Glutamate decarboxylase

{{Short description|Enzyme}} {{infobox enzyme | Name = glutamate decarboxylase | EC_number = 4.1.1.15 | CAS_number = 9024-58-2m | GO_code = 0004351 | image = | width = | caption = }} {{infobox protein | Name = Glutamic acid decarboxylase 1 | caption = GAD67 derived from {{PDB|2okj}} | image = PDB GAD67.jpg | width = | HGNCid = 4092 | Symbol = GAD1 | AltSymbols = glutamate decarboxylase 1<br />(brain, 67kD); GAD67 | EntrezGene = 2571 | OMIM = 605363 | RefSeq = NM_000817 | UniProt = Q99259 | PDB = | ECnumber = 4.1.1.15 | Chromosome = 2 | Arm = q | Band = 31 | LocusSupplementaryData = }} {{infobox protein | Name = glutamic acid decarboxylase 2 | caption = | image = | width = | HGNCid = 11284 | Symbol = GAD2 | AltSymbols = GAD65 | EntrezGene = 2572 | OMIM = 4093 | RefSeq = NM_001047 | UniProt = Q05329 | PDB = | ECnumber = 4.1.1.15 | Chromosome = 10 | Arm = p | Band = 11.23 | LocusSupplementaryData = }} '''Glutamate decarboxylase''' or '''glutamic acid decarboxylase''' ('''GAD''') is an enzyme that catalyzes the decarboxylation of glutamate to gamma-aminobutyric acid (GABA) and carbon dioxide ({{chem2|CO2}}). GAD uses pyridoxal-phosphate (PLP) as a cofactor. The reaction proceeds as follows:

: {{chem2|HOOC\sCH2\sCH2\sCH(NH2)\sCOOH -> CO2 + HOOC\sCH2\sCH2\sCH2NH2}}

In mammals, GAD exists in two isoforms with molecular weights of 67 and 65 kDa (GAD₆₇ and GAD₆₅), which are encoded by two different genes on different chromosomes (''GAD1'' and ''GAD2'' genes, chromosomes 2 and 10 in humans, respectively).<ref name="pmid2069816">{{cite journal | vauthors = Erlander MG, Tillakaratne NJ, Feldblum S, Patel N, Tobin AJ | title = Two genes encode distinct glutamate decarboxylases | journal = Neuron | volume = 7 | issue = 1 | pages = 91–100 | date = July 1991 | pmid = 2069816 | doi = 10.1016/0896-6273(91)90077-D | s2cid = 15863479 }}</ref><ref name="ReferenceA">{{cite journal | vauthors = Langendorf CG, Tuck KL, Key TL, Fenalti G, Pike RN, Rosado CJ, Wong AS, Buckle AM, Law RH, Whisstock JC | display-authors = 6 | title = Structural characterization of the mechanism through which human glutamic acid decarboxylase auto-activates | journal = Bioscience Reports | volume = 33 | issue = 1 | pages = 137–44 | date = January 2013 | pmid = 23126365 | pmc = 3546353 | doi = 10.1042/BSR20120111 }}</ref> GAD₆₇ and GAD₆₅ are expressed in the brain where GABA is used as a neurotransmitter, and they are also expressed in the insulin-producing β-cells of the pancreas, in varying ratios depending upon the species.<ref name="pmid8243826">{{cite journal | vauthors = Kim J, Richter W, Aanstoot HJ, Shi Y, Fu Q, Rajotte R, Warnock G, Baekkeskov S | display-authors = 6 | title = Differential expression of GAD65 and GAD67 in human, rat, and mouse pancreatic islets | journal = Diabetes | volume = 42 | issue = 12 | pages = 1799–808 | date = December 1993 | pmid = 8243826 | doi = 10.2337/diab.42.12.1799 | s2cid = 29615710 }}</ref> Together, these two enzymes maintain the major physiological supply of GABA in mammals,<ref name="ReferenceA"/> though it may also be synthesized from putrescine in the enteric nervous system,<ref name=":0">{{cite journal | vauthors = Krantis A | title = GABA in the Mammalian Enteric Nervous System | journal = News in Physiological Sciences | volume = 15 | issue = 6 | pages = 284–290 | date = December 2000 | pmid = 11390928 | doi = 10.1152/physiologyonline.2000.15.6.284 }}</ref> brain,<ref>{{cite journal | vauthors = Sequerra EB, Gardino P, Hedin-Pereira C, de Mello FG | title = Putrescine as an important source of GABA in the postnatal rat subventricular zone | journal = Neuroscience | volume = 146 | issue = 2 | pages = 489–93 | date = May 2007 | pmid = 17395389 | doi = 10.1016/j.neuroscience.2007.01.062 | s2cid = 43003476 }}</ref><ref name=":1">{{cite journal | vauthors = Kim JI, Ganesan S, Luo SX, Wu YW, Park E, Huang EJ, Chen L, Ding JB | display-authors = 6 | title = Aldehyde dehydrogenase 1a1 mediates a GABA synthesis pathway in midbrain dopaminergic neurons | journal = Science | volume = 350 | issue = 6256 | pages = 102–6 | date = October 2015 | pmid = 26430123 | doi = 10.1126/science.aac4690 | url = https://escholarship.org/content/qt6rc4c1hw/qt6rc4c1hw.pdf?t=nw99wr | pmc = 4725325 | bibcode = 2015Sci...350..102K }}</ref> and elsewhere by the actions of diamine oxidase and aldehyde dehydrogenase 1a1.<ref name=":0" /><ref name=":1" />

Several truncated transcripts and polypeptides of GAD₆₇ are detectable in the developing brain,<ref name="pmid7935469">{{cite journal | vauthors = Szabo G, Katarova Z, Greenspan R | title = Distinct protein forms are produced from alternatively spliced bicistronic glutamic acid decarboxylase mRNAs during development | journal = Molecular and Cellular Biology | volume = 14 | issue = 11 | pages = 7535–45 | date = November 1994 | pmid = 7935469 | pmc = 359290 | doi = 10.1128/mcb.14.11.7535 }}</ref> however their function, if any, is unknown.

