Does the colonic H,K-ATPase also act as an Na,K-ATPase?

Airway Surface Liquid pH Regulation in Airway Epithelium Current Understandings and Gaps in Knowledge

Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO3-) entry occurs via the electrogenic, coupled transport of sodium (Na+) and HCO3-, and, together with carbonic anhydrase enzymatic activity, provides HCO3- for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H+) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H+ apical secretion, mainly involving H+/K+ ATPase and vacuolar H+-ATPase that carry H+ against the electrochemical potential gradient. Paracellular HCO3- transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H+ and HCO3- is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

Increased colonic K+ excretion through inhibition of the H,K-ATPase type 2 helps reduce plasma K+ level in a murine model of nephronic reduction

Hyperkalemia is frequently observed in patients at the end-stage of chronic kidney disease (CKD), and has possible harmful consequences on cardiac function. Many strategies are currently used to manage hyperkalemia, one consisting of increasing fecal K⁺ excretion through the administration of cation-exchange resins. In this study, we explored another more specific method of increasing intestinal K⁺ secretion by inhibiting the H,K-ATPase type 2 (HKA2), which is the main colonic K⁺ reabsorptive pathway. We hypothetised that the absence of this pump could impede the increase of plasma K⁺ levels following nephronic reduction (N5/6) by favoring fecal K⁺ secretion. In N5/6 WT and HKA2KO mice under normal K⁺ intake, the plasma K⁺ level remained within the normal range, however, a load of K⁺ induced strong hyperkalemia in N5/6 WT mice (9.1 ± 0.5 mM), which was significantly less pronounced in N5/6 HKA2KO mice (7.9 ± 0.4 mM, p < 0.01). This was correlated to a higher capacity of HKA2KO mice to excrete K⁺ in their feces. The absence of HKA2 also increased fecal Na⁺ excretion by inhibiting its colonic ENaC-dependent absorption. We also showed that angiotensin-converting-enzyme inhibitor like enalapril, used to treat hypertension during CKD, induced a less severe hyperkalemia in N5/6 HKA2KO than in N5/6 WT mice. This study therefore provides the proof of concept that the targeted inhibition of HKA2 could be a specific therapeutic maneuver to reduce plasma K⁺ levels in CKD patients.

H,K-ATPase type 2 regulates gestational extracellular compartment expansion and blood pressure in mice

The modifications of the hemodynamic system and hydromineral metabolism are physiological features characterizing a normal gestation. Thus, the ability to expand plasma volume without increasing the level of blood pressure is necessary for the correct perfusion of the placenta. The kidney is essential in this adaptation by reabsorbing avidly sodium and fluid. In this study, we observed that the H,K-ATPase type 2 (HKA2), an ion pump expressed in kidney and colon and already involved in the control of the K⁺ balance during gestation, is also required for the correct plasma volume expansion and to maintain normal blood pressure. Indeed, compared with WT pregnant mice that exhibit a 1.6-fold increase of their plasma volume, pregnant HKA2-null mice (HKA2KO) only modestly expand their extracellular volume (×1.2). The renal expression of the epithelial Na channel (ENaC) α- and γ-subunits and that of the pendrin are stimulated in gravid WT mice, whereas the Na/Cl^- cotransporter (NCC) expression is downregulated. These modifications are all blunted in HKA2KO mice. This impeded renal adaptation to gestation is accompanied by the development of hypotension in the pregnant HKA2KO mice. Altogether, our results showed that the absence of the HKA2 during gestation leads to an "underfilled" situation and has established this transporter as a key player of the renal control of salt and potassium metabolism during gestation.

ANP-stimulated Na+ secretion in the collecting duct prevents Na+ retention in the renal adaptation to acid load

We have recently reported that type A intercalated cells of the collecting duct secrete Na⁺ by a mechanism coupling the basolateral type 1 Na⁺-K⁺-2Cl^- cotransporter with apical type 2 H⁺-K⁺-ATPase (HKA2) functioning under its Na⁺/K⁺ exchange mode. The first aim of the present study was to evaluate whether this secretory pathway is a target of atrial natriuretic peptide (ANP). Despite hyperaldosteronemia, metabolic acidosis is not associated with Na⁺ retention. The second aim of the present study was to evaluate whether ANP-induced stimulation of Na⁺ secretion by type A intercalated cells might account for mineralocorticoid escape during metabolic acidosis. In Xenopus oocytes expressing HKA2, cGMP, the second messenger of ANP, increased the membrane expression, activity, and Na⁺-transporting rate of HKA2. Feeding mice with a NH₄Cl-enriched diet increased urinary excretion of aldosterone and induced a transient Na⁺ retention that reversed within 3 days. At that time, expression of ANP mRNA in the collecting duct and urinary excretion of cGMP were increased. Reversion of Na⁺ retention was prevented by treatment with an inhibitor of ANP receptors and was absent in HKA2-null mice. In conclusion, paracrine stimulation of HKA2 by ANP is responsible for the escape of the Na⁺-retaining effect of aldosterone during metabolic acidosis.

Increased expression of ATP12A proton pump in cystic fibrosis airways

Proton secretion mediated by ATP12A protein on the surface of the airway epithelium may contribute to cystic fibrosis (CF) lung disease by favoring bacterial infection and airway obstruction. We studied ATP12A in fresh bronchial samples and in cultured epithelial cells. In vivo, ATP12A expression was found almost exclusively at the apical side of nonciliated cells of airway epithelium and in submucosal glands, with much higher expression in CF samples. This could be due to bacterial infection and inflammation, since treating cultured cells with bacterial supernatants or with IL-4 (a cytokine that induces goblet cell hyperplasia) increased the expression of ATP12A in nonciliated cells. This observation was associated with upregulation and translocation of ATP1B1 protein from the basal to apical epithelial side, where it colocalizes with ATP12A. ATP12A function was evaluated by measuring the pH of the apical fluid in cultured epithelia. Under resting conditions, CF epithelia showed more acidic values. This abnormality was minimized by inhibiting ATP12A with ouabain. Following treatment with IL-4, ATP12A function was markedly increased, as indicated by strong acidification occurring under bicarbonate-free conditions. Our study reveals potentially novel aspects of ATP12A and remarks its importance as a possible therapeutic target in CF and other respiratory diseases.

