Kinetics of brain glutamate decarboxylase. Interactions with glutamate, pyridoxal 5'-phosphate and glutamate-pyridoxal 5'-phosphate Schiff base

Epilepsy and hippocampal neurodegeneration induced by glutamate decarboxylase inhibitors in awake rats

Glutamic acid decarboxylase (GAD), the enzyme responsible for GABA synthesis, requires pyridoxal phosphate (PLP) as a cofactor. Thiosemicarbazide (TSC) and γ-glutamyl-hydrazone (PLPGH) inhibit the free PLP-dependent isoform (GAD65) activity after systemic administration, leading to epilepsy in mice and in young, but not in adult rats. However, the competitive GAD inhibitor 3-mercaptopropionic acid (MPA) induces convulsions in both immature and adult rats. In the present study we tested comparatively the epileptogenic and neurotoxic effects of PLPGH, TSC and MPA, administered by microdialysis in the hippocampus of adult awake rats. Cortical EEG and motor behavior were analyzed during the next 2h, and aspartate, glutamate and GABA were measured by HPLC in the microdialysis-collected fractions. Twenty-four hours after drug administration rats were fixed for histological analysis of the hippocampus. PLPGH or TSC did not affect the motor behavior, EEG or cellular morphology, although the extracellular concentration of GABA was decreased. In contrast, MPA produced intense wet-dog shakes, EEG epileptiform discharges, a >75% reduction of extracellular GABA levels and remarkable neurodegeneration of the CA1 region, with >80% neuronal loss. The systemic administration of the NMDA glutamate receptor antagonist MK-801 30 min before MPA did not prevent the MPA-induced epilepsy but significantly protected against its neurotoxic effect, reducing neuronal loss to <30%. We conclude that in adult awake rats, drugs acting on PLP availability have only a weak effect on GABA neurotransmission, whereas direct GAD inhibition produced by MPA induces hyperexcitation leading to epilepsy and hippocampal neurodegeneration. Because this degeneration was prevented by the blockade of NMDA receptors, we conclude that it is due to glutamate-mediated excitotoxicity consequent to disinhibition of the hippocampal excitatory circuits.

Molecular Identification of the 62 kd Form of Glutamic Acid Decarboxylase from the Mouse

A series of overlapping clones coding for L-glutamic acid decarboxylase was purified from a mouse brain cDNA library, the longest of which contains a 1869 bp open reading frame and 913 bp of non-coding sequence. By comparison with the corresponding sequences from the mouse genome, it was determined that the first methionine in the longest cDNA represents the initiation codon. Expression of this cDNA in eukaryotic cells produces a 62 kd protein that is recognized by antiserum against rat GAD and which displays GAD activity commensurate with the amount of protein produced. Antibodies raised against the purified product of this cDNA recognize a 62 kd protein from mouse brain on immunoblots, specifically stain GABA-ergic neurons in brain sections, and are capable of immunoprecipitating most GAD activity from mouse brain extracts. These results provide the first definitive identification of a cDNA coding for the larger of two forms of GAD in mouse brain, and suggest that the two forms are closely related.

Biochemical characteristics of the gamma-aminobutyric acid system in the insulinoma cell lines HIT-T15, RIN-m5F, betaTC3, and comparison with rat brain

BACKGROUND

gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the mammalian brain. Both GABA and its synthesizing enzyme, L-glutamate decarboxylase (GAD), are also present in the insulin-secreting pancreatic beta cells, in which its physiologic role is unclear. We have studied several aspects of the GABA system in the insulinoma cell lines HIT-T15, RIN-m5F, and betaTC3 in comparison with rat brain tissue.

METHODS

Insulinoma cell lines and embryonic rat brain cortex neurons were cultured. GAD activity was determined by a radioenzymatic method and the presence of GAD(67) protein was assessed by immunocytochemistry. Amino acid content and the effect of different conditions on the release of endogenous GABA were measured by HPLC and fluorometric detection after o-phthaldialdehyde derivatization. [3H]GABA was used for measuring the uptake of the amino acid in the insulinoma cultures and in rat forebrain synaptosomes.

