501
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Bie B, Pan ZZ. Trafficking of central opioid receptors and descending pain inhibition. Mol Pain 2007; 3:37. [PMID: 18053223 PMCID: PMC2219988 DOI: 10.1186/1744-8069-3-37] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2007] [Accepted: 12/04/2007] [Indexed: 12/02/2022] Open
Abstract
The delta-opioid receptor (DOR) belongs to the superfamily of G-protein-coupled receptors (GPCRs) with seven transmembrane domains, and its membrane trafficking is regulated by intracellular sorting processes involving its C-tail motifs, intracellular sorting proteins, and several intracellular signaling pathways. In the quiescent state, DOR is generally located in the intracellular compartments in central neurons. However, chronic stimulation, such as chronic pain and sustained opioid exposure, may induce membrane trafficking of DOR and its translocation to surface membrane. The emerged functional DOR on cell membrane is actively involved in pain modulation and opioid analgesia. This article reviews current understanding of the mechanisms underlying GPCRs and DOR membrane trafficking, and the analgesic function of emerged DOR through membrane trafficking under certain pathophysiological circumstances.
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Affiliation(s)
- Bihua Bie
- Department of Anesthesiology and Pain Medicine, The University of Texas-MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
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502
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Gripentrog JM, Miettinen HM. Formyl peptide receptor-mediated ERK1/2 activation occurs through G(i) and is not dependent on beta-arrestin1/2. Cell Signal 2007; 20:424-31. [PMID: 18060741 DOI: 10.1016/j.cellsig.2007.11.002] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2007] [Revised: 11/01/2007] [Accepted: 11/01/2007] [Indexed: 02/04/2023]
Abstract
Formyl peptide receptor (FPR) and C5a receptor (C5aR) are chemoattractant G protein-coupled receptors (GPCRs) involved in the innate immune response against bacterial infections and tissue injury. Like other GPCRs, they recruit beta-arrestin1/2 to the plasma membrane and activate the extracellular signal-regulated kinases 1 and 2 (ERK1/2). Previous studies with several GPCRs have suggested that beta-arrestins play an important role as signal transducers by scaffolding signaling molecules such as ERK1/2. This function of the beta-arrestins was not discovered until several years after their role in desensitization and endocytosis had been reported. In this study, we investigated the role of the beta-arrestins in the activation of ERK1/2 and receptor endocytosis. We took advantage of previously described mutants of FPR that have defects in G(i) coupling or beta-arrestin recruitment. The results obtained with the mutant FPRs, as well as experiments using an inhibitor of G(i) and cells overexpressing beta-arrestin2, showed that activation of ERK1/2 takes place through G(i) and is not affected by beta-arrestins. However, overexpression of beta-arrestin2 does enhance FPR sequestration from the cell surface, suggesting a role in desensitization, as shown for many other GPCRs. Experiments with CHO C5aR cells showed similar sensitivity to the G(i) inhibitor as CHO FPR cells, suggesting that the predominant activation of ERK1/2 through G protein may be a common characteristic among chemoattractant receptors.
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Affiliation(s)
- Jeannie M Gripentrog
- Department of Microbiology, Montana State University, 109 Lewis Hall, Bozeman, MT 59717-3520, USA
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503
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Liu Q, Dewi DA, Liu W, Bee MS, Schonbrunn A. Distinct phosphorylation sites in the SST2A somatostatin receptor control internalization, desensitization, and arrestin binding. Mol Pharmacol 2007; 73:292-304. [PMID: 17981995 DOI: 10.1124/mol.107.038570] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
The somatostatin subtype 2A (sst2A) receptor, a member of the G protein-coupled receptor superfamily, mediates many of the neuroendocrine and neuromodulatory actions of somatostatin, and it represents the primary target for somatostatin analogs used in cancer therapy and tumor localization. Agonist stimulation leads to the rapid phosphorylation, endocytosis, and desensitization of the sst2A receptor; however, little is known about the role of phosphorylation in sst2A regulation. sst2A phosphorylation occurs on serine and threonine residues in the third intracellular loop and carboxyl terminus. Therefore, we generated mutant receptors in which serine (Ser-), threonine (Thr-), or both (Ser-/Thr-) residues in these regions were mutated to alanine. In contrast to the wild-type receptor, somatostatin treatment did not stimulate the phosphorylation of the Ser-/Thr- mutant, and it did not produce desensitization. Furthermore, internalization of the Ser-/Thr- mutant occurred 5 times more slowly than with the wild-type receptor. Mutating only the Ser residues did not inhibit either internalization or desensitization. In contrast, mutating only the Thr residues inhibited receptor endocytosis to the same extent as in the full mutant, but it did not affect receptor desensitization. In both the wild-type and Ser- receptors, agonist binding produced a stable arrestin-receptor complex that was maintained during receptor trafficking, whereas arrestin was not recruited to either the Thr- or the Ser-/Thr- receptors. These results demonstrate that agonist-stimulated receptor phosphorylation is necessary for both desensitization and rapid internalization of the sst2A receptor. However, sst2A receptor internalization and uncoupling can occur independently, involve different receptor phosphorylation sites, and exhibit different requirements for stable arrestin association.
