INTRODUCTION

Glutamatergic system is a primary neurotransmitter system in cognitive function. ¹ Although there is accumulating evidence that glutamate (Glu) in the cingulate cortices play a crucial role in the pathophysiology of Alzheimer's disease (AD) ², the associations between aggregated tau and amyloid-β (Aβ), and glutamatergic dysfunction in the regions remain elusive. Since most of the single-voxel spectroscopy (SVS) studies have investigated alterations of Glu levels in the posterior cingulate cortex (PCC), regional alterations of Glu in the cingulate cortices and the surrounding white matter (WM) are unclear. ³ This study aimed to evaluate regional alterations of Glu at the level of the cingulate gyrus in relation to tau and Aβ depositions in AD and mild cognitive impairment (MCI) patients using magnetic resonance spectroscopic imaging (MRSI) and PET imaging.

METHODS

We enrolled 16 MCI/AD patients (6 MCI and 10 AD patients) with positive Aβ pathology confirmed by ¹¹C-PiB PET and 15 healthy controls (HCs) without Aβ pathology. We evaluated Glu, total N-acetylaspartate (tNAA), myo-inositol (mI), and total choline (tCho) to total creatine (tCr) ratio using single-plane MRSI to investigate spatial information at the level of the cingulate gyrus. We acquired MRSI data with point-resolved spectroscopy (PRESS) pulse sequence (TR/TE/average = 2000 ms / 30 ms / 3) in every single volume of interest (VOI) of 10 × 10 × 15 mm³ [the measurement volume of the central 8 × 8 voxels (white line voxels in Figure 1)]. We analyzed MRSI data using LCModel software. We set the exclusion criteria of spectra with a signal-to-noise < 5, full width at half maximum > 0.143 ppm, or Cramér–Rao lower bound > 30% for Glu, tNAA, mI, tCho, and tCr in each voxel ⁴ and excluded the 8 voxels in the most dorsal row. We examined tau and Aβ PET with ¹⁸F-PM-PBB3 and ¹¹C-PiB, respectively. Using the cerebellar cortex as a reference region, we quantified PET probe retentions as standardized uptake value ratio (SUVR). We made heatmaps to visualize Z scores of ¹⁸F-PM-PBB3 and ¹¹C-PiB SUVRs and the metabolites to tCr ratios using data of HCs as a reference, and a heatmap of Pearson correlation coefficients between the metabolites to tCr ratios and SUVRs of ¹⁸F-PM-PBB3 and ¹¹C-PiB. Based on the heatmaps, we chose combined voxels for evaluating the metabolites to tCr ratios between AD/MCI patients and HCs and correlations with ¹⁸F-PM-PBB3 and ¹¹C-PiB SUVRs and cognitive functions of mini-mental state examination (MMSE). The Certified Review Board approved this study.

RESULTS

The heatmaps visualized decreases in Glu/tCr ratios in GM, especially in the PCC, and moderate increases in WM, while tNAA/tCr ratios were reduced in both GM and WM voxels (Figure 2). The mI/tCr and tCho/tCr ratios were increased only in some voxels of PCC (Figure 2). While both ¹⁸F-PM-PBB3 and ¹¹C-PiB SUVRs were markedly increased in cingulate cortices and other GM regions (Figure 2), Glu/tCr ratios in the PCC regions were negatively associated with ¹⁸F-PM-PBB3 SUVR (Figure 3), revealing the vulnerabilities of the PCC region to tau deposits. Subsequently, we analyzed the combined voxels covering PCC, and found the reduction of Glu/tCr ratios (P < 0.05), increase of ¹⁸F-PM-PBB3, and ¹¹C-PiB SUVRs in AD/MCI patients compared with HCs (¹⁸F-PM-PBB3: P < 0.001; ¹¹C-PiB SUVR: P < 0.001) (Figure 4). While tNAA, mI, and tCho to tCr ratios were not significantly different in AD/MCI patients from HCs (tNAA/tCr: P = 0.063; mI/tCr: P = 0.76; tCho/tCr: P = 0.86) (Figure 4), there was a significant decrease in tNAA/tCr ratios in AD patients (p < 0.05). The Glu/tCr ratios were negatively correlated with tau deposits in the AD/MCI patients (r = -0.53, p < 0.05) and positively correlated with MMSE scores (r = 0.74, p < 0.05) in AD patients (Figure 5).

DISCUSSION and CONCLUSION

Combining MRSI and PET, we demonstrated for the first time that the glutamatergic system in the PCC region is vulnerable to tau deposits but not Aβ. This result supports the rationale of many SVS studies which have focused on the PCC. The fact that Glu levels did not correlate with SUVR of amyloid PET but did correlate with SUVR of tau PET was also consistent with the assumption that tau is more strongly associated with neurodegeneration. This study suggests that Glu is an early marker of disease progression in AD. Glu levels differed from the HC group in AD/MCI patients, whereas tNAA levels in HCs differed only from the AD patients. tNAA levels showed a downward trend in GM and WM, whereas Glu levels showed an upward trend in WM. This may reflect the elevation of Glu in the WM with abundant extracellular space at the early stage of the AD pathology. ⁵ We found correlations between Glu/Cr ratios in the PCC and cognitive function. PCC is a core hub of default-mode network relating to several cognitive processes ⁶ and tau accumulations associated with disrupted Glu transmissions in PCC may play a crucial role in aggravating cognitive functions in AD. Collectively, MRSI combined with tau PET revealed the regionally variable vulnerability of the glutamatergic system to tau depositions in AD brains.

Acknowledgements

The authors thank all patients and their caregivers for participation in this study, clinical research coordinators, PET and MRI operators, and research ethics advisers at QST for their assistance to the current projects.