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Glutaric acidemia type I (GA I) is a neurometabolic disorder of lysine (Lys) catabolism caused by glutaryl-CoA dehydrogenase (GCDH) deficiency. Patients are susceptible to develop acute striatum degeneration during catabolic stress situations whose underlying mechanisms are not fully established. Thus, in the present work we investigated the effects of a single intrastriatal Lys administration (1.5-4 μmol) to 30-day-old wild type (WT) and GCDH deficient (Gcdh-/-) mice on brain morphology, neuronal injury, astrocyte reactivity and myelin structure, as well as signaling pathways of redox homeostasis. We observed a marked vacuolation/edema in striatum and at higher doses also in cerebral cortex of Gcdh-/-, but not of WT mice. Lys also provoked a reduction of NeuN and synaptophysin, as well as an increase of astrocytic GFAP, in the striatum of Gcdh-/- mice, indicating neuronal loss and astrocyte reactivity. Furthermore, we verified an increase of Nrf2 and NF-κB expression in the nuclear fraction, and a decrease of heme oxygenase-1 (HO-1) content in the striatum of Lys-injected Gcdh-/- mice, implying disruption of redox homeostasis. Finally, it was found that Lys provoked alterations of myelin structure reflected by decreased myelin basic protein (MBP) in the cerebral cortex of Gcdh-/- mice. Taken together, the present data demonstrate neuronal loss, gliosis, altered redox homeostasis and demyelination caused by acute Lys overload in brain of Gcdh-/- mice, supporting the hypothesis that increased brain concentrations of glutaric and 3-hydroxyglutaric acids formed from Lys may be responsible for the acute brain degeneration observed in GA I patients during episodes of metabolic decompensation., (Copyright © 2019. Published by Elsevier Ltd.)

In this study, we developed an in vivo method to determine drug effects on oxidation-induced apoptosis in the zebrafish brain caused by treatment with L-hydroxyglutaric acid (LGA). We confirmed that LGA-induced apoptosis was caused by oxidation by examining the presence of an oxidative product, nitrotyrosine. Next, we examined the effects of 14 characterized neuroprotectants on LGA-treated zebrafish, including: D-methionine (D-Met), Indole-3-carbinol, deferoxamine (DFO), dihydroxybenzoate (DHB), deprenyl, L-NAME (N(G)-nitro-L-arginine methyl ester), n-acetyl L-cysteine (L-NAC), 2-oxothiazolidine-4-carboxylate (OTC), lipoic acid, minocycline, isatin, cortisone, ascorbic acid and alpha-tocopherol. Eleven of 14 neuroprotectants and 7 of 7 synthetic anti-oxidants exhibit significant protection in zebrafish. Buthionine sulfoximine (BSO), used as a negative control, exhibited no significant protective effects. In addition, three blood-brain barrier (BBB) impermeable compounds exhibited no significant effects. Our results in zebrafish were similar to results reported in mammals supporting the utility of this in vivo method for identifying potential neuroprotective anti-oxidants.

Glutaric acidemia type I (GA I) is an inherited neurometabolic disorder caused by glutaryl-CoA dehydrogenase deficiency, which leads to accumulation in body fluids and in brain of predominantly glutaric acid (GA), and to a lesser extent of 3-hydroxyglutaric and glutaconic acids. Neurological presentation is common in patients with GA I. Although the mechanisms underlying brain damage in this disorder are not yet well established, there is growing evidence that excitotoxicity may play a central role in the neuropathogenesis of this disease. In the present study, preparations of synaptosomes, synaptic plasma membranes and synaptic vesicles, as well as cultured astrocytes from rat forebrain were exposed to various concentrations of GA for the determination of the basal and potassium-induced release of [(3)H]glutamate by synaptosomes, Na(+)-independent glutamate binding to synaptic membranes and vesicular glutamate uptake and Na(+)-dependent glutamate uptake into astrocytes, respectively. GA (1-100 nM) significantly stimulated [(3)H]glutamate binding to brain plasma membranes (40-70%) in the absence of extracellular Na(+) concentrations, reflecting glutamate binding to receptors. Furthermore, this stimulatory effect was totally abolished by the metabotropic glutamate ligands DHPG, DCG-IV and l-AP4, attenuated by the ionotropic non-NMDA glutamate receptor agonist AMPA and had no interference of the NMDA receptor antagonist MK-801. Moreover, [(3)H]glutamate uptake into synaptic vesicles was inhibited by approximately 50% by 10 and 100 nM GA and Na(+)-dependent [(3)H]glutamate uptake by astrocytes was significantly increased (up to 50%) in a dose-dependent manner (maximal stimulation at 100 microM GA). In contrast, synaptosomal glutamate release was not affected by the acid at concentrations as high as 1 mM. These results indicate that the inhibition of glutamate uptake into synaptic vesicles by low concentrations GA may result in elevated concentrations of the excitatory neurotransmitter in the cytosol and the stimulatory effect of this organic acid on glutamate binding may potentially cause excitotoxicity to neural cells. Finally, taken together these results and previous findings showing that GA markedly decreases synaptosomal glutamate uptake, it is possible that the stimulatory effect of GA on astrocyte glutamate uptake might indicate that astrocytes may protect neurons from excitotoxic damage caused by GA by increasing glutamate uptake and therefore reducing the concentration of this excitatory neurotransmitter in the synaptic cleft.

