Antiidiotypic antibody recognizes an amiloride binding domain within the alpha subunit of the epithelial Na+ channel

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

Engineered mutant α-ENaC subunit mRNA delivered by lipid nanoparticles reduces amiloride currents in cystic fibrosis-based cell and mice models

Cystic fibrosis (CF) results from mutations in the chloride-conducting CF transmembrane conductance regulator (CFTR) gene. Airway dehydration and impaired mucociliary clearance in CF is proposed to result in tonic epithelial sodium channel (ENaC) activity, which drives amiloride-sensitive electrogenic sodium absorption. Decreasing sodium absorption by inhibiting ENaC can reverse airway surface liquid dehydration. Here, we inhibit endogenous heterotrimeric ENaC channels by introducing inactivating mutant ENaC α mRNA (α_mutENaC). Lipid nanoparticles carrying α_mutENaC were transfected in CF-based airway cells in vitro and in vivo. We observed a significant decrease in macroscopic as well as amiloride-sensitive ENaC currents and an increase in airway surface liquid height in CF airway cells. Similarly, intranasal transfection of α_mutENaC mRNA decreased amiloride-sensitive nasal potential difference in CFTRKO mice. These data suggest that mRNA-based ENaC inhibition is a powerful strategy for reducing mucus dehydration and has therapeutic potential for treating CF in all patients, independent of genotype.

N-linked glycans are required on epithelial Na+ channel subunits for maturation and surface expression

Epithelial Na⁺ channel (ENaC) subunits undergo N-linked glycosylation in the endoplasmic reticulum where they assemble into an αβγ complex. Six, 13, and 5 consensus sites (Asn-X-Ser/Thr) for N-glycosylation reside in the extracellular domains of the mouse α-, β-, and γ-subunits, respectively. Because the importance of ENaC N-linked glycans has not been fully addressed, we examined the effect of preventing N-glycosylation of specific subunits on channel function, expression, maturation, and folding. Heterologous expression in Xenopus oocytes or Fischer rat thyroid cells with αβγ-ENaC lacking N-linked glycans on a single subunit reduced ENaC activity as well as the inhibitory response to extracellular Na⁺. The lack of N-linked glycans on the β-subunit also precluded channel activation by trypsin. However, channel activation by shear stress was N-linked glycan independent, regardless of which subunit was modified. We also discovered that the lack of N-linked glycans on any one subunit reduced the total and surface levels of cognate subunits. The lack of N-linked glycans on the β-subunit had the largest effect on total levels, with the lack of N-linked glycans on the γ- and α-subunits having intermediate and modest effects, respectively. Finally, channels with wild-type β-subunits were more sensitive to limited trypsin proteolysis than channels lacking N-linked glycans on the β-subunit. Our results indicate that N-linked glycans on each subunit are required for proper folding, maturation, surface expression, and function of the channel.

ENaC structure and function in the wake of a resolved structure of a family member

Our understanding of epithelial Na(+) channel (ENaC) structure and function has been profoundly impacted by the resolved structure of the homologous acid-sensing ion channel 1 (ASIC1). The structure of the extracellular and pore regions provide insight into channel assembly, processing, and the ability of these channels to sense the external environment. The absence of intracellular structures precludes insight into important interactions with intracellular factors that regulate trafficking and function. The primary sequences of ASIC1 and ENaC subunits are well conserved within the regions that are within or in close proximity to the plasma membrane, but poorly conserved in peripheral domains that may functionally differentiate family members. This review examines functional data, including ion selectivity, gating, and amiloride block, in light of the resolved ASIC1 structure.

Amiloride docking to acid-sensing ion channel-1

Amiloride is a small molecule diuretic, which has been used to dissect sodium transport pathways in many different systems. This drug is known to interact with the epithelial sodium channel and acid-sensing ion channel proteins, as well as sodium/hydrogen antiporters and sodium/calcium exchangers. The exact structural basis for these interactions has not been elucidated as crystal structures of these proteins have been challenging to obtain, though some involved residues and domains have been mapped. This work examines the interaction of amiloride with acid-sensing ion channel-1, a protein whose structure is available using computational and experimental techniques. Using molecular docking software, amiloride and related molecules were docked to model structures of homomeric human ASIC-1 to generate potential interaction sites and predict which analogs would be more or less potent than amiloride. The predictions made were experimentally tested using whole-cell patch clamp. Drugs previously classified as NCX or NHE inhibitors are shown to also inhibit hASIC-1. Potential docking sites were re-examined against experimental data to remove spurious interaction sites. The voltage sensitivity of inhibitors was also examined. Using the aggregated data from these computational and experimental experiments, putative interaction sites for amiloride and hASIC-1 have been defined. Future work will experimentally verify these interaction sites, but at present this should allow for virtual screening of drug libraries at these putative interaction sites.

