[Show abstract][Hide abstract] ABSTRACT: The renal epithelial sodium channel (ENaC) provides regulated sodium transport in the distal nephron. The effects of intracellular
calcium ([Ca2+]i) on this channel are only beginning to be elucidated. It appears from previous studies that the [Ca2+]i increases downstream of ATP administration may have a polarized effect on ENaC, where apical application of ATP and the subsequent
[Ca2+]i increase have an inhibitory effect on the channel, whereas basolateral ATP and [Ca2+]i have a stimulatory effect. We asked whether this polarized effect of ATP is, in fact, reflective of a polarized effect of
increased [Ca2+]i on ENaC and what underlying mechanism is responsible. We began by performing patch clamp experiments in which ENaC activity
was measured during apical or basolateral application of ionomycin to increase [Ca2+]i near the apical or basolateral membrane, respectively. We found that ENaC does indeed respond to increased [Ca2+]i in a polarized fashion, with apical increases being inhibitory and basolateral increases stimulating channel activity. In
other epithelial cell types, mitochondria sequester [Ca2+]i, creating [Ca2+]i signaling microdomains within the cell that are dependent on mitochondrial localization. We found that mitochondria localize
in bands just beneath the apical and basolateral membranes in two different cortical collecting duct principal cell lines
and in cortical collecting duct principal cells in mouse kidney tissue. We found that inhibiting mitochondrial [Ca2+]i uptake destroyed the polarized response of ENaC to [Ca2+]i. Overall, our data suggest that ENaC is regulated by [Ca2+]i in a polarized fashion and that this polarization is maintained by mitochondrial [Ca2+]i sequestration.
Full-text · Article · Oct 2015 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The polarized nature of epithelial cells allows for different responses to luminal or serosal stimuli. In kidney tubules, ATP is produced luminally in response to changes in luminal flow. Luminal increases in ATP have been previously shown to inhibit the renal epithelial sodium channel (ENaC). On the other hand, ATP is increased basolaterally in renal epithelia in response to aldosterone. We tested the hypothesis that basolateral ATP can stimulate ENaC function through activation of the P2X4 receptor/channel. Using single channel cell attached patch clamp techniques, we demonstrate the existence of a basolaterally expressed channel stimulated by the P2X4 agonist meSATP in Xenopus A6 cells, a renal collecting duct principal cell line. This channel had a similar reversal potential and conductance to that of P2X4 channels. Cell surface biotinylation of the basolateral side of these cells confirmed the basolateral presence of the P2X4 receptor. Basolateral addition of meSATP enhanced the activity of ENaC in single channel patch clamp studies, an effect which was absent in cells transfected with a dominant negative P2X4 receptor construct, indicating that activation of P2X4 channels stimulates ENaC activity in these cells. The effect of meSATP on ENaC activity was reduced following chelation of basolateral calcium with EGTA or inhibition of PI3 kinase with LY294002. Overall, our results show that ENaC is stimulated by P2X4 receptor activation and that the stimulation is dependent on increases in intracellular calcium and PI3 kinase activation.
Preview · Article · Aug 2014 · American journal of physiology. Renal physiology
[Show abstract][Hide abstract] ABSTRACT: Numerous reports have linked cytoskeleton associated proteins with the regulation of ENaC activity. The purpose of this study was to determine the effect of actin cytoskeleton disruption by cytochalasin E on ENaC activity in Xenopus 2F3 cells. Here we show cytochalasin E treatment for 60 minutes can disrupt the integrity of the actin cytoskeleton in cultured Xenopus 2F3 cells. We show by single-channel patch clamp studies and measurements of short circuit current that ENaC activity, but not its density is altered by cytochalasin E induced disruption of the cytoskeleton. In non-treated cells 8 out of 33 patches (24%) had no measurable ENaC activity while in cytochalasin E treated cells 17 out of 32 patches (53%) had no activity. Analysis of those patches that did contain ENaC activity showed channel open probability significantly decreased from 0.081 ± 0.01 in non-treated cells to 0.043 ± 0.01 in cells treated with cytochalasin E. Transepithelial current from mpkCCD cells treated with cytochalasin E, cytochalasin D, or latrunculin B for 60 minutes was decreased compared to vehicle treated cells. The subcellular expression of fodrin changed significantly and several protein elements of the cytoskeleton decreased at least two fold after 60 minutes of cytochalasin E treatment. Cytochalasin E treatment disrupted the association between ENaC and MARCKS. The results presented here suggest disruption of the actin cytoskeleton by different compounds can attenuate ENaC activity through a mechanism involving changes in the subcellular expression of fodrin, several elements of the cytoskeleton, and destabilization of the ENaC-MARCKS complex.
