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

The hormonal control of uterine luminal fluid secretion and absorption

The secretion of uterine luminal fluid initially provides a transport and support medium for spermatozoa and unimplanted embryos, while the absorption of uterine luminal fluid in early pregnancy results in the closure of the lumen and allows blastocysts to establish intimate contact with the uterine epithelium. We have established an in vivo perfusion technique of the lumen to study the hormonal control of the events in the peri-implantation period. Fluorescein-labelled dextran was included in the perfusion medium to monitor fluid movements and the concentrations of Na(+) and CI(-) ions in the effluent were monitored. Using an established regimen of steroid treatment of ovariectomized rats mimicking early pregnancy, oestradiol caused fluid secretion, while progesterone resulted in an amiloride-sensitive fluid absorption. Fluid absorption peaked at about the expected time of implantation. The effect of progesterone could be inhibited by treatment with a high dose of oestradiol, by the anti-progestin RU486, and by the presence of an intra-uterine contraceptive device. Studies of expression of Na(+) and CI(-) channels (ENaC, CFTR) indicated that these channels were subject to tissue-specific regulation within the uterus, but more work is required to determine their role and the factors controlling their abundance and localization in early pregnancy.

Liddle's syndrome mutations increase Na+ transport through dual effects on epithelial Na+ channel surface expression and proteolytic cleavage

Liddle's syndrome, an inherited form of hypertension, is caused by mutations that delete or disrupt a C-terminal PY motif in the epithelial Na+ channel (ENaC). Previous work indicates that these mutations increase expression of ENaC at the cell surface by disrupting its binding to Nedd4-2, an E3 ubiquitin-protein ligase that targets ENaC for degradation. However, it remains uncertain whether this mechanism alone is responsible; increased activity of ENaC channels could also contribute to excessive Na+ transport in Liddle's syndrome. ENaC activity is controlled in part by its cleavage state; proteolytic cleavage produces channels with a high open-state probability, whereas uncleaved channels are inactive. Here, we found that Liddle's syndrome mutations have two distinct effects of ENaC surface expression, both of which contribute to increased Na+ transport. First, these mutations increased ENaC expression at the cell surface; second, they increased the fraction of ENaC at the cell surface that was cleaved (active). This disproportionate increase in cleavage was reproduced by expression of a dominant-negative Nedd4-2 or mutation of ENaC ubiquitination sites, interventions that disrupt ENaC endocytosis and lysosomal degradation. Conversely, overexpression of Nedd4-2 had the opposite effect, decreasing the fraction of cleaved ENaC at the cell surface. Thus, the data not only suggest that Nedd4-2 regulates epithelial Na+ transport in part by controlling the relative expression of cleaved and uncleaved ENaC at the cell surface but also provide a mechanism by which Liddle's syndrome mutations alter ENaC activity.

Differential modulation of a polymorphism in the COOH terminus of the α-subunit of the human epithelial sodium channel by protein kinase Cδ

The A663T polymorphism of the α-subunit of the human epithelial sodium channel (hENaC) increases the functional and surface expression of αβγ-hENaC in Xenopus laevis oocytes. The context of this residue in the COOH terminus of α-hENaC is important for this effect, as a homologous change in murine ENaC (mENaC), A692T, does not alter functional and surface expression of mENaC. Query of a phosphoprotein database suggested that the α-T663 residue might be phosphorylated by PKCδ. General inhibition of PKC with calphostin C decreased the functional and surface expression of αT663-hENaC and not αA663-hENaC, and was without effect on αA692-mENaC, αT692-mENaC, and a chimeric m(1–678)/h(650–669)αT663, mβγ-ENaC. These data suggest that residues outside of the α-hENaC COOH terminus are important for modulation of αT663-hENaC trafficking by PKC. In contrast, expression of PKCδ decreased the functional and surface expression of αT663-hENaC and the functional expression of m(1–678)/h(650–669)αT663, mβγ-ENaC, and was without effect on αA663-hENaC, αA692-mENaC, or αT692-mENaC. PKCδ did not phosphorylate the COOH terminus of either αT663-hENaC or αA663-hENaC in vitro, suggesting that it acts indirectly to regulate hENaC trafficking. αT663-hENaC was retrieved from the oocyte membrane more slowly than αA663-hENaC, and calphostin C increased the rate of αT663-hENaC removal from the oocyte membrane to a rate similar to that of αA663-hENaC. In contrast, PKCδ did not alter the rate of removal of αT663-hENaC from the oocyte membrane, suggesting that PKCδ altered rates of αT663-hENaC biosynthesis and/or delivery to the plasma membrane. These data are consistent with PKC isoform-specific effects on the intracellular trafficking of αT663- vs. αA663-hENaC.

