Implementation of X-ray fluorescence microscopy for investigation of elemental abnormalities in amyotrophic lateral sclerosis

A review of the “metallome” within neurons and glia, as revealed by elemental mapping of brain tissue

It is now well established that transition metals, such as Iron (Fe), Copper (Cu), and Zinc (Zn) are necessary for healthy brain function. Although Fe, Cu, and Zn are essential to the brain, imbalances in the amount, distribution, or chemical form ("metallome") of these metals is linked to the pathology of numerous brain diseases or disorders. Despite the known importance of metal ions for both brain health and disease, the metallome that exists within specific types of brain cells is yet to be fully characterised. The aim of this mini-review is to present an overview of the current knowledge of the metallome found within specific brain cells (oligodendrocytes, astrocytes, microglia, and neurons), as revealed by direct elemental mapping techniques. It is hoped this review will foster continued research using direct elemental mapping techniques to fully characterise the brain cell metallome.

Crossroads between membrane trafficking machinery and copper homeostasis in the nerve system

Imbalanced copper homeostasis and perturbation of membrane trafficking are two common symptoms that have been associated with the pathogenesis of neurodegenerative and neurodevelopmental diseases. Accumulating evidence from biophysical, cellular and in vivo studies suggest that membrane trafficking orchestrates both copper homeostasis and neural functions-however, a systematic review of how copper homeostasis and membrane trafficking interplays in neurons remains lacking. Here, we summarize current knowledge of the general trafficking itineraries for copper transporters and highlight several critical membrane trafficking regulators in maintaining copper homeostasis. We discuss how membrane trafficking regulators may alter copper transporter distribution in different membrane compartments to regulate intracellular copper homeostasis. Using Parkinson's disease and MEDNIK as examples, we further elaborate how misregulated trafficking regulators may interplay parallelly or synergistically with copper dyshomeostasis in devastating pathogenesis in neurodegenerative diseases. Finally, we explore multiple unsolved questions and highlight the existing challenges to understand how copper homeostasis is modulated through membrane trafficking.

A novel hypothesis on metal dyshomeostasis and mitochondrial dysfunction in amyotrophic lateral sclerosis: Potential pathogenetic mechanism and therapeutic implications

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder characterized by motor dysfunctions resulting from the loss of upper (UMNs) and lower (LMNs) motor neurons. While ALS symptoms are coincidental with pathological changes in LMNs and UMNs, the causal relationship between the two is unclear. For example, research on the extra-motor symptoms associated with this condition suggests that an imbalance of metals, including copper, zinc, iron, and manganese, is initially induced in the sensory ganglia due to a malfunction of metal binding proteins and transporters. It is proposed that the resultant metal dyshomeostasis may promote mitochondrial dysfunction in the satellite glial cells of these sensory ganglia, causing sensory neuron disturbances and sensory symptoms. Sensory neuron hyperactivation can result in LMN impairments, while metal dyshomeostasis in spinal cord and brain stem parenchyma induces mitochondrial dysfunction in LMNs and UMNs. These events could prompt intracellular calcium dyshomeostasis, pathological TDP-43 formation, and reactive microglia with neuroinflammation, which in turn activate the apoptosis signaling pathways within the LMNs and UMNs. Our model suggests that the degeneration of LMNs and UMNs is incidental to the metal-induced changes in the spinal cord and brain stem. Over time psychiatric symptoms may appear as the metal dyshomeostasis and mitochondrial dysfunction affect other brain regions, including the reticular formation, hippocampus, and prefrontal cortex. It is proposed that metal dyshomeostasis in combination with mitochondrial dysfunction could be the underlying mechanism responsible for the initiation and progression of the pathological changes associated with both the motor and extra-motor symptoms of ALS.

