Specific protonation of acidic residues confers K+ selectivity to the gastric proton pump

The gastric proton pump (H+,K+-ATPase) transports a proton into the stomach lumen for every K+ ion exchanged in the opposite direction. In the lumen-facing state of the pump (E2), the pump selectively binds K+ despite the presence of a 10-fold higher concentration of Na+. The molecular basis for the ion selectivity of the pump is unknown. Using molecular dynamics simulations, free energy calculations, and Na+ and K+-dependent ATPase activity assays, we demonstrate that the K+ selectivity of the pump depends upon the simultaneous protonation of the acidic residues E343 and E795 in the ion-binding site. We also show that when E936 is protonated, the pump becomes Na+ sensitive. The protonation-mimetic mutant E936Q exhibits weak Na+-activated ATPase activity. A 2.5-Å resolution cryo-EM structure of the E936Q mutant in the K+-occluded E2-Pi form shows, however, no significant structural difference compared with wildtype except less-than-ideal coordination of K+ in the mutant. The selectivity toward a specific ion correlates with a more rigid and less fluctuating ion-binding site. Despite being exposed to a pH of 1, the fundamental principle driving the K+ ion selectivity of H+,K+-ATPase is similar to that of Na+,K+-ATPase: the ionization states of the acidic residues in the ion-binding sites determine ion selectivity. Unlike the Na+,K+-ATPase, however, protonation of an ion-binding glutamate residue (E936) confers Na+ sensitivity.

The revolution of personalized pharmacotherapies for cystic fibrosis: what does the future hold?

INTRODUCTION

Cystic fibrosis (CF), a potentially fatal genetic disease, is caused by loss-of-function mutations in the gene encoding for the CFTR chloride/bicarbonate channel. Modulator drugs rescuing mutant CFTR traffic and function are now in the clinic, providing unprecedented breakthrough therapies for people with CF (PwCF) carrying specific genotypes. However, several CFTR variants are unresponsive to these therapies.

AREA COVERED

We discussed several therapeutic approaches that are under development to tackle the fundamental cause of CF, including strategies targeting defective CFTR mRNA and/or protein expression and function. Alternatively, defective chloride secretion and dehydration in CF epithelia could be restored by exploiting pharmacological modulation of alternative targets, i.e., ion channels/transporters that concur with CFTR to maintain the airway surface liquid homeostasis (e.g., ENaC, TMEM16A, SLC26A4, SLC26A9, and ATP12A). Finally, we assessed progress and challenges in the development of gene-based therapies to replace or correct the mutant CFTR gene.

EXPERT OPINION

CFTR modulators are benefiting many PwCF responsive to these drugs, yielding substantial improvements in various clinical outcomes. Meanwhile, the CF therapy development pipeline continues to expand with the development of novel CFTR modulators and alternative therapeutic strategies with the ultimate goal of providing effective therapies for all PwCF in the foreseeable future.

Differential expression of KCNQ1 and ATP4A genes according to the sex and age in the stomach

We compared the expression of six genes in stomach tissue samples between healthy men and women in different age groups to study sexually dimorphic gene expression. Real-Time RT-PCR was used to compare gene expression between men and women. Our results showed that the expression of KCNQ1 (p = 0.01) was significantly higher in non-menopausal women compared to post-menopausal women. In addition, the expression level of the ATP4A gene in men under 35 years was significantly higher than in men above 50 (p = 0.026). Sexually and age dimorphic gene expression in some genes throughout life may affect gastric function.

Targeting ATP12A, a Nongastric Proton Pump α Subunit, for Idiopathic Pulmonary Fibrosis Treatment

Idiopathic pulmonary fibrosis (IPF) is a pathological condition of unknown etiology that results from injury to the lung and an ensuing fibrotic response that leads to the thickening of the alveolar walls and obliteration of the alveolar space. The pathogenesis is not clear, and there are currently no effective therapies for IPF. Small airway disease and mucus accumulation are prominent features in IPF lungs, similar to cystic fibrosis lung disease. The ATP12A gene encodes the α-subunit of the nongastric H+, K+-ATPase, which functions to acidify the airway surface fluid and impairs mucociliary transport function in patients with cystic fibrosis. It is hypothesized that the ATP12A protein may play a role in the pathogenesis of IPF. The authors' studies demonstrate that ATP12A protein is overexpressed in distal small airways from the lungs of patients with IPF compared with normal human lungs. In addition, overexpression of the ATP12A protein in mouse lungs worsened bleomycin induced experimental pulmonary fibrosis. This was prevented by a potassium competitive proton pump blocker, vonoprazan. These data support the concept that the ATP12A protein plays an important role in the pathogenesis of lung fibrosis. Inhibition of the ATP12A protein has potential as a novel therapeutic strategy in IPF treatment.

Extra-gastric expression of the proton pump H+/K+-ATPase in the gills and kidney of the teleost Oreochromis niloticus

Potassium regulation is essential for the proper functioning of excitable tissues in vertebrates. The H+/K+-ATPase (HKA), which is composed of the HKα1 (gene: atp4a) and HKβ (gene: atp4b) subunits, has an established role in potassium and acid-base regulation in mammals and is well known for its role in gastric acidification. However, the role of HKA in extra-gastric organs such as the gill and kidney is less clear, especially in fishes. In the present study in Nile tilapia, Oreochromis niloticus, uptake of the K+ surrogate flux marker rubidium (Rb+) was demonstrated in vivo; however, this uptake was not inhibited with omeprazole, a potent inhibitor of the gastric HKA. This contrasts with gill and kidney ex vivo preparations, where tissue Rb+ uptake was significantly inhibited by omeprazole and SCH28080, another gastric HKA inhibitor. The cellular localization of this pump in both the gill and kidney was demonstrated using immunohistochemical techniques with custom-made antibodies specific for Atp4a and Atp4b. Antibodies against the two subunits showed the same apical ionocyte distribution pattern in the gill and collecting tubules/ducts in the kidney. Atp4a antibody specificity was confirmed by western blotting. RT-PCT was used to confirm the expression of both subunits in the gill and kidney. Taken together, these results indicate for the first time K+ (Rb+) uptake in O. niloticus and that HKA is implicated, as shown through the ex vivo uptake inhibition by omeprazole and SCH28080, verifying a role for HKA in K+ absorption in the gill's ionocytes and collecting tubule/duct segments of the kidney.

A genetic origin for acid-base imbalance triggers the mitochondrial damage that explains the autoimmune response and drives to gastric neuroendocrine tumours

BACKGROUND

Type I gastric neuroendocrine tumors (gNETs) arise from hypergastrinemia in patients with autoimmune chronic atrophic gastritis. According to the classical model, the gastric H+/K+ ATPase was the causative autoantigen recognized by CD4+ T cells in chronic autoimmune scenario that secretes IL-17 and correlates with parietal cell (PC) atrophy, which drives to gastric achlorhydria and increases the risk for gastric neoplasms. However, the mechanism by which the inflammatory response correlates with PC atrophy is not clearly defined.

METHODS

Recently, we found that the ATP4A^p.R703C mutation impaired PC function and gastric acidification, which drove familial gNET. Our group constructed a knock-in mouse model for the ATP4A mutation, which has served us to better understand the relation between impaired capability to export protons across the plasma membrane of PCs and tumor progression.

RESULTS

The ATP4A^p.R703C mutation drives gastric achlorhydria, but also deregulates the acid-base balance within PCs, affecting mitochondrial biogenesis. Mitochondrial malfunction activates ROS signaling, which triggers caspase-3-mediated apoptosis of parietal cells. In addition, when gastric euchlorhydria was restored, mitochondrial function is recovered. Infection by H. pylori promotes destabilization of the mitochondria of the PCs by a mechanism similar to that described for APT4A^p.R703C carriers.

CONCLUSIONS

A genetic origin that drives mitochondria alteration would initiate the gastric chronic inflammation instead of the classical IL-17 secretion-mediated mechanism explanation. Gastric euchlorhydria restoration is suggested to be indicated for mitochondrial recover. Our results open a new window to understand gastric neoplasms formation but also the inflammatory mechanisms and autoimmune disorders conducted by genetic origin that composes a premalignant scenario.

