Regulation of the amiloride-sensitive epithelial sodium channel by syntaxin 1A

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.

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.

Mechanotransduction in the muscle spindle

The focus of this review is on the principal sensory ending of the mammalian muscle spindle, known as the primary ending. The process of mechanosensory transduction in the primary ending is examined under five headings: (i) action potential responses to defined mechanical stimuli—representing the ending's input–output properties; (ii) the receptor potential—including the currents giving rise to it; (iii) sensory-terminal deformation—measurable changes in the shape of the primary-ending terminals correlated with intrafusal sarcomere length, and what may cause them; (iv) putative stretch-sensitive channels—pharmacological and immunocytochemical clues to their identity; and (v) synaptic-like vesicles—the physiology and pharmacology of an intrinsic glutamatergic system in the primary and other mechanosensory endings, with some thoughts on the possible role of the system. Thus, the review highlights spindle stretch-evoked output is the product of multi-ionic receptor currents plus complex and sophisticated regulatory gain controls, both positive and negative in nature, as befits its status as the most complex sensory organ after the special senses.

Organization of the ENaC-regulatory machinery

The control of fluid and electrolyte homeostasis in vertebrates requires the integration of a diverse set of signaling inputs, which control epithelial Na(+) transport, the principal ionic component of extracellular fluid. The key site of regulation is a segment of the kidney tubules, frequently termed the aldosterone-sensitive distal nephron, wherein the epithelial Na(+) channel (or ENaC) mediates apical ion entry. Na(+) transport in this segment is strongly regulated by the salt-retaining hormone, aldosterone, which acts through the mineralocorticoid receptor (MR) to influence the expression of a selected set of target genes, most notably the serine-threonine kinase SGK1, which phosphorylates and inhibits the E3 ubiquitin ligase Nedd4-2. It has long been known that ENaC activity is tightly regulated in vertebrate epithelia. Recent evidence suggests that SGK1 and Nedd4-2, along with other ENaC-regulatory proteins, physically associate with each other and with ENaC in a multi-protein complex. The various components of the complex are regulated by diverse signaling networks, including steroid receptor-, PI3-kinase-, mTOR-, and Raf-MEK-ERK-dependent pathways. In this review, we focus on the organization of the targets of these pathways by multi-domain scaffold proteins and lipid platforms into a unified complex, thereby providing a molecular basis for signal integration in the control of ENaC.

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.

Amiloride-sensitive channels are a major contributor to mechanotransduction in mammalian muscle spindles

We investigated whether channels of the epithelial sodium/amiloride-sensitive degenerin (ENaC/DEG) family are a major contributor to mechanosensory transduction in primary mechanosensory afferents, using adult rat muscle spindles as a model system. Stretch-evoked afferent discharge was reduced in a dose-dependent manner by amiloride and three analogues - benzamil, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and hexamethyleneamiloride (HMA), reaching > or = 85% inhibition at 1 mm. Moreover, firing was slightly but significantly increased by ENaC delta subunit agonists (icilin and capsazepine). HMA's profile of effects was distinct from that of the other drugs. Amiloride, benzamil and EIPA significantly decreased firing (P < 0.01 each) at 1 microm, while 10 microm HMA was required for highly significant inhibition (P < 0.0001). Conversely, amiloride, benzamil and EIPA rarely blocked firing entirely at 1 mm, whereas 1 mm HMA blocked 12 of 16 preparations. This pharmacology suggests low-affinity ENaCs are the important spindle mechanotransducer. In agreement with this, immunoreactivity to ENaC alpha, beta and gamma subunits was detected both by Western blot and immunocytochemistry. Immunofluorescence intensity ratios for ENaC alpha, beta or gamma relative to the vesicle marker synaptophysin in the same spindle all significantly exceeded controls (P < 0.001). Ratios for the related brain sodium channel ASIC2 (BNaC1alpha) were also highly significantly greater (P < 0.005). Analysis of confocal images showed strong colocalisation within the terminal of ENaC/ASIC2 subunits and synaptophysin. This study implicates ENaC and ASIC2 in mammalian mechanotransduction. Moreover, within the terminals they colocalise with synaptophysin, a marker for the synaptic-like vesicles which regulate afferent excitability in these mechanosensitive endings.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

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.

Calcium/calmodulin-dependent kinase II regulates the interaction between the serotonin transporter and syntaxin 1A

Plasma membrane serotonin transporters (SERTs) regulate serotonin (5HT) levels in brain and are a site of action of antidepressants and psychostimulant drugs of abuse. Syntaxin 1A is a component of the synaptic vesicle docking and fusion apparatus and has been shown to interact with multiple plasma membrane neurotransmitter transporters including SERT. Previously, we showed that syntaxin 1A regulates the transport stoichiometry of SERT. When not bound to syntaxin 1A, SERT shows both substrate-independent Na(+) fluxes and substrate-dependent Na(+) fluxes of variable stoichiometry; these fluxes are eliminated in the presence of syntaxin 1A as Na(+) flux becomes strictly coupled to 5HT uptake. However, not known are the endogenous signaling molecules that determine the conducting states that SERT exhibits. In the present experiments, we show that inhibitors of calcium/calmodulin-dependent kinase II (CaM kinase II) modulate the stoichiometry of 5HT flux and that this effect requires syntaxin 1A. The modulation correlates with a shift in the affinity of SERT for syntaxin 1A binding. The regulation by CaM kinase II is eliminated by a mutation in the N-terminal domain of SERT. In neonatal thalomocortical neurons that endogenously express SERT and syntaxin 1A, inhibition of CaM kinase II reveals SERT-mediated currents. These data suggest that calcium-mediated signals can serve as a trigger for regulating protein-protein interactions that control SERT conducting states.

