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Suslova M, Kortzak D, Machtens JP, Kovermann P, Fahlke C. Apo state pore opening as functional basis of increased EAAT anion channel activity in episodic ataxia 6. Front Physiol 2023; 14:1147216. [PMID: 37538371 PMCID: PMC10394623 DOI: 10.3389/fphys.2023.1147216] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2023] [Accepted: 07/07/2023] [Indexed: 08/05/2023] Open
Abstract
SLC1A2 and SLC1A3 encode the glial glutamate transporters EAAT2 and EAAT1, which are not only the predominant glutamate uptake carriers in our brain, but also function as anion channels. Two homologous mutations, which predict substitutions of prolines in the center of the fifth transmembrane helix by arginine (P289R EAAT2, P290R EAAT1), have been identified in patients with epileptic encephalopathy (SLC1A2) or with episodic ataxia type 6 (SLC1A3). Both mutations have been shown to impair glutamate uptake and to increase anion conduction. The molecular processes that link the disease-causing mutations to two major alterations of glutamate transporter function remain insufficiently understood. The mutated proline is conserved in every EAAT. Since the pathogenic changes mainly affect the anion channel function, we here study the functional consequences of the homologous P312R mutation in the neuronal glutamate transporter EAAT4, a low capacity glutamate transporter with predominant anion channel function. To assess the impact of charge and structure of the inserted amino acid for the observed functional changes, we generated and functionally evaluated not only P312R, but also substitutions of P312 with all other amino acids. However, only exchange of proline by arginine, lysine, histidine and asparagine were functionally tolerated. We compared WT, P312R and P312N EAAT4 using a combination of cellular electrophysiology, fast substrate application and kinetic modelling. We found that WT and mutant EAAT4 anion currents can be described with a 11-state model of the transport cycle, in which several states are connected to branching anion channel states to account for the EAAT anion channel function. Substitutions of P312 modify various transitions describing substrate binding/unbinding, translocation or anion channel opening. Most importantly, P312R generates a new anion conducting state that is accessible in the outward facing apo state and that is the main determinant of the increased anion conduction of EAAT transporters carrying this mutation. Our work provides a quantitative description how a naturally occurring mutation changes glutamate uptake and anion currents in two genetic diseases.
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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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Kovermann P, Engels M, Müller F, Fahlke C. Cellular Physiology and Pathophysiology of EAAT Anion Channels. Front Cell Neurosci 2022; 15:815279. [PMID: 35087380 PMCID: PMC8787812 DOI: 10.3389/fncel.2021.815279] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2021] [Accepted: 12/13/2021] [Indexed: 11/17/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) optimize the temporal resolution and energy demand of mammalian excitatory synapses by quickly removing glutamate from the synaptic cleft into surrounding neuronal and glial cells and ensuring low resting glutamate concentrations. In addition to secondary active glutamate transport, EAATs also function as anion channels. The channel function of these transporters is conserved in all homologs ranging from archaebacteria to mammals; however, its physiological roles are insufficiently understood. There are five human EAATs, which differ in their glutamate transport rates. Until recently the high-capacity transporters EAAT1, EAAT2, and EAAT3 were believed to conduct only negligible anion currents, with no obvious function in cell physiology. In contrast, the low-capacity glutamate transporters EAAT4 and EAAT5 are thought to regulate neuronal signaling as glutamate-gated channels. In recent years, new experimental approaches and novel animal models, together with the discovery of a human genetic disease caused by gain-of-function mutations in EAAT anion channels have enabled identification of the first physiological and pathophysiological roles of EAAT anion channels.
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Danbolt NC, López-Corcuera B, Zhou Y. Reconstitution of GABA, Glycine and Glutamate Transporters. Neurochem Res 2022; 47:85-110. [PMID: 33905037 PMCID: PMC8763731 DOI: 10.1007/s11064-021-03331-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2021] [Revised: 04/13/2021] [Accepted: 04/15/2021] [Indexed: 10/25/2022]
Abstract
In contrast to water soluble enzymes which can be purified and studied while in solution, studies of solute carrier (transporter) proteins require both that the protein of interest is situated in a phospholipid membrane and that this membrane forms a closed compartment. An additional challenge to the study of transporter proteins has been that the transport depends on the transmembrane electrochemical gradients. Baruch I. Kanner understood this early on and first developed techniques for studying plasma membrane vesicles. This advanced the field in that the experimenter could control the electrochemical gradients. Kanner, however, did not stop there, but started to solubilize the membranes so that the transporter proteins were taken out of their natural environment. In order to study them, Kanner then had to find a way to reconstitute them (reinsert them into phospholipid membranes). The scope of the present review is both to describe the reconstitution method in full detail as that has never been done, and also to reveal the scientific impact that this method has had. Kanner's later work is not reviewed here although that also deserves a review because it too has had a huge impact.
