Organic Solutes Rescue the Functional Defect in ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator

The most common defect in cystic fibrosis, deletion of phenylalanine from position 508 of the cystic fibrosis transmembrane conductance regulator (ΔF508 CFTR), decreases the trafficking of this protein to the cell surface membrane. Previous studies have shown that low temperature and high concentrations of glycerol or trimethylamine N-oxide can partially counteract the processing defect of ΔF508 CFTR. The present study investigates whether physiologically relevant concentrations of organic solutes, accumulated by cotransporter proteins, can rescue the misprocessing of ΔF508 CFTR. Myoinositol alone or myoinositol, betaine, and taurine given sequentially increased the processing of core-glycosylated, endoplasmic reticulum-arrested ΔF508 CFTR into the fully glycosylated form of CFTR in IB3 cells or NIH 3T3 cells stably expressing ΔF508 CFTR. Pulse-chase experiments using transiently transfected COS7 cells demonstrated that organic solutes also increased the processing of the core-glycosylated form of green fluorescent protein-ΔF508 CFTR. Moreover, the prolonged half-life of the complex-glycosylated form of GFP-ΔF508 CFTR suggests that this treatment stabilized the mature form of the protein. In vitro studies of purified NBD1 stability and aggregation showed that myoinositol stabilized both the ΔF508 and wild type CFTR and inhibited AF508 misfolding. Most significantly, treatment of CF bronchial airway cells with these transportable organic solutes restores cAMP-stimulated single channel activity of both CFTR and outwardly rectifying chloride channel in the cell surface membrane and also restores a forskolin-stimulated macroscopic ³⁶Cl^- efflux. We conclude that organic solutes can repair CFTR functions by enhancing the processing of ΔF508 CFTR to the plasma membrane by stabilizing the complex-glycosylated form of ΔF508 CFTR.