EXPRESSION OF THE CYSTIC-FIBROSIS PHENOTYPE IN A RENAL AMPHIBIAN EPITHELIAL-CELL LINE

Mutations in a Cl- channel (cystic fibrosis transmembrane conductance regulator or CFTR) are responsible for the cystic fibrosis (CF) phenot ype, Increased Na+ transport rates are observed in CF airway epitheliu m, and recent studies suggest that this is due to an increase in Na+ c hannel open probability (P-o), The Xenopus renal epithelial cell line, A6, expresses both cAMP-activated 8-picosiemen (pS) Cl- channels and amiloride-sensitive 4-pS Na+ channels, and provides a model system for examining the interactions of CFTR and epithelial Na+ channels, A6 ce lls express CFTR mRNA, as demonstrated by reverse transcriptase-polyme rase chain reaction and partial sequence analysis, A phosphorothioate antisense oligonucleotide, complementary to the 5' end of the open rea ding frame of Xenopus CFTR, was used to inhibit functional expression of CFTR in A6 cells, Parallel studies utilized the corresponding sense oligonucleotide as a control, CFTR protein expression was markedly re duced in cells incubated with the antisense oligonucleotide. Incubatio n of A6 cells with the antisense oligonucleotide led to inhibition of forskolin-activated amiloride-insensitive short circuit current (I-sc) , After a 30-min exposure to 10 mu M forskolin, 8-pS Cl- channel activ ity was detected in only 1 of 31 (3%) cell-attached patches on cells t reated with antisense oligonucleotide, compared to 5 of 19 (26%) patch es from control cells, A shift in the single-channel current-voltage r elationship derived from antisense-treated cells was also consistent w ith a reduction in Cl- reabsorption, Both amiloride-sensitive I-sc and Na+ channel P-o were significantly increased in antisense-treated, fo rskolin-stimulated A6 cells, when compared with forskolin-stimulated c ontrols, These data suggest that the regulation of Na+ channels by CFT R is not limited to respiratory epithelia and to epithelial cells in c ulture overexpressing CFTR and epithelial Na+ channels.