Abstract
Serotonin (5-HT)2C receptor is a Gq-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-HT2C 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 HT2C-INI receptors in HEK-293 cells resulted in a marked increase in ERK1,2 phosphorylation, compared with non-transfected cells. No increase in ERK1,2 phosphorylation was measured in cells expressing fully edited (5-HT2C-VGV) receptors. Basal ERK1,2 phosphorylation in 5 HT2C-INI receptor-expressing cells was abolished by SB206,553, a 5-HT2C inverse agonist toward phospholipase C. This effect was prevented by the neutral antagonist 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-HT2C receptors. Reminiscent to agonist-induced ERK1,2 phosphorylation, basal activity in HEK-293 cells was unaffected by cellular depletion of Gαq/11 and Gα13 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 HT2C-INI receptor mutant not capable of interacting with calmodulin. The tetracyclic antidepressants mirtazapine and mianserin likewise reduced basal ERK activation. Conversely, the m-chlorophenylpiperazine derivative trazodone and the SSRI fluoxetine were inactive alone but blocked 5-HT-induced ERK1,2 phosphorylation. Collectively, these data provide the first evidence of constitutive activity of a G protein-coupled receptor toward G-independent, β-arrestin-dependent, receptor signaling.
Footnotes
- The American Society for Pharmacology and Experimental Therapeutics