LY-294002-inhibitable PI 3-kinase and regulation of baseline rates of Na(+) transport in A6 epithelia

Dexamethasone and insulin activate serum and glucocorticoid-inducible kinase 1 (SGK1) via different molecular mechanisms in cortical collecting duct cells

Serum and glucocorticoid-inducible kinase 1 (SGK1) is a protein kinase that contributes to the hormonal control of renal Na(+) retention by regulating the abundance of epithelial Na(+) channels (ENaC) at the apical surface of the principal cells of the cortical collecting duct (CCD). Although glucocorticoids and insulin stimulate Na(+) transport by activating SGK1, the responses follow different time courses suggesting that these hormones act by different mechanisms. We therefore explored the signaling pathways that allow dexamethasone and insulin to stimulate Na(+) transport in mouse CCD cells (mpkCCDcl4). Dexamethasone evoked a progressive augmentation of electrogenic Na(+) transport that became apparent after ~45 min latency and was associated with increases in SGK1 activity and abundance and with increased expression of SGK1 mRNA Although the catalytic activity of SGK1 is maintained by phosphatidylinositol-OH-3-kinase (PI3K), dexamethasone had no effect upon PI3K activity. Insulin also stimulated Na(+) transport but this response occurred with no discernible latency. Moreover, although insulin also activated SGK1, it had no effect upon SGK1 protein or mRNA abundance. Insulin did, however, evoke a clear increase in cellular PI3K activity. Our data are consistent with earlier work, which shows that glucocorticoids regulate Na(+) retention by inducing sgk1 gene expression, and also establish that this occurs independently of increased PI3K activity. Insulin, on the other hand, stimulates Na(+) transport via a mechanism independent of sgk1 gene expression that involves PI3K activation. Although both hormones act via SGK1, our data show that they activate this kinase by distinct physiological mechanisms.

Phosphoinositide 3-kinase pathway mediates early aldosterone action on morphology and epithelial sodium channel in mammalian renal epithelia

Involvement of phosphoinositide 3-kinases (PI3Ks) in early aldosterone action on epithelial sodium channel (ENaC) in mammalian renal epithelia was investigated by hopping probe ion conductance microscopy combined with patch-clamping in this study. Aldosterone treatment enlarged the cell volume and elevated the apical membrane of renal mpkCCDc14 epithelia, which resulted in enhancing the open probability of ENaC. Inhibition of PI3K pathway by LY294002 obviously suppressed these aldosterone-induced changes in both cell morphology and ENaC activity. These results indicated the important role of PI3K pathway in early aldosterone action and the close relationship between cell morphology and ENaC activity in mammalian renal epithelia.

Cholera toxin enhances Na(+) absorption across MCF10A human mammary epithelia

Cellular mechanisms to account for the low Na(+) concentration in human milk are poorly defined. MCF10A cells, which were derived from human mammary epithelium and grown on permeable supports, exhibit amiloride- and benzamil-sensitive short-circuit current (Isc; a sensitive indicator of net ion transport), suggesting activity of the epithelial Na(+) channel ENaC. When cultured in the presence of cholera toxin (Ctx), MCF10A cells exhibit greater amiloride-sensitive Isc at all time points tested (2 h to 7 days), an effect that is not reduced with Ctx washout for 12 h. Amiloride-sensitive Isc remains elevated by Ctx in the presence of inhibitors for PKA (H-89, Rp-cAMP), PI3K (LY294002), and protein trafficking (brefeldin A). Additionally, the Ctx B subunit, alone, does not replicate these effects. RT-PCR and Western blot analyses indicate no significant increase in either the mRNA or protein expression for α-, β-, or, γ-ENaC subunits. Ctx increases the abundance of both β- and γ-ENaC in the apical membrane. Additionally, Ctx increases both phosphorylated and nonphosphorylated Nedd4-2 expression. These results demonstrate that human mammary epithelia express ENaC, which can account for the low Na(+) concentration in milk. Importantly, the results suggest that Ctx increases the expression but reduces the activity of the E3 ubiquitin ligase Nedd4-2, which would tend to reduce the ENaC retrieval and increase steady-state membrane residency. The results reveal a novel mechanism in human mammary gland epithelia by which Ctx regulates ENaC-mediated Na(+) transport, which may have inferences for epithelial ion transport regulation in other tissues throughout the body.

Epithelial Na⁺ channel activity in human airway epithelial cells: the role of serum and glucocorticoid-inducible kinase 1

BACKGROUND AND PURPOSE

Glucocorticoids appear to control Na⁺ absorption in pulmonary epithelial cells via a mechanism dependent upon serum and glucocorticoid-inducible kinase 1 (SGK1), a kinase that allows control over the surface abundance of epithelial Na⁺ channel subunits (α-, β- and γ-ENaC). However, not all data support this model and the present study re-evaluates this hypothesis in order to clarify the mechanism that allows glucocorticoids to control ENaC activity.

EXPERIMENTAL APPROACH

Electrophysiological studies explored the effects of agents that suppress SGK1 activity upon glucocorticoid-induced ENaC activity in H441 human airway epithelial cells, whilst analyses of extracted proteins explored the associated changes to the activities of endogenous protein kinase substrates and the overall/surface expression of ENaC subunits.

KEY RESULTS

Although dexamethasone-induced (24 h) ENaC activity was dependent upon SGK1, prolonged exposure to this glucocorticoid did not cause sustained activation of this kinase and neither did it induce a coordinated increase in the surface abundance of α-, β- and γ-ENaC. Brief (3 h) exposure to dexamethasone, on the other hand, did not evoke Na⁺ current but did activate SGK1 and cause SGK1-dependent increases in the surface abundance of α-, β- and γ-ENaC.

CONCLUSIONS AND IMPLICATIONS

Although glucocorticoids activated SGK1 and increased the surface abundance of α-, β- and γ-ENaC, these responses were transient and could not account for the sustained activation of ENaC. The maintenance of ENaC activity did, however, depend upon SGK1 and this protein kinase must therefore play an important but permissive role in glucocorticoid-induced ENaC activation.

Regulation of ENaC-mediated alveolar fluid clearance by insulin via PI3K/Akt pathway in LPS-induced acute lung injury

BACKGROUND

Stimulation of epithelial sodium channel (ENaC) increases Na(+) transport, a driving force of alveolar fluid clearance (AFC) to keep alveolar spaces free of edema fluid that is beneficial for acute lung injury (ALI). It is well recognized that regulation of ENaC by insulin via PI3K pathway, but the mechanism of this signaling pathway to regulate AFC and ENaC in ALI remains unclear. The aim of this study was to investigate the effect of insulin on AFC in ALI and clarify the pathway in which insulin regulates the expression of ENaC in vitro and in vivo.

