The membrane-recruitment-and-recycling hypothesis of gastric HCl secretion

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Functional characterization of the sodium/hydrogen exchanger 8 and its role in proliferation of colonic epithelial cells

Intestinal NaCl, HCO₃^-, and fluid absorption are strongly dependent on apical Na⁺/H⁺ exchange. The intestine expresses three presumably apical sodium-hydrogen exchanger (NHE) isoforms: NHE2, NHE3, and NHE8. We addressed the role of NHE8 [solute carrier 9A8 (SLC9A8)] and its interplay with NHE2 (SLC9A2) in luminal proton extrusion during acute and chronic enterocyte acidosis and studied the differential effects of NHE8 and NHE2 on enterocyte proliferation. In contrast to NHE3, which was upregulated in differentiated versus undifferentiated colonoids, the expression of NHE2 and NHE8 remained constant during differentiation of colonoids and Caco2Bbe cells. Heterogeneously expressed Flag-tagged rat (r)Nhe8 and human (h)NHE8 translocated to the apical membrane of Caco2Bbe cells. rNhe8 and hNHE8, when expressed in NHE-deficient PS120 fibroblasts showed higher sensitivity to HOE642 compared to NHE2. Lentiviral shRNA knockdown of endogenous NHE2 in Caco2Bbe cells (C2Bbe/shNHE2) resulted in a decreased steady-state intracellular pH (pH_i), an increased NHE8 mRNA expression, and augmented NHE8-mediated apical NHE activity. Lentiviral shRNA knockdown of endogenous NHE8 in Caco2Bbe cells (C2Bbe/shNHE8) resulted in a decreased steady-state pH_i as well, accompanied by decreased NHE2 mRNA expression and activity, which together contributed to reduced apical NHE activity in the NHE8-knockdown cells. Chronic acidosis increased NHE8 but not NHE2 mRNA expression. Alterations in NHE2 and NHE8 expression/activity affected proliferation, with C2Bbe/shNHE2 cells having lower and C2Bbe/shNHE8 having higher proliferative capacity, accompanied by amplified ERK1/2 signaling pathway and increased EGFR expression in the latter cell line. Thus, both Na⁺/H⁺ exchangers have distinct functions during cellular homeostasis by triggering different signaling pathways to regulate cellular proliferation and pH_i control.

Mitotic motor CENP-E cooperates with PRC1 in temporal control of central spindle assembly

Error-free cell division depends on the accurate assembly of the spindle midzone from dynamic spindle microtubules to ensure chromatid segregation during metaphase-anaphase transition. However, the mechanism underlying the key transition from the mitotic spindle to central spindle before anaphase onset remains elusive. Given the prevalence of chromosome instability phenotype in gastric tumorigenesis, we developed a strategy to model context-dependent cell division using a combination of light sheet microscope and 3D gastric organoids. Light sheet microscopic image analyses of 3D organoids showed that CENP-E inhibited cells undergoing aberrant metaphase-anaphase transition and exhibiting chromosome segregation errors during mitosis. High-resolution real-time imaging analyses of 2D cell culture revealed that CENP-E inhibited cells undergoing central spindle splitting and chromosome instability phenotype. Using biotinylated syntelin as an affinity matrix, we found that CENP-E forms a complex with PRC1 in mitotic cells. Chemical inhibition of CENP-E in metaphase by syntelin prevented accurate central spindle assembly by perturbing temporal assembly of PRC1 to the midzone. Thus, CENP-E-mediated PRC1 assembly to the central spindle constitutes a temporal switch to organize dynamic kinetochore microtubules into stable midzone arrays. These findings reveal a previously uncharacterized role of CENP-E in temporal control of central spindle assembly. Since CENP-E is absent from yeast, we reasoned that metazoans evolved an elaborate central spindle organization machinery to ensure accurate sister chromatid segregation during anaphase and cytokinesis.

Adenosine: Direct and Indirect Actions on Gastric Acid Secretion

Composed by a molecule of adenine and a molecule of ribose, adenosine is a paradigm of recyclable nucleoside with a multiplicity of functions that occupies a privileged position in the metabolic and regulatory contexts. Adenosine is formed continuously in intracellular and extracellular locations of all tissues. Extracellular adenosine is a signaling molecule, able to modulate a vast range of physiologic responses in many cells and organs, including digestive organs. The adenosine A1, A2A, A2B, and A3 receptors are P1 purinergic receptors, G protein-coupled proteins implicated in tissue protection. This review is focused on gastric acid secretion, a process centered on the parietal cell of the stomach, which contains large amounts of H⁺/K⁺-ATPase, the proton pump responsible for proton extrusion during acid secretion. Gastric acid secretion is regulated by an extensive collection of neural stimuli and endocrine and paracrine agents, which act either directly at membrane receptors of the parietal cell or indirectly through other regulatory cells of the gastric mucosa, as well as mechanic and chemic stimuli. In this review, after briefly introducing these points, we condense the current body of knowledge about the modulating action of adenosine on the pathophysiology of gastric acid secretion and update its significance based on recent findings in gastric mucosa and parietal cells in humans and animal models.

MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.

Apical vacuole formation by gastric parietal cells in primary culture: effect of low extracellular Ca2+

In primary culture, the gastric parietal cell's deeply invaginated apical membrane, seen in microscopy by phalloidin binding to F-actin (concentrated in microvilli and a subapical web), is engulfed into the cell, separated from the basolateral membrane (which then becomes the complete plasma membrane), and converted, from a lacy interconnected system of canaliculi, into several separate vacuoles. In this study, vacuolar morphology was achieved by 71% of parietal cells 8 h after typical collagenase digestion of rabbit gastric mucosa, but the tight-junctional protein zonula occludens-1 (ZO-1) was completely delocalized after ∼2 h, when cells were ready for culturing. Use of low-Ca(2+) medium (4 mM EGTA) to release cells quickly from gastric glands yielded parietal cells in which ZO-1 was seen in a small spot or ring, a localization quickly lost if these cells were then cultured in normal Ca(2+) but remaining up to 20 h if they were cultured in low Ca(2+). The cells in low Ca(2+) mostly retained, at 20 h, an intermediate morphology of many bulbous canalicular expansions ("prevacuoles"), seemingly with narrow interconnections. Histamine stimulation of 20-h cells with intermediate morphology caused colocalization of proton-pumping H-K-ATPase with canaliculi and prevacuoles but little swelling of those structures, consistent with a remaining apical pore through which secreted acid could escape. Apparent canalicular interconnections, lack of stimulated swelling, and lingering ZO-1 staining indicate inhibition of membrane fission processes that separate apical from basolateral membrane and vacuoles from each other, suggesting an important role for extracellular Ca(2+) in these, and possibly other, endocytotic processes.

Functional transformation of gastric parietal cells and intracellular trafficking of ion channels/transporters in the apical canalicular membrane associated with acid secretion

The parietal cell of the gastric gland is a highly differentiated cell responsible for the gastric hydrochloric acid secretion into the lumen of the stomach. In response to stimulation of acid secretion, the parietal cells undergo well-characterized morphological transformations to recruit H⁺/K⁺-ATPase from the cytoplasmic tubulovesicles to the apical canalicular membrane. Besides H⁺ extrusion via H⁺/K⁺-ATPase, Cl⁻ efflux and K⁺ recycling across the apical canalicular membrane are necessary via chloride and potassium channels/transporters, respectively. In the last decade, a number of molecular candidates for the Cl⁻ efflux and K⁺ recycling have been identified in the apical canalicular membrane of the parietal cell. This review focuses on the functional transformation of gastric parietal cells and intracellular trafficking of ion channels/transporters expressed in the apical canalicular membrane associated with gastric acid secretion.

A role for the Ca2+ channel TRPML1 in gastric acid secretion, based on analysis of knockout mice

BACKGROUND & AIMS

Mutations in TRPML1, a lysosomal Ca(2+)-permeable TRP channel, lead to mucolipidosis type IV, a neurodegenerative lysosomal storage disease. An unusual feature of mucolipidosis type IV is constitutive achlorhydria. We produced Trpml1(-/-) (null) mice to investigate the requirement for this protein in gastric acid secretion.

