Abnormal brain iron homeostasis in human and animal prion disorders

Low copper and high manganese levels in prion protein plaques

Accumulation of aggregates rich in an abnormally folded form of the prion protein characterize the neurodegeneration caused by transmissible spongiform encephalopathies (TSEs). The molecular triggers of plaque formation and neurodegeneration remain unknown, but analyses of TSE-infected brain homogenates and preparations enriched for abnormal prion protein suggest that reduced levels of copper and increased levels of manganese are associated with disease. The objectives of this study were to: (1) assess copper and manganese levels in healthy and TSE-infected Syrian hamster brain homogenates; (2) determine if the distribution of these metals can be mapped in TSE-infected brain tissue using X-ray photoelectron emission microscopy (X-PEEM) with synchrotron radiation; and (3) use X-PEEM to assess the relative amounts of copper and manganese in prion plaques in situ. In agreement with studies of other TSEs and species, we found reduced brain levels of copper and increased levels of manganese associated with disease in our hamster model. We also found that the in situ levels of these metals in brainstem were sufficient to image by X-PEEM. Using immunolabeled prion plaques in directly adjacent tissue sections to identify regions to image by X-PEEM, we found a statistically significant relationship of copper-manganese dysregulation in prion plaques: copper was depleted whereas manganese was enriched. These data provide evidence for prion plaques altering local transition metal distribution in the TSE-infected central nervous system.

Age-related differences in iron content of subcortical nuclei observed in vivo: a meta-analysis

Accumulation of non-heme iron in the brain has been proposed as a biomarker of the progressive neuroanatomical and cognitive declines in healthy adult aging. Postmortem studies indicate that iron content and lifespan differences therein are regionally specific, with a predilection for the basal ganglia. However, the reported in vivo estimates of adult age differences in iron content within subcortical nuclei are highly variable. We present a meta-analysis of 20 in vivo magnetic resonance imaging (MRI) studies that estimated iron content in the caudate nucleus, globus pallidus, putamen, red nucleus, and substantia nigra. The results of the analyses support a robust association between advanced age and high iron content in the substantia nigra and striatum, with a smaller effect noted in the globus pallidus. The magnitude of age differences in estimated iron content of the caudate nucleus and putamen partially depended on the method of estimation, but not on the type of design (continuous age vs. extreme age groups).

Effect of divalent metals on the neuronal proteasomal system, prion protein ubiquitination and aggregation

The role of normal cellular prion protein (PrP) remains to be fully elucidated; however, the protein is crucial for the infection and progression of prion diseases. Recent evidence indicates that PrP is a metalloprotein since the octapeptide repeat sequences in the protein have high affinity for various divalent cations and the binding sites appear to play a role in the pathogenesis of prion diseases. In our present study, we tested several divalent metals including manganese and cadmium and determined their effects on protein degradation and protein aggregation in mouse neuronal cells expressing PrP. Cadmium was more neurotoxic than manganese following 24h exposure. Manganese did not show any significant effect on the inhibition of proteasomal activity or formation of high molecular weight ubiquitinated PrPs. Interestingly, treatment with cadmium profoundly inhibited proteasomal activity, which resulted in greatly increased formation of high molecular weight ubiquitinated PrPs. Immunohistochemical analysis also revealed a dramatic increase in formation of oligomers after cadmium treatment. Cadmium also increased the formation of ubiquitinated PrP, but it did not lead to the formation of proteinase-K resistant PrP. Collectively, our results show that a divalent metal, cadmium affects proteasomal function and PrP aggregation, which promote neurotoxicity.

Change in the characteristics of ferritin induces iron imbalance in prion disease affected brains

