Thermal instability of ΔF508 cystic fibrosis transmembrane conductance regulator (CFTR) channel function: protection by single suppressor mutations and inhibiting channel activity

Thermal Ring-Based Heat Switches in Hyperthermophilic Class II Bacterial Fructose Aldolase

Both thermophilic and hyperthermophilic enzymes in bacterial and archaeal species are activated above a specific temperature threshold but inactivated at another higher temperature. However, the underlying structural basis for these two heat switches is still unresolved. Here, graph theory was used to test if the temperature-dependent noncovalent interactions and metal bridges as identified in a series of crystal structures of the class II bacterial fructose 1,6-bisphosphate aldolase homodimer or homotetramer with or without natural substrates and products bound could form systematic fluidic grid-like mesh networks with topological grids as thermal rings to regulate their structural thermostability and functional thermoactivity. The results indicated that the second biggest grid in the Thermus aquaticus fructose 1,6-diphosphate aldolase dimer may control the specific temperature threshold to release the swapping flexible active sites at the dimeric interface for heat-evoked activation. Meanwhile, the third biggest grid may serve as a necessary structural motif against heat inactivation. Finally, the smallest grid may act as a stiff thermostable anchor. Its dissociation at the maximal melting temperature threshold may stop the catalytic activity. Taken as a whole, this computational study may render the structural motifs for the optimal growth temperature and the extreme heat stability of hyperthermophiles.

Network Basis for the Heat-Adapted Structural Thermostability of Bacterial Class II Fructose Bisphosphate Aldolase

The sufficient structural thermostability of a biological macromolecule is an overriding need for green nanoreactors and nanofactories to secure high activity. However, little is still known about what specific structural motif is responsible for it. Here, graph theory was employed to examine if the temperature-dependent noncovalent interactions and metal bridges, as identified in the structures of Escherichia coli class II fructose 1,6-bisphosphate aldolase, could shape a systematic fluidic grid-like mesh network with topological grids to regulate the structural thermostability of the wild-type construct and its evolved variants in each generation upon decyclization. The results indicated that the biggest grids may govern the temperature thresholds for their tertiary structural perturbations but without affecting the catalytic activities. Moreover, lower grid-based systematic thermal instability may facilitate structural thermostability, but a highly independent thermostable grid may still be required to serve as a critical anchor to secure the stereospecific thermoactivity. Its end melting temperature thresholds, together with the start ones of the biggest grids in the evolved variants, may confer high temperature sensitivity against thermal inactivation. Collectively, this computational study may have widespread significance in advancing our complete understanding and biotechnology of the thermoadaptive mechanism of the structural thermostability of a biological macromolecule.

A small molecule CFTR potentiator restores ATP-dependent channel gating to the cystic fibrosis mutant G551D-CFTR

BACKGROUND AND PURPOSE

Cystic fibrosis transmembrane conductance regulator (CFTR) potentiators are small molecules developed to treat the genetic disease cystic fibrosis (CF). They interact directly with CFTR Cl^- channels at the plasma membrane to enhance channel gating. Here, we investigate the action of a new CFTR potentiator, CP-628006 with a distinct chemical structure.

EXPERIMENTAL APPROACH

Using electrophysiological assays with CFTR-expressing heterologous cells and CF patient-derived human bronchial epithelial (hBE) cells, we compared the effects of CP-628006 with the marketed CFTR potentiator ivacaftor.

KEY RESULTS

CP-628006 efficaciously potentiated CFTR function in epithelia from cultured hBE cells. Its effects on the predominant CFTR variant F508del-CFTR were larger than those with the gating variant G551D-CFTR. In excised inside-out membrane patches, CP-628006 potentiated wild-type, F508del- and G551D-CFTR by increasing the frequency and duration of channel openings. CP-628006 increased the affinity and efficacy of F508del-CFTR gating by ATP. In these respects, CP-628006 behaved like ivacaftor. CP-628006 also demonstrated notable differences with ivacaftor. Its potency and efficacy were lower than those of ivacaftor. CP-628006 conferred ATP-dependent gating on G551D-CFTR, whereas the action of ivacaftor was ATP-independent. For G551D-CFTR, but not F508del-CFTR, the action of CP-628006 plus ivacaftor was greater than ivacaftor alone. CP-628006 delayed, but did not prevent, the deactivation of F508del-CFTR at the plasma membrane, whereas ivacaftor accentuated F508del-CFTR deactivation.

CONCLUSIONS AND IMPLICATIONS

CP-628006 has distinct effects compared to ivacaftor, suggesting a different mechanism of CFTR potentiation. The emergence of CFTR potentiators with diverse modes of action makes therapy with combinations of potentiators a possibility.

Functional stability of CFTR depends on tight binding of ATP at its degenerate ATP-binding site

Opening of the cystic fibrosis transmembrane conductance regulator (CFTR) channel is coupled to the motion of its two nucleotide-binding domains: they form a heterodimer sandwiching two functionally distinct ATP-binding sites (sites 1 and 2). While active ATP hydrolysis in site 2 triggers rapid channel closure, the functional role of stable ATP binding in the catalysis-incompetent (or degenerate) site 1, a feature conserved in many other ATP-binding cassette (ABC) transporter proteins, remains elusive. Here, we found that CFTR loses its prompt responsiveness to ATP after the channel is devoid of ATP for tens to hundreds of seconds. Mutants with weakened ATP binding in site 1 and the most prevalent disease-causing mutation, F508del, are more vulnerable to ATP depletion. In contrast, strengthening ligand binding in site 1 with N⁶ -(2-phenylethyl)-ATP, a high-affinity ATP analogue, or abolishing ATP hydrolysis in site 2 by the mutation D1370N, helps sustain a durable function of the otherwise unstable mutant channels. Thus, tight binding of ATP in the degenerate ATP-binding site is crucial to the functional stability of CFTR. Small molecules targeting site 1 may bear therapeutic potential to overcome the membrane instability of F508del-CFTR. KEY POINTS: During evolution, many ATP-binding cassette transporters - including the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, whose dysfunction causes cystic fibrosis (CF) - lose the ability to hydrolyse ATP in one of the two ATP-binding sites. Here we show that tight ATP binding at this degenerate site in CFTR is central for maintaining the stable, robust function of normal CFTR. We also demonstrate that membrane instability of the most common CF-causing mutant, F508del-CFTR, can be rescued by strengthening ATP binding at CFTR's degenerate site. Our data thus explain an evolutionary puzzle and offer a potential therapeutic strategy for CF.

