Iron, neuromelanin and ferritin content in the substantia nigra of normal subjects at different ages: consequences for iron storage and neurodegenerative processes

A critical review of the function of neuromelanin and an attempt to provide a unified theory

This paper provides a critical review of the numerous and various biological functions so far attributed to neuromelanin and an attempt to provide a unified theory based on the peculiar physical and chemical properties of the black particle (the neuromelanin cage). It is stressed that neuromelanin is not homogeneous, as is commonly accepted, but is made up of different substrate specific black pigments formed by the oxidation of o.diphenols or other oxygenated precursors (substantia nigra melanin, locus coeruleus melanin, retinal pigmented epithelium or ocular melanin, inner-ear melanin, and so on). Ocular melanin is believed to protect the eye by trapping metals and free radicals. The paper shows that this unconfirmed mechanism is a rather fortuitous irreversible molecular accident, which at times may prove itself deleterious. Albinism often leads to deafness in animals, indicating a genetic correlation. These two conditions appear to be correlated at a molecular level to eye/ear pigmentation and suggest verifying this hypothesis in normal and albino human individuals. Skin and ocular melanin are chemically different. However, they are both involved in light absorption/dissipation. The black particle structure (melanin cage) is believed to be fundamental to this process because there is a common bioelectric mechanism. The latter is worth of further investigation. It is also proposed checking how ocular melanin dissipates the excessive absorbed light (as heat or as current?). It has been claimed that inner-ear melanin mutes acoustic waves. This paper suggests investigating the underlying mechanism and also studying whether this pigment is bio-electrically involved in audiology. According to numerous authors, substantia nigra melanin is only biological garbage. This view is rejected, and it is stressed that intracellular melanogenesis is a fundamental and genetically controlled physiological process. It has been repeatedly claimed that the binding of iron, heavy metals, free radicals and harmful chemicals by substantia nigra melanin is fundamental to body detoxification/protection. Presumably, such irreversible and generic binding mechanisms have no physiological foundation; it is suggested the alternative that, substantia nigra melanin acts as semiconductor, transmitting and modulating nervous impulses, in a reversible way. In fact, substantia nigra melanin is absent or significantly scarce in two conditions of life in which the coordination of movement is either inefficient (newborn babies) or strongly compromised (Parkinson). To check this assumption, further investigation of nucleus caudatus, putamen, globus pallidus, substantia nigra pars compacta and reticulata, nucleus hypothalamicus is recommended.

Correlation of R2 with total iron concentration in the brains of rhesus monkeys

PURPOSE

To estimate the relationship between R2 = 1/T2 as measured with a double echo spin echo sequence and total iron concentration in gray matter structures in the brains of aging rhesus monkeys.

MATERIALS AND METHODS

Using a 1.5-T magnetic resonance (MR) imager, we collected double echo spin echo images of the brains of 12 female rhesus monkeys aged between 9 and 23 years. From the double echo images, the transverse relaxation rate R2 = 1/T2 was calculated in selected gray matter regions. After the animals were euthanized, their brains were excised and tissue punches were taken of the substantia nigra, globus pallidus, and gray matter regions of the cerebellum. Some of the tissue punches were assayed for total iron using atomic absorption spectroscopy.

RESULTS

The range of tissue iron concentration spanned from 15 to 450 microg/g wet weight, with the highest levels in the globus pallidus and the lowest levels in the cerebellum. The results show that R2 was highly correlated with the total iron concentration and that the relationship between R2 and tissue iron concentration appeared to depend upon the iron concentration. For concentrations above approximately 150 microg/g wet weight, R2 increased with a sensitivity of 0.0484 +/- 0.0023 second(-1)(microg/g)(-1). In contrast, where the iron concentration was below 150 microg/g, R2 increased at 0.0013 +/- 0.0073 second(-1)(microg/g)(-1). The bilinear behavior may reflect changes with age in the relative amounts of iron distributed diffusely and in granular form in the globus pallidus and substantia nigra. Histological sections of the tissues stained for iron and ferritin support this hypothesis and indicate that the distribution of ferritin is similar to the distribution of iron.

CONCLUSION

This study reaffirms the value of measuring the MR relaxation rate R2 for a noninvasive estimate of iron content in the brain and identified limitations in the relationship at low tissue iron concentrations.

Ferritin light chain down-modulation generates depigmentation in human metastatic melanoma cells by influencing tyrosinase maturation

Recently, after the identification of ferritin light chain (L-ferritin) gene and protein over-expression in human metastatic melanoma cells, we engineered, starting from the LM metastatic melanoma cell line, clones in which L-ferritin gene expression was down-regulated by the stable expression of a specific antisense construct. The present investigation started from the observation that L-ferritin down-regulated LM cells displayed a less pigmented phenotype, confirmed by a major decrease of total melanin, when compared to control LM cells. This finding was accompanied by a dramatic decrease in tyrosinase activity, which was not paralleled by a concomitant reduction of the amount of tyrosinase specific mRNA. Western blot analysis of tyrosinase in control LM cells displayed a pattern, which corresponds to the progressive glycosylation of the native protein up to the 80 kDa form, considered the functional one. Tyrosinase pattern assayed in L-ferritin down-regulated LM cells showed the remarkable absence of the 80 kDa form and a prevalence of endoglycosidase H (endo H)-sensitive immature (70 kDa) tyrosinase, accumulated in the endoplasmic reticulum (ER), as confirmed by confocal microscopy analysis. These results demonstrate that, in a human metastatic melanoma cell line, the stress condition promoted by L-ferritin down-modulation, can substantially influence proper maturation of tyrosinase.

