Intracellular metabolic compartmentation assessed by 13C magnetic resonance spectroscopy

Fine Astrocyte Processes Contain Very Small Mitochondria: Glial Oxidative Capability May Fuel Transmitter Metabolism

The peripheral astrocyte process (PAP) is the glial compartment largely handling inactivation of transmitter glutamate, and supplying glutamate to the axon terminal. It is not clear how these energy demanding processes are fueled, and whether the PAP exhibits oxidative capability. Whereas the GFAP-positive perinuclear cytoplasm and stem process are rich in mitochondria, the PAP is often considered too narrow to contain mitochondria and might thus not rely on oxidative metabolism. Applying high resolution light microscopy, we investigate here the presence of mitochondria in the PAPs of freshly dissociated, isolated astrocytes. We provide an overview of the subcellular distribution and the approximate size of astrocytic mitochondria. A substantial proportion of the astrocyte's mitochondria are contained in the PAPs and, on the average, they are smaller there than in the stem processes. The majority of mitochondria in the stem and peripheral processes are surprisingly small (0.2-0.4 µm), spherical and not elongate, or tubular, which is supported by electron microscopy. The density of mitochondria is two to several times lower in the PAPs than in the stem processes. Thus, PAPs do not constitute a mitochondria free glial compartment but contain mitochondria in large numbers. No juxtaposition of mitochondria-containing PAPs and glutamatergic synapses has been reported. However, the issue of sufficient ATP concentrations in perisynaptic PAPs can be seen in the light of (1) the rapid, activity dependent PAP motility, and (2) the recently reported activity-dependent mitochondrial transport and immobilization leading to spatial, subcellular organisation of glutamate uptake and oxidative metabolism.

Acetyl-CoA the key factor for survival or death of cholinergic neurons in course of neurodegenerative diseases

Glucose-derived pyruvate is a principal source of acetyl-CoA in all brain cells, through pyruvate dehydogenase complex (PDHC) reaction. Cholinergic neurons like neurons of other transmitter systems and glial cells, utilize acetyl-CoA for energy production in mitochondria and diverse synthetic pathways in their extramitochondrial compartments. However, cholinergic neurons require additional amounts of acetyl-CoA for acetylcholine synthesis in their cytoplasmic compartment to maintain their transmitter functions. Characteristic feature of several neurodegenerating diseases including Alzheimer’s disease and thiamine diphosphate deficiency encephalopathy is the decrease of PDHC activity correlating with cholinergic deficits and losses of cognitive functions. Such conditions generate acetyl-CoA deficits that are deeper in cholinergic neurons than in noncholinergic neuronal and glial cells, due to its additional consumption in the transmitter synthesis. Therefore, any neuropathologic conditions are likely to be more harmful for the cholinergic neurons than for noncholinergic ones. For this reason attempts preserving proper supply of acetyl-CoA in the diseased brain, should attenuate high susceptibility of cholinergic neurons to diverse neurodegenerative conditions. This review describes how common neurodegenerative signals could induce deficts in cholinergic neurotransmission through suppression of acetyl-CoA metabolism in the cholinergic neurons.

Quantitative importance of the pentose phosphate pathway determined by incorporation of 13C from [2-13C]- and [3-13C]glucose into TCA cycle intermediates and neurotransmitter amino acids in functionally intact neurons

The brain is highly susceptible to oxidative injury, and the pentose phosphate pathway (PPP) has been shown to be affected by pathological conditions, such as Alzheimer's disease and traumatic brain injury. While this pathway has been investigated in the intact brain and in astrocytes, little is known about the PPP in neurons. The activity of the PPP was quantified in cultured cerebral cortical and cerebellar neurons after incubation in the presence of [2-(13)C]glucose or [3-(13)C]glucose. The activity of the PPP was several fold lower than glycolysis in both types of neurons. While metabolism of (13)C-labeled glucose via the PPP does not appear to contribute to the production of releasable lactate, it contributes to labeling of tricarboxylic acid (TCA) cycle intermediates and related amino acids. Based on glutamate isotopomers, it was calculated that PPP activity accounts for ~6% of glucose metabolism in cortical neurons and ~4% in cerebellar neurons. This is the first demonstration that pyruvate generated from glucose via the PPP contributes to the synthesis of acetyl CoA for oxidation in the TCA cycle. Moreover, the fact that (13)C labeling from glucose is incorporated into glutamate proves that both the oxidative and the nonoxidative stages of the PPP are active in neurons.

