Properties of CFTR activated by the xanthine derivative X-33 in human airway Calu-3 cells

Engineered fluidic device to achieve multiplexed monitoring of cell cultures with digital holographic microscopy

We present a low-cost, 3D-printed, and biocompatible fluidic device, engineered to produce laminar and homogeneous flow over a large field-of-view. Such a fluidic device allows us to perform multiplexed temporal monitoring of cell cultures compatible with the use of various pharmacological protocols. Therefore, specific properties of each of the observed cell cultures can be discriminated simultaneously during the same experiment. This was illustrated by monitoring the agonists-mediated cellular responses, with digital holographic microscopy, of four different cell culture models of cystic fibrosis. Quantitatively speaking, this multiplexed approach provides a time saving factor of around four to reveal specific cellular features.

Development and Characterization of Dapsone Cocrystal Prepared by Scalable Production Methods

In this study, the formation of caffeine/dapsone (CAF/DAP) cocrystals by scalable production methods, such as liquid-assisted grinding (LAG) and spray drying, was investigated in the context of the potential use of processed cocrystal powder for pulmonary delivery. A CAF/DAP cocrystal (1:1 M ratio) was successfully prepared by slow evaporation from both acetone and ethyl acetate. Acetone, ethyl acetate, and ethanol were all successfully used to prepare cocrystals by LAG and spray drying. The powders obtained were characterized by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), thermogravimetry (TGA), and Fourier transform infrared spectroscopy (FTIR). Laser diffraction analysis indicated a median particle size (D₅₀) for spray-dried powders prepared from acetone, ethanol, and ethyl acetate of 5.4 ± 0.7, 5.2 ± 0.1, and 5.1 ± 0.0 μm respectively, which are appropriate sizes for pulmonary delivery by means of a dry powder inhaler. The solubility of the CAF/DAP cocrystal in phosphate buffer pH 7.4, prepared by spray drying using acetone, was 506.5 ± 31.5 μg/mL, while pure crystalline DAP had a measured solubility of 217.1 ± 7.8 μg/mL. In vitro cytotoxicity studies using Calu-3 cells indicated that the cocrystals were not toxic at concentrations of 0.1 and of 1 mM of DAP, while an in vitro permeability study suggested caffeine may contribute to the permeation of DAP by hindering the efflux effect. The results obtained indicate that the CAF/DAP cocrystal, particularly when prepared by the spray drying method, represents a possible suitable approach for inhalation formulations with applications in pulmonary pathologies.

The human CFTR protein expressed in CHO cells activates aquaporin-3 in a cAMP-dependent pathway: study by digital holographic microscopy

The transmembrane water movements during cellular processes and their relationship to ionic channel activity remain largely unknown. As an example, in epithelial cells it was proposed that the movement of water could be directly linked to cystic fibrosis transmembrane conductance regulator (CFTR) protein activity through a cAMP-stimulated aqueous pore, or be dependent on aquaporin. Here, we used digital holographic microscopy (DHM) an interferometric technique to quantify in situ the transmembrane water fluxes during the activity of the epithelial chloride channel, CFTR, measured by patch-clamp and iodide efflux techniques. We showed that the water transport measured by DHM is fully inhibited by the selective CFTR blocker CFTRinh172 and is absent in cells lacking CFTR. Of note, in cells expressing the mutated version of CFTR (F508del-CFTR), which mimics the most common genetic alteration encountered in cystic fibrosis, we also show that the water movement is profoundly altered but restored by pharmacological manipulation of F508del-CFTR-defective trafficking. Importantly, whereas activation of this endogenous water channel required a cAMP-dependent stimulation of CFTR, activation of CFTR or F508del-CFTR by two cAMP-independent CFTR activators, genistein and MPB91, failed to trigger water movements. Finally, using a specific small-interfering RNA against the endogenous aquaporin AQP3, the water transport accompanying CFTR activity decreased. We conclude that water fluxes accompanying CFTR activity are linked to AQP3 but not to a cAMP-stimulated aqueous pore in the CFTR protein.

