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

Deletion of the serine protease CAP2/Tmprss4 leads to dysregulated renal water handling upon dietary potassium depletion

The kidney needs to adapt daily to variable dietary K⁺ contents via various mechanisms including diuretic, acid-base and hormonal changes that are still not fully understood. In this study, we demonstrate that following a K⁺-deficient diet in wildtype mice, the serine protease CAP2/Tmprss4 is upregulated in connecting tubule and cortical collecting duct and also localizes to the medulla and transitional epithelium of the papilla and minor calyx. Male CAP2/Tmprss4 knockout mice display altered water handling and urine osmolality, enhanced vasopressin response leading to upregulated adenylate cyclase 6 expression and cAMP overproduction, and subsequently greater aquaporin 2 (AQP2) and Na⁺-K⁺-2Cl⁻ cotransporter 2 (NKCC2) expression following K⁺-deficient diet. Urinary acidification coincides with significantly increased H⁺,K⁺-ATPase type 2 (HKA2) mRNA and protein expression, and decreased calcium and phosphate excretion. This is accompanied by increased glucocorticoid receptor (GR) protein levels and reduced 11β-hydroxysteroid dehydrogenase 2 activity in knockout mice. Strikingly, genetic nephron-specific deletion of GR leads to the mirrored phenotype of CAP2/Tmprss4 knockouts, including increased water intake and urine output, urinary alkalinisation, downregulation of HKA2, AQP2 and NKCC2. Collectively, our data unveil a novel role of the serine protease CAP2/Tmprss4 and GR on renal water handling upon dietary K⁺ depletion.

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.

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.

Decreased hydrogen–potassium-activated ATPase (H+–K+-ATPase) expression and gastric acid secretory capacity in aged mice

Gastric acid secretion in response to chemical stimulation and to mechanical stimulation was investigated in adult and old mice. The protein expression of a proton pump (H(+)-K(+)-ATPase), a marker of parietal cell function, was determined by Western blotting. Acid secretion was stimulated by histamine (500 and 1000 microg/kg) or carbachol (10 and 20 microg/kg). To investigate the response to mechanical stimulation, the stomach was distended by an intragastric injection of isotonic saline (0.5, 1.0, 1.5, and 2.0 ml). Administration of two doses of histamine produced a dose-dependent increase in acid secretion in adult mice, whereas a higher dose of histamine failed to produce a further increase in old mice. Gastric acid secretion, whether produced by carbachol or mechanical stimulation, did not differ between the two age groups. The protein expression of H(+)-K(+)-ATPase was significantly lower in old mice than in adult. Insofar as histamine increases acid secretion via the cyclic AMP (cAMP) pathway in parietal cells, while carbachol and gastric distention do so via the calcium signaling pathway, the cAMP signaling pathway may be more susceptible to aging than the calcium signaling pathway. The decrease in the secretory capacity of acid secretion in the old mice may be partly attributable to a decrease in parietal cell function, as shown by decrease in H(+)-K(+)-ATPase protein expression.

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

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

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.

Nitric Oxide-dependent Negative Feedback of PARP-1 trans-Activation of the Inducible Nitric-oxide Synthase Gene

Nitric oxide (NO) participates in a variety of physiologic and pathophysiologic processes in diverse tissues, including the kidney. Although mechanisms for cytokine induction of inducible nitric-oxide synthase (iNOS) have been increasingly clarified, the controls for termination of NO production remain unclear. Because excessive NO production can be cytotoxic to host cells, feedback inhibition of iNOS transcription would represent a means of cytoprotection. Many of the cGMP-independent functions of NO are mediated by S-nitrosylation of cysteine thiols of target proteins. We hypothesized that NO-mediated S-nitrosylation of transcription factors might serve to feedback inhibit their trans-activation potential and deactivate iNOS gene transcription. Transient transfection of murine mesangial cells with iNOS promoter deletion-luciferase constructs revealed the region -915 to -849 to be NO sensitive with respect to IL-1beta-induced promoter activity. In vitro DNase I footprinting identified a footprint at -865/-842 in the absence of NO, but not in the presence of endogenous or exogenously delivered NO. Southwestern blotting using this probe coupled with partial peptide sequencing of the protein bands revealed that poly(ADP-ribose) polymerase isoform 1 (PARP-1) bound the probe in a sequence-specific manner. Gel shift/supershift experiments and chromatin immunoprecipitation assay analysis confirmed this binding in vitro and in vivo. Functionally, mutation of the -859/-850 site to prevent PARP-1 binding or PARP-1 knockdown by RNA interference relieved the inhibitory effects of NO on iNOS promoter activity. Biotin-switch assays and co-immunoprecipitation with an anti-nitrocysteine antibody indicated that PARP-1 was S-nitrosylated. We conclude that NO feedback inhibits iNOS gene transcription by S-nitrosylating the trans-activator PARP-1 and decreasing its binding and/or action at the iNOS promoter.

