CFTR is a conductance regulator as well as a chloride channel

Steady-state interactions of glibenclamide with CFTR: evidence for multiple sites in the pore

The objective of the present study was to clarify the mechanism by which the sulfonylurea drug, glibenclamide, inhibits single CFTR channels in excised patches from Xenopus oocytes. Glibenclamide blocks the open pore of the channel via binding at multiple sites with varying kinetics. In the absence of glibenclamide, open-channel bursts exhibited a flickery intraburst closed state (C1); this is due to block of the pore by the pH buffer, TES. Application of 25 microM glibenclamide to the cytoplasmic solution resulted in the appearance of two drug-induced intraburst closed states (C2, C3) of widely different duration, which differed in pH-dependence. The kinetics of interaction with the C3 state, but not the C2 state, were strongly voltage-dependent. The durations of both the C2 and C3 states were concentration-dependent, indicating a non-linear reaction scheme. Application of drug also increased the burst duration, which is consistent with an open-channel blocking mechanism. A kinetic model is proposed. These results indicate that glibenclamide interacts with open CFTR channels in a complex manner, involving interactions with multiple binding sites in the channel pore.

ClC-5 chloride channel alters expression of the epithelial sodium channel (ENaC)

ClC-5 chloride channels and epithelial sodium channels (ENaC) are present in many cell types including airway and retinal epithelia. Since ENaC activity is known to be affected by chloride transport, we co-injected Xenopus oocytes with cRNAs encoding ENaC and ClC-5 to investigate whether channel currents are impacted by heterologous co-expression of these proteins. ClC-5 currents were not detectably affected by co-expression with ENaC, whereas amiloride-sensitive ENaC currents were significantly lower compared to control oocytes expressing ENaC alone. Co-expression of ENaC with cRNA sequences encoding non-conducting fragments of ClC-5 revealed that the amino acid sequence region between positions 347 and 647 was sufficient for inhibition of ENaC currents. Co-expression of ENaC and another transport protein, the sodium dicarboxylate co-transporter (NaDC-1), did not affect ENaC currents. To test whether the inhibitory effects of ClC-5 were specific for ENaC, ClC-5 was also co-expressed with CFTR. CFTR currents were also inhibited by co-expression with ClC-5, whereas ClC-5 currents were unaffected. Western blot analysis of biotinylated oocyte surface membranes revealed that the co-expression of ClC-5 with ENaC, CFTR, or NaDC-1 decreased the abundance of these proteins at the surface membrane. We conclude that overexpression of ClC-5, specifically amino acids 347-647, can alter the normal translation or trafficking of ENaC and other ion transport proteins by a mechanism that is independent of the chloride conductance of ClC-5.

Relapsing pancreatitis due to a novel compound heterozygosity in the CFTR gene involving the second most common mutation in central and eastern Europe [CFTRdele2,3(21 kb)]

A 43-year-old otherwise healthy female patient presented with mild pancreatitis. Her family history revealed that her only son had cystic fibrosis. Genotyping of the patient demonstrated CFTR compound heterozygosity CFTRdele2,3(21 kb) and R117H and wild type alleles of the poly-T-tract in intron 8 (7T/7T). No mutations were detected in the cationic pancreatic trypsinogen (PRSS1) and the pancreatic secretory trypsinogen inhibitor (SPINK1) genes. CFTRdele2,3(21 kb) has only been described in 2000 and is the second most frequent severe CFTR mutation after DeltaF508 in central and eastern Europe. This haplotype should be included in the genetic panel when evaluating patients of central or eastern European genetic background for possible CFTR related pancreatitis.

CFTR fails to inhibit the epithelial sodium channel ENaC expressed in Xenopus laevis oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) plays a crucial role in regulating fluid secretion by the airways, intestines, sweat glands and other epithelial tissues. It is well established that the CFTR is a cAMP-activated, nucleotide-dependent anion channel, but additional functions are often attributed to it, including regulation of the epithelial sodium channel (ENaC). The absence of CFTR-dependent ENaC inhibition and the resulting sodium hyperabsorption were postulated to be a major electrolyte transport abnormality in cystic fibrosis (CF)-affected epithelia. Several ex vivo studies, including those that used the Xenopus oocyte expression system, have reported ENaC inhibition by activated CFTR, but contradictory results have also been obtained. Because CFTR-ENaC interactions have important implications in the pathogenesis of CF, the present investigation was undertaken by our three independent laboratories to resolve whether CFTR regulates ENaC in oocytes and to clarify potential sources of previously reported dissimilar observations. Using different experimental protocols and a wide range of channel expression levels, we found no evidence that activated CFTR regulates ENaC when oocyte membrane potential was carefully clamped. We determined that an apparent CFTR-dependent ENaC inhibition could be observed when resistance in series with the oocyte membrane was not low enough or the feedback voltage gain was not high enough. We suggest that the inhibitory effect of CFTR on ENaC reported in some earlier oocyte studies could be attributed to problems arising from high levels of channel expression and suboptimal recording conditions, that is, large series resistance and/or insufficient feedback voltage gain.

Expression of the unconventional myosin Myo1c alters sodium transport in M1 collecting duct cells

Epithelial cells rely on proper targeting of cellular components to perform their physiological function. This dynamic process utilizes the cytoskeleton and involves movement of vesicles to and from the plasma membrane, thus traversing the actin cortical cytoskeleton. Studies support both direct interaction of actin with channels and an indirect mechanism whereby actin may serve as a track in the final delivery of the channel to the plasma membrane. Actin-dependent processes are often mediated via a member of the myosin family of proteins. Myosin I family members have been implicated in multiple cellular events occurring at the plasma membrane. In these studies, we investigated the function of the unconventional myosin I Myo1c in the M1 mouse collecting duct cell line. Myo1c was observed to be concentrated at or near the plasma membrane, often in discrete membrane domains. To address the possible role of Myo1c in channel regulation, we expressed a truncated Myo1c, lacking ATP and actin domains, in M1 cells and compared electrophysiological responses to control M1 cells, M1 cells expressing the empty vector, and M1 cells expressing the full-length Myo1c construct. Interestingly, cells expressing the Myo1c constructs had modulated antidiuretic hormone (ADH)-stimulated short-circuit current and showed little inhibition of short-circuit current with amiloride addition. Evaluation of enhanced green fluorescent protein-Myo1c constructs supports the importance of the IQ region in targeting the Myo1c to its respective cellular domain. These data are consistent with Myo1c participating in the regulation of the Na+ channel after ADH stimulation.

ADP inhibits function of the ABC transporter cystic fibrosis transmembrane conductance regulator via its adenylate kinase activity

ADP interacts with the nucleotide-binding domains (NBDs) of the cystic fibrosis transmembrane conductance regulator (CFTR) to inhibit its Cl- channel activity. Because CFTR NBD2 has reversible adenylate kinase activity (ATP + AMP<==> ADP + ADP) that gates the channel, we asked whether ADP might inhibit current through this enzymatic activity. In adenylate kinases, binding of the two ADP molecules is cooperative. Consistent with this hypothesis, CFTR current inhibition showed positive cooperativity for ADP. We also found that ADP inhibition of current was attenuated when we prevented adenylate kinase activity with P1,P5-di(adenosine-5') pentaphosphate. Additional studies suggested that adenylate kinase-dependent inhibition involved phosphotransfer between two nucleotide diphosphates. These data indicate that the adenylate kinase reaction at NBD2 contributed to the inhibitory effect of ADP. Finding that ADP inhibits function via an adenylate kinase activity also helps explain the earlier observation that mutations that disrupt adenylate kinase activity also disrupt ADP inhibition. Thus, the results reveal a previously unrecognized mechanism by which ADP inhibits an ABC transporter.

