2020

APPswe/PS1dE9 mice with cortical amyloid pathology show a reduced NAA/Cr ratio without apparent brain atrophy: A MRS and MRI study
Angela Kuhla1, Fatemah Sakr2, Claire Ruehlmann1, Tobias Lindner3, Stefan Polei3, Stefan Hadlich4, Bernd J Krause5, Brigitte Vollmar1, and Stefan Teipel2,6

1Institute for Experimental Surgery, Rostock University Medical Center, Rostock, Germany, 2Rostock University Medical Center, Rostock, Germany, 3Core Facility Multimodal Small Animal imaging, Rostock University Medical Center, Rostock University Medical Center, Rostock, Germany, 4Institute of Diagnostic Radiology & Neuroradiology, University Medicine Greifswald, Gerifswald, Germany, 5Department of Nuclear Medicine, Rostock University Medical Center, Rostock, Germany, 6German Center for Neurodegenerative Diseases (DZNE), Rostock, Germany

Synopsis

Amyloid-ß deposition is one of the hallmarks of Alzheimer’s disease (AD) that starts to progress decades before the onset of cognitive impairment. With the rise of the new diagnostic criteria of AD that considers the neuropathological changes as the main aspects for explaining the extent of the disease regardless the cognitive status of the patient & further highlighted the importance of finding reliable in-vivo biological markers to identify those in the preclinical stage of AD. Through the use of the transgenic mice models, particularly APPswe/PS1dE9 we could study the different pathomechanics contributing to the development of AD. So, in this study, we assumed an approach combining morphometry based on high-resolution MRI as a measure for the brain atrophy & proton magnetic resonance spectroscopy as a measure of neuronal functional viability. Then compare these data with a well known & standardized method as the histopathological assessments of neuron & amyloid plaques load. Using the quantitative neuroimaging allows us to translate these mechanistic findings in transgenic models to human phenotypes of brain morphology and function.

Introduction

Amyloid-ß deposition is one of the hallmarks of Alzheimer’s disease (AD) that starts to progress decades before the onset of cognitive impairment. With rise of the new diagnostic criteria of AD that considers the neuro-pathological changes as the main aspects for explaining the extent of the disease regardless the cognitive status of the patient & further highlighted the importance of finding reliable in-vivo biological markers to identify those in the preclinical stage of AD (6). Transgenic mice models offers a good opportunity for studying the different pathomechanics contributing to the development of AD, particularly APPswe/PS1dE9 which is one of the most widely used models, in which amyloid plaques deposition is initiated at the age of 4-6 months followed by other morphological alterations (6). So, we assumed an approach combining morphometry based on high resolution MRI & proton magnetic resonance spectroscopy together with the histopathological assessments of neuron & amyloid plaques load in APPswe/PS1dE9 mouse, to assess the interaction between histological & neuroimaging markers of neuronal loss , neuronal function & brain atrophy (2).

Methods

We used for this study double transgenic female APPswe/PS1dE9 (tg) mice on the genetic background C57BL/6J & C3H/HeJ (n=10) at the age of 12 months. For the controls, wild type (wt) age-matched littermates (n=9) were used. All mice were anesthetized with 1-3% isoflurane in 100% oxygen then scanned in a 7T small animal MRI (Brucker). The imaging protocol included morphological high resolution T2-weighted (RARE sequence) followed MRS using stimulation echo acquisition method (STEAM) with outer volume suppression sequence. All the mice are sacrificed after experimental procedures then brain tissue was harvested for histopathological analysis. The voxel-based morphometric analysis of the T2-weighted RARE sequence were carried out using SPM 8 implemented within MATLAB (1, 4) & MRS data were analyzed using jMRU software package to compute metabolites ratios as NAA/Cr ratio.

