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Kosuge A, Kunisawa K, Iida T, Wulaer B, Kawai T, Tanabe M, Saito K, Nabeshima T, Mouri A. Chronic social defeat stress induces the down-regulation of the Nedd4L-GLT-1 ubiquitination pathway in the prefrontal cortex of mice. J Neurochem 2024. [PMID: 38497582 DOI: 10.1111/jnc.16100] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2023] [Revised: 02/25/2024] [Accepted: 02/29/2024] [Indexed: 03/19/2024]
Abstract
Stressful life events contribute to the onset of major depressive disorder (MDD). We recently demonstrated abnormalities in ubiquitination in the pathophysiology of MDD. However, the underlying molecular mechanisms remain unclear. We investigated the involvement of the ubiquitination system-mediated glutamatergic dysfunction in social impairment induced by chronic social defeat stress (CSDS). Adult C57BL/6J mice were exposed to aggressor ICR male mice for 10 consecutive days. Social impairment was induced by CSDS in the social interaction test 1 days after the last stress exposure. In terms of brain microdialysis, CSDS reduced depolarization-evoked glutamate release in the prefrontal cortex (PFC), which was reversed by a glutamate transporter 1 (GLT-1) inhibitor. Interestingly, the expression of ubiquitinated, but not total GLT-1, was decreased in the PFC of mice exposed to CSDS. The expression of neural precursor cells expressing developmentally downregulated gene 4-like (Nedd4L: E3 ligase for GLT-1), and ubiquitin-conjugating enzyme E2D2 (Ube2d2: E2 ubiquitin-conjugating enzyme for Nedd4L) was also reduced in CSDS mice. Furthermore, the downregulation of the Nedd4L-GLT-1 ubiquitination pathway decreased SIT ratio, but up-regulation increased it even in non-CSDS mice. Taken together, the decrease in GLT-1 ubiquitination may reduce the release of extracellular glutamate induced by high-potassium stimulation, which may lead to social impairment, while we could not find differences in GLT-1 ubiquitination between susceptible and resistant CSDS mice. In conclusion, GLT-1 ubiquitination could play a crucial role in the pathophysiology of MDD and is an attractive target for the development of novel antidepressants.
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Affiliation(s)
- Aika Kosuge
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Kazuo Kunisawa
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Tsubasa Iida
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Bolati Wulaer
- Advanced Diagnostic System Research Laboratory, Fujita Health University Graduate School of Health Science, Toyoake, Aichi, Japan
- Department of Advanced Diagnostic System Development, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Tomoki Kawai
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Moeka Tanabe
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
| | - Kuniaki Saito
- Advanced Diagnostic System Research Laboratory, Fujita Health University Graduate School of Health Science, Toyoake, Aichi, Japan
- Department of Advanced Diagnostic System Development, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
- Laboratory of Health and Medical Science Innovation, Fujita Health University Graduate School of Health Science, Toyoake, Aichi, Japan
- Japanese Drug Organization of Appropriate Use and Research, Toyoake, Aichi, Japan
| | - Toshitaka Nabeshima
- Advanced Diagnostic System Research Laboratory, Fujita Health University Graduate School of Health Science, Toyoake, Aichi, Japan
- Laboratory of Health and Medical Science Innovation, Fujita Health University Graduate School of Health Science, Toyoake, Aichi, Japan
- Japanese Drug Organization of Appropriate Use and Research, Toyoake, Aichi, Japan
| | - Akihiro Mouri
- Department of Regulatory Science for Evaluation & Development of Pharmaceuticals & Devices, Fujita Health University Graduate School of Health Sciences, Toyoake, Aichi, Japan
- Japanese Drug Organization of Appropriate Use and Research, Toyoake, Aichi, Japan
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2
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Iovino L, Giusti V, Pischedda F, Giusto E, Plotegher N, Marte A, Battisti I, Di Iacovo A, Marku A, Piccoli G, Bandopadhyay R, Perego C, Bonifacino T, Bonanno G, Roseti C, Bossi E, Arrigoni G, Bubacco L, Greggio E, Hilfiker S, Civiero L. Trafficking of the glutamate transporter is impaired in LRRK2-related Parkinson's disease. Acta Neuropathol 2022; 144:81-106. [PMID: 35596783 PMCID: PMC9217889 DOI: 10.1007/s00401-022-02437-0] [Citation(s) in RCA: 19] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2022] [Revised: 05/11/2022] [Accepted: 05/11/2022] [Indexed: 12/02/2022]
Abstract
The Excitatory Amino Acid Transporter 2 (EAAT2) accounts for 80% of brain glutamate clearance and is mainly expressed in astrocytic perisynaptic processes. EAAT2 function is finely regulated by endocytic events, recycling to the plasma membrane and degradation. Noteworthy, deficits in EAAT2 have been associated with neuronal excitotoxicity and neurodegeneration. In this study, we show that EAAT2 trafficking is impaired by the leucine-rich repeat kinase 2 (LRRK2) pathogenic variant G2019S, a common cause of late-onset familial Parkinson’s disease (PD). In LRRK2 G2019S human brains and experimental animal models, EAAT2 protein levels are significantly decreased, which is associated with elevated gliosis. The decreased expression of the transporter correlates with its reduced functionality in mouse LRRK2 G2019S purified astrocytic terminals and in Xenopus laevis oocytes expressing human LRRK2 G2019S. In LRRK2 G2019S knock-in mouse brain, the correct surface localization of the endogenous transporter is impaired, resulting in its interaction with a plethora of endo-vesicular proteins. Mechanistically, we report that pathogenic LRRK2 kinase activity delays the recycling of the transporter to the plasma membrane via Rabs inactivation, causing its intracellular re-localization and degradation. Taken together, our results demonstrate that pathogenic LRRK2 interferes with the physiology of EAAT2, pointing to extracellular glutamate overload as a possible contributor to neurodegeneration in PD.
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3
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Hirschberg S, Dvorzhak A, Rasooli-Nejad SMA, Angelov S, Kirchner M, Mertins P, Lättig-Tünnemann G, Harms C, Schmitz D, Grantyn R. Uncoupling the Excitatory Amino Acid Transporter 2 From Its C-Terminal Interactome Restores Synaptic Glutamate Clearance at Corticostriatal Synapses and Alleviates Mutant Huntingtin-Induced Hypokinesia. Front Cell Neurosci 2022; 15:792652. [PMID: 35173582 PMCID: PMC8841566 DOI: 10.3389/fncel.2021.792652] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2021] [Accepted: 12/21/2021] [Indexed: 02/05/2023] Open
Abstract
Rapid removal of glutamate from the sites of glutamate release is an essential step in excitatory synaptic transmission. However, despite many years of research, the molecular mechanisms underlying the intracellular regulation of glutamate transport at tripartite synapses have not been fully uncovered. This limits the options for pharmacological treatment of glutamate-related motor disorders, including Huntington’s disease (HD). We therefore investigated the possible binding partners of transgenic EAAT2 and their alterations under the influence of mutant huntingtin (mHTT). Mass spectrometry analysis after pull-down of striatal YFP-EAAT2 from wild-type (WT) mice and heterozygote (HET) Q175 mHTT-knock-in mice identified a total of 148 significant (FDR < 0.05) binders to full-length EAAT2. Of them 58 proteins exhibited mHTT-related differences. Most important, in 26 of the 58 mHTT-sensitive cases, protein abundance changed back toward WT levels when the mice expressed a C-terminal-truncated instead of full-length variant of EAAT2. These findings motivated new attempts to clarify the role of astrocytic EAAT2 regulation in cortico-basal movement control. Striatal astrocytes of Q175 HET mice were targeted by a PHP.B vector encoding EAAT2 with different degree of C-terminal modification, i.e., EAAT2-S506X (truncation at S506), EAAT2-4KR (4 lysine to arginine substitutions) or EAAT2 (full-length). The results were compared to HET and WT injected with a tag-only vector (CTRL). It was found that the presence of a C-terminal-modified EAAT2 transgene (i) increased the level of native EAAT2 protein in striatal lysates and perisynaptic astrocyte processes, (ii) enhanced the glutamate uptake of transduced astrocytes, (iii) stimulated glutamate clearance at individual corticostriatal synapses, (iv) increased the glutamate uptake of striatal astrocytes and (iv) alleviated the mHTT-related hypokinesia (open field indicators of movement initiation). In contrast, over-expression of full-length EAAT2 neither facilitated glutamate uptake nor locomotion. Together, our results support the new hypothesis that preventing abnormal protein-protein interactions at the C-terminal of EAAT2 could eliminate the mHTT-related deficits in corticostriatal synaptic glutamate clearance and movement initiation.
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Affiliation(s)
- Stefan Hirschberg
- Synaptic Dysfunction Lab, Neuroscience Research Center, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, Berlin Institute of Health, Berlin, Germany
| | - Anton Dvorzhak
- Synaptic Dysfunction Lab, Neuroscience Research Center, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, Berlin Institute of Health, Berlin, Germany
| | - Seyed M. A. Rasooli-Nejad
- Synaptic Dysfunction Lab, Neuroscience Research Center, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, Berlin Institute of Health, Berlin, Germany
| | - Svilen Angelov
- Synaptic Dysfunction Lab, Neuroscience Research Center, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, Berlin Institute of Health, Berlin, Germany
| | - Marieluise Kirchner
- Proteomics Platform, Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
- Berlin Institute of Health (BIH), Berlin, Germany
| | - Philipp Mertins
- Proteomics Platform, Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
- Berlin Institute of Health (BIH), Berlin, Germany
| | - Gilla Lättig-Tünnemann
- Department of Experimental Neurology, Charité – Universitätsmedizin Berlin, Berlin, Germany
- Center for Stroke Research Berlin, Charité – Universitätsmedizin Berlin, Berlin, Germany
| | - Christoph Harms
- Department of Experimental Neurology, Charité – Universitätsmedizin Berlin, Berlin, Germany
- Center for Stroke Research Berlin, Charité – Universitätsmedizin Berlin, Berlin, Germany
| | - Dietmar Schmitz
- German Center for Neurodegenerative Diseases (DZNE), Berlin, Germany
- Cluster of Excellence NeuroCure, Berlin, Germany
- Einstein Center for Neurosciences Berlin, Berlin, Germany
| | - Rosemarie Grantyn
- Synaptic Dysfunction Lab, Neuroscience Research Center, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, Berlin Institute of Health, Berlin, Germany
- Department of Experimental Neurology, Charité – Universitätsmedizin Berlin, Berlin, Germany
- Cluster of Excellence NeuroCure, Berlin, Germany
- Einstein Center for Neurosciences Berlin, Berlin, Germany
- *Correspondence: Rosemarie Grantyn,
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Rapid Regulation of Glutamate Transport: Where Do We Go from Here? Neurochem Res 2022; 47:61-84. [PMID: 33893911 PMCID: PMC8542062 DOI: 10.1007/s11064-021-03329-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2021] [Revised: 04/08/2021] [Accepted: 04/13/2021] [Indexed: 01/03/2023]
Abstract
Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na+-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.
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5
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The presynaptic glycine transporter GlyT2 is regulated by the Hedgehog pathway in vitro and in vivo. Commun Biol 2021; 4:1197. [PMID: 34663888 PMCID: PMC8523746 DOI: 10.1038/s42003-021-02718-6] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2020] [Accepted: 09/22/2021] [Indexed: 01/20/2023] Open
Abstract
The identity of a glycinergic synapse is maintained presynaptically by the activity of a surface glycine transporter, GlyT2, which recaptures glycine back to presynaptic terminals to preserve vesicular glycine content. GlyT2 loss-of-function mutations cause Hyperekplexia, a rare neurological disease in which loss of glycinergic neurotransmission causes generalized stiffness and strong motor alterations. However, the molecular underpinnings controlling GlyT2 activity remain poorly understood. In this work, we identify the Hedgehog pathway as a robust controller of GlyT2 expression and transport activity. Modulating the activation state of the Hedgehog pathway in vitro in rodent primary spinal cord neurons or in vivo in zebrafish embryos induced a selective control in GlyT2 expression, regulating GlyT2 transport activity. Our results indicate that activation of Hedgehog reduces GlyT2 expression by increasing its ubiquitination and degradation. This work describes a new molecular link between the Hedgehog signaling pathway and presynaptic glycine availability. By modulating the activation state of the Hedgehog pathway, de la Rocha-Muñoz et al demonstrate that Hedgehog signaling controls the expression and transport activity of the neuronal glycine transporter GlyT2. This work begins to reveal a potential link between the Hedgehog signaling pathway and presynaptic glycine availability.