Ischemia-induced enhancement of CFTR expression on the plasma membrane in neonatal rat ventricular myocytes

Pathophysiological functions of cardiac cystic fibrosis transmembrane conductance regulator (cCFTR) in ischemia are not well known. Using neonatal rat ventricular cardiomyocytes in primary culture in this study, we thus examined whether the CFTR protein is expressed and is functioning as a cAMP-activated anion channel on the plasma membrane under ischemic conditions. After the cells were subjected to simulated ischemia (O(2) and glucose deprivation), an up-regulation of the CFTR expression was transiently observed in the membrane fraction by Western blot. A peak expression of mature CFTR protein was found at 3 h of ischemia, and thereafter the signal diminished gradually. In contrast, the results of Northern blot indicated that the expression level of CFTR mRNA changed little until 3 h of ischemia, whereas the level slightly decreased after 8 h of ischemia. An immunohistochemical examination showed, in agreement with the results of Western blot analysis, that the expression of CFTR protein on the plasma membrane became most prominent at 3 h of ischemia, whereas the plasmalemmal CFTR signal was markedly reduced after 8 h of ischemia. Whole-cell recordings showed that the cardiomyocytes responded to cAMP with an activation of time- and voltage-independent currents that contained an anion-selective component sensitive to CFTR Cl(-) channel blockers (NPPB and glibenclamide) but not to a stilbene-derivative conventional Cl(-) channel blocker (SITS). This cAMP-activated Cl(-) channel current was found to be enhanced after an application of ischemic stress for 3 to 4 h. These findings indicate that a plasmalemmal expression of CFTR is transiently enhanced under glucose-free hypoxic conditions presumably because of a posttranslational control.

Molecular pathology of the CFTR locus in male infertility

Congenital bilateral absence of the vas deferens (CBAVD) is a form of infertility with an autosomal recessive genetic background in otherwise healthy males. CBAVD is caused by cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations on both alleles in approximately 80% of cases. Striking CFTR genotypic differences are observed in cystic fibrosis (CF) and in CBAVD. The 5T allele is a CBAVD mutation with incomplete penetrance. Recent evidence confirmed that a second polymorphic locus exists and is a major CFTR modifier. The development of minigene models have led to results suggesting that CFTR exon 9 is skipped in humans because of unusual suboptimal 5' splice sites. An extremely rare T3 allele has been reported and it has recently been confirmed that the T3 allele dramatically increases exon 9 skipping and should be considered as a 'CF' mutation. Routine testing for the most prevalent mutations in the CF Caucasian population will miss most CFTR gene alterations, which can be detected only through exhaustive scanning of CFTR sequences. Finally, a higher than expected frequency of CFTR mutations and/or polymorphisms is now found in a growing number of monosymptomatic disorders, which creates a dilemma for setting nosologic boundaries between CF and diseases related to CFTR.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Reevaluating gel-forming mucins' roles in cystic fibrosis lung disease

The existence of mucus plugs, containing mucins, bacteria, and neutrophils, blocking the lower airways in the lung of cystic fibrosis (CF) patients has raised the possibility that production of "abnormal" mucins is a critical characteristic of this disease. The molecular nature, if any, of this abnormality is unknown. Recent studies suggest that CF lung disease progression is characterized by an early phase in which airway surface liquid (ASL) increased dehydration is accompanied by altered pH and levels of reduced glutathione (GSH). In a later phase, bacterial infection and neutrophil invasion lead to increased ASL of concentrations myeloperoxidase and hypochlorous acid (HOCl). Independent studies indicate that gel-forming mucins, the key components of airway mucus, form disulfide-linked polymers through a pH-dependent, likely self-catalyzed mechanism. In this article, we present the hypothesis that increased mucus concentration (dehydration) and altered pH, and levels of GSH, myeloperoxidase, and/or HOCl result in the extracellular formation of additional interchain bonds among airway mucins. These novel interactions would create an atypical mucin network with abnormal viscoelastic and adhesive properties.

Purinergic signaling underlies CFTR control of human airway epithelial cell volume

BACKGROUND

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function in cystic fibrosis (CF) causes dysregulation of multiple ion channels, water channels, and acid-base transporters in epithelia. As such, we hypothesized that dysregulation of many critical ion channels and transporters may cause defects in human airway epithelial cell volume regulation.

METHODS

Cell volume, regulatory volume decrease, and its regulation was assessed in real-time via Coulter Counter Multisizer III-driven electronic cell sizing in non-CF, CF, and CFTR-complemented CF human airway epithelial cells. SPQ halide fluorescence assay of hypotonicity-induced chloride efflux provided indirect validation of the cell volume assays.

RESULTS

CFTR, via autocrine ATP signaling, governs human airway epithelial cell volume regulation. Non-CF cells and wild-type (WT)-CFTR-transfected CF cells had normal regulatory volume decrease (RVD) responses that were attenuated by blockade of autocrine and paracrine purinergic signaling. In contrast, parental IB3-1 CF cells or IB3-1 cells expressing CFTR mutants (DeltaF508, G551D, and S1455X) failed to RVD. CF cell RVD was rescued by agonists to P2Y G protein-coupled receptors and, more robustly, by agonists to P2X purinergic receptor channels.

CONCLUSIONS

Loss of CFTR and CFTR-driven autocrine ATP signaling may underlie defective cell volume regulation and dysregulated ion, water, and acid-base transport in CF airway epithelia.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Swelling-activated Ca2+ entry via TRPV4 channel is defective in cystic fibrosis airway epithelia

The vertebrate transient receptor potential cationic channel TRPV4 has been proposed as an osmo- and mechanosensor channel. Studies using knock-out animal models have further emphasized the relevance of the TRPV4 channel in the maintenance of the internal osmotic equilibrium and mechanosensation. However, at the cellular level, there is still one important question to answer: does the TRPV4 channel generate the Ca(2+) signal in those cells undergoing a Ca(2+)-dependent regulatory volume decrease (RVD) response? RVD in human airway epithelia requires the generation of a Ca(2+) signal to activate Ca(2+)-dependent K(+) channels. The RVD response is lost in airway epithelia affected with cystic fibrosis (CF), a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator channel. We have previously shown that the defective RVD in CF epithelia is linked to the lack of swelling-dependent activation of Ca(2+)-dependent K(+) channels. In the present study, we show the expression of TRPV4 in normal human airway epithelia, where it functions as the Ca(2+) entry pathway that triggers the RVD response after hypotonic stress, as demonstrated by TRPV4 antisense experiments. However, cell swelling failed to trigger Ca(2+) entry via TRPV4 channels in CF airway epithelia, although the channel's response to a specific synthetic activator, 4 alpha-phorbol 12,13-didecanoate, was maintained. Furthermore, RVD was recovered in CF airway epithelia treated with 4 alpha-phorbol 12,13-didecanoate. Together, these results suggest that defective RVD in CF airway epithelia might be caused by the absence of a TRPV4-mediated Ca(2+) signal and the subsequent activation of Ca(2+)-dependent K(+) channels.

Toward the pharmacogenomics of cystic fibrosis – an update

Cystic fibrosis (CF) is the most common autosomal recessive disorder in Caucasians, with a frequency of ∼ 1 in 3000 live births. The mutated gene is a defective chloride channel in epithelial cells, named cystic fibrosis transmembrane conductance regulator (CFTR). Several different protocols for the scanning of the entire gene have aided molecular diagnosis and improved our understanding of the disorder’s pathophysiology, but also showed the disease’s complexity. Therefore, CF phenotype remains difficult to predict from CFTR mutation data alone: several studies have suggested that additional genes could modulate its clinical outcome. Gene replacement therapy is still far from being used in patients with CF, mostly due to the difficulties with targeting the appropriate cells. In this review, we summarize recent advances, both in the pharmacological and gene therapy field, aimed for the treatment of the disease.

