1
|
Liu WX, Wang J, Xie ZM, Xu N, Zhang GF, Jia M, Zhou ZQ, Hashimoto K, Yang JJ. Regulation of glutamate transporter 1 via BDNF-TrkB signaling plays a role in the anti-apoptotic and antidepressant effects of ketamine in chronic unpredictable stress model of depression. Psychopharmacology (Berl) 2016; 233:405-15. [PMID: 26514555 DOI: 10.1007/s00213-015-4128-2] [Citation(s) in RCA: 108] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/15/2015] [Accepted: 10/12/2015] [Indexed: 01/28/2023]
Abstract
RATIONALE Growing evidence suggests that downregulated clearance of glutamate and signaling pathways involving brain-derived neurotrophic factor (BDNF) and its receptor TrkB play a role in morphological changes in the hippocampus of depressed patients. The N-methyl-D-aspartate (NMDA) receptor antagonist ketamine is the most attractive antidepressant, although precise mechanisms are unknown. OBJECTIVE In this study, we examined whether hippocampal BDNF-TrkB signaling underlies the antidepressant effects of ketamine via upregulating glutamate transporter 1 (GLT-1) in rats, subjected to the chronic unpredictable stress (CUS) for 42 days. The rats received a single injection of ketamine (10 mg/kg, i.p.) and/or a TrkB inhibitor, K252a (1 μl, 2 mM, intracerebroventicular (i.c.v.)) on day 43. Behavioral tests and brain sample collection were evaluated 24 h later. RESULTS The CUS-exposed rats exhibited depression- and anxiety-like behaviors; decreased number of glial fibrillary acidic protein (GFAP)-positive (but not NeuN-positive) cells in the dentate gyrus (DG), CA1, and CA3 areas; increased number of cleaved caspase-3-positive astrocytes; reduced spine density; lower ratio of Bcl2 to Bax; and decreased levels of BDNF, phosphorylated cAMP response element binging protein (CREB), GLT-1, and postsynaptic density 95 (PSD95) proteins in the hippocampus. Ketamine alleviated the CUS-induced abnormalities. The effects of ketamine were antagonized by pretreatment with K252a. CONCLUSIONS Our findings suggest that regulation of GLT-1 on astrocytes, responsible for 90 % of glutamate reuptake from the synapse, through BDNF-TrkB signaling is involved in mediation of the therapeutic effects of ketamine on behavioral abnormalities and morphological changes in the hippocampus of the CUS-exposed rats.
Collapse
Affiliation(s)
- Wen-Xue Liu
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
| | - Jing Wang
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
| | - Ze-Min Xie
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
- Jiangsu Province Key Laboratory of Anesthesiology, Xuzhou Medical College, Xuzhou, China
- Jiangsu Province Key Laboratory of Anesthesia and Analgesia Application Technology, Xuzhou, China
| | - Ning Xu
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
- Jiangsu Province Key Laboratory of Anesthesiology, Xuzhou Medical College, Xuzhou, China
- Jiangsu Province Key Laboratory of Anesthesia and Analgesia Application Technology, Xuzhou, China
| | - Guang-Fen Zhang
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
| | - Min Jia
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
| | - Zhi-Qiang Zhou
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China
| | - Kenji Hashimoto
- Division of Clinical Neuroscience, Chiba University Center for Forensic Mental Health, Chiba, Japan.
| | - Jian-Jun Yang
- Department of Anesthesiology, Jinling Hospital, Medical School of Nanjing University, Nanjing, China.
- Jiangsu Province Key Laboratory of Anesthesiology, Xuzhou Medical College, Xuzhou, China.
