Effect of NMDA antagonists on the activity of glutaminase and aspartate aminotransferase in the developing rat cerebellum

Febuxostat ameliorates secondary progressive experimental autoimmune encephalomyelitis by restoring mitochondrial energy production in a GOT2-dependent manner

Oxidative stress and mitochondrial dysfunction are important determinants of neurodegeneration in secondary progressive multiple sclerosis (SPMS). We previously showed that febuxostat, a xanthine oxidase inhibitor, ameliorated both relapsing-remitting and secondary progressive experimental autoimmune encephalomyelitis (EAE) by preventing neurodegeneration in mice. In this study, we investigated how febuxostat protects neuron in secondary progressive EAE. A DNA microarray analysis revealed that febuxostat treatment increased the CNS expression of several mitochondria-related genes in EAE mice, most notably including GOT2, which encodes glutamate oxaloacetate transaminase 2 (GOT2). GOT2 is a mitochondrial enzyme that oxidizes glutamate to produce α-ketoglutarate for the Krebs cycle, eventually leading to the production of adenosine triphosphate (ATP). Whereas GOT2 expression was decreased in the spinal cord during the chronic progressive phase of EAE, febuxostat-treated EAE mice showed increased GOT2 expression. Moreover, febuxostat treatment of Neuro2a cells in vitro ameliorated ATP exhaustion induced by rotenone application. The ability of febuxostat to preserve ATP production in the presence of rotenone was significantly reduced by GOT2 siRNA. GOT2-mediated ATP synthesis may be a pivotal mechanism underlying the protective effect of febuxostat against neurodegeneration in EAE. Accordingly, febuxostat may also have clinical utility as a disease-modifying drug in SPMS.

Enzyme Complexes Important for the Glutamate-Glutamine Cycle

Transient multienzyme and/or multiprotein complexes (metabolons) direct substrates toward specific pathways and can significantly influence the metabolism of glutamate and glutamine in the brain. Glutamate is the primary excitatory neurotransmitter in brain. This neurotransmitter has essential roles in normal brain function including learning and memory. Metabolism of glutamate involves the coordinated activity of astrocytes and neurons and high affinity transporter proteins that are selectively distributed on these cells. This chapter describes known and possible metabolons that affect the metabolism of glutamate and related compounds in the brain, as well as some factors that can modulate the association and dissociation of such complexes, including protein modifications by acylation reactions (e.g., acetylation, palmitoylation, succinylation, SUMOylation, etc.) of specific residues. Development of strategies to modulate transient multienzyme and/or enzyme-protein interactions may represent a novel and promising therapeutic approach for treatment of diseases involving dysregulation of glutamate metabolism.

Biochemical and structural characterization of mouse mitochondrial aspartate aminotransferase, a newly identified kynurenine aminotransferase-IV

Mammalian mAspAT (mitochondrial aspartate aminotransferase) is recently reported to have KAT (kynurenine aminotransferase) activity and plays a role in the biosynthesis of KYNA (kynurenic acid) in rat, mouse and human brains. This study concerns the biochemical and structural characterization of mouse mAspAT. In this study, mouse mAspAT cDNA was amplified from mouse brain first stand cDNA and its recombinant protein was expressed in an Escherichia coli expression system. Sixteen oxo acids were tested for the co-substrate specificity of mouse mAspAT and 14 of them were shown to be capable of serving as co-substrates for the enzyme. Structural analysis of mAspAT by macromolecular crystallography revealed that the cofactor-binding residues of mAspAT are similar to those of other KATs. The substrate-binding residues of mAspAT are slightly different from those of other KATs. Our results provide a biochemical and structural basis towards understanding the overall physiological role of mAspAT in vivo and insight into controlling the levels of endogenous KYNA through modulation of the enzyme in the mouse brain.

Glutamate dehydrogenase in brain mitochondria: do lipid modifications and transient metabolon formation influence enzyme activity?

