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

Demonstration of pyruvate recycling in primary cultures of neocortical astrocytes but not in neurons

Pyruvate recycling was studied in primary cultures of mouse cerebrocortical astrocytes, GABAergic cerebrocortical interneurons, and co-cultures consisting of both cell types by measuring production of [4-(13)C]glutamate from [3-(13)C]glutamate by aid of nuclear magnetic resonance spectroscopy. This change in the position of the label can only occur by entry of [3-(13)C]glutamate into the tricarboxylic acid (TCA) cycle, conversion of labeled alpha-ketoglutarate to malate or oxaloacetate, malic enzyme-mediated decarboxylation of malate to pyruvate or phosphoenolpyruvate carboxykinase-mediated conversion of oxaloacetate to phosphoenolpyruvate and subsequent hydrolysis of the latter to pyruvate, and introduction of the labeled pyruvate into the TCA cycle, i.e., after exit of the carbon skeleton of pyruvate from the TCA cycle followed by re-entry of the same pyruvate molecules via acetyl CoA. In agreement with earlier observations, pyruvate recycling was demonstrated in astrocytes, indicating the ability of these cells to undertake complete oxidative degradation of glutamate. The recycled [4-(13)C]glutamate was not further converted to glutamine, showing compartmentation of astrocytic metabolism. Thus, absence of recycling into glutamine in the brain in vivo cannot be taken as indication that pyruvate recycling is absent in astrocytes. No recycling could be demonstrated in the cerebrocortical neurons. This is consistent with a previously demonstrated lack of incorporation of label from glutamate into lactate, and it also indicates that mitochondrial malic enzyme is not operational. Nor was there any indication of pyruvate recycling in the co-cultures. Although this may partly be due to more rapid depletion of glutamate in the co-cultures, this observation at the very least indicates that pyruvate recycling is not up-regulated in the neuronal-astrocytic co-cultures.

Time-dependence of the contribution of pyruvate carboxylase versus pyruvate dehydrogenase to rat brain glutamine labelling from [1-(13) C]glucose metabolism

[1-(13) C]glucose metabolism in the rat brain was investigated after intravenous infusion of the labelled substrate. Incorporation of the label into metabolites was analysed by NMR spectroscopy as a function of the infusion time: 10, 20, 30 or 60 min. Specific enrichments in purified mono- and dicarboxylic amino acids were determined from (1) H-observed/(13) C-edited and (13) C-NMR spectroscopy. The relative contribution of pyruvate carboxylase versus pyruvate dehydrogenase (PC/PDH) to amino acid labelling was evaluated from the enrichment difference between either C2 and C3 for Glu and Gln, or C4 and C3 for GABA, respectively. No contribution of pyruvate carboxylase to aspartate, glutamate or GABA labelling was evidenced. The pyruvate carboxylase contribution to glutamine labelling varied with time. PC/PDH decreased from around 80% after 10 min to less than 30% between 20 and 60 min. This was interpreted as reflecting different labelling kinetics of the two glutamine precursor glutamate pools: the astrocytic glutamate and the neuronal glutamate taken up by astrocytes through the glutamate-glutamine cycle. The results are discussed in the light of the possible occurrence of neuronal pyruvate carboxylation. The methods previously used to determine PC/PDH in brain were re-evaluated as regards their capacity to discriminate between astrocytic (via pyruvate carboxylase) and neuronal (via malic enzyme) pyruvate carboxylation.

Effects of peroxisome proliferator-activated receptor agonists on LPS-induced neuronal death in mixed cortical neurons: associated with iNOS and COX-2

In neurodegenerative disease, the use of non-steroidal anti-inflammatory drugs (NSAIDs) has been regarded as beneficial. The NSAID, an inhibitor of cyclooxygenase (COX), has been also suggested as a ligand of the peroxisome proliferator-activated receptor (PPAR). In cortical neuron-glial co-cultures, we examined the effect of PPAR agonists on lipopolysaccharide(LPS)-induced neuronal death, which has been known to be NO-dependent. LPS induced iNOS expression and the release of nitric oxide in microglia, and COX-2 expression in neurons. PPAR-gamma agonists such as 15d-PGJ(2), ciglitazone and troglitazone prevented LPS-induced neuronal death and abolished LPS-induced NO and PGE(2) release, however PPAR-alpha agonists such as clofibrate and WY14,643 did not produce the same results. PPAR-gamma agonists also reduced LPS-induced iNOS and COX-2 expression, which suggested by interfering with the NF-kappaB signal pathway.

Substrate-dependence of reduction of MTT: a tetrazolium dye differs in cultured astroglia and neurons

3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction is widely used to evaluate cell proliferation and viability. MTT reduction is interpreted to be indicative of cellular metabolic activity, and the site of reduction includes both mitochondrial and cytosolic redox reactions. Astrocytes are believed to rely mainly on glycolysis for ATP generation, whereas neurons are considered to depend more on oxidative metabolism. The present study, therefore, tested the substrate-preference of glucose and its metabolites for MTT reduction in cultures of rat type 1 astroglia and neurons.MTT specific activity of astroglia was much higher than that of neurons. Astroglial MTT reducing activity in glucose-free medium or 2mM glucose with iodoacetate (5mM) was completely blocked. In glucose-depleted medium, 2mM lactate, pyruvate, malate, or acetate elicited minimal increases in MTT reduction by astroglia. In contrast, MTT reducing activity in neurons was enhanced two-fold by pyruvate and the reducing activity of lactate was equivalent to that of glucose, while malate had a small and acetate had no effect on MTT reduction. These results indicate that these two cell types differ markedly in their substrate-preferences for MTT reduction. In astroglia, MTT reduction reflects mainly cytosolic redox activity and is dependent on glyceraldehyde-3-phosphate dehydrogenase. In neurons, pyruvate dehydrogenase supports MTT reduction more effectively than glucose or lactate, even though both of these substrates can produce NADH and pyruvate.