Endosomal SNARE proteins regulate CFTR activity and trafficking in epithelial cells

The Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein is a chloride channel localized at the apical plasma membrane of epithelial cells. We previously described that syntaxin 8, an endosomal SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) protein, interacts with CFTR and regulates its trafficking to the plasma membrane and hence its channel activity. Syntaxin 8 belongs to the endosomal SNARE complex which also contains syntaxin 7, vti1b and VAMP8. Here, we report that these four endosomal SNARE proteins physically and functionally interact with CFTR. In LLC-PK1 cells transfected with CFTR and in Caco-2 cells endogenously expressing CFTR, we demonstrated that endosomal SNARE protein overexpression inhibits CFTR activity but not swelling- or calcium-activated iodide efflux, indicating a specific effect upon CFTR activity. Moreover, co-immunoprecipitation experiments in LLC-PK1-CFTR cells showed that CFTR and SNARE proteins belong to a same complex and pull-down assays showed that VAMP8 and vti1b preferentially interact with CFTR N-terminus tail. By cell surface biotinylation and immunofluorescence experiments, we evidenced that endosomal SNARE overexpression disturbs CFTR apical targeting. Finally, we found a colocalization of CFTR and endosomal SNARE proteins in Rab11-positive recycling endosomes, suggesting a new role for endosomal SNARE proteins in CFTR trafficking in epithelial cells.

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.

Modulation of the Kv4.3 channel by syntaxin 1A

The SNARE protein syntaxin 1A (Syn1A) is known to inhibit delayed rectifier K(+) channels of the K(v)1 and K(v)2 families with heterogeneous effects on their gating properties. In this study, we explored whether Syn1A could directly modulate K(v)4.3, a rapidly inactivating K(v) channel with important roles in neuroendocrine cells and cardiac myocytes. Immunoprecipitation studies in HEK293 cells coexpressing Syn1A and K(v)4.3 revealed a direct interaction with increased trafficking to the plasma membrane without a change in channel synthesis. Paradoxically, Syn1A inhibited K(v)4.3 current density. In particular, Syn1A produced a left-shift in steady-state inactivation of K(v)4.3 without affecting either voltage dependence of activation or gating kinetics, a pattern distinct from other K(v) channels. Combined with our previous reports, our results further verify the notion that the mechanisms involved in Syn1A-K(v) interactions vary significantly between K(v) channels, thus providing a wide scope for Syn1A modulation of exocytosis and membrane excitability.

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

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

Quantifying RhoA facilitated trafficking of the epithelial Na+ channel toward the plasma membrane with total internal reflection fluorescence-fluorescence recovery after photobleaching

The epithelial Na(+) channel (ENaC) plays a central role in control of epithelial surface hydration and vascular volume. Similar to other ion channels, ENaC activity is set, in part, by its membrane levels. The small G protein RhoA increases ENaC activity by increasing the membrane levels of this channel. We hypothesize that RhoA increases ENaC activity by promoting channel trafficking to the plasma membrane. Few experimental methods are available to directly visualize trafficking of ion channels to the plasma membrane. Here we combine electrophysiology with two complementary imaging methods, total internal reflection fluorescence microscopy and fluorescence recovery after photobleaching, to study the mechanistic basis of RhoA actions on ENaC. Patch clamp results demonstrate that RhoA increases ENaC activity in an additive manner with dominant-negative dynamin. This is consistent with a mechanism of increased ENaC trafficking to the membrane. Direct visualization of ENaC movement near the plasma membrane with total internal reflection fluorescence-fluorescence recovery after photobleaching revealed that RhoA accelerates ENaC trafficking toward the membrane. RhoA-facilitated movement of the channel was sensitive to disrupting the endomembrane system. Moreover, facilitating retrieval decreased ENaC activity but not trafficking toward the membrane. ENaC at the plasma membrane clustered and was laterally immobile suggesting that the cytoskeleton tethers or corrals membrane resident channels or membrane-directed vesicles containing ENaC. Disrupting microtubules but not microfilaments led to reorganization of ENaC clusters and slowed trafficking toward the membrane. The cytoskeleton is an established target for RhoA signaling. We conclude that RhoA, likely through effects on the cytoskeleton, promotes ENaC trafficking to the plasma membrane to increase channel membrane levels and activity.

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.

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.

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.

Syntaxin 1A has a specific binding site in the H3 domain that is critical for targeting of H+-ATPase to apical membrane of renal epithelial cells

H+ transport in the collecting duct is regulated by exocytic insertion of H+-ATPase-laden vesicles into the apical membrane. The soluble N-ethylmaleimide-sensitive fusion protein attachment protein (SNAP) receptor (SNARE) proteins are critical for exocytosis. Syntaxin 1A contains three main domains, SNARE N, H3, and carboxy-terminal transmembrane domain. Several syntaxin isoforms form SNARE fusion complexes through the H3 domain; only syntaxin 1A, through its H3 domain, also binds H+-ATPase. This raised the possibility that there are separate binding sites within the H3 domain of syntaxin 1A for H+-ATPase and for SNARE proteins. A series of truncations in the H3 domain of syntaxin 1A were made and expressed as glutathione S-transferase (GST) fusion proteins. We determined the amount of H+-ATPase and SNARE proteins in rat kidney homogenate that complexed with GST-syntaxin molecules. Full-length syntaxin isoforms and syntaxin-1AΔC [amino acids (aa) 1–264] formed complexes with H+-ATPase and SNAP23 and vesicle-associated membrane polypeptide (VAMP). A cassette within the H3 portion was found that bound H+-ATPase (aa 235–264) and another that bound SNAP23 and VAMP (aa 190–234) to an equivalent degree as full-length syntaxin. However, the aa 235–264 cassette alone without the SNARE N (aa 1–160) does not bind but requires ligation to the SNARE N to bind H+-ATPase. When this chimerical construct was transected into inner medullary collecting duct cells it inhibited intracellular pH recovery, an index of H+-ATPase mediated secretion. We conclude that within the H3 domain of syntaxin 1A is a unique cassette that participates in the binding of the H+-ATPase to the apical membrane and confers specificity of syntaxin 1A in the process of H+-ATPase exocytosis.

