Interaction between permeation and gating in a putative pore domain mutant in the cystic fibrosis transmembrane conductance regulator

The molecular evolution of function in the CFTR chloride channel

The ATP-binding cassette (ABC) transporter superfamily includes many proteins of clinical relevance, with genes expressed in all domains of life. Although most members use the energy of ATP binding and hydrolysis to accomplish the active import or export of various substrates across membranes, the cystic fibrosis transmembrane conductance regulator (CFTR) is the only known animal ABC transporter that functions primarily as an ion channel. Defects in CFTR, which is closely related to ABCC subfamily members that bear function as bona fide transporters, underlie the lethal genetic disease cystic fibrosis. This article seeks to integrate structural, functional, and genomic data to begin to answer the critical question of how the function of CFTR evolved to exhibit regulated channel activity. We highlight several examples wherein preexisting features in ABCC transporters were functionally leveraged as is, or altered by molecular evolution, to ultimately support channel function. This includes features that may underlie (1) construction of an anionic channel pore from an anionic substrate transport pathway, (2) establishment and tuning of phosphoregulation, and (3) optimization of channel function by specialized ligand–channel interactions. We also discuss how divergence and conservation may help elucidate the pharmacology of important CFTR modulators.

STRUCTURE, GATING, AND REGULATION OF THE CFTR ANION CHANNEL

The cystic fibrosis transmembrane conductance regulator (CFTR) belongs to the ATP binding cassette (ABC) transporter superfamily but functions as an anion channel crucial for salt and water transport across epithelial cells. CFTR dysfunction, because of mutations, causes cystic fibrosis (CF). The anion-selective pore of the CFTR protein is formed by its two transmembrane domains (TMDs) and regulated by its cytosolic domains: two nucleotide binding domains (NBDs) and a regulatory (R) domain. Channel activation requires phosphorylation of the R domain by cAMP-dependent protein kinase (PKA), and pore opening and closing (gating) of phosphorylated channels is driven by ATP binding and hydrolysis at the NBDs. This review summarizes available information on structure and mechanism of the CFTR protein, with a particular focus on atomic-level insight gained from recent cryo-electron microscopic structures and on the molecular mechanisms of channel gating and its regulation. The pharmacological mechanisms of small molecules targeting CFTR's ion channel function, aimed at treating patients suffering from CF and other diseases, are briefly discussed.

Transmembrane helical interactions in the CFTR channel pore

Mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene affect CFTR protein biogenesis or its function as a chloride channel, resulting in dysregulation of epithelial fluid transport in the lung, pancreas and other organs in cystic fibrosis (CF). Development of pharmaceutical strategies to treat CF requires understanding of the mechanisms underlying channel function. However, incomplete 3D structural information on the unique ABC ion channel, CFTR, hinders elucidation of its functional mechanism and correction of cystic fibrosis causing mutants. Several CFTR homology models have been developed using bacterial ABC transporters as templates but these have low sequence similarity to CFTR and are not ion channels. Here, we refine an earlier model in an outward (OWF) and develop an inward (IWF) facing model employing an integrated experimental-molecular dynamics simulation (200 ns) approach. Our IWF structure agrees well with a recently solved cryo-EM structure of a CFTR IWF state. We utilize cysteine cross-linking to verify positions and orientations of residues within trans-membrane helices (TMHs) of the OWF conformation and to reconstruct a physiologically relevant pore structure. Comparison of pore profiles of the two conformations reveal a radius sufficient to permit passage of hydrated Cl- ions in the OWF but not the IWF model. To identify structural determinants that distinguish the two conformations and possible rearrangements of TMHs within them responsible for channel gating, we perform cross-linking by bifunctional reagents of multiple predicted pairs of cysteines in TMH 6 and 12 and 6 and 9. To determine whether the effects of cross-linking on gating observed are the result of switching of the channel from open to close state, we also treat the same residue pairs with monofunctional reagents in separate experiments. Both types of reagents prevent ion currents indicating that pore blockage is primarily responsible.

Timing of CFTR pore opening and structure of its transition state

In CFTR, the chloride ion channel mutated in cystic fibrosis (CF) patients, pore opening is coupled to ATP-binding-induced dimerization of two cytosolic nucleotide binding domains (NBDs) and closure to dimer disruption following ATP hydrolysis. CFTR opening rate, unusually slow because of its high-energy transition state, is further slowed by CF mutation ΔF508. Here, we exploit equilibrium gating of hydrolysis-deficient CFTR mutant D1370N and apply rate-equilibrium free-energy relationship analysis to estimate relative timing of opening movements in distinct protein regions. We find clear directionality of motion along the longitudinal protein axis and identify an opening transition-state structure with the NBD dimer formed but the pore still closed. Thus, strain at the NBD/pore-domain interface, the ΔF508 mutation locus, underlies the energetic barrier for opening. Our findings suggest a therapeutic opportunity to stabilize this transition-state structure pharmacologically in ΔF508-CFTR to correct its opening defect, an essential step toward restoring CFTR function.

