Adaptation to low-K+ media increases H(+)-K(+)-ATPase but not H(+)-ATPase-mediated pHi recovery in OMCD1 cells

The study of intercalated cells using ex vivo techniques: primary cell culture, cell lines, kidney slices, and organoids

This review summarizes methods to study kidney intercalated cell (IC) function ex vivo. While important for acid-base homeostasis, IC dysfunction is often not recognized clinically until it becomes severe. The advantage of using ex vivo techniques is that they allow for the differential evaluation of IC function in controlled environments. Although in vitro kidney tubular perfusion is a classical ex vivo technique to study IC, here we concentrate on primary cell cultures, immortalized cell lines, and ex vivo kidney slices. Ex vivo techniques are useful in evaluating IC signaling pathways that allow rapid responses to extracellular changes in pH, CO2, and bicarbonate (HCO3-). However, these methods for IC work can also be challenging, as cell lines that recapitulate IC do not proliferate easily in culture. Moreover, a "pure" IC population in culture does not necessarily replicate its collecting duct (CD) environment, where ICs are surrounded by the more abundant principal cells (PCs). It is reassuring that many findings obtained in ex vivo IC systems signaling have been largely confirmed in vivo. Some of these newly identified signaling pathways reveal that ICs are important for regulating NaCl reabsorption, thus suggesting new frontiers to target antihypertensive treatments. Moreover, recent single-cell characterization studies of kidney epithelial cells revealed a dual developmental origin of IC, as well as the presence of novel CD cell types with certain IC characteristics. These exciting findings present new opportunities for the study of IC ex vivo and will likely rediscover the importance of available tools in this field.NEW & NOTEWORTHY The study of kidney intercalated cells has been limited by current cell culture and kidney tissue isolation techniques. This review is to be used as a reference to select ex vivo techniques to study intercalated cells. We focused on the use of cell lines and kidney slices as potential useful models to study membrane transport proteins. We also review how novel collecting duct organoids may help better elucidate the role of these intriguing cells.

Intercalated Cells of the Kidney Collecting Duct in Kidney Physiology

The epithelium of the kidney collecting duct (CD) is composed mainly of two different types of cells with distinct and complementary functions. CD principal cells traditionally have been considered to have a major role in Na⁺ and water regulation, while intercalated cells (ICs) were thought to largely modulate acid-base homeostasis. In recent years, our understanding of IC function has improved significantly owing to new research findings. Thus, we now have a new model for CD transport that integrates mechanisms of salt and water reabsorption, K⁺ homeostasis, and acid-base status between principal cells and ICs. There are three main types of ICs (type A, type B, and non-A, non-B), which first appear in the late distal convoluted tubule or in the connecting segment in a species-dependent manner. ICs can be detected in CD from cortex to the initial part of the inner medulla, although some transport proteins that are key components of ICs also are present in medullary CD, cells considered inner medullary. Of the three types of ICs, each has a distinct morphology and expresses different complements of membrane transport proteins that translate into very different functions in homeostasis and contributions to CD luminal pro-urine composition. This review includes recent discoveries in IC intracellular and paracrine signaling that contributes to acid-base regulation as well as Na⁺, Cl^-, K⁺, and Ca²⁺ homeostasis. Thus, these new findings highlight the potential role of ICs as targets for potential hypertension treatments.