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.

The epidermal barrier function is dependent on the serine protease CAP1/Prss8

Serine proteases are proteolytic enzymes that are involved in the regulation of various physiological processes. We generated mice lacking the membrane-anchored channel-activating serine protease (CAP) 1 (also termed protease serine S1 family member 8 [Prss8] and prostasin) in skin, and these mice died within 60 h after birth. They presented a lower body weight and exhibited severe malformation of the stratum corneum (SC). This aberrant skin development was accompanied by an impaired skin barrier function, as evidenced by dehydration and skin permeability assay and transepidermal water loss measurements leading to rapid, fatal dehydration. Analysis of differentiation markers revealed no major alterations in CAP1/Prss8-deficient skin even though the epidermal deficiency of CAP1/Prss8 expression disturbs SC lipid composition, corneocyte morphogenesis, and the processing of profilaggrin. The examination of tight junction proteins revealed an absence of occludin, which did not prevent the diffusion of subcutaneously injected tracer (∼600 D) toward the skin surface. This study shows that CAP1/Prss8 expression in the epidermis is crucial for the epidermal permeability barrier and is, thereby, indispensable for postnatal survival.

Oxygen and glucocorticoids modulate alphaENaC mRNA translation in fetal distal lung epithelium

Glucocorticoid hormones play an important role in fetal lung maturation. It is unknown how they interact with changes in O2 tension, which play an important role in converting the lung from a fluid-secreting to a fluid-absorbing organ at birth. Airspace fluid absorption arises from active transepithelial Na+ transport with the amiloride-sensitive epithelial Na channel (ENaC), consisting of alpha, beta, and gamma subunits, representing the rate-limiting step under nonpathologic conditions. We investigated the individual and combined effects of dexamethasone (DEX) and PO2 on alphaENaC mRNA levels, rate of alphaENaC protein synthesis, and amiloride-sensitive short-circuit current in primary cultures of rat fetal distal lung epithelial cells. DEX significantly induced alphaENaC mRNA in fetal (3%) and postnatal (21%) O2, but increases in alphaENaC protein synthesis and function occurred only when epithelia were grown under a postnatal PO2. Sucrose density gradient analyses showed that DEX treatment of cells cultured at 3% O2 decreased the association of alphaENaC mRNA with large polysomes and enhanced the association with small polysomes. Conversely, incubation of DEX-treated cells in 21% O2 restored alphaENaC mRNA association with large polysomes. No significant changes were seen in the overall polyribosome profiles or in the distribution of mRNAs encoding beta and gamma subunits of ENaC or cytokeratin 18, indicating specific modulation of alphaENaC mRNA translation. These data suggest that postnatal O2 exposure may be important for efficient translation of the alphaENaC mRNA.

New insights into epithelial sodium channel function in the kidney: site of action, regulation by ubiquitin ligases, serum- and glucocorticoid-inducible kinase and proteolysis

PURPOSE OF REVIEW

The epithelial sodium channel (ENaC) sets the rate of Na+ reabsorption in the collecting duct. This review describes recent advances in our understanding of ENaC function.