Genomic Comparison of the P-ATPase Gene Family in Four Cotton Species and Their Expression Patterns in Gossypium hirsutum

Plant P-type H⁺-ATPase (P-ATPase) is a membrane protein existing in the plasma membrane that plays an important role in the transmembrane transport of plant cells. To understand the variety and quantity of P-ATPase proteins in different cotton species, we combined four databases from two diploid cotton species (Gossypium raimondii and G. arboreum) and two tetraploid cotton species (G. hirsutum and G. barbadense) to screen the P-ATPase gene family and resolved the evolutionary relationships between the former cotton species. We identified 53, 51, 99 and 98 P-ATPase genes from G. arboretum, G. raimondii, G. barbadense and G. hirsutum, respectively. The structural and phylogenetic analyses revealed that the gene structure was consistent between P-ATPase genes, with a close evolutionary relationship. The expression analysis of P-ATPase genes showed that many P-ATPase genes were highly expressed in various tissues and at different fiber developmental stages in G. hirsutum, suggesting that they have potential functions during growth and fiber development in cotton.

Renal and colonic potassium transporters in the pregnant rat

Gestational potassium retention, most of which occurs during late pregnancy, is essential for fetal development. The purpose of this study was to examine mechanisms underlying changes in potassium handling by the kidney and colon in pregnancy. We found that potassium intake and renal excretion increased in late pregnancy while fecal potassium excretion remained unchanged and that pregnant rats exhibited net potassium retention. By quantitative PCR we found markedly increased H+-K+-ATPase type 2 (HKA2) mRNA expression in the cortex and outer medullary of late pregnant vs. virgin. Renal outer medullary potassium channel (ROMK) mRNA was unchanged in the cortex, but apical ROMK abundance (by immunofluorescence) was decreased in pregnant vs. virgin in the distal convoluted tubule (DCT) and connecting tubule (CNT). Big potassium-α (BKα) channel-α protein abundance in intercalated cells in the cortex and outer medullary collecting ducts (by immunohistochemistry) fell in late pregnancy. In the distal colon we found increased HKA2 mRNA and protein abundance (Western blot) and decreased BKα protein with no observed changes in mRNA. Therefore, the potassium retention of pregnancy is likely to be due to increased collecting duct potassium reabsorption (via increased HKA2), decreased potassium secretion (via decreased ROMK and BK), as well as increased colonic reabsorption via HKA2.

Plasmid-free CRISPR/Cas9 genome editing in Plasmodium falciparum confirms mutations conferring resistance to the dihydroisoquinolone clinical candidate SJ733

Genetic manipulation of the deadly malaria parasite Plasmodium falciparum remains challenging, but the rise of CRISPR/Cas9-based genome editing tools is increasing the feasibility of altering this parasite’s genome in order to study its biology. Of particular interest is the investigation of drug targets and drug resistance mechanisms, which have major implications for fighting malaria. We present a new method for introducing drug resistance mutations in P. falciparum without the use of plasmids or the need for cloning homologous recombination templates. We demonstrate this method by introducing edits into the sodium efflux channel PfATP4 by transfection of a purified CRISPR/Cas9-guide RNA ribonucleoprotein complex and a 200-nucleotide single-stranded oligodeoxynucleotide (ssODN) repair template. Analysis of whole genome sequencing data with the variant-finding program MinorityReport confirmed that only the intended edits were made, and growth inhibition assays confirmed that these mutations confer resistance to the antimalarial SJ733. The method described here is ideally suited for the introduction of mutations that confer a fitness advantage under selection conditions, and the novel finding that an ssODN can function as a repair template in P. falciparum could greatly simplify future editing attempts regardless of the nuclease used or the delivery method.

Study of the effects of nesfatin-1 on gastric function in obese rats

AIM

To investigate the effects of nesfatin-1 on gastric function in obese rats.

METHODS

The obese rat model was induced by a high-fat diet. The gastric emptying rate and gastric acid secretory capacity of the rats were determined after treatment with different drug concentrations of nesfatin-1 and administration routes. Based on this, the expression of H+/K+-ATPase was measured using RT-PCR and western blot to preliminarily explore the mechanism of gastric acid secretion changes.

RESULTS

Body weight, body length, and Lee’s index of the rats significantly increased in the high-fat diet-induced obese rat model. Two hours after lateral intracerebroventricular injection of nesfatin-1, the gastric emptying rate and gastric acid secretory capacity of rats decreased. Four hours after injection, both were restored to normal levels. In addition, the expression of H+/K+-ATPase decreased and moved in line with changes in gastric acid secretory capacity. This in vivo experiment revealed that intracerebroventricular injection of nesfatin-1, rather than intravenous injection, could suppress gastric function in obese rats. Moreover, its effect on the gastric emptying and gastric acid secretory capacity of rats is dose-dependent within a certain period of time.

CONCLUSION

Through this research, we provide a theoretical basis for further studies on nesfatin-1, a potential anti-obesity drug.

Expression of Na,K-ATPase and H,K-ATPase Isoforms with the Baculovirus Expression System

P-type ATPases can be expressed in several cell systems. The baculovirus expressions system uses an insect virus to enter and express proteins in Sf9 insect cells. This expression system is a lytic system in which the cells will die a few days after viral infection. Subsequently, the expressed proteins can be isolated. Insect cells are a perfect system to study P-type ATPases as they have little or no endogenous Na,K-ATPase activity and other ATPase activities can be inhibited easily. Here we describe in detail the expression and isolation of Na,K-ATPase and H,K-ATPase isoforms with the baculovirus expression system.

Skeletal effects of a gastrin receptor antagonist in H+/K+ATPase beta subunit KO mice

Epidemiological studies suggest an increased fracture risk in patients taking proton pump inhibitors (PPIs) for long term. The underlying mechanism, however, has been disputed. By binding to the gastric proton pump, PPIs inhibit gastric acid secretion. We have previously shown that proton pump (H(+)/K(+)ATPase beta subunit) KO mice exhibit reduced bone mineral density (BMD) and inferior bone strength compared with WT mice. Patients using PPIs as well as these KO mice exhibit gastric hypoacidity, and subsequently increased serum concentrations of the hormone gastrin. In this study, we wanted to examine whether inhibition of the gastrin/CCK2 receptor influences bone quality in these mice. KO and WT mice were given either the gastrin/CCK2 receptor antagonist netazepide dissolved in polyethylene glycol (PEG) or only PEG for 1year. We found significantly lower bone mineral content and BMD, as well as inferior bone microarchitecture in KO mice compared with WT. Biomechanical properties by three-point bending test also proved inferior in KO mice. KO mice receiving netazepide exhibited significantly higher cortical thickness, cortical area fraction, trabecular thickness and trabecular BMD by micro-CT compared with the control group. Three-point bending test also showed higher Young's modulus of elasticity in the netazepide KO group compared with control mice. In conclusion, we observed that the gastrin receptor antagonist netazepide slightly improved bone quality in this mouse model, suggesting that hypergastrinemia may contribute to deteriorated bone quality during acid inhibition.

PAI-1 deficiency increases the trophic effects of hypergastrinemia in the gastric corpus mucosa

The gastric hormone gastrin plays a role in organizing the gastric mucosa. Gastrin also regulates the expression of genes that have important actions in extracellular matrix modelling, including plasminogen activator inhibitor (PAI)-1 which is part of the urokinase plasminogen activator (uPA) system. The uPA system (including PAI-1) is associated with cancer progression, fibrosis and thrombosis. Its biological role in the stomach and molecular mechanisms of action are not well understood. The aim of this study was to examine the effect of PAI-1 on the trophic changes observed in gastric corpus mucosa in hypergastrinemia using PAI-1 and/or HK-ATPase beta subunit knockout (KO) mice. HK-ATPase beta subunit KO mice were used as a model of hypergastrinemia. In 12 month old female mice, intragastric acidity and plasma gastrin were measured. The stomachs were examined for macroscopic and histological changes. In mice null for both PAI-1 and HK-ATPase beta (double KO), there was exaggerated hypergastrinemia, increased stomach weight and corpus mucosal thickness, and more pronounced trophic and architectural changes in the corpus compared with HK-ATPase beta KO mice. Genome-wide microarray expression data for the gastric corpus mucosa showed a distinct gene expression profile for the HK-ATPase beta KO mice; moreover, enrichment analysis revealed changes in expression of genes regulating intracellular processes including cytoskeleton remodelling, cell adhesion, signal transduction and epithelial-to-mesenchymal transition (EMT). Genes differentially expressed in the double KO compared with HK-ATPase beta KO mice included the transcription factor Barx2 and the chromatin remodeler gene Tet2, which may be involved in both normal gastric physiology and development of gastric cancer. Based on the present data, we suggest that PAI-1 plays a role in maintaining gastric mucosal organization in hypergastrinemia.