Using Ex Vivo Kidney Slices to Study AMPK Effects on Kidney Proteins

The ex vivo kidney slice technique has been used extensively in the fields of kidney physiology and cell biology. Our group and others have used this method to study epithelial traffic of transport proteins in situ in kidney tissue. In this methodology chapter, we summarize our adaptation of this classic protocol for the study of the effect of AMPK in the modulation of transport protein regulation, especially in kidney epithelial cells. Briefly, slices were obtained by sectioning freshly harvested rodent (rat or mouse) kidneys using a Stadie-Riggs tissue slicer. The harvested kidney and the kidney slices are kept in a physiological buffer equilibrated with 5% CO₂ at body temperature (37 °C) in the presence of different AMPK activating agents vs. vehicle control followed by rapid freezing or fixation of the slices to prevent non-specific AMPK activation. Thus, homogenates of these frozen slices can be used to study AMPK activation status in the tissue as well as the downstream effects of AMPK on kidney proteins via biochemical techniques, such as immunoblotting and immunoprecipitation. Alternatively, the fixed slices can be used to evaluate AMPK-mediated subcellular traffic changes of epithelial transport proteins via immunolabeling followed by confocal microscopy. The resulting micrographs can then be used for systematic quantification of AMPK-induced changes in subcellular localization of transport proteins.

Aldosterone alters the chromatin structure of the murine endothelin-1 gene

Aldosterone increases sodium reabsorption in the renal collecting duct and systemic blood pressure. Paradoxically, aldosterone also induces transcription of the endothelin-1 (Edn1) gene to increase protein (ET-1) levels, which inhibits sodium reabsorption.

AIMS

Here we investigated changes in the chromatin structure of the Edn1 gene of collecting duct cell lines in response to aldosterone treatment. The Edn1 gene has a CpG island that encompasses the transcription start site and four sites in the 5' regulatory region previously linked to transcriptional regulation.

MATERIALS AND METHODS

The chromatin structure of the Edn1 gene was investigated using a quantitative PCR-based DNaseI hypersensitivity assay in murine hepatocyte (AML12), renal cortical collecting duct (mpkCCDC14), outer medullary collecting duct1 (OMCD1), and inner medullary collecting duct-3 (IMCD-3) cell lines.

KEY FINDINGS

The CpG island was uniformly accessible. One calcium-responsive NFAT element remained at low chromatin accessibility in all cell lines under all conditions tested. However, the second calcium responsive NFAT element located at -1563bp upstream became markedly more accessible in IMCD-3 cells exposed to aldosterone. Importantly, one established aldosterone hormone response element HRE at -671bp relative to the transcription start site was highly accessible, and another HRE (-551bp) became more accessible in aldosterone-treated IMCD-3 and OMCD1 cells.

SIGNIFICANCE

The evidence supports a model in which aldosterone activation of the mineralocorticoid receptor (MR) results in the MR-hormone complex binding at HRE at -671bp to open chromatin structure around other regulatory elements in the Edn1 gene.

Molecular mechanisms and regulation of urinary acidification

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

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

Pyk2 regulates H+-ATPase-mediated proton secretion in the outer medullary collecting duct via an ERK1/2 signaling pathway

Acid-secreting intercalated cells respond to changes in systemic pH through regulation of apical H(+) transporters. Little is known about the mechanism by which these cells sense changes in extracellular pH (pH(o)). Pyk2 is a nonreceptor tyrosine kinase activated by autophosphorylation at Tyr402 by cell-specific stimuli, including decreased pH, and is involved in the regulation of MAPK signaling pathways and transporter activity. We examined whether the Pyk2 and MAPK signaling pathway mediates the response of transport proteins to decreased pH in outer medullary collecting duct cells. Immunoblot analysis of phosphorylated Pyk2 (Tyr402), ERK1/2 (Thr202/Tyr204), and p38 (Thr180/Tyr182) was used to assay protein activation. To examine specificity of kinase activation and its effects, we used Pyk2 small interfering RNA to knockdown Pyk2 expression levels, the Src kinase inhibitor 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP 1) to inhibit Pyk2 phosphorylation, and the MEK inhibitor U0126 to inhibit ERK1/2 phosphorylation. The pH-sensitive fluorescent probe 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (BCECF-AM) was used to assay H(+) transporter activity. The activity of H(+) transporters was measured as the rate of intracellular pH (pH(i)) recovery after an NH(4)Cl prepulse. We show that Pyk2 is endogenously expressed and activated by acid pH in mouse-derived outer medullary collecting duct (mOMCD1) cells. Incubation of mOMCD1 cells in acid media [extracellular pH (pH(o)) 6.7] increased the phosphorylation of Pyk2, ERK1/2, and p38. Reduction in pH(i) induced by an NH(4)Cl prepulse also increased the phosphorylation of Pyk2, ERK1/2, and p38. Consistent with our previous studies, we found that mOMCD1 cells exhibit H(+)-ATPase and H(+),K(+)-ATPase activity. Pyk2 inhibition by Pyk2 siRNA and PP 1 prevented Pyk2 phosphorylation as well as H(+)-ATPase-mediated recovery in mOMCD1 cells. In addition, ERK1/2 inhibition by U0126 prevented acid-induced ERK1/2 phosphorylation and H(+)-ATPase-mediated pH(i) recovery but not phosphorylation of p38. We conclude that Pyk2 and ERK1/2 are required for increasing H(+)-ATPase, but not H(+),K(+)-ATPase, activity at decreased pH(i) in mOMCD1 cells.

