Traumatic Brain Injury, Chronic Traumatic Encephalopathy, and Alzheimer’s Disease: Common Pathologies Potentiated by Altered Zinc Homeostasis

A Timm-Nissl multiplane microscopic atlas of rat brain zincergic terminal fields and metal-containing glia

The ability of Timm's sulphide silver method to stain zincergic terminal fields has made it a useful neuromorphological marker. Beyond its roles in zinc-signalling and neuromodulation, zinc is involved in the pathophysiology of ischemic stroke, epilepsy, degenerative diseases and neuropsychiatric conditions. In addition to visualising zincergic terminal fields, the method also labels transition metals in neuronal perikarya and glial cells. To provide a benchmark reference for planning and interpretation of experimental investigations of zinc-related phenomena in rat brains, we have established a comprehensive repository of serial microscopic images from a historical collection of coronally, horizontally and sagittally oriented rat brain sections stained with Timm's method. Adjacent Nissl-stained sections showing cytoarchitecture, and customised atlas overlays from a three-dimensional rat brain reference atlas registered to each section image are included for spatial reference and guiding identification of anatomical boundaries. The Timm-Nissl atlas, available from EBRAINS, enables experimental researchers to navigate normal rat brain material in three planes and investigate the spatial distribution and density of zincergic terminal fields across the entire brain.

Characterization of the spatial distribution of metals and profile of metalloprotein complexes in a mouse model of repetitive mild traumatic brain injury

Metal dyshomeostasis is a well-established consequence of neurodegenerative diseases and traumatic brain injury. While the significance of metals continues to be uncovered in many neurological disorders, their implication in repetitive mild traumatic brain injury remains uncharted. To address this gap, we characterized the spatial distribution of metal levels (iron, zinc, and copper) using laser ablation-inductively coupled plasma-mass spectrometry, the profile of metal-binding proteins via size exclusion chromatography-inductively coupled plasma-mass spectrometry and the expression of the major iron storing protein ferritin via western blotting. Using a mouse model of repetitive mild traumatic brain injury, 3-month-old male and female C57Bl6 mice received one or five impacts (48 h apart). At 1 month following 5× TBI (traumatic brain injury), iron and ferritin levels were significantly elevated in the contralateral cortex. There was a trend toward increased iron levels in the entire contralateral hemisphere and a reduction in contralateral cortical iron-binding proteins following 1× TBI. No major changes in zinc levels were seen in both hemispheres following 5× or 1× TBI, although there was a reduction in ipsilateral zinc-binding proteins following 5× TBI and a contralateral increase in zinc-binding proteins following 1× TBI. Copper levels were significantly increased in both hemispheres following 5× TBI, without changes in copper-binding proteins. This study shows for the first time that repetitive mild TBI (r-mTBI) leads to metal dyshomeostasis, highlighting its potential involvement in promoting neurodegeneration, which provides a rationale for examining the benefit of metal-targeting drugs, which have shown promising results in neurodegenerative conditions and single TBI, but have yet to be tested following r-mTBI.

Zinc enhances liquid-liquid phase separation of Tau protein and aggravates mitochondrial damages in cells

Intraneuronal neurofibrillary tangles composed of Tau aggregates have been widely accepted as an important pathological hallmark of Alzheimer's disease. Liquid-liquid phase separation (LLPS) of Tau can lead to its aggregation, and Tau aggregation can then be enhanced by zinc. However, it is unclear whether zinc modulates the formation of Tau stress granules in cells. We herein report that zinc promotes the formation of stress granules containing a pathological mutant ΔK280 of full-length human Tau. Furthermore, zinc promotes LLPS of ΔK280 of full-length Tau, shifting the equilibrium phase boundary to a lower protein concentration, and modulates the liquid nature of droplets formed by this pathological mutation. Zinc also promotes pathological phosphorylation of ΔK280 in neuronal cells, and aggravates mitochondrial damage and elevates reactive oxygen species production induced by Tau aggregation. Importantly, we show that treatment of cells with zinc increases the interaction between full-length Tau and G3BP1 inside stress granules to promote the formation of Tau filaments and increase Tau toxicity in neuronal cells. Collectively, these results demonstrate how Tau condensation and mitochondrial damages induced by Tau aggregation are enhanced by zinc to deteriorate the pathogenesis of Alzheimer's disease, bridging the gap between Tau LLPS and aggregation in neuronal cells.

