Intracellular trafficking of core-glycosylated forms of CFTR

Abstract

The cystic fibrosis transmembrane conductance regulator (CFTR) is an apically localized chloride channel that mediates electrolyte transport in epithelial cells. CFTR is a typical glycoprotein which maturates during the conventional secretory pathway. Mutations in CFTR cause cystic fibrosis and among them, the most prevalent disease-causing mutation is loss of the phenylalanine residue at position 508 (ΔF508). ΔF508 results in protein misfolding and retention in the endoplasmic reticulum (ER). Recent studies suggest that immature, core-glycosylated ΔF508-CFTR can reach the plasma membrane under some conditions. The present study demonstrates that Golgi reassembly stacking protein (GRASP) is required for an unconventional secretory pathway of core-glycosylated CFTR that bypasses the usual route for Golgi-dependent membrane traffic. Integrated molecular and physiological analyses indicate that the core-glycosylated CFTR, but not the mature complex-glycosylated CFTR, can be expressed at the cell surface through a GRASP-dependent mechanism when the conventional route of ER-to-Golgi transport is blocked or ER stress is imposed. In addition, the core-glycosylated CFTR on the plasma membrane can exhibit Cl- channel activity. Overexpression of GRASPs rescued surface expression of ΔF508-CFTR and Cl- channel activity in heterologous systems, and this is mediated by the first PDZ (PSD-95/discs large/ZO-1) domain of GRASP that associates with the C-terminus of CFTR. Furthermore, transgenic expression of GRASP55 restored epithelial Cl- current and rescued mouse survival of ΔF508-CFTR model mice. These results suggest that GRASP-mediated translocation of CFTR could offer a novel therapeutic target for the treatment of CF and could help to understand how therapeutics for ΔF508-CFTR rescue the trafficking of mutant CFTR.