Structure and function of amyloid in Alzheimer's disease

This review is focused on the structure and function of Alzheimer's amyloid deposits. Amyloid formation is a process in which normal well-folded cellular proteins undergo a self-assembly process that leads to the formation of large and ordered protein structures. Amyloid deposition, oligomerization, and higher order polymerization, and the structure adopted by these assemblies, as well as their functional relationship with cell biology are underscored. Numerous efforts have been directed to elucidate these issues and their relation with senile dementia. Significant advances made in the last decade in amyloid structure, dynamics and cell biology are summarized and discussed. The mechanism of amyloid neurotoxicity is discussed with emphasis on the Wnt signaling pathway. This review is focused on Alzheimer's amyloid fibrils in general and has been divided into two parts dealing with the structure and function of amyloid.

The Neuromelanin of Human Substantia Nigra: Physiological and Pathogenic Aspects

Neuromelanin (NM) accumulates as a function of age in normal human substantia nigra (SN) but is relatively depleted in the SN of patients with Parkinson disease (PD). Several studies have been performed to further our understanding of the role of NM in neuronal aging and neurodegenerative mechanisms of PD. To this purpose, NM from human SN was isolated and its structure and molecular interactions were investigated. Cysteinyl-dopamine was shown to be one precursor of NM synthesis. A striking affinity of NM for specific metals, lipids, drugs and pesticides was found in vitro, and in animal and human brain postmortem studies. Because of these affinities, NM seems to play a protective role in the human brain by blocking toxic molecules. On the other hand, experiments in cell culture indicate that NM can activate microglia, eliciting the release of cytotoxic factors that can induce neurodegeneration.

Ion-exchange and adsorption of Fe(III) by Sepia melanin

Sepia eumelanin is associated with many metal ions, yet little is known about its metal binding capacity and the chemical nature of the binding site(s). Herein, the natural concentrations of metal ions are presented and the ability to remove metals by exposure of the melanin granules to EDTA is quantified. The results reveal that the binding constants of melanin at pH 5.8 for Mg(II), Ca(II), Sr(II) and Cu(II) are, respectively, 5, 4, 14 and 34 times greater than the corresponding binding constants of these ions with EDTA. By exposing Sepia eumelanin to aqueous solutions of FeCl(3), the content of bound Fe(III) can be increased from a natural concentration of approximately 180 ppm to a saturation limit of approximately 80 000 ppm or 1.43 mmol/g of melanin. Similar saturation limits are found for Mg(II) and Ca(II). Exposure of Sepia melanin granules to aqueous solutions containing Ca(II) results in the stoichiometric replacement of the initially bound Mg(II), arguing that these two ions occupy the same binding site(s) in the pigment. The pH-dependent binding of Mg(II) and Ca(II) suggests coordination of these ions to carboxylic acid groups in the pigment. Mg(II) and Ca(II) can be added to a Fe(III)-saturated melanin sample without affecting the amount of Fe(III) pre-adsorbed, clearly establishing Fe(III) and Mg(II)/Ca(II) occupy different binding sites. Taking recent Raman spectroscopic data into account, the binding of Fe(III) is concluded to involve coordination to o-dihydroxyl groups. The effects of metal ion content on the surface morphology were analyzed. No significant changes were found over the full range of Fe(III) concentration studied, which is supported by the Brunauer-Emmett-Teller surface area analysis. These observations imply the existence of channels within the melanin granules that can serve to transport metal ions.

Iron, brain ageing and neurodegenerative disorders

There is increasing evidence that iron is involved in the mechanisms that underlie many neurodegenerative diseases. Conditions such as neuroferritinopathy and Friedreich ataxia are associated with mutations in genes that encode proteins that are involved in iron metabolism, and as the brain ages, iron accumulates in regions that are affected by Alzheimer's disease and Parkinson's disease. High concentrations of reactive iron can increase oxidative-stress induced neuronal vulnerability, and iron accumulation might increase the toxicity of environmental or endogenous toxins. By studying the accumulation and cellular distribution of iron during ageing, we should be able to increase our understanding of these neurodegenerative disorders and develop new therapeutic strategies.

