In vivo 2D L-COSY MR spectroscopy at ultra-high field to probe membrane degradation during Alzheimer's disease

Introduction: The increase in glycerophosphocholine (GPC) an indicator of increased breakdown of phospholipids has been shown to increase in CSF of Alzheimer's disease (AD) patients¹. Till now the detection of phosphocholine and GPC in disease affected regions in AD brain has not been possible due to lack of sensitive detection methods. In vivo localized MR spectroscopy cannot separate the signal of GPC and phosphocholine (PC). In the present study we have optimized and employed, for the first time, two dimensional correlated spectroscopy (L-COSY) in mouse model of AD to separately detect resonances of phosphocholine and GPC.
Methods: The TG2576 transgenic AD ² and wild type mice were used in this study. All MR measurements were performed using a 9.4-T vertical wide-bore imaging systems equipped with a Bruker Avance console and 1000-mT/m gradients. The MRS voxel was located in the hippocampus/cortex region (4x4x1.7 mm3; 27 µl - Fig. 1b) which was selected on the basis of high Aβ plaques deposited areas of the AD brain (Fig. 1c). Field homogeneity was optimized using the Fastmap sequence, which typically yielded a water linewidth of ~16-20 Hz in live mouse brain. For 2D MRS, a localized 2D shift correlated spectroscopic sequence (L-COSY) was optimized (Fig. 1a). 2D spectra were recorded using TR=1500 ms, TE=15 ms, 2048 complex points along F2, and 192 points along F1.
Results and Discussion: Figure 1 (d,e) show high resolution 2D MR spectra obtained from the 27µl voxel placed in cortex/hippocampus region of control and TG mouse. The combination of the high field strength, strong gradient system, efficient water suppression and short echo time allowed clear detection of cross-peaks of GPC, PC allowing their direct unambiguous assignments in vivo (Fig. 1e). Comparison of 2D MRS spectra in control and TG mice show ~4 fold increase in GPC at 18 month of age. Thus L-COSY method allows clear and unambiguous identification of GPC signal in vivo and provides a clear means to differentiate AD mouse brain from control brain. Increase in GPC reflects membrane breakdown which was correlated with the severity of A β -amyloid plaque deposition.

References: ¹Walter A, Korth U, Hilgert M et al Neurobiol Aging 2004, 25:1299-1303; ²Hsiao K, Chapman P, Nilsen S, et al. Science 1996, 274:99-103.

Acknowledgements: This work was supported by grants from Alzheimer Forschung Initiative e.V. (AFI, Grant Nr 13810) and Centre for Medical Systems Biology (CMSB).

Fig. 1. (a) 2D L-COSY pulse sequence, (b) Voxel position in cortex/hippocampus of mouse brain for MRS. (c) Amyloid-plaque measured by immunohistology. (d) 2D MRS data from WT and (e) AD mouse. (f) Separation of GPC and PC is shown in enlarge view.