Mechanisms of selective G protein-coupled receptor localization and trafficking.

The trafficking of G protein-coupled receptors (GPCRs) to different membrane compartments has recently emerged as being a critical determinant of the signaling profiles of activation. GPCRs, which share many structural and functional similarities, also share many mechanisms that traffic them between compartments. This sharing raises the question of how the trafficking of individual GPCRs is selectively regulated. Here, we will discuss recent studies addressing the mechanisms that contribute to selectivity in endocytic and biosynthetic trafficking of GPCRs.

Compartment-specific opioid receptor signaling is selectively modulated by different dynorphin peptides.

Many signal transduction systems have an apparent redundancy built into them, where multiple physiological agonists activate the same receptors. Whether this is true redundancy, or whether this provides an as-yet unrecognized specificity in downstream signaling, is not well understood. We address this question using the kappa opioid receptor (KOR), a physiologically relevant G protein-coupled receptor (GPCR) that is activated by multiple members of the Dynorphin family of opioid peptides. We show that two related peptides, Dynorphin A and Dynorphin B, bind and activate KOR to similar extents in mammalian neuroendocrine cells and rat striatal neurons, but localize KOR to distinct intracellular compartments and drive different post-endocytic fates of the receptor. Strikingly, localization of KOR to the degradative pathway by Dynorphin A induces sustained KOR signaling from these compartments. Our results suggest that seemingly redundant endogenous peptides can fine-tune signaling by regulating the spatiotemporal profile of KOR signaling.

Homologous Regulation of Mu Opioid Receptor Recycling by G ßγ , Protein Kinase C, and Receptor Phosphorylation.

Membrane trafficking and receptor signaling are two fundamental cellular processes that interact constantly. Although how trafficking regulates signaling is well studied, how signaling pathways regulate trafficking is less well understood. Here, we use the mu opioid receptor (MOR), the primary target for opioid analgesics, to define a signaling pathway that dynamically regulates postendocytic receptor recycling. By directly visualizing individual MOR recycling events, we show that agonist increases MOR recycling. Inhibition of G ßγ, phospholipase C, or protein kinase C mimicked agonist removal, whereas activation of G ßγ increased recycling even after agonist removal. Phosphorylation of serine 363 on the C-terminal tail of MOR was required and sufficient for agonist-mediated regulation of MOR recycling. Our results identify a feedback loop that regulates MOR recycling via G ßγ , protein kinase C, and receptor phosphorylation. This could serve as a general model for how signaling regulates postendocytic trafficking of G protein-coupled receptors. SIGNIFICANCE STATEMENT: G protein-coupled receptor (GPCR) localization in the endosome is being increasingly recognized as an important and distinct component of GPCR signaling and physiology. This study identifies a G protein-dependent and protein kinase C-dependent signaling pathway that dynamically regulates the endosomal localization of the mu opioid receptor, the primary target of opioid analgesics and abused drugs. This pathway could provide a mechanism to manipulate spatial encoding of opioid signaling and physiology.