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More than 2000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) have been described that confer a range of molecular cell biological and functional phenotypes. Most of these mutations lead to compromised anion conductance at the apical plasma membrane of secretory epithelia and cause cystic fibrosis (CF) with variable disease severity. Based on the molecular phenotypic complexity of CFTR mutants and their susceptibility to pharmacotherapy, it has been recognized that mutations may impose combinatorial defects in CFTR channel biology. This notion led to the conclusion that the combination of pharmacotherapies addressing single defects (e.g., transcription, translation, folding, and/or gating) may show improved clinical benefit over available low-efficacy monotherapies. Indeed, recent phase 3 clinical trials combining ivacaftor (a gating potentiator) and lumacaftor (a folding corrector) have proven efficacious in CF patients harboring the most common mutation (deletion of residue F508, ΔF508, or Phe508del). This drug combination was recently approved by the U.S. Food and Drug Administration for patients homozygous for ΔF508. Emerging studies of the structural, cell biological, and functional defects caused by rare mutations provide a new framework that reveals a mixture of deficiencies in different CFTR alleles. Establishment of a set of combinatorial categories of the previously defined basic defects in CF alleles will aid the design of even more efficacious therapeutic interventions for CF patients.

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

Dysfunction of the epithelial anion channel cystic fibrosis transmembrane conductance regulator (CFTR) causes a wide spectrum of disease, including cystic fibrosis (CF) and CFTR‐related diseases (CFTR‐RDs). Here, we investigate genotype–phenotype–CFTR function relationships using human nasal epithelial (hNE) cells from a small cohort of non‐CF subjects and individuals with CF and CFTR‐RDs and genotypes associated with either residual or minimal CFTR function using electrophysiological techniques. Collected hNE cells were either studied directly with the whole‐cell patch‐clamp technique or grown as primary cultures at an air–liquid interface after conditional reprogramming. The properties of cAMP‐activated whole‐cell Cl− currents in freshly isolated hNE cells identified them as CFTR‐mediated. Their magnitude varied between hNE cells from individuals within the same genotype and decreased in the rank order: non‐CF > CFTR residual function > CFTR minimal function. CFTR‐mediated whole‐cell Cl− currents in hNE cells isolated from fully differentiated primary cultures were identical to those in freshly isolated hNE cells in both magnitude and behaviour, demonstrating that conditional reprogramming culture is without effect on CFTR expression and function. For the cohort of subjects studied, CFTR‐mediated whole‐cell Cl− currents in hNE cells correlated well with CFTR‐mediated transepithelial Cl− currents measured in vitro with the Ussing chamber technique, but not with those determined in vivo with the nasal potential difference assay. Nevertheless, they did correlate with the sweat Cl− concentration of study subjects. Thus, this study highlights the complexity of genotype–phenotype–CFTR function relationships, but emphasises the value of conditionally reprogrammed hNE cells in CFTR research and therapeutic testing. Key points: The genetic disease cystic fibrosis is caused by pathogenic variants in the cystic fibrosis transmembrane conductance regulator (CFTR), an ion channel, which controls anion flow across epithelia lining ducts and tubes in the body.This study investigated CFTR function in nasal epithelial cells from people with cystic fibrosis and CFTR variants with a range of disease severity.CFTR function varied widely in nasal epithelial cells depending on the identity of CFTR variants, but was unaffected by conditional reprogramming culture, a cell culture technique used to grow large numbers of patient‐derived cells.Assessment of CFTR function in vitro in nasal epithelial cells and epithelia, and in vivo in the nasal epithelium and sweat gland highlights the complexity of genotype–phenotype–CFTR function relationships. [ABSTRACT FROM AUTHOR]

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) C1- channel is associated with a wide spectrum of disease. In the search for modulators of CFTR, pharmacological agents that interact directly with the CFTR C1-channel have been identified. Some agents stimulate CFTR by interacting with the nucleotide-binding domains that control channel gating, whereas others inhibit CFTR by binding within the channel pore and preventing C1- permeation. Knowledge of the molecular pharmacology of CFTR might lead to new treatments for diseases caused by the dysfunction of CFTR.