Iron-mediated aggregation and a localized structural change characterize ferritin from a mutant light chain polypeptide that causes neurodegeneration

Alteration of Iron Concentration in Alzheimer's Disease as a Possible Diagnostic Biomarker Unveiling Ferroptosis

Proper functioning of all organs, including the brain, requires iron. It is present in different forms in biological fluids, and alterations in its distribution can induce oxidative stress and neurodegeneration. However, the clinical parameters normally used for monitoring iron concentration in biological fluids (i.e., serum and cerebrospinal fluid) can hardly detect the quantity of circulating iron, while indirect measurements, e.g., magnetic resonance imaging, require further validation. This review summarizes the mechanisms involved in brain iron metabolism, homeostasis, and iron imbalance caused by alterations detectable by standard and non-standard indicators of iron status. These indicators for iron transport, storage, and metabolism can help to understand which biomarkers can better detect iron imbalances responsible for neurodegenerative diseases.

Pathogenic mechanism and modeling of neuroferritinopathy

Neuroferritinopathy is a rare autosomal dominant inherited movement disorder caused by alteration of the L-ferritin gene that results in the production of a ferritin molecule that is unable to properly manage iron, leading to the presence of free redox-active iron in the cytosol. This form of iron has detrimental effects on cells, particularly severe for neuronal cells, which are highly sensitive to oxidative stress. Although very rare, the disorder is notable for two reasons. First, neuroferritinopathy displays features also found in a larger group of disorders named Neurodegeneration with Brain Iron Accumulation (NBIA), such as iron deposition in the basal ganglia and extrapyramidal symptoms; thus, the elucidation of its pathogenic mechanism may contribute to clarifying the incompletely understood aspects of NBIA. Second, neuroferritinopathy shows the characteristic signs of an accelerated process of aging; thus, it can be considered an interesting model to study the progress of aging. Here, we will review the clinical and neurological features of neuroferritinopathy and summarize biochemical studies and data from cellular and animal models to propose a pathogenic mechanism of the disorder.

Cryo-EM structures and functional characterization of homo- and heteropolymers of human ferritin variants

The role of abnormal brain iron metabolism in neurodegenerative diseases is still insufficiently understood. Here, we investigate the molecular basis of the neurodegenerative disease hereditary ferritinopathy (HF), in which dysregulation of brain iron homeostasis is the primary cause of neurodegeneration. We mutagenized ferritin’s three-fold pores (3FPs), i.e. the main entry route for iron, to investigate ferritin’s iron management when iron must traverse the protein shell through the disrupted four-fold pores (4FPs) generated by mutations in the ferritin light chain (FtL) gene in HF. We assessed the structure and properties of ferritins using cryo-electron microscopy and a range of functional analyses in vitro. Loss of 3FP function did not alter ferritin structure but led to a decrease in protein solubility and iron storage. Abnormal 4FPs acted as alternate routes for iron entry and exit in the absence of functional 3FPs, further reducing ferritin iron-storage capacity. Importantly, even a small number of MtFtL subunits significantly compromises ferritin solubility and function, providing a rationale for the presence of ferritin aggregates in cell types expressing different levels of FtLs in patients with HF. These findings led us to discuss whether modifying pores could be used as a pharmacological target in HF.

Neuropathological and biochemical investigation of Hereditary Ferritinopathy cases with ferritin light chain mutation: Prominent protein aggregation in the absence of major mitochondrial or oxidative stress

AIMS

Neuroferritinopathy (NF) or hereditary ferritinopathy (HF) is an autosomal dominant movement disorder due to mutation in the light chain of the iron storage protein ferritin (FTL). HF is the only late-onset neurodegeneration with brain iron accumulation disorder and study of HF offers a unique opportunity to understand the role of iron in more common neurodegenerative syndromes.

METHODS

We carried out pathological and biochemical studies of six individuals with the same pathogenic FTL mutation.

RESULTS

CNS pathological changes were most prominent in the basal ganglia and cerebellar dentate, echoing the normal pattern of brain iron accumulation. Accumulation of ferritin and iron was conspicuous in cells with a phenotype suggesting oligodendrocytes, with accompanying neuronal pathology and neuronal loss. Neurons still survived, however, despite extensive adjacent glial iron deposition, suggesting neuronal loss is a downstream event. Typical age-related neurodegenerative pathology was not normally present. Uniquely, the extensive aggregates of ubiquitinated ferritin identified indicate that abnormal FTL can aggregate, reflecting the intrinsic ability of FTL to self-assemble. Ferritin aggregates were seen in neuronal and glial nuclei showing parallels with Huntington's disease. There was neither evidence of oxidative stress activation nor any significant mitochondrial pathology in the affected basal ganglia.

CONCLUSIONS

HF shows hallmarks of a protein aggregation disorder, in addition to iron accumulation. Degeneration in HF is not accompanied by age-related neurodegenerative pathology and the lack of evidence of oxidative stress and mitochondrial damage suggests that these are not key mediators of neurodegeneration in HF, casting light on other neurodegenerative diseases characterized by iron deposition.