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.

Inhibition of ROMK potassium channel by syntaxin 1A

ROMK potassium channels are present in the cortical collecting duct (CCD) of the kidney and serve as apical exit pathways for K+secretion in this nephron segment. K+secretion in the CCD is regulated by multiple factors. In this study, we show that syntaxin 1A, but not syntaxin 3 or 4, inhibited whole cell ROMK currents in Xenopus laevis oocytes. Syntaxin 1A, but not syntaxin 3 or 4, interacted with the COOH-terminal cytoplasmic domain of ROMK in intro. Coexpression with synaptobrevin 2 reversed inhibition of whole cell ROMK currents by syntaxin 1A. In excised inside-out membranes of oocytes, application of fusion proteins containing the cytoplasmic region of syntaxin 1A to the cytoplasmic face caused a dose-dependent inhibition of ROMK. We further examined regulation of the K+channels in the CCD by syntaxin 1A. Application of botulinum toxin C1 to the excised inside-out membranes of the CCD caused an increase in the activity of K+channels. In contrast, application of toxin B had no effects. These results suggest that syntaxin 1A causes a tonic inhibition of the K+channels in the apical membrane of the CCD. Binding of synaptobrevin 2 to syntaxin 1A during docking and fusion of transport vesicles to the plasma membranes of CCD may lead to activation of these channels.

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

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

Expression and transport function of the glutamate transporter EAAC1 inXenopus oocytes is regulated by syntaxin 1A

The function of several membrane proteins is regulated by interaction with the SNARE protein syntaxin 1A; this includes regulation of GAT1, the transporter for the dominating inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Here we demonstrate that also EAAC1, the transporter for the dominating excitatory neurotransmitter, is down-regulated by interaction with syntaxin 1A. This is shown by coexpression of EAAC1 and syntaxin 1A in Xenopus oocytes. Total EAAC1 expression is not significantly affected by the coexpression of syntaxin 1A, but more proteins become targeted to the membrane as demonstrated by biotinylation. Colocalization by coimmunoprecipitation suggests direct interaction between the two proteins. In contrast to the number of transporters, the glutamate transport activity becomes reduced, and even stronger inhibition is observed for the EAAC1-mediated conductance uncoupled from glutamate translocation. We conclude that the interaction of syntaxin 1A with EAAC1 particularly disrupts the structure of the conductance pathway of EAAC1.

ENaC subunit-subunit interactions and inhibition by syntaxin 1A

Amiloride-sensitive epithelial Na+ channels (ENaCs) are subject to modulation by many factors. Recent data have also linked the N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) machinery to this regulation of ENaC, but the molecular mechanisms that underlie this modulation are poorly understood. In this study, we demonstrate that syntaxin 1A physically interacts with ENaC and functionally regulates ENaC activity. Syntaxin 1A was able to coimmunoprecipitate in vitro-translated γ-ENaC, but not α- or β-ENaC. Also, using antibodies raised against α-, β-, or γ-ENaC, we detected syntaxin 1A in immunoprecipitates from Madin-Darby canine kidney cells stably transfected with αβγ-ENaC. In bilayers, syntaxin 1A inhibited ENaC, and this syntaxin 1A modulation of ENaC activity was eliminated by truncations of cytoplasmic domains of the ENaC subunits. Our findings provide evidence for a direct physical interaction between ENaC and syntaxin 1A and suggest involvement of ENaC's cytoplasmic domains in functional modulation of ENaC activity by syntaxin 1A.

Traffic jams affect plant development and signal transduction

Analysis of the Arabidopsis thaliana endomembrane system has shown that plant cell viability depends on a properly functioning vacuole and intact vesicular trafficking. The endomembrane system is also essential for various aspects of plant development and signal transduction. In this review, we discuss examples of these newly discovered roles for the endomembrane system in plants, and new experimental approaches and technologies that are based on high-throughput screens, which combine chemical genetics and automated confocal microscopy.

Syntaxin 1A regulates ENaC channel activity

Na(+) entry across the apical membranes of many absorptive epithelia is determined by the number (N) and open probability (P(o)) of epithelial sodium channels (ENaC). Previous results showed that the H3 domain of syntaxin-1A (S1A) binds to ENaC to reduce N, supporting a role for S1A in the regulation of ENaC trafficking. The aim of this study was to determine whether S1A-induced reductions in ENaC current also result from interactions between cell surface ENaC and S1A that alter ENaC P(o). Injection of a glutathione S-transferase (GST)-H3 S1A fusion protein into ENaC-expressing Xenopus oocytes inhibited whole cell Na(+) current (I(Na)) by 33% within 5 min. This effect was dose-dependent, with a K(i) of 7 ng/microl (approximately 200 nm). In contrast, injection of GST alone or a H3 domain-deleted GST-S1A fusion protein had no effect on I(Na). In cell-attached patch clamp experiments, GST-H3 acutely decreased ENaC P(o) by 30%, whereas GST-S1A Delta H3 was without effect. Further analysis revealed that ENaC mean closed time was significantly prolonged by S1A. Interestingly, GST-H3 had no effect on channel activity of an ENaC pore mutant that constitutively gates open (P(o) approximately equal 1.0), supporting the idea that S1A alters the closed state of ENaC and indicating that the actions of S1A on ENaC trafficking and gating can be separated experimentally. This study indicates that, in addition to a primary effect on ENaC trafficking, S1A interacts with cell surface ENaC to rapidly decrease channel gating. This rapid effect of S1A may modulate Na(+) entry rate during rapid increases in ENaC N.

