In vitro modulator responsiveness of 655 CFTR variants found in people with cystic fibrosis

BACKGROUND

In 2017, the US Food and Drug Administration initiated expansion of drug labels for the treatment of cystic fibrosis (CF) to include CF transmembrane conductance regulator (CFTR) gene variants based on in vitro functional studies. This study aims to identify CFTR variants that result in increased chloride (Cl-) transport function by the CFTR protein after treatment with the CFTR modulator combination elexacaftor/tezacaftor/ivacaftor (ELX/TEZ/IVA). These data may benefit people with CF (pwCF) who are not currently eligible for modulator therapies.

METHODS

Plasmid DNA encoding 655 CFTR variants and wild-type (WT) CFTR were transfected into Fisher Rat Thyroid cells that do not natively express CFTR. After 24 h of incubation with control or TEZ and ELX, and acute addition of IVA, CFTR function was assessed using the transepithelial current clamp conductance assay. Each variant's forskolin/cAMP-induced baseline Cl- transport activity, responsiveness to IVA alone, and responsiveness to the TEZ/ELX/IVA combination were measured in three different laboratories. Western blots were conducted to evaluate CFTR protein maturation and complement the functional data.

RESULTS AND CONCLUSIONS

253 variants not currently approved for CFTR modulator therapy showed low baseline activity (<10 % of normal CFTR Cl- transport activity). For 152 of these variants, treatment with ELX/TEZ/IVA improved the Cl- transport activity by ≥10 % of normal CFTR function, which is suggestive of clinical benefit. ELX/TEZ/IVA increased CFTR function by ≥10 percentage points for an additional 140 unapproved variants with ≥10 % but <50 % of normal CFTR function at baseline. These findings significantly expand the number of rare CFTR variants for which ELX/TEZ/IVA treatment should result in clinical benefit.

KCa3.1 potentiation stimulates Cl- secretion in F508del and G551D CFTR corrected primary human bronchial epithelial cells

We previously identified potentiators of KCa3.1 (DCEBIO) that stimulate Cl^- secretion across HBEs expressing wild type (WT) CFTR. However, these compounds failed to stimulate Cl^- secretion in F508del CFTR HBEs. Drug discovery efforts identified CFTR potentiators (VX-770) and correctors (VX-445, VX-661) for CF disease causing mutations, including F508del and G551D. Herein, we evaluated the effect of KCa3.1 potentiation on Cl^- equivalent current (I_Cl) across primary HBEs expressing WT, F508del and G551D CFTR. Transepithelial impedance analysis was used to obtain estimates of apical (R_a) and basolateral membrane (BLM; R_b) resistances. In WT CFTR HBEs, DCEBIO stimulated I_Cl, which was increased by forskolin. Similarly, forskolin stimulated I_Cl, and this was increased by DCEBIO. The KCa3.1 blocker, TRAM-34 inhibited I_Cl. DCEBIO decreased R_b, whereas TRAM-34 increased R_b, consistent with BLM localization of KCa3.1. Following correction of F508del CFTR with VX-445+VX-661, DCEBIO failed to stimulate I_Cl, although the subsequent addition of forskolin+VX-770 increased I_Cl. Importantly, following stimulation of I_Cl with forskolin+VX-770, DCEBIO induced a further significant increase in I_Cl. As above, DCEBIO reduced R_b, whereas TRAM-34 increased R_b, consistent with BLM localized KCa3.1. Finally, we assessed KCa3.1 potentiation on I_Cl in G551D/F508del CFTR HBEs in the absence or presence of VX-445+VX-661. In both cases, DCEBIO failed to stimulate I_Cl. However, following stimulation with forskolin+VX-770, DCEBIO nearly doubled I_Cl. Our results demonstrate, following correction/potentiation of F508del and G551D CFTR, potentiation of KCa3.1 increases the Cl^- secretory response, suggesting this class of compounds may represent a novel means of further increasing Cl^- secretion across CF airway.

CFTR bearing variant p.Phe312del exhibits function inconsistent with phenotype and negligible response to ivacaftor

The chloride channel dysfunction caused by deleterious cystic fibrosis transmembrane conductance regulator (CFTR) variants generally correlates with severity of cystic fibrosis (CF). However, 3 adults bearing the common severe variant p.Phe508del (legacy: F508del) and a deletion variant in an ivacaftor binding region of CFTR (p.Phe312del; legacy: F312del) manifested only elevated sweat chloride concentration (sw[Cl-]; 87-105 mEq/L). A database review of 25 individuals with F312del and a CF-causing variant revealed elevated sw[Cl-] (75-123 mEq/L) and variable CF features. F312del occurs at a higher-than-expected frequency in the general population, confirming that individuals with F312del and a CF-causing variant do not consistently develop overt CF features. In primary nasal cells, CFTR bearing F312del and F508del generated substantial chloride transport (66.0% ± 4.5% of WT-CFTR) but did not respond to ivacaftor. Single-channel analysis demonstrated that F312del did not affect current flow through CFTR, minimally altered gating, and ablated the ivacaftor response. When expressed stably in CF bronchial epithelial (CFBE41o-) cells, F312del-CFTR demonstrated residual function (50.9% ± 3.3% WT-CFTR) and a subtle decrease in forskolin response compared with WT-CFTR. F312del provides an exception to the established correlation between CFTR chloride transport and CF phenotype and informs our molecular understanding of ivacaftor response.

Translating in vitro CFTR rescue into small molecule correctors for cystic fibrosis using the Library of Integrated Network-based Cellular Signatures drug discovery platform

Cystic fibrosis (CF) is a lethal autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The common ΔF508-CFTR mutation results in protein misfolding and proteasomal degradation. If ΔF508-CFTR trafficks to the cell surface, its anion channel function may be partially restored. Several in vitro strategies can partially correct ΔF508-CFTR trafficking and function, including low-temperature, small molecules, overexpression of miR-138, or knockdown of SIN3A. The challenge remains to translate such interventions into therapies and to understand their mechanisms. One approach for connecting such interventions to small molecule therapies that has previously succeeded for CF and other diseases is via mRNA expression profiling and iterative searches of small molecules with similar expression signatures. Here, we query the Library of Integrated Network-based Cellular Signatures using transcriptomic signatures from previously generated CF expression data, including RNAi- and low temperature-based rescue signatures. This LINCS in silico screen prioritized 135 small molecules that mimicked our rescue interventions based on their genomewide transcriptional perturbations. Functional screens of these small molecules identified eight compounds that partially restored ΔF508-CFTR function, as assessed by cAMP-activated chloride conductance. Of these, XL147 rescued ΔF508-CFTR function in primary CF airway epithelia, while also showing cooperativity when administered with C18. Improved CF corrector therapies are needed and this integrative drug prioritization approach offers a novel method to both identify small molecules that may rescue ΔF508-CFTR function and identify gene networks underlying such rescue.

Open reading frame correction using splice-switching antisense oligonucleotides for the treatment of cystic fibrosis

CFTR gene mutations that result in the introduction of premature termination codons (PTCs) are common in cystic fibrosis (CF). This mutation type causes a severe form of the disease, likely because of low CFTR messenger RNA (mRNA) expression as a result of nonsense-mediated mRNA decay, as well as the production of a nonfunctional, truncated CFTR protein. Current therapeutics for CF, which target residual protein function, are less effective in patients with these types of mutations due in part to low CFTR protein levels. Splice-switching antisense oligonucleotides (ASOs), designed to induce skipping of exons in order to restore the mRNA open reading frame, have shown therapeutic promise preclinically and clinically for a number of diseases. We hypothesized that ASO-mediated skipping of CFTR exon 23 would recover CFTR activity associated with terminating mutations in the exon, including CFTR p.W1282X, the fifth most common mutation in CF. Here, we show that CFTR lacking the amino acids encoding exon 23 is partially functional and responsive to corrector and modulator drugs currently in clinical use. ASO-induced exon 23 skipping rescued CFTR expression and chloride current in primary human bronchial epithelial cells isolated from a homozygote CFTR-W1282X patient. These results support the use of ASOs in treating CF patients with CFTR class I mutations in exon 23 that result in unstable CFTR mRNA and truncations of the CFTR protein.

