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Jiang H, Galtes D, Wang J, Rockman HA. G protein-coupled receptor signaling: transducers and effectors. Am J Physiol Cell Physiol 2022; 323:C731-C748. [PMID: 35816644 PMCID: PMC9448338 DOI: 10.1152/ajpcell.00210.2022] [Citation(s) in RCA: 19] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2022] [Revised: 06/27/2022] [Accepted: 07/10/2022] [Indexed: 01/14/2023]
Abstract
G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.
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Affiliation(s)
- Haoran Jiang
- Department of Medicine, Duke University Medical Center, Durham, North Carolina
| | - Daniella Galtes
- Department of Medicine, Duke University Medical Center, Durham, North Carolina
| | - Jialu Wang
- Department of Medicine, Duke University Medical Center, Durham, North Carolina
| | - Howard A Rockman
- Department of Medicine, Duke University Medical Center, Durham, North Carolina
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina
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2
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Hanson J. [G proteins: privileged transducers of 7-transmembrane spanning receptors]. Biol Aujourdhui 2022; 215:95-106. [PMID: 35275054 DOI: 10.1051/jbio/2021011] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2021] [Indexed: 06/14/2023]
Abstract
G protein-coupled receptors or GPCR are the most abundant membrane receptors in our genome with around 800 members. They play an essential role in most physiological and pathophysiological phenomena. In addition, they constitute 30% of the targets of currently marketed drugs and remain an important reservoir for new innovative therapies. Their main effectors are heterotrimeric G proteins. These are composed of 3 subunits, α, β and γ, which, upon coupling with a GPCR, dissociate into Gα and Gβγ to activate numerous signaling pathways. This article describes some of the recent advances in understanding the function and role of heterotrimeric G proteins. After a short introduction to GPCRs, the history of the discovery of G proteins is briefly described. Then, the fundamental mechanisms of activation, signaling and regulation of G proteins are reviewed. New paradigms concerning intracellular signaling, specific recognition of G proteins by GPCRs as well as biased signaling are also discussed.
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Affiliation(s)
- Julien Hanson
- Laboratoire de Pharmacologie Moléculaire, GIGA-Molecular Biology of Diseases, Université de Liège, CHU, B34, Tour GIGA (+4), Avenue de l'Hôpital 11, B-4000 Liège, Belgique
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3
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Mitochondrial GTP Links Nutrient Sensing to β Cell Health, Mitochondrial Morphology, and Insulin Secretion Independent of OxPhos. Cell Rep 2020; 28:759-772.e10. [PMID: 31315053 DOI: 10.1016/j.celrep.2019.06.058] [Citation(s) in RCA: 34] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2017] [Revised: 02/15/2019] [Accepted: 06/14/2019] [Indexed: 12/18/2022] Open
Abstract
Mechanisms coordinating pancreatic β cell metabolism with insulin secretion are essential for glucose homeostasis. One key mechanism of β cell nutrient sensing uses the mitochondrial GTP (mtGTP) cycle. In this cycle, mtGTP synthesized by succinyl-CoA synthetase (SCS) is hydrolyzed via mitochondrial PEPCK (PEPCK-M) to make phosphoenolpyruvate, a high-energy metabolite that integrates TCA cycling and anaplerosis with glucose-stimulated insulin secretion (GSIS). Several strategies, including xenotopic overexpression of yeast mitochondrial GTP/GDP exchanger (GGC1) and human ATP and GTP-specific SCS isoforms, demonstrated the importance of the mtGTP cycle. These studies confirmed that mtGTP triggers and amplifies normal GSIS and rescues defects in GSIS both in vitro and in vivo. Increased mtGTP synthesis enhanced calcium oscillations during GSIS. mtGTP also augmented mitochondrial mass, increased insulin granule number, and membrane proximity without triggering de-differentiation or metabolic fragility. These data highlight the importance of the mtGTP signal in nutrient sensing, insulin secretion, mitochondrial maintenance, and β cell health.
