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Serrano J, Meshram NN, Soundarapandian MM, Smith KR, Mason C, Brown IS, Tyrberg B, Kyriazis GA. Saccharin Stimulates Insulin Secretion Dependent on Sweet Taste Receptor-Induced Activation of PLC Signaling Axis. Biomedicines 2022; 10:biomedicines10010120. [PMID: 35052799 PMCID: PMC8773316 DOI: 10.3390/biomedicines10010120] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2021] [Revised: 12/27/2021] [Accepted: 01/04/2022] [Indexed: 02/05/2023] Open
Abstract
Background: Saccharin is a common artificial sweetener and a bona fide ligand for sweet taste receptors (STR). STR can regulate insulin secretion in beta cells, so we investigated whether saccharin can stimulate insulin secretion dependent on STR and the activation of phospholipase C (PLC) signaling. Methods: We performed in vivo and in vitro approaches in mice and cells with loss-of-function of STR signaling and specifically assessed the involvement of a PLC signaling cascade using real-time biosensors and calcium imaging. Results: We found that the ingestion of a physiological amount of saccharin can potentiate insulin secretion dependent on STR. Similar to natural sweeteners, saccharin triggers the activation of the PLC signaling cascade, leading to calcium influx and the vesicular exocytosis of insulin. The effects of saccharin also partially require transient receptor potential cation channel M5 (TRPM5) activity. Conclusions: Saccharin ingestion may transiently potentiate insulin secretion through the activation of the canonical STR signaling pathway. These physiological effects provide a framework for understanding the potential health impact of saccharin use and the contribution of STR in peripheral tissues.
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Affiliation(s)
- Joan Serrano
- Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, OH 43210, USA; (J.S.); (N.N.M.); (C.M.); (I.S.B.)
| | - Nishita N. Meshram
- Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, OH 43210, USA; (J.S.); (N.N.M.); (C.M.); (I.S.B.)
| | | | - Kathleen R. Smith
- Sanford Burnham Prebys Medical Discovery Institute, Lake Nona, FL 32827, USA; (M.M.S.); (K.R.S.)
| | - Carter Mason
- Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, OH 43210, USA; (J.S.); (N.N.M.); (C.M.); (I.S.B.)
| | - Ian S. Brown
- Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, OH 43210, USA; (J.S.); (N.N.M.); (C.M.); (I.S.B.)
| | - Björn Tyrberg
- Department of Physiology, Sahlgrenska Academy, University of Gothenburg, 405 30 Gothenburg, Sweden;
| | - George A. Kyriazis
- Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, OH 43210, USA; (J.S.); (N.N.M.); (C.M.); (I.S.B.)
- Correspondence: or
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2
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Lubberding AF, Zhang J, Lundh M, Nielsen TS, Søndergaard MS, Villadsen M, Skovhøj EZ, Boer GA, Hansen JB, Thomsen MB, Treebak JT, Holst JJ, Kanters JK, Mandrup-Poulsen T, Jespersen T, Emanuelli B, Torekov SS. Age-dependent transition from islet insulin hypersecretion to hyposecretion in mice with the long QT-syndrome loss-of-function mutation Kcnq1-A340V. Sci Rep 2021; 11:12253. [PMID: 34112814 PMCID: PMC8192901 DOI: 10.1038/s41598-021-90452-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2020] [Accepted: 05/05/2021] [Indexed: 11/24/2022] Open
Abstract
Loss-of-function (LoF) mutations in KCNQ1, encoding the voltage-gated K+ channel Kv7.1, lead to long QT syndrome 1 (LQT1). LQT1 patients also present with post-prandial hyperinsulinemia and hypoglycaemia. In contrast, KCNQ1 polymorphisms are associated with diabetes, and LQTS patients have a higher prevalence of diabetes. We developed a mouse model with a LoF Kcnq1 mutation using CRISPR-Cas9 and hypothesized that this mouse model would display QT prolongation, increased glucose-stimulated insulin secretion and allow for interrogation of Kv7.1 function in islets. Mice were characterized by electrocardiography and oral glucose tolerance tests. Ex vivo, islet glucose-induced insulin release was measured, and beta-cell area quantified by immunohistochemistry. Homozygous mice had QT prolongation. Ex vivo, glucose-stimulated insulin release was increased in islets from homozygous mice at 12–14 weeks, while beta-cell area was reduced. Non-fasting blood glucose levels were decreased at this age. In follow-up studies 8–10 weeks later, beta-cell area was similar in all groups, while glucose-stimulated insulin secretion was now reduced in islets from hetero- and homozygous mice. Non-fasting blood glucose levels had normalized. These data suggest that Kv7.1 dysfunction is involved in a transition from hyper- to hyposecretion of insulin, potentially explaining the association with both hypoglycemia and hyperglycemia in LQT1 patients.
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Affiliation(s)
- Anniek F Lubberding
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Jinyi Zhang
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark.,Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Morten Lundh
- Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Thomas Svava Nielsen
- Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Mathilde S Søndergaard
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Maria Villadsen
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Emil Z Skovhøj
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Geke A Boer
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark.,Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Jakob B Hansen
- Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Morten B Thomsen
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Jonas T Treebak
- Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Jens J Holst
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark.,Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Jørgen K Kanters
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Thomas Mandrup-Poulsen
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Thomas Jespersen
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark
| | - Brice Emanuelli
- Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Signe S Torekov
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200, Copenhagen, Denmark. .,Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
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3
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Conventional and Unconventional Mechanisms by which Exocytosis Proteins Oversee β-cell Function and Protection. Int J Mol Sci 2021; 22:ijms22041833. [PMID: 33673206 PMCID: PMC7918544 DOI: 10.3390/ijms22041833] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2021] [Revised: 02/02/2021] [Accepted: 02/08/2021] [Indexed: 02/06/2023] Open
Abstract
Type 2 diabetes (T2D) is one of the prominent causes of morbidity and mortality in the United States and beyond, reaching global pandemic proportions. One hallmark of T2D is dysfunctional glucose-stimulated insulin secretion from the pancreatic β-cell. Insulin is secreted via the recruitment of insulin secretory granules to the plasma membrane, where the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and SNARE regulators work together to dock the secretory granules and release insulin into the circulation. SNARE proteins and their regulators include the Syntaxins, SNAPs, Sec1/Munc18, VAMPs, and double C2-domain proteins. Recent studies using genomics, proteomics, and biochemical approaches have linked deficiencies of exocytosis proteins with the onset and progression of T2D. Promising results are also emerging wherein restoration or enhancement of certain exocytosis proteins to β-cells improves whole-body glucose homeostasis, enhances β-cell function, and surprisingly, protection of β-cell mass. Intriguingly, overexpression and knockout studies have revealed novel functions of certain exocytosis proteins, like Syntaxin 4, suggesting that exocytosis proteins can impact a variety of pathways, including inflammatory signaling and aging. In this review, we present the conventional and unconventional functions of β-cell exocytosis proteins in normal physiology and T2D and describe how these insights might improve clinical care for T2D.
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4
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Thurmond DC, Gaisano HY. Recent Insights into Beta-cell Exocytosis in Type 2 Diabetes. J Mol Biol 2020; 432:1310-1325. [PMID: 31863749 PMCID: PMC8061716 DOI: 10.1016/j.jmb.2019.12.012] [Citation(s) in RCA: 35] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2019] [Revised: 11/26/2019] [Accepted: 12/05/2019] [Indexed: 01/26/2023]
Abstract
As one of the leading causes of morbidity and mortality worldwide, diabetes affects an estimated 422 million adults, and it is expected to continue expanding such that by 2050, 30% of the U.S. population will become diabetic within their lifetime. Out of the estimated 422 million people currently afflicted with diabetes worldwide, about 5% have type 1 diabetes (T1D), while the remaining ~95% of diabetics have type 2 diabetes (T2D). Type 1 diabetes results from the autoimmune-mediated destruction of functional β-cell mass, whereas T2D results from combinatorial defects in functional β-cell mass plus peripheral glucose uptake. Both types of diabetes are now believed to be preceded by β-cell dysfunction. T2D is increasingly associated with numerous reports of deficiencies in the exocytosis proteins that regulate insulin release from β-cells, specifically the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE protein's functionality is further regulated by a variety of accessory factors such as Sec1/Munc18 (SM), double C2-domain proteins (DOC2), and additional interacting proteins at the cell surface that influence the fidelity of insulin release. As new evidence emerges about the detailed mechanisms of exocytosis, new questions and controversies have come to light. This emerging information is also contributing to dialogue in the islet biology field focused on how to correct the defects in insulin exocytosis. Herein we present a balanced review of the role of exocytosis proteins in T2D, with thoughts on novel strategies to protect functional β-cell mass.
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Affiliation(s)
- Debbie C Thurmond
- Department of Molecular and Cellular Endocrinology, Beckman Research Institute of City of Hope, CA, USA.
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5
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Nie J, Sun C, Chang Z, Musi N, Shi Y. SAD-A Promotes Glucose-Stimulated Insulin Secretion Through Phosphorylation and Inhibition of GDIα in Male Islet β Cells. Endocrinology 2018; 159:3036-3047. [PMID: 29873699 PMCID: PMC6693047 DOI: 10.1210/en.2017-03243] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/18/2017] [Accepted: 05/26/2018] [Indexed: 02/06/2023]
Abstract
Rho GDP-dissociation inhibitor (GDIα) inhibits glucose-stimulated insulin secretion (GSIS) in part by locking Rho GTPases in an inactive GDP-bound form. The onset of GSIS causes phosphorylation of GDIα at Ser174, a critical inhibitory site for GDIα, leading to the release of Rho GTPases and their subsequent activation. However, the kinase regulator(s) that catalyzes the phosphorylation of GDIα in islet β cells remains elusive. We propose that SAD-A, a member of AMP-activated protein kinase-related kinases that promotes GSIS as an effector kinase for incretin signaling, interacts with and inhibits GDIα through phosphorylation of Ser174 during the onset GSIS from islet β cells. Coimmunoprecipitation and phosphorylation analyses were carried out to identify the physical interaction and phosphorylation site of GDIα by SAD-A in the context of GSIS from INS-1 β cells and primary islets. We identified GDIα directly binds to SAD-A kinase domain and phosphorylated by SAD-A on Ser174, leading to dissociation of Rho GTPases from GDIα complexes. Accordingly, overexpression of SAD-A significantly stimulated GDIα phosphorylation at Ser174 in response to GSIS, which is dramatically potentiated by glucagonlike peptide-1, an incretin hormone. Conversely, SAD-A deficiency, which is mediated by short hairpin RNA transfection in INS-1 cells, significantly attenuated endogenous GDIα phosphorylation at Ser174. Consequently, coexpression of SAD-A completely prevented the inhibitory effect of GDIα on insulin secretion in islets. In summary, glucose and incretin stimulate insulin secretion through the phosphorylation of GDIα at Ser174 by SAD-A, which leads to the activation of Rho GTPases, culminating in insulin exocytosis.