Aldosterone-sensitive repression of ENaCalpha transcription by a histone H3 lysine-79 methyltransferase

Aldosterone is a major regulator of epithelial Na(+) absorption. One of its principal targets is the epithelial Na(+) channel alpha-subunit (ENaCalpha), principally expressed in the kidney collecting duct, lung, and colon. Models of aldosterone-mediated trans-activation of the ENaCalpha gene have focused primarily on interactions of liganded nuclear receptors with the ENaCalpha gene promoter. Herein, we demonstrate that the murine histone H3 lysine-79 methyltransferase, murine disruptor of telomeric silencing alternative splice variant "a" (mDot1a), is a novel component in the aldosterone signaling network controlling transcription of the ENaCalpha gene. Aldosterone downregulated mDot1a mRNA levels in murine inner medullary collecting ducts cells, which was associated with histone H3 K79 hypomethylation in bulk histones and at specific sites in the ENaCalpha 5'-flanking region, and trans-activation of ENaCalpha. Knockdown of mDot1a by RNA interference increased activity of a stably integrated ENaCalpha promoter-luciferase construct and expression of endogenous ENaCalpha mRNA. Conversely, overexpression of EGFP-tagged mDot1a resulted in hypermethylation of histone H3 K79 at the endogenous ENaCalpha promoter, repression of endogenous ENaCalpha mRNA expression, and decreased activity of the ENaCalpha promoter-luciferase construct. mDot1a-mediated histone H3 K79 hypermethylation and repression of ENaCalpha promoter activity was abolished by mDot1a mutations that eliminate its methyltransferase activity. Collectively, our data identify mDot1a as a novel aldosterone-regulated histone modification enzyme, and, through binding the ENaCalpha promoter and hypermethylating histone H3 K79 associated with the ENaCalpha promoter, a negative regulator of ENaCalpha transcription.

Plasticity of mouse renal collecting duct in response to potassium depletion

Plasticity of mouse renal collecting duct in response to potassium depletion. —Renal collecting ducts are the main sites for regulation of whole body potassium balance. Changes in dietary intake of potassium induce pleiotropic adaptations of collecting duct cells, which include alterations of ion and water transport properties along with an hypertrophic response. To study the pleiotropic adaptation of the outer medullary collecting duct (OMCD) to dietary potassium depletion, we combined functional studies of renal function (ion, water, and acid/base handling), analysis of OMCD hypertrophy (electron microscopy) and hyperplasia (PCNA labeling), and large scale analysis of gene expression (transcriptome analysis). The transcriptome of OMCD was compared in mice fed either a normal or a potassium-depleted diet for 3 days using serial analysis of gene expression (SAGE) adapted for downsized extracts. SAGE is based on the generation of transcript-specific tag libraries. Approximately 20,000 tags corresponding to 10,000 different molecular species were sequenced in each library. Among the 186 tags differentially expressed ( P < 0.05) between the two libraries, 120 were overexpressed and 66 were downregulated. The SAGE expression profile obtained in the control library was representative of different functional classes of proteins and of the two cell types (principal and α-intercalated cells) constituting the OMCD. Combined with gene expression analysis, results of functional and morphological studies allowed us to identify candidate genes for distinct physiological processes modified by potassium depletion: sodium, potassium, and water handling, hyperplasia and hypertrophy. Finally, comparison of mouse and human OMCD transcriptomes allowed us to address the question of the relevance of the mouse as a model for human physiology and pathophysiology.