Prion disease associated neurotoxicity is mainly attributed to PrP-scrapie (PrP(Sc)), the disease associated isoform of a normal protein, the prion protein (PrP(C)). Participation of other proteins and processes is suspected, but their identity and contribution to the pathogenic process is unclear. Emerging evidence implicates imbalance of brain iron homeostasis as a significant cause of prion disease-associated neurotoxicity. The underlying cause of this change, however, remains unclear. We demonstrate that iron is sequestered in heat and SDS-stable protein complexes in sporadic-Creutzfeldt-Jakob-disease (sCJD) brains, creating a phenotype of iron deficiency. The underlying cause is change in the characteristics of ferritin, an iron storage protein that becomes aggregated, detergent-insoluble, and partitions with denatured ferritin using conventional methods of ferritin purification. A similar phenotype of iron deficiency is noted in the lumbar spinal cord (SC) tissue of scrapie infected hamsters, a site unlikely to be affected by massive neuronal death and non-specific iron deposition. As a result, the iron uptake protein transferrin (Tf) is upregulated in scrapie infected SC tissue, and increases with disease progression. A direct correlation between Tf and PrP(Sc) suggests sequestration of iron in dysfunctional ferritin that either co-aggregates with PrP(Sc) or is rendered dysfunctional by PrP(Sc) through an indirect process. Surprisingly, amplification of PrP(Sc)in vitro by the protein-misfolding-cyclic-amplification (PMCA) reaction using normal brain homogenate as substrate does not increase the heat and SDS-stable pool of iron even though both PrP(Sc) and ferritin aggregate by this procedure. These observations highlight important differences between PrP(Sc)-protein complexes generated in vivo during disease progression and in vitro by the PMCA reaction, and the significance of these complexes in PrP(Sc)-associated neurotoxicity.

Role of manganese in neurodegenerative diseases

Manganese (Mn) is an essential ubiquitous trace element that is required for normal growth, development and cellular homeostasis. Exposure to high Mn levels causes a clinical disease characterized by extrapyramidal symptom resembling idiopathic Parkinson's disease (IPD). The present review focuses on the role of various transporters in maintaining brain Mn homeostasis along with recent methodological advances in real-time measurements of intracellular Mn levels. We also provide an overview on the role for Mn in IPD, discussing the similarities (and differences) between manganism and IPD, and the relationship between α-synuclein and Mn-related protein aggregation, as well as mitochondrial dysfunction, Mn and PD. Additional sections of the review discuss the link between Mn and Huntington's disease (HD), with emphasis on huntingtin function and the potential role for altered Mn homeostasis and toxicity in HD. We conclude with a brief survey on the potential role of Mn in the etiologies of Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) and prion disease. Where possible, we discuss the mechanistic commonalities inherent to Mn-induced neurotoxicity and neurodegenerative disorders.

Functional roles of transferrin in the brain

BACKGROUND

Transferrin is synthesized in the brain by choroid plexus and oligodendrocytes, but only that in the choroid plexus is secreted. Transferrin is a major iron delivery protein to the brain, but the amount transcytosed across the brain microvasculature is minimal. Transferrin is the major source of iron delivery to neurons. It may deliver iron to immature oligodendrocytes but this trophic effect declines over time while iron requirements for maintaining myelination continue. Finally, transferrin may play an important role in neurodegenerative diseases through its ability to mobilize iron.

SCOPE OF REVIEW

The role of transferrin in maintaining brain iron homeostasis and the mechanism by which it enters the brain and delivers iron will be discussed. Its relevance to neurological disorders will also be addressed.

MAJOR CONCLUSIONS

Transferrin is the major iron delivery protein for neurons and the microvasculature, but has a limited role for glial cells. The main source of transferrin in the brain is likely from the choroid plexus although the concentration of transferrin at any given time in the brain includes that synthesized in oligodendrocytes. Little is known about brain iron egress or the role of transferrin in this process.

GENERAL SIGNIFICANCE

Neuron survival requires iron, which is predominantly delivered by transferrin. The concentration of transferrin in the cerebrospinal fluid is reflective of brain iron availability and can function as a biomarker in disease. Accumulation of iron in the brain contributes to neurodegenerative processes, thus an understanding of the role that transferrin plays in regulating brain iron homeostasis is essential. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Instability of the octarepeat region of the human prion protein gene

