Identification of extracellular domain residues required for epithelial Na+ channel activation by acidic pH

Extracellular intersubunit interactions modulate epithelial Na+ channel gating

Epithelial Na+ channels (ENaCs) and related channels have large extracellular domains where specific factors interact and induce conformational changes, leading to altered channel activity. However, extracellular structural transitions associated with changes in ENaC activity are not well defined. Using crosslinking and two-electrode voltage clamp in Xenopus oocytes, we identified several pairs of functional intersubunit contacts where mouse ENaC activity was modulated by inducing or breaking a disulfide bond between introduced Cys residues. Specifically, crosslinking E499C in the β-subunit palm domain and N510C in the α-subunit palm domain activated ENaC, whereas crosslinking βE499C with αQ441C in the α-subunit thumb domain inhibited ENaC. We determined that bridging βE499C to αN510C or αQ441C altered the Na+ self-inhibition response via distinct mechanisms. Similar to bridging βE499C and αQ441C, we found that crosslinking palm domain αE557C with thumb domain γQ398C strongly inhibited ENaC activity. In conclusion, we propose that certain residues at specific subunit interfaces form microswitches that convey a conformational wave during ENaC gating and its regulation.

Accessibility of ENaC extracellular domain central core residues

The epithelial Na+ channel (ENaC)/degenerin family has a similar extracellular architecture, where specific regulatory factors interact and alter channel gating behavior. The extracellular palm domain serves as a key link to the channel pore. In this study, we used cysteine-scanning mutagenesis to assess the functional effects of Cys-modifying reagents on palm domain β10 strand residues in mouse ENaC. Of the 13 ENaC α subunit mutants with Cys substitutions examined, only mutants at sites in the proximal region of β10 exhibited changes in channel activity in response to methanethiosulfonate reagents. Additionally, Cys substitutions at three proximal sites of β and γ subunit β10 strands also rendered mutant channels methanethiosulfonate-responsive. Moreover, multiple Cys mutants were activated by low concentrations of thiophilic Cd2+. Using the Na+ self-inhibition response to assess ENaC gating behavior, we identified four α, two β, and two γ subunit β10 strand mutations that changed the Na+ self-inhibition response. Our results suggest that the proximal regions of β10 strands in all three subunits are accessible to small aqueous compounds and Cd2+ and have a role in modulating ENaC gating. These results are consistent with a structural model of mouse ENaC that predicts the presence of aqueous tunnels adjacent to the proximal part of β10 and with previously resolved structures of a related family member where palm domain structural transitions were observed with channels in an open or closed state.

Atomic Force Microscopy (AFM) Applications in Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited heart muscle disorder characterized by progressive replacement of cardiomyocytes by fibrofatty tissue, ventricular dilatation, cardiac dysfunction, arrhythmias, and sudden cardiac death. Interest in molecular biomechanics for these disorders is constantly growing. Atomic force microscopy (AFM) is a well-established technic to study the mechanobiology of biological samples under physiological and pathological conditions at the cellular scale. However, a review which described all the different data that can be obtained using the AFM (cell elasticity, adhesion behavior, viscoelasticity, beating force, and frequency) is still missing. In this review, we will discuss several techniques that highlight the potential of AFM to be used as a tool for assessing the biomechanics involved in ACM. Indeed, analysis of genetically mutated cells with AFM reveal abnormalities of the cytoskeleton, cell membrane structures, and defects of contractility. The higher the Young’s modulus, the stiffer the cell, and it is well known that abnormal tissue stiffness is symptomatic of a range of diseases. The cell beating force and frequency provide information during the depolarization and repolarization phases, complementary to cell electrophysiology (calcium imaging, MEA, patch clamp). In addition, original data is also presented to emphasize the unique potential of AFM as a tool to assess fibrosis in cardiac tissue.

Analyses of epithelial Na+ channel variants reveal that an extracellular β-ball domain critically regulates ENaC gating

Epithelial Na⁺ channel (ENaC)-mediated Na⁺ transport has a key role in the regulation of extracellular fluid volume, blood pressure, and extracellular [K⁺]. Among the thousands of human ENaC variants, only a few exist whose functional consequences have been experimentally tested. Here, we used the Xenopus oocyte expression system to investigate the functional roles of four nonsynonymous human ENaC variants located within the β7-strand and its adjacent loop of the α-subunit extracellular β-ball domain. αR350Wβγ and αG355Rβγ channels exhibited 2.5- and 1.8-fold greater amiloride-sensitive currents than WT αβγ human ENaCs, respectively, whereas αV351Aβγ channels conducted significantly less current than WT. Currents in αH354Rβγ-expressing oocytes were similar to those expressing WT. Surface expression levels of three mutants (αR350Wβγ, αV351Aβγ, and αG355Rβγ) were similar to that of WT. However, three mutant channels (αR350Wβγ, αH354Rβγ, and αG355Rβγ) exhibited a reduced Na⁺ self-inhibition response. Open probability of αR350Wβγ was significantly greater than that of WT. Moreover, other Arg-350 variants, including αR350G, αR350L, and αR350Q, also had significantly increased channel activity. A direct comparison of αR350W and two previously reported gain-of-function variants revealed that αR350W increases ENaC activity similarly to αW493R, but to a much greater degree than does αC479R. Our results indicate that αR350W along with αR350G, αR350L, and αR350Q, and αG355R are novel gain-of-function variants that function as gating modifiers. The location of these multiple functional variants suggests that the αENaC β-ball domain portion that interfaces with the palm domain of βENaC critically regulates ENaC gating.

