Glutamate transporters are a family OF neurotransmitter transporter proteins that move glutamate – the principal excitatory neurotransmitter – across a membrane. The family of glutamate transporters is composed of two primary subclasses: the excitatory amino acid transporter (EAAT) family and vesicular glutamate transporter (VGLUT) family. In the brain, EAATs remove glutamate from the synaptic cleft and extrasynaptic sites via glutamate reuptake into glial cells and neurons, while VGLUTs move glutamate from the cell cytoplasm into synaptic vesicles. Glutamate transporters also transport aspartate and are present in virtually all peripheral tissues, including the heart, liver, testes, and bone. They exhibit stereoselectivity for L-glutamate but transport both L-aspartate and D-aspartate.
The EAATs are membrane-bound secondary transporters that superficially resemble ion channels. These transporters play the important role of regulating concentrations of glutamate in the extracellular space by transporting it along with other ions across cellular membranes. After glutamate is released as the result of an action potential, glutamate transporters quickly remove it from the extracellular space to keep its levels low, thereby terminating the synaptic transmission.
Without the activity of glutamate transporters, glutamate would build up and kill cells in a process called excitotoxicity, in which excessive amounts of glutamate acts as a toxin to neurons by triggering a number of biochemical cascades. The activity of glutamate transporters also allows glutamate to be recycled for repeated release.
|EAAT2||SLC1A2||Mainly astroglia; mediates >90% of CNS glutamate reuptake|
|EAAT3||SLC1A1||all neurons – located on dendrites and axon terminals|
There are two general classes of glutamate transporters, those that are dependent on an electrochemical gradient of sodium ions (the EAATs) and those that are not (VGLUTs and xCT). The cystine-glutamate antiporter (xCT) is localised to the plasma membrane of cells whilst vesicular glutamate transporters (VGLUTs) are found in the membrane of glutamate-containing synaptic vesicles. Na+-dependent EAATs are also dependent on transmembrane K+ and H+concentration gradients, and so are also known as 'sodium and potassium coupled glutamate transporters'. Na+-dependent transporters have also been called 'high-affinity glutamate transporters', though their glutamate affinity actually varies widely. EAATs are antiporters which carry one molecule of glutamate in along with three Na+ and one H+, while export one K+. EAATs are transmembrane integral proteins which traverse the plasmalemma 8 times.
In humans (as well as in rodents), five subtypes have been identified and named EAAT1-5 (SLC1A3, SLC1A2, SLC1A1, SLC1A6, SLC1A7). Subtypes EAAT1-2 are found in membranes of glial cells (astrocytes, microglia, and oligodendrocytes). However, low levels of EAAT2 are also found in the axon-terminals of hippocampal CA3 pyramidal cells. EAAT2 is responsible for over 90% of glutamate reuptake within the central nervous system (CNS). The EAAT3-4 subtypes are exclusively neuronal, and are expressed in axon terminals, cell bodies, and dendrites. Finally, EAAT5 is only found in the retina where it is principally localized to photoreceptors and bipolar neurons in the retina.
When glutamate is taken up into glial cells by the EAATs, it is converted to glutamine and subsequently transported back into the presynaptic neuron, converted back into glutamate, and taken up into synaptic vesicles by action of the VGLUTs. This process is named the glutamate–glutamine cycle.
Three types of vesicular glutamate transporters are known, VGLUTs 1–3 (SLC17A7, SLC17A6, and SLC17A8 respectively) and the novel glutamate/aspartate transporter sialin. These transporters pack the neurotransmitter into synaptic vesicles so that they can be released into the synapse. VGLUTs are dependent on the proton gradient that exists in the secretory system (vesicles being more acidic than the cytosol). VGLUTs have only between one hundredth and one thousandth the affinity for glutamate that EAATs have. Also unlike EAATs, they do not appear to transport aspartate.
VGluT3 (Vesicular Glutamate Transporter 3) that is encoded by the SLC17A8 gene is a member of the vesicular glutamate transporter family that transports glutamate into the cells. It is involved in neurological and pain diseases.