The CFTR P67L variant reveals a key role for N-terminal lasso helices in channel folding, maturation, and pharmacologic rescue

Patients with cystic fibrosis (CF) harboring the P67L variant in the cystic fibrosis transmembrane conductance regulator (CFTR) often exhibit a typical CF phenotype, including severe respiratory compromise. This rare mutation (reported in <300 patients worldwide) responds robustly to CFTR correctors, such as lumacaftor and tezacaftor, with rescue in model systems that far exceed what can be achieved for the archetypical CFTR mutant F508del. However, the specific molecular consequences of the P67L mutation are poorly characterized. In this study, we conducted biochemical measurements following low-temperature growth and/or intragenic suppression, which suggest a mechanism underlying P67L that (1) shares key pathogenic features with F508del, including off-pathway (non-native) folding intermediates, (2) is linked to folding stability of nucleotide-binding domains 1 and 2, and (3) demonstrates pharmacologic rescue that requires domains in the carboxyl half of the protein. We also investigated the "lasso" helices 1 and 2, which occur immediately upstream of P67. Based on limited proteolysis, pulse chase, and molecular dynamics analysis of full-length CFTR and a series of deletion constructs, we argue that P67L and other maturational processing (class 2) defects impair the integrity of the lasso motif and confer misfolding of downstream domains. Thus, amino-terminal missense variants elicit a conformational change throughout CFTR that abrogates maturation while providing a robust substrate for pharmacologic repair.

Short-term consequences of F508del-CFTR thermal instability on CFTR-dependent transepithelial currents in human airway epithelial cells

Loss-of-function mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) channel in human airway epithelial cells are responsible for Cystic Fibrosis. A deleterious impact of physiological temperature on CFTR plasma membrane expression, residence and channel activity is characteristic of the most common and severe CF mutation, F508del. Using primary human F508del-airway epithelial cells and CF bronchial epithelial CFBE41o- cell lines expressing F508del- or WT-CFTR, we examined the effects of temperature (29 °C-39 °C) on the amplitude and stability of short-circuit CFTR-dependent currents over time and the efficiency of pharmacological strategies to stably restore F508del-CFTR function. We show that F508del-CFTR functional instability at 37 °C is not prevented by low temperature or VX-809 correction, genistein and VX-770 potentiators, nor by the combination VX-809/VX-770. Moreover, F508del-CFTR-dependent currents 30 minutes after CFTR activation at 37 °C did not significantly differ whether a potentiator was used or not. We demonstrate that F508del-CFTR function loss is aggravated at temperatures above 37 °C while limited by a small decrease of temperature and show that the more F508del-CFTR is stimulated, the faster the current loss happens. Our study highlights the existence of a temperature-dependent process inhibiting the function of F508del-CFTR, possibly explaining the low efficacy of pharmacological drugs in clinic.

A defective flexible loop contributes to the processing and gating defects of the predominant cystic fibrosis-causing mutation

People with the genetic disease cystic fibrosis (CF) often carry a deletion mutation ΔF508 on the gene encoding the CF transmembrane conductance regulator (CFTR) Cl^- channel. This mutation greatly reduces the CFTR maturation process and slows the channel opening rate. Here, we investigate whether residues near F508 contribute to these defects in ΔF508-CFTR. Most deletion mutations, but not alanine substitutions, of individual residues from positions 503 to 513 impaired CFTR maturation. Interestingly, only protein processing of ΔY512-CFTR, like that of ΔF508-CFTR, was greatly improved by low-temperature culture at 27°C or small-molecule corrector C18. The 2 mutant Cl^- channels were equally slow to open, suggesting that they may share common structural flaws. Studies on the H3-H4 loop that links residues F508 and Y512 demonstrate that G509A/V510G mutations, moving G509 1 position backward in the loop, markedly enhanced ΔF508-CFTR maturation and opening rate while promoting protein stability and persistence of the H3 helix in ΔF508 nucleotide-binding domain 1. Moreover, V510A/S511A mutations noticeably increased ΔY512-CFTR maturation at 27°C and its opening rate. Thus, loop abnormalities may contribute to ΔF508- and ΔY512-CFTR defects. Importantly, correcting defects from G509 displacement in ΔF508-CFTR may offer a new avenue for drug discovery and CF treatments.-Chen, X., Zhu, S., Zhenin, M., Xu, W., Bose, S. J., Wong, M. P.-F., Leung, G. P. H., Senderowitz, H., Chen, J.-H. A defective flexible loop contributes to the processing and gating defects of the predominant cystic fibrosis-causing mutation.

Potentiation of the cystic fibrosis transmembrane conductance regulator Cl− channel by ivacaftor is temperature independent

Ivacaftor is the first drug to target directly defects in the cystic fibrosis transmembrane conductance regulator (CFTR), which causes cystic fibrosis (CF). To understand better how ivacaftor potentiates CFTR channel gating, here we investigated the effects of temperature on its action. As a control, we studied the benzimidazolone UCCF-853, which potentiates CFTR by a different mechanism. Using the patch-clamp technique and cells expressing recombinant CFTR, we studied the single-channel behavior of wild-type and F508del-CFTR, the most common CF mutation. Raising the temperature of the intracellular solution from 23 to 37°C increased the frequency but reduced the duration of wild-type and F508del-CFTR channel openings. Although the open probability ( Po) of wild-type CFTR increased progressively as temperature was elevated, the relationship between Po and temperature for F508del-CFTR was bell-shaped with a maximum Po at ~30°C. For wild-type CFTR and to a greatly reduced extent F508del-CFTR, the temperature dependence of channel gating was asymmetric with the opening rate demonstrating greater temperature sensitivity than the closing rate. At all temperatures tested, ivacaftor and UCCF-853 potentiated wild-type and F508del-CFTR. Strikingly, ivacaftor but not UCCF-853 abolished the asymmetric temperature dependence of CFTR channel gating. At all temperatures tested, Po values of wild-type CFTR in the presence of ivacaftor were approximately double those of F508del-CFTR, which were equivalent to or greater than those of wild-type CFTR at 37°C in the absence of the drug. We conclude that the principal effect of ivacaftor is to promote channel opening to abolish the temperature dependence of CFTR channel gating.

