Regulation of CFTR Cl- channel gating by ATP binding and hydrolysis

Molecular dynamics study of Cl- permeation through cystic fibrosis transmembrane conductance regulator (CFTR)

The recent elucidation of atomistic structures of Cl^- channel CFTR provides opportunities for understanding the molecular basis of cystic fibrosis. Despite having been activated through phosphorylation and provided with ATP ligands, several near-atomistic cryo-EM structures of CFTR are in a closed state, as inferred from the lack of a continuous passage through a hydrophobic bottleneck region located in the extracellular portion of the pore. Here, we present repeated, microsecond-long molecular dynamics simulations of human CFTR solvated in a lipid bilayer and aqueous NaCl. At equilibrium, Cl^- ions enter the channel through a lateral intracellular portal and bind to two distinct cationic sites inside the channel pore but do not traverse the narrow, de-wetted bottleneck. Simulations conducted in the presence of a strong hyperpolarizing electric field led to spontaneous Cl^- translocation events through the bottleneck region of the channel, suggesting that the protein relaxed to a functionally open state. Conformational changes of small magnitude involving transmembrane helices 1 and 6 preceded ion permeation through diverging exit routes at the extracellular end of the pore. The pore bottleneck undergoes wetting prior to Cl^- translocation, suggesting that it acts as a hydrophobic gate. Although permeating Cl^- ions remain mostly hydrated, partial dehydration occurs at the binding sites and in the bottleneck. The observed Cl^- pathway is largely consistent with the loci of mutations that alter channel conductance, anion binding, and ion selectivity, supporting the model of the open state of CFTR obtained in the present study.

Pharmacological Effects of Panduratin A on Renal Cyst Development in In Vitro and In Vivo Models of Polycystic Kidney Disease

Renal cyst expansion in polycystic kidney disease (PKD) involves abnormalities in both cyst-lining-cell proliferation and fluid accumulation. Suppression of these processes may retard the progression of PKD. Evidence suggests that the activation of 5' AMP-activated protein kinase (AMPK) inhibits cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride secretion, leading to reduced progression of PKD. Here we investigated the pharmacological effects of panduratin A, a bioactive compound known as an AMPK activator, on CFTR-mediated chloride secretion and renal cyst development using in vitro and animal models of PKD. We demonstrated that AMPK was activated in immortalized normal renal cells and autosomal dominant polycystic kidney disease (ADPKD) cells following treatment with panduratin A. Treatment with panduratin A reduced the number of renal cyst colonies corresponding with a decrease in cell proliferation and phosphorylated p70/S6K, a downstream target of mTOR signaling. Additionally, panduratin A slowed cyst expansion via inhibition of the protein expression and transport function of CFTR. In heterozygous Han:Sprague-Dawley (Cy/+) rats, an animal model of PKD, intraperitoneal administration of panduratin A (25 mg/kg BW) for 5 weeks significantly decreased the kidney weight per body weight ratios and the cystic index. Panduratin A also reduced collagen deposition in renal tissue. Intraperitoneal administration of panduratin A caused abdominal bleeding and reduced body weight. However, 25 mg/kg BW of panduratin A via oral administration in the PCK rats, another non-orthologous PKD model, showed a significant decrease in the cystic index without severe adverse effects, indicating that the route of administration is critical in preventing adverse effects while still slowing disease progression. These findings reveal that panduratin A might hold therapeutic properties for the treatment of PKD.

Protein kinase A phosphorylation potentiates cystic fibrosis transmembrane conductance regulator gating by relieving autoinhibition on the stimulatory C terminus of the regulatory domain

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel activated by protein kinase A (PKA) phosphorylation on the regulatory (R) domain. Phosphorylation at several R domain residues stimulates ATP-dependent channel openings and closings, termed channel gating. To explore the protein segment responsible for channel potentiation and PKA-dependent activation, deletion mutations were constructed by removing one to three protein segments of the R domain including residues 708-759 (ΔR_708-759), R_760-783, and R_784-835, each of which contains one or two PKA phosphorylation sites. Deletion of R_708-759 or R_760-783 had little effect on CFTR gating, whereas all mutations lacking R_784-835 reduced CFTR activity by decreasing the mean burst duration and increasing the interburst interval (IBI). The data suggest that R_784-835 plays a major role in stimulating CFTR gating. For ATP-associated regulation, ΔR_784-835 had minor impact on gating potentiation by 2'dATP, CaATP, and pyrophosphate. Interestingly, introducing a phosphorylated peptide matching R_809-835 shortened the IBI of ΔR_708-835-CFTR. Consistently, ΔR_815-835, but not ΔR_784-814, enhanced IBI, whereas both reduced mean burst duration. These data suggest that the entirety of R_784-835 is required for stabilizing the open state of CFTR; however, R_815-835, through interactions with the channel, is dominant for enhancing the opening rate. Of note, PKA markedly decreased the IBI of ΔR_708-783-CFTR. Conversely, the IBI of ΔR_708-814-CFTR was short and PKA-independent. These data reveal that for stimulating CFTR gating, PKA phosphorylation may relieve R_784-814-mediated autoinhibition that prevents IBI shortening by R_815-835 This mechanism may elucidate how the R domain potentiates channel gating and may unveil CFTR stimulation by other protein kinases.

The structural basis of cystic fibrosis

CFTR (ABCC7) is a phospho-regulated chloride channel that is found in the apical membranes of epithelial cells, is gated by ATP and the activity of the protein is crucial in the homeostasis of the extracellular liquid layer in many organs [Annu. Rev. Biochem. (2008) 77, 701-726; Science (1989) 245, 1066-1073]. Mutations in CFTR cause the inherited disease cystic fibrosis (CF), the most common inherited condition in humans of European descent [Science (1989) 245, 1066-1073; Pflugers Arch. (2007) 453, 555-567]. The structural basis of CF will be discussed in this article.

Glucose-Sensitive CFTR Suppresses Glucagon Secretion by Potentiating KATP Channels in Pancreatic Islet α Cells

The secretion of glucagon by islet α cells is normally suppressed by high blood glucose, but this suppressibility is impaired in patients with diabetes or cystic fibrosis (CF), a disease caused by mutations in the gene encoding CF transmembrane conductance regulator (CFTR), a cyclic adenosine monophosphate-activated Cl- channel. However, precisely how glucose regulates glucagon release remains controversial. Here we report that elevated glucagon secretion, together with increased glucose-induced membrane depolarization and Ca2+ response, is found in CFTR mutant (DF508) mice/islets compared with the wild-type. Overexpression of CFTR in AlphaTC1-9 cells results in membrane hyperpolarization and reduced glucagon release, which can be reversed by CFTR inhibition. CFTR is found to potentiate the adenosine triphosphate-sensitive K+ (KATP) channel because membrane depolarization and whole-cell currents sensitive to KATP blockers are significantly greater in wild-type/CFTR-overexpressed α cells compared with that in DF508/non-overexpressed cells. KATP knockdown also reverses the suppressive effect of CFTR overexpression on glucagon secretion. The results reveal that by potentiating KATP channels, CFTR acts as a glucose-sensing negative regulator of glucagon secretion in α cells, a defect of which may contribute to glucose intolerance in CF and other types of diabetes.

The safety of lumacaftor and ivacaftor for the treatment of cystic fibrosis

INTRODUCTION

Lumacaftor-ivacaftor is indicated for treatment of cystic fibrosis (CF) in patients homozygous for the Phe-508del cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations. In clinical trials, treated patients showed improved pulmonary function, reduced pulmonary exacerbations, and other benefits. This article reviews safety of this therapy. Areas covered: Safety findings in ivacaftor, lumacaftor and combined therapy trials, and reported subsequently through post-approval evaluation, were accessed by PubMed and Google searches using key words 'VX-770', 'ivacaftor', 'VX-809', and 'lumacaftor'. Transaminitis was seen in ivacaftor and combination trials. Non-congenital cataracts were seen in pre-clinical animal studies and in children taking ivacaftor and combined therapy. Dyspnea occurs in some patients taking lumacaftor and combined therapy and usually resolves without stopping treatment. Lumacaftor is a strong inducer of CYP3A while ivacaftor is a CYP3A sensitive substrate. Combination therapy can decrease systemic exposure of medications that are substrates of CYP3A, decreasing therapeutic effect. Co-administration of lumacaftor-ivacaftor with sensitive CYP3A substrates or CYP3A substrates with narrow therapeutic index is not recommended. Expert opinion: Lumacaftor-ivacaftor therapy may be associated with ocular and hepatic side effects. Specific recommendations for monitoring are available. Dyspnea occurs, especially during initiation of treatment. Potential drug interactions should be evaluated in patients taking combination therapy. The risk benefit ratio of lumacaftor-ivacaftor favors therapy.

