FXYD5 (dysadherin) may mediate metastatic progression through regulation of the β-Na+-K+-ATPase subunit in the 4T1 mouse breast cancer model

FXYD5 is a Na+-K+-ATPase regulator, expressed in a variety of normal epithelia. In parallel, it has been found to be associated with several types of cancer and effect lethal outcome by promoting metastasis. However, the molecular mechanism underlying FXYD5 mediated invasion has not yet been identified. In this study, using in vivo 4T1 murine breast cancer model, we found that FXYD5-specific shRNA significantly inhibited lung cancer metastasis, without having a substantial effect on primary tumor growth. Our study reveals that FXYD5 participates in multiple stages of metastatic development and exhibits more than one mode of E-cadherin regulation. We provide the first evidence that FXYD5-related morphological changes are mediated through its interaction with Na+-K+-ATPase. Experiments in cultured 4T1 cells have indicated that FXYD5 expression may downregulate the β1 isoform of the pump. This behavior could have implications on both transcellular interactions and intracellular events. Further studies suggest that differential localization of the adaptor protein Annexin A2 in FXYD5-expressing cells may correlate with matrix metalloproteinase 9 secretion and adhesion changes in 4T1 wild-type cells.

FXYD5 Protein Has a Pro-inflammatory Role in Epithelial Cells

The FXYD proteins are a family of small membrane proteins that share an invariant four amino acid signature motif F-X-Y-D and act as tissue-specific regulatory subunits of the Na,K-ATPase. FXYD5 (also termed dysadherin or RIC) is a structurally and functionally unique member of the FXYD family. As other FXYD proteins, FXYD5 specifically interacts with the Na,K-ATPase and alters its kinetics by increasing Vmax However, unlike other family members FXYD5 appears to have additional functions, which cannot be readily explained by modulation of transport kinetics. Knockdown of FXYD5 in MDA-MB-231 breast cancer cells largely decreases expression and secretion of the chemokine CCL2 (MCP-1). A related effect has also been observed in renal cell carcinoma cells. The current study aims to further characterize the relationship between the expression of FXYD5 and CCL2 secretion. We demonstrate that transfection of M1 epithelial cell line with FXYD5 largely increases lipopolysaccharide (LPS) stimulated CCL2 mRNA and secretion of the translated protein. We have completed a detailed analysis of the molecular events leading to the above response. Our key findings indicate that FXYD5 generates a late response by increasing the surface expression of the TNFα receptor, without affecting its total protein level, or mRNA transcription. LPS administration to mice demonstrates induced secretion of CCL2 and TNFα in FXYD5-expressing lung peripheral tissue, which suggests a possible role for FXYD5 in normal epithelia during inflammation.

Cardiac glycosides induced toxicity in human cells expressing α1-, α2-, or α3-isoforms of Na-K-ATPase

The Na+-K+-ATPase is specifically inhibited by cardiac glycosides, some of which may also function as endogenous mammalian hormones. Previous studies using Xenopus oocytes, yeast cells, or purified isoforms demonstrated that affinities of various cardiac glycosides for three isoforms of the Na+-K+-ATPase (α1-α3β1) may differ, a finding with potential clinical implication. The present study investigates isoform selectivity and effects of cardiac glycosides on cultured mammalian cells under more physiological conditions. H1299 cells (non-small cell lung carcinoma) were engineered to express only one α-isoform (α1, α2, or α3) by combining stable transfection of isoforms and silencing endogenous α1. Cardiac glycoside binding was measured by displacement of bound 3H-ouabain. The experiments confirm moderate α1/α3:α2 selectivity of ouabain, moderate α2:α1 selectivity of digoxin, and enhanced α2:α1 selectivity of synthetic derivatives (Katz A, Tal DM, Heller D, Haviv H, Rabah B, Barkana Y, Marcovich AL, Karlish SJD. J Biol Chem 289: 21153-21162, 2014). Relative α2:α1 selectivity of digoxin vs. ouabain was also manifested by enhanced internalization of α2 in response to digoxin. Cellular proliferation assays of H1299 cells confirmed the patterns of α2:α1 selectivity for ouabain, digoxin, and a synthetic derivative and reveal a crucial role of surface pump density on sensitivity to cardiac glycosides. Because cardiac glycosides are being considered as drugs for treatment of cancer, effects of ouabain on proliferation of 12 cancer and noncancer cell lines, with variable plasma membrane expression of α1, have been tested. These demonstrated that sensitivity to ouabain indeed depends linearly on the plasma membrane surface density of Na+-K+-ATPase irrespective of status, malignant or nonmalignant.

Modulation of cell polarization by the Na+-K+-ATPase-associated protein FXYD5 (dysadherin)

FXYD5 (dysadherin or also called a related to ion channel, RIC) is a transmembrane auxiliary subunit of the Na(+)-K(+)-ATPase shown to increase its maximal velocity (Vmax). FXYD5 has also been identified as a cancer-associated protein whose expression in tumor-derived cell lines impairs cytoskeletal organization and increases cell motility. Previously, we have demonstrated that the expression of FXYD5 in M1 cells derived from mouse kidney collecting duct impairs the formation of tight and adherence junctions. The current study aimed to further explore effects of FXYD5 at a single cell level. It was found that in M1, as well as three other cell lines, FXYD5 inhibits transformation of adhered single cells from the initial radial shape to a flattened, elongated shape in the first stage of monolayer formation. This is also correlated to less ordered actin cables and fewer focal points. Structure-function analysis has demonstrated that the transmembrane domain of FXYD5, and not its unique extracellular segment, mediates the inhibition of change in cell shape. This domain has been shown before to be involved in the association of FXYD5 with the Na(+)-K(+)-ATPase, which leads to the increase in Vmax. Furthermore, specific transmembrane point mutations in FXYD5 that either increase or decrease its effect on cell elongation had a corresponding effect on the coimmunoprecipitation of FXYD5 with α Na(+)-K(+)-ATPase. These findings lend support to the possibility that FXYD5 affects cell polarization through its transmembrane domain interaction with the Na(+)-K(+)-ATPase. Yet interaction of FXYD5 with other proteins cannot be excluded.

Induction of FKBP51 by aldosterone in intestinal epithelium

Screening female rat distal colon preparations for aldosterone-induced genes identified the Hsp90-binding immunophilin FKBP51 as a major aldosterone-induced mRNA and protein. Limited induction of FKBP51 was observed also in other aldosterone-responsive tissues such as kidney medulla and heart. Ex vivo measurements in colonic tissue have characterized time course, dose response and receptor specificity of the induction of FKBP51. FKBP51 mRNA and protein were strongly up regulated by physiological concentrations of aldosterone in a late (greater than 2.5h) response to the hormone. Maximal increase in FKBP51 mRNA requires aldosterone concentrations that are higher than those needed to fully occupy the mineralocorticoid receptor (MR). Yet, the response is fully inhibited by the MR antagonist spironolactone and not inhibited and even stimulated by the glucocorticoid receptor (GR) antagonist RU486. These and related findings cannot be explained by a simple activation and dimerization of either MR or GR but are in agreement with response mediated by an MR-GR heterodimer. Overexpression or silencing FKBP51 in the kidney collecting duct cell line M1 had little or no effect on the aldosterone-induced increase in transepithelial Na(+) transport.