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Affiliation(s)
- Jia Nie
- Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, San Antonio, Texas
- Correspondence: Jia Nie, PhD, Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, San Antonio, Texas 78245. E-mail:
| | - Chao Sun
- Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, San Antonio, Texas
| | - Zhijie Chang
- State Key Laboratory of Biomembrane and Membrane Biotechnology, School of Medicine, Tsinghua University, Beijing, China
| | - Nicolas Musi
- Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, San Antonio, Texas
| | - Yuguang Shi
- Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, San Antonio, Texas
- School of Basic Medical Sciences, Department of Biochemistry and Molecular Biology, Nanjing Medical University, Nanjing, Jiangsu, China
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6
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Zhu YH, Hyun J, Pan YZ, Hopper JE, Rizo J, Wu JQ. Roles of the fission yeast UNC-13/Munc13 protein Ync13 in late stages of cytokinesis. Mol Biol Cell 2018; 29:2259-2279. [PMID: 30044717 PMCID: PMC6249806 DOI: 10.1091/mbc.e18-04-0225] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Cytokinesis is a complicated yet conserved step of the cell-division cycle that requires the coordination of multiple proteins and cellular processes. Here we describe a previously uncharacterized protein, Ync13, and its roles during fission yeast cytokinesis. Ync13 is a member of the UNC-13/Munc13 protein family, whose animal homologues are essential priming factors for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex assembly during exocytosis in various cell types, but no roles in cytokinesis have been reported. We find that Ync13 binds to lipids in vitro and dynamically localizes to the plasma membrane at cell tips during interphase and at the division site during cytokinesis. Deletion of Ync13 leads to defective septation and exocytosis, uneven distribution of cell-wall enzymes and components of cell-wall integrity pathway along the division site and massive cell lysis during cell separation. Interestingly, loss of Ync13 compromises endocytic site selection at the division plane. Collectively, we find that Ync13 has a novel function as an UNC-13/Munc13 protein in coordinating exocytosis, endocytosis, and cell-wall integrity during fission yeast cytokinesis.
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Affiliation(s)
- Yi-Hua Zhu
- Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210
| | - Joanne Hyun
- Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210
| | - Yun-Zu Pan
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX 75390.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - James E Hopper
- Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210.,Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210
| | - Josep Rizo
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX 75390.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Jian-Qiu Wu
- Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210.,Department of Biological Chemistry and Pharmacology, The Ohio State University, Columbus, OH 43210
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7
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Gaisano HY. Recent new insights into the role of SNARE and associated proteins in insulin granule exocytosis. Diabetes Obes Metab 2017; 19 Suppl 1:115-123. [PMID: 28880475 DOI: 10.1111/dom.13001] [Citation(s) in RCA: 47] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/27/2017] [Revised: 04/23/2017] [Accepted: 05/02/2017] [Indexed: 01/22/2023]
Abstract
Initial work on the exocytotic machinery of predocked insulin secretory granules (SGs) in pancreatic β-cells mimicked the SNARE hypothesis work in neurons, which includes SM/SNARE complex and associated priming proteins, fusion clamps and Ca2+ sensors. However, β-cell SGs, unlike neuronal synaptic vesicles, exhibit a biphasic secretory response that requires additional distinct features in exocytosis including newcomer SGs that undergo minimal docking time at the plasma membrane (PM) before fusion and multi-SG (compound) fusion. These exocytotic events are mediated by Munc18/SNARE complexes distinct from that which mediates predocked SG fusion. We review some recent insights in SNARE complex assembly and the promiscuity in SM/SNARE complex formation, whereby both contribute to conferring different insulin SG fusion kinetics. Some SNARE and associated proteins play non-fusion roles, including tethering SGs to Ca2+ channels, SG recruitment from cell interior to PM, and inhibitory SNAREs that block the action of profusion SNAREs. We discuss new insights into how sub-PM cytoskeletal mesh gates SG access to the PM and the targeting of SG exocytosis to PM domains in functionally polarized β-cells within intact islets. These recent developments have major implications on devising clever SNARE replacement therapies that could restore the deficient insulin secretion in diabetic islet β-cells.
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8
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Li Y, Li F, Bai B, Wu Z, Hou X, Shen Y, Wang Y. Protein interacting with C‑kinase 1 modulates exocytosis and KATP conductance in pancreatic β cells. Mol Med Rep 2017; 16:4247-4252. [PMID: 28731156 DOI: 10.3892/mmr.2017.7056] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2016] [Accepted: 04/06/2017] [Indexed: 11/06/2022] Open
Abstract
It has been previously identified that α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs) are expressed in pancreatic β cells and regulate exocytosis and insulin release. It is known that protein interacting with C‑kinase 1 (PICK1) regulates trafficking and synaptic targeting of AMPARs in the central nervous system. However, it is unknown whether PICK1 regulates glutamate‑induced insulin release in β cells. The present study demonstrated that glutamate‑induced exocytosis was increased in β cells derived from PICK1‑knockout mice. In agreement with this result, adding PICK1 in β cells reduced glutamate‑induced exocytosis, whereas adding EVKI, a peptide that interrupts the interaction between AMPARs and PICK1, increased the exocytosis of β cells with the application of glutamate. Furthermore, the conductance of ATP‑sensitive potassium (KATP) channels was reduced in PICK1‑knockout mice, which was reversed by the overexpression of PICK1. In addition, PICK1 application reduced voltage oscillation induced by the closure of KATP. Taken together, the results indicate that PICK1 regulates glutamate‑induced exocytosis in β cells.
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Affiliation(s)
- Yunhong Li
- Key Laboratory of Craniocerebral Diseases of Ningxia Hui Autonomous Region, Basic Medical College of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
| | - Fan Li
- Key Laboratory of Craniocerebral Diseases of Ningxia Hui Autonomous Region, Basic Medical College of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
| | - Bin Bai
- Department of Endocrinology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
| | - Zhenyong Wu
- Department of Neurobiology, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, P.R. China
| | - Xiaolin Hou
- Department of Endocrinology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
| | - Ying Shen
- Department of Neurobiology, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, P.R. China
| | - Yin Wang
- Key Laboratory of Craniocerebral Diseases of Ningxia Hui Autonomous Region, Basic Medical College of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
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9
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Alonso B, Bartolomé-Martín D, Ferrero JJ, Ramírez-Franco J, Torres M, Sánchez-Prieto J. CB1 receptors down-regulate a cAMP/Epac2/PLC pathway to silence the nerve terminals of cerebellar granule cells. J Neurochem 2017; 142:350-364. [PMID: 28445587 DOI: 10.1111/jnc.14059] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2017] [Revised: 04/19/2017] [Accepted: 04/19/2017] [Indexed: 01/06/2023]
Abstract
Cannabinoid receptors mediate short-term retrograde inhibition of neurotransmitter release, as well as long-term depression of synaptic transmission at excitatory synapses. The responses of individual nerve terminals in VGLUT1-pHluorin transfected cerebellar granule cells to cannabinoids have shown that prolonged activation of cannabinoid type 1 receptors (CB1Rs) silences a subpopulation of previously active synaptic boutons. Adopting a combined pharmacological and genetic approach to study the molecular mechanisms of CB1R-induced silencing, we found that adenylyl cyclase inhibition decreases cAMP levels while it increases the number of silent synaptic boutons and occludes the induction of further silencing by the cannabinoid agonist HU-210. Guanine nucleotide exchange proteins directly activated by cAMP (Epac proteins) mediate some of the presynaptic effects of cAMP in the potentiation of synaptic transmission. ESI05, a selective Epac2 inhibitor, and U-73122, the specific inhibitor of phospholipase C (PLC), both augment the number of silent synaptic boutons. Moreover, they abolish the capacity of the Epac activator, 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate, to prevent HU-210-induced silencing consistent with PLC signaling lying downstream of Epac2 proteins. Furthermore, Rab3-interacting molecule (RIM)1α KO cells have many more basally silent synaptic boutons (12.9 ± 3.5%) than wild-type cells (1.1 ± 0.5%). HU-210 induced further silencing in these mutant cells, although 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate only awoke the HU-210-induced silence and not the basally silent synaptic boutons. This behavior can be rescued by expressing RIM1α in RIM1α KO cells, these cells behaving very much like wild-type cells. These findings support the hypothesis that a cAMP/Epac/PLC signaling pathway targeting the release machinery appears to mediate cannabinoid-induced presynaptic silencing.
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Affiliation(s)
- Beatris Alonso
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
| | - David Bartolomé-Martín
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
| | - José Javier Ferrero
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
| | - Jorge Ramírez-Franco
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
| | - Magdalena Torres
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
| | - José Sánchez-Prieto
- Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.,Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, Madrid, Spain
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10
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Wang P, Liu Z, Chen H, Ye N, Cheng X, Zhou J. Exchange proteins directly activated by cAMP (EPACs): Emerging therapeutic targets. Bioorg Med Chem Lett 2017; 27:1633-1639. [PMID: 28283242 PMCID: PMC5397994 DOI: 10.1016/j.bmcl.2017.02.065] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2016] [Revised: 02/07/2017] [Accepted: 02/26/2017] [Indexed: 11/22/2022]
Abstract
Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway mediators. The discovery of EPAC proteins has significantly facilitated understanding on cAMP-dependent signaling pathway and efforts along this line open new avenues for developing novel therapeutics for cancer, diabetes, heart failure, inflammation, infections, neurological disorders and other human diseases. Over the past decade, important progress has been made in the identification of EPAC agonists, antagonists and their biological and pharmacological applications. In this review, we briefly summarize recently reported novel functions of EPACs and the discovery of their small molecule modulators. The challenges and future perspectives are also discussed.
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Affiliation(s)
- Pingyuan Wang
- Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555, United States
| | - Zhiqing Liu
- Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555, United States
| | - Haiying Chen
- Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555, United States
| | - Na Ye
- Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555, United States
| | - Xiaodong Cheng
- Department of Integrative Biology and Pharmacology, Texas Therapeutics Institute, University of Texas Health Science Center, Houston, TX 77030, United States
| | - Jia Zhou
- Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555, United States.
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11
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Zhu D, Xie L, Kang Y, Dolai S, Bondo Hansen J, Qin T, Xie H, Liang T, Rubin DC, Osborne L, Gaisano HY. Syntaxin 2 Acts as Inhibitory SNARE for Insulin Granule Exocytosis. Diabetes 2017; 66:948-959. [PMID: 28115395 PMCID: PMC5860373 DOI: 10.2337/db16-0636] [Citation(s) in RCA: 19] [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: 05/18/2016] [Accepted: 12/24/2016] [Indexed: 01/03/2023]
Abstract
Of the four syntaxins specialized for exocytosis, syntaxin (Syn)-2 is the least understood. In this study, we used Syn-2/epimorphin knockout mice to examine the role of Syn-2 in insulin secretory granule (SG) exocytosis. Unexpectedly, Syn-2 knockout mice exhibited paradoxical superior glucose homeostasis resulting from an enhanced insulin secretion. This was confirmed in vitro by pancreatic islet perifusion showing an amplified biphasic glucose-stimulated insulin secretion arising from an increase in size of the readily releasable pool of insulin SGs and enhanced SG pool refilling. The increase in insulin exocytosis was attributed mainly to an enhanced recruitment of the larger pool of newcomer SGs that undergoes no residence time on plasma membrane before fusion and, to a lesser extent, also the predocked SGs. Consistently, Syn-2 depletion resulted in a stimulation-induced increase in abundance of exocytotic complexes we previously demonstrated as mediating the fusion of newcomer SGs (Syn-3/VAMP8/SNAP25/Munc18b) and predocked SGs (Syn-1A/VAMP2/SNAP25/Muncn18a). This work is the first to show in mammals that Syn-2 could function as an inhibitory SNARE protein that, when relieved, could promote exocytosis in pancreatic islet β-cells. Thus, Syn-2 may serve as a potential target to treat diabetes.
