Mechanism by which Liddle's syndrome mutations increase activity of a human epithelial Na+ channel

Role of the UPS in Liddle syndrome

Hypertension is a serious medical problem affecting a large population worldwide. Liddle syndrome is a hereditary form of early onset hypertension caused by mutations in the epithelial Na⁺ channel (ENaC). The mutated region, called the PY (Pro-Pro-x-Tyr) motif, serves as a binding site for Nedd4-2, an E3 ubiquitin ligase from the HECT family. Nedd4-2 binds the ENaC PY motif via its WW domains, normally leading to ENaC ubiquitylation and endocytosis, reducing the number of active channels at the plasma membrane. In Liddle syndrome, this endocytosis is impaired due to the inability of the mutated PY motif in ENaC to properly bind Nedd4-2. This leads to accumulation of active channels at the cell surface and increased Na⁺ (and fluid) absorption in the distal nephron, resulting in elevated blood volume and blood pressure. Small molecules/compounds that destabilize cell surface ENaC, or enhance Nedd4-2 activity in the kidney, could potentially serve to alleviate hypertension.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

The ubiquitin system, disease, and drug discovery

The ubiquitin system of protein modification has emerged as a crucial mechanism involved in the regulation of a wide array of cellular processes. As our knowledge of the pathways in this system has grown, so have the ties between the protein ubiquitin and human disease. The power of the ubiquitin system for therapeutic benefit blossomed with the approval of the proteasome inhibitor Velcade in 2003 by the FDA. Current drug discovery activities in the ubiquitin system seek to (i) expand the development of new proteasome inhibitors with distinct mechanisms of action and improved bioavailability, and (ii) validate new targets. This review summarizes our current understanding of the role of the ubiquitin system in various human diseases ranging from cancer, viral infection and neurodegenerative disorders to muscle wasting, diabetes and inflammation. I provide an introduction to the ubiquitin system, highlight some emerging relationships between the ubiquitin system and disease, and discuss current and future efforts to harness aspects of this potentially powerful system for improving human health.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

Regulation of the epithelial Na+ channel by endothelin-1 in rat collecting duct

We used patch-clamp electrophysiology to investigate regulation of the epithelial Na+ channel (ENaC) by endothelin-1 (ET-1) in isolated, split-open rat collecting ducts. ET-1 significantly decreases ENaC open probability by about threefold within 5 min. ET-1 decreases ENaC activity through basolateral membrane ETB but not ETA receptors. In rat collecting duct, we find no role for phospholipase C or protein kinase C in the rapid response of ENaC to ET-1. ET-1, although, does activate src family tyrosine kinases and their downstream MAPK1/2 effector cascade in renal principal cells. Both src kinases and MAPK1/2 signaling are necessary for ET-1-dependent decreases in ENaC open probability in the split-open collecting duct. We conclude that ET-1 in a physiologically relevant manner rapidly suppresses ENaC activity in native, mammalian principal cells. These findings may provide a potential mechanism for the natriuresis observed in vivo in response to ET-1, as well as a potential cause for the salt-sensitive hypertension found in animals with impaired endothelin signaling.

Small molecule activator of the human epithelial sodium channel

The epithelial sodium channel (ENaC), a heterotrimeric complex composed of alpha, beta, and gamma subunits, belongs to the ENaC/degenerin family of ion channels and forms the principal route for apical Na(+) entry in many reabsorbing epithelia. Although high affinity ENaC blockers, including amiloride and derivatives, have been described, potent and specific small molecule ENaC activators have not been reported. Here we describe compound S3969 that fully and reversibly activates human ENaC (hENaC) in an amiloride-sensitive and dose-dependent manner in heterologous cells. Mechanistically, S3969 increases hENaC open probability through interactions requiring the extracellular domain of the beta subunit. hENaC activation by S3969 did not require cleavage by the furin protease, indicating that nonproteolyzed channels can be opened. Function of alphabetaG37Sgamma hENaC, a channel defective in gating that leads to the salt-wasting disease pseudohypoaldosteronism type I, was rescued by S3969. Small molecule activation of hENaC may find application in alleviating human disease, including pseudohypoaldosteronism type I, hypotension, and neonatal respiratory distress syndrome, when improved Na(+) flux across epithelial membranes is clinically desirable.

Regulation of epithelial Na+ channels by aldosterone: role of Sgk1

1. The epithelial sodium channel (ENaC) is tightly regulated by hormonal and humoral factors, including cytosolic ion concentration and glucocorticoid and mineralocorticoid hormones. Many of these regulators of ENaC control its activity by regulating its surface expression via neural precursor cell-expressed developmentally downregulated (gene 4) protein (Nedd4-2). 2. During the early phase of aldosterone action, Nedd4-2-dependent downregulation of ENaC is inhibited by the serum- and glucocorticoid-induced kinase 1 (Sgk1). 3. Sgk1 phosphorylates Nedd4-2. Subsequently, phosphorylated Nedd4-2 binds to the 14-3-3 protein and, hence, reduces binding of Nedd4-2 to ENaC. 4. Nedd4-2 is also phosphorylated by protein kinase B (Akt1). Both Sgk1 and Akt1 are part of the insulin signalling pathway that increases transepithelial Na(+) absorption by inhibiting Nedd4-2 and activating ENaC.

Nedd4-2 induces endocytosis and degradation of proteolytically cleaved epithelial Na+ channels

As a pathway for Na(+) reabsorption, the epithelial Na(+) channel ENaC is critical for Na(+) homeostasis and blood pressure control. Na(+) transport is regulated by Nedd4-2, an E3 ubiquitin ligase that decreases ENaC expression at the cell surface. To investigate the underlying mechanisms, we proteolytically cleaved/activated ENaC at the cell surface and then quantitated the rate of disappearance of cleaved channels using electrophysiological and biochemical assays. We found that cleaved ENaC channels were rapidly removed from the cell surface. Deletion or mutation of the Nedd4-2 binding motifs in alpha, beta, and gammaENaC dramatically reduced endocytosis, whereas a mutation that disrupts a YXXØ endocytosis motif had no effect. ENaC endocytosis was also decreased by silencing of Nedd4-2 and by expression of a dominant negative Nedd4-2 construct. Conversely, Nedd4-2 overexpression increased ENaC endocytosis in human embryonic kidney 293 cells but had no effect in Fischer rat thyroid epithelia. In addition to its effect on endocytosis, Nedd4-2 also increased the rate of degradation of the cell surface pool of cleaved alphaENaC. Together the data indicate that Nedd4-2 reduces ENaC surface expression by altering its trafficking at two distinct sites in the endocytic pathway, inducing endocytosis of cleaved channels and targeting them for degradation.

Acute downregulation of ENaC by EGF involves the PY motif and putative ERK phosphorylation site

The epithelial sodium channel (ENaC) is expressed in a variety of tissues, including the renal collecting duct, where it constitutes the rate-limiting step for sodium reabsorption. Liddle's syndrome is caused by gain-of-function mutations in the β and γ subunits of ENaC, resulting in enhanced Na reabsorption and hypertension. Epidermal growth factor (EGF) causes acute inhibition of Na absorption in collecting duct principal cells via an extracellular signal–regulated kinase (ERK)–dependent mechanism. In experiments with primary cultures of collecting duct cells derived from a mouse model of Liddle's disease (β-ENaC truncation), it was found that EGF inhibited short-circuit current (Isc) by 24 ± 5% in wild-type cells but only by 6 ± 3% in homozygous mutant cells. In order to elucidate the role of specific regions of the β-ENaC C terminus, Madin-Darby canine kidney (MDCK) cell lines that express β-ENaC with mutation of the PY motif (P616L), the ERK phosphorylation site (T613A), and C terminus truncation (R564stop) were created using the Phoenix retroviral system. All three mutants exhibited significant attenuation of the EGF-induced inhibition of sodium current. In MDCK cells with wild-type β-ENaC, EGF-induced inhibition of Isc (<30 min) was fully reversed by exposure to an ERK kinase inhibitor and occurred with no change in ENaC surface expression, indicative of an effect on channel open probability (P_o). At later times (>30 min), EGF-induced inhibition of Isc was not reversed by an ERK kinase inhibitor and was accompanied by a decrease in ENaC surface expression. Our results are consistent with an ERK-mediated decrease in ENaC open probability and enhanced retrieval of sodium channels from the apical membrane.

