Biospectroscopic Imaging Provides Evidence of Hippocampal Zn Deficiency and Decreased Lipid Unsaturation in an Accelerated Aging Mouse Model

The application of X-ray fluorescence microscopy and micro-XANES spectroscopy to study neuro-metallomics

This early career research highlight provides a review of my own research program over the last decade, a time frame that encompasses my transition from postdoctoral fellowships to independent researcher. As an analytical chemist and applied spectroscopist, the central theme of my research program over this time has been protocol development at synchrotron facilities, with the main objective to investigate brain metal homeostasis during both brain health and brain disease. I will begin my review with an overview of brain metal homeostasis, before introducing analytical challenges associated with its study. I will then provide a brief summary of the two main X-ray techniques I have used to study brain metal homeostasis, X-ray fluorescence microscopy (XFM) and X-ray absorption near edge structure spectroscopy (XANES). The review then finishes with a summary of my main research contributions using these two techniques, put in the context of the results from others in the field.

Insight into brain metallothioneins from bidirectional Zn2+ signaling in synaptic dynamics

The basal levels as the labile Zn2+ pools in the extracellular and intracellular compartments are in the range of ∼10 nM and ∼100 pM, respectively. The influx of extracellular Zn2+ is used for memory via cognitive activity and is regulated for synaptic plasticity, a cellular mechanism of memory. When Zn2+ influx into neurons excessively occurs, however, it becomes a critical trigger for cognitive decline and neurodegeneration, resulting in acute and chronic pathogenesis. Aging, a biological process, generally accelerates vulnerability to neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). The basal level of extracellular Zn2+ is age relatedly increased in the rat hippocampus, and the influx of extracellular Zn2+ contributes to accelerating vulnerability to the AD and PD pathogenesis in experimental animals with aging. Metallothioneins (MTs) are Zn2+-binding proteins for cellular Zn2+ homeostasis and involved in not only supplying functional Zn2+ required for cognitive activity, but also capturing excess (toxic) Zn2+ involved in cognitive decline and neurodegeneration. Therefore, it is estimated that regulation of MT synthesis is involved in both neuronal activity and neuroprotection. The present report provides recent knowledge regarding the protective/preventive potential of MT synthesis against not only normal aging but also the AD and PD pathogenesis in experimental animals, focused on MT function in bidirectional Zn2+ signaling in synaptic dynamics.

A commentary on studies of brain iron accumulation during ageing

Brain iron content is widely reported to increase during "ageing", across multiple species from nematodes, rodents (mice and rats) and humans. Given the redox-active properties of iron, there has been a large research focus on iron-mediated oxidative stress as a contributor to tissue damage during natural ageing, and also as a risk factor for neurodegenerative disease. Surprisingly, however, the majority of published studies have not investigated brain iron homeostasis during the biological time period of senescence, and thus knowledge of how brain homeostasis changes during this critical stage of life largely remains unknown. This commentary examines the literature published on the topic of brain iron homeostasis during ageing, providing a critique on limitations of currently used experimental designs. The commentary also aims to highlight that although much research attention has been given to iron accumulation or iron overload as a pathological feature of ageing, there is evidence to support functional iron deficiency may exist, and this should not be overlooked in studies of ageing or neurodegenerative disease.

Systematic review of brain and blood lipidomics in Alzheimer's disease mouse models

Alzheimer's disease (AD) diagnosis is based on invasive and expensive biomarkers. Regarding AD pathophysiological mechanisms, there is evidence of a link between AD and aberrant lipid homeostasis. Alterations in lipid composition have been observed in blood and brain samples, and transgenic mouse models represent a promising approach. Nevertheless, there is great variability among studies in mice for the determination of different types of lipids in targeted and untargeted methods. It could be explained by the different variables (model, age, sex, analytical technique), and experimental conditions used. The aim of this work is to review the studies on lipid alteration in brain tissue and blood samples from AD mouse models, focusing on different experimental parameters. As result, great disparity has been observed among the reviewed studies. Brain studies showed an increase in gangliosides, sphingomyelins, lysophospholipids and monounsaturated fatty acids and a decrease in sulfatides. In contrast, blood studies showed an increase in phosphoglycerides, sterols, diacylglycerols, triacylglycerols and polyunsaturated fatty acids, and a decrease in phospholipids, lysophospholipids and monounsaturated fatty acids. Thus, lipids are closely related to AD, and a consensus on lipidomics studies could be used as a diagnostic tool and providing insight into the mechanisms involved in AD.