Mutant L-chain ferritins that cause neuroferritinopathy alter ferritin functionality and iron permeability

In mammals, the iron storage and detoxification protein ferritin is composed of two functionally and genetically distinct subunit types, H (heavy) and L (light). The two subunits co-assemble in various ratios, with a tissue specific distribution, to form shell-like protein structures of 24 subunits within which a mineralized iron core is stored. The H-subunits possess ferroxidase centers that catalyze the rapid oxidation of ferrous ions, whereas the L-subunit does not have such centers and is believed to play an important role in electron transfer reactions that occur during the uptake and release of iron. Pathogenic mutations on the L-chain lead to neuroferritinopathy, a neurodegenerative disease characterized by abnormal accumulation of ferritin inclusion bodies and iron in the central nervous system. Here, we have characterized the thermal stability, iron loading capacity, iron uptake, and iron release properties of ferritin heteropolymers carrying the three pathogenic L-ferritin mutants (L154fs, L167fs, and L148fs, which for simplicity we named Ln1, Ln2 and Ln3, respectively), and a non-pathogenic variant (L135P) bearing a single substitution on the 3-fold axes of L-subunits. The UV-Vis data show a similar iron loading capacity (ranging between 1800 to 2400 Fe(iii)/shell) for all ferritin samples examined in this study, with Ln2 holding the least amount of iron (i.e. 1800 Fe(iii)/shell). The three pathogenic L-ferritin mutants revealed higher rates of iron oxidation and iron release, suggesting that a few mutated L-chains on the heteropolymer have a significant effect on iron permeability through the ferritin shell. DSC thermograms showed a strong destabilization effect, the severity of which depends on the location of the frameshift mutations (i.e. wt heteropolymer ferritin ≅ homopolymer H-chain > L135P > Ln2 > Ln1 > Ln3). Variant L135P had only minor effects on the protein functionality and stability, suggesting that local melting of the 3-fold axes in this variant may not be responsible for neuroferritinopathy-like disorders. The data support the hypothesis that hereditary neuroferritinopathies are due to alterations of ferritin functionality and lower physical stability which correlate with the frameshifts introduced at the C-terminal sequence and explain the dominant transmission of the disorder.

Activation of integrated stress response and disordered iron homeostasis upon combined exposure to cadmium and PCB77

Both cadmium and polychlorinated biphenyls (PCBs) can induce diverse detrimental effects on human health. Though these compounds co-exist in various environmental contexts and in the human body, studies on their joint toxicities are limited. Activation of the integrated stress response (ISR) and iron homeostasis are crucial for erythropoiesis. The impact of cadmium and PCBs on the ISR activation and iron homeostasis of erythroid progenitors is unknown. We investigated the adverse effects and mechanisms of CdCl₂ and PCB77 on HEL cells, a human cell model of erythroid progenitors. We found that at high concentrations of CdCl₂ and PCB77, cytotoxicity and apoptosis of HEL cells were mainly induced by CdCl₂. At low concentrations of CdCl₂ and PCB77, iron homeostasis inside HEL cells was disturbed by both of these two compounds. Both CdCl₂ and PCB77 activated ISR to combat stress, which at high concentration was mainly induced by ROS, leading to apoptosis, and at low concentration was partly induced by disordered iron homeostasis. The patterns of ISR activation and iron homeostasis disorder were different between CdCl₂ and PCB77. Their combined exposure exhibited synergetic effect on activating ISR but antagonistic effect on disturbing iron homeostasis. Our study demonstrates some previously unrecognized harmful characteristics and mechanisms of cadmium and PCB77.

Iron, Ferritin, Hereditary Ferritinopathy, and Neurodegeneration

Cellular growth, function, and protection require proper iron management, and ferritin plays a crucial role as the major iron sequestration and storage protein. Ferritin is a 24 subunit spherical shell protein composed of both light (FTL) and heavy chain (FTH1) subunits, possessing complimentary iron-handling functions and forming three-fold and four-fold pores. Iron uptake through the three-fold pores is well-defined, but the unloading process somewhat less and generally focuses on lysosomal ferritin degradation although it may have an additional, energetically efficient pore mechanism. Hereditary Ferritinopathy (HF) or neuroferritinopathy is an autosomal dominant neurodegenerative disease caused by mutations in the FTL C-terminal sequence, which in turn cause disorder and unraveling at the four-fold pores allowing iron leakage and enhanced formation of toxic, improperly coordinated iron (ICI). Histopathologically, HF is characterized by iron deposition and formation of ferritin inclusion bodies (IBs) as the cells overexpress ferritin in an attempt to address iron accumulation while lacking the ability to clear ferritin and its aggregates. Overexpression and IB formation tax cells materially and energetically, i.e., their synthesis and disposal systems, and may hinder cellular transport and other spatially dependent functions. ICI causes cellular damage to proteins and lipids through reactive oxygen species (ROS) formation because of high levels of brain oxygen, reductants and metabolism, taxing cellular repair. Iron can cause protein aggregation both indirectly by ROS-induced protein modification and destabilization, and directly as with mutant ferritin through C-terminal bridging. Iron release and ferritin degradation are also linked to cellular misfunction through ferritinophagy, which can release sufficient iron to initiate the unique programmed cell death process ferroptosis causing ROS formation and lipid peroxidation. But IB buildup suggests suppressed ferritinophagy, with elevated iron from four-fold pore leakage together with ROS damage and stress leading to a long-term ferroptotic-like state in HF. Several of these processes have parallels in cell line and mouse models. This review addresses the roles of ferritin structure and function within the above-mentioned framework, as they relate to HF and associated disorders characterized by abnormal iron accumulation, protein aggregation, oxidative damage, and the resulting contributions to cumulative cellular stress and death.

Hepcidin and its therapeutic potential in neurodegenerative disorders

Abnormally high brain iron, resulting from the disrupted expression or function of proteins involved in iron metabolism in the brain, is an initial cause of neuronal death in neuroferritinopathy and aceruloplasminemia, and also plays a causative role in at least some of the other neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Friedreich's ataxia. As such, iron is believed to be a novel target for pharmacological intervention in these disorders. Reducing iron toward normal levels or hampering the increases in iron associated with age in the brain is a promising therapeutic strategy for all iron-related neurodegenerative disorders. Hepcidin is a crucial regulator of iron homeostasis in the brain. Recent studies have suggested that upregulating brain hepcidin levels can significantly reduce brain iron content through the regulation of iron transport protein expression in the blood-brain barrier and in neurons and astrocytes. In this review, we focus on the discussion of the therapeutic potential of hepcidin in iron-associated neurodegenerative diseases and also provide a systematic overview of recent research progress on how misregulated brain iron metabolism is involved in the development of multiple neurodegenerative disorders.

