Regulation of mitochondrial and cytosolic malic enzymes from cultured rat brain astrocytes

The Roles of White Adipose Tissue and Liver NADPH in Dietary Restriction-Induced Longevity

Dietary restriction (DR) protocols frequently employ intermittent fasting. Following a period of fasting, meal consumption increases lipogenic gene expression, including that of NADPH-generating enzymes that fuel lipogenesis in white adipose tissue (WAT) through the induction of transcriptional regulators SREBP-1c and CHREBP. SREBP-1c knockout mice, unlike controls, did not show an extended lifespan on the DR diet. WAT cytoplasmic NADPH is generated by both malic enzyme 1 (ME1) and the pentose phosphate pathway (PPP), while liver cytoplasmic NADPH is primarily synthesized by folate cycle enzymes provided one-carbon units through serine catabolism. During the daily fasting period of the DR diet, fatty acids are released from WAT and are transported to peripheral tissues, where they are used for beta-oxidation and for phospholipid and lipid droplet synthesis, where monounsaturated fatty acids (MUFAs) may activate Nrf1 and inhibit ferroptosis to promote longevity. Decreased WAT NADPH from PPP gene knockout stimulated the browning of WAT and protected from a high-fat diet, while high levels of NADPH-generating enzymes in WAT and macrophages are linked to obesity. But oscillations in WAT [NADPH]/[NADP+] from feeding and fasting cycles may play an important role in maintaining metabolic plasticity to drive longevity. Studies measuring the WAT malate/pyruvate as a proxy for the cytoplasmic [NADPH]/[NADP+], as well as studies using fluorescent biosensors expressed in the WAT of animal models to monitor the changes in cytoplasmic [NADPH]/[NADP+], are needed during ad libitum and DR diets to determine the changes that are associated with longevity.

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

Revealing the contribution of astrocytes to glutamatergic neuronal transmission

Research on glutamatergic neurotransmission has focused mainly on the function of presynaptic and postsynaptic neurons, leaving astrocytes with a secondary role only to ensure successful neurotransmission. However, recent evidence indicates that astrocytes contribute actively and even regulate neuronal transmission at different levels. This review establishes a framework by comparing glutamatergic components between neurons and astrocytes to examine how astrocytes modulate or otherwise influence neuronal transmission. We have included the most recent findings about the role of astrocytes in neurotransmission, allowing us to understand the complex network of neuron-astrocyte interactions. However, despite the knowledge of synaptic modulation by astrocytes, their contribution to specific physiological and pathological conditions remains to be elucidated. A full understanding of the astrocyte's role in neuronal processing could open fruitful new frontiers in the development of therapeutic applications.

Astrocytic reactivity triggered by defective autophagy and metabolic failure causes neurotoxicity in frontotemporal dementia type 3

Frontotemporal dementia type 3 (FTD3), caused by a point mutation in the charged multivesicular body protein 2B (CHMP2B), affects mitochondrial ultrastructure and the endolysosomal pathway in neurons. To dissect the astrocyte-specific impact of mutant CHMP2B expression, we generated astrocytes from human induced pluripotent stem cells (hiPSCs) and confirmed our findings in CHMP2B mutant mice. Our data provide mechanistic insights into how defective autophagy causes perturbed mitochondrial dynamics with impaired glycolysis, increased reactive oxygen species, and elongated mitochondrial morphology, indicating increased mitochondrial fusion in FTD3 astrocytes. This shift in astrocyte homeostasis triggers a reactive astrocyte phenotype and increased release of toxic cytokines, which accumulate in nuclear factor kappa b (NF-κB) pathway activation with increased production of CHF, LCN2, and C3 causing neurodegeneration.

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FTD3 iPSC-derived astrocytes display impaired autophagy

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Impaired autophagy affects mitochondria turnover, glucose hypometabolism and TCA cycle

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FTD3 astrocytes contribute to reactive gliosis by increased C3, LCN2, IL6, and IL8

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Reactive astrocyte phenotypes are present in both in vitro and in vivo models

A rotary mechanism for allostery in bacterial hybrid malic enzymes

Bacterial hybrid malic enzymes (MaeB grouping, multidomain) catalyse the transformation of malate to pyruvate, and are a major contributor to cellular reducing power and carbon flux. Distinct from other malic enzyme subtypes, the hybrid enzymes are regulated by acetyl-CoA, a molecular indicator of the metabolic state of the cell. Here we solve the structure of a MaeB protein, which reveals hybrid enzymes use the appended phosphotransacetylase (PTA) domain to form a hexameric sensor that communicates acetyl-CoA occupancy to the malic enzyme active site, 60 Å away. We demonstrate that allostery is governed by a large-scale rearrangement that rotates the catalytic subunits 70° between the two states, identifying MaeB as a new model enzyme for the study of ligand-induced conformational change. Our work provides the mechanistic basis for metabolic control of hybrid malic enzymes, and identifies inhibition-insensitive variants that may find utility in synthetic biology.