METHODS

Trpml1-null mice were generated by gene targeting. The expression of Trpml1 and its role in acid secretion by gastric parietal cells were analyzed using biochemical, histologic, and ultrastructural approaches.

RESULTS

Trpml1 is expressed by parietal cells and localizes predominantly to the lysosomes; it was dynamically palmitoylated and dephosphorylated in vivo following histamine stimulation of acid secretion. Trpml1-null mice had significant impairments in basal and histamine-stimulated gastric acid secretion and markedly reduced levels of the gastric proton pump. Histologic and ultrastructural analyses revealed that Trpml1(-/-) parietal cells were enlarged, had multivesicular and multi-lamellated lysosomes, and maintained an abnormal intracellular canalicular membrane. The intralysosomal Ca(2+) content and receptor-mediated Ca(2+) signaling were, however, unaffected in Trpml1(-/-) gastric glands, indicating that Trpml1 does not function in the regulation of lysosomal Ca(2+).

CONCLUSIONS

Loss of Trpml1 causes reduced levels and mislocalization of the gastric proton pump and alters the secretory canaliculi, causing hypochlorhydria and hypergastrinemia. The lysosomal enlargement and defective intracellular canaliculi formation observed in Trpml1(-/-) parietal cells indicate that Trpml1 functions in the formation and trafficking of the tubulovesicles. This study provides direct evidence for the regulation of gastric acid secretion by a TRP channel; TRPML1 is an important protein in parietal cell apical membrane trafficking.

Helicobacter pylori and gastric cancer: factors that modulate disease risk

Helicobacter pylori is a gastric pathogen that colonizes approximately 50% of the world's population. Infection with H. pylori causes chronic inflammation and significantly increases the risk of developing duodenal and gastric ulcer disease and gastric cancer. Infection with H. pylori is the strongest known risk factor for gastric cancer, which is the second leading cause of cancer-related deaths worldwide. Once H. pylori colonizes the gastric environment, it persists for the lifetime of the host, suggesting that the host immune response is ineffective in clearing this bacterium. In this review, we discuss the host immune response and examine other host factors that increase the pathogenic potential of this bacterium, including host polymorphisms, alterations to the apical-junctional complex, and the effects of environmental factors. In addition to host effects and responses, H. pylori strains are genetically diverse. We discuss the main virulence determinants in H. pylori strains and the correlation between these and the diverse clinical outcomes following H. pylori infection. Since H. pylori inhibits the gastric epithelium of half of the world, it is crucial that we continue to gain understanding of host and microbial factors that increase the risk of developing more severe clinical outcomes.

Phospho-regulated ACAP4-Ezrin interaction is essential for histamine-stimulated parietal cell secretion

The ezrin-radixin-moesin proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton and also participate in signal transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the protein kinase A activation cascade to the regulated HCl secretion. Our recent proteomic study revealed a protein complex of ezrin-ACAP4-ARF6 essential for volatile membrane remodeling (Fang, Z., Miao, Y., Ding, X., Deng, H., Liu, S., Wang, F., Zhou, R., Watson, C., Fu, C., Hu, Q., Lillard, J. W., Jr., Powell, M., Chen, Y., Forte, J. G., and Yao, X. (2006) Mol. Cell Proteomics 5, 1437-1449). However, knowledge of whether ACAP4 physically interacts with ezrin and how their interaction is integrated into membrane-cytoskeletal remodeling has remained elusive. Here we provide the first evidence that ezrin interacts with ACAP4 in a protein kinase A-mediated phosphorylation-dependent manner through the N-terminal 400 amino acids of ACAP4. ACAP4 locates in the cytoplasmic membrane in resting parietal cells but translocates to the apical plasma membrane upon histamine stimulation. ACAP4 was precipitated with ezrin from secreting but not resting parietal cell lysates, suggesting a phospho-regulated interaction. Indeed, this interaction is abolished by phosphatase treatment and validated by an in vitro reconstitution assay using phospho-mimicking ezrin(S66D). Importantly, ezrin specifies the apical distribution of ACAP4 in secreting parietal cells because either suppression of ezrin or overexpression of non-phosphorylatable ezrin prevents the apical localization of ACAP4. In addition, overexpressing GTPase-activating protein-deficient ACAP4 results in an inhibition of apical membrane-cytoskeletal remodeling and gastric acid secretion. Taken together, these results define a novel molecular mechanism linking ACAP4-ezrin interaction to polarized epithelial secretion.

Volume density, distribution, and ultrastructure of secretory and basolateral membranes and mitochondria predict parietal cell secretory (dys)function

Acid secretion in gastric parietal cells requires highly coordinated membrane transport and vesicle trafficking. Histologically, consensus defines acid secretion as the ratio of the volume density (Vd) of canalicular and apical membranes (CAMs) to tubulovesicular (TV) membranes, a value which varies widely under normal conditions. Examination of numerous achlorhydric mice made it clear that this paradigm is discrepant when used to assess most mice with genetic mutations affecting acid secretion. Vd of organelles in parietal cells of 6 genetically engineered mouse strains was obtained to identify a stable histological phenotype of acid secretion. We confirmed that CAM to TV ratio fairly represented secretory activity in untreated and secretion-inhibited wild-type (WT) mice and in NHE2−/− mice as well, though the response was significantly attenuated in the latter. However, high CAM to TV ratios wrongly posed as active acid secretion in AE2−/−, GHKAα−/−, and NHE4−/− mice. Achlorhydric genotypes also had a significantly higher Vd of basolateral membrane than WT mice, and reduced Vd of mitochondria and canaliculi. The Vd of mitochondria, and ratio of the Vd of basolateral membranes/Vd of mitochondria were preferred predictors of the level of acid secretion. Alterations in acid secretion, then, cause significant changes not only in the Vd of secretory membranes but also in mitochondria and basolateral membranes.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Lasp1 gene disruption is linked to enhanced cell migration and tumor formation

Lasp1 is an actin-binding, signaling pathway-regulated phosphoprotein that is overexpressed in several cancers. siRNA knockdown in cell lines retards cell migration, suggesting the possibility that Lasp1 upregulation influences cancer metastasis. Herein, we utilized a recently developed gene knockout model to assess the role of Lasp1 in modulating nontransformed cell functions. Wound healing and tumor initiation progressed more rapidly in Lasp1(-/-) mice compared with Lasp1(+/+) controls. Embryonic fibroblasts (MEFs) derived from Lasp1(-/-) mice also migrated more rapidly in vitro. These MEFs characteristically possessed increased focal adhesion numbers and displayed more rapid attachment compared with wild-type MEFs. Differential microarray analyses revealed alterations in message expression for proteins implicated in cell migration, adhesion, and cytoskeletal organization. Notably, the focal adhesion protein, lipoma preferred partner (LPP), a zyxin family member and putative Lasp1 binding protein, was increased about twofold. Because LPP gene disruption reduces cell migration, we hypothesize that LPP plays a role in enhancing the migratory capacity of Lasp1(-/-) MEFs, perhaps by modifying the subcellular localization of other motility-associated proteins. The striking contrast in the functional effects of loss of Lasp1 in innate cells compared with cell lines reveals distinct differences in mechanisms of motility and attachment in these models.