Preview · Article · May 2014 · AJP Renal Physiology
[Show abstract][Hide abstract] ABSTRACT: Female sex predisposes individuals to poorer outcomes during respiratory disorders like cystic fibrosis and influenza-associated pneumonia. A common link between these disorders is dysregulation of alveolar fluid clearance via disruption of epithelial sodium channel (ENaC) activity. Recent evidence suggests that female sex hormones directly regulate expression and activity of alveolar ENaC. In our study, we identified the mechanism by which estradiol (E2) or progesterone (P4) independently regulates alveolar ENaC. Using cell-attached patch clamp, we measured ENaC single channel activity in a rat alveolar cell line (L2) in response to overnight exposure to either E2 or P4. In contrast to P4, E2 increased ENaC channel activity (NPo) through an increase in channel open probability (Po) and an increased number of patches with observable channel activity. Apical plasma membrane abundance of the ENaC α subunit (αENaC) more than doubled in response to E2 as determined by cell surface biotinylation. αENaC membrane abundance was ~3 fold greater in lungs from female rats in proestrus, when serum E2 is greatest, compared to diestrus, when it is lowest. Our results also revealed a significant role for the G-protein coupled estrogen receptor (Gper) to mediate E2's effects on ENaC. Overall, our results demonstrate that E2 signaling through Gper selectively activates alveolar ENaC through an effect on channel gating and channel density, the latter via greater trafficking of channels to the plasma membrane. The results presented herein implicate E2-mediated regulation of alveolar sodium channels in the sex differences observed in the pathogenesis of several pulmonary diseases.
No preview · Article · Oct 2013 · AJP Lung Cellular and Molecular Physiology
[Show abstract][Hide abstract] ABSTRACT: Epithelial Na+ channel (ENaC) activity, which determines the rate of renal Na(+) reabsorption, can be regulated by G-protein coupled receptors. Regulation of ENaC by Gα mediated downstream effectors has been studied extensively, but the effect of Gβγ dimers on ENaC is unclear. A6 cells endogenously contain high levels of Gβ1 but low levels of Gβ3 ,Gβ4 and Gβ5 were detected by Q-PCR. We tested Gγ2 combined individually with Gβ1 through Gβ5 expressed in A6 cells, after which we recorded single ENaC activity. Among the five β and γ2 combinations, β1γ2 strongly inhibits ENaC activity by reducing both ENaC channel number (N) and open probability (Po) compared to control cells. In contrast, the other four β isoforms combined with γ2 have no significant effect on ENaC activity. By using various inhibitors to probe Gβ1γ2 effects on ENaC regulation, we found that Gβ1γ2-mediated ENaC inhibition involved activation of phospholipase C-β and its enzymatic products that induce protein kinase C and ERK1/2 signaling pathways.
[Show abstract][Hide abstract] ABSTRACT: A serine-threonine protein kinase, WNK4, reduces Na(+) reabsorption and K(+) secretion in the distal convoluted tubule by reducing trafficking of thiazide-sensitive Na-Cl co-transporter to and enhancing renal outer medullary potassium channel retrieval from the apical membrane. Epithelial sodium channels (ENaC) in the distal nephron also play a role in regulating Na(+) reabsorption and are also regulated by WNK4, but the mechanism is unclear. In A6 distal nephron cells, transepithelial current measurement and single channel recording show that WNK4 inhibits ENaC activity. Analysis of the number of channel per patch shows that WNK4 reduces channel number but has no effect on channel open probability. Western blots of apical and total ENaC provide additional evidence that WNK4 reduces apical as well as total ENaC expression. WNK4 enhances ENaC internalization independent of Nedd4-2-mediated ENaC ubiquitination. WNK4 also reduced the amount of ENaC available for recycling, but has no effect on the rate of transepithelial current increase to forskolin. In contrast, Nedd4-2 not only reduced ENaC in the recycling pool but also decreased the rate of increase of current after forskolin. WNK4 associates with wild type as well as Liddle's mutated ENaC, and WNK4 reduces both wild type and mutated ENaC expressed in HEK293 cells.
