Iron dysregulation in Huntington's disease

A Golgi-targeting fluorescent probe for labile Fe(ii) to reveal an abnormal cellular iron distribution induced by dysfunction of VPS35

Iron is involved in numerous physiologically essential processes in our body. However, excessive iron is a pathogenic factor in neurodegenerative diseases, causing aberrant oxidative stress. Divalent metal transporter 1 (DMT1) acts as a primary transporter of Fe(ii) ions. The intracellular delivery of DMT1 toward the cellular membrane via the trans-Golgi network during the endocytotic process is partially regulated by a retromer-mediated protein-sorting system comprising vacuolar protein-sorting proteins (VPSs). Thus, together with DMT1, the Golgi-apparatus acts as a hub organelle in the delivery system for intracellular Fe(ii) ions. Dysfunction of the VPS-relevant protein sorting system can induce the abnormal delivery of DMT1 toward lysosomes concomitantly with Fe(ii) ions. To explore this issue, we developed a fluorescent probe, Gol-SiRhoNox, for the Golgi-specific detection of Fe(ii) ions by integrating our original N-oxide-based Fe(ii)-specific chemical switch, a new Golgi-localizable chemical motif, and polarity-sensitive fluorogenic scaffold. Our synchronous imaging study using Gol-SiRhoNox and LysoRhoNox, a previously developed fluorescent probe for lysosomal Fe(ii), revealed that the intracellular distribution balance of Fe(ii) ions between the Golgi apparatus and lysosomes is normally Golgi-dominant, whereas the lysosome-specific elevation of Fe(ii) ions was observed in cells with induced dysfunction of VPS35, a member of the retromer complex. Treatment of cells with dysfunctional VPS35 with R55, a molecular chaperone, resulted in the restoration of the subcellular distribution of Fe(ii) ions to the Golgi-dominant state. These results indicate that the impairment of the DMT1 traffic machinery affects subcellular iron homeostasis, promoting Fe(ii) leakage at the Golgi and lysosomal accumulation of Fe(ii) through missorting of DMT1.

The Emerging Roles of Ferroptosis in Huntington's Disease

Huntington's disease (HD) is an autosomal dominant and fatal neurodegenerative disorder, which is caused by an abnormal CAG repeat in the huntingtin gene. Despite its well-defined genetic origin, the molecular mechanisms of neuronal death are unclear yet, thus there are no effective strategies to block or postpone the process of HD. Ferroptosis, a recently identified iron-dependent cell death, attracts considerable attention due to its putative involvement in neurodegenerative diseases. Accumulative data suggest that ferroptosis is very likely to participate in HD, and inhibition of the molecules and signaling pathways involved in ferroptosis can significantly eliminate the symptoms and pathology of HD. This review first describes evidence for the close relevance of ferroptosis and HD in patients and mouse models, then summarizes advances for the mechanisms of ferroptosis involved in HD, finally outlines some therapeutic strategies targeted ferroptosis. Comprehensive understanding of the emerging roles of ferroptosis in the occurrence of HD will help us to explore effective therapies for slowing the progression of this disease.

Crosstalk between mitochondrial dysfunction, oxidative stress, and age related neurodegenerative disease: Etiologies and therapeutic strategies

Mitochondrial function is vital for normal cellular processes. Mitochondrial damage and oxidative stress have been greatly implicated in the progression of aging, along with the pathogenesis of age-related neurodegenerative diseases (NDs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Although antioxidant therapy has been proposed for the prevention and treatment of age-related NDs, unraveling the molecular mechanisms of mitochondrial dysfunction can lead to significant progress in the development of effective treatments against such diseases. Aging is associated with the generation and accumulation of reactive oxygen species (ROS) that are the major contributors to oxidative stress. Oxidative stress is caused because of the imbalance between the production of ROS and their oxidation, which can affect the mitochondrial respiratory chain function, thereby altering the membrane permeability and calcium homeostasis, along with increasing the heteroplasmic mtDNA and weakening the mitochondrial defense systems. Mitochondrial dysfunction mainly affects mitochondrial biogenesis and dynamics that are prominent in several age-related NDs. Mitochondrial dysfunction has a crucial role in the pathophysiology of age-related NDs. Several mitochondria targeted strategies, such as enhancing the antioxidant bioavailability via novel delivery systems, identifying unique mitochondrial proteins as specific drug targets, investigating the signaling pathways of mitochondrial biogenesis and dynamics, and identifying effective natural products are potentially effective to counteract mitochondrial dysfunction-related NDs.

