Why are amyloid-β plaques detected by X-ray phase-contrast imaging? Role of metals revealed through combined synchrotron infrared and X-ray fluorescence microscopies

Abstract

Amyloid-β (Aβ) plaques from Alzheimer's Disease (AD) can be visualized ex vivo in label-free brain samples using synchrotron X-ray phase-contrast tomography (XPCT). However, for XPCT to be useful as a screening method for amyloid pathology, it is mandatory to understand which factors drive the detection of Aβ plaques. The current study was designed to test the hypothesis that the Aβ-related contrast in XPCT could be caused by the Aβ fibrils and/or by metal entrapment within the Aβ plaques. This study probed the fibrillar and elemental compositions of Aβ plaques in brain samples from different types of AD patients and different AD animal models to establish a relationship between XPCT contrast and Aβ plaque characteristics. XPCT, micro-Fourier-Transform Infrared (μFTIR) spectroscopy and micro-X-Ray Fluorescence (μXRF) spectroscopy were conducted under synchrotron radiation on human samples (genetic and sporadic cases) and on four transgenic rodent strains (mouse: APPPS1, ArcAβ, J20; rat: TgF344). Aβ plaques from the genetic AD patient were visible using XPCT, and had higher β-sheet content and higher metal levels than those from the sporadic AD patient, which remained undetected by XPCT. Aβ plaques in J20 mice and TgF344 rats appeared hyperintense on XPCT images, while they were hypointense in the case of APPPS1 and ArcAβ mice. In all four transgenic strains, β-sheet content was similar, while metal levels were highly variable: J20 mice (zinc and iron), and TgF344 (copper) showed greater metal accumulation than APPPS1 and ArcAβ mice. In humans, the greater and more diffuse metal accumulation led to a positive contrast in the genetic case of AD. Hence, contrast formation of Aβ plaques in XPCT images depended mostly on biometal entrapment.