==Structure and mechanism==

Both isoforms of GAD are homodimeric structures, consisting of three primary domains: the PLP, C-terminal and N-terminal domains. The PLP-binding domain of this enzyme adopts a type I PLP-dependent transferase-like fold.<ref name="Reingold 1979">{{cite journal | vauthors = Reingold DF, Orlowski M | title = Inhibition of brain glutamate decarboxylase by 2-keto-4-pentenoic acid, a metabolite of allylglycine | journal = J Neurochem | volume = 32 | issue = 3 | pages = 907–13 | date = Mar 1979 | pmid = 430066 | doi = 10.1111/j.1471-4159.1979.tb04574.x | s2cid = 31823191 }}</ref> The reaction proceeds via the canonical mechanism, involving Schiff base linkage between PLP and Lys405. PLP is held in place through base-stacking with an adjacent histidine residue, and GABA is positioned such that its carboxyl group forms a salt bridge with arginine and a hydrogen bond with glutamine.

thumb|center|upright=1.5|GAD67 active site containing PLP-glutamate complex (shown in green), with Schiff base linkage at Lys405. Side chain residues shown in red.

Dimerization is essential to maintaining function as the active site is found at this interface, and mutations interfering with optimal association between the 2 chains has been linked to pathology, such as schizophrenia.<ref name="Magri 2018">{{cite journal | vauthors = Magri C, Giacopuzzi E, La Via L, Bonini D, Ravasio V, Elhussiny ME, Orizio F, Gangemi F, Valsecchi P, Bresciani R, Barbon A, Vita A, Gennarelli M | title = A novel homozygous mutation in GAD1 gene described in a schizophrenic patient impairs activity and dimerization of GAD67 enzyme | journal = Sci Rep | volume = 8 | issue = 1 | page = 15470 | date = Oct 2018 | pmid = 30341396 | pmc=6195539 | doi = 10.1038/s41598-018-33924-8| bibcode = 2018NatSR...815470M }}</ref><ref name="Giacopuzzi 2017">{{cite journal | vauthors = Giacopuzzi E, Gennarelli M, Minelli A, Gardella R, Valsecchi P, Traversa M, Bonvicini C, Vita A, Sacchetti E, Magri C | title = Exome sequencing in schizophrenic patients with high levels of homozygosity identifies novel and extremely rare mutations in the GABA/glutamatergic pathways | journal = PLOS ONE | volume = 12 | issue = 8 | article-number = e0182778 | date = Aug 2017 | pmid = 28787007 |pmc=5546675 | doi = 10.1371/journal.pone.0182778 | bibcode = 2017PLoSO..1282778G | doi-access = free }}</ref> Interference of dimerization by GAD inhibitors such as 2-keto-4-pentenoic acid (KPA) and ethyl ketopentenoate (EKP) were also shown to lead to dramatic reductions in GABA production and incidence of seizures.<ref name="Zhang 2017">{{cite journal | vauthors = Zhang Y, Vanmeert M, Siekierska A, Ny A, John J, Callewaert G, Lescrinier E, Dehaen W, de Witte PA, Kaminski RM | title = Inhibition of glutamate decarboxylase (GAD) by ethyl ketopentenoate (EKP) induces treatment-resistant epileptic seizures in zebrafish | journal = Sci Rep | volume = 7 | issue = 1 | page = 7195 | date = Aug 2017 | pmid = 28775328 |pmc=5543107 | doi = 10.1038/s41598-017-06294-w | bibcode = 2017NatSR...7.7195Z }}</ref><ref name="Reingold 1979"/>

Catalytic activity is mediated by a short flexible loop at the dimer interface (residues 432–442 in GAD67, and 423–433 in GAD65). In GAD67 this loop remains tethered, covering the active site and providing a catalytic environment to sustain GABA production; its mobility in GAD65 promotes a side reaction that results in release of PLP, leading to autoinactivation.<ref name="Fenalti 2007"/> The conformation of this loop is intimately linked to the C-terminal domain, which also affects the rate of autoinactivation.<ref name="Langendorf 2013">{{cite journal | vauthors = Langendorf CG, Tuck KL, Key TL, Fenalti G, Pike RN, Rosado CJ, Wong AS, Buckle AM, Law RH, Whisstock JC | title = Structural characterization of the mechanism through which human glutamic acid decarboxylase auto-activates | journal = Biosci Rep | volume = 33 | issue = 1 | pages = 137–44 | date = Jan 2013 | pmid = 23126365 |pmc=3546353 | doi = 10.1042/BSR20120111 }}</ref> Moreover, GABA-bound GAD65 is intrinsically more flexible and exists as an ensemble of states, thus providing more opportunities for autoantigenicity as seen in Type 1 diabetes.<ref name="Kass 2014">{{cite journal | vauthors = Kass I, Hoke DE, Costa MG, Reboul CF, Porebski BT, Cowieson NP, Leh H, Pennacchietti E, McCoey J, Kleifeld O, Borri Voltattorni C, Langley D, Roome B, Mackay IR, Christ D, Perahia D, Buckle M, Paiardini A, De Biase D, Buckle AM | title = Cofactor-dependent conformational heterogeneity of GAD65 and its role in autoimmunity and neurotransmitter homeostasis | journal = Proc Natl Acad Sci U S A | volume = 111 | issue = 25 | pages = E2524-9 | date = Jun 2019 | pmid = 24927554 |pmc=4078817 | doi = 10.1073/pnas.1403182111 | doi-access = free }}</ref><ref name="Ellis 1996">{{cite journal | vauthors = Ellis TM, Atkinson MA | title = The clinical significance of an autoimmune response against glutamic acid decarboxylase | journal = Nat Med | volume = 2 | issue = 2 | pages = 148–53 | date = Feb 1996 | pmid = 8574952 | doi = 10.1038/nm0296-148 | s2cid = 12788084 }}</ref> GAD derived from ''Escherichia coli'' shows additional structural intricacies, including a pH-dependent conformational change. This behavior is defined by the presence of a triple helical bundle formed by the N-termini of the hexameric protein in acidic environments.<ref name="Capitani 2003">{{cite journal | vauthors = Capitani G, De Biase D, Aurizi C, Gut H, Bossa F, Grütter MG | title = Crystal structure and functional analysis of Escherichia coli glutamate decarboxylase | journal = EMBO J | volume = 22 | issue = 16 | pages = 4027–37 | date = Aug 2003 | pmid=12912902 |pmc=175793 | doi = 10.1093/emboj/cdg403 }}</ref> thumb|center|upright=2.5|Hexameric E. coli GAD conformational transition: low-pH (left), neutral pH (right).

