Effects of chronic hypokalemia on renal expression of the "gastric" H(+)-K(+)-ATPase alpha-subunit gene

Chronic potassium restriction leads to active potassium reabsorption in the late distal nephron and collecting duct, segments known to express "gastric" H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) alpha-subunit mRNA. In this study, the cellular distribution and relative abundance of mRNA encoding this isoform were examined in kidneys of normal and potassium-deprived (2 wk) rats. In situ hybridization with isoform-specific cRNA probes demonstrated prominent expression of this gene in the connecting segment (CNT), entire collecting duct, and renal papillary surface epithelium in a comparable distribution in both groups of rats. Hypertrophy of the outer medullary collecting ducts in the inner stripe of potassium-restricted rats was observed. Competitive polymerase chain reaction analysis revealed twofold greater levels of gastric H(+)-K(+)-ATPase alpha-subunit mRNA (normalized to the level of beta-actin mRNA) in the cortex, but roughly comparable levels in the outer and inner medulla, of potassium-restricted rats compared with controls. These data suggest that chronic potassium restriction results in modestly enhanced renal cortical expression of the gastric H(+)-K(+)-ATPase alpha-subunit gene and that this isoform may participate in potassium conversation by the CNT and cortical collecting duct during potassium deprivation.

[Gastric proton pump: genes and their expression]

The gastric proton pump (H+/K(+)-ATPase) is a typical P-type ATPase consisting of the alpha and beta subunits and secreting acid into stomach. This enzyme is similar to Na+/K(+)-ATPase in primary structure and gene organization. The exon/intron organizations of the genes for the two subunits of H+/K(+)-ATPase are highly similar to those of the corresponding subunits of Na+/K(+)-ATPase. In contrast to Na+/K(+)-ATPase subunits, alpha and beta subunits of H+/K(+)-ATPase are expressed specifically in gastric parietal cells. Consistently, a sequence motif [(G/C)PuPu(G/C)NGAT(A/T)PuPy] in the 5'upstream regions of the H+/K(+)-ATPase subunit genes was recognized by the gastric mucosal nuclear protein(s). We found that two novel zinc-finger proteins (GATA-GT1 and GATA-GT2) are present in the gastric parietal cells and bind to this motif. These proteins activate the transcription of the reporter gene connected with the 5'-upstream regions of the H+/K(+)-ATPase subunit genes. These results suggest that gastric GATA-DNA binding proteins have roles in activating transcription of H+/K(+)-ATPase genes in parietal cells.

Genetic ablation of parietal cells in transgenic mice: a new model for analyzing cell lineage relationships in the gastric mucosa

The gastric mucosa of mammalian stomach contains several differentiated cell types specialized for the secretion of acid, digestive enzymes, mucus, and hormones. Understanding whether each of these cell lineages is derived from a common stem cell has been a challenging problem. We have used a genetic approach to analyze the ontogeny of progenitor cells within mouse stomach. Herpes simplex virus 1 thymidine kinase was targeted to parietal cells within the gastric mucosa of transgenic mice, and parietal cells were ablated by treatment of animals with the antiherpetic drug ganciclovir. Ganciclovir treatment produced complete ablation of parietal cells, dissolution of gastric glands, and loss of chief and mucus-producing cells. Termination of drug treatment led to the reemergence of all major gastric epithelial cell types and restoration of glandular architecture. Our results imply the existence of a pluripotent stem cell for the gastric mucosa. Parietal cell ablation should provide a model for analyzing cell lineage relationships within the stomach as well as mechanisms underlying gastric injury and repair.

Genomic organization of the human ATP1AL1 gene encoding a ouabain-sensitive H,K-ATPase

The human ATP1AL1 gene belongs to the family of Na,K-ATPase and H,K-ATPase (X,K-ATPases) genes. It encodes a catalytic subunit of hitherto unknown human ouabain-sensitive H,K-ATPase that represents a novel third group of X,K-ATPases distinct from the known Na,K-ATPase and gastric H,K-ATPase. Cloning of the ATP1AL1 gene is described in this report. The exon-intron structure of ATP1AL1 was found to be very similar to that of related genes. It contains 23 exons and spans approximately 32 kb of genomic DNA. All ATP1AL1 exons and 12 of its 22 introns were entirely sequenced. A total of nine Alu repeats were identified in introns. The transcription initiation site was mapped 187 bp upstream of the ATG initiation codon by primer extension and S1 nuclease protection analyses of RNA from human skin and colon. Sequence analysis of the 5'-flanking region (1.48 kb) revealed numerous potential binding sites for transcription factors Sp1 and AP2 and one putative NF-kappa B binding site. The 0.85-kb region from position -484 (5'-flanking region) to position +369 (intron 1) meets the structural criteria of a CpG island. It is suggested that the ATP1AL1 gene contains two poly(A) addition sites that may function in a tissue-specific manner.