The Dual Role of Hepcidin in Brain Iron Load and Inflammation

Hepcidin is the major regulator of systemic iron metabolism, while the role of this peptide in the brain has just recently been elucidated. Studies suggest a dual role of hepcidin in neuronal iron load and inflammation. This is important since neuronal iron load and inflammation are pathophysiological processes frequently associated with neurodegeneration. Furthermore, manipulation of hepcidin activity has recently been used to recover neuronal damage due to brain inflammation in animal models and cultured cells. Therefore, understanding the mechanistic insights of hepcidin action in the brain is important to uncover its role in treating neuronal damage in neurodegenerative diseases.

Structural Magnetic Resonance Imaging in Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by expansion of the CAG repeat in the huntingtin gene. HD is characterized clinically by progressive motor, cognitive and neuropsychiatric symptoms. There are currently no disease modifying treatments available for HD, and there is a great need for biomarkers to monitor disease progression and identify new targets for therapeutic intervention. Neuroimaging techniques provide a powerful tool for assessing disease pathology and progression in premanifest stages, before the onset of overt motor symptoms. Structural magnetic resonance imaging (MRI) is non-invasive imaging techniques which have been employed to study structural and microstructural changes in premanifest and manifest HD gene carriers. This chapter described structural imaging techniques and analysis methods employed across HD MRI studies. Current evidence for structural MRI abnormalities in HD, and associations between atrophy, structural white matter changes, iron deposition and clinical performance are discussed; together with the use of structural MRI measures as a diagnostic tool, to assess longitudinal changes, and as potential biomarkers and endpoints for clinical trials.

Studying Huntington's Disease in Yeast: From Mechanisms to Pharmacological Approaches

Huntington's disease (HD) is a neurodegenerative disorder that leads to progressive neuronal loss, provoking impaired motor control, cognitive decline, and dementia. So far, HD remains incurable, and available drugs are effective only for symptomatic management. HD is caused by a mutant form of the huntingtin protein, which harbors an elongated polyglutamine domain and is highly prone to aggregation. However, many aspects underlying the cytotoxicity of mutant huntingtin (mHTT) remain elusive, hindering the efficient development of applicable interventions to counteract HD. An important strategy to obtain molecular insights into human disorders in general is the use of eukaryotic model organisms, which are easy to genetically manipulate and display a high degree of conservation regarding disease-relevant cellular processes. The budding yeast Saccharomyces cerevisiae has a long-standing and successful history in modeling a plethora of human maladies and has recently emerged as an effective tool to study neurodegenerative disorders, including HD. Here, we summarize some of the most important contributions of yeast to HD research, specifically concerning the elucidation of mechanistic features of mHTT cytotoxicity and the potential of yeast as a platform to screen for pharmacological agents against HD.

Dysregulation of Neuronal Iron Homeostasis as an Alternative Unifying Effect of Mutations Causing Familial Alzheimer's Disease

The overwhelming majority of dominant mutations causing early onset familial Alzheimer’s disease (EOfAD) occur in only three genes, PSEN1, PSEN2, and APP. An effect-in-common of these mutations is alteration of production of the APP-derived peptide, amyloid β (Aβ). It is this key fact that underlies the authority of the Amyloid Hypothesis that has informed Alzheimer’s disease research for over two decades. Any challenge to this authority must offer an alternative explanation for the relationship between the PSEN genes and APP. In this paper, we explore one possible alternative relationship – the dysregulation of cellular iron homeostasis as a common effect of EOfAD mutations in these genes. This idea is attractive since it provides clear connections between EOfAD mutations and major characteristics of Alzheimer’s disease such as dysfunctional mitochondria, vascular risk factors/hypoxia, energy metabolism, and inflammation. We combine our ideas with observations by others to describe a “Stress Threshold Change of State” model of Alzheimer’s disease that may begin to explain the existence of both EOfAD and late onset sporadic (LOsAD) forms of the disease. Directing research to investigate the role of dysregulation of iron homeostasis in EOfAD may be a profitable way forward in our struggle to understand this form of dementia.

Ferritinophagy/ferroptosis: Iron-related newcomers in human diseases

Nuclear receptor coactivator 4 mediated ferritinophagy is an autophagic phenomenon that specifically involves ferritin to release intracellular free iron. Ferritinophagy is implicated in maintaining efficient erythropoiesis. Notably, ferritinophagy also plays a central role in driving some pathological processes, including Parkinson's disease (PD) and urinary tract infections. Some evidence has demonstrated that ferritinophagy is critical to induce ferroptosis. Ferroptosis is a newly nonapoptotic form of cell death, characterized by the accumulation of iron-based lipid reactive oxygen species. Ferroptosis plays an important role in inhibiting some types of cancers, such as hepatocellular carcinoma, pancreatic carcinoma, prostate cancer, and breast cancer. Conversely, the activation of ferroptosis accelerates neurodegeneration diseases, including PD and Alzheimer's disease. Therefore, in this review, we summarize the regulatory mechanisms related to ferritinophagy and ferroptosis. Moreover, the distinctive effects of ferritinophagy in human erythropoiesis and some pathologies, coupled with the promotive or inhibitory role of tumorous and neurodegenerative diseases mediated by ferroptosis, are elucidated. Obviously, activating or inhibiting ferroptosis could be exploited to achieve desirable therapeutic effects on diverse cancers and neurodegeneration diseases. Interrupting ferritinophagy to control iron level might provide a potentially therapeutic avenue to suppress urinary tract infections.