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Single-Channel Properties of the ROMK-Pore-Forming Subunit of the Mitochondrial ATP-Sensitive Potassium Channel. Int J Mol Sci 2019; 20:ijms20215323. [PMID: 31731540 PMCID: PMC6862428 DOI: 10.3390/ijms20215323] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2019] [Revised: 10/21/2019] [Accepted: 10/23/2019] [Indexed: 12/11/2022] Open
Abstract
An increased flux of potassium ions into the mitochondrial matrix through the ATP-sensitive potassium channel (mitoKATP) has been shown to provide protection against ischemia-reperfusion injury. Recently, it was proposed that the mitochondrial-targeted isoform of the renal outer medullary potassium channel (ROMK) protein creates a pore-forming subunit of mitoKATP in heart mitochondria. Our research focuses on the properties of mitoKATP from heart-derived H9c2 cells. For the first time, we detected single-channel activity and describe the pharmacology of mitoKATP in the H9c2 heart-derived cells. The patch-clamping of mitoplasts from wild type (WT) and cells overexpressing ROMK2 revealed the existence of a potassium channel that exhibits the same basic properties previously attributed to mitoKATP. ROMK2 overexpression resulted in a significant increase of mitoKATP activity. The conductance of both channels in symmetric 150/150 mM KCl was around 97 ± 2 pS in WT cells and 94 ± 3 pS in cells overexpressing ROMK2. The channels were inhibited by 5-hydroxydecanoic acid (a mitoKATP inhibitor) and by Tertiapin Q (an inhibitor of both the ROMK-type channels and mitoKATP). Additionally, mitoKATP from cells overexpressing ROMK2 were inhibited by ATP/Mg2+ and activated by diazoxide. We used an assay based on proteinase K to examine the topology of the channel in the inner mitochondrial membrane and found that both termini of the protein localized to the mitochondrial matrix. We conclude that the observed activity of the channel formed by the ROMK protein corresponds to the electrophysiological and pharmacological properties of mitoKATP.
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Dyar KA, Lutter D, Artati A, Ceglia NJ, Liu Y, Armenta D, Jastroch M, Schneider S, de Mateo S, Cervantes M, Abbondante S, Tognini P, Orozco-Solis R, Kinouchi K, Wang C, Swerdloff R, Nadeef S, Masri S, Magistretti P, Orlando V, Borrelli E, Uhlenhaut NH, Baldi P, Adamski J, Tschöp MH, Eckel-Mahan K, Sassone-Corsi P. Atlas of Circadian Metabolism Reveals System-wide Coordination and Communication between Clocks. Cell 2019; 174:1571-1585.e11. [PMID: 30193114 DOI: 10.1016/j.cell.2018.08.042] [Citation(s) in RCA: 207] [Impact Index Per Article: 41.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2018] [Revised: 05/20/2018] [Accepted: 08/20/2018] [Indexed: 12/13/2022]
Abstract
Metabolic diseases are often characterized by circadian misalignment in different tissues, yet how altered coordination and communication among tissue clocks relate to specific pathogenic mechanisms remains largely unknown. Applying an integrated systems biology approach, we performed 24-hr metabolomics profiling of eight mouse tissues simultaneously. We present a temporal and spatial atlas of circadian metabolism in the context of systemic energy balance and under chronic nutrient stress (high-fat diet [HFD]). Comparative analysis reveals how the repertoires of tissue metabolism are linked and gated to specific temporal windows and how this highly specialized communication and coherence among tissue clocks is rewired by nutrient challenge. Overall, we illustrate how dynamic metabolic relationships can be reconstructed across time and space and how integration of circadian metabolomics data from multiple tissues can improve our understanding of health and disease.