RNAi-mediated Ghrelin affects gastric H(+)-K(+)-ATPase activity and expression of GOAT-Ghrelin system in vitro

Ghrelin has been implicated in the regulation of gastric functional development, and its physiological functions are mediated by Ghrelin-O-acyltransferase (GOAT) which is capable of generating the active form of this polypeptide hormone. However, whether and how ghrelin gene silencing may modify gastric acid secretion and GOAT-Ghrelin system is yet to be explored. The study was performed in gastric mucosal cells from weanling piglets in vitro. We evaluated the effect of ghrelin on gastric acid secretion, gene expression of GOAT and ghrelin as well as ghrelin levels by RNA interference assay. shGhrelin triggered the down-regulation of ghrelin mRNA expression (P<0.05) via an RNAi mechanism, as observed by real-time RT-PCR. In addition, shGhrelin showed reduced total ghrelin production and secretion (P<0.05) using ELISA in vitro. We also detected that GOAT mRNA expression was reduced in shGhrelin group (P<0.05), compared with control groups. In accordance with the GOAT expression, acylated ghrelin production and secretion were reduced in gastric mucosal cells and culture medium (P<0.05). Silencing of ghrelin gene achieved by RNAi-mediation inhibited the activity of H(+)-K(+)-ATPase and pepsin (P<0.05) in gastric mucosal cells. These results indicated that RNAi of Ghrelin gene inhibited the gastric acid secretion with decreased GOAT mRNA and acylated Ghrelin in gastric mucosal cells.

Voltage Clamp Fluorometry of P-Type ATPases

Voltage clamp fluorometry has become a powerful tool to compare partial reactions of P-type ATPases such as the Na(+),K(+)-ATPase and H(+),K(+)-ATPase with conformational dynamics of these ion pumps. Here, we describe the methodology to heterologously express membrane proteins in X. laevis oocytes and site-specifically label these proteins with one or more fluorophores. Fluorescence changes are measured simultaneously with current measurements under two-electrode voltage clamp conditions.

Proton Pump Inhibitors Inhibit Pancreatic Secretion: Role of Gastric and Non-Gastric H+/K+-ATPases

The mechanism by which pancreas secretes high HCO₃^- has not been fully resolved. This alkaline secretion, formed in pancreatic ducts, can be achieved by transporting HCO₃^- from serosa to mucosa or by moving H⁺ in the opposite direction. The aim of the present study was to determine whether H⁺/K⁺-ATPases are expressed and functional in human pancreatic ducts and whether proton pump inhibitors (PPIs) have effect on those. Here we show that the gastric HKα1 and HKβ subunits (ATP4A; ATP4B) and non-gastric HKα2 subunits (ATP12A) of H⁺/K⁺-ATPases are expressed in human pancreatic cells. Pumps have similar localizations in duct cell monolayers (Capan-1) and human pancreas, and notably the gastric pumps are localized on the luminal membranes. In Capan-1 cells, PPIs inhibited recovery of intracellular pH from acidosis. Furthermore, in rats treated with PPIs, pancreatic secretion was inhibited but concentrations of major ions in secretion follow similar excretory curves in control and PPI treated animals. In addition to HCO₃^-, pancreas also secretes K⁺. In conclusion, this study calls for a revision of the basic model for HCO₃^- secretion. We propose that proton transport is driving secretion, and that in addition it may provide a protective pH buffer zone and K⁺ recirculation. Furthermore, it seems relevant to re-evaluate whether PPIs should be used in treatment therapies where pancreatic functions are already compromised.

New aspects in the pathomechanism and diagnosis of the laryngopharyngeal reflux-clinical impact of laryngeal proton pumps and pharyngeal pH metry in extraesophageal gastroesophageal reflux disease

AIM: To determine the laryngeal H+K+-ATPase and pharyngeal pH in patients with laryngopharyngeal reflux (LPR)-symptoms as well as to assess the symptom scores during PPI therapy.

METHODS: Endoscopy was performed to exclude neoplasia and to collect biopsies from the posterior cricoid area (immunohistochemistry and PCR analysis). Immunohistochemical staining was performed with monoclonal mouse antibodies against human H+K+-ATPase. Quantitative real-time RT-PCR for each of the H+K+-ATPase subunits was performed. The pH values were assessed in the aerosolized environment of the oropharynx (DxpH Catheter) and compared to a subsequently applied combined pH/MII measurement.

RESULTS: Twenty patients with LPR symptoms were included. In only one patient, the laryngeal H+K+-ATPase was verified by immunohistochemical staining. In another patient, real-time RT-PCR for each H+K+-ATPase subunit was positive. Fourteen out of twenty patients had pathological results in DxpH, and 6/20 patients had pathological results in pH/MII. Four patients had pathological results in both functional tests. Nine out of twenty patients responded to PPIs.

CONCLUSION: The laryngeal H+K+-ATPase can only be sporadically detected in patients with LPR symptoms and is unlikely to cause the LPR symptoms. Alternative hypotheses for the pathomechanism are needed. The role of pharyngeal pH-metry remains unclear and its use can only be recommended for patients in a research study setting.

Symmetry breakage in the vertebrate embryo: when does it happen and how does it work?

Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.

Reciprocal signaling between the ectoderm and a mesendodermal left-right organizer directs left-right determination in the sea urchin embryo

During echinoderm development, expression of nodal on the right side plays a crucial role in positioning of the rudiment on the left side, but the mechanisms that restrict nodal expression to the right side are not known. Here we show that establishment of left-right asymmetry in the sea urchin embryo relies on reciprocal signaling between the ectoderm and a left-right organizer located in the endomesoderm. FGF/ERK and BMP2/4 signaling are required to initiate nodal expression in this organizer, while Delta/Notch signaling is required to suppress formation of this organizer on the left side of the archenteron. Furthermore, we report that the H(+)/K(+)-ATPase is critically required in the Notch signaling pathway upstream of the S3 cleavage of Notch. Our results identify several novel players and key early steps responsible for initiation, restriction, and propagation of left-right asymmetry during embryogenesis of a non-chordate deuterostome and uncover a functional link between the H(+)/K(+)-ATPase and the Notch signaling pathway.

How Helicobacter pylori infection controls gastric acid secretion

Infection of the human stomach mucosa by Helicobacter pylori induces strong inflammatory responses and a transitory hypochlorhydria which can progress in ~2 % of patients to atrophic gastritis, dysplasia, or gastric adenocarcinoma. H. pylori infection of gastric biopsies or cultured gastric epithelial cells in vitro represses the activity of endogenous or transfected promoter of the alpha-subunit (HKα) of gastric H,K-adenosine triphosphatase (H,K-ATPase), the parietal cell enzyme mediating acid secretion. Some mechanistic details of H. pylori-mediated repression of HKα and ensuing hypochlorhydria have been recently elucidated. H. pylori strains expressing a type IV secretion system (T4SS) encoded by the cag pathogenicity island are known to upregulate the transcription factor nuclear factor (NF)-κB. The NF-κB-binding regions in the HKα promoter were identified and shown to repress its transcriptional activity. Interaction studies have indicated that although active phosphorylated NF-κB p65 is present in infected cells, an NF-κB p50/p65 heterodimeric complex fails to bind to the HKα promoter. Point mutations at -159 and -161 bp in the HKα promoter NF-κB binding sequence prevent the binding of NF-κB p50 and prevent H. pylori repression of point-mutated HKα promoter activity. The T4SS factors CagL, CagE, CagM, and possibly CagA and the lytic transglycosylase Slt, are mechanistically involved in NF-κB activation and repression of HKα transcription. CagL, a T4SS pilus component, binds to the integrin α(5)β(1) to mediate translocation of virulence factors into the host cell and initiate signaling. During acute H. pylori infection, CagL dissociates ADAM 17 (a disintegrin and a metalloprotease 17) from the integrin α(5)β(1) complex and stimulates ADAM17-dependent release of heparin-binding epidermal growth factor (HB-EGF), EGF receptor (EGFR) stimulation, ERK1/2 kinase activation, and NF-κB-mediated repression of HKα. These studies suggest that H. pylori inhibits HKα gene expression by an integrin α(5)β(1) → ADAM17 → HB-EGF → EGFR → ERK1/2 → NF-κB pathway mediating NF-κB p50 homodimer binding to the HKα promoter. Here we review the molecular basis and recent progress of this novel pathogen-dependent mechanism of H,K-ATPase inhibition, which contributes significantly to our current understanding of H. pylori pathophysiology.

