The epithelial amiloride-sensitive sodium channel (ENaC) controls transepithelial Na ؉ movement in Na ؉ -transporting epithelia and is associated with Liddle syndrome, an autosomal dominant form of salt-sensitive hypertension. Detailed analysis of ENaC channel properties and the functional consequences of mutations causing Liddle syndrome has been, so far, limited by lack of a method allowing specific and quantitative detection of cell-surfaceexpressed ENaC. We have developed a quantitative assay based on the binding of 125 I-labeled M 2 anti-FLAG monoclonal antibody (M 2 Ab*) directed against a FLAG reporter epitope introduced in the extracellular loop of each of the ␣, ␤, and ␥ ENaC subunits. Insertion of the FLAG epitope into ENaC sequences did not change its functional and pharmacological properties. The binding specificity and affinity (K d ‫؍‬ 3 nM) allowed us to correlate in individual Xenopus oocytes the macroscopic amiloride-sensitive sodium current (I Na ) with the number of ENaC wild-type and mutant subunits expressed at the cell surface. These experiments demonstrate that: (i) only heteromultimeric channels made of ␣, ␤, and ␥ ENaC subunits are maximally and efficiently expressed at the cell surface; (ii) the overall ENaC open probability is one order of magnitude lower than previously observed in single-channel recordings; (iii) the mutation causing Liddle syndrome (␤ R564stop) enhances channel activity by two mechanisms, i.e., by increasing ENaC cell surface expression and by changing channel open probability. This quantitative approach provides new insights on the molecular mechanisms underlying one form of salt-sensitive hypertension.The amiloride-sensitive epithelial sodium channel (ENaC) is a heteromultimeric protein composed of three homologous subunits, ␣, ␤, and ␥ (1, 2, 4, 5), exhibiting Ϸ30% identity at the amino acid level. Predicted protein topology reveals a large (Ϸ500 amino acids) extracellular hydrophilic loop with several putative N-linked glycosylation sites flanked by two hydrophobic domains (M1 and M2) that span the membrane.In aldosterone target epithelia, ENaC represents the ratelimiting step for Na ϩ reabsorption (6, 7). The control of Na ϩ movements in these epithelia is critical for the maintenance of extracellular fluid and electrolyte balance, and the central role of ENaC in this regulation is exemplified by the recent discoveries of several heritable human mutations in the genes encoding the ENaC subunits that lead to abnormal regulation of blood pressure and electrolyte balance (8, 9). Therefore, identification of the molecular and cellular mechanisms involved in the regulation of ENaC channel activity at the cell surface is critical for our understanding of the pathogenesis of salt-sensitive hypertension. ENaC is characterized by its high ionic selectivity for sodium and lithium, by its low single-channel conductance (5 pS in the presence of Na and 8 pS in the presence of Li), by its long open and closed times, and by its high affinity for amiloride. Our knowled...

Liddle syndrome is an autosomal dominant form of hypertension, resulting from mutations in the cytoplasmic C‐terminus of either the beta or gamma subunits of the amiloride‐sensitive epithelial Na channel (ENaC) which lead to constitutively increased channel activity. Most mutations reported to date result in the elimination of 45–75 normal amino acids from these segments, leaving open the question of the identity of the precise amino acids in which mutation can lead to an enhanced channel activity. To address this question, we have performed a systematic mutagenesis study of the C‐termini of the alpha, beta and gamma ENaC subunits of the rat channel and have analyzed their function by expression in Xenopus oocytes. The results demonstrate that a short proline‐rich segment present in the cytoplasmic C‐terminus of each subunit is required for the normal regulation of channel activity. Missense mutations altering a consensus PPPXY sequence of the alpha, beta or gamma subunits reproduced the increase in channel activity found in mutants in which the entire cytoplasmic C‐termini are deleted. This proline‐rich sequence, referred to as the PY motif, is known to be a site of binding by proteins bearing a WW domain. These findings show that the three PY motifs in the C‐termini of ENaC are involved in the regulation of channel activity, probably via protein‐protein interactions. This new regulatory mechanism of channel function is critical for the maintenance of normal Na reabsorption in the kidney and of Na+ balance and blood pressure.