Sites associated with Kalydeco binding on human Cystic Fibrosis Transmembrane Conductance Regulator revealed by Hydrogen/Deuterium Exchange

Cystic Fibrosis (CF) is caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Mutations associated with CF cause loss-of-function in CFTR leading to salt imbalance in epithelial tissues. Kalydeco (also called VX-770 or ivacaftor) was approved for CF treatment in 2012 but little is known regarding the compound’s interactions with CFTR including the site of binding or mechanisms of action. In this study we use hydrogen/deuterium exchange (HDX) coupled with mass spectrometry to assess the conformational dynamics of a thermostabilized form of CFTR in apo and ligand-bound states. We observe HDX protection at a known binding site for AMPPNP and significant protection for several regions of CFTR in the presence of Kalydeco. The ligand-induced changes of CFTR in the presence of Kalydeco suggest a potential binding site.

Partial rescue of F508del-cystic fibrosis transmembrane conductance regulator channel gating with modest improvement of protein processing, but not stability, by a dual-acting small molecule

BACKGROUND AND PURPOSE

Rescue of F508del-cystic fibrosis (CF) transmembrane conductance regulator (CFTR), the most common CF mutation, requires small molecules that overcome protein processing, stability and channel gating defects. Here, we investigate F508del-CFTR rescue by CFFT-004, a small molecule designed to independently correct protein processing and channel gating defects.

EXPERIMENTAL APPROACH

Using CFTR-expressing recombinant cells and CF patient-derived bronchial epithelial cells, we studied CFTR expression by Western blotting and channel gating and stability with the patch-clamp and Ussing chamber techniques.

KEY RESULTS

Chronic treatment with CFFT-004 improved modestly F508del-CFTR processing, but not its plasma membrane stability. By contrast, CFFT-004 rescued F508del-CFTR channel gating better than C18, an analogue of the clinically used CFTR corrector lumacaftor. Subsequent acute addition of CFFT-004, but not C18, potentiated F508del-CFTR channel gating. However, CFFT-004 was without effect on A561E-CFTR, a CF mutation with a comparable mechanism of CFTR dysfunction as F508del-CFTR. To investigate the mechanism of action of CFFT-004, we used F508del-CFTR revertant mutations. Potentiation by CFFT-004 was unaffected by revertant mutations, but correction was abolished by the revertant mutation G550E. These data suggest that correction, but not potentiation, by CFFT-004 might involve nucleotide-binding domain 1 of CFTR.

CONCLUSIONS AND IMPLICATIONS

CFFT-004 is a dual-acting small molecule with independent corrector and potentiator activities that partially rescues F508del-CFTR in recombinant cells and native airway epithelia. The limited efficacy and potency of CFFT-004 suggests that combinations of small molecules targeting different defects in F508del-CFTR might be a more effective therapeutic strategy than a single agent.

Structural stability of purified human CFTR is systematically improved by mutations in nucleotide binding domain 1

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is an ABC transporter containing two transmembrane domains forming a chloride ion channel, and two nucleotide binding domains (NBD1 and NBD2). CFTR has presented a formidable challenge to obtain monodisperse, biophysically stable protein. Here we report a comprehensive study comparing effects of single and multiple NBD1 mutations on stability of both the NBD1 domain alone and on purified full length human CFTR. Single mutations S492P, A534P, I539T acted additively, and when combined with M470V, S495P, and R555K cumulatively yielded an NBD1 with highly improved structural stability. Strategic combinations of these mutations strongly stabilized the domain to attain a calorimetric T_m > 70 °C. Replica exchange molecular dynamics simulations on the most stable 6SS-NBD1 variant implicated fluctuations, electrostatic interactions and side chain packing as potential contributors to improved stability. Progressive stabilization of NBD1 directly correlated with enhanced structural stability of full-length CFTR protein. Thermal unfolding of the stabilized CFTR mutants, monitored by changes in intrinsic fluorescence, demonstrated that Tm could be shifted as high as 67.4 °C in 6SS-CFTR, more than 20 °C higher than wild-type. H1402S, an NBD2 mutation, conferred CFTR with additional thermal stability, possibly by stabilizing an NBD-dimerized conformation. CFTR variants with NBD1-stabilizing mutations were expressed at the cell surface in mammalian cells, exhibited ATPase and channel activity, and retained these functions to higher temperatures. The capability to produce enzymatically active CFTR with improved structural stability amenable to biophysical and structural studies will advance mechanistic investigations and future cystic fibrosis drug development.

Substitution of Yor1p NBD1 residues improves the thermal stability of Human Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a plasma membrane chloride channel protein that regulates vertebrate fluid homeostasis. The inefficiency of wild type human CFTR protein folding/trafficking is exacerbated by genetic mutations that can cause protein misfolding in the endoplasmic reticulum (ER) and subsequent degradation. This project investigates small changes in protein sequence that can alter the thermal stability of the large multi-domain CFTR protein. We target a conserved 70-residue α-subdomain located in the first nucleotide-binding domain that hosts the common misfolding mutation ∆F508. To investigate substitutions that can stabilize this domain, we constructed chimeras between human CFTR and its closest yeast homolog Yor1p. The α-subdomain of Yor1p was replaced with that of CFTR in Saccharomyces cerevisiae. Cellular localization of green fluorescence protein-tagged Yor1p-CFTR chimeras was analyzed by fluorescence microscopy and quantitative multispectral imaging flow cytometry, steady-state protein levels were compared by SDS-PAGE and protein function probed by a phenotypic oligomycin resistance assay. The chimeras exhibited ER retention in yeast characteristic of defective protein folding/processing. Substitution of seven CFTR α-subdomain residues that are highly conserved in Yor1p and other transporters but differ in CFTR (S495P/R516K/F533L/A534P/K536G/I539T/R553K) improved Yor1p-CFTR chimera localization to the yeast plasma membrane. When introduced into human CFTR expressed in mammalian cells, the same substitutions improve the purified protein thermal stability. This stabilized human CFTR protein will be directly useful for structural and biophysical studies that have been limited by the thermal sensitivity of wild type CFTR. The insights into critical structural residues within CFTR could facilitate development of effective therapeutics for CF-causing mutations.