RECENT FINDINGS

First, collecting duct-specific deletion of alphaENaC does not cause Na wasting in mice, suggesting that other regions can compensate. Second, Nedd4 and Nedd4-2 are ubiquitin ligases that reduce surface expression of ENaC and inhibit Na+ transport. Nedd4-2, but not Nedd4, is negatively regulated by serum- and glucocorticoid-inducible kinase 1, an aldosterone-induced kinase, providing an attractive mechanism for the stimulatory effect of aldosterone on Na+ transport. However, mice with germline ablation of serum- and glucocorticoid-inducible kinase 1 show only modest hypotension and are able to decrease Na+ excretion rates substantially. Third, maturation of ENaC is associated with processing at consensus furin cleavage sites and this cleavage is critical for channel activity. A separate class of serine proteases, the channel-activating proteases, also stimulates ENaC activity.

SUMMARY

The connecting tubule of the kidney has abundant ENaC and Na(+)- and K(+)-transport capacity and may provide much of ENaC-mediated Na+ transport in the kidney. Aldosterone may increase Na transport, in part, by serum- and glucocorticoid-inducible kinase 1-mediated inhibition of Nedd4-2 but this has not been demonstrated in the native collecting duct or connecting tubule. The mild phenotype of the serum- and glucocorticoid-inducible kinase 1-knockout mouse points to serum- and glucocorticoid-inducible kinase 1-independent mechanisms that regulate Na+ transport. Two separate classes of protease appear to regulate Na+ transport: one is furin or furin-like and cleaves ENaC subunits to stimulate transport; the other, the channel-activating proteases, may act on ENaC or a regulatory molecule.

Inhibition of prostasin expression by TGF-beta1 in renal epithelial cells

BACKGROUND

Prostasin has been shown to be involved in the regulation of sodium handling in the kidney. TGF-beta1 has been demonstrated to suppress alphaENaC expression and sodium uptake. Therefore, we hypothesized that TGF-beta1 may regulate prostasin expression to modulate sodium reabsorption in the kidney.

METHODS

To determine if TGF-beta1 has an effect on prostasin expression, we isolated 2.9 kb of the rat prostasin promoter, and measured its transcriptional activity with a luciferase assay in mouse cortical collecting duct cell line (M-1). The effect of TGF-beta1 on the mRNA and protein abundance of prostasin, and amiloride-sensitive (22)Na uptake was determined.

RESULTS

Treatment of M-1 cells with 20 ng/mL of TGF-beta1 for 24 hours significantly decreased the promoter activity by 50 +/- 1%, and the inhibitory effect was dose dependent over the range of 0.1 to 20 ng/mL. We identified a 50 bp region (-410 to -360) containing c-Rel-like sequence in prostasin promoter that is responsible for the TGF-beta1-mediated inhibition, and found that TGF-beta1 increases IkappaBalpha expression in M-1 cells. TGF-beta1 reduced endogenous prostasin mRNA and protein expression in M-1 cells by 50 +/- 12% and 44 +/- 12%, respectively, and the amiloride-sensitive (22)Na uptake by 35.9 +/- 4.8%.

CONCLUSION

Our findings indicate the possibility that TGF-beta1 transcriptionally inhibits prostasin expression by the induction of IkappaBalpha and the subsequent inhibition of NF-kappaB/Rel activity in M-1 cells, and also suggest the possibility that TGF-beta1 might inhibit sodium reabsorption through a reduction in prostasin expression and subsequent inhibition of ENaC activity.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

Cystic fibrosis results from disruption of the biosynthesis or function of the cystic fibrosis transmembrane conductance regulator. Cystic fibrosis transmembrane conductance regulator plays a critical role in the regulation of epithelial ion transport. Restoration of cystic fibrosis transmembrane conductance regulator function should improve the cystic fibrosis phenotype.

