Local modulation of cystic fibrosis conductance regulator: cytoskeleton and compartmentalized cAMP signalling

Abstract

The cystic fibrosis conductance regulator (CFTR) is a cAMP-regulated Cl(-) channel expressed predominantly at the apical membrane of secreting epithelial cells. Mutations in the CFTR gene lead to cystic fibrosis, the most frequent genetic disease in the Caucasian population. The most common mutation, a deletion of phenylalanine at position 508 (F508del), impairs CFTR folding and chloride channel function. Although an intense effort is under way to identify compounds that target the F508del CFTR structural defect and promote its expression and stability at the plasma membrane, so far their clinical efficacy has proven to be poor, highlighting the necessity to better understand the molecular mechanism of CFTR regulation and of the pathogenesis of the disease. Accumulating evidence suggests that the inclusion of the CFTR in macromolecular complexes and its interaction with the cortical cytoskeleton may play a key role in fine-tuning the regulation of channel function. Here we review some recent findings that support a critical role for protein-protein interactions involving CFTR and for the cytoskeleton in promoting local control of channel activity. These findings indicate that compounds that rescue and stabilize CFTR at the apical membrane may not be sufficient to restore its function unless the appropriate intracellular milieu is also reconstituted.

Figure 1

(A) Cells expressing wtCFTR show a well-organized subcortical cytoskeleton that limits cAMP diffusion away from the plasma membrane. As a result, in response to AC activation, the concentration of cAMP is higher in the sub-membrane compartment compared with the bulk cytosol. The local increase in cAMP activates ezrin-bound PKA, resulting in efficient phosphorylation of the CFTR and increased Cl⁻ efflux. (B) Cells expressing F508del CFTR show a disorganized cytoskeleton and, consequently, diffusion of cAMP away from the membrane. Loss of an organized cortical cytoskeleton releases the PKA-anchoring protein ezrin with consequent relocalization of PKA to the cytosol. RD = regulatory domain; R = PKA regulatory subunit.