AKAP2-anchored extracellular signal-regulated kinase 1 (ERK1) regulates cardiac myofibroblast migration.

Cardiac fibrosis is a major cause of dysfunctions and arrhythmias in failing hearts. At the cellular level fibrosis is mediated by cardiac myofibroblasts, which display an increased migratory capacity and secrete large amounts of extracellular matrix. These properties allow myofibroblasts to invade, remodel and stiffen the myocardium and eventually alter cardiac function. While the enhanced ability of cardiac myofibroblasts to migrate has been proposed to contribute to the initiation of the fibrotic process, the molecular mechanisms controlling their motile function have been poorly defined. In this context, our current findings indicate that A-kinase anchoring protein 2 (AKAP2) associates with actin at the leading edge of migrating cardiac myofibroblasts. Proteomic analysis of the AKAP2 interactome revealed that this anchoring protein assembles a signaling complex composed of the extracellular regulated kinase 1 (ERK1) and its upstream activator Grb2 that mediates the activation of ERK in cardiac myofibroblasts. Silencing AKAP2 expression results in a significant reduction in the phosphorylation of ERK1 and its downstream effector WAVE2, a protein involved in actin polymerization, and impairs the ability of cardiac myofibroblasts to migrate. Importantly, disruption of the interaction between AKAP2 and F-actin using cell-permeant competitor peptides, inhibits the activation of the ERK-WAVE2 signaling axis, resulting in a reduction of the translocation of Arp2 to the leading-edge membrane and in inhibition of cardiac myofibroblast migration. Collectively, these findings suggest that AKAP2 functions as an F-actin bound molecular scaffold mediating the activation of an ERK1-dependent promigratory transduction pathway in cardiac myofibroblasts.

A mild and transient form of autosomal recessive pseudohypoaldosteronism type 1 caused by a novel mutation in the SCNN1A gene.

We investigate the genetic etiology in a cohort of patients with a clinical, biochemical, and hormonal profile suggestive of a mild and transient form of pseudohypoaldosteronism type 1 (PHA1). Twelve patients with PHA1 from four different families with clinical and biochemical data were analyzed. The coding regions of NR3C2 and SCNN1A genes were sequenced. Human α-epithelial sodium channel (ENaC) wild-type (wt), αPhe226Cys and αPhe226Ser ENaC variants were expressed in Xenopus laevis oocytes to evaluate ENaC activity. The protein expression of α-ENaC wt and mutants was determined by Western blot. All patients were homozygotes for the p.Phe226Cys mutation of the α subunit of ENaC. In functional studies in X. laevis oocytes, p.Phe226Cys caused a significant reduction of ENaC activity (83% reduction), reduced the number of active ENαC mutant channels, and reduced the basal open probability compared with wt. Quantitative Western blot analysis revealed that the reduced activity of ENαC mutant channels was due to a reduced ENaC protein expression for the αPhe226Cys compared with wt. We present 12 patients from four different families with a mild and transient autosomal recessive PHA1 due to a novel homozygous missense mutation in the SCNN1A gene. Functional studies showed that the p.Phe226Cys substitution mutation in ENaC leads to a partial loss of function resulting mainly from both a decrease in the intrinsic ENaC activity and a reduction in channel expression at the protein level. The partial loss of ENaC function could explain the mild phenotype, variable expressivity, and the transient course of the disorder in these patients.NEW & NOTEWORTHY This paper demonstrates that mild autosomal recessive pseudohypoaldosteronism type 1 (PHA1) due to p.Phe226Cys missense mutation in the extracellular domain of ENαC α subunit can be transient, with phenotypic variability even with the normal sweat test, and incomplete penetrance. Functional studies explain the phenotype and denote the importance of the location on the extracellular domain of the SCNN1A p.Phe226Cys mutation for the intrinsic ENaC activity and the channel expression at the protein level.

Calcium regulates acid-sensing ion channel 3 activation by competing with protons in the channel pore and at an allosteric binding site.

