Expression of epithelial Na channels in Xenopus oocytes

Effect of SARS-CoV-2 S protein on the proteolytic cleavage of the epithelial Na+ channel ENaC

Severe cases of COVID-19 are characterized by development of acute respiratory distress syndrome (ARDS). Water accumulation in the lungs is thought to occur as consequence of an exaggerated inflammatory response. A possible mechanism could involve decreased activity of the epithelial Na+ channel, ENaC, expressed in type II pneumocytes. Reduced transepithelial Na+ reabsorption could contribute to lung edema due to reduced alveolar fluid clearance. This hypothesis is based on the observation of the presence of a novel furin cleavage site in the S protein of SARS-CoV-2 that is identical to the furin cleavage site present in the alpha subunit of ENaC. Proteolytic processing of αENaC by furin-like proteases is essential for channel activity. Thus, competition between S protein and αENaC for furin-mediated cleavage in SARS-CoV-2-infected cells may negatively affect channel activity. Here we present experimental evidence showing that coexpression of the S protein with ENaC in a cellular model reduces channel activity. In addition, we show that bidirectional competition for cleavage by furin-like proteases occurs between 〈ENaC and S protein. In transgenic mice sensitive to lethal SARS-CoV-2, however, a significant decrease in gamma ENaC expression was not observed by immunostaining of lungs infected as shown by SARS-CoV2 nucleoprotein staining.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Expression and Analysis of Flow-regulated Ion Channels in Xenopus Oocytes

Mechanically-gated ion channels play key roles in mechanotransduction, a process that translates physical forces into biological signals. Epithelial and endothelial cells are exposed to laminar shear stress (LSS), a tangential force exerted by flowing fluids against the wall of vessels and epithelia. The protocol outlined herein has been used to examine the response of ion channels expressed in Xenopus oocytes to LSS (Hoger et al., 2002; Carattino et al., 2004; Shi et al., 2006). The Xenopus oocyte is a reliable system that allows for the expression and chemical modification of ion channels and regulatory proteins (George et al., 1989; Palmer et al., 1990; Sheng et al., 2001; Carattino et al., 2003). Therefore, this technique is suitable for studying the molecular mechanisms that allow flow-activated channels to respond to LSS.

Cistrome of the aldosterone-activated mineralocorticoid receptor in human renal cells

Aldosterone exerts its effects mainly by activating the mineralocorticoid receptor (MR), a transcription factor that regulates gene expression through complex and dynamic interactions with coregulators and transcriptional machinery, leading to fine-tuned control of vectorial ionic transport in the distal nephron. To identify genome-wide aldosterone-regulated MR targets in human renal cells, we set up a chromatin immunoprecipitation (ChIP) assay by using a specific anti-MR antibody in a differentiated human renal cell line expressing green fluorescent protein (GFP)-MR. This approach, coupled with high-throughput sequencing, allowed identification of 974 genomic MR targets. Computational analysis identified an MR response element (MRE) including single or multiple half-sites and palindromic motifs in which the AGtACAgxatGTtCt sequence was the most prevalent motif. Most genomic MR-binding sites (MBSs) are located >10 kb from the transcriptional start sites of target genes (84%). Specific aldosterone-induced recruitment of MR on the first most relevant genomic sequences was further validated by ChIP-quantitative (q)PCR and correlated with concomitant and positive aldosterone-activated transcriptional regulation of the corresponding gene, as assayed by RT-qPCR. It was notable that most MBSs lacked MREs but harbored DNA recognition motifs for other transcription factors (FOX, EGR1, AP1, PAX5) suggesting functional interaction. This work provides new insights into aldosterone MR-mediated renal signaling and opens relevant perspectives for mineralocorticoid-related pathophysiology.

ENaC in the Rabbit Lacrimal Gland and its Changes During Sjögren Syndrome and Pregnancy

PURPOSE

Epithelial sodium channel (ENaC) plays a critical role in the control of Na(+) balance and the development and progression of exocrine gland pathologic condition. The aim of the present study was to investigate the presence of ENaC in the rabbit lacrimal gland (LG) and its potential changes during induced autoimmune dacryoadenitis (IAD) and pregnancy.

METHODS

Total messenger RNA (mRNA) of α, β, and γ subunits was extracted from whole LG, acinar cells, and ductal cells by laser capture microdissection (LCM) for real-time reverse-transcriptase polymerase chain reaction. Lacrimal glands were processed for Western blot and immunofluorescence.

RESULTS

Messenger RNA for both α and γ was expressed in whole LG lysates, whereas β was undetectable. In rabbits with IAD, the levels of mRNA for α and γ were 20.9% and 58.9% lower (P<0.05), whereas no significant changes were observed in term-pregnant rabbits (P=0.152). However, we were unable to detect mRNA of any subunit in LCM specimens of ductal cells because of their low levels. Western blot demonstrated bands for both α (90 kDa) and γ (85 kDa) but β was undetectable. In rabbits with IAD, densitometry analysis showed that expression of α decreased 22%, whereas γ decreased 26% (P<0.05). In pregnant rabbits, however, α expression was 31% lower, whereas γ expression was 34% lower (P<0.05). From immunofluorescence studies, all subunits were present in ductal cells, whereas virtually no immunoreactivity was detected in acini. No noticeable changes of their distribution pattern and intensity were found in rabbits with IAD or during pregnancy.