Ouabain-induced internalization and lysosomal degradation of the Na+/K+-ATPase

Internalization of the Na(+)/K(+)-ATPase (the Na(+) pump) has been studied in the human lung carcinoma cell line H1299 that expresses YFP-tagged α1 from its normal genomic localization. Both real-time imaging and surface biotinylation have demonstrated internalization of α1 induced by ≥100 nm ouabain which occurs in a time scale of hours. Unlike previous studies in other systems, the ouabain-induced internalization was insensitive to Src or PI3K inhibitors. Accumulation of α1 in the cells could be augmented by inhibition of lysosomal degradation but not by proteosomal inhibitors. In agreement, the internalized α1 could be colocalized with the lysosomal marker LAMP1 but not with Golgi or nuclear markers. In principle, internalization could be triggered by a conformational change of the ouabain-bound Na(+)/K(+)-ATPase molecule or more generally by the disruption of cation homeostasis (Na(+), K(+), Ca(2+)) due to the partial inhibition of active Na(+) and K(+) transport. Overexpression of ouabain-insensitive rat α1 failed to inhibit internalization of human α1 expressed in the same cells. In addition, incubating cells in a K(+)-free medium did not induce internalization of the pump or affect the response to ouabain. Thus, internalization is not the result of changes in the cellular cation balance but is likely to be triggered by a conformational change of the protein itself. In physiological conditions, internalization may serve to eliminate pumps that have been blocked by endogenous ouabain or other cardiac glycosides. This mechanism may be required due to the very slow dissociation of the ouabain·Na(+)/K(+)-ATPase complex.

Intracellular trafficking of FXYD1 (phospholemman) and FXYD7 proteins in Xenopus oocytes and mammalian cells

FXYD proteins are a group of short single-span transmembrane proteins that interact with the Na(+)/K(+) ATPase and modulate its kinetic properties. This study characterizes intracellular trafficking of two FXYD family members, FXYD1 (phospholemman (PLM)) and FXYD7. Surface expression of PLM in Xenopus oocytes requires coexpression with the Na(+)/K(+) ATPase. On the other hand, the Na(+)/Ca(2+) exchanger, another PLM-interacting protein could not drive it to the cell surface. The Na(+)/K(+) ATPase-dependent surface expression of PLM could be facilitated by either a phosphorylation-mimicking mutation at Thr-69 or a truncation of three terminal arginine residues. Unlike PLM, FXYD7 could translocate to the cell surface of Xenopus oocytes independently of the coexpression of α1β1 Na(+)/K(+) ATPase. The Na(+)/K(+) ATPase-independent membrane translocation of FXYD7 requires O-glycosylation of at least two of three conserved threonines in its ectodomain. Subsequent experiments in mammalian cells confirmed the role of conserved extracellular threonine residues and demonstrated that FXYD7 protein, in which these have been mutated to alanine, is trapped in the endoplasmic reticulum and Golgi apparatus.

FXYD5 (dysadherin) regulates the paracellular permeability in cultured kidney collecting duct cells

FXYD5 (dysadherin or RIC) is a member of the FXYD family of single-span transmembrane proteins associated with the Na(+)-K(+)-ATPase. Several studies have demonstrated enhanced expression of FXYD5 during metastasis and effects on cell adhesion and motility. The current study examines effects of FXYD5 on the paracellular permeability in the mouse kidney collecting duct cell line M1. Expressing FXYD5 in these cells leads to a large decrease in amiloride-insensitive transepithelial electrical resistance as well as increased permeability to 4-kDa dextran. Impairment of cell-cell contact was also demonstrated by staining cells for the tight and adherence junction markers zonula occludens-1 and β-catenin, respectively. This is further supported by large expansions of the interstitial spaces, visualized in electron microscope images. Expressing FXYD5 in M1 cells resulted in a decrease in N-glycosylation of β1 Na(+)-K(+)-ATPase, while silencing it in H1299 cells had an opposite effect. This may provide a mechanism for the above effects, since normal glycosylation of β1 plays an important role in cell-cell contact formation (Vagin O, Tokhtaeva E, Sachs G. J Biol Chem 281: 39573-39587, 2006).

Phospholemman (FXYD1) raises the affinity of the human α1β1 isoform of Na,K-ATPase for Na ions

The human α(1)/His(10)-β(1) isoform of the Na,K-ATPase has been expressed in Pichia pastoris, solubilized in n-dodecyl-β-maltoside, and purified by metal chelate chromatography. The α(1)β(1) complex spontaneously associates in vitro with the detergent-solubilized purified human FXYD1 (phospholemman) expressed in Escherichia coli. It has been confirmed that FXYD1 spontaneously associates in vitro with the α(1)/His(10)-β(1) complex and stabilizes it in an active mode. The functional properties of the α(1)/His(10)-β(1) and α(1)/His(10)-β(1)/FXYD1 complexes have been investigated by fluorescence methods. The electrochromic dye RH421 which monitors binding to and release of ions from the binding sites has been applied in equilibrium titration experiments to determine ion binding affinities and revealed that FXYD1 induces an ∼30% increase of the Na(+)-binding affinity in both the E(1) and P-E(2) conformations. By contrast, it does not affect the affinities for K(+) and Rb(+) ions. Phosphorylation induced partial reactions of the enzyme have been studied as backdoor phosphorylation by inorganic phosphate and in kinetic experiments with caged ATP in order to evaluate the ATP-binding affinity and the time constant of the conformational transition, Na(3)E(1)-P → P-E(2)Na(3). No significant differences with or without FXYD1 could be detected. Rate constants of the conformational transitions Rb(2)E(1) → E(2)(Rb(2)) and E(2)(Rb(2)) → Na(3)E(1), investigated with fluorescein-labeled Na,K-ATPase, showed only minor or no effects of FXYD1, respectively. The conclusion from all these experiments is that FXYD1 raises the binding affinity of α(1)β(1) for Na ions, presumably at the third Na-selective binding site. In whole cell expression studies FXYD1 reduces the apparent affinity for Na ions. Possible reasons for the difference from this study using the purified recombinant Na,K-ATPase are discussed.