Prion diseases are a family of unique fatal transmissible neurodegenerative diseases that affect humans and many animals. Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common prion disease in humans, accounting for 85–90% of all human prion cases, and exhibits a high degree of diversity in phenotypes. The etiology of sCJD remains to be elucidated. The human prion protein gene has an octapeptide repeat region (octarepeats) that normally contains 5 repeats of 24–27 bp (1 nonapeptide and 4 octapeptide coding sequences). An increase of the octarepeat numbers to six or more or a decrease of the octarepeat number to three is linked to genetic prion diseases with heterogeneous phenotypes in humans. Here we report that the human octarepeat region is prone to either contraction or expansion when subjected to PCR amplification in vitro using Taq or Pwo polymerase and when replicated in wild type E. coli cells. Octarepeat insertion mutants were even less stable, and the mutation rate for the wild type octarepeats was much higher when replicated in DNA mismatch repair-deficient E.coli cells. All observed octarepeat mutants resulting from DNA replication in E.coli were contained in head-to-head plasmid dimers and DNA mfold analysis (http://mfold.rna.albany.edu/?q=mfold/DNA-Folding-Form) indicates that both DNA strands of the octarepeat region would likely form multiple stable hairpin structures, suggesting that the octarepeat sequence may form stable hairpin structures during DNA replication or repair to cause octarepeat instability. These results provide the first evidence supporting a somatic octarepeat mutation-based model for human sCJD etiology: 1) the instability of the octarepeat region leads to accumulation of somatic octarepeat mutations in brain cells during development and aging, 2) this instability is augmented by compromised DNA mismatch repair in aged cells, and 3) eventually some of the octarepeat mutation-containing brain cells start spontaneous de novo prion formation and replication to initiate sCJD.

Infectious prion protein alters manganese transport and neurotoxicity in a cell culture model of prion disease

Protein misfolding and aggregation are considered key features of many neurodegenerative diseases, but biochemical mechanisms underlying protein misfolding and the propagation of protein aggregates are not well understood. Prion disease is a classical neurodegenerative disorder resulting from the misfolding of endogenously expressed normal cellular prion protein (PrP(C)). Although the exact function of PrP(C) has not been fully elucidated, studies have suggested that it can function as a metal binding protein. Interestingly, increased brain manganese (Mn) levels have been reported in various prion diseases indicating divalent metals also may play a role in the disease process. Recently, we reported that PrP(C) protects against Mn-induced cytotoxicity in a neural cell culture model. To further understand the role of Mn in prion diseases, we examined Mn neurotoxicity in an infectious cell culture model of prion disease. Our results show CAD5 scrapie-infected cells were more resistant to Mn neurotoxicity as compared to uninfected cells (EC(50)=428.8 μM for CAD5 infected cells vs. 211.6 μM for uninfected cells). Additionally, treatment with 300 μM Mn in persistently infected CAD5 cells showed a reduction in mitochondrial impairment, caspase-3 activation, and DNA fragmentation when compared to uninfected cells. Scrapie-infected cells also showed significantly reduced Mn uptake as measured by inductively coupled plasma-mass spectrometry (ICP-MS), and altered expression of metal transporting proteins DMT1 and transferrin. Together, our data indicate that conversion of PrP to the pathogenic isoform enhances its ability to regulate Mn homeostasis, and suggest that understanding the interaction of metals with disease-specific proteins may provide further insight to protein aggregation in neurodegenerative diseases.

Gender and iron genes may modify associations between brain iron and memory in healthy aging

Brain iron increases with age and is abnormally elevated early in the disease process in several neurodegenerative disorders that impact memory including Alzheimer's disease (AD). Higher brain iron levels are associated with male gender and presence of highly prevalent allelic variants in genes encoding for iron metabolism proteins (hemochromatosis H63D (HFE H63D) and transferrin C2 (TfC2)). In this study, we examined whether in healthy older individuals memory performance is associated with increased brain iron, and whether gender and gene variant carrier (IRON+) vs noncarrier (IRON-) status (for HFE H63D/TfC2) modify the associations. Tissue iron deposited in ferritin molecules can be measured in vivo with magnetic resonance imaging utilizing the field-dependent relaxation rate increase (FDRI) method. FDRI was assessed in hippocampus, basal ganglia, and white matter, and IRON+ vs IRON- status was determined in a cohort of 63 healthy older individuals. Three cognitive domains were assessed: verbal memory (delayed recall), working memory/attention, and processing speed. Independent of gene status, worse verbal-memory performance was associated with higher hippocampal iron in men (r=-0.50, p=0.003) but not in women. Independent of gender, worse verbal working memory performance was associated with higher basal ganglia iron in IRON- group (r=-0.49, p=0.005) but not in the IRON+ group. Between-group interactions (p=0.006) were noted for both of these associations. No significant associations with white matter or processing speed were observed. The results suggest that in specific subgroups of healthy older individuals, higher accumulations of iron in vulnerable gray matter regions may adversely impact memory functions and could represent a risk factor for accelerated cognitive decline. Combining genetic and MRI biomarkers may provide opportunities to design primary prevention clinical trials that target high-risk groups.