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Affiliation(s)
- Niels Christian Danbolt
- Neurotransporter Group, Department of Molecular Medicine, Institute of Basic Medical Sciences, University of Oslo, 0317, Oslo, Norway.
| | - Beatriz López-Corcuera
- Departamento de Biología Molecular, Universidad Autónoma de Madrid, Madrid, Spain
- Centro de Biología Molecular "Severo Ochoa" Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain
- IdiPAZ, Hospital Universitario La Paz, Madrid, Spain
| | - Yun Zhou
- Neurotransporter Group, Department of Molecular Medicine, Institute of Basic Medical Sciences, University of Oslo, 0317, Oslo, Norway
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Kovermann P, Kolobkova Y, Franzen A, Fahlke C. Mutations associated with epileptic encephalopathy modify EAAT2 anion channel function. Epilepsia 2021; 63:388-401. [PMID: 34961934 DOI: 10.1111/epi.17154] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2021] [Revised: 12/09/2021] [Accepted: 12/09/2021] [Indexed: 11/28/2022]
Abstract
OBJECTIVE Mutations in the gene solute carrier family member 1A2 (SLC1A2) encoding the excitatory amino acid transporter 2 (EAAT2) are associated with severe forms of epileptic encephalopathy. EAAT2 is expressed in glial cells and presynaptic nerve terminals and represents the main l-glutamate uptake carrier in the mammalian brain. It does not only function as a secondary active glutamate transporter, but also as an anion channel. How naturally occurring mutations affect these two transport functions of EAAT2 and how such alterations cause epilepsy is insufficiently understood. METHODS Here we studied the functional consequences of three disease-associated mutations, which predict amino acid exchanges p.Gly82Arg (G82R), p.Leu85Pro (L85P), and p.Pro289Arg (P289R), by heterologous expression in mammalian cells, biochemistry, confocal imaging, and whole-cell patch-clamp recordings of EAAT2 l-glutamate transport and anion current. RESULTS G82R and L85P exchange amino acid residues contribute to the formation of the EAAT anion pore. They enlarge the pore diameter sufficiently to permit the passage of l-glutamate and thus function as l-glutamate efflux pathways. The mutation P289R decreases l-glutamate uptake, but increases anion currents despite a lower membrane expression. SIGNIFICANCE l-glutamate permeability of the EAAT anion pore is an unexpected functional consequence of naturally occurring single amino acid substitutions. l-glutamate efflux through mutant EAAT2 anion channels will cause glutamate excitotoxicity and neuronal hyperexcitability in affected patients. Antagonists that selectively suppress the EAAT anion channel function could serve as therapeutic agents in the future.
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Affiliation(s)
- Peter Kovermann
- Molekular- und Zellphysiologie (IBI-1) Forschungszentrum Jülich, Institute of Biological Information Processing, Jülich, Germany
| | - Yulia Kolobkova
- Molekular- und Zellphysiologie (IBI-1) Forschungszentrum Jülich, Institute of Biological Information Processing, Jülich, Germany
| | - Arne Franzen
- Molekular- und Zellphysiologie (IBI-1) Forschungszentrum Jülich, Institute of Biological Information Processing, Jülich, Germany
| | - Christoph Fahlke
- Molekular- und Zellphysiologie (IBI-1) Forschungszentrum Jülich, Institute of Biological Information Processing, Jülich, Germany
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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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Kolen B, Kortzak D, Franzen A, Fahlke C. An amino-terminal point mutation increases EAAT2 anion currents without affecting glutamate transport rates. J Biol Chem 2020; 295:14936-14947. [PMID: 32820048 DOI: 10.1074/jbc.ra120.013704] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2020] [Revised: 08/03/2020] [Indexed: 12/16/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) are prototypical dual function proteins that function as coupled glutamate/Na+/H+/K+ transporters and as anion-selective channels. Both transport functions are intimately intertwined at the structural level: Secondary active glutamate transport is based on elevator-like movements of the mobile transport domain across the membrane, and the lateral movement of this domain results in anion channel opening. This particular anion channel gating mechanism predicts the existence of mutant transporters with changed anion channel properties, but without alteration in glutamate transport. We here report that the L46P mutation in the human EAAT2 transporter fulfills this prediction. L46 is a pore-forming residue of the EAAT2 anion channels at the cytoplasmic entrance into the ion conduction pathway. In whole-cell patch clamp recordings, we observed larger macroscopic anion current amplitudes for L46P than for WT EAAT2. Rapid l-glutamate application under forward transport conditions demonstrated that L46P does not reduce the transport rate of individual transporters. In contrast, changes in selectivity made gluconate permeant in L46P EAAT2, and nonstationary noise analysis revealed slightly increased unitary current amplitudes in mutant EAAT2 anion channels. We used unitary current amplitudes and individual transport rates to quantify absolute open probabilities of EAAT2 anion channels from ratios of anion currents by glutamate uptake currents. This analysis revealed up to 7-fold increased absolute open probability of L46P EAAT2 anion channels. Our results reveal an important determinant of the diameter of EAAT2 anion pore and demonstrate the existence of anion channel gating processes outside the EAAT uptake cycle.
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Affiliation(s)
- Bettina Kolen
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Daniel Kortzak
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Arne Franzen
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Christoph Fahlke
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany.
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Qu S, Zhang W, He S, Zhang X. Paired-Cysteine Scanning Reveals Conformationally Sensitive Proximity between the TM4b-4c Loop and TM8 of the Glutamate Transporter EAAT1. ACS Chem Neurosci 2019; 10:2541-2550. [PMID: 30802031 DOI: 10.1021/acschemneuro.9b00048] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) take up the neurotransmitter glutamate from the synaptic cleft and maintain glutamate concentrations below neurotoxic levels. Recently, the crystal structures of thermostable EAAT1 variants have been reported; however, little is understood regarding the functional mechanism of the transmembrane domain (TM) 4b-4c loop, which contains more than 50 amino acids in mammalian EAATs that are absent in prokaryotic homologues. To explore the spatial position and function of TM4 during the transport cycle, we introduced pairwise cysteine substitutions between the TM4b-4c loop and TM8 in a cysteine-less version of EAAT1, CL-EAAT1. We observed pronounced inhibition of transport by Cu(II)(1,10-phenanthroline)3 (CuPh) for doubly substituted V238C/I469C and A243C/I469C variants, but not for corresponding singly substituted CL-EAAT1 or for more than 20 other double-cysteine variants. Dithiothreitol treatment partially restored the uptake activity of the CuPh-treated V238C/I469C and A243C/I469C doubly substituted variants, confirming that the effects of CuPh on these variants were due to the formation of intramolecular disulfide bonds. Glutamate, KCl, and d,l-threo-β-benzyloxy-aspartate weakened CuPh inhibition of the V238C/I469C variant, but only KCl weakened CuPh inhibition of the V243C/I469C variant, suggesting that the TM4b-4c loop and TM8 are separated from each other in the inward-facing conformations of EAAT1. Our results suggest that the TM4b-4c loop and TM8 are positioned in close proximity during the transport cycle and are less closely spaced in the inward-facing conformation.