Diphtheria toxin-mediated ablation of parietal cells in the stomach of transgenic mice

The self-renewing epithelial populations present in the gastric units of the mouse stomach are descended from a multipotent stem cell and undergo an orderly migration-associated differentiation followed by apoptosis. The steady state census of the three principal cell types (acid-producing parietal cells, mucus-producing pit cells, and pepsinogen and intrinsic factor-producing zymogenic cells) is accurately controlled, despite marked differences in the rates of migration of each lineage. A transgenic mouse model has been created to define functional interrelationships between the proliferation, differentiation, and death programs of these lineages. Nucleotides -1035 to +24 of the noncatalytic beta subunit gene of mouse H+/K+-ATPase were used to direct expression of an attenuated diphtheria toxin A subunit in the parietal cell lineage. These transcriptional regulatory elements are not active in members of the pit and zymogenic lineages. Stomachs, prepared from postnatal day 28-80 transgenic mice and their normal littermates, were subjected to single- and multilabel immunohistochemical studies as well as qualitative and quantitative light and electron microscopic morphologic analyses. The toxin produced complete ablation of differentiated parietal cells. Loss of parietal cells was accompanied by a 5-fold increase in the number of undifferentiated granule-free cells located in the proliferative compartment of gastric units. This amplified population of granule-free cells included the multipotent stem cell as well as committed precursors of the pit and zymogenic lineages. Loss of mature parietal cells was also associated with (i) a block in the differentiation program of the zymogenic lineage with an accumulation of pre-neck cells and a depletion of their neck and mature zymogenic cell descendants, and (ii) an approximately 2-fold amplification of pit cells. These findings are consistent with the notion that epithelial homeostasis within gastric units is maintained by instructive interactions between their different cell lineages. Unlike pit and zymogenic cells, parietal cells complete their differentiation in the gastric unit's proliferative compartment before undergoing a bipolar migration along the unit. Thus, the mature parietal cell is in a strategic position to influence decision-making among gastric epithelial cell precursors and to modulate the migration-associated terminal differentiation programs of the pit and zymogenic lineages.

Functional expression of gastric H+,K(+)-ATPase and site-directed mutagenesis of the putative cation binding site and catalytic center

Gastric H+,K(+)-ATPase was functionally expressed in the human kidney HEK293 cell line. The expressed enzyme catalyzed ouabain-resistant K(+)-dependent ATP hydrolysis. The K(+)-ATPase activity was inhibited by SCH 28090, a specific inhibitor of gastric proton pump, in a dose-dependent manner. By using this functional expression system in combination with site-directed mutagenesis, we investigated effects of mutations in the putative cation binding site and the catalytic center of the gastric H+,K(+)-ATPase. In Na+,K(+)-ATPase, the glutamic acid residue in the 4th transmembrane segment is regarded as one of the residues responsible for the K(+)-induced conformational change (Kuntzweiler, T. A., Wallick, E. T., Johnson, C. L., and Lingrel, J. B. (1995) J. Biol. Chem. 270, 2993-3000). When the corresponding glutamic acid (Glu-345) of H+,K(+)-ATPase was mutated to aspartic acid, lysine, or valine, the SCH 28080-sensitive K(+)-ATPase activity was abolished. However, when this residue was replaced by glutamine, about 50% of the activity was retained. This mutant showed a 10-fold lower affinity for K+ (Km = 2.6 mM) compared with the wild-type enzyme (Km = 0.24 mm). Thus, Glu-345 is important in determining the K+ affinity of H+,K(+)-ATPase. When the aspartic acid residue in the phosphorylation site was mutated to glutamic acid, this mutant showed no SCH 28080-sensitive K(+)-ATPase activity. Thus, amino acid replacement of the phosphorylation site is not tolerated and a stringent structure appears to be required for enzyme activity. When the lysine residue in the fluorescein isothiocyanate binding site (part of ATP binding site) was mutated to arginine, asparagine, or glutamic acid, the SCH 28080-sensitive K(+)-ATPase activity was eliminated. However, the mutant in which this residue was changed to glutamine had about 30% of the activity, suggesting that amino acid replacement of this site is tolerated to a certain extent.