Progressive iron accumulation induces a biphasic change in the glutathione content of neuroblastoma cells

Glutathione (GSH) constitutes the single most important antioxidant in neurons, whereas iron causes oxidative stress that leads to cell damage and death. Although GSH and iron produce opposite effects on redox cell status, no mechanistic relationships between iron and GSH metabolism are known. In this work, we evaluated in SH-SY5Y neuroblastoma cells the effects of iron accumulation on intracellular GSH metabolism. After 2 d exposure to increasing concentrations of iron, cells underwent concentration-dependent iron accumulation and a biphasic change in intracellular GSH levels. Increasing iron from 1 to 5 microM resulted in a marked increase in intracellular oxidative stress and increased GSH levels. Increased GSH levels were due to increased synthesis. Further increases in iron concentration led to significant reduction in both reduced (GSH) and total (GSH + (2 x GSSG)) glutathione. Cell exposure to high iron concentrations (20-80 microM) was associated with a marked decrease in the GSH/GSSG molar ratio and the GSH half-cell reduction potential. Moreover, increasing iron from 40 to 80 microM resulted in loss of cell viability. Iron loading did not change GSH reductase activity but induced significant increases in GSH peroxidase and GSH transferase activities. The changes in GSH homeostasis reported here recapitulate several of those observed in Parkinson's disease substantia nigra. These results support a model by which progressive iron accumulation leads to a progressive decrease in GSH content and cell reduction potential, which finally results in impaired cell integrity.

Redox imbalance

Substantial evidence implies that redox imbalance attributable to an overproduction of reactive oxygen species or reactive nitrogen species that overwhelm the protective defense mechanism of cells contributes to all forms of Parkinson's disease. Factors such as dopamine, neuromelanin, and transition metals may, under certain circumstances, contribute to the formation of oxygen species such as H(2)O(2), superoxide radicals, and hydroxyl radicals and react with reactive nitrogen species such as nitric oxide or peroxinitrite. Mitochodrial dysfunction and excitotoxicity may be a cause and a result of oxidative stress. Consequences of this redox imbalance are lipid peroxidation, oxidation of proteins, DNA damage, and interference of reactive oxygen species with signal transduction pathways. These consequences become even more harmful when genetic variations impair the normal degradation of altered proteins. Therefore, therapeutic strategies must aim at reducing free-radical formation and scavenging free-radicals.

Decreased transferrin receptor expression by neuromelanin cells in restless legs syndrome

BACKGROUND

Restless legs syndrome (RLS) is a sensory-movement disorder affecting 5 to 10% of the population. Its etiology is unknown, but MRI analyses and immunohistochemical studies on autopsy tissue suggest the substantia nigra (SN) of patients with RLS has subnormal amounts of iron.

METHODS

Neuromelanin cells from the SN of four RLS and four control brains were isolated by laser capture microdissection, and a profile of iron-management protein expression was obtained by immunoblot analysis. Binding assays for iron regulatory protein activity were performed on cell homogenates.

RESULTS

Ferritin, divalent metal transporter 1, ferroportin, and transferrin receptor (TfR) were decreased in RLS neuromelanin cells compared with control. Transferrin was increased in RLS neuromelanin cells. This protein profile in RLS neuromelanin cells is consistent with iron deficiency with the exception that TfR expression was decreased rather than increased. The concentration and activity of the iron regulatory proteins (IRP1 and IRP2) were analyzed to determine whether there was a functional deficit in the post-transcriptional regulatory mechanism for TfR expression. Total IRP activity, IRP1 activity, and IRP1 protein levels were decreased in RLS, but total IRP2 protein levels were not decreased in RLS.

CONCLUSION

Restless legs syndrome may result from a defect in iron regulatory protein 1 in neuromelanin cells that promotes destabilization of the transferrin receptor mRNA, leading to cellular iron deficiency.

Isolation and biophysical studies of natural eumelanins: applications of imaging technologies and ultrafast spectroscopy

The major pigments found in the skin, hair, and eyes of humans and other animals are melanins. Despite significant research efforts, the current understanding of the molecular structure of melanins, the assembly of the pigment within its organelle, and the structural consequences of the association of melanins with protein and metal cations is limited. Likewise, a detailed understanding of the photochemical and photophysical properties of melanins has remained elusive. Many types of melanins have been studied to date, including natural and synthetic model pigments. Such studies are often contradictory and to some extent the diversity of systems studied may have detracted from the development of a basic understanding of the structure and function of the natural pigment. Advances in the understanding of the structure and function of melanins require careful characterization of the pigments examined so as to assure the data obtained may be relevant to the properties of the pigment in vivo. To address this issue, herein the influence of isolation procedures on the resulting structure of the pigment is examined. Sections describing the applications of new technologies to the study of melanins follow this. Advanced imaging technologies such as scanning probe microscopies are providing new insights into the morphology of the pigment assembly. Recent photochemical studies on photoreduction of cytochrome c by different mass fraction of sonicated natural melanins reveal that the photogeneration of reactive oxygen species (ROS) depends upon aggregation of melanin. Specifically, aggregation mitigates ROS photoproduction by UV-excitation, suggesting the integrity of melanosomes in tissue may play an important role in the balance between the photoprotective and photodamaging behaviors attributed to melanins. Ultrafast laser spectroscopy studies of melanins are providing insights into the time scales and mechanisms by which melanin dissipates absorbed light energy.

Iron and ageing: an introduction to iron regulatory mechanisms

While there have been significant advances made in our understanding of the cellular and molecular mechanisms that regulate iron absorption, transport, storage, and utilization, the effect of ageing on these mechanisms and the role of iron in the ageing process is not fully understood. Thus, this review will provide an overview of the iron regulatory mechanisms that may be a factor in the ageing process. Additional reviews in this volume represent an attempt to explore the very latest information on the regulation of iron with a particular emphasis on age-related pathology including mitochondrial function, Parkinson's disease, Alzheimer's disease, stroke, and cardiovascular disease.

