A domain mimic increases ΔF508 CFTR trafficking and restores cAMP-stimulated anion secretion in cystic fibrosis epithelia

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Abstract
The major disease-causing mutation of the cystic fibrosis transmembrane conductance regulator (CFTR) is deletion of phenylalanine 508 (ΔF508), which adversely affects processing and plasma membrane targeting of CFTR. Under conditions predicted to stabilize protein folding, ΔF508 CFTR is capable of trafficking to the plasma membrane and retains cAMP-regulated anion channel activity. Overexpression is one factor that increases CFTR trafficking; therefore, we hypothesized that expression of a domain mimic of the first nucleotide-binding fold (NBF1) of CFTR, i.e., the site of F508, may be sufficient to overwhelm the quality control process or otherwise stabilize ΔF508 CFTR and thereby restore cAMP-stimulated anion secretion. In epithelial cells expressing recombinant ΔF508 human (h)CFTR, expression of wild-type NBF1 increased the amount of both core-glycosylated and mature protein to a greater extent than expression of ΔF508 NBF1. Expression of wild-type NBF1 in the ΔF508 hCFTR cells increased whole cell Cl−current density to ∼50% of that in cells expressing wild-type hCFTR. Expression of NBF1 in polarized epithelial monolayers from a ΔF508/ΔF508 cystic fibrosis mouse (MGEF) restored cAMP-stimulated transepithelial anion secretion but not in monolayers from a CFTR-null mouse (MGEN). Restoration of anion secretion was sustained in NBF1-expressing MGEF for >30 passages, whereas MGEN corrected with hCFTR progressively lost anion secretion capability. We conclude that expression of a NBF1 domain mimic may be useful for correction of the ΔF508 CFTR protein trafficking defect in cystic fibrosis epithelia.