In this study, we characterized the effects of activation of cyclic AMP-dependent protein kinase (PKA) on the internalization and functional coupling of the metabotropic glutamate receptor (mGluR1) splice variants mGluR1a and mGluR1b. Using an enzyme-linked immunosorbent assay technique to assess receptor internalization, we found that the glutamate-induced internalization of mGluR1a or mGluR1b transiently expressed in human embryonic kidney (HEK) 293 cells was inhibited by coactivation of endogenous β2-adrenoceptors with isoprenaline or by direct activation of adenylyl cyclase with forskolin. The PKA inhibitor N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H89) blocked the effects of both isoprenaline and forskolin. The heterologous internalization of the mGluR1 splice variants triggered by carbachol was also inhibited by isoprenaline and forskolin in a PKA-sensitive fashion, whereas the constitutive (agonist-independent) internalization of mGluR1a was inhibited only modestly by PKA activation. Using inositol phosphate (IP) accumulation in cells prelabeled with [3H]inositol to assess receptor coupling, PKA activation increased basal IP accumulation in mGluR1a receptor-expressing cells and also increased glutamate-stimulated IP accumulation in both mGluR1a- and mGluR1b-expressing cells, but only at short times of glutamate addition. Furthermore, PKA activation completely blocked the carbachol-induced heterologous desensitization of glutamate-stimulated IP accumulation in both mGluR1a- and mGluR1b-expressing cells. In coimmunoprecipitation experiments, the ability of glutamate to increase association of GRK2 and arrestin-2 with mGluR1a and mGluR1b was inhibited by PKA activation with forskolin. Together, these results indicate that PKA activation inhibits the agonist-induced internalization and desensitization of mGluR1a and mGluR1b, probably by reducing their interaction with GRK2 and nonvisual arrestins.
Mundell, S. J., Pula, G., More, J. C. A., Jane, D. E., Roberts, P. J., & Kelly, E. (2004). Activation of Cyclic AMP-Dependent Protein Kinase Inhibits the Desensitization and Internalization of Metabotropic Glutamate Receptors 1a and 1b. Molecular Pharmacology, 65(6), 1507-1516. https://doi.org/10.1124/mol.65.6.1507