Ligand-induced activation of ERK1/2 signaling by constitutively active G-coupled 5-HT receptors.

5-HTR, 5-HTR, and 5-HTR are three constitutively active G-coupled 5-HT receptors that have key roles in brain development, learning, memory, cognition, and other physiological processes in the central nervous system. In addition to G signaling cascade mediated by these three 5-HT receptors, the ERK1/2 signaling which is dependent on cyclic adenosine monophosphate (cAMP) production and protein kinase A (PKA) activation downstream of G signaling has also been widely studied. In this study, we investigated these two signaling pathways originating from the three G-coupled 5-HT receptors in AD293 cells. We found that the phosphorylation and activation of ERK1/2 are ligand-induced, in contrast to the constitutively active G signaling. This indicates that G signaling alone is not sufficient for ERK1/2 activation in these three 5-HT receptors. In addition to G, we found that β-arrestin and Fyn are essential for the activation of ERK1/2. Together, these results put forth a novel mechanism for ERK1/2 activation involving the cooperative action of G, β-arrestin, and Fyn.