METHODS

A model of ALI (LPS at a dose of 5.0 mg/kg) with non-hyperglycemia was established in Sprague-Dawley rats receiving continuous exogenous insulin by micro-osmotic pumps and wortmannin. The lungs were isolated for measurement of bronchoalveolar lavage fluid(BALF), total lung water content(TLW), and AFC after ALI for 8 hours. Alveolar epithelial type II cells were pre-incubated with LY294002, Akt inhibitor and SGK1 inhibitor 30 minutes before insulin treatment for 2 hours. The expressions of α-,β-, and γ-ENaC were detected by immunocytochemistry, reverse transcriptase polymerase chain reaction (RT-PCR) and western blotting.

RESULTS

In vivo, insulin decreased TLW, enchanced AFC, increased the expressions of α-,β-, and γ-ENaC and the level of phosphorylated Akt, attenuated lung injury and improved the survival rate in LPS-induced ALI, the effects of which were blocked by wortmannin. Amiloride, a sodium channel inhibitor, significantly reduced insulin-induced increase in AFC. In vitro, insulin increased the expressions of α-,β-, and γ-ENaC as well as the level of phosphorylated Akt but LY294002 and Akt inhibitor significantly prevented insulin-induced increase in the expression of ENaC and the level of phosphorylated Akt respectively. Immunoprecipitation studies showed that levels of Nedd4-2 binding to ENaC were decreased by insulin via PI3K/Akt pathway.

CONCLUSIONS

Our study demonstrated that insulin alleviated pulmonary edema and enhanced AFC by increasing the expression of ENaC that dependent upon PI3K/Akt pathway by inhibition of Nedd4-2.

Dysregulation of epithelial Na+ absorption induced by inhibition of the kinases TORC1 and TORC2

BACKGROUND AND PURPOSE

Although the serum and glucocorticoid-inducible protein kinase 1 (SGK1) appears to be involved in controlling epithelial Na(+) absorption, its role in this physiologically important ion transport process is undefined. As SGK1 activity is dependent upon target of rapamycin complex 2 (TORC2)-catalysed phosphorylation of SGK1-Ser(422) , we have explored the effects of inhibiting TORC2 and/or TORC1 upon the hormonal control of Na(+) absorption.

EXPERIMENTAL APPROACH

Na(+) absorption was quantified electrometrically in mouse cortical collecting duct cells (mpkCCD) grown to confluence on permeable membranes. Kinase activities were assessed by monitoring endogenous protein phosphorylation, with or without TORC1/2 inhibitors (TORIN1 and PP242) and the TORC1 inhibitor: rapamycin.

KEY RESULTS

Inhibition of TORC1/2 (TORIN1, PP242) suppressed basal SGK1 activity, prevented insulin- and dexamethasone-induced SGK1 activation, and caused modest (10-20%) inhibition of basal Na(+) absorption and substantial (∼80%) inhibition of insulin/dexamethasone-induced Na(+) transport. Inhibition of TORC1 did not impair SGK1 activation or insulin-induced Na(+) transport, but did inhibit (∼80%) dexamethasone-induced Na(+) absorption. Arginine vasopressin stimulated Na(+) absorption via a TORC1/2-independent mechanism.

CONCLUSION AND IMPLICATIONS

Target of rapamycin complex 2, but not TORC1, is important to SGK1 activation. Signalling via phosphoinositide-3-kinase/TORC2/SGK1 can explain insulin-induced Na(+) absorption. TORC2, but not TORC1, is also involved in glucocorticoid-induced SGK1 activation but its role is permissive. Glucocorticoid-induced Na(+) transport displayed a requirement for TORC1 activity. Therefore, TORC1 and TORC2 contribute to the regulation of Na(+) absorption. Pharmacological manipulation of TORC1/2 signalling may provide novel therapies for Na(+)-sensitive hypertension.

Effects of nominally selective inhibitors of the kinases PI3K, SGK1 and PKB on the insulin-dependent control of epithelial Na+ absorption

BACKGROUND AND PURPOSE

Insulin-induced Na(+) retention in the distal nephron may contribute to the development of oedema/hypertension in patients with type 2 diabetes. This response to insulin is usually attributed to phosphatidylinositol-3-kinase (PI3K)/serum and glucocorticoid-inducible kinase 1 (SGK1) but a role for protein kinase B (PKB) has been proposed. The present study therefore aimed to clarify the way in which insulin can evoke Na(+) retention.

EXPERIMENTAL APPROACH

We examined the effects of nominally selective inhibitors of PI3K (wortmannin, PI103, GDC-0941), SGK1 (GSK650394A) and PKB (Akti-1/2) on Na(+) transport in hormone-deprived and insulin-stimulated cortical collecting duct (mpkCCD) cells, while PI3K, SGK1 and PKB activities were assayed by monitoring the phosphorylation of endogenous proteins.

KEY RESULTS

Wortmannin substantially inhibited basal Na(+) transport whereas PI103 and GDC-0941 had only very small effects. However, these PI3K inhibitors all abolished insulin-induced Na(+) absorption and inactivated PI3K, SGK1 and PKB fully. GSK650394A and Akti-1/2 also inhibited insulin-evoked Na(+) absorption and while GSK650394A inhibited SGK1 without affecting PKB, Akti-1/2 inactivated both kinases.

CONCLUSION AND IMPLICATIONS

While studies undertaken using PI103 and GDC-0941 show that hormone-deprived cells can absorb Na(+) independently of PI3K, PI3K seems to be essential for insulin induced Na(+) transport. Akti-1/2 does not act as a selective inhibitor of PKB and data obtained using this compound must therefore be treated with caution. GSK650394A, on the other hand, selectively inhibits SGK1 and the finding that GSK650394A suppressed insulin-induced Na(+) absorption suggests that this response is dependent upon signalling via PI3K/SGK1.

