title: Dysfunctional TCA-Cycle Metabolism in Glutamate Dehydrogenase Deficient Astrocytes.
abstract: Astrocytes take up glutamate in the synaptic area subsequent to glutamatergic transmission by the aid of high affinity glutamate transporters. Glutamate is converted to glutamine or metabolized to support intermediary metabolism and energy production. Glutamate dehydrogenase (GDH) and aspartate aminotransferase (AAT) catalyze the reversible reaction between glutamate and α-ketoglutarate, which is the initial step for glutamate to enter TCA cycle metabolism. In contrast to GDH, AAT requires a concomitant interconversion of oxaloacetate and aspartate. We have investigated the role of GDH in astrocyte glutamate and glucose metabolism employing siRNA mediated knock down (KD) of GDH in cultured astrocytes using stable and radioactive isotopes for metabolic mapping. An increased level of aspartate was observed upon exposure to [U-(13) C]glutamate in astrocytes exhibiting reduced GDH activity. (13) C Labeling of aspartate and TCA cycle intermediates confirmed that the increased amount of aspartate is associated with elevated TCA cycle flux from α-ketoglutarate to oxaloacetate, i.e. truncated TCA cycle. (13) C Glucose metabolism was elevated in GDH deficient astrocytes as observed by increased de novo synthesis of aspartate via pyruvate carboxylation. In the absence of glucose, lactate production from glutamate via malic enzyme was lower in GDH deficient astrocytes. In conclusions, our studies reveal that metabolism via GDH serves an important anaplerotic role by adding net carbon to the TCA cycle. A reduction in GDH activity seems to cause the astrocytes to up-regulate activity in pathways involved in maintaining the amount of TCA cycle intermediates such as pyruvate carboxylation as well as utilization of alternate substrates such as branched chain amino acids.
authors.affiliation: Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jakob D
authors.initials: JD
authors.lastname: Nissen
authors.affiliation: Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Kamilla
authors.initials: K
authors.lastname: Pajęcka
authors.affiliation: Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Malin H
authors.initials: MH
authors.lastname: Stridh
authors.affiliation: Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Dorte M
authors.initials: DM
authors.lastname: Skytt
authors.affiliation: Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Helle S
authors.initials: HS
authors.lastname: Waagepetersen
mesh_terms: D000818:Animals; D001219:Aspartate Aminotransferases / Q000378:metabolism; D001224:Aspartic Acid / Q000378:metabolism; D001253:Astrocytes / Q000201:enzymology*; D002245:Carbon Dioxide / Q000378:metabolism; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000378:metabolism; D002952:Citric Acid Cycle / Q000502:physiology*; D055785:Gene Knockdown Techniques; D005947:Glucose / Q000378:metabolism; D005969:Glutamate Dehydrogenase / Q000172:deficiency* / Q000235:genetics; D018698:Glutamic Acid / Q000378:metabolism; D007532:Isoleucine / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism; D051379:Mice; D034741:RNA, Small Interfering / Q000378:metabolism; D014233:Tricarboxylic Acids / Q000378:metabolism
chemical_list: D034741:RNA, Small Interfering; D014233:Tricarboxylic Acids; D007532:Isoleucine; D002245:Carbon Dioxide; D001224:Aspartic Acid; D019344:Lactic Acid; D018698:Glutamic Acid; D005969:Glutamate Dehydrogenase; D001219:Aspartate Aminotransferases; D005947:Glucose
keywords: aspartate aminotransferase; glucose; metabolism; truncated TCA cycle