Syntaxin 1A regulates ENaC channel activity

Epigenetics of the far northern Yakutian population

BACKGROUND

Yakuts are one of the indigenous populations of the subarctic and arctic territories of Siberia characterized by a continental subarctic climate with severe winters, with the regular January average temperature in the regional capital city of Yakutsk dipping below - 40 °C. The epigenetic mechanisms of adaptation to such ecologies and environments and, in particular, epigenetic age acceleration in the local population have not been studied before.

RESULTS

This work reports the first epigenetic study of the Yakutian population using whole-blood DNA methylation data, supplemented with the comparison to the residents of Central Russia. Gene set enrichment analysis revealed, among others, geographic region-specific differentially methylated regions associated with adaptation to climatic conditions (water consumption, digestive system regulation), aging processes (actin filament activity, cell fate), and both of them (channel activity, regulation of steroid and corticosteroid hormone secretion). Further, it is demonstrated that the epigenetic age acceleration of the Yakutian representatives is significantly higher than that of Central Russia counterparts. For both geographic regions, we showed that epigenetically males age faster than females, whereas no significant sex differences were found between the regions.

CONCLUSIONS

We performed the first study of the epigenetic data of the Yakutia cohort, paying special attention to region-specific features, aging processes, age acceleration, and sex specificity.

Role of SNARE Proteins in the Insertion of KCa3.1 in the Plasma Membrane of a Polarized Epithelium

Targeting proteins to a specific membrane is crucial for proper epithelial cell function. KCa3.1, a calcium-activated, intermediate-conductance potassium channel, is targeted to the basolateral membrane (BLM) in epithelial cells. Surprisingly, the mechanism of KCa3.1 membrane targeting is poorly understood. We previously reported that targeting of KCa3.1 to the BLM of epithelial cells is Myosin-Vc-, Rab1-and Rab8-dependent. Here, we examine the role of the SNARE proteins VAMP3, SNAP-23 and syntaxin 4 (STX-4) in the targeting of KCa3.1 to the BLM of Fischer rat thyroid (FRT) epithelial cells. We carried out immunoblot, siRNA and Ussing chamber experiments on FRT cells, stably expressing KCa3.1-BLAP/Bir-A-KDEL, grown as high-resistance monolayers. siRNA-mediated knockdown of VAMP3 reduced BLM expression of KCa3.1 by 57 ± 5% (p ≤ 0.05, n = 5). Measurements of BLM-localized KCa3.1 currents, in Ussing chambers, demonstrated knockdown of VAMP3 reduced KCa3.1 current by 70 ± 4% (p ≤ 0.05, n = 5). Similarly, siRNA knockdown of SNAP-23 reduced the expression of KCa3.1 at the BLM by 56 ± 7% (p ≤ 0.01, n = 6) and reduced KCa3.1 current by 80 ± 11% (p ≤ 0.05, n = 6). Also, knockdown of STX-4 lowered the BLM expression of KCa3.1 by 54 ± 6% (p ≤ 0.05, n = 5) and reduced KCa3.1 current by 78 ± 11% (p ≤ 0.05, n = 5). Finally, co-immunoprecipitation experiments demonstrated associations between KCa3.1, VAMP3, SNAP-23 and STX-4. These data indicate that VAMP3, SNAP-23 and STX-4 are critical for the targeting KCa3.1 to BLM of polarized epithelial cells.

Accessibility of ENaC extracellular domain central core residues

The epithelial Na+ channel (ENaC)/degenerin family has a similar extracellular architecture, where specific regulatory factors interact and alter channel gating behavior. The extracellular palm domain serves as a key link to the channel pore. In this study, we used cysteine-scanning mutagenesis to assess the functional effects of Cys-modifying reagents on palm domain β10 strand residues in mouse ENaC. Of the 13 ENaC α subunit mutants with Cys substitutions examined, only mutants at sites in the proximal region of β10 exhibited changes in channel activity in response to methanethiosulfonate reagents. Additionally, Cys substitutions at three proximal sites of β and γ subunit β10 strands also rendered mutant channels methanethiosulfonate-responsive. Moreover, multiple Cys mutants were activated by low concentrations of thiophilic Cd2+. Using the Na+ self-inhibition response to assess ENaC gating behavior, we identified four α, two β, and two γ subunit β10 strand mutations that changed the Na+ self-inhibition response. Our results suggest that the proximal regions of β10 strands in all three subunits are accessible to small aqueous compounds and Cd2+ and have a role in modulating ENaC gating. These results are consistent with a structural model of mouse ENaC that predicts the presence of aqueous tunnels adjacent to the proximal part of β10 and with previously resolved structures of a related family member where palm domain structural transitions were observed with channels in an open or closed state.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Effects of syntaxins 2, 3, and 4 on rat and human epithelial sodium channel (ENaC) in Xenopus laevis oocytes

Syntaxins are SNARE proteins and may play a role in epithelial sodium channel (ENaC) trafficking. The aim of the present study was to investigate the effects of syntaxin 2 (STX2), syntaxin 3 (STX3), and syntaxin 4 (STX4) on rat (rENaC) and human ENaC (hENaC). Co-expression of rENaC and STX3 or STX4 in Xenopus laevis oocytes increased amiloride-sensitive whole-cell currents (ΔI_ami) on average by 50% and 135%, respectively, compared to oocytes expressing rENaC alone. In contrast, STX2 had no significant effect on rENaC. Similar to its effect on rENaC, STX3 stimulated hENaC by 48%. In contrast, STX2 and STX4 inhibited hENaC by 51% and 44%, respectively. Using rENaC carrying a FLAG tag in the extracellular loop of the β-subunit, we demonstrated that the stimulatory effects of STX3 and STX4 on ΔI_ami were associated with an increased expression of the channel at the cell surface. Co-expression of STX3 or STX4 did not significantly alter the degree of proteolytic channel activation by chymotrypsin. STX3 had no effect on the inhibition of rENaC by brefeldin A, and the stimulatory effect of STX3 was preserved in the presence of dominant negative Rab11. This indicates that the stimulatory effect of STX3 is not mediated by inhibiting channel retrieval or by stimulating fusion of recycling endosomes. Our results suggest that the effects of syntaxins on ENaC are isoform and species dependent. Furthermore, our results demonstrate that STX3 increases ENaC expression at the cell surface, probably by enhancing insertion of vesicles carrying newly synthesized channels.

Membrane trafficking pathways regulating the epithelial Na+ channel

Renal Na⁺ reabsorption, facilitated by the epithelial Na⁺ channel (ENaC), is subject to multiple forms of control to ensure optimal body blood volume and pressure through altering both the ENaC population and activity at the cell surface. Here, the focus is on regulating the number of ENaCs present in the apical membrane domain through pathways of ENaC synthesis and targeting to the apical membrane as well as ENaC removal, recycling, and degradation. Finally, the mechanisms by which ENaC trafficking pathways are regulated are summarized.

