Differential Signaling of the Endogenous Agonists at the β2-Adrenergic Receptor

The concept of "functional selectivity" or "biased signaling" suggests that a ligand can have distinct efficacies with regard to different signaling pathways. We have investigated the question of whether biased signaling may be related to distinct agonist-induced conformational changes in receptors using the beta(2)-adrenergic receptor (beta(2)AR) and its two endogenous ligands epinephrine and norepinephrine as a model system. Agonist-induced conformational changes were determined in a fluorescently tagged beta(2)ARFRET sensor. In this beta(2)AR sensor, norepinephrine caused signals that amounted to only approximate to 50% of those induced by epinephrine and the standard "full" agonist isoproterenol. Furthermore, norepinephrine-induced changes in the beta(2)AR FRET sensor were slower than those induced by epinephrine (rate constants, 47 versus 128 ms). A similar partial beta(2)AR activation signal was revealed for the synthetic agonists fenoterol and terbutaline. However, norepinephrine was almost as efficient as epinephrine (and isoproterenol) in causing activation of G(s) and adenylyl cyclase. In contrast, fenoterol was quite efficient in triggering beta-arrestin2 recruitment to the cell surface and its interaction with beta(2)AR, as well as internalization of the receptors, whereas norepinephrine caused partial and slow changes in these assays. We conclude that partial agonism of norepinephrine at the beta(2)AR is related to the induction of a different active conformation and that this conformation is efficient in signaling to Gs and less efficient in signaling to beta-arrestin2. These observations extend the concept of biased signaling to the endogenous agonists of the beta(2)AR and link it to distinct conformational changes in the receptor.