No effects without causes: the Iron Dysregulation and Dormant Microbes hypothesis for chronic, inflammatory diseases

Since the successful conquest of many acute, communicable (infectious) diseases through the use of vaccines and antibiotics, the currently most prevalent diseases are chronic and progressive in nature, and are all accompanied by inflammation. These diseases include neurodegenerative (e.g. Alzheimer's, Parkinson's), vascular (e.g. atherosclerosis, pre-eclampsia, type 2 diabetes) and autoimmune (e.g. rheumatoid arthritis and multiple sclerosis) diseases that may appear to have little in common. In fact they all share significant features, in particular chronic inflammation and its attendant inflammatory cytokines. Such effects do not happen without underlying and initially 'external' causes, and it is of interest to seek these causes. Taking a systems approach, we argue that these causes include (i) stress-induced iron dysregulation, and (ii) its ability to awaken dormant, non-replicating microbes with which the host has become infected. Other external causes may be dietary. Such microbes are capable of shedding small, but functionally significant amounts of highly inflammagenic molecules such as lipopolysaccharide and lipoteichoic acid. Sequelae include significant coagulopathies, not least the recently discovered amyloidogenic clotting of blood, leading to cell death and the release of further inflammagens. The extensive evidence discussed here implies, as was found with ulcers, that almost all chronic, infectious diseases do in fact harbour a microbial component. What differs is simply the microbes and the anatomical location from and at which they exert damage. This analysis offers novel avenues for diagnosis and treatment.

Oxidative stress and neurodegeneration: the involvement of iron

Many evidences indicate that oxidative stress plays a significant role in a variety of human disease states, including neurodegenerative diseases. Iron is an essential metal for almost all living organisms due to its involvement in a large number of iron-containing proteins and enzymes, though it could be also toxic. Actually, free iron excess generates oxidative stress, particularly in brain, where anti-oxidative defences are relatively low. Its accumulation in specific regions is associated with pathogenesis in a variety of neurodegenerative diseases (i.e., Parkinson's disease, Alzheimer's disease, Huntington's chorea, Amyotrophic Lateral Sclerosis and Neurodegeneration with Brain Iron Accumulation). Anyway, the extent of toxicity is dictated, in part, by the localization of the iron complex within the cell (cytosolic, lysosomal and mitochondrial), its biochemical form, i.e., ferritin or hemosiderin, as well as the ability of the cell to prevent the generation and propagation of free radical by the wide range of antioxidants and cytoprotective enzymes in the cell. Particularly, ferrous iron can act as a catalyst in the Fenton reaction that potentiates oxygen toxicity by generating a wide range of free radical species, including hydroxyl radicals (·OH). The observation that patients with neurodegenerative diseases show a dramatic increase in their brain iron content, correlated with the production of reactive oxigen species in these areas of the brain, conceivably suggests that disturbances in brain iron homeostasis may contribute to the pathogenesis of these disorders. The aim of this review is to describe the chemical features of iron in human beings and iron induced toxicity in neurodegenerative diseases. Furthermore, the attention is focused on metal chelating drugs therapeutic strategies.

Mutant huntingtin induces iron overload via up-regulating IRP1 in Huntington's disease

Background

Iron accumulation in basal ganglia accompanies neuronal loss in Huntington’s disease (HD) patients and mouse disease models. Disruption of HD brain iron homeostasis occurs before the onset of clinical signs. Therefore, investigating the mechanism of iron accumulation is essential to understanding its role in disease pathogenesis.

Methods

N171-82Q HD transgenic mice brain iron was detected by using Diaminobenzidine-enhanced Perls’ stain. Iron homeostatic proteins including iron response protein 1 (IRP1), transferrin (Tf), ferritin and transferrin receptor (TfR) were determined by using western blotting and immunohistochemistry, and their relative expression levels of RNA were measured by RT-PCR in both N171-82Q HD transgenic mice and HEK293 cells expressing N-terminal of huntingtin.

Results

Iron was increased in striatum and cortex of N171-82Q HD transgenic mice. Analysis of iron homeostatic proteins revealed increased expression of IRP1, Tf, ferritin and TfR in N171-82Q mice striatum and cortex. The same results were obtained in HEK293 cells expressing N-terminal of mutant huntingtin containing 160 CAG repeats.