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6
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Boender AJ, Bontempi L, Nava L, Pelloux Y, Tonini R. Striatal Astrocytes Shape Behavioral Flexibility via Regulation of the Glutamate Transporter EAAT2. Biol Psychiatry 2021; 89:1045-1057. [PMID: 33516457 DOI: 10.1016/j.biopsych.2020.11.015] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/25/2020] [Revised: 10/21/2020] [Accepted: 11/13/2020] [Indexed: 12/31/2022]
Abstract
BACKGROUND Striatal circuits must be modulated for behavioral flexibility, the ability to adapt to environmental changes. Striatal astrocytes contribute to circuit neuromodulation by controlling the activity of ambient neurotransmitters. In particular, extracellular glutamate levels are tightly controlled by the astrocytic glutamate transporter EAAT2, influencing synaptic functioning and neural network activity. However, it remains unclear if EAAT2 responds to environmental cues to specifically shape action control. METHODS To investigate the relationship between behavioral flexibility and experience-dependent regulation of EAAT2 expression in the dorsal striatum, mice were trained on an instrumental task. We manipulated EAAT2 expression using chemogenetic activation of astrocytic Gq signaling or in vivo morpholinos and determined the ability to adapt to novel environmental contingencies. RESULTS The loss of behavioral flexibility with task overtraining is associated with the upregulation of EAAT2, which results in enhanced glutamate clearance and altered modulation of glutamatergic neurotransmission in the lateral part of the dorsal striatum. Interfering with EAAT2 upregulation in this striatal area preserves behavioral flexibility. CONCLUSIONS Astrocytes are emerging as critical regulators of striatal functions. This work demonstrates that plasticity of EAAT2 expression in the lateral part of the dorsal striatum shapes behavior, thus providing novel mechanistic insights into how flexibility in action control is regulated.
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Affiliation(s)
- Arjen J Boender
- Neuromodulation of Cortical and Subcortical Circuits Laboratory, Fondazione Istituto Italiano di Tecnologia, Genova, Italy
| | - Leonardo Bontempi
- Neuromodulation of Cortical and Subcortical Circuits Laboratory, Fondazione Istituto Italiano di Tecnologia, Genova, Italy
| | - Luca Nava
- Neuromodulation of Cortical and Subcortical Circuits Laboratory, Fondazione Istituto Italiano di Tecnologia, Genova, Italy
| | - Yann Pelloux
- Neuromodulation of Cortical and Subcortical Circuits Laboratory, Fondazione Istituto Italiano di Tecnologia, Genova, Italy
| | - Raffaella Tonini
- Neuromodulation of Cortical and Subcortical Circuits Laboratory, Fondazione Istituto Italiano di Tecnologia, Genova, Italy.
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7
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Meng X, Zhong J, Zeng C, Yung KKL, Zhang X, Wu X, Qu S. MiR-30a-5p Regulates GLT-1 Function via a PKCα-Mediated Ubiquitin Degradation Pathway in a Mouse Model of Parkinson's Disease. ACS Chem Neurosci 2021; 12:1578-1592. [PMID: 33882234 DOI: 10.1021/acschemneuro.1c00076] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Glutamate excitotoxicity is caused by dysfunctional glutamate transporters and plays an important role in the pathogenesis of Parkinson's disease (PD); however, the mechanisms that underlie the regulation of glutamate transporters in PD are still not fully elucidated. MicroRNAs(miRNA), which are abundant in astrocytes and neurons, have been reported to play key roles in regulating the translation of glutamate-transporter mRNA. In this study, we hypothesized that the miR-30a-5p contributes to the pathogenesis of PD by regulating the ubiquitin-mediated degradation of glutamate transporter 1 (GLT-1). We demonstrated that short-hairpin RNA-mediated knockdown of miR-30a-5p ameliorated motor deficits and pathological changes like astrogliosis and reactive microgliosis in a mouse model of PD (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice). Western blotting and immunofluorescent labeling revealed that miR-30a-5p suppressed the expression and function of GLT-1 in MPTP-treated mice and specifically in astrocytes treated with 1-methyl-4-phenylpyridinium (MPP+) (cell model of PD). Both in vitro and in vivo, we found that miR-30a-5p knockdown promoted glutamate uptake and increased GLT-1 expression by hindering GLT-1 ubiquitination and subsequent degradation in a PKCα-dependent manner. Therefore, we conclude that miR-30a-5p represents a potential therapeutic target for the treatment of PD.
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Affiliation(s)
- Xingjun Meng
- Department of Neurology, Nanfang Hospital, Southern Medical University, Guangzhou, Guangdong 510515, China
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
- Guangdong-Hong Kong-Macao Greater Bay Area Center for Brain Science and Brain-Inspired Intelligence, Guangzhou, Guangdong 510515, China
- Key Laboratory of Mental Health of the Ministry of Education, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Jianping Zhong
- Department of Neurology, Shunde Hospital of Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
| | - Chong Zeng
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
| | - Ken Kin Lam Yung
- Department of Biology, Faculty of Science, Hong Kong Baptist University, Hong Kong, China
| | - Xiuping Zhang
- Teaching Center of Experimental Medicine, School of Basic Medical Sciences, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Xiaojuan Wu
- Department of Immunology, School of Basic Medical Sciences, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Shaogang Qu
- Department of Neurology, Nanfang Hospital, Southern Medical University, Guangzhou, Guangdong 510515, China
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
- Guangdong-Hong Kong-Macao Greater Bay Area Center for Brain Science and Brain-Inspired Intelligence, Guangzhou, Guangdong 510515, China
- Key Laboratory of Mental Health of the Ministry of Education, Southern Medical University, Guangzhou, Guangdong 510515, China
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8
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Michaluk P, Heller JP, Rusakov DA. Rapid recycling of glutamate transporters on the astroglial surface. eLife 2021; 10:e64714. [PMID: 33860761 PMCID: PMC8079145 DOI: 10.7554/elife.64714] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2020] [Accepted: 04/15/2021] [Indexed: 12/17/2022] Open
Abstract
Glutamate uptake by astroglial transporters confines excitatory transmission to the synaptic cleft. The efficiency of this mechanism depends on the transporter dynamics in the astrocyte membrane, which remains poorly understood. Here, we visualise the main glial glutamate transporter GLT1 by generating its pH-sensitive fluorescent analogue, GLT1-SEP. Fluorescence recovery after photobleaching-based imaging shows that 70-75% of GLT1-SEP dwell on the surface of rat brain astroglia, recycling with a lifetime of ~22 s. Genetic deletion of the C-terminus accelerates GLT1-SEP membrane turnover while disrupting its surface pattern, as revealed by single-molecule localisation microscopy. Excitatory activity boosts surface mobility of GLT1-SEP, involving its C-terminus, metabotropic glutamate receptors, intracellular Ca2+, and calcineurin-phosphatase activity, but not the broad-range kinase activity. The results suggest that membrane turnover, rather than lateral diffusion, is the main 'redeployment' route for the immobile fraction (20-30%) of surface-expressed GLT1. This finding reveals an important mechanism helping to control extrasynaptic escape of glutamate.
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Affiliation(s)
- Piotr Michaluk
- UCL Queen Square Institute of Neurology, University College LondonLondonUnited Kingdom
- BRAINCITY, Laboratory of Neurobiology, Nencki Institute of Experimental Biology PASWarsawPoland
| | - Janosch Peter Heller
- UCL Queen Square Institute of Neurology, University College LondonLondonUnited Kingdom
- School of Biotechnology and National Institute for Cellular Biotechnology (NICB), Dublin City UniversityGlasnevinIreland
| | - Dmitri A Rusakov
- UCL Queen Square Institute of Neurology, University College LondonLondonUnited Kingdom
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9
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Ryan RM, Ingram SL, Scimemi A. Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression. Front Cell Neurosci 2021; 15:670346. [PMID: 33927596 PMCID: PMC8076567 DOI: 10.3389/fncel.2021.670346] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2021] [Accepted: 03/15/2021] [Indexed: 01/31/2023] Open
Abstract
Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.
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Affiliation(s)
- Renae M. Ryan
- School of Medical Sciences, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia
| | - Susan L. Ingram
- Department of Neurological Surgery, Oregon Health & Science University, Portland, OR, United States
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10
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Perez-Jimenez E, Viana R, Muñoz-Ballester C, Vendrell-Tornero C, Moll-Diaz R, Garcia-Gimeno MA, Sanz P. Endocytosis of the glutamate transporter 1 is regulated by laforin and malin: Implications in Lafora disease. Glia 2020; 69:1170-1183. [PMID: 33368637 DOI: 10.1002/glia.23956] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2020] [Revised: 11/10/2020] [Accepted: 12/10/2020] [Indexed: 01/13/2023]
Abstract
Lafora disease (LD) is a fatal rare type of progressive myoclonus epilepsy that appears during early adolescence. The disease is caused by mutations in EPM2A or EPM2B genes, which encode laforin, a glucan phosphatase, and malin, an E3-ubiquitin ligase, respectively. Although the exact roles of laforin and malin are still not well understood, it is known that they work as a complex in which laforin recruits targets that will be ubiquitinated by malin. Recently, we suggested that the type of epilepsy that accompanies LD could be due to deficiencies in the function of the astrocytic glutamate transporter GLT-1. We described that astrocytes from LD mouse models presented decreased levels of GLT-1 at the plasma membrane, leading to increased levels of glutamate in the brain parenchyma. In this work, we present evidence indicating that in the absence of a functional laforin/malin complex (as in LD cellular models) there is an alteration in the ubiquitination of GLT-1, which could be the cause of the reduction in the levels of GLT-1 at the plasma membrane. On the contrary, overexpression of the laforin/malin complex promotes the retention of GLT-1 at the plasma membrane. This retention may be due to the direct ubiquitination of GLT-1 and/or to an opposite effect of this complex on the dynamics of the Nedd4.2-mediated endocytosis of the transporter. This work, therefore, presents new pieces of evidence on the regulation of GLT-1 by the laforin/malin complex, highlighting its value as a therapeutic target for the amelioration of the type of epilepsy that accompanies LD.
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Affiliation(s)
- Eva Perez-Jimenez
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
| | - Rosa Viana
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
| | - Carmen Muñoz-Ballester
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
| | - Carlos Vendrell-Tornero
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
| | - Raquel Moll-Diaz
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
| | | | - Pascual Sanz
- Consejo Superior de Investigaciones Científicas, Instituto de Biomedicina de Valencia, Valencia, Spain.,CIBERER, Centro de Investigación Biomédica en Red de Enfermedades Raras, Instituto de Biomedicina de Valencia, Valencia, Spain
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11
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Voss TD, Gerget M, Linkus B, von Einem B, Landwehrmeyer GB, Lewerenz J. Ubiquitination and the proteasome rather than caspase-3-mediated C-terminal cleavage are involved in the EAAT2 degradation by staurosporine-induced cellular stress. J Neurochem 2020; 157:1284-1299. [PMID: 33180957 DOI: 10.1111/jnc.15237] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2020] [Revised: 03/09/2020] [Accepted: 11/03/2020] [Indexed: 01/01/2023]
Abstract
Diminished glutamate (Glu) uptake via the excitatory amino acid transporter EAAT2, which normally accounts for ~90% of total forebrain EAAT activity, may contribute to neurodegeneration via Glu-mediated excitotoxicity. C-terminal cleavage by caspase-3 (C3) was reported to mediate EAAT2 inactivation and down-regulation in the context of neurodegeneration. For a detailed analysis of C3-dependent EAAT2 degradation, we employed A172 glioblastoma as well as hippocampal HT22 cells and murine astrocytes over-expressing VSV-G-tagged EAAT2 constructs. C3 activation was induced by staurosporine (STR). In HT22 cells, STR-induced C3 activation-induced rapid EAAT2 protein degradation. The mutation of asparagine 504 to aspartate (D504N), which should inactivate the putative C3 cleavage site, increased EAAT2 activity in A172 cells. In contrast, the D504N mutation did not protect EAAT2 protein against STR-induced degradation in HT22 cells, whereas inhibition of caspases, ubiquitination and the proteasome did. Similar results were obtained in astrocytes. Phylogenetic analysis showed that C-terminal ubiquitin acceptor sites-but not the putative C3 cleavage site-exhibit a high degree of conservation. Moreover, C-terminal truncation mimicking C3 cleavage increased rather than decreased EAAT2 activity and stability as well as protected EAAT2 against STR-induced ubiquitination-dependent degradation. We conclude that cellular stress associated with endogenous C3 activation degrades EAAT2 via a pathway involving ubiquitination and the proteasome but not direct C3-mediated cleavage. In addition, C3 cleavage of EAAT2, described to occur in other models, is unlikely to inactivate EAAT2. However, mutation of the highly conserved D504 within the putative C3 cleavage site increases EAAT2 activity via an unknown mechanism.