Ionic Homeostasis Maintenance in ALS: Focus on New Therapeutic Targets

Amyotrophic lateral sclerosis (ALS) is one of the most threatening neurodegenerative disease since it causes muscular paralysis for the loss of Motor Neurons in the spinal cord, brainstem and motor cortex. Up until now, no effective pharmacological treatment is available. Two forms of ALS have been described so far: 90% of the cases presents the sporadic form (sALS) whereas the remaining 10% of the cases displays the familiar form (fALS). Approximately 20% of fALS is associated with inherited mutations in the Cu, Zn-superoxide dismutase 1 (SOD1) gene. In the last decade, ionic homeostasis dysregulation has been proposed as the main trigger of the pathological cascade that brings to motor-neurons loss. In the light of these premises, the present review will analyze the involvement in ALS pathophysiology of the most well studied metal ions, i.e., calcium, sodium, iron, copper and zinc, with particular focus to the role of ionic channels and transporters able to contribute in the regulation of ionic homeostasis, in order to propose new putative molecular targets for future therapeutic strategies to ameliorate the progression of this devastating neurodegenerative disease.

Imaging of glial cell morphology, SOD1 distribution and elemental composition in the brainstem and hippocampus of the ALS hSOD1G93A rat

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor and cognitive domains of the CNS. Mutations in the Cu,Zn-superoxide dismutase (SOD1) cause 20% of familial ALS and provoke formation of intracellular aggregates and copper and zinc unbinding, leading to glial activation and neurodegeneration. Therefore, we investigated glial cell morphology, intracellular SOD1 distribution, and elemental composition in the brainstem and hippocampus of the hSOD1^G93A transgenic rat model of ALS. Immunostaining for astrocytes, microglia and SOD1 revealed glial proliferation and progressive tissue accumulation of SOD1 in both brain regions of ALS rats starting already at the presymptomatic stage. Glial cell morphology analysis in the brainstem of ALS rats revealed astrocyte activation occurring before disease symptoms onset, followed by activation of microglia. Hippocampal ALS astrocytes exhibited an identical reactive profile, while microglial morphology was unchanged. Additionally, ALS brainstem astrocytes demonstrated progressive SOD1 accumulation in the cell body and processes, while microglial SOD1 levels were reduced and its distribution limited to distal cell processes. In the hippocampus both glial cell types exhibited SOD1 accumulation in the cell body. X-ray fluorescence imaging revealed decreased P and increased Ca, Cl, K, Ni, Cu and Zn in the brainstem, and higher levels of Cl, Ni and Cu, but lower levels of Zn in the hippocampus of symptomatic ALS rats. These results bring new insights into the glial response during disease development and progression in motor as well as in non-motor CNS structures, and indicate disturbed tissue elemental homeostasis as a prominent hallmark of disease pathology.

Bioanalytics in Quantitive (Bio)imaging/Mapping of Metallic Elements in Biological Samples

The aim of this article is to describe selected analytical techniques and their applications in the quantitative mapping/(bio)imaging of metals in biological samples. This work presents the advantages and disadvantages as well as the appropriate methods of scope for research. Distribution of metals in biological samples is currently one of the most important issues in physiology, toxicology, pharmacology, and other disciplines where functional information about the distribution of metals is essential. This issue is a subject of research in (bio)imaging/mapping studies, which use a variety of analytical techniques for the identification and determination of metallic elements. Increased interest in analytical techniques enabling the (bio)imaging of metals in a variety of biological material has been observed more recently. Measuring the distribution of trace metals in tissues after a drug dose or ingestion of poison-containing metals allows for the studying of pathomechanisms and the pathophysiology of various diseases and disorders related to the management of metals in human and animal systems.

