Epithelial Na+ Channel Channel Research Articles

The epithelial Na+ channel (ENaC) in ovarian granulosa cells modulates Ca2+ mobilization and gonadotrophin signaling for estrogen homeostasis and female fertility

Ovarian granulosa cells are essential to gonadotrophin-regulated estrogen production, female cycle maintenance and fertility. The epithelial Na+ channel (ENaC) is associated with female fertility; however, whether and how it plays a role in ovarian cell function(s) remained unexplored. Here, we report patch-clamp and Na+ imaging detection of ENaC expression and channel activity in both human and mouse ovarian granulosa cells, which are promoted by pituitary gonadotrophins, follicle stimulating hormone (FSH) or luteinizing hormone (LH). Cre-recombinase- and CRISPR-Cas9-based granulosa-specific knockout of ENaC α subunit (Scnn1a) in mice resulted in failed estrogen elevation at early estrus, reduced number of corpus luteum, abnormally extended estrus phase, reduced litter size and subfertility in adult female mice. Further analysis using technologies including RNA sequencing and Ca2+ imaging revealed that pharmacological inhibition, shRNA-based knockdown or the knockout of ENaC diminished spontaneous or stimulated Ca2+ oscillations, lowered the capacity of intracellular Ca2+ stores and impaired FSH/LH-stimulated transcriptome changes for estrogen production in mouse and/or human granulosa cells. Together, these results have revealed a previously undefined role of ENaC in modulating gonadotrophin signaling in granulosa cells for estrogen homeostasis and thus female fertility.

Epithelial Sodium Channels in Monocyte‐Macrophage Migration and Regulation by Pro‐inflammatory Cytokines TNFα and IFNγ

Migration of monocytes‐macrophages play an important role in phagocytosis of pathogens and cellular debris in a variety of pathophysiological conditions. While Epithelial Na+ Channels (ENaC) are required for normal migratory responses in glial, trophoblast, and vascular smooth muscle cells, their role in monocyte‐macrophage migration signaling is unknown. The Human Protein Atlas data base (www.proteinatlas.org) shows ENaC message in peripheral blood mononuclear cells and tissue resident macrophages in human populations. We used an in vitro model to determine the importance of ENaC in chemotactic migration of monocytes‐macrophages using a standard Boyden chamber assay. Cells were suspended in 0.4% FBS ± amiloride (0.1‐10 µM) or benzamil (0.01‐1 µM) to inhibit ENaC channels, then migrated for 4 hr towards a 10% FBS gradient. In mouse RAW cells, migration responses were inhibited by amiloride (65±6 – 46±4% of control at 0.1 and 10 µM, p=0.0001) and benzamil (55±6 – 55±5% of control at 0.01 and 1.0 µM, p<0.0001). In human THP‐1 cells, migration responses were similarly inhibited by amiloride (73±5 ‐ 40±6% of control at 0.1 and 10 µM, p<0.0001) and benzamil (68±7 – 53±4% of control at 0.01 and 1.0 µM, p<0.0001). Since benzamil and amiloride are selective for ENaC at sub and low micromolar concentrations, respectively, our findings indicate ENaC signaling is required for migration of unstimulated monocyte‐macrophage cells. To determine if pro‐inflammatory cytokines, such as tumor necrosis factor alpha (TNFα) and interferon gamma (INFγ), regulate ENaC expression and function, we used TaqMan quantitative PCR, western blotting and chemotactic migration. Cells were exposed to a standard protocol to activate macrophages: INFγ for 48 hr plus TNFα (10 ng/mL each) during the last 24 hr. Expression of α and βENaC and GAPDH was calculated as fold change using the delta delta CT method (2‐∆∆CT). INFγ48hr + TNFα24hr increased α and βENaC message (9±2 and 5±1, n=4‐6, p=0.01). γENaC was undetectable. Despite increased message, whole cell αENaC protein expression was inhibited 50% by western blotting. The mechanism underlying message stimulation and protein inhibition remains to be determined. A similar effect on message upregulation and migration inhibition also occurred with 18 hr INFγ or TNFα alone (0‐10 ng/mL). Activation with INFγ/TNFα inhibited migration up to 70% and abolished the amiloride (1 µM) sensitive component. In unstimulated cells, amiloride (1 µM) coculture induced a rebound increase in migration following withdrawal, which was abolished by INFγ/TNFα treatment. This finding suggests cytokine induced ENaC loss mediates the migration inhibition. In summary, ENaC mediates monocyte migration, however, monocyte activation by INFγ/TNFα inhibits migration by inhibiting ENaC. Since pro‐inflammatory cytokines initiate monocyte differentiation and polarization to the phagocytic M1 phenotype, future studies will determine if ENaC is required for the phenotypic switch.