Altering intracellular pH reveals the kinetic basis of intraburst gating in the CFTR Cl- channel

KEY POINTS

The cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in the genetic disease cystic fibrosis (CF), forms a gated pathway for chloride movement regulated by intracellular ATP. To understand better CFTR function, we investigated the regulation of channel openings by intracellular pH. We found that short-lived channel closures during channel openings represent subtle changes in the structure of CFTR that are regulated by intracellular pH, in part, at ATP-binding site 1 formed by the nucleotide-binding domains. Our results provide a framework for future studies to understand better the regulation of channel openings, the dysfunction of CFTR in CF and the action of drugs that repair CFTR gating defects.

ABSTRACT

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated Cl^- channel defective in the genetic disease cystic fibrosis (CF). The gating behaviour of CFTR is characterized by bursts of channel openings interrupted by brief, flickery closures, separated by long closures between bursts. Entry to and exit from an open burst is controlled by the interaction of ATP with two ATP-binding sites, sites 1 and 2, in CFTR. To understand better the kinetic basis of CFTR intraburst gating, we investigated the single-channel activity of human CFTR at different intracellular pH (pH_i ) values. When compared with the control (pH_i 7.3), acidifying pH_i to 6.3 or alkalinizing pH_i to 8.3 and 8.8 caused small reductions in the open-time constant (τ_o ) of wild-type CFTR. By contrast, the fast closed-time constant (τ_cf ), which describes the short-lived closures that interrupt open bursts, was greatly increased at pH_i 5.8 and 6.3. To analyse intraburst kinetics, we used linear three-state gating schemes. All data were satisfactorily modelled by the C₁ ↔ O ↔ C₂ kinetic scheme. Changing the intracellular ATP concentration was without effect on τ_o , τ_cf and their responses to pH_i changes. However, mutations that disrupt the interaction of ATP with ATP-binding site 1, including K464A, D572N and the CF-associated mutation G1349D all abolished the prolongation of τ_cf at pH_i 6.3. Taken together, our data suggest that the regulation of CFTR intraburst gating is distinct from the ATP-dependent mechanism that controls channel opening and closing. However, our data also suggest that ATP-binding site 1 modulates intraburst gating.

Function and regulation of TRPM7, as well as intracellular magnesium content, are altered in cells expressing ΔF508-CFTR and G551D-CFTR

Cystic fibrosis (CF), one of the most common fatal hereditary disorders, is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The CFTR gene product is a multidomain adenosine triphosphate-binding cassette (ABC) protein that functions as a chloride (Cl(-)) channel that is regulated by intracellular magnesium [Mg(2+)]i. The most common mutations in CFTR are a deletion of a phenylalanine residue at position 508 (ΔF508-CFTR, 70-80 % of CF phenotypes) and a Gly551Asp substitution (G551D-CFTR, 4-5 % of alleles), which lead to decreased or almost abolished Cl(-) channel function, respectively. Magnesium ions have to be finely regulated within cells for optimal expression and function of CFTR. Therefore, the melastatin-like transient receptor potential cation channel, subfamily M, member 7 (TRPM7), which is responsible for Mg(2+) entry, was studies and [Mg(2+)]i measured in cells stably expressing wildtype CFTR, and two mutant proteins (ΔF508-CFTR and G551D-CFTR). This study shows for the first time that [Mg(2+)]i is decreased in cells expressing ΔF508-CFTR and G551D-CFTR mutated proteins. It was also observed that the expression of the TRPM7 protein is increased; however, membrane localization was altered for both ΔF508del-CFTR and G551D-CFTR. Furthermore, both the function and regulation of the TRPM7 channel regarding Mg(2+) is decreased in the cells expressing the mutated CFTR. Ca(2+) influx via TRPM7 were also modified in cells expressing a mutated CFTR. Therefore, there appears to be a direct involvement of TRPM7 in CF physiopathology. Finally, we propose that the TRPM7 activator Naltriben is a new potentiator for G551D-CFTR as the function of this mutant increases upon activation of TRPM7 by Naltriben.

Characterization of mitochondrial function in cells with impaired cystic fibrosis transmembrane conductance regulator (CFTR) function

Evidence supporting the occurrence of oxidative stress in Cystic Fibrosis (CF) is well established and the literature suggests that oxidative stress is inseparably linked to mitochondrial dysfunction. Here, we have characterized mitochondrial function, in particular as it regards the steps of oxidative phosphorylation and ROS production, in airway cells either homozygous for the F508del-CFTR allele or stably expressing wt-CFTR. We find that oxygen consumption, ΔΨ generation, adenine nucleotide translocator-dependent ADP/ATP exchange and both mitochondrial Complex I and IV activities are impaired in CF cells, while both mitochondrial ROS production and membrane lipid peroxidation increase. Importantly, treatment of CF cells with the small molecules VX-809 and 4,6,4'-trimethylangelicin, which act as "correctors" for F508del CFTR by rescuing the F508del CFTR-dependent chloride secretion, while having no effect per sè on mitochondrial function in wt-CFTR cells, significantly improved all the above mitochondrial parameters towards values found in the airway cells expressing wt-CFTR. This novel study on mitochondrial bioenergetics provides a springboard for future research to further understand the molecular mechanisms responsible for the involvement of mitochondria in CF and identify the proteins primarily responsible for the F508del-CFTR-dependent mitochondrial impairment and thus reveal potential novel targets for CF therapy.

Lumacaftor and ivacaftor in the management of patients with cystic fibrosis: current evidence and future prospects

Cystic fibrosis (CF) is a genetic disorder that causes multiorgan morbidity and premature death, most commonly from pulmonary dysfunction. Mutations in the CF transmembrane conductance regulator (CFTR) gene, of which almost 2000 have been described, result in a dysfunctional CFTR protein. This protein is an adenosine triphosphate binding anion channel, present primarily at the surface of epithelial cells. Loss of function mutations in this anion channel result in decreased or absent chloride/bicarbonate transport. The subsequent abnormal salt and water transport at epithelial cell surfaces leads to thickened secretions, and infection or inflammation in affected organs. In the last 20 years, therapeutics have been developed to treat the signs and symptoms of CF. However, in 2012, the small molecule drug, ivacaftor, became the first approved therapy that addresses the basic defect in CF. Ivacaftor is a potentiator of CFTR channels defective in their chloride/bicarbonate gating/conductance, but present at the epithelial cell surface. It is only approved for 10 mutations carried by approximately 7% of the population of patients with CF. F508del is the most common CFTR mutation, present in homozygosity in approximately 50% of patients with CF. The F508del mutation results in multiple CFTR channel defects that require both correction (stabilization of misfolded CFTR and trafficking to the epithelial cell membrane) and potentiation. This article reviews the in vitro and clinical trial data for the potential use of the potentiator, ivacaftor, and the corrector, lumacaftor, in patients with CF.

Cystic fibrosis-related oxidative stress and intestinal lipid disorders

SIGNIFICANCE

Cystic fibrosis (CF) is the most common lethal genetic disorder in the Caucasian people. It is due to the mutation of cystic fibrosis transmembrane conductance regulator (CFTR) gene located on the long arm of the chromosome 7, which encodes for CFTR protein. The latter, an adenosine triphosphate binding cassette, is a transmembrane chloride channel that is also involved in glutathione transport. As glutathione/glutathione disulfide constitutes the most important pool of cellular redox systems, CFTR defects could thus disrupt the intracellular redox balance. Resulting multisystemic diseases are essentially characterized by a chronic respiratory failure, a pancreatic insufficiency, an essential fatty acid deficiency (EFAD), and inadequate levels of antioxidant vitamins.

RECENT ADVANCES

The pathophysiology of CF is complex; however, several mechanisms are proposed, including oxidative stress (OxS) whose implication is recognized and has been clearly demonstrated in CF airways.

