Acid secretion by the gastric parietal cell is regulated by paracrine, endocrine, and neural pathways. The physiological stimuli include histamine, acetylcholine, and gastrin via their receptors located on the basolateral plasma membranes. Stimulation of acid secretion typically involves an initial elevation of intracellular calcium and/or cAMP followed by activation of a cAMP-dependent protein kinase cascade that triggers the translocation and insertion of the proton pump enzyme, H,K-ATPase, into the apical plasma membrane of parietal cells. Whereas the H,K-ATPase contains a plasma membrane targeting motif, the stimulation-mediated relocation of the H,K-ATPase from the cytoplasmic membrane compartment to the apical plasma membrane is mediated by a SNARE protein complex and its regulatory proteins. This review summarizes the progress made toward an understanding of the cell biology of gastric acid secretion. In particular we have reviewed the early signaling events following histaminergic and cholinergic activation, the identification of multiple factors participating in the trafficking and recycling of the proton pump, and the role of the cytoskeleton in supporting the apical pole remodeling, which appears to be necessary for active acid secretion by the parietal cell. Emphasis is placed on identifying protein factors that serve as effectors for the mechanistic changes associated with cellular activation and the secretory response.

Helicobacter pylori is involved in gastritis, gastric and duodenal ulcers, gastric adenocarcinoma, and mucosaassociated lymphoid tissue lymphoma. Earlier studies already suggested a role for autoimmune phenomena in H. pylori-linked disease. We now report that lipopolysaccharides (LPS) of H. pylori express Lewis y, Lewis x, and H type I blood group structures similar to those commonly occurring in gastric mucosa. Immunization of mice and rabbits with H. pylori cells or purified LPS induced an anti-Lewis x or y or anti-H type I response, yielding antibodies that bound human and murine gastric glandular tissue, granulocytes, adenocarcinoma, and mucosa-associated lymphoid tissue lymphoma cells. Experimental oral infections in mice or natural infection in humans yielded anti-Lewis antibodies also. The ␤ chain of gastric H ؉ ,K ؉ -ATPase, the parietal cell proton pump involved in acid secretion, contained Lewis y epitopes; gastric mucin contained Lewis x and y antigenic determinants. Growth in mice of a hybridoma that secretes H. pylori-induced anti-Lewis y monoclonal antibodies resulted in histopathological evidence of gastritis, which indicates a direct pathogenic role for anti-Lewis antibodies. In conclusion, our observations demonstrate that molecular mimicry between H. pylori LPS and the host, based on Lewis antigens, and provide understanding of an autoimmune mechanism for H. pylori-associated type B gastritis.

Gastric ezrin was initially identified as a phosphoprotein associated with parietal cell activation. To explore the nature of ezrin phosphorylation, proteins from resting and secreting gastric glands were subjected to two-dimensional SDS-PAGE. Histamine triggers acid secretion and a series of acidic isoforms of ezrin on two-dimensional SDS-PAGE. Mass spectrometric analysis of these acidic ezrin spots induced by stimulation suggests that Ser 66 is phosphorylated. To determine whether Ser 66 is a substrate of protein kinase A (PKA), recombinant proteins of ezrin, both wild type and S66A mutant, were incubated with the catalytic subunit of PKA and [ Ezrin is an actin-binding protein of the ezrin/radixin/moesin (ERM) 1 family of cytoskeleton membrane linker proteins (1). Within the gastric epithelium, ezrin has been localized exclusively to parietal cells and primarily to the apical canalicular membrane of these cells (e.g. Refs. 2 and 3). Because of its cytolocalization and observed stimulation-dependent phosphorylation, an implied role for ezrin was suggested in the apical surface membrane remodeling associated with parietal cell activation via the protein kinase A pathway. Phosphorylation of ezrin has also been shown to be associated with surface membrane remodeling of A431 cells stimulated by epidermal growth factor, although activation in this case was via protein tyrosine kinase (4, 5). Our previous studies showed that gastric ezrin is co-distributed with the ␤-actin isoform in vivo (6) and preferentially bound to the ␤-actin isoform in vitro (7). However, it is still not clear how ezrin is involved in the membrane cytoskeletal dynamics triggered by histamine stimulation.Using fluorescence resonance energy transfer monitored by fluorescence lifetime imaging microscopy and chemotaxis assays (8), it has been shown that protein kinase C-mediated phosphorylation of CD44 and ezrin modulates the interaction between these two proteins in vivo and that this phosphorylation was critical for CD44-directed cell motility, suggesting that phosphorylation of ezrin and its accessory proteins provides means to regulate the membrane cytoskeletal dynamics in response to stimulation. Whereas protein kinase C-mediated phosphorylation of CD44 was mapped to Ser 291 , the nature of ezrin phosphorylation is not characterized.Phosphorylation has been proposed to regulate ERM activation, since phosphorylation of ERM proteins correlates with their cytoskeletal association, whereas dephosphorylation of ezrin is parallel to its liberation from actin-based cytoskeleton (e.g. Refs. 9 and 10). Ezrin is phosphorylated on tyrosine residues upon growth factor stimulation (11-13). In response to epidermal growth factor, ezrin phosphorylation on tyrosines 145 and 353 is concomitant with an increase in dimer formation, suggesting a causal relationship between phosphorylation and oligomerization (14, 15). However, mutations of these tyrosines into phenylalanines does not alter ezrin localization in microvilli, and production of this mutated ezrin d...

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.