External nickel inhibits epithelial sodium channel by binding to histidine residues within the extracellular domains of alpha and gamma subunits and reducing channel open probability

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.

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.

Unravelling the molecular mechanisms of nickel in woodlice

During the last few years, there has been an alarming increase in the amount of nickel (Ni) being released into the environment, primarily due to its use in the production of stainless steel but also from other sources such as batteries manufacturing and consequent disposal. The established biotic ligand models provide precise estimates for Ni bioavailability, in contrast, studies describing the mechanisms underpinning toxicological effect of Ni are scarce. This study exploits RNA-seq to determine the transcriptomic responses of isopods using Porcellionides pruinosus as an example of a terrestrial metal-resistant woodlouse. Furthermore, the recently proposed model for Ni adverse outcome pathways (Ni-AOP) presents an unprecedented opportunity to fit isopod responses to Ni toxicity and define Porcellionides pruinosus as a metalomic model. Prior to this study, P. pruinosus represented an important environmental sentinel, though lacking genetic/omic data. The reference transcriptome generated here thus represents a major advance and a novel resource. A detailed annotation of the transcripts obtained is presented together with the homology to genes/gene products from Metazoan and Arthropoda phylum, Gene Ontology (GO) classification, clusters of orthologous groups (COG) and assignment to KEGG metabolic pathways. The differential gene expression comparison was determined in response to nickel (Ni) exposure and used to derive the enriched pathways and processes. It revealed a significant impact on ion trafficking and storage, oxidative stress, neurotoxicity, reproduction impairment, genetics and epigenetics. Many of the processes observed support the current Ni-AOP although the data highlights that the current model can be improved by including epigenetic endpoints, which represents key chronic risks under a scenario of Ni toxicity.

Thumb domains of the three epithelial Na+ channel subunits have distinct functions

The epithelial Na+ channel (ENaC) possesses a large extracellular domain formed by a β-strand core enclosed by three peripheral α-helical subdomains, which have been dubbed thumb, finger, and knuckle. Here we asked whether the ENaC thumb domains play specific roles in channel function. To this end, we examined the characteristics of channels lacking a thumb domain in an individual ENaC subunit (α, β, or γ). Removing the γ subunit thumb domain had no effect on Na+ currents when expressed in Xenopus oocytes, but moderately reduced channel surface expression. In contrast, ENaCs lacking the α or β subunit thumb domain exhibited significantly reduced Na+ currents along with a large reduction in channel surface expression. Moreover, channels lacking an α or γ thumb domain exhibited a diminished Na+ self-inhibition response, whereas this response was retained in channels lacking a β thumb domain. In turn, deletion of the α thumb domain had no effect on the degradation rate of the immature α subunit as assessed by cycloheximide chase analysis. However, accelerated degradation of the immature β subunit and mature γ subunit was observed when the β or γ thumb domain was deleted, respectively. Our results suggest that the thumb domains in each ENaC subunit are required for optimal surface expression in oocytes and that the α and γ thumb domains both have important roles in the channel's inhibitory response to external Na+ Our findings support the notion that the extracellular helical domains serve as functional modules that regulate ENaC biogenesis and activity.

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.

The Epithelial Sodium Channel Is a Modifier of the Long-Term Nonprogressive Phenotype Associated with F508del CFTR Mutations

Cystic fibrosis (CF) remains the most lethal genetic disease in the Caucasian population. However, there is great variability in clinical phenotypes and survival times, even among patients harboring the same genotype. We identified five patients with CF and a homozygous F508del mutation in the CFTR gene who were in their fifth or sixth decade of life and had shown minimal changes in lung function over a longitudinal period of more than 20 years. Because of the rarity of this long-term nonprogressive phenotype, we hypothesized these individuals may carry rare genetic variants in modifier genes that ameliorate disease severity. Individuals at the extremes of survival time and lung-function trajectory underwent whole-exome sequencing, and the sequencing data were filtered to include rare missense, stopgain, indel, and splicing variants present with a mean allele frequency of <0.2% in general population databases. Epithelial sodium channel (ENaC) mutants were generated via site-directed mutagenesis and expressed for Xenopus oocyte assays. Four of the five individuals carried extremely rare or never reported variants in the SCNN1D and SCNN1B genes of the ENaC. Separately, an independently enriched rare variant in SCNN1D was identified in the Exome Variant Server database associated with a milder pulmonary disease phenotype. Functional analysis using Xenopus oocytes revealed that two of the three variants in δ-ENaC encoded by SCNN1D exhibited hypomorphic channel activity. Our data suggest a potential role for δ-ENaC in controlling sodium reabsorption in the airways, and advance the plausibility of ENaC as a therapeutic target in CF.

Altered ion transport in normal human bronchial epithelial cells following exposure to chemically distinct metal welding fume particles

Welding fume inhalation causes pulmonary toxicity, including susceptibility to infection. We hypothesized that airway epithelial ion transport is a target of fume toxicity, and investigated the effects of fume particulates from manual metal arc-stainless steel (MMA-SS) and gas metal arc-mild steel (GMA-MS) on ion transport in normal human bronchial epithelium (NHBE) cultured in air-interface. MMA-SS particles, more soluble than GMA-MS particles, contain Cr, Ni, Fe and Mn; GMA-MS particles contain Fe and Mn. MMA-SS or GMA-MS particles (0.0167-166.7μg/cm²) were applied apically to NHBEs. After 18h transepithelial potential difference (V_t), resistance (R_t), and short circuit current (I_sc) were measured. Particle effects on Na⁺ and Cl¯ channels and the Na⁺,K⁺,2Cl¯-cotransporter were evaluated using amiloride (apical), 5-nitro-2-[(3-phenylpropyl)amino]benzoic acid (NPPB, apical), and bumetanide (basolateral), respectively. MMA-SS (0.0167-16.7μg/cm²) increased basal V_t. Only 16.7μg/cm² GMA-MS increased basal V_t significantly. MMA-SS or GMA-MS exposure potentiated I_sc responses (decreases) to amiloride and bumetanide, while not affecting those to NPPB, GMA-MS to a lesser degree than MMA-SS. Variable effects on R_t were observed in response to amiloride, and bumetanide. Generally, MMA-SS was more potent in altering responses to amiloride and bumetanide than GMA-MS. Hyperpolarization occurred in the absence of LDH release, but decreases in V_t, R_t, and I_sc at higher fume particulate doses accompanied LDH release, to a greater extent for MMA-SS. Thus, Na⁺ transport and Na⁺,K⁺,2Cl¯-cotransport are affected by fume exposure; MMA-MS is more potent than GMA-MS. Enhanced Na⁺ absorption and decreased airway surface liquid could compromise defenses against infection.

Gadolinium released by the linear gadolinium-based contrast-agent Gd-DTPA decreases the activity of human epithelial Na+ channels (ENaCs)

BACKGROUND

Gadolinium-based-contrast-agents (GBCAs) are used for magnetic-resonance-imaging and associated with renal and cardiovascular adverse reactions caused by released Gd³⁺ ions. Gd³⁺ is also a modulator of mechano-gated ion channels, including the epithelial Na⁺ channel (ENaC) that is expressed in kidney epithelium and the vasculature. ENaC is important for salt-/water homeostasis and blood pressure regulation and a likely target of released Gd³⁺ from GBCAs causing the above-mentioned adverse reactions. Therefore this study examined the effect of Gd³⁺ and GBCAs on ENaC's activity.

METHODS

Human αβγENaC was expressed in Xenopus laevis oocytes and exposed to Gd³⁺, linear (Gd-DTPA, Magnevist) or cyclic (Dotarem) GBCAs. Transmembrane ion-currents (I_M) were recorded by the two-electrode-voltage-clamp technique and Gd³⁺-release by Gd-DTPA was confirmed by inductively coupled plasma-mass spectrometry.