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Affiliation(s)
- Shaogang Qu
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
- Key Laboratory of Mental Health of the Ministry of Education, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Wenlong Zhang
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
- Key Laboratory of Mental Health of the Ministry of Education, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Suifen He
- Central Laboratory and Department of Neurology, Shunde Hospital, Southern Medical University (The First People’s Hospital of Shunde Foshan), Foshan, Guangdong 528300, China
- Key Laboratory of Mental Health of the Ministry of Education, Southern Medical University, Guangzhou, Guangdong 510515, China
| | - Xiuping Zhang
- Teaching Center of Experimental Medicine, School of Basic Medical Sciences, Southern Medical University, Guangzhou, Guangdong 510515, China
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Single Synapse Indicators of Impaired Glutamate Clearance Derived from Fast iGlu u Imaging of Cortical Afferents in the Striatum of Normal and Huntington (Q175) Mice. J Neurosci 2019; 39:3970-3982. [PMID: 30819797 DOI: 10.1523/jneurosci.2865-18.2019] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2018] [Revised: 02/05/2019] [Accepted: 02/19/2019] [Indexed: 01/23/2023] Open
Abstract
Changes in the balance between glutamate (Glu) release and uptake may stimulate synaptic reorganization and even synapse loss. In the case of neurodegeneration, a mismatch between astroglial Glu uptake and presynaptic Glu release could be detected if both parameters were assessed independently and at a single-synapse level. This has now become possible due to a new imaging assay with the genetically encoded ultrafast Glu sensor iGlu u We report findings from individual corticostriatal synapses in acute slices prepared from mice of either sex that were >1 year of age. Contrasting patterns of short-term plasticity and a size criterion identified two classes of terminals, presumably corresponding to the previously defined IT (intratelencephalic) and PT (pyramidal tract) synapses. The latter exhibited a higher degree of frequency potentiation/residual Glu accumulation and were selected for our first iGlu u single-synapse study in Q175 mice, a model of Huntington's disease (HD). In HD mice, the decay time constant of the perisynaptic Glu concentration (TauD), as an indicator of uptake, and the peak iGlu u amplitude, as an indicator of release, were prolonged and reduced, respectively. Treatment of WT preparations with the astrocytic Glu uptake blocker TFB-TBOA (100 nm) mimicked the TauD changes in homozygotes. Considering the largest TauD values encountered in WT, ∼40% of PT synapses tested in Q175 heterozygotes can be classified as dysfunctional. Moreover, HD but not WT synapses exhibited a positive correlation between TauD and the peak amplitude of iGlu u Finally, EAAT2 (excitatory amino acid transport protein 2) immunoreactivity was reduced next to corticostriatal terminals. Thus, astrocytic Glu transport remains a promising target for therapeutic intervention.SIGNIFICANCE STATEMENT Alterations in astrocytic Glu uptake can play a role in synaptic plasticity and neurodegeneration. Until now, the sensitivity of synaptic responses to pharmacological transport block and the resulting activation of NMDA receptors were regarded as reliable evidence for a mismatch between synaptic uptake and release. But the latter parameters are interdependent. Using a new genetically encoded sensor to monitor extracellular glutamate concentration ([Glu]) at individual corticostriatal synapses, we can now quantify the time constant of perisynaptic [Glu] decay (as an indicator of uptake) and the maximal [Glu] elevation next to the active zone (as an indicator of Glu release). The results provide a positive answer to the hitherto unresolved question of whether neurodegeneration (e.g., Huntington's disease) associates with a glutamate uptake deficit at tripartite excitatory synapses.
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Pérez-Mato M, Iglesias-Rey R, Vieites-Prado A, Dopico-López A, Argibay B, Fernández-Susavila H, da Silva-Candal A, Pérez-Díaz A, Correa-Paz C, Günther A, Ávila-Gómez P, Isabel Loza M, Baumann A, Castillo J, Sobrino T, Campos F. Blood glutamate EAAT 2-cell grabbing therapy in cerebral ischemia. EBioMedicine 2018; 39:118-131. [PMID: 30555045 PMCID: PMC6354443 DOI: 10.1016/j.ebiom.2018.11.024] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2018] [Revised: 11/13/2018] [Accepted: 11/13/2018] [Indexed: 01/10/2023] Open
Abstract
Background Excitatory amino acid transporter 2 (EAAT2) plays a pivotal role in glutamate clearance in the adult brain, thereby preventing excitotoxic effects. Considering the high efficacy of EAAT2 for glutamate uptake, we hypothesized that the expression of this transporter in mesenchymal stem cells (MSCs) for systemic administration could yield a cell-based glutamate-grabbing therapy, combining the intrinsic properties of these cells with excitotoxic protection. Methods To address this hypothesis, EAAT2-encoding cDNA was introduced into MSCs and human embryonic kidney 293 cells (HEK cells) as the control cell line. EAAT2 expression and functionality were evaluated by in vitro assays. Blood glutamate-grabbing activity was tested in healthy and ischemic rat models treated with 3 × 106 and 9 × 106 cells/animal. Findings The expression of EAAT2 in both cell types conferred the expected glutamate-grabbing activity in in vitro and in vivo studies. The functional improvement observed in ischemic rats treated with EAAT2–HEK at low dose, confirmed that this effect was indeed mediated by the glutamate-grabbing activity associated with EAAT2 functionality. Unexpectedly, both cell doses of non-transfected MSCs induced higher protection than transfected EAAT2–MSCs by another mechanism independent of the glutamate-grabbing capacity. Interpretation Although the transfection procedure most likely interferes with some of the intrinsic protective mechanisms of mesenchymal cells, the results show that the induced expression of EAAT2 in cells represents a novel alternative to mitigate the excitotoxic effects of glutamate and paves the way to combine this strategy with current cell therapies for cerebral ischemia.
