A conservative, single-amino acid substitution in the second cytoplasmic domain of the human serotonin2C receptor alters both ligand-dependent and -independent receptor signaling

The post-transcriptional process of mRNA editing changes up to three amino acids in the second intracellular domain (i2) of the serotonin(2C) (5-HT2C) receptor and alters some signaling characteristics of the receptor. Here, we report that the substitution of valine for isoleucine (I156V; 5-HT2C-VNI), which occurs naturally as a result of mRNA editing, alters both ligand-dependent and -independent signaling. Agonist functional selectivity at the 5-HT2C-VNI receptor differed from the nonedited 5-HT2C-INI receptor. Ligands with selectivity for phospholipase C (PLC) signaling in 5-HT2C-INI cells retained this selectivity in 5-HT2C-VNI-expressing cells. However, ligands with selectivity for phospholipase A2 (PLA2) signaling in 5-HT2C-INI cells lost the capacity for preferential PLA2 activation in 5-HT2C-VNI cells. Maximal PLC responses elicited by 5-HT (full agonist) and lysergic acid diethylamide and 2,5-dimethoxy-4-iodophenylisopropylamine (partial agonists) at edited receptors (5-HT2C-VNI, 5-HT2C-VSV, and 5-HT2C-VGV) were not different from 5-HT2C-INI receptors, suggesting that the capacity of the agonist-occupied receptor to couple to G(q/11) proteins was not different. Ligand-independent (i.e.,constitutive) receptor activity toward PLC for the 5-HT2C-VNI receptor was markedly reduced to a level similar to that for the fully edited 5-HT2C-VSV isoform. However, there was no difference in the thermal stability of the edited receptors, suggesting that mRNA editing does not alter the capacity of receptors to adopt active conformations. These results indicate that a conservative change in one amino acid (I156V) located in i2 of the 5-HT2C receptor produces profound changes in receptor function that differ depending upon whether the receptor is unoccupied or occupied by agonist.