CRITICAL ISSUES

Little is known about OxS intrinsic triggers and its own involvement in intestinal lipid disorders. Despite the regular administration of pancreatic supplements, high-fat high-calorie diets, and antioxidant fat-soluble vitamins, there is a persistence of steatorrhea, EFAD, and harmful OxS. Intriguingly, several trials with elevated doses of antioxidant vitamins have not yielded significant improvements.

FUTURE DIRECTIONS

The main sources and self-maintenance of OxS in CF should be clarified to improve treatment of patients. Therefore, this review will discuss the potential sources and study the mechanisms of OxS in the intestine, known to develop various complications, and its involvement in intestinal lipid disorders in CF patients.

CFTR structure and cystic fibrosis

CFTR (cystic fibrosis transmembrane conductance regulator) is a member of the ATP-binding cassette family of membrane proteins. Although almost all members of this family are transporters, CFTR functions as a channel with specificity for anions, in particular chloride and bicarbonate. In this review we look at what is known about CFTR structure and function within the context of the ATP-binding cassette family. We also review current strategies aimed at obtaining the high resolution structure of the protein.

TRPM7

The channel kinases TRPM6 and TRPM7 are fusion proteins with an ion transport domain and an enzymatically active kinase domain. TRPM7 has been found in every mammalian tissue investigated to date. The two-in-one protein structure, the ubiquitous expression profile, and the protein's unique biophysical characteristics that enable divalent ion transport involve TRPM7 in a plethora of (patho)physiological processes. With its prominent role in cellular and systemic magnesium homeostasis, TRPM7 emerges as a key player in embryonic development, global ischemia, cardiovascular disease, and cancer.

Intracellular ATP binding is required to activate the slowly activating K+ channel I(Ks)

Gating of ion channels by ligands is fundamental to cellular function, and ATP serves as both an energy source and a signaling molecule that modulates ion channel and transporter functions. The slowly activating K(+) channel I(Ks) in cardiac myocytes is formed by KCNQ1 and KCNE1 subunits that conduct K(+) to repolarize the action potential. Here we show that intracellular ATP activates heterologously coexpressed KCNQ1 and KCNE1 as well as I(Ks) in cardiac myocytes by directly binding to the C terminus of KCNQ1 to allow the pore to open. The channel is most sensitive to ATP near its physiological concentration, and lowering ATP concentration in cardiac myocytes results in I(Ks) reduction and action potential prolongation. Multiple mutations that suppress I(Ks) by decreasing the ATP sensitivity of the channel are associated with the long QT (interval between the Q and T waves in electrocardiogram) syndrome that predisposes afflicted individuals to cardiac arrhythmia and sudden death. A cluster of basic and aromatic residues that may form a unique ATP binding site are identified; ATP activation of the wild-type channel and the effects of the mutations on ATP sensitivity are consistent with an allosteric mechanism. These results demonstrate the activation of an ion channel by intracellular ATP binding, and ATP-dependent gating allows I(Ks) to couple myocyte energy state to its electrophysiology in physiologic and pathologic conditions.

VX-809 corrects folding defects in cystic fibrosis transmembrane conductance regulator protein through action on membrane-spanning domain 1

Misfolding of cystic fibrosis transmembrane conductance regulator protein (CFTR) causes the fatal lung disease cystic fibrosis. VX-809 was developed to suppress disease-related folding defects in CFTR. VX-809 suppresses folding defects in CFTR by modulating the conformation of membrane-spanning domain 1. VX-808 is thereby able to partially restore function to F508del-CFTR and other disease-related mutants.

Cystic fibrosis (CF) is a fatal genetic disorder associated with defective hydration of lung airways due to the loss of chloride transport through the CF transmembrane conductance regulator protein (CFTR). CFTR contains two membrane-spanning domains (MSDs), two nucleotide-binding domains (NBDs), and a regulatory domain, and its channel assembly requires multiple interdomain contacts. The most common CF-causing mutation, F508del, occurs in NBD1 and results in misfolding and premature degradation of F508del-CFTR. VX-809 is an investigational CFTR corrector that partially restores CFTR function in people who are homozygous for F508del-CFTR. To identify the folding defect(s) in F508del-CFTR that must be repaired to treat CF, we explored the mechanism of VX-809 action. VX-809 stabilized an N-terminal domain in CFTR that contains only MSD1 and efficaciously restored function to CFTR forms that have missense mutations in MSD1. The action of VX-809 on MSD1 appears to suppress folding defects in F508del-CFTR by enhancing interactions among the NBD1, MSD1, and MSD2 domains. The ability of VX-809 to correct F508del-CFTR is enhanced when combined with mutations that improve F508del-NBD1 interaction with MSD2. These data suggest that the use of VX-809 in combination with an additional CFTR corrector that suppresses folding defects downstream of MSD1 may further enhance CFTR function in people with F508del-CFTR.

Electrophysiological evidence for the presence of cystic fibrosis transmembrane conductance regulator (CFTR) in mouse sperm

Mammalian sperm must undergo a maturational process, named capacitation, in the female reproductive tract to fertilize the egg. Sperm capacitation is regulated by a cAMP/protein kinase A (PKA) pathway and involves increases in intracellular Ca(2+), pH, Cl(-), protein tyrosine phosphorylation, and in mouse and some other mammals a membrane potential hyperpolarization. The cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) channel modulated by cAMP/PKA and ATP, was detected in mammalian sperm and proposed to modulate capacitation. Our whole-cell patch-clamp recordings from testicular mouse sperm now reveal a Cl(-) selective component to membrane current that is ATP-dependent, stimulated by cAMP, cGMP, and genistein (a CFTR agonist, at low concentrations), and inhibited by DPC and CFTR(inh) -172, two well-known CFTR antagonists. Furthermore, the Cl(-) current component activated by cAMP and inhibited by CFTR(inh) -172 is absent in recordings on testicular sperm from mice possessing the CFTR ΔF508 loss-of-function mutation, indicating that CFTR is responsible for this component. A Cl(-) selective like current component displaying CFTR characteristics was also found in wild type epididymal sperm bearing the cytoplasmatic droplet. Capacitated sperm treated with CFTR(inh) -172 undergo a shape change, suggesting that CFTR is involved in cell volume regulation. These findings indicate that functional CFTR channels are present in mouse sperm and their biophysical properties are consistent with their proposed participation in capacitation.

Calcium-modulated chloride pathways contribute to chloride flux in murine cystic fibrosis-affected macrophages

Cystic fibrosis (CF), a common lethal inherited disorder defined by ion transport abnormalities, chronic infection, and robust inflammation, is the result of mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a cAMP-activated chloride (Cl-) channel. Macrophages are reported to have impaired activity in CF. Previous studies suggest that Cl- transport is important for macrophage function; therefore, impaired Cl- secretion may underlie CF macrophage dysfunction. To determine whether alterations in Cl- transport exist in CF macrophages, Cl- efflux was measured using N-[ethoxycarbonylmethyl]- 6-methoxy-quinolinium bromide (MQAE), a fluorescent indicator dye. The contribution of CFTR was assessed by calculating Cl- flux in the presence and absence of cftr(inh)-172. The contribution of calcium (Ca(2+))-modulated Cl- pathways was assessed by examining Cl- flux with varied extracellular Ca(2+) concentrations or after treatment with carbachol or thapsigargin, agents that increase intracellular Ca(2+) levels. Our data demonstrate that CFTR contributed to Cl- efflux only in WT macrophages, while Ca(2+)-mediated pathways contributed to Cl- transport in CF and WT macrophages. Furthermore, CF macrophages demonstrated augmented Cl- efflux with increases in extracellular Ca(2+). Taken together, this suggests that Ca(2+)-mediated Cl- pathways are enhanced in CF macrophages compared with WT macrophages.

