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

Dot1l deficiency leads to increased intercalated cells and upregulation of V-ATPase B1 in mice

The collecting duct in the mammalian kidney consists of principal cells (PCs) and intercalated cells (ICs), which regulate electrolyte/fluid and acid/base balance, respectively. The epigenetic regulators of PC and IC differentiation remain obscure. We previously used Aqp2 and V-ATPase B1B2 to label PCs and ICs, respectively. We found that mice with histone H3 K79 methyltransferase Dot1l disrupted in Aqp2-expressing cells (Dot1l(AC)) vs. Dot1l(f/f) possessed ~20% more ICs coupled with a similar decrease in PCs. Here, we performed multiple double immunofluorescence staining using various PC and IC markers and confirmed that this finding. Both α-IC and β-IC populations were significantly expanded in Dot1l(AC) vs. Dot1l(f/f). These changes are associated with significantly upregulated V-ATPase B1 and B2, but not Aqp2, AE1, and Pendrin. Chromatin immunoprecipitation assay unveiled a significant reduction of Dot1l and H3K79 di-methylation bound at the Atp6v1b1 5' flanking region. Overexpression of Dot1a significantly downregulated a stably-transfected luciferase reporter driven by the Atp6v1b1 promoter in IMCD3 cells. This downregulation was impaired, but not completely abolished when a methyltransferase-dead mutant was overexpressed. Taken together, our data suggest that Dot1l is a new epigenetic regulator of PC and IC differentiation and Atp6v1b1 is a new transcriptional target of Dot1l.

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.

Aqp2-expressing cells give rise to renal intercalated cells

The mammalian collecting duct comprises principal and intercalated cells, which maintain sodium/water and acid/base balance, respectively, but the epigenetic contributors to the differentiation of these cell types remain unknown. Here, we investigated whether the histone H3 K79 methyltransferase Dot1l, which is highly expressed in principal cells, participates in this process. Taking advantage of the distribution of aquaporin 2 (Aqp2), which localizes to principal cells of the collecting duct, we developed mice lacking Dot1l in Aqp2-expressing cells (Dot1l(AC)) and found that these mice had approximately 20% fewer principal cells and 13%-16% more intercalated cells than control mice. This deletion of Dot1l in principal cells abolished histone H3 K79 methylation in these cells, but unexpectedly, most intercalated cells also had undetectable di-methyl K79, suggesting that Aqp2(+) cells give rise to intercalated cells. These Aqp2(+) cell-derived intercalated cells were present in both developing and mature kidneys. Furthermore, compared with control mice, Dot1l(AC) mice had 40% higher urine volume and 18% lower urine osmolarity with relatively normal electrolyte and acid-base homeostasis. In conclusion, these data suggest that Dot1l deletion facilitates the differentiation of some α- and β-intercalated cells from Aqp2-expressing progenitor cells or mature principal cells.

AF17 facilitates Dot1a nuclear export and upregulates ENaC-mediated Na+ transport in renal collecting duct cells

Our previous work in 293T cells and AF17^-/- mice suggests that AF17 upregulates expression and activity of the epithelial Na⁺ channel (ENaC), possibly by relieving Dot1a-AF9-mediated repression. However, whether and how AF17 directly regulates Dot1a cellular distribution and ENaC function in renal collecting duct cells remain unaddressed. Here, we report our findings in mouse cortical collecting duct M-1 cells that overexpression of AF17 led to preferential distribution of Dot1a in the cytoplasm. This effect could be blocked by nuclear export inhibitor leptomycin B. siRNA-mediated depletion of AF17 caused nuclear accumulation of Dot1a. AF17 overexpression elicited multiple effects that are reminiscent of aldosterone action. These effects include 1) increased mRNA and protein expression of the three ENaC subunits (α, β and γ) and serum- and glucocorticoid inducible kinase 1, as revealed by real-time RT-qPCR and immunoblotting analyses; 2) impaired Dot1a-AF9 interaction and H3 K79 methylation at the αENaC promoter without affecting AF9 binding to the promoter, as evidenced by chromatin immunoprecipitation; and 3) elevated ENaC-mediated Na⁺ transport, as analyzed by measurement of benzamil-sensitive intracellular [Na⁺] and equivalent short circuit current using single-cell fluorescence imaging and an epithelial Volt-ohmmeter, respectively. Knockdown of AF17 elicited opposite effects. However, combination of AF17 overexpression or depletion with aldosterone treatment did not cause an additive effect on mRNA expression of the ENaC subunits. Taken together, we conclude that AF17 promotes Dot1a nuclear export and upregulates basal, but not aldosterone-stimulated ENaC expression, leading to an increase in ENaC-mediated Na⁺ transport in renal collecting duct cells.