Molecular determinants of PI(4,5)P2 and PI(3,4,5)P3 regulation of the epithelial Na+ channel

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) are physiologically important second messengers. These molecules bind effector proteins to modulate activity. Several types of ion channels, including the epithelial Na⁺ channel (ENaC), are phosphoinositide effectors capable of directly interacting with these signaling molecules. Little, however, is known of the regions within ENaC and other ion channels important to phosphoinositide binding and modulation. Moreover, the molecular mechanism of this regulation, in many instances, remains obscure. Here, we investigate modulation of ENaC by PI(3,4,5)P₃ and PI(4,5)P₂ to begin identifying the molecular determinants of this regulation. We identify intracellular regions near the inner membrane interface just following the second transmembrane domains in β- and γ- but not α-ENaC as necessary for PI(3,4,5)P₂ but not PI(4,5)P₂ modulation. Charge neutralization of conserved basic amino acids within these regions demonstrated that these polar residues are critical to phosphoinositide regulation. Single channel analysis, moreover, reveals that the regions just following the second transmembrane domains in β- and γ-ENaC are critical to PI(3,4,5)P₃ augmentation of ENaC open probability, thus, defining mechanism. Unexpectedly, intracellular domains within the extreme N terminus of β- and γ-ENaC were identified as being critical to down-regulation of ENaC activity and P_o in response to depletion of membrane PI(4,5)P₂. These regions of the channel played no identifiable role in a PI(3,4,5)P₃ response. Again, conserved positive-charged residues within these domains were particularly important, being necessary for exogenous PI(4,5)P₂ to increase open probability. We conclude that β and γ subunits bestow phosphoinositide sensitivity to ENaC with distinct regions of the channel being critical to regulation by PI(3,4,5)P₃ and PI(4,5)P₂. This argues that these phosphoinositides occupy distinct ligand-binding sites within ENaC to modulate open probability.

A novel epithelial sodium channel gamma-subunit de novo frameshift mutation leads to Liddle syndrome

OBJECTIVE

Liddle syndrome is a rare autosomal-dominant monogenic form of hypertension caused by mutations in the C-termini of the epithelial sodium channel beta- or gamma-subunit encoded by SCNN1B and SCNN1G, respectively, and often presenting with a familial history of hypertension. The purpose of this study was to determine whether mutations of SCNN1B or SCNN1G were present in a patient clinically suspected to have Liddle syndrome with no familial history of hypertension.

DESIGN AND PATIENTS

We screened the C-terminus of SCNN1B and SCNN1G in the patient, and also screened for the mutation in his parents, 50 hypertensive patients and 50 controls.

RESULTS

In this patient, no mutations were found in the C-terminus of SCNN1B. However, we found a frameshift mutation caused by an 'AGCTC' deletion at the 583 codon in SCNN1G. The frameshift resulted in a new termination site at the 585 codon of the gamma-subunit and the deletion of its PY motif. Neither his parents nor 50 randomly selected patients with hypertension nor 50 controls have the mutation, indicating that this is a de novo mutation and not a common genetic polymorphism.

CONCLUSION

The de novo mutation is the first reported frameshift of the gamma-subunit causing Liddle syndrome. These data imply that a familial history of hypertension is not an essential criterion for the diagnosis of Liddle syndrome.

Mechano-sensitivity of ENaC: may the (shear) force be with you

The epithelial Na+ channel (ENaC) is the rate-limiting step for Na+ absorption in various vertebrate epithelia and deeply enmeshed in the control of salt and water homeostasis. The phylogenetic relationship of ENaC molecules to mechano-sensitive Degenerins from Caenorhabditis elegans indicates that ENaC might be mechano-sensitive as well. Primarily, it was suggested that ENaC might be activated by membrane stretch. However, this issue still remains to be clarified because controversial results were published. Recent publications indicate that shear stress represents an adequate stimulus, activating ENaC via increasing the single-channel open probability. Basing on the experimental evidence published within the past years and integrating this knowledge into a model related to the mechano-sensitive receptor complex known from C. elegans, we introduce a putative mechanism concerning the mechano-sensitivity of ENaC. We suggest that mechano-sensitive ENaC activation represents a nonhormonal regulatory mechanism. This feature could be of considerable physiological significance because many Na+-absorbing epithelia are exposed to shear forces. Furthermore, it may explain the wide distribution of ENaC proteins in nonepithelial tissues. Nevertheless, it remains a challenge for future studies to explore the mechanism how ENaC is controlled by mechanical forces and shear stress in particular.

Epithelial sodium channel exit from the endoplasmic reticulum is regulated by a signal within the carboxyl cytoplasmic domain of the alpha subunit

Epithelial sodium channels (ENaCs) are assembled in the endoplasmic reticulum (ER) from alpha, beta, and gamma subunits, each with two transmembrane domains, a large extracellular loop, and cytoplasmic amino and carboxyl termini. ENaC maturation involves transit through the Golgi complex where Asn-linked glycans are processed to complex type and the channel is activated by furin-dependent cleavage of the alpha and gamma subunits. To identify signals in ENaC for ER retention/retrieval or ER exit/release, chimera were prepared with the interleukin alpha subunit (Tac) and each of the three cytoplasmic carboxyl termini of mouse ENaC (Tac-Ct) or with gamma-glutamyltranspeptidase and each of the three cytoplasmic amino termini (Nt-GGT). By monitoring acquisition of endoglycosidase H resistance after metabolic labeling, we found no evidence of ER retention of any chimera when compared with control Tac or GGT, but we did observe enhanced exit of Tac-alphaCt when compared with Tac. ER exit of ENaC was assayed after metabolic labeling by following the appearance of cleaved alpha as cleaved alpha subunit, but not non-cleaved alpha, is endoglycosidase H-resistant. Interestingly ER exit of epitope-tagged and truncated alpha (alphaDelta624-699-V5) with full-length betagamma was similar to wild type alpha (+betagamma), whereas ER exit of ENaC lacking the entire cytoplasmic carboxyl tail of alpha (alphaDelta613-699-V5 +betagamma) was significantly reduced. Subsequent analysis of ER exit for ENaCs with mutations within the intervening sequence (613)HRFRSRYWSPG(623) within the context of the full-length alpha revealed that mutation alphaRSRYW(620) to AAAAA significantly reduced ER exit. These data indicate that ER exit of ENaC is regulated by a signal within the alpha subunit carboxyl cytoplasmic tail.