RECENT FINDINGS

Recent investigations affording a better understanding of the mechanism of dysfunction of mutant cystic fibrosis transmembrane conductance regulators, as well as the roles of cystic fibrosis transmembrane conductance regulator in regulating epithelial ion transport, have led to development of therapeutic strategies based on repair or bypass of mutant cystic fibrosis transmembrane conductance regulator dysfunction. The former strategy, coined 'protein repair therapy,' is aimed at improving or restoring the function of mutant cystic fibrosis transmembrane conductance regulators, whereas the latter approach aims to augment epithelial ion transport to compensate for the absent function mutant cystic fibrosis transmembrane conductance regulator.

SUMMARY

Strategies to improve mutant cystic fibrosis transmembrane conductance regulator function or to bypass mutant cystic fibrosis transmembrane conductance regulator function hold great promise for development of novel therapies aimed at correcting the underlying pathophysiology of cystic fibrosis.

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.

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.

Epithelial Sodium Channel Inhibition by AMP-activated Protein Kinase in Oocytes and Polarized Renal Epithelial Cells

The epithelial Na(+) channel (ENaC) regulates epithelial salt and water reabsorption, processes that require significant expenditure of cellular energy. To test whether the ubiquitous metabolic sensor AMP-activated kinase (AMPK) regulates ENaC, we examined the effects of AMPK activation on amiloride-sensitive currents in Xenopus oocytes and polarized mouse collecting duct mpkCCD(c14) cells. Microinjection of oocytes expressing mouse ENaC (mENaC) with either active AMPK protein or an AMPK activator inhibited mENaC currents relative to controls as measured by two-electrode voltage-clamp studies. Similarly, pharmacological AMPK activation or overexpression of an activating AMPK mutant in mpkCCD(c14) cells inhibited amiloride-sensitive short circuit currents. Expression of a degenerin mutant beta-mENaC subunit (S518K) along with wild type alpha and gamma increased the channel open probability (P(o)) to approximately 1. However, AMPK activation inhibited currents similarly with expression of either degenerin mutant or wild type mENaC. Single channel recordings under these conditions demonstrated that neither P(o) nor channel conductance was affected by AMPK activation. Moreover, expression of a Liddle's syndrome-type beta-mENaC mutant (Y618A) greatly enhanced ENaC whole cell currents relative to wild type ENaC controls and prevented AMPK-dependent inhibition. These findings indicate that AMPK-dependent ENaC inhibition is mediated through a decrease in the number of active channels at the plasma membrane (N), presumably through enhanced Nedd4-2-dependent ENaC endocytosis. The AMPK-ENaC interaction appears to be indirect; AMPK did not bind ENaC in cells, as assessed by in vivo pull-down assays, nor did it phosphorylate ENaC in vitro. In summary, these results suggest a novel mechanism for coupling ENaC activity and renal Na(+) handling to cellular metabolic status through AMPK, which may help prevent cellular Na(+) loading under hypoxic or ischemic conditions.

In vitro and in vivo regulation of transepithelial lung alveolar sodium transport by serine proteases

The amiloride-sensitive epithelial sodium channel (ENaC) constitutes a rate-limiting step for sodium (Na+) and water absorption across lung alveolar epithelium. Recent reports suggested that ENaC is regulated by membrane-bound extracellular serine proteases, such as channel-activating proteases (CAPs). The objectives of this study were to examine the role of serine proteases in the regulation of transepithelial alveolar Na+ and water transport in vitro and in vivo and the expression of CAPs in rodent distal lung. In vitro experiments showed that inhibition of endogenous serine proteases by apical aprotinin 1) decreased ENaC-mediated currents in primary cultures of rat and mouse alveolar epithelial cells without affecting the abundance nor the electrophoretic migration pattern of biotinylated alpha- and beta-ENaC expressed at the cell surface and 2) suppressed the increase in amiloride-sensitive short-circuit current induced by the beta2-agonist terbutaline. RT-PCR experiments indicated that CAP1, CAP2, and CAP3 mRNAs were expressed in mouse alveolar epithelial cells, whereas CAP1 was also expressed in alveolar macrophages recovered by bronchoalveolar lavage. CAP1 protein was detected by Western blotting in rat and mouse alveolar epithelial cells, alveolar macrophages and bronchoalveolar lavage fluid. Finally, in vivo experiments revealed that intra-alveolar treatment with aprotinin abolished the increase in Na+-driven alveolar fluid clearance (AFC) induced by terbutaline in an in situ mouse lung model, whereas trypsin potentiated it. These results show that endogenous membrane-bound and/or secreted serine proteases such as CAPs regulate alveolar Na+ and fluid transport in vitro and in vivo in rodent lung.