Versatility of NaCl transport mechanisms in the cortical collecting duct

The cortical collecting duct (CCD) forms part of the aldosterone-sensitive distal nephron and plays an essential role in maintaining the NaCl balance and acid-base status. The CCD epithelium comprises principal cells as well as different types of intercalated cells. Until recently, transcellular Na⁺ transport was thought to be restricted to principal cells, whereas (acid-secreting) type A and (bicarbonate-secreting) type B intercalated cells were associated with the regulation of acid-base homeostasis. This review describes how this traditional view has been upended by several discoveries in the past decade. A series of studies has shown that type B intercalated cells can mediate electroneutral NaCl reabsorption by a mechanism involving Na⁺-dependent and Na⁺-independent Cl^-/[Formula: see text] exchange, and that is energetically driven by basolateral vacuolar H⁺-ATPase pumps. Other research indicates that type A intercalated cells can mediate NaCl secretion, through a bumetanide-sensitive pathway that is energized by apical H⁺,K⁺-ATPase type 2 pumps operating as Na⁺/K⁺ exchangers. We also review recent findings on the contribution of the paracellular route to NaCl transport in the CCD. Last, we describe cross-talk processes, by which one CCD cell type impacts Na⁺/Cl^- transport in another cell type. The mechanisms that have been identified to date demonstrate clearly the interdependence of NaCl and acid-base transport systems in the CCD. They also highlight the remarkable versatility of this nephron segment.

Iron is a substrate of the Plasmodium falciparum chloroquine resistance transporter PfCRT in Xenopus oocytes

The chloroquine resistance transporter of the human malaria parasite Plasmodium falciparum, PfCRT, is an important determinant of resistance to several quinoline and quinoline-like antimalarial drugs. PfCRT also plays an essential role in the physiology of the parasite during development inside erythrocytes. However, the function of this transporter besides its role in drug resistance is still unclear. Using electrophysiological and flux experiments conducted on PfCRT-expressing Xenopus laevis oocytes, we show here that both wild-type PfCRT and a PfCRT variant associated with chloroquine resistance transport both ferrous and ferric iron, albeit with different kinetics. In particular, we found that the ability to transport ferrous iron is reduced by the specific polymorphisms acquired by the PfCRT variant as a result of chloroquine selection. We further show that iron and chloroquine transport via PfCRT is electrogenic. If these findings in the Xenopus model extend to P. falciparum in vivo, our data suggest that PfCRT might play a role in iron homeostasis, which is essential for the parasite's development in erythrocytes.

Retinoschisin is linked to retinal Na/K-ATPase signaling and localization

Retinoschisin binds to the extracellular domain of Na/K-ATPase subunit β2. Retinoschisin inhibits Na/K-ATPase–associated signaling cascades and affects Na/K-ATPase localization. The retinoschisin-Na/K-ATPase complex overlaps with signaling mediators. Defective Na/K-ATPase signaling by retinoschisin deficiency may promote retinal dystrophy.

Mutations in the RS1 gene cause X-linked juvenile retinoschisis (XLRS), a hereditary retinal dystrophy. We recently showed that retinoschisin, the protein encoded by RS1, regulates ERK signaling and apoptosis in retinal cells. In this study, we explored an influence of retinoschisin on the functionality of the Na/K-ATPase, its interaction partner at retinal plasma membranes. We show that retinoschisin binding requires the β2-subunit of the Na/K-ATPase, whereas the α-subunit is exchangeable. Our investigations revealed no effect of retinoschisin on Na/K-ATPase–mediated ATP hydrolysis and ion transport. However, we identified an influence of retinoschisin on Na/K-ATPase–regulated signaling cascades and Na/K-ATPase localization. In addition to the known ERK deactivation, retinoschisin treatment of retinoschisin-deficient (Rs1h^-/Y) murine retinal explants decreased activation of Src, an initial transmitter in Na/K-ATPase signal transduction, and of Ca²⁺ signaling marker Camk2. Immunohistochemistry on murine retinae revealed an overlap of the retinoschisin–Na/K-ATPase complex with proteins involved in Na/K-ATPase signaling, such as caveolin, phospholipase C, Src, and the IP3 receptor. Finally, retinoschisin treatment altered Na/K-ATPase localization in photoreceptors of Rs1h^-/Y retinae. Taken together, our results suggest a regulatory effect of retinoschisin on Na/K-ATPase signaling and localization, whereas Na/K-ATPase-dysregulation caused by retinoschisin deficiency could represent an initial step in XLRS pathogenesis.

The renal cortical collecting duct: a secreting epithelium?

KEY POINTS

The cortical collecting duct (CCD) plays an essential role in sodium homeostasis by fine-tuning the amount of sodium that is excreted in the urine. Ex vivo, the microperfused CCD reabsorbs sodium in the absence of lumen-to-bath concentration gradients. In the present study, we show that, in the presence of physiological lumen-to-bath concentration gradients, and in the absence of endocrine, paracrine and neural regulation, the mouse CCD secretes sodium, which represents a paradigm shift. This secretion occurs via the paracellular route, as well as a transcellular pathway that is energized by apical H+ /K+ -ATPase type 2 pumps operating as Na+ /K+ exchangers. The newly identified transcellular secretory pathway represents a physiological target for the regulation of sodium handling and for anti-hypertensive therapeutic agents.

ABSTRACT

In vitro microperfusion experiments have demonstrated that cortical collecting ducts (CCDs) reabsorb sodium via principal and type B intercalated cells under sodium-depleted conditions and thereby contribute to sodium and blood pressure homeostasis. However, these experiments were performed in the absence of the transepithelial ion concentration gradients that prevail in vivo and determine paracellular transport. The present study aimed to characterize Na+ , K+ and Cl- fluxes in the mouse CCD in the presence of physiological transepithelial concentration gradients. For this purpose, we combined in vitro measurements of ion fluxes across microperfused CCDs of sodium-depleted mice with the predictions of a mathematical model. When NaCl transport was inhibited in all cells, CCDs secreted Na+ and reabsorbed K+ ; Cl- transport was negligible. Removing inhibitors of type A and B intercalated cells increased Na+ secretion in wild-type (WT) mice but not in H+ /K+ -ATPase type 2 (HKA2) knockout mice. Further inhibition of basolateral NaCl entry via the Na+ -K+ -2Cl- cotransporter in type A intercalated cells reduced Na+ secretion in WT mice to the levels observed in HKA2-/- mice. With no inhibitors, WT mouse CCDs still secreted Na+ and reabsorbed K+ . In vivo, HKA2-/- mice excreted less Na+ than WT mice after switching to a high-salt diet. Taken together, our results indicate that type A intercalated cells secrete Na+ via basolateral Na+ -K+ -2Cl- cotransporters in tandem with apical HKA2 pumps. They also suggest that the CCD can mediate overall Na+ secretion, and that its ability to reabsorb NaCl in vivo depends on the presence of acute regulatory factors.