CONCLUSIONS

The present study demonstrated the presence of ENaC in the rabbit LG and its alterations in IAD and pregnancy, suggesting that ENaC may contribute to the pathogenesis of altered LG secretion and ocular surface symptoms in these animals.

Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited

Transcription and translation require a high concentration of potassium across the entire tree of life. The conservation of a high intracellular potassium was an absolute requirement for the evolution of life on Earth. This was achieved by the interplay of P- and V-ATPases that can set up electrochemical gradients across the cell membrane, an energetically costly process requiring the synthesis of ATP by F-ATPases. In animals, the control of an extracellular compartment was achieved by the emergence of multicellular organisms able to produce tight epithelial barriers creating a stable extracellular milieu. Finally, the adaptation to a terrestrian environment was achieved by the evolution of distinct regulatory pathways allowing salt and water conservation. In this review we emphasize the critical and dual role of Na(+)-K(+)-ATPase in the control of the ionic composition of the extracellular fluid and the renin-angiotensin-aldosterone system (RAAS) in salt and water conservation in vertebrates. The action of aldosterone on transepithelial sodium transport by activation of the epithelial sodium channel (ENaC) at the apical membrane and that of Na(+)-K(+)-ATPase at the basolateral membrane may have evolved in lungfish before the emergence of tetrapods. Finally, we discuss the implication of RAAS in the origin of the present pandemia of hypertension and its associated cardiovascular diseases.

Rapid fluidic exchange microsystem for recording of fast ion channel kinetics in Xenopus oocytes

We present a new lab-on-a-chip system for electrophysiological measurements on Xenopus oocytes. Xenopus oocytes are widely used host cells in the field of pharmacological studies and drug development. We developed a novel non-invasive technique using immobilized non-devitellinized cells that replaces the traditional "two-electrode voltage-clamp" (TEVC) method. In particular, rapid fluidic exchange was implemented on-chip to allow recording of fast kinetic events of exogenous ion channels expressed in the cell membrane. Reducing fluidic exchange times of extracellular reagent solutions is a great challenge with these large millimetre-sized cells. Fluidic switching is obtained by shifting the laminar flow interface in a perfusion channel under the cell by means of integrated poly-dimethylsiloxane (PDMS) microvalves. Reagent solution exchange times down to 20 ms have been achieved. An on-chip purging system allows to perform complex pharmacological protocols, making the system suitable for screening of ion channel ligand libraries. The performance of the integrated rapid fluidic exchange system was demonstrated by investigating the self-inhibition of human epithelial sodium channels (ENaC). Our results show that the response time of this ion channel to a specific reactant is about an order of magnitude faster than could be estimated with the traditional TEVC technique.

Integrated microsystem for non-invasive electrophysiological measurements on Xenopus oocytes

We propose a new non-invasive integrated microsystem for electrophysiological measurements on Xenopus laevis oocytes. Xenopus oocyte is a well-known expression system for various kinds of ion channels, that are potential tools in drug screening. In the traditional "Two Electrode Voltage Clamp" (TEVC) method, delicate micromanipulation is required to impale an oocyte with two microelectrodes. In our system, a non-invasive electrical access to the cytoplasm is provided by permeabilizing the cell membrane with an ionophore (e.g. nystatin). Unlike the classical patch-clamp or "macropatch" techniques, this method does not require removal of the vitelline membrane. Cell handling is significantly simplified, resulting in more robust recordings with increased throughput. Moreover, because only part of the oocyte surface is exposed to reagents, the required volume of reagent solutions could be reduced by an order of magnitude compared to the TEVC method. The fabrication process for this disposable microchip, based on poly-dimethylsiloxane (PDMS) molding and glass/PDMS bonding, is cost-efficient and simple. We tested this new microdevice by recording currents in oocytes expressing the human Epithelial Sodium Channel (hENaC) for membrane potentials between -100 and +50 mV. We recorded benzamil-sensitive currents with a large signal-to-noise ratio and we also obtained a benzamil concentration-inhibition curve displaying an inhibition constant IC(50) of about 50 nM, comparable to previously published values obtained with the TEVC technique.

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.

Epithelial sodium channel regulatory proteins identified by functional expression cloning

We describe here our current strategy for identifying and cloning proteins involved in the regulation of the epithelial sodium channel (ENaC). We have set up a complementation functional assay in the Xenopus laevis oocyte expression system. Using this assay, we have been able to identify a channel-activating protease (CAP-1) that can increase ENaC activity threefold. We propose a novel extracellular signal transduction pathway controlling ionic channels of the ENaC gene family that include genes involved in mechanotransduction (degenerins), in peptide-gated channels involved in neurotransmission (FaNaCh), in proton-gated channels involved in pH sensing (ASIC) or pain sensation (DRASIC).

