Trafficking and cell surface stability of the epithelial Na+ channel expressed in epithelial Madin-Darby canine kidney cells

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

Epithelial sodium channel (ENaC) is multi-ubiquitinated at the cell surface

The human ENaC (epithelial sodium channel), a complex of three subunits, provides the rate-limiting step for sodium uptake in the distal nephron, and therefore plays a key role in salt homoeostasis and in regulating blood pressure. The number of active sodium channel complexes present at the plasma membrane appears to be tightly controlled. In Liddle's syndrome, a form of hypertension caused by an increase in the number of active sodium channels at the cell membrane, the betaENaC or gammaENaC subunit gene contains a mutation that disrupts the binding site for the Nedd4 (neuronal precursor cell expressed developmentally down-regulated gene 4) family of ubiquitin-protein ligases. Therefore ubiquitination of channel subunits may be involved in altering cell surface ENaC. Here, we provide evidence that the ENaC subunits located at the cell surface are modified with multiple mono-ubiquitins (multi-ubiquitination) and that Nedd4-2 modulates this ubiquitination. We confirm that ENaC is associated with the mu2 subunit of the AP-2 (adaptor protein 2) clathrin adaptor. Since mono- or multi-ubiquitination of other membrane proteins is a signal for their internalization by clathrin-mediated endocytosis and subsequent trafficking, our results support a model whereby ubiquitin and clathrin adaptor binding sites act in concert to remove ENaC from the cell surface.

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.

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.

CFTR stabilizes ENaC at the plasma membrane

CFTR was reported to regulate ENaC channel opening, decreasing ENaC activity in airways and increasing it in sweat ducts. We generated MDCK-I cell lines stably expressing tagged alphabetagammaENaC+CFTR or ENaC alone, and developed an assay to quantify cell-surface half-life of ENaC. Surprisingly, we found that co-expressed CFTR stabilizes ENaC at the plasma membrane, suggesting that CFTR regulates ENaC stability, not just opening.

Trafficking of ENaC subunits in response to acute insulin in mouse kidney

Studies done in cell culture have demonstrated that insulin activates the epithelial sodium channel (ENaC) via a variety of mechanisms. However, to date, upregulation of ENaC in native renal tissue by in vivo administration of insulin has not been demonstrated. To address this, we injected 6-mo-old male C57BL/CBA mice (n = 14/group) intraperitoneally with vehicle or 0.5 U/kg body wt insulin and examined short-term (1-2 h) sodium excretion and kidney ENaC subunits (alpha, beta, and gamma) and serum and glucocorticoid-induced kinase (SGK-1) regulation. Insulin resulted in a significant reduction in urine sodium (by approximately 80%) that was restored by intraperitoneal administration of the ENaC antagonist, benzamil (1.4 mg/kg body wt). Differential centrifugation followed by Western blotting of whole kidney revealed significantly increased band densities (by 26-103%) for insulin- relative to vehicle-treated mice for alpha- and gamma-ENaC in the homogenate (H), and plasma membrane-enriched fraction (MF), with no difference in the vesicle-enriched fraction (VF). Similarly, beta-ENaC was significantly increased in MF (by 45%) but no change in the H. It was, however, significantly decreased in the VF (by 28%) with insulin. In agreement, immunoperoxidase labeling demonstrated relatively stronger apical, relative to cytosolic, localization of alpha-, beta-, and gamma-ENaC with insulin, whereas, with vehicle, labeling was fairly evenly dispersed throughout collecting duct principal cells. Furthermore, Western blotting showed insulin increased SGK-1 (by 75%) and phosphorylated-SGK band densities (by 30%) but only in the MF. These studies demonstrate novel in vivo regulation of renal ENaC activity and subunit proteins and SGK-1 by insulin in the acute time frame in the mouse.

Missense mutations of dual oxidase 2 (DUOX2) implicated in congenital hypothyroidism have impaired trafficking in cells reconstituted with DUOX2 maturation factor

Dual oxidase 2 (DUOX2), a reduced NAD phosphate:O2 oxidoreductase flavoprotein, is a component of the thyrocyte H2O2 generator required for hormone synthesis at the apical plasma membrane. We recently identified a specific DUOX2 maturation factor (DUOXA2) that is necessary and sufficient for expression of functional DUOX2 in mammalian cell lines. We have now used a DUOXA2 reconstituted system to provide the first characterization of natural DUOX2 missense variants (Q36H, R376W, D506N) at the molecular level, analyzing their impact on H2O2 generation, trafficking, stability, folding, and DUOXA2 interaction. The Q36H and R376W mutations completely prevent routing of DUOX2 to the cell surface. The mutant proteins are predominantly present as core N-glycosylated, thiol-reduced folding intermediates, which are retained by the quality control system within the endoplasmic reticulum (ER) as indicated by increased complexation with the lectin calnexin. D506N displays a partial deficiency phenotype with reduced surface expression of a mutant protein with normal intrinsic activity in generating H2O2. D506N N-glycan moieties are not subject to normal modification in the Golgi apparatus, suggesting that nonnative protein can escape the quality control in the ER. Oxidative folding of DUOX2 in the ER appears to be the rate-limiting step in the maturation of DUOX2, but is not facilitated by DUOXA2. Rather, DUOXA2 allows rapid ER exit of folded DUOX2 or enhanced degradation of mutant DUOX2 proteins not competent for ER exit. DUOXA2 may thus be part of a secondary quality control system specific for DUOX2.

Cell confluence-induced activation of signal transducer and activator of transcription-3 (Stat3) triggers epithelial dome formation via augmentation of sodium hydrogen exchanger-3 (NHE3) expression

Cell confluence induces the activation of signal transducer and activator of transcription-3 (Stat3) in various cancer and epithelial cells, yet the biological implications and the associated regulatory mechanisms remain unclear. Because confluent polarized epithelia demonstrate dome formation and sodium influx that mimic the onset of differentiation, we sought to elucidate the role of Stat3 in association with the regulation of selective epithelial transporters in this biological phenomenon. This study established the correlation between Stat3 activation and cell confluence-induced dome formation in Madin-Darby canine kidney cells (MDCK) by following Stat3 activation events in dome-forming cells. Epifluorescent and confocal microscopy provided evidence showing specific localization of phosphorylated Stat3 Tyr(705) in the nuclei of dome-forming cells at initial stages. The relationship was further elucidated by the establishment of tetracycline-inducible expression of constitutive Stat3 mutant (Stat3-C) in MDCK cells or expression of dominant negative Stat3 (Stat3-D) stable cell lines (MDCK and NMuMG). Dome formation was promoted by the expression of Stat3-C but inhibited by Stat3-D. Two trans-epithelial transporters, NHE3 and ENaC alpha-subunit, were found to be increased during cell confluence. Interestingly, NHE3 expression could be specifically up-regulated by Stat3-C but inhibited by Stat3-D through promoter regulation, whereas NHE1 and ENaC alpha-subunit were not affected by Stat3 expression. Application of NHE3 shRNA, NHE3 inhibitors (EIPA and S3226) suppressed confluence-induced dome formation in MDCK or NMuMG cells. These results demonstrate a cell confluence-induced Stat3 signaling pathway in epithelial cells in triggering dome formation through NHE3 augmentation.