Neurodegeneration with Brain Iron Accumulation Disorders: Valuable Models Aimed at Understanding the Pathogenesis of Iron Deposition

Neurodegeneration with brain iron accumulation (NBIA) is a set of neurodegenerative disorders, which includes very rare monogenetic diseases. They are heterogeneous in regard to the onset and the clinical symptoms, while the have in common a specific brain iron deposition in the region of the basal ganglia that can be visualized by radiological and histopathological examinations. Nowadays, 15 genes have been identified as causative for NBIA, of which only two code for iron-proteins, while all the other causative genes codify for proteins not involved in iron management. Thus, how iron participates to the pathogenetic mechanism of most NBIA remains unclear, essentially for the lack of experimental models that fully recapitulate the human phenotype. In this review we reported the recent data on new models of these disorders aimed at highlight the still scarce knowledge of the pathogenesis of iron deposition.

L-Ferritin: One Gene, Five Diseases; from Hereditary Hyperferritinemia to Hypoferritinemia-Report of New Cases

Ferritin is a multimeric protein composed of light (L-ferritin) and heavy (H-ferritin) subunits that binds and stores iron inside the cell. A variety of mutations have been reported in the L-ferritin subunit gene (FTL gene) that cause the following five diseases: (1) hereditary hyperferritinemia with cataract syndrome (HHCS), (2) neuroferritinopathy, a subtype of neurodegeneration with brain iron accumulation (NBIA), (3) benign hyperferritinemia, (4) L-ferritin deficiency with autosomal dominant inheritance, and (5) L-ferritin deficiency with autosomal recessive inheritance. Defects in the FTL gene lead to abnormally high levels of serum ferritin (hyperferritinemia) in HHCS and benign hyperferritinemia, while low levels (hypoferritinemia) are present in neuroferritinopathy and in autosomal dominant and recessive L-ferritin deficiency. Iron disturbances as well as neuromuscular and cognitive deficits are present in some, but not all, of these diseases. Here, we identified two novel FTL variants that cause dominant L-ferritin deficiency and HHCS (c.375+2T > A and 36_42delCAACAGT, respectively), and one previously reported variant (Met1Val) that causes dominant L-ferritin deficiency. Globally, genetic changes in the FTL gene are responsible for multiple phenotypes and an accurate diagnosis is useful for appropriate treatment. To help in this goal, we included a diagnostic algorithm for the detection of diseases caused by defects in FTL gene.

Effect of Systemic Iron Overload and a Chelation Therapy in a Mouse Model of the Neurodegenerative Disease Hereditary Ferritinopathy

Mutations in the ferritin light chain (FTL) gene cause the neurodegenerative disease neuroferritinopathy or hereditary ferritinopathy (HF). HF is characterized by a severe movement disorder and by the presence of nuclear and cytoplasmic iron-containing ferritin inclusion bodies (IBs) in glia and neurons throughout the central nervous system (CNS) and in tissues of multiple organ systems. Herein, using primary mouse embryonic fibroblasts from a mouse model of HF, we show significant intracellular accumulation of ferritin and an increase in susceptibility to oxidative damage when cells are exposed to iron. Treatment of the cells with the iron chelator deferiprone (DFP) led to a significant improvement in cell viability and a decrease in iron content. In vivo, iron overload and DFP treatment of the mouse model had remarkable effects on systemic iron homeostasis and ferritin deposition, without significantly affecting CNS pathology. Our study highlights the role of iron in modulating ferritin aggregation in vivo in the disease HF. It also puts emphasis on the potential usefulness of a therapy based on chelators that can target the CNS to remove and redistribute iron and to resolubilize or prevent ferritin aggregation while maintaining normal systemic iron stores.

Neuroferritinopathy: From ferritin structure modification to pathogenetic mechanism

Neuroferritinopathy is a rare, late-onset, dominantly inherited movement disorder caused by mutations in L-ferritin gene. It is characterized by iron and ferritin aggregate accumulation in brain, normal or low serum ferritin levels and high variable clinical feature. To date, nine causative mutations have been identified and eight of them are frameshift mutations determined by nucleotide(s) insertion in the exon 4 of L-ferritin gene altering the structural conformation of the C-terminus of the L-ferritin subunit. Acting in a dominant negative manner, mutations are responsible for an impairment of the iron storage efficiency of ferritin molecule. Here, we review the main characteristics of neuroferritinopathy and present a computational analysis of some representative recently defined mutations with the purpose to gain new information about the pathogenetic mechanism of the disorder. This is particularly important as neuroferritinopathy can be considered an interesting model to study the relationship between iron, oxidative stress and neurodegeneration.

Systemic and cerebral iron homeostasis in ferritin knock-out mice

Ferritin, a 24-mer heteropolymer of heavy (H) and light (L) subunits, is the main cellular iron storage protein and plays a pivotal role in iron homeostasis by modulating free iron levels thus reducing radical-mediated damage. The H subunit has ferroxidase activity (converting Fe(II) to Fe(III)), while the L subunit promotes iron nucleation and increases ferritin stability. Previous studies on the H gene (Fth) in mice have shown that complete inactivation of Fth is lethal during embryonic development, without ability to compensate by the L subunit. In humans, homozygous loss of the L gene (FTL) is associated with generalized seizure and atypical restless leg syndrome, while mutations in FTL cause a form of neurodegeneration with brain iron accumulation. Here we generated mice with genetic ablation of the Fth and Ftl genes. As previously reported, homozygous loss of the Fth allele on a wild-type Ftl background was embryonic lethal, whereas knock-out of the Ftl allele (Ftl^-/-) led to a significant decrease in the percentage of Ftl^-/- newborn mice. Analysis of Ftl^-/- mice revealed systemic and brain iron dyshomeostasis, without any noticeable signs of neurodegeneration. Our findings indicate that expression of the H subunit can rescue the loss of the L subunit and that H ferritin homopolymers have the capacity to sequester iron in vivo. We also observed that a single allele expressing the H subunit is not sufficient for survival when both alleles encoding the L subunit are absent, suggesting the need of some degree of complementation between the subunits as well as a dosage effect.