Tau pathology, metal dyshomeostasis and repetitive mild traumatic brain injury: an unexplored link paving the way for neurodegeneration

Repetitive mild traumatic brain injury (r-mTBI), commonly experienced by athletes and military personnel, causes changes in multiple intracellular pathways, one of which involves the tau protein. Tau phosphorylation plays a role in several neurodegenerative conditions including chronic traumatic encephalopathy (CTE), a progressive neurodegenerative disorder linked to repeated head trauma. There is now mounting evidence suggesting that tau phosphorylation may be regulated by metal ions (such as iron, zinc and copper), which themselves are implicated in ageing and neurodegenerative disorders such as Alzheimer's disease (AD). Recent work has also shown that a single TBI can result in age-dependent and region-specific modulation of metal ions. As such, this review explores the link between TBI, CTE, ageing and neurodegeneration with a specific focus on the involvement of (and interaction between) tau pathology and metal dyshomeostasis. The authors highlight that metal dyshomeostasis has yet to be investigated in the context of repeat head trauma or CTE. Given the evidence that metal dyshomeostasis contributes to the onset and/or progression of neurodegeneration, and that CTE itself is a neurodegenerative condition, this brings to light an uncharted link that should be explored. The development of adequate models of r-mTBI and/or CTE will be crucial in deepening our understanding of the pathological mechanisms that drive the clinical manifestations in these conditions and also in the development of effective therapeutics targeted towards slowing progressive neurodegenerative disorders.

Nanodelivery of oxiracetam enhances memory, functional recovery and induces neuroprotection following concussive head injury

Military personnel are the most susceptible to concussive head injury (CHI) caused by explosion, blast or missile or blunt head trauma. Mild to moderate CHI could induce lifetime functional and cognitive disturbances causing significant decrease in quality of life. Severe CHI leads to instant death and lifetime paralysis. Thus, further exploration of novel therapeutic agents or new features of known pharmacological agents are needed to enhance quality of life of CHI victims. Previous reports from our laboratory showed that mild CHI induced by weight drop technique causing an impact of 0.224N results in profound progressive functional deficit, memory impairment and brain pathology from 5h after trauma that continued over several weeks of injury. In this investigation we report that TiO2 nanowired delivery of oxiracetam (50mg/kg, i.p.) daily for 5 days after CHI resulted in significant improvement of functional deficit on the 8th day. This was observed using Rota Rod treadmill, memory improvement assessed by the time spent in finding hidden platform under water. The motor function improvement is seen in oxiracetam treated CHI group by placing forepaw on an inclined mesh walking and foot print analysis for stride length and distance between hind feet. TiO2-nanowired oxiracetam also induced marked improvements in the cerebral blood flow, reduction in the BBB breakdown and edema formation as well as neuroprotection of neuronal, glial and myelin damages caused by CHI at light and electron microscopy on the 7th day after 5 days TiO2 oxiracetam treatment. Adverse biochemical events such as upregulation of CSF nitrite and nitrate, IL-6, TNF-a and p-Tau are also reduced significantly in oxiracetam treated CHI group. On the other hand post treatment of 100mg/kg dose of normal oxiracetam in identical conditions after CHI is needed to show slight but significant neuroprotection together with mild recovery of memory function and functional deficits on the 8th day. These observations are the first to point out that nanowired delivery of oxiracetam has superior neuroprotective ability in CHI. These results indicate a promising clinical future of TiO2 oxiracetam in treating CHI patients for better quality of life and neurorehabilitation, not reported earlier.

Imaging Tissue Physiology In Vivo by Use of Metal Ion-Responsive MRI Contrast Agents

Paramagnetic metal ion complexes, mostly based on gadolinium (Gd3+), have been used for over 30 years as magnetic resonance imaging (MRI) contrast agents. Gd3+-based contrast agents have a strong influence on T1 relaxation times and are consequently the most commonly used agents in both the clinical and research environments. Zinc is an essential element involved with over 3000 different cellular proteins, and disturbances in tissue levels of zinc have been linked to a wide range of pathologies, including Alzheimer's disease, prostate cancer, and diabetes mellitus. MR contrast agents that respond to the presence of Zn2+ in vivo offer the possibility of imaging changes in Zn2+ levels in real-time with the superior spatial resolution offered by MRI. Such responsive agents, often referred to as smart agents, are typically composed of a paramagnetic metal ion with a ligand encapsulating it and one or more chelating units that selectively bind with the analyte of interest. Translation of these agents into clinical radiology is the next goal. In this review, we discuss Gd3+-based MR contrast agents that respond to a change in local Zn2+ concentration.

