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Bischof J, Rosenova G, Woldrich V, Streubel MK, Richter K, Duranton A, Rinnerthaler M. The Role of EAAT4 in Epidermal Differentiation and Calcium Homeostasis during Aging. J Invest Dermatol 2023; 143:2515-2517.e4. [PMID: 37257638 DOI: 10.1016/j.jid.2023.05.010] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2022] [Revised: 05/10/2023] [Accepted: 05/15/2023] [Indexed: 06/02/2023]
Affiliation(s)
- Johannes Bischof
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria; EB House Austria, Research Program for Molecular Therapy of Genodermatoses, Department of Dermatology and Allergology, University Hospital of the Paracelsus Medical University, Salzburg, Austria
| | - Germina Rosenova
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria
| | - Vitus Woldrich
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria
| | - Maria Karolin Streubel
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria
| | - Klaus Richter
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria
| | | | - Mark Rinnerthaler
- Department of Biosciences and Medical Biology, Paris-Lodron University Salzburg, Salzburg, Austria.
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2
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Ryan RM, Ingram SL, Scimemi A. Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression. Front Cell Neurosci 2021; 15:670346. [PMID: 33927596 PMCID: PMC8076567 DOI: 10.3389/fncel.2021.670346] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2021] [Accepted: 03/15/2021] [Indexed: 01/31/2023] Open
Abstract
Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.
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Affiliation(s)
- Renae M. Ryan
- School of Medical Sciences, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia
| | - Susan L. Ingram
- Department of Neurological Surgery, Oregon Health & Science University, Portland, OR, United States
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3
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Niello M, Gradisch R, Loland CJ, Stockner T, Sitte HH. Allosteric Modulation of Neurotransmitter Transporters as a Therapeutic Strategy. Trends Pharmacol Sci 2020; 41:446-463. [PMID: 32471654 PMCID: PMC7610661 DOI: 10.1016/j.tips.2020.04.006] [Citation(s) in RCA: 41] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2020] [Revised: 04/21/2020] [Accepted: 04/22/2020] [Indexed: 12/11/2022]
Abstract
Neurotransmitter transporters (NTTs) are involved in the fine-tuning of brain neurotransmitter homeostasis. As such, they are implicated in a plethora of complex behaviors, including reward, movement, and cognition. During recent decades, compounds that modulate NTT functions have been developed. Some of them are in clinical use for the management of different neuropsychiatric conditions. The majority of these compounds have been found to selectively interact with the orthosteric site of NTTs. Recently, diverse allosteric sites have been described in a number of NTTs, modulating their function. A more complex NTT pharmacology may be useful in the development of novel therapeutics. Here, we summarize current knowledge on such modulatory allosteric sites, with specific focus on their pharmacological and therapeutic potential.
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Affiliation(s)
- Marco Niello
- Centre for Physiology and Pharmacology, Institute of Pharmacology, Medical University of Vienna, Vienna, Austria
| | - Ralph Gradisch
- Centre for Physiology and Pharmacology, Institute of Pharmacology, Medical University of Vienna, Vienna, Austria
| | - Claus Juul Loland
- Laboratory for Membrane Protein Dynamics. Department of Neuroscience. University of Copenhagen, Copenhagen, Denmark
| | - Thomas Stockner
- Centre for Physiology and Pharmacology, Institute of Pharmacology, Medical University of Vienna, Vienna, Austria
| | - Harald H Sitte
- Centre for Physiology and Pharmacology, Institute of Pharmacology, Medical University of Vienna, Vienna, Austria; AddRess, Centre for Addiction Research and Science, Medical University of Vienna, Vienna, Austria.
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Olivares-Bañuelos TN, Chí-Castañeda D, Ortega A. Glutamate transporters: Gene expression regulation and signaling properties. Neuropharmacology 2019; 161:107550. [PMID: 30822498 DOI: 10.1016/j.neuropharm.2019.02.032] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2018] [Revised: 02/22/2019] [Accepted: 02/24/2019] [Indexed: 12/24/2022]
Abstract
Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. During synaptic activity, glutamate is released and binds to specific membrane receptors and transporters activating, in the one hand, a wide variety of signal transduction cascades, while in the other hand, its removal from the synaptic cleft. Extracellular glutamate concentrations are maintained within physiological levels mainly by glia glutamate transporters. Inefficient clearance of this amino acid is neurotoxic due to a prolonged hyperactivation of its postsynaptic receptors, exacerbating a wide array of intracellular events linked to an ionic imbalance, that results in neuronal cell death. This process is known as excitotoxicity and is the underlying mechanisms of an important number of neurodegenerative diseases. Therefore, it is important to understand the regulation of glutamate transporters function. The transporter activity can be regulated at different levels: gene expression, transporter protein targeting and trafficking, and post-translational modifications of the transporter protein. The identification of these mechanisms has paved the way to our current understanding the role of glutamate transporters in brain physiology and will certainly provide the needed biochemical information for the development of therapeutic strategies towards the establishment of novel therapeutic approaches for the treatment and/or prevention of pathologies associated with excitotoxicity insults. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.
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Affiliation(s)
- Tatiana N Olivares-Bañuelos
- Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California, Carretera Tijuana-Ensenada No. 3917, Fraccionamiento Playitas, 22860, Ensenada, Baja California, Mexico
| | - Donají Chí-Castañeda
- Departamento de Toxicología, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado Postal 14-740, Ciudad de México, 07000, Mexico
| | - Arturo Ortega
- Departamento de Toxicología, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado Postal 14-740, Ciudad de México, 07000, Mexico.