TGF-beta 1 downregulates CFTR expression and function in nasal polyps of non-CF patients

Nasal polyposis is a chronic inflammatory disease of the upper airways. It has been suggested that ion transports and CFTR expression could be modified in epithelial cells from nasal polyps of non-cystic fibrosis patients. We compared human nasal epithelial cells from nasal polyps (NP) with control nasal mucosa (CM). The level of CFTR mRNA was studied by Northern blot analysis and protein expression was studied by immunoprecipitation both ex vivo and in vitro in primary cultures of human nasal epithelial cells at the air-liquid interface. Ion transports were evaluated by short-circuit measurements in vitro. CFTR gene and protein expressions were significantly decreased in NP native tissues and in culture on day 4, when a global defect of ion transports was observed in NP cultures, but not in CM. We evaluated the effect of transforming growth factor (TGF)-beta 1 on CFTR expression and function in NP cultures on day 14 and showed, for the first time, that TGF-beta 1 was able to significantly downregulate the level of CFTR mRNA and cAMP-dependent current in NP cultures. Finally, we showed that the effects of TGF-beta 1 on ion transports could be reversed after 48-h removal of TGF-beta1 in NP cultures. In conclusion, our data strongly suggest that chronic inflammation in nasal polyposis downregulates CFTR gene and protein expression.

Zinc potentiates an uncoupled anion conductance associated with the dopamine transporter

Binding of Zn(2+) to an endogenous binding site in the dopamine transporter (DAT) leads to inhibition of dopamine (DA) uptake and enhancement of carrier-mediated substrate efflux. To elucidate the molecular mechanism for this dual effect, we expressed the DAT and selected mutants in Xenopus laevis oocytes and applied the two-electrode voltage clamp technique together with substrate flux studies employing radiolabeled tracers. Under voltage clamp conditions we found that Zn(2+) (10 mum) enhanced the current induced by both DA and amphetamine. This was not accompanied by a change in the uptake rate but by a marked increase in the charge/DA flux coupling ratio as assessed from concomitant measurements of [(3)H]DA uptake and currents in voltage-clamped oocytes. These data suggest that Zn(2+) facilitates an uncoupled ion conductance mediated by DAT. Whereas this required substrate in the wild type (WT), we observed that Zn(2+) by itself activated such a conductance in a previously described mutant (Y335A). This signifies that the conductance is not strictly dependent on an active transport process. Ion substitution experiments in Y335A, as well as in WT, indicated that the uncoupled conductance activated by Zn(2+) was mainly carried by Cl(-). Experiments in oocytes under non-voltage-clamped conditions revealed furthermore that Zn(2+) could enhance the depolarizing effect of substrates in oocytes expressing WT. The data suggest that by potentiating an uncoupled Cl(-) conductance, Zn(2+) is capable of modulating the membrane potential of cells expressing DAT and as a result cause simultaneous inhibition of uptake and enhancement of efflux.

Involvement of anion channel(s) in the modulation of the transient outward K+ channel in rat ventricular myocytes

The cardiac Ca2+-independent transient outward K+ current ( Ito), a major repolarizing ionic current, is markedly affected by Cl− substitution and anion channel blockers. We reexplored the mechanism of the action of anions on Ito by using whole cell patch-clamp in single isolated rat cardiac ventricular myocytes. The transient outward current was sensitive to blockade by 4-aminopyridine (4-AP) and was abolished by Cs+ substitution for intracellular K+. Replacement of most of the extracellular Cl− with less permeant anions, aspartate (Asp−) and glutamate (Glu−), markedly suppressed the current. Removal of external Na+ or stabilization of F-actin with phalloidin did not significantly affect the inhibitory action of less permeant anions on Ito. In contrast, the permeant Cl− substitute Br− did not markedly affect the current, whereas F− substitution for Cl− induced a slight inhibition. The Ito elicited during Br− substitution for Cl− was also sensitive to blockade by 4-AP. The ability of Cl− substitutes to induce rightward shifts of the steady-state inactivation curve of Ito was in the following sequence: NO3− > Cl− ≈ Br− > gluconate− > Glu− > Asp−. Depolymerization of actin filaments with cytochalasin D (CytD) induced an effect on the steady-state inactivation of Ito similar to that of less permeant anions. Fluorescent phalloidin staining experiments revealed that CytD-pretreatment significantly decreased the intensity of FITC-phalloidin staining of F-actin, whereas Asp− substitution for Cl− was without significant effect on the intensity. These results suggest that the Ito channel is modulated by anion channel(s), in which the actin cytoskeleton may be implicated.

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus ( cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This "early secretory pathway" includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.

Activation of plasma membrane reduced glutathione transport in death receptor apoptosis of HepG2 cells

Cells undergoing apoptosis release reduced glutathione (GSH) into the extracellular space; however, the physiological significance and the mechanism behind the GSH export remain unclear. The present study demonstrates that GSH is released by HepG2 cells undergoing Fas, tumor necrosis factor alpha (TNFalpha), or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-stimulated cell death. GSH release was observed at times when extracellular lactate dehydrogenase (LDH) activity and propidium iodide (PI) incorporation were low, suggesting that the GSH release does not occur because of nonspecific cell damage, but is occurring through a specific transport system. Caspase 3-like proteases were activated before GSH was released, indicating that protease may be involved in signaling GSH release. To investigate the mechanism of GSH release, studies were performed in the presence of GSH transport inhibitors, as well as 25 mM GSH in the media. Two organic anion transporter inhibitors, probenecid and dibromosulfophthalein (DBSP), were effective in inhibiting Fas-stimulated GSH release. The addition of 25 mM GSH to the extracellular media also prevented the loss of intracellular GSH and delayed cell death. These findings suggest that an organic anion transporter is involved in GSH release during apoptosis, and that maintenance of intracellular GSH levels during apoptosis provides protection for the cell.

Modulation of calcium-dependent chloride secretion by basolateral SK4-like channels in a human bronchial cell line

The human bronchial cell line16HBE14o- was used as a model of airway epithelial cells to study the Ca(2+)-dependent Cl(-) secretion and the identity of K(Ca) channels involved in the generation of a favorable driving force for Cl(-) exit. After ionomycin application, a calcium-activated short-circuit current ( I(sc)) developed, presenting a transient peak followed by a plateau phase. Both phases were inhibited to different degrees by NFA, glybenclamide and NPPB but DIDS was only effective on the peak phase. (86)Rb effluxes through both apical and basolateral membranes were stimulated by calcium, blocked by charybdotoxin, clotrimazole and TPA. 1-EBIO, a SK-channel opener, stimulated (86)Rb effluxes. Block of basolateral K(Ca) channels resulted in I(sc) inhibition but, while reduced, I(sc) was still observed if mucosal Cl(-) was lowered. Among SK family members, only SK4 and SK1 mRNAs were detected by RT-PCR. KCNQ1 mRNAs were also identified, but involvement of K(cAMP) channels in Cl(-) secretion was unlikely, since cAMP application had no effect on (86)Rb effluxes. Moreover, chromanol 293B or clofilium, specific inhibitors of KCNQ1 channels, had no effect on cAMP-dependent I(sc). In conclusion, two distinct components of Cl(-) secretion were identified by a pharmacological approach after a Cai2+ rise. K(Ca) channels presenting the pharmacology of SK4 channels are present on both apical and basolateral membranes, but it is the basolateral SK4-like channels that play a major role in calcium-dependent chloride secretion in 16HBE14o- cells.

