Mammalian homologues of Drosophila transient receptor potential (TRP) proteins are responsible for receptor-activated Ca2+ influx in vertebrate cells. We previously reported the involvement of intracellular Ca2+ in the receptor-mediated activation of mammalian canonical transient receptor potential 5 (TRPC5) channels. Here we investigated the role of calmodulin, an important sensor of changes in intracellular Ca2+, and its downstream cascades in the activation of recombinant TRPC5 channels in human embryonic kidney (HEK) 293 cells. Ca2+ entry through TRPC5 channels, induced upon stimulation of the G-protein-coupled ATP receptor, was abolished by treatment with W-13, an inhibitor of calmodulin. ML-9 and wortmannin, inhibitors of Ca2+-calmodulin-dependent myosin light chain kinase (MLCK), and the expression of a dominant-negative mutant of MLCK inhibited the TRPC5 channel activity, revealing an essential role of MLCK in maintaining TRPC5 channel activity. It is important to note that ML-9 impaired the plasma membrane localization of TRPC5 channels. Furthermore, TRPC5 channel activity measured using the whole-cell patch-clamp technique was inhibited by ML-9, whereas TRPC5 channel activity observed in the cell-excised, inside-out patch was unaffected by ML-9. An antibody that recognizes phosphorylated myosin light chain (MLC) revealed that the basal level of phosphorylated MLC under unstimulated conditions was reduced by ML-9 in HEK293 cells. These findings strongly suggest that intracellular Ca2+-calmodulin constitutively activates MLCK, thereby maintaining TRPC5 activity through the promotion of plasma membrane TRPC5 channel distribution under the control of phosphorylation/dephosphorylation equilibrium of MLC.
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