Secreted PCSK9 decreases the number of LDL receptors in hepatocytes and in livers of parabiotic mice

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a member of the proteinase K subfamily of subtilases that reduces the number of LDL receptors (LDLRs) in liver through an undefined posttranscriptional mechanism. We show that purified PCSK9 added to the medium of HepG2 cells reduces the number of cell-surface LDLRs in a dose- and time-dependent manner. This activity was approximately 10-fold greater for a gain-of-function mutant, PCSK9(D374Y), that causes hypercholesterolemia. Binding and uptake of PCSK9 were largely dependent on the presence of LDLRs. Coimmunoprecipitation and ligand blotting studies indicated that PCSK9 and LDLR directly associate; both proteins colocalized to late endocytic compartments. Purified PCSK9 had no effect on cell-surface LDLRs in hepatocytes lacking autosomal recessive hypercholesterolemia (ARH), an adaptor protein required for endocytosis of the receptor. Transgenic mice overexpressing human PCSK9 in liver secreted large amounts of the protein into plasma, which increased plasma LDL cholesterol concentrations to levels similar to those of LDLR-knockout mice. To determine whether PCSK9 was active in plasma, transgenic PCSK9 mice were parabiosed with wild-type littermates. After parabiosis, secreted PCSK9 was transferred to the circulation of wild-type mice and reduced the number of hepatic LDLRs to nearly undetectable levels. We conclude that secreted PCSK9 associates with the LDLR and reduces hepatic LDLR protein levels.

Membrane cholesterol content modulates ClC-2 gating and sensitivity to oxidative stress

ClC-2 is a broadly expressed member of the voltage-gated ClC chloride channel family. In this study, we aimed to evaluate the role of the membrane lipid environment in ClC-2 function, and in particular the effect of cholesterol and ClC-2 distribution in membrane microdomains. Detergent-resistant and detergent-soluble microdomains (DSM) were isolated from stably transfected HEK293 cells by a discontinuous OptiPrep gradient. ClC-2 was found concentrated in detergent-insoluble membranes in basal conditions and relocalized to DSM upon cholesterol depletion by methyl-beta-cyclodextrin. As assessed by patch clamp recordings, relocalization was accompanied by acceleration of the activation kinetics of the channel. A similar distribution and activation pattern were obtained when cells were treated with the oxidant tert-butyl hydroperoxide and after ATP depletion. In both cases activation was prevented by cholesterol enrichment of cells. We conclude that the cholesterol environment regulates ClC-2 activity, and we provide evidence that the increase in ClC-2 activity in response to acute oxidative or metabolic stress involves relocalization of this channel to DSM.

Functional links between membrane transport and the spectrin cytoskeleton

Membrane transporters precisely regulate which molecules cross the plasma membrane and when they can cross. In many cases it is also important to regulate where substances can cross the plasma membrane. Consequently, cells have evolved mechanisms to confine and stabilize membrane transport proteins within specific subdomains of the plasma membrane. A number of different transporters (including ion pumps, channels and exchangers) are known to physically associate with the spectrin cytoskeleton, a submembrane complex of spectrin and ankyrin. These proteins form a protein scaffold that assembles within discrete subdomains of the plasma membrane in polarized cells. Recent genetic studies in humans and model organisms have provided the opportunity to test the hypothesis that the spectrin cytoskeleton has a direct role in restricting transporters to specialized domains. Remarkably, genetic defects in spectrin and ankyrin can produce effects on cell physiology that are comparable to knockouts of the transporters themselves.

Solution structure of the COMMD1 N-terminal domain

COMMD1 is the prototype of a new protein family that plays a role in several important cellular processes, including NF-kappaB signaling, sodium transport, and copper metabolism. The COMMD proteins interact with one another via a conserved C-terminal domain, whereas distinct functions are predicted to result from a variable N-terminal domain. The COMMD proteins have not been characterized biochemically or structurally. Here, we present the solution structure of the N-terminal domain of COMMD1 (N-COMMD1, residues 1-108). This domain adopts an alpha-helical structure that bears little resemblance to any other helical protein. The compact nature of N-COMMD1 suggests that full-length COMMD proteins are modular, consistent with specific functional properties for each domain. Interactions between N-COMMD1 and partner proteins may occur via complementary electrostatic surfaces. These data provide a new foundation for biochemical characterization of COMMD proteins and for probing COMMD1 protein-protein interactions at the molecular level.

Role of NH2 and COOH termini in targeting, stability, and activity of sodium bicarbonate cotransporter 1

Sodium bicarbonate cotransporter 1 (NBC1) mediates 80% of bicarbonate reabsorption by the kidney, but the molecular determinants for activity, targeting, and cell membrane stability are poorly understood. We generated truncation mutants involving the entire NH2 (ΔN424) or the entire COOH (ΔC92) terminus and examined the effects of these truncations on targeting, cell membrane stability, and NBC1 activity. ΔN424 and ΔC92 targeted to the plasma membrane of HEK293 cells or to the basolateral membrane of opossum kidney (OK) cells at 24 h but did not display NBC1 activity. Unlike the NBC1 wild-type and the ΔN424, ΔC92 expression was significantly decreased in the basolateral membrane at 48 h and yet the total ΔC92 expression in the cell was constant. We found that decreased ΔC92 expression in the basolateral membrane was due to increased endocytosis and mistargeting to the apical membrane. Increased endocytosis was prevented when both ΔN424 and ΔC92 were cotransfected together and more stable expression of ΔC92 was observed. Immunoprecipitation studies using NBC1 antibody specific for the COOH epitope were able to detect the COOH truncated NBC1 when probed with NH2 epitope-specific antibody or vice versa. Similar findings were observed with Ni-NTA pull-down assay. Cotransfection of both mutants partially restored NBC1 activity. In summary, NBC1 targets to the basolateral membrane of OK cells by a default mechanism and the COOH terminus plays a role on NBC1 stability in the basolateral membrane.

Rab27a negatively regulates CFTR chloride channel function in colonic epithelia: Involvement of the effector proteins in the regulatory mechanism

Cystic fibrosis, an autosomal recessive disorder, is caused by the disruption of biosynthesis or function of CFTR. CFTR regulatory mechanisms include channel transport to plasma membrane and protein-protein interactions. Rab proteins are small GTPases involved in vesicle transport, docking, and fusion. The colorectal epithelial HT-29 cells natively express CFTR and respond to cAMP with an increase in CFTR-mediated currents. DPC-inhibited currents could be completely eliminated with CFTR-specific SiRNA. Over-expression of Rab27a inhibited, while isoform specific SiRNA and Rab27a antibody stimulated CFTR-mediated currents in HT-29 cells. CFTR activity is inhibited both by Rab27a (Q78L) (constitutive active GTP-bound form of Rab27a) and Rab27a (T23N) (constitutive negative form that mimics the GDP-bound form). Rab27a mediated effects could be reversed by Rab27a-binding proteins, the synaptotagmin-like protein (SLP-5) and Munc13-4 accessory protein (a putative priming factor for exocytosis). The SLP reversal of Rab27a effect was restricted to C2A/C2B domains while the SHD motif imparted little more inhibition. The CFTR-mediated currents remain unaffected by Rab3 though SLP-5 appears to weakly bind it. The immunoprecipitation experiments suggest protein-protein interactions between Rab27a and CFTR. Rab27a appears to impair CFTR appearance at the cell surface by trapping CFTR in the intracellular compartments. Munc13-4 and SLP-5, on the other hand, limit Rab27a availability to CFTR, thus minimizing its effect on channel function. These observations decisively prove that Rab27a is involved in CFTR channel regulation through protein-protein interactions involving Munc13-4 and SLP-5 effector proteins, and thus could be a potential target for cystic fibrosis therapy.

