Defective regulation of the epithelial Na+ channel by Nedd4 in Liddle's syndrome

Liddle syndrome: genetics and mechanisms of Na+ channel defects

Our current understanding of Na+ transport defects has been greatly expanded over the last several years and has provided new insights into unusual clinical syndromes resulting from mutations of specific ion transporters. These genetic disorders affect Na+ balance, with both Na+ retaining and Na+ wasting conditions being the consequence. A major focus of these studies has been the epithelial sodium channel (ENaC), which can be directly affected by mutations (eg, Liddle syndrome, autosomal recessive pseudohypoaldosteronism, type I) or by changes in the response to (autosomal recessive pseudohypoaldosteronism, type I), or production of mineralocorticoids (apparent mineralocorticoid excess syndrome, glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined syndromes in which ENaC activity is "dysregulated" with subsequent development of disorders of systemic blood pressure that can be attributed to a primary renal mechanisms. The focus of the current review is on Liddle syndrome ("pseudoaldosteronism").

Regulation of epithelial sodium channel activity through a region of the carboxyl terminus of the alpha -subunit. Evidence for intracellular kinase-mediated reactions

The epithelial sodium channel (ENaC) is a heteromultimer composed of three subunits, each having two membrane-spanning domains with intracellular amino and carboxyl termini. Several hormones and proteins regulate channel activity, but the molecular nature of this regulation is unknown. We conducted experiments to determine a possible new site within the carboxyl terminus of the alpha-subunit involved in enhanced channel activity through endogenous kinases. When an alpha-subunit that was truncated to remove a PY motif was expressed in Xenopus oocytes with wild type human beta- and gamma-ENaC subunits, channel activity was greatly enhanced. The removal of the entire intracellular carboxyl terminus of the alpha-subunit eliminated this enhanced basal activity. Using several point mutations, we localized this site to two amino acid residues (Pro(595)-Gly(596)) near the second membrane-spanning domain. The nonspecific kinase inhibitor staurosporine inhibits basal channel activity of wild type ENaC but was ineffective in inhibiting channels mutated at this site. The major effect of these mutations was not on channel kinetics but was largely, if not entirely, on the number of active channels on the cell surface. This region is potentially important in effecting kinase-mediated increases in ENaC activity.

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.

Distinct characteristics of two human Nedd4 proteins with respect to epithelial Na(+) channel regulation

The epithelial Na(+) channel (ENaC) is regulated via PY motif-WW domain interaction by the mouse (m) ubiquitin-protein ligase mNedd4-2 but not by its close relative mNedd4-1. Whereas mNedd4-1 is composed of one C2, three WW, and one HECT domain, mNedd4-2 comprises four WW domains and one HECT domain. Both proteins have human (h) homologs, hNedd4-1 and hNedd4-2; however, both of them include four WW domains. Therefore, we characterized hNedd4-1 and hNedd4-2 in Xenopus laevis oocytes with respect to ENaC binding and interaction. We found that hNedd4-2 binds to and abrogates ENaC activity, whereas hNedd4-1 does not coimmunoprecipitate with ENaC and has only modest effects on ENaC activity. Structure-function studies revealed that the C2 domain of hNedd4-1 prevents this protein from downregulating ENaC and that WW domains 3 and 4, involved in interaction with ENaC, do not by themselves provide specificity for ENaC recognition. Taken together, our data demonstrate that hNedd4-2 inhibits ENaC, implying that this protein is a modulator of salt homeostasis, whereas hNedd4-1 is not primarily involved in ENaC regulation.

Trafficking and cell surface stability of ENaC

The epithelial Na(+) channel (ENaC) plays a key role in the regulation of Na(+) and water absorption in several epithelia, including those of the distal nephron, distal colon, and lung. Accordingly, mutations in ENaC leading to reduced or increased channel activity cause human diseases such as pseudohypoaldosteronism type I or Liddle's syndrome, respectively. The gain of ENaC function in Liddle's syndrome is associated with increased activity and stability of the channel at the plasma membrane. Thus understanding the regulation of channel processing and trafficking to and stability at the cell surface is of fundamental importance. This review describes some of the recent advances in our understanding of ENaC trafficking, including the role of glycosylation, ENaC solubility in nonionic detergent, targeting signal(s) and hormones. It also describes the regulation of ENaC stability at the cell surface and the roles of the ubiquitin ligase Nedd4 (and ubiquitination) and clathrin-mediated endocytosis in that regulation.