Biochemical Alterations in White Matter Tracts of the Aging Mouse Brain Revealed by FTIR Spectroscopy Imaging

White matter degeneration in the central nervous system (CNS) has been correlated with a decline in cognitive function during aging. Ultrastructural examination of the aging human brain shows a loss of myelin, yet little is known about molecular and biochemical changes that lead to myelin degeneration. In this study, we investigate myelination across the lifespan in C57BL/6 mice using electron microscopy and Fourier transform infrared (FTIR) spectroscopic imaging to better understand the relationship between structural and biochemical changes in CNS white matter tracts. A decrease in the number of myelinated axons was associated with altered lipid profiles in the corpus callosum of aged mice. FTIR spectroscopic imaging revealed alterations in functional groups associated with phospholipids, including the lipid acyl, lipid ester and phosphate vibrations. Biochemical changes in white matter were observed prior to structural changes and most predominant in the anterior regions of the corpus callosum. This was supported by biochemical analysis of fatty acid composition that demonstrated an overall trend towards increased monounsaturated fatty acids and decreased polyunsaturated fatty acids with age. To further explore the molecular mechanisms underlying these biochemical alterations, gene expression profiles of lipid metabolism and oxidative stress pathways were investigated. A decrease in the expression of several genes involved in glutathione metabolism suggests that oxidative damage to lipids may contribute to age-related white matter degeneration.

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.

Imaging the structural organization of chemical elements in growth cones of developing hippocampal neurons

During neurodevelopment, neurons form growth cones, F-actin rich extensions located at the distal end of the neurites. Growth cones allow dendrites and axons to build synaptic connections through a process of neurite guidance whose mechanisms have not been fully elucidated. Calcium is an important element in this process by inducing F-actin reorganization. We hypothesized that other biologically active elements might be involved in the growth cone-mediated neurite guidance mechanisms. We performed super resolution and confocal microscopy of F-actin, followed by synchrotron X-ray fluorescence microscopy of phosphorous, sulfur, chlorine, potassium, calcium, iron and zinc on growth cones from primary rat hippocampal neurons. We identified two main patterns of element organization. First, active growth cones presenting an asymmetric distribution of Ca co-localized with the cytoskeleton protein F-actin. In active growth cones, we found that the distributions of P, S, Cl, K and Zn are correlated with Ca. This correlation is lost in the second pattern, quiescent growth cones, exhibiting a spread elemental distribution. These results suggest that Ca is not the only element required in the F-actin rich active regions of growth cones. In addition, highly concentrated Fe spots of sub-micrometer size were observed in calcium-rich areas of active growth cones. These results reveal the need for biological active elements in growth cones during neural development and may help explain why early life deficiencies of elements, such as Fe or Zn, induce learning and memory deficits in children.

Synthesis of human amyloid restricted to liver results in an Alzheimer disease-like neurodegenerative phenotype

Several lines of study suggest that peripheral metabolism of amyloid beta (Aß) is associated with risk for Alzheimer disease (AD). In blood, greater than 90% of Aß is complexed as an apolipoprotein, raising the possibility of a lipoprotein-mediated axis for AD risk. In this study, we report that genetic modification of C57BL/6J mice engineered to synthesise human Aß only in liver (hepatocyte-specific human amyloid (HSHA) strain) has marked neurodegeneration concomitant with capillary dysfunction, parenchymal extravasation of lipoprotein-Aß, and neurovascular inflammation. Moreover, the HSHA mice showed impaired performance in the passive avoidance test, suggesting impairment in hippocampal-dependent learning. Transmission electron microscopy shows marked neurovascular disruption in HSHA mice. This study provides causal evidence of a lipoprotein-Aß /capillary axis for onset and progression of a neurodegenerative process.

It has been suggested that peripheral metabolism of amyloid-beta is associated with risk for Alzheimer’s disease. This study reveals that the expression of human amyloid exclusively in the liver induces Alzheimer’s disease-like pathologies in mice, potentially indicating a completely novel pathway of Alzheimer’s disease aetiology and therapies.

[Mapping the Metallo-maze to Memory Loss: Does Neuronal Metal Ion Deficiency Contribute to Dementia?]