- Jiangsu Province Key Laboratory of Anesthesia and Analgesia Application Technology, Xuzhou, China.
| |
Collapse
|
2
|
Huo TG, Li WK, Zhang YH, Yuan J, Gao LY, Yuan Y, Yang HL, Jiang H, Sun GF. Excitotoxicity Induced by Realgar in the Rat Hippocampus: the Involvement of Learning Memory Injury, Dysfunction of Glutamate Metabolism and NMDA Receptors. Mol Neurobiol 2014; 51:980-94. [PMID: 24865513 DOI: 10.1007/s12035-014-8753-2] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2014] [Accepted: 05/13/2014] [Indexed: 11/28/2022]
Abstract
Realgar is a type of mineral drug containing arsenic. The nervous system toxicity of realgar has received extensive attention. However, the underlying mechanisms of realgar-induced neurotoxicity have not been clearly elucidated. To explore the mechanisms that contribute to realgar-induced neurotoxicity, weanling rats were exposed to realgar (0, 0.3, 0.9, 2.7 g/kg) for 6 weeks, and cognitive ability was tested using the Morris water maze (MWM) test and object recognition task (ORT). The levels of arsenic in the blood and hippocampus were monitored. The ultrastructures of hippocampal neurons were observed. The levels of glutamate (Glu) and glutamine (Gln) in the hippocampus and hippocampal CA1 region; the activities of glutamine synthetase (GS) and phosphate-activated glutaminase (PAG); the mRNA and protein expression of glutamate transporter 1 (GLT-1), glutamate/aspartate transporter (GLAST), and N-methyl-D-aspartate (NMDA) receptors; and the level of intracellular Ca(2+) were also investigated. The results indicate that the rats developed deficiencies in cognitive ability after a 6-week exposure to realgar. The arsenic contained in realgar and the arsenic metabolites passed through the blood-brain barrier (BBB) and accumulated in the hippocampus, which resulted in the excessive accumulation of Glu in the extracellular space. The excessive accumulation of Glu in the extracellular space induced excitotoxicity, which was shown by enhanced GS and PAG activities, inhibition of GLT-1 mRNA and protein expression, alterations in NMDA receptor mRNA and protein expression, disturbance of intracellular Ca(2+) homeostasis, and ultrastructural changes in hippocampal neurons. In conclusion, the findings from our study indicate that exposure to realgar induces excitotoxicity and that the mechanism by which this occurs may be associated with disturbances in Glu metabolism and transportation and alterations in NMDA receptor expression.
Collapse
Affiliation(s)
- Tao-guang Huo
- Department of Health Laboratory Technology, School of Public Health, China Medical University, No. 92 of Bei Er Road, Shenyang, 110001, People's Republic of China
| | | | | | | | | | | | | | | | | |
Collapse
|
3
|
Shabaneh M, Rosental N, Kanner BI. Disulfide cross-linking of transport and trimerization domains of a neuronal glutamate transporter restricts the role of the substrate to the gating of the anion conductance. J Biol Chem 2014; 289:11175-11182. [PMID: 24584931 DOI: 10.1074/jbc.m114.550277] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Excitatory amino acid transporters remove synaptically released glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. A structure of an archeal homologue, which reflects an early intermediate on the proposed substrate translocation path, has been suggested to be similar to an anion conducting conformation. To probe this idea by functional studies, we have introduced two cysteine residues in the neuronal glutamate transporter EAAC1 at positions predicted to be close enough to form a disulfide bond only in outward-facing and early intermediate conformations of the homologue. Upon treatment of Xenopus laevis oocytes expressing the W441C/K269C double mutant with dithiothreitol, radioactive transport was stimulated >2-fold but potently inhibited by low micromolar concentrations of the oxidizing reagent copper(II)(1,10-phenanthroline)3. The substrate-induced currents by the untreated double mutant, reversed at approximately -20 mV, close to the reversal potential of chloride, but treatment with dithiothreitol resulted in transport currents with the same voltage dependence as the wild type. It appears therefore that in the oocyte expression system the introduced cysteine residues in many of the mutant transporters are already cross-linked and are only capable of mediating the substrate-gated anion conductance. Reduction of the disulfide bond now allows these transporters to execute the full transport cycle. Our functional data support the idea that the anion conducting conformation of the neuronal glutamate transporter is associated with an early step of the transport cycle.