Metabolic complications associated with use of diuretics

Diuretics are commonly used therapeutic agents that act to inhibit sodium transport systems along the length of the renal tubule. The most effective diuretics are inhibitors of sodium chloride transport in the thick ascending limb of Henle. Loop diuretics mobilize large amounts of sodium chloride and water and produce a copious diuresis with a sharp reduction of extracellular fluid volume. As the site of action of diuretics moves downstream (thiazide and potassium-sparing diuretics), their effectiveness declines because the transport systems they inhibit have low transport capacity. Depending on the site of action diuretics can influence the renal handling of electrolyte-free water, calcium, potassium, protons, sodium bicarbonate, and uric acid. As a result, electrolyte and acid-base disorders commonly accompany diuretic use. Glucose and lipid abnormalities also can occur, particularly with the use of thiazide diuretics. This review focuses on the biochemical complications associated with the use of diuretics. The development of these complications can be minimized with careful monitoring, dosage adjustment, and replacement of electrolyte losses.

pH-dependent regulation of the α-subunit of H+-K+-ATPase (HKα2)

The H(+)-K(+)-ATPase α-subunit (HKα(2)) participates importantly in systemic acid-base homeostasis and defends against metabolic acidosis. We have previously shown that HKα(2) plasma membrane expression is regulated by PKA (Codina J, Liu J, Bleyer AJ, Penn RB, DuBose TD Jr. J Am Soc Nephrol 17: 1833-1840, 2006) and in a separate study demonstrated that genetic ablation of the proton-sensing G(s)-coupled receptor GPR4 results in spontaneous metabolic acidosis (Sun X, Yang LV, Tiegs BC, Arend LJ, McGraw DW, Penn RB, Petrovic S. J Am Soc Nephrol 21: 1745-1755, 2010). In the present study, we investigated the ability of chronic acidosis and GPR4 to regulate HKα(2) expression in HEK-293 cells. Chronic acidosis was modeled in vitro by using multiple methods: reducing media pH by adjusting bicarbonate concentration, adding HCl, or by increasing the ambient concentration of CO(2). PKA activity and HKα(2) protein were monitored by immunoblot analysis, and HKα(2) mRNA, by real-time PCR. Chronic acidosis did not alter the expression of HKα(2) mRNA; however, PKA activity and HKα(2) protein abundance increased when media pH decreased from 7.4 to 6.8. Furthermore, this increase was independent of the method used to create chronic acidosis. Heterologous expression of GPR4 was sufficient to increase both basal and acid-stimulated PKA activity and similarly increase basal and acid-stimulated HKα(2) expression. Collectively, these results suggest that chronic acidosis and GPR4 increase HKα(2) protein by increasing PKA activity without altering HKα(2) mRNA abundance, implicating a regulatory role of pH-activated GPR4 in homeostatic regulation of HKα(2) and acid-base balance.