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Affiliation(s)
- Dan Zhu
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Li Xie
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Youhou Kang
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Subhankar Dolai
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | | | - Tairan Qin
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Huanli Xie
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Tao Liang
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Deborah C Rubin
- Department of Medicine, Washington University School of Medicine in St. Louis, St. Louis, MO
- Department of Developmental Biology, Washington University School of Medicine in St. Louis, St. Louis, MO
| | - Lucy Osborne
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, Ontario, Canada
| | - Herbert Y Gaisano
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
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12
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Graaf CD, Donnelly D, Wootten D, Lau J, Sexton PM, Miller LJ, Ahn JM, Liao J, Fletcher MM, Yang D, Brown AJH, Zhou C, Deng J, Wang MW. Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes. Pharmacol Rev 2017; 68:954-1013. [PMID: 27630114 PMCID: PMC5050443 DOI: 10.1124/pr.115.011395] [Citation(s) in RCA: 219] [Impact Index Per Article: 31.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.
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Affiliation(s)
- Chris de Graaf
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Dan Donnelly
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Denise Wootten
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Jesper Lau
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Patrick M Sexton
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Laurence J Miller
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Jung-Mo Ahn
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Jiayu Liao
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Madeleine M Fletcher
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Dehua Yang
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Alastair J H Brown
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Caihong Zhou
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Jiejie Deng
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
| | - Ming-Wei Wang
- Division of Medicinal Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands (C.d.G.); School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom (D.D.); Drug Discovery Biology Theme and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia (D.W., P.M.S., M.M.F.); Protein and Peptide Chemistry, Global Research, Novo Nordisk A/S, Måløv, Denmark (J.La.); Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona (L.J.M.); Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas (J.-M.A.); Department of Bioengineering, Bourns College of Engineering, University of California at Riverside, Riverside, California (J.Li.); National Center for Drug Screening and CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China (D.Y., C.Z., J.D., M.-W.W.); Heptares Therapeutics, BioPark, Welwyn Garden City, United Kingdom (A.J.H.B.); and School of Pharmacy, Fudan University, Zhangjiang High-Tech Park, Shanghai, China (M.-W.W.)
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13
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Liu J, Pang ZP. Glucagon-like peptide-1 drives energy metabolism on the synaptic highway. FEBS J 2016; 283:4413-4423. [DOI: 10.1111/febs.13785] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2016] [Revised: 06/04/2016] [Accepted: 06/16/2016] [Indexed: 01/17/2023]
Affiliation(s)
- Ji Liu
- Child Health Institute of New Jersey; Rutgers University Robert Wood Johnson Medical School; New Brunswick NJ USA
- Department of Neuroscience and Cell Biology; Rutgers University Robert Wood Johnson Medical School; New Brunswick NJ USA
| | - Zhiping P. Pang
- Child Health Institute of New Jersey; Rutgers University Robert Wood Johnson Medical School; New Brunswick NJ USA
- Department of Neuroscience and Cell Biology; Rutgers University Robert Wood Johnson Medical School; New Brunswick NJ USA
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14
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Zhao S, Poursharifi P, Mugabo Y, Levens EJ, Vivot K, Attane C, Iglesias J, Peyot ML, Joly E, Madiraju SM, Prentki M. α/β-Hydrolase domain-6 and saturated long chain monoacylglycerol regulate insulin secretion promoted by both fuel and non-fuel stimuli. Mol Metab 2015; 4:940-50. [PMID: 26909310 PMCID: PMC4731734 DOI: 10.1016/j.molmet.2015.09.012] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/03/2015] [Revised: 09/21/2015] [Accepted: 09/28/2015] [Indexed: 01/15/2023] Open
Abstract
Objective α/β-Hydrolase domain-6 (ABHD6) is a newly identified monoacylglycerol (MAG) lipase. We recently reported that it negatively regulates glucose stimulated insulin secretion (GSIS) in the β cells by hydrolyzing lipolysis-derived MAG that acts as a metabolic coupling factor and signaling molecule via exocytotic regulator Munc13-1. Whether ABHD6 and MAG play a role in response to all classes of insulin secretagogues, in particular various fuel and non-fuel stimuli, is unknown. Methods Insulin secretion in response to various classes of secretagogues, exogenous MAG and pharmacological agents was measured in islets of mice deficient in ABHD6 specifically in the β cell (BKO). Islet perifusion experiments and determinations of glucose and fatty acid metabolism, cytosolic Ca2+ and MAG species levels were carried out. Results Deletion of ABHD6 potentiated insulin secretion in response to the fuels glutamine plus leucine and α-ketoisocaproate and to the non-fuel stimuli glucagon-like peptide 1, carbamylcholine and elevated KCl. Fatty acids amplified GSIS in control and BKO mice to the same extent. Exogenous 1-MAG amplified insulin secretion in response to fuel and non-fuel stimuli. MAG hydrolysis activity was greatly reduced in BKO islets without changes in total diacylglycerol and triacylglycerol lipase activity. ABHD6 deletion induced insulin secretion independently from KATP channels and did not alter the glucose induced rise in intracellular Ca2+. Perifusion studies showed elevated insulin secretion during second phase of GSIS in BKO islets that was not due to altered cytosolic Ca2+ signaling or because of changes in glucose and fatty acid metabolism. Glucose increased islet saturated long chain 1-MAG species and ABHD6 deletion caused accumulation of these 1-MAG species at both low and elevated glucose. Conclusion ABHD6 regulates insulin secretion in response to fuel stimuli at large and some non-fuel stimuli by controlling long chain saturated 1-MAG levels that synergize with other signaling pathways for secretion. ABHD6 is the major monoacylglycerol (MAG) hydrolase in pancreatic β cells. 1-MAG level is elevated in islets from β cell specific ABHD6-KO mice (BKO). BKO islets show enhanced fuel and non-fuel induced insulin secretion. ABHD6 accessible 1-MAG synergizes with other signals for insulin secretion.
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Key Words
- 1-OG, 1-oleoylglycerol
- 1-PG, 1-palmitoylglycerol
- 1-SG, 1-stearoylglycerol
- ABHD6, α/β-hydrolase domain-6
- ATGL, adipose triglyceride lipase
- BKO, β cell specific ABHD6-knockout
- Carb, carbamylcholine
- Cytosolic Ca2+
- DAG, diacylglycerol
- FFA, free fatty acid
- Flox, flox/flox
- GL/FFA, glycerolipid/ free fatty acid
- GLP1, glucagon-like peptide 1
- GPCR, G-protein coupled receptor
- GSIS, glucose stimulated insulin secretion
- HSL, hormone sensitive lipase
- Insulin secretion
- KO, knockout
- Kic, α-ketoisocaproate
- MAG, monoacylglycerol
- Monoacylglycerol
- OGTT, oral glucose tolerance test
- Pancreatic islets
- ROS, reactive oxygen species
- TG, triacylglycerol
- WT, wild type
- α/β-Hydrolase domain-6
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Affiliation(s)
| | | | | | | | | | | | | | | | | | - S.R. Murthy Madiraju
- Corresponding author. Montreal Diabetes Research Center, CRCHUM, 900 St-Denis (Viger Tower), Rm R08-414, Montreal, QC H1W 4A4, Canada. Tel.: +1 514 890 8000x23610; fax: +1 514 412 7648.
| | - Marc Prentki
- Corresponding author. Montreal Diabetes Research Center, CRCHUM, 900 St-Denis (Viger Tower), Rm R08-412, Montreal, QC H1W 4A4, Canada. Tel.: +1 514 890 8000x23642; fax: +1 514 412 7648.
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15
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Epac2 Mediates cAMP-Dependent Potentiation of Neurotransmission in the Hippocampus. J Neurosci 2015; 35:6544-53. [PMID: 25904804 DOI: 10.1523/jneurosci.0314-14.2015] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Presynaptic terminal cAMP elevation plays a central role in plasticity at the mossy fiber-CA3 synapse of the hippocampus. Prior studies have identified protein kinase A as a downstream effector of cAMP that contributes to mossy fiber LTP (MF-LTP), but the potential contribution of Epac2, another cAMP effector expressed in the MF synapse, has not been considered. We investigated the role of Epac2 in MF-CA3 neurotransmission using Epac2(-/-) mice. The deletion of Epac2 did not cause gross alterations in hippocampal neuroanatomy or basal synaptic transmission. Synaptic facilitation during short trains was not affected by loss of Epac2 activity; however, both long-term plasticity and forskolin-mediated potentiation of MFs were impaired, demonstrating that Epac2 contributes to cAMP-dependent potentiation of transmitter release. Examination of synaptic transmission during long sustained trains of activity suggested that the readily releasable pool of vesicles is reduced in Epac2(-/-) mice. These data suggest that cAMP elevation uses an Epac2-dependent pathway to promote transmitter release, and that Epac2 is required to maintain the readily releasable pool at MF synapses in the hippocampus.
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16
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Parnell E, Palmer TM, Yarwood SJ. The future of EPAC-targeted therapies: agonism versus antagonism. Trends Pharmacol Sci 2015; 36:203-14. [PMID: 25744542 PMCID: PMC4392396 DOI: 10.1016/j.tips.2015.02.003] [Citation(s) in RCA: 66] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2014] [Revised: 02/03/2015] [Accepted: 02/05/2015] [Indexed: 02/06/2023]
Abstract
Although tractable to drug development, targeting of cAMP signalling has side effects. Selectively targeting EPAC1 and EPAC2 cAMP sensor enzymes may limit some of these off-target effects. EPAC agonists could be used to treat vascular inflammation (EPAC1) or type 2 diabetes (EPAC2). EPAC1 and EPAC2 antagonists could be used to treat heart disease.
Pharmaceutical manipulation of cAMP levels exerts beneficial effects through the regulation of the exchange protein activated by cAMP (EPAC) and protein kinase A (PKA) signalling routes. Recent attention has turned to the specific regulation of EPAC isoforms (EPAC1 and EPAC2) as a more targeted approach to cAMP-based therapies. For example, EPAC2-selective agonists could promote insulin secretion from pancreatic β cells, whereas EPAC1-selective agonists may be useful in the treatment of vascular inflammation. By contrast, EPAC1 and EPAC2 antagonists could both be useful in the treatment of heart failure. Here we discuss whether the best way forward is to design EPAC-selective agonists or antagonists and the current strategies being used to develop isoform-selective, small-molecule regulators of EPAC1 and EPAC2 activity.