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5
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Regulation of Glutamate Transporter Expression in Glial Cells. ADVANCES IN NEUROBIOLOGY 2017; 16:199-224. [DOI: 10.1007/978-3-319-55769-4_10] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
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Quinta-Ferreira ME, Sampaio Dos Aidos FDS, Matias CM, Mendes PJ, Dionísio JC, Santos RM, Rosário LM, Quinta-Ferreira RM. Modelling zinc changes at the hippocampal mossy fiber synaptic cleft. J Comput Neurosci 2016; 41:323-337. [PMID: 27696002 DOI: 10.1007/s10827-016-0620-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2016] [Revised: 08/11/2016] [Accepted: 08/15/2016] [Indexed: 01/18/2023]
Abstract
Zinc, a transition metal existing in very high concentrations in the hippocampal mossy fibers from CA3 area, is assumed to be co-released with glutamate and to have a neuromodulatory role at the corresponding synapses. The synaptic action of zinc is determined both by the spatiotemporal characteristics of the zinc release process and by the kinetics of zinc binding to sites located in the cleft area, as well as by their concentrations. This work addresses total, free and complexed zinc concentration changes, in an individual synaptic cleft, following single, short and long periods of evoked zinc release. The results estimate the magnitude and time course of the concentrations of zinc complexes, assuming that the dynamics of the release processes are similar to those of glutamate. It is also considered that, for the cleft zinc concentrations used in the model (≤ 1 μM), there is no postsynaptic zinc entry. For this reason, all released zinc ends up being reuptaken in a process that is several orders of magnitude slower than that of release and has thus a much smaller amplitude. The time derivative of the total zinc concentration in the cleft is represented by the difference between two alpha functions, corresponding to the released and uptaken components. These include specific parameters that were chosen assuming zinc and glutamate co-release, with similar time courses. The peak amplitudes of free zinc in the cleft were selected based on previously reported experimental cleft zinc concentration changes evoked by single and multiple stimulation protocols. The results suggest that following a low amount of zinc release, similar to that associated with one or a few stimuli, zinc clearance is mainly mediated by zinc binding to the high-affinity sites on the NMDA receptors and to the low-affinity sites on the highly abundant GLAST glutamate transporters. In the case of higher zinc release brought about by a larger group of stimuli, most zinc binding occurs essentially to the GLAST transporters, having the corresponding zinc complex a maximum concentration that is more than one order of magnitude larger than that for the high and low affinity NMDA sites. The other zinc complexes considered in the model, namely those formed with sites on the AMPA receptors, calcium and KATP channels and with ATP molecules, have much smaller contributions to the synaptic zinc clearance.
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Affiliation(s)
- M E Quinta-Ferreira
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal.
- Department of Physics, University of Coimbra, P-3004-516, Coimbra, Portugal.
| | - F D S Sampaio Dos Aidos
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal
- Department of Physics, University of Coimbra, P-3004-516, Coimbra, Portugal
- CFisUC, Department of Physics, University of Coimbra, P-3004-516, Coimbra, Portugal
| | - C M Matias
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal
- UTAD- University of Trás-os-montes and Alto Douro, P-5000-801, Vila Real, Portugal
| | - P J Mendes
- Department of Physics, University of Coimbra, P-3004-516, Coimbra, Portugal
- LIP- Laboratory of Instrumentation and Experimental Particles Physics, P-3004-516, Coimbra, Portugal
| | - J C Dionísio
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal
- Department of Animal Biology, University of Lisbon, P-1749-016, Lisbon, Portugal
| | - R M Santos
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal
- Department of Life Sciences, University of Coimbra, P-3004-516, Coimbra, Portugal
| | - L M Rosário
- CNC- Center for Neuroscience and Cell Biology, University of Coimbra, P-3004-504, Coimbra, Portugal
- Department of Life Sciences, University of Coimbra, P-3004-516, Coimbra, Portugal
| | - R M Quinta-Ferreira
- CIEPQPF - Research Centre of Chemical Process Engineering and Forest Products, Department of Chemical Engineering, University of Coimbra, P-3030-790, Coimbra, Portugal
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Jensen AA, Fahlke C, Bjørn-Yoshimoto WE, Bunch L. Excitatory amino acid transporters: recent insights into molecular mechanisms, novel modes of modulation and new therapeutic possibilities. Curr Opin Pharmacol 2014; 20:116-23. [PMID: 25466154 DOI: 10.1016/j.coph.2014.10.008] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2014] [Revised: 10/05/2014] [Accepted: 10/21/2014] [Indexed: 11/24/2022]
Abstract
The five excitatory amino acid transporters (EAAT1-5) mediating the synaptic uptake of the major excitatory neurotransmitter glutamate are differently expressed throughout the CNS and at the synaptic level. Although EAATs are crucial for normal excitatory neurotransmission, explorations into the physiological functions mediated by the different transporter subtypes and their respective therapeutic potential have so far been sparse, in no small part due to the limited selection of pharmacological tools available. In the present update, we outline important new insights into the molecular compositions of EAATs and their intricate transport process, the novel approaches to pharmacological modulation of the transporters that have emerged, and interesting new perspectives in EAAT as drug targets proposed in recent years.
