Imaging zinc speciation in the mouse hippocampus with µXANES Spectroscopic mapping

Zinc ions (Zn2+) are the second most abundant trace metal ion in the brain of rodents and primates, often serving functions as a structure-stabilizing element or catalytic role. There is an additional pool of Zn2+, ∼15% of total brain Zn2+, which exists in a labile chemical form in a specific subset of glutamatergic neurons ('zinergic' or 'zincergic' neurons). The labile pool of Zn2+ is now well established to be critical for healthy memory function, with disturbance to the labile Zn2+ pool implicated in diminished memory performance during the ageing process or neurodegeneration. The chemical form of Zn2+ in the labile Zn2+ pool has however, remained unknown, largely due to the difficulty of imaging metal speciation for 'spectroscopically silent' metals such as Zn2+. In this study, we have developed X-ray absorption near edge structure (XANES) spectroscopic protocols to enable chemically specific imaging of Zn2+ speciation in murine brain (hippocampal) tissue. The protocols capitalise on the unique sensitivity of the XANES spectral region to metal ion coordination environment, enabling a direct in situ measurement of metal speciation. Key findings of our method development are characterisation of the effects of sample preparation on metal speciation, and revelation that Zn2+ coordination with histidine is likely to be the dominant coordination environment of the labile Zn2+ pool in the murine hippocampus.