A novel neuroferritinopathy mouse model (FTL 498InsTC) shows progressive brain iron dysregulation, morphological signs of early neurodegeneration and motor coordination deficits

Neuroferritinopathy is a rare genetic disease with a dominant autosomal transmission caused by mutations of the ferritin light chain gene (FTL). It belongs to Neurodegeneration with Brain Iron Accumulation, a group of disorders where iron dysregulation is tightly associated with neurodegeneration. We studied the 498–499InsTC mutation which causes the substitution of the last 9 amino acids and an elongation of extra 16 amino acids at the C-terminus of L-ferritin peptide. An analysis with cyclic voltammetry on the purified protein showed that this structural modification severely reduces the ability of the protein to store iron. In order to analyze the impact of the mutation in vivo, we generated mouse models for the some pathogenic human FTL gene in FVB and C57BL/6J strains.

Transgenic mice in the FVB background showed high accumulation of the mutated ferritin in brain where it correlated with increased iron deposition with age, as scored by magnetic resonance imaging. Notably, the accumulation of iron–ferritin bodies was accompanied by signs of oxidative damage. In the C57BL/6 background, both the expression of the mutant ferritin and the iron levels were lower than in the FVB strain. Nevertheless, also these mice showed oxidative alterations in the brain. Furthermore, post-natal hippocampal neurons obtained from these mice experienced a marked increased cell death in response to chronic iron overload and/or acute oxidative stress, in comparison to wild-type neurons. Ultrastructural analyses revealed an accumulation of lipofuscin granules associated with iron deposits, particularly enriched in the cerebellum and striatum of our transgenic mice. Finally, experimental subjects were tested throughout development and aging at 2-, 8- and 18-months for behavioral phenotype. Rotarod test revealed a progressive impaired motor coordination building up with age, FTL mutant old mice showing a shorter latency to fall from the apparatus, according to higher accumulation of iron aggregates in the striatum. Our data show that our 498–499InsTC mouse models recapitulate early pathological and clinical traits of the human neuroferritinopathy, thus providing a valuable model for the study of the disease. Finally, we propose a mechanistic model of lipofuscine formation that can account for the etiopathogenesis of human neuroferritinopathy.

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We developed two new neuroferritinopathy mice models (NF).

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NF brains are characterized by iron/ferritin accumulation and oxidative damage.

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NF brains show granules of lipofuscine associated with iron.

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A mechanism of lipofuscine formation is proposed.

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NF mice show impaired motor coordination increasing with age.

Biology of ferritin in mammals: an update on iron storage, oxidative damage and neurodegeneration

Iron is an abundant transition metal that is essential for life, being associated with many enzyme and oxygen carrier proteins involved in a variety of fundamental cellular processes. At the same time, the metal is potentially toxic due to its capacity to engage in the catalytic production of noxious reactive oxygen species. The control of iron availability in the cells is largely dependent on ferritins, ubiquitous proteins with storage and detoxification capacity. In mammals, cytosolic ferritins are composed of two types of subunits, the H and the L chain, assembled to form a 24-mer spherical cage. Ferritin is present also in mitochondria, in the form of a complex with 24 identical chains. Even though the proteins have been known for a long time, their study is a very active and interesting field yet. In this review, we will focus our attention to mammalian cytosolic and mitochondrial ferritins, describing the most recent advancement regarding their storage and antioxidant function, the effects of their genetic mutations in human pathology, and also the possible involvement in non-iron-related activities. We will also discuss recent evidence connecting ferritins and the toxicity of iron in a set of neurodegenerative disorder characterized by focal cerebral siderosis.

Neurodegeneration with brain iron accumulation: update on pathogenic mechanisms

Perturbation of iron distribution is observed in many neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease, but the comprehension of the metal role in the development and progression of such disorders is still very limited. The combination of more powerful brain imaging techniques and faster genomic DNA sequencing procedures has allowed the description of a set of genetic disorders characterized by a constant and often early accumulation of iron in specific brain regions and the identification of the associated genes; these disorders are now collectively included in the category of neurodegeneration with brain iron accumulation (NBIA). So far 10 different genetic forms have been described but this number is likely to increase in short time. Two forms are linked to mutations in genes directly involved in iron metabolism: neuroferritinopathy, associated to mutations in the FTL gene and aceruloplasminemia, where the ceruloplasmin gene product is defective. In the other forms the connection with iron metabolism is not evident at all and the genetic data let infer the involvement of other pathways: Pank2, Pla2G6, C19orf12, COASY, and FA2H genes seem to be related to lipid metabolism and to mitochondria functioning, WDR45 and ATP13A2 genes are implicated in lysosomal and autophagosome activity, while the C2orf37 gene encodes a nucleolar protein of unknown function. There is much hope in the scientific community that the study of the NBIA forms may provide important insight as to the link between brain iron metabolism and neurodegenerative mechanisms and eventually pave the way for new therapeutic avenues also for the more common neurodegenerative disorders. In this work, we will review the most recent findings in the molecular mechanisms underlining the most common forms of NBIA and analyze their possible link with brain iron metabolism.