Helicobacter pylori VacA disrupts apical membrane-cytoskeletal interactions in gastric parietal cells

Helicobacter pylori persistently colonize the human stomach and have been linked to atrophic gastritis and gastric carcinoma. Although it is well known that H. pylori infection can result in hypochlorhydria, the molecular mechanisms underlying this phenomenon remain poorly understood. Here we show that VacA permeabilizes the apical membrane of gastric parietal cells and induces hypochlorhydria. The functional consequences of VacA infection on parietal cell physiology were studied using freshly isolated rabbit gastric glands and cultured parietal cells. Secretory activity of parietal cells was judged by an aminopyrine uptake assay and confocal microscopic examination. VacA permeabilization induces an influx of extracellular calcium, followed by activation of calpain and subsequent proteolysis of ezrin at Met(469)-Thr(470), which results in the liberation of ezrin from the apical membrane of the parietal cells. VacA treatment inhibits acid secretion by preventing the recruitment of H,K-ATPase-containing tubulovesicles to the apical membrane of gastric parietal cells. Electron microscopic examination revealed that VacA treatment disrupts the radial arrangement of actin filaments in apical microvilli due to the loss of ezrin integrity in parietal cells. Significantly, expression of calpain-resistant ezrin restored the functional activity of parietal cells in the presence of VacA. Proteolysis of ezrin in VacA-infected parietal cells is a novel mechanism underlying H. pylori-induced inhibition of acid secretion. Our results indicate that VacA disrupts the apical membrane-cytoskeletal interactions in gastric parietal cells and thereby causes hypochlorhydria.

Targeted disruption of the Lasp-1 gene is linked to increases in histamine-stimulated gastric HCl secretion

Lasp-1 (LIM and SH3 domain protein 1) is a multidomain actin-binding protein that is differentially expressed within epithelial tissues and brain. In the gastric mucosa, Lasp-1 is highly expressed in the HCl-secreting parietal cell, where it is prominently localized within the F-actin-rich subcellular regions. Histamine-induced elevation of parietal cell [cAMP]i increases Lasp-1 phosphorylation, which is correlated with activation of HCl secretion. To determine whether Lasp-1 is involved in the regulation of HCl secretion in vivo, we generated a murine model with a targeted disruption of the Lasp-1 gene. Lasp-1-null mice had slightly lower body weights but developed normally and had no overt phenotypic abnormalities. Basal HCl secretion was unaffected by loss of Lasp-1, but histamine stimulation induced a more robust acid secretory response in Lasp-1-null mice compared with wild-type littermates. A similar effect of histamine was observed in isolated gastric glands on the basis of measurements of accumulation of the weak base [14C]aminopyrine. In addition, inhibition of the acid secretory response to histamine by H2 receptor blockade with ranitidine proceeded more slowly in glands from Lasp-1-null mice. These findings support the conclusion that Lasp-1 is involved in the regulation of parietal HCl secretion. We speculate that cAMP-dependent phosphorylation of Lasp-1 alters interactions with F-actin and/or endocytic proteins that interact with Lasp-1, thereby regulating the trafficking/activation of the H+, K+-ATPase (proton pump).

Exfoliation of gastric pit-parietal cells into the gastric lumen associated with a stimulation of isolated rat gastric mucosa in vitro: a morphological study by the application of cryotechniques

It is clinicopathologically important to elucidate the cell kinetics for the maintenance of normal gastric epithelium. In a rat gastric mucosa isolated after stimulation, a number of cells were exfoliated into the gastric lumen of the pit region. The present study was undertaken to clarify the origin of exfoliated cells and their histochemical profiles by taking the advantages of cryotechniques. As results, most of the exfoliated cells were identified as pit-parietal cells labeled with both peanut-lectin and anti-H+/K+-ATPase antibody. Quantitative analysis verified a time-dependent increase in the number of exfoliated cells in the gastric mucosa isolated after stimulation. The exfoliated cells exhibited a diffuse intracellular staining for E-cadherin, suggesting a dissociation of the adhesion molecule prior to the cell exfoliation. It should be noted that most of the exfoliated cells were negative to the apoptotic markers (TUNEL staining and caspase-3). Ultrastructurally, autophagosome-like structures consisting of H+/K+-ATPase positive membranes were frequently seen in the exfoliated pit-parietal cells. In addition, the pit-parietal cell exfoliation was accompanied by sealing of their basal portion with the cytoplasmic processes of adjacent surface mucous cells. The present morphological findings provide a new insight into the cell kinetics in the gastric epithelium in vitro.

Molecular dissection of HCl secretion in gastric parietal cells using streptolysin O permeabilization

Histamine-stimulated gastric acid secretion involves a transient elevation of intracellular Ca(2+) and the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) cascade through phosphorylation, the actions of which ultimately result in the fusion of vesicles containing H,K-ATPase (adenosine triphosphatase) to the apical plasma membrane of parietal cells. To dissect the signaling events underlying gastric acid secretion, we have developed a permeabilized gastric gland model using streptolysin O (SLO). The advantage of this model is its ability to retain cytosolic components that are required for the secretory machinery while granting accessibility for the introduction of macromolecules into the cytoplasm. Our studies showed that acid secretion in SLO-permeabilized glands is a cAMP-dependent process and involves the recruitment of H,K-ATPase-rich tubulovesicles into the apical plasma membrane as judged by biochemical assays. These studies established a functional permeabilized gland model in which the resting-to-secreting transition can be triggered by second messengers, while the manipulation of the cytoplasmic environment can be achieved with ease.

Influence of gastric acid on susceptibility to infection with ingested bacterial pathogens

Despite the widely held belief that gastric acid serves as a barrier to bacterial pathogens, there are almost no experimental data to support this hypothesis. We have developed a mouse model to quantify the effectiveness of gastric acid in mediating resistance to infection with ingested bacteria. Mice that were constitutively hypochlorhydric due to a mutation in a gastric H(+)/K(+)-ATPase (proton pump) gene were infected with Yersinia enterocolitica, Salmonella enterica serovar Typhimurium, Citrobacter rodentium, or Clostridium perfringens cells or spores. Significantly greater numbers of Yersinia, Salmonella, and Citrobacter cells (P < OR = 0.006) and Clostridium spores (P = 0.02) survived in hypochlorhydric mice, resulting in reduced median infectious doses. Experiments involving intraperitoneal infection or infection of mice treated with antacids indicated that the increased sensitivity of hypochlorhydric mice to infection was entirely due to the absence of stomach acid. Apart from establishing the role of gastric acid in nonspecific immunity to ingested bacterial pathogens, our model provides an excellent system with which to investigate the effects of hypochlorhydria on susceptibility to infection and to evaluate the in vivo susceptibility to gastric acid of orally administered therapies, such as vaccines and probiotics.

A mechanism of Munc18b-syntaxin 3-SNAP25 complex assembly in regulated epithelial secretion

Syntaxin and Munc18 are essential for regulated exocytosis in all eukaryotes. It was shown that Munc18 inhibition of neuronal syntaxin 1 can be overcome by CDK5 phosphorylation, indicating that structural change disrupts the syntaxin-Munc18 interaction. Here, we show that this phosphorylation promotes the assembly of Munc18b-syntaxin 3-SNAP25 tripartite complex and membrane fusion machinery SNARE. Using siRNAs to screen for genes required for regulated epithelial secretion, we identified the requirements of CDK5 and Munc18b in cAMP-dependent gastric acid secretion. Biochemical characterization revealed that Munc18b bears a syntaxin 3-selective binding site located at its most C-terminal 53 amino acids. Significantly, the phosphorylation of Thr572 by CDK5 attenuates Munc18b-syntaxin 3 interaction and promotes formation of Munc18b-syntaxin 3-SNAP25 tripartite complex, leading to an assembly of functional Munc18b-syntaxin 3-SNAP25-VAMP2 membrane fusion machinery. Thus, our studies suggest a novel regulatory mechanism in which phosphorylation of Munc18b operates vesicle docking and fusion in regulated exocytosis.