[Genes for gastric proton pump and their transcriptional regulation]

The highly differentiated gastric parietal cell has a characteristic morphology and plays a specialized role in the hydrochloric acid secretion into the stomach lumen. The major enzyme in this system is an ATP-driven proton pump, the H+/K(+)-ATPase, which is responsible for proton translocation across the apical plasma membrane. The primary structures of the catalytic alpha and glycosylated beta subunits, and their transmembrane topology are similar to those of the corresponding subunits of Na+/K(+)-ATPase, suggesting that the reaction mechanism of both ATPases would be essentially the same if not identical. Most of the positions of introns in the H+/K(+)-ATPase subunit gene. These findings suggest that the alpha and beta subunit genes, respectively, of the two ATPases were derived from the common ancestors. We found that a DNA sequence motif, (G/C)PuPu(G/C)NGAT(A/T)PuPy, was located in the upstream regions of both alpha and beta subunit genes from human and rat. This motif may be a binding site for a positive transcriptional regulator that functions specifically in the parietal cells. cDNA cloning and in situ hybridization demonstrated that novel zinc finger proteins (GATA-GT1 and GATA-GT2) are present in the gastric parietal cells. These proteins bind to the (G/C)PuPu(G/C)NGAT(A/T)PuPy motif. Furthermore, they activate the transcription of the reporter gene with the 5'-upstream region of the alpha or beta subunit gene. These results suggest that gastric GATA DNA-binding proteins play important roles in transcriptional activation of H+/K(+)-ATPase genes in the parietal cells.

Renal H,K-ATPase: structure, function and regulation

The H,K-ATPase comprises a family of isoenzymes with unique biochemical, pharmacological, and regulatory properties. This review explores recent advances in discovery of the molecular biology, biochemistry, and function of these ion pumps, with particular emphasis on the implications for renal potassium and proton handling. Structure-function correlations governing the ion transport mechanisms, inhibitor binding sites, oligomerization, and transcriptional control of the H,K-ATPase isoforms are examined. Functional studies of H,K-ATPase in renal tubules and cultured epithelial cells are analyzed and related to data concerning the expression and distribution of the H,K-ATPase gene products in the kidney. Functional and molecular biological evidence for adaptive changes in renal H,K-ATPase expression during the evolution of potassium and acid-base disturbances are discussed. Current investigative challenges and avenues for future research of these enzymes are presented.

[Molecular and functional diversity of NA,K-ATPase and renal H,K-ATPases]

Potassium homeostasis is a determinant factor in the maintenance of many vital functions. Cell excitability, for instance, in striate and cardiac muscle, as well as in neurons, is dependent upon the ratio of potassium levels on either side of the plasmic membrane. Acute or chronic mechanisms of adjustment to disorders of bodily potassium balance exist in muscle, the kidney and distal colon. Na+K(+)-ATPase is involved in potassium transfers between the extracellular and intracellular compartments, in particular in muscle, enabling the creation of an appropriate trans-membrane K gradient. Na+K(+)-ATPase also participates in the development and maintenance of a transmembrane potassium electrochemical gradient necessary for potassium secretion processes in the kidney or distal colon. Colonic and renal H+K(+)-ATPases, so-called non-gastric H+K(+)-ATPases, are involved in the absorption of potassium from the gastrointestinal lumen or urinary fluid. They have an important role to play during chronic disorders, e.g. chronic bodily potassium depletion. Renal H+K(+)-ATPases and Na+K-ATPase are P-ATPases, consisting of a heterodimer of two alpha and beta sub-units. Several isoforms have been identified, on both a molecular and functional basis, for both the alpha and beta sub-unit. These two ATPases form part of the Na+K(+)-ATPase/H+K(+)-ATPase gene group. These pumps share many structural and functional similarities, but also particular functional specificities, probably involved in separate physiological roles for each isoform. Four isoforms of the alpha sub-unit and two isoforms of the beta sub-unit of Na+K(+)-ATPase have been identified. Sensitivity to ouabain, a Na+K(+)-ATPase inhibitor, differs according to the alpha isoform present in the alpha beta heterodimer. It is also involved in the catalytic cycle and influences pump potassium affinity. Several H+K(+)-ATPases have been identified from a molecular standpoint: gastric H+K(+)-ATPases and a colonic H+K(+)-ATPase found more recently. Recent studies have shown that both these H+K(+)-ATPases exist in the kidney. "Gastric" H+K(+)-ATPase is active along the entire length of the collecting tubule, in rats exposed to a normal potassium intake. In contrast, colonic H+K(+)-ATPase is active only in the cells of the external medullary collecting duct. This activity cannot be detected in animals on a standard diet but is very powerfully induced by potassium depletion. Activity is independent of steroidal status and of aldosterone in particular. Identification of a molecular homologue in the bladder of the amphibian Bufo marinus (the functional equivalent of the cortical collecting duct of mammals) has enabled the development of functional tests by activity in the oocyte of Xenopus laevis. The use this functional approach has shown that bladder H+K(+)-ATPase, just like that of rat distal colon, is sensitive to ouabain, an inhibitor considered up to now to be specific to Na+K(+)-ATPase. In contrast, this H+K(+)-ATPase shows little or no sensitivity to Sch 28080, a "classical" gastric H+K(+)-ATPase inhibitor. It thus seems that two H+K(+)-ATPases, different from a molecular standpoint, exist in rat kidney. They differ in terms of their cellular activity, regulation and functional properties. This is strongly suggestive of a specific role of each of them in potassium homeostasis, a role which remains to be defined. The use of genetically modified animals, as well as of physiological studies more focussed on this question, should provide clarification of the specific functional role of each isoform of the alpha and beta sub-units of renal H+K(+)-ATPases and Na+K(+)-ATPase. Extrapolation of these results to human pathophysiology is quite another challenge. Control of Na+K(+)-ATPase activity by endoouabain and its effects on cardiovascular pathophysiology must be identified. An H+K(+)-ATPase with molecular and functional characteristics similar to those of amphibian bladder and rat colon H+K(+)-A

