Identification of cytoplasmic domains within the epithelial Na+ channel reactive at the plasma membrane

The role of the ENaC-regulatory complex in aldosterone-mediated sodium transport

The mineralocorticoid aldosterone is indispensable for the control of blood pressure and fluid volume in mammals. It acts in large part to increase the abundance and activity of the epithelial Na(+) channel (ENaC), which mediates apical Na(+) entry in the distal parts of the kidney tubules. Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1. Recent evidence suggests that these key aldosterone-regulated factors function within a unique multi-protein ENaC-regulatory-complex that governs the net cell surface expression and activity of the channel. Another aldosterone-induced protein, CNK3 (connector enhancer of kinase suppressor of Ras 3), also stimulates ENaC and has all of the features of a scaffolding protein. With these observations in mind, we discuss the possibility that CNK3 coordinates the dynamic assembly of the ENaC-regulatory-complex, and promotes context-appropriate aldosterone signal transduction in the regulation of epithelial Na(+) transport.

Activation of a latent nuclear localization signal in the NH2 terminus of γ-ENaC initiates feedback regulation of channel activity

Proteolytic enzymes cleave the epithelial Na(+) channel (ENaC) at several positions releasing, in part, the NH(2) terminus of the γ-subunit. Cleavage increases ENaC activity by increasing open probability; however, the role of polypeptides cleaved from the channel core remains unclear. We find that the cytosolic NH(2) terminus of γ-ENaC unexpectedly targets to the nucleus being particularly strong in nucleoli. In contrast, the cytosolic COOH terminus targets to the cytoplasm and plasma membrane in a manner similar to full-length subunits. Targeting of the cytosolic NH(2) terminus of γ-ENaC to the nucleus has functional consequences for coexpression of eGFP-fusion proteins containing this segment of the channel, but not the COOH terminus, decrease ENaC activity in a dose-dependent manner. The mechanism of this negative regulation is associated with a decrease in the functional half-life of ENaC at the plasma membrane. Inspection of the primary amino acid sequence of γ-ENaC reveals possible nuclear localization signals (NLS) conserved at the extreme NH(2) terminus and just preceding the first transmembrane domain. Disruption of the putative NLS preceding the first transmembrane domain in γ-ENaC but not that at the extreme NH(2) terminus abolishes both targeting to the nucleus and negative regulation of ENaC activity. These findings are consistent with the release of the NH(2) terminus of γ-ENaC following cleavage being functionally important for signaling to the nucleus in a manner similar to Notch signaling and release of the cytosolic COOH-terminal tail of polycystin-1.

The antiepileptic drug carbamazepine affects sodium transport in toad epithelium

The present work investigates the effects of the antiepileptic drug carbamazepine (CBZ) on sodium transport in the isolated skin of the toad Pleurodema thaul. A submaximal concentration of the drug (0.2 mM) applied to the outer surface of the epithelium increased the electrical parameters short-circuit current (Isc) and potential difference (PD) by over 28%, whereas only a higher concentration (1 mM) induced over a 45% decrease in these parameters when applied to the inner surface. The amiloride test showed that the outer surface stimulatory effect was accompanied by an increase and the inner surface inhibitory effect by a decrease in the sodium electromotive force (ENa). Exploration of these effects of CBZ on the outer surface showed that 0.2 mM increased net Na+ (22Na) influx by 20% and 0.6 mM CBZ decreased Na+ mucosa-serosa flux by 19%, a result in agreement with the finding that higher concentrations of CBZ applied to the inner surface not only decreased ENa but also sodium conductance (GNa).

Disinhibitory pathways for control of sodium transport: regulation of ENaC by SGK1 and GILZ

Regulation of ENaC occurs at several levels. The principal hormonal regulator of ENaC, aldosterone, acts through the mineralocorticoid receptor to modulate ENaC-mediated sodium transport, and considerable attention has focused on defining the components of the early phase of this response. Two genes, SGK1 and GILZ, have now been implicated in this regulation. While the functional significance of SGK1 in mediating aldosterone effects is well established, new evidence has enhanced our understanding of the mechanisms of SGK1 action. In addition, recent work demonstrates a novel role for GILZ in the stimulation of ENaC-mediated sodium transport. Interestingly, both SGK1 and GILZ appear to negatively regulate tonic inhibition of ENaC and thus use disinhibition to propagate the rapid effects of aldosterone to increase sodium reabsorption in tight epithelia.

The antiepileptic drug phenytoin affects sodium transport in toad epithelium

The effects of phenytoin on isolated Pleurodema thaul toad skin were investigated. Low (micromolar) concentrations of the antiepileptic agent applied to the outside surface of the toad epithelium increased the electrical parameters (short-circuit current and potential difference) by over 40%, reflecting stimulation of Na(+) transport, whereas higher (millimolar concentrations, outside and inside surface) decreased both electric parameters, the effect being greater at the inside surface (40% and 80% decrease, respectively). The amiloride test showed that the stimulatory effect was accompanied by an increase and the inhibitory effect by a decrease in the sodium electromotive force (ENa). It is concluded that the drug interaction with membrane lipid bilayers might result in a distortion of the lipid-protein interface contributing to disturbance of Na(+) epithelial channel activity. After applying the Na(+)-K(+)-ATPase blocker ouabain and replacing the Na(+) ions in the outer Ringer's solution by choline, it was concluded that both active and passive transport are involved in sodium absorption, although active transport predominates.

