The Na(+)-K(+)-ATPase beta 1 subunit is associated with the HK alpha 2 protein in the rat kidney

The Non-Gastric H+/K+ ATPase (ATP12A) Is Expressed in Mammalian Spermatozoa

H⁺/K⁺ ATPase Type 2 is an heteromeric membrane protein involved in cation transmembrane transport and consists of two subunits: a specific α subunit (ATP12A) and a non-specific β subunit. The aim of this study was to demonstrate the presence and establish the localization of ATP12A in spermatozoa from Bubalus bubalis, Bos taurus and Ovis aries. Immunoblotting revealed, in all three species, a major band (100 kDa) corresponding to the expected molecular mass. The ATP12A immunolocalization pattern showed, consistently in the three species, a strong signal at the acrosome. These results, described here for the first time in spermatozoa, are consistent with those observed for the β₁ subunit of Na⁺/K⁺ ATPase, suggesting that the latter may assemble with the α subunit to produce a functional ATP12A dimer in sperm cells. The above scenario appeared to be nicely supported by 3D comparative modeling and interaction energy calculations. The expression of ATP12A during different stages of bovine sperm maturation progressively increased, moving from epididymis to deferent ducts. Based on overall results, we hypothesize that ATP12A may play a role in acrosome reactions. Further studies will be required in order to address the functional role of this target protein in sperm physiology.

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.

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

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

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.

Pharmacological profiles of the murine gastric and colonic H,K-ATPases

BACKGROUND

The H,K-ATPase, consisting of α and ß subunits, belongs to the P-type ATPase family. There are two isoforms of the α subunit, HKα₁ and HKα₂ encoded by different genes. The ouabain-resistant gastric HKα₁-H,K-ATPase is Sch28080-sensitive. However, the colonic HKα₂-H,K-ATPase from different species shows poor primary structure conservation of the HKα₂ subunit between species and diverse pharmacological sensitivity to ouabain and Sch28080. This study sought to determine the contribution of each gene to functional activity and its pharmacological profile using mouse models with targeted disruption of HKα₁, HKα₂, or HKbeta genes.

METHODS

Membrane vesicles from gastric mucosa and distal colon in wild-type (WT), HKα₁, HKα₂, or HKß knockout (KO) mice were extracted. K-ATPase activity and pharmacological profiles were examined.

RESULTS

The colonic H,K-ATPase demonstrated slightly greater affinity for K(+) than the gastric H,K-ATPase. This K-ATPase activity was not detected in the colon of HKα₂ KO but was observed in HKß KO with properties indistinguishable from WT. Neither ouabain nor Sch28080 had a significant effect on the WT colonic K-ATPase activity, but orthovanadate abolished this activity. Amiloride and its analogs benzamil and 5-N-ethyl-N-isopropylamiloride inhibited K-ATPase activity of HKα₁-containing H,K-ATPase; the dose dependence of inhibition was similar for all three inhibitors. In contrast, the colonic HKα₂-H,K-ATPase was not inhibited by these compounds.

CONCLUSIONS

These data demonstrate that the mouse colonic H,K-ATPase exhibits a ouabain- and Sch28080-insensitive, orthovanadate-sensitive K-ATPase activity. Interestingly, pharmacological studies suggested that the mouse gastric H,K-ATPase is sensitive to amiloride.

GENERAL SIGNIFICANCE

Characterization of the pharmacological profiles of the H,K-ATPases is important for understanding the relevant knockout animals and for considering the specificity of the inhibitors.

