Synchrotron soft X-ray nano-imaging and spectromicroscopy reveals iron and calcium biomineralization in Alzheimer's disease amyloid plaques.

The chemistry of copper and iron plays a critical role in normal brain function. A variety of enzymes and proteins containing positively charged Cu+, Cu2+, Fe2+, and Fe3+ control key processes, catalyzing oxidative metabolism and neurotransmitter and neuropeptide production. Here, we report the discovery of elemental (zero–oxidation state) metallic Cu0 accompanying ferromagnetic elemental Fe0 in the human brain. These nanoscale biometal deposits were identified within amyloid plaque cores isolated from Alzheimer’s disease subjects, using synchrotron x-ray spectromicroscopy. The surfaces of nanodeposits of metallic copper and iron are highly reactive, with distinctly different chemical and magnetic properties from their predominant oxide counterparts. The discovery of metals in their elemental form in the brain raises new questions regarding their generation and their role in neurochemistry, neurobiology, and the etiology of neurodegenerative disease.

Establishing the composition and distribution of chemical compounds within biological materials is fundamental to understanding the chemistry that drives life. Additionally, region-specific changes in chemical homeostasis have been linked to disease states [1]. Traditional techniques used to explore these relationships rely on the use of fixatives and dyes, which can significantly alter the native chemistry of the sample material [2]. Here we describe methodology for the preparation and examination of biological sample materials, including human tissues, using the X-ray Spectromicroscopy technique Scanning Transmission X-ray Microscopy (STXM). Through STXM we show the distribution and speciation of organic and inorganic materials within biological samples at a nanoscale resolution. Importantly this approach does not require the use of dyes, aldehyde fixatives or contrast agents and therefore offers an unprecedented insight into the native chemistry of biological samples. Human brain tissues were ethanol dehydrated and embedded in a resin comprised of equimolar trimethylolpropane triglycidyl ether: 4,4'-methylenebis (2-methylcyclohexylamine). Semi-thin (typically 200-500 nm) sections from embedded samples were cut using a non-metallic blade, before being mounted onto a support. STXM was performed at the carbon K-edge, oxygen K-edge, calcium L-edge and iron Ledge. To map distributions of chemical species, paired images were taken at the energy corresponding to a peak feature of interest and an off-peak energy a few eV away from this feature. The off-peak image is then subtracted from the peak image to give a difference map. This process is demonstrated in Figure 1, for a 200 nm thick section of human substantia nigra tissue imaged at the carbon K-edge. Here the peak image (288.3 eV; left) corresponds to the 1s to π* transition for amide groups (proteins), whereas the offpeak image (290 eV; middle) corresponds to the principal carbon K-edge absorption feature for the resin (see also [1]). This image processing allows artifacts and background resin absorption features to be removed, revealing the true tissue structure. From these difference maps, tissue structure is shown to be well preserved over the 160 µm 2 area presented. The high resolution map taken at ca. 40 nm resolution (right; inset) shows cellular structures, also displaying subcellular organelles.Further speciation maps and a carbon K-edge absorption spectrum from a 200 nm thick section of human amygdala tissue are shown in Figure 2. X-ray absorption spectra were obtained from a series of images taken at energies spanning the carbon K-edge edge. The resulting spectrum from tissue area A1 ( Figure 2f; green spectrum) was consistent with a synthetic peptide standard (Figure 2f; black spectrum), indicating the tissue to have been preserved through the embedding process. Further dense areas of tissue (as evidenced by enhanced contrast in Figure 2a) were found to be loaded with calcium (Figure 2b), carbonates ( Figure 2c) and iron (Figure 2d). This demonstra...