The role of iron in neurodegenerative disorders: insights and opportunities with synchrotron light

There is evidence for iron dysregulation in many forms of disease, including a broad spectrum of neurodegenerative disorders. In order to advance our understanding of the pathophysiological role of iron, it is helpful to be able to determine in detail the distribution of iron as it relates to metabolites, proteins, cells, and tissues, the chemical state and local environment of iron, and its relationship with other metal elements. Synchrotron light sources, providing primarily X-ray beams accompanied by access to longer wavelengths such as infra-red, are an outstanding tool for multi-modal non-destructive analysis of iron in these systems. The micro- and nano-focused X-ray beams that are generated at synchrotron facilities enable measurement of iron and other transition metal elements to be performed with outstanding analytic sensitivity and specificity. Recent developments have increased the scope for methods such as X-ray fluorescence mapping to be used quantitatively rather than semi-quantitatively. Burgeoning interest, coupled with technical advances and beamline development at synchrotron facilities, has led to substantial improvements in resources and methodologies in the field over the past decade. In this paper we will consider how the field has evolved with regard to the study of iron in proteins, cells, and brain tissue, and identify challenges in sample preparation and analysis. Selected examples will be used to illustrate the contribution, and future potential, of synchrotron X-ray analysis for the characterization of iron in model systems exhibiting iron dysregulation, and for human cases of neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Friedreich's ataxia, and amyotrophic lateral sclerosis.

Experimental demonstration of novel imaging geometries for x-ray fluorescence computed tomography

PURPOSE

X-ray fluorescence computed tomography (XFCT) is an emerging imaging modality that maps the three-dimensional distribution of elements, generally metals, in ex vivo specimens and potentially in living animals and humans. At present, it is generally performed at synchrotrons, taking advantage of the high flux of monochromatic x rays, but recent work has demonstrated the feasibility of using laboratory-based x-ray tube sources. In this paper, the authors report the development and experimental implementation of two novel imaging geometries for mapping of trace metals in biological samples with ∼50-500 μm spatial resolution.

METHODS

One of the new imaging approaches involves illuminating and scanning a single slice of the object and imaging each slice's x-ray fluorescent emissions using a position-sensitive detector and a pinhole collimator. The other involves illuminating a single line through the object and imaging the emissions using a position-sensitive detector and a slit collimator. They have implemented both of these using synchrotron radiation at the Advanced Photon Source.

RESULTS

The authors show that it is possible to achieve 250 eV energy resolution using an electron multiplying CCD operating in a quasiphoton-counting mode. Doing so allowed them to generate elemental images using both of the novel geometries for imaging of phantoms and, for the second geometry, an osmium-stained zebrafish.

CONCLUSIONS

The authors have demonstrated the feasibility of these two novel approaches to XFCT imaging. While they use synchrotron radiation in this demonstration, the geometries could readily be translated to laboratory systems based on tube sources.

Localization of copper and copper transporters in the human brain

Disturbances in brain copper result in rare and severe neurological disorders and may play a role in the pathogenesis and progression of multiple neurodegenerative diseases. Our current understanding of mammalian brain copper transport is based on model systems outside the central nervous system and no data are available regarding copper transport systems in the human brain. To address this deficit, we quantified regional copper concentrations and examined the distribution and cellular localization of the copper transport proteins Copper transporter 1, Atox1, ATP7A, and ATP7B in multiple regions of the human brain using inductively coupled plasma-mass spectrometry, Western blot and immunohistochemistry. We identified significant relationships between copper transporter levels and brain copper concentrations, supporting a role for these proteins in copper transport in the human brain. Interestingly, the substantia nigra contained twice as much copper than that in other brain regions, suggesting an important role for copper in this brain region. Furthermore, ATP7A levels were significantly greater in the cerebellum, compared with other brain regions, supporting an important role for ATP7A in cerebellar neuronal health. This study provides novel data regarding copper regulation in the human brain, critical to understand the mechanisms by which brain copper levels can be altered, leading to neurological disease.

Metal imaging in neurodegenerative diseases

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.