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Affiliation(s)
- Euan Parnell
- Institute of Molecular, Cell, and Systems Biology, College of Medical, Veterinary, and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK
| | - Timothy M Palmer
- School of Pharmacy, University of Bradford, Bradford BD7 1DP, UK
| | - Stephen J Yarwood
- Institute of Molecular, Cell, and Systems Biology, College of Medical, Veterinary, and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK.
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Pulsatile insulin secretion, impaired glucose tolerance and type 2 diabetes. Mol Aspects Med 2015; 42:61-77. [PMID: 25637831 DOI: 10.1016/j.mam.2015.01.003] [Citation(s) in RCA: 151] [Impact Index Per Article: 16.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2014] [Revised: 01/09/2015] [Accepted: 01/10/2015] [Indexed: 12/28/2022]
Abstract
Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.
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18
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Gaisano HY. Here come the newcomer granules, better late than never. Trends Endocrinol Metab 2014; 25:381-8. [PMID: 24746186 DOI: 10.1016/j.tem.2014.03.005] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/29/2013] [Revised: 03/06/2014] [Accepted: 03/14/2014] [Indexed: 01/03/2023]
Abstract
Exocytosis in pancreatic β-cells employs Munc18/soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes that mediate the priming and docking onto the plasma membrane (PM) of insulin granules, called predocked granules, that sit on the PM until Ca(2+) influx evokes fusion. This accounts for most of the initial peak secretory response. However, the subsequent sustained phase of glucose-stimulated insulin secretion arises from newcomer granules that have a minimal residence time at the PM before fusion. In this Opinion I discuss recent work that has begun to decipher the components of the exocytotic machinery of newcomer granules, including a Munc18/SNARE complex that is different from that mediating the fusion of predocked granules and which can potentially rescue defective insulin secretion in diabetes. These insights are applicable to other neuroendocrine cells that exhibit sustained secretion.
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Affiliation(s)
- Herbert Y Gaisano
- Department of Medicine, University of Toronto, M5S 1A8, Toronto, Canada.
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19
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Holz GG, Leech CA, Chepurny OG. New insights concerning the molecular basis for defective glucoregulation in soluble adenylyl cyclase knockout mice. Biochim Biophys Acta Mol Basis Dis 2014; 1842:2593-600. [PMID: 24980705 DOI: 10.1016/j.bbadis.2014.06.023] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2014] [Revised: 06/17/2014] [Accepted: 06/18/2014] [Indexed: 01/09/2023]
Abstract
Recently published findings indicate that a knockout (KO) of soluble adenylyl cyclase (sAC, also known as AC-10) gene expression in mice leads to defective glucoregulation that is characterized by reduced pancreatic insulin secretion and reduced intraperitoneal glucose tolerance. Summarized here are current concepts regarding the molecular basis for this phenotype, with special emphasis on the potential role of sAC as a determinant of glucose-stimulated insulin secretion. Highlighted is new evidence that in pancreatic beta cells, oxidative glucose metabolism stimulates mitochondrial CO₂production that in turn generates bicarbonate ion (HCO(3)(-)). Since HCO(3)(-) binds to and directly stimulates the activity of sAC, we propose that glucose-stimulated cAMP production in beta cells is mediated not simply by transmembrane adenylyl cyclases (TMACs), but also by sAC. Based on evidence that sAC is expressed in mitochondria, there exists the possibility that beta-cell glucose metabolism is linked to mitochondrial cAMP production with consequent facilitation of oxidative phosphorylation. Since sAC is also expressed in the cytoplasm, sAC catalyzed cAMP production may activate cAMP sensors such as PKA and Epac2 to control ion channel function, intracellular Ca²⁺ handling, and Ca²⁺-dependent exocytosis. Thus, we propose that the existence of sAC in beta cells provides a new and unexpected explanation for previously reported actions of glucose metabolism to stimulate cAMP production. It seems possible that alterations of sAC activity might be of importance when evaluating new strategies for the treatment of type 2 diabetes (T2DM), or when evaluating why glucose metabolism fails to stimulate insulin secretion in patients diagnosed with T2DM. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.
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Affiliation(s)
- George G Holz
- Department of Medicine, State University of New York (SUNY), Upstate Medical University, Syracuse, NY 13210, USA; Department of Pharmacology, State University of New York (SUNY), Upstate Medical University, Syracuse, NY 13210, USA.
| | - Colin A Leech
- Department of Medicine, State University of New York (SUNY), Upstate Medical University, Syracuse, NY 13210, USA
| | - Oleg G Chepurny
- Department of Medicine, State University of New York (SUNY), Upstate Medical University, Syracuse, NY 13210, USA
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20
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Rondas D, D'Hertog W, Overbergh L, Mathieu C. Glucagon-like peptide-1: modulator of β-cell dysfunction and death. Diabetes Obes Metab 2013; 15 Suppl 3:185-92. [PMID: 24003936 DOI: 10.1111/dom.12165] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/21/2013] [Accepted: 04/17/2013] [Indexed: 12/16/2022]
Abstract
Glucagon-like peptide-1 (GLP-1) is one of the hormones responsible for the incretin effect, a term that refers to the observation that orally administered glucose results in a larger increase in plasma insulin levels and insulin-dependent decrease in blood glucose concentration when compared to the same amount of glucose given intravenously. GLP-1 is secreted mainly by gut endocrine L-cells and is released under the control of carbohydrates, proteins and lipids. Upon secretion, GLP-1 targets different cell types and exerts a wide variety of actions such as potentiation of glucose-stimulated insulin secretion, reduction of appetite, delay of gastric emptying and increase in β-cell mass. These beneficial effects have resulted in the application of GLP-1-based therapies in patients with type 2 diabetes, but also exploitation of its effects in type 1 diabetes is being envisaged. In this review, we focus on the different, short- and long-term action mechanisms of GLP-1 with specific emphasis on its role as a modulator of β-cell function and survival.
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Affiliation(s)
- D Rondas
- Laboratory for Clinical and Experimental Endocrinology, 3000, Leuven, Belgium
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21
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Mourad NI, Nenquin M, Henquin JC. Amplification of insulin secretion by acetylcholine or phorbol ester is independent of β-cell microfilaments and distinct from metabolic amplification. Mol Cell Endocrinol 2013; 367:11-20. [PMID: 23246352 DOI: 10.1016/j.mce.2012.12.002] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/18/2012] [Revised: 11/23/2012] [Accepted: 12/04/2012] [Indexed: 11/30/2022]
Abstract
Insulin secretion (IS) triggered by β-cell [Ca(2+)](c) is amplified by metabolic and receptor-generated signals. Diacylglycerol largely mediates acetylcholine (ACh) effects through protein-kinase C and other effectors, which can be directly activated by phorbol-ester (PMA). Using mouse islets, we investigated the possible role of microfilaments in ACh/PMA-mediated amplification of IS. PMA had no steady-state impact on actin microfilaments. Although ACh slightly augmented and PMA diminished glucose- and tolbutamide-induced increases in β-cell [Ca(2+)](c), both amplified IS in control islets and after microfilament disruption (latrunculin) or stabilization (jasplakinolide). Both phases of IS were larger in response to glucose than tolbutamide, although [Ca(2+)](c) was lower. This difference in secretion, which reflects metabolic amplification, persisted in presence of ACh/PMA and was independent of microfilaments. Amplification of IS by ACh/PMA is thus distinct from metabolic amplification, but both pathways promote acquisition of release competence by insulin granules, which can access exocytotic sites without intervention of microfilaments.
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Affiliation(s)
- Nizar I Mourad
- Unit of Endocrinology and Metabolism, University of Louvain, Faculty of Medicine, B-1200 Brussels, Belgium
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22
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Xie L, Kang Y, Liang T, Dolai S, Xie H, Parsaud L, Lopez JA, He Y, Chidambaram S, Lam PP, James DE, Sugita S, Gaisano HY. RalA GTPase tethers insulin granules to L- and R-type calcium channels through binding α2 δ-1 subunit. Traffic 2013; 14:428-39. [PMID: 23346930 DOI: 10.1111/tra.12047] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2012] [Revised: 01/21/2013] [Accepted: 01/24/2013] [Indexed: 01/05/2023]
Abstract
RalA GTPase has been implicated in the regulated delivery of exocytotic vesicles to the plasma membrane (PM) in mammalian cells. We had reported that RalA regulates biphasic insulin secretion, which we have now determined to be contributed by RalA direct interaction with voltage-gated calcium (Cav ) channels. RalA knockdown (KD) in INS-1 cells and primary rat β-cells resulted in a reduction in Ca(2+) currents arising specifically from L-(Cav 1.2 and Cav 1.3) and R-type (Cav 2.3) Ca(2+) channels. Restoration of RalA expression in RalA KD cells rescued these defects in Ca(2+) currents. RalA co-immunoprecipitated with the Cav α2 δ-1 auxiliary subunit known to bind the three Cav s. Moreover, the functional molecular interactions between Cav α2 δ-1 and RalA on the PM shown by total internal reflection fluorescent microscopy/FRET analysis could be induced by glucose stimulation. KD of RalA inhibited trafficking of α2 δ-1 to insulin granules without affecting the localization of the other Cav subunits. Furthermore, we confirmed that RalA and α2 δ-1 functionally interact since RalA KD-induced inhibition of Cav currents could not be recovered by RalA when α2 δ-1 was simultaneously knocked down. These data provide a mechanism for RalA function in insulin secretion, whereby RalA binds α2 δ-1 on insulin granules to tether these granules to PM Ca(2+) channels. This acts as a chaperoning step prior to and in preparation for sequential assembly of exocyst and excitosome complexes that mediate biphasic insulin secretion.
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Affiliation(s)
- Li Xie
- Department of Physiology and Medicine, University of Toronto, Toronto, Ontario, M5S 1A8, Canada
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Zhu D, Koo E, Kwan E, Kang Y, Park S, Xie H, Sugita S, Gaisano HY. Syntaxin-3 regulates newcomer insulin granule exocytosis and compound fusion in pancreatic beta cells. Diabetologia 2013; 56:359-69. [PMID: 23132338 DOI: 10.1007/s00125-012-2757-0] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/10/2012] [Accepted: 10/01/2012] [Indexed: 12/23/2022]
Abstract
AIMS/HYPOTHESIS The molecular basis of the exocytosis of secretory insulin-containing granules (SGs) during biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells remains unclear. Syntaxin (SYN)-1A and SYN-4 have been shown to mediate insulin exocytosis. The insulin-secretory function of SYN-3, which is particularly abundant in SGs, is unclear. METHODS Mouse pancreatic islets and INS-1 cells were treated with adenovirus carrying Syn-3 (also known as Stx3) or small interfering RNA targeting Syn-3 in order to examine insulin secretion by radioimmunoassay. The localisation and distribution of insulin granules were examined by confocal and electron microscopy. Dynamic single-granule fusion events were assessed using total internal reflection fluorescence microscopy (TIRFM). RESULTS Depletion of endogenous SYN-3 inhibited insulin release. TIRFM showed no change in the number or fusion competence of previously docked SGs but, instead, a marked reduction in the recruitment of newcomer SGs and their subsequent exocytotic fusion during biphasic GSIS. Conversely, overexpression of Syn-3 enhanced both phases of GSIS, owing to the increase in newcomer SGs and, remarkably, to increased SG-SG fusion, which was confirmed by electron microscopy. CONCLUSIONS/INTERPRETATION In insulin secretion, SYN-3 plays a role in the mediation of newcomer SG exocytosis and SG-SG fusion that contributes to biphasic GSIS.
