Molecular determinants of PI(4,5)P2 and PI(3,4,5)P3 regulation of the epithelial Na+ channel

P2Y2 receptor decreases blood pressure by inhibiting ENaC

Stimulating the Gq-coupled P2Y2 receptor (P2ry2) lowers blood pressure. Global knockout of P2ry2 increases blood pressure. Vascular and renal mechanisms are believed to participate in P2ry2 effects on blood pressure. To isolate the role of the kidneys in P2ry2 effects on blood pressure and to reveal the molecular and cellular mechanisms of this action, we test here the necessity of the P2ry2 and the sufficiency of Gq-dependent signaling in renal principal cells to the regulation of the epithelial Na+ channel (ENaC), sodium excretion, and blood pressure. Activating P2ry2 in littermate controls but not principal cell-specific P2ry2-knockout mice decreased the activity of ENaC in renal tubules. Moreover, deletion of P2ry2 in principal cells abolished increases in sodium excretion in response to stimulation of P2ry2 and compromised the normal ability to excrete a sodium load. Consequently, principal cell-specific knockout of P2ry2 prevented decreases in blood pressure in response to P2ry2 stimulation in the deoxycorticosterone acetate-salt (DOCA-salt) model of hypertension. In wild-type littermate controls, such stimulation decreased blood pressure in this model of hypertension by promoting a natriuresis. Pharmacogenetic activation of Gq exclusively in principal cells using targeted expression of Gq-designer receptors exclusively activated by designer drugs and clozapine N-oxide decreased the activity of ENaC in renal tubules, promoting a natriuresis that lowered elevated blood pressure in the DOCA-salt model of hypertension. These findings demonstrate that the kidneys play a major role in decreasing blood pressure in response to P2ry2 activation and that inhibition of ENaC activity in response to P2ry2-mediated Gq signaling lowered blood pressure by increasing renal sodium excretion.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

PIP2 Interacts Electrostatically with MARCKS-like Protein-1 and ENaC in Renal Epithelial Cells

We examined the interaction of a membrane-associated protein, MARCKS-like Protein-1 (MLP-1), and an ion channel, Epithelial Sodium Channel (ENaC), with the anionic lipid, phosphatidylinositol 4, 5-bisphosphate (PIP₂). We found that PIP₂ strongly activates ENaC in excised, inside-out patches with a half-activating concentration of 21 ± 1.17 µM. We have identified 2 PIP₂ binding sites in the N-terminus of ENaC β and γ with a high concentration of basic residues. Normal channel activity requires MLP-1's strongly positively charged effector domain to electrostatically sequester most of the membrane PIP₂ and increase the local concentration of PIP₂. Our previous data showed that ENaC covalently binds MLP-1 so PIP₂ bound to MLP-1 would be near PIP₂ binding sites on the cytosolic N terminal regions of ENaC. We have modified the charge structure of the PIP₂ -binding domains of MLP-1 and ENaC and showed that the changes affect membrane localization and ENaC activity in a way consistent with electrostatic theory.

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.

Epac1-/- and Epac2-/- mice exhibit deficient epithelial Na+ channel regulation and impaired urinary Na+ conservation

Exchange proteins directly activated by cAMP (Epacs) are abundantly expressed in the renal tubules. We used genetic and pharmacological tools in combination with balance, electrophysiological, and biochemical approaches to examine the role of Epac1 and Epac2 in renal sodium handling. We demonstrate that Epac1^–/– and Epac2^–/– mice exhibit a delayed anti-natriuresis to dietary sodium restriction despite augmented aldosterone levels. This was associated with a significantly lower response to the epithelial Na⁺ channel (ENaC) blocker amiloride, reduced ENaC activity in split-opened collecting ducts, and defective posttranslational processing of α and γENaC subunits in the KO mice fed with a Na⁺-deficient diet. Concomitant deletion of both isoforms led to a marginally greater natriuresis but further increased aldosterone levels. Epac2 blocker ESI-05 and Epac1&2 blocker ESI-09 decreased ENaC activity in Epac WT mice kept on the Na⁺-deficient diet but not on the regular diet. ESI-09 injections led to natriuresis in Epac WT mice on the Na⁺-deficient diet, which was caused by ENaC inhibition. In summary, our results demonstrate similar but nonredundant actions of Epac1 and Epac2 in stimulation of ENaC activity during variations in dietary salt intake. We speculate that inhibition of Epac signaling could be instrumental in treatment of hypertensive states associated with ENaC overactivation.

ENaC regulation by phospholipids and DGK explained through mathematical modeling

Cystic fibrosis is a condition caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). It is also thought to increase the activity of epithelial sodium channels (ENaC). The altered function of these ion channels is one of the causes of the thick dehydrated mucus that characterizes the disease and is partially responsible for recurrent pulmonary infections and inflammation events that ultimately destroy the lungs of affected subjects. Phosphoinositides are signaling lipids that regulate numerous cellular processes and membrane proteins, including ENaC. Inhibition of diacylglycerol kinase (DGK), an enzyme of the phosphoinositide pathway, reduces ENaC function. We propose a computational analysis that is based on the combination of two existing mathematical models: one representing the dynamics of phosphoinositides and the other explaining how phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) influences ENaC activity and, consequently, airway surface liquid. This integrated model permits, for the first time, a detailed assessment of the intricate interactions between DGK and ENaC and is consistent with available literature data. In particular, the computational approach allows comparisons of two competing hypotheses regarding the regulation of ENaC. The results strongly suggest that the regulation of ENaC is primarily exerted through the control of PI(4,5)P2 production by type-I phosphatidylinositol-4-phosphate 5-kinase (PIP5KI), which in turn is controlled by phosphatidic acid (PA), the product of the DGK reaction.

Myristoylated alanine-rich C kinase substrate-like protein-1 regulates epithelial sodium channel activity in renal distal convoluted tubule cells

The epithelial sodium channel (ENaC) regulates blood pressure by fine-tuning distal nephron sodium reabsorption. Our previous work has shown that ENaC gating is regulated by anionic phospholipid phosphates, including phosphatidylinositol 4,5-bisphosphate (PIP₂). The PIP₂-dependent regulation of ENaC is mediated by the myristoylated alanine-rich protein kinase C substrate-like protein-1 (MLP-1). MLP-1 binds to and is a reversible source of PIP₂ at the plasma membrane. We examined MLP-1 regulation of ENaC in distal convoluted tubule clonal cell line DCT-15 cells. Wild-type MLP-1 runs at an apparent molecular mass of 52 kDa despite having a predicted molecular mass of 21 kDa. Native MLP-1 consists of several distinct structural elements: an effector domain that is highly positively charged, sequesters PIP₂, contains serines that are the target of PKC, and controls MLP-1 association with the membrane; a myristoylation domain that promotes association with the membrane; and a multiple homology 2 domain of previously unknown function. To further examine MLP-1 in DCT-15 cells, we constructed several MLP-1 mutants: WT, a full-length wild-type protein; S3A, three substitutions in the effector domain to prevent phosphorylation; S3D mimicked constitutive phosphorylation by replacing three serines with aspartates; and GA replaced the myristoylation site glycine with alanine, so GA could not be myristoylated. Each mutant was tagged with either NH₂-terminal 3XFLAG or COOH-terminal mCherry or V5. Transfection with MLP mutants modified ENaC activity in DCT-15 cells: activity was highest in S3A and lowest in S3D, and the activity after transfection with either construct was significantly different from WT. In Western blots, when transfected with 3XFLAG-tagged MLP-1 mutants, the expression of the full length of MLP-1 at 52 kDa increased in mutant S3A-MLP-1-transfected DCT-15 cells and decreased in S3D-MLP-1-transfected DCT-15 cells. Several lower molecular mass bands were also detected that correspond to potential presumptive calpain cleavage products. Confocal imaging shows that the different mutants localize in different subcellular compartments consistent with their preferred location in the membrane or in the cytosol. Activation of protein kinase C increases phosphorylation of endogenous MLP-1 and reduces ENaC activity. Our results suggest a complicated role for proteolytic processing in MLP-1 regulation of ENaC.

