Endosomal b2-adrenergic receptor controls nuclear cAMP signal in learning and memory via sequestration of phosphodiesterase 4D.

Learning and memory-related behaviors depend on GPCR signaling to the nucleus to induce Immediate early gene (IEG) expression. However, the molecular mechanisms mediating this process remain unclear. We have previously identified a subpopulation of β AR which is preferentially phosphorylated by GRK sites and internalized to the endosome upon stimulation. Here, we demonstrate that GRK phosphorylated endosomal β Adrenergic Receptor (β AR) indirectly facilitates nuclear cAMP signaling through sequestration of a PDE4D/β-arrestin complex. Combining subcellular cAMP biosensors and transgenic mice, we found deleting GRK phosphorylation sites on β AR (GRKD) blocked agonist-induced receptor endocytosis and nuclear cAMP signaling in hippocampal neurons. This resulted in impaired PKA-mediated IEGs expression in vitro,reduced learning induced IEG expression in vivo, and impaired long-term memory in a Morris water maze. Mechanistically, while β AR stimulation promoted β-arrestin-dependent recruitment of PDE4D5, inhibition of β-arrestin-PDE4D5 association alone prevented β AR-induced increases in nuclear cAMP signaling without affecting receptor endocytosis. Furthermore, direct PDE4 inhibition was sufficient to rescue the β AR-induced nuclear cAMP signal in GRKD neurons and ameliorate the long-term memory deficits in GRKD mice. This work indicates that the sequestration of the β-arrestin/PDE4D complex by the GRK-phosphorylated β AR critically controls the receptor-induced nuclear cAMP signals. This constitutes a novel mechanism by which GPCR activation promotes nuclear cAMP signaling through endocytosis dependent relocalization of PDE4D isoforms, IEG expression, and learning and memory-related behaviors.