Brain iron homeostasis: from molecular mechanisms to clinical significance and therapeutic opportunities

Iron has emerged as a significant cause of neurotoxicity in several neurodegenerative conditions, including Alzheimer's disease (AD), Parkinson's disease (PD), sporadic Creutzfeldt-Jakob disease (sCJD), and others. In some cases, the underlying cause of iron mis-metabolism is known, while in others, our understanding is, at best, incomplete. Recent evidence implicating key proteins involved in the pathogenesis of AD, PD, and sCJD in cellular iron metabolism suggests that imbalance of brain iron homeostasis associated with these disorders is a direct consequence of disease pathogenesis. A complete understanding of the molecular events leading to this phenotype is lacking partly because of the complex regulation of iron homeostasis within the brain. Since systemic organs and the brain share several iron regulatory mechanisms and iron-modulating proteins, dysfunction of a specific pathway or selective absence of iron-modulating protein(s) in systemic organs has provided important insights into the maintenance of iron homeostasis within the brain. Here, we review recent information on the regulation of iron uptake and utilization in systemic organs and within the complex environment of the brain, with particular emphasis on the underlying mechanisms leading to brain iron mis-metabolism in specific neurodegenerative conditions. Mouse models that have been instrumental in understanding systemic and brain disorders associated with iron mis-metabolism are also described, followed by current therapeutic strategies which are aimed at restoring brain iron homeostasis in different neurodegenerative conditions. We conclude by highlighting important gaps in our understanding of brain iron metabolism and mis-metabolism, particularly in the context of neurodegenerative disorders.

Abnormal iron homeostasis and neurodegeneration

Abnormal iron metabolism is observed in many neurodegenerative diseases, however, only two have shown dysregulation of brain iron homeostasis as the primary cause of neurodegeneration. Herein, we review one of these - hereditary ferritinopathy (HF) or neuroferritinopathy, which is an autosomal dominant, adult onset degenerative disease caused by mutations in the ferritin light chain (FTL) gene. HF has a clinical phenotype characterized by a progressive movement disorder, behavioral disturbances, and cognitive impairment. The main pathologic findings are cystic cavitation of the basal ganglia, the presence of ferritin inclusion bodies (IBs), and substantial iron deposition. Mutant FTL subunits have altered sequence and length but assemble into soluble 24-mers that are ultrastructurally indistinguishable from those of the wild type. Crystallography shows substantial localized disruption of the normally tiny 4-fold pores between the ferritin subunits because of unraveling of the C-termini into multiple polypeptide conformations. This structural alteration causes attenuated net iron incorporation leading to cellular iron mishandling, ferritin aggregation, and oxidative damage at physiological concentrations of iron and ascorbate. A transgenic murine model parallels several features of HF, including a progressive neurological phenotype, ferritin IB formation, and misregulation of iron metabolism. These studies provide a working hypothesis for the pathogenesis of HF by implicating (1) a loss of normal ferritin function that triggers iron accumulation and overproduction of ferritin polypeptides, and (2) a gain of toxic function through radical production, ferritin aggregation, and oxidative stress. Importantly, the finding that ferritin aggregation can be reversed by iron chelators and oxidative damage can be inhibited by radical trapping may be used for clinical investigation. This work provides new insights into the role of abnormal iron metabolism in neurodegeneration.

Neuroferritinopathy

Neuroferritinopathy is an autosomal dominantly inherited disorder caused by mutations in the gene encoding the ferritin light chain polypeptide. It leads to iron deposition particularly in the cerebellum, basal ganglia and motor cortex. The disease becomes clinically apparent in adulthood mainly with extrapyramidal signs and progresses slowly over decades. Patients usually have intact cognition until the very late stages of this disorder. Neuroimaging is the most helpful investigation and shows a very distinctive picture. So far no medication has been shown to have a disease-modifying effect. We present five new cases of this condition and review the current understanding of the pathogenesis and its clinical findings.

Brain, blood, and iron: perspectives on the roles of erythrocytes and iron in neurodegeneration

The terms "neuroacanthocytosis" (NA) and "neurodegeneration with brain iron accumulation" (NBIA) both refer to groups of genetically heterogeneous disorders, classified together due to similarities of their phenotypic or pathological findings. Even collectively, the disorders that comprise these sets are exceedingly rare and challenging to study. The NBIA disorders are defined by their appearance on brain magnetic resonance imaging, with iron deposition in the basal ganglia. Clinical features vary, but most include a movement disorder. New causative genes are being rapidly identified; however, the mechanisms by which mutations cause iron accumulation and neurodegeneration are not well understood. NA syndromes are also characterized by a progressive movement disorder, accompanied by cognitive and psychiatric features, resulting from mutations in a number of genes whose roles are also basically unknown. An overlapping feature of the two groups, NBIA and NA, is the occurrence of acanthocytes, spiky red cells with a poorly-understood membrane dysfunction. In this review we summarise recent developments in this field, specifically insights into cellular mechanisms and from animal models. Cell membrane research may shed light upon the significance of the erythrocyte abnormality, and upon possible connections between the two sets of disorders. Shared pathophysiologic mechanisms may lead to progress in the understanding of other types of neurodegeneration.

A mutant light-chain ferritin that causes neurodegeneration has enhanced propensity toward oxidative damage

Intracellular inclusion bodies (IBs) containing ferritin and iron are hallmarks of hereditary ferritinopathy (HF). This neurodegenerative disease is caused by mutations in the coding sequence of the ferritin light chain (FTL) gene that generate FTL polypeptides with a C-terminus that is altered in amino acid sequence and length. Previous studies of ferritin formed with p.Phe167SerfsX26 mutant FTL (Mt-FTL) subunits found disordered 4-fold pores, iron mishandling, and proaggregative behavior, as well as a general increase in cellular oxidative stress when expressed in vivo. Herein, we demonstrate that Mt-FTL is also a target of iron-catalyzed oxidative damage in vitro and in vivo. Incubation of recombinant Mt-FTL ferritin with physiological concentrations of iron and ascorbate resulted in shell structural disruption and polypeptide cleavage not seen with the wild type, as well as a 2.5-fold increase in carbonyl group formation. However, Mt-FTL shell disruption and polypeptide cleavage were completely inhibited by the addition of the radical trap 5,5-dimethyl-1-pyrroline N-oxide. These results indicate an enhanced propensity of Mt-FTL toward free radical-induced oxidative damage in vitro. We also found evidence of extensive carbonylation in IBs from a patient with HF together with isolation of a C-terminal Mt-FTL fragment, which are both indicative of oxidative ferritin damage in vivo. Our data demonstrate an enhanced propensity of mutant ferritin to undergo iron-catalyzed oxidative damage and support this as a mechanism causing disruption of ferritin structure and iron mishandling that contribute to the pathology of HF.