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Affiliation(s)
- D Zhu
- Department of Medicine, University of Toronto, Toronto, Ontario, Canada
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24
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Specific actions of GLP-1 receptor agonists and DPP4 inhibitors for the treatment of pancreatic β-cell impairments in type 2 diabetes. Cell Signal 2013; 25:570-9. [DOI: 10.1016/j.cellsig.2012.11.009] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Accepted: 11/08/2012] [Indexed: 01/09/2023]
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25
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Canepa E, Domenicotti C, Marengo B, Passalacqua M, Marinari UM, Pronzato MA, Fedele E, Ricciarelli R. Cyclic adenosine monophosphate as an endogenous modulator of the amyloid-β precursor protein metabolism. IUBMB Life 2013; 65:127-33. [PMID: 23297063 DOI: 10.1002/iub.1109] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2012] [Accepted: 10/05/2012] [Indexed: 01/23/2023]
Abstract
Besides playing a pathogenic role in Alzheimer disease, amyloid-beta peptides are normally produced in low amounts in the brain, and several lines of evidence suggest that they can modulate synaptic plasticity and memory. As cyclic adenosine monophosphate (cAMP) is known to be involved in the same processes and the blockade of its degradation by phosphodiesterase 4 inhibitors has consistently shown beneficial effects on cognition, we investigated the possible correlation between this second messenger and Aβ peptides in neuronal N2a cells overexpressing the amyloid-β precursor protein (APP). We herein report that the elevation of endogenous cAMP by rolipram increased APP protein expression and both its amyloidogenic and nonamyloidogenic processing. The effects of rolipram were reproduced by both the cAMP membrane-permeant analog 8Br-cAMP and the forskolin-induced activation of adenylyl cyclase but were not affected by the PKA inhibitor H-89. Our results demonstrate that, in neuronal cells, APP metabolism is physiologically modulated by cAMP and suggest that this might represent an additional mechanism through which the second messenger could influence memory functions.
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Affiliation(s)
- Elisa Canepa
- Department of Experimental Medicine, University of Genoa, Genoa, Italy
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26
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Thulé PM. Mechanisms of current therapies for diabetes mellitus type 2. ADVANCES IN PHYSIOLOGY EDUCATION 2012; 36:275-83. [PMID: 23209008 PMCID: PMC3776431 DOI: 10.1152/advan.00094.2012] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/29/2012] [Accepted: 09/21/2012] [Indexed: 06/12/2023]
Abstract
The array of medications available for the treatment of hyperglycemia has increased rapidly in the previous decade, and recent investigations have clarified novel mechanisms underlying the antihyperglycemic efficacy of these drugs. This article reviews the mechanisms of action for medications currently approved to treat diabetes mellitus in the United States, with the exception of insulin and its analogs. Finally, it attempts to integrate these mechanisms into the schema of pathophysiological factors that combine to produce hyperglycemia in patients with diabetes mellitus.
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Affiliation(s)
- Peter M Thulé
- Emory University School of Medicine, Decatur, Georgia, USA.
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27
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Hansen JB, Tonnesen MF, Madsen AN, Hagedorn PH, Friberg J, Grunnet LG, Heller RS, Nielsen AØ, Størling J, Baeyens L, Anker-Kitai L, Qvortrup K, Bouwens L, Efrat S, Aalund M, Andrews NC, Billestrup N, Karlsen AE, Holst B, Pociot F, Mandrup-Poulsen T. Divalent metal transporter 1 regulates iron-mediated ROS and pancreatic β cell fate in response to cytokines. Cell Metab 2012; 16:449-61. [PMID: 23000401 DOI: 10.1016/j.cmet.2012.09.001] [Citation(s) in RCA: 122] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/09/2011] [Revised: 06/27/2012] [Accepted: 08/27/2012] [Indexed: 02/08/2023]
Abstract
Reactive oxygen species (ROS) contribute to target-cell damage in inflammatory and iron-overload diseases. Little is known about iron transport regulation during inflammatory attack. Through a combination of in vitro and in vivo studies, we show that the proinflammatory cytokine IL-1β induces divalent metal transporter 1 (DMT1) expression correlating with increased β cell iron content and ROS production. Iron chelation and siRNA and genetic knockdown of DMT1 expression reduce cytokine-induced ROS formation and cell death. Glucose-stimulated insulin secretion in the absence of cytokines in Dmt1 knockout islets is defective, highlighting a physiological role of iron and ROS in the regulation of insulin secretion. Dmt1 knockout mice are protected against multiple low-dose streptozotocin and high-fat diet-induced glucose intolerance, models of type 1 and type 2 diabetes, respectively. Thus, β cells become prone to ROS-mediated inflammatory damage via aberrant cellular iron metabolism, a finding with potential general cellular implications.
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Affiliation(s)
- Jakob Bondo Hansen
- Center for Medical Research Methodology, Department of Biomedical Sciences, University of Copenhagen, 2200 Copenhagen, Denmark
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28
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Xie L, Zhu D, Gaisano HY. Role of mammalian homologue of Caenorhabditis elegans unc-13-1 (Munc13-1) in the recruitment of newcomer insulin granules in both first and second phases of glucose-stimulated insulin secretion in mouse islets. Diabetologia 2012; 55:2693-2702. [PMID: 22814762 DOI: 10.1007/s00125-012-2640-z] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/23/2011] [Accepted: 06/08/2012] [Indexed: 12/30/2022]
Abstract
AIMS/HYPOTHESIS We have previously reported that the haplodeficient Munc13-1(+/-) mouse exhibits impaired biphasic glucose-stimulated insulin secretion (GSIS), causing glucose intolerance mimicking type 2 diabetes. Glucagon-like peptide-1 (GLP-1) can bypass these insulin-secretory defects in type 2 diabetes, but the mechanism of exocytotic events mediated by GLP-1 in rescuing insulin secretion is unclear. METHODS The total internal reflection fluorescence microscopy (TIRFM) technique was used to examine single insulin granule fusion events in mouse islet beta cells. RESULTS There was no difference in the density of docked granules in the resting state between Munc13-1(+/+) and Munc13-1(+/-) mouse islet beta cells. While exocytosis of previously docked granules in Munc13-1(+/-) beta cells is reduced during high-K(+) stimulation as expected, we now find a reduction in additional exocytosis events that account for the major portion of GSIS, namely two types of newcomer granules, one which has a short docking time (short-dock) and another undergoing no docking before exocytosis (no-dock). As mammalian homologue of Caenorhabditis elegans unc-13-1 (Munc13-1) is a phorbol ester substrate, phorbol ester could partially rescue biphasic GSIS in Munc13-1-deficient beta cells by enhancing recruitment of short-dock newcomer granules for exocytosis. The more effective rescue of biphasic GSIS by GLP-1 than by phorbol was due to increased recruitment of both short-dock and no-dock newcomer granules. CONCLUSIONS/INTERPRETATION Phorbol ester and GLP-1 potentiation of biphasic GSIS are brought about by recruitment of distinct populations of newcomer granules for exocytosis, which may be mediated by Munc13-1 interaction with syntaxin-SNARE complexes other than that formed by syntaxin-1A.
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Affiliation(s)
- L Xie
- Departments of Medicine & Physiology, University of Toronto, Room 7368, Medical Sciences Building, 1 King's College Circle, Toronto, ON, M5S 1A8, Canada
| | - D Zhu
- Departments of Medicine & Physiology, University of Toronto, Room 7368, Medical Sciences Building, 1 King's College Circle, Toronto, ON, M5S 1A8, Canada
| | - H Y Gaisano
- Departments of Medicine & Physiology, University of Toronto, Room 7368, Medical Sciences Building, 1 King's College Circle, Toronto, ON, M5S 1A8, Canada.
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29
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Kasai H, Takahashi N, Tokumaru H. Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis. Physiol Rev 2012; 92:1915-64. [DOI: 10.1152/physrev.00007.2012] [Citation(s) in RCA: 120] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.
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Affiliation(s)
- Haruo Kasai
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Noriko Takahashi
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Hiroshi Tokumaru
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
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30
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Zhu D, Zhang Y, Lam PPL, Dolai S, Liu Y, Cai EP, Choi D, Schroer SA, Kang Y, Allister EM, Qin T, Wheeler MB, Wang CC, Hong WJ, Woo M, Gaisano HY. Dual role of VAMP8 in regulating insulin exocytosis and islet β cell growth. Cell Metab 2012; 16:238-49. [PMID: 22841572 DOI: 10.1016/j.cmet.2012.07.001] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/02/2011] [Revised: 04/17/2012] [Accepted: 06/26/2012] [Indexed: 12/22/2022]
Abstract
Optimal insulin secretion required to maintain glucose homeostasis is the summation of total pancreatic islet β cell mass and intrinsic secretory capacity of individual β cells, which are regulated by distinct mechanisms that could be amplified by glucagon-like-peptide-1 (GLP-1). Because of these actions of GLP-1 on islet β cells, GLP-1 has been deployed to treat diabetes. We employed SNARE protein VAMP8-null mice to demonstrate that VAMP8 mediates insulin granule recruitment to the plasma membrane, which partly accounts for GLP-1 potentiation of glucose-stimulated insulin secretion. VAMP8-null mice also exhibited increased islet β cell mass from increased β cell mitosis, with β cell proliferative activity greatly amplified by GLP-1. Thus, despite the β cell exocytotic defect, VAMP8-null mice have an increased total insulin secretory capacity, which improved glucose homeostasis. We conclude that these VAMP8-mediated events partly underlie the therapeutic actions of GLP-1 on insulin secretion and β cell growth.
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Affiliation(s)
- Dan Zhu
- Department of Medicine, University of Toronto, Toronto, ON M5S 1A8, Canada
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31
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Wu ZY, Zhu LJ, Zou N, Bombek LK, Shao CY, Wang N, Wang XX, Liang L, Xia J, Rupnik M, Shen Y. AMPA receptors regulate exocytosis and insulin release in pancreatic β cells. Traffic 2012; 13:1124-39. [PMID: 22540213 DOI: 10.1111/j.1600-0854.2012.01373.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2011] [Revised: 04/25/2012] [Accepted: 04/27/2012] [Indexed: 01/06/2023]
Abstract
Ionotropic glutamate receptors (iGluRs) are expressed in islets and insulinoma cells and involved in insulin secretion. However, the exact roles that iGluRs play in β cells remain unclear. Here, we demonstrated that GluR2-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) were expressed in mouse β cells. Glutamate application increased both cytosolic calcium and the number of docked insulin-containing granules, which resulted in augmentation of depolarization-induced exocytosis and high-glucose-stimulated insulin release. While glutamate application directly depolarized β cells, it also induced an enormous depolarization when K(ATP) channels were available. Glutamate application reduced the conductance of K(ATP) channels and increased voltage oscillations. Moreover, actions of AMPARs were absent in Kir6.2 knock-out mice. The effects of AMPARs on K(ATP) channels were mediated by cytosolic cGMP. Taken together, our experiments uncovered a novel mechanism by which AMPARs participate in insulin release.