In-depth quantitative proteomic characterization of organotypic hippocampal slice culture reveals sex-specific differences in biochemical pathways

Sex differences in the brain of mammals range from neuroarchitecture through cognition to cellular metabolism. The hippocampus, a structure mostly associated with learning and memory, presents high vulnerability to neurodegeneration and aging. Therefore, we explored basal sex-related differences in the proteome of organotypic hippocampal slice culture, a major in vitro model for studying the cellular and molecular mechanisms related to neurodegenerative disorders. Results suggest a greater prevalence of astrocytic metabolism in females and significant neuronal metabolism in males. The preference for glucose use in glycolysis, pentose phosphate pathway and glycogen metabolism in females and high abundance of mitochondrial respiration subunits in males support this idea. An overall upregulation of lipid metabolism was observed in females. Upregulation of proteins responsible for neuronal glutamate and GABA synthesis, along with synaptic associated proteins, were observed in males. In general, the significant spectrum of pathways known to predominate in neurons or astrocytes, together with the well-known neuronal and glial markers observed, revealed sex-specific metabolic differences in the hippocampus. TEM qualitative analysis might indicate a greater presence of mitochondria at CA1 synapses in females. These findings are crucial to a better understanding of how sex chromosomes can influence the physiology of cultured hippocampal slices and allow us to gain insights into distinct responses of males and females on neurological diseases that present a sex-biased incidence.

In Vivo Metabolism of [1,6-13C2]Glucose Reveals Distinct Neuroenergetic Functionality between Mouse Hippocampus and Hypothalamus

Glucose is a major energy fuel for the brain, however, less is known about specificities of its metabolism in distinct cerebral areas. Here we examined the regional differences in glucose utilization between the hypothalamus and hippocampus using in vivo indirect ¹³C magnetic resonance spectroscopy (¹H-[¹³C]-MRS) upon infusion of [1,6-¹³C₂]glucose. Using a metabolic flux analysis with a 1-compartment mathematical model of brain metabolism, we report that compared to hippocampus, hypothalamus shows higher levels of aerobic glycolysis associated with a marked gamma-aminobutyric acid-ergic (GABAergic) and astrocytic metabolic dependence. In addition, our analysis suggests a higher rate of ATP production in hypothalamus that is accompanied by an excess of cytosolic nicotinamide adenine dinucleotide (NADH) production that does not fuel mitochondria via the malate-aspartate shuttle (MAS). In conclusion, our results reveal significant metabolic differences, which might be attributable to respective cell populations or functional features of both structures.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

Oligodendrocytes: Development, Physiology and Glucose Metabolism

The glutamate-glutamine cycle is an outstanding example of how essential neuronal-glial interactions are for brain function. For several decades, this and other metabolic cycles in the brain have only included neurons and astrocytes but not oligodendrocytes, the myelinating cells of the central nervous system (CNS). Recent data revealed that oligodendrocytes are highly metabolically active cells in the brain and, therefore, should not be ignored. Using ¹³C-labelled glucose in combination with nuclear magnetic resonance spectroscopy (MRS) and/or mass spectrometry (MS) it is possible to characterize metabolic functions in primary oligodendrocyte cultures. Mature rat oligodendrocytes avidly metabolize glucose in the cytosol and pyruvate derived from glucose in mitochondria. Moreover, they seem to have the ability of performing anaplerosis from pyruvate, which might enable them to synthesize metabolites de novo and transfer them to neighbouring cells. All these original findings highlight the importance of investigating oligodendrocyte metabolism separately from that of astrocytes and neurons to be able to discern the roles played by the individual partners. This is of particular importance in the white matter where the number of oligodendrocytes is considerable. The present book chapter provides some background on oligodendrocyte biology and physiology and summarizes the not very extensive information published on glucose metabolism in oligodendrocytes.

Free-choice high-fat diet alters circadian oscillation of energy intake in adolescent mice: role of prefrontal cortex

PURPOSE

Our aim was to characterize the effect of an unfamiliar high-fat diet (HFD) on circadian feeding behaviour, plasma parameters, body weight (BW), and gene expression in the prefrontal cortex (PFC) of adolescent male mice. To this end, mice were allowed to consume a HFD during 48 h, but one group was allowed a free choice of HFD or normal chow (FC-HFD), while the other was restricted to a non-optional unfamiliar HFD feeding (NOP-HFD).

METHODS

Energy intake was monitored at 6-h intervals during 48 h. Mice cohorts were killed at 6-h intervals after 48-h dietary treatment, and PFC samples dissected for RT-PCR analysis.

RESULTS

Mice on the FC-HFD protocol avoided eating the standard chow, showed lower energy intake and lower BW increase than NOP-HFD mice. All animals with access to HFD exhibited nocturnal overeating, but diurnal hyperphagia was more prominent in the FC-HFD cohort. A robust increase in tyrosine hydroxylase (Th) gene expression was detected specifically during the light period of the circadian cycle in FC-HFD mice. In contrast, both protocols similarly up-regulated the expression of cytosolic malic enzyme (Me1), which is very sensitive to HFD.

CONCLUSION

Our data show that the PFC participates in driving motivational feeding during HFD-evoked hyperphagia and also suggest that sensory neural pathways might be relevant for the onset of eating disorders and overweight. Moreover, we have observed that animals that had the possibility of choosing between standard chow and HFD were more hyperphagic and specifically displayed an overexpression of the tyrosine hydroxylase gene.

Nrf2 activation in the treatment of neurodegenerative diseases: a focus on its role in mitochondrial bioenergetics and function

The nuclear factor erythroid-derived 2 (NF-E2)-related factor 2 (Nrf2) is a transcription factor well-known for its function in controlling the basal and inducible expression of a variety of antioxidant and detoxifying enzymes. As part of its cytoprotective activity, increasing evidence supports its role in metabolism and mitochondrial bioenergetics and function. Neurodegenerative diseases are excellent candidates for Nrf2-targeted treatments. Most neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia and Friedreich's ataxia are characterized by oxidative stress, misfolded protein aggregates, and chronic inflammation, the common targets of Nrf2 therapeutic strategies. Together with them, mitochondrial dysfunction is implicated in the pathogenesis of most neurodegenerative disorders. The recently recognized ability of Nrf2 to regulate intermediary metabolism and mitochondrial function makes Nrf2 activation an attractive and comprehensive strategy for the treatment of neurodegenerative disorders. This review aims to focus on the potential therapeutic role of Nrf2 activation in neurodegeneration, with special emphasis on mitochondrial bioenergetics and function, metabolism and the role of transporters, all of which collectively contribute to the cytoprotective activity of this transcription factor.