The amiloride-sensitive Na+ channel: from primary structure to function

Three homologous subunits of the amiloride-sensitive Na+ channel, entitled alpha, beta, and gamma, have been cloned either from distal colon of a steroid-treated rat or from human lung. The alpha, beta, and gamma subunits have similarities with degenerins, a family of proteins found in the mechanosensory neurons of the nematode Caenorhabditis elegans. All these proteins are characterized by the presence of a large extracellular domain, located between two transmembrane alpha-helices, and by short NH2 and COOH terminal cytoplasmic segments. They constitute the first members of a new gene super-family of ionic channels. The epithelial Na+ channel is specifically expressed at the apical membrane of Na(+)-reabsorbing epithelial cells. Its activity is controlled by several distinct hormones, especially by corticosteroids. These hormones act either transcriptionally (such as aldosterone in distal colon, or glucocorticoids in lung) and/or post-transcriptionally (such as aldosterone in kidney). Recent works have provided new insights in the function of that important osmoregulatory system.

Novel isoforms of the beta and gamma subunits of the Xenopus epithelial Na channel provide information about the amiloride binding site and extracellular sodium sensing

We have previously identified three homologous subunits alpha, beta, and gamma of the highly selective amiloride-sensitive Na channel from the Xenopus laevis kidney A6 cell line, which forms a tight epithelium in culture. We report here two novel genes, termed beta2 and gamma2, which share 90 and 92% sequence identity with the previously characterized beta and gamma XENaC, respectively. beta2 and gamma2 transcripts were detected in lung, kidney, and A6 cells grown on porous substrate. The physiological and pharmacological profile of the Na channel expressed after alphabeta2gamma XENaC cRNA injection in Xenopus oocyte did not differ from alphabetagamma XENaC. By contrast, the channel expressed after alphabetagamma2 injection showed: (i) a lower maximal amiloride-sensitive sodium current, (ii) a higher apparent affinity for external sodium and inactivation of the sodium current by high sodium concentrations, and (iii) a lower apparent affinity for amiloride (KI alphabetagamma2; 1.34 microM versus alphabetagamma 0.35 microM). These data indicate that the gamma (and/or gamma2) subunit participates in amiloride binding and the sensing of the extracellular sodium concentration. The close homology between gamma and gamma2 will help to define the domains involved in sensing external sodium and in the structure of this important drug receptor.

Expression of human pICln and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

pICln is a protein that induces an outwardly rectifying, nucleotide-sensitive chloride current (ICln) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to ICln is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pICln and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO3- > I- > Br- > Cl- > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 degrees C) prior to the analysis. In addition, the ICln current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the ICln current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pICln resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pICln forms the channel that is responsible for the ICln current in Xenopus oocytes.

Regulation of epithelial sodium channels by short actin filaments

Cytoskeletal elements play an important role in the regulation of ion transport in epithelia. We have studied the effects of actin filaments of different length on the alpha, beta, gamma-rENaC (rat epithelial Na+ channel) in planar lipid bilayers. We found the following. 1) Short actin filaments caused a 2-fold decrease in unitary conductance and a 2-fold increase in open probability (Po) of alpha,beta,gamma-rENaC. 2) alpha,beta,gamma-rENaC could be transiently activated by protein kinase A (PKA) plus ATP in the presence, but not in the absence, of actin. 3) ATP in the presence of actin was also able to induce a transitory activation of alpha, beta,gamma-rENaC, although with a shortened time course and with a lower magnitude of change in Po. 4) DNase I, an agent known to prohibit elongation of actin filaments, prevented activation of alpha,beta,gamma-rENaC by ATP or PKA plus ATP. 5) Cytochalasin D, added after rundown of alpha,beta,gamma-rENaC activity following ATP or PKA plus ATP treatment, produced a second transient activation of alpha,beta,gamma-rENaC. 6) Gelsolin, a protein that stabilizes polymerization of actin filaments at certain lengths, evoked a sustained activation of alpha,beta,gamma-rENaC at actin/gelsolin ratios of <32:1, with a maximal effect at an actin/gelsolin ratio of 2:1. These results suggest that short actin filaments activate alpha, beta,gamma-rENaC. PKA-mediated phosphorylation augments activation of this channel by decreasing the rate of elongation of actin filaments. These results are consistent with the hypothesis that cloned alpha,beta,gamma-rENaCs form a core conduction unit of epithelial Na+ channels and that interaction of these channels with other associated proteins, such as short actin filaments, confers regulation to channel activity.