The structural stability and chaperone activity of artemin, a ferritin homologue from diapause-destined Artemia embryos, depend on different cysteine residues

Diapause-destined embryos of the crustacean, Artemia franciscana, accumulate large amounts of an oligomeric, heat-stable, molecular chaperone termed artemin, a cysteine-enriched ferritin homologue. In this study, cysteines 22, 61, 166, and 172 of artemin were substituted with alanines, respectively yielding ArtC22A, ArtC61A, ArtC166A, and ArtC172A. Wild-type and modified artemins were synthesized in transformed bacteria and purified. As measured by heat-induced denaturation of citrate synthase in vitro, each substitution reduced chaperone activity, with ArtC172A the least active. Protein modeling indicated that C172 is close to a region of surface hydrophobicity, also present in ferritin, suggesting that this site contributes to chaperone activity. Only slight differences in oligomer molecular mass were apparent between artemin variants, but ArtC22A and ArtC61A displayed significantly reduced thermostability, perhaps due to the disruption of an inter-subunit disulphide bridge. In contrast, ArtC172A was thermostable, reflecting the location of C172 on the oligomer surface and that it contributes minimally to artemin stabilization. To our knowledge, this is the initial study of structure/function relationships within a ferritin homologue of importance in diapause and the first to indicate that a defined region of hydrophobicity contributes to artemin and ferritin chaperoning.

Abnormal iron metabolism in fibroblasts from a patient with the neurodegenerative disease hereditary ferritinopathy

Background

Nucleotide duplications in exon 4 of the ferritin light polypeptide (FTL) gene cause the autosomal dominant neurodegenerative disease neuroferritinopathy or hereditary ferritinopathy (HF). Pathologic examination of patients with HF has shown abnormal ferritin and iron accumulation in neurons and glia in the central nervous system (CNS) as well as in cells of other organ systems, including skin fibroblasts. To gain some understanding on the molecular basis of HF, we characterized iron metabolism in primary cultures of human skin fibroblasts from an individual with the FTL c.497_498dupTC mutation.

Results

Compared to normal controls, HF fibroblasts showed abnormal iron metabolism consisting of increased levels of ferritin polypeptides, divalent metal transporter 1, basal iron content and reactive oxygen species, and decreased levels of transferrin receptor-1 and IRE-IRP binding activity.

Conclusions

Our data indicates that HF fibroblasts replicate the abnormal iron metabolism observed in the CNS of patients with HF. We propose that HF fibroblasts are a unique cellular model in which to study the role of abnormal iron metabolism in the pathogenesis of HF without artifacts derived from over-expression or lack of endogenous translational regulatory elements.

Iron loading-induced aggregation and reduction of iron incorporation in heteropolymeric ferritin containing a mutant light chain that causes neurodegeneration

Hereditary ferritinopathy (HF) is a neurodegenerative disease characterized by intracellular ferritin inclusion bodies (IBs) and iron accumulation throughout the central nervous system. Ferritin IBs are composed of mutant ferritin light chain as well as wild-type light (Wt-FTL) and heavy chain (FTH1) polypeptides. In vitro studies have shown that the mutant light chain polypeptide p.Phe167SerfsX26 (Mt-FTL) forms soluble ferritin 24-mer homopolymers having a specific structural disruption that explains its functional problems of reduced ability to incorporate iron and aggregation during iron loading. However, because ferritins are usually 24-mer heteropolymers and all three polypeptides are found in IBs, we investigated the properties of Mt-FTL/FTH1 and Mt-FTL/Wt-FTL heteropolymeric ferritins. We show here the facile assembly of Mt-FTL and FTH1 subunits into soluble ferritin heteropolymers, but their ability to incorporate iron was significantly reduced relative to Wt-FTL/FTH1 heteropolymers. In addition, Mt-FTL/FTH1 heteropolymers formed aggregates during iron loading, contrasting Wt-FTL/FTH1 heteropolymers and similar to what was seen for Mt-FTL homopolymers. The resulting precipitate contained both Mt-FTL and FTH1 polypeptides as do ferritin IBs in patients with HF. The presence of Mt-FTL subunits in Mt-FTL/Wt-FTL heteropolymers also caused iron loading-induced aggregation relative to Wt-FTL homopolymers, with the precipitate containing Mt- and Wt-FTL polypeptides again paralleling HF. Our data demonstrate that co-assembly with wild-type subunits does not circumvent the functional problems caused by mutant subunits. Furthermore, the functional problems characterized here in heteropolymers that contain mutant subunits parallel those problems previously reported in homopolymers composed exclusively of mutant subunits, which strongly suggests that the structural disruption characterized previously in Mt-FTL homopolymers occurs in a similar manner and to a significant extent in both Mt-FTL/FTH1 and Mt-FTL/Wt-FTL heteropolymers.

