Regulation of CFTR chloride channels by syntaxin and Munc18 isoforms

The EF-hand Ca(2+)-binding protein p22 associates with microtubules in an N-myristoylation-dependent manner

Proteins containing the EF-hand Ca(2+)-binding motif, such as calmodulin and calcineurin B, function as regulators of various cellular processes. Here we focus on p22, an N-myristoylated, widely expressed EF-hand Ca(2+)-binding protein conserved throughout evolution, which was shown previously to be required for membrane traffic. Immunofluorescence studies show that p22 distributes along microtubules during interphase and mitosis in various cell lines. Moreover, we report that p22 associates with the microtubule cytoskeleton indirectly via a cytosolic microtubule-binding factor. Gel filtration studies indicate that the p22-microtubule-binding activity behaves as a 70- to 30-kDa globular protein. Our results indicate that p22 associates with microtubules via a novel N-myristoylation-dependent mechanism that does not involve classic microtubule-associated proteins and motor proteins. The association of p22 with microtubules requires the N-myristoylation of p22 but does not involve p22's Ca(2+)-binding activity, suggesting that the p22-microtubule association and the role of p22 in membrane traffic are functionally related, because N-myristoylation is required for both events. Therefore, p22 is an excellent candidate for a protein that can mediate interactions between the microtubule cytoskeleton and membrane traffic.

Syntaxin 3 and Munc-18-2 in epithelial cells during kidney development

BACKGROUND

Differentiation of epithelial cells involves the assembly of polarized membrane transport machineries necessary for the generation and maintenance of the apical and basolateral membrane domains characteristic of this cell type. We have analyzed the expression patterns of vesicle-docking proteins of the syntaxin family in mouse kidney, focusing on syntaxin 3 and its interaction partner, the Sec1-related Munc-18-2.

METHODS

Expression patterns were studied by in situ hybridization and immunocytochemistry and the complex formation of syntaxin 3 and Munc-18-2 by coimmunoprecipitation and Western blotting.

RESULTS

We have previously shown by in situ hybridization that Munc-18-2 is present in the proximal tubules and collecting ducts of embryonic day 17 mouse kidney. We compared this with the expression patterns of syntaxin 1A, 2, 3, 4, and 5, and found that syntaxin 3 was enriched in the same epithelial structures in which Munc-18-2 was abundant. By immunocytochemistry, the two proteins colocalized at the apical plasma membrane of proximal tubule and collecting duct epithelial cells, and they were shown to form a physical complex in the kidney. The expression of both proteins was up-regulated during kidney development. The most prominent changes in expression levels coincided with the differentiation of proximal tubules, suggesting a role in the generation of the highly active reabsorption machinery characterizing this segment of the nephron.

CONCLUSION

The results show that Munc-18-2 and syntaxin 3 form a complex in vivo and suggest that they participate in epithelial cell differentiation and targeted vesicle transport processes in the developing kidney.

Inhibition of matrix metalloproteinase MMP-2 activates chloride current in human airway epithelial cells

Matrix metalloproteinases (MMPs) are involved in the remodeling and degradation of the extracellular matrix. Recently, it has been found that MMPs also contribute to processes not directly related to tissue remodeling, such as platelet aggregation or degranulation of airway gland cells. Since mucus secretion is closely related to ion channel function, we investigated whether MMPs could also be involved in the regulation of ion channels. We used human airway submucosal cell line Calu-3 to study the effects of MMPs on whole-cell current and transepithelial short-circuit current (Isc). Phenanthroline, a specific inhibitor of MMPs, increased whole-cell current with the half-maximally effective dose of 5.2 µM, and reversibly activated Isc in transepithelial measurements. Current stimulated by phenanthroline displayed linear current-voltage relationships and had inhibitor pharmacology and ion selectivity consistent with cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel activity. Zymography and Western blot showed significant expression of MMP-2 in Calu-3 cells. Moreover, anti-MMP-2 antibodies (1 µg/mL) increased whole-cell current and Isc, whereas human recombinant MMP-2 (10 ng/mL) reduced it. We also studied the expression of MMPs and the effects of phenanthroline on whole-cell current in A549 cells, which are derived from airway surface epithelium and do not express CFTR Cl- channels. While these cells also showed significant expression of MMP-2, inhibition of this enzyme with phenanthroline exerted no significant effect on whole-cell current. It is concluded that MMP-2 is involved in the regulation of CFTR Cl- channels in human airways.Key words: matrix metalloproteinases, Cl- current, Calu-3 cells, zymography, phenanthroline.