Neurons are able to express VGluT3 when they use a neurotransmitter different to Glutamate, for example in the specific case of central 5-HT neurons. The role of this unconventional transporter (VGluT3) still remains unknown but, at the moment, has been demonstrated that, in auditory system, the VGluT3 is involved in fast excitatory glutamatergic transmission very similar to the another two vesicular glutamate transporter, VGluT1 and VGluT2.
There are behavioral and physiological consequences of VGluT3 ablation because it modulates a wide range of neuronal and physiological processes like anxiety, mood regulation, impulsivity, aggressive behavior, pain perception, sleep–wake cycle, appetite, body temperature and sexual behavior. Certainly, no significant change was found in aggression and depression-like behaviors, but in contrast, the loss of VGluT3 resulted in a specific anxiety-related phenotype.
The sensory nerve fibers have different ways to detect the pain hypersensivity throughout their sensory modalities and conduction velocities, but at the moment is still unknown which types of sensory is related to the different forms of inflammatory and neuropathic pain hypersensivity. In this case, Vesicular glutamate transporter 3 (VGluT3), have been implicated in mechanical hypersensitivity after inflammation, but their role in neuropathic pain still remains under debate.
VGluT3 has extensive somatic throughout development, which could be involved in non-synaptic modulation by glutamate in developing retina, and could influence trophic and extra-synaptic neuronal signaling by glutamate in the inner retina.
Molecular Structure of EAATs
Like all glutamate transporters, EAATs are trimers, with each protomer consisting of two domains : the central scaffold domain (Figure 1A, wheat) and the peripheral transport domain (Figure 1A, blue). The transport conformational path is as follows. First, the outward facing conformation occurs (OF, open) which allows the glutamate to bind. Then the HP2 region closes after uptake (OF, closed) and the elevator like movement carries the substrate to the intracellular side of the membrane. It worth nothing that this elevator motion consists of several yet to be categorized/identified conformational changes. After the elevator motion brings the substrate to the IC side of the membrane, EAAT adopts the inward facing (IF, closed) state in which the transport domain is lowered, but the HP2 gate is still closed with the glutamate still bound to the transporter. Lastly, the HP2 gate opens and the glutamate diffuses into the cytoplasm of the cell. 
During injury processes such as ischemia and traumatic brain injury, the action of glutamate transporters may fail, leading to toxic buildup of glutamate. In fact, their activity may also actually be reversed due to inadequate amounts of adenosine triphosphate to power ATPase pumps, resulting in the loss of the electrochemical ion gradient. Since the direction of glutamate transport depends on the ion gradient, these transporters release glutamate instead of removing it, which results in neurotoxicity due to overactivation of glutamate receptors.
Loss of the Na+-dependent glutamate transporter EAAT2 is suspected to be associated with neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and ALS–parkinsonism dementia complex. Also, degeneration of motor neurons in the disease amyotrophic lateral sclerosis has been linked to loss of EAAT2 from patients' brains and spinal cords.
Addiction to certain addictive drugs (e.g., cocaine, heroin, alcohol, and nicotine) is correlated with a persistent reduction in the expression of EAAT2 in the nucleus accumbens (NAcc); the reduced expression of EAAT2 in this region is implicated in addictive drug-seeking behavior. In particular, the long-term dysregulation of glutamate neurotransmission in the NAcc of addicts is associated with an increase in vulnerability to relapse after re-exposure to the addictive drug or its associated drug cues. Drugs which help to normalize the expression of EAAT2 in this region, such as N-acetylcysteine, have been proposed as an adjunct therapy for the treatment of addiction to cocaine, nicotine, alcohol, and other drugs.
