Distribution of the branched-chain α-ketoacid dehydrogenase complex E1α subunit and glutamate dehydrogenase in the human brain and their role in neuro-metabolism

Abstract

Glutamate is the major excitatory neurotransmitter of the central nervous system, with the branched-chain amino acids (BCAAs) acting as key nitrogen donors for de novo glutamate synthesis. Despite the importance of these major metabolites, their metabolic pathway in the human brain is still not well characterised. The metabolic pathways that influence the metabolism of BCAAs have been well characterised in rat models. However, the expression of key proteins such as the branched-chain α-ketoacid dehydrogenase (BCKD) complex and glutamate dehydrogenase isozymes (GDH) in the human brain is still not well characterised. We have used specific antibodies to these proteins to analyse their distribution within the human brain and report, for the first time, that the E1α subunit of the BCKD is located in both neurons and vascular endothelial cells. We also demonstrate that GDH is localised to astrocytes, although vascular immunolabelling does occur. The labelling of GDH was most intense in astrocytes adjacent to the hippocampus, in keeping with glutamatergic neurotransmission in this region. GDH was also present in astrocyte processes abutting vascular endothelial cells. Previously, we demonstrated that the branched-chain aminotransferase (hBCAT) proteins were most abundant in vascular cells (hBCATm) and neurons (hBCATc). Present findings are further evidence that BCAAs are metabolised within both the vasculature and neurons in the human brain. We suggest that GDH, hBCAT and the BCKD proteins operate in conjunction with astrocytic glutamate transporters and glutamine synthetase to regulate the availability of glutamate. This has important implications given that the dysregulation of glutamate metabolism, leading to glutamate excitotoxicity, is an important contributor to the pathogenesis of several neurodegenerative conditions such as Alzheimer's disease.

Severe Spinal Cord Injury in Rats Induces Chronic Changes in the Spinal Cord and Cerebral Cortex Metabolism, Adjusted by Thiamine That Improves Locomotor Performance

“…Physiological importance of the brain metabolism of glutamate after SCI is supported by the most significant and strongest correlation of the cortex glutamate level with the BBB score, added by the positive correlation trend of BBB with the glutamate-generated GABA. The trends of positive correlations of BBB score with the branched chain amino acids leucine and isoleucine agree with the known role of these amino acids in supporting both the TCA cycle flux at insufficient OGDHC function (Santos et al, 2006;Trofimova et al, 2012) and the glutamate homeostasis during the neurotransmission (Yudkoff, 2017;Hull et al, 2018).…”

Alzheimer’s disease: targeting the glutamatergic system

“…Until the study by Helms et al (2012), the potential for Glu metabolism within endothelial cells was not considered. Our group have shown that GDH and the BCKD complex are also expressed in endothelial cells, indicating that Glu oxidation can occur and will dependent on concentration and cellular redox state (Hull et al 2018). The role of BCATm in endothelial cells has yet to be confirmed but most likely will be important in regulating brain glutamate through metabolism through the BCATm/ BCKD/GDH metabolon or additional cellular roles yet to be defined.…”

Role of branched‐chain amino acid–catabolizing enzymes in intertissue signaling, metabolic remodeling, and energy homeostasis

“…The tissue distribution of total BCKDH activity in humans was reported to be 164.5 6 18 mU per gram wet weight, with kidneys showing the highest total activity, small intestine the lowest, and undetectable activity in the pancreas (22). BCKDH is most active in the liver and brain and shows about half as much activity in the kidney and heart (2, 3) ( Table 2), with the E1-a subunit located in the neurons as well as in the endothelial cells throughout the brain vasculature (24). Unlike BCAT, BCKDH activity is significantly low in resting skeletal muscle (2), with the majority of the enzyme complex residing in its inactive, phosphorylated state (33).…”

“…Notably, BCAA oxidation is significantly less than BCAA transamination, suggesting efficient amino group transfer from BCAAs to generate alanine and glutamine, important gluconeogenic substrates in the liver and kidney, respectively (23). In the human neurons, BCATm forms a complex with the BCKA dehydrogenase (BCKDH) E1-a subunit and glutamate dehydrogenase 1 (GDH1) (24), resulting in a 12-fold increase in BCKA decarboxylation (24). GDH1 protein subsequently binds to the pyridoxamine-59-phosphate form of BCATm, thus replenishing the a-ketoglutarate (24).…”