Annexin II Light Chain p11 Interacts With ENaC to Increase Functional Activity at the Membrane

The epithelial Na⁺ channel (ENaC) provides for Na⁺ absorption in various types of epithelia including the kidney, lung, and colon where ENaC is localized to the apical membrane to enable Na⁺ entry into the cell. The degree of Na⁺ entry via ENaC largely depends on the number of active channels localized to the cell membrane, and is tightly controlled by interactions with ubiquitin ligases, kinases, and G-proteins. While regulation of ENaC endocytosis has been well-studied, relatively little is understood of the proteins that govern ENaC exocytosis. We hypothesized that the annexin II light chain, p11, could participate in the transport of ENaC along the exocytic pathway. Our results demonstrate that all three ENaC channel subunits interacted with p11 in an in vitro binding assay. Furthermore, p11 was able to immunoprecipitate ENaC in epithelial cells. Quantitative mass spectrometry of affinity-purified ENaC-p11 complexes recovered several other trafficking proteins including HSP-90 and annexin A6. We also report that p11 exhibits a robust protein expression in cortical collecting duct epithelial cells. However, the expression of p11 in these cells was not influenced by either short-term or long-term exposure to aldosterone. To determine whether the p11 interaction affected ENaC function, we measured amiloride sensitive Na⁺ currents in Xenopus oocytes or mammalian epithelia co-expressing ENaC and p11 or a siRNA to p11. Results from these experiments showed that p11 significantly augmented ENaC current, whereas knockdown of p11 decreased current. Further, knockdown of p11 reduced ENaC cell surface population suggesting p11 promotes membrane insertion of ENaC. Overall, our findings reveal a novel protein interaction that controls the number of ENaC channels inserted at the membrane via the exocytic pathway.

Silencing of Syntaxin 1A in the Dopaminergic Neurons Decreases the Activity of the Dopamine Transporter and Prevents Amphetamine-Induced Behaviors in C. elegans

The dopamine transporter (DAT) is a cell membrane protein whose main function is to reuptake the dopamine (DA) released in the synaptic cleft back into the dopaminergic neurons. Previous studies suggested that the activity of DAT is regulated by allosteric proteins such as Syntaxin-1A and is altered by drugs of abuse such as amphetamine (Amph). Because Caenorhabditis elegans expresses both DAT (DAT-1) and Syntaxin-1A (UNC-64), we used this model system to investigate the functional and behavioral effects caused by lack of expression of unc-64 in cultured dopaminergic neurons and in living animals. Using an inheritable RNA silencing technique, we were able to knockdown unc-64 specifically in the dopaminergic neurons. This cell-specific knockdown approach avoids the pleiotropic phenotypes caused by knockout mutations of unc-64 and ensures the transmission of dopaminergic specific unc-64 silencing to the progeny. We found that, similarly to dat-1 knockouts and dat-1 silenced lines, animals with reduced unc-64 expression in the dopaminergic neurons did not respond to Amph treatment when tested for locomotor behaviors. Our in vitro data demonstrated that in neuronal cultures derived from animals silenced for unc-64, the DA uptake was reduced by 30% when compared to controls, and this reduction was similar to that measured in neurons isolated from animals silenced for dat-1 (40%). Moreover, reduced expression of unc-64 in the dopaminergic neurons significantly reduced the DA release elicited by Amph. Because in C. elegans DAT-1 is the only protein capable to reuptake DA, these data show that reduced expression of unc-64 in the dopaminergic neurons decreases the capability of DAT in re-accumulating synaptic DA. Moreover, these results demonstrate that decreased expression of unc-64 in the dopaminergic neurons abrogates the locomotor behavior induced by Amph. Taken together these data suggest that Syntaxin-1A plays an important role in both functional and behavioral effects caused by Amph.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

Regulation of αENaC transcription

Aldosterone is a major regulator of Na(+) absorption and acts primarily by controlling the epithelial Na(+) channel (ENaC) function at multiple levels including transcription. ENaC consists of α, β, and γ subunits. In the classical model, aldosterone enhances transcription primarily by activating mineralocorticoid receptor (MR). However, how aldosterone induces chromatin alternation and thus leads to gene activation or repression remains largely unknown. Emerging evidence suggests that Dot1a-Af9 complex plays an important role in repression of αENaC by directly binding and modulating targeted histone H3 K79 hypermethylation at the specific subregions of αENaC promoter. Aldosterone impairs Dot1a-Af9 formation by decreasing expression of Dot1a and Af9 and by inducing Sgk1, which, in turn, phosphorylates Af9 at S435 to weaken Dot1a-Af9 interaction. MR counterbalances Dot1a-Af9 action by competing with Dot1a for binding Af9. Af17 derepresses αENaC by competitively interacting with Dot1a and facilitating Dot1a nuclear export. Consistently, MR(-/-) mice have impaired ENaC expression at day 5 after birth, which may contribute to progressive development of pseudohypoaldosteronism type 1 in a later stage. Af17(-/-) mice have decreased ENaC expression, renal Na(+) retention, and blood pressure. In contrast, Dot1l(AC) mice have increased αENaC expression, despite a 20% reduction of the principal cells. This chapter reviews these findings linking aldosterone action to ENaC transcription through chromatin modification. Future direction toward the understanding the role of Dot1a-Af9 complex beyond ENaC regulation, in particular, in renal fibrosis is also briefly discussed.

Na+ inhibits the epithelial Na+ channel by binding to a site in an extracellular acidic cleft

The epithelial Na(+) channel (ENaC) has a key role in the regulation of extracellular fluid volume and blood pressure. ENaC belongs to a family of ion channels that sense the external environment. These channels have large extracellular regions that are thought to interact with environmental cues, such as Na(+), Cl(-), protons, proteases, and shear stress, which modulate gating behavior. We sought to determine the molecular mechanism by which ENaC senses high external Na(+) concentrations, resulting in an inhibition of channel activity. Both our structural model of an ENaC α subunit and the resolved structure of an acid-sensing ion channel (ASIC1) have conserved acidic pockets in the periphery of the extracellular region of the channel. We hypothesized that these acidic pockets host inhibitory allosteric Na(+) binding sites. Through site-directed mutagenesis targeting the acidic pocket, we modified the inhibitory response to external Na(+). Mutations at selected sites altered the cation inhibitory preference to favor Li(+) or K(+) rather than Na(+). Channel activity was reduced in response to restraining movement within this region by cross-linking structures across the acidic pocket. Our results suggest that residues within the acidic pocket form an allosteric effector binding site for Na(+). Our study supports the hypothesis that an acidic cleft is a key ligand binding locus for ENaC and perhaps other members of the ENaC/degenerin family.

Characterization of SNARE proteins in human pituitary adenomas: targeted secretion inhibitors as a new strategy for the treatment of acromegaly?