Sortase independent and dependent systems for acquisition of haem and haemoglobin in Listeria monocytogenes

We studied three Fur-regulated systems of Listeria monocytogenes: the srtB region, that encodes sortase-anchored proteins and a putative ABC transporter, and the fhu and hup operons, that produce putative ABC transporters for ferric hydroxamates and haemin (Hn)/haemoglobin (Hb) respectively. Deletion of lmo2185 in the srtB region reduced listerial [(59) Fe]-Hn transport, and purified Lmo2185 bound [(59) Fe]-Hn (K(D) = 12 nM), leading to its designation as a Hn/Hb binding protein (hbp2). Purified Hbp2 also acted as a haemophore, capturing and supplying Hn from the environment. Nevertheless, Hbp2 only functioned in [(59) Fe]-Hn transport at external concentrations less than 50 nM: at higher Hn levels its uptake occurred with equivalent affinity and rate without Hbp2. Similarly, deletion of sortase A had no effect on ferric siderophore or Hn/Hb transport at any concentration, and the srtA-independence of listerial Hn/Hb uptake distinguished it from comparable systems of Staphylococcus aureus. In the cytoplasmic membrane, the Hup transporter was specific for Hn: its lipoprotein (HupD) only showed high affinity for the iron porphyrin (K(D) = 26 nM). Conversely, the FhuD lipoprotein encoded by the fhu operon had broad specificity: it bound both ferric siderophores and Hn, with the highest affinity for ferrioxamine B (K(D) = 123 nM). Deletions of Hup permease components hupD, hupG or hupDGC reduced Hn/Hb uptake, and complementation of ΔhupC and ΔhupG by chromosomal integration of hupC(+) and hupG(+) alleles on pPL2 restored growth promotion by Hn/Hb. However, ΔhupDGC did not completely eliminate [(59) Fe]-Hn transport, implying the existence of another cytoplasmic membrane Hn transporter. The overall K(M) of Hn uptake by wild-type strain EGD-e was 1 nM, and it occurred at similar rates (V(max) = 23 pmol 10(9) cells(-1) min(-1)) to those of ferric siderophore transporters. In the ΔhupDGC strain uptake occurred at a threefold lower rate (V(max) = 7 pmol 10(9) cells(-1) min(-1)). The results show that at low (< 50 nM) levels of Hn, SrtB-dependent peptidoglycan-anchored proteins (e.g. Hbp2) bind the porphyrin, and HupDGC or another transporter completes its uptake into the cytoplasm. However, at higher concentrations Hn uptake is SrtB-independent: peptidoglycan-anchored binding proteins are dispensable because HupDGC directly absorbs and internalizes Hn. Finally, ΔhupDGC increased the LD(50) of L. monocytogenes 100-fold in the mouse infection model, reiterating the importance of this system in listerial virulence.

Decreased CSF transferrin in sCJD: a potential pre-mortem diagnostic test for prion disorders