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Affiliation(s)
- María Pérez-Mato
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Ramón Iglesias-Rey
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Alba Vieites-Prado
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Antonio Dopico-López
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Bárbara Argibay
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Héctor Fernández-Susavila
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Andrés da Silva-Candal
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Amparo Pérez-Díaz
- Drug Screening Platform/Biofarma Research Group, Centro Singular de Investigación en Medicina Molecular y Enfermedades Crónicas (CIMUS), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
| | - Clara Correa-Paz
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Anne Günther
- Institute of Complex Systems-Cellular Biophysics (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Paulo Ávila-Gómez
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - M Isabel Loza
- Drug Screening Platform/Biofarma Research Group, Centro Singular de Investigación en Medicina Molecular y Enfermedades Crónicas (CIMUS), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
| | - Arnd Baumann
- Institute of Complex Systems-Cellular Biophysics (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - José Castillo
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain
| | - Tomás Sobrino
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain.
| | - Francisco Campos
- Clinical Neurosciences Research Laboratory (LINC), Department of Neurology, Health Research Institute of Santiago de Compostela (IDIS), Clinical University Hospital, Santiago de Compostela, Spain.
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11
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Kovermann P, Hessel M, Kortzak D, Jen JC, Koch J, Fahlke C, Freilinger T. Impaired K + binding to glial glutamate transporter EAAT1 in migraine. Sci Rep 2017; 7:13913. [PMID: 29066757 PMCID: PMC5654970 DOI: 10.1038/s41598-017-14176-4] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2017] [Accepted: 10/06/2017] [Indexed: 12/28/2022] Open
Abstract
SLC1A3 encodes the glial glutamate transporter hEAAT1, which removes glutamate from the synaptic cleft via stoichiometrically coupled Na+-K+-H+-glutamate transport. In a young man with migraine with aura including hemiplegia, we identified a novel SLC1A3 mutation that predicts the substitution of a conserved threonine by proline at position 387 (T387P) in hEAAT1. To evaluate the functional effects of the novel variant, we expressed the wildtype or mutant hEAAT1 in mammalian cells and performed whole-cell patch clamp, fast substrate application, and biochemical analyses. T387P diminishes hEAAT1 glutamate uptake rates and reduces the number of hEAAT1 in the surface membrane. Whereas hEAAT1 anion currents display normal ligand and voltage dependence in cells internally dialyzed with Na+-based solution, no anion currents were observed with internal K+. Fast substrate application demonstrated that T387P abolishes K+-bound retranslocation. Our finding expands the phenotypic spectrum of genetic variation in SLC1A3 and highlights impaired K+ binding to hEAAT1 as a novel mechanism of glutamate transport dysfunction in human disease.
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Affiliation(s)
- Peter Kovermann
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Margarita Hessel
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Daniel Kortzak
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Joanna C Jen
- Departments of Neurology and Neurobiology, UCLA School of Medicine, Los Angeles, USA
| | - Johannes Koch
- Department of Paediatrics, Salzburger Universitätsklinikum, Salzburg, Austria
| | - Christoph Fahlke
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Tobias Freilinger
- Department of Neurology and Epileptology, Hertie-Institute for Clinical Brain Research (HIH), Tübingen, Germany.
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12
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Divito CB, Borowski JE, Glasgow NG, Gonzalez-Suarez AD, Torres-Salazar D, Johnson JW, Amara SG. Glial and Neuronal Glutamate Transporters Differ in the Na + Requirements for Activation of the Substrate-Independent Anion Conductance. Front Mol Neurosci 2017; 10:150. [PMID: 28611584 PMCID: PMC5447070 DOI: 10.3389/fnmol.2017.00150] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2016] [Accepted: 05/04/2017] [Indexed: 01/12/2023] Open
Abstract
Excitatory amino acid transporters (EAATs) are secondary active transporters of L-glutamate and L- or D-aspartate. These carriers also mediate a thermodynamically uncoupled anion conductance that is gated by Na+ and substrate binding. The activation of the anion channel by binding of Na+ alone, however, has only been demonstrated for mammalian EAAC1 (EAAT3) and EAAT4. To date, no difference has been observed for the substrate dependence of anion channel gating between the glial, EAAT1 and EAAT2, and the neuronal isoforms EAAT3, EAAT4 and EAAT5. Here we describe a difference in the Na+-dependence of anion channel gating between glial and neuronal isoforms. Chloride flux through transporters without glutamate binding has previously been described as substrate-independent or "leak" channel activity. Choline or N-methyl-D-glucamine replacement of external Na+ ions significantly reduced or abolished substrate-independent EAAT channel activity in EAAT3 and EAAT4 yet has no effect on EAAT1 or EAAT2. The interaction of Na+ with the neuronal carrier isoforms was concentration dependent, consistent with previous data. The presence of substrate and Na+-independent open states in the glial EAAT isoforms is a novel finding in the field of EAAT function. Our results reveal an important divergence in anion channel function between glial and neuronal glutamate transporters and highlight new potential roles for the EAAT-associated anion channel activity based on transporter expression and localization in the central nervous system.
