GABA transporter type 1 explained

GABA transporter 1 (GAT1) also known as sodium- and chloride-dependent GABA transporter 1 is a protein that in humans is encoded by the SLC6A1 gene and belongs to the solute carrier 6 (SLC6) family of transporters.^{[1] [2] [3]} It mediates gamma-aminobutyric acid's translocation from the extracellular to intracellular spaces within brain tissue and the central nervous system as a whole.^{[4] [5]}

Structure

GAT1 is a 599 amino acid protein that consists of 12 transmembrane domains with an intracellular N-terminus and C-terminus.^[6]

Function

GAT1 is a gamma-aminobutyric acid (GABA) transporter, which removes GABA from the synaptic cleft by shuttling it to presynaptic neurons (where GABA can be recycled) and astrocytes (where GABA can be broken down).^{[7] [8]} GABA Transporter 1 uses energy from the dissipation of a Na⁺ gradient, aided by the presence of a Cl⁻ gradient, to translocate GABA across CNS neuronal membranes. The stoichiometry for GABA Transporter 1 is 2 Na⁺: 1 Cl⁻: 1 GABA.^[9] The presence of a Cl⁻/Cl⁻ exchange is also proposed because the Cl⁻ transported across the membrane does not affect the net charge.^[10] GABA is also the primary inhibitory neurotransmitter in the cerebral cortex and has the highest level of expression within it.^[11] The GABA affinity (K_m) of the mouse isoform of GAT1 is 8 μM.^[12]

In the brain of a mature mammal, glutamate is converted to GABA by the enzyme glutamate decarboxylase (GAD) along with the addition of vitamin B6. GABA is then packed and released into the post-synaptic terminals of neurons after synthesis. GABA can also be used to form succinate, which is involved in the citric acid cycle.^[13] Vesicle uptake has been shown to prioritize newly synthesized GABA over preformed GABA, though the reasoning behind this mechanism is currently not completely understood.

The regulation of the modular functioning of GATs is highly dependent on a multitude of second messengers and synaptic proteins.

Translocation cycle

Throughout the translocation cycle, GAT1 assumes three different conformations:

1. Open-to-out. In this conformation, 2 extracellular Na⁺ ions are co-transported into the neuron along with 1 GABA and 1 Cl⁻ that bind to the empty transporter, thus making it fully loaded. In prokaryotes, it has been found that transport does not require Cl⁻. In mammals, the Cl⁻ ion is required to offset the positive charge of the Na⁺ in order to maintain the proper membrane potential.
2. Occluded-out. Once fully loaded, this conformation prevents the release of ions/substrate into the cytoplasm or the extracellular space/synapse. The Na⁺, Cl⁻, and GABA are bound to the transporter until it changes conformation.
3. Open-to-in. The transporter, which was previously facing the synapse, becomes inward facing and can now release the ions and GABA into the neuron's cytoplasm. Once empty, the transporter occludes its binding site and flips to become outward facing so a new translocation cycle can begin.

Clinical significance

Research has shown that schizophrenia patients have GABA synthesis and expression altered, leading to the conclusion that GABA Transporter-1, which adds and removes GABA from the synaptic cleft, plays a role in the development of neurological disorders such as schizophrenia.^{[14] [15]} GABA and its precursor glutamate have opposite functions within the nervous system. Glutamate is considered an excitatory neurotransmitter, while GABA is an inhibitory neurotransmitter. Glutamate and GABA imbalances contribute to different neurological pathologies..

Imbalance in the GABAergic neurotransmission is involved in the pathophysiology of various neurological diseases such as epilepsy, Alzheimer's and stroke.^[16]

A study on genetic absence epilepsy rats from Strasbourg (GAERS) found that poor GABA uptake by GAT1 caused an increase in tonic current of GABA_A. In the two most understood forms of absence epilepsy, synaptic GABA_A receptors including GAT1 play a major role in seizure development. Blocking GAT1 in non-epileptic control (NEC) rats caused tonic current to increase to a rate similar to that of GAERS of the same age. This common cellular control site shows a possible target for future seizure treatments.^[17]

