Ferret CFTR processing and function

The most common cystic fibrosis transmembrane conductance regulator (CFTR) gene mutation is δF508 and this causes cystic fibrosis (CF). Animal models that recapitulate the human disease phenotype are critical to understanding pathophysiologic mechanisms in CF and developing therapies. New CF models in the pig and ferret have been generated that develop lung, pancreatic, liver, and intestinal pathologies that reflect disease in CF patients. Species-specific biology in the processing of CFTR has demonstrated that pig and mouse δF508-CFTR proteins are more effectively processed to the apical membrane of airway epithelia than human δF508-CFTR. The processing behavior of ferret wild-type (WT) and δF508-CFTR proteins remain unknown and such information is important to predicting the utility of a δF508-CFTR ferret. To this end, we sought to compare processing, membrane stability, and function of human and ferret WT- and δF508-CFTR proteins in a heterologous expression system using HT1080, HEK293T, BHK21, and Cos7 cells, as well as human and ferret CF polarized airway epithelia. Analysis of the protein processing and stability by metabolic pulse-chase and surface On-Cell Western blots revealed that WT-fCFTR half-life and membrane stability were increased relative to WT-hCFTR. Furthermore, in BHK21, Cos7, and CuFi cells, human and ferret δF508-CFTR processing was negligible, while low levels of processing of δF508-fCFTR could be seen in HT1080 and HEK293T cells. Only the WT-fCFTR, but not δF508-fCFTR, produced functional cAMP-inducible chloride currents in both CF human and ferret airway epithelia. Further elucidation of the mechanism responsible for elevated fCFTR protein stability may lead to new therapeutic approaches to augment CFTR function. These findings also suggest that generation of a ferret CFTR^{δF508/δF508} animal model may be useful.

Furthermore, in the CFTR and CFTR^+/+ ferret model we have characterized abnormalities in the bioelectric properties of the trachea, stomach, intestine and gallbladder of newborn CF ferrets. Short circuit current (I_SC) analysis of CF and WT tracheas revealed the following similarities and differences: 1) amiloride sensitive sodium currents were similar between genotypes, 2) responses to 4,4'-diisothiocyano-2,2'-stilbene disulphonic acid (DIDS) were ~4-fold greater in CF animals, suggesting elevated baseline chloride transport through non-CFTR channels, and 3) as expected, there was a lack of IBMX/forskolin-stimulated and GlyH-101-inhibited currents in CF animals due to the lack of CFTR. CFTR mRNA and protein was present throughout all levels of the WT ferret and IBMX/forskolin-inducible I_SC was only observed in WT animals. Interestingly, IBMX/forskolin-inducible intestinal I_SC in WT animals was not inhibited by the CFTR inhibitor GlyH-101 or bumetanide. The luminal pH of the CF ferret stomach was significantly decreased relative to the controls, while both genotypes maintained near neutral pH along the length of the intestine. The WT stomach and gallbladder exhibited significantly enhanced IBMX/forskolin I_SC responses and inhibition by GlyH-101 relative to CF samples. These findings demonstrate that multiple organs affected by disease in the CF ferret have bioelectric abnormalities consistent with the lack of cAMP-mediated chloride transport.