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Affiliation(s)
- Kenneth A Dyar
- Institute for Diabetes and Obesity (IDO), Helmholtz Diabetes Center (HDC), Helmholtz Zentrum München, 85764 Neuherberg, Germany; German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany
| | - Dominik Lutter
- Institute for Diabetes and Obesity (IDO), Helmholtz Diabetes Center (HDC), Helmholtz Zentrum München, 85764 Neuherberg, Germany; German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany
| | - Anna Artati
- Institute of Experimental Genetics, Genome Analysis Center, Helmholtz Zentrum München, 85764 Neuherberg Germany
| | - Nicholas J Ceglia
- Institute for Genomics and Bioinformatics, School of Information and Computer Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Yu Liu
- Institute for Genomics and Bioinformatics, School of Information and Computer Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Danny Armenta
- Institute for Genomics and Bioinformatics, School of Information and Computer Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Martin Jastroch
- Institute for Diabetes and Obesity (IDO), Helmholtz Diabetes Center (HDC), Helmholtz Zentrum München, 85764 Neuherberg, Germany; German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany
| | - Sandra Schneider
- Department of Genetics, University of Cambridge, Cambridge, CB2 3EH, UK
| | - Sara de Mateo
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Marlene Cervantes
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Serena Abbondante
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Paola Tognini
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Ricardo Orozco-Solis
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Kenichiro Kinouchi
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Christina Wang
- Harbor-UCLA Medical Center and Los Angeles Biomedical Research Institute, Torrance, CA 90509, USA
| | - Ronald Swerdloff
- Harbor-UCLA Medical Center and Los Angeles Biomedical Research Institute, Torrance, CA 90509, USA
| | - Seba Nadeef
- BESE Division, KAUST Environmental Epigenetics Program, King Abdullah University Science and Technology, Thuwal, Saudi Arabia
| | - Selma Masri
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - Pierre Magistretti
- BESE Division, KAUST Environmental Epigenetics Program, King Abdullah University Science and Technology, Thuwal, Saudi Arabia
| | - Valerio Orlando
- BESE Division, KAUST Environmental Epigenetics Program, King Abdullah University Science and Technology, Thuwal, Saudi Arabia
| | - Emiliana Borrelli
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA
| | - N Henriette Uhlenhaut
- Institute for Diabetes and Obesity (IDO), Helmholtz Diabetes Center (HDC), Helmholtz Zentrum München, 85764 Neuherberg, Germany; German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany
| | - Pierre Baldi
- Institute for Genomics and Bioinformatics, School of Information and Computer Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Jerzy Adamski
- German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany; Institute of Experimental Genetics, Genome Analysis Center, Helmholtz Zentrum München, 85764 Neuherberg Germany; Chair of Experimental Genetics, Technical University of Munich, 85350 Freising-Weihenstephan, Germany.
| | - Matthias H Tschöp
- Institute for Diabetes and Obesity (IDO), Helmholtz Diabetes Center (HDC), Helmholtz Zentrum München, 85764 Neuherberg, Germany; German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany; Division of Metabolic Diseases, Technical University of Munich, 80333 Munich, Germany.
| | - Kristin Eckel-Mahan
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA; The Brown Foundation Institute of Molecular Medicine, University of Texas Health Science Center at Houston, Houston, TX 77030, USA.
| | - Paolo Sassone-Corsi
- Center for Epigenetics and Metabolism, U1233 INSERM, Department of Biological Chemistry, University of California, Irvine, Irvine, CA 92697, USA.
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Mahoney JP, Sunahara RK. Mechanistic insights into GPCR-G protein interactions. Curr Opin Struct Biol 2016; 41:247-254. [PMID: 27871057 DOI: 10.1016/j.sbi.2016.11.005] [Citation(s) in RCA: 79] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2016] [Revised: 10/24/2016] [Accepted: 11/04/2016] [Indexed: 01/24/2023]
Abstract
G protein-coupled receptors (GPCRs) respond to extracellular stimuli and interact with several intracellular binding partners to elicit cellular responses, including heterotrimeric G proteins. Recent structural and biophysical studies have highlighted the dynamic nature of GPCRs and G proteins and have identified specific conformational changes important for receptor-mediated nucleotide exchange on Gα. While domain separation within Gα is necessary for GDP release, opening the inter-domain interface is insufficient to stimulate nucleotide exchange. Rather, an activated receptor promotes GDP release by allosterically disrupting the nucleotide-binding site via interactions with the Gα N-termini and C-termini. Highlighting the allosteric nature of GPCRs, recent studies suggest that agonist binding alone poorly stabilizes an active conformation of several receptors. Rather, full stabilization of the receptor in an active state requires formation of the agonist-receptor-G protein ternary complex. In turn, nucleotide-free Gα is able to stabilize conformational changes around the receptor's agonist-binding site to enhance agonist affinity.