The role of iron and copper molecules in the neuronal vulnerability of locus coeruleus and substantia nigra during aging

In this study, a comparative analysis of metal-related neuronal vulnerability was performed in two brainstem nuclei, the locus coeruleus (LC) and substantia nigra (SN), known targets of the etiological noxae in Parkinson's disease and related disorders. LC and SN pars compacta neurons both degenerate in Parkinson's disease and other Parkinsonisms; however, LC neurons are comparatively less affected and with a variable degree of involvement. In this study, iron, copper, and their major molecular forms like ferritins, ceruloplasmin, neuromelanin (NM), manganese-superoxide dismutase (SOD), and copper/zinc-SOD were measured in LC and SN of normal subjects at different ages. Iron content in LC was much lower than that in SN, and the ratio heavy-chain ferritin/iron in LC was higher than in the SN. The NM concentration was similar in LC and SN, but the iron content in NM of LC was much lower than SN. In both regions, heavy- and light-chain ferritins were present only in glia and were not detectable in neurons. These data suggest that in LC neurons, the iron mobilization and toxicity is lower than that in SN and is efficiently buffered by NM. The bigger damage occurring in SN could be related to the higher content of iron. Ferritins accomplish the same function of buffering iron in glial cells. Ceruloplasmin levels were similar in LC and SN, but copper was higher in LC. However, the copper content in NM of LC was higher than that of SN, indicating a higher copper mobilization in LC neurons. Manganese-SOD and copper/zinc-SOD had similar age trend in LC and SN. These results may explain at least one of the reasons underlying lower vulnerability of LC compared to SN in Parkinsonian syndromes.

Mössbauer spectroscopy and ELISA studies reveal differences between Parkinson's disease and control substantia nigra

The possible role of iron in the degeneration of nervous cells in Parkinson's disease (PD) was studied with the use of Mössbauer spectroscopy (MS) and enzyme-linked immunoabsorbent assay (ELISA). Mössbauer data were obtained at 90 and 4.1 K from 21 samples of control and 9 samples of parkinsonian substantia nigra (SN). Mössbauer spectra were very similar to those observed in ferritin. Small differences were detected between the spectra obtained from PD and from control SN, and could be due to a slight difference in the composition of the ferritin-like iron cores or due to the presence of about 8% of non-ferritin-like iron in parkinsonian SN. ELISA studies from 11 controls and 6 parkinsonian SN showed a decrease in the concentration of L-chains in wet tissues of PD-SN compared to control SN. The decrease in the amount of L subunits may correspond to a decreased ability of this ferritin to keep iron in a safe form. Iron released from ferritin or neuromelanin (NM) may be the source of such iron, which may cause the difference in the Mössbauer spectra and may trigger oxidative stress leading to cell death.

The Relevance of Iron in the Pathogenesis of Parkinson's Disease

Investigations that revealed increased levels of iron in postmortem brains from patients with Parkinson's disease (PD) as compared to those from individuals not suffering from neurological disorders are reported. The chemical natures in which iron predominates in the brain and the relevance of neuromelanin for neuronal iron binding are discussed. Major findings have been that iron levels increase with the severity of neuropathological changes in PD, presumably due to increased transport through the blood-brain barrier in late stages of parkinsonism. Glial iron is mainly stored as ferric iron in ferritin, while neuronal iron is predominantly bound to neuromelanin. Iron overload may induce progressive degeneration of nigrostriatal neurons by facilitating the formation of reactive biological intermediates, including reactive oxygen species, and the formation of cytotoxic protein aggregates. There are indications that iron-mediated neuronal death in PD proceeds retrogradely. These results are also discussed with respect to their relevance for disease progression in relation to cytotoxic alpha-synuclein protofibril formation.

Binding of Metal Ions to Melanin and Their Effects on the Aerobic Reactivity¶

The binding of Mg(II), Ca(II), Zn(II), Cu(II) and Fe(III) to ETDA-washed Sepia melanin is quantified by inductively coupled plasma mass spectrometry. By monitoring the solution pH change associated with metal uptake, it is concluded that Mg(II), Ca(II) and Zn(II) bind to carboxylic acid groups in melanin, Cu(II) binds to hydroxyl (OH) groups and Fe(III) binds to OH or amine groups. The aerobic reactivity of melanins with different metal contents is analyzed by examining their ability to cause strand breaks in supercoiled pUC18 DNA. Cu(II)- and Fe(III)-enriched melanins induce the most damage. Hydroxyl radical, *OH, is proposed to be one of the reactive oxygen species responsible.

Characterization of the Fe(III)-binding Site in Sepia Eumelanin by Resonance Raman Confocal Microspectroscopy¶

The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)-like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm(-1); the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm(-1) bands are assigned to Fe-OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin-binding site in melanin is composed of CAT-like structural units.