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Affiliation(s)
- Q Liu
- Department of Integrative Biology and Pharmacology, School of Medicine, University of Texas-Houston, P.O. Box 20708, Houston, TX 77225, USA
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504
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Alexander SPH, Kendall DA. The complications of promiscuity: endocannabinoid action and metabolism. Br J Pharmacol 2007; 152:602-23. [PMID: 17876303 PMCID: PMC2190010 DOI: 10.1038/sj.bjp.0707456] [Citation(s) in RCA: 102] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2007] [Revised: 08/15/2007] [Accepted: 08/16/2007] [Indexed: 01/27/2023] Open
Abstract
In this review, we present our understanding of the action and metabolism of endocannabinoids and related endogenous molecules. It is clear that the interactions between the multiple endocannabinoid-like molecules (ECLs) are highly complex, both at the level of signal transduction and metabolism. Thus, ECLs are a group of ligands active at 7-transmembrane and nuclear receptors, as well as transmitter-gated and ion channels. ECLs and their metabolites can converge on common endpoints (either metabolic or signalling) through contradictory or reinforcing pathways. We highlight the complexity of the endocannabinoid system, based on the promiscuous nature of ECLs and their metabolites, as well as the synthetic modulators of the endocannabinoid system.
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Affiliation(s)
- S P H Alexander
- School of Biomedical Sciences and Institute of Neuroscience, University of Nottingham Medical School, Nottingham NG7 7LP, UK.
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505
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von Degenfeld G, Wehrman TS, Hammer MM, Blau HM. A universal technology for monitoring G-protein-coupled receptor activation in vitro and noninvasively in live animals. FASEB J 2007; 21:3819-26. [PMID: 17942828 DOI: 10.1096/fj.07-9597com] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
G-protein coupled receptors (GPCRs) are a versatile and ubiquitous family of membrane receptors that transmit extracellular signals to mammalian cells and constitute the most important class of drug targets. Yet, sensitive and specific methods are lacking that would allow quantitative comparisons of pharmacologic properties of these receptors in physiological or pathological settings in live animals. We sought to overcome these limitations by employing low affinity, reversible beta-galactosidase complementation to quantify GPCR activation via interaction with beta-arrestin. A panel of cell lines was engineered expressing different GPCRs together with the reporter system. In vitro evaluation revealed highly sensitive, dynamic, and specific assessment of GPCR agonists and antagonists. Following implantation of the cells into mice, it was possible for the first time to monitor pharmacological GPCR activation and inhibition in their physiological context by noninvasive bioluminescence imaging in living animals. This technology has unique advantages that enable novel applications in the functional investigation of GPCR modulation in live animals in biological research and drug discovery.