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Affiliation(s)
- Anders A Jensen
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, DK-2100 Copenhagen OE, Denmark.
| | - Christoph Fahlke
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Germany
| | - Walden E Bjørn-Yoshimoto
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, DK-2100 Copenhagen OE, Denmark
| | - Lennart Bunch
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, DK-2100 Copenhagen OE, Denmark
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8
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Zhou Y, Danbolt NC. Glutamate as a neurotransmitter in the healthy brain. J Neural Transm (Vienna) 2014; 121:799-817. [PMID: 24578174 PMCID: PMC4133642 DOI: 10.1007/s00702-014-1180-8] [Citation(s) in RCA: 527] [Impact Index Per Article: 52.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2013] [Accepted: 02/11/2014] [Indexed: 12/13/2022]
Abstract
Glutamate is the most abundant free amino acid in the brain and is at the crossroad between multiple metabolic pathways. Considering this, it was a surprise to discover that glutamate has excitatory effects on nerve cells, and that it can excite cells to their death in a process now referred to as "excitotoxicity". This effect is due to glutamate receptors present on the surface of brain cells. Powerful uptake systems (glutamate transporters) prevent excessive activation of these receptors by continuously removing glutamate from the extracellular fluid in the brain. Further, the blood-brain barrier shields the brain from glutamate in the blood. The highest concentrations of glutamate are found in synaptic vesicles in nerve terminals from where it can be released by exocytosis. In fact, glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. It took, however, a long time to realize that. The present review provides a brief historical description, gives a short overview of glutamate as a transmitter in the healthy brain, and comments on the so-called glutamate-glutamine cycle. The glutamate transporters responsible for the glutamate removal are described in some detail.
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Affiliation(s)
- Y. Zhou
- The Neurotransporter Group, Department of Anatomy, Institute of Basic Medical Sciences, University of Oslo, Blindern, P.O. Box 1105, 0317 Oslo, Norway
| | - N. C. Danbolt
- The Neurotransporter Group, Department of Anatomy, Institute of Basic Medical Sciences, University of Oslo, Blindern, P.O. Box 1105, 0317 Oslo, Norway
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9
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Abstract
L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.
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10
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Bahar I, Lezon TR, Bakan A, Shrivastava IH. Normal mode analysis of biomolecular structures: functional mechanisms of membrane proteins. Chem Rev 2010; 110:1463-97. [PMID: 19785456 PMCID: PMC2836427 DOI: 10.1021/cr900095e] [Citation(s) in RCA: 377] [Impact Index Per Article: 26.9] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Affiliation(s)
- Ivet Bahar
- Department of Computational Biology, School of Medicine, University of Pittsburgh, 3064 BST3, 3501 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA.
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11
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Belloni-Olivi L, Marshall C, Laal B, Andrews GK, Bressler J. Localization of zip1 and zip4 mRNA in the adult rat brain. J Neurosci Res 2009; 87:3221-30. [PMID: 19530166 DOI: 10.1002/jnr.22144] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The localization of two members of the Slc39a (zip1 and zip4) family of zinc transporters was examined in the brains of adult mice. Zip1 was highly enriched in brain regions with high densities of neuronal cell bodies, including the hippocampus, thalamus, and perifontal cortex. Zip1 was also expressed in commissural fiber tracts such as the corpus callosum and anterior commissure, but little was found in the internal and external capsules. Also, very low amounts of zip1 mRNA were detected in resting astrocytes and reactive astrocytes that were examined at 14 days after inflicting a stab wound. Zip1 mRNA was detected in ependymal cells lining the third and lateral ventricles and epithelium cells in the choroid plexus. Interestingly, zip4 mRNA was detected in the choroid plexus but not in the ependymal cells or other neural elements. Zip4 mRNA was also detected in brain capillaries, but zip1 mRNA was not. In zip4 knockout heterozygotes that express green fluorescent protein regulated by the zip4 promoter, green fluorescent protein was detected in brain capillaries. Because zip4 levels are regulated by dietary Zn, our studies suggest that the brain has the potential of adapting to changes in Zn status.
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12
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Modulation of Glutamate and Glycine Transporters by Niflumic, Flufenamic and Mefenamic Acids. Neurochem Res 2009; 34:1738-47. [DOI: 10.1007/s11064-009-9983-y] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2008] [Accepted: 04/20/2009] [Indexed: 12/21/2022]
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13
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Beart PM, O'Shea RD. Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement. Br J Pharmacol 2006; 150:5-17. [PMID: 17088867 PMCID: PMC2013845 DOI: 10.1038/sj.bjp.0706949] [Citation(s) in RCA: 299] [Impact Index Per Article: 16.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.
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Affiliation(s)
- P M Beart
- Howard Florey Institute, The University of Melbourne, Parkville, Victoria 3800, Australia.
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14
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Abstract
Neurotransmitter transporters located at the presynaptic or glial cell membrane are responsible for the stringent and rapid clearance of the transmitter from the synapse, and hence they terminate signaling and control the duration of synaptic inputs in the brain. Two distinct families of neurotransmitter transporters have been identified based on sequence homology: (1) the neurotransmitter sodium symporter family (NSS), which includes the Na+/C1(-)-dependent transporters for dopamine, norepinephrine, and serotonin; and (2) the dicarboxylate/amino acid cation symporter family (DAACS), which includes the Na(+)-dependent glutamate transporters (excitatory amino acid transporters; EAAT). In this chapter, we describe how the identification of endogenous Zn2(+)-binding sites, as well as engineering of artificial Zn2(+)-binding sites both in the Na+/Cl(-)-dependent transporters and in the EAATs, have proved to be an important tool for studying the molecular function of these proteins. We also interpret the current available data on Zn2(+)-binding sites in the context of the recently published crystal structures. Moreover, we review how the identification of endogenous Zn2(+)-binding sites has indirectly suggested the possibility that several of the transporters are modulated by Zn2+ in vivo, and thus that Zn2+ can play a role as a neuromodulator by affecting the function of neurotransmitter transporters.