Optogenetic Control of PIP2 Interactions Shaping ENaC Activity.

The activity of the epithelial Na+ Channel (ENaC) is strongly dependent on the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 binds two distinct cationic clusters within the N termini of β- and γ-ENaC subunits (βN1 and γN2). The affinities of these sites were previously determined using short synthetic peptides, yet their role in sensitizing ENaC to changes in PIP2 levels in the cellular system is not well established. We addressed this question by comparing the effects of PIP2 depletion and recovery on ENaC channel activity and intracellular Na+ levels [Na+]i. We tested effects on ENaC activity with mutations to the PIP2 binding sites using the optogenetic system CIBN/CRY2-OCRL to selectively deplete PIP2. We monitored changes of [Na+]i by measuring the fluorescent Na+ indicator, CoroNa Green AM, and changes in channel activity by performing patch clamp electrophysiology. Whole cell patch clamp measurements showed a complete lack of response to PIP2 depletion and recovery in ENaC with mutations to βN1 or γN2 or both sites, compared to wild type ENaC. Whereas mutant βN1 also had no change in CoroNa Green fluorescence in response to PIP2 depletion, γN2 did have reduced [Na+]i, which was explained by having shorter CoroNa Green uptake and half-life. These results suggest that CoroNa Green measurements should be interpreted with caution. Importantly, the electrophysiology results show that the βN1 and γN2 sites on ENaC are each necessary to permit maximal ENaC activity in the presence of PIP2.

ENaC and ROMK channels in the connecting tubule regulate renal K+ secretion.

We measured the activities of epithelial Na channels (ENaC) and ROMK channels in the distal nephron of the mouse kidney and assessed their role in the process of K+ secretion under different physiological conditions. Under basal dietary conditions (0.5% K), ENaC activity, measured as amiloride-sensitive currents, was high in cells at the distal end of the distal convoluted tubule (DCT) and proximal end of the connecting tubule (CNT), a region we call the early CNT (CNTe). In more distal parts of the CNT (aldosterone-sensitive portion [CNTas]), these currents were minimal. This functional difference correlated with alterations in the intracellular location of ENaC, which was at or near the apical membrane in CNTe and more cytoplasmic in the CNTas. ROMK activity, measured as TPNQ-sensitive currents, was substantial in both segments. A mathematical model of the rat nephron suggested that K+ secretion by the CNTe predicted from these currents provides much of the urinary K+ required for K balance on this diet. In animals fed a K-deficient diet (0.1% K), both ENaC and ROMK currents in the CNTe decreased by ∼50%, predicting a 50% decline in K+ secretion. Enhanced reabsorption by a separate mechanism is required to avoid excessive urinary K+ losses. In animals fed a diet supplemented with 3% K, ENaC currents increased modestly in the CNTe but strongly in the CNTas, while ROMK currents tripled in both segments. The enhanced secretion of K+ by the CNTe and the recruitment of secretion by the CNTas account for the additional transport required for K balance. Therefore, adaptation to increased K+ intake involves the extension of robust K+ secretion to more distal parts of the nephron.

CRISPR/Cas9 Mediated Knock Down of δ-ENaC Blunted the TNF-Induced Activation of ENaC in A549 Cells.

Tumor necrosis factor (TNF) is known to activate the epithelial Na+ channel (ENaC) in A549 cells. A549 cells are widely used model for ENaC research. The role of δ-ENaC subunit in TNF-induced activation has not been studied. In this study we hypothesized that δ-ENaC plays a major role in TNF-induced activation of ENaC channel in A549 cells which are widely used model for ENaC research. We used CRISPR/Cas 9 approach to knock down (KD) the δ-ENaC in A549 cells. Western blot and immunofluorescence assays were performed to analyze efficacy of δ-ENaC protein KD. Whole-cell patch clamp technique was used to analyze the TNF-induced activation of ENaC. Overexpression of wild type δ-ENaC in the δ-ENaC KD of A549 cells restored the TNF-induced activation of whole-cell Na+ current. Neither N-linked glycosylation sites nor carboxyl terminus domain of δ-ENaC was necessary for the TNF-induced activation of whole-cell Na+ current in δ-ENaC KD of A549 cells. Our data demonstrated that in A549 cells the δ-ENaC plays a major role in TNF-induced activation of ENaC.