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

Despite its key role in potassium homeostasis, transcriptional control of the H(+)-K(+)-ATPase alpha(2)-subunit (HKalpha(2)) gene in the collecting duct remains poorly characterized. cAMP increases H(+)-K(+)-ATPase activity in the collecting duct, but its role in activating HKalpha(2) transcription has not been explored. Previously, we demonstrated that the proximal 177 bp of the HKalpha(2) promoter confers basal collecting duct-selective expression. This region contains several potential cAMP/Ca(2+)-responsive elements (CRE). Accordingly, we examined the participation of CRE-binding protein (CREB) in HKalpha(2) transcriptional control in murine inner medullary collecting duct (mIMCD)-3 cells. Forskolin and vasopressin induced HKalpha(2) mRNA levels, and CREB overexpression stimulated the activity of HKalpha(2) promoter-luciferase constructs. Serial deletion analysis revealed that CREB inducibility was retained in a construct containing the proximal 100 bp of the HKalpha(2) promoter. In contrast, expression of a dominant negative inhibitor (A-CREB) resulted in 60% lower HKalpha(2) promoter-luciferase activity, suggesting that constitutive CREB participates in basal HKalpha(2) transcriptional activity. A constitutively active CREB mutant (CREB-VP16) strongly induced HKalpha(2) promoter-luciferase activity, whereas overexpression of CREBdLZ-VP16, which lacks the CREB DNA-binding domain, abolished this activation. In vitro DNase I footprinting and gel shift/supershift analysis of the proximal promoter with recombinant glutathione S-transferase (GST)-CREB-1 and mIMCD-3 cell nuclear extracts revealed sequence-specific DNA-CREB-1 complexes at -86/-60. Mutation at three CRE-like sequences within this region abolished CREB-1 DNA-binding activity and abrogated CREB-VP16 trans-activation of the HKalpha(2) promoter. In contrast, mutation of the neighboring -104/-94 kappabeta element did not alter CREB-VP16 trans-activation of the HKalpha(2) promoter. Thus CREB-1, binding to one or more CRE-like elements in the -86/-60 region, trans-activates the HKalpha(2) gene and may represent an important link between rapid and delayed effects of cAMP on HKalpha(2) activity.

Lipopolysaccharide-induced changes in rat gastric H/K-ATPase expression

OBJECTIVE

To evaluate lipopolysaccharide (LPS)-induced inhibition of gastric acid secretion.

SUMMARY BACKGROUND DATA

Endotoxemia from LPS inhibits gastric acid secretion by an unknown mechanism. Bacterial overgrowth in the stomach caused by decreased acid secretion could be responsible for nosocomial pneumonia developing in critically ill intensive care unit patients. Because acid secretion is via the H/K-ATPase and the effects of LPS on this enzyme are unknown, we hypothesized that LPS causes inhibition of gastric acid secretion by down-regulating the H/K-ATPase.

METHODS

A rat model to study gastric acid secretion was created. Saline or LPS (0.05-20 mg/kg IP) was given for 1 hour, after which basal acid secretion was determined for 1 hour. Pentagastrin (PG; 10 microg/kg IV) or saline was then given and gastric acid output collected for another 2 hours.

RESULTS

LPS dose dependently inhibited basal and PG stimulated acid secretion. LPS increased alpha- and beta-H/K-ATPase subunit mRNA expression (Northern blot) in the absence of PG compared with saline. In the presence of PG, LPS did not have this effect. Western blot analysis did not show any difference in alpha- or beta-subunit immunoreactivity. Immunofluorescence analysis demonstrated that PG increased staining in the secretory membranes for H/K-ATPase subunits whereas in all LPS-treated rats, it appeared that H/K-ATPase subunits remained within the tubulovesicles. Furthermore, changes in H/K-ATPase mRNA expression may not be related to changes in NF-kappaB activity.

CONCLUSIONS

These data suggest that inhibition of gastric acid secretion by LPS is due to inhibition of H/K-ATPase enzymatic function or changes in cytoskeletal rearrangements in H/K-ATPase subunits rather than by down-regulation of transcriptional or translational events.