State-dependent regulation of cystic fibrosis transmembrane conductance regulator (CFTR) gating by a high affinity Fe3+ bridge between the regulatory domain and cytoplasmic loop 3

The unique regulatory (R) domain differentiates the human CFTR channel from other ATP-binding cassette transporters and exerts multiple effects on channel function. However, the underlying mechanisms are unclear. Here, an intracellular high affinity (2.3 × 10(-19) M) Fe(3+) bridge is reported as a novel approach to regulating channel gating. It inhibited CFTR activity by primarily reducing an open probability and an opening rate, and inhibition was reversed by EDTA and phenanthroline. His-950, His-954, Cys-832, His-775, and Asp-836 were found essential for inhibition and phosphorylated Ser-768 may enhance Fe(3+) binding. More importantly, inhibition by Fe(3+) was state-dependent. Sensitivity to Fe(3+) was reduced when the channel was locked in an open state by AMP-PNP. Similarly, a K978C mutation from cytoplasmic loop 3 (CL3), which promotes ATP-independent channel opening, greatly weakened inhibition by Fe(3+) no matter whether NBD2 was present or not. Therefore, although ATP binding-induced dimerization of NBD1-NBD2 is required for channel gating, regulation of CFTR activity by Fe(3+) may involve an interaction between the R domain and CL3. These findings may support proximity of the R domain to the cytoplasmic loops. They also suggest that Fe(3+) homeostasis may play a critical role in regulating pathophysiological CFTR activity because dysregulation of this protein causes cystic fibrosis, secretary diarrhea, and infertility.

State-dependent modulation of CFTR gating by pyrophosphate

Cystic fibrosis transmembrane conductance regulator (CFTR) is an adenosine triphosphate (ATP)-gated chloride channel. ATP-induced dimerization of CFTR's two nucleotide-binding domains (NBDs) has been shown to reflect the channel open state, whereas hydrolysis of ATP is associated with channel closure. Pyrophosphate (PPi), like nonhydrolytic ATP analogues, is known to lock open the CFTR channel for tens of seconds when applied with ATP. Here, we demonstrate that PPi by itself opens the CFTR channel in a Mg²⁺-dependent manner long after ATP is removed from the cytoplasmic side of excised membrane patches. However, the short-lived open state (τ ∼1.5 s) induced by MgPPi suggests that MgPPi alone does not support a stable NBD dimer configuration. Surprisingly, MgPPi elicits long-lasting opening events (τ ∼30 s) when administrated shortly after the closure of ATP-opened channels. These results indicate the presence of two different closed states (C₁ and C₂) upon channel closure and a state-dependent effect of MgPPi on CFTR gating. The relative amount of channels entering MgPPi-induced long-open bursts during the ATP washout phase decreases over time, indicating a time-dependent dissipation of the closed state (C₂) that can be locked open by MgPPi. The stability of the C₂ state is enhanced when the channel is initially opened by N⁶-phenylethyl-ATP, a high affinity ATP analogue, but attenuated by W401G mutation, which likely weakens ATP binding to NBD1, suggesting that an ATP molecule remains bound to the NBD1 site in the C₂ state. Taking advantage of the slow opening rate of Y1219G-CFTR, we are able to identify a C₂-equivalent state (C₂*), which exists before the channel in the C₁ state is opened by ATP. This closed state responds to MgPPi much more inefficiently than the C₂ state. Finally, we show that MgAMP-PNP exerts its effects on CFTR gating via a similar mechanism as MgPPi. The structural and functional significance of our findings is discussed.

Direct sensing of intracellular pH by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel

In cystic fibrosis (CF), dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel disrupts epithelial ion transport and perturbs the regulation of intracellular pH (pH(i)). CFTR modulates pH(i) through its role as an ion channel and by regulating transport proteins. However, it is unknown how CFTR senses pH(i). Here, we investigate the direct effects of pH(i) on recombinant CFTR using excised membrane patches. By altering channel gating, acidic pH(i) increased the open probability (P(o)) of wild-type CFTR, whereas alkaline pH(i) decreased P(o) and inhibited Cl(-) flow through the channel. Acidic pH(i) potentiated the MgATP dependence of wild-type CFTR by increasing MgATP affinity and enhancing channel activity, whereas alkaline pH(i) inhibited the MgATP dependence of wild-type CFTR by decreasing channel activity. Because these data suggest that pH(i) modulates the interaction of MgATP with the nucleotide-binding domains (NBDs) of CFTR, we examined the pH(i) dependence of site-directed mutations in the two ATP-binding sites of CFTR that are located at the NBD1:NBD2 dimer interface (site 1: K464A-, D572N-, and G1349D-CFTR; site 2: G551D-, K1250M-, and D1370N-CFTR). Site 2 mutants, but not site 1 mutants, perturbed both potentiation by acidic pH(i) and inhibition by alkaline pH(i), suggesting that site 2 is a critical determinant of the pH(i) sensitivity of CFTR. The effects of pH(i) also suggest that site 2 might employ substrate-assisted catalysis to ensure that ATP hydrolysis follows NBD dimerization. We conclude that the CFTR Cl(-) channel senses directly pH(i). The direct regulation of CFTR by pH(i) has important implications for the regulation of epithelial ion transport.

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.

A mathematical model of calcium-induced fluid secretion in airway epithelium

Regulation of periciliary liquid (PCL) depth is of central importance to mucociliary clearance by the airway epithelium. Without adequate hydration mucociliary transport would cease, leading to build up of mucus in the airways, and impairing the clearance of any trapped inhaled particulates. Airway epithelial cells are known to release ATP under a number of stress conditions. Cell surface receptors bind ATP and trigger an intracellular calcium response which regulates the gating of specific ion channels on the apical and basolateral cell membranes. This shifts the electrochemical balance, resulting in the accumulation of Na(+) and Cl(-) in the periciliary liquid, and providing an osmotic driving force for water flux. In this study, we present a mathematical model of a single airway epithelial cell which describes the fluid secretion elicited after a rise in intracellular calcium. The model provides a basis to quantitatively analyse the influence of intracellular calcium signalling on fluid movement. The model demonstrates behaviour consistent with a number of experimental data on manipulating periciliary liquid volume and tonicity, and provides a quantitative basis for analysing the role of the different membrane ion channels in determining water flux following different physiological stimuli.

Glutathione and its transporters in ocular surface defense

Glutathione (GSH) is an abundant antioxidant ubiquitous in nearly all cell types. Deficiency of GSH has been linked to ocular disease and viral infection. Other established vital roles of GSH include detoxification and immunoprotection. Endogenous GSH plays a protagonist's role in safeguarding active transport processes compartmentalized at the interface between conjunctival mucosa and the tear film. Optimal electrokinetic transport across the conjunctival epithelium requires the mucosal presence of GSH. Glutathione is the most abundant known endogenous antioxidant molecule in tear fluid, mainly derived from conjunctival secretion. Conjunctival GSH transport, a major kinetic component of GSH turnover, occurs through multiple functionally distinct mechanisms. Cell membrane potential regulates conjunctival GSH efflux, while conjunctival GSH uptake requires extracellular Na(+). Significant modulation of GSH, its constituent amino acids, and functions of associated transporters occurs in the conjunctival epithelium with viral inflammatory disease. Topical conjunctival delivery of GSH, its metabolic precursors, or pharmacologic stimulation of endogenous conjunctival GSH secretion carry potential in alleviating viral-inflammatory conjunctivitis.

A mutation in CFTR modifies the effects of the adenylate kinase inhibitor Ap5A on channel gating

Mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. The CFTR anion channel is controlled by ATP binding and enzymatic activity at the two nucleotide-binding domains. CFTR exhibits two types of enzymatic activity: 1), ATPase activity in the presence of ATP and 2), adenylate kinase activity in the presence of ATP plus physiologic concentrations of AMP or ADP. Previous work showed that P(1),P(5)-di(adenosine-5')pentaphosphate (Ap(5)A), a specific adenylate kinases inhibitor, inhibited wild-type CFTR. In this study, we report that Ap(5)A increased activity of CFTR with an L1254A mutation. This mutation increased the EC50 for ATP by >10-fold and reduced channel activity by prolonging the closed state. Ap(5)A did not elicit current on its own nor did it alter ATP EC50 or maximal current. However, it changed the relationship between ATP concentration and current. At submaximal ATP concentrations, Ap(5)A stimulated current by stabilizing the channel open state. Whereas previous work indicated that adenylate kinase activity regulated channel opening, our data suggest that Ap(5)A binding may also influence channel closing. These results also suggest that a better understanding of the adenylate kinase activity of CFTR may be of value in developing new therapeutic strategies for cystic fibrosis.

