Feedback inhibition of epithelial Na(+) channels in Xenopus oocytes does not require G(0) or G(i2) proteins

Regulation of ion transport from within ion transit pathways

All cells must control the activities of their ion channels and transporters to maintain physiologically appropriate gradients of solutes and ions. The complexity of underlying regulatory mechanisms is staggering, as exemplified by insulin regulation of transporter trafficking. Simpler strategies occur in single-cell organisms, where subsets of transporters act as solute sensors to regulate expression of their active homologues. This Viewpoint highlights still simpler mechanisms by which Na transporters use their own transport sites as sensors for regulation. The underlying principle is inherent to Na/K pumps in which aspartate phosphorylation and dephosphorylation are controlled by occupation of transport sites for Na and K, respectively. By this same principle, Na binding to transport sites can control intrinsic inactivation reactions that are in turn modified by extrinsic signaling factors. Cardiac Na/Ca exchangers (NCX1s) and Na/K pumps are the best examples. Inactivation of NCX1 occurs when cytoplasmic Na sites are fully occupied and is regulated by lipid signaling. Inactivation of cardiac Na/K pumps occurs when cytoplasmic Na-binding sites are not fully occupied, and inactivation is in turn regulated by Ca signaling. Potentially, Na/H exchangers (NHEs) and epithelial Na channels (ENaCs) are regulated similarly. Extracellular protons and cytoplasmic Na ions oppose secondary activation of NHEs by cytoplasmic protons. ENaCs undergo inactivation as cytoplasmic Na rises, and small diffusible molecules of an unidentified nature are likely involved. Multiple other ion channels have recently been shown to be regulated by transiting ions, thereby underscoring that ion permeation and channel gating need not be independent processes.

Cl- interference with the epithelial Na+ channel ENaC

The cystic fibrosis transmembrane conductance regulator (CFTR) is a protein kinase A and ATP-regulated Cl- channel that also controls the activity of other membrane transport proteins, such as the epithelial Na+ channel ENaC. Previous studies demonstrated that cytosolic domains of ENaC are critical for down-regulation of ENaC by CFTR, whereas others suggested a role of cytosolic Cl- ions. We therefore examined in detail the anion dependence of ENaC and the role of its cytosolic domains for the inhibition by CFTR and the Cl- channel CLC-0. Coexpression of rat ENaC with human CFTR or the human Cl- channel CLC-0 caused inhibition of amiloride-sensitive Na+ currents after cAMP-dependent stimulation and in the presence of a 100 mM bath Cl- concentration. After activation of CFTR by 3-isobutyl-1-methylxanthine and forskolin or expression of CLC-0, the intracellular Cl- concentration was increased in Xenopus oocytes in the presence of a high bath Cl- concentration, which inhibited ENaC without changing surface expression of alpha beta gammaENaC. In contrast, a 5 mM bath Cl- concentration reduced the cytosolic Cl- concentration and enhanced ENaC activity. ENaC was also inhibited by injection of Cl- into oocytes and in inside/out macropatches by exposure to high cytosolic Cl- concentrations. The effect of Cl- was mimicked by Br-, Br-, NO3(-), and I-. Inhibition by Cl- was reduced in trimeric channels with a truncated COOH terminus of betaENaC and gammaENaC, and it was no longer detected in dimeric alpha deltaCbeta ENaC channels. Deletion of the NH2 terminus of alpha-, beta-, or gammaENaC, mutations in the NH2-terminal phosphatidylinositol bisphosphate-binding domain of betaENaC and gammaEnaC, and activation of phospholipase C, all reduced ENaC activity but allowed for Cl(-)-dependent inhibition of the remaining ENaC current. The results confirm a role of the carboxyl terminus of betaENaC for Cl(-)-dependent inhibition of the Na+ channel, which, however, may only be part of a complex regulation of ENaC by CFTR.