RESULTS

The three insulinoma lines possess GABA and GAD activity at levels of approximately 20% compared with adult rat brain cortex. Dissimilar from the latter, in insulinoma cultures enzyme activity was not enhanced by addition of an excess of the coenzyme pyridoxal-5'-phosphate. Immunocytochemical visualization of GAD showed that the cells in both neuronal cultures and insulinoma lines were GAD(67)-positive, similar to Purkinje cell somata of adult rat cerebellar cortex. [3H]GABA uptake in the cell lines was approximately 10% of that in rat forebrain synaptosomes and showed less ionic and temperature dependence. In both cultured cerebral neurons and RINm5F cells, the addition of arginine induced the release of GABA, whereas neither high K(+) concentration nor glucose had any effect.

CONCLUSIONS

The insulinoma cell lines studied possess the same GAD(67) form of the enzyme present in brain. RIN line cells are capable of transporting glutamate. In these cells as well as in cultured cortical neurons, arginine stimulates the release of GABA and glutamate probably as the result of its electrogenic transport. Insulinoma cell lines may therefore be useful to study GABA metabolism and function in pancreatic beta cells.

Convulsions and inhibition of glutamate decarboxylase by pyridoxal phosphate-gamma-glutamyl hydrazone in the developing rat

We have previously shown that in the adult rat the inhibition of brain glutamate decarboxylase (GAD) activity by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH) administration does not result in convulsions, whereas in the adult mouse intense convulsions invariably occur. In the present study we report that, surprisingly, immature rats from 2 to 20 days of age treated with PLPGH (80 mg/kg) showed generalized tonic-clonic convulsions, whereas no convulsions at all were present in 30 days-old or older rats. GAD activity, measured by enzymic determination of GABA formed in forebrain homogenates, was inhibited by about 60% at the time of convulsions in 15 days-old and younger rats, whereas the inhibition was between 40 and 50% in older animals. The addition of the coenzyme pyridoxal 5'-phosphate to the incubation medium completely reversed this inhibition. In all treated animals GABA levels were lower compared to controls. The results indicate that the susceptibility of GAD in vivo to a diminished cofactor concentration decreases with age. It seems possible that changes in the expression of enzyme forms are reflected in developmental variations in the susceptibility to seizures induced by vitamin B6 depletion, but alterations of other B6-dependent biochemical pathways cannot be discarded.

Comparative localization of mRNAs encoding two forms of glutamic acid decarboxylase with nonradioactive in situ hybridization methods

Nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes were used to localize two glutamic acid decarboxylase (GAD) mRNAs in rat brain. These mRNAs encode two forms of GAD that both synthesize GABA but differ in a number of characteristics including their molecular size (65 and 67 kDa). For each GAD mRNA, discrete neuronal labeling with high cellular resolution and low background staining was obtained in most populations of known GABA neurons. In addition, the current methods revealed differences in the intensity of labeling among neurons for each GAD mRNA, suggesting that the relative concentrations of each GAD mRNA may be higher in some groups of GABA neurons than in others. Most major classes of GABA neurons were labeled for each GAD mRNA. In some groups of GABA neurons, the labeling for the two mRNAs was virtually identical, as in the reticular nucleus of the thalamus. In other groups of neurons, although there was substantial labeling for each GAD mRNA, labeling for one of the mRNAs was noticeably stronger than for the other. In most brain regions, such as the cerebellar cortex, labeling for GAD67 mRNA was stronger than for GAD65 mRNA, but there were a few brain regions in which labeling for GAD65 mRNA was more pronounced, and these included some regions of the hypothalamus. Finally, some groups of GABA neurons were predominantly labeled for one of the GAD mRNAs and showed little or no detectable labeling for the other GAD mRNA, as, for example, in neurons of the tuberomammillary nucleus of the hypothalamus where labeling for GAD67 mRNA was very strong but no labeling for GAD65 mRNA was evident. The findings suggest that most classes of GABA neurons in the central nervous system (CNS) contain mRNAs for at least two forms of GAD, and thus, have dual enzyme systems for the synthesis of GABA. Higher levels of one or the other GAD mRNA in certain groups of GABA neurons may be related to differences in the functional properties of these neurons and their means of regulating GABA synthesis.