Epithelial sodium channel (ENaC) is multi-ubiquitinated at the cell surface

The human ENaC (epithelial sodium channel), a complex of three subunits, provides the rate-limiting step for sodium uptake in the distal nephron, and therefore plays a key role in salt homoeostasis and in regulating blood pressure. The number of active sodium channel complexes present at the plasma membrane appears to be tightly controlled. In Liddle's syndrome, a form of hypertension caused by an increase in the number of active sodium channels at the cell membrane, the betaENaC or gammaENaC subunit gene contains a mutation that disrupts the binding site for the Nedd4 (neuronal precursor cell expressed developmentally down-regulated gene 4) family of ubiquitin-protein ligases. Therefore ubiquitination of channel subunits may be involved in altering cell surface ENaC. Here, we provide evidence that the ENaC subunits located at the cell surface are modified with multiple mono-ubiquitins (multi-ubiquitination) and that Nedd4-2 modulates this ubiquitination. We confirm that ENaC is associated with the mu2 subunit of the AP-2 (adaptor protein 2) clathrin adaptor. Since mono- or multi-ubiquitination of other membrane proteins is a signal for their internalization by clathrin-mediated endocytosis and subsequent trafficking, our results support a model whereby ubiquitin and clathrin adaptor binding sites act in concert to remove ENaC from the cell surface.

The PY motif of ENaC, mutated in Liddle syndrome, regulates channel internalization, sorting and mobilization from subapical pool

The epithelial-Na(+)-channel (alphabetagammaENaC) regulates kidney salt-transport and blood pressure. Each ENaC subunit contains a PY motif (PPxY) and its mutation in beta/gammaENaC causes Liddle syndrome, a hereditary hypertension. These (extended) PY motifs (PP(616)xY(618)xxL(621)) serve as binding sites for the ubiquitin ligase Nedd4-2, which decreases cell-surface expression of ENaC by unknown route(s). Using polarized kidney epithelia [Madin-Darby canine kidney I (MDCK-I)] cells stably expressing extracellularly myc-tagged wild type (WT) or PY-motif mutants of betaENaC (P616A, Y618A or L621A, with WT-alphagammaENaC), and live-imaging plus enzyme-linked immunosorbent assay (ELISA)-type assays to analyze routes/rates of ENaC internalization/recycling, we show here that cell-surface half-life of all PY mutants was fourfold longer than WT-ENaC (approximately 120 versus 30 minutes), reflecting primarily reduced channel internalization but also attenuated replenishment of cell-surface ENaC from a large subapical pool. The Y618A mutant revealed more severe internalization and replenishment defects than the other PY mutants. Internalized WT-ENaC was detected in sorting/recycling and late endosomes/lysosomes, while the Y618A mutant accumulated in the former. Nedd4-2 ubiquitinated ENaC at the apical membrane causing channel internalization and degradation. Cyclic AMP (cAMP) accelerated mobilization of subapical ENaC to the cell surface and long-term ENaC recycling, but only mobilization, not recycling, was inhibited in the PY mutants. These results suggest that the ENaC PY motifs (and Nedd4-2) primarily regulate channel internalization but also affect cAMP-dependent replenishment, providing important insight into the Liddle syndrome defects.

Determination of epithelial Na+ channel subunit stoichiometry from single-channel conductances

The epithelial Na⁺ channel (ENaC) is a multimeric membrane protein consisting of three subunits, α, β, and γ. The total number of subunits per functional channel complex has been described variously to follow either a tetrameric arrangement of 2α:1β:1γ or a higher-ordered stoichiometry of 3α:3β:3γ. Therefore, while it is clear that all three ENaC subunits are required for full channel activity, the number of the subunits required remains controversial. We used a new approach, based on single-channel measurements in Xenopus oocytes to address this issue. Individual mutations that alter single-channel conductance were made in pore-lining residues of ENaC α, β, or γ subunits. Recordings from patches in oocytes expressing a single species, wild type or mutant, of α, β, and γ showed a well-defined current transition amplitude with a single Gaussian distribution. When cRNAs for all three wild-type subunits were mixed with an equimolar amount of a mutant α-subunit (either S589D or S592T), amplitudes corresponding to pure wild-type or mutant conductances could be observed in the same patch, along with a third intermediate amplitude most likely arising from channels with at least one wild-type and at least 1 mutant α-subunit. However, intermediate or hybrid conductances were not observed with coexpression of wild-type and mutant βG529A or γG534E subunits. Our results support a tetrameric arrangement of ENaC subunits where 2α, 1β, and 1γ come together around central pore.

Nedd4-2 catalyzes ubiquitination and degradation of cell surface ENaC

Epithelial Na(+) absorption is regulated by Nedd4-2, an E3 ubiquitin-protein ligase that reduces expression of the epithelial Na(+) channel ENaC at the cell surface. Defects in this regulation cause Liddle syndrome, an inherited form of hypertension. Previous work found that Nedd4-2 binds to ENaC via PY motifs located in the C termini of alpha-, beta-, and gammaENaC. However, little is known about the mechanism by which Nedd4-2 regulates ENaC surface expression. Here we found that Nedd4-2 catalyzes ubiquitination of alpha-, beta-, and gammaENaC; Nedd4-2 overexpression increased ubiquitination, whereas Nedd4-2 silencing decreased ubiquitination. Although Nedd4-2 increased both mono/oligoubiquitinated and multiubiquitinated forms of ENaC, monoubiquitination was sufficient for Nedd4-2 to reduce ENaC surface expression and reduce ENaC current. Ubiquitination was disrupted by Liddle syndrome-associated mutations in ENaC or mutation of the catalytic HECT domain in Nedd4-2. Several findings suggest that the interaction between Nedd4-2 and ENaC is localized to the cell surface. First, Nedd4-2 bound to a population of ENaC at the cell surface. Second, Nedd4-2 catalyzed ubiquitination of cell surface ENaC. Third, Nedd4-2 selectively reduced ENaC expression at the cell surface but did not alter the quantity of immature ENaC in the biosynthetic pathway. Finally, Nedd4-2 induced degradation of the cell surface pool of ENaC. Together, the data suggest a model in which Nedd4-2 binds to and ubiquitinates ENaC at the cell surface, which targets surface ENaC for degradation, and thus, reduces epithelial Na(+) transport.

Mechano-sensitivity of epithelial sodium channels (ENaCs): laminar shear stress increases ion channel open probability

Epithelial cells are exposed to a variety of mechanical forces, but little is known about the impact of these forces on epithelial ion channels. Here we show that mechanical activation of epithelial sodium channels (ENaCs), which are essential for electrolyte and water balance, occurs via an increased ion channel open probability. ENaC activity of heterologously expressed rat (rENaC) and Xenopus (xENaC) orthologs was measured by whole-cell as well as single-channel recordings. Laminar shear stress (LSS), producing shear forces in physiologically relevant ranges, was used to mechanically stimulate ENaCs and was able to activate ENaC currents in whole-cell recordings. Preceding pharmacological activation of rENaC with Zn2+ and xENaC with gadolinium and glibenclamide largely prevented LSS-activated currents. In contrast, proteolytic cleavage with trypsin potentiated the LSS effect on rENaC whereas the LSS effect on xENaC was reversed (inhibition of xENaC current). Further, we found that exposure of excised outside-out patches to LSS led to an increased ion channel open probability without affecting the number of active channels. We suggest that mechano-sensitivity of ENaC may represent a ubiquitous feature for the physiology of epithelia, providing a putative mechanism for coupling transepithelial Na+ reabsorption to luminal transport.

Genetic mapping of a new heart rate QTL on chromosome 8 of spontaneously hypertensive rats

Background

Tachycardia is commonly observed in hypertensive patients, predominantly mediated by regulatory mechanisms integrated within the autonomic nervous system. The genetic loci and genes associated with increased heart rate in hypertension, however, have not yet been identified.

Methods

An F2 intercross of Spontaneously Hypertensive Rats (SHR) × Brown Norway (BN) linkage analysis of quantitative trait loci mapping was utilized to identify candidate genes associated with an increased heart rate in arterial hypertension.

Results

Basal heart rate in SHR was higher compared to that of normotensive BN rats (365 ± 3 vs. 314 ± 6 bpm, p < 0.05 for SHR and BN, respectively). A total genome scan identified one quantitative trait locus in a 6.78 cM interval on rat chromosome 8 (8q22–q24) that was responsible for elevated heart rate. This interval contained 241 genes, of which 65 are known genes.

