Identification of a second blocker binding site at the cytoplasmic mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Structural identification of a selectivity filter in CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that regulates transepithelial salt and fluid homeostasis. CFTR dysfunction leads to reduced chloride secretion into the mucosal lining of epithelial tissues, thereby causing the inherited disease cystic fibrosis. Although several structures of CFTR are available, our understanding of the ion-conduction pathway is incomplete. In particular, the route that connects the cytosolic vestibule with the extracellular space has not been clearly defined, and the structure of the open pore remains elusive. Furthermore, although many residues have been implicated in altering the selectivity of CFTR, the structure of the "selectivity filter" has yet to be determined. In this study, we identify a chloride-binding site at the extracellular ends of transmembrane helices 1, 6, and 8, where a dehydrated chloride is coordinated by residues G103, R334, F337, T338, and Y914. Alterations to this site, consistent with its function as a selectivity filter, affect ion selectivity, conductance, and open channel block. This selectivity filter is accessible from the cytosol through a large inner vestibule and opens to the extracellular solvent through a narrow portal. The identification of a chloride-binding site at the intra- and extracellular bridging point leads us to propose a complete conductance path that permits dehydrated chloride ions to traverse the lipid bilayer.

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.

Functionally Additive Fixed Positive and Negative Charges in the CFTR Channel Pore Control Anion Binding and Conductance

Ion channels use charged amino-acid residues to attract oppositely charged permeant ions into the channel pore. In the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel, a number of arginine and lysine residues have been shown to be important for Cl⁻ permeation. Among these, two in close proximity in the pore—Lys⁹⁵ and Arg¹³⁴—are indispensable for anion binding and high Cl⁻ conductance, suggesting that high positive charge density is required for pore function. Here we used mutagenesis and functional characterization to show that a nearby pore-lining negatively charged residue (Glu⁹²) plays a functionally additive role with these two positive charges. While neutralization of this negative charge had little effect on anion binding or Cl⁻ conductance, such neutralization was able to reverse the detrimental effects of removing the positive charge at either Lys⁹⁵ or Arg¹³⁴, as well as the similar effects of introducing a negative charge at a neighboring residue (Ser¹¹⁴¹). Furthermore, neutralization of Glu⁹² greatly increased the susceptibility of the channel to blockage by divalent S₂O₃²⁻ anions, mimicking the effect of introducing additional positive charge in this region; this effect was reversed by concurrent neutralization of either Lys⁹⁵ or Arg¹³⁴. Across a panel of mutant channels that introduced or removed fixed charges at these four positions, we found that many pore properties are dependent on the overall charge or charge density. We propose that the CFTR pore uses a combination of positively and negatively charged residues to optimize the anion binding and Cl⁻ conductance properties of the channel.

Suramin exposure alters cellular metabolism and mitochondrial energy production in African trypanosomes

Introduced about a century ago, suramin remains a frontline drug for the management of early-stage East African trypanosomiasis (sleeping sickness). Cellular entry into the causative agent, the protozoan parasite Trypanosoma brucei, occurs through receptor-mediated endocytosis involving the parasite's invariant surface glycoprotein 75 (ISG75), followed by transport into the cytosol via a lysosomal transporter. The molecular basis of the trypanocidal activity of suramin remains unclear, but some evidence suggests broad, but specific, impacts on trypanosome metabolism (i.e. polypharmacology). Here we observed that suramin is rapidly accumulated in trypanosome cells proportionally to ISG75 abundance. Although we found little evidence that suramin disrupts glycolytic or glycosomal pathways, we noted increased mitochondrial ATP production, but a net decrease in cellular ATP levels. Metabolomics highlighted additional impacts on mitochondrial metabolism, including partial Krebs' cycle activation and significant accumulation of pyruvate, corroborated by increased expression of mitochondrial enzymes and transporters. Significantly, the vast majority of suramin-induced proteins were normally more abundant in the insect forms compared with the blood stage of the parasite, including several proteins associated with differentiation. We conclude that suramin has multiple and complex effects on trypanosomes, but unexpectedly partially activates mitochondrial ATP-generating activity. We propose that despite apparent compensatory mechanisms in drug-challenged cells, the suramin-induced collapse of cellular ATP ultimately leads to trypanosome cell death.

Electrostatic Tuning of Anion Attraction from the Cytoplasm to the Pore of the CFTR Chloride Channel

Anions enter from the cytoplasm into the channel pore of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel not via a central pathway but via a single lateral portal or fenestration. High Cl^- conductance is dependent on electrostatic attraction of cytoplasmic Cl^- ions by four positively charged amino acid side-chains located within this portal. Here we use a mutagenic approach to investigate the functional effects of transplanting or supplementing these positive charges at nearby portal-lining sites. Using patch clamp recording, we find that the functionally important positive charges at K190 and R303 can be transplanted to four nearby sites (N186, L197, W356, and A367) with little loss of Cl^- conductance. Introduction of additional positive charge at these sites had almost no effect on Cl^- conductance, but did increase the sensitivity to channel block by intracellular suramin and Pt(NO₂)₄^2- anions. We suggest that it is the number of positive charges within the portal, rather than their exact location, that is the most important factor influencing Cl^- conductance. The portal appears well optimized in terms of charge distribution to maximize Cl^- conductance.

