Endogenous surface expression of ΔF508‐CFTR mediates cAMP‐stimulated Cl− current in CFTRΔF508/ΔF508 pig thyroid epithelial cells

Abstract

The cystic fibrosis transmembrane conductance regulator (CFTR) is both an anion channel and a regulator of other transport proteins. Mutations in the CFTR gene underlie the human disease, cystic fibrosis. The most common CFTR mutation, ΔF508, produces a misfolded protein which traffics improperly. The availability of transgenic CFTR(ΔF508/ΔF508) pigs allows measurement of the impact of ΔF508 in native tissue. Thyroid epithelia respond to cAMP-elevating agents by increasing anion transport, a process reliant on functional CFTR. To assess whether endogenous levels of ΔF508-CFTR mediate thyroid transport, primary thyroid epithelial cultures (pThECs) were grown from newborn CFTR(+/+) (wild-type) and CFTR(ΔF508/ΔF508) (ΔF) pig thyroids and the stimulated, secretory components of short-circuit current (I(sc)) compared. Surface biotinylation studies assessed the surface presentation of ΔF508-CFTR. Baseline I(sc) levels of both wild-type and ΔF pThECs consisted of an amiloride-sensitive component. In ΔF pThECs, this mirrored previous measurements in CFTR(-/-) (knockout) pThECs. Surprisingly, elevation of cAMP transiently increased I(sc) to peak levels ∼65% of those achieved by wild-type. In contrast, knockout pThECs were indifferent to cAMP activation. In ΔF pThECs, total ΔF508-CFTR expression was ∼9% that of wild-type, consistent with misfolding and enhanced degradation. Surface biotinylation studies indicated that ∼4% of the total ΔF508 resided at the surface and did not increase with cAMP elevation. The present findings show that low endogenous levels of pig ΔF508-CFTR can mediate substantial anion transport by thyroid epithelia. These data suggest that both wild-type and ΔF508-CFTR regulate additional thyroid transporters, and together co-ordinate the overall I(sc) response.