==Regulation of GAD65 and GAD67==

Despite an extensive sequence similarity between the two genes, GAD65 and GAD67 fulfill very different roles within the human body. Additionally, research suggests that GAD65 and GAD67 are regulated by distinctly different cellular mechanisms.

GAD₆₅ and GAD₆₇ synthesize GABA at different locations in the cell, at different developmental times, and for functionally different purposes.<ref name="pmid9777629">{{cite journal | vauthors = Pinal CS, Tobin AJ | title = Uniqueness and redundancy in GABA production | journal = Perspectives on Developmental Neurobiology | volume = 5 | issue = 2–3 | pages = 109–18 | year = 1998 | pmid = 9777629 }}</ref><ref name="Soghomonian_1998">{{cite journal | vauthors = Soghomonian JJ, Martin DL | title = Two isoforms of glutamate decarboxylase: why? | journal = Trends in Pharmacological Sciences | volume = 19 | issue = 12 | pages = 500–5 | date = December 1998 | pmid = 9871412 | doi = 10.1016/s0165-6147(98)01270-x }}</ref> GAD₆₇ is spread evenly throughout the cell while GAD₆₅ is localized to nerve terminals.<ref name="pmid9777629" /><ref name="pmid1988566">{{cite journal | vauthors = Kaufman DL, Houser CR, Tobin AJ | title = Two forms of the gamma-aminobutyric acid synthetic enzyme glutamate decarboxylase have distinct intraneuronal distributions and cofactor interactions | journal = Journal of Neurochemistry | volume = 56 | issue = 2 | pages = 720–3 | date = February 1991 | pmid = 1988566 | doi = 10.1111/j.1471-4159.1991.tb08211.x | s2cid = 35743434 | pmc = 8194030 }}</ref><ref name="pmid25647668">{{cite journal | vauthors = Kanaani J, Cianciaruso C, Phelps EA, Pasquier M, Brioudes E, Billestrup N, Baekkeskov S | title = Compartmentalization of GABA synthesis by GAD67 differs between pancreatic beta cells and neurons | journal = PLOS ONE | volume = 10 | issue = 2 | article-number = e0117130 | year = 2015 | pmid = 25647668 | pmc = 4315522 | doi = 10.1371/journal.pone.0117130 | bibcode = 2015PLoSO..1017130K | doi-access = free }}</ref> GAD₆₇ synthesizes GABA for neuron activity unrelated to neurotransmission, such as synaptogenesis and protection from neural injury.<ref name="pmid9777629" /><ref name="Soghomonian_1998" /> This function requires widespread, ubiquitous presence of GABA. GAD₆₅, however, synthesizes GABA for neurotransmission,<ref name="pmid9777629" /> and therefore is only necessary at nerve terminals and synapses. In order to aid in neurotransmission, GAD₆₅ forms a complex with heat shock cognate 70 (HSC₇₀), cysteine string protein (CSP) and vesicular GABA transporter VGAT, which, as a complex, helps package GABA into vesicles for release during neurotransmission.<ref>{{cite journal | vauthors = Jin H, Wu H, Osterhaus G, Wei J, Davis K, Sha D, Floor E, Hsu CC, Kopke RD, Wu JY | display-authors = 6 | title = Demonstration of functional coupling between gamma -aminobutyric acid (GABA) synthesis and vesicular GABA transport into synaptic vesicles | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 7 | pages = 4293–8 | date = April 2003 | pmid = 12634427 | pmc = 153086 | doi = 10.1073/pnas.0730698100 | bibcode = 2003PNAS..100.4293J | doi-access = free }}</ref> GAD₆₇ is transcribed during early development, while GAD₆₅ is not transcribed until later in life.<ref name="pmid9777629" /> This developmental difference in GAD₆₇ and GAD₆₅ reflects the functional properties of each isoform; GAD₆₇ is needed throughout development for normal cellular functioning, while GAD₆₅ is not needed until slightly later in development when synaptic inhibition is more prevalent.<ref name="pmid9777629" /> thumb|Gad65 in red, Gad67 in green, and tyrosine hydroxylase (blue) in the ventral tegmental area of the mouse brain

