Characterization and imaging of A6 epithelial cell clones expressing fluorescently labeled ENaC subunits

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.

A review of reagents for fluorescence microscopy of cellular compartments and structures, Part III: reagents for actin, tubulin, cellular membranes, and whole cell and cytoplasm

Non-antibody commercial fluorescent reagents for imaging of cytoskeletal structures have been limited primarily to tubulin and actin, with the main factor in choice based mainly on whether cells are live or fixed and permeabilized. A wider range of options exist for cell membrane dyes, and the choice of reagent primarily depends on the preferred localization in the cell (i.e., all membranes or only the plasma membrane) and usage (i.e., whether the protocol involves fixation and permeabilization). For whole-cell or cytoplasmic imaging, the choice of reagent is determined mostly by the length of time that the cells need to be visualized (hours or days) and by fixation status. Presented here is a discussion on choosing commercially available reagents for these cellular structures, with an emphasis on use for microscopic imaging, with a featured reagent for each structure, a recommended protocol, troubleshooting guide, and example image.

Lipid coated mesoporous silica nanoparticles as photosensitive drug carriers

This paper presents a strategy for the biofunctionalization of novel photosensitizer carriers, mesoporous silica nanoparticles (MSNs). After being calcined and absorbed with photosensitizers (hypocrellin B, HB), MSNs can be coated with a lipid layer. Transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) results proved that HB molecules can be loaded into MSN porous and lipid can coated on the surface of the nanoparticles. When co-cultured with cancer cells (MCF-7), MSNs can transport HB molecules into cells and present low cytotoxicity. With the introduction of a lipid layer, the efficiency of MSN uptake by cells can be improved. These intracellular HB-loaded MSN materials also present cytotoxicity to MCF-7 cells after light irradiation which indicates the materials can be used as good photosensitizer carriers in photodynamic therapy.

A human polymorphism affects NEDD4L subcellular targeting by leading to two isoforms that contain or lack a C2 domain

Background

Ubiquitination serves multiple cellular functions, including proteasomal degradation and the control of stability, function, and intracellular localization of a wide variety of proteins. NEDD4L is a member of the HECT class of E3 ubiquitin ligases. A defining feature of NEDD4L protein isoforms is the presence or absence of an amino-terminal C2 domain, a class of subcellular, calcium-dependent targeting domains. We previously identified a common variant in human NEDD4L that generates isoforms that contain or lack a C2 domain.

Results

To address the potential functional significance of the NEDD4L common variant on NEDD4L subcellular localization, NEDD4L isoforms that either contained or lacked a C2 domain were tagged with enhanced green fluorescent protein, transfected into Xenopus laevis kidney epithelial cells, and imaged by performing confocal microscopy on live cells. We report that the presence or absence of this C2 domain exerts differential effects on the subcellular distribution of NEDD4L, the ability of C2 containing and lacking NEDD4L isoforms to mobilize in response to a calcium stimulus, and the intracellular transport of subunits of the NEDD4L substrate, ENaC. Furthermore, the ability of the C2-containing isoform to influence β-ENaC mobilization from intracellular pools involves the NEDD4L active site for ubiquitination. We propose a model to account for the potential impact of this common genetic variant on protein function at the cellular level.

Conclusion

NEDD4L isoforms that contain or lack a C2 domain target different intracellular locations. Additionally, whereas the C2-containing NEDD4L isoform is capable of shuttling between the plasma membrane and intracellular compartments in response to calcium stimulus the C2-lacking isoform can not. The C2-containing isoform differentially affects the mobilization of ENaC subunits from intracellular pools and this trafficking step requires NEDD4L ubiquitin ligase activity. This observation suggests a new mechanism for the requirement for the PY motif in cAMP-mediated exocytosis of ENaC. We have elucidated how a common genetic variant can underlie significant functional diversity in NEDD4L at the cellular level. We propose a model that describes how that functional variation may influence blood pressure. Moreover, our observations regarding differential function of the NEDD4L isoforms may impact other aspects of physiology that involve this ubiquitin ligase.