Metabolism of glutamate, the primary excitatory neurotransmitter in brain, is complex and of paramount importance to overall brain function. Thus, understanding the regulation of enzymes involved in formation and disposal of glutamate and related metabolites is crucial to understanding glutamate metabolism. Glutamate dehydrogenase (GDH) is a pivotal enzyme that links amino acid metabolism and TCA cycle activity in brain and other tissues. The allosteric regulation of GDH has been extensively studied and characterized. Less is known about the influence of lipid modifications on GDH activity, and the participation of GDH in transient heteroenzyme complexes (metabolons) that can greatly influence metabolism by altering kinetic parameters and lead to channeling of metabolites. This review summarizes evidence for palmitoylation and acylation of GDH, information on protein binding, and information regarding the participation of GDH in transient heteroenzyme complexes. Recent studies suggest that a number of other proteins can bind to GDH altering activity and overall metabolism. It is likely that these modifications and interactions contribute additional levels of regulation of GDH activity and glutamate metabolism.

Structure, expression, and function of kynurenine aminotransferases in human and rodent brains

Kynurenine aminotransferases (KATs) catalyze the synthesis of kynurenic acid (KYNA), an endogenous antagonist of N-methyl-D: -aspartate and alpha 7-nicotinic acetylcholine receptors. Abnormal KYNA levels in human brains are implicated in the pathophysiology of schizophrenia, Alzheimer's disease, and other neurological disorders. Four KATs have been reported in mammalian brains, KAT I/glutamine transaminase K/cysteine conjugate beta-lyase 1, KAT II/aminoadipate aminotransferase, KAT III/cysteine conjugate beta-lyase 2, and KAT IV/glutamic-oxaloacetic transaminase 2/mitochondrial aspartate aminotransferase. KAT II has a striking tertiary structure in N-terminal part and forms a new subgroup in fold type I aminotransferases, which has been classified as subgroup Iepsilon. Knowledge regarding KATs is vast and complex; therefore, this review is focused on recent important progress of their gene characterization, physiological and biochemical function, and structural properties. The biochemical differences of four KATs, specific enzyme activity assays, and the structural insights into the mechanism of catalysis and inhibition of these enzymes are discussed.

Memantine decreases hippocampal glutamate levels: a magnetic resonance spectroscopy study

Glutamate (Glu) is associated with excitotoxic cell damage. Memantine modulates the glutamate induced excitotoxicity in Alzheimer's disease (AD). No information is available as to the influence of memantine on in vivo brain glutamate levels. Hippocampal Glu levels were measured in cognitively impaired and normal individuals (n=10) before and after 6 months of memantine treatment, using three dimensional high spatial resolution (0.5 cm(3) voxels) proton magnetic resonance spectroscopy at 3 T. These measurements were also repeated in a non-treated cognitively normal group (n=6). Treatment with memantine decreased Glu/Cr (creatine) ratio in the left hippocampal region. Memantine reduced hippocampal glutamate levels, which may be consistent with its anti-excitotoxic property.

Effect of N-methyl-d-aspartate receptor blockade on caspase activation and neuronal death in the developing rat cerebellum

In vitro studies have demonstrated that N-methyl-D-aspartate (NMDA) receptor activation rescue cerebellar granule neurons (CGN) from apoptotic death. It has been suggested that this effect mimics the transient glutamate receptors activation by mossy fibers during cerebellar development. We reported previously that CGN from postnatal days 2-4 (P2-P4) rats increased cell survival in response to NMDA treatment. In this study, we evaluated the effect of dizocilpine (MK-801) administrated for three consecutive days on the apoptotic death of CGN during development. MK-801 treatment decreased the large number of CGN condensed nuclei found at P8, but this drug increased the proportion of condensed nuclei at P16. We also found a high activity of caspases during the first postnatal week that decreased during development. MK-801 treatment did not modify the activity of caspase-8 at any age, but decreased caspase-9 activity at P8 and increased the activity of caspase-1 (76%) at P8, caspase-3 (160%) at P16 and caspase-9 (50%) at P12. These results suggest that NMDA receptor stimulation regulates the activity of caspases in a differential way and plays an important role in the in vivo CGN death during postnatal development.