Plasma membrane glutathione transporters and their roles in cell physiology and pathophysiology

Reduced glutathione (GSH) is critical for many cellular processes, and both its intracellular and extracellular concentrations are tightly regulated. Intracellular GSH levels are regulated by two main mechanisms: by adjusting the rates of synthesis and of export from cells. Some of the proteins responsible for GSH export from mammalian cells have recently been identified, and there is increasing evidence that these GSH exporters are multispecific and multifunctional, regulating a number of key biological processes. In particular, some of the multidrug resistance-associated proteins (Mrp/Abcc) appear to mediate GSH export and homeostasis. The Mrp proteins mediate not only GSH efflux, but they also export oxidized glutathione derivatives (e.g., glutathione disulfide (GSSG), S-nitrosoglutathione (GS-NO), and glutathione-metal complexes), as well as other glutathione S-conjugates. The ability to export both GSH and oxidized derivatives of GSH, endows these transporters with the capacity to directly regulate the cellular thiol-redox status, and therefore the ability to influence many key signaling and biochemical pathways. Among the many processes that are influenced by the GSH transporters are apoptosis, cell proliferation, and cell differentiation. This report summarizes the evidence that Mrps contribute to the regulation of cellular GSH levels and the thiol-redox state, and thus to the many biochemical processes that are influenced by this tripeptide.

State-dependent inhibition of cystic fibrosis transmembrane conductance regulator chloride channels by a novel peptide toxin

Peptide toxins from animal venom have been used for many years for the identification and study of cation-permeable ion channels. However, no peptide toxins have been identified that interact with known anion-selective channels, including cystic fibrosis transmembrane conductance regulator (CFTR), the protein defective in cystic fibrosis and a member of the ABC transporter superfamily. Here, we describe the identification and initial characterization of a novel 3.7-kDa peptide toxin, GaTx1, which is a potent and reversible inhibitor of CFTR, acting from the cytoplasmic side of the membrane. Thus, GaTx1 is the first peptide toxin identified that inhibits a chloride channel of known molecular identity. GaTx1 exhibited high specificity, showing no effect on a panel of nine transport proteins, including Cl(-) and K(+) channels, and ABC transporters. GaTx1-mediated inhibition of CFTR channel activity is strongly state-dependent; both potency and efficacy are reduced under conditions of elevated [ATP], suggesting that GaTx1 may function as a non-competitive inhibitor of ATP-dependent channel gating. This tool will allow the application of new quantitative approaches to study CFTR structure and function, particularly with respect to the conformational changes that underlie transitions between open and closed states.

Restoration of W1282X CFTR activity by enhanced expression

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Premature termination codons represent a common minority of CFTR mutations, and are caused by base pair substitutions that produce abnormal stop codons in the coding sequence. Select aminoglycosides induce "translational readthrough" of premature stop codons and have been shown to restore full-length functional protein in a number of preclinical and clinical settings. We studied two well-described premature termination codons found in the distal open reading frame of CFTR, W1282X and R1162X, expressed in polarizing and nonpolarizing cells. Our findings indicate that W1282X CFTR-expressing cells demonstrate significantly greater CFTR activity when overexpressed compared with R1162X CFTR cells, even when truncated protein is the predominant form. In addition, our results show that the combination of stimulated expression and stop codon suppression produces additive effects on CFTR-mediated ion transport. These findings provide evidence that W1282X CFTR exhibits membrane localization and retained chloride channel function after enhanced expression, and suggest that patients harboring this mutation may be more susceptible to CFTR rescue.

Angiotensin II inhibits native bTREK-1 K+ channels through a PLC-, kinase C-, and PIP2-independent pathway requiring ATP hydrolysis

Angiotensin II (ANG II) inhibits bTREK-1 (bovine KCNK2) K(+) channels in bovine adrenocortical cells through a Gq-coupled AT(1) receptor by activation of separate Ca(2+)- and ATP hydrolysis-dependent signaling pathways. Whole cell patch-clamp recording from bovine adrenal zona fasciculata (AZF) cells was used to characterize the ATP-dependent signaling mechanism for inhibition of bTREK-1 by ANG II. We discovered that ATP-dependent inhibition of bTREK-1 by ANG II occurred through a novel mechanism that was independent of PLC and its established downstream effectors. The ATP-dependent inhibition of bTREK-1 by ANG II was not reduced by the PLC antagonists edelfosine and U73122, or by the PKC antagonists bisindolylmaleimide I (BIM) or calphostin C. bTREK-1 was partially inhibited ( approximately 25%) by the PKC activator phorbol 12,13 dibutyrate (PDBu) through an ATP-dependent mechanism that was blocked by BIM. Addition of Phosphatidylinositol(4,5) bisphosphate diC8 [DiC(8)PI(4,5)P(2)], a water-soluble derivative of phosphotidyl inositol 4,5 bisphosphate (PIP(2)) to the pipette solution failed to alter inhibition by ANG II. bTREK-1 inhibition by ANG II was also insensitive to antagonists of other protein kinases activated by ANG II in adrenocortical cells but was completely blocked by inorganic polytriphosphate PPPi. DiC(8)PI(4,5)P(2) was a weak activator of bTREK-1 channels, compared with the high-affinity ATP analog N(6)-(2-phenylethyl)adenosine-5'-O-triphosphate (6-PhEt-ATP). These results demonstrate that the modulation of bTREK-1 channels in bovine AZF cells is distinctive with respect to activation by phosphoinositides and nucleotides and inhibition by Gq-coupled receptors. Importantly, ANG II inhibits bTREK-1 channels through a novel pathway that is different from that described for inhibition of native TREK-1 channels in neurons, or cloned channels expressed in cell lines. They also indicate that, under physiological conditions, ANG II inhibits bTREK-1 and depolarizes AZF cells by two, novel, independent pathways that diverge proximal to the activation of PLC.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

CFTR (ABCC7) is a hydrolyzable-ligand-gated channel

As the product of the gene mutated in cystic fibrosis, the most common genetic disease of Caucasians, CFTR is an atypical ABC protein. From an evolutionary perspective, it is apparently a relatively young member of the ABC family, present only in metazoans where it plays a critical role in epithelial salt and fluid homeostasis. Functionally, the membrane translocation process it mediates, the passive bidirectional diffusion of small inorganic anions, is simpler than the vectorial transport of larger more complex substrates ("allocrites") by most ABC transporters. However, the control of the permeation pathway which cannot go unchecked is necessarily more stringent than in the case of the transporters. There is tight regulation by the phosphorylation/dephosphorylation of the unique CFTR R domain superimposed on the basic ABC regulation mode of ATP binding and hydrolysis at the dual nucleotide binding sites. As with other ABCC subfamily members, only the second of these sites is hydrolytic in CFTR. The phosphorylation and ATP binding/hydrolysis events do not strongly influence each other; rather, R domain phosphorylation appears to enable transduction of the nucleotide binding allosteric signal to the responding channel gate. ATP hydrolysis is not required for either the opening or closing gating transitions but efficiently clears the ligand-binding site enabling a new gating cycle to be initiated.

The block of CFTR by scorpion venom is state-dependent

Cystic fibrosis transmembrane conductance regulator (CFTR) adenosine triphosphate-dependent chloride channels are expressed in epithelial cells and are associated with a number of genetic disorders, including cystic fibrosis. Venom of the scorpion Leirus quinquestriatus hebraeus reversibly inhibits CFTR when applied to its cytoplasmic surface. To examine the state-dependence of inhibition we recorded wild-type and mutant CFTR channel currents using inside-out membrane patches from Xenopus oocytes. Application of either venom or diphenylamine-2-carboxylate to channels that were either activated (open) or resting (closed) indicate primarily closed state-dependent inhibition of CFTR by venom, whereas diphenylamine-2-carboxylate showed no state-dependence of block. Efficacy of venom-mediated macroscopic current inhibition was inversely related to channel activity. Analysis of single-channel and macropatch data indicated that venom could either inhibit channel opening, if it binds during an interburst closed state or in the absence of cytosolic adenosine triphosphate, or introduce new intraburst closed states, if it binds during an open event. The on-rate of venom binding for intraburst block could be modulated by changing CFTR activity with vanadate or adenylyl-imidodiphosphate, or by introducing the Walker A mutation K1250A. These findings represent the first description of state-dependent inhibition of CFTR and suggest that the active toxin could be used as a tool to study the conformational changes that occur during CFTR gating.