Biochemical characterization of prostasin, a channel activating protease

Human prostasin was recently identified as a potential regulator of epithelial sodium channel (ENaC) function. Through the use of positional scanning combinatorial substrate libraries, prostasin was shown to have a preference for poly-basic substrates: in position P4 preference was for arginine or lysine; in P3 preference was for histidine, lysine or arginine; in P2 preference was for basic or large hydrophobic amino acids; and in P1 preference was for arginine and lysine. P1', P2', and P3' displayed broad selectivity with the exception of a lack of activity for isoleucine, and P4' had a preference for small, unbranched, amino acids such as alanine and serine. A prostasin-preferred poly-basic cleavage site was found in the extracellular domains of the ENaC alpha- and beta-subunits, and may present a mechanism for prostasin activation. The absence of activity seen with substrates containing isoleucine in position P1' explains the inability of prostasin to autoactivate and suggests that prostasin proteolytic activity is regulated by an upstream protease. Prostasin activity was highly influenced by mono- and divalent metal ions which were potent inhibitors and substrate specific modulators of enzymatic activity. In the presence of sub-inhibitory concentrations of zinc, the activity of prostasin increased several-fold and its substrate specificity was significantly altered in favor of a strong preference for histidine in positions P3 or P4 of the substrate.

Distinct Pools of Epithelial Sodium Channels Are Expressed at the Plasma Membrane

The epithelial Na+ channel (ENaC) is assembled in the endoplasmic reticulum from three structurally related subunits (alpha, beta, and gamma). Channel maturation within the biosynthetic pathway involves cleavage of the alpha and gamma subunits by furin and processing of N-linked glycans on alpha, beta, and gamma to complex type. Both mature and immature subunits have been observed at the surface of stably transfected Madin-Darby canine kidney cells. We have examined whether channel maturation is an all-or-none event or whether heterogeneous processing of channel subunits occurs within an individual channel complex. Using an immobilized lectin to isolate proteins with complex type N-glycans, we found that individual channel complexes with mature subunits lack immature subunits. Furthermore, terminal processing of N-glycans on ENaC subunits was not dependent on cleavage of ENaC subunits, and proteolysis of channel subunits was not dependent on prior processing of N-glycans. Our results suggest that processing of subunits within an individual channel complex is an all-or-none event such that channels present on the cell surface contain either all mature or all immature subunits. The presence of immature channel complexes at the plasma membrane provides epithelial cells with a reserve of poorly functional channels that can be activated by proteases in post-Golgi compartments.

Prostasin, a membrane-anchored serine peptidase, regulates sodium currents in JME/CF15 cells, a cystic fibrosis airway epithelial cell line