Glutamate and GABA have also been found to interact within the nucleus tractus solitarii (NTS), paraventricular nucleus (PVN), and rostral ventrolateral medulla (RVLM) of the brain to modulate blood pressure.^[18]

Interactions

SLC6A1 has been shown to interact with STX1A.^{[19] [20] [21]}

See also

• Solute carrier family
• GABA transporter 2
• GABA transporter 3
• SLC6A1 epileptic encephalopathy

Further reading

• Nelson H, Mandiyan S, Nelson N . Cloning of the human brain GABA transporter . FEBS Letters . 269 . 1 . 181–184 . August 1990 . 2387399 . 10.1016/0014-5793(90)81149-I . 34636220 . free .
• Bennett ER, Kanner BI . The membrane topology of GAT-1, a (Na+ + Cl-)-coupled gamma-aminobutyric acid transporter from rat brain . The Journal of Biological Chemistry . 272 . 2 . 1203–1210 . January 1997 . 8995422 . 10.1074/jbc.272.2.1203 . free .
• Bismuth Y, Kavanaugh MP, Kanner BI . Tyrosine 140 of the gamma-aminobutyric acid transporter GAT-1 plays a critical role in neurotransmitter recognition . The Journal of Biological Chemistry . 272 . 26 . 16096–16102 . June 1997 . 9195904 . 10.1074/jbc.272.26.16096 . free .
• DeFelipe J, González-Albo MC . Chandelier cell axons are immunoreactive for GAT-1 in the human neocortex . NeuroReport . 9 . 3 . 467–470 . February 1998 . 9512391 . 10.1097/00001756-199802160-00020 . 27446580 .
• Conti F, Melone M, De Biasi S, Minelli A, Brecha NC, Ducati A . Neuronal and glial localization of GAT-1, a high-affinity gamma-aminobutyric acid plasma membrane transporter, in human cerebral cortex: with a note on its distribution in monkey cortex . The Journal of Comparative Neurology . 396 . 1 . 51–63 . June 1998 . 9623887 . 10.1002/(SICI)1096-9861(19980622)396:1<51::AID-CNE5>3.0.CO;2-H . 33438310 .
• Augood SJ, Waldvogel HJ, Münkle MC, Faull RL, Emson PC . Localization of calcium-binding proteins and GABA transporter (GAT-1) messenger RNA in the human subthalamic nucleus . Neuroscience . 88 . 2 . 521–534 . January 1999 . 10197772 . 10.1016/S0306-4522(98)00226-7 . 2514970 .
• Ong WY, Yeo TT, Balcar VJ, Garey LJ . A light and electron microscopic study of GAT-1-positive cells in the cerebral cortex of man and monkey . Journal of Neurocytology . 27 . 10 . 719–730 . October 1998 . 10640187 . 10.1023/A:1006946717065 . 39552099 .
• Whitworth TL, Quick MW . Substrate-induced regulation of gamma-aminobutyric acid transporter trafficking requires tyrosine phosphorylation . The Journal of Biological Chemistry . 276 . 46 . 42932–42937 . November 2001 . 11555659 . 10.1074/jbc.M107638200 . free .
• Hachiya Y, Takashima S . Development of GABAergic neurons and their transporter in human temporal cortex . Pediatric Neurology . 25 . 5 . 390–396 . November 2001 . 11744314 . 10.1016/S0887-8994(01)00348-4 .
• Kanner BI . Transmembrane domain I of the gamma-aminobutyric acid transporter GAT-1 plays a crucial role in the transition between cation leak and transport modes . The Journal of Biological Chemistry . 278 . 6 . 3705–3712 . February 2003 . 12446715 . 10.1074/jbc.M210525200 . free .
• Zomot E, Kanner BI . The interaction of the gamma-aminobutyric acid transporter GAT-1 with the neurotransmitter is selectively impaired by sulfhydryl modification of a conformationally sensitive cysteine residue engineered into extracellular loop IV . The Journal of Biological Chemistry . 278 . 44 . 42950–42958 . October 2003 . 12925537 . 10.1074/jbc.M209307200 . free .
• Zhou Y, Bennett ER, Kanner BI . The aqueous accessibility in the external half of transmembrane domain I of the GABA transporter GAT-1 Is modulated by its ligands . The Journal of Biological Chemistry . 279 . 14 . 13800–13808 . April 2004 . 14744863 . 10.1074/jbc.M311579200 . free .
• Hu JH, Ma YH, Jiang J, Yang N, Duan SH, Jiang ZH, Mei ZT, Fei J, Guo LH . 6 . Cognitive impairment in mice over-expressing gamma-aminobutyric acid transporter 1 (GAT1) . NeuroReport . 15 . 1 . 9–12 . January 2004 . 15106822 . 10.1097/00001756-200401190-00003 . 6407617 .
• Korkhov VM, Farhan H, Freissmuth M, Sitte HH . Oligomerization of the -aminobutyric acid transporter-1 is driven by an interplay of polar and hydrophobic interactions in transmembrane helix II . The Journal of Biological Chemistry . 279 . 53 . 55728–55736 . December 2004 . 15496410 . 10.1074/jbc.M409449200 . free .