Phosphatidylinositol 4,5-bisphosphate directly interacts with the β and γ subunits of the sodium channel ENaC

The plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) regulates the activity of diverse ion channels to include the epithelial Na+ channel ENaC. Whether PIP2 regulation of ENaC is due to a direct phospholipid-protein interaction, remains obscure. To date, possible interaction of PIP2 with ENaC primarily has been tested indirectly through assays of channel function. A fragment-based biochemical analysis approach is used here to directly quantify possible PIP2-ENaC interactions. We find using the CIBN-CRY2 optogenetic dimerization system that the phosphoryl group positioned at carbon 5 of PIP2 is necessary for interaction with ENaC. Previous studies have implicated conserved basic residues in the cytosolic portions of β- and γ-ENaC subunits as being important for PIP2-ENaC interactions. To test this, we used synthetic peptides of these regions of β- and γ-ENaC. Steady-state intrinsic fluorescence spectroscopy demonstrated that phosphoinositides change the local conformation of the N terminus of β-ENaC, and two sites of γ-ENaC adjacent to the plasma membrane, suggesting direct interactions of PIP2 with these three regions. Microscale thermophoresis elaborated PIP2 interactions with the N termini of β- (Kd ∼5.2 μm) and γ-ENaC (Kd ∼13 μm). A weaker interaction site within the carboxyl terminus of γ-ENaC (Kd ∼800 μm) was also observed. These results support that PIP2 regulates ENaC activity by directly interacting with at least three distinct regions within the cytoplasmic domains of the channel that contain conserved basic residues. These interactions are probably electrostatic in nature, and are likely to bear a key structural role in support of channel activity.

Regulating ENaC's gate

Epithelial Na⁺ channels (ENaCs) are members of a family of cation channels that function as sensors of the extracellular environment. ENaCs are activated by specific proteases in the biosynthetic pathway and at the cell surface and remove embedded inhibitory tracts, which allows channels to transition to higher open-probability states. Resolved structures of ENaC and an acid-sensing ion channel revealed highly organized extracellular regions. Within the periphery of ENaC subunits are unique domains formed by antiparallel β-strands containing the inhibitory tracts and protease cleavage sites. ENaCs are inhibited by Na⁺ binding to specific extracellular site(s), which promotes channel transition to a lower open-probability state. Specific inositol phospholipids and channel modification by Cys-palmitoylation enhance channel open probability. How these regulatory factors interact in a concerted manner to influence channel open probability is an important question that has not been resolved. These various factors are reviewed, and the impact of specific factors on human disorders is discussed.

Thickness of the airway surface liquid layer in the lung is affected in cystic fibrosis by compromised synergistic regulation of the ENaC ion channel

The lung epithelium is lined with a layer of airway surface liquid (ASL) that is crucial for healthy lung function. ASL thickness is controlled by two ion channels: epithelium sodium channel (ENaC) and cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Here, we present a minimal mathematical model of ENaC, CFTR and ASL regulation that sheds light on the control of ENaC by the short palate lung and nasal epithelial clone 1 (SPLUNC1) protein and by phosphatidylinositol 4,5-biphosphate (PI(4,5)P₂). The model, despite its simplicity, yields a good fit to experimental observations and is an effective tool for exploring the interplay between ENaC, CFTR and ASL. Steady-state data and dynamic information constrain the model's parameters without ambiguities. Testing the hypothesis that PI(4,5)P₂ protects ENaC from ubiquitination suggests that this protection does not improve the model results and that the control of the ENaC opening probability by PI(4,5)P₂ is sufficient to explain all available data. The model analysis further demonstrates that ASL at the steady state is sensitive to small changes in PI(4,5)P₂ abundance, particularly in CF conditions, which suggests that manipulation of phosphoinositide metabolism may promote therapeutic benefits for CF patients.

Intracellular cholesterol stimulates ENaC by interacting with phosphatidylinositol‑4,5‑bisphosphate and mediates cyclosporine A-induced hypertension

We have previously shown that blockade of ATP-binding cassette transporter A1 (ABCA1) with cyclosporine A (CsA) stimulates the epithelial sodium channel (ENaC) in cultured distal nephron cells. Here we show that CsA elevated systolic blood pressure in both wild-type and apolipoprotein E (ApoE) knockout (KO) mice to a similar level. The elevated systolic blood pressure was completely reversed by inhibition of cholesterol (Cho) synthesis with lovastatin. Inside-out patch-clamp data show that intracellular Cho stimulated ENaC in cultured distal nephron cells by interacting with phosphatidylinositol‑4,5‑bisphosphate (PIP2), an ENaC activator. Confocal microscopy data show that both α‑ENaC and PIP2 were localized in microvilli via a Cho-dependent mechanism. Deletion of membrane Cho reduced the levels of γ‑ENaC in the apical membrane. Reduced ABCA1 expression and elevated intracellular Cho were observed in old mice, compared to young mice. In parallel, cell-attached patch-clamp data from the split-open cortical collecting ducts (CCD) show that ENaC activity was significantly increased in old mice. These data suggest that elevation of intracellular Cho due to blockade of ABCA1 stimulates ENaC, which may contribute to CsA-induced hypertension. This study also implies that reduced ABCA1 expression may mediate age-related hypertension by increasing ENaC activity via elevation of intracellular Cho.

Phosphoinositides modulate the voltage dependence of two-pore channel 3

Two-pore channels, or two-pore Na⁺ channels (TPCs), contain two homologous domains, each containing a functional unit typical of voltage-dependent cation channels. Each domain is considered to be responsible for either phosphoinositide (PI) binding or voltage sensing. Among the three members of the TPC family, TPC1 and TPC2 are activated by PI(3,5)P₂, while TPC3 has been thought not to be affected by any PIs. Here, we report that TPC3 is sensitive to PI(3,4)P₂ and PI(3,5)P₂, but not to PI(4,5)P₂, and that the extremely slow increase in TPC3 currents induced by depolarization in Xenopus oocytes is due to the production of PI(3,4)P₂ Similarly to TPC1, the cluster of basic amino acid residues in domain I is critical for PI sensitivity, but with a slight variation that may allow TPC3 to be sensitive to both PI(3,4)P₂ and PI(3,5)P₂ We also found that TPC3 has a unique PI-dependent modulation mechanism of voltage dependence, which is achieved by a specific bridging interaction between domain I and domain II. Taken together, these findings show that TPC3 is a unique member of the TPC family that senses PIs and displays a strong coupling between PI binding and voltage-dependent gating.