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Affiliation(s)
- Jacob P Mahoney
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI 48109, United States
| | - Roger K Sunahara
- Department of Pharmacology, University of California at San Diego, La Jolla, CA 92093, United States.
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Different effects of guanine nucleotides (GDP and GTP) on protein-mediated mitochondrial proton leak. PLoS One 2014; 9:e98969. [PMID: 24904988 PMCID: PMC4056835 DOI: 10.1371/journal.pone.0098969] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2013] [Accepted: 05/08/2014] [Indexed: 11/19/2022] Open
Abstract
In this study, we compared the influence of GDP and GTP on isolated mitochondria respiring under conditions favoring oxidative phosphorylation (OXPHOS) and under conditions excluding this process, i.e., in the presence of carboxyatractyloside, an adenine nucleotide translocase inhibitor, and/or oligomycin, an FOF1-ATP synthase inhibitor. Using mitochondria isolated from rat kidney and human endothelial cells, we found that the action of GDP and GTP can differ diametrically depending on the conditions. Namely, under conditions favoring OXPHOS, both in the absence and presence of linoleic acid, an activator of uncoupling proteins (UCPs), the addition of 1 mM GDP resulted in the state 4 (non-phosphorylating respiration)-state 3 (phosphorylating respiration) transition, which is characteristic of ADP oxidative phosphorylation. In contrast, the addition of 1 mM GTP resulted in a decrease in the respiratory rate and an increase in the membrane potential, which is characteristic of UCP inhibition. The stimulatory effect of GDP, but not GTP, was also observed in inside-out submitochondrial particles prepared from rat kidney mitochondria. However, the effects of GDP and GTP were more similar in the presence of OXPHOS inhibitors. The importance of these observations in connection with the action of UCPs, adenine nucleotide translocase (or other carboxyatractyloside-sensitive carriers), carboxyatractyloside- and purine nucleotide-insensitive carriers, as well as nucleoside-diphosphate kinase (NDPK) are considered. Because the measurements favoring oxidative phosphorylation better reflect in vivo conditions, our study strongly supports the idea that GDP cannot be considered a significant physiological inhibitor of UCP. Moreover, it appears that, under native conditions, GTP functions as a more efficient UCP inhibitor than GDP and ATP.
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8
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Stark R, Kibbey RG. The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: has it been overlooked? BIOCHIMICA ET BIOPHYSICA ACTA 2014; 1840:1313-30. [PMID: 24177027 PMCID: PMC3943549 DOI: 10.1016/j.bbagen.2013.10.033] [Citation(s) in RCA: 64] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/09/2013] [Revised: 10/13/2013] [Accepted: 10/18/2013] [Indexed: 01/03/2023]
Abstract
BACKGROUND Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis. SCOPE OF REVIEW A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways. MAJOR CONCLUSIONS PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis. GENERAL SIGNIFICANCE The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.
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Affiliation(s)
- Romana Stark
- Department of Physiology, Monash University, Clayton, Victoria 3800, Australia.
| | - Richard G Kibbey
- Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8020, USA.