GAD₆₇ and GAD₆₅ are also regulated differently post-translationally. Both GAD₆₅ and GAD₆₇ are regulated via phosphorylation of a dynamic catalytic loop,<ref>{{cite journal | vauthors = Wei J, Davis KM, Wu H, Wu JY | title = Protein phosphorylation of human brain glutamic acid decarboxylase (GAD)65 and GAD67 and its physiological implications | journal = Biochemistry | volume = 43 | issue = 20 | pages = 6182–9 | date = May 2004 | pmid = 15147202 | doi = 10.1021/bi0496992 }}</ref><ref name="Fenalti 2007">{{cite journal | vauthors = Fenalti G, Law RH, Buckle AM, Langendorf C, Tuck K, Rosado CJ, Faux NG, Mahmood K, Hampe CS, Banga JP, Wilce M, Schmidberger J, Rossjohn J, El-Kabbani O, Pike RN, Smith AI, Mackay IR, Rowley MJ, Whisstock JC | display-authors = 6 | title = GABA production by glutamic acid decarboxylase is regulated by a dynamic catalytic loop | journal = Nature Structural & Molecular Biology | volume = 14 | issue = 4 | pages = 280–6 | date = April 2007 | pmid = 17384644 | doi = 10.1038/nsmb1228 | s2cid = 20265911 }}</ref> but the regulation of these isoforms differs; GAD₆₅ is activated by phosphorylation while GAD₆₇ is inhibited by phosphorylation. GAD67 is predominantly found activated (~92%), whereas GAD65 is predominantly found inactivated (~72%).<ref name="pmid12887686"/> GAD₆₇ is phosphorylated at threonine 91 by protein kinase A (PKA), while GAD₆₅ is phosphorylated, and therefore regulated by, protein kinase C (PKC). Both GAD₆₇ and GAD₆₅ are also regulated post-translationally by pyridoxal 5'-phosphate (PLP); GAD is activated when bound to PLP and inactive when not bound to PLP.<ref name="pmid12887686">{{cite journal | vauthors = Battaglioli G, Liu H, Martin DL | title = Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis | journal = Journal of Neurochemistry | volume = 86 | issue = 4 | pages = 879–87 | date = August 2003 | pmid = 12887686 | doi = 10.1046/j.1471-4159.2003.01910.x | s2cid = 23640198 }}</ref> Majority of GAD₆₇ is bound to PLP at any given time, whereas GAD₆₅ binds PLP when GABA is needed for neurotransmission.<ref name="pmid12887686" /> This reflects the functional properties of the two isoforms; GAD₆₇ must be active at all times for normal cellular functioning, and is therefore constantly activated by PLP, while GAD₆₅ must only be activated when GABA neurotransmission occurs, and is therefore regulated according to the synaptic environment.

Studies with mice also show functional differences between Gad67 and Gad65. GAD67−/− mice are born with cleft palate and die within a day after birth while GAD65−/− mice survive with a slightly increased tendency in seizures. Additionally, GAD65± have symptoms defined similarly to attention deficit hyperactivity disorder (ADHD) in humans.<ref>{{Cite journal | vauthors = Ueno H | title = Enzymatic and structural aspects on glutamate decarboxylase. | journal = Journal of Molecular Catalysis B: Enzymatic | date = October 2000 | volume = 10 | issue = 1–3 | pages = 67–79 | doi = 10.1016/S1381-1177(00)00114-4 }}</ref>

== Role in the nervous system ==

Both GAD67 and GAD65 are present in all types of synapses within the human nervous system. This includes dendrodendritic, axosomatic, and axodendritic synapses. Preliminary evidence suggests that GAD65 is dominant in the visual and neuroendocrine systems, which undergo more phasic changes. It is also believed that GAD67 is present at higher amounts in tonically active neurons.<ref>{{cite journal | vauthors = Feldblum S, Erlander MG, Tobin AJ | title = Different distributions of GAD65 and GAD67 mRNAs suggest that the two glutamate decarboxylases play distinctive functional roles | journal = Journal of Neuroscience Research | volume = 34 | issue = 6 | pages = 689–706 | date = April 1993 | pmid = 8315667 | doi = 10.1002/jnr.490340612 | s2cid = 19314092 }}</ref>

== Role in pathology ==

=== Autism ===

Both GAD65 and GAD67 experience significant downregulation in cases of autism. In a comparison of autistic versus control brains, GAD65 and GAD67 experienced a downregulation average of 50% in parietal and cerebellar cortices of autistic brains.<ref name = "Fatemi_2002">{{cite journal | vauthors = Fatemi SH, Halt AR, Stary JM, Kanodia R, Schulz SC, Realmuto GR | title = Glutamic acid decarboxylase 65 and 67 kDa proteins are reduced in autistic parietal and cerebellar cortices | language = en | journal = Biological Psychiatry | volume = 52 | issue = 8 | pages = 805–10 | date = October 2002 | pmid = 12372652 | doi = 10.1016/S0006-3223(02)01430-0 | s2cid = 30140735 }}</ref> Cerebellar Purkinje cells also reported a 40% downregulation, suggesting that affected cerebellar nuclei may disrupt output to higher order motor and cognitive areas of the brain.<ref name="Soghomonian_1998" />