CONTEXT

Targeted secretion inhibitors (TSIs), a new class of recombinant biotherapeutic proteins engineered from botulinum toxin, represent a novel approach for treating diseases with excess secretion. They inhibit hormone secretion from targeted cell types through cleavage of SNARE (soluble N-ethylmaleimide-sensitive factor-activating protein receptor) proteins. qGHRH-LH(N)/D is a TSI targeting pituitary somatotroph through binding to the GHRH-receptor and cleavage of the vesicle-associated membrane protein (VAMP) family of SNARE proteins.

OBJECTIVE

Our objective was to study SNARE protein expression in pituitary adenomas and to inhibit GH secretion from somatotropinomas using qGHRH-LH(N)/D.

DESIGN

We analyzed human pituitary adenoma analysis for SNARE expression and response to qGHRH-LH(N)/D treatment.

SETTING

The study was conducted in University Hospitals.

PATIENTS

We used pituitary adenoma samples from 25 acromegaly and 47 nonfunctioning pituitary adenoma patients.

OUTCOME

Vesicle-SNARE (VAMP1-3), target-SNARE (syntaxin1, SNAP-23, and SNAP-25), and GHRH-receptor detection with RT-qPCR, immunocytochemistry, and immunoblotting. Assessment of TSI catalytic activity on VAMPs and release of GH from adenoma cells.

RESULTS

SNARE proteins were variably expressed in pituitary samples. In vitro evidence using recombinant GFP-VAMP2&3 or pituitary adenoma lysates suggested sufficient catalytic activity of qGHRH-LH(N)/D to degrade VAMPs, but was unable to inhibit GH secretion in somatotropinoma cell cultures.

CONCLUSIONS

SNARE proteins are present in human pituitary somatotroph adenomas that can be targeted by TSIs to inhibit GH secretion. qGHRH-LH(N)/D was unable to inhibit GH secretion from human somatotroph adenoma cells. Further studies are required to understand how the SNARE proteins drive GH secretion in human somatotrophs to allow the development of novel TSIs with a potential therapeutic benefit.

Evaluation of common variants in 16 genes involved in the regulation of neurotransmitter release in ADHD

Attention-deficit hyperactivity disorder (ADHD) is a neurobehavioral disorder characterized by inappropriate difficulties to sustain attention, control impulses and modulate activity level. Although ADHD is one of the most prevalent childhood psychiatric disorders, it also persists into adulthood in around 30-50% of the cases. Based on the effect of psychostimulants used in the pharmacological treatment of ADHD, dysfunctions in neuroplasticity mechanisms and synapses have been postulated to be involved in the pathophysiology of ADHD. With this background, we evaluated, both in childhood and adulthood ADHD, the role of several genes involved in the control of neurotransmitter release through synaptic vesicle docking, fusion and recycling processes by means of a population-based association study. We analyzed single nucleotide polymorphisms across 16 genes in a clinical sample of 950 ADHD patients (506 adults and 444 children) and 905 controls. Single and multiple-marker analyses identified several significant associations after correcting for multiple testing with a false discovery rate (FDR) of 15%: (i) the SYT2 gene was strongly associated with both adulthood and childhood ADHD (p=0.001, OR=1.49 (1.18-1.89) and p=0.007, OR=1.37 (1.09-1.72), respectively) and (ii) STX1A was found associated with ADHD only in adults (p=0.0041; OR=1.28 (1.08-1.51)). These data provide preliminary evidence for the involvement of genes that participate in the control of neurotransmitter release in the genetic predisposition to ADHD through a gene-system association study. Further follow-up studies in larger cohorts and deep-sequencing of the associated genomic regions are required to identify sequence variants directly involved in ADHD.

Annexin A7 and SNAP23 interactions in alveolar type II cells and in vitro: a role for Ca(2+) and PKC

Lung surfactant secretion involves lamellar body docking and fusion with the plasma membrane in alveolar type II cells. Annexin A7 (A7) is postulated to play a role in membrane fusion during exocytosis. Our recent studies demonstrated increased co-localization of A7 with ABCA3 in lamellar bodies in type II cells stimulated with established secretagogues of lung surfactant. In this study, we investigated in vivo and in vitro interactions of A7 with the t-SNARE protein, SNAP23. Immuno-fluorescence studies showed time-dependent increases in co-localization of A7 with SNAP23 in PMA- and in A23187-stimulated cells. PMA and A23187 also caused a time-dependent increase in co-localization of ABCA3 with SNAP23. The relocation of A7 to SNAP23 domains was inhibited in the presence of PKC inhibitor, similar to that previously reported for co-localization of A7 with ABCA3. The interaction of A7 and SNAP23 was confirmed by affinity binding and by in vitro interaction of recombinant A7 and SNAP23 proteins. The in vitro binding of recombinant A7 (rA7) to GST-SNAP23 fusion protein was calcium-dependent. Phosphorylation of rA7 with PKC increased its in vitro binding to SNAP23 suggesting that a similar mechanism may operate during A7 relocation to t-SNARE domains. Thus, our studies demonstrate that annexin A7 may function in co-ordination with SNARE proteins and that protein kinase activation may be required for annexin A7 trafficking to the interacting membranes (lamellar bodies and plasma membrane) to facilitate membrane fusion during surfactant secretion.

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

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

Secretagogues of lung surfactant increase annexin A7 localization with ABCA3 in alveolar type II cells

Membrane fusion between the lamellar bodies and plasma membrane is an obligatory event in the secretion of lung surfactant. Previous studies have postulated a role for annexin A7 (A7) in membrane fusion during exocytosis in some cells including alveolar type II cells. However, the intracellular trafficking of A7 during such fusion is not described. In this study, we investigated association of endogenous A7 with lamellar bodies in alveolar type II cells following treatment with several secretagogues of lung surfactant. Biochemical studies with specific antibodies showed increased membrane-association of cell A7 in type II cells stimulated with agents that increase secretion through different signaling mechanisms. Immuno-fluorescence studies showed increased co-localization of A7 with ABCA3, the lamellar body marker protein. Because these agents increase surfactant secretion through activation of PKC and PKA, we also investigated the effects of PKC and PKA inhibitors, bisindolylmaleimideI (BisI) and H89, respectively, on A7 partitioning. Western blot analysis showed that these inhibitors prevented secretagogue-mediated A7 increase in the membrane fractions. These inhibitors also blocked increased co-localization of A7 with ABCA3 in secretagogue-treated cells, as revealed by immuno-fluorescence studies. In vitro studies with recombinant A7 showed phosphorylation with PKC and PKA. The cell A7 was also phosphorylated in cells treated with surfactant secretagogues. Thus, our studies demonstrate that annexin A7 relocates to lamellar bodies in a phosphorylation-dependent manner. We suggest that activation of protein kinase promotes phosphorylation and membrane-association of A7 presumably to facilitate membrane fusion during lung surfactant secretion.

