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 (DeltaF508 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 DeltaF508 CFTR. The present study investigates whether physiologically relevant concentrations of organic solutes, accumulated by cotransporter proteins, can rescue the misprocessing of DeltaF508 CFTR. Myoinositol alone or myoinositol, betaine, and taurine given sequentially increased the processing of core-glycosylated, endoplasmic reticulum-arrested DeltaF508 CFTR into the fully glycosylated form of CFTR in IB3 cells or NIH 3T3 cells stably expressing DeltaF508 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-DeltaF508 CFTR. Moreover, the prolonged half-life of the complex-glycosylated form of GFP-DeltaF508 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 DeltaF508 and wild type CFTR and inhibited DeltaF508 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 Cl-36(-) efflux. We conclude that organic solutes can repair CFTR functions by enhancing the processing of DeltaF508 CFTR to the plasma membrane by stabilizing the complex-glycosylated form of DeltaF508 CFTR.