Conclusion

We conclude that mutant huntingtin may cause abnormal iron homeostatic pathways by increasing IRP1 expression in Huntington’s disease, suggesting potential therapeutic target.

Disease-modifying effects of ganglioside GM1 in Huntington's disease models

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by motor, cognitive and psychiatric problems. Previous studies indicated that levels of brain gangliosides are lower than normal in HD models and that administration of exogenous ganglioside GM1 corrects motor dysfunction in the YAC128 mouse model of HD In this study, we provide evidence that intraventricular administration of GM1 has profound disease-modifying effects across HD mouse models with different genetic background. GM1 administration results in decreased levels of mutant huntingtin, the protein that causes HD, and in a wide array of beneficial effects that include changes in levels of DARPP32, ferritin, Iba1 and GFAP, modulation of dopamine and serotonin metabolism, and restoration of normal levels of glutamate, GABA, L-Ser and D-Ser. Treatment with GM1 slows down neurodegeneration, white matter atrophy and body weight loss in R6/2 mice. Motor functions are significantly improved in R6/2 mice and restored to normal in Q140 mice, including gait abnormalities that are often resistant to treatments. Psychiatric-like and cognitive dysfunctions are also ameliorated by GM1 administration in Q140 and YAC128 mice. The widespread benefits of GM1 administration, at molecular, cellular and behavioural levels, indicate that this ganglioside has strong therapeutic and disease-modifying potential in HD.

Mitochondrial Quality Control in Neurodegenerative Diseases: Focus on Parkinson's Disease and Huntington's Disease

In recent years, several important advances have been made in our understanding of the pathways that lead to cell dysfunction and death in Parkinson's disease (PD) and Huntington's disease (HD). Despite distinct clinical and pathological features, these two neurodegenerative diseases share critical processes, such as the presence of misfolded and/or aggregated proteins, oxidative stress, and mitochondrial anomalies. Even though the mitochondria are commonly regarded as the "powerhouses" of the cell, they are involved in a multitude of cellular events such as heme metabolism, calcium homeostasis, and apoptosis. Disruption of mitochondrial homeostasis and subsequent mitochondrial dysfunction play a key role in the pathophysiology of neurodegenerative diseases, further highlighting the importance of these organelles, especially in neurons. The maintenance of mitochondrial integrity through different surveillance mechanisms is thus critical for neuron survival. Mitochondria display a wide range of quality control mechanisms, from the molecular to the organellar level. Interestingly, many of these lines of defense have been found to be altered in neurodegenerative diseases such as PD and HD. Current knowledge and further elucidation of the novel pathways that protect the cell through mitochondrial quality control may offer unique opportunities for disease therapy in situations where ongoing mitochondrial damage occurs. In this review, we discuss the involvement of mitochondrial dysfunction in neurodegeneration with a special focus on the recent findings regarding mitochondrial quality control pathways, beyond the classical effects of increased production of reactive oxygen species (ROS) and bioenergetic alterations. We also discuss how disturbances in these processes underlie the pathophysiology of neurodegenerative disorders such as PD and HD.

Hepcidin, an emerging and important player in brain iron homeostasis

Hepcidin is emerging as a new important factor in brain iron homeostasis. Studies suggest that there are two sources of hepcidin in the brain; one is local and the other comes from the circulation. Little is known about the molecular mediators of local hepcidin expression, but inflammation and iron-load have been shown to induce hepcidin expression in the brain. The most important source of hepcidin in the brain are glial cells. Role of hepcidin in brain functions has been observed during neuronal iron-load and brain hemorrhage, where secretion of abundant hepcidin is related with the severity of brain damage. This damage can be reversed by blocking systemic and local hepcidin secretion. Studies have yet to unveil its role in other brain conditions, but the rationale exists, since these conditions are characterized by overexpression of the factors that stimulate brain hepcidin expression, such as inflammation, hypoxia and iron-overload.

Metallomics Applied to the Study of Neurodegenerative and Mental Diseases

Biochemical imbalances, provoked by aging or a secondary illness, might directly affect the brain, causing severe problems, such as loss of memory or alteration of behavior patterns. Brain disorders are usually classified as injuries (such as stroke, hematomas, and concussions), tumors, and neurodegenerative (such as Parkinson's and Alzheimer's diseases) and mental (such as depression, bipolar disorder, schizophrenia) diseases. As the pathophysiology of these illnesses is not completely established and multiple factors are involved, metallomics, a bioanalytical strategy that allows the detection of metal ions and metalloproteins in diverse biological matrices, is of extreme relevance in identifying which elements are affected by a disease and/or treatment. Thus, determining which element ions suffer disturbances in their homeostasis during the disease progress is relevant to understand the biochemical changes and propose new drug targets. In addition, it is well known that oxidative stress plays an important role in the development of pathological neurodegenerative and mental diseases, which may be caused by metal ion dyshomeostasis, so it is also important to understand endogenous antioxidant metalloprotein and metalloenzyme mechanisms in this regard. In this context, recent applications of metallomics in the study of neurodegenerative and mental disorders are discussed in this chapter, as well as future trends in this research area.

