Exocytosis is not involved in activation of Cl- secretion via CFTR in Calu-3 airway epithelial cells

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Chronic β2AR stimulation limits CFTR activation in human airway epithelia

Traditional pulmonary therapies for cystic fibrosis (CF) target the downstream effects of CF transmembrane conductance regulator (CFTR) dysfunction (the cause of CF). Use of one such therapy, β-adrenergic bronchodilators (such as albuterol), is nearly universal for airway clearance. Conversely, novel modulator therapies restore function to select mutant CFTR proteins, offering a disease-modifying treatment. Recent trials of modulators targeting F508del-CFTR, the most common CFTR mutation, suggest that chronic β-agonist use may undermine clinical modulator benefits. We therefore sought to understand the impact of chronic or excess β-agonist exposure on CFTR activation in human airway epithelium. The present studies demonstrate a greater than 60% reduction in both wild-type and modulator-corrected F508del-CFTR activation following chronic exposure to short- and long-acting β-agonists. This reduction was due to reduced cellular generation of cAMP downstream of the β-2 adrenergic receptor-G protein complex. Our results point towards a posttranscriptional reduction in adenylyl cyclase function as the mechanism of impaired CFTR activation produced by prolonged β-agonist exposure. β-Agonist-induced CFTR dysfunction was sufficient to abrogate VX809/VX770 modulation of F508del-CFTR in vitro. Understanding the clinical relevance of our observations is critical for CF patients using these drugs, and for investigators to inform future CFTR modulator drug trials.

Thymosin α1 represents a potential potent single-molecule-based therapy for cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) that compromise its chloride-channel activity. The most common mutation, p.Phe508del, results in the production of a misfolded CFTR protein, which has residual channel activity but is prematurely degraded. Because of the inherent complexity of the pathogenetic mechanisms involved in CF —which include impaired chloride permeability and persistent lung inflammation—a multidrug approach is required for efficacious CF therapy. To date, no individual, drug with pleiotropic beneficial effects for CF is available. Here we report on the ability of thymosin alpha 1 (Tα1)—a naturally occurring polypeptide with an excellent safety profile in the clinic when used as an adjuvant or an immunotherapeutic agent—to rectify the multiple tissue defects in CF mice as well as in cells from subjects with the p.Phe508del mutation. Tα1 displayed two combined properties that favorably opposed CF symptomatology; namely, it reduced inflammation and increased CFTR maturation, stability and activity. By virtue of this two-pronged action, Tα1 offers a strong potential to be an efficacious single molecule-based therapeutic agent in CF.

HEK-293 cells expressing the cystic fibrosis transmembrane conductance regulator (CFTR): a model for studying regulation of Cl- transport

The Human Embryonic Kidney 293 cell line (HEK‐293) readily lends itself to genetic manipulation and is a common tool for biologists to overexpress proteins of interest and study their function and molecular regulation. Although these cells have some limitations, such as an inability to form resistive monolayers necessary for studying transepithelial ion transport, they are nevertheless valuable in studying individual epithelial ion transporters. We report the use of HEK‐293 cells to study the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. While HEK‐293 cells endogenously express mRNA for the Cl⁻ channels, ClC‐2 and TMEM16A, they neither express CFTR mRNA nor protein. Therefore, we stably transfected HEK‐293 cells with EGFP‐CFTR (HEK‐CFTR) and demonstrated CFTR function by measuring forskolin‐stimulated iodide efflux. This efflux was inhibited by CFTR_inh172, and the protein kinase A inhibitor H89, but not by Ca²⁺ chelation. In contrast to intestinal epithelia, forskolin stimulation does not increase surface CFTR expression and does not require intact microtubules in HEK‐CFTR. To investigate the role of an endogenous Gα_S‐coupled receptor, we examined the bile acid receptor, TGR5. Although HEK‐CFTR cells express TGR5, the potent TGR5 agonist lithocholic acid (LCA; 5–500 μmol/L) did not activate CFTR. Furthermore, forskolin, but not LCA, increased [cAMP]_i in HEK‐CFTR suggesting that endogenous TGR5 may not be functionally linked to Gα_S. However, LCA did increase [Ca²⁺]_i and interestingly, abolished forskolin‐stimulated iodide efflux. Thus, we propose that the stable HEK‐CFTR cell line is a useful model to study the multiple signaling pathways that regulate CFTR.

In this study, we characterize a HEK‐293 cell line, stably transfected with EGFP‐CFTR (HEK‐CFTR). We examined its regulation by endogenously expressed signaling pathways, in particular the cAMP and the GαS‐coupled bile acid receptor, TGR5, signaling pathways.

Molecular motors and apical CFTR traffic in epithelia

Intracellular protein traffic plays an important role in the regulation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) chloride channels. Microtubule and actin-based motor proteins direct CFTR movement along trafficking pathways. As shown for other regulatory proteins such as adaptors, the involvement of protein motors in CFTR traffic is cell-type specific. Understanding motor specificity provides insight into the biology of the channel and opens opportunity for discovery of organ-specific drug targets for treating CFTR-mediated diseases.

Endogenous surface expression of ΔF508-CFTR mediates cAMP-stimulated Cl(-) current in CFTR(ΔF508/ΔF508) pig thyroid epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is both an anion channel and a regulator of other transport proteins. Mutations in the CFTR gene underlie the human disease, cystic fibrosis. The most common CFTR mutation, ΔF508, produces a misfolded protein which traffics improperly. The availability of transgenic CFTR(ΔF508/ΔF508) pigs allows measurement of the impact of ΔF508 in native tissue. Thyroid epithelia respond to cAMP-elevating agents by increasing anion transport, a process reliant on functional CFTR. To assess whether endogenous levels of ΔF508-CFTR mediate thyroid transport, primary thyroid epithelial cultures (pThECs) were grown from newborn CFTR(+/+) (wild-type) and CFTR(ΔF508/ΔF508) (ΔF) pig thyroids and the stimulated, secretory components of short-circuit current (I(sc)) compared. Surface biotinylation studies assessed the surface presentation of ΔF508-CFTR. Baseline I(sc) levels of both wild-type and ΔF pThECs consisted of an amiloride-sensitive component. In ΔF pThECs, this mirrored previous measurements in CFTR(-/-) (knockout) pThECs. Surprisingly, elevation of cAMP transiently increased I(sc) to peak levels ∼65% of those achieved by wild-type. In contrast, knockout pThECs were indifferent to cAMP activation. In ΔF pThECs, total ΔF508-CFTR expression was ∼9% that of wild-type, consistent with misfolding and enhanced degradation. Surface biotinylation studies indicated that ∼4% of the total ΔF508 resided at the surface and did not increase with cAMP elevation. The present findings show that low endogenous levels of pig ΔF508-CFTR can mediate substantial anion transport by thyroid epithelia. These data suggest that both wild-type and ΔF508-CFTR regulate additional thyroid transporters, and together co-ordinate the overall I(sc) response.