System-level insights into yeast metabolism by thermodynamic analysis of elementary flux modes

One of the most obvious phenotypes of a cell is its metabolic activity, which is defined by the fluxes in the metabolic network. Although experimental methods to determine intracellular fluxes are well established, only a limited number of fluxes can be resolved. Especially in eukaryotes such as yeast, compartmentalization and the existence of many parallel routes render exact flux analysis impossible using current methods. To gain more insight into the metabolic operation of S. cerevisiae we developed a new computational approach where we characterize the flux solution space by determining elementary flux modes (EFMs) that are subsequently classified as thermodynamically feasible or infeasible on the basis of experimental metabolome data. This allows us to provably rule out the contribution of certain EFMs to the in vivo flux distribution. From the 71 million EFMs in a medium size metabolic network of S. cerevisiae, we classified 54% as thermodynamically feasible. By comparing the thermodynamically feasible and infeasible EFMs, we could identify reaction combinations that span the cytosol and mitochondrion and, as a system, cannot operate under the investigated glucose batch conditions. Besides conclusions on single reactions, we found that thermodynamic constraints prevent the import of redox cofactor equivalents into the mitochondrion due to limits on compartmental cofactor concentrations. Our novel approach of incorporating quantitative metabolite concentrations into the analysis of the space of all stoichiometrically feasible flux distributions allows generating new insights into the system-level operation of the intracellular fluxes without making assumptions on metabolic objectives of the cell.

Fluxes in metabolic pathways are a highly informative aspect of an organism's phenotype. The experimental determination of such fluxes is well established and has proven very useful. To address some of the limitations of experimental flux analysis, such as when the cell is divided in multiple compartments, stoichiometric modeling provides a valuable addition. The approach that we take is based on stoichiometric modeling where we consider the thermodynamic feasibility of many different possible routes through the metabolic network of Saccharomyces cerevisiae using experimentally determined metabolite concentrations. We show that next to conclusions on single biochemical reactions in the metabolic network, we obtain system-level insights on thermodynamically infeasible flux patterns. We found that the compartmental concentrations of and NADH are the causes for the system-level infeasibilities. With the current advances in quantitative metabolomics and biochemical thermodynamics, we envision that the presented method will help gaining more insight into complex metabolic systems.

Direct measurement of backflux between oxaloacetate and fumarate following pyruvate carboxylation

Pyruvate carboxylation (PC) is thought to be the major anaplerotic reaction for the tricarboxylic acid cycle and is necessary for de novo synthesis of amino acid neurotransmitters. In the brain, the main enzyme involved is pyruvate carboxylase, which is predominantly located in astrocytes. Carboxylation leads to the formation of oxaloacetate, which condenses with acetyl coenzyme A to form citrate. However, oxaloacetate may also be converted to malate and fumarate before being regenerated. This pathway is termed the oxaloacetate-fumarate-flux or backflux. Carbon isotope-based methods for quantification of activity of PC lead to underestimation when backflux is not taken into account and critical errors have been made in the interpretation of results from metabolic studies. This study was conducted to establish the degree of backflux after PC in cerebellar and neocortical astrocytes. Astrocyte cultures from cerebellum or neocortex were incubated with either [3-(13) C] or [2-(13) C]glucose, and extracts were analyzed using mass spectrometry or nuclear magnetic resonance spectroscopy. Substantial PC compared with pyruvate dehydrogenase activity was observed, and extensive backflux was demonstrated in both types of astrocytes. The extent of backflux varied between the metabolites, reaffirming that metabolism is highly compartmentalized. By applying our calculations to published data, we demonstrate the existence of backflux in vivo in cat, rat, mouse, and human brain. Thus, backflux should be taken into account when calculating the magnitude of PC to allow for a more precise evaluation of cerebral metabolism.

A comprehensive metabolic profile of cultured astrocytes using isotopic transient metabolic flux analysis and C-labeled glucose