AF17 competes with AF9 for binding to Dot1a to up-regulate transcription of epithelial Na+ channel alpha

We previously reported that Dot1a.AF9 complex represses transcription of the epithelial Na(+) channel subunit alpha (alpha-ENaC) gene in mouse inner medullary collecting duct mIMCD3 cells and mouse kidney. Aldosterone relieves this repression by down-regulating the complex through various mechanisms. Whether these mechanisms are sufficient and conserved in human cells or can be applied to other aldosterone-regulated genes remains largely unknown. Here we demonstrate that human embryonic kidney 293T cells express the three ENaC subunits and all of the ENaC transcriptional regulators examined. These cells respond to aldosterone and display benzamil-sensitive Na(+) currents, as measured by whole-cell patch clamping. We also show that AF17 and AF9 competitively bind to the same domain of Dot1a in multiple assays and have antagonistic effects on expression of an alpha-ENaC promoter-luciferase construct. Overexpression of Dot1a or AF9 decreased mRNA expression of the ENaC subunits and their transcriptional regulators and reduced benzamil-sensitive Na(+) currents. AF17 overexpression caused the opposite effects, accompanied by redirection of Dot1a from the nucleus to the cytoplasm and reduction in histone H3 K79 methylation. The nuclear export inhibitor leptomycin B blocked the effect of AF17 overexpression on H3 K79 hypomethylation. RNAi-mediated knockdown of AF17 yielded nuclear enrichment of Dot1a and histone H3 K79 hypermethylation. As with AF9, AF17 displays nuclear and cytoplasmic co-localization with Sgk1. Therefore, AF17 competes with AF9 to bind Dot1a, decreases Dot1a nuclear expression by possibly facilitating its nuclear export, and relieves Dot1a.AF9-mediated repression of alpha-ENaC and other target genes.

Heterogeneity of H-K-ATPase-mediated acid secretion along the mouse collecting duct

In the collecting duct (CD), H-K-ATPases function in cation reabsorption and H secretion. This study evaluated H-K-ATPase-mediated H secretion along the mouse CD, measured as EIPA- and luminal bafilomycin A(1)-insensitive intracellular pH (pH(i)) recovery from acute H loading (NH(4)) using BCECF. pH(i) recovery was measured in 1) microperfused cortical, outer medullary, and inner medullary CDs (CCD, OMCD, and IMCD) from C57BL/6J mice fed a normal diet and 2) common murine CD cell lines. H-K-ATPase activity along the native, microperfused CD was greatest in the CCD, less in the OMCD, and least in the IMCD (0.10 +/- 0.02, 0.04 +/- 0.01, and 0.01 +/- 0.002 U/min, respectively). H-K-ATPase activity was 0.30 +/- 0.03 and 0.26 +/- 0.03 in A- and B-type ICs, respectively, and was sensitive to Sch-28080 or ouabain. pH(i) recovery was greatest in the OMCD(1) cell line (0.25 +/- 0.01) and less in mpkCCD(c14) (0.17 +/- 0.01), mIMCD-K2 (0.12 +/- 0.01), and mIMCD-3 (0.05 +/- 0.01) cells. EIPA inhibited the majority of pH(i) recovery in these cells (100%, 64%, 75%, and 80% in mpkCCD(c14), OMCD(1), mIMCD-K2, and mIMCD-3, respectively). In OMCD(1) cells, where EIPA-insensitive pH(i) recovery was greatest, H-K-ATPase activity was 0.10 +/- 0.01 and was significantly inhibited (80%) by Sch-28080. We conclude that 1) H-K-ATPase-mediated H secretion in the native mouse CD is greatest in the ICs of the CCD, 2) A- and B-type ICs possess HKalpha(1) and HKalpha(2) H-K-ATPase activity, and 3) the OMCD(1) cell line best exhibits H-K-ATPase.

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.

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.

Dot1a-AF9 complex mediates histone H3 Lys-79 hypermethylation and repression of ENaCalpha in an aldosterone-sensitive manner

Aldosterone is a major regulator of epithelial Na(+) absorption and acts in large part through induction of the epithelial Na(+) channel (ENaC) gene in the renal collecting duct. We previously identified Dot1a as an aldosterone early repressed gene and a repressor of ENaCalpha transcription through mediating histone H3 Lys-79 methylation associated with the ENaCalpha promoter. Here, we report a novel aldosterone-signaling network involving AF9, Dot1a, and ENaCalpha. AF9 and Dot1a interact in vitro and in vivo as evidenced in multiple assays and colocalize in the nuclei of mIMCD3 renal collecting duct cells. Overexpression of AF9 results in hypermethylation of histone H3 Lys-79 at the endogenous ENaCalpha promoter at most, but not all subregions examined, repression of endogenous ENaCalpha mRNA expression and acts synergistically with Dot1a to inhibit ENaCalpha promoter-luciferase constructs. In contrast, RNA interference-mediated knockdown of AF9 causes the opposite effects. Chromatin immunoprecipitation assays reveal that overexpressed FLAG-AF9, endogenous AF9, and Dot1a are each associated with the ENaCalpha promoter. Aldosterone negatively regulates AF9 expression at both mRNA and protein levels. Thus, Dot1a-AF9 modulates histone H3 Lys-79 methylation at the ENaCalpha promoter and represses ENaCalpha transcription in an aldosterone-sensitive manner. This mechanism appears to be more broadly applicable to other aldosterone-regulated genes because overexpression of AF9 alone or in combination with Dot1a inhibited mRNA levels of three other known aldosterone-inducible genes in mIMCD3 cells.

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.