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.

Structural mechanisms of CFTR function and dysfunction

Hwang et al. integrate new structural insights with prior functional studies to reveal the functional anatomy of CFTR chloride channels.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel plays a critical role in regulating transepithelial movement of water and electrolyte in exocrine tissues. Malfunction of the channel because of mutations of the cftr gene results in CF, the most prevalent lethal genetic disease among Caucasians. Recently, the publication of atomic structures of CFTR in two distinct conformations provides, for the first time, a clear overview of the protein. However, given the highly dynamic nature of the interactions among CFTR’s various domains, better understanding of the functional significance of these structures requires an integration of these new structural insights with previously established biochemical/biophysical studies, which is the goal of this review.

Functional organization of cytoplasmic portals controlling access to the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl^- channel that apparently has evolved from an ancestral active transporter. Key to the CFTR's switch from pump to channel function may have been the appearance of one or more "lateral portals." Such portals connect the cytoplasm to the transmembrane channel pore, allowing a continuous pathway for the electrodiffusional movement of Cl^- ions. However, these portals remain the least well-characterized part of the Cl^- transport pathway; even the number of functional portals is uncertain, and if multiple portals do exist, their relative functional contributions are unknown. Here, we used patch-clamp recording to identify the contributions of positively charged amino acid side chains located in CFTR's cytoplasmic transmembrane extensions to portal function. Mutagenesis-mediated neutralization of several charged side chains reduced single-channel Cl^- conductance. However, these same mutations differentially affected channel blockade by cytoplasmic suramin and Pt(NO₂)₄^2- anions. We considered and tested several models by which the contribution of these positively charged side chains to one or more independent or non-independent portals to the pore could affect Cl^- conductance and interactions with blockers. Overall, our results suggest the existence of a single portal that is lined by several positively charged side chains that interact electrostatically with both Cl^- and blocking anions. We further propose that mutations at other sites indirectly alter the function of this single portal. Comparison of our functional results with recent structural information on CFTR completes our picture of the overall molecular architecture of the Cl^- permeation pathway.

Molecular modelling and molecular dynamics of CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a member of the ATP-binding cassette (ABC) transporter superfamily that functions as an ATP-gated channel. Considerable progress has been made over the last years in the understanding of the molecular basis of the CFTR functions, as well as dysfunctions causing the common genetic disease cystic fibrosis (CF). This review provides a global overview of the theoretical studies that have been performed so far, especially molecular modelling and molecular dynamics (MD) simulations. A special emphasis is placed on the CFTR-specific evolution of an ABC transporter framework towards a channel function, as well as on the understanding of the effects of disease-causing mutations and their specific modulation. This in silico work should help structure-based drug discovery and design, with a view to develop CFTR-specific pharmacotherapeutic approaches for the treatment of CF in the context of precision medicine.

Architecture and functional properties of the CFTR channel pore

The main function of the cystic fibrosis transmembrane conductance regulator (CFTR) is as an ion channel for the movement of small anions across epithelial cell membranes. As an ion channel, CFTR must form a continuous pathway across the cell membrane-referred to as the channel pore-for the rapid electrodiffusional movement of ions. This review summarizes our current understanding of the architecture of the channel pore, as defined by electrophysiological analysis and molecular modeling studies. This includes consideration of the characteristic functional properties of the pore, definition of the overall shape of the entire extent of the pore, and discussion of how the molecular structure of distinct regions of the pore might control different facets of pore function. Comparisons are drawn with closely related proteins that are not ion channels, and also with structurally unrelated proteins with anion channel function. A simple model of pore function is also described.

Double Blockade of Glioma Cell Proliferation and Migration by Temozolomide Conjugated with NPPB, a Chloride Channel Blocker

Glioblastoma is the most common and aggressive primary malignant brain tumor. Temozolomide (TMZ), a chemotherapeutic agent combined with radiation therapy, is used as a standard treatment. The infiltrative nature of glioblastoma, however, interrupts effective treatment with TMZ and increases the tendency to relapse. Voltage-gated chloride channels have been identified as crucial regulators of glioma cell migration and invasion by mediating cell shape and volume change. Accordingly, chloride current inhibition by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), a chloride channel blocker, suppresses cell movement by diminishing the osmotic cell volume regulation. In this study, we developed a novel compound, TMZ conjugated with NPPB (TMZ-NPPB), as a potential anticancer drug. TMZ-NPPB blocked chloride currents in U373MG, a severely invasive human glioma cell line, and suppressed migration and invasion of U373MG cells. Moreover, TMZ-NPPB exhibited DNA modification activity similar to that of TMZ, and surprisingly showed remarkably enhanced cytotoxicity relative to TMZ by inducing apoptotic cell death via DNA damage. These findings indicate that TMZ-NPPB has a dual function in blocking both proliferation and migration of human glioma cells, thereby suggesting its potential to overcome challenges in current glioblastoma therapy.