Variability, compensation and homeostasis in neuron and network function

Neurons in most animals live a very long time relative to the half-lives of all of the proteins that govern excitability and synaptic transmission. Consequently, homeostatic mechanisms are necessary to ensure stable neuronal and network function over an animal's lifetime. To understand how these homeostatic mechanisms might function, it is crucial to understand how tightly regulated synaptic and intrinsic properties must be for adequate network performance, and the extent to which compensatory mechanisms allow for multiple solutions to the production of similar behaviour. Here, we use examples from theoretical and experimental studies of invertebrates and vertebrates to explore several issues relevant to understanding the precision of tuning of synaptic and intrinsic currents for the operation of functional neuronal circuits.

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.

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.

Rab proteins regulate epithelial sodium channel activity in colonic epithelial HT-29 cells

ENaC, the sodium-selective amiloride-sensitive epithelial channel, mediates electrogenic sodium re-absorption in tight epithelia and is deeply associated with human hypertension. The ENaC expression at plasma membrane requires the regulated transport, processing, and macromolecular assembly in a defined and highly compartmentalized manner. Ras-related Rab GTPases regulate intracellular trafficking during endocytosis, regulated exocytosis, and secretion. To evaluate the role of these proteins in regulating amiloride-sensitive sodium channel activity, multiple Rab isoforms 3, 5, 6, and Rab27a were expressed in HT-29 cells. Rab3 and Rab27a inhibited ENaC currents, while the expression of other Rab isoforms failed to elicit any statistically significant effect on amiloride-sensitive currents. The immunoprecipitation experiments suggest protein-protein interaction of Rab3 and Rab27a with epithelial sodium channel. Biotinylation studies revealed that modulation of ENaC function is due to the reduced apical expression of channel proteins. Study also indicates that Rabs do not appear to affect the steady-state level of total cellular ENaC. Alternatively, introduction of isoform-specific small inhibitory RNA (SiRNA) reversed the Rab-dependent inhibition of amiloride-sensitive currents. These observations point to the involvement of multiple Rab proteins in ENaC transport through intracellular routes like exocytosis, recycling from ER to plasma membrane or degradation and thus serve as potential target for human hypertension.

Impact of Nedd4 proteins and serum and glucocorticoid-induced kinases on epithelial Na+ transport in the distal nephron

The precise control of BP occurs via Na(+) homeostasis and involves the precise regulation of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. This has been corroborated by the linkage of mutations in the genes encoding ENaC subunits and Liddle's syndrome, a heritable form of human hypertension. Mapping of these mutations on ENaC indicated that inactivation of PY motifs is responsible and leads to the proposition that the channel interacts via its PY motifs with the WW domains of the Nedd4/Nedd4-like ubiquitin-protein ligase family. It is now well established that the cell surface expression of ENaC is controlled via ubiquitylation by this protein family and that this ubiquitylation is regulated by the aldosterone-induced protein serum and glucocorticoid induced kinase 1.

Intracellular Trafficking of Thyroid Peroxidase to the Cell Surface

For thyroid hormone synthesis, thyroid peroxidase (TPO) molecules must be transported from the endoplasmic reticulum via the Golgi complex to be delivered at the cell surface to catalyze iodination of secreted thyroglobulin. Like other glycoproteins, TPO molecules in transit to the cell surface have the potential to acquire endoglycosidase H resistance as a consequence of Golgi-based modification of their N-linked carbohydrates, and measurement of the intracellular distribution of TPO has often relied on this assumption. To examine TPO surface distribution in thyrocyte cell lines, we prepared new antibodies against rat TPO. Antibody reactivity was first established upon expression of recombinant rat (r) TPO in 293 cells, which were heterogeneous for surface expression as determined by flow cytometry. By cell fractionation, surface rTPO fractionated distinctly from internal pools of TPO (that co-fractionate with calnexin), yet surface TPO molecules remained endoglycosidase H (endo H)-sensitive. Although the FRTL5 (and PC Cl3) rat thyrocyte cell line also exhibits almost no endo H-resistant TPO, much of the endogenous rTPO is localized to the cell surface by immunofluorescence. Similar results were obtained by fractionation of FRTL5 cell membranes on sucrose gradients. We conclude that in FRTL5 cells, a large fraction of rTPO is delivered to the plasma membrane yet does not acquire Golgi-type processing of its N-glycans. Rat and mouse thyroid tissue TPO also shows little or no endo H resistance, although cell fractionation still needs to be optimized for these tissues.

Characterization of a Madin-Darby canine kidney cell line stably expressing TRPV5

To provide a cell model for studying specifically the regulation of Ca2+ entry by the epithelial calcium channel transient receptor potential-vanilloid-5 (TRPV5), green fluorescent protein (GFP)-tagged TRPV5 was expressed stably in Madin-Darby canine kidney type I (MDCK) cells. The localization of GFP-TRPV5 in this cell line showed an intracellular granular distribution. Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on plastic supports was threefold higher than in non-transfected cells. Moreover, apical Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on permeable supports was eightfold higher than basolateral Ca2+ uptake, indicating that GFP-TRPV5 is expressed predominantly in the apical membrane. Patch-clamp analysis showed the presence of typical electrophysiological features of GFP-TRPV5, such as inwardly rectifying currents, inhibition by divalent cations and Ca2+-dependent inactivation. Moreover, the TRPV5 inhibitor ruthenium red completely inhibited Ca2+ uptake in GFP-TRPV5-MDCK cells, whereas Ca2+ uptake in non-transfected cells was not inhibited. The characterized GFP-TRPV5-MDCK cell line was used to assess the regulation of TRPV5. The protein kinase C activator phorbol 12-myristate 13-acetate and the cAMP-elevating compounds forskolin/3-isobutyl-1-methylxanthine, 8-Br-cAMP and PGE2 stimulated TRPV5 activity in GFP-TRPV5-MDCK cells by 121+/-7, 79+/-5, 55+/-4 and 61+/-7%, respectively. These compounds did not affect Ca2+ uptake in non-transfected cells. In conclusion, the GFP-TRPV5-MDCK cell line provides a model to specifically study the regulation of TRPV5 activity.

Acute ENaC stimulation by cAMP in a kidney cell line is mediated by exocytic insertion from a recycling channel pool

Acute hormonal regulation of the epithelial sodium channel (ENaC) in tight epithelia increases transcellular Na⁺ transport via trafficking of intracellular channels to the apical surface. The fate of the channels removed from the apical surface following agonist washout is less clear. By repetitively stimulating polarized mouse cortical collecting duct (mCCD, _MPKCCD₁₄) epithelia, we evaluated the hypothesis that ENaC recycles through an intracellular pool to be available for reinsertion into the apical membrane. Short circuit current (I_SC), membrane capacitance (C_T), and conductance (G_T) were recorded from mCCD epithelia mounted in modified Ussing chambers. Surface biotinylation of ENaC demonstrated an increase in channel number in the apical membrane following cAMP stimulation. This increase was accompanied by a 83 ± 6% (n = 31) increase in I_SC and a 15.3 ± 1.5% (n = 15) increase in C_T. Selective membrane permeabilization demonstrated that the C_T increase was due to an increase in apical membrane capacitance. I_SC and C_T declined to basal levels on stimulus washout. Repetitive cAMP stimulation and washout (∼1 h each cycle) resulted in response fatigue; ΔI_SC decreased ∼10% per stimulation–recovery cycle. When channel production was blocked by cycloheximide, ΔI_SC decreased ∼15% per stimulation cycle, indicating that newly synthesized ENaC contributed a relatively small fraction of the channels mobilized to the apical membrane. Selective block of surface ENaC by benzamil demonstrated that channels inserted from a subapical pool made up >90% of the stimulated I_SC, and that on restimulation a large proportion of channels retrieved from the apical surface were reinserted into the apical membrane. Channel recycling was disrupted by brefeldin A, which inhibited ENaC exocytosis, by chloroquine, which inhibited ENaC endocytosis and recycling, and by latrunculin A, which blocked ENaC exocytosis. A compartment model featuring channel populations in the apical membrane and intracellular recycling pool provided an adequate kinetic description of the I_SC responses to repetitive stimulation. The model supports the concept of ENaC recycling in response to repetitive cAMP stimulation.