Metabolic compartmentation in cortical synaptosomes: influence of glucose and preferential incorporation of endogenous glutamate into GABA

Metabolism of glutamine was determined under a variety of conditions to study compartmentation in cortical synaptosomes. The combined intracellular and extracellular amounts of [U-13C] GABA, [U-13C]glutamate and [U-13C]glutamine were the same in synaptosomes incubated with U-13C]glutamine in the presence and absence of glucose. However, the concentration of these amino acids was decreased in the latter group, demonstrating the requirement for glucose to maintain the size of neurotransmitter pools. In hypoglycemic synaptosomes more [U-13C]glutamine was converted to [U-13C]aspartate, and less glutamate was re-synthesized from the tricarboxylic acid (TCA) cycle, suggesting use of the partial TCA cycle from alpha-ketoglutarate to oxaloacetate for energy. Compartmentation was studied in synaptosomes incubated with glucose plus labeled and unlabeled glutamine and glutamate. Incubation with [U-13C]glutamine plus unlabeled glutamate gave rise to [U-13C]GABA but not labeled aspartate; however, incubation with [U-13C]glutamate plus unlabeled glutamine gave rise to [U-13C]aspartate, but not labeled GABA. Thus the endogenous glutamate formed via glutaminase in synaptic terminals is preferentially used for GABA synthesis, and is metabolized differently than glutamate taken up from the extracellular milieu.

Membrane architecture of mitochondria in neurons of the central nervous system

Electron tomography was used to help redefine the membrane architecture of mitochondria in neurons of the brain. Investigations were conducted on unexplored questions of structural homogeneity between mitochondria in the four intensely studied regions of the brain and in the functionally distinct neuronal sub-compartments. These mitochondria have the majority of cristae composed of both tubular and lamellar segments with the tubes arranged more peripherally and the lamellae more centrally located. Cristae that are entirely tubular were not commonly seen and those that are entirely lamellar were rare. It was determined that cristae connect through narrow, sometimes very long tubular regions to the peripheral surface of the inner membrane. A structurally distinct type of contact site was revealed in brain mitochondria, which we named the bridge contact site. These bridges may play a role in the structural integrity of the outer and inner membrane systems. It was found that the membrane architecture in the various brain regions and neuronal compartments was strikingly uniform, including consistently tubular crista junctions. The functional consequences of this junctional architecture are discussed in relation to the segregation of proteins between the inner boundary membrane and the cristae membranes, and in relation to the model of microcompartmentation of macromolecules inside cristae.

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.

Elucidation of the quantitative significance of pyruvate carboxylation in cultured cerebellar neurons and astrocytes

Pyruvate carboxylation was studied in cerebellar astrocytes and granule neurons. The cells were incubated in medium containing [U-(13)C]glucose (2.5 mM) and [U-(13)C]lactate (1 mM) and varying amounts of 3-nitropropionic acid (3-NPA) plus/minus aspartate. 3-NPA alone clearly stopped tricarboxylic acid (TCA) cycle activity at the succinate dehydrogenase step in both culture types as evidenced by a buildup of succinate. Labeling of aspartate and glutamate was abolished in neurons in the presence of 3-NPA. In astrocytes, however, labeled glutamate and glutamine derived from pyruvate carboxylation was detected. Unchanged glucose and lactate metabolism in the absence of a functioning malate aspartate shuttle indicates the importance of the glycerol-3-phosphate shuttle in brain cells. To compensate for the loss of oxaloacetate in the presence of 3-NPA, unlabeled aspartate (0.25 mM) was added. In this case [1,2-(13)C] and [3,4-(13)C]aspartate were observed in neurons but not in astrocytes. This labeling pattern in aspartate occurs after a full turn of the TCA cycle and thus indicates only partial inhibition by 3-NPA in the neurons when aspartate is present. In astrocytes, however, aspartate derived from uniformly labeled pyruvate was observed clearly indicating pyruvate carboxylation. The present study has unequivocally demonstrated a quantitatively important pyruvate carboxylation in astrocytes but it was not possible to demonstrate the presence of such carboxylation in neurons. Based on the present results it may be safely concluded that neuronal pyruvate carboxylation is unlikely to be of quantitative significance.

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.

Effects of L-glutamate, D-aspartate, and monensin on glycolytic and oxidative glucose metabolism in mouse astrocyte cultures: further evidence that glutamate uptake is metabolically driven by oxidative metabolism

The hypothesis was tested that oxidative metabolism, mainly fueled by glutamate itself, provides the energy for active, Na(+),K(+)-ATPase-catalyzed Na(+) extrusion following glutamate uptake in conjunction with Na(+). This hypothesis was supported by the following observations: (i) glutamate had either no effect or caused a slight reduction in glycolytic rate, measured as deoxyglucose phosphorylation; (ii) D-aspartate, which is accumulated by the L-glutamate carrier, but cannot be metabolized by the cells, caused an increase in glycolytic rate; (iii) monensin which, like D-aspartate, stimulates the intracellular, Na(+)-activated site of the Na, K-ATPase and thus energy metabolism, but provides no metabolic substrate, stimulated both glycolysis and glucose oxidation; and (iv) oxidation of glucose was potently inhibited by glutamate, although glutamate is known to stimulate oxygen consumption in primary cultures of astrocytes, a combination showing that oxidation of a non-glucose substrate is increased in the presence of glutamate. These findings should be considered in attempts to understand metabolic interactions between neurons and astrocytes and regulation of energy metabolism in brain.

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).