Abstract
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. 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. 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. 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.
Highlights
An adhesion of the mucus glycoprotein, mucin, to gastrointestinal lumenal surfaces was viewed as a physical property of the large viscous molecules discharged by the mucosal surfaces [1,2,3]
The mucin binding protein (MBP) of gastric mucosal membranes prepared by affinity chromatography on wheat germ agglutinin (WGA)-Sepharose column
MO) [11,12], and the proteins isolated from the Triton X-100-insoluble apical membranes of gastric mucosa that were partitioned into Triton X-114 detergent phase and on SDS-PAGE migrated as a 97 kDa glycoprotein (Fig. 1)
Summary
An adhesion of the mucus glycoprotein, mucin, to gastrointestinal lumenal surfaces was viewed as a physical property of the large viscous molecules discharged by the mucosal surfaces [1,2,3]. The investigations focused on mucin as the product of epithelial cells responsible for lubrication of the lumenal gastrointestinal surfaces, protection from hydrogen ion diffusion, and elimination of the discards and byproducts [4]. Over the years, this trend overpowered other conclusions on mucus functions [5] and the research on gastric pathologies concentrated on the acid-suppression therapy [6] or mucosal immunodefenses reflected in the production of immunoglobulin A (IgA) [7,8,9]. 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
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