Development of Automated Patch Clamp Technique to Investigate CFTR Chloride Channel Function

The chloride (Cl^-) channel cystic fibrosis transmembrane conductance regulator (CFTR) is defective in cystic fibrosis (CF), and mutation of its encoding gene leads to various defects such as retention of the misfolded protein in the endoplasmic reticulum, reduced stability at the plasma membrane, abnormal channel gating with low open probability, and thermal instability, which leads to inactivation of the channel at physiological temperature. Pharmacotherapy is one major therapeutic approach in the CF field and needs sensible and fast tools to identify promising compounds. The high throughput screening assays available are often fast and sensible techniques but with lack of specificity. Few works used automated patch clamp (APC) for CFTR recording, and none have compared conventional and planar techniques and demonstrated their capabilities for different types of experiments. In this study, we evaluated the use of planar parallel APC technique for pharmacological search of CFTR-trafficking correctors and CFTR function modulators. Using optimized conditions, we recorded both wt- and corrected F508del-CFTR Cl^- currents with automated whole-cell patch clamp and compared the data to results obtained with conventional manual whole-cell patch clamp. We found no significant difference in patch clamp parameters such as cell capacitance and series resistance between automated and manual patch clamp. Also, the results showed good similarities of CFTR currents recording between the two methods. We showed that similar stimulation protocols could be used in both manual and automatic techniques allowing precise control of temperature, classic I/V relationship, and monitoring of current stability in time. In conclusion, parallel patch-clamp recording allows rapid and efficient investigation of CFTR currents with a variety of tests available and could be considered as new tool for medium throughput screening in CF pharmacotherapy.

Two Small Molecules Restore Stability to a Subpopulation of the Cystic Fibrosis Transmembrane Conductance Regulator with the Predominant Disease-causing Mutation

Cystic fibrosis (CF) is caused by mutations that disrupt the plasma membrane expression, stability, and function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel. Two small molecules, the CFTR corrector lumacaftor and the potentiator ivacaftor, are now used clinically to treat CF, although some studies suggest that they have counteracting effects on CFTR stability. Here, we investigated the impact of these compounds on the instability of F508del-CFTR, the most common CF mutation. To study individual CFTR Cl^- channels, we performed single-channel recording, whereas to assess entire CFTR populations, we used purified CFTR proteins and macroscopic CFTR Cl^- currents. At 37 °C, low temperature-rescued F508del-CFTR more rapidly lost function in cell-free membrane patches and showed altered channel gating and current flow through open channels. Compared with purified wild-type CFTR, the full-length F508del-CFTR was about 10 °C less thermostable. Lumacaftor partially stabilized purified full-length F508del-CFTR and slightly delayed deactivation of individual F508del-CFTR Cl^- channels. By contrast, ivacaftor further destabilized full-length F508del-CFTR and accelerated channel deactivation. Chronic (prolonged) co-incubation of F508del-CFTR-expressing cells with lumacaftor and ivacaftor deactivated macroscopic F508del-CFTR Cl^- currents. However, at the single-channel level, chronic co-incubation greatly increased F508del-CFTR channel activity and temporal stability in most, but not all, cell-free membrane patches. We conclude that chronic lumacaftor and ivacaftor co-treatment restores stability in a small subpopulation of F508del-CFTR Cl^- channels but that the majority remain destabilized. A fuller understanding of these effects and the characterization of the small F508del-CFTR subpopulation might be crucial for CF therapy development.

Potentiation of ΔF508- and G551D-CFTR-Mediated Cl- Current by Novel Hydroxypyrazolines

The most common mutation of CFTR, affecting approximately 90% of CF patients, is a deletion of phenylalanine at position 508 (F508del, ΔF508). Misfolding of ΔF508-CFTR impairs both its trafficking to the plasma membrane and its chloride channel activity. To identify small molecules that can restore channel activity of ΔF508-CFTR, we synthesized and evaluated eighteen novel hydroxypyrazoline analogues as CFTR potentiators. To elucidate potentiation activities of hydroxypyrazolines for ΔF508-CFTR, CFTR activity was measured using a halide-sensitive YFP assay, Ussing chamber assay and patch-clamp technique. Compounds 7p, 7q and 7r exhibited excellent potentiation with EC₅₀ value <10 μM. Among the compounds, 7q (a novel CFTR potentiator, CP7q) showed the highest potentiation activity with EC₅₀ values of 0.88 ± 0.11 and 4.45 ± 0.31 μM for wild-type and ΔF508-CFTR, respectively. In addition, CP7q significantly potentiated chloride conductance of G551D-CFTR, a CFTR gating mutant; its maximal potentiation activity was 1.9 fold higher than the well-known CFTR potentiator genistein. Combination treatment with CP7q and VX-809, a corrector of ΔF508-CFTR, significantly enhanced functional rescue of ΔF508-CFTR compared with VX-809 alone. CP7q did not alter the cytosolic cAMP level and showed no cytotoxicity at the concentration showing maximum efficacy. The hydroxypyrazolines may be potential development candidates for drug therapy of cystic fibrosis.

Thermal stability of purified and reconstituted CFTR in a locked open channel conformation

CFTR is unique among ABC transporters as the only one functioning as an ion channel and from a human health perspective because mutations in its gene cause cystic fibrosis. Although considerable advances have been made towards understanding CFTR's mechanism of action and the impact of mutations, the lack of a high-resolution 3D structure has hindered progress. The large multi-domain membrane glycoprotein is normally present at low copy number and when over expressed at high levels it aggregates strongly, limiting the production of stable mono-disperse preparations. While the reasons for the strong self-association are not fully understood, its relatively low thermal stability seems likely to be one. The major CF causing mutation, ΔF508, renders the protein very thermally unstable and therefore a great deal of attention has been paid to this property of CFTR. Multiple second site mutations of CFTR in NBD1 where F508 normally resides and small molecule binders of the domain increase the thermal stability of the mutant. These manipulations also stabilize the wild-type protein. Here we have applied ΔF508-stabilizing changes and other modifications to generate wild-type constructs that express at much higher levels in scaled-up suspension cultures of mammalian cells. After purification and reconstitution into liposomes these proteins are active in a locked-open conformation at temperatures as high as 50 °C and remain monodisperse at 4 °C in detergent or lipid for at least a week. The availability of adequate amounts of these and related stable active preparations of homogeneous CFTR will enable stalled structural and ligand binding studies to proceed.