The renal H+-K+-ATPases: physiology, regulation, and structure

The H(+)-K(+)-ATPases are ion pumps that use the energy of ATP hydrolysis to transport protons (H(+)) in exchange for potassium ions (K(+)). These enzymes consist of a catalytic alpha-subunit and a regulatory beta-subunit. There are two catalytic subunits present in the kidney, the gastric or HKalpha(1) isoform and the colonic or HKalpha(2) isoform. In this review we discuss new information on the physiological function, regulation, and structure of the renal H(+)-K(+)-ATPases. Evaluation of enzymatic functions along the nephron and collecting duct and studies in HKalpha(1) and HKalpha(2) knockout mice suggest that the H(+)-K(+)-ATPases may function to transport ions other than protons and potassium. These reports and recent studies in mice lacking both HKalpha(1) and HKalpha(2) suggest important roles for the renal H(+)-K(+)-ATPases in acid/base balance as well as potassium and sodium homeostasis. Molecular modeling studies based on the crystal structure of a related enzyme have made it possible to evaluate the structures of HKalpha(1) and HKalpha(2) and provide a means to study the specific cation transport properties of H(+)-K(+)-ATPases. Studies to characterize the cation specificity of these enzymes under different physiological conditions are necessary to fully understand the role of the H(+)-K(+) ATPases in renal physiology.

Molecular regulation and physiology of the H+,K+ -ATPases in kidney

Two H(+), K(+)-adenosine triphosphatase (ATPase) proteins participate in K(+) absorption and H(+) secretion in the renal medulla. Both the gastric (HKalpha(1)) and colonic (HKalpha(2)) H(+),K(+)-ATPases have been localized and characterized by a number of techniques, and are known to be highly regulated in response to acid-base and electrolyte disturbances. Both ATPases are dimers of composition alpha/beta that localize to the apical membrane and both interact with the tetraspanin protein CD63. Although CD63 interacts with the carboxy-terminus of the alpha-subunit of the colonic H(+),K(+)-ATPase, it interacts with the beta-subunit of the gastric H(+),K(+)-ATPase. Pharmacologically, both ATPases are distinct; for example, the gastric H(+),K(+)-ATPase is inhibited by Sch-28080, but the colonic H(+),K(+)-ATPase is inhibited by ouabain (a classic inhibitor of the Na(+)-pump) and is completely insensitive to Sch-28080. The alpha-subunit of the colonic H(+),K(+)-ATPase is the only subunit of the X(+),K(+)-ATPase superfamily that has 3 different splice variants that emerge by deletion or elongation of the amino-terminus. The messenger RNA and protein of one of these splice variants (HKalpha(2C)) is specifically up-regulated in newborn rats and becomes undetectable in adult rats. Therefore, HKalpha(2), in addition to its role in potassium and acid-base homeostasis, appears to play a significant role in early growth and development. Finally, because chronic hypokalemia appears to be the most potent stimulus for upregulation of HKalpha(2), we propose that the HKalpha(2) participates importantly in the maintenance of chronic metabolic alkalosis.

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.

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

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

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

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

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.

Role of muscle in regulating extracellular [K+]

There is a positive association between diets rich in potassium, control of blood pressure, and prevention of stroke. Extracellular [K+] is regulated closely to maintain normal membrane excitability by the concerted regulatory responses of muscle and kidney. Although kidney is responsible for ultimately matching K+ output to K+ intake each day, muscle contains more than 90% of the body's K+ and can buffer changes in extracellular fluid [K+] by either acutely taking up extracellular fluid K+ or releasing intracellular fluid K+ from muscle. It long has been assumed that the changes in muscle K+ transport are a function of sodium pump (Na,K-adenosine triphosphatase [Na, K-ATPasel]) abundance, especially that of the alpha2 isoform, which predominates in skeletal muscle. To test the physiologic significance of changes in muscle Na,K-ATPase expression, we developed the K+ clamp, which measures insulin-stimulated cellular K+ uptake in vivo in the conscious rat. By using the K+ clamp we discovered that significant insulin resistance to cell K+ uptake occurs as follows: (1) early in K+ deprivation before a decrease in muscle sodium pump pool size, and (2) during glucocorticoid treatment, which increases muscle Na,K-ATPase alpha2 levels greater than 50%. We also discovered that adaptation of renal and extrarenal K+ handling to altered K+ balance often occurs without changes in plasma [K+], supporting a feedforward mechanism involving K+ sensing in the splanchnic bed and adjustment of K+ handling. These findings establish the advantage of combining molecular analyses of Na,K-ATPase expression and activity with systems analyses of cellular K+ uptake and excretion in vivo to reveal regulatory mechanisms operating to control K+ homeostasis.