Collapse
Affiliation(s)
- Mustafa Shabaneh
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel
| | - Noa Rosental
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel
| | - Baruch I Kanner
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel.
| |
Collapse
|
4
|
Murugan M, Ling EA, Kaur C. Dysregulated glutamate uptake by astrocytes causes oligodendroglia death in hypoxic perventricular white matter damage. Mol Cell Neurosci 2013; 56:342-54. [PMID: 23859823 DOI: 10.1016/j.mcn.2013.07.005] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2012] [Revised: 07/04/2013] [Accepted: 07/08/2013] [Indexed: 01/31/2023] Open
Abstract
Excess glutamate mediates damage to oligodendroglia, resulting in myelination disturbances characteristic of hypoxic periventricular white matter (PWM) damage. We sought to examine if hypoxia altered the expression of astroglial excitatory amino acid transporters (EAAT1, EAAT2 and EAAT3) in the PWM, and, if so, whether it activated astroglial N-methyl D-aspartate receptors (NMDAR) which might lead to apoptosis of oligodendroglia. EAAT expression in the PWM of neonatal rats was measured at different time points after hypoxic exposure; it was attenuated at 7 and 14 d following hypoxia. Hypoxia prevented the uptake of glutamate by astroglial EAATs causing increased levels of extracellular glutamate. Excess glutamate augmented the expression of functional astroglial NMDAR. Following hypoxia, an increase in gap junction proteins between astroglia and oligodendroglia aided in the spreading of NMDAR-mediated excitotoxic calcium signals into the latter cell type triggering its apoptosis. Hence, dysregulated glutamate homeostasis is believed to contribute to hypoxia-induced death of oligodendroglia leading to neonatal PWM damage.
Collapse
Affiliation(s)
- Madhuvika Murugan
- Department of Anatomy, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117597, Singapore
| | | | | |
Collapse
|
5
|
Stolzenberg S, Khelashvili G, Weinstein H. Structural intermediates in a model of the substrate translocation path of the bacterial glutamate transporter homologue GltPh. J Phys Chem B 2012; 116:5372-83. [PMID: 22494242 PMCID: PMC3350225 DOI: 10.1021/jp301726s] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2012] [Revised: 04/10/2012] [Indexed: 01/05/2023]
Abstract
Excitatory amino acid transporters (EAATs) are membrane proteins responsible for reuptake of glutamate from the synaptic cleft to terminate neurotransmission and help prevent neurotoxically high, extracellular glutamate concentrations. Important structural information about these proteins emerged from crystal structures of GltPh, a bacterial homologue of EAATs, in conformations facing outward and inward. These remarkably different conformations are considered to be end points of the substrate translocation path (STP), suggesting that the transport mechanism involves major conformational rearrangements that remain uncharted. To investigate possible steps in the structural transitions of the STP between the two end-point conformations, we applied a combination of computational modeling methods (motion planning, molecular dynamics simulations, and mixed elastic network models). We found that the conformational changes in the transition involve mainly the repositioning the "transport domain" and the "trimerization domain" identified previously in the crystal structures. The two domains move in opposite directions along the membrane normal, and the transport domain also tilts by ∼17° with respect to this axis. Moreover, the TM3-4 loop undergoes a flexible, "restraining bar"-like conformational change with respect to the transport domain. As a consequence of these conformational rearrangements along the transition path we calculated a significant decrease of nearly 20% in the area of the transport-to-trimerization domain interface (TTDI). Water penetrates parts of the TTDI in the modeled intermediates but very much less in the end-point conformations. We show that these characteristics of the modeled intermediate states agree with experimental results from residue-accessibility studies in individual monomers and identify specific residues that can be used to test the proposed STP. Moreover, MD simulations of complete GltPh trimers constructed from initially identical monomer intermediates suggest that asymmetry can appear in the trimer, consonant with available experimental data showing independent transport kinetics by individual monomers in the trimers.