The Fifth Transmembrane Segment of Cystic Fibrosis Transmembrane Conductance Regulator Contributes to Its Anion Permeation Pathway

Previous studies have identified several transmembrane segments (TMs), including TM1, TM3, TM6, TM9, TM11, and TM12, as pore-lining segments in cystic fibrosis transmembrane conductance regulator (CFTR), but the role of TM5 in pore construction remains controversial. In this study, we employed substituted cysteine accessibility methodology (SCAM) to screen the entire TM5 defined by the original topology model and its cytoplasmic extension in a Cysless background. We found six positions (A299, R303, N306, S307, F310, and F311) where engineered cysteines react to intracellular 2-sulfonatoethyl methanethiosulfonate (MTSES⁻). Quantification of the modification rate of engineered cysteines in the presence or absence of ATP suggests that these six residues are accessible in both the open and closed states. Whole-cell experiments with external MTSES⁻ identified only two positive positions (L323 and A326), resulting in a segment containing 11 consecutive amino acids, where substituted cysteines respond to neither internal nor external MTSES⁻, a unique feature not seen previously in CFTR's pore-lining segments. The observation that these positions are inaccessible to channel-permeant thiol-specific reagent [Au(CN)₂]⁻ suggests that this segment of TM5 between F311 and L323 is concealed from the pore by other TMs and/or lipid bilayers. In addition, our data support the idea that the positively charged arginine at position 303 poses a pure electrostatic action in determining the single-channel current amplitude of CFTR and the effect of an open-channel blocker glibencalmide. Collectively, we conclude that the cytoplasmic portion of CFTR's TM5 lines the pore. Our functional data are remarkably consistent with predicted structural arrangements of TM5 in some homology models of CFTR.

Functional Architecture of the Cytoplasmic Entrance to the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel Pore

As an ion channel, the cystic fibrosis transmembrane conductance regulator must form a continuous pathway for the movement of Cl(-) and other anions between the cytoplasm and the extracellular solution. Both the structure and the function of the membrane-spanning part of this pathway are well defined. In contrast, the structure of the pathway that connects the cytoplasm to the membrane-spanning regions is unknown, and functional roles for different parts of the protein forming this pathway have not been described. We used patch clamp recording and substituted cysteine accessibility mutagenesis to identify positively charged amino acid side chains that attract cytoplasmic Cl(-) ions to the inner mouth of the pore. Our results indicate that the side chains of Lys-190, Arg-248, Arg-303, Lys-370, Lys-1041, and Arg-1048, located in different intracellular loops of the protein, play important roles in the electrostatic attraction of Cl(-) ions. Mutation and covalent modification of these residues have charge-dependent effects on the rate of Cl(-) permeation, demonstrating their functional role in maximization of Cl(-) flux. Other nearby positively charged side chains were not involved in electrostatic interactions with Cl(-). The location of these Cl(-)-attractive residues suggests that cytoplasmic Cl(-) ions enter the pore via a lateral portal located between the cytoplasmic extensions to the fourth and sixth transmembrane helices; a secondary, functionally less relevant portal might exist between the extensions to the 10th and 12th transmembrane helices. These results define the cytoplasmic mouth of the pore and show how it attracts Cl(-) ions from the cytoplasm.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Full-open and closed CFTR channels, with lateral tunnels from the cytoplasm and an alternative position of the F508 region, as revealed by molecular dynamics

In absence of experimental 3D structures, several homology models, based on ABC exporter 3D structures, have provided significant insights into the molecular mechanisms underlying the function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel whose defects are associated with cystic fibrosis (CF). Until now, these models, however, did not furnished much insights into the continuous way that ions could follow from the cytosol to the extracellular milieu in the open form of the channel. Here, we have built a refined model of CFTR, based on the outward-facing Sav1866 experimental 3D structure and integrating the evolutionary and structural information available today. Molecular dynamics simulations revealed significant conformational changes, resulting in a full-open channel, accessible from the cytosol through lateral tunnels displayed in the long intracellular loops (ICLs). At the same time, the region of nucleotide-binding domain 1 in contact with one of the ICLs and carrying amino acid F508, the deletion of which is the most common CF-causing mutation, was found to adopt an alternative but stable position. Then, in a second step, this first stable full-open conformation evolved toward another stable state, in which only a limited displacement of the upper part of the transmembrane helices leads to a closure of the channel, in a conformation very close to that adopted by the Atm1 ABC exporter, in an inward-facing conformation. These models, supported by experimental data, provide significant new insights into the CFTR structure-function relationships and into the possible impact of CF-causing mutations.