The membrane topology of the rat sarcoplasmic and endoplasmic reticulum calcium ATPases by in vitro translation scanning

The membrane topology of the rat endoplasmic reticulum (ER) and sarcoplasmic reticulum (SR) Ca2+ ATPases were investigated using in vitro transcription/translation of fusion vectors containing DNA sequences encoding putative membrane-spanning domains. The sequences of these Ca2+ ATPases are identical except for the COOH-terminal end, which contains an additional predicted transmembrane segment in the ER ATPase. The M0 and M1 fusion vectors (Bamberg, K., and Sachs, G. (1994) J. Biol. Chem. 269, 16909-16919) encode the NH2-terminal 101 (M0 vector) or 139 (M1 vector) amino acids of the H,K-ATPase alpha subunit followed by a linker region for insertion of putative transmembrane sequences and, finally, the COOH-terminal 177 amino acids of the H,K-ATPase beta subunit containing five N-linked glycosylation consensus sequences. The linker region was replaced by the putative transmembrane domains of the Ca2+ ATPases, either individually or in pairs. Transcription and translation were performed using [35S]methionine in a reticulocyte lysate system in the absence or presence of canine pancreatic microsomes. The translated fusion protein was identified by autoradiography following separation using SDS-polyacrylamide gel electrophoresis. When testing single transmembrane segments, this method detects signal anchor activity with M0 or stop transfer activity with M1. The first four predicted SERCA transmembrane domains acted as both signal anchor and stop transfer sequences. A construct containing the fifth predicted transmembrane segment was able to act only as a stop transfer sequence. The sixth transmembrane segment did not insert cotranslationally into the membrane. The seventh was able to act as both a signal anchor and stop transfer sequence, and the eighth showed stop transfer ability in the M1 vector. The ninth transmembrane segment had both signal anchor and stop transfer capacity, whereas the tenth transmembrane segment showed only stop transfer sequence properties. The eleventh transmembrane sequence, unique to the ER Ca2+ ATPase, had both signal anchor and stop transfer properties. These translation data provide direct experimental evidence for 8 or 9 of the 10 or 11 predicted transmembrane sequences in the current topological models for the SR or ER Ca2+ ATPases, respectively.

Human ATP1AL1 gene encodes a ouabain-sensitive H-K-ATPase

The cDNA for ATP1AL1, the fifth member of the human Na-K-adenosinetriphosphatase (ATPase)/H-K-ATPase gene family, was recently cloned (A. V. Grishin, V. E. Sverdlov, M. B. Kostina, and N. N. Modyanov. FEBS Lett. 349: 144-150, 1994). The encoded protein (ATP1AL1) has all the primary structural features common to the catalytic alpha-subunit of ion-transporting P-type ATPases and is similar (63-64% identity) to the Na-K-ATPase alpha-subunit isoforms and the gastric H-K-ATPase alpha-subunit. In this study, ATP1AL1 was expressed in Xenopus laevis oocytes in combination with the beta-subunit of rabbit gastric H-K-ATPase. The functional properties of the stable alpha/beta-complex were studied by 86Rb+ uptake and demonstrated that ATP1AL1 is a novel human K(+)-dependent ATPase [apparent half-constant activation/(K1/2) for K+ approximately 375 microM)]. ATP1AL1-mediated inward K+ transport was inhibited by ouabain (inhibition constant approximately 13 microM) and was found to be inhibited by high concentrations of SCH-28080 (approximately 70% at 500 microM). ATP1AL1 expression resulted in the alkalinization of the oocytes' cytoplasm and ouabain-sensitive proton extrusion, as measured with pH-sensitive microelectrodes. These data argue that ATP1AL1 is the catalytic alpha-subunit of a human nongastric P-type ATPase capable of exchanging extracellular potassium for intracellular protons.

