Heterotrimeric G‐protein serves as scaffold for Mu‐opioid receptor mediated signal transduction in lipid rafts

Abstract

Mu-opioid receptor (MOR) belongs to the G-protein coupled receptors, which couples specifically to Gi/Go heterotrimeric G proteins. Our recent studies suggest that Gi2 not only is responsible for opioid agonist inhibition of adenylyl cyclase activity, but also performs as a scaffold to recruit the receptor and post-receptor signaling molecules to form a signaling complex for adenylyl cyclase superactivation after chronic agonist treatment. Our current results show that in neuroblastoma Neuro2A-MOR (N2AMOR) cells, MOR and Gi2 are both located in lipid rafts fractions, the plasma membrane microdomains that are rich in sphingolipid and cholesterol. Using co-immunoprecipitation and confocal fluorescent microscopy techniques, MOR and Gi2 are colocalized and interacted with each other in lipid rafts. Disruption of lipid rafts by methyl-beta-cyclodextrin causes the loss of colocalization of mu-receptors and Gi2. More importantly, disruption of lipid rafts also causes the loss of MOR activities, including the inhibition of agonist-induced intracellular cAMP level and mobilization of the intracellular Ca2+. Furthermore these activities can be restored after adding back cholesterol. Taken together, these observations suggest that G-protein scaffolding and lipid raft localization are essential for the receptor to regulate the signal transduction precisely and efficiently. (Research supported by NIDA grants DA0166774 and DA007339)