Collapse
Affiliation(s)
- Sebastian Stolzenberg
- Department of Physiology and
Biophysics, Weill Cornell Medical College, Cornell University, 1300 York Avenue, New York, New York 10065, United States
- Department of Physics, Cornell University, 109 Clark Hall, Ithaca, New York
14853-2501, United States
| | - George Khelashvili
- Department of Physiology and
Biophysics, Weill Cornell Medical College, Cornell University, 1300 York Avenue, New York, New York 10065, United States
| | - Harel Weinstein
- Department of Physiology and
Biophysics, Weill Cornell Medical College, Cornell University, 1300 York Avenue, New York, New York 10065, United States
- HRH Prince Alwaleed Bin Talal
Bin Abdulaziz Alsaud Institute for Computational Biomedicine, Weill
Cornell Medical College, Cornell University, New York, New York 10065, United States
| |
Collapse
|
6
|
Crystal structure of an asymmetric trimer of a bacterial glutamate transporter homolog. Nat Struct Mol Biol 2012; 19:355-7. [PMID: 22343718 DOI: 10.1038/nsmb.2233] [Citation(s) in RCA: 132] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2011] [Accepted: 12/16/2011] [Indexed: 11/08/2022]
Abstract
We report a structure of a trimeric glutamate transporter homolog from Pyrococcus horikoshii with two protomers in an inward facing state and the third in an intermediate conformation between the outward and inward facing states. The intermediate shows a cavity in the thinnest region of the transporter, which is potentially accessible to extracellular and cytoplasmic solutions. Our findings suggest a structural principle by which transport intermediates may mediate uncoupled permeation of polar solutes.
Collapse
|
7
|
Regulation of ethanol-sensitive EAAT2 expression through adenosine A1 receptor in astrocytes. Biochem Biophys Res Commun 2011; 406:47-52. [PMID: 21291865 DOI: 10.1016/j.bbrc.2011.01.104] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2011] [Accepted: 01/27/2011] [Indexed: 01/20/2023]
Abstract
Adenosine-regulated glutamate signaling in astrocytes is implicated in many neurological and neuropsychiatric disorders. In this study, we examined whether adenosine A1 receptor regulates EAAT2 expression in astrocytes using pharmacological agents and siRNAs. We found that adenosine A1 receptor-specific antagonist DPCPX or PSB36 decreased EAAT2 expression in a dose-dependent manner. Consistently, knockdown of A1 receptor in astrocytes decreased EAAT2 mRNA expression while overexpression of A1 receptor upregulated EAAT2 expression and function. Since A1 receptor activation is mainly coupled to inhibitory G-proteins and inhibits the activity of adenylate cyclase, we investigated the effect of forskolin, which activates adenylate cyclase activity, on EAAT2 mRNA levels. Interestingly, we found that forskolin reduced EAAT2 expression in dose- and time-dependent manners. In contrast, adenylate cyclase inhibitor SQ22536 increased EAAT2 expression in dose- and time-dependent manners. In addition, forskolin blocked ethanol-induced EAAT2 upregulation. Taken together, these results suggest that A1 receptor-mediated signaling regulates EAAT2 expression in astrocytes.