Amiloride-sensitive sodium channels in confluent M-1 mouse cortical collecting duct cells

Confluent M-1 cells show electrogenic Na+ absorption and possess an amiloride-sensitive Na(+)-conductance (Korbmacher et al., J. Gen. Physiol. 102:761-793, 1993). In the present study, we further characterized this conductance and identified the underlying single channels using conventional patch clamp technique. Moreover, we isolated poly(A)+ RNA from M-1 cells to express the channels in Xenopus laevis oocytes, and to check for the presence of transcripts related to the epithelial Na+ channel recently cloned from rat colon (Canessa et al., Nature 361:467-470, 1993). Patch clamp experiments were performed in 6-13-day-old confluent M-1 cells at 37 degrees C. In whole-cell experiments application of 10(-5) M amiloride caused a hyperpolarization of 24.9, SEM +/- 2.2 mV (n = 35) and a reduction of the inward current by 107 +/- 10 pA (n = 51) at a holding potential of -60 mV. Complete removal of bath Na+ had similar effects, indicating that the amiloride-sensitive component of the inward current is a Na+ current. The effect of amiloride was concentration-dependent with half-inhibition at 0.22 microM. The Na+ current saturated with increasing extracellular Na+ concentrations with an apparent Km of 24 mM. Na+ replacement for Li+ demonstrated a higher apical membrane conductance for Li+ than for Na+. In excised inside-out (i/o) or outside-out (o/o) patches from the apical membrane, we observed single-channels which showed slow kinetics and were reversibly inhibited by amiloride. Their average conductance for Na+ was 6.8 +/- 0.5 pS (n = 15) and for Li+ 11.2 +/- 1.0 pS (n = 14). They had no measurable conductance for K+. In o/o patches, channel activity was slightly voltage dependent with an open probability (NPo) of 0.46 +/- 0.14 and 0.16 +/- 0.05 at a holding potential of -100 and 0 mV, respectively (n = 8, P < 0.05). Using the two-microelectrode voltage-clamp technique, we assayed defolliculated stage V-VI Xenopus oocytes for an amiloride-sensitive inward current 1-6 days after injection with H2O or with 20-50 ng of M-1 poly(A)+ RNA. In poly(A)+ RNA-injected oocytes held at -60 or -100 mV application of amiloride (2 microM) reduced the Na-inward current by 25.5 +/- 4.6 nA (n = 25) while it had no effect in H2O-injected oocytes (n = 19). Northern blot analysis of M-1 poly(A+) RNA revealed the presence of transcripts related to the three known subunits of the rat colon Na+ channel (Canessa et al., Nature 367:463-467, 1994). We conclude that the channel in M-1 cells is closely related to the amiloride-sensitive epithelial Na+ channel in the rat colon and that the M-1 cell line provides a useful tool to investigate the biophysical and molecular properties of the corresponding channel in the cortical collecting duct.

Whole cell sodium conductance of principal cells freshly isolated from rat cortical collecting duct

Cortical collecting duct fragments were manually dissected from 6-wk-old Sprague-Dawley rats. The fragments were enzymatically digested (collagenase A) into single cells, washed, and resuspended in serum-free RPMI 1640. Individual cells were examined electrophysiologically using the whole cell patch-clamp technique. Two morphologically distinct cell types were present in the cell suspension. Small round cells that had a capacitance of 7 pF and larger oval cells with a capacitance of 29 pF were consistently observed. Whole cell electrophysiological examination revealed that the small round cells had virtually no plasma membrane ionic conductance, whereas both inward and outward currents were observed in the larger oval-type cells. Also, superfusion of 250 pM arginine vasopressin specifically increased the inward conductance of only the larger cells. The effect could be completely inhibited by 2 microM amiloride or 100 mumol of the Rp diastereomer of 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (a specific adenosine 3',5'-cyclic monophosphate inhibitor). These findings are consistent with the hypothesis that the larger cells are principal cells and the smaller cells are intercalated cells and directly demonstrate that an amiloride-sensitive whole cell conductance is readily observable in freshly isolated cortical collecting duct cells. Thus the whole cell configuration of the patch-clamp technique appears to be well suited for assessing cellular mechanisms that regulate the ionic conductances of cortical collecting duct cells.

The highly selective low-conductance epithelial Na channel of Xenopus laevis A6 kidney cells

In Na-reabsorbing tight epithelia, the rate-limiting step for Na transport is the highly selective low-conductance amiloride-sensitive epithelial Na channel (type 1 ENaC). In rat distal colon, type 1 ENaC is made of three homologous subunits. The aim of this study was to identify the corresponding genes of the renal channel from the kidney-derived A6 cell line of Xenopus laevis. Three homologous subunits were identified and coexpressed in the Xenopus oocyte system. The reconstituted channel had all the characteristics of the native type 1 ENaC described in A6 cells: 1) high selectivity, 2) low single-channel conductance, 3) slow gating kinetics, and 4) high affinity for amiloride. Transcripts for alpha-, beta-, and gamma-subunits of the Xenopus epithelial Na channel (xENaC) were detected in A6 kidney cells, Xenopus kidney, lung, and to a lesser extent in stomach and skin. Each subunit of the xENaC shares approximately 60% overall identity with the corresponding rat homologue (alpha, beta, and gamma rENaC). Our data suggest that the triplication of the ENaC subunits occurred before the divergence between mammalian and amphibian lineages.