=== Diabetes ===

Both GAD₆₇ and GAD₆₅ are targets of autoantibodies in people who later develop type 1 diabetes mellitus or latent autoimmune diabetes.<ref name="pmid1697648">{{cite journal | vauthors = Baekkeskov S, Aanstoot HJ, Christgau S, Reetz A, Solimena M, Cascalho M, Folli F, Richter-Olesen H, De Camilli P, Camilli PD | title = Identification of the 64K autoantigen in insulin-dependent diabetes as the GABA-synthesizing enzyme glutamic acid decarboxylase | journal = Nature | volume = 347 | issue = 6289 | pages = 151–6 | date = September 1990 | pmid = 1697648 | doi = 10.1038/347151a0 | bibcode = 1990Natur.347..151B | s2cid = 4317318 }}</ref><ref name="pmid1370298">{{cite journal | vauthors = Kaufman DL, Erlander MG, Clare-Salzler M, Atkinson MA, Maclaren NK, Tobin AJ | title = Autoimmunity to two forms of glutamate decarboxylase in insulin-dependent diabetes mellitus | journal = The Journal of Clinical Investigation | volume = 89 | issue = 1 | pages = 283–92 | date = January 1992 | pmid = 1370298 | pmc = 442846 | doi = 10.1172/JCI115573 }}</ref> Injections with GAD₆₅ in ways that induce immune tolerance have been shown to prevent type 1 diabetes in rodent models.<ref name="pmid8232539">{{cite journal | vauthors = Tisch R, Yang XD, Singer SM, Liblau RS, Fugger L, McDevitt HO | title = Immune response to glutamic acid decarboxylase correlates with insulitis in non-obese diabetic mice | journal = Nature | volume = 366 | issue = 6450 | pages = 72–5 | date = November 1993 | pmid = 8232539 | doi = 10.1038/366072a0 | bibcode = 1993Natur.366...72T | s2cid = 4273636 }}</ref><ref name="pmid7694152">{{cite journal | vauthors = Kaufman DL, Clare-Salzler M, Tian J, Forsthuber T, Ting GS, Robinson P, Atkinson MA, Sercarz EE, Tobin AJ, Lehmann PV | title = Spontaneous loss of T-cell tolerance to glutamic acid decarboxylase in murine insulin-dependent diabetes | journal = Nature | volume = 366 | issue = 6450 | pages = 69–72 | date = November 1993 | pmid = 7694152 | doi = 10.1038/366069a0 | bibcode = 1993Natur.366...69K | s2cid = 4370149 | pmc = 8216222 }}</ref><ref name="pmid8946834">{{cite journal | vauthors = Tian J, Clare-Salzler M, Herschenfeld A, Middleton B, Newman D, Mueller R, Arita S, Evans C, Atkinson MA, Mullen Y, Sarvetnick N, Tobin AJ, Lehmann PV, Kaufman DL | title = Modulating autoimmune responses to GAD inhibits disease progression and prolongs islet graft survival in diabetes-prone mice | journal = Nature Medicine | volume = 2 | issue = 12 | pages = 1348–53 | date = December 1996 | pmid = 8946834 | doi = 10.1038/nm1296-1348 | s2cid = 27692555 }}</ref> In clinical trials, injections with GAD₆₅ have been shown to preserve some insulin production for 30 months in humans with type 1 diabetes.<ref name="pmid18843118">{{cite journal | vauthors = Ludvigsson J, Faresjö M, Hjorth M, Axelsson S, Chéramy M, Pihl M, Vaarala O, Forsander G, Ivarsson S, Johansson C, Lindh A, Nilsson NO, Aman J, Ortqvist E, Zerhouni P, Casas R | title = GAD treatment and insulin secretion in recent-onset type 1 diabetes | journal = The New England Journal of Medicine | volume = 359 | issue = 18 | pages = 1909–20 | date = October 2008 | pmid = 18843118 | doi = 10.1056/NEJMoa0804328 | doi-access =free }}</ref><ref name="url_Diamyd">{{cite web | url = http://www.diamyd.com/docs/pressClip.aspx?section=investor&ClipID=420 | title = Diamyd announces completion of type 1 diabetes vaccine trial with long term efficacy demonstrated at 30 months | date = 2008-01-28 | work = Press Release | publisher = Diamyd Medical AB | access-date = 2010-01-13 }}</ref> A Cochrane systematic review also examined 1 study showing improvement of C-peptide levels in cases of Latent Autoimmune Diabetes in adults, 5 years following treatment with GAD₆₅. Still, it is important to highlight that the studies available to be included in this review presented considerable flaws in quality and design.<ref>{{Cite journal |last1=Brophy |first1=Sinead |last2=Davies |first2=Helen |last3=Mannan |first3=Sopna |last4=Brunt |first4=Huw |last5=Williams |first5=Rhys |date=2011-09-07 |title=Interventions for latent autoimmune diabetes (LADA) in adults |journal=Cochrane Database of Systematic Reviews |volume=2011 |issue=9 |article-number=CD006165 |doi=10.1002/14651858.cd006165.pub3 |issn=1465-1858 |pmc=6486159 |pmid=21901702}}</ref>

===Stiff person syndrome===

thumb|Healthy human cerebellum stained with a reference anti-GAD65 monoclonal antibody. Thin arrows show presynaptic terminals staining with the anti-GAD65 monoclonal antibody High titers of autoantibodies to glutamic acid decarboxylase (GAD) are well documented in association with stiff person syndrome (SPS).<ref>{{cite journal | vauthors = Dalakas MC, Fujii M, Li M, Lutfi B, Kyhos J, McElroy B | title = High-dose intravenous immune globulin for stiff-person syndrome | journal = The New England Journal of Medicine | volume = 345 | issue = 26 | pages = 1870–6 | date = December 2001 | pmid = 11756577 | doi = 10.1056/NEJMoa01167 | doi-access = free }}</ref> Glutamic acid decarboxylase is the rate-limiting enzyme in the synthesis of γ-aminobutyric acid (GABA), and impaired function of GABAergic neurons has been implicated in the pathogenesis of SPS. Autoantibodies to GAD might be the causative agent or a disease marker.<ref>{{cite journal | vauthors = Chang T, Alexopoulos H, McMenamin M, Carvajal-González A, Alexander SK, Deacon R, Erdelyi F, Szabó G, Gabor S, Lang B, Blaes F, Brown P, Vincent A | title = Neuronal surface and glutamic acid decarboxylase autoantibodies in Nonparaneoplastic stiff person syndrome | journal = JAMA Neurology | volume = 70 | issue = 9 | pages = 1140–9 | date = September 2013 | pmid = 23877118 | doi = 10.1001/jamaneurol.2013.3499 | pmc = 6055982 }}</ref>

