Stabilizing rescued surface-localized δf508 CFTR by potentiation of its interaction with Na(+)/H(+) exchanger regulatory factor 1

Cystic fibrosis (CF) is a recessive genetic disease caused by mutations in CFTR, a plasma-membrane-localized anion channel. The most common mutation in CFTR, deletion of phenylalanine at residue 508 (ΔF508), causes misfolding of CFTR resulting in little or no protein at the plasma membrane. The CFTR corrector VX-809 shows promise for treating CF patients homozygous for ΔF508. Here, we demonstrate the significance of protein–protein interactions in enhancing the stability of the ΔF508 CFTR mutant channel protein at the plasma membrane. We determined that VX-809 prolongs the stability of ΔF508 CFTR at the plasma membrane. Using competition-based assays, we demonstrated that ΔF508 CFTR interacts poorly with Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) compared to wild-type CFTR, and VX-809 significantly increased this binding affinity. We conclude that stabilized CFTR–NHERF1 interaction is a determinant of the functional efficiency of rescued ΔF508 CFTR. Our results demonstrate the importance of macromolecular-complex formation in stabilizing rescued mutant CFTR at the plasma membrane and suggest this to be foundational for the development of a new generation of effective CFTR-corrector-based therapeutics.

Compartmentalized cyclic adenosine 3',5'-monophosphate at the plasma membrane clusters PDE3A and cystic fibrosis transmembrane conductance regulator into microdomains

Formation of multiple-protein macromolecular complexes at specialized subcellular microdomains increases the specificity and efficiency of signaling in cells. In this study, we demonstrate that phosphodiesterase type 3A (PDE3A) physically and functionally interacts with cystic fibrosis transmembrane conductance regulator (CFTR) channel. PDE3A inhibition generates compartmentalized cyclic adenosine 3',5'-monophosphate (cAMP), which further clusters PDE3A and CFTR into microdomains at the plasma membrane and potentiates CFTR channel function. Actin skeleton disruption reduces PDE3A-CFTR interaction and segregates PDE3A from its interacting partners, thus compromising the integrity of the CFTR-PDE3A-containing macromolecular complex. Consequently, compartmentalized cAMP signaling is lost. PDE3A inhibition no longer activates CFTR channel function in a compartmentalized manner. The physiological relevance of PDE3A-CFTR interaction was investigated using pig trachea submucosal gland secretion model. Our data show that PDE3A inhibition augments CFTR-dependent submucosal gland secretion and actin skeleton disruption decreases secretion.

Clinical and molecular characterization of S1118F-CFTR

BACKGROUND

Cystic fibrosis is a lethal autosomal recessive disorder usually associated with lung disease, pancreatic insufficiency and high sweat chloride levels.

CLINICAL CASE

A patient admitted to Le Bonheur Children's Medical Center (LBCMC, Memphis, TN) showed symptoms of meconium ileus which required exploratory laparotomy, bowel resection and ileostomy. Genotyping showed DeltaF508/I1027T on one chromosome and S1118F on the other. Sweat testing on three different occasions gave negative and intermediate results (22.7, 24.6 mmol/L; 55.1, 58.6 mmol/L and 55.1, 58 mmol/L) and pancreatic elastase testing showed normal levels.

OBJECTIVE

To characterize S1118F-CFTR mutation at a molecular level to help understand the associated CF-phenotype.

METHODS

Molecular characterization of S1118F-CFTR mutant was studied in HEK-293 cells at 37 degrees C. Various biochemical methods such as Western blotting, real-time PCR, Pulse chase labeling and iodide efflux assay were employed.

RESULTS

S1118F-CFTR makes less than 10-15% of mature CFTR (band C) compared to WT-CFTR. The mRNA levels of S1118F-CFTR and WT-CFTR are comparable. S1118F-CFTR is functional but shows about 10-15% of WT-CFTR activity. S1118F-CFTR shows impaired maturation and CF-correctors can increase the amount of mature and functional CFTR by three- to fourfold.

CONCLUSION

S1118F-CFTR shows impaired maturation and an individual with S1118F-CFTR paired with DeltaF508-CFTR exhibits atypical CF symptoms with intermediate sweat chloride level and meconium ileus despite documented pancreatic sufficiency.

Spatiotemporal coupling of cAMP transporter to CFTR chloride channel function in the gut epithelia

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized at apical cell membranes and exists in macromolecular complexes with a variety of signaling and transporter molecules. Here, we report that the multidrug resistance protein 4 (MRP4), a cAMP transporter, functionally and physically associates with CFTR. Adenosine-stimulated CFTR-mediated chloride currents are potentiated by MRP4 inhibition, and this potentiation is directly coupled to attenuated cAMP efflux through the apical cAMP transporter. CFTR single-channel recordings and FRET-based intracellular cAMP dynamics suggest that a compartmentalized coupling of cAMP transporter and CFTR occurs via the PDZ scaffolding protein, PDZK1, forming a macromolecular complex at apical surfaces of gut epithelia. Disrupting this complex abrogates the functional coupling of cAMP transporter activity to CFTR function. Mrp4 knockout mice are more prone to CFTR-mediated secretory diarrhea. Our findings have important implications for disorders such as inflammatory bowel disease and secretory diarrhea.