Nongenomic effects of fluticasone propionate and budesonide on human airway anion secretion

This study investigated the physiological effects of inhaled corticosteroids, which are used widely to treat asthma. The application of fluticasone propionate (FP, 100 μM) induced sustained increases in the short-circuit current (I(SC)) in human airway Calu-3 epithelial cells. The FP-induced I(SC) was prevented by the presence of H89 (10 μM, a protein kinase A inhibitor) and SQ22536 (100 μM, an adenylate cyclase inhibitor). The FP-induced responses involved bumetanide (a Na(+)-K(+)-2Cl(-) cotransporter inhibitor)-sensitive and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of HCO(3)(-)-dependent anion transporters)-sensitive components, both of which reflect basolateral anion transport. Further, FP augmented apical membrane Cl(-) current (I(Cl)), reflecting cystic fibrosis transmembrane conductance regulator (CFTR)-mediated conductance, in the nystatin-permeabilized monolayer. In I(SC) and I(Cl) responses, FP failed to enhance the responses to forskolin (10 μM, an adenylate cyclase activator). Nevertheless, we found that FP synergistically increased cytosolic cAMP concentrations in combination with forskolin. All these effects of FP were reproduced with the use of budesonide. Collectively, inhaled corticosteroids such as FP and budesonide stimulate CFTR-mediated anion transport through adenylate cyclase-mediated mechanisms in a nongenomic fashion, thus sharing elements of a common pathway with forskolin. However, the corticosteroids cooperate with forskolin for synergistic cAMP production, suggesting that the corticosteroids and forskolin do not compete with each other to exert their effects on adenylate cyclase. Considering that such synergism was also observed in the FP/salmeterol combination, these nongenomic aspects may play therapeutic roles in mucus congestive airway diseases, in addition to genomic aspects that are generally recognized.

Metabolomic profiling reveals biochemical pathways and biomarkers associated with pathogenesis in cystic fibrosis cells

Cystic fibrosis (CF) is a life-shortening disease caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. To gain an understanding of the epithelial dysfunction associated with CF mutations and discover biomarkers for therapeutics development, untargeted metabolomic analysis was performed on primary human airway epithelial cell cultures from three separate cohorts of CF patients and non-CF subjects. Statistical analysis revealed a set of reproducible and significant metabolic differences between the CF and non-CF cells. Aside from changes that were consistent with known CF effects, such as diminished cellular regulation against oxidative stress and osmotic stress, new observations on the cellular metabolism in the disease were generated. In the CF cells, the levels of various purine nucleotides, which may function to regulate cellular responses via purinergic signaling, were significantly decreased. Furthermore, CF cells exhibited reduced glucose metabolism in glycolysis, pentose phosphate pathway, and sorbitol pathway, which may further exacerbate oxidative stress and limit the epithelial cell response to environmental pressure. Taken together, these findings reveal novel metabolic abnormalities associated with the CF pathological process and identify a panel of potential biomarkers for therapeutic development using this model system.

Mutation-specific potency and efficacy of cystic fibrosis transmembrane conductance regulator chloride channel potentiators

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. The mutations G551D and G1349D, which affect the nucleotide-binding domains (NBDs) of CFTR protein, reduce channel activity. This defect can be corrected pharmacologically by small molecules called potentiators. CF mutations residing in the intracellular loops (ICLs), connecting the transmembrane segments of CFTR, may also reduce channel activity. We have investigated the extent of loss of function caused by ICL mutations and the sensitivity to pharmacological stimulation. We found that E193K and G970R (in ICL1 and ICL3, respectively) cause a severe loss of CFTR channel activity that can be rescued by the same potentiators that are effective on NBD mutations. We compared potency and efficacy of three different potentiators for E193K, G970R, and G551D. The 1,4-dihydropyridine felodipine and the phenylglycine PG-01 [2-[(2-1H-indol-3-yl-acetyl)-methylamino]-N-(4-isopropylphenyl)-2-phenylacetamide] were strongly effective on the three CFTR mutants. The efficacy of sulfonamide SF-01 [6-(ethylphenylsulfamoyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid cycloheptylamide], another CFTR potentiator, was instead significantly lower than felodipine and PG-01 for the E193K and G970R mutations, and almost abolished for G551D. Furthermore, SF-01 modified the response of G551D and G970R to the other two potentiators, an effect that may be explained by an allosteric antagonistic effect. Our results indicate that CFTR potentiators correct the basic defect caused by CF mutations residing in different CFTR domains. However, there are differences among potentiators, with felodipine and PG-01 having a wider pharmacological activity, and SF-01 being more mutation specific. Our observations are useful in the prioritization and development of drugs targeting the CF basic defect.

