Imaging zinc speciation in the mouse hippocampus with µXANES Spectroscopic mapping

TL;DR

This study uses XANES spectroscopy to image zinc speciation in the mouse hippocampus, revealing that labile zinc is likely coordinated with histidine.

Contribution

The study introduces XANES protocols for imaging Zn2+ speciation in brain tissue and identifies histidine coordination in the labile Zn2+ pool.

Findings

XANES spectroscopy enables chemically specific imaging of Zn2+ in murine hippocampal tissue.

Sample preparation significantly affects metal speciation measurements.

Histidine coordination is likely the dominant form of labile Zn2+ in the hippocampus.

Abstract

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.

Graphical Abstract

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