Nedd4-2 isoforms differentially associate with ENaC and regulate its activity

Mutations that disrupt a PY motif in epithelial Na(+) channel (ENaC) subunits increase surface expression of Na(+) channels in the collecting duct, resulting in greater Na(+) reabsorption. Nedd4 and Nedd4-2 have been identified as ubiquitin ligases that can interact with ENaC via its PY motifs to regulate channel activity. We recently reported that human Nedd4-2 (hNedd4-2) is expressed as many isoforms because of alternative promoter usage and/or variable splicing. To understand the relevance of hNedd4-2 isoforms for collecting duct Na(+) transport, we studied the interaction with ENaC and the intracellular localization and function of the following three naturally occurring hNedd4-2 isoforms: full-length Nedd4-2 (Nedd4-2), Nedd4-2 lacking the NH(2)-terminal C2 domain (Nedd4-2DeltaC2), and Nedd4-2 lacking the C2 domain and WW domains 2 and 3 (Nedd4-2DeltaWW2,3). Nedd4-2 and Nedd4-2DeltaC2 associate with ENaC and robustly reduce Na(+) transport in Xenopus oocytes, whereas the interaction with and functional effect of Nedd4-2DeltaWW2,3 on ENaC is weak. Nedd4-2 is expressed in the mouse collecting duct, and overexpression of Nedd4-2 reduces endogenous ENaC activity in a collecting duct cell line. This reduction in ENaC activity can be reversed early with exposure to dexamethasone, an effect that is associated with an increase in sgk1 abundance. The C2 domain is required to target Nedd4-2 to the plasma membrane in response to elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in MDCK cells, although it does not appear to mediate the inhibitory effect of [Ca(2+)](i) on Na(+) transport. Our data illustrate that naturally occurring hNedd4-2 isoforms differentially associate with ENaC to regulate its activity.

Phosphatidylinositol 3-kinase-mediated effects of glucose on vacuolar H+-ATPase assembly, translocation, and acidification of intracellular compartments in renal epithelial cells

Vacuolar H+-ATPases (V-ATPases) are a family of ATP-driven proton pumps. They maintain pH gradients between intracellular compartments and are required for proton secretion out of the cytoplasm. Mechanisms of extrinsic control of V-ATPase are poorly understood. Previous studies showed that glucose is an important regulator of V-ATPase assembly in Saccharomyces cerevisiae. Human V-ATPase directly interacts with aldolase, providing a coupling mechanism for glucose metabolism and V-ATPase function. Here we show that glucose is a crucial regulator of V-ATPase in renal epithelial cells and that the effect of glucose is mediated by phosphatidylinositol 3-kinase (PI3K). Glucose stimulates V-ATPase-dependent acidification of the intracellular compartments in human proximal tubular cells HK-2 and porcine renal epithelial cells LLC-PK1. Glucose induces rapid ATP-independent assembly of the V1 and Vo domains of V-ATPase and extensive translocation of the V-ATPase V1 and Vo domains between different membrane pools and between membranes and the cytoplasm. In HK-2 cells, glucose stimulates polarized translocation of V-ATPase to the apical plasma membrane. The effects of glucose on V-ATPase trafficking and assembly can be abolished by pretreatment with the PI3K inhibitor LY294002 and can be reproduced in glucose-deprived cells by adenoviral expression of the constitutively active catalytic subunit p110alpha of PI3K. Taken together these data provide evidence that, in renal epithelial cells, glucose plays an important role in the control of V-ATPase-dependent acidification of intracellular compartments and V-ATPase assembly and trafficking and that the effects of glucose are mediated by PI3K-dependent signaling.

Role of Nedd4-2 and polyubiquitination in epithelial sodium channel degradation in untransfected renal A6 cells expressing endogenous ENaC subunits

Amiloride-sensitive epithelial sodium channels (ENaC) are responsible for transepithelial Na(+) transport in the kidney, lung, and colon. The channel consists of three subunits (alpha, beta, and gamma). In Madin-Darby canine kidney (MDCK) cells and Xenopus laevis oocytes transfected with all three ENaC subunits, neural precursor cell-expressed developmentally downregulated protein (Nedd4-2) promotes ubiquitin conjugation of ENaC. For native proteins in some cells, ubiquitin conjugation is a signal for their degradation by the ubiquitin-proteasome pathway, whereas in other cell types ubiquitin conjugation is a signal for endocytosis and lysosomal protein degradation. When ENaC are transfected into MDCK cells, ubiquitin conjugation leads to lysosomal degradation. In this paper, we characterize the involvement of the ubiquitin-proteasome proteolytic pathway in the regulation of functional ENaC in untransfected renal A6 cells expressing native ENaC subunits. In contrast to transfected cells, we show that total cellular alpha-, beta-, and gamma-ENaC subunits are polyubiquitinated and that ubiquitin conjugation of subunits increases when the cells are treated with a proteasome inhibitor. We show that Nedd4-2 is associated with alpha- and beta-subunits and is associated with the apical membrane. We also show the Nedd4-2 can regulate the number of functional ENaC subunits in the apical membrane. The results reported here suggest that the ubiquitin-proteasome proteolytic pathway is an important determinant of ENaC function in untransfected renal cells expressing endogenous ENaC.

Neutrophil elastase activates near-silent epithelial Na+ channels and increases airway epithelial Na+ transport

Neutrophil elastase is a serine protease that is abundant in the airways of individuals with cystic fibrosis (CF), a genetic disease manifested by excessive airway Na(+) absorption and consequent depletion of the airway surface liquid layer. Although endogenous epithelium-derived serine proteases regulate epithelial Na(+) transport, the effects of neutrophil elastase on epithelial Na(+) transport and epithelial Na(+) channel (ENaC) activity are unknown. Low micromolar concentrations of human neutrophil elastase (hNE) applied to the apical surface of a human bronchial cell line (16HBE14o-/beta gamma) increased Na(+) transport about twofold. Similar effects were observed with trypsin, also a serine protease. Proteolytic inhibitors of hNE or trypsin selectively abolished the enzyme-induced increase of epithelial Na(+) transport. At the level of the single channel, submicromolar concentrations of hNE increased activity of near-silent ENaC approximately 108-fold in patches from NIH-3T3 cells expressing rat alpha-, beta-, and gamma-ENaC subunits. However, no enzyme effects were observed on basally active ENaCs. Trypsin exposure following hNE revealed no additional increase in amiloride-sensitive short-circuit current or in ENaC activity, suggesting these enzymes share a common mode of action for increasing Na(+) transport, likely through proteolytic activation of ENaC. The hNE-induced increase of near-silent ENaC activity in CF airways could contribute to Na(+) hyperabsorption, reduced airway surface liquid height, and dehydrated mucus culminating in inefficient mucociliary clearance.