Sporadic Creutzfeldt-Jakob-disease (sCJD) is a fatal neurodegenerative condition that escapes detection until autopsy. Recently, brain iron dyshomeostasis accompanied by increased transferrin (Tf) was reported in sCJD cases. The consequence of this abnormality on cerebrospinal-fluid (CSF) levels of Tf is uncertain. We evaluated the accuracy of CSF Tf, a ‘new’ biomarker, as a pre-mortem diagnostic test for sCJD when used alone or in combination with the ‘current’ biomarker total-tau (T-tau). Levels of total-Tf (T-Tf), isoforms of Tf (Tf-1 and Tf-β2), and iron saturation of Tf were quantified in CSF collected 0.3–36 months before death (duration) from 99 autopsy confirmed sCJD (CJD+) and 75 confirmed cases of dementia of non-CJD origin (CJD-). Diagnostic accuracy was estimated by non-parametric tests, logistic regression, and receiver operating characteristic (ROC) analysis. Area under the ROC curve (AUC), sensitivity, specificity, positive and negative predictive values (PV), and likelihood ratios (LR) of each biomarker and biomarker combination were calculated. We report that relative to CJD-, CJD+ cases had lower median CSF T-Tf (125,7093 vs. 217,7893) and higher T-tau (11530 vs. 1266) values. AUC was 0.90 (95% confidence interval (CI), 0.85–0.94) for T-Tf, and 0.93 (95% CI, 0.89–0.97) for T-Tf combined with T-tau. With cut-offs defined to achieve a sensitivity of ∼85%, T-Tf identified CJD+ cases with a specificity of 71.6% (95% CI, 59.1–81.7), positive LR of 3.0 (95% CI, 2.1–4.5), negative LR of 0.2 (95% CI, 0.1–0.3), and accuracy of 80.1%. The effect of patient age and duration was insignificant. T-Tf combined with T-tau identified CJD+ with improved specificity of 87.5% (95%CI, 76.3–94.1), positive LR of 6.8 (95% CI, 3.5–13.1), negative LR of 0.2 (95% CI, 0.1–0.3), positive-PV of 91.0%, negative-PV of 80.0%, and accuracy of 86.2%. Thus, CSF T-Tf, a new biomarker, when combined with the current biomarker T-tau, is a reliable pre-mortem diagnostic test for sCJD.

Transcriptional modulation in a leukocyte-depleted splenic cell population during prion disease

Prion replication in the periphery precedes neuroinvasion in many experimental rodent scrapie models, and in natural sheep scrapie and chronic wasting disease (CWD) in cervids. Prions propagate in the germinal centers of secondary lymphoid organs and are strongly associated with follicular dendritic cells (FDC) and possibly circulating dendritic cells and macrophages. Given the importance of lymphoid organs in prion disease transmission and pathogenesis, gene expression studies may reveal host factors or biological pathways related to prion replication and accumulation. A procedure was developed to enrich for FDC, dendritic cells, and macrophages prior to the investigation of transcriptional alterations in murine splenic cells during prion pathogenesis. In total, 1753 transcripts exhibited fold changes greater than three (false discovery rates less than 2%) in this population isolated from spleens of prion-infected versus uninfected mice. The gene for the small leucine-rich proteoglycan decorin (DCN) was one of the genes most overexpressed in infected mice, and the splenic protein levels mirrored this in mice infected with scrapie as well as bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD). A number of groups of functionally related genes were also significantly decreased in infected spleens. These included genes related to iron metabolism and homeostasis, pathways that have also been implicated in prion pathogenesis in the brain. These gene expression alterations provide insights into the molecular mechanisms underlying prion disease pathogenesis and may serve as a pool of potential surrogate markers for the early detection and diagnosis of some prion diseases.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Manganese (Mn) and iron (Fe): interdependency of transport and regulation

Manganese (Mn) and iron (Fe) are transition metals that are crucial to the appropriate growth, development, function, and maintenance of biological organisms. Because of their chemical similarity, in organisms ranging from bacteria to mammals they share and compete for many protein transporters, such as the divalent metal transporter-1. As such, during conditions of low Fe, abnormal Mn accumulation occurs. Conversely, when Mn concentrations are altered, the homeostasis and deposition of Fe and other transition metals are disrupted. Our lab has undertaken a series of studies in rats involving pregnant dams, neo- and perinatal pups, and adult animals. Animals were exposed to various concentrations of dietary Fe and/or Mn, and protein transporter expression, blood Mn and Fe concentrations, brain transition metal concentrations, and temporal brain deposition patterns were examined. As a result, we have demonstrated the importance of the interdependence of the transport of Mn and Fe, and established brain metal concentrations in several longitudinal studies. The purpose of this review is to examine these studies in their entirety and highlight the importance of monitoring the deposition and accumulation of both Mn and Fe when designing future studies related to either dietary or environmental changes in transition metal levels. Finally, this review will provide information about various transport proteins currently under investigation in the research community related to Fe and Mn regulation and transport.