β-Naphthoflavone, an exogenous ligand of aryl hydrocarbon receptor, disrupts zinc homeostasis in human hepatoma HepG2 cells

Recent studies have demonstrated a relationship between the disruption of zinc homeostasis and the onset of diseases. However, little is known about the factors that disrupt zinc homeostasis. Here, we investigated the effects of β-naphthoflavone, an exogenous ligand of aryl hydrocarbon receptor (AHR), on intracellular zinc levels. Human hepatoma HepG2 cells were treated with β-naphthoflavone for 3 days, and intracellular labile and total zinc levels were assessed through flow cytometry and inductively coupled plasma atom emission spectroscopy, respectively. The mRNA levels of zinc transporters were determined by real-time PCR. Treatment of cells with β-naphthoflavone induced a decrease in intracellular labile zinc in a dose-dependent manner, with significantly decreased levels observed at 1 µM compared with controls. Additionally, intracellular total zinc levels demonstrated a decreasing trend with 10 µM β-naphthoflavone. Zinc pyrithione recovered the decrease in intracellular labile zinc levels induced by β-naphthoflavone, while zinc sulfate had no effect. Moreover, significant decreases in the mRNA levels of zinc transporters ZnT10 and ZIP5 were observed in response to 10 µM β-naphthoflavone. These results demonstrated that β-naphthoflavone has the potential to disrupt zinc homeostasis in hepatocytes. Although the underlying mechanism remains to be determined, suppression of zinc transporter transcription through AHR activation may be involved in the β-naphthoflavone-induced disruption of intracellular zinc levels.

Targeting metals rescues the phenotype in an animal model of tauopathy

Tauopathies are characterized by the pathological accumulation of the microtubule associated protein tau within the brain. We demonstrate here that a copper/zinc chaperone (PBT2, Prana Biotechnology) has rapid and profound effects in the rTg(tauP301L)4510 mouse model of tauopathy. This was evidenced by significantly improved cognition, a preservation of neurons, a decrease in tau aggregates and a decrease in other forms of "pathological" tau (including phosphorylated tau and sarkosyl-insoluble tau). Our data demonstrate that one of the primary mechanisms of action of PBT2 in this model may be driven by an interaction on the pathways responsible for the dephosphorylation of tau. Specifically, PBT2 increased protein levels of both the structural and catalytic subunits of protein phosphatase 2A (PP2A), decreased levels of the methyl esterase (PME1) that dampens PP2A activity, and increased levels of the prolyl isomerase (Pin1) that stimulates the dephosphorylation activity of PP2A. None of these effects were observed when the metal binding site of PBT2 was blocked. This highlights the potential utility of targeting metal ions as a novel therapeutic strategy for diseases in which tau pathology is a feature, which includes conditions such as frontotemporal dementia and Alzheimer's disease.

Uncoupled Endothelial Nitric Oxide Synthase Enhances p-Tau in Chronic Traumatic Encephalopathy Mouse Model

AIMS

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disease thought to be caused by repetitive traumatic brain injury (TBI) and subconcussive injuries. While hyperphosphorylation of tau (p-Tau), which is attributed to astrocytic tangles (ATs) and neurofibrillary tangles, is known to be involved in CTE, there are limited neuropathological or molecular data. By utilizing repetitive mild TBI (rmTBI) mouse models, our aim was to examine the pathological changes of CTE-associated structures, specifically the ATs.