Amiloride-sensitive apical membrane sodium channels of everted Ambystoma collecting tubule

Patch clamp methods were used to characterize sodium channels on the apical membrane of Ambystoma distal nephron. The apical membranes were exposed by everting and perfusing initial collecting tubules in vitro. In cell-attached patches, we observed channels whose mean inward unitary current averaged 0.39 +/- 0.05 pA (9 patches). The conductance of these channels was 4.3 +/- 0.2 pS. The unitary current approached zero at a pipette voltage of -92 mV. When clamped at the membrane potential the channel expressed a relatively high open probability (0.46). These characteristics, together with observation that doses of 0.5 to 2 microM amiloride reversibly inhibited the channel activity, are consistent with the presence of the high amiloride affinity, high sodium selectivity channel reported for rat cortical collecting tubule and cultured epithelial cell lines. We used antisodium channel antibodies to identify biochemically the epithelial sodium channels in the distal nephron of Ambystoma. Polyclonal antisodium channel antibodies generated against purified bovine renal, high amiloride affinity epithelial sodium channel specifically recognized 110, 57, and 55 kDa polypeptides in Ambystoma and localized the channels to the apical membrane of the distal nephron. A polyclonal antibody generated against a synthetic peptide corresponding to the C-terminus of Apx, a protein associated with the high amiloride affinity epithelial sodium channel expressed in A6 cells, specifically recognized a 170 kDa polypeptide. These data corroborate that the apically restricted sodium channels in Ambystoma are similar to the high amiloride affinity, sodium selective channels expressed in both A6 cells and the mammalian kidney.

Structure and function of amiloride-sensitive Na+ channels

A new molecular biological epoch in amiloride-sensitive Na+ channel physiology has begun. With the application of these new techniques, undoubtedly a plethora of new information and new questions will be forthcoming. First and foremost, however, is the question of how many discrete amiloride-sensitive Na+ channels exist. This question is important not only for elucidating structure-function relationships, but also for developing strategies for pharmacological or, ultimately, genetic intervention in such diseases as obstructive nephropathy, Liddle's syndrome, or salt-sensitive hypertension where amiloride-sensitive Na+ channel dysfunction has been implicated [17, 62]. Epithelia Na+ channels purified from kidney are multimeric. However, it is not yet clear which subunits are regulatory and which participate directly as a part of the Na+ conducting core and what is the nature of the gate. The combination of electrophysiologic techniques such as patch clamp and the ability to study reconstituted channels in planar lipid bilayers along with molecular biology techniques to potentially manipulate the individual subunits should provide the answers to questions that have puzzled physiologists for decades. It seems clear that the robust versatility of the channel in responding to a wide range of differing and potentially synergistic regulatory inputs must be a function of its multimeric structure and relation to the cytoskeleton. Multiple mechanisms of regulation imply multiple regulatory sites. This hypothesis has been validated by the demonstration that enzymatic carboxyl methylation and phosphorylation have both individual and synergistic effects on the purified channel in planar lipid bilayers. Of the multiple mechanisms proposed for channel regulation, evidence is now available to support the ideas that channels may be activated (or inactivated) by direct modifications including phosphorylation and carboxyl methylation, by activation or association of regulatory proteins such as G proteins, and by recruitment from subapical membrane domains. The observation that channel gating is achieved primarily through regulation of open probability without alterations in conductance may simplify future understanding of the molecular events involved in gating once the regulatory sites have been identified. As more Na+ channels or Na+ channel subunits are cloned from different epithelia, it will become possible to piece together the puzzle of epithelial Na+ channels. It is interesting to observe that renal Na+ channel proteins contain a subunit which falls into the 70 kD range. This size protein is in the range reported for the aldosterone-induced proteins [12, 46, 153].(ABSTRACT TRUNCATED AT 400 WORDS)

Amiloride-sensitive epithelial Na+ channel is made of three homologous subunits

The amiloride-sensitive epithelial sodium channel constitutes the rate-limiting step for sodium reabsorption in epithelial cells that line the distal part of the renal tubule, the distal colon, the duct of several exocrine glands, and the lung. The activity of this channel is upregulated by vasopressin and aldosterone, hormones involved in the maintenance of sodium balance, blood volume and blood pressure. We have identified the primary structure of the alpha-subunit of the rat epithelial sodium channel by expression cloning in Xenopus laevis oocytes. An identical subunit has recently been reported. Here we identify two other subunits (beta and gamma) by functional complementation of the alpha-subunit of the rat epithelial Na+ channel. The ion-selective permeability, the gating properties and the pharmacological profile of the channel formed by coexpressing the three subunits in oocytes are similar to that of the native channel.