Amiloride-sensitive NaCl taste responses are associated with genetic variation of ENaC alpha-subunit in mice

An epithelial Na(+) channel (ENaC) is expressed in taste cells and may be involved in the salt taste transduction. ENaC activity is blocked by amiloride, which in several mammalian species also inhibits taste responses to NaCl. In mice, lingual application of amiloride inhibits NaCl responses in the chorda tympani (CT) gustatory nerve much stronger in the C57BL/6 (B6) strain than in the 129P3/J (129) strain. We examined whether this strain difference is related to gene sequence variation or mRNA expression of three ENaC subunits (alpha, beta, gamma). Real-time RT-PCR and in situ hybridization detected no significant strain differences in expression of all three ENaC subunits in fungiform papillae. Sequences of the beta- and gammaENaC subunit genes were also similar in the B6 and 129 strains, but alphaENaC gene had three single nucleotide polymorphisms (SNPs). One of these SNPs resulted in a substitution of arginine in the B6 strain to tryptophan in the 129 strain (R616W) in the alphaENaC protein. To examine association of this SNP with amiloride sensitivity of CT responses to NaCl, we produced F(2) hybrids between B6 and 129 strains. Amiloride inhibited CT responses to NaCl in F(2) hybrids with B6/129 and B6/B6 alphaENaC R616W genotypes stronger than in F(2) hybrids with 129/129 genotype. This suggests that the R616W variation in the alphaENaC subunit affects amiloride sensitivity of the ENaC channel and provides evidence that ENaC is involved in amiloride-sensitive salt taste responses in mice.

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.

Extracellular histidine residues crucial for Na+ self-inhibition of epithelial Na+ channels

Epithelial Na(+) channels (ENaC) participate in the regulation of extracellular fluid volume homeostasis and blood pressure. Channel activity is regulated by both extracellular and intracellular Na(+). The down-regulation of ENaC activity by external Na(+) is referred to as Na(+) self-inhibition. We investigated the structural determinants of Na(+) self-inhibition by expressing wild-type or mutant ENaCs in Xenopus oocytes and analyzing changes in whole-cell Na(+) currents following a rapid increase of bath Na(+) concentration. Our results indicated that wild-type mouse alphabetagammaENaC has intrinsic Na(+) self-inhibition similar to that reported for human, rat, and Xenopus ENaCs. Mutations at His(239) (gammaH239R, gammaH239D, and gammaH239C) in the extracellular loop of the gammaENaC subunit prevented Na(+) self-inhibition whereas mutations of the corresponding His(282) in alphaENaC (alphaH282D, alphaH282R, alphaH282W, and alphaH282C) significantly enhanced Na(+) self-inhibition. These results suggest that these two histidine residues within the extracellular loops are crucial structural determinants for Na(+) self-inhibition.

Proliferation and differentiation of Xenopus A6 cells under hypergravity as revealed by time-lapse imaging

Xenopus laevis A6 cells, which are cloned epithelial cells from the Xenopus kidney, differentiate into a dome structure when the cells reach confluence. We investigated the gravitational responses of A6 cellular motility during normal differentiation and differentiation under hypergravity conditions using centrifugation (1-100 x g). Progression to dome formation was analyzed by time-lapse micrography. Dome formation and increased expression of Na(+)/K(+)-adenosine triphosphatase were used as markers of differentiation. Interestingly, a high rate of cellular proliferation was observed at a low level of hypergravity (5 x g). Despite this, there was no difference in the time to dome formation between the control cells at primary cell density and those that differentiated under hyper- or hypogravity conditions. In conclusion, this experiment on amphibian cells revealed that the proliferation of A6 cells was strongly affected by gravity conditions, but the differentiation step appears to be controlled by an intra- or intercellular clock.