Metabolic models have been used to elucidate important aspects of brain metabolism in recent years. This work applies for the first time the concept of isotopic transient 13C metabolic flux analysis (MFA) to estimate intracellular fluxes in primary cultures of astrocytes. This methodology comprehensively explores the information provided by 13C labeling time-courses of intracellular metabolites after administration of a 13C-labeled substrate. Cells were incubated with medium containing [1-13C]glucose for 24 h and samples of cell supernatant and extracts collected at different time points were then analyzed by mass spectrometry and/or high performance liquid chromatography. Metabolic fluxes were estimated by fitting a carbon labeling network model to isotopomer profiles experimentally determined. Both the fast isotopic equilibrium of glycolytic metabolite pools and the slow labeling dynamics of TCA cycle intermediates are described well by the model. The large pools of glutamate and aspartate which are linked to the TCA cycle via reversible aminotransferase reactions are likely to be responsible for the observed delay in equilibration of TCA cycle intermediates. Furthermore, it was estimated that 11% of the glucose taken up by astrocytes was diverted to the pentose phosphate pathway. In addition, considerable fluxes through pyruvate carboxylase [PC; PC/pyruvate dehydrogenase (PDH) ratio = 0.5], malic enzyme (5% of the total pyruvate production), and catabolism of branched-chained amino acids (contributing with ∼40% to total acetyl-CoA produced) confirmed the significance of these pathways to astrocytic metabolism. Consistent with the need of maintaining cytosolic redox potential, the fluxes through the malate–aspartate shuttle and the PDH pathway were comparable. Finally, the estimated glutamate/α-ketoglutarate exchange rate (∼0.7 μmol mg prot−1 h−1) was similar to the TCA cycle flux. In conclusion, this work demonstrates the potential of isotopic transient MFA for a comprehensive analysis of energy metabolism.

Mitochondrial configurations in peripheral nerve suggest differential ATP production

Physiological states of mitochondria often correlate with distinctive morphology. Electron microscopy and tomographic reconstruction were used to investigate the three-dimensional structure of axonal mitochondria and mitochondria in the surrounding Schwann cells of the peripheral nervous system (PNS), both in the vicinity of nodes of Ranvier and far from these nodes. Condensed mitochondria were found to be abundant in the axoplasm, but not in the Schwann cell. Uncharacteristic of the classical morphology of condensed mitochondria, the outer and inner boundary membranes are in close apposition and the crista junctions are narrow, consistent with their function as gates for the diffusion of macromolecules. There is also less cristae surface area and lower density of crista junctions in these mitochondria. The density of mitochondria was greater at the paranode-node-paranode (PNP) as was the crista junction opening, yet there were fewer cristae in these organelles compared to those in the internodal region. The greater density of condensed mitochondria in the PNS axoplasm and in particular at the PNP suggests a need for these organelles to operate at a high workload of ATP production.

Redox dependence and compartmentation of [13C]pyruvate in the brain of deuterated rats bearing implanted C6 gliomas

We investigated the redox dependence and compartmentation of the pyruvate pool in the brain of partially deuterated rats bearing C6 gliomas. Adult male Wistar rats implanted with C6 cells drunk 2H2O (50% v/v) or tap water for 9 days before they were infused with solutions containing [1-13C]glucose and [2-13C]pyruvate or [U-13C3]lactate. Their brains were fixed with high-power focused microwaves and biopsies prepared from the contralateral, ipsilateral, and tumor regions. 13C NMR analysis of the extracts allowed the determination of the relative amounts of [13C]lactate isotopomers derived from infused glucose or monocarboxylates. The relative amounts of [3-13C]lactate derived from glucose to the [2-13C] or [U-13C3]lactate isotopomers derived from monocarboxylates decreased in the order contralateral > ipsilateral > tumor regions, revealing a progressive reduction in glycolysis for regions containing increasing endogenous lactate concentrations. Cortical astrocytes depicted similar reductions in glycolysis when incubated with [1,2-13C2]glucose and increasing concentrations of [3-13C]lactate. Deuterated animals bearing C6 tumors, infused with [1-13C]glucose and [2-13C]pyruvate, showed different deuterium enrichments in the methyl groups of cerebral [3-13C] and [2-13C]lactate, revealing a slow mixing of the [3-13C] and [2-13C]pyruvate precursors in the 2H exchange timescale of their methyl groups. Together, these evidences reveal the role of the redox state of the pyruvate pool derived from monocarboxylates in the modulation of cerebral glycolytic flux in vivo and in vitro.

Bidirectionality and compartmentation of metabolic fluxes are revealed in the dynamics of isotopomer networks

Isotope labeling is one of the few methods of revealing the in vivo bidirectionality and compartmentalization of metabolic fluxes within metabolic networks. We argue that a shift from steady state to dynamic isotopomer analysis is required to deal with these cellular complexities and provide a review of dynamic studies of compartmentalized energy fluxes in eukaryotic cells including cardiac muscle, plants, and astrocytes. Knowledge of complex metabolic behaviour on a molecular level is prerequisite for the intelligent design of genetically modified organisms able to realize their potential of revolutionizing food, energy, and pharmaceutical production. We describe techniques to explore the bidirectionality and compartmentalization of metabolic fluxes using information contained in the isotopic transient, and discuss the integration of kinetic models with MFA. The flux parameters of an example metabolic network were optimized to examine the compartmentalization of metabolites and and the bidirectionality of fluxes in the TCA cycle of Saccharomyces uvarum for steady-state respiratory growth.