Mechanistic analysis and significance of sphingomyelinase-mediated decreases in transepithelial CFTR currents in nHBEs

Loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) causes cystic fibrosis (CF). In the lungs, this manifests as immune cell infiltration and bacterial infections, leading to tissue destruction. Previous work has determined that acute bacterial sphingomyelinase (SMase) decreases CFTR function in bronchial epithelial cells from individuals without CF (nHBEs) and with CF (cfHBEs, homozygous ΔF508-CFTR mutation). This study focuses on exploring the mechanisms underlying this effect. SMase increased the abundance of dihydroceramides, a result mimicked by blockade of ceramidase enzyme using ceranib-1, which also decreased CFTR function. The SMase-mediated inhibitory mechanism did not involve the reduction of cellular CFTR abundance or removal of CFTR from the apical surface, nor did it involve the activation of 5' adenosine monophosphate-activated protein kinase. In order to determine the pathological relevance of these sphingolipid imbalances, we evaluated the sphingolipid profiles of cfHBEs and cfHNEs (nasal) as compared to non-CF controls. Sphingomyelins, ceramides, and dihydroceramides were largely increased in CF cells. Correction of ΔF508-CFTR trafficking with VX445 + VX661 decreased some sphingomyelins and all ceramides, but exacerbated increases in dihydroceramides. Additional treatment with the CFTR potentiator VX770 did not affect these changes, suggesting rescue of misfolded CFTR was sufficient. We furthermore determined that cfHBEs express more acid-SMase protein than nHBEs. Lastly, we determined that airway-like neutrophils, which are increased in the CF lung, secrete acid-SMase. Identifying the mechanism of SMase-mediated inhibition of CFTR will be important, given the imbalance of sphingolipids in CF cells and the secretion of acid-SMase from cell types relevant to CF.

Sphingomyelinase decreases transepithelial anion secretion in airway epithelial cells in part by inhibiting CFTR-mediated apical conductance

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel whose dysfunction causes cystic fibrosis (CF). The loss of CFTR function in pulmonary epithelial cells causes surface dehydration, mucus build-up, inflammation, and bacterial infections that lead to lung failure. Little has been done to evaluate the effects of lipid perturbation on CFTR activity, despite CFTR residing in the plasma membrane. This work focuses on the acute effects of sphingomyelinase (SMase), a bacterial virulence factor secreted by CF relevant airway bacteria which degrades sphingomyelin into ceramide and phosphocholine, on the electrical circuitry of pulmonary epithelial monolayers. We report that basolateral SMase decreases CFTR-mediated transepithelial anion secretion in both primary bronchial and tracheal epithelial cells from explant tissue, with current CFTR modulators unable to rescue this effect. Focusing on primary cells, we took a holistic ion homeostasis approach to determine a cause for reduced anion secretion following SMase treatment. Using impedance analysis, we determined that basolateral SMase inhibits apical and basolateral conductance in non-CF primary cells without affecting paracellular permeability. In CF primary airway cells, correction with clinically relevant CFTR modulators did not prevent SMase-mediated inhibition of CFTR currents. Furthermore, SMase was found to inhibit only apical conductance in these cells. Future work should determine the mechanism for SMase-mediated inhibition of CFTR currents, and further explore the clinical relevance of SMase and sphingolipid imbalances.

Integrative chemogenomic analysis identifies small molecules that partially rescue ΔF508-CFTR for cystic fibrosis

Rare diseases affect 10% of the first‐world population, yet over 95% lack even a single pharmaceutical treatment. In the present age of information, we need ways to leverage our vast data and knowledge to streamline therapeutic development and lessen this gap. Here, we develop and implement an innovative informatic approach to identify therapeutic molecules, using the Connectivity Map and LINCS L1000 databases and disease‐associated transcriptional signatures and pathways. We apply this to cystic fibrosis (CF), the most common genetic disease in people of northern European ancestry leading to chronic lung disease and reduced lifespan. We selected and tested 120 small molecules in a CF cell line, finding 8 with activity, and confirmed 3 in primary CF airway epithelia. Although chemically diverse, the transcriptional profiles of the hits suggest a common mechanism associated with the unfolded protein response and/or TNFα signaling. This study highlights the power of informatics to help identify new therapies and reveal mechanistic insights while moving beyond target‐centric drug discovery.

Antisense oligonucleotide-mediated correction of CFTR splicing improves chloride secretion in cystic fibrosis patient-derived bronchial epithelial cells

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, encoding an anion channel that conducts chloride and bicarbonate across epithelial membranes. Mutations that disrupt pre-mRNA splicing occur in >15% of CF cases. One common CFTR splicing mutation is CFTR c.3718-2477C>T (3849+10 kb C>T), which creates a new 5′ splice site, resulting in splicing to a cryptic exon with a premature termination codon. Splice-switching antisense oligonucleotides (ASOs) have emerged as an effective therapeutic strategy to block aberrant splicing. We test an ASO targeting the CFTR c.3718-2477C>T mutation and show that it effectively blocks aberrant splicing in primary bronchial epithelial (hBE) cells from CF patients with the mutation. ASO treatment results in long-term improvement in CFTR activity in hBE cells, as demonstrated by a recovery of chloride secretion and apical membrane conductance. We also show that the ASO is more effective at recovering chloride secretion in our assay than ivacaftor, the potentiator treatment currently available to these patients. Our findings demonstrate the utility of ASOs in correcting CFTR expression and channel activity in a manner expected to be therapeutic in patients.

System Xc- Antiporter Inhibitors: Azo-Linked Amino-Naphthyl-Sulfonate Analogues of Sulfasalazine

The cystine/glutamate antiporter system Xc- (SXc-) mediates the exchange of intracellular L-glutamate (L-Glu) with extracellular L-cystine (L-Cys2). Both the import of L-Cys2 and the export of L-Glu take on added significance in CNS cells, especially astrocytes. When the relative activity of SXc- overwhelms the regulatory capacity of the EAATs, the efflux of L-Glu through the antiporter can be significant enough to trigger excitotoxic pathology, as is thought to occur in glioblastoma. This has prompted considerable interest in the pharmacological specificity of SXc- and the development of inhibitors. The present study explores a series of analogues that are structurally related to sulfasalazine, a widely employed inhibitor of SXc-. We identify a number of novel aryl-substituted amino-naphthylsulfonate analogues that inhibit SXc- more potently than sulfasalazine. Interestingly, the inhibitors switch from a competitive to noncompetitive mechanism with increased length and lipophilic substitutions, a structure-activity relationship that was previously observed with aryl-substituted isoxazole. These results suggest that the two classes of inhibitors may interact with some of the same domains on the antiporter protein and that the substrate and inhibitor binding sites may be in close proximity to one another. Molecular modeling is used to explore this possibility.

CFTR modulator theratyping: Current status, gaps and future directions

BACKGROUND

New drugs that improve the function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein with discreet disease-causing variants have been successfully developed for cystic fibrosis (CF) patients. Preclinical model systems have played a critical role in this process, and have the potential to inform researchers and CF healthcare providers regarding the nature of defects in rare CFTR variants, and to potentially support use of modulator therapies in new populations.

METHODS

The Cystic Fibrosis Foundation (CFF) assembled a workshop of international experts to discuss the use of preclinical model systems to examine the nature of CF-causing variants in CFTR and the role of in vitro CFTR modulator testing to inform in vivo modulator use. The theme of the workshop was centered on CFTR theratyping, a term that encompasses the use of CFTR modulators to define defects in CFTR in vitro, with application to both common and rare CFTR variants.

RESULTS

Several preclinical model systems were identified in various stages of maturity, ranging from the expression of CFTR variant cDNA in stable cell lines to examination of cells derived from CF patients, including the gastrointestinal tract, the respiratory tree, and the blood. Common themes included the ongoing need for standardization, validation, and defining the predictive capacity of data derived from model systems to estimate clinical outcomes from modulator-treated CF patients.

CONCLUSIONS

CFTR modulator theratyping is a novel and rapidly evolving field that has the potential to identify rare CFTR variants that are responsive to approved drugs or drugs in development.

Cystic Fibrosis, Cystic Fibrosis Transmembrane Conductance Regulator and Drugs: Insights from Cellular Trafficking

The eukaryotic cell is organized into membrane-delineated compartments that are characterized by specific cadres of proteins sustaining biochemically distinct cellular processes. The appropriate subcellular localization of proteins is key to proper organelle function and provides a physiological context for cellular processes. Disruption of normal trafficking pathways for proteins is seen in several genetic diseases, where a protein's absence for a specific subcellular compartment leads to organelle disruption, and in the context of an individual, a disruption of normal physiology. Importantly, several drug therapies can also alter protein trafficking, causing unwanted side effects. Thus, a deeper understanding of trafficking pathways needs to be appreciated as novel therapeutic modalities are proposed. Despite the promising efficacy of novel therapeutic agents, the intracellular bioavailability of these compounds has proved to be a potential barrier, leading to failures in treatments for various diseases and disorders. While endocytosis of drug moieties provides an efficient means of getting material into cells, the subsequent release and endosomal escape of materials into the cytosol where they need to act has been a barrier. An understanding of cellular protein/lipid trafficking pathways has opened up strategies for increasing drug bioavailability. Approaches to enhance endosomal exit have greatly increased the cytosolic bioavailability of drugs and will provide a means of investigating previous drugs that may have been shelved due to their low cytosolic concentration.

