Unlocking the DOR to the Surface: Regulated Surface Trafficking of the δ-Opioid Receptor

Pain is a major global health problem affecting 1.5 billion people. More Americans suffer from chronic pain than from diabetes, heart disease, and cancer combined (Institute on Medicine Report, 2011). Opioid analgesics that target the μ-Opioid Receptor (MOR), such as morphine or oxycodone, are the primary treatment options for chronic pain. These MOR agonists are initially effective, but have extreme adverse effects, lose their efficacy over time, and lead to dependence and addiction. Therefore, identifying alternative targets for pain remediation is of extreme significance. The δ-Opioid Receptor (DOR) is a highly attractive alternate target. DOR activates similar cellular pathways as MOR, and can mediate attenuation of pain under specific experimental conditions. However, in the clinical context, DOR agonists have been minimally effective as an analgesic target, and the precise reason for this is not clear. I hypothesize that DOR agonists have limited analgesic efficacy because of the decreased delivery of DOR to the plasma membrane in neurons. In neurons, newly synthesized DOR is retained in intracellular compartments, and may require specific signals for release from intracellular stores and insertion into the cell membrane. Recent data have localized DOR within subsets of neurons that mediate modalities of pain similarly to MOR, increasing the potential impact of targeting DOR agonists. However, the clinical inefficacy of DOR agonists, and the significance of DOR’s regulated surface trafficking, are topics of debate. This thesis will investigate the mechanism controlling the regulated surface trafficking of DOR in neuronal cells in order to determine the mechanism of DOR retention, and subsequently reverse the retention to induce DOR trafficking to the surface. Chapter 2 studies the mechanism controlling surface trafficking of DOR, and shows that a phosphoinositide-regulated Golgi checkpoint regulates the bioavailability of δ-opioid receptors. Chapter 3 elaborates on the phosphoinositide-regulated Golgi checkpoint controlling DOR trafficking, and demonstrates that optogenetic recruitment of PI3K C2α to the trans-Golgi network is sufficient to promote surface trafficking of the δ-opioid receptor. Chapter 4 explores the amino acid sequence required for Golgi retention of DOR, and reveals that NGF-induced golgi retention of δ-Opioid receptors requires dual RXR retention/retrieval motifs. Chapter 5 investigates the role of the Rho-associated GTPase TC10 in DOR surface trafficking, and shows that TC10 GTP is sufficient to drive constitutive surface trafficking of the δ-Opioid receptor. Together, the data presented in this thesis reveal a novel mechanism for the regulated delivery of δ-opioid receptors to the cell surface, and demonstrate a proof of principle for induced trafficking of DOR to provide analgesic benefits under circumstances of chronic pain.