Prevalent iron metabolism gene variants associated with increased brain ferritin iron in healthy older men

Prevalent gene variants involved in iron metabolism [hemochromatosis (HFE) H63D and transferrin C2 (TfC2)] have been associated with higher risk and earlier age at onset of Alzheimer's disease (AD), especially in men. Brain iron increases with age, is higher in men, and is abnormally elevated in several neurodegenerative diseases, including AD and Parkinson's disease, where it has been reported to contribute to younger age at onset in men. The effects of the common genetic variants (HFE H63D and/or TfC2) on brain iron were studied across eight brain regions (caudate, putamen, globus pallidus, thalamus, hippocampus, white matter of frontal lobe, genu, and splenium of corpus callosum) in 66 healthy adults (35 men, 31 women) aged 55 to 76. The iron content of ferritin molecules (ferritin iron) in the brain was measured with MRI utilizing the Field Dependent Relaxation Rate Increase (FDRI) method. 47% of the sample carried neither genetic variant (IRON-) and 53% carried one and/or the other (IRON+). IRON+ men had significantly higher FDRI compared to IRON- men (p=0.013). This genotype effect was not observed in women who, as expected, had lower FDRI than men. This is the first published evidence that these highly prevalent genetic variants in iron metabolism genes can influence brain iron levels in men. Clinical phenomena such as differential gender-associated risks of developing neurodegenerative diseases and age at onset may be associated with interactions between iron genes and brain iron accumulation. Clarifying mechanisms of brain iron accumulation may help identify novel interventions for age-related neurodegenerative diseases.

Paradoxical role of prion protein aggregates in redox-iron induced toxicity

BACKGROUND

Imbalance of iron homeostasis has been reported in sporadic Creutzfeldt-Jakob-disease (sCJD) affected human and scrapie infected animal brains, but the contribution of this phenotype to disease associated neurotoxicity is unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using cell models of familial prion disorders, we demonstrate that exposure of cells expressing normal prion protein (PrP(C)) or mutant PrP forms to a source of redox-iron induces aggregation of PrP(C) and specific mutant PrP forms. Initially this response is cytoprotective, but becomes increasingly toxic with time due to accumulation of PrP-ferritin aggregates. Mutant PrP forms that do not aggregate are not cytoprotective, and cells show signs of acute toxicity. Intracellular PrP-ferritin aggregates induce the expression of LC3-II, indicating stimulation of autophagy in these cells. Similar observations are noted in sCJD and scrapie infected hamster brains, lending credence to these results. Furthermore, phagocytosis of PrP-ferritin aggregates by astrocytes is cytoprotective, while culture in astrocyte conditioned medium (CM) shows no measurable effect. Exposure to H(2)O(2), on the other hand, does not cause aggregation of PrP, and cells show acute toxicity that is alleviated by CM.

CONCLUSIONS/SIGNIFICANCE

These observations suggest that aggregation of PrP in response to redox-iron is cytoprotective. However, subsequent co-aggregation of PrP with ferritin induces intracellular toxicity unless the aggregates are degraded by autophagosomes or phagocytosed by adjacent scavenger cells. H(2)O(2), on the other hand, does not cause aggregation of PrP, and induces toxicity through extra-cellular free radicals. Together with previous observations demonstrating imbalance of iron homeostasis in prion disease affected brains, these observations provide insight into the mechanism of neurotoxicity by redox-iron, and the role of PrP in this process.

Redox control of prion and disease pathogenesis

Imbalance of brain metal homeostasis and associated oxidative stress by redox-active metals like iron and copper is an important trigger of neurotoxicity in several neurodegenerative conditions, including prion disorders. Whereas some reports attribute this to end-stage disease, others provide evidence for specific mechanisms leading to brain metal dyshomeostasis during disease progression. In prion disorders, imbalance of brain-iron homeostasis is observed before end-stage disease and worsens with disease progression, implicating iron-induced oxidative stress in disease pathogenesis. This is an unexpected observation, because the underlying cause of brain pathology in all prion disorders is PrP-scrapie (PrP(Sc)), a beta-sheet-rich conformation of a normal glycoprotein, the prion protein (PrP(C)). Whether brain-iron dyshomeostasis occurs because of gain of toxic function by PrP(Sc) or loss of normal function of PrP(C) remains unclear. In this review, we summarize available evidence suggesting the involvement of oxidative stress in prion-disease pathogenesis. Subsequently, we review the biology of PrP(C) to highlight its possible role in maintaining brain metal homeostasis during health and the contribution of PrP(Sc) in inducing brain metal imbalance with disease progression. Finally, we discuss possible therapeutic avenues directed at restoring brain metal homeostasis and alleviating metal-induced oxidative stress in prion disorders.