Functional architecture of the CFTR chloride channel

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of membrane transport proteins. CFTR is unique among ABC proteins in that it functions not as an active transporter but as an ATP-gated Cl(-) channel. As an ion channel, the function of the CFTR transmembrane channel pore that mediates Cl(-) movement has been studied in great detail. On the other hand, only low resolution structural data is available on the transmembrane parts of the protein. The structure of the channel pore has, however, been modeled on the known structure of active transporter ABC proteins. Currently, significant barriers exist to building a unified view of CFTR pore structure and function. Reconciling functional data on the channel with indirect structural data based on other proteins with very different transport functions and substrates has proven problematic. This review summarizes current structural and functional models of the CFTR Cl(-) channel pore, including a comprehensive review of previous electrophysiological investigations of channel structure and function. In addition, functional data on the three-dimensional arrangement of pore-lining helices, as well as contemporary hypotheses concerning conformational changes in the pore that occur during channel opening and closing, are discussed. Important similarities and differences between different models of the pore highlight current gaps in our knowledge of CFTR structure and function. In order to fill these gaps, structural and functional models of the membrane-spanning pore need to become better integrated.

Relative contribution of different transmembrane segments to the CFTR chloride channel pore

The membrane-spanning part of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel comprises 12 transmembrane (TM) α-helices, arranged in 2 symmetrical groups of 6. However, those TMs that line the channel pore are not completely defined. We used patch clamp recording to compare the accessibility of cysteine-reactive reagents to cysteines introduced into different TMs. Several residues in TM11 were accessible to extracellular and/or intracellular cysteine reactive reagents; however, no reactive cysteines were identified in TMs 5 or 11. Two accessible residues in TM11 (T1115C and S1118C) were found to be more readily modified from the extracellular solution in closed channels, but more readily modified from the intracellular solution in open channels, as previously reported for T338C in TM6. However, the effects of mutagenesis at S1118 (TM11) on a range of pore functional properties were relatively minor compared to the large effects of mutagenesis at T338 (TM6). Our results suggest that the CFTR pore is lined by TM11 but not by TM5 or TM7. Comparison with previous works therefore suggests that the pore is lined by TMs 1, 6, 11, and 12, suggesting that the structure of the open channel pore is asymmetric in terms of the contributions of different TMs. Although TMs 6 and 11 appear to undergo similar conformational changes during channel opening and closing, the influence of these two TMs on the functional properties of the narrowest region of the pore is clearly unequal.

TRPP2 and TRPV4 form an EGF-activated calcium permeable channel at the apical membrane of renal collecting duct cells

Objective

Regulation of apical calcium entry is important for the function of principal cells of the collecting duct. However, the molecular identity and the regulators of the transporter/channel, which is responsible for apical calcium entry and what factors regulate the calcium conduction remain unclear.

Methods and Results

We report that endogenous TRPP2 and TRPV4 assemble to form a 23-pS divalent cation-permeable non-selective ion channel at the apical membrane of renal principal cells of the collecting duct. TRPP2\TRPV4 channel complex was identified by patch-clamp, immunofluorescence and co-immunprecipitation studies in both principal cells that either possess normal cilia (cilia (+)) or in which cilia are absent (cilia (-)). This channel has distinct biophysical and pharmacological and regulatory profiles compared to either TRPP2 or TRPV4 channels. The rate of occurrence detected by patch clamp was higher in cilia (-) compared to cilia (+) cells. In addition, shRNA knockdown of TRPP2 increased the prevalence of TRPV4 channel activity while knockdown of TRPV4 resulted in TRPP2 activity and knockdown of both proteins vastly decreased the 23-pS channel activity. Epidermal growth factor (EGF) stimulated TRPP2\TRPV4 channel through the EGF receptor (EGFR) tyrosine kinase-dependent signaling. With loss of cilia, apical EGF treatment resulted in 64-fold increase in channel activity in cilia (-) but not cilia (+) cells. In addition EGF increased cell proliferation in cilia (-) cell that was dependent upon TRPP2\TRPV4 channel mediated increase in intracellular calcium.

Conclusion

We conclude that in the absence of cilia, an EGF activated TRPP2\TRPV4 channel may play an important role in increased cell proliferation and cystogenesis.

The CFTR ion channel: gating, regulation, and anion permeation

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated anion channel with two remarkable distinctions. First, it is the only ATP-binding cassette (ABC) transporter that is known to be an ion channel--almost all others function as transport ATPases. Second, CFTR is the only ligand-gated channel that consumes its ligand (ATP) during the gating cycle--a consequence of its enzymatic activity as an ABC transporter. We discuss these special properties of CFTR in the context of its evolutionary history as an ABC transporter. Other topics include the mechanisms by which CFTR gating is regulated by phosphorylation of its unique regulatory domain and our current view of the CFTR permeation pathway (or pore). Understanding these basic operating principles of the CFTR channel is central to defining the mechanisms of action of prospective cystic fibrosis drugs and to the development of new, rational treatment strategies.

