CFTR facilitates fluid secretion by ferret alveolar type 2 cells

The function of the cystic fibrosis transmembrane conductance regulator (CFTR) in alveolar type 2 (AT2) cells and its role in maintaining alveolar fluid balance remain controversial. Certain studies suggest that CFTR expression in the alveolus facilitates fluid absorption, while others document a role in fluid secretion.

We established organoid and air-liquid interface (ALI) cultures from primary ferret AT2 cells. The cellular composition of organoids was characterized by scRNA-seq. Using these models, we examined how CFTR functions and pharmacologic potentiation of the G551D mutant influences chloride and fluid transport, and how these processes regulate the homeostatic volume of apical surface liquid (ASL).

scRNA-seq confirmed that the organoids contained >99% AT2 and transitional AT2 cells. Forskolin-induced swelling revealed strong CFTR-dependent fluid secretion in wild-type organoids that was absent in CFTR-G551D cystic fibrosis (CF) organoids but rescued by the CFTR potentiator VX-770 (Ivacaftor). Electrophysiologic studies confirmed impaired CFTR-mediated chloride secretion in CF ALI cultures, which was partially restored by VX-770. CFTR-mediated currents were prominent under conditions favoring secretion but minimal under gradients favoring absorption. The ASL height of ALI cultures was significantly reduced in CF cultures and partially restored by VX-770 treatment.

Our results from ferret primary AT2-derived cultures provide evidence that CFTR primarily contributes to chloride and fluid secretion rather than absorption. This in vitro model system may help elucidate CFTR functions in the distal lung, providing insights into CF pathogenesis and serving as a platform for therapeutic testing.