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Dobolyi A, Ostergaard E, Bagó AG, Dóczi T, Palkovits M, Gál A, Molnár MJ, Adam-Vizi V, Chinopoulos C. Exclusive neuronal expression of SUCLA2 in the human brain. Brain Struct Funct 2013; 220:135-51. [PMID: 24085565 DOI: 10.1007/s00429-013-0643-2] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2013] [Accepted: 09/18/2013] [Indexed: 11/24/2022]
Abstract
SUCLA2 encodes the ATP-forming β subunit (A-SUCL-β) of succinyl-CoA ligase, an enzyme of the citric acid cycle. Mutations in SUCLA2 lead to a mitochondrial disorder manifesting as encephalomyopathy with dystonia, deafness and lesions in the basal ganglia. Despite the distinct brain pathology associated with SUCLA2 mutations, the precise localization of SUCLA2 protein has never been investigated. Here, we show that immunoreactivity of A-SUCL-β in surgical human cortical tissue samples was present exclusively in neurons, identified by their morphology and visualized by double labeling with a fluorescent Nissl dye. A-SUCL-β immunoreactivity co-localized >99 % with that of the d subunit of the mitochondrial F0-F1 ATP synthase. Specificity of the anti-A-SUCL-β antiserum was verified by the absence of labeling in fibroblasts from a patient with a complete deletion of SUCLA2. A-SUCL-β immunoreactivity was absent in glial cells, identified by antibodies directed against the glial markers GFAP and S100. Furthermore, in situ hybridization histochemistry demonstrated that SUCLA2 mRNA was present in Nissl-labeled neurons but not glial cells labeled with S100. Immunoreactivity of the GTP-forming β subunit (G-SUCL-β) encoded by SUCLG2, or in situ hybridization histochemistry for SUCLG2 mRNA could not be demonstrated in either neurons or astrocytes. Western blotting of post mortem brain samples revealed minor G-SUCL-β immunoreactivity that was, however, not upregulated in samples obtained from diabetic versus non-diabetic patients, as has been described for murine brain. Our work establishes that SUCLA2 is expressed exclusively in neurons in the human cerebral cortex.
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Affiliation(s)
- Arpád Dobolyi
- Department of Anatomy, Histology and Embryology, Semmelweis University, Budapest, 1094, Hungary
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10
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Yao XJ, Vélez Ruiz G, Whorton MR, Rasmussen SGF, DeVree BT, Deupi X, Sunahara RK, Kobilka B. The effect of ligand efficacy on the formation and stability of a GPCR-G protein complex. Proc Natl Acad Sci U S A 2009; 106:9501-6. [PMID: 19470481 PMCID: PMC2685739 DOI: 10.1073/pnas.0811437106] [Citation(s) in RCA: 194] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2008] [Indexed: 12/13/2022] Open
Abstract
G protein-coupled receptors (GPCRs) mediate the majority of physiologic responses to hormones and neurotransmitters. However, many GPCRs exhibit varying degrees of agonist-independent G protein activation. This phenomenon is referred to as basal or constitutive activity. For many of these GPCRs, drugs classified as inverse agonists can suppress basal activity. There is a growing body of evidence that basal activity is physiologically relevant, and the ability of a drug to inhibit basal activity may influence its therapeutic properties. However, the molecular mechanism for basal activation and inhibition of basal activity by inverse agonists is poorly understood and difficult to study, because the basally active state is short-lived and represents a minor fraction of receptor conformations. Here, we investigate basal activation of the G protein Gs by the beta(2) adrenergic receptor (beta(2)AR) by using purified receptor reconstituted into recombinant HDL particles with a stoichiometric excess of Gs. The beta(2)AR is site-specifically labeled with a small, environmentally sensitive fluorophore enabling direct monitoring of agonist- and Gs-induced conformational changes. In the absence of an agonist, the beta(2)AR and Gs can be trapped in a complex by enzymatic depletion of guanine nucleotides. Formation of the complex is enhanced by the agonist isoproterenol, and it rapidly dissociates on exposure to concentrations of GTP and GDP found in the cytoplasm. The inverse agonist ICI prevents formation of the beta(2)AR-Gs complex, but has little effect on preformed complexes. These results provide insights into G protein-induced conformational changes in the beta(2)AR and the structural basis for ligand efficacy.
