Biomedical subjects

Helle S Waagepetersen

Publications and source records attributed to Helle S Waagepetersen.

At least 19 recordsLinked to original sources

PubMed2007

Glutamine in the central nervous system: function and dysfunction.

Glutamine (Gln) abounds in the central nervous system (CNS), and its interstitial and cerebrospinal fluid (CSF) concentrations are at least one order of magnitude higher than of any other amino acid. Gln transport from blood to the brain is insufficient to meet the demand of the brain tissues for this amino acid. This demand is met by intracerebral Gln synthesis from glutamate (Glu), a reaction carried out by glutamine synthetase (GS), an enzyme residing in astrocytes. A major proportion of astroglia-derived Gln is shuttled to neurons where it is degraded by phosphate-activated glutaminase (PAG) giving rise to the excitatory neurotransmitter amino acid Glu, which is also a precursor of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Glu released from neurons is taken up by astrocytes, and reconverted to Gln, closing the so called "glutamate-glutamine" cycle. A portion of Gln serves as an energy metabolite, and part of it leaves the brain to blood. Gln efflux from astrocytes, its neuronal uptake and egress to the blood via the cerebral capillary endothelial cells is mediated by different amino acid carriers showing i) considerable preference for Gln, ii) distribution between astrocytes and neurons that favors astrocyte-to-neuron fluxes of the amino acid. The Gln-specific carriers also largely contribute to Gln efflux from the brain to the vascular bed. Excessive accumulation of Gln in brain cells may be deleterious to brain function. In hyperammonemia associated with acute liver failure, excess Gln leads to cerebral edema, which largely results from its interference with mitochondrial function and partly from its osmotic action. Future analyses of the roles of Gln in both normal and abnormal cerebral metabolism and function will have to account for its newly recognized direct involvement in the regulation of gene transcription and/or translation.

Animals↗

PubMed2006

Complex glutamate labeling from [U-13C]glucose or [U-13C]lactate in co-cultures of cerebellar neurons and astrocytes.

Glutamate metabolism was studied in co-cultures of mouse cerebellar neurons (predominantly glutamatergic) and astrocytes. One set of cultures was superfused (90 min) in the presence of either [U-(13)C]glucose (2.5 mM) and lactate (1 mM) or [U-(13)C]lactate (1 mM) and glucose (2.5 mM). Other sets of cultures were incubated in medium containing [U-(13)C]lactate (1 mM) and glucose (2.5 mM) for 4 h. Regardless of the experimental conditions cell extracts were analyzed using mass spectrometry and nuclear magnetic resonance spectroscopy. (13)C labeling of glutamate was much higher than that of glutamine under all experimental conditions indicating that acetyl-CoA from both lactate and glucose was preferentially metabolized in the neurons. Aspartate labeling was similar to that of glutamate, especially when [U-(13)C]glucose was the substrate. Labeling of glutamate, aspartate and glutamine was lower in the cells incubated with [U-(13)C]lactate. The first part of the pyruvate recycling pathway, pyruvate formation, was detected in singlet and doublet labeling of alanine under all experimental conditions. However, full recycling, detectable in singlet labeling of glutamate in the C-4 position was only quantifiable in the superfused cells both from [U-(13)C]glucose and [U-(13)C]lactate. Lactate and alanine were mostly uniformly labeled and labeling of alanine was the same regardless of the labeled substrate present and higher than that of lactate when superfused in the presence of [U-(13)C]glucose. These results show that metabolism of pyruvate, the precursor for lactate, alanine and acetyl-CoA is highly compartmentalized.

Alanine↗

PubMed2006

The glutamate/GABA-glutamine cycle: aspects of transport, neurotransmitter homeostasis and ammonia transfer.

