Correctors Promote Maturation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)-processing Mutants by Binding to the Protein

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.

ABCB19/PGP19 stabilises PIN1 in membrane microdomains in Arabidopsis

Auxin transport is mediated at the cellular level by three independent mechanisms that are characterised by the PIN-formed (PIN), P-glycoprotein (ABCB/PGP) and AUX/LAX transport proteins. The PIN and ABCB transport proteins, best represented by PIN1 and ABCB19 (PGP19), have been shown to coordinately regulate auxin efflux. When PIN1 and ABCB19 coincide on the plasma membrane, their interaction enhances the rate and specificity of auxin efflux and the dynamic cycling of PIN1 is reduced. However, ABCB19 function is not regulated by the dynamic cellular trafficking mechanisms that regulate PIN1 in apical tissues, as localisation of ABCB19 on the plasma membrane was not inhibited by short-term treatments with latrunculin B, oryzalin, brefeldin A (BFA) or wortmannin--all of which have been shown to alter PIN1 and/or PIN2 plasma membrane localisation. When taken up by endocytosis, the styryl dye FM4-64 labels diffuse rather than punctuate intracellular bodies in abcb19 (pgp19), and some aggregations of PIN1 induced by short-term BFA treatment did not disperse after BFA washout in abcb19. Although the subcellular localisations of ABCB19 and PIN1 in the reciprocal mutant backgrounds were like those in wild type, PIN1 plasma membrane localisation in abcb19 roots was more easily perturbed by the detergent Triton X-100, but not other non-ionic detergents. ABCB19 is stably associated with sterol/sphingolipid-enriched membrane fractions containing BIG/TIR3 and partitions into Triton X-100 detergent-resistant membrane (DRM) fractions. In the wild type, PIN1 was also present in DRMs, but was less abundant in abcb19 DRMs. These observations suggested a rationale for the observed lack of auxin transport activity when PIN1 is expressed in a non-plant heterologous system. PIN1 was therefore expressed in Schizosaccharomyces pombe, which has plant-like sterol-enriched microdomains, and catalysed auxin transport in these cells. These data suggest that ABCB19 stabilises PIN1 localisation at the plasma membrane in discrete cellular subdomains where PIN1 and ABCB19 expression overlaps.

Processing mutations disrupt interactions between the nucleotide binding and transmembrane domains of P-glycoprotein and the cystic fibrosis transmembrane conductance regulator (CFTR)

P-glycoprotein (P-gp, ABCB1) is an ATP-dependent drug pump. Each of its two homologous halves contains a transmembrane domain (TMD) that has six transmembrane (TM) segments and a nucleotide-binding domain (NBD). Determining how the two halves interact may provide insight into the folding of P-gp as the drug-binding pocket and nucleotide-binding sites are predicted to be at the interface between the two halves. Here, we present evidence for NBD1-TMD2 and NBD2-TMD1 interactions. We also show that TMD-NBD interactions in immature and mature P-gp can be affected by the presence of a processing mutation. We found that the NBD-TMD mutants L443C(NBD1)/S909C(TMD2) and A266C(TMD1)/F1086C(NBD2) could be cross-linked at 0 degrees C with oxidant (copper phenanthroline). Cross-linking was inhibited by vanadate-trapping of nucleotide. The presence of a processing mutation (G268V/L443C(NBD1)/S909C(TMD2); L1260A/A266C(TMD1)/F1086C(NBD2)) resulted in the synthesis of the immature (150 kDa) protein as the major product and the mutants could not be cross-linked with copper phenanthroline. Expression of the processing mutants in the presence of a pharmacological chaperone (cyclosporin A), however, resulted in the expression of mature (170 kDa) protein at the cell surface that could be cross-linked. Similarly, CFTR mutants A274C(TMD1)/L1260C(NBD2) and V510C(NBD1)/A1067C(TMD2) could be cross-linked at 0 degrees C with copper phenanthroline. Introduction of DeltaF508 mutation in these mutants, however, resulted in the synthesis of immature CFTR that could not be cross-linked. These results suggest that establishment of NBD interactions with the opposite TMD is a key step in folding of ABC transporters.

Multiple membrane-cytoplasmic domain contacts in the cystic fibrosis transmembrane conductance regulator (CFTR) mediate regulation of channel gating

The cystic fibrosis transmembrane conductance regulator (CFTR) is a unique ATP-binding cassette (ABC) ion channel mutated in patients with cystic fibrosis. The most common mutation, deletion of phenylalanine 508 (DeltaF508) and many other disease-associated mutations occur in the nucleotide binding domains (NBD) and the cytoplasmic loops (CL) of the membrane-spanning domains (MSD). A recently constructed computational model of the CFTR three-dimensional structure, supported by experimental data (Serohijos, A. W., Hegedus, T., Aleksandrov, A. A., He, L., Cui, L., Dokholyan, N. V., and Riordan, J. R. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 3256-3261) revealed that several of these mutations including DeltaF508 disrupted interfaces between these domains. Here we have used cysteine cross-linking experiments to verify all NBD/CL interfaces predicted by the structural model and observed that their cross-linking has a variety of different effects on channel gating. The interdomain contacts comprise aromatic clusters important for stabilization of the interfaces and also involve the Q-loops and X-loops that are in close proximity to the ATP binding sites. Cross-linking of all domain-swapping contacts between NBDs and MSD cytoplasmic loops in opposite halves of the protein rapidly and reversibly arrest single channel gating while those in the same halves have lesser impact. These results reinforce the idea that mediation of regulatory signals between cytoplasmic- and membrane-integrated domains of the CFTR channel apparently relies on an array of precise but highly dynamic interdomain structural joints.