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Affiliation(s)
- Georges von Degenfeld
- Baxter Laboratory in Genetic Pharmacology, Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California 94305-5175, USA
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506
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Delhaye M, Gravot A, Ayinde D, Niedergang F, Alizon M, Brelot A. Identification of a postendocytic sorting sequence in CCR5. Mol Pharmacol 2007; 72:1497-507. [PMID: 17855654 DOI: 10.1124/mol.107.038422] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
The chemokine receptor 5 (CCR5), a member of the G protein-coupled receptor family (GPCR), is used by human immunodeficiency virus type 1 (HIV-1) with a R5 tropism as an entry receptor in addition to CD4. It is a key target for an antiviral action aiming at inhibiting the HIV-1 entry process. Only few data are available today regarding the mechanism involved in the intracellular trafficking process of CCR5. Understanding how CCR5 cell surface expression is regulated is particularly important with regard to HIV-1 entry inhibition. We set out to investigate whether CCR5 molecular determinants were involved in the postendocytic recycling and degradative pathways. We constructed progressive deletion mutants of the C-terminal domain of CCR5 that we stably expressed in HEK293 cells. All of the deletion mutants were expressed at the cell surface and were functional HIV-1 receptors. The deletion mutants were internalized after stimulation, but they lost their ability to recycle to the plasma membrane. They were rerouted toward a lysosomal degradative pathway. We identified here a sequence of four amino acids, present at the extreme C terminus of CCR5, that is necessary for the recycling of the internalized receptor, independently of its phosphorylation. A detailed analysis of this sequence indicated that the four amino acids acted as a postsynaptic density 95/discs-large/zona occludens (PDZ) interacting sequence. These results show that the CCR5 cytoplasmic domain bears a sequence similar to the "recycling signals" previously identified in other GPCRs. Drugs able to disrupt the recycling pathway of CCR5 may constitute promising tools for therapeutic treatment.
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Affiliation(s)
- Maurine Delhaye
- Institut Cochin, Université Paris Descartes, Centre National de la Recherche Scientifique (Unité Mixte de Recherche 8104), Paris, France
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507
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Gousseva V, Simaan M, Laporte SA, Swain PS. Inferring the lifetime of endosomal protein complexes by fluorescence recovery after photobleaching. Biophys J 2007; 94:679-87. [PMID: 17827242 PMCID: PMC2157253 DOI: 10.1529/biophysj.107.115188] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cellular signal transduction is dynamic, with signaling proteins continually associating and dissociating into and from protein complexes. Here we present a fluorescence recovery after photobleaching technique to determine the lifetime of protein complexes on intracellular vesicles. We use Bayesian inference based on a model that includes the diffusion of cytosolic proteins and their interaction with membrane-bound receptors. Our analysis is general: we incorporate prior information on protein diffusion, measurement error in determining fluorescence intensities, corrections for photobleaching, and variation in the concentration of receptors between vesicles. We apply our method to the complexes formed on endosomes by G-protein-coupled receptors and the protein beta-arrestin. The lifetime of these complexes determines the recycling rate of the receptors. We find in mammalian cells that the bradykinin type 2 receptor and beta-arrestin2 complex has a lifetime of approximately 2 min, while the angiotensin II type 1A receptor and beta-arrestin2 complex has a lifetime of approximately 6 min. As well as allowing quantitative comparisons between experiments, our method provides in vivo parameters for systems biology simulations of signaling networks.
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Affiliation(s)
- Veronika Gousseva
- Centre for Non-Linear Dynamics, Department of Physiology, McGill University, Montreal, Quebec, Canada
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508
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Holliday ND, Holst B, Rodionova EA, Schwartz TW, Cox HM. Importance of constitutive activity and arrestin-independent mechanisms for intracellular trafficking of the ghrelin receptor. Mol Endocrinol 2007; 21:3100-12. [PMID: 17717076 DOI: 10.1210/me.2007-0254] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
The ghrelin receptor (GhrelinR) and its related orphan GPR39 each display constitutive signaling, but only GhrelinRs undergo basal internalization. Here we investigate these differences by considering the roles of the C tail receptor domains for constitutive internalization and activity. Furthermore the interaction between phosphorylated receptors and beta-arrestin adaptor proteins has been examined. Replacement of the FLAG-tagged GhrelinR C tail with the equivalent GPR39 domain (GhR-39 chimera) preserved G(q) signaling. However in contrast to the GhrelinR, GhR-39 receptors exhibited no basal and substantially decreased agonist-induced internalization in transiently transfected HEK293 cells. Internalized GhrelinR and GhR-39 were predominantly localized to recycling compartments, identified with transferrin and the monomeric G proteins Rab5 and Rab11. Both the inverse agonist [d-Arg(1), d-Phe(5), d-Trp(7,9), Leu(11)] substance P and a naturally occurring mutant GhrelinR (A204E) with eliminated constitutive activity inhibited basal GhrelinR internalization. Surprisingly, we found that noninternalizing GPR39 was highly phosphorylated and that basal and agonist-induced phosphorylation of the GhR-39 chimera was elevated compared with GhrelinRs. Moreover, basal GhrelinR endocytosis occurred without significant phosphorylation, and it was not prevented by cotransfection of a dominant-negative beta-arrestin1(319-418) fragment or by expression in beta-arrestin1/2 double-knockout mouse embryonic fibroblasts. In contrast, agonist-stimulated GhrelinRs recruited the clathrin adaptor green fluorescent protein-tagged beta-arrestin2 to endosomes, coincident with increased receptor phosphorylation. Thus, GhrelinR internalization to recycling compartments depends on C-terminal motifs and constitutive activity, but the high levels of GPR39 phosphorylation, and of the GhR-39 chimera, are not sufficient to drive endocytosis. In addition, basal GhrelinR internalization occurs independently of beta-arrestins.
