Abstract

The mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing to generate an array of splice variants. One group of splice variants exclude the first transmembrane (TM) domain and contain six TM domains. These 6TM variants are essential for the action of a novel class of analgesic drugs, including 3-iodobenzoyl-6β-naltrexamide (IBNtxA), which is potent against a spectrum of pain models without exhibiting the adverse side effects of traditional opiates. The 6TM variants are also involved in analgesic action through other drug classes, including delta and kappa opioids, and α2-adrenergic drugs. Of the five 6TM variants in mouse, mMOR-1G is abundant and conserved from rodent to human. In the present study, we demonstrate a new function of mMOR-1G in enhancing expression of the full-length 7TM mu opioid receptor, mMOR-1. When co-expressed with mMOR-1 in a Tet-Off inducible Chinese Hamster Ovary cell line, mMOR-1G has no effect on mMOR-1 mRNA expression, but greatly increases mMOR-1 protein expression in a dose-dependent manner, determined by opioid receptor binding and [35S]GTPγS binding. Subcellular fractionation analysis using OptiPrep density gradient centrifugation shows an increase of functional mMOR-1 receptor in plasma membrane-enriched fractions. Using a co-immunoprecipitation approach, we further demonstrate that mMOR-1G physically associates with mMOR-1 starting at the endoplasmic reticulum, suggesting a chaperone-like function. These data provide a molecular mechanism for how mMOR-1G regulates expression and function of the full-length 7TM mu opioid receptor.