N-acetylcysteine inhibits Na+ absorption across human nasal epithelial cells

N-acetylcysteine (NAC) is a widely used mucolytic drug in patients with a variety of respiratory disorders. The mechanism of action is based on rupture of the disulfide bridges of the high molecular glycoproteins present in the mucus, resulting in smaller subunits of the glycoproteins and reduced viscosity of the mucus. Because Na(+) absorption regulates airway surface liquid volume and thus the efficiency of mucociliary clearance, we asked whether NAC affects the bioelectric properties of human nasal epithelial cells. A 24-h basolateral treatment with 10 mM of NAC decreased the transepithelial potential difference and short-circuit current (I(SC)) by 40%, and reduced the amiloride-sensitive current by 50%, without affecting the transepithelial resistance. After permeabilization of the basolateral membranes of cells with amphotericin B in the presence of a mucosal-to-serosal Na(+) gradient (135:25 mM), NAC inhibited 45% of the amiloride-sensitive current. The Na(+)-K(+)-ATPase pump activity and the basolateral K(+) conductance were not affected by NAC treatment. NAC did not alter total cell mRNA and protein levels of alpha-epithelial Na(+) channel (EnaC) subunit, but reduced abundance of alpha-ENaC subunits in the apical cell membrane as quantified by biotinylation. This effect can be ascribed to the sulphydryl (SH) group of NAC, since N-acetylserine and S-carboxymethyl-l-cysteine were ineffective. Given the importance of epithelial Na(+) channels in controlling the thin layer of fluid that covers the surface of the airways, the increase in the fluidity of the airway mucus following NAC treatment in vivo might be in part related to downregulation of Na(+) absorption and consequently water transport.

Differential Current Decay Profiles of Epithelial Sodium Channel Subunit Combinations in Polarized Renal Epithelial Cells

In many epithelial tissues in the body, the rate of Na(+) reabsorption is governed by the activity of the epithelial sodium channel (ENaC). The assembly, trafficking, and turnover of the three ENaC subunits (alpha, beta, and gamma) is complex and not well understood. Recent experiments suggest that ENaC must be proteolytically cleaved for maximal activity and may explain the discrepancies reported in prior biochemical approaches focused on quantitating the trafficking and half-life of full-length subunits. As an alternative approach to examining the dynamics of ENaC subunits, we have generated doxycycline-repressible replication-defective recombinant adenoviruses encoding individual epitope-tagged mouse ENaC subunits and expressed these in polarized MDCK I cells. Co-infection with these viruses encoding all three subunits generates robust amiloride-sensitive currents in polarized MDCK cells. Significant current was also observed in cells expressing alpha- and gamma-mENaC in the absence of beta-mENaC. These currents did not appear to result from association with endogenous canine beta-ENaC. Treatment of alpha beta gamma-expressing cells with cycloheximide (CHX) resulted in the rapid inhibition (within 3 h) of approximately 50-80% of the initial current; however, a sizable fraction of the initial current remained even after 6 h of CHX. By contrast, CHX addition to cells expressing only alpha- and gamma-mENaC resulted in rapid decay in current with no residual fraction. Our data suggest that ENaC channels of differing stoichiometries are differentially trafficked and degraded and provide support for the possibility that noncoordinate trafficking of ENaC subunits may function in vivo as a mechanism to modulate ENaC activity.

Functional proteomic screens reveal an essential extracellular role for hsp90 alpha in cancer cell invasiveness

Tumour cell invasiveness is crucial for cancer metastasis and is not yet understood. Here we describe two functional screens for proteins required for the invasion of fibrosarcoma cells that identified the molecular chaperone heat shock protein 90 (hsp90). The hsp90 alpha isoform, but not hsp90 beta, is expressed extracellularly where it interacts with the matrix metalloproteinase 2 (MMP2). Inhibition of extracellular hsp90 alpha decreases both MMP2 activity and invasiveness. This role for extracellular hsp90 alpha in MMP2 activation indicates that cell-impermeant anti-hsp90 drugs might decrease invasiveness without the concerns inherent in inhibiting intracellular hsp90.

The epithelial sodium channel: from molecule to disease

Genetic analysis has demonstrated that Na absorption in the aldosterone-sensitive distal nephron (ASDN) critically determines extracellular blood volume and blood pressure variations. The epithelial sodium channel (ENaC) represents the main transport pathway for Na+ absorption in the ASDN, in particular in the connecting tubule (CNT), which shows the highest capacity for ENaC-mediated Na+ absorption. Gain-of-function mutations of ENaC causing hypertension target an intracellular proline-rich sequence involved in the control of ENaC activity at the cell surface. In animal models, these ENaC mutations exacerbate Na+ transport in response to aldosterone, an effect that likely plays an important role in the development of volume expansion and hypertension. Recent studies of the functional consequences of mutations in genes controlling Na+ absorption in the ASDN provide a new understanding of the molecular and cellular mechanisms underlying the pathogenesis of salt-sensitive hypertension.

A stomatin and a degenerin interact in lipid rafts of the nervous system of Caenorhabditis elegans

In Caenorhabditis elegans, the gene unc-1 controls anesthetic sensitivity and normal locomotion. The protein UNC-1 is a close homolog of the mammalian protein stomatin and is expressed primarily in the nervous system. Genetic studies in C. elegans have shown that the UNC-1 protein interacts with a sodium channel subunit, UNC-8. In humans, absence of stomatin is associated with abnormal sodium and potassium levels in red blood cells. Stomatin also has been postulated to participate in the formation of lipid rafts, which are membrane microdomains associated with protein complexes, cholesterol, and sphingolipids. In this study, we isolated a low-density, detergent-resistant fraction from cell membranes of C. elegans. This fraction contains cholesterol, sphingolipids, and protein consistent with their identification as lipid rafts. We then probed Western blots of protein from the rafts and found that the UNC-1 protein is almost totally restricted to this fraction. The UNC-8 protein is also found in rafts and coimmunoprecipitates UNC-1. A second stomatin-like protein, UNC-24, also affects anesthetic sensitivity, is found in lipid rafts, and regulates UNC-1 distribution. Mutations in the unc-24 gene alter the distribution of UNC-1 in lipid rafts. Each of these mutations alters anesthetic sensitivity in C. elegans. Because lipid rafts contain many of the putative targets of volatile anesthetics, they may represent a novel class of targets for volatile anesthetics.

Noncoordinate regulation of ENaC: paradigm lost?

The epithelial sodium channel (ENaC) is composed of the three homologous subunits α, β, and γ. The basic oligomerization process inferred from all studies in heterologous systems is preferential assembly of the three subunits into a single oligomeric form. However, there is also considerable evidence that channels composed of only α-, αβ-, or αγ-subunits can form under some circumstances and that individual subunits expressed in heterologous systems can traffic to the cell membrane. In cells that express endogenous ENaC, the three subunits are often synthesized in a differential fashion, with one or two subunits expressed constitutively while the other(s) are induced by different physiological stimuli in parallel with increased ENaC activity. This phenomenon, which we term noncoordinate regulation, has been observed for both whole cell and apical membrane ENaC subunit expression. Several other heteromeric membrane proteins have also been observed to have differential rates of either turnover or trafficking of individual subunits after biosynthesis and membrane localization. Here, we examine the possibility that noncoordinate regulation of ENaC subunits may represent another mechanism in the arsenal of physiological responses to diverse stimuli.