Neurons are metabolically handicapped in the sense that they are not able to perform de novo synthesis of neurotransmitter glutamate and gamma-aminobutyric acid (GABA) from glucose. A metabolite shuttle known as the glutamate/GABA-glutamine cycle describes the release of neurotransmitter glutamate or GABA from neurons and subsequent uptake into astrocytes. In return, astrocytes release glutamine to be taken up into neurons for use as neurotransmitter precursor. In this review, the basic properties of the glutamate/GABA-glutamine cycle will be discussed, including aspects of transport and metabolism. Discussions of stoichiometry, the relative role of glutamate vs. GABA and pathological conditions affecting the glutamate/GABA-glutamine cycling are presented. Furthermore, a section is devoted to the accompanying ammonia homeostasis of the glutamate/GABA-glutamine cycle, examining the possible means of intercellular transfer of ammonia produced in neurons (when glutamine is deamidated to glutamate) and utilized in astrocytes (for amidation of glutamate) when the glutamate/GABA-glutamine cycle is operating. A main objective of this review is to endorse the view that the glutamate/GABA-glutamine cycle must be seen as a bi-directional transfer of not only carbon units but also nitrogen units.

Ammonia↗

PubMed2006

Demonstration of extensive GABA synthesis in the small population of GAD positive neurons in cerebellar cultures by the use of pharmacological tools.

Cultures of dissociated cerebella from 7-day-old mice were maintained in vitro for 1-13 days. GABA biosynthesis and degradation were studied during development in culture and pharmacological agents were used to identify the enzymes involved. The amount of GABA increased, whereas that of glutamate was unchanged during the first 5 days and both decreased thereafter. The presence of aminooxyacetic acid (AOAA, 10 microM) which inhibits transaminases and other pyridoxal phosphate dependent enzymes including GABA-transaminase (GABA-T), in the culture medium caused an increase in the intracellular amount of GABA and a decrease in glutamate. The GABA content was also increased following exposure to the specific GABA-T inhibitor gamma-vinyl GABA. From day 6 in culture (day 4 when cultured in the presence of AOAA) GABA levels in the medium were increased compared to that in medium from 1-day-old cultures. Synthesis of GABA during the first 3 days was demonstrated by the finding that incubation with either [1-(13)C]glucose or [U-(13)C]glutamine led to formation of labeled GABA. Synthesis of GABA after 1 week in culture, when the enzymatic machinery is considered to be at a more differentiated level, was shown by labeling from [U-(13)C]glutamine added on day 7. Altogether the findings show continuous GABA synthesis and degradation throughout the culture period in the cerebellar neurons. At 10 microM AOAA, GABA synthesis from [U-(13)C]glutamine was not affected, indicating that transaminases are not involved in GABA synthesis and thus excluding the putrescine pathway. At a concentration of 5 mM AOAA GABA labeling was, however, abolished, showing that glutamate decarboxylase, which is inhibited at this level of AOAA, is responsible for GABA synthesis in the cerebellar cultures. In conclusion, the present study shows that GABA synthesis is taking place via GAD in a subpopulation of the cerebellar neurons, throughout the culture period.

4-Aminobutyrate Transaminase↗

PubMed2006

Glucose is necessary to maintain neurotransmitter homeostasis during synaptic activity in cultured glutamatergic neurons.