Two genes encode distinct glutamate decarboxylases

gamma-Aminobutyric acid (GABA) is the most widely distributed known inhibitory neurotransmitter in the vertebrate brain. GABA also serves regulatory and trophic roles in several other organs, including the pancreas. The brain contains two forms of the GABA synthetic enzyme glutamate decarboxylase (GAD), which differ in molecular size, amino acid sequence, antigenicity, cellular and subcellular location, and interaction with the GAD cofactor pyridoxal phosphate. These forms, GAD65 and GAD67, derive from two genes. The distinctive properties of the two GADs provide a substrate for understanding not only the multiple roles of GABA in the nervous system, but also the autoimmune response to GAD in insulin-dependent diabetes mellitus.

The structural and functional heterogeneity of glutamic acid decarboxylase: a review

Studies of the GABA-synthetic enzyme glutamate decarboxylase (glutamic acid decarboxylase; GAD; E.C.4.1.1.15) began in 1951 with the work of Roberts and his colleagues. Since then, many investigators have demonstrated the structural and functional heterogeneity of brain GAD. At least part of this heterogeneity derives from the existence of two GAD genes.

Cofactor interactions and the regulation of glutamate decarboxylase activity

More than 50% of glutamate decarboxylase (GAD) in brain is present as apoenzyme. Recent work has opened the possibility that apoGAD can be studied in brain by labeling with radioactive cofactor. Such studies would be aided by a compound that inhibits specific binding. One possibility is 4-deoxy-pyridoxine 5'-phosphate, a close structural analog of the cofactor pyridoxal 5'-phosphate. The effects of deoxypyridoxine-P on the cyclic series of reactions that interconverts apo- and holoGAD was investigated and found to be consistent with simple competitive inhibition of the activation of apoGAD by pyridoxal-P. As expected from the cycle GAD was inactivated when incubated with glutamate and deoxypyridoxine-P even though cofactor was present, but no inactivation was observed with deoxypyridoxine-P in the absence of glutamate. Deoxypyridoxine-P also stabilized apoGAD against heat denaturation. These effects were quantitatively accounted for by a kinetic model of the apo-holoGAD cycle. Deoxypyridoxine-P inhibited the labeling by [32P]pyridoxal-P of GAD isolated from rat brain. Hippocampal extracts were labeled with [32P]pyridoxal-P and analyzed by SDS-polyacrylamide gel electrophoresis. Remarkably few bands were strongly labeled. The major labeled band (at 63 kDa) corresponded to one of the forms of GAD. Other strongly-labeled bands were observed at 65 kDa (corresponding to the higher molecular weight form of GAD) and at 69--72 kDa. Labeling of the 63- and 65-kDa bands was inhibited by deoxypyridoxine-P, but the 69-72 kDa bands were unaffected, suggesting that the latter were non-specifically labeled. The results suggest that the 63-kDa form of GAD makes up the majority of apoGAD in hippocampus.

Glucose and synaptosomal glutamate metabolism: studies with [15N]glutamate

The metabolism of [15N]glutamate was studied with gas chromatography-mass spectrometry in rat brain synaptosomes incubated with and without glucose. [15N]Glutamate was taken up rapidly by the preparation, reaching a steady-state level in less than 5 min. 15N was incorporated predominantly into aspartate and, to a much lesser extent, into gamma-aminobutyrate. The amount of [15N]ammonia formed was very small, and the enrichment of 15N in alanine and glutamine was below the level of detection. Omission of glucose substantially increased the rate and amount of [15N]aspartate generated. It is proposed that in synaptosomes (a) the predominant route of glutamate nitrogen disposal is through the aspartate aminotransferase reaction; (b) the aspartate aminotransferase pathway generates 2-oxoglutarate, which then serves as the metabolic fuel needed to produce ATP; (c) utilization of glutamate via transamination to aspartate is greatly accelerated when flux through the tricarboxylic acid cycle is diminished by the omission of glucose; (d) the metabolism of glutamate via glutamate dehydrogenase in intact synaptosomes is slow, most likely reflecting restriction of enzyme activity by some unknown factor(s), which suggests that the glutamate dehydrogenase reaction may not be near equilibrium in neurons; and (e) the activities of alanine aminotransferase and glutamine synthetase in synaptosomes are very low.