7T MRI for neurodegenerative dementias in vivo: a systematic review of the literature

The spatial resolution of 7T MRI approaches the scale of pathologies of interest in degenerative brain diseases, such as amyloid plaques and changes in cortical layers and subcortical nuclei. It may reveal new information about neurodegenerative dementias, although challenges may include increased artefact production and more adverse effects. We performed a systematic review of papers investigating Alzheimer's disease (AD), Lewy body dementia (LBD), frontotemporal dementia (FTD) and Huntington's disease (HD) in vivo using 7T MRI. Of 19 studies identified, 15 investigated AD (the majority of which examined hippocampal subfield changes), and 4 investigated HD. Ultrahigh resolution revealed changes not visible using lower field strengths, such as hippocampal subfield atrophy in mild cognitive impairment. Increased sensitivity to susceptibility-enhanced iron imaging, facilitating amyloid and microbleed examination; for example, higher microbleed prevalence was found in AD than previously recognised. Theoretical difficulties regarding image acquisition and scan tolerance were not reported as problematic. Study limitations included small subject groups, a lack of studies investigating LBD and FTD and an absence of longitudinal data. In vivo 7T MRI may illuminate disease processes and reveal new biomarkers and therapeutic targets. Evidence from AD and HD studies suggest that other neurodegenerative dementias would also benefit from imaging at ultrahigh resolution.

Iron accumulation, glutathione depletion, and lipid peroxidation must occur simultaneously during ferroptosis and are mutually amplifying events

Ferroptosis is a recently discovered form of regulated necrosis that involves iron-dependent lipid peroxidation. How cells die once ferroptosis is triggered remains unclear. Ferroptosis is hypothesized to require three critical events: (1) accumulation of redox-active iron, (2) glutathione depletion, and (3) lipid peroxidation. It is proposed that these three events must unfold simultaneously because stopping any critical event also stops ferroptosis. These events are hypothesized to amplify in severity through positive feedback loops. The cause of death in ferroptosis is therefore the synergistic combination of antioxidant depletion, iron toxicity, and membrane denaturation. The relevance of these feedback loops for cancer and neurodegenerative therapies is discussed.

Discerning the antioxidant mechanism of rapanone: A naturally occurring benzoquinone with iron complexing and radical scavenging activities

Oxidative stress resulting from iron and reactive oxygen species (ROS) homeostasis breakdown has been implicated in several diseases. Therefore, molecules capable of binding iron and/or scavenging ROS may be reasonable strategies for protecting cells. Rapanone is a naturally occurring hydroxyl-benzoquinone with a privileged chelating structure. In this work, we addressed the antioxidant properties of rapanone concerning its iron-chelating and scavenging activities, and its protective potential against iron and tert-butyl hydroperoxide-induced damage to mitochondria. Experimental determinations revealed the formation of rapanone-Fe(II)/Fe(III) complexes. Additionally, the electrochemical assays indicated that rapanone oxidized Fe(II) and O₂^-, thus inhibiting Fenton-Haber-Weiss reactions. Furthermore, rapanone displayed an increased 2,2-diphenyl-1-picrylhydrazyl radical scavenging ability in the presence of Fe(II). The above results explained the capacity of rapanone to provide near-full protection against iron and tert-butyl hydroperoxide induced mitochondrial lipid peroxidation in energized organelles, which fail under non-energized condition. We postulate that rapanone affords protection against iron and reactive oxygen species by means of both iron chelating and iron-stimulated free radical scavenging activity.

Trace elements during primordial plexiform network formation in human cerebral organoids

Systematic studies of micronutrients during brain formation are hindered by restrictions to animal models and adult post-mortem tissues. Recently, advances in stem cell biology have enabled recapitulation of the early stages of human telencephalon development in vitro. In the present work, we analyzed cerebral organoids derived from human pluripotent stem cells by synchrotron radiation X-ray fluorescence in order to measure biologically valuable micronutrients incorporated and distributed into the exogenously developing brain. Our findings indicate that elemental inclusion in organoids is consistent with human brain tissue and involves P, S, K, Ca, Fe and Zn. Occurrence of different concentration gradients also suggests active regulation of elemental transmembrane transport. Finally, the analysis of pairs of elements shows interesting elemental interaction patterns that change from 30 to 45 days of development, suggesting short- or long-term associations, such as storage in similar compartments or relevance for time-dependent biological processes. These findings shed light on which trace elements are important during human brain development and will support studies aimed to unravel the consequences of disrupted metal homeostasis for neurodevelopmental diseases, including those manifested in adulthood.