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Affiliation(s)
- Zhen-Yong Wu
- Department of Neurobiology, Key Laboratory of Medical Neurobiology of Ministry of Health, Zhejiang Province Key Laboratory of Neurobiology, Zhejiang University School of Medicine, Hangzhou, Zhejiang, 310058, P. R. China
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32
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Gustavsson N, Wu B, Han W. Calcium sensing in exocytosis. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2012; 740:731-57. [PMID: 22453967 DOI: 10.1007/978-94-007-2888-2_32] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.
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Affiliation(s)
- Natalia Gustavsson
- Laboratory of Metabolic Medicine, Singapore Bioimaging Consortium, Agency for Science, Technology and Research, Singapore.
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Lamont BJ, Li Y, Kwan E, Brown TJ, Gaisano H, Drucker DJ. Pancreatic GLP-1 receptor activation is sufficient for incretin control of glucose metabolism in mice. J Clin Invest 2011; 122:388-402. [PMID: 22182839 DOI: 10.1172/jci42497] [Citation(s) in RCA: 129] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2011] [Accepted: 11/02/2011] [Indexed: 12/11/2022] Open
Abstract
Glucagon-like peptide-1 (GLP-1) circulates at low levels and acts as an incretin hormone, potentiating glucose-dependent insulin secretion from islet β cells. GLP-1 also modulates gastric emptying and engages neural circuits in the portal region and CNS that contribute to GLP-1 receptor-dependent (GLP-1R-dependent) regulation of glucose homeostasis. To elucidate the importance of pancreatic GLP-1R signaling for glucose homeostasis, we generated transgenic mice that expressed the human GLP-1R in islets and pancreatic ductal cells (Pdx1-hGLP1R:Glp1r-/- mice). Transgene expression restored GLP-1R-dependent stimulation of cAMP and Akt phosphorylation in isolated islets, conferred GLP-1R-dependent stimulation of β cell proliferation, and was sufficient for restoration of GLP-1-stimulated insulin secretion in perifused islets. Systemic GLP-1R activation with the GLP-1R agonist exendin-4 had no effect on food intake, hindbrain c-fos expression, or gastric emptying but improved glucose tolerance and stimulated insulin secretion in Pdx1-hGLP1R:Glp1r-/- mice. i.c.v. GLP-1R blockade with the antagonist exendin(9-39) impaired glucose tolerance in WT mice but had no effect in Pdx1-hGLP1R:Glp1r-/- mice. Nevertheless, transgenic expression of the pancreatic GLP-1R was sufficient to normalize both oral and i.p. glucose tolerance in Glp1r-/- mice. These findings illustrate that low levels of endogenous GLP-1 secreted from gut endocrine cells are capable of augmenting glucoregulatory activity via pancreatic GLP-1Rs independent of communication with neural pathways.
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Affiliation(s)
- Benjamin J Lamont
- Department of Medicine, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, University of Toronto, Toronto, Ontario, Canada
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Leech CA, Dzhura I, Chepurny OG, Kang G, Schwede F, Genieser HG, Holz GG. Molecular physiology of glucagon-like peptide-1 insulin secretagogue action in pancreatic β cells. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2011; 107:236-47. [PMID: 21782840 DOI: 10.1016/j.pbiomolbio.2011.07.005] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/01/2011] [Accepted: 07/08/2011] [Indexed: 10/18/2022]
Abstract
Insulin secretion from pancreatic β cells is stimulated by glucagon-like peptide-1 (GLP-1), a blood glucose-lowering hormone that is released from enteroendocrine L cells of the distal intestine after the ingestion of a meal. GLP-1 mimetics (e.g., Byetta) and GLP-1 analogs (e.g., Victoza) activate the β cell GLP-1 receptor (GLP-1R), and these compounds stimulate insulin secretion while also lowering levels of blood glucose in patients diagnosed with type 2 diabetes mellitus (T2DM). An additional option for the treatment of T2DM involves the administration of dipeptidyl peptidase-IV (DPP-IV) inhibitors (e.g., Januvia, Galvus). These compounds slow metabolic degradation of intestinally released GLP-1, thereby raising post-prandial levels of circulating GLP-1 substantially. Investigational compounds that stimulate GLP-1 secretion also exist, and in this regard a noteworthy advance is the demonstration that small molecule GPR119 agonists (e.g., AR231453) stimulate L cell GLP-1 secretion while also directly stimulating β cell insulin release. In this review, we summarize what is currently known concerning the signal transduction properties of the β cell GLP-1R as they relate to insulin secretion. Emphasized are the cyclic AMP, protein kinase A, and Epac2-mediated actions of GLP-1 to regulate ATP-sensitive K⁺ channels, voltage-dependent K⁺ channels, TRPM2 cation channels, intracellular Ca⁺ release channels, and Ca⁺-dependent exocytosis. We also discuss new evidence that provides a conceptual framework with which to understand why GLP-1R agonists are less likely to induce hypoglycemia when they are administered for the treatment of T2DM.
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Affiliation(s)
- Colin A Leech
- Department of Medicine, State University of New York Upstate Medical University, Syracuse, NY, USA.
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Activation of the GLP-1 receptor signalling pathway: a relevant strategy to repair a deficient beta-cell mass. EXPERIMENTAL DIABETES RESEARCH 2011; 2011:376509. [PMID: 21716694 PMCID: PMC3118608 DOI: 10.1155/2011/376509] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/30/2010] [Accepted: 02/25/2011] [Indexed: 12/18/2022]
Abstract
Recent preclinical studies in rodent models of diabetes suggest that exogenous GLP-1R agonists and DPP-4 inhibitors have the ability to increase islet mass and preserve beta-cell function, by immediate reactivation of beta-cell glucose competence, as well as enhanced beta-cell proliferation and neogenesis and promotion of beta-cell survival. These effects have tremendous implication in the treatment of T2D because they directly address one of the basic defects in T2D, that is, beta-cell failure. In human diabetes, however, evidence that the GLP-1-based drugs alter the course of beta-cell function remains to be found. Several questions surrounding the risks and benefits of GLP-1-based therapy for the diabetic beta-cell mass are discussed in this review and require further investigation.
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Dzhura I, Chepurny OG, Leech CA, Roe MW, Dzhura E, Xu X, Lu Y, Schwede F, Genieser HG, Smrcka AV, Holz GG. Phospholipase C-ε links Epac2 activation to the potentiation of glucose-stimulated insulin secretion from mouse islets of Langerhans. Islets 2011; 3:121-8. [PMID: 21478675 PMCID: PMC3116928 DOI: 10.4161/isl.3.3.15507] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is potentiated by cAMP-elevating agents, such as the incretin hormone glucagon-like peptide-1 (GLP-1), and cAMP exerts its insulin secretagogue action by activating both protein kinase A (PKA) and the cAMP-regulated guanine nucleotide exchange factor designated as Epac2. Although prior studies of mouse islets demonstrated that Epac2 acts via Rap1 GTPase to potentiate GSIS, it is not understood which downstream targets of Rap1 promote the exocytosis of insulin. Here, we measured insulin secretion stimulated by a cAMP analog that is a selective activator of Epac proteins in order to demonstrate that a Rap1-regulated phospholipase C-epsilon (PLC-ε) links Epac2 activation to the potentiation of GSIS. Our analysis demonstrates that the Epac activator 8-pCPT-2'-O-Me-cAMP-AM potentiates GSIS from the islets of wild-type (WT) mice, whereas it has a greatly reduced insulin secretagogue action in the islets of Epac2 (-/-) and PLC-ε (-/-) knockout (KO) mice. Importantly, the insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM in WT mouse islets cannot be explained by an unexpected action of this cAMP analog to activate PKA, as verified through the use of a FRET-based A-kinase activity reporter (AKAR3) that reports PKA activation. Since the KO of PLC-ε disrupts the ability of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS, while also disrupting its ability to stimulate an increase of β-cell [Ca2+]i, the available evidence indicates that it is a Rap1-regulated PLC-ε that links Epac2 activation to Ca2+-dependent exocytosis of insulin.
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Affiliation(s)
- Igor Dzhura
- Department of Medicine, State University of New York, Upstate Medical University, Syracuse, NY, USA
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Skelin M, Rupnik M. cAMP increases the sensitivity of exocytosis to Ca²+ primarily through protein kinase A in mouse pancreatic beta cells. Cell Calcium 2011; 49:89-99. [PMID: 21242000 DOI: 10.1016/j.ceca.2010.12.005] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2010] [Revised: 12/20/2010] [Accepted: 12/21/2010] [Indexed: 11/19/2022]
Abstract
Cyclic AMP regulates the late step of Ca²+-dependent exocytosis in many secretory cells through two major mechanisms: a protein kinase A-dependent and a cAMP-GEF/Epac-dependent pathway. We designed a protocol to characterize the role of these two cAMP-dependent pathways on the Ca²+ sensitivity and kinetics of regulated exocytosis in mouse pancreatic beta cells, using a whole-cell patch-clamp based capacitance measurements. A train of depolarizing pulses or slow photo-release of caged Ca²+ were stimuli for the exocytotic activity. In controls, due to exocytosis after slow photo-release, the C(m) change had typically two phases. We observed that the Ca²+-dependency of the rate of the first C(m) change follows saturation kinetics with high cooperativity and half-maximal rate at 2.9±0.2 μM. The intracellular depletion of cAMP did not change amp1, while rate1 and amp2 were strongly reduced. This manipulation pushed the Ca²+-dependency of the exocytotic burst to significantly lower [Ca²+](i). To address the question of which of the cAMP-dependent mechanisms regulates the observed shifts in Ca²+ dependency we included regulators of PKA and Epac2 activity in the pipette solution. PKA activation with 100 μM 6-Phe-cAMP or inhibition with 500 μM Rp-cAMPs in beta cells significantly shifted the EC(50) in the opposite directions. Specific activation of Epac2 did not change Ca²+ sensitivity. Our findings suggest that cAMP modulates Ca²+-dependent exocytosis in mouse beta cells mainly through a PKA-dependent mechanism by sensitizing the insulin releasing machinery to [Ca²+](i); Epac2 may contribute to enhance the rates of secretory vesicle fusion.