- Dopamine transporters
- Norepinephrine transporters
- Serotonin transporters
- NMDA receptors
- AMPA receptors
- Kainate receptors
- Metabotropic glutamate receptors
- Ganel R, Rothstein JD (1999). "Chapter 15, Glutamate transporter dysfunction and neuronal death". In Monyer, Hannah, Gabriel A. Adelmann, Jonas, Peter (eds.). Ionotropic glutamate receptors in the CNS. Berlin: Springer. pp. 472–493. ISBN 3-540-66120-4.
- Zerangue, N, Kavanaugh, MP (1996). "Flux coupling in a neuronal glutamate transporter". Nature. 383 (6601): 634–37. Bibcode:1996Natur.383..634Z. doi:10.1038/383634a0. PMID 8857541. S2CID 4266755.
- Shigeri Y, Seal RP, Shimamoto K (2004). "Molecular pharmacology of glutamate transporters, EAATs and VGLUTs". Brain Res. Brain Res. Rev. 45 (3): 250–65. doi:10.1016/j.brainresrev.2004.04.004. PMID 15210307. S2CID 41057787.
- Zou JY, Crews FT (2005). "TNF alpha potentiates glutamate neurotoxicity by inhibiting glutamate uptake in organotypic brain slice cultures: neuroprotection by NF kappa B inhibition". Brain Res. 1034 (1–2): 11–24. doi:10.1016/j.brainres.2004.11.014. PMID 15713255. S2CID 54316877.
- Beardsley PM, Hauser KF (2014). "Glial modulators as potential treatments of psychostimulant abuse". Emerging Targets & Therapeutics in the Treatment of Psychostimulant Abuse. Adv. Pharmacol. Advances in Pharmacology. Vol. 69. pp. 1–69. doi:10.1016/B978-0-12-420118-7.00001-9. ISBN 9780124201187. PMC 4103010. PMID 24484974.
- Cisneros IE, Ghorpade A (October 2014). "Methamphetamine and HIV-1-induced neurotoxicity: role of trace amine associated receptor 1 cAMP signaling in astrocytes". Neuropharmacology. 85: 499–507. doi:10.1016/j.neuropharm.2014.06.011. PMC 4315503. PMID 24950453.
TAAR1 overexpression significantly decreased EAAT-2 levels and glutamate clearance ... METH treatment activated TAAR1 leading to intracellular cAMP in human astrocytes and modulated glutamate clearance abilities. Furthermore, molecular alterations in astrocyte TAAR1 levels correspond to changes in astrocyte EAAT-2 levels and function.
- Rao P, Yallapu MM, Sari Y, Fisher PB, Kumar S (July 2015). "Designing Novel Nanoformulations Targeting Glutamate Transporter Excitatory Amino Acid Transporter 2: Implications in Treating Drug Addiction". J. Pers. Nanomed. 1 (1): 3–9. PMC 4666545. PMID 26635971.
The glutamate transporter 1 (GLT1)/ excitatory amino acid transporter 2 (EAAT2) is responsible for the reuptake of more than 90% glutamate in the CNS [12–14].
- Underhill SM, Wheeler DS, Li M, Watts SD, Ingram SL, Amara SG (July 2014). "Amphetamine modulates excitatory neurotransmission through endocytosis of the glutamate transporter EAAT3 in dopamine neurons". Neuron. 83 (2): 404–16. doi:10.1016/j.neuron.2014.05.043. PMC 4159050. PMID 25033183.
The dependence of EAAT3 internalization on the DAT also suggests that the two transporters might be internalized together. We found that EAAT3 and DAT are expressed in the same cells, as well as in axons and dendrites. However, the subcellular co-localization of the two neurotransmitter transporters remains to be established definitively by high resolution electron microscopy.
- Holmseth S, Dehnes Y, Huang YH, Follin-Arbelet VV, Grutle NJ, Mylonakou MN, Plachez C, Zhou Y, Furness DN, Bergles DE, Lehre KP, Danbolt NC (2012). "The density of EAAC1 (EAAT3) glutamate transporters expressed by neurons in the mammalian CNS". J Neurosci. 32 (17): 6000–13. doi:10.1523/JNEUROSCI.5347-11.2012. PMC 4031369. PMID 22539860.