Iron mediated toxicity and programmed cell death: A review and a re-examination of existing paradigms

Iron is an essential micronutrient that is problematic for biological systems since it is toxic as it generates free radicals by interconverting between ferrous (Fe²⁺) and ferric (Fe³⁺) forms. Additionally, even though iron is abundant, it is largely insoluble so cells must treat biologically available iron as a valuable commodity. Thus elaborate mechanisms have evolved to absorb, re-cycle and store iron while minimizing toxicity. Focusing on rarely encountered situations, most of the existing literature suggests that iron toxicity is common. A more nuanced examination clearly demonstrates that existing regulatory processes are more than adequate to limit the toxicity of iron even in response to iron overload. Only under pathological or artificially harsh situations of exposure to excess iron does it become problematic. Here we review iron metabolism and its toxicity as well as the literature demonstrating that intracellular iron is not toxic but a stress responsive programmed cell death-inducing second messenger.

Significant glial alterations in response to iron loading in a novel organotypic hippocampal slice culture model

Aberrant iron deposition in the brain is associated with neurodegenerative disorders including Multiple Sclerosis, Alzheimer’s disease and Parkinson’s disease. To study the collective response to iron loading, we have used hippocampal organotypic slices as a platform to develop a novel ex vivo model of iron accumulation. We demonstrated differential uptake and toxicity of iron after 12 h exposure to 10 μM ferrous ammonium sulphate, ferric citrate or ferrocene. Having established the supremacy of ferrocene in this model, the cultures were then loaded with 0.1–100 μM ferrocene for 12 h. One μM ferrocene exposure produced the maximal 1.6-fold increase in iron compared with vehicle. This was accompanied by a 1.4-fold increase in ferritin transcripts and mild toxicity. Using dual-immunohistochemistry, we detected ferritin in oligodendrocytes, microglia, but rarely in astrocytes and never in neurons in iron-loaded slice cultures. Moreover, iron loading led to a 15% loss of olig2-positive cells and a 16% increase in number and greater activation of microglia compared with vehicle. However, there was no appreciable effect of iron loading on astrocytes. In what we believe is a significant advance on traditional mono- or dual-cultures, our novel ex vivo slice-culture model allows characterization of the collective response of brain cells to iron-loading.

Oxidative Stress and Huntington's Disease: The Good, The Bad, and The Ugly

Redox homeostasis is crucial for proper cellular functions, including receptor tyrosine kinase signaling, protein folding, and xenobiotic detoxification. Under basal conditions, there is a balance between oxidants and antioxidants. This balance facilitates the ability of oxidants, such as reactive oxygen species, to play critical regulatory functions through a direct modification of a small number of amino acids (e.g. cysteine) on signaling proteins. These signaling functions leverage tight spatial, amplitude, and temporal control of oxidant concentrations. However, when oxidants overwhelm the antioxidant capacity, they lead to a harmful condition of oxidative stress. Oxidative stress has long been held to be one of the key players in disease progression for Huntington's disease (HD). In this review, we will critically review this evidence, drawing some intermediate conclusions, and ultimately provide a framework for thinking about the role of oxidative stress in the pathophysiology of HD.

Quantitative Susceptibility Mapping Suggests Altered Brain Iron in Premanifest Huntington Disease

BACKGROUND AND PURPOSE

In patients with premanifest (nonsymptomatic) and advanced Huntington disease, changes in brain iron levels in the basal ganglia have been previously reported, especially in the striatum. Quantitative susceptibility mapping by using MR phase imaging allows in vivo measurements of tissue magnetic susceptibility, which has been shown to correlate well with iron levels in brain gray matter and is believed to be more specific than other imaging-based iron measures. The purpose of this study was to investigate the use of magnetic susceptibility as a biomarker of disease progression.

MATERIALS AND METHODS

Fifteen subjects with premanifest Huntington disease and 16 age-matched healthy controls were scanned at 7T. Magnetic susceptibility, effective relaxation, and tissue volume in deep gray matter structures were quantified and compared with genetic and clinical measures.