Lipidomic analysis and electron transport chain activities in C57BL/6J mouse brain mitochondria

The objective of this study was to characterize the lipidome and electron transport chain activities in purified non-synaptic (NS) and synaptic (Syn) mitochondria from C57BL/6J mouse cerebral cortex. Contamination from subcellular membranes, especially myelin, has hindered past attempts to accurately characterize the lipid composition of brain mitochondria. An improved Ficoll and sucrose discontinuous gradient method was employed that yielded highly enriched mitochondrial populations free of myelin contamination. The activities of Complexes I, II, III, and II/III were lower in Syn than in NS mitochondria, while Complexes I/III and IV activities were similar in both populations. Shotgun lipidomics showed that levels of cardiolipin (Ptd(2)Gro) were lower, whereas levels of ceramide and phosphatidylserine were higher in Syn than in NS mitochondria. Coenzyme Q(9) and Q(10) was also lower in Syn than in NS mitochondria. Gangliosides, phosphatidic acid, sulfatides, and cerebrosides were undetectable in brain mitochondria. The distribution of Ptd(2)Gro molecular species was similar in both populations and formed a unique pattern, consisting of seven major molecular species groups, when arranged according to mass to charge ratios. Remodeling involving choline and ethanolamine phosphoglycerides could explain Ptd(2)Gro heterogeneity. NS and Syn mitochondrial lipidomic heterogeneity could influence energy metabolism, which may contribute to metabolic compartmentation of the brain.

The in-depth evaluation of suspected mitochondrial disease

Mitochondrial disease confirmation and establishment of a specific molecular diagnosis requires extensive clinical and laboratory evaluation. Dual genome origins of mitochondrial disease, multi-organ system manifestations, and an ever increasing spectrum of recognized phenotypes represent the main diagnostic challenges. To overcome these obstacles, compiling information from a variety of diagnostic laboratory modalities can often provide sufficient evidence to establish an etiology. These include blood and tissue histochemical and analyte measurements, neuroimaging, provocative testing, enzymatic assays of tissue samples and cultured cells, as well as DNA analysis. As interpretation of results from these multifaceted investigations can become quite complex, the Diagnostic Committee of the Mitochondrial Medicine Society developed this review to provide an overview of currently available and emerging methodologies for the diagnosis of primary mitochondrial disease, with a focus on disorders characterized by impairment of oxidative phosphorylation. The aim of this work is to facilitate the diagnosis of mitochondrial disease by geneticists, neurologists, and other metabolic specialists who face the challenge of evaluating patients of all ages with suspected mitochondrial disease.

Pyruvate recycling in cultured neurons from cerebellum

Pyruvate recycling is a pathway for complete oxidation of glutamate. The cellular location and the physiological significance of such recycling has been debated during the last decade. The present study was aimed at elucidating whether recycling takes place in neuron-enriched cultures of dissociated cerebella, consisting mainly of glutamatergic granule cells, some GABAergic neurons, and few astrocytes. These cultures and cultures of astrocytes from cerebellum were incubated in medium containing [U-(13)C]glutamate, and cell extracts were analyzed by gas chromatography and mass spectrometry. Additionally, in the case of the neuron-enriched cultures, a magnetic resonance (MR) spectrum was obtained. It could be shown that the atom percentage excess of the isotopomer representing pyruvate recycling in glutamate (M + 4) was similar for astrocytes and neuron-enriched cultures. However, the latter showed more recycling in glutamine (synthesized in the small fraction of astrocytes) than the pure astrocyte cultures, whereas the reverse was the case for aspartate. In fact, the atom percentage excess of the isotopomer representing pyruvate recycling in glutamine was slightly but significantly higher than that in glutamate in the neuron-enriched cultures. It can be concluded that pyruvate recycling is clearly present in neurons, and this was verified by MR spectroscopy.

The ketogenic diet and brain metabolism of amino acids: relationship to the anticonvulsant effect

In many epileptic patients, anticonvulsant drugs either fail adequately to control seizures or they cause serious side effects. An important adjunct to pharmacologic therapy is the ketogenic diet, which often improves seizure control, even in patients who respond poorly to medications. The mechanisms that explain the therapeutic effect are incompletely understood. Evidence points to an effect on brain handling of amino acids, especially glutamic acid, the major excitatory neurotransmitter of the central nervous system. The diet may limit the availability of oxaloacetate to the aspartate aminotransferase reaction, an important route of brain glutamate handling. As a result, more glutamate becomes accessible to the glutamate decarboxylase reaction to yield gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter and an important antiseizure agent. In addition, the ketogenic diet appears to favor the synthesis of glutamine, an essential precursor to GABA. This occurs both because ketone body carbon is metabolized to glutamine and because in ketosis there is increased consumption of acetate, which astrocytes in the brain quickly convert to glutamine. The ketogenic diet also may facilitate mechanisms by which the brain exports to blood compounds such as glutamine and alanine, in the process favoring the removal of glutamate carbon and nitrogen.