Collapse
|
8
|
Leinenweber A, Machtens JP, Begemann B, Fahlke C. Regulation of glial glutamate transporters by C-terminal domains. J Biol Chem 2010; 286:1927-37. [PMID: 21097502 DOI: 10.1074/jbc.m110.153486] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Excitatory amino acid transporter 2 (EAAT2) is a high affinity glutamate transporter predominantly expressed in astroglia. Human EAAT2 encompasses eight transmembrane domains and a 74-amino acid C-terminal domain that resides in the cytoplasm. We examined the role of this region by studying various C-terminal truncations and mutations using heterologous expression in mammalian cells, whole-cell patch clamp recording and confocal imaging. Removal of the complete C terminus (K498X EAAT2) results in loss of function because of intracellular retention of truncated proteins in the cytoplasm. However, a short stretch of amino acids (E500X EAAT2) within the C terminus results in correctly processed transporters. E500X reduced glutamate transport currents by 90%. Moreover, the voltage and substrate dependence of E500X EAAT2 anion currents was significantly altered. WT and mutant EAAT2 anion channels are modified by external Na(+) in the presence as well as in the absence of L-glutamate. Whereas Na(+) stimulates EAAT2 anion currents in the presence of L-glutamate, increased [Na(+)] reduces such currents without glutamate. In cells internally dialyzed with Na(+), WT, and truncated EAAT2 display comparable Na(+) dependence. With K(+) as main internal cation, E500X drastically increased the apparent dissociation constant for external Na(+). The effects of E500X can be represented by a kinetic model that allows translocation of the empty transporter from the outward- to the inward-facing conformation and stabilization of the inward-facing conformation by internal K(+). Our results demonstrate that the C terminus modifies the glutamate uptake cycle, possibly affecting the movements of the translocation domain of EAAT2 glutamate transporter.
Collapse
Affiliation(s)
- Ariane Leinenweber
- Institut für Neurophysiologie, Medizinische Hochschule, D-30625 Hannover, Germany
| | | | | | | |
Collapse
|
9
|
Kovermann P, Machtens JP, Ewers D, Fahlke C. A conserved aspartate determines pore properties of anion channels associated with excitatory amino acid transporter 4 (EAAT4). J Biol Chem 2010; 285:23676-86. [PMID: 20519505 DOI: 10.1074/jbc.m110.126557] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
Excitatory amino acid transporter (EAAT) glutamate transporters function not only as secondary active glutamate transporters but also as anion channels. Recently, a conserved aspartic acid (Asp(112)) within the intracellular loop near to the end of transmembrane domain 2 was proposed as a major determinant of substrate-dependent gating of the anion channel associated with the glial glutamate transporter EAAT1. We studied the corresponding mutation (D117A) in another EAAT isoform, EAAT4, using heterologous expression in mammalian cells, whole cell patch clamp, and noise analysis. In EAAT4, D117A modifies unitary conductances, relative anion permeabilities, as well as gating of associated anion channels. EAAT4 anion channel gating is characterized by two voltage-dependent gating processes with inverse voltage dependence. In wild type EAAT4, external l-glutamate modifies the voltage dependence as well as the minimum open probabilities of both gates, resulting in concentration-dependent changes of the number of open channels. Not only transport substrates but also anions affect wild type EAAT4 channel gating. External anions increase the open probability and slow down relaxation constants of one gating process that is activated by depolarization. D117A abolishes the anion and glutamate dependence of EAAT4 anion currents and shifts the voltage dependence of EAAT4 anion channel activation by more than 200 mV to more positive potentials. D117A is the first reported mutation that changes the unitary conductance of an EAAT anion channel. The finding that mutating a pore-forming residue modifies gating illustrates the close linkage between pore conformation and voltage- and substrate-dependent gating in EAAT4 anion channels.
Collapse
Affiliation(s)
- Peter Kovermann
- Institut für Neurophysiologie, Medizinische Hochschule, D-30625 Hannover, Germany.