Activation of CFTR trafficking and gating by vasoactive intestinal peptide in human bronchial epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is an apical membrane chloride channel critical to the regulation of fluid, chloride, and bicarbonate transport in epithelia and other cell types. The most common cause of cystic fibrosis (CF) is the abnormal trafficking of CFTR mutants. Therefore, understanding the cellular machineries that transit CFTR from the endoplasmic reticulum to the cell surface is important. Vasoactive intestinal polypeptide (VIP) plays an important role in CFTR-dependent chloride transport. The present study was designed to observe the affection of VIP on the trafficking of CFTR, and channel gating in human bronchial epithelium cells (HBEC). Confocal microscopy revealed CFTR immunofluorescence extending from the apical membrane deeply into the cell cytoplasm. After VIP treatment, apical extension of CFTR immunofluorescence into the cell was reduced and the peak intensity of CFTR fluorescence shifted towards the apical membrane. Western blot showed VIP increased cell surface and total CFTR. Compared with the augmented level of total CFTR, the surface CFTR increased more markedly. Immunoprecipitation founded that the mature form of CFTR had a marked increase in HBEC treated with VIP. VIP led to a threefold increase in Cl(-) efflux in HBEC. Glibenclamide-sensitive and DIDS-insensitive CFTR Cl(-) currents were consistently observed after stimulation with VIP (10(-8) mol/L). The augmentation of CFTR Cl(-) currents enhanced by VIP (10(-8) mol/L) was reversed, at least in part, by the protein kinase A (PKA) inhibitor, H-89 and the protein kinase C (PKC) inhibitor, H-7, suggesting PKA and PKC participate in the VIP-promoted CFTR Cl(-) currents.

Chemotoxicity of doxorubicin and surface expression of P-glycoprotein (MDR1) is regulated by the Pseudomonas aeruginosa toxin Cif

P-glycoprotein (Pgp), a member of the adenosine triphosphate-binding cassette (ABC) transporter superfamily, is a major drug efflux pump expressed in normal tissues, and is overexpressed in many human cancers. Overexpression of Pgp results in reduced intracellular drug concentration and cytotoxicity of chemotherapeutic drugs and is thought to contribute to multidrug resistance of cancer cells. The involvement of Pgp in clinical drug resistance has led to a search for molecules that block Pgp transporter activity to improve the efficacy and pharmacokinetics of therapeutic agents. We have recently identified and characterized a secreted toxin from Pseudomonas aeruginosa, designated cystic fibrosis transmembrane conductance regulator (CFTR) inhibitory factor (Cif). Cif reduces the apical membrane abundance of CFTR, also an ABC transporter, and inhibits the CFTR-mediated chloride ion secretion by human airway and kidney epithelial cells. We report presently that Cif also inhibits the apical membrane abundance of Pgp in kidney, airway, and intestinal epithelial cells but has no effect on plasma membrane abundance of multidrug resistance protein 1 or 2. Cif increased the drug sensitivity to doxorubicin in kidney cells expressing Pgp by 10-fold and increased the cellular accumulation of daunorubicin by 2-fold. Thus our studies show that Cif increases the sensitivity of Pgp-overexpressing cells to doxorubicin, consistent with the hypothesis that Cif affects Pgp functional expression. These results suggest that Cif may be useful to develop a new class of specific inhibitors of Pgp aimed at increasing the sensitivity of tumors to chemotherapeutic drugs, and at improving the bioavailability of Pgp transport substrates.

Vasoactive intestinal peptide increases cystic fibrosis transmembrane conductance regulator levels in the apical membrane of Calu-3 cells through a protein kinase C-dependent mechanism

Noncholinergic neurons contribute to innate airway defenses by releasing vasoactive intestinal peptides (VIP), which stimulates the submucosal glands to produce a bicarbonate-rich fluid containing mucins and antimicrobial factors. VIP elevates cAMP and activates cystic fibrosis transmembrane conductance regulator (CFTR) channels; however, its effects on surface expression have not been investigated. We studied CFTR levels in the apical membrane of polarized Calu-3 cell monolayers, a widely used model for submucosal gland serous cells. Biotinylation during VIP exposure revealed a significant increase in apical CFTR within 10 min, which reached a maximal 3.3-fold increase after 30 min. Total CFTR content of cell lysates was not altered during this time period; therefore, the increase in surface CFTR reflects redistribution from intracellular pools. Internalization assays revealed that apical accumulation was due, at least in part, to a reduction in the rate of CFTR endocytosis. VIP-induced accumulation of apical CFTR was mimicked by phorbol ester but not by forskolin, and it was blocked by the protein kinase (PK)C inhibitors bisindolylmaleimide X (BisX) or chelerythrine chloride but not by the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H89). Increases in surface expression were paralleled by enhanced iodide effluxes during cAMP stimulation. BisX inhibition of VIP responses was abrogated when monolayers were pretreated with tannic acid to inhibit endosome recycling. Thus, PKC increases the surface expression of CFTR channels in addition to potentiating their responsiveness to PKA phosphorylation. Integrated regulation through multiple signaling pathways may be a common feature of VIP and other physiological secretagogues.

The DeltaF508-CFTR mutation results in increased biofilm formation by Pseudomonas aeruginosa by increasing iron availability

Enhanced antibiotic resistance of Pseudomonas aeruginosa in the cystic fibrosis (CF) lung is thought to be due to the formation of biofilms. However, there is no information on the antibiotic resistance of P. aeruginosa biofilms grown on human airway epithelial cells or on the effects of airway cells on biofilm formation by P. aeruginosa. Thus we developed a coculture model and report that airway cells increase the resistance of P. aeruginosa to tobramycin (Tb) by >25-fold compared with P. aeruginosa grown on abiotic surfaces. Therefore, the concentration of Tb required to kill P. aeruginosa biofilms on airway cells is 10-fold higher than the concentration achievable in the lungs of CF patients. In addition, CF airway cells expressing DeltaF508-CFTR significantly enhanced P. aeruginosa biofilm formation, and DeltaF508 rescue with wild-type CFTR reduced biofilm formation. Iron (Fe) content of the airway in CF is elevated, and Fe is known to enhance P. aeruginosa growth. Thus we investigated whether enhanced biofilm formation on DeltaF508-CFTR cells was due to increased Fe release by airway cells. We found that airway cells expressing DeltaF508-CFTR released more Fe than cells rescued with WT-CFTR. Moreover, Fe chelation reduced biofilm formation on airway cells, whereas Fe supplementation enhanced biofilm formation on airway cells expressing WT-CFTR. These data demonstrate that human airway epithelial cells promote the formation of P. aeruginosa biofilms with a dramatically increased antibiotic resistance. The DeltaF508-CFTR mutation enhances biofilm formation, in part, by increasing Fe release into the apical medium.