ENaC channels in oocytes from Xenopus laevis and their regulation by xShroom1 protein

Shroom is a family of related proteins linked to the actin cytoskeleton. xShroom1 is constitutively expressed in X. oocytes and is required for the expression of amiloride sensitive sodium channels (ENaC). Oocytes were injected with α, β, and γ mENaC and xShroom1 sense or antisense oligonucleotides. We used voltage clamp techniques to study the amiloride-sensitive Na(+) currents (INa((amil))). We observed a marked reduction in INa((amil)) in oocytes co-injected with xShroom1 antisense. Oocytes expressing a DEG mutant β-mENaC subunit (β-S518K) with an open probability of 1 had enhanced INa((amil)) although these currents were also reduced when co-injected with xShroom1 antisense. Addition of low concentration (20 ng/ml) of trypsin which activates the membrane-resident ENaC channels led to a slow increase in INa((amil)) in oocytes with xShroom1 sense but had no effect on the currents in oocytes coinjected with ENaC and xShroom1 antisense. The same results were obtained with higher concentrations of trypsin (2 μg/ml) exposed during 2.5 min. In addition, fluorescence positive staining of plasma membrane in the oocytes expressing α, β and γ mENaC and xShroom1 sense were observed but not in oocytes coinjected with ENaC and xShroom1 antisense oligonucleotides. On this basis, we suggest that xShroom1-dependent ENaC inhibition may be through the number of channels inserted in the membrane.

Cortical actin binding protein cortactin mediates ENaC activity via Arp2/3 complex

Epithelial Na(+) channel (ENaC) activity is regulated, in part, by the cortical cytoskeleton. Here we demonstrate that cortactin is highly expressed in the kidney cortex and polarized epithelial cells, and is localized to the cortical collecting duct. Coexpression of cortactin with ENaC decreases ENaC activity, as measured in patch-clamp experiments. Biotinylation experiments and single-channel analysis reveal that cortactin decreases ENaC activity via affecting channel open probability (P(o)). Knockdown of cortactin in mpkCCD(c14) principal cells results in an increase in ENaC activity and sodium reabsorption. Coimmunoprecipitation analysis shows direct interactions between cortactin and all three ENaC subunits in cultured and native cells. To address the question of what mechanism underlies the action of cortactin on ENaC activity, we assayed the effects of various mutants of cortactin. The data show that only a cortactin mutant unable to bind Arp2/3 complex does not influence ENaC activity. Furthermore, inhibitor of the Arp2/3 complex CK-0944666 precludes the effect of cortactin. Depolymerization of the actin microfilaments and inhibition of the Arp2/3 complex does not result in the loss of association between ENaC and cortactin. Thus, these results indicate that cortactin is functionally important for ENaC activity and that Arp2/3 complex is involved in this mechanism.

Syntaxin 1A regulates surface expression of beta-cell ATP-sensitive potassium channels

The pancreatic ATP-sensitive potassium (K(ATP)) channel consisting of four inwardly rectifying potassium channel 6.2 (Kir6.2) and four sulfonylurea receptor SUR1 subunits plays a key role in insulin secretion by linking glucose metabolism to membrane excitability. Syntaxin 1A (Syn-1A) is a plasma membrane protein important for membrane fusion during exocytosis of insulin granules. Here, we show that Syn-1A and K(ATP) channels endogenously expressed in the insulin-secreting cell INS-1 interact. Upregulation of Syn-1A by overexpression in INS-1 leads to a decrease, whereas downregulation of Syn-1A by small interfering RNA (siRNA) leads to an increase, in surface expression of K(ATP) channels. Using COSm6 cells as a heterologous expression system for mechanistic investigation, we found that Syn-1A interacts with SUR1 but not Kir6.2. Furthermore, Syn-1A decreases surface expression of K(ATP) channels via two mechanisms. One mechanism involves accelerated endocytosis of surface channels. The other involves decreased biogenesis and processing of channels in the early secretory pathway. This regulation is K(ATP) channel specific as Syn-1A has no effect on another inward rectifier potassium channel Kir3.1/3.4. Our results demonstrate that in addition to a previously documented role in modulating K(ATP) channel gating, Syn-1A also regulates K(ATP) channel expression in β-cells. We propose that physiological or pathological changes in Syn-1A expression may modulate insulin secretion by altering glucose-secretion coupling via changes in K(ATP) channel expression.

Allosteric inhibition of the epithelial Na+ channel through peptide binding at peripheral finger and thumb domains

The epithelial Na(+) channel (ENaC) mediates the rate-limiting step in transepithelial Na(+) transport in the distal segments of the nephron and in the lung. ENaC subunits are cleaved by proteases, resulting in channel activation due to the release of inhibitory tracts. Peptides derived from these tracts inhibit channel activity. The mechanism by which these intrinsic inhibitory tracts reduce channel activity is unknown, as are the sites where these tracts interact with other residues within the channel. We performed site-directed mutagenesis in large portions of the predicted periphery of the extracellular region of the α subunit and measured the effect of mutations on an 8-residue inhibitory tract-derived peptide. Our data show that the inhibitory peptide likely binds to specific residues within the finger and thumb domains of ENaC. Pairwise interactions between the peptide and the channel were identified by double mutant cycle experiments. Our data suggest that the inhibitory peptide has a specific peptide orientation within its binding site. Extended to the intrinsic inhibitory tract, our data suggest that proteases activate ENaC by removing residues that bind at the finger-thumb domain interface.

Extracellular allosteric regulatory subdomain within the gamma subunit of the epithelial Na+ channel

The activity of the epithelial Na(+) channel (ENaC) is modulated by Na(+) self-inhibition, a down-regulation of the open probability of ENaC by extracellular Na(+). A His residue within the extracellular domain of gammaENaC (gammaHis(239)) was found to have a critical role in Na(+) self-inhibition. We investigated the functional roles of residues in the vicinity of this His by mutagenesis and analyses of Na(+) self-inhibition responses in Xenopus oocytes. Significant changes in the speed and magnitude of Na(+) self-inhibition were observed in 16 of the 47 mutants analyzed. These 16 mutants were distributed within a 22-residue tract. We further characterized this scanned region by examining the accessibility of introduced Cys residues to the sulfhydryl reagent MTSET. External MTSET irreversibly increased or decreased currents in 13 of 47 mutants. The distribution patterns of the residues where substitutions significantly altered Na(+) self-inhibition or/and conferred sensitivity to MTSET were consistent with the existence of two helices within this region. In addition, single channel recordings of the gammaH239F mutant showed that, in the absence of Na(+) self-inhibition and with an increased open probability, ENaCs still undergo transitions between open and closed states. We conclude that gammaHis(239) functions within an extracellular allosteric regulatory subdomain of the gamma subunit that has an important role in conferring the response of the channel to external Na(+).