| | | | | | | |
Collapse
|
10
|
Wu J, Lee MR, Choi S, Kim T, Choi DS. ENT1 regulates ethanol-sensitive EAAT2 expression and function in astrocytes. Alcohol Clin Exp Res 2010; 34:1110-7. [PMID: 20374202 DOI: 10.1111/j.1530-0277.2010.01187.x] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
BACKGROUND Equilibrative nucleoside transporter 1 (ENT1) and excitatory amino acid transporter 2 (EAAT2) are predominantly expressed in astrocytes where they are thought to regulate synaptic adenosine and glutamate levels. Because mice lacking ENT1 display increased glutamate levels in the ventral striatum, we investigated whether ENT1 regulates the expression and function of EAAT2 in astrocytes, which could contribute to altered glutamate levels in the striatum. METHODS We examined the effect of ENT1 inhibition and overexpression on the expression of EAAT2 using quantitative real-time PCR and measured glutamate uptake activity in cultured astrocytes. We also examined the effect of 0 to 200 mM ethanol doses for 0 to 24 hours of ethanol exposure on EAAT2 expression and glutamate uptake activity. We further examined the effect of ENT1 knockdown by a specific siRNA on ethanol-induced EAAT2 expression. RESULTS An ENT1-specific antagonist and siRNA treatments significantly reduced both EAAT2 expression and glutamate uptake activity while ENT1 overexpression up-regulated EAAT2 mRNA expression. Interestingly, 100 or 200 mM ethanol exposure increased EAAT2 mRNA expression as well as glutamate uptake activity. Moreover, we found that ENT1 knockdown inhibited the ethanol-induced EAAT2 up-regulation. CONCLUSIONS Our results suggest that ENT1 regulates glutamate uptake activity by altering EAAT2 expression and function, which might be implicated in ethanol intoxication and preference.
Collapse
Affiliation(s)
- Jinhua Wu
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA
| | | | | | | | | |
Collapse
|
11
|
Reyes N, Ginter C, Boudker O. Transport mechanism of a bacterial homologue of glutamate transporters. Nature 2009; 462:880-5. [PMID: 19924125 DOI: 10.1038/nature08616] [Citation(s) in RCA: 336] [Impact Index Per Article: 22.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2009] [Accepted: 10/29/2009] [Indexed: 11/09/2022]
Abstract
Glutamate transporters are integral membrane proteins that catalyse a thermodynamically uphill uptake of the neurotransmitter glutamate from the synaptic cleft into the cytoplasm of glia and neuronal cells by harnessing the energy of pre-existing electrochemical gradients of ions. Crucial to the reaction is the conformational transition of the transporters between outward and inward facing states, in which the substrate binding sites are accessible from the extracellular space and the cytoplasm, respectively. Here we describe the crystal structure of a double cysteine mutant of a glutamate transporter homologue from Pyrococcus horikoshii, Glt(Ph), which is trapped in the inward facing state by cysteine crosslinking. Together with the previously determined crystal structures of Glt(Ph) in the outward facing state, the structure of the crosslinked mutant allows us to propose a molecular mechanism by which Glt(Ph) and, by analogy, mammalian glutamate transporters mediate sodium-coupled substrate uptake.
Collapse
Affiliation(s)
- Nicolas Reyes
- Department of Physiology and Biophysics, Weill Cornell Medical College, 1300 York Avenue, Box 75, New York, New York 10065, USA
| | | | | |
Collapse
|
12
|
Affiliation(s)
- Baruch I. Kanner
- Department of Biochemistry, Hebrew University, Hadassah Medical School, Post Office Box 12272, Jerusalem 91120, Israel
| | - Elia Zomot
- Department of Biochemistry, Hebrew University, Hadassah Medical School, Post Office Box 12272, Jerusalem 91120, Israel
| |
Collapse
|
13
|
Teichman S, Kanner BI. Aspartate-444 is essential for productive substrate interactions in a neuronal glutamate transporter. ACTA ACUST UNITED AC 2007; 129:527-39. [PMID: 17535962 PMCID: PMC2151622 DOI: 10.1085/jgp.200609707] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
In the central nervous system, electrogenic sodium- and potassium-coupled glutamate transporters terminate the synaptic actions of this neurotransmitter. In contrast to acidic amino acids, dicarboxylic acids are not recognized by glutamate transporters, but the related bacterial DctA transporters are capable of transporting succinate and other dicarboxylic acids. Transmembrane domain 8 contains several residues that differ between these two types of transporters. One of these, aspartate-444 of the neuronal glutamate transporter EAAC1, is conserved in glutamate transporters, but a serine residue occupies this position in DctA transporters. When aspartate-444 is mutated to serine, cysteine, alanine, or even to glutamate, uptake of d-[3H]-aspartate as well as the inwardly rectifying steady-state currents induced by acidic amino acids is impaired. Even though succinate was not capable of inducing any steady-state transport currents, the dicarboxylic acid inhibited the sodium-dependent transient currents by the mutants with a neutral substitution at position 444. In the neutral substitution mutants inhibition of the transients was also observed with acidic amino acids. In the D444E mutant, acidic amino acids were potent inhibitors of the transient currents, whereas the apparent affinity for succinate was lower by at least three orders of magnitude. Even though L-aspartate could bind to D444E with a high apparent affinity, this binding resulted in inhibition rather than stimulation of the uncoupled anion conductance. Thus, a carboxylic acid–containing side chain at position 444 prevents the interaction of glutamate transporters with succinate, and the presence of aspartate itself at this position is crucial for productive substrate binding compatible with substrate translocation.
