The roles of phospholipase C activation and alternative ADAR1 and ADAR2 pre-mRNA splicing in modulating serotonin 2C-receptor editing in vivo

The serotonin 2C receptor (5-HT2CR), a Gq-protein-coupled neurotransmitter receptor, exists in multiple isoforms that result from RNA editing of five exonic adenosines that are converted to inosines. In the adult brain, editing of 5-HT2C pre-mRNA exhibits remarkable plasticity in response to environmental and neurochemical stimuli. Here, we investigated two potential mechanisms underlying these plastic changes in adult 5-HT2CR editing phenotypes in vivo: activation of phospholipase C (PLC) and alternative splicing of pre-mRNA encoding the editing enzymes ADAR1 and ADAR2. Studies on two inbred strains of mice (C57Bl/6 and Balb/c) revealed that sustained stimulation of PLC—a downstream effector of activated Gαq protein—increased editing of forebrain neocortical 5-HT2C pre-mRNA at two sites known to be targeted by ADAR2. Moreover, changes in relative expression of the alternatively spliced “a” and “b” mRNA isoforms of ADAR1 and ADAR2 also correlate with changes in 5-HT2CR editing. The site-specific changes in 5-HT2CR editing detected in mice with different “a” over “b” ADAR mRNA isoform ratios only partially overlap with those evoked by sustained PLC activation and are best explained by the increased editing efficiency of ADAR1. Thus, activation of PLC and alternative splicing of ADAR pre-mRNA have both overlapping and specific roles in modulating 5-HT2CR editing phenotypes.