RESULTS

Subjects with premanifest Huntington disease showed significantly higher susceptibility values in the caudate nucleus, putamen, and globus pallidus, indicating increased iron levels in these structures. Significant decreases in magnetic susceptibility were found in the substantia nigra and hippocampus. In addition, significant volume loss (atrophy) and an increase effective relaxation were observed in the caudate nucleus and putamen. Susceptibility values in the caudate nucleus and putamen were found to be inversely correlated with structure volumes and directly correlated with the genetic burdens, represented by cytosine-adenine-guanine repeat age-product-scaled scores.

CONCLUSIONS

The significant magnetic susceptibility differences between subjects with premanifest Huntington disease and controls and their correlation with genetic burden scores indicate the potential use of magnetic susceptibility as a biomarker of disease progression in premanifest Huntington disease.

Brain Iron Metabolism Dysfunction in Parkinson's Disease

Dysfunction of iron metabolism, which includes its uptake, storage, and release, plays a key role in neurodegenerative disorders, including Parkinson's disease (PD), Alzheimer's disease, and Huntington's disease. Understanding how iron accumulates in the substantia nigra (SN) and why it specifically targets dopaminergic (DAergic) neurons is particularly warranted for PD, as this knowledge may provide new therapeutic avenues for a more targeted neurotherapeutic strategy for this disease. In this review, we begin with a brief introduction describing brain iron metabolism and its regulation. We then provide a detailed description of how iron accumulates specifically in the SN and why DAergic neurons are especially vulnerable to iron in PD. Furthermore, we focus on the possible mechanisms involved in iron-induced cell death of DAergic neurons in the SN. Finally, we present evidence in support that iron chelation represents a plausable therapeutic strategy for PD.

Role of iron in neurodegenerative diseases

Currently, we still lack effective measures to modify disease progression in neurodegenerative diseases. Iron-containing proteins play an essential role in many fundamental biological processes in the central nervous system. In addition, iron is a redox-active ion and can induce oxidative stress in the cell. Although the causes and pathology hallmarks of different neurodegenerative diseases vary, iron dyshomeostasis, oxidative stress and mitochondrial injury constitute a common pathway to cell death in several neurodegenerative diseases. MRI is capable of depicting iron content in the brain, and serves as a potential biomarker for early and differential diagnosis, tracking disease progression and evaluating the effectiveness of neuroprotective therapy. Iron chelators have shown their efficacy against neurodegeneration in a series of animal models, and been applied in several clinical trials. In this review, we summarize recent developments on iron dyshomeostasis in Parkinson's disease, Alzheimer's disease, Friedreich ataxia, and Huntington's disease.

Ferroptosis: Death by Lipid Peroxidation

Ferroptosis is a regulated form of cell death driven by loss of activity of the lipid repair enzyme glutathione peroxidase 4 (GPX4) and subsequent accumulation of lipid-based reactive oxygen species (ROS), particularly lipid hydroperoxides. This form of iron-dependent cell death is genetically, biochemically, and morphologically distinct from other cell death modalities, including apoptosis, unregulated necrosis, and necroptosis. Ferroptosis is regulated by specific pathways and is involved in diverse biological contexts. Here we summarize the discovery of ferroptosis, the mechanism of ferroptosis regulation, and its increasingly appreciated relevance to both normal and pathological physiology.

Optimized real-time monitoring of glutathione redox status in single pyramidal neurons in organotypic hippocampal slices during oxygen-glucose deprivation and reperfusion

A redox-sensitive Grx1-roGFP2 fusion protein was introduced by transfection into single pyramidal neurons in the CA1 subfield of organotypic hippocampal slice cultures (OHSCs). We assessed changes in the GSH system in neuronal cytoplasm and mitochondria during oxygen-glucose deprivation and reperfusion (OGD/RP), an in vitro model of stroke. Pyramidal cells in a narrow range of depths below the surface of the OHSC were transfected by gene gun or single-cell electroporation with cyto- or mito-Grx1-roGFP2. To mimic the conditions of acute stroke, we developed an optimized superfusion system with the capability of rapid and reproducible exchange of the solution bathing the OHSCs. Measurements of pO2 as a function of tissue depth show that in the region containing the transfected cells, the pO2 is well-controlled. We also found that the pO2 changes on the same time scale as changes in intracranial pressure, cerebral blood flow, and pO2 during acute stroke. Determining the reduction potential, EGSH, from the ratiometric fluorescence signal requires an absolute intensity measurement during calibration of the Grx1-roGFP2. Using the signal from cotransfected tdTomato as an internal standard during calibration improves quantitative measurements of Grx1-roGFP2 redox status and allows EGSH to be determined. EGSH becomes more reducing during OGD and more oxidizing during RP in mitochondria while changes in cytoplasm are not significant compared with controls.

Iron and Neurodegeneration: Is Ferritinophagy the Link?