Regulation of the epithelial sodium channel (ENaC) by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in salt-sensitive hypertension. ENaC activity in epithelial cells is regulated both by open probability and channel number. This review focuses on the regulation of ENaC in the cells of the kidney cortical collecting duct by trafficking and recycling. The trafficking of ENaC is discussed in the broader context of epithelial cell vesicle trafficking. Well-characterized pathways and protein interactions elucidated using epithelial model cells are discussed, and the known overlap with ENaC regulation is highlighted. In following the life of ENaC in CCD epithelial cells the apical delivery, internalization, recycling, and destruction of the channel will be discussed. While a number of pathways presented still need to be linked to ENaC regulation and many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

Syntaxin modulates Kv1.1 through dual action on channel surface expression and conductance

The Kv1.1 channel that is expressed throughout the central and peripheral nervous system is known to interact with syntaxin 1A, a member of the exocytosis machinery protein complex. This interaction was previously shown to increase the macroscopic currents of the presynaptic Kv1.1 channel when coexpressed in Xenopus oocytes, while it decreased the unitary channel conductance and open probability. This apparent discrepancy has been resolved in this work, using electrophysiological, biochemical, and immunohistochemical analyses in oocytes by overexpression and antisense knockdown of syntaxin. Here, we demonstrate that syntaxin plays a dual role in the modulation of Kv1.1 function: enhancement of the channel's surface expression along with attenuation of single channel ion flux. These findings broaden the scope of channels and transporters that are dually modulated by syntaxin. Although the dual functioning of syntaxin in modulation of Kv1.1 channel activity may seem antagonistic, the combination of the two mechanisms may provide a useful means for fine-tuning axonal excitability and synaptic efficacy.

Novel determinants of epithelial sodium channel gating within extracellular thumb domains

Activity of the epithelial Na(+) channel (ENaC) is modulated by Na(+) self-inhibition, an allosteric down-regulation of channel open probability by extracellular Na(+). We searched for determinants of Na(+) self-inhibition by analyzing changes in this inhibitory response resulting from specific mutations within the extracellular domains of mouse ENaC subunits. Mutations at gammaMet(438) altered the Na(+) self-inhibition response in a substitution-specific manner. Fourteen substitutions (Ala, Arg, Asp, Cys, Gln, Glu, His, Ile, Phe, Pro, Ser, Thr, Tyr, and Val) significantly suppressed Na(+) self-inhibition, whereas three mutations (Asn, Gly, and Leu) moderately enhanced the inhibition. Met to Lys mutation did not alter Na(+) self-inhibition. Mutations at the homologous site in the alpha subunit (G481A, G481C, and G481M) dramatically increased the magnitude and speed of Na(+) self-inhibition. Mutations at the homologous betaAla(422) resulted in minimal or no change in Na(+) self-inhibition. Low, high, and intermediate open probabilities were observed in oocytes expressing alphaG481Mbetagamma, alphabetagammaM438V, and alphaG481M/betagammaM438V, respectively. This pair of residues map to thealpha5 helix in the extracellular thumb domain in the chicken acid sensing ion channel 1 structure. Both residues likely reside near the channel surface because both alphaG481Cbetagamma and alphabetagammaM438C channels were inhibited by an externally applied and membrane-impermeant sulfhydryl reagent. Our results demonstrate that alphaGly(481) and gammaMet(438) are functional determinants of Na(+) self-inhibition and of ENaC gating and suggest that the thumb domain contributes to the channel gating machinery.

Assessment of the CFTR and ENaC association

Cystic fibrosis (CF) is one of the most common lethal genetic disorders. It results primarily from mutations in the cystic fibrosis transmembrane conductance regulator (cftr) gene. These mutations cause inadequate functioning of CFTR, which in turn leads to the severe disruption of transport function in several epithelia across various organs. Affected organs include the sweat glands, the intestine, and the reproductive system, with the most devastating consequences due to the effects of the disease on airways. Despite aggressive treatment, gradual lung failure is the major life limiting factor in patients with CF. Understanding of the exact manner by which defects in the CFTR lead to lung failure is thus critical. In the CF airway, decreased chloride secretion and increased salt absorption is observed. The decreased chloride secretion appears to be a direct consequence of defective CFTR; however, the increased salt absorption is believed to result from the failure of CFTR to restrict salt absorption through a sodium channel named the epithelial Na(+) channel, ENaC. The mechanism by which CFTR modulates the function of ENaC proteins is still obscure and somewhat controversial. In this short review we will focus on recent findings of a possible direct CFTR and ENaC association.

Syntaxin 1A interaction with the dopamine transporter promotes amphetamine-induced dopamine efflux

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein syntaxin 1A (SYN1A) interacts with and regulates the function of transmembrane proteins, including ion channels and neurotransmitter transporters. Here, we define the first 33 amino acids of the N terminus of the dopamine (DA) transporter (DAT) as the site of direct interaction with SYN1A. Amphetamine (AMPH) increases the association of SYN1A with human DAT (hDAT) in a heterologous expression system (hDAT cells) and with native DAT in murine striatal synaptosomes. Immunoprecipitation of DAT from the biotinylated fraction shows that the AMPH-induced increase in DAT/SYN1A association occurs at the plasma membrane. In a superfusion assay of DA efflux, cells overexpressing SYN1A exhibited significantly greater AMPH-induced DA release with respect to control cells. By combining the patch-clamp technique with amperometry, we measured DA release under voltage clamp. At -60 mV, a physiological resting potential, AMPH did not induce DA efflux in hDAT cells and DA neurons. In contrast, perfusion of exogenous SYN1A (3 microM) into the cell with the whole-cell pipette enabled AMPH-induced DA efflux at -60 mV in both hDAT cells and DA neurons. It has been shown recently that Ca2+/calmodulin-dependent protein kinase II (CaMKII) is activated by AMPH and regulates AMPH-induced DA efflux. Here, we show that AMPH-induced association between DAT and SYN1A requires CaMKII activity and that inhibition of CaMKII blocks the ability of exogenous SYN1A to promote DA efflux. These data suggest that AMPH activation of CaMKII supports DAT/SYN1A association, resulting in a mode of DAT capable of DA efflux.

Conformational switch of syntaxin-1 controls synaptic vesicle fusion

During synaptic vesicle fusion, the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) protein syntaxin-1 exhibits two conformations that both bind to Munc18-1: a "closed" conformation outside the SNARE complex and an "open" conformation in the SNARE complex. Although SNARE complexes containing open syntaxin-1 and Munc18-1 are essential for exocytosis, the function of closed syntaxin-1 is unknown. We generated knockin/knockout mice that expressed only open syntaxin-1B. Syntaxin-1B(Open) mice were viable but succumbed to generalized seizures at 2 to 3 months of age. Binding of Munc18-1 to syntaxin-1 was impaired in syntaxin-1B(Open) synapses, and the size of the readily releasable vesicle pool was decreased; however, the rate of synaptic vesicle fusion was dramatically enhanced. Thus, the closed conformation of syntaxin-1 gates the initiation of the synaptic vesicle fusion reaction, which is then mediated by SNARE-complex/Munc18-1 assemblies.