Study of the relationship of small variations of the molecular structure and the iron state in iron containing proteins by Mössbauer spectroscopy: biomedical approach

This review considers the results of experimental Mössbauer studies and theoretical calculations of the effect of small variations of protein molecular structure on the iron electronic structure and stereochemistry in order to understand the proteins structural heterogeneity and functional variety. Structural changes in iron containing proteins during various diseases are also considered. These results show the relationship of the small structural variations and Mössbauer parameters of iron containing proteins and demonstrate the possibilities of Mössbauer spectroscopy to obtain new information at the molecular level in biomedical research.

Mössbauer spectroscopy in biomedical research

This work discusses the main directions and results of the application of Mössbauer spectroscopy of iron containing species in biomedical research. These studies demonstrate the wide possibilities of Mössbauer spectroscopy to obtain physical parameters and information about the iron electronic structure in normal and pathological biomolecules, model compounds and pharmaceutical samples as well as about qualitative and quantitative changes of iron containing biomolecules during pathological processes or the effect of environmental factors. The results obtained may be useful for further understanding of the molecular nature of diseases and pathological processes.

Neuromelanin of the substantia nigra: a neuronal black hole with protective and toxic characteristics

Neuromelanin accumulates in dopaminergic neurons during normal aging, and in Parkinson's disease, neurons with this pigment are those that selectively degenerate. Intraneuronal neuromelanin could play a protective role during its synthesis by preventing the toxic accumulation of cytosolic catechol derivatives and, in addition, by its ability to scavenge reactive metals, pesticides and other toxins to form stable adducts. However, dying neurons in Parkinson's disease that release neuromelanin might induce a vicious cycle of chronic neuroinflammation and neuronal loss.

The neuromelanin of human substantia nigra: structure, synthesis and molecular behaviour

The pigmented neurons of the substantia nigra (SN) are typically lost in Parkinson's disease: however the possible relationship between neuronal vulnerability and the presence of neuromelanin (NM) has not been elucidated. Early histological studies revealed the presence of increasing amounts of NM in the SN with aging in higher mammals, showed that NM granules are surrounded by membrane, and comparatively evaluated the pigmentation of SN in different animal species. Histochemical studies showed the association of NM with lipofuscins. However, systematic investigations of NM structure, synthesis and molecular interactions have been undertaken only during the last decade. In these latter studies, NM was identified as a genuine melanin with a strong chelating ability for iron and affinity for compounds such as lipids, pesticides, and MPP+. The affinity of NM for a variety of inorganic and organic toxins is consistent with a postulated protective function for NM. Moreover, the neuronal accumulation of NM during aging, and the link between its synthesis and high cytosolic concentration of catechols suggests a protective role. However, its putative neuroprotective effects could be quenched in conditions of toxin overload.

Neuromelanin associated redox-active iron is increased in the substantia nigra of patients with Parkinson's disease

Degeneration of dopaminergic neurones during Parkinson's disease is most extensive in the subpopulation of melanized-neurones located in the substantia nigra pars compacta. Neuromelanin is a dark pigment produced in the dopaminergic neurones of the human substantia nigra and has the ability to bind a variety of metal ions, especially iron. Post-mortem analyses of the human brain have established that oxidative stress and iron content are enhanced in association with neuronal death. As redox-active iron (free Fe2+ form) and other transition metals have the ability to generate highly reactive hydroxyl radicals by a catalytic process, we investigated the redox activity of neuromelanin (NM)-aggregates in a group of parkinsonian patients, who presented a statistically significant reduction (- 70%) in the number of melanized-neurones and an increased non-heme (Fe3+) iron content as compared with a group of matched-control subjects. The level of redox activity detected in neuromelanin-aggregates was significantly increased (+ 69%) in parkinsonian patients and was highest in patients with the most severe neuronal loss. This change was not observed in tissue in the immediate vicinity of melanized-neurones. A possible consequence of an overloading of neuromelanin with redox-active elements is an increased contribution to oxidative stress and intraneuronal damage in patients with Parkinson's disease.

Comparison of the structural and physical properties of human hair eumelanin following enzymatic or acid/base extraction

Eumelanin was isolated from a sample of black, Indonesian human hair using three different published procedures: two different acid/base extractions and an enzymatic extraction. The morphology and spectroscopic properties of the isolated pigments differ significantly. The acid/base procedures both yield an amorphous material, while enzymatic extraction yields ellipsoidal melanosomes. Amino acid analysis shows that there is protein associated with the isolated pigments, accounting for 52, 40 and 14% of the total mass for the two acid/base extractions and the enzymatic extraction, respectively. The amino acid compositions do not correlate with those of keratin or tyrosinase. Metal elemental analysis shows that the acid/base extraction removes a majority of many metal ions bound to the pigment. Chemical degradation analysis by KMnO4/H+ and H2O2/OH- indicates significant differences between the pigments isolated by acid/base and enzymatic extraction. After correction for the protein mass in the two pigments, the lower yields of both pyrrole-2,3,5-tricarboxylic acid and pyrrole-2,3-dicarboxylic acid, eumelanin degradation products, indicate acid/base extraction modifies the chemical structure of the melanin, consistent with the result of Soluene solubilization assay. While the optical absorption spectra of the bulk pigments are similar, the spectra of the molecular weight less than 1000 mass fractions differ significantly. The data clearly indicate that pigment obtained from human hair by acid/base extraction contains significant protein, exhibits destruction of the melanosome, and possesses altered molecular structure. The acid/base extracted hair melanin is not representative of the natural material and is a poor model system for studying the physical and biological properties of melanins. The enzymatically extracted hair melanin, on the contrary, retains the morphology of intact melanosomes and is an excellent source of human melanin.