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Affiliation(s)
- K Nørgaard-Nielsen
- Molecular Neuropharmacology Group, Department of Pharmacology, The Panum Institute, University of Copenhagen, 2200 Copenhagen N, Denmark
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15
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Abstract
Excitatory amino acid transporters (EAATs) play a central role in the termination of synaptic transmission and in extracellular glutamate homeostasis in the mammalian CNS. A functional transporter is assembled as oligomer consisting of three subunits, each of which appears to transport glutamate independently from the neighboring subunits. EAATs do not only sustain a secondary-active glutamate transport but also function as anion channel. We here address the question whether intersubunit interactions play a role in pore-mediated anion conduction. We expressed a neuronal isoform, EAAT4, heterologously in Xenopus oocytes and mammalian cells and measured glutamate flux and anion currents under various concentrations of Na+ and glutamate. EAAT4 anion channels are active in the absence of both substrates, and increasing concentrations activate EAAT4 anion currents with a sigmoidal concentration dependence. Because only one glutamate molecule is cotransported per uptake cycle, the cooperativity between glutamate binding sites most likely arises from an interaction between different carrier domains. This interaction is modified by two point mutations close to the putative glutamate binding site, G464S and Q467S. Both mutations alter the dissociation constants and Hill coefficient of the substrate dependence of anion currents, leaving the concentration dependence of glutamate uptake unaffected. Our results demonstrate that glutamate carriers cooperatively interact during anion channel activation.
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Mim C, Balani P, Rauen T, Grewer C. The glutamate transporter subtypes EAAT4 and EAATs 1-3 transport glutamate with dramatically different kinetics and voltage dependence but share a common uptake mechanism. ACTA ACUST UNITED AC 2006; 126:571-89. [PMID: 16316976 PMCID: PMC2266596 DOI: 10.1085/jgp.200509365] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Here, we report the application of glutamate concentration jumps and voltage jumps to determine the kinetics of rapid reaction steps of excitatory amino acid transporter subtype 4 (EAAT4) with a 100-μs time resolution. EAAT4 was expressed in HEK293 cells, and the electrogenic transport and anion currents were measured using the patch-clamp method. At steady state, EAAT4 was activated by glutamate and Na+ with high affinities of 0.6 μM and 8.4 mM, respectively, and showed kinetics consistent with sequential binding of Na+-glutamate-Na+. The steady-state cycle time of EAAT4 was estimated to be >300 ms (at −90 mV). Applying step changes to the transmembrane potential, Vm, of EAAT4-expressing cells resulted in the generation of transient anion currents (decaying with a τ of ∼15 ms), indicating inhibition of steady-state EAAT4 activity at negative voltages (<−40 mV) and activation at positive Vm (>0 mV). A similar inhibitory effect at Vm < 0 mV was seen when the electrogenic glutamate transport current was monitored, resulting in a bell-shaped I-Vm curve. Jumping the glutamate concentration to 100 μM generated biphasic, saturable transient transport and anion currents (Km ∼ 5 μM) that decayed within 100 ms, indicating the existence of two separate electrogenic reaction steps. The fast electrogenic reaction was assigned to Na+ binding to EAAT4, whereas the second reaction is most likely associated with glutamate translocation. Together, these results suggest that glutamate uptake of EAAT4 is based on the same molecular mechanism as transport by the subtypes EAATs 1–3, but that its kinetics and voltage dependence are dramatically different from the other subtypes. EAAT4 kinetics appear to be optimized for high affinity binding of glutamate, but not rapid turnover. Therefore, we propose that EAAT4 is a high-affinity/low-capacity transport system, supplementing low-affinity/high-capacity synaptic glutamate uptake by the other subtypes.
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Affiliation(s)
- Carsten Mim
- University of Miami School of Medicine, Miami, FL 33136, USA
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17
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Abstract
Glutamate transporters are a family of transporters that regulate extracellular glutamate concentrations so as to maintain a dynamic and high-fidelity cell signalling process in the brain. Site-directed mutagenesis has been used to investigate various aspects of the structural and functional properties of these transporters to gain insights into how they work. This field of research has recently undergone a major development with the determination of the crystal structure of a bacterial glutamate transporter, and this chapter relates the results from mutagenesis experiments with what we now know about glutamate transporter structure.
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Affiliation(s)
- R J Vandenberg
- Department of Pharmacology, Institute for Biomedical Research, University of Sydney, 2006 New South Wales, Australia.
