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Demonstration of pyruvate recycling in primary cultures of neocortical astrocytes but not in neurons
Authors:Waagepetersen Helle S  Qu Hong  Hertz Leif  Sonnewald Ursula  Schousboe Arne
Affiliation:(1) Department of Pharmacology, Neuroscience PharmaBiotec Research Center, Royal Danish School of Pharmacy, DK-2100 Copenhagen, Denmark;(2) Department of Clinical Neurosciences, Norwegian University of Science and Technology (NTNU), N-7489 Trondheim, Norway;(3) HK DNA Chips Ltd., Kowloon, Hong Kong, China
Abstract:Pyruvate recycling was studied in primary cultures of mouse cerebrocortical astrocytes, GABAergic cerebrocortical interneurons, and co-cultures consisting of both cell types by measuring production of [4-13C]glutamate from [3-13C]glutamate by aid of nuclear magnetic resonance spectroscopy. This change in the position of the label can only occur by entry of [3-13C]glutamate into the tricarboxylic acid (TCA) cycle, conversion of labeled agr-ketoglutarate to malate or oxaloacetate, malic enzyme-mediated decarboxylation of malate to pyruvate or phosphoenolpyruvate carboxykinase-mediated conversion of oxaloacetate to phosphoenolpyruvate and subsequent hydrolysis of the latter to pyruvate, and introduction of the labeled pyruvate into the TCA cycle, i.e., after exit of the carbon skeleton of pyruvate from the TCA cycle followed by re-entry of the same pyruvate molecules via acetyl CoA. In agreement with earlier observations, pyruvate recycling was demonstrated in astrocytes, indicating the ability of these cells to undertake complete oxidative degradation of glutamate. The recycled [4-13C]glutamate was not further converted to glutamine, showing compartmentation of astrocytic metabolism. Thus, absence of recycling into glutamine in the brain in vivo cannot be taken as indication that pyruvate recycling is absent in astrocytes. No recycling could be demonstrated in the cerebrocortical neurons. This is consistent with a previously demonstrated lack of incorporation of label from glutamate into lactate, and it also indicates that mitochondrial malic enzyme is not operational. Nor was there any indication of pyruvate recycling in the co-cultures. Although this may partly be due to more rapid depletion of glutamate in the co-cultures, this observation at the very least indicates that pyruvate recycling is not up-regulated in the neuronal-astrocytic co-cultures.
Keywords:Astrocytes  glutamate metabolism  glutamine  malic enzyme  metabolic compartmentation  neurons
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