High affinity ATP/ADP analogues as new tools for studying CFTR gating

Previous studies using non-hydrolysable ATP analogues and hydrolysis-deficient cystic fibrosis transmembrane conductance regulator (CFTR) mutants have indicated that ATP hydrolysis precedes channel closing. Our recent data suggest that ATP binding is also important in modulating the closing rate. This latter hypothesis predicts that ATP analogues with higher binding affinities should stabilize the open state more than ATP. Here we explore the possibility of using N6-modified ATP/ADP analogues as high-affinity ligands for CFTR gating, since these analogues have been shown to be more potent than native ATP/ADP in other ATP-binding proteins. Among the three N6-modified ATP analogues tested, N6-(2-phenylethyl)-ATP (P-ATP) was the most potent, with a K(1/2) of 1.6 +/- 0.4 microm (>50-fold more potent than ATP). The maximal open probability (P(o)) in the presence of P-ATP was approximately 30% higher than that of ATP, indicating that P-ATP also has a higher efficacy than ATP. Single-channel kinetic analysis showed that as [P-ATP] was increased, the opening rate increased, whereas the closing rate decreased. The fact that these two kinetic parameters have different sensitivities to changes of [P-ATP] suggests an involvement of two different ATP-binding sites, a high-affinity site modulating channel closing and a low affinity site controlling channel opening. The effect of P-ATP on the stability of open states was more evident when ATP hydrolysis was abolished, either by mutating the nucleotide-binding domain 2 (NBD2) Walker B glutamate (i.e. E1371) or by using the non-hydrolysable ATP analogue AMP-PNP. Similar strategies to develop nucleotide analogues with a modified adenine ring could be valuable for future studies of CFTR gating.

Molecular mechanisms of reduced glutathione transport: role of the MRP/CFTR/ABCC and OATP/SLC21A families of membrane proteins

The initial step in reduced glutathione (GSH) turnover in all mammalian cells is its transport across the plasma membrane into the extracellular space; however, the mechanisms of GSH transport are not clearly defined. GSH export is required for the delivery of its constituent amino acids to other tissues, detoxification of drugs, metals, and other reactive compounds of both endogenous and exogenous origin, protection against oxidant stress, and secretion of hepatic bile. Recent studies indicate that some members of the multidrug resistance-associated protein (MRP/CFTR or ABCC) family of ATP-binding cassette (ABC) proteins, as well as some members of the organic anion transporting polypeptide (OATP or SLC21A) family of transporters contribute to this process. In particular, five of the 12 members of the MRP/CFTR family appear to mediate GSH export from cells namely, MRP1, MRP2, MRP4, MRP5, and CFTR. Additionally, two members of the OATP family, rat Oatp1 and Oatp2, have been identified as GSH transporters. For the Oatp1 transporter, efflux of GSH may provide the driving force for the uptake of extracellular substrates. In humans, OATP-B and OATP8 do not appear to transport GSH; however, other members of this family have yet to be characterized in regards to GSH transport. In yeast, the ABC proteins Ycf1p and Bpt1p transport GSH from the cytosol into the vacuole, whereas Hgt1p mediates GSH uptake across the plasma membrane. Because transport is a key step in GSH homeostasis and is intimately linked to its biological functions, GSH export proteins are likely to modulate essential cellular functions.

CFTR gating I: Characterization of the ATP-dependent gating of a phosphorylation-independent CFTR channel (DeltaR-CFTR)

The CFTR chloride channel is activated by phosphorylation of serine residues in the regulatory (R) domain and then gated by ATP binding and hydrolysis at the nucleotide binding domains (NBDs). Studies of the ATP-dependent gating process in excised inside-out patches are very often hampered by channel rundown partly caused by membrane-associated phosphatases. Since the severed ΔR-CFTR, whose R domain is completely removed, can bypass the phosphorylation-dependent regulation, this mutant channel might be a useful tool to explore the gating mechanisms of CFTR. To this end, we investigated the regulation and gating of the ΔR-CFTR expressed in Chinese hamster ovary cells. In the cell-attached mode, basal ΔR-CFTR currents were always obtained in the absence of cAMP agonists. Application of cAMP agonists or PMA, a PKC activator, failed to affect the activity, indicating that the activity of ΔR-CFTR channels is indeed phosphorylation independent. Consistent with this conclusion, in excised inside-out patches, application of the catalytic subunit of PKA did not affect ATP-induced currents. Similarities of ATP-dependent gating between wild type and ΔR-CFTR make this phosphorylation-independent mutant a useful system to explore more extensively the gating mechanisms of CFTR. Using the ΔR-CFTR construct, we studied the inhibitory effect of ADP on CFTR gating. The Ki for ADP increases as the [ATP] is increased, suggesting a competitive mechanism of inhibition. Single channel kinetic analysis reveals a new closed state in the presence of ADP, consistent with a kinetic mechanism by which ADP binds at the same site as ATP for channel opening. Moreover, we found that the open time of the channel is shortened by as much as 54% in the presence of ADP. This unexpected result suggests another ADP binding site that modulates channel closing.

Normal gating of CFTR requires ATP binding to both nucleotide-binding domains and hydrolysis at the second nucleotide-binding domain

ATP interacts with the two nucleotide-binding domains (NBDs) of CFTR to control gating. However, it is unclear whether gating involves ATP binding alone, or also involves hydrolysis at each NBD. We introduced phenylalanine residues into nonconserved positions of each NBD Walker A motif to sterically prevent ATP binding. These mutations blocked [alpha-(32)P]8-N(3)-ATP labeling of the mutated NBD and reduced channel opening rate without changing burst duration. Introducing cysteine residues at these positions and modifying with N-ethylmaleimide produced the same gating behavior. These results indicate that normal gating requires ATP binding to both NBDs, but ATP interaction with one NBD is sufficient to support some activity. We also studied mutations of the conserved Walker A lysine residues (K464A and K1250A) that prevent hydrolysis. By combining substitutions that block ATP binding with Walker A lysine mutations, we could differentiate the role of ATP binding vs. hydrolysis at each NBD. The K1250A mutation prolonged burst duration; however, blocking ATP binding prevented the long bursts. These data indicate that ATP binding to NBD2 allowed channel opening and that closing was delayed in the absence of hydrolysis. The corresponding NBD1 mutations showed relatively little effect of preventing ATP hydrolysis but a large inhibition of blocking ATP binding. These data suggest that ATP binding to NBD1 is required for normal activity but that hydrolysis has little effect. Our results suggest that both NBDs contribute to channel gating, NBD1 binds ATP but supports little hydrolysis, and ATP binding and hydrolysis at NBD2 are key for normal gating.

4-Chloro-benzo[F]isoquinoline (CBIQ) activates CFTR chloride channels and KCNN4 potassium channels in Calu-3 human airway epithelial cells

1 Calu-3 cells have been used to investigate the actions of 4-chloro-benzo[F]isoquinoline (CBIQ) on short-circuit current (SCC) in monolayers, whole-cell recording from single cells and by patch clamping. 2 CBIQ caused a sustained, reversible and repeatable increase in SCC in Calu-3 monolayers with an EC50 of 4.0 microm. Simultaneous measurements of SCC and isotopic fluxes of 36Cl- showed that CBIQ caused electrogenic chloride secretion. 3 Apical membrane permeabilisation to allow recording of basolateral membrane conductance in the presence of a K+ gradient suggested that CBIQ activated the intermediate-conductance calcium-sensitive K(+)-channel (KCNN4). Permeabilisation of the basolateral membranes of epithelial monolayers in the presence of a Cl- gradient suggested that CBIQ activated the Cl(-)-channel CFTR in the apical membrane. 4 Whole-cell recording in the absence of ATP/GTP of Calu-3 cells showed that CBIQ generated an inwardly rectifying current sensitive to clotrimazole. In the presence of the nucleotides, a more complex I/V relation was found that was partially sensitive to glibenclamide. The data are consistent with the presence of both KCNN4 and CFTR in Calu-3. 5 Isolated inside-out patches from Calu-3 cells revealed clotrimazole-sensitive channels with a conductance of 12 pS at positive potentials after activation with CBIQ and demonstrating inwardly rectifying properties, consistent with the known properties of KCNN4. Cell-attached patches showed single channel events with a conductance of 7 pS and a linear I/V relation that were further activated by CBIQ by an increase in open state probability, consistent with known properties of CFTR. It is concluded that CBIQ activates CFTR and KCNN4 ion channels in Calu-3 cells.

