Neuronal Iron Accumulation Research Articles

6-Hydroxydopamine promotes iron traffic in primary cultured astrocytes

It is well known that disrupted brain iron homeostasis was involved in Parkinson's disease. We previously reported 6-hydroxydopamine (6-OHDA) could enhance iron influx and attenuate iron efflux process, thus promote iron accumulation in neurons. Astrocytes, the major glial cell type in the central nervous system, are largely responsible for iron distribution in the brain. However, how iron metabolism changes in astrocytes with 6-OHDA treatment are not fully elucidated. In the present study, we first observed that both iron influx and efflux were enhanced with 10 μM 6-OHDA treatment for 24 h in primary cultured astrocytes. In accordance with these iron traffic modulations, both mRNA and protein levels of iron importer divalent metal transporter 1 with iron responsive element (DMT1+IRE) and exporter ferroportin 1 (FPN1) were up-regulated in these cells. L-ferritin mRNA levels were increased. Iron regulatory protein 1 (IRP1) showed a dynamic regulation with 6-OHDA treatment, as indicated by a moderate up-regulation at 12 h, however, down-regulation at 24 h. We further demonstrated that 6-OHDA treatment could induce activation of nuclear factor-kappaB (NF-κB) p65. IκBα activation inhibitor BAY11-7082 fully blocked 6-OHDA induced NF-κB p65 phosphorylation and DMT1 + IRE up-regulation. These results suggest that 6-OHDA might promote iron transport rate in astrocytes by regulating iron transporters, IRP1 expression and NF-κB p65 activation, indicating a different response between neurons and astrocytes.

Inflammation alters the expression of DMT1, FPN1 and hepcidin, and it causes iron accumulation in central nervous system cells

Inflammation and iron accumulation are present in a variety of neurodegenerative diseases that include Alzheimer's disease and Parkinson's disease. The study of the putative association between inflammation and iron accumulation in central nervous system cells is relevant to understand the contribution of these processes to the progression of neuronal death. In this study, we analyzed the effects of the inflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) and of lipopolysaccharide on total cell iron content and on the expression and abundance of the iron transporters divalent metal transporter 1 (DMT1) and Ferroportin 1 (FPN1) in neurons, astrocytes and microglia obtained from rat brain. Considering previous reports indicating that inflammatory stimuli induce the systemic synthesis of the master iron regulator hepcidin, we identified brain cells that produce hepcidin in response to inflammatory stimuli, as well as hepcidin-target cells. We found that inflammatory stimuli increased the expression of DMT1 in neurons, astrocytes, and microglia. Inflammatory stimuli also induced the expression of hepcidin in astrocytes and microglia, but not in neurons. Incubation with hepcidin decreased the expression of FPN1 in the three cell types. The net result of these changes was increased iron accumulation in neurons and microglia but not in astrocytes. The data presented here establish for the first time a causal association between inflammation and iron accumulation in brain cells, probably promoted by changes in DMT1 and FPN1 expression and mediated in part by hepcidin. This connection may potentially contribute to the progression of neurodegenerative diseases by enhancing iron-induced oxidative damage.

Neuronal iron accumulation associated with tau deficiency in parkinsonism

Neuronal iron accumulation associated with tau deficiency in parkinsonism

Tau deficiency induces parkinsonism with dementia by impairing APP-mediated iron export

The microtubule-associated protein tau has risk alleles for both Alzheimer's disease and Parkinson's disease and mutations that cause brain degenerative diseases termed tauopathies. Aggregated tau forms neurofibrillary tangles in these pathologies, but little is certain about the function of tau or its mode of involvement in pathogenesis. Neuronal iron accumulation has been observed pathologically in the cortex in Alzheimer's disease, the substantia nigra (SN) in Parkinson's disease and various brain regions in the tauopathies. Here we report that tau-knockout mice develop age-dependent brain atrophy, iron accumulation and SN neuronal loss, with concomitant cognitive deficits and parkinsonism. These changes are prevented by oral treatment with a moderate iron chelator, clioquinol. Amyloid precursor protein (APP) ferroxidase activity couples with surface ferroportin to export iron, but its activity is inhibited in Alzheimer's disease, thereby causing neuronal iron accumulation. In primary neuronal culture, we found loss of tau also causes iron retention, by decreasing surface trafficking of APP. Soluble tau levels fall in affected brain regions in Alzheimer's disease and tauopathies, and we found a similar decrease of soluble tau in the SN in both Parkinson's disease and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model. These data suggest that the loss of soluble tau could contribute to toxic neuronal iron accumulation in Alzheimer's disease, Parkinson's disease and tauopathies, and that it can be rescued pharmacologically.