Characterization of glucose-related metabolic pathways in differentiated rat oligodendrocyte lineage cells

Although oligodendrocytes constitute a significant proportion of cells in the central nervous system (CNS), little is known about their intermediary metabolism. We have, therefore, characterized metabolic functions of primary oligodendrocyte precursor cell cultures at late stages of differentiation using isotope-labelled metabolites. We report that differentiated oligodendrocyte lineage cells avidly metabolize glucose in the cytosol and pyruvate derived from glucose in the mitochondria. The labelling patterns of metabolites obtained after incubation with [1,2-(13)C]glucose demonstrated that the pentose phosphate pathway (PPP) is highly active in oligodendrocytes (approximately 10% of glucose is metabolized via the PPP as indicated by labelling patterns in phosphoenolpyruvate). Mass spectrometry and magnetic resonance spectroscopy analyses of metabolites after incubation of cells with [1-(13)C]lactate or [1,2-(13)C]glucose, respectively, demonstrated that anaplerotic pyruvate carboxylation, which was thought to be exclusive to astrocytes, is also active in oligodendrocytes. Using [1,2-(13)C]acetate, we show that oligodendrocytes convert acetate into acetyl CoA which is metabolized in the tricarboxylic acid cycle. Analysis of labelling patterns of alanine after incubation of cells with [1,2-(13)C]acetate and [1,2-(13)C]glucose showed catabolic oxidation of malate or oxaloacetate. In conclusion, we report that oligodendrocyte lineage cells at late differentiation stages are metabolically highly active cells that are likely to contribute considerably to the metabolic activity of the CNS.

Anaplerosis for Glutamate Synthesis in the Neonate and in Adulthood

A central task of the tricarboxylic acid (TCA, Krebs, citric acid) cycle in brain is to provide precursors for biosynthesis of glutamate, GABA, aspartate and glutamine. Three of these amino acids are the partners in the intricate interaction between astrocytes and neurons and form the so-called glutamine-glutamate (GABA) cycle. The ketoacids α-ketoglutarate and oxaloacetate are removed from the cycle for this process. When something is removed from the TCA cycle it must be replaced to permit the continued function of this essential pathway, a process termed anaplerosis. This anaplerotic process in the brain is mainly carried out by pyruvate carboxylation performed by pyruvate carboxylase. The present book chapter gives an introduction and overview into this carboxylation and additionally anaplerosis mediated by propionyl-CoA carboxylase under physiological conditions in the adult and in the developing rodent brain. Furthermore, examples are given about pathological conditions in which anaplerosis is disturbed.

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.

Identification of active elementary flux modes in mitochondria using selectively permeabilized CHO cells

Metabolic compartmentation is a key feature of mammalian cells. Mitochondria are the powerhouse of eukaryotic cells, responsible for respiration and the TCA cycle. We accessed the mitochondrial metabolism of the economically important Chinese hamster ovary (CHO) cells using selective permeabilization. We tested key substrates without and with addition of ADP. Based on quantified uptake and production rates, we could determine the contribution of different elementary flux modes to the metabolism of a substrate or substrate combination. ADP stimulated the uptake of most metabolites, directly by serving as substrate for the respiratory chain, thus removing the inhibitory effect of NADH, or as allosteric effector. Addition of ADP favored substrate metabolization to CO2 and did not enhance the production of other metabolites. The controlling effect of ADP was more pronounced when we supplied metabolites to the first part of the TCA cycle: pyruvate, citrate, α-ketoglutarate and glutamine. In the second part of the TCA cycle, the rates were primarily controlled by the concentrations of C4-dicarboxylates. Without ADP addition, the activity of the pyruvate carboxylase-malate dehydrogenase-malic enzyme cycle consumed the ATP produced by oxidative phosphorylation, preventing its accumulation and maintaining metabolic steady state conditions. Aspartate was taken up only in combination with pyruvate, whose uptake also increased, a fact explained by complex regulatory effects. Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase were identified as the key regulators of the TCA cycle, confirming existent knowledge from other cells. We have shown that selectively permeabilized cells combined with elementary mode analysis allow in-depth studying of the mitochondrial metabolism and regulation.

Glutamate synthesis has to be matched by its degradation - where do all the carbons go?

The central process in energy production is the oxidation of acetyl-CoA to CO2 by the tricarboxylic acid (TCA, Krebs, citric acid) cycle. However, this cycle functions also as a biosynthetic pathway from which intermediates leave to be converted primarily to glutamate, GABA, glutamine and aspartate and to a smaller extent to glucose derivatives and fatty acids in the brain. When TCA cycle ketoacids are removed, they must be replaced to permit the continued function of this essential pathway, by a process termed anaplerosis. Since the TCA cycle cannot act as a carbon sink, anaplerosis must be coupled with cataplerosis; the exit of intermediates from the TCA cycle. The role of anaplerotic reactions for cellular metabolism in the brain has been studied extensively. However, the coupling of this process with cataplerosis and the roles that both pathways play in the regulation of amino acid, glucose, and fatty acid homeostasis have not been emphasized. The concept of a linkage between anaplerosis and cataplerosis should be underscored, because the balance between these two processes is essential. The hypothesis that cataplerosis in the brain is achieved by exporting the lactate generated from the TCA cycle intermediates into the blood and perivascular area is presented. This shifts the generally accepted paradigm of lactate generation as simply derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis. Intermediates leave the tricarboxylic acid cycle and must be replaced by a process termed anaplerosis that must be coupled to cataplerosis. We hypothesize that cataplerosis is achieved by exporting the lactate generated from the cycle into the blood and perivascular area. This shifts the paradigm of lactate generation as solely derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis.