Regulatory Interdependence of Cloned Epithelial Na+Channels and P2X Receptors

Epithelial Na+ channels (ENaC) coexist with a family of ATP-gated ion channels known as P2X receptors in the renal collecting duct. Although ENaC is itself insensitive to extracellular ATP, tubular perfusion of ATP can modify the activity of ENaC. To investigate a possible regulatory relationship between P2X receptors and ENaC, coexpression studies were performed in Xenopus oocytes. ENaC generated a persistent inward Na+ current that was sensitive to the channel blocker amiloride (I(am-s)). Exogenous ATP transiently activated all cloned isoforms of P2X receptors, which in some cases irreversibly inhibited I(am-s). The degree of inhibition depended on the P2X receptor subtype present. Activation of P2X2, P2X(2/6), P2X4, and P2X(4/6) receptor subtypes inhibited I(am-s), whereas activation of P2X1, P2X3, and P2X5 receptors had no significant effect. The degree of inhibition of I(am-s) correlated positively with the amount of ionic charge conducted by P2X receptor subtypes. ENaC inhibition required Na+ influx through I(am-s)-inhibiting P2X ion channels but also Ca2+ influx through P2X4 and P2X(4/6) ion channels. P2X-mediated inhibition of I(am-s) was found to be due to retrieval of ENaC from the plasma membrane. Maximum amplitudes of ATP-evoked P2X-mediated currents (I(ATP)) were significantly increased for P2X2, P2X(2/6), and P2X5 receptor subtypes after coexpression of ENaC. The increase in I(ATP) was due to increased levels of plasma membrane-bound P2X receptor protein, suggesting that ENaC modulates protein trafficking. In summary, ENaC was downregulated by the activation of P2X2, P2X(2/6), P2X4, and P2X(4/6) receptors. Conversely, ENaC increased the plasma membrane expression of P2X2, P2X(2/6), and P2X5 receptors.

Sensitivity of oocyte-expressed epithelial Na+ channel to glibenclamide

The effect of glibenclamide on heterologously expressed amiloride-sensitive sodium channels (ENaCs) was investigated in Xenopus oocytes. The ENaC is a heteromer and consists of alpha-, beta- and gamma-subunits and the alpha- and beta-subunits have previously been shown to confer sensitivity to glibenclamide. We coexpressed either colonic rat alpha- (ralpha) or guinea-pig alpha-subunit (gpalpha) with Xenopus betagamma-subunits. The gpalphaxbetagamma was significantly stimulated by glibenclamide (100 microM) (184+/-15%), whereas the ralpha-combination was slightly down-regulated by the sulfonylurea (79+/-4%). The stimulating effect did not interfere with Na(+)-self-inhibition resulting from intracellular accumulation of Na(+)-ions. We exchanged cytosolic termini between both orthologs but the gpalpha-chimera with the termini from rat retained sensitivity to glibenclamide. The effect of glibenclamide on Xenopus ENaC (xENaC) was inhibited by ADP-beta-S but not by ATP-gamma-S, when applied intracellularly. Intracellular loading with Na(+)-ions after inhibition of Na(+)/K(+)-ATPases with ouabain prevented an up-regulation of ENaC activity by glibenclamide. Pretreatment of oocytes expressing xENaC with edelfosine (ET-18-OCH(3)) slightly reduced stimulation of I(ami) (118+/-12%; control: 132+/-9%) while phosphatidylinositol-4,5-biphosphate (PIP(2)) significantly reduced the effect of glibenclamide to 101+/-3%.