MTOR Modulates Sodium and Potassium Conduction in Heterologous Expression System

The role of mammalian target of rapamycin (mTOR) complexes mTORC1 and mTORC2 in ion transport in kidney tubule has been an area of increasing interest in epithelial research. We and others have shown both in cultured cells and in vivo that mTORC2 is the hydrophobic motif kinase for serum and glucocorticoid kinase 1 (SGK1) and that its activity is required for epithelial Na+ channel (ENaC)‐dependent sodium reabsorption in the aldosterone‐sensitive distal nephron (ASDN). However, recent evidence has shown that mTORC2 can also modulate the renal outer medullary K+ channel (ROMK) activity directly. Electrogenic sodium reabsorption through ENaC generates the main driving force in ASDN for potassium secretion through ROMK, and is a key locus of acute regulation of K+ secretion. We suspected that differences observed in various studies may be due to acute vs. chronic regulation. We therefore compared the effects on ENaC and ROMK activity of chronic (through gene‐knockout) and acute (through a pharmacological approach) mTOR inhibition. We used CRISPR‐Cas9 to generate Sin1 (an essential component of mTORC2)‐deficient HEK‐293T cells, which were compared with wild type cells. The cells were transiently transfected with ENaC (alpha, beta gamma subunits) and ROMK1 channels. After recovery, the cells were treated with either vehicle or the mTOR inhibitor AZD8055 for 30 min and subjected to whole cell patch clamp to assess ENaC and ROMK currents. We found that in Sin1 KO cells (chronic loss of mTORC2) ENaC current was unaffected, while ROMK was markedly reduced. In contrast, acute mTOR inhibition with AZD8055 inhibited ENaC but not ROMK whole cell currents. These data provide initial support for the idea that acute pharmacologic inhibition of mTORC2 and chronic selective inhibition of mTORC2 have opposite effects on ENaC and ROMK activities. However, the results are also consistent with the possibility that global inhibition of mTOR (blocking mTORC1 and mTORC2) has distinct effects from selective inhibition of mTORC2. These observations may provide further insight into such diverse processes as aldosterone escape and the distinct plasma K+ concentrations seen in various types of hypertension.Support or Funding InformationThis work was supported by Relypsa Nephrolgy and Cardiology fellowship 2019–2020 to WS and NIH RO1‐DK 56695 to DP

Shear force sensing of epithelial Na+ channel (ENaC) relies on N-glycosylated asparagines in the palm and knuckle domains of αENaC

Mechanosensitive ion channels are crucial for normal cell function and facilitate physiological function, such as blood pressure regulation. So far little is known about the molecular mechanisms of how channels sense mechanical force. Canonical vertebrate epithelial Na+ channel (ENaC) formed by α-, β-, and γ-subunits is a shear force (SF) sensor and a member of the ENaC/degenerin protein family. ENaC activity in epithelial cells contributes to electrolyte/fluid-homeostasis and blood pressure regulation. Furthermore, ENaC in endothelial cells mediates vascular responsiveness to regulate blood pressure. Here, we provide evidence that ENaC's ability to mediate SF responsiveness relies on the "force-from-filament" principle involving extracellular tethers and the extracellular matrix (ECM). Two glycosylated asparagines, respectively their N-glycans localized in the palm and knuckle domains of αENaC, were identified as potential tethers. Decreased SF-induced ENaC currents were observed following removal of the ECM/glycocalyx, replacement of these glycosylated asparagines, or removal of N-glycans. Endothelial-specific overexpression of αENaC in mice induced hypertension. In contrast, expression of αENaC lacking these glycosylated asparagines blunted this effect. In summary, glycosylated asparagines in the palm and knuckle domains of αENaC are important for SF sensing. In accordance with the force-from-filament principle, they may provide a connection to the ECM that facilitates vascular responsiveness contributing to blood pressure regulation.

