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

ATP12A Proton Pump as an Emerging Therapeutic Target in Cystic Fibrosis and Other Respiratory Diseases

ATP12A encodes the catalytic subunit of the non-gastric proton pump, which is expressed in many epithelial tissues and mediates the secretion of protons in exchange for potassium ions. In the airways, ATP12A-dependent proton secretion contributes to complex mechanisms regulating the composition and properties of the fluid and mucus lining the respiratory epithelia, which are essential to maintain the airway host defense and the respiratory health. Increased expression and activity of ATP12A in combination with the loss of other balancing activities, such as the bicarbonate secretion mediated by CFTR, leads to excessive acidification of the airway surface liquid and mucus dysfunction, processes that play relevant roles in the pathogenesis of cystic fibrosis and other chronic inflammatory respiratory disorders. In this review, we summarize the findings dealing with ATP12A expression, function, and modulation in the airways, which led to the consideration of ATP12A as a potential therapeutic target for the treatment of cystic fibrosis and other airway diseases; we also highlight the current advances and gaps regarding the development of therapeutic strategies aimed at ATP12A inhibition.

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.

Left ventricular diastolic function associated with common genetic variation in ATP12A in a general population

Background

Left ventricular (LV) function depends on the activity of transmembrane electrolyte transporters. Failing human myocardium has lower Na⁺/K⁺ ATPase expression and higher intracellular sodium concentrations. The ATP12A gene encodes a catalytic subunit of an ATPase that can function as a Na⁺/K⁺ pump. We, therefore, investigated the association between LV function and common genetic variants in ATP12A.

Methods

A random sample of 1166 participants (53.7% women; mean age 49.5 years, 44.8% hypertensive) was recruited in Belgium, Poland, Italy and Russia. We measured transmitral early and late diastolic velocities (E and A) by pulsed wave Doppler, and mitral annular velocities (e’ and a’) by tissue Doppler. Using principal component analysis, we summarized 7 Doppler indexes – namely, E, A, e’ and a’ velocities, and their ratios (E/A, e’/a’, and E/e’) – into a single diastolic score. We genotyped 5 tag SNPs (rs963984, rs9553395, rs10507337, rs12872010, rs2071490) in ATP12A. In our analysis we focused on rs10507337 because it is located within a transcription factor binding site.

Results

In the population-based analyses while adjusting for covariables and accounting for family clusters and country, rs10507337 C allele carriers had significantly higher E/A (P = 0.003), e’ (P = 5.8×10⁻⁵), e’/a’ (P = 0.003) and diastolic score (P = 0.0001) compared to TT homozygotes. Our findings were confirmed in the haplotype analysis and in the family-based analyses in 74 informative offspring.

Conclusions

LV diastolic function as assessed by conventional and tissue Doppler indexes including a composite diastolic score was associated with genetic variation in ATP12A. Further experimental studies are necessary to clarify the role of ATP12A in myocardial relaxation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-014-0121-6) contains supplementary material, which is available to authorized users.

Sp1 trans-activates the murine H(+)-K(+)-ATPase alpha(2)-subunit gene

The H(+)-K(+)-ATPase alpha(2) (HKalpha2) gene of the renal collecting duct and distal colon plays a central role in potassium and acid-base homeostasis, yet its transcriptional control remains poorly characterized. We previously demonstrated that the proximal 177 bp of its 5'-flanking region confers basal transcriptional activity in murine inner medullary collecting duct (mIMCD3) cells and that NF-kappaB and CREB-1 bind this region to alter transcription. In the present study, we sought to determine whether the -144/-135 Sp element influences basal HKalpha2 gene transcription in these cells. Electrophoretic mobility shift and supershift assays using probes for -154/-127 revealed Sp1-containing DNA-protein complexes in nuclear extracts of mIMCD3 cells. Chromatin immunoprecipitation (ChIP) assays demonstrated that Sp1, but not Sp3, binds to this promoter region of the HKalpha2 gene in mIMCD3 cells in vivo. HKalpha2 minimal promoter-luciferase constructs with point mutations in the -144/-135 Sp element exhibited much lower activity than the wild-type promoter in transient transfection assays. Overexpression of Sp1, but not Sp3, trans-activated an HKalpha2 proximal promoter-luciferase construct in mIMCD3 cells as well as in SL2 insect cells, which lack Sp factors. Conversely, small interfering RNA knockdown of Sp1 inhibited endogenous HKalpha2 mRNA expression, and binding of Sp1 to chromatin associated with the proximal HKalpha2 promoter without altering the binding or regulatory influence of NF-kappaB p65 or CREB-1 on the proximal HKalpha2 promoter. We conclude that Sp1 plays an important and positive role in controlling basal HKalpha2 gene expression in mIMCD3 cells in vivo and in vitro.