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.

Isoflurane strongly affects the diffusion of intracellular metabolites, as shown by 1H nuclear magnetic resonance spectroscopy of the monkey brain

Isoflurane is a volatile anesthetic commonly used for animal studies. In particular, diffusion nuclear magnetic resonance (NMR) spectroscopy is frequently performed under isoflurane anesthesia. However, isoflurane is known to affect the phase transition of lipid bilayer, possibly resulting in increased permeability to metabolites. Resulting decreased restriction may affect metabolite apparent diffusion coefficient (ADC). In the present work, the effect of isoflurane dose on metabolite ADC is evaluated using diffusion tensor spectroscopy in the monkey brain. For the five detected intracellular metabolites, the ADC exhibits a significant increase when isoflurane dose varies from 1% to 2%: 13%+/-8% for myo-inositol, 14%+/-13% for total N-acetyl-aspartate, 20%+/-18% for glutamate, 27%+/-7% for total creatine and 53%+/-17% for total choline. Detailed analysis of ADC changes experienced by the five different metabolites argues in favor of facilitated metabolite exchange between subcellular structures at high isoflurane dose. This work strongly supports the idea of metabolite diffusion in vivo being significantly restricted in subcellular structures at long diffusion time, and provides new insights for interpreting ADC values as measured by diffusion NMR spectroscopy.

Modeling of spatial metabolite distributions in the cardiac sarcomere

Although a high ATP diffusion rate implies homogeneous distribution of the principal energetic currency in the cytosol, local diffusion barriers represented by macromolecular structures can render ATP concentrations to be inhomogeneous. A method is presented here that provides apparent diffusion coefficient values in local intracellular regions and allows the estimation of spatial metabolite distribution. The apparent local diffusion coefficient for ATP in cardiac myofibrils was determined from the analysis of diffusion-dependent rightward shift of the substrate dependence for actomyosin ATPase activity using the reaction-diffusion model, which accounted for the properties of phosphotransfer reactions. This functional analysis, which took into account the local diffusional ATP delivery to the active sites, provided an apparent value that was three orders of magnitude lower than that defined by direct methods for the cytosol. The low value of the diffusion coefficient was shown to define unusual properties of the intracellular space in working heart, where small reductions in ATP levels in the surrounding cytosol result in a large drop in [ATP] inside myofibrils. This drop is critical for vital cellular functions, and the analysis presented here defines its physical basis. The diffusion barriers thus defined explain the coexistence of pathological energy deficit with almost normal average ATP levels.

Global reconstruction of the human metabolic network based on genomic and bibliomic data

Metabolism is a vital cellular process, and its malfunction is a major contributor to human disease. Metabolic networks are complex and highly interconnected, and thus systems-level computational approaches are required to elucidate and understand metabolic genotype-phenotype relationships. We have manually reconstructed the global human metabolic network based on Build 35 of the genome annotation and a comprehensive evaluation of >50 years of legacy data (i.e., bibliomic data). Herein we describe the reconstruction process and demonstrate how the resulting genome-scale (or global) network can be used (i) for the discovery of missing information, (ii) for the formulation of an in silico model, and (iii) as a structured context for analyzing high-throughput biological data sets. Our comprehensive evaluation of the literature revealed many gaps in the current understanding of human metabolism that require future experimental investigation. Mathematical analysis of network structure elucidated the implications of intracellular compartmentalization and the potential use of correlated reaction sets for alternative drug target identification. Integrated analysis of high-throughput data sets within the context of the reconstruction enabled a global assessment of functional metabolic states. These results highlight some of the applications enabled by the reconstructed human metabolic network. The establishment of this network represents an important step toward genome-scale human systems biology.