Gd3+-sensitive Na+ transport across the integument of Hirudo medicinalis

Leeches Hirudo medicinalis were exposed to either artificial pond water (APW; 1 mM NaCl) or to high-salinity conditions (HS; 200 mM NaCl) for several days. The aim of the study was to assess whether transepithelial ion conductances in their dorsal integuments were affected by this long-term acclimation. In voltage-clamp experiments using Ussing-type chambers, the transepithelial potential V(T) was clamped to 0 mV, and amiloride-sensitive currents (I(ami)) and total Na(+) transport (I(Na)) were determined. Apical Ca(2+)-free conditions strongly increased I(ami) to a similar magnitude in both differently acclimated integuments. Apical application of the lanthanide gadolinium <0.1 mM decreased the short-circuit current (I(sc)). In contrast, higher concentrations up to 10 mM Gd(3+) upregulated I(sc) by an additional 90% in APW integuments and by an additional 300% in HS integuments. This Gd(3+) effect was due to a doubling of I(Na) in APW and a more than sixfold increase of I(Na) in HS integuments. In summary, the macroscopic electrophysiological variables, including I(Na), were generally not affected by long-term exposure to high salinity. However, the presence of Gd(3+)-sensitive Na(+) conductances or regulating structures were greatly upregulated during HS acclimation.

No evidence for inhibition of ENaC through CFTR-mediated release of ATP

Both purinergic stimulation and activation of cystic fibrosis transmembrane conductance regulator (CFTR) increases Cl(-) secretion and inhibit amiloride-sensitive Na(+) transport. CFTR has been suggested to conduct adenosine 5'-triphosphate (ATP) or to control ATP release to the luminal side of epithelial tissues. Therefore, a possible mechanism on how CFTR controls the activity of epithelial Na(+) channels (ENaC) could be by release of ATP or uridine 5'-triphosphate (UTP), which would then bind to P2Y receptors and inhibit ENaC. We examined this question in native tissues from airways and colon and in Xenopus oocytes. Inhibition of amiloride-sensitive transport by both CFTR and extracellular nucleotides was observed in colon and trachea. However, nucleotides did not inhibit ENaC in Xenopus oocytes, even after coexpression of P2Y(2) receptors. Using different tools such as hexokinase, the P2Y inhibitor suramin or the Cl(-) channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), we did not detect any role of a putative ATP secretion in activation of Cl(-) transport or inhibition of amiloride sensitive short circuit currents by CFTR. In addition, N(2),2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate (cGMP) and protein kinase G (PKG)-dependent phosphorylation or the nucleoside diphosphate kinase (NDPK) do not seem to play a role for the inhibition of ENaC by CFTR, which, however, requires the presence of extracellular Cl(-).

Effects of extracellular purines on ion transport across the integument of Hirudo medicinalis

Little is known about the long-term regulation of epithelial ion transport in invertebrates and the specific mediators involved. For some years, we have been investigating the short-term regulation of transepithelial ion transport across the dorsal integument of the leech Hirudo medicinalis, and we have established a model of Na+ uptake. In the present study, we investigated the effect of long-term acclimation on transintegumental ion transport by adapting leeches to high-salinity conditions. We dissected segments of dorsal integument and measured ion currents in Ussing chamber experiments. Electrophysiological variables, such as transepithelial potential(VT) and short-circuit-current (Isc),were profoundly affected by adaptation to high-salinity conditions. The total transepithelial Na+ current (INa) decreased from 7.66±0.82 to 4.6±0.54 μA cm-2 in preparations adapted to high salinity. The involvement of epithelial Na+channels was determined as current inhibition (Iami) by apical application of amiloride; Na+ channels were equally active in control epithelia and epithelia from leeches adapted to high salinity. Removal of Ca2+ from the apical solutions, which is believed to reduce intracellular Ca2+ concentrations, equalized transepithelial variables between high-salt-adapted integuments and control integuments.

Extracellular purines regulate transepithelial Cl- secretion and Na+ absorption. In a variety of tissues we tested ATP and adenosine for their effects on epithelial transport. Examination of integuments from pondwater- and high-salinity-adapted leeches revealed different sensitivities for these purines. Apical and basolateral application of ATP both stimulated transepithelial Na+ uptake and Iami. Adenosine upregulated non-Na+ currents and acted from the basolateral side only. Apical Ca2+-free conditions attenuated these effects of purines on transepithelial currents. Extracellular UTP had no effect on ion transport.