Differential contribution of TM6 and TM12 to the pore of CFTR identified by three sulfonylurea-based blockers

Previous studies suggested that four transmembrane domains 5, 6, 11, 12 make the greatest contribution to forming the pore of the CFTR chloride channel. We used excised, inside-out patches from oocytes expressing CFTR with alanine-scanning mutagenesis in amino acids in TM6 and TM12 to probe CFTR pore structure with four blockers: glibenclamide (Glyb), glipizide (Glip), tolbutamide (Tolb), and Meglitinide. Glyb and Glip blocked wildtype (WT)-CFTR in a voltage-, time-, and concentration-dependent manner. At V (M) = -120 mV with symmetrical 150 mM Cl(-) solution, fractional block of WT-CFTR by 50 μM Glyb and 200 μM Glip was 0.64 ± 0.03 (n = 7) and 0.48 ± 0.02 (n = 7), respectively. The major effects on block by Glyb and Glip were found with mutations at F337, S341, I344, M348, and V350 of TM6. Under similar conditions, fractional block of WT-CFTR by 300 μM Tolb was 0.40 ± 0.04. Unlike Glyb, Glip, and Meglitinide, block by Tolb lacked time-dependence (n = 7). We then tested the effects of alanine mutations in TM12 on block by Glyb and Glip; the major effects were found at N1138, T1142, V1147, N1148, S1149, S1150, I1151, and D1152. From these experiments, we infer that amino acids F337, S341, I344, M348, and V350 of TM6 face the pore when the channel is in the open state, while the amino acids of TM12 make less important contributions to pore function. These data also suggest that the region between F337 and S341 forms the narrow part of the CFTR pore.

ATP hydrolysis-dependent asymmetry of the conformation of CFTR channel pore

Despite substantial efforts, the entire cystic fibrosis transmembrane conductance regulator (CFTR) protein proved to be difficult for structural analysis at high resolution, and little is still known about the actual dimensions of the anion-transporting pathway of CFTR channel. In the present study, we therefore gauged geometrical features of the CFTR Cl(-) channel pore by a nonelectrolyte exclusion technique. Polyethylene glycols with a hydrodynamic radius (R (h)) smaller than 0.95 nm (PEG 300-1,000) added from the intracellular side greatly suppressed the inward unitary anionic conductance, whereas only molecules with R (h) ≤ 0.62 nm (PEG 200-400) applied extracellularly were able to affect the outward unitary anionic currents. Larger molecules with R (h) = 1.16-1.84 nm (PEG 1,540-3,400) added from either side were completely excluded from the pore and had no significant effect on the single-channel conductance. The cut-off radius of the inner entrance of CFTR channel pore was assessed to be 1.19 ± 0.02 nm. The outer entrance was narrower with its cut-off radius of 0.70 ± 0.16 nm and was dilated to 0.93 ± 0.23 nm when a non-hydrolyzable ATP analog, 5'-adenylylimidodiphosphate (AMP-PNP), was added to the intracellular solution. Thus, it is concluded that the structure of CFTR channel pore is highly asymmetric with a narrower extracellular entrance and that a dilating conformational change of the extracellular entrance is associated with the channel transition to a non-hydrolytic, locked-open state.

Clinical and molecular characterization of S1118F-CFTR

BACKGROUND

Cystic fibrosis is a lethal autosomal recessive disorder usually associated with lung disease, pancreatic insufficiency and high sweat chloride levels.

CLINICAL CASE

A patient admitted to Le Bonheur Children's Medical Center (LBCMC, Memphis, TN) showed symptoms of meconium ileus which required exploratory laparotomy, bowel resection and ileostomy. Genotyping showed DeltaF508/I1027T on one chromosome and S1118F on the other. Sweat testing on three different occasions gave negative and intermediate results (22.7, 24.6 mmol/L; 55.1, 58.6 mmol/L and 55.1, 58 mmol/L) and pancreatic elastase testing showed normal levels.

OBJECTIVE

To characterize S1118F-CFTR mutation at a molecular level to help understand the associated CF-phenotype.

METHODS

Molecular characterization of S1118F-CFTR mutant was studied in HEK-293 cells at 37 degrees C. Various biochemical methods such as Western blotting, real-time PCR, Pulse chase labeling and iodide efflux assay were employed.

RESULTS

S1118F-CFTR makes less than 10-15% of mature CFTR (band C) compared to WT-CFTR. The mRNA levels of S1118F-CFTR and WT-CFTR are comparable. S1118F-CFTR is functional but shows about 10-15% of WT-CFTR activity. S1118F-CFTR shows impaired maturation and CF-correctors can increase the amount of mature and functional CFTR by three- to fourfold.

CONCLUSION

S1118F-CFTR shows impaired maturation and an individual with S1118F-CFTR paired with DeltaF508-CFTR exhibits atypical CF symptoms with intermediate sweat chloride level and meconium ileus despite documented pancreatic sufficiency.

Novel residues lining the CFTR chloride channel pore identified by functional modification of introduced cysteines

Substituted cysteine accessibility mutagenesis (SCAM) has been used widely to identify pore-lining amino acid side chains in ion channel proteins. However, functional effects on permeation and gating can be difficult to separate, leading to uncertainty concerning the location of reactive cysteine side chains. We have combined SCAM with investigation of the charge-dependent effects of methanethiosulfonate (MTS) reagents on the functional permeation properties of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. We find that cysteines substituted for seven out of 21 continuous amino acids in the eleventh and twelfth transmembrane (TM) regions can be modified by external application of positively charged [2-(trimethylammonium)ethyl] MTS bromide (MTSET) and negatively charged sodium [2-sulfonatoethyl] MTS (MTSES). Modification of these cysteines leads to changes in the open channel current-voltage relationship at both the macroscopic and single-channel current levels that reflect specific, charge-dependent effects on the rate of Cl(-) permeation through the channel from the external solution. This approach therefore identifies amino acid side chains that lie within the permeation pathway. Cysteine mutagenesis of pore-lining residues also affects intrapore anion binding and anion selectivity, giving more information regarding the roles of these residues. Our results demonstrate a straightforward method of screening for pore-lining amino acids in ion channels. We suggest that TM11 contributes to the CFTR pore and that the extracellular loop between TMs 11 and 12 lies close to the outer mouth of the pore.

