Changed iron regulation in scrapie-infected neuroblastoma cells

Dietary and Sentinel Factors Leading to Hemochromatosis

Although hereditary hemochromatosis is associated with the mutation of genes involved in iron transport and metabolism, secondary hemochromatosis is due to external factors, such as intended or unintended iron overload, hemolysis-linked iron exposure or other stress-impaired iron metabolism. The present review addresses diet-linked etiologies of hemochromatosis and their pathogenesis in the network of genes and nutrients. Although the mechanistic association to diet-linked etiologies can be complicated, the stress sentinels are pivotally involved in the pathological processes of secondary hemochromatosis in response to iron excess and other external stresses. Moreover, the mutations in these sentineling pathway-linked genes increase susceptibility to secondary hemochromatosis. Thus, the crosstalk between nutrients and genes would verify the complex procedures in the clinical outcomes of secondary hemochromatosis and chronic complications, such as malignancy. All of this evidence provides crucial insights into comprehensive clinical or nutritional interventions for hemochromatosis.

The prion-ZIP connection: From cousins to partners in iron uptake

Converging observations from disparate lines of inquiry are beginning to clarify the cause of brain iron dyshomeostasis in sporadic Creutzfeldt-Jakob disease (sCJD), a neurodegenerative condition associated with the conversion of prion protein (PrP^C), a plasma membrane glycoprotein, from α-helical to a β-sheet rich PrP-scrapie (PrP^Sc) isoform. Biochemical evidence indicates that PrP^C facilitates cellular iron uptake by functioning as a membrane-bound ferrireductase (FR), an activity necessary for the transport of iron across biological membranes through metal transporters. An entirely different experimental approach reveals an evolutionary link between PrP^C and the Zrt, Irt-like protein (ZIP) family, a group of proteins involved in the transport of zinc, iron, and manganese across the plasma membrane. Close physical proximity of PrP^C with certain members of the ZIP family on the plasma membrane and increased uptake of extracellular iron by cells that co-express PrP^C and ZIP14 suggest that PrP^C functions as a FR partner for certain members of this family. The connection between PrP^C and ZIP proteins therefore extends beyond common ancestry to that of functional cooperation. Here, we summarize evidence supporting the facilitative role of PrP^C in cellular iron uptake, and implications of this activity on iron metabolism in sCJD brains.

Iron in neurodegenerative disorders of protein misfolding: a case of prion disorders and Parkinson's disease

SIGNIFICANCE

Intracellular and extracellular aggregation of a specific protein or protein fragments is the principal pathological event in several neurodegenerative conditions. We describe two such conditions: sporadic Creutzfeldt-Jakob disease (sCJD), a rare but potentially infectious and invariably fatal human prion disorder, and Parkinson's disease (PD), a common neurodegenerative condition second only to Alzheimer's disease in prevalence. In sCJD, a cell surface glycoprotein known as the prion protein (PrP(C)) undergoes a conformational change to PrP-scrapie, a pathogenic and infectious isoform that accumulates in the brain parenchyma as insoluble aggregates. In PD, α-synuclein, a cytosolic protein, forms insoluble aggregates that accumulate in neurons of the substantia nigra and cause neurotoxicity.

RECENT ADVANCES

Although distinct processes are involved in the pathogenesis of sCJD and PD, both share brain iron dyshomeostasis as a common associated feature that is reflected in the cerebrospinal fluid in a disease-specific manner.

CRITICAL ISSUES

Since PrP(C) and α-synuclein play a significant role in maintaining cellular iron homeostasis, it is important to understand whether the aggregation of these proteins and iron dyshomeostasis are causally related. Here, we discuss recent information on the normal function of PrP(C) and α-synuclein in cellular iron metabolism and the cellular and biochemical processes that contribute to iron imbalance in sCJD and PD.

FUTURE DIRECTIONS

Improved understanding of the relationship between brain iron imbalance and protein aggregation is likely to help in the development of therapeutic strategies that can restore brain iron homeostasis and mitigate neurotoxicity.

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

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

Prion protein conversion induced by trivalent iron in vesicular trafficking

Iron dyshomeostasis has been observed in prion diseases; however, little is known regarding the contribution of the oxidation state of iron to prion protein (PrP) conversion. In this study, PrP(C)-deficient HpL3-4 cells were exposed to divalent [Fe(II)] or trivalent [Fe(III)] iron, followed by exogenous recombinant PrP (rPrP) treatment. We then analyzed the accumulation of internalized rPrP and its biochemical properties, including its resistance to both proteinase K (PK) digestion and detergent solubility. Fe(III), but not Fe(II), induced the accumulation of internalized rPrP, which was partially converted to detergent-insoluble and PK-resistant PrP (PrP(res)). The Fe(III)-induced PrP(res) generation required an intact cell structure, and it was hindered by U18666A, an inhibitor of vesicular trafficking, but not by NH4Cl, an inhibitor of endolysosomal acidification. These observations implicated that the Fe(III)-mediated PrP(res) conversion likely occurs during endosomal vesicular trafficking rather than in the acidic environment of lysosomes.

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.

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.

The depletion of α and β PrP from complex mixtures

Prion disorders occur when endogenous prion protein (PrP(C)) undergoes a conformational change from a predominantly α-helix