Interaction of syntaxins with the amiloride-sensitive epithelial sodium channel

Amiloride-sensitive sodium channels mediate sodium entry across the apical membrane of epithelial cells in variety of tissues. The rate of Na(+) entry is controlled by the regulation of the epithelial sodium channel (ENaC) complex. Insertion/retrieval of the ENaC complex into the apical membrane as well as direct kinetic effects at the single channel level are recognized mechanisms of regulation. Recent data suggest that the syntaxin family of targeting proteins interact with and functionally regulate a number of ion channels and pumps. To evaluate the role of these proteins in regulating ENaC activity, we co-expressed rat ENaC cRNA (alpha, beta, gamma subunits) with syntaxin 1A or 3 cRNAs in Xenopus oocytes. Basal ENaC currents were inhibited by syntaxin 1A and stimulated by syntaxin 3. Both syntaxin 1A and syntaxin 3 could be co-immunoprecipitated with ENaC subunit proteins, suggesting physical interaction. Interestingly, immunofluorescence data suggest that with either syntaxin isoform the ENaC-associated epifluorescence on the oocyte surface is enhanced. These data indicate that (i) both syntaxin isoforms increase the net externalization of the ENaC channel complex, (ii) that the functional regulation is isoform specific, and (iii) suggest that ENaC may be regulated through mechanisms involving protein-protein interactions.

Syntaxin 1A inhibits regulated CFTR trafficking in xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial cell Cl channel, whose gating activity and membrane trafficking are controlled by cAMP/protein kinase A (PKA)-mediated phosphorylation. CFTR Cl currents are regulated also by syntaxin 1A (A. P. Naren, D. J. Nelson, W. W. Xie, B. Jovov, J. Pevsner, M. K. Bennett, D. J. Benos, M. W. Quick, and K. L. Kirk. Nature 390: 302-305, 1997), a protein best known for its role in membrane trafficking and neurosecretion. To examine the mechanism of syntaxin 1A inhibition, we expressed these proteins in Xenopus oocytes and monitored agonist-induced changes in plasma membrane capacitance and cell surface fluorescence of CFTR that contains an external epitope tag. cAMP stimulation elicited large increases in membrane capacitance and in cell surface labeling of flag-tagged CFTR. Coexpression of CFTR with syntaxin 1A, but not syntaxin 3, inhibited cAMP-induced increases in membrane capacitance and plasma membrane CFTR content. Injection of botulinum toxin/C1 rapidly reversed syntaxin's effects on current and capacitance, indicating that they cannot be explained by an effect on CFTR synthesis. Functional expression of other integral membrane proteins, including Na-coupled glucose transporter hSGLT1, inwardly rectified K channel hIK1, P2Y2 nucleotide receptor, and viral hemagglutinin protein, was not affected by syntaxin 1A coexpression. These findings indicate that acute regulation of the number of CFTR Cl channels in plasma membrane is one mechanism by which cAMP/PKA regulates Cl currents. Inhibition of plasma membrane CFTR content by syntaxin 1A is consistent with the concept that syntaxin and other components of the SNARE machinery are involved in regulated trafficking of CFTR.