RESULTS

Gd³⁺ exerts biphasic effects on ENaC's activity: ≤0.3mmol/l decreased I_M which was preventable by DEPC (modifies histidines). Strikingly Gd³⁺≥0.4mmol/l increased I_M and this effect was prevented by cysteine-modifying MTSEA. Linear Gd-DTPA and Magnevist mimicked the effect of ≤0.3mmol/l Gd³⁺, whereas the chelator DTPA showed no effect. Gd³⁺ and Gd-DTPA increased the IC₅₀ for amiloride, but did not affect ENaC's self-inhibition. Interestingly, cyclic Gd-DOTA (Dotarem) increased I_M to a similar extent as its chelator DOTA, suggesting that the chelator rather than released Gd³⁺ is responsible for this effect.

CONCLUSION

These results confirm Gd³⁺-release from linear Gd-DTPA and indicate that the released Gd³⁺ amount is sufficient to interfere with ENaC's activity to provide putative explanations for GBCA-related adverse effects.

Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response

The epithelial sodium channel (ENaC) is composed of three homologous subunits that form a triangular pyramid-shaped funnel, anchored in the membrane with a stem of six transmembrane domains. We examined the structure-function relationships of 17 conserved charged residues on the surface of the ectodomain of human γ-ENaC subunit by alanine mutagenesis and co-expression with α- and β-ENaC subunits in Xenopus oocytes. The results showed that Na(+) conductance of cells expressing these mutants can be accounted for by two parameters: (a) the ENaC density on the cell surface as measured by the fluorescence of an α-EnaC-yellow fluorescent protein hybrid and (b) the sodium self-inhibition (SSI) response that reflects the open probability of the channel (Po). Overall, the activity of all 17 mutants was correlated with surface levels of ENaC. There was no significant correlation between these parameters measured for α- and γ-ENaC subunit mutants at nine homologous positions. Thus, the functions of most of the homologous surface residues examined differ between the two subunits. Only four mutants (K328, D510, R514 and E518) significantly reduced the SSI response. The α-ENaC homologs of three of these (R350, E530 and E538) also severely affected the SSI response. The cASIC1 homologs of these (K247, E417, Q421) are located at the interface between subunits, on or about the ion pathway at the rotational symmetry axis in the center of the trimer. Thus, it is likely that these residues are involved in conformational changes that lead to channel constriction and the SSI response upon Na(+) ion flooding.

Deletion of α-subunit exon 11 of the epithelial Na+ channel reveals a regulatory module

Epithelial Na(+) channel (ENaC) subunits (α, β, and γ) found in functional complexes are translated from mature mRNAs that are similarly processed by the inclusion of 13 canonical exons. We examined whether individual exons 3-12, encoding the large extracellular domain, are required for functional channel expression. Human ENaCs with an in-frame deletion of a single α-subunit exon were expressed in Xenopus oocytes, and their functional properties were examined by two-electrode voltage clamp. With the exception of exon 11, deletion of an individual exon eliminated channel activity. Channels lacking α-subunit exon 11 were hyperactive. Oocytes expressing this mutant exhibited fourfold greater amiloride-sensitive whole cell currents than cells expressing wild-type channels. A parallel fivefold increase in channel open probability was observed with channels lacking α-subunit exon 11. These mutant channels also exhibited a lost of Na(+) self-inhibition, whereas we found similar levels of surface expression of mutant and wild-type channels. In contrast, in-frame deletions of exon 11 from either the β- or γ-subunit led to a significant loss of channel activity, in association with a marked decrease in surface expression. Our results suggest that exon 11 within the three human ENaC genes encodes structurally homologous yet functionally diverse domains and that exon 11 in the α-subunit encodes a module that regulates channel gating.

Ni(2+) effects on Nicotiana tabacum L. pollen germination and pollen tube growth

To investigate the mechanisms of Ni(2+) effects on initiation and maintenance of polar cell growth, we used a well-studied model system-germination of angiosperm pollen grains. In liquid medium tobacco pollen grain forms a long tube, where the growth is restricted to the very tip. Ni(2+) did not prevent the formation of pollen tube initials, but inhibited their subsequent growth with IC(50) = 550 μM. 1 mM Ni(2+) completely blocked the polar growth, but all pollen grains remained viable, their respiration was slightly affected and ROS production did not increase. Addition of Ni(2+) after the onset of germination had a bidirectional effect on the tubes development: there was a considerable amount of extra-long tubes, which appeared to be rapidly growing, but the growth of many tubes was impaired. Studying the localization of possible targets of Ni(2+) influence, we found that they may occur both in the wall and in the cytoplasm, as confirmed by specific staining. Ni(2+) disturbed the segregation of transport vesicles in the tips of these tubes and significantly reduced the relative content of calcium in the aperture area of pollen grains, as measured by X-ray microanalysis. These factors are considered being critical for normal polar cell growth. Ni(2+) also causes the deposition of callose in the tips of the tube initials and the pollen tubes that had stopped their growth. We can assume that Ni(2+)-induced disruption of calcium homeostasis can lead to vesicle traffic impairment and abnormal callose deposition and, consequently, block the polar growth.

Gain-of-function variant of the human epithelial sodium channel

Epithelial Na(+) channel (ENaC) mutations are associated with several human disorders, underscoring the importance of these channels in human health. Recent human genome sequencing projects have revealed a large number of ENaC gene variations, several of which have been found in individuals with salt-sensitive hypertension, cystic fibrosis, and other disorders. However, the functional consequences of most variants are unknown. In this study, we used the Xenopus oocyte expression system to examine the functional properties of a human ENaC variant. Oocytes expressing αβγL511Q human ENaCs showed 4.6-fold greater amiloride-sensitive currents than cells expressing wild-type channels. The γL511Q variant did not significantly alter channel surface expression. Single channel recordings revealed that the variant had fourfold higher open probability than wild type. In addition, γL511Q largely eliminated the Na(+) self-inhibition response, which reflects a downregulation of ENaC open probability by extracellular Na(+). Moreover, γL511Q diminished chymotrypsin-induced activation of the mutant channel. We conclude that γL511Q is a gain-of-function human ENaC variant. Our results suggest that γL511Q enhances ENaC activity by increasing channel open probability and dampens channel regulation by extracellular Na(+) and proteases.

Probing the structural basis of Zn2+ regulation of the epithelial Na+ channel

Extracellular Zn(2+) activates the epithelial Na(+) channel (ENaC) by relieving Na(+) self-inhibition. However, a biphasic Zn(2+) dose response was observed, suggesting that Zn(2+) has dual effects on the channel (i.e. activating and inhibitory). To investigate the structural basis for this biphasic effect of Zn(2+), we examined the effects of mutating the 10 extracellular His residues of mouse γENaC. Four mutations within the finger subdomain (γH193A, γH200A, γH202A, and γH239A) significantly reduced the maximal Zn(2+) activation of the channel. Whereas γH193A, γH200A, and γH202A reduced the apparent affinity of the Zn(2+) activating site, γH239A diminished Na(+) self-inhibition and thus concealed the activating effects of Zn(2+). Mutation of a His residue within the palm subdomain (γH88A) abolished the low-affinity Zn(2+) inhibitory effect. Based on structural homology with acid-sensing ion channel 1, γAsp(516) was predicted to be in close proximity to γHis(88). Ala substitution of the residue (γD516A) blunted the inhibitory effect of Zn(2+). Our results suggest that external Zn(2+) regulates ENaC activity by binding to multiple extracellular sites within the γ-subunit, including (i) a high-affinity stimulatory site within the finger subdomain involving His(193), His(200), and His(202) and (ii) a low-affinity Zn(2+) inhibitory site within the palm subdomain that includes His(88) and Asp(516).