A novel human Cl(-) channel family related to Drosophila flightless locus

Large conductance chloride (maxi-Cl(-)) currents have been recorded in some cells, but there is still little information on the molecular nature of the channel underlying this conductance. We report here that tweety, a gene located in Drosophila flightless, has a structure similar to those of known channels and that human homologues of tweety (hTTYH1-3) are novel maxi-Cl(-) channels. hTTYH3 mRNA was found to be distributed in excitable tissues. The whole cell current of hTTYH3 was large enough to be discriminated from the control but emerged only after treatment with ionomycin. Analysis of pore mutants suggested that positively charged amino acids contributed to anion selectivity. Like a maxi-Cl(-) channel in situ, the hTTYH3 single channel showed 26-picosiemen linear current voltage, complex kinetics, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid sensitivity, subconductance, and the permeability order of I(-) > Br(-) > Cl(-). Similarly, hTTYH2 encoded an ionomycin-induced maxi-Cl(-) channel, but TTYH1 encoded a Ca(2+)-independent and swelling-activated maxi-Cl(-) channel. Therefore, the hTTYH family encoded maxi-Cl(-) channels of mammals. Further studies on the hTTYH family should lead to the elucidation of physiological and pathophysiological roles of novel Cl(-) channel molecules.

Guanylin regulates chloride secretion in the human gallbladder via the bile fluid

BACKGROUND & AIMS

The biliary epithelium of bile ducts and gallbladder modifies the composition of primary hepatic bile by absorption and secretion of an electrolyte-rich fluid. The underlying transport mechanisms, however, are still incompletely understood. We investigated the expression, the cellular localization, and the functional role of guanylin, a bioactive intestinal peptide involved in the cystic fibrosis transmembrane conductance regulator (CFTR)-regulated electrolyte/water secretion, in the human gallbladder.

METHODS

Peptide-specific antibodies were raised to localize guanylin and its affiliated signaling proteins, i.e., the guanylin receptor, guanylate cyclase C (GC-C), cGMP-dependent protein kinase type II (cGKII), and CFTR in the human gallbladder and cholangiocarcinoma cells (Mz-Cha-1) by RT-PCR, Western blot, and immunocytochemistry. A sensitive ELISA was used to assess the range of guanylin concentration in human bile fluid. The functional role of guanylin was investigated in subconfluent Mz-Cha-1 cell monolayers by isotope efflux experiments.

RESULTS

Guanylin and its affiliated signaling proteins are highly expressed in the human gallbladder. Guanylin is localized to secretory epithelial cells of the gallbladder and is present in the bile, whereas GC-C, cGKII, and CFTR are confined exclusively to the apical membrane of the same epithelial cells. Functional studies in Mz-Cha-1 cells identify guanylin as a specific regulator of biliary Cl(-) secretion that very likely is mediated by an intracellular increase of cGMP-concentration.

CONCLUSIONS

Based on the present findings and on the functional role of guanylin in other epithelia, it is likely that gallbladder epithelial cells synthesize and release guanylin into the bile to regulate electrolyte secretion by a paracrine/luminocrine signaling pathway.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

A Phase I Trial of Intranasal Moli1901 for Cystic Fibrosis

BACKGROUND

The peptide drug Moli1901 activates an alternative chloride channel that is present in cystic fibrosis (CF) nasal and airway epithelia. Doing so bypasses the dysfunctional CF transmembrane regulator.

STUDY OBJECTIVE

To determine whether intranasal Moli1901 is safe, tolerable, and will induce chloride transport in healthy volunteers and CF subjects.

DESIGN

A single-blind (to the participant), randomized, placebo-controlled, dose-escalation study of intranasal Moli1901 was performed in four healthy non-CF participants and four participants with CF. Drug or placebo was administered by intranasal superfusion, and nasal potential difference responses were continuously monitored during sequential dose escalations at 1-min intervals from 0.01 through 10 micro mol/L.

RESULTS

Neither Moli1901 nor placebo were associated with visible changes such as edema, erythema, drainage, secretions, or ulcer formation. No elevations in lactate dehydrogenase, albumin, or cell counts were observed in nasal lavage fluid after administration. No clinically significant changes in FEV(1) or other toxicity parameters occurred. Changes in the nasal potential difference (NPD) induced by chloride-free, amiloride-containing Ringers solution and by subsequent superfusion with the same solution plus 10 micro mol/L isoproterenol were consistent with both an acute and a sustained change in chloride transport in response to Moli1901. A similar analysis of NPD in the four CF participants demonstrated an acute response that resolved more quickly. A dose-response relationship to Moli1901 was observed in non-CF participants, but a greater range of variability within the CF participants contributed to the lack of a clear dose-response relationship in this group.

CONCLUSION

Moli1901 stimulates chloride transport in normal and CF nasal epithelia in vivo, but may have a shorter duration of action in CF participants.

Inhibitory regulation of cystic fibrosis transmembrane conductance regulator anion-transporting activities by Shank2

Accumulating evidence suggests that protein-protein interactions play an important role in transepithelial ion transport. In the present study, we report on the biochemical and functional association between cystic fibrosis transmembrane conductance regulator (CFTR) and a PDZ domain-containing protein Shank2. Exploratory reverse transcription-PCR screening revealed mRNAs for several members of PDZ domain-containing proteins in epithelial cells. Shank2, one of these scaffolding proteins, showed a strong interaction with CFTR by yeast two-hybrid assays. Shank2-CFTR interaction was verified by co-immunoprecipitation experiments in mammalian cells. Notably, this interaction was abolished by mutations in the PDZ domain of Shank2. Protein phosphorylation, HCO(3)(-) transport and Cl(-) current by CFTR were measured in NIH 3T3 cells with heterologous expression of Shank2. Of interest, expression of Shank2 suppressed cAMP-induced phosphorylation and activation of CFTR. Importantly, loss of Shank2 by stable transfection of antisense-hShank2 plasmid strongly increased CFTR currents in colonic T84 cells, in which CFTR and Shank2 were natively expressed. Our results indicate that Shank2 negatively regulates CFTR and that this may play a significant role in maintaining epithelial homeostasis under normal and diseased conditions such as those presented by secretory diarrhea.

Increased diffusional mobility of CFTR at the plasma membrane after deletion of its C-terminal PDZ binding motif

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated Cl- channel expressed at the apical plasma membrane. It has been proposed that the C-terminal PDZ binding motif of CFTR is required for its apical membrane targeting and that PDZ-domain interactions may tether CFTR to the actin cytoskeleton via soluble proteins including EBP50/NHERF1 and ezrin. We measured the diffusional mobility of human CFTR in the plasma membrane of Madin-Darby canine kidney cells by photobleaching of green fluorescent protein (GFP)-CFTR chimeras. After bleaching by a focused laser beam, GFP-CFTR fluorescence in the bleached membrane region recovered to approximately 90% of its initial level, indicating that nearly all of the CFTR was mobile. The GFP-CFTR diffusion coefficient (D) was 0.99 +/- 0.09 x 10(-10) cm2/s at 37 degrees C, similar to that of other membrane proteins. GFP-CFTR diffusion was not altered by protein kinase A or C activators but was blocked by paraformaldehyde and filipin. CFTR mutants lacking functional PDZ-binding domains (GFPCFTR-DeltaTRL and GFP-CFTR-DeltaTRA) were also mobile with D significantly increased by approximately 60% compared with GFP-CFTR. However, GFP-CFTR, GFP-CFTR-Delta TRL, and GFP-CFTR-DeltaTRA had similar mobilities (D approximately 12 x 10(-10) cm2/s) at the endoplasmic reticulum in brefeldin A-treated cells. Agents that modulate the actin cytoskeleton (cytochalasin D and jasplakinolide) altered the plasma membrane mobility of CFTR but not CFTR- DeltaTRL. EBP50 (NHERF1), a PDZ domain-containing protein that interacts with the C terminus of CFTR, diffused freely in the cytoplasm with a diffusion coefficient of 0.9 +/- 0.1 x 10(-7) cm2/s. EBP50 diffusion increased by approximately 2-fold after deletion of its ezrin-binding domain. These results indicate that wild-type CFTR is not tethered statically at the plasma membrane but that its diffusion is dependent on PDZ-domain interactions and an intact actin skeleton. PDZ-domain interactions of CFTR are thus dynamic and occur on a time scale of seconds or faster.