=== Schizophrenia and bipolar disorder ===

Substantial dysregulation of GAD mRNA expression, coupled with downregulation of reelin, is observed in schizophrenia and bipolar disorder.<ref name="Guidotti2000">{{cite journal | last1 = Guidotti | first1 = Alessandro | last2 = Auta | first2 = James | last3 = Davis | first3 = John M. | last4 = Gerevini | first4 = Valeria DiGiorgi | last5 = Dwivedi | first5 = Yogesh | last6 = Grayson | first6 = Dennis R. | last7 = Impagnatiello | first7 = Francesco | last8 = Pandey | first8 = Ghanshyam | last9 = Pesold | first9 = Christine | last10 = Sharma | first10 = Rajiv | last11 = Uzunov | first11 = Doncho | last12 = Costa | first12 = Erminio | title = Decrease in reelin and glutamic acid decarboxylase₆₇ (GAD₆₇) expression in schizophrenia and bipolar disorder: a postmortem brain study | journal = Archives of General Psychiatry | volume = 57 | issue = 11 | pages = 1061–1069 | year = 2000 | doi = 10.1001/archpsyc.57.11.1061 | pmid = 11074872 }}</ref><ref name="Akbarian2006">{{cite journal | last1 = Akbarian | first1 = Schahram | last2 = Huang | first2 = Hsien-Sung | title = Molecular and cellular mechanisms of altered GAD1/GAD67 expression in schizophrenia and related disorders | journal = Brain Research Reviews | volume = 52 | issue = 2 | pages = 293–304 | year = 2006 | doi = 10.1016/j.brainresrev.2006.04.001 | pmid = 16759710 | s2cid = 25771139 }}</ref> The most pronounced downregulation of GAD₆₇ was found in hippocampal stratum oriens layer in both disorders and in other layers and structures of hippocampus with varying degrees.<ref name="pmid17553960">{{cite journal | vauthors = Benes FM, Lim B, Matzilevich D, Walsh JP, Subburaju S, Minns M | title = Regulation of the GABA cell phenotype in hippocampus of schizophrenics and bipolars | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 24 | pages = 10164–9 | date = June 2007 | pmid = 17553960 | pmc = 1888575 | doi = 10.1073/pnas.0703806104 | bibcode = 2007PNAS..10410164B | doi-access = free }}</ref>

GAD₆₇ is a key enzyme involved in the synthesis of inhibitory neurotransmitter GABA and people with schizophrenia have been shown to express lower amounts of GAD₆₇ in the dorsolateral prefrontal cortex compared to healthy controls.<ref name="pmid24874453">{{cite journal | vauthors = Kimoto S, Bazmi HH, Lewis DA | title = Lower expression of glutamic acid decarboxylase 67 in the prefrontal cortex in schizophrenia: contribution of altered regulation by Zif268 | journal = The American Journal of Psychiatry | volume = 171 | issue = 9 | pages = 969–78 | date = September 2014 | pmid = 24874453 | pmc = 4376371 | doi = 10.1176/appi.ajp.2014.14010004 }}</ref> The mechanism underlying the decreased levels of GAD₆₇ in people with schizophrenia remains unclear.<ref name="Georgiev2016">{{cite journal | last1 = Georgiev | first1 = Danko | last2 = Yoshihara | first2 = Toru | last3 = Kawabata | first3 = Rika | last4 = Matsubara | first4 = Takurou | last5 = Tsubomoto | first5 = Makoto | last6 = Minabe | first6 = Yoshio | last7 = Lewis | first7 = David A. | last8 = Hashimoto | first8 = Takanori | title = Cortical gene expression after a conditional knockout of 67 kDa glutamic acid decarboxylase in parvalbumin neurons | journal = Schizophrenia Bulletin | volume = 42 | issue = 4 | pages = 992–1002 | year = 2016 | doi = 10.1093/schbul/sbw022 | pmid = 26980143 | pmc = 4903066 | s2cid = 24197087 }}</ref> Some have proposed that an immediate early gene, Zif268, which normally binds to the promoter region of GAD₆₇ and increases transcription of GAD₆₇, is lower in schizophrenic patients, thus contributing to decreased levels of GAD₆₇.<ref name="pmid24874453"/> Since the dorsolateral prefrontal cortex (DLPFC) is involved in working memory, and GAD₆₇ and Zif268 mRNA levels are lower in the DLPFC of schizophrenic patients, this molecular alteration may account, at least in part, for the working memory impairments associated with the disease.

=== Parkinson disease ===

The bilateral delivery of glutamic acid decarboxylase (GAD) by an adeno-associated viral vector into the subthalamic nucleus of patients between 30 and 75 years of age with advanced, progressive, levodopa-responsive Parkinson disease resulted in significant improvement over baseline during the course of a six-month study.<ref name="pmid21419704">{{cite journal | vauthors = LeWitt PA, Rezai AR, Leehey MA, Ojemann SG, Flaherty AW, Eskandar EN, Kostyk SK, Thomas K, Sarkar A, Siddiqui MS, Tatter SB, Schwalb JM, Poston KL, Henderson JM, Kurlan RM, Richard IH, Van Meter L, Sapan CV, During MJ, Kaplitt MG, Feigin A | title = AAV2-GAD gene therapy for advanced Parkinson's disease: a double-blind, sham-surgery controlled, randomised trial | journal = The Lancet. Neurology | volume = 10 | issue = 4 | pages = 309–19 | date = April 2011 | pmid = 21419704 | doi = 10.1016/S1474-4422(11)70039-4 | s2cid = 37154043 }}</ref>