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Otsuguro K, Ohta T, Ito S. Zinc modulates primary afferent fiber-evoked responses of ventral roots in neonatal rat spinal cord in vitro. Neuroscience 2005; 138:281-91. [PMID: 16360285 DOI: 10.1016/j.neuroscience.2005.11.007] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2005] [Revised: 10/28/2005] [Accepted: 11/04/2005] [Indexed: 10/25/2022]
Abstract
Zinc ions (Zn(2+)) are known to modulate the functions of a variety of channels, receptors and transporters. We examined the effects of Zn(2+) on the reflex potentials evoked by electrical stimulation and responses to depolarizing agents in the isolated spinal cord of the neonatal rat in vitro. Zn(2+) at low concentrations (0.5-2 microM) inhibited, but at high concentrations (5 and 10 microM) augmented, a slow depolarizing component (slow ventral root potential). Zn(2+) had no effect on fast components (monosynaptic reflex potential; fast polysynaptic reflex potential). Unlike Zn(2+), strychnine (5 microM), a glycine receptor antagonist, and (S),9(R)-(-)-bicuculline methobromide (10 microM), a GABA(A) receptor antagonist, potentiated both fast polysynaptic reflex potential and slow ventral root potential. Zn(2+) (5 microM) did not affect depolarizing responses to glutamate and N-methyl-D-aspartate. Zn(2+) enhanced the substance P-evoked depolarization in the absence of tetrodotoxin (0.3 microM) but not in its presence. The dorsal root potential was inhibited by (S),9(R)-(-)-bicuculline methobromide (10 microM) but not by Zn(2+) (5 microM). The Zn(2+)-potentiated slow ventral root potential was inhibited by the N-methyl-D-aspartate receptor antagonists, ketamine (10 microM) and DL-2-amino-5-phosphaonovaleric acid (50 microM) but not by P2X receptor antagonists, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (30 microM) and 2',3'-O-(2,4,6-trinitrophenyl)ATP (10 microM). Ketamine (10 microM) and DL-2-amino-5-phosphaonovaleric acid (50 microM) almost abolished spontaneous activities increased by Zn(2+). It is concluded that Zn(2+) potentiated slow ventral root potential induced by primary afferent stimulation, which was mediated by the activation of N-methyl-D-aspartate receptors but not by activation of P2X receptors or blockade of glycinergic and GABAergic inhibition. Zn(2+) does not seem to directly affect N-methyl-D-aspartate receptors. The release of glutamate from interneurons may play an important role in Zn(2+)-induced potentiation of slow ventral root potential in the spinal cord of the neonatal rat.
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Affiliation(s)
- K Otsuguro
- Laboratory of Pharmacology, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan.
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Abstract
1. Glutamate transporters (or excitatory amino acid transporters (EAAT)) are responsible for removing synaptically released glutamate from the extracellular space. The failure of EAAT to carry out this role will lead to excessive stimulation of glutamatergic receptors, causing excitotoxicity and cell death. 2. Glutamate is cotransported into the cell with three Na+ and one H+, followed by the counter-transport of one K+. In addition, glutamate and Na+ binding activates an uncoupled chloride conductance. Thus, glutamate transporters can function as both a transporter and an ion channel. At present, there is no clear understanding of the structural basis for the dual functions of glutamate transporters and, in the present review, we shall discuss some recent studies that have started to address this question. 3. It is possible to modulate one function of glutamate transporters without affecting the other, which suggests that the two functions have separate molecular determinants, and a number of models have been suggested to account for the dual functions of the EAAT that predict both single and dual pores for transporter function. 4. It appears that the two functions of glutamate transporters arise from separate transmembrane domains. The C-terminal region of the transporters forms the glutamate translocation domain, whereas the second transmembrane domain in the N-terminal half of the protein plays a crucial role in chloride channel function. Although the two functions arise from separate molecular determinants, the two functional domains are likely to be in close proximity. The significance of these observations will be discussed in terms of likely functional models for the transport and channel processes.
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Affiliation(s)
- Renae M Ryan
- Department of Pharmacology, Institute for Biomedical Research, University of Sydney, Sydney, New South Wales, Australia
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20
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Qian J, Noebels JL. Visualization of transmitter release with zinc fluorescence detection at the mouse hippocampal mossy fibre synapse. J Physiol 2005; 566:747-58. [PMID: 15919713 PMCID: PMC1464795 DOI: 10.1113/jphysiol.2005.089276] [Citation(s) in RCA: 160] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Exocytosis of synaptic vesicle contents defines the quantal nature of neurotransmitter release. Here we developed a technique to directly assess exocytosis by measuring vesicular zinc release with the zinc-sensitive dye FluoZin-3 at the hippocampal mossy fibre (MF) synapse. Using a photodiode, we were able to clearly resolve the zinc fluorescence transient ([Zn2+]t) with a train of five action potentials in mouse hippocampal brain slices. The vesicular origin of [Zn2+]t was verified by the lack of zinc signal in vesicular zinc transporter Znt3-deficient mice. Manipulating release probability with the application of neuromodulators such as DCG IV, 4-aminopyridine and forskolin as well as a paired train stimulation protocol altered both the [Zn2+]t and the field excitatory postsynaptic potential (fEPSP) coordinately, strongly indicating that zinc is co-released with glutamate during exocytosis. Since zinc ions colocalize with glutamate in small clear vesicles and modulate postsynaptic excitability at NMDA and GABA receptors, the findings establish zinc as a cotransmitter during physiological signalling at the mossy fibre synapse. The ability to directly visualize release dynamics with zinc imaging will facilitate the exploration of the molecular pharmacology and plasticity of exocytosis at MF synapses.