Regulation of Epithelial Na+ Channel Activity by Conserved Serine/Threonine Switches within Sorting Signals

The PY and YXXphi motifs are canonical sorting signals involved in trafficking. Nedd4-2 and the mu(2)-subunit of the AP-2 complex target these motifs to facilitate internalization. Epithelial Na(+) channel (ENaC) subunits contain both motifs in their cytosolic COOH termini where they overlap ((S/T)PPPXYX(S/T)phi). Just preceding the PY and embedded within the YXXphi motifs are conserved serine/threonine. We test here whether these conserved Ser/Thr modulate ENaC activity by influencing the function of the internalization domains. We find that co-expression of dominant-negative dynamin (K44A) with ENaC increases channel activity. Conversely, co-expression of Nedd4-2 and epsin with ENaC decrease activity. Alanine substitution of the conserved Thr(628) preceding the PY motif in gamma-mENaC had no effect on basal activity. Channels with this mutation, however, responded to K44A and epsin but not Nedd4-2. Similarly, mutation of the proline repeat in the PY motif of gamma-mENaC disrupted only Nedd4-2 regulation having no effect on regulation by K44A and epsin. Alanine substitution of the conserved Thr within the YXX motif of gamma-mENaC (T635A) increased basal activity. Channels containing this mutation responded to Nedd4-2 but not K44A and epsin. Channels containing the T635(D/E) substitution in gamma-mENaC did not have increased basal activity and responded to Nedd4-2 but not K44A. The double mutant T628A,T635A did not respond to Nedd4-2 or K44A. Mutation of Thr(628) and Thr(635) also disrupted ENaC precipitation with the mu(2)-subunit of the AP-2 complex. Moreover, the YXXphi motif, independent of the PY motif, was sufficient to target degradation with T635A disrupting this effect. These results demonstrate that the overlapping PY and YXXphi motifs in ENaC are, in some instances, capable of independent function and that the Ser/Thr just preceding and within these domains impact this function.

Identification of a functional phosphatidylinositol 3,4,5-trisphosphate binding site in the epithelial Na+ channel

Membrane phospholipids, such as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), are signaling molecules that can directly modulate the activity of ion channels, including the epithelial Na(+) channel (ENaC). Whereas PI(3,4,5)P(3) directly activates ENaC, its binding site within the channel has not been identified. We identify here a region of gamma-mENaC just following the second trans-membrane domain (residues 569-583) important to PI(3,4,5)P(3) binding and regulation. Deletion of this track decreases activity of ENaC heterologously expressed in Chinese hamster ovary cells. K-Ras and its first effector phosphoinositide 3-OH kinase (PI3-K), as well as RhoA and its effector phosphatidylinositol 4-phosphate 5-kinase increase ENaC activity. Whereas the former, via generation of PI(3,4,5)P(3), increases ENaC open probability, the latter increases activity by increasing membrane levels of the channel. Deletion of the region just distal to the second trans-membrane domain disrupted regulation by K-Ras and PI3-K but not RhoA and phosphatidylinositol 4-phosphate 5-kinase. Moreover, PI(3,4,5)P(3) binds ENaC with deletion of the region following the second transmembrane domain disrupting this interaction and disrupting direct activation of the channel by PI(3,4,5)P(3). Mutation analysis revealed the importance of conserved positive and negative charged residues as well as bulky amino acids within this region to modulation of ENaC by PI3-K. The current results identify the region just distal to the second trans-membrane domain within gamma-mENaC as being part of a functional PI(3,4,5)P(3) binding site that directly impacts ENaC activity. Phospholipid binding to this site is probably mediated by the positively charged amino acids within this track, with negatively charged and bulky residues also influencing specificity of interactions.

A region directly following the second transmembrane domain in gamma ENaC is required for normal channel gating

We used a yeast one-hybrid complementation screen to identify regions within the cytosolic tails of the mouse alpha, beta, and gamma epithelial Na+ channel (ENaC) important to protein-protein and/or protein-lipid interactions at the plasma membrane. The cytosolic COOH terminus of alphaENaC contained a strongly interactive domain just distal to the second transmembrane region (TM2) between Met610 and Val632. Likewise, gammaENaC contained such a domain just distal to TM2 spanning Gln573-Pro600. Interactive domains were also localized within Met1-Gln54 and the last 17 residues of alpha- and betaENaC, respectively. Confocal images of Chinese hamster ovary cells transfected with enhanced green fluorescent fusion proteins of the cytosolic tails of mENaC subunits were consistent with results in yeast. Fusion proteins of the NH2 terminus of alphaENaC and the COOH termini of all three subunits co-localized with a plasma membrane marker. The functional importance of the membrane interactive domain in the COOH terminus of gammaENaC was established with whole-cell patch clamp experiments of wild type (alpha, beta, and gamma) and mutant (alpha, beta, and gammadeltaQ573-P600) mENaC reconstituted in Chinese hamster ovary cells. Mutant channels had about 13% of the activity of wild type channels with 0.33 +/- 0.14 versus 2.5 +/- 0.80 nA of amiloridesensitive inward current at -80 mV. Single channel analysis of recombinant channels demonstrated that mutant channels had a decrease in Po with 0.16 +/- 0.03 versus 0.67 +/- 0.07 for wild type. Mutant gammaENaC associated normally with the other two subunits in co-immunoprecipitation studies and localized to the plasma membrane in membrane labeling experiments and when visualized with evanescent-field fluorescence microscopy. Similar to deletion of Gln573-Pro600, deletion of Gln573-Arg583 but not Thr584-Pro600 decreased ENaC activity. The current results demonstrate that residues within Gln573-Arg583 of gammaENaC are necessary for normal channel gating.