Characterization of a novel splice variant of δ ENaC subunit in human lungs

Salt absorption via apical epithelial sodium channels (ENaC) is a critical rate-limiting process in maintaining airway and lung lining fluid at the physiological level. δ ENaC (termed δ1 in this article) has been detected in human lung epithelial cells in addition to α, β, and γ subunits (Ji HL, Su XF, Kedar S, Li J, Barbry P, Smith PR, Matalon S, Benos DJ. J Biol Chem 281: 8233-8241, 2006; Nie HG, Chen L, Han DY, Li J, Song WF, Wei SP, Fang XH, Gu X, Matalon S, Ji HL, J Physiol 587: 2663-2676, 2009) and may contribute to the differences in the biophysical properties of amiloride-inhibitable cation channels in pulmonary epithelial cells. Here we cloned a splicing variant of the δ1 ENaC, namely, δ2 ENaC in human bronchoalveolar epithelial cells (16HBEo). δ2 ENaC possesses 66 extra amino acids attached to the distal amino terminal tail of the δ1 ENaC. δ2 ENaC was expressed in both alveolar type I and II cells of human lungs as revealed by in situ hybridization and real-time RT-PCR. To characterize the biophysical and pharmacological features of the splicing variant, we injected Xenopus oocytes with human ENaC cRNAs and measured whole cell and single channel currents of δ1βγ, δ2βγ, and αβγ channels. Oocytes injected with δ2βγ cRNAs exhibited whole cell currents significantly greater than those expressing δ1βγ and αβγ channels. Single channel activity, unitary conductance, and open probability of δ2βγ channels were significantly greater compared with δ1βγ and αβγ channels. In addition, δ2βγ and δ1βγ channels displayed significant differences in apparent Na(+) affinity, dissociation constant for amiloride (K(i)(amil)), the EC(50) for capsazepine activation, and gating kinetics by protons. Channels comprising of this novel splice variant may contribute to the diversities of native epithelial Na(+) channels.

Epithelial Na(+) channel regulation by cytoplasmic and extracellular factors

Electrogenic Na(+) transport across high resistance epithelial is mediated by the epithelial Na(+) channel (ENaC). Our understanding of the mechanisms of ENaC regulation has continued to evolve over the two decades following the cloning of ENaC subunits. This review highlights many of the cellular and extracellular factors that regulate channel trafficking or gating.

External Cu2+ inhibits human epithelial Na+ channels by binding at a subunit interface of extracellular domains

Epithelial Na(+) channels (ENaCs) play an essential role in the regulation of body fluid homeostasis. Certain transition metals activate or inhibit the activity of ENaCs. In this study, we examined the effect of extracellular Cu(2+) on human ENaC expressed in Xenopus oocytes and investigated the structural basis for its effects. External Cu(2+) inhibited human αβγ ENaC with an estimated IC(50) of 0.3 μM. The slow time course and a lack of change in the current-voltage relationship were consistent with an allosteric (non pore-plugging) inhibition of human ENaC by Cu(2+). Experiments with mixed human and mouse ENaC subunits suggested that both the α and β subunits were primarily responsible for the inhibitory effect of Cu(2+) on human ENaC. Lowering bath solution pH diminished the inhibition by Cu(2+). Mutations of two α, two β, and two γ His residues within extracellular domains significantly reduced the inhibition of human ENaC by Cu(2+). We identified a pair of residues as potential Cu(2+)-binding sites at the subunit interface between thumb subdomain of αhENaC and palm subdomain of βhENaC, suggesting a counterclockwise arrangement of α, β, and γ ENaC subunits in a trimeric channel complex when viewed from above. We conclude that extracellular Cu(2+) is a potent inhibitor of human ENaC and binds to multiple sites within the extracellular domains including a subunit interface.

8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate-Na stimulates human alveolar fluid clearance by releasing external Na+ self-inhibition of epithelial Na+ channels

Salt absorption via alveolar epithelial Na(+) channels (ENaC) is a critical step for maintaining an airspace free of flooding. Previously, we found that 8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate-Na (CPT-cGMP) activated native and heterologous ENaC. To investigate the potential pharmacological relevance, we applied this compound intratracheally to human lungs and found that ex vivo alveolar fluid clearance was increased significantly. Furthermore, this compound eliminated self-inhibition in human lung H441 cells and in oocytes expressing human αβγ but not δβγ channels. To further elucidate this novel mechanism, we constructed mutants abolishing (β(ΔV348) and γ(H233R)) or augmenting (α(Y458A) and γ(M432G)) self-inhibition. The mutants eliminating self-inhibition lost their responses to CPT-cGMP, whereas those enhancing self-inhibition facilitated the stimulatory effects of this compound. CPT-cGMP was unable to activate a high P(o) mutant (β(S520C)) and plasmin proteolytically cleaved channels. Our data suggest that elimination of self-inhibition of αβγ ENaC may be a novel mechanism for CPT-cGMP to stimulate salt reabsorption in human lungs.

Nickel toxicity in embryos and larvae of the South American toad: effects on cell differentiation, morphogenesis, and oxygen consumption

Nickel, a widely distributed heavy metal in the biosphere, produces systemic, carcinogenic, and teratogenic effects. The objectives of the present study are to report the acute, short-term chronic, and chronic toxicity of Ni in Rhinella arenarum embryos as well as the stage-dependent susceptibility to this heavy metal, including oxygen consumption, teratogenesis, and adverse effects on cell differentiation processes. The stages evaluated were blastula (S.7), gastrula (S.11), tail bud (S.17), fin circulation (S.22), and complete operculum (S.25), in this last case by means of toxicity profile curves. Nickel increases its adverse effects gradually, with a maximum value after 96 h. The 50% lethal concentrations (LC50s) for 96, 168, and 240 h at S.25 were 1.14, 0.60, and 0.48 mg Ni²(+) /L, respectively; S.11 and S.22 were the least and most susceptible to Ni with, LC50s 96 h of 6.12 and 0.19 mg Ni²(+) /L, respectively. A reduction of approximately 25% in oxygen consumption anticipates lethal effects from S.17 onward. The main teratogenic effects were retarded growth and development, extremely severe axis incurvations, persistent yolk plug, asymmetry, microcephaly and mouth and gill agenesia, and limited neuromuscular activity. Ciliated cells were not functional. The possibility of associating the remarkable stage-dependent susceptibility to Ni with environmental changes during the evolutionary process is also considered.

Cpt-cAMP activates human epithelial sodium channels via relieving self-inhibition

External Na(+) self-inhibition is an intrinsic feature of epithelial sodium channels (ENaC). Cpt-cAMP regulates heterologous guinea pig but not rat αβγ ENaC in a ligand-gated manner. We hypothesized that cpt-cAMP may eliminate the self-inhibition of human ENaC thereby open channels. Regulation of self-inhibition by this compound in oocytes was analyzed using the two-electrode voltage clamp and Ussing chamber setups. External cpt-cAMP stimulated human but not rat and murine αβγ ENaC in a dose- and external Na(+) concentration-dependent fashion. Intriguingly, cpt-cAMP activated human δβγ more potently than αβγ channels, suggesting that structural diversity in ectoloop between human α, δ, and those ENaC of other species determines the stimulating effects of cpt-cAMP. Cpt-cAMP increased the ratio of stationary and maximal currents. Mutants having abolished self-inhibition (β(ΔV348) and γ(H233R)) almost completely eliminated cpt-cAMP mediated activation of ENaC. On the other hand, mutants both enhancing self-inhibition and elevating cpt-cAMP sensitivity increased the stimulating effects of the compound. This compound, however, could not activate already fully opened channels, e.g., degenerin mutation (αβ(S520C)γ) and the proteolytically cleaved ENaC by plasmin. Cpt-cAMP activated native ENaC to the same extent as that for heterologous ENaC in human lung epithelial cells. Our data demonstrate that cpt-cAMP, a broadly used PKA activator, stimulates human αβγ and δβγ ENaC channels by relieving self-inhibition.