Renal Na+ excretion consequent to pharmacogenetic activation of Gq-DREADD in principal cells

Stimulation of metabotropic Gq-coupled purinergic P2Y2 receptors decreases activity of the epithelial Na+ channel (ENaC) in renal principal cells of the distal nephron. The physiological consequences of P2Y2 receptor signaling disruption in the P2Y2 receptor knockout mouse are decreased Na+ excretion and increased arterial blood pressure. However, because of the global nature of this knockout model, the quantitative contribution of ENaC and distal nephron compared with that of upstream renal vascular and tubular elements to changes in urinary excretion and arterial blood pressure is obscure. Moreover, it is uncertain whether stimulation of P2Y2 receptor inhibition of ENaC is sufficient to drive renal (urinary) Na+ excretion (UNaV). Here, using a pharmacogenetic approach and selective agonism of the P2Y2 receptor, we test the sufficiency of targeted stimulation of Gq signaling in principal cells of the distal nephron and P2Y2 receptors to increase UNaV. Selective stimulation of the P2Y2 receptor with the ligand MRS2768 decreased ENaC activity in freshly isolated tubules (as assessed by patch-clamp electrophysiology) and increased UNaV (as assessed in metabolic cages). Similarly, selective agonism of hM3Dq-designer receptors exclusively activated by designer drugs (DREADD) restrictively expressed in principal cells of the distal nephron with clozapine- N-oxide decreased ENaC activity and, consequently, increased UNaV. Clozapine- N-oxide, when applied to control littermates, failed to affect ENaC and UNaV. This study represents the first use of pharmacogenetic (DREADD) technology in the renal tubule and demonstrated that selective activation of the P2Y2 receptor and Gq signaling in principal cells is sufficient to promote renal salt excretion.

New insights regarding epithelial Na+ channel regulation and its role in the kidney, immune system and vasculature

PURPOSE OF REVIEW

This review describes recent findings regarding the epithelial Na channel (ENaC) and its roles in physiologic and pathophysiologic states. We discuss new insights regarding ENaC's structure, its regulation by various factors, its potential role in hypertension and nephrotic syndrome, and its roles in the immune system and vasculature.

RECENT FINDINGS

A recently resolved structure of ENaC provides clues regarding mechanisms of ENaC activation by proteases. The use of amiloride in nephrotic syndrome, and associated complications are discussed. ENaC is expressed in dendritic cells and contributes to immune system activation and increases in blood pressure in response to NaCl. ENaC is expressed in endothelial ENaC and has a role in regulating vascular tone.

SUMMARY

New findings have emerged regarding ENaC and its role in the kidney, immune system, and vasculature.

Palmitate Stimulates the Epithelial Sodium Channel by Elevating Intracellular Calcium, Reactive Oxygen Species, and Phosphoinositide 3-Kinase Activity

Previous studies indicate that the epithelial sodium channel (ENaC) in the kidney is upregulated in diabetes mellitus. Here, we show that ENaC single-channel activity in distal nephron cells was significantly increased by palmitate, a free fatty acid which is elevated in diabetes mellitus. We also show that palmitate increased intracellular Ca²⁺ and that after chelating intracellular Ca²⁺ with BAPTA-AM, palmitate failed to affect ENaC activity. Treatment of the cells with 2-aminoethoxydiphenyl borate (2-APB, an inhibitor of IP₃ receptors) abolished the elevation of both intracellular Ca²⁺ and ENaC activity. Treatment of the cells with apocynin (an NADPH oxidase inhibitor), dithiothreitol/NaHS (reducing agents), or LY294002 (a phosphoinositide 3-kinase (PI3K) inhibitor) prevented palmitate-induced ENaC activity, whereas thimerosal (an oxidizing agent) mimicked the effects of palmitate on ENaC activity. However, these treatments did not alter the levels of intracellular Ca²⁺, indicating that elevation of reactive oxygen species (ROS) and activation of PI3K are downstream of the signaling cascade. Since we have shown that ROS stimulate ENaC by activating PI3K, these data together suggest that palmitate first elevates intracellular Ca²⁺, then activates an NADPH oxidase to elevate intracellular ROS and PI3K activity, and finally increases ENaC activity via the activated PI3K.

Distal tubule basolateral potassium channels: cellular and molecular mechanisms of regulation

PURPOSE OF REVIEW

Multiple clinical and translational evidence support benefits of high potassium diet; however, there many uncertainties underlying the molecular and cellular mechanisms determining effects of dietary potassium. Kir4.1 and Kir5.1 proteins form a functional heteromer (Kir4.1/Kir5.1), which is the primary inwardly rectifying potassium channel on the basolateral membrane of both distal convoluted tubule (DCT) and the collecting duct principal cells. The purpose of this mini-review is to summarize latest advances in our understanding of the evolution, physiological relevance and mechanisms controlling these channels.

RECENT FINDINGS

Kir4.1 and Kir5.1 channels play a critical role in determining electrolyte homeostasis in the kidney and blood pressure, respectively. It was reported that Kir4.1/Kir5.1 serves as potassium sensors in the distal nephron responding to variations in dietary intake and hormonal stimuli. Global and kidney specific knockouts of either channel resulted in hypokalemia and severe cardiorenal phenotypes. Furthermore, knock out of Kir5.1 in Dahl salt-sensitive rat background revealed the crucial role of the Kir4.1/Kir5.1 channel in salt-induced hypertension.

SUMMARY

Here, we focus on reviewing novel experimental evidence of the physiological function, expression and hormonal regulation of renal basolateral inwardly rectifying potassium channels. Further investigation of molecular and cellular mechanisms controlling Kir4.1 and Kir4.1/Kir5.1-mediating pathways and development of specific compounds targeting these channels function is essential for proper control of electrolyte homeostasis and blood pressure.

Deficient transient receptor potential vanilloid type 4 function contributes to compromised [Ca2+]i homeostasis in human autosomal-dominant polycystic kidney disease cells

Autosomal-dominant polycystic kidney disease (ADPKD) is a devastating disorder that is characterized by a progressive decline in renal function as a result of the development of fluid-filled cysts. Defective flow-mediated [Ca²⁺]_i responses and disrupted [Ca²⁺]_i homeostasis have been repeatedly associated with cyst progression in ADPKD. We have previously demonstrated that the transient receptor potential vanilloid type 4 (TRPV4) channel is imperative for flow-mediated [Ca²⁺]_i responses in murine distal renal tubule cells. To determine whether compromised TRPV4 function contributes to aberrant Ca²⁺ regulation in ADPKD, we assessed TRPV4 function in primary cells that were cultured from ADPKD and normal human kidneys (NHKs). Single-channel TRPV4 activity and TRPV4-dependent Ca²⁺ influxes were drastically reduced in ADPKD cells, which correlated with distorted [Ca²⁺]_i signaling. Whereas total TRPV4 protein levels were comparable in NHK and ADPKD cells, we detected a marked decrease in TRPV4 glycosylation in ADPKD cells. Tunicamycin-induced deglycosylation inhibited TRPV4 activity and compromised [Ca²⁺]_i signaling in NHK cells. Overall, we demonstrate that TRPV4 glycosylation and channel activity are diminished in human ADPKD cells compared with NHK cells, and that this contributes significantly to the distorted [Ca²⁺]_i dynamics. We propose that TRPV4 stimulation may be beneficial for restoring [Ca²⁺]_i homeostasis in cyst cells, thereby interfering with ADPKD progression.-Tomilin, V., Reif, G. A., Zaika, O., Wallace, D. P., Pochynyuk, O. Deficient transient receptor potential vanilloid type 4 function contributes to compromised [Ca²⁺]_i homeostasis in human autosomal-dominant polycystic kidney disease cells.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Dietary K+ and Cl- independently regulate basolateral conductance in principal and intercalated cells of the collecting duct