Identification of Murr1 as a regulator of the human delta epithelial sodium channel

The human delta epithelial sodium channel (deltaENaC) subunit is related to the alpha-, beta-, and gammaENaC subunits that control salt homeostasis. DeltaENaC forms an amiloride-sensitive Na+ channel with the beta and gamma subunits. However, the in vivo function of deltaENaC is not known. To gain insight into the function of deltaENaC, a yeast two-hybrid screen of a human brain cDNA library was carried out using the C- and N-terminal domains of deltaENaC. A novel deltaENaC-interacting protein called Murr1 (mouse U2af1-rs1 region) was isolated in the C-terminal domain screen. Murr1 is a 21-kDa protein mutated in Bedlington terriers suffering from copper toxicosis. The interaction of Murr1 and deltaENaC was confirmed by glutathione S-transferase pulldown assay and coimmunoprecipitation. To test the functional significance of the interaction, Murr1 was coexpressed with deltabetagammaENaC in Xenopus oocytes. Murr1 inhibited amiloride-sensitive sodium current in a dose-dependent manner. In addition, deletion of the last 59 amino acids of deltaENaC abolished the inhibition. Murr1 also bound to the beta- and gammaENaC subunits and inhibited alphabetagammaENaC sodium current. Therefore, these results suggest that Murr1 is a novel regulator of ENaC.

Syntaxin 1A co-associates with native rat brain and cloned large conductance, calcium-activated potassium channels in situ

Large conductance, calcium-activated potassium channels (BKCa channels) are regulated by several distinct mechanisms, including phosphorylation/dephosphorylation events and protein-protein interactions. In this study, we have examined the interaction between BKCa channels and syntaxin 1A, a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) that is reported to modulate the activity and/or localization of different classes of ion channels. Using a reciprocal co-immunoprecipitation strategy, we observed that native BKCa channels in rat hippocampus co-associate with syntaxin 1A, but not the closely related homologue syntaxin 3. This BKCa channel-syntaxin 1A interaction could be further demonstrated in a non-neuronal cell line (human embryonic kidney (HEK) 293 cells) following co-expression of rat syntaxin 1A and BKCa channels cloned from either mouse brain or bovine aorta. However, co-expression of these same channels with syntaxin 3 did not lead to a detectable protein-protein interaction. Immunofluorescent co-staining of HEK 293 cells expressing BKCa channels and syntaxin 1A demonstrated overlapping distribution of these two proteins in situ. Functionally, co-expression of BKCa channels with syntaxin 1A, but not syntaxin 3, was observed to enhance channel gating and kinetics at low concentrations (1-4 microM) of free cytosolic calcium, but not at higher concentrations (< or = 10 microM), as judged by macroscopic current recordings in excised membrane patches. Interactions between BKCa channels and neighbouring membrane proteins may thus play important roles in regulating the activity and/or distribution of these channels within specialized cellular compartments.

Syntaxin 1A Inhibits GABA Flux, Efflux, and Exchange Mediated by the Rat Brain GABA Transporter GAT1

GABA transporters control extracellular GABA levels by coupling transmitter uptake to the sodium and chloride cotransport. The rat brain GABA transporter GAT1 and other members of this family are regulated by direct interactions with syntaxin 1A, a protein involved in vesicle docking and in the regulation of several ion channels and transporters. We have shown previously that syntaxin 1A exerts its effects on GAT1 by decreasing the net uptake of GABA and its associated ions through interactions with aspartic acid residues in the N-terminal tail of GAT1. This reduction in net uptake could be caused by many steps in the transport cycle, including substrate binding, substrate flux, substrate efflux, or reorientation of the unliganded transporter. To address this question, we performed GABA flux assays, measured flux- and efflux-associated ion currents, and assessed GABA exchange in multiple experimental systems expressing syntaxin 1A and wild-type GAT1 or GAT1 mutants. Syntaxin 1A caused similar reductions in forward and reverse transport that did not involve changes in apparent transport affinities for sodium, chloride, or GABA. The syntaxin 1A-mediated reduction in GABA flux and efflux was mimicked by mutations in GAT1 at the syntaxin 1A binding site. The binding site GAT1 mutant also caused a reduction in exchange. These data suggest that syntaxin 1A exerts its effects, directly or indirectly, on GAT1 function through interactions with GAT1's N-terminal tail and that the inhibition occurs at a step in the translocation process after substrate binding but which involves both unidirectional transport and transmitter exchange.

False interaction of syntaxin 1A with a Ca(2+)-activated K(+) channel revealed by co-immunoprecipitation and pull-down assays: implications for identification of protein-protein interactions

The techniques of co-immunoprecipitation and immunocytochemical co-labelling are classically used to identify protein-protein interactions. We have used an antibody to the rat small conductance calcium-activated potassium channel subtype 1 (rSK1) to immunoprecipitate proteins from rat brain. A 35 kDa protein was recognized by two monoclonal antibodies to syntaxin 1 and a polyclonal antibody to syntaxin 1A, but not by antibodies to syntaxins 2, 3 or 4. These data suggested that syntaxin 1A is specifically associated with rSK1 in rat brain. A GST construct of the carboxyl terminus of rSK1 was able to pull-down syntaxin 1A from rat brain. Immunocytochemistry showed somatic labelling for both rSK1 and syntaxin 1A in acutely dissociated hippocampal CA1 neurons, confirming that these proteins could interact in vivo. However, control immunoprecipitations showed that antibodies to eight potassium channels could also immunoprecipitate syntaxin, even though some of these channels would not be expected to reside in the same subcellular compartment. Mock immunoprecipitations and pull-down assays showed that syntaxin 1 could directly interact with sepharose and agarose resins. Hence immunoprecipitation and pull-down assays do not provide evidence that syntaxin is specifically associating with a protein, placing doubt on a number of reported interactions with syntaxin 1A.