A synonymous codon change alters the drug sensitivity of ΔF508 cystic fibrosis transmembrane conductance regulator

Synonymous mutations, such as I507-ATC→ATT, in deletion of Phe508 in cystic fibrosis transmembrane conductance regulator (ΔF508 CFTR), the most frequent disease-associated mutant of CFTR, may affect protein biogenesis, structure, and function and contribute to an altered disease phenotype. Small-molecule drugs are being developed to correct ΔF508 CFTR. To understand correction mechanisms and the consequences of synonymous mutations, we analyzed the effect of mechanistically distinct correctors, corrector 4a (C4) and lumacaftor (VX-809), on I507-ATT and I507-ATC ΔF508 CFTR biogenesis and function. C4 stabilized I507-ATT ΔF508 CFTR band B, but without considerable biochemical and functional correction. VX-809 biochemically corrected ∼10% of both of the variants, leading to stable, forskolin+3-isobutyl-1-methylxanthine (IBMX)-activated whole-cell currents in the presence of the corrector. Omitting VX-809 during whole-cell recordings led to a spontaneous decline of the currents, suggesting posttranslational stabilization by VX-809. Treatment of cells with the C4+VX-809 combination resulted in enhanced rescue and 2-fold higher forskolin+IBMX-activated currents of both I507-ATT and I507-ATC ΔF508 CFTR, compared with VX-809 treatment alone. The lack of an effect of C4 on I507-ATC ΔF508 CFTR, but its additive effect in combination with VX-809, implies that C4 acted on VX-809-modified I507-ATC ΔF508 CFTR. Our results suggest that binding of C4 and VX-809 to ΔF508 CFTR is conformation specific and provide evidence that synonymous mutations can alter the drug sensitivity of proteins.

Generation of Distal Airway Epithelium from Multipotent Human Foregut Stem Cells

Collectively, lung diseases are one of the largest causes of premature death worldwide and represent a major focus in the field of regenerative medicine. Despite significant progress, only few stem cell platforms are currently available for cell-based therapy, disease modeling, and drug screening in the context of pulmonary disorders. Human foregut stem cells (hFSCs) represent an advantageous progenitor cell type that can be used to amplify large quantities of cells for regenerative medicine applications and can be derived from any human pluripotent stem cell line. Here, we further demonstrate the application of hFSCs by generating a near homogeneous population of early pulmonary endoderm cells coexpressing NKX2.1 and FOXP2. These progenitors are then able to form cells that are representative of distal airway epithelium that express NKX2.1, GATA6, and cystic fibrosis transmembrane conductance regulator (CFTR) and secrete SFTPC. This culture system can be applied to hFSCs carrying the CFTR mutation Δf508, enabling the development of an in vitro model for cystic fibrosis. This platform is compatible with drug screening and functional validations of small molecules, which can reverse the phenotype associated with CFTR mutation. This is the first demonstration that multipotent endoderm stem cells can differentiate not only into both liver and pancreatic cells but also into lung endoderm. Furthermore, our study establishes a new approach for the generation of functional lung cells that can be used for disease modeling as well as for drug screening and the study of lung development.

Potentiator ivacaftor abrogates pharmacological correction of ΔF508 CFTR in cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR). Newly developed "correctors" such as lumacaftor (VX-809) that improve CFTR maturation and trafficking and "potentiators" such as ivacaftor (VX-770) that enhance channel activity may provide important advances in CF therapy. Although VX-770 has demonstrated substantial clinical efficacy in the small subset of patients with a mutation (G551D) that affects only channel activity, a single compound is not sufficient to treat patients with the more common CFTR mutation, ΔF508. Thus, patients with ΔF508 will likely require treatment with both correctors and potentiators to achieve clinical benefit. However, whereas the effectiveness of acute treatment with this drug combination has been demonstrated in vitro, the impact of chronic therapy has not been established. In studies of human primary airway epithelial cells, we found that both acute and chronic treatment with VX-770 improved CFTR function in cells with the G551D mutation, consistent with clinical studies. In contrast, chronic VX-770 administration caused a dose-dependent reversal of VX-809-mediated CFTR correction in ΔF508 homozygous cultures. This result reflected the destabilization of corrected ΔF508 CFTR by VX-770, markedly increasing its turnover rate. Chronic VX-770 treatment also reduced mature wild-type CFTR levels and function. These findings demonstrate that chronic treatment with CFTR potentiators and correctors may have unexpected effects that cannot be predicted from short-term studies. Combining these drugs to maximize rescue of ΔF508 CFTR may require changes in dosing and/or development of new potentiator compounds that do not interfere with CFTR stability.

Impact of the F508del mutation on ovine CFTR, a Cl- channel with enhanced conductance and ATP-dependent gating

KEY POINTS

Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR), a gated pathway for chloride movement, causes the common life-shortening genetic disease cystic fibrosis (CF). Towards the development of a sheep model of CF, we have investigated the function of sheep CFTR. We found that sheep CFTR was noticeably more active than human CFTR, while the most common CF mutation, F508del, had reduced impact on sheep CFTR function. Our results demonstrate that subtle changes in protein structure have marked effects on CFTR function and the consequences of the CF mutation F508del.

