A Chemical Corrector Modifies the Channel Function of F508del-CFTR

The deletion of Phe-508 (F508del) constitutes the most prevalent cystic fibrosis-causing mutation. This mutation leads to cystic fibrosis transmembrane conductance regulator (CFTR) misfolding and retention in the endoplasmic reticulum and altered channel activity in mammalian cells. This folding defect can however be partially overcome by growing cells expressing this mutant protein at low (27 degrees C) temperature. Chemical correctors have been identified that are also effective in rescuing the biosynthetic defect in F508del-CFTR, thereby permitting its functional expression at the cell surface. The mechanism of action of chemical correctors remains unclear, but it has been suggested that certain correctors [including 4-cyclohexyloxy-2-(1-[4-(4-methoxy-benzenesulfonyl)-piperazin-1-yl]-ethyl)-quinazoline (VRT-325)] may act to promote trafficking by interacting directly with the mutant protein. To test this hypothesis, we assessed the effect of VRT-325 addition on the channel activity of F508del-CFTR after its surface expression had been rescued by low temperature. It is noteworthy that short-term pretreatment with VRT-325 [but not with an inactive analog, 4-hydroxy-2-(1-[4-(4-methoxy-benzenesulfonyl)-piperazin-1-yl]-ethyl)-quinazoline (VRT-186)], caused a modest but significant inhibition of cAMP-mediated halide flux. Furthermore, VRT-325 decreased the apparent ATP affinity of purified and reconstituted F508del-CFTR in our ATPase activity assay, an effect that may account for the decrease in channel activity by temperature-rescued F508del-CFTR. These findings suggest that biosynthetic rescue mediated by VRT-325 may be conferred (at least in part) by direct modification of the structure of the mutant protein, leading to a decrease in its ATP-dependent conformational dynamics. Therefore, the challenge for therapy discovery will be the design of small molecules that bind to promote biosynthetic maturation of the major mutant without compromising its activity in vivo.