Expression of colonic H-K-ATPase mRNA in cortical collecting duct: regulation by acid/base balance

In addition to the gastric isoform of H-K-ATPase, the colonic isoform is also expressed in the kidney, but its intrarenal localization and exact function are not known. The goal of this study was to determine whether the colonic H-K-ATPase is expressed in the rabbit cortical collecting duct (CCD) and whether it is regulated by changes in acid/base balance. With quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) with RNA isolated from immunodissected rabbit CCD cells and degenerate oligonucleotide primers, a PCR product of the predicted size (approximately 430 bp) was amplified. The amplified DNA was further characterized by nested PCR and sequencing. Direct sequencing of the 434-bp PCR product revealed 83% identity at the nucleotide level and an 80.4% identity at the deduced amino acid level to the rat colonic H-K-ATPase. With the same primers and cDNA originating from rabbit distal colon, a DNA fragment with a size and nucleotide sequence identical to that originating from CCD cells was amplified. Furthermore, using PCR screening, we isolated and sequenced a 1.5-kb cDNA clone from a rabbit CCD library. The predicted amino acid sequence of the protein encoded by this cDNA is 85 and 82% identical to the corresponding regions of the guinea pig and rat colonic H-K-ATPase, respectively, and 70% identical to the H-K-ATPase recently cloned from Bufo marinus, whereas it shows only 45 and 42% homology to the rat Na-K-ATPase alpha 1-subunit and the rat gastric H-K-ATPase, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of H(+)-K(+)-ATPase expression in kidney

It is now widely accepted that proton secretion by the collecting duct is mediated, in part, by an H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase). Controversy persists regarding which H(+)-K(+)-ATPase isoform is expressed in kidney. Several laboratories have reported preliminarily the amplification from kidney of stomach and/or colon-identical products using gastric or colonic-specific primers in the polymerase chain reaction (PCR). We have developed highly specific probes for the catalytic subunit using reverse transcriptase-PCR with gastric- or colonic-specific primers. The resulting cDNAs were verified by sequencing and were then used in Northern analysis of whole kidney total RNA obtained from one of the following three groups of rats: 1) controls, 2) chronic hypokalemia, or 3) chronic metabolic acidosis. Probes for both the colonic and gastric alpha-subunit H(+)-K(+)-ATPase isoforms hybridized to whole kidney total RNA derived from potassium-replete control rats. A marked elevation of colonic mRNA abundance, but not gastric message, was observed in response to chronic hypokalemia induced by dietary potassium deprivation. Elevation of either gastric or colonic mRNA was not observed with chronic metabolic acidosis. Under the conditions of the present study, it appears that the mRNA encoding the colonic alpha-isoform of the H(+)-K(+)-ATPase in kidney is upregulated by chronic hypokalemia but not by chronic metabolic acidosis. The observation that the gastric H(+)-K(+)-ATPase alpha-isoform does not appear to be regulated in either condition suggests that this isoform is expressed constitutively in kidney.

In situ hybridization of H-K-ATPase beta-subunit mRNA in rat and rabbit kidney

Through a variety of techniques, several investigators have demonstrated the presence of an H-K-adenosinetriphosphatase (H-K-ATPase) enzyme in the renal collecting duct, suggesting that this enzyme serves an important physiological role in the regulation of acid-base balance and potassium excretion by the kidney. The present study was designed to localize cells expressing H-K-ATPase beta-subunit mRNA in rat and rabbit kidney by nonradioactive in situ hybridization. A 570-bp DNA fragment of rabbit renal H-K-ATPase beta-subunit was used to produce digoxigenin-labeled riboprobes by in vitro transcription. Northern blot hybridization demonstrated transcripts in rat gastric oxyntic mucosa and kidney. In situ hybridization on kidney tissue sections demonstrated H-K-ATPase beta-subunit mRNA localization in epithelial cells, including intercalated cells in the connecting segment and cortical and medullary collecting duct, principal cells in the inner stripe of the outer medullary collecting duct, and inner medullary collecting duct cells in both the rat and the rabbit. These observations provide evidence that H-K-ATPase beta-subunit mRNA is present throughout the collecting duct of the kidney. The distribution of this message is consistent with a role for H-K-ATPase in bicarbonate absorption in both the outer and inner medullary collecting duct.

Famotidine, a histamine-2-receptor antagonist, inhibits the increase in rat gastric H+/K(+)-ATPase mRNA induced by intravenous infusion of gastrin 17 and histamine

We examined the effects of gastrin and histamine on rat gastric H+/K(+)-ATPase, the enzyme responsible for H+ secretion, gene expression in vivo. Gastrin 17 (G 17) or histamine dihydrochloride (histamine) was continuously infused through the femoral vein of anesthetized rats. Gastric H+/K(+)-ATPase mRNA levels were measured using northern blot analysis. Infusion of G 17 and histamine increased the H+/K(+)-ATPase mRNA level significantly compared with basal control level or vehicle control level (P < 0.01). However, pretreatment with famotidine, a potent histamine-2 (H2)-receptor antagonist, inhibited the increase of rat gastric H+/K(+)-ATPase mRNA following G 17 and histamine infusion. These findings indicate that both histamine and G 17 increase expression of H+/K(+)-ATPase mRNA by activating H2 receptor on the parietal cell.