Extracellular allosteric regulatory subdomain within the gamma subunit of the epithelial Na+ channel

The activity of the epithelial Na(+) channel (ENaC) is modulated by Na(+) self-inhibition, a down-regulation of the open probability of ENaC by extracellular Na(+). A His residue within the extracellular domain of gammaENaC (gammaHis(239)) was found to have a critical role in Na(+) self-inhibition. We investigated the functional roles of residues in the vicinity of this His by mutagenesis and analyses of Na(+) self-inhibition responses in Xenopus oocytes. Significant changes in the speed and magnitude of Na(+) self-inhibition were observed in 16 of the 47 mutants analyzed. These 16 mutants were distributed within a 22-residue tract. We further characterized this scanned region by examining the accessibility of introduced Cys residues to the sulfhydryl reagent MTSET. External MTSET irreversibly increased or decreased currents in 13 of 47 mutants. The distribution patterns of the residues where substitutions significantly altered Na(+) self-inhibition or/and conferred sensitivity to MTSET were consistent with the existence of two helices within this region. In addition, single channel recordings of the gammaH239F mutant showed that, in the absence of Na(+) self-inhibition and with an increased open probability, ENaCs still undergo transitions between open and closed states. We conclude that gammaHis(239) functions within an extracellular allosteric regulatory subdomain of the gamma subunit that has an important role in conferring the response of the channel to external Na(+).

Inhibition of lung fluid clearance and epithelial Na+ channels by chlorine, hypochlorous acid, and chloramines

We investigated the mechanisms by which chlorine (Cl(2)) and its reactive byproducts inhibit Na(+)-dependent alveolar fluid clearance (AFC) in vivo and the activity of amiloride-sensitive epithelial Na(+) channels (ENaC) by measuring AFC in mice exposed to Cl(2) (0-500 ppm for 30 min) and Na(+) and amiloride-sensitive currents (I(Na) and I(amil), respectively) across Xenopus oocytes expressing human alpha-, beta-, and gamma-ENaC incubated with HOCl (1-2000 microm). Both Cl(2) and HOCl-derived products decreased AFC in mice and whole cell and single channel I(Na) in a dose-dependent manner; these effects were counteracted by serine proteases. Mass spectrometry analysis of the oocyte recording medium identified organic chloramines formed by the interaction of HOCl with HEPES (used as an extracellular buffer). In addition, chloramines formed by the interaction of HOCl with taurine or glycine decreased I(Na) in a similar fashion. Preincubation of oocytes with serine proteases prevented the decrease of I(Na) by HOCl, whereas perfusion of oocytes with a synthetic 51-mer peptide corresponding to the putative furin and plasmin cleaving segment in the gamma-ENaC subunit restored the ability of HOCl to inhibit I(Na). Finally, I(Na) of oocytes expressing wild type alpha- and gamma-ENaC and a mutant form of beta ENaC (S520K), known to result in ENaC channels locked in the open position, were not altered by HOCl. We concluded that HOCl and its reactive intermediates (such as organic chloramines) inhibit ENaC by affecting channel gating, which could be relieved by proteases cleavage.

Airway surface liquid contains endogenous DNase activity which can be activated by exogenous magnesium

Introduction

The removal of highly viscous mucus from the airways is an important task in the treatment of chronic lung disease like in cystic fibrosis. The inhalation of recombinant human DNase-I (rhDNase-I) is used to facilitate the removal of tenacious airway secretions in different lung diseases and especially in CF. Little is known about endogenous DNase activity in the airway surface liquid. Therefore, we analysed bronchoalveolar lavage fluid (BAL) and exhaled breath condensate (EBC) for the presence of endogenous DNase activity.

Methods

The degradation of plasmid DNA by BAL from patients who had diagnostic bronchoscopy and bronchoalveolar lavage was analyzed. In a group of CF patients and healthy control volunteers the exhaled breath condensate was obtained and also analyzed for the ability to degrade plasmid DNA. In addition, the ability of magnesium to activate endogenous DNase activity in BAL and exhaled breath condensate was investigated.

Results

The analyzed BAL samples degraded plasmid DNA only after preincubation with magnesium. When analyzing the exhaled breath condensate the samples obtained from the healthy volunteers showed no DNase activity even after preincubation with magnesium, whereas in one of the two samples obtained from CF patients we found a DNase activity after preincubation with magnesium.

Conclusion

Increasing the magnesium concentration in the airway surface liquid by aerosolisation of magnesium solutions or oral magnesium supplements could improve the removal of highly viscous mucus in chronic lung disease by activating endogenous DNase activity.

Extracellular chloride regulates the epithelial sodium channel

The extracellular domain of the epithelial sodium channel ENaC is exposed to a wide range of Cl(-) concentrations in the kidney and in other epithelia. We tested whether Cl(-) alters ENaC activity. In Xenopus oocytes expressing human ENaC, replacement of Cl(-) with SO4(2-), H2PO4(-), or SCN(-) produced a large increase in ENaC current, indicating that extracellular Cl(-) inhibits ENaC. Extracellular Cl(-) also inhibited ENaC in Na+-transporting epithelia. The anion selectivity sequence was SCN(-) < SO4(2-) < H2PO4(-) < F(-) < I(-) < Cl(-) < Br(-). Crystallization of ASIC1a revealed a Cl(-) binding site in the extracellular domain. We found that mutation of corresponding residues in ENaC (alpha(H418A) and beta(R388A)) disrupted the response to Cl(-), suggesting that Cl(-) might regulate ENaC through an analogous binding site. Maneuvers that lock ENaC in an open state (a DEG mutation and trypsin) abolished ENaC regulation by Cl(-). The response to Cl(-) was also modulated by changes in extracellular pH; acidic pH increased and alkaline pH reduced ENaC inhibition by Cl(-). Cl(-) regulated ENaC activity in part through enhanced Na+ self-inhibition, a process by which extracellular Na+ inhibits ENaC. Together, the data indicate that extracellular Cl(-) regulates ENaC activity, providing a potential mechanism by which changes in extracellular Cl(-) might modulate epithelial Na+ absorption.

ASIC1 contributes to pulmonary vascular smooth muscle store-operated Ca(2+) entry

Acid-sensing ion channels (ASIC) are voltage-insensitive, cationic channels that have recently been identified in vascular smooth muscle (VSM). It is possible that ASIC contribute to vascular reactivity via Na(+) and Ca(2+) conductance; however, their function in VSM is largely unknown. In pulmonary VSM, store-operated Ca(2+) entry (SOCE) plays a significant role in vasoregulatory mechanisms such as hypoxic pulmonary vasoconstriction and receptor-mediated arterial constriction. Therefore, we hypothesized that ASIC contribute to SOCE in pulmonary VSM. We examined SOCE resulting from depletion of intracellular Ca(2+) stores with cyclopiazonic acid in isolated small pulmonary arteries and primary cultured pulmonary arterial smooth muscle cells by measuring 1) changes in VSM [Ca(2+)](i) using fura-2 indicator dye, 2) Mn(2+) quenching of fura-2 fluorescence, and 3) store-operated Ca(2+) and Na(+) currents using conventional whole cell patch-clamp configuration in voltage-clamp mode. The role of ASIC was assessed by the use of the ASIC inhibitors, amiloride, benzamil, and psalmotoxin 1, or siRNA directed towards ASIC1, ASIC2, or ASIC3 isoforms. We found that store-operated VSM [Ca(2+)](i) responses, Mn(2+) influx, and inward cationic currents were attenuated by either pharmacological ASIC inhibition or treatment with ASIC1 siRNA. These data establish a unique role for ASIC1 in mediating SOCE in pulmonary VSM and provide new insight into mechanisms of VSM Ca(2+) entry and pulmonary vasoregulation.

Novel determinants of epithelial sodium channel gating within extracellular thumb domains

Activity of the epithelial Na(+) channel (ENaC) is modulated by Na(+) self-inhibition, an allosteric down-regulation of channel open probability by extracellular Na(+). We searched for determinants of Na(+) self-inhibition by analyzing changes in this inhibitory response resulting from specific mutations within the extracellular domains of mouse ENaC subunits. Mutations at gammaMet(438) altered the Na(+) self-inhibition response in a substitution-specific manner. Fourteen substitutions (Ala, Arg, Asp, Cys, Gln, Glu, His, Ile, Phe, Pro, Ser, Thr, Tyr, and Val) significantly suppressed Na(+) self-inhibition, whereas three mutations (Asn, Gly, and Leu) moderately enhanced the inhibition. Met to Lys mutation did not alter Na(+) self-inhibition. Mutations at the homologous site in the alpha subunit (G481A, G481C, and G481M) dramatically increased the magnitude and speed of Na(+) self-inhibition. Mutations at the homologous betaAla(422) resulted in minimal or no change in Na(+) self-inhibition. Low, high, and intermediate open probabilities were observed in oocytes expressing alphaG481Mbetagamma, alphabetagammaM438V, and alphaG481M/betagammaM438V, respectively. This pair of residues map to thealpha5 helix in the extracellular thumb domain in the chicken acid sensing ion channel 1 structure. Both residues likely reside near the channel surface because both alphaG481Cbetagamma and alphabetagammaM438C channels were inhibited by an externally applied and membrane-impermeant sulfhydryl reagent. Our results demonstrate that alphaGly(481) and gammaMet(438) are functional determinants of Na(+) self-inhibition and of ENaC gating and suggest that the thumb domain contributes to the channel gating machinery.

