Astrocytic Control of Biosynthesis and Turnover of the Neurotransmitters Glutamate and GABA

Abstract

Glutamate and GABA are the quantitatively major neurotransmitters in the brain mediating excitatory and inhibitory signaling, respectively. These amino acids are metabolically interrelated and at the same time they are tightly coupled to the intermediary metabolism including energy homeostasis. Astrocytes play a pivotal role in the maintenance of the neurotransmitter pools of glutamate and GABA since only these cells express pyruvate carboxylase, the enzyme required for de novo synthesis of the two amino acids. Such de novo synthesis is obligatory to compensate for catabolism of glutamate and GABA related to oxidative metabolism when the amino acids are used as energy substrates. This, in turn, is influenced by the extent to which the cycling of the amino acids between neurons and astrocytes may occur. This cycling is brought about by the glutamate/GABA - glutamine cycle the operation of which involves the enzymes glutamine synthetase (GS) and phosphate-activated glutaminase together with the plasma membrane transporters for glutamate, GABA, and glutamine. The distribution of these proteins between neurons and astrocytes determines the efficacy of the cycle and it is of particular importance that GS is exclusively expressed in astrocytes. It should be kept in mind that the operation of the cycle is associated with movement of ammonia nitrogen between the two cell types and different mechanisms which can mediate this have been proposed. This review is intended to delineate the above mentioned processes and to discuss quantitatively their relative importance in the homeostatic mechanisms responsible for the maintenance of optimal conditions for the respective neurotransmission processes to operate.

Keywords: GABA; astrocyte; energy; glutamate; homeostasis; neurotransmitter.

Figure 1

Schematic representation of the TCA cycle and the GABA shunt which provides an alternative pathway for conversion of αKG to succinate. (*) Please note that glutamate decarboxylase is localized in the cytosol whereas all other enzymes depicted are mitochondrial. αKG, α-ketoglutarate; SSADH, succinate-semialdehyde; GABA-T, GABA-transaminase; GABA, γ-aminobutyrate; GAD, glutamate decarboxylase; Glu, glutamate.

Figure 2

Schematic drawing of the reactions involved in the biosynthesis of neurotransmitter glutamate. Reactions involve the elements of the malate-aspartate-shuttle (43), i.e., the glutamate/aspartate translocator and the dicarboxylate transporter as indicated. Notice that a significant fraction (about 50%) of glutamate derived from the reaction catalyzed by phosphate-activated glutaminase (PAG) is directly entering mitochondria presumably via the glutamate/aspartate translocater. It has also been suggested that PAG is catalytically active in the mitochondrial matrix (23, 24) although this is controversial [see text and Ref. (22)]. MDHc, cytosolic malate dehydrogenase; MDHm, mitochondrial malate dehydrogenase; AATc, cytosolic aspartate aminotransferase; AATm, mitochondrial aspartate aminotransferase; OAA, oxaloacetate; Mal, malate; Glu, glutamate; Asp, aspartate; GLN, glutamine; α-KG, α-ketoglutarate.

Figure 3

An illustration of two pathways for GABA synthesis from glutamine. The bold arrow illustrates the predominant pathway where the carbon skeleton of glutamine is metabolized via the TCA cycle prior to synthesis of GABA. The thin arrow illustrates the direct synthesis of GABA from glutamine without involvement of the TCA cycle. Note that AAT is present in both the cytosol and the mitochondrial matrix. αKG, α-ketoglutarate; PAG, phosphate-activated glutaminase; ATT, aspartate aminotransferase; GAD, glutamate decarboxylase.