Iron content of ferritin modulates its uptake by intestinal epithelium: implications for co-transport of prions

The spread of Chronic Wasting Disease (CWD) in the deer and elk population has caused serious public health concerns due to its potential to infect farm animals and humans. Like other prion disorders such a sporadic Creutzfeldt-Jakob-disease of humans and Mad Cow Disease of cattle, CWD is caused by PrP-scrapie (PrP^Sc), a β-sheet rich isoform of a normal cell surface glycoprotein, the prion protein (PrP^C). Since PrP^Sc is sufficient to cause infection and neurotoxicity if ingested by a susceptible host, it is important to understand the mechanism by which it crosses the stringent epithelial cell barrier of the small intestine. Possible mechanisms include co-transport with ferritin in ingested food and uptake by dendritic cells. Since ferritin is ubiquitously expressed and shares considerable homology among species, co-transport of PrP^Sc with ferritin can result in cross-species spread with deleterious consequences. We have used a combination of in vitro and in vivo models of intestinal epithelial cell barrier to understand the role of ferritin in mediating PrP^Sc uptake and transport. In this report, we demonstrate that PrP^Sc and ferritin from CWD affected deer and elk brains and scrapie from sheep resist degradation by digestive enzymes, and are transcytosed across a tight monolayer of human epithelial cells with significant efficiency. Likewise, ferritin from hamster brains is taken up by mouse intestinal epithelial cells in vivo, indicating that uptake of ferritin is not limited by species differences as described for prions. More importantly, the iron content of ferritin determines its efficiency of uptake and transport by Caco-2 cells and mouse models, providing insight into the mechanism(s) of ferritin and PrP^Sc uptake by intestinal epithelial cells.

Alzheimer's disease as homeostatic responses to age-related myelin breakdown

The amyloid hypothesis (AH) of Alzheimer's disease (AD) posits that the fundamental cause of AD is the accumulation of the peptide amyloid beta (Aβ) in the brain. This hypothesis has been supported by observations that genetic defects in amyloid precursor protein (APP) and presenilin increase Aβ production and cause familial AD (FAD). The AH is widely accepted but does not account for important phenomena including recent failures of clinical trials to impact dementia in humans even after successfully reducing Aβ deposits. Herein, the AH is viewed from the broader overarching perspective of the myelin model of the human brain that focuses on functioning brain circuits and encompasses white matter and myelin in addition to neurons and synapses. The model proposes that the recently evolved and extensive myelination of the human brain underlies both our unique abilities and susceptibility to highly prevalent age-related neuropsychiatric disorders such as late onset AD (LOAD). It regards oligodendrocytes and the myelin they produce as being both critical for circuit function and uniquely vulnerable to damage. This perspective reframes key observations such as axonal transport disruptions, formation of axonal swellings/sphenoids and neuritic plaques, and proteinaceous deposits such as Aβ and tau as by-products of homeostatic myelin repair processes. It delineates empirically testable mechanisms of action for genes underlying FAD and LOAD and provides "upstream" treatment targets. Such interventions could potentially treat multiple degenerative brain disorders by mitigating the effects of aging and associated changes in iron, cholesterol, and free radicals on oligodendrocytes and their myelin.