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.

The effect of beta-subunit assembly on function and localization of the colonic H+,K+-ATPase alpha-subunit

BACKGROUND

Previous experiments from our laboratory have demonstrated that HKalpha(2) coimmunoprecipitated with beta(1)-Na(+),K(+)-ATPase. Although HKalpha(2) is expressed abundantly in the apical membrane of distal colon, the demonstration that beta(1) localizes to this same membrane in distal colon has not been demonstrated previously.

METHODS

Immunolocalization was performed in distal colon using a polyclonal antibody against HKalpha(2) and a monoclonal antibody against beta(1).

RESULTS

The results demonstrate that HKalpha(2) localizes to the apical membrane. Two pools of beta(1)-Na(+),K(+)-ATPase were detected. The first localized to the apical membrane. The second pool was detected in the basolateral membrane when distal colon sections were deglycosylated with glycosidase F. Therefore, our results demonstrate that beta(1) localizes to the apical membrane with HKalpha(2), and supports the view that beta(1) is the physiologic beta-subunit for HKalpha(2). We tested, therefore, the efficiency of the two beta-subunits expressed in distal colon (beta(1) and beta(3)) to support the activity of HKalpha(2). Human embryonic kidney HEK-293 cells were transiently cotransfected with HKalpha(2) plus beta(1) or HKalpha(2) plus beta(3). Subsequently, (86)Rb(+)-uptake and plasma membrane localization were evaluated. The results demonstrate that both HKalpha(2)/beta(1) and HKalpha(2)/beta(3) support (86)Rb(+)-uptake. However, (86)Rb(+)-uptake measured in the cells cotransfected with HKalpha(2) plus beta(1) exceeded that measured in cells expressing HKalpha(2)/beta(3). Fluorescence microscopy using enhanced green fluorescent protein cloned at the amino-terminus of HKalpha(2) demonstrated protein migration to the plasma membrane in cells cotransfected with EGFP-HKalpha(2) plus beta(1). In contrast, in cells cotransfected with EGFP-HKalpha(2) plus beta(3), the vast majority of the protein remained confined to intracellular compartments. The significantly higher (86)Rb(+)-uptake corresponded to additional localization of HKalpha(2) to the plasma membrane when coexpressed with beta(1) compared to beta(3).

CONCLUSION

Taken together, these and previous results from our laboratory indicate that beta(1)-Na(+),K(+)-ATPase is likely to represent the most physiologic and efficient subunit for HKalpha(2) assembly in distal colon.

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.

In vivo expression profile of a H+-K+-ATPase alpha2-subunit promoter-reporter transgene

Because little is known about the molecular basis of transcriptional regulation of the murine H(+)-K(+)-ATPase alpha(2) (HKalpha(2)) gene or other genes whose expression is restricted in part to the collecting duct, especially in vivo, we developed transgenic mice carrying an insertional HKalpha(2) promoter-reporter gene construct. In these mice, the region -7,264/+253 of the HKalpha(2) 5'-flanking region controls expression of the reporter gene enhanced green fluorescent protein (EGFP). Patterns of HKalpha(2)/EGFP transgene expression were examined by fluorescence microscopy and immunoblotting. Of 10 major organs examined, EGFP immunoreactivity was detected abundantly in the kidney, and to a far lesser extent, in the brain and lung. Within the kidney, EGFP fluorescence was detected exclusively in the collecting ducts of transgenic mice and colocalized with the cellular distribution of both endogenous HKalpha(2) and aquaporin-2, consistent with the known expression pattern of endogenous HKalpha(2) in principal cells. Surprisingly, no transgene expression was evident by immunoblotting or fluorescence microscopy in the distal colon, the site of the highest endogenous HKalpha(2) expression. Although previous studies of steady-state mRNA levels suggested differences in HKalpha(2) gene regulation in the kidney and colon, our results provide the first direct evidence of differential transcriptional control of the HKalpha(2) gene in these organs and suggest that regions outside the 5'-flanking region or other regulatory factors play a role in HKalpha(2) expression in the distal colon.