The role of the phosphoinositide pathway in hormonal regulation of the epithelial sodium channel

In summary, insulin and aldosterone stimulate phosphatidylinositol phosphorylation, thus indicating the existence of a regulated protein at or before the PI3-kinase step. Aldosterone induces the synthesis of sgk, a downstream element of the PI pathway. Sgk is necessary, but not rate-limiting, for aldosterone- and insulin-stimulated Na+ transport. However, the enzyme appears to be rate-limiting for the natriferic action of ADH. Insulin-stimulated Na+ transport, an acute response, is dependent on PI3-kinase activity but the magnitude of the response is not altered by a cellular excess of sgk. ADH-stimulated transport is not dependent on PI3-kinase but is potentiated by an excess of sgk. The foregoing data indicate that the PI pathway is involved in several steps of the natriferic action of hormones and intersects with other pathways which regulate ENaC. Furthermore, the data are consistent with the hypothesis that activation of PI3-kinase may ultimately stimulate channel insertion as well as regulate channel endocytosis. Both of these phenomena can result in an increase of ENaC-mediated Na+ transport.

Phosphatidylinositol 4-phosphate 5-kinase reduces cell surface expression of the epithelial sodium channel (ENaC) in cultured collecting duct cells

Ubiquitination of ENaC subunits has been shown to negatively regulate the cell surface expression of ENaC channels. We have previously demonstrated that epsin links ubiquitinated ENaC to clathrin adaptors for clathrin-mediated endocytosis. Epsin is thought to directly modify the curvature of membranes upon binding to phosphatidylinositol 4,5-bisphosphate (PIP2) where it recruits clathrin and stimulates lattice assembly. Murine phosphatidylinositol 4-phosphate 5-kinase alpha (PI5KIalpha) has been shown to enhance endocytosis in a PIP2-dependent manner. We tested the hypothesis that PI5KIalpha-mediated PIP2 production would negatively regulate ENaC current by enhancing epsin-mediated endocytosis of the channel. Expression of PI5KIalpha decreased ENaC currents in Xenopus oocytes by 80%, entirely because of a decrease in cell surface ENaC levels. Catalytically inactive mutants of PI5Kalpha had no effect on ENaC activity. Expression of the PIP2 binding region of epsin increased ENaC current in oocytes, an effect completely reversed by co-expression of PI5KIalpha. Overexpression of epsin reduced amiloride-sensitive current in CCD cells. Overexpression of PI5KIalpha enhanced membrane PIP2 levels and reduced apical surface expression of ENaC in CCD cells, down-regulating amiloride-sensitive current. Knockdown of PI5KIalpha with isoform-specific siRNA resulted in a 4-fold enhancement of ENaC activity. PI5KIalpha localized exclusively to the apical plasma membrane domain when overexpressed in mouse CCD cells, consistent for a role in regulating PIP2 production at the apical plasma membrane. We conclude that membrane turnover events regulating ENaC surface expression and activity in oocytes and CCD cells can be regulated by PI5KIalpha.

The PY motif of ENaC, mutated in Liddle syndrome, regulates channel internalization, sorting and mobilization from subapical pool