Characterization of the rabbit HKalpha2 gene promoter

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

Nongastric H,K-ATPase: structure and functional properties

Nongastric H,K-ATPases whose catalytic subunits (AL1) encoded by human ATP1AL1 and homologous animal genes comprise the third distinct group within the X,K-ATPase family. No unique nongastric beta has been identified. Precise in situ colocalization and strong association of AL1 with beta1 of Na,K-ATPase was detected in apical membranes of rodent prostate epithelium. In this tissue, beta1NK serves as an authentic subunit of both the Na,K- and nongastric H,K-pumps. Upon expression in Xenopus oocytes the human AL1 can assemble with beta1NK, and more efficiently with gastric betaHK, into functional H,K-pumps. Both AL1/beta complexes exhibit a similar K-affinity, and their K-transport depends on intra- and extracellular Na. These data provide new evidence that nongastric H,K-ATPase can perform Na/K-exchange, and indicate that beta does not significantly affect this ion-pump function. Analysis of human nongastric H,K-ATPase expressed in Sf-21 insect cells revealed that AL1/betaHK exhibits substantial enzymatic activities in K-free medium and K stimulates, but Na has inhibitory effect on ATP hydrolysis. Thus, although the nongastric H,K-ATPase can function as Na/K exchanger, its reaction mechanism is different from that of the Na,K-ATPase. Human nongastric H,K-ATPase is highly sensitive to bufalin, digoxin, and digitoxin, but almost resistant to digoxigenin and ouabagenin.

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.

Structure, promoter analysis, and chromosomal localization of the murine H(+)/K(+)-ATPase alpha 2 subunit gene

The H(+)/K(+)-ATPase alpha2 subunit (HK alpha 2) of distal colon and renal collecting ducts plays a critical role in potassium and acid-base homeostasis. The isolation and complete sequence of the murine HK alpha 2 gene are reported. The HK alpha 2 gene contains 23 exons and spans 23.5 kb of genomic DNA. The exon/intron organization is comparable to that of the human ATP1AL1 gene. Primer extension and 5'-rapid amplification of cDNA ends of distal colon RNA were used to map the transcription initiation site. Fluorescence in situ hybridization analysis localized the HK alpha 2 gene to murine chromosome 14C3. Sequence analysis of 7.2 kb of the 5'-flanking region revealed numerous consensus sites for transcription factors, including two potential glucocorticoid response elements. Transient transfection of promoter-luciferase constructs demonstrated strong basal HK alpha 2 promoter activity in renal collecting duct cells but not in fibroblasts or in a medullary thick ascending limb of Henle's loop cell line. Deletion analysis revealed that the proximal 0.2 kb of the promoter was sufficient to confer activity in collecting duct cells. These data should prove important in elucidation of the mechanisms controlling the differential, tissue-specific expression of the HK alpha 2 gene.

The betam protein, a member of the X,K-ATPase beta-subunits family, is located intracellularly in pig skeletal muscle

The sequence of the pig cDNA encoding the muscle-specific betam-protein, a member of the X,K-ATPase beta-subunits family, was determined. Two alternatively spliced transcripts encoding polypeptide chains of 355 and 351 residues were identified. The tissue specificity of expression of betam and other X,K-ATPase beta-subunit genes was studied by RT-PCR performed on 24 tissues from newborn pigs. The betam expression was shown to be highly tissue-specific, being detected at the highest level in skeletal muscle, at a lower level in heart, and at much lower level in skin. The betam transcripts are more abundant in the tissues from the newborn than adult. Immunoblotting and deglycosylation shift assay indicated that skeletal muscle membranes of newborn pigs contain betam protein with an electrophoretic mobility and carbohydrate content very similar to that of human betam. Fractionation of membranes from both newborn and adult pig skeletal muscles by isopycnic centrifugation revealed that the majority of the betam protein is concentrated in the sarcoplasmic reticulum-containing fractions. This intracellular location is a unique property that distinguishes the betam protein from other members of the X,K-ATPase beta-subunit family.

Identification of a novel gene of the X,K-ATPase beta-subunit family that is predominantly expressed in skeletal and heart muscles

We have identified the fifth member of the mammalian X,K-ATPase beta-subunit gene family. The human and rat genes are largely expressed in skeletal muscle and at a lower level in heart. The deduced human and rat proteins designated as beta(muscle) (beta(m)) consist of 357 and 356 amino acid residues, respectively, and exhibit 89% identity. The sequence homology of beta(m) proteins with known Na,K- and H,K-ATPase beta-subunits are 30.5-39.4%. Unlike other beta-subunits, putative beta(m) proteins have large N-terminal cytoplasmic domains containing long Glu-rich sequences. The data obtained indicate the existence of hitherto unknown X,K-ATPase (most probably Na,K-ATPase) isozymes in muscle cells.

