Glutamate decarboxylase

Glutamic acid decarboxylase 1
GAD67 derived from PDB: 2okj​
Identifiers
Symbol	GAD1
Alt. symbols	glutamate decarboxylase 1 (brain, 67kD); GAD67
Entrez	2571
HUGO	4092
OMIM	605363
RefSeq	NM_000817
UniProt	Q99259
Other data
EC number	4.1.1.15
Locus	Chr. 2 q31

Glutamate decarboxylase or glutamic acid decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of glutamate to GABA and CO₂. GAD uses PLP as a cofactor. The reaction proceeds as follows:

HOOC-CH₂-CH₂-CH(NH₂)-COOH → CO₂ + HOOC-CH₂-CH₂-CH₂NH₂

In mammals, GAD exists in two isoforms encoded by two different genes - GAD1 and GAD2. These isoforms are GAD₆₇ and GAD₆₅ with molecular weights of 67 and 65 kDa, respectively.^[1] GAD1 and GAD2 are expressed in the brain where GABA is used as a neurotransmitter, GAD2 is also expressed in the pancreas.

At least two more forms, GAD₂₅ and GAD₄₄ (embryonic; EGAD) are described in the developing brain. They are coded by the alternative transcripts of GAD1, I-80 and I-86: GAD₂₅ is coded by both, GAD₄₄ - only by I-80.^[2]

Regulation of GAD65 and GAD67

GAD₆₅ and GAD₆₇ synthesize GABA at different locations in the cell, at different developmental times, and for functionally different purposes.^[3] GAD₆₇ is spread evenly throughout the cell while GAD₆₅ is localized to nerve terminals.^[3] This difference is thought to reflect a functional difference; GAD₆₇ synthesizes GABA for neuron activity unrelated to neurotransmission, such as synaptogenesis and protection from neural injury.^[3] This function requires widespread, ubiquitous presence of GABA. GAD₆₅, however, synthesizes GABA for neurotransmission,^[3] 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.^[4] GAD₆₇ is transcribed during early development, while GAD₆₅ is not transcribed until later in life.^[3] 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.^[3]

GAD₆₇ and GAD₆₅ are also regulated differently post-translationally. Both GAD₆₅ and GAD₆₇ are regulated via phosphorylation,^[5] but the regulation of these isoforms differs; GAD₆₅ is activated by phosphorylation while GAD₆₇ is inhibited by phosphorylation. 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.^[6] Majority of GAD₆₇ is bound to PLP at any given time, whereas GAD₆₅ binds PLP when GABA is needed for neurotransmission.^[6] 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.

Role in pathology

Diabetes

Both GAD₆₇ and GAD₆₅ are targets of autoantibodies in people who later develop type 1 diabetes mellitus or latent autoimmune diabetes.^[7][8] Injections with GAD₆₅ has been shown to preserve some insulin production for 30 months in humans with type 1 diabetes.^[9][10]

Stiff Person Syndrome

High titers of autoantibodies to glutamic acid decarboxylase (GAD) are well documented in association with stiff person syndrome (SPS). 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.^[11]

Schizophrenia and bipolar disorder

Substantial dysregulation of GAD mRNA expression, coupled with downregulation of reelin, is observed in schizophrenia and bipolar disorder.^[12] 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.^[13]

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.^[14] The mechanism underlying the decreased levels of GAD₆₇ in people with schizophrenia remains unclear. 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.^[14] 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.^[15]

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.^[16] 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.^[17]

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.^[18]

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.^[19]

References

External links

• Media related to Glutamate decarboxylase at Wikimedia Commons
• Genetics, Expression Profiling Support GABA Deficits in Schizophrenia - Schizophrenia Research Forum, 25 June 2007.