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

Epithelial sodium channels (ENaC) are regulated by various intracellular and extracellular factors including divalent cations. We studied the inhibitory effect and mechanism of external Ni(2+) on cloned mouse alpha-beta-gamma ENaC expressed in Xenopus oocytes. Ni(2+) reduced amiloride-sensitive Na(+) currents of the wild type mouse ENaC in a dose-dependent manner. The Ni(2+) block was fast and partially reversible at low concentrations and irreversible at high concentrations. ENaC inhibition by Ni(2+) was accompanied by moderate inward rectification at concentrations higher than 0.1 mm. ENaC currents were also blocked by the histidine-reactive reagent diethyl pyrocarbonate. Pretreatment of the oocytes with the reagent reduced Ni(2+) inhibition of the remaining current. Mutations at alphaHis(282) and gammaHis(239) located within the extracellular loops significantly decreased Ni(2+) inhibition of ENaC currents. The mutation alphaH282D or double mutations alphaH282R/gammaH239R eliminated Ni(2+) block. All mutations at gammaHis(239) eliminated Ni(2+)-induced inward current rectification. Ni(2+) block was significantly enhanced by introduction of a histidine at alphaArg(280). Lowering extracellular pH to 5.5 and 4.4 decreased or eliminated Ni(2+) block. Although alphaH282C-beta-gamma channels were partially inhibited by the sulfhydryl-reactive reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET), alpha-beta-gamma H239C channels were insensitive to MTSET. From patch clamp studies, Ni(2+) did not affect unitary current but decreased open probability when perfused into the recording pipette. Our results suggest that external Ni(2+) reduces ENaC open probability by binding to a site consisting of alphaHis(282) and gammaHis(239) and that these histidine residues may participate in ENaC gating.

Altering airway surface liquid volume: inhalation therapy with amiloride and hyperosmotic agents

The thin layer of liquid lining the entire respiratory tract is the first line of defense against the continuous insult of inhaled bacteria and noxious chemicals. Many chronic obstructive diseases of the airway may reflect decreased airway surface liquid, which results from imbalances in ion transport and mucus production. Reduction in the thickness of airway surface liquid leads to reduced mucociliary clearance rates, causing mucus accumulation and infection in the airway. In this chapter, two inhalation therapies to replenish airway surface liquid and enhance mucociliary clearance are discussed: (1) aerosolized hyperosmotic agents; and (2) aerosolized sodium channel blockers. The advantages and disadvantages of each therapy are discussed, as well as future directions for improving airway surface liquid volume by inhalation pharmacotherapy.

Four-state equilibrium unfolding of an scFv antibody fragment

The conformational stability of a single-chain Fv antibody fragment against a hepatitis B surface antigen (anti-HBsAg scFv) has been studied by urea and temperature denaturation followed by fluorescence and circular dichroism. At neutral pH and low protein concentration, it is a well-folded monomer, and its urea and thermal denaturations are reversible. The noncoincidence of the fluorescence and circular dichroism transitions indicates the accumulation in the urea denaturation of an intermediate (I(1)) not previously described in scFv molecules. In addition, at higher urea concentrations, a red-shift in the fluorescence emission maximum reveals an additional intermediate (I(2)), already reported in the denaturation of other scFvs. The urea equilibrium unfolding of the anti-HBsAg scFv is thus four-state. A similar four-state behavior is observed in the thermal unfolding although the intermediates involved are not identical to those found in the urea denaturation. Global analysis of the thermal unfolding data suggests that the first intermediate displays substantial secondary structure and some well-defined tertiary interactions while the second one lacks well-defined tertiary interactions but is compact and unfolds at higher temperature in a noncooperative fashion. Global analysis of the urea unfolding data (together with the modeled structure of the scFv) provides insights into the conformation of the chemical denaturation intermediates and allows calculation of the N-I(1), I(1)-I(2), and I(2)-D free energy differences. Interestingly, although the N-D free energy difference is very large, the N-I(1) one, representing the "relevant" conformational stability of the scFv, is small.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.

