Regulation of G Protein-Coupled Receptor Trafficking by Downstream Signaling Kinases

Our cells rely on several diverse, extracellular signals to sense and interact with our environment. Many of these signals, such as hormones, neurotransmitters, odorants, taste, and light are transduced by the large family of signaling receptors the G protein-coupled receptors (GPCRs). The role of membrane trafficking of GPCRs in regulating cellular sensitivity to signals has been well described. However, many questions remain about the functional consequences of post-endocytic sorting of GPCRs, the molecular mechanisms that govern this process, and how it is regulated in a physiological context. An emerging paradigm in GPCR biology is that GPCRs undergo endosomal signaling, in addition to cell surface signaling, and the role of GPCR endosomal sorting in regulating this process is unknown. This thesis investigates how phosphorylation of GPCRs by downstream signaling kinases regulates GPCR endosomal sorting and activity. Chapter 2 shows that hierarchical sorting of GPCRs by signaling kinases switches receptors between endosomal microdomains to control initiation of endosomal G protein signaling. Chapter 3 suggests that the mu-opioid receptor (MOR), the target of endogenous endorphins and clinical opiates, undergoes agonist-selective hierarchical sorting via PKC phosphorylation of the receptor. Chapter 4 of this thesis shows that PKC-dependent sorting of MOR and opioid sensitivity is regulated by substance P signaling in physiologically relevant sensory neurons. Together, the data in this thesis suggest that hierarchical sorting of GPCRs spatially encodes GPCR signaling and that heterologous signaling pathways can regulate GPCR membrane trafficking via receptor phosphorylation.