Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

Abstract

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Integration between Glycolysis and Glutamate-Glutamine Cycle Flux May Explain Preferential Glycolytic Increase during Brain Activation, Requiring Glutamate

“…Under resting conditions brain metabolism in the adult brain occurs almost exclusively as complete oxidative metabolism of glucose (Siesjö, 1978 ; Shulman et al, 2001 ; Patel et al, 2014 ) in both neurons and astrocytes, which show similar rates of oxidative metabolism of glucose (reviewed by Hertz, 2011 ). These two cell types are connected by the glutamate-glutamine cycle, which carries glutamate synthesized in astrocytes to neurons to cover their entire supply of transmitter glutamate and GABA, which cannot be synthesized in neurons (Bringmann et al, 2013 ; Schousboe et al, 2013 ; Hertz and Rothman, 2016 , 2017 ). This cycle provides a prime example how specific metabolic processes in different brain cell types are integrated to enable complex behaviors, and it will be described in more detail later.…”

“…Shulman et al ( 2001 ) were the first to consider that the integration between glycolysis and the glutamate-glutamine cycle might play an important role in the reduction of OGI. As shown in Figure 2 approximately one glucose molecule is known to be degraded during the flux of the glutamate-glutamine cycle at all levels of its activity between deep anesthesia and wake activity (Sibson et al, 1998 ; Hertz and Rothman, 2017 ).…”

“…Glutamate re-accumulation occurs almost exclusively in astrocytes (Danbolt et al, 2016 ). Its oxidative metabolism via αKG gives rise to the second oxidation in the TCA cycle (Figure 3 ) where αKG is decarboxylated to 4-carbon compounds, including malate, which can leave the TCA cycle and be converted by malic enzyme to pyruvate which is further metabolized (Hertz and Rothman, 2016 , 2017 ). This process provides a substantial amount of energy (Hertz et al, 2007 ) without any utilization of glucose.…”

“…Glutamate metabolism may thus contribute to the increased OGI found by Madsen et al ( 1999 ) after the stimulation and probably also to the reduced increase in OGI found by several authors during long-lasting stimulation. At the same time the content of aspartate is reduced (Mangia et al, 2007 , 2012 ), reflecting that glutamate oxidation via aspartate aminotransferase produces aspartate (Hertz and Rothman, 2017 ). Increased glutamate content suggests that de novo astrocytic glutamate production is increased and thus contributes more to glycolysis than under resting conditions when de novo synthesis and degradation of glutamate are similar.…”

The Pivotal Role of the Glutamate - glutamine Cycle in Minimal Hepatic Encephalopathy. An experimental magnetic resonance spectroscopy study

“…Glutamate is the major mediator of excitatory signals as well as of nervous system plasticity, including cell elimination, and is continuously being released to the extracellular fluid [10]. Neurons cannot synthesize glutamate, however, astrocytes can accordingly synthesize glutamate and transport it, together with accumulated neuronal-released glutamate and GABA, after conversion to glutamine to neurons in a very active pathway: the glutamine-glutamate cycle [11]. Glutamate is converted to glutamine in the astrocytes by glutamine synthetase, an enzyme specific to astrocytes and absent in the neurons [12].…”