Elemental micro-imaging and quantification of human substantia nigra using synchrotron radiation based x-ray fluorescence--in relation to Parkinson's disease

Synchrotron radiation based x-ray fluorescence (SRXRF) was applied to the quantitative evaluation of elemental changes in substantia nigra pars compacta (SNc) in Parkinson's disease (PD) in the framework of a study on the role of chemical elements in the pathophysiology of PD. The analysis was carried out for dopaminergic nerve cells and extraneuronal spaces. The mass fractions of P, S, Cl, K, Ca, Fe, Cu, Zn, Br and Rb were determined. The application of standard samples developed especially for the determination of elemental mass fractions in thin tissue sections using the SRXRF technique is presented. Two-dimensional maps of elemental distribution show that the location of nerve cells in SNc sections is precisely visualized by the high levels of most elements. It was found that statistically significant differences between control and PD neurons are observed for S (p = 0.04), Cl (p = 0.02), Ca (p = 0.08), Fe (p = 0.04) and Zn (p = 0.04). The mass fractions of P (p = 0.08), S (p = 0.07), Cl (p = 0.04), Zn (p = 0.08) and Rb (p = 0.08) in areas outside the nerve cell bodies differed significantly between PD and control groups. A clear cluster separation between the PD nerve cells and neurons representing the control group was noticed. It was found that Cl, Fe, Ca and Zn are the most significant elements in the general discrimination between PD nerve cells and the control. The comparison between the extraneuronal spaces showed that Cl, Fe and Cu differentiate the PD and control group the most. The evident contribution of chemical elements to the pathophysiology of PD was shown.

Copper and zinc metallation status of copper-zinc superoxide dismutase from amyotrophic lateral sclerosis transgenic mice

Mutations in the metalloenzyme copper-zinc superoxide dismutase (SOD1) cause one form of familial amyotrophic lateral sclerosis (ALS), and metals are suspected to play a pivotal role in ALS pathology. To learn more about metals in ALS, we determined the metallation states of human wild-type or mutant (G37R, G93A, and H46R/H48Q) SOD1 proteins from SOD1-ALS transgenic mice spinal cords. SOD1 was gently extracted from spinal cord and separated into insoluble (aggregated) and soluble (supernatant) fractions, and then metallation states were determined by HPLC inductively coupled plasma MS. Insoluble SOD1-rich fractions were not enriched in copper and zinc. However, the soluble mutant and WT SOD1s were highly metallated except for the metal-binding-region mutant H46R/H48Q, which did not bind any copper. Due to the stability conferred by high metallation of G37R and G93A, it is unlikely that these soluble SOD1s are prone to aggregation in vivo, supporting the hypothesis that immature nascent SOD1 is the substrate for aggregation. We also investigated the effect of SOD1 overexpression and disease on metal homeostasis in spinal cord cross-sections of SOD1-ALS mice using synchrotron-based x-ray fluorescence microscopy. In each mouse genotype, except for the H46R/H48Q mouse, we found a redistribution of copper between gray and white matters correlated to areas of high SOD1. Interestingly, a disease-specific increase of zinc was observed in the white matter for all mutant SOD1 mice. Together these data provide a picture of copper and zinc in the cell as well as highlight the importance of these metals in understanding SOD1-ALS pathology.

The use of cluster and discriminant analysis in the investigations of the role of trace metals in the pathogenesis of Parkinson's disease

X-ray fluorescence microscopy was applied for two-dimensional elemental analysis of substantia nigra (SN) tissue. The samples representing Parkinson's disease (PD) and control cases were examined at HASYLAB beamline L and at ESRF beamline ID22. Two-dimensional mapping of P, S, Cl, K, Ca, Fe, Cu, Zn, Se and Br was done with the spatial resolution of 15 and 5 microm. The masses per unit area of elements in neuromelanin reach nerve cells of SN were determined. The elemental data were processed using two multivariate techniques, namely cluster and discriminant analysis. The statistical methods were used for data reduction, both unsupervised and supervised classification as well as for the creation of a model that would simplify case identification based on the elemental analysis of SN tissue. The results of cluster analysis confirmed the statistical significance of the differences in elemental composition of PD and control SN nerve cells. Based on the results of discriminant analysis, the elements (P, Cl, Fe, Cu and Zn) that played the greatest role in the process of differentiation between neurons from examined groups were determined.