Parameter estimation for mathematical models of a nongastric H+(Na+)-K(+)(NH4+)-ATPase

The role of nongastric H(+)-K(+)-ATPase (HKA) in ion homeostasis of macula densa (MD) cells is an open question. To begin to explore this issue, we developed two mathematical models that describe ion fluxes through a nongastric HKA. One model assumes a 1H(+):1K(+)-per-ATP stoichiometry; the other assumes a 2H(+):2K(+)-per-ATP stoichiometry. Both models include Na+ and NH4+ competitive binding with H+ and K+, respectively, a characteristic observed in vitro and in situ. Model rate constants were obtained by minimizing the distance between model and experimental outcomes. Both 1H(+)(1Na(+)):1K(+)(1NH4 (+))-per-ATP and 2H(+)(2Na(+)):2K(+)(2NH4 (+))-per-ATP models fit the experimental data well. Using both models, we simulated ion net fluxes as a function of cytosolic or luminal ion concentrations typical for the cortical thick ascending limb and MD region. We observed that (1) K+ and NH4+ flowed in the lumen-to-cytosol direction, (2) there was competitive behavior between luminal K+ and NH4+ and between cytosolic Na+ and H+, 3) ion fluxes were highly sensitive to changes in cytosolic Na+ or H+ concentrations, and 4) the transporter does mostly Na+ / K+ exchange under physiological conditions. These results support the concept that nongastric HKA may contribute to Na+ and pH homeostasis in MD cells. Furthermore, in both models, H+ flux reversed at a luminal pH that was <5.6. Such reversal led to Na+ / H+ exchange for a luminal pH of <2 and 4 in the 1:1-per-ATP and 2:2-per-ATP models, respectively. This suggests a novel role of nongastric HKA in cell Na+ homeostasis in the more acidic regions of the renal tubules.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

H-K-ATPase type 2: relevance for renal physiology and beyond

H-K-ATPase type 2 (HKA2), also known as the "nongastric" or "colonic" H-K-ATPase, is broadly expressed, and its presence in the kidney has puzzled experts in the field of renal ion transport systems for many years. One of the most important and robust characteristics of this transporter is that it is strongly stimulated after dietary K(+) restriction. This result prompted many investigators to propose that it should play a role in allowing the kidney to efficiently retain K(+) under K(+) depletion. However, the apparent absence of a clear renal phenotype in HKA2-null mice has led to the idea that this transporter is an epiphenomenon. This review summarizes past and recent findings regarding the functional, structural and physiological characteristics of H-K-ATPase type 2. The findings discussed in this review suggest that, as in the famous story, the ugly duckling of the X-K-ATPase family is actually a swan.

The parietal cell gastric H, K-ATPase also functions as the Na, K-ATPase and Ca-ATPase in altered states

This article offers an explanation for the apparent lack of Na, K-ATPase activity in parietal cells although ouabain has been known to inhibit gastric acid secretion since 1962. The gastric H, K-ATPase (proton-pump) seems to be acting in altered states, thus behaving like a Na, K-ATPase (Na-pump) and/or Ca-ATPase (Ca-pump) depending on cellular needs.  This conclusion is based on the following findings. First, parietal cell fractions do not exhibit Na, K-ATPase activity at pH 7.0 but do at pH 8.5. Second, the apical plasma membrane (APM) fraction exhibits a (Ca or Mg)-ATPase activity with negligible H, K-ATPase activity. However, when assayed with Mg alone in presence of the 80 k Da cytosolic proton-pump activator (HAF), the APM fraction reveals remarkably high H, K-ATPase activity, suggesting the observed low affinity of Ca (or Mg)-ATPase is an altered state of the latter. Third, calcium (between 1 and 4 µM) shows both stimulation and inhibition of the HAF-stimulated H, K-ATPase depending on its concentration, revealing a close interaction between the  proton-pump activator and local Ca concentration in gastric H, K-ATPase function. Such interactions suggest that Ca is acting as a terminal member of the intracellular signaling system for the HAF-regulated proton-pump. It appears that during resting state, the HAF-associated H, K-ATPase remains inhibited by Ca (>1 µM) and, prior to resumption of acid secretion the gastric H, K-ATPase acts temporarily as a Ca-pump for removing excess Ca from its immediate environment. This conclusion is consistent with the recent reports of immunochemical co-localization of the gastric H, K-ATPase and Ca-ATPase by superimposition in parietal cells, and a transitory efflux of Ca immediately preceding the onset of acid secretion. These new perspectives on proton-pump function would open new avenues for a fuller understanding of the intracellular regulation of the ubiquitous Na-pump.

Deceleration of the E1P-E2P transition and ion transport by mutation of potentially salt bridge-forming residues Lys-791 and Glu-820 in gastric H+/K+-ATPase

A lysine residue within the highly conserved center of the fifth transmembrane segment in P(IIC)-type ATPase α-subunits is uniquely found in H,K-ATPases instead of a serine in all Na,K-ATPase isoforms. Because previous studies suggested a prominent role of this residue in determining the electrogenicity of non-gastric H,K-ATPase and in pK(a) modulation of the proton-translocating residues in the gastric H,K-ATPases as well, we investigated its functional significance for ion transport by expressing several Lys-791 variants of the gastric H,K-ATPase in Xenopus oocytes. Although the mutant proteins were all detected at the cell surface, none of the investigated mutants displayed any measurable K(+)-induced stationary currents. In Rb(+) uptake measurements, replacement of Lys-791 by Arg, Ala, Ser, and Glu substantially impaired transport activity and reduced the sensitivity toward the E(2)-specific inhibitor SCH28080. Furthermore, voltage clamp fluorometry using a reporter site in the TM5/TM6 loop for labeling with tetra-methylrhodamine-6-maleimide revealed markedly changed fluorescence signals. All four investigated mutants exhibited a strong shift toward the E(1)P state, in agreement with their reduced SCH28080 sensitivity, and an about 5-10-fold decreased forward rate constant of the E(1)P ↔ E(2)P conformational transition, thus explaining the E(1)P shift and the reduced Rb(+) transport activity. When Glu-820 in TM6 adjacent to Lys-791 was replaced by non-charged or positively charged amino acids, severe effects on fluorescence signals and Rb(+) transport were also observed, whereas substitution by aspartate was less disturbing. These results suggest that formation of an E(2)P-stabilizing interhelical salt bridge is essential to prevent futile proton exchange cycles of H(+) pumping P-type ATPases.

Discovery and validation of colonic tumor-associated proteins via metabolic labeling and stable isotopic dilution

The unique biology of a neoplasm is reflected by its distinct molecular profile compared with normal tissue. To understand tumor development better, we have undertaken a quantitative proteomic search for abnormally expressed proteins in colonic tumors from Apc(Min/+) (Min) mice. By raising pairs of Min and wild-type mice on diets derived from natural-abundance or (15)N-labeled algae, we used metabolic labeling to compare protein levels in colonic tumor versus normal tissue. Because metabolic labeling allows internal control throughout sample preparation and analysis, technical error is minimized as compared with in vitro labeling. Several proteins displayed altered expression, and a subset was validated via stable isotopic dilution using synthetic peptide standards. We also compared gene and protein expression among tumor and nontumor tissue, revealing limited correlation. This divergence was especially pronounced for species showing biological change, highlighting the complementary perspectives provided by transcriptomics and proteomics. Our work demonstrates the power of metabolic labeling combined with stable isotopic dilution as an integrated strategy for the identification and validation of differentially expressed proteins using rodent models of human disease.