On the measurement of the functional properties of the CFTR

A number of methods are currently employed to assess the functional properties of CFTR channels and their response to pharmacological potentiators, correction of the defective CFTR trafficking, and vectorial introduction of new proteins. Here we review the most common methods used to assess CFTR channel function. The suitability of each technique to various experimental conditions is discussed.

Discovery of α-Aminoazaheterocycle-Methylglyoxal Adducts as a New Class of High-Affinity Inhibitors of Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channels

The cystic fibrosis transmembrane conductance regulator (CFTR) represents the main Cl(-) channel in the apical membrane of epithelial cells for cAMP-dependent Cl(-) secretion. Here we report on the synthesis and screening of a small library of nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts, inhibitors of wild-type (WT) CFTR and G551D-, G1349D-, and F508del-CFTR Cl(-) channels. In whole-cell patch-clamp experiments of Chinese hamster ovary (CHO) cells expressing WT-CFTR, we recorded rapid and reversible inhibition of forskolin-activated CFTR currents in the presence of the adducts 5a and 8a,b at 10 pM concentrations. Using iodide efflux experiments, we compared concentration-dependent inhibition of CFTR with glibenclamide (IC(50) = 14.7 microM), 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl-)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172) (IC(50) = 1.2 microM), and alpha-aminoazaheterocycle-methylglyoxal adducts and identified compounds 5a (IC(50) = 71 pM), 8a,b (IC(50) = 2.5 nM), and 7a,b (IC(50) = 3.4 nM) as the most potent inhibitors of WT-CFTR channels. Similar ranges of inhibition were also found when these compounds were evaluated on CFTR channels with the cystic fibrosis mutations F508del (in temperature-corrected human airway epithelial F508del/F508del CF15 cells)-, G551D-, and G1349D-CFTR (expressed in CHO and COS-7 cells). No effect of compound 5a was detected on the volume-regulated or calcium-regulated iodide efflux. Picomolar inhibition of WT-CFTR with adduct 5a was also found using a 6-methoxy-N-(3-sulfopropyl)-quinolinium fluorescent probe applied to the human tracheobronchial epithelial cell line 16HBE14o-. Finally, we found comparable inhibition by 5a or by CFTR(inh)-172 of forskolin-dependent short-circuit currents in mouse colon. To the best of our knowledge, these new nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts represent the most potent compounds reported to inhibit CFTR chloride channels.

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.

Regulation of apical surface fluid and protein secretion in human airway epithelial cell line Calu-3

We evaluated the relationship between apical surface fluid (ASF) and protein secretion in Calu-3 cells grown at an air-liquid interface. Calu-3 monolayers responded to forskolin, a cystic fibrosis transmembrane regulator (CFTR) channel agonist, by secreting a significant amount of ASF. Such a response from Calu-3 monolayers was not observed with CFTR channel blockers glybenclamide and DPC. Other ion channel mediators, PGF-2alpha, PMA, DNDS, and DIDS, had no effect on Calu-3 ASF secretion. Forskolin decreased Calu-3 protein secretion and glybenclamide increased protein secretion. Similarly, forskolin decreased Calu-3 lysozyme secretion, whereas glybenclamide and DPC increased lysozyme secretion. We observed significant changes in Calu-3 fluid and protein secretions with ion channel mediators known to alter CFTR activity. Our results demonstrate a functional link between fluid and protein secretions in Calu-3 apical surface and suggested a possible involvement of CFTR in these processes.

Antibodies for CFTR studies

For most expression studies focusing on the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein, sensitive and specific antibodies (Abs) are critically needed. Several Abs have been produced commercially or by research laboratories for CFTR detection in both cell lines with heterologous or endogenous expression and native cells/tissues. Here, we review the applicability of most Abs currently in use in CF research for the biochemical and/or immunocytochemical detection of CFTR.