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Affiliation(s)
| | - Maria Gerget
- Department of Neurology, Ulm University, Ulm, Germany
| | - Birgit Linkus
- Department of Neurology, Ulm University, Ulm, Germany
| | | | | | - Jan Lewerenz
- Department of Neurology, Ulm University, Ulm, Germany
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12
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Robinson MB, Lee ML, DaSilva S. Glutamate Transporters and Mitochondria: Signaling, Co-compartmentalization, Functional Coupling, and Future Directions. Neurochem Res 2020; 45:526-540. [PMID: 32002773 DOI: 10.1007/s11064-020-02974-8] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2019] [Revised: 01/21/2020] [Accepted: 01/22/2020] [Indexed: 12/12/2022]
Abstract
In addition to being an amino acid that is incorporated into proteins, glutamate is the most abundant neurotransmitter in the mammalian CNS, the precursor for the inhibitory neurotransmitter γ-aminobutyric acid, and one metabolic step from the tricarboxylic acid cycle intermediate α-ketoglutarate. Extracellular glutamate is cleared by a family of Na+-dependent transporters. These transporters are variably expressed by all cell types in the nervous system, but the bulk of clearance is into astrocytes. GLT-1 and GLAST (also called EAAT2 and EAAT1) mediate this activity and are extremely abundant proteins with their expression enriched in fine astrocyte processes. In this review, we will focus on three topics related to these astrocytic glutamate transporters. First, these transporters co-transport three Na+ ions and a H+ with each molecule of glutamate and counter-transport one K+; they are also coupled to a Cl- conductance. The movement of Na+ is sufficient to cause profound astrocytic depolarization, and the movement of H+ is linked to astrocytic acidification. In addition, the movement of Na+ can trigger the activation of Na+ co-transporters (e.g. Na+-Ca2+ exchangers). We will describe the ways in which these ionic movements have been linked as signals to brain function and/or metabolism. Second, these transporters co-compartmentalize with mitochondria, potentially providing a mechanism to supply glutamate to mitochondria as a source of fuel for the brain. We will provide an overview of the proteins involved, discuss the evidence that glutamate is oxidized, and then highlight some of the un-resolved issues related to glutamate oxidation. Finally, we will review evidence that ischemic insults (stroke or oxygen/glucose deprivation) cause changes in these astrocytic mitochondria and discuss the ways in which these changes have been linked to glutamate transport, glutamate transport-dependent signaling, and altered glutamate metabolism. We conclude with a broader summary of some of the unresolved issues.
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Affiliation(s)
- Michael B Robinson
- Departments of Pediatrics and Systems Pharmacology & Translational Therapeutics, Children's Hospital of Philadelphia, University of Pennsylvania, 502N, Abramson Pediatric Research Building, 3615 Civic Center Boulevard, Philadelphia, PA, 19104-4318, USA.
| | - Meredith L Lee
- Departments of Pediatrics and Systems Pharmacology & Translational Therapeutics, Children's Hospital of Philadelphia, University of Pennsylvania, 502N, Abramson Pediatric Research Building, 3615 Civic Center Boulevard, Philadelphia, PA, 19104-4318, USA
| | - Sabrina DaSilva
- Departments of Pediatrics and Systems Pharmacology & Translational Therapeutics, Children's Hospital of Philadelphia, University of Pennsylvania, 502N, Abramson Pediatric Research Building, 3615 Civic Center Boulevard, Philadelphia, PA, 19104-4318, USA
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13
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Fortinguerra S, Sorrenti V, Giusti P, Zusso M, Buriani A. Pharmacogenomic Characterization in Bipolar Spectrum Disorders. Pharmaceutics 2019; 12:E13. [PMID: 31877761 PMCID: PMC7022469 DOI: 10.3390/pharmaceutics12010013] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2019] [Revised: 12/14/2019] [Accepted: 12/19/2019] [Indexed: 12/15/2022] Open
Abstract
The holistic approach of personalized medicine, merging clinical and molecular characteristics to tailor the diagnostic and therapeutic path to each individual, is steadily spreading in clinical practice. Psychiatric disorders represent one of the most difficult diagnostic challenges, given their frequent mixed nature and intrinsic variability, as in bipolar disorders and depression. Patients misdiagnosed as depressed are often initially prescribed serotonergic antidepressants, a treatment that can exacerbate a previously unrecognized bipolar condition. Thanks to the use of the patient's genomic profile, it is possible to recognize such risk and at the same time characterize specific genetic assets specifically associated with bipolar spectrum disorder, as well as with the individual response to the various therapeutic options. This provides the basis for molecular diagnosis and the definition of pharmacogenomic profiles, thus guiding therapeutic choices and allowing a safer and more effective use of psychotropic drugs. Here, we report the pharmacogenomics state of the art in bipolar disorders and suggest an algorithm for therapeutic regimen choice.
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Affiliation(s)
- Stefano Fortinguerra
- Maria Paola Belloni Center for Personalized Medicine, Data Medica Group (Synlab Limited), 35131 Padova, Italy; (S.F.); (V.S.)
- Department of Pharmaceutical & Pharmacological Sciences, University of Padova, 35131 Padova, Italy; (P.G.); (M.Z.)
| | - Vincenzo Sorrenti
- Maria Paola Belloni Center for Personalized Medicine, Data Medica Group (Synlab Limited), 35131 Padova, Italy; (S.F.); (V.S.)
- Department of Pharmaceutical & Pharmacological Sciences, University of Padova, 35131 Padova, Italy; (P.G.); (M.Z.)
- Bendessere™ Study Center, Solgar Italia Multinutrient S.p.A., 35131 Padova, Italy
| | - Pietro Giusti
- Department of Pharmaceutical & Pharmacological Sciences, University of Padova, 35131 Padova, Italy; (P.G.); (M.Z.)
| | - Morena Zusso
- Department of Pharmaceutical & Pharmacological Sciences, University of Padova, 35131 Padova, Italy; (P.G.); (M.Z.)
| | - Alessandro Buriani
- Maria Paola Belloni Center for Personalized Medicine, Data Medica Group (Synlab Limited), 35131 Padova, Italy; (S.F.); (V.S.)
- Department of Pharmaceutical & Pharmacological Sciences, University of Padova, 35131 Padova, Italy; (P.G.); (M.Z.)
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Pajarillo E, Rizor A, Lee J, Aschner M, Lee E. The role of astrocytic glutamate transporters GLT-1 and GLAST in neurological disorders: Potential targets for neurotherapeutics. Neuropharmacology 2019; 161:107559. [PMID: 30851309 PMCID: PMC6731169 DOI: 10.1016/j.neuropharm.2019.03.002] [Citation(s) in RCA: 190] [Impact Index Per Article: 38.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2018] [Revised: 02/28/2019] [Accepted: 03/02/2019] [Indexed: 12/12/2022]
Abstract
Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS) which initiates rapid signal transmission in the synapse before its re-uptake into the surrounding glia, specifically astrocytes. The astrocytic glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2), respectively, are the major transporters which take up synaptic glutamate to maintain optimal extracellular glutamic levels, thus preventing accumulation in the synaptic cleft and ensuing excitotoxicity. Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism. While the mechanisms of neurological disorders are not well understood, the dysregulation of GLAST/GLT-1 may play a significant role in excitotoxicity and associated neuropathogenesis. The expression and function of GLAST/GLT-1 may be dysregulated at the genetic, epigenetic, transcriptional or translational levels, leading to high levels of extracellular glutamate and excitotoxicity. Consequently, understanding the regulatory mechanisms of GLAST/GLT-1 has been an area of interest in developing therapeutics for the treatment of neurological disorders. Pharmacological agents including β-lactam antibiotics, estrogen/selective estrogen receptor modulators (SERMs), growth factors, histone deacetylase inhibitors (HDACi), and translational activators have shown significant efficacy in enhancing the expression and function of GLAST/GLT-1 and glutamate uptake both in vitro and in vivo. This comprehensive review will discuss the regulatory mechanisms of GLAST/GLT-1, their association with neurological disorders, and the pharmacological agents which mediate their expression and function. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.
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Affiliation(s)
- Edward Pajarillo
- Department of Pharmaceutical Sciences, College of Pharmacy, Florida A&M University, Tallahassee, FL, 32301, USA
| | - Asha Rizor
- Department of Pharmaceutical Sciences, College of Pharmacy, Florida A&M University, Tallahassee, FL, 32301, USA
| | - Jayden Lee
- Department of Speech, Language & Hearing Sciences, Boston University, Boston, MA, 02215, USA
| | - Michael Aschner
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY, 10461, USA
| | - Eunsook Lee
- Department of Pharmaceutical Sciences, College of Pharmacy, Florida A&M University, Tallahassee, FL, 32301, USA.
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de la Rocha-Muñoz A, Núñez E, Arribas-González E, López-Corcuera B, Aragón C, de Juan-Sanz J. E3 ubiquitin ligases LNX1 and LNX2 are major regulators of the presynaptic glycine transporter GlyT2. Sci Rep 2019; 9:14944. [PMID: 31628376 PMCID: PMC6802383 DOI: 10.1038/s41598-019-51301-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2018] [Accepted: 09/27/2019] [Indexed: 12/15/2022] Open
Abstract
The neuronal glycine transporter GlyT2 is an essential regulator of glycinergic neurotransmission that recaptures glycine in presynaptic terminals to facilitate transmitter packaging in synaptic vesicles. Alterations in GlyT2 expression or activity result in lower cytosolic glycine levels, emptying glycinergic synaptic vesicles and impairing neurotransmission. Lack of glycinergic neurotransmission caused by GlyT2 loss-of-function mutations results in Hyperekplexia, a rare neurological disease characterized by generalized stiffness and motor alterations that may cause sudden infant death. Although the importance of GlyT2 in pathology is known, how this transporter is regulated at the molecular level is poorly understood, limiting current therapeutic strategies. Guided by an unbiased screening, we discovered that E3 ubiquitin ligase Ligand of Numb proteins X1/2 (LNX1/2) modulate the ubiquitination status of GlyT2. The N-terminal RING-finger domain of LNX1/2 ubiquitinates a cytoplasmic C-terminal lysine cluster in GlyT2 (K751, K773, K787 and K791), and this process regulates the expression levels and transport activity of GlyT2. The genetic deletion of endogenous LNX2 in spinal cord primary neurons causes an increase in GlyT2 expression and we find that LNX2 is required for PKC-mediated control of GlyT2 transport. This work identifies, to our knowledge, the first E3 ubiquitin-ligases acting on GlyT2, revealing a novel molecular mechanism that controls presynaptic glycine availability. Providing a better understanding of the molecular regulation of GlyT2 may help future investigations into the molecular basis of human disease states caused by dysfunctional glycinergic neurotransmission, such as hyperekplexia and chronic pain.