Cellular mechanisms underlying the laxative effect of flavonol naringenin on rat constipation model

BACKGROUND & AIMS

Symptoms of constipation are extremely common, especially in the elderly. The present study aim to identify an efficacious treatment strategy for constipation by evaluating the secretion-promoting and laxative effect of a herbal compound, naringenin, on intestinal epithelial anion secretion and a rat constipation model, respectively.

METHODS/PRINCIPAL FINDINGS

In isolated rat colonic crypts, mucosal addition of naringenin (100 microM) elicited a concentration-dependent and sustained increase in the short-circuit current (I(SC)), which could be inhibited in Cl- free solution or by bumetanide and DPC (diphenylamine-2-carboxylic acid), but not by DIDS (4, 4'- diisothiocyanatostilbene-2, 2'-disulfonic acid). Naringenin could increase intracellular cAMP content and PKA activity, consisted with that MDL-12330A (N-(Cis-2-phenyl-cyclopentyl) azacyclotridecan-2-imine-hydrochloride) pretreatment reduced the naringenin-induced I(SC). In addition, significant inhibition of the naringenin-induced I(SC) by quinidine indicated that basolateral K+ channels were involved in maintaining this cAMP-dependent Cl- secretion. Naringenin-evoked whole cell current which exhibited a linear I-V relationship and time-and voltage- independent characteristics was inhibited by DPC, indicating that the cAMP activated Cl- conductance most likely CFTR (cystic fibrosis transmembrane conductance regulator) was involved. In rat constipation model, administration of naringenin restored the level of fecal output, water content and mucus secretion compared to loperamide-administrated group.

CONCLUSIONS

Taken together, our data suggest that naringenin could stimulate Cl- secretion in colonic epithelium via a signaling pathway involving cAMP and PKA, hence provide an osmotic force for subsequent colonic fluid secretion by which the laxative effect observed in the rat constipation model. Naringenin appears to be a novel alternative treatment strategy for constipation.

A synthetic prostone activates apical chloride channels in A6 epithelial cells

The bicyclic fatty acid lubiprostone (formerly known as SPI-0211) activates two types of anion channels in A6 cells. Both channel types are rarely, if ever, observed in untreated cells. The first channel type was activated at low concentrations of lubiprostone (<100 nM) in >80% of cell-attached patches and had a unit conductance of approximately 3-4 pS. The second channel type required higher concentrations (>100 nM) of lubiprostone to activate, was observed in approximately 30% of patches, and had a unit conductance of 8-9 pS. The properties of the first type of channel were consistent with ClC-2 and the second with CFTR. ClC-2's unit current strongly inwardly rectified that could be best fit by models of the channel with multiple energy barrier and multiple anion binding sites in the conductance pore. The open probability and mean open time of ClC-2 was voltage dependent, decreasing dramatically as the patches were depolarized. The order of anion selectivity for ClC-2 was Cl > Br > NO(3) > I > SCN, where SCN is thiocyanate. ClC-2 was a "double-barreled" channel favoring even numbers of levels over odd numbers as if the channel protein had two conductance pathways that opened independently of one another. The channel could be, at least, partially blocked by glibenclamide. The properties of the channel in A6 cells were indistinguishable from ClC-2 channels stably transfected in HEK293 cells. CFTR in the patches had a selectivity of Cl > Br >> NO(3) congruent with SCN congruent with I. It outwardly rectified as expected for a single-site anion channel. Because of its properties, ClC-2 is uniquely suitable to promote anion secretion with little anion reabsorption. CFTR, on the other hand, could promote either reabsorption or secretion depending on the anion driving forces.

Mutations at arginine 352 alter the pore architecture of CFTR

Arginine 352 (R352) in the sixth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) previously was reported to form an anion/cation selectivity filter and to provide positive charge in the intracellular vestibule. However, mutations at this site have nonspecific effects, such as inducing susceptibility of endogenous cysteines to chemical modification. We hypothesized that R352 stabilizes channel structure and that charge-destroying mutations at this site disrupt pore architecture, with multiple consequences. We tested the effects of mutations at R352 on conductance, anion selectivity and block by the sulfonylurea drug glipizide, using recordings of wild-type and mutant channels. Charge-altering mutations at R352 destabilized the open state and altered both selectivity and block. In contrast, R352K-CFTR was similar to wild-type. Full conductance state amplitude was similar to that of wild-type CFTR in all mutants except R352E, suggesting that R352 does not itself form an anion coordination site. In an attempt to identify an acidic residue that may interact with R352, we found that permeation properties were similarly affected by charge-reversing mutations at D993. Wild-type-like properties were rescued in R352E/D993R-CFTR, suggesting that R352 and D993 in the wild-type channel may interact to stabilize pore architecture. Finally, R352A-CFTR was sensitive to modification by externally applied MTSEA+, while wild-type and R352E/D993R-CFTR were not. These data suggest that R352 plays an important structural role in CFTR, perhaps reflecting its involvement in forming a salt bridge with residue D993.