Altered expression of aquaporin 4 and H+/K+-ATPase in the stomachs of peptide YY (PYY) transgenic mice

BACKGROUND INFORMATION

The hormone PYY (peptide YY), synthesized by endocrine cells in the pancreas, ileum, colon and stomach has widespread inhibitory effects on gastrointestinal and pancreatic fluid secretion. Transgenic mice expressing a viral oncoprotein under the control of the PYY gene 5'-flanking region develop well-differentiated colonic endocrine tumours producing mainly PYY and enteroglucagon. In the present study, we investigated the expression of AQP4 (aquaporin 4) water channel and H(+)/K(+)-ATPase in stomachs from both control and transgenic mice.

RESULTS

Semi-quantitative RT (reverse transcriptase)-PCR showed an increase in the AQP4 transcript compared with control mice. Quantitative Western-blot analysis of stomachs from control and transgenic mice confirmed a significant increase in the 30 kDa AQP4 protein in transgenic mice. In control mice, AQP4 is specifically expressed in the basolateral membrane of gastric parietal cells, located in the basal region of the fundic glands. This particular location suggests that parietal cells in the base region of gastric pits might have a major role in water transport when compared with the more superficial parietal cells. Interestingly, immunofluorescence studies on transgenic mice revealed that the quantitative increase of AQP4 expression was actually due to an increase in the number of AQP4-expressing epithelial cells rather than to a higher expression of AQP4 in parietal cells. In fact, immunofluorescence experiments using the specific antibody raised against the AE2 isoform of Cl(-)/HCO3- exchanger specifically expressed in parietal cells confirmed that the number of parietal cells was comparable in both PYY and control stomachs. Moreover, in transgenic mice, a parallel significant decrease in the expression of H(+)/K(+)-ATPase was observed, as revealed by RT-PCR, quantitative immunoblotting and immunofluorescence.

CONCLUSIONS

In the present study, we demonstrate that the sustained inhibition of gastric secretion due to tumours producing PYY/enteroglucagon in transgenic mice is associated with an increase in AQP4 expression and a down-regulation of H(+)/K(+)-ATPase in parietal cells that acquire the characteristics of basal parietal cells. The absence of H2 receptors-mediated signalling due to the inhibition of histamine release from ECL (enterochromaffin-like) cells by PYY may be in part responsible for the observed increase in the number of parietal cells expressing AQP4.

A convenient model of severe, high incidence autoimmune gastritis caused by polyclonal effector T cells and without perturbation of regulatory T cells

Autoimmune gastritis results from the breakdown of T cell tolerance to the gastric H(+)/K(+) ATPase. The gastric H(+)/K(+) ATPase is responsible for the acidification of gastric juice and consists of an α subunit (H/Kα) and a β subunit (H/Kβ). Here we show that CD4(+) T cells from H/Kα-deficient mice (H/Kα(-/-)) are highly pathogenic and autoimmune gastritis can be induced in sublethally irradiated wildtype mice by adoptive transfer of unfractionated CD4(+) T cells from H/Kα(-/-) mice. All recipient mice consistently developed the most severe form of autoimmune gastritis 8 weeks after the transfer, featuring hypertrophy of the gastric mucosa, complete depletion of the parietal and zymogenic cells, and presence of autoantibodies to H(+)/K(+) ATPase in the serum. Furthermore, we demonstrated that the disease significantly affected stomach weight and stomach pH of recipient mice. Depletion of parietal cells in this disease model required the presence of both H/Kα and H/Kβ since transfer of H/Kα(-/-) CD4(+) T cells did not result in depletion of parietal cells in H/Kα(-/-) or H/Kβ(-/-) recipient mice. The consistency of disease severity, the use of polyclonal T cells and a specific T cell response to the gastric autoantigen make this an ideal disease model for the study of many aspects of organ-specific autoimmunity including prevention and treatment of the disease.

[Study on activities and protein and gene expression of renal H(+)-K(+)-ATPase in rats subchronic exposed to trimethyltin chloride]

OBJECTIVE

To study the activity, protein and gene expression of renal HK-ATPase (HKA) in rats subchronic exposed to trimethyltin chloride (TMT).

METHODS

In subchronic toxic test (14-week), 55 female SD rats (age, 6 weeks) were divided randomly into 5 groups: control, low, medium, high and super high dosage, respectively, which drank water with TMT of 0, 8.20, 32.81, 131.25 and 262.50 microg x kg(-1) x d(-1) for 14 weeks. Then serum K+ levels were measured; the activities of HK-ATPase (HKA) in kidneys were detected by the method of determinated phosphorus content; Western Blot assay and real-time PCR were used to exam the protein and mRNA expression levels of HKA in kidneys, respectively.

RESULTS

The serum K+ level in super-high dosage group was (5.6 +/- 0.4) mmol/L, which was significantly lower than that [(6.9 +/- 0.3) mmol/L] in control group (P < 0.01). The HKA enzymatic activity of kidneys in low and super high dosage groups was 4.50 +/- 1.45 and 4.55 +/- 0.72 micromolPi x mg prot(-1)h(-1), respectively, which were significantly lower than that (6.55 +/- 0.77 micromol Pi x mg prot(-1) h(-1)) in control group (P < 0.05).

CONCLUSION

When rats were exposed subchronic to TMT, the renal HKA activity could reduce, but the expression levels of HKA protein and mRNA did not decrease.

Targeted deletion of Kcne2 causes gastritis cystica profunda and gastric neoplasia

Gastric cancer is the second leading cause of cancer death worldwide. Predisposing factors include achlorhydria, Helicobacter pylori infection, oxyntic atrophy and TFF2-expressing metaplasia. In parietal cells, apical potassium channels comprising the KCNQ1 α subunit and the KCNE2 β subunit provide a K⁺ efflux current to facilitate gastric acid secretion by the apical H⁺K⁺ATPase. Accordingly, genetic deletion of murine Kcnq1 or Kcne2 impairs gastric acid secretion. Other evidence has suggested a role for KCNE2 in human gastric cancer cell proliferation, independent of its role in gastric acidification. Here, we demonstrate that 1-year-old Kcne2^−/− mice in a pathogen-free environment all exhibit a severe gastric preneoplastic phenotype comprising gastritis cystica profunda, 6-fold increased stomach mass, increased Ki67 and nuclear Cyclin D1 expression, and TFF2- and cytokeratin 7-expressing metaplasia. Some Kcne2^−/−mice also exhibited pyloric polypoid adenomas extending into the duodenum, and neoplastic invasion of thin walled vessels in the sub-mucosa. Finally, analysis of human gastric cancer tissue indicated reduced parietal cell KCNE2 expression. Together with previous findings, the results suggest KCNE2 disruption as a possible risk factor for gastric neoplasia.

Reversing the Autoimmune Condition: Experience with Experimental Autoimmune Gastritis

Autoimmune diseases remain a significant health problem in our society, despite the best efforts to understand and treat these conditions. Current clinical treatments are aimed at alleviating the consequences of these diseases, with limited prospects for cure. Our studies with the experimental model of autoimmune gastritis have led us to explore potential curative strategies that can reverse the autoimmune condition. Using mouse models, we have shown that expression of the known gastric autoantigen in the thymus results in immunological tolerance and resistance to the induction of autoimmune gastritis. Also, induced tolerance in donor mice can be transferred to syngeneic recipient mice by bone marrow cells. Strategies based on these observations could lead to reversal of established disease. Transfer of ensuing knowledge to the cure of serious human autoimmune diseases is our ultimate goal.

Molecular Specificity and Functional Properties of Autoreactive T-Cell Response in Human Gastric Autoimmunity

Human autoimmune gastritis (AIG) is a chronic inflammatory disorder of the gastric corpus. We have defined the antigen repertoire and the functional properties of in vivo activated CD4+ T cells derived from the gastric mucosa of patients with AIG. A remarkable proportion of the CD4+ T cell clones proliferated in response to H+,K+-ATPase. Six epitopes identified in the alpha chain, and 5 in the beta chain, of gastric K+,K+-ATPase were recognized by autoreactive gastric T cell clones. The majority of the autoreactive T cell clones secreted IFN-gamma and showed a T helper 1 (Th1) profile. All clones produced TNF-alpha,provided help for B cell immunoglobulin production, expressed perforin-mediated cytotoxicity, and most induced Fas-Fas ligand-mediated apoptosis. Data suggest that activation of gastric H+,K+-ATPase-specific Th1 T cells is crucial in the pathogenesis of human gastric autoimmunity and atrophy.