Accumulation of oxidative DNA damage in brain mitochondria in mouse model of hereditary ferritinopathy

Tissue iron content is strictly regulated to concomitantly satisfy specialized metabolic requirements and avoid toxicity. Ferritin, a multi-subunit iron storage protein, is central to maintenance of iron homeostasis in the brain. Mutations in the ferritin light chain (FTL)-encoding gene underlie the autosomal dominant, neurodegenerative disease, neuroferritinopathy/hereditary ferritinopathy (HF). HF is characterized by progressive accumulation of ferritin and iron. To gain insight into mechanisms by which FTL mutations promote neurodegeneration, a transgenic mouse, expressing human mutant form of FTL, was recently generated. The FTL mouse exhibits buildup of iron in the brain and presents manifestations of oxidative stress reminiscent of the human disease. Here, we asked whether oxidative DNA damage accumulates in the FTL mouse brain. Long-range PCR (L-PCR) amplification-mediated DNA damage detection assays revealed that the integrity of mitochondrial DNA (mtDNA) in the brain was significantly compromised in the 12- but not 6-month-old FTL mice. Furthermore, L-PCR employed in conjunction with DNA modifying enzymes, which target specific DNA adducts, revealed the types of oxidative adducts accumulating in mtDNA in the FTL brain. Consistently with DNA damage predicted to form under conditions of excessive oxidative stress, detected adducts include, oxidized guanines, abasic sites and strand breaks. Elevated mtDNA damage may impair mitochondrial function and brain energetics and in the long term contribute to neuronal loss and exacerbate neurodegeneration in HF.

Cytosolic and mitochondrial ferritins in the regulation of cellular iron homeostasis and oxidative damage

BACKGROUND

Ferritin structure is designed to maintain large amounts of iron in a compact and bioavailable form in solution. All ferritins induce fast Fe(II) oxidation in a reaction catalyzed by a ferroxidase center that consumes Fe(II) and peroxides, the reagents that produce toxic free radicals in the Fenton reaction, and thus have anti-oxidant effects. Cytosolic ferritins are composed of the H- and L-chains, whose expression are regulated by iron at a post-transcriptional level and by oxidative stress at a transcriptional level. The regulation of mitochondrial ferritin expression is presently unclear.

SCOPE OF REVIEW

The scope of the review is to update recent progress regarding the role of ferritins in the regulation of cellular iron and in the response to oxidative stress with particular attention paid to the new roles described for cytosolic ferritins, to genetic disorders caused by mutations of the ferritin L-chain, and new findings on mitochondrial ferritin.

MAJOR CONCLUSIONS

The new data on the adult conditional knockout (KO) mice for the H-chain and on the hereditary ferritinopathies with mutations that reduce ferritin functionality strongly indicate that the major role of ferritins is to protect from the oxidative damage caused by iron deregulation. In addition, the study of mitochondrial ferritin, which is not iron-regulated, indicates that it participates in the protection against oxidative damage, particularly in cells with high oxidative activity.

GENERAL SIGNIFICANCE

Ferritins have a central role in the protection against oxidative damage, but they are also involved in non-iron-dependent processes.

Mutant ferritin L-chains that cause neurodegeneration act in a dominant-negative manner to reduce ferritin iron incorporation

Nucleotide insertions that modify the C terminus of ferritin light chain (FTL) cause neurodegenerative movement disorders named neuroferritinopathies, which are inherited with dominant transmission. The disorders are characterized by abnormal brain iron accumulation. Here we describe the biochemical and crystallographic characterization of pathogenic FTL mutant p.Phe167SerfsX26 showing that it is a functional ferritin with an altered conformation of the C terminus. Moreover we analyze functional and stability properties of ferritin heteropolymers made of 20-23 H-chains and 1-4 L-chains with representative pathogenic mutations or the last 10-28 residues truncated. All the heteropolymers containing the pathogenic or truncated mutants had a strongly reduced capacity to incorporate iron, both when expressed in Escherichia coli, and in vitro when iron was supplied as Fe(III) in the presence of ascorbate. The mutations also reduced the physical stability of the heteropolymers. The data indicate that even a few mutated L-chains are sufficient to alter the permeability of 1-2 of the 6 hydrophobic channels and modify ferritin capacity to incorporate iron. The dominant-negative action of the mutations explains the dominant transmission of the disorder. The data support the hypothesis that hereditary ferritinopathies are due to alterations of ferritin functionality and provide new input on the mechanism of the function of isoferritins.

Unraveling of the E-helices and disruption of 4-fold pores are associated with iron mishandling in a mutant ferritin causing neurodegeneration

Mutations in the coding sequence of the ferritin light chain (FTL) gene cause a neurodegenerative disease known as neuroferritinopathy or hereditary ferritinopathy, which is characterized by the presence of intracellular inclusion bodies containing the mutant FTL polypeptide and by abnormal accumulation of iron in the brain. Here, we describe the x-ray crystallographic structure and report functional studies of ferritin homopolymers formed from the mutant FTL polypeptide p.Phe167SerfsX26, which has a C terminus that is altered in amino acid sequence and length. The structure was determined and refined to 2.85 A resolution and was very similar to the wild type between residues Ile-5 and Arg-154. However, instead of the E-helices normally present in wild type ferritin, the C-terminal sequences of all 24 mutant subunits showed substantial amounts of disorder, leading to multiple C-terminal polypeptide conformations and a large disruption of the normally tiny 4-fold axis pores. Functional studies underscored the importance of the mutant C-terminal sequence in iron-induced precipitation and revealed iron mishandling by soluble mutant FTL homopolymers in that only wild type incorporated iron when in direct competition in solution with mutant ferritin. Even without competition, the amount of iron incorporation over the first few minutes differed severalfold. Our data suggest that disruption at the 4-fold pores may lead to direct iron mishandling through attenuated iron incorporation by the soluble form of mutant ferritin and that the disordered C-terminal polypeptides may play a major role in iron-induced precipitation and formation of ferritin inclusion bodies in hereditary ferritinopathy.