ATP release triggered by activation of the Ca2+-activated K+ channel in human airway Calu-3 cells

Airway mucociliary clearance is subject to the autocrine/paracrine regulation of extracellular nucleotides released from the airway epithelial cells. The present study was performed in pursuit of effective modulators of ATP release under physiologic conditions in polarized human airway epithelial cells (Calu-3). Neither isoproterenol, forskolin, nor ionomycin augmented extracellular ATP release detected by luciferase assay. However, direct activation of the human intermediate conductance, Ca(2+)-activated K(+) channel (hIK-1) by 1-ethyl-2-benzimdazolinone (1-EBIO, 1 mM) and chlorzoxazone (CZ, 1 mM) increased ATP release predominantly in the apical compartment. Measurement of fluo-3 signals revealed that 1-EBIO- and CZ-stimulated cytosolic Ca(2+) mobilization was suppressed by the presence of MRS-2179, a specific P2Y(1) receptor antagonist. The hIK-1-mediated ATP release was inhibited by a hIK-1 blocker (charybdotoxin), and an Na(+)-K(+)-2Cl(-) cotransport blocker (bumetanide) without interruption by GdCl(3), an inhibitor of stretch-activated nonselective cation (SA) channels, or glybenclamide, a blocker of the cystic fibrosis transmembrane conductance regulator (CFTR). These results suggest that a cell volume decrease via the hIK-1-mediated KCl loss and the resultant induction of a regulatory volume increase via the Na(+)-K(+)-2Cl(-) transporter may trigger release of ATP, which causes P2Y(1)-mediated Ca(2+) mobilization, through mechanisms unrelated to the CFTR and SA channels.

Synthesis, SAR, Crystal Structure, and Biological Evaluation of Benzoquinoliziniums as Activators of Wild-Type and Mutant Cystic Fibrosis Transmembrane Conductance Regulator Channels

Chloride channels play important roles in homeostasis and regulate cell volume, transepithelial transport, and electrical excitability. Despite recent progress made in the genetic and molecular aspect of chloride channels, their pharmacology is still poorly understood. The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated epithelial chloride channel for which mutations cause cystic fibrosis. Here we have synthesized benzo[c]quinolizinium and benzo[f]indolo[2,3-a]quinolizinium salts (MPB) and performed a SAR to identify the structural basis for activation of the CFTR chloride channel. Synthesized compounds were evaluated on wild-type CFTR and on CFTR having the glycine-to-aspartic acid missense mutation at codon 551 (G551D-CFTR), using a robot and cell-based assay. The presence of an hydroxyl group at position 6 of the benzo[c]quinolizinium skeleton associated with a chlorine atom at position 10 or 7 and an alkyl chain at position 5 determined the highest activity. The most potent product is 5-butyl-7-chloro-6-hydroxybenzo[c]quinolizinium chloride (8u, MPB-104). 8u is 100 times more potent than the parent compound 8a (MPB-07).

Activation of VPAC1 receptors by VIP and PACAP-27 in human bronchial epithelial cells induces CFTR-dependent chloride secretion

1. In the human airway epithelium, VIP/PACAP receptors are distributed in nerve fibers and in epithelial cells but their role in transepithelial ion transport have not been reported. Here, we show that human bronchial epithelial Calu-3 cells expressed the VPAC(1) receptor subtype which shares similar high affinity for VIP and PACAP-27. 2. The stoichiometric binding parameters characterizing the (125)I-VIP and (125)I-PACAP-27 binding to these receptors were determined. 3. We found that VIP (EC(50) approximately 7.6 nM) and PACAP-27 (EC(50) approximately 10 nM) stimulated glibenclamide-sensitive and DIDS-insensitive iodide efflux in Calu-3 cells. 4. The protein kinase A (PKA) inhibitor, H-89 and the protein kinase C (PKC) inhibitor, chelerythrine chloride prevented activation by both peptides demonstrating that PKA and PKC are part of the signaling pathway. This profile corresponds to the pharmacological signature of CFTR. 5. In the cystic fibrosis airway epithelial IB3-1 cell lacking functional CFTR but expressing VPAC(1) receptors, neither VIP, PACAP-27 nor forskolin stimulated chloride transport. 6. Ussing chamber experiments demonstrated stimulation of CFTR-dependent short-circuit currents by VIP or PACAP-27 applied to the basolateral but not to the apical side of Calu-3 cells monolayers. 7. This study shows the stimulation in human bronchial epithelial cells of CFTR-dependent chloride secretion following activation by VIP and PACAP-27 of basolateral VPAC(1) receptors.