SARS-CoV proteins decrease levels and activity of human ENaC via activation of distinct PKC isoforms

Among the multiple organ disorders caused by the severe acute respiratory syndrome coronavirus (SARS-CoV), acute lung failure following atypical pneumonia is the most serious and often fatal event. We hypothesized that two of the hydrophilic structural coronoviral proteins (S and E) would regulate alveolar fluid clearance by decreasing the cell surface expression and activity of amiloride-sensitive epithelial sodium (Na(+)) channels (ENaC), the rate-limiting protein in transepithelial Na(+) vectorial transport across distal lung epithelial cells. Coexpression of either S or E protein with human alpha-, beta-, and gamma-ENaC in Xenopus oocytes led to significant decreases of both amiloride-sensitive Na(+) currents and gamma-ENaC protein levels at their plasma membranes. S and E proteins decreased the rate of ENaC exocytosis and either had no effect (S) or decreased (E) rates of endocytosis. No direct interactions among SARS-CoV E protein with either alpha- or gamma-ENaC were indentified. Instead, the downregulation of ENaC activity by SARS proteins was partially or completely restored by administration of inhibitors of PKCalpha/beta1 and PKCzeta. Consistent with the whole cell data, expression of S and E proteins decreased ENaC single-channel activity in oocytes, and these effects were partially abrogated by PKCalpha/beta1 inhibitors. Finally, transfection of human airway epithelial (H441) cells with SARS E protein decreased whole cell amiloride-sensitive currents. These findings indicate that lung edema in SARS infection may be due at least in part to activation of PKC by SARS proteins, leading to decreasing levels and activity of ENaC at the apical surfaces of lung epithelial cells.

Extracellular protons regulate human ENaC by modulating Na+ self-inhibition

The epithelial Na(+) channel, ENaC, is exposed to a wide range of proton concentrations in the kidney, lung, and sweat duct. We, therefore, tested whether pH alters ENaC activity. In Xenopus oocytes expressing human alpha-, beta-, and gammaENaC, amiloride-sensitive current was altered by protons in the physiologically relevant range (pH 8.5-6.0). Compared with pH 7.4, acidic pH increased ENaC current, whereas alkaline pH decreased current (pH(50) = 7.2). Acidic pH also increased ENaC current in H441 epithelia and in human primary airway epithelia. In contrast to human ENaC, pH did not alter rat ENaC current, indicating that there are species differences in ENaC regulation by protons. This resulted predominantly from species differences in gammaENaC. Maneuvers that lock ENaC in a high open-probability state ("DEG" mutation, proteolytic cleavage) abolished the effect of pH on human ENaC, indicating that protons alter ENaC current by modulating channel gating. Previous work showed that ENaC gating is regulated in part by extracellular Na(+) ("Na(+) self-inhibition"). Based on several observations, we conclude that protons regulate ENaC by altering Na(+) self-inhibition. First, protons reduced Na(+) self-inhibition in a dose-dependent manner. Second, ENaC regulation by pH was abolished by removing Na(+) from the extracellular bathing solution. Third, mutations that alter Na(+) self-inhibition produced corresponding changes in ENaC regulation by pH. Together, the data support a model in which protons modulate ENaC gating by relieving Na(+) self-inhibition. We speculate that this may be an important mechanism to facilitate epithelial Na(+) transport under conditions of acidosis.

Effects of pH buffering on horizontal and ganglion cell light responses in primate retina: evidence for the proton hypothesis of surround formation

Negative feedback from horizontal cells to cone photoreceptors is regarded as the critical pathway for the formation of the antagonistic surround of retinal neurons, yet the mechanism by which horizontal cells accomplish negative feedback has been difficult to determine. Recent evidence suggests that feedback uses a novel, non-GABAergic pathway that directly modulates the calcium current in cones. In non-mammalian vertebrates, enrichment of retinal pH buffering capacity attenuates horizontal cell feedback, supporting one model in which feedback occurs by horizontal cell modulation of the extracellular pH in the cone synaptic cleft. Here we test the effect of exogenous pH buffering on the response dynamics of H1 horizontal cells and the center-surround receptive field structure of parasol ganglion cells in the macaque monkey retina. Enrichment of the extracellular buffering capacity with HEPES selectively attenuates surround antagonism in parasol ganglion cells. The H1 horizontal cell light response includes a slow, depolarizing component that is attributed to negative feedback to cones. This part of the response is attenuated by HEPES and other pH buffers in a dose-dependent manner that is correlated with predicted buffering capacity. The selective effects of pH buffering on the parasol cell surround and H1 cell light response suggests that, in primate retina, horizontal cell feedback to cones is mediated via a pH-dependent mechanism and is a major determinant of the ganglion cell receptive field surround.

Role of proteolysis in the activation of epithelial sodium channels

PURPOSE OF REVIEW

Epithelial sodium channels mediate Na+ transport across high resistance, Na+-transporting epithelia. This review describes recent findings that indicate that epithelial sodium channels are activated by the proteolytic release of inhibitory peptides from the alpha and gamma subunits.

RECENT FINDINGS

Non-cleaved channels have a low intrinsic open probability that may reflect enhanced channel inhibition by external Na+--a process referred to as Na+ self-inhibition. Cleavage at a minimum of two sites within the alpha or gamma subunits is required to activate the channel, presumably by releasing inhibitory fragments. The extent of epithelial sodium channel proteolysis is dependent on channel residency time at the plasma membrane, as well as on the balance between levels of expression of proteases that activate epithelial sodium channels and inhibitors of these proteases. Regulated epithelial sodium channel proteolysis has been observed in rat kidney and in human airway epithelia.

SUMMARY

Proteolysis of epithelial sodium channel subunits plays a key role in modulating epithelial sodium channel activity through changes in channel open probability.

Differential regulation of the Cdk5-dependent phosphorylation sites of inhibitor-1 and DARPP-32 by depolarization

While cyclin-dependent kinase 5 (Cdk5) is of growing importance to neuronal signaling, its regulation remains relatively unexplored. Examination of the mechanism by which NMDA modulates the phosphorylation of protein phosphatase inhibitor-1 at Ser6 and Ser67 and dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 at Thr75 revealed that generalized depolarization, rather than specific activation of NMDA receptors, was sufficient to induce decreases in these Cdk5 sites. Although no evidence for the involvement of the Cdk5 cofactors p35 or p39, or for L- and T-type voltage-gated Ca(2+) channels, was found, evaluation of the role of phosphatases and extracellular cations revealed differential regulation of the three sites. NMDA-induced decreases in the phosphorylation of Thr75 of dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 required protein phosphatase 1/2A activity and extracellular Ca(2+). In contrast, the effects on Ser6 and Ser67 of inhibitor-1 were not cation specific; either Na(+) or Ca(2+) sufficed. Furthermore, while the decrease in phosphorylation of Ser6 was partially dependent on protein phosphatase 2B, that of Ser67 was independent of the major protein serine/threonine phosphatases, likely indicating the presence of a pathway by which NMDA inhibits Cdk5 activity. Thus, in the striatum the regulation of phosphorylation of Cdk5-dependent sites by NMDA occurs through multiple distinct pathways.