An extracellular acidic cleft confers profound H+-sensitivity to epithelial sodium channels containing the δ-subunit in Xenopus laevis

The limited sodium availability of freshwater and terrestrial environments was a major physiological challenge during vertebrate evolution. The epithelial sodium channel (ENaC) is present in the apical membrane of sodium-absorbing vertebrate epithelia and evolved as part of a machinery for efficient sodium conservation. ENaC belongs to the degenerin/ENaC protein family and is the only member that opens without an external stimulus. We hypothesized that ENaC evolved from a proton-activated sodium channel present in ionocytes of freshwater vertebrates and therefore investigated whether such ancestral traits are present in ENaC isoforms of the aquatic pipid frog Xenopus laevis Using whole-cell and single-channel electrophysiology of Xenopus oocytes expressing ENaC isoforms assembled from αβγ- or δβγ-subunit combinations, we demonstrate that Xenopus δβγ-ENaC is profoundly activated by extracellular acidification within biologically relevant ranges (pH 8.0-6.0). This effect was not observed in Xenopus αβγ-ENaC or human ENaC orthologs. We show that protons interfere with allosteric ENaC inhibition by extracellular sodium ions, thereby increasing the probability of channel opening. Using homology modeling of ENaC structure and site-directed mutagenesis, we identified a cleft region within the extracellular loop of the δ-subunit that contains several acidic amino acid residues that confer proton-sensitivity and enable allosteric inhibition by extracellular sodium ions. We propose that Xenopus δβγ-ENaC can serve as a model for investigating ENaC transformation from a proton-activated toward a constitutively-active ion channel. Such transformation might have occurred during the evolution of tetrapod vertebrates to enable bulk sodium absorption during the water-to-land transition.

The epithelial Na+ channel γ subunit autoinhibitory tract suppresses channel activity by binding the γ subunit's finger-thumb domain interface

Epithelial Na⁺ channel (ENaC) maturation and activation require proteolysis of both the α and γ subunits. Cleavage at multiple sites in the finger domain of each subunit liberates their autoinhibitory tracts. Synthetic peptides derived from the proteolytically released fragments inhibit the channel, likely by reconstituting key interactions removed by the proteolysis. We previously showed that a peptide derived from the α subunit's autoinhibitory sequence (α-8) binds at the α subunit's finger-thumb domain interface. Despite low sequence similarity between the α and γ subunit finger domains, we hypothesized that a peptide derived from the γ subunit's autoinhibitory sequence (γ-11) inhibits the channel through an analogous mechanism. Using Xenopus oocytes, we found here that channels lacking a γ subunit thumb domain were no longer sensitive to γ-11, but remained sensitive to α-8. We identified finger domain sites in the γ subunit that dramatically reduced γ-11 inhibition. Using cysteines and sulfhydryl reactive cross-linkers introduced into both the peptide and the subunit, we also could cross-link γ-11 to both the finger domain and the thumb domain of the γ subunit. Our results suggest that α-8 and γ-11 occupy similar binding pockets within their respective subunits, and that proteolysis of the α and γ subunits activate the channel through analogous mechanisms.

Incorporation of the δ-subunit into the epithelial sodium channel (ENaC) generates protease-resistant ENaCs in Xenopus laevis

The epithelial sodium channel (ENaC) is a critical regulator of vertebrate electrolyte homeostasis. ENaC is the only constitutively open ion channel in the degenerin/ENaC protein family, and its expression, membrane abundance, and open probability therefore are tightly controlled. The canonical ENaC is composed of three subunits (α, β, and γ), but a fourth δ-subunit may replace α and form atypical δβγ-ENaCs. Using Xenopus laevis as a model, here we found that mRNAs of the α- and δ-subunits are differentially expressed in different tissues and that δ-ENaC predominantly is present in the urogenital tract. Using whole-cell and single-channel electrophysiology of oocytes expressing Xenopus αβγ- or δβγ-ENaC, we demonstrate that the presence of the δ-subunit enhances the amount of current generated by ENaC due to an increased open probability, but also changes current into a transient form. Activity of canonical ENaCs is critically dependent on proteolytic processing of the α- and γ-subunits, and immunoblotting with epitope-tagged ENaC subunits indicated that, unlike α-ENaC, the δ-subunit does not undergo proteolytic maturation by the endogenous protease furin. Furthermore, currents generated by δβγ-ENaC were insensitive to activation by extracellular chymotrypsin, and presence of the δ-subunit prevented cleavage of γ-ENaC at the cell surface. Our findings suggest that subunit composition constitutes an additional level of ENaC regulation, and we propose that the Xenopus δ-ENaC subunit represents a functional example that demonstrates the importance of proteolytic maturation during ENaC evolution.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