The stimulatory action of tolbutamide on Ca2+-dependent exocytosis in pancreatic beta cells is mediated by a 65-kDa mdr-like P-glycoprotein

Intracellular application of the sulfonylurea tolbutamide during whole-cell patch-clamp recordings stimulated exocytosis >5-fold when applied at a cytoplasmic Ca2+ concentration of 0.17 microM. This effect was not detectable in the complete absence of cytoplasmic Ca2+ and when exocytosis was elicited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The stimulatory action could be antagonized by the sulfonamide diazoxide, by the Cl--channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), by intracellular application of the antibody JSB1 [originally raised against a 170-kDa multidrug resistance (mdr) protein], and by tamoxifen (an inhibitor of the mdr- and volume-regulated Cl- channels). Immunocytochemistry and Western blot analyses revealed that JSB1 recognizes a 65-kDa protein in the secretory granules. This protein exhibited no detectable binding of sulfonylureas and is distinct from the 140-kDa sulfonylurea high-affinity sulfonylurea receptors also present in the granules. We conclude that (i) tolbutamide stimulates Ca2+-dependent exocytosis secondary to its binding to a 140-kDa high-affinity sulfonylurea receptor in the secretory granules; and (ii) a granular 65-kDa mdr-like protein mediates the action. The processes thus initiated culminate in the activation of a granular Cl- conductance. We speculate that the activation of granular Cl- fluxes promotes exocytosis (possibly by providing the energy required for membrane fusion) by inducing water uptake and an increased intragranular hydrostatic pressure.

The cystic fibrosis transmembrane conductance regulator and its function in epithelial transport

CF is a well characterized disease affecting a variety of epithelial tissues. Impaired function of the cAMP activated CFTR Cl- channel appears to be the basic defect detectable in epithelial and non-epithelial cells derived from CF patients. Apart from cAMP-dependent Cl- channels also Ca2+ and volume activated Cl- currents may be changed in the presence of CFTR mutations. This is supported by recent additional findings showing that different intracellular messengers converge on the CFTR Cl- channel. Analysis of the ion transport in CF airways and intestinal epithelium identified additional defects in Na+ transport. It became clear recently that mutations of CFTR may also affect the activity of other membrane conductances including epithelial Na+ channels, KvLQT-1 K+ channels and aquaporins (Fig. 7). Several additional, initially unexpected effects of CFTR on cellular functions, such as exocytosis, mucin secretion and regulation of the intracellular pH were reported during the past. Taken together, these results clearly indicate that CFTR not only acts as a cAMP regulated Cl- channel, but may fulfill several other cellular functions, particularly by regulating other membrane conductances. Failure in CFTR dependent regulation of these membrane conductances is likely to contribute to the defects observed in CF. Currently, no general concept is available that can explain how CFTR controls this variety of cellular functions. Further studies will have to verify whether direct protein interaction, specific effects on membrane turnover, changes of the intracellular ion concentration or additional proteins are involved in these regulatory loops. At the end of this review one cannot share the provocative and reassuring title "CFTR!" of a review written a few years ago [114]. Today one might rather finish with the statement "CFTR?".

Molecular cloning, expression analysis, and chromosomal localization of human syntaxin 8 (STX8)

We report the cloning of a cDNA encoding human syntaxin 8 (STX8), using the regulator (R) domain of the cystic fibrosis transmembrane conductance regulator (CFTR) as a bait to screen a human fetal lung cDNA library by the yeast two-hybrid system. This gene was found broadly transcribed and its mRNA size is about 1.3 kb. The STX8 gene maps to chromosomal band 17p12 and it encodes a 236-amino-acid protein. Syntaxin 8 contains in its C-terminal half a coiled-coil domain found highly conserved in the t-SNARE (SNAP receptor on target membrane) superfamily of proteins, which are involved in vesicular trafficking and docking. In syntaxin 8, a C-terminal hydrophobic domain may constitute a transmembrane anchor. It was recently shown that CFTR-mediated chloride currents can be regulated by syntaxin 1A, a t-SNARE family member, through direct protein-protein interaction. This raises the possibility that syntaxin 8 may also be involved in such regulations.