HK-ATPase Expression in the Susceptible BALB/c and the Resistant DBA/2 Strains of Mice to Autoimmune Gastritis

Neonatal thymectomy (NTx) in mice induces a group of alterations in the immune system homeostasis that results in the development of a variety of organ-specific autoimmune diseases such as gastritis, thyroiditis, oophoritis and orchitis. Given the importance of self-antigen expression in thymus for the control of autoreactive cells and generation of regulatory cells, we have compared the expression of parietal cell antigen in two strains of mice with the same H-2: BALB/c (susceptible to develop gastritis after NTx) and DBA/2 (resistant). We detected mRNA of HK-ATPase alpha and beta chains in day 1 thymi of both strains. Fifty percent of BALB/c mice presented mRNA levels similar to DBA/2. However, lower mRNA levels were found in the remaining BALB/c mice that may correspond to those that would develop AIG after NTx. Since the presence of the antigen in periphery is also necessary for the induction of regulatory cells, we have compared both strains observing in day 1 stomachs from resistant DBA/2 strain, a significantly higher content of positive cells for HK-ATPase subunits than stomachs from susceptible BALB/c strain. Also, the presence of antinuclear Abs in NTx BALB/c mice makes this model a useful experimental system for analyzing the responsible mechanisms breaking the non-specific self-tolerance.

Evidence for the regulation of left-right asymmetry in Ciona intestinalis by ion flux

Vertebrate embryos develop distinct left-right asymmetry under the control of a conserved pathway involving left-sided deployment of the nodal and Pit x 2 genes. The mechanism that initiates asymmetric expression of these genes is less clear, with cilia, ion flux, and signalling molecules all implicated. Vertebrates share the chordate phylum with urochordates such as the sea squirt Ciona intestinalis. We have explored the role of ion flux in regulating left-right asymmetry in Ciona, using an assay in which perturbation of left-sided Ci-Pitx expression provides a read-out for the disruption of asymmetry. Our data show that omeprazole, which specifically inhibits H(+)K(+)ATPase activity, disrupts asymmetry in Ciona. The vertebrate H(+)K(+)ATPase is composed of two subunits, alpha and beta. We identified one Ciona beta ortholog and two Ciona alpha orthologs of the vertebrate H(+)K(+)ATPase genes, and show that one of these is expressed in dorsal and ventral embryonic midline cells shortly before the activation of left-sided Ci-Pitx expression. Furthermore, we show that omeprazole exerts its effect on asymmetry at this point in development, and additionally implicate K(+) channels in the regulation of asymmetry in Ciona. These experiments demonstrate a role for ion flux in the regulation of asymmetry in Ciona, and show a conserved, ancestral role for the H(+)K(+)ATPase ion pump in this process.

Systematic expression profiling of the gastric H+/K+ ATPase in human tissue

OBJECTIVE

The potassium-competitive acid blockers (P-CABs), comprise a new, innovative group of competitive and reversible inhibitors of the gastric H+/K+ ATPase. Our aim was to identify sites of expression of the H+/K+ ATPase that are potential targets of these compounds by examining the expression profile of the gastric H+/K+ ATPase in the human body from a broad range of tissues.

MATERIAL AND METHODS

Expression profiling was done by quantitative mRNA analysis (TaqMan PCR). Tissues that were mRNA-positive for the alpha subunit were investigated further by Western blot and immunohistochemistry (IHC) for the presence of gastric H+/K+ ATPase protein.

RESULTS

In addition to the very high expression levels in the stomach, the adrenal gland, cerebellum and pancreas gave unexpectedly positive mRNA signals for the alpha subunit of gastric H +/K+ ATPase. However, they were negative for mRNA of the beta subunit, and Western blot and IHC were negative for alpha and beta subunit protein. Another group of tissues with low alpha subunit mRNA expression including the frontal cortex, cortex grey matter, testis, thymus and larynx submucosa were also found negative for both alpha and beta subunit protein. In contrast to mouse kidney, no gastric H+/K+ ATPase could be detected in human kidney.

CONCLUSIONS

We therefore conclude that the only organ in humans expressing significant levels of the P-CAB target gastric H+/K+ ATPase is the stomach.

Functional role of bone morphogenetic protein-4 in isolated canine parietal cells

Bone morphogenetic protein (BMP)-4 is an important regulator of cellular growth and differentiation. Expression of BMP-4 has been documented in the gastric mucosa. We reported that incubation of canine parietal cells with EGF for 72 h induced both parietal cell morphological transformation and inhibition of H(+)/K(+)-ATPase gene expression through MAPK-dependent mechanisms. We explored the role of BMP-4 in parietal cell maturation and differentiation. Moreover, we investigated if BMP-4 modulates the actions of EGF in parietal cells. H(+)/K(+)-ATPase gene expression was examined by Northern blots and quantitative RT-PCR. Acid production was assessed by measuring the uptake of [(14)C]aminopyrine. Parietal cell apoptosis was quantitated by Western blots with anti-cleaved caspase 3 antibodies and by counting the numbers of fragmented, propidium iodide-stained nuclei. MAPK activation and Smad1 phosphorylation were measured by Western blots with anti-phospho-MAPK and anti-phospho-Smad1 antibodies. Parietal cell morphology was examined by immunohistochemical staining of cells with anti-H(+)/K(+)-ATPase alpha-subunit antibodies. BMP-4 stimulated Smad1 phosphorylation and induced H(+)/K(+)-ATPase gene expression. BMP-4 attenuated EGF-mediated inhibition of H(+)/K(+)-ATPase gene expression and blocked EGF induction of both parietal cell morphological transformation and MAPK activation. Incubation of cells with BMP-4 enhanced histamine-stimulated [(14)C]aminopyrine uptake. BMP-4 had no effect on parietal cell apoptosis, whereas TGF-beta stimulated caspase-3 activation and nuclear fragmentation. In conclusion, BMP-4 promotes the induction and maintenance of a differentiated parietal cell phenotype. These findings may provide new clues for a better understanding of the mechanisms that regulate gastric epithelial cell growth and differentiation.

The renal H+, K+-ATPases as therapeutic targets

The kidney is an important regulatory organ responsible for maintaining constant blood volume and composition despite wide variations in the intake of food and water. Throughout the nephron, the functional unit of the kidney, there is a wide variety of proteins that function to add additional waste products and to recover needed materials from the lumen filtrate. The collecting duct of the nephron is the primary renal location for the H+, K+-ATPases, a group of ion pumps that function in both acid/base balance and potassium homeostasis. This review summarizes the present understanding of the structure and functions for the different subtypes of the H+, K+-ATPases under specific physiologic conditions. The obstacles in determining the pharmacologic properties of the different subtypes are considered and future directions for the inhibition and/or stimulation of the H+, K+-ATPases are evaluated.

Prevention of autoimmune gastritis in mice requires extra-thymic T-cell deletion and suppression by regulatory T cells

BACKGROUND AND AIMS

Autoimmune gastritis is one of the most common autoimmune diseases and is caused by a CD4(+) T-cell response to the gastric H(+)/K(+) ATPase encoded by Atp4a and Atp4b (H(+)/K(+) ATPase). Here, we have elucidated events that result in immunological tolerance to the H(+)/K(+) ATPase and thus the prevention of autoimmune gastritis.

METHODS

T cells from H(+)/K(+) ATPase-deficient mice and H(+)/K(+) ATPase-specific T-cell receptor transgenic mice were purified and transferred to wild-type (WT) or H(+)/K(+) ATPase-deficient recipients to assess the impact of exposure to antigen on pathogenicity.

RESULTS

The CD4(+) T-cell population from H(+)/K(+) ATPase-deficient mice was highly effective at inducing gastritis when compared with T cells from WT mice and, as a population, was comparatively resistant to the suppressive activity of regulatory T cells. Exposing T cells from H(+)/K(+) ATPase-deficient mice to H(+)/K(+) ATPase in WT mice decreased their ability to induce gastritis and resulted in a population that could be more easily suppressed by T(reg) cells. Transfer of clonotypic antigen-inexperienced H(+)/K(+) ATPase-specific T cells into WT mice resulted in extra-thymic clonal deletion.

CONCLUSIONS

Prevention of autoimmune gastritis requires the extra-thymic purging of highly autoaggressive H(+)/K(+) ATPase-specific T cells to produce a T-cell repertoire that is more susceptible to the suppressive activity of regulatory T cells. Taken together with recent published data describing the role of T-cell receptor signalling in the maintenance of regulatory T-cell populations, we propose that exposure of T cells to antigen in the periphery is able to both delete autoaggressive specificities and maintain regulatory T-cell activity, establishing a balance between pathogenicity and regulation.

[Selective COX-2 inhibitor delays experimental gastric ulcer healing by stimulating gastric acid secretion in rats]

OBJECTIVE

To observe the effect of selective cyclooxygenase-2 (COX-2) inhibitor on the healing of experimental gastric ulcer in rats and explore its mechanisms in light of gastric acid secretion.

