X-ray fluorescence imaging reveals subcellular biometal disturbances in a childhood neurodegenerative disorder

Biometals such as zinc, iron, copper and calcium play key roles in diverse physiological processes in the brain, but can be toxic in excess. A hallmark of neurodegeneration is a failure of homeostatic mechanisms controlling the concentration and distribution of these elements, resulting in overload, deficiency or mislocalization. A major roadblock to understanding the impact of altered biometal homeostasis in neurodegenerative disease is the lack of rapid, specific and sensitive techniques capable of providing quantitative subcellular information on biometal homeostasis in situ. Recent advances in X-ray fluorescence detectors have provided an opportunity to rapidly measure biometal content at subcellular resolution in cell populations using X-ray Fluorescence Microscopy (XFM). We applied this approach to investigate subcellular biometal homeostasis in a cerebellar cell line isolated from a natural mouse model of a childhood neurodegenerative disorder, the CLN6 form of neuronal ceroid lipofuscinosis, commonly known as Batten disease. Despite no global changes to whole cell concentrations of zinc or calcium, XFM revealed significant subcellular mislocalization of these important biological second messengers in cerebellar Cln6 (CbCln6 ) cells. XFM revealed that nuclear-to-cytoplasmic trafficking of zinc was severely perturbed in diseased cells and the subcellular distribution of calcium was drastically altered in CbCln6 cells. Subtle differences in the zinc K-edge X-ray Absorption Near Edge Structure (XANES) spectra of control and CbCln6 cells suggested that impaired zinc homeostasis may be associated with an altered ligand set in CbCln6 cells. Importantly, a zinc-complex, Zn^II(atsm), restored the nuclear-to-cytoplasmic zinc ratios in CbCln6 cells via nuclear zinc delivery, and restored the relationship between subcellular zinc and calcium levels to that observed in healthy control cells. Zn^II(atsm) treatment also resulted in a reduction in the number of calcium-rich puncta observed in CbCln6 cells. This study highlights the complementarities of bulk and single cell analysis of metal content for understanding disease states. We demonstrate the utility and broad applicability of XFM for subcellular analysis of perturbed biometal metabolism and mechanism of action studies for novel therapeutics to target neurodegeneration.