Anionic phospholipids differentially regulate the epithelial sodium channel (ENaC) by interacting with alpha, beta, and gamma ENaC subunits

Anionic phospholipids (APs) present a variety of lipids in the cytoplasmic leaflet of the plasma membrane, including phosphatidylinositol (PI), PI-4-phosphate (PI(4)P), phosphatidylserine (PS), PI-4,5-bisphosphate (PI(4,5)P(2)), PI-3,4,5-trisphosphate (PI(3,4,5)P(3)), and phosphatidic acid (PA). We previously showed that PI(4,5)P(2) and PI(3,4,5)P(3) upregulate the renal epithelial sodium channel (ENaC). Further studies from others suggested that PI(4,5)P(2) and PI(3,4,5)P(3) respectively target beta- and gamma-ENaC subunit. To determine whether PI(4,5)P(2) and PI(3,4,5)P(3) selectively bind to beta and gamma subunit, we performed lipid-protein overlay experiments. Surprisingly, the results reveal that most APs, including PI(4)P, PS, PI(4,5)P(2), PI(3,4,5)P(3), and PA, but not PI, non-selectively bind to not only beta and gamma but also alpha subunit. To determine how these APs regulate ENaC, we performed inside-out patch-clamp experiments and found that PS, but not PI or PI(4)P, maintained ENaC activity, that PI(4,5)P(2) and PI(3,4,5)P(3) stimulated ENaC, and that PA, however, inhibited ENaC. These data together suggest that APs differentially regulate ENaC by physically interacting with alpha-, beta-, and gamma-ENaC. Further, the data from cell-attached patch-clamp and confocal microscopy experiments indicate that PA, a product of phospholipase D, may provide one of the pathways for inhibition of ENaC by endothelin receptors.

SGK1 activity in Na+ absorbing airway epithelial cells monitored by assaying NDRG1-Thr346/356/366 phosphorylation

Studies of HeLa cells and serum- and glucocorticoid-regulated kinase 1 (SGK1) knockout mice identified threonine residues in the n-myc downstream-regulated gene 1 protein (NDRG1-Thr(346/356/366)) that are phosphorylated by SGK1 but not by related kinases (Murray et al., Biochem J 385:1-12, 2005). We have, therefore, monitored the phosphorylation of NDRG1-Thr(346/356/366) in order to explore the changes in SGK1 activity associated with the induction and regulation of the glucocorticoid-dependent Na(+) conductance (G (Na)) in human airway epithelial cells. Transient expression of active (SGK1-S422D) and inactive (SGK1-K127A) SGK1 mutants confirmed that activating SGK1 stimulates NDRG1-Thr(346/356/366) phosphorylation. Although G (Na) is negligible in hormone-deprived cells, these cells displayed basal SGK1 activity that was sensitive to LY294002, an inhibitor of 3-phosphatidylinositol phosphate kinase (PI3K). Dexamethasone (0.2 muM) acutely activated SGK1 and the peak of this response (2-3 h) coincided with the induction of G (Na), and both responses were PI3K-dependent. While these data suggest that SGK1 might mediate the rise in G (Na), transient expression of the inactive SGK1-K127A mutant did not affect the hormonal induction of G (Na) but did suppress the activation of SGK1. Dexamethasone-treated cells grown on permeable supports formed confluent epithelial sheets that generated short circuit current due to electrogenic Na(+) absorption. Forskolin and insulin both stimulated this current and the response to insulin, but not forskolin, was LY294002-sensitive and associated with the activation of SGK1. While these data suggest that SGK1 is involved in the control of G (Na), its role may be minor, which could explain why sgk1 knockout has different effects upon different tissues.

Physiologic regulation of the epithelial sodium channel by phosphatidylinositides

PURPOSE OF REVIEW

Epithelial sodium channel (ENaC) activity is limiting for sodium reabsorption in the distal nephron. Humans regulate blood pressure by fine-tuning sodium balance through control of ENaC. ENaC dysfunction causes some hypertensive and renal salt wasting diseases. Thus, it is critical to understand the cellular mechanisms controlling ENaC activity.

RECENT FINDINGS

ENaC is sensitive to phosphatidylinositol 4,5-bisphosphate (PIP2), the target of phospholipase C-mediated metabolism, and phosphatidylinositiol 3,4,5-trisphosphate (PIP3), the product of phosphatidylinositide 3-OH kinase (PI3-K). PIP2 is permissive for ENaC gating possibly interacting directly with the channel. Activation of distal nephron P2Y receptors tempers ENaC activity by promoting PIP2 metabolism. This is important because gene deletion of P2Y2 receptors causes hypertension associated with hyperactive ENaC. Aldosterone, the final hormone in a negative-feedback cascade activated by decreases in blood pressure, increases ENaC activity. PIP3 sits at a critical bifurcation in the aldosterone-signaling cascade, increasing ENaC open probability and number. PIP3-effectors mediate increases in ENaC number by suppressing channel retrieval. PIP3 binds ENaC, at a site distinct from that important to PIP2 regulation, to modulate directly open probability.

SUMMARY

Phosphoinositides play key roles in physiologic control of ENaC and perhaps dysregulation plays a role in disease associated with abnormal renal sodium handling.

The regulation of selective and nonselective Na+ conductances in H441 human airway epithelial cells

Analysis of membrane currents recorded from hormone-deprived H441 cells showed that the membrane potential (V(m)) in single cells (approximately -80 mV) was unaffected by lowering [Na+]o or [Cl(-)]o, indicating that cellular Na+ and Cl(-) conductances (GNa and GCl, respectively) are negligible. Although insulin (20 nM, approximately 24 h) and dexamethasone (0.2 microM, approximately 24 h) both depolarized Vm by approximately 20 mV, the response to insulin reflected a rise in GCl mediated via phosphatidylinositol 3-kinase (PI3K) whereas dexamethasone acted by inducing a serum- and glucocorticoid-regulated kinase 1 (SGK1)-dependent rise in GNa. Although insulin stimulation/PI3K-P110 alpha expression did not directly increase GNa, these maneuvers augmented the dexamethasone-induced conductance. The glucocorticoid/SGK1-induced GNa in single cells discriminated poorly between Na+ and K+ (PNa/PK approximately 0.6), was insensitive to amiloride (1 mM), but was partially blocked by LaCl3 (La3+; 1 mM, approximately 80%), pimozide (0.1 mM, approximately 40%), and dichlorobenzamil (15 microM, approximately 15%). Cells growing as small groups, on the other hand, expressed an amiloride-sensitive (10 microM), selective GNa that displayed the same pattern of hormonal regulation as the nonselective conductance in single cells. These data therefore 1) confirm that H441 cells can express selective or nonselective GNa (14, 48), 2) show that these conductances are both induced by glucocorticoids/SGK1 and subject to PI3K-dependent regulation, and 3) establish that cell-cell contact is vitally important to the development of Na+ selectivity and amiloride sensitivity.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Acute regulation of the epithelial Na+ channel by phosphatidylinositide 3-OH kinase signaling in native collecting duct principal cells

Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) reabsorption in the aldosterone-sensitive distal nephron. Hormones, including aldosterone and insulin, increase ENaC activity, in part by stimulating phosphatidylinositide 3-OH kinase (PI3-K) signaling. Recent studies in heterologous expression systems reveal a close spatiotemporal coupling between PI3-K signaling and ENaC activity with the phospholipid product of this kinase, PI(3,4,5)P(3), in some cases, directly binding the channel and increasing open probability (P(o)). This study tested whether this tight coupling plays a physiologic role in modulating ENaC activity in native tissue and polarized epithelial cells. IGF-I was found to increase Na(+) reabsorption across mpkCCD(c14) principal cell monolayers in a PI3-K-sensitive manner. Inhibition of PI3-K signaling, moreover, rapidly decreased Na(+) reabsorption and ENaC activity in mpkCCD(c14) cells that were treated with corticosteroids and IGF-I. These decreases paralleled changes in apical membrane PI(3,4,5)P(3) levels, demonstrating tight spatiotemporal coupling between ENaC activity and PI3-K/PI(3,4,5)P(3) signaling within this membrane. For further probing of the mechanism underpinning this coupling, cortical collecting ducts (CCD) were isolated from rat and split open to expose the apical membrane for patch-clamp analysis. Inhibition of PI3-K signaling with wortmannin and LY294002 but not its inactive analogue rapidly and markedly decreased the P(o) of ENaC. Moreover, IGF-I acutely increased P(o) of ENaC in CCD principal cells in a PI3-K-sensitive manner. Together, these observations stress the importance of tight spatiotemporal coupling between PI3-K signaling and ENaC within the apical membrane of principal cells to the physiologic control of this ion channel.

Regulation of the epithelial Na+ channel (ENaC) by phosphatidylinositides

The epithelial Na(+) channel (ENaC) is an end-effector of diverse cellular signaling cascades, including those with phosphatidylinositide second messengers. Recent evidence also suggests that in some instances, phospatidylinositides can directly interact with ENaC to increase channel activity by increasing channel open probability and/or membrane localization. We review here findings relevant to regulation of ENaC by phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-triphosphate (PIP(3)). Similar to its actions on other ion channels, PIP(2) is permissive for ENaC openings having a direct effect on gating. The PIP(2) binding site in ENaC involved in this regulation is most likely localized to the NH(2) terminus of beta-ENaC. PIP(3) also affects ENaC gating but, rather than being permissive, augments open probability. The PIP(3) binding site in ENaC involved in this regulation is localized to the proximal region of the COOH terminus of gamma-ENaC just following the second transmembrane domain. In complementary pathways, PIP(3) also impacts ENaC membrane levels through both direct actions on the channel and via a signaling cascade involving phosphoinositide 3-OH kinase (PI3-K) and the aldosterone-induced gene product serum and glucocorticoid-inducible kinase. The putative PIP(3) binding site in ENaC involved in direct regulation of channel membrane levels has not yet been identified.

Endogenous protease activation of ENaC: effect of serine protease inhibition on ENaC single channel properties

Endogenous serine proteases have been reported to control the reabsorption of Na⁺ by kidney- and lung-derived epithelial cells via stimulation of electrogenic Na⁺ transport mediated by the epithelial Na⁺ channel (ENaC). In this study we investigated the effects of aprotinin on ENaC single channel properties using transepithelial fluctuation analysis in the amphibian kidney epithelium, A6. Aprotinin caused a time- and concentration-dependent inhibition (84 ± 10.5%) in the amiloride-sensitive sodium transport (I_Na) with a time constant of 18 min and half maximal inhibition constant of 1 μM. Analysis of amiloride analogue blocker–induced fluctuations in I_Na showed linear rate–concentration plots with identical blocker on and off rates in control and aprotinin-inhibited conditions. Verification of open-block kinetics allowed for the use of a pulse protocol method (Helman, S.I., X. Liu, K. Baldwin, B.L. Blazer-Yost, and W.J. Els. 1998. Am. J. Physiol. 274:C947–C957) to study the same cells under different conditions as well as the reversibility of the aprotinin effect on single channel properties. Aprotinin caused reversible changes in all three single channel properties but only the change in the number of open channels was consistent with the inhibition of I_Na. A 50% decrease in I_Na was accompanied by 50% increases in the single channel current and open probability but an 80% decrease in the number of open channels. Washout of aprotinin led to a time-dependent restoration of I_Na as well as the single channel properties to the control, pre-aprotinin, values. We conclude that protease regulation of I_Na is mediated by changes in the number of open channels in the apical membrane. The increase in the single channel current caused by protease inhibition can be explained by a hyperpolarization of the apical membrane potential as active Na⁺ channels are retrieved. The paradoxical increase in channel open probability caused by protease inhibition will require further investigation but does suggest a potential compensatory regulatory mechanism to maintain I_Na at some minimal threshold value.

Acute regulation of epithelial sodium channel by anionic phospholipids

Anionic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) are normally located in the inner leaflet of the plasma membrane, where these anionic phospholipids can regulate transmembrane proteins, including ion channels and transporters. Recent work has demonstrated that (1) ATP inhibits the renal epithelial sodium channel (ENaC) via a phospholipase C-dependent pathway that reduces PIP(2), (2) aldosterone stimulates ENaC via phosphoinositide 3-kinase, and (3) PIP(2) and PIP(3) regulate ENaC. Several lines of evidence show that ATP stimulation of purinergic P2Y receptors hydrolyzes PIP(2) and that aldosterone stimulation of steroid receptors induces PIP(3) formation. These studies together suggest that one primary mechanism for regulating ENaC is by alteration of anionic phospholipids and that the receptor-mediated and hormonal regulation of ENaC works through a variety of signaling pathways, but many of these pathways finally alter ENaC activity by regulating the formation or degradation of anionic phospholipids. Therefore, changes in the concentration of PIP(2) and PIP(3) are hypothesized to participate in the regulation of ENaC by purinergic and corticoid receptors. The underlying mechanism may be associated with a physical interaction of the positively charged cytoplasmic domains of the beta- and gamma-ENaC with the negatively charged membrane phospholipids. The exact nature of this interaction will require further investigation.