ABSTRACT

Cross-species comparative studies are a powerful approach to understanding the epithelial Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in the genetic disease cystic fibrosis (CF). Here, we investigate the single-channel behaviour of ovine CFTR and the impact of the most common CF mutation, F508del-CFTR, using excised inside-out membrane patches from transiently transfected CHO cells. Like human CFTR, ovine CFTR formed a weakly inwardly rectifying Cl(-) channel regulated by PKA-dependent phosphorylation, inhibited by the open-channel blocker glibenclamide. However, for three reasons, ovine CFTR was noticeably more active than human CFTR. First, single-channel conductance was increased. Second, open probability was augmented because the frequency and duration of channel openings were increased. Third, with enhanced affinity and efficacy, ATP more strongly stimulated ovine CFTR channel gating. Consistent with these data, the CFTR modulator phloxine B failed to potentiate ovine CFTR Cl(-) currents. Similar to its impact on human CFTR, the F508del mutation caused a temperature-sensitive folding defect, which disrupted ovine CFTR protein processing and reduced membrane stability. However, the F508del mutation had reduced impact on ovine CFTR channel gating in contrast to its marked effects on human CFTR. We conclude that ovine CFTR forms a regulated Cl(-) channel with enhanced conductance and ATP-dependent channel gating. This phylogenetic analysis of CFTR structure and function demonstrates that subtle changes in structure have pronounced effects on channel function and the consequences of the CF mutation F508del.

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.

CFTR potentiators partially restore channel function to A561E-CFTR, a cystic fibrosis mutant with a similar mechanism of dysfunction as F508del-CFTR

BACKGROUND AND PURPOSE

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel causes the genetic disease cystic fibrosis (CF). Towards the development of transformational drug therapies for CF, we investigated the channel function and action of CFTR potentiators on A561E, a CF mutation found frequently in Portugal. Like the most common CF mutation F508del, A561E causes a temperature-sensitive folding defect that prevents CFTR delivery to the cell membrane and is associated with severe disease.

EXPERIMENTAL APPROACH

Using baby hamster kidney cells expressing recombinant CFTR, we investigated CFTR expression by cell surface biotinylation, and function and pharmacology with the iodide efflux and patch-clamp techniques.

KEY RESULTS

Low temperature incubation delivered a small proportion of A561E-CFTR protein to the cell surface. Like F508del-CFTR, low temperature-rescued A561E-CFTR exhibited a severe gating defect characterized by brief channel openings separated by prolonged channel closures. A561E-CFTR also exhibited thermoinstability, losing function more quickly than F508del-CFTR in cell-free membrane patches and intact cells. Using the iodide efflux assay, CFTR potentiators, including genistein and the clinically approved small-molecule ivacaftor, partially restored function to A561E-CFTR. Interestingly, ivacaftor restored wild-type levels of channel activity (as measured by open probability) to single A561E- and F508del-CFTR Cl(-) channels. However, it accentuated the thermoinstability of both mutants in cell-free membrane patches.

CONCLUSIONS AND IMPLICATIONS

Like F508del-CFTR, A561E-CFTR perturbs protein processing, thermostability and channel gating. CFTR potentiators partially restore channel function to low temperature-rescued A561E-CFTR. Transformational drug therapy for A561E-CFTR is likely to require CFTR correctors, CFTR potentiators and special attention to thermostability.

Cystic fibrosis transmembrane conductance regulator (CFTR) potentiators protect G551D but not ΔF508 CFTR from thermal instability

The G551D cystic fibrosis transmembrane conductance regulator (CFTR) mutation is associated with severe disease in ∼5% of cystic fibrosis patients worldwide. This amino acid substitution in NBD1 results in a CFTR chloride channel characterized by a severe gating defect that can be at least partially overcome in vitro by exposure to a CFTR potentiator. In contrast, the more common ΔF508 mutation is associated with a severe protein trafficking defect, as well as impaired channel function. Recent clinical trials demonstrated a beneficial effect of the CFTR potentiator, Ivacaftor (VX-770), on lung function of patients bearing at least one copy of G551D CFTR, but no comparable effect on ΔF508 homozygotes. This difference in efficacy was not surprising in view of the established difference in the molecular phenotypes of the two mutant channels. Recently, however, it was shown that the structural defect introduced by the deletion of F508 is associated with the thermal instability of ΔF508 CFTR channel function in vitro. This additional mutant phenotype raised the possibility that the differences in the behavior of ΔF508 and G551D CFTR, as well as the disparate efficacy of Ivacaftor, might be a reflection of the differing thermal stabilities of the two channels at 37 °C. We compared the thermal stability of G551D and ΔF508 CFTR in Xenopus oocytes in the presence and absence of CTFR potentiators. G551D CFTR exhibited a thermal instability that was comparable to that of ΔF508 CFTR. G551D CFTR, however, was protected from thermal instability by CFTR potentiators, whereas ΔF508 CFTR was not. These results suggest that the efficacy of VX-770 in patients bearing the G551D mutation is due, at least in part, to the ability of the small molecule to protect the mutant channel from thermal instability at human body temperature.

Restoration of NBD1 thermal stability is necessary and sufficient to correct ∆F508 CFTR folding and assembly

Cystic fibrosis transmembrane conductance regulator (CFTR) (ABCC7), unique among ABC exporters as an ion channel, regulates ion and fluid transport in epithelial tissues. Loss of function due to mutations in the cftr gene causes cystic fibrosis. The most common cystic-fibrosis-causing mutation, the deletion of F508 (ΔF508) from the first nucleotide binding domain (NBD1) of CFTR, results in misfolding of the protein and clearance by cellular quality control systems. The ΔF508 mutation has two major impacts on CFTR: reduced thermal stability of NBD1 and disruption of its interface with membrane-spanning domains (MSDs). It is unknown if these two defects are independent and need to be targeted separately. To address this question, we varied the extent of stabilization of NBD1 using different second-site mutations and NBD1 binding small molecules with or without NBD1/MSD interface mutation. Combinations of different NBD1 changes had additive corrective effects on ∆F508 maturation that correlated with their ability to increase NBD1 thermostability. These effects were much larger than those caused by interface modification alone and accounted for most of the correction achieved by modifying both the domain and the interface. Thus, NBD1 stabilization plays a dominant role in overcoming the ΔF508 defect. Furthermore, the dual target approach resulted in a locked-open ion channel that was constitutively active in the absence of the normally obligatory dependence on phosphorylation by protein kinase A. Thus, simultaneous targeting of both the domain and the interface, as well as being non-essential for correction of biogenesis, may disrupt normal regulation of channel function.