Commensal bacteria increase invasion of intestinal epithelium by Salmonella enterica serovar Typhi

The intestinal microflora consists of a heterogeneous population of microorganisms and has many effects on the health status of its human host. Here, it is shown that the products of certain strains of bacteria normally present in the intestinal microflora are able to trigger redistribution of the cystic fibrosis transmembrane conductance regulator (CFTR) protein in epithelial cells. CFTR is used by Salmonella enterica serovar Typhi as a receptor on epithelial cells which mediate the translocation of this microorganism to the gastric submucosa. Serovar Typhi-epithelial cell adhesion and CFTR-dependent invasion by serovar Typhi of epithelial cells were increased following commensal-mediated CFTR redistribution. These data suggest that commensal microorganisms present in the intestinal lumen can affect the efficiency of serovar Typhi invasion of the intestinal submucosa. This could be a key factor influencing host susceptibility to typhoid fever.

The engine of ABC proteins

Proteins that belong to the ATP-binding cassette superfamily span from bacteria to humans and comprise one of the largest protein families. These proteins are characterized by the presence of two nucleotide-binding domains, and recent studies suggest that association and dissociation of these domains is a common basic molecular mechanism of operation that couples ATP binding/hydrolysis to substrate transport across membranes.

Assembly and trafficking of a multiprotein ROMK (Kir 1.1) channel complex by PDZ interactions

The ROMK subtypes of inward rectifier K+ channels (Kir 1.1, KCNJ1) mediate potassium secretion and regulate NaCl reabsorption in the kidney. In the present study, the role of the PDZ binding motif in ROMK function is explored. Here we identify the Na/H exchange regulatory factors, NHERF-1 and NHERF-2, as PDZ domain interaction partners of the ROMK channel. Characterization of the basis and consequences of NHERF association with ROMK reveals a PDZ interaction-dependent trafficking process and a coupling mechanism for linking ROMK to a channel modifier protein, the cystic fibrosis transmembrane regulator (CFTR). As measured by antibody binding of external epitope-tagged forms of Kir 1.1 in intact cells, NHERF-1 or NHERF-2 coexpression increased cell surface expression of ROMK. Channel interaction with NHERF proteins and effects of NHERF on ROMK localization were dependent on the presence of the PDZ domain binding motif in ROMK. Both NHERF proteins contain two PDZ domains; recombinant protein-protein binding assays and yeast-two-hybrid studies revealed that ROMK preferentially associates with the second PDZ domain of NHERF-1 and with the first PDZ domain of NHERF-2, precisely opposite of what has been reported for CFTR. Consistent with the scaffolding capacity of the NHERF proteins, coexpression of NHERF-2 with ROMK and CFTR dramatically increases the amount of ROMK protein that coimmunopurifies and functionally interacts with CFTR. Thus NHERF facilitates assembly of a ternary complex containing ROMK and CFTR. These observations raise the possibility that PDZ-based interactions may underscore physiological regulation and membrane targeting of ROMK in the kidney.

A tale of two tails: cytosolic termini and K(+) channel function

The enormous variety of neuronal action potential waveforms can be ascribed, in large part, to the sculpting of their falling phases by currents through voltage-gated potassium channels. These proteins play several additional roles in other tissues such as the regulation of heartbeat and of insulin release from pancreatic cells as well as auditory signal processing in the cochlea. The functional channel is a tetramer with either six or two transmembrane segments per monomer. Selectivity filters, voltage sensors and gating elements have been mapped to residues within the transmembrane region. Cytoplasmic residues, which are accessible targets for signal transduction cascades and provide attractive means of regulation of channel activity, are now seen to be capable of modulating various aspects of channel function. Here we review structural studies on segments of the cytoplasmic tails of K(+) channels, as well as the range of modulatory activities of these tails.

Evaluation of MRP1-5 gene expression in cystic fibrosis patients homozygous for the delta F508 mutation

Cystic fibrosis (CF), due to mutations of the cystic fibrosis transmembrane conductance regulator (CFTR), exhibits a wide range of disease severity, even among deltaF508 homozygous patients, and the mechanisms of this variability have yet to be elucidated. In view of the close structural homology and possible functional overlap between CFTR and Multidrug Resistance-associated Proteins (MRPs), MRPs were investigated as potentially relevant factors in CF pathophysiology. MRP1-5 gene expression was analyzed in nasal respiratory epithelial cells from deltaF508 homozygous patients (n = 19) and control subjects (n = 20) using semiquantitative RT-PCR. Significantly lower MRP1 and MRP5 transcript levels were found in CF patients than in control subjects. MRP1 and MRP5 transcript levels were strongly correlated (r = 0.71). In CF patients, low MRP1 transcript levels were associated with more severe disease as assessed by the Shwachman score. A relation was also observed between MRP1 levels and presence of a cAMP-independent chloride conductive pathway, as determined by a halide-sensitive fluorescent assay. These results suggest that MRPs, especially MRP1, might play a role in CF phenotype and might therefore constitute a target for a novel pharmacotherapy of CF.

Modulation of mature cystic fibrosis transmembrane regulator protein by the PDZ domain protein CAL

We have previously identified the cystic fibrosis transmembrane regulator (CFTR)-interacting protein CAL and demonstrated that CAL modulates CFTR plasma membrane expression by retaining CFTR within the cell. Here, we report that in addition to regulating membrane expression, CAL also regulates the expression of mature CFTR. The co-expression of hemagglutinin-tagged or Myc-tagged CAL with green fluorescent protein (GFP)-CFTR in COS-7 cells causes a dose-dependent reduction in mature GFP-CFTR, independent of its tags. Bafilomycin A1, a lysosomal proton pump inhibitor, increases mature GFP-CFTR, confirming previous reports of lysosomal degradation of mature CFTR. Importantly, bafilomycin A1 reverses CAL-mediated CFTR degradation. The proteasome inhibitor, MG132, on the other hand, does not reverse the effect of CAL. CAL has no effect on CFTR maturation, suggesting that it exerts its effects on mature CFTR. Co-expression of CAL enhances the degradation of CFTR. We showed previously that CAL reduces the half-life of CFTR at the cell surface. Here we show that expression of dominant-negative dynamin 2 K44A, a large GTPase inhibitor that is known to inhibit clathrin-mediated endocytosis and vesicle formation in the Golgi, increases cell surface CFTR as measured by surface biotinylation. More importantly, dynamin 2 K44A also restores cell surface CFTR in CAL-overexpressing cells and partially blocks the CAL-mediated degradation of mature CFTR. These data suggest a model in which CAL retains CFTR in the cell and targets CFTR for degradation.