Protein kinase D2 regulates epithelial sodium channel activity and aldosterone non-genomic responses in renal cortical collecting duct cells

Protein kinase D2 (PKD2) is a serine/threonine protein kinase which plays an important role in vesicle fission at the trans-Golgi network (TGN) to coordinate subcellular trafficking with gene expression. We found that in the rat kidney, PKD2 is specifically expressed in collecting duct principal cells predominantly at the apical membrane and with lower basal expression in cytosolic compartments. When rats were maintained on a Na+ depleted diet (<0.87 mmol Na+/kg) to increase plasma aldosterone levels, PKD2 became internalized to a cytoplasmic compartment. Treatment of murine M1 cortical collecting duct (M1-CCD) cells with aldosterone (10 nM) promoted PKD2 co-localization with the trans-Golgi network within 30 min. PKD2 underwent autophosphorylation at Ser876 within 10 min of aldosterone treatment and remained phosphorylated (active) for at least 24 h. A stable PKD2 shRNA knock-down (PKD2 KD) M1-CCD cell line was developed to study the role of PKD2 in epithelial Na+ channel (ENaC) trafficking and transepithelial Na+ transport (SCC) in epithelial monolayers grown in Ussing chambers. The PKD2 KD cells developed transepithelial resistance with kinetics equivalent to wild-type cells, however the transepithelial voltage and Na+ current were significantly elevated in PKD2 knock-down CCD epithelia. The higher basal SCC was due to increased ENaC activity. Aldosterone treatment for 24 h resulted in a decline in ENaC activity in the PKD2 KD cells as opposed to the increase observed in the wild-type cells. The paradoxical inhibition of SCC by aldosterone in PKD2 KD epithelium was attributed to a reduction in ENaC current and lower membrane abundance of ENaC, demonstrating that PKD2 plays a critical tonic role in ENaC trafficking and channel subunit stability. The rapid activation of PKD2 by aldosterone is synergistic with the transcriptional activity of MR and contributes to increased ENaC activity.

Increased ENaC activity during kidney preservation in Wisconsin solution

BackgroundThe invention of an effective kidney preservation solution capable of prolonging harvested kidney viability is the core of kidney transplantation procedure. Researchers have been working on upgrading the preservation solution quality aiming at prolonging storage time while maintaining utmost organ viability and functionality. For many years, the University of Wisconsin (UW) solution has been considered the gold standard solution for kidney preservation. However, the lifespan of kidney preservation in the UW solution is still limited. Its impact on the epithelial Na+ channel (ENaC) activity and its mediated processes is unknown and the primary goal of this study.MethodsKidneys harvested from 8 weeks old Sprague Dawley rats were divided into 4 groups depending upon the period of preservation in UW solution. Additional analysis was performed using dogs’ kidneys. ENaC activity was measured using patch clamp technique; protein expression and mRNA transcription were tested through Western blot and RT-qPCR, respectively. A colorimetric LDH level estimation was performed at different time points during UW solution preservation.ResultsKidney preservation in Wisconsin solution caused reduction of the kidney size and weight and elevation of LDH level. ENaC activity increased in both rat and dog kidneys preserved in the UW solution as assessed by patch clamp analysis. On the contrary, ENaC channel mRNA levels remained unchanged.ConclusionsENaC activity is significantly elevated in the kidneys during preservation in UW solution, which might affect the immediate post-implantation allograft function and trajectory post-transplant.

Annexin II Light Chain p11 Interacts With ENaC to Increase Functional Activity at the Membrane.

The epithelial Na+ channel (ENaC) provides for Na+ absorption in various types of epithelia including the kidney, lung, and colon where ENaC is localized to the apical membrane to enable Na+ entry into the cell. The degree of Na+ entry via ENaC largely depends on the number of active channels localized to the cell membrane, and is tightly controlled by interactions with ubiquitin ligases, kinases, and G-proteins. While regulation of ENaC endocytosis has been well-studied, relatively little is understood of the proteins that govern ENaC exocytosis. We hypothesized that the annexin II light chain, p11, could participate in the transport of ENaC along the exocytic pathway. Our results demonstrate that all three ENaC channel subunits interacted with p11 in an in vitro binding assay. Furthermore, p11 was able to immunoprecipitate ENaC in epithelial cells. Quantitative mass spectrometry of affinity-purified ENaC-p11 complexes recovered several other trafficking proteins including HSP-90 and annexin A6. We also report that p11 exhibits a robust protein expression in cortical collecting duct epithelial cells. However, the expression of p11 in these cells was not influenced by either short-term or long-term exposure to aldosterone. To determine whether the p11 interaction affected ENaC function, we measured amiloride sensitive Na+ currents in Xenopus oocytes or mammalian epithelia co-expressing ENaC and p11 or a siRNA to p11. Results from these experiments showed that p11 significantly augmented ENaC current, whereas knockdown of p11 decreased current. Further, knockdown of p11 reduced ENaC cell surface population suggesting p11 promotes membrane insertion of ENaC. Overall, our findings reveal a novel protein interaction that controls the number of ENaC channels inserted at the membrane via the exocytic pathway.