Structure and regulation of the mDot1 gene, a mouse histone H3 methyltransferase

The nucleotide sequence data reported have been deposited in the DDBJ, EMBL, GenBank(R) and GSDB Nucleotide Sequence Databases under accession numbers AY196089, AY196090, AY376663, AY377920 and AY376664. Recently, a new class of histone methyltransferases that plays an indirect role in chromatin silencing by targeting a conserved lysine residue in the nucleosome core was described, namely the Dot1 (disruptor of telomeric silencing) family [Feng, Wang, Ng, Erdjument-Bromage, Tempst, Struhl and Zhang (2002) Curr. Biol. 12, 1052-1058; van Leeuwen, Gafken and Gottschling (2002) Cell (Cambridge, Mass.) 109, 745-756; Ng, Feng, Wang, Erdjument-Bromage, Tempst, Zhang and Struhl (2002) Genes Dev. 16, 1518-1527]. In the present study, we report the isolation, genomic organization and in vivo expression of a mouse Dot1 homologue (mDot1). Expressed sequence tag analysis identified five mDot1 mRNAs (mDot1a-mDot1e) derived from alternative splicing. mDot1a and mDot1b encode 1540 and 1114 amino acids respectively, whereas mDot1c-mDot1e are incomplete at the 5'-end. mDot1a is closest to its human counterpart (hDot1L), sharing 84% amino acid identity. mDot1b is truncated at its N- and C-termini and contains an internal deletion. The five mDot1 isoforms are encoded by 28 exons on chromosome 10qC1, with exons 24 and 28 further divided into two and four sections respectively. Alternative splicing occurs in exons 3, 4, 12, 24, 27 and 28. Northern-blot analysis with probes corresponding to the methyltransferase domain or the mDot1a-coding region detected 7.6 and 9.5 kb transcripts in multiple tissues, but only the 7.6 kb transcript was evident in mIMCD3-collecting duct cells. Transfection of mDot1a-EGFP constructs (where EGFP stands for enhanced green fluorescent protein) into human embryonic kidney (HEK)-293T or mIMCD3 cells increased the methylation of H3-K79 but not H3-K4, -K9 or -K36. Furthermore, DMSO induced mDot1 gene expression and methylation specifically at H3-K79 in mIMCD3 cells in a time- and dose-dependent manner. Collectively, these results add new members to the Dot1 family and show that mDot1 is involved in a DMSO-mediated signal-transduction pathway in collecting duct cells.

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.

NF-kappaB inhibits transcription of the H(+)-K(+)-ATPase alpha(2)-subunit gene: role of histone deacetylases

The H(+)-K(+)-ATPase alpha(2) (HKalpha(2)) gene plays a central role in potassium homeostasis, yet little is known about its transcriptional control. We recently demonstrated that the proximal promoter confers basal transcriptional activity in mouse inner medullary collecting duct 3 cells. We sought to determine whether the kappaB DNA binding element at -104 to -94 influences basal HKalpha(2) gene transcription in these cells. Recombinant NF-kappaB p50 footprinted the region -116/-94 in vitro. Gel shift and supershift analysis revealed NF-kappaB p50- and p65-containing DNA-protein complexes in nuclear extracts of mouse inner medullary collecting duct 3 cells. A promoter-luciferase construct with a mutated -104/-94 NF-kappaB element exhibited higher activity than the wild-type promoter in transfection assays. Overexpression of NF-kappaB p50, p65, or their combination trans-repressed the HKalpha(2) promoter. The histone deacetylase (HDAC) inhibitor trichostatin A partially reversed NF-kappaB-mediated trans-repression of the HKalpha(2) promoter. HDAC6 overexpression inhibited HKalpha(2) promoter activity, and HDAC6 coimmunoprecipitated with NF-kappaB p50 and p65. These results suggest that HDAC6, recruited to the DNA protein complex, acts with NF-kappaB to suppress HKalpha(2) transcription and identify NF-kappaB p50 and p65 as novel binding partners for HDAC6.

Plasminogen activator inhibitor-1 and the kidney

Plasminogen activator inhibitor-1 (PAI-1) is a serine protease inhibitor that was isolated 20 years ago. First recognized as an inhibitor of intravascular fibrinolysis, it is now evident that PAI-1 is a multifunctional protein with actions that may be dependent on or independent of its protease inhibitory effects. The latter often involve interactions between PAI-1 and vitronectin or the urokinase receptor. The protease-inhibitory actions of PAI-1 extend beyond fibrinolysis and include extracellular matrix turnover and activation of several proenzymes and latent growth factors. PAI-1 has been implicated in several renal pathogenetic processes, including thrombotic microangiopathies and proliferative and/or crescentic glomerulopathies. Most recently, it has become clear that PAI-1 also plays a pivotal role in progressive renal disease, both glomerulosclerosis and tubulointerstitial fibrosis. An active area of present research interest, untold stories are likely to be uncovered soon.