Lactate storm marks cerebral metabolism following brain trauma

Brain metabolism is thought to be maintained by neuronal-glial metabolic coupling. Glia take up glutamate from the synaptic cleft for conversion into glutamine, triggering glial glycolysis and lactate production. This lactate is shuttled into neurons and further metabolized. The origin and role of lactate in severe traumatic brain injury (TBI) remains controversial. Using a modified weight drop model of severe TBI and magnetic resonance (MR) spectroscopy with infusion of (13)C-labeled glucose, lactate, and acetate, the present study investigated the possibility that neuronal-glial metabolism is uncoupled following severe TBI. Histopathology of the model showed severe brain injury with subarachnoid and hemorrhage together with glial cell activation and positive staining for Tau at 90 min post-trauma. High resolution MR spectroscopy of brain metabolites revealed significant labeling of lactate at C-3 and C-2 irrespective of the infused substrates. Increased (13)C-labeled lactate in all study groups in the absence of ischemia implied activated astrocytic glycolysis and production of lactate with failure of neuronal uptake (i.e. a loss of glial sensing for glutamate). The early increase in extracellular lactate in severe TBI with the injured neurons rendered unable to pick it up probably contributes to a rapid progression toward irreversible injury and pan-necrosis. Hence, a method to detect and scavenge the excess extracellular lactate on site or early following severe TBI may be a potential primary therapeutic measure.

Synaptosomal lactate dehydrogenase isoenzyme composition is shifted toward aerobic forms in primate brain evolution

With the evolution of a relatively large brain size in haplorhine primates (i.e. tarsiers, monkeys, apes, and humans), there have been associated changes in the molecular machinery that delivers energy to the neocortex. Here we investigated variation in lactate dehydrogenase (LDH) expression and isoenzyme composition of the neocortex and striatum in primates using quantitative Western blotting and isoenzyme analysis of total homogenates and synaptosomal fractions. Analysis of isoform expression revealed that LDH in synaptosomal fractions from both forebrain regions shifted towards a predominance of the heart-type, aerobic isoform LDH-B among haplorhines as compared to strepsirrhines (i.e. lorises and lemurs), while in the total homogenate of the neocortex and striatum there was no significant difference in LDH isoenzyme composition between the primate suborders. The largest increase occurred in synapse-associated LDH-B expression in the neocortex, with an especially remarkable elevation in the ratio of LDH-B/LDH-A in humans. The phylogenetic variation in the ratio of LDH-B/LDH-A was correlated with species-typical brain mass but not the encephalization quotient. A significant LDH-B increase in the subneuronal fraction from haplorhine neocortex and striatum suggests a relatively higher rate of aerobic glycolysis that is linked to synaptosomal mitochondrial metabolism. Our results indicate that there is a differential composition of LDH isoenzymes and metabolism in synaptic terminals that evolved in primates to meet increased energy requirements in association with brain enlargement.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex

The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states involving energetic imbalance. In spite of the fact that it is being branded as a "cycle", during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high-energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this Mini-Review, these segments are appraised, pointing to the critical importance of the α-ketoglutarate dehydrogenase complex dictating their directionalities.

Metabolic aspects of neuron-oligodendrocyte-astrocyte interactions

Whereas astrocytes have been in the limelight of scientific interest in brain energy metabolism for a while, oligodendrocytes are still waiting for a place on the metabolic stage. We propose to term the interaction of oligodendrocytes with astrocytes and neurons: NOA (neuron-oligodendrocyte-astrocyte) interactions. One of the reasons to find out more about metabolic interactions between oligodendrocytes, neurons, and astrocytes is to establish markers of healthy oligodendrocyte metabolism that could be used for the diagnosis and assessment of white matter disease. The vesicular release of glutamate in the white matter has received considerable attention in the past. Oligodendrocyte lineage cells express glutamate receptors and glutamate toxicity has been implicated in diseases affecting oligodendrocytes such as hypoxic-ischaemic encephalopathy, inflammatory diseases and trauma. As oligodendrocyte precursor cells vividly react to injury it is also important to establish whether cells recruited into damaged areas are able to regenerate lost myelin sheaths or whether astrocytic scarring occurs. It is therefore important to consider metabolic aspects of astrocytes and oligodendrocytes separately. The present review summarizes the limited evidence available on metabolic cycles in oligodendrocytes and so hopes to stimulate further research interests in this important field.