Expression of a gastric autoantigen in pancreatic islets results in non-destructive insulitis after neonatal thymectomy

Autoimmune gastritis, induced by day-3 thymectomy of BALB/c mice, is a destructive CD4+ T cell-mediated disease characterized by leukocyte infiltrates in the gastric mucosa, loss of parietal and chief cells and anti-gastric H/K ATPase autoantibodies. Our previous studies have indicated that a T cell response to the H/K ATPase beta subunit is required for the onset of autoimmune gastritis (Alderuccio, F., Toh, B. H., Tan, S. S., Gleeson, P. A. and van Driel, I. R., J. Exp. Med. 1993. 178: 419). To determine whether a response to the beta subunit autoantigen is alone sufficient to induce autoimmunity, or whether other tissue-specific factors are required, we have generated transgenic mice expressing the gastric H/K ATPase beta subunit in beta islet cells of the pancreas (RIP-H/K beta). RIP-H/K beta mice developed autoimmune gastritis and insulitis after day-3 thymectomy. Significantly, insulitis, observed as a peri-islet infiltrate, was only detected in thymectomized mice with autoimmune gastritis. There was no apparent immune destruction of the pancreas as insulitis did not progress to invasion of the islets or diabetes. Double transgenic mice, expressing the gastric H/K ATPase beta subunit in the thymus and in the pancreas, were protected from both gastritis and insulitis after day-3 thymectomy. Therefore, insulitis in the RIP-H/K beta mice appears to be dependent on a T cell response to the H/K ATPase beta subunit. This is the first example where an organ-specific initiating autoantigen has been expressed in another peripheral tissue. Autoimmune destruction in the stomach, but not the pancreas, indicates that tissue-specific factors play a fundamental role in the development of organ-specific autoimmunity.

Hepatocyte growth factor induces gastric H+/K(+)-ATPase expression

The processes regulating the development of the fetal gastrointestinal tract are largely unknown, but are likely dependent, in part, on peptide growth factors. The purpose of this study was to determine the contribution of hepatocyte growth factor (HGF) to the development of the fetal gastric epithelium, with particular reference to the parietal cell. Fifty-six fetal rabbits from 18 time-mated pregnant New Zealand White rabbit does were divided into four groups at Day 23 of gestation (term is 31 days): (1) unoperated control littermates, (2) those prevented from swallowing amniotic fluid by esophageal ligation (EL), (3) those with EL plus intragastric carrier infusion, and (4) those with EL plus intragastric HGF infusion. At Day 28 of gestation, fetal stomachs were harvested and analyzed for gastric weight, DNA content, and H+/K(+)-ATPase expression. In control fetuses, gastric weight was 470 +/- 30 mg, gastric DNA content was 741 +/- 59 micrograms, and gastric H+/K(+)-ATPase expression was 25.4 +/- 2.7 micrograms. EL resulted in a 45% decrease in gastric weight (P = 0.001), a 34% decrease in DNA content (P = 0.04), and a 43% decrease in H+/K(+)-ATPase expression (P = 0.007). These inhibitory effects were not reversed by intragastric carrier infusion. Although intragastric HGF infusion did not significantly restore gastric weight or gastric DNA content, it restored gastric H+/K(+)-ATPase expression to levels no different from those of unoperated controls (23.9 +/- 2.8 micrograms), but significantly greater than those of the EL or carrier infusion groups (P = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Primary structure and functional expression of the mouse and frog alpha-subunit of the gastric H(+)-K(+)-ATPase

The H(+)-K(+)-ATPase of the gastric parietal cells is responsible for the acidification of the stomach lumen. This heterodimeric protein belongs to the family of cation-translocating P-type ATPases, which includes the closely related Na(+)-ATPase. We have cloned the alpha-subunit cDNA of the Xenopus and murine gastric H(+)-K(+)-ATPase (alpha H-K). We have expressed Xenopus and murine alpha H-K along with the previously cloned gastric H(+)-K(+)-ATPase beta-subunit of rabbit (beta H-K) in Xenopus oocytes by cRNA injection. An antibody directed against the beta H-K coimmunoprecipitates under nondenaturing conditions the alpha H-K of both species, demonstrating assembly of the alpha/beta complex. Additionally, we demonstrate the presence of K(+)-transporting H(+)-K(+)-ATPase in the plasma membrane of oocytes by 86Rb- uptake. The H(+)-K(+)-ATPase-mediated K+ uptake was inhibited by the gastric H(+)-K(+)-ATPase inhibitor Sch-28080, but not by ouabain, and shows K(+)-dependent activation (K1/2 approximately 2 mM). Furthermore, H(+)-K(+)-ATPase-expressing oocytes show a Sch-28080 inhibitable proton extrusion. Our data indicate that the expressed H(+)-K(+)-ATPase behaves functionally in oocytes as in the gastric gland.