Insight in eukaryotic ABC transporter function by mutation analysis

With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

ASSEMBLY OF FUNCTIONAL CFTR CHLORIDE CHANNELS

The assembly of the cystic fibrosis transmembrane regulator (CFTR) chloride channel is of interest from the broad perspective of understanding how ion channels and ABC transporters are formed as well as dealing with the mis-assembly of CFTR in cystic fibrosis. CFTR is functionally distinct from other ABC transporters because it permits bidirectional permeation of anions rather than vectorial transport of solutes. This adaptation of the ABC transporter structure can be rationalized by considering CFTR as a hydrolyzable-ligand-gated channel with cytoplasmic ATP as ligand. Channel gating is initiated by ligand binding when the protein is also phosphorylated by protein kinase A and made reversible by ligand hydrolysis. The two nucleotide-binding sites play different roles in channel activation. CFTR self-associates, possibly as a function of its activation, but most evidence, including the low-resolution three-dimensional structure, indicates that the channel is monomeric. Domain assembly and interaction within the monomer is critical in maturation, stability, and function of the protein. Disease-associated mutations, including the most common, DeltaF508, interfere with domain folding and association, which occur both co- and post-translationally. Intermolecular interactions of mature CFTR have been detected primarily with the N- and C-terminal tails, and these interactions have some impact not only on channel function but also on localization and processing within the cell. The biosynthetic processing of the nascent polypeptide leading to channel assembly involves transient interactions with numerous chaperones and enzymes on both sides of the endoplasmic reticulum membrane.

CFTR: a cysteine at position 338 in TM6 senses a positive electrostatic potential in the pore

We investigated the accessibility to protons and thiol-directed reagents of a cysteine substituted at position 338 in transmembrane segment 6 (TM6) of CFTR to test the hypothesis that T338 resides in the pore. Xenopus oocytes expressing T338C CFTR exhibited pH-dependent changes in gCl and I-V shape that were specific to the substituted cysteine. The apparent pKa of T338C CFTR was more acidic than that expected for a cysteine or similar simple thiols in aqueous solution. The pKa was shifted toward alkaline values when a nearby positive charge (R334) was substituted with neutral or negatively charged residues, consistent with the predicted influence of the positive charge of R334, and perhaps other residues, on the titration of a cysteine at 338. The relative rates of chemical modification of T338C CFTR by MTSET+ and MTSES- were also altered by the charge at 334. These observations support a model for CFTR that places T338 within the anion conduction path. The apparent pKa of a cysteine substituted at 338 and the relative rates of reaction of charged thiol-directed reagents provide a crude measure of a positive electrostatic potential that may be due to R334 and other residues near this position in the pore.

Time-dependent Interactions of Glibenclamide with CFTR: Kinetically Complex Block of Macroscopic Currents

Blockade of the CFTR chloride channel by glibenclamide was studied in Xenopus oocytes using two-electrode voltage-clamp recordings, macropatch recordings, and summations of single-channel currents, in order to test a kinetic model recently developed by us from single-channel experiments. Both the forward and reverse macroscopic reactions, at negative and positive membrane potential V(M), respectively, were slow in comparison to those reactions for other CFTR pore blockers such as DPC and NPPB, resulting in prominent relaxations on the order of tens of milliseconds. The rate of the reverse reaction was voltage-dependent, and dependent on the Cl(-) driving force, while that of the forward reaction was not. In inside-out macropatches, block and relief from block occurred in two distinct phases that differed in apparent affinity. The results are consistent with the presence of multiple glibenclamide binding sites in CFTR, with varying affinity and voltage dependence; they support the kinetic model and suggest experimental approaches for identification of those sites by mutagenesis.

Direct comparison of the functional roles played by different transmembrane regions in the cystic fibrosis transmembrane conductance regulator chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel contains 12 transmembrane (TM) regions that are presumed to form the channel pore. However, little is known about the relative functional contribution of different TM regions to the pore. We have used patch clamp recording to investigate the functional consequences of point mutations throughout the six transmembrane regions in the N-terminal part of the CFTR protein (TM1-TM6). A range of specific functional assays compared the single channel conductance, anion binding, and anion selectivity properties of different channel variants. Overall, our results suggest that TM1 and -6 play dominant roles in forming the channel pore and determining its functional properties, with TM5 perhaps playing a lesser role. In contrast, TM2, -3, and -4 appear to play only minor supporting roles. These results define transmembrane regions 1 and 6 as major contributors to the CFTR channel pore and have strong implications for emerging structural models of CFTR and related ATP-binding cassette proteins.