The epithelial-Na(+)-channel (alphabetagammaENaC) regulates kidney salt-transport and blood pressure. Each ENaC subunit contains a PY motif (PPxY) and its mutation in beta/gammaENaC causes Liddle syndrome, a hereditary hypertension. These (extended) PY motifs (PP(616)xY(618)xxL(621)) serve as binding sites for the ubiquitin ligase Nedd4-2, which decreases cell-surface expression of ENaC by unknown route(s). Using polarized kidney epithelia [Madin-Darby canine kidney I (MDCK-I)] cells stably expressing extracellularly myc-tagged wild type (WT) or PY-motif mutants of betaENaC (P616A, Y618A or L621A, with WT-alphagammaENaC), and live-imaging plus enzyme-linked immunosorbent assay (ELISA)-type assays to analyze routes/rates of ENaC internalization/recycling, we show here that cell-surface half-life of all PY mutants was fourfold longer than WT-ENaC (approximately 120 versus 30 minutes), reflecting primarily reduced channel internalization but also attenuated replenishment of cell-surface ENaC from a large subapical pool. The Y618A mutant revealed more severe internalization and replenishment defects than the other PY mutants. Internalized WT-ENaC was detected in sorting/recycling and late endosomes/lysosomes, while the Y618A mutant accumulated in the former. Nedd4-2 ubiquitinated ENaC at the apical membrane causing channel internalization and degradation. Cyclic AMP (cAMP) accelerated mobilization of subapical ENaC to the cell surface and long-term ENaC recycling, but only mobilization, not recycling, was inhibited in the PY mutants. These results suggest that the ENaC PY motifs (and Nedd4-2) primarily regulate channel internalization but also affect cAMP-dependent replenishment, providing important insight into the Liddle syndrome defects.

Hormonal regulation of the epithelial Na+ channel: from amphibians to mammals

High-resistance epithelia derived from amphibian sources such as frog skin, toad urinary bladder, and the A6 Xenopus laevis kidney cell line have been widely used to elucidate the underlying mechanisms involved in the regulation of vectorial ion transport. More recently, the isolation of high-resistance mammalian cell lines has provided model systems in which to study differences and similarities between the regulation of ion transporter function in amphibian and mammalian renal epithelia. In the present study, we have compared the natriferic (Na+ retaining) responses to aldosterone, insulin, and vasotocin/vasopressin in the A6 and mpkCCDcl4 (mouse principal cells of the kidney cortical collecting duct) cell lines. The functional responses of the epithelial Na+ channel (ENaC) to hormonal stimulation were remarkably similar in both the amphibian and mammalian lines. In addition, insulin- and aldosterone-stimulated, reabsorptive Na+ transport in both cell lines requires the presence of functional PI3-kinase.

Modulation of basal and peptide hormone-stimulated Na transport by membrane cholesterol content in the A6 epithelial cell line

These studies examined the effect of altering plasma membrane cholesterol on basal Na+ flux as well as on the natriferic responses to the peptide hormones, insulin and anti-diuretic hormone (ADH) in the A6 model renal cell line. Membrane cholesterol concentrations were depleted or enriched using methyl-beta-cyclodextrin (MbetaCD) or a MbetaCD/cholesterol inclusion complex respectively. Effects of changes in the apical and basolateral plasma membranes were examined independently. Apical membrane cholesterol removal or supplementation had no effect on the basal Na+ transport rate. Short-term apical membrane cholesterol supplementation also had no effect on insulin-stimulated Na+ transport or on the initial phase of the ADH response. Interestingly, the additional apical membrane cholesterol had an inhibitory effect on the ADH response after 30 minutes. Apical membrane cholesterol depletion partially inhibited the responses to both insulin and ADH. Conversely, supplementation of basolateral cholesterol caused a significant increase in basal Na+ flux. Removal of cholesterol from the basolateral plasma membrane caused a decrease in basal Na+ flux with a time course analogous to channel turnover and completely inhibited peptide hormone responses. None of the changes in membrane cholesterol content decreased transcellular resistance. These results indicate an important role for membrane cholesterol content in the regulation of ENaC-mediated Na+ uptake.

