(13)C NMR spectroscopy applications to brain energy metabolism

Abstract

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

Keywords: 13C NMR spectroscopy; astrocyte; brain metabolism; hyperpolarized NMR; metabolic modeling; neuroglial coupling; neuron.

FIGURE 1

Transfer of individual ¹³C nuclei during the main steps of oxidative metabolism. (A) Glycolysis; (B) acetyl-CoA synthesis through pyruvate dehydrogenase (PDH) and TCA cycle; (C) pyruvate carboxylase (PC). AcCoA, acetyl-CoA; αKG, α-ketoglutarate; Asp, aspartate; Glu, glutamate; OAA, oxaloacetate; Suc, succinate.

FIGURE 2

Metabolic models consist in an ensemble of metabolic pools, connected by metabolic fluxes respecting the steady-state condition for metabolic pools.(A) Example of a simple metabolic model used to illustrate how to write ¹³C-labeling equations; metabolic pools A and B yield C at rates V₁ and V₂, respectively; (B) a “classical” two-compartment metabolic model (see main text for model’s description). αKG, α-ketoglutarate; Asp, aspartate; Glc, glucose; Gln, glutamine; Glu, glutamate; Lac, lactate; OAA, oxaloacetate; Pyr, pyruvate; Suc, succinate.

FIGURE 3

The traditional metabolic coupling hypothesis between neurons and astrocytes during glutamatergic neurotransmission. Glutamate released to the synaptic cleft during glutamatergic neurotransmission is co-transported with Na⁺ to the astrocytes. Astrocytic Na⁺ is exchanged by extracellular K⁺ through the Na⁺/K⁺ ATPase, consuming one ATP molecule. Astrocytic glutamate produces glutamine through glutamine synthetase, consuming one additional ATP molecule. Lactate produced exclusively in astrocytic glycolysis to support these energy demands, is extruded to the extracellular medium, taken up by the surrounding neurons and oxidized as their main metabolic fuel. Note the apparent stoichiometric coupling between glutamate–glutamine cycling and glucose uptake as well as the exclusive glycolytic or oxidative metabolisms in astrocytes and neurons, respectively. Gln, glutamine; Glu, glutamate; GLUT 1 and GLUT 3, glucose transporters 1 and 3; Lac, lactate; MCT1 and MCT2, monocarboxylate transporters 1 and 2; PGK, phosphoglycerate kinase; Pyr, pyruvate. Adapted with permission from Tsacopoulos and Magistretti (1996).

FIGURE 4

The subcellular compartmentation of pyruvate and glutamate and the redox switch/redox coupling hypothesis. Two pools ofpyruvate exist in neurons and astrocytes derived from extracellular monocarboxylates (Pp) or glucose (Pg). A lactate/pyruvate redox shuttle is able to transfer continuously lactate from astrocytes to neurons, taking advantage of the kinetics of plasma membrane transporters and lactate dehydrogenase isoenzymes. High cytosolic lactate concentration inhibits neuronal glycolysis at the glyceraldehyde-3-phosphate dehydrogenase step by competition with cytosolic NAD⁺, favoring the oxidation of extracellular Lac. Neuronal pyruvate is transferred back to the astrocyte to close the transcellular exchange of reducing equivalents. Two α-ketoglutarate/glutamate pools exist in neurons and astrocytes, associated probably to cytosolic and mitochondrial compartments. Exchange of α-ketoglutarate/glutamate between mitochondria and cytosol appears to be slow in the H3 glutamate hydrogen exchange timescale and dependent of the cytosolic and mitochondrial NAD(P)⁺/NAD(P)H ratios, as determined by the malate–aspartate shuttle. Both glycolysis and oxidative astrocytic metabolism contribute the energy for glutamine production in the astrocytes, indicating that this coupling involves both transcellular and intracellular redox coupling mechanisms that allow the simultaneous operation of glycolysis and oxidation in astrocytes. Asp, aspartate; Glc, glucose; Gln, glutamine; Glu, glutamate; GLUT1 and GLUT3, glutamate transporters 1 and 3; α-KG, α-ketoglutarate; Lac, lactate; LDH1 and LDH5, lactate dehydrogenase 1 and 5; Mal, malate. Reproduced with permission from Rodrigues etal. (2012).