Conclusion

Our data suggest that an influential genetic region located on the rat chromosome 8 contributes to the regulation of heart rate. Candidate genes that have previously been associated with tachycardia and/or hypertension were found within this QTL, strengthening our hypothesis that these genes are, potentially, associated with the increase in heart rate in a hypertension rat model.

Genetics of arterial hypertension and hypotension

Human hypertension affects affects more than 20% of the adult population in industrialized countries, and it is implicated in millions of deaths worldwide each year from stroke, heart failure and ischemic heart disease. Available evidence suggests a major genetic impact on blood pressure regulation. Studies in monogenic hypertension revealed that renal salt and volume regulation systems are predominantly involved in the genesis of these disorders. Mutations here affect the synthesis of mineralocorticoids, the function of the mineralocorticoid receptor, epithelial sodium channels and their regulation by a new class of kinases, termed WNK kinases. It has been learned from monogenic hypotension that almost all ion transporters involved in the renal uptake of Na(+) have a major impact on blood pressure regulation. For essential hypertension as a complex disease, many candidate genes have been analysed. These include components of the renin-angiotensin-aldosterone system, adducin, beta-adrenoceptors, G protein subunits, regulators of G protein signalling (RGS) proteins, Rho kinases and G protein receptor kinases. At present, the individual impact of common polymorphisms in these genes on the observed blood pressure variation, on risk for stroke and as predictors of antihypertensive responses remains small and clinically irrelevant. Nevertheless, these studies have greatly augmented our knowledge on the regulation of renal functions, cellular signal transduction and the integration of both. Together, this provides the basis for the identification of novel drug targets and, hopefully, innovative antihypertensive drugs.

AMP-activated Kinase Inhibits the Epithelial Na+ Channel through Functional Regulation of the Ubiquitin Ligase Nedd4-2

We recently found that the metabolic sensor AMP-activated kinase (AMPK) inhibits the epithelial Na+ channel (ENaC) through decreased plasma membrane ENaC expression, an effect requiring the presence of a binding motif in the cytoplasmic tail of the beta-ENaC subunit for the ubiquitin ligase Nedd4-2. To further examine the role of Nedd4-2 in the regulation of ENaC by AMPK, we studied the effects of AMPK activation on ENaC currents in Xenopus oocytes co-expressing ENaC and wild-type (WT) or mutant forms of Nedd4-2. ENaC inhibition by AMPK was preserved in oocytes expressing WT Nedd4-2 but blocked in oocytes expressing either a dominant-negative (DN) or constitutively active (CA) Nedd4-2 mutant, suggesting that AMPK-dependent modulation of Nedd4-2 function is involved. Similar experiments utilizing WT or mutant forms of the serum- and glucocorticoid-regulated kinase (SGK1), modulators of protein kinase A (PKA), or extracellular-regulated kinase (ERK) did not affect ENaC inhibition by AMPK, suggesting that these pathways known to modulate the Nedd4-2-ENaC interaction are not responsible. AMPK-dependent phosphorylation of Nedd4-2 expressed in HEK-293 cells occurred both in vitro and in vivo, suggesting a potential mechanism for modulation of Nedd4-2 and thus cellular ENaC activity. Moreover, cellular AMPK activation significantly enhanced the interaction of the beta-ENaC subunit with Nedd4-2, as measured by co-immunoprecipitation assays in HEK-293 cells. In summary, these results suggest a novel mechanism for ENaC regulation in which AMPK promotes ENaC-Nedd4-2 interaction, thereby inhibiting ENaC by increasing Nedd4-2-dependent ENaC retrieval from the plasma membrane. AMPK-dependent ENaC inhibition may limit cellular Na+ loading under conditions of metabolic stress when AMPK becomes activated.

Structural determinants for high-affinity binding in a Nedd4 WW3* domain-Comm PY motif complex

Interactions between the WW domains of Drosophila Nedd4 (dNedd4) and Commissureless (Comm) PY motifs promote axon crossing at the CNS midline and muscle synaptogenesis. Here we report the solution structure of the dNedd4 WW3* domain complexed to the second PY motif (227'TGLPSYDEALH237') of Comm. Unexpectedly, there are interactions between WW3* and ligand residues both N- and C-terminal to the PY motif. Residues Y232'-L236' form a helical turn, following the PPII helical PY motif. Mutagenesis and binding studies confirm the importance of these extensive contacts, not simultaneously observed in other WW domain complexes, and identify a variable loop in WW3* responsible for its high-affinity interaction. These studies expand our general understanding of the molecular determinants involved in WW domain-ligand recognition. In addition, they provide insights into the specific regulation of dNedd4-mediated ubiquitination of Comm and subsequent internalization of Comm or the Comm/Roundabout complex, critical for CNS and muscle development.

Sgk kinases and their role in epithelial transport

The serum/glucocorticoid-induced kinase Sgk1 plays an important role in the regulation of epithelial ion transport. This kinase is very rapidly regulated at the transcriptional level as well as via posttranslational modifications involving phosphorylation by the MAP or PI-3 kinase pathways and/or ubiquitylation. Although Sgk1 is a cell survival kinase, its primary role likely concerns the regulation of epithelial ion transport, as suggested by the phenotype of Sgk1-null mice, which display a defect in Na( homeostasis owing to disturbed renal tubular Na+ handling. In this review we first discuss the molecular, cellular, and regulatory aspects of Sgk1 and its paralogs. We then discuss its roles in the physiology and pathophysiology of epithelial ion transport.

Regulation of the epithelial Na+ channel (ENaC) by phosphatidylinositides

The epithelial Na(+) channel (ENaC) is an end-effector of diverse cellular signaling cascades, including those with phosphatidylinositide second messengers. Recent evidence also suggests that in some instances, phospatidylinositides can directly interact with ENaC to increase channel activity by increasing channel open probability and/or membrane localization. We review here findings relevant to regulation of ENaC by phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-triphosphate (PIP(3)). Similar to its actions on other ion channels, PIP(2) is permissive for ENaC openings having a direct effect on gating. The PIP(2) binding site in ENaC involved in this regulation is most likely localized to the NH(2) terminus of beta-ENaC. PIP(3) also affects ENaC gating but, rather than being permissive, augments open probability. The PIP(3) binding site in ENaC involved in this regulation is localized to the proximal region of the COOH terminus of gamma-ENaC just following the second transmembrane domain. In complementary pathways, PIP(3) also impacts ENaC membrane levels through both direct actions on the channel and via a signaling cascade involving phosphoinositide 3-OH kinase (PI3-K) and the aldosterone-induced gene product serum and glucocorticoid-inducible kinase. The putative PIP(3) binding site in ENaC involved in direct regulation of channel membrane levels has not yet been identified.

Open probability of the epithelial sodium channel is regulated by intracellular sodium

The regulation of epithelial Na(+) channel (ENaC) activity by Na(+) was studied in Xenopus oocytes using two-electrode voltage clamp and patch-clamp recording techniques. Here we show that amiloride-sensitive Na(+) current (I(Na)) is downregulated when ENaC-expressing cells are exposed to high extracellular [Na(+)]. The reduction in macroscopic Na(+) current is accompanied by an increase in the concentration of intracellular Na(+) ([Na(+)](i)) and is only slowly reversible. At the single-channel level, incubating oocytes in high-Na(+) solution reduces open probability (P(o)) approximately twofold compared to when [Na(+)] is kept low, by increasing mean channel closed times. However, increasing P(o) by introducing a mutation in the beta-subunit (S518C) which, in the presence of [2-(trimethylammonium) ethyl] methane thiosulfonate (MTSET), locks the channel in an open state, could not alone abolish the downregulation of macroscopic current measured with exposure to high external [Na(+)]. Inhibition of the insertion of new channels into the plasma membrane using Brefeldin A revealed that surface channel lifetime is also markedly reduced under these conditions. In channels harbouring a beta-subunit mutation, R564X, associated with Liddle's syndrome, open probability in both high- and low-Na(+) conditions is significantly higher than in wild-type channels. Increasing the P(o) of these channels with an activating mutation abrogated the difference in macroscopic current observed between groups of oocytes incubated in high- and low-Na(+) conditions. These findings demonstrate that reduction of ENaC P(o) is a physiological mechanism limiting Na(+) entry when [Na(+)](i) is high.

