| Research Abstract |
Many G protein-coupled receptors (GPCRs) undergo functional and distributional changes on 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+>-mobilizing receptors coupled to the Gq family of G proteins, such as histamine H_1, receptors (H_1,Rs), has been much less studied, in particular, with respect to feedback modulation of the desensitization/internalization process via the Ca^<2+> signaling. We have found in human U373 MG astrocytoma cells that agonist-induced, clathrin-mediated internalization of H_1Rs is transiently inhibited by Ca^<2+>/calmodulin (CaM), where neither Ca^<2+>/CaM-dependent enzymes, such as CaM kinase II and calcineurin (PP2B), nor protein kinase C (PKCs) is involved. Since Ca^<2+>/CaM is known to inhibit receptor Phosphorylation by GRKs, this might be responsible for the Ca^<2+>/CaM-mediated inhibition of receptor internalization. As a result, H_1Rs remain on the cell surface membrane during the early stage of agonist stimulation. In contrast to the receptor internalization, the affinity of histamine for the H,Rs was dually regulated by desensitizing CaM kinase II and resensitizing PP2B before initiation of the receptor internalization. The subsequent decrease in the intracellular Ca^<2+> concentration even in the presence of agonist may allow H_1Rs to be internalized, and the internalized receptors showed a reduced affinity for histamine. Thus, it is suggested that Ca^<2+>/CaM play a crucial role in determining both function and distribution of Gq protein-coupled receptors by regulating activity of CaM kinase II, PP2B and GRKs.
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