The natriuretic mechanism of Gamma-Melanocyte-Stimulating Hormone

Gamma-Melanocyte Stimulating Hormone (Gamma-MSH) regulates sodium (Na(+)) balance and blood pressure through activation of the melanocortin receptor 3 (MC3-R). The mechanism of the natriuretic effect is proposed to involve binding of MC3-R either in the kidney to directly inhibit tubular Na(+) transport or in the brain to inhibit central neural pathways that control renal tubular Na(+) absorption. This study aimed to clarify the mechanism involved in the natriuretic effect of Gamma-MSH on MC3-R in kidney cells. In Ussing chamber studies, we observed no effects of Gamma-MSH on NaCl transport in the mouse inner medullary collecting duct cell line (mIMCD-K2). We also found that neither MC3-R protein nor mRNA was expressed in mouse kidney, suggesting that renal Gamma-MSH action may not be mediated through direct effects on tubular Na(+) transport but rather through effects on central neural pathways that innervate the kidney.

Deletion of the pH sensor GPR4 decreases renal acid excretion

Proton receptors are G protein-coupled receptors that accept protons as ligands and function as pH sensors. One of the proton receptors, GPR4, is relatively abundant in the kidney, but its potential role in acid-base homeostasis is unknown. In this study, we examined the distribution of GPR4 in the kidney, its function in kidney epithelial cells, and the effects of its deletion on acid-base homeostasis. We observed GPR4 expression in the kidney cortex, in the outer and inner medulla, in isolated kidney collecting ducts, and in cultured outer and inner medullary collecting duct cells (mOMCD1 and mIMCD3). Cultured mOMCD1 cells exhibited pH-dependent accumulation of intracellular cAMP, characteristic of GPR4 activation; GPR4 knockdown attenuated this accumulation. In vivo, deletion of GPR4 decreased net acid secretion by the kidney and resulted in a nongap metabolic acidosis, indicating that GPR4 is required to maintain acid-base homeostasis. Collectively, these findings suggest that GPR4 is a pH sensor with an important role in regulating acid secretion in the kidney collecting duct.

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.

Aldosterone modulates steroid receptor binding to the endothelin-1 gene (edn1)

Aldosterone and endothelin-1 (ET-1) act on collecting duct cells of the kidney and are important regulators of renal sodium transport and cardiovascular physiology. We recently identified the ET-1 gene (edn1) as a novel aldosterone-induced transcript. However, aldosterone action on edn1 has not been characterized at the present time. In this report, we show that aldosterone stimulated edn1 mRNA in acutely isolated rat inner medullary collecting duct cells ex vivo and ET-1 peptide in rat inner medulla in vivo. Aldosterone induction of edn1 mRNA occurred in cortical, outer medullary, and inner medullary collecting duct cells in vitro. Inspection of the edn1 promoter revealed two putative hormone response elements. Levels of heterogeneous nuclear RNA synthesis demonstrated that edn1 mRNA stimulation occurred at the level of transcription. RNA knockdowns corroborated pharmacological studies and demonstrated both mineralocorticoid receptor and glucocorticoid receptor participated in this response. Aldosterone resulted in dose-dependent nuclear translocation and binding of mineralocorticoid receptor and glucocorticoid receptor to the edn1 hormone response elements. Hormone receptors mediated the association of chromatin remodeling complexes, histone modification, and RNA polymerase II at the edn1 promoter. Direct interaction between aldosterone and ET-1 has important implications for renal and cardiovascular function.