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Affiliation(s)
- Christopher B Divito
- Center for Neuroscience, Department of Neurobiology, University of PittsburghPittsburgh, PA, United States
| | - Jenna E Borowski
- Center for Neuroscience, Department of Neurobiology, University of PittsburghPittsburgh, PA, United States
| | - Nathan G Glasgow
- Center for Neuroscience, Department of Neuroscience, University of PittsburghPittsburgh, PA, United States
| | - Aneysis D Gonzalez-Suarez
- Laboratory of Cellular and Molecular Neurobiology, National Institute of Mental Health, National Institutes of HealthBethesda, MD, United States
| | - Delany Torres-Salazar
- Laboratory of Cellular and Molecular Neurobiology, National Institute of Mental Health, National Institutes of HealthBethesda, MD, United States
| | - Jon W Johnson
- Center for Neuroscience, Department of Neuroscience, University of PittsburghPittsburgh, PA, United States
| | - Susan G Amara
- Laboratory of Cellular and Molecular Neurobiology, National Institute of Mental Health, National Institutes of HealthBethesda, MD, United States
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13
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Canul-Tec JC, Assal R, Cirri E, Legrand P, Brier S, Chamot-Rooke J, Reyes N. Structure and allosteric inhibition of excitatory amino acid transporter 1. Nature 2017; 544:446-451. [PMID: 28424515 PMCID: PMC5410168 DOI: 10.1038/nature22064] [Citation(s) in RCA: 154] [Impact Index Per Article: 22.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2016] [Accepted: 03/10/2017] [Indexed: 12/24/2022]
Abstract
Human members of the solute carrier 1 (SLC1) family of transporters take up excitatory neurotransmitters in the brain and amino acids in peripheral organs. Dysregulation of the function of SLC1 transporters is associated with neurodegenerative disorders and cancer. Here we present crystal structures of a thermostabilized human SLC1 transporter, the excitatory amino acid transporter 1 (EAAT1), with and without allosteric and competitive inhibitors bound. The structures reveal architectural features of the human transporters, such as intra- and extracellular domains that have potential roles in transport function, regulation by lipids and post-translational modifications. The coordination of the allosteric inhibitor in the structures and the change in the transporter dynamics measured by hydrogen-deuterium exchange mass spectrometry reveal a mechanism of inhibition, in which the transporter is locked in the outward-facing states of the transport cycle. Our results provide insights into the molecular mechanisms underlying the function and pharmacology of human SLC1 transporters.
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Affiliation(s)
- Juan C Canul-Tec
- Molecular Mechanisms of Membrane Transport Laboratory, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
| | - Reda Assal
- Molecular Mechanisms of Membrane Transport Laboratory, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
| | - Erica Cirri
- Molecular Mechanisms of Membrane Transport Laboratory, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
| | - Pierre Legrand
- Synchrotron SOLEIL, L'Orme des Merisiers, 91192 Gif-sur-Yvette, France
| | - Sébastien Brier
- Structural Mass Spectrometry and Proteomics Unit, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
| | - Julia Chamot-Rooke
- Structural Mass Spectrometry and Proteomics Unit, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
| | - Nicolas Reyes
- Molecular Mechanisms of Membrane Transport Laboratory, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France.,UMR 3528, CNRS, Institut Pasteur, 25-28 rue du Docteur Roux, 75015 Paris, France
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14
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Krycer JR, Fazakerley DJ, Cater RJ, C Thomas K, Naghiloo S, Burchfield JG, Humphrey SJ, Vandenberg RJ, Ryan RM, James DE. The amino acid transporter, SLC1A3, is plasma membrane-localised in adipocytes and its activity is insensitive to insulin. FEBS Lett 2017; 591:322-330. [PMID: 28032905 DOI: 10.1002/1873-3468.12549] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2016] [Revised: 11/25/2016] [Accepted: 12/23/2016] [Indexed: 12/21/2022]
Abstract
The hormone insulin coordinates the catabolism of nutrients by protein phosphorylation. Phosphoproteomic analysis identified insulin-responsive phosphorylation of the Glu/Asp transporter SLC1A3/EAAT1 in adipocytes. The role of SLC1A3 in adipocytes is not well-understood. We show that SLC1A3 is localised to the plasma membrane and the major regulator of acidic amino acid uptake in adipocytes. However, its localisation and activity were unaffected by insulin or mutation of the insulin-regulated phosphosite. The latter was also observed using a heterologous expression system in Xenopus laevis oocytes. Thus, SLC1A3 maintains a constant import of acidic amino acids independently of nutritional status in adipocytes.