Iron-binding characteristics of neuromelanin of the human substantia nigra

The vulnerability of the dopaminergic neurons of the substantia nigra (SN) in Parkinson's disease has been related to the presence of the pigment neuromelanin (NM) in these neurons. It is hypothesised that NM may act as an endogenous storage molecule for iron, an interaction suggested to influence free radical production. The current study quantified and characterised the interaction between NM and iron. Iron-binding studies demonstrated that both NM and synthetically-produced dopamine melanin contain equivalent numbers of high and low-affinity binding sites for iron but that the affinity of NM for iron is higher than that of synthetic melanin. Quantification of the total iron content in iron-loaded NM and synthetic melanin demonstrated that the iron-binding capacity of NM is 10-fold greater than that of the model melanin. This data was in agreement with the larger iron cluster size demonstrated by Mössbauer spectroscopy in the native pigment compared with the synthetic melanin. These findings are consistent with the hypothesis that NM may act as an endogenous iron-binding molecule in dopaminergic neurons of the SN in the human brain. The interaction between NM and iron has implications for disorders such as Parkinson's disease where an increase in iron in the SN is associated with increased indices of oxidative stress.

Genetic or pharmacological iron chelation prevents MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease

Studies on postmortem brains from Parkinson's patients reveal elevated iron in the substantia nigra (SN). Selective cell death in this brain region is associated with oxidative stress, which may be exacerbated by the presence of excess iron. Whether iron plays a causative role in cell death, however, is controversial. Here, we explore the effects of iron chelation via either transgenic expression of the iron binding protein ferritin or oral administration of the bioavailable metal chelator clioquinol (CQ) on susceptibility to the Parkinson's-inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrapyridine (MPTP). Reduction in reactive iron by either genetic or pharmacological means was found to be well tolerated in animals in our studies and to result in protection against the toxin, suggesting that iron chelation may be an effective therapy for prevention and treatment of the disease.

Activation of microglia by human neuromelanin is NF-kappaB dependent and involves p38 mitogen-activated protein kinase: implications for Parkinson's disease

It has been suggested that microglial inflammation augments the progression of Parkinson's disease (PD). However, endogenous factors initiating microglial activation are largely unknown. We therefore investigated the effects of human neuromelanin (NM) on the release of neurotoxic mediators and the underlying signaling pathways from rat microglia in vitro. The addition of NM to microglial cultures induced positive chemotactic effects, activated the proinflammatory transcription factor nuclear factor kappaB (NF-kappaB) via phosphorylation and degradation of the inhibitor protein kappaB (IkappaB), and led to an up-regulation of tumor necrosis factor alpha, interleukin-6, and nitric oxide. The impairment of NF-kappaB function by the IkappaB kinase inhibitor sulfasalazine was paralleled by a decline in neurotoxic mediators. NM also activated p38 mitogen-activated protein kinase (MAPK), the inhibition of this pathway by SB203580 diminished phosphorylation of the transactivation domain of the p65 subunit of NF-kappaB. These findings demonstrate a crucial role of NM in the pathogenesis of PD by augmentation of microglial activation, leading to a vicious cycle of neuronal death, exposure of additional neuromelanin, and chronification of inflammation. The antagonization of microglial activation by a pharmacological intervention targeting microglial NF-kappaB or p38 MAPK could point to additional venues in the treatment of PD.

Effect of retinoic acid on ferritin H expression during brain development and neuronal differentiation

We have previously shown that brain ferritin H expression, which has been associated with iron utilization, is developmentally regulated. Because retinoic acid (RA) regulates gene expression and is involved in cellular differentiation, we tested the hypothesis that RA regulates ferritin H during brain development and neuronal differentiation. RA, administered to rats on postnatal day 1, produced a 4-fold increase in brain ferritin H mRNA (p < 0.01) after 24 h. To examine whether RA-stimulated neuronal differentiation contributed to this up-regulation, ferritin and ferritin H mRNA were measured in human neuronal precursor cells (NTera-2, NT2) before and after 4-weeks of RA-stimulated differentiation into post-mitotic neurons. Differentiation resulted in a 2-fold increase in both ferritin and ferritin H mRNA (p < 0.05). Immunocytochemistry and Northern analysis showed significant elevations in ferritin expression that began as early as 24 h after RA treatment. While there was also a significant increase in the labile iron pool after RA treatment, this did not occur until 72 h. These data show that RA regulates ferritin H expression during rat brain development and neuronal differentiation and suggests a new role for RA in brain iron metabolism.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.

Is there a subtype of developmental Parkinson's disease?

Evidence accumulates suggesting that environmental factors including viral and risk factors associated with pregnancy and birth-giving, may increase the incidence of hypokinesia/parkinsonism in early life, or PD in later life. Such environmental pre-, peri- or postnatal stress may lead to disturbances in the developing brain and malformations in regions of particular interest and associated with PD. Genetic predisposition to hypoactivity plus environmental effects may lead to reorganization of brain circuitry including changes in monoaminergic and/or EAA systems, leading to a subtype of PD, i.e. genetic, drug induced, viral, developmental and other possible subtypes. The spectrum disorder of PD is going to be further substantiated into various etiopathologically verifyable subgroups.