The circadian clock protein Period 1 regulates expression of the renal epithelial sodium channel in mice

The mineralocorticoid aldosterone is a major regulator of sodium transport in target epithelia and contributes to the control of blood pressure and cardiac function. It specifically functions to increase renal absorption of sodium from tubular fluid via regulation of the alpha subunit of the epithelial sodium channel (alphaENaC). We previously used microarray technology to identify the immediate transcriptional targets of aldosterone in a mouse inner medullary collecting duct cell line and found that the transcript induced to the greatest extent was the circadian clock gene Period 1. Here, we investigated the role of Period 1 in mediating the downstream effects of aldosterone in renal cells. Aldosterone treatment stimulated expression of Period 1 (Per1) mRNA in renal collecting duct cell lines and in the rodent kidney. RNA silencing of Period 1 dramatically decreased expression of mRNA encoding alphaENaC in the presence or absence of aldosterone. Furthermore, expression of alphaENaC-encoding mRNA was attenuated in the renal medulla of mice with disruption of the Per1 gene, and these mice exhibited increased urinary sodium excretion. Renal alphaENaC-encoding mRNA was expressed in an apparent circadian pattern, and this pattern was dramatically altered in mice lacking functional Period genes. These results suggest a role for Period 1 in the regulation of the renal epithelial sodium channel and more broadly implicate the circadian clock in control of sodium balance.

Regulation of potassium (K) handling in the renal collecting duct

This review provides an overview of the molecular mechanisms of K transport in the mammalian connecting tubule (CNT) and cortical collecting duct (CCD), both nephron segments responsible for the regulation of renal K secretion. Aldosterone and dietary K intake are two of the most important factors regulating K secretion in the CNT and CCD. Recently, angiotensin II (AngII) has also been shown to play a role in the regulation of K secretion. In addition, genetic and molecular biological approaches have further identified new mechanisms by which aldosterone and dietary K intake regulate K transport. Thus, the interaction between serum-glucocorticoid-induced kinase 1 (SGK1) and with-no-lysine kinase 4 (WNK4) plays a significant role in mediating the effect of aldosterone on ROMK (Kir1.1), an important apical K channel modulating K secretion. Recent evidence suggests that WNK1, mitogen-activated protein kinases such as P38, ERK, and Src family protein tyrosine kinase are involved in mediating the effect of low K intake on apical K secretory channels.

Impaired acid secretion in cortical collecting duct intercalated cells from H-K-ATPase-deficient mice: role of HKalpha isoforms

Two classes of H pumps, H-K-ATPase and H-ATPase, contribute to luminal acidification and HCO(3) transport in the collecting duct (CD). At least two H-K-ATPase alpha-subunits are expressed in the CD: HKalpha(1) and HKalpha(2). Both exhibit K dependence but have different inhibitor sensitivities. The HKalpha(1) H-K-ATPase is Sch-28080 sensitive, whereas the pharmacological profile of the HKalpha(2) H-K-ATPase is not completely understood. The present study used a nonpharmacological, genetic approach to determine the contribution of HKalpha(1) and HKalpha(2) to cortical CD (CCD) intercalated cell (IC) proton transport in mice fed a normal diet. Intracellular pH (pH(i)) recovery was determined in ICs using in vitro microperfusion of CCD after an acute intracellular acid load in wild-type mice and mice of the same strain lacking expression of HKalpha(1), HKalpha(2), or both H-K-ATPases (HKalpha(1,2)). A-type and B-type ICs were differentiated by luminal loading with BCECF-AM and peritubular chloride removal from CO(2)/HCO(3)-buffered solutions to identify the membrane locations of Cl/HCO(3) exchange activity. H-ATPase- and Na/H exchange-mediated H transport were inhibited with bafilomycin A(1) (100 nM) and EIPA (10 microM), respectively. Here, we report 1) initial pH(i) and buffering capacity were not significantly altered in the ICs of HKalpha-deficient mice, 2) either HKalpha(1) or HKalpha(2) deficiency resulted in slower acid extrusion, and 3) A-type ICs from HKalpha(1,2)-deficient mice had significantly slower acid extrusion compared with A-type ICs from HKalpha(1)-deficient mice alone. These studies are the first nonpharmacological demonstration that both HKalpha(1) and HKalpha(2) contribute to H secretion in both A-type and B-type ICs in animals fed a normal diet.