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Affiliation(s)
- James R Krycer
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - Daniel J Fazakerley
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - Rosemary J Cater
- Discipline of Pharmacology, Sydney Medical School, The University of Sydney, NSW, Australia
| | - Kristen C Thomas
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - Sheyda Naghiloo
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - James G Burchfield
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - Sean J Humphrey
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia
| | - Robert J Vandenberg
- Discipline of Pharmacology, Sydney Medical School, The University of Sydney, NSW, Australia
| | - Renae M Ryan
- Discipline of Pharmacology, Sydney Medical School, The University of Sydney, NSW, Australia
| | - David E James
- School of Life and Environmental Sciences and Charles Perkins Centre, The University of Sydney, NSW, Australia.,Sydney Medical School, The University of Sydney, NSW, Australia
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15
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LeVine MV, Cuendet MA, Khelashvili G, Weinstein H. Allosteric Mechanisms of Molecular Machines at the Membrane: Transport by Sodium-Coupled Symporters. Chem Rev 2016; 116:6552-87. [PMID: 26892914 DOI: 10.1021/acs.chemrev.5b00627] [Citation(s) in RCA: 59] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Solute transport across cell membranes is ubiquitous in biology as an essential physiological process. Secondary active transporters couple the unfavorable process of solute transport against its concentration gradient to the energetically favorable transport of one or several ions. The study of such transporters over several decades indicates that their function involves complex allosteric mechanisms that are progressively being revealed in atomistic detail. We focus on two well-characterized sodium-coupled symporters: the bacterial amino acid transporter LeuT, which is the prototype for the "gated pore" mechanism in the mammalian synaptic monoamine transporters, and the archaeal GltPh, which is the prototype for the "elevator" mechanism in the mammalian excitatory amino acid transporters. We present the evidence for the role of allostery in the context of a quantitative formalism that can reconcile biochemical and biophysical data and thereby connects directly to recent insights into the molecular structure and dynamics of these proteins. We demonstrate that, while the structures and mechanisms of these transporters are very different, the available data suggest a common role of specific models of allostery in their functions. We argue that such allosteric mechanisms appear essential not only for sodium-coupled symport in general but also for the function of other types of molecular machines in the membrane.
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Affiliation(s)
- Michael V LeVine
- Department of Physiology and Biophysics, ‡HRH Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University , New York, New York 10065, United States
| | - Michel A Cuendet
- Department of Physiology and Biophysics, ‡HRH Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University , New York, New York 10065, United States
| | - George Khelashvili
- Department of Physiology and Biophysics, ‡HRH Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University , New York, New York 10065, United States
| | - Harel Weinstein
- Department of Physiology and Biophysics, ‡HRH Prince Alwaleed Bin Talal Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill Cornell Medical College of Cornell University , New York, New York 10065, United States
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16
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Fahlke C, Kortzak D, Machtens JP. Molecular physiology of EAAT anion channels. Pflugers Arch 2015; 468:491-502. [PMID: 26687113 DOI: 10.1007/s00424-015-1768-3] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2015] [Revised: 11/24/2015] [Accepted: 11/26/2015] [Indexed: 12/25/2022]
Abstract
Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic nerve terminals, glutamate is quickly removed from the synaptic cleft by a family of five glutamate transporters, the so-called excitatory amino acid transporters (EAAT1-5). EAATs are prototypic members of the growing number of dual-function transport proteins: they are not only glutamate transporters, but also anion channels. Whereas the mechanisms underlying secondary active glutamate transport are well understood at the functional and at the structural level, mechanisms and cellular roles of EAAT anion conduction have remained elusive for many years. Recently, molecular dynamics simulations combined with simulation-guided mutagenesis and experimental analysis identified a novel anion-conducting conformation, which accounts for all experimental data on EAAT anion currents reported so far. We here review recent findings on how EAATs accommodate a transporter and a channel in one single protein.
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Affiliation(s)
- Christoph Fahlke
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, 52425, Jülich, Germany.
| | - Daniel Kortzak
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, 52425, Jülich, Germany
| | - Jan-Philipp Machtens
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, 52425, Jülich, Germany
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17
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The Split Personality of Glutamate Transporters: A Chloride Channel and a Transporter. Neurochem Res 2015; 41:593-9. [DOI: 10.1007/s11064-015-1699-6] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2015] [Revised: 07/21/2015] [Accepted: 08/13/2015] [Indexed: 02/07/2023]
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18
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The transmembrane transporter domain of glutamate transporters is a process tip localizer. Sci Rep 2015; 5:9032. [PMID: 25761899 PMCID: PMC4357008 DOI: 10.1038/srep09032] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2014] [Accepted: 02/10/2015] [Indexed: 01/09/2023] Open
Abstract
Glutamate transporters in the central nervous system remove glutamate released from neurons to terminate the signal. These transporters localize to astrocyte process tips approaching neuronal synapses. The mechanisms underlying the localization of glutamate transporters to these processes, however, are not known. In this study, we demonstrate that the trimeric transmembrane transporter domain fragment of glutamate transporters, lacking both N- and C-terminal cytoplasmic regions, localized to filopodia tips. This is a common property of trimeric transporters including a neutral amino acid transporter ASCT1. Astrocyte specific proteins are not required for the filopodia tip localization. An extracellular loop at the centre of the 4th transmembrane helices, unique for metazoans, is required for the localization. Moreover, a C186S mutation at the 4th transmembrane region of EAAT1, found in episodic ataxia patients, significantly decreased its process tip localization. The transmembrane transporter domain fragments of glutamate transporters also localized to astrocyte process tips in cultured hippocampal slice. These results indicate that the transmembrane transporter domain of glutamate transporters have an additional function as a sorting signal to process tips.