Regulation of CFTR trafficking by its R domain

Phosphorylation of the R domain is required for cystic fibrosis transmembrane conductance regulator (CFTR) channel gating, and cAMP/protein kinase A (PKA) simulation can also elicit insertion of CFTR into the plasma membrane from intracellular compartments (Bertrand, C. A., and Frizzell, R. A. (2003) Am. J. Physiol. 285, C1-C18). We evaluated the structural basis of regulated CFTR trafficking by determining agonist-evoked increases in plasma membrane capacitance (Cm) of Xenopus oocytes expressing CFTR deletion mutants. Expression of CFTR as a split construct that omitted the R domain (Deltaamino acids 635-834) produced a channel with elevated basal current (Im) and no DeltaIm or trafficking response (DeltaCm) upon cAMP/PKA stimulation, indicating that the structure(s) required for regulated CFTR trafficking are contained within the R domain. Additional deletions showed that removal of amino acids 817-838, a 22-amino acid conserved helical region having a net charge of -9, termed NEG2 (Xie, J., Adams, L. M., Zhao, J., Gerken, T. A., Davis, P. B., and Ma, J. (2002) J. Biol. Chem. 277, 23019-23027), produced a channel with regulated gating that lacked the agonist-induced increase in CFTR trafficking. Injection of NEG2 peptides into oocytes expressing split DeltaNEG2 CFTR prior to stimulation restored the agonist-evoked DeltaCm, consistent with the concept that this sequence mediates the regulated trafficking event. In support of this idea, DeltaNEG2 CFTR escaped from the inhibition of wild type CFTR trafficking produced by overexpression of syntaxin 1A. These observations suggest that the NEG2 region at the C terminus of the R domain allows stabilization of CFTR in a regulated intracellular compartment from which it traffics to the plasma membrane in response to cAMP/PKA stimulation.

Regulation of the epithelial sodium channel by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle's syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in states as diverse as salt-sensitive hypertension, nephrosis, and pulmonary edema. ENaC activity in epithelial cells is highly regulated both by open probability and number of channels. Open probability is regulated by a number of factors, including proteolytic processing, while ENaC number is regulated by cellular trafficking. This review discusses current understanding of apical membrane delivery, cell surface stability, endocytosis, retrieval, and recycling of ENaC and the molecular partners that have so far been shown to participate in these processes. We review known sites and mechanisms of hormonal regulation of trafficking by aldosterone, vasopressin, and insulin. While many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

Acute downregulation of ENaC by EGF involves the PY motif and putative ERK phosphorylation site

The epithelial sodium channel (ENaC) is expressed in a variety of tissues, including the renal collecting duct, where it constitutes the rate-limiting step for sodium reabsorption. Liddle's syndrome is caused by gain-of-function mutations in the β and γ subunits of ENaC, resulting in enhanced Na reabsorption and hypertension. Epidermal growth factor (EGF) causes acute inhibition of Na absorption in collecting duct principal cells via an extracellular signal–regulated kinase (ERK)–dependent mechanism. In experiments with primary cultures of collecting duct cells derived from a mouse model of Liddle's disease (β-ENaC truncation), it was found that EGF inhibited short-circuit current (Isc) by 24 ± 5% in wild-type cells but only by 6 ± 3% in homozygous mutant cells. In order to elucidate the role of specific regions of the β-ENaC C terminus, Madin-Darby canine kidney (MDCK) cell lines that express β-ENaC with mutation of the PY motif (P616L), the ERK phosphorylation site (T613A), and C terminus truncation (R564stop) were created using the Phoenix retroviral system. All three mutants exhibited significant attenuation of the EGF-induced inhibition of sodium current. In MDCK cells with wild-type β-ENaC, EGF-induced inhibition of Isc (<30 min) was fully reversed by exposure to an ERK kinase inhibitor and occurred with no change in ENaC surface expression, indicative of an effect on channel open probability (P_o). At later times (>30 min), EGF-induced inhibition of Isc was not reversed by an ERK kinase inhibitor and was accompanied by a decrease in ENaC surface expression. Our results are consistent with an ERK-mediated decrease in ENaC open probability and enhanced retrieval of sodium channels from the apical membrane.

The epithelial sodium channel (ENaC) traffics to apical membrane in lipid rafts in mouse cortical collecting duct cells

We previously showed that ENaC is present in lipid rafts in A6 cells, a Xenopus kidney cell line. We now demonstrate that ENaC can be detected in lipid rafts in mouse cortical collecting duct ((MPK)CCD(14)) cells by detergent insolubility, buoyancy on density gradients using two distinct approaches, and colocalization with caveolin 1. Less than 30% of ENaC subunits were found in raft fractions. The channel subunits also colocalized on sucrose gradients with known vesicle targeting and fusion proteins syntaxin 1A, Vamp 2, and SNAP23. Hormonal stimulation of ENaC activity by either forskolin or aldosterone, short or long term, did not alter the lipid raft distribution of ENaC. Methyl-beta-cyclodextrin added apically to (MPK)CCD(14) cells resulted in a slow decline in amiloride-sensitive sodium transport with short circuit current reductions of 38.1 +/- 9.6% after 60 min. The slow decline in ENaC activity in response to apical cyclodextrin was identical to the rate of decline seen when protein synthesis was inhibited by cycloheximide. Apical biotinylation of (MPK)CCD(14) cells confirmed the loss of ENaC at the cell surface following cyclodextrin treatment. Acute stimulation of the recycling pool of ENaC was unaffected by apical cyclodextrin application. Expression of dominant negative caveolin isoforms (CAV1-eGFP and CAV3-DGV) which disrupt caveolae, reduced basal ENaC currents by 72.3 and 78.2%, respectively; but, as with cyclodextrin, the acute response to forskolin was unaffected. We conclude that ENaC is present in and regulated by lipid rafts. The data are consistent with a model in which rafts mediate the constitutive apical delivery of ENaC.

Melanophilin, a novel aldosterone-induced gene in mouse cortical collecting duct cells

The molecular mechanisms of aldosterone-regulated Na+ transport are not entirely clear. The goal of this study was to identify aldosterone-induced genes potentially involved in the trafficking of the epithelial Na+ channel (ENaC). We report that the transcript levels of melanophilin (MLPH), a protein involved in vesicular trafficking in melanocytes, are rapidly increased by aldosterone in cortical collecting duct (CCD) cells. This effect was near maximal at physiological aldosterone concentrations, indicating that it is mediated by the mineralocorticoid receptor. De novo protein synthesis is not required for the induction of MLPH mRNA by aldosterone. To determine whether this induction has functional consequences on transepithelial Na+ current, we generated clonal CCD cell lines that express a tetracycline-inducible MLPH. Induction of MLPH in these cells led to a relatively modest, but statistically significant, increase in amiloride-sensitive Na+ current, suggesting the MLPH may be involved in ENaC trafficking. MyosinVc, the epithelial-specific class V myosin that is highly homologous to MyosinVa, another component of the melanosome trafficking complex, has putative consensus sites for serum and glucocorticoid-induced kinase 1 (SGK1), an early aldosterone-induced kinase that mediates some of aldosterone's effects on Na+ transport. Our results indicate that MyosinVc is phosphorylated by endogenous SGK1, suggesting that this complex may be involved in the aldosterone-regulated trafficking of ENaC in the CCD. These results suggest potential mechanisms by which aldosterone may regulate Na+ transport both directly, by increasing the abundance of MLPH, and indirectly by increasing the transcription of SGK1, which in turn regulates the activity of MyosinVc.