Liddle's syndrome mutations increase Na+ transport through dual effects on epithelial Na+ channel surface expression and proteolytic cleavage

Liddle's syndrome, an inherited form of hypertension, is caused by mutations that delete or disrupt a C-terminal PY motif in the epithelial Na+ channel (ENaC). Previous work indicates that these mutations increase expression of ENaC at the cell surface by disrupting its binding to Nedd4-2, an E3 ubiquitin-protein ligase that targets ENaC for degradation. However, it remains uncertain whether this mechanism alone is responsible; increased activity of ENaC channels could also contribute to excessive Na+ transport in Liddle's syndrome. ENaC activity is controlled in part by its cleavage state; proteolytic cleavage produces channels with a high open-state probability, whereas uncleaved channels are inactive. Here, we found that Liddle's syndrome mutations have two distinct effects of ENaC surface expression, both of which contribute to increased Na+ transport. First, these mutations increased ENaC expression at the cell surface; second, they increased the fraction of ENaC at the cell surface that was cleaved (active). This disproportionate increase in cleavage was reproduced by expression of a dominant-negative Nedd4-2 or mutation of ENaC ubiquitination sites, interventions that disrupt ENaC endocytosis and lysosomal degradation. Conversely, overexpression of Nedd4-2 had the opposite effect, decreasing the fraction of cleaved ENaC at the cell surface. Thus, the data not only suggest that Nedd4-2 regulates epithelial Na+ transport in part by controlling the relative expression of cleaved and uncleaved ENaC at the cell surface but also provide a mechanism by which Liddle's syndrome mutations alter ENaC activity.

Regulation of ROMK (Kir1.1) channels: new mechanisms and aspects

This brief review attempts to provide an overview regarding recent developments in the regulation of ROMK channels. Studies performed in ROMK null mice suggest that ROMK cannot only form hometetramers such as the small-conductance (30-pS) K channels but also construct heterotetramers such as the 70-pS K channel in the thick ascending limb (TAL). The expression of ROMK channels in the plasma membrane is regulated by protein tyrosine kinase (PTK), serum and glucorticoid-induced kinase (SGK), and with-no-lysine-kinase 4. PTK is involved in mediating the effect of low K intake on ROMK channel activity. Increases in superoxide anions induced by low dietary K intake are responsible for the stimulation of PTK expression and tyrosine phosphorylation of ROMK channels. Finally, a recent study indicated that ROMK channels can be monoubiquitinated and monoubiquitination regulates the surface expression of ROMK channels.

Ubiquitylation of Ion Channels

Ubiquitylation (i.e., covalent attachment of ubiquitin moieties to proteins) of ion channels allows regulation of their activity and fate. Nedd4/Nedd4-like ubiquitin-protein ligases bind to, ubiquitylate, and modulate the internalization of several channels bearing PY motifs, whereas endoplasmic reticulum-associated degradation (involving ubiquitylation) plays an important role in the biogenesis of normal and defective channels.

Minireview: regulation of epithelial Na+ channel trafficking

The epithelial Na(+) channel (ENaC) is a pathway for Na(+) transport across epithelia, including the kidney collecting duct, lung, and distal colon. ENaC is critical for Na(+) homeostasis and blood pressure control; defects in ENaC function and regulation are responsible for inherited forms of hypertension and hypotension and may contribute to the pathogenesis of cystic fibrosis and other lung diseases. An emerging theme is that epithelial Na(+) transport is regulated in large part through trafficking mechanisms that control ENaC expression at the cell surface. ENaC trafficking is regulated at multiple steps. Delivery of channels to the cell surface is regulated by aldosterone (and corticosteroids) and vasopressin, which increase ENaC synthesis and exocytosis, respectively. Conversely, endocytosis and degradation is controlled by a sequence located in the C terminus of alpha, beta, and gammaENaC (PPPXYXXL). This sequence functions as an endocytosis motif and as a binding site for Nedd4-2, an E3 ubiquitin protein ligase that targets ENaC for degradation. Mutations that delete or disrupt this motif cause accumulation of channels at the cell surface, resulting in Liddle's syndrome, an inherited form of hypertension. Nedd4-2 is a central convergence point for ENaC regulation by aldosterone and vasopressin; both induce phosphorylation of a common set of three Nedd4-2 residues, which blocks Nedd4-2 binding to ENaC. Thus, aldosterone and vasopressin regulate epithelial Na(+) transport in part by altering ENaC trafficking to and from the cell surface.

Regulation of Epithelial Na+ Channel Activity by Conserved Serine/Threonine Switches within Sorting Signals

The PY and YXXphi motifs are canonical sorting signals involved in trafficking. Nedd4-2 and the mu(2)-subunit of the AP-2 complex target these motifs to facilitate internalization. Epithelial Na(+) channel (ENaC) subunits contain both motifs in their cytosolic COOH termini where they overlap ((S/T)PPPXYX(S/T)phi). Just preceding the PY and embedded within the YXXphi motifs are conserved serine/threonine. We test here whether these conserved Ser/Thr modulate ENaC activity by influencing the function of the internalization domains. We find that co-expression of dominant-negative dynamin (K44A) with ENaC increases channel activity. Conversely, co-expression of Nedd4-2 and epsin with ENaC decrease activity. Alanine substitution of the conserved Thr(628) preceding the PY motif in gamma-mENaC had no effect on basal activity. Channels with this mutation, however, responded to K44A and epsin but not Nedd4-2. Similarly, mutation of the proline repeat in the PY motif of gamma-mENaC disrupted only Nedd4-2 regulation having no effect on regulation by K44A and epsin. Alanine substitution of the conserved Thr within the YXX motif of gamma-mENaC (T635A) increased basal activity. Channels containing this mutation responded to Nedd4-2 but not K44A and epsin. Channels containing the T635(D/E) substitution in gamma-mENaC did not have increased basal activity and responded to Nedd4-2 but not K44A. The double mutant T628A,T635A did not respond to Nedd4-2 or K44A. Mutation of Thr(628) and Thr(635) also disrupted ENaC precipitation with the mu(2)-subunit of the AP-2 complex. Moreover, the YXXphi motif, independent of the PY motif, was sufficient to target degradation with T635A disrupting this effect. These results demonstrate that the overlapping PY and YXXphi motifs in ENaC are, in some instances, capable of independent function and that the Ser/Thr just preceding and within these domains impact this function.

Impact of Nedd4 proteins and serum and glucocorticoid-induced kinases on epithelial Na+ transport in the distal nephron

The precise control of BP occurs via Na(+) homeostasis and involves the precise regulation of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. This has been corroborated by the linkage of mutations in the genes encoding ENaC subunits and Liddle's syndrome, a heritable form of human hypertension. Mapping of these mutations on ENaC indicated that inactivation of PY motifs is responsible and leads to the proposition that the channel interacts via its PY motifs with the WW domains of the Nedd4/Nedd4-like ubiquitin-protein ligase family. It is now well established that the cell surface expression of ENaC is controlled via ubiquitylation by this protein family and that this ubiquitylation is regulated by the aldosterone-induced protein serum and glucocorticoid induced kinase 1.