Glucose is the primary energy substrate for the adult mammalian brain. However, lactate produced within the brain might be able to serve this purpose in neurons. In the present study, the relative significance of glucose and lactate as substrates to maintain neurotransmitter homeostasis was investigated. Cultured cerebellar (primarily glutamatergic) neurons were superfused in medium containing [U-13C]glucose (2.5 mmol/L) and lactate (1 or 5 mmol/L) or glucose (2.5 mmol/L) and [U-13C]lactate (1 mmol/L), and exposed to pulses of N-methyl-D-aspartate (300 micromol/L), leading to synaptic activity including vesicular release. The incorporation of 13C label into intracellular lactate, alanine, succinate, glutamate, and aspartate was determined by mass spectrometry. The metabolism of [U-13C]lactate under non-depolarizing conditions was high compared with that of [U-13C]glucose; however, it decreased significantly during induced depolarization. In contrast, at both concentrations of extracellular lactate, the metabolism of [U-13C]glucose was increased during neuronal depolarization. The role of glucose and lactate as energy substrates during vesicular release as well as transporter-mediated influx and efflux of glutamate was examined using preloaded D-[3H]aspartate as a glutamate tracer and DL-threo-beta-benzyloxyaspartate to inhibit glutamate transporters. The results suggest that glucose is essential to prevent depolarization-induced reversal of the transporter (efflux), whereas vesicular release was unaffected by the choice of substrate. In conclusion, the present study shows that glucose is a necessary substrate to maintain neurotransmitter homeostasis during synaptic activity and that synaptic activity does not induce an upregulation of lactate metabolism in glutamatergic neurons.

Animals↗

PubMed2006

Cellular mitochondrial heterogeneity in cultured astrocytes as demonstrated by immunogold labeling of alpha-ketoglutarate dehydrogenase.

In brain cells, various metabolites and metabolic pathways, largely of mitochondrial origin, have been shown to be compartmentalized. Attention has therefore been focused on the possible existence of mitochondrial heterogeneity in the brain at the cellular level. To determine whether mitochondria in cultured cortical and cerebellar astrocytes are heterogeneous at the single cell level, immunogold electron microscopy and an antibody against the alpha-ketoglutarate dehydrogenase component of the alpha-ketoglutarate dehydrogenase complex, a marker enzyme for the tricarboxylic acid (TCA) cycle, were employed. The number of gold particles was counted in the mitochondria of 36 and 42 cells from cultured cerebellar and cortical astrocytes, respectively. A test for random distribution (Poisson distribution) of mitochondria according to the number of gold particles was subsequently performed for every one of the 36 and 42 cells as the ratio variance/mean (= index of dispersion). This should be approximately distributed as chi2/degrees of freedom (df) = n - 1, n = number of mitochondria), if the observations obeyed a Poisson distribution. For 26 of the 36 (cerebellar astrocytes) distributions and for 28 of the 42 (cortical astrocytes) distributions a random distribution had to be rejected. These findings therefore strongly indicate that alpha-ketoglutarate dehydrogenase is heterogeneously distributed in mitochondria within individual astrocytes originating either from cerebellum or cerebral cortex. In conclusion, this study underlines the probability that mitochondrial heterogeneity at the single cell level might be extended to involve other metabolic pathways and metabolites.

Algorithms↗

PubMed2006

Glial modulation of GABAergic and glutamat ergic neurotransmission.

Function of astroglia in the modulation of availability, release and clearance (inactivation) of Glu and GABA within the central nervous system is reviewed. Net synthesis of Glu through Gln synthetase exclusively localized in astrocytes can only occur by a metabolic coupling between neurons and astrocytes. Two (GLAST and GLT-1) of the five Glu transporters cloned preferentially expressed in astrocytes perform the astroglial Glu uptake of very high capacity. Moreover, astrocytes have been shown to mediate Glu release by a mechanism mimicking vesicular release. Biosynthesis of GABA in neurons is brought about by decarboxylation of Glu catalyzed by a pyridoxal phosphate requiring enzyme (GAD) that exists in two isoforms (GAD65 and GAD67) exhibiting different subcellular localization and regulatory properties. Detailed studies of GABA synthesis in GABAergic neurons using (13)C NMR spectroscopy have provided evidence for direct involvement of the tricarboxylic acid cycle. Synaptically released GABA taken up into surrounding astrocytes is converted to either CO(2) or Gln. Two reports on the release of GABA in rat dorsal root ganglia indicated that glial cells may perform GABA release as well. Gln formed from GABA in astrocytes can be transferred to GABAergic neurons and subsequently converted to GABA. Inhibition of either degradation or clearance of GABA has been successfully applied in the development of antiepileptics such as vigabatrin or tiagabine. So far, no specific strategy has been developed aimed at stimulating Glu transport.