Pharmacological profile of novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methyl cyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526)

The pharmacological profiles of the novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methylcyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526) were investigated in terms of hog gastric H+,K+-ATPase activity, gastric acid secretion, and acute gastroesophageal lesions in comparison with other proton pump inhibitors (PPIs). CS-526 inhibited H+,K+-ATPase activity in a concentration-dependent manner, with an IC50 value of 61 nM. The inhibitory effect of CS-526 on H+,K+-ATPase activity was more potent than that of any of the other PPIs examined. The inhibitory mechanism of CS-526 on H+,K+-ATPase was a competitive antagonism to the K+ binding site of H+,K+-ATPase, and it was also a reversible inhibition. In pylorus-ligated rats, intraduodenal or oral administration of CS-526 inhibited gastric acid secretion in a dose-dependent manner, and the ID50 values were 2.8 or 0.7 mg/kg, respectively. In Heidenhain pouch dogs, intrapouch administration of CS-526 inhibited histamine-stimulated gastric acid secretion in a dose- and retention time-dependent manner. In a reflux esophagitis model, intraduodenal and oral administration of CS-526 prevented esophageal lesions with ID50 values of 5.4 and 1.9 mg/kg, respectively. Lansoprazole prevented esophagitis only by intraduodenal administration (ID50 = 2.2 mg/kg). Furthermore, CS-526 inhibited acute gastric mucosal lesions. These data demonstrate that the novel acid pump antagonist CS-526 has potent antisecretory and antiulcer effects. These findings indicate that CS-526 would have a curative effect on gastroesophageal reflux disease via its potent antisecretory and antiulcer actions.

Role of endosomal Na+-K+-ATPase and cardiac steroids in the regulation of endocytosis

Plasma membrane Na(+)-K(+)-ATPase, which drives potassium into and sodium out of the cell, has important roles in numerous physiological processes. Cardiac steroids (CS), such as ouabain and bufalin, specifically interact with the pump and affect ionic homeostasis, signal transduction, and endocytosed membrane traffic. CS-like compounds are present in mammalian tissues, synthesized in the adrenal gland, and considered to be new family of steroid hormones. In this study, the mechanism of Na(+)-K(+)-ATPase involvement in the regulation of endocytosis is explored. We show that the effects of various CS on changes in endosomal pH are mediated by the pump and correspond to their effects on endosomal membrane traffic. In addition, it was found that CS-induced changes in endocytosed membrane traffic were dependent on alterations in [Na(+)] and [H(+)] in the endosome. Furthermore, we show that various CS differentially regulate endosomal pH and membrane traffic. The results suggest that these differences are due to specific binding characteristics. Based on our observations, we propose that Na(+)-K(+)-ATPase is a key player in the regulation of endosomal pH and endocytosed membrane traffic. Furthermore, our results raise the possibility that CS-like hormones regulate differentially intracellular membrane traffic.

Real time measurements of water flow in amphibian gastric glands: modulation via the extracellular Ca2+-sensing receptor

The mechanisms for the formation of the osmotic gradient driving water movements in the gastric gland and its modulation via the extracellular Ca(2+)-sensing receptor (CaR) were investigated. Real time measurements of net water flux in the lumen of single gastric glands of the intact amphibian stomach were performed using ion-selective double-barreled microelectrodes. Water movement was measured by recording changes in the concentration of impermeant TEA(+) ions ([TEA(+)](gl)) with TEA(+)-sensitive microelectrodes inserted in the lumen of individual gastric glands. Glandular K(+) (K(+)(gl)) and H(+) (pH(gl)) were also measured by using K(+)- and H(+)-sensitive microelectrodes, respectively. Stimulation with histamine significantly decreased [TEA](gl), indicating net water flow toward the gland lumen. This response was inhibited by the H(+)/K(+)-ATPase inhibitor, SCH 28080. Histamine also elicited a significant and reversible increase in [K(+)](gl) that was blocked by chromanol 293B, a blocker of KCQN1 K(+) channels. Histamine failed to induce net water flow in the presence of chromanol 293B. In the "resting state," stimulation of CaR with diverse agonists resulted in significant increase in [TEA](gl). CaR activation also significantly reduced histamine-induced water secretion and apical K(+) transport. Our data validate the strong link between histamine-stimulated acid secretion and water transport. We also show that cAMP-dependent [K(+)](gl) elevation prior to the onset of acid secretion generates the osmotic gradient initially driving water into the gastric glands and that CaR activation inhibits this process, probably through reduction of intracellular cAMP levels.

ΔF508 Mutation Results in Impaired Gastric Acid Secretion

The cystic fibrosis transmembrane conductance regulator (CFTR) is recognized as a multifunctional protein that is involved in Cl(-) secretion, as well as acting as a regulatory protein. In order for acid secretion to take place a complex interaction of transport proteins and channels must occur at the apical pole of the parietal cell. Included in this process is at least one K(+) and Cl(-) channel, allowing for both recycling of K(+) for the H,K-ATPase, and Cl(-) secretion, necessary for the generation of concentrated HCl in the gastric gland lumen. We have previously shown that an ATP-sensitive potassium channel (K(ATP)) is expressed in parietal cells. In the present study we measured secretagogue-induced acid secretion from wild-type and DeltaF508-deficient mice in isolated gastric glands and whole stomach preparations. Secretagogue-induced acid secretion in wild-type mouse gastric glands could be significantly reduced with either glibenclamide or the specific inhibitor CFTR-inh172. In DeltaF508-deficient mice, however, histamine-induced acid secretion was significantly less than in wild-type mice. Furthermore, immunofluorescent localization of sulfonylurea 1 and 2 failed to show expression of a sulfonylurea receptor in the parietal cell, thus further implicating CFTR as the ATP-binding cassette transporter associated with the K(ATP) channels. These results demonstrate a regulatory role for the CFTR protein in normal gastric acid secretion.

Understanding ERM proteins--the awesome power of genetics finally brought to bear

In epithelial cells, the Ezrin, Radixin and Moesin (ERM) proteins are involved in many cellular functions, including regulation of actin cytoskeleton, control of cell shape, adhesion and motility, and modulation of signaling pathways. However, discerning the specific cellular roles of ERMs has been complicated by redundancy between these proteins. Recent genetic studies in model organisms have identified unique roles for ERM proteins. These include the regulation of morphogenesis and maintenance of integrity of epithelial cells, stabilization of intercellular junctions, and regulation of the Rho small GTPase. These studies also suggest that ERMs have roles in actomyosin contractility and vesicular trafficking in the apical domain of epithelial cells. Thus, genetic analysis has enhanced our understanding of these widely expressed membrane-associated proteins.

Characteristics of the K+-competitive H+,K+-ATPase inhibitor AZD0865 in isolated rat gastric glands

The gastric H+,K+-ATPase of the parietal cell is responsible for acid secretion in the stomach and is the main target in the pharmacological treatment of acid-related diseases. Omeprazole and other benzimidazole drugs, although having delayed efficacy if taken orally, have high success rates in the treatment of peptic ulcer disease. Potassium competitive acid blockers (P-CAB) compete with K+ for binding to the H+,K+-ATPase and thereby they inhibit acid secretion. In this study, the in vitro properties of AZD0865, a reversible H+,K+-ATPase inhibitor of gastric acid secretion, are described. We used a digital-imaging system and the pH sensitive dye BCECF to observe proton efflux from hand-dissected rat gastric glands. Glands were stimulated with histamine (100 microM) and exposed to a bicarbonate- and Na+-free perfusate to induce an acid load. H+,K+-ATPase inhibition was determined by calculating pHi recovery (dpH/dT) in the presence of omeprazole (10-200 microM) or AZD0865 (0.01-100 microM). The efficacies of both drugs were compared. Our data show that acid secretion is inhibited by both the proton pump inhibitor omeprazole and the P-CAB AZD0865. Complete inhibition of acid secretion by AZD0865 had a rapid onset of activation, was reversible, and occurred at a 100-fold lower dose than omeprazole (1 microM AZD0865 vs. 100 microM omeprazole). This study demonstrates that AZD0865 is a potent, fast-acting inhibitor of gastric acid secretion, effective at lower concentrations than drugs of the benzimidazole class. Therefore, these data strongly suggest that AZD0865 has great potential as a fast-acting, low-dose inhibitor of acid secretion.

