Involvement of G protein -subunits in diverse signaling induced by Gi/o-coupled receptors: study using the Xenopus oocyte expression system

Uezono, Yasuhito, Muneshige Kaibara, Osamu Murasaki, and Kohtaro Taniyama. Involvement of G protein -subunits in diverse signaling induced by Gi/o-coupled receptors: study using the Xenopus oocyte expression system. Am J Physiol Cell Physiol 287: C885– C894, 2004. First published May 19, 2004; 10.1152/ajpcell.00125. 2004.—We studied the functions of -subunits of Gi/o protein using the Xenopus oocyte expression system. Isoproterenol (ISO) elicited cAMP production and slowly activating Cl currents in oocytes expressing 2-adrenoceptor and the protein kinase A-dependent Cl channel encoded by the cystic fibrosis transmembrane conductance regulator (CFTR) gene. 5-Hydroxytryptamine (5-HT), [D-Ala, D-Leu]-enkephalin (DADLE), and baclofen enhanced ISO-induced cAMP levels and CFTR currents in oocytes expressing 2-adrenoceptor-CFTR and 5-HT1A receptor (5-HT1AR), -opioid receptor, or GABAB receptor, respectively. 5-HT also enhanced pituitary adenylate cyclase activating peptide (PACAP) 38-induced cAMP levels and CFTR currents in oocytes expressing PACAP receptor, CFTR and 5-HT1AR. The 5-HT-induced enhancement of Gs-coupled receptormediated currents was abrogated by pretreatment with pertussis toxin (PTX) and coexpression of G transducin (Gt ). The 5-HT-induced enhancement was further augmented by coexpression of the G activated form of adenylate cyclase (AC) type II but not AC type III. Thus -subunits of Gi/o protein contribute to the enhancement of Gs-coupled receptor-mediated responses. 5-HT and DADLE did not elicit any currents in oocytes expressing 5-HT1AR or -opioid receptor alone. They elicited Ca -activated Cl currents in oocytes coexpressing these receptors with the G -activated form of phospholipase C (PLC)2 but not with PLC1. These currents were inhibited by pretreatment with PTX and coexpression of Gt , suggesting that -subunits of Gi/o protein activate PLC2 and then cause intracellular Ca mobilization. Our results indicate that subunits of Gi/o protein participate in diverse intracellular signals, enhancement of Gs-coupled receptor-mediated responses, and intracellular Ca mobilization.