Amiloride-sensitive sodium conductance in human B lymphoid cells

The Na+ conductance of RPMI 8226 human B lymphoblastoid cells was examined using whole cell patch clamp. When the bath solution contained RPMI 1640 and the pipette solution contained (in mM) 100 potassium gluconate, 30 KCl, 10 NaCl, 0.5 ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), and 20 N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), as well as < 10 nM free Ca2+, pH 7.25, the mean membrane potential was -58 +/- 4.5 mV (n = 18). Cells were voltage clamped from -160 to +40 mV in 20-mV increments. The inward conductance was 767 +/- 221 pS/10 pF, and the outward conductance was 1,212 +/- 272 pS/10 pF (n = 12). Superfusion with 2 microM amiloride significantly hyperpolarized the cells by 7.4 +/- 2.2 mV (P = 0.004), significantly reduced the inward conductance to 221 +/- 65 pS/10 pF (P = 0.028), but had no effect on the outward conductance (1,294 +/- 236 pS/10 pF, P = 0.820, after amiloride). Next, the pipette and bath solutions were changed to (in mM) 150 sodium gluconate, 0.5 EGTA, and 20 HEPES, as well as < 10 nM free Ca2+, pH 7.25. Under these conditions amiloride significantly reduced (50%, P < 0.05; n = 7) the whole cell currents. When potassium gluconate was substituted for sodium gluconate, amiloride had no effect. Thus amiloride inhibited a Na(+)-specific conductance expressed by B lymphoid cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression cloning of an epithelial amiloride-sensitive Na+ channel. A new channel type with homologies to Caenorhabditis elegans degenerins

A complementary DNA encoding an amiloride-sensitive Na+ channel has been cloned and characterized from rat colon. The protein encoded by the cDNA has a sequence of 699 amino acids (79 kDa) containing several putative membrane spanning domains and potential phosphorylation sites. It forms a channel that has the electrophysiological and pharmacological properties characteristic of the epithelial Na+ channel. Homologies (including in transmembrane domains) have been found between a part of the channel sequence and the Mec4 gene product of Caenorhabditis elegans, a protein associated with mutation-induced neuronal degeneration.

Epithelial sodium channel related to proteins involved in neurodegeneration

The epithelial amiloride-sensitive sodium channel constitutes the rate limiting step for sodium reabsorbtion by the epithelial lining the distal part of the kidney tubule, the urinary bladder and the distal colon. Reabsorbtion of sodium through this channel, which is regulated by hormones such as aldosterone and vasopressin, is one of the essential mechanisms involved in the regulation of sodium balance, blood volume and blood pressure. Here we isolate a DNA from epithelial cells of rat distal colon and identify it by functional expression of an amiloride-sensitive sodium current in Xenopus oocyte. The deduced polypeptide (698 amino acids) has at least two putative transmembrane segments. Expression of this protein in Xenopus oocytes reconstitutes the functional properties of the highly selective amiloride-sensitive, epithelial sodium channel. The gene encoding this rat sodium channel subunit shares significant sequence similarity with mec-4 and deg-1, members of a family of Caenorhabditis elegans genes involved in sensory touch transduction and, when mutated, neuronal degeneration. We propose that the gene products of these three genes are members of a gene family coding for cation channels.

Primary structure of an apical protein from Xenopus laevis that participates in amiloride-sensitive sodium channel activity

High resistance epithelia express on their apical side an amiloride-sensitive sodium channel that controls sodium reabsorption. A cDNA was found to encode a 1,420-amino acid long polypeptide with no signal sequence, a putative transmembrane segment, and three predicted amphipathic alpha helices. A corresponding 5.2-kb mRNA was detected in Xenopus laevis kidney, intestine, and oocytes, with weak expression in stomach and eyes. An antibody directed against a fusion protein containing a COOH-terminus segment of the protein and an antiidiotypic antibody known to recognize the amiloride binding site of the epithelial sodium channel (Kleyman, T. R., J.-P. Kraehenbuhl, and S. A. Ernst. 1991. J. Biol. Chem. 266:3907-3915) immunoprecipitated a similar protein complex from [35S]methionine-labeled and from apically radioiodinated Xenopus laevis kidney-derived A6 cells. A single integral of 130-kD protein was recovered from samples reduced with DTT. The antibody also cross-reacted by ELISA with the putative amiloride-sensitive sodium channel isolated from A6 cells (Benos, D. J., G. Saccomani, and S. Sariban-Sohraby. 1987. J. Biol. Chem. 262:10613-10618). Although the protein is translated, cRNA injected into oocytes did not reconstitute amiloride-sensitive sodium transport, while antisense RNA or antisense oligodeoxynucleotides specific for two distinct sequences of the cloned cDNA inhibited amiloride-sensitive sodium current induced by injection of A6 cell mRNA. We propose that the cDNA encodes an apical plasma membrane protein that plays a role in the functional expression of the amiloride-sensitive epithelial sodium channel. It may represent a subunit of the Xenopus laevis sodium channel or a regulatory protein essential for sodium channel function.