FXYD proteins stabilize Na,K-ATPase: amplification of specific phosphatidylserine-protein interactions

FXYD proteins are a family of seven small regulatory proteins, expressed in a tissue-specific manner, that associate with Na,K-ATPase as subsidiary subunits and modulate kinetic properties. This study describes an additional property of FXYD proteins as stabilizers of Na,K-ATPase. FXYD1 (phospholemman), FXYD2 (γ subunit), and FXYD4 (CHIF) have been expressed in Escherichia coli and purified. These FXYD proteins associate spontaneously in vitro with detergent-soluble purified recombinant human Na,K-ATPase (α1β1) to form α1β1FXYD complexes. Compared with the control (α1β1), all three FXYD proteins strongly protect Na,K-ATPase activity against inactivation by heating or excess detergent (C(12)E(8)), with effectiveness FXYD1 > FXYD2 ≥ FXYD4. Heating also inactivates E(1) ↔ E(2) conformational changes and cation occlusion, and FXYD1 protects strongly. Incubation of α1β1 or α1β1FXYD complexes with guanidinium chloride (up to 6 m) causes protein unfolding, detected by changes in protein fluorescence, but FXYD proteins do not protect. Thus, general protein denaturation is not the cause of thermally mediated or detergent-mediated inactivation. By contrast, the experiments show that displacement of specifically bound phosphatidylserine is the primary cause of thermally mediated or detergent-mediated inactivation, and FXYD proteins stabilize phosphatidylserine-Na,K-ATPase interactions. Phosphatidylserine probably binds near trans-membrane segments M9 of the α subunit and the FXYD protein, which are in proximity. FXYD1, FXYD2, and FXYD4 co-expressed in HeLa cells with rat α1 protect strongly against thermal inactivation. Stabilization of Na,K-ATPase by three FXYD proteins in a mammalian cell membrane, as well the purified recombinant Na,K-ATPase, suggests that stabilization is a general property of FXYD proteins, consistent with a significant biological function.

Purification of the human alpha2 Isoform of Na,K-ATPase expressed in Pichia pastoris. Stabilization by lipids and FXYD1

Human alpha1 and alpha2 isoforms of Na,K-ATPase have been expressed with porcine 10*Histidine-tagged beta1 subunit in Pichia pastoris. Methanol-induced expression of alpha2 is optimal at 20 degrees C, whereas at 25 degrees C, which is optimal for expression of alpha1, alpha2 is not expressed. Detergent-soluble alpha2beta1 and alpha1beta1 complexes have been purified in a stable and functional state. alpha2beta1 shows a somewhat lower Na,K-ATPase activity and higher K0.5K compared to alpha1beta1, while values of K0.5Na and KmATP are similar. Ouabain inhibits both alpha1beta1 (K0.5 24.6 +/- 6 nM) and alpha2beta1 (K0.5 102 +/- 14 nM) with high affinity. A striking difference between the isoforms is that alpha2beta1 is unstable. Both alpha1beta1 and alpha2beta1 complexes, prepared in C12E8 with an added phosphatidyl serine, are active, but alpha2beta1 is rapidly inactivated at 0 degrees C. Addition of low concentrations of cholesterol with 1-stearoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (SOPS) stabilizes strongly, maintaining alpha2beta1 active up to two weeks at 0 degrees C. By contrast, alpha1beta1 is stable at 0 degrees C without added cholesterol. Both alpha1beta1 and alpha2beta1 complexes are stabilized by cholesterol at 37 degrees C. Human FXYD1 spontaneously associates in vitro with either alpha1beta1 or alpha2beta1, to form alpha1beta1/FXYD1 and alpha2beta1/FXYD1 complexes. The reconstituted FXYD1 protects both alpha1beta1 and alpha2beta1 very strongly against thermal inactivation. Instability of alpha2 is attributable to suboptimal phophatidylserine-protein interactions. Residues within TM8, TM9 and TM10, near the alphabeta subunit interface, may play an important role in differential interactions of lipid with alpha1 and alpha2, and affect isoform stability. Possible physiological implications of isoform interactions with phospholipids and FXYD1 are discussed.

Structural and functional interactions between FXYD5 and the Na+-K+-ATPase

FXYD5 is a member of a family of tissue-specific regulators of the Na(+)-K(+)-ATPase expressed in kidney tubules. Previously, we have shown that FXYD5 interacts with the alphabeta-subunits of the Na(+)-K(+)-ATPase and increases its V(max) (Lubarski I, Pihakaski-Maunsbach K, Karlish SJ, Maunsbach AB, Garty H. J Biol Chem 280: 37717-37724, 2005). The current study further characterizes structural interaction and structure-function relationships of FXYD5. FXYD5/FXYD4 chimeras expressed in Xenopus laevis oocytes have been used to demonstrate that both the high-affinity association with the pump and the increase in V(max) are mediated by the transmembrane domain of FXYD5. Several amino acids that participate in the high-affinity interaction between FXYD5 and the alpha-subunit of the Na(+)-K(+)-ATPase have been identified. The data suggest that different FXYD proteins interact similarly with the Na(+)-K(+)-ATPase and their transmembrane domains play a key role in both the structural interactions and functional effects. Other experiments have identified at least one splice variant of FXYD5 with 10 additional amino acids at the COOH terminus, suggesting the possibility of other functional effects not mediated by the transmembrane domain. FXYD5 could be specifically bound to wheat germ agglutinin beads, indicating that it is glycosylated. However, unlike previous findings in metastatic cells, such glycosylation does not evoke a large increase in the size of the protein expressed in native epithelia and X. laevis oocytes.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

Locations, abundances, and possible functions of FXYD ion transport regulators in rat renal medulla

The gamma-subunit of Na-K-ATPase (FXYD2) and corticosteroid hormone-induced factor (CHIF; FXYD4) are considered pump regulators in kidney tubules. The aim of this study was to expand the information about their locations in the kidney medulla and to evaluate their importance for electrolyte excretion in an animal model. The cellular and subcellular locations and abundances of gamma and CHIF in the medulla of control and sodium-depleted rats were analyzed by immunofluorescence and immunoelectron microscopy and semiquantitative Western blotting. The results showed that antibodies against the gamma-subunit COOH terminus and splice variant gamma(a), but not splice variant gamma(b), labeled intercalated cells, but not principal cells, in the initial part of the inner medullary collecting duct (IMCD1). In subsequent segments (IMCD2 and IMCD3), all principal cells exhibited distinct basolateral labeling for both the gamma-subunit COOH terminus, splice variant gamma(a), and CHIF. Splice variant gamma(b) was abundant in the inner stripe of the outer medulla but absent in the inner medulla (IM). Double labeling by high-resolution immunoelectron microscopy showed close structural association between CHIF and the Na-K-ATPase alpha(1)-subunit in basolateral membranes. The present observations provide new information about the cellular and subcellular locations of gamma and CHIF in the renal medulla and show a new gamma variant in the IM. Extensive NaCl depletion did not induce significant changes in the locations or abundances of the gamma-subunit COOH terminus and CHIF in different kidney zones. We conclude that the unchanged levels of these two FXYD proteins suggest that they are not primary determinants for urine electrolyte composition during NaCl depletion.

