β2-Adrenergic Receptor Signaling and Desensitization Elucidated by Quantitative Modeling of Real Time cAMP Dynamics

TLDR

GPCR signaling is rapidly attenuated by multiple feedback mechanisms that cause desensitization, yet the individual contributions of these mechanisms remain poorly understood. The study employs an improved fluorescent cAMP biosensor to monitor endogenous β2‑adrenergic receptor signaling dynamics in living cells. We examined the roles of GRK6, β‑arrestin, and cAMP‑dependent kinase in β2‑AR signal kinetics using pharmacological inhibitors, siRNAs, and mouse embryonic fibroblasts, and developed a quantitative model fit to the data. β2‑AR activation produces a brief cAMP pulse (~10 µM) lasting less than five minutes, limited by GRK6/β‑arrestin‑mediated receptor inactivation and cAMP‑dependent kinase‑induced phosphodiesterase activity, with the model showing GRK6/β‑arrestin inactivation dominates with a half‑time of 70 s.

Abstract

G protein-coupled receptor signaling is dynamically regulated by multiple feedback mechanisms, which rapidly attenuate signals elicited by ligand stimulation, causing desensitization. The individual contributions of these mechanisms, however, are poorly understood. Here, we use an improved fluorescent biosensor for cAMP to measure second messenger dynamics stimulated by endogenous beta(2)-adrenergic receptor (beta(2)AR) in living cells. beta(2)AR stimulation with isoproterenol results in a transient pulse of cAMP, reaching a maximal concentration of approximately 10 microm and persisting for less than 5 min. We investigated the contributions of cAMP-dependent kinase, G protein-coupled receptor kinases, and beta-arrestin to the regulation of beta(2)AR signal kinetics by using small molecule inhibitors, small interfering RNAs, and mouse embryonic fibroblasts. We found that the cAMP response is restricted in duration by two distinct mechanisms in HEK-293 cells: G protein-coupled receptor kinase (GRK6)-mediated receptor phosphorylation leading to beta-arrestin mediated receptor inactivation and cAMP-dependent kinase-mediated induction of cAMP metabolism by phosphodiesterases. A mathematical model of beta(2)AR signal kinetics, fit to these data, revealed that direct receptor inactivation by cAMP-dependent kinase is insignificant but that GRK6/beta-arrestin-mediated inactivation is rapid and profound, occurring with a half-time of 70 s. This quantitative system analysis represents an important advance toward quantifying mechanisms contributing to the physiological regulation of receptor signaling.