The renal collecting duct contains two distinct cell types, principal and intercalated cells, expressing potassium K_ir4.1/5.1 (KCNJ10/16) and chloride ClC-K2 (ClC-Kb in humans) channels on their basolateral membrane, respectively. Both channels are thought to play important roles in controlling systemic water-electrolyte balance and blood pressure. However, little is known about mechanisms regulating activity of K_ir4.1/5.1 and ClC-K2/b. Here, we employed patch clamp analysis at the single channel and whole cell levels in freshly isolated mouse collecting ducts to investigate regulation of K_ir4.1/5.1 and ClC-K2/b by dietary K⁺ and Cl^- intake. Treatment of mice with high K⁺ and high Cl^- diet (6% K⁺, 5% Cl^-) for 1 week significantly increased basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity and induced hyperpolarization of basolateral membrane potential in principal cells when compared to values in mice on a regular diet (0.9% K⁺, 0.5% Cl^-). In contrast, basolateral Cl^--selective current and single channel ClC-K2/b activity was markedly decreased in intercalated cells under this condition. Substitution of dietary K⁺ to Na⁺ in the presence of high Cl^- exerted a similar inhibiting action of ClC-K2/b suggesting that the channel is sensitive to variations in dietary Cl^- per se. Cl^--sensitive with-no-lysine kinase (WNK) cascade has been recently proposed to orchestrate electrolyte transport in the distal tubule during variations of dietary K⁺. However, co-expression of WNK1 or its major downstream effector Ste20-related proline-alanine-rich kinase (SPAK) had no effect on ClC-Kb over-expressed in Chinese hamster ovary (CHO) cells. Treatment of mice with high K⁺ diet without concomitant elevations in dietary Cl^- (6% K⁺, 0.5% Cl^-) elicited a comparable increase in basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity in principal cells, but had no significant effect on ClC-K2/b activity in intercalated cells. Furthermore, stimulation of aldosterone signaling by Deoxycorticosterone acetate (DOCA) recapitulated the stimulatory actions of high K⁺ intake on K_ir4.1/5.1 channels in principal cells but was ineffective to alter ClC-K2/b activity and basolateral Cl^- conductance in intercalated cells. In summary, we report that variations of dietary K⁺ and Cl^- independently regulate basolateral potassium and chloride conductance in principal and intercalated cells. We propose that such discrete mechanism might contribute to fine-tuning of urinary excretion of electrolytes depending on dietary intake.

Specific Palmitoyltransferases Associate with and Activate the Epithelial Sodium Channel

The epithelial sodium channel (ENaC) has an important role in regulating extracellular fluid volume and blood pressure, as well as airway surface liquid volume and mucociliary clearance. ENaC is a trimer of three homologous subunits (α, β, and γ). We previously reported that cytoplasmic residues on the β (βCys-43 and βCys-557) and γ (γCys-33 and γCys-41) subunits are palmitoylated. Mutation of Cys that blocked ENaC palmitoylation also reduced channel open probability. Furthermore, γ subunit palmitoylation had a dominant role over β subunit palmitoylation in regulating ENaC. To determine which palmitoyltransferases (termed DHHCs) regulate the channel, mouse ENaCs were co-expressed in Xenopus oocytes with each of the 23 mouse DHHCs. ENaC activity was significantly increased by DHHCs 1, 2, 3, 7, and 14. ENaC activation by DHHCs was lost when γ subunit palmitoylation sites were mutated, whereas DHHCs 1, 2, and 14 still activated ENaC lacking β subunit palmitoylation sites. β subunit palmitoylation was increased by ENaC co-expression with DHHC 7. Both wild type ENaC and channels lacking β and γ palmitoylation sites co-immunoprecipitated with the five activating DHHCs, suggesting that ENaC forms a complex with multiple DHHCs. RT-PCR revealed that transcripts for the five activating DHHCs were present in cultured mCCD_cl1 cells, and DHHC 3 was expressed in aquaporin 2-positive principal cells of mouse aldosterone-sensitive distal nephron where ENaC is localized. Treatment of polarized mCCD_cl1 cells with a general inhibitor of palmitoylation reduced ENaC-mediated Na⁺ currents within minutes. Our results indicate that specific DHHCs have a role in regulating ENaC.

The sodium chloride cotransporter (NCC) and epithelial sodium channel (ENaC) associate

The thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC) are two of the most important determinants of salt balance and thus systemic blood pressure. Abnormalities in either result in profound changes in blood pressure. There is one segment of the nephron where these two sodium transporters are coexpressed, the second part of the distal convoluted tubule. This is a key part of the aldosterone-sensitive distal nephron, the final regulator of salt handling in the kidney. Aldosterone is the key hormonal regulator for both of these proteins. Despite these shared regulators and coexpression in a key nephron segment, associations between these proteins have not been investigated. After confirming apical localization of these proteins, we demonstrated the presence of functional transport proteins and native association by blue native PAGE. Extensive coimmunoprecipitation experiments demonstrated a consistent interaction of NCC with α- and γ-ENaC. Mammalian two-hybrid studies demonstrated direct binding of NCC to ENaC subunits. Fluorescence resonance energy transfer and immunogold EM studies confirmed that these transport proteins are within appropriate proximity for direct binding. Additionally, we demonstrate that there are functional consequences of this interaction, with inhibition of NCC affecting the function of ENaC. This novel finding of an association between ENaC and NCC could alter our understanding of salt transport in the distal tubule.

The Epithelial Sodium Channel and the Processes of Wound Healing

The epithelial sodium channel (ENaC) mediates passive sodium transport across the apical membranes of sodium absorbing epithelia, like the distal nephron, the intestine, and the lung airways. Additionally, the channel has been involved in the transduction of mechanical stimuli, such as hydrostatic pressure, membrane stretch, and shear stress from fluid flow. Thus, in vascular endothelium, it participates in the control of the vascular tone via its activity both as a sodium channel and as a shear stress transducer. Rather recently, ENaC has been shown to participate in the processes of wound healing, a role that may also involve its activities as sodium transporter and as mechanotransducer. Its presence as the sole channel mediating sodium transport in many tissues and the diversity of its functions probably underlie the complexity of its regulation. This brief review describes some aspects of ENaC regulation, comments on evidence about ENaC participation in wound healing, and suggests possible regulatory mechanisms involved in this participation.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

Pseudohypoaldosteronism type 1 and Liddle's syndrome mutations that affect the single-channel properties of the epithelial Na+ channel

These studies test whether three disease-causing mutations in genes (SCNN1A and SCNN1G) encoding subunits of the epithelial Na⁺ channel, ENaC, affect the biophysical and gating properties of this important renal ion channel. The S562P missense mutation in αENaC and the K106_S108delinsN mutation in γENaC are associated with pseudohypoaldosteronism type 1 (PHA1). The N530S missense mutation in γENaC causes Liddle’s syndrome. Incorporation of S562P into αENaC and K106_S108N into γENaC resulted in significant decreases in macroscopic ENaC currents. Conversely, incorporation of N530S into γENaC increased macroscopic ENaC current. The S562P substitution resulted in a nonfunctional channel. The K106_S108N mutation produced a functional channel having a normal macroscopic current–voltage relation, there was a slight but significant decrease in unitary conductance and a marked decrease in single-channel open probability. The N530S substitution increased single-channel open probability having no effect on the macroscopic current–voltage relation or unitary conductance of the channel. These findings are consistent with mutation of residues at 562 in αENaC and 530 in γENaC, and a 3′ splice site in SCNN1G (318-1 G→A; K106_108SdelinsN) resulting in aberrant ENaC activity due to changes in the biophysical and gating properties of the channel. Such changes likely contribute to the cellular mechanism underpinning the PHA1 and Liddle’s syndrome caused by these mutations in ENaC subunits.