Functional effects of a monoclonal antibody on mechanoelectrical transduction in outer hair cells

The functional effect of a monoclonal antibody, RA6.3, on mechanoelectrical transduction (MET) of outer hair cells (OHCs) isolated from the adult guinea-pig cochlea was investigated. This antibody was raised by an antiidiotypic approach against amiloride binding sites. RA6.3 irreversibly reduced the receptor current, independent of membrane potential. The time course of the functional block was independent of dilution (1:100, 50 and 10), beginning 1.2+/-0.5 min after the start of application and decreasing exponentially with a time constant of 0.29+/-0.18 min to 53+/-8% of the control current. The residual current was reversibly blocked by amiloride (300 microM), mainly at negative membrane potentials. Block of control current by amiloride was competitively inhibited by simultaneous application of RA6.3. These results suggest that RA6.3 binds to or in close proximity to amiloride receptor sites associated with the MET channels. Irreversibility, incompleteness, independence of membrane potential and independence of antibody dilution of the functional block can all be explained by irreversible binding of one antibody molecule to a receptor site, yielding a non-blocked state, followed by a relatively slow, reversible transition to a blocked state. It is proposed that the reversible transition might represent intramolecular binding of the second antibody combining site to the second receptor site.

Directing the immune response to carbohydrate antigens

Peptide mimetics may substitute for carbohydrate antigens in vaccine design applications. At present, the structural and immunological aspects of antigenic mimicry, which translate into immunologic mimicry, as well as the functional correlates of each, are unknown. In contrast to screening peptide display libraries, we demonstrate the feasibility of a structure-assisted vaccine design approach to identify functional mimeotopes. By using concanavalin A (ConA), as a recognition template, peptide mimetics reactive with ConA were identified. Designed peptides were observed to compete with synthetic carbohydrate probes for ConA binding, as demonstrated by enzyme-linked immunosorbent assay and isothermal titration calorimetry (ITC) analysis. ITC measurements indicate that a multivalent form of one particular mimetic binds to ConA with similar affinity as does trimannoside. Splenocytes from mimeotope-immunized mice display a peptide-specific cellular response, confirming a T-cell-dependent nature for the mimetic. As ConA binds to the Envelope protein of the human immunodeficiency virus, type 1 (HIV-1), we observed that mimeotope-induced serum also binds to HIV-1-infected cells, as assessed by flow cytometry, and could neutralize T-cell line adapted HIV-1 isolates in vitro, albeit at low titers. These studies emphasize that mimicry is based more upon functional rather than structural determinants that regulate mimeotope-induced T-dependent antibody responses to polysaccharide and emphasize that rational approaches can be employed to develop further vaccine candidates.

Cell surface expression and turnover of the alpha-subunit of the epithelial sodium channel

The renal epithelial cell line A6, derived from Xenopus laevis, expresses epithelial Na(+) channels (ENaCs) and serves as a model system to study hormonal regulation and turnover of ENaCs. Our previous studies suggest that the alpha-subunit of Xenopus ENaC (alpha-xENaC) is detectable as 150- and 180-kDa polypeptides, putative immature and mature alpha-subunit heterodimers. The 150- and 180-kDa alpha-xENaC were present in distinct fractions after sedimentation of A6 cell lysate through a sucrose density gradient. Two anti-alpha-xENaC antibodies directed against distinct domains demonstrated that only 180-kDa alpha-xENaC was expressed at the apical cell surface. The half-life of cell surface-expressed alpha-xENaC was 24-30 h, suggesting that once ENaC matures and is expressed at the plasma membrane, its turnover is similar to that reported for mature cystic fibrosis transmembrane conductance regulator. No significant changes in apical surface expression of alpha-xENaC were observed after treatment of A6 cells with aldosterone for 24 h, despite a 5.3-fold increase in short-circuit current. This lack of change in surface expression is consistent with previous observations in A6 cells and suggests that aldosterone regulates ENaC gating and increases channel open probability.

Detection of aquaporin-2 in the plasma membranes of oocytes: a novel isolation method with improved yield and purity

Aquaporin-2 (AQP2) water channel mutations cause autosomal recessive and dominant nephrogenic diabetes insipidus (NDI). Expressed in oocytes, a mutant in dominant (AQP2-E258K), but not in recessive (AQP2-R187C), NDI conferred a specific dominant-negative effect on wild-type (wt) AQP2 water permeability (Pf) only at low expression levels. Since at these levels, the yield of conventional-isolated plasma membranes was too low, an improved technique to semiquantify AQP2 in the plasma membrane was needed. Antibodies against the C-loop of AQP2 were not applicable since they were unspecific and introduction of a tag into this loop caused misfolding and ER retardation. Membrane-impermeable biotin analogues turned out to label intracellular AQP2 proteins. Therefore, a method has been developed which generates a high yield of nearly pure plasma membranes, which enables semiquantification of plasma membrane proteins expressed at low levels in oocytes. Our new method allows for phenotype-genotype correlation studies in a wide range of channelopathies.