Acid-sensing ion channels in malignant gliomas

High grade glioma cells derived from patient biopsies express an amiloride-sensitive sodium conductance that has properties attributed to the human brain sodium channel family, also known as acid-sensing ion channels (ASICs). This amiloride-sensitive conductance was not detected in cells obtained from normal brain tissue or low grade or benign tumors. Differential gene profiling data showed that ASIC1 and ASIC2 mRNA were present in normal and low grade tumor cells. Although ASIC1 was present in all of the high grade glial cells examined, ASIC2 mRNA was detected in less than half. The main purpose of our work was to examine the molecular mechanisms that may underlie the constitutively activated sodium currents present in high grade glioma cells. Our results show that 1) gain-of-function mutations of ASIC1 were not present in a number of freshly resected and cultured high grade gliomas, 2) syntaxin 1A inhibited ASIC currents only when ASIC1 and ASIC2 were co-expressed, and 3) the inhibition of ASIC currents by syntaxin 1A had an absolute requirement for either gamma- or delta-hENaC. Transfection of cultured cells originally derived from high grade gliomas (U87-MG and SK-MG1) with ASIC2 abolished basal amiloride-sensitive sodium conductance; this inhibition was reversed by dialysis of the cell interior with Munc-18, a syntaxin-binding protein that typically blocks the interaction of syntaxin with other proteins. Thus, syntaxin 1A cannot inhibit Na(+) permeability in the absence of adequate plasma membrane ASIC2 expression, accounting for the observed functional expression of amiloride-sensitive currents in high grade glioma cells.

Syntaxin 1A regulates ENaC via domain-specific interactions

The epithelial sodium channel (ENaC) is a heterotrimeric protein responsible for Na(+) absorption across the apical membranes of several absorptive epithelia. The rate of Na(+) absorption is governed in part by regulated membrane trafficking mechanisms that control the apical membrane ENaC density. Previous reports have implicated a role for the t-SNARE protein, syntaxin 1A (S1A), in the regulation of ENaC current (I(Na)). In the present study, we examine the structure-function relations influencing S1A-ENaC interactions. In vitro pull-down assays demonstrated that S1A directly interacts with the C termini of the alpha-, beta-, and gamma-ENaC subunits but not with the N terminus of any ENaC subunit. The H3 domain of S1A is the critical motif mediating S1A-ENaC binding. Functional studies in ENaC expressing Xenopus oocytes revealed that deletion of the H3 domain of co-expressed S1A eliminated its inhibition of I(Na), and acute injection of a GST-H3 fusion protein into ENaC expressing oocytes inhibited I(Na) to the same extent as S1A co-expression. In cell surface ENaC labeling experiments, reductions in plasma membrane ENaC accounted for the H3 domain inhibition of I(Na). Individually substituting C terminus-truncated alpha-, beta-, or gamma-ENaC subunits for their wild-type counterparts reversed the S1A-induced inhibition of I(Na), and oocytes expressing ENaC comprised of three C terminus-truncated subunits showed no S1A inhibition of I(Na). C terminus truncation or disruption of the C terminus beta-subunit PY motif increases I(Na) by interfering with ENaC endocytosis. In contrast to subunit truncation, a beta-ENaC PY mutation did not relieve S1A inhibition of I(Na), suggesting that S1A does not perturb Nedd4 interactions that lead to ENaC endocytosis/degradation. This study provides support for the concept that S1A inhibits ENaC-mediated Na(+) transport by decreasing cell surface channel number via direct protein-protein interactions at the ENaC C termini.

Regulation of the epithelial sodium channel by accessory proteins

The epithelial sodium channel (ENaC) is of fundamental importance in the control of sodium fluxes in epithelial cells. Modulation of sodium reabsorption through the distal nephron ENaC is an important component in the overall control of sodium balance, blood volume and thereby of blood pressure. This is clearly demonstrated by rare genetic disorders of sodium-channel activity (Liddle's syndrome and pseudohypoaldosteronism type 1), associated with contrasting effects on blood pressure. The mineralocorticoid aldosterone is a well-established modulator of sodium-channel activity. Considerable insight has now been gained into the intracellular signalling pathways linking aldosterone-mediated changes in gene transcription with changes in ion transport. Activating pathways include aldosterone-induced proteins and especially the serum- and glucocorticoid-inducible kinase (SGK) and the small G-protein, K-Ras 2A. Targeting of the ENaC for endocytosis and degradation is now emerging as a major mechanism for the down-regulation of channel activity. Several proteins acting in concert are an intrinsic part of this process but Nedd4 (neural precursor cell expressed developmentally down-regulated 4) is of central importance. Other mechanisms known to interact with ENaC and affect sodium transport include channel-activating protease 1 (CAP-1), a membrane-anchored protein, and the cystic fibrosis transmembrane regulator. The implications of research on accessory factors controlling ENaC activity are wide-ranging. Understanding cellular mechanisms controlling ENaC activity may provide a more detailed insight not only of ion-channel abnormalities in cystic fibrosis but also of the link between abnormal renal sodium transport and essential hypertension.

Tetanus toxin abolishes exocytosis of ROMK1 induced by inhibition of protein tyrosine kinase

We used confocal microscopy, patch-clamp, and biotin-labeling techniques to examine the role of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in mediating the effect of inhibition of PTK on ROMK1 trafficking in HEK-293 cells transfected with c-Src and green fluorescent protein (GFP)-ROMK1. Inhibition of c-Src with herbimycin A significantly decreased the tyrosine phosphorylation level of ROMK1. Patch-clamp studies demonstrated that addition of herbimycin A increased the activity of ROMK1 in cell-attached patches. Confocal microscopic imaging showed that herbimycin A decreased the intracellular intensity of GFP-ROMK1. The biotin-labeling technique demonstrated that the inhibition of c-Src increased surface ROMK1 by 110%. In contrast, inhibition of c-Src did not increase the K channel number in HEK cells transfected with R1Y337A, a ROMK1 mutant in which tyrosine residue 337 was mutated to alanine. This suggests that tyrosine residue 337 is essential for the herbimycin A-induced increase in surface ROMK1 channels. To determine whether SNARE proteins are involved in mediating exocytosis of ROMK1 induced by the inhibition of c-Src, we examined the effect of herbimycin A on ROMK1 trafficking in cells treated with tetanus toxin. The incubation of cells in a medium containing tetanus toxin abolished the herbimycin A-induced increase in the number of surface ROMK1. In contrast, inhibition of c-Src still increased the numbers of surface ROMK1 in cells treated with boiled tetanus toxin. We conclude that tyrosine dephosphorylation enhances the exocytosis of ROMK1 and that SNARE proteins are required for exocytosis induced by inhibition of PTK.