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.

Skeletal muscle regulates extracellular potassium

Maintaining extracellular fluid (ECF) K(+) concentration ([K(+)]) within a narrow range is accomplished by the concerted responses of the kidney, which matches K(+) excretion to K(+) intake, and skeletal muscle, the main intracellular fluid (ICF) store of K(+), which can rapidly buffer ECF [K(+)]. In both systems, homologous P-type ATPase isoforms are key effectors of this homeostasis. During dietary K(+) deprivation, these P-type ATPases are regulated in opposite directions: increased abundance of the H,K-ATPase "colonic" isoform in the renal collecting duct drives active K(+) conservation while decreased abundance of the plasma membrane Na,K-ATPase alpha(2)-isoform leads to the specific shift of K(+) from muscle ICF to ECF. The skeletal muscle response is isoform and muscle specific: alpha(2) and beta(2), not alpha(1) and beta(1), levels are depressed, and fast glycolytic muscles lose >90% alpha(2), whereas slow oxidative muscles lose ~50%; however, both muscle types have the same fall in cellular [K(+)]. To understand the physiological impact, we developed the "K(+) clamp" to assess insulin-stimulated cellular K(+) uptake in vivo in the conscious rat by measuring the exogenous K(+) infusion rate needed to maintain constant plasma [K(+)] during insulin infusion. Using the K(+) clamp, we established that K(+) deprivation leads to near-complete insulin resistance of cellular K(+) uptake and that this insulin resistance can occur before any decrease in plasma [K(+)] or muscle Na(+) pump expression. These studies establish the advantage of combining molecular analyses of P-type ATPase expression with in vivo analyses of cellular K(+) uptake and excretion to determine mechanisms in models of disrupted K(+) homeostasis.

The gamma-Na+,K+-ATPase subunit assembles selectively with alpha1/beta1-Na+,K+-ATPase but not with the colonic H+,K+-ATPase

BACKGROUND

The ubiquitous Na+-pump (Na+,K+-ATPase) assembles as a heterodimer of composition alpha/beta in some nephron segments, while in other segments it may exist as a heterotrimer of composition alpha/beta/gamma. The gamma-subunit has been reported to increase the affinity of the Na+-pump for adenosine 5'-triphosphate (ATP), and decrease affinity for both Na+ and K+. The alpha-subunit of the colonic H+,K+-ATPase (cHK) shares 75% sequence similarity with alpha1-Na+,K+-ATPase (alpha1) and assembles with beta1-Na+,K+-ATPase (beta1) in distal colon and renal medulla. Differences in pharmacological properties have been ascribed to when heterologously expressed function has been compared to function in vitro. The purpose of this study was to determine if cHK might associate with the gamma-subunit of the Na+,K+-ATPase (gamma) as a possible explanation for these variations in function.

METHODS

An antibody specific for the gamma was used in coimmunoprecipitation experiments to determine if the gamma assembles stably in vitro with cHK and beta1 in rat renal medulla or distal colon.

RESULTS

Our results demonstrate that the gamma-subunit assembles specifically with the Na+-pump, but not with cHK. Furthermore, the gamma-subunit assembly was specific for rat kidney and was not observed in distal colon.

CONCLUSION

Since the gamma-subunit did not assemble with the cHK/beta1 complex, gamma-subunit assembly cannot explain those variations in ex vivo and in vitro pharmacologic properties ascribed to cHK.

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.

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.

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.

NH4+ secretion in inner medullary collecting duct in potassium deprivation: role of colonic H+-K+-ATPase

NH4+ secretion in inner medullary collecting duct in potassium deprivation: Role of colonic H+-K+-ATPase.