Myosin Vb is required for trafficking of the cystic fibrosis transmembrane conductance regulator in Rab11a-specific apical recycling endosomes in polarized human airway epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(-) secretion across fluid-transporting epithelia is regulated, in part, by modulating the number of CFTR Cl(-) channels in the plasma membrane by adjusting CFTR endocytosis and recycling. However, the mechanisms that regulate CFTR recycling in airway epithelial cells remain unknown, at least in part, because the recycling itineraries of CFTR in these cells are incompletely understood. In a previous study, we demonstrated that CFTR undergoes trafficking in Rab11a-specific apical recycling endosomes in human airway epithelial cells. Myosin Vb is a plus-end-directed, actin-based mechanoenzyme that facilitates protein trafficking in Rab11a-specific recycling vesicles in several cell model systems. There are no published studies examining the role of myosin Vb in airway epithelial cells. Thus, the goal of this study was to determine whether myosin Vb facilitates CFTR recycling in polarized human airway epithelial cells. Endogenous CFTR formed a complex with endogenous myosin Vb and Rab11a. Silencing myosin Vb by RNA-mediated interference decreased the expression of wild-type CFTR and DeltaF508-CFTR in the apical membrane and decreased CFTR-mediated Cl(-) secretion across polarized human airway epithelial cells. A recombinant tail domain fragment of myosin Vb attenuated the plasma membrane expression of CFTR by arresting CFTR recycling. The dominant-negative effect was dependent on the ability of the myosin Vb tail fragment to interact with Rab11a. Taken together, these data indicate that myosin Vb is required for CFTR recycling in Rab11a-specific apical recycling endosomes in polarized human airway epithelial cells.

Endocytic trafficking of CFTR in health and disease

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl-selective anion channel expressed in epithelial tissues. Mutations in CFTR lead to the genetic disease cystic fibrosis (CF). Within each epithelial cell, CFTR interacts with a large number of transient macromolecular complexes, many of which are involved in the trafficking and targeting of CFTR. Understanding how these complexes regulate the trafficking and fate of CFTR, provides a singular insight not only into the patho-physiology of cystic fibrosis, but also provides potential drug targets to help cure this debilitating disease.

New insights into cystic fibrosis: molecular switches that regulate CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-)-selective ion channel, is a prototypic member of the ATP-binding cassette transporter superfamily that is expressed in several organs. In these organs, CFTR assembles into large, dynamic macromolecular complexes that contain signalling molecules, kinases, transport proteins, PDZ-domain-containing proteins, myosin motors, Rab GTPases, and SNAREs. Understanding how these complexes regulate the intracellular trafficking and activity of CFTR provides a unique insight into the aetiology of cystic fibrosis and other diseases.

Ambroxol-induced modification of ion transport in human airway Calu-3 epithelia

Ambroxol is often used as a mucolytic agent in various lung diseases. However, it is unclear how ambroxol acts on bronchial epithelial cells. To clarify the action of ambroxol, we studied the effects of ambroxol on the ion transport in human Calu-3 cells, a human submucosal serous cell line, measuring the transepithelial short-circuit current and conductance across monolayers of Calu-3 cells. Ambroxol of 100 microM diminished the terbutaline (a beta2-adrenergic agonist)-stimulated Cl-/HCO3(-)-dependent secretion without any decreases in the conductance of cystic fibrosis transmembrane conductance regulator (CFTR) channel locating on the apical membrane. On the other hand, under the basal (unstimulated) condition ambroxol increased the Cl(-)-dependent secretion with no significant change in the apical CFTR channel conductance and decreased the HCO3- secretion associated with a decrease in the apical CFTR channel conductance. Ambroxol had no major action on the epithelial Na+ channel (ENaC) or the ENaC-mediated Na+ absorption. These results indicate that in Calu-3 cells: (1) under the basal (unstimulated) condition ambroxol increases Cl- secretion by stimulating the entry step of Cl- and decreases HCO3- secretion by diminishing the activity of the CFTR channel and/or the Na+/HCO3(-)-dependent cotransporter, (2) under the adrenergic agonist-stimulated condition, ambroxol decreases Cl- secretion by acting on the Cl-/HCO3- exchanger, and (3) ambroxol has a more powerful action than the adrenergic agonist on the Cl-/HCO3- exchanger, leading fluid secretion to a moderately stimulated level from a hyper-stimulated level.

The Short Apical Membrane Half-life of Rescued ΔF508-Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Results from Accelerated Endocytosis of ΔF508-CFTR in Polarized Human Airway Epithelial Cells

The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene in individuals with cystic fibrosis, DeltaF508, causes retention of DeltaF508-CFTR in the endoplasmic reticulum and leads to the absence of CFTR Cl(-) channels in the apical plasma membrane. Rescue of DeltaF508-CFTR by reduced temperature or chemical means reveals that the DeltaF508 mutation reduces the half-life of DeltaF508-CFTR in the apical plasma membrane. Because DeltaF508-CFTR retains some Cl(-) channel activity, increased expression of DeltaF508-CFTR in the apical membrane could serve as a potential therapeutic approach for cystic fibrosis. However, little is known about the mechanisms responsible for the short apical membrane half-life of DeltaF508-CFTR in polarized human airway epithelial cells. Accordingly, the goal of this study was to determine the cellular defects in the trafficking of rescued DeltaF508-CFTR that lead to the decreased apical membrane half-life of DeltaF508-CFTR in polarized human airway epithelial cells. We report that in polarized human airway epithelial cells (CFBE41o-) the DeltaF508 mutation increased endocytosis of CFTR from the apical membrane without causing a global endocytic defect or affecting the endocytic recycling of CFTR in the Rab11a-specific apical recycling compartment.

Pseudomonas aeruginosa inhibits endocytic recycling of CFTR in polarized human airway epithelial cells

The most common mutation in the CFTR gene in individuals with cystic fibrosis (CF), DeltaF508, leads to the absence of CFTR Cl(-) channels in the apical plasma membrane, which in turn results in impairment of mucociliary clearance, the first line of defense against inhaled bacteria. Pseudomonas aeruginosa is particularly successful at colonizing and chronically infecting the lungs and is responsible for the majority of morbidity and mortality in patients with CF. Rescue of DeltaF508-CFTR by reduced temperature or chemical means reveals that the protein is at least partially functional as a Cl(-) channel. Thus current research efforts have focused on identification of drugs that restore the presence of CFTR in the apical membrane to alleviate the symptoms of CF. Because little is known about the effects of P. aeruginosa on CFTR in the apical membrane, whether P. aeruginosa will affect the efficacy of new drugs designed to restore the plasma membrane expression of CFTR is unknown. Accordingly, the objective of the present study was to determine whether P. aeruginosa affects CFTR-mediated Cl(-) secretion in polarized human airway epithelial cells. We report herein that a cell-free filtrate of P. aeruginosa reduced CFTR-mediated transepithelial Cl(-) secretion by inhibiting the endocytic recycling of CFTR and thus the number of WT-CFTR and DeltaF508-CFTR Cl(-) channels in the apical membrane in polarized human airway epithelial cells. These data suggest that chronic infection with P. aeruginosa may interfere with therapeutic strategies aimed at increasing the apical membrane expression of DeltaF508-CFTR.

