Synthesis of vesicular GABA from glutamine involves TCA cycle metabolism in neocortical neurons

Abstract

In contrast to the classic concept of direct conversion of glutamine to gamma-aminobutyric acid (GABA; via glutamate), this process may involve alpha-ketoglutarate as an intermediary metabolite and tricarboxylic acid (TCA) cycle activity. To obtain information about a possible differential role of these pathways for the synthesis of cytosolic and vesicular GABA, cultured neocortical neurons were incubated in medium containing [U-(13)C]glucose (0.5 mM) and in some cases unlabeled glutamine (0.5 mM). Subsequently, the cells were "superfused" for investigation of the effect of depolarization by 55 mM K+. To make sure that depolarization by 55 mM K+ released only vesicular GABA, tiagabin, a nontransportable inhibitor of the plasma membrane GABA carriers, was included in the medium to prevent GABA release from the cytoplasmic pool by reversal of the carriers. The importance of the TCA cycle for conversion of the carbon skeleton of glutamine to GABA was evident from the effect of glutamine on the labeling pattern of GABA. Percentage of labeling by GABA released into the depolarizing medium was the same as that in the corresponding cell extracts and was unaffected by the presence of glutamine during incubation. Despite the existence of multiple forms of glutamate decarboxylase, compartmentation of glutamate pools, and functionally different compartments within neurons, there appears to be full equilibration between the vesicular and cytosolic pools of GABA. However, during depolarization, the newly synthesized pool of GABA from glutamine does not rapidly equilibrate with the vesicular pool.

Expression of glutamine transporter isoforms in cerebral cortex of rats with chronic hepatic encephalopathy

“…In line with these findings, we have previously reported that the synthesis of neurotransmitter GABA is altered during chronic HE (Leke et al, 2011a), albeit no change was observed in the expression of GAD65, which is the GAD isoform closely associated with the biosynthesis of GABA in the vesicular neurotransmitter pool Waagepetersen et al, 1999Waagepetersen et al, , 2001Walls et al, 2011). Considering that glutamine is important in the restoration of GABA pools and that synthesis of this amino acid is disturbed during HE, it is possible that during this neurologic disorder the expression of the glutamine transporter isoforms might be altered which would consequently affect the function of the glutamate/GABA-glutamine cycle and thus likely contribute to the observed disturbance of GABA neurotransmitter synthesis (for further discussion, see Walls et al, 2015).…”

“…In this context it is of interest that using bileduct ligated rats (BDL), an experimental model of chronic HE, it has been demonstrated that the biosynthetic pathway for GABA was altered to occur preferentially via the tricarboxylic acid (TCA) cycle relative to the direct decarboxylation of glutamate not involving the TCA cycle (Leke et al, 2011a). However, no differences in the gene expression were found for the glutamate decarboxylase (GAD) enzyme isoforms GAD65 and GAD67 , which have different roles in the two GABA biosynthetic pathways mentioned earlier (Waagepetersen et al, 1999(Waagepetersen et al, , 2001Walls et al, 2011). Therefore, it can be hypothesized that glutamine transfer between astrocytes and neurons may be altered leading to changes in the biosynthesis of neurotransmitter GABA.…”

Ischemic Cerebral Damage

“…A high density of mitochondria has been found around the synaptic cleft. 23,24 Besides the fact that critical steps in the metabolism of the neurotransmitters glutamate and GABA are located in the mitochondrial tricarboxylic acid cycle, 25 this high concentration of mitochondria probably reflects high energy consumption.…”

Succinic Semialdehyde Dehydrogenase: Biochemical–Molecular–Clinical Disease Mechanisms, Redox Regulation, and Functional Significance

“…This was observed in whole brain homogenates, as well as in extracts from different brain regions (including frontal and parietal cortices, hippocampus and cerebellum). Glutamine is a major storage form and intermediate for both GABA and glutamate, and the metabolism of this species is well-regulated through localization of specific synthetic=degradative enzymes in various neural intracellular compartments (193). For example, glutamine synthase is localized specifically to astrocytes, whereas the glutamate forming enzyme, glutaminase (converting glutamine to glutamate), is regionalized to the neuronal terminals (71).…”