Phosphoinositides regulate ion channels

Phosphoinositides serve as signature motifs for different cellular membranes and often are required for the function of membrane proteins. Here, we summarize clear evidence supporting the concept that many ion channels are regulated by membrane phosphoinositides. We describe tools used to test their dependence on phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, and consider mechanisms and biological meanings of phosphoinositide regulation of ion channels. This lipid regulation can underlie changes of channel activity and electrical excitability in response to receptors. Since different intracellular membranes have different lipid compositions, the activity of ion channels still in transit towards their final destination membrane may be suppressed until they reach an optimal lipid environment. This article is part of a Special Issue entitled Phosphoinositides.

Control of ENaC-mediated sodium reabsorption in the distal nephron by Bradykinin

Kinins, such as Bradykinin (BK), are peptide hormones of the kallikrein-kinin system. Apart from being a vasodilator, BK also increases urinary sodium excretion to reduce systemic blood pressure. It is becoming appreciated that BK modulates function of the epithelial Na(+) channel in the distal part of the renal nephron to affect tubular sodium reabsorption. In this chapter, we outline the molecular details, as well as discuss the physiological relevance of this regulation for the whole organism sodium homeostasis and setting chronic blood pressure.

Differential regulation of proton-sensitive ion channels by phospholipids: a comparative study between ASICs and TRPV1

Protons are released in pain-generating pathological conditions such as inflammation, ischemic stroke, infection, and cancer. During normal synaptic activities, protons are thought to play a role in neurotransmission processes. Acid-sensing ion channels (ASICs) are typical proton sensors in the central nervous system (CNS) and the peripheral nervous system (PNS). In addition to ASICs, capsaicin- and heat-activated transient receptor potential vanilloid 1 (TRPV1) channels can also mediate proton-mediated pain signaling. In spite of their importance in perception of pH fluctuations, the regulatory mechanisms of these proton-sensitive ion channels still need to be further investigated. Here, we compared regulation of ASICs and TRPV1 by membrane phosphoinositides, which are general cofactors of many receptors and ion channels. We observed that ASICs do not require membrane phosphatidylinositol 4-phosphate (PI(4)P) or phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for their function. However, TRPV1 currents were inhibited by simultaneous breakdown of PI(4)P and PI(4,5)P₂. By using a novel chimeric protein, CF-PTEN, that can specifically dephosphorylate at the D3 position of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃), we also observed that neither ASICs nor TRPV1 activities were altered by depletion of PI(3,4,5)P₃ in intact cells. Finally, we compared the effects of arachidonic acid (AA) on two proton-sensitive ion channels. We observed that AA potentiates the currents of both ASICs and TRPV1, but that they have different recovery aspects. In conclusion, ASICs and TRPV1 have different sensitivities toward membrane phospholipids, such as PI(4)P, PI(4,5)P₂, and AA, although they have common roles as proton sensors. Further investigation about the complementary roles and respective contributions of ASICs and TRPV1 in proton-mediated signaling is necessary.

Regulation of Na+ excretion and arterial blood pressure by purinergic signalling intrinsic to the distal nephron: consequences and mechanisms

Discretionary control of Na(+) excretion is a key component of the regulation of arterial blood pressure in mammals. Sodium excretion is fine-tuned in the aldosterone-sensitive distal nephron by the activity of the epithelial Na(+) channel (ENaC). Here, ENaC functions as a final effector of the renin-angiotensin-aldosterone system (RAAS) during negative feedback control of blood pressure. Mutations affecting ENaC activity and abnormal regulation of this channel affect blood pressure through pathological changes to Na(+) excretion. Recent evidence demonstrates that powerful signalling pathways function in parallel with the RAAS to modulate ENaC activity and blood pressure. An inclusive paradigm is emerging with respect to regulation of blood pressure where ENaC serves as a critical point of convergence for several important signalling systems that affect renal Na(+) excretion. A robust inhibitory purinergic signalling system intrinsic to the distal nephron dynamically regulates ENaC through paracrine ATP signalling via the metabotropic P2Y2 purinergic receptor to properly match urinary Na(+) excretion to dietary Na(+) intake. This enables blood pressure to be maintained within a normal range despite broad changes in dietary Na(+) consumption. Loss of purinergic inhibition of ENaC increases blood pressure by causing inappropriate Na(+) excretion. In contrast, stimulation of the P2Y2 receptor promotes natriuresis and a decrease in blood pressure. Such observations identify purinergic signalling in the distal nephron as possibly causative, when dysfunctional, for certain forms of elevated blood pressure, and as a possible therapeutic target for the treatment of elevated blood pressure particularly that associated with salt sensitivity.

Lovastatin-Induced Phosphatidylinositol-4-Phosphate 5-Kinase Diffusion from Microvilli Stimulates ROMK Channels

We recently showed that lovastatin attenuates cyclosporin A (CsA)-induced damage of cortical collecting duct (CCD) principal cells by reducing intracellular cholesterol. Previous studies showed that, in cell expression models or artificial membranes, exogenous cholesterol directly inhibits inward rectifier potassium channels, including Kir1.1 (Kcnj1; the gene locus for renal outer medullary K(+) [ROMK1] channels). Therefore, we hypothesized that lovastatin might stimulate ROMK1 by reducing cholesterol in CCD cells. Western blots showed that mpkCCDc14 cells express ROMK1 channels with molecular masses that approximate the molecular masses of ROMK1 in renal tubules detected before and after treatment with DTT. Confocal microscopy showed that ROMK1 channels were not in the microvilli, where cholesterol-rich lipid rafts are located, but rather, the planar regions of the apical membrane of mpkCCDc14 cells. Furthermore, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], an activator of ROMK channels, was detected mainly in the microvilli under resting conditions along with the kinase responsible for PI(4,5)P2 synthesis, phosphatidylinositol-4-phosphate 5-kinase, type I γ [PI(4)P5K I γ], which may explain the low basal open probability and increased sensitivity to tetraethylammonium observed here for this channel. Notably, lovastatin induced PI(4)P5K I γ diffusion into planar regions and elevated PI(4,5)P2 and ROMK1 open probability in these regions through a cholesterol-associated mechanism. However, exogenous cholesterol alone did not induce these effects. These results suggest that lovastatin stimulates ROMK1 channels, at least in part, by inducing PI(4,5)P2 synthesis in planar regions of the renal CCD cell apical membrane, suggesting that lovastatin could reduce cyclosporin-induced nephropathy and associated hyperkalemia.