Searching for combinations of small-molecule correctors to restore f508del-cystic fibrosis transmembrane conductance regulator function and processing

The mutated protein F508del-cystic fibrosis transmembrane conductance regulator (CFTR) failed to traffic properly as a result of its retention in the endoplasmic reticulum and functions as a chloride (Cl(-)) channel with abnormal gating and endocytosis. Small chemicals (called correctors) individually restore F508del-CFTR trafficking and Cl(-) transport function, but recent findings indicate that synergistic pharmacology should be considered to address CFTR defects more clearly. We studied the function and maturation of F508del-CFTR expressed in HeLa cells using a combination of five correctors [miglustat, IsoLAB (1,4-dideoxy-2-hydroxymethyl-1,4-imino-l-threitol), Corr4a (N-[2-(5-chloro-2-methoxy-phenylamino)-4'-methyl-[4,5']bithiazolyl-2'-yl]-benzamide), VX-809 [3-(6-(1-(2,2-difluorobenzo[d][1,3]dioxol-5-yl)cyclopropanecarboxamido)-3-methylpyridin-2-yl)benzoic acid], and suberoylamilide hydroxamic acid (SAHA)]. Using the whole-cell patch-clamp technique, the current density recorded in response to CFTR activators (forskolin + genistein) was significantly increased in the presence of the following combinations: VX-809 + IsoLAB; VX-809 + miglustat + SAHA; VX-809 + miglustat + IsoLAB; VX-809 + IsoLAB + SAHA; VX-809 + miglustat + IsoLAB + SAHA. These combinations restored the activity of F508del-CFTR but with a differential effect on the appearance of mature c-band of F508del-CFTR proteins. Focusing on the VX-809 + IsoLAB cocktail, we recorded a level of correction higher at 37°C versus room temperature, but without amelioration of the thermal instability of CFTR. The level of functional rescue with VX-809 + IsoLAB after 4 hours of incubation was maximal and similar to that obtained in optimal conditions of use for each compound (i.e., 24 hours for VX-809 + 4 hours for IsoLAB). Finally, we compared the stimulation of F508del-CFTR by forskolin or forskolin + VX-770 [N-(2,4-di-tert-butyl-5-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide] with cells corrected by VX-809 + IsoLAB. Our results open new perspectives for the development of a synergistic polypharmacology to rescue F508del-CFTR and show the importance of temperature on the effect of correctors and on the level of correction, suggesting that optimized combination of correctors could lead to a better rescue of F508del-CFTR function.

Ractopamine, a livestock feed additive, is a full agonist at trace amine-associated receptor 1

Ractopamine (RAC) is fed to an estimated 80% of all beef, swine, and turkey raised in the United States. It promotes muscle mass development, limits fat deposition, and reduces feed consumption. However, it has several undesirable behavioral side effects in livestock, especially pigs, including restlessness, agitation, excessive oral-facial movements, and aggressive behavior. Numerous in vitro and in vivo studies suggest RAC's physiological actions begin with its stimulation of β1- and β2-adrenergic receptor-mediated signaling in skeletal muscle and adipose tissue; however, the molecular pharmacology of RAC's psychoactive effects is poorly understood. Using human cystic fibrosis transmembrane conductance regulator (hCFTR) chloride channels as a sensor for intracellular cAMP, we found that RAC and p-tyramine (TYR) produced concentration-dependent increases in chloride conductance in oocytes coexpressing hCFTR and mouse trace amine-associated receptor 1 (mTAAR1), which was completely reversed by the trace amine-associated receptor 1 (TAAR1)-selective antagonist EPPTB [N-(3-ethoxyphenyl)-4-pyrrolidin-1-yl-3-trifluoromethylbenzamide]. Oocytes coexpressing hCFTR and the human β2-adrenergic receptor showed no response to RAC or TYR. These studies demonstrate that, contrary to expectations, RAC is not an agonist of the human β2-adrenergic receptor but rather a full agonist for mTAAR1. Since TAAR1-mediated signaling can influence cardiovascular tone and behavior in several animal models, our finding that RAC is a full mTAAR1 agonist supports the idea that this novel mechanism of action influences the physiology and behavior of pigs and other species. These findings should stimulate future studies to characterize the pharmacological, physiological, and behavioral actions of RAC in humans and other species exposed to this drug.

Complement yourself: Transcomplementation rescues partially folded mutant proteins

Cystic Fibrosis (CF) is an autosomal disease associated with malfunction in fluid and electrolyte transport across several mucosal membranes. The most common mutation in CF is an in-frame three-base pair deletion that removes a phenylalanine at position 508 in the first nucleotide-binding domain of the cystic fibrosis conductance regulator (CFTR) chloride channel. This mutation has been studied extensively and leads to biosynthetic arrest of the protein in the endoplasmic reticulum and severely reduced channel activity. This review discusses a novel method of rescuing ΔF508 with transcomplementation, which occurs when smaller fragments of CFTR containing the wild-type nucleotide binding domain are co-expressed with the ΔF508 deletion mutant. Transcomplementation rescues the processing and channel activity of ΔF508 and reduces its rate of degradation in airway epithelial cells. To apply transcomplementation as a therapy would require that the cDNA encoding the truncated CFTR be delivered to cells. We also discuss a gene therapeutic approach based on delivery of a truncated form of CFTR to airway cells using adeno-associated viral vectors.

The silent codon change I507-ATC->ATT contributes to the severity of the ΔF508 CFTR channel dysfunction

The most common disease-causing mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is the out-of-frame deletion of 3 nucleotides (CTT). This mutation leads to the loss of phenylalanine-508 (ΔF508) and a silent codon change (SCC) for isoleucine-507 (I507-ATC→ATT). ΔF508 CFTR is misfolded and degraded by endoplasmic reticulum-associated degradation (ERAD). We have demonstrated that the I507-ATC→ATT SCC alters ΔF508 CFTR mRNA structure and translation dynamics. By comparing the biochemical and functional properties of the I507-ATT and I507-ATC ΔF508 CFTR, we establish that the I507-ATC→ATT SCC contributes to the cotranslational misfolding, ERAD, and to the functional defects associated with ΔF508 CFTR. We demonstrate that the I507-ATC ΔF508 CFTR is less susceptible to the ER quality-control machinery during translation than the I507-ATT, although 27°C correction is necessary for sufficient cell-surface expression. Whole-cell patch-clamp recordings indicate sustained, thermally stable cAMP-activated Cl(-) transport through I507-ATC and unstable function of the I507-ATT ΔF508 CFTR. Single-channel recordings reveal improved gating properties of the I507-ATC compared to I507-ATT ΔF508 CFTR (NPo=0.45±0.037 vs. NPo=0.09±0.002; P<0.001). Our results signify the role of the I507-ATC→ATT SCC in the ΔF508 CFTR defects and support the importance of synonymous codon choices in determining the function of gene products.