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19
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Foran E, Rosenblum L, Bogush A, Pasinelli P, Trotti D. Sumoylation of the astroglial glutamate transporter EAAT2 governs its intracellular compartmentalization. Glia 2014; 62:1241-53. [PMID: 24753081 DOI: 10.1002/glia.22677] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2014] [Revised: 03/24/2014] [Accepted: 04/02/2014] [Indexed: 12/13/2022]
Abstract
EAAT2 is a predominantly astroglial glutamate transporter responsible for the majority of synaptic glutamate clearance in the mammalian central nervous system (CNS). Its dysfunction has been linked with many neurological disorders, including amyotrophic lateral sclerosis (ALS). Decreases in EAAT2 expression and function have been implicated in causing motor neuron excitotoxic death in ALS. Nevertheless, increasing EAAT2 expression does not significantly improve ALS phenotype in mouse models or in clinical trials. In the SOD1-G93A mouse model of inherited ALS, the cytosolic carboxy-terminal domain is cleaved from EAAT2, conjugated to SUMO1, and accumulated in astrocytes where it triggers astrocyte-mediated neurotoxic effects as disease progresses. However, it is not known whether this fragment is sumoylated after cleavage or if full-length EAAT2 is already sumoylated prior to cleavage as part of physiological regulation. In this study, we show that a fraction of full-length EAAT2 is constitutively sumoylated in primary cultures of astrocytes in vitro and in the CNS in vivo. Furthermore, the extent of sumoylation of EAAT2 does not change during the course of ALS in the SOD1-G93A mouse and is not affected by the expression of ALS-causative mutant SOD1 proteins in astrocytes in vitro, indicating that EAAT2 sumoylation is not driven by pathogenic mechanisms. Most interestingly, sumoylated EAAT2 localizes to intracellular compartments, whereas non-sumoylated EAAT2 resides on the plasma membrane. In agreement, promoting desumoylation in primary astrocytes causes increased EAAT2-mediated glutamate uptake. These findings could have implications for optimizing therapeutic approaches aimed at increasing EAAT2 activity in the dysfunctional or diseased CNS.
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Affiliation(s)
- E Foran
- Frances and Joseph Weinberg Unit for ALS Research, Farber Institute for Neuroscience, Department of Neuroscience, Thomas Jefferson University, Philadelphia, Pennsylvania
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20
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Schneider N, Cordeiro S, Machtens JP, Braams S, Rauen T, Fahlke C. Functional properties of the retinal glutamate transporters GLT-1c and EAAT5. J Biol Chem 2013; 289:1815-24. [PMID: 24307171 DOI: 10.1074/jbc.m113.517177] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the mammalian retina, glutamate uptake is mediated by members of a family of glutamate transporters known as "excitatory amino acid transporters (EAATs)." Here we cloned and functionally characterized two retinal EAATs from mouse, the GLT-1/EAAT2 splice variant GLT-1c, and EAAT5. EAATs are glutamate transporters and anion-selective ion channels, and we used heterologous expression in mammalian cells, patch-clamp recordings and noise analysis to study and compare glutamate transport and anion channel properties of both EAAT isoforms. We found GLT-1c to be an effective glutamate transporter with high affinity for Na(+) and glutamate that resembles original GLT-1/EAAT2 in all tested functional aspects. EAAT5 exhibits glutamate transport rates too low to be accurately measured in our experimental system, with significantly lower affinities for Na(+) and glutamate than GLT-1c. Non-stationary noise analysis demonstrated that GLT-1c and EAAT5 also differ in single-channel current amplitudes of associated anion channels. Unitary current amplitudes of EAAT5 anion channels turned out to be approximately twice as high as single-channel amplitudes of GLT-1c. Moreover, at negative potentials open probabilities of EAAT5 anion channels were much larger than for GLT-1c. Our data illustrate unique functional properties of EAAT5, being a low-affinity and low-capacity glutamate transport system, with an anion channel optimized for anion conduction in the negative voltage range.
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Affiliation(s)
- Nicole Schneider
- From the Institute of Complex Systems, Zelluläre Biophysik, Forschungszentrum Jülich, Leo-Brandt-Straβe, 52428 Jülich, Germany
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21
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Winter N, Kovermann P, Fahlke C. A point mutation associated with episodic ataxia 6 increases glutamate transporter anion currents. ACTA ACUST UNITED AC 2012; 135:3416-25. [PMID: 23107647 DOI: 10.1093/brain/aws255] [Citation(s) in RCA: 62] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Episodic ataxia is a human genetic disease characterized by paroxysmal cerebellar incoordination. There are several genetically and clinically distinct forms of this disease, and one of them, episodic ataxia type 6, is caused by mutations in the gene encoding a glial glutamate transporter, the excitatory amino acid transporter-1. So far, reduced glutamate uptake by mutant excitatory amino acid transporter-1 has been thought to be the main pathophysiological process in episodic ataxia type 6. However, excitatory amino acid transporter-1 does not only mediate secondary-active glutamate transport, but also functions as an ion channel. Here, we examined the effects of a disease-associated point mutation, P290R, on glutamate transport, anion current as well as on the subcellular distribution of excitatory amino acid transporter-1 using heterologous expression in mammalian cells. P290R reduces the number of excitatory amino acid transporter-1 in the surface membrane and impairs excitatory amino acid transporter-1-mediated glutamate uptake. Cells expressing P290R excitatory amino acid transporter-1 exhibit larger anion currents than wild-type cells in the absence as well as in the presence of external l-glutamate, despite a lower number of mutant transporters in the surface membrane. Noise analysis revealed unaltered unitary current amplitudes, indicating that P290R modifies opening and closing, and not anion permeation through mutant excitatory amino acid transporter-1 anion channels. These findings identify gain-of-function of excitatory amino acid transporter anion conduction as a pathological process in episodic ataxia. Episodic ataxia type 6 represents the first human disease found to be associated with altered function of excitatory amino acid transporter anion channels and illustrates possible physiological and pathophysiological impacts of this functional mode of this class of glutamate transporters.