Loop diuretics are open-channel blockers of the cystic fibrosis transmembrane conductance regulator with distinct kinetics

BACKGROUND AND PURPOSE

Loop diuretics are widely used to inhibit the Na(+), K(+), 2Cl(-) co-transporter, but they also inhibit the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. Here, we investigated the mechanism of CFTR inhibition by loop diuretics and explored the effects of chemical structure on channel blockade.

EXPERIMENTAL APPROACH

Using the patch-clamp technique, we tested the effects of bumetanide, furosemide, piretanide and xipamide on recombinant wild-type human CFTR.

KEY RESULTS

When added to the intracellular solution, loop diuretics inhibited CFTR Cl(-) currents with potency approaching that of glibenclamide, a widely used CFTR blocker with some structural similarity to loop diuretics. To begin to study the kinetics of channel blockade, we examined the time dependence of macroscopic current inhibition following a hyperpolarizing voltage step. Like glibenclamide, piretanide blockade of CFTR was time and voltage dependent. By contrast, furosemide blockade was voltage dependent, but time independent. Consistent with these data, furosemide blocked individual CFTR Cl(-) channels with 'very fast' speed and drug-induced blocking events overlapped brief channel closures, whereas piretanide inhibited individual channels with 'intermediate' speed and drug-induced blocking events were distinct from channel closures.

CONCLUSIONS AND IMPLICATIONS

Structure-activity analysis of the loop diuretics suggests that the phenoxy group present in bumetanide and piretanide, but absent in furosemide and xipamide, might account for the different kinetics of channel block by locking loop diuretics within the intracellular vestibule of the CFTR pore. We conclude that loop diuretics are open-channel blockers of CFTR with distinct kinetics, affected by molecular dimensions and lipophilicity.

Cystic fibrosis transmembrane conductance regulator chloride channel blockers: Pharmacological, biophysical and physiological relevance

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel causes cystic fibrosis, while inappropriate activity of this channel occurs in secretory diarrhea and polycystic kidney disease. Drugs that interact directly with CFTR are therefore of interest in the treatment of a number of disease states. This review focuses on one class of small molecules that interacts directly with CFTR, namely inhibitors that act by directly blocking chloride movement through the open channel pore. In theory such compounds could be of use in the treatment of diarrhea and polycystic kidney disease, however in practice all known substances acting by this mechanism to inhibit CFTR function lack either the potency or specificity for in vivo use. Nevertheless, this theoretical pharmacological usefulness set the scene for the development of more potent, specific CFTR inhibitors. Biophysically, open channel blockers have proven most useful as experimental probes of the structure and function of the CFTR chloride channel pore. Most importantly, the use of these blockers has been fundamental in developing a functional model of the pore that includes a wide inner vestibule that uses positively charged amino acid side chains to attract both permeant and blocking anions from the cell cytoplasm. CFTR channels are also subject to this kind of blocking action by endogenous anions present in the cell cytoplasm, and recently this blocking effect has been suggested to play a role in the physiological control of CFTR channel function, in particular as a novel mechanism linking CFTR function dynamically to the composition of epithelial cell secretions. It has also been suggested that future drugs could target this same pathway as a way of pharmacologically increasing CFTR activity in cystic fibrosis. Studying open channel blockers and their mechanisms of action has resulted in significant advances in our understanding of CFTR as a pharmacological target in disease states, of CFTR channel structure and function, and of how CFTR activity is controlled by its local environment.

Acute inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel by thyroid hormones involves multiple mechanisms

The chemical structures of the thyroid hormones triiodothyronine (T3) and thyroxine (T4) resemble those of small-molecules that inhibit the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. We therefore tested the acute effects of T3, T4 and reverse T3 (rT3) on recombinant wild-type human CFTR using the patch-clamp technique. When added directly to the intracellular solution bathing excised membrane patches, T3, T4, and rT3 (all tested at 50 μM) inhibited CFTR in several ways: they strongly reduced CFTR open probability by impeding channel opening; they moderately decreased single-channel current amplitude, and they promoted transitions to subconductance states. To investigate the mechanism of CFTR inhibition, we studied T3. T3 (50 μM) had multiple effects on CFTR gating kinetics, suggestive of both allosteric inhibition and open-channel blockade. Channel inhibition by T3 was weakly voltage dependent and stronger than the allosteric inhibitor genistein, but weaker than the open-channel blocker glibenclamide. Raising the intracellular ATP concentration abrogated T3 inhibition of CFTR gating, but not the reduction in single-channel current amplitude nor the transitions to subconductance states. The decrease in single-channel current amplitude was relieved by membrane depolarization, but not the transitions to subconductance states. We conclude that T3 has complex effects on CFTR consistent with both allosteric inhibition and open-channel blockade. Our results suggest that there are multiple allosteric mechanisms of CFTR inhibition, including interference with ATP-dependent channel gating and obstruction of conformational changes that gate the CFTR pore. CFTR inhibition by thyroid hormones has implications for the development of innovative small-molecule CFTR inhibitors.

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.