Functional interactions of phospholemman (PLM) (FXYD1) with Na+,K+-ATPase. Purification of alpha1/beta1/PLM complexes expressed in Pichia pastoris

Human FXYD1 (phospholemman, PLM) has been expressed in Pichia pastoris with porcine alpha1/His10-beta1 subunits of Na+,K+-ATPase or alone. Dodecyl-beta-maltoside-soluble complexes of alpha1/beta1/PLM have been purified by metal chelate chromatography, either from membranes co-expressing alpha1,His10-beta1, and PLM or by in vitro reconstitution of PLM with alpha1/His10-beta1 subunits. Comparison of functional properties of purified alpha1/His10-beta1 and alpha1/His10-beta1/PLM complexes show that PLM lowered K0.5 for Na+ ions moderately (approximately 30%) but did not affect the turnover rate or Km of ATP for activating Na+,K+-ATPase activity. PLM also stabilized the alpha1/His10-beta1 complex. In addition, PLM markedly (>3-fold) reduced the K0.5 of Na+ ions for activating Na+-ATPase activity. In membranes co-expressing alpha1/His10-beta1 with PLM the K0.5 of Na+ ions was also reduced, compared with the control, excluding the possibility that detergent or lipid in purified complexes compromise functional interactions. When expressed in HeLa cells with rat alpha1, rat PLM significantly raised the K0.5 of Na+ ions, whereas for a chimeric molecule consisting of transmembranes segments of PLM and extramembrane segments of FXYD4, the K0.5 of Na+ ions was significantly reduced, compared with the control. The opposite functional effects in P. pastoris and HeLa cells are correlated with endogenous phosphorylation of PLM at Ser68 or unphosphorylated PLM, respectively, as detected with antibodies, which recognize PLM phosphorylated at Ser68 (protein kinase A site) or unphosphorylated PLM. We hypothesize that PLM interacts with alpha1/His10-beta1 subunits at multiple locations, the different functional effects depending on the degree of phosphorylation at Ser68. We discuss the role of PLM in regulation of Na+,K+-ATPase in cardiac or skeletal muscle cells.

Structural interactions between FXYD proteins and Na+,K+-ATPase: alpha/beta/FXYD subunit stoichiometry and cross-linking

Interactions of rat FXYD4 (corticosteroid hormone-induced factor (CHIF)), FXYD2 (gamma), or FXYD1 (phospholemman (PLM)) proteins with rat alpha1 subunits of Na(+),K(+)-ATPase have been analyzed by co-immunoprecipitation and covalent cross-linking. In detergent-solubilized membranes from HeLa cells expressing both gamma and CHIF or CHIF and hemagglutinin A-tagged CHIF, mixed complexes of CHIF and gamma or CHIF and hemagglutinin A-tagged CHIF with alpha/beta subunits are undetectable. This implies that the alpha/beta/FXYD protomer is the major species in detergent solution. A lipid-soluble cysteine-cysteine bifunctional reagent, dibromobimane, cross-links CHIF to alpha in colonic membranes but not gamma or PLM to alpha in kidney or heart membranes, respectively. Sequence comparisons of the FXYD proteins suggested that Cys-49 in the trans-membrane segment of CHIF could be involved. In detergent-solubilized HeLa cell membranes, dibromobimane cross-links wild-type CHIF to alpha but not the C49F mutant, and also the corresponding F36C mutant but not wild-type gammab, and F48C but not wild-type PLM. C140S, C338A, C804A, and C966S mutants of the alpha subunit have been expressed. Only the C140S mutant prevents cross-linking with CHIF. The data demonstrated the proximity of trans-membrane segments of CHIF, gamma, and PLM to M2 of alpha. Molecular modeling is consistent with location of the trans-membrane segment of all FXYD proteins between M2, M6, and M9 and the proximity of Cys-49 of CHIF or Phe-36 of gamma with Cys-140 of M2. Cross-linking also demonstrated CHIF-alpha and CHIF-beta proximities in extra-membrane regions, similar to the evidence for gamma-alpha and gamma-beta cross-links.

Interaction with the Na,K-ATPase and Tissue Distribution of FXYD5 (Related to Ion Channel)

FXYD5 (related to ion channel, dysadherin) is a member of the FXYD family of single span type I membrane proteins. Five members of this group have been shown to interact with the Na,K-ATPase and to modulate its properties. However, FXYD5 is structurally different from other family members and has been suggested to play a role in regulating E-cadherin and promoting metastasis (Ino, Y., Gotoh, M., Sakamoto, M., Tsukagoshi, K., and Hirohashi, S. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 365-370). The goal of this study was to determine whether FXYD5 can modulate the Na,K-ATPase activity, establish its cellular and tissue distribution, and characterize its biochemical properties. Anti-FXYD5 antibodies detected a 24-kDa polypeptide that was preferentially expressed in kidney, intestine, spleen, and lung. In kidney, FXYD5 resides in the basolateral membrane of the connecting tubule, the collecting tubule, and the intercalated cells of the collecting duct. However, there is also labeling of the apical membrane in long thin limb of Henle's loop. FXYD5 was effectively immunoprecipitated by antibodies to the alpha subunit of Na,K-ATPase and the anti-FXYD5 antibody immunoprecipitates alpha. Co-expressing FXYD5 with the alpha1 and beta1 subunits of the Na,K-ATPase in Xenopus oocytes elicited a more than 2-fold increase in pump activity, measured either as ouabain-blockable outward current or as ouabain-sensitive (86)Rb(+) uptake. Thus, as found with other FXYD proteins, FXYD5 interacts with the Na,K-ATPase and modulates its properties.

FXYD Proteins: Tissue-Specific Regulators of the Na,K-ATPase

Work in several laboratories has led to the identification of a family of short single-span transmembrane proteins named after the invariant extracellular motif: FXYD. Four members of this group have been shown to interact with the Na,K-adenosine triphosphatase (ATPase) and alter the pump kinetics. Thus, it is assumed that FXYD proteins are tissue-specific regulatory subunits, which adjust the kinetic properties of the pump to the specific needs of the relevant tissue, cell type, or physiologic state, without affecting it elsewhere. A number of studies have provided evidence for additional and possibly unrelated functions of the FXYD proteins. This review summarizes current knowledge on the structure, function, and cellular distribution of FXYD proteins with special emphasis on their role in kidney electrolyte homeostasis.

Covalent Cross-links between the γ Subunit (FXYD2) and α and β Subunits of Na,K-ATPase

This study describes specific intramolecular covalent cross-linking of the gamma to alpha and gamma to beta subunits of pig kidney Na,K-ATPase and rat gamma to alpha co-expressed in HeLa cells. For this purpose pig gammaa and gammab sequences were determined by cloning and mass spectrometry. Three bifunctional reagents were used: N-hydroxysuccinimidyl-4-azidosalicylic acid (NHS-ASA), disuccinimidyl tartrate (DST), and 1-ethyl-3-[3dimethylaminopropyl]carbodiimide (EDC). NHS-ASA induced alpha-gamma, DST induced alpha-gamma and beta-gamma, and EDC induced primarily beta-gamma cross-links. Specific proteolytic and Fe(2+)-catalyzed cleavages located NHS-ASA- and DST-induced alpha-gamma cross-links on the cytoplasmic surface of the alpha subunit, downstream of His(283) and upstream of Val(440). Additional considerations indicated that the DST-induced and NHS-ASA-induced cross-links involve either Lys(347) or Lys(352) in the S4 stalk segment. Mutational analysis of the rat gamma subunit expressed in HeLa cells showed that the DST-induced cross-link involves Lys(55) and Lys(56) in the cytoplasmic segment. DST and EDC induced two beta-gamma cross-links, a major one at the extracellular surface within the segment Gly(143)-Ser(302) of the beta subunit and another within Ala(1)-Arg(142). Based on the cross-linking and other data on alpha-gamma proximities, we modeled interactions of the transmembrane alpha-helix and an unstructured cytoplasmic segment SKRLRCGGKKHR of gamma with a homology model of the pig alpha1 subunit. According to the model, the transmembrane segment fits in a groove between M2, M6, and M9, and the cytoplasmic segment interacts with loops L6/7 and L8/9 and stalk S5.