Phosphoinositide control of membrane protein function: a frontier led by studies on ion channels

Anionic phospholipids are critical constituents of the inner leaflet of the plasma membrane, ensuring appropriate membrane topology of transmembrane proteins. Additionally, in eukaryotes, the negatively charged phosphoinositides serve as key signals not only through their hydrolysis products but also through direct control of transmembrane protein function. Direct phosphoinositide control of the activity of ion channels and transporters has been the most convincing case of the critical importance of phospholipid-protein interactions in the functional control of membrane proteins. Furthermore, second messengers, such as [Ca(2+)]i, or posttranslational modifications, such as phosphorylation, can directly or allosterically fine-tune phospholipid-protein interactions and modulate activity. Recent advances in structure determination of membrane proteins have allowed investigators to obtain complexes of ion channels with phosphoinositides and to use computational and experimental approaches to probe the dynamic mechanisms by which lipid-protein interactions control active and inactive protein states.

Direct regulation of ENaC by bradykinin in the distal nephron. Implications for renal sodium handling

PURPOSE OF REVIEW

Locally produced peptide hormones kinins, such as bradykinin, are thought to oppose many of the prohypertensive actions of the renin-angiotensin-aldosterone system. In the kidney, bradykinin, via stimulation of B2 receptors (B2R), favors natriuresis mostly due to the inhibition of tubular Na reabsorption. Recent experimental evidence identifies the epithelial Na channel (ENaC) as a key end effector of bradykinin actions in the distal tubular segments. The focus of this review is the physiological relevance and molecular details of the bradykinin signal to ENaC.

RECENT FINDINGS

The recent epidemiological GenSalt study demonstrated that genetic variants of the gene encoding B2R show significant associations with the salt sensitivity of blood pressure. Bradykinin was shown to have an inhibitory effect on the distal nephron sodium transport via stimulation of B2 receptor-phospholipase C (B2R-PLC) cascade to decrease ENaC open probability. Genetic ablation of bradykinin receptors in mice led to an augmented ENaC function, particularly during elevated sodium intake, likely contributing to the salt-sensitive hypertensive phenotype. Furthermore, augmentation of bradykinin signaling in the distal nephron was demonstrated to be an important component of the natriuretic and antihypertensive effects of angiotensin converting enzyme inhibition.

SUMMARY

Salt-sensitive inhibition of ENaC activity by bradykinin greatly advances our understanding of the molecular mechanisms that are responsible for shutting down distal tubule sodium reabsorption during volume expanded conditions to avoid salt-sensitive hypertension.

Cyp2c44 epoxygenase in the collecting duct is essential for the high K+ intake-induced antihypertensive effect

Cytochrome P-450, family 2, subfamily c, polypeptide 44 (Cyp2c44) epoxygenase metabolizes arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs) in kidney and vascular tissues. In the present study, we used real-time quantitative PCR techniques to examine the effect of high salt or high K(+) (HK) intake on the expression of Cyp2c44, a major Cyp2c epoxygenase in the mouse kidney. We detected Cyp2c44 in the proximal convoluted tubule, thick ascending limb, distal convoluted tubule (DCT)/connecting tubule (CNT), and collecting duct (CD). A high-salt diet increased the expression of Cyp2c44 in the thick ascending limb and DCT/CNT but not in the proximal convoluted tubule and CD. In contrast, an increase in dietary K(+) intake augmented Cyp2c44 expression only in the DCT/CNT and CD. Neither high salt nor HK intake had a significant effect on the blood pressure (BP) of wild-type mice. However, HK but not high salt intake increased BP in CD-specific, Cyp2c44 conditional knockout (KO) mice. Amiloride, an epithelial Na(+) channel (ENaC) inhibitor, normalized the BP of KO mice fed HK diets, suggesting that lack of Cyp2c44 in the CD enhances ENaC activity and increases Na(+) absorption in KO mice fed HK diets. This notion was supported by metabolic cage experiments demonstrating that renal Na(+) excretion was compromised in KO mice fed HK diets. Also, patch-clamp experiments demonstrated that 11,12-EET, a major Cyp2c44 product, but not AA inhibited ENaC activity in the cortical CD of KO mice. We conclude that Cyp2c44 in the CD is required for preventing the excessive Na(+) absorption induced by HK intake by inhibition of ENaC and facilitating renal Na(+) excretion.

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn²⁺, and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel’s large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB₁ cannabinoid receptors and 5-HT₂. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated. [BMB Reports 2013; 46(6): 295-304]

Resveratrol inhibits the epithelial sodium channel via phopshoinositides and AMP-activated protein kinase in kidney collecting duct cells

Resveratrol, a naturally occurring phytoalexin, has reported cardioprotective, anti-inflammatory, chemopreventative and antidiabetic properties. Several studies indicate the multiple effects of resveratrol on cellular function are due to its inhibition of class 1A phosphoinositide 3-kinase (PI3K) mediated signaling pathways, but it also activates AMP-activated protein kinase (AMPK). As sodium transport in the kidney via the Epithelial Sodium Channel (ENaC) is highly sensitive to changes in phosphoinositide signaling in the membrane and AMPK, we employed resveratrol to probe the relative effects of phosphatidylinositol species in the plasma membrane and AMPK activity and their impact on ENaC activity in mouse cortical collecting duct (mpkCCD_c14) cells. Here we demonstrate that resveratrol acutely reduces amiloride-sensitive current in mpkCCD_c14 cells. The time course and dose dependency of this inhibition paralleled depletion of the PI(3,4,5)P₃ reporter (AKT-PH) in live-cell microscopy, indicating the early inhibition is likely mediated by resveratrol's known effects on PI3K activity. Additionally, resveratrol induces a late inhibitory effect (4–24 hours) that appears to be mediated via AMPK activation. Resveratrol treatment induces significant AMPK activation compared with vehicle controls after 4 h, which persists through 16 h. Knockdown of AMPK or treatment with the AMPK inhibitor Compound C reduced the late phase of current reduction but had no effect on the early inhibitory activity of resveratrol. Collectively, these data demonstrate that resveratrol inhibits ENaC activity by a dual effect: an early reduction in activity seen within 5 minutes related to depletion of membrane PIP₃, and a sustained late (4–24 h) effect secondary to activation of AMPK.

The inhibitory effect of Gβγ and Gβ isoform 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.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Recording ion channels in isolated, split-opened tubules

Ion channels play key roles in physiology. They function as protein transducers able to transform stimuli and chemical gradients into electrical signals. They also are critical for cell signaling and play a particularly important role in epithelial transport acting as gateways for the movement of electrolytes across epithelial cell membranes. Experimental limitations, though, have hampered the recording of ion channel activity in many types of tissue. This has slowed progress in understanding the cellular and physiological function of these channels with most function inferred from in vitro systems and cell culture models. In many cases, such inferences have clouded rather than clarified the picture. Here, we describe a contemporary method for isolating and patch-clamping renal tubules for ex vivo analysis of ion channel function in native tissue. Focus is placed on quantifying the activity of the epithelial Na(+) channel (ENaC) in the aldosterone--sensitive distal nephron (ASDN). This isolated, split-open tubule preparation enables recording of renal ion channels in the close-to-native environment under the control of native cell signaling pathways and receptors. When combined with complementary measurements of organ and system function, and contemporary molecular genetics and pharmacology used to manipulate function and regulation, patch-clamping renal channels in the isolated, split-open tubule enables understanding to emerge about the physiological function of these key proteins from the molecule to the whole animal.