Restoration of CFTR Activity in Ducts Rescues Acinar Cell Function and Reduces Inflammation in Pancreatic and Salivary Glands of Mice

BACKGROUND & AIMS

Sjögren's syndrome and autoimmune pancreatitis are disorders with decreased function of salivary, lacrimal glands, and the exocrine pancreas. Nonobese diabetic/ShiLTJ mice and mice transduced with the cytokine BMP6 develop Sjögren's syndrome and chronic pancreatitis and MRL/Mp mice are models of autoimmune pancreatitis. Cystic fibrosis transmembrane conductance regulator (CFTR) is a ductal Cl^- channel essential for ductal fluid and HCO₃^- secretion. We used these models to ask the following questions: is CFTR expression altered in these diseases, does correction of CFTR correct gland function, and most notably, does correcting ductal function correct acinar function?

METHODS

We treated the mice models with the CFTR corrector C18 and the potentiator VX770. Glandular, ductal, and acinar cells damage, infiltration, immune cells and function were measured in vivo and in isolated duct/acini.

RESULTS

In the disease models, CFTR expression is markedly reduced. The salivary glands and pancreas are inflamed with increased fibrosis and tissue damage. Treatment with VX770 and, in particular, C18 restored salivation, rescued CFTR expression and localization, and nearly eliminated the inflammation and tissue damage. Transgenic overexpression of CFTR exclusively in the duct had similar effects. Most notably, the markedly reduced acinar cell Ca²⁺ signaling, Orai1, inositol triphosphate receptors, Aquaporin 5 expression, and fluid secretion were restored by rescuing ductal CFTR.

CONCLUSIONS

Our findings reveal that correcting ductal function is sufficient to rescue acinar cell function and suggests that CFTR correctors are strong candidates for the treatment of Sjögren's syndrome and pancreatitis.

Identification of the amino acids inserted during suppression of CFTR nonsense mutations and determination of their functional consequences

In-frame premature termination codons (PTCs) account for ∼11% of all disease-associated mutations. PTC suppression therapy utilizes small molecules that suppress translation termination at a PTC to restore synthesis of a full-length protein. PTC suppression is mediated by the base pairing of a near-cognate aminoacyl-tRNA with a PTC and subsequently, the amino acid becomes incorporated into the nascent polypeptide at the site of the PTC. However, little is known about the identity of the amino acid(s) inserted at a PTC during this process in mammalian cells, or how the surrounding sequence context influences amino acid incorporation. Here, we determined the amino acids inserted at the cystic fibrosis transmembrane conductance regulator (CFTR) W1282X PTC (a UGA codon) in the context of its three upstream and downstream CFTR codons during G418-mediated suppression. We found that leucine, cysteine and tryptophan are inserted during W1282X suppression. Interestingly, these amino acids (and their proportions) are significantly different from those recently identified following G418-mediated suppression of the CFTR G542X UGA mutation. These results demonstrate for the first time that local mRNA sequence context plays a key role in near-cognate aminoacyl-tRNA selection during PTC suppression. We also found that some variant CFTR proteins generated by PTC suppression exhibit reduced maturation and activity, indicating the complexity of nonsense suppression therapy. However, both a CFTR corrector and potentiator enhanced activity of protein variants generated by G418-mediated suppression. These results suggest that PTC suppression in combination with CFTR modulators may be beneficial for the treatment of CF patients with PTCs.

Fatty Acid Cysteamine Conjugates as Novel and Potent Autophagy Activators That Enhance the Correction of Misfolded F508del-Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

A depressed autophagy has previously been reported in cystic fibrosis patients with the common F508del-CFTR mutation. This report describes the synthesis and preliminary biological characterization of a novel series of autophagy activators involving fatty acid cysteamine conjugates. These molecular entities were synthesized by first covalently linking cysteamine to docosahexaenoic acid. The resulting conjugate 1 synergistically activated autophagy in primary homozygous F508del-CFTR human bronchial epithelial (hBE) cells at submicromolar concentrations. When conjugate 1 was used in combination with the corrector lumacaftor and the potentiator ivacaftor, it showed an additive effect, as measured by the increase in the chloride current in a functional assay. In order to obtain a more stable form for oral dosing, the sulfhydryl group in conjugate 1 was converted into a functionalized disulfide moiety. The resulting conjugate 5 is orally bioavailable in the mouse, rat, and dog and allows a sustained delivery of the biologically active conjugate 1.

Evidence against resveratrol as a viable therapy for the rescue of defective ΔF508 CFTR

BACKGROUND

Resveratrol, a natural phenolic compound, has been reported to rescue mutant ΔF508 CFTR in expression systems and primary epithelial cells. Although this implies a therapeutic benefit to patients with CF, investigations were performed using resveratrol concentrations greatly in excess of those achievable in plasma. We evaluated the efficacy of resveratrol as a CFTR corrector in relevant primary airway cells, using physiologically achievable resveratrol concentrations.

METHODS

Cells expressing wt or ΔF508 CFTR were exposed to chronic or acute resveratrol. CFTR mRNA and protein expression were monitored. The effects of resveratrol on primary ΔF508 human airway cells were evaluated by equivalent current analysis using modified Ussing chambers.

RESULTS

Consistent with previously published data in heterologous expression systems, high doses of resveratrol increased CFTR expression; however physiologically relevant concentrations were without effect. In contrast to heterologous expression systems, resveratrol was unable to increase mutant CFTR channel activity in primary airway cells. Elevated amiloride-sensitive currents, indicative of sodium transport and characteristically elevated in CF airway cells, were also unaffected by resveratrol.

CONCLUSIONS

High concentrations of resveratrol can increase CFTR mRNA and protein in some cell types. In addition, acute resveratrol exposure can stimulate CFTR mediated chloride secretion, probably by increasing cellular cAMP levels. Resveratrol at physiologically achievable levels yielded no benefit in primary ΔF508 airway cells, either in terms of amiloride-sensitive currents of CFTR currents.

GENERAL SIGNIFICANCE

Taken together, our results do not support the use of resveratrol supplements as a therapy for patients with cystic fibrosis. It is possible that further modifications of the resveratrol backbone would yield a more efficacious compound.

Synthetic aminoglycosides efficiently suppress cystic fibrosis transmembrane conductance regulator nonsense mutations and are enhanced by ivacaftor

New drugs are needed to enhance premature termination codon (PTC) suppression to treat the underlying cause of cystic fibrosis (CF) and other diseases caused by nonsense mutations. We tested new synthetic aminoglycoside derivatives expressly developed for PTC suppression in a series of complementary CF models. Using a dual-luciferase reporter system containing the four most prevalent CF transmembrane conductance regulator (CFTR) nonsense mutations (G542X, R553X, R1162X, and W1282X) within their local sequence contexts (the three codons on either side of the PTC), we found that NB124 promoted the most readthrough of G542X, R1162X, and W1282X PTCs. NB124 also restored full-length CFTR expression and chloride transport in Fischer rat thyroid cells stably transduced with a CFTR-G542XcDNA transgene, and was superior to gentamicin and other aminoglycosides tested. NB124 restored CFTR function to roughly 7% of wild-type activity in primary human bronchial epithelial (HBE) CF cells (G542X/delF508), a highly relevant preclinical model with endogenous CFTR expression. Efficacy was further enhanced by addition of the CFTR potentiator, ivacaftor (VX-770), to airway cells expressing CFTR PTCs. NB124 treatment rescued CFTR function in a CF mouse model expressing a human CFTR-G542X transgene; efficacy was superior to gentamicin and exhibited favorable pharmacokinetic properties, suggesting that in vitro results translated to clinical benefit in vivo. NB124 was also less cytotoxic than gentamicin in a tissue-based model for ototoxicity. These results provide evidence that NB124 and other synthetic aminoglycosides provide a 10-fold improvement in therapeutic index over gentamicin and other first-generation aminoglycosides, providing a promising treatment for a wide array of CFTR nonsense mutations.

Novel di-aryl-substituted isoxazoles act as noncompetitive inhibitors of the system Xc(-) cystine/glutamate exchanger

The system xc(-) antiporter is a plasma membrane transporter that mediates the exchange of extracellular l-cystine with intracellular l-glutamate. This exchange is significant within the context of the CNS because the import of l-cystine is required for the synthesis of the antioxidant glutathione, while the efflux of l-glutamate has the potential to contribute to either excitatory signaling or excitotoxic pathology. Changes in the activity of the transport system have been linked to the underlying pathological mechanisms of a variety of CNS disorders, one of the most prominent of which is its highly enriched expression in glial brain tumors. In an effort to produce more potent system xc(-) blockers, we have been using amino-3-carboxy-5-methylisoxazole propionic acid (ACPA) as a scaffold for inhibitor development. We previously demonstrated that the addition of lipophilic aryl groups to either the #4 or #5 position on the isoxazole ring markedly increased the inhibitory activity at system xc(-). In the present work a novel series of analogues has been prepared in which aryl groups have been introduced at both the #4 and #5 positions. In contrast to the competitive action of the mono-substituted analogues, kinetic analyses indicate that the di-substituted isoxazoles block system xc(-)-mediated uptake of (3)H-l-glutamate into SNB-19 cells by a noncompetitive mechanism. These new analogues appear to be the first noncompetitive inhibitors identified for this transport system, as well as being among the most potent blockers identified to date. These diaryl-isoxazoles should be of value in assessing the physiological roles and molecular pharmacology of system xc(-).