Kidney and colon electrolyte transport in CHIF knockout mice

Corticosteroid hormone induced factor (CHIF) is a small epithelial-specific protein regulated by aldosterone and K+ intake. It is a member of the FXYD family of single span transmembrane proteins involved in the regulation of ion transport. Recent data have suggested that CHIF interacts with the a subunit of the Na+-K+-ATPase and increases the pump's affinity to cell Na+. CHIF knockout (KO) mice have mild renal phenotype under low Na+ or high K+ diets. The present study further characterizes kidney electrolyte metabolism in CHIF KO mice and describes abnormalities in the colonic ion transport function. Kidney: KO mice were not compromised in salt and water balance under resting conditions. Fractional excretions (FE) of Na+ and K+ were normal and the animals had no deficit in the adaptation to low Na+ or high K+ intake. Glucocorticoid treatment did not unmask any difference. The effects of amiloride on Na+ absorption were not different at any treatment protocol. In contrast, FEK+ was reduced by 35% in KO mice under low Na+ intake. COLON: Amiloride inhibitable Na+ absorption was reduced in distal colon by 42%, 54% and 58% under control conditions, glucocorticoid treatment and low Na+ intake, respectively. Also, the cAMP dependent ion transport was significantly diminished. Forskolin induced equivalent short circuit current (I'SC) was reduced by 41%, 32% and 58%, under control conditions, high K+, and low Na+ intake, respectively. The present findings support a role of CHIF as an indirect modulator of several different ion transport mechanisms and are consistent with regulation of the Na+-K+-ATPase as the common denominator.

Structure-function relations of interactions between Na,K-ATPase, the gamma subunit, and corticosteroid hormone-induced factor

Corticosteroid hormone-induced factor (CHIF) and the gamma subunit of the Na,K-ATPase (gamma) are two members of the FXYD family whose function has been elucidated recently. CHIF and gamma interact with the Na+ pump and alter its kinetic properties, in different ways, which appear to serve their specific physiological roles. Although functional interactions with the Na,K-ATPase have been clearly demonstrated, it is not known which domains and which residues interact with the alpha and/or beta subunits and affect the pump kinetics. The current study provides the first systematic analysis of structure-function relations of CHIF and gamma. It is demonstrated that the stability of detergent-solubilized complexes of CHIF and gamma with alpha and/or beta subunits is determined by the trans-membrane segments, especially three residues that may be involved in hydrophobic interactions. The transmembrane segments also determine the opposite effects of CHIF and gamma on the Na+ affinity of the pump, but the amino acids involved in this functional effect are different from those responsible for stable interactions with alpha.

Characterization of the interactions between Nedd4-2, ENaC, and sgk-1 using surface plasmon resonance

Previous studies have characterized interactions between the ubiquitin ligase Nedd4-1 and the epithelial Na(+) channel (ENaC). Such interactions control the channel cell surface expression and activity. Recently, evidence has been provided that a related protein, termed Nedd4-2, is likely to be the true physiological regulator of the channel. Unlike Nedd4-1, Nedd4-2 also interacts with the aldosterone-induced channel activating kinase sgk-1. The current study uses surface plasmon resonance to quantify the binding of the four WW domains of Nedd4-2 to synthetic peptides corresponding to the PY motifs of ENaC and sgk-1. The measurements demonstrate that WW3 and WW4 are the only Nedd4-2 domains interacting with both ENaC and sgk-1 and that their binding constants are in the 1-6 microM range.

FXYD proteins: new tissue- and isoform-specific regulators of Na,K-ATPase

The recently defined FXYD protein family contains seven members that are small, single-span membrane proteins characterized by a signature sequence containing an FXYD motif and three other conserved amino acid residues. Until recently, the functional role of FXYD proteins was largely unknown, with the exception of the gamma subunit of Na,K-ATPase, which was shown to be a specific regulator of renal alpha1-beta1 isozymes. We have investigated whether other members of the FXYD family may have a similar role as the gamma subunit and have found that CHIF (corticosteroid hormone-induced factor, FXYD4), FXYD7, as well as phospholemman (FXYD1) specifically associate with Na,K-ATPase and preferentially with alpha1-beta isozymes in native tissues, and produce distinct effects on the transport properties of Na,K-ATPase that are adapted to the physiological demands of the tissues in which they are expressed. These results provide evidence for a unique and novel mode of regulation of Na,K-ATPase by FXYD proteins that involves a tissue-specific expression of an auxiliary subunit of distinct Na,K-ATPase isozymes.

FXYD7, the first brain- and isoform-specific regulator of Na,K-ATPase: biosynthesis and function of its posttranslational modifications

The FXYD protein family has recently been defined as a result of the search for homologues of the Na,K-ATPase gamma subunit, CHIF, and phospholemman in EST and gene data banks. FXYD7 has been seen to have a role as a brain- and isozyme-specific regulator of Na/K-ATPase. In this study, the biosynthesis, membrane topology, nature, and role of the processing of FXYD7 are investigated.

A specific functional interaction between CHIF and Na,K-ATPase: role of FXYD proteins in the cellular regulation of the pump

CHIF (corticosteroid hormone-induced factor) is a member of the FXYD family that shares approximately 50% homology with the gamma subunit of Na,K-ATPase. It is expressed in renal collecting duct and distal colon, and is upregulated by Na(+) deprivation and high K(+) diet. Both CHIF and gamma are coimmunoprecipitated by an anti-alpha subunit antibody, and alpha is immunoprecipitated by anti-gamma and anti-CHIF antibodies. (86)Rb(+) flux experiments in CHIF-transfected HeLa cells demonstrate that CHIF increases the affinity for cytoplasmic Na(+), but does not affect the affinity for extracellular K(Rb). A physiological role of CHIF in kidney function is further elucidated by the phenotypic analysis of CHIF knockout mice. Taken together with data by others, it appears that FXYD proteins are tissue-specific subunits or regulators of the Na,K-ATPase whose function is to adjust the pump kinetics to particular physiological needs.

Immunocytochemical localization of Na,K-ATPase gamma subunit and CHIF in inner medulla of rat kidney

The gamma subunit of Na,K-ATPase and CHIF both belong to the FXYD single-membrane-spanning protein family and have been suggested to have regulatory functions in kidney tubules. CHIF is known to be present in the collecting duct, and gamma has been demonstrated in several segments of the rat kidney tubule, but never clearly in the inner medullary collecting duct (IMCD). Here, we demonstrate the cellular and subcellular localization of the gamma subunit and CHIF in the IMCD in inner medulla by using Western blotting, laser-scanning confocal immunofluorescence, and immunoelectron microscopy. In the initial quarter of the IMCD (next to the outer medulla), antibodies against the C-terminal of gamma as well as splice variant gammaa labeled the basolateral surface of intercalated cells (ICs), while principal cells (PCs) remained unlabeled. In the middle segment of the IMCD, all PCs exhibited distinct basolateral staining for the gammaC-terminal as well as gammaa and CHIF. Immunoelectron microscopy showed that the gammaC-terminal and CHIF were associated with the inner leaflet of the basolateral plasma membrane in the labeled cells. Immunoblotting demonstrated the presence of both the gammaC-terminal and gammaa in inner medullary tissue. However, splice variant gammab was not detected in inner medulla by immunocytochemistry or immunoblotting. The present observations demonstrate that the Na,K-ATPase gamma subunit and CHIF are strategically located in the inner medulla to participate in the fine-tuning of urine ion composition through the regulation of the Na,K-ATPase activity in the IMCD.