Palytoxin acts on Na(+),K (+)-ATPase but not nongastric H(+),K (+)-ATPase

Palytoxin (PTX) opens a pathway for ions to pass through Na,K-ATPase. We investigate here whether PTX also acts on nongastric H,K-ATPases. The following combinations of cRNA were expressed in Xenopus laevis oocytes: Bufo marinus bladder H,K-ATPase alpha(2)- and Na,K-ATPase beta(2)-subunits; Bufo Na,K-ATPase alpha(1)- and Na,K-ATPase beta(2)-subunits; and Bufo Na,K-ATPase beta(2)-subunit alone. The response to PTX was measured after blocking endogenous Xenopus Na,K-ATPase with 10 microM ouabain. Functional expression was confirmed by measuring (86)Rb uptake. PTX (5 nM: ) produced a large increase of membrane conductance in oocytes expressing Bufo Na,K-ATPase, but no significant increase occurred in oocytes expressing Bufo H,K-ATPase or in those injected with Bufo beta(2)-subunit alone. Expression of the following combinations of cDNA was investigated in HeLa cells: rat colonic H,K-ATPase alpha(1)-subunit and Na,K-ATPase beta(1)-subunit; rat Na,K-ATPase alpha(2)-subunit and Na,K-ATPase beta(2)-subunit; and rat Na,K-ATPase beta(1)- or Na,K-ATPase beta(2)-subunit alone. Measurement of increases in (86)Rb uptake confirmed that both rat Na,K and H,K pumps were functional in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) and NKalpha(2)/NKbeta(2). Whole-cell patch-clamp measurements in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) exposed to 100 nM PTX showed no significant increase of membrane current, and there was no membrane conductance increase in HeLa cells transfected with rat NKbeta(1)- or rat NKbeta(2)-subunit alone. However, in HeLa cells expressing rat NKalpha(2)/NKbeta(2), outward current was observed after pump activation by 20 mM K(+) and a large membrane conductance increase occurred after 100 nM PTX. We conclude that nongastric H,K-ATPases are not sensitive to PTX when expressed in these cells, whereas PTX does act on Na,K-ATPase.

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.

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.

Loss of acidification of anterior prostate fluids in Atp12a-null mutant mice indicates that nongastric H-K-ATPase functions as proton pump in vivo

The physiological functions of nongastric (colonic) H-K-ATPase (gene symbol Atp12a), unlike those of Na-K-ATPase and gastric H-K-ATPase, are poorly understood. It has been suggested that it pumps Na+ more efficiently than H+; however, so far, there is no direct evidence that it pumps H+ in vivo. Previously, we found that the nongastric H-K-ATPase alpha-subunit is expressed in apical membranes of rodent anterior prostate epithelium, in a complex with the Na-K-ATPase beta1-subunit. Here we report the effects of Atp12a gene ablation on polarization of the beta1-subunit and secretory function of the anterior prostate. In nongastric H-K-ATPase-deficient prostate, the Na-K-ATPase alpha-subunit resided exclusively in basolateral membranes; however, the beta1-subunit disappeared from apical membranes, demonstrating that beta1 is an authentic subunit of nongastric H-K-ATPase in vivo and that apical localization of beta1 in the prostate is completely dependent on its association with the nongastric H-K-ATPase alpha-subunit. A remarkable reduction in acidification of anterior prostate fluids was observed: pH 6.38 +/- 0.14 for wild-type mice and 6.96 +/- 0.10 for homozygous mutants. These results show that nongastric H-K-ATPase is required for acidification of luminal prostate fluids, thereby providing a strong in vivo correlate of previous functional expression studies demonstrating that it operates as a proton pump.

Molecular identification of Sch28080-sensitive K-ATPase activities in the mouse kidney

Rat collecting ducts display either an ouabain-insensitive or an ouabain-sensitive K-ATPase activity inhibited by Sch28080 according as animals are fed a normal or a potassium-depleted diet (types I and III K-ATPase, respectively). Two isoforms of H,K-ATPase have been cloned from rat gastric mucosa and colon, respectively. Gastric and colonic H,K-ATPase are expressed in the kidney, suggesting that they might account for types I and III K-ATPases. However, this hypothesis is not fully supported by segmental expression of gastric and colonic H,K-ATPase along the rat collecting duct, as well as by comparison of the pharmacological properties of gastric and colonic H,K-ATPase expressed in Xenopus ovocyte and types I and III K-ATPases in rat collecting ducts. The aim of the present work is to address directly the molecular origin of types I and III K-ATPases in the mouse collecting duct by measuring K-ATPase activities in collecting ducts of wild-type mice and mice genetically deficient in either gastric or colonic H,K-ATPase fed either a regular or a potassium-depleted diet. Like the rat, mouse collecting ducts display type I or III K-ATPase activity when fed a regular or a potassium-depleted diet, respectively. Type I K-ATPase activity is detected in colonic H,K-ATPase-deficient mice but not in gastric H,K-ATPase-deficient animals. Conversely, type III K-ATPase activity disappears in colonic H,K-ATPase-deficient but not in gastric H,K-ATPase-deficient mice. In conclusion, types I and III K-ATPases measured in collecting ducts of normal and potassium-depleted mice reflect the functional expression of gastric and colonic H,K-ATPase, respectively.

The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase

We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.

CD63 interacts with the carboxy terminus of the colonic H+-K+-ATPase to decrease [corrected] plasma membrane localization and 86Rb+ uptake

The carboxy terminus (CT) of the colonic H(+)-K(+)-ATPase is required for stable assembly with the beta-subunit, translocation to the plasma membrane, and efficient function of the transporter. To identify protein-protein interactions involved in the localization and function of HKalpha(2), we selected 84 amino acids in the CT of the alpha-subunit of mouse colonic H(+)-K(+)-ATPase (CT-HKalpha(2)) as the bait in a yeast two-hybrid screen of a mouse kidney cDNA library. The longest identified clone was CD63. To characterize the interaction of CT-HKalpha(2) with CD63, recombinant CT-HKalpha(2) and CD63 were synthesized in vitro and incubated, and complexes were immunoprecipitated. CT-HKalpha(2) protein (but not CT-HKalpha(1)) coprecipitated with CD63, confirming stable assembly of HKalpha(2) with CD63. In HEK-293 transfected with HKalpha(2) plus beta(1)-Na(+)-K(+)-ATPase, suppression of CD63 by RNA interference increased cell surface expression of HKalpha(2)/NKbeta(1) and (86)Rb(+) uptake. These studies demonstrate that CD63 participates in the regulation of the abundance of the HKalpha(2)-NKbeta(1) complex in the cell membrane.