Liddle's syndrome: a novel mouse Nedd4 isoform regulates the activity of the epithelial Na(+) channel

The epithelial Na(+) channel (ENaC), which plays an essential role in renal Na(+) handling, is composed of three subunits (alpha beta gamma), each containing a conserved PY motif at the C terminus. In Liddle's syndrome, an inherited form of salt-sensitive hypertension, the PY motifs of either beta or gamma ENaC are deleted or modified. We have recently shown that a ubiquitin-protein ligase Nedd4 binds via its WW domains to these PY motifs on ENaC, that ENaC is regulated by ubiquitination, and that Xenopus laevis Nedd4 (xNedd4) controls the cell surface pool of ENaC when coexpressed in Xenopus oocytes. Interestingly, Na(+) transporting cells, derived from mouse cortical collecting duct, express two different Nedd4 isoforms, which we have termed mNedd4-1 and mNedd4-2. Only mNedd4-2, which is orthologous to xNedd4, but not mNedd4-1, is able to regulate ENaC activity, and this property correlates with the capability to bind to the ENaC complex. Hence, Nedd4-2 may be encoded by a novel susceptibility gene for arterial hypertension.

Nuclear import/export of hRPF1/Nedd4 regulates the ubiquitin-dependent degradation of its nuclear substrates

The ubiquitin-protein ligase (E3), hRPF1/Nedd4, is a component of the ubiquitin-proteasome pathway responsible for substrate recognition and specificity. Although previously characterized as a regulator of the stability of cytoplasmic proteins, hRPF1/Nedd4 has also been suggested to have a role in the nucleus. However, in light of the cytoplasmic localization of hRPF1/Nedd4, it is unclear whether bona fide nuclear substrates of hRPF1/Nedd4 exist, and if so, what mechanism may allow a cytoplasmic ubiquitin ligase to manifest nuclear activity. Our search for nuclear substrates led to the identification of the human proline-rich transcript, brain-expressed (hPRTB) protein, the ubiquitination and degradation of which is regulated by hRPF1/Nedd4. Interestingly, hPRTB colocalizes with the splicing factor SC35 in nuclear speckles. Finally, we demonstrate that hRPF1/Nedd4 is indeed capable of entering the nucleus; however, the presence of a functional Rev-like nuclear export sequence in hRPF1/Nedd4 ensures a predominant cytoplasmic localization. Cumulatively, these findings highlight a nuclear role for the ubiquitin ligase hRPF1/Nedd4 and underscore cytoplasmic/nuclear localization as an important regulatory component of hRPF1/Nedd4-substrate recognition.

Genetic forms of human hypertension

Our basic understanding of sodium mechanisms provides unique insights into epithelial transport processes, and unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect sodium balance, with both sodium-retaining and sodium-wasting conditions being the consequence. A major focus of such studies has been the epithelial sodium channel, which can be activated by mutations in the channel subunits or mineralocorticoid receptor, and changes in the response to or production of mineralocorticoids. As a result, there are now clearly defined Mendelian syndromes in which epithelial sodium channel activity is 'dysregulated', with the subsequent development of systemic hypertension with suppressed plasma renin activity that can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension, and more clearly define the interactions of dietary constituents such as sodium and potassium in the regulation of blood pressure.

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+.

Multiple WW domains, but not the C2 domain, are required for inhibition of the epithelial Na+ channel by human Nedd4

The epithelial Na+ channel (ENaC) absorbs Na+ across the apical membrane of epithelia. The activity of ENaC is controlled by its interaction with Nedd4; mutations that disrupt this interaction increase Na+ absorption, causing an inherited form of hypertension (Liddle's syndrome). Nedd4 contains an N-terminal C2 domain, a C-terminal ubiquitin ligase domain, and multiple WW domains. The C2 domain is thought to be involved in the Ca2+-dependent localization of Nedd4 at the cell surface. However, we found that the C2 domain was not required for human Nedd4 (hNedd4) to inhibit ENaC in both Xenopus oocytes and Fischer rat thyroid epithelia. Rather, hNedd4 lacking the C2 domain inhibited ENaC more potently than wild-type hNedd4. Earlier work indicated that the WW domains bind to PY motifs in the C terminus of ENaC. However, it is not known which WW domains mediate this interaction. Glutathione S-transferase-fusion proteins of WW domains 2-4 each bound to alpha, beta, and gammaENaC in vitro. The interactions were abolished by mutation of two residues. WW domain 3 (but not the other WW domains) was both necessary and sufficient for the binding of hNedd4 to alphaENaC. WW domain 3 was also required for the inhibition of ENaC by hNedd4; inhibition was nearly abolished when WW domain 3 was mutated. However, the interaction between ENaC and WW domain 3 alone was not sufficient for inhibition. Moreover, inhibition was decreased by mutation of WW domain 2 or WW domain 4. Thus, WW domains 2-4 each participate in the functional interaction between hNedd4 and ENaC in intact cells.