Effect of apical hyperosmotic sodium challenge and amiloride on sodium transport in human bronchial epithelial cells from cystic fibrosis donors

Hypertonic saline (HS) inhalation therapy benefits cystic fibrosis (CF) patients [Donaldson SH, Bennet WD, Zeman KL, Knowles MR, Tarran R, Boucher RC. N Engl J Med 354: 241-250, 2006; Elkins MR, Robinson M, Rose BR, Harbour C, Moriarty CP, Marks GB, Belousova EG, Xuan W, Bye PT; the National Hypertonic Saline in Cystic Fibrosis (NHSCF) Study Group. N Engl J Med 354: 229-240, 2006]. Surprisingly, these benefits are long-lasting and are diminished by the epithelial Na(+) channel blocker amiloride (Donaldson SH, Bennet WD, Zeman KL, Knowles MR, Tarran R, Boucher RC. N Engl J Med 354: 241-250, 2006). Our aim was to explain these effects. Human bronchial epithelial (hBE) cells from CF lungs were grown in inserts and were used in three experimental approaches: 1) Ussing chambers to measure amiloride-sensitive short-circuit currents (INa); 2) continuous perfusion Ussing chambers; and 3) near "thin-film" conditions in which the airway surface of the inserts was exposed to a small volume (30 μl) of isosmotic or HS solution as the inserts were kept in their incubation tray and were subsequently used to measure INa under isosmotic conditions (near thin-film experiments; Tarran R, Boucher RC. Methods Mol Med 70: 479-492, 2002). HS solutions (660 mosmol/kgH2O) were prepared by adding additional NaCl to the isosmotic buffer. The transepithelial short-circuit current (ISC), conductance (GT), and capacitance (CT) were measured by transepithelial impedance analysis (Danahay H, Atherton HC, Jackson AD, Kreindler JL, Poll CT, Bridges RJ. Am J Physiol Lung Cell Mol Physiol 290: L558-L569, 2006; Singh AK, Singh S, Devor DC, Frizzell RA, van Driessche W, Bridges RJ. Methods Mol Med 70: 129-142, 2002). Exposure to apical HS inhibited INa, GT, and CT. The INa inhibition required 60 min of reexposure to the isosmotic solution to recover 75%. The time of exposure to HS required to inhibit INa was <2.5 min. Under near thin-film conditions, apical exposure to HS inhibited INa, but as osmotically driven water moved to the apical surface, the aqueous apical volume increased, leading to an amiloride-insensitive decrease in its osmolality and to recovery of INa that lagged behind the osmotic recovery. Amiloride significantly accelerated the recovery of INa following exposure to HS. Our conclusions are that exposure to HS inhibits hBE INa and that amiloride diminishes this effect.

Mechanisms of bicarbonate secretion: lessons from the airways

Early studies showed that airway cells secrete HCO(3)(-) in response to cAMP-mediated agonists and HCO(3)(-) secretion was impaired in cystic fibrosis (CF). Studies with Calu-3 cells, an airway serous model with high expression of CFTR, also show the secretion of HCO(3)(-) when cells are stimulated with cAMP-mediated agonists. Activation of basolateral membrane hIK-1 K(+) channels inhibits HCO(3)(-) secretion and stimulates Cl(-) secretion. CFTR mediates the exit of both HCO(3)(-) and Cl(-) across the apical membrane. Entry of HCO(3)(-) on a basolateral membrane NBC or Cl(-) on the NKCC determines which anion is secreted. Switching between these two secreted anions is determined by the activity of hIK-1 K(+) channels.

Requirements for efficient correction of ΔF508 CFTR revealed by analyses of evolved sequences

Misfolding of ΔF508 cystic fibrosis (CF) transmembrane conductance regulator (CFTR) underlies pathology in most CF patients. F508 resides in the first nucleotide-binding domain (NBD1) of CFTR near a predicted interface with the fourth intracellular loop (ICL4). Efforts to identify small molecules that restore function by correcting the folding defect have revealed an apparent efficacy ceiling. To understand the mechanistic basis of this obstacle, positions statistically coupled to 508, in evolved sequences, were identified and assessed for their impact on both NBD1 and CFTR folding. The results indicate that both NBD1 folding and interaction with ICL4 are altered by the ΔF508 mutation and that correction of either individual process is only partially effective. By contrast, combination of mutations that counteract both defects restores ΔF508 maturation and function to wild-type levels. These results provide a mechanistic rationale for the limited efficacy of extant corrector compounds and suggest approaches for identifying compounds that correct both defective steps.

Small molecule correctors of F508del-CFTR discovered by structure-based virtual screening

Folding correctors of F508del-CFTR were discovered by in silico structure-based screening utilizing homology models of CFTR. The intracellular segment of CFTR was modeled and three cavities were identified at inter-domain interfaces: (1) Interface between the two Nucleotide Binding Domains (NBDs); (2) Interface between NBD1 and Intracellular Loop (ICL) 4, in the region of the F508 deletion; (3) multi-domain interface between NBD1:2:ICL1:2:4. We hypothesized that compounds binding at these interfaces may improve the stability of the protein, potentially affecting the folding yield or surface stability. In silico structure-based screening was performed at the putative binding-sites and a total of 496 candidate compounds from all three sites were tested in functional assays. A total of 15 compounds, representing diverse chemotypes, were identified as F508del folding correctors. This corresponds to a 3% hit rate, ~tenfold higher than hit rates obtained in corresponding high-throughput screening campaigns. The same binding sites also yielded potentiators and, most notably, compounds with a dual corrector-potentiator activity (dual-acting). Compounds harboring both activity types may prove to be better leads for the development of CF therapeutics than either pure correctors or pure potentiators. To the best of our knowledge this is the first report of structure-based discovery of CFTR modulators.

Report from the Canadian Association of Gastroenterology Board

On behalf of the Canadian Association of Gastroenterology (CAG) Board, I am pleased to provide you with this report summarizing the activities and directions of the organization on behalf of its members. It is an honour to participate in the affairs of the organization and interact with groups and individuals from across the country dedicated to advancing science and care in the field of digestive health and disease. This is a challenging time in medicine, and the organization has been working hard to enhance the benefits, programs and services available to its members. The goal is to provide the highest level of services possible to meet your needs.

A segment of gamma ENaC mediates elastase activation of Na+ transport

The epithelial Na⁺ channel (ENaC) that mediates regulated Na⁺ reabsorption by epithelial cells in the kidney and lungs can be activated by endogenous proteases such as channel activating protease 1 and exogenous proteases such as trypsin and neutrophil elastase (NE). The mechanism by which exogenous proteases activate the channel is unknown. To test the hypothesis that residues on ENaC mediate protease-dependent channel activation wild-type and mutant ENaC were stably expressed in the FRT epithelial cell line using a tripromoter human ENaC construct, and protease-induced short-circuit current activation was measured in aprotinin-treated cells. The amiloride-sensitive short circuit current (I_Na) was stimulated by aldosterone (1.5-fold) and dexamethasone (8-fold). Dexamethasone-treated cells were used for all subsequent studies. The serum protease inhibitor aprotinin decreased baseline I_Na by approximately 50% and I_Na could be restored to baseline control values by the exogenous addition of trypsin, NE, and porcine pancreatic elastase (PE) but not by thrombin. All protease experiments were thus performed after exposure to aprotinin. Because NE recognition of substrates occurs with a preference for binding valines at the active site, several valines in the extracellular loops of α and γ ENaC were sequentially substituted with glycines. This scan yielded two valine residues in γ ENaC at positions 182 and 193 that resulted in inhibited responses to NE when simultaneously changed to other amino acids. The mutations resulted in decreased rates of activation and decreased activated steady-state current levels. There was an ∼20-fold difference in activation efficiency of NE against wild-type ENaC compared to a mutant with glycine substitutions at positions 182 and 193. However, the mutants remain susceptible to activation by trypsin and the related elastase, PE. Alanine is the preferred P₁ position residue for PE and substitution of alanine 190 in the γ subunit eliminated I_Na activation by PE. Further, substitution with a novel thrombin consensus sequence (LVPRG) beginning at residue 186 in the γ subunit (γ_Th) allowed for I_Na activation by thrombin, whereas wild-type ENaC was unresponsive. MALDI-TOF mass spectrometric evaluation of proteolytic digests of a 23-mer peptide encompassing the identified residues (T¹⁷⁶-S¹⁹⁸) showed that hydrolysis occurred between residues V193 and M194 for NE and between A190 and S191 for PE. In vitro translation studies demonstrated thrombin cleaved the γ_Th but not the wild-type γ subunit. These results demonstrate that γ subunit valines 182 and 193 are critical for channel activation by NE, alanine 190 is critical for channel activation by PE, and that channel activation can be achieved by inserting a novel thrombin consensus sequence. These results support the conclusion that protease binding and perhaps cleavage of the γ subunit results in ENaC activation.