Novel p-type ATPases mediate high-affinity potassium or sodium uptake in fungi

Fungi have an absolute requirement for K+, but K+ may be partially replaced by Na+. Na+ uptake in Ustilago maydis and Pichia sorbitophila was found to exhibit a fast rate, low Km, and apparent independence of the membrane potential. Searches of sequences with similarity to P-type ATPases in databases allowed us to identify three genes in these species, Umacu1, Umacu2, and PsACU1, that could encode P-type ATPases of a novel type. Deletion of the acu1 and acu2 genes proved that they encoded the transporters that mediated the high-affinity Na+ uptake of U. maydis. Heterologous expressions of the Umacu2 gene in K+ transport mutants of Saccharomyces cerevisiae and transport studies in the single and double Deltaacu1 and Deltaacu2 mutants of U. maydis revealed that the acu1 and acu2 genes encode transporters that mediated high-affinity K+ uptake in addition to Na+ uptake. Other fungi also have genes or pseudogenes whose translated sequences show high similarity to the ACU proteins of U. maydis and P. sorbitophila. In the phylogenetic tree of P-type ATPases all the identified ACU ATPases define a new cluster, which shows the lowest divergence with type IIC, animal Na+,K(+)-ATPases. The fungal high-affinity Na+ uptake mediated by ACU ATPases is functionally identical to the uptake that is mediated by some plant HKT transporters.

Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT

A large body of genetic, reverse genetic, and epidemiological data has linked chloroquine-resistant malaria to polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum. To investigate the biological function of the chloroquine resistance transporter, PfCRT, as well as its role in chloroquine resistance, we functionally expressed this protein in Xenopus laevis oocytes. Our data show that PfCRT-expressing oocytes exhibit a depolarized resting membrane potential and a higher intracellular pH compared with control oocytes. Pharmacological and electrophysiological studies link the higher intracellular pH to an enhanced amiloride-sensitive H(+) extrusion and the low membrane potential to an activated nonselective cation conductance. The finding that both properties are independent of each other, together with the fact that they are endogenously present in X. laevis oocytes, supports a model in which PfCRT activates transport systems. Our data suggest that PfCRT plays a role as a direct or indirect activator or modulator of other transporters.

Identification of the β-subunit for nongastric H-K-ATPase in rat anterior prostate

The structural organization of nongastric H-K-ATPase, unlike that of closely related Na-K-ATPase and gastric H-K-ATPase, is not well characterized. Recently, we demonstrated that nongastric H-K-ATPase α-subunit (αng) is expressed in apical membranes of rodent prostate. Its highest level, as well as relative abundance, with respect to α1-isoform of Na-K-ATPase, was observed in anterior lobe. Here, we aimed to determine the subunit composition of nongastric H-K-ATPase through the detailed analysis of the expression of all known X-K-ATPase β-subunits in rat anterior prostate (AP). RT-PCR detects transcripts of β-subunits of Na-K-ATPase only. Measurement of absolute protein content of these three β-subunit isoforms, with the use of quantitative Western blotting of AP membrane proteins, indicates that the abundance order is β1> β3≫ β2. Immunohistochemical experiments demonstrate that β1is present predominantly in apical membranes, coinciding with αng, whereas β3is localized in the basolateral compartment, coinciding with α1. This is the first direct demonstration of the αng-β1colocalization in situ indicating that, in rat AP, αngassociates only with β1. The existence of αng-β1complex has been confirmed by immunoprecipitation experiments. These results indicate that β1-isoform functions as the authentic subunit of Na-K-ATPase and nongastric H-K-ATPase. Putatively, the intracellular polarization of X-K-ATPase isoforms depends on interaction with other proteins.

The carboxy terminus of the colonic H+,K+-ATPase α-subunit is required for stable β subunit assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of colonic H(+), K(+)-ATPase alpha-subunit (HKalpha(2)), for both function as well as integrity of assembly with beta1-Na(+), K(+)-ATPase.

METHODS

For this purpose, a mutation of 84 amino acids in the carboxy-terminus was created (DeltaHKalpha(2)) and HEK-293 cells were used as expression systems for functional studies using (86)Rb(+)-uptake, coimmunoprecipitation using specific antibodies and fluorescence microscopy using green fluorescent protein.

RESULTS

The results demonstrate that comparable levels of expression of HKalpha(2) and DeltaHKalpha(2) mRNA were observed when cells were cotransfected with beta1 subunit. However, the abundance of expression of full length HKalpha(2) protein exceeded that of the truncated protein DeltaHKalpha(2). Ouabain-sensitive (86)Rb(+)-uptake was present only in cells cotransfected with HKalpha(2)/beta(1), indicating that the mutation was incapable of sustaining functionality. Coimmunoprecipitation experiments demonstrated that HKalpha(2) protein was immunoprecipitated more abundantly than DeltaHKalpha(2) when coexpressed with beta1. The use of sucrose gradients and green fluorescence protein immunofluorescence demonstrated that while the DeltaHKalpha(2)/beta(1) complex was confined to the endoplasmic reticulum, the HKalpha(2)/beta(1) complex translocated to the plasma membrane.

CONCLUSION

Taken together, our results are consistent with the view that the carboxy-terminus of HKalpha(2) facilitates the proper folding of the HKalpha(2)/beta(1) complex allowing translocation of the heterodimer to the plasma membrane where potassium uptake occurs. Otherwise, the alpha/beta complex is destined for degradation.

NH3 is involved in the NH4+ transport induced by the functional expression of the human Rh C glycoprotein

Renal ammonium (NH3 + NH4+) transport is a key process for body acid-base balance. It is well known that several ionic transport systems allow NH4+ transmembrane translocation without high specificity NH4+, but it is still debated whether NH3, and more generally, gas, may be transported by transmembrane proteins. The human Rh glycoproteins have been proposed to mediate ammonium transport. Transport of NH4+ and/or NH3 by the epithelial Rh C glycoprotein (RhCG) may be of physiological importance in renal ammonium excretion because RhCG is mainly expressed in the distal nephron. However, RhCG function is not yet established. In the present study, we search for ammonium transport by RhCG. RhCG function was investigated by electrophysiological approaches in RhCG-expressing Xenopus laevis oocytes. In the submillimolar concentration range, NH4Cl exposure induced inward currents (IAM) in voltage-clamped RhCG-expressing cells, but not in control cells. At physiological extracellular pH (pHo) = 7.5, the amplitude of IAM increased with NH4Cl concentration and membrane hyperpolarization. The amplitude of IAM was independent of external Na+ or K+ concentrations but was enhanced by alkaline pHo and decreased by acid pHo. The apparent affinity of RhCG for NH4+ was affected by NH3 concentration and by changing pHo, whereas the apparent affinity for NH3 was unchanged by pHo, consistent with direct NH3 involvement in RhCG function. The enhancement of methylammonium-induced current by NH3 further supported this conclusion. Exposure to 500 microm NH4Cl induced a biphasic intracellular pH change in RhCG-expressing oocytes, consistent with both NH3 and NH4+ enhanced influx. Our results support the hypothesis of a specific role for RhCG in NH3 and NH4+ transport.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.