METHODS

Gastric ulcers were induced in rats by an application of acetic acid to the serosal surface of the anterior gastric body. The effects of selective COX-2 inhibitor, celecoxib, on the healing of gastric ulcer, the total acidity of gastric juice, the expressions of H+, K+-ATPase mRNA and protein, and the ultrastructure of the parietal cell were observed in comparison with the effects of normal saline.

RESULTS

Nine days after ulcer induction, the ulcer area was 11.9-/+3.1 mm square and 19.7-/+3.8 mm square in rats with normal saline and celecoxib treatments, respectively (P<0.01). The total acidity of gastric juice and the expressions of H+, K+-ATPase mRNA and protein in celecoxib group were significantly higher than that in normal saline group at both 6 and 9 days after ulcer induction, but no significant difference was found between the two groups in the amount of secretary canaliculus and microvillus.

CONCLUSION

Selective COX-2 inhibitor can significantly delay the healing of experimental gastric ulcer in rats, the mechanism of which might be associated with enhanced digestive action of gastric acid on the new granulation tissue at the ulcer base as a result of celecoxib-stimulated gastric acid secretion of the parietal cells.

Role of homologous ASP334 and GLU319 in human non-gastric H,K- and Na,K-ATPases in cardiac glycoside binding

Cardiac steroids inhibit Na,K-ATPase and the related non-gastric H,K-ATPase, while they do not interact with gastric H,K-ATPase. Introducing an arginine, the residue present in the gastric H,K-ATPase, in the second extracellular loop at the corresponding position 334 in the human non-gastric H,K-ATPase (D334R mutation) rendered it completely resistant to 2mM ouabain. The corresponding mutation (E319R) in alpha1 Na,K-ATPase produced a approximately 2-fold increase of the ouabain IC(50) in the ouabain-resistant rat alpha1 Na,K-ATPase and a large decrease of the ouabain affinity of human alpha1 Na,K-ATPase, on the other hand this mutation had no effect on the affinity for the aglycone ouabagenin. These results provide a strong support for the orientation of ouabain in its biding site with its sugar moiety interacting directly with the second extracellular loop.

Analysis of the gastric H,K ATPase for ion pathways and inhibitor binding sites

New models of the gastric H,K ATPase in the E1K and E2P states are presented as the first structures of a K+ counter-transport P2-type ATPase exhibiting ion entry and exit paths. Homology modeling was first used to generate a starting conformation from the srCa ATPase E2P form (PDB code 1wpg) that contains bound MgADP. Energy minimization of the model showed a conserved adenosine site but nonconserved polyphosphate contacts compared to the srCa ATPase. Molecular dynamics was then employed to expand the luminal entry sufficiently to allow access of the rigid K+ competitive naphthyridine inhibitor, Byk99, to its binding site within the membrane domain. The new E2P model had increased separation between transmembrane segments M3 through M8, and addition of water in this space showed not only an inhibitor entry path to the luminal vestibule but also a channel leading to the ion binding site. Addition of K+ to the hydrated channel with molecular dynamics modeling of ion movement identified a pathway for K+ from the lumen to the ion binding site to give E2K. A K+ exit path to the cytoplasm operating during the normal catalytic cycle is also proposed on the basis of an E1K homology model derived from the E12Ca2+ form of the srCa ATPase (PDB code 1su4). Autodock analyses of the new E2P model now correctly discriminate between high- and low-affinity K+ competitive inhibitors. Finally, the expanded luminal vestibule of the E2P model explains high-affinity ouabain binding in a mutant of the H,K ATPase [Qiu et al. (2005) J. Biol. Chem. 280, 32349-32355].

IL-1beta modulation of H,K-ATPase alpha-subunit gene transcription in Helicobacter pylori infection

Helicobacter pylori infection of the human gastric body induces hypochlorhydria by perturbing acid secretion. H. pylori inhibits parietal cell H,K-ATPase alpha-subunit (HKalpha) gene and protein expression, providing a mechanistic basis for clinical hypochlorhydria. Given that H. pylori infection increases gastric mucosal IL-1beta, an acid secretory inhibitor, we investigated the role of IL-1beta in H. pylori-mediated inhibition of HKalpha transcription. Human gastric adenocarcinoma (AGS) cells were transfected with promoter-reporter constructs containing human HKalpha 5'-flanking sequence deletions. IL-1beta (10 ng/ml) had no effect on the transcriptional activity of six progressively shorter deletion constructs of the HKalpha promoter (HKalpha2179-HKalpha340) and significantly stimulated the activity of HKalpha206, HKalpha177, HKalpha165, and HKalpha102 deletion constructs (80%, 100%, 46%, and 35%, respectively). H. pylori inhibited the transcriptional activity of HKalpha2179, HKalpha206, HKalpha177, and HKalpha165; IL-1beta relieved the H. pylori inhibition of HKalpha2179 and HKalpha206 activity but not HKalpha177 and HKalpha165 activity. AGS cell pretreatment with a MEK1/2 inhibitor prevented the IL-1beta-mediated stimulation, but p38 and JNK pathway inhibitors did not. IL-1beta mRNA levels in AGS cells were low and unaffected by H. pylori, and ELISAs of H. pylori-conditioned AGS culture media showed no measurable IL-1beta secretion. These data indicate that an IL-1beta-dependent cis-response element lies downstream of -206 nt in the HKalpha promoter and that IL-1beta-mediated upregulation of HKalpha transcription is affected by an ERK1/2 kinase signal pathway. We conclude that an IL-1beta-responsive HKalpha cis element positively regulates HKalpha gene transcription in shortened deletion constructs and that H. pylori-induced inhibition of HKalpha transcription is not mediated by IL-1beta.

Soraprazan: setting new standards in inhibition of gastric acid secretion

After treatment of millions of patients suffering from gastroesophageal reflux disease (GERD) and other acid-related ailments with proton pump inhibitors, there are still unmet medical needs such as rapid and reliable pain relief, especially for nocturnal acid breakthrough. In this work, we introduce and characterize the biochemistry and pharmacology of the potassium-competitive acid blocker (P-CAB) soraprazan, a novel, reversible, and fast-acting inhibitor of gastric H,K-ATPase. Inhibitory and binding properties of soraprazan were analyzed together with its mode of action, its selectivity, and its in vivo potency. This P-CAB has an IC(50) of 0.1 microM if measured with ion leaky vesicles and of 0.19 microM in isolated gastric glands. With a K(i) of 6.4 nM, a K(d) of 26.4 nM, and a B(max) of 2.89 nmol/mg, this compound is a highly potent and reversible inhibitor of the H,K-ATPase. Soraprazan shows immediate inhibition of acid secretion in various in vitro models and in vivo and was found to be more than 2000-fold selective for H,K-ATPase over Na,K- and Ca-ATPases. Soraprazan is superior to esomeprazole in terms of onset of action and the extent and duration of pH elevation in vivo in the dog. Rapid and consistent inhibition of acid secretion by soraprazan renders the P-CABs a promising group of compounds for therapy of GERD.

The human non-gastric H,K-ATPase has a different cation specificity than the rat enzyme

The primary sequence of non-gastric H,K-ATPase differs much more between species than that of Na,K-ATPase or gastric H,K-ATPase. To investigate whether this causes species-dependent differences in enzymatic properties, we co-expressed the catalytic subunit of human non-gastric H,K-ATPase in Sf9 cells with the beta(1) subunit of rat Na,K-ATPase and compared its properties with those of the rat enzyme (Swarts et al., J. Biol. Chem. 280, 33115-33122, 2005). Maximal ATPase activity was obtained with NH(4)(+) as activating cation. The enzyme was also stimulated by Na(+), but in contrast to the rat enzyme, hardly by K(+). SCH 28080 inhibited the NH(4)(+)-stimulated activity of the human enzyme much more potently than that of the rat enzyme. The steady-state phosphorylation level of the human enzyme decreased with increasing pH, [K(+)], and [Na(+)] and nearly doubled in the presence of oligomycin. Oligomycin increased the sensitivity of the phosphorylated intermediate to ADP, demonstrating that it inhibited the conversion of E(1)P to E(2)P. All three cations stimulated the dephosphorylation rate dose-dependently. Our studies support a role of the human enzyme in H(+)/Na(+) and/or H(+)/NH(4)(+) transport but not in Na(+)/K(+) transport.