Electrogenic bicarbonate secretion by prairie dog gallbladder

Pathological rates of gallbladder salt and water transport may promote the formation of cholesterol gallstones. Because prairie dogs are widely used as a model of this event, we characterized gallbladder ion transport in animals fed control chow by using electrophysiology, ion substitution, pharmacology, isotopic fluxes, impedance analysis, and molecular biology. In contrast to the electroneutral properties of rabbit and Necturus gallbladders, prairie dog gallbladders generated significant short-circuit current (I(sc); 171 +/- 21 microA/cm(2)) and lumen-negative potential difference (-10.1 +/- 1.2 mV) under basal conditions. Unidirectional radioisotopic fluxes demonstrated electroneutral NaCl absorption, whereas the residual net ion flux corresponded to I(sc). In response to 2 microM forskolin, I(sc) exceeded 270 microA/cm(2), and impedance estimates of the apical membrane resistance decreased from 200 Omega.cm(2) to 13 Omega.cm(2). The forskolin-induced I(sc) was dependent on extracellular HCO(3)(-) and was blocked by serosal 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS) and acetazolamide, whereas serosal bumetanide and Cl(-) ion substitution had little effect. Serosal trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chroman and Ba(2+) reduced I(sc), consistent with the inhibition of cAMP-dependent K(+) channels. Immunoprecipitation and confocal microscopy localized cystic fibrosis transmembrane conductance regulator protein (CFTR) to the apical membrane and subapical vesicles. Consistent with serosal DNDS sensitivity, pancreatic sodium-bicarbonate cotransporter protein pNBC1 expression was localized to the basolateral membrane. We conclude that prairie dog gallbladders secrete bicarbonate through cAMP-dependent apical CFTR anion channels. Basolateral HCO(3)(-) entry is mediated by DNDS-sensitive pNBC1, and the driving force for apical anion secretion is provided by K(+) channel activation.

Methods for detecting internalized, FM 1-43 stained particles in epithelial cells and monolayers

The membrane dye FM 1-43 has frequently been used to quantify exocytosis in neurons. In epithelia, intense lateral intracellular space staining and fluctuations in baseline labeling produced inconsistent results. Membrane retrieved in the presence of FM 1-43 retains the dye, however, and cells that undergo compensatory endocytosis during and following evoked exocytosis contain punctate, fluorescent particles after washout of external stain. As an alternative measure of trafficking, we quantified the fluorescent puncta retained after dye washout and tested our method on both coverslip-grown cell clusters and filter-grown intact monolayers. Images for analysis were acquired using serial sectioning with either epifluorescence or confocal microscopy. Tests with an intestinal goblet cell line that exhibits basal and ATP-stimulated granule trafficking confirmed that 1), the algorithm identified the same number of internalized particles with either epifluorescence or confocal microscopy acquired images; 2), low density clusters exhibited significantly more internalized particles per cell than either filter-grown monolayers or high density clusters; 3), ATP stimulation significantly increased the number of internalized particles in all preparations; and 4), the number of particles internalized was comparable to capacitance measurements of exocytosis. This method provides a single technique for quantifying membrane trafficking in both monolayers and unpolarized cells.

Identification and membrane localization of electrogenic sodium bicarbonate cotransporters in Calu-3 cells

Cystic fibrosis (CF) is a severely life-shortening genetic disease resulting from mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR). Impaired bicarbonate secretion is a key component of CF-related pancreatic disease, but the role of impaired bicarbonate secretion in CF lung disease is less well understood. The submucosal glands of the conducting airways produce and secrete a complex airway surface liquid that lines the airway epithelium and plays a significant role in mucociliary clearance. The serous cell is the predominant cell type of the submucosal gland and a predominant site of CFTR expression. Calu-3 cells are a model of airway submucosal gland serous cells that demonstrates vectorial bicarbonate secretion in response to elevations in cAMP. Based on previously published measurements of unidirectional ion flux, pharmacological inhibition of short-circuit current and ion substitution studies, one can hypothesize the existence of an electrogenic sodium bicarbonate cotransporter (NBC) in the basolateral membrane of Calu-3 cells that mediates bicarbonate entry from the interstitium. To test this hypothesis, we performed reverse-transcriptase PCR, western blotting, and surface biotinylation to identify and localize electrogenic NBCs in Calu-3 cells. Our data demonstrate that both pNBC1 and NBC4 mRNAs can be identified and that their protein products are expressed at the basolateral membrane of polarized Calu-3 cells. These data suggest that these transporters contribute to regulated bicarbonate secretion across Calu-3 cells and perhaps human airway submucosal glands.

Endogenous protease activation of ENaC: effect of serine protease inhibition on ENaC single channel properties

Endogenous serine proteases have been reported to control the reabsorption of Na⁺ by kidney- and lung-derived epithelial cells via stimulation of electrogenic Na⁺ transport mediated by the epithelial Na⁺ channel (ENaC). In this study we investigated the effects of aprotinin on ENaC single channel properties using transepithelial fluctuation analysis in the amphibian kidney epithelium, A6. Aprotinin caused a time- and concentration-dependent inhibition (84 ± 10.5%) in the amiloride-sensitive sodium transport (I_Na) with a time constant of 18 min and half maximal inhibition constant of 1 μM. Analysis of amiloride analogue blocker–induced fluctuations in I_Na showed linear rate–concentration plots with identical blocker on and off rates in control and aprotinin-inhibited conditions. Verification of open-block kinetics allowed for the use of a pulse protocol method (Helman, S.I., X. Liu, K. Baldwin, B.L. Blazer-Yost, and W.J. Els. 1998. Am. J. Physiol. 274:C947–C957) to study the same cells under different conditions as well as the reversibility of the aprotinin effect on single channel properties. Aprotinin caused reversible changes in all three single channel properties but only the change in the number of open channels was consistent with the inhibition of I_Na. A 50% decrease in I_Na was accompanied by 50% increases in the single channel current and open probability but an 80% decrease in the number of open channels. Washout of aprotinin led to a time-dependent restoration of I_Na as well as the single channel properties to the control, pre-aprotinin, values. We conclude that protease regulation of I_Na is mediated by changes in the number of open channels in the apical membrane. The increase in the single channel current caused by protease inhibition can be explained by a hyperpolarization of the apical membrane potential as active Na⁺ channels are retrieved. The paradoxical increase in channel open probability caused by protease inhibition will require further investigation but does suggest a potential compensatory regulatory mechanism to maintain I_Na at some minimal threshold value.

Membrane capacitance and conductance changes parallel mucin secretion in the human airway epithelium

Measurement of the magnitude and kinetics of exocytosis from intact epithelia has historically been difficult. Using well-differentiated cultures of human bronchial epithelial cells, we describe the use of transepithelial impedance analysis to enable the real-time quantification of mucin secretagogue-induced changes in membrane capacitance (surface area) and conductance. ATPgammaS, UTP, ionomycin, and PMA induced robust increases in total cellular capacitance that were demonstrated to be dominated by a specific increase in apical membrane surface area. The UTP-induced increase in capacitance occurred in parallel with goblet cell emptying and the secretion of mucin and was associated with decreases in apical and basolateral membrane resistances. The magnitude and kinetics of the capacitance increases were dependent on the agonist and the sidedness of the stimulation. The peak increase in capacitance induced by UTP was approximately 30 mucin granule fusions per goblet cell. Secretagogue-induced decreases in apical membrane resistance were independent of exocytosis, although each of the secretagogues induced profound reductions in basolateral membrane resistance. Transepithelial impedance analysis offers the potential to study morphological and conductance changes in cultured human bronchial epithelial cells.