Benzoflavone activators of the cystic fibrosis transmembrane conductance regulator: towards a pharmacophore model for the nucleotide-binding domain

Our previous screen of flavones and related heterocycles for the ability to activate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel indicated that UCCF-029, a 7,8-benzoflavone, was a potent activator. In the present study, we describe the synthesis and evaluation, using cell-based assays, of a series of benzoflavone analogues to examine structure-activity relationships and to identify compounds having greater potency for activation of both wild type CFTR and a mutant CFTR (G551D-CFTR) that causes cystic fibrosis in some human subjects. Using UCCF-029 as a structural guide, a panel of 77 flavonoid analogues was prepared. Analysis of the panel in FRT cells indicated that benzannulation of the flavone A-ring at the 7,8-position greatly improved compound activity and potency for several flavonoids. Incorporation of a B-ring pyridyl nitrogen either at the 3- or 4-position also elevated CFTR activity, but the influence of this structural modification was not as uniform as the influence of benzannulation. The most potent new analogue, UCCF-339, activated wild-type CFTR with a K(d) of 1.7 microM, which is more active than the previous most potent flavonoid activator of CFTR, apigenin. Several compounds in the benzoflavone panel also activated G551D-CFTR, but none were as active as apigenin. Pharmacophore modeling suggests a common binding mode for the flavones and other known CFTR activators at one of the nucleotide-binding sites, allowing for the rational development of more potent flavone analogues.

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.

Xanthine derivatives without PDE effect stimulate voltage-activated chloride conductance of toad skin

The effect of xanthine derivatives on the voltage-activated Cl(-) conductance (G(Cl)) of amphibian skin was analyzed. 3-Isobutyl-1-methylxanthine (IBMX) and the recently synthesized xanthine derivatives 3,7-dimethyl-1-propyl xanthine (X-32) and 3,7-dimethyl-1-isobutyl xanthine (X-33), which lack inhibitory effects on phosphodiesterases in CHO and Calu-3 cells, increased voltage-activated G(Cl) without effect on baseline conductance at inactivating voltage. Half-maximal stimulation of G(Cl) occurred at 108 +/- 9 microM for X-32 and X-33 after apical or basolateral application. The stimulation of G(Cl), which occurs only in the presence of Cl(-) in the mucosal solution, is caused by a shift of the voltage sensitivity to lower clamp potentials and an increase of the maximally activated level. Furosemide reversed both the shift of sensitivity and the increase in magnitude. These patterns are fundamentally different from those seen after application of membrane-permeant, nonmetabolized analogs of cAMP, and they indicate that the xanthines stimulate G(Cl) directly. This notion is strengthened by the lack of influence on intracellular cAMP content, which is consistent with the observations in CHO and Calu-3 cells. We propose that the xanthine derivatives increase the voltage sensitivity of a regulative component in the conductive Cl(-) pathway across amphibian skin.

Correction of G551D-CFTR transport defect in epithelial monolayers by genistein but not by CPX or MPB-07

1. This study compares the effect of three chemically unrelated cystic fibrosis transmembrane conductance regulator (CFTR) activators on epithelial cell monolayers expressing the G551D-CFTR mutant. 2. We measured Cl(-) transport as the amplitude of short-circuit current in response to the membrane permeable cAMP analogue 8-(4-chlorophenylthio)adenosine-3'-5'-cyclic monophosphate (CPT-cAMP) alone or in combination with a CFTR opener. The correction of G551D-CFTR defect was quantified by comparison with maximal activity elicited in cells expressing wild type CFTR. To this end we used Fisher rat thyroid (FRT) cells transfected with wild type or G551D CFTR, and primary cultures of human nasal epithelial cells. 3. In both types of epithelia, cAMP caused activation of Cl(-) transport that was inhibited by glibenclamide and not by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid. After normalising for CFTR expression, the response of FRT-G551D epithelia was 1% that of wild type monolayers. 4. Addition of genistein (10-200 micro M), but not of 8-cyclopentyl-1,3-dipropylxanthine (CPX, 1-100 micro M) or of the benzo[c]quinolizinium MPB-07 (10-200 micro M) to FRT-G551D epithelia pre-treated with cAMP, stimulated a sustained current that at maximal genistein concentration corresponded to 30% of the response of wild type epithelia. 5. The genistein dose-response curve was bell-shaped due to inhibitory activity at the highest concentrations. The dose-dependence in G551D cells was shifted with respect to wild type CFTR so that higher genistein concentrations were required to observe activation and inhibition, respectively. 6. On human nasal epithelia the correction of G551D-CFTR defective conductance obtained with genistein was 20% that of wild type. The impressive effect of genistein suggests that it might correct the Cl(-) transport defect on G551D patients.