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Affiliation(s)
- Maša Skelin
- Faculty of Medicine University of Maribor, Slomskov trg 15, 2000 Maribor, Slovenia
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Abstract
Plasmalemmal repair is necessary for survival of damaged eukaryotic cells. Ca(2+) influx through plasmalemmal disruptions activates calpain, vesicle accumulation at lesion sites, and membrane fusion proteins; Ca(2+) influx also initiates competing apoptotic pathways. Using the formation of a dye barrier (seal) to assess plasmalemmal repair, we now report that B104 hippocampal cells with neurites transected nearer (<50 μm) to the soma seal at a lower frequency and slower rate compared to cells with neurites transected farther (>50 μm) from the soma. Analogs of cAMP, including protein kinase A (PKA)-specific and Epac-specific cAMP, each increase the frequency and rate of sealing and can even initiate sealing in the absence of Ca(2+) influx at both transection distances. Furthermore, Epac activates a cAMP-dependent, PKA-independent, pathway involved in plasmalemmal sealing. The frequency and rate of plasmalemmal sealing are decreased by a small molecule inhibitor of PKA targeted to its catalytic subunit (KT5720), a peptide inhibitor targeted to its regulatory subunits (PKI), an inhibitor of a novel PKC (an nPKCη pseudosubstrate fragment), and an antioxidant (melatonin). Given these and other data, we propose a model for redundant parallel pathways of Ca(2+)-dependent plasmalemmal sealing of injured neurons mediated in part by nPKCs, cytosolic oxidation, and cAMP activation of PKA and Epac. We also propose that the evolutionary origin of these pathways and substances was to repair plasmalemmal damage in eukaryotic cells. Greater understanding of vesicle interactions, proteins, and pathways involved in plasmalemmal sealing should suggest novel neuroprotective treatments for traumatic nerve injuries and neurodegenerative disorders.
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Dzhura I, Chepurny OG, Kelley GG, Leech CA, Roe MW, Dzhura E, Afshari P, Malik S, Rindler MJ, Xu X, Lu Y, Smrcka AV, Holz GG. Epac2-dependent mobilization of intracellular Ca²+ by glucagon-like peptide-1 receptor agonist exendin-4 is disrupted in β-cells of phospholipase C-ε knockout mice. J Physiol 2010; 588:4871-89. [PMID: 21041529 DOI: 10.1113/jphysiol.2010.198424] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Calcium can be mobilized in pancreatic β-cells via a mechanism of Ca(2+)-induced Ca(2+) release (CICR), and cAMP-elevating agents such as exendin-4 facilitate CICR in β-cells by activating both protein kinase A and Epac2. Here we provide the first report that a novel phosphoinositide-specific phospholipase C- (PLC-) is expressed in the islets of Langerhans, and that the knockout (KO) of PLC- gene expression in mice disrupts the action of exendin-4 to facilitate CICR in the β-cells of these mice. Thus, in the present study, in which wild-type (WT) C57BL/6 mouse β-cells were loaded with the photolabile Ca(2+) chelator NP-EGTA, the UV flash photolysis-catalysed uncaging of Ca(2+) generated CICR in only 9% of the β-cells tested, whereas CICR was generated in 82% of the β-cells pretreated with exendin-4. This action of exendin-4 to facilitate CICR was reproduced by cAMP analogues that activate protein kinase A (6-Bnz-cAMP-AM) or Epac2 (8-pCPT-2'-O-Me-cAMP-AM) selectively. However, in β-cells of PLC- KO mice, and also Epac2 KO mice, these test substances exhibited differential efficacies in the CICR assay such that exendin-4 was partly effective, 6-Bnz-cAMP-AM was fully effective, and 8-pCPT-2'-O-Me-cAMP-AM was without significant effect. Importantly, transduction of PLC- KO β-cells with recombinant PLC- rescued the action of 8-pCPT-2'-O-Me-cAMP-AM to facilitate CICR, whereas a K2150E PLC- with a mutated Ras association (RA) domain, or a H1640L PLC- that is catalytically dead, were both ineffective. Since 8-pCPT-2'-O-Me-cAMP-AM failed to facilitate CICR in WT β-cells transduced with a GTPase activating protein (RapGAP) that downregulates Rap activity, the available evidence indicates that a signal transduction 'module' comprised of Epac2, Rap and PLC- exists in β-cells, and that the activities of Epac2 and PLC- are key determinants of CICR in this cell type.
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Affiliation(s)
- Igor Dzhura
- Department of Medicine, State University of New York Upstate Medical University, Syracuse, NY, USA
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40
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Zhao Y, Fang Q, Straub SG, Lindau M, Sharp GWG. Noradrenaline inhibits exocytosis via the G protein βγ subunit and refilling of the readily releasable granule pool via the α(i1/2) subunit. J Physiol 2010; 588:3485-98. [PMID: 20643776 DOI: 10.1113/jphysiol.2010.190090] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The molecular mechanisms responsible for the 'distal' effect by which noradrenaline (NA) blocks exocytosis in the β-cell were examined by whole-cell and cell-attached patch clamp capacitance measurements in INS 832/13 β-cells. NA inhibited Ca(2+)-evoked exocytosis by reducing the number of exocytotic events, without modifying vesicle size. Fusion pore properties also were unaffected. NA-induced inhibition of exocytosis was abolished by a high level of Ca(2+) influx, by intracellular application of antibodies against the G protein subunit Gβ and was mimicked by the myristoylated βγ-binding/activating peptide mSIRK. NA-induced inhibition was also abolished by treatment with BoNT/A, which cleaves the C-terminal nine amino acids of SNAP-25, and also by a SNAP-25 C-terminal-blocking peptide containing the BoNT/A cleavage site. These data indicate that inhibition of exocytosis by NA is downstream of increased [Ca(2+)](i) and is mediated by an interaction between Gβγ and the C-terminus of SNAP-25, as is the case for inhibition of neurotransmitter release. Remarkably, in the course of this work, a novel effect of NA was discovered. NA induced a marked retardation of the rate of refilling of the readily releasable pool (RRP) of secretory granules. This retardation was specifically abolished by a Gα(i1/2) blocking peptide demonstrating that the effect is mediated via activation of Gα(i1) and/or Gα(i2).
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Affiliation(s)
- Ying Zhao
- Department of Molecular Medicine, Cornell University, Ithaca, NY 14853-6401, USA
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Cyclic AMP signaling in pancreatic islets. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2010; 654:281-304. [PMID: 20217503 DOI: 10.1007/978-90-481-3271-3_13] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.
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Shao W, Yu Z, Fantus IG, Jin T. Cyclic AMP signaling stimulates proteasome degradation of thioredoxin interacting protein (TxNIP) in pancreatic beta-cells. Cell Signal 2010; 22:1240-6. [PMID: 20385228 DOI: 10.1016/j.cellsig.2010.04.001] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2010] [Accepted: 04/05/2010] [Indexed: 01/03/2023]
Abstract
Thioredoxin interacting protein (TxNIP) functions as an effector of glucotoxicity in pancreatic beta-cells. Exendin-4 (Ex-4), a long-term effective GLP-1 receptor agonist, reduces TxNIP level in pancreatic beta-cells. Mechanisms underlying this reduction, however, remain largely unknown. We show here that Ex-4, 8-bromo-cAMP, the cAMP promoting agent forskolin, as well as activators of protein kinase A (PKA) and exchange protein activated by cAMP (Epac), all attenuated the effect of high glucose (20mM) on TxNIP level in the pancreatic beta-cell line Ins-1. Forskolin and Ex-4 also reduced TxNIP level in cultured primary rat islets. This repressive effect is at least partially mediated via stimulating proteasome-dependent TxNIP degradation, since the proteasomal inhibitor MG132, but not the lysosomal inhibitor chloroquine, significantly blocked the repressive effect of forskolin. Furthermore, forskolin enhanced TxNIP ubiquitination. Both PKA inhibition and Epac inhibition partially blocked the repressive effect of forskolin on TxNIP level. In addition, forskolin and Ex-4 protected Ins-1 cells from high glucose-induced apoptotic activity, assessed by measuring caspase 3 activity. Finally, knockdown of TxNIP expression led to reduced caspase 3 expression levels and blunted response to forskolin treatment. We suggest that proteasome-dependent TxNIP degradation is a novel mechanism by which Ex-4-cAMP signaling protects pancreatic beta cells.
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Affiliation(s)
- Weijuan Shao
- Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network, Canada; Banting and Best Diabetes Centre, Faculty of Medicine, University of Toronto, Canada; Dept of Medicine, University of Toronto, Canada
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Chepurny OG, Kelley GG, Dzhura I, Leech CA, Roe MW, Dzhura E, Li X, Schwede F, Genieser HG, Holz GG. PKA-dependent potentiation of glucose-stimulated insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM in human islets of Langerhans. Am J Physiol Endocrinol Metab 2010; 298:E622-33. [PMID: 20009023 PMCID: PMC2838523 DOI: 10.1152/ajpendo.00630.2009] [Citation(s) in RCA: 62] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Potential insulin secretagogue properties of an acetoxymethyl ester of a cAMP analog (8-pCPT-2'-O-Me-cAMP-AM) that activates the guanine nucleotide exchange factors Epac1 and Epac2 were assessed using isolated human islets of Langerhans. RT-QPCR demonstrated that the predominant variant of Epac expressed in human islets was Epac2, although Epac1 was detectable. Under conditions of islet perifusion, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) potentiated first- and second-phase 10 mM glucose-stimulated insulin secretion (GSIS) while failing to influence insulin secretion measured in the presence of 3 mM glucose. The insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM was associated with depolarization and an increase of [Ca(2+)](i) that reflected both Ca(2+) influx and intracellular Ca(2+) mobilization in islet beta-cells. As expected for an Epac-selective cAMP analog, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) failed to stimulate phosphorylation of PKA substrates CREB and Kemptide in human islets. Furthermore, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) had no significant ability to activate AKAR3, a PKA-regulated biosensor expressed in human islet cells by viral transduction. Unexpectedly, treatment of human islets with an inhibitor of PKA activity (H-89) or treatment with a cAMP antagonist that blocks PKA activation (Rp-8-CPT-cAMPS) nearly abolished the action of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS. It is concluded that there exists a permissive role for PKA activity in support of human islet insulin secretion that is both glucose dependent and Epac regulated. This permissive action of PKA may be operative at the insulin secretory granule recruitment, priming, and/or postpriming steps of Ca(2+)-dependent exocytosis.
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Affiliation(s)
- Oleg G Chepurny
- Departments of Medicine, State University of New York, Syracuse, 13210, USA
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44
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Kasai H, Hatakeyama H, Ohno M, Takahashi N. Exocytosis in islet beta-cells. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2010; 654:305-38. [PMID: 20217504 DOI: 10.1007/978-90-481-3271-3_14] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The development of technologies that allow for live optical imaging of exocytosis from beta-cells has greatly improved our understanding of insulin secretion. Two-photon imaging, in particular, has enabled researchers to visualize the exocytosis of large dense-core vesicles (LDCVs) containing insulin from beta-cells in intact islets of Langerhans. These studies have revealed that high glucose levels induce two phases of insulin secretion and that this release is dependent upon cytosolic Ca(2+) and cAMP. This technology has also made it possible to examine the spatial profile of insulin exocytosis in these tissues and compare that profile with those of other secretory glands. Such studies have led to the discovery of the massive exocytosis of synaptic-like microvesicles (SLMVs) in beta-cells. These imaging studies have also helped clarify facets of insulin exocytosis that cannot be properly addressed using the currently available electrophysiological techniques. This chapter provides a concise introduction to the field of optical imaging for those researchers who wish to characterize exocytosis from beta-cells in the islets of Langerhans.
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Affiliation(s)
- Haruo Kasai
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, The University of Tokyo, Hongo, Tokyo 113-0033, Japan.
