Mammalian Osmolytes and S-Nitrosoglutathione Promote ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Protein Maturation and Function*

In cystic fibrosis, the absence of functional CFTR results in thick mucous secretions in the lung and intestines, as well as pancreatic deficiency. Although expressed at high levels in the kidney, mutations in CFTR result in little or no apparent kidney dysfunction. In an effort to understand this phenomenon, we analyzed ΔF508 CFTR maturation and function in kidney cells under conditions that are common to the kidney, namely osmotic stress. Kidney cells were grown in culture and adapted to 250 mm NaCl and 250 mm urea. High performance liquid chromatography analysis of lysates from kidney cells adapted to these conditions identified an increase in the cellular osmolytes glycerophosphorylcholine, myo-inositol, sorbitol, and taurine. In contrast to isoosmotic conditions, hyperosmotic stress led to the proper folding and processing of ΔF508 CFTR. Furthermore, three of the cellular osmolytes, when added individually to cells, proved effective in promoting the proper folding and processing of the ΔF508 CFTR protein in both epithelial and fibroblast cells. Whole-cell patch clamping of osmolyte-treated cells showed that ΔF508 CFTR had trafficked to the plasma membrane and was activated by forskolin. Encouraged by these findings, we looked at other features common to the kidney that may impact ΔF508 maturation and function. Interestingly, a small molecule, S-nitrosoglutathione, which is a substrate for gamma glutamyltranspeptidase, an abundant enzyme in the kidney, likewise promoted ΔF508 CFTR maturation and function. S-Nitrosoglutathione-corrected ΔF508 CFTR exhibited a shorter half-life as compared with wild type CFTR. These results demonstrate the feasibility of a small molecule approach as a therapeutic treatment in promoting ΔF508 CFTR maturation and function and suggest that an additional treatment may be required to stabilize ΔF508 CFTR protein once present at the plasma membrane. Finally, our observations may help to explain why ΔF508 homozygous patients do not present with kidney dysfunction.