Neuromelanin and its interaction with iron as a potential risk factor for dopaminergic neurodegeneration underlying Parkinson's disease

Neuromelanin (NM) is a granular, dark brown pigment produced in some but not all of the dopaminergic neurons of the human substantia nigra (SN). In Parkinson's disease (PD) the pigmented dopaminergic neurons of the SN degenerate, suggesting that this process is related to the presence of NM. As yet it is unknown whether NM in the parkinsonian brain differs from that found in healthy tissue and thus may fulfil a different role within this tissue. The function of NM within the pigmented neurons is unknown but other melanins are believed to play a protective role via attenuation of free radical damage. Experimental evidence suggests that NM may also exhibit this characteristic, possibly by direct inactivation of free radical species or via its ability to chelate transition metals, such as iron. NM has the ability to bind a variety of metals, seven per cent of isolated NM is reported to consist of Fe, Cu, Zn and Cr. Iron is of particular interest as this metal is highly concentrated within the SN. Up to 20 per cent of the total iron contained in the SN from normal subjects is bound within NM. Further, it was demonstrated that NM contains a protein component and that iron is bound to NM in the ferric form. Increased tissue iron found in the parkinsonian SN may saturate iron-chelating sites on NM, and a looser association between iron and NM may result in an increased, rather than decreased, production of free radical species. It is hypothesized that this redox-active iron could be released and involved in a Fenton-like reaction leading to an increased production of oxidative radicals. The resultant radical-mediated cytotoxicity may contribute to cellular damage observed in PD.

Overexpression of Alzheimer's disease amyloid-beta opposes the age-dependent elevations of brain copper and iron

Increased brain metal levels have been associated with normal aging and a variety of diseases, including Alzheimer's disease (AD). Copper and iron levels both show marked increases with age and may adversely interact with the amyloid-beta (Abeta) peptide causing its aggregation and the production of neurotoxic hydrogen peroxide (H(2)O(2)), contributing to the pathogenesis of AD. Amyloid precursor protein (APP) possesses copper/zinc binding sites in its amino-terminal domain and in the Abeta domain. Here we demonstrate that overexpression of the carboxyl-terminal fragment of APP, containing Abeta, results in significantly reduced copper and iron levels in transgenic mouse brain, while overexpression of the APP in Tg2576 transgenic mice results in significantly reduced copper, but not iron, levels prior to the appearance of amyloid neuropathology and throughout the lifespan of the mouse. Concomitant increases in brain manganese levels were observed with both transgenic strains. These findings, complemented by our previous findings of elevated copper levels in APP knock-out mice, support roles for APP and Abeta in physiological metal regulation.

Iron in neurodegenerative disorders

Increasing evidence implicates a role of iron in the pathogenesis of numerous neurodegenerative diseases due to its capacity to enhance production of toxic reactive radicals and to induce protein aggregation. The underlying mechanism of iron accumulation in areas of the brain specific for the respective disease, however, is still unknown. Recent molecular and biochemical studies provide new insights into the consequences of impairment of brain iron metabolism. This review summarizes our understanding of the regulation of iron in the brain and defines the current knowledge on the involvement of iron metabolism in neurodegenerative diseases with genetically determined iron accumulation in the brain.

Lack of up-regulation of ferritin is associated with sustained iron regulatory protein-1 binding activity in the substantia nigra of patients with Parkinson's disease

Dopaminergic neurones degenerate during Parkinson's disease and cell loss is most extensive in the subpopulation of melanized neurones located in the substantia nigra pars compacta. Iron accumulation, together with a lack of up-regulation of the iron-storing protein, ferritin, has been reported and may contribute to increased oxidative stress in this region. We investigated the binding activity of iron regulatory protein-1 (IRP1) to the iron-responsive element that precludes ferritin mRNA translation, in the substantia nigra of a group of parkinsonian patients who presented a statistically significant reduction in the number of nigral melanized-neurones and an increased iron content, together with unchanged H-ferritin and L-ferritin subunit levels as compared to matched controls. The levels of ferritin mRNAs and the binding activity of IRP1 to the iron-responsive element of ferritin mRNA did not differ significantly between the two groups. Moreover, there was no detectable contribution of the iron regulatory protein-2 (IRP2) binding activity. No change in IRP1 control of ferritin mRNA translation explains the lack of up-regulation of ferritin expression in cytoplasmic extracts of SNpc that would be normally expected with cytosolic iron accumulation. The data of this study do not favor changes in transcription and post-transcriptional regulation of ferritin expression in Parkinson's disease and suggest a 'compartmentalized' iron accumulation.