=== Cerebellar disorders ===

Intracerebellar administration of GAD autoantibodies to animals increases the excitability of motoneurons and impairs the production of nitric oxide (NO), a molecule involved in learning. Epitope recognition contributes to cerebellar involvement.<ref>{{cite journal | vauthors = Manto MU, Hampe CS, Rogemond V, Honnorat J | title = Respective implications of glutamate decarboxylase antibodies in stiff person syndrome and cerebellar ataxia | journal = Orphanet Journal of Rare Diseases | volume = 6 | issue = 3 | page = 3 | date = February 2011 | pmid = 21294897 | pmc = 3042903 | doi = 10.1186/1750-1172-6-3 | doi-access = free }}</ref> Reduced GABA levels increase glutamate levels as a consequence of lower inhibition of subtypes of GABA receptors. Higher glutamate levels activate microglia and activation of xc(−) increases the extracellular glutamate release.<ref>{{cite journal | vauthors = Mitoma H, Manto M, Hampe CS | title = Pathogenic Roles of Glutamic Acid Decarboxylase 65 Autoantibodies in Cerebellar Ataxias | journal = Journal of Immunology Research | volume = 2017 | article-number = 2913297 | date = 2017-03-12 | pmid = 28386570 | doi = 10.1155/2017/2913297 | pmc = 5366212 | doi-access = free }}</ref>

=== Neuropathic pain ===

Peripheral nerve injury of the sciatic nerve (a neuropathic pain model) induces a transient loss of GAD₆₅ immunoreactive terminals in the spinal cord dorsal horn and suggests a potential involvement for these alterations in the development and amelioration of pain behaviour.<ref name="pmid 25189404">{{cite journal | vauthors = Lorenzo LE, Magnussen C, Bailey AL, St Louis M, De Koninck Y, Ribeiro-da-Silva A | title = Spatial and temporal pattern of changes in the number of GAD65-immunoreactive inhibitory terminals in the rat superficial dorsal horn following peripheral nerve injury | journal = Molecular Pain | volume = 10 | issue = 1 | pages = 1744-8069-10-57 | date = September 2014 | pmid = 25189404 | pmc = 4164746 | doi = 10.1186/1744-8069-10-57 | doi-access = free }}</ref>

==Other anti-GAD-associated neurologic disorders==

Antibodies directed against glutamic acid decarboxylase (GAD) are increasingly found in patients with other symptoms indicative of central nervous system (CNS) dysfunction, such as ataxia, progressive encephalomyelitis with rigidity and myoclonus (PERM), limbic encephalitis, and epilepsy.<ref>{{cite journal | vauthors = Dayalu P, Teener JW | title = Stiff Person syndrome and other anti-GAD-associated neurologic disorders | journal = Seminars in Neurology | volume = 32 | issue = 5 | pages = 544–9 | date = November 2012 | pmid = 23677666 | doi = 10.1055/s-0033-1334477 | s2cid = 35562171 }}</ref> The pattern of anti-GAD antibodies in epilepsy differs from type 1 diabetes and stiff-person syndrome.<ref>{{cite journal | vauthors = Liimatainen S, Honnorat J, Pittock SJ, McKeon A, Manto M, Radtke JR, Hampe CS | title = GAD65 autoantibody characteristics in patients with co-occurring type 1 diabetes and epilepsy may help identify underlying epilepsy etiologies | journal = Orphanet Journal of Rare Diseases | volume = 13 | issue = 1 | page = 55 | date = April 2018 | pmid = 29636076 | pmc = 5892043 | doi = 10.1186/s13023-018-0787-5 | doi-access = free }}</ref>

== Role of glutamate decarboxylase in other organisms ==

Besides the synthesis of GABA, GAD has additional functions and structural variations that are organism-dependent. In ''Saccharomyces cerevisiae'', GAD binds the Ca²⁺ regulatory protein calmodulin (CaM) and is also involved in responding to oxidative stress.<ref name="Coleman 2001">{{cite journal | vauthors = Coleman ST, Fang TK, Rovinsky SA, Turano FJ, Moye-Rowley WS | title = Expression of a glutamate decarboxylase homologue is required for normal oxidative stress tolerance in Saccharomyces cerevisiae | journal = J Biol Chem | volume = 276 | issue = 1 | pages = 244–50 | date = Jan 2001 | pmid = 11031268 | doi = 10.1074/jbc.M007103200 | doi-access = free }}</ref> Similarly, GAD in plants binds calmodulin as well.<ref name="Baum 1996">{{cite journal | vauthors = Baum G, Lev-Yadun S, Fridmann Y, Arazi T, Katsnelson H, Zik M, Fromm H | title = Calmodulin binding to glutamate decarboxylase is required for regulation of glutamate and GABA metabolism and normal development in plants | journal = EMBO J | volume = 15 | issue = 12 | pages = 2988–96 | date = Jun 1996 | doi = 10.1002/j.1460-2075.1996.tb00662.x | pmid = 8670800 |pmc=450240 }}</ref> This interaction occurs at the 30-50bp CAM-binding domain (CaMBD) in its C terminus and is necessary for proper regulation of GABA production.<ref name="Baum 1993">{{cite journal | vauthors = Baum G, Chen Y, Arazi T, Takatsuji H, Fromm H | title = A plant glutamate decarboxylase containing a calmodulin binding domain. Cloning, sequence, and functional analysis | journal = J Biol Chem | volume = 268 | issue = 26 | pages = 19610–7 | date = Sep 1993 | doi = 10.1016/S0021-9258(19)36560-3 | pmid = 8366104 | doi-access = free }}</ref> Unlike vertebrates and invertebrates, the GABA produced by GAD is used in plants to signal abiotic stress by controlling levels of intracellular Ca²⁺ via CaM. Binding to CaM opens Ca²⁺ channels and leads to an increase in Ca²⁺ concentrations in the cytosol, allowing Ca²⁺ to act as a secondary messenger and activate downstream pathways. When GAD is not bound to CaM, the CaMBD acts as an autoinhibitory domain, thus deactivating GAD in the absence of stress.<ref name="Baum 1993"/> Interesting, in two plant species, rice and apples, Ca2+ /CAM-independent GAD isoforms have been discovered.<ref name="Akama 2001">{{cite journal | vauthors = Akama K, Akihiro T, Kitagawa M, Takaiwa F | title = Rice (Oryza sativa) contains a novel isoform of glutamate decarboxylase that lacks an authentic calmodulin-binding domain at the C-terminus | journal = Biochim Biophys Acta | volume = 1522 | issue = 3 | pages = 143–50 | date = Dec 2001 | pmid = 11779628 | doi = 10.1016/s0167-4781(01)00324-4 }}</ref><ref name="Trobacher 2013">{{cite journal | vauthors = Trobacher CP, Zarei A, Liu J, Clark SM, Bozzo GG, Shelp | title = Calmodulin-dependent and calmodulin-independent glutamate decarboxylases in apple fruit | journal = BMC Plant Biol | volume = 144 | issue = 13 | date = Sep 2013 | page = 144 | doi = 10.1186/1471-2229-13-144 | pmid = 24074460 | pmc = 3849887 | doi-access = free | bibcode = 2013BMCPB..13..144T }}</ref> The C-terminus of these isoforms contain substitutions at key residues necessary to interact with CaM in the CaMBD, preventing the protein from binding to GAD. Whereas CaMBD of the isoform in rice still functions as an autoinhibitory domain,<ref name="Akama 2001"/> the C-terminus in the isoform in apples does not.<ref name="Trobacher 2013"/> Finally, the structure of plant GAD is a hexamer and has pH-dependent activity, with the optimal pH of 5.8 in multiple species.<ref name="Baum 1993"/><ref name="Zik 1998">{{cite journal | vauthors = Zik M, Arazi T, Snedden WA, Fromm H |title=Two isoforms of glutamate decarboxylase in Arabidopsis are regulated by calcium/calmodulin and differ in organ distribution | journal = Plant Mol Biol | volume = 37 | issue = 6 | pages = 967–75 | date = Aug 1998 | pmid = 9700069 | doi = 10.1023/a:1006047623263 |s2cid=28501096 }}</ref> but also significant activity at pH 7.3 in the presence of CaM.<ref name="Capitani 2003"/>