[Show abstract][Hide abstract] ABSTRACT: Phosphatidylinositol phosphates (e.g. PIP2) regulate the activity of ENaC at the apical membrane. Here, we investigate a role for protein kinase C (PKC) and calmodulin (CaM) in the PIP dependent regulation of ENaC by MARCKS. After treating Xenopus 2F3 cells with PMA, immunoblot studies showed an increase in MARCKS phosphorylation, confocal microscopy revealed displacement of MARCKS from the apical membrane, and amiloride-sensitive transepithelial current decreased significantly. Single-channel patch clamp studies showed that ENaC activity increased after treating the apical side of Xenopus 2F3 cells with a pharmacological inhibitor of PKC (GF109203X). We mutated the CaM binding site within the basic effector domain of MARCKS by site-directed mutagenesis and co-transfected the wildtype or mutant MARCKS CFP-tagged construct and an ENaC gamma GFP-tagged construct into Xenopus 2F3 cells. Live cell imaging showed that in response to an increase in calcium, CaM translocated to the apical membrane and MARCKS translocated to the cytoplasm in cells transfected with wildtype MARCKS, but not with mutant MARCKS. We show by single-channel patch clamp studies that application of the calmodulin inhibitor, calmidazolium, to the apical side of Xenopus 2F3 cells causes an increase in the open probability of ENaC. The findings presented here suggest that the cycling of MARCKS between the apical membrane and cytoplasm is critical for the PIP2 dependent regulation of ENaC.
[Show abstract][Hide abstract] ABSTRACT: Phosphatidylinositol phosphates (PIPs) are known to regulate epithelial sodium channels (ENaC). Lipid binding assays and coimmunoprecipitation showed that the amino-terminal domain of the β- and γ-subunits of Xenopus ENaC can directly bind to phosphatidylinositol 4,5-bisphosphate (PIP(2)), phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), and phosphatidic acid (PA). Similar assays demonstrated various PIPs can bind strongly to a native myristoylated alanine-rich C-kinase substrate (MARCKS), but weakly or not at all to a mutant form of MARCKS. Confocal microscopy demonstrated colocalization between MARCKS and PIP(2). Confocal microscopy also showed that MARCKS redistributes from the apical membrane to the cytoplasm after PMA-induced MARCKS phosphorylation or ionomycin-induced intracellular calcium increases. Fluorescence resonance energy transfer studies revealed ENaC and MARCKS in close proximity in 2F3 cells when PKC activity and intracellular calcium concentrations are low. Transepithelial current measurements from Xenopus 2F3 cells treated with PMA and single-channel patch-clamp studies of Xenopus 2F3 cells treated with a PKC inhibitor altered Xenopus ENaC activity, which suggest an essential role for MARCKS in the regulation of Xenopus ENaC activity.
[Show abstract][Hide abstract] ABSTRACT: β-Adrenergic receptors (β-AR) increase epithelial sodium channel (ENaC) activity to promote lung fluid clearance. However, the effect of selective β-AR agonist on highly selective cation (HSC) channels or nonselective cation (NSC) channels in alveolar type 1 (T1) and type 2 (T2) cells is unknown. We hypothesized that stimulation with β(1)-AR agonist (denopamine) or β(2)-AR agonist (terbutaline) would increase HSC and/or NSC channel activity in alveolar epithelial cells. We performed single-channel measurements from T1 and T2 cells accessed from rat lung slices. Terbutaline (20 μM) increased HSC ENaC activity (open probability, NP(o)) in T1 (from 0.96 ± 0.61 to 1.25 ± 0.71, n = 5, P <0.05) and T2 cells (from 0.28 ± 0.14 to 1.0 ± 0.30, n = 8, P = 0.02). Denopamine (20 μM) increased NSC NP(o) in T1 cells (from 0.34 ± 0.09 to 0.63 ± 0.14, n = 7, P = 0.02) and in T2 cells (from 0.47 ± 0.09 to 0.68 ± 0.10, P = 0.004). In vivo X-ray imaging of lung fluid clearance and ICI 118,551 selective inhibition of β(2)-ARs confirmed patch-clamp findings. cAMP concentrations increased following treatment with denopamine or terbutaline (n = 3, P < 0.002). The effects of systemic (intraperitoneal, IP) and local (intratracheal, IT) modes of delivery on lung fluid clearance were assessed. IT delivery of denopamine promoted alveolar flooding, whereas IP delivery promoted delayed fluid clearance. In summary, β-AR agonists differentially regulate HSC and NSC in T1 and T2 cells to promote lung fluid clearance in vivo, and the mode of drug delivery is critical for maximizing β-AR agonist efficacy.