Inhibition of chloride secretion in human bronchial epithelial cells by cigarette smoke extract

Chronic bronchitis, a disease mainly of cigarette smokers, shares many clinical features with cystic fibrosis, a disease of altered ion transport, suggesting that the negative effects of cigarette smoke on mucociliary clearance may be mediated through alterations in ion transport. We tested the hypothesis that cigarette smoke extract would inhibit chloride secretion in human bronchial epithelial cells. In agreement with studies in canine trachea, cigarette smoke extract inhibited net chloride secretion without affecting sodium transport. We performed microelectrode impalements and impedance analysis studies to investigate the physiological mechanisms of this inhibition. These data demonstrated that cigarette smoke extract caused an acute increase in membrane resistances in conjunction with apical membrane hyperpolarization, an effect consistent with inhibition of an apical membrane anion conductance. After this acute phase, both membrane resistances decreased while membrane potentials continued to hyperpolarize, indicating that cigarette smoke extract also inhibited the basolateral entry of chloride into the cell. Furthermore, cigarette smoke extract caused an increase in mucin secretion. Therefore, the ion transport phenotype of human bronchial epithelial cells exposed to cigarette smoke extract is similar to that of cystic fibrosis epithelia in which there is sodium absorption out of proportion to chloride secretion in the setting of increased mucus secretion.

Transepithelial fluctuation analysis of chloride secretion

Transepithelial fluctuation analysis (noise analysis) provides valuable information about the density and single-channel properties of ion channels in intact epithelia. Here we investigate cystic fibrosis transmembrane conductance regulator (CFTR)-dependent chloride (Cl-) secretion in T84 human colonic epithelia by inducing noise using the diarylsulfonylurea DASU-01, a low-affinity open-channel blocker of CFTR. Our data indicate that the apical membrane of maximally stimulated T84 epithelia has a very high Cl- conductance generated by approximately 7000 active CFTR channels per cell with open probability (Po) of approximately 0.4 and single-channel amplitude (i) of approximately 0.1 pA. Similar experiments might provide important information about how drugs regulate CFTR in intact epithelia.

Niflumic acid inhibits ATP-stimulated exocytosis in a mucin-secreting epithelial cell line

ATP is an efficacious secretagogue for mucin and chloride in the epithelial cell line HT29-Cl.16E. Mucin release has been measured as [3H]glucosamine-labeled product in extracellular medium and as single-cell membrane capacitance increases indicative of exocytosis-related increases in membrane area. The calcium-activated chloride channel blocker niflumic acid, also reported to modulate secretion, was used to probe for divergence in the purinergic signaling of mucin exocytosis and channel activation. With the use of whole cell patch clamping, ATP stimulated a transient capacitance increase of 15 +/- 4%. Inclusion of niflumic acid significantly reduced the ATP-stimulated capacitance change to 3 +/- 1%, although normalized peak currents were not significantly different. Ratiometric imaging was used to assess intracellular calcium (Cai2+) dynamics during stimulation. In the presence of niflumic acid, the ATP-stimulated peak change in Cai2+ was unaffected, but the initial response and overall time to Cai2+ peak were significantly affected. Excluding external calcium before ATP stimulation or including the capacitative calcium entry blocker LaCl3 during stimulation muted the initial calcium transient similar to that observed with niflumic acid and significantly reduced peak capacitance change, suggesting that a substantial portion of the ATP-stimulated mucin exocytosis in HT29-Cl.16E depends on a rapid, brief calcium influx through the plasma membrane. Niflumic acid interferes with this influx independent of a chloride channel blockade effect.

A mutation in the cystic fibrosis transmembrane conductance regulator generates a novel internalization sequence and enhances endocytic rates

Cystic fibrosis is a common lethal genetic disease among Caucasians. The cystic fibrosis gene encodes a cyclic adenosine monophosphate-activated chloride channel (cystic fibrosis transmembrane conductance regulator (CFTR)) that mediates electrolyte transport across the luminal surfaces of a variety of epithelial cells. Mutations in CFTR fall into two broad categories; those that affect protein biosynthesis/stability and traffic to the cell surface and those that cause altered channel kinetics in proteins that reach the cell surface. Here we report a novel mechanism by which mutations in CFTR give rise to disease. N287Y, a mutation within an intracellular loop of CFTR, increases channel endocytosis from the cell surface without affecting either biosynthesis or channel gating. The sole consequence of this novel mutation is to generate a novel tyrosine-based endocytic sequence within an intracellular loop in CFTR leading to increased removal from the cell surface and a reduction in the steady-state level of CFTR at the cell surface.

CFTR and bicarbonate secretion by [correction of to] epithelial cells

Defective HCO(3)(-) and fluid secretion are hallmarks of the pathophysiology of the pancreas of cystic fibrosis patients. Recently, impaired HCO(3)(-) secretion has been shown in most tissues known to express the cystic fibrosis transmembrane conductance regulator (CFTR). New results suggest that CFTR plays an important role in the transcellular secretion of HCO(3)(-).

Interleukin-13 induces a hypersecretory ion transport phenotype in human bronchial epithelial cells

Interleukin (IL)-13 has been associated with asthma, allergic rhinitis, and chronic sinusitis, all conditions where an imbalance in epithelial fluid secretion and absorption could impact upon the disease. We have investigated the effects of IL-13 on the ion transport characteristics of human bronchial epithelial cells cultured at an apical-air interface. Ussing chamber studies indicated that 48 h pretreatment with IL-13 or IL-4 significantly reduced the basal short-circuit current (I(sc)) and inhibited the amiloride-sensitive current by >98%. Furthermore, the I(sc) responses were increased by more than six- and twofold over control values when stimulated with UTP or forskolin, respectively, after cytokine treatment. The IL-13-enhanced response to UTP/ionomycin was sensitive to bumetanide and DIDS and was reduced in a low-chloride, bicarbonate-free solution. Membrane permeablization studies indicated that IL-13 induced the functional expression of an apical Ca(2+)-activated anion conductance and that changes in apical or basolateral K(+) conductances could not account for the increased I(sc) responses to UTP or ionomycin. The results indicate that IL-13 converts the human bronchial epithelium from an absorptive to a secretory phenotype that is the result of loss of amiloride-sensitive current and an increase in a DIDS-sensitive apical anion conductance.

Differing effects of substrate and non-substrate transport inhibitors on glutamate uptake reversal

Na(+)-dependent excitatory amino acid transporters (EAATs) normally function to remove extracellular glutamate from brain extracellular space, but EAATs can also increase extracellular glutamate by reversal of uptake. Effects of inhibitors on EAATs can be complex, depending on cell type, whether conditions favor glutamate uptake or uptake reversal and whether the inhibitor itself is a substrate for the transporters. The present study assessed EAAT inhibitors for their ability to inhibit glutamate uptake, act as transporter substrates and block uptake reversal in astrocyte and neuron cultures. L-threo-beta-hydroxyaspartate (L-TBHA), DL-threo-beta-benzyloxyaspartate (DL-TBOA), L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-2,4-PDC) (+/-)-cis-4-methy-trans-pyrrolidine-2,4-dicarboxylic acid (cis-4-methy-trans-2,4-PDC) and L-antiendo-3,4-methanopyrrolidine-2,4-dicarboxylic acid (L-antiendo-3,4-MPDC) inhibited L-[14C]glutamate uptake in astrocytes with equilibrium binding constants ranging from 17 microM (DL-TBOA and L-TBHA) - 43 microM (cis-4-methy-trans-2,4-PDC). Transportability of inhibitors was assessed in astrocytes and neurons. While L-TBHA, L-trans-2,4-PDC, cis-4-methy-trans-2,4-PDC and L-antiendo-3,4-MPDC displayed significant transporter substrate activities in neurons and astrocytes, DL-TBOA was a substrate only in astrocytes. This effect of DL-TBOA was concentration-dependent, leading to complex effects on glutamate uptake reversal. At concentrations low enough to produce minimal DL-TBOA uptake velocity (< or = 10 microM), DL-TBOA blocked uptake reversal in ATP-depleted astrocytes; this blockade was negated at concentrations that drove substantial DL-TBOA uptake (> 10 microM). These findings indicate that the net effects of EAAT inhibitors can vary with cell type and exposure conditions.

