Most cases of cystic fibrosis (CF) are attributable to the F508del allele of CFTR, which causes the protein to be retained in the endoplasmic reticulum (ER) and subsequently degraded. One strategy for CF therapy is to identify corrector compounds that help traffic F508del-CFTR to the cell surface. Pharmacological chaperones, or correctors that bind specifically to F508del-CFTR and restore function, would be the most promising drug development candidates, but few pharmacological chaperones exist for F508del-CFTR. Using differential scanning fluorimetry (DSF), we have surveyed corrector compounds and identified one, RDR1, which binds directly to the first nucleotide binding domain (NBD1) of F508del-CFTR. We show that RDR1 treatment partially rescues F508del-CFTR function in both cells and in an F508del-CF mouse model. Thus, RDR1 is a pharmacological chaperone of F508del-CFTR and represents a novel scaffold for drug development.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a plasma-membrane anion channel that, when mutated, causes the disease cystic fibrosis. Although CFTR has been detected in a detergent-resistant membrane fraction prepared from airway epithelial cells, suggesting that it may partition into cholesterol-rich membrane microdomains (lipid rafts), its compartmentalization has not been demonstrated in intact cells and the influence of microdomains on CFTR lateral mobility is unknown. We used live-cell imaging, spatial image correlation spectroscopy, and k-space image correlation spectroscopy to examine the aggregation state of CFTR and its dynamics both within and outside microdomains in the plasma membrane of primary human bronchial epithelial cells. These studies were also performed during treatments that augment or deplete membrane cholesterol. We found two populations of CFTR molecules that were distinguishable based on their dynamics at the cell surface. One population showed confinement and had slow dynamics that were highly cholesterol dependent. The other, more abundant population was less confined and diffused more rapidly. Treatments that deplete the membrane of cholesterol caused the confined fraction and average number of CFTR molecules per cluster to decrease. Elevating cholesterol had the opposite effect, increasing channel aggregation and the fraction of channels displaying confinement, consistent with CFTR recruitment into cholesterol-rich microdomains with dimensions below the optical resolution limit. Viral infection caused the nanoscale microdomains to fuse into large platforms and reduced CFTR mobility. To our knowledge, these results provide the first biophysical evidence for multiple CFTR populations and have implications for regulation of their surface expression and channel function.

BACKGROUND AND PURPOSEThe most common cystic fibrosis (CF) mutation F508del inhibits the gating and surface expression of CFTR, a plasma membrane anion channel. Optimal pharmacotherapies will probably require both a 'potentiator' to increase channel open probability and a 'corrector' that improves folding and trafficking of the mutant protein and its stability at the cell surface. Interaction between CF drugs has been reported but remains poorly understood. EXPERIMENTAL APPROACHCF bronchial epithelial cells were exposed to the corrector VX-809 (lumacaftor) and potentiator VX-770 (ivacaftor) individually or in combination. Functional expression of CFTR was assayed as the forskolin-stimulated short-circuit current (I sc ) across airway epithelial monolayers expressing F508del CFTR. KEY RESULTSThe potentiated I sc response during forskolin stimulation was increased sixfold after pretreatment with VX-809 alone and reached 11% that measured across non-CF monolayers. VX-770 (100 nM) and genistein (50 μM) caused similar levels of potentiation, which were not additive and were abolished by the CFTR inhibitor CFTR inh -172. The unbound fraction of VX-770 in plasma was 0.13 ± 0.04%, which together with previous measurements in patients given 250 mg p.o. twice daily, suggests a peak free plasma concentration of 1.5-8.5 nM. Chronic exposure to high VX-770 concentrations (>1 μM) inhibited functional correction by VX-809 but not in the presence of physiological protein levels (20-40 mg·mL À1 ). Chronic exposure to a low concentration of VX-770 (100 nM) together with VX-809 (1 μM) also did not reduce the forskolin-stimulated I sc , relative to cells chronically exposed to VX-809 alone, provided it was assayed acutely using the same, clinically relevant concentration of potentiator. CONCLUSIONS AND IMPLICATIONSChronic exposure to clinically relevant concentrations of VX-770 did not reduce F508del CFTR function. Therapeutic benefit of VX-770 + VX-809 (Orkambi) is probably limited by the efficacy of VX-809 rather than by inhibition by VX-770. Abbreviations

Bicarbonate facilitates mucin unpacking and bacterial killing however its transport mechanisms in the airways are not well understood. cAMP stimulates anion efflux through the CFTR (ABCC7) anion channel and this is defective in CF. The anion exchanger pendrin (SLC26A4) also mediates HCO3− efflux and is upregulated by proinflammatory cytokines. Here we examined pendrin and CFTR expression and their contributions to HCO3− secretion by human nasal and bronchial epithelia. In native tissue, both proteins were most abundant at the apical pole of ciliated surface cells with little expression in submucosal glands. In well‐differentiated primary nasal and bronchial cell cultures, IL‐4 dramatically increased pendrin mRNA levels and apical immunostaining. Exposure to low‐Cl− apical solution caused intracellular alkalinization (ΔpHi) that was enhanced 4‐fold by IL‐4 pretreatment. ΔpHi was unaffected by DIDS or CFTRinh‐172 but was reduced by adenoviral shRNA targeting pendrin. Forskolin increased ΔpHi and this stimulation was prevented by CFTRinh‐172 implicating CFTR, yet forskolin only increased ΔpHi after pendrin expression had been induced by IL‐4. The dependence of ΔpHi on pendrin suggests there is minimal electrical coupling between Cl− and HCO3− fluxes and that CFTR activation increases anion exchange‐mediated HCO3− influx. Conversely, inducing pendrin expression increased forskolin‐stimulated, CFTRinh‐172‐sensitive current by ~2‐fold in epithelial and non‐epithelial cells. We conclude that pendrin mediates most HCO3− secretion across airway surface epithelium during inflammation and enhances electrogenic Cl− secretion via CFTR as described for other SLC26A transporters. Support or Funding Information Support: Cystic Fibrosis Canada, Canadian Institutes of Health Research, Canada Foundation for Innovation This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The ubiquitous bacterium Pseudomonas aeruginosa frequently causes hospital-acquired infections. P. aeruginosa also infects the lungs of cystic fibrosis (CF) patients and secretes N-(3-oxo-dodecanoyl)-S-homoserine lactone (3O-C12) to regulate bacterial gene expression critical for P. aeruginosa persistence. In addition to its effects as a quorum-sensing gene regulator in P. aeruginosa, 3O-C12 elicits cross-kingdom effects on host cell signaling leading to both pro-or anti-inflammatory effects. We find that in addition to these slow effects mediated through changes in gene expression, 3O-C12 also rapidly increases Cl ؊ and fluid secretion in the cystic fibrosis trans-