G protein-coupled receptors (GPCRs) are classically characterized as cell-surface receptors transmitting extracellular signals into cells. Here we show that central components of a GPCR signaling system comprised of the melatonin type 1 receptor (MT), its associated G protein, and β-arrestins are on and within neuronal mitochondria. We discovered that the ligand melatonin is exclusively synthesized in the mitochondrial matrix and released by the organelle activating the mitochondrial MT signal-transduction pathway inhibiting stress-mediated cytochrome release and caspase activation. These findings coupled with our observation that mitochondrial MT overexpression reduces ischemic brain injury in mice delineate a mitochondrial GPCR mechanism contributing to the neuroprotective action of melatonin. We propose a new term, "automitocrine," analogous to "autocrine" when a similar phenomenon occurs at the cellular level, to describe this unexpected intracellular organelle ligand-receptor pathway that opens a new research avenue investigating mitochondrial GPCR biology.

The parathyroid hormone receptor-1 (PTH1R) is a class B G protein–coupled receptor central to calcium homeostasis and a therapeutic target for osteoporosis and hypoparathyroidism. Here we report the cryo–electron microscopy structure of human PTH1R bound to a long-acting PTH analog and the stimulatory G protein. The bound peptide adopts an extended helix with its amino terminus inserted deeply into the receptor transmembrane domain (TMD), which leads to partial unwinding of the carboxyl terminus of transmembrane helix 6 and induces a sharp kink at the middle of this helix to allow the receptor to couple with G protein. In contrast to a single TMD structure state, the extracellular domain adopts multiple conformations. These results provide insights into the structural basis and dynamics of PTH binding and receptor activation.

It has been widely assumed that the production of the ubiquitous second messenger cyclic AMP, which is mediated by cell surface G protein–coupled receptors (GPCRs), and its termination take place exclusively at the plasma membrane. Recent studies reveal that diverse GPCRs do not always follow this conventional paradigm. In the new model, GPCRs mediate G-protein signaling not only from the plasma membrane but also from endosomal membranes. This model proposes that following ligand binding and activation, cell surface GPCRs internalize and redistribute into early endosomes, where trimeric G protein signaling can be maintained for an extended period of time. This Perspective discusses the molecular and cellular mechanistic subtleties as well as the physiological consequences of this unexpected process, which is considerably changing how we think about GPCR signaling and regulation and how we study drugs that target this receptor family.

Background: Following ligand-induced internalization, GPCRs are sorted by diverse receptor motifs and protein interactions.Results: Distinct GPCRs are targeted to a pre-early endosome compartment for their sorting and MAPK signaling.Conclusion: GPCR sorting motifs and their interacting proteins provide specificity in endosomal targeting and receptor signaling.Significance: We describe a system to reprogram GPCR signaling at an unprecedented spatial level.

SummaryEndocytic trafficking is a critical mechanism for cells to decode complex signaling pathways, including those activated by G-protein-coupled receptors (GPCRs). Heterogeneity in the endosomal network enables GPCR activity to be spatially restricted between early endosomes (EEs) and the recently discovered endosomal compartment, the very early endosome (VEE). However, the molecular machinery driving GPCR activity from the VEE is unknown. Using luteinizing hormone receptor (LHR) as a prototype GPCR for this compartment, along with additional VEE-localized GPCRs, we identify a role for the adaptor protein APPL1 in rapid recycling and endosomal cAMP signaling without impacting the EE-localized β2-adrenergic receptor. LHR recycling is driven by receptor-mediated Gαs/cAMP signaling from the VEE and PKA-dependent phosphorylation of APPL1 at serine 410. Receptor/Gαs endosomal signaling is localized to microdomains of heterogeneous VEE populations and regulated by APPL1 phosphorylation. Our study uncovers a highly integrated inter-endosomal communication system enabling cells to tightly regulate spatially encoded signaling.

G-protein-coupled receptors (GPCRs) are a superfamily of cell surface signaling proteins that act as central molecular activators and integrators in all endocrine systems. Membrane trafficking of GPCRs is a fundamental process in shaping extensive signaling networks activated by these receptors. Mounting evidence has identified an increasingly complex network of pathways and protein interactions that a GPCR can traverse and associate with, indicating a multi-level system of regulation. This review will discuss the recent developments in how GPCRs are trafficked to the cell surface as newly synthesized receptors, their recruitment to the clathrin-mediated pathway for endocytosis, and their sorting to subsequent divergent post-endocytic fates, focusing primarily on hormone-activated GPCRs. Current models depicting the classic roles membrane trafficking plays in GPCR signaling have evolved to a highly regulated and complex system than previously appreciated. These developments impart key mechanistic information on how spatial and temporal aspects of GPCR signaling may be integrated and could provide pathway-specific targets to be exploited for therapeutic intervention.

X-linked congenital nephrogenic diabetes insipidus (cNDI) results from inactivating mutations of the human arginine vasopressin (AVP) V2 receptor (hV 2 R). Most of these mutations lead to intracellular retention of the hV 2 R, preventing its interaction with AVP and thereby limiting water reabsorption and concentration of urine. Because the majority of cNDI-hV 2 Rs exhibit protein misfolding, molecular chaperones hold promise as therapeutic agents; therefore, we sought to identify pharmacochaperones for hV 2 R that also acted as agonists. Here, we describe high-affinity nonpeptide compounds that promoted maturation and membrane rescue of L44P, A294P, and R337X cNDI mutants and restored a functional AVP-dependent cAMP signal. Contrary to pharmacochaperone antagonists, these compounds directly activated a cAMP signal upon binding to several cNDI mutants. In addition, these molecules displayed original functionally selective properties (biased agonism) toward the hV 2 R, being unable to recruit arrestin, trigger receptor internalization, or stimulate mitogen-activated protein kinases. These characteristics make these hV 2 R agonist pharmacochaperones promising therapeutic candidates for cNDI. The antidiuretic hormone arginine-vasopressin (AVP) is crucial for osmoregulation, cardiovascular control, and water homeostasis. The human AVP V 2 receptor (hV 2 R), localized in the principal cells of the kidney collecting duct, mediates AVP antidiuretic effect and therefore helps in maintaining physiologic plasma osmolality, blood volume, and arterial pressure. Binding of AVP to hV 2 R first triggers a cAMP signal through activation of the G protein ␣ s (Gs) subunit and adenylyl cyclase (AC). Then, the cAMP-activated protein kinase A phosphorylates aquaporin 2 water channels, resulting in their insertion into the luminal membrane of principal cells and finally to water reabsorption. 1 AVP binding to hV 2 R also induces arrestin recruitment, receptor internalization, 2 and mitogen-activated protein kinase (MAPK) activation. 3 Mutations in the hV 2 R gene lead to the X-linked congenital nephrogenic diabetes insipidus (cNDI), a rare disease characterized by the kidney's inability to concentrate urine despite normal or elevated plasma concentrations of AVP. 4 More than 200 different mutations have been described and are responsible for polyuria, a main consequence of the disease. Most of the mutant receptors (cNDIhV 2 Rs), trapped in the endoplasmic reticulum

The PTH receptor is one of the first GPCR found to sustain cAMP signaling after internalization of the ligand–receptor complex in endosomes. This unexpected model is adding a new dimension on how we think about GPCR signaling, but its mechanism is incompletely understood. We report here that endosomal acidification mediated by the PKA action on the v-ATPase provides a negative feedback mechanism by which endosomal receptor signaling is turned-off.