Effect of cytochrome P450 1A induction on oxidative damage in rat brain

Polycyclic and halogenated aromatic hydrocarbons (PAHs and HAHs) can enhance the generation of reactive oxygen species (ROS) by inducing cytochrome P450 1A (CYP 1A) in vivo and in vitro. While the brain is vulnerable to oxidative injury, whether or not CYP 1A induction in the brain can produce measurable levels of oxidative damage has not been reported. The objective of this study was to investigate the effect of this induction on oxidative damage to the CNS. Time course changes in rat brain CYP 1A activity were determined by measurement of ethoxyresorufin O-deethylase (EROD) activity in whole brain homogenates. Three days after exposure of rats to five daily injections of 3-methylcholanthrene (3-MC) an approximately sevenfold increase in EROD activity was observed. Hepatic levels were increased 60-100 fold. This increase in CYP 1A activity was not accompanied by increased protein or lipid oxidation as measured by tryptophan fluorescence and TBAR formation, or decreased glutamine synthetase (GS) activity. These findings indicate that if increased CYP 1A activity in the brain following 3-MC treatment leads to increased ROS generation, the increase is insufficient to overwhelm the endogenous antioxidant defense system, produce detectable oxidative damage, and alter glutamate homeostasis.

pH regulation and bicarbonate transport of isolated porcine submucosal glands

We have previously demonstrated that the airway serous cell line Calu-3 employs a number of pH regulatory mechanisms required for bicarbonate secretion by these cells. The aim of the present study was to investigate the pH regulatory mechanisms of serous cells of freshly isolated submucosal glands (SMG). Porcine SMG were dissected out of pig tracheas obtained from a local slaughterhouse. Single glands were transferred into the chamber of an inverted microscope, immobilized by two holding pipettes and the serous cells loaded with the fluorescent pH probe 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). Fluorescence was monitored from small areas consisting of up to 20 cells. The fluorescence ratio of the emission after excitation at 488 nm and 436 nm respectively was used to estimate cytosolic pH (pH(i)). Resting pH(i) of SMG cells in the absence of HCO(3)(-)/CO(2) was 7.1 +/- 0.16 (n=24). Addition of a solution buffered with HCO(3)(-)/CO(2) to the bath transiently acidified the cells by 0.18 +/- 0.03 (n=18). pH(i) rapidly recovered to a slightly more alkaline value than baseline pH(i). Removal of the HCO(3)(-)/CO(2) buffer strongly alkalinized SMG cells by 0.2 +/- 0.03 (n=18). To challenge pH regulatory mechanisms we exposed the cells to 20 mmol/L NH4(+) in the absence and presence of HCO(3)(-)/CO(2). In both cases we observed a rapid increase in pH(i) followed by a slight recovery. Washout of NH4(+) strongly acidified the cells. Realkalinization of pH(i) could only be observed in the presence of Na(+). This effect was inhibited by the addition of the specific Na(+)/H(+) exchanger isoform 1 (NHE1) blocker 3-methylsulfonyl-4-piperidinobenzoyl guanidine hydrochloride (HOE 694, 10-100 micromol/L) with an half maximal inhibitory concentration (IC(50)) of approximately 20 micromol/L. Full recovery of pH(i) in the presence of HOE 694 was observed when the cells were bathed in HCO(3)(-)/CO(2) solution. Addition of forskolin (5 micromol/L) in the presence of HCO(3)(-)/CO(2) did not significantly alter pH(i) or change pH(i) recovery after acid loading. We conclude that SMG cells possess both HCO(3)(-) dependent and HCO(3)(-) independent pH(i); regulatory mechanisms that require the presence of extracellular Na(+). Further studies are required to understand whether bicarbonate is only transported to regulate pH(i) or whether this transport determines the overall secretory capacity of SMG serous cells.

Na+ transport in normal and CF human bronchial epithelial cells is inhibited by BAY 39-9437

To test the hypothesis that Na+ transport in human bronchial epithelial (HBE) cells is regulated by a protease-mediated mechanism, we investigated the effects of BAY 39-9437, a recombinant Kunitz-type serine protease inhibitor, on amiloride-sensitive short-circuit current of normal [non-cystic fibrosis (CF) cells] and CF HBE cells. Mucosal treatment of non-CF and CF HBE cells with BAY 39-9437 decreased the short-circuit current, with a half-life of approximately 45 min. At 90 min, BAY 39-9437 (470 nM) reduced Na+ transport by approximately 70%. The inhibitory effect of BAY 39-9437 was concentration dependent, with a half-maximal inhibitory concentration of approximately 25 nM. Na+ transport was restored to control levels, with a half-life of approximately 15 min, on washout of BAY 39-9437. In addition, trypsin (1 microM) rapidly reversed the inhibitory effect of BAY 39-9437. These data indicate that Na+ transport in HBE cells is activated by a BAY 39-9437-inhibitable, endogenously expressed serine protease. BAY 39-9437 inhibition of this serine protease maybe of therapeutic potential for the treatment of Na+ hyperabsorption in CF.

Microelectrode and impedance analysis of anion secretion in Calu-3 cells

Calu-3 cells secrete HCO(3)(-) in response to cAMP agonists but can be stimulated to secrete Cl(-) with K(+) channel activating agonists. Microelectrode and impedance analysis experiments were performed to obtain a better understanding of the conductances and driving forces involved in these different modes of anion secretion in Calu-3 cells. Microelectrode studies revealed apical and basolateral membrane depolarizations upon the addition of forskolin (V(ap) -52 mV vs. -21 mV; V(bl) -60 mV vs. -44 mV) that paralleled the hyperpolarization of the mucosal negative transepithelial voltage (V(T) -8 mV vs. -23 mV). These changes were accompanied by a decrease in the apical membrane fractional resistance (F(Rap)) from approximately 0.50 to 0.08, consistent with the activation of an apical membrane conductance. The subsequent addition of 1-ethyl-2-benzimidazolinone (1-EBIO), a K(+) channel activator, hyperpolarized V(ap) to -27 mV, V(bl) to -60 mV and V(T) to -33 mV. Impedance analysis revealed the apical membrane resistance (R(ap)) of the forskolin-stimulated cells was less than 20 ohm cm(2), indeed in most monolayers R(ap) fell to less than 5 ohm cm(2). The impedance derived estimate of the basolateral membrane resistance (R(bl)) was approximately 170 ohm cm(2) in forskolin treated cells and fell to 50 ohm cm(2) with the addition of 1-EBIO. Using these values for the R(bl) and the F(Rap) value of 0.08 yields a R(ap) of approximately 14 ohm cm(2) in the presence of forskolin and 4 ohm cm(2) in the presence of forskolin plus 1-EBIO. Thus, by two independent methods, forskolin-stimulated Calu-3 cells are seen to have a very high apical membrane conductance of 50 to 200 mS/cm(2). Therefore, we would assert that even at one-tenth the anion selectivity for Cl(-), this high conductance could support the conductive exit of HCO(3)(-) across the apical membrane. We further propose that this high apical membrane conductance serves to clamp the apical membrane potential near the equilibrium potential for Cl(-) and thereby provides the driving force for HCO(3)(-) secretion in forskolin-stimulated Calu-3 cells. The hyperpolarization of V(ap) and V(bl) caused by 1-EBIO provides a driving force for Cl(-) exit across the apical membrane, inhibits the influx of HCO(3)(-) on the Na(+):HCO(3)(-) cotransporter across the basolateral membrane, activates the basolateral membrane Na(+):K:2Cl(-) cotransporter and thereby provides the switch from HCO(3)(-) secretion to Cl(-) secretion.

Organotins disrupt components of glutamate homeostasis in rat astrocyte cultures

A rat cortical astrocyte preparation was used to investigate the effects of organotins on glutamate regulation by astrocytes. Exposure of astrocytes to low levels of organotins produced significant changes in two key components of glutamate homeostasis: glutamine synthetase (CS) activity and the high-affinity transport of L-glutamate. Trimethyltin (TMT), triethyltin (TET), and triphenyltin (TPT) exhibited differential abilities to reduce GS activity and glutamate uptake. Cultures incubated with 1 microM TET or TPT, but not TMT, exhibited a marked decrease in GS activity. Exposure to TET or TPT also produced a significant decrease in glutamate transport activity that was not observed with TMT. These declines in activity were not attributable to cell loss as measured by MTT reduction and lactate dehydrogenase (LDH) leakage. Since the loss of GS activity and transporter activity was not seen with acute organotin exposure, it is most likely attributable to a decreased presence of fully functioning protein. While the attenuation of GS and glutamate transporter activities by organotins does not match their pattern of neurotoxicity, the results indicate the potential for subtoxic concentrations of these compounds to increase extracellular glutamate and interact with other excitotoxic episodes.

Protease-activated receptor-2-mediated inhibition of ion transport in human bronchial epithelial cells

A cytoprotective role for protease-activated receptor-2 (PAR2) has been suggested in a number of systems including the airway, and to this end, we have studied the role that PARs play in the regulation of airway ion transport, using cultures of normal human bronchial epithelial cells. PAR2 activators, added to the basolateral membrane, caused a transient, Ca2+-dependent increase in short-circuit current ( I sc), followed by a sustained inhibition of amiloride-sensitive I sc. These phases corresponded with a transient increase in intracellular Ca2+ concentration and then a transient increase, followed by decrease, in basolateral K+ permeability. After PAR2 activation and the addition of amiloride, the forskolin-stimulated increase in I sc was also attenuated. By contrast, PAR2 activators added to the apical surface of the epithelia or PAR1 activators added to both the apical and basolateral surfaces were without effect. PAR2 may, therefore, play a role in the airway, regulating Na+ absorption and anion secretion, processes that are central to the control of airway surface liquid volume and composition.

