Oxidation of [U-13 C]glucose in the human brain at 7T under steady state conditions

Abstract

Purpose: Disorders of brain energy metabolism and neurotransmitter recycling have been implicated in multiple neurological conditions. ¹³ C magnetic resonance spectroscopy (¹³ C MRS) during intravenous administration of ¹³ C-labeled compounds has been used to measure turnover rates of brain metabolites. This approach, however, requires prolonged infusion inside the magnet. Proton decoupling is typically required but may be difficult to implement with standard equipment. We examined an alternative approach to monitor glucose metabolism in the human brain.

Methods: ¹³ C-enriched glucose was infused in healthy subjects outside the magnet to a steady-state level of ¹³ C enrichment. Subsequently, the subjects were scanned at 7T for 60 min without ¹ H decoupling. Metabolic modeling was used to calculate anaplerosis.

Results: Biomarkers of energy metabolism and anaplerosis were detected. The glutamate C5 doublet provided information about glucose-derived acetyl-coenzyme A flux into the tricarboxylic acid (TCA) cycle via pyruvate dehydrogenase, and the bicarbonate signal reflected overall TCA cycle activity. The glutamate C1/C5 ratio is sensitive to anaplerosis.

Conclusion: Brain ¹³ C MRS at 7T provides information about glucose oxidation and anaplerosis without the need of prolonged ¹³ C infusions inside the scanner and without technical challenges of ¹ H decoupling, making it a feasible approach for clinical research. Magn Reson Med 78:2065-2071, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: 13C MRS; 13C bicarbonate; 7 Tesla; TCA cycle; brain energy metabolism.

Figure 1

A) Proton image showing the occipital brain area investigated by ¹³C MRS. B) Representative post-infusion proton-coupled ¹³C spectrum (in red) of a healthy subject showing detection of infused [U-¹³C]glucose products; same experiment with no glucose infusion (in blue); (Zoom-in) The glutamate C5 singlet, S, and doublet D45 fitting. The high SNR bicarbonate signal at 160.8ppm is a prominent global biomarker for TCA-cycle activity.

Figure 2

Simplified model of the TCA cycle showing the sources of ¹³C bicarbonate and site of entry of ¹²CO₂. In the presence of [U-¹³C]glucose, acetyl-CoA is either unlabeled or [1,2-¹³C]acetyl-CoA. If pyruvate carboxylation is inactive (anaplerosis = 0) and the CO₂ pool is not ¹³C enriched, then the glutamate C1/C5 = 1 regardless of the fractional enrichment in acetyl-CoA. If pyruvate carboxylation is active, the C5 of glutamate remains labelled (carbons in red) equal to the ¹³C enrichment in acetyl-CoA. However, the C1 labelling in glutamate is diluted by unlabeled carbon (in white) that enters the cycle at the point of pyruvate carboxylation. Consequently, C1/C5 must equal 1 in neurons since pyruvate carboxylation is 0 in that compartment. Thus the measured ratio of C1/C5 of glutamate is less than 1 when anaplerosis (y) is active in astroglia. This figure illustrates conditions when all glucose is [U-¹³C]glucose (100% enrichment). The same principles apply with lower levels of enrichment in glucose. Abbreviations: αKG, α-ketoglutarate; OAA, oxaloacetate; y, anaplerosis.

Figure 3

A) Plasma glucose fractional ¹³C enrichment measured by mass spec. Steady-state is achieved in approximately 120 min with an enrichment level of about 50%. B) Blood glucose concentration as measured by blood draws and glucometer. Peak glucose concentration corresponds to approximately 50% increase compared to base level which is in the normal physiological loading range. Return to base physiological level is achieved in approximately 120 min. Lactate concentration is stable at about 0.7 mM throughout the infusion. ¹³C MRS acquisition started at around the 120 min, as indicated by the black arrows.

Figure 4

Stable time course of ¹³C signals after infusion cessation (black arrow) was observed for approximately one hour.