A functional interaction between CHIF and Na-K-ATPase: implication for regulation by FXYD proteins

Like the gamma-subunit of Na-K-ATPase, the corticosteroid hormone-induced factor (CHIF) is a member of the FXYD family of one-transmembrane-segment proteins. Both CHIF and two splice variants of gamma, gamma(a) and gamma(b), are expressed in the kidney. Immunolocalization experiments demonstrate mutually exclusive expression of CHIF and gamma in different nephron segments. Specific coimmunoprecipitation experiments demonstrate the existence in kidney membranes of the complexes alpha/beta/gamma(a), alpha/beta/gamma(b), and alpha/beta/CHIF and exclude mixed complexes such as alpha/beta/gamma(a)/gamma(b) and alpha/beta/gamma/CHIF. CHIF has been expressed in HeLa cells harboring the rat alpha(1)-subunit of Na-K-ATPase. (86)Rb flux experiments demonstrate that CHIF induces a two- to threefold increase in apparent affinity for cytoplasmic Na (K'(Na)) but does not affect affinity for extracellular K (Rb) ions (K'(K)) or V(max). Measurements of Na-K-ATPase using isolated membranes show similar but smaller effects of CHIF on K'(Na), whereas K'(K) and K'(ATP) are unaffected. The functional effects of CHIF differ from those of gamma. An implication of these findings is that other FXYD proteins could act as tissue-specific modulators of Na-K-ATPase.

Casein kinase 2 specifically binds to and phosphorylates the carboxy termini of ENaC subunits

A number of findings have suggested the involvement of protein phosphorylation in the regulation of the epithelial Na+ channel (ENaC). A recent study has demonstrated that the C tails of the beta and gamma subunits of ENaC are subject to phosphorylation by at least three protein kinases [Shi, H., Asher, C., Chigaev, A., Yung, Y., Reuveny, E., Seger, R. & Garty, H. (2002) J. Biol. Chem. 277, 13539-13547]. One of them was identified as ERK which phosphorylates betaT613 and gammaT623 and affects the channel interaction with Nedd4. The current study identifies a second protein kinase as casein kinase 2 (CK2), or CK-2-like kinase. It phosphorylates betaS631, a well-conserved serine on the beta subunit. Such phosphorylation is observed both in vitro using glutathione-S-transferase-ENaC fusion proteins and in vivo in ENaC-expressing Xenopus oocytes. The gamma subunit is weakly phosphorylated by this protein kinase on another residue (gammaT599), and the C tail of alpha is not significantly phosphorylated by this kinase. Thus, CK2 may be involved in the regulation of the epithelial Na+ channel.

Generation and phenotypic analysis of CHIF knockout mice

Corticosteroid hormone-induced factor (CHIF) is a short epithelial-specific protein that is independently induced by aldosterone and a high-K(+) diet. It is a member of the FXYD family of single-span transmembrane proteins that include phospholemman, Mat-8, and the gamma-subunit of Na(+)-K(+)-ATPase. A number of studies have suggested that these proteins are involved in the regulation of ion transport and, in particular, functionally interact with the Na(+)-K(+)-ATPase. The present study describes the characterization, targeted disruption, and phenotypic analysis of the mouse CHIF gene. The CHIF knockout mice are viable and not distinguishable from wild-type littermates under normal conditions. Under K(+) loading, they have a twofold higher urine volume and an increased glomerular filtration rate. Similar but smaller effects are observed in mice fed a low-Na(+) diet. Treating K(+)-loaded mice for 10 days with furosemide resulted in lethality in the knockout mice (17 of 39) but not in the wild-type group (1 of 39). The data are consistent with an effect of CHIF on the Na(+)-K(+)-ATPase that is specific to the outer and inner medullary duct, its major expression site.

Phospholemman (FXYD1) associates with Na,K-ATPase and regulates its transport properties

A family of small, single-span membrane proteins (the FXYD family) has recently been defined based on their sequence and structural homology. Some members of this family have already been identified as tissue-specific regulators of Na,K-ATPase (NKA). In the present study, we demonstrate that phospholemman (PLM) (FXYD1), so far considered to be a heart- and muscle-specific channel or channel-regulating protein, associates specifically and stably with six different alpha-beta isozymes of NKA after coexpression in Xenopus oocytes, and with alpha1-beta, and less efficiently with alpha2-beta isozymes, in native cardiac and skeletal muscles. Stoichiometric association of PLM with NKA occurs posttranslationally either in the Golgi or the plasma membrane. Interaction of PLM with NKA induces a small decrease in the external K+ affinity of alpha1-beta1 and alpha2-beta1 isozymes and a nearly 2-fold decrease in the internal Na+ affinity. In conclusion, this study demonstrates that PLM is a tissue-specific regulator of NKA that may play an essential role in muscle contractility.

FXYD7 is a brain-specific regulator of Na,K-ATPase alpha 1-beta isozymes

Recently, corticosteroid hormone-induced factor (CHIF) and the gamma-subunit, two members of the FXYD family of small proteins, have been identified as regulators of renal Na,K-ATPase. In this study, we have investigated the tissue distribution and the structural and functional properties of FXYD7, another family member which has not yet been characterized. Expressed exclusively in the brain, FXYD7 is a type I membrane protein bearing N-terminal, post-translationally added modifications on threonine residues, most probably O-glycosylations that are important for protein stabilization. Expressed in Xenopus oocytes, FXYD7 can interact with Na,K-ATPase alpha 1-beta 1, alpha 2-beta 1 and alpha 3-beta 1 but not with alpha-beta 2 isozymes, whereas, in brain, it is only associated with alpha 1-beta isozymes. FXYD7 decreases the apparent K(+) affinity of alpha 1-beta 1 and alpha 2-beta 1, but not of alpha 3-beta1 isozymes. These data suggest that FXYD7 is a novel, tissue- and isoform-specific Na,K-ATPase regulator which could play an important role in neuronal excitability.