Characterization of the rabbit HKalpha2 gene promoter

The HKalpha2 gene directs synthesis of the HKalpha2 subunit of the H(+), K(+)-ATPase. In the kidney and colon, the gene is highly expressed and is thought to play a role in potassium (K(+)) conservation. The rabbit has been an important experimental system for physiological studies of ion transport in the kidney, so the rabbit HKalpha2 gene has been cloned and characterized. The genomic clones and the previously reported HKalpha2a and HKalpha2c subunit cDNAs provided a means to address several issues regarding the structure and expression of the HKalpha2 gene. First, the genomic organization established that the rabbit HKalpha2 gene was unambiguously homologous to the mouse HKalpha2 gene and the human ATP1AL1 gene. Second, the mapping of the transcription start site for the alternate transcript, HKalpha2c, confirmed that it was an authentic rabbit transcript. Finally, isolation of DNA from the 5' end of the HKalpha2 gene enabled us to initiate studies on its regulation in the rabbit cortical collecting duct. The promoter and two putative negative regulatory regions were identified and the effect of cell confluency on gene expression was studied.

Expression of gastric ghrelin and H(+)-K(+)-ATPase mRNA in weanling piglets and effect of ghrelin on H(+)-K(+)-ATPase expression and activity in gastric mucosal cells in vitro

This study was designed to investigate the effect of ghrelin on gastric acid secretion in weaning piglets both in vivo and in vitro. Thirty newborn piglets were selected from six litters and on 28, 35 (weaning), 38, 42 and 45d of age, respectively, one piglet from each litter was killed and the mucosal tissue from gastric fundus was collected for detecting ghrelin mRNA as well as H(+)-K(+)-ATPase mRNA expression and activity. Primary cultures of gastric mucosal cells from 5-week-old weaning piglets were challenged with 3x10(-5), 3x10(-4), 3x10(-3), 3x10(-2) and 3x10(-1)nmol/ml h-ghrelin, respectively, for 4h in order to further clarify the effect of ghrelin on gastric H(+)-K(+)-ATPase mRNA expression and activity. Ghrelin mRNA expression in gastric fundus kept stable from 28d to 42d, followed by a sudden increase on 45d, exhibiting a peak that was significantly higher than any other age groups investigated. H(+)-K(+)-ATPase activity and mRNA expression showed similar trends of increase with slightly different timing. H(+)-K(+)-ATPase mRNA expression tended to increase on 42d, while H(+)-K(+)-ATPase activity started to rise from 35d, but neither of them reached significantly higher levels until 45d. In vitro, ghrelin significantly increased H(+)-K(+)-ATPase activity of gastric mucosal cells at 3x10(-4), 3x10(-3), and 3x10(-2)nmol/ml, but augmented H(+)-K(+)-ATPase mRNA expression only at 3x10(-4)nmol/ml. The results indicate that ghrelin mRNA expression is up-regulated 10 days after weaning in the gastric fundus of piglets, coinciding with the increase of H(+)-K(+)-ATPase mRNA expression and activity. Ghrelin acts on gastric mucosal cells to stimulate both mRNA expression and activity of H(+)-K(+)-ATPase in vitro.

An extracellular loop of the human non-gastric H,K-ATPase alpha-subunit is involved in apical plasma membrane polarization

The human non-gastric H,K-ATPase, ATP1AL1, belongs to the gene family of P-type ATPases. Consistent with their physiological roles in ion transport, members of this group, including the Na,KATPase and the gastric and non-gastric H,K-ATPases, are differentially polarized to either the basolateral or apical plasma membrane in epithelial cells. However, their polarized distribution is highly complex and depends on specific sorting signals or motifs which are recognized by the subcellular targeting machinery. For the gastric H,K-ATPase it has been suggested that the 4(th) transmembrane spanning domain (TM4) and its flanking regions induce conformational sorting motifs which direct the ion pump exclusively to the epithelial apical membrane. Here, we show in transfected Madin-Darby canine kidney (MDCK) cells that the related non-gastric H,KATPase, ATP1AL1, does contain similar sorting motifs in close proximity to TM4. A short extracellular loop between TM3 and TM4 is critical for this pump's apical delivery. A single point mutation in the corresponding region redirects ATP1AL1 to the basolateral membrane. In conclusion, our work provides further evidence that the cellular distribution of P-type ATPases is determined by conformational sorting motifs.

Gastric type H+,K+-ATPase in the cochlear lateral wall is critically involved in formation of the endocochlear potential

Cochlear endolymph has a highly positive potential of approximately +80 mV known as the endocochlear potential (EP). The EP is essential for hearing and is maintained by K(+) circulation from perilymph to endolymph through the cochlear lateral wall. Various K(+) transport apparatuses such as the Na(+),K(+)-ATPase, the Na(+)-K(+)-2Cl(-) cotransporter, and the K(+) channels Kir4.1 and KCNQ1/KCNE1 are expressed in the lateral wall and are known to play indispensable roles in cochlear K(+) circulation. The gastric type of the H(+),K(+)-ATPase was also shown to be expressed in the cochlear lateral wall (Lecain E, Robert JC, Thomas A, and Tran Ba Huy P. Hear Res 149: 147-154, 2000), but its functional role has not been well studied. In this study we examined the precise localization of H(+),K(+)-ATPase in the cochlea and its involvement in formation of EP. RT-PCR analysis showed that the cochlea expressed mRNAs of gastric alpha(1)-, but not colonic alpha(2)-, and beta-subunits of H(+),K(+)-ATPase. Immunolabeling of an antibody specific to the alpha(1) subunit was detected in type II, IV, and V fibrocytes distributed in the spiral ligament of the lateral wall and in the spiral limbus. Strong immunoreactivity was also found in the stria vascularis. Immunoelectron microscopic examination exhibited that the H(+),K(+)-ATPase was localized exclusively at the basolateral site of strial marginal cells. Application of Sch-28080, a specific inhibitor of gastric H(+),K(+)-ATPase, to the spiral ligament as well as to the stria vascularis caused prominent reduction of EP. These results may imply that the H(+),K(+)-ATPase in the cochlear lateral wall is crucial for K(+) circulation and thus plays a critical role in generation of EP.

Phosphorylation of S955 at the protein kinase A consensus promotes maturation of the alpha subunit of the colonic H+,K+ -ATPase

All the alpha subunits of the Na+,K+ -ATPases and H+,K+ -ATPases have a protein kinase A (PKA) consensus sequence near or in the ninth transmembrane domain. The role of this domain in influencing alpha subunit synthesis/degradation, plasma membrane localization, and 86Rb+ uptake has not been established for the alpha subunit of the colonic H+,K+ -ATPase. This study examined the effect of mutating S955 (within the PKA consensus site of the alpha subunit of the colonic H+,K+ -ATPase [HKalpha2]) to alanine (S955/A) or aspartic acid (S955/D) on alpha subunit expression and function. The results demonstrate that a negatively charged amino acid at position 955 of HKalpha2 promotes higher expression levels of both whole-cell and plasma membrane-localized HKalpha2. Moreover, inhibition of PKA reduced expression of wild-type HKalpha2 and associated 86Rb+ uptake. Last, the activity of the HKalpha2 S955/A was rescued by treatment with 4-phenylbutyric acid, a compound that was shown previously to restore function to the cystic fibrosis transmembrane conductance regulator.

A continuous dietary supply of free calcium formate negatively affects the parietal cell population and gastric RNA expression for H+/K+-ATPase in weaning pigs

Baby formula acidification can be used to reduce diarrhea. Calcium formate is a dietary acidifier frequently used in animal weaning diets; it is also a source of available calcium. Gastric acidification reduces gastrin release and hydrochloric acid (HCl) secretion. To study the medium-term effects on fundic gastric mucosa, we fed weaning pigs control diets or diets supplemented with free or fat-protected calcium formate. We evaluated the following: 1) the number of HCl-secreting parietal cells, by immunohistochemistry using an antibody against H(+)/K(+)-ATPase; 2) the number of enteroendocrine cells immunohistochemically stained with chromogranin A (CGA), somatostatin, and histamine (HIS); and 3) the expression of the H(+)/K(+)-ATPase gene, by real-time RT-PCR in the oxyntic mucosa. Cells co-staining for CGA and HIS were defined as enterochromaffin-like (ECL) cells. Pigs fed calcium formate had fewer parietal cells and a lower expression of the H(+)/K(+)-ATPase gene than the controls (P < 0.05). This reduction did not occur in pigs fed fat-protected calcium formate. Somatostatin immune-reactive cells were also more numerous in pigs fed free calcium formate than in controls (P < 0.05). The number of ECL cells was not affected. Using covariance analysis, the number of parietal cells explained part of the differences in the expression of H(+)/K(+)-ATPase gene (positive correlation, r = 0.385, P < 0.01), and excluded the statistical significance of the diet. In the future, the effects on the oxyntic mucosa should be checked when the diet supplemented with calcium formate is discontinued. Furthermore, a reduction in the number of parietal cells could impair the absorption of vitamin B-12 due to a reduced secretion of the intrinsic factor by these cells.

