title: Glutamate decarboxylase protects neurons against excitotoxic injury.
abstract: Although the majority of agents with antiexcitotoxic action act as glutamate receptor antagonists, enzymatic degradation of glutamate can also be neuroprotective. The very low specific activity of the mammalian form of glutamate decarboxylase (GAD), the enzyme that catalyzes the formation of gamma-aminobutyric acid (GABA) from glutamate in neurons, is likely to limit its utility as an antiglutamate neuroprotectant. In contrast, the bacterial form of GAD can be isolated with relatively high specific activity and is most active in acidic environments. We have expressed and purified GAD from Escherichia coli (bGAD) and tested the ability of the enzyme to protect against glutamate excitotoxicity. Incubation of rat hipppocampal slices with the potassium channel antagonist tetraethyl ammonium (TEA) resulted in widespread excitotoxic death of pyramidal and granule cell neurons. bGAD alone showed no significant neurotoxicity and significantly reduced excitotoxicity induced by TEA. We hypothesize that bGAD may be internalized into the synaptic vesicle compartment by nonspecific endocytosis, where both the appropriate pH and high glutamate concentrations are present. Targeting of this enzyme to the interior of synaptic vesicles may enhance its potency as a neuroprotectant against excitotoxicity.
authors.affiliation: Research Service, Maryland VA Healthcare System, Baltimore, Maryland 21201, USA. ccmatthe@umaryland.edu
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Matthews
authors.forename: H Ronald
authors.initials: HR
authors.lastname: Zielke
authors.forename: Paul S
authors.initials: PS
authors.lastname: Fishman
authors.forename: Mary P
authors.initials: MP
authors.lastname: Remington
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Bowen
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001426:Bacterial Proteins / Q000494:pharmacology; D016923:Cell Death / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005968:Glutamate Decarboxylase / Q000494:pharmacology*; D006624:Hippocampus / Q000293:injuries* / Q000473:pathology*; D009474:Neurons / Q000166:cytology / Q000187:drug effects*; D018696:Neuroprotective Agents / Q000494:pharmacology*; D009924:Organ Culture Techniques; D026902:Potassium Channel Blockers / Q000633:toxicity; D051381:Rats; D017207:Rats, Sprague-Dawley; D019789:Tetraethylammonium / Q000633:toxicity
chemical_list: D001426:Bacterial Proteins; D018696:Neuroprotective Agents; D026902:Potassium Channel Blockers; D019789:Tetraethylammonium; D005968:Glutamate Decarboxylase