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QSM meets MRS: The influence of subcortical iron on glutamatergic neurotransmission in a movement disorder population

Ahmad Seif Kanaan^1,2, Alfred Anwander¹, Andreas Schäfer³, Berkin Bilgic⁴, Torsten Schlumm¹, Jamie Near⁵, Kirsten Müller-Vahl², and Harald E. Möller¹

¹Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ²Department of Psychiatry, Social Psychiatry and Psychotherpay, Hannover Medical School, Hannover, Germany, ³Diagnostic Imaging, Magnetic Resonance, Research & Development, Siemens Healthcare GmbH, Erlangen, Germany, ⁴Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ⁵Douglas Mental Health University Institute and Department of Psychiatry, McGill University, Montreal, QC, Canada

Synopsis

We use a combination of Quantitative Susceptibility Mapping and 1H-MRS to examine the role of iron and its association with glutamatergic signalling in Gilles de la Tourette syndrome (GTS). GTS is a neurodevelopmental movement disorder with abnormalities in the neurotransmission of dopamine and GABA and, as shown more recently, also in subcortical glutamate (Glu) and glutamine (Gln). In this work, we observed that GTS patients exhibit reductions in cerebral iron levels and report a general association between iron and the Gln:Glu ratio. This work provides a good example of utilizing multi-modal neuroimaging methods to interrogate pathophyiology at multiple scales.

INTRODUCTION

Iron is a trace element that is essential to the vitality of an organism, as it is ideally suited for the catalysis of many biochemical reactions due to its ability to transition between two thermodynamically stable oxidation states. Within the brain, non-heme iron is abundant in subcortical nuclei, overlapping with regions that contain large proportions of dopamine, GABA and glutamate (Glu)¹. The atypical homeostasis of iron during development may influence the biochemical mechanisms that sustain typical subcortical neurochemical signalling^2,3, providing a possible biological basis for deficits exhibited by varied neuropsychiatric and movement disorders^4,5. Gilles de la Tourette Syndrome (GTS) is an example of a neurodevelopmental movement disorder with reported reductions in serum ferritin levels^6–8 and abnormalities in the neurotransmission of dopamine and GABA⁹. It was recently shown that GTS patients also exhibit reductions in striatal Glu and glutamine (Gln)¹⁰, indicating possible abnormalities in GABA-Glu-Gln cycling, which may lead to alterations in the spatio-temporal dynamics of excitatory, inhibitory and modulatory subcortical neurochemical signalling. Along this line, a comprehensive body of literature has indicated that iron deficiency alters the neurotransmission of dopamine and GABA by influencing the mechanisms involved in receptor function and neurotransmitter synthesis/transport³. Here, we used a combination of Quantitative Susceptibility Mapping (QSM) and Magnetic Resonance Spectroscopy (1H-MRS) to investigate (a) whether GTS patients exhibit alterations in iron metabolism, and (b) whether iron levels exhibit a general influence on the Gln:Glu ratio.

METHODS

QSM, 1H-MRS and MP2RAGE¹¹ data were acquired from 28 GTS patients and 22 age/gender matched healthy controls on a 3T Siemens MAGNETOM Verio using a 32-channel head coil. A 10ml blood sample was also collected from each subject for the quantitation of serum Ferritin. Susceptibility-weighted data were acquired using FLASH with =30ms; =17ms; 256x256 matrix; flip-angle=13°; 0.8mm isotropic nominal resolution. High-quality phase maps were reconstructed using an automated, data-driven coil combination method^12,13. QSM images were computed using the superfast dipole inversion approach via SHARP bias removal and TKD susceptibility calculation¹⁴, with referencing to lateral ventricle CSF¹⁵ (Figure 1). An MP2RAGE-QSM hybrid contrast image was created to yield more robust/accurate segmentations of basal ganglia nuclei via FSL-FIRST¹⁶. Brainstem and cerebellar nuclei were mapped onto native QSM space by non-linear transformation of masks that were carefully delineated on a QSM group-average template. Median susceptibility values were extracted from each ROI and Wilcoxon rank-sum tests were used to assess group differences. Motion-affected data were removed using a multivariate robust Mahalanobis distance framework based on two structural image quality indices^17,18. 1H-MRS data were acquired from a left striatal voxel (TE= 30ms, TR=3000ms, nav=128) employing automated voxel localization; frequency and phase-drift correction; and LCModel absolute metabolite quantitation with consideration of within-voxel compartmentation¹⁰. A multiple linear regression model accounting for age, gender and two indices of image quality was used to examine the relationship of susceptibility with Ferritin and the Gln:Glu ratio. The variance inflation factor (VIF) was used to assess multi-collinearity between predictor variables.