Impact of recovery from desensitization on acid-sensing ion channel-1a (ASIC1a) current and response to high frequency stimulation

BACKGROUND

Consecutive proton stimulation reduces ASIC1a peak currents leading to silencing of channels.

RESULTS

Kinetic analysis using a fast perfusion system shows that human ASIC1a has two desensitized states with markedly different stabilities.

CONCLUSION

High frequency trains of short stimuli prevent desensitization.

SIGNIFICANCE

The results predict steady ASIC1a responses to high frequency release of protons as in synaptic transmission. ASIC1a is a neuronal sodium channel activated by external H(+) ions. To date, all the characterization of ASIC1a has been conducted applying long H(+) stimuli lasting several seconds. Such experimental protocols weaken and even silence ASIC1a currents to repetitive stimulation. In this work, we examined ASIC1a currents by methods that use rapid application and removal of H(+). We found that brief H(+) stimuli, <100 ms, even if applied at high frequency, prevent desensitization thereby generate full and steady peak currents of human ASIC1a. Kinetic analysis of recovery from desensitization of hASIC1a revealed two desensitized states: short- and long-lasting with time constants of τ(Ds) ≤0.5 and τ(Dl) = 229 s, while in chicken ASIC1a the two desensitized states have similar values τ(D) 4.5 s. It is the large difference in stability of the two desensitized states that makes hASIC1a desensitization more pronounced and complex than in cASIC1a. Furthermore, recovery from desensitization was unrelated to cytosolic variations in pH, ATP, PIP(2), or redox state but was dependent on the hydrophobicity of key residues in the first transmembrane segment (TM1). In conclusion, brief H(+)-stimuli maintain steady the magnitude of peak currents thereby the ASIC1a channel is well poised to partake in high frequency signals in the brain.

Flow-sensitive K+-coupled ATP secretion modulates activity of the epithelial Na+ channel in the distal nephron

The epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) is under tonic inhibition by a local purinergic signaling system responding to changes in dietary sodium intake. Normal BK(Ca) channel function is required for flow-sensitive ATP secretion in the ASDN. We tested here whether ATP secreted through connexin channels in a coupled manner with K(+) efflux through BK(Ca) channels is required for inhibitory purinergic regulation of ENaC in response to increases in sodium intake. Inhibition of connexin channels relieves purinergic inhibition of ENaC. Deletion of the BK-β4 regulatory subunit, which is required for normal BK(Ca) channel function and flow-sensitive ATP secretion in the ASDN, suppresses increases in urinary ATP in response to increases in sodium intake. As a consequence, ENaC activity, particularly in the presence of high sodium intake, is inappropriately elevated in BK-β4 null mice. ENaC in BK-β4 null mice, however, responds normally to exogenous ATP, indicating that increases in activity do not result from end-organ resistance but rather from lowered urinary ATP. Consistent with this, disruption of purinergic regulation increases ENaC activity in wild type but not BK-β4 null mice. Consequently, sodium excretion is impaired in BK-β4 null mice. These results demonstrate that the ATP secreted in the ASDN in a BK(Ca) channel-dependent manner is physiologically available for purinergic inhibition of ENaC in response to changes in sodium homeostasis. Impaired sodium excretion resulting form loss of normal purinergic regulation of ENaC in BK-β4 null mice likely contributes to their elevated blood pressure.

Phosphatidylinositol phosphate-dependent regulation of Xenopus ENaC by MARCKS protein

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.

Intrinsic control of sodium excretion in the distal nephron by inhibitory purinergic regulation of the epithelial Na(+) channel

PURPOSE OF REVIEW

This review summarizes the new evidence for an intrinsic control system in the aldosterone-sensitive distal nephron in which purinergic signaling regulates sodium transport and governs renal sodium excretion.

RECENT FINDINGS

Electrophysiological studies identify epithelial Na(+) channels (ENaC) as final effectors of purinergic signaling via P2Y(2) receptors in the distal nephron. Inhibition of ENaC by autocrine/paracrine purinergic signaling reduces sodium reabsorption allowing an appropriately graded pressure-natriuresis response when delivery of sodium to the distal nephron is high. Disruption of this intrinsic control mechanism decreases sodium excretion and therefore has a prohypertensive effect. Because purinergic inhibition of ENaC is tonic yet submaximal, its enhancement increases sodium excretion and therefore has an antihypertensive action.

SUMMARY

Purinergic inhibitory regulation of ENaC is a key component of an intrinsic control system that enables the distal nephron to respond appropriately to the delivered load of sodium. This control system is physiologically important and functions in parallel with extrinsic control by the renin-angiotensin-aldosterone system, enabling sodium excretion to keep pace with sodium intake, especially when intake is high, and thereby maintaining arterial blood pressure. Disruption of intrinsic control of sodium transport by the distal nephron likely contributes to diseases such as arterial hypertension.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Angiotensin II stimulates epithelial sodium channels in the cortical collecting duct of the rat kidney

We examined the effect of angiotensin II (ANG II) on epithelial Na(+) channel (ENaC) in the rat cortical collecting duct (CCD) with single-channel and the perforated whole cell patch-clamp recording. Application of 50 nM ANG II increased ENaC activity, defined by NP(o) (a product of channel numbers and open probability), and the amiloride-sensitive whole cell Na currents by twofold. The stimulatory effect of ANG II on ENaC was absent in the presence of losartan, suggesting that the effect of ANG II on ENaC was mediated by ANG II type 1 receptor. Moreover, depletion of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM failed to abolish the stimulatory effect of ANG II on ENaC but inhibiting protein kinase C (PKC) abolished the effect of ANG II, suggesting that the effect of ANG II was the result of stimulating Ca(2+)-independent PKC. This notion was also suggested by the experiments in which stimulation of PKC with phorbol ester derivative mimicked the effect of ANG II and increased amiloride-sensitive Na currents in the principal cell, an effect that was not abolished by treatment of the CCD with BAPTA-AM. Also, inhibition of NADPH oxidase (NOX) with diphenyleneiodonium chloride abolished the stimulatory effect of ANG II on ENaC and application of superoxide donors, pyrogallol or xanthine and xanthine oxidase, significantly increased ENaC activity. Moreover, addition of ANG II or H(2)O(2) diminished the arachidonic acid (AA)-induced inhibition of ENaC in the CCD. We conclude that ANG II stimulates ENaC in the CCD through a Ca(2+)-independent PKC pathway that activates NOX thereby increasing superoxide generation. The stimulatory effect of ANG II on ENaC may be partially the result of blocking AA-induced inhibition of ENaC.

Epithelial Na(+) channel regulation by cytoplasmic and extracellular factors

Electrogenic Na(+) transport across high resistance epithelial is mediated by the epithelial Na(+) channel (ENaC). Our understanding of the mechanisms of ENaC regulation has continued to evolve over the two decades following the cloning of ENaC subunits. This review highlights many of the cellular and extracellular factors that regulate channel trafficking or gating.