Subunit-dependent cadmium and nickel inhibition of acid-sensing ion channels

Acid-sensing ion channels (ASIC) are ligand-gated cation channels that are highly expressed in peripheral sensory and central neurons. ASIC are transiently activated by decreases in extracellular pH and are thought to play important roles in sensory perception, neuronal transmission, and excitability, and in the pathology of neurological conditions, such as brain ischemia. We demonstrate here that the heavy metals Ni(2+) and Cd(2+) dose-dependently inhibit ASIC currents in hippocampus CA1 neurons and in Chinese hamster ovary (CHO) cells heterologously expressing these channels. The effects of both Ni(2+) and Cd(2+) were voltage-independent, fast, and reversible. Neither metal affected activation and desensitization kinetics but rather decreased pH-sensitivity. Moreover, distinct ASIC isoforms were differentially inhibited by Ni(2+) and Cd(2+). External application of 1 mM Ni(2+) rapidly inhibited homomeric ASIC1a and heteromeric ASIC1a/2a channels without affecting ASIC1b, 2a, and ASIC3 homomeric channels and ASIC1a/3 and 2a/3 heteromeric channels. In contrast, external Cd(+) (1 mM) inhibited ASIC2a and ASIC3 homomeric channels and ASIC1a/2a, 1a/3, and 2a/3 heteromeric channels but not ASIC1a homomeric channels. The acid-sensing current in isolated rat hippocampus CA1 neurons, thought to be carried primarily by ASIC1a and 1a/2a, was inhibited by 1 mM Ni(2+). The current study identifies ASIC as a novel target for the neurotoxic heavy metals Cd(2+) and Ni(2+).

Amiloride-sensitive sodium absorption is different in vertebrates and invertebrates

Amiloride-sensitive Na+absorption is a well-described feature of numerous transporting epithelia in vertebrates. Yet, very little is known about this important physiological process regarding invertebrates. In the present paper, we compare vertebrate Na+absorption mediated by the amiloride-sensitive epithelial Na+channel (ENaC) and its invertebrate counterpart. We used the dorsal skin of the annelid Hirudo medicinalis as a model for the Na+absorption of invertebrate epithelia. In applying electrophysiological, molecular, and biochemical techniques we found striking functional and structural differences between vertebrate and invertebrate amiloride-sensitive Na+absorption. Using modified Ussing chambers, we analyzed the influence of different known blockers and effectors of vertebrate ENaC on leech epithelial Na+absorption. We demonstrate that the serine protease trypsin had no effect on the Na+transport across leech integument, while it strongly activates vertebrate ENaC. While protons, and the divalent cations Ni2+and Zn2+stimulate vertebrate ENaC, amiloride-sensitive Na+currents in leech integument were substantially reduced. For molecular studies, we constructed a cDNA library of Hirudo medicinalis and screened it with specific ENaC antibodies. We performed numerous PCR approaches using a vast number of different degenerated and specific ENaC primers to identify ENaC-like structures. Yet, both strategies did not reveal any ENaC-like sequence in leech integument. From these data we conclude that amiloride-sensitive Na+absorption in leech skin is not mediated by an ENaC-like Na+channel but by a still unknown invertebrate member of the ENaC/DEG family that we termed lENaTP (leech epithelial Na+transporting protein).

Functional role of extracellular loop cysteine residues of the epithelial Na+ channel in Na+ self-inhibition

The epithelial Na(+) channel (ENaC) is typically formed by three homologous subunits (alpha, beta, and gamma) that possess a characteristic large extracellular loop (ECL) containing 16 conserved cysteine (Cys) residues. We investigated the functional role of these Cys residues in Na(+) self-inhibition, an allosteric inhibition of ENaC activity by extracellular Na(+). All 16 Cys residues within alpha and gamma ECLs and selected beta ECL Cys residues were individually mutated to alanine or serine residues. The Na(+) self-inhibition response of wild type and mutant channels expressed in Xenopus oocytes was determined by whole cell voltage clamp. Individual mutation of eight alpha (Cys-1, -4, -5, -6, -7, -10, -13, or -16), one beta (Cys-7), and nine gamma (Cys-3, -4, -6, -7, -10, -11, -12, -13, or -16) residues significantly reduced the magnitude of Na(+) self-inhibition. Na(+) self-inhibition was eliminated by simultaneous mutations of either the last three alpha ECL Cys residues (Cys-14, -15, and -16) or Cys-7 within both alpha and gamma ECLs. By analyzing the Na(+) self-inhibition responses and the effects of a methanethiosulfonate reagent on channel currents in single and double Cys mutants, we identified five Cys pairs within the alphaECL (alphaCys-1/alphaCys-6, alphaCys-4/alphaCys-5, alphaCys-7/alphaCys-16, alphaCys-10/alphaCys-13, and alphaCys-11/alphaCys-12) and one pair within the gammaECL (gammaCys-7/gammaCys-16) that likely form intrasubunit disulfide bonds. We conclude that approximately half of the ECL Cys residues in the alpha and gamma ENaC subunits are required to establish the tertiary structure that ensures a proper Na(+) self-inhibition response, likely by formation of multiple intrasubunit disulfide bonds.

Effect of divalent heavy metals on epithelial Na+ channels in A6 cells

To better understand how renal Na(+) reabsorption is altered by heavy metal poisoning, we examined the effects of several divalent heavy metal ions (Zn(2+), Ni(2+), Cu(2+), Pb(2+), Cd(2+), and Hg(2+)) on the activity of single epithelial Na(+) channels (ENaC) in a renal epithelial cell line (A6). None of the cations changed the single-channel conductance. However, ENaC activity [measured as the number of channels (N) x open probability (P(o))] was decreased by Cd(2+) and Hg(2+) and increased by Cu(2+), Zn(2+), and Ni(2+) but was not changed by Pb(2+). Of the cations that induced an increase in Na(+) channel function, Zn(2+) increased N, Ni(2+) increased P(o), and Cu(2+) increased both. The cysteine modification reagent [2-(trimethylammonium)ethyl]methanethiosulfonate bromide also increased N, whereas diethylpyrocarbonate, which covalently modifies histidine residues, affected neither P(o) nor N. Cu(2+) increased N and stimulated P(o) by reducing Na(+) self-inhibition. Furthermore, we observed that ENaC activity is slightly voltage dependent and that the voltage dependence of ENaC is insensitive to extracellular Na(+) concentration; however, apical application of Ni(2+) or diethylpyrocarbonate reduced the channel voltage dependence. Thus the voltage sensor of Xenopus ENaC is different from that of typical voltage-gated channels, since voltage appears to be sensed by histidine residues in the extracellular loops of ENaC, rather than by charged amino acids in a transmembrane domain.

Distinct structural elements in the first membrane-spanning segment of the epithelial sodium channel

Epithelial Na+ channels (ENaCs) comprise three subunits that have been proposed to be arranged in either an alpha2betagamma or a higher ordered configuration. Each subunit has two putative membrane-spanning segments (M1 and M2), intracellular amino and carboxyl termini, and a large extracellular loop. We have used the TOXCAT assay (a reporter assay for transmembrane segment homodimerization) to identify residues within the transmembrane segments of ENaC that may participate in important structural interactions within ENaC, with which we identified a candidate site within alphaM1. We performed site-directed mutagenesis at this site and found that, although the mutants reduced channel activity, ENaC protein expression at the plasma membrane was unaffected. To deduce the role of alphaM1 in the pore structure of ENaC, we performed tryptophan-scanning mutagenesis throughout alphaM1 (residues 110-130). We found that mutations within the amino-terminal part of alphaM1 had effects on activity and selectivity with a periodicity consistent with a helical structure but no effect on channel surface expression. We also observed that mutations within the carboxyl-terminal part of alphaM1 had effects on activity and selectivity but with no apparent periodicity. Additionally, these mutants reduced channel surface expression. Our data support a model in which the amino-terminal half of alphaM1 is alpha-helical and packs against structural element(s) that contribute to the ENaC pore. Furthermore, these data suggest that the carboxyl-terminal half of alphaM1 may be helical or assume a different conformation and may be involved in tertiary interactions essential to proper channel folding or assembly. Together, our data suggest that alphaM1 is divided into two distinct regions.