BACKGROUND

In K+ deprivation (KD), gastric (g) H+-K+-ATPase (HKA) is suppressed, whereas colonic (c) HKA is induced in the terminal inner medullary collecting duct (IMCD). We hypothesized that in KD, cHKA is induced and can mediate the secretion of NH4+.

METHODS

Rats were sacrificed after 2, 3, 6, or 14 days on regular (NML) or K+-free (KD) diet. mRNA expression of HKA isoforms in terminal inner medulla was examined and correlated with NH4+ secretion in perfused IMCD in vitro.

RESULTS

Urinary NH4+ excretion increased after K+-free diet for six days. In terminal inner medulla, cHKA expression was strongly induced, whereas gHKA expression was decreased. NH4+ secretion increased by 62% in KD (JtNH4+ 0.57 vs. 0.92 pmol/min/mm tubule length, P < 0.001). Ouabain (1 mM) in perfusate inhibited NH4+ secretion in KD by 45% (P < 0.002) but not in NML. At luminal pH 7.7, which inhibits NH3 diffusion, NH4+ secretion in IMCD was 140% higher in KD (0.36 vs. 0.15, P < 0.03) and was sensitive to ouabain. ROMK-1 mRNA expression was induced in parallel with cHKA in inner medulla.

CONCLUSIONS

These data suggest that in KD, cHKA replaces gHKA and mediates enhanced secretion of NH4+ (and H+) into the lumen facilitated by K+ recycling through ROMK-1.

Gastric H(+),K(+)-adenosine triphosphatase beta subunit is required for normal function, development, and membrane structure of mouse parietal cells

BACKGROUND & AIMS

Parietal cells of the gastric mucosa contain a complex and extensive secretory membrane system that harbors gastric H(+),K(+)-adenosine triphosphatase (ATPase), the enzyme primarily responsible for acidification of the gastric lumen. We have produced mice deficient in the H(+),K(+)-ATPase beta subunit to determine the role of the protein in the biosynthesis of this membrane system and the biology of gastric mucosa.

METHODS

Mice deficient in the H(+), K(+)-ATPase beta subunit were produced by gene targeting.

RESULTS

The stomachs of H(+),K(+)-ATPase beta subunit-deficient mice were achlorhydric. Histological and immunocytochemical analyses with antibodies to the H(+),K(+)-ATPase alpha subunit revealed that parietal cell development during ontogeny was retarded in H(+), K(+)-ATPase beta subunit-deficient mice. In 15-day-old mice, cells with secretory canaliculi were observed in wild-type but not in H(+), K(+)-ATPase beta subunit-deficient mice. Parietal cells of H(+), K(+)-ATPase beta subunit-deficient mice 17 days and older contained an abnormal canaliculus that was dilated and contained fewer and shorter microvilli than normal. In older parietal cells, the abnormal canaliculus was massive (25 micrometer in diameter) and contained few microvilli. We did not observe typical tubulovesicular membranes in any parietal cell from H(+),K(+)-ATPase beta subunit-deficient mice. Histopathologic alterations were only observed in the stomach.

CONCLUSIONS

The H(+),K(+)-ATPase beta subunit is required for acid-secretory activity of parietal cells in vivo, normal development and cellular homeostasis of the gastric mucosa, and attainment of the normal structure of the secretory membranes.