The extracellular Ca2+ concentration changes locally under certain physiological and pathological conditions. Such variations affect the function of ion channels of the nervous system and consequently also neuronal signalling. We investigated here the mechanisms by which Ca2+ controls the activity of acid-sensing ion channel (ASIC) 3. ASICs are neuronal, H+-gated Na+ channels involved in several physiological and pathological processes, including the expression of fear, learning, pain sensation and neurodegeneration after ischaemic stroke. It was previously shown that Ca2+ negatively modulates the ASIC pH dependence. While protons are default activators of ASIC3, this channel can also be activated at pH7.4 by the removal of the extracellular Ca2+. Two previous studies concluded that low pH opens ASIC3 by displacing Ca2+ ions that block the channel pore at physiological pH. We show here that an acidic residue, distant from the pore, together with pore residues, controls the modulation of ASIC3 by Ca2+. Our study identifies a new regulatory site in ASIC3 and demonstrates that ASIC3 activation involves an allosteric mechanism together with Ca2+ unbinding from the channel pore. We provide a molecular analysis of a regulatory mechanism found in many ion channels.

Photoresponsive Nanocarriers Based on Lithium Niobate Nanoparticles for Harmonic Imaging and On-Demand Release of Anticancer Chemotherapeutics.

Nanoparticle-based drug delivery systems have the potential for increasing the efficiency of chemotherapeutics by enhancing the drug accumulation at specific target sites, thereby reducing adverse side effects and mitigating patient acquired resistance. In particular, photo-responsive nanomaterials have attracted much interest due to their ability to release molecular cargos on demand upon light irradiation. In some settings, they can also provide complementary information by optical imaging on the (sub)cellular scale. We herein present a system based on lithium niobate harmonic nanoparticles (LNO HNPs) for the decoupled multi-harmonic cell imaging and near-infrared light-triggered delivery of an erlotinib derivative (ELA) for the treatment of epidermal growth factor receptor (EGFR)-overexpressing carcinomas. The ELA cargo was covalently conjugated to the surface of silica-coated LNO HNPs through a coumarinyl photo-cleavable linker, achieving a surface loading of the active molecule of 27 nmol/mg NPs. The resulting nanoconjugates (LNO-CM-ELA NPs) were successfully imaged upon pulsed laser excitation at 1250 nm in EGFR-overexpressing human prostate cancer cells DU145 by detecting the second harmonic emission at 625 nm, in the tissue transparency window. Tuning the laser at 790 nm resulted in the uncaging of the ELA cargo as a result of the second harmonic emission of the inorganic HNP core at 395 nm. This protocol induced a significant growth inhibition in DU145 cells, which was only observed upon specific irradiation at 790 nm, highlighting the promising capabilities of LNO-CM-ELA NPs for theranostic applications.

Kidney-Specific CAP1/Prss8-Deficient Mice Maintain ENaC-Mediated Sodium Balance through an Aldosterone Independent Pathway.

The serine protease prostasin (CAP1/Prss8, channel-activating protease-1) is a confirmed in vitro and in vivo activator of the epithelial sodium channel ENaC. To test whether proteolytic activity or CAP1/Prss8 abundance itself are required for ENaC activation in the kidney, we studied animals either hetero- or homozygous mutant at serine 238 (S238A; Prss8cat/+ and Prss8cat/cat), and renal tubule-specific CAP1/Prss8 knockout (Prss8PaxLC1) mice. When exposed to varying Na+-containing diets, no changes in Na+ and K+ handling and only minor changes in the expression of Na+ and K+ transporting protein were found in both models. Similarly, the α- or γENaC subunit cleavage pattern did not differ from control mice. On standard and low Na+ diet, Prss8cat/+ and Prss8cat/cat mice exhibited standard plasma aldosterone levels and unchanged amiloride-sensitive rectal potential difference indicating adapted ENaC activity. Upon Na+ deprivation, mice lacking the renal CAP1/Prss8 expression (Prss8PaxLC1) exhibit significantly decreased plasma aldosterone and lower K+ levels but compensate by showing significantly higher plasma renin activity. Our data clearly demonstrated that the catalytic activity of CAP1/Prss8 is dispensable for proteolytic ENaC activation. CAP1/Prss8-deficiency uncoupled ENaC activation from its aldosterone dependence, but Na+ homeostasis is maintained through alternative pathways.