Forskolin-induced cell shrinkage and apical translocation of functional enhanced green fluorescent protein-human alphaENaC in H441 lung epithelial cell monolayers

Elevation of intracellular cAMP increases fluid re-absorption in the lung by raising amiloride-sensitive Na+ transport through the apically localized epithelial, amiloride-sensitive Na+ channel (ENaC). However, the signaling pathways mediating this response are still not fully understood. We show that inhibition of protein-tyrosine kinase (PTK) with Genistein and protein kinase A (PKA) with KT5720, decreased forskolin-stimulated amiloride-sensitive short circuit current (I(sc)) across H441 adult human lung epithelial cell monolayers. KT5720 also decreased basal I(sc). Stable expression of green fluorescent protein (GFP)-labeled human alphaENaC in H441 cells was used to investigate dynamic changes in the cellular localization of this protein in response to forskolin. Reverse transcription-PCR and immunoblotting analysis revealed two clones expressing a truncated (alphaC3-5) and full-length (alphaC3-3) EGFP-halphaENaC protein. Only the alphaC3-3 clone displayed dome formation and exhibited a 50% increase in basal and forskolin-stimulated amiloride-sensitive I(sc) indicating that the full-length protein was required for functional activity. Apical surface biotinylation and real-time confocal microscopy demonstrated that EGFP-halphaENaC (alphaC3-3) translocated to the apical membrane in response to forskolin in a Brefeldin A-sensitive manner. This effect was completely inhibited by Genistein but only partially inhibited by KT5720. Forskolin also induced a reduction in the height of cells within alphaC3-3 monolayers, indicative of cell shrinkage. This effect was inhibited by KT5720 but not by Genistein or Brefeldin A. These data show that forskolin activates PKA-sensitive cell shrinkage in adult human H441 lung epithelial cell monolayers, which induces a PTK-sensitive translocation of EGFP-halphaENaC subunits to the apical membrane and increases amiloride-sensitive Na+ transport.

The carboxyl terminus of the alpha-subunit of the amiloride-sensitive epithelial sodium channel binds to F-actin

The activity of the amiloride-sensitive epithelial sodium channel (ENaC) is modulated by F-actin. However, it is unknown if there is a direct interaction between alpha-ENaC and actin. We have investigated the hypothesis that the actin cytoskeleton directly binds to the carboxyl terminus of alpha-ENaC using a combination of confocal microscopy, co-immunoprecipitation, and protein binding studies. Confocal microscopy of Madin-Darby canine kidney cell monolayers stably transfected with wild type, rat isoforms of alpha-, beta-, and gamma-ENaC revealed co-localization of alpha-ENaC with the cortical F-actin cytoskeleton both at the apical membrane and within the subapical cytoplasm. F-actin was found to co-immunoprecipitate with alpha-ENaC from whole cell lysates of this cell line. Gel overlay assays demonstrated that F-actin specifically binds to the carboxyl terminus of alpha-ENaC. A direct interaction between F-actin and the COOH terminus of alpha-ENaC was further corroborated by F-actin co-sedimentation studies. This is the first study to report a direct and specific biochemical interaction between F-actin and ENaC.

Overexpression of the epithelial Na+ channel gamma subunit in collecting duct cells: interactions of Liddle's mutations and steroids on expression and function

The epithelial Na(+) channel (ENaC) has three subunits; the expression of each can be regulated. Liddle's syndrome is caused by an activating mutation in the C terminus of either the beta or gamma subunit. We used a doxycycline-regulated adenovirus system to express varying levels of human gammaENaC in renal collecting duct (M1 cell) monolayers. Increasing levels of wild type human gamma ENaC (gammahENaC) produced a 2.5-fold enhancement of Na(+) transport. Expression of a truncated C terminus produced less protein than wild type or a gammaY627A missense mutation. However, either of these mutations produced a approximately 4-fold increase in Na(+) transport despite the different levels of protein expression. Unexpectedly, overexpression of a marginally detectable amount of gammahENaC was sufficient to produce a full increase in Na(+) transport; a further increase in protein expression produced no further increase in Na(+) transport. Steroid treatment increased Na(+) transport to a similar absolute magnitude in control monolayers and in monolayers expressing all types of gammahENaC. Withdrawal of steroids after 24 h produced a decline in Na(+) transport over 8 h in monolayers expressing wild type but not the Liddle's mutation. Using treatment with brefeldin A to estimate the disappearance rate constants, we found progressively slower disappearance rates in monolayers overexpressing gammahENaC and the Liddle's mutant. Calculated insertion rates were slower for the Liddle's mutant than for wild type despite increasing rates of Na(+) transport. These results raise questions regarding previously held assumptions about the behavior of ENaC.