Benzimidazolone activators of chloride secretion: potential therapeutics for cystic fibrosis and chronic obstructive pulmonary disease

The diseases of cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD) are characterized by mucus-congested airways. Agents that stimulate the secretion of Cl- are anticipated to facilitate mucociliary clearance and thus be of benefit in the treatment of CF and COPD. Recently 1-EBIO (1-ethyl-2-benzimidazolinone or 1-ethyl-1,3-dihydro-2H-benzimidazol-2-one) was shown to stimulate chloride secretion albeit at relatively high concentrations (0.6-1 mM). The studies reported here were undertaken to develop a more potent benzimidazolone. Structure activity studies with 30 benzimidazolone derivatives revealed that ethyl and hydrogen groups at the 1 and 3 nitrogen positions, respectively, were critical for the activation of hIK1 K+ channels and that other alkyl groups were not tolerated at these positions without some loss in potency. Substitutions at the 5 and 6 positions improved the potency of 1-EBIO. Compared with 1-EBIO, the most potent of these derivatives, DCEBIO (5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one) was severalfold better in a 86Rb+ uptake assay, 20-fold better in short circuit current measurements on T84 monolayers, and 100-fold better in patch-clamp assays of hIK1 activity. Short circuit current studies revealed DCEBIO stimulates Cl- secretion via the activation of hIK1 K+ channels and the activation of an apical membrane Cl- conductance. The improved potency of DCEBIO strengthens the possibility that compounds in this class may be of therapeutic benefit in the treatment of CF and COPD.

Estrogen inhibition of cystic fibrosis transmembrane conductance regulator-mediated chloride secretion

Cystic fibrosis (CF) is an autosomal genetic disease associated with impaired epithelial ion transport. Mutations in the CF gene alter the primary sequence of the CF transmembrane conductance regulator (CFTR). Several therapeutic modalities have been proposed for CF patients, including the phytoestrogen genistein. Experiments were completed in cellular and subcellular systems to evaluate the impact of naturally occurring and synthetic estrogens on epithelial ion transport, and specifically on the CF protein CFTR. 17beta-Estradiol, a naturally occurring estrogen, caused a rapid and reversible inhibition of forskolin-stimulated chloride secretion across T84 epithelial cell monolayers with a K(i) of 8 microM. In addition, 17alpha-estradiol, a stereoisomer that fails to bind and activate nuclear estrogen receptors was equipotent with 17beta-estradiol, arguing against a genomic-mediated mechanism of action. Synthetic estrogens, including diethylstilbesterol and the antiestrogen tamoxifen likewise inhibited forskolin-stimulated ion transport. Aldosterone, dexamethasone, and cholesterol were without effect at the highest concentrations tested (>/=1 mM). Studies indicated that diethylstilbesterol and other synthetic estrogens that inhibited anion secretion in intact monolayers likewise inhibited CFTR chloride channel activity with similar concentration dependencies in excised membrane patches. Experiments with radioactive photoactivatable estrogen derivatives demonstrated that these compounds bind directly to CFTR expressed in insect cells. Taken together, the data suggest that estrogens can interact directly with CFTR to alter anion transport.

Pharmacological modulation of ion transport across wild-type and DeltaF508 CFTR-expressing human bronchial epithelia

Forskolin, UTP, 1-ethyl-2-benzimidazolinone (1-EBIO), NS004, 8-methoxypsoralen (Methoxsalen; 8-MOP), and genistein were evaluated for their effects on ion transport across primary cultures of human bronchial epithelium (HBE) expressing wild-type (wt HBE) and DeltaF508 (DeltaF-HBE) cystic fibrosis transmembrane conductance regulator. In wt HBE, the baseline short-circuit current (I(sc)) averaged 27.0 +/- 0.6 microA/cm(2) (n = 350). Amiloride reduced this I(sc) by 13.5 +/- 0.5 microA/cm(2) (n = 317). In DeltaF-HBE, baseline I(sc) was 33.8 +/- 1.2 microA/cm(2) (n = 200), and amiloride reduced this by 29.6 +/- 1.5 microA/cm(2) (n = 116), demonstrating the characteristic hyperabsorption of Na(+) associated with cystic fibrosis (CF). In wt HBE, subsequent to amiloride, forskolin induced a sustained, bumetanide-sensitive I(sc) (DeltaI(sc) = 8.4 +/- 0.8 microA/cm(2); n = 119). Addition of acetazolamide, 5-(N-ethyl-N-isopropyl)-amiloride, and serosal 4, 4'-dinitrostilben-2,2'-disulfonic acid further reduced I(sc), suggesting forskolin also stimulates HCO(3)(-) secretion. This was confirmed by ion substitution studies. The forskolin-induced I(sc) was inhibited by 293B, Ba(2+), clofilium, and quinine, whereas charybdotoxin was without effect. In DeltaF-HBE the forskolin I(sc) response was reduced to 1.2 +/- 0.3 microA/cm(2) (n = 30). In wt HBE, mucosal UTP induced a transient increase in I(sc) (Delta I(sc) = 15. 5 +/- 1.1 microA/cm(2); n = 44) followed by a sustained plateau, whereas in DeltaF-HBE the increase in I(sc) was reduced to 5.8 +/- 0. 7 microA/cm(2) (n = 13). In wt HBE, 1-EBIO, NS004, 8-MOP, and genistein increased I(sc) by 11.6 +/- 0.9 (n = 20), 10.8 +/- 1.7 (n = 18), 10.0 +/- 1.6 (n = 5), and 7.9 +/- 0.8 microA/cm(2) (n = 17), respectively. In DeltaF-HBE, 1-EBIO, NS004, and 8-MOP failed to stimulate Cl(-) secretion. However, addition of NS004 subsequent to forskolin induced a sustained Cl(-) secretory response (2.1 +/- 0.3 microA/cm(2), n = 21). In DeltaF-HBE, genistein alone stimulated Cl(-) secretion (2.5 +/- 0.5 microA/cm(2), n = 11). After incubation of DeltaF-HBE at 26 degrees C for 24 h, the responses to 1-EBIO, NS004, and genistein were all potentiated. 1-EBIO and genistein increased Na(+) absorption across DeltaF-HBE, whereas NS004 and 8-MOP had no effect. Finally, Ca(2+)-, but not cAMP-mediated agonists, stimulated K(+) secretion across both wt HBE and DeltaF-HBE in a glibenclamide-dependent fashion. Our results demonstrate that pharmacological agents directed at both basolateral K(+) and apical Cl(-) conductances directly modulate Cl(-) secretion across HBE, indicating they may be useful in ameliorating the ion transport defect associated with CF.

Stimulation of Cl(-) secretion by chlorzoxazone

We previously demonstrated that 1-ethyl-2-benzimidazolone (1-EBIO) directly activates basolateral membrane calcium-activated K(+) channels (K(Ca)), thereby stimulating Cl(-) secretion across several epithelia. In our pursuit to identify potent modulators of Cl(-) secretion that may be useful to overcome the Cl(-) secretory defect in cystic fibrosis (CF), we have identified chlorzoxazone [5-chloro-2(3H)-benzoxazolone], a clinically used centrally acting muscle relaxant, as a stimulator of Cl(-) secretion in several epithelial cell types, including T84, Calu-3, and human bronchial epithelium. The Cl(-) secretory response induced by chlorzoxazone was blocked by charybdotoxin (CTX), a known blocker of K(Ca). In nystatin-permeabilized monolayers, chlorzoxazone stimulated a basolateral membrane I(K), which was inhibited by CTX and also stimulated an apical I(Cl) that was inhibited by glibenclamide, indicating that the G(Cl) responsible for this I(Cl) may be cystic fibrosis transmembrane conductance regulator (CFTR). In membrane vesicles prepared from T84 cells, chlorzoxazone stimulated (86)Rb(+) uptake in a CTX-sensitive manner. In excised, inside-out patches, chlorzoxazone activated an inwardly-rectifying K(+) channel, which was inhibited by CTX. 6-Hydroxychlorzoxazone, the major metabolite of chlorzoxazone, did not activate K(Ca), whereas zoxazolamine (2-amino-5-chlorzoxazole) showed a similar response profile as chlorzoxazone. In normal human nasal epithelium, chlorzoxazone elicited hyperpolarization of the potential difference that was similar in magnitude to isoproterenol. However, in the nasal epithelium of CF patients with the DeltaF508 mutation of CFTR, there was no detectable Cl(-) secretory response to chlorzoxazone. These studies demonstrate that chlorzoxazone stimulates transepithelial Cl(-) secretion in normal airway epithelium in vitro and in vivo, and suggest that stimulation requires functional CFTR in the epithelia.