Electrogenicity of Na,K- and H,K-ATPase activity and presence of a positively charged amino acid in the fifth transmembrane segment

The transport activity of the Na,K-ATPase (a 3 Na+ for 2 K+ ion exchange) is electrogenic, whereas the closely related gastric and non-gastric H,K-ATPases perform electroneutral cation exchange. We have studied the role of a highly conserved serine residue in the fifth transmembrane segment of the Na,K-ATPase, which is replaced with a lysine in all known H,K-ATPases. Ouabain-sensitive 86Rb uptake and K+-activated currents were measured in Xenopus oocytes expressing the Bufo bladder H,K-ATPase or the Bufo Na,K-ATPase in which these residues, Lys800 and Ser782, respectively, were mutated. Mutants K800A and K800E of the H,K-ATPase showed K+-stimulated and ouabain-sensitive electrogenic transport. In contrast, when the positive charge was conserved (K800R), no K+-induced outward current could be measured, even though rubidium transport activity was present. Conversely, the S782R mutant of the Na,K-ATPase had non-electrogenic transport activity, whereas the S782A mutant was electrogenic. The K800S mutant of the H,K-ATPase had a more complex behavior, with electrogenic transport only in the absence of extracellular Na+. Thus, a single positively charged residue in the fifth transmembrane segment of the alpha-subunit can determine the electrogenicity and therefore the stoichiometry of cation transport by these ATPases.

Stimulation of protein kinase C pathway mediates endocytosis of human nongastric H+-K+-ATPase, ATP1AL1

The human nongastric H+-K+-ATPase, ATP1AL1, shown to reabsorb K+ in exchange for H+ or Na+, is localized in the luminal plasma membrane of renal epithelial cells. It is presumed that renal H+-K+-ATPases can be regulated by endocytosis. However, little is known about the molecular mechanisms that control plasma membrane expression of renal H+-K+-ATPases. In our study, activation of protein kinase C (PKC) using phorbol esters (phorbol 12-myristate 13-acetate) leads to clathrin-dependent internalization and intracellular accumulation of the ion pump in stably transfected Madin-Darby canine kidney cells. Functional inactivation of the H+-K+-ATPase by PKC activation is shown by intracellular pH measurements. Proton extrusion capacity of ATP1AL1-transfected cells is drastically reduced after phorbol 12-myristate 13-acetate incubation and can be prevented with the PKC blocker bisindolylmaleimide. Ion pump internalization and inactivation are specifically mediated by the PKC pathway, whereas activation of the protein kinase A pathway has no influence. Our results show that the nongastric H+-K+-ATPase is a specific target for the PKC pathway. Therefore, PKC-mediated phosphorylation is a potential regulatory mechanism for apical nongastric H+-K+-ATPase plasma membrane expression.

Action of palytoxin on apical H+/K+-ATPase in rat colon

Palytoxin stimulated a cation-dependent short-circuit current (Isc) in rat distal and proximal colon in a concentration-dependent fashion when applied to the mucosal surface of the tissue. The distal colon exhibited a higher sensitivity to the toxin. The palytoxin-induced Isc was blocked by vanadate but was resistant to ouabain or scilliroside, suggesting the conversion of a vanadate-sensitive H+/K+-ATPase into an electrogenic cation transporter. Cation substitution experiments with basolaterally depolarized tissues suggested an apparent permeability of the palytoxin-induced conductance of Na+>K+>Li+. Immunohistochemical control experiments confirmed the absence of the Na+/K+-ATPase in the apical membrane. Consequently, the pore-forming action of palytoxin is not restricted to Na+/K+-ATPase but is also observed with the colonic H+/K+-ATPase.

Human nongastric H+-K+-ATPase: transport properties of ATP1al1 assembled with different beta-subunits

To investigate whether nongastric H+-K+-ATPases transport Na+ in exchange for K+ and whether different beta-isoforms influence their transport properties, we compared the functional properties of the catalytic subunit of human nongastric H+-K+-ATPase, ATP1al1 (AL1), and of the Na+-K+-ATPase alpha1-subunit (alpha1) expressed in Xenopus oocytes, with different beta-subunits. Our results show that betaHK and beta1-NK can produce functional AL1/beta complexes at the oocyte cell surface that, in contrast to alpha1/beta1 NK and alpha1/betaHK complexes, exhibit a similar apparent K+ affinity. Similar to Na+-K+-ATPase, AL1/beta complexes are able to decrease intracellular Na+ concentrations in Na+-loaded oocytes, and their K+ transport depends on intra- and extracellular Na+ concentrations. Finally, controlled trypsinolysis reveals that beta-isoforms influence the protease sensitivity of AL1 and alpha1 and that AL1/beta complexes, similar to the Na+-K+-ATPase, can undergo distinct K+-Na+- and ouabain-dependent conformational changes. These results provide new evidence that the human nongastric H+-K+-ATPase interacts with and transports Na+ in exchange for K+ and that beta-isoforms have a distinct effect on the overall structural integrity of AL1 but influence its transport properties less than those of the Na+-K+-ATPase alpha-subunit.

Nongastric H-K-ATPase in rodent prostate: lobe-specific expression and apical localization

The molecular basis of active ion transport in secretory glands such as the prostate is not well characterized. Rat nongastric H-K-ATPase is expressed at high levels in distal colon surface cell apical membranes and thus is referred to as "colonic." Here we show that the ATPase is expressed in rodent prostate complex in a lobe-specific manner. RT-PCR and Western blot analyses indicate that rat nongastric H-K-ATPase alpha-subunit (alpha(ng)) mRNA and protein are present in coagulating gland (anterior prostate) and lateral and dorsal prostate and absent from ventral lobe, whereas Na-K-ATPase alpha-subunit is present in all lobes. RT-PCR analysis shows that Na-K-ATPase alpha(4) and alpha(3) and gastric H-K-ATPase alpha-subunit are not present in significant amounts in all prostate lobes. Relatively low levels of Na-K-ATPase alpha(2) were found in lateral, dorsal, and anterior lobes. alpha(ng) protein expression is anteriodorsolateral: highest in coagulating gland, somewhat lower in dorsal lobe, and even lower in lateral lobe. Na-K-ATPase protein abundance has the reverse order: expression in ventral lobe is higher than in coagulating gland. alpha(ng) protein abundance is higher in coagulating gland than distal colon membranes. Immunohistochemistry shows that in rat and mouse coagulating gland epithelium alpha(ng) protein has an apical polarization and Na-K-ATPase alpha(1) is localized in basolateral membranes. The presence of nongastric H-K-ATPase in rodent prostate apical membranes may indicate its involvement in potassium concentration regulation in secretions of these glands.