Expression of the unconventional myosin Myo1c alters sodium transport in M1 collecting duct cells

Epithelial cells rely on proper targeting of cellular components to perform their physiological function. This dynamic process utilizes the cytoskeleton and involves movement of vesicles to and from the plasma membrane, thus traversing the actin cortical cytoskeleton. Studies support both direct interaction of actin with channels and an indirect mechanism whereby actin may serve as a track in the final delivery of the channel to the plasma membrane. Actin-dependent processes are often mediated via a member of the myosin family of proteins. Myosin I family members have been implicated in multiple cellular events occurring at the plasma membrane. In these studies, we investigated the function of the unconventional myosin I Myo1c in the M1 mouse collecting duct cell line. Myo1c was observed to be concentrated at or near the plasma membrane, often in discrete membrane domains. To address the possible role of Myo1c in channel regulation, we expressed a truncated Myo1c, lacking ATP and actin domains, in M1 cells and compared electrophysiological responses to control M1 cells, M1 cells expressing the empty vector, and M1 cells expressing the full-length Myo1c construct. Interestingly, cells expressing the Myo1c constructs had modulated antidiuretic hormone (ADH)-stimulated short-circuit current and showed little inhibition of short-circuit current with amiloride addition. Evaluation of enhanced green fluorescent protein-Myo1c constructs supports the importance of the IQ region in targeting the Myo1c to its respective cellular domain. These data are consistent with Myo1c participating in the regulation of the Na+ channel after ADH stimulation.

Phosphoinositide lipid second messengers: new paradigms for transepithelial signal transduction

Multiple forms of phosphatidylinositol are generated by differential phosphorylation of the inositol headgroup. These phosphoinositides, specifically PI(4,5)P2, have been implicated as modulators in a variety of transport processes. The data indicate that phosphoinositides can modulate transporters directly or via the activation of down-stream signaling components. The phosphoinositide pathway has been linked to changes in transporter kinetics, intracellular signaling, membrane targeting and membrane stability. Recent results obtained for several of the well-characterized transport systems suggest the need to reassess the role of PI(4,5)P2 and question whether lower abundance forms of the phosphoinositides, notably PI(3,4,5)P3 (PIP3) and PI(3,4)P2, are the pertinent transport regulators. In contrast to PI(4,5)P2, these latter forms represent a dynamic, regulated pool, the characteristics of which are more compatible with the nature of signaling intermediates. A recently described, novel transepithelial signaling pathway has been demonstrated for PIP3 in which a signal initiated on the basolateral membrane is transduced to the apical membrane entirely within the planar face of the inner leaflet of the plasma membrane. The new paradigms emerging from recent studies may be widely applicable to transporter regulation in other cell types and are particularly relevant for signaling in polarized cells.

cAMP and serum and glucocorticoid-inducible kinase (SGK) regulate the epithelial Na(+) channel through convergent phosphorylation of Nedd4-2

The epithelial Na(+) channel (ENaC) functions as a pathway for epithelial Na(+) transport, contributing to Na(+) homeostasis and blood pressure control. Vasopressin increases ENaC expression at the cell surface through a pathway that includes cAMP and cAMP-dependent protein kinase (PKA), but the mechanisms that link PKA to ENaC are unknown. Here we found that cAMP regulates Na(+) transport in part by inhibiting the function of Nedd4-2, an E3 ubiquitin-protein ligase that targets ENaC for degradation. Consistent with this model, we found that cAMP inhibited Nedd4-2 by decreasing its binding to ENaC. Moreover, decreased Nedd4-2 expression (RNA interference) or overexpression of a dominant negative Nedd4-2 construct disrupted ENaC regulation by cAMP. Nedd4-2 was a substrate for phosphorylation by PKA in vitro and in cells; three Nedd4-2 residues were phosphorylated by PKA and were required for cAMP to inhibit Nedd4-2 (relative functional importance Ser-327 > Ser-221 > Thr-246). Previous work found that these residues are also phosphorylated by serum and glucocorticoid-inducible kinase (SGK), a downstream mediator by which aldosterone regulates epithelial Na(+) transport. Consistent with a functional interaction between these pathways, overexpression of SGK blunted ENaC stimulation by cAMP, whereas inhibition of SGK increased stimulation. Conversely, cAMP agonists decreased ENaC stimulation by SGK. The data suggest that cAMP regulates ENaC in part by phosphorylation and inhibition of Nedd4-2. Moreover, Nedd4-2 is a central convergence point for kinase regulation of Na(+) transport.

