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.