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

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.