Biased signaling at mu opioid receptor full‐length C‐terminal splice variants

Abstract

Acting on the samee G-protein coupled receptor, various ligands produce distinct activities by differentially activating downstream signaling pathways, a phenomenon referred to as biased agonism or functional selectivity. The original mu opioid receptor (MOR-1) has largely been studied in biased agonism mainly through G protein and β-arrestin2 signaling pathways. Mu opioid-induced G protein activation is often associated with its analgesic action, while mu opioid-induced β-arrestin recruitment is commonly linked to the side effects related to traditional opiates. However, there exist multiple full-length 7 transmembrane (7TM) C-terminal variants generated by alternative 3′ splicing of the mu opioid receptor gene, OPRM1, which is conserved from rodent to human. These 7TM C-terminal variants have identical receptor structures including the ligand binding pocket except for a variable intracellular C-terminal sequence. Several in vitro studies indicated marked differences among the 7TM C-terminal variants in mu agonist-induced G protein activation, receptor phosphorylation, internalization, desensitization, and post-endocytic sorting. Using a set of gene targeting mouse models in which a specific C-terminus was truncated at translational level, we demonstrated the functional importance of the C-terminal tails in various morphine actions. In the current study, we examined the role of different 7TM C-terminal variants on mu agonist-induced G protein activation and β-arrestin2 recruitment, which were used to determine signaling bias. The results showed that mu agonists induced significant differences in G protein activation and β-arrestin recruitment among various C-terminal variants. Especially, exon 7-associated 7TM variants displayed greater β-arrestin2 bias for most mu agonists compared to exon 4-associated 7TM MOR-1. Our finding provides new insights into mu agonist-induced biased signaling in vivo where these 7TM C-terminal variants are co-expressed.