Salt-deficient diet exacerbates cystogenesis in ARPKD via epithelial sodium channel (ENaC).

BackgroundAutosomal Recessive Polycystic Kidney Disease (ARPKD) is marked by cyst formation in the renal tubules, primarily in the collecting duct (CD) system, ultimately leading to end-stage renal disease. Patients with PKD are generally advised to restrict their dietary sodium intake. This study was aimed at testing the outcomes of dietary salt manipulation in ARPKD.MethodsPCK/CrljCrlPkhd1pck/CRL (PCK) rats, a model of ARPKD, were fed a normal (0.4% NaCl; NS), high salt (4% NaCl; HS), and sodium-deficient (0.01% NaCl; SD) diets for 8 weeks. Immunohistochemistry, GFR measurements, balance studies, and molecular biology approaches were applied to evaluate the outcomes of the protocol. Renin-angiotensin-aldosterone system (RAAS) levels were assessed using LC-MS/MS, and renal miRNA profiles were studied.FindingsBoth HS and SD diets resulted in an increase in cystogenesis. However, SD diet caused extensive growth of cysts in the renal cortical area, and hypertrophy of the tissue; RAAS components were enhanced in the SD group. We observed a reduction in epithelial Na+ channel (ENaC) expression in the SD group, accompanied with mRNA level increase. miRNA assay revealed that renal miR-9a-5p level was augmented in the SD group; we showed that this miRNA decreases ENaC channel number in CD cells.InterpretationOur data demonstrate a mechanism of ARPKD progression during salt restriction that involves activity of ENaC. We further show that miR-9a-5p potentially implicated in this mechanism and that miR-9a-5p downregulates ENaC in cultured CD cells. Our findings open new therapeutic possibilities and highlight the importance of understanding salt reabsorption in ARPKD.

Lack of urea transporters, UT-A1 and UT-A3, increases nitric oxide accumulation to dampen medullary sodium reabsorption through ENaC.

Although the role of urea in urine concentration is known, the effect of urea handling by the urea transporters (UTs), UT-A1 and UT-A3, on sodium balance remains elusive. Serum and urinary sodium concentration is similar between wild-type mice (WT) and UT-A3 null (UT-A3 KO) mice; however, mice lacking both UT-A1 and UT-A3 (UT-A1/A3 KO) have significantly lower serum sodium and higher urinary sodium. Protein expression of renal sodium transporters is unchanged among all three genotypes. WT, UT-A3 KO, and UT-A1/A3 KO acutely respond to hydrochlorothiazide and furosemide; however, UT-A1/A3 KO fail to show a diuretic or natriuretic response following amiloride administration, indicating that baseline epithelial Na+ channel (ENaC) activity is impaired. UT-A1/A3 KO have more ENaC at the apical membrane than WT mice, and single-channel analysis of ENaC in split-open inner medullary collecting duct (IMCD) isolated in saline shows that ENaC channel density and open probability is higher in UT-A1/A3 KO than WT. UT-A1/A3 KO excrete more urinary nitric oxide (NO), a paracrine inhibitor of ENaC, and inner medullary nitric oxide synthase 1 mRNA expression is ~40-fold higher than WT. Because endogenous NO is unstable, ENaC activity was reassessed in split-open IMCD with the NO donor PAPA NONOate [1-propanamine-3-(2-hydroxy-2-nitroso-1-propylhydrazine)], and ENaC activity was almost abolished in UT-A1/A3 KO. In summary, loss of both UT-A1 and UT-A3 (but not UT-A3 alone) causes elevated medullary NO production and salt wasting. NO inhibition of ENaC, despite elevated apical accumulation of ENaC in UT-A1/A3 KO IMCD, appears to be the main contributor to natriuresis in UT-A1/A3 KO mice.