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Affiliation(s)
- Nicholas D Holliday
- Institute of Cell Signalling, Queen's Medical Centre, Nottingham NG7 2UH, United Kingdom.
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509
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Shenoy SK, Barak LS, Xiao K, Ahn S, Berthouze M, Shukla AK, Luttrell LM, Lefkowitz RJ. Ubiquitination of beta-arrestin links seven-transmembrane receptor endocytosis and ERK activation. J Biol Chem 2007; 282:29549-62. [PMID: 17666399 PMCID: PMC2216744 DOI: 10.1074/jbc.m700852200] [Citation(s) in RCA: 106] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Beta-arrestin2 and its ubiquitination play crucial roles in both internalization and signaling of seven-transmembrane receptors (7TMRs). To understand the connection between ubiquitination and the endocytic and signaling functions of beta-arrestin, we generated a beta-arrestin2 mutant that is defective in ubiquitination (beta-arrestin2(0K)), by mutating all of the ubiquitin acceptor lysines to arginines and compared its properties with the wild type and a stably ubiquitinated beta-arrestin2-ubiquitin (Ub) chimera. In vitro translated beta-arrestin2 and beta-arrestin2(0K) displayed equivalent binding to recombinant beta(2)-adrenergic receptor (beta(2)AR) reconstituted in vesicles, whereas beta-arrestin2-Ub bound approximately 4-fold more. In cellular coimmunoprecipitation assays, beta-arrestin2(0K) bound nonreceptor partners, such as AP-2 and c-Raf and scaffolded phosphorylated ERK robustly but displayed weak binding to clathrin. Moreover, beta-arrestin2(0K) was recruited only transiently to activated receptors at the membrane, did not enhance receptor internalization, and decreased the amount of phosphorylated ERK assimilated into isolated beta(2)AR complexes. Although the wild type beta-arrestin2 formed ERK signaling complexes with the beta(2)AR at the membrane, a stably ubiquitinated beta-arrestin2-Ub chimera not only stabilized the ERK signalosomes but also led to their endosomal targeting. Interestingly, in cellular fractionation assays, the ubiquitination state of beta-arrestin2 favors its distribution in membrane fractions, suggesting that ubiquitination increases the propensity of beta-arrestin for membrane association. Our findings suggest that although beta-arrestin ubiquitination is dispensable for beta-arrestin cytosol to membrane translocation and its "constitutive" interactions with some cytosolic proteins, it nevertheless is a prerequisite both for the formation of tight complexes with 7TMRs in vivo and for membrane compartment interactions that are crucial for downstream endocytic and signaling processes.
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Affiliation(s)
- Sudha K Shenoy
- Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA.
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510
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von Zastrow M, Sorkin A. Signaling on the endocytic pathway. Curr Opin Cell Biol 2007; 19:436-45. [PMID: 17662591 PMCID: PMC1992519 DOI: 10.1016/j.ceb.2007.04.021] [Citation(s) in RCA: 261] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2007] [Accepted: 04/16/2007] [Indexed: 10/23/2022]
Abstract
Endocytosis regulates many cellular signaling processes by controlling the number of functional receptors available at the cell surface. Conversely, some signaling processes regulate the endocytic pathway. Furthermore, various cellular signaling events appear to occur on endosome membranes. The endocytic pathway, by providing a set of dynamic and biochemically specialized endomembrane structures that physically communicate with the plasma membrane, is increasingly viewed as a highly flexible scaffold for mediating precise spatiotemporal control and transport of diverse biological signals. General principles of endosome-based signaling are beginning to emerge but, in many cases, the physiological significance of signaling on the endocytic pathway remains poorly understood.
