Quantitative gradient recalled echo (qGRE) MRI enables in vivo measurement of pre-atrophic neurodegeneration in a mouse model of Alzheimer's disease

Abstract

Robust methods are needed for preclinical evaluation of novel Alzheimer Disease (AD) therapies to accelerate drug discovery. Quantitative Gradient Recalled Echo (qGRE) MRI has shown promise to provide insight into neurodegeneration in AD prior to atrophy development in humans, highlighting areas of low neuronal density. In this study a novel qGRE method (20 echoes, TE=2-40ms) is shown to non-invasively measure the longitudinal neuronal loss in the hippocampus of a mouse model of AD tauopathy Tg4510. Tg4510 (n=10) and wild type (WT, n=6) mice underwent MRI (7T field strength) at 3-7 months old. 3D qGRE approach was used to generate brain-specific R₂* maps free of magnetic field inhomogeneity artifacts. Light-sheet microscopy of the brains stained with NeuN and MBP served to visualize neuronal nuclei and myelin content respectively. Significant decrease in NeuN staining between 3mo and 5mo was observed in the hippocampus of Tg4510, validating the mouse AD model. Longitudinal analysis showed clear decreases in R₂* metric of qGRE signal in the Tg4510 mice hippocampus undergoing neurodegeneration between 3 and 5 months old. Histogram analysis revealed an upward trend in patterns of low R₂* value (Dark Matter, DM), and broadening of R₂* distribution. These were quantified as significant increase in both DM Volume Fraction (DMVF) and R₂* Standard Deviation (SD) in Tg4510 mice (p=0.004/p=0.016 DMVF/SD) but not in WT controls (p>0.25). Further monotonical increase was also observed in both metrics in time. A significant negative correlation was observed between the DMVF and myelin content (p=0.01, r=-0.76), suggesting sensitivity of the technique to the loss of myelinated axons. The presented qGRE technique, validated by histological measurements, can be readily applied as in vivo tool in preclinical models of neurodegeneration for pharmacodynamics and mechanism of action assessment.

Keywords: Alzheimer's disease; MRI; Neurodegeneration; Neuronal density; QGRE; Tg4510.

Fig. 1.

Histological analysis reveals neurodegeneration in Tg4510 mice. Example NeuN staining pattern indicative of neuronal density at 5mo is shown in (A, left) together with anatomical True-FISP MRI. Quantification of mean neuronal densities in hippocampus of Tg4510 mice (B) reveals a decrease between 3 and 5 months of age (n=6 hemispheres from n=3 brains at each time-point). Green lines indicate atlas regions overlaid over the image. Grayscale presentation was chosen to facilitate visual comparison with MRI and emphasize the more binary black/white negative/positive relevance of the cellular staining, especially in the case of NeuN.

Fig. 2.

Changes observed longitudinally in R₂* maps in Tg4510 mice. Differences in both brain anatomy (TrueFISP scan, left) and R₂* relaxation maps (right) are shown for a representative Tg4510 mouse brain between 3 (top) and 5 (bottom) months of age. Black arrows indicate example areas of low R₂* which appear in 5mo but not 3mo brain.

Fig. 3.

Quantification of R₂* changes (Paired comparison of n=7). Evolution of the R₂* value distribution in a representative Tg4510 hippocampus is shown in (A), reflected in a trend towards lower mean R2* (B) and significantly increased incidence of low R₂* values, quantified in (C) as Dark Matter Volume Ratio (DMVF), as well as increased spatial R₂* heterogeneity quantified in (D) as standard deviation. Note that bright red color represents overlap of 3 months old and 5 months old histograms.

Fig. 4.

Temporal evolution of R₂* metrics in Tg4510 mice. Monotonic increase past 5 months of age is shown both for the Dark Matter Volume Fraction (A) and R2* standard deviation (B). n=10/7/4/4 at 3/5/6/7 months old. Error bars indicate standard error of the mean.

Fig. 5.

Brain R₂* changes are associated with de-myelination. (A) a representative image of Myelin Basic Protein staining in the Tg4510 brain, (B) relationship between hippocampal R₂*-defined Dark Matter Volume Fraction (DMVF) and the average intensity of MBP signal show negative correlation (p=0.01, B). Each point represents data from one mouse. Green lines indicate atlas regions overlaid over the image. WT – wild type.