Collapse
Affiliation(s)
- Shlomit Teichman
- Department of Biochemistry, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel
| | | |
Collapse
|
14
|
Kanner BI. Structure and function of sodium-coupled GABA and glutamate transporters. J Membr Biol 2007; 213:89-100. [PMID: 17417704 DOI: 10.1007/s00232-006-0877-5] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2006] [Indexed: 11/25/2022]
Abstract
Neurotransmitter transporters are key elements in the termination of the synaptic actions of the neurotransmitters. They use the energy stored in the electrochemical ion gradients across the plasma membrane of neurons and glial cells for uphill transport of the transmitters into the cells surrounding the synapse. Therefore specific transporter inhibitors can potentially be used as novel drugs for neurological disease. Sodium-coupled neurotransmitter transporters belong to either of two distinct families. The glutamate transporters belong to the SLC1 family, whereas the transporters of the other neurotransmitters belong to the SLC6 family. An exciting and recent development is the emergence of the first high-resolution structures of archeal and bacterial members belonging to these two families. In this review the functional results on prototypes of the two families, the GABA transporter GAT-1 and the glutamate transporters GLT-1 and EAAC1, are described and discussed within the perspective provided by the novel structures.
Collapse
Affiliation(s)
- Baruch I Kanner
- Dept. of Biochemistry, Hebrew University, Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel.
| |
Collapse
|
15
|
Zhang Z, Grewer C. The sodium-coupled neutral amino acid transporter SNAT2 mediates an anion leak conductance that is differentially inhibited by transported substrates. Biophys J 2007; 92:2621-32. [PMID: 17237199 PMCID: PMC1864845 DOI: 10.1529/biophysj.106.100776] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The sodium-coupled neutral amino acid transporter SNAT2 mediates cellular uptake of glutamine and other small, neutral amino acids. Here, we report the existence of a leak anion pathway associated with SNAT2. The leak anion conductance was increased by, but did not require the presence of, extracellular sodium. The transported substrates L-alanine, L-glutamine, and alpha-(methylamino)isobutyrate inhibited the anion leak conductance, each with different potency. A transporter with the mutation H-304A did not catalyze alanine transport but still catalyzed anion leak current, demonstrating that substrate transport is not required for anion current inhibition. Both the substrate and Na+ were able to bind to the SNAT2H-304A transporter normally. The selectivity sequence of the SNAT2H-304A anion conductance was SCN->>NO3->I->Br->Cl->Mes-. Anion flux mediated by the more hydrophobic anion SCN- was not saturable, whereas nitrate flux demonstrated saturation kinetics with an apparent Km of 29 mM. SNAT2, which belongs to the SLC38 family of transporters, has to be added to the growing number of secondary, Na+-coupled transporters catalyzing substrate-gated or leak anion conductances. Therefore, we can speculate that such anion-conducting pathways are general features of Na+-transporting systems.
Collapse
Affiliation(s)
- Zhou Zhang
- University of Miami School of Medicine, Miami, Florida 33136, USA
| | | |
Collapse
|