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45
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Kelley GG, Chepurny OG, Schwede F, Genieser HG, Leech CA, Roe MW, Li X, Dzhura I, Dzhura E, Afshari P, Holz GG. Glucose-dependent potentiation of mouse islet insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM. Islets 2009; 1:260-5. [PMID: 21099281 PMCID: PMC2859731 DOI: 10.4161/isl.1.3.9645] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Epac2 is a cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF) that is proposed to mediate stimulatory actions of the second messenger cAMP on mouse islet insulin secretion. Here we have used methods of islet perifusion to demonstrate that the acetoxymethyl ester (AM-ester) of an Epac-selective cAMP analog (ESCA) penetrates into mouse islets and is capable of potentiating both first and second phases of glucose-stimulated insulin secretion (GSIS). When used at low concentrations (1-10 μM), 8-pCPT-2'-O-Me-cAMP-AM activates Rap1 GTPase but exhibits little or no ability to activate protein kinase A (PKA), as validated in assays of in vitro PKA activity (phosphorylation of Kemptide), Ser (133) CREB phosphorylation status, RIP1-CRE-Luc reporter gene activity, and PKA-dependent AKAR3 biosensor activation. Since quantitative PCR demonstrates Epac2 mRNA to be expressed at levels ca. 5.3-fold greater than that of Epac1, available evidence indicates that Epac2 does in fact mediate stimulatory actions of cAMP on mouse islet GSIS.
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Affiliation(s)
- Grant G. Kelley
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | - Oleg G. Chepurny
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | | | | | - Colin A. Leech
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | - Michael W. Roe
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
- Department of Cell and Developmental Biology; State University of New York Upstate Medical University; Syracuse, NY USA
| | - Xiangquan Li
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | - Igor Dzhura
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | - Elvira Dzhura
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
| | - Parisa Afshari
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
- Department of Pharmacology, State University of New York Upstate Medical University; Syracuse, NY USA
| | - George G. Holz
- Department of Medicine, State University of New York Upstate Medical University; Syracuse, NY USA
- Department of Pharmacology, State University of New York Upstate Medical University; Syracuse, NY USA
- Correspondence to: George G. Holz;
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Yu Z, Jin T. New insights into the role of cAMP in the production and function of the incretin hormone glucagon-like peptide-1 (GLP-1). Cell Signal 2009; 22:1-8. [PMID: 19772917 DOI: 10.1016/j.cellsig.2009.09.032] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2009] [Revised: 09/10/2009] [Accepted: 09/13/2009] [Indexed: 12/25/2022]
Abstract
The proglucagon gene (gcg) encodes both glucagon and glucagon-like peptide-1 (GLP-1), produced in pancreatic alpha cells and intestinal endocrine L cells, respectively. The incretin hormone GLP-1 stimulates insulin secretion and pro-insulin gene transcription. GLP-1 also enhances pancreatic beta-cell proliferation, inhibits cell apoptosis, and has been utilized in the trans-differentiation of insulin producing cells. A long-term effective GLP-1 receptor agonist, Byetta, has now been developed as the drug in treating type II diabetes and potentially other metabolic disorders. The expression of gcg and the production of GLP-1 can be activated by the elevation of the second messenger cyclic AMP (cAMP). Recent studies suggest that in addition to protein kinase A (PKA), exchange protein activated by cAMP (Epac), another effector of cAMP, and the crosstalk between PKA and the Wnt signaling pathway, are involved in cAMP-stimulated gcg transcription and GLP-1 production as well. Finally, functions of GLP-1 in pancreatic beta cells are also mediated by PKA, Epac, as well as the effector of the Wnt signaling pathway. Together, these novel findings bring us a new insight into the role of cAMP in the production and function of the incretin hormone GLP-1.
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Affiliation(s)
- Zhiwen Yu
- Banting and Best Diabetes Centre, University of Toronto, Canada
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Vikman J, Svensson H, Huang YC, Kang Y, Andersson SA, Gaisano HY, Eliasson L. Truncation of SNAP-25 reduces the stimulatory action of cAMP on rapid exocytosis in insulin-secreting cells. Am J Physiol Endocrinol Metab 2009; 297:E452-61. [PMID: 19509185 DOI: 10.1152/ajpendo.90585.2008] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Synaptosomal protein of 25 kDa (SNAP-25) is important for Ca(2+)-dependent fusion of large dense core vesicles (LDCVs) in insulin-secreting cells. Exocytosis is further enhanced by cAMP-increasing agents such as glucagon-like peptide-1 (GLP-1), and this augmentation includes interaction with both PKA and cAMP-GEFII. To investigate the coupling between SNAP-25- and cAMP-dependent stimulation of insulin exocytosis, we have used capacitance measurements, protein-binding assays, and Western blot analysis. In insulin-secreting INS-1 cells overexpressing wild-type SNAP-25 (SNAP-25(WT)), rapid exocytosis was stimulated more than threefold by cAMP, similar to the situation in nontransfected cells. However, cAMP failed to potentiate rapid exocytosis in INS-1 cells overexpressing a truncated form of SNAP-25 (SNAP-25(1-197)) or Botulinum neurotoxin A (BoNT/A). Close dissection of the exocytotic response revealed that the inability of cAMP to stimulate exocytosis in the presence of a truncated SNAP-25 was confined to the release of primed LDCVs within the readily releasable pool, especially from the immediately releasable pool, whereas cAMP enhanced mobilization of granules from the reserve pool in both SNAP-25(1-197) (P < 0.01) and SNAP-25(WT) (P < 0.05) cells. This was supported by hormone release measurements. Augmentation of the immediately releasable pool by cAMP has been suggested to act through the cAMP-GEFII-dependent, PKA-independent pathway. Indeed, we were able to verify an interaction between SNAP-25 with both cAMP-GEFII and RIM2, two proteins involved in the PKA-independent pathway. Thus we hypothesize that SNAP-25 is a necessary partner in the complex mediating cAMP-enhanced rapid exocytosis in insulin-secreting cells.
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Affiliation(s)
- Jenny Vikman
- Department of Clinical Sciences Lund, Biomedical Center, Lund University Diabetes Centre, Lund, Sweden
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Branham MT, Bustos MA, De Blas GA, Rehmann H, Zarelli VEP, Treviño CL, Darszon A, Mayorga LS, Tomes CN. Epac activates the small G proteins Rap1 and Rab3A to achieve exocytosis. J Biol Chem 2009; 284:24825-39. [PMID: 19546222 DOI: 10.1074/jbc.m109.015362] [Citation(s) in RCA: 77] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Exocytosis of the acrosome (the acrosome reaction) relies on cAMP production, assembly of a proteinaceous fusion machinery, calcium influx from the extracellular medium, and mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Addition of cAMP to human sperm suspensions bypasses some of these requirements and elicits exocytosis in a protein kinase A- and extracellular calcium-independent manner. The relevant cAMP target is Epac, a guanine nucleotide exchange factor for the small GTPase Rap. We show here that a soluble adenylyl cyclase synthesizes the cAMP required for the acrosome reaction. Epac stimulates the exchange of GDP for GTP on Rap1, upstream of a phospholipase C. The Epac-selective cAMP analogue 8-pCPT-2'-O-Me-cAMP induces a phospholipase C-dependent calcium mobilization in human sperm suspensions. In addition, our studies identify a novel connection between cAMP and Rab3A, a secretory granule-associated protein, revealing that the latter functions downstream of soluble adenylyl cyclase/cAMP/Epac but not of Rap1. Challenging sperm with calcium or 8-pCPT-2'-O-Me-cAMP boosts the exchange of GDP for GTP on Rab3A. Recombinant Epac does not release GDP from Rab3A in vitro, suggesting that the Rab3A-GEF activation by cAMP/Epac in vivo is indirect. We propose that Epac sits at a critical point during the exocytotic cascade after which the pathway splits into two limbs, one that assembles the fusion machinery into place and another that elicits intracellular calcium release.
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Affiliation(s)
- María T Branham
- Laboratorio de Biología Celular y Molecular, Instituto de Histología y Embriología-Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Médicas, CC 56, Universidad Nacional de Cuyo, 5500 Mendoza, Argentina
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Wang Z, Thurmond DC. Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins. J Cell Sci 2009; 122:893-903. [PMID: 19295123 DOI: 10.1242/jcs.034355] [Citation(s) in RCA: 268] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
The release of insulin from pancreatic islets requires negative regulation to ensure low levels of insulin release under resting conditions, as well as positive regulation to facilitate robust responsiveness to conditions of elevated fuel or glucose. The first phase of release involves the plasma-membrane fusion of a small pool of granules, termed the readily releasable pool; these granules are already at the membrane under basal conditions, and discharge their cargo in response to nutrient and also non-nutrient secretagogues. By contrast, second-phase secretion is evoked exclusively by nutrients, and involves the mobilization of intracellular granules to t-SNARE sites at the plasma membrane to enable the distal docking and fusion steps of insulin exocytosis. Nearly 40 years ago, the actin cytoskeleton was first recognized as a key mediator of biphasic insulin release, and was originally presumed to act as a barrier to block granule docking at the cell periphery. More recently, however, the discovery of cycling GTPases that are involved in F-actin reorganization in the islet beta-cell, combined with the availability of reagents that are more specific and tools with which to study the mechanisms that underlie granule movement, have contributed greatly to our understanding of the role of the cytoskeleton in regulating biphasic insulin secretion. Herein, we provide historical perspective and review recent progress that has been made towards integrating cytoskeletal reorganization and cycling of small Rho-, Rab- and Ras-family GTPases into our current models of stimulus-secretion coupling and second-phase insulin release.
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Affiliation(s)
- Zhanxiang Wang
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA
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Zhang Y, Kang YH, Chang N, Lam PPL, Liu Y, Olkkonen VM, Gaisano HY. Cab45b, a Munc18b-interacting partner, regulates exocytosis in pancreatic beta-cells. J Biol Chem 2009; 284:20840-7. [PMID: 19487699 DOI: 10.1074/jbc.m109.017467] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cab45b is a cytosolic Ca(2+)-binding protein reported to regulate zymogen secretion in pancreatic acini. We now show that Cab45b is also expressed in pancreatic islet beta-cells and interacts there with the Sec1-Munc18 protein Munc18b. We employed patch clamp cell capacitance measurements to show that antibodies against Cab45b inhibited depolarization-evoked membrane capacitance increments, suggesting an impact on beta-cell granule exocytosis, both the readily releasable granule pool and refilling of this pool. Site-specific mutants in the Cab45b EF-hands were used to dissect the molecular interactions involved in Cab45b function. Mutants in EF-hands 2 and 3 had no detectable effects on interaction of Cab45b with Munc18b and did not affect the depolarization-evoked calcium currents, but remarkably, they facilitated the complex formation of Munc18b with syntaxin-2 and -3. As a result, these two EF-hand mutants inhibited beta-cell membrane capacitance increments. This inhibition is mediated via Munc18b because Munc18b silencing with small interfering RNA abolished the effects of these two mutants. The results suggest a mechanism for Cab45b action that involves regulating the dynamic association of Munc18b with SNAREs to impact beta-cell granule exocytosis.
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Affiliation(s)
- Yi Zhang
- Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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