Integration of enzyme kinetic models and isotopomer distribution analysis for studies of in situ cell operation

A current trend in neuroscience research is the use of stable isotope tracers in order to address metabolic processes in vivo. The tracers produce a huge number of metabolite forms that differ according to the number and position of labeled isotopes in the carbon skeleton (isotopomers) and such a large variety makes the analysis of isotopomer data highly complex. On the other hand, this multiplicity of forms does provide sufficient information to address cell operation in vivo. By the end of last millennium, a number of tools have been developed for estimation of metabolic flux profile from any possible isotopomer distribution data. However, although well elaborated, these tools were limited to steady state analysis, and the obtained set of fluxes remained disconnected from their biochemical context. In this review we focus on a new numerical analytical approach that integrates kinetic and metabolic flux analysis. The related computational algorithm estimates the dynamic flux based on the time-dependent distribution of all possible isotopomers of metabolic pathway intermediates that are generated from a labeled substrate. The new algorithm connects specific tracer data with enzyme kinetic characteristics, thereby extending the amount of data available for analysis: it uses enzyme kinetic data to estimate the flux profile, and vice versa, for the kinetic analysis it uses in vivo tracer data to reveal the biochemical basis of the estimated metabolic fluxes.

Short-term fasting, seizure control and brain amino acid metabolism

The ketogenic diet is an effective treatment for seizures, but the mechanism of action is unknown. It is uncertain whether the anti-epileptic effect presupposes ketosis, or whether the restriction of calories and/or carbohydrate might be sufficient. We found that a relatively brief (24 h) period of low glucose and low calorie intake significantly attenuated the severity of seizures in young Sprague-Dawley rats (50-70 gms) in whom convulsions were induced by administration of pentylenetetrazole (PTZ). The blood glucose concentration was lower in animals that received less dietary glucose, but the brain glucose level did not differ from control blood [3-OH-butyrate] tended to be higher in blood, but not in brain, of animals on a low-glucose intake. The concentration in brain of glutamine increased and that of alanine declined significantly with low-glucose intake. The blood alanine level fell more than that of brain alanine, resulting in a marked increase ( approximately 50%) in the brain:blood ratio for alanine. In contrast, the brain:blood ratio for leucine declined by about 35% in the low-glucose group. When animals received [1-(13)C]glucose, a metabolic precursor of alanine, the appearance of (13)C in alanine and glutamine increased significantly relative to control. The brain:blood ratio for [(13)C]alanine exceeded 1, indicating that the alanine must have been formed in brain and not transported from blood. The elevated brain(alanine):blood(alanine) could mean that a component of the anti-epileptic effect of low carbohydrate intake is release of alanine from brain-to-blood, in the process abetting the disposal of glutamate, excess levels of which in the synaptic cleft would contribute to the development of seizures.

Isolation of mitochondria with high respiratory control from primary cultures of neurons and astrocytes using nitrogen cavitation

To study neurons or glia-specific mitochondria one needs to isolate these organelles from primary neuronal or astrocytic cell culture. This work provides novel method for isolation of functional and morphologically intact mitochondria from neurons and astrocytes in cell cultures. In the first step, mitochondria are released from cells by disruption of cell membranes using a nitrogen cavitation technique. This technique is based on rapid decompression of a cell suspension from within a pressure vessel. Mitochondria released from cell bodies are then separated from the rest of cell homogenate by Percoll gradient centrifugation. This is a relatively rapid technique that yields to very well coupled mitochondria that exhibited functional and morphological characteristics comparable to mitochondria isolated from brain tissue using common techniques. This technique thus will allow examination of mitochondria that are exclusively cell specific in origin.

In vivo magnetic resonance spectroscopy in cancer

Magnetic resonance spectroscopy (MRS) has been used for more than two decades to interrogate metabolite distributions in living cells and tissues. Techniques have been developed that allow multiple spectra to be obtained simultaneously with individual volume elements as small as 1 uL of tissue (i.e., 1 x 1 x 1 mm(3)). The most common modern applications of in vivo MRS use endogenous signals from (1)H, (31)P, or (23)Na. Important contributions have also been made using exogenous compounds containing (19)F, (13)C, or (17)O. MRS has been used to investigate cardiac and skeletal muscle energetics, neurobiology, and cancer. This review focuses on the latter applications, with specific reference to the measurement of tissue choline, which has proven to be a tumor biomarker that is significantly affected by anticancer therapies.