Tuning of CFTR chloride channel function by location of positive charges within the pore

High unitary Cl(-) conductance in the cystic fibrosis transmembrane conductance regulator Cl(-) channel requires a functionally unique, positively charged lysine residue (K95) in the inner vestibule of the channel pore. Here we used a mutagenic approach to investigate the ability of other sites in the pore to host this important positive charge. The loss of conductance observed in the K95Q mutation was >50% rescued by substituting a lysine for each of five different pore-lining amino acids, suggesting that the exact location of the fixed positive charge is not crucial to support high conductance. Moving the positive charge also restored open-channel blocker interactions that are lost in K95Q. Introducing a second positive charge in addition to that at K95 did not increase conductance at any site, but did result in a striking increase in the strength of block by divalent Pt(NO(2))(4)(2-) ions. Based on the site dependence of these effects, we propose that although the exact location of the positive charge is not crucial for normal pore properties, transplanting this charge to other sites results in a diminution of its effectiveness that appears to depend on its location along the axis of the pore.

Divergent CFTR orthologs respond differently to the channel inhibitors CFTRinh-172, glibenclamide, and GlyH-101

Comparison of diverse orthologs is a powerful tool to study the structure and function of channel proteins. We investigated the response of human, killifish, pig, and shark cystic fibrosis transmembrane conductance regulator (CFTR) to specific inhibitors of the channel: CFTR(inh)-172, glibenclamide, and GlyH-101. In three systems, including organ perfusion of the shark rectal gland, primary cultures of shark rectal gland tubules, and expression studies of each ortholog in cRNA microinjected Xenopus laevis oocytes, we observed fundamental differences in the sensitivity to inhibition by these channel blockers. In organ perfusion studies, shark CFTR was insensitive to inhibition by CFTR(inh)-172. This insensitivity was also seen in short-circuit current experiments with cultured rectal gland tubular epithelial cells (maximum inhibition 4 ± 1.3%). In oocyte expression studies, shark CFTR was again insensitive to CFTR(inh)-172 (maximum inhibition 10.3 ± 2.5% at 25 μM), pig CFTR was insensitive to glibenclamide (maximum inhibition 18.4 ± 4.4% at 250 μM), and all orthologs were sensitive to GlyH-101. The amino acid residues considered responsible by previous site-directed mutagenesis for binding of the three inhibitors are conserved in the four CFTR isoforms studied. These experiments demonstrate a profound difference in the sensitivity of different orthologs of CFTR proteins to inhibition by CFTR blockers that cannot be explained by mutagenesis of single amino acids. We believe that the potency of the inhibitors CFTR(inh)-172, glibenclamide, and GlyH-101 on the CFTR chloride channel protein is likely dictated by the local environment and the three-dimensional structure of additional residues that form the vestibules, the chloride pore, and regulatory regions of the channel.

Functional differences in pore properties between wild-type and cysteine-less forms of the CFTR chloride channel

Studies of the structure and function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel have been advanced by the development of functional channel variants in which all 18 endogenous cysteine residues have been mutated ("cys-less" CFTR). However, cys-less CFTR has a slightly higher single-channel conductance than wild-type CFTR, raising questions as to the suitability of cys-less as a model of the wild-type CFTR pore. We used site-directed mutagenesis and patch-clamp recording to investigate the origin of this conductance difference and to determine the extent of functional differences between wild-type and cys-less CFTR channel permeation properties. Our results suggest that the conductance difference is the result of a single substitution, of C343: the point mutant C343S has a conductance similar to cys-less, whereas the reverse mutation, S343C in a cys-less background, restores wild-type conductance levels. Other cysteine substitutions (C128S, C225S, C376S, C866S) were without effect. Substitution of other residues for C343 suggested that conductance is dependent on amino acid side chain volume at this position. A range of other functional pore properties, including interactions with channel blockers (Au[CN] (2) (-) , 5-nitro-2-[3-phenylpropylamino]benzoic acid, suramin) and anion permeability, were not significantly different between wild-type and cys-less CFTR. Our results suggest that functional differences between these two CFTR constructs are of limited scale and scope and result from a small change in side chain volume at position 343. These results therefore support the use of cys-less as a model of the CFTR pore region.

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.

Regulation of CFTR chloride channel macroscopic conductance by extracellular bicarbonate

The CFTR contributes to Cl⁻ and HCO₃⁻ transport across epithelial cell apical membranes. The extracellular face of CFTR is exposed to varying concentrations of Cl⁻ and HCO₃⁻ in epithelial tissues, and there is evidence that CFTR is sensitive to changes in extracellular anion concentrations. Here we present functional evidence that extracellular Cl⁻ and HCO₃⁻ regulate anion conduction in open CFTR channels. Using cell-attached and inside-out patch-clamp recordings from constitutively active mutant E1371Q-CFTR channels, we show that voltage-dependent inhibition of CFTR currents in intact cells is significantly stronger when the extracellular solution contains HCO₃⁻ than when it contains Cl⁻. This difference appears to reflect differences in the ability of extracellular HCO₃⁻ and Cl⁻ to interact with and repel intracellular blocking anions from the pore. Strong block by endogenous cytosolic anions leading to reduced CFTR channel currents in intact cells occurs at physiologically relevant HCO₃⁻ concentrations and membrane potentials and can result in up to ∼50% inhibition of current amplitude. We propose that channel block by cytosolic anions is a previously unrecognized, physiologically relevant mechanism of channel regulation that confers on CFTR channels sensitivity to different anions in the extracellular fluid. We further suggest that this anion sensitivity represents a feedback mechanism by which CFTR-dependent anion secretion could be regulated by the composition of the secretions themselves. Implications for the mechanism and regulation of CFTR-dependent secretion in epithelial tissues are discussed.

