Selective interactions of spinophilin with the C-terminal domains of the δ- and μ-opioid receptors and G proteins differentially modulate opioid receptor signaling.

Previous studies have shown that the intracellular domains of opioid receptors serve as platforms for the formation of a multi-component signaling complex consisting of various interacting partners (Leontiadis et al., 2009, Cell Signal. 21, 1218-1228; Georganta et al., 2010, Neuropharmacology, 59(3), 139-148). In the present study we demonstrate that spinophilin a dendritic-spine enriched scaffold protein associates with δ- and μ-opioid receptors (δ-ΟR, μ-OR) constitutively in HEK293 an interaction that is altered upon agonist administration and enhanced upon forskolin treatment for both μ-OR and δ-ΟR. Spinophilin association with the opioid receptors is mediated via the third intracellular loop and a conserved region of the C-terminal tails. The portion of spinophilin responsible for interaction with the δ-OR and μ-OR is narrowed to a region encompassing amino acids 151-444. Spinophilin, RGS4, Gα and Gβγ subunits of G proteins form a multi-protein complex using specific regions of spinophilin and a conserved amino acid stretch of the C-terminal tails of both δ-μ-ORs. Expression of spinophilin in HEK293 cells potentiated DPDPE-mediated adenylyl-cyclase inhibition of δ-OR leaving unaffected the levels of cAMP accumulation mediated by the μ-OR. Moreover, measurements of extracellular signal regulated kinase (ERK1,2) phosphorylation indicated that the presence of spinophilin attenuated agonist-driven ERK1,2 phosphorylation mediated upon activation of the δ-OR but not the μ-OR. Collectively, these findings suggest that spinophilin associates with both δ- and μ-ΟR and G protein subunits in HEK293 cells participating in a multimeric signaling complex that displays a differential regulatory role in opioid receptor signaling.