RESULTS

We report significant reductions in the susceptibility of brainstem (p=0.001), basal ganglia (p=0.04) and all subcortical (p=0.0007) nuclei of GTS patients in comparison to controls. These effects were mainly driven by significant reductions of susceptibility in the subthalamic nucleus, substantia nigra, striatum, and red nucleus (Figure 2). Subcortical susceptibility reductions were mirrored by significant reductions in serum Ferritin (Welch’s test, p=0.011; Figure 3). Correlation analyses in the whole sample revealed significant associations between Ferritin and susceptibility in all subcortical (R=0.65, p=0.009) and basal-ganglia (R=0.44,p=0.02) nuclei. Significant associations were also observed between striatal Gln:Glu and susceptibility in the striatum (R=0.65, p=0.002) and all subcortical nuclei (R=0.59, p=0.033). All correlations exhibited an approximate VIF of 1.5 indicating little to no multi-collinearity between predictor variables.

DISCUSSION

Our results indicate that GTS patients exhibit abnormalities in cerebral iron homeostasis; and provide in-vivo evidence for a relationship between iron metabolism and glutamatergic neurotransmission. This result is supported by previous work indicating that gestational and lactational iron deficiency suppresses striatal¹⁹ and hippocampal²⁰ glutamatergic neurotransmission, in addition to influencing enzymes involved in GABA-Glu-Gln cycling^21–23. As iron is involved in multiple biochemical pathways that regulate neurochemical signalling, alterations of its hemostasis in GTS provides new clues on the biological basis of the observed disruptions in excitatory, inhibitory and modulatory signalling. Although GTS is considered a polygenic hereditary disorder with multiple affected systems, it is plausible to suggest that iron holds an important role in the pathophysiology of GTS, as its deficiency has been related to a neurochemically driven influence on the acquisition motor and behavioural functions during development⁵.

Acknowledgements

This work was funded by the FP7 Marie Curie Actions of the European Commission (“TS-EUROTRAIN FP7-PEOPLE-2012-ITN, Grant No. 316978”) and, in part, by the Helmholtz Alliance ("ICEMED: Imaging and Curing EnvironmentalMetabolic Diseases”).

References

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Figures

Figure 1. Processing and analysis framework utilized to obtain high-quality quantitative susceptibility maps and 1H-MR spectra.

Figure 2. Illustration of the quality of nucleus segmentation and group comparison statistics. GTS patients exhibited significant susceptibility reductions, passing false-discovery rate multiple comparison correction in the red nucleus, substantia nigra, subthalamic nucleus and striatum. These effects were mainly driven by reductions in left hemisphere nuclei as illustrated by the split violin plots.

Figure 3. (A) Bar-charts illustrating the significant reductions of serum Ferritin observed in GTS patients in comparison to controls. (B,C,D) Correlation plots illustrating the significant correlations observed between serum Ferritin and susceptibility in all subcortical, basal ganglia and brainstem nuclei. The strong correlations were observed in the whole sample and in each of the sub-samples separately. These results indicate the GTS patients exhibit reductions in iron and that susceptibility is a good index of iron in the brain.

Figure 4. (A) An exemplary striatal 1H-MRS spectrum of frequency and phase-drift corrected data collected from one GTS patients. The inset image illustrates the localization of the region of interest on an axial section. Significant correlations were observed between striatal Gln:Glu and susceptibility in the same striatal region of interest (B), as well as the combination of all subcortical nuclei (C). These results provide in-vivo support for a relationship between iron metabolism and glutamatergic neurotransmission.

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

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