β-Adrenergic receptors couple to CFTR chloride channels of intercalated mitochondria-rich cells in the heterocellular toad skin epithelium

In the heterocellular toad skin epithelium the beta-adrenergic receptor agonist isoproterenol activates cyclic AMP-dependent Cl(-) channels that are not located in the principal cells. With four experimental approaches, in the present study, we tested the hypothesis that the signalling pathway targets apical CFTR-chloride channels of mitochondria-rich cells. (i) Serosal application of isoproterenol (log(10)EC50=-7.1+/0.2; Hill coefficient=1.1+/0.2), as well as nor-adrenaline, activated an anion pathway with an apical selectivity sequence, G(Cl)>G(Br)> or =G(NO(3))>G(I), comparable to the published selectivity sequence of cloned human CFTR expressed in Xenopus oocytes. (ii) Known modulators of human CFTR, glibenclamide (200 micromol/l) and genistein (50 micromol/l), depressed and activated, respectively, the receptor-stimulated G(Cl). Genistein did not modify the anion selectivity. (iii) Transcellular voltage clamp studies of single isolated mitochondria-rich cells revealed functional beta-adrenergic receptors on the basolateral membrane. With approximately 60,000 mitochondria-rich cells per cm(2), the saturating activation of 11.9+/-1.6 ns/cell accounted for the measured isoproterenol-activated transepithelial conductance of 600-900 micros/cm(2). In forskolin-stimulated cells, glibenclamide (200 micromol/l) reversibly inhibited the transcellular conductance by 9.6+/1.6 ns/cell. (iv) A nucleotide sequence of one third of the Bufo bufo CFTR gene corresponding to the R-domain and part of the first nucleotide binding domain (NBD1) including its Walker motif was amplified from gallbladder epithelium. Somewhat smaller sequences of the BbCFTR were cloned from lung and isolated skin epithelium. The above new results taken together with our previously identified small-conductance CFTR-like Cl(-) channel in the apical membrane of isolated mitochondria-rich cells provide compelling evidence that the toad's CFTR gene codes for a functional Cl(-) channel in the apical plasma membrane of this minority cell type.

Stable dimeric assembly of the second membrane-spanning domain of CFTR (cystic fibrosis transmembrane conductance regulator) reconstitutes a chloride-selective pore

Structural information is required to define the molecular basis for chloride conduction through CFTR (cystic fibrosis transmembrane conductance regulator). Towards this goal, we expressed MSD2, the second of the two MSDs (membrane-spanning domains) of CFTR, encompassing residues 857-1158 in Sf9 cells using the baculovirus system. In Sf9 plasma membranes, MSD2 migrates as expected for a dimer in non-dissociative PAGE, and confers the appearance of an anion permeation pathway suggesting that dimeric MSD2 mediates anion flux. To assess directly the function and quaternary structure of MSD2, we purified it from Sf9 cells by virtue of its polyhistidine tag and nickel affinity. Reconstitution of MSD2 into liposomes conferred a 4,4'-di-isothiocyanostilbene-2,2'-disulphonate-inhibitable, chloride-selective electrodiffusion pathway. Further, this activity is probably mediated directly by MSD2 as reaction of its single cysteine residue (Cys866) with the thiol modifying reagent, N(alpha)(3-maleimidylpropionyl)biocytin, inhibited chloride flux. Only MSD2 dimers were labelled by N(alpha)(3-maleimidylpropionyl)biocytin, supporting the idea that only dimeric MSD2 can mediate anion flux. As a further test of this hypothesis, we conducted a second purification procedure, wherein purified dimeric and monomeric MSD2 proteins were reconstituted separately. Only proteoliposomes containing stable MSD2 dimers mediated chloride electrodiffusion, providing direct evidence that dimeric MSD2 mediates chloride channel function. In summary, we have shown that the second membrane domain of CFTR can be purified and functionally reconstituted as a chloride channel, providing a tool for probing the structural basis of chloride conduction through CFTR.

Voltage-dependent gating of the cystic fibrosis transmembrane conductance regulator Cl- channel

When excised inside-out membrane patches are bathed in symmetrical Cl--rich solutions, the current-voltage (I-V) relationship of macroscopic cystic fibrosis transmembrane conductance regulator (CFTR) Cl- currents inwardly rectifies at large positive voltages. To investigate the mechanism of inward rectification, we studied CFTR Cl- channels in excised inside-out membrane patches from cells expressing wild-type human and murine CFTR using voltage-ramp and -step protocols. Using a voltage-ramp protocol, the magnitude of human CFTR Cl- current at +100 mV was 74 +/- 2% (n = 10) of that at -100 mV. This rectification of macroscopic CFTR Cl- current was reproduced in full by ensemble currents generated by averaging single-channel currents elicited by an identical voltage-ramp protocol. However, using a voltage-step protocol the single-channel current amplitude (i) of human CFTR at +100 mV was 88 +/- 2% (n = 10) of that at -100 mV. Based on these data, we hypothesized that voltage might alter the gating behavior of human CFTR. Using linear three-state kinetic schemes, we demonstrated that voltage has marked effects on channel gating. Membrane depolarization decreased both the duration of bursts and the interburst interval, but increased the duration of gaps within bursts. However, because the voltage dependencies of the different rate constants were in opposite directions, voltage was without large effect on the open probability (Po) of human CFTR. In contrast, the Po of murine CFTR was decreased markedly at positive voltages, suggesting that the rectification of murine CFTR is stronger than that of human CFTR. We conclude that inward rectification of CFTR is caused by a reduction in i and changes in gating kinetics. We suggest that inward rectification is an intrinsic property of the CFTR Cl- channel and not the result of pore block.