Parietal cell hyperstimulation and autoimmune gastritis in cholera toxin transgenic mice

The stimulation of gastric acid secretion from parietal cells involves both intracellular calcium and cAMP signaling. To understand the effect of increased cAMP on parietal cell function, we engineered transgenic mice expressing cholera toxin (Ctox), an irreversible stimulator of adenylate cyclase. The parietal cell-specific H(+),K(+)-ATPase beta-subunit promoter was used to drive expression of the cholera toxin A1 subunit (CtoxA1). Transgenic lines were established and tested for Ctox expression, acid content, plasma gastrin, tissue morphology, and cellular composition of the gastric mucosa. Four lines were generated, with Ctox-7 expressing approximately 50-fold higher Ctox than the other lines. Enhanced cAMP signaling in parietal cells was confirmed by observation of hyperphosphorylation of the protein kinase A-regulated proteins LASP-1 and CREB. Basal acid content was elevated and circulating gastrin was reduced in Ctox transgenic lines. Analysis of gastric morphology revealed a progressive cellular transformation in Ctox-7. Expanded patches of mucous neck cells were observed as early as 3 mo of age, and by 15 mo, extensive mucous cell metaplasia was observed in parallel with almost complete loss of parietal and chief cells. Detection of anti-parietal cell antibodies, inflammatory cell infiltrates, and increased expression of the Th1 cytokine IFN-gamma in Ctox-7 mice suggested that autoimmune destruction of the tissue caused atrophic gastritis. Thus constitutively high parietal cell cAMP results in high acid secretion and a compensatory reduction in circulating gastrin. High Ctox in parietal cells can also induce progressive changes in the cellular architecture of the gastric glands, corresponding to the development of anti-parietal cell antibodies and autoimmune gastritis.

Conversion of the Low Affinity Ouabain-binding Site of Non-gastric H,K-ATPase into a High Affinity Binding Site by Substitution of Only Five Amino Acids

P-type ATPases of the IIC subfamily exhibit large differences in sensitivity toward ouabain. This allows a strategy in which ouabain-insensitive members of this subfamily are used as template for mutational elucidation of the ouabain-binding site. With this strategy, we recently identified seven amino acids in Na,K-ATPase that conferred high affinity ouabain binding to gastric H,K-ATPase (Qiu, L. Y., Krieger, E., Schaftenaar, G., Swarts, H. G. P., Willems, P. H. G. M., De Pont, J. J. H. H. M., and Koenderink, J. B. (2005) J. Biol. Chem. 280, 32349-32355). Because important, but identical, amino acids were not recognized in that study, here we used the non-gastric H,K-ATPase, which is rather ouabain-insensitive, as template. The catalytic subunit of this enzyme, in which several amino acids from Na,K-ATPase were incorporated, was expressed with the Na,K-ATPase beta1 subunit in Xenopus laevis oocytes. A chimera containing 14 amino acids, located in M4, M5, and M6, which are unique to Na,K-ATPase, displayed high affinity ouabain binding. Four of these residues, all located in M5, appeared dispensable for high affinity binding. Individual mutation of the remaining 10 residues to their non-gastric H,K-ATPase counterparts yielded five amino acids (Glu312,Gly319, Pro778, Leu795, and Cys802) whose mutation resulted in a loss of ouabain binding. In a final gain-of-function experiment, we introduced these five amino acids in different combinations in non-gastric H,K-ATPase and demonstrated that all five were essential for high affinity ouabain binding. The non-gastric H,K-ATPase with these five mutations had a similar apparent affinity for ouabain as the wild type Na,K-ATPase and showed a 2000 times increased affinity for ouabain in the NH4+-stimulated ATPase activity in membranes of transfected Sf9 cells.

Helicobacter pylori eradication induces marked increase in H+/K+-adenosine triphosphatase expression without altering parietal cell number in human gastric mucosa

BACKGROUND AND AIMS

Gastric acid secretion is downregulated by Helicobacter pylori infection and upregulated after its eradication, but the mechanisms are still unclear. We examined the effects of H pylori eradication on the number of parietal cells and on expression of molecules functioning in acid secretion in the human gastric mucosa.

METHODS

We enrolled 111 consecutive men with chronic gastritis induced by H pylori. Biopsy specimens were endoscopically obtained before and 12 weeks after successful eradication of H pylori and parietal cell numbers were counted. mRNA expression levels of H+/K+-adenosine triphosphatase (H+/K+-ATPase), anion exchanger 2, M3 muscarinic receptor, intrinsic factor, and interleukin 1beta were determined with a real time reverse transcriptase-polymerase chain reaction method. The severity of gastric atrophy was evaluated using the serum pepsinogen I/II ratio.

RESULTS

No significant difference was observed in parietal cell numbers before and after H pylori eradication. Median mRNA expression levels of H+/K+-ATPase in the gastric mucosa increased 250-fold after H pylori eradication accompanied by attenuation of interleukin 1beta. A large increase in H+/K+-ATPase expression was observed even in patients with severe atrophic gastritis. In contrast, fold increases in mRNA expression levels, including intrinsic factor, anion exchanger 2, and M3 muscarinic receptor, after eradication therapy, were limited to 1.4, 2.3, and 2.5 times, respectively.

CONCLUSIONS

In the absence of alteration of parietal cell number, gastric H+/K+-ATPase mRNA expression was markedly restored after successful H pylori eradication, suggesting a central role for the restoration of H+/K+-ATPase expression in gastric acid secretion recovery after H pylori eradication.

H+-ATPase activity in selective disruption of H+-K+-ATPase alpha 1 gene of mice under normal and K-depleted conditions

The outer medullary collecting duct (OMCD) plays an important role in acid-base homeostasis by two luminal proton ATPases, H(+)-ATPase and H(+)-K(+)-ATPase (HKA), both of which are in the intercalated cells (ICs) of OMCD. We showed previously that HKAalpha1 (gastric H(+)-K(+)-ATPase) activity is the essential H(+)-K(+)-ATPase activity under normal conditions, and that HKAalpha2 (colonic H(+)-K(+)-ATPase) is induced and mediates increased proton-secretion under K-depleted conditions. To better understand the role of H(+)-ATPase (potassium-independent) in acid secretion and the relationship between H(+)-ATPase and a specific HKA isoform, we examined H(+)-ATPase activity in the H(+)-K(+)-ATPasealpha1 knockout (KO) mice under normal and K-depleted conditions. Mice were fed a potassium-free diet and studied after 7 days. Segments of the OMCD were perfused in vitro, and intracellular pH (pH(i)) was measured by ratiometric fluorescence microscopy using the pH-sensitive indicator BCECF-AM. The isolated OMCD tubules obtained from mice fed a potassium-free diet were examined by fluorescent immunocytochemistry with an antibody to the 31-kDa subunit of H(+)-ATPase (E-11) and were compared with those obtained from a normal diet. In the absence of Na(+) and K(+), the H(+)-ATPase-mediate pH(i) recovery rates were 6.7 +/- 1.1 x 10(-4) units/s (n = 7 ICs) in wild-type (WT) mice and increased to 8.7 +/- 1.8 x 10(-4) (P < 0.05; n = 6) in HKAalpha1 KO mice. K-independent proton transport activity was significantly inhibited by the H(+)-ATPase inhibitor bafilomycin A(1) (BAF, 10 nM) with luminal applied in both WT and KO mice. Comparison of the results indicated upregulation of BAF-sensitive H(+)-ATPase activity in KO mice. To determine the intracellular localization of H(+)-ATPase in the intercalated cells of OMCD, we dissected the OMCD and performed fluorescent immunocytochemistry with the H(+)-ATPase antibody in the WT and KO mice. In the WT mice, on normal diet, H(+)-ATPase staining distributed diffusely throughout the intercalated cells and was slightly polarized to the apical plasma membrane in the KO mice, consistent with increase in the H(+)-ATPase-mediate pH(i) recovery in the KO mice. One week of a potassium-free diet resulted in a significant increase in the degree of H(+)-ATPase polarization at the apical plasma membrane in both WT and KO mice. Hypokalemia stimulates H(+)-ATPase in the intercalated cells of OMCD of both WT and KO mice. The enhanced activity of H(+)-ATPase plays an important role in compensatory proton secretion in the HKAalpha1 KO mice under normal conditions.