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Affiliation(s)
- A de la Rocha-Muñoz
- Centro de Biología Molecular "Severo Ochoa", Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, 28049, Madrid, Spain
- IdiPAZ, Hospital Universitario La Paz, Madrid, Spain
| | - E Núñez
- Centro de Biología Molecular "Severo Ochoa", Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, 28049, Madrid, Spain
- IdiPAZ, Hospital Universitario La Paz, Madrid, Spain
| | - E Arribas-González
- Instituto Cajal, Consejo Superior de Investigaciones Científicas, 28002, Madrid, Spain
| | - B López-Corcuera
- Centro de Biología Molecular "Severo Ochoa", Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, 28049, Madrid, Spain
- IdiPAZ, Hospital Universitario La Paz, Madrid, Spain
| | - C Aragón
- Centro de Biología Molecular "Severo Ochoa", Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, 28049, Madrid, Spain.
- IdiPAZ, Hospital Universitario La Paz, Madrid, Spain.
| | - J de Juan-Sanz
- Sorbonne Université and Institut du Cerveau et de la Moelle Epinière (ICM) - Hôpital Pitié-Salpêtrière, Inserm, CNRS, Paris, France.
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16
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The Effect of Glutamatergic Modulators on Extracellular Glutamate: How Does this Information Contribute to the Discovery of Novel Antidepressants? Curr Ther Res Clin Exp 2019; 91:25-32. [PMID: 31871505 PMCID: PMC6911922 DOI: 10.1016/j.curtheres.2019.100566] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2019] [Accepted: 08/29/2019] [Indexed: 01/19/2023] Open
Abstract
The complexity of glutamatergic signaling challenges glutamate modulator usage. Functional biomarkers are needed to understand the MOA of glutamate modulators. Evaluating drug effect on EAATs' kinetics may add to antidepressant discovery.
Background In the search for new antidepressants, clinical researchers have been using drugs that simultaneously modulate multiple targets. During preclinical and clinical trials, the glutamatergic modulators riluzole and ketamine have received particular attention. Glutamatergic agents have a modulatory effect on synaptic transmission, so they can act on both neurons and astrocytes. In addition to influencing the quantity of glutamate released, these modulators can also affect the expression, localization, and functionality of glutamate-binding sites. Objective This review discusses the complexity of the glutamatergic system, the ambiguity of data regarding glutamate levels in patients with depression, as well as the mechanisms of action for riluzole and ketamine, which includes their relation to the physiology of glutamatergic transmission. The principal aim is to contribute to the development of novel glutamatergic antidepressant medications whilst emphasizing the need for innovative approaches that evaluate their effects on extracellular glutamate. Methods Literature was obtained via PubMed by searching the term depression in combination with each of the following terms: riluzole, ketamine, and glutamate. The search was restricted to full-text articles published in English between 1985 and 2018 relating to both the modulatory mechanisms of glutamatergic-binding proteins and the antidepressant actions of these medicines. Articles about mechanisms associated with synaptic plasticity and antidepressant effects were excluded. Results Although experimental data relates glutamatergic signaling to the pathophysiology of major depression and bipolar disorder, the role of glutamate—as well as its extracellular concentration in patients with said disorders—is still unclear. Riluzole's antidepressant action is ascribed to its capacity to reduce glutamate levels in the synaptic cleft, and ketamine's effect has been associated with increased extracellular glutamate levels. Conclusions The strategy of using glutamatergic modulators as therapeutic agents requires a better understanding of the role of glutamate in the pathophysiology of depression. Gaining such understanding is a challenge because it entails evaluating different targets as well as the effects of these modulators on the kinetics of glutamate uptake. Essentially, glutamate transport is a dynamic process and, currently, it is still necessary to develop new approaches to assay glutamate in the synaptic cleft. ORCID: 0000-0002-3358-6939.
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Piniella D, Martínez-Blanco E, Ibáñez I, Bartolomé-Martín D, Porlan E, Díez-Guerra J, Giménez C, Zafra F. Identification of novel regulatory partners of the glutamate transporter GLT-1. Glia 2018; 66:2737-2755. [PMID: 30394597 DOI: 10.1002/glia.23524] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2018] [Revised: 07/27/2018] [Accepted: 07/30/2018] [Indexed: 01/23/2023]
Abstract
We used proximity-dependent biotin identification (BioID) to find proteins that potentially interact with the major glial glutamate transporter, GLT-1, and we studied how these interactions might affect its activity. GTPase Rac1 was one protein identified, and interfering with its GTP/GDP cycle in mixed primary rat brain cultures affected both the clustering of GLT-1 at the astrocytic processes and the transport kinetics, increasing its uptake activity at low micromolar glutamate concentrations in a manner that was dependent on the effector kinase PAK1 and the actin cytoskeleton. Interestingly, the same manipulations had a different effect on another glial glutamate transporter, GLAST, inhibiting its activity. Importantly, glutamate acts through metabotropic receptors to stimulate the activity of Rac1 in astrocytes, supporting the existence of cross-talk between extracellular glutamate and the astrocytic form of the GLT-1 regulated by Rac1. CDC42EP4/BORG4 (a CDC42 effector) was also identified in the BioID screen, and it is a protein that regulates the assembly of septins and actin fibers, influencing the organization of the cytoskeleton. We found that GLT-1 interacts with septins, which reduces its lateral mobility at the cell surface. Finally, the G-protein subunit GNB4 dampens the activity of GLT-1, as revealed by its response to the activator peptide mSIRK, both in heterologous systems and in primary brain cultures. This effect occurs rapidly and thus, it is unlikely to depend on cytoskeletal dynamics. These novel interactions shed new light on the events controlling GLT-1 activity, thereby helping us to better understand how glutamate homeostasis is maintained in the brain.
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Affiliation(s)
- Dolores Piniella
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Elena Martínez-Blanco
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Ignacio Ibáñez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - David Bartolomé-Martín
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Eva Porlan
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Javier Díez-Guerra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain
| | - Cecilio Giménez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Francisco Zafra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
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18
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Lafora Disease: A Ubiquitination-Related Pathology. Cells 2018; 7:cells7080087. [PMID: 30050012 PMCID: PMC6116066 DOI: 10.3390/cells7080087] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2018] [Revised: 07/23/2018] [Accepted: 07/24/2018] [Indexed: 11/17/2022] Open
Abstract
Lafora disease (LD, OMIM254780) is a rare and fatal form of progressive myoclonus epilepsy (PME). Among PMEs, LD is unique because of the rapid neurological deterioration of the patients and the appearance in brain and peripheral tissues of insoluble glycogen-like (polyglucosan) inclusions, named Lafora bodies (LBs). LD is caused by mutations in the EPM2A gene, encoding the dual phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Laforin and malin form a functional complex that is involved in the regulation of glycogen synthesis. Thus, in the absence of a functional complex glycogen accumulates in LBs. In addition, it has been suggested that the laforin-malin complex participates in alternative physiological pathways, such as intracellular protein degradation, oxidative stress, and the endoplasmic reticulum unfolded protein response. In this work we review the possible cellular functions of laforin and malin with a special focus on their role in the ubiquitination of specific substrates. We also discuss here the pathological consequences of defects in laforin or malin functions, as well as the therapeutic strategies that are being explored for LD.
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19
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Yokukansan, a Traditional Japanese Medicine, Enhances the Glutamate Transporter GLT-1 Function in Cultured Rat Cortical Astrocytes. EVIDENCE-BASED COMPLEMENTARY AND ALTERNATIVE MEDICINE 2018; 2018:6804017. [PMID: 29853967 PMCID: PMC5960509 DOI: 10.1155/2018/6804017] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/13/2017] [Revised: 02/16/2018] [Accepted: 03/18/2018] [Indexed: 12/21/2022]
Abstract
Astrocytes carry two glutamate transporters—GLAST and GLT-1—the latter of which is responsible for >90% of glutamate uptake activity in the brain; however, under culture conditions, the GLT-1 expression in astrocytes is exceedingly low, as is the glutamate uptake activity mediated by GLT-1. This study aimed to elucidate the effects of yokukansan (YKS) in relation to the GLT-1-mediated regulation of extracellular glutamate concentrations. Thus, we treated cultured astrocytes with tumor necrosis factor-α (TNF-α) and dibutyryl-cAMP (dBcAMP) (hereinafter, referred to as “TA”) to increase GLT-1 expression and then functionally examined how YKS would affect glutamate uptake ability derived from GLT-1. Contrary to expectations, although the TA treatments did not affect the uptake activity, YKS significantly augmented it. Conversely, GLAST-derived glutamate uptake was significantly reduced by TA treatments but was unaffected by YKS. Subsequently, we analyzed the GLT-1 protein and mRNA levels and found that TA treatments had significantly increased them, which were then further augmented by YKS. These findings suggest that YKS enhances GLT-1-derived glutamate transport functions in TA-treated cultured astrocytes and that this process entails increased GLT-1 protein and mRNA levels. This type of mechanism may contribute to the YKS-mediated regulation of extracellular glutamate concentrations.
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20
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Kv1.3 activity perturbs the homeostatic properties of astrocytes in glioma. Sci Rep 2018; 8:7654. [PMID: 29769580 PMCID: PMC5955950 DOI: 10.1038/s41598-018-25940-5] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2017] [Accepted: 05/02/2018] [Indexed: 01/06/2023] Open
Abstract
Glial cells actively maintain the homeostasis of brain parenchyma, regulating neuronal excitability and preserving the physiological composition of the extracellular milieu. Under pathological conditions, some functions of glial cells could be compromised, exacerbating the neurotoxic processes. We investigated if the homeostatic activities of astrocytes and microglia could be modulated by the voltage-gated K+ channel Kv1.3. To this end we used in vitro and in vivo systems to model cell-to-cell interactions in tumoral conditions, using a specific inhibitor of Kv1.3 channels, 5-(4-phenoxybutoxy) psoralen (PAP-1). We demonstrated that PAP-1 increases astrocytic glutamate uptake, reduces glioma-induced neurotoxicity, and decreases microglial migration and phagocytosis. We also found in a tumor blood brain barrier model that Kv1.3 activity is required for its integrity. The crucial role of Kv1.3 channels as modulators of glial cell activity was confirmed in a mouse model of glioma, where PAP-1 treatment reduces tumor volume only in the presence of active glutamate transporters GLT-1. In the same mouse model, PAP-1 reduces astrogliosis and microglial infiltration. PAP-1 also reduces tumor cell invasion. All these findings point to Kv1.3 channels as potential targets to re-instruct glial cells toward their homeostatic functions, in the context of brain tumors.
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21
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Dong W, Todd AC, Bröer A, Hulme SR, Bröer S, Billups B. PKC-Mediated Modulation of Astrocyte SNAT3 Glutamine Transporter Function at Synapses in Situ. Int J Mol Sci 2018; 19:ijms19040924. [PMID: 29561757 PMCID: PMC5979592 DOI: 10.3390/ijms19040924] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2018] [Revised: 03/07/2018] [Accepted: 03/15/2018] [Indexed: 01/13/2023] Open
Abstract
Astrocytes are glial cells that have an intimate physical and functional association with synapses in the brain. One of their main roles is to recycle the neurotransmitters glutamate and gamma-aminobutyric acid (GABA), as a component of the glutamate/GABA-glutamine cycle. They perform this function by sequestering neurotransmitters and releasing glutamine via the neutral amino acid transporter SNAT3. In this way, astrocytes regulate the availability of neurotransmitters and subsequently influence synaptic function. Since many plasma membrane transporters are regulated by protein kinase C (PKC), the aim of this study was to understand how PKC influences SNAT3 glutamine transport in astrocytes located immediately adjacent to synapses. We studied SNAT3 transport by whole-cell patch-clamping and fluorescence pH imaging of single astrocytes in acutely isolated brainstem slices, adjacent to the calyx of the Held synapse. Activation of SNAT3-mediated glutamine transport in these astrocytes was reduced to 77 ± 6% when PKC was activated with phorbol 12-myristate 13-acetate (PMA). This effect was very rapid (within ~20 min) and eliminated by application of bisindolylmaleimide I (Bis I) or 7-hydroxystaurosporine (UCN-01), suggesting that activation of conventional isoforms of PKC reduces SNAT3 function. In addition, cell surface biotinylation experiments in these brain slices show that the amount of SNAT3 in the plasma membrane is reduced by a comparable amount (to 68 ± 5%) upon activation of PKC. This indicates a role for PKC in dynamically controlling the trafficking of SNAT3 transporters in astrocytes in situ. These data demonstrate that PKC rapidly regulates the astrocytic glutamine release mechanism, which would influence the glutamine availability for adjacent synapses and control levels of neurotransmission.