The use of human nasal in vitro cell systems during drug discovery and development

The nasal route is widely used for the administration of drugs for both topical and systemic action. At an early stage in drug discovery and during the development process, it is essential to gain a thorough insight of the nasal absorption potential, metabolism and toxicity of the active compound and the components of the drug formulation. Human nasal epithelial cell cultures may provide a reliable screening tool for pharmaco-toxicological assessment of potential nasal drug formulations. The aim of this review is to give an overview of the information relevant for the development of a human nasal epithelial cell culture model useful during drug discovery and development. A primary goal in the development of in vitro cell culture systems is to maintain differentiated morphology and biochemical features, resembling the original tissue as closely as possible. The potential and limitations of the existing in vitro human nasal models are summarized. The following topics related to cell culture methodology are discussed: (i) primary cultures versus cell lines; (ii) cell-support substrate; (iii) medium and medium supplements; and (iv) the air-liquid interface model versus liquid-liquid. Several considerations with respect to the use of in vitro systems for pharmaceutical applications (transport, metabolism, assessment of ciliary toxicity) are also discussed.

TGFbeta down-regulation of the CFTR: a means to limit epithelial chloride secretion

Transforming growth factor beta (TGFbeta) is a multifunctional cytokine with effects on many cell types. We recently showed that in addition to epithelial barrier enhancing properties, TGFbeta causes diminished cAMP-driven chloride secretion in colonic epithelia, in a manner that is p38 MAPK-dependent. In this study, we sought to further delineate the mechanism behind TGFbeta diminution of chloride secretion. Using colonic and kidney epithelial cell lines, we found that exposure to TGFbeta causes dramatic changes in the expression and localization of the apical membrane chloride channel, cystic fibrosis transmembrane conductance regulator (CFTR). In TGFbeta-treated colonic epithelia (T84 and HT-29), CFTR mRNA was significantly reduced 2-24 h post-cytokine exposure. At a time consistent with decreased colonic epithelial secretory responses (16 h), TGFbeta treatment caused diminished intracellular CFTR protein expression (confocal microscopy) and reduced channel expression in the apical membrane during stimulated chloride secretion (biotinylation assay). In comparison, polarized kidney epithelia (MDCK) treated with TGFbeta displayed similarly reduced secretory responses to cAMP stimulating agents; however, a perinuclear accumulation of CFTR was observed, contrasting the diffuse cytoplasmic CFTR expression of control cells. Our data indicate that TGFbeta has profound effects on the expression and subcellular localization of an important channel involved in cAMP-driven chloride secretion, and thus suggest TGFbeta represents a key regulator of fluid movement.

Potentiation of effect of PKA stimulation of Xenopus CFTR by activation of PKC: role of NBD2

Activity of the human (h) cystic fibrosis transmembrane conductance regulator (CFTR) channel is predominantly regulated by PKA-mediated phosphorylation. In contrast, Xenopus (X)CFTR is more responsive to PKC than PKA stimulation. We investigated the interaction between the two kinases in XCFTR. We expressed XCFTR in Xenopus oocytes and maximally stimulated it with PKA agonists. The magnitude of activation after PKC stimulation was about eightfold that without pretreatment with PKC agonist. hCFTR, expressed in the same system, lacked this response. We name this phenomenon XCFTR-specific PKC potentiation effect. To ascertain its biophysical mechanism, we first tested for XCFTR channel insertion into the plasma membrane by a substituted-cysteine-accessibility method. No insertion was detected during kinase stimulation. Next, we studied single-channel properties and found that the single-channel open probability (Po) with PKA stimulation subsequent to PKC stimulation was 2.8-fold that observed in the absence of PKC preactivation and that single-channel conductance (gamma) was increased by approximately 22%. To ascertain which XCFTR regions are responsible for the potentiation, we constructed several XCFTR-hCFTR chimeras, expressed them in Xenopus oocytes, and tested them electrophysiologically. Two chimeras [hCFTR NH2-terminal region or regulatory (R) domain in XCFTR] showed a significant decrease in potentiation. In the chimera in which XCFTR nucleotide-binding domain (NBD)2 was replaced with the hCFTR sequence there was no potentiation whatsoever. The converse chimera (hCFTR with Xenopus NBD2) did not exhibit potentiation. These results indicate that potentiation by PKC involves a large increase in Po (with a small change in gamma) without CFTR channel insertion into the plasma membrane, that XCFTR NBD2 is necessary but not sufficient for the effect, and that the potentiation effect is likely to involve other CFTR domains.

Myosin VI regulates endocytosis of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-regulated Cl(-) channel expressed in the apical plasma membrane in fluid-transporting epithelia. Although CFTR is rapidly endocytosed from the apical membrane of polarized epithelial cells and efficiently recycled back to the plasma membrane, little is known about the molecular mechanisms regulating CFTR endocytosis and endocytic recycling. Myosin VI, an actin-dependent, minus-end directed mechanoenzyme, has been implicated in clathrin-mediated endocytosis in epithelial cells. The goal of this study was to determine whether myosin VI regulates CFTR endocytosis. Endogenous, apical membrane CFTR in polarized human airway epithelial cells (Calu-3) formed a complex with myosin VI, the myosin VI adaptor protein Disabled 2 (Dab2), and clathrin. The tail domain of myosin VI, a dominant-negative recombinant fragment, displaced endogenous myosin VI from interacting with Dab2 and CFTR and increased the expression of CFTR in the plasma membrane by reducing CFTR endocytosis. However, the myosin VI tail fragment had no effect on the recycling of endocytosed CFTR or on fluid-phase endocytosis. CFTR endocytosis was decreased by cytochalasin D, an actin-filament depolymerizing agent. Taken together, these data indicate that myosin VI and Dab2 facilitate CFTR endocytosis by a mechanism that requires actin filaments.