A molecular basis for functional peptide mimicry of a carbohydrate antigen

Peptides may substitute for carbohydrate antigens in carbohydrate-specific immunological reactions. Using the recognition properties of an anti-Lewis Y (LeY) antibody, BR55-2, as a model system, we establish a molecular perspective for peptide mimicry by comparing the three-dimensional basis of BR55-2 binding to LeY with the binding of the same antibody to peptides. The peptides compete with LeY, as demonstrated by enzyme-linked immunosorbent assay and Biacore analysis. The computer program LUDI was used to epitope map the antibody-combining site, correlating peptide reactivity patterns. This approach identified amino acids interacting with the same BR55-2 functional residue groups that recognize the Fucalpha(1-3) moiety of LeY. Molecular modeling indicates that the peptides adopt an extended turn conformation within the BR55-2 combining site, serving to overlap the peptides with the LeY spatial position. Peptide binding is associated with only minor changes in BR55-2, relative to the BR55-2-LeY complex. Anti-peptide serum distinguishes the Fucalpha(1-3) from the Fucalpha(1-4) linkage, therefore differentiating difucosylated neolactoseries antigens. These results further confirm that peptides and carbohydrates can bind to the same antibody-binding site and that peptides can structurally and functionally mimic salient features of carbohydrate epitopes.

Regulation of epithelial Na(+) channels by actin in planar lipid bilayers and in the Xenopus oocyte expression system

The hypothesis that actin interactions account for the signature biophysical properties of cloned epithelial Na(+) channels (ENaC) (conductance, ion selectivity, and long mean open and closed times) was tested using planar lipid bilayer reconstitution and patch clamp techniques. We found the following. 1) In bilayers, actin produced a more than 2-fold decrease in single channel conductance, a 5-fold increase in Na(+) versus K(+) permselectivity, and a substantial increase in mean open and closed times of wild-type alphabetagamma-rENaC but had no effect on a mutant form of rENaC in which the majority of the C terminus of the alpha subunit was deleted (alpha(R613X)betagamma-rENaC). 2) When alpha(R613X)betagamma-rENaC was heterologously expressed in oocytes and single channels examined by patch clamp, 12.5-pS channels of relatively low cation permeability were recorded. These characteristics were identical to those recorded in bilayers for either alpha(R613X)betagamma-rENaC or wild-type alphabetagamma-rENaC in the absence of actin. Moreover, we show that rENaC subunits tightly associate, forming either homo- or heteromeric complexes when prepared by in vitro translation or when expressed in oocytes. Finally, we show that alpha-rENaC is properly assembled but retained in the endoplasmic reticulum compartment. We conclude that actin subserves an important regulatory function for ENaC and that planar bilayers are an appropriate system in which to study the biophysical and regulatory properties of these cloned channels.

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

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

Association of the epithelial sodium channel with Apx and alpha-spectrin in A6 renal epithelial cells

Recent molecular cloning of the epithelial sodium channel (ENaC) provides the opportunity to identify ENaC-associated proteins that function in regulating its cell surface expression and activity. We have examined whether ENaC is associated with Apx (apical protein Xenopus) and the spectrin-based membrane cytoskeleton in Xenopus A6 renal epithelial cells. We have also addressed whether Apx is required for the expression of amiloride-sensitive Na(+) currents by cloned ENaC. Sucrose density gradient centrifugation of A6 cell detergent extracts showed co-sedimentation of xENaC, alpha-spectrin, and Apx. Immunoblot analysis of proteins co-immunoprecipitating under high stringency conditions from peak Xenopus ENaC/Apx-containing gradient fractions indicate that ENaC, Apx, and alpha-spectrin are associated in a macromolecular complex. To examine whether Apx is required for the functional expression of ENaC, alphabetagamma mENaC cRNAs were coinjected into Xenopus oocytes with Apx sense or antisense oligodeoxynucleotides. The two-electrode voltage clamp technique showed there was a marked reduction in amiloride-sensitive current in oocytes coinjected with antisense oligonucleotides when to compared with oocytes coinjected with sense oligonucleotides. These studies indicate that ENaC is associated in a macromolecular complex with Apx and alpha-spectrin in A6 cells and suggest that Apx is required for the functional expression of ENaC in Xenopus epithelia.