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Affiliation(s)
- Natalie Winter
- Institute for Neurophysiology, Hannover Medical School, Carl-Neuberg-Straße 1, 30625 Hannover, Germany
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22
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Li Y, Zhou Y, Danbolt NC. The rates of postmortem proteolysis of glutamate transporters differ dramatically between cells and between transporter subtypes. J Histochem Cytochem 2012; 60:811-21. [PMID: 22859703 DOI: 10.1369/0022155412458589] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Glutamate transporters (GLT-1, GLAST, EAAC1) limit the actions of excitatory amino acids. Because a disturbed transporter operation can cause or aggravate neurological diseases, transporters are of considerable neuropathological interest. Human samples, however, are seldom obtained fresh. Here, we used mice brains to study how fast glutamate transporters are degraded after death. Immunoblots showed that terminal GLT-1 epitopes (within residues 1-26 and 518-573) had mostly disappeared after 24 hr. GLAST termini (1-25 and 522-543) degraded slightly slower. In contrast, epitopes within central parts of GLT-1 (493-508) and the EAAC1 C-terminus (510-523) were readily detectable after 72 hr. The decline in immunoreactivity of the GLT-1 and GLAST termini was also seen in tissue sections, but proteolysis did not happen synchronously in all cells. At 24 hr, scattered cells remained strongly immunopositive, while the majority of cells were completely immunonegative. GLAST and GLT-1 co-localized in neocortical tissue, but at 12 hr, many GLAST-positive cells had lost the GLT-1 termini. The uneven disappearance of labeling was not observed with the antibodies to GLT-1 residues 493-508. The immunoreactivity to this epitope correlated better with the reported glutamate uptake activity. Thus, postmortem delay may affect epitopes differently, possibly causing erroneous conclusions about relative expression levels.
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Affiliation(s)
- Yuchuan Li
- Department 3 E.N.T., 1st Affiliated Hospital of Zhengzhou University, Zhengzhou, China
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23
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Hotzy J, Machtens JP, Fahlke C. Neutralizing aspartate 83 modifies substrate translocation of excitatory amino acid transporter 3 (EAAT3) glutamate transporters. J Biol Chem 2012; 287:20016-26. [PMID: 22532568 DOI: 10.1074/jbc.m112.344077] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) terminate glutamatergic synaptic transmission by removing glutamate from the synaptic cleft into neuronal and glial cells. EAATs are not only secondary active glutamate transporters but also function as anion channels. Gating of EAAT anion channels is tightly coupled to transitions within the glutamate uptake cycle, resulting in Na(+)- and glutamate-dependent anion currents. A point mutation neutralizing a conserved aspartic acid within the intracellular loop close to the end of transmembrane domain 2 was recently shown to modify the substrate dependence of EAAT anion currents. To distinguish whether this mutation affects transitions within the uptake cycle or directly modifies the opening/closing of the anion channel, we used voltage clamp fluorometry. Using three different sites for fluorophore attachment, V120C, M205C, and A430C, we observed time-, voltage-, and substrate-dependent alterations of EAAT3 fluorescence intensities. The voltage and substrate dependence of fluorescence intensities can be described by a 15-state model of the transport cycle in which several states are connected to branching anion channel states. D83A-mediated changes of fluorescence intensities, anion currents, and secondary active transport can be explained by exclusive modifications of substrate translocation rates. In contrast, sole modification of anion channel opening and closing is insufficient to account for all experimental data. We conclude that D83A has direct effects on the glutamate transport cycle and that these effects result in changed anion channel function.
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Affiliation(s)
- Jasmin Hotzy
- Institut für Neurophysiologie, Medizinische Hochschule Hannover, D-30625 Hannover, Germany
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24
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Water and urea permeation pathways of the human excitatory amino acid transporter EAAT1. Biochem J 2011; 439:333-40. [DOI: 10.1042/bj20110905] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Glutamate transport is coupled to the co-transport of 3 Na+ and 1 H+ followed by the counter-transport of 1 K+. In addition, glutamate and Na+ binding to glutamate transporters generates an uncoupled anion conductance. The human glial glutamate transporter EAAT1 (excitatory amino acid transporter 1) also allows significant passive and active water transport, which suggests that water permeation through glutamate transporters may play an important role in glial cell homoeostasis. Urea also permeates EAAT1 and has been used to characterize the permeation properties of the transporter. We have previously identified a series of mutations that differentially affect either the glutamate transport process or the substrate-activated channel function of EAAT1. The water and urea permeation properties of wild-type EAAT1 and two mutant transporters were measured to identify which permeation pathway facilitates the movement of these molecules. We demonstrate that there is a significant rate of L-glutamate-stimulated passive and active water transport. Both the passive and active L-glutamate-stimulated water transport is most closely associated with the glutamate transport process. In contrast, L-glutamate-stimulated [14C]urea permeation is associated with the anion channel of the transporter. However, there is also likely to be a transporter-specific, but glutamate independent, flux of water via the anion channel.
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Lewis BP, Burge CB, Bartel DP. Conserved seed pairing, often flanked by adenosines, indicates that thousands of human genes are microRNA targets. Cell 2005; 160:542-53. [PMID: 15652477 DOI: 10.1016/j.cell.2014.12.035] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2014] [Revised: 08/27/2014] [Accepted: 12/19/2014] [Indexed: 12/12/2022]
Abstract
We predict regulatory targets of vertebrate microRNAs (miRNAs) by identifying mRNAs with conserved complementarity to the seed (nucleotides 2-7) of the miRNA. An overrepresentation of conserved adenosines flanking the seed complementary sites in mRNAs indicates that primary sequence determinants can supplement base pairing to specify miRNA target recognition. In a four-genome analysis of 3' UTRs, approximately 13,000 regulatory relationships were detected above the estimate of false-positive predictions, thereby implicating as miRNA targets more than 5300 human genes, which represented 30% of our gene set. Targeting was also detected in open reading frames. In sum, well over one third of human genes appear to be conserved miRNA targets.
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