Indazole inhibition of cystic fibrosis transmembrane conductance regulator Cl(-) channels in rat epididymal epithelial cells

Previous studies have shown that two indazole compounds, lonidamine [1-(2,4-dichlorobenzyl)-indazole-3-carboxylic acid] and its analogue AF2785 [(1-(2,4-dichlorobenzyl)-indazol-3-acrylic acid], suppress fertility in male rats. We also found that these compounds inhibit the cystic fibrosis transmembrane conductance regulator chloride (CFTR-Cl(-)) current in epididymal epithelial cells. To further investigate how lonidamine and AF2785 inhibit the current, we used a spectral analysis protocol to study whole-cell CFTR current variance. Application of lonidamine or AF2785 to the extracellular membrane of rat epididymal epithelial cells introduced a new component to the whole-cell current variance. Spectral analysis of this variance suggested a block at a rate of 3.68 micro mol(-1)/sec(-1) and an off rate of 69.01 sec(-1) for lonidamine, and an on rate of 3.27 micro mol(-1)/sec(-1) and an off rate of 108 sec(-1) for AF2785. Single CFTR-Cl(-) channel activity using excised inside-out membrane patches from rat epididymal epithelial cells revealed that addition of lonidamine to the intracellular solution caused a flickery block (a reduction in channel-open time) at lower concentration (10 micro M) without any effect on open channel probability or single-channel current amplitude. At higher concentrations (50 and 100 micro M), lonidamine showed a flickery block and a decrease in open-channel probability. The flickery block by lonidamine was both voltage-dependent and concentration-dependent. These results suggest that lonidamine and AF2785, which are open-channel blockers of CFTR at low concentrations, also affect CFTR gating at high concentrations. We conclude that these indazole compounds provide new pharmacological tools for the investigation of CFTR. By virtue of their interference with reproductive processes, these drugs have the potential for being developed into novel male contraceptives.

Voltage-sensitive gating induced by a mutation in the fifth transmembrane domain of CFTR

A mutation in the fifth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel (V317E) resulted in whole cell currents that exhibited marked outward rectification on expression in Xenopus oocytes. However, the single-channel unitary current (i)-voltage (V) relationship failed to account for the rectification of whole cell currents. In excised patches containing one to a few channels, the time-averaged single-channel current (I)-V relationship (I = N x P(o) x i, where N is the number of active channels and P(o) is open probability) of V317E CFTR displayed outward rectification, whereas that of wild-type CFTR was linear, indicating that the P(o) of V317E CFTR is voltage dependent. The decrease in P(o) at negative potentials was due to both a decreased burst duration and a decreased opening rate that could not be ameliorated by a 10-fold increase in ATP concentration. This behavior appears to reflect a true voltage dependence of the gating process because the P(o)-V relationship did not depend on the direction of Cl(-) movement. The results are consistent with the introduction, by a point mutation, of a novel voltage-dependent gating mode that may provide a useful tool for probing the portions of the protein that move in response to ATP-dependent gating.

CFTR: covalent and noncovalent modification suggests a role for fixed charges in anion conduction

The goal of the experiments described here was to explore the possible role of fixed charges in determining the conduction properties of CFTR. We focused on transmembrane segment 6 (TM6) which contains four basic residues (R334, K335, R347, and R352) that would be predicted, on the basis of their positions in the primary structure, to span TM6 from near the extracellular (R334, K335) to near the intracellular (R347, R352) end. Cysteines substituted at positions 334 and 335 were readily accessible to thiol reagents, whereas those at positions 347 and 352 were either not accessible or lacked significant functional consequences when modified. The charge at positions 334 and 335 was an important determinant of CFTR channel function. Charge changes at position 334--brought about by covalent modification of engineered cysteine residues, pH titration of cysteine and histidine residues, and amino acid substitution--produced similar effects on macroscopic conductance and the shape of the I-V plot. The effect of charge changes at position 334 on conduction properties could be described by electrodiffusion or rate-theory models in which the charge on this residue lies in an external vestibule of the pore where it functions to increase the concentration of Cl adjacent to the rate-limiting portion of the conduction path. Covalent modification of R334C CFTR increased single-channel conductance determined in detached patches, but did not alter open probability. The results are consistent with the hypothesis that in wild-type CFTR, R334 occupies a position where its charge can influence the distribution of anions near the mouth of the pore.

Expression of cystic fibrosis transmembrane conductance regulator in the skin of the toad, Bufo bufo and possible role for Cl- transport across the heterocellular epithelium

Evidence is discussed that apical CFTR Cl- channels of mitochondria-rich (MR) cells of Bufo bufo skin conduct beta-adrenergic receptor-activated Cl- currents. Ussing chambers studies revealed the following selectivity sequence of the receptor activated conductance, Cl- > Br- > NO3- > I-. With ion selective microelectrode-techniques, it was shown that receptor-coupled Cl- channels are not located in principal cells. A small conductance (7-10 pS) CFTR-like Cl- channel is located in the apical plasma membrane of MR cells. Short life times of sealed patches prevented detailed study of its selectivity to other halide ions and its molecular regulation. With monoclonal hCFTR-antibodies, selective expression in MR cells of the targeted antigens could be demonstrated. A transcript of CFTR was amplified in the skin, and a bbCFTR cDNA clone was generated from toad skin mRNA that exhibits 89% amino acid identity with the human homologue. The frequency of obtaining channels in patch clamp studies was too low for accounting quantitatively for the macroscopic conductance. Since MR cells were isolated by trypsin, and a putative extracellular loop of the deduced bbCFTR protein contains a target peptide bond for trypsin, enzyme treatment may have destroyed apical CFTR molecules.