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

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

Hydrogen peroxide and epidermal growth factor activate phosphatidylinositol 3-kinase and increase sodium transport in A6 cell monolayers

Activation of phosphatidylinositol 3-kinase (PI 3-kinase) is required for insulin stimulation of sodium transport in A6 cell monolayers. In this study, we investigate whether stimulation of the PI 3-kinase by other agents also provoked an increase in sodium transport. Both epidermal growth factor (EGF) and H2O2provoked a rise in sodium transport that was inhibited by LY-294002, an inhibitor of PI 3-kinase activity. PI 3-kinase activity was estimated in extracts from A6 cell monolayers directly by performance of a PI 3-kinase assay. We also estimated the relative importance of the PI 3-kinase pathway by two different methods: 1) coprecipitation of the p85 regulatory subunit with anti-phosphotyrosine antibodies and 2) phosphorylation of PKB on both Ser 473 and Thr 308 residues observed by Western blotting. Since the mitogen-activated protein kinase (MAPK) pathway has also been implicated in the regulation of sodium transport, we also investigated whether this pathway is turned on by insulin, H2O2, or EGF. Phosphorylation of ERK1/2 was increased only transiently by insulin and H2O2but quite sustainedly by EGF. Inhibitors of this pathway (U-0126 and PD-98059) failed to affect the insulin and H2O2stimulation of sodium transport but increased substantially the stimulation induced by EGF. The latter effect was associated with an increase in PKB phosphorylation, thus suggesting that the stimulation of the MAPK pathway prevents, in part, the stimulation of the PI 3-kinase pathway in the transport of sodium stimulated by EGF.

Mechanisms of regulation of epithelial sodium channel by SGK1 in A6 cells

The serum and glucocorticoid induced kinase 1 (SGK1) participates in the regulation of sodium reabsorption in the distal segment of the renal tubule, where it may modify the function of the epithelial sodium channel (ENaC). The molecular mechanism underlying SGK1 regulation of ENaC in renal epithelial cells remains controversial. We have addressed this issue in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible system. Expression of a constitutively active mutant of SGK1 (SGK1^T_S425D) induced a sixfold increase in amiloride-sensitive short-circuit current (I_sc). Using noise analysis we demonstrate that SGK1 effect on I_sc is due to a fourfold increase in the number of functional ENaCs in the membrane and a 43% increase in channel open probability. Impedance analysis indicated that SGK1^T_S425D increased the absolute value of cell equivalent capacitance by an average of 13.7%. SGK1^T_S425D also produced a 1.6–1.9-fold increase in total and plasma membrane subunit abundance, without changing the half-life of channels in the membrane. We conclude that in contrast to aldosterone, where stimulation of transport can be explained simply by an increase in channel synthesis, SGK1 effects are more complex and involve at least three actions: (1) increase of ENaC open probability; (2) increase of subunit abundance within apical membranes and intracellular compartments; and (3) activation of one or more pools of preexistent channels within the apical membranes and/or intracellular compartments.

Ras couples phosphoinositide 3-OH kinase to the epithelial Na+ channel

Aldosterone induces the expression of the small G protein K-Ras. Both K-Ras and its 1st effector phosphoinositide 3-OH kinase (PI3-K) are necessary and sufficient for the activation of ENaC increasing channel open probability. The cell signaling mechanism by which K-Ras enhances ENaC activity, however, is uncertain. We demonstrate here that K-Ras significantly activates human ENaC reconstituted in Chinese hamster ovary cells approximately 3-fold. Activation in response to K-Ras was sensitive to the irreversible PI3-K inhibitor wortmannin but not the competitive LY294002 inhibitor of this phospholipid kinase. Similarly, a PI3-K 1st effector-specific Ras mutant (G12:C40) enhanced ENaC activity in a wortmannin but not LY294002 sensitive manner. Constitutively active PI3-K also enhanced ENaC activity but in a wortmannin and LY294002 sensitive manner with the effects of PI3-K and K-Ras not being additive. The activation of ENaC by PI3-K was also sensitive to intracellular GDPbetaS. Constitutively active PI3-K that is incapable of interacting with K-Ras (K227E p110alpha) acted as dominant negative with respect to the regulation of ENaC even in the presence of K-Ras. K-Ras is known to directly interact with PI3-K with aldosterone promoting this interaction. Here we demonstrate that K-Ras also interacts with ENaC through an, as yet, undetermined mechanism. We conclude that K-Ras enhances ENaC activity by localizing PI3-K near the channel and stimulating of PI3-K activity.

Phosphatidylinositol 3,4,5-trisphosphate: an early mediator of insulin-stimulated sodium transport in A6 cells

Insulin stimulates sodium transport across A6 epithelial cell monolayers. Activation of phosphatidylinositol 3-kinase (PI 3-kinase) was suggested as an early step in the insulin-stimulated sodium reabsorption (Ref. 35). To establish that the stimulation of the PI 3-kinase signaling cascade is causing stimulation of apical epithelial Na channel, we added permeant forms of phosphatidylinositol (PI) phosphate (P) derivatives complexed with a histone carrier to A6 epithelium. Only PIP3and PI( 3 , 4 )P2but not PI( 4 , 5 )P2stimulated sodium transport, although each of them penetrated into A6 cell monolayers as assessed using fluorescent permeant phosphoinositides derivatives. By Western blot analysis of A6 cell extracts, the inositol 3-phosphatase PTEN and the protein kinase B PKB were both detected. To further establish that the stimulation of sodium transport induced by insulin is related to PIP3levels, we transfected A6 cells with human PTEN cDNA and observed a 30% decrease in the natriferic effect of insulin. Similarly, the increase in sodium transport observed by addition of permeant PIP3was also reduced by 30% in PTEN-overexpressing cells. PKB, a main downstream effector of PI 3-kinase, was phosphorylated at both Thr 308 and Ser 473 residues upon insulin stimulation of the A6 cell monolayer. PKB phosphorylation in response to insulin stimulation was reduced in PTEN-overexpressing cells. Permeant PIP3also increased PKB phosphorylation. Taken together, the present results establish that the d-3-phosphorylated phosphoinositides PIP3and PI( 3 , 4 )P2mediate the effect of insulin on sodium transport across A6 cell monolayers.