Multiorder Nonlinear Mixing in Metal Oxide Nanoparticles.

Whereas most of the reports on the nonlinear properties of micro- and nanostructures address the generation of distinct signals, such as second or third harmonic, here we demonstrate that the novel generation of dual output lasers recently developed for microscopy can readily increase the accessible parameter space and enable the simultaneous excitation and detection of multiple emission orders such as several harmonics and signals stemming from various sum and difference frequency mixing processes. This rich response, which in our case features 10 distinct emissions and encompasses the whole spectral range from the deep ultraviolet to the short-wave infrared region, is demonstrated using various nonlinear oxide nanomaterials while being characterized and simulated temporally and spectrally. Notably, we show that the response is conserved when the particles are embedded in biological media opening the way to novel biolabeling and phototriggering strategies.

Structural and Functional Analysis of Gly212 Mutants Reveals the Importance of Intersubunit Interactions in ASIC1a Channel Function.

Acid-sensing ion channels (ASICs) act as pH sensors in neurons. ASICs contribute to pain sensation, learning, fear behavior and to neuronal death after ischemic stroke. Extracellular acidification induces a transient activation and subsequent desensitization of these Na+-selective channels. ASICs are trimeric channels made of identical or homologous subunits. We have previously shown that mutation of the highly conserved Gly212 residue of human ASIC1a to Asp affects the channel function. Gly212 is located in the proximity of a predicted Cl- binding site at a subunit interface. Here, we have measured the function of a series of Gly212 mutants. We show that substitution of Gly212 affects the ASIC1a pH dependence and current decay kinetics. Intriguingly, the mutations to the acidic residues Asp and Glu have opposing effects on the pH dependence and the current decay kinetics. Analysis of molecular dynamics simulation trajectories started with the coordinates of the closed conformation indicates that the immediate environment of residue 212 in G212E, which shifts the pH dependence to more alkaline values, adopts a conformation closer to the open state. The G212D and G212E mutants have a different pattern of intersubunit salt bridges, that, in the case of G212E, leads to an approaching of neighboring subunits. Based on the comparison of crystal structures, the conformational changes in this zone appear to be smaller during the open-desensitized transition. Nevertheless, MD simulations highlight differences between mutants, suggesting that the changed function upon substitution of residue 212 is due to differences in intra- and intersubunit interactions in its proximity.

Human Mutations in SLC2A9 (Glut9) Affect Transport Capacity for Urate.

SLC2A9 or Glut9 is a voltage sensitive urate transporter, mainly expressed in the kidneys, the liver, and the intestine. Human Glut9 loss-of-function mutations were identified in familial hypouricemia, and several single nucleotide polymorphisms (SNPs) were associated with lower serum urate, further indicating that Glut9 is a major determinant of serum uric acid level. To get insights in Glut9 transport characteristics, we systematically analyzed the function of known human Glut9 mutants using 14C-urate uptake assay and two-electrode voltage clamp (TEVC) in the Xenopus laevis oocyte expression system. Surface expression was assessed by immunostaining and biotinylation. We found decreased urate transport by flux studies for most of the variants. No variant was permissive for glucose transport. We could further differentiate two behaviors among the mutants: those harboring poor overall and cell-surface expression leading to low activity and those fully expressed at the cell surface, but presenting decreased activity. We studied the latter by TEVC and observed, in depolarized conditions, decreased inward currents measured in presence of 400 µM urate, partially reversed in 1 mM urate. In addition, we showed that C210F displays lower transport ability. By contrast, N333S showed decreased urate transport activity and urate affinity, suggesting that it may belong to the urate binding pocket. Systematic analysis of Glut9 mutants confirms Glut9 as putative target for the treatment of hyperuricemia and brings new insights in Glut9 structure - function.