Dementia has no cure and is an international health crisis. In addition to the immeasurable loss of QOL caused by dementia, the global economic cost is predicted to reach $2 trillion (USD) by 2030. Although much remains unknown about the biochemical pathways driving cognitive decline and memory loss during dementia, metals have been implicated in neurodegenerative disease. For example, total levels of Fe and Cu increase, which has been proposed to drive oxidative stress; and Fe, Cu, and Zn can bind amyloid-β, catalysing aggregation and formation of amyloid plaques. Unfortunately, despite these known facets through which metal ions may induce pathology, studies in greater detail have been hampered by a lack of microscopy methods to directly visualise metal ions, and their chemical form, within brain cells. Herein we report the use of synchrotron X-ray fluorescence microscopy to simultaneously image Fe, Cu, and Zn within neurons in ex vivo brain tissue sections. Using animal models of dementia, we now demonstrate for the first time that despite global increases in brain metal content and metal ion accumulation within amyloid plaques, key brain regions may also become metal ion deficient. Such deficiency could contribute to cognitive decline because of the essential roles metal ions play in neurotransmitter synthesis and energy metabolism. These recent findings are discussed in the context of memory loss, and the impact that metal ion dis-homeostasis may have on diagnostic and therapeutic development.

Blood-brain barrier disruption and ventricular enlargement are the earliest neuropathological changes in rats with repeated sub-concussive impacts over 2 weeks

Repeated sub-concussive impact (e.g. soccer ball heading), a significantly lighter form of mild traumatic brain injury, is increasingly suggested to cumulatively alter brain structure and compromise neurobehavioural function in the long-term. However, the underlying mechanisms whereby repeated long-term sub-concussion induces cerebral structural and neurobehavioural changes are currently unknown. Here, we utilised an established rat model to investigate the effects of repeated sub-concussion on size of lateral ventricles, cerebrovascular blood-brain barrier (BBB) integrity, neuroinflammation, oxidative stress, and biochemical distribution. Following repeated sub-concussion 3 days per week for 2 weeks, the rats showed significantly enlarged lateral ventricles compared with the rats receiving sham-only procedure. The sub-concussive rats also presented significant BBB dysfunction in the cerebral cortex and hippocampal formation, whilst neuromotor function assessed by beamwalk and rotarod tests were comparable to the sham rats. Immunofluorescent and spectroscopic microscopy analyses revealed no significant changes in neuroinflammation, oxidative stress, lipid distribution or protein aggregation, within the hippocampus and cortex. These data collectively indicate that repeated sub-concussion for 2 weeks induce significant ventriculomegaly and BBB disruption, preceding neuromotor deficits.

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

Sample preparation with sucrose cryoprotection dramatically alters Zn distribution in the rodent hippocampus, as revealed by elemental mapping

Transition metal ions (Fe, Mn, Cu, Zn) are essential for healthy brain function, but altered concentration, distribution, or chemical form of the metal ions has been implicated in numerous brain pathologies. Currently, it is not possible to image the cellular or sub-cellular distribution of metal ions in vivo and therefore, studying brain-metal homeostasis largely relies on ex vivo in situ elemental mapping. Sample preparation methods that accurately preserve the in vivo elemental distribution are essential if one wishes to translate the knowledge of elemental distributions measured ex vivo toward increased understanding of chemical and physiological pathways of brain disease. The choice of sample preparation is particularly important for metal ions that exist in a labile or mobile form, for which the in vivo distribution could be easily distorted by inappropriate sample preparation. One of the most widely studied brain structures, the hippocampus, contains a large pool of labile and mobile Zn. Herein, we describe how sucrose cryoprotection, the gold standard method of preparing tissues for immuno-histochemistry or immuno-fluorescence, which is also often used as a sample preparation method for elemental mapping studies, drastically alters hippocampal Zn distribution. Based on the results of this study, in combination with a comparison against the strong body of published literature that has used either rapid plunge freezing of brain tissue, or sucrose cryo-protection, we strongly urge investigators in the future to cease using sucrose cryoprotection as a method of sample preparation for elemental mapping, especially if Zn is an analyte of interest.