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Affiliation(s)
- Wuxing Dong
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Canberra ACT 2601, Australia.
| | - Alison C Todd
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Canberra ACT 2601, Australia.
- Centre for Discovery Brain Sciences, School of Biomedical Sciences, University of Edinburgh, Edinburgh EH8 9XD, UK.
| | - Angelika Bröer
- Research School of Biology, The Australian National University, Linnaeus Way 134, Canberra ACT 2601, Australia.
| | - Sarah R Hulme
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Canberra ACT 2601, Australia.
| | - Stefan Bröer
- Research School of Biology, The Australian National University, Linnaeus Way 134, Canberra ACT 2601, Australia.
| | - Brian Billups
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Canberra ACT 2601, Australia.
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22
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Al Awabdh S, Gupta-Agarwal S, Sheehan DF, Muir J, Norkett R, Twelvetrees AE, Griffin LD, Kittler JT. Neuronal activity mediated regulation of glutamate transporter GLT-1 surface diffusion in rat astrocytes in dissociated and slice cultures. Glia 2018; 64:1252-64. [PMID: 27189737 PMCID: PMC4915597 DOI: 10.1002/glia.22997] [Citation(s) in RCA: 53] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2015] [Revised: 04/03/2016] [Accepted: 04/13/2016] [Indexed: 11/17/2022]
Abstract
The astrocytic GLT‐1 (or EAAT2) is the major glutamate transporter for clearing synaptic glutamate. While the diffusion dynamics of neurotransmitter receptors at the neuronal surface are well understood, far less is known regarding the surface trafficking of transporters in subcellular domains of the astrocyte membrane. Here, we have used live‐cell imaging to study the mechanisms regulating GLT‐1 surface diffusion in astrocytes in dissociated and brain slice cultures. Using GFP‐time lapse imaging, we show that GLT‐1 forms stable clusters that are dispersed rapidly and reversibly upon glutamate treatment in a transporter activity‐dependent manner. Fluorescence recovery after photobleaching and single particle tracking using quantum dots revealed that clustered GLT‐1 is more stable than diffuse GLT‐1 and that glutamate increases GLT‐1 surface diffusion in the astrocyte membrane. Interestingly, the two main GLT‐1 isoforms expressed in the brain, GLT‐1a and GLT‐1b, are both found to be stabilized opposed to synapses under basal conditions, with GLT‐1b more so. GLT‐1 surface mobility is increased in proximity to activated synapses and alterations of neuronal activity can bidirectionally modulate the dynamics of both GLT‐1 isoforms. Altogether, these data reveal that astrocytic GLT‐1 surface mobility, via its transport activity, is modulated during neuronal firing, which may be a key process for shaping glutamate clearance and glutamatergic synaptic transmission. GLIA 2016;64:1252–1264
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Affiliation(s)
- Sana Al Awabdh
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - Swati Gupta-Agarwal
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - David F Sheehan
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - James Muir
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - Rosalind Norkett
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - Alison E Twelvetrees
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
| | - Lewis D Griffin
- Department of Computer Science, University College London, United Kingdom
| | - Josef T Kittler
- Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom
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23
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Bai W, Zhou YG. Homeostasis of the Intraparenchymal-Blood Glutamate Concentration Gradient: Maintenance, Imbalance, and Regulation. Front Mol Neurosci 2017; 10:400. [PMID: 29259540 PMCID: PMC5723322 DOI: 10.3389/fnmol.2017.00400] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2017] [Accepted: 11/20/2017] [Indexed: 12/25/2022] Open
Abstract
It is widely accepted that glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS). However, there is also a large amount of glutamate in the blood. Generally, the concentration gradient of glutamate between intraparenchymal and blood environments is stable. However, this gradient is dramatically disrupted under a variety of pathological conditions, resulting in an amplifying cascade that causes a series of pathological reactions in the CNS and peripheral organs. This eventually seriously worsens a patient’s prognosis. These two “isolated” systems are rarely considered as a whole even though they mutually influence each other. In this review, we summarize what is currently known regarding the maintenance, imbalance and regulatory mechanisms that control the intraparenchymal-blood glutamate concentration gradient, discuss the interrelationships between these systems and further explore their significance in clinical practice.
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Affiliation(s)
- Wei Bai
- Molecular Biology Center, State Key Laboratory of Trauma, Burn, and Combined Injury, Research Institute of Surgery and Daping Hospital, Third Military Medical University, Chongqing, China
| | - Yuan-Guo Zhou
- Molecular Biology Center, State Key Laboratory of Trauma, Burn, and Combined Injury, Research Institute of Surgery and Daping Hospital, Third Military Medical University, Chongqing, China
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24
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Amyloid-beta peptide decreases expression and function of glutamate transporters in nervous system cells. Int J Biochem Cell Biol 2017; 85:75-84. [DOI: 10.1016/j.biocel.2017.01.017] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2016] [Revised: 12/26/2016] [Accepted: 01/29/2017] [Indexed: 12/26/2022]
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25
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Murphy-Royal C, Dupuis J, Groc L, Oliet SHR. Astroglial glutamate transporters in the brain: Regulating neurotransmitter homeostasis and synaptic transmission. J Neurosci Res 2017; 95:2140-2151. [PMID: 28150867 DOI: 10.1002/jnr.24029] [Citation(s) in RCA: 104] [Impact Index Per Article: 14.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2016] [Revised: 12/20/2016] [Accepted: 01/02/2017] [Indexed: 12/29/2022]
Abstract
Astrocytes, the major glial cell type in the central nervous system (CNS), are critical for brain function and have been implicated in various disorders of the central nervous system. These cells are involved in a wide range of cerebral processes including brain metabolism, control of central blood flow, ionic homeostasis, fine-tuning synaptic transmission, and neurotransmitter clearance. Such varied roles can be efficiently carried out due to the intimate interactions astrocytes maintain with neurons, the vasculature, as well as with other glial cells. Arguably, one of the most important functions of astrocytes in the brain is their control of neurotransmitter clearance. This is particularly true for glutamate whose timecourse in the synaptic cleft needs to be controlled tightly under physiological conditions to maintain point-to-point excitatory transmission, thereby limiting spillover and activation of more receptors. Most importantly, accumulation of glutamate in the extracellular space can trigger excessive activation of glutamatergic receptors and lead to excitotoxicity, a trademark of many neurodegenerative diseases. It is thus of utmost importance for both physiological and pathophysiological reasons to understand the processes that control glutamate time course within the synaptic cleft and regulate its concentrations in the extracellular space. © 2017 Wiley Periodicals, Inc.
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Affiliation(s)
- Ciaran Murphy-Royal
- Neurocentre Magendie, Inserm U1215, Bordeaux, France.,Université de Bordeaux, Bordeaux, France
| | - Julien Dupuis
- Université de Bordeaux, Bordeaux, France.,Interdisciplinary Institute for Neuroscience, CNRS UMR 5297, Bordeaux, France
| | - Laurent Groc
- Université de Bordeaux, Bordeaux, France.,Interdisciplinary Institute for Neuroscience, CNRS UMR 5297, Bordeaux, France
| | - Stéphane H R Oliet
- Neurocentre Magendie, Inserm U1215, Bordeaux, France.,Université de Bordeaux, Bordeaux, France
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26
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Karklin Fontana AC, Fox DP, Zoubroulis A, Valente Mortensen O, Raghupathi R. Neuroprotective Effects of the Glutamate Transporter Activator (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153) following Traumatic Brain Injury in the Adult Rat. J Neurotrauma 2016; 33:1073-83. [PMID: 26200170 PMCID: PMC4892232 DOI: 10.1089/neu.2015.4079] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Traumatic brain injury (TBI) in humans and in animals leads to an acute and sustained increase in tissue glutamate concentrations within the brain, triggering glutamate-mediated excitotoxicity. Excitatory amino acid transporters (EAATs) are responsible for maintaining extracellular central nervous system glutamate concentrations below neurotoxic levels. Our results demonstrate that as early as 5 min and up to 2 h following brain trauma in brain-injured rats, the activity (Vmax) of EAAT2 in the cortex and the hippocampus was significantly decreased, compared with sham-injured animals. The affinity for glutamate (KM) and the expression of glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST) were not altered by the injury. Administration of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a GLT-1 activator, beginning immediately after injury and continuing for 24 h, significantly decreased neurodegeneration, loss of microtubule-associated protein 2 and NeuN (+) immunoreactivities, and attenuated calpain activation in both the cortex and the hippocampus at 24 h after the injury; the reduction in neurodegeneration remained evident up to 14 days post-injury. In synaptosomal uptake assays, MS-153 up-regulated GLT-1 activity in the naïve rat brain but did not reverse the reduced activity of GLT-1 in traumatically-injured brains. This study demonstrates that administration of MS-153 in the acute post-traumatic period provides acute and long-term neuroprotection for TBI and suggests that the neuroprotective effects of MS-153 are related to mechanisms other than GLT-1 activation, such as the inhibition of voltage-gated calcium channels.
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Affiliation(s)
| | - Douglas P. Fox
- Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Argie Zoubroulis
- Department of Pharmacology and Physiology, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Ole Valente Mortensen
- Department of Pharmacology and Physiology, Drexel University College of Medicine, Philadelphia, Pennsylvania
| | - Ramesh Raghupathi
- Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania
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27
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Ibáñez I, Díez-Guerra FJ, Giménez C, Zafra F. Activity dependent internalization of the glutamate transporter GLT-1 mediated by β-arrestin 1 and ubiquitination. Neuropharmacology 2016; 107:376-386. [PMID: 27044663 DOI: 10.1016/j.neuropharm.2016.03.042] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2015] [Revised: 03/07/2016] [Accepted: 03/24/2016] [Indexed: 10/22/2022]
Abstract
GLT-1 is the main glutamate transporter in the brain and undergoes trafficking processes that control its concentration on the cell surface thereby shaping glutamatergic neurotransmission. We have investigated how the traffic of GLT-1 is regulated by transporter activity. We report that internalization of GLT-1 from the cell surface is accelerated by transportable substrates like glutamate or aspartate, as well as by the transportable inhibitor L-trans-2,4-PDC, but not by the non-substrate inhibitor WAY 213613 in primary mixed cultures and in transiently transfected HEK293 cells. Analysis of the mechanism of endocytosis in HEK293 cells revealed that glutamate promoted the association with the transporter of the adaptor protein β-arrestin and the ubiquitin ligase Nedd4-2. The addition of glutamate is accompanied by an increase in the transporter ubiquitination, and the internalization is suppressed by an ubiquitination inhibitor (PYR41), and in a mutant defective in C-terminal lysines. The glutamate triggered endocytosis was also suppressed by siRNA for β-arrestin. This regulatory mechanism might be relevant in controlling the amount of transporter on the cell surface in conditions such as ischemia or traumatic brain injury, where extracellular concentrations of glutamate are persistently elevated.
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Affiliation(s)
- Ignacio Ibáñez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Raras, Spain; IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - F Javier Díez-Guerra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain
| | - Cecilio Giménez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Raras, Spain; IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Francisco Zafra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Raras, Spain; IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain.
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28
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Homeostasis of the astrocytic glutamate transporter GLT-1 is altered in mouse models of Lafora disease. Biochim Biophys Acta Mol Basis Dis 2016; 1862:1074-83. [PMID: 26976331 DOI: 10.1016/j.bbadis.2016.03.008] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2015] [Revised: 02/04/2016] [Accepted: 03/10/2016] [Indexed: 11/20/2022]
Abstract
Lafora disease (LD, OMIM 254780) is a fatal rare disorder characterized by epilepsy and neurodegeneration. Although in recent years a lot of information has been gained on the molecular basis of the neurodegeneration that accompanies LD, the molecular basis of epilepsy is poorly understood. Here, we present evidence indicating that the homeostasis of glutamate transporter GLT-1 (EAAT2) is compromised in mouse models of LD. Our results indicate that primary astrocytes from LD mice have reduced capacity of glutamate transport, probably because they present a reduction in the levels of the glutamate transporter at the plasma membrane. On the other hand, the overexpression in cellular models of laforin and malin, the two proteins related to LD, results in an accumulation of GLT-1 (EAAT2) at the plasma membrane and in a severe reduction of the ubiquitination of the transporter. All these results suggest that the laforin/malin complex slows down the endocytic recycling of the GLT-1 (EAAT2) transporter. Since, defects in the function of this transporter lead to excitotoxicity and epilepsy, we suggest that the epilepsy that accompanies LD could be due, at least in part, to deficiencies in the function of the GLT-1 (EAAT2) transporter.