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Affiliation(s)
- Xiao Jie Yao
- Department of Molecular and Cellular Physiology, Stanford University Medical School Department of Chemistry, Stanford University, Stanford, CA 94305
| | - Gisselle Vélez Ruiz
- Department of Pharmacology, University of Michigan Medical School, 1301 Medical Sciences Research Building III, Ann Arbor, MI 48109; and
| | - Matthew R. Whorton
- Department of Pharmacology, University of Michigan Medical School, 1301 Medical Sciences Research Building III, Ann Arbor, MI 48109; and
| | - Søren G. F. Rasmussen
- Department of Molecular and Cellular Physiology, Stanford University Medical School Department of Chemistry, Stanford University, Stanford, CA 94305
| | - Brian T. DeVree
- Department of Pharmacology, University of Michigan Medical School, 1301 Medical Sciences Research Building III, Ann Arbor, MI 48109; and
| | - Xavier Deupi
- Laboratori de Medicina Computacional, Unitat de Bioestadística, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalunya, Spain
| | - Roger K. Sunahara
- Department of Pharmacology, University of Michigan Medical School, 1301 Medical Sciences Research Building III, Ann Arbor, MI 48109; and
| | - Brian Kobilka
- Department of Molecular and Cellular Physiology, Stanford University Medical School Department of Chemistry, Stanford University, Stanford, CA 94305
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11
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Kibbey RG, Pongratz RL, Romanelli AJ, Wollheim CB, Cline GW, Shulman GI. Mitochondrial GTP regulates glucose-stimulated insulin secretion. Cell Metab 2007; 5:253-64. [PMID: 17403370 PMCID: PMC1876711 DOI: 10.1016/j.cmet.2007.02.008] [Citation(s) in RCA: 130] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/11/2006] [Revised: 12/01/2006] [Accepted: 02/15/2007] [Indexed: 11/15/2022]
Abstract
Nucleotide-specific isoforms of the tricarboxylic acid (TCA) cycle enzyme succinyl-CoA synthetase (SCS) catalyze substrate-level synthesis of mitochondrial GTP (mtGTP) and ATP (mtATP). While mtATP yield from glucose metabolism is coupled with oxidative phosphorylation and can vary, each molecule of glucose metabolized within pancreatic beta cells produces approximately one mtGTP, making mtGTP a potentially important fuel signal. In INS-1 832/13 cells and cultured rat islets, siRNA suppression of the GTP-producing pathway (DeltaSCS-GTP) reduced glucose-stimulated insulin secretion (GSIS) by 50%, while suppression of the ATP-producing isoform (DeltaSCS-ATP) increased GSIS 2-fold. Insulin secretion correlated with increases in cytosolic calcium, but not with changes in NAD(P)H or the ATP/ADP ratio. These data suggest a role for mtGTP in controlling pancreatic GSIS through modulation of mitochondrial metabolism, possibly involving mitochondrial calcium. Furthermore, in light of its tight coupling to TCA oxidation rates, mtGTP production may serve as an important molecular signal of TCA-cycle activity.
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Affiliation(s)
- Richard G. Kibbey
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Rebecca L. Pongratz
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Anthony J. Romanelli
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Claes B. Wollheim
- Department of Cell Physiology and Metabolism University Medical Center CH-1211 Geneva 4, Switzerland
| | - Gary W. Cline
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Gerald I. Shulman
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- Department of Cellular, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- Department of Molecular Physiology, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- *To whom correspondence should be addressed. E-mail:
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Gordon D, Lyver E, Lesuisse E, Dancis A, Pain D. GTP in the mitochondrial matrix plays a crucial role in organellar iron homoeostasis. Biochem J 2006; 400:163-8. [PMID: 16842238 PMCID: PMC1635451 DOI: 10.1042/bj20060904] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Mitochondria are the major site of cellular iron utilization for the synthesis of essential cofactors such as iron-sulfur clusters and haem. In the present study, we provide evidence that GTP in the mitochondrial matrix is involved in organellar iron homoeostasis. A mutant of yeast Saccharomyces cerevisiae lacking the mitochondrial GTP/GDP carrier protein (Ggc1p) exhibits decreased levels of matrix GTP and increased levels of matrix GDP [Vozza, Blanco, Palmieri and Palmieri (2004) J. Biol. Chem. 279, 20850-20857]. This mutant (previously called yhm1) also manifests high cellular iron uptake and tremendous iron accumulation within mitochondria [Lesuisse, Lyver, Knight and Dancis (2004) Biochem. J. 378, 599-607]. The reason for these two very different phenotypic defects of the same yeast mutant has so far remained elusive. We show that in vivo targeting of a human nucleoside diphosphate kinase (Nm23-H4), which converts ATP into GTP, to the matrix of ggc1 mutants restores normal iron regulation. Thus the role of Ggc1p in iron metabolism is mediated by effects on GTP/GDP levels in the mitochondrial matrix.