Changes in accessibility of cytoplasmic substances to the pore associated with activation of the cystic fibrosis transmembrane conductance regulator chloride channel

Opening of the cystic fibrosis transmembrane conductance regulator Cl(-) channel is dependent both on phosphorylation and on ATP binding and hydrolysis. However, the mechanisms by which these cytoplasmic regulatory factors open the Cl(-) channel pore are not known. We have used patch clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of the pore-lining sixth transmembrane region (TM6) of a cysteine-less variant of cystic fibrosis transmembrane conductance regulator. We find that methanethiosulfonate (MTS) reagents modify irreversibly cysteines substituted for TM6 residues Phe-337, Thr-338, Ser-341, Ile-344, Val-345, Met-348, Ala-349, Arg-352, and Gln-353 when applied to the cytoplasmic side of open channels. However, the apparent rate of modification by internal [2-sulfonatoethyl] methanethiosulfonate (MTSES), a negatively charged MTS reagent, is dependent on the activation state of the channels. In particular, cysteines introduced far along the axis of TM6 from the inside (T338C, S341C, I344C) showed no evidence of significant modification even after prolonged pretreatment of non-activated channels with internal MTSES. In contrast, cysteines introduced closer to the inside of TM6 (V345C, M348C) were readily modified in both activated and non-activated channels. Access of a permeant anion, Au(CN)(2)(-), to T338C was similarly dependent upon channel activation state. The pattern of MTS modification we observe allows us to designate different pore-lining amino acid side chains to distinct functional regions of the channel pore. One logical interpretation of these findings is that cytoplasmic access to residues at the narrowest region of the pore changes concomitant with activation.

Calcium activates a chloride conductance likely involved in olfactory receptor neuron repolarization in the moth Spodoptera littoralis

The response of insect olfactory receptor neurons (ORNs) to odorants involves the opening of Ca(2+)-permeable channels, generating an increase in intracellular Ca(2+) concentration. Here, we studied the downstream effect of this Ca(2+) rise in cultured ORNs of the moth Spodoptera littoralis. Intracellular dialysis of Ca(2+) from the patch pipette in whole-cell patch-clamp configuration activated a conductance with a K(1/2) of 2.8 microm. Intracellular and extracellular anionic and cationic substitutions demonstrated that Cl(-) carries this current. The anion permeability sequence I(-) > NO(3)(-) > Br(-) > Cl(-) > CH(3)SO(3)(-) >> gluconate(-) of the Ca(2+)-activated Cl(-) channel suggests a weak electrical field pore of the channel. The Ca(2+)-activated current partly inactivated over time and did not depend on protein kinase C (PKC) and CaMKII activity or on calmodulin. Application of Cl(-) channel blockers, flufenamic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or niflumic acid reversibly blocked the Ca(2+)-activated current. In addition, lowering Cl(-) concentration in the sensillar lymph bathing the ORN outer dendrites caused a significant delay in pheromone response termination in vivo. The present work identifies a new Cl(-) conductance activated by Ca(2+) in insect ORNs presumably required for ORN repolarization.

Therapeutic potential of cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors in polycystic kidney disease

In the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD), kidney function is disrupted by multiple fluid-filled epithelial cysts. Cyst growth in ADPKD involves fluid accumulation within the cyst lumen driven by cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial Cl- secretion. This suggests that inhibitors of the CFTR Cl- channel might retard cyst growth. This review considers how knowledge of CFTR structure and function and its role in transepithelial salt and water movements provides insight into the mechanism of action of CFTR inhibitors. Some small molecules, termed open-channel blockers, inhibit directly the CFTR Cl- channel by physically obstructing the CFTR pore and preventing Cl- flow. By contrast, other small molecules, termed allosteric inhibitors, bind to CFTR at a site remote from the channel pore and interfere with conformational changes that open the pore. The application of high-throughput screening to CFTR drug discovery has led to the identification of new inhibitors of the CFTR Cl- channel including the thiazolidinone CFTR(inh)-172 and the glycine hydrazide GlyH-101. The demonstration that CFTR inhibitors retard cyst expansion and kidney enlargement in mouse models of ADPKD provides proof of concept for the use of small-molecule CFTR inhibitors in the treatment of ADPKD.