It is also believed that the control of glutamate decarboxylase has the prospect of improving citrus produce quality post-harvest. In Citrus plants, research has shown that glutamate decarboxylase plays a key role in citrate metabolism. With the increase of glutamate decarboxylase via direct exposure, citrate levels have been seen to significantly increase within plants, and in conjunction post-harvest quality maintenance was significantly improved, and rot rates decreased.<ref>{{cite journal | vauthors = Sheng L, Shen D, Luo Y, Sun X, Wang J, Luo T, Zeng Y, Xu J, Deng X, Cheng Y | title = Exogenous γ-aminobutyric acid treatment affects citrate and amino acid accumulation to improve fruit quality and storage performance of postharvest citrus fruit | journal = Food Chemistry | volume = 216 | pages = 138–45 | date = February 2017 | pmid = 27596402 | doi = 10.1016/j.foodchem.2016.08.024 }}</ref>

Just like GAD in plants, GAD in ''E. coli'' has a hexamer structure and is more active under acidic pH; the pH optimum for ''E. coli'' GAD is 3.8-4.6. However, unlike plants and yeast, GAD in ''E. coli'' does not require calmodulin binding to function. There are also two isoforms of GAD, namely GadA and GadB, encoded by separate genes in ''E. coli'',<ref name="Smith 1992">{{cite journal | vauthors = Smith DK, Kassam T, Singh B, Elliott JF | title = Escherichia coli has two homologous glutamate decarboxylase genes that map to distinct loci | journal = J Bacteriol | volume = 174 | issue = 18 | pages = 5820–6 | date = Sep 1992 | pmid = 1522060 |pmc=207112 | doi = 10.1128/jb.174.18.5820-5826.1992 }}</ref> although both isoforms are biochemically identical.<ref name="De Biase 1996">{{cite journal | vauthors = De Biase D, Tramonti A, John RA, Bossa F | title = Isolation, overexpression, and biochemical characterization of the two isoforms of glutamic acid decarboxylase from Escherichia coli | journal = Protein Expr Purif | volume = 8 | issue = 4 | pages = 430–8 | date = Dec 1996 | pmid = 8954890 | doi = 10.1006/prep.1996.0121 }}</ref> The enzyme plays a major role in conferring acid resistance and allows bacteria to temporarily survive in highly acidic environments (pH < 2.5) like the stomach.<ref name="Lin 1995">{{cite journal | vauthors = Lin J, Lee IS, Frey J, Slonczewski JL, Foster JW | title = Comparative analysis of extreme acid survival in Salmonella typhimurium, Shigella flexneri, and Escherichia coli | journal = J Bacteriol | volume = 177 | issue = 14 | pages = 4097–104 | date = Jul 1995 | pmid = 7608084 |pmc=177142 | doi = 10.1128/jb.177.14.4097-4104.1995 }}</ref> This is done by GAD decarboxylating glutamate to GABA, which requires H+ to be uptaken as a reactant and raises the pH inside the bacteria. GABA can then be exported out of ''E. coli'' cells and contribute to increasing the pH of the nearby extracellular environments.<ref name="Capitani 2003"/>

== References == {{reflist|33em}}

== External links == * {{commons-inline}} * [https://web.archive.org/web/20070815162255/http://www.schizophreniaforum.org/new/detail.asp?id=1358 Genetics, Expression Profiling Support GABA Deficits in Schizophrenia] - Schizophrenia Research Forum, 25 June 2007. * {{PDBe-KB2|Q99259|Glutamate decarboxylase 1}} * {{PDBe-KB2|Q05329|Glutamate decarboxylase 2}}

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Category:EC 4.1.1 Category:Molecular neuroscience Category:Biology of bipolar disorder Category:GABA Category:Glutamate (neurotransmitter)