SLC6A14 Enhances CFTR Channel Activity in the Cystic Fibrosis Affected Lung Epithelium

Abstract

Cystic fibrosis (CF) is a common genetic disorder caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. CFTR protein functions apically on epithelial cells as an anion channel, helping to keep the surface hydrated. F508del-mutation in CFTR is the most common CF-causing mutation, which leads to decreased functional expression of the protein. However, patients homozygous for F508del mutation show a large variation in disease severity. To address this variation, previous genome-wide association studies have shown that SLC6A14 is a major modifier of CF disease severity. SLC6A14 is an electrogenic neutral/cationic amino-acid transporter expressed in colonic and lung epithelia. We examined the hypothesis that SLC6A14 has a positive impact on the functional expression of Wt and F508del-CFTR. Consistent with this hypothesis, we found that CFTR mediated anion-flux was enhanced by over-expression of SLC6A14 and addition of its transported substrate: arginine in a heterologous expression system. Similarly, in airway cell cultures endogenously expressing both SLC6A14 and CFTR, we observed that shRNA mediated knockdown of SLC6A14 reduced CFTR channel function. These findings could be translated to primary lung epithelial cultures derived from CF patients. The mechanism underlying the positive effect of SLC6A14 on CFTR function is currently unclear. It unlikely to be mediated via a direct interaction, since the two proteins could not be co-immunoprecipitated. However, it could be due to activation of intracellular signaling pathways leading to change in CFTR channel activity. Future studies will focus on the exploration of these pathways, and whether they affect the phosphorylation status of CFTR. Taken together, these findings give biologic insight to support that SLC6A14 is a modifier of CF disease severity. Hence, it can be used as an alternative drug target, especially in patients resistant to currently available therapies.