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29
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Barrera SP, Castrejon-Tellez V, Trinidad M, Robles-Escajeda E, Vargas-Medrano J, Varela-Ramirez A, Miranda M. PKC-Dependent GlyT1 Ubiquitination Occurs Independent of Phosphorylation: Inespecificity in Lysine Selection for Ubiquitination. PLoS One 2015; 10:e0138897. [PMID: 26418248 PMCID: PMC4587969 DOI: 10.1371/journal.pone.0138897] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2014] [Accepted: 09/06/2015] [Indexed: 01/08/2023] Open
Abstract
Neurotransmitter transporter ubiquitination is emerging as the main mechanism for endocytosis and sorting of cargo into lysosomes. In this study, we demonstrate PKC-dependent ubiquitination of three different isoforms of the glycine transporter 1 (GlyT1). Incubation of cells expressing transporter with the PKC activator phorbol ester induced a dramatic, time-dependent increase in GlyT1 ubiquitination, followed by accumulation of GlyT1 in EEA1 positive early endosomes. This occurred via a mechanism that was abolished by inhibition of PKC. GlyT1 endocytosis was confirmed in both retinal sections and primary cultures of mouse amacrine neurons. Replacement of only all lysines in the N-and C-termini to arginines prevented ubiquitination and endocytosis, displaying redundancy in the mechanism of ubiquitination. Interestingly, a 40–50% reduction in glycine uptake was detected in phorbol-ester stimulated cells expressing the WT-GlyT1, whereas no significant change was for the mutant protein, demonstrating that endocytosis participates in the reduction of uptake. Consistent with previous findings for the dopamine transporter DAT, ubiquitination of GlyT1 tails functions as sorting signal to deliver transporter into the lysosome and removal of ubiquitination sites dramatically attenuated the rate of GlyT1 degradation. Finally, we showed for the first time that PKC-dependent GlyT1 phosphorylation was not affected by removal of ubiquitination sites, suggesting separate PKC-dependent signaling events for these posttranslational modifications.
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Affiliation(s)
- Susana P. Barrera
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Vicente Castrejon-Tellez
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Margarita Trinidad
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Elisa Robles-Escajeda
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Javier Vargas-Medrano
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Armando Varela-Ramirez
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
| | - Manuel Miranda
- Department of Biological Sciences and Border Biomedical Research Center, University of Texas at El Paso, El Paso, TX, 79968, United States of America
- * E-mail:
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30
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Takahashi K, Foster JB, Lin CLG. Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease. Cell Mol Life Sci 2015; 72:3489-506. [PMID: 26033496 PMCID: PMC11113985 DOI: 10.1007/s00018-015-1937-8] [Citation(s) in RCA: 108] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2015] [Revised: 05/22/2015] [Accepted: 05/26/2015] [Indexed: 12/12/2022]
Abstract
Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.
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Affiliation(s)
- Kou Takahashi
- Department of Neuroscience, The Ohio State University, 333 West 10th Avenue, Columbus, OH 43210 USA
| | - Joshua B. Foster
- Department of Neuroscience, The Ohio State University, 333 West 10th Avenue, Columbus, OH 43210 USA
| | - Chien-Liang Glenn Lin
- Department of Neuroscience, The Ohio State University, 333 West 10th Avenue, Columbus, OH 43210 USA
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31
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Activity-Dependent Plasticity of Astroglial Potassium and Glutamate Clearance. Neural Plast 2015; 2015:109106. [PMID: 26346563 PMCID: PMC4539499 DOI: 10.1155/2015/109106] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2015] [Accepted: 04/17/2015] [Indexed: 12/21/2022] Open
Abstract
Recent evidence has shown that astrocytes play essential roles in synaptic transmission and plasticity. Nevertheless, how neuronal activity alters astroglial functional properties and whether such properties also display specific forms of plasticity still remain elusive. Here, we review research findings supporting this aspect of astrocytes, focusing on their roles in the clearance of extracellular potassium and glutamate, two neuroactive substances promptly released during excitatory synaptic transmission. Their subsequent removal, which is primarily carried out by glial potassium channels and glutamate transporters, is essential for proper functioning of the brain. Similar to neurons, different forms of short- and long-term plasticity in astroglial uptake have been reported. In addition, we also present novel findings showing robust potentiation of astrocytic inward currents in response to repetitive stimulations at mild frequencies, as low as 0.75 Hz, in acute hippocampal slices. Interestingly, neurotransmission was hardly affected at this frequency range, suggesting that astrocytes may be more sensitive to low frequency stimulation and may exhibit stronger plasticity than neurons to prevent hyperexcitability. Taken together, these important findings strongly indicate that astrocytes display both short- and long-term plasticity in their clearance of excess neuroactive substances from the extracellular space, thereby regulating neuronal activity and brain homeostasis.
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32
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Zhang Y, Zhang X, Qu S. Ceftriaxone Protects Astrocytes from MPP(+) via Suppression of NF-κB/JNK/c-Jun Signaling. Mol Neurobiol 2014; 52:78-92. [PMID: 25112679 DOI: 10.1007/s12035-014-8845-z] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2014] [Accepted: 07/31/2014] [Indexed: 12/23/2022]
Abstract
Ceftriaxone has been shown to attenuate the dopaminergic neuron death and alleviate behavioral disorders in Parkinson's disease models via upregulation of glutamate transporter-1 (GLT-1) and decreases in extracellular glutamate. However, details of how this neuroprotection occurs are uncertain. We hypothesized that cytoprotection by ceftriaxone in astrocytes exposed to 1-methyl-4-phenylpyridinium (MPP(+)) involves suppression of the NF-κB/JNK/c-Jun signaling pathway. Here, we observed a protective effect of ceftriaxone in primary astrocytes exposed to MPP(+). Ceftriaxone enhanced glutamate uptake and promoted primary astrocyte viability after MPP(+) exposure. Ceftriaxone enhances glutamate uptake via upregulation of GLT-1 in the plasma membrane, and alleviates MPP(+)-induced neurotoxicity via suppression of NF-κB/JNK/c-Jun signaling. Collectively, our data offer evidence that increased expression and function of GLT-1 are involved in the protective mechanism of ceftriaxone in astrocytes exposed to MPP(+) in vitro, and we offer insight into the potential therapeutic role of ceftriaxone in treatment of Parkinson's disease.
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Affiliation(s)
- Yunlong Zhang
- Department of Immunology, School of Basic Medical Sciences, Southern Medical University, Guangzhou, Guangdong, 510515, China
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33
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Genetic dys-regulation of astrocytic glutamate transporter EAAT2 and its implications in neurological disorders and manganese toxicity. Neurochem Res 2014; 40:380-8. [PMID: 25064045 DOI: 10.1007/s11064-014-1391-2] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2014] [Revised: 07/09/2014] [Accepted: 07/15/2014] [Indexed: 12/14/2022]
Abstract
Astrocytic glutamate transporters, the excitatory amino acid transporter (EAAT) 2 and EAAT1 (glutamate transporter 1 and glutamate aspartate transporter in rodents, respectively), are the main transporters for maintaining optimal glutamate levels in the synaptic clefts by taking up more than 90% of glutamate from extracellular space thus preventing excitotoxic neuronal death. Reduced expression and function of these transporters, especially EAAT2, has been reported in numerous neurological disorders, including amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, schizophrenia and epilepsy. The mechanism of down-regulation of EAAT2 in these diseases has yet to be fully established. Genetic as well as transcriptional dys-regulation of these transporters by various modes, such as single nucleotide polymorphisms and epigenetics, resulting in impairment of their functions, might play an important role in the etiology of neurological diseases. Consequently, there has been an extensive effort to identify molecular targets for enhancement of EAAT2 expression as a potential therapeutic approach. Several pharmacological agents increase expression of EAAT2 via nuclear factor κB and cAMP response element binding protein at the transcriptional level. However, the negative regulatory mechanisms of EAAT2 have yet to be identified. Recent studies, including those from our laboratory, suggest that the transcriptional factor yin yang 1 plays a critical role in the repressive effects of various neurotoxins, such as manganese (Mn), on EAAT2 expression. In this review, we will focus on transcriptional epigenetics and translational regulation of EAAT2.
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34
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Adenosine and glutamate in neuroglial interaction: implications for circadian disorders and alcoholism. ADVANCES IN NEUROBIOLOGY 2014; 11:103-19. [PMID: 25236726 DOI: 10.1007/978-3-319-08894-5_6] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Recent studies have demonstrated that the function of glia is not restricted to the support of neuronal function. In fact, astrocytes are essential for neuronal activity in the brain and play an important role in the regulation of complex behavior. Astrocytes actively participate in synapse formation and brain information processing by releasing and uptaking glutamate, D-serine, adenosine 5'-triphosphate (ATP), and adenosine. In the central nervous system, adenosine-mediated neuronal activity modulates the actions of other neurotransmitter systems. Adenosinergic fine-tuning of the glutamate system in particular has been shown to regulate circadian rhythmicity and sleep, as well as alcohol-related behavior and drinking. Adenosine gates both photic (light-induced) glutamatergic and nonphotic (alerting) input to the circadian clock located in the suprachiasmatic nucleus of the hypothalamus. Astrocytic, SNARE-mediated ATP release provides the extracellular adenosine that drives homeostatic sleep. Acute ethanol increases extracellular adenosine, which mediates the ataxic and hypnotic/sedative effects of alcohol, while chronic ethanol leads to downregulated adenosine signaling that underlies insomnia, a major predictor of relapse. Studies using mice lacking the equilibrative nucleoside transporter 1 have illuminated how adenosine functions through neuroglial interactions involving glutamate uptake transporter GLT-1 [referred to as excitatory amino acid transporter 2 (EAAT2) in human] and possibly water channel aquaporin 4 to regulate ethanol sensitivity, reward-related motivational processes, and alcohol intake.
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35
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Rauen T, Tanui R, Grewer C. Structural and functional dynamics of Excitatory Amino Acid Transporters (EAAT). AIMS MOLECULAR SCIENCE 2014. [DOI: 10.3934/molsci.2014.3.99] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
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36
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Grewer C, Gameiro A, Rauen T. SLC1 glutamate transporters. Pflugers Arch 2013; 466:3-24. [PMID: 24240778 DOI: 10.1007/s00424-013-1397-7] [Citation(s) in RCA: 82] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2013] [Revised: 10/21/2013] [Accepted: 10/22/2013] [Indexed: 12/13/2022]
Abstract
The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.
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Affiliation(s)
- Christof Grewer
- Department of Chemistry, Binghamton University, PO Box 6000, Binghamton, 13902-6000, NY, USA,
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37
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Sogaard R, Borre L, Braunstein TH, Madsen KL, MacAulay N. Functional modulation of the glutamate transporter variant GLT1b by the PDZ domain protein PICK1. J Biol Chem 2013; 288:20195-207. [PMID: 23697999 DOI: 10.1074/jbc.m113.471128] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The dominant glutamate transporter isoform in the mammalian brain, GLT1, exists as at least three splice variants, GLT1a, GLT1b, and GLT1c. GLT1b interacts with the scaffold protein PICK1 (protein interacting with kinase C1), which is implicated in glutamatergic neurotransmission via its regulatory effect on trafficking of AMPA-type glutamate receptors. The 11 extreme C-terminal residues specific for the GLT1b variant are essential for its specific interaction with the PICK1 PDZ domain, but a functional consequence of this interaction has remained unresolved. To identify a functional effect of PICK1 on GLT1a or GLT1b separately, we employed the Xenopus laevis expression system. GLT1a and GLT1b displayed similar electrophysiological properties and EC50 for glutamate. Co-expressed PICK1 localized efficiently to the plasma membrane and resulted in a 5-fold enhancement of the leak current in GLT1b-expressing oocytes with only a minor effect on [(3)H]glutamate uptake. Three different GLT1 substrates all caused a slow TBOA-sensitive decay in the membrane current upon prolonged application, which provides support for the leak current being mediated by GLT1b itself. Leak and glutamate-evoked currents in GLT1a-expressing oocytes were unaffected by PICK1 co-expression. PKC activation down-regulated GLT1a and GLT1b activity to a similar extent, which was not affected by co-expression of PICK1. In conclusion, PICK1 may not only affect glutamatergic neurotransmission by its regulatory effect on glutamate receptors but may also affect neuronal excitability via an increased GLT1b-mediated leak current. This may be particularly relevant in pathological conditions such as amyotrophic lateral sclerosis and cerebral hypoxia, which are associated with neuronal GLT1b up-regulation.