- Danbolt NC (2001). "Glutamate uptake". Prog. Neurobiol. 65 (1): 1–105. doi:10.1016/S0301-0082(00)00067-8. PMID 11369436. S2CID 27347413.
- E. R. Kandel, J. H. Schwartz, T. M. Jessell, S. A. Siegelbaum, A. J. Hudpseth, Principles of neural science, 5th ed., The McGraw-Hill Companies, Inc., 2013, p. 304
- Holmseth S, Scott HA, Real K, Lehre KP, Leergaard TB, Bjaalie JG, Danbolt NC (2009). "The concentrations and distributions of three C-terminal variants of the GLT1 (EAAT2; slc1a2) glutamate transporter protein in rat brain tissue suggest differential regulation". Neuroscience. 162 (4): 1055–71. doi:10.1016/j.neuroscience.2009.03.048. PMID 19328838. S2CID 41615013.
Since then, a family of five high-affinity glutamate transporters has been characterized that is responsible for the precise regulation of glutamate levels at both synaptic and extrasynaptic sites, although the glutamate transporter 1 (GLT1) is responsible for more than 90% of glutamate uptake in the brain.3 The importance of GLT1 is further highlighted by the large number of neuropsychiatric disorders associated with glutamate-induced neurotoxicity. Clarification of nomenclature: The major glial glutamate transporter is referred to as GLT1 in the rodent literature and excitatory amino acid transporter 2 (EAAT2) in the human literature.
- Lehre KP, Levy LM, Ottersen OP, Storm-Mathisen J, Danbolt NC (1995). "Differential expression of two glial glutamate transporters in the rat brain: quantitative and immunocytochemical observations". J Neurosci. 15 (3): 1835–53. doi:10.1523/JNEUROSCI.15-03-01835.1995. PMC 6578153. PMID 7891138.
- Furness DN, Dehnes Y, Akhtar AQ, Rossi DJ, Hamann M, Grutle NJ, Gundersen V, Holmseth S, Lehre KP, Ullensvang K, Wojewodzic M, Zhou Y, Attwell D, Danbolt NC (2008). "A quantitative assessment of glutamate uptake into hippocampal synaptic terminals and astrocytes: new insights into a neuronal role for excitatory amino acid transporter 2 (EAAT2)". Neuroscience. 157 (1): 80–94. doi:10.1016/j.neuroscience.2008.08.043. PMC 2775085. PMID 18805467.
- Anderson CM, Swanson RA (2000). "Astrocyte glutamate transport: review of properties, regulation, and physiological functions". Glia. 32 (1): 1–14. doi:10.1002/1098-1136(200010)32:1<1::aid-glia10>3.3.co;2-n. PMID 10975906.
- Pow DV, Barnett NL (2000). "Developmental expression of excitatory amino acid transporter 5: a photoreceptor and bipolar cell glutamate transporter in rat retina". Neurosci. Lett. 280 (1): 21–4. doi:10.1016/S0304-3940(99)00988-X. PMID 10696802. S2CID 13184375.
- Pow DV, Robinson SR (1994). "Glutamate in some retinal neurons is derived solely from glia". Neuroscience. 60 (2): 355–66. doi:10.1016/0306-4522(94)90249-6. PMID 7915410. S2CID 28256349.
- Naito S, Ueda T (January 1983). "Adenosine triphosphate-dependent uptake of glutamate into protein I-associated synaptic vesicles". J. Biol. Chem. 258 (2): 696–9. doi:10.1016/S0021-9258(18)33100-4. PMID 6130088.
- Miyaji T, Echigo N, Hiasa M, Senoh S, Omote H, Moriyama Y (August 2008). "Identification of a vesicular aspartate transporter". Proc. Natl. Acad. Sci. U.S.A. 105 (33): 11720–4. Bibcode:2008PNAS..10511720M. doi:10.1073/pnas.0804015105. PMC 2575331. PMID 18695252.
