Mapping metals in brain tissue with X-ray fluorescence and X-ray absorption spectroscopy at synchrotron light sources

Abstract

The hippocampus (Figure 1A) is a brain region critical to spatial learning and memory. The hippocampal formation contains a highly organised architecture of neurons and neuron–neuron connections, which have been intensively studied by neuroscientists for many decades. In fact, the hippocampus is often referred to as the “Rosetta Stone” of neuroscience, with many believing elucidation of hippocampus circuitry and cell function will unravel the inner workings of the brain. A fasc inat ing fact of the hippocampus is that it appears to be relatively enriched in transition metal ions, particularly Fe, Cu and Zn (Figure 1B). The Zn enrichment was discovered by scientists developing histochemical methods to detect labile metals in brain tissue (e.g., works of Danscher and others),1,2 with work led by Frederickson definitively demonstrating that the characteristic pool of labile metal ions observed in the hippocampus was Zn.2–4 Of great interest, experiments aimed at depleting the labile Zn pool in the hippocampus subsequently revealed behavioural and cognitive deficits in mice,2 consistent with facets of memory loss observed during neurodegenerative diseases of ageing, such as Alzheimer’s disease.5 Consequently, a plethora of lines of research enquiries emerged, aiming to uncover the physiological and chemical pathways through which transition metal ions might be implicated in healthy memory function, and also memory loss. While the classical Timm’s histochemical stain has been invaluable to study labile Zn in the hippocampus (and brain in general), a number of important advances in this field have now been made using direct spectroscopic mapping. Specifically, the provision of intense (bright) and tuneable X-ray sources at synchrotron facilities has revolutionised the biological applications of X-ray techniques, especially X-ray fluorescence spectroscopy (XRF) and X-ray absorption spectroscopy (XAS). Key advantages of XRF are its ability to simultaneously and directly detect (map) elemental distribution at cellular resolution (and sometimes sub-cellular resolution), in situ. The direct in situ detection capabilities of XRF are