GRIN1 Regulates micro-opioid Receptor Activities by Tethering the Receptor and G Protein in the Lipid Raft.

The lipid raft location of mu-opioid receptor (MOR) determines the receptor activities. However, the manner in which MOR is anchored within the lipid rafts is undetermined. Using the targeted proteomic approach and mass spectrometry analyses, we have identified GRIN1 (G protein-regulated inducer of neurite outgrowth 1) can tether MOR with the G protein alpha-subunit and subsequently regulate the receptor distribution within the lipid rafts. Glutathione S-transferase fusion pulldown and receptor mutational analyses indicate that GRIN1-MOR interaction involves a receptor sequence (267)GSKEK(271) within the MOR third intracellular loop that is not involved in Galpha interaction. The GRIN1 domains involved in MOR interaction are also distinct from those involved in Galpha interaction. Pertussis toxin pretreatment reduced the amount of GRIN1 co-immunoprecipitated with MOR but not the amount with Galpha. Furthermore, overexpression of GRIN1 significantly enhanced the amount of MOR in lipid raft and the receptor signaling magnitude as measured by Src kinase activation. Such increase in MOR signaling was demonstrated further by determining the GRIN1-dependent pertussis toxin-sensitive neurite outgrowth. In contrast to minimal neurite outgrowth induced by etorphine in control neuroblastoma N2A cells, overexpression of GRIN1 resulted in the increase in etorphine- and non-morphine-induced neurite outgrowth in these cells. Knocking down endogenous GRIN1 by small interfering RNA attenuated the agonist-induced neurite outgrowth. Disrupting lipid raft by methyl-beta-cyclodextrin also blocked neurite outgrowth. Hence, by tethering Galpha with MOR, GRIN1 stabilizes the receptor within the lipid rafts and potentiates the receptor signaling in the neurite outgrowth processes.