The cystic fibrosis transmembrane conductance regulator (CFTR) inhibits ENaC through an increase in the intracellular Cl- concentration

Activation of the CFTR Cl- channel inhibits epithelial Na+ channels (ENaC), according to studies on epithelial cells and overexpressing recombinant cells. Here we demonstrate that ENaC is inhibited during stimulation of the cystic fibrosis transmembrance conductance regulator (CFTR) in Xenopus oocytes, independent of the experimental set-up and the magnitude of the whole-cell current. Inhibition of ENaC is augmented at higher CFTR Cl- currents. Similar to CFTR, ClC-0 Cl- currents also inhibit ENaC, as well as high extracellular Na+ and Cl- in partially permeabilized oocytes. Thus, inhibition of ENaC is not specific to CFTR and seems to be mediated by Cl-.

Modulation of aldosterone-induced stimulation of ENaC synthesis by changing the rate of apical Na+ entry

Primary cultures of immunodissected rabbit connecting tubule and cortical collecting duct cells were used to investigate the effect of apical Na+ entry rate on aldosterone-induced transepithelial Na+ transport, which was measured as benzamil-sensitive short-circuit current (I(sc)). Stimulation of the apical Na+ entry, by long-term short-circuiting of the monolayers, suppressed the aldosterone-stimulated benzamil-sensitive I(sc) from 320 +/- 49 to 117 +/- 14%, whereas in the presence of benzamil this inhibitory effect was not observed (335 +/- 74%). Immunoprecipitation of [(35)S]methionine-labeled beta-rabbit epithelial Na+ channel (rbENaC) revealed that the effects of modulation of apical Na+ entry on transepithelial Na+ transport are exactly mirrored by beta-rbENaC protein levels, because short-circuiting the monolayers decreased aldosterone-induced beta-rbENaC protein synthesis from 310 +/- 51 to 56 +/- 17%. Exposure to benzamil doubled the beta-rbENaC protein level to 281 +/- 68% in control cells but had no significant effect on aldosterone-stimulated beta-rbENaC levels (282 +/- 68%). In conclusion, stimulation of apical Na+ entry suppresses the aldosterone-induced increase in transepithelial Na+ transport. This negative-feedback inhibition is reflected in a decrease in beta-rbENaC synthesis or in an increase in beta-rbENaC degradation.

Roles of the C termini of alpha -, beta -, and gamma -subunits of epithelial Na+ channels (ENaC) in regulating ENaC and mediating its inhibition by cytosolic Na+

The amiloride-sensitive epithelial Na(+) channels (ENaC) in the intralobular duct cells of mouse mandibular glands are inhibited by the ubiquitin-protein ligase, Nedd4, which is activated by increased intracellular Na(+). In this study we have used whole-cell patch clamp methods in mouse mandibular duct cells to investigate the role of the C termini of the alpha-, beta-, and gamma-subunits of ENaC in mediating this inhibition. We found that peptides corresponding to the C termini of the beta- and gamma-subunits, but not the alpha-subunit, inhibited the activity of the Na(+) channels. This mechanism did not involve Nedd4 and probably resulted from the exogenous C termini interfering competitively with the protein-protein interactions that keep the channels active. In the case of the C terminus of mouse beta-ENaC, the interacting motif included betaSer(631), betaAsp(632), and betaSer(633). In the C terminus of mouse gamma-ENaC, it included gammaSer(640). Once these motifs were deleted, we were able to use the C termini of beta- and gamma-ENaC to prevent Nedd4-mediated down-regulation of Na(+) channel activity. The C terminus of the alpha-subunit, on the contrary, did not prevent Nedd4-mediated inhibition of the Na(+) channels. We conclude that mouse Nedd4 interacts with the beta- and gamma-subunits of ENaC.