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Single-channel behavior of a purified epithelial Na+ channel subunit that binds amiloride

The apical membrane of high electrical resistance epithelia, which is selectively permeable to Na+, plays an essential role in the maintenance of salt balance. Na+ entry from the apical fluid into the cells is mediated by amiloride-blockable Na(+)-specific channels. The channel protein, purified from both amphibian and mammalian sources, is composed of several subunits, only one of which the 150-kDa polypeptide, specifically binds the Na+ transport inhibitor amiloride. The goal of the present study was to investigate whether the isolated amiloride-binding subunit of the channel could conduct Na+. The patch-clamp technique was used to study the 150-kDa polypeptide incorporated into a lipid bilayer formed on the tip of a glass pipette. Unitary conductance jumps averaged 4.8 pS at 100 mM Na2HPO4. Open times ranged from 24 ms to several seconds. The channel spent most of the time in the closed state. Channel conductance and gating were independent of voltage between -60 and +100 mV. Amiloride (0.1 microM) decreased the mean open time of the channel by 98%. We conclude that the 150-kDa subunit of the amiloride-blockable Na+ channel conducts current and may be sufficient for the Na+ transport function of the whole channel.

Aldosterone regulation of gene transcription leading to control of ion transport

Aldosterone, like other steroid hormones, initiates its effects by binding to intracellular receptors; these receptors are then able to control the transcription of several genes. The products of these genes eventually modulate the activity of ionic transport systems located in the apical and the basolateral membrane of specialized epithelial cells, thereby modulating the excretion of Na+ and K+ ions. Considerable progress has been made recently in understanding these mechanisms and the structure of the proteins involved in these processes. A novel principle has been discovered to explain the selective effect of aldosterone on its target epithelia. These tissues exclude competing glucocorticoid hormones by the activity of the 11 beta-hydroxysteroid dehydrogenase to allow aldosterone, an enzyme-resistant steroid, to bind to its receptors. Aldosterone induces numerous changes in the activity of membrane ion transport systems and enzymes and cell morphology. Although the enhancement of Na,K-ATPase synthesis and the increase of the number of active Na+ channels in the apical membrane appear as both direct and primary effects, the mechanisms of the other effects remain to be determined. The knowledge of the primary structure of several elements of the aldosterone response system (e.g., mineralocorticoid receptor and Na,K-ATPase) allows us to understand abnormal regulation of Na+ balance at the molecular level and, potentially, to identify genetic alterations responsible for these defects.

Toad bladder amiloride-sensitive channels reconstituted into planar lipid bilayers

In the present study we used established methods to obtain apical membrane vesicles from the toad urinary bladder and incorporated these membrane fragments to solvent-free planar lipid bilayer membranes. This resulted in the appearance of a macroscopic conductance highly sensitive to the diuretic amiloride added to the cis side. The blockage is voltage dependent and well described by a model which assumes that the drug binds to sites in the channel lumen. This binding site is localized at about 15% of the electric field across the membrane. The apparent inhibition constant (K(0)) is equal to 0.98 microM. Ca2+, in the micromolar range on the cis side, is a potent blocker of this conductance. The effect of the divalent has a complex voltage dependence and is modulated by pH. At the unitary level we have found two distinct amiloride-blockable channels with conductances of 160 pS (more frequent) and 120 pS. In the absence of the drug the mean open time is around 0.5 sec for both channels and is not dependent on voltage. The channels are cation selective (PNa/PCl = 15) and poorly discriminate between Na+ and K+ (PNa/PK = 2). Amiloride decreases the lifetime in the open state of both channels and also the conductance of the 160-pS channel.

Barium- or quinine-induced depolarization activates K+, Na+ and cationic conductances in frog proximal tubular cells

1. Frog proximal tubular cells were fused into giant cells. We measured membrane potential (Vm), its changes (delta Vm), and current-induced voltage changes (delta psi) in single cells, during control and experimental states. Each cell served as its own control. 2. In the presence of a physiological Ringer solution, the transference number for potassium (tK) was 0.50. Barium (3 mM) reduced membrane conductance (Gm) by 50%; low-Cl- solutions and low-Na+ solutions also diminished Gm, by 52 and 30%, respectively. The association of barium and low-NaCl solutions decreased Gm to approximately 38% of control, indicating that the impermeant substitute of a physiological ion may interact with other pathways; alternatively, blockade of steady-state conductances may activate physiologically silent processes. 3. In an attempt to enhance the contribution of the partial K+ conductance (GK) to Gm, fused cells were exposed to low-Cl- solutions, containing in addition 0.1 mM-methazolamide, to inhibit the rheogenic Na(+)-HCO3-symport, and 1 microM-amiloride, to block Na+ conductance (GNa). tK went up to 0.83. 4. The high tK preparation was challenged with barium (3 mM) or quinine (Quin, 1 mM). These blockers produced large depolarizations (approximately 60 mV), however, although Gm decreased along early- and mid-depolarization, Gm plateaued and eventually it increased with larger and larger depolarization. 5. Depolarization-associated increase in Gm reflects activation of other conductances. These are Na+, cationic, and K+ conductance(s) poorly sensitive to quinine or barium. In the presence of Ba(2+)- or Quin-induced depolarization, injection of depolarizing current produces delayed increase in conductance. 6. Depolarization-induced activation of cationic conductance (Gcat) and GNa results in enlargement of the K+ electrochemical potential difference, to about 70 mV; this difference allows recycling of K+ ions outwards, since a GK is still detected and may contribute up to 38% of the total conductance.