CFTR/ENaC-dependent regulation of membrane potential during human sperm capacitation is initiated by bicarbonate uptake through NBC

To fertilize an egg, sperm must reside in the female reproductive tract to undergo several maturational changes that are collectively referred to as capacitation. From a molecular point of view, the HCO3--dependent activation of the atypical soluble adenylyl cyclase (ADCY10) is one of the first events that occurs during capacitation and leads to the subsequent cAMP-dependent activation of protein kinase A (PKA). Capacitation is also accompanied by hyperpolarization of the sperm plasma membrane. We previously reported that PKA activation is necessary for CFTR (cystic fibrosis transmembrane conductance regulator channel) activity and for the modulation of membrane potential (Em). However, the main HCO3- transporters involved in the initial transport and the PKA-dependent Em changes are not well known nor characterized. Here, we analyzed how the activity of CFTR regulates Em during capacitation and examined its relationship with an electrogenic Na+/HCO3- cotransporter (NBC) and epithelial Na+ channels (ENaCs). We observed that inhibition of both CFTR and NBC decreased HCO3- influx, resulting in lower PKA activity, and that events downstream of the cAMP activation of PKA are essential for the regulation of Em. Addition of a permeable cAMP analog partially rescued the inhibitory effects caused by these inhibitors. HCO3- also produced a rapid membrane hyperpolarization mediated by ENaC channels, which contribute to the regulation of Em during capacitation. Altogether, we demonstrate for the first time, that NBC cotransporters and ENaC channels are essential in the CFTR-dependent activation of the cAMP/PKA signaling pathway and Em regulation during human sperm capacitation.

Annexin II light chain p11 interacts with ENaC to increase functional activity at the membrane

The epithelial Na+ channel (ENaC) provides for Na+ absorption in various types of epithelia including the kidney, lung and colon where ENaC is localized to the apical membrane to enable Na+ entry into the cell. The degree of Na+ entry via ENaC largely depends on the number of active channels localized to the cell membrane. While regulation of ENaC endocytosis has been well studied, relatively little is understood of the proteins that govern ENaC exocytosis. We hypothesized that the annexin II light chain, p11 could participate in the transport of ENaC along the exocytic pathway. Our results demonstrate that all three ENaC channel subunits interacted with p11 in an in vitro binding assay. Quantitative mass spectrometry of affinity‐purified ENaC‐p11 complexes recovered several other trafficking proteins including HSP‐90, COMMD10 and annexin A6. We also found that p11 exhibits a robust protein expression in cortical collecting duct epithelial cells. However, the expression of p11 in these cells was not influenced by either short‐term or long‐term exposure to aldosterone. To determine whether the p11 interaction affected ENaC function, we measured amiloride sensitive Na+ currents in Xenopus oocytes or mammalian epithelia coexpressing ENaC and p11, or a siRNA to p11. Results from these experiments showed that p11 significantly augmented amiloride‐sensitive current, whereas knockdown of p11 decreased current. Further, knockdown of p11 reduced the ENaC cell surface population suggesting p11 promotes membrane insertion of ENaC. Overall, our findings reveal a novel protein‐protein interaction of p11 with ENaC that controls the number of ENaC channels inserted at the plasma membrane via the exocytic pathway.Support or Funding InformationThis work was supported by a University of Otago Research Grant, and an AIM grant from the Department of Physiology, University of Otago.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Collecting duct principal, but not intercalated, cell (pro)renin receptor modulates sodium and water transport