Regulation of the epithelial Na+ channel by cytosolic ATP

The epithelial Na+ channel (ENaC), composed of three subunits (alphabetagamma), is expressed in various Na(+)-absorbing epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. By using patch clamp techniques, we have examined the effect of cytosolic ATP on the activity of the rat alphabetagammaENaC (rENaC) stably expressed in NIH-3T3 cells and in Madin-Darby canine kidney epithelial cells. The inward whole-cell current attributable to rENaC activity ran down when these cells were dialyzed with an ATP-free pipette solution in the conventional whole-cell voltage-clamping technique. This run down was prevented by 2 mM ATP (but not by AMP or ADP) in the pipette solution or by the poorly or non-hydrolyzable analogues of ATP (adenosine 5'-O-(thiotriphosphate) and adenosine 5'-(beta,gamma-imino)triphosphate) in both cell lines, suggesting that protection from run down was mediated through non-hydrolytic nucleotide binding. Accordingly, we demonstrate binding of ATP (but not AMP) to alpharENaC expressed in Madin-Darby canine kidney cells, which was inhibited upon mutation of the two putative nucleotide-binding motifs of alpharENaC. Single channel analyses indicated that the run down of currents observed in the whole-cell recording was attributable to run down of channel activity, defined as NPo (the product of the number of channels and open probability). We propose that this novel ATP regulation of ENaC may be, at least in part, involved in the fine-tuning of ENaC activity under physiologic and pathophysiologic conditions.

PPARgamma activation enhances cell surface ENaCalpha via up-regulation of SGK1 in human collecting duct cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-dependent transcription factor that belongs to the nuclear receptor family that plays a critical role in adipocyte differentiation and lipid metabolism. Here we report for the first time that PPARgamma is expressed in human renal cortical collecting ducts (CCD), segments of the nephor involved in regulation of sodium and water homeostasis via action of the epithelial sodium channel (ENaC). ENaC activity is regulated by the hormones aldosterone and insulin, primarily through co-ordinate actions on serum and glucocorticoid regulated kinase 1 (SGK1). We show that SGK1 activity is stimulated by treatment of a human CCD cell line with PPARgamma agonists, paralleled by an increase in SGK1 mRNA that is abolished by pretreatment with a specific PPARgamma antagonist, and that this leads to increased levels of cell surface ENaCalpha. Electrophoretic mobility shift assays suggest that these effects are caused by binding of PPARgamma to a specific response element in the SGK1 promoter. Our results identify SGK1 as a target for PPARgamma and suggest a novel role for PPARgamma in regulation of sodium re-absorption in the CCD via stimulation of ENaC activity. This pathway may play a role in sodium retention caused by activation of PPARgamma in man.

Multiple epithelial Na+ channel domains participate in subunit assembly

Epithelial sodium channels (ENaCs) are composed of three structurally related subunits that form a tetrameric channel. The Xenopus laevis oocyte expression system was used to identify regions within the ENaC alpha-subunit that confer a dominant negative phenotype on functional expression of alphabetagamma-ENaC to define domains that have a role in subunit-subunit interactions. Coexpression of full-length mouse alphabetagamma-ENaC with either 1) the alpha-subunit first membrane-spanning domain and short downstream hydrophobic domain (alpha-M1H1); 2) alpha-M1H1 and its downstream hydrophilic extracellular loop (alpha-M1H1-ECL); 3) the membrane-spanning domain of a control type 2 transmembrane protein (glutamyl transpeptidase; gamma-GT) fused to the alpha-ECL (gamma-GT-alpha-ECL); 4) the extracellular domain of a control type 1 transmembrane protein (Tac) fused to the alpha-subunit second membrane-spanning domain and short upstream hydrophobic domain (Tac-alpha-H2M2); or 5) the alpha-subunit cytoplasmic COOH terminus (alpha-Ct) significantly reduced amiloride-sensitive Na+ currents in X. laevis oocytes. Functional expression of Na+ channels was not inhibited when full-length alphabetagamma-ENaC was coexpressed with either 1) the alpha-ECL lacking a signal-anchor sequence, 2) alpha-M1H1 and alpha-Ct expressed as a fusion protein, 3) full-length gamma-GT, or 4) full-length Tac. Furthermore, the expression of ROMK channels was not inhibited when full-length ROMK was coexpressed with either alpha-M1H1-ECL or alpha-Ct. Full-length FLAG-tagged alpha-, beta-, or gamma-ENaC coimmunoprecipitated with myc-tagged alpha-M1H1-ECL, whereas wild-type gamma-GT did not. These data suggest that multiple sites within the alpha-subunit participate in subunit-subunit interactions that are required for proper assembly of the heterooligomeric ENaC complex.

Maturation of the epithelial Na+ channel involves proteolytic processing of the alpha- and gamma-subunits

The epithelial Na+ channel (ENaC) is a tetramer of two alpha-, one beta-, and one gamma-subunit, but little is known about its assembly and processing. Because co-expression of mouse ENaC subunits with three different carboxyl-terminal epitope tags produced an amiloride-sensitive sodium current in oocytes, these tagged subunits were expressed in both Chinese hamster ovary or Madin-Darby canine kidney type 1 epithelial cells for further study. When expressed alone alpha-(95 kDa), beta-(96 kDa), and gamma-subunits (93 kDa) each produced a single band on SDS gels by immunoblotting. However, co-expression of alphabetagammaENaC subunits revealed a second band for each subunit (65 kDa for alpha, 110 kDa for beta, and 75 kDa for gamma) that exhibited N-glycans that had been processed to complex type based on sensitivity to treatment with neuraminidase, resistance to cleavage by endoglycosidase H, and GalNAc-independent labeling with [3H]Gal in glycosylation-defective Chinese hamster ovary cells (ldlD). The smaller size of the processed alpha- and gamma-subunits is also consistent with proteolytic cleavage. By using alpha- and gamma-subunits with epitope tags at both the amino and carboxyl termini, proteolytic processing of the alpha- and gamma-subunits was confirmed by isolation of an additional epitope-tagged fragment from the amino terminus (30 kDa for alpha and 18 kDa for gamma) consistent with cleavage within the extracellular loop. The fragments remain stably associated with the channel as shown by immunoblotting of co-immunoprecipitates, suggesting that proteolytic cleavage represents maturation rather than degradation of the channel.

Downregulation of ENaC activity and expression by TNF-alpha in alveolar epithelial cells

Sodium absorption by an amiloride-sensitive channel is the main driving force of lung liquid clearance at birth and lung edema clearance in adulthood. In this study, we tested whether tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine involved in several lung pathologies, could modulate sodium absorption in cultured alveolar epithelial cells. We found that TNF-alpha decreased the expression of the alpha-, beta-, and gamma-subunits of epithelial sodium channel (ENaC) mRNA to 36, 43, and 16% of the controls after 24-h treatment and reduced to 50% the amount of alpha-ENaC protein in these cells. There was no impact, however, on alpha(1) and beta(1) Na(+)-K(+)-ATPase mRNA expression. Amiloride-sensitive current and ouabain-sensitive Rb(+) uptake were reduced, respectively, to 28 and 39% of the controls. A strong correlation was found at different TNF-alpha concentrations between the decrease of amiloride-sensitive current and alpha-ENaC mRNA expression. All these data show that TNF-alpha, a proinflammatory cytokine present during lung infection, has a profound influence on the capacity of alveolar epithelial cells to transport sodium.