Neurological symptoms are common in patients with glutaric acidemia type I (GA-I). Although the pathophysiology of this disorder is not yet fully established, 3-hydroxyglutaric acid (3-HGA), which accumulates in affected patients, has recently been demonstrated to be excitotoxic to embryonic chick and neonatal rat neurons probably via NMDA glutamate receptors. In the present study, we investigated the in vitro effects of 3-HGA on the [(3)H]glutamate and [(3)H]MK-801 (dizocilpine) binding to rat synaptic plasma membranes from cerebral cortex of young rats in order to elucidate the interactions of 3-HGA with glutamate receptors and its possible contribution to the in vitro excitotoxic properties of 3-HGA. 3-HGA (10-100 microM) significantly decreased Na(+)-dependent (up to 62%) and Na(+)-independent (up to 30%) [(3)H]glutamate binding to synaptic membranes, reflecting a possible competition between glutamate and 3-HGA for the glutamate transporter and receptor sites, respectively. Since a decrease in Na(+)-independent glutamate binding might represent an interaction of 3-HGA with glutamate receptors, we next investigated whether 3-HGA interacts with NMDA receptors by adding NMDA alone or combined with 3-HGA and measuring Na(+)-independent [(3)H]glutamate binding to synaptic membranes (binding to receptors). We verified that 3-HGA and NMDA, at 10 and 100 microM concentrations, decreased glutamate binding by up to 20 and 45%, respectively, and that the simultaneous addition of both substances did not provoke an additive effect, implying that they bind to NMDA receptors at the same site. Furthermore, the binding of the NMDA-channel blocker [(3)H ]MK-801 was significantly increased (approximately 32-40%) by 10 and 100 microM 3-HGA, implying that 3-HGA was able to open the NMDA channel allowing MK-801 binding, which is a characteristic of NMDA agonists. On the other hand, glutamate had a much higher stimulatory effect on this binding (180% increase), reflecting its strong NMDA agonist property. Furthermore, the simultaneous addition of 3-HGA and glutamate provoked an additive stimulatory effect on [(3)H]MK-801 binding to the NMDA receptor. These data indicate that, relatively to glutamate, 3-HGA is a weak agonist of NMDA receptors. Finally, we demonstrated that 3-HGA provoked a significant increase of extracellular calcium uptake by cerebral cortex slices, strengthening therefore, the view that 3-HGA activates NMDA receptors. The present study therefore, demonstrates at the molecular level that 3-HGA modulates glutamatergic neurotransmission and may explain previous findings relating the neurotoxic actions of this organic acid with excitotoxicity.

A predominantly neurological presentation is common in patients with glutaric acidemia type I (GA-I). 3-hydroxyglutaric acid (3-OHGA), which accumulates in affected patients, has recently been demonstrated to play a central role in the neuropathogenesis of this disease. In the present study, we investigated the in vitro effects of 3-OHGA at concentrations ranging from 10 to 1000 microM on various parameters of the glutamatergic system, such as the basal and potassium-induced release of [3H]glutamate by synaptosomes, as well as on Na+-dependent [3H]glutamate uptake by synaptosomes and astrocytes and Na+-independent [3H]glutamate uptake by synaptic vesicles from cerebral cortex of 30-day-old Wistar rats. First, we observed that exposure of cultured astrocytes to 3-OHGA for 20 h did not reduce their viability. Furthermore, 3-OHGA significantly increased Na+-dependent [3H]glutamate uptake by astrocytes by up to 80% in a dose-dependent manner at doses as low as 30 microM. This effect was not dependent on the presence of the metabolite during the uptake assay, since it occurred even when 3-OHGA was withdrawn from the medium after cultured cells had been exposed to the acid for approximately 1 h. All other parameters investigated were not influenced by this organic acid, indicating a selective action of 3-OHGA on astrocyte transporters. Although the exact mechanisms involved in 3-OHGA-stimulatory effect on astrocyte glutamate uptake are unknown, the present findings contribute to the understanding of the pathophysiology of GA-I, suggesting that astrocytes may protect neurons against excitotoxic damage caused by 3-OHGA by increasing glutamate uptake and therefore reducing the concentration of this excitatory neurotransmitter in the synaptic cleft.

