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Hyperpolarized Imaging of Lithium-Induced Modulation of [1-13C]Pyruvate Metabolism in the Heart
Jonas Steinhauser1, Grzegorz Kwiatkowski 1, Patrick Wespi1, and Sebastian Kozerke1

1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Synopsis

Lithium is the first-line drug used in bipolar disorder, a chronic psychiatric illness characterized by severe biphasic changes in mood. Lithium has a narrow therapeutic window and has shown cardiac side effects. The present work demonstrates the detectability of lithium induced changes of mitochondrial metabolism in cardiomyocytes by employing hyperpolarized [1-13C]pyruvate magnetic resonance imaging of the in-vivo heart. In healthy rats, lithium is shown to increase mitochondrial metabolism and decrease glycolytic lactate production indicating a potential role of lithium in the heart.

Introduction

Bipolar disorder (BD) is a chronic psychiatric illness, characterized by severe biphasic changes in mood. With a global prevalence of up to 4.8%, it results in more disability-adjusted life-years lost than major neurological conditions or cancer.1 Several pathophysiological mechanisms have been hypothesized to underpin the neurobiology of BD, including the presence of mitochondrial dysfunction. Mitochondrial dysfunction is proposed to have a central role in the pathophysiology of BD through mechanisms involving impaired oxidative phosphorylation, a shift to glycolytic production of energy, general decrease of energy, and abnormalities in the morphology and intracellular distribution of mitochondria.2 Lithium (Li+) is a first-line treatment for BD with a narrow therapeutic window.3 One mechanism of action of Li+ is an increase in mitochondrial aerobic energy production.4 However, this effect is not tissue specific. In the heart, metabolic deterioration and accompanying processes eventually lead to myocarditis, cardiomyopathy, endothelial cell death, and consequently cardiovascular dysfunction.5 Thus it is of great importance to provide patients with the right dose of Li+, facilitating mitochondrial metabolism in the brain on one hand, but sparing cardiomyocytes from damage on the other. The objective of the present study was to examine whether modulation of mitochondrial metabolism by Li+ can be monitored with hyperpolarized [1-13C]pyruvate magnetic resonance imaging.

Methods

Hyperpolarization

A home-built multisample dissolution dynamic nuclear polarization (DNP) system was used to polarize samples consisting of 50.8µL [1-13C]pyruvic acid and 13.5mM trityl, doped with 1mM Dotarem yielding an 80mM [1-13C]pyruvate solution.6

Animal Preparation

All animal experiments were performed with adherence to the Swiss Animal Protection law and were approved by the regional veterinary office. Four female Spargue-Dawley rats weighing 360-380g were anesthetized with 4% isoflurane in a mixture of air and oxygen (4:1), endotracheal intubation was performed, and ventilation was initiated. Anesthesia was maintained by using 1-2% isoflurane. Body temperature was kept at 37-38°C using a warm water heating system. Two 26-gauge intravenous cannulas were placed in opposite sides of the tail, one to allow injection of the dynamic nuclear polarization substrate and one for continuous glucose infusion (15mg/kg/min)7.

Study Protocol

Following an initial baseline measurement, rats were injected with 1ml sodium chloride 0.9% solution containing either lithium citrate (treatment, 20mg lithium citrate/animal, n=2), or an equivalent amount pH adjusted citric acid (n=2). Additional measurements were taken at t=20, 40, and 60min after the injection of Li/citrate (Figure 1).

Magnetic Resonance Imaging

Imaging experiments were performed with a 9.4-T MR imaging system (Biospec 94/30, Bruker Biospin, Ettlingen, Germany). A birdcage dual 1H/13C coil (Rapid Biomedical, Wurzburg, Germany) was used for excitation. A rectangular 13C surface coil with a sensitive coil area of 40x30mm2 (Rapid Biomedical) was placed over the thorax for signal reception. Metabolic data were acquired with a multiband radiofrequency pulse in combination with a multiecho single-shot echo-planar readout8. The imaging field of view was 60x40mm2, in-plane spatial resolution was 1.25x1.25mm2, and section thickness was 4mm. Seven echoes were acquired. Metabolic imaging was triggered to end systole, and the seven readouts were repeated every 1.5s during a total imaging duration of 2min.