N-Glycosylation and conserved cysteine residues in RAMP3 play a critical role for the functional expression of CRLR/RAMP3 adrenomedullin receptor

The calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein-3 (RAMP3) can assemble into a CRLR/RAMP3 heterodimeric receptor that exhibits the characteristics of a high affinity adrenomedullin receptor. RAMP3 participates in adrenomedullin (AM) binding via its extracellular N-terminus characterized by the presence of six highly conserved cysteine residues and four N-glycosylation consensus sites. Here, we assessed the usage of these conserved residues in cotranslational modifications of RAMP3 and addressed their role in functional expression of the CRLR/RAMP3 receptor. Using a Xenopus oocyte expression system, we show that (i) RAMP3 is assembled with CRLR as a multiple N-glycosylated species in which two, three, or four consensus sites are used; (ii) elimination of all N-glycans in RAMP3 results in a significant inhibition of receptor [(125)I]AM binding and an increase in the EC(50) value for AM; (iii) several lines of indirect evidence indicate that each of the six cysteines is involved in disulfide bond formation; (iv) when all cysteines are mutated to serines, RAMP3 is N-glycosylated at all four consensus sites, suggesting that disulfide bond formation inhibits N-gylcosylation; and (v) elimination of all cysteines abolishes adrenomedullin binding and leads to a complete loss of receptor function. Our data demonstrate that cotranslational modifications of RAMP3 play a critical role in the function of the CRLR/RAMP3 adrenomedullin receptor.

Epithelial Na+ channel mutants causing Liddle's syndrome retain ability to respond to aldosterone and vasopressin

Liddle's syndrome is a monogenic form of hypertension caused by mutations in the PY motif of the COOH terminus of beta- and gamma-epithelial Na+ channel (ENaC) subunits. These mutations lead to retention of active channels at the cell surface. Because of the critical role of this PY motif in the stability of ENaCs at the cell surface, we have investigated its contribution to the ENaC response to aldosterone and vasopressin. Mutants of the PY motif in beta- and gamma-ENaC subunits (beta-Y618A, beta-P616L, beta-R564stop, and gamma-K570stop) were stably expressed by retroviral gene transfer in a renal cortical collecting duct cell line (mpkCCDcl4), and transepithelial Na+ transport was assessed by measurements of the benzamil-sensitive short-circuit current (Isc). Cells that express ENaC mutants of the PY motif showed a five- to sixfold higher basal Isc compared with control cells and responded to stimulation by aldosterone (10(-6) M) or vasopressin (10(-9) M) with a further increase in Isc. The rates of the initial increases in Isc after aldosterone or vasopressin stimulation were comparable in cells transduced with wild-type and mutant ENaCs, but reversal of the effects of aldosterone and vasopressin was slower in cells that expressed the ENaC mutants. The conserved sensitivity of ENaC mutants to stimulation by aldosterone and vasopressin together with the prolonged activity at the cell surface likely contribute to the increased Na+ absorption in the distal nephron of patients with Liddle's syndrome.

Endoplasmic reticulum retention, degradation, and aggregation of olfactory G-protein coupled receptors

The mammalian olfactory G-protein coupled receptor family is comprised of hundreds of proteins that mediate odorant binding and initiate signal transduction cascades leading to the sensation of smell. However, efforts to functionally express olfactory receptors and identify specific odorant ligand-olfactory receptor interactions have been severely impeded by poor olfactory receptor surface expression in heterologous systems. Therefore, experiments were performed to elucidate the cellular mechanism(s) responsible for inefficient olfactory receptor cell surface expression. We determined that the mouse odorant receptors mI7 and mOREG are not selected for export from the ER and therefore are not detectable at the Golgi apparatus or plasma membrane. Specifically, olfactory receptors interact with the ER chaperone calnexin, are excluded from ER export sites, do not accumulate in ER-Golgi transport intermediates at 15 degrees C, and contain endoglycosidase H-sensitive oligosaccharides, consistent with olfactory receptor exclusion from post-ER compartments. A labile pool of ER-retained olfactory receptors are post-translationally modified by polyubiquitination and targeted for degradation by the proteasome. In addition, olfactory receptors are sequestered into ER aggregates that are degraded by autophagy. Collectively, these data demonstrate that poor surface expression of olfactory receptors in heterologous cells is attributable to a combination of ER retention due to inefficient folding and poor coupling to ER export machinery, aggregation, and degradation via both proteasomal and autophagic pathways.

Targeted degradation of ENaC in response to PKC activation of the ERK1/2 cascade

Renal A6 epithelial cells were used to determine the mechanism by which protein kinase C (PKC) decreases epithelial Na(+) channel (ENaC) activity. Activation of PKC reduced relative Na(+) reabsorption to <20% within 60 min. This decrease was sustained over the next 24-48 h. In response to PKC signaling, alpha-, beta-, and gamma-ENaC levels were 0.97, 0.36, and 0.39, respectively, after 24 h, with the levels of the latter two subunits being significantly decreased. The PKC-mediated decreases in beta- and gamma-ENaC were significantly reversed by simultaneous addition of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase-1/2 inhibitors U-0126 and PD-98059. These inhibitors, in addition, protected Na(+) reabsorption from PKC, demonstrating that the MAPK1/2 cascade, in some instances, plays a central role in downregulation of ENaC activity. The effects of PKC on beta- and gamma-ENaC levels were additive with those of inhibitors of transcription (actinomycin D) and translation (emetine and cycloheximide), suggesting that PKC promotes subunit degradation and does not affect subunit synthesis. The bulk of whole cell gamma-ENaC was degraded within 1 h after treatment with inhibitors of synthesis; however, a significant pool was "protected" from inhibitors for up to 12 h. PKC affected this protected pool of gamma-ENaC. Moreover, proteosome inhibitors (MG-132 and lactacystin) reversed PKC effects on this protected pool of gamma-ENaC. Thus PKC signaling via MAPK1/2 cascade activation in A6 cells promotes degradation of gamma-ENaC.

Inhibitors of glycosphingolipid biosynthesis reduce transepithelial electrical resistance in MDCK I and FRT cells

Madin-Darby canine kidney (MDCK) I and Fisher rat thyroid (FRT) cells exhibit transepithelial electrical resistance (TER) values in excess of 5,000 Omega. cm(2). When these cells were incubated in the presence of various inhibitors of sphingolipid biosynthesis, a >5-fold reduction of TER was observed without changes in the gate function for uncharged solutes or the fence function for apically applied fluorescent lipids. The localization of ZO-1 and occludin was not altered between control and inhibitor-treated cells, indicating that the tight junction was still intact. Furthermore, the complexity of tight junction strands, analyzed by freeze-fracture microscopy, was not reduced. Once the inhibitor was removed and the cells were allowed to synthesize sphingolipids, a gradual recovery of the TER was observed. Interestingly, these inhibitors did not attenuate the TER of MDCK II cells, a cell line that typically exhibits values below 800 omega x cm(2.) These results suggest that glycosphingolipids play a role in regulating the electrical properties of epithelial cells.

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.

Epithelial sodium channel activity in detergent-resistant membrane microdomains

The activity of epithelial Na(+) selective channels is modulated by various factors, with growing evidence that membrane lipids also participate in the regulation. In the present study, Triton X-100 extracts of whole cells and of apical membrane-enriched preparations from cultured A6 renal epithelial cells were floated on continuous-sucrose-density gradients. Na(+) channel protein, probed by immunostaining of Western blots, was detected in the high-density fractions of the gradients (between 18 and 30% sucrose), which contain the detergent-soluble material but also in the lighter, detergent-resistant 16% sucrose fraction. Single amiloride-sensitive Na(+) channel activity, recorded after incorporation of reconstituted proteoliposomes into lipid bilayers, was exclusively localized in the 16% sucrose fraction. In accordance with other studies, high- and low-density fractions of sucrose gradients likely represent membrane domains with different lipid contents. However, exposure of the cells to cholesterol-depleting or sphingomyelin-depleting agents did not affect transepithelial Na(+) current, single-Na(+) channel activity, or the expression of Na(+) channel protein. This is the first reconstitution study of native epithelial Na(+) channels, which suggests that functional channels are compartmentalized in discrete domains within the plane of the apical cell membrane.