Functional evaluation of Dent's disease-causing mutations: implications for ClC-5 channel trafficking and internalization

ClC-5 is a member of the ClC family of voltage-gated chloride channels. Loss-of-function mutations of its corresponding gene (CLCN5) cause Dent's disease, an X-linked kidney disorder, characterized by low-molecular weight proteinuria, hypercalciuria, nephrocalcinosis/nephrolithiasis, and progressive renal failure. Here, we examined the effect of different mutations on function and cellular trafficking of the recombinant protein. Mutant CLCN5 cDNAs were generated by site directed mutagenesis for two premature stop codon variants (R347X and M517IfsX528), and several missense mutations (C221R, L324R, G462 V, and R516 W). We also tested L521R (instead of L521RfsX526 observed) and mutants G506E and R648X (previously reported by others). After heterologous expression in Xenopus oocytes, ClC-5 channel activity and surface expression were determined by two-electrode voltage-clamp analysis and ClC-5 surface ELISA, respectively. Except for the R516 W and R648X variants, none of the mutated proteins induced functional chloride currents or reached the plasma membrane. This is readily understandable for the truncation mutations. Yet, the tested missense mutations are distributed over different transmembrane regions, implying that correct channel structure and orientation in the membrane is not only a prerequisite for proper ClC-5 function but also for Golgi exit. Interestingly, the R648X mutant although functionally compromised, displayed a significant increase in surface expression. This finding might be explained by the deletion of a ClC-5 carboxy-terminal PY-like internalization signal, which in turn impairs channel removal from the membrane. Our observations further imply that recruitment of ClC-5 to alternative routes (plasma membrane or early endosomes) in the trans-Golgi network is mediated via different signal sequences.

Overexpression of the epithelial Na+ channel gamma subunit in collecting duct cells: interactions of Liddle's mutations and steroids on expression and function

The epithelial Na(+) channel (ENaC) has three subunits; the expression of each can be regulated. Liddle's syndrome is caused by an activating mutation in the C terminus of either the beta or gamma subunit. We used a doxycycline-regulated adenovirus system to express varying levels of human gammaENaC in renal collecting duct (M1 cell) monolayers. Increasing levels of wild type human gamma ENaC (gammahENaC) produced a 2.5-fold enhancement of Na(+) transport. Expression of a truncated C terminus produced less protein than wild type or a gammaY627A missense mutation. However, either of these mutations produced a approximately 4-fold increase in Na(+) transport despite the different levels of protein expression. Unexpectedly, overexpression of a marginally detectable amount of gammahENaC was sufficient to produce a full increase in Na(+) transport; a further increase in protein expression produced no further increase in Na(+) transport. Steroid treatment increased Na(+) transport to a similar absolute magnitude in control monolayers and in monolayers expressing all types of gammahENaC. Withdrawal of steroids after 24 h produced a decline in Na(+) transport over 8 h in monolayers expressing wild type but not the Liddle's mutation. Using treatment with brefeldin A to estimate the disappearance rate constants, we found progressively slower disappearance rates in monolayers overexpressing gammahENaC and the Liddle's mutant. Calculated insertion rates were slower for the Liddle's mutant than for wild type despite increasing rates of Na(+) transport. These results raise questions regarding previously held assumptions about the behavior of ENaC.

Epithelial Sodium Channel Inhibition by AMP-activated Protein Kinase in Oocytes and Polarized Renal Epithelial Cells

The epithelial Na(+) channel (ENaC) regulates epithelial salt and water reabsorption, processes that require significant expenditure of cellular energy. To test whether the ubiquitous metabolic sensor AMP-activated kinase (AMPK) regulates ENaC, we examined the effects of AMPK activation on amiloride-sensitive currents in Xenopus oocytes and polarized mouse collecting duct mpkCCD(c14) cells. Microinjection of oocytes expressing mouse ENaC (mENaC) with either active AMPK protein or an AMPK activator inhibited mENaC currents relative to controls as measured by two-electrode voltage-clamp studies. Similarly, pharmacological AMPK activation or overexpression of an activating AMPK mutant in mpkCCD(c14) cells inhibited amiloride-sensitive short circuit currents. Expression of a degenerin mutant beta-mENaC subunit (S518K) along with wild type alpha and gamma increased the channel open probability (P(o)) to approximately 1. However, AMPK activation inhibited currents similarly with expression of either degenerin mutant or wild type mENaC. Single channel recordings under these conditions demonstrated that neither P(o) nor channel conductance was affected by AMPK activation. Moreover, expression of a Liddle's syndrome-type beta-mENaC mutant (Y618A) greatly enhanced ENaC whole cell currents relative to wild type ENaC controls and prevented AMPK-dependent inhibition. These findings indicate that AMPK-dependent ENaC inhibition is mediated through a decrease in the number of active channels at the plasma membrane (N), presumably through enhanced Nedd4-2-dependent ENaC endocytosis. The AMPK-ENaC interaction appears to be indirect; AMPK did not bind ENaC in cells, as assessed by in vivo pull-down assays, nor did it phosphorylate ENaC in vitro. In summary, these results suggest a novel mechanism for coupling ENaC activity and renal Na(+) handling to cellular metabolic status through AMPK, which may help prevent cellular Na(+) loading under hypoxic or ischemic conditions.

14-3-3 proteins modulate the expression of epithelial Na+ channels by phosphorylation-dependent interaction with Nedd4-2 ubiquitin ligase

The ubiquitin E3 protein ligase Nedd4-2 is a physiological regulator of the epithelial sodium channel ENaC, which is essential for transepithelial Na+ transport and is linked to Liddle's syndrome, an autosomal dominant disorder of human salt-sensitive hypertension. Nedd4-2 function is negatively regulated by phosphorylation via a serum- and glucocorticoid-inducible protein kinase (Sgk1), which serves as a mechanism to inhibit the ubiquitination-dependent degradation of ENaC. We report here that 14-3-3 proteins participate in this regulatory process through a direct interaction with a phosphorylated form of human Nedd4-2 (a human gene product of KIAA0439, termed hNedd4-2). The interaction is dependent on Sgk1-catalyzed phosphorylation of hNedd4-2 at Ser-468. We found that this interaction preserved the activity of the Sgk1-stimulated ENaC-dependent Na+ current while disrupting the interaction decreased ENaC density on the Xenopus laevis oocytes surface possibly by enhancing Nedd4-2-mediated ubiquitination that leads to ENaC degradation. Our findings suggest that 14-3-3 proteins modulate the cell surface density of ENaC cooperatively with Sgk1 kinase by maintaining hNedd4-2 in an inactive phosphorylated state.

Common variants of the beta and gamma subunits of the epithelial sodium channel and their relation to plasma renin and aldosterone levels in essential hypertension

Background

Rare mutations of the epithelial sodium channel (ENaC) result in the monogenic hypertension form of Liddle's syndrome. We decided to screen for common variants in the ENaC βand γ subunits in patients with essential hypertension and to relate their occurrence to the activity of circulating renin-angiotensin-aldosterone system.

Methods

Initially, DNA samples from 27 patients with low renin/low aldosterone hypertension were examined. The DNA variants were subsequently screened for in 347 patients with treatment-resistant hypertension, 175 male subjects with documented long-lasting normotension and 301 healthy Plasma renin and aldosterone levels were measured under baseline conditions and during postural and captopril challenge tests.