The nongastric H+-K+-ATPases: molecular and functional properties

The Na-K/H-K-ATPase gene family is divided in three subgroups including the Na-K-ATPases, mainly involved in whole body and cellular ion homeostasis, the gastric H-K-ATPase involved in gastric fluid acidification, and the newly described nongastric H-K-ATPases for which the identification of physiological roles is still in its infancy. The first member of this last subfamily was first identified in 1992, rapidly followed by the molecular cloning of several other members. The relationship between each member remains unclear. The functional properties of these H-K-ATPases have been studied after their ex vivo expression in various functional expression systems, including the Xenopus laevis oocyte, the insect Sf9 cell line, and the human HEK 293 cells. All these H-K-ATPase alpha-subunits appear to encode H-K-ATPases when exogenously expressed in such expression systems. Recent data suggest that these H-K-ATPases could also transport Na+ in exchange for K+, revealing a complex cation transport selectivity. Moreover, they display a unique pharmacological profile compared with the canonical Na-K-ATPases or the gastric H-K-ATPase. In addition to their molecular and functional characterizations, a major goal is to correlate the molecular expression of these cloned H-K-ATPases with the native K-ATPases activities described in vivo. This appears to be more complex than anticipated. The discrepancies between the functional data obtained by exogenous expression of the nongastric H-K-ATPases and the physiological data obtained in native organs could have several explanations as discussed in the present review. Extensive studies will be required in the future to better understand the physiological role of these H-K-ATPases, especially in disease processes including ionic or acid-base disorders.

H-K-ATPase in the RCCT-28A rabbit cortical collecting duct cell line

In the present study, we demonstrate that the rabbit cortical collecting duct cell line RCCT-28A possesses three distinct H-K-ATPase catalytic subunits (HKalpha). Intracellular measurements of RCCT-28A cells using the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) indicated that the mechanism accounting for recovery from an acid load exhibited both K+ dependence and sensitivity to Sch-28080 characteristic of H-K-ATPases. Recovery rates were 0.022 +/- 0.005 pH units/min in the presence of K+, 0.004 +/- 0.002 in the absence of K+, and 0.002 +/- 0.002 in the presence of Sch-28080. The mRNAs encoding the HKalpha1 subunit and the H-K-ATPase beta-subunit (HKbeta) were detected by RT-PCR. In addition, two HKalpha2 species were found by RT-PCR and 5' rapid amplification of cDNA ends (5'-RACE) in the rabbit renal cortex. One was homologous to HKalpha2 cDNAs generated from other species, and the second was novel. The latter, referred to as HKalpha2c, encoded an apparent 61-residue amino-terminal extension that bore no homology to reported sequences. Antipeptide antibodies were designed on the basis of this extension, and these antibodies recognized a protein of the appropriate mass in both rabbit renal tissue samples and RCCT-28A cells. Such findings constitute very strong evidence for expression of the HKalpha2c subunit in vivo. The results suggest that the rabbit kidney and RCCT-28A cells express at least three distinct H-K-ATPases.

ATP1AL1, a member of the non-gastric H,K-ATPase family, functions as a sodium pump

The human ATP1AL1-encoded protein (an alpha subunit of the human non-gastric H,K-ATPase) has previously been shown to assemble with the gastric H,K-ATPase beta subunit (gH,Kbeta) to form a functionally active ionic pump in HEK 293 cells. This pump has been found to be sensitive to both SCH 28080 and ouabain. However, the 86Rb+-influx mediated by the ATP1AL1-gH,Kbeta heterodimer in HEK 293 cells is at least 1 order of magnitude larger than the maximum ouabain-sensitive proton efflux detected in the same cells. In this study we find that the intracellular Na+ content in cells expressing ATP1AL1 and gH,Kbeta is two times lower than that in control HEK 293 cells in response to incubation for 3 h in the presence of 1 microM ouabain. Moreover, analysis of net Na+ efflux in HEK 293 expressing the ATP1AL1-gH,Kbeta heterodimer reveals the presence of Na+ extrusion activity that is not sensitive to 1 microM ouabain but can be inhibited by 1 mM of this drug. In contrast, ouabain-inhibitable Na+ efflux in control HEK 293 cells is similarly sensitive to either 1 microM or 1 mM ouabain. Finally, 86Rb+ influx through the ATP1AL1-gH,Kbeta complex is comparable to the 1 mM ouabain-sensitive Na+ efflux in the same cells. The data presented here suggest that the enzyme formed by ATP1AL1 and the gastric H,K-ATPase beta subunit in HEK 293 cells mediates primarily Na+,K+ rather than H+,K+ exchange.