Abstract
A fundamental question in G protein coupled receptor biology is how a single ligand acting at a specific receptor is able to induce a range of signaling that results in a variety of physiological responses. We focused on Type 1 cannabinoid receptor (CB&sub1;R) as a model GPCR involved in a variety of processes spanning from analgesia and euphoria to neuronal development, survival and differentiation. We examined receptor dimerization as a possible mechanism underlying expanded signaling responses by a single ligand and focused on interactions between CB&sub1;R and delta opioid receptor (DOR). Using co-immunoprecipitation assays as well as analysis of changes in receptor subcellular localization upon co-expression, we show that CB&sub1;R and DOR form receptor heteromers. We find that heteromerization affects receptor signaling since the potency of the CB&sub1;R ligand to stimulate G-protein activity is increased in the absence of DOR, suggesting that the decrease in CB&sub1;R activity in the presence of DOR could, at least in part, be due to heteromerization. We also find that the decrease in activity is associated with enhanced PLC-dependent recruitment of arrestin3 to the CB&sub1;R-DOR complex, suggesting that interaction with DOR enhances arrestin-mediated CB&sub1;R desensitization. Additionally, presence of DOR facilitates signaling via a new CB&sub1;R-mediated anti-apoptotic pathway leading to enhanced neuronal survival. Taken together, these results support a role for CB&sub1;R-DOR heteromerization in diversification of endocannabinoid signaling and highlight the importance of heteromer-directed signal trafficking in enhancing the repertoire of GPCR signaling.