Prion protein and metal interaction: physiological and pathological implications

Metal induced free radicals are important mediators of neurotoxicity in several neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. Similar evidence is now emerging for prion diseases, a group of neurodegenerative disorders of humans and animals. The main pathogenic agent in all prion disorders is PrP-scrapie (PrP(Sc)), a beta-sheet rich isoform of a normal cell surface glycoprotein known as the prion protein (PrP(C)). Deposits of PrP(Sc) in the brain parenchyma are believed to induce neurotoxicity through poorly understood mechanisms. Recent reports suggest that imbalance of brain metal homeostasis is a significant cause of PrP(Sc)-associated neurotoxicity, though the underlying mechanisms are difficult to explain based on existing information. Proposed hypotheses include a functional role for PrP(C) in metal metabolism, and loss of this function due to aggregation to the disease associated PrP(Sc) form as the cause of brain metal imbalance. Other views suggest gain of toxic function by PrP(Sc) due to sequestration of PrP(C)-associated metals within the aggregates, resulting in the generation of redox-active PrP(Sc) complexes. The physiological implications of some PrP(C)-metal interactions are known, while others are still unclear. The pathological implications of PrP(C)-metal interaction include metal-induced oxidative damage, and in some instances conversion of PrP(C) to a PrP(Sc)-like form. Despite its significance, only limited information is available on PrP-metal interaction and its implications on prion disease pathogenesis. In this review, we summarize the physiological significance and pathological implications of PrP-metal interaction on prion disease pathogenesis.

Prion protein (PrP) knock-out mice show altered iron metabolism: a functional role for PrP in iron uptake and transport

Despite overwhelming evidence implicating the prion protein (PrP) in prion disease pathogenesis, the normal function of this cell surface glycoprotein remains unclear. In previous reports we demonstrated that PrP mediates cellular iron uptake and transport, and aggregation of PrP to the disease causing PrP-scrapie (PrP^Sc) form results in imbalance of iron homeostasis in prion disease affected human and animal brains. Here, we show that selective deletion of PrP in transgenic mice (PrP^KO) alters systemic iron homeostasis as reflected in hematological parameters and levels of total iron and iron regulatory proteins in the plasma, liver, spleen, and brain of PrP^KO mice relative to matched wild type controls. Introduction of radiolabeled iron (⁵⁹FeCl₃) to Wt and PrP^KO mice by gastric gavage reveals inefficient transport of ⁵⁹Fe from the duodenum to the blood stream, an early abortive spike of erythropoiesis in the long bones and spleen, and eventual decreased ⁵⁹Fe content in red blood cells and all major organs of PrP^KO mice relative to Wt controls. The iron deficient phenotype of PrP^KO mice is reversed by expressing Wt PrP in the PrP^KO background, demonstrating a functional role for PrP in iron uptake and transport. Since iron is required for essential metabolic processes and is also potentially toxic if mismanaged, these results suggest that loss of normal function of PrP due to aggregation to the PrP^Sc form induces imbalance of brain iron homeostasis, resulting in disease associated neurotoxicity.

Prion protein modulates cellular iron uptake: a novel function with implications for prion disease pathogenesis

Converging evidence leaves little doubt that a change in the conformation of prion protein (PrP^C) from a mainly α-helical to a β-sheet rich PrP-scrapie (PrP^Sc) form is the main event responsible for prion disease associated neurotoxicity. However, neither the mechanism of toxicity by PrP^Sc, nor the normal function of PrP^C is entirely clear. Recent reports suggest that imbalance of iron homeostasis is a common feature of prion infected cells and mouse models, implicating redox-iron in prion disease pathogenesis. In this report, we provide evidence that PrP^C mediates cellular iron uptake and transport, and mutant PrP forms alter cellular iron levels differentially. Using human neuroblastoma cells as models, we demonstrate that over-expression of PrP^C increases intra-cellular iron relative to non-transfected controls as indicated by an increase in total cellular iron, the cellular labile iron pool (LIP), and iron content of ferritin. As a result, the levels of iron uptake proteins transferrin (Tf) and transferrin receptor (TfR) are decreased, and expression of iron storage protein ferritin is increased. The positive effect of PrP^C on ferritin iron content is enhanced by stimulating PrP^C endocytosis, and reversed by cross-linking PrP^C on the plasma membrane. Expression of mutant PrP forms lacking the octapeptide-repeats, the membrane anchor, or carrying the pathogenic mutation PrP^102L decreases ferritin iron content significantly relative to PrP^C expressing cells, but the effect on cellular LIP and levels of Tf, TfR, and ferritin is complex, varying with the mutation. Neither PrP^C nor the mutant PrP forms influence the rate or amount of iron released into the medium, suggesting a functional role for PrP^C in cellular iron uptake and transport to ferritin, and dysfunction of PrP^C as a significant contributing factor of brain iron imbalance in prion disorders.