Glutamate decarboxylase inhibition and vitamin B6 metabolism in brain of cirrhotic rats chronically treated with carbon tetrachloride

In a previous work we found that the activity of glutamate decarboxylase (GAD), the enzyme responsible for the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), is decreased in comatose cirrhotic rats after chronic treatment with CCl4. In the present report we studied the participation of pyridoxal phosphate in the inhibition of GAD, as well as the concentration of this coenzyme and the activity of its synthesizing enzyme, pyridoxal kinase, in the brain of the cirrhotic rats. Furthermore, cirrhotic animals were treated with three inhibitors of GAD, and the effects of such treatment were compared to those of ammonium. Liver failure resulted in a 25% inhibition of GAD activity when measured in the absence of added pyridoxal phosphate. Treatment with the GAD inhibitors thiosemicarbazide or 3-mercaptopropionic acid enhanced this inhibition and produced convulsions at a dose that had no behavioral effects in control rats. Treatment with ammonia resulted in a comatose state and in a 25-40% inhibition of GAD. Both pyridoxal kinase activity and pyridoxal phosphate levels were found to be decreased by 15-20% in the brain of the cirrhotic rats. We concluded that chronic liver failure results in a decreased pyridoxal phosphate and GABA synthesis in brain, with a consequent diminished efficiency of GABAergic neurotransmission; these effects are probably related to the manifestations of neuronal hyperexcitability that are frequently seen in human hepatic encephalopathy.

Kinetically different, multiple forms of glutamate decarboxylase in rat brain

Four molecular forms of rat-brain glutamate decarboxylase were resolved by hydrophobic interaction chromatography on phenyl-Sepharose and affinity chromatography on ATP-agarose. SDS-polyacrylamide gel electrophoresis of purified enzyme and immunoblots of SDS gels indicated a subunit molecular weight of approximately 60,000 for each form of the enzyme, and cross-linking with dimethyl suberimidate prior to electrophoresis indicated that each form has dimeric subunit structure. Immunoblots of non-denaturing gels showed differing electrophoretic mobilities among the forms. The kinetic properties of the 4 enzyme forms were found to be significantly different. The Km for glutamate ranged from 0.17 +/- 0.05 to 1.18 +/- 0.08 mM, and there was a greater than two-fold range in their rates of inactivation by glutamate and GABA in the absence of pyridoxal 5'-phosphate. In subcellular fractionation experiments the forms with greater electrophoretic mobility were recovered in the synaptosomal fraction, and the form with the lowest electrophoretic mobility was the most abundant in the postmicrosomal supernatant. Calcium-dependent binding of glutamate decarboxylase in crude enzyme preparations to phospholipid vesicles was observed, but none of the purified enzyme forms showed an appreciable degree of binding to the vesicles.

Regulatory properties of brain glutamate decarboxylase

1. Glutamate decarboxylase is a focal point for controlling gamma-aminobutyric acid (GABA) synthesis in brain. Several factors that appear to be important in the regulation of GABA synthesis have been identified by relating studies of purified glutamate decarboxylase to conditions in vivo. 2. The interaction of glutamate decarboxylase with its cofactor, pyridoxal 5'-phosphate, is a regulated process and appears to be one of the major means of controlling enzyme activity. The enzyme is present in brain predominantly as apoenzyme (inactive enzyme without bound cofactor). Studies with purified enzyme indicate that the relative amounts of apo- and holoenzyme are determined by the balance in a cycle that continuously interconverts the two. 3. The cycle that interconverts apo- and holoenzyme is part of the normal catalytic mechanism of the enzyme and is strongly affected by several probable regulatory compounds including pyridoxal 5'-phosphate, ATP, inorganic phosphate, and the amino acids glutamate, GABA, and aspartate. ATP and the amino acids promote apoenzyme formation and pyridoxal 5'-phosphate and inorganic phosphate promote holoenzyme formation. 4. Numerous studies indicate that brain contains multiple molecular forms of glutamate decarboxylase. Multiple forms that differ markedly in kinetic properties including their interactions with the cofactor have been isolated and characterized. The kinetic differences among the forms suggest that they play a significant role in the regulation of GABA synthesis.