Notes and References

1. Huang F, Shi LJ, Heng HH, Fei J, Guo LH . Assignment of the human GABA transporter gene (GABATHG) locus to chromosome 3p24-p25 . Genomics . 29 . 1 . 302–304 . September 1995 . 8530094 . 10.1006/geno.1995.1253 .
2. Web site: Entrez Gene: SLC6A1 solute carrier family 6 (neurotransmitter transporter, GABA), member 1.
3. Scimemi A . Structure, function, and plasticity of GABA transporters . Frontiers in Cellular Neuroscience . 8 . 161 . 2014 . 24987330 . 10.3389/fncel.2014.00161 . 4060055 . free .
4. Book: Gonzalez-Burgos G . GABABReceptor Pharmacology - A Tribute to Norman Bowery . GABA transporter GAT1: a crucial determinant of GABAB receptor activation in cortical circuits? . Advances in Pharmacology . 58 . 175–204 . 2010 . 20655483 . 10.1016/S1054-3589(10)58008-6 . 9780123786470 .
5. Johannesen KM, Gardella E, Linnankivi T, Courage C, de Saint Martin A, Lehesjoki AE, Mignot C, Afenjar A, Lesca G, Abi-Warde MT, Chelly J, Piton A, Merritt JL, Rodan LH, Tan WH, Bird LM, Nespeca M, Gleeson JG, Yoo Y, Choi M, Chae JH, Czapansky-Beilman D, Reichert SC, Pendziwiat M, Verhoeven JS, Schelhaas HJ, Devinsky O, Christensen J, Specchio N, Trivisano M, Weber YG, Nava C, Keren B, Doummar D, Schaefer E, Hopkins S, Dubbs H, Shaw JE, Pisani L, Myers CT, Tang S, Tang S, Pal DK, Millichap JJ, Carvill GL, Helbig KL, Mecarelli O, Striano P, Helbig I, Rubboli G, Mefford HC, Møller RS . 6 . Defining the phenotypic spectrum of SLC6A1 mutations . Epilepsia . 59 . 2 . 389–402 . February 2018 . 29315614 . 5912688 . 10.1111/epi.13986 .
6. Zafar S, Jabeen I . Structure, Function, and Modulation of γ-Aminobutyric Acid Transporter 1 (GAT1) in Neurological Disorders: A Pharmacoinformatic Prospective . Frontiers in Chemistry . 6 . 397 . 2018 . 30255012 . 10.3389/fchem.2018.00397 . 6141625 . 2018FrCh....6..397Z . free .
7. Hirunsatit R, George ED, Lipska BK, Elwafi HM, Sander L, Yrigollen CM, Gelernter J, Grigorenko EL, Lappalainen J, Mane S, Nairn AC, Kleinman JE, Simen AA . 6 . Twenty-one-base-pair insertion polymorphism creates an enhancer element and potentiates SLC6A1 GABA transporter promoter activity . Pharmacogenetics and Genomics . 19 . 1 . 53–65 . January 2009 . 19077666 . 2791799 . 10.1097/FPC.0b013e328318b21a .
8. Madsen KK, Hansen GH, Danielsen EM, Schousboe A . The subcellular localization of GABA transporters and its implication for seizure management . Neurochemical Research . 40 . 2 . 410–419 . February 2015 . 25519681 . 10.1007/s11064-014-1494-9 . 19008879 .
9. Jin XT, Galvan A, Wichmann T, Smith Y . Localization and Function of GABA Transporters GAT-1 and GAT-3 in the Basal Ganglia . Frontiers in Systems Neuroscience . 5 . 63 . 28 July 2011 . 21847373 . 3148782 . 10.3389/fnsys.2011.00063 . free .