Animals↗

PubMed2005

Neuronal and astrocytic shuttle mechanisms for cytosolic-mitochondrial transfer of reducing equivalents: current evidence and pharmacological tools.

The malate-aspartate shuttle and the glycerol phosphate shuttle act to transfer reducing equivalents from NADH in the cytosol to the mitochondria since the inner mitochondrial membrane is impermeable to NADH and NAD+. This transfer of reducing equivalents is essential for maintaining a favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of neurotransmitters in brain. There is evidence that both the malate-aspartate shuttle and glycerol phosphate shuttle function in brain; however, there is controversy about the relative importance and cellular localization of these shuttles. The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons and may be extremely low, or even non-existent in brain astrocytes. Several studies provide evidence of glycerol phosphate shuttle activity in brain cells; however, the activity of this shuttle in brain has been questioned. A number of pharmacological tools, including aminooxyacetic acid, beta-methyleneaspartate, phenylsuccinate, and 3-nitropropionic acid, have been used to inhibit the four enzymes and two carrier proteins that participate in the malate-aspartate shuttle. Although no drugs completely inhibit the glycerol phosphate shuttle, evidence for the existence of this shuttle is provided by studies using drugs to inhibit the malate-aspartate shuttle. This report evaluates the evidence for each shuttle in brain cells and the drugs that can be used as pharmacological tools to study these shuttles.

Animals↗

PubMed2005

Role of astrocytes in glutamate homeostasis: implications for excitotoxicity.

Glutamate homeostasis in the brain is maintained by its well balanced release, uptake and metabolism. It appears that astrocytes play a prominent role in this context since they possess a very powerful battery of glutamate transporters. Thus, malfunction of astrocytic glutamate transporters will lead to an excessively high extracellular glutamate concentration which may result in neurodegeneration caused by the excitotoxic action of glutamate.

Animals↗

PubMed2005

Compartmentation of lactate originating from glycogen and glucose in cultured astrocytes.

Brain glycogen metabolism was investigated by employing isofagomine, an inhibitor of glycogen phosphorylase. Cultured cerebellar and neocortical astrocytes were incubated in medium containing [U-(13C)]glucose in the absence or presence of isofagomine and the amounts and percent labeling of intra- and extracellular metabolites were determined by mass spectrometry (MS). The percent labeling in glycogen was markedly decreased in the presence of isofagomine. Surprisingly, the percent labeling of intracellular lactate was also decreased demonstrating the importance of glycogen turnover. The decrease was limited to the percent labeling in the intracellular pool of lactate, which was considerably lower compared to that observed in the medium in which it was close to 100%. These findings indicate compartmentation of lactate derived from glycogenolysis and that derived from glycolysis. Inhibiting glycogen degradation had no effect on the percent labeling in citrate. However, the percent labeling of extracellular glutamine was slightly decreased in neocortical astrocytes exposed to isofagomine, indicating an importance of glycogen turnover in the synthesis of releasable glutamine. In conclusion, the results demonstrate that glycogen in cultured astrocytes is continuously synthesized and degraded. Moreover, it was found that lactate originating from glycogen is compartmentalized from that derived from glucose, which lends further support to a compartmentalized metabolism in astrocytes.

Amino Acids↗

PubMed2005

Role of glutamine and neuronal glutamate uptake in glutamate homeostasis and synthesis during vesicular release in cultured glutamatergic neurons.