Prostasin is a tryptic peptidase expressed in prostate, kidney, lung, and airway. Mammalian prostasins are related to Xenopus channel-activating protease, which stimulates epithelial Na+channel (ENaC) activity in frogs. In human epithelia, prostasin is one of several membrane peptidases proposed to regulate ENaC. This study tests the hypothesis that prostasin can regulate ENaC in cystic fibrosis epithelia in which excessive Na+uptake contributes to salt and water imbalance. We show that prostasin mRNA and protein are strongly expressed by human airway epithelial cell lines, including immortalized JME/CF15 nasal epithelial cells homozygous for the ΔF508 cystic fibrosis mutation. Epithelial cells transfected with vectors encoding recombinant soluble prostasin secrete active, tryptic peptidase that is highly sensitive to inactivation by aprotinin. When studied as monolayers in Ussing chambers, JME/CF15 cells exhibit amiloride-sensitive, transepithelial Na+currents that are markedly diminished by aprotinin, suggesting regulation by serine-class peptidases. Overproduction of membrane-anchored prostasin in transfected JME/CF15 cells does not augment Na+currents, and trypsin-induced increases are small, suggesting that baseline serine peptidase-dependent ENaC activation is maximal in these cells. To probe prostasin’s involvement in basal ENaC activity, we silenced expression of prostasin using short interfering RNA targeting of prostasin mRNA’s 3′-untranslated region. This drops ENaC currents to 26 ± 9% of baseline. These data predict that prostasin is a major regulator of ENaC-mediated Na+current in ΔF508 cystic fibrosis epithelia and suggest that airway prostasin is a target for therapeutic inhibition to normalize ion current in cystic fibrosis airway.

Selective regulation of acid-sensing ion channel 1 by serine proteases

Acid-sensing ion channels (ASICs) are neuronal Na(+) channels that belong to the epithelial Na(+) channel/degenerin family. ASICs are transiently activated by a rapid drop in extracellular pH. Conditions of low extracellular pH, such as ischemia and inflammation in which ASICs are thought to be active, are accompanied by increased protease activity. We show here that serine proteases modulate the function of ASIC1a and ASIC1b but not of ASIC2a and ASIC3. We show that protease exposure shifts the pH dependence of ASIC1a activation and steady-state inactivation to more acidic pH. As a consequence, protease exposure leads to a decrease in current response if ASIC1a is activated by a pH drop from pH 7.4. If, however, acidification occurs from a basal pH of approximately 7, protease-exposed ASIC1a shows higher activity than untreated ASIC1a. We provide evidence that this bi-directional regulation of ASIC1a function also occurs in neurons. Thus, we have identified a mechanism that modulates ASIC function and may allow ASIC1a to adapt its gating to situations of persistent extracellular acidification.

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 Polymorphism in the Carboxyl Terminus of the α-Subunit of the Human Epithelial Sodium Channel

A common human epithelial sodium channel (ENaC) polymorphism, alphaT663A, is present in the cytoplasmic C terminus of the alpha-subunit, although it is unclear whether this polymorphism segregates with blood pressure. We examined whether this polymorphism was associated with differences in functional Na(+) channel expression. Whole cell amiloride-sensitive currents in Xenopus oocytes expressing wild type channels (alphaT663betagamma) were significantly approximately 1.3-2.0-fold higher than currents measured in oocytes expressing channels with an Ala, Gly or Leu, or Lys at position alpha663. In contrast, differences in functional human ENaC expression were not observed with oocytes expressing channels having Thr (wild type), Ser, or Asp at this position. The surface expression of channels, measured using an epitope-tagged beta-subunit, was significantly reduced in oocytes expressing alphaT663Abetagamma when compared with oocytes expressing alphaT663betagamma. The corresponding polymorphism was generated in the mouse alpha-subunit (malphaA692T) and was not associated with differences in functional alphabetagamma-mouse ENaC expression. The polymorphism is present in a region that is not well conserved between human and mouse. We generated a mouse/human chimera by replacement of the distal C terminus of the mouse alpha-subunit with the distal C terminus of the human alpha-subunit. Co-expression of this m(1-678)/h(650-669)T663A chimera with mouse betagamma led to a significant reduction in whole cell Na(+) currents and surface expression when compared with m(1-678)/h(650-669)T663-mbetagamma. Our results suggest that halphaT663A is a functional polymorphism that affects human ENaC surface expression.

Epithelial sodium channel: a ligand-gated channel?