A Missense Mutation in the Extracellular Domain of αENaC Causes Liddle Syndrome.

Liddle syndrome is an autosomal dominant form of hypokalemic hypertension due to mutations in the ß- or γ-subunit of the epithelial sodium channel (ENaC). Here, we describe a family with Liddle syndrome due to a mutation in αENaC. The proband was referred because of resistant hypokalemic hypertension, suppressed renin and aldosterone, and no mutations in the genes encoding ß- or γENaC. Exome sequencing revealed a heterozygous, nonconservative T>C single-nucleotide mutation in αENaC that substituted Cys479 with Arg (C479R). C479 is a highly conserved residue in the extracellular domain of ENaC and likely involved in a disulfide bridge with the partner cysteine C394. In oocytes, the C479R and C394S mutations resulted in similar twofold increases in amiloride-sensitive ENaC current. Quantification of mature cleaved αENaC in membrane fractions showed that the number of channels did not increase with these mutations. Trypsin, which increases open probability of the channel by proteolytic cleavage, resulted in significantly higher currents in the wild type than in C479R or C394S mutants. In summary, a mutation in the extracellular domain of αENaC causes Liddle syndrome by increasing intrinsic channel activity. This mechanism differs from that of the ß- and γ-mutations, which result in an increase in channel density at the cell surface. This mutation may explain other cases of patients with resistant hypertension and also provides novel insight into ENaC activation, which is relevant for kidney sodium reabsorption and salt-sensitive hypertension.

Proton and non-proton activation of ASIC channels.

The Acid-Sensing Ion Channels (ASIC) exhibit a fast desensitizing current when activated by pH values below 7.0. By contrast, non-proton ligands are able to trigger sustained ASIC currents at physiological pHs. To analyze the functional basis of the ASIC desensitizing and sustained currents, we have used ASIC1a and ASIC2a mutants with a cysteine in the pore vestibule for covalent binding of different sulfhydryl reagents. We found that ASIC1a and ASIC2a exhibit two distinct currents, a proton-induced desensitizing current and a sustained current triggered by sulfhydryl reagents. These currents differ in their pH dependency, their sensitivity to the sulfhydryl reagents, their ionic selectivity and their relative magnitude. We propose a model for ASIC1 and ASIC2 activity where the channels can function in two distinct modes, a desensitizing mode and a sustained mode depending on the activating ligands. The pore vestibule of the channel represents a functional site for binding non-proton ligands to activate ASIC1 and ASIC2 at neutral pH and to prevent channel desensitization.

Renal Fanconi Syndrome and Hypophosphatemic Rickets in the Absence of Xenotropic and Polytropic Retroviral Receptor in the Nephron.

Tight control of extracellular and intracellular inorganic phosphate (Pi) levels is critical to most biochemical and physiologic processes. Urinary Pi is freely filtered at the kidney glomerulus and is reabsorbed in the renal tubule by the action of the apical sodium-dependent phosphate transporters, NaPi-IIa/NaPi-IIc/Pit2. However, the molecular identity of the protein(s) participating in the basolateral Pi efflux remains unknown. Evidence has suggested that xenotropic and polytropic retroviral receptor 1 (XPR1) might be involved in this process. Here, we show that conditional inactivation of Xpr1 in the renal tubule in mice resulted in impaired renal Pi reabsorption. Analysis of Pi transport in primary cultures of proximal tubular cells or in freshly isolated renal tubules revealed that this Xpr1 deficiency significantly affected Pi efflux. Further, mice with conditional inactivation of Xpr1 in the renal tubule exhibited generalized proximal tubular dysfunction indicative of Fanconi syndrome, characterized by glycosuria, aminoaciduria, calciuria, and albuminuria. Dramatic alterations in the renal transcriptome, including a significant reduction in NaPi-IIa/NaPi-IIc expression, accompanied these functional changes. Additionally, Xpr1-deficient mice developed hypophosphatemic rickets secondary to renal dysfunction. These results identify XPR1 as a major regulator of Pi homeostasis and as a potential therapeutic target in bone and kidney disorders.