Phosphatidylinositol 4,5-bisphosphate stimulates alveolar epithelial fluid clearance in male and female adult rats

Cell membrane phospholipids, like phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], can regulate epithelial Na channel (ENaC) activity. Gender differences in lung ENaC expression have also been demonstrated. However, the effects in vivo on alveolar fluid clearance are uncertain. Thus PI(4,5)P(2) effects on alveolar fluid clearance were studied in male and female rats. An isosmolar 5% albumin solution was intrapulmonary instilled; alveolar fluid clearance was studied for 1 h. Female rats had a 37 ± 19% higher baseline alveolar fluid clearance than male rats. Bilateral ovariectomy attenuated this gender difference. Compared with controls, PI(4,5)P(2) instillation (300 μM) increased alveolar fluid clearance by ∼93% in both genders. Amiloride or the specific αENaC small-interfering RNA inhibited baseline and PI(4,5)P(2)-stimulated alveolar fluid clearance in both genders, indicating a dependence on amiloride-sensitive pathways. The fraction of amiloride inhibition was greater in PI(4,5)P(2)-instilled rats (male: 64 ± 10%; female: 70 ± 11%) than in controls (male: 30 ± 6%; female: 44 ± 8%). PI(4,5)P(2) instillation lacked additional alveolar fluid clearance stimulation above that of terbutaline, nor did propranolol inhibit alveolar fluid clearance after PI(4,5)P(2) instillation, indicating that PI(4,5)P(2) stimulation was not secondary to endogenous β-adrenoceptor activation. PI(4,5)P(2) amine instillation resulted in an intermediate alveolar fluid clearance stimulation, suggesting that, to reach maximal alveolar fluid clearance stimulation, PI(4,5)P(2) must reside in cell membranes. In summary, PI(4,5)P(2) instillation upregulated in vivo alveolar fluid clearance similar to short-term β-adrenoceptor upregulation of alveolar fluid clearance. PI(4,5)P(2) stimulation was mediated partly by increased amiloride-sensitive Na transport. There exist important gender-related effects suggesting a female advantage that may have clinical implications for resolution of acute lung injury.

Voltage-dependent gating underlies loss of ENaC function in Pseudohypoaldosteronism type 1

Here we explore the mechanism and associated structure-function implications of loss of function for epithelial Na(+) channel (ENaC) containing a pseudohypoaldosteronism type 1 (PHA-1)-causing missense point mutation. As expected, human ENaC that contained subunits harboring PHA-1-causing substitutions within an absolutely conserved, cytosolic Gly residue (e.g., βG37S) had significantly less activity. Unexpectedly, though, such substitution also results in voltage sensitivity with greater activity at hyperpolarizing potentials. This is a consequence of voltage-dependent changes in the single-channel open probability and is not species- or subunit-dependent. Voltage sensitivity in PHA-1 mutants stems from the disruption of critical structure, rather than the development of new properties resulting from the introduction of novel side chains. Residues near the conserved His-Gly sequence of G95 in α-mENaC are particularly important for voltage sensing. Although substitution of I93 in α-mENaC results in voltage sensing, it also slows the activation and deactivation kinetics enough to enable capture of the dynamic changes in single-channel open probability that account for changes in macroscopic activity. This provides definitive proof of the mechanism that underlies loss of function. In addition, the voltage dependence of ENaC with PHA-1 substitutions is akin to that which results from substitution of a critical, interfacial Trp residue conserved at the intracellular base of TM1 (e.g., W112 in α-mENaC). Dynamic interactions between similarly positioned His and Trp residues are essential for gating and the girdle-like structure that lines the intracellular mouth of the M2 proton channel. The similar residues in ENaC may serve a shared function, suggesting the possibility of an intracellular girdle just below the mouth of the ENaC pore.

Bradykinin acutely inhibits activity of the epithelial Na+ channel in mammalian aldosterone-sensitive distal nephron

Activation of the renal kallikrein-kinin system results in natriuresis and diuresis, suggesting its possible role in renal tubular sodium transport regulation. Here, we used patch-clamp electrophysiology to directly assess the effects of bradykinin (BK) on the epithelial Na(+) channel (ENaC) activity in freshly isolated split-opened murine aldosterone-sensitive distal nephrons (ASDNs). BK acutely inhibits ENaC activity by reducing channel open probability (P(o)) in a dose-dependent and reversible manner. Inhibition of B2 receptors with icatibant (HOE-140) abolished BK actions on ENaC. In contrast, activation of B1 receptors with the selective agonist Lys-des-Arg(9)-BK failed to reproduce BK actions on ENaC. This is consistent with B2 receptors playing a critical role in mediating BK signaling to ENaC. BK has little effect on ENaC P(o) when G(q/11) was inhibited with Gp antagonist 2A. Moreover, inhibition of phospholipase C (PLC) with U73122, but not saturation of cellular cAMP levels with the membrane-permeable nonhydrolysable cAMP analog 8-cpt-cAMP, prevents BK actions on ENaC activity. This argues that BK stimulates B2 receptors with subsequent activation of G(q/11)-PLC signaling cascade to acutely inhibit ENaC activity. Activation of BK signaling acutely depletes apical PI(4,5)P(2) levels. However, inhibition of Ca(2+) pump SERCA of the endoplasmic reticulum with thapsigargin does not prevent BK signaling to ENaC. Furthermore, caffeine, while producing a similar rise in [Ca(2+)](i) as in response to BK stimulation, fails to recapitulate BK actions on ENaC. Therefore, we concluded that BK acutely inhibits ENaC P(o) in mammalian ASDN via stimulation of B2 receptors and following depletion of PI(4,5)P(2), but not increases in [Ca(2+)](i).

Insulin activates epithelial sodium channel (ENaC) via phosphoinositide 3-kinase in mammalian taste receptor cells

Diabetes is a profound disease that results in a severe lack of regulation of systemic salt and water balance. From our earlier work on the endocrine regulation of salt taste at the level of the epithelial sodium channel (ENaC), we have begun to investigate the ability of insulin to alter ENaC function with patch-clamp recording on isolated mouse taste receptor cells (TRCs). In fungiform and vallate TRCs that exhibit functional ENaC currents (e.g., amiloride-sensitive Na(+) influx), insulin (5-20 nM) caused a significant increase in Na(+) influx at -80 mV (EC(50) = 7.53 nM). The insulin-enhanced currents were inhibited by amiloride (30 μM). Similarly, in ratiometric Na(+) imaging using SBFI, insulin treatment (20 nM) enhanced Na(+) movement in TRCs, consistent with its action in electrophysiological assays. The ability of insulin to regulate ENaC function is dependent on the enzyme phosphoinositide 3-kinase since treatment with the inhibitor LY294002 (10 μM) abolished insulin-induced changes in ENaC. To test the role of insulin in the regulation of salt taste, we have characterized behavioral responses to NaCl using a mouse model of acute hyperinsulinemia. Insulin-treated mice show significant avoidance of NaCl at lower concentrations than the control group. Interestingly, these differences between groups were abolished when amiloride (100 μM) was added into NaCl solutions, suggesting that insulin was regulating ENaC. Our results are consistent with a role for insulin in maintaining functional expression of ENaC in mouse TRCs.