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Affiliation(s)
- Donna M. Gordon
- *Department of Pharmacology and Physiology, UMDNJ, New Jersey Medical School, Newark, NJ 07103, U.S.A
| | - Elise R. Lyver
- †Department of Medicine, Division of Haematology–Oncology, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Emmanuel Lesuisse
- ‡Laboratoire d'Ingénierie des Protéines et Contrôle Métabolique, Institut Jacques Monod, Tour 43, Université Paris 7/Paris 6, Paris, France
| | - Andrew Dancis
- †Department of Medicine, Division of Haematology–Oncology, University of Pennsylvania, Philadelphia, PA 19104, U.S.A
| | - Debkumar Pain
- *Department of Pharmacology and Physiology, UMDNJ, New Jersey Medical School, Newark, NJ 07103, U.S.A
- To whom correspondence should be addressed (email )
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McKee EE, Bentley AT, Hatch M, Gingerich J, Susan-Resiga D. Phosphorylation of thymidine and AZT in heart mitochondria: elucidation of a novel mechanism of AZT cardiotoxicity. Cardiovasc Toxicol 2005; 4:155-67. [PMID: 15371631 PMCID: PMC1472705 DOI: 10.1385/ct:4:2:155] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/23/2003] [Revised: 02/16/2004] [Accepted: 02/18/2004] [Indexed: 11/11/2022]
Abstract
Antiretroviral nucleoside analogs used in highly active antiretroviral therapy (HAART) are associated with cardiovascular and other tissue toxicity associated with mitochondrial DNA depletion, suggesting a block in mitochondrial (mt)-DNA replication. Because the triphosphate forms of these analogs variably inhibit mt-DNA polymerase, this enzyme has been promoted as the major target of toxicity associated with HAART. We have used isolated mitochondria from rat heart to study the mitochondrial transport and phosphorylation of thymidine and AZT (azidothymidine, or zidovudine), a component used in HAART. We demonstrate that isolated mitochondria readily transport thymidine and phosphorylate it to thymidine 5'-triphosphate (TTP) within the matrix. Under identical conditions, AZT is phosphorylated only to AZT-5'-monophosphate (AZT-MP). The kinetics of thymidine and AZT suggest negative cooperativity of substrate interaction with the enzyme, consistent with work by others on mitochondrial thymidine kinase 2. Results show that TMP and AZT-MP are not transported across the inner membrane, suggesting that AZT-MP may accumulate with time in the matrix. Given the lack of AZT-5'-triphosphate (AZT-TP), it seems unlikely that the toxicity of AZT in the heart is mediated by AZT-TP inhibition of DNA polymerase gamma. Rather, our work shows that AZT is a potent inhibitor of thymidine phosphorylation in heart mitochondria, having an inhibitory concentration (IC)(50) of 7.0 +/- 0.9 microM. Thus, the toxicity of AZT in some tissues may be mediated by disrupting the substrate supply of TTP for mt-DNA replication.
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Affiliation(s)
- Edward E McKee
- Indiana University School of Medicine, South Bend Center for Medical Education, Notre Dame, IN 46556, USA.
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Passarella S, Atlante A, Valenti D, de Bari L. The role of mitochondrial transport in energy metabolism. Mitochondrion 2003; 2:319-43. [PMID: 16120331 DOI: 10.1016/s1567-7249(03)00008-4] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2002] [Revised: 01/21/2003] [Accepted: 01/22/2003] [Indexed: 11/29/2022]
Abstract
Since mitochondria are closed spaces in the cell, metabolite traffic across the mitochondrial membrane is needed to accomplish energy metabolism. The mitochondrial carriers play this function by uniport, symport and antiport processes. We give here a survey of about 50 transport processes catalysed by more than 30 carriers with a survey of the methods used to investigate metabolite transport in isolated mammalian mitochondria. The role of mitochondria in metabolic pathways including ammoniogenesis, amino acid metabolism, mitochondrial shuttles etc. is also reported in more detail, mainly in the light of the existence of new transport processes.