Expression of gastric and colonic H(+)-K(+)-ATPase in the rat kidney

Enzymatic and microperfusion studies have indicated that an ATP-dependent H+/K+ exchange process is present in the collecting duct of the mammalian kidney. Immunochemical staining has also provided evidence for expression of a gastric-type H(+)-K+ adenosine triphosphatase (H(+)-K(+)-ATPase). Rat kidney mRNA was probed with use of the polymerase chain reaction (PCR) to determine the presence of an H(+)-K(+)-ATPase. cDNA made with mRNA isolated from the kidneys of rats maintained on a low-K diet was used as template in PCR reactions with primers encompassing the cDNA sequence of the alpha-subunit of the gastric H(+)-K(+)-ATPase and the 5' and 3' ends of the colonic H(+)-K(+)-ATPase. The resulting products, 300-700 bp in size, hybridized with probes directed against either the gastric or colonic sequences of the H(+)-K(+)-ATPase. Sequencing of the individual PCR products showed identity with the appropriate regions of the alpha-subunits of the gastric H(+)-K(+)-ATPase and colonic H(+)-K(+)-ATPase. These data indicate that the rat kidney expresses mRNAs encoding both gastric and colonic H(+)-K(+)-ATPases.

Association of Helicobacter pylori and gastric autoimmunity: a population-based study

Based on clinical studies, a negative association between Helicobacter pylori and autoimmune corpus gastritis is described. In the present investigation of an unselected population of 1461 adults we can state, however, that there exists a relationship between H. pylori infection and the development of gastric corpus autoimmunity. As confirmation for the gastric autoantibody development through molecular mimicry, a high homology (72% in 25 amino acid overlap) between the beta subunit of H. pylori urease and that of H + K + ATPase, the gastric parietal cell autoantigen, was revealed.

Renal expression of the gene encoding the gastric H(+)-K(+)-ATPase beta-subunit

The gastric mucosal parietal cells and cells of the renal collecting duct both possess H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) activities. In the stomach, the H(+)-K(+)-ATPase (EC 3.6.1.3) is responsible for acidification of luminal contents. The kidney H(+)-K(+)-ATPase protein(s) contribute to potassium reabsorption and secretion of hydrogen ions to maintain potassium and acid-base homeostasis. The stomach H(+)-K(+)-ATPase is well defined and consists of an alpha-catalytic subunit of apparent molecular mass of 95 kDa and a highly glycosylated beta-subunit of 60-90 kDa. The molecular identity of the protein that mediates the H(+)-K(+)-ATPase activity in the kidney has been addressed in this paper. A combination of RNA hybridizations, polymerase chain reaction analysis of kidney RNA, and sequence analysis of cDNAs indicated that gastric H(+)-K(+)-ATPase beta-subunit mRNA is present in kidney. Immunoblotting with antibodies specific for the gastric H(+)-K(+)-ATPase beta-subunit detected proteins, which, after deglycosylation, had the same molecular mass as the gastric beta-subunit in membrane protein preparations from rabbit, pig, rat, and mouse kidneys. Furthermore, we have used transgenic mice to demonstrate that the gastric H(+)-K(+)-ATPase beta-subunit gene contains cis-acting regulatory sequences that are active in both gastric parietal cells and the renal collecting ducts. Overall, these data indicate that the gastric H(+)-K(+)-ATPase beta-subunit is found in the kidney and probably associates with the gastric H(+)-K(+)-ATPase alpha-subunit and/or other P-type ATPase alpha-subunits, thus contributing to acid-base and potassium homeostasis.

[Enzyme structure and specific gene expression of gastric proton pump]

Progress in molecular biological studies on the H+/K(+)-ATPase (gastric proton pump) now enables us to discuss not only its subunit protein structures and catalysis but also the organizations of its subunit genes and their cell-specific transcription. The primary structures of the catalytic alpha and glycosylated beta subunits and their transmembrane topology are similar to those of the corresponding subunits of Na+/K(+)-ATPase. The exon/intron organizations of the genes for the H+/K(+)-ATPase alpha and beta subunits are closely similar to those of the corresponding subunits of Na+/K(+)-ATPase, suggesting that the alpha and beta subunit genes of the two ATPases were respectively derived from common ancestors. In contrast to ubiquitous Na+/K(+)-ATPase, the H+/K(+)-ATPase is expressed specifically in gastric parietal cells. Consistent with this fact, we found novel zinc finger proteins which are present in the gastric parietal cells and recognize a gastric sequence motif in the 5'-upstream regions of the H+/K(+)-ATPase alpha and beta subunit genes. The proteins are likely to play important roles in the transcriptional regulation of the parietal cell specific genes.