Roles of the C termini of alpha -, beta -, and gamma -subunits of epithelial Na+ channels (ENaC) in regulating ENaC and mediating its inhibition by cytosolic Na+

The amiloride-sensitive epithelial Na(+) channels (ENaC) in the intralobular duct cells of mouse mandibular glands are inhibited by the ubiquitin-protein ligase, Nedd4, which is activated by increased intracellular Na(+). In this study we have used whole-cell patch clamp methods in mouse mandibular duct cells to investigate the role of the C termini of the alpha-, beta-, and gamma-subunits of ENaC in mediating this inhibition. We found that peptides corresponding to the C termini of the beta- and gamma-subunits, but not the alpha-subunit, inhibited the activity of the Na(+) channels. This mechanism did not involve Nedd4 and probably resulted from the exogenous C termini interfering competitively with the protein-protein interactions that keep the channels active. In the case of the C terminus of mouse beta-ENaC, the interacting motif included betaSer(631), betaAsp(632), and betaSer(633). In the C terminus of mouse gamma-ENaC, it included gammaSer(640). Once these motifs were deleted, we were able to use the C termini of beta- and gamma-ENaC to prevent Nedd4-mediated down-regulation of Na(+) channel activity. The C terminus of the alpha-subunit, on the contrary, did not prevent Nedd4-mediated inhibition of the Na(+) channels. We conclude that mouse Nedd4 interacts with the beta- and gamma-subunits of ENaC.

Enac degradation in A6 cells by the ubiquitin-proteosome proteolytic pathway

Amiloride-sensitive epithelial Na(+) channels (ENaC) are responsible for trans-epithelial Na(+) transport in the kidney, lung, and colon. The channel consists of three subunits (alpha, beta, gamma) each containing a proline rich region (PPXY) in their carboxyl-terminal end. Mutations in this PPXY domain cause Liddle's syndrome, an autosomal dominant, salt-sensitive hypertension, by preventing the channel's interactions with the ubiquitin ligase Neural precursor cell-expressed developmentally down-regulated protein (Nedd4). It is postulated that this results in defective endocytosis and lysosomal degradation of ENaC leading to an increase in ENaC activity. To show the pathway that degrades ENaC in epithelial cells that express functioning ENaC channels, we used inhibitors of the proteosome and measured sodium channel activity. We found that the inhibitor, MG-132, increases amiloride-sensitive trans-epithelial current in Xenopus distal nephron A6 cells. There also is an increase of total cellular as well as membrane-associated ENaC subunit molecules by Western blotting. MG-132-treated cells also have increased channel density in patch clamp experiments. Inhibitors of lysosomal function did not reproduce these findings. Our results suggest that in native renal cells the proteosomal pathway is an important regulator of ENaC function.

An internalization signal in ClC-5, an endosomal Cl-channel mutated in dent's disease

The ClC-5 chloride channel resides mainly in vesicles of the endocytotic pathway and contributes to their acidification. Its disruption in mice entails a broad defect in renal endocytosis and causes secondary changes in calciotropic hormone levels. Inactivating mutations in Dent's disease lead to proteinuria and kidney stones. Possibly by recycling, a small fraction of ClC-5 also reaches the plasma membrane. Here we identify a carboxyl-terminal internalization motif in ClC-5. It resembles the PY motif, which is crucial for the endocytosis and degradation of epithelial Na(+) channels. Mutating this motif increases surface expression and currents about 2-fold. This is probably because of interactions with WW domains, because dominant negative mutants of the ubiquitin-protein ligase WWP2 increased surface expression and currents of ClC-5 only when its PY motif was intact. Stimulating endocytosis by expressing rab5 or its GTPase-deficient Q79L mutant decreased WT ClC-5 currents but did not affect channels with mutated motifs. Similarly, decreasing endocytosis by expressing the inactive S34N mutant of rab5 increased ClC-5 currents only if its PY-like motif was intact. Thus, the endocytosis of ClC-5, which itself is crucial for the endocytosis of other proteins, depends on the interaction of a carboxyl-terminal internalization signal with ubiquitin-protein ligases containing WW domains.

The Nedd4-like protein KIAA0439 is a potential regulator of the epithelial sodium channel

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and consists of alpha, beta, and gamma subunits. The carboxyl terminus of each ENaC subunit contains a PPxY, motif which is believed to be important for interaction with the WW domains of the ubiquitin-protein ligase, Nedd4. Disruption of this interaction, as in Liddle's syndrome, where mutations delete or alter the PPxY motif of either the beta or gamma subunits, has been proposed to result in increased ENaC activity. Here we present evidence that KIAA0439 protein, a close relative of Nedd4, is also a potential regulator of ENaC. We demonstrate that KIAA0439 WW domains bind all three ENaC subunits. We show that a recombinant KIAA0439 WW domain protein acts as a dominant negative mutant that can interfere with the Na(+)-dependent feedback inhibition of ENaC in whole-cell patch clamp experiments. We propose that KIAA0439 and Nedd4 proteins either play a redundant role in ENaC regulation or function in a tissue- and/or signal-specific manner to down-regulate ENaC.