Brush border spectrin is required for early endosome recycling in Drosophila

An apical brush border is a characteristic of many mature epithelia. This dynamic structure consists of dense microvilli supported by F-actin bundles that protrude into the apical cytoplasm, where they are crosslinked by spectrin and myosin II to form the terminal web. Little is known about the terminal web, through which vesicles transit to and from the apical membrane. Analysis of mutations in beta(Heavy)-spectrin, the Drosophila brush border spectrin, reveals that this protein is necessary for the maintenance of Rab5 endosomes in the midgut. As a consequence, an apical H+ V-ATPase that is probably responsible for lumenal acidification is lost both from the brush border and Rab5 endosomes. Epistasis tests indicate that beta(Heavy)-spectrin is required during endocytosis after Dynamin and before Rab5-mediated endosome activities. These data are consistent with the location of spectrin in the terminal web, and suggest that this molecule is required for correct sorting decisions at the early endosome.

PKA-mediated protein phosphorylation regulates ezrin–WWOX interaction

The ezrin-radixin-moesin proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton, and also participate in signal-transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the cAMP-dependent protein kinase activation cascade to the regulated HCl secretion in gastric parietal cells. Our recent studies have mapped the PKA-mediated phosphorylation site to Ser(66) and established its functional role in parietal cell activation [R. Zhou et al., Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation, J. Biol. Chem. 278 (2003) 35651-35659], but the underlying basis for this regulation is not known. Here, we provide the first evidence that PKA-mediated phosphorylation of Ser(66)regulates the interaction of ezrin with WWOX, a WW domain-containing protein. Our biochemical study reveals that ezrin directly binds to the first WW domain of WWOX via its C-terminal tyrosine-containing polyproline sequence (470)PPPPPPVY(477). Mutational analyses further demonstrate that tyrosine(477) is essential for the ezrin-WWOX interaction. In addition, our study shows that PKA-mediated phosphorylation of ezrin is essential and sufficient for the apical localization of WWOX protein as disruption of ezrin-WWOX interaction eliminated the apical localization of WWOX. Finally, our study demonstrates the essential role of ezrin-WWOX interaction in the apical membrane remodeling associated with H,K-ATPase recruitment. Taken together, these results define a novel molecular mechanism underlying phospho-regulation of ezrin function by PKA in parietal cell activation.

The cryofixation of isolated rat gastric mucosa provides new insights into the functional transformation of gastric parietal cells: an in vitro experimental model study

Cryofixation is currently accepted as the best initial fixation step to preserve not only the fine structure but also the antigenicity of biological samples. To elucidate the functional transformation of gastric parietal cells, we have newly developed an in vitro experimental model, named the isolated gastric mucosa. In this study, acid secretion of the parietal cell was stimulated with histamine or inhibited with cimetidine, and the samples were cryofixed by plunge freezing for light microscopy or high-pressure freezing for electron microscopy. As a result, the organization of glandular cells was well-preserved and quite similar to freshly excised rat gastric mucosa for at least 2 h after isolation. Immunohistochemistry of H+/K+-ATPase demonstrated a translocation of H+/K+-ATPase from the cytoplasm to the apical membrane associated with histamine-stimulation. In cimetidine-treated mucosa, most of the parietal cells were morphologically in the resting state, showing numerous tubulovesicles in their cytoplasm. In contrast, histamine-stimulated parietal cells exhibited well-developed intracellular canaliculi lined with long microvilli. To the best of our knowledge, the present study is first to demonstrate an electron micrograph that strongly suggests a membrane fusion between the tubulovescile and the apical membrane. Moreover, a stimulation-associated translocation of ezrin was clearly shown from the cytoplasm to the apical region, corresponding to apical microvilli development in the isolated gastric mucosa model. We here describe the preparation of the isolated rat gastric mucosa model, which provides new insights into the functional transformation of parietal cells by the application of cryotechniques.

ECL cell histamine mobilization and parietal cell stimulation in the rat stomach studied by microdialysis and electron microscopy

AIM

Gastrin stimulates acid secretion by mobilizing histamine from enterochromaffin-like (ECL) cells that occur predominantly at the base of the gastric glands. The parietal cells occur higher up in the glands nearer to the gastric lumen. The present study was performed to assess whether histamine is transported from the ECL cell via the microcirculation (endocrine route) or local diffusion (paracrine route).

METHODS

Totally isolated, vascularly perfused, rat stomachs were examined both in basal and gastrin-stimulated state. Histamine concentrations, determined by radioimmunoassay in venous effluent and microdialysate from an indwelling probe in the submucosa, were monitored over a period of 240 min. Gastrin-17 was infused through an arterial catheter for 120 min. The parietal cells were examined by electron microscopy, and the percentage of actively secreting parietal cells (displaying secretory canaliculi) in four regions along the glands (basal to surface, zones I-IV) was determined.

RESULTS

Gastrin stimulated acid secretion and histamine release as well as parietal cell activation. Upon gastrin stimulation, histamine concentration in the microdialysate was 2.5-fold higher than in the venous effluent (P = 0.008). The parietal cells in the upper part of the gland (zone III) were found to be activated the most.

CONCLUSION

As the histamine concentrations were higher in the tissue (microdialysate) than in blood, histamine seems to reach the parietal cells via the paracrine route. The fraction of active parietal cells seems to depend more on the age of the parietal cells than on the distance from the ECL cell.

Drebrin E2 is differentially expressed and phosphorylated in parietal cells in the gastric mucosa

Developmentally regulated brain proteins (drebrins) are highly expressed in brain where they may regulate actin filament formation in dendritic spines. Recently, the drebrin E2 isoform was detected in certain epithelial cell types including the gastric parietal cell. In gastric parietal cells, activation of HCl secretion is correlated with actin filament formation and elongation within intracellular canaliculi, which are the sites of acid secretion. The aim of this study was to define the pattern of drebrin expression in gland units in the intact rabbit oxyntic gastric mucosa and to initiate approaches to define the functions of this protein in parietal cells. Drebrin E2 expression was limited entirely or almost entirely to parietal cells and depended upon the localization of parietal cells along the gland axis. Rabbit drebrin E2 was cloned and found to share 86% identity with human drebrin 1a and to possess a number of cross-species conserved protein-protein interaction and phosphorylation consensus sites. Two-dimensional Western blot and phosphoaffinity column analyses confirmed that drebrin is phosphorylated in parietal cells, and several candidate phosphorylation sites were identified by mass spectrometry. Overexpression of epitope-tagged drebrin E2 led to the formation of microspikes and F-actin-rich ring-like structures in cultured parietal cells and suppressed cAMP-dependent acid secretory responses. In Madin-Darby canine kidney cells, coexpression of epitope-tagged drebrin and the Rho family GTPase Cdc42, which induces filopodial extension, produced an additive increase in the length of microspike projections. Coexpression of dominant negative Cdc42 with drebrin E2 did not prevent drebrin-induced microspike formation. These findings suggest that 1) drebrin can induce the formation of F-actin-rich membrane projections by Cdc42-dependent and -independent mechanisms; and that 2) drebrin plays an active role in directing the secretagogue-dependent formation of F-actin-rich filaments on the parietal cell canalicular membrane. Finally, the differential distribution of drebrin in parietal cells along the gland axis suggests that drebrin E2 may be an important marker of parietal cell differentiation and functionality.

Cellular Localization and Stimulation-Associated Distribution Dynamics of Syntaxin-1 and Syntaxin-3 in Gastric Parietal Cells

Syntaxins are differentially localized in polarized cells and play an important role in vesicle trafficking and membrane fusion. These soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are believed to be involved in tubulovesicle trafficking and membrane fusion during the secretory cycle of the gastric parietal cell. We examined the cellular localization and distribution of syntaxin-1 and syntaxin-3 in rabbit parietal cells. Fractionation of gastric epithelial cell membranes showed that syntaxin-1 was more abundant in a fraction enriched in apical plasma membranes, whereas syntaxin-3 was found predominantly in the H,K-ATPase-rich tubulovesicle fraction. We also examined the cellular localization of syntaxins in cultured parietal cells. Parietal cells were infected with CFP-syntaxin-1 and CFP-syntaxin-3 adenoviral constructs. Fluorescence microscopy of live and fixed cells demonstrated that syntaxin-1 was primarily on the apical membrane vacuoles of infected cells, but there was also the expression of syntaxin-1 in a subadjacent cytoplasmic compartment. In resting, non-secreting parietal cells, syntaxin-3 was distributed throughout the cytoplasmic compartment; after stimulation, syntaxin-3 translocated to the apical membrane vacuoles, there co-localizing with H,K-ATPase, syntaxin-1 and F-actin. The differential location of these syntaxin isoforms in gastric parietal cells suggests that these proteins may be critical for maintaining membrane compartment identity and that they may play important, but somewhat different, roles in the membrane recruitment processes associated with secretory activation.