The cystic fibrosis mutation G551D alters the non-Michaelis-Menten behavior of the cystic fibrosis transmembrane conductance regulator (CFTR) channel and abolishes the inhibitory Genistein binding site

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) channel activity explains most of the manifestations of the cystic fibrosis (CF) disease. To understand the consequences of CF mutations on CFTR channel activity, we compared the pharmacological properties of wild-type (wt) and G551D-CFTR. Dose-dependent relationships of wt-CFTR activated by genistein follows a non-Michaelis-Menten behavior consistent with the presence of two binding sites. With phosphorylated CFTR, a high affinity site for genistein is the activator (K(s) approximately 3 microm), whereas a second site of low affinity (K(i) approximately 75 microm) is the inhibitor. With non-phosphorylated CFTR, K(s) was increased (K(s) approximately 12 microm), but K(i) was not affected (K(i) approximately 70 microm). In G551D-CFTR cells, channel activity was recovered by co-application of forskolin and genistein in a dose-dependent manner. A further stimulation of G551D-CFTR channel activity was measured at concentrations from 30 microm to 1 mm. The dose response is described by a classical Michaelis-Menten kinetics with only a single apparent site (K(m) approximately 11 microm). Our results suggest glycine 551 in NBD1 as an important location within the low affinity inhibitory site for genistein and offers new evidence for pharmacological alteration caused by an NBD1 mutation of CFTR. This study also reveals how a mutation of an ion channel converts a non-Michaelis-Menten behavior (two binding sites) into a classical Michaelis-Menten model (one binding site).

Polarized expression of cystic fibrosis transmembrane conductance regulator and associated epithelial proteins during the regeneration of human airway surface epithelium in three-dimensional culture

We have previously shown that, in normal human airway tissue, localization of the cystic fibrosis transmembrane conductance regulator (CFTR) can be affected by epithelial maturation, polarity, and differentiation and that CFTR trafficking and apical localization depend on the integrity of the airway epithelium. In this study, we addressed the question of whether the three-dimensional (3-D) organization of adult human airway epithelial cells in suspension culture under rotation, leading to spheroid-like structures, could mimic the in vivo phenomenon of differentiation and polarization. The kinetics of the differentiation, polarity, and formation of the CFTR-ZO-1-ezrin complex was analyzed by transmission, scanning, and immunofluorescence microscopy. Functional activity of the airway surface epithelium was assessed by monitoring the degree of cAMP-stimulated chloride efflux from cultured cells. Our results show that after the initial step of dedifferentiation, characterized by a loss of ciliated cells and disappearance of epithelial subapical CFTR-ezrin-ZO-1 complex, the isolated cells formed 3-D spheroid structures within 24 hours. After 15 days, progressive ciliogenesis was observed and secretory cells could be identified. After 35 days of 3-D culture, ZO-1, CFTR, ezrin, and CD59 were apically or subapically located, and well-differentiated secretory and ciliated cells were identified. CFTR functionality was assessed by analyzing the Cl(-) secretion after amiloride and forskolin perfusion. After 35 days of culture of spheroids in suspension, a significant increase in Cl(-) efflux was observed in well-differentiated ciliated cells.