Regulation of conductance by the number of fixed positive charges in the intracellular vestibule of the CFTR chloride channel pore

Rapid chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is dependent on the presence of fixed positive charges in the permeation pathway. Here, we use site-directed mutagenesis and patch clamp recording to show that the functional role played by one such positive charge (K95) in the inner vestibule of the pore can be "transplanted" to a residue in a different transmembrane (TM) region (S1141). Thus, the mutant channel K95S/S1141K showed Cl(-) conductance and open-channel blocker interactions similar to those of wild-type CFTR, thereby "rescuing" the effects of the charge-neutralizing K95S mutation. Furthermore, the function of K95C/S1141C, but not K95C or S1141C, was inhibited by the oxidizing agent copper(II)-o-phenanthroline, and this inhibition was reversed by the reducing agent dithiothreitol, suggesting disulfide bond formation between these two introduced cysteine side chains. These results suggest that the amino acid side chains of K95 (in TM1) and S1141 (in TM12) are functionally interchangeable and located closely together in the inner vestibule of the pore. This allowed us to investigate the functional effects of increasing the number of fixed positive charges in this vestibule from one (in wild type) to two (in the S1141K mutant). The S1141K mutant had similar Cl(-) conductance as wild type, but increased susceptibility to channel block by cytoplasmic anions including adenosine triphosphate, pyrophosphate, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and Pt(NO(2))(4)(2-) in inside-out membrane patches. Furthermore, in cell-attached patch recordings, apparent voltage-dependent channel block by cytosolic anions was strengthened by the S1141K mutation. Thus, the Cl(-) channel function of CFTR is maximal with a single fixed positive charge in this part of the inner vestibule of the pore, and increasing the number of such charges to two causes a net decrease in overall Cl(-) transport through a combination of failure to increase Cl(-) conductance and increased susceptibility to channel block by cytosolic substances.

Molecular models of the open and closed states of the whole human CFTR protein

Cystic fibrosis transmembrane conductance regulator (CFTR), involved in cystic fibrosis (CF), is a chloride channel belonging to the ATP-binding cassette (ABC) superfamily. Using the experimental structure of Sav1866 as template, we previously modeled the human CFTR structure, including membrane-spanning domains (MSD) and nucleotide-binding domains (NBD), in an outward-facing conformation (open channel state). Here, we constructed a model of the CFTR inward-facing conformation (closed channel) on the basis of the recent corrected structures of MsbA and compared the structural features of those two states of the channel. Interestingly, the MSD:NBD coupling interfaces including F508 (DeltaF508 being the most common CF mutation) are mainly left unchanged. This prediction, completed by the modeling of the regulatory R domain, is supported by experimental data and provides a molecular basis for a better understanding of the functioning of CFTR, especially of the structural features that make CFTR the unique channel among the ABC transporters.

Cysteine-independent inhibition of the CFTR chloride channel by the cysteine-reactive reagent sodium (2-sulphonatoethyl) methanethiosulphonate

BACKGROUND AND PURPOSE

Methanethiosulphonate (MTS) reagents are used extensively to modify covalently cysteine side chains in ion channel structure-function studies. We have investigated the interaction between a widely used negatively charged MTS reagent, (2-sulphonatoethyl) methanethiosulphonate (MTSES), and the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel.

EXPERIMENTAL APPROACH

Patch clamp recordings were used to study a 'cys-less' variant of human CFTR, in which all 18 endogenous cysteine residues have been removed by mutagenesis, expressed in mammalian cell lines. Use of excised inside-out membrane patches allowed MTS reagents to be applied to the cytoplasmic face of active channels.

KEY RESULTS

Intracellular application of MTSES, but not the positively charged MTSET, inhibited the function of cys-less CFTR. Inhibition was voltage dependent, with a K(d) of 1.97 mmol x L(-1) at -80 mV increasing to 36 mmol x L(-1) at +80 mV. Inhibition was completely reversed on washout of MTSES, inconsistent with covalent modification of the channel protein. At the single channel level, MTSES caused a concentration-dependent reduction in unitary current amplitude. This inhibition was strengthened when extracellular Cl(-) concentration was decreased.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that MTSES inhibits the function of CFTR in a manner that is independent of its ability to modify cysteine residues covalently. Instead, we suggest that MTSES functions as an open channel blocker that enters the CFTR channel pore from its cytoplasmic end to physically occlude Cl(-) permeation. Given the very widespread use of MTS reagents in functional studies, our findings offer a broadly applicable caveat to the interpretation of results obtained from such studies.

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.

Mechanism of direct bicarbonate transport by the CFTR anion channel

BACKGROUND

CFTR contributes to HCO(3)(-) transport in epithelial cells both directly (by HCO(3)(-) permeation through the channel) and indirectly (by regulating Cl(-)/HCO(3)(-) exchange proteins). While loss of HCO(3)(-) transport is highly relevant to cystic fibrosis, the relative importance of direct and indirect HCO(3)(-) transport it is currently unknown.