Substrate competition studies demonstrate oxidative metabolism of glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate in cortical astrocytes from rat brain

It is well established that astrocytes can utilize many substrates to support oxidative energy metabolism; however, use of energy substrates in the presence of other substrates, as would occur in vivo, has not been systematically evaluated. Substrate competition studies were used to determine changes in the rates of (14)CO(2) production since little is known about the interaction of energy substrates in astrocytes. The rates of (14)CO(2) production from 1 mM D-[6-(14)C]glucose, L-[U-(14)C]glutamate, L-[U-(14)C]glutamine, D-3-hydroxy[3-(14)C]butyrate, L-[U-(14)C]lactate and L-[U-(14)C]malate by primary cultures of astrocytes from rat brain were determined to be 1.17 ± 0.19, 85.30 ± 12.25, 28.04 ± 2.84, 13.55 ± 4.56, 14.84 ± 2.40 and 5.20 ± 1.20 nmol/h/mg protein (mean ± SEM), respectively. The rate of (14)CO(2) production from glutamate oxidation was higher than that of the other substrates Addition of unlabeled glutamate significantly decreased the rates of (14)CO(2) production from all other substrates studied; however, glutamate oxidation was not altered by the addition of any of the other substrates. The rate of (14)CO(2) production of glutamine was decreased by glutamate, but not altered by other substrates. The rate of (14)CO(2) production from glucose was significantly decreased by the addition of unlabeled glutamate, glutamine or lactate, but not by 3-hydroxybutyrate or malate. Addition of unlabeled glucose did not significantly alter the (14)CO(2) production from any other substrate. (14)CO(2) production from lactate was decreased by the addition of unlabeled glutamine or glutamate and increased by addition of malate. The (14)CO(2) production from malate was decreased by the addition of unlabeled glutamate or lactate, but was not altered by the other substrates. The substrate utilization for oxidative energy metabolism in astrocytes is very different than the profile previously reported for synaptic terminals. These studies demonstrate the potential use of multiple substrates including glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate as energy substrates for astrocytes. The data also provide evidence of interactions of substrates and multiple compartments of TCA cycle activity in cultured astrocytes.

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.

Pyruvate carboxylation in different model systems studied by (13)C MRS

Pyruvate carboxylation is of great importance in the brain since it is responsible for adding net carbons to the tricarboxylic acid cycle following removal of carbon backbone for synthesis of the two most abundant neurotransmitters, glutamate and GABA. Despite having such a pivotal role, there is still much uncertainty in the exact metabolic details about where and how this carbon is returned. Pyruvate carboxylation has been studied in various model systems of the brain and ¹³C magnetic resonance spectroscopy is an excellent tool for doing this. This review will focus on results dealing with the extent and cellular location of pyruvate carboxylation and its role in pathophysiology and concludes that pyruvate carboxylation is an extraordinarily important predominantly astrocytic pathway which plays a pivotal part in a number of diseases.

Pupal cuticle protein is abundant during early adult diapause in the mosquito Culex pipiens

Using a proteomics approach, we identified 13 differentially abundant proteins in heads of nondiapause, early, and mid-diapause Culex pipiens L. adult females. Three proteins that were more abundant during early diapause showed highest identities to pupal cuticle protein (PCP). The majority of less abundant proteins were involved in energy metabolism and cytoskeletonal reorganization. The increase of PCP in early diapause may be involved in enhancing stress resistance in the cuticle, a response akin to the general enhancement of stress resistance associated with overwintering insects. PCP also may have utility as a biomarker for early diapause in this species.

Proposed cycles for functional glutamate trafficking in synaptic neurotransmission

To date, the glutamate-glutamine cycle has been the dominant paradigm for understanding the coordinated, compartmentalized activities of phosphate-activated glutaminase (PAG) and glutamine synthetase (GS) in support of functional glutamate trafficking in vivo. However, studies in cell cultures have repeatedly challenged the notion that functional glutamate trafficking is accomplished via the glutamate-glutamine cycle alone. The present study introduces and elaborates alternative cycles for functional glutamate trafficking that integrate glucose metabolism, glutamate anabolism, transport, and catabolism, and trafficking of TCA cycle intermediates from astrocytes to presynaptic neurons. Detailed stoichiometry for each of these alternative cycles is established by strict application of the principle of conservation of atomic species to cytosolic and mitochondrial compartments in both presynaptic neurons and astrocytes. In contrast to the glutamate-glutamine cycle, which requires ATP, but not necessarily oxidative metabolism, to function, cycles for functional glutamate trafficking based on intercellular transport of TCA cycle intermediates require oxidative processes to function. These proposed alternative cycles are energetically more efficient than, and incorporate an inherent mechanism for transporting nitrogen from presynaptic neurons to astrocytes in support of the coordinated activities of PAG and GS that is absent in, the glutamate-glutamine cycle. In light of these newly elaborated alternative cycles, it is premature to presuppose that functional glutamate trafficking in synaptic neurotransmission in vivo is sustained by the glutamate-glutamine cycle alone.

The glutamate-glutamine cycle is not stoichiometric: fates of glutamate in brain

Although glutamate is usually thought of as the major excitatory neurotransmitter in brain, it is important to note that glutamate has many other fates in brain, including oxidation for energy, incorporation into proteins, and formation of glutamine, gamma-aminobutyric acid (GABA), and glutathione. The compartmentation of glutamate in brain cells is complex and modulated by the presence and concentration of glutamate per se as well as by other metabolites. Both astrocytes and neurons distinguish between exogenous glutamate and glutamate formed endogenously from glutamine via glutaminase. There is evidence of multiple subcellular compartments of glutamate within both neurons and astrocytes, and the carbon skeleton of glutamate can be derived from other amino acids and many energy substrates including glucose, lactate, and 3-hydroxybutyrate. Both astrocytes and neurons utilize glutamate, albeit for cell-specific metabolic fates. Glutamate is readily formed in neurons from glutamine synthesized in astrocytes, released into the extracellular space, and taken up by neurons. However, the glutamate-glutamine cycle is not a stoichiometric cycle but rather an open pathway that interfaces with many other metabolic pathways to varying extents depending on cellular requirements and priorities.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.

