Differential coupling of β3A- and β3B-adrenergic receptors to endogenous and chimeric Gαs and Gαi

Abstract

Chimeric G proteins made by replacing the COOH-terminal heptapeptide of Gαq with the COOH-terminal heptapeptide of Gαs or Gαi were used to assess the relative coupling of β3-adrenergic receptor (β3-AR) splice variants (β3Aand β3B) to Gαs and Gαi. The Gαq/s and Gαq/i chimeras transformed the response to receptor activation from regulation of adenylyl cyclase to mobilization of intracellular calcium (Ca2+i). Complementary high-throughput and single-cell approaches were used to evaluate agonist-induced coupling of the receptor to the G protein chimeras. In cells stably transformed with rat β3-AR, transfected with the G protein chimeras, and evaluated using a scanning fluorometer, β3-AR-induced coupling to Gαq/s produced a rapid eightfold increase in Ca2+ifollowed by a slow decay to levels 25% above baseline. Gαq/i also linked rat β3-AR to mobilization of Ca2+iin a similar time- and agonist-dependent manner, but the net 2.5-fold increase in Ca2+iwas only 30% of the response obtained with Gαq/s. Activation of the rat β3-AR also increased GTP binding to endogenous Gαi threefold in membranes from CHO cells stably transformed with the receptor. A complementary single-cell imaging approach was used to assess the relative coupling of mouse β3A- and β3B-AR to Gαi under conditions established to produce equivalent agonist-dependent coupling of the receptor splice variants to Gαq/s and to increases in intracellular cAMP through endogenous Gαs. The β3A- and β3B-AR coupled equivalently to Gαq/i, but the temporal patterns of Ca2+imobilization indicated that coupling was significantly less efficient than coupling to Gαq/s. Collectively, these findings indicate less efficient but equivalent coupling of β3A- and β3B-AR to Gαi vs. Gαs and suggest that differential expression of the splice variants would not produce local differences in signaling networks linked to β3-AR activation.