Results

Two commonly occurring βENaC variants (G589S and a novel intronic i12-17CT substitution) and one novel γENaC variant (V546I) were detected. One of these variants occurred in a heterozygous form in 32 patients, a prevalence (9.2%) significantly higher than that in normotensive males (2.9%, p = 0.007) and blood donors (3.0%, p = 0.001). βENaC i12-17CT was significantly more prevalent in the hypertension group than in the two control groups combined (4.6% vs. 1.1%, p = 0.001). When expressed in Xenopus oocytes, neither of the two ENaC amino acid-changing variants showed a significant difference in activity compared with ENaC wild-type. No direct evidence for a mRNA splicing defect could be obtained for the βENaC intronic variant. The ratio of daily urinary potassium excretion to upright and mean (of supine and upright values) plasma renin activity was higher in variant allele carriers than in non-carriers (p = 0.034 and p = 0.048).

Conclusions

At least 9% of Finnish patients with hypertension admitted to a specialized center carry genetic variants of β and γENaC, a three times higher prevalence than in the normotensive individuals or in random healthy controls. Patients with the variant alleles showed an increased urinary potassium excretion rate in relation to their renin levels.

Nedd4-2 phosphorylation induces serum and glucocorticoid-regulated kinase (SGK) ubiquitination and degradation

Serum and glucocorticoid-regulated kinase (SGK) plays a key role in the regulation of epithelial Na+ transport. SGK phosphorylates Nedd4-2, an E3 ubiquitin-protein ligase that targets the epithelial Na+ channel (ENaC) for degradation. Phosphorylation increases Na+ transport by reducing Nedd4-2 binding to ENaC, which increases ENaC expression at the cell surface. Thus, SGK expression must be tightly controlled to maintain Na+ homeostasis. This occurs in part by regulation of SGK transcription; a variety of signals including steroid hormones (aldosterone and glucocorticoids) increase SGK levels by inducing transcription of SGK. However, SGK has a short half-life, suggesting that SGK levels might also be controlled by regulation of SGK degradation. Here we found that SGK degradation is mediated in part by Nedd4-2. Consistent with this model, overexpression of Nedd4-2 decreased steady-state levels of SGK in a dose-dependent manner by increasing SGK ubiquitination and degradation in the 26S proteasome. Conversely, silencing of Nedd4-2 by RNA interference stabilized SGK. Nedd4-2 phosphorylation potentiates SGK degradation; degradation was reduced by Nedd4-2 and SGK mutations that disrupt phosphorylation or by inhibition of SGK kinase activity. Together with previous work, the data support a model in which SGK and Nedd4-2 regulate one another in a reciprocal manner. SGK phosphorylates Nedd4-2, which reduces Nedd4-2 binding and inhibition of ENaC. Conversely, phosphorylation increases Nedd4-2-mediated degradation of SGK. Thus, by phosphorylating Nedd4-2, SGK induces its own degradation. This feedback inhibition may fine-tune the regulation of epithelial Na+ absorption.

Arachidonic acid inhibits epithelial Na channel via cytochrome P450 (CYP) epoxygenase-dependent metabolic pathways

We used the patch-clamp technique to study the effect of arachidonic acid (AA) on epithelial Na channels (ENaC) in the rat cortical collecting duct (CCD). Application of 10 μM AA decreased the ENaC activity defined by NPo from 1.0 to 0.1. The dose–response curve of the AA effect on ENaC shows that 2 μM AA inhibited the ENaC activity by 50%. The effect of AA on ENaC is specific because neither 5,8,11,14-eicosatetraynoic acid (ETYA), a nonmetabolized analogue of AA, nor 11,14,17-eicosatrienoic acid mimicked the inhibitory effect of AA on ENaC. Moreover, inhibition of either cyclooxygenase (COX) with indomethacin or cytochrome P450 (CYP) ω-hydroxylation with N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) failed to abolish the effect of AA on ENaC. In contrast, the inhibitory effect of AA on ENaC was absent in the presence of N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide (MS-PPOH), an agent that inhibits CYP-epoxygenase activity. The notion that the inhibitory effect of AA is mediated by CYP-epoxygenase–dependent metabolites is also supported by the observation that application of 200 nM 11,12-epoxyeicosatrienoic acid (EET) inhibited ENaC in the CCD. In contrast, addition of 5,6-, 8,9-, or 14,15-EET failed to decrease ENaC activity. Also, application of 11,12-EET can still reduce ENaC activity in the presence of MS-PPOH, suggesting that 11,12-EET is a mediator for the AA-induced inhibition of ENaC. Furthermore, gas chromatography mass spectrometry analysis detected the presence of 11,12-EET in the CCD and CYP2C23 is expressed in the principal cells of the CCD. We conclude that AA inhibits ENaC activity in the CCD and that the effect of AA is mediated by a CYP-epoxygenase–dependent metabolite, 11,12-EET.

Vasopressin-stimulated CFTR Cl- currents are increased in the renal collecting duct cells of a mouse model of Liddle's syndrome

Liddle's syndrome is a genetic form of hypertension linked to Na(+) retention caused by activating mutations in the COOH terminus of the beta or gamma subunit of the epithelial sodium channel (ENaC). In this study, we used the short-circuit current (I(sc)) method to investigate the effects of deamino-8-d-arginine vasopressin (dDAVP) on Na(+) and Cl(-) fluxes in primary cultures of cortical collecting ducts (CCDs) microdissected from the kidneys of mice with Liddle's syndrome carrying a stop codon mutation, corresponding to the beta-ENaC R(566) stop mutation (L) found in the original pedigree. Compared to wild-type (+/+) CCD cells, untreated L/+ and L/L CCD cells exhibited 2.7- and 4.2-fold increases, respectively, in amiloride-sensitive (Ams) I(sc), reflecting ENaC-dependent Na(+) absorption. Short-term incubation with dDAVP caused a rapid and significant increase (approximately 2-fold) in Ams I(sc) in +/+, but not in L/+ or L/L CCD cells. In sharp contrast, dDAVP induced a greater increase in 5-nitro-2-(3-phenylpropamino)benzoate (NPPB)-inhibited apical Cl(-) currents in amiloride-treated L/L and L/+ cells than in their +/+ counterparts. I(sc) recordings performed under apical ion substituted conditions revealed that the dDAVP-stimulated apical secretion of Cl(-), which was absent in cultured CCDs lacking CFTR, was 1.8-fold greater in L/+ and 3.7-fold greater in L/L CCD cells than in their +/+ CCD counterparts. After the basal membrane had been permeabilized with nystatin and a basal-to-apical Cl(-) gradient had been imposed, dDAVP also stimulated larger Cl(-) currents across L/L and L/+ CCD layers than +/+ CCD layers. These findings demonstrate that vasopressin stimulates greater apical CFTR Cl(-) conductance in the renal CCD cells of mice with Liddle's syndrome than in wild-type mice. This effect could contribute to the enhanced NaCl reabsorption observed in the distal nephron of patients with Liddle's syndrome.

Regulation of glucose transporter SGLT1 by ubiquitin ligase Nedd4-2 and kinases SGK1, SGK3, and PKB

OBJECTIVE

Serum- and glucocorticoid-inducible kinase 1 (SGK1) inhibits the ubiquitin ligase neuronal cell expressed developmentally downregulated 4-2 (Nedd4-2), which retards the retrieval of the epithelial Na+ channel ENaC. Accordingly, SGK1 enhances ENaC abundance in the cell membrane. The significance of this effect is shown by an association of an E8CC/CT;I6CC polymorphism in the SGK1 gene with increased blood pressure. However, strong expression of SGK1 in enterocytes not expressing ENaC points to further functions of SGK1. This study was performed to test for regulation of Na+-coupled glucose transporter 1 (SGLT1) by Nedd4-2, SGK1, and/or the related kinases SGK3 and PKB. Additional studies searched for an association of the SGK1 gene with BMI.