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.

Glutamine-induced free radical production in cultured astrocytes

Ammonia is a neurotoxin implicated in the pathogenesis of hepatic encephalopathy, Reye's syndrome, inborn errors of the urea cycle, glutaric aciduria, and other metabolic encephalopathies. Brain ammonia is predominantly metabolized to glutamine in astrocytes by glutamine synthetase. While the synthesis of glutamine has generally been viewed as the principal means of ammonia detoxification, this presumed beneficial effect has been questioned as growing evidence suggest that some of the deleterious effects of ammonia may be mediated by glutamine rather than ammonia per se. Since ammonia is known to induce the production of free radicals in cultured astrocytes, we investigated whether such production might be mediated by glutamine. Treatment of astrocytes with glutamine (4.5 mM) increased free radical production at 2-3 min (95%; P < 0.05), as well as at 1 and 3 h (42% and 49%, respectively; P < 0.05). Similarly treated cultured neurons failed to generate free radicals. Free radical production by glutamine was blocked by the antioxidants deferoxamine (40 microM) and alpha-phenyl-N-tert-butyl-nitrone (250 microM), as well as by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (500 microM). Free radical production was also blocked by 6-diazo-5-oxo-L-norleucine (1 mM), an inhibitor of glutaminase, suggesting that ammonia released by glutamine hydrolysis may be responsible for the generation of free radicals. Additionally, the mitochondrial permeability transition inhibitor, cyclosporin A, blocked free radical production by glutamine. The results indicate that astrocytes, but not neurons, generate free radicals following glutamine exposure. Glutamine-induced oxidative and/or nitrosative stress may represent a key mechanism in ammonia neurotoxicity.

Cataplerotic TCA cycle flux determined as glutamate-sustained oxygen consumption in primary cultures of astrocytes

Utilization of glucose by adult brain as its metabolic substrate does not mean that glutamate cannot be synthesized from glucose and subsequently oxidatively degraded. Between 10 and 20% of total pyruvate metabolism in brain occurs as formation of oxaloacetate (OAA), a tricarboxylic acid (TCA) cycle intermediate, from pyruvate plus CO(2). This anaplerotic ('pool-filling') process occurs in astrocytes, which in contrast to neurons express pyruvate carboxylase (PC) activity. Equivalent amounts of pyruvate are converted to acetylcoenzyme A and condensed with oxaloacetate to form citrate (Cit), which is metabolized to alpha-ketoglutarate (generating oxidatively-derived energy), glutamate and glutamine and transferred to neurons in the glutamate-glutamine cycle and used as precursor for transmitter glutamate. Since the blood-brain barrier is poorly permeable to glutamate and its metabolites, net synthesis of glutamate must be followed by degradation of equivalent amounts of glutamate, a cataplerotic ('pool-emptying') process, in which glutamate is converted in the TCA cycle to malate or oxaloacetate (generating additional energy), which exit the cycle to form one molecule pyruvate. To obtain an estimate of the rate of astrocytic oxidation of glutamate the rate of oxygen consumption was measured in primary cultures of mouse astrocytes metabolizing glutamate in the absence of other metabolic substrates. The observed rate is compatible with complete oxidative degradation of glutamate.

Effects on free radical generation by ligands of the peripheral benzodiazepine receptor in cultured neural cells

The effect of peripheral benzodiazepine receptor (PBR) ligands on free radical production was investigated in primary cultures of rat brain astrocytes and neurons as well as in BV-2 microglial cell lines using the fluorescent dye dichlorofluorescein-diacetate. Free radical production was measured at 2, 30, 60 and 120 min of treatment with the PBR ligands 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195), 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864) and protoporphyrin IX (PpIX) (all at 10 nm). In astrocytes, all ligands showed a significant increase in free radical production at 2 min. The increase was short-lived with PK11195, whereas with Ro5-4864 it persisted for at least 2 h. PpIX caused an increase at 2 and 30 min, but not at 2 h. Similar results were observed in microglial cells. In neurons, PK11195 and PpIX showed an increase in free radical production only at 2 min; Ro5-4864 had no effect. The central-type benzodiazepine receptor ligand, clonazepam, was ineffective in eliciting free radical production in all cell types. As the PBR may be a component of the mitochondrial permeability transition (MPT) pore, and free radical production may occur following induction of the MPT, we further investigated whether cyclosporin A (CsA), an inhibitor of the MPT, could prevent free radical formation by PBR ligands. CsA (1 micro m) completely blocked free radical production following treatment with PK11195 and Ro5-4864 in all cell types. CsA was also effective in blocking free radical production in astrocytes following PpIX treatment, but it failed to do so in neurons and microglia. Our results indicate that exposure of neural cells to PBR ligands generates free radicals, and that the MPT may be involved in this process.

Pyruvate carboxylation in neurons

Carboxylation of pyruvate in the brain was for many years thought to occur only in glia, an assumption that formed much of the basis for the concept of the glutamine cycle. It was shown recently, however, that carboxylation of pyruvate to malate occurs in neurons and that it supports formation of transmitter glutamate. The role of pyruvate carboxylation in neurons is to ensure tricarboxylic acid cycle activity by compensating for losses of alpha-ketoglutarate that occur through release of transmitter glutamate and GABA; these amino acids are alpha-ketoglutarate derivatives. Available data suggest that neuronal pyruvate carboxylation is quantitatively important. But because there is no net CO(2) fixation in the brain, pyruvate carboxylation must be balanced by decarboxylation of malate or oxaloacetate. Such decarboxylation occurs in both neurons and astrocytes. Several in vitro studies have shown a neuroprotective effect of pyruvate supplementation. Pyruvate carboxylation may be one mechanism through which such treatment is effective, because pyruvate carboxylation through malic enzyme is active during energy deficiency and leads to an increase in the level of dicarboxylates that can be metabolized through the tricarboxylic acid cycle for ATP production.