The role of pendrin in blood pressure regulation

Pendrin is a Na(+)-independent Cl(-)/HCO3(-) exchanger found in the apical regions of type B and non-A, non-B intercalated cells within the aldosterone-sensitive region of the nephron, i.e., the distal convoluted tubule (DCT), the connecting tubule (CNT), and the cortical collecting duct (CCD). Type B intercalated cells mediate Cl(-) absorption and HCO3(-) secretion primarily through pendrin-mediated Cl(-)/HCO3(-) exchange. This exchanger is upregulated with angiotensin II administration and in models of metabolic alkalosis, such as following administration of aldosterone or NaHCO3. In the absence of pendrin-mediated HCO3(-) secretion, an enhanced alkalosis is observed following aldosterone or NaHCO3 administration. However, probably of more significance is the role of pendrin in the pressor response to aldosterone. Pendrin mediates Cl(-) absorption and modulates aldosterone-induced Na(+) absorption mediated by the epithelial Na channel (ENaC). Pendrin changes ENaC activity by changing both channel open probability (Po) and surface density (N), at least partly by altering luminal HCO3(-) and ATP concentration. Thus aldosterone and angiotensin II stimulate pendrin expression and function, which stimulates ENaC activity, thereby contributing to the pressor response of these hormones. However, pendrin may modulate blood pressure partly through its extrarenal effects. For example, pendrin is expressed in the adrenal medulla, where it modulates catecholamine release. The increase in catecholamine release observed with pendrin gene ablation likely contributes to the increment in vascular contractile force observed in the pendrin null mouse. This review summarizes the signaling mechanisms that regulate pendrin abundance and function as well as the contribution of pendrin to distal nephron function.

Changes in pH and NADPH regulate the DNA binding activity of neuronal PAS domain protein 2, a mammalian circadian transcription factor

Neuronal PAS domain protein 2 (NPAS2) is a core clock transcription factor that forms a heterodimer with BMAL1 to bind the E-box in the promoter of clock genes and is regulated by various environmental stimuli such as heme, carbon monoxide, and NAD(P)H. In this study, we investigated the effects of pH and NADPH on the DNA binding activity of NPAS2. In an electrophoretic mobility shift (EMS) assay, the pH of the reaction mixture affected the DNA binding activity of the NPAS2/BMAL1 heterodimer but not that of the BMAL1/BMAL1 homodimer. A change in pH from 7.0 to 7.5 resulted in a 1.7-fold increase in activity in the absence of NADPH, and NADPH additively enhanced the activity up to 2.7-fold at pH 7.5. The experiments using truncated mutants revealed that N-terminal amino acids 1-61 of NPAS2 were sufficient to sense the change in both pH and NADPH. We further analyzed the kinetics of formation and DNA binding of the NPAS2/BMAL1 heterodimer at various pH values. In the absence of NADPH, a change in pH from 6.5 to 8.0 decreased the KD(app) value of the E-box from 125 to 22 nM, with an 8-fold increase in the maximal level of DNA binding for the NPAS2/BMAL1 heterodimer. The addition of NADPH resulted in a further decrease in KD(app) to 9 nM at pH 8.0. Furthermore, NPAS2-dependent transcriptional activity in a luciferase assay using NIH3T3 cells also increased with the pH of the culture medium. These results suggest that NPAS2 has a role as a pH and metabolite sensor in regulating circadian rhythms.

Na+ inhibits the epithelial Na+ channel by binding to a site in an extracellular acidic cleft

The epithelial Na(+) channel (ENaC) has a key role in the regulation of extracellular fluid volume and blood pressure. ENaC belongs to a family of ion channels that sense the external environment. These channels have large extracellular regions that are thought to interact with environmental cues, such as Na(+), Cl(-), protons, proteases, and shear stress, which modulate gating behavior. We sought to determine the molecular mechanism by which ENaC senses high external Na(+) concentrations, resulting in an inhibition of channel activity. Both our structural model of an ENaC α subunit and the resolved structure of an acid-sensing ion channel (ASIC1) have conserved acidic pockets in the periphery of the extracellular region of the channel. We hypothesized that these acidic pockets host inhibitory allosteric Na(+) binding sites. Through site-directed mutagenesis targeting the acidic pocket, we modified the inhibitory response to external Na(+). Mutations at selected sites altered the cation inhibitory preference to favor Li(+) or K(+) rather than Na(+). Channel activity was reduced in response to restraining movement within this region by cross-linking structures across the acidic pocket. Our results suggest that residues within the acidic pocket form an allosteric effector binding site for Na(+). Our study supports the hypothesis that an acidic cleft is a key ligand binding locus for ENaC and perhaps other members of the ENaC/degenerin family.