CFTR chloride channels are regulated by a SNAP-23/syntaxin 1A complex

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion reactions in eukaryotic cells by assembling into complexes that link vesicle-associated SNAREs with SNAREs on target membranes (t-SNAREs). Many SNARE complexes contain two t-SNAREs that form a heterodimer, a putative intermediate in SNARE assembly. Individual t-SNAREs (e.g., syntaxin 1A) also regulate synaptic calcium channels and cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial chloride channel that is defective in cystic fibrosis. Whether the regulation of ion channels by individual t-SNAREs is related to SNARE complex assembly and membrane fusion is unknown. Here we show that CFTR channels are coordinately regulated by two cognate t-SNAREs, SNAP-23 (synaptosome-associated protein of 23 kDa) and syntaxin 1A. SNAP-23 physically associates with CFTR by binding to its amino-terminal tail, a region that modulates channel gating. CFTR-mediated chloride currents are inhibited by introducing excess SNAP-23 into HT29-Cl.19A epithelial cells. Conversely, CFTR activity is stimulated by a SNAP-23 antibody that blocks the binding of this t-SNARE to the CFTR amino-terminal tail. The physical and functional interactions between SNAP-23 and CFTR depend on syntaxin 1A, which binds to both proteins. We conclude that CFTR channels are regulated by a t-SNARE complex that may tune CFTR activity to rates of membrane traffic in epithelial cells.

Regulation of Na+ channel density at the apical surface of rabbit urinary bladder epithelium

We have investigated the effects of various manipulations on Na(+) transport across the rabbit urinary bladder epithelium. After bladders were mounted in Ussing chambers there was a spontaneous and significant (>4-fold) increase in amiloride-sensitive short-circuit current (equivalent to net Na(+) transport) over a 6-h period. The increase in current was almost abolished by brefeldin A, an inhibitor of anterograde vesicular transport, and reduced after a 3-h delay by cycloheximide, an inhibitor of protein synthesis. The spontaneous increase in short-circuit current was potentiated by treatment of bladders with either forskolin, which causes an elevation in cAMP levels, or aldosterone. Acting together, these two agents produced a significant synergistic effect on short-circuit current. The short-circuit current recovered rapidly after reduction in intracellular Na(+) levels, achieved either by lowering the extracellular Na(+) concentration or blockade of epithelial Na(+) channels with the sulphydryl modifying reagent p-chloromercuribenzenesulphonic acid (PCMBS). Recovery after PCMBS treatment was partially sensitive to brefeldin A. Short-circuit current saturated as the extracellular Na(+) concentration was increased (EC(50) = 51 mM). Saturation occurred over a range of Na(+) concentrations in which single channel permeability is known to remain constant, indicating that it depends on a reduction in epithelial Na(+) channel density at the apical plasma membrane. Exposure of bladders to a high Na(+) concentration caused an increase in endocytotic activity, detected through an increase in the uptake of the fluid-phase marker fluorescein isothiocyanate (FITC)-dextran into vesicles located beneath the apical plasma membrane. We conclude that the urinary bladder epithelium is able to respond rapidly and efficiently to changes in its environment by regulating the density of epithelial Na(+) channels in its apical surface.

The 25-kDa synaptosome-associated protein (SNAP-25) binds and inhibits delayed rectifier potassium channels in secretory cells

Delayed-rectifier K(+) channels (K(DR)) are important regulators of membrane excitability in neurons and neuroendocrine cells. Opening of these voltage-dependent K(+) channels results in membrane repolarization, leading to the closure of the Ca(2+) channels and cessation of insulin secretion in neuroendocrine islet beta cells. Using patch clamp techniques, we have demonstrated that the activity of the K(DR) channel subtype, K(V)1.1, identified by its specific blocker dendrodotoxin-K, is inhibited by SNAP-25 in insulinoma HIT-T15 beta cells. A co-precipitation study of rat brain confirmed that SNAP-25 interacts with the K(V)1.1 protein. Cleavage of SNAP-25 by expression of botulinum neurotoxin A in HIT-T15 cells relieved this SNAP-25-mediated inhibition of K(DR). This inhibitory effect of SNAP-25 is mediated by the N terminus of K(V)1.1, likely by direct interactions with K(Valpha)1.1 and/or K(V)beta subunits, as revealed by co-immunoprecipitation performed in the Xenopus oocyte expression system and in vitro binding. Taken together we have concluded that SNAP-25 mediates secretion not only through its participation in the exocytotic SNARE complex but also by regulating membrane potential and calcium entry through its interaction with K(DR) channels.

Role of syntaxin 1A on serotonin transporter expression in developing thalamocortical neurons

Neurotransmitter transporters are regulated through a variety of signal transduction mechanisms which may operate in order to maintain appropriate levels of transmitter in the synaptic cleft. GABA and glycine transporters both interact with components of the neurotransmitter release, such as the SNARE protein syntaxin 1A, suggesting that protein-protein interactions are a common method for regulating members of the neurotransmitter transporter family, and thus, linking the release of transmitter to its subsequent re-uptake. In the present report, the interaction of syntaxin 1A with endogenous serotonin transporters (SERT) expressed in developing thalamocortical neurons is examined. Incubation of thalamocortical cultures with botulinum toxin C1, which specifically cleaves syntaxin 1A, decreased SERT function. Serotonin (5HT) saturation analysis showed that the effect of the toxin was to decrease maximum transport capacity with little change to the affinity of the transporter for 5HT. The 5HT uptake data were consistent with biotinylation experiments showing a decrease in the surface expression of SERT following toxin treatment. In addition, co-immunoprecipitation experiments showed that SERT and syntaxin 1A form a protein complex in these neurons. These data show that components of the transmitter release machinery interact with endogenously expressed amine transporters, and suggest a mechanism for the control of transmitter levels in disorders related to aminergic signaling.

