Biomarkers.

Background: Myelin integrity is central to healthy brains and is increasingly shown to be compromised in neurodegenerative diseases. Diffusion- and susceptibility-based MRI metrics can detect myelin changes. We show advanced diffusion and susceptibility metrics can detect degenerative myelin changes in ex vivo AD and HD mouse models.

Method: AD mice included 2 groups: 8-month-old male hAPP (Lond/Swe mutations, n=4) and wild-type (WT, n=4). HD mice included 3 groups: 12-week-old vehicle-treated controls (WT, n=4), R6/2 HD treated for 7 weeks with vehicle (HD, n=4) or LM11A-31 (HD-treated, n=4). After perfusion-fixation and brain extraction, 4-shell diffusion (b=1,2,5,10ms/μm, 100 directions) and multi-echo gradient echo (MGE) MRI (10 echoes, GRE=4-40ms) were performed on a Bruker 7T scanner. We used DESIGNER and MEDI to generate diffusion- and susceptibility-based myelin-sensitive metrics, respectively (diffusion: mean/radial/axial diffusivities -MD/RD/AD, kurtoses -MK/RK/AK, axonal water fraction -AWF; susceptibility: R2*, quantitative susceptibility mapping-QSM) (Figure 1A). Datasets were registered to the Allen Atlas through linear/non-linear transformations using FSL flirt/ANTs (Figure 1B). Median values were derived for two Allen Atlas white matter regions-of-interest -ROIs (corpus callosum and fornix). For AD, scan timing effects were included in a 2-way ANOVA analysis. For HD, two-sample t-tests were performed between the 3 groups. For histological validation, a second set of R6/2 HD brains (WT, HD, HD-treated n=10-13 mice/group) were myelin basic protein (MBP)-stained, and the percent area of striatum immunostaining determined by ImageJ thresholding.

Results: AD mice showed increased RD and reduced AWF and RK compared to controls in both ROIs, suggesting a loss of myelinated axons. Susceptibility-based metrics show no clear pattern. HD mice showed an increasing trend in AWF and RK and decreases in RD in both ROIs vs WT, suggesting increased myelination. Interestingly, LM11A-31-treated HD mice showed a significant reversal in RD and AWF. Histologically, myelin immunostaining was increased in the striatum of HD mice, reversed by LM11A-31.

Conclusion: Diffusion MRI myelin-sensitive metrics detected possibly compromised myelin in AD mouse white matter (corpus callosum and fornix). In HD mice, both diffusion MRI and histology showed a trend towards increased myelin, which was reversed by LM11A-31 treatment.