Figure 4

Schematic cartoon depicting the metabolic interactions between a glutamatergic neuron, a GABAergic neuron, and a nearby astrocyte. In all three cell types, glucose (Glc) is metabolized to pyruvate via the multi-step process of glycolysis and either reduced to lactate (by lactate dehydrogenase, LDH) or oxidized to acetyl-CoA (by pyruvate dehydrogenase complex, PDH) which will subsequently be oxidized in the TCA cycle. In astrocytes, pyruvate may also undergo carboxylation to form oxaloacetate (OAA), an anaplerotic reaction catalyzed by pyruvate carboxylase (PC). At the glutamatergic synapse, glutamate (GLU) released as neurotransmitter will be taken up by nearby astrocytes and amidated to glutamine (GLN) by glutamine synthetase (GS) and returned to the neuron for re-use as neurotransmitter, the so-called glutamate-glutamine cycle. In neurons, GLU is re-formed from GLN by the mitochondrial enzyme phosphate-activated glutaminase (PAG). A similar cycle exists at the GABAergic synapse; however, the carbon skeleton of GABA enters the TCA cycle as indicated. GABA is transformed to the TCA cycle intermediate succinate via two reactions catalyzed by GABA-transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). The GABA-T catalyzed reaction produces GLU from α-KG that may then be used as precursor for GLN synthesis and eventual synthesis of GABA in the neuron. This process is known as the GABA shunt since it by-passes two reactions of the (astrocytic) TCA cycle. In GABAergic neurons, GABA is synthesized from GLU by the enzyme glutamate decarboxylase (GAD). Notice that in all three cell types, GLU is in transamination equilibrium (catalyzed by aminotransferases, AT) with α-KG linking TCA cycle metabolism with GLU and GABA homeostasis. Also notice that re-uptake of released GLU and GABA takes place to some extent as well and that the GABA shunt works in GABAergic neurons when GABA is taken up pre-synaptically.

Figure 5

The astrocytic part of the synapse provides a net synthesis of glutamine (GLN) via the concerted action of pyruvate carboxylase (PC) and pyruvate dehydrogenase (PDH) generating OAA and acetyl-CoA, the combination of which leads to synthesis of CIT. This subsequently leads to a net synthesis of α-ketoglutarate (α-KG) allowing synthesis of glutamate (GLU) catalyzed by either glutamate dehydrogenase (GDH) or an amino acid aminotransferase (AA). Glutamate is used for synthesis of glutamine (GLN) catalyzed by glutamine synthetase (GS). Glutamine is transferred to the glutamatergic neuron to be used for synthesis of glutamate catalyzed by phosphate-activated glutaminase (PAG). Released glutamate is taken up into the astrocyte and transformed into glutamine completing the glutamate-glutamine cycle. Alternatively the glutamate taken up may be oxidatively metabolized which subsequently requires de novo synthesis of glutamine via the anaplerotic processes indicated in bold arrows.

Figure 6

The extent to which glutamate (GLU) is oxidized in astrocytes seems to increase particularly during higher glutamate concentrations. A net synthesis of tricarboxylic acid (TCA) cycle intermediates occurs when the initial reaction is catalyzed by glutamate dehydrogenase (GDH) which paves the way for the complete oxidation of the carbon skeleton of glutamate. This requires pyruvate recycling via the concerted action of malic enzyme (ME) and pyruvate dehydrogenase (PDH) converting malate into acetyl CoA producing NAD(P)H. Acetyl CoA is oxidized completely in one turn of the TCA cycle. A partial oxidation of glutamate is acquired when pyruvate (PYR) is reduced to lactate (LAC) instead of being oxidized to acetyl CoA. The redox state of the cell is likely important in the regulation of the destiny of the glutamate molecule. As an alternative to the activity of GDH, aspartate aminotransferase (AAT) facilitates the formation of α-ketoglutarate (α-KG) from glutamate at the expense of oxaloacetate (OAA); thus no net synthesis of TCA cycle intermediates is obtained. In contrast to the complete oxidation initiated by the activity of GDH, AAT enables the truncated TCA cycle which refers to the net synthesis of aspartate from glutamate, a pathway shown to accelerate during hypoglycemic conditions. All enzymes except ME and LDH are located in the mitochondria. PC, pyruvate carboxylase; CIT, citrate.

Figure 7

A simplified schematic representation of glucose metabolism via glycolysis or via the “glycogen-shunt” illustrating how glucose units may be metabolized via incorporation into and subsequent hydrolysis from the branched glycogen molecule preceding metabolism to pyruvate and lactate, i.e., glycogenolysis. Glucose-6-P, glucose-6-phosphate; TCA, tricarboxylic acid; ETC, electron transport chain.