[Clinical features of neuroferritinopathy]

Neuroferritinopathy is an autosomal dominant basal ganglia disease with iron accumulation caused by a mutation of the gene encoding ferritin light polypeptide (FTL). Six pathogenic mutations in the FTL gene have so far been reported. One such mutation was found in a Japanese family, thus suggesting that a new mutation in the FTL gene can therefore occur anywhere in the world. The typical clinical features of neuroferritinopathy are dystonia (especially orofacial dystonia related to speech and leading to dysarthrophonia) and involuntary movement, but such features vary greatly among the affected individuals. The findings of excess iron storage and cystic changes involving the globus pallidus and the putamen on brain MRI. and low serum ferritin levels are characteristic in neuroferritinopathy. Brain histochemistry shows abnormal aggregates of ferritin and iron throughout the central nervous system. Iron atoms are stored in the central cavity of the ferritin polymer and the E-helices of ferritin play an important role in maintaining the central cavity. A mutation in exon 4 of the FTL gene is known to alter the structure of E-helices, thereby leading to the release of free iron and excessive oxidative stress. Iron depletion therapy by iron chelation in symptomatic patients has not been shown to be beneficial, however before the nset of clinical symptoms, such a treatment strategy may still have some benefit. Neuroferritinopathy should therefore be considered in all patients presenting with basal ganglia disorders of unknown origin. These characteristic MRI findings may help to differentiate neuroferritinopathy from other diseases showing similar clinical features.

Oxidative stress and cell death in cells expressing L-ferritin variants causing neuroferritinopathy

Neuroferritinopathies are dominantly inherited movement disorders associated with nucleotide insertions in the L-ferritin gene that modify the protein's C-terminus. The insertions alter physical and functional properties of the ferritins, causing an imbalance in brain iron homeostasis. We describe the effects produced by the over-expression in HeLa and neuroblastoma SH-SY5Y cells of two pathogenic L-ferritin variants, 460InsA and 498InsTC. Both peptides co-assembled with endogenous ferritins, producing molecules with reduced iron incorporation capacity, acting in a dominant negative manner. The cells showed an increase in cell death and a decrease in proteasomal activity. The formation of iron-ferritin aggregates became evident after 10 days of variant expression and was not associated with increased cell death. The addition of iron chelators or antioxidants restored proteasomal activity and reduced aggregate formation. The data indicate that cellular iron imbalance and oxidative damage are primary causes of cell death, while aggregate formation is a secondary effect.

Sequence and structural analysis of artemin based on ferritin: a comparative study

Artemia cysts can tolerate extreme environments, partly due to a heat-stable protein called artemin. According to previous studies, artemin shares structural similarity with ferritins. Actually, there is still no strong structural information about artemin three-dimensional (3-D) structure. In this research, the artemin encoding gene from Artemia urmiana was cloned and sequenced. A reliable 3-D model of artemin was initially built using ferritin as template and refined using Molecular Dynamic (MD) Simulation. It is interesting that the proposed model, confirmed by circular dichroism (CD), shows significant differences in secondary structure contents with ferritin. Three conserved regions (ferroxidase center, iron nucleation center and 3-fold channel) in ferritins, cooperating in iron-interaction, have been substantially changed in artemin. Analysis of C-terminal region of the model revealed its major role in preventing artemin from iron-binding due to some suitable interactions. Finally, it is concluded that significant differences between artemin and ferritin, both in conserved regions related to iron-interaction and three-dimensional structure, can justify their functional differences.

Magnetic Resonance Imaging of Cells Overexpressing MagA, an Endogenous Contrast Agent for Live Cell Imaging

Molecular imaging with magnetic resonance imaging (MRI) may benefit from the ferrimagnetic properties of magnetosomes, membrane-enclosed iron biominerals whose formation in magnetotactic bacteria is encoded by multiple genes. One such gene is MagA, a putative iron transporter. We have examined expression of MagA in mouse neuroblastoma N2A cells and characterized their response to iron loading and cellular imaging by MRI. MagA expression augmented both Prussian blue staining and the elemental iron content of N2A cells, without altering cell proliferation, in cultures grown in the presence of iron supplements. Despite evidence for iron incorporation in both MagA and a variant, MagAE137V, only MagA expression produced intracellular contrast detectable by MRI at 11 Tesla. We used this stable expression system to model a new sequence for cellular imaging with MRI, using the difference between gradient and spin echo images to distinguish cells from artifacts in the field of view. Our results show that MagA activity in mammalian cells responds to iron supplementation and functions as a contrast agent that can be deactivated by a single point mutation. We conclude that MagA is a candidate MRI reporter gene that can exploit more fully the superior resolution of MRI in noninvasive medical imaging.

Abnormal iron metabolism and oxidative stress in mice expressing a mutant form of the ferritin light polypeptide gene

Insertional mutations in exon 4 of the ferritin light chain (FTL) gene are associated with hereditary ferritinopathy (HF) or neuroferritinopathy, an autosomal dominant neurodegenerative disease characterized by progressive impairment of motor and cognitive functions. To determine the pathogenic mechanisms by which mutations in FTL lead to neurodegeneration, we investigated iron metabolism and markers of oxidative stress in the brain of transgenic (Tg) mice that express the mutant human FTL498-499InsTC cDNA. Compared with wild-type mice, brain extracts from Tg (FTL-Tg) mice showed an increase in the cytoplasmic levels of both FTL and ferritin heavy chain polypeptides, a decrease in the protein and mRNA levels of transferrin receptor-1, and a significant increase in iron levels. Transgenic mice also showed the presence of markers for lipid peroxidation, protein carbonyls, and nitrone-protein adducts in the brain. However, gene expression analysis of iron management proteins in the liver of Tg mice indicates that the FTL-Tg mouse liver is iron deficient. Our data suggest that disruption of iron metabolism in the brain has a primary role in the process of neurodegeneration in HF and that the pathogenesis of HF is likely to result from a combination of reduction in iron storage function and enhanced toxicity associated with iron-induced ferritin aggregates in the brain.