INVESTIGATION OF DESENSITIZATION/RESENSITIZATION MECHANISMS FOR Gq PROTEIN-COUPLED RECEPTORS

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Many G protein-coupled receptors (GPCRs) undergo functional and distributional changes upon stimulation with agonist, i.e., desensitization/internalization of receptors. Phosphorylation of agonist-occupied GPCRs by G protein-coupled receptor kinases (GRKs) is a key event for induction of receptor desensitization and the subsequent arrestin-mediated internalization. However, desensitization/internalization mechanisms of Ca^<2+> receptors coupled to the Gq family of G proteins, such as histamine H_1 receptors (H_1Rs), has been much less studied, in particular, with respect to feedback modulation of the desensitization/internalization process via the Ca^<2+> We investigated Ca~<2+> (CaM)-mediated regulation of H_1Rs in human U373 MG astrocytoma cells, and found that the onset of agonist-induced, clathrin-mediated internalization of H_1Rs was delayed by activation of Ca^<2+>/CaM, possibly due to inhibition of GRK-mediated internalization process. While the H_1Rs remained on the cell surface membrane owing to the Ca^<2+>/CaM-mediated inhibition of receptor internalization, the agonist affinity for the cell surface H_1Rs was regulated by Ca^<2+>/CaM via actiyation of CaM kinase II (for desensitization) and protein phosphatase 2B (for resenisitization). The subsequent decrease in the intracellular Ca^<2+> concentration even in the presence of agonist may allow H_1Rs to be internalized. In contrast, a receptor-non-mediated and sustained increase in the intracellular Ca^<2+> by a Ca^<2+> ionophore, ionomycin, failed to inhibit histamine-mediated internalization of H_1Rs. Thus, it is suggested that the receptor-mediated activation of Ca^<2+>/CaM plays a crucial role in determining both function and distribution of Gq protein-coupled receptors by regulating activity of CaM kinase II, PP2B and GRKs.