The amiloride-sensitive epithelial Nachannel (ENaC) is a heteromultimeric channel made of three αβγ subunits. The structures involved in the ion permeation pathway have only been partially identified, and the respective contributions of each subunit in the formation of the conduction pore has not yet been established. Using a site-directed mutagenesis approach, we have identified in a short segment preceding the second membrane-spanning domain (the pre-M2 segment) amino acid residues involved in ion permeation and critical for channel block by amiloride. Cys substitutions of Gly residues in β and γ subunits at position βG525 and γG537 increased the apparent inhibitory constant (K i) for amiloride by >1,000-fold and decreased channel unitary current without affecting ion selectivity. The corresponding mutation S583 to C in the α subunit increased amiloride K i by 20-fold, without changing channel conducting properties. Coexpression of these mutated αβγ subunits resulted in a nonconducting channel expressed at the cell surface. Finally, these Cys substitutions increased channel affinity for block by externalZn2+ ions, in particular the αS583C mutant showing a K i for Zn2+of 29 μM. Mutations of residues αW582L or βG522D also increased amiloride K i, the later mutation generating a Ca2+blocking site located 15% within the membrane electric field. These experiments provide strong evidence that αβγ ENaCs are pore-forming subunits involved in ion permeation through the channel. The pre-M2 segment of αβγ subunits may form a pore loop structure at the extracellular face of the channel, where amiloride binds within the channel lumen. We propose that amiloride interacts with Na+ions at an external Na+binding site preventing ion permeation through the channel pore.

The epithelial Na+ channel (ENaC) is highly selective for Na+ and Li+ over K+ and is blocked by the diuretic amiloride. ENaC is a heterotetramer made of two α, one β, and one γ homologous subunits, each subunit comprising two transmembrane segments. Amino acid residues involved in binding of the pore blocker amiloride are located in the pre-M2 segment of β and γ subunits, which precedes the second putative transmembrane α helix (M2). A residue in the α subunit (αS589) at the NH2 terminus of M2 is critical for the molecular sieving properties of ENaC. ENaC is more permeable to Li+ than Na+ ions. The concentration of half-maximal unitary conductance is 38 mM for Na+ and 118 mM for Li+, a kinetic property that can account for the differences in Li+ and Na+ permeability. We show here that mutation of amino acid residues at homologous positions in the pre-M2 segment of α, β, and γ subunits (αG587, βG529, γS541) decreases the Li+/Na+ selectivity by changing the apparent channel affinity for Li+ and Na+. Fitting single-channel data of the Li+ permeation to a discrete-state model including three barriers and two binding sites revealed that these mutations increased the energy needed for the translocation of Li+ from an outer ion binding site through the selectivity filter. Mutation of βG529 to Ser, Cys, or Asp made ENaC partially permeable to K+ and larger ions, similar to the previously reported αS589 mutations. We conclude that the residues αG587 to αS589 and homologous residues in the β and γ subunits form the selectivity filter, which tightly accommodates Na+ and Li+ ions and excludes larger ions like K+.

The epithelial Na+ channel (ENaC), located in the apical membrane of tight epithelia, allows vectorial Na+ absorption. The amiloride-sensitive ENaC is highly selective for Na+ and Li+ ions. There is growing evidence that the short stretch of amino acid residues (preM2) preceding the putative second transmembrane domain M2 forms the outer channel pore with the amiloride binding site and the narrow ion-selective region of the pore. We have shown previously that mutations of the αS589 residue in the preM2 segment change the ion selectivity, making the channel permeant to K+ ions. To understand the molecular basis of this important change in ionic selectivity, we have substituted αS589 with amino acids of different sizes and physicochemical properties. Here, we show that the molecular cutoff of the channel pore for inorganic and organic cations increases with the size of the amino acid residue at position α589, indicating that αS589 mutations enlarge the pore at the selectivity filter. Mutants with an increased permeability to large cations show a decrease in the ENaC unitary conductance of small cations such as Na+ and Li+. These findings demonstrate the critical role of the pore size at the αS589 residue for the selectivity properties of ENaC. Our data are consistent with the main chain carbonyl oxygens of the αS589 residues lining the channel pore at the selectivity filter with their side chain pointing away from the pore lumen. We propose that the αS589 side chain is oriented toward the subunit–subunit interface and that substitution of αS589 by larger residues increases the pore diameter by adding extra volume at the subunit–subunit interface.