[Show abstract][Hide abstract] ABSTRACT: Amiloride-sensitive epithelial sodium (Na(+)) channels (ENaC) play a crucial role in Na(+) transport and fluid reabsorption in the kidney, lung, and colon. The magnitude of ENaC-mediated Na(+) transport in epithelial cells depends on the average open probability of the channels and the number of channels on the apical surface of epithelial cells. The number of channels in the apical membrane, in turn, depends upon a balance between the rate of ENaC insertion and the rate of removal from the apical membrane. ENaC is made up of three homologous subunits, alpha, beta, and gamma. The C-terminal domain of all three subunits is intracellular and contains a proline rich motif (PPxY). Mutations or deletion of this PPxY motif in the beta and gamma subunits prevent the binding of one isoform of a specific ubiquitin ligase, neural precursor cell expressed developmentally down-regulated protein (Nedd4-2) to the channel in vitro and in transfected cell systems, thereby impeding ubiquitin conjugation of the channel subunits. Ubiquitin conjugation would seem to imply that ENaC turnover is determined by the ubiquitin-proteasome system, but when MDCK cells are transfected with ENaC, ubiquitin conjugation apparently leads to lysosomal degradation. However, in untransfected epithelial cells (A6) expressing endogenous ENaC, ENaC appears to be degraded by the ubiquitin-proteasome system. Nonetheless, in both transfected and untransfected cells, the rate of ENaC degradation is apparently controlled by the rate of Nedd4-2-mediated ENaC ubiquitination. Controlling the rate of degradation is apparently important enough to have multiple, redundant pathways to control Nedd4-2 and ENaC ubiquitination.
Full-text · Article · Feb 2010 · Proceedings of the American Thoracic Society
[Show abstract][Hide abstract] ABSTRACT: Epithelial sodium channels (ENaC) play an essential role in maintaining total body fluid and electrolyte homeostasis. As such, abnormal expression of ENaC at the cell surface is linked to several important human diseases. Although the stability of ENaC subunits has been extensively studied by protein biochemical analysis, the half-life of the functional channel in the apical membrane remains controversial. Because the functional stability of the multisubunit channel may be more physiologically relevant than the stability of individual subunit proteins, we performed studies of functional ENaC channels using A6 epithelial cells, a Xenopus laevis distal nephron cell line. We recorded single-channel activity in over 400 cells with the translation blockers cycloheximide (CHX) or puromycin, as well as the intracellular protein trafficking inhibitors brefeldin A (BFA) or nocodazole. Our cell-attached, single-channel recordings allow us to quantify the channel density in the apical membrane, as well as to determine channel open probability (Po) from control (untreated) cells and from cells at different times of drug treatment. The data suggest that the half-life of ENaC channels is approximately 3.5 h following puromycin, BFA, and nocodazole treatment. Furthermore, these three drugs had no significant effect on the Po of ENaC for at least 6 h after exposure. A decrease in apical channel number and Po was observed following 2 h of CHX inhibition of protein synthesis, and the apparent channel half-life was closer to 1.5 h following CHX treatment. Treatment of cells with the translation inhibitors does not alter the expression of the protease furin, and therefore changes in protease activity cannot explain changes in ENaC Po. Confocal images show that BFA and nocodazole both disrupt most of the Golgi apparatus after 1-h exposure. In cells with the Golgi totally disrupted by overnight exposure to BFA, 20% of apical ENaC channels remained functional. This result suggests that ENaC is delivered to the apical membrane via a pathway that might bypass the Golgi vesicular trafficking pathway, or that there might be two pools of channels with markedly different half-lives in the apical membrane.