Real-time three-dimensional imaging of lipid signal transduction: apical membrane insertion of epithelial Na(+) channels

In the distal tubule, Na(+) resorption is mediated by epithelial Na(+) channels (ENaC). Hormones such as aldosterone, vasopressin, and insulin modulate ENaC membrane targeting, assembly, and/or kinetic activity, thereby regulating salt and water homeostasis. Insulin binds to a receptor on the basal membrane to initiate a signal transduction cascade that rapidly results in an increase in apical membrane ENaC. Current models of this signaling pathway envision diffusion of signaling intermediates from the basal to the apical membrane. This necessitates diffusion of several high-molecular-weight signaling elements across a three-dimensional space. Transduction of the insulin signal involves the phosphoinositide pathway, but how and where this lipid-based signaling pathway controls ENaC activity is not known. We used tagged channels, biosensor lipid probes, and intravital imaging to investigate the role of lipids in insulin-stimulated Na(+) flux. Insulin-stimulated delivery of intracellular ENaC to apical membranes was concurrent with plasma membrane-limited changes in lipid composition. Notably, in response to insulin, phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) formed in the basolateral membrane, rapidly diffused within the bilayer, and crossed the tight junction to enter the apical membrane. This novel signaling pathway takes advantage of the fact that the lipids of the plasma membrane's inner leaflet are not constrained by the tight junction. Therefore, diffusion of PIP(3) as a signal transduction intermediate occurs within a planar surface, thus facilitating swift responses and confining and controlling the signaling pathway.

Subcellular redistribution of the renal betaine transporter during hypertonic stress

The betaine transporter (BGT1) protects cells in the hypertonic renal inner medulla by mediating uptake and accumulation of the osmolyte betaine. Transcriptional regulation plays an essential role in upregulation of BGT1 transport when renal cells are exposed to hypertonic medium for 24 h. Posttranscriptional regulation of the BGT1 protein is largely unexplored. We have investigated the distribution of BGT1 protein in live cells after transfection with BGT1 tagged with enhanced green fluorescent protein (EGFP). Fusion of EGFP to the NH2 terminus of BGT1 produced a fusion protein (EGFP-BGT) with transport properties identical to normal BGT1, as determined by ion dependence, inhibitor sensitivity, and apparent Km for GABA. Confocal microscopy of EGFP-BGT fluorescence in transfected Madin-Darby canine kidney (MDCK) cells showed that hypertonic stress for 24 h induced a shift in subcellular distribution from cytoplasm to plasma membrane. This was confirmed by colocalization with anti-BGT1 antibody staining. In fibroblasts, transfected EGFP-BGT caused increased transport in response to hypertonic stress. The activation of transport was not accompanied by increased expression of EGFP-BGT, as determined by Western blotting. Membrane insertion of EGFP-BGT protein in MDCK cells began within 2-3 h after onset of hypertonic stress and was blocked by cycloheximide. We conclude that posttranscriptional regulation of BGT1 is essential for adaptation to hypertonic stress and that insertion of BGT1 protein to the plasma membrane may require accessory proteins.