Potassium-competitive acid blockade: a new therapeutic strategy in acid-related diseases

Current therapies to treat gastroesophageal reflux disease (GERD), peptic ulcer disease (PUD), and other acid-related diseases either prevent stimulation of the parietal cell (H2 receptor antagonists, H2RAs) or inhibit gastric H+,K+-ATPase (e.g., proton pump inhibitors, PPIs). Of the 2 approaches, the inhibition of the final step in acid production by PPIs provides more effective relief of symptoms and healing. Despite the documented efficacy of the PPIs, therapeutic doses have a gradual onset of effect and do not provide complete symptom relief in all patients. There is scope for further improvements in acid suppressive therapy to maximize healing and offer more complete symptom relief. It is unlikely that cholecystokinin2 (CCK2, gastrin) receptor antagonists, a class in clinical trials, will be superior to H2RAs or PPIs. However, a new class of acid suppressant, the potassium-competitive acid blockers (P-CABs), is undergoing clinical trials in GERD and other acid-related diseases. These drugs block gastric H+,K+-ATPase by reversible and K+-competitive ionic binding. After oral doses, P-CABs rapidly achieve high plasma concentrations and have linear, dose-dependent pharmacokinetics. The pharmacodynamic properties reflect the pharmacokinetics of this group (i.e., the effect on acid secretion is correlated with plasma concentrations). These agents dose dependently inhibit gastric acid secretion with a fast onset of action and have similar effects after single and repeated doses (i.e., full effect from the first dose). Animal studies comparing P-CABs with PPIs suggest some important pharmacodynamic differences (e.g., faster and better control of 24-hr intragastric acidity). Studies in humans comparing PPIs with P-CABs will help to define the place of this new class in the management of acid-related diseases.

Achlorhydria by ezrin knockdown: defects in the formation/expansion of apical canaliculi in gastric parietal cells

Loss of gastric acid secretion is pathologically known as achlorhydria. Acid-secreting parietal cells are characterized by abundant expression of ezrin (Vil2), one of ezrin/radixin/moesin proteins, which generally cross-link actin filaments with plasma membrane proteins. Here, we show the direct in vivo involvement of ezrin in gastric acid secretion. Ezrin knockout (Vil2^−/−) mice did not survive >1.5 wk after birth, making difficult to examine gastric acid secretion. We then generated ezrin knockdown (Vil2^kd/kd) mice by introducing a neomycin resistance cassette between exons 2 and 3. Vil2^kd/kd mice born at the expected Mendelian ratio exhibited growth retardation and a high mortality. Approximately 7% of Vil2^kd/kd mice survived to adulthood. Ezrin protein levels in Vil2^kd/kd stomachs decreased to <5% of the wild-type levels without compensatory up-regulation of radixin or moesin. Adult Vil2^kd/kd mice suffered from severe achlorhydria. Immunofluorescence and electron microscopy revealed that this achlorhydria was caused by defects in the formation/expansion of canalicular apical membranes in gastric parietal cells.

PALS1 specifies the localization of ezrin to the apical membrane of gastric parietal cells

The ERM (ezrin/radixin/moesin) proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton and also participate in signal transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the protein kinase A activation cascade to regulated HCl secretion in gastric parietal cells. Here, we show that the integrity of ezrin is essential for parietal cell activation and provide the first evidence that ezrin interacts with PALS1, an evolutionarily conserved PDZ and SH3 domain-containing protein. Our biochemical study verifies that ezrin binds to PALS1 via its N terminus and is co-localized with PALS1 to the apical membrane of gastric parietal cells. Furthermore, our study shows that PALS1 is essential for the apical localization of ezrin, as either suppression of PALS1 protein accumulation or deletion of the PALS1-binding domain of ezrin eliminated the apical localization of ezrin. Finally, our study demonstrates the essential role of ezrin-PALS1 interaction in the apical membrane remodeling associated with parietal cell secretion. Taken together, these results define a novel molecular mechanism linking ezrin to the conserved apical polarity complexes and their roles in polarized epithelial secretion of gastric parietal cells.

Gastric parietal cell acid secretion in mice can be regulated independently of H/K ATPase endocytosis

BACKGROUND & AIMS

Gastric parietal cells secrete acid into the lumen of the stomach. They express a proton pump, the gastric H(+)/K(+) ATPase, the activity of which is tightly regulated. The H(+)/K(+) ATPase traffics between an intracytoplasmic compartment (tubulovesicles) in quiescent parietal cells and the apical plasma membrane in activated cells. These trafficking events are considered to contribute to the control of acid secretion by modulating access to apical K(+) and Cl(-) conductances that are required for transmembrane H(+) ion transport by the H(+)/K(+) ATPase. Here, we have determined whether the control of acid secretion in vivo requires membrane trafficking of the H(+)/K(+) ATPase.

METHODS

We developed mice that only express an H(+)/K(+) ATPase beta subunit in which a putative tyrosine-based endocytosis motif in the cytoplasmic tail is mutated. Location of the H(+)/K(+) ATPase and parietal cell ultrastructure and gastric acid secretion were then examined.

RESULTS

Parietal cells of these mice lacked a tubulovesicular compartment, and the H(+)/K(+) ATPase was resident exclusively on the apical plasma membrane. Despite the inability of the H(+)/K(+) ATPase to be endocytosed, the gastric acid secretory response to histamine or an antagonist was very similar to that of wild-type mice, indicating that control of H(+)/K(+) ATPase activity can occur independently of intracellular trafficking.

CONCLUSIONS

We were able to dissociate the regulation of H(+)/K(+) ATPase activity from intracellular trafficking of the protein. Thus, it is likely that direct regulation of apical K(+) and Cl(-) conductances are sufficient to control gastric acid secretion.

High-pressure freezing of isolated gastric glands provides new insight into the fine structure and subcellular localization of H+/K+-ATPase in gastric parietal cells

High-pressure freezing (HPF) is currently the most reliable method to obtain an adequately frozen sample for high-resolution morphological evaluation. Here we applied the HPF technique to isolated rabbit gastric glands to reveal structural evidence that may be correlated with functional activity of gastric parietal cells. This approach provided well-preserved fine structure and excellent antigenicity of several parietal cell proteins. Microtubules were abundant in the cytoplasm and frequently appeared to be associating with tubulovesicles. Interestingly, many electron-dense coated vesicles were apparent around the intracellular canaliculi (IC) of resting parietal cells, consistent with active membrane retrieval from the apical membranes. Immunolabeling of H+/K+-ATPase was evident on the endocytic components (e.g., multivesicular bodies) and tubulovesicles. After histamine stimulation, the parietal cells characteristically showed expanded IC membranes with varied features of their apical microvilli. The labeling density of H+/K+-ATPase was four-fold higher on the IC membrane of stimulated parietal cells than on that of resting parietal cells. Immunolabeling of ezrin was clearly identified on the IC and basolateral membranes of parietal cells, corresponding to their F-actin-rich sites. The present findings provide a new insight into the correlation of cell structure and function in gastric parietal cells.