Novel regulation of cell [Na(+)] in macula densa cells: apical Na(+) recycling by H-K-ATPase

Na-K-ATPase is the nearly ubiquitous enzyme that maintains low-Na(+), high-K(+) concentrations in cells by actively extruding Na(+) in exchange for K(+). The prevailing paradigm in polarized absorbing epithelial cells, including renal nephron segments and intestine, has been that Na-K-ATPase is restricted to the basolateral membrane domain, where it plays a prominent role in Na(+) absorption. We have found, however, that macula densa (MD) cells lack functionally and immunologically detectable amounts of Na-K-ATPase protein. In fact, these cells appear to regulate their cytosolic [Na(+)] via another member of the P-type ATPase family, the colonic form of H-K-ATPase, which is located at the apical membrane in these cells. We now report that this constitutively expressed apical MD colonic H-K-ATPase can function as a Na(H)-K-ATPase and regulate cytosolic [Na(+)] in a novel manner. This apical Na(+)-recycling mechanism may be important as part of the sensor function of MD cells and represents a new paradigm in cell [Na(+)] regulation.

Novel Schering and ouabain-insensitive potassium-dependent proton secretion in the mouse cortical collecting duct

The intercalated (IC) cells of the cortical collecting duct (CCD) are important to acid-base homeostasis by secreting acid and reabsorbing bicarbonate. Acid secretion is mediated predominantly by apical membrane Schering (SCH-28080)-sensitive H(+)-K(+)- ATPase (HKA) and bafilomycin-sensitive H(+)-ATPase. The SCH-28080-sensitive HKA is believed to be the gastric HKA (HKAg). Here we examined apical membrane potassium-dependent proton secretion in IC cells of wild-type HKAg (+/+) and HKAg knockout (-/-) mice to determine relative contribution of HKAg to luminal proton secretion. The results demonstrated that HKAg (-/-) and wild-type mice had comparable rates of potassium-dependent proton secretion, with HKAg (-/-) mice having 100% of K(+)-dependent H(+) secretion vs. wild-type mice. Potassium-dependent proton secretion was resistant to ouabain and SCH-28080 in HKAg knockout mice but was sensitive to SCH-28080 in wild-type animals. Northern hybridizations did not demonstrate any upregulation of colonic HKA in HKAg knockout mice. These data indicate the presence of a previously unrecognized K(+)-dependent SCH-28080 and ouabain-insensitive proton secretory mechanism in the cortical collecting tubule that may play an important role in acid-base homeostasis.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Colonic H(+)-K(+)-ATPase in K(+) conservation and electrogenic Na(+) absorption during Na(+) restriction

Upregulation of the colonic H(+)-K(+)- ATPase (cHKA) during hyperaldosteronism suggests that it functions in both K(+) conservation and electrogenic Na(+) absorption in the colon when Na(+)-conserving mechanisms are activated. To test this hypothesis, wild-type (cHKA(+/+)) and cHKA-deficient (cHKA(-/-)) mice were fed Na(+)-replete and Na(+)-restricted diets and their responses were analyzed. In both genotypes, Na(+) restriction led to reduced plasma Na(+) and increased serum aldosterone, and mRNAs for the epithelial Na(+) channel (ENaC) beta- and gamma-subunits, channel-inducing factor, and cHKA were increased in distal colon. Relative to wild-type controls, cHKA(-/-) mice on a Na(+)-replete diet had elevated fecal K(+) excretion. Dietary Na(+) restriction led to increased K(+) excretion in knockout but not in wild-type mice. The amiloride-sensitive, ENaC-mediated short-circuit current in distal colon was significantly reduced in knockout mice maintained on either the Na(+)-replete or Na(+)-restricted diet. These results demonstrate that cHKA plays an important role in K(+) conservation during dietary Na(+) restriction and suggest that cHKA-mediated K(+) recycling across the apical membrane is required for maximum electrogenic Na(+) absorption.

Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase subfamily of oligomeric P-type ATPases and is closely related to rat and human nongastric H-K-ATPases. It has been demonstrated that this ATPase transports K(+) into the cell in exchange for protons and sodium ions, but the stoichiometry of this cation exchange is not yet known. We studied the electrogenic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevis oocytes. In a HEPES-buffered solution, K(+) activation of the H-K-ATPase induced a slow-onset inward current that reached an amplitude of approximately 20 nA after 1-2 min. When measurements were performed in a solution containing 25 mM HCO at a PCO(2) of 40 Torr, the negative current activated by K(+) was reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude that the transport activity of the nongastric B. marinus H-K-ATPase is not intrinsically electrogenic but that the inward current observed in oocytes expressing this ion pump is secondary to intracellular alkalization induced by proton transport.

The functional role of beta subunits in oligomeric P-type ATPases

Na,K-ATPase and gastric and nongastric H,K-ATPases are the only P-type ATPases of higher organisms that are oligomeric and are associated with a beta subunit, which is obligatory for expression and function of enzymes. Topogenesis studies suggest that beta subunits have a fundamental and unique role in K+-transporting P-type ATPases in that they facilitate the correct membrane integration and packing of the catalytic a subunit of these P-type ATPases, which is necessary for their resistance to cellular degradation, their acquisition of functional properties, and their routing to the cell surface. In addition to this chaperone function, beta subunits also participate in the determination of intrinsic transport properties of Na,K- and H,K-ATPases. Increasing experimental evidence suggests that beta assembly is a highly ordered, beta isoform-specific process, which is mediated by multiple interaction sites that contribute in a coordinate, multistep process to the structural and functional maturation of Na,K- and H,K-ATPases.

Ion pumps in polarized cells: sorting and regulation of the Na+, K+- and H+, K+-ATPases

The physiologic function of an ion transport protein is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na(+),K(+)-ATPase and the H(+),K(+)-ATPases are closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells, and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intramolecular interactions govern trafficking, distribution, and catalytic capacities of these proteins.