Hypoxia and beta 2-agonists regulate cell surface expression of the epithelial sodium channel in native alveolar epithelial cells

Alveolar hypoxia may impair sodium-dependent alveolar fluid transport and induce pulmonary edema in rat and human lung, an effect that can be prevented by the inhalation of beta(2)-agonists. To investigate the mechanism of beta(2)-agonist-mediated stimulation of sodium transport under conditions of moderate hypoxia, we examined the effect of terbutaline on epithelial sodium channel (ENaC) expression and activity in cultured rat alveolar epithelial type II cells exposed to 3% O(2) for 24 h. Hypoxia reduced transepithelial sodium current and amiloride-sensitive sodium channel activity without decreasing ENaC subunit mRNA or protein levels. The functional decrease was associated with reduced abundance of ENaC subunits (especially beta and gamma) in the apical membrane of hypoxic cells, as quantified by biotinylation. cAMP stimulation with terbutaline reversed the hypoxia-induced decrease in transepithelial sodium transport by stimulating sodium channel activity and markedly increased the abundance of beta-and gamma-ENaC in the plasma membrane of hypoxic cells. The effect of terbutaline was prevented by brefeldin A, a blocker of anterograde transport. These novel results establish that hypoxia-induced inhibition of amiloride-sensitive sodium channel activity is mediated by decreased apical expression of ENaC subunits and that beta(2)-agonists reverse this effect by enhancing the insertion of ENaC subunits into the membrane of hypoxic alveolar epithelial cells.

Reciprocal modulation between the alpha and beta 4 subunits of hSlo calcium-dependent potassium channels

Large conductance Ca(2+)-dependent potassium (K(Ca) or maxi K) channels are composed of a pore-forming alpha subunit and an auxiliary beta subunit. We have shown that the brain-specific beta4 subunit modulates the voltage dependence, activation kinetics, and toxin sensitivity of the hSlo channel (Weiger, T. M., Holmqvist, M. H., Levitan, I. B., Clark, F. T., Sprague, S., Huang, W. J., Ge, P., Wang, C., Lawson, D., Jurman, M. E., Glucksmann, M. A., Silos-Santiago, I., DiStefano, P. S., and Curtis, R. (2000) J. Neurosci. 20, 3563-3570). We investigated here the N-linked glycosylation of the beta4 subunit and its effect on the modulation of the hSlo alpha subunit. When expressed alone in HEK293 cells, the beta4 subunit runs as a single molecular weight band on an SDS gel. However, when coexpressed with the hSlo alpha subunit, the beta4 subunit appears as two different molecular weight bands. Enzymatic deglycosylation or mutation of the N-linked glycosylation residues in beta4 converts it to a single lower molecular weight band, even in the presence of the hSlo alpha subunit, suggesting that the beta4 subunit can be present as an immature, core glycosylated form and a mature, highly glycosylated form. Blockage of protein transport from the endoplasmic reticulum to the Golgi compartment with brefeldin A abolishes the mature, highly glycosylated beta4 band. Glycosylation of the beta4 subunit is not required for its binding to the hSlo channel alpha subunit. It also is not necessary for cell membrane targeting of the beta4 subunit, as demonstrated by surface biotinylation experiments. However, the double glycosylation site mutant beta4 (beta4 N53A/N90A) protects the channel less against toxin blockade, as compared with the hSlo channel coexpressed with wild type beta4 subunit. Taken together, these data show that the pore-forming alpha subunit of the hSlo channel promotes N-linked glycosylation of its auxiliary beta4 subunit, and this in turn influences the modulation of the channel by the beta4 subunit.

Endogenously expressed epithelial sodium channel is present in lipid rafts in A6 cells

The epithelial sodium channel (ENaC) present in the kidney collecting duct, distal colon, and the lung is responsible for salt reabsorption and whole body volume regulation. It is composed of three homologous subunits, alpha, beta, and gamma, and mutations to these subunits can lead to the salt wasting disease pseudohypoaldosteronism type I, associated with decreased channel density at the plasma membrane or to the hypertensive disorder, Liddle's syndrome, in which channel residency time at the plasma membrane is enhanced. Regulation of ENaC trafficking and turnover is therefore critical to sodium homeostasis. In this study we examined whether ENaC is present in the cholesterol-enriched microdomains commonly called lipid rafts, in the endogenously expressing A6 cell line. We demonstrate that a fraction of alpha, beta, and gamma ENaC is present in detergent-insoluble membranes, that subunits exist in membranes that float on discontinuous sucrose density gradients, and that methyl-beta-cyclodextrin treatment causes a redistribution of ENaC subunits to higher density membranes. Furthermore, chronic aldosterone stimulation results in a shift in the membrane density of all three subunits. Biotinylation of apical membrane proteins revealed that ENaC is present in lipid rafts on the plasma membrane. In conclusion, these results show that ENaC is present in lipid rafts both intracellularly and on the cell surface. Raft association may be important for trafficking and/or function of the channel.

Concerted action of ENaC, Nedd4-2, and Sgk1 in transepithelial Na(+) transport

The epithelial Na(+) channel (ENaC), located in the apical membrane of renal aldosterone-responsive epithelia, plays an essential role in controlling the Na(+) balance of extracellular fluids and hence blood pressure. As of now, ENaC is the only Na(+) transport protein for which genetic evidence exists for its involvement in the genesis of both hypertension (Liddle's syndrome) and hypotension (pseudohypoaldosteronism type 1). The regulation of ENaC involves a variety of hormonal signals (aldosterone, vasopressin, insulin), but the molecular mechanisms behind this regulation are mostly unknown. Two regulatory proteins have gained interest in recent years: the ubiquitin-protein ligase neural precursor cell-expressed, developmentally downregulated gene 4 isoform Nedd4-2, which negatively controls ENaC cell surface expression, and serum glucocorticoid-inducible kinase 1 (Sgk1), which is an aldosterone- and insulin-dependent, positive regulator of ENaC density at the plasma membrane. Here, we summarize present ideas about Sgk1 and Nedd4-2 and the lines of experimental evidence, suggesting that they act sequentially in the regulatory pathways governed by aldosterone and insulin and regulate ENaC number at the plasma membrane.

Effects of aldosterone on biosynthesis, traffic, and functional expression of epithelial sodium channels in A6 cells

The collecting duct regulates Na(+) transport by adjusting the abundance/activity of epithelial Na(+) channels (ENaC). In this study we have investigated the synthesis, degradation, endocytosis, and activity of ENaC and the effects of aldosterone on these processes using endogenous channels expressed in the A6 cell line. Biochemical studies were performed with a newly raised set of specific antibodies against each of the three subunits of the amphibian ENaC. Our results indicate simultaneous transcription and translation of alpha, beta, and gamma subunits and enhancement of both processes by aldosterone: two- and fourfold increase, respectively. The biosynthesis of new channels can be followed by acquisition of endoglycosidase H-resistant oligosacharides in alpha and beta subunits and, in the case of alpha, by the appearance of a form resistant to reducing agents. The half-life of the total pool of subunits (t(1/2) 40-70 min) is longer than the fraction of channels in the apical membrane (t(1/2) 12-17 min). Aldosterone induces a fourfold increase in the abundance of the three subunits in the apical membrane without significant changes in the open probability, kinetics of single channels, or in the rate of degradation of ENaC subunits. Accordingly, the aldosterone response could be accounted by an increase in the abundance of apical channels due, at least in part, to de novo synthesis of subunits.