Protein kinase A regulates ATP hydrolysis and dimerization by a CFTR (cystic fibrosis transmembrane conductance regulator) domain

Gating of the CFTR Cl- channel is associated with ATP hydrolysis at the nucleotide-binding domains (NBD1, NBD2) and requires PKA (protein kinase A) phosphorylation of the R domain. The manner in which the NBD1, NBD2 and R domains of CFTR (cystic fibrosis transmembrane conductance regulator) interact to achieve a properly regulated ion channel is largely unknown. In this study we used bacterially expressed recombinant proteins to examine interactions between these soluble domains of CFTR in vitro. PKA phosphorylated a fusion protein containing NBD1 and R (NBD1-R-GST) on CFTR residues Ser-660, Ser-700, Ser-712, Ser-737, Ser-768, Ser-795 and Ser-813. Phosphorylation of these serine residues regulated ATP hydrolysis by NBD1-R-GST by increasing the apparent K(m) for ATP (from 70 to 250 microM) and the Hill coefficient (from 1 to 1.7) without changing the V(max). When fusion proteins were photolabelled with 8-azido-[alpha-32P]ATP, PKA phosphorylation increased the apparent k(d) for nucleotide binding and it caused binding to become co-operative. PKA phosphorylation also resulted in dimerization of NBD1-R-GST but not of R-GST, a related fusion protein lacking the NBD1 domain. Finally, an MBP (maltose-binding protein) fusion protein containing the NBD2 domain (NBD2-MBP) associated with and regulated the ATPase activity of PKA-phosphorylated NBD1-R-GST. Thus when the R domain in NBD1-R-GST is phosphorylated by PKA, ATP binding and hydrolysis becomes co-operative and NBD dimerization occurs. These findings suggest that during the activation of native CFTR, phosphorylation of the R domain by PKA can control the ability of the NBD1 domain to hydrolyse ATP and to interact with other NBD domains.

An intrinsic adenylate kinase activity regulates gating of the ABC transporter CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP binding cassette (ABC) transporter family. Like other ABC transporters, it can hydrolyze ATP. Yet while ATP hydrolysis influences channel gating, it has long seemed puzzling that CFTR would require this reaction because anions flow passively through CFTR. Moreover, no other ion channel is known to require the large energy of ATP hydrolysis to gate. We found that CFTR also has adenylate kinase activity (ATP + AMP <=> ADP + ADP) that regulates gating. When functioning as an adenylate kinase, CFTR showed positive cooperativity for ATP suggesting its two nucleotide binding domains may dimerize. Thus, channel activity could be regulated by two different enzymatic reactions, ATPase and adenylate kinase, that share a common ATP binding site in the second nucleotide binding domain. At physiologic nucleotide concentrations, adenylate kinase activity, rather than ATPase activity may control gating, and therefore involve little energy consumption.

CFTR directly mediates nucleotide-regulated glutathione flux

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

Cystic fibrosis transmembrane conductance regulator: the NBF1+R (nucleotide-binding fold 1 and regulatory domain) segment acting alone catalyses a Co2+/Mn2+/Mg2+-ATPase activity markedly inhibited by both Cd2+ and the transition-state analogue orthovanadate

Cystic fibrosis (CF) is caused by mutations in the gene encoding CFTR (cystic fibrosis transmembrane conductance regulator), a regulated anion channel and member of the ATP-binding-cassette transporter (ABC transporter) superfamily. Of CFTR's five domains, the first nucleotide-binding fold (NBF1) has been of greatest interest both because it is the major 'hotspot' for mutations that cause CF, and because it is connected to a unique regulatory domain (R). However, attempts have failed to obtain a catalytically active NBF1+R protein in the absence of a fusion partner. Here, we report that such a protein can be obtained following its overexpression in bacteria. The pure NBF1+R protein exhibits significant ATPase activity [catalytic-centre activity (turnover number) 6.7 min(-1)] and an apparent affinity for ATP ( K (m), 8.7 microM) higher than reported previously for CFTR or segments thereof. As predicted, the ATPase activity is inhibited by mutations in the Walker A motif. It is also inhibited by vanadate, a transition-state analogue. Surprisingly, however, the best divalent metal activator is Co(2+), followed by Mn(2+) and Mg(2+). In contrast, Ca(2+) is ineffective and Cd(2+) is a potent inhibitor. These novel studies, while demonstrating clearly that CFTR's NBF1+R segment can act independently as an active, vanadate-sensitive ATPase, also identify its unique cation activators and a new inhibitor, thus providing insight into the nature of its active site.

On the mechanism of MgATP-dependent gating of CFTR Cl- channels

CFTR, the product of the gene mutated in cystic fibrosis, is an ATPase that functions as a Cl(-) channel in which bursts of openings separate relatively long interburst closed times (tauib). Channel gating is controlled by phosphorylation and MgATP, but the underlying molecular mechanisms remain controversial. To investigate them, we expressed CFTR channels in Xenopus oocytes and examined, in excised patches, how gating kinetics of phosphorylated channels were affected by changes in [MgATP], by alterations in the chemical structure of the activating nucleotide, and by mutations expected to impair nucleotide hydrolysis and/or diminish nucleotide binding affinity. The rate of opening to a burst (1/tauib) was a saturable function of [MgATP], but apparent affinity was reduced by mutations in either of CFTR's nucleotide binding domains (NBDs): K464A in NBD1, and K1250A or D1370N in NBD2. Burst duration of neither wild-type nor mutant channels was much influenced by [MgATP]. Poorly hydrolyzable nucleotide analogs, MgAMPPNP, MgAMPPCP, and MgATPgammaS, could open CFTR channels, but only to a maximal rate of opening approximately 20-fold lower than attained by MgATP acting on the same channels. NBD2 catalytic site mutations K1250A, D1370N, and E1371S were found to prolong open bursts. Corresponding NBD1 mutations did not affect timing of burst termination in normal, hydrolytic conditions. However, when hydrolysis at NBD2 was impaired, the NBD1 mutation K464A shortened the prolonged open bursts. In light of recent biochemical and structural data, the results suggest that: nucleotide binding to both NBDs precedes channel opening; at saturating nucleotide concentrations the rate of opening to a burst is influenced by the structure of the phosphate chain of the activating nucleotide; normal, rapid exit from bursts occurs after hydrolysis of the nucleotide at NBD2, without requiring a further nucleotide binding step; if hydrolysis at NBD2 is prevented, exit from bursts occurs through a slower pathway, the rate of which is modulated by the structure of the NBD1 catalytic site and its bound nucleotide. Based on these and other results, we propose a mechanism linking hydrolytic and gating cycles via ATP-driven dimerization of CFTR's NBDs.

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).

Functional analysis of the C-terminal boundary of the second nucleotide binding domain of the cystic fibrosis transmembrane conductance regulator and structural implications

The cystic fibrosis transmembrane conductance regulator (CFTR) contains two nucleotide-binding domains (NBDs) or ATP-binding cassettes (ABCs) that characterize a large family of membrane transporters. Although the three-dimensional structures of these domains from several ABC proteins have been determined, this is not the case for CFTR, and hence the domains are defined simply on the basis of sequence alignment. The functional C-terminal boundary of NBD1 of CFTR was located by analysis of chloride channel function [Chan, Csanady, Seto-Young, Nairn and Gadsby (2000) J. Gen. Physiol. 116, 163-180]. However, the boundary between the C-terminal end of NBD2 and sequences further downstream in the whole protein, that are important for its cellular localization and endocytotic turnover, has not been defined. We have now done this by assaying the influence of progressive C-terminal truncations on photolabelling of NBD2 by 8-azido-ATP, which reflects hydrolysis, as well as binding, at that domain, and on NBD2-dependent channel gating itself. The boundary defined in this way is between residues 1420 and 1424, which corresponds to the final beta-strand in aligned NBDs whose structures have been determined. Utilization of this information should facilitate the generation of monodisperse NBD2 polypeptides for structural analysis, which until now has not been possible. The established boundary includes within NBD2 a hydrophobic patch of four residues (1413-1416) previously shown to be essential for CFTR maturation and stability [Gentzsch and Riordan (2001) J. Biol. Chem. 276, 1291-1298]. This hydrophobic cluster is conserved in most ABC proteins, and on alignment with ones of known structure constitutes the penultimate beta-strand of the domain which is likely to participate in essential structure-stabilizing beta-sheet formation.