Interactions of beta and gamma ENaC with Nedd4 can be facilitated by an ERK-mediated phosphorylation

Phosphorylation of the epithelial Na(+) channel (ENaC) has been suggested to play a role in its regulation. Here we demonstrate that phosphorylating the carboxyl termini of the beta and gamma subunits facilitates their interactions with the ubiquitin ligase Nedd4 and inhibits channel activity. Three protein kinases, which phosphorylate the carboxyl termini of beta and gammaENaC, have been identified by an in vitro assay. One of these phosphorylates betaThr-613 and gammaThr-623, well-conserved C-tail threonines in the immediate vicinity of the PY motifs. Phosphorylation of gammaThr-623 has also been demonstrated in vivo in channels expressed in Xenopus oocytes, and mutating betaThr-613 and gammaThr-623 into alanine increased the channel activity by 3.5-fold. Effects of the above phosphorylations on interactions between ENaC and Nedd4 have been studied using surface plasmon resonance. Peptides having phospho-threonine at positions beta613 or gamma623 bind the WW domains of Nedd4 two to three times better than the non-phosphorylated analogues, due to higher association rate constants. Using a number of different approaches it was demonstrated that the protein kinase acting on betaThr-613 and gammaThr-623 is the extracellular regulated kinase (ERK). It is suggested that an ERK-mediated phosphorylation of betaThr-613 and gammaThr-623 down-regulates the channel by facilitating its interaction with Nedd4.

Characterization of interactions between Nedd4 and beta and gammaENaC using surface plasmon resonance

Cell surface expression of the epithelial Na(+) channel ENaC is regulated by the ubiquitin ligase Nedd4. Binding of the WW domains of Nedd4 to the PY region in the carboxy tails of beta and gammaENaC, results in channel ubiquitination and degradation. Kinetic analysis of these interactions has been done using surface plasmon resonance. Synthetic peptides corresponding to the PY regions of beta and gammaENaC were immobilized on a sensor chip and "real-time" kinetics of their binding to recombinant WW proteins was determined. Specificity of the interactions was established by competition experiment, as well as by monitoring effects of a point mutation known to impair Nedd4/ENaC binding. These data provides the first determination of association, dissociation and equilibrium constants for the interactions between WW2 and beta or gammaENaC.

CHIF, a member of the FXYD protein family, is a regulator of Na,K-ATPase distinct from the gamma-subunit

The biological role of small membrane proteins of the new FXYD family is largely unknown. The best characterized FXYD protein is the gamma-subunit of the Na,K-ATPase (NKA) that modulates the Na,K-pump function in the kidney. Here, we report that, similarly to gamma(a) and gamma(b) splice variants, the FXYD protein CHIF (corticosteroid-induced factor) is a type I membrane protein which is associated with NKA in renal tissue, and modulates the Na,K-pump transport when expressed in Xenopus oocytes. In contrast to gamma(a) and gamma(b), which both decrease the apparent Na+ affinity of the Na,K-pump, CHIF significantly increases the Na+ affinity and decreases the apparent K+ affinity due to an increased Na+ competition at external binding sites. The extracytoplasmic FXYD motif is required for stable gamma-subunit and CHIF interaction with NKA, while cytoplasmic, positively charged residues are necessary for the gamma-subunit's association efficiency and for CHIF's functional effects. These data document that CHIF is a new tissue-specific regulator of NKA which probably plays a crucial role in aldosterone-responsive tissues responsible for the maintenance of body Na+ and K+ homeostasis.

In vitro phosphorylation of COOH termini of the epithelial Na+ channel and its effects on channel activity inXenopus oocytes

Recent findings have suggested the involvement of protein phosphorylation in the regulation of the epithelial Na+ channel (ENaC). This study reports the in vitro phosphorylation of the COOH termini of ENaC subunits expressed as glutathione S-transferase fusion proteins. Channel subunits were specifically phosphorylated by kinase-enriched cytosolic fractions derived from rat colon. The phosphorylation observed was not mediated by the serum- and glucocorticoid-regulated kinase sgk. For the γ-subunit, phosphorylation occurred on a single, well-conserved threonine residue located in the immediate vicinity of the PY motif (T630). The analogous residue on β(S620) was phosphorylated as well. The possible role of γT630 and βS620 in channel function was studied in Xenopus laevis oocytes. Mutating these residues to alanine had no effect on the basal channel-mediated current. They do, however, inhibit the sgk-induced increase in channel activity but only in oocytes that were preincubated in low Na+ and had a high basal Na+ current. Thus mutating γT630 or βS620 may limit the maximal channel activity achieved by a combination of sgk and low Na+.

Membrane topology and immunolocalization of CHIF in kidney and intestine

Corticosteroid hormone-induced factor (CHIF) is an aldosterone-induced gene, the function of which is yet unknown. It is specifically expressed in kidney collecting duct (CD) and distal colon and is upregulated by either Na+ deprivation or K+ loading. Hence, it may play a role in epithelial electrolyte transport. Previous studies have characterized regulation and tissue distribution of CHIF mRNA but provided no information on the protein itself. The present paper addresses this issue by using Western blotting, immunochemistry, and in vitro translation. CHIF is an approximately 8-kDa membranal protein, and protease digestion experiments suggest that its COOH tail faces the cell interior. The protein is abundant in distal colon, kidney medulla, and papilla but cannot be detected in a variety of other tissues. Confocal immunocytochemistry demonstrates that CHIF is present in the basolateral membrane of CD principal cells and distal colon surface cells, with occasional intracellular staining. Dexamethasone and low Na+ intake increase the abundance of CHIF. Unlike previous Northern data, induction of CHIF protein by low-Na+ intake was apparent not only in the distal colon but also in the kidney.

Cloning and function of the rat colonic epithelial K+ channel KVLQT1

KVLQT1 (KCNQ1) is a voltage-gated K+ channel essential for repolarization of the heart action potential that is defective in cardiac arrhythmia. The channel is inhibited by the chromanol 293B, a compound that blocks cAMP-dependent electrolyte secretion in rat and human colon, therefore suggesting expression of a similar type of K+ channel in the colonic epithelium. We now report cloning and expression of KVLQT1 from rat colon. Overlapping clones identified by cDNA-library screening were combined to a full length cDNA that shares high sequence homology to KVLQT1 cloned from other species. RT-PCR analysis of rat colonic musoca demonstrated expression of KVLQT1 in crypt cells and surface epithelium. Expression of rKVLQT1 in Xenopus oocytes induced a typical delayed activated K+ current, that was further activated by increase of intracellular cAMP but not Ca2+ and that was blocked by the chromanol 293B. The same compound blocked a basolateral cAMP-activated K+ conductance in the colonic mucosal epithelium and inhibited whole cell K+ currents in patch-clamp experiments on isolated colonic crypts. We conclude that KVLQT1 is forming an important component of the basolateral cAMP-activated K+ conductance in the colonic epithelium and plays a crucial role in diseases like secretory diarrhea and cystic fibrosis.

Expression and function of colonic epithelial KvLQT1 K+ channels

1. KvLQT1 (KCNQ1) is a voltage-gated K+ channel essential for repolarization of the heart action potential. Defects in ion channels have been demonstrated in cardiac arrhythmia. This channel is inhibited potently by the chromanol 293B. The same compound has been shown to block cAMP-dependent electrolyte secretion in rat and human colon. Therefore, it was suggested that a K+ channel similar to KvLQT1 is expressed in the colonic epithelium. 2. In the present paper, expression of KvLQT1 and its function in colonic epithelial cells is described. Reverse transcription-polymerase chain reaction analysis of rat colonic mucosa demonstrated expression of KvLQT1 in both crypt cells and surface epithelium. When expressed in Xenopus oocytes, KvLQT1 induced a typical delayed activated K+ current. 3. As demonstrated, the channel activity could be further activated by increases in intracellular cAMP. These and other data support the concept that KvLQT1 is forming a component of the basolateral cAMP-activated K+ conductance in the colonic epithelium.