Glutamate exists in a vesicular as well as a cytoplasmic pool and is metabolically closely related to the tricarboxylic acid (TCA) cycle. Glutamate released during neuronal activity is most likely to a large extent accumulated by astrocytes surrounding the synapse. A compensatory flux from astrocytes to neurons of suitable precursors is obligatory as neurons are incapable of performing a net synthesis of glutamate from glucose. Glutamine appears to play a major role in this context. Employing cultured cerebellar granule cells, as a model system for glutamatergic neurons, details of the biosynthetic machinery have been investigated during depolarizing conditions inducing vesicular release. [U-13C]Glucose and [U-13C]glutamine were used as labeled precursors for monitoring metabolic pathways by nuclear magnetic resonance (NMR) spectroscopy and liquid chromatography-mass spectrometry (LC-MS) technologies. To characterize release mechanisms and influence of glutamate transporters on maintenance of homeostasis in the glutamatergic synapse, a quantification was performed by HPLC analysis of the amounts of glutamate and aspartate released in response to depolarization by potassium (55 mM) in the absence and presence of DL-threo-beta-benzyloxyaspartate (TBOA) and in response to L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC), a substrate for the glutamate transporter. Based on labeling patterns of glutamate the biosynthesis of the intracellular pool of glutamate from glutamine was found to involve the TCA cycle to a considerable extent (approximately 50%). Due to the mitochondrial localization of PAG this is unlikely only to reflect amino acid exchange via the cytosolic aspartate aminotransferase reaction. The involvement of the TCA cycle was significantly lower in the synthesis of the released vesicular pool of glutamate. However, in the presence of TBOA, inhibiting glutamate uptake, the difference between the intracellular and the vesicular pool with regard to the extent of involvement of the TCA cycle in glutamate synthesis from glutamine was eliminated. Surprisingly, the intracellular pool of glutamate was decreased after repetitive release from the vesicular pool in the presence of TBOA indicating that neuronal reuptake of released glutamate is involved in the maintenance of the neurotransmitter pool and that 0.5 mM glutamine exogenously supplied is inadequate to sustain this pool.

Amino Acid Transport System X-AG↗

PubMed2004

Role of astrocytes in depolarization-coupled release of glutamate in cerebellar cultures.

Release of preloaded D-[3H]aspartate in response to depolarization induced by high potassium, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or the endogenous agonist glutamate was studied using cultured glutamatergic cerebellar granule neurons, cerebellar astrocytes, and corresponding cocultures. Release from the vesicular and the cytoplasmic glutamate pools, respectively, was distinguished employing the competitive, non-transportable glutamate transport inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA). The results indicate that the release in response to AMPA (30 microM) in the presence of cyclothiazide (50 microM) to block desensitization, was of a vesicular origin. Pulses of 55 mM K+ caused a DL-TBOA resistant efflux of preloaded D-[3H]aspartate from astrocytes, indicating that this release was not mediated by glutamate transporters. The results furthermore support the notion of an important function of the astrocytes in the uptake of released glutamate, because DL-TBOA caused a large, apparent increase in the depolarization-coupled release of preloaded D-[3H]aspartate in the cocultures, compared to neuronal monocultures.

Animals↗

PubMed2003

Compartmentation of glutamine, glutamate, and GABA metabolism in neurons and astrocytes: functional implications.

The classical concept regarding compartmentation of brain metabolism pertinent to the two neurotransmitter amino acids, glutamate and GABA (gamma-aminobutyrate), operates with different pools of glutamate and glutamine in different cell types, that is, pools that have different sizes and turnover rates. As a result of more sophisticated technology (e.g., nuclear magnetic resonance spectroscopy and mass spectrometry used in relation to cultured neurons and astrocytes), a more complex scenario is emerging. Hence, both neurons and astrocytes exhibit a compartmentalized metabolism that very likely relates to individual cells containing mitochondrial populations having different metabolic roles. Models for this in neurons and astrocytes, respectively, are presented. The functional implications of this for the homeostatic mechanisms regulating the levels of neurotransmitter glutamate and GABA are discussed in relation to development of therapeutic strategies for neurological disorders in which these transmitters are believed to play important roles.

Animals↗

PubMed2002

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.

Animals↗

PubMedDate not supplied

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.

