Functional cystic fibrosis transmembrane conductance regulator tagged with an epitope of the vesicular stomatis virus glycoprotein can be addressed to the apical domain of polarized cells

Abstract

The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation-activated chloride channel apically localized in epithelial cells. In cystic fibrosis patients, the gene encoding this N-linked glycoprotein is mutated. About 70% of CF patients express a mutated form of CFTR, deleted at the phenylalanine residue at position 508 (ΔF508). CFTR-ΔF508 fails to exit the endoplasmic reticulum; it remains incompletely glycosylated and is rapidly degraded. To optimize CFTR detection for membrane localization studies and biochemical studies, we tagged wild-type and ΔF508 CFTR with the VSV-G epitope at their carboxyterminal ends. We have generated pig kidney epithelial cell clones (LLCPK1) expressing VSV-G-tagged human wild-type and ΔF508-CFTR. In CFTR-expressing cells, the transfected protein is maturated and transported to the apical membrane where it is concentrated. The cells exhibit a strong anion channel activity after stimulation by cAMP, as demonstrated by a halide sensitive fluorescent dye assay (6-methoxy-N-ethylquinominium, SPQ), and whole-cell patch-clamp approach. This activity of CFTR-VSV-G is indistinguishable from the wild-type CFTR. In contrast, in cells expressing tagged ΔF508-CFTR or in non-transfected cells, no anion channel activity could be detected after stimulation by cAMP. In ΔF508-CFTR-VSV-G-expressing cells, the mutated CFTR remained in the incompletely glycosylated form and was localized in the endoplasmic reticulum. These cell lines reproduce the cellular fate of wild-type and mutated CFTR-ΔF508. To our knowledge, they are the first differentiated epithelial cell lines stably expressing tagged CFTR and CFTR-ΔF508 in which cellular processing and functional activity of these two proteins are reproduced. Thus the addition of the VSV-G epitope does not impair the localization and function of CFTR, and these cell lines can be used to examine CFTR function in vitro.