A novel mouse Nedd4 protein suppresses the activity of the epithelial Na+ channel

Liddle's syndrome is a form of inherited hypertension linked to mutations in the genes encoding the epithelial Na+ channel (ENaC). These mutations alter or delete PY motifs involved in protein-protein interactions with a ubiquitin-protein ligase, Nedd4. Here we show that Na+ transporting cells, derived from mouse cortical collecting duct, express two Nedd4 proteins with different structural organization and characteristics of ENaC regulation: 1) the classical Nedd4 (herein referred to as Nedd4-1) containing one amino-terminal C2, three WW, and one HECT-ubiquitin protein ligase domain and 2) a novel Nedd4 protein (Nedd4-2), homologous to Xenopus Nedd4 and comprising four WW, one HECT, yet lacking a C2 domain. Nedd4-2, but not Nedd4-1, inhibits ENaC activity when coexpressed in Xenopus oocytes and this property correlates with the ability to bind to ENaC, as only Nedd4-2 coimmunoprecipitates with ENaC. Furthermore, this interaction depends on the presence of at least one PY motif in the ENaC complex and on WW domains 3 and 4 in Nedd4-2. Thus, these results suggest that the novel suppressor protein Nedd4-2 is the regulator of ENaC and hence a potential susceptibility gene for arterial hypertension.

Involvement of renal apical Na transport systems in the control of blood pressure

Human genetic studies and gene targeting techniques in mice suggest that the genes encoding renal apical Na transport proteins play an essential role in the control of extracellular fluid volume and blood pressure. Particularly, very significant advancements in understanding the role of these genes in Mendelian forms of hypertension or hypotension have been achieved in recent years. However, much progress still needs to be made in understanding the more common forms of human essential hypertension. In addition to the mouse models that should be very useful for investigating the mechanisms by which a mutation provokes the hypertensive phenotype, improved clinical phenotyping of patients is needed as well as the use of DNA chip techniques to unravel global gene interactions. Indeed, it is likely that most chronic blood pressure disturbances in a given environment result from a specific combination of polymorphisms or mutations rather than from unique genetic variants. Of equal importance will be definition of the various factors that regulate the expression and activity of the Na transport systems. These regulatory pathways and the responses to environmental factors such as dietary salt, stress, etc, may play a central role in determining the appearance, severity, and complications of essential hypertension.

Structure and regulation of amiloride-sensitive sodium channels

Amiloride-sensitive Na+ channels constitute a new class of proteins known as the ENaC-Deg family of ion channels. All members in this family share a common protein structure but differ in their ion selectivity, their affinity for the blocker amiloride, and in their gating mechanisms. These channels are expressed in many tissues of invertebrate and vertebrate organisms where they serve diverse functions varying from Na+ absorption across epithelia to being the receptors for neurotransmitters in the nervous system. Here, we review progress made during the last years in the characterization, regulation, and cloning of new amiloride-sensitive Na+ channels.

Regulation of the epithelial sodium channel (ENaC) by accessory proteins

The epithelial sodium channel (ENaC) plays a key role in the regulation of fluid absorption in the kidney, lung, colon and exocrine glands, and in the regulation of blood pressure. Abnormal functioning of ENaC is associated with several human diseases, including pseudohypoaldosteronism type I, Liddle's syndrome, pulmonary edema, and cystic fibrosis. ENaC is regulated by several hormones, ions and accessory proteins. This review focuses on the regulation of ENaC by recently described accessory proteins, mainly Nedd4, syntaxin 1A, CFTR, sgk, K-Ras2A and Cap-1.

The molecular basis of renal tubular transport disorders

Sodium and water homeostasis are key to the survival of organisms. Reabsorption of sodium and water occurs throughout the tubule structure of the nephron, the basic functional unit of the kidney, by various transport mechanisms. Altered transport protein function can lead to renal tubular disorders resulting in metabolic alkalosis, hypokalemia, hypertension, and decreased capacity to concentrate urine, for instance. However, recent advances in molecular physiology, molecular genetics and expression cloning systems have aided in unraveling the molecular basis of some renal tubular disorders. This review will examine the molecular basis of Bartter's syndrome, Gitelman's syndrome, Liddle's syndrome, and autosomal nephrogenic diabetes insipidus. An understanding of the molecular basis of these disorders of the human kidney can give us a better understanding of basic renal function of lower mammals and other vertebrates.