Characterization of the internalization pathways for the cystic fibrosis transmembrane conductance regulator

Mutations in the gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel give rise to the most common lethal genetic disease of Caucasian populations, CF. Although the function of CFTR is primarily related to the regulation of apical membrane chloride permeability, biochemical, immunocytochemical, and functional studies indicate that CFTR is also present in endosomal and trans Golgi compartments. The molecular pathways by which CFTR is internalized into intracellular compartments are not fully understood. To define the pathways for CFTR internalization, we investigated the association of CFTR with two specialized domains of the plasma membrane, clathrin-coated pits and caveolae. Internalization of CFTR was monitored after cell surface biotinylation and quantitation of cell surface CFTR levels after elution of cell lysates from a monomeric avidin column. Cell surface levels of CFTR were determined after disruption of caveolae or clathrin-coated vesicle formation. Biochemical assays revealed that disrupting the formation of clathrin-coated vesicles inhibited the internalization of CFTR from the plasma membrane, resulting in a threefold increase in the steady-state levels of cell surface CFTR. In contrast, the levels of cell surface CFTR after disruption of caveolae were not different from those in control cells. In addition, although our studies show the presence of caveolin at the apical membrane domain of human airway epithelial cells, we were unable to detect CFTR in purified caveolae. These results suggest that CFTR is constitutively internalized from the apical plasma membrane via clathrin-coated pits and that CFTR is excluded from caveolae.

Efficient endocytosis of the cystic fibrosis transmembrane conductance regulator requires a tyrosine-based signal

We previously demonstrated that the cystic fibrosis transmembrane conductance regulator (CFTR) is rapidly endocytosed in epithelial cells (Prince, L. S., Workman, R. B., Jr., and Marchase, R. B. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 5192-5196). To determine the structural features of CFTR required for endocytosis, we prepared chimeric molecules consisting of the amino-terminal (residues 2-78) and carboxyl-terminal tail regions (residues 1391-1476) of CFTR, each fused to the transmembrane and extracellular domains of the transferrin receptor. Functional analysis of the CFTR-(2-78) and CFTR-(1391-1476) indicated that both chimeras were rapidly internalized. Deletion of residues 1440-1476 had no effect on chimera internalization. Mutations of potential internalization signals in both cytoplasmic domains reveal that only one mutation inhibits internalization, Y1424A. Using a surface biotinylation reaction, we also examined internalization rates of wild type and mutant CFTRs expressed in COS-7 cells. We found that both wild type and A1440X CFTR were rapidly internalized, whereas the Y1424A CFTR mutant, like the chimeric protein, had approximately 40% reduced internalization activity. Deletions in the amino-terminal tail region of CFTR resulted in defective trafficking of CFTR out of the endoplasmic reticulum to the cell surface, suggesting that an intact amino terminus is critical for biosynthesis. In summary, our results suggest that both tail regions of CFTR are sufficient to promote rapid internalization of a reporter molecule and that tyrosine 1424 is required for efficient CFTR endocytosis.

Control of CFTR channel gating by phosphorylation and nucleotide hydrolysis

Control of CTFR Channel Gating by Phosphorylation and Nucleotide Hydrolysis. Physiol. Rev. 79, Suppl.: S77-S107, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is the protein product of the gene defective in cystic fibrosis, the most common lethal genetic disease among Caucasians. Unlike any other known ion channel, CFTR belongs to the ATP-binding cassette superfamily of transporters and, like all other family members, CFTR includes two cytoplasmic nucleotide-binding domains (NBDs), both of which bind and hydrolyze ATP. It appears that in a single open-close gating cycle, an individual CFTR channel hydrolyzes one ATP molecule at the NH2-terminal NBD to open the channel, and then binds and hydrolyzes a second ATP molecule at the COOH-terminal NBD to close the channel. This complex coordinated behavior of the two NBDs is orchestrated by multiple protein kinase A-dependent phosphorylation events, at least some of which occur within the third large cytoplasmic domain, called the regulatory domain. Two or more kinds of protein phosphatases selectively dephosphorylate distinct sites. Under appropriately controlled conditions of progressive phosphorylation or dephosphorylation, three functionally different phosphoforms of a single CFTR channel can be distinguished on the basis of channel opening and closing kinetics. Recording single CFTR channel currents affords an unprecedented opportunity to reproducibly examine, and manipulate, individual ATP hydrolysis cycles in a single molecule, in its natural environment, in real time.