Data Analysis

Metabolic images were reconstructed using the IDEAL approach9 encoding pyruvate, lactate, bicarbonate, pyruvate hydrate, and alanine resonances. Lactate, bicarbonate, and lactate-to-bicarbonate ratio were quantified based on the area under the curve (AUC) of the signal intensity–time curves. Metabolite AUCs were normalized by the total myocardial carbon (TmC) signal acquired in each scan corresponding to the sum of the lactate, bicarbonate and alanine resonances.

Results

Metabolite AUC ratios showed a time dependent change after injection of lithium treatment, with effects becoming the strongest at t=60min (Figure 2). Lactate/TmC decreased over the course of 60min (down to finally -11 and -26% relative to baseline). Bicarbonate/TmC reproducibly increased (up to +20 and +33%) for the treatment group, leading to changes of -26 and -45% in lactate-to-bicarbonate comparing baseline and t=60min. The trends were not observed for animals injected with the control solution (citric acid). Representative time-courses are shown in Figure 3.

Discussion

This preliminary data shows that lithium modulates mitochondrial activity in the heart as resolved by hyperpolarized [1-13C]pyruvate MR imaging. The decrease in lactate and increase in bicarbonate signal was found to evolve with time. These observations match findings for BD, where bicarbonate production is impaired and energy is generated by glycolytic lactate production.2 Probing metabolism of the heart and other tissues of interest (e.g. brain) using hyperpolarized [1-13C]pyruvate MR imaging might thus prove to be a useful tool to monitor effects of lithium treatment in-vivo.

Acknowledgements

The authors acknowledge funding from the Swiss National Science Foundation, grant 320030_153014.

References

1. Merikangas, K. R. et al. Prevalence and correlates of bipolar spectrum disorder in the world mental health survey initiative. Arch. Gen. Psychiatry 68, 241–51 (2011).

2. Scaini, G. et al. Mitochondrial dysfunction in bipolar disorder: Evidence, pathophysiology and translational implications. Neurosci. Biobehav. Rev. 68, 694–713 (2016).

3. Mohandas, E. & Rajmohan, V. Lithium use in special populations. Indian J. Psychiatry 49, 211–8 (2007).

4. Salimi, A., Gholamifar, E., Naserzadeh, P., Hosseini, M.-J. & Pourahmad, J. Toxicity of lithium on isolated heart mitochondria and cardiomyocyte: A justification for its cardiotoxic adverse effect. J. Biochem. Mol. Toxicol. 1–8 (2016). doi:10.1002/jbt.21836

5. Kang, Y. J. Molecular and cellular mechanisms of cardiotoxicity. Environ. Health Perspect. 109, 27–34 (2001).

6. Krajewski, M. et al. A multisample dissolution dynamic nuclear polarization system for serial injections in small animals. Magn. Reson. Med. (2016). doi:10.1002/mrm.26147

7. Lauritzen, M. H. et al. Enhancing the [13C]bicarbonate signal in cardiac hyperpolarized [1-13C]pyruvate MRS studies by infusion of glucose, insulin and potassium. NMR Biomed. 26, 1496–1500 (2013).

8. Sigfridsson, A. et al. Hybrid multiband excitation multiecho acquisition for hyperpolarized 13 C spectroscopic imaging. Magn. Reson. Med. 73, 1713–1717 (2015).

9. Reeder, S. B. et al. Multicoil Dixon chemical species separation with an iterative least-squares estimation method. Magn. Reson. Med. 51, 35–45 (2004).

Figures

Figure1: Depiction of the experimental protocol. After an initial baseline, three subsequent DNP measurements were taken at t = 20, 40, and 60 minutes after injection of treatment/control (t=0min). The treatment group (n=2) was injected with 20mg lithium citrate dissolved in sodium chloride 0.9% whereas the control group (n=2) received an equimolar amount of citrate in sodium chloride 0.9%.

Figure 2: Summary of metabolic changes (AUC ratios) at baseline (BL), t=20min, t=40min and t=60min: lactate-to-bicarbonate-ratio (Lac / Bic), lactate (Lac), bicarbonate (Bic) and alanine (Ala) after treatment (lithium citrate in sodium chloride) and control (citrate in sodium chloride 0.9 %) injection. Values are displayed relative to baseline measurement. Error bars of each group (n=2) display the two individuals.

Figure 3: Exemplary dynamic data of baseline- and post-injection (t=60min) metabolism. Lactate, bicarbonate, and alanine signal were extracted from the myocardial region of the left ventricle (area between orange and blue line). Pyruvate signal was sampled in the left ventricular blood pool (enclosed by blue circle).

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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