Efficient intracellular processing of the endogenous cystic fibrosis transmembrane conductance regulator in epithelial cell lines

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent protein kinase A-activated chloride channel that resides on the apical surface of epithelial cells. One unusual feature of this protein is that during biogenesis, approximately 75% of wild type CFTR is degraded by the endoplasmic reticulum (ER)-associated degradative (ERAD) pathway. Examining the biogenesis and structural instability of the molecule has been technically challenging due to the limited amount of CFTR expressed in epithelia. Consequently, investigators have employed heterologous overexpression systems. Based on recent results that epithelial specific factors regulate both CFTR biogenesis and function, we hypothesized that CFTR biogenesis in endogenous CFTR expressing epithelial cells may be more efficient. To test this, we compared CFTR biogenesis in two epithelial cell lines endogenously expressing CFTR (Calu-3 and T84) with two heterologous expression systems (COS-7 and HeLa). Consistent with previous reports, 20 and 35% of the newly synthesized CFTR were converted to maturely glycosylated CFTR in COS-7 and HeLa cells, respectively. In contrast, CFTR maturation was virtually 100% efficient in Calu-3 and T84 cells. Furthermore, inhibition of the proteasome had no effect on CFTR biogenesis in Calu-3 cells, whereas it stabilized the immature form of CFTR in HeLa cells. Quantitative reverse transcriptase-PCR indicated that CFTR message levels are approximately 4-fold lower in Calu-3 than HeLa cells, yet steady-state protein levels are comparable. Our results question the structural instability model of wild type CFTR and indicate that epithelial cells endogenously expressing CFTR efficiently process this protein to post-Golgi compartments.

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.

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.

Stimulation of beta 2-adrenergic receptor increases cystic fibrosis transmembrane conductance regulator expression in human airway epithelial cells through a cAMP/protein kinase A-independent pathway

PSD-95/Dlg-A/ZO-1 (PDZ) domains play an essential role in determining cell polarity. The Na(+)/H(+) exchanger regulatory factor (NHERF), also known as EBP50, contains two PDZ domains that mediate the assembly of transmembrane and cytosolic proteins into functional signal transduction complexes. Moreover, it has been shown that cystic fibrosis transmembrane conductance regulator (CFTR) and beta(2)-adrenergic receptor (beta(2)AR) bind equally well to the PDZ1 domain of EBP50. We hypothesized that beta(2)AR activation may regulate CFTR protein expression. To verify this, we evaluated the effects of a pharmacologically relevant concentration of salmeterol (2.10(-7) m), a long acting beta(2)AR agonist, on CFTR expression in primary human airway epithelial cells (HAEC). beta(2)AR stimulation induced a time-dependent increase in apical CFTR protein expression, with a maximal response reached after treatment for 24 h. This effect was post-transcriptional, dependent upon the beta(2)AR agonist binding to beta(2)AR and independent of the known beta(2)AR agonist-mediated cAMP/PKA pathway. We demonstrated by immunohistochemistry that CFTR, beta(2)AR, and EBP50 localize to the apical membrane of HAEC. Analyses of anti-EBP50 protein immunoprecipitate showed that salmeterol induced an increase in the amount of CFTR that binds to EBP50. These data suggest that beta(2)AR activation regulates the association of CFTR with EBP50 in polarized HAEC.

cAMP-dependent exocytosis and vesicle traffic regulate CFTR and fluid transport in rat jejunum in vivo

The cystic fibrosis transmembrane conductance regulator (CFTR) channel is regulated by cAMP-dependent vesicle traffic and exocytosis to the apical membrane in some cell types, but this has not been demonstrated in the intestinal crypt. The distribution of CFTR, lactase (control), and fluid secretion were determined in rat jejunum after cAMP activation in the presence of nocodazole and primaquine to disrupt vesicle traffic. CFTR and lactase were localized by immunofluorescence, and surface proteins were detected by biotinylation of enterocytes. Immunoprecipitates from biotinylated and nonbiotinylated cells were analyzed by streptavidin detection and immunoblots. Immunolocalization confirmed a cAMP-dependent shift of CFTR but not lactase from a subapical compartment to the apical surface associated with fluid secretion that was reduced in the presence of primaquine and nocodazole. Analysis of immunoblots from immunoprecipitates after biotinylation revealed a 3.8 +/- 1.7-fold (P < 0.005) increase of surface-exposed CFTR after vasoactive intestinal peptide (VIP). These measurements provide independent corroboration supporting a role for vesicle traffic in regulating CFTR and cAMP-induced fluid transport in the intestine.

PDZ domain interaction controls the endocytic recycling of the cystic fibrosis transmembrane conductance regulator

The C terminus of CFTR contains a PDZ interacting domain that is required for the polarized expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the apical plasma membrane of polarized epithelial cells. To elucidate the mechanism whereby the PDZ interacting domain mediates the polarized expression of CFTR, Madin-Darby canine kidney cells were stably transfected with wild type (wt-CFTR) or C-terminally truncated human CFTR (CFTR-DeltaTRL). We tested the hypothesis that the PDZ interacting domain regulates sorting of CFTR from the Golgi to the apical plasma membrane. Pulse-chase studies in combination with domain-selective cell surface biotinylation revealed that newly synthesized wt-CFTR and CFTR-DeltaTRL were targeted equally to the apical and basolateral membranes in a nonpolarized fashion. Thus, the PDZ interacting domain is not an apical sorting motif. Deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane from approximately 24 to approximately 13 h but had no effect on the half-life of CFTR in the basolateral membrane. Thus, the PDZ interacting domain is an apical membrane retention motif. Next, we examined the hypothesis that the PDZ interacting domain affects the apical membrane half-life of CFTR by altering its endocytosis and/or endocytic recycling. Endocytosis of wt-CFTR and CFTR-DeltaTRL did not differ. However, endocytic recycling of CFTR-DeltaTRL was decreased when compared with wt-CFTR. Thus, deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane by decreasing CFTR endocytic recycling. Our results identify a new role for PDZ proteins in regulating the endocytic recycling of CFTR in polarized epithelial cells.

Non-conventional trafficking of the cystic fibrosis transmembrane conductance regulator through the early secretory pathway

The mechanism(s) of cystic fibrosis transmembrane conductance regulator (CFTR) trafficking from the endoplasmic reticulum (ER) through the Golgi apparatus, the step impaired in individuals afflicted with the prevalent CFTR-DeltaF508 mutation leading to cystic fibrosis, is largely unknown. Recent morphological observations suggested that CFTR is largely absent from the Golgi in situ (Bannykh, S. I., Bannykh, G. I., Fish, K. N., Moyer, B. D., Riordan, J. R., and Balch, W. E. (2000) Traffic 1, 852-870), raising the possibility of a novel trafficking pathway through the early secretory pathway. We now report that export of CFTR from the ER is regulated by the conventional coat protein complex II (COPII) in all cell types tested. Remarkably, in a cell type-specific manner, processing of CFTR from the core-glycosylated (band B) ER form to the complex-glycosylated (band C) isoform followed a non-conventional pathway that was insensitive to dominant negative Arf1, Rab1a/Rab2 GTPases, or the SNAp REceptor (SNARE) component syntaxin 5, all of which block the conventional trafficking pathway from the ER to the Golgi. Moreover, CFTR transport through the non-conventional pathway was potently blocked by overexpression of the late endosomal target-SNARE syntaxin 13, suggesting that recycling through a late Golgi/endosomal system was a prerequisite for CFTR maturation. We conclude that CFTR transport in the early secretory pathway can involve a novel pathway between the ER and late Golgi/endosomal compartments that may influence developmental expression of CFTR on the cell surface in polarized epithelial cells.

