Enhanced expression of the full‐length seven transmembrane mu opioid receptor by MOR‐1G, a truncated six transmembrane splice variants through heterodimerization

Abstract

The mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, generating an array of splice variants. One type of the splice variants are truncated variants that have six transmembrane (TM) domains lacking the first TM. Of five 6TM variants in mouse, MOR‐1G was quite abundant and conserved from rodent to humans. The 6TM variants are essential for analgesic actions of a novel type of analgesic drugs such as 3‐iodobenzoyl‐6β‐naltrexamide (IBNtxA) that is potent analgesic against a spectrum of pain models without many traditional opiate side effects. The 6TM variants also involve analgesia of delta and kappa opioids and α2‐adrenergic drugs. In the present study, we demonstrate a new function of MOR‐1G in enhancing expression of the full‐length 7TM mu opioid receptor, MOR‐1, using a Tet‐Off inducible system in Chinese Hamster Ovary cells. When co‐expressed, MOR‐1G had no effect on expression of MOR‐1 mRNA, but greatly increased MOR‐1 expression at protein level in a dose‐dependent manner determined by opioid receptor binding and [35S]GTPγS binding. Interestingly, co‐expression of MOR‐1 with MOR‐1G significantly altered mu binding profile for several mu agonists, including M6G, β‐endorphin and dynorphin A, determined by competition assay. We further show that MOR‐1G can physically interact with MOR‐1 through heterodimerization using co‐immunoprecipitation approach, providing a molecular mechanism of how MOR‐1G regulates expression and function of the full‐length 7TM mu opioid receptor.Support or Funding InformationThis work was supported by the National Institute on Drug Abuse (DA042888, DA046714 and DA007242), the Mayday Foundation, the Peter F. McManus Charitable Trust and a core grant from the National Cancer Institute to Memorial Sloan‐Kettering Cancer Center (CA08748).