Aspartate aminotransferase in synaptic and nonsynaptic mitochondria: differential effect of compounds that influence transient hetero-enzyme complex (metabolon) formation

The enzyme aspartate aminotransferase (AAT) has a number of key roles in astrocytes and neurons in brain. An understanding of the regulation of AAT is important since AAT is involved in many aspects of glutamate metabolism including the synthesis of neurotransmitter glutamate. Mitochondrial AAT binds to a protein and lipids on the inner mitochondrial membrane and also forms a number of transient hetero-enzyme complexes with other enzymes. These complexes serve to facilitate metabolism by essentially channeling substrates and cofactors to other enzymes within the complex. The association and dissociation of transiently formed hetero-enzyme complexes may modulate enzyme activity in "real time" since these complexes are dynamically influenced by changes in the concentration of a number of key metabolites. The influence of several effectors that modulate AAT activity, either directly, or by altering the binding of AAT to mitochondrial lipids, or the association/dissociation into transient hetero-enzyme complexes was determined. The addition of palmitate, malate, citrate, glutamate, bovine serum albumin and Mg(2+) modulated AAT activity differently in synaptic and nonsynaptic mitochondria from brain. These findings suggest that AAT activity and also glutamate metabolism, may be regulated in part, by metabolites that influence binding of the enzyme to lipids or proteins in the inner mitochondrial membrane and/or the association/dissociation of transient hetero-enzyme complexes. This may have a role in the compartmentation of glutamate metabolism in brain.

Regulation of glutamate-synthesizing enzymes by NMDA and potassium in cerebellar granule cells

The presence of 25 mm potassium (KCl) or N-methyl-d-aspartate (NMDA) in cultured cerebellar granule neurons (CGN) induces a trophic effect, including a specific regulation of the enzymes involved in the glutamate neurotransmitter synthesis. In this study we explored the effect of these conditions on the cytosolic and mitochondrial isoenzymes of aspartate aminotransferase (AAT), and phosphate-activated glutaminase (PAG) in CGN. We found that NMDA and KCl increased the AAT total activity by 40% and 70%, respectively. This effect was mediated by an augmentation in the protein levels (68% by NMDA, 58% by KCl). NMDA raised the Vmax and KCl raised both the maximol velocity (Vmax) and Michaelis constant (Km) of AAT. NMDA increased cytosolic AAT activity by 30% and mitochondrial activity by 70%; KCl increased cytosolic and mitochondrial AAT activity by 60% and 100%, respectively. This activation was also related to an increase in the protein levels. The effect of both conditions on the activity and protein levels were more pronounced in mitochondrial than cytosolic AAT and the increment elicited by KCl was higher in both isoforms than that produced by NMDA. The PAG and AAT mRNA levels were also regulated by incubation with NMDA and KCl similarly to the observed changes in the protein levels. These results suggest that NMDA receptor stimulation during CGN development differentially regulates the two AAT isoenzymes involved in the maturation of CGN and that the regulation of both AAT and PAG occurs also at the mRNA expression level, suggesting the involvement of a mechanism of gene expression regulation.

Phosphate-activated glutaminase and mitochondrial glutamine transport in the brain

A review of the properties of purified and tissue bound phosphate activated glutaminase (PAG) in brain and kidney (pig and rat) is presented, based on kinetic, electron microscopic and immunocytochemical studies. PAG is a mitochondrial enzyme and two pools can be separated, a soluble and membrane associated one. Intact mitochondria appear to express PAG accessible only to the outer phase of the inner mitochondrial membrane. This PAG has properties similar to that of the membrane fraction and polymeric form of purified enzyme. PAG in the soluble fraction has properties similar to that of the monomeric form of purified enzyme and is assumed to be dormant due to the high matrix concentration of the inhibitor glutamate. A hypothetical model for the localization of PAG in the mitochondria is presented. The activity of PAG in vivo is assumed to be regulated by cytosolic glutamate and other compounds, that affect the activation by phosphate. Glutamine is transported into brain and kidney mitochondria by a protein catalyzed energy requiring process, which may be mediated by more than one protein. There is no correlation between glutamine hydrolysis and transport.