formation of innate protective barrier.