Elemental characterisation of the pyramidal neuron layer within the rat and mouse hippocampus

A unique combination of sensitivity, resolution, and penetration make X-ray fluorescence imaging (XFI) ideally suited to investigate trace elemental distributions in the biological context. XFI has gained widespread use as an analytical technique in the biological sciences, and in particular enables exciting new avenues of research in the field of neuroscience. In this study, elemental mapping by XFI was applied to characterise the elemental content within neuronal cell layers of hippocampal sub-regions of mice and rats. Although classical histochemical methods for metal detection exist, such approaches are typically limited to qualitative analysis. Specifically, histochemical methods are not uniformly sensitive to all chemical forms of a metal, often displaying variable sensitivity to specific "pools" or chemical forms of a metal. In addition, histochemical methods require fixation and extensive chemical treatment of samples, creating the strong likelihood for metal redistribution, leaching, or contamination. Direct quantitative elemental mapping of total elemental pools, in situ within ex vivo tissue sections, without the need for chemical fixation or addition of staining reagents is not possible with traditional histochemical methods; however, such a capability, which is provided by XFI, can offer an enormous analytical advantage. The results we report herein demonstrate the analytical advantage of XFI elemental mapping for direct, label-free metal quantification, in situ within ex vivo brain tissue sections. Specifically, we definitively characterise for the first time, the abundance of Fe within the pyramidal cell layers of the hippocampus. Localisation of Fe to this cell layer is not reproducibly achieved with classical Perls histochemical Fe stains. The ability of XFI to directly quantify neuronal elemental (P, S, Cl, K, Ca, Fe, Cu, Zn) distributions, revealed unique profiles of Fe and Zn within anatomical sub-regions of the hippocampus i.e., cornu ammonis 1, 2 or 3 (CA1, CA2 or CA3) sub-regions. Interestingly, our study reveals a unique Fe gradient across neuron populations within the non-degenerating and pathology free rat hippocampus, which curiously mirrors the pattern of region-specific vulnerability of the hippocampus that has previously been established to occur in various neurodegenerative diseases.

Integrated molecular imaging technologies for investigation of metals in biological systems: A brief review

Metals play an essential role in biological systems and are required as structural or catalytic co-factors in many proteins. Disruption of the homeostatic control and/or spatial distributions of metals can lead to disease. Imaging technologies have been developed to visualize elemental distributions across a biological sample. Measurement of elemental distributions by imaging mass spectrometry and imaging X-ray fluorescence are increasingly employed with technologies that can assess histological features and molecular compositions. Data from several modalities can be interrogated as multimodal images to correlate morphological, elemental, and molecular properties. Elemental and molecular distributions have also been axially resolved to achieve three-dimensional volumes, dramatically increasing the biological information. In this review, we provide an overview of recent developments in the field of metal imaging with an emphasis on multimodal studies in two and three dimensions. We specifically highlight studies that present technological advancements and biological applications of how metal homeostasis affects human health.

Characterization of Ionic and Lipid Gradients within Corpus Callosum White Matter after Diffuse Traumatic Brain Injury in the Rat

There is increased recognition of the effects of diffuse traumatic brain injury (dTBI), which can initiate yet unknown biochemical cascades, resulting in delayed secondary brain degeneration and long-term neurological sequela. There is limited availability of therapies that minimize the effect of secondary brain damage on the quality of life of people who have suffered TBI, many of which were otherwise healthy adults. Understanding the cascade of biochemical events initiated in specific brain regions in the acute phase of dTBI and how this spreads into adjacent brain structures may provide the necessary insight into drive development of improved therapies. In this study, we have used direct biochemical imaging techniques (Fourier transform infrared spectroscopic imaging) and elemental mapping (X-ray fluorescence microscopy) to characterize biochemical and elemental alterations that occur in corpus callosum white matter in the acute phase of dTBI. The results provide direct visualization of differential biochemical and ionic changes that occur in the highly vulnerable medial corpus callosum white matter relative to the less vulnerable lateral regions of the corpus callosum. Specifically, the results suggest that altered ionic gradients manifest within mechanically damaged medial corpus callosum, potentially spreading to and inducing lipid alterations to white matter structures in lateral brain regions.

