Control of Opioid Receptor Signaling and Pharmacological Tolerance through Endosomal Recycling and Retromer

About This Grant

Project Summary Opioids play a frontline role in acute pain management in the clinical setting. Prolonged use of opioids causes multiple adaptations including tolerance—a phenomenon in which increasing opioid dosage is required to maintain drug efficacy. The pharmacological component of opioid tolerance is largely driven on the cellular level and is the result of cellular protein networks terminating the activity of the mu opioid receptor (µOR). A critical step in this regulatory process is membrane trafficking. High efficacy opioids cause the µOR to rapidly traffic from the cell surface to internal cellular structures called endosomes. At endosomes, the µOR is either sorted back to the cell surface for continued activity (resensitization) or sent to the lysosome for destruction (downregulation). A long-standing model is that downregulation can drive pharmacological opioid tolerance through proteolysis of opioid receptors outpacing new receptor synthesis. One challenge in testing this model is that we don’t know the identity of key cellular protein networks regulating mu opioid receptor trafficking at endosomes. Here we leverage our recently developed proteomic and genomic methods for identifying cellular proteins which regulate opioid receptors. In Aim 1 we focus on the trafficking of the µOR at endosomes and, using biochemical and cellular techniques, test the hypothesis that the endosomal Retromer complex rescues mu opioid receptor from lysosomal degradation in a process called recycling. We will use functional genomics to identify potential druggable targets—kinases and ubiquitin-ligases—that regulate µOR through endosomes, and test the related hypothesis that opioid-induced downregulation of the µOR can be blocked by engineered “super-recycling” variants of the receptor. In Aim 2 we test the hypothesis that endosomal recycling and the endosomal Retromer complex controls three different phases of µOR signaling: endosomal signaling, resensitization, and the development of pharmacological opioid tolerance at the cellular level. Together, these studies seek to define a critical regulatory process controlling µOR function and identify cellular proteins which could be targeted to control the development pharmacological opioid tolerance.