Ion pump sorting in polarized renal epithelial cells

The plasma membranes of renal epithelial cells are divided into distinct apical and basolateral domains, which contain different inventories of ion transport proteins. Without this polarity vectorial ion and fluid transport would not be possible. Little is known of the signals and mechanisms that renal epithelial cells use to establish and maintain polarized distributions of their ion transport proteins. Analysis of ion pump sorting reveals that multiple complex signals participate in determining and regulating these proteins' subcellular localizations.

Cellular origin and hormonal regulation of K(+)-ATPase activities sensitive to Sch-28080 in rat collecting duct

Rat collecting ducts exhibit type I or type III K(+)-ATPase activities when animals are fed a normal (NK) or a K(+)-depleted diet (LK). This study aimed at determining functionally the cell origin of these two K(+)-ATPases. For this purpose, we searched for an effect on K(+)-ATPases of hormones that trigger cAMP production in a cell-specific fashion. The effects of 1-deamino-8-D-arginine vasopressin (dD-AVP), calcitonin, and isoproterenol in principal cells, alpha-intercalated cells, and beta-intercalated cells of cortical collecting duct (CCD), respectively, and of dD-AVP and glucagon in principal and alpha-intercalated cells of outer medullary collecting duct (OMCD), respectively, were examined. In CCDs, K(+)-ATPase was stimulated by calcitonin and isoproterenol in NK rats (type I K(+)-ATPase) and by dD-AVP in LK rats (type III K(+)-ATPase). In OMCDs, dD-AVP and glucagon stimulated type III but not type I K(+)-ATPase. These hormone effects were mimicked by the cAMP-permeant analog dibutyryl-cAMP. In conclusion, in NK rats, cAMP stimulates type I K(+)-ATPase activity in alpha- and beta-intercalated CCD cells, whereas in LK rats it stimulates type III K(+)-ATPase in principal cells of both CCD and OMCD and in OMCD intercalated cells.

Differential localization of human nongastric H(+)-K(+)-ATPase ATP1AL1 in polarized renal epithelial cells

The human H(+)-K(+)-ATPase, ATP1AL1, belongs to the subgroup of nongastric, K(+)-transporting ATPases. In concert with the structurally related gastric H(+)-K(+)-ATPase, it plays a major role in K(+) reabsorption in various tissues, including colon and kidney. Physiological and immunocytochemical data suggest that the functional heteromeric ion pumps are usually found in the apical plasma membranes of renal epithelial cells. However, the low expression levels of characteristic nongastric ion pumps makes it difficult to verify their spatial distribution in vivo. To investigate the sorting behavior of ATP1AL1, we expressed this pump by stable transfection in MDCK and LLC-PK(1) renal epithelial cell lines. Stable interaction of ATP1AL1 with either the endogenous Na(+)-K(+)-ATPase beta-subunit or the gastric H(+)-K(+)-ATPase beta-subunit was tested by confocal immunofluorescence microscopy and surface biotinylation. In cells transfected with ATP1AL1 alone, the alpha-subunit accumulated intracellularly, consistent with its inability to assemble and travel to the plasma membrane with the endogenous Na(+)-K(+)-ATPase beta-subunit. Cotransfection of ATP1AL1 with the gastric H(+)-K(+)-ATPase beta-subunit resulted in plasma membrane localization of both pump subunits. In cotransfected MDCK cells the heteromeric ion pump was predominantly polarized to the apical plasma membrane. Functional expression of ATP1AL1 was confirmed by (86)Rb(+) uptake measurements. In contrast, cotransfected LLC-PK(1) cells accumulate ATP1AL1 at the lateral membrane. The distinct polarization of ATP1AL1 indicates that the alpha-subunit encodes sorting information that is differently interpreted by cell type-specific sorting mechanisms.

The cell biology of ion pumps: sorting and regulation

The physiologic function of an ion pump is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na,K-ATPase and the gastric H,K-ATPase are two closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intra-molecular interactions govern these proteins' trafficking, distribution and catalytic capacity.

Ouabain-sensitive H,K-ATPase functions as Na,K-ATPase in apical membranes of rat distal colon

Na,K-ATPase activity has been identified in the apical membrane of rat distal colon, whereas ouabain-sensitive and ouabain-insensitive H,K-ATPase activities are localized solely to apical membranes. This study was designed to determine whether apical membrane Na,K-ATPase represented contamination of basolateral membranes or an alternate mode of H,K-ATPase expression. An antibody directed against the H, K-ATPase alpha subunit (HKcalpha) inhibited apical Na,K-ATPase activity by 92% but did not alter basolateral membrane Na,K-ATPase activity. Two distinct H,K-ATPase isoforms exist; one of which, the ouabain-insensitive HKcalpha, has been cloned. Because dietary sodium depletion markedly increases ouabain-insensitive active potassium absorption and HKcalpha mRNA and protein expression, Na, K-ATPase and H,K-ATPase activities and protein expression were determined in apical membranes from control and sodium-depleted rats. Sodium depletion substantially increased ouabain-insensitive H, K-ATPase activity and HKcalpha protein expression by 109-250% but increased ouabain-sensitive Na,K-ATPase and H,K-ATPase activities by only 30% and 42%, respectively. These studies suggest that apical membrane Na,K-ATPase activity is an alternate mode of ouabain-sensitive H,K-ATPase and does not solely represent basolateral membrane contamination.

A transmembrane segment determines the steady-state localization of an ion-transporting adenosine triphosphatase

The H,K-adenosine triphosphatase (ATPase) of gastric parietal cells is targeted to a regulated membrane compartment that fuses with the apical plasma membrane in response to secretagogue stimulation. Previous work has demonstrated that the alpha subunit of the H, K-ATPase encodes localization information responsible for this pump's apical distribution, whereas the beta subunit carries the signal responsible for the cessation of acid secretion through the retrieval of the pump from the surface to the regulated intracellular compartment. By analyzing the sorting behaviors of a number of chimeric pumps composed of complementary portions of the H, K-ATPase alpha subunit and the highly homologous Na,K-ATPase alpha subunit, we have identified a portion of the gastric H,K-ATPase, which is sufficient to redirect the normally basolateral Na,K-ATPase to the apical surface in transfected epithelial cells. This motif resides within the fourth of the H,K-ATPase alpha subunit's ten predicted transmembrane domains. Although interactions with glycosphingolipid-rich membrane domains have been proposed to play an important role in the targeting of several apical membrane proteins, the apically located chimeras are not found in detergent-insoluble complexes, which are typically enriched in glycosphingolipids. Furthermore, a chimera incorporating the Na, K-ATPase alpha subunit fourth transmembrane domain is apically targeted when both of its flanking sequences derive from H,K-ATPase sequence. These results provide the identification of a defined apical localization signal in a polytopic membrane transport protein, and suggest that this signal functions through conformational interactions between the fourth transmembrane spanning segment and its surrounding sequence domains.