Improving Cell Surface Functional Expression of F508 CFTR: A Quest for Therapeutic Targets

Abstract

Cystic fibrosis (CF) is largely a protein misfolding disease. The deletion of a phenylalanine at residue 508 (F508) in the cystic fibrosis transmembrane conductance regulator (CFTR) accounts for 70% of all disease-causing alleles and is present in at least one copy in 90% of CF patients (Kerem et al., 1989). The F508 mutation impairs the conformational maturation of nascent CFTR (Lukacs et al., 1994), and arrests it in an early folding intermediate (Zhang et al., 1998). As a result, the mutant CFTR is retained in the endoplasmic reticulum (ER) (Cheng et al., 1990) in a chaperone-bound state (Yang et al., 1993), The ER-accumulated mutant CFTR fails to efficiently couple to the coatomer complex II (COPII) ER export machinery (Wang et al., 2004), and is degraded by the ubiquitin proteasome system through the ER-associated degradation (ERAD) pathway (Jensen et al., 1995; Ward et al., 1995), leading to loss of CFTR function at the cell surface. The folding defect of F508 CFTR appears kinetic in nature (Qu et al., 1997). A small fraction of F508 CFTR is able to exit the ER but the escaped mutant protein is not stable at the cell periphery and is rapidly cleared through lysosomal degradation (Lukacs et al., 1993). This second defect further reduces the cell surface localization of this mutant CFTR. Aside from localization defect, the F508 mutation also impairs the channel gating of CFTR, leading to reduced open probability (Dalemans et al., 1991). The threefold defect of F508 CFTR stems from its defective conformation, and impairs the CFTR functional expression at the cell surface, leading to severe clinical phenotype. Given the autosomal recessive inheritance of the disease, improving plasma membrane functional expression of F508 CFTR will benefit the vast majority of CF patients (Gelman & Kopito, 2002). Numerous research efforts have been made to improve F508 CFTR cell surface functional expression, including elevating its expression, reducing its degradation, enhancing the efficiency of its maturation, increasing its post-ER stability and improving its channel gating. Restoring F508 CFTR conformation will potentially improve its ER folding, its cell surface stability and its channel gating, leading to efficient F508 CFTR rescue. In this chapter, multiple approaches for F508 CFTR rescue will be reviewed, and their advantages as well as limitations will be discussed.