Serotonin 5-HT1A Receptor-mediated Erk Activation Requires Calcium/Calmodulin-dependent Receptor Endocytosis

Many receptors that couple to heterotrimeric guanine nucleotide-binding (G) proteins mediate rapid activation of the mitogen-activated protein kinases, Erk1 and Erk2. The G i -coupled serotonin (5-hydroxytryptamine (5-HT)) 5-HT 1A receptor, heterologously expressed in Chinese hamster ovary or human embryonic kidney 293 cells, mediated rapid activation of Erk1/2 via a mechanism dependent upon both Ras activation and clathrin-mediated endocytosis. This activation was attenuated by chelation of intracellular Ca 2+ and Ca 2+ /calmodulin (CAM) inhibitors or the CAM sequestrant protein calspermin. The CAM-dependent step in the Erk1/2 activation cascade is downstream of Ras activation, because inhibitors of CAM antagonize Erk1/2 activation induced by constitutively activated mutants of Ras and c-Src but not by constitutively activated mutants of Raf and MEK ( m itogen and e xtracellular signal-regulated k inase). Inhibitors of the classical CAM effectors myosin light chain kinase, CAM-dependent protein kinases II and IV, PP2B, and CAM-sensitive phosphodiesterase had no effect upon 5-HT 1A receptor-mediated Erk1/2 activation. Because clathrin-mediated endocytosis was required for 5-HT 1A receptor-mediated Erk1/2 activation, we postulated a role for CAM in receptor endocytosis. Inhibition of receptor endocytosis by use of sequestration-defective mutants of β-arrestin 1 and dynamin attenuated 5-HT 1A receptor-stimulated Erk1/2 activation. Inhibition of CAM prevented agonist-dependent endocytosis of epitope-tagged 5-HT 1A receptors. We conclude that CAM-dependent activation of Erk1/2 through the 5-HT 1A receptor reflects its role in endocytosis of the receptor, which is a required step in the activation of MEK and subsequently Erk1/2.