Glycerol reverses the misfolding phenotype of the most common cystic fibrosis mutation

The common Delta F508 mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) interferes with the biosynthetic folding of nascent CFTR polypeptides, leading to their retention and rapid degradation in an intracellular compartment proximal to the Gels apparatus, Neither the pathway by which wild-type CFTR folds nor the mechanism by which the Phe(508) deletion interferes with this process is well understood, We have investigated the effect of glycerol, a polyhydric alcohol known to stabilize protein conformation, on the folding of CFTR and Delta F508 in vivo. Incubation of transient and stable Delta F508 tranfectants with 10% glycerol induced a significant accumulation of Delta F508 protein bearing complex N-linked oligosaccharides, indicative of their transit to a compartment distal to the endoplasmic reticulum (ER). This accumulation was accompanied by an increase in mean whole cell cAMP activated chloride conductance, suggesting that the glycerol-rescued Delta F508 polypeptides form functional plasma membrane CFTR channels, These effects were dose- and time-dependent and fully reversible, Glycerol treatment also stabilized immature (core-glycosylated) Delta F508 and CFTR molecules that are normally degraded rapidly. These effects of glycerol were not due to a general disruption of ER quality control processes but appeared to correlate with the degree of temperature sensitivity of specific CFTR mutations, These data suggest a model in which glycerol serves to stabilize an otherwise unstable intermediate in CFTR biosynthesis, maintaining it in a conformation that is competent for folding and subsequent release from the ER quality control apparatus.