D-2-Hydroxyglutaric acid (DGA) is the biochemical hallmark of patients affected by the neurometabolic disorder known as D-2-hydroxyglutaric aciduria (DHGA). Although this disease is predominantly characterized by severe neurological findings, the underlying mechanisms of brain injury are virtually unknown. In the present study, we investigated the effect of DGA on total, cytosolic, and mitochondrial creatine kinase (CK) activities from cerebral cortex of 30-day-old Wistar rats. Total CK activity (tCK) was measured in whole cell homogenates, whereas cytosolic and mitochondrial activities were measured in the cytosolic and mitochondrial preparations from cerebral cortex. We verified that CK activities were significantly inhibited by DGA (11-34% inhibition) at concentrations as low as 0.25 mM, being the mitochondrial fraction the most affected activity. Kinetic studies revealed that the inhibitory effect of DGA was non-competitive in relation to phosphocreatine. We also observed that this inhibition was fully prevented by pre-incubation of the homogenates with reduced glutathione, suggesting that the inhibitory effect of DGA on tCK activity is possibly mediated by oxidation of essential thiol groups of the enzyme. Considering the importance of CK activity for brain metabolism homeostasis, our results suggest that inhibition of this enzyme by increased levels of DGA may be related to the neurodegeneration of patients affected by DHGA.

The activity of a gamma-substituted glutamate analogue, (2S, 4R, 6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434) and (2S,4R)-4-methylglutamic acid at ionotropic glutamate receptors has been examined. Ligand binding studies were performed using [3H] AMPA binding to membranes expressing either homomeric recombinant GluR1, GluR2, GluR4 receptors, and [3H] kainate binding to GluR5 and GluR6 kainate receptors. LY339434 and (2S,4R)-4-methylglutamic acid showed selectivity in ligand binding studies for kainate receptors over AMPA receptors. Within the kainate class of glutamate receptors, LY339434 showed selectivity for GluR5 over GluR6 whereas (2S,4R)-4-methylglutamic acid showed high affinity for both GluR5 and GluR6 kainate receptors. Examination of the functional activity of LY339434 and (2S,4R)-4-methylglutamic acid showed that both compounds evoked inward currents in dorsal root ganglion neurons (DRG) with estimated EC50 values of 0.8 +/- 0.2 microM and 0.17 +/- 0.04 microM, respectively. In GluR5 expressing HEK 293 cells, LY339434 evoked inward currents with an estimated EC50 value of 2.5 +/- 0.9 microM but had little effect on GluR6 expressing cells at concentrations less than 100 microM. LY339434 was a weak AMPA receptor agonist (EC50 values > 300 microM) as determined by activity in acutely isolated cerebellar Purkinje neurons. LY339434 and (2S,4R)-4-methylglutamic acid had agonist activity at NMDA receptors studied in cultured hippocampal neurons with EC50s of 2.5 microM and 11.7 microM, respectively. These results indicate that both LY339434 and (2S,4R)-4-methyl glutamic acid may be useful pharmacological tools for the examination of kainate receptors.

Glucose (10-50 mM) added to samples of citrated whole blood increased the spontaneous platelet aggregation (SPA) that was observed upon stirring the samples, in a dose-dependent but time-independent manner. EDTA prevented and also reversed the SPA that occurred in the presence of glucose. Galactose, fructose and 2-deoxyglucose, but not sorbitol and sucrose, had effects on SPA similar to those of glucose. The effect of glucose on SPA in whole blood could not be specifically inhibited by tetramethylene glutaric acid (an inhibitor of aldose reductase). SPA was not affected by pentoxifylline, but was markedly reduced by chlorpromazine. We previously found that glucose, galactose, fructose and 2-deoxyglucose, but not sorbitol and sucrose, render red blood cells more fragile, and the results obtained in the present study provide further evidence that glucose as well as other reducing sugars potentiate SPA in whole blood by liberating ADP or other aggregating material from red blood cells.