The Human Acid-Sensing Ion Channel ASIC1a: Evidence for a Homotetrameric Assembly State at the Cell Surface.

The chicken acid-sensing ion channel ASIC1 has been crystallized as a homotrimer. We address here the oligomeric state of the functional ASIC1 in situ at the cell surface. The oligomeric states of functional ASIC1a and mutants with additional cysteines introduced in the extracellular pore vestibule were resolved on SDS-PAGE. The functional ASIC1 complexes were stabilized at the cell surface of Xenopus laevis oocytes or CHO cells either using the sulfhydryl crosslinker BMOE, or sodium tetrathionate (NaTT). Under these different crosslinking conditions ASIC1a migrates as four distinct oligomeric states that correspond by mass to multiples of a single ASIC1a subunit. The relative importance of each of the four ASIC1a oligomers was critically dependent on the availability of cysteines in the transmembrane domain for crosslinking, consistent with the presence of ASIC1a homo-oligomers. The expression of ASIC1a monomers, trimeric or tetrameric concatemeric cDNA constructs resulted in functional channels. The resulting ASIC1a complexes are resolved as a predominant tetramer over the other oligomeric forms, after stabilization with BMOE or NaTT and SDS-PAGE/western blot analysis. Our data identify a major ASIC1a homotetramer at the surface membrane of the cell expressing functional ASIC1a channel.

Congenital ataxia and hemiplegic migraine with cerebral edema associated with a novel gain of function mutation in the calcium channel CACNA1A.

Mutations in the CACNA1A gene, encoding the α1 subunit of the voltage-gated calcium channel Ca(V)2.1 (P/Q-type), have been associated with three neurological phenotypes: familial and sporadic hemiplegic migraine type 1 (FHM1, SHM1), episodic ataxia type 2 (EA2), and spinocerebellar ataxia type 6 (SCA6). We report a child with congenital ataxia, abnormal eye movements and developmental delay who presented severe attacks of hemiplegic migraine triggered by minor head traumas and associated with hemispheric swelling and seizures. Progressive cerebellar atrophy was also observed. Remission of the attacks was obtained with acetazolamide. A de novo 3 bp deletion was found in heterozygosity causing loss of a phenylalanine residue at position 1502, in one of the critical transmembrane domains of the protein contributing to the inner part of the pore. We characterized the electrophysiology of this mutant in a Xenopus oocyte in vitro system and showed that it causes gain of function of the channel. The mutant Ca(V)2.1 activates at lower voltage threshold than the wild type. These findings provide further evidence of this molecular mechanism as causative of FHM1 and expand the phenotypic spectrum of CACNA1A mutations with a child exhibiting severe SHM1 and non-episodic ataxia of congenital onset.

Functional analysis of a missense mutation in the serine protease inhibitor SPINT2 associated with congenital sodium diarrhea.