Interaction of syntaxin with a single Kv1.1 channel: a possible mechanism for modulating neuronal excitability

Voltage-gated K(+) channels are crucial for intrinsic neuronal plasticity and present a target for modulations by protein-protein interactions, notably, by exocytotic proteins demonstrated by us in several systems. Here, we investigated the interaction of a single Kv1.1 channel with syntaxin 1A. Syntaxin decreased the unitary conductance of all conductance states (two subconductances and a full conductance) and decreased their open probabilities by prolongation of mean closed dwell-times at depolarized potentials. However, at subthreshold potentials syntaxin 1A increased the probabilities of the subconductance states. Consequently, the macroscopic conductance is decreased at potentials above threshold and increased at threshold potentials. Numerical modeling based on steady-state and kinetic analyses suggests: (1) a mechanism whereby syntaxin controls activation gating by forcing the conductance pathway only via a sequence of discrete steps through the subconductance states, possibly via a breakdown of cooperative movements of voltage sensors that exist in Kv1.1; (2) a physiological effect, apparently paradoxical for an agent that reduces K(+) current, of attenuating neuronal firing frequency via an increase in K(+) shunting conductance. Such modulation of the gain of neuronal output in response to different levels of syntaxin is in accord with the suggested role for Kv1.1 in axonal excitability and synaptic efficacy.

Distinct domain-dependent effect of syntaxin1A on amiloride-sensitive sodium channel (ENaC) currents in HT-29 colonic epithelial cells

The amiloride-sensitive epithelial sodium channel (ENaC), a plasma membrane protein mediates sodium reabsorption in epithelial tissues, including the distal nephron and colon. Syntaxin1A, a trafficking protein of the t-SNARE family has been reported to inhibit ENaC in the Xenopus oocyte expression and artificial lipid bilayer systems. The present report describes the regulation of the epithelial sodium channel by syntaxin1A in a human cell line that is physiologically relevant as it expresses both components and also responds to aldosterone stimulation. In order to evaluate the physiological significance of syntaxin1A interaction with natively expressed ENaC, we over-expressed HT-29 with syntaxin1A constructs comprising various motifs. Unexpectedly, we observed the augmentation of amiloride-sensitive currents with wild-type syntaxin1A full-length construct (1-288) in this cell line. Both γENaC and neutralizing syntaxin1A antibodies blocked native expression as amiloride-sensitive sodium currents were inhibited while munc18-1 antibody reversed this effect. The coiled-coiled domain H3 (194-266) of syntaxin1A inhibited, however the inclusion of the transmembrane domain to this motif (194-288) augmented amiloride sensitive currents. More so, data suggest that ENaC interacts with multiple syntaxin1A domains, which differentially regulate channel function. This functional modulation is the consequence of the physical enhancement of ENaC at the cell surface in cells over-expressed with syntaxin(s). Our data further suggest that syntaxin1A up-regulates ENaC function by multiple mechanisms that include PKA, PLC, PI3 and MAP Kinase (p42/44) signaling systems. We propose that syntaxin1A possesses distinct inhibitory and stimulatory domains that interact with ENaC subunits, which critically determines the overall ENaC functionality/regulation under distinct physiological conditions.

The epithelial Na+ channel is inhibited by a peptide derived from proteolytic processing of its alpha subunit

Epithelial sodium channels (ENaCs) mediate Na(+) entry across the apical membrane of high resistance epithelia that line the distal nephron, airway and alveoli, and distal colon. These channels are composed of three homologous subunits, termed alpha, beta, and gamma, which have intracellular amino and carboxyl termini and two membrane-spanning domains connected by large extracellular loops. Maturation of ENaC subunits involves furin-dependent cleavage of the extracellular loops at two sites within the alpha subunit and at a single site within the gamma subunit. The alpha subunits must be cleaved twice, immediately following Arg-205 and Arg-231, in order for channels to be fully active. Channels lacking alpha subunit cleavage are inactive with a very low open probability. In contrast, channels lacking both alpha subunit cleavage and the tract alphaAsp-206-Arg-231 are active when expressed in oocytes, suggesting that alphaAsp-206-Arg-231 functions as an inhibitor that stabilizes the channel in the closed conformation. A synthetic 26-mer peptide (alpha-26), corresponding to alphaAsp-206-Arg-231, reversibly inhibits wild-type mouse ENaCs expressed in Xenopus oocytes, as well as endogenous Na(+) channels expressed in either a mouse collecting duct cell line or primary cultures of human airway epithelial cells. The IC(50) for amiloride block of ENaC was not affected by the presence of alpha-26, indicating that alpha-26 does not bind to or interact with the amiloride binding site. Substitution of Arg residues within alpha-26 with Glu, or substitution of Pro residues with Ala, significantly reduced the efficacy of alpha-26. The peptide inhibits ENaC by reducing channel open probability. Our results suggest that proteolysis of the alpha subunit activates ENaC by disassociating an inhibitory domain (alphaAsp-206-Arg-231) from its effector site within the channel complex.

Epithelial sodium channel is regulated by SNAP-23/syntaxin 1A interplay

Sodium-selective amiloride-sensitive epithelial channel (ENaC) located in the apical membrane is involved in the reabsorption of sodium in tight epithelia. The soluble N-ethylmaleimide-sensitive attachment receptors (SNAREs) mediate vesicle trafficking in a variety of cell systems. Syntaxin (a t-SNARE) has been shown to interact with and functionally regulate a number of ion channels including ENaC. In this study, we investigated the role of SNAP-23, another SNARE protein, on ENaC activity in the HT-29 colonic epithelial cell system and Xenopus oocytes. Recording of amiloride-sensitive currents in both systems suggest that SNAP-23 modulates channel function, though a much higher concentration is required to inhibit ENaC in Xenopus oocytes. The introduction of Botulinum toxin A (a neurotoxin which cleaves SNAP-23), but not Botulinum toxin B or heat-inactivated Botulinum toxin A, reversed the inhibitory effect of SNAP-23 on amiloride-sensitive currents. However, syntaxin 1A and SNAP-23 combined portray a complex scenario that suggests that this channel interacts within a quaternary complex. Synaptotagmin expression neither interacts with, nor showed any effect on amiloride-sensitive currents when co-expressed with ENaC. Pull down assays suggest mild interaction between ENaC and SNAP-23, which gets stronger in the presence of syntaxin 1A. Data further suggest that SNAP-23 possibly interacts with the N-terminal alphaENaC. These functional and biochemical approaches provide evidence for a complex relationship between ENaC and the exocytotic machinery. Our data suggest that SNARE protein interplay defines the fine regulation of sodium channel function.

