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Longitudinal monitoring of brain glutamate levels using gluCEST in a rat model of Huntington’s disease1MIRCen, CEA, Fontenay-aux-Roses, France
Synopsis
Huntington’s disease (HD) is an inherited neurodegenerative disease characterized by motor, cognitive and psychiatric symptoms. As glutamate has been shown to be a potential biomarker of neurodegenerative diseases, we used Chemical Exchange Saturation Transfer imaging of glutamate (gluCEST) to map cerebral glutamate distribution in a rat model of HD. The longitudinal follow-up of brain glutamate levels reveals different variations between HD and control animals, suggesting that gluCEST may serve as a potential biomarker of HD, especially at asymptomatic stage.
Introduction
Huntington’s disease (HD) is an inherited neurodegenerative disease characterized by motor, cognitive and psychiatric symptoms [1]. Atrophy of the striatum is currently the best biomarker of disease progression in HD gene carriers. However, there is an urgent need to identify novel functional biomarkers of disease progression to better understand pathological processes and to monitor HD patients in clinical trials. Changes in brain metabolites have been also consistently seen in HD patients and animal models using MRS [2], but metabolite measurements are generally limited to a single voxel. We have already reported that Chemical Exchange Saturation Transfer imaging of glutamate, or gluCEST [3], can be highly valuable to assess metabolic alterations in a mouse model of HD [4] with a high anatomical resolution. In this study, we propose to monitor glutamate levels longitudinally in a slowly progressive rat model of HD in order to track disease progression and to identify potential biomarkers for HD, especially during asymptomatic stage.Methods
Rat model: Transgenic rats were obtained using a human Bacterial Artificial Chromosome containing 97 CAG/CAA repeats were used (BACHD [5]). Two cohorts were compared: Wild Type (n=6) and BACHD (n=7) rats. Animals were scanned at 2, 4, 6 and 12 months old.
GluCEST: GluCEST images were acquired on a horizontal 11.7T Bruker magnet using TSE sequence preceded by a frequency-selective continuous wave saturation pulse of 1s with a B1 intensity of 5µT applied at frequencies ranging from -5 to 5ppm by 0.5ppm steps. B0 inhomogeneity was corrected using WASSR [6]. GluCEST images were calculated using asymmetric Magnetization Transfer Ratio (MTRasym) at ±3ppm. Variation maps of gluCEST contrast were calculated between WT and BACHD rats as already described [4].
Results
In order to monitor brain glutamate levels longitudinally, mean MTRasym were calculated for each rat cohort at each timepoint. Mean glutamate levels measured in WT rats seemed to decrease over time (Fig.1.a). However, even if variation maps of gluCEST contrast calculated between 2 months and other timepoints confirmed this decrease, they did not reach statistical significance (Fig.1.b). Interestingly, glutamate levels seemed to be preserved in BACHD rats up to 6 months and then decreased at 12 months (Fig.2.a), especially in the striatum and the corpus callosum (Fig.2.b).
In order to highlight differential evolutions of glutamate levels, we calculated variation maps of gluCEST contrasts between WT and BACHD rats at ea ch timepoint (Fig.3). At 2 months, glutamate levels are significantly lower in BACHD as compared to WT, especially in the right cortex, left striatum and pallidum. At 4 months, any statistical difference was measured in any region. The variation map measured at 6 months seemed to suggest an increase of glutamate level but it was mostly due to a smaller decrease of gluCEST contrast in BACHD than in WT rats. At 12 months, strong and significant decreases of glutamate levels were measured in the cortex, striatum and corpus callosum (about 15%) of BACHD rats.
Discussion
The decrease of glutamate levels in the brain of WT rats can be attributed to normal aging without neuronal alteration, as it has already been reported in healthy young adulthood [7]. This suggests that glutamate level is more likely correlated to a modified metabolic activity than neuronal alteration [8, 9]. In addition, changes in glutaminase regulation activity have been reported during aging [10] and may lead to a modification of the glutamate-glutamine cycle [11].
Interestingly, BACHD rats exhibited very different kinetic of glutamate levels variations, especially in the striatum, which is known to be altered in HD, and in the corpus callosum as observed previously in a mouse model of HD [4]. This suggests a reorganization of metabolic fluxes and energy metabolism in BACHD animals. One can hypothesize that during first months, BACHD animals set up compensatory mechanisms to maintain sufficient energy production. Then, such mechanisms run out faster after 12 months leading to a sudden failure of the energy machinery.
Conclusion
In this study, we monitored for the first time brain glutamate levels longitudinally in a slowly progressive rat model of HD. We highlighted various evolutions of glutamate levels in HD rats as compared to control animals, suggesting metabolic adaptation of HD animals during first months of life. Longitudinal follow-up of glutamate levels using gluCEST imaging could be a valuable tool to monitor disease progression in HD patients, especially during asymptomatic stage.Acknowledgements
This work was supported by a grant from Agence Nationale pour la Recherche (“HDeNERGY” project, ANR-14-15CE-0007-01). The 11.7 T scanner was funded by a grant from NeurATRIS: A Translational Research Infrastructure for Biotherapies in Neurosciences (“Investissements d'Avenir”, ANR-11-INBS-0011).References
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