10. Loo DD, Eskandari S, Boorer KJ, Sarkar HK, Wright EM . Role of Cl- in electrogenic Na+-coupled cotransporters GAT1 and SGLT1 . English . The Journal of Biological Chemistry . 275 . 48 . 37414–37422 . December 2000 . 10973981 . 10.1074/jbc.M007241200 . free .
11. Conti F, Minelli A, Melone M . GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications . Brain Research. Brain Research Reviews . 45 . 3 . 196–212 . July 2004 . 15210304 . 10.1016/j.brainresrev.2004.03.003 . 19003675 .
12. Zhou Y, Danbolt NC . GABA and Glutamate Transporters in Brain . Frontiers in Endocrinology . 4 . 165 . 2013 . 24273530 . 10.3389/fendo.2013.00165 . 3822327 . free .
13. Book: Allen MJ, Sabir S, Sharma S . GABA Receptor . 2022 . http://www.ncbi.nlm.nih.gov/books/NBK526124/ . StatPearls . Treasure Island (FL) . StatPearls Publishing . 30252380 . 2022-04-11 .
14. Volk D, Austin M, Pierri J, Sampson A, Lewis D . GABA transporter-1 mRNA in the prefrontal cortex in schizophrenia: decreased expression in a subset of neurons . The American Journal of Psychiatry . 158 . 2 . 256–265 . February 2001 . 11156808 . 10.1176/appi.ajp.158.2.256 .
15. Hashimoto T, Matsubara T, Lewis DA . [Schizophrenia and cortical GABA neurotransmission] . Seishin Shinkeigaku Zasshi = Psychiatria et Neurologia Japonica . 112 . 5 . 439–452 . 2010 . 20560363 .
16. Kickinger S, Hellsberg E, Frølund B, Schousboe A, Ecker GF, Wellendorph P . Structural and molecular aspects of betaine-GABA transporter 1 (BGT1) and its relation to brain function . Neuropharmacology . 161 . 107644 . December 2019 . 31108110 . 10.1016/j.neuropharm.2019.05.021 . Neurotransmitter Transporters . 156055973 .
17. Cope DW, Di Giovanni G, Fyson SJ, Orbán G, Errington AC, Lorincz ML, Gould TM, Carter DA, Crunelli V . 6 . Enhanced tonic GABAA inhibition in typical absence epilepsy . Nature Medicine . 15 . 12 . 1392–1398 . December 2009 . 19966779 . 10.1038/nm.2058 . 2824149 .
18. Dupont AG, Légat L . GABA is a mediator of brain AT₁ and AT₂ receptor-mediated blood pressure responses . Hypertension Research . 43 . 10 . 995–1005 . October 2020 . 32451494 . 10.1038/s41440-020-0470-9 . 218864718 .
19. Beckman ML, Bernstein EM, Quick MW . Protein kinase C regulates the interaction between a GABA transporter and syntaxin 1A . The Journal of Neuroscience . 18 . 16 . 6103–6112 . August 1998 . 9698305 . 6793212 . 10.1523/JNEUROSCI.18-16-06103.1998 . free .
20. Quick MW . Substrates regulate gamma-aminobutyric acid transporters in a syntaxin 1A-dependent manner . Proceedings of the National Academy of Sciences of the United States of America . 99 . 8 . 5686–5691 . April 2002 . 11960023 . 122832 . 10.1073/pnas.082712899 . free . 2002PNAS...99.5686Q .
21. Deken SL, Beckman ML, Boos L, Quick MW . Transport rates of GABA transporters: regulation by the N-terminal domain and syntaxin 1A . Nature Neuroscience . 3 . 10 . 998–1003 . October 2000 . 11017172 . 10.1038/79939 . 11312913 .