The collecting duct (CD) is the predominant site for prorenin receptor (PRR) expression within the nephron. We have previously demonstrated that the CD PRR regulates epithelial Na+ channel (ENaC) activity and water transport; however, which CD cell type is involved is unclear. Hence, we examined the effects of principal cell (PC) or intercalated cell (IC) PRR deletion on renal Na+ and water handling. PC or IC PRR KO mice were obtained by crossing floxed PRR mice with mice harboring Cre recombinase under the control of the AQP2 or B‐1 subunit of the H+ ATPase promoters, respectively. In‐situ hybridization using RNAscope demonstrated abundant PRR RNA in IC with modest staining in PC in control mice with reduced cell‐specific PRR RNA in PC and IC KO mice. Compared to controls, PC KO, but not IC KO mice, had increased urinary volume (Control: 1.2 ± 0.4 vs PC KO: 2.0 ± 0.3, P &lt; 0.05; IC KO: 1.4 ± 0.2 ml/day), reduced urine osmolality (Control: 3160 ± 279 vs PC KO: 2343 ± 247, P &lt; 0.05; IC KO: 2748 ± 356 mosm/Kg H2O) and reduced expression of renal medullary AQP2 (50% of control). Further, PC KO mice had reduced renal medullary ENaC‐α expression with normal and low Na+ diet and lower Na+ balance (24‐hour intake – excretion) following low Na+ intake (controls: 60.5 ± 7.5 vs PC KO: 25.7 ± 10.6 μmol/day, P &lt;0.05). In contrast, no differences in Na+ balance or ENaC expression was observed between control or IC KO mice. In acutely isolated cortical CD, under basal conditions, no significant differences in ENaC activity were observed between control, PC or IC specific PRR KO mice. Treatment with mouse prorenin (10 nM for 30 min) increased ENaC channel number and open probability by 2‐fold in control mice (basal: 0.15 ± 0.04; prorenin treatment: 0.74 ± 0.25). Addition of prorenin did not alter ENaC activity in PC specific PRR KO mice (basal: 0.28 ± 0.06; prorenin treatment: 0.33 ± 0.13) but increased ENaC activity in IC specific PRR KO mice (basal: 0.17 ± 0.08; prorenin treatment 1.29 ± 0.13). However, mean arterial pressure as measured by telemetry (Controls: 104 ± 1, PC KO: 106 ± 2, IC KO: 101 ± 4 mm Hg) was similar between controls and PC or IC KO mice on normal and low Na+ diets. Taken together, these findings indicate that under physiologic conditions, the PC, but not IC, PRR modulates ENaC expression and activity, urinary Na+ excretion and water transport. Further studies are needed to examine the role of PC and IC PRR in pathological states such as Ang‐II induced hypertension.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Alveolar nonselective channels are ASIC1a/α-ENaC channels and contribute to AFC.

A thin fluid layer in alveoli is normal and results from a balance of fluid entry and fluid uptake by transepithelial salt and water reabsorption. Conventional wisdom suggests the reabsorption is via epithelial Na+ channels (ENaC), but if all Na+ reabsorption were via ENaC, then amiloride, an ENaC inhibitor, should block alveolar fluid clearance (AFC). However, amiloride blocks only half of AFC. The reason for failure to block is clear from single-channel measurements from alveolar epithelial cells: ENaC channels are observed, but another channel is present at the same frequency that is nonselective for Na+ over K+, has a larger conductance, and has shorter open and closed times. These two channel types are known as highly selective channels (HSC) and nonselective cation channels (NSC). HSC channels are made up of three ENaC subunits since knocking down any of the subunits reduces HSC number. NSC channels contain α-ENaC since knocking down α-ENaC reduces the number of NSC (knocking down β- or γ-ENaC has no effect on NSC, but the molecular composition of NSC channels remains unclear). We show that NSC channels consist of at least one α-ENaC and one or more acid-sensing ion channel 1a (ASIC1a) proteins. Knocking down either α-ENaC or ASIC1a reduces both NSC and HSC number, and no NSC channels are observable in single-channel patches on lung slices from ASIC1a knockout mice. AFC is reduced in knockout mice, and wet wt-to-dry wt ratio is increased, but the percentage increase in wet wt-to-dry wt ratio is larger than expected based on the reduction in AFC.

ENaC and ROMK activity are inhibited in the DCT2/CNT of TgWnk4PHAII mice.

