Synopsis

Keywords: Alzheimer's Disease, CEST & MT

Our study characterized the hippocampal glucose uptake in an Alzheimer’s Disease rat model (TgF344AD) comorbid with metabolic syndrome modeled by cafeteria (CAF) diet. Glucose uptake was evaluated by detecting 2-deoxy-D-glucose (2DG) using chemical exchange saturation transfer (CEST) MRI in 6- and 12-month-old cohorts, reflecting pre-clinical and established disease stages, respectively. CAF diet attenuated glucose uptake of both non-transgenic and transgenic-AD at 6 months of age; whereas at 12 months of age, CAF diet enhanced the hippocampal glucose uptake of TgAD but attenuated it in chow-fed TgAD, when compared to that in correspondingly fed nTg littermates.

Introduction

By the time a patient is diagnosed with Alzheimer’s disease (AD), neuronal loss has already occurred, yet intervening before significant atrophy is thought key for abating cognitive decline. There is thus a pressing need for the identification of sensitive and specific biomarkers of initial AD pathology to enable earlier diagnosis. Disruption of brain glucose metabolism has been observed in AD-susceptible brain regions (most prominently hippocampus)^1,2 before the onset of significant cognitive symptoms in patients, making glucose metabolism impairment of interest for early detection of AD and a potential target of early interventions. 2-deoxy-D-glucose (2DG) is a labeled glucose that gets taken up by cells via glucose transporters yet remains trapped for hours intracellularly³. 2DG is detected using chemical exchange saturation transfer (CEST) MRI, a highly sensitive technique that can detect 2DG at low concentrations (µM - nM)⁴. This project utilized the TgF344-AD rat model, exhibiting amyloidogenesis, tau pathology, frank neuronal loss, and progressive cognitive decline⁵. Since the translation of preclinical findings has been long confounded by the lack of incorporation of prominent AD comorbidities, we have presently incorporated a model of metabolic syndrome in the TgF344-AD rats. Cafeteria (CAF) diet comprises grocery store-purchased food items with variations of nutrients, energy-dense, and highly palatable food to animals that imitate obesogenic features of the human diet⁶. TgF344-AD rats and their non-transgenic littermates were subjected to CAF diet to model human metabolic syndrome and their hippocampal glucose uptake was investigated using 2DG CEST MRI.

Method

TgF344-AD rats and their non-transgenic (nTg) littermates were subjected to CAF diet for 3 months prior to imaging. The rats were imaged at 6 and 12 months of age, representing early amyloid and established amyloid, tau pathology, and neuronal loss stages, respectively. Animals were anesthetized using isoflurane delivered via nose cone and scanned on a 7T MRI scanner BioSpec 70/30 USR with BGA-12SHP gradients running ParaVision 6.0.1, Bruker BioSpin, Billerica, MA, USA). The saturation transfer-weighted images were acquired using one 490-ms block RF saturation pulse with amplitude B₁ = 1.5µT per k-space line followed by a single-slice FLASH acquisition (TR/TE = 500/3 ms; flip angle = 30°; FOV = 20 × 20 mm²; in-plane resolution = 0.31 × 0.31 mm², slice thickness = 1.5 mm)^7,8. Dummy scan, reference scan (at 667 ppm frequency offset to avoid saturation transfer effects), 12 repetitions of three-offset acquisition (at 1.2, 2.0, 2.9 ppm reflecting OH at third, fourth, and first carbon in 2DG), and another reference scan compromised the 20.7 min dynamic acquisition segment⁹. One such segment was acquired during baseline and three more were acquired following the onset of 2DG injection. The mean magnetization transfer ratio (MTR; defined as 1 – S/S0 where S and S0 are the reduced water signal and the signal without saturation respectively) of the baseline segment was subtracted to produce the MTR change (ΔMTR) time course⁷. To quantify the uptake of 2DG, the mean ΔMTR between the onset and offset times was calculated where onset and offset times for the time series were defined for each frequency offset as the time when the ΔMTR rose and fell to half of its maximum level respectively. Two-way unbalanced ANOVA with genotype and sex as the main effects was used to evaluate the differences in glucose uptake.