Influence of neuromelanin on oxidative pathways within the human substantia nigra

Neuromelanin (NM) is a dark-coloured pigment produced in the dopaminergic neurons of the human substantia nigra (SN). The function of NM within the pigmented neurons is unknown but other melanins are believed to play a protective role via attenuation of free radical damage. Experimental evidence suggests that NM may also exhibit this characteristic, possibly by directly inactivating free radical species or via its ability to chelate transition metals, such as iron. Increased tissue iron, however, may saturate iron-chelating sites on NM and a looser association between iron and NM may result in an increased, rather than decreased, production of free radical species. The death of NM-pigmented neurons in Parkinson's disease (PD) is associated with both a measurable increase in tissue iron concentrations and indices of free radical mediated damage, suggesting that NM is involved in the aetiology of this disorder. As yet, it is unknown whether NM in the parkinsonian brain differs to that found in healthy tissue and thus may fulfil a different role within this tissue.

Molecular and cellular mechanisms of iron homeostasis and toxicity in mammalian cells

Iron is an essential metal for almost all living organisms due to its involvement in a large number of iron-containing enzymes and proteins, yet it is also toxic. The mechanisms involved in iron absorption across the intestinal tract, its transport in serum and delivery to cells and iron storage within cells is briefly reviewed. Current views on cellular iron homeostasis involving the iron regulatory proteins IRP1 and IRP2 and their interactions with the iron regulatory elements, affecting either mRNA translation (ferritin and erythroid cell delta-aminolaevulinate synthase) or mRNA stability (transferrin receptor) are discussed. The potential of Fe(II) to catalyse hydroxyl radical formation via the Fenton reaction means that iron is potentially toxic. The toxicity of iron in specific tissues and cell types (liver, macrophages and brain) is illustrated by studies with appropriate cellular and animal models. In liver, the high levels of cyoprotective enzymes and antioxidants, means that to observe toxic effects substantial levels of iron loading are required. In reticuloendothelial cells, such as macrophages, relatively small increases in cellular iron (2-3-fold) can affect cellular signalling, as measured by NO production and activation of the nuclear transcription factor NF kappa B, as well as cellular function, as measured by the capacity of the cells to produce reactive oxygen species when stimulated. The situation in brain, where anti-oxidative defences are relatively low, is highly regionally specific, where iron accumulation in specific brain regions is associated with a number of neurodegenerative diseases. In the brains of animals treated with either trimethylhexanoylferrocene or aluminium gluconate, iron and aluminium accumulate, respectively. With the latter compound, iron also increases, which may reflect an effect of aluminium on the IRP2 protein. Chelation therapy can reduce brain aluminium levels significantly, while iron can also be removed, but with greater difficulty. The prospects for chelation therapy in the treatment and possible prevention of neurodegenerative diseases is reviewed.

Magnetic investigations of human mesencephalic neuromelanin

Pigmentation of neurons in substantia nigra is due to neuromelanin, a pigment that stores large amounts of iron. Human mesencephalic neuromelanin has been investigated by means of magnetic susceptibility measurements as a function of temperature. Magnetic measurements provide a physico-chemical characterization of the iron cluster buried in the organic melanin matrix and support the view that iron is not simply chelated, but rather is organized in a three-dimensional network. The paramagnetism of isolated iron ions is observed, in agreement with electron paramagnetic resonance spectroscopy. Furthermore, antiferromagnetic grains with a large size distribution function are present. These grains contain N spins coupled antiferromagnetically; however, N(1/2) spins are decoupled from the grain bulk and parallelly aligned. The latter subgrains are superparamagnetic with a blocking temperature ranging between 5 K and room temperature. This behavior has not been observed in synthetic melanin, where the paramagnetic contribution is strongly enhanced. Preliminary results on pigment isolated from patients affected by Parkinson's disease, a neurodegenerative pathology involving primarily pigmented neurons in substantia nigra pars compacta, show a lower total magnetization compared to control neuromelanin. The temperature behavior of zero field cooling and field cooling magnetizations is similar for both. The significant depletion of iron content in Parkinson's disease neuromelanin could indicate a progressive Fe migration from its storage environment to the cytosol.

Iron and Parkinson's disease

Multiple studies implicate iron in the pathophysiology of Parkinson's disease (PD). In the brains of patients with PD, iron levels are elevated and the levels of iron-binding proteins are abnormal. Iron has been suspected to contribute to PD because Fe(II) is known to promote oxidative damage. Recent studies suggest that an additional mechanism by which iron might contribute to PD is by inducing aggregation of the alpha-synuclein, which is a protein that accumulates in Lewy bodies in PD.

The absolute concentration of nigral neuromelanin, assayed by a new sensitive method, increases throughout the life and is dramatically decreased in Parkinson's disease

The concentration of neuromelanin (NM) in substantia nigra pars compacta (SNPC) has been measured in male and female normal subjects at different ages in the range 1-97 years old and in SNPC of parkinsonian patients. A very similar age trend of NM concentration was found in both sexes. In the first year of life NM was not detectable, between 10 and 20 years the NM levels were 0.3-0.8 microg/mg of SNPC, between 20 and 50 years were 0.8-2.3 microg/mg SNPC and between 50 and 90 were 2.3-3.7 microg/mg of SNPC. In parkinsonian subjects, the NM levels were 1.2-1.5 microg/mg of SNPC, which is less than 50% with respect to the age-matched controls. These data demonstrate a continuous NM accumulation in SNPC neurons during aging, the presence of large amounts of NM in SNPC and severe depletion of NM in Parkinson's disease.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.