Metal imaging in neurodegenerative diseases

Abstract

Metal ions are known to play an important role in many neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and prion diseases. In these diseases, aberrant metal binding or improper regulation of redox active metal ions can induce oxidative stress by producing cytotoxic reactive oxygen species (ROS). Altered metal homeostasis is also frequently seen in the diseased state. As a result, the imaging of metals in intact biological cells and tissues has been very important for understanding the role of metals in neurodegenerative diseases. A wide range of imaging techniques have been utilized, including X-ray fluorescence microscopy (XFM), particle induced X-ray emission (PIXE), energy dispersive X-ray spectroscopy (EDS), laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS), all of which allow for the imaging of metals in biological specimens with high spatial resolution and detection sensitivity. These techniques represent unique tools for advancing the understanding of the disease mechanisms and for identifying possible targets for developing treatments. In this review, we will highlight the advances in neurodegenerative disease research facilitated by metal imaging techniques.

Fig. 1

XFM reveals the metal content within the plaques and surrounding tissue of PSAPP mice, a model of AD. (A) Thioflavin S-stained PSAPP mouse brain tissue. XFM microprobe images of (B) zinc, (C) copper, (D) iron, and (E) calcium distribution in the same tissue. (F) XFM microprobe spectra from the center of a plaque (cyan) and from the surrounding normal tissue (red). All scale bars are 5 μm. (From Leskovjan et al.).

Fig. 2

XFM analysis of iron, copper, and zinc from the PSAPP mouse hippocampus. Spatial distribution of iron, copper, and zinc in the hippocampus of PSAPP and CNT mice measured using XFM. (A) H&E stained hippocampal brain section from a PSAPP mouse. XFM images of (B) iron, (C) zinc, and (D) copper in a serial tissue section. Units are mM. Scale bar=300 μm. (E) Hierarchical cluster analysis (HCA) was used to create unsupervised regions of interest (ROIs) based on iron, copper, and zinc content in order to compare metal content in separate regions of the hippocampus. On average, four clusters were required to separate the images into histological ROIs where the dendritic layer is gray, the PCL is green, CA3 is blue, and the hilus is magenta. These areas corresponded to distinct anatomical regions of the hippocampus defining four distinct regions of the hippocampus. (F) Average XFM spectra from each ROI (from Leskovjan et al.).

Fig. 3

Visible light image (left) and PIXE elemental maps for phosphorus, sulfur, iron, calcium, copper and nickel from two neuromelanin containing dopaminergic neurons, from a control specimen (top) and from a PD specimen (bottom). The iron image shows that there is no difference in concentration between the control and the PD specimen. Reprinted from Nucl. Instrum. Methods Phys. Res., Sect. B, 260, Reinert et al., High resolution quantitative element mapping of neuromelanin-containing neurons, pages 227–230, copyright 2007, with permission from Elsevier.

Fig. 4

LA-ICP-MS metal images of copper, iron, zinc and manganese representative of each group (control, 2 h, 7 d, and 28 d after the last of five daily MPTP injections). Sections are on a posterior level crossing the substantia nigra, the interpeduncular nucleus, and the hippocampus. Reprinted from J. Am. Soc. Mass Spectrom., 21, Matusch et al., Cerebral bioimaging of Cu, Fe, Zn, and Mn in the MPTP mouse model of Parkinson’s disease using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), pages 161–171, copyright 2010, with permission from Elsevier.

Fig. 5

XFM images of iron, copper, and zinc from a portion of the ventral horn of a healthy WT mouse (top) and a symptomatic G83R mouse (bottom). Several motor neurons can be seen in the zinc image from the WT mice, while no intact motor neurons are visible in the G85R mice. The G85R mice also show an increase in iron and zinc in the gray matter.