Regulation of Na+ channels in the cortical collecting duct by AVP and mineralocorticoids

A variety of experimental approaches have shown that AVP and mineralocorticoids stimulate Na+ transport through their effects on the number and kinetic properties of amiloride-sensitive Na+ channels in the apical membrane. The different mechanisms by which AVP and mineralocorticoid act on the Na+ channel provide a basis for synergism in their actions, perhaps by a scheme such as that proposed in Figure 5. However, the details of this interaction will require a better understanding of the molecular details involved in activating quiescent channels, increasing their open probability, and reorientating or inserting channels to an operational position in the apical membrane. Electrophysiological and biochemical approaches have gone a long way toward elucidating some of these molecular details. But the latter approach in particular has indicated that the Na+ channel may have multiple regulatory subunits and thus be a target for several intracellular second messengers and autacoids other than those involved in the actions of AVP and aldosterone. The challenges for future research in this area are multiple. It seems likely that the primary amino acid sequence of the channel subunits will soon become available from cloning and sequencing approaches, but the application of this knowledge to understanding how the subunits are integrated into the complete protein and mediate regulatory signals will be a formidable task. It will be important to determine the normal extracellular signals (other than aldosterone and AVP) and the associated intracellular second messengers that alter channel activity. It will also be important to understand how some species such as the rabbit may "turn off" the stimulatory effect of AVP on Na+ reabsorption in the CCD, and how this regulatory process is altered when these cells are cultured. At the whole animal level, it will also be important to investigate whether changes in one or more of the normal regulatory pathways that impinge on the Na+ channel might be involved in a diminished ability to excrete a salt load, as is observed in some models of hypertension. All of these issues need to be understood at the molecular level, and it seems likely they will provide exciting physiological insights at all levels.

Voltage dependent membrane conductances in cultured renal distal cells

Cultured Na(+)-transporting epithelia from amphibian renal distal tubule (A6) were impaled with microelectrodes and analyzed at short-circuit and after transepithelial voltage perturbation to evaluate the influence of voltage on apical and basolateral membrane conductances. For equivalent circuit analysis, amiloride was applied at each setting of transepithelial potential. At short-circuit, apical and basolateral membrane conductances averaged 88 and 497 microS/cm2, respectively (n = 10). Apical membrane conductance, essentially due to Na(+)-specific pathways, decreased after depolarization of the apical membrane. The drop was considerably larger than predicted by the Goldman-Hodgkin-Katz (GHK) constant-field equation. This suggests decrease in permeability of the apical Na+ channels upon depolarization. Basolateral membrane conductance, preferentially determined by K+ channels, increased after hyperpolarization of the basolateral membrane. This behavior is contrary to the prediction of the GHK constant field equation and reflects inward rectification of the K+ channels. The observed rectification patterns can be valuable for maintenance of cellular homeostasis.

Sodium dependence of the epithelial sodium conductance expressed in Xenopus laevis oocytes

The epithelial Na+ conductance was expressed in Xenopus laevis oocytes by injection of size-fractionated mRNA of bovine tracheal epithelium. Fractionation was achieved by sucrose density gradient centrifugation. Successful expression was analysed by recording current/voltage (I/V) curves in the presence and absence of amiloride (10 mumol/l). The newly expressed conductance was half-maximally inhibited by 44 nmol/l amiloride and exhibited a selectivity for Na+ over K+ of 140:1. I/V curves obtained at different extracellular Na+ concentrations ([Na+]o) were subjected to a Goldman-fit analysis to obtain the relation between Na+ permeability (PNa) and [Na+]o. The data show that decreasing [Na+]o from 85 mmol/l to 0.85 mmol/l increased PNa by more than threefold, which is thought to reflect Na+ channel inhibition by increasing [Na+]o. This effect clearly exceeded what can be attributed to concentration saturation of single Na+ channel conductance (Palmer and Frindt (1986) Proc Natl Acad Sci USA 83:2767). No correlation of inhibition with intracellular Na+ concentration was observed. Preservation of the [Na+]o-dependent self-inhibition by the newly expressed Na+ conductance suggests that it is an intrinsic property of the Na+ channel protein, probably mediated by an extracellular Na+ binding site.