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Affiliation(s)
- Jing Qian
- Department of Neurology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
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21
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Vandenberg RJ, Ju P, Aubrey KR, Ryan RM, Mitrovic AD. Allosteric modulation of neurotransmitter transporters at excitatory synapses. Eur J Pharm Sci 2004; 23:1-11. [PMID: 15324920 DOI: 10.1016/j.ejps.2004.05.006] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2003] [Revised: 05/10/2004] [Accepted: 05/18/2004] [Indexed: 10/26/2022]
Abstract
The regulation of glutamate and glycine concentrations within excitatory synapses plays an important role in maintaining a dynamic signalling process between neurones, but the failure to regulate the concentrations of these neurotransmitters has been implicated in the pathogenesis of various neurological disorders. In this review we shall discuss how glutamate and glycine transporters regulate synaptic concentrations of these neurotransmitters and how endogenous allosteric modulators influence transporter function. Whilst glutamate transport inhibitors are unlikely to be of therapeutic value because their potential to cause excitoxicity and cell death, a greater understanding of how endogenous compounds allosterically modulate glutamate transporters may provide alternate drug targets. On the other hand, there are some promising drugs that inhibit glycine transporters, which are being trialled as an alternate treatment for schizophrenia. We shall discuss how the activity of one such compound may be expected to influence excitatory neurotransmission.
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Affiliation(s)
- Robert J Vandenberg
- Department of Pharmacology, Institute for Biomedical Research, University of Sydney, Sydney 2006, NSW, Australia.
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22
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Okamura M, Terada T, Katsura T, Saito H, Inui KI. Inhibitory effect of zinc on PEPT1-mediated transport of glycylsarcosine and beta-lactam antibiotics in human intestinal cell line Caco-2. Pharm Res 2004; 20:1389-93. [PMID: 14567632 DOI: 10.1023/a:1025797808703] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
PURPOSE The aim of this study was to examine the effects of zinc on the intestinal peptide transporters (PEPT1 and basolateral peptide transporter) and to elucidate the mechanism of the interactions. METHODS Caco-2 cells were pretreated with zinc, and the uptake studies were carried out. RESULTS Zinc treatment resulted in the inhibition of [14C]glycylsarcosine (Gly-Sar) uptake via PEPT1 in a concentration-dependent manner, whereas it showed moderate inhibitory effect on the basolateral peptide transporter. Zinc also inhibited the uptake of oral beta-lactam antibiotics such as ceftibuten and cephradine by PEPT1. Kinetic analysis showed that zinc treatment increased Km values without affecting Vmax values of the [14C]Gly-Sar uptake. The inhibition of [14C]Gly-Sar uptake induced by zinc was observed in the presence of an H+ gradient but not in the absence of an H+ gradient. CONCLUSIONS These results indicate that zinc is a competitive inhibitor of PEPT1. Zinc inhibited the PEPT1 function, possibly by interacting with histidine residues of PEPT1 that are part of an H+-binding site. These findings would provide important information for clinical, physiologic, and biochemical aspects of peptide transporters.
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Affiliation(s)
- Miyako Okamura
- Department of Pharmacy, Kyoto University Hospital, Faculty of Medicine, Kyoto University, Kyoto 606-8507, Japan
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23
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Wong KY, Adolph AR, Dowling JE. Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis. J Neurophysiol 2004; 93:84-93. [PMID: 15229213 DOI: 10.1152/jn.00259.2004] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
UNLABELLED Electroretinograms (ERGs) were recorded from the giant danio (Danio aequipinnatus) to study glutamatergic input mechanisms onto bipolar cells. Glutamate analogs were applied to determine which receptor types mediate synaptic transmission from rods and cones to on and off bipolar cells. Picrotoxin, strychnine, and tetrodotoxin were used to isolate the effects of the glutamate analogs to the photoreceptor-bipolar cell synapse. Under photopic conditions, the group III metabotropic glutamate receptor (mGluR) antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) only slightly reduced the b-wave, whereas the excitatory amino acid transporter (EAAT) blocker dl-threo-beta-benzyl-oxyaspartate (TBOA) removed most of it. Complete elimination of the b-wave required both antagonists. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) blocked the d-wave. Under scotopic conditions, rod and cone inputs onto on bipolar cells were studied by comparing the sensitivities of the b-wave to photopically matched green and red stimuli. The b-wave was >1 log unit more sensitive to the green than to the red stimulus under control conditions. In CPPG or l-AP4 (l-(+)-2-amino-4-phosphonobutyric acid, a group III mGluR agonist), the sensitivity of the b-wave to the green stimulus was dramatically reduced and the b-waves elicited by the 2 stimuli became nearly matched. The d-wave elicited by dim green stimuli, which presumably could be detected only by the rods, was eliminated by NBQX. IN CONCLUSION 1) cone signals onto on bipolar cells involve mainly EAATs but also mGluRs (presumably mGluR6) to a lesser extent; 2) rods signal onto on bipolars by mainly mGluR6; 3) off bipolar cells receive signals from both photoreceptor types by AMPA/kainate receptors.
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Affiliation(s)
- Kwoon Y Wong
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA.
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24
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Ju P, Aubrey KR, Vandenberg RJ. Zn2+ Inhibits Glycine Transport by Glycine Transporter Subtype 1b. J Biol Chem 2004; 279:22983-91. [PMID: 15031290 DOI: 10.1074/jbc.m312484200] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the central nervous system, glycine is a co-agonist with glutamate at the N-methyl-D-aspartate subtype of glutamate receptors and also an agonist at inhibitory, strychnine-sensitive glycine receptors. The GLYT1 subtypes of glycine transporters (GLYTs) are responsible for regulation of glycine at excitatory synapses, whereas a combination of GLYT1 and GLYT2 subtypes of glycine transporters are used at inhibitory glycinergic synapses. Zn2+ is stored in synaptic vesicles with glutamate in a number of regions of the brain and is believed to play a role in modulation of excitatory neurotransmission. In this study we have investigated the actions of Zn2+ on the glycine transporters, GLYT1b and GLYT2a, expressed in Xenopus laevis oocytes and we demonstrate that Zn2+ is a noncompetitive inhibitor of GLYT1 but has no effect on GLYT2. We have also investigated the molecular basis for these differences and the relationship between the Zn2+ and proton binding sites on GLYT1. Using site-directed mutagenesis, we identified 2 histidine residues, His-243 in the large second extracellular loop (ECL2) and His-410 in the fourth extracellular loop (ECL4), as two coordinates in the Zn2+ binding site of GLYT1b. In addition, our study suggests that the molecular determinants of proton regulation of GLYT1b are localized to the 2 histidine residues (His-410 and His-421) of ECL4. The ability of Zn2+ and protons to regulate the rate of glycine transport by interacting with residues situated in ECL4 of GLYT1b suggests that this region may influence the substrate translocation mechanism.