Role of Rho in rabbit parietal cell

Rho is known as an important regulator of actin microfilament formation. We were led to study it because a dynamic rearrangement of actin filaments occurs during activation of gastric acid secretion. In order to use specific probes, the rabbit gastric gland culture system was employed and the various genes were expressed using adenovirus vector. When the constitutive active mutant of Rho (RhoAV14) was expressed, histamine- or carbachol-stimulated acid secretion monitored by (14)C-aminopyrine accumulation was inhibited. Conversely, expression of C3 toxin, the specific inhibitor of Rho, and expression of G(12/13)-specific regulator of G-protein signaling domain, the specific inhibitor of G(12/13) which is considered to be an upstream mediator of Rho, both potentiated acid secretion stimulated by the agonists. F-actin staining of parietal cell expressing RhoAV14 revealed that the microfilament supporting the intracellular canaliculi (not on the basolateral membrane) almost disappeared. No clear changes in the intracellular localization of Rho were observed during stimulation of parietal cell. In resting glands, the endogenous active form of Rho was relatively high, and it decreased during histamine stimulation. The finding that any treatment which inhibit Rho augment acid secretion whereas those that activate Rho inhibit secretion strongly suggests that the Rho-pathway conducts a negatively regulating signal in parietal cell activation, possibly via site-specific regulation of actin microfilaments.

Rab11b resides in a vesicular compartment distinct from Rab11a in parietal cells and other epithelial cells

The Rab11 family of small GTPases is composed of three members, Rab11a, Rab11b, and Rab25. While recent work on Rab11a and Rab25 has yielded some insights into their function, Rab11b has received little attention. Therefore, we sought to examine the distribution of endogenous Rab11b in epithelial cells. In rabbit gastric parietal cells, unlike Rab11a, Rab11b did not colocalize or coisolate with H(+)/K(+)-ATPase. In MDCK cells, endogenous Rab11b localized to an apical pericentrisomal region distinct from Rab11a. The microtubule agents nocodazole and taxol dramatically alter Rab11a's localization in the cell, while effects on Rab11b's distribution were less apparent. These results indicate that in contrast to Rab11a, the Rab11b compartment in the apical region is not as dependent upon microtubules. While Rab11a is known to regulate transferrin trafficking in nonpolarized cells and IgA trafficking in polarized cells, Rab11b exhibited little colocalization with either of these cargoes. Thus, while Rab11a and Rab11b share high sequence homology, they appear to reside within distinct vesicle compartments.

Characterization of protein kinase A-mediated phosphorylation of ezrin in gastric parietal cell activation

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 Ser66 is phosphorylated. To determine whether Ser66 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 [32P]ATP. Incorporation of 32P into wild type but not the mutant ezrin verified that Ser66 is a substrate of PKA. In addition, expression of S66A mutant ezrin in cultured parietal cells attenuates the dilation of apical vacuolar membrane associated with stimulation by histamine, indicating that PKA-mediated phosphorylation of ezrin is necessary for acid secretion. In fact, expression of phosphorylation-like S66D mutant in parietal cells mimics histamine-stimulated apical vacuole remodeling. Further examination of H,K-ATPase distribution revealed a blockade of stimulation-induced proton pump mobilization in S66A but not S66D ezrin-expressing parietal cells. These data suggest that PKA-mediated phosphorylation of ezrin plays an important role in mediating the remodeling of the apical membrane cytoskeleton associated with acid secretion in parietal cells.

Myosin II is present in gastric parietal cells and required for lamellipodial dynamics associated with cell activation

Nonmuscle myosin II has been shown to participate in organizing the actin cytoskeleton in polarized epithelial cells. Vectorial acid secretion in cultured parietal cells involves translocation of proton pumps from cytoplasmic vesicular membranes to the apical plasma membrane vacuole with coordinated lamellipodial dynamics at the basolateral membrane. Here we identify nonmuscle myosin II in rabbit gastric parietal cells. Western blots with isoform-specific antibodies indicate that myosin IIA is present in both cytosolic and particulate membrane fractions whereas the IIB isoform is associated only with particulate fractions. Immunofluorescent staining demonstrates that myosin IIA is diffusely located throughout the cytoplasm of resting parietal cells. However, after stimulation, myosin IIA is rapidly redistributed to lamellipodial extensions at the cell periphery; virtually all the cytoplasmic myosin IIA joins the newly formed basolateral membrane extensions. 2,3-Butanedione monoximine (BDM), a myosin-ATPase inhibitor, greatly diminishes the lamellipodial dynamics elicited by stimulation and retains the pattern of myosin IIA cytoplasmic staining. However, BDM had no apparent effect on the stimulation associated redistribution of H,K-ATPase from a cytoplasmic membrane compartment to apical membrane vacuoles. The myosin light chain kinase inhibitor 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-7) also did not alter the stimulation-associated recruitment of H,K-ATPase to apical membrane vacuoles, but unlike BDM it had relatively minor inhibitory effects on lamellipodial dynamics. We conclude that specific disruption of the basolateral actomyosin cytoskeleton has no demonstrable effect on recruitment of H,K-ATPase-rich vesicles into the apical secretory membrane. However, myosin II plays an important role in regulating lamellipodial dynamics and cortical actomyosin associated with parietal cell activation.

Cell biology of acid secretion by the parietal cell

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.

Polarized distribution of IQGAP proteins in gastric parietal cells and their roles in regulated epithelial cell secretion

Actin cytoskeleton plays an important role in the establishment of epithelial cell polarity. Cdc42, a member of Rho GTPase family, modulates actin dynamics via its regulators, such as IQGAP proteins. Gastric parietal cells are polarized epithelial cells in which regulated acid secretion occurs in the apical membrane upon stimulation. We have previously shown that actin isoforms are polarized to different membrane domains and that the integrity of the actin cytoskeleton is essential for acid secretion. Herein, we show that Cdc42 is preferentially distributed to the apical membrane of gastric parietal cells. In addition, we revealed that two Cdc42 regulators, IQGAP1 and IQGAP2, are present in gastric parietal cells. Interestingly, IQGAP2 is polarized to the apical membrane of the parietal cells, whereas IQGAP1 is mainly distributed to the basolateral membrane. An IQGAP peptide that competes with full-length IQGAP proteins for Cdc42-binding in vitro also inhibits acid secretion in streptolysin-O-permeabilized gastric glands. Furthermore, this peptide disrupts the association of IQGAP and Cdc42 with the apical actin cytoskeleton and prevents the apical membrane remodeling upon stimulation. We propose that IQGAP2 forms a link that associates Cdc42 with the apical cytoskeleton and thus allows for activation of polarized secretion in gastric parietal cells.

Localization and function of soluble N-ethylmaleimide-sensitive factor attachment protein-25 and vesicle-associated membrane protein-2 in functioning gastric parietal cells

The soluble N-ethylmaleimide-sensitive factor attachment protein of 25 kDa (SNAP-25) plays an important role in vesicle trafficking. Together with vesicle-associated membrane protein-2 (VAMP-2) and syntaxin, SNAP-25 forms a ternary complex implicated in docking and fusion of secretory vesicles with the plasma membrane during exocytosis. These so-called SNARE proteins are believed to regulate tubulovesicle trafficking and fusion during the secretory cycle of the gastric parietal cell. Here we examined the cellular localization and functional importance of SNAP-25 in parietal cell cultures. Adenoviral constructs were used to express SNAP-25 tagged with cyan fluorescent protein, VAMP-2 tagged with yellow fluorescent protein, and SNAP-25 in which the C-terminal 25 amino acids were deleted (SNAP-25 Delta181-206). Membrane fractionation experiments and fluorescent imaging showed that SNAP-25 is localized to the apical plasma membrane. The expression of the mutant SNAP-25 Delta181-226 inhibited the acid secretory response of parietal cells. Also, SNAP Delta181-226 bound poorly in vitro with recombinant syntaxin-1 compared with wild type SNAP-25, indicating that pairing between syntaxin-1 and SNAP-25 is required for parietal cell activation. Dual expression of SNAP-25 tagged with cyan fluorescent protein and VAMP-2 tagged with yellow fluorescent protein revealed a dynamic change in distribution associated with acid secretion. In resting cells, SNAP-25 is at the apical plasma membrane and VAMP-2 is associated with cytoplasmic H,K-ATPase-rich tubulovesicles. After stimulation, the two proteins co-localize on the apical plasma membrane. These data demonstrate the functional significance of SNAP-25 as a SNARE protein in the parietal cell and show the dynamic stimulation-associated redistribution of VAMP-2 from H,K-ATPase-rich tubulovesicles to co-localize with SNAP-25 on the apical plasma membrane.

