Mechanism of Regulation of Casein Kinase I Activity by Group I Metabotropic Glutamate Receptors*

Previously, we reported that (S)-3,5-dihydroxypenylglycine (DHPG), an agonist for group I metabotropic glutamate receptors (mGluRs), stimulates CK1 and Cdk5 kinase activities in neostriatal neurons, leading to enhanced phosphorylation, respectively, of Ser-137 and Thr-75 of DARPP-32 (dopamine and cAMP-regulatedphosphoprotein, 32 kDa). We have now investigated the signaling pathway that leads from mGluRs to casein kinase 1 (CK1) activation. In mouse neostriatal slices, the effect of DHPG on phosphorylation of Ser-137 or Thr-75 of DARPP-32 was blocked by the phospholipase Cβ inhibitor U73122, the Ca2+chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA/AM), and the calcineurin inhibitor cyclosporin A. In neuroblastoma N2a cells, the effect of DHPG on the activity of transfected HA-tagged CK1ε was blocked by BAPTA/AM and cyclosporin A. In neostriatal slices, the effect of DHPG on Cdk5 activity was also abolished by BAPTA/AM and cyclosporin A, presumably through blocking activation of CK1. Metabolic labeling studies and phosphopeptide mapping revealed that a set of C-terminal sites in HA-CK1ε were transiently dephosphorylated in N2a cells upon treatment with DHPG, and this was blocked by cyclosporin A. A mutant CK1ε with a nonphosphorylatable C-terminal domain was not activated by DHPG. Together, these studies suggest that DHPG activates CK1ε via Ca2+-dependent stimulation of calcineurin and subsequent dephosphorylation of inhibitory C-terminal autophosphorylation sites.