Dietary pyridoxine and the susceptibility to limbic motor seizures in rats

Dietary pyridoxine (PN) deficiency in adult female rats produced a 32% decrease in hippocampal gamma-aminobutyric acid content, measured by a radioreceptor assay. No spontaneous seizures were observed in pyridoxine-deficient animals, but the seizure latency after a systemic kainic acid challenge decreased by 35%. The results suggest that latency is a useful measure of limbic seizure susceptibility; this susceptibility can be manipulated by diet; and in adults pyridoxine-dependent mechanisms normally participate in preventing, rather than initiating, limbic seizures.

Characterization of three kinetically distinct forms of glutamate decarboxylase from pig brain

Pig brain contains three forms of glutamate decarboxylase with pI values of 5.3, 5.5 and 5.8, referred to as the alpha-, beta- and gamma-forms respectively. These forms were purified and kinetically characterized. The major synaptic form of glutamate decarboxylase (the beta-form) migrated as a single band on electrophoresis in sodium dodecyl sulphate/polyacrylamide gels with an apparent Mr of 60 000. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis followed by immunoblotting with an affinity-purified antibody to the enzyme indicated a subunit Mr of 60 000 for the alpha- and gamma-forms as well. An extensive kinetic analysis, aided by an integrated equation that describes the inactivation and re-activation cycle of the enzyme, revealed that the three forms of the enzyme differ markedly in kinetic properties. The Km values for L-glutamate were 0.17, 0.45 and 1.24 mM respectively for the alpha-, beta- and gamma-forms. The Ki for 4-aminobutyrate, the first-order rate constants for inactivation by L-glutamate and 4-aminobutyrate, the rate constant for re-activation of the apoenzyme by pyridoxal 5'-phosphate and the dissociation constant for pyridoxal 5'-phosphate also differed in a similar way among the three forms; the values were in the order alpha-form less than beta-form less than gamma-form.

Two forms of rat brain glutamic acid decarboxylase differ in their dependence on free pyridoxal phosphate

There are two forms of glutamate decarboxylase (GAD) found in the rat brain. One form (form A) does not require exogenous pyridoxal-5'-phosphate (PLP) for activity whereas another form (form B) requires exogenous PLP for activity. These two forms differ greatly in temperature sensitivity, inactivation, and reactivation by the removal and readdition of PLP, electrophoretic mobility, and regional distribution. For instance, forms A and B are inactivated to an extent of 91% and 10%, respectively, by the treatment at 45 degrees C for 30 min; form A is greatly inactivated (77%) by the removal of PLP by aminooxyacetic acid and the readdition of PLP, whereas form B is only slightly inactivated (7%). Forms A and B can be clearly separated by 5% polyacrylamide gel electrophoresis in which form A migrates faster than form B. In all 10 brain regions studied, form A is present in smaller amounts than form B. This difference is greatest in the superior colliculus (the ratio of B to A is about 5), while in the locus coeruleus and cerebellum, forms A and B are present in nearly equal proportion. Forms A and B are similar with respect to relative abundance in hypotonic, isotonic, and hypertonic preparations, inhibition of catalytic activity by a carbonyl-trapping agent, immunochemical properties, and chromatographic patterns in a variety of systems. The significance of forms A and B and PLP in the regulation of gamma-amino-butyric acid (GABA) level is also discussed.

Pyridoxal phosphate-unrelated inhibition of hippocampal glutamic acid decarboxylase by convulsant pyridoxal sulphate

Previous studies from this laboratory have shown that pyridoxal-5'-sulphate, the synthetic analogue of pyridoxal phosphate, causes epileptic seizures including tonic-clonic convulsions. These seizure activities are prevented or reversed by GABA or muscimol. In an attempt to delineate the biochemical basis of these seizure processes further, we have studied and shown that pyridoxal sulphate is a competitive inhibitor of glutamic acid decarboxylase. In addition, the chronic administration of pyridoxal sulphate was shown to reduce the concentration of pyridoxal phosphate in the cerebellum, the cerebrum, and basal ganglion, but not in the hippocampus. The activity of hippocampal glutamic acid decarboxylase was reduced after 1, 3, and 5 days of chronic application of pyridoxal sulphate. The inhibition was demonstrated, whether glutamic acid decarboxylase was assayed in the presence or absence of its coenzyme pyridoxal phosphate. Unlike findings in the hippocampus, the activity of glutamic acid decarboxylase in other brain regions was unaffected following chronic application of pyridoxal sulphate. The selective toxic effects of pyridoxal sulfate to the hippocampus, a brain area well known for its high susceptibility to seizure discharges, deserve additional indepth investigation.