- Fremeau RT, Burman J, Qureshi T, Tran CH, Proctor J, Johnson J, Zhang H, Sulzer D, Copenhagen DR, Storm-Mathisen J, Reimer RJ, Chaudhry FA, Edwards RH (2002). "The identification of vesicular glutamate transporter 3 suggests novel modes of signaling by glutamate". Proceedings of the National Academy of Sciences of the United States of America. 99 (22): 14488–93. Bibcode:2002PNAS...9914488F. doi:10.1073/pnas.222546799. PMC 137910. PMID 12388773.
- Gras C, Herzog E, Bellenchi GC, Bernard V, Ravassard P, Pohl M, Gasnier B, Giros B, El Mestikawy S (2002). "A third vesicular glutamate transporter expressed by cholinergic and serotoninergic neurons". The Journal of Neuroscience. 22 (13): 5442–51. doi:10.1523/jneurosci.22-13-05442.2002. PMC 6758212. PMID 12097496.
- Schäfer MK, Varoqui H, Defamie N, Weihe E, Erickson JD (2002). "Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons". The Journal of Biological Chemistry. 277 (52): 50734–48. doi:10.1074/jbc.M206738200. PMID 12384506.
- Takamori S, Malherbe P, Broger C, Jahn R (2002). "Molecular cloning and functional characterization of human vesicular glutamate transporter 3". EMBO Reports. 3 (8): 798–803. doi:10.1093/embo-reports/kvf159. PMC 1084213. PMID 12151341.
- Ruel J, Emery S, Nouvian R, Bersot T, Amilhon B, Van Rybroek JM, Rebillard G, Lenoir M, Eybalin M, Delprat B, Sivakumaran TA, Giros B, El Mestikawy S, Moser T, Smith RJ, Lesperance MM, Puel JL (2008). "Impairment of SLC17A8 encoding vesicular glutamate transporter-3, VGLUT3, underlies nonsyndromic deafness DFNA25 and inner hair cell dysfunction in null mice". American Journal of Human Genetics. 83 (2): 278–92. doi:10.1016/j.ajhg.2008.07.008. PMC 2495073. PMID 18674745.
- Seal RP, Akil O, Yi E, Weber CM, Grant L, Yoo J, Clause A, Kandler K, Noebels JL, Glowatzki E, Lustig LR, Edwards RH (2008). "Sensorineural deafness and seizures in mice lacking vesicular glutamate transporter 3". Neuron. 57 (2): 263–75. doi:10.1016/j.neuron.2007.11.032. PMC 2293283. PMID 18215623.
- Mortensen, Ole V.; Liberato, José L.; Coutinho-Netto, Joaquim; Santos, Wagner F. dos; Fontana, Andréia C. K. (2015). "Molecular determinants of transport stimulation of EAAT2 are located at interface between the trimerization and substrate transport domains". Journal of Neurochemistry. 133 (2): 199–210. doi:10.1111/jnc.13047. ISSN 1471-4159. PMID 25626691. S2CID 206090609.
- Kim AH, Kerchner GA, Choi DW (2002). "Chapter 1, Blocking Excitotoxicity". In Marcoux, Frank W. (ed.). CNS neuroprotection. Berlin: Springer. pp. 3–36. ISBN 3-540-42412-1.
- Yi JH, Hazell AS (2006). "Excitotoxic mechanisms and the role of astrocytic glutamate transporters in traumatic brain injury". Neurochem. Int. 48 (5): 394–403. doi:10.1016/j.neuint.2005.12.001. PMID 16473439. S2CID 44719394.
- McClure EA, Gipson CD, Malcolm RJ, Kalivas PW, Gray KM (2014). "Potential role of N-acetylcysteine in the management of substance use disorders". CNS Drugs. 28 (2): 95–106. doi:10.1007/s40263-014-0142-x. PMC 4009342. PMID 24442756.