Organic solutes rescue the functional defect in delta F508 cystic fibrosis transmembrane conductance regulator

The most common defect in cystic fibrosis, deletion of phenylalanine from position 508 of the cystic fibrosis transmembrane conductance regulator (Delta F508 CFTR), decreases the trafficking of this protein to the cell surface membrane. Previous studies have shown that low temperature and high concentrations of glycerol or trimethylamine N-oxide can partially counteract the processing defect of Delta F508 CFTR. The present study investigates whether physiologically relevant concentrations of organic solutes, accumulated by cotransporter proteins, can rescue the misprocessing of Delta F508 CFTR. Myoinositol alone or myoinositol, betaine, and taurine given sequentially increased the processing of core-glycosylated, endoplasmic reticulum-arrested Delta F508 CFTR into the fully glycosylated form of CFTR in IB3 cells or NIH 3T3 cells stably expressing Delta F508 CFTR. Pulse-chase experiments using transiently transfected COS7 cells demonstrated that organic solutes also increased the processing of the core-glycosylated form of green fluorescent protein-Delta F508 CFTR. Moreover, the prolonged half-life of the complex-glycosylated form of GFP-Delta F508 CFTR suggests that this treatment stabilized the mature form of the protein. In vitro studies of purified NBD1 stability and aggregation showed that myoinositol stabilized both the Delta F508 and wild type CFTR and inhibited Delta F508 misfolding. Most significantly, treatment of CF bronchial airway cells with these transportable organic solutes restores cAMP-stimulated single channel activity of both CFTR and outwardly rectifying chloride channel in the cell surface membrane and also restores a forskolin-stimulated macroscopic 36Cl- efflux. We conclude that organic solutes can repair CFTR functions by enhancing the processing of Delta F508 CFTR to the plasma membrane by stabilizing the complex-glycosylated form of Delta F508 CFTR.

A haplotype-based molecular analysis of CFTR mutations associated with respiratory and pancreatic diseases

Aberrant membrane transport caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with a wide spectrum of respiratory and digestive diseases as well as cystic fibrosis. Using a gene scanning method, we found 11 polymorphisms and mutations of the CFTR gene in the Korean population. Individual variants at these sites were analyzed by conventional DNA screening in 117 control and 75 patients having bronchiectasis or chronic pancreatitis. In a haplotype determination based on a Bayesian algorithm, 15 haplotypes were assembled in the 192 individuals tested. Several haplotypes, especially with Q1352H, IVS8 T5, and E217G, were found to have disease associations in a case-control study. Notably, a common polymorphism of M470V appears to affect the intensity of the disease association. Among the two haplotypes having IVS8 T5, the T5-V470 haplotype showed higher disease association than the T5-M470 haplotype. In addition, a Q1352H mutation found in a V470 background showed the strongest disease association. The physiological significances of the identified mutations were rigorously analyzed. Non-synonymous E217G and Q1352H mutations in the M470 background caused a 60-80% reduction in CFTR-dependent Cl(-) currents and HCO3(-) -transport activities. Surprisingly, the additional M470V polymorphic variant with the Q1352H mutation completely abolished CFTR-dependent anion transport activities. These findings provide the first evidence on the importance of CFTR mutations in the Asian population. Importantly, the results also reveal that interactions between multiple genetic variants in cis affect the final function of the gene products.

Novel SIN-1 reactive intermediates modulate chloride secretion across murine airway cells

We examined the effects of reactive oxygen-nitrogen intermediates on chloride (Cl-) currents across murine tracheal epithelial (MTE) cells isolated from CD-1 mice. MTE cells were cultured on permeable supports until they formed water-tight monolayers with transepithelial resistances (Rt)>500 Omega/cm2 and then were mounted in Ussing chambers. Baseline short-circuit current (ISC) values, prior to and following the addition of 10 microM amiloride (an inhibitor of sodium-transport pathways) into the apical side, were 65 +/- 1.9 microA/cm2 and 7.6 +/- 0.51 microA/cm2, respectively (X +/- 1 SE, n=32). The addition of 3-morpholinosydnominine (SIN-1, 1 mM), which generates both superoxide and nitric oxide anions, to amiloride-treated monolayers resulted in a transient increase of ISC to a peak value of 35 +/- 1.3 microA/cm2 (X +/- SE, n=14) within the next 30-60 min. After this, the ISC decreased gradually and returned to its pre-SIN-1 value. These changes were blocked by glibenclamide (200 microM), an inhibitor of cystic fibrosis transmembrane regulator, or reduced by glutathione (GSH, 5 mM), a scavenger of peroxynitrite. Forskolin (10 microM) augmented the SIN-1 effect when added at the peak of the SIN-1 response but not when ISC had returned to its baseline value. Perfusion of MTE cells with SIN-1 also increased whole cell Cl- currents 4-fold and the open probability of CFTR-type single-channel currents from 0.041 to 0.92 in a transient fashion. Decomposed SIN-1, but not pure SIN-1c (the stable decomposition product of SIN-1), also increased ISC with an EC50 of 5 microM. Electrospray mass spectroscopy revealed several unique and uncharacterized compounds formed during the decomposition of SIN-1 as well as the reaction of SIN-1c with peroxynitrite. Formation of these compounds was inhibited by GSH. We conclude that compounds formed by the reaction of peroxynitrite with by-products of SIN-1, rather than reactive oxygen-nitrogen species per se, were responsible for the modulation of Cl- secretion across primary cultures of MTE cells.

The phenotypic consequences of CFTR mutations

Cystic fibrosis is a common autosomal recessive disorder that primarily affects the epithelial cells in the intestine, respiratory system, pancreas, gall bladder and sweat glands. Over one thousand mutations have currently been identified in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene that are associated with CF disease. There have been many studies on the correlation of the CFTR genotype and CF disease phenotype; however, this relationship is still not well understood. A connection between CFTR genotype and disease manifested in the pancreas has been well described, but pulmonary disease appears to be highly variable even between individuals with the same genotype. This review describes the current classification of CFTR mutation classes and resulting CF disease phenotypes. Complex disease alleles and modifier genes are discussed along with alternative disorders, such as disseminated bronchiectasis and pancreatitis, which are also thought to result from CFTR mutations.

Adenosine receptors and phosphodiesterase inhibitors stimulate Cl- secretion in Calu-3 cells

We investigated cystic fibrosis transmembrane conductance regulator (CFTR) activation by clinically used phosphodiesterase inhibitors (PDEis) in Calu-3 cell monolayers alone and in combination with A2B adenosine receptor stimulation. This receptor pathway has previously been shown to activate wild-type and mutant CFTR molecules. Several PDEis, including milrinone, cilostazol (Pletal), papaverine, rolipram, and sildenafil (Viagra), produced a short circuit current (Isc) that was glibenclamide-sensitive, achieving 20-85% of forskolin-stimulated Isc. Papaverine, cilostazol, and rolipram also augmented both the magnitude and the duration of Isc following low dose stimulation of adenosine receptors with Ado (0.1-1.0 microM, P < 0.01). Subsequent studies demonstrated that very low concentrations of cilostazol or papaverine (approximately 1/2 peak serum concentrations) were sufficient to activate Isc, and both agents markedly augmented Ado-stimulated Isc (1 microM, P < 0.01). Our results provide evidence that select PDEis, at concentrations achieved as part of systemic therapies, can activate CFTR-dependent Isc in Calu-3 cell monolayers. These studies also indicate that PDEis have the capacity to augment an endogenous CFTR-activating pathway in an "in vivo"-like model system, and supports future investigations of these agents relevant to cystic fibrosis.