METHODS

Patch clamp recordings from membrane patches excised from cells heterologously expressing wild type and mutant forms of human CFTR were used to isolate directly CFTR-mediated HCO(3)(-) transport and characterize its functional properties.

RESULTS

The permeability of HCO(3)(-) was approximately 25% that of Cl(-) and was invariable under all ionic conditions studied. CFTR-mediated HCO(3)(-) currents were inhibited by open channel blockers DNDS, glibenclamide and suramin, and these inhibitions were affected by mutations within the channel pore. Cystic fibrosis mutations previously associated with disrupted cellular HCO(3)(-) transport did not affect direct HCO(3)(-) permeability.

CONCLUSIONS

Cl(-) and HCO(3)(-) share a common transport pathway in CFTR, and selectivity between Cl(-) and HCO(3)(-) is independent of ionic conditions. The mechanism of transport is therefore effectively identical for both ions. We suggest that mutations in CFTR that cause cystic fibrosis by selectively disrupting HCO(3)(-) transport do not impair direct CFTR-mediated HCO(3)(-) transport, but may predominantly alter CFTR regulation of other HCO(3)(-) transport pathways.

Identification of positive charges situated at the outer mouth of the CFTR chloride channel pore

We have used site-directed mutagenesis and functional analysis to identify positively charged amino acid residues in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel that interact with extracellular anions. Mutation of two positively charged arginine residues in the first extracellular loop (ECL) of CFTR, R104, and R117, as well as lysine residue K335 in the sixth transmembrane region, leads to inward rectification of the current-voltage relationship and decreased single channel conductance. These effects are dependent on the charge of the substituted side chain and on the Cl(-) concentration, suggesting that these positive charges normally act to concentrate extracellular Cl(-) ions near the outer mouth of the pore. Side chain charge-dependent effects are mimicked by manipulating charge in situ by mutating these amino acids to cysteine followed by covalent modification with charged cysteine-reactive reagents, confirming the location of these side chains within the pore outer vestibule. State-independent modification of R104C and R117C suggests that these residues are located at the outermost part of the pore. We suggest that ECL1 contributes to the CFTR pore external vestibule and that positively charged amino acid side chains in this region act to attract Cl(-) ions into the pore. In contrast, we find no evidence that fixed positive charges in other ECLs contribute to the permeation properties of the pore.

Direct and indirect effects of mutations at the outer mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore is thought to contain multiple binding sites for permeant and impermeant anions. Here, we investigate the effects of mutation of different positively charged residues in the pore on current inhibition by impermeant Pt(NO(2)) (4) (2-) and suramin anions. We show that mutations that remove positive charges (K95, R303) influence interactions with intracellular, but not extracellular, Pt(NO(2))(4)(2-) ions, consistent with these residues being situated within the pore inner vestibule. In contrast, mutation of R334, supposedly located in the outer vestibule of the pore, affects block by both extracellular and intracellular Pt(NO(2))(4)(2-). Inhibition by extracellular Pt(NO(2))(4)(2-) requires a positive charge at position 334, consistent with a direct electrostatic interaction resulting in either open channel block or surface charge screening. In contrast, inhibition by intracellular Pt(NO(2))(4)(2-) is weakened in all R334-mutant forms of the channel studied, inconsistent with a direct interaction. Furthermore, mutation of R334 had similar effects on block by intracellular suramin, a large organic molecule that is apparently unable to enter deeply into the channel pore. Mutation of R334 altered interactions between intracellular Pt(NO(2))(4)(2-) and extracellular Cl(-) but not those between intracellular Pt(NO(2))(4)(2-) and extracellular Pt(NO(2))(4)(2-). We propose that while the positive charge of R334 interacts directly with extracellular anions, mutation of this residue also alters interactions with intracellular anions by an indirect mechanism, due to mutation-induced conformational changes in the protein that are propagated some distance from the site of the mutation in the outer mouth of the pore.

CFTR inhibition by glibenclamide requires a positive charge in cytoplasmic loop three

The sulfonylurea glibenclamide is widely used as an open-channel blocker of the CFTR chloride channel. Here, we used site-directed mutagenesis to identify glibenclamide site of interaction: a positively charged residue K978, located in the cytoplasmic loop 3. Charge-neutralizing mutations K978A, K978Q, K978S abolished the inhibition of forskolin-activated CFTR chloride current by glibenclamide but not by CFTR(inh)-172. The charge-conservative mutation K978R did not alter glibenclamide sensitivity of CFTR current. Mutations of the neighbouring R975 (R975A, R975S, R975Q) did not affect electrophysiological and pharmacological properties of CFTR. No alteration of halide selectivity was observed with any of these CFTR mutant channels. This study identifies a novel potential inhibitor site within the CFTR molecule, and suggests a novel role of cytoplasmic loop three, within the second transmembrane domain of CFTR protein. This work is the first to report on the role of a residue in a cytoplasmic loop in the mechanism of action of the channel blocker glibenclamide.