Alpha-cyano-4-hydroxycinnamate decreases both glucose and lactate metabolism in neurons and astrocytes: implications for lactate as an energy substrate for neurons

The rates of uptake and oxidation of [U-(14)C]lactate and [U-(14)C]glucose were determined in primary cultures of astrocytes and neurons from rat brain, in the presence and absence of the monocarboxylic acid transport inhibitor alpha-cyano-4-hydroxycinnamate (4-CIN). The rates of uptake for 1 mM lactate and glucose were 7.45 +/- 1.35 and 8.80 +/- 1.0 nmol/30 sec/mg protein in astrocytes and 2.36 +/- 0.19 and 1.93 +/- 0.16 nmol/30 sec/mg protein in neuron cultures, respectively. Lactate transport into both astrocytes and neurons was significantly decreased by 0.25-1.0 mM 4-CIN; however, glucose uptake was not affected. The rates of (14)CO(2) formation from 1 mM lactate and glucose were 12.49 +/- 0.77 and 3.42 +/- 0.67 nmol/hr/mg protein in astrocytes and 29.32 +/- 2.81 and 10.04 +/- 1.79 nmol/hr/mg protein in neurons, respectively. Incubation with 0.25 mM 4-CIN decreased the oxidation of lactate and glucose to 57.1% and 54.1% of control values in astrocytes and to 13.2% and 41.6% of the control rates in neurons, respectively. Preincubation with 4-CIN further decreased the oxidation of both glucose and lactate. Studies with glucose specifically labeled in the one and six positions demonstrated that 4-CIN decreased mitochondrial glucose oxidation but did not impair the metabolism of glucose via the pentose phosphate pathway in the cytosol. The lack of effect of 4-CIN on glutamate oxidation demonstrated that overall mitochondrial metabolism was not impaired. These findings suggest that the impaired neuronal function and tissue damage in the presence of 4-CIN observed in other studies may be due in part to decreased uptake of lactate; however, the effects of 4-CIN on mitochondrial transport would significantly decrease the oxidative metabolism of pyruvate derived from both glucose and lactate.

BH3 death domain peptide induces cell type-selective mitochondrial outer membrane permeability

The BH3 domain is essential for the release of cytochrome c from mitochondria by pro-apoptotic Bcl-2 family proteins during apoptosis. This study tested the hypothesis that a Bax peptide that includes the BH3 domain can permeabilize the mitochondrial outer membrane and release cytochrome c in the absence of a permeability transition at the mitochondrial inner membrane. BH3 peptide (0.1-60 microm) released cytochrome c from mitochondria in the presence of physiological concentrations of ions in a cell type-selective manner, whereas a BH3 peptide with a single amino acid substitution was ineffective. The release of cytochrome c by BH3 peptide correlated with the presence of endogenous Bax at the mitochondria and its integral membrane insertion. Cytochrome c release was accompanied by adenylate kinase release, was not associated with mitochondrial swelling or substantial loss of electrical potential across the inner membrane, and was unaffected by inhibitors of the permeability transition pore. Cytochrome c release was, however, inhibited by Bcl-2. Although energy-coupled respiration was inhibited after the release of cytochrome c, mitochondria maintained membrane potential in the presence of ATP due to the reversal of the ATP synthase. Overall, results support the hypothesis that BH3 peptide releases cytochrome c by a Bax-dependent process that is independent of the mitochondrial permeability transition pore but regulated by Bcl-2.

In vivo nuclear magnetic resonance studies of glutamate-gamma-aminobutyric acid-glutamine cycling in rodent and human cortex: the central role of glutamine

It has been recognized for many years that the metabolism of brain glutamate and gamma-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters, is linked to a substrate cycle between neurons and astrocytes involving glutamine. However, the quantitative significance of these fluxes in vivo was not known. Recent in vivo 13C and 15N NMR studies in rodents and 13C NMR in humans indicate that glutamine synthesis is substantial and that the total glutamate-GABA-glutamine cycling flux, necessary to replenish neurotransmitter glutamate and GABA, accounts for >80% of net glutamine synthesis. In studies of the rodent cortex, a linear relationship exists between the rate of glucose oxidation and total glutamate-GABA-glutamine cycling flux over a large range of cortical electrical activity. The molar stoichiometric relationship (approximately 1:1) found between these fluxes suggests that they share a common mechanism and that the glutamate-GABA-glutamine cycle is coupled to a major fraction of cortical glucose utilization. Thus, glutamine appears to play a central role in the normal functional energetics of the cerebral cortex.