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Affiliation(s)
- Pengchu Ju
- Department of Pharmacology, Institute for Biomedical Research, University of Sydney, Sydney, New South Wales 2006, Australia
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25
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Ryan RM, Mitrovic AD, Vandenberg RJ. The chloride permeation pathway of a glutamate transporter and its proximity to the glutamate translocation pathway. J Biol Chem 2004; 279:20742-51. [PMID: 14982939 DOI: 10.1074/jbc.m304433200] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) regulate glutamate concentrations in the brain to maintain normal excitatory synaptic transmission. A widely accepted view of transporters is that they consist of a pore with alternating access to the intracellular and extracellular solutions, which serves to couple ion movement to the movement of substrate. However, recent observations that EAATs, and also a number of other neurotransmitter transporters, can also function as ligand-gated chloride channels have blurred the distinctions between transporters and ion channels. Here we show that mutations in the second transmembrane domain (TM2) of EAAT1 alter anion permeation properties without affecting glutamate transport and that a number of TM2 residues are accessible to the external aqueous solution. Furthermore, we demonstrate that the extracellular edge of TM2 is in close proximity to a membrane-associated domain that influences glutamate transport. This study will provide the foundation for beginning to understand how transporters can function as both transporters and ion channels.
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Affiliation(s)
- Renae M Ryan
- Department of Pharmacology, Institute for Biomedical Research, University of Sydney, Sydney, New South Wales 2006, Australia
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26
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Abstract
Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.
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Affiliation(s)
- Thomas E Decoursey
- Department of Molecular Biophysics and Physiology, Rush Presbyterian St. Luke's Medical Center, Chicago, Illinois 60612, USA.
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Disease progression in a transgenic model of familial amyotrophic lateral sclerosis is dependent on both neuronal and non-neuronal zinc binding proteins. J Neurosci 2002. [PMID: 12388585 DOI: 10.1523/jneurosci.22-20-08790.2002] [Citation(s) in RCA: 87] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Mutations in the Cu/Zn superoxide dismutase (SOD1) gene cause one form of familial amyotrophic lateral sclerosis, a progressive disorder of motor neurons leading to weakness and death of affected individuals. Experiments using both transgenic mice expressing mutant SOD1 and SOD1 knock-out mice have demonstrated that disease is caused by a toxic gain of function and not by a loss of normal SOD1 activity. Precise mechanisms underlying mutant SOD1 toxicity are unclear but may involve abnormal interactions between zinc and SOD1. The metallothioneins (MTs) represent a family of zinc binding proteins that can function as zinc chaperones for apo-SOD1 in vitro. We hypothesized that manipulation of metallothioneins in vivo might alter the disease phenotype of transgenic mice expressing G93A SOD1 and therefore crossed this line with MT-I and MT-II or MT-III knock-out mice. G93A SOD1 mice deficient of either MT-I and MT-II or MT-III exhibited significant reductions in survival compared with G93A SOD1 mice. In addition, motor dysfunction was markedly accelerated in G93A SOD1 mice deficient in metallothioneins with regard to onset (MT-I and MT-II) or progression (MT-III). These results indicate that the disease course in G93A SOD1 mice is dependent on levels of metallothionein expression. Because MT-I and MT-II are expressed in glia whereas MT-III is found in neurons, these results also indicate that primary changes within non-neuronal cells can affect mutant SOD1-induced disease and do so in ways distinct from primary neuronal changes.
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Laube B. Potentiation of inhibitory glycinergic neurotransmission by Zn2+: a synergistic interplay between presynaptic P2X2 and postsynaptic glycine receptors. Eur J Neurosci 2002; 16:1025-36. [PMID: 12383231 DOI: 10.1046/j.1460-9568.2002.02170.x] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The divalent cation zinc is known to modulate chloride currents carried by native and recombinant mammalian glycine receptors (GlyRs). To unravel the effect of Zn2+ on glycinergic neurotransmission, inhibitory postsynaptic currents (IPSC) of rat spinal neurons grown in culture were analysed in the absence and presence of Zn2+. Low concentrations of Zn2+ (0.5 and 5 micro m) augmented the mean amplitude of miniature IPSCs by approximately 40% over values obtained in the absence of zinc, whereas higher concentrations of Zn2+ (50 micro m) significantly decreased mean amplitude values. Remarkably, low concentrations of Zn2+ also significantly increased the mean frequency of miniature IPSCs. This effect was blocked by the P2X receptor antagonists PPADS and suramin, indicating the presence of Zn2+-sensitive presynaptic P2X receptors on glycinergic terminals. Immunostaining with antibodies against different P2X receptor subtypes revealed that P2X2 receptors partially colocalize with the GlyR. Potentiating concentrations of Zn2+ also affected the kinetics of miniature and evoked IPSCs by significantly prolonging their decay time constants. Electrophysiological analysis of heterologously expressed glycine transporters (GlyT) revealed for GlyT2 zero, and for GlyT1 a modest (< 20%), reduction of glycine uptake in the presence of 5 micro m Zn2+, indicating that prolongation of glycinergic IPSCs by Zn2+ is not due to inhibition of glycine removal from the synaptic cleft. Together, these results suggest that Zn2+ is a potent modulator of glycinergic synaptic transmission which increases in a synergistic manner the agonist affinity of both presynaptic P2X2 receptors and postsynaptic GlyRs.
