Increases in cytosolic Ca 2+ induce dynamin- and calcineurin-dependent internalisation of CFTR

Abstract

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated, apical anion channel that regulates ion and fluid transport in many epithelia including the airways. We have previously shown that cigarette smoke (CS) exposure to airway epithelia causes a reduction in plasma membrane CFTR expression which correlated with a decrease in airway surface hydration. The effect of CS on CFTR was dependent on an increase in cytosolic Ca²⁺. However, the underlying mechanism for this Ca²⁺-dependent, internalisation of CFTR is unknown. To gain a better understanding of the effect of Ca²⁺ on CFTR, we performed whole cell current recordings to study the temporal effect of raising cytosolic Ca²⁺ on CFTR function. We show that an increase in cytosolic Ca²⁺ induced a time-dependent reduction in whole cell CFTR conductance, which was paralleled by a loss of cell surface CFTR expression, as measured by confocal and widefield fluorescence microscopy. The decrease in CFTR conductance and cell surface expression were both dynamin-dependent. Single channel reconstitution studies showed that raising cytosolic Ca²⁺ per se had no direct effect on CFTR. In fact, the loss of CFTR plasma membrane activity correlated with activation of calcineurin, a Ca²⁺-dependent phosphatase, suggesting that dephosphorylation of CFTR was linked to the loss of surface expression. In support of this, the calcineurin inhibitor, cyclosporin A, prevented the Ca²⁺-induced decrease in cell surface CFTR. These results provide a hitherto unrecognised role for cytosolic Ca²⁺ in modulating the residency of CFTR at the plasma membrane through a dynamin- and calcineurin-dependent mechanism.

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