Effect of Annexin A5 on CFTR: regulated traffic or scaffolding?

Previous studies have implicated annexins in regulating ion channels and in particular annexin A5 (AnxA5) in the traffic of the cystic fibrosis transmembrane conductance regulator (CFTR). In the present study, we further investigated the role of AnxA5 in regulating CFTR function and intracellular trafficking in both Xenopus oocytes and mammalian cells. Although we could confirm the previously reported CFTR/AnnxA5 interaction, we found that in oocytes AnxA5 inhibits CFTR-mediated whole-cell membrane conductance presumably by a mechanism independent of PDZ-binding domain at the C-terminus of CFTR but protein kinase C (PKC)-dependent and results from either endocytosis activation and/or exocytosis block. In contrast, in human cells, co-expression of AnxA5 augmented CFTR whole-cell currents, an effect that was independent of CFTR PDZ-binding domain. We conclude that annexin A5 has multiple effects on CFTR, so that the net effect observed is cell system-dependent. Nevertheless, both effects observed here are consistent with the described role of annexins forming scaffolding platforms at cell membranes, thus contributing to a decrease in their dynamics. Finally, we could not confirm that AnxA5 overexpression rescues traffic/function of the most frequent disease-causing mutant F508del-CFTR, thus concluding that AnxA5 is not a promising tool for correction of the F508del-CFTR defect.