Transforming growth factor alpha disrupts the normal program of cellular differentiation in the gastric mucosa of transgenic mice

Transforming growth factor alpha (TGF alpha) evokes diverse responses in transgenic mouse tissues in which it is over-expressed, including the gastric mucosa, which experiences aberrant growth and a coincident repression of hydrochloric acid production. Here we show that ectopically expressed TGF alpha induces an age-dependent cellular reorganization of the transgenic stomach, in which the surface mucous cell population in the gastric pit is greatly expanded at the expense of cells in the glandular base. Immunohistochemical analysis of BrdU incorporation into DNA demonstrated that although mature surface mucous cells were not proliferating, DNA synthesis was enhanced by approximately 67% in the glandular base and isthmus, where progenitor cells reside. RNA blot and in situ hybridization were employed to determine temporal and spatial expression patterns of specific markers representing a variety of exocrine and endocrine gastric cell types. Mature parietal and chief cells were specifically depleted from the glandular mucosa, as judged by a 6- to 7-fold decrease in the expression of genes encoding H+,K(+)-ATPase, which is required for acid secretion, and pepsinogen C, respectively. The reduction of these markers coincided in time with the activation of TGF alpha transgene expression in the neonatal stomach. The rate of cell death in the glandular region was not overtly different. Significantly, the loss of parietal and chief cells occurred without a concomitant loss of their respective cellular precursors. In contrast to exocrine cells, D and G endocrine cells were much less severely affected, based on analysis of somatostatin and gastrin expression. Analysis of these dynamic changes indicates that TGF alpha can induce selective alterations in terminal differentiation and proliferation in the gastric mucosa, and suggests that TGF alpha plays an important physiological role in the normal regulation of epithelial cell renewal.

Functional significance of the beta-subunit for heterodimeric P-type ATPases

We have reviewed the structural and functional role of the beta-subunit in a subfamily of the P-ATPases known as the alpha/beta-heterodimeric, cation-exchange ATPases. The subfamily consists of the various isoforms of Na+/K(+)-ATPase and H+/K(+)-ATPase, both of which pump a cation out of the cell (Na+ or H+, respectively) in recycle exchange for K+. Much of the earlier work has emphasized the functional activities of the alpha-subunit, which shares many characteristics with the broader P-ATPase family. It is now clear that the glycosylated beta-subunit is an essential component of the cation-exchange ATPase subfamily. All beta-subunit isoforms have three highly conserved disulfide bonds within the extracellular domain that serve to stabilize the alpha-subunit, alpha/beta interaction and functional activity of the holoenzyme. Evidence strongly suggests that the beta-subunit is involved in the K(+)-dependent reactions of the enzymes, such as the E1-E2 transition and K+ occlusion, and that the extracellular domain of the beta-subunit plays an important role in determining the kinetics of K+ interaction. In most vertebrate cells, the unassociated alpha-subunit is restricted to the endoplasmic reticulum (ER), and assembly of the alpha/beta complex occurs within the ER. Signals for exiting the ER and directing the correct intracellular trafficking are primarily determined by the beta-subunit; Na+/K(+)-ATPase typically terminates in the plasma membrane facing the basolateral membrane, whereas all isoforms of H+/K(+)-ATPase terminate in the apical membrane. The C-terminal extracellular domain of the beta-subunit is important for proper interaction with the alpha-subunit and for correct intracellular trafficking. Oligosaccharides on the beta-subunit are not essential for enzyme function, but do serve to enhance the efficiency of alpha/beta association by increasing the lifetime of the unassociated beta-subunit and the stability of the alpha/beta complex to tryptic attack. We propose that highly specialized glycosylation on the beta-subunit of the gastric H+/K(+)-ATPase may help to protect that enzyme from the harsh extracellular environment of the stomach.

Gastrin receptor genes are expressed in gastric parietal and enterochromaffin-like cells of Mastomys natalensis

Although gastric enterochromaffin-like (ECL) carcinoid tumors are known to develop in patients with long-standing hypergastrinemia, the expression of the gastrin receptor gene in ECL cells has not yet been demonstrated. Therefore, this study was designed to examine gastrin receptor gene expression in ECL cells. Mastomys gastric mucosal cells isolated by enzyme dispersion were separated into 10 fractions (F1-10) by centrifugal elutriation. Each fraction was examined histologically to determine whether they contained ECL and/or parietal cells and Northern blot analysis was used to confirm the presence of histidine decarboxylase and H+, K(+)-ATPase gene expression. ECL cells were found only in fractions 1 and 2, whereas parietal cells were detected in fractions 6-10. Gastrin receptor gene expression was demonstrated in both parietal cell-rich and ECL cell-rich fractions. In addition, the gastrin receptor cDNA sequences obtained from the two of the fractions (F1 and 8) were identical. These results suggest that gastrin receptor genes are expressed in ECL cells as well as in parietal cells and that these receptors are identical.