Substrates regulate gamma-aminobutyric acid transporters in a syntaxin 1A-dependent manner

Several ion channels and pumps are regulated by syntaxin 1A, a component of the synaptic vesicle docking and fusion apparatus. One such regulated protein is the rat brain gamma-aminobutyric acid (GABA) transporter GAT1. The N-terminal cytoplasmic domain of GAT1 directly interacts with syntaxin 1A; this interaction induces a decrease in the rate at which GABA and associated ions are transported. GAT1 function also is regulated by transporter substrates, raising the possibility that substrates mediate at least some of their effects by regulating the interaction between GAT1 and syntaxin 1A. In oocytes expressing GAT1 and syntaxin 1A, superfusion of transporter substrates increases the GAT1 transport rate. The substrate-induced rate change (i) is prevented by coapplication of GAT1 antagonists, (ii) does not occur in oocytes expressing GAT1 alone, and (iii) does not occur in oocytes expressing interaction-deficient syntaxin 1A mutants. In oocytes, and in hippocampal neurons that endogenously express both GAT1 and syntaxin 1A, substrate application results in a decrease in the fraction of syntaxin 1A that is bound to GAT1 on a time-scale comparable to the substrate-induced change in transport rates. These data suggest that substrate translocation regulates GAT1-syntaxin 1A interactions and provide a mechanism by which GABA transport can be increased during times of rising synaptic GABA concentrations.

The epithelial Na+ channel: cell surface insertion and retrieval in Na+ homeostasis and hypertension

The epithelial Na+ channel (ENaC) forms the pathway for Na+ absorption in the kidney collecting duct and other epithelia. Dominant gain-of-function mutations cause Liddle's syndrome, an inherited form of hypertension resulting from excessive renal Na+ absorption. Conversely, loss-of-function mutations cause pseudohypoaldosteronism type I, a disorder of salt wasting and hypotension. Thus, ENaC has a critical role in the maintenance of Na+ homeostasis and blood pressure control. Altered Na+ absorption in the lung may also contribute to the pathogenesis of cystic fibrosis. Epithelial Na+ absorption is regulated in large part by mechanisms that control the expression of ENaC at the cell surface. Nedd4, a ubiquitin protein ligase, binds to ENaC and targets the channel for endocytosis and degradation. Liddle's syndrome mutations disrupt the interaction between ENaC and Nedd4, resulting in an increase in the number of ENaC channels at the cell surface. Aldosterone and vasopressin also regulate Na+ absorption to defend against hypotension and hypovolemia. Both hormones increase the expression of ENaC at the cell surface. The goal of this review is to summarize recent data on the regulation of ENaC expression at the cell surface.

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

The apically located epithelial Na(+) channel (alphabetagamma-ENaC) plays a key role in the regulation of salt and fluid transport in the kidney and other epithelia, yet its mode of trafficking to the plasma membrane and its cell surface stability in mammalian cells are poorly understood. Because the expression of ENaC in native tissues/cells is very low, we generated epithelial Madin-Darby canine kidney (MDCK) cells stably expressing alphabetagamma-ENaC, where each subunit is tagged differentially at the intracellular C terminus and the beta-subunit is also Myc-tagged at the ectodomain (alpha(HA)beta(Myc,T7)gamma(FLAG)). ENaC expression in these cells was verified by immunoblotting with antibodies to the tags, and patch clamp analysis has confirmed that the tagged channel is functional. Moreover, using electron microscopy, we demonstrated apical, but not basal, membrane localization of ENaC in these cells. The glycosylation pattern of the intracellular pool of ENaC revealed peptide N-glycosidase F and endoglycosidase H sensitivity. Surprisingly, the cell surface pool of ENaC, analyzed by surface biotinylation, was also core glycosylated and lacked detectable endoglycosidase H-resistant channels. Extraction of the channel from cells in Triton X-100 demonstrated that both intracellular and cell surface pools of ENaC are largely soluble. Moreover, floatation assays to analyze the presence of ENaC in lipid rafts showed that both intracellular and cell surface pools of this channel are not associated with rafts. We have shown previously that the total cellular pool of ENaC is turned over rapidly (t(1/2) approximately 1-2 h). Using cycloheximide treatment and surface biotinylation we now demonstrate that the cell surface pool of ENaC has a similarly short half-life (t(1/2) approximately 1 h), unlike the long half-life reported recently for the Xenopus A6 cells. Collectively, these results help elucidate key aspects of ENaC trafficking and turnover rates in mammalian kidney epithelial cells.

Mechanisms of CFTR regulation by syntaxin 1A and PKA

Activation of the chloride selective anion channel CFTR is stimulated by cAMP-dependent phosphorylation and is regulated by the target membrane t-SNARE syntaxin 1A. The mechanism by which SNARE proteins modulate CFTR in secretory epithelia is controversial. In addition, controversy exists as to whether PKA activates CFTR-mediated Cl- currents (ICFTR) by increasing the number of channels in the plasma membrane and/or by stimulating membrane-resident channels. SNARE proteins play a well known role in exocytosis and have recently been implicated in the regulation of ion channels; therefore this investigation sought to resolve two related issues:(a) is PKA activation or SNARE protein modulation of CFTR linked to changes in membrane turnover and (b) does syntaxin 1A modulate CFTR via direct effects on the gating of channels residing in the plasma membrane versus alterations in membrane traffic. Our data demonstrate that syntaxin 1A inhibits CFTR as a result of direct protein-protein interactions that decrease channel open probability (Po) and serves as a model for other SNARE protein-ion channel interactions. We also show that PKA activation can enhance membrane trafficking in some epithelial cell types, and this is independent from CFTR activation or syntaxin 1A association.

