RNA-editing of the 5-HT2C receptor alters agonist-receptor-effector coupling specificity

1 The serotonin(2C) (5-HT2C) receptor couples to both phospholipase C (PLC) -inositol phosphate (IP) and phospholipase A(2) (PLA(2))-arachidonic acid ( AA) signalling cascades. Agonists can differentially activate these effecto rs (i.e. agonist-directed trafficking of receptor stimulus) perhaps due to agonist-specific receptor conformations which differentially couple to/acti vate transducer molecules (e.g. G proteins). Since editing of RNA transcrip ts of the human 5-HT2C receptor leads to substitution of amino acids at pos itions 156, 158 and 160 of the putative second intracellular loop, a region important for G protein coupling, we examined the capacity of agonists to activate both the PLC-IP and PLA(2)-AA pathways in CHO cells stably express ing two major, fully RNA-edited isoforms (5-HT2C-VSV, 5-HT2C-VGV) of the h5 -HT2C receptor. 2 5-HT increased AA release and IP accumulation in both 5-HT2C-VSV and 5-HT 2C-VGV expressing cells. As expected, the potency of 5-HT for both RNA-edit ed isoforms for both responses was 10 fold lower relative to that of the no n-edited receptor (5-HT2C-INI) when receptors were expressed at similar lev els. 3 Consistent with our previous report, the efficacy order of two 5-HT recep tor agonists (TFMPP and bufotenin) was reversed for AA release and IP accum ulation at the non-edited receptor thus demonstrating agonist trafficking o f receptor stimulus. However, with the RNA-edited receptor isoforms there w as no difference in the relative efficacies of TFMPP or bufotenin for AA re lease and IP accumulation suggesting that the capacity for 5-HT2C agonists to traffic receptor stimulus is lost as a result of RNA editing. 4 These results suggest an important role for the second intracellular loop in transmitting agonist-specific information to signalling molecules.