Measuring mitochondrial metabolism in rat brain in vivo using MR Spectroscopy of hyperpolarized [2-¹³C]pyruvate

Abstract

Hyperpolarized [1-(13) C]pyruvate ([1-(13) C]Pyr) has been used to assess metabolism in healthy and diseased states, focusing on the downstream labeling of lactate (Lac), bicarbonate and alanine. Although hyperpolarized [2-(13) C]Pyr, which retains the labeled carbon when Pyr is converted to acetyl-coenzyme A, has been used successfully to assess mitochondrial metabolism in the heart, the application of [2-(13) C]Pyr in the study of brain metabolism has been limited to date, with Lac being the only downstream metabolic product reported previously. In this study, single-time-point chemical shift imaging data were acquired from rat brain in vivo. [5-(13) C]Glutamate, [1-(13) C]acetylcarnitine and [1-(13) C]citrate were detected in addition to resonances from [2-(13) C]Pyr and [2-(13) C]Lac. Brain metabolism was further investigated by infusing dichloroacetate, which upregulates Pyr flux to acetyl-coenzyme A. After dichloroacetate administration, a 40% increase in [5-(13) C]glutamate from 0.014 ± 0.004 to 0.020 ± 0.006 (p = 0.02), primarily from brain, and a trend to higher citrate (0.002 ± 0.001 to 0.004 ± 0.002) were detected, whereas [1-(13) C]acetylcarnitine was increased in peripheral tissues. This study demonstrates, for the first time, that hyperpolarized [2-(13) C]Pyr can be used for the in vivo investigation of mitochondrial function and tricarboxylic acid cycle metabolism in brain.

Keywords: MRS; brain; dichloroacetate; glutamate; hyperpolarized 13C; pyruvate; tricarboxylic acid cycle metabolism.

Figure 1

(A) Time-averaged (3-60 s) spectrum from a slice through the brain of Rat1, and its zoomed spectrum, showing [5-¹³C]glutamate (Glu), [1-¹³C]citrate (Cit), [1-¹³C]acetoacetate, [1-¹³C]acetylcarnitine (ALCAR), and [1-¹³C]pyruvate (Pyr). (B) Time courses of [2-¹³C]Pyr (black--), [5-¹³C]Glu (blue*), [1-¹³C]ALCAR (redΔ), and [2-¹³C]pyruvate hydrate (PyH, green□) from the average of the pre- and post-dichloroacetate (DCA) data. The shaded area in the time course plot indicates the acquisition time window for single time-point 2D CSI.

Figure 2

Representative metabolite maps of (A) [2-¹³C]Pyr, (B) [5-¹³C]Glu, (C) [1-¹³C]ALCAR, acquired before and after DCA infusion from Rat2 using the single time-point 2D FIDCSI sequence. Each map was averaged over data from two injections. (E) Spectra of pre- and post-DCA from brain ROI shown in (D).

Figure 3

Metabolic pathways of [2-¹³C]Pyr and its observed downstream products (red). Glu and Cit can be biomarkers for the TCA cycle metabolism. The fate of the labeled carbon can be in ALCAR, which connects acetyl-CoA to β-oxidation of fatty acids in mitochondria, or acetoacetate in ketone-body metabolism.