Furin cleavage activates the epithelial Na+channel by relieving Na+self-inhibition

Epithelial Na+channels (ENaC) are inhibited by extracellular Na+, a process referred to as Na+self-inhibition. We previously demonstrated that mutation of key residues within two furin cleavage consensus sites in α, or one site in γ, blocked subunit proteolysis and inhibited channel activity when mutant channels were expressed in Xenopus laevis oocytes (Hughey RP, Bruns JB, Kinlough CL, Harkleroad KL, Tong Q, Carattino MD, Johnson JP, Stockand JD, and Kleyman TR. J Biol Chem 279: 18111–18114, 2004). Cleavage of subunits was also blocked by these mutations when expressed in Madin-Darby canine kidney cells, and both subunit cleavage and channel activity were blocked when wild-type subunits were expressed in furin-deficient Chinese hamster ovary cells. We now report that channels with mutant α-subunits lacking either one or both furin cleavage sites exhibited a marked enhancement of the Na+self-inhibition response, while channels with a mutant γ-subunit showed a modestly enhanced Na+self-inhibition response. Analysis of Na+self-inhibition at varying [Na+] indicates that channels containing mutant α-subunits exhibit an increased Na+affinity. At the single-channel level, channels with a mutant α-subunit had a low open probability ( Po) in the presence of a high external [Na+] in the patch pipette. Podramatically increased when trypsin was also present, or when a low external [Na+] was in the patch pipette. Our results suggest that furin cleavage of ENaC subunits activates the channels by relieving Na+self-inhibition and that activation requires that the α-subunit be cleaved twice. Moreover, we demonstrate for the first time a clear relationship between ENaC Poand extracellular [Na+], supporting the notion that Na+self-inhibition reflects a Poreduction due to high extracellular [Na+].

Proton-mediated feedback inhibition of presynaptic calcium channels at the cone photoreceptor synapse

Generation of center-surround antagonistic receptive fields in the outer retina occurs via inhibitory feedback modulation of presynaptic voltage-gated calcium channels in cone photoreceptor synaptic terminals. Both conventional and unconventional neurotransmitters, as well as an ephaptic effect, have been proposed, but the intercellular messaging that mediates the inhibitory feedback signal from postsynaptic horizontal cells (HCs) to cones remains unknown. We examined the possibility that proton concentration in the synaptic cleft is regulated by HCs and that it carries the feedback signal to cones. In isolated, dark-adapted goldfish retina, we assessed feedback in the responses of HCs to light and found that strengthened pH buffering reduced both rollback and the depolarization to red light. In zebrafish retinal slices loaded with Fluo-4, depolarization with elevated K(+) increased Ca signals in the synaptic terminals of cone photoreceptors. Kainic acid, which depolarizes HCs but has no direct effect on cones, depressed the K(+)-induced Ca signal, whereas CNQX, which hyperpolarizes HCs, increased the Ca signals, suggesting that polarization of HCs alters inhibitory feedback to cones. We found that these feedback signals were blocked by elevated extracellular pH buffering, as well as amiloride and divalent cations. Voltage clamp of isolated HCs revealed an amiloride-sensitive conductance that could mediate modulation of cleft pH dependent on the membrane potential of these postsynaptic cells.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

The epithelial Na+ channel (ENaC) is modulated by various extracellular factors, including Na+, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni2+ cation on ENaCs, we compared the Na+ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni2+, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na+ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni2+ on amiloride binding is not species dependent, because Ni2+ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na+. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni2+, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni2+ was obtained. To further study the origin of the xENaC stimulation by Ni2+, and based on the rationale of the well-known high affinity of Ni2+ for histidine residues, we designed alpha-subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT beta- and gamma-subunits. Changing His215 to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na+ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni2+, and neither did complete deletion of this segment. Next, we mutated His416 flanked by His411 and Cys417, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (approximately 100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His416 with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na+ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni2+ or reverse it to an inhibitory type.

On the interaction between amiloride and its putative alpha-subunit epithelial Na+ channel binding site

The epithelial Na+ channel (ENaC) belongs to the structurally conserved ENaC/Degenerin superfamily. These channels are blocked by amiloride and its analogues. Several amino acid residues have been implicated in amiloride binding. Primary among these are alphaSer-583, betaGly-525, and gammaGly-542, which are present at a homologous site within the three subunits of ENaC. Mutations of the beta and gamma glycines greatly weakened amiloride block, but, surprisingly, mutation of the serine of the alpha subunit resulted in moderate (<5-fold) weakening of amiloride K(i). We investigated the role of alphaSer-583 in amiloride binding by systematically mutating alphaSer-583 and analyzing the mutant channels with two-electrode voltage clamp. We observed that most mutations had moderate effects on amiloride block, whereas those introducing rings showed dramatic effects on amiloride block. In addition, mutations introducing a beta-methyl group at this site altered the electric field of ENaC, affecting both amiloride binding and the voltage dependence of channel gating. We also found that the His mutation, in addition to greatly weakening amiloride binding, appends a voltage-sensitive gate within the pore of ENaC at low pH. Because diverse residues at alpha583, such as Asn, Gln, Ser, Gly, Thr, and Ala, have similar amiloride binding affinities, our results suggest that the wild type Ser side chain is not important for amiloride binding. However, given that some alphaSer-583 mutations affect the electrical properties of the channel whereas those introducing rings greatly weaken amiloride block, we conclude that amiloride binds at or near this site and that alphaSer-583 may have a role in ion permeation through ENaC.

Acute ENaC stimulation by cAMP in a kidney cell line is mediated by exocytic insertion from a recycling channel pool

Acute hormonal regulation of the epithelial sodium channel (ENaC) in tight epithelia increases transcellular Na⁺ transport via trafficking of intracellular channels to the apical surface. The fate of the channels removed from the apical surface following agonist washout is less clear. By repetitively stimulating polarized mouse cortical collecting duct (mCCD, _MPKCCD₁₄) epithelia, we evaluated the hypothesis that ENaC recycles through an intracellular pool to be available for reinsertion into the apical membrane. Short circuit current (I_SC), membrane capacitance (C_T), and conductance (G_T) were recorded from mCCD epithelia mounted in modified Ussing chambers. Surface biotinylation of ENaC demonstrated an increase in channel number in the apical membrane following cAMP stimulation. This increase was accompanied by a 83 ± 6% (n = 31) increase in I_SC and a 15.3 ± 1.5% (n = 15) increase in C_T. Selective membrane permeabilization demonstrated that the C_T increase was due to an increase in apical membrane capacitance. I_SC and C_T declined to basal levels on stimulus washout. Repetitive cAMP stimulation and washout (∼1 h each cycle) resulted in response fatigue; ΔI_SC decreased ∼10% per stimulation–recovery cycle. When channel production was blocked by cycloheximide, ΔI_SC decreased ∼15% per stimulation cycle, indicating that newly synthesized ENaC contributed a relatively small fraction of the channels mobilized to the apical membrane. Selective block of surface ENaC by benzamil demonstrated that channels inserted from a subapical pool made up >90% of the stimulated I_SC, and that on restimulation a large proportion of channels retrieved from the apical surface were reinserted into the apical membrane. Channel recycling was disrupted by brefeldin A, which inhibited ENaC exocytosis, by chloroquine, which inhibited ENaC endocytosis and recycling, and by latrunculin A, which blocked ENaC exocytosis. A compartment model featuring channel populations in the apical membrane and intracellular recycling pool provided an adequate kinetic description of the I_SC responses to repetitive stimulation. The model supports the concept of ENaC recycling in response to repetitive cAMP stimulation.

