CFTR and Airway obstruction: Neutrophil elastase may worsen mucus-driven airway obstruction via two processes: activation of the sodium channel ENaC on the apical surface of epithelial cells (via degradation of SPLUNC1, the endogenous inhibitor of ENaC [64]) and indirect degradation of the cystic fibrosis transmembrane conductance regulator (CFTR) [65].