Characterization of CFTR expression and chloride channel activity in human endothelia

The cystic fibrosis transmembrane conductance regulator (CFTR) functions as a low-conductance, cAMP-regulated chloride (Cl-) channel in a variety of cell types, such as exocrine epithelial cells. Our results demonstrate that human primary endothelial cells isolated from umbilical vein (HUVEC) and lung microvasculature (HLMVEC) also express CFTR as determined via RT-PCR and immunohistochemical and immunoprecipitation analyses. Moreover, Cl- efflux and whole cell patch-clamp analyses reveal that HUVEC (n = 6 samples, P < 0.05) and HLMVEC (n = 5 samples, P < 0.05) display cyclic nucleotide-stimulated Cl- transport that is inhibited by the CFTR selective Cl- channel blocker glibenclamide but not by the blocker DIDS, indicative of CFTR Cl- channel activity. Taken together, these findings demonstrate that human endothelial cells derived from multiple organ systems express CFTR and that CFTR functions as a cyclic nucleotide-regulated Cl- channel in human endothelia.

Protein kinase C regulates the interaction between a GABA transporter and syntaxin 1A

Syntaxin 1A inhibits GABA uptake of an endogenous GABA transporter in neuronal cultures from rat hippocampus and in reconstitution systems expressing the cloned rat brain GABA transporter GAT1. Evidence of interactions between syntaxin 1A and GAT1 comes from three experimental approaches: botulinum toxin cleavage of syntaxin 1A, syntaxin 1A antisense treatments, and coimmunoprecipitation of a complex containing GAT1 and syntaxin 1A. Protein kinase C (PKC), shown previously to modulate GABA transporter function, exerts its modulatory effects by regulating the availability of syntaxin 1A to interact with the transporter, and a transporter mutant that fails to interact with syntaxin 1A is not regulated by PKC. These results suggest a new target for regulation by syntaxin 1A and a novel mechanism for controlling the machinery involved in both neurotransmitter release and reuptake.

Syntaxin 1A inhibits CFTR chloride channels by means of domain-specific protein-protein interactions

Previously we showed that the functional activity of the epithelial chloride channel that is encoded by the cystic fibrosis gene (CFTR) is reciprocally modulated by two components of the vesicle fusion machinery, syntaxin 1A and Munc-18. Here we report that syntaxin 1A inhibits CFTR chloride channels by means of direct and domain-specific protein-protein interactions. Syntaxin 1A stoichiometrically binds to the N-terminal cytoplasmic tail of CFTR, and this binding is blocked by Munc-18. The modulation of CFTR currents by syntaxin 1A is eliminated either by deletion of this tail or by injecting this tail as a blocking peptide into coexpressing Xenopus oocytes. The CFTR binding site on syntaxin 1A maps to the third predicted helical domain (H3) of this membrane protein. Moreover, CFTR Cl- currents are effectively inhibited by a minimal syntaxin 1A construct (i.e., the membrane-anchored H3 domain) that cannot fully substitute for wild-type syntaxin 1A in membrane fusion reactions. We also show that syntaxin 1A binds to and inhibits the activities of disease-associated mutants of CFTR, and that the chloride current activity of recombinant DeltaF508 CFTR (i.e., the most common cystic fibrosis mutant) can be potentiated by disrupting its interaction with syntaxin 1A in cultured epithelial cells. Our results provide evidence for a direct physical interaction between CFTR and syntaxin 1A that limits the functional activities of normal and disease-associated forms of this chloride channel.

Molecular physiology of renal chloride channels

Chloride channels are present in all cells of the kidney. Physiological studies have revealed a bewildering variety of kidney chloride channels, but only in the past few years has molecular information on some of these channels emerged. This review will focus on cloned chloride channels expressed in renal cells.

