RESEARCH ARTICLE Expression of the Human Isoform of Glutamate Dehydrogenase, hGDH2, Augments TCA Cycle Capacity and Oxidative Metabolism of Glutamate During Glucose Deprivation in Astrocytes Jakob D. Nissen, 1 Kasper Lykke, 1 Jaroslaw Bryk, 2 Malin H. Stridh, 1 Ioannis Zaganas, 3 Dorte M. Skytt, 1 Arne Schousboe, 1 Lasse K. Bak, 1 Wolfgang Enard, 2 Svante P€ a€ abo, 2 and Helle S. Waagepetersen 1 A key enzyme in brain glutamate homeostasis is glutamate dehydrogenase (GDH) which links carbohydrate and amino acid metabolism mediating glutamate degradation to CO 2 and expanding tricarboxylic acid (TCA) cycle capacity with intermediates, i.e. anaplerosis. Humans express two GDH isoforms, GDH1 and 2, whereas most other mammals express only GDH1. hGDH1 is widely expressed in human brain while hGDH2 is confined to astrocytes. The two isoforms display different enzymatic properties and the nature of these supports that hGDH2 expression in astrocytes potentially increases glutamate oxidation and supports the TCA cycle during energy-demanding processes such as high intensity glutamatergic signaling. However, little is known about how expression of hGDH2 affects the handling of glutamate and TCA cycle metabolism in astrocytes. Therefore, we cultured astrocytes from cerebral cortical tissue of hGDH2-expressing transgenic mice. We measured glutamate uptake and metabolism using [ 3 H]glutamate, while the effect on metabolic pathways of glutamate and glucose was evaluated by use of 13 C and 14 C substrates and analysis by mass spectrometry and determination of radioactively labeled metabolites including CO 2 , respective- ly. We conclude that hGDH2 expression increases capacity for uptake and oxidative metabolism of glutamate, particularly dur- ing increased workload and aglycemia. Additionally, hGDH2 expression increased utilization of branched-chain amino acids (BCAA) during aglycemia and caused a general decrease in oxidative glucose metabolism. We speculate, that expression of hGDH2 allows astrocytes to spare glucose and utilize BCAAs during substrate shortages. These findings support the proposed role of hGDH2 in astrocytes as an important fail-safe during situations of intense glutamatergic activity. GLIA 2017;65:474–488 Key words: anaplerosis, isotopic labeling, amino acid, hypoglycemic, brain Introduction M aintenance of glutamate homeostasis is essential to brain function and to avoid excitotoxicity (Anderson and Swanson, 2000). Astrocytes take up the major part of neuro- transmitter glutamate from the synapse in which glutamate may be converted to glutamine or enter the mitochondria for View this article online at wileyonlinelibrary.com. DOI: 10.1002/glia.23105 Published online in Wiley Online Library (wileyonlinelibrary.com). Received Aug 19, 2016, Accepted for publication Nov 30, 2016. Address correspondence to Helle S. Waagepetersen, Department of Drug Design and Pharmacology, Faculty of Health and Medical Science, University of Copenhagen, Universitetsparken 2, 2100 Copenhagen, Denmark. E-mail: helle.waagepetersen@sund.ku.dk Jaroslaw Bryk is currently at School of Applied Sciences, University of Huddersfield, Queensgate, HD1 3DH, Huddersfield, United Kingdom Dorte M. Skytt is currently at Department of Neuroscience and Pharmacology, the Panum Institute, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen 2200, Denmark Wolfgang Enard is currently at Department of Biology, II Ludwig Maximilian University, Munich, Martinsried 82152, Germany From the 1 Department of Drug Design and Pharmacology, Faculty of Health and Medical Science, University of Copenhagen, Copenhagen 2100, Denmark; 2 Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Anthropology, Leipzig 02109, Germany; 3 Neurology Laboratory, School of Health Sciences, Faculty of Medicine, University of Crete, Heraklion, Crete, Greece 474 V C 2016 Wiley Periodicals, Inc.