Apical membrane targeting of Nedd4 is mediated by an association of its C2 domain with annexin XIIIb

Nedd4 is a ubiquitin protein ligase (E3) containing a C2 domain, three or four WW domains, and a ubiquitin ligase HECT domain. We have shown previously that the C2 domain of Nedd4 is responsible for its Ca(2+)-dependent targeting to the plasma membrane, particularly the apical region of epithelial MDCK cells. To investigate this apical preference, we searched for Nedd4-C2 domain-interacting proteins that might be involved in targeting Nedd4 to the apical surface. Using immobilized Nedd4-C2 domain to trap interacting proteins from MDCK cell lysate, we isolated, in the presence of Ca(2+), a approximately 35-40-kD protein that we identified as annexin XIII using mass spectrometry. Annexin XIII has two known isoforms, a and b, that are apically localized, although XIIIa is also found in the basolateral compartment. In vitro binding and coprecipitation experiments showed that the Nedd4-C2 domain interacts with both annexin XIIIa and b in the presence of Ca(2+), and the interaction is direct and optimal at 1 microM Ca(2+). Immunofluorescence and immunogold electron microscopy revealed colocalization of Nedd4 and annexin XIIIb in apical carriers and at the apical plasma membrane. Moreover, we show that Nedd4 associates with raft lipid microdomains in a Ca(2+)-dependent manner, as determined by detergent extraction and floatation assays. These results suggest that the apical membrane localization of Nedd4 is mediated by an association of its C2 domain with the apically targeted annexin XIIIb.

Aldosterone-related genetic effects in hypertension

Our basic understanding of Na(+) transport mechanisms provides unique insights into epithelial transport processes. Unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect Na(+) balance, resulting in both N(a+) retaining and Na(+) wasting conditions. A major focus has been the epithelial sodium channel (ENaC), which can be activated by mutations (eg, Liddle's syndrome), changes in the response to mineralocorticoids (apparent mineralocorticoid excess syndrome), or production of mineralocorticoids (glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined Mendelian syndromes in which ENaC activity is "dysregulated." This dysregulation leads to systemic hypertension associated with suppressed plasma renin activity, which can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low-renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension.

Ubiquitin-mediated proteolysis: biological regulation via destruction

The ubiquitin proteolytic system plays an important role in a broad array of basic cellular processes. Among these are regulation of cell cycle, modulation of the immune and inflammatory responses, control of signal transduction pathways, development and differentiation. These complex processes are controlled via specific degradation of a single or a subset of proteins. Degradation of a protein by the ubiquitin system involves two successive steps, conjugation of multiple moieties of ubiquitin and degradation of the tagged protein by the 26S proteasome. An important question concerns the identity of the mechanisms that underlie the high degree of specificity of the system. Substrate recognition is governed by a large family ubiquitin ligases that recognize the substrates, bind them and catalyze/facilitate their interaction with ubiquitin.

Molecular genetics of human hypertension

The year 1999 saw considerable activity in the area of hypertension-related molecular genetics. Several new monogenic hypertensive disorders, as well as a monogenic form of hypotension, were elucidated. Molecular genetics has made significant inroads in explaining basic mechanisms of magnesium homeostasis. Linkage strategies have been applied in family studies, sib-pair analyses, and twin studies. More stringent criteria for association studies have been formulated. The 11 beta-hydroxysteroid dehydrogenase gene, the prostacyclin synthase gene, genes coding for variants in G proteins, and adrenergic receptor genes have received particular attention. On the horizon are better phenotyped patient and subject collectives, expanded genotyping with the availability of a 300,000 genome-wide single-nucleotide polymorphism map, multigenic studies in the form of metabolic control analyses, and new bioinformatic strategies including neural networks.

Renal proximal tubule sodium transport and genetic mechanisms of essential hypertension

Renal sodium re-absorption is a closely regulated process serving to maintain both extracellular fluid volume and arterial blood pressure. Proteins participating in sodium re-absorption and its regulation are therefore important candidate proteins whose genes may contain sequence variation contributing to the inherited tendency for increased arterial blood pressure (essential hypertension). Important insight has come from rare forms of single-gene hypertension in human subjects and from polygenic animal models of genetic hypertension. Both indicate the primacy of altered renal function in the genesis of hypertension, and suggest that genes contributing to the disease are members of the subset of genes expressed in the kidney. This review examines evidence for abnormalities in renal sodium re-absorption in hypertension and focuses on the proximal tubule as a site of relevant dysfunction. Identification of the proteins participating in renal sodium re-absorption and its regulation, particularly those involved in the renal pressure-natriuresis mechanism, will allow gene cloning and sequencing which in turn may lead to the identification of novel gene sequence variation participating in hypertension.