Distinct Mg(2+)-dependent steps rate limit opening and closing of a single CFTR Cl(-) channel

The roles played by ATP binding and hydrolysis in the complex mechanisms that open and close cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels remain controversial. In this work, the contributions made by ATP and Mg(2+) ions to the gating of phosphorylated cardiac CFTR channels were evaluated separately by measuring the rates of opening and closing of single channels in excised patches exposed to solutions in which [ATP] and [Mg(2+)] were varied independently. Channel opening was found to be rate-limited not by the binding of ATP alone, but by a Mg(2+)-dependent step that followed binding of both ATP and Mg(2+). Once a channel had opened, sudden withdrawal of all Mg(2+) and ATP could prevent it from closing for tens of seconds. But subsequent exposure of such an open channel to Mg(2+) ions alone could close it, and the closing rate increased with [Mg(2+)] over the micromolar range (half maximal at approximately 50 microM [Mg(2+)]). A simple interpretation is that channel closing is stoichiometrically coupled to hydrolysis of an ATP molecule that remains tightly associated with the open CFTR channel despite continuous washing. If correct, that ATP molecule appears able to reside for over a minute in the catalytic site that controls channel closing, implying that the site must entrap, or have an intrinsically high apparent affinity for, ATP, even without a Mg(2+) ion. Such stabilization of the open-channel conformation of CFTR by tight binding, or occlusion, of an ATP molecule echoes the stabilization of the active conformation of a G protein by GTP.

The First Nucleotide Binding Domain of Cystic Fibrosis Transmembrane Conductance Regulator Is a Site of Stable Nucleotide Interaction, whereas the Second Is a Site of Rapid Turnover

As in other adenine nucleotide binding cassette (ABC) proteins the nucleotide binding domains of the cystic fibrosis transmembrane conductance regulator (CFTR) bind and hydrolyze ATP and in some manner regulate CFTR ion channel gating. Unlike some other ABC proteins, however, there are preliminary indications that the two domains of CFTR are nonequivalent in their nucleotide interactions (Szabo, K., Szakacs, G., Hegeds, T., and Sarkadi, B. (1999) J. Biol. Chem. 274, 12209-12212; Aleksandrov, L., Mengos, A., Chang, X., Aleksandrov, A., and Riordan, J. R. (2001) J. Biol. Chem. 276, 12918-12923). We have now characterized the interactions of the 8-azido-photoactive analogues of ATP, ADP, and 5'-adenyl-beta,gamma-imidodiphosphate (AMP-PNP) with the two domains of functional membrane-bound CFTR. The results show that the two domains appear to act independently in the binding and hydrolysis of 8-azido-ATP. At NBD1 binding does not require a divalent cation. This binding is followed by minimal Mg(2+)-dependent hydrolysis and retention of the hydrolysis product, 8-azido-ADP, but not as a vanadate stabilized post-hydrolysis transition state complex. In contrast, at NBD2, MgN(3)ATP is hydrolyzed as rapidly as it is bound and the nucleoside diphosphate hydrolysis product dissociates immediately. Confirming this characterization of NBD1 as a site of more stable nucleotide interaction and NBD2 as a site of fast turnover, the non-hydrolyzable N(3)AMP-PNP bound preferentially to NBD1. This demonstration of NBD2 as the rapid nucleotide turnover site is consistent with the strong effect on channel gating kinetics of inactivation of this domain by mutagenesis.

Mutation of Walker-A lysine 464 in cystic fibrosis transmembrane conductance regulator reveals functional interaction between its nucleotide-binding domains

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel bears two nucleotide-binding domains (NBD1 and NBD2) that control its ATP-dependent gating. Exactly how these NBDs control gating is controversial. To address this issue, we examined channels with a Walker-A lysine mutation in NBD1 (K464A) using the patch clamp technique. K464A mutants have an ATP dependence (EC(50) approximate 60 microM) and opening rate at 2.75 mM ATP (approximately 2.1 s(-1)) similar to wild type (EC(50) approximate 97 microM; approximately 2.0 s(-1)). However, K464A's closing rate at 2.75 mM ATP (approximately 3.6 s(-1)) is faster than that of wild type (approximately 2.1 s(-1)), suggesting involvement of NBD1 in nucleotide-dependent closing. Delay of closing in wild type by adenylyl imidodiphosphate (AMP-PNP), a non-hydrolysable ATP analogue, is markedly diminished in K464A mutants due to reduction in AMP-PNP's apparent on-rate and acceleration of its apparent off-rate (approximately 2- and approximately 10-fold, respectively). Since the delay of closing by AMP-PNP is thought to occur via NBD2, K464A's effect on the NBD2 mutant K1250A was examined. In sharp contrast to K464A, K1250A single mutants exhibit reduced opening (approximately 0.055 s(-1)) and closing (approximately 0.006 s(-1)) rates at millimolar [ATP], suggesting a role for K1250 in both opening and closing. At millimolar [ATP], K464A-K1250A double mutants close approximately 5-fold faster (approximately 0.029 s(-1)) than K1250A but open with a similar rate (approximately 0.059 s(-1)), indicating an effect of K464A on NBD2 function. In summary, our results reveal that both of CFTR's functionally asymmetric NBDs participate in nucleotide-dependent closing, which provides important constraints for NBD-mediated gating models.

Cysteine residues in the nucleotide binding domains regulate the conductance state of CFTR channels

Gating of cystic fibrosis transmembrane conductance regulator (CFTR) channels requires intermolecular or interdomain interactions, but the exact nature and physiological significance of those interactions remains uncertain. Subconductance states of the channel may result from alterations in interactions among domains, and studying mutant channels enriched for a single conductance type may elucidate those interactions. Analysis of CFTR channels in inside-out patches revealed that mutation of cysteine residues in NBD1 and NBD2 affects the frequency of channel opening to the full-size versus a 3-pS subconductance. Mutating cysteines in NBD1 resulted in channels that open almost exclusively to the 3-pS subconductance, while mutations of cysteines in NBD2 decreased the frequency of subconductance openings. Wild-type channels open to both size conductances and make fast transitions between them within a single open burst. Full-size and subconductance openings of both mutant and wild-type channels are similarly activated by ATP and phosphorylation. However, the different size conductances open very differently in the presence of a nonhydrolyzable ATP analog, with subconductance openings significantly shortened by ATPgammaS, while full-size channels are locked open. In wild-type channels, reducing conditions increase the frequency and decrease the open time of subconductance channels, while oxidizing conditions decrease the frequency of subconductance openings. In contrast, in the cysteine mutants studied, altering redox potential has little effect on gating of the subconductance.

Mutations that change the position of the putative gamma-phosphate linker in the nucleotide binding domains of CFTR alter channel gating

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is an ATP-binding cassette transporter that contains conserved nucleotide-binding domains (NBDs). In CFTR, the NBDs bind and hydrolyze ATP to open and close the channel. Crystal structures of related NBDs suggest a structural model with an important signaling role for a gamma-phosphate linker peptide that couples bound nucleotide to movement of an alpha-helical subdomain. We mutated two residues in CFTR that the structural model predicts will uncouple effects of nucleotide binding from movement of the alpha-helical subdomain. These residues are Gln-493 and Gln-1291, which may directly connect the ATP gamma-phosphate to the gamma-phosphate linker, and residues Asn-505 and Asn-1303, which may form hydrogen bonds that stabilize the linker. In NBD1, Q493A reduced the frequency of channel opening, suggesting a role for this residue in coupling ATP binding to channel opening. In contrast, N505C increased the frequency of channel opening, consistent with a role for Asn-505 in stabilizing the inactive state of the NBD. In NBD2, Q1291A decreased the effects of pyrophosphate without altering other functions. Mutations of Asn-1303 decreased the rate of channel opening and closing, suggesting an important role for NBD2 in controlling channel burst duration. These findings are consistent with both the bacterial NBD structural model and gating models for CFTR. Our results extend models of nucleotide-induced structural changes from bacterial NBDs to a functional mammalian ATP-binding cassette transporter.

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.