The epithelial sodium channel (ENaC) is a key component of the transepithelial Na+ transport. In epithelia, it is responsible for the maintenance of Na+ balance (which in turn controls extracellular fluid volume and arterial blood pressure) and the regulation of airway surface fluid. While the regulation of channel synthesis and surface density have been well described, the control of channel opening is still poorly understood. The channel has a large extracellular domain of as yet unknown function; a number of extracellular factors have been shown to modulate ENaC activity, including extracellular Na+ itself (through a phenomenon called 'self-inhibition'), several other organic or inorganic cations, which seem to interfere with self-inhibition, and serine proteases. Although a direct interaction with the extracellular domain of ENaC has not yet been demonstrated for each of these modulators, the available data strongly suggest that ENaC behaves as a ligand-gated channel similar to several other members of the ENaC/degenerin family.

Cystic Fibrosis Transmembrane Conductance Regulator Differentially Regulates Human and Mouse Epithelial Sodium Channels in Xenopus Oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its well defined Cl- channel properties, regulates other ion channels. CFTR inhibits murine or rat epithelial Na+ channel (mENaC or rENaC) currents in many epithelial and non-epithelial cells, whereas murine or rat ENaC increases CFTR functional expression. These regulatory interactions are reproduced in Xenopus oocytes where both the open probability and surface expression of wild type CFTR Cl- channels are increased when CFTR is co-expressed with alphabetagamma mENaC, and conversely the activity of mENaC is inhibited after wild type CFTR activation. Using the Xenopus oocyte expression system, differences in functional regulatory interactions were observed when CFTR was co-expressed with either alphabetagamma mENaC or alphabetagamma human ENaC (hENaC). Co-expression of CFTR and alphabetagamma mENaC or hENaC resulted in an approximately 3-fold increase in CFTR Cl- current compared with oocytes expressing CFTR alone. Oocytes co-injected with both CFTR and mENaC or hENaC expressed an amiloride-sensitive whole cell current that was decreased compared with that observed with the injection of mENaC or hENaC alone before CFTR activation with forskolin/3-isobutyl-1-methylxanthine. CFTR activation resulted in a further 50% decrease in mENaC-mediated currents, an approximately 20% decrease in alpha-T663-hENaC-mediated currents, and essentially no change in alpha-A663-hENaC-mediated currents. Changes in ENaC functional expression correlated with ENaC surface expression by oocyte surface biotinylation experiments. Assessment of regulatory interactions between CFTR and chimeric mouse/human ENaCs suggest that the 20 C-terminal amino acid residues of alpha ENaC confer species specificity regarding ENaC inhibition by activated CFTR.

Epithelial sodium channels are activated by furin-dependent proteolysis

Epithelial Na(+) channels (ENaCs) are activated by extracellular trypsin or by co-expression with channel-activating proteases, although there is no direct evidence that these proteases activate ENaC by cleaving the channel. We previously demonstrated that the alpha and gamma subunits of ENaC are cleaved during maturation near consensus sites for furin cleavage. Using site-specific mutagenesis of channel subunits, ENaC expression in furin-deficient cells, and furin-specific inhibitors, we now report that ENaC cleavage correlates with channel activity. Channel activity in furin-deficient cells was rescued by expression of furin. Our data provide the first example of a vertebrate ion channel that is a substrate for furin and whose activity is dependent on its proteolysis.