Brain amino acid requirements and toxicity: the example of leucine

Glutamic acid is an important excitatory neurotransmitter of the brain. Two key goals of brain amino acid handling are to maintain a very low intrasynaptic concentration of glutamic acid and also to provide the system with precursors from which to synthesize glutamate. The intrasynaptic glutamate level must be kept low to maximize the signal-to-noise ratio upon the release of glutamate from nerve terminals and to minimize the risk of excitotoxicity consequent to excessive glutamatergic stimulation of susceptible neurons. The brain must also provide neurons with a constant supply of glutamate, which both neurons and glia robustly oxidize. The branched-chain amino acids (BCAAs), particularly leucine, play an important role in this regard. Leucine enters the brain from the blood more rapidly than any other amino acid. Astrocytes, which are in close approximation to brain capillaries, probably are the initial site of metabolism of leucine. A mitochondrial branched-chain aminotransferase is very active in these cells. Indeed, from 30 to 50% of all alpha-amino groups of brain glutamate and glutamine are derived from leucine alone. Astrocytes release the cognate ketoacid [alpha-ketoisocaproate (KIC)] to neurons, which have a cytosolic branched-chain aminotransferase that reaminates the KIC to leucine, in the process consuming glutamate and providing a mechanism for the "buffering" of glutamate if concentrations become excessive. In maple syrup urine disease, or a congenital deficiency of branched-chain ketoacid dehydrogenase, the brain concentration of KIC and other branched-chain ketoacids can increase 10- to 20-fold. This leads to a depletion of glutamate and a consequent reduction in the concentration of brain glutamine, aspartate, alanine, and other amino acids. The result is a compromise of energy metabolism because of a failure of the malate-aspartate shuttle and a diminished rate of protein synthesis.

Cysteine and keto acids modulate mosquito kynurenine aminotransferase catalyzed kynurenic acid production

Kynurenine aminotransferase (KAT) catalyzes the formation of kynurenic acid (KYNA), the natural antagonist of ionotropic glutamate receptors. This study tests potential substrates and assesses the effects of amino acids and keto acids on the activity of mosquito KAT. Various keto acids, when simultaneously present in the same reaction mixture, display a combined effect on KAT catalyzed KYNA production. Moreover, methionine and glutamine show inhibitory effects on KAT activity, while cysteine functions as either an antagonist or an inhibitor depending on the concentration. Therefore, the overall level of keto acids and cysteine might modulate the KYNA synthesis. Results from this study will be useful in the study of KAT regulation in other animals.

Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought?

Functional activation of astrocytic metabolism is believed, according to one hypothesis, to be closely linked to excitatory neurotransmission and to provide lactate as fuel for oxidative metabolism in neighboring neurons. However, review of emerging evidence suggests that the energetic demands of activated astrocytes are higher and more complex than recognized and much of the lactate presumably produced by astrocytes is not locally oxidized during activation. In vivo activation studies in normal subjects reveal that the rise in consumption of blood-borne glucose usually exceeds that of oxygen, especially in retina compared to brain. When the contribution of glycogen, the brain's major energy reserve located in astrocytes, is taken into account the magnitude of the carbohydrate-oxygen utilization mismatch increases further because the magnitude of glycogenolysis greatly exceeds the incremental increase in utilization of blood-borne glucose. Failure of local oxygen consumption to equal that of glucose plus glycogen in vivo is strong evidence against stoichiometric transfer of lactate from astrocytes to neighboring neurons for oxidation. Thus, astrocytes, not nearby neurons, use the glycogen for energy during physiological activation in normal brain. These findings plus apparent compartmentation of metabolism of glycogen and blood-borne glucose during activation lead to our working hypothesis that activated astrocytes have high energy demands in their fine perisynaptic processes (filopodia) that might be met by glycogenolysis and glycolysis coupled to rapid lactate clearance. Tissue culture studies do not consistently support the lactate shuttle hypothesis because key elements of the model, glutamate-induced increases in glucose utilization and lactate release, are not observed in many astrocyte preparations, suggesting differences in their oxidative capacities that have not been included in the model. In vivo nutritional interactions between working neurons and astrocytes are not as simple as implied by "sweet (glucose-glycogen) and sour (lactate) food for thought."

GABA alters the metabolic fate of [U-13C]glutamate in cultured cortical astrocytes