NaCl and fluid secretion by the intestine of the teleostFundulus heteroclitus: involvement of CFTR

Sections of posterior intestine of the euryhaline killifish Fundulus heteroclitus adapted to sea water were stimulated by the calcium ionophore ionomycin (1 μmol l–1) in combination with agents to elevate intracellular cyclic AMP levels, 0.5 mmol l–1 dibutyryl-cyclic AMP (db-cAMP) with 0.1 mmol l–1 3-isobutyl-1-methylxanthine (IBMX). Intestinal bag preparations from recently fed animals (but not from overnight unfed animals) changed from fluid absorption (+18.9±8.30 μl cm–2 h–1 , N=8) in the untreated control period to net fluid secretion after stimulation (–7.43±1.30 μl cm–2 h–1, N=8, P&lt;0.01; means ± s.e.m.), indicative of the capacity of teleost intestine to undergo secretion. Posterior intestinal pieces mounted in vitro in Ussing-style membrane chambers showed net Cl– uptake (+2.245±0.633 μequiv cm–2 h–1, N=7) that turned to net secretion following stimulation by ionomycin + db-cAMP + IBMX (–3.809±1.22 μequiv cm–2 h–1, N=7, P&lt;0.01). Mucosal application of the anion channel blocker 1 mmol l–1 diphenylamine-2-carboxylate (DPC) after ionomycin + db-cAMP + IBMX treatment significantly reduced serosal-to-mucosal unidirectional Cl– flux (P&lt;0.001), net Cl– flux (P&lt;0.05), short-circuit current (Isc, P&lt;0.001) and tissue conductance (Gt, P&lt;0.001), while 0.1 mmol l–1 4,4′-diisothiocyano-2,2′-stilbene-disulphonic acid (DIDS, a blocker of anion exchange) was without effect. Stimulation by db-cAMP + IBMX (no ionomycin) significantly increased unidirectional fluxes, Isc and Gt but did not produce net Cl– secretion. Ionomycin alone produced a transient increase in Isc but had no effect on Gt and caused no significant changes in unidirectional or net Cl– fluxes. Addition of db-cAMP + IBMX after ionomycin treatment produced net secretion of Cl– and large increases in unidirectional fluxes and Gt. Cystic fibrosis transmembrane conductance regulator (CFTR) was immunocytochemically localized with a monoclonal mouse antibody to the carboxy terminus and found to be present in the cytoplasm and basolateral membranes of all enterocytes and in the brush-border membrane of some cells, whereas NKCC immunofluorescence, demonstrating the presence of the Na+/K+/2Cl– cotransporter, was present in the cytoplasm and brush-border membrane. We conclude that the teleost intestine is capable of salt and fluid secretion only if intracellular Ca2+ and cyclic AMP pathways are stimulated together and that this secretion appears to involve activation of CFTR ion channels in the apical membrane of a subpopulation of enterocytes.

Mechanisms of CFTR regulation by syntaxin 1A and PKA

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

Leptin signaling deficiency impairs humoral and cellular immune responses and attenuates experimental arthritis

Leptin is produced almost exclusively by adipocytes and regulates body weight at the hypothalamic level. In addition, recent studies showed that leptin plays an important role in T lymphocyte responses. To examine the role of leptin in Ag-induced arthritis, the development of joint inflammation was assessed in immunized leptin-deficient mice (ob/ob), +/?, and wild-type mice (+/+) following the administration of methylated BSA into the knees. The results showed that ob/ob mice developed less severe arthritis compared with control mice. The levels of IL-1beta and TNF-alpha mRNA in the synovium of arthritic knees were lower in ob/ob than in +/? mice. In vitro Ag-specific T cell proliferative responses were significantly decreased in ob/ob mice with lower IFN-gamma and higher IL-10 production, suggesting a shift toward a Th2-type response in ob/ob mice. The serum levels of anti-methylated BSA Abs of any isotype were significantly decreased in arthritic ob/ob mice compared with controls. Essentially identical results were obtained in db/db mice, which lack the expression of the long isoform of leptin receptor. By RT-PCR, we observed that B lymphocytes express leptin receptor mRNA, indicating that in addition to its effect on the cellular response, leptin may exert a direct effect on B cell function. In conclusion, leptin contributes to the mechanisms of joint inflammation in Ag-induced arthritis by regulating both humoral and cell-mediated immune responses.

CFTR: covalent modification of cysteine-substituted channels expressed in Xenopus oocytes shows that activation is due to the opening of channels resident in the plasma membrane

Some studies of CFTR imply that channel activation can be explained by an increase in open probability (P(o)), whereas others suggest that activation involves an increase in the number of CFTR channels (N) in the plasma membrane. Using two-electrode voltage clamp, we tested for changes in N associated with activation of CFTR in Xenopus oocytes using a cysteine-substituted construct (R334C CFTR) that can be modified by externally applied, impermeant thiol reagents like [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET+). Covalent modification of R334C CFTR with MTSET+ doubled the conductance and changed the I-V relation from inward rectifying to linear and was completely reversed by 2-mercaptoethanol (2-ME). Thus, labeled and unlabeled channels could be differentiated by noting the percent decrease in conductance brought about by exposure to 2-ME. When oocytes were briefly (20 s) exposed to MTSET+ before CFTR activation, the subsequently activated conductance was characteristic of labeled R334C CFTR, indicating that the entire pool of CFTR channels activated by cAMP was accessible to MTSET+. The addition of unlabeled, newly synthesized channels to the plasma membrane could be monitored on-line during the time when the rate of addition was most rapid after cRNA injection. The addition of new channels could be detected as early as 5 h after cRNA injection, occurred with a half time of approximately 24-48 h, and was disrupted by exposing oocytes to Brefeldin A, whereas activation of R334C CFTR by cAMP occurred with a half time of tens of minutes, and did not appear to involve the addition of new channels to the plasma membrane. These findings demonstrate that in Xenopus oocytes, the major mechanism of CFTR activation by cAMP is by means of an increase in the open probability of CFTR channels.

The relationship between cAMP, Ca(2)+, and transport of CFTR to the plasma membrane

The mechanism whereby cAMP stimulates Cl(-) flux through CFTR ion channels in secretory epithelia remains controversial. It is generally accepted that phosphorylation by cAMP-dependent protein kinase increases the open probability of the CFTR channel. A more controversial hypothesis is that cAMP triggers the translocation of CFTR from an intracellular pool to the cell surface. We have monitored membrane turnover in Calu-3 cells, a cell line derived from human airway submucosal glands that expresses high levels of CFTR using membrane capacitance and FM1-43 fluorescence measurements. Using a conventional capacitance measurement technique, we observe an apparent increase in membrane capacitance in most cells that exhibit an increase in Cl(-) current. However, after we carefully correct our recordings for changes in membrane conductance, the apparent changes in capacitance are eliminated. Measurements using the fluorescent membrane marker FM1-43 also indicate that no changes in membrane turnover accompany the activation of CFTR. Robust membrane insertion can be triggered with photorelease of caged Ca(2)+ in Calu-3 cells. However, no increase in Cl(-) current accompanies Ca(2)+-evoked membrane fusion. We conclude that neither increases in cAMP or Ca(2)+ lead to transport of CFTR to the plasma membrane in Calu-3 cells. In addition, we conclude that membrane capacitance measurements must be interpreted with caution when large changes in membrane conductance occur.