Results

At 6 months of age, there was no significant interaction between diet and genotype. However, diet had a significant effect on the mean ΔMTR (at 2.9ppm: 24% of the variation, p = 0.0312) indicating that the CAF diet decreases the hippocampal glucose uptake in both 6-month-old nTg and TgAD rats. There was no significant difference in glucose uptake between nTg and TgAD rats exposed to either diet. In contrast, at 12 months of age, the interaction of diet and genotype was significant (at 2.9ppm: 17% of the variation, p = 0.0487) indicating that the CAF diet affects the hippocampal glucose uptake of nTg vs. TgAD rats differently. While chow-fed TgAD rats showed lower ΔMTR compared to chow-fed nTg (at 2.9ppm: difference of -0.1275 ± 0.4163, p = 0.7672), CAF-fed TgAD rats showed significantly higher mean ΔMTR compared to CAF-fed nTg rats (at 2.9ppm: difference of 0.9928 ± 0.2840, p = 0.0058). The higher glucose uptake in CAF-fed TgAD rats suggests beneficial effects of the CAF diet in the early stages of AD pathology.

Conclusion

We postulate that at the pre-clinical stage (6 months of age), CAF diet is detrimental to both non-transgenic and AD rats as demonstrated by attenuated glucose uptake. However, the high caloric intake afforded by CAF diet exposure may be beneficial in AD-challenged rats (12 months of age) due to their incipient mitochondrial dysfunction and hence decreased brain metabolic efficiency. The contrast between CAF- and chow-fed TgAD rats’ metabolism supports diet as a modifiable risk factor for AD.

Acknowledgements

This study had funding support from Canadian Consortium on Neurodegeneration in Aging (CCNA, Canada; Grant ID: CAN 163902); Canadian Institutes of Health Research (CIHR, Canada; Grant ID: PJT173409) and National Institutes of Health (NIH, USA; Grant ID: 1R01AG057665- 01A1).

References

1. Mosconi L, De Santi S, Brys M, et al. Hypometabolism and Altered Cerebrospinal Fluid Markers in Normal Apolipoprotein E E4 Carriers with Subjective Memory Complaints. Biol Psychiatry. 2008;63(6):609-618.
2. Mosconi L, De Santi S, Li J, et al. Hippocampal hypometabolism predicts cognitive decline from normal aging. Neurobiol Aging. 2008;29(5):676-692.
3. Jin T, Mehrens H, Wang P, Kim SG. Glucose metabolism-weighted imaging with chemical exchange-sensitive MRI of 2-deoxyglucose (2DG) in brain: Sensitivity and biological sources. NeuroImage. 2016;143:82-90. doi:10.1016/j.neuroimage.2016.08.040
4. Nasrallah FA, Pagès G, Kuchel PW, Golay X, Chuang KH. Imaging brain deoxyglucose uptake and metabolism by glucoCEST MRI. J Cereb Blood Flow Metab. 2013;33(8):1270-1278.
5. Cohen RM, Rezai-Zadeh K, Weitz TM, et al. A transgenic Alzheimer rat with plaques, tau pathology, behavioral impairment, oligomeric aβ, and frank neuronal loss. J Neurosci. 2013;33(15):6245-6256.
6. Gomez-Smith M, Karthikeyan S, Jeffers MS, et al. A physiological characterization of the Cafeteria diet model of metabolic syndrome in the rat. Physiol Behav. 2016;167:382-391.
7. Lam WW, Oakden W, Murray L, et al. Differentiation of Normal and Radioresistant Prostate Cancer Xenografts Using Magnetization Transfer-Prepared MRI. Sci Rep. 2018;8(1):10447.
8. Tan Z, Lam WW, Oakden W, et al. Saturation transfer properties of tumour xenografts derived from prostate cancer cell lines 22Rv1 and DU145. Sci Rep. 2020;10(1):21315.
9. Joo IL, Lam WW, Oakden W, et al. Early alterations in brain glucose metabolism and vascular function in a transgenic rat model of Alzheimer’s disease. Progress in Neurobiology. 2022;217:102327. doi:10.1016/j.pneurobio.2022.102327