Lasp-1 binds to non-muscle F-actin in vitro and is localized within multiple sites of dynamic actin assembly in vivo

Lasp-1 has been identified as a signaling molecule that is phosphorylated upon elevation of [cAMP]i in pancreas, intestine and gastric mucosa and is selectively expressed in cells within epithelial tissues. In the gastric parietal cell, cAMP-dependent phosphorylation induces the partial translocation of lasp-1 to the apically directed F-actin-rich canalicular membrane, which is the site of active HCl secretion. Lasp-1 is an unusual modular protein that contains an N-terminal LIM domain, a C-terminal SH3 domain and two internal nebulin repeats. Domain-based analyses have recently categorized this protein as an epithelial representative of the nebulin family, which also includes the actin binding, muscle-specific proteins, nebulin, nebulette and N-RAP. In this study, we show that lasp-1 binds to non-muscle filamentous (F) actin in vitro in a phosphorylation-dependent manner. In addition, we provide evidence that lasp-1 is concentrated within focal complexes as well as in the leading edges of lamellipodia and the tips of filopodia in non-transformed gastric fibroblasts. In actin pull-down assays, the apparent K(d) of bacterially expressed his-tagged lasp-1 binding to F-actin was 2 micro M with a saturation stoichiometry of approximately 1:7. Phosphorylation of recombinant lasp-1 with recombinant PKA increased the K(d) and decreased the B(max) for lasp-1 binding to F-actin. Microsequencing and site-directed mutagenesis localized the major in vivo and in vitro PKA-dependent phosphorylation sites in rabbit lasp-1 to S(99) and S(146). BLAST searches confirmed that both sites are conserved in human and chicken homologues. Transfection of lasp-1 cDNA encoding for alanine substitutions at S(99) and S(146), into parietal cells appeared to suppress the cAMP-dependent translocation of lasp-1 to the intracellular canalicular region. In gastric fibroblasts, exposure to the protein kinase C activator, PMA, was correlated with the translocation of lasp-1 into newly formed F-actin-rich lamellipodial extensions and nascent focal complexes. Since lasp-1 does not appear to be phosphorylated by PKC, these data suggest that other mechanisms in addition to cAMP-dependent phosphorylation can mediate the translocation of lasp-1 to regions of dynamic actin turnover. The localization of lasp-1 to these subcellular regions under a range of experimental conditions and the phosphorylation-dependent regulation of this protein in F-actin rich epithelial cells suggests an integral and possibly cell-specific role in modulating cytoskeletal/membrane-based cellular activities.

Intracellular distribution and functional importance of vesicle-associated membrane protein 2 in gastric parietal cells

BACKGROUND & AIMS

Acid secretion by parietal cells involves secretagogue-dependent recycling of the H+-K+-ATPase. Proteins called soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) have been implicated as participants in membrane trafficking, docking, and fusing processes. Here we studied the intracellular distribution and functional importance of one SNARE protein, vesicle associated membrane protein-2 (VAMP-2), in gastric parietal cells.

METHODS

Using an adenoviral recombinant expression system encoding VAMP-2 (synaptobrevin-2) fused to the green fluorescent protein (GFP), we expressed the GFP-VAMP-2 protein in primary cultures of rabbit parietal cells, which enables us to visualize the dynamics of GFP-VAMP-2 in a variety of functional states by fluorescence microscopy. To ascertain the function of VAMP-2 in parietal cell activation, streptolysin-O permeabilized gastric glands were treated with tetanus toxin, a potent and preferential protease for VAMP-2, and acid secretion was measured.

RESULTS

In resting parietal cells GFP was detected throughout the cytoplasm in a pattern of distribution that was very similar to that of H+-K+-ATPase. After stimulation, we observed that the GFP-VAMP-2 translocated to the apical plasma membrane along with the H+-K+-ATPase. A relatively high degree of co-localization was detected between GFP-VAMP-2 and H+-K+-ATPase. Tetanus toxin inhibited cAMP/ATP-stimulated acid secretion by about 45% in permeabilized gastric glands with a concomitant reduction in the level of immunoreactive VAMP-2.

CONCLUSIONS

Adenovirus-based GFP reporter fusion proteins can be used to efficiently study the functional dynamics of SNAREs. VAMP-2 is associated with tubulovesicle membranes in the parietal cell and plays a role in stimulation-associated membrane recruitment and acid secretion.

Perturbation of gastric mucosa in mice expressing the temperature-sensitive mutant of SV40 large T antigen. Potential for establishment of an immortalised parietal cell line

Gastric parietal cells have a unique secretory membrane system that undergoes a profound transformation when the parietal cell is stimulated to secrete acid. Understanding this process has been hindered by the lack of an immortalised parietal cell line. Here we have explored a strategy for the development of a parietal cell line by the generation of transgenic mice bearing the temperature-sensitive mutant of the SV40 large T antigen (SV40 tsA58) under the control of the regulatory sequences of the gastric H+/K+ ATPase beta-subunit (H/Kbeta-tsA58). Three H/ Kbeta-tsA58 transgenic mouse lines were established, namely 218, 224 and 228, all of which expressed the tsA58 T antigen in the gastric mucosa. Unexpectedly, the gastric mucosae of all lines were hypertrophic indicating that the temperature-sensitive large T antigen was partially active at 37 degrees C. Immunofluorescence together with light and electron microscopic studies revealed that mature parietal and zymogenic cells were absent in H/Kbeta-tsA58 transgenic lines 218 and 224, and small undifferentiated cells were the dominant cell type in the gastric units. On the other hand, a few mature parietal cells were detected in line 228 together with an increased proportion of undifferentiated cells and, normally rare, pre-parietal cells. As line 228 represented a rich source of pre-parietal cells, gastric cells from line 228 were isolated and cultured at 33 degrees C, the permissive temperature for tsA58. Gastric epithelial cells, expressing the T antigen, were maintained in culture for over 6 weeks. Upon a temperature shift to 39 C the cultured gastric cells developed characteristics of differentiated parietal cells, including the presence of a nascent canaliculus and dramatically increased production of the gastric H+/K+ ATPase beta-subunit. Therefore, this system shows the potential to generate an immature parietal cell line that can be induced to differentiate in vitro.

Ca(2+) and Mg(2+)/ATP independently trigger homotypic membrane fusion in gastric secretory membranes

Exocytic activation of gastric parietal cells represents a massive transformation. We studied a step in this process, homotypic fusion of H,K-ATPase-containing tubulovesicles, using R18 dequenching. Ca(2+) and Mg(2+)/ATP each caused dramatic dequenching, reflecting a change in R18 distribution from 5% to 65-90% of the assay's membranes in 2.5 min. These stimuli also triggered fusion between tubulovesicles and liposomes. Independent confirmation that dequenching represented membrane fusion was established by separating tubulovesicle-liposome fusion products on density gradients. Only agents that trigger fusion allowed the transmembrane H,K-ATPase to move to low-density fractions along with R18. EC(50) for Ca(2+)-triggered fusion was 150 nm and for Mg(2+)/ATP-triggered fusion 1 mm, the latter having a Hill coefficient of 2.5. ATP-triggered fusion was specific for Mg(2+)/ATP, required ATP hydrolysis, and was insensitive to inhibition of NSF and/or H,K-ATPase. Fusion initiated by either trigger caused tubulovesicles to become resistant to subsequent challenge by either trigger. Ca(2+) and Mg(2+)/ATP-triggered fusion required protein component(s) in tubulovesicles, though this was required in only one of the fusing membranes since tubulovesicles fused well with liposomes containing no proteins. Our data suggest that exocytosis in parietal cells is triggered by separate but interacting pathways and is regulated by self-inhibition.