Results

Regarding the morphometry, few scattered clusters of reduced grey matter volume could be detected in caudate, hippocampus & cerebellum in tg mice, however, the effects didn’t survive a p-value of p < 0.05 using false discovery rate. On the other hand, the MRS revealed a difference of 10% of NAA/Cr ratio in wt when compared to age-matched tg mice (3, 5). Immunohistochemically analyses showed that the tg mice revealed an up to 27% decrease in the number of cortical neuronal cells along with raised cortical gliosis when compared to wt mice. However, there was no appreciable difference detected in the hippocampus regions between the tg & wt mice.

Conclusion

APPswe/PS1dE9 mice, as a model of AD showed an increased Aß plaques, loss of neurons & an impairment of NAA/Cr ratio (3) , which wasn’t accompanied with significant brain atrophy & subsequently this implies caution when projecting these experiments form mice models to humans.

Keywords

APPswe/PS1 dE9 mice, neuronal loss, Aß-plaques, NAA/Cr ratio, brain atrophy

Acknowledgements

No acknowledgement found.

References

1- Ashburner, J., Friston, K., 1997. Multimodal image coregistration and partitioning–a unified framework. NeuroImage 6, 209–217.

2- Biedermann, S.V., Fuss, J., Steinle, J., Auer, M.K., Dormann, C., Falfán-Melgoza, C., Ende, G., Gass, P., Weber-Fahr, W., 2016. The hippocampus and exercise: histological correlates of MR-detected volume changes. Brain Struct. Funct. 221, 1353–1363.

3- Chen, S.Q., Cai, Q., Shen, Y.Y., Wang, P.J., Teng, G.J., Zhang, W., Zang, F.C., 2012. Age- related changes in brain metabolites and cognitive function in APP/PS1 transgenic mice. Behav. Brain Res. 235, 1–6.

4- Lau, J.C., Lerch, J.P., Sled, J.G., Henkelman, R.M., Evans, A.C., Bedell, B.J., 2008. Longitudinal neuroanatomical changes determined by deformation-based morphometry in a mouse model of Alzheimer's disease. NeuroImage 42, 19–27.

5- Mlynárik, V., Cacquevel, M., Sun-Reimer, L., Janssens, S., Cudalbu, C., Lei, H., Schneider, B.L., Aebischer, P., Gruetter, R., 2012. Proton and phosphorus magnetic resonance spectroscopy of a mouse model of Alzheimer's disease. J. Alzheimers Dis. 31 (Suppl.3), S87–S99.

6- Teipel, S.J., Buchert, R., Thome, J., Hampel, H., Pahnke, J., 2011. Development of Alzheimer-disease neuroimaging-biomarkers using mouse models with amyloid-precursor protein-transgene expression. Prog. Neurobiol. 95, 547–556.

Figures

Significant reductions of gray matter volumes in transgenic APPswe/PS1dE9 (tg) mice compared to wild type (wt) mice indicated by the colored regions where the colon intensity corresponds to the voxel-wise T-value (see color bar in the right). Cluster extension set at > 5 contiguous voxels passing the significance threshold of p < 0.01, uncorrected. Effects are projected on the group-specific averaged anatomical image in standard space representing the template brain. The right side of the image corresponds to the right side of the brain (a: anterior, p: posterior, rt: right, lt: left

(A): Transversal T2 weighted MRI image including position of the spectroscopy voxel (red dashed box). (B) MRS spectrum as derived from the voxel of interest shown in A. Two prominent metabolites, e.g. N-acetylaspartate (NAA resonates at 2.0 ppm) and creatine (Cr at 3.0 ppm) from a tg (APPswe/PS1dE9) mouse are evident and were further evaluated. (C). Diagram of the NAA/Cr ratio in wild type (wt) and transgenic APPswe/PS1dE9 (tg) mice. Values are given as mean±SEM.

Quantitative analysis of the number of NeuN positive neurons (A) and of GFAP positive glial cells (B) per mm2 in the cortex and in the hippocampus of wild type (wt) and of transgenic APPswe/PS1dE9 (tg) mice. Values are given as mean±SEM; ANOVA on ranks (Mann-Whitney Rank Sum test): * p < 0.05 vs. wt.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
2020