title: Constitutive activity of serotonin 2C receptors at G protein-independent signaling: modulation by RNA editing and antidepressants.
abstract: Serotonin (5-HT)(2C) receptor is a G(q)-coupled receptor exhibiting a high degree of constitutive activity toward phospholipase C effector pathway, a process regulated by receptor mRNA editing. In addition to G protein-dependent signaling, 5-HT(2C) receptors also activate the extracellular signal-regulated kinase (ERK) 1/2 pathway independently of receptor coupling to G proteins. Constitutive activity at ERK signaling has not yet been explored. Transient expression of unedited 5-HT(2C-INI) receptors in human embryonic kidney (HEK) 293 cells resulted in a marked increase in ERK1/2 phosphorylation compared with nontransfected cells. No increase in ERK1/2 phosphorylation was measured in cells expressing fully edited (5-HT(2C-VGV)) receptors. Basal ERK1/2 phosphorylation in 5-HT(2C-INI) receptor-expressing cells was abolished by 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole (SB206,553), a 5-HT(2C) inverse agonist toward phospholipase C. This effect was prevented by the neutral antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-ylcarbamoyl]indoline (SB242,084), which alone did not alter basal activity. Similar observations were made in vivo in mouse choroid plexus, a structure rich in constitutively active 5-HT(2C) receptors. Reminiscent of agonist-induced ERK1/2 phosphorylation, basal activity in HEK 293 cells was unaffected by cellular depletion of Gα(q/11) and Gα(13) proteins but strongly reduced in cells expressing a dominant-negative β-arrestin or depleted of β-arrestin by RNA interference and in cells expressing a dominant-negative calmodulin or a 5-HT(2C-INI) receptor mutant not capable of interacting with calmodulin. The tetracyclic antidepressants mirtazapine and mianserin likewise reduced basal ERK activation. On the other hand, the m-chlorophenylpiperazine derivative trazodone and the selective serotonin reuptake inhibitor fluoxetine were inactive alone but blocked 5-HT-induced ERK1/2 phosphorylation. Together, these data provide the first evidence of constitutive activity of a G protein-coupled receptor toward G-independent, β-arrestin-dependent, receptor signaling.
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5203, Institut de Génomique Fonctionnelle, Université Montpellier 1 and 2, Montpellier, France.
authors.forename: Marilyne
authors.initials: M
authors.lastname: Labasque
authors.forename: Julie
authors.initials: J
authors.lastname: Meffre
authors.forename: Gaelle
authors.initials: G
authors.lastname: Carrat
authors.forename: Carine
authors.initials: C
authors.lastname: Becamel
authors.forename: Joël
authors.initials: J
authors.lastname: Bockaert
authors.forename: Philippe
authors.initials: P
authors.lastname: Marin
mesh_terms: D000818:Animals; D000928:Antidepressive Agents / Q000494:pharmacology*; D019390:Arrestins / Q000378:metabolism; D002147:Calmodulin / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D002831:Choroid Plexus / Q000187:drug effects / Q000378:metabolism; D054314:Drug Inverse Agonism; D004789:Enzyme Activation; D019204:GTP-Binding Proteins / Q000502:physiology*; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism; D009154:Mutation; D010766:Phosphorylation; D055503:Protein Multimerization; D017393:RNA Editing / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D044405:Receptor, Serotonin, 5-HT2C / Q000235:genetics / Q000502:physiology*; D058826:Serotonin 5-HT2 Receptor Agonists / Q000494:pharmacology; D058830:Serotonin 5-HT2 Receptor Antagonists / Q000494:pharmacology; D015398:Signal Transduction; D000071557:beta-Arrestins
chemical_list: D000928:Antidepressive Agents; D019390:Arrestins; D002147:Calmodulin; D044405:Receptor, Serotonin, 5-HT2C; D058826:Serotonin 5-HT2 Receptor Agonists; D058830:Serotonin 5-HT2 Receptor Antagonists; D000071557:beta-Arrestins; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D019204:GTP-Binding Proteins