Membrane-bound serine proteases play important roles in different biological processes. Their regulation by endogenous inhibitors is poorly understood. A Y163C mutation in the SPINT2 gene encoding the serine protease inhibitor Hepatocyte Growth Factor Inhibitor HAI-2 is associated with a congenital sodium diarrhea. The functional consequences of this mutation on HAI-2 activity and its physiological targets are unknown. We established a cellular assay in Xenopus laevis oocytes to study functional interactions between HAI-2 and candidate membrane-bound serine proteases expressed in the gastro-intestinal tract. We found that the wild-type form of HAI-2 is a potent inhibitor of nine gastro-intestinal serine proteases. The Y163C mutation in the second Kunitz domain of HAI-2 resulted in a complete loss of inhibitory activity on two intestinal proteases, prostasin and tmprss13. The effect of the mutation of the homologous Y68C in the first Kunitz domain of HAI-2 is consistent with a differential contribution of the two Kunitz domains of HAI-2 in the inhibition of serine proteases. By contrast to the Tyr to Cys, the Tyr to Ser substitution did not change the inhibitory potency of HAI-2, indicating that the thiol-group of the cysteine rather than the Tyr deletion is responsible for the HAI-2 loss of function. Our functional assay allowed us to identify membrane-bound serine proteases as cellular target for inhibition by HAI-2 wild type and mutants, and to better define the role of the Tyr in the second Kunitz domain in the inhibitory activity of HAI-2.

Licorice-induced hypertension and common variants of genes regulating renal sodium reabsorption.

AIM: To study if gene alterations affecting renal sodium reabsorption associate with susceptibility to licorice-induced hypertension. METHODS: Finnish subjects (n = 30) with a previously documented incident of licorice-induced hypertension were recruited for the study using a newspaper announcement. Their previous clinical and family histories as well as serum electrolyte levels were examined. DNA samples from all individuals were screened for variants of the genes encoding 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2) and alpha-, beta-, and gamma-subunits of the epithelial sodium channel (ENaC). RESULTS: Upon licorice predisposition, the patients had a mean blood pressure of 201/118 mmHg. Circulating potassium, renin, and aldosterone levels were low. No significant DNA variations were identified in the 11betaHSD2 gene. Four subjects were heterozygous for beta- and gammaENaC variants previously shown to be associated with hypertension. Furthermore, a novel G insertion (2004-2005insG) in the SCNN1A gene encoding the alphaENaC was identified in two subjects. The frequency of these ENaC variants was significantly higher in subjects with licorice-induced hypertension (6/30 i.e. 20%) than in blood donors (11/301 i.e. 3.7%, P = 0.002). CONCLUSIONS: Defects of the 11betaHSD2 gene do not constitute a likely cause for licorice-induced hypertension. Variants of the ENaC subunits may render some individuals sensitive to licorice-induced metabolic alterations and hypertension.

Revealing a subclinical salt-losing phenotype in heterozygous carriers of the novel S562P mutation in the alpha subunit of the epithelial sodium channel.

OBJECTIVE: Pseudohypoaldosteronism type I (PHA1) is a rare inborn disease causing severe salt loss. Mutations in the three coding genes of the epithelial sodium channel (ENaC) are responsible for the systemic autosomal recessive form. So far, no phenotype has been reported in heterozygous carriers. PATIENTS: A consanguineous family from Somalia giving birth to a neonate suffering from PHA1 was studied including clinical and hormonal characteristics of the family, mutational analysis of the SCNN1A, SCNN1B, SCNN1G and CFTR genes and in vitro analysis of the functional consequences of a mutant ENaC channel. RESULTS: CFTR mutations have been excluded. SCNN1A gene analysis revealed a novel homozygous c.1684T > C mutation resulting in a S562P substitution in the alphaENaC protein of the patient. Functional analysis showed a significantly reduced S562P channel function compared to ENaC wild type. Protein synthesis and channel subunit assembly were not altered by the S562P mutation. Co-expression of mutant and wild-type channels revealed a dominant negative effect. In heterozygote carriers, sweat sodium and chloride concentrations were increased without additional hormonal or clinical phenotypes. CONCLUSION: Hence, the novel mutation S562P is causing systemic PHA1 in the homozygous state. A thorough clinical investigation of the heterozygote SCNN1A mutation carriers revealed increased sweat sodium and chloride levels consistent with a dominant effect of the mutant S562P allele. Whether this subclinical phenotype is of any consequence for the otherwise asymptomatic heterozygous carriers has to be elucidated.

Cadmium trapping in an epithelial sodium channel pore mutant.