Interaction of the synaptic protein PICK1 (protein interacting with C kinase 1) with the non-voltage gated sodium channels BNC1 (brain Na+ channel 1) and ASIC (acid-sensing ion channel)

Neuronal members of the degenerin/epithelial Na(+) channel (DEG/ENaC) family of cation channels include the mammalian brain Na(+) channel 1 (BNC1), acid-sensing ion channel (ASIC) and dorsal-root acid-sensing ion channel (DRASIC). Their response to acidic pH, their sequence similarity to nematode proteins involved in mechanotransduction and their modulation by neuropeptides suggest that they may function as receptors for a number of different stimuli. Using the yeast two-hybrid assay, we found that the PDZ domain-containing protein PICK1 (protein interacting with C kinase) interacts specifically with the C-termini of BNC1 and ASIC, but not DRASIC or the related alphaENaC or betaENaC. In both the yeast two-hybrid system and mammalian cells, deletion of the BNC1 and ASIC C-termini abolished the interaction with PICK1. Likewise, mutating the PDZ domain in PICK1 abolished its interaction with BNC1 and ASIC. In addition, in a heterologous expression system PICK1 altered the distribution of BNC1 channels; this effect was dependent on the PDZ domain of PICK1 and the C-terminus of BNC1. We found crude synaptosomal fractions of brain to be enriched in ASIC, suggesting a possible synaptic localization. Moreover, in transfected hippocampal neurons ASIC co-localized with PICK1 in a punctate pattern at synapses. These data suggest that PICK1 binds ASIC and BNC1 via its PDZ domain. This interaction may be important for the localization and/or function of these channels in both the central and peripheral nervous systems.

Regulation of epithelial sodium channel activity through a region of the carboxyl terminus of the alpha -subunit. Evidence for intracellular kinase-mediated reactions

The epithelial sodium channel (ENaC) is a heteromultimer composed of three subunits, each having two membrane-spanning domains with intracellular amino and carboxyl termini. Several hormones and proteins regulate channel activity, but the molecular nature of this regulation is unknown. We conducted experiments to determine a possible new site within the carboxyl terminus of the alpha-subunit involved in enhanced channel activity through endogenous kinases. When an alpha-subunit that was truncated to remove a PY motif was expressed in Xenopus oocytes with wild type human beta- and gamma-ENaC subunits, channel activity was greatly enhanced. The removal of the entire intracellular carboxyl terminus of the alpha-subunit eliminated this enhanced basal activity. Using several point mutations, we localized this site to two amino acid residues (Pro(595)-Gly(596)) near the second membrane-spanning domain. The nonspecific kinase inhibitor staurosporine inhibits basal channel activity of wild type ENaC but was ineffective in inhibiting channels mutated at this site. The major effect of these mutations was not on channel kinetics but was largely, if not entirely, on the number of active channels on the cell surface. This region is potentially important in effecting kinase-mediated increases in ENaC activity.

Trafficking and cell surface stability of ENaC

The epithelial Na(+) channel (ENaC) plays a key role in the regulation of Na(+) and water absorption in several epithelia, including those of the distal nephron, distal colon, and lung. Accordingly, mutations in ENaC leading to reduced or increased channel activity cause human diseases such as pseudohypoaldosteronism type I or Liddle's syndrome, respectively. The gain of ENaC function in Liddle's syndrome is associated with increased activity and stability of the channel at the plasma membrane. Thus understanding the regulation of channel processing and trafficking to and stability at the cell surface is of fundamental importance. This review describes some of the recent advances in our understanding of ENaC trafficking, including the role of glycosylation, ENaC solubility in nonionic detergent, targeting signal(s) and hormones. It also describes the regulation of ENaC stability at the cell surface and the roles of the ubiquitin ligase Nedd4 (and ubiquitination) and clathrin-mediated endocytosis in that regulation.

Functional and physical coupling of voltage-sensitive calcium channels with exocytotic proteins: ramifications for the secretion mechanism

The secretion of neurotransmitters is a rapid Ca(2+)-regulated process that brings about vesicle fusion with the plasma membrane. This rapid process (< 100 microseconds) involves multiple proteins located at the plasma and vesicular membranes. Because of their homology to proteins participating in constitutive secretion and protein trafficking, they have been characterized extensively. The sequential events that lead these proteins to vesicle docking and fusion are still unclear. We will review recent studies that demonstrate the operative role played by voltage-sensitive Ca(2+) channels and discuss the relevance for the process of evoked transmitter release. The regulation of Ca(2+) influx by syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and synaptotagmin, and the reciprocity of these proteins in controlling the kinetic properties of the channel will be discussed. Calcium channel and synaptic proteins expressed in Xenopus oocytes demonstrate a strong functional interaction, which could be pertinent to the mechanism of secretion. First, the voltage-sensitive Ca(2+) channels are negatively modulated by syntaxin: this inhibition is reversed by synaptotagmin. Second, the modulation of N-type Ca(2+) channel activation kinetics strongly suggests that the vesicle could be docked at the plasma membrane through direct interaction with synaptotagmin. Finally, these interactions provide evidence for the assembly of the voltage-sensitive Ca(2+) channel with syntaxin 1A, SNAP-25 and synaptotagmin into an excitosome complex: a putative fusion complex with a potential role in the final stages of secretion. Studies suggest that cross-talk between the synaptic proteins and the channel in a tightly organized complex may enable a rapid secretory response to an incoming signal such as membrane depolarization.