Ras activates the epithelial Na(+) channel through phosphoinositide 3-OH kinase signaling

Aldosterone induces expression and activation of the GTP-dependent signaling switch K-Ras. This small monomeric G protein is both necessary and sufficient for activation of the epithelial Na(+) channel (ENaC). The mechanism by which K-Ras enhances ENaC activity, however, is uncertain. We demonstrate here that K-Ras activates human ENaC reconstituted in Chinese hamster ovary cells in a GTP-dependent manner. K-Ras influences ENaC activity most likely by affecting open probability. Inhibition of phosphoinositide 3-OH kinase (PI3K) abolished K-Ras actions on ENaC. In contrast, inhibition of other K-Ras effector cascades, including the MAPK and Ral/Rac/Rho cascades, did not affect K-Ras actions on ENaC. Activation of ENaC by K-Ras, moreover, was sensitive to co-expression of dominant negative p85(PI3K). The G12:C40 effector-specific double mutant of Ras, which preferentially activates PI3K, enhanced ENaC activity in a manner sensitive to inhibition of PI3K. Other effector-specific mutants preferentially activating MAPK and RalGDS signaling had no effect. Constitutively active PI3K activated ENaC independent of K-Ras with the effects of PI3K and K-Ras on ENaC not being additive. We conclude that K-Ras activates ENaC via the PI3K cascade.

Regulation of Na+ transport by aldosterone: signaling convergence and cross talk between the PI3-K and MAPK1/2 cascades

Cross talk between the phosphatidylinositol 3-kinase (PI3-K) and mitogen-activating protein kinase (MAPK)1/2 signaling cascades in response to aldosterone-induced K-RasA was investigated in renal A6 epithelial cells. In addition, the contribution of these signaling pathways to aldosterone-stimulated Na+ transport was investigated. Aldosterone increased active K-RasA levels in A6 cells resulting in activation of downstream effectors in both the MAPK1/2 and PI3-K cascades with K-RasA directly interacting with the catalytic p110 subunit of PI3-K in a steroid-dependent manner. Aldosterone-stimulated PI3-K signaling impinged on the MAPK1/2 cascade at the level of Akt-mediated phosphorylation of c-Raf at an established negative regulatory site. Aldosterone also increased Sgk levels as well as stimulated phosphorylation of this kinase in a PI3-K- and K-RasA-dependent manner. Blockade of MAPK1/2 signaling had little effect on Na+ transport. Conversely, inhibition of PI3-K markedly suppressed transport. Likewise, suppression of K-RasA induction decreased transport. However, Na+ transport was subsequently stimulated under these conditions with the PLA2 inhibitor aristolochic acid, an established positive modulator of Na+ transport, suggesting that K-RasA signaling through PI3-K does not directly affect epithelial sodium channel (ENaC) levels but the activity of this channel. Consistent with this possibility, activity of ENaC reconstituted in Chinese hamster ovary cells was increased by coexpression of constitutively active PI3-K. The current study demonstrates that aldosterone increases Na+ transport, in part, by stimulating PI3-K signaling and that during aldosterone actions, there is both signaling convergence between the two aldosterone-induced proteins, K-RasA and Sgk, as well as cross talk between the PI3-K and MAPK1/2 cascades with the prior but not latter cascade enhancing ENaC activity.

Direct Activation of the Epithelial Na+ Channel by Phosphatidylinositol 3,4,5-Trisphosphate and Phosphatidylinositol 3,4-Bisphosphate Produced by Phosphoinositide 3-OH Kinase

The phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) is accepted to be a direct modulator of ion channel activity. The products of phosphoinositide 3-OH kinase (PI3K), PtdIns(3,4)P(2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), in contrast, are not. We report here activation of the epithelial Na(+) channel (ENaC) reconstituted in Chinese hamster ovary cells by PI3K. Insulin-like growth factor-I also activated reconstituted ENaC and increased Na(+) reabsorption across renal A6 epithelial cell monolayers via PI3K. Neither IGF-I nor PI3K affected the levels of ENaC in the plasma membrane. The effects of PI3K and IGF-I on ENaC activity paralleled changes in the plasma membrane levels of the PI3K product phospholipids, PtdIns(3,4)P(2)/PtdIns(3,4,5)P(3), as measured by evanescent field fluorescence microscopy. Both PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3) activated ENaC in excised patches. Activation of ENaC by PI3K and its phospholipid products corresponded to changes in channel open probability. We conclude that PI3K directly modulates ENaC activity via PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3). This represents a novel transduction pathway whereby growth factors, such as IGF-I, rapidly modulate target proteins independent of signaling elicited by kinases downstream of PI3K.

Distribution and regulation of expression of serum- and glucocorticoid-induced kinase-1 in the rat kidney

The serum- and glucocorticoid-induced kinase-1 (sgk1) increases the activity of a number of epithelial ion channels and transporters. The present study examines the distribution and subcellular localization of sgk1 protein in the rat kidney and the regulation of levels of expression induced by steroids. The results indicate that the kidney expresses predominantly the sgk1 isoform with a distribution restricted to the thick ascending limb of Henle, distal convoluted, connecting and cortical collecting tubules. Within cells, sgk1 strongly associates with the microsomal fraction of homogenates and it colocalizes with the Na+,K+-ATPase to the basolateral membrane. Analysis of the levels of expression of sgk1 by Western blotting and immunohistochemistry indicates constitutive high expression under basal conditions. Approximately half of the basal level is maintained by glucocorticoids whereas physiological fluctuations of aldosterone produce minor changes in sgk1 abundance in adrenal-intact animals. These results do not support the notion that physiological changes of aldosterone concentration turn the expression of sgk1 'on and off' in the mammalian kidney. Additionally, localization of sgk1 to the basolateral membrane indicates that the effects mediated by sgk1 do not require a direct interaction with the ion channels and transporters whose activity is modulated, since most of these proteins are located in the apical membrane of renal epithelial cells.

Insulin-stimulated trafficking of ENaC in renal cells requires PI 3-kinase activity

AlphaENaC-EGFP (enhanced green fluorescent protein-tagged alpha-subunit of the epithelial Na(+) channel) stably transfected clonal lines derived from the A6 parental cell line were used to study the physical mechanisms of insulin-stimulated Na(+) transport. Within 1 min of insulin stimulation, ENaC migrates from a diffuse cytoplasmic localization to the apical and lateral membranes. Concurrently, after insulin stimulation, phosphatidylinositol 3-kinase (PI 3-kinase) is colocalized with ENaC on the lateral but not apical membrane. An inhibitor of PI 3-kinase, LY-294002, does not inhibit ENaC/PI 3-kinase colocalization but does alter the intracellular site of the colocalization, preventing the translocation of ENaC to the lateral and apical membranes. These data show that insulin stimulation causes the migration of ENaC to the lateral and apical cell membranes and that this trafficking is dependent on PI 3-kinase activity.