The putative selectivity filter of the epithelial sodium channel (ENaC) comprises a three-residue sequence G/SXS, but it remains uncertain whether the backbone atoms of this sequence or whether their side chains are lining the pore. It has been reported that the S589C mutation in the selectivity filter of alphaENaC renders the channel sensitive to block by externally applied Cd2+; this was interpreted as evidence for Cd2+ coordination with the thiol group of the side chain of alpha589C, pointing toward the pore lumen. Because the alphaS589C mutation alters the monovalent to divalent cation selectivity ratio of ENaC and because internally applied Cd2+ blocks wild-type ENaC with high affinity, we hypothesized that the inhibition of alphaS589C ENaC by Cd2+ results rather from the coordination of this cation with native cysteine residues located in the internal pore of ENaC. We show here that Cd2+ inhibits not only ENaC alphaS589C and alphaS589D but also alphaS589N mutants and that Ca2+ weakly interacts with the S589D mutant. The block of alphaS589C, -D, and -N mutants is characterized by a slow on-rate, is nearly irreversible, is voltage-dependent, and can be prevented by amiloride. The C546S mutation in the second transmembrane helix of gamma subunit in the background of the ENaC alphaS589C, -D, or -N mutants reduces the sensitivity to block by Cd2+ and renders the block rapidly reversible. We conclude therefore that the block by Cd2+ of the alphaS589C, -D, and -N mutants results from the trapping of Cd2+ ions in the internal pore of the channel and involves Cys-546 in the second transmembrane helix of the gammaENaC subunit.

A gating mutation in the internal pore of ASIC1a.

Using a substituted cysteine accessibility scan, we have investigated the structures that form the internal pore of the acid-sensing ion channel 1a. We have identified the amino acid residues Ala-22, Ile-33, and Phe-34 in the amino terminus and Arg-43 in the first transmembrane helix, which when mutated into cysteine, were modified by intracellular application of MTSET, resulting in channel inhibition. The inhibition of the R43C mutant by internal MTSET requires opening of the channel. In addition, binding of Cd2+ ions to R43C slows the channel inactivation. This indicates that the first transmembrane helix undergoes conformational changes during channel inactivation. The effect of Cd2+ on R43C can be obtained with Cd2+ applied at either the extracellular or the intracellular side, indicating that R43C is located in the channel pore. The block of the A22C, I33C, and F34C mutants by MTSET suggests that these residues in the amino terminus of the channel also participate to the internal pore.

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 beta and gamma 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 betaENaC variants (G589S and a novel intronic i12-17CT substitution) and one novel gammaENaC 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). betaENaC 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 betaENaC 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 beta and gammaENaC, 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.

Intracellular thiol-mediated modulation of epithelial sodium channel activity.

The epithelial sodium channel ENaC is physiologically important in the kidney for the regulation of the extracellular fluid volume, and in the lungs for the maintenance of the appropriate airway surface liquid volume that lines the pulmonary epithelium. Besides the regulation of ENaC by hormones, intracellular factors such as Na(+) ions, pH, or Ca(2+) are responsible for fast adaptive responses of ENaC activity to changes in the intracellular milieu. In this study, we show that ENaC is rapidly and reversibly inhibited by internal sulfhydryl-reactive molecules such as methanethiosulfonate derivatives of different sizes, the metal cations Cd(2+) and Zn(2+), or copper(II) phenanthroline, a mild oxidizing agent that promotes the formation of disulfide bonds. At the single channel level, these agents applied intracellularly induce the appearance of long channel closures, suggesting an effect on ENaC gating. The intracellular reducing agent dithiothreitol fully reverses the rundown of ENaC activity in inside-out patches. Our observations suggest that changes in intracellular redox potential modulate ENaC activity and may regulate ENaC-mediated Na(+) transport in epithelia. Finally, substitution experiments reveal that multiple cysteine residues in the amino and carboxyl termini of ENaC subunits are responsible for this thiol-mediated inhibition of ENaC.