Effect of L-dopa on glutamate decarboxylase activity in the hypothalamic and amygdaloid nuclei

Repeated administration of L-dopa methylester produced a significant increase in glutamate decarboxylase (GAD) activity without pyridoxal-5'-phosphate in the lateral hypothalamic area and medial amygdaloid nucleus. The effect of L-dopa on GAD activity was opposite to that of haloperidol in the lateral hypothalamic area.

Multiple forms of glutamate decarboxylase in porcine brain

Three forms of glutamate decarboxylase from hog brain (termed alpha-, beta-, and gamma-GAD) were separated by hydrophobic interaction chromatography on phenyl-Sepharose, by isoelectric focusing, and by polyacrylamide gel electrophoresis. When rechromatographed on phenyl-Sepharose, each form migrated as a single entity, indicating that the forms are not readily interconvertible. The three forms are not different-sized aggregates of one form, since all three have the same approximate molecular weight (100,000) as determined by Sephadex G-200 chromatography. The pIs of the three forms separated by phenyl-Sepharose were determined by isoelectric focusing. The values obtained (5.3, 5.5, and 5.8 for alpha-, beta-, and gamma-GAD, respectively) were comparable to the pIs of the three peaks of activity observed upon focusing of enzyme that had been subjected to phenyl-Sepharose chromatography. These results indicate that phenyl-Sepharose chromatography and isoelectric focusing separate the same three components. When synaptosomal extracts were analyzed by phenyl-Sepharose chromatography without intervening purification steps, all three forms were present, but the proportion of beta-GAD was somewhat higher and that of gamma-GAD somewhat lower than in the usual preparations.

The selective inhibition of hippocampal glutamic acid decarboxylase in zinc-induced epileptic seizures

The intracerebroventricular administration of Zn2+ (0.3 mumol/10 microliters) causes epileptic seizures characterized by running fits, jumping, vocalization, fasiculation of facial muscles, myoclonic movements of the limbs and tonic-clonic convulsions. These episodes are blocked or reversed by gamma-aminobutyric acid (0.4 mumol/10 microliters). When assayed under conditions where pyridoxal phosphate was not added, the activity of glutamic acid decarboxylase decreased significantly in hippocampus from 18.9 to 15.3 and 9.7 mumols 14CO2 formed/gram proteins/20 min, 15 and 30 min following administration of Zn2+. The inhibition of glutamic acid decarboxylase by Zn2+ was selective occurring only in hippocampus and not in the hypothalamus, amygdala, caudate or thalamus. The inhibition of glutamic acid decarboxylase was not due to a reduction in the concentration of endogenous pyridoxal phosphate which remained unaltered in hippocampus following Zn2+ administration.

Effects of GABA-analogues on the high-affinity uptake of GABA in astrocytes in primary cultures

Employing primary cultures of astrocytes which seem to constitute a valid model of their in vivo counterparts, it has been demonstrated that this cell type is likely to be of importance in terminating the transmitter activity of GABA. It has been shown that the transport carrier in astrocytes is stereospecific for the C-4 hydrogens of the GABA molecule and that its structural requirements are distinct from those exhibited by the neuronal GABA carrier. beta-Proline was a selective inhibitor of GABA transport in astrocytes, whereas R-trans-4-methyl-4-aminocrotonic acid and S-nipecotic acid seemed to be selective inhibitors of neuronal GABA transport, as studied using very thin slices ("prisms") of brain cortex. These findings may be important for studies on the relative significance of the two transport systems in GABA-mediated neurotransmission, and thus for future pharmacological manipulations of the GABA system. This may eventually be beneficial for the treatment of neurological disorders such as epilepsy, Huntington's chorea and Parkinson's disease in which the GABA system seems to be disturbed (34,60,62).