Metabolic Alkalosis, Bedside and Bench

Although significant contributions to the understanding of metabolic alkalosis have been made recently, much of our knowledge rests on data from clearance studies performed in humans and animals many years ago. This article reviews the contributions of these studies, as well as more recent work relating to the control of renal acid-base transport by mineralocorticoid hormones, angiotensin, endothelin, nitric oxide, and potassium balance. Finally, clinical aspects of metabolic alkalosis are considered.

A carboxy-terminus motif of HKalpha2 is necessary for assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of HKalpha2, for both function and integrity of assembly with beta1-Na+,K+-ATPase.

METHODS

For this purpose, stop codons were created, by polymerase chain reaction (PCR), at different positions in the carboxy-terminus of HKalpha2. Subsequently, chimeras between HKalpha2 and the carboxy-terminus of alpha1-Na+,K+-ATPase or with the carboxy-terminus of the gastric H+,K+-ATPase were created. Human embryonic kidney HEK-293 cells were used as expression systems for functional studies using 86Rb+ uptake and alpha/beta assembly using specific antibodies.

RESULTS

The results demonstrate that the entire carboxy-terminus of HKalpha2 is required for optimal protection of the alpha/beta complex from degradation and for functionality as evidenced by 86Rb+ uptake. The results also demonstrate that there was flexibility in the sequence of the carboxy-terminus. The last two tyrosines (Y1035Y1036) of HKalpha2 could be mutated to alanines and the carboxy-terminus of HKalpha2 could be replaced by the carboxy-terminus of alpha1-Na+,K+-ATPase while preserving transport activity.

CONCLUSION

The entire carboxy-terminus of HKalpha2 is required for stable assembly with beta1-Na+,K+-ATPase and functionality.

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.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Regulation of renal K transport by dietary K intake

Extracellular K must be kept within a narrow concentration range for the normal function of neurons, skeletal muscle, and cardiac myocytes. Maintenance of normal plasma K is achieved by a dual mechanism that includes extrarenal factors such as insulin and beta-adrenergic agonists, which stimulate the movement of K from extracellular to intracellular fluid and modulate renal K excretion. Dietary K intake is an important factor for the regulation of K secretion: An increase in K intake stimulates secretion, whereas a decrease inhibits K secretion and enhances absorption. This effect of changes in dietary K intake on tubule K transport is mediated by aldosterone-dependent and -independent mechanisms. Recently, it has been demonstrated that the protein tyrosine kinase (PTK)-dependent signal transduction pathway is an important aldosterone-independent regulatory mechanism that mediates the effect of altered K intake on K secretion. A low-K intake stimulates PTK activity, which leads to increase in phosphorylation of cloned inwardly rectifying renal K (ROMK) channels, whereas a high-K intake has the opposite effect. Stimulation of tyrosine phosphorylation also suppresses K secretion in principal cell by facilitating the internalization of apical K channels in the collecting duct.

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.

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.

Sch-28080 depletes intracellular ATP selectively in mIMCD-3 cells

Two H(+)-K(+)-ATPase isoforms are present in kidney: the gastric, highly sensitive to Sch-28080, and the colonic, partially sensitive to ouabain. Upregulation of Sch-28080-sensitive H(+)-K(+)-ATPase, or "gastric" H(+)-K(+)-ATPase, has been demonstrated in hypokalemic rat inner medullary collecting duct cells (IMCDs). Nevertheless, only colonic H(+)-K(+)-ATPase mRNA and protein abundance increase in this condition. This study was designed to determine whether Sch-28080 inhibits transporters other than the gastric H(+)-K(+)-ATPase. In the presence of bumetanide, Sch-28080 (200 microM) and ouabain (2 mM) inhibited (86)Rb(+) uptake (>90%). That (86)Rb(+) uptake was almost completely abolished by Sch-28080 indicates an effect of this agent on the Na(+)-K(+)-ATPase. ATPase assays in membranes, or lysed cells, demonstrated sensitivity to ouabain but not Sch-28080. Thus the inhibitory effect of Sch-28080 was dependent on cell integrity. (86)Rb(+)-uptake studies without bumetanide demonstrated that ouabain inhibited activity by only 50%. Addition of Sch-28080 (200 microM) blocked all residual activity. Intracellular ATP declined after Sch-28080 (200 microM) but recovered after removal of this agent. In conclusion, high concentrations of Sch-28080 inhibit K(+)-ATPase activity in mouse IMCD-3 (mIMCD-3) cells as a result of ATP depletion.