CFTR et les anomalies des transports ioniques dans la mucoviscidose

The genetic disease cystic fibrosis (CF) is caused by mutations of the CF gene and generates defective Cl- transport across the affected epithelium. Recent progress have been made to understand CFTR activity and regulation in epithelia and its role in the muco-ciliary clearance of airway. This revue-overviews the mechanisms of transepithelial ion transport, the role of CFTR in that process and the consequences for CF of CFTR mutations.

3-(2-Benzyloxyphenyl)isoxazoles and isoxazolines: synthesis and evaluation as CFTR activators

A novel class of activators for chloride conductance in the cystic fibrosis transmembrane conductance regulator (CFTR) protein has been identified. These 3-(2-benzyloxyphenyl)isoxazoles and 3-(2-benzyloxyphenyl)isoxazolines were synthesized employing the 1,3-dipolar cycloaddition of nitrile oxides with various alkene and alkyne dipolarophiles. Utilizing a fluorescence cell-based assay of halide transport, the best compounds increased CFTR-dependent chloride transport with half-maximal stimulation at 20-50 microM.

Pseudomonas aeruginosa-induced apoptosis is defective in respiratory epithelial cells expressing mutant cystic fibrosis transmembrane conductance regulator

Chronic lung infection with Pseudomonas aeruginosa constitutes the most severe manifestation of cystic fibrosis, a scenario that results from defects in early clearance of the microbe. Early clearance involves epithelial cell ingestion of bacteria, rapid activation of nuclear factor-kappa B and cellular desquamation within minutes of P. aeruginosa infection, processes that are deficient in cells with mutant alleles of Cftr. Analyzing the effect of Cftr genotype on the apoptotic response of airway epithelial cells to P. aeruginosa, we found that human bronchial epithelial cells expressing Delta F508 cystic fibrosis transmembrane conductance regulator (CFTR) underwent significantly delayed apoptosis compared with cells expressing wild-type (WT) CFTR. Mice with a WT Cftr allele had apoptotic cells in their lungs after P. aeruginosa infections, whereas mice homozygous for the Delta F508 or G551D Cftr alleles showed little apoptosis in response to acute infection. Pseudomonal infection induced expression of CD95 and CD95 ligand, a response that was also delayed in cells homozygous for mutant Cftr alleles. Thus, WT CFTR expression promotes a rapid expression of CD95/CD95 ligand and apoptotic response to P. aeruginosa infection. Prompt apoptosis of infected epithelial cells may be critical for clearance of P. aeruginosa, and CFTR-associated defects in apoptosis may contribute to the pathogenesis of the lung disease in cystic fibrosis.

The role of regulated CFTR trafficking in epithelial secretion

The focus of this review is the regulated trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in distal compartments of the protein secretory pathway and the question of how changes in CFTR cellular distribution may impact on the functions of polarized epithelial cells. We summarize data concerning the cellular localization and activity of CFTR and attempt to synthesize often conflicting results from functional studies of regulated endocytosis and exocytosis in CFTR-expressing cells. In some instances, findings that are inconsistent with regulated CFTR trafficking may result from the use of overexpression systems or nonphysiological experimental conditions. Nevertheless, judging from data on other transporters, an appropriate cellular context is necessary to support regulated CFTR trafficking, even in epithelial cells. The discovery that disease mutations can influence CFTR trafficking in distal secretory and recycling compartments provides support for the concept that regulated CFTR recycling contributes to normal epithelial function, including the control of apical CFTR channel density and epithelial protein secretion. Finally, we propose molecular mechanisms for regulated CFTR endocytosis and exocytosis that are based on CFTR interactions with other proteins, particularly those whose primary function is membrane trafficking. These models provide testable hypotheses that may lead to elucidation of CFTR trafficking mechanisms and permit their experimental manipulation in polarized epithelial cells.

Upregulation of swelling-activated Cl- channel sensitivity to cell volume by activation of EGF receptors in murine mammary cells

Whole-cell recordings showed that, in mouse mammary C127 cells transfected with the full genome of the bovine papilloma virus (BPV), a hypotonic challenge induced the activation of outwardly rectifying Cl- currents with a peak amplitude 2.7 times greater than that in control C127 cells. Cell-attached single-channel recordings showed that BPV-induced augmentation of the peak amplitude of the whole-cell current could not chiefly be explained by a small increase (1.2 times) in unitary conductance. There was no difference between control and BPV-transfected cells in the osmotic cell swelling rate, and hence, osmotic water permeability. However, a plot of the whole-cell current density as a function of cell volume, which was measured simultaneously, showed that the BPV-transfected cells had a strikingly greater volume sensitivity than control cells. Since the E5 protein of BPV has been reported to induce constitutive activation of the epidermal growth factor (EGF) receptor and platelet-derived growth factor (PDGF) receptor in a variety of cell lines including C127 cells, effects of the growth factors on volume-sensitive outwardly rectifying (VSOR) Cl- currents were examined in C127 cells. Application of PDGF peptides failed to affect the Cl- currents in control and BPV-transfected cells, although C127 cells are known to endogenously express PDGF receptors. In contrast, EGF peptides significantly increased the VSOR Cl- current in control cells. However, they failed to induce further augmentation of the current in BPV-transfected cells. VSOR Cl- currents were inhibited by tyrphostin B46, an inhibitor of the EGF receptor tyrosine kinase, in both control and BPV-transfected cells. The IC50 value in BPV-transfected cells (12 micro M) was lower than that in control cells (31 micro M). However, the VSOR Cl- currents in both cell types were insensitive to tyrphostin AG1296, an inhibitor of the PDGF receptor tyrosine kinase. The rate of regulatory volume decrease (RVD) was markedly diminished by tyrphostin B46 but not significantly affected by tyrphostin AG1296. We thus conclude that the EGF receptor tyrosine kinase upregulates the activity of the VSOR Cl- channel, mainly by enhancing the volume sensitivity.

The role of the C terminus and Na+/H+ exchanger regulatory factor in the functional expression of cystic fibrosis transmembrane conductance regulator in nonpolarized cells and epithelia

The conserved C-terminal peptide motif (1476DTRL) of the cystic fibrosis transmembrane conductance regulator (CFTR) ensures high affinity binding to different PSD-95/Disc-large/zonula occludens-1 (PDZ) domain-containing molecules, including the Na+/H+ exchanger regulatory factor (NHERF)/ezrin-radixin-moesin-binding phosphoprotein of 50 kDa. The physiological relevance of NHERF binding to CFTR is not fully understood. Individuals with mutations resulting in premature termination of CFTR (S1455X or Delta26 CFTR) have moderately elevated sweat Cl- concentration, without an obvious lung and pancreatic phenotype, implying that the CFTR function is largely preserved. Surprisingly, when expressed heterologously, the Delta26 mutation was reported to abrogate channel activity by destabilizing the protein at the apical domain and inducing its accumulation at the basolateral membrane (Moyer, B., Denton, J., Karlson, K., Reynolds, D., Wang, S., Mickle, J., Milewski, M., Cutting, G., Guggino, W., Li, M., and Stanton, B. (1999) J. Clin. Invest. 104, 1353-1361). The goals of this study were to resolve the contrasting clinical and cellular phenotype of the Delta26 CFTR mutation and evaluate the role of NHERF in the functional expression of CFTR at the plasma membrane. Complex formation between CFTR and NHERF was disrupted by C-terminal deletions, C-terminal epitope tag attachments, or overexpression of a dominant negative NHERF mutant. These perturbations did not alter CFTR expression, metabolic stability, or function in nonpolarized cells. Likewise, inhibition of NHERF binding had no discernible effect on the apical localization of CFTR in polarized tracheal, pancreatic, intestinal, and kidney epithelia and did not influence the metabolic stability or the cAMP-dependent protein kinase-activated chloride channel conductance in polarized pancreatic epithelia. On the other hand, electrophysiological studies demonstrated that NHERF is able to stimulate the cAMP-dependent protein kinase-phosphorylated CFTR channel activity in intact cells. These results help to reconcile the discordant genotype-phenotype relationship in individuals with C-terminal truncations and indicate that apical localization of CFTR involves sorting signals other than the C-terminal 26 amino acid residues and the PDZ-binding motif in differentiated epithelia.