Revealing the Elemental Distribution within Latent Fingermarks Using Synchrotron Sourced X-ray Fluorescence Microscopy

Fingermarks are an important form of crime-scene trace evidence; however, their usefulness may be hampered by a variation in response or a lack of robustness in detection methods. Understanding the chemical composition and distribution within fingermarks may help explain variation in latent fingermark detection with existing methods and identify new strategies to increase detection capabilities. The majority of research in the literature describes investigation of organic components of fingermark residue, leaving the elemental distribution less well understood. The relative scarcity of information regarding the elemental distribution within fingermarks is in part due to previous unavailability of direct, micron resolution elemental mapping techniques. This capability is now provided at third generation synchrotron light sources, where X-ray fluorescence microscopy (XFM) provides micron or submicron spatial resolution and direct detection with sub-μM detection limits. XFM has been applied in this study to reveal the distribution of inorganic components within fingermark residue, including endogenous trace metals (Fe, Cu, Zn), diffusible ions (Cl^-, K⁺, Ca²⁺), and exogeneous metals (Ni, Ti, Bi). This study incorporated a multimodal approach using XFM and infrared microspectroscopy analyses to demonstrate colocalization of endogenous metals within the hydrophilic organic components of fingermark residue. Additional experiments were then undertaken to investigate how sources of exogenous metals (e.g., coins and cosmetics) may be transferred to, and distributed within, latent fingermarks. Lastly, this study reports a preliminary assessment of how environmental factors such as exposure to aqueous environments may affect elemental distribution within fingermarks. Taken together, the results of this study advance our current understanding of fingermark composition and its spatial distribution of chemical components and may help explain detection variation observed during detection of fingermarks using standard forensic protocols.

Multimodal Imaging Analyses of Brain Hippocampal Formation Reveal Reduced Cu and Lipid Content and Increased Lactate Content in Non-Insulin-Dependent Diabetic Mice

Non-insulin-dependent diabetes mellitus (NIDDM) is reported to increase the risk of cognitive impairment and dementia. However, the underlying mechanisms are not fully understood. While the brain homeostasis of metals and lipids is pivotal to maintaining energy metabolism and redox homeostasis for healthy brain function, no studies have reported hippocampal metal and biochemical changes in NIDDM. Therefore, we here utilized direct spectroscopic imaging to reveal the elemental distribution within the hippocampal subregions of an established murine model of NIDDM, db/db mice. In 26-week-old insulin resistant db/db mice, X-ray fluorescence microscopy revealed that the Cu content within the dentate gyrus and CA3 was significantly greater than that of the age-matched nondiabetic control mice. In addition, Fourier transform infrared (FTIR) spectroscopy analysis indicated a significant increase in the abundance of lactate within the corpus callosum (CC), dentate gyrus, CA1, and CA3 regions of diabetic db/db mice compared to that of the control, indicating altered energy metabolism. FTIR analysis also showed a significant decrease in the level of lipid methylene and ester within the CC of db/db mice. Furthermore, immunomicroscopy analyses demonstrated the increase in the level of glial fibrillary acidic protein expression and peri-vascular extravasation of IgG, indicating astrogliosis and blood-brain barrier dysfunction, respectively. These data suggest that astrogliosis-induced alterations in the supply of Cu, lipids, and energy substrates may be involved in the mechanisms of NIDDM-associated cognitive decline.

Biochemical Changes Indicate Developmental Stage in the Hippocampal Formation

The literature showing how age of humans or animals influences the IR absorption spectra recorded in different brain regions is very poor. A very limited number of studies used FTIR microspectroscopy for analysis of the aging process, however there is lack of data concerning the biomolecular changes occurring in the course of postnatal development of the central nervous system. Therefore, in this paper the topographic and semiquantitative biochemical changes occurring within the rat hippocampus during postnatal development were examined. To achieve the goal of the study, three groups of normal male rats differing in age were investigated. These were 6, 30, and 60 day old animals, and the chosen ages correspond to the neonatal period, childhood, and early adulthood in humans, respectively. Already, preliminary topographic analysis identified a number of significant changes in the accumulation of biomolecules within the hippocampal formation occurring during brain development. Such observation was confirmed by further semiquantitative analysis of intensities of selected absorption bands or ratios of their intensities. The detailed examinations were done for four hippocampal cellular layers (multiform, molecular, pyramidal, and granular layers), and the results showed that the accumulation of most biomolecules, including both saturated and unsaturated lipids as well as compounds containing phosphate and carbonyl groups, was significantly higher in adulthood comparing to the neonatal period. What is more, the increases in their levels were observed mostly between 6th and 30th days of animals' life. The unsaturation level of lipids did not change during postnatal development, although the differences in unsaturated and saturated lipids contents were noticed between examined animal groups. Significant differences in relative secondary structure of proteins were found between young adult rats and animals in neonatal period for which the relative level of proteins with β-type secondary structure was the highest.