Identification of a region of strong discrimination in the pore of CFTR

The variety of methods used to identify the structural determinants of anion selectivity in the cystic fibrosis transmembrane conductance regulator Cl(-) channel has made it difficult to assemble the data into a coherent framework that describes the three-dimensional structure of the pore. Here, we compare the relative importance of sites previously studied and identify new sites that contribute strongly to anion selectivity. We studied Cl(-) and substitute anions in oocytes expressing wild-type cystic fibrosis transmembrane conductance regulator or 12-pore-domain mutants to determine relative permeability and relative conductance for 9 monovalent anions and 1 divalent anion. The data indicate that the region of strong discrimination resides between T338 and S341 in transmembrane 6, where mutations affected selectivity between Cl(-) and both large and small anions. Mutations further toward the extracellular end of the pore only strongly affected selectivity between Cl(-) and larger anions. Only mutations at S341 affected selectivity between monovalent and divalent anions. The data are consistent with a narrowing of the pore between the extracellular end and a constriction near the middle of the pore.

Expression of cystic fibrosis transmembrane conductance regulator in rat efferent duct epithelium

The expression of cystic fibrosis transmembrane conductance regulator (CFTR) was studied in rat efferent ducts. Under whole-cell patch-clamp condition, efferent duct cells responded to intracellular cAMP with a rise in inward current. The cAMP-activated current exhibited a linear I-V relationship and time- and voltage-independent characteristics. The current was inhibited by the Cl(-) channel blocker diphenylamine 2,2'-dicarboxylic acid (DPC) in a voltage-dependent manner and reversed at 24 +/- 0.5 mV, close to the equilibrium potential for Cl(-) (30 mV), suggesting that the current was Cl(-) selective. The cAMP-activated current displayed a permeability sequence of Br(-) > Cl(-) > I(-). Short-circuit current measurement in cultured rat efferent duct epithelia also revealed a cAMP-activated inward current inhibitable by DPC. These electrophysiological properties of the cAMP-activated Cl(-) conductance in the efferent duct were consistent with those reported for CFTR. In support of the functional studies, reverse transcription polymerase chain reaction revealed the presence of CFTR message in cultured efferent duct epithelium. Immunohistochemical studies in intact rats also demonstrated CFTR protein at the apical membrane of the principal cells of efferent duct. CFTR may play a role in modulating fluid transport in the efferent duct.

Perturbation of the pore of the cystic fibrosis transmembrane conductance regulator (CFTR) inhibits its atpase activity

Mutations in the cystic fibrosis gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) lead to altered chloride (Cl(-)) flux in affected epithelial tissues. CFTR is a Cl(-) channel that is regulated by phosphorylation, nucleotide binding, and hydrolysis. However, the molecular basis for the functional regulation of wild type and mutant CFTR remains poorly understood. CFTR possesses two nucleotide binding domains, a phosphorylation-dependent regulatory domain, and two transmembrane domains that comprise the pore through which Cl(-) permeates. Mutations of residues lining the channel pore (e.g. R347D) are typically thought to cause disease by altering the interaction of Cl(-) with the pore. However, in the present study we show that the R347D mutation and diphenylamine-2-carboxylate (an open pore inhibitor) also inhibit CFTR ATPase activity, revealing a novel mechanism for cross-talk from the pore to the catalytic domains. In both cases, the reduction in ATPase correlates with a decrease in nucleotide turnover rather than affinity. Finally, we demonstrate that glutathione (GSH) inhibits CFTR ATPase and that this inhibition is altered in the CFTR-R347D variant. These findings suggest that cross-talk between the pore and nucleotide binding domains of CFTR may be important in the in vivo regulation of CFTR in health and disease.

Relationship between anion binding and anion permeability revealed by mutagenesis within the cystic fibrosis transmembrane conductance regulator chloride channel pore

1. Anion binding within the pores of wild-type and mutant cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, expressed in two different mammalian cell lines, was assayed using patch clamp recording. Specifically, experiments measured both the conductance of different anions and the ability of other permeant anions to block Cl- permeation through the pore. 2. Under symmetrical ionic conditions, wild-type CFTR channels showed the conductance sequence Cl- > NO3- > Br- > or = formate > F- > SCN- congruent to ClO4-. 3. High SCN- conductance was not observed, nor was there an anomalous mole fraction effect of SCN- on conductance under the conditions used. Iodide currents could not be measured under symmetrical ionic conditions, but under bi-ionic conditions I- conductance appeared low. 4. Chloride currents through CFTR channels were blocked by low concentrations (10 mM) of SCN-, I- and ClO4-, implying relatively tight binding of these anions within the pore. 5. Two mutations in CFTR which alter the anion permeability sequence, F337S and T338A, also altered the anion conductance sequence. Furthermore, block by SCN-, I- and ClO4- were weakened in both mutants. Both these effects are consistent with altered anion binding within the pore. 6. The effects of mutations on anion permeability and relative anion conductance suggested that, for most anions, increased permeability was associated with increased conductance. This indicates that the CFTR channel pore does not achieve its anion selectivity by selective anion binding within the mutated region. Instead, it is suggested that entry of anions into the region around F337 and T338 facilitates their passage through the pore. In wild-type CFTR channels, anion entry into this crucial pore region is probably dominated by anion hydration energies.