The effect of gamma-aminobutyric acid (GABA) on glutamate metabolism was studied by (13)C-nuclear magnetic resonance (NMR) spectroscopy. Cerebral cortical astrocytes were incubated with 0.5 mM [U-(13)C]glutamate and 5 mM glucose in the presence or absence of 0.2 mM GABA for 2 hr. (13)C-labeled glutamate, glutamine, and aspartate were observed in cell extracts, and (13)C-labeled glutamine and lactate were present in the media. Both uniformly labeled glutamate and [1,2,3-(13)C]glutamate derived from the tricarboxylic acid (TCA) cycle were present in the cells. The consumption of [U-(13)C]glutamate and glucose was unchanged in the presence of GABA; however, the formation of [U-(13)C]lactate and [U-(13)C]aspartate from metabolism of [U-(13)C]glutamate was increased in cells incubated with GABA. The total concentration of aspartate was increased to the same extent as the (13)C-labeled aspartate, suggesting increased entry of [U-(13)C]glutamate into the TCA cycle to allow for the transamination of GABA. Although the concentrations of unlabeled glucose and lactate in the media were unchanged in the presence of GABA, the concentration of alanine was decreased, indicating that there was decreased transamination of the unlabeled pyruvate from glucose metabolism. The amount of [U-(13)C]glutamate converted to [U-(13)C]glutamine and [U-(13)C]lactate was increased in the presence of GABA. However, since the overall consumption of [U-(13)C]glutamate was not different, it can be concluded that the amount of [U-(13)C]glutamate used for energy was decreased. This suggests that exogenous GABA could substitute for glutamate as an energy source for astrocytes. The results indicate that the presence of GABA influences the metabolic fate of both glutamate and glucose in astrocytes, suggesting that fluctuations in the concentration of GABA in normal and pathological conditions can alter the compartmentation of glial metabolism in brain.

Activity of the lactate-alanine shuttle is independent of glutamate-glutamine cycle activity in cerebellar neuronal-astrocytic cultures

The glutamate-glutamine cycle describes the neuronal release of glutamate into the synaptic cleft, astrocytic uptake, and conversion into glutamine, followed by release for use as a neuronal glutamate precursor. This only explains the fate of the carbon atoms, however, and not that of the ammonia. Recently, a role for alanine has been proposed in transfer of ammonia between glutamatergic neurons and astrocytes, denoted the lactate-alanine shuttle (Waagepetersen et al. [ 2000] J. Neurochem. 75:471-479). The role of alanine in this context has been studied further using cerebellar neuronal cultures and corresponding neuronal-astrocytic cocultures. A superfusion paradigm was used to induce repetitively vesicular glutamate release by N-methyl-D-aspartate (NMDA) in the neurons, allowing the relative activity dependency of the lactate-alanine shuttle to be assessed. [(15)N]Alanine (0.2 mM), [2-(15)N]/[5-(15)N]glutamine (0.25 mM), and [(15)N]ammonia (0.3 mM) were used as precursors and cell extracts were analyzed by mass spectrometry. Labeling from [(15)N]alanine in glutamine, aspartate, and glutamate in cerebellar cocultures was independent of depolarization of the neurons. Employing glutamine with the amino group labeled ([2-(15)N]glutamine) as the precursor, an activity-dependent increase in the labeling of both glutamate and aspartate (but not alanine) was observed in the cerebellar neurons. When the amide group of glutamine was labeled ([5-(15)N]glutamine), no labeling could be detected in the analyzed metabolites. Altogether, the results of this study support the existence of the lactate-alanine shuttle and the associated glutamate-glutamine cycle. No direct coupling of the two shuttles was observed, however, and only the glutamate-glutamine cycle seemed activity dependent.

Pentylenetetrazole affects metabolism of astrocytes in culture

Cortical and cerebellar astrocytes were cultured in medium containing pentylenetetrazole (PTZ), a gamma-aminobutyric acid (GABA)(A) receptor antagonist, for 3 weeks (up to 6 mM) or 2 hr (10 mM). Cells were incubated in medium containing [U-(13)C]glutamate (0.5 mM) and unlabeled glucose (3 mM) for 2 hr and cell extracts and media were analyzed by (13)C magnetic resonance (MR) spectroscopy and high-performance liquid chromatography (HPLC). When cerebellar astrocytes were incubated with PTZ for 2 hr, the amount of glucose removed from the medium and glucose and [U-(13)C]glutamate oxidation were decreased. Metabolism in cortical astrocytes was affected only slightly; amounts of glutathione and aspartate were decreased. When cerebellar and cortical cells were cultured in the presence of PTZ for 3 weeks, the amount of glucose removed from the medium and lactate formed were increased, indicating increased glycolytic activity. Despite the increased intracellular [U-(13)C]glutamate concentration in both types of astrocytes cultured with PTZ, labeled glutamine and glutathione were unchanged, indicating intracellular compartmentation. The amount of cellular protein was decreased at 6 mM PTZ for cerebellar astrocytes and 1 mM for cortical astrocytes, indicating a differential sensitivity to the effects of PTZ. In conclusion, mitochondrial metabolism and glycolysis were decreased by short-term incubation with PTZ in cerebellar astrocytes, whereas long-term incubation affected both types of astrocytes, leading to increased glycolysis.