Preview · Article · Oct 2008 · American journal of physiology. Renal physiology
[Show abstract][Hide abstract] ABSTRACT: Oxygen radicals play an important role in signal transduction and have been shown to influence epithelial sodium channel (ENaC) activity. We show that aldosterone, the principal hormone regulating renal ENaC activity, increases superoxide (O2*) production in A6 distal nephron cells. Aldosterone (50 nM to 1.5 microM) induced increases in dihydroethidium fluorescence in a dose-dependent manner in confluent A6 epithelial cells. Using single-channel measurements, we showed that sequestering endogenous O2* (with the O2* scavenger 2,2,6,6-tetramethylpiperidine 1-oxyl) significantly decreased ENaC open probability from 0.10 +/- 0.03 to 0.03 +/- 0.01. We also found that increasing endogenous O2* in A6 cells, by applying a superoxide dismutase inhibitor, prevented nitric oxide (NO) inhibition of ENaC activity. ENaC open probability values did not significantly change from control values (0.23 +/- 0.05) after superoxide dismutase and 1.5 microM NO coincubation (0.21 +/- 0.04). We report that xanthine oxidase and hypoxanthine compounds increase local concentrations of O2* by approximately 30%; with this mix, an increase in ENaC number of channels times the open probability (from 0.1 to 0.3) can be achieved in a cell-attached patch. Our data also suggest that O2* alters NO activity in a cGMP-independent mechanism, since pretreating A6 cells with ODQ compound (a selective inhibitor of NO-sensitive guanylyl cyclase) failed to block 2,2,6,6-tetramethylpiperidine 1-oxyl inhibition of ENaC activity.
Full-text · Article · Dec 2007 · American journal of physiology. Renal physiology
[Show abstract][Hide abstract] ABSTRACT: To better understand how renal Na(+) reabsorption is altered by heavy metal poisoning, we examined the effects of several divalent heavy metal ions (Zn(2+), Ni(2+), Cu(2+), Pb(2+), Cd(2+), and Hg(2+)) on the activity of single epithelial Na(+) channels (ENaC) in a renal epithelial cell line (A6). None of the cations changed the single-channel conductance. However, ENaC activity [measured as the number of channels (N) x open probability (P(o))] was decreased by Cd(2+) and Hg(2+) and increased by Cu(2+), Zn(2+), and Ni(2+) but was not changed by Pb(2+). Of the cations that induced an increase in Na(+) channel function, Zn(2+) increased N, Ni(2+) increased P(o), and Cu(2+) increased both. The cysteine modification reagent [2-(trimethylammonium)ethyl]methanethiosulfonate bromide also increased N, whereas diethylpyrocarbonate, which covalently modifies histidine residues, affected neither P(o) nor N. Cu(2+) increased N and stimulated P(o) by reducing Na(+) self-inhibition. Furthermore, we observed that ENaC activity is slightly voltage dependent and that the voltage dependence of ENaC is insensitive to extracellular Na(+) concentration; however, apical application of Ni(2+) or diethylpyrocarbonate reduced the channel voltage dependence. Thus the voltage sensor of Xenopus ENaC is different from that of typical voltage-gated channels, since voltage appears to be sensed by histidine residues in the extracellular loops of ENaC, rather than by charged amino acids in a transmembrane domain.
No preview · Article · Aug 2007 · American journal of physiology. Renal physiology
[Show abstract][Hide abstract] ABSTRACT: Several studies have shown that nitric oxide (NO) inhibits Na(+) transport in renal and alveolar monolayers. However, the mechanisms by which NO alters epithelial Na(+) channel (ENaC) activity is unclear. Therefore, we examined the effect of applying the NO donor drug l-propanamine 3,2-hydroxy-2-nitroso-1-propylhidrazino (PAPA-NONOate) to cultured renal epithelial cells. A6 and M1 cells were maintained on permeable supports in medium containing 1.5 microM dexamethasone and 10% bovine serum. After 1.5 microM PAPA-NONOate was applied, amiloride-sensitive short-circuit current measurements decreased 29% in A6 cells and 44% in M1 cells. This differed significantly from the 3% and 19% decreases in A6 and M1 cells, respectively, treated with control donor compound (P < 0.0005). Subsequent application of PAPA-NONOate to amiloride-treated control (no NONOate) A6 and M1 cells did not further decrease transepithelial current. In single-channel patch-clamp studies, NONOate significantly decreased ENaC open probability (P(o)) from 0.186 +/- 0.043 to 0.045 +/- 0.009 (n = 7; P < 0.05) without changing the unitary current. We also showed that aldosterone significantly decreased NO production in primary cultures of alveolar type II (ATII) epithelial cells. Because inducible nitric oxide synthase (iNOS) coimmunoprecipitated with the serum- and glucocorticoid-inducible kinase (SGK1) and both proteins colocalized in the cytoplasm (as shown in our studies in mouse ATII cells), SGK1 may also be important in regulating NO production in the alveolar epithelium. Our study also identified iNOS as a novel SGK1 phosphorylated protein (at S733 and S903 residues in miNOS) suggesting that one way in which SGK1 could increase Na(+) transport is by altering iNOS production of NO.