Expression and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in the normal and CFTR-deficient murine colon

Defective regulation and/or reduced expression of the Na+-K+-2Cl- cotransporter NKCC1 may contribute to the severe secretory defect that is observed in cystic fibrosis, but data concerning the expression and function of NKCC1 in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient cells are equivocal. We therefore investigated NKCC1 mRNA expression, Na+-K+-2Cl- cotransport activity and regulation by cAMP in crypts isolated from the proximal colon of CFTR-containing (CFTR (+/+)) and CFTR-deficient (CFTR (-/-)) mice. mRNA expression levels were determined by semiquantitative PCR, transport rates were measured fluorometrically in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetomethylester (BCECF)-loaded crypts, cytoplasmic volume changes were assessed by confocal microscopy, and [Cl-]i changes were examined by N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) quenching. NKCC1 mRNA expression levels were not significantly reduced in CFTR (-/-) crypts compared to controls. Azosemide-sensitive NH4+ influx (used as a measure of Na+-K+-2Cl- cotransport) was 2.23 +/- 0.72 vs. 1.56 +/- 0.16 mM min-1, and increased by 63.6 % in (+/+) and 87.3 % in (-/-) crypts upon stimulation for 5 min with forskolin. After 20 min of stimulation with forskolin, the Na+-K+-2Cl- cotransport rates in (-/-) and (+/+) crypts were identical. Crypt cross-sectional area and [Cl-]i decreased only in (+/+) crypts upon stimulation. In conclusion, normal NKCC1 expression levels, somewhat reduced Na+-K+-2Cl- cotransport rates, but preserved activation by cAMP were found in colonic crypts from CFTR (-/-) mice, ruling out a severe dysfunction of the Na+-K+-2Cl- cotransporter in the CF intestine. Furthermore, these studies establish the existence of a direct, cell-volume- and [Cl-]i-independent activation of colonic NKCC1 by an increase in intracellular cAMP.

Functional analysis of CFTR chloride channel activity in cells with elevated MDR1 expression

Using the patch-clamp method, we investigated a relationship between MDR1 expression and its effects on the CFTR channel function. Incubation of CaCo-2 cells with increasing concentrations of doxorubicin resulted in a reduction of CFTR chloride channel activity in a dose-dependent manner. This reduction was associated with a decrease of CFTR mRNA and simultaneous up-regulation of MDR1 mRNA in the presence of doxorubicin. Similar alteration of the CFTR function was observed in CaCo-2 cells transiently overexpressing MDR1. No alterations of the cAMP-dependent chloride currents were observed in COS-1 cells transiently co-expressing CFTR and MDR1 from strong CMV promoters. This indicated that repression of CFTR by MDR1 induction requires the presence of the native CFTR promoter.

PTH stimulates a Cl(-)-dependent and EIPA-sensitive current in chick proximal tubule cells in culture

The electrophysiological effects of parathyroid hormone (PTH) were studied in a primary cell culture model of the chick (Gallus domesticus) proximal tubule. In this model, confluent monolayers are grown on permeable filters and exhibit vectorial transport, including glucose-stimulated current. Under short-circuit conditions, PTH, at 10(-9) M, induced a positive current [short-circuit current (I(sc))] response, with an average 2-min peak response of 14.30 +/- 1.58 microA/cm(2) over the baseline I(sc,) followed by a slow decay. The PTH response was dose dependent, with a half-maximal response at 5 x 10(-9) M and maximal response at 5 x 10(-8) M. Forskolin and dibutyryl-cAMP also stimulated I(sc), as did the phosphodiesterase inhibitor IBMX. In contrast, the phorbol ester PMA inhibited baseline I(sc). The PTH response was nearly abolished by apical addition of 100 microM EIPA, an inhibitor of Na(+)/H(+) exchangers, and partially blocked by the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 100 microM) and glibenclamide (300 microM). Higher doses of EIPA or NPPB alone (500 microM) were almost fully effective, with no or slight additional effects of NPPB or EIPA, respectively. The anion exchange inhibitor DIDS (100 microM) and the Na(+) channel blocker amiloride (10 microM) had no effect. Bilateral reduction of Cl(-) in the buffer, from 137 to 2.6 mM, abolished the PTH response; increasing Cl(-) concentration restored the I(sc) response, with a half-maximal effect at 50 mM. These data suggest that, in the chick proximal tubule, PTH activates both an Na(+)/H(+) exchanger and a Cl(-) channel that may be functionally linked.

CFTR directly mediates nucleotide-regulated glutathione flux

Studies have shown that expression of cystic fibrosis transmembrane conductance regulator (CFTR) is associated with enhanced glutathione (GSH) efflux from airway epithelial cells, implicating a role for CFTR in the control of oxidative stress in the airways. To define the mechanism underlying CFTR-associated GSH flux, we studied wild-type and mutant CFTR proteins expressed in Sf9 membranes, as well as purified and reconstituted CFTR. We show that CFTR-expressing membrane vesicles mediate nucleotide-activated GSH flux, which is disrupted in the R347D pore mutant, and in the Walker A K464A and K1250A mutants. Further, we reveal that purified CFTR protein alone directly mediates nucleotide-dependent GSH flux. Interestingly, although ATP supports GSH flux through CFTR, this activity is enhanced in the presence of the non-hydrolyzable ATP analog AMP-PNP. These findings corroborate previous suggestions that CFTR pore properties can vary with the nature of the nucleotide interaction. In conclusion, our data demonstrate that GSH flux is an intrinsic function of CFTR and prompt future examination of the role of this function in airway biology in health and disease.

Sustained calcium entry through P2X nucleotide receptor channels in human airway epithelial cells

Purinergic receptor stimulation has potential therapeutic effects for cystic fibrosis (CF). Thus, we explored roles for P2Y and P2X receptors in stably increasing [Ca(2+)](i) in human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Cytosolic Ca(2+) was measured by fluorospectrometry using the fluorescent dye Fura-2/AM. Expression of P2X receptor (P2XR) subtypes was assessed by immunoblotting and biotinylation. In IB3-1 cells, ATP and other P2Y agonists caused only a transient increase in [Ca(2+)](i) derived from intracellular stores in a Na(+)-rich environment. In contrast, ATP induced an increase in [Ca(2+)](i) that had transient and sustained components in a Na(+)-free medium; the sustained plateau was potentiated by zinc or increasing extracellular pH. Benzoyl-benzoyl-ATP, a P2XR-selective agonist, increased [Ca(2+)](i) only in Na(+)-free medium, suggesting competition between Na(+) and Ca(2+) through P2XRs. Biochemical evidence showed that the P2X(4) receptor is the major subtype shared by these airway epithelial cells. A role for store-operated Ca(2+) channels, voltage-dependent Ca(2+) channels, or Na(+)/Ca(2+) exchanger in the ATP-induced sustained Ca(2+) signal was ruled out. In conclusion, these data show that epithelial P2X(4) receptors serve as ATP-gated calcium entry channels that induce a sustained increase in [Ca(2+)](i). In airway epithelia, a P2XR-mediated Ca(2+) signal may have therapeutic benefit for CF.