Intersubunit conformational changes mediate epithelial sodium channel gating

Residues forming interfaces between the three ENaC subunits participate in conformational changes required for transition between open and closed states.

The epithelial Na⁺ channel (ENaC) functions as a pathway for Na⁺ absorption in the kidney and lung, where it is crucial for Na⁺ homeostasis and blood pressure regulation. However, the basic mechanisms that control ENaC gating are poorly understood. Here we define a role in gating for residues forming interfaces between the extracellular domains of the three ENaC subunits. Using cysteine substitution combined with chemical cross-linking, we determined that residues located at equivalent positions in the three subunits (α_K477, β_E446, and γ_E455) form interfaces with residues in adjacent subunits (β_V85, γ_V87, and α_L120, respectively). Cross-linking of these residues altered ENaC activity in a length-dependent manner; long cross-linkers increased ENaC current by increasing its open probability, whereas short cross-linkers reduced ENaC open probability. Cross-linking also disrupted ENaC gating responses to extracellular pH and Na⁺, signals which modulate ENaC activity during shifts in volume status. Introduction of charged side chains at the interfacing residues altered ENaC activity in a charge-dependent manner. Current increased when like charges were present at both interfacing residues, whereas opposing charges reduced current. Together, these data indicate that conformational changes at intersubunit interfaces participate in ENaC transitions between the open and closed states; movements that increase intersubunit distance favor the open state, whereas the closed state is favored when the distance is reduced. This provides a mechanism to modulate ENaC gating in response to changing extracellular conditions that threaten Na⁺ homeostasis.

Inhibition of ASICs reduces rat hepatic stellate cells activity and liver fibrosis: an in vitro and in vivo study

Hepatic fibrosis is a chronic inflammation-associated disease, which is involved in the infiltration of inflammatory cells and releasing of proinflammatory cytokines. In the pathological process, protons are released by damaged cells and acidosis is considered to play a critical role in cell injury. Although the underlying mechanism (s) remain ill-defined, ASICs (acid-sensing ion channels) are assumed to be involved in this process. The diuretic, amiloride, is neuroprotective in models of cerebral ischemia, a property attributable to the inhibition of central ASICs by the drug. However, the effect of inhibition of ASICs by amiloride in the liver fibrotic process remains unclear. We found that amiloride (25, 50, or 100 μM) could restrain acid-induced HSCs at pH6 in vitro. In vivo experiments showed that amiloride could significantly alleviate liver injury, decreasing levels of profibrogenic cytokines, collagen deposition, and reducing pathological tissue damage. In summary, amiloride inhibits hepatic fibrosis in vivo and in vitro, which is probably associated with the downregulation of ASICs.

Developmental changes of ENaC expression and function in the inner ear of pendrin knock-out mice as a perspective on the development of endolymphatic hydrops

Pendrin mutations cause enlarged vestibular aqueducts and various degrees of sensorineural hearing loss. The selective abolition of pendrin causes dilation of the membranous labyrinth known as endolymphatic hydrops, loss of the endocochlear potential, and consequently loss of hearing function. Because Na+ transport is one of the most important driving forces for fluid transport, the epithelial Na+ channel (ENaC) is believed to play an important role in fluid volume regulation in the inner ear. Therefore, the dysfunction of Na+ transport through ENaC by the acidification of endolymph in Pendred syndrome is one of the potential causes of endolymphatic hydrops. We investigated the changes of ENaC expression and function during the development of the pendrin knock-out mouse. In the cochlea, the expression of β and γENaC was significantly increased at P56 in Pds-/- mice compared with Pds+/+ mice. In the vestibule, the expression of βENaC was significantly increased at P56, and γENaC expression significantly increased from P6 to P56 in Pds-/- mice. The ENaC-dependent trans-epithelial current was not significantly different between Pds+/+ and Pds-/- mice in Reissner's membrane or the saccular extramacular roof epithelium at P0, but the current was significantly increased in Pds-/- mice at P56 compared with Pds+/+ mice. These findings indicate that the expression and function of ENaC were enhanced in Pds-/- mice after the development of endolymphatic hydrops as a compensatory mechanism. This result provides insight into the role of Na+ transport in the development and regulation of endolymphatic hydrops due to pendrin mutations.