Carboxylation and anaplerosis in neurons and glia

Anaplerosis, or de novo formation of intermediates of the tricarboxylic acid (TCA) cycle, compensates for losses of TCA cycle intermediates, especially alpha-ketoglutarate, from brain cells. Loss of alpha-ketoglutarate occurs through release of glutamate and GABA from neurons and through export of glutamine from glia, because these amino acids are alpha-ketoglutarate derivatives. Anaplerosis in the brain may involve four different carboxylating enzymes: malic enzyme, phosphoenopyruvate carboxykinase (PEPCK), propionyl-CoA carboxylase, and pyruvate carboxylase. Anaplerotic carboxylation was for many years thought to occur only in glia through pyruvate carboxylase; therefore, loss of transmitter glutamate and GABA from neurons was thought to be compensated by uptake of glutamine from glia. Recently, however, anaplerotic pyruvate carboxylation was demonstrated in glutamatergic neurons, meaning that these neurons to some extent can maintain transmitter synthesis independently of glutamine. Malic enzyme, which may carboxylate pyruvate, was recently detected in neurons. The available data suggest that neuronal and glial pyruvate carboxylation could operate at as much as 30% and 40-60% of the TCA cycle rate, respectively. Cerebral carboxylation reactions are probably balanced by decarboxylation reactions,, because cerebral CO2 formation equals O2 consumption. The finding of pyruvate carboxylation in neurons entails a major revision of the concept of the glutamine cycle.

Differential distribution of the enzymes glutamate dehydrogenase and aspartate aminotransferase in cortical synaptic mitochondria contributes to metabolic compartmentation in cortical synaptic terminals

There have been numerous studies on the activity and localization of aspartate aminotransferase (AAT) and glutamate dehydrogenase (GDH) in brain tissue. However, there is still a controversy as to the specific roles and relative importance of these enzymes in glutamate and glutamine metabolism in astrocytes and neurons or synaptic terminals. There are many reports documenting GDH activity in synaptic terminals, yet the misconception that it is a glial enzyme persists. Furthermore, there is evidence that this tightly regulated enzyme may have an increased role in synaptic metabolism in adverse conditions such as low glucose and hyperammonemia that could compromise synaptic function. In the present study, we report high activity of both AAT and GDH in mitochondrial subfractions from cortical synaptic terminals. The relative amount of GDH/AAT activity was higher in SM2 mitochondria, compared to SM1 mitochondria. Such a differential distribution of enzymes can contribute significantly to the compartmentation of metabolism. There is evidence that the metabolic capabilities of the SM1 and SM2 subfractions of synaptic mitochondria are compatible with the compartments A and B of neuronal metabolism proposed by Waagepetersen et al. (1998b. Dev. Neurosci. 20, 310-320).

Monocarboxylic acid transporters, MCT1 and MCT2, in cortical astrocytes in vitro and in vivo

We used sequence-specific antibodies to characterize two monocarboxylic acid transporters, MCT1 and MCT2, in astrocytes. Both proteins are expressed in primary cultures of cortical astrocytes, as indicated by immunoblotting and immunofluorescence. Both MCT1 and MCT2 are present in small, punctate structures in the cytoplasm and at the cell membrane. Cells showing very low levels of labeling for glial fibrillary acidic protein (GFAP) also label more dimly for MCT2, but not for MCT1. In vivo, double-label immunofluorescence studies coupled with confocal microscopy indicate that MCT1 and MCT2 are rare in astrocytes in the cortex. However, they are specifically labeled in astrocytes of the glial limiting membrane and in white matter tracts. Both transporters are also present in the microvasculature. Comparison of labeling for MCT1 and MCT2 with markers of the blood-brain barrier shows that the transporters are not always limited to the astrocytic endfeet in vivo. Our results suggest that the level of expression of monocarboxylic acid transporters MCT1 and MCT2 by cortical astrocytes in vivo is significantly lower than in vitro but that astrocytes in some other regions of the brain can express one or both proteins in significant amounts.

Mitochondrial malic enzyme activity is much higher in mitochondria from cortical synaptic terminals compared with mitochondria from primary cultures of cortical neurons or cerebellar granule cells

Most of the malic enzyme activity in the brain is found in the mitochondria. This isozyme may have a key role in the pyruvate recycling pathway which utilizes dicarboxylic acids and substrates such as glutamine to provide pyruvate to maintain TCA cycle activity when glucose and lactate are low. In the present study we determined the activity and kinetics of malic enzyme in two subfractions of mitochondria isolated from cortical synaptic terminals, as well as the activity and kinetics in mitochondria isolated from primary cultures of cortical neurons and cerebellar granule cells. The synaptic mitochondrial fractions had very high mitochondrial malic enzyme (mME) activity with a Km and a Vmax of 0.37 mM and 32.6 nmol/min/mg protein and 0.29 mM and 22.4 nmol/min mg protein, for the SM2 and SM1 fractions, respectively. The Km and Vmax for malic enzyme activity in mitochondria isolated from cortical neurons was 0.10 mM and 1.4 nmol/min/mg protein and from cerebellar granule cells was 0.16 mM and 5.2 nmol/min/mg protein. These data show that mME activity is highly enriched in cortical synaptic mitochondria compared to mitochondria from cultured cortical neurons. The activity of mME in cerebellar granule cells is of the same magnitude as astrocyte mitochondria. The extremely high activity of mME in synaptic mitochondria is consistent with a role for mME in the pyruvate recycling pathway, and a function in maintaining the intramitochondrial reduced glutathione in synaptic terminals.

Malic enzyme isoforms in astrocytes: comparative study on activities in rat brain tissue and astroglia-rich primary cultures

Anion exchange chromatography on diethylaminoethyl cellulose was optimized to separate the cytosolic and mitochondrial isoforms of malic enzyme from rat brain. Extracts of adult rat brain and of astroglia-rich primary cultures derived from the brains of newborn rats were analyzed for their content of the two isozymes. In the case of brain tissue 45% of malic enzyme activity was due to the cytosolic isoform. In contrast, in extracts from astroglia-rich primary cultures more than 95% of the total activity was associated with the cytosolic isozyme. From these data it is concluded that the cytosolic rather than the mitochondrial isoform of malic enzyme has prominent functions in astroglial metabolism.