Dysfunction of epithelial sodium transport: from human to mouse

The highly amiloride-sensitive epithelial sodium channel (ENaC) is an apical membrane constituent of cells of many salt-absorbing epithelia. In the kidney, the functional relevance of ENaC expression has been well established. ENaC mediates the aldosterone-dependent sodium reabsorption in the distal nephron and is involved in the regulation of blood pressure. Mutations in genes encoding ENaC subunits are causative for two human inherited diseases: Liddle's syndrome, a severe form of hypertension associated with ENaC hyperfunction, and pseudohypoaldosteronism (PHA-1), a salt-wasting syndrome caused by decreased ENaC function. Transgenic mouse technologies provide a useful tool to study the role of ENaC in vivo. Different mouse lines have been established in which each of the ENaC subunits was affected. The phenotypes observed in these mice demonstrated that each subunit is essential for survival and for regulation of sodium transport in kidney and colon. Moreover, the alpha subunit plays a specific role in the control of fluid absorption in the airways at birth. Such mice can now be used to study the role of ENaC in various organs and can serve as models to understand the pathophysiology of these human diseases.

Regulation of the epithelial Na+ channel by Nedd4 and ubiquitination

The epithelial Na+ channel (ENaC) is comprised of three subunits, alpha, beta and gamma, and plays an essential role in Na+ and fluid absorption in the kidney, colon and lung. We had identified proline-rich sequences at the C termini of alpha beta gamma ENaC, which include the sequence PPxY, the PY motif. This sequence in beta or gamma ENaC is deleted or mutated in Liddle's syndrome, a hereditary form of arterial hypertension. Our previous work demonstrated that these PY motifs bind to the WW domains of Nedd4, a ubiquitin protein ligase containing a C2 domain, three or four WW domains and a ubiquitin protein ligase Hect domain. Accordingly, we have recently demonstrated that Nedd4 regulates ENaC function by controlling the number of channels at the cell surface, that this regulation is impaired in ENaC bearing Liddle's syndrome mutations, and that ENaC stability and function are regulated by ubiquitination. The C2 domain is responsible for localizing Nedd4 to the plasma membrane in a Ca(2+)-dependent manner, and in polarized epithelial MDCK cells this localization is primarily apical. In accordance, electrophysiological characterization of ENaC expressed in MDCK cells revealed inhibition of channel activity by elevated intracellular Ca2+ levels. Thus, in response to Ca2+, Nedd4 may be mobilized to the apical membrane via its C2 domain, where it binds ENaC via Nedd4-WW:ENaC-PY motifs' interactions, leading to ubiquitination of the channel by the Nedd4-Hect domain and subsequent channel endocytosis and lysosomal degradation. This process may be at least partially impaired in Liddle's syndrome due to reduced Nedd4 binding, leading to increased retention of ENaC at the cell surface.

Human Nedd4 interacts with the human epithelial Na+ channel: WW3 but not WW1 binds to Na+-channel subunits

The epithelial Na(+) channel (ENaC) regulates Na(+) absorption in epithelial tissues including the lung, colon and sweat gland, and in the distal nephrons of the kidney. When Na(+)-channel function is disrupted, salt and water homoeostasis is affected. The cytoplasmic regions of the Na(+)-channel subunits provide binding sites for other proteins to interact with and potentially regulate Na(+)-channel activity. Previously we showed that a proline-rich region of the alpha subunit of the Na(+) channel bound to a protein of 116 kDa from human lung cells. Here we report the identification of this protein as human Nedd4, a ubiquitin-protein ligase that binds to the Na(+)-channel subunits via its WW domains. Further, we show that WW domains 2, 3 and 4 of human Nedd4 bind to the alpha, beta and gamma Na(+)-channel subunits but not to a mutated beta subunit. In addition, when co-expressed in Xenopus oocytes, human Nedd4 down-regulates Na(+)-channel activity.

Mineralocorticoid action

The physiology of mineralocorticoid action, particularly with respect to epithelial sodium transport, is well defined. A full understanding of the molecular basis of mineralocorticoid action has however proven to be more elusive. In the last decade insights into structural and functional aspects of the mineralocorticoid receptor combined with emerging details of the components of the mediators of the sodium flux has resulted in a clearer picture. This review focuses on two aspects of these new developments; the mineralocorticoid receptor and putative aldosterone induced proteins.

