Decoupling the effect of mutant amyloid precursor protein (APP) from the effect of plaque on axonal transport dynamics in the living mouse brain: A correlation MRI-microscopy study

Abstract

The parent protein for amyloid plaques, amyloid precursor protein (APP), mediates cargo‐motor attachments for intracellular transport. Axonal transport is decreased and the distal location of accumulation is altered in transgenic mice expressing human APP with the Swedish and Indiana mutations (APPSwInd) linked to Familial Alzheimer's Disease, as detected by time‐lapse magnetic resonance imaging (MRI) of transport in living mouse brains (Bearer et al. 2017). Transport is also altered in brains of Down syndrome mice with 3 copies of APP gene. Questions now become whether expression of mutated APP effects transport dynamics independent of plaque, and do plaques alone contribute to transport defects? To address these we used the Tet‐Off system to decouple expression of APPSwInd from presence of plaques, and then studied transport using our MRI technique in three experimental groups of transgenic mice in which the timing and duration of APPSwInd expression, and thereby plaque formation, was altered with doxycycline: Group A (+ plaques, + APPSwInd); Group B (+ plaques, no APPSwInd), and group C (no plaques, + APPSwInd). Manganese‐enhanced MRI (MEMRI) allows us to perform cell biological experiments in live animals with T1‐weighted MRI in a Bruker 11.7T scanner (Medina et al 2016). Time‐lapse MR images were captured before and after stereotactic injection of Mn2+ (3‐5nL) into CA3 of the hippocampus at successive time‐points. Images of multiple individuals were aligned and processed with our automated computational pipeline (Medina et al. 2017) and statistical parametric mapping (SPM) performed. After MRI brains were harvested for histopathology or biochemistry. Results show that within group between time‐point have altered transport locations as well as diminished transport in all groups compared to wildtype (p<0.05 FDR n= 36). Preliminary ANOVA between‐group comparisons both by SPM and by region of interest measurements of images support the visual impression that APPSwInd expression alone may compromise transport. Groups A and B displayed plaques, but not C, and Western blots showed APPSwInd expressed 3.2‐fold over normal at sacrifice in Groups A and C but not B, with Aβ detected only in Groups A and B, where phospho‐tau was also present in dystrophic neurites surrounding plaques. Cholinergic neurons that project to hippocampus from the medial septal nucleus were decreased in Group C (p=0.0006 by ANOVA, n=15). Isolated hippocampal vesicles contained Mn2+, as well as Trk (NGF receptor), Rab 5 and 7 (associated with transport vesicles), suggesting a distinct vesicle population is affected by these APP mutations. These surprising results implicate mutated APPSwInd in transport defects, separable from the effect of plaque.

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