Activation of G551D CFTR channel with MPB-91: regulation by ATPase activity and phosphorylation

We have designed and synthesized benzo[c]quinolizinium derivatives and evaluated their effects on the activity of G551D cystic fibrosis transmembrane conductance regulator (CFTR) expressed in Chinese hamster ovary and Fisher rat thyroid cells. We demonstrated, using iodide efflux, whole cell patch clamp, and short-circuit recordings, that 5-butyl-6-hydroxy-10-chlorobenzo[c]quinolizinium chloride (MPB-91) restored the activity of G551D CFTR (EC(50) = 85 microM) and activated CFTR in Calu-3 cells (EC(50) = 47 microM). MPB-91 has no effect on the ATPase activity of wild-type and G551D NBD1/R/GST fusion proteins or on the ATPase, GTPase, and adenylate kinase activities of purified NBD2. The activation of CFTR by MPB-91 is independent of phosphorylation because 1) kinase inhibitors have no effect and 2) the compound still activated CFTR having 10 mutated protein kinase A sites (10SA-CFTR). The new pharmacological agent MPB-91 may be an important candidate drug to ameliorate the ion transport defect associated with CF and to point out a new pathway to modulate CFTR activity.

Regulation of anion secretion by nitric oxide in human airway epithelial cells

Nitric oxide (NO) is continuously produced and released in human airways, but the biological significance of this process is unknown. In this study, we have used Calu-3 cells to investigate the effects of NO on transepithelial anion secretion. An inhibitor of NO synthase, NG-nitro-L-arginine methyl ester, reduced short- circuit current (I(sc)), whereas an NO donor, S-nitrosoglutathione (GSNO), increased I(sc), with an EC50 approximately 1.2 microM. The NO-activated current was inhibited by diphenylamine-2-carboxylate, clotrimazole, and charybdotoxin. Selective permeabilization of cell membranes indicated that NO activated both apical anion channels and basolateral potassium channels. An inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, prevented activation of I(sc) by NO but not by 8-bromo-cGMP, suggesting that NO acts via a cGMP-dependent pathway. Sequential treatment of cells with forskolin and GSNO or 1-ethyl-2-benzimidazolinone and GSNO showed additive effects of these chemicals on I(sc). Interestingly, GSNO elevated intracellular Ca2+ concentration ([Ca2+]i) but had no effect on I(sc) activated by thapsigargin. These results show that NO activates transepithelial anion secretion via a cGMP-dependent pathway that involves cross talk between NO and [Ca2+]i.

Novel CFTR chloride channel activators identified by screening of combinatorial libraries based on flavone and benzoquinolizinium lead compounds

The flavonoid genistein and the benzo[c]quinolizinium MPB-07 have been shown to activate the cystic fibrosis transmembrane conductance regulator (CFTR), the protein that is defective in cystic fibrosis. Lead-based combinatorial and parallel synthesis yielded 223 flavonoid, quinolizinium, and related heterocyclic compounds. The compounds were screened for their ability to activate CFTR at 50 microm concentration by measurement of the kinetics of iodide influx in Fisher rat thyroid cells expressing wild-type or G551D CFTR together with the green fluorescent protein-based halide indicator YFP-H148Q. Duplicate screenings revealed that 204 compounds did not significantly affect CFTR function. Compounds of the 7,8-benzoflavone class, which are structurally intermediate between flavones and benzo[c]quinoliziniums, were effective CFTR activators with the most potent being 2-(4-pyridinium)benzo[h]4H-chromen-4-one bisulfate (UCcf-029). Compounds of the novel structural class of fused pyrazolo heterocycles were also strong CFTR activators with the most potent being 3-(3-butynyl)-5-methoxy-1-phenylpyrazole-4-carbaldehyde (UCcf-180). A CFTR inhibitor was also identified. The active compounds did not induce iodide influx in null cells deficient in CFTR. Short-circuit current measurements showed that the CFTR activators identified by screening induced strong anion currents in the transfected cell monolayers grown on porous supports. Compared with genistein, the most active compounds had up to 10 times greater potency in activating wild-type and/or G551D-CFTR. The activators had low cellular toxicity and did not elevate cellular cAMP concentration or inhibit phosphatase activity, suggesting that CFTR activation may involve a direct interaction. These results establish an efficient screening procedure to identify CFTR activators and inhibitors and have identified 7,8-benzoflavones and pyrazolo derivatives as novel classes of CFTR activators.