Hormone-regulated transepithelial Na+ transport in mammalian CCD cells requires SGK1 expression

To study the role of serum and glucocorticoid-inducible kinase-1 (SGK1) in mammalian cells, we compared Na(+) transport rates in wild-type (WT) M1 cortical collecting duct cells with M1 populations stably expressing human full-length SGK1, NH(2)-terminal truncated (DeltaN-60) SGK1, "kinase-dead" (K127M) SGK1, and cells that have downregulated levels of SGK1 mRNA (antisense SGK1). Basal rates of transepithelial Na(+) transport were highest in full-length SGK1 populations, compared among the above populations. Dexamethasone treatment increased Na(+) transport in WT and full-length SGK1 cells 2.7- and 2-fold, respectively. Modest stimulation of Na(+) absorption was detected after dexamethasone treatment in DeltaN-60 SGK1 populations. However, DeltaN-60 SGK1 transport rates remained substantially lower than WT values. Importantly, a combination of high insulin, dexamethasone, and serum failed to significantly stimulate Na(+) transport in antisense or K127M SGK1 cells. Additionally, expression of antisense SGK1 significantly decreased transepithelial resistance values. Overall, we concluded that SGK1 is a critical component in corticosteroid-regulated Na(+) transport in mammalian cortical collecting duct cells. Furthermore, our data suggest that the NH(2) terminus of SGK1 may contain a Phox homology-like domain that may be necessary for effective Na(+) transport.

Role of SGK in hormonal regulation of epithelial sodium channel in A6 cells

The purpose of this study was to examine the role of the serum- and glucocorticoid-induced kinase (SGK) in the activation of the epithelial sodium channel (ENaC) by aldosterone, arginine vasopressin (AVP), and insulin. We used a tetracycline-inducible system to control the expression of wild-type (SGK(wt)(T)), constitutively active (S425D mutation; SGK(S425D)(T)), or inactive (K130M mutation; SGK(K130M)(T)) SGK in A6 cells independently of hormonal stimulation. The effect of SGK expression on ENaC activity was monitored by measuring transepithelial amiloride-sensitive short-circuit current (I(sc)) of transfected A6 cell lines. Expression of SGK(wt)(T) or SGK(S425D)(T) and aldosterone stimulation have additive effects on I(sc). Although SGK could play some role in the aldosterone response, our results suggest that other mechanisms take place. SGK(S425D)(T) abrogates the responses to AVP and insulin; hence, in the signaling pathways of these hormones there is a shared step that is stimulated by SGK. Because AVP and insulin induce fusion of vesicles to the apical membrane, our results support the notion that SGK promotes incorporation of channels in the apical membrane.

Role of PKCalpha in feedback regulation of Na(+) transport in an electrically tight epithelium

It has long been known that Na(+) channels in electrically tight epithelia are regulated by homeostatic mechanisms that maintain a steady state and allow new levels of transport to be sustained in hormonally challenged cells. Little is known about the potential pathways involved in these processes. In addition to short-term effect, recent evidence also indicates the involvement of PKC in the long-term regulation of the epithelial Na(+) channel (ENaC) at the protein level (40). To determine whether stimulation of ENaC involves feedback regulation of PKC levels, we utilized Western blot analysis to determine the distribution of PKC isoforms in polarized A6 epithelia. We found the presence of PKC isoforms in the conventional (alpha and gamma), novel (delta, eta, and epsilon), and atypical (iota, lambda, and zeta) groups. Steady-state stimulation of Na(+) transport with aldosterone was accompanied by a specific decrease of PKCalpha protein levels in both the cytoplasmic and membrane fractions. Similarly, overnight treatment with an uncharged amiloride analog (CDPC), a procedure that through feedback regulation causes a stimulation of Na(+) transport, also decreased PKCalpha levels. These effects were additive, indicating separate mechanisms that converge at the level of PKCalpha. These effects were not accompanied by changes of PKCalpha mRNA levels as determined by Northern blot analysis. We propose that this may represent a novel regulatory feedback mechanism necessary for sustaining an increase of Na(+) transport.

Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates epithelial sodium channel activity in A6 cells

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a membrane lipid found in all eukaryotic cells, which regulates many important cellular processes, including ion channel activity. In this study, we used inside-out patch clamp technique, immunoprecipitation, and Western blot analysis to investigate the effect of PIP(2) on epithelial sodium channel activity in A6 cells. A6 cells were cultured in media supplemented with 1.5 microm aldosterone. Single sodium channel activity in excised, inside-out patches was increased by perfusion of the bath solution with 30 microm PIP(2) plus 100 microm GTP (NP(o) = 1.34 +/- 0.14) compared with the paired control (NP(o) = 0.09 +/- 0.02). However, neither 30 microm PIP(2) (NP(o) = 0.11 +/- 0.02) nor 100 microm GTP (NP(o) = 0.10 +/- 0.02) alone stimulated the sodium channels. The PIP(2)-stimulated channel activity was abolished by application of 10 nm G protein betagamma subunits (NP(o) = 0.14 +/- 0.05). However, 10 nm Galpha(i-3) + 30 microm PIP(2) increased both NP(o) and P(o). The stimulating effect of 10 nm Galpha(i-3) + 30 microm PIP(2) is similar to that of 30 microm PIP(2) plus 100 microm GTP. Immunoprecipitation and Western blot analysis show that both Gi(alpha-3) and PIP(2) bind beta and gamma epithelial Na(+) channels (ENaC), but not alpha ENaC. These results indicate that PIP(2) increases ENaC activity by direct interaction with beta or gamma xENaC in the presence of Galpha(i-3).