Alanine↗

PubMedDate not supplied

Astrocyte glycogen metabolism is required for neural activity during aglycemia or intense stimulation in mouse white matter.

We tested the hypothesis that inhibiting glycogen degradation accelerates compound action potential (CAP) failure in mouse optic nerve (MON) during aglycemia or high-intensity stimulation. Axon function was assessed as the evoked CAP, and glycogen content was measured biochemically. Isofagomine, a novel inhibitor of central nervous system (CNS) glycogen phosphorylase, significantly increased glycogen content under normoglycemic conditions. When MONs were bathed in artificial cerebrospinal fluid (aCSF) containing 10 mM glucose, the CAP failed 16 min after exposure to glucose-free aCSF. MONs bathed in aCSF plus isofagomine displayed accelerated CAP failure on glucose removal. Similar results were obtained in MONs bathed in 30 mM glucose, which increased baseline glycogen concentration. The ability of isofagomine to increase glycogen content thus was not translated into delayed CAP failure. This is likely due to the inability of the tissue to metabolize glycogen in the presence of isofagomine, highlighting the importance of glycogen in sustaining neural function during aglycemia. The hypothesis that glycogen breakdown supports intense neural activity was tested by blocking glycogen breakdown during periods of high-frequency stimulation. The CAP area declined more rapidly when glycogen metabolism was inhibited by isofagomine, explicitly showing an important physiological role for glycogen metabolism during neural activity.

Action Potentials↗

PubMedDate not supplied

Intracellular metabolic compartmentation assessed by 13C magnetic resonance spectroscopy.

Our understanding of the brain has developed from the theory that it is one continuous cell to the knowledge that there are many brain cells originally termed neurons and, furthermore to the discovery of glial cells and their multiple functions. Thus, an increasing complexity was unraveled and we have not reached a complete understanding of the phenomenon which comprises the compartmentation of metabolic pathways and metabolites. This is an important principle needed to fully understand the metabolic processes of the brain. At the cellular level this concept is well established whereas intracellular compartmentation has yet to be explored. Using magnetic resonance spectroscopy (MRS) for analysis of isotopomer composition combined with quantification of amino acid contents it is possible to construct models that describe intracellular compartmentation. Results of studies of cultures of astrocytes and neurons incubated in media containing [U- 13C]glutamate in the presence or absence of thiopental may be used to propose an intracellular three compartment model of mitochondrial function. Due to the experimental paradigm only certain aspects of metabolism can be described. The present model consists of compartments assigned to CO(2) production, glutamate synthesis from ketoglutarate and finally synthesis of a four-carbon metabolite which is shuttled between compartments. It is likely that metabolism may be far more complex than this and we are only beginning to glimpse some aspects of compartmentation at the cellular level.

Animals↗

PubMedDate not supplied

Differential roles of alanine in GABAergic and glutamatergic neurons.

Studies in different preparations of neurons and astrocytes of alanine transport and activities of its metabolizing enzyme alanine aminotransferase have led to the proposal that this amino acid is preferentially synthesized in astrocytes and transferred from the astrocytic to the neuronal compartment. From a functional point of view this may well be the case in a GABAergic synapse since theoretically alanine can be utilized as a metabolic fuel in GABAergic neurons where the GABA shunt is operating. Thus, a metabolic scheme is proposed, according to which alanine catabolism is coupled to the TCA cycle where the GABA shunt replaces the alpha-ketoglutarate dehydrogenase/succinyl CoA synthetase reactions. In a glutamatergic synapse in which the large demand for synthesis of neurotransmitter glutamate leads to a large production of ammonia, it is possible that alanine could play a completely different role. Hence, experimental evidence is reviewed suggesting that alanine may serve as a carrier of ammonia nitrogen from the neuronal compartment to the astrocytic compartment using a flux of lactate in the opposite direction to account for transfer of the C-3 carbon skeleton.

Alanine↗