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.

Ouabain-sensitive H,K-ATPase: tissue-specific expression of the mammalian genes encoding the catalytic alpha subunit

Human ATP1AL1 and corresponding genes of other mammals encode the catalytic alpha subunit of a non-gastric ouabain-sensitive H,K-ATPases, the ion pump presumably involved in maintenance of potassium homeostasis. The tissue specificity of the expression of these genes in different species has not been analyzed in detail. Here we report comparative RT-PCR screening of mouse, rat, rabbit, human, and dog tissues. Significant expression levels were observed in the skin, kidney and distal colon of all species (with the exception of the human colon). Analysis of rat urogenital organs also revealed strong expression in coagulating and preputial glands. Relatively lower expression levels were detected in many other tissues including brain, placenta and lung. In rabbit brain the expression was found to be specific to choroid plexus and cortex. Prominent similarity of tissue-specific expression patterns indicates that animal and human non-gastric H,K-ATPases are indeed products of homologous genes. This is also consistent with the high sequence similarity of non-gastric H,K-ATPases (including partial sequences of hitherto unknown cDNAs for mouse and dog proteins).

A novel N-terminal splice variant of the rat H+-K+-ATPase alpha2 subunit. Cloning, functional expression, and renal adaptive response to chronic hypokalemia

The H+-K+-ATPase of renal collecting duct mediates K+ conservation during chronic hypokalemia. K+ deprivation promotes H+-K+-ATPase alpha2 (HKalpha2) gene expression in the medullary collecting duct, the principal site of active K+ reabsorption, suggesting that this isozyme contributes to renal K+ reclamation. We report here that alternative transcriptional initiation and mRNA splicing give rise to distinct N-terminal variants of the HKalpha2 subunit. Sequence analysis and in vitro translation revealed that HKalpha2a corresponds to the known HKalpha2 cDNA (Crowson, M. S., and Shull, G. E. (1992) J. Biol. Chem. 267, 13740-13748), whereas HKalpha2b represents a novel variant truncated by 108 amino acids at its N terminus. HKalpha2b mRNA contains a complex 5'-untranslated region with eight upstream open reading frames, features implicated in translational regulation of other genes. Heterologous expression of HKalpha2b with and without the gastric H+-K+-ATPase beta subunit in HEK 293 cells indicated that this variant encodes a K+ uptake mechanism that is relatively Sch 28080-resistant, partially sensitive to ouabain, and appears to require coexpression with the gastric H+-K+-ATPase beta subunit for optimal functional activity. Northern analysis demonstrated that both subtypes (HKalpha2b > HKalpha2a) are expressed abundantly in distal colon and modestly in proximal colon and kidney. Moreover, the abundance of the two mRNAs increases coordinately among the renal zones, but not in colon, with chronic K+ deprivation. These results demonstrate the potential for complex control of HKalpha2 gene expression by transcriptional and posttranscriptional mechanisms not recognized in other members of the Na+-K+-ATPase/H+-K+-ATPase family.

Enzymatic properties of human Na,K-ATPase alpha1beta3 isozyme

Recent results of a wide-scale human cDNA sequencing project have identified a cDNA which encodes a hitherto unknown human protein sequence exhibiting structural similarities with beta-subunits of the Na,K- and H,K-ATPase family and with the amphibian Na,KATPase beta3-subunit, in particular. In this study the ability of the putative human beta3-subunit to assemble with the human alpha1-subunit in functionally active Na,KATPase was examined using the baculovirus expression system. The recombinant baculovirus simultaneously expressing both alpha1 and beta3 human proteins was produced using the dual-promoter transfer vector p2Bac. The expression of both human proteins in baculovirus-infected Sf-9 cell membranes detected with specific antibodies resulted in the formation of a catalytically competent alpha1beta3 ATPase complex. Characterization of the recombinant ATPase complex involved the analysis of Na+, K+, and ATP dependencies of enzyme activity and its sensitivity toward ouabain. Preparations of HeLa cell membranes containing alpha1beta1 isozyme of human Na,K-ATPase were used as control. The data obtained clearly demonstrated that alpha1beta3 ATPase exhibits enzymatic properties which are characteristic of Na, K-ATPase. The recombinant alpha1beta3 isozyme displayed significantly lower sensitivity to ouabain than native alpha1beta1. These findings indicate that the hitherto unknown alpha1beta3 isozyme of human Na,K-ATPase is likely to exist in vivo, thus suggesting further expansion of human Na,K-ATPase isozyme diversity. The present studies are the first in which heterologous expression has been used for the characterization of an isozyme of human Na, K-ATPase.