Regulation of the epithelial Na+ channel by aldosterone: open questions and emerging answers

Aldosterone is the principal adrenal steroid controlling Na+ retention in amphibians and mammalians. It acts primarily by increasing the apical Na+ permeability through activation of the epithelial Na+ channel (ENaC). The cellular events mediating the hormonal action are mostly unknown. Early studies have provided evidence that the hormone functions to activate or translocate pre-existing channels by a yet undefined mechanism. In addition, enhanced de novo channel synthesis appears to take place as well. The molecular cloning of the three ENaC subunits has provided new powerful tools for testing and confirming this hypothesis, as well as for characterizing mechanisms by which ENaC is regulated. Another important development is the recent identification of several cDNAs corresponding to aldosterone-induced and suppressed mRNAs. The study of these genes and their putative interactions with ENaC is likely to provide important clues to the mechanisms by which aldosterone controls the apical Na+ permeability of tight epithelia. This article reviews recent developments in the field that may lead to the elucidation of the mechanisms by which the hormone controls Na+ transport.

Regulation of sgk by aldosterone and its effects on the epithelial Na(+) channel

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

The ontogeny of the expression of K+ channel-like gene (CHIF) in the rat kidney papilla

Recently, an IsK-like potassium (K+) channel corticosteroid-induced gene (CHIF) was cloned. A high-K+ diet enhances, while a low-K+ diet decreases the expression of this gene. The major expression of CHIF in the adult rat kidney is in the papilla, where it is constitutive, in contrast to its inducibility by corticosteroids and a low-salt diet in the rat colon. In order to further understand the ontogeny of K+ clearance, we studied the presence of CHIF in the kidney papilla in different stages of rat development. Total RNA from rat kidney papillae of 1- to 3-day pre-labor unborn offspring, 2- to 3-day-old newborns, 10-day-old, 6-week-old, and 43-week-old rats underwent northern hybridization for CHIF and the alpha-subunit of the Na+-K+-ATPase mRNA. Minor expression of CHIF mRNA was found in fetal and newborn rat papillae, while older rats showed an age-related increase in gene expression. The expression of the alpha-sub unit of the Na+-K+-ATPase was not age related. We conclude that CHIF is present in the rat kidney papilla and the expression is related to age. The relative deficiency of CHIF in the newborn may be one of the factors responsible for the reduced K+ clearance in is period.

Differential regulation of ROMK expression in kidney cortex and medulla by aldosterone and potassium

This study explores the role of K+ and aldosterone in the regulation of mRNA of the ATP-sensitive, inwardly rectifying K+ channel, ROMK, in the rat kidney. K+ deficiency downregulated ROMK mRNA in cortex to 47.1 +/- 5.1% of control (P < 0.001) and in medulla to 56.1 +/- 3. 4% (P < 0.001). High-K+ diet slightly increased ROMK mRNA in medulla to 122 +/- 9% (P < 0.05 vs. control). Adrenalectomy (Adx) downregulated cortical ROMK mRNA to 30.7 +/- 6.8% (P < 0.001 vs. control), and increased it in medulla to 138 +/- 12.9% (P < 0.02 vs. control). In Adx rats, K+ deficiency decreased ROMK mRNA in cortex and medulla similar to intact rats. The alpha1- and beta1-Na-K-ATPase subunits were regulated in parallel to that of ROMK. In medulla, ROMK mRNA correlated with serum K+ concentration at R = 0.9406 (n = 6, P < 0.001) and alpha1-Na-K-ATPase mRNA at R = 0.9756 (n = 6, P < 0.001). ROMK2 also correlated with serum K+ concentration (R = 0.895; n = 6, P < 0.01). These results show that cortical ROMK expression is regulated by aldosterone and K+, whereas the medullary ROMK mRNA is regulated by serum K+.

An aldosterone regulated chicken intestine protein with high affinity to amiloride

The pattern of chicken intestine amiloride-binding proteins was determined using the photoreactive amiloride analogue 2'-methoxy-5'-nitrobenzamil (NMBA) and a polyclonal anti-amiloride antibody. At 10(-7)M, NMBA inhibits approximately 62% of the Na+ channel activity. At this concentration the amiloride analogue labels a number of membrane proteins, and in particular a 40-45 kDa polypeptide denoted ABP40. Incorporation of NMBA into ABP40 could be prevented by a 100-fold excess of benzamil, but not by a 1000-fold excess of 5-(N-ethyl-N-isopropyl)-amiloride. Labeling of ABP40 was intense in membranes derived from salt-deprived chickens and approximately 5-fold weaker in membranes from salt-repleted animals. Because of its small size, ABP40 is not likely to be an avian Na+ channel subunit, yet this amiloride-binding protein could be involved in the response to aldosterone.

Differential regulation of CHIF mRNA by potassium intake and aldosterone

The channel-inducing factor (CHIF) is an epithelial-specific transmembrane protein, which is induced by aldosterone in distal colon (but not in kidney) and can evoke K+ conductance in Xenopus oocytes. The current study examined the possibility that CHIF participates in maintaining K+ balance by assessing its regulation during variations in K+ intake. In adrenal-intact rats, high-K+ diet stimulated, whereas K+ deficiency downregulated, CHIF mRNA both in kidney and colon. The downregulation of CHIF observed in rats fed a low-K+ diet for different periods of time closely correlated with a decrease in plasma K+ but also with changes in aldosterone levels. To differentiate between the two, modulation of CHIF has been studied in adrenalectomized rats with and without corticosteroid supplementation. These experiments have demonstrated that a low-K+ intake suppresses CHIF mRNA, irrespective of aldosterone level. On the other hand, the upregulation evoked by a high-K+ load is apparent only in adrenal-intact rats. This is despite the fact that infusing rats with aldosterone and corticosterone does not increase the expression of this mRNA in kidney. These findings may suggest a role for CHIF in preserving K+ balance.

Epithelial sodium channels: function, structure, and regulation

The apical (outward-facing) membranes of high-resistance epithelia contain Na+ channels, traditionally identified by their sensitivity to block by the K(+)-sparing diuretic amiloride. Such channels have been characterized in amphibian skin and urinary bladder, renal collecting duct, distal colon, sweat and salivary glands, lung, and taste buds. They mediate the first step of active Na+ reabsorption and play a major role in the maintenance of electrolyte and water homeostasis in all vertebrates. In the past, these channels were classified according to their biophysical and pharmacological properties. The recent cloning of the three homologous channel subunits denoted alpha-, beta-, and gamma-epithelial Na+ channels (ENaC) has provided a molecular definition of at least one class of amiloride-blockable channels. Subsequent studies have established that ENaC is a major Na(+)-conducting pathway in both absorbing and secretory epithelia and is related to one type of channel involved in mechanosensation. This review summarizes the biophysical characteristics, molecular properties, and regulatory mechanisms of epithelial amiloride-blockable Na+ channels. Special emphasis is given to recent studies utilizing cloned ENaC subunits and purified amiloride-binding proteins.