Rab4 GTP/GDP modulates amiloride-sensitive sodium channel (ENaC) function in colonic epithelia

The sodium-selective amiloride-sensitive epithelial sodium channel (ENaC) mediates electrogenic sodium re-absorption in tight epithelia. ENaC expression at the plasma membrane requires regulated transport, processing, and macromolecular assembly of subunit proteins in a defined and highly compartmentalized manner. Ras-related Rab GTPases monitor these processes in a highly regulated sequence of events. In order to evaluate the role of Rab proteins in ENaC function, Rab4 wild-type (WT), the GTPase-deficient mutant Rab4Q67L, and the dominant negative GDP-locked mutant Rab4S22N were over-expressed in the colon cancer cell line, HT-29 and amiloride-sensitive currents were recorded. Rab4 over-expression inhibited amiloride-sensitive currents. The effect was reversed by introducing Rab4-neutralizing antibody and Rab4 specific SiRNA. The GDP-locked Rab4 mutant inhibited, while GTPase-deficient mutant moderately stimulated amiloride-sensitive currents. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. Immunoprecipitation and pull-down assay suggest protein-protein interaction between Rab4 and ENaC. In addition, the functional modulation coincides with concomitant changes in ENaC expression at the cell surface and in intracellular pool. We propose that Rab4 is a critical element that regulates ENaC function by mechanisms that include GTP-GDP status, recycling, and expression level. Our observations imply that channel expression in apical membranes of epithelial cell system incorporates RabGTPase as an essential determinant of channel function and adds an exciting paradigm to ENaC therapeutics.

Minireview: regulation of epithelial Na+ channel trafficking

The epithelial Na(+) channel (ENaC) is a pathway for Na(+) transport across epithelia, including the kidney collecting duct, lung, and distal colon. ENaC is critical for Na(+) homeostasis and blood pressure control; defects in ENaC function and regulation are responsible for inherited forms of hypertension and hypotension and may contribute to the pathogenesis of cystic fibrosis and other lung diseases. An emerging theme is that epithelial Na(+) transport is regulated in large part through trafficking mechanisms that control ENaC expression at the cell surface. ENaC trafficking is regulated at multiple steps. Delivery of channels to the cell surface is regulated by aldosterone (and corticosteroids) and vasopressin, which increase ENaC synthesis and exocytosis, respectively. Conversely, endocytosis and degradation is controlled by a sequence located in the C terminus of alpha, beta, and gammaENaC (PPPXYXXL). This sequence functions as an endocytosis motif and as a binding site for Nedd4-2, an E3 ubiquitin protein ligase that targets ENaC for degradation. Mutations that delete or disrupt this motif cause accumulation of channels at the cell surface, resulting in Liddle's syndrome, an inherited form of hypertension. Nedd4-2 is a central convergence point for ENaC regulation by aldosterone and vasopressin; both induce phosphorylation of a common set of three Nedd4-2 residues, which blocks Nedd4-2 binding to ENaC. Thus, aldosterone and vasopressin regulate epithelial Na(+) transport in part by altering ENaC trafficking to and from the cell surface.

PKA-dependent ENaC trafficking requires the SNARE-binding protein complexin

Acute regulation of epithelial sodium channel (ENaC) function at the apical surface of polarized kidney cortical collecting duct (CCD) epithelial cells occurs in large part by changes in channel number, mediated by membrane vesicle trafficking. Several soluble N-ethyl-maleimide-sensitive factor attachment protein receptors (SNARE) have been implicated in this process. A novel SNARE-binding protein, complexin, has been identified in nervous tissue which specifically binds to and stabilizes SNARE complexes at synaptic membranes to promote vesicle fusion. To test whether this protein is present in mouse CCD (mCCD) cells and its possible involvement in acute ENaC regulation, we cloned complexin (isoform II) from a mouse kidney cDNA library. Complexin II mRNA coexpressed with alpha-, beta-, and gamma-ENaC subunits in Xenopus laevis oocytes reduced sodium currents to 16 +/- 3% (n = 19) of control values. Short-circuit current (I(sc)) measurements on mCCD cell lines stably over- or underexpressing complexin produced similar results. Basal I(sc) was reduced from 12.0 +/- 1.0 (n = 15) to 2.0 +/- 0.4 (n = 15) and 1.8 +/- 0.3 (n = 17) microA/cm(2), respectively. Similarly forskolin-stimulated I(sc) was reduced from control values of 20.0 +/- 2 to 2.7 +/- 0.5 and 2.3 +/- 0.4 microA/cm(2) by either increasing or decreasing complexin expression. Surface biotinylation demonstrated that the complexin-induced reduction in basal I(sc)was due to a reduction in apical membrane-resident ENaC and the inhibition in forskolin stimulation was due to the lack of ENaC insertion into the apical membrane to increase surface channel number. Immunofluorescent localization of SNARE proteins in polarized mCCD epithelia detected the presence of syntaxins 1 and 3 and synaptosomal-associated protein of 23 kDa (SNAP-23) at the apical membrane, and vesicle-associated membrane protein (VAMP2) was localized to intracellular compartments. These findings identify SNAREs that may mediate ENaC-containing vesicle insertion in mCCD epithelia and suggest that stabilization of SNARE interactions by complexin is an essential aspect of the regulated trafficking events that increase apical membrane ENaC density either by constitutive or regulated trafficking pathways.

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.

Capsazepine Is a Novel Activator of the δ Subunit of the Human Epithelial Na+ Channel

The amiloride-sensitive epithelial Na+ channel (ENaC) regulates Na+ homeostasis into cells and across epithelia. So far, four homologous subunits of mammalian ENaC have been isolated and are denoted as alpha, beta, gamma, and delta. The chemical agents acting on ENaC are, however, largely unknown, except for amiloride and benzamil as ENaC inhibitors. In particular, there are no agonists currently known that are selective for ENaCdelta, which is mainly expressed in the brain. Here we demonstrate that capsazepine, a competitive antagonist for transient receptor potential vanilloid subfamily 1, potentiates the activity of human ENaCdeltabetagamma (hENaCdeltabetagamma) heteromultimer expressed in Xenopus oocytes. The inward currents at a holding potential of -60 mV in hENaCdeltabetagamma-expressing oocytes were markedly enhanced by the application of capsazepine (> or =1 microM), and the capsazepine-induced current was mostly abolished by the addition of 100 microM amiloride. The stimulatory effects of capsazepine on the inward current were concentration-dependent with an EC50 value of 8 microM. Neither the application of other vanilloid compounds (capsaicin, resiniferatoxin, and olvanil) nor a structurally related compound (dopamine) modulated the inward current. Although hENaCdelta homomer was also significantly activated by capsazepine, unexpectedly, capsazepine had no effect on hENaCalpha and caused a slight decrease on the hENaCalphabetagamma current. In conclusion, capsazepine acts on ENaCdelta and acts together with protons. Other vanilloids tested do not have any effect. These findings identify capsazepine as the first known chemical activator of ENaCdelta.