Trends in Molecular Medicine
ReviewCFTR: folding, misfolding and correcting the ΔF508 conformational defect
Section snippets
Cystic fibrosis (CF) and approaches to alleviate the CF clinical phenotype
The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cyclic AMP (cAMP)-regulated chloride channel expressed in the plasma membrane (PM) of secretory epithelia in the airways, intestine, pancreas, testis and exocrine glands, as well in some non-epithelial cell types. Each CFTR molecule contains two membrane-spanning domains (MSD1 and MSD2), two nucleotide binding domains that participate in ATP binding and hydrolysis (NBD1 and NBD2), and a regulatory domain (R) whose
CFTR domain assembly
In most heterologously expressing cells only 20–40% of newly synthesized CFTR nascent chains attain their native conformation, exit the ER, and undergo complex glycosylation in the Golgi compartment 14, 15. Interestingly, however, the maturation efficiency of CFTR endogenously expressed in two epithelial cell lines was reported to be nearly 100% [16]. Partially folded channels are disposed of by ER-associated degradation (ERAD) via the ubiquitin–proteasome system (UPS) [17]. Although the
Membrane trafficking and functional defects of ΔF508 CFTR
The activity of cellular protein homeostasis networks limits the escape of non-native ΔF508 CFTR from the ER and reduces its metabolic stability at the PM. Modulation of these protein quality control systems by PRs may thus alleviate the ΔF508 CFTR loss-of-function phenotype and be exploited therapeutically [13].
Implications for CF therapy
Correction of defective ΔF508 CFTR folding at the ER by a small molecule is an attractive approach to treat CF because it targets the underlying defect in the appropriate CFTR-expressing cells. In principle, if a PC or PR can modify the ΔF508 CFTR structure to resemble that of WT CFTR, then downstream consequences of defective folding may also be corrected. However, compounds identified thus far have limited efficacy. Because of the complexity of ΔF508 CFTR folding defects discussed above, it
Future perspectives
From the basic science perspective, although there is now a substantial body of data on the biophysics and cellular mechanisms of CFTR folding and ΔF508 CFTR misfolding in vitro, there remain major gaps in our knowledge of how critical components of the quality control machinery recognize the folding defect in tissues. Correctors of ΔF508 CFTR misprocessing have rapidly moved to clinical trials despite even a rudimentary understanding of their mechanism of action. From the translational
Acknowledgments
Work performed in the authors’ laboratories was supported by the Cystic Fibrosis Foundation Therapeutics Inc., Cystic Fibrosis Canada, NIH–NIDDK (National Institutes of Health–National Institute of Diabetes and Digestive and Kidney Diseases), CIHR (Canadian Institute of Health Research) and Canadian Foundation for Innovation. G.L. is holder of a Canada Research Chair. We thank past and present lab members for their invaluable contributions and T. Okiyoneda for careful reading of the manuscript.
Glossary
- Complex glycosylated CFTR
- N-glycan chain containing fucose, galactose and sialic acid instead of terminal mannose residues, a modification that occurs in Golgi compartments.
- Computational screening
- virtual screening of compounds by molecular docking computations in which the energetics of compound interaction with a target are calculated using high-resolution crystal structure data for the target.
- Core glycosylated CFTR
- containing N-glycan chains with terminal mannose residues.
- Cysteine (Cys)
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