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Affiliation(s)
- Bodo Laube
- Department of Neurochemistry, Max-Planck-Institute for Brain Research, Deutschordenstrasse 46, 60528 Frankfurt, Germany.
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29
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Ryan RM, Vandenberg RJ. Distinct conformational states mediate the transport and anion channel properties of the glutamate transporter EAAT-1. J Biol Chem 2002; 277:13494-500. [PMID: 11815608 DOI: 10.1074/jbc.m109970200] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Glutamate transport by the excitatory amino acid transporters (EAATs) is coupled to the co-transport of 3 Na(+), 1 H(+), and the counter-transport of 1 K(+) ion. In addition to coupled ion fluxes, glutamate and Na(+) binding to the transporter activates a thermodynamically uncoupled anion conductance through the transporter. In this study, we have distinguished between these two conductance states of the EAAT-1 transporter using a [2-(trimethylammonium)ethyl]methanethiosulfonate-modified V452C mutant transporter. Glutamate binds to the modified mutant transporter and activates the uncoupled anion conductance but is not transported. The selective alteration of the transport function without altering the anion channel function of the V452C mutant transporter suggests that the two functions are generated by distinct conformational states of the transporter.
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Affiliation(s)
- Renae M Ryan
- Department of Pharmacology, University of Sydney, Sydney, New South Wales 2006, Australia
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30
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Borre L, Kavanaugh MP, Kanner BI. Dynamic equilibrium between coupled and uncoupled modes of a neuronal glutamate transporter. J Biol Chem 2002; 277:13501-7. [PMID: 11823462 DOI: 10.1074/jbc.m110861200] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the brain, the neurotransmitter glutamate is removed from the synaptic cleft by (Na(+) + K(+))-coupled transporters by an electrogenic process. Moreover, these transporters mediate a sodium- and glutamate-dependent uncoupled chloride conductance. In contrast to the wild type, the uptake of radiolabeled substrate by the I421C mutant is inhibited by the membrane-impermeant [2-(trimethylammonium)ethyl]methanethiosulfonate and also by other sulfhydryl reagents. In the wild-type and the unmodified mutant, substrate-induced currents are inwardly rectifying and reflect the sum of the coupled electrogenic flux and the anion conductance. Remarkably, the I421C mutant modified by sulfhydryl reagents exhibits currents that are non-rectifying and reverse at the equilibrium potential for chloride. Strikingly, almost 10-fold higher concentrations of d-aspartate are required to activate the currents in the modified mutant as compared with untreated I421C. Under conditions in which only the coupled currents are observed, the modified mutant does not exhibit any currents. However, when the uncoupled current is dominant, sulfhydryl reagents cause >4-fold stimulation of this current. Thus, the modification of the cysteine introduced at position 421 impacts the coupled but not the uncoupled fluxes. Although both fluxes are activated by substrate, they behave as independent processes that are in dynamic equilibrium.
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Affiliation(s)
- Lars Borre
- Department of Biochemistry, Hadassah Medical School, The Hebrew University, P. O. Box 12272, Jerusalem 91120, Israel
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31
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Shigeri Y, Shimamoto K, Yasuda-Kamatani Y, Seal RP, Yumoto N, Nakajima T, Amara SG. Effects of threo-beta-hydroxyaspartate derivatives on excitatory amino acid transporters (EAAT4 and EAAT5). J Neurochem 2001; 79:297-302. [PMID: 11677257 DOI: 10.1046/j.1471-4159.2001.00588.x] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
D,L-threo-beta-Benzyloxyaspartate (D,L-TBOA), an analog of threo-beta-hydroxyaspartate (THA) possessing a bulky substituent, is a potent non-transportable blocker for the excitatory amino acid transporters, EAAT1, 2 and 3, while L-threo-beta-methoxyaspartate (L-TMOA) is a blocker for EAAT2, but a substrate for EAAT1 and EAAT3. To characterize the actions of these THA analogs and the function of EAAT4 and EAAT5, we performed electrophysiological analyses in EAAT4 or EAAT5 expressed on Xenopus oocytes. In EAAT4-expressing oocytes, D,L-TBOA acted as a non-transportable blocker, while L-TMOA like D,L-THA was a competitive substrate. In contrast, D,L-THA, D,L-TBOA and L-TMOA all strongly attenuated the glutamate-induced currents generated by EAAT5. Among them, L-TMOA showed the most potent inhibitory action. Moreover, D,L-THA, D,L-TBOA and L-TMOA themselves elicited outward currents at negative potentials and remained inward at positive potentials suggesting that D,L-TBOA and L-TMOA, as well as D,L-THA, not only act as non-transportable blockers, but also block the EAAT5 leak currents. These results indicate that EAATs 4 and 5 show different sensitivities to THA analogs although they share properties of a glutamate-gated chloride channel.
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Affiliation(s)
- Y Shigeri
- National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan.
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