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Affiliation(s)
- Rikke Sogaard
- Department of Cellular and Molecular Medicine, University of Copenhagen, DK-2200 Copenhagen N, Denmark
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Sidoryk-Wegrzynowicz M, Aschner M. Manganese toxicity in the central nervous system: the glutamine/glutamate-γ-aminobutyric acid cycle. J Intern Med 2013; 273:466-77. [PMID: 23360507 PMCID: PMC3633698 DOI: 10.1111/joim.12040] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Manganese (Mn) is an essential trace element that is required for maintaining proper function and regulation of numerous biochemical and cellular reactions. Despite its essentiality, at excessive levels Mn is toxic to the central nervous system (CNS). Increased accumulation of Mn in specific brain regions, such as the substantia nigra, globus pallidus and striatum, triggers neurotoxicity resulting in a neurological brain disorder, termed manganism. Mn has been also implicated in the pathophysiology of several other neurodegenerative diseases. Its toxicity is associated with disruption of the glutamine (Gln)/glutamate (Glu)-γ-aminobutyric acid (GABA) cycle (GGC) between astrocytes and neurons, thus leading to changes in Glu-ergic and/or GABAergic transmission and Gln metabolism. Here we discuss the common mechanisms underlying Mn-induced neurotoxicity and their relationship to CNS pathology and GGC impairment.
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de Juan-Sanz J, Núñez E, López-Corcuera B, Aragón C. Constitutive endocytosis and turnover of the neuronal glycine transporter GlyT2 is dependent on ubiquitination of a C-terminal lysine cluster. PLoS One 2013; 8:e58863. [PMID: 23484054 PMCID: PMC3590132 DOI: 10.1371/journal.pone.0058863] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Accepted: 02/07/2013] [Indexed: 12/13/2022] Open
Abstract
Inhibitory glycinergic neurotransmission is terminated by sodium and chloride-dependent plasma membrane glycine transporters (GlyTs). The mainly glial glycine transporter GlyT1 is primarily responsible for the completion of inhibitory neurotransmission and the neuronal glycine transporter GlyT2 mediates the reuptake of the neurotransmitter that is used to refill synaptic vesicles in the terminal, a fundamental role in the physiology and pathology of glycinergic neurotransmission. Indeed, inhibitory glycinergic neurotransmission is modulated by the exocytosis and endocytosis of GlyT2. We previously reported that constitutive and Protein Kinase C (PKC)-regulated endocytosis of GlyT2 is mediated by clathrin and that PKC accelerates GlyT2 endocytosis by increasing its ubiquitination. However, the role of ubiquitination in the constitutive endocytosis and turnover of this protein remains unexplored. Here, we show that ubiquitination of a C-terminus four lysine cluster of GlyT2 is required for constitutive endocytosis, sorting into the slow recycling pathway and turnover of the transporter. Ubiquitination negatively modulates the turnover of GlyT2, such that increased ubiquitination driven by PKC activation accelerates transporter degradation rate shortening its half-life while decreased ubiquitination increases transporter stability. Finally, ubiquitination of GlyT2 in neurons is highly responsive to the free pool of ubiquitin, suggesting that the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1 (UCHL1), as the major regulator of neuronal ubiquitin homeostasis, indirectly modulates the turnover of GlyT2. Our results contribute to the elucidation of the mechanisms underlying the dynamic trafficking of this important neuronal protein which has pathological relevance since mutations in the GlyT2 gene (SLC6A5) are the second most common cause of human hyperekplexia.
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Affiliation(s)
- Jaime de Juan-Sanz
- Centro de Biología Molecular ‘‘Severo Ochoa’’, Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, Madrid, Spain
- Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Madrid, Spain
- IdiPAZ-Hospital Universitario La Paz, Madrid, Spain
| | - Enrique Núñez
- Centro de Biología Molecular ‘‘Severo Ochoa’’, Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, Madrid, Spain
- Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Madrid, Spain
- IdiPAZ-Hospital Universitario La Paz, Madrid, Spain
| | - Beatriz López-Corcuera
- Centro de Biología Molecular ‘‘Severo Ochoa’’, Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, Madrid, Spain
- Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Madrid, Spain
- IdiPAZ-Hospital Universitario La Paz, Madrid, Spain
| | - Carmen Aragón
- Centro de Biología Molecular ‘‘Severo Ochoa’’, Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, Madrid, Spain
- Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Madrid, Spain
- IdiPAZ-Hospital Universitario La Paz, Madrid, Spain
- * E-mail:
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Ubiquitination of neurotransmitter receptors and postsynaptic scaffolding proteins. Neural Plast 2013; 2013:432057. [PMID: 23431475 PMCID: PMC3574743 DOI: 10.1155/2013/432057] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2012] [Accepted: 12/26/2012] [Indexed: 12/19/2022] Open
Abstract
The human brain is made up of an extensive network of neurons that communicate by forming specialized connections called synapses. The amount, location, and dynamic turnover of synaptic proteins, including neurotransmitter receptors and synaptic scaffolding molecules, are under complex regulation and play a crucial role in synaptic connectivity and plasticity, as well as in higher brain functions. An increasing number of studies have established ubiquitination and proteasome-mediated degradation as universal mechanisms in the control of synaptic protein homeostasis. In this paper, we focus on the role of the ubiquitin-proteasome system (UPS) in the turnover of major neurotransmitter receptors, including glutamatergic and nonglutamatergic receptors, as well as postsynaptic receptor-interacting proteins.
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Lee MR, Ruby CL, Hinton DJ, Choi S, Adams CA, Young Kang N, Choi DS. Striatal adenosine signaling regulates EAAT2 and astrocytic AQP4 expression and alcohol drinking in mice. Neuropsychopharmacology 2013; 38:437-45. [PMID: 23032072 PMCID: PMC3547194 DOI: 10.1038/npp.2012.198] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Adenosine signaling is implicated in several neuropsychiatric disorders, including alcoholism. Among its diverse functions in the brain, adenosine regulates glutamate release and has an essential role in ethanol sensitivity and preference. However, the molecular mechanisms underlying adenosine-mediated glutamate signaling in neuroglial interaction remain elusive. We have previously shown that mice lacking the ethanol-sensitive adenosine transporter, type 1 equilibrative nucleoside transporter (ENT1), drink more ethanol compared with wild-type mice and have elevated striatal glutamate levels. In addition, ENT1 inhibition or knockdown reduces glutamate transporter expression in cultured astrocytes. Here, we examined how adenosine signaling in astrocytes contributes to ethanol drinking. Inhibition or deletion of ENT1 reduced the expression of type 2 excitatory amino-acid transporter (EAAT2) and the astrocyte-specific water channel, aquaporin 4 (AQP4). EAAT2 and AQP4 colocalization was also reduced in the striatum of ENT1 null mice. Ceftriaxone, an antibiotic compound known to increase EAAT2 expression and function, elevated not only EAAT2 but also AQP4 expression in the striatum. Furthermore, ceftriaxone reduced ethanol drinking, suggesting that ENT1-mediated downregulation of EAAT2 and AQP4 expression contributes to excessive ethanol consumption in our mouse model. Overall, our findings indicate that adenosine signaling regulates EAAT2 and astrocytic AQP4 expressions, which control ethanol drinking in mice.
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Affiliation(s)
- Moonnoh R Lee
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA,Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - Christina L Ruby
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - David J Hinton
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - Sun Choi
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - Chelsea A Adams
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - Na Young Kang
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA
| | - Doo-Sup Choi
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, MN, USA,Department of Psychiatry and Psychology, Mayo Clinic College of Medicine, Rochester, MN, USA,Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905, USA. Tel: +1 507 284-5602, Fax: +1 507 266-0824, E-mail:
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Sidoryk-Wegrzynowicz M, Lee E, Aschner M. Mechanism of Mn(II)-mediated dysregulation of glutamine-glutamate cycle: focus on glutamate turnover. J Neurochem 2012; 122:856-67. [PMID: 22708868 DOI: 10.1111/j.1471-4159.2012.07835.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
Manganese (Mn) has been implicated in the impairment of the glutamate-glutamine cycling (GGC) by deregulation of Glu and glutamine (Gln) turnover in astrocytes. Here, we have examined possible mechanisms involved in the Mn(II)-mediated disruption of Glu turnover, including those related to protein degradation, such as the proteasomal and lysosomal machinery. Our study revealed that lysosome but not proteasomal inhibition is responsible for down-regulation of the Glu transporter after Mn(II) treatment. Because protein kinase C (PKC) activation leads to the down-regulation of Glu carriers, and Mn(II) increases PKC activity, we hypothesized that the PKC signaling contributes to the Mn(II)-mediated disruption of Glu turnover. Our results show that PKC activation causes a decrease in Glu uptake and that inhibition of PKC reverses Mn(II)-dependent down-regulation of Glu influx as well as glutamate transporter 1 (GLT1) and glutamate-aspartate transporter (GLAST) protein level. Co-immunoprecipitation studies show association of GLT1 with the PKCδ and PKCα isoforms and Mn(II)-induced specific increase in PKCδ-GLT1 interaction. In addition, astrocytes transfected with shRNA against PKCδ show decreased sensitivity to Mn(II) compared with those transfected with control shRNA or shRNA targeted against PKCα. Taken together, these findings demonstrate that PKCδ signaling is involved in the Mn(II)-induced deregulation of Glu turnover in astrocytes.
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43
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Glisic D, Lehmann C, Figiel M, Ödemis V, Lindner R, Engele J. A novel cross-talk between endothelin and ErbB receptors controlling glutamate transporter expression in astrocytes. J Neurochem 2012; 122:844-55. [PMID: 22671705 DOI: 10.1111/j.1471-4159.2012.07819.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
The endothelin and epidermal growth factor (EGF) systems are central to the control of reactive brain processes and are thought to partly exert these tasks by endothelin-induced transactivation of the epidermal growth factor receptor (EGFR) Here we show that beyond EGFR transactivation, endothelins prevent the ligand-induced internalization of the EGFR. We unravel that endothelins abrogate internalization of the EGFR by either promoting the formation of "internalization-deficient" EGFR/ErB2-heterodimers or by activating c-Abl kinase, a negative regulator of EGFR internalization. We further provide evidence that this cross-talk is operational in the control of astrocytic glutamate transport. Specifically, we establish that the inhibitory effects exerted by endothelins on basal as well as EGF-induced expression of the major astroglial glutamate transporter subtype, glutamate transporter 1, are a direct consequence of the endothelin-dependent retention of the EGFR at the cell surface. Together our findings unravel a previously unknown cross-talk between endothelin and epidermal growth factor receptors, which may have implications for a variety of pathological conditions.