Mounting evidence indicates that the lysosome-autophagy pathway plays a critical role in iron release from ferritin, the main iron storage cellular protein, hence in the distribution of iron to the cells. The recent identification of nuclear receptor co-activator 4 as the receptor for ferritin delivery to selective autophagy sheds further light on the understanding of the mechanisms underlying this pathway. The emerging view is that iron release from ferritin through the lysosomes is a general mechanism in normal and tumour cells of different tissue origins, but it has not yet been investigated in brain cells. Defects in the lysosome-autophagy pathway are often involved in the pathogenesis of neurodegenerative disorders, and brain iron homeostasis disruption is a hallmark of many of these diseases. However, in most cases, it has not been established whether iron dysregulation is directly involved in the pathogenesis of the diseases or if it is a secondary effect derived from other pathogenic mechanisms. The recent evidence of the crucial involvement of autophagy in cellular iron handling offers new perspectives about the role of iron in neurodegeneration, suggesting that autophagy dysregulation could cause iron dyshomeostasis. In this review, we recapitulate our current knowledge on the routes through which iron is released from ferritin, focusing on the most recent advances. We summarise the current evidence concerning lysosome-autophagy pathway dysfunctions and those of iron metabolism and discuss their potential interconnections in several neurodegenerative disorders, such as Alzheimer's, Parkinson's and Huntington's diseases; amyotrophic lateral sclerosis; and frontotemporal lobar dementia.

Targeting mitochondrial metal dyshomeostasis for the treatment of neurodegeneration

Mitochondrial impairment and metal dyshomeostasis are suggested to be associated with many neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and Friedreich's ataxia. Treatments aimed at restoring metal homeostasis are highly effective in models of these diseases, and clinical trials hold promise. However, in general, the effect of these treatments on mitochondrial metal homeostasis is unclear, and the contribution of mitochondrial metal dyshomeostasis to disease pathogenesis requires further investigation. This review describes the role of metals in mitochondria in health, how mitochondrial metals are disrupted in neurodegenerative diseases, and potential therapeutics aimed at restoring mitochondrial metal homeostasis and function.

Substantia nigra hyperechogenicity in hypokinetic Huntington's disease patients

Substantia nigra (SN) hyperechogenicity can be detected by transcranial sonography (TCS) to assist the early diagnosis of idiopathic Parkinson's disease (IPD). This study prospectively investigated whether SN hyperechogenicity is also present in Huntington's disease (HD) patients with symptoms of hypokinesia and/or rigidity. All patients recruited to the study (n = 15) were characterised by hypokinesia and/or rigidity while nine of these patients also displayed chorea and/or dystonia. The control group included 15 individuals. Clinical examination was documented using the Unified Huntington's Disease Rating Scale (UHDRS). TCS examination revealed SN hyperechogenicity in 14/15 (93.3 %) patients (9/14 unilateral, 5/14 bilateral). Hyperechogenicity of the caudate and lentiform nuclei (CN, LN) was less frequent (CN: 80 % total, LN: 53.3 % total). This is the first study to assess SN hyperechogenicity in hypokinetic HD patients. Assuming that the primary sites of pathology in IPD and HD are the SN and the striatum, respectively, our observations suggest a functional impairment of the nigrostriatal system in HD, an effect that is potentially independent of the primarily-affected basal ganglia nucleus.

Echogenicity of basal ganglia structures in different Huntington's disease phenotypes

In Huntington's disease (HD), a neurodegenerative-inherited disease, chorea as the typical kind of movement disorder is described. Beside chorea, however, all other kinds of movement disturbances, such as bradykinesia, dystonia, tremor or myoclonus can occur. Aim of the current study was to investigate alterations in the echogenicity of basal ganglia structures in different Huntington's disease phenotypes. 47 patients with manifest and genetically confirmed HD were recruited. All participants underwent a thorough neurological examination. According to a previously described method, classification into predominantly choreatic, mixed or bradykinetic-rigid motor phenotypes was performed depending on subscores of the Unified Huntington's Disease Rating Scale. In addition, findings in juvenile HD were compared to adult HD. Transcranial sonography was performed by investigators blinded to clinical classification. There were no significant differences in basal ganglia echogenicities between the three phenotypes. Size of echogenic area of substantia nigra (SN) correlated positively with CAG repeat and bradykinesia subscore, and negatively with age of onset and chorea subscore. Comparing juvenile and adult HD subtypes, SN hyperechogenicity was significantly more often detectable in the juvenile form (100 vs. 29.3 %, p = 0.002). Regarding echogenicity of caudate or lentiform nuclei, no significant differences were detected. HD patients with the juvenile variant exhibit marked hyperechogenicity of substantia nigra. No significant differences in basal ganglia echogenicities between predominantly choreatic, mixed or bradykinetic-rigid motor phenotypes were detected.