New Insights into CFTR as GSH Transporter

The cystic fibrosis transmembrane regulator (CFTR) chloride channel is an important member of the ATP-binding cassette superfamily because dysfunction or low expression of CFTR was found in patients with human cystic fibrosis. Further studies show low mitochondrial GSH levels in the CFTR-knockout mouse lung. Because impaired mitochondrial oxidative metabolism plays a critical role in lung diseases, CFTR has been proposed to be expressed on the mitochondria to promote GSH transportation across mitochondria to protect the lung from oxidative damage. However, it has not been well-established that CFTR functions as GSH transporter under the physiological conditions. Here, patch clamp studies demonstrate that only outward GSH currents across inside-out membrane patches were found with WT CFTR while both inward and outward GSH currents were observed with the constitutively active K190C/K978C CFTR construct with a high open probability. Thus, asymmetric GSH permeation may be state-dependent. Further studies show that the constitutively active CFTR mutant T338C/K190C/K978C was completely inhibited by extracellular Cu2+ and inhibition was fractionally reversed by 10mM intracellular membrane-impermeant GSH and completely by intracellular membrane-permeant TPEN. In contrast, extracellular Cu2+ failed to suppress the activity of the CFTR mutant K190C/K978C. The application of 10mM GSH or TPEN to the intracellular side of this construct did not increase the channel activity, either. These results suggest that native CFTR may function as GSH transport under the physiological condition and designed transition metal binding sites at the channel pore can be used to define its GSH permeability.