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Affiliation(s)
- Darko Glisic
- Institute of Anatomy, University of Leipzig, Medical Faculty, Leipzig, Germany
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Martínez-Villarreal J, García Tardón N, Ibáñez I, Giménez C, Zafra F. Cell surface turnover of the glutamate transporter GLT-1 is mediated by ubiquitination/deubiquitination. Glia 2012; 60:1356-65. [PMID: 22593014 DOI: 10.1002/glia.22354] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2012] [Accepted: 04/23/2012] [Indexed: 11/11/2022]
Abstract
The main glutamate transporter in the brain, GLT-1, mediates glutamatergic neurotransmission in both physiological and pathological conditions. GLT-1 activity is controlled by both constitutive and regulated trafficking, and although recent evidence indicates that the turnover of this protein in the plasma membrane is accelerated by protein kinase C via an ubiquitin-dependent process, the mechanisms driving the constitutive trafficking of GLT-1 remain unexplored. Here, we used a heterologous system and primary astrocytes to investigate the turnover of GLT-1 and the role of ubiquitin attachment in this process. We show that GLT-1 is endocytosed constitutively in a clathrin-dependent manner, recycling the transporter into endosomes containing EEA1 and Rab4, a marker of rapidly recycling endosomes, and not Rab11 or Rab7, markers of the slow recycling and late endosomal compartments, respectively. We also show that this process is dependent on ubiquitination, because the inhibitor of the ubiquitin-activating enzyme E1, 4[4-(5-nitro-furan-2-ylmethylene)-3,5-dioxo-pyrazolidin-1-yl]-benzoic acid ethyl ester, promotes the retention of GLT-1 at the plasma membrane. Moreover, site-directed mutagenesis demonstrated the involvement of lysines 517 and 526 of GLT-1 in the constitutive internalization of the transporter. The translocation of GLT-1 from the recycling endosomes to the plasma membrane was blocked by LDN-57444, a specific inhibitor to the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1, but not by an inhibitor of the related DUB ubiquitin C-terminal hydrolase-L3, supporting the existence of specific ubiquitination/deubiquitination cycles that ensure the correct concentrations of GLT-1 at the cell surface.
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Affiliation(s)
- Jaime Martínez-Villarreal
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain
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García-Tardón N, González-González IM, Martínez-Villarreal J, Fernández-Sánchez E, Giménez C, Zafra F. Protein kinase C (PKC)-promoted endocytosis of glutamate transporter GLT-1 requires ubiquitin ligase Nedd4-2-dependent ubiquitination but not phosphorylation. J Biol Chem 2012; 287:19177-87. [PMID: 22505712 DOI: 10.1074/jbc.m112.355909] [Citation(s) in RCA: 77] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Glutamate transporter-1 (GLT-1) is the main glutamate transporter in the central nervous system, and its concentration severely decreases in neurodegenerative diseases. The number of transporters in the plasma membrane reflects the balance between their insertion and removal, and it has been reported that the regulated endocytosis of GLT-1 depends on its ubiquitination triggered by protein kinase C (PKC) activation. Here, we identified serine 520 of GLT-1 as the primary target for PKC-dependent phosphorylation, although elimination of this serine did not impair either GLT-1 ubiquitination or endocytosis in response to phorbol esters. In fact, we present evidence indicating that the ubiquitin ligase Nedd4-2 mediates the PKC-dependent ubiquitination and down-regulation of GLT-1. Overexpression of Nedd4-2 increased the ubiquitination of the transporter and promoted its degradation. Moreover, phorbol myristate acetate enhanced Nedd4-2 phosphorylation and the formation of GLT-1·Nedd4-2 complexes, whereas siRNA knockdown of Nedd4-2 prevented ubiquitination, endocytosis, and the concomitant decrease in GLT-1 activity triggered by PKC activation. These results indicate that GLT-1 endocytosis is independent of its phosphorylation and that Nedd4-2 mediates PKC-dependent down-regulation of the transporter.
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Affiliation(s)
- Noemí García-Tardón
- Departamento de Biología Molecular and Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid-Consejo Superior de Investigaciones Científicas, 28049 Madrid, Spain
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Chan TJ, Her LS, Liaw HJ, Chen MC, Tzeng SF. Retinoic acid mediates the expression of glutamate transporter-1 in rat astrocytes through genomic RXR action and non-genomic protein kinase C signaling pathway. J Neurochem 2012; 121:537-50. [PMID: 22380620 DOI: 10.1111/j.1471-4159.2012.07715.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Astrocytic glutamate transporter-1 (GLT-1) is responsible for 90% of forebrain glutamate uptake in the adult CNS. Retinoic acid (RA) is a potent regulator of neural cell differentiation and neuronal maturation in the developing CNS through activation of RA receptors/retinoic X receptors (RXRs) or non-genomic mechanisms. Although rat GLT-1 contains several RXR binding regions, RA-triggered RXR mechanisms regulating GLT-1 expression remain unknown. RA applied at submicromolar concentrations for 24 h significantly reduced GLT-1 mRNA and membrane levels in astrocytes and dibutyryl cAMP (dbcAMP)-primed astrocytes. An RXR agonist reduced astrocytic GLT-1 mRNA expression, whereas an RXR antagonist blocked the effects of RA on the reduction of astrocytic GLT-1 mRNA expression. Electrophoresis motility shift assay indicated that RA-treatment increased astrocytic RXR-DNA binding activity. RA-induced reduction in GLT-1 mRNA expression was also observed in dbcAMP-primed astrocytes. Through lentivirus-mediated astrocytic over-expression of rat GLT-1, levels of GLT-1 in the processes of dbcAMP-treated astrocytes were attenuated by exposure to RA. The protein kinase C inhibitor, Bis I, restored GLT-1 distribution in the processes of RA-treated dbcAMP-primed astrocytes. These results suggest that RA reduces astrocytic GLT-1 levels through both RXR-mediated inhibition at the transcriptional level and triggering activation of protein kinase C which reduces cell surface GLT-1 levels.
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Affiliation(s)
- Ti-Ju Chan
- Department of Life Sciences, College of Bioscience and Biotechnology, National Cheng Kung University, Tainan, Taiwan
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de Juan-Sanz J, Zafra F, López-Corcuera B, Aragón C. Endocytosis of the neuronal glycine transporter GLYT2: role of membrane rafts and protein kinase C-dependent ubiquitination. Traffic 2011; 12:1850-67. [PMID: 21910806 DOI: 10.1111/j.1600-0854.2011.01278.x] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Glycinergic neurotransmission is terminated by sodium- and chloride-dependent plasma membrane transporters. The neuronal glycine transporter 2 (GLYT2) supplies the terminal with substrate to refill synaptic vesicles containing glycine. This crucial process is defective in human hyperekplexia, a condition that can be caused by mutations in GLYT2. Inhibitory glycinergic neurotransmission is modulated by the GLYT2 exocytosis/endocytosis equilibrium, although the mechanisms underlying the turnover of this transporter remain elusive. We studied GLYT2 internalization pathways and the role of ubiquitination and membrane raft association of the transporter in its endocytosis. Using pharmacological tools, dominant-negative mutants and small-interfering RNAs, we show that the clathrin-mediated pathway is the primary mechanism for constitutive and regulated GLYT2 endocytosis in heterologous cells and neurons. We show that GLYT2 is constitutively internalized from cell surface lipid rafts, remaining associated with rafts in subcellular recycling structures. Protein kinase C (PKC) negatively modulates GLYT2 via rapid and dynamic redistribution of GLYT2 from raft to non-raft membrane subdomains and increasing ubiquitinated GLYT2 endocytosis. This biphasic mechanism is a versatile means to modulate GLYT2 behavior and hence, inhibitory glycinergic neurotransmission. These findings may reveal new therapeutic targets to address glycinergic pathologies associated with alterations in GLYT2 trafficking.
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Affiliation(s)
- Jaime de Juan-Sanz
- Departamento de Biología Molecular and Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid, Madrid, Spain
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Lei JT, Mazumdar T, Martinez-Moczygemba M. Three lysine residues in the common β chain of the interleukin-5 receptor are required for Janus kinase (JAK)-dependent receptor ubiquitination, endocytosis, and signaling. J Biol Chem 2011; 286:40091-103. [PMID: 21965659 DOI: 10.1074/jbc.m111.273482] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
Eosinophils are multifunctional leukocytes implicated in the pathogenesis of numerous inflammatory diseases including allergic asthma and hypereosinophilic syndrome. Eosinophil physiology is critically dependent on IL-5 and the IL-5 receptor (IL-5R), composed of a ligand binding α chain (IL-5Rα), and a common β chain, βc. Previously, we demonstrated that the βc cytoplasmic tail is ubiquitinated and degraded by proteasomes following IL-5 stimulation. However, a complete understanding of the role of βc ubiquitination in IL-5R biology is currently lacking. By using a well established, stably transduced HEK293 cell model system, we show here that in the absence of ubiquitination, βc subcellular localization, IL-5-induced endocytosis, turnover, and IL-5R signaling were significantly impaired. Whereas ubiquitinated IL-5Rs internalized into trafficking endosomes for their degradation, ubiquitination-deficient IL-5Rs accumulated on the cell surface and displayed blunted signaling even after IL-5 stimulation. Importantly, we identified a cluster of three membrane-proximal βc lysine residues (Lys(457), Lys(461), and Lys(467)) whose presence was required for both JAK1/2 binding to βc and receptor ubiquitination. These findings establish that JAK kinase binding to βc requires the presence of three critical βc lysine residues, and this binding event is essential for receptor ubiquitination, endocytosis, and signaling.
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Affiliation(s)
- Jonathan T Lei
- Department of Microbial and Molecular Pathogenesis, College of Medicine, Texas A&M Health Science Center, Houston, Texas 77030, USA
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Bertelsen V, Sak MM, Breen K, Rødland MS, Johannessen LE, Traub LM, Stang E, Madshus IH. A chimeric pre-ubiquitinated EGF receptor is constitutively endocytosed in a clathrin-dependent, but kinase-independent manner. Traffic 2011; 12:507-20. [PMID: 21226813 DOI: 10.1111/j.1600-0854.2011.01162.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The roles of EGF receptor (EGFR) kinase activity and ubiquitination in EGFR endocytosis have been controversial. The adaptor protein and ubiquitin ligase Cbl has reportedly been required. Consistently, we now report that siRNA-mediated knock-down of c-Cbl and Cbl-b significantly slowed clathrin-dependent internalization of activated wild-type (wt) EGFR by inhibiting recruitment of the EGFR to clathrin-coated pits. However, a chimeric protein consisting of wt-EGFR, a C-terminal linker and four linearly connected ubiquitins was found to interact with Eps15 and epsin 1 and to be constitutively endocytosed in a clathrin-dependent manner. Interestingly, endocytosis of this fusion protein did not require binding of EGF. Nor was kinase activity required, and the fusion protein was endocytosed in the presence of an EGFR kinase inhibitor, which efficiently counteracted tyrosine phosphorylation. This demonstrates that ubiquitination over-rides the requirement for kinase activity in recruitment of the EGFR to clathrin-coated pits.
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Affiliation(s)
- Vibeke Bertelsen
- Institute of Pathology, Faculty of Medicine, University of Oslo, 0027 Oslo, Norway
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50
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Madshus IH, Stang E. Internalization and intracellular sorting of the EGF receptor: a model for understanding the mechanisms of receptor trafficking. J Cell Sci 2009; 122:3433-9. [PMID: 19759283 DOI: 10.1242/jcs.050260] [Citation(s) in RCA: 112] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
The epidermal growth factor receptor (EGFR; also known as ErbB1) is one of four related receptor tyrosine kinases. These receptors (EGFR, ErbB2, ErbB3 and ErbB4) are frequently overexpressed in cancer and such overexpression is associated with poor clinical outcome. Understanding the mechanisms involved in growth-factor-receptor downregulation is medically important, as several drugs that interfere with the function and trafficking of ErbB proteins are currently being developed or are already in clinical trials. EGFR has become a model protein for understanding the biology and endocytosis of related growth-factor receptors, and the mechanisms involved in its endocytosis and degradation have been scrutinized for several decades. Nevertheless, the details and principles of these processes are still poorly understood and often controversial. In particular, the literature describing how the ubiquitylation and recruitment of EGFR to clathrin-coated pits are connected is inconsistent and confusing. In this Opinion article, we discuss the impact of signaling motifs, kinase activity and ubiquitylation on clathrin-dependent endocytosis and lysosomal sorting of EGFR. In addition, we discuss potential explanations for contradicting reports, and propose models for the recruitment of ligand-activated EGFR to clathrin-coated pits as well as for lysosomal sorting of ligand-activated EGFR.
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Affiliation(s)
- Inger Helene Madshus
- University of Oslo, Institute of Pathology, Rikshospitalet, N-0027 Oslo, Norway.
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