Implications of aberrant temperature-sensitive glucose transport via the glucose transporter deficiency mutant (GLUT1DS) T295M for the alternate-access and fixed-site transport models

In silico glucose docking to the transporter GLUT1 templated to the crystal structure of Escherichia coli XylE, a bacterial homolog of GLUT1-4 (4GBZ.pdb), reveals multiple docking sites. One site in the external vestibule in the exofacial linker between TM7 and -8 is adjacent to a missense T295M and a 4-mer insertion mutation. Glucose docking to the adjacent site is occluded in these mutants. These mutants cause an atypical form of glucose transport deficiency syndrome (GLUT1DS), where transport into the brain is deficient, although unusually transport into erythrocytes at 4 °C appears normal. A model in which glucose traverses the transporter via a network of saturable fixed sites simulates the temperature sensitivity of normal and mutant glucose influx and the mutation-dependent alterations of influx and efflux asymmetry when expressed in Xenopus oocytes at 37 °C. The explanation for the temperature sensitivity is that at 4 °C glucose influx between the external and internal vestibules is slow and causes glucose to accumulate in the external vestibule. This retards net glucose uptake from the external solution via two parallel sites into the external vestibule, consequently masking any transport defect at either one of these sites. At 37 °C glucose transit between the external and internal vestibules is rapid, with no significant glucose buildup in the external vestibule, and thereby unmasks any transport defect at one of the parallel input sites. Monitoring glucose transport in patients' erythrocytes at higher temperatures may improve the diagnostic accuracy of the functional test of GLUT1DS.

Mechanism-based corrector combination restores ΔF508-CFTR folding and function

The most common cystic fibrosis mutation, ΔF508 in nucleotide binding domain 1 (NBD1), impairs cystic fibrosis transmembrane conductance regulator (CFTR)-coupled domain folding, plasma membrane expression, function and stability. VX-809, a promising investigational corrector of ΔF508-CFTR misprocessing, has limited clinical benefit and an incompletely understood mechanism, hampering drug development. Given the effect of second-site suppressor mutations, robust ΔF508-CFTR correction most likely requires stabilization of NBD1 energetics and the interface between membrane-spanning domains (MSDs) and NBD1, which are both established primary conformational defects. Here we elucidate the molecular targets of available correctors: class I stabilizes the NBD1-MSD1 and NBD1-MSD2 interfaces, and class II targets NBD2. Only chemical chaperones, surrogates of class III correctors, stabilize human ΔF508-NBD1. Although VX-809 can correct missense mutations primarily destabilizing the NBD1-MSD1/2 interface, functional plasma membrane expression of ΔF508-CFTR also requires compounds that counteract the NBD1 and NBD2 stability defects in cystic fibrosis bronchial epithelial cells and intestinal organoids. Thus, the combination of structure-guided correctors represents an effective approach for cystic fibrosis therapy.

Mechanisms of CFTR Folding at the Endoplasmic Reticulum

In the past decade much has been learned about how Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) folds and misfolds as the etiologic cause of cystic fibrosis (CF). CFTR folding is complex and hierarchical, takes place in multiple cellular compartments and physical environments, and involves several large networks of folding machineries. Insertion of transmembrane (TM) segments into the endoplasmic reticulum (ER) membrane and tertiary folding of cytosolic domains begin cotranslationally as the nascent polypeptide emerges from the ribosome, whereas posttranslational folding establishes critical domain-domain contacts needed to form a physiologically stable structure. Within the membrane, N- and C-terminal TM helices are sorted into bundles that project from the cytosol to form docking sites for nucleotide binding domains, NBD1 and NBD2, which in turn form a sandwich dimer for ATP binding. While tertiary folding is required for domain assembly, proper domain assembly also reciprocally affects folding of individual domains analogous to a jig-saw puzzle wherein the structure of each interlocking piece influences its neighbors. Superimposed on this process is an elaborate proteostatic network of cellular chaperones and folding machineries that facilitate the timing and coordination of specific folding steps in and across the ER membrane. While the details of this process require further refinement, we finally have a useful framework to understand key folding defect(s) caused by ΔF508 that provides a molecular target(s) for the next generation of CFTR small molecule correctors aimed at the specific defect present in the majority of CF patients.

Correctors of ΔF508 CFTR restore global conformational maturation without thermally stabilizing the mutant protein

Most cystic fibrosis is caused by the deletion of a single amino acid (F508) from CFTR and the resulting misfolding and destabilization of the protein. Compounds identified by high-throughput screening to improve ΔF508 CFTR maturation have already entered clinical trials, and it is important to understand their mechanisms of action to further improve their efficacy. Here, we showed that several of these compounds, including the investigational drug VX-809, caused a much greater increase (5- to 10-fold) in maturation at 27 than at 37°C (<2-fold), and the mature product remained short-lived (T(1/2)∼4.5 h) and thermally unstable, even though its overall conformational state was similar to wild type, as judged by resistance to proteolysis and interdomain cross-linking. Consistent with its inability to restore thermodynamic stability, VX-809 stimulated maturation 2-5-fold beyond that caused by several different stabilizing modifications of NBD1 and the NBD1/CL4 interface. The compound also promoted maturation of several disease-associated processing mutants on the CL4 side of this interface. Although these effects may reflect an interaction of VX-809 with this interface, an interpretation supported by computational docking, it also rescued maturation of mutants in other cytoplasmic loops, either by allosteric effects or via additional sites of action. In addition to revealing the capabilities and some of the limitations of this important investigational drug, these findings clearly demonstrate that ΔF508 CFTR can be completely assembled and evade cellular quality control systems, while remaining thermodynamically unstable. He, L., Kota, P., Aleksandrov, A. A., Cui, L., Jensen, T., Dokholyan, N. V., Riordan, J. R. Correctors of ΔF508 CFTR restore global conformational maturation without thermally stabilizing the mutant protein.