Mice transgenic for genomic segments harboring PHAII (pseudohypoaldosteronism type II) mutant Wnk4 (with-No-Lysine kinase 4) (TgWnk4PHAII) have hyperkalemia which is currently believed to be the result of high activity of Na-Cl cotransporter (NCC). This leads to decreasing Na+ delivery to the distal nephron segment including late distal convoluted tubule (DCT) and connecting tubule (CNT). Since epithelial Na+ channel (ENaC) and renal outer medullary K+ channel (ROMK or Kir4.1) are expressed in the late DCT and play an important role in mediating K+ secretion, the aim of the present study is to test whether ROMK and ENaC activity in the DCT/CNT are also compromised in the mice expressing PHAII mutant Wnk4. Western blot analysis shows that the expression of βENaC and γENaC subunits but not αENaC subunit was lower in TgWnk4PHAII mice than that in wild-type (WT) and TgWnk4WT mice. Patch-clamp experiments detected amiloride-sensitive Na+ currents and TPNQ-sensitive K+ currents in DCT2/CNT, suggesting the activity of ENaC and ROMK. However, both Na+ and ROMK currents in DCT2/CNT of TgWnk4PHAII mice were significantly smaller than those in WT and TgWnk4WT mice. In contrast, the basolateral K+ currents in the DCT were similar among three groups, despite higher NCC expression in TgWnk4PHAII mice than those of WT and TgWnk4WTmice. An increase in dietary K+ intake significantly increased both ENaC and ROMK currents in the DCT2/CNT of all three groups. However, high-K+ (HK) intake-induced stimulation of Na+ and K+ currents was smaller in TgWnk4PHAII mice than those in WT and TgWnk4WT mice. We conclude that ENaC and ROMK channel activity in DCT2/CNT are inhibited in TgWnk4PHAII mice and that Wnk4PHAII-induced inhibition of ENaC and ROMK may contribute to the suppression of K+ secretion in the DCT2/CNT in addition to increased NCC activity.

Pendrin gene ablation alters ENaC subcellular distribution and open probability.

The present study explored whether the intercalated cell Cl(-)/HCO3(-) exchanger pendrin modulates epithelial Na(+) channel (ENaC) function by changing channel open probability and/or channel density. To do so, we measured ENaC subunit subcellular distribution by immunohistochemistry, single channel recordings in split open cortical collecting ducts (CCDs), as well as transepithelial voltage and Na(+) absorption in CCDs from aldosterone-treated wild-type and pendrin-null mice. Because pendrin gene ablation reduced 70-kDa more than 85-kDa γ-ENaC band density, we asked if pendrin gene ablation interferes with ENaC cleavage. We observed that ENaC-cleaving protease application (trypsin) increased the lumen-negative transepithelial voltage in pendrin-null mice but not in wild-type mice, which raised the possibility that pendrin gene ablation blunts ENaC cleavage, thereby reducing open probability. In mice harboring wild-type ENaC, pendrin gene ablation reduced ENaC-mediated Na(+) absorption by reducing channel open probability as well as by reducing channel density through changes in subunit total protein abundance and subcellular distribution. Further experiments used mice with blunted ENaC endocytosis and degradation (Liddle's syndrome) to explore the significance of pendrin-dependent changes in ENaC open probability. In mouse models of Liddle's syndrome, pendrin gene ablation did not change ENaC subunit total protein abundance, subcellular distribution, or channel density, but markedly reduced channel open probability. We conclude that in mice harboring wild-type ENaC, pendrin modulates ENaC function through changes in subunit abundance, subcellular distribution, and channel open probability. In a mouse model of Liddle's syndrome, however, pendrin gene ablation reduces channel activity mainly through changes in open probability.

Feedback inhibition of ENaC: Acute and chronic mechanisms

Intracellular [Na+] ([Na+]i) modulates the activity of the epithelial Na channel (ENaC) to help prevent cell swelling and regulate epithelial Na+ transport, but the underlying mechanisms remain unclear. We show here that short-term (60–80 min) incubation of ENaC-expressing oocytes in high Na+ results in a 75% decrease in channel activity. When the β subunit was truncated, corresponding to a gain-of-function mutation found in Liddle's syndrome, the same maneuver reduced activity by 45% despite a larger increase in [Na+]i. In both cases the inhibition occurred with little to no change in cell-surface expression of γENaC. Long-term incubation (18 hours) in high Na+ reduced activity by 92% and 75% in wild-type channels and Liddle's mutant, respectively, with concomitant 70% and 52% decreases in cell-surface γENaC. In the presence of Brefeldin A to inhibit forward protein trafficking, high-Na+ incubation decreased wt ENaC activity by 52% and 88% after 4 and 8 hour incubations, respectively. Cleaved γENaC at the cell surface had lifetimes at the surface of 6 hrs in low Na+ and 4 hrs in high Na+, suggesting that [Na+]i increased the rate of retrieval of cleaved γ ENaC by 50%. This implies that enhanced retrieval of ENaC channels at the cell surface accounts for part, but not all, of the downregulation of ENaC activity shown with chronic increases in [Na+]i.

The inhibitory effect of Gβγand Gβisoform specificity on ENaC activity

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-channel 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 with 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.