Regulation of the cardiac voltage-gated Na+ channel (H1) by the ubiquitin-protein ligase Nedd4

The cardiac voltage-gated Na+ channel H1, involved in the generation of cardiac action potential, contains a C-terminal PY motif (xPPxY). Since PY motifs are known ligands to WW domains, we investigated their role for H1 regulation and the possible involvement of the WW domain containing ubiquitin-protein ligase Nedd4, taking advantage of the Xenopus oocyte system. Mutation of the PY motif leads to higher peak currents when compared to wild-type channel. Moreover, co-expression of Nedd4 reduced the peak currents, whereas an enzymatically inactive Nedd4 mutant increased them, likely by competing with endogenous Nedd4. The effect of Nedd4 was not observed in the PY motif mutated channel or in the skeletal muscle voltage-gated Na+ channel, which lacks a PY motif. We conclude that H1 may be regulated by Nedd4 depending on WW-PY interaction, and on an active ubiquitination site.

Modes of regulation of ubiquitin-mediated protein degradation

The ubiquitin-proteasome pathway is responsible for the major portion of specific cellular protein degradation. Ubiquitin-mediated degradation is involved in physiological regulation of many cellular processes, including cell cycle progression, differentiation, and signal transduction. Here, we review the basic mechanisms of the ubiquitin system and the various ways in which ubiquitin-mediated degradation can be modulated by physiological signals.

The hereditary stomatocytoses and allied disorders: congenital disorders of erythrocyte membrane permeability to Na and K

The hereditary stomatocytoses and allied disorders are a set of dominantly inherited haemolytic anaemias in which the plasma membrane of the red cell 'leaks' sodium and potassium. There are about 10 different forms of these conditions, ranging from a moderately severe haemolytic anaemia to minor conditions in which the haematology is essentially normal, but where the patients present with pseudohyperkalaemia, due to leakage of K from the red cells on cooling to room temperature. Frequently misdiagnosed as atypical hereditary spherocytosis, these conditions can show marked thrombotic complications after splenectomy, which should be avoided. Laboratory studies of these conditions have drawn attention to a 32 kDa membrane protein, stomatin, which seems to act as a regulator of Na and K transport in human and animal tissues generally, but mutations in this gene do not cause these diseases. Genetic mapping in some kindreds, but not all, points to a mutation locus on chromosome 16.

Functional domains within the degenerin/epithelial sodium channel (Deg/ENaC) superfamily of ion channels

Application of recombinant DNA technology and electrophysiology to the study of amiloride-sensitive Na+ channels has resulted in an enormous increase in the understanding of the structure-function relationships of these channels. Moreover, this knowledge has permitted the elucidation of the physiological roles of these ion channels in cellular processes as diverse as transepithelial salt and water movement, taste perception, volume regulation, nociception, neuronal function, mechanosensation, and even defaecation. Although members of this ever-growing superfamily of ion channels (the Deg/ENaC superfamily) share little amino acid identity, they are all organized similarly, namely, two short N- and C-termini, two short membrane-spanning segments, and a very large extracellular loop domain. In this brief Topical Review, we discuss the structural features of each domain of this Deg/ENaC superfamily and, using ENaC as a model, show how each domain relates to overall channel function.

Feedback inhibition of epithelial Na(+) channels in Xenopus oocytes does not require G(0) or G(i2) proteins

Regulation of amiloride-sensitive epithelial Na(+) channels (ENaC) is a prerequisite for coordination of electrolyte transport in epithelia. Downregulation of Na(+) conductance occurs when the intracellular Na(+) concentration is increased during reabsorption of electrolytes, known as feedback inhibition. Recent studies have demonstrated the involvement of alphaG(0) and alphaG(i2) proteins in the feedback control of ENaC in mouse salivary duct cells. In this report, we demonstrate that Na(+) feedback inhibition is also present in Xenopus oocytes after expression of rat alpha,beta, gamma-ENaC. Interfering with intracellular alphaG(0) or alphaG(i2) signaling by coexpression of either constitutively active alphaG(0)/alphaG(i2) or dominant negative alphaG(0)/alphaG(i2) and by coinjecting sense or antisense oligonucleotides for alphaG(0) had no impact on Na(+) feedback. Moreover, no evidence for involvement of the intracellular G protein cascade was found in experiments in which a regulator of G protein signaling (RGS3) or beta-adrenergic receptor kinase (betaARK) was coexpressed together with alpha,beta, gamma-ENaC. Although some experiments suggest the presence of an intracellular Na(+) receptor, we may conclude that Na(+) feedback in Xenopus oocytes is different from that described for salivary duct cells in that it does not require G protein signaling.