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Affiliation(s)
- Salvatore Passarella
- Dipartimento di Scienze Animali, Vegetali e dell'Ambiente, Università del Molise, Via De Sanctis, 86100 Campobasso, Italy.
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McKee EE, Bentley AT, Smith RM, Kraas JR, Ciaccio CE. Guanine nucleotide transport by atractyloside-sensitive and -insensitive carriers in isolated heart mitochondria. Am J Physiol Cell Physiol 2000; 279:C1870-9. [PMID: 11078702 DOI: 10.1152/ajpcell.2000.279.6.c1870] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In previous work (McKee EE, Bentley AT, Smith RM Jr, and Ciaccio CE, Biochem Biophys Res Commun 257: 466-472, 1999), the transport of guanine nucleotides into the matrix of intact isolated heart mitochondria was demonstrated. In this study, the time course and mechanisms of guanine nucleotide transport are characterized. Two distinct mechanisms of transport were found to be capable of moving guanine nucleotides across the inner membrane. The first carrier was saturable, displayed temperature dependence, preferred GDP to GTP, and did not transport GMP or IMP. When incubated in the absence of exogenous ATP, this carrier had a V(max) of 946 +/- 53 pmol. mg(-1). min(-1) with a K(m) of 2.9 +/- 0.3 mM for GDP. However, transport of GTP and GDP on this carrier was completely inhibited by physiological concentrations of ATP, suggesting that this carrier was not involved with guanine nucleotide transport in vivo. Because transport on this carrier was also inhibited by atractyloside, this carrier was consistent with the well-characterized ATP/ADP translocase. The second mechanism of guanine nucleotide uptake was insensitive to atractyloside, displayed temperature dependence, and was capable of transporting GMP, GDP, and GTP at approximately equal rates but did not transport IMP, guanine, or guanosine. GTP transport via this mechanism was slow, with a V(max) of 48.7 +/- 1.4 pmol. mg(-1). min(-1) and a K(m) = 4.4 +/- 0.4 mM. However, because the requirement for guanine nucleotide transport is low in nondividing tissues such as the heart, this transport process is nevertheless sufficient to account for the matrix uptake of guanine nucleotides and may represent the physiological mechanism of transport.
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Affiliation(s)
- E E McKee
- South Bend Center for Medical Education, Indiana University School of Medicine, University of Notre Dame, Notre Dame, Indiana 46556, USA.
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Cai YC, Bullard JM, Thompson NL, Spremulli LL. Interaction of mitochondrial elongation factor Tu with aminoacyl-tRNA and elongation factor Ts. J Biol Chem 2000; 275:20308-14. [PMID: 10801827 DOI: 10.1074/jbc.m001899200] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Elongation factor (EF) Tu promotes the binding of aminoacyl-tRNA (aa-tRNA) to the acceptor site of the ribosome. This process requires the formation of a ternary complex (EF-Tu.GTP.aa-tRNA). EF-Tu is released from the ribosome as an EF-Tu.GDP complex. Exchange of GDP for GTP is carried out through the formation of a complex with EF-Ts (EF-Tu.Ts). Mammalian mitochondrial EF-Tu (EF-Tu(mt)) differs from the corresponding prokaryotic factors in having a much lower affinity for guanine nucleotides. To further understand the EF-Tu(mt) subcycle, the dissociation constants for the release of aa-tRNA from the ternary complex (K(tRNA)) and for the dissociation of the EF-Tu.Ts(mt) complex (K(Ts)) were investigated. The equilibrium dissociation constant for the ternary complex was 18 +/- 4 nm, which is close to that observed in the prokaryotic system. The kinetic dissociation rate constant for the ternary complex was 7.3 x 10(-)(4) s(-)(1), which is essentially equivalent to that observed for the ternary complex in Escherichia coli. The binding of EF-Tu(mt) to EF-Ts(mt) is mutually exclusive with the formation of the ternary complex. K(Ts) was determined by quantifying the effects of increasing concentrations of EF-Ts(mt) on the amount of ternary complex formed with EF-Tu(mt). The value obtained for K(Ts) (5.5 +/- 1.3 nm) is comparable to the value of K(tRNA).
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Affiliation(s)
- Y C Cai
- Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA
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