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Affiliation(s)
- Mark von Zastrow
- Departments of Psychiatry and Cellular & Molecular Pharmacology, University of California at San Francisco, N212E Genentech Hall, Box 2140, UCSF Mission Bay Campus, 600 16th Street, San Francisco, CA 94158, USA.
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511
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Force T, Woulfe K, Koch WJ, Kerkelä R. Molecular Scaffolds Regulate Bidirectional Crosstalk Between Wnt and Classical Seven-Transmembrane Domain Receptor Signaling Pathways. ACTA ACUST UNITED AC 2007; 2007:pe41. [PMID: 17666710 DOI: 10.1126/stke.3972007pe41] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Signaling downstream of classical seven-transmembrane domain receptors (7TMRs) had generally been thought to recruit factors that are in large part separate from those recruited by atypical 7TMRs, such as Frizzleds (Fzs), receptors for the Wnt family of glycoproteins. Classical 7TMRs are also known as G protein-coupled receptors (GPCRs) and are mediated by signaling factors such as heterotrimeric guanine nucleotide-binding proteins (G proteins), GPCR kinases (GRKs), and beta-arrestins. Over the past few years, it has become increasingly apparent that classical and atypical 7TMRs share these factors, which are often associated with mediating classical 7TMR signaling, as well as the scaffolding proteins that were initially thought to be involved in transmitting atypical 7TMR signals. This sharing of signaling components by agonists that bind classical 7TMRs and those binding to atypical 7TMRs establishes the possibility of extensive crosstalk between these receptor classes. We discuss the evidence for, and against, crosstalk, and examine mechanisms by which this can occur.
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Affiliation(s)
- Thomas Force
- The Center for Translational Medicine, Thomas Jefferson University, Philadelphia, PA 19107, USA.
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512
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Pfleger KDG, Dalrymple MB, Dromey JR, Eidne KA. Monitoring interactions between G-protein-coupled receptors and β-arrestins. Biochem Soc Trans 2007; 35:764-6. [PMID: 17635143 DOI: 10.1042/bst0350764] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
β-Arrestins 1 and 2 are ubiquitously expressed intracellular adaptor and scaffolding proteins that play important roles in GPCR (G-protein-coupled receptor) desensitization, internalization, intracellular trafficking and G-protein-independent signalling. Recent developments in BRET (bioluminescence resonance energy transfer) technology enable novel insights to be gained from real-time monitoring of GPCR–β-arrestin complexes in live cells for prolonged periods. In concert with confocal microscopy, assays for studying internalization and recycling kinetics such as ELISAs, and techniques for measuring downstream signalling pathways such as those involving MAPKs (mitogen-activated protein kinases), investigators can now use a range of experimental tools to elucidate the ever-expanding roles of β-arrestins in mediating GPCR function.
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Affiliation(s)
- K D G Pfleger
- 7TM Laboratory/Laboratory for Molecular Endocrinology, Western Australian Institute for Medical Research (WAIMR) and Centre for Medical Research, University of Western Australia, Nedlands, Perth, WA 6009, Australia.
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513
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Thompson D, Pusch M, Whistler JL. Changes in G protein-coupled receptor sorting protein affinity regulate postendocytic targeting of G protein-coupled receptors. J Biol Chem 2007; 282:29178-85. [PMID: 17635908 DOI: 10.1074/jbc.m704014200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
After activation, most G protein-coupled receptors (GPCRs) are regulated by a cascade of events involving desensitization and endocytosis. Internalized receptors can then be recycled to the plasma membrane, retained in an endosomal compartment, or targeted for degradation. The GPCR-associated sorting protein, GASP, has been shown to preferentially sort a number of native GPCRs to the lysosome for degradation after endocytosis. Here we show that a mutant beta(2) adrenergic receptor and a mutant mu opioid receptor that have previously been described as lacking "recycling signals" due to mutations in their C termini in fact bind to GASP and are targeted for degradation. We also show that a mutant dopamine D1 receptor, which has likewise been described as lacking a recycling signal, does not bind to GASP and is therefore not targeted for degradation. Together, these results indicate that alteration of receptors in their C termini can expose determinants with affinity for GASP binding and consequently target receptors for degradation.
