Activation of the TRPV4 Ion Channel Is Enhanced by Phosphorylation

The TRPV4 (transient receptor potential vanilloid 4) ion channel, a member of the vanilloid subfamily of the transient receptor potential channels, is activated by membrane stretch, by non-noxious warm temperatures, and by a range of chemical activators. In the present study we examined the role of phosphorylation in modulating the activation of TRPV4. We expressed TRPV4 in HEK293 cells and activated the channel by cell swelling in a hypotonic solution. TRPV4 channel activation and serine phosphorylation were enhanced by exposure to the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate or by application of bradykinin, which activates PKC via a G-protein-coupled mechanism. The enhancement was inhibited by the PKC inhibitors staurosporine, bisindolylmaleimide I, and rottlerin or by mutation of the serine/threonine residues Ser(162), Thr(175), and Ser(189). The adenylate cyclase activator forskolin also enhanced activation of TRPV4, and the enhancement was antagonized by the selective cyclic AMP-dependent protein kinase (PKA) inhibitor H89 or by mutation of serine residue Ser(824). Sensitization of TRPV4 by both PKC and PKA depended on the scaffolding protein AKAP79, because channel activation and phosphorylation were enhanced by co-transfection of AKAP79 and were antagonized by removal of AKAP79 using small interfering RNA. We conclude that the serine/threonine kinases PKC and PKA enhance activation of the TRPV4 ion channel by phosphorylation at specific sites and that phosphorylation depends on assembly of PKC and PKA by AKAP79 into a signaling complex with TRPV4.