RESULTS

Our rmTBI mouse models showed symptoms of depressive behavior and memory deficit, alongside an increased p-Tau expression in their neurons and astrocytes in both the hippocampus and cortex. rmTBI induced oxidative stress in endothelial cells and nitric oxide (NO) generation in astrocytes, which were mediated by hypoxia and increased hypoxia-inducible factor 1-α (HIF1α). There was also correlated decreased regional cerebral tissue perfusion units, mild activation of astrocytes and NFκB phosphorylation, increased expression of inducible nitric oxide synthase (iNOS), increased endothelial nitric oxide synthase (eNOS) uncoupling with decreased tetrahydrobiopterin, and increased expression of nitrotyrosine, NADPH oxidase 2 (Nox2)/nuclear factor (erythroid-derived 2) factor 2 (Nrf2) signaling proteins. Combined, these effects induced peroxynitrite formation and hyperphosphorylation of tau in the hippocampus and cortex toward the formation of ATs.

INNOVATION

Our model features molecular pathogenesis events of CTE with clinically relevant latency periods. In particular, this is the first demonstration of an increased astrocytic iNOS expression in an in vivo model.

CONCLUSION

We propose a novel mechanism of uncoupled eNOS and NO contribution to Tau phosphorylation and AT formation in rmTBI brain, toward an increased molecular understanding of the pathophysiology of human CTE.

Trehalose elevates brain zinc levels following controlled cortical impact in a mouse model of traumatic brain injury

Zinc (Zn) deficiency is a clinical consequence of brain injury that can result in neuropathological outcomes that are exacerbated with age. Here, we present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) imaging data showing modulation of brain Zn levels by the disaccharide trehalose in aged mice following a controlled cortical impact model of traumatic brain injury. In this proof-of-concept study, trehalose induced an increase in brain zinc levels, providing important preliminary data for larger studies using this simple carbohydrate as a modulator of this essential micronutrient in traumatic brain injury. Our results may have further implications for the treatment of a variety of neurodegenerative diseases and other disorders of the nervous system.

Metal chaperones: a novel therapeutic strategy for brain injury?

OBJECTIVE

This study sought to assess the potential efficacy of a novel class of metal chaperone on the outcomes in an animal model of a controlled cortical impact. This work was predicated on previous observations that this class of compound has exhibited neuroprotective potential in other models of aging and neurodegeneration.

RESEARCH DESIGN

The study employed a controlled cortical impact traumatic brain injury in three month old mice with subsequent behavioral and cellular assessments to determine therapeutic efficacy.

METHODS

Cognitive (Y-maze) and motor assessments (Rotarod and Open Field) were employed to determine behavioral end points. Histological-based methods were utilized to assess neuronal integrity, astrocytosis, and lesion volume.

OUTCOMES

We demonstrate here that acute post-injury treatment with PBT2 (Prana Biotechnology) is sufficient to maintain neuronal integrity (evidenced by decreased lesion area and increased numbers of neurons; decreased astrocytosis was also present) and to normalize performance in cognitive testing (Y-maze). These effects occurred within days and were maintained for the entire duration of the study (26 days post-injury). These data support the further interrogation of the utility of metal chaperones for the treatment and/or prevention of the neuroanatomical, biochemical, and behavioral deficits that occur following brain injuries of different etiologies.

Crosstalk between zinc and free fatty acids in plasma

In mammalian blood plasma, serum albumin acts as a transport protein for free fatty acids, other lipids and hydrophobic molecules including neurodegenerative peptides, and essential metal ions such as zinc to allow their systemic distribution. Importantly, binding of these chemically extremely diverse entities is not independent, but linked allosterically. One particularly intriguing allosteric link exists between free fatty acid and zinc binding. Albumin thus mediates crosstalk between energy status/metabolism and organismal zinc handling. In recognition of the fact that even small changes in extracellular zinc concentration and speciation modulate the function of many cell types, the albumin-mediated impact of free fatty acid concentration on zinc distribution may be significant for both normal physiological processes including energy metabolism, insulin activity, heparin neutralisation, blood coagulation, and zinc signalling, and a range of disease states, including metabolic syndrome, cardiovascular disease, myocardial ischemia, diabetes, and thrombosis.