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Affiliation(s)
- Dawn Thompson
- Ernest Gallo Clinic and Research Center, University of California, San Francisco, Emeryville, California 94608, USA
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514
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Traub LM, Lukacs GL. Decoding ubiquitin sorting signals for clathrin-dependent endocytosis by CLASPs. J Cell Sci 2007; 120:543-53. [PMID: 17287393 DOI: 10.1242/jcs.03385] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Cargo selectivity is a hallmark of clathrin-mediated endocytosis. A wide range of structurally unrelated internalization signals specify the preferential clustering of transmembrane cargo into clathrin coats forming on the plasma membrane. Intriguingly, the classical endocytic adaptor AP-2 appears to recognize only a subset of these endocytic sorting signals. New data now reveal the molecular basis for recognition of other internalization signals, including post-translationally appended ubiquitin, by clathrin-coat-associated sorting proteins (CLASPs). Curiously, structurally related ubiquitin-recognition modules are shared by select CLASPs and the 26S proteasome, and recent work indicates that both display similar requirements for ubiquitin binding. During endocytosis, these modules engage oligoubiquitylated cargo in the form of polyubiquitin chains and/or multiple single ubiquitin molecules appended to different acceptor lysines. Functional separation between clathrin-mediated endocytosis and proteasome-dependent proteolysis is probably ensured by temporally regulated, local assembly of ubiquitin-tagged membrane cargo at sorting stations on the cell surface, shielding ubiquitin sorting signals from the proteasome. Thus, an expanded repertoire of CLASPs couples the process of clathrin-coat assembly with high-fidelity incorporation of assorted, cargo-specific sorting signals.
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Affiliation(s)
- Linton M Traub
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, PA 15261, USA, and Program in Cell and Lung Biology, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada.
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515
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Abstract
Upon their discovery, beta-arrestins 1 and 2 were named for their capacity to sterically hinder the G protein coupling of agonist-activated seven-transmembrane receptors, ultimately resulting in receptor desensitization. Surprisingly, recent evidence shows that beta-arrestins can also function to activate signaling cascades independently of G protein activation. By serving as multiprotein scaffolds, the beta-arrestins bring elements of specific signaling pathways into close proximity. beta-Arrestin regulation has been demonstrated for an ever-increasing number of signaling molecules, including the mitogen-activated protein kinases ERK, JNK, and p38 as well as Akt, PI3 kinase, and RhoA. In addition, investigators are discovering new roles for beta-arrestins in nuclear functions. Here, we review the signaling capacities of these versatile adapter molecules and discuss the possible implications for cellular processes such as chemotaxis and apoptosis.
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Affiliation(s)
- Scott M DeWire
- Howard Hughes Medical Institute and Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA
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516
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Abstract
The heptahelical G protein-coupled receptor (GPCR) family includes approximately 900 members and is the largest family of signaling receptors encoded in the mammalian genome. G protein-coupled receptors elicit cellular responses to diverse extracellular stimuli at the plasma membrane and some internalized receptors continue to signal from intracellular compartments. In addition to rapid desensitization, receptor trafficking is critical for regulation of the temporal and spatial aspects of GPCR signaling. Indeed, GPCR internalization functions to control signal termination and propagation as well as receptor resensitization. Our knowledge of the mechanisms that regulate mammalian GPCR endocytosis is based predominantly on arrestin regulation of receptors through a clathrin- and dynamin-dependent pathway. However, multiple clathrin adaptors, which recognize distinct endocytic signals, are now known to function in clathrin-mediated endocytosis of diverse cargo. Given the vast number and diversity of GPCRs, the complexity of clathrin-mediated endocytosis and the discovery of multiple clathrin adaptors, a single universal mechanism controlling endocytosis of all mammalian GPCRs is unlikely. Indeed, several recent studies now suggest that endocytosis of different GPCRs is regulated by distinct mechanisms and clathrin adaptors. In this review, we discuss the diverse mechanisms that regulate clathrin-dependent GPCR endocytosis.
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Affiliation(s)
- Breann L Wolfe
- Department of Pharmacology, School of Medicine, University of North Carolina at Chapel Hill, 1106 Mary Ellen Jones Building, CB#7365, Chapel Hill, NC 27599-7563, USA
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