Brefeldin A block of integrin-dependent mechanosensitive ATP release from Xenopus oocytes reveals a novel mechanism of mechanotransduction

Many animal cells release ATP into the extracellular medium, and often this release is mechanosensitive. However, the mechanisms underlying this release are not well understood. Using the luciferin-luciferase bioluminescent assay we demonstrate that a Xenopus oocyte releases ATP at a basal rate approximately 0.01 fmol/s, and gentle mechanical stimulation can increase this to 50 fmol/s. Brefeldin A, nocodazole, and progesterone-induced- maturation block basal and mechanosensitive ATP release. These treatments share the common feature of disrupting the Golgi complex and vesicle trafficking to the cell surface and thereby block protein secretion and membrane protein insertion. We propose that ATP release occurs when protein transport vesicles enriched in ATP fuse with the plasma membrane. Collagenase, integrin-binding peptides, and cytochalasin D also block ATP release, indicating that extracellular, membrane and cytoskeletal elements are involved in the release process. Elevation of intracellular Ca(2+) does not evoke ATP release but potentiates mechanosensitive ATP release. Our study indicates a novel mechanism of mechanotransduction that would allow cells to regulate membrane trafficking and protein transport/secretion in response to mechanical loading.

Differential regulation of Cl- transport proteins by PKC in Calu-3 cells

Cl- transport proteins expressed in a Calu-3 airway epithelial cell line were differentiated by function and regulation by protein kinase C (PKC) isotypes. mRNA expression of Cl- transporters was semiquantitated by RT-PCR after transfection with a sense or antisense oligonucleotide to the PKC isotypes that modulate the activity of the cystic fibrosis transmembrane conductance regulator [CFTR (PKC-epsilon)] or of the Na/K/2Cl (NKCC1) cotransporter (PKC-delta). Expression of NKCC1 and CFTR mRNAs and proteins was independent of antisense oligonucleotide treatment. Transport function was measured in cell monolayers grown on a plastic surface or on filter inserts. With both culture methods, the antisense oligonucleotide to PKC-epsilon decreased the amount of PKC-epsilon and reduced cAMP-dependent activation of CFTR but not alpha(1)-adrenergic activation of NKCC1. The antisense oligonucleotide to PKC-delta did not affect CFTR function but did block alpha(1)-adrenergic activation of NKCC1 and reduce PKC-delta mass. These results provide the first evidence for mRNA and protein expression of NKCC1 in Calu-3 cells and establish the differential regulation of CFTR and NKCC1 function by specific PKC isotypes at a site distal to mRNA expression and translation in airway epithelial cells.

Control of the cystic fibrosis transmembrane conductance regulator by alphaG(i) and RGS proteins

The cystic fibrosis transmembrane conductance regulator (CFTR) has been shown previously to be regulated by inhibitory G proteins. In the present study, we demonstrate inhibition of CFTR by alphaG(i2) and alphaG(i1), but not alphaG(0), in Xenopus oocytes. We further examined whether regulators of G protein signaling (RGS) proteins interfere with alphaG(i)-dependent inhibition of CFTR. Activation of CFTR by IBMX and forskolin was attenuated in the presence of alphaG(i2), indicating inhibition of CFTR by alphaG(i2) in Xenopus oocytes. Coexpression of the proteins RGS3 and RGS7 together with CFTR and alphaG(i2) partially recovered activation by IBMX/forskolin. 14-3-3, a protein that is known to interfere with RGS proteins, counteracted the effects of RGS3. These data demonstrate the regulation of CFTR by alphaG(i) in Xenopus oocytes. Because RGS proteins interfere with the G protein-dependent regulation of CFTR, this may offer new potential pathways for pharmacological intervention in cystic fibrosis.

Functional and molecular characterization of an anion exchanger in airway serous epithelial cells

Serous cells secrete Cl(-) and HCO(3)(-) and play an important role in airway function. Recent studies suggest that a Cl(-)/HCO(3)(-) anion exchanger (AE) may contribute to Cl(-) secretion by airway epithelial cells. However, the molecular identity, the cellular location, and the contribution of AEs to Cl(-) secretion in serous epithelial cells in tracheal submucosal glands are unknown. The goal of the present study was to determine the molecular identity, the cellular location, and the role of AEs in the function of serous epithelial cells. To this end, Calu-3 cells, a human airway cell line with a serous-cell phenotype, were studied by RT-PCR, immunoblot, and electrophysiological analysis to examine the role of AEs in Cl(-) secretion. In addition, the subcellular location of AE proteins was examined by immunofluorescence microscopy. Calu-3 cells expressed mRNA and protein for AE2 as determined by RT-PCR and Western blot analysis, respectively. Immunofluorescence microscopy identified AE2 in the basolateral membrane of Calu-3 cells in culture and rat tracheal serous cells in situ. In Cl(-)/HCO(3)(-)/Na(+)-containing media, the 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP)-stimulated short-circuit anion current (I(sc)) was reduced by basolateral but not by apical application of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (50 microM) and 4, 4'-dinitrostilbene-2,2'-disulfonic acid [DNDS (500 microM)], inhibitors of AEs. In the absence of Na(+) in the bath solutions, to eliminate the contributions of the Na(+)/HCO(3)(-) and Na(+)/K(+)/2Cl(-) cotransporters to I(sc), CPT-cAMP stimulated a small DNDS-sensitive I(sc). Taken together with previous studies, these observations suggest that a small component of cAMP-stimulated I(sc) across serous cells may be referable to Cl(-) secretion and that uptake of Cl(-) across the basolateral membrane may be mediated by AE2.

Chloride secretion by the intestinal epithelium: molecular basis and regulatory aspects

Chloride secretion is the major determinant of mucosal hydration throughout the gastrointestinal tract, and chloride transport is also pivotal in the regulation of fluid secretion by organs that drain into the intestine. Moreover, there are pathological consequences if chloride secretion is either reduced or increased such as in cystic fibrosis and secretory diarrhea, respectively. With the molecular cloning of many of the proteins and regulatory factors that make up the chloride secretory mechanism, there have been significant advances in our understanding of this process at the cellular level. Similarly, emerging data have clarified the intercellular relationships that govern the extent of chloride secretion. The goal of our article is to review this area of investigation, with an emphasis on recent developments and their implications for the physiology and pathophysiology of chloride transport.