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Serum albumin binds and transports both free fatty acids and Zn²⁺ ions

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Elevated plasma free fatty acids impair Zn²⁺ binding by albumin through an allosteric mechanism

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The resulting changes in plasma zinc speciation are thought to impact blood coagulation and may promote thrombosis

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Increased free Zn²⁺ may lead to enhanced zinc export from plasma and dysregulation of zinc homeostasis in multiple tissues

Trehalose improves traumatic brain injury-induced cognitive impairment

Traumatic brain Injury (TBI) is a significant cause of death and long-term disability for which there are currently no effective pharmacological treatment options. In this study then, we utilized a mouse model of TBI to assess the therapeutic potential of the stable disaccharide trehalose, which is known to protect against oxidative stress, increase levels of chaperone molecules and enhance autophagy. Furthermore, trehalose has demonstrated neuroprotective properties in numerous animal models and has been proposed as a potential treatment for neurodegeneration. As TBI (and associated neurodegenerative disorders) is complicated by a sudden and dramatic change in brain metal concentrations, including iron (Fe) and zinc (Zn), the collective accumulation and translocation of which has been hypothesized to contribute to the pathogenesis of TBI, then we also sought to determine whether trehalose modulated the metal dyshomeostasis associated with TBI. In this study three-month-old C57Bl/6 wildtype mice received a controlled cortical impact TBI, and were subsequently treated for one month with trehalose. During this time animals were assessed on multiple behavioral tasks prior to tissue collection. Results showed an overall significant improvement in the Morris water maze, Y-maze and open field behavioral tests in trehalose-treated mice when compared to controls. These functional benefits occurred in the absence of any change in lesion volume or any significant modulation of biometals, as assessed by laser ablation inductively coupled plasma mass spectrometry. Western blot analysis, however, revealed an upregulation of synaptophysin, doublecortin and brain derived neurotrophic factor protein in trehalose treated mice in the contralateral cortex. These results indicate that trehalose may be efficacious in improving functional outcomes following TBI by a previously undescribed mechanism of action that has relevance to multiple disorders of the central nervous system.

A time-course analysis of changes in cerebral metal levels following a controlled cortical impact

Traumatic brain injury (TBI) is complicated by a sudden and dramatic change in brain metal levels, including iron (Fe), copper (Cu) and zinc (Zn). Specific 'metallo-pathological' features of TBI include increased non-heme bound Fe and the liberation of free Zn ions, both of which may contribute to the pathogenesis of TBI. To further characterise the metal dyshomeostasis that occurs following brain trauma, we performed a quantitative time-course survey of spatial Fe, Cu and Zn distribution in mice receiving a controlled cortical impact TBI. Images of brain metal levels produced using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in the upper quadrant of the ipsilateral hemisphere were compared to the corresponding contralateral hemisphere, together with regional areas radiating toward the center of the brain from the site of lesion. Significant regional and time point specific elevations in Fe, Zn and Cu were detected immediately and up to 28 days after TBI. The magnitude and timeframe of many of these changes suggest that TBI results in a pronounced and sustained alteration in normal metal levels within the brain. Such alterations are likely to play a role in both the short- and long-term consequences of head trauma and suggest that pharmacological modulation to normalize these metal levels may be efficacious in improving functional outcome.

Metalloneurochemistry and the Pierian Spring: 'Shallow Draughts Intoxicate the Brain'

Metal ions perform critical and diverse functions in nervous system physiology and pathology. The field of metalloneurochemistry aims to understand the mechanistic bases for these varied roles at the molecular level. Here, we review several areas of research that illustrate progress toward achieving this ambitious goal and identify key challenges for the future. We examine the use of lithium as a mood stabilizer, the roles of mobile zinc and copper in the synapse, the interplay of nitric oxide and metals in retrograde signaling, and the regulation of iron homeostasis in the brain. These topics were chosen to demonstrate not only the breadth of the field, but also to highlight opportunities for discovery by studying such complex systems in greater detail. We are beginning to uncover the principles by which receptors and transmitters utilize metal ions to modulate neurotransmission. These advances have revealed exciting new insights into the intricate mechanisms that give rise to learning, memory, and sensory perception, while opening many new avenues for further exploration.

Novel zinc-targeted therapeutic options for cognitive decline

Zinc (Zn2+) is an essential metal in the brain, having critical roles in many proteins and cellular pathways, including synaptic plasticity and the normal functioning of long-term potentiation, the in vitro correlate of learning and memory. Importantly, several neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease, show alterations in Zn2+ metabolism that may be involved in the pathogenesis of these disorders. Therefore, the modulation of Zn2+ levels and/or distribution may prove an important target for the treatment of neurodegenerative diseases. This review aims to evaluate the current efforts directed at therapeutic Zn2+ modulation as it relates to disease-associated cognitive decline.