Functional activation of pyruvate dehydrogenase in human brain using hyperpolarized [1-13 C]pyruvate

Abstract

Purpose: Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [¹³ C]bicarbonate in the brain from hyperpolarized [1-¹³ C]pyruvate using carbon-13 (¹³ C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo ¹³ C MRS with hyperpolarized [1-¹³ C]pyruvate.

Methods: From seven sedentary adults in good general health, time-resolved [¹³ C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1-¹³ C]pyruvate and [1-¹³ C]lactate, to test the hypothesis that the appearance of [¹³ C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus-injected [1-¹³ C]pyruvate being converted to [1-¹³ C]lactate prior to decarboxylation was investigated by measuring [¹³ C]bicarbonate production with and without [1-¹³ C]lactate saturation.

Results: In the course of visual stimulation, the measured [¹³ C]bicarbonate signal normalized to the total ¹³ C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1-¹³ C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate-to-pyruvate ratio by 44.4% ± 9.3% (p < 0.01).

Conclusion: We demonstrated the utility of ¹³ C MRS with hyperpolarized [1-¹³ C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [¹³ C]bicarbonate production.

Keywords: bicarbonate; brain activation; dissolution dynamic nuclear polarization; fMRS; hyperpolarized pyruvate; pyruvate dehydrogenase.

Figure 1.. Functional Activation of Cerebral Metabolism.

In response to stimulation, oxidative metabolism and the activity of one of its key regulating enzymes, PDH, increase preceding concomitant changes in blood oxygen consumption, which results in changes in the BOLD signal. PDH: pyruvate dehydrogenase; BOLD: blood oxygen level dependent.

Figure 2.. Experimental Protocol and Setup.

(a) The experimental MRI protocol that consists of ¹H BOLD fMRI, ¹H MPRAGE, and two dynamic ¹³C MRS with hyperpolarized [1-¹³C]pyruvate. (b) ¹³C/¹H dual-frequency RF coil. Signals detected in the two posterior ¹³C receive channels (colored in blue) of the ¹³C/¹H dual-frequency RF coil were used to assess the metabolic changes due to visual stimulation. The activation map shown in the center of the coil is the averaged BOLD response to the visual stimuli. (c) Spectral-spatial excitation profiles of two RF pulses used to excite the [1-¹³C]lactate resonance with 10° (RF1) and 90° (RF2) and their spectral profiles (RF1 as red line; RF2 as blue line) at the central position (z = 0). (d) ¹H BOLD fMRI and one of the ¹³C MRS scans were acquired with block-designed checkerboard stimuli while the other ¹³C MRS was without stimulation. AUC: area under the curve; BOLD: blood oxygenation level dependent.

Figure 3.. Activation of Pyruvate Dehydrogenase Flux in Response to Visual Stimulation.

(a) Time-averaged ¹³C spectra acquired from a prescribed 3-cm slab that includes the visual cortex using the RF coil and the non-lactate-saturating RF pulse (RF1) with and without visual stimulation. (b) Bicarbonate AUC, normalized to the total ¹³C signal, increased with the stimuli as compared to the baseline in all participants. The presented time-courses of hyperpolarized (c) [1-¹³C]pyruvate, (d) [1-¹³C]lactate, and (e) [¹³C]bicarbonate signals with and without visual stimulation are from a representative participant (#1, 23 y.o. male). A coefficient was applied to the time-courses to match the maximum pyruvate signals between the injections. *: P < 0.05

Figure 4.. Effect of Saturating [1-¹³C]Lactate on [¹³C]Bicarbonate Formation.

(a) Time-averaged ¹³C spectra from a representative healthy participant (#9, 32 y.o. male) using the RF pulses. (b) Decreased [¹³C]bicarbonate and increased [1-¹³C]lactate were measured with the lactate-saturating RF pulse (RF2) as compared to the measurements without lactate saturation (RF1). Time-courses of (c) [¹³C]bicarbonate and (d) [1-¹³C]lactate acquired from the participant. *: P < 0.05; **: P < 0.01

Figure 5.. Pathway to Hyperpolarized [¹³C]Bicarbonate Formation in The Brain.

Intravenously injected [1-¹³C]pyruvate bolus can be converted to [1-¹³C]lactate by red blood cells or other organs before being delivered to the brain. ¹³C-labeled pyruvate and subsequent metabolic products are in blue font. GLUT: glucose transporter; LDH; lactate dehydrogenase; MCT: monocarboxylate transporter; MPC; mitochondrial pyruvate carrier; PC: pyruvate carboxylase; PDH: pyruvate dehydrogenase; RBC: red blood cell