Aldosterone-mediated regulation of ENaC alpha, beta, and gamma subunit proteins in rat kidney

Aldosterone stimulates sodium transport in the renal collecting duct by activating the epithelial sodium channel (ENaC). To investigate the basis of this effect, we have developed a novel set of rabbit polyclonal antibodies to the 3 subunits of ENaC and have determined the abundance and distribution of ENaC subunits in the principal cells of the rat renal collecting duct. Elevated circulating aldosterone (due to either dietary NaCl restriction or aldosterone infusion) markedly increased the abundance of alphaENaC protein without increasing the abundance of the beta and gamma subunits. Thus, alphaENaC is selectively induced by aldosterone. In addition, immunofluorescence immunolocalization showed a striking redistribution in ENaC labeling to the apical region of the collecting duct principal cells. Finally, aldosterone induced a shift in molecular weight of gammaENaC from 85 kDa to 70 kDa, consistent with physiological proteolytic clipping of the extracellular loop as postulated previously. Thus, at the protein level, the response of ENaC to aldosterone stimulation is heterogenous, with both quantitative and qualitative changes that can explain observed increases in ENaC-mediated sodium transport.

Na(+)-H(+) exchange in salivary secretory cells is controlled by an intracellular Na(+) receptor

It recently has been shown that epithelial Na(+) channels are controlled by a receptor for intracellular Na(+), a G protein (G(o)), and a ubiquitin-protein ligase (Nedd4). Furthermore, mutations in the epithelial Na(+) channel that underlie the autosomal dominant form of hypertension known as Liddle's syndrome inhibit feedback control of Na(+) channels by intracellular Na(+). Because all epithelia, including those such as secretory epithelia, which do not express Na(+) channels, need to maintain a stable cytosolic Na(+) concentration ([Na(+)](i)) despite fluctuating rates of transepithelial Na(+) transport, these discoveries raise the question of whether other Na(+) transporting systems in epithelia also may be regulated by this feedback pathway. Here we show in mouse mandibular secretory (endpiece) cells that the Na(+)-H(+) exchanger, NHE1, which provides a major pathway for Na(+) transport in salivary secretory cells, is inhibited by raised [Na(+)](i) acting via a Na(+) receptor and G(o). This inhibition involves ubiquitination, but does not involve the ubiquitin protein ligase, Nedd4. We conclude that control of membrane transport systems by intracellular Na(+) receptors may provide a general mechanism for regulating intracellular Na(+) concentration.

Interaction of syntaxins with the amiloride-sensitive epithelial sodium channel

Amiloride-sensitive sodium channels mediate sodium entry across the apical membrane of epithelial cells in variety of tissues. The rate of Na(+) entry is controlled by the regulation of the epithelial sodium channel (ENaC) complex. Insertion/retrieval of the ENaC complex into the apical membrane as well as direct kinetic effects at the single channel level are recognized mechanisms of regulation. Recent data suggest that the syntaxin family of targeting proteins interact with and functionally regulate a number of ion channels and pumps. To evaluate the role of these proteins in regulating ENaC activity, we co-expressed rat ENaC cRNA (alpha, beta, gamma subunits) with syntaxin 1A or 3 cRNAs in Xenopus oocytes. Basal ENaC currents were inhibited by syntaxin 1A and stimulated by syntaxin 3. Both syntaxin 1A and syntaxin 3 could be co-immunoprecipitated with ENaC subunit proteins, suggesting physical interaction. Interestingly, immunofluorescence data suggest that with either syntaxin isoform the ENaC-associated epifluorescence on the oocyte surface is enhanced. These data indicate that (i) both syntaxin isoforms increase the net externalization of the ENaC channel complex, (ii) that the functional regulation is isoform specific, and (iii) suggest that ENaC may be regulated through mechanisms involving protein-protein interactions.

Seven lessons from two candidate genes in human essential hypertension: angiotensinogen and epithelial sodium channel

The candidate gene approach to understanding the genetics of human essential hypertension is discussed by analyzing the contribution of 2 genes, angiotensinogen (AGT) and epithelial amiloride-sensitive sodium channel (ENaC). From a large series of studies conducted in humans and animals, it appears that the AGT gene plays a significant but modest role in human blood pressure variance. Mutations of the beta- and gamma-ENaC subunits are responsible for Liddle's syndrome, but the implication of the 3 ENaC subunits in essential hypertension is still questionable. Several lessons can be learned from these studies and applied to other candidate genes in essential hypertension: (1) Many linkage or association studies have a limited statistical power; (2) The genetic findings may vary greatly according to the populations studied; (3) There is a need for better phenotyping of the hypertensive population; (4) The causal relationship between molecular variants and hypertension is and will be difficult to establish firmly; (5) The contribution of genetic studied in rodents to the molecular genetics of human hypertension must be re-examined; (6) Most molecular variants lead to a low attributable risk in the population or a low individual effect at the individual level; and (7) It is too early to propose dietary recommendations and specific drug treatment according to patients' genotypes.