Changes in iron-regulatory gene expression occur in human cell culture models of Parkinson’s disease

Background Neuronal iron accumulation is thought to be relevant to the pathogenesis of Parkinson’s disease (PD), although the mechanism remains elusive. We hypothesized that neuronal iron uptake may be stimulated by functional mitochondrial iron deficiency. Objective To determine firstly whether the mitochondrial toxin, 1-methyl-4-phenylpyridinium iodide (MPP +), results in upregulation of iron-import proteins and transporters of iron into the mitochondria, and secondly whether similar changes in expression are induced by toxins with different mechanisms of action. Methods We used quantitative PCR and Western blotting to investigate expression of the iron importers, divalent metal transporter, transferrin receptor 1 and 2 (TfR1 and TfR2) and mitoferrin-2 and the iron exporter ferroportin in differentiated SH-SY5Y cells exposed to three different toxins relevant to PD, MPP +, paraquat (a free radical generator) and lactacystin (an inhibitor of the ubiquitin–proteasome system (UPS)). Results MPP + resulted in increased mRNA and protein levels of genes involved in cellular iron import and transport into the mitochondria. Similar changes occurred following exposure to paraquat, another inducer of oxidative stress. Lactacystin also resulted in increased TfR1 mRNA levels, although the other changes were not found. Conclusion Our results support the hypothesis of a functional mitochondrial iron deficit driving neuronal iron uptake but also suggest that differences exist in neuronal iron handling induced by different toxins.

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

BackgroundNucleotide 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.ResultsCompared 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.ConclusionsOur 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-Export Ferroxidase Activity of β-Amyloid Precursor Protein Is Inhibited by Zinc in Alzheimer's Disease

Alzheimer's Disease (AD) is complicated by pro-oxidant intraneuronal Fe(2+) elevation as well as extracellular Zn(2+) accumulation within amyloid plaque. We found that the AD β-amyloid protein precursor (APP) possesses ferroxidase activity mediated by a conserved H-ferritin-like active site, which is inhibited specifically by Zn(2+). Like ceruloplasmin, APP catalytically oxidizes Fe(2+), loads Fe(3+) into transferrin, and has a major interaction with ferroportin in HEK293T cells (that lack ceruloplasmin) and in human cortical tissue. Ablation of APP in HEK293T cells and primary neurons induces marked iron retention, whereas increasing APP695 promotes iron export. Unlike normal mice, APP(-/-) mice are vulnerable to dietary iron exposure, which causes Fe(2+) accumulation and oxidative stress in cortical neurons. Paralleling iron accumulation, APP ferroxidase activity in AD postmortem neocortex is inhibited by endogenous Zn(2+), which we demonstrate can originate from Zn(2+)-laden amyloid aggregates and correlates with Aβ burden. Abnormal exchange of cortical zinc may link amyloid pathology with neuronal iron accumulation in AD.

Dysregulation of iron homeostasis in the CNS contributes to disease progression in a mouse model of amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS), characterized by degeneration of spinal motor neurons, consists of sporadic and familial forms. One cause of familial ALS is missense mutations in the superoxide dismutase 1 (SOD1) gene. Iron accumulation occurs in the CNS of both forms of ALS; however, its contribution to the pathogenesis of ALS is not known. We examined the role of iron in a transgenic mouse line overexpressing the human SOD1(G37R) mutant. We show that multiple mechanisms may underlie the iron accumulation in neurons and glia in SOD1(G37R) transgenic mice. These include dysregulation of proteins involved in iron influx and sensing of intracellular iron; iron accumulation in ventral motor neurons secondary to blockage of anterograde axonal transport; and increased mitochondrial iron load in neurons and glia. We also show that treatment of SOD1(G37R) mice with an iron chelator extends life span by 5 weeks, accompanied by increased survival of spinal motor neurons and improved locomotor function. These data suggest that iron chelator therapy might be useful for the treatment of ALS.

Iron and glutathione at the crossroad of redox metabolism in neurons

Neurons, as non-dividing cells, encounter a myriad of stressful conditions throughout their lifespan. In particular, there is increasing evidence that iron progressively accumulates in the brain with age and that iron induced oxidative stress is the cause of several forms of neurodegeneration. Here, we review recent evidence that gives support to the following notions: 1) neuronal iron accumulation leads to oxidative stress and cell death; 2) neuronal survival to iron accumulation associates with decreased expression of the iron import transporter DMT1 and increased expression of the efflux transporter IREG1; and 3) the adaptive process of neurons towards iron-induced oxidative stress includes a marked increase in both the expression of the catalytic subunit of gamma glutamate-cysteine ligase and glutathione. These findings may help to understand aging-related neurodegeneration hallmarks: oxidative damage, functional impairment and cell death.

Is increased redox-active iron in Alzheimer disease a failure of the copper-binding protein ceruloplasmin?

One of the most striking features of Alzheimer disease (AD) is an accumulation of iron in neurofibrillary tangles and senile plaques. Intriguingly, this iron is found as both iron (II) and iron (III) and is redox-active. To address the issue of whether such iron participates in redox cycling, it was essential to investigate how iron (II) accumulates, since oxidation of iron (II) can lead to the generation of reactive oxygen species. To begin to address this issue, here we investigated ceruloplasmin, a key protein involved in the regulation of the redox state of iron by converting iron (II) to iron (III). Cases of AD and age-matched controls, obtained at autopsy with similar postmortem intervals, display similar levels of ceruloplasmin immunoreactivity that is mainly confined to neurons. However, in marked contrast, cases of AD show a significant increase in ceruloplasmin within the neuropil determined by immunoblot analysis of tissue homogenates as well as a generalized increased neuropil staining. Together, these findings suggest that neuronal induction of ceruloplasmin is feeble in AD, even while there is an increase in tissue ceruloplasmin. Therefore, a failure of neuronal ceruloplasmin to respond to iron may be an important factor that then leads to an accumulation of redox-active iron in neurons in AD.