Mutations in the Pore Region Modify Epithelial Sodium Channel Gating by Shear Stress

Previous studies have shown that epithelial Na+ channels (ENaCs) are activated by laminar shear stress (LSS). ENaCs with a high intrinsic open probability because of a mutation (betaS518K) or covalent modification of an introduced Cys residue (alphaS580C) in the pre-second transmembrane domain (pre-M2) were not activated by LSS, suggesting that the pre-M2 region participates in conformational rearrangements during channel activation. We examined the role of the pore region of the alpha-subunit in channel gating by studying the kinetics of activation by LSS of wild-type ENaC and channels with Cys mutations in the tract Ser576-Ser592. Whole cell Na+ currents were monitored in oocytes expressing wild-type or mutant ENaCs prior to and following application of LSS. Following a 2.2-s delay, a monoexponential increase in Na+ currents was observed with a time constant (tau) of 8.1 s in oocytes expressing wild-type ENaC. Cys substitutions within the alpha-subunit in the tract Ser580-Ser589 resulted in: (i) a reduction (Ser580-Trp585, Gly587) or increase (Ser589) in delay times preceding channel activation by LSS, (ii) an increase (Gln581, Leu584, Trp585, Phe586, Ser588) or decrease (Ser589) in the rate of channel activation, or (iii) a decrease in the magnitude of the response (Ser583, Gly587, Leu584). Cys substitutions at a putative amiloride-binding site (alphaSer583 or betaGly525) or within the selectivity filter (alphaGly587) resulted in a reduction in the LSS response, and exhibited a multiexponential time course of activation. The corresponding gamma-subunit mutant (alphabetagammaG542C) had a minimal response to LSS and exhibited a high intrinsic open probability. These data suggest that residues in the pore region participate in the sensing and/or transduction of the mechanical stimulus that results in channel activation and are consistent with the hypothesis that the ENaC pore region has a key role in modulating channel gating.

Extracellular Zn2+ activates epithelial Na+ channels by eliminating Na+ self-inhibition

Inhibition of epithelial Na(+) channel (ENaC) activity by high concentrations of extracellular Na(+) is referred to as Na(+) self-inhibition. We investigated the effects of external Zn(2+) on whole cell Na(+) currents and on the Na(+) self-inhibition response in Xenopus oocytes expressing mouse alphabetagamma ENaC. Na(+) self-inhibition was examined by analyzing inward current decay from a peak current to a steady-state current following a fast switching of a low Na(+) (1 mm) bath solution to a high Na(+) (110 mm) solution. Our results indicate that external Zn(2+) rapidly and reversibly activates ENaC in a dose-dependent manner with an estimated EC(50) of 2 microm. External Zn(2+) in the high Na(+) bath also prevents or reverses Na(+) self-inhibition with similar affinity. Zn(2+) activation is dependent on extracellular Na(+) concentration and is absent in ENaCs containing gammaH239 mutations that eliminate Na(+) self-inhibition and in alphaS580Cbetagamma following covalent modification by a sulfhydryl-reactive reagent that locks the channels in a fully open state. In contrast, external Ni(2+) inhibition of ENaC currents appears to be additive to Na(+) self-inhibition when Ni(2+) is present in the high Na(+) bath. Pretreatment of oocytes with Ni(2+) in a low Na(+) bath also prevents the current decay following a switch to a high Na(+) bath but rendered the currents below the control steady-state level measured in the absence of Ni(2+) pretreatment. Our results suggest that external Zn(2+) activates ENaC by relieving the channel from Na(+) self-inhibition, and that external Ni(2+) mimics or masks Na(+) self-inhibition.

Epithelial sodium channels are activated by furin-dependent proteolysis

Epithelial Na(+) channels (ENaCs) are activated by extracellular trypsin or by co-expression with channel-activating proteases, although there is no direct evidence that these proteases activate ENaC by cleaving the channel. We previously demonstrated that the alpha and gamma subunits of ENaC are cleaved during maturation near consensus sites for furin cleavage. Using site-specific mutagenesis of channel subunits, ENaC expression in furin-deficient cells, and furin-specific inhibitors, we now report that ENaC cleavage correlates with channel activity. Channel activity in furin-deficient cells was rescued by expression of furin. Our data provide the first example of a vertebrate ion channel that is a substrate for furin and whose activity is dependent on its proteolysis.

External Ni2 + and ENaC in A6 cells: Na+ current stimulation by competition at a binding site for amiloride and Na+

In cultured A6 monolayers from distal Xenopus kidney, external Ni2+ stimulated active Na+ uptake via the epithelial Na+ channel, ENaC. Transepithelial capacitance measurements ruled out exocytosis of ENaC-containing vesicles underlying the Ni2+ effect. Na+ current noise analysis was performed using the neutral Na(+) -channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC) and amiloride. The analysis of CDPC-induced noise in terms of a three-state channel model revealed that Ni2+ elicits an increase in the number of open channels as well as in the spontaneous open probability. While Ni2+ had no influence on CDPC-blocker kinetics, the macroscopic and microscopic blocking kinetics of amiloride were affected. Ni2+ turned out to compete with amiloride for a putative binding site but not with CDPC. Moreover, external Na(+)--known to compete with amiloride and so producing the "self-inhibition" phenomenon--and Ni2+ exerted mutually exclusive analogous effects on amiloride kinetics. Na+ current kinetics revealed that Ni2+ prevents ENaC to be downregulated by self-inhibition. Co2+ behaved similarly to Ni2+, whereas Zn2+ did not. Attempts to disclose the chemical nature of the site reacting with Ni2+ suggested cysteine but not histidine as reaction partner.

Mutations in the extracellular loop of alpha-rENaC alter sensitivity to amiloride and reactive species

We studied the effects of two mutations of the extracellular loop of the alpha-subunit of the (ENaC) on amiloride-sensitive current in Xenopus laevis oocytes and the inhibition of this current by 3-morpholinosydnonimine (SIN-1). Injection of oocytes with wild-type (wt) alpha-,beta-,gamma-rENaC cRNA (8.3 ng/subunit) resulted 48-72 h later in inward Na(+) currents (-5.5 +/- 0.8 microA; means +/- SE at -100 mV; n = 21), which were completely inhibited by amiloride. Oocytes injected with either alpha(Y279A)- or alpha(Y283A)- and beta-,gamma-rENaC cRNAs had significantly lower Na(+) currents. Furthermore, alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes had a higher K(i) for amiloride (0.54 +/- 0.97 vs. 0.10 +/- 0.04 microM; P < 0.01). Exposure of oocytes to SIN-1 (1 mM) for 5 min decreased both total Na(+) and amiloride-sensitive currents across wt and alpha(Y279A)- but not alpha(Y283A)-,beta-,gamma-rENaC. Furthermore, exposure to SIN-1 increased the K(i) for amiloride across wt but not alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes. These data indicate that both tyrosines are important for proper ENaC function and their oxidative modifications contribute to altered ENaC function.

Nucleotide-mediated inhibition of alveolar fluid clearance in BALB/c mice after respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract disease in infants and children worldwide. Intranasal infection of BALB/c mice with RSV strain A2, but not ultraviolet-inactivated RSV, for 2 or 4 days reduced basal alveolar fluid clearance (AFC), a seminal function of bronchoalveolar epithelium, and caused loss of AFC sensitivity to amiloride inhibition. Reduced AFC was temporally associated with increased lung water content but was not a consequence of increased epithelial permeability or cell death. Reduced AFC was also not due to decreased transcription of epithelial Na+ channel subunit genes in lung tissue. RSV-mediated inhibition of AFC 2 days after infection was rapidly prevented by addition to the instillate of P2Y receptor antagonists (suramin and XAMR-0721) or enzymes that degrade UTP, but not those that degrade ATP. After UTP degradation, AFC returned to control levels but was no longer sensitive to amiloride. UTP at nanomolar concentrations recapitulated the AFC inhibitory effect of RSV in normal mice and mice infected with RSV for 6 days, indicating that normalization of AFC at this time point is a consequence of cessation of UTP release, rather than P2Y receptor desensitization. We conclude that RSV infection of the bronchoalveolar epithelium results in reduced AFC as a consequence of autocrine feedback inhibition mediated by UTP. These studies are the first to demonstrate AFC inhibition by an important pulmonary viral pathogen. Reduced AFC may result in formation of an increased volume of fluid mucus, airway congestion, and rhinorrhea, all features of severe RSV disease.