Rescuing protein conformation: prospects for pharmacological therapy in cystic fibrosis (2002)

Cystic fibrosis (CF), a fatal autosomal recessive genetic disease that affects over 60,000 people worldwide, is caused by mutations in CFTR (1).This gene encodes the cystic fibrosis transmembrane conductance regulator protein, which functions as a Cl -channel at the apical membranes of pulmonary epithelial cells (2).The CFTR channel is also found in certain other epithelia, such as the sweat ducts and part of the gastrointestinal tract, but lung pathology is by far the most prominent cause of clinical disease in CFTR homozygotes and compound heterozygotes.Precisely how the loss of functional, surface-expressed CFTR channels and the consequent decrease in Cl -conductance lead to CF pathogenesis is controversial (3).Still, the recognition that the majority of cases of CF are the result of a defect in biogenesis or intracellular trafficking of the protein, and that the mutant protein retains at least partial function, has stimulated an intensive search for therapeutic strategies aimed at rescuing the function of the mutant CFTR.Herein, we discuss some of the results of this search and the prospects of such a protein-repair strategy in the context of CFTR biogenesis and intracellular trafficking. CFTR biogenesisCFTR is a polytopic integral membrane glycoprotein composed of 1,480 amino acids (Figure 1).Biogenesis of CFTR begins with the targeting of nascent chainribosome complexes to the endoplasmic reticulum (ER) membrane, followed by translocation and integration of transmembrane domains into the lipid bilayer (4) (Figure 2a, step 1).Conformational maturation of wild-type CFTR in the ER (step 2) is an inefficient process; approximately 75% of newly synthe-sized CFTR molecules are degraded by cytoplasmic proteasomes shortly after synthesis (step 3).Maturation of CFTR to post-ER compartments (step 4) can be readily detected as an approximately 20-kDa decrease in electrophoretic mobility.This decrease reflects conversion by enzymes in the Golgi apparatus of the two Asn-linked glycans in the fourth extracellular loop from immature, high-mannose forms into mature, complex oligosaccharides.Once delivered to the plasma membrane, CFTR is subject to rapid internalization to a pool of subapical vesicles (step 5) that can be recycled to the plasma membrane (step 6) or delivered to lysosomes for degradation (5) (step 8).Over 1,200 mutations and sequence variants in the CFTR gene have been linked to CF (Cystic Fibrosis Mutation Data Base, http://www.genet.sickkids.on.ca/ cftr/).These mutations have been grouped into four classes (2): class I mutations abrogate the synthesis of CFTR protein (Figure 2a, step 1), class II mutants are defective in protein trafficking (steps 2 and 4), class III mutations lead to the presence of unstable or nonfunctional protein at the plasma membrane (steps 5, 6, and 8), and class IV mutations interfere with channel activation and regulation by physiological agonists (step 7).Despite this large number of CFTR disease alleles, the vast majority (>90%) of CF patients of Northern European origin have at least one copy of a single mutant allele, ∆F508, which encodes a CFTR molecule lacking a phenylalanine at position 508 (1, 6) (Figure 1).