RESEARCH METHODS AND PROCEDURES

mRNA encoding SGLT1, wild-type Nedd4-2, inactive (C938S)Nedd4-2, wild type SGK1, constitutively active (S422D)SGK1 or inactive (K127N)SGK1, wild-type SGK3, and constitutively active (T308DS473D)PKB or inactive (T308AS473A)PKB were injected into Xenopus oocytes, and glucose transport was quantified from glucose-induced current (I(glc)). BMI was determined in individuals with or without the E8CC/CT;I6CC polymorphism.

RESULTS

I(glc) was significantly decreased by coexpression of Nedd4-2 but not of (C938S)Nedd4-2. Coexpression of SGK1, (S422D)SGK1, SGK3, or (T308DS473D)PKB, but not of (K127N)SGK1 or (T308AS473A)PKB, enhanced I(glc) and reversed the effect of Nedd4-2. SGK1 and SGK3 phosphorylated Nedd4-2. Deletion of the SGK/PKB phosphorylation sites in Nedd4-2 blunted the kinase effects. BMI was significantly (p < 0.008) greater in individuals with the E8CC/CT;I6CC polymorphism than in individuals without.

DISCUSSION

Overactivity of SGK1 may lead not only to excessive ENaC activity and hypertension but also to enhanced SGLT1 activity and obesity.

A genética das síndromes hipertensivas endócrinas

A hipertensão arterial sistêmica está associada a altos índices de morbi-mortalidade e constitui um dos grandes problemas de saúde pública no mundo, dada sua alta prevalência e baixa porcentagem de controle com os tratamentos adotados. Este último problema é justificado, pelo menos em parte, porque ainda utilizamos medidas empíricas para o tratamento, ao invés de uma abordagem específica para cada caso. Os determinantes primários da hipertensão permanecem desconhecidos na maioria dos pacientes, ao qual damos o nome genérico de hipertensão essencial ou primária. Estas limitações estão alicerçadas pelo conceito de que a hipertensão é uma doença complexa, poligênica em sua maioria e com direta interação com fatores ambientais, tais como dieta, ingestão de sal e obesidade, entre outras. A utilização de técnicas de biologia molecular tem trazido uma enorme contribuição para a compreensão de fenômenos biológicos complexos. Sabe-se que em uma minoria dos casos a hipertensão arterial ocorre pela presença de mutações específicas, ditas formas mendelianas, que resultam em ganho de função de transportadores do néfron distal, bem como de vários componentes do sistema renina-angiotensina-aldosterona com conseqüente retenção excessiva de sal. De interesse particular na endocrinologia, estas síndromes podem ser divididas em aumento na produção ou na atividade dos mineralocorticóides e estarão expostas nesta revisão.

Identification of new partners of the epithelial sodium channel alpha subunit

A fine regulation of the amiloride-sensitive Epithelial Sodium Channel (ENaC), made of alpha, beta and gamma subunits, is crucial for maintenance of Na+ balance and blood pressure. Both beta- and gamma-ENaC participate in negative regulation by interacting with Nedd4-2, an E3 ubiquitin-ligase. Disruption of this interaction results in increased ENaC activity (Liddle syndrome). By two-hybrid screenings, we identified new potential partners of alpha-ENaC: WWP1 (E3 ubiquitin-ligase protein), UBC9 and TSG101 (E2 ubiquitin/SUMO-conjugating enzymes) and confirmed these interactions in GST pull-down assays. All these partners are implicated in protein trafficking and could be involved in the regulation of ENaC activity.

cAMP and serum and glucocorticoid-inducible kinase (SGK) regulate the epithelial Na(+) channel through convergent phosphorylation of Nedd4-2

The epithelial Na(+) channel (ENaC) functions as a pathway for epithelial Na(+) transport, contributing to Na(+) homeostasis and blood pressure control. Vasopressin increases ENaC expression at the cell surface through a pathway that includes cAMP and cAMP-dependent protein kinase (PKA), but the mechanisms that link PKA to ENaC are unknown. Here we found that cAMP regulates Na(+) transport in part by inhibiting the function of Nedd4-2, an E3 ubiquitin-protein ligase that targets ENaC for degradation. Consistent with this model, we found that cAMP inhibited Nedd4-2 by decreasing its binding to ENaC. Moreover, decreased Nedd4-2 expression (RNA interference) or overexpression of a dominant negative Nedd4-2 construct disrupted ENaC regulation by cAMP. Nedd4-2 was a substrate for phosphorylation by PKA in vitro and in cells; three Nedd4-2 residues were phosphorylated by PKA and were required for cAMP to inhibit Nedd4-2 (relative functional importance Ser-327 > Ser-221 > Thr-246). Previous work found that these residues are also phosphorylated by serum and glucocorticoid-inducible kinase (SGK), a downstream mediator by which aldosterone regulates epithelial Na(+) transport. Consistent with a functional interaction between these pathways, overexpression of SGK blunted ENaC stimulation by cAMP, whereas inhibition of SGK increased stimulation. Conversely, cAMP agonists decreased ENaC stimulation by SGK. The data suggest that cAMP regulates ENaC in part by phosphorylation and inhibition of Nedd4-2. Moreover, Nedd4-2 is a central convergence point for kinase regulation of Na(+) transport.

The kinase Grk2 regulates Nedd4/Nedd4-2-dependent control of epithelial Na+ channels

Epithelial Na(+) channels mediate the transport of Na across epithelia in the kidney, gut, and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4 and Nedd4-2, with loss of this inhibition leading to hypertension. Here, we report that these channels are maintained in the active state by the G protein-coupled receptor kinase, Grk2, which has been previously implicated in the development of essential hypertension. We also show that Grk2 phosphorylates the C terminus of the channel beta subunit and renders the channels insensitive to inhibition by Nedd4-2. This mechanism has not been previously reported to regulate epithelial Na(+) channels and provides a potential explanation for the observed association of Grk2 overactivity with hypertension. Here, we report a G protein-coupled receptor kinase regulating a membrane protein other than a receptor and provide a paradigm for understanding how the interaction between membrane proteins and ubiquitin protein ligases is controlled.

Functional Polymorphism in the Carboxyl Terminus of the α-Subunit of the Human Epithelial Sodium Channel

A common human epithelial sodium channel (ENaC) polymorphism, alphaT663A, is present in the cytoplasmic C terminus of the alpha-subunit, although it is unclear whether this polymorphism segregates with blood pressure. We examined whether this polymorphism was associated with differences in functional Na(+) channel expression. Whole cell amiloride-sensitive currents in Xenopus oocytes expressing wild type channels (alphaT663betagamma) were significantly approximately 1.3-2.0-fold higher than currents measured in oocytes expressing channels with an Ala, Gly or Leu, or Lys at position alpha663. In contrast, differences in functional human ENaC expression were not observed with oocytes expressing channels having Thr (wild type), Ser, or Asp at this position. The surface expression of channels, measured using an epitope-tagged beta-subunit, was significantly reduced in oocytes expressing alphaT663Abetagamma when compared with oocytes expressing alphaT663betagamma. The corresponding polymorphism was generated in the mouse alpha-subunit (malphaA692T) and was not associated with differences in functional alphabetagamma-mouse ENaC expression. The polymorphism is present in a region that is not well conserved between human and mouse. We generated a mouse/human chimera by replacement of the distal C terminus of the mouse alpha-subunit with the distal C terminus of the human alpha-subunit. Co-expression of this m(1-678)/h(650-669)T663A chimera with mouse betagamma led to a significant reduction in whole cell Na(+) currents and surface expression when compared with m(1-678)/h(650-669)T663-mbetagamma. Our results suggest that halphaT663A is a functional polymorphism that affects human ENaC surface expression.