Correctors promote folding of the CFTR in the endoplasmic reticulum

Cystic fibrosis (CF) is most commonly caused by deletion of a residue (DeltaF508) in the CFTR (cystic fibrosis transmembrane conductance regulator) protein. The misfolded mutant protein is retained in the ER (endoplasmic reticulum) and is not trafficked to the cell surface (misprocessed mutant). Corrector molecules such as corr-2b or corr-4a are small molecules that increase the amount of functional CFTR at the cell surface. Correctors may function by stabilizing CFTR at the cell surface or by promoting folding in the ER. To test whether correctors promoted folding of CFTR in the ER, we constructed double-cysteine CFTR mutants that would be retained in the ER and only undergo cross-linking when the protein folds into a native structure. The mature form, but not the immature forms, of M348C(TM6)/T1142C(TM12) (where TM is transmembrane segment), T351C(TM6)/T1142C(TM12) and W356C(TM6)/W1145C(TM12) mutants were efficiently cross-linked. Mutations to the COPII (coatamer protein II) exit motif (Y(563)KDAD(567)) were then made in the cross-linkable cysteine mutants to prevent the mutant proteins from leaving the ER. Membranes were prepared from the mutants expressed in the absence or presence of correctors and subjected to disulfide cross-linking analysis. The presence of correctors promoted folding of the mutants as the efficiency of cross-linking increased from approx. 2-5% to 22-35%. The results suggest that correctors interact with CFTR in the ER to promote folding of the protein into a native structure.

Chaperone displacement from mutant cystic fibrosis transmembrane conductance regulator restores its function in human airway epithelia

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF). The most common mutation, DeltaF508, omits the phenylalanine residue at position 508 in the first nucleotide binding domain (NBD1) of CFTR. The mutant protein is retained in the endoplasmic reticulum and degraded by the ubiquitin-proteasome system. We demonstrate that expression of NBD1 plus the regulatory domain (RD) of DeltaF508 CFTR (DeltaFRD) restores the biogenesis of mature DeltaF508 CFTR protein. In addition, DeltaFRD elicited a cAMP-stimulated anion conductance response in primary human bronchial epithelial (HBE) cells isolated from homozygous DeltaF508 CF patients. A protein transduction domain (PTD) could efficiently transduce (approximately 90%) airway epithelial cells. When fused to a PTD, direct addition of the DeltaFRD peptide conferred a dose-dependent, cAMP-stimulated anion efflux to DeltaF508 HBE cells. Hsp70 and Hsp90 associated equally with WT and DeltaF508 CFTR, whereas nearly twice as much of the Hsp90 cochaperone, Aha1, associated with DeltaF508 CFTR. Expression of DeltaFRD produced a dose-dependent removal of Aha1 from DeltaF508 CFTR that correlated with its functional rescue. These findings indicate that disruption of the excessive association of the cochaperone, Aha1, with DeltaF508 CFTR is associated with the correction of its maturation, trafficking and regulated anion channel activity in human airway epithelial cells. Thus, PTD-mediated DeltaFRD fragment delivery may provide a therapy for CF.

Enhanced cell-surface stability of rescued DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) by pharmacological chaperones

Misfolded proteins destined for the cell surface are recognized and degraded by the ERAD [ER (endoplasmic reticulum) associated degradation] pathway. TS (temperature-sensitive) mutants at the permissive temperature escape ERAD and reach the cell surface. In this present paper, we examined a TS mutant of the CFTR [CF (cystic fibrosis) transmembrane conductance regulator], CFTR DeltaF508, and analysed its cell-surface trafficking after rescue [rDeltaF508 (rescued DeltaF508) CFTR]. We show that rDeltaF508 CFTR endocytosis is 6-fold more rapid (approximately 30% per 2.5 min) than WT (wild-type, approximately 5% per 2.5 min) CFTR at 37 degrees C in polarized airway epithelial cells (CFBE41o-). We also investigated rDeltaF508 CFTR endocytosis under two further conditions: in culture at the permissive temperature (27 degrees C) and following treatment with pharmacological chaperones. At low temperature, rDeltaF508 CFTR endocytosis slowed to WT rates (20% per 10 min), indicating that the cell-surface trafficking defect of rDeltaF508 CFTR is TS. Furthermore, rDeltaF508 CFTR is stabilized at the lower temperature; its half-life increases from <2 h at 37 degrees C to >8 h at 27 degrees C. Pharmacological chaperone treatment at 37 degrees C corrected the rDeltaF508 CFTR internalization defect, slowing endocytosis from approximately 30% per 2.5 min to approximately 5% per 2.5 min, and doubled DeltaF508 surface half-life from 2 to 4 h. These effects are DeltaF508 CFTR-specific, as pharmacological chaperones did not affect WT CFTR or transferrin receptor internalization rates. The results indicate that small molecular correctors may reproduce the effect of incubation at the permissive temperature, not only by rescuing DeltaF508 CFTR from ERAD, but also by enhancing its cell-surface stability.