E3KARP mediates the association of ezrin and protein kinase A with the cystic fibrosis transmembrane conductance regulator in airway cells

Although it is generally recognized that cystic fibrosis transmembrane conductance regulator (CFTR) contains a PSD-95/Disc-large/ZO-1 (PDZ)-binding motif at its COOH terminus, the identity of the PDZ domain protein(s) that interact with CFTR is uncertain, and the functional impact of this interaction is not fully understood. By using human airway epithelial cells, we show that CFTR associates with Na(+)/H(+) exchanger (NHE) type 3 kinase A regulatory protein (E3KARP), an EBP50/NHE regulatory factor (NHERF)-related PDZ domain protein. The PDZ binding motif located at the COOH terminus of CFTR interacts preferentially with the second PDZ domain of E3KARP, with nanomolar affinity. In contrast to EBP50/NHERF, E3KARP is predominantly localized (>95%) in the membrane fractions of Calu-3 and T84 cells, where CFTR is located. Moreover, confocal immunofluorescence microscopy of polarized Calu-3 monolayers shows that E3KARP and CFTR are co-localized at the apical membrane domain. We also found that ezrin associates with E3KARP in vivo. Co-expression of CFTR with E3KARP and ezrin in Xenopus oocytes potentiated cAMP-stimulated CFTR Cl(-) currents. These results support the concept that E3KARP functions as a scaffold protein that links CFTR to ezrin. Since ezrin has been shown previously to function as a protein kinase A anchoring protein, we suggest that one function served by the interaction of E3KARP with both ezrin and CFTR is to localize protein kinase A in the vicinity of the R-domain of CFTR. Since ezrin is also an actin-binding protein, the formation of a CFTR.E3KARP.ezrin complex may be important also in stabilizing CFTR at the apical membrane domain of airway cells.

Forskolin-induced apical membrane insertion of virally expressed, epitope-tagged CFTR in polarized MDCK cells

Channel gating of the cystic fibrosis transmembrane conductance regulator (CFTR) is activated in response to cAMP stimulation. In addition, CFTR activation may also involve rapid insertion of a subapical pool of CFTR into the plasma membrane (PM). However, this issue has been controversial, in part because of the difficulty in distinguishing cell surface vs. intracellular CFTR. Recently, a fully functional, epitope-tagged form of CFTR (M2-901/CFTR) that can be detected immunologically in nonpermeabilized cells was characterized (Howard M, Duvall MD, Devor DC, Dong J-Y, Henze K, and Frizzell RA. Am J Physiol Cell Physiol 269: C1565-C1576, 1995; and Schultz BD, Takahashi A, Liu C, Frizzell RA, and Howard M. Am J Physiol Cell Physiol 273: C2080-C2089, 1997). We have developed replication-defective recombinant adenoviruses that express M2-901/CFTR and used them to probe cell surface CFTR in forskolin (FSK)-stimulated polarized Madin-Darby canine kidney (MDCK) cells. Virally expressed M2-901/CFTR was functional and was readily detected on the apical surface of FSK-stimulated polarized MDCK cells. Interestingly, at low multiplicity of infection, we observed FSK-stimulated insertion of M2901/CFTR into the apical PM, whereas at higher M2-901/CFTR expression levels, no increase in surface expression was detected using indirect immunofluorescence. Immunoelectron microscopy of unstimulated and FSK-stimulated cells confirmed the M2-901/CFTR redistribution to the PM upon FSK stimulation and demonstrates that the apically inserted M2-901/CFTR originates from a population of subapical vesicles. Our observations may reconcile previous conflicting reports regarding the effect of cAMP stimulation on CFTR trafficking.

PBA increases CFTR expression but at high doses inhibits Cl(-) secretion in Calu-3 airway epithelial cells

Sodium 4-phenylbutyrate (PBA), a short-chain fatty acid, has been approved to treat patients with urea cycle enzyme deficiencies and is being evaluated in the management of sickle cell disease, thalassemia, cancer, and cystic fibrosis (CF). Because relatively little is known about the effects of PBA on the expression and function of the wild-type CF transmembrane conductance regulator (wt CFTR), the goal of this study was to examine the effects of PBA and related compounds on wt CFTR-mediated Cl(-) secretion. To this end, we studied Calu-3 cells, a human airway cell line that expresses endogenous wt CFTR and has a serous cell phenotype. We report that chronic treatment of Calu-3 cells with a high concentration (5 mM) of PBA, sodium butyrate, or sodium valproate but not of sodium acetate reduced basal and 8-(4-chlorophenylthio)-cAMP-stimulated Cl(-) secretion. Paradoxically, PBA enhanced CFTR protein expression 6- to 10-fold and increased the intensity of CFTR staining in the apical plasma membrane. PBA also increased protein expression of Na(+)-K(+)-ATPase. PBA reduced CFTR Cl(-) currents across the apical membrane but had no effect on Na(+)-K(+)-ATPase activity in the basolateral membrane. Thus a high concentration of PBA (5 mM) reduces Cl(-) secretion by inhibiting CFTR Cl(-) currents across the apical membrane. In contrast, lower therapeutic concentrations of PBA (0.05-2 mM) had no effect on cAMP-stimulated Cl(-) secretion across Calu-3 cells. We conclude that PBA concentrations in the therapeutic range are unlikely to have a negative effect on Cl(-) secretion. However, concentrations >5 mM might reduce transepithelial Cl(-) secretion by serous cells in submucosal glands in individuals expressing wt CFTR.

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.

CFTR channel insertion to the apical surface in rat duodenal villus epithelial cells is upregulated by VIP in vivo

cAMP activated insertion of the cystic fibrosis transmembrane conductance regulator (CFTR) channels from endosomes to the apical plasma membrane has been hypothesized to regulate surface expression and CFTR function although the physiologic relevance of this remains unclear. We previously identified a subpopulation of small intestinal villus epithelial cells or CFTR high expressor (CHE) cells possessing very high levels of apical membrane CFTR in association with a prominent subapical vesicular pool of CFTR. We have examined the subcellular redistribution of CFTR in duodenal CHE cells in vivo in response to the cAMP activated secretagogue vasoactive intestinal peptide (VIP). Using anti-CFTR antibodies against the C terminus of rodent CFTR and indirect immunofluorescence, we show by quantitative confocal microscopy that CFTR rapidly redistributes from the cytoplasm to the apical surface upon cAMP stimulation by VIP and returns to the cytoplasm upon removal of VIP stimulation of intracellular cAMP levels. Using ultrastructural and confocal immunofluorescence examination in the presence or absence of cycloheximide, we also show that redistribution was not dependent on new protein synthesis, changes in endocytosis, or rearrangement of the apical cytoskeleton. These observations suggest that physiologic cAMP activated apical membrane insertion and recycling of CFTR channels in normal CFTR expressing epithelia contributes to the in vivo regulation of CFTR mediated anion transport.