Effect of altered Na+ entry on expression of apical and basolateral transport proteins in A6 epithelia

In several in vivo settings, prolonged alterations in the rate of apical Na+ entry into epithelial cells alter the ability of these cells to reabsorb Na+. We previously modeled this load dependence of transport in A6 cells by either decreasing Na+ entry via apical Na+ removal or amiloride or enhancing Na+ entry by chronic short-circuiting (Rokaw MD, Sarac E, Lechman E, West M, Angeski J, Johnson JP, and Zeidel ML. Am J Physiol Cell Physiol 270: C600-C607, 1996). Inhibition of Na+ entry by either method was associated with striking downregulation of transport rate as measured by short-circuit current (Isc), which recovered to basal levels of transport over a period of hours. Conversely, upregulation of Na+ entry by short-circuiting resulted in a sustained increase in transport rate that also returned to basal levels over a period of hours. The current studies were undertaken to determine whether these conditions were associated with alterations in either the whole cell content or apical membrane distribution of sodium channel (ENaC) subunits or on basolateral expression of either of the subunits of the Na+-K+-ATPase. We compared these effects to those achieved by chronic upregulation of Na+ transport by aldosterone. Whole cell levels of ENaC subunits were measured by immunoblot following 18-h inhibition of Na+ entry achieved by either tetramethylammonium replacement of Na+ or apical amiloride or after an 18-h increase in Na+ entry achieved by chronic short-circuiting. None of these maneuvers significantly altered the whole cell content of any of the ENaC subunits compared with control cells. We then examined the effects of these maneuvers on apical membrane ENaC expression using domain-specific biotinylation and immunoblot. Inhibition of Na+ entry by either method was associated with a profound decrease in apical membrane beta-ENaC without significant changes in apical membrane alpha-or gamma-ENaC amounts. Restoration of apical Na+ and/or removal of amiloride resulted in return of Isc to control levels over 2 h and coincided with return of apical beta-ENaC to control levels without change in apical alpha- or gamma-ENaC. Stimulation of Na+ transport by short-circuiting, in contrast, did not significantly alter apical membrane composition of any of the ENaC subunits. Basolateral expression of Na+-K+-ATPase was also measured by biotinylation and immunoblot and was unchanged under all conditions. Aldosterone increased basolateral expression of the alpha-subunit of Na+-K+-ATPase. These results suggest that chronic downregulation of transport is mediated, in part, by a selective decrease in apical membrane ENaC expression, consistent with our previous observations of noncoordinate regulation of ENaC expression under varying transport conditions in A6 cells. The chronic increase in the rate of Na+ entry is not associated with any of the changes in transporter density at either apical or basolateral membrane seen with aldosterone, suggesting that these two mechanisms of augmenting transport are completely distinct.

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.

sgk: an essential convergence point for peptide and steroid hormone regulation of ENaC-mediated Na+ transport

To study the role of sgk (serum, glucocorticoid-induced kinase) in hormonal regulation of Na+ transport mediated by the epithelial Na+ channel (ENaC), clonal cell lines stably expressing human sgk, an S422A sgk mutant, or a D222A sgk mutant were created in the background of the A6 model renal epithelial cell line. Expression of normal sgk results in a 3.5-fold enhancement of basal transport and potentiation of the natriferic response to antidiuretic hormone (ADH). Transfection of a S422A mutant form of sgk, which cannot be phosphorylated by phosphatidylinositol-dependent kinase (PDK)-2, results in a cell line that is indistinguishable from the parent line in basal and hormone-stimulated Na+ transport. The D222A sgk mutant, which lacks kinase activity, functions as a dominant-negative mutant inhibiting basal as well as peptide- and steroid hormone-stimulated Na+ transport. Thus sgk activity is necessary for ENaC-mediated Na+ transport. Phosphorylation and activation by PDK-2 are necessary for sgk stimulation of ENaC. Expression of normal sgk over endogenous levels results in a potentiated natriferic response to ADH, suggesting that the enzyme is a rate-limiting step for the hormone response. In contrast, sgk does not appear to be the rate-limiting step for the cellular response to aldosterone or insulin.