Region of interest reconstruction in x-ray fluorescence computed tomography for negligible attenuation

X-ray fluorescence computed tomography (XFCT) is a synchrotron-based imaging modality employed for mapping the distribution of elements within slices or volumes of intact specimens. A pencil beam of external radiation is used to stimulate emission of characteristic X-rays from within a sample, which is scanned and rotated through the pencil beam in a first-generation tomographic geometry. One limitation of XFCT is the long image acquisition time required to acquire a complete set of line integrals one-by-one. Typically, even if only a portion of a slice through the object is of interest, measurement lines are acquired spanning the entire object at every projection view over 180 degrees to avoid reconstructing images with so-called truncation artifacts. In this work, we show that when attenuation is negligible, recent developments in tomographic reconstruction theory can be used to reduce the scanning effort required to reconstruct regions of interest within the slice. The new theory provides explicit guidance as to which line integrals must be measured for a given ROI and also provides a backprojection-filtration reconstruction algorithm that averts the truncation artifacts that typically plague filtered backprojection reconstructions from truncated data. This is demonstrated through simulation studies and with real synchrotron-based XFCT data.

The role of trace elements in the pathogenesis and progress of pilocarpine-induced epileptic seizures

X-ray fluorescence microscopy was applied for topographic and quantitative elemental analysis within the areas of the rat brain that undergo neurodegenerative changes in consequence of pilocarpine-induced seizures. Significant changes in levels of selected elements were observed in epileptic animals. They included an increased tissue content of Ca in the CA1 and CA3 regions of the hippocampus and in the cerebral cortex. The opposite relation was observed for the Cu level in the dentate gyrus and for Zn in the CA3 region of the hippocampus and in the dentate gyrus.

Biological applications of X-ray fluorescence microscopy: exploring the subcellular topography and speciation of transition metals

Synchrotron X-ray fluorescence microscopy (SXRF) is a microanalytical technique for the quantitative mapping of elemental distributions. Among currently available imaging modalities, SXRF is the only technique that is compatible with fully hydrated biological samples such as whole cells or tissue sections, while simultaneously offering trace element sensitivity and submicron spatial resolution. Combined with the ability to provide information regarding the oxidation state and coordination environment of metal cations, SXRF is ideally suited to study the intracellular distribution and speciation of trace elements, toxic heavy metals and therapeutic or diagnostic metal complexes.

A new sample substrate for imaging and correlating organic and trace metal composition in biological cells and tissues

Many disease processes involve alterations in the chemical makeup of tissue. Synchrotron-based infrared (IR) and X-ray fluorescence (XRF) microscopes are becoming increasingly popular tools for imaging the organic and trace metal compositions of biological materials, respectively, without the need for extrinsic labels or stains. Fourier transform infrared microspectroscopy (FTIRM) provides chemical information on the organic components of a material at a diffraction-limited spatial resolution of 2-10 microm in the mid-infrared region. The synchrotron X-ray fluorescence (SXRF) microprobe is a complementary technique used to probe trace element content in the same systems with a similar spatial resolution. However to be most beneficial, it is important to combine the results from both imaging techniques on a single sample, which requires precise overlap of the IR and X-ray images. In this work, we have developed a sample substrate containing a gold grid pattern on its surface, which can be imaged with both the IR and X-ray microscopes. The substrate consists of a low trace element glass slide that has a gold grid patterned on its surface, where the major and minor parts of the grid contain 25 and 12 nm gold, respectively. This grid pattern can be imaged with the IR microscope because the reflectivity of gold differs as a function of thickness. The pattern can also be imaged with the SXRF microprobe because the Au fluorescence intensity changes with gold thickness. The tissue sample is placed on top of the patterned substrate. The grid pattern's IR reflectivity image and the gold SXRF image are used as fiducial markers for spatially overlapping the IR and SXRF images from the tissue. Results show that IR and X-ray images can be correlated precisely, with a spatial resolution of less than one pixel (i.e., 2-3 microns). The development of this new tool will be presented along with applications to paraffin-embedded metalloprotein crystals, Alzheimer's disease, and hair composition.