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

Anionic phospholipids regulate native and expressed epithelial sodium channel (ENaC)

Using patch clamp techniques, we found that the epithelial sodium channel (ENaC) activity in the apical membrane of A6 distal nephron cells showed a sudden rundown beginning at 4 min after forming the inside-out configuration. This sudden rundown was prevented by addition of anionic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)), phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), and phosphatidylserine (PS) to the "cytoplasmic" bath. Conversely, chelation of endogenous PIP(2) with anti-PIP(2) antibody, hydrolysis of PIP(2) with either exogenous phospholipase C (PLC) or activation of endogenous PLC by extracellular ATP, or application of the positively charged molecule, poly-L-lysine, accelerated channel rundown. However, neutral phosphatidylcholine had no effect on ENaC activity. By two-electrode voltage clamp recordings, we demonstrated that PIP(2) and PIP(3) significantly increased amiloride-sensitive current in Xenopus oocytes injected with cRNAs of rat alpha-, beta-, and gamma-ENaC. However, PIP(2) and PIP(3) did not affect surface expression of ENaC, indicating that PIP(2) and PIP(3) regulate ENaC at the level of the inner plasma membrane through a mechanism that is independent of ENaC trafficking. These data suggest that anionic phospholipids may mediate the regulation of ENaC by PLC- or phosphoinositide 3-kinase-coupled receptors.

Mediators of aldosterone action in the renal tubule

The aldosterone-sensitive distal nephron extends from the second part of the distal convoluted tubule to the inner medullary collecting duct. As recently shown, aldosterone increases within two hours the abundance of the alpha-subunit of the epithelial sodium channel along the entire aldosterone-sensitive distal nephron, whereas it induces only in an initial portion of the aldosterone-sensitive distal nephron an apical translocation of all three epithelial sodium channel subunits. This suggests that another factor or factors determines the length of the aldosterone-sensitive distal nephron portion in which aldosterone controls epithelial sodium channel surface expression. Since the glucocorticoid-induced kinase SGK1 was identified as aldosterone-induced protein in 1999, it has been postulated to play a key regulatory role. The in-vivo localization of its induction to segment-specific cells of the aldosterone-sensitive distal nephron, and the in-vitro correlation of the amount of its hyperphosphorylated form with transepithelial sodium transport, support this hypothesis. Other recent studies unravel pathways other than those activated by aldosterone and insulin that impact on SGK1 expression and/or function, and thus shed some light onto the complex network that appears to control sodium transport. In view of the ongoing research, the question of how, and formally also whether, SGK1 acts on the epithelial sodium channel should be resolved in the near future.

PGE(2) activation of apical membrane Cl(-) channels in A6 epithelia: impedance analysis

Measurements of transepithelial electrical impedance of continuously short-circuited A6 epithelia were made at audio frequencies (0.244 Hz to 10.45 kHz) to investigate the time course and extent to which prostaglandin E(2) (PGE(2)) modulates Cl(-) transport and apical membrane capacitance in this cell-cultured model epithelium. Apical and basolateral membrane resistances were determined by nonlinear curve-fitting of the impedance vectors at relatively low frequencies (<50 Hz) to equations (Păunescu, T. G., and S. I. Helman. 2001. Biophys. J. 81:838--851) where depressed Nyquist impedance semicircles were characteristic of the membrane impedances under control Na(+)-transporting and amiloride-inhibited conditions. In all tissues (control, amiloride-blocked, and amiloride-blocked and furosemide-pretreated), PGE(2) caused relatively small (< approximately 3 microA/cm(2)) and rapid (<60 s) maximal increase of chloride current due to activation of a rather large increase of apical membrane conductance that preceded significant activation of Na(+) transport through amiloride-sensitive epithelial Na(+) channels (ENaCs). Apical membrane capacitance was frequency-dependent with a Cole-Cole dielectric dispersion whose relaxation frequency was near 150 Hz. Analysis of the time-dependent changes of the complex frequency-dependent equivalent capacitance of the cells at frequencies >1.5 kHz revealed that the mean 9.8% increase of capacitance caused by PGE(2) was not correlated in time with activation of chloride conductance, but rather correlated with activation of apical membrane Na(+) transport.

cAmp activation of apical membrane Cl(-) channels: theoretical considerations for impedance analysis

Transepithelial electrical impedance analysis provides a sensitive method to evaluate the conductances and capacitances of apical and basolateral plasma membranes of epithelial cells. Impedance analysis is complicated, due not only to the anatomical arrangement of the cells and their paracellular shunt pathways, but also in particular to the existence of audio frequency-dependent capacitances or dispersions. In this paper we explore implications and consequences of anatomically related Maxwell-Wagner and Cole-Cole dielectric dispersions that impose limitations, approximations, and pitfalls of impedance analysis when tissues are studied under widely ranging spontaneous rates of transport, and in particular when apical membrane sodium and chloride channels are activated by adenosine 3',5'-cyclic monophosphate (cAMP) in A6 epithelia. We develop the thesis that capacitive relaxation processes of any origin lead not only to dependence on frequency of the impedance locus, but also to the appearance of depressed semicircles in Nyquist transepithelial impedance plots, regardless of the tightness or leakiness of the paracellular shunt pathways. Frequency dependence of capacitance precludes analysis of data in traditional ways, where capacitance is assumed constant, and is especially important when apical and/or basolateral membranes exhibit one or more dielectric dispersions.

Sodium reabsorption in aldosterone-sensitive distal nephron: news and contributions from genetically engineered animals

The precise adaptation of renal sodium excretion to systemic needs is to a large extent achieved by the regulation of sodium re-absorption in the aldosterone-sensitive distal nephron. Transcellular sodium re-absorption by the segment-specific cells of the aldosterone-sensitive distal nephron (often called principal cells) is mainly controlled at the level of the expression and activity levels of the epithelial sodium channel, the apical amiloride-sensitive sodium influx pathway. Recent investigations have identified the first early aldosterone-induced proteins that act on epithelial sodium channel function in expression systems. Indirect evidence suggests that one of these aldosterone-induced proteins, the serum- and glucocorticoid-inducible protein kinase SGK1, plays a central integratory role in the control of epithelial sodium channel surface expression and activity, also in the mammalian kidney. Gene-modified animals lacking epithelial sodium channel subunits or expressing mutant subunits have substantiated the central role of the epithelial sodium channel in sodium re-absorption and blood pressure control, as well as for neonatal lung liquid clearance. Mice overexpressing or lacking specific hormones or their receptors have been used to study their role in sodium transport regulation, but the study of mouse physiology appears to lag behind the generation of gene-modified mice. Nonetheless, these new animal models have had a strong impact on research, by stimulating the integration of knowledge and techniques learned from reductionistic molecular approaches into tissue and animal studies, thus breaking down barriers and stimulating collaborations.