Serine protease activation of near-silent epithelial Na+ channels

The regulation of epithelial Na+ channel (ENaC) function is critical for normal salt and water balance. This regulation is achieved through cell surface insertion/retrieval of channels, by changes in channel open probability (Po), or through a combination of these processes. Epithelium-derived serine proteases, including channel activating protease (CAP) and prostasin, regulate epithelial Na+ transport, but the molecular mechanism is unknown. We tested the hypothesis that extracellular serine proteases activate a near-silent ENaC population resident in the plasma membrane. Single-channel events were recorded in outside-out patches from fibroblasts (NIH/3T3) stably expressing rat alpha-, beta-, and gamma-subunits (rENaC), before and during exposure to trypsin, a serine protease homologous to CAP and prostasin. Under baseline conditions, near-silent patches were defined as having rENaC activity (NPo) < 0.03, where N is the number of channels. Within 1-5 min of 3 microg/ml bath trypsin superfusion, NPo increased approximately 66-fold (n = 7). In patches observed to contain a single functional channel, trypsin increased Po from 0.02 +/- 0.01 to 0.57 +/- 0.03 (n = 3, mean +/- SE), resulting from the combination of an increased channel open time and decreased channel closed time. Catalytic activity was required for activation of near-silent ENaC. Channel conductance and the Na+/Li+ current ratio with trypsin were similar to control values. Modulation of ENaC Po by endogenous epithelial serine proteases is a potentially important regulator of epithelial Na+ transport, distinct from the regulation achieved by hormone-induced plasma membrane insertion of channels.

Extracellular histidine residues crucial for Na+ self-inhibition of epithelial Na+ channels

Epithelial Na(+) channels (ENaC) participate in the regulation of extracellular fluid volume homeostasis and blood pressure. Channel activity is regulated by both extracellular and intracellular Na(+). The down-regulation of ENaC activity by external Na(+) is referred to as Na(+) self-inhibition. We investigated the structural determinants of Na(+) self-inhibition by expressing wild-type or mutant ENaCs in Xenopus oocytes and analyzing changes in whole-cell Na(+) currents following a rapid increase of bath Na(+) concentration. Our results indicated that wild-type mouse alphabetagammaENaC has intrinsic Na(+) self-inhibition similar to that reported for human, rat, and Xenopus ENaCs. Mutations at His(239) (gammaH239R, gammaH239D, and gammaH239C) in the extracellular loop of the gammaENaC subunit prevented Na(+) self-inhibition whereas mutations of the corresponding His(282) in alphaENaC (alphaH282D, alphaH282R, alphaH282W, and alphaH282C) significantly enhanced Na(+) self-inhibition. These results suggest that these two histidine residues within the extracellular loops are crucial structural determinants for Na(+) self-inhibition.

Regulation of amiloride-sensitive Na(+) transport by basal nitric oxide

We investigated the mechanisms of endogenous nitric oxide (NO) modulation of lung sodium (Na(+)) transport. C57BL/6 mice injected intraperitoneally with the specific inducible NO synthase (iNOS) inhibitor 1400W (10 mg/kg every 8 h for 72 h) exhibited decreased alveolar nitrite levels and Na(+)-dependent amiloride-sensitive alveolar fluid clearance as compared with mice injected with vehicle. Similarly, pretreatment of mouse tracheal epithelial cells with 1400W abolished the inhibitory effects of amiloride on their Na(+) short circuit currents. On the other hand, mouse tracheal epithelial cells pretreated with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a specific inhibitor of guanylate cyclase, had lower levels of cGMP, but normal values of amiloride-sensitive Na(+) currents. Amiloride also inhibited whole-cell Na(+) currents across A549 cells treated with vehicle (K(i) = 249 nM), but had no effect in A549 cells treated with 1400W. Western blotting studies showed significantly lower levels of alpha and gammaENaC in lung tissues and alveolar type II (ATII) cells from iNOS(-/-) as well as iNOS(+/+) mice treated with 1400W, as compared with the corresponding values from vehicle-treated iNOS(+/+) mice. Similar values for ratios of alpha, beta, and gammaenac to gapdh were obtained by real-time polymerase chain reaction for iNOS(+/+) mice and iNOS(-/-) mice. We concluded that NO derived from iNOS under basal conditions is necessary for amiloride-sensitive Na(+) transport across lung epithelial cells and modulates the amount of alpha and gammaENaC via post-transcriptional, cGMP-independent mechanisms.