Abstract
Oligodendroglial cells are known to de-acetylate the N-acetylaspartate (NAA) synthesized and released by neurons and use it for lipid synthesis. However, the role of NAA regarding their intermediary metabolism remains poorly understood. Two hypotheses were proposed regarding the fate of aspartate after being released by de-acetylation: (1) aspartate is metabolized in the mitochondria of oligodendrocyte lineage cells; (2) aspartate is released to the medium. We report here that aspartoacylase mRNA expression increases when primary rat oligodendrocyte progenitor cells (OPCs) differentiate into mature cells in culture. Moreover, characterising metabolic functions of acetyl coenzyme A and aspartate from NAA catabolism in mature oligodendrocyte cultures after 5 days using isotope-labelled glucose after 5-days of differentiation we found evidence of extensive NAA metabolism. Incubation with [1,6-13C]glucose followed by gas chromatography–mass spectrometry and high performance liquid chromatography analyses of cell extracts and media in the presence and absence of NAA established that the acetate moiety produced by hydrolysis of NAA does not enter mitochondrial metabolism in the form of acetyl coenzyme A. We also resolved the controversy concerning the possible release of aspartate to the medium: aspartate is not released to the medium by oligodendrocytes in amounts detectable by our methods. Therefore we propose that: aspartate released from NAA joins the cytosolic aspartate pool rapidly and takes part in the malate–aspartate shuttle, which transports reducing equivalents from glycolysis into the mitochondria for ATP production and enters the tricarboxylic acid cycle at a slow rate.
Highlights
The brain is an organ with exceptionally high energy demands and relies on an uninterrupted supply of substrates for oxidative phosphorylation in mitochondria
Cell culture reagents were purchased from Sigma (Dorset, UK)—Dulbecco’s modified Eagle’s medium (DMEM), minimum essential medium Eagle (MEM), l-glutamine, poly-l-lysine (PLL), papain, NAA—or Life Technologies (Paisley, UK)—fetal bovine serum (FBS), penicillin–streptomycin, trypsin–EDTA, phosphate buffered saline (PBS). 13C-labelled compounds were obtained from Cambridge Isotope Laboratories, MA, USA
To investigate how NAA is metabolised by mature oligodendrocytes, we incubated primary oligodendrocyte progenitor cells (OPCs) cultures maintained in differentiation medium for 5 days with [1,6-13C]glucose in the presence or absence of NAA
Summary
The brain is an organ with exceptionally high energy demands and relies on an uninterrupted supply of substrates for oxidative phosphorylation in mitochondria. Around 25 % of the body’s total glucose budget is spent on processes in the brain, including the generation of action potentials and synaptic transmission [1]. Glucose-derived energy is of utmost importance for maintaining physiological function of the brain. Many neurodegenerative diseases including Alzheimer’s disease [2] are associated with compromised glucose metabolism and markers of low energy status. Quantification of N-acetylaspartate (NAA) has often been used to assess the metabolic integrity of neurons [3, 4]. NAA can be detected by 1H-magnetic resonance spectroscopy, and has been applied as a non-invasive
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