Regulated phosphorylation and trafficking of antidepressant-sensitive serotonin transporter proteins

Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) mediate antidepressant-sensitive clearance of 5-HT following release. Although we have been aware for decades that SERT-mediated 5-HT clearance can be modulated by exogenous agents including serotonin-selective reuptake inhibitors, amphetamines, and cocaine, we have had little reason to speculate that SERT activity was actively controlled through endogenous pathways. Recent studies indicate that SERTs are likely to be trafficked to specific plasma membrane subdomains to achieve localized clearance of 5-HT, and that the number of SERTs resident in the plasma membrane is controlled through kinase- and phosphatase-linked pathways. In particular, roles for protein kinase C and phosphatase 2A become apparent through studies with enzyme activators and inhibitors in SERT-transfected cells, where SERT proteins are rapidly phosphorylated in parallel with transporter redistribution and loss of functional uptake capacity. Based on our findings, and the studies of others in native tissues and transfected cells, we propose a model whereby SERTs are organized in a macromolecular complex in the plasma membrane that may serve to locate reuptake activity near release sites. Although many elements of this model remain hypothetical, our findings suggest a much more dynamic picture of transporter-mediated 5-HT reuptake than typically described and suggest opportunities both for the development of new SERT regulatory agents and for the identification of regulatory pathways that may be compromised in mental illness.

An apical PDZ protein anchors the cystic fibrosis transmembrane conductance regulator to the cytoskeleton

The function of the cystic fibrosis transmembrane conductance regulator (CFTR) as a Cl- channel in the apical membrane of epithelial cells is extensively documented. However, less is known about the molecular determinants of CFTR residence in the apical membrane, basal regulation of its Cl- channel activity, and its reported effects on the function of other transporters. These aspects of CFTR function likely require specific interactions between CFTR and unknown proteins in the apical compartment of epithelial cells. Here we report that CFTR interacts with the recently discovered protein, EBP50 (ERM-binding phosphoprotein 50). EBP50 is concentrated at the apical membrane in human airway epithelial cells, in vivo, and CFTR and EBP50 associate in in vitro binding assays. The CFTR-EBP50 interaction requires the COOH-terminal DTRL sequence of CFTR and utilizes either PDZ1 or PDZ2 of EBP50, although binding to PDZ1 is of greater affinity. Through formation of a complex, the interaction between CFTR and EBP50 may influence the stability and/or regulation of CFTR Cl- channel function in the cell membrane and provides a potential mechanism through which CFTR can affect the activity of other apical membrane proteins.

A C-terminal motif found in the beta2-adrenergic receptor, P2Y1 receptor and cystic fibrosis transmembrane conductance regulator determines binding to the Na+/H+ exchanger regulatory factor family of PDZ proteins

The Na+/H+ exchanger regulatory factor (NHERF) binds to the tail of the beta2-adrenergic receptor and plays a role in adrenergic regulation of Na+/H+ exchange. NHERF contains two PDZ domains, the first of which is required for its interaction with the beta2 receptor. Mutagenesis studies of the beta2 receptor tail revealed that the optimal C-terminal motif for binding to the first PDZ domain of NHERF is D-S/T-x-L, a motif distinct from those recognized by other PDZ domains. The first PDZ domain of NHERF-2, a protein that is 52% identical to NHERF and also known as E3KARP, SIP-1, and TKA-1, exhibits binding preferences very similar to those of the first PDZ domain of NHERF. The delineation of the preferred binding motif for the first PDZ domain of the NHERF family of proteins allows for predictions for other proteins that may interact with NHERF or NHERF-2. For example, as would be predicted from the beta2 receptor tail mutagenesis studies, NHERF binds to the tail of the purinergic P2Y1 receptor, a seven-transmembrane receptor with an intracellular C-terminal tail ending in D-T-S-L. NHERF also binds to the tail of the cystic fibrosis transmembrane conductance regulator, which ends in D-T-R-L. Because the preferred binding motif of the first PDZ domain of the NHERF family of proteins is found at the C termini of a variety of intracellular proteins, NHERF and NHERF-2 may be multifunctional adaptor proteins involved in many previously unsuspected aspects of intracellular signaling.