H+,K+-ATPase

The H+,K+-ATPases comprise a group of integral membrane proteins that belong to the X+,K+-ATPase subfamily of P-type cation-transporting ATPases. Although these H+,K+-ATPase isoforms share approximately 60-70% amino acid identity, they exhibit discrete kinetic and pharmacological properties when expressed in heterologous systems. HK alpha2 has been categorized by its insensitivity to Sch-28080, an inhibitor of the gastric H+,K+-ATPase, and partial sensitivity to ouabain, an inhibitor of the Na+,K+-ATPase. This functional profile contrasts with the pharmacological sensitivities ascribed to HK alpha2 in transport studies in rat isolated medullary collecting ducts perfused in vitro and in mouse medullary collecting duct cell lines. HK alpha2 mRNA and protein abundance appears to be both tissue and site-specifically upregulated in response to chronic hypokalemia. This regulatory response has been localized to the outer and inner medulla. To reconcile these expressed sensitivities to those reported in vitro in isolated tubules and cells in culture, it would be necessary to invoke modification of the pharmacologic insensitivity of the colonic H+,K+-ATPase to Sch-28080. Although a 'unique' beta-subunit has been reported recently, this beta-subunit (beta(c)) is identical at the amino acid level to the recently cloned beta3-Na+,K+-ATPase. Moreover, while HK alpha2 can assemble indiscriminately with any X+,K+-ATPase beta-subunit, HK alpha2 has been reported to assemble stably with beta1-Na+,K+-ATPase in the renal medulla and in the distal colon. It remains conceivable that subunit assembly could be tissue specific and might respond to different physiological and pathophysiological stimuli. Furthermore, recent studies have suggested that the H+,K+-ATPase is both Na+-dependent and localized to the apical membrane in the distal colon. Therefore, future studies will need to resolve these discrepancies by determining if a unique, yet undiscovered H+,K+-ATPase isoform exists in kidney, or if post-translational modifications of the alpha- and/or beta-subunits could account for these functional diversities.

H+-K+-ATPases: regulation and role in pathophysiological states

Molecular cloning experiments have identified the existence of two H+-K+-ATPases (HKAs), colonic and gastric. Recent functional and molecular studies indicate the presence of both transporters in the kidney, which are presumed to mediate the exchange of intracellular H+ for extracellular K+. On the basis of these studies, a picture is evolving that indicates differential regulation of HKAs at the molecular level in acid-base and electrolyte disorders. Of the two transporters, gastric HKA is expressed constitutively along the length of the collecting duct and is responsible for H+ secretion and K+ reabsorption under normal conditions and may be stimulated with acid-base perturbations and/or K+ depletion. This regulation may be species specific. To date there are no data to indicate that the colonic HKA (HKAc) plays a role in H+ secretion or K+ reabsorption under normal conditions. However, HKAc shows adaptive regulation in pathophysiological conditions such as K+ depletion, NaCl deficiency, and proximal renal tubular acidosis, suggesting an important role for this exchanger in potassium, HCO-3, and sodium (or chloride) reabsorption in disease states. The purpose of this review is to summarize recent functional and molecular studies on the regulation of HKAs in physiological and pathophysiological states. Possible signals responsible for regulation of HKAs in these conditions will be discussed. Furthermore, the role of these transporters in acid-base and electrolyte homeostasis will be evaluated in the context of genetically altered animals deficient in HKAc.

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.

Dietary K+ restriction upregulates total and Sch-28080-sensitive bicarbonate absorption in rat tIMCD

In tubules from the terminal segment of the inner medullary collecting duct (tIMCD) from rats with chronic metabolic acidosis, our laboratory has shown that bicarbonate absorption (JtCO2) is inhibited by removal of K+ from the luminal fluid or by the addition of Sch-28080 to the perfusate. The present study asked whether total and/or Sch-28080-sensitive JtCO2 is regulated by changes in systemic K+ homeostasis. Rat tIMCD tubules were perfused in vitro in symmetrical, HCO-3/CO2-buffered solutions containing 10 mM KCl + 6 mM NH4Cl. Total and Sch-28080-sensitive JtCO2 were measured in rats with varying K+ intake. In K+-replete rats, baseline JtCO2 was 2.1 +/- 0.3 pmol . mm-1 . min-1 (n = 6). In rats fed a K+-deficient diet for 3 days, JtCO2 was 5.4 +/- 0.7 pmol . mm-1 . min-1 (n = 16, P < 0. 05). To determine the mechanism for the increase in HCO-3 absorption observed with K+ restriction, the Sch-28080-sensitive component of JtCO2 was measured in each treatment group. Following the addition of Sch-28080 (10 microM) to the perfusate, a 40% reduction in JtCO2 was observed in K+-restricted rats. JtCO2 was not reduced following the addition of Sch-28080 in rats with normal K+ intake. Because Sch-28080-sensitive JtCO2 was increased in K+-restricted rats, Sch-28080-sensitive JtCO2 was studied further in tIMCD tubules from rats in this treatment group. In K+-restricted rats, JtCO2 decreased by 20% following the addition of 5 mM ouabain to the perfusate. This ouabain-induced decline in JtCO2 was observed both in the presence and in the absence of Sch-28080. We conclude that total and Sch-28080-sensitive net acid secretion is increased with dietary K+ restriction. However, since approximately 50% of JtCO2 is insensitive to both Sch-28080 and ouabain, future studies will be necessary to define other mechanisms of luminal acidification in the rat tIMCD.

Expression of HKalpha2 protein is increased selectively in renal medulla by chronic hypokalemia

Our laboratory has demonstrated by Northern analysis that chronic hypokalemia increases HKalpha2 (i.e., alpha-subunit of the colonic H+-K+-ATPase) mRNA abundance in the rat. To determine whether the increase in mRNA correlated with an increase in HKalpha2 protein, an antibody was raised against a synthetic peptide derived from amino acids 686-698 of the HKalpha2 sequence. The anti-HKalpha2 antibody hybridized to rat distal colon membranes which migrated at approximately 100 kDa (expected mobility of HKalpha2). HKalpha2 protein was not detected in plasma membranes from rat whole kidney or stomach (100 microg) derived from control animals. The antibody was then used to investigate changes in expression of HKalpha2 in renal cortex, renal medulla, and distal colon in two pathophysiological conditions: 1) chronic hypokalemia (LK) and 2) chronic metabolic acidosis (CMA). In LK rats there was a marked, but selective, increase in the abundance of HKalpha2 protein in membranes prepared from renal medulla. Nevertheless, a corresponding increase in HKalpha2 protein abundance was not observed in membranes prepared from the distal colon of LK rats. HKalpha2 protein abundance in CMA was indistinguishable from controls. Moreover, chronic hypokalemia had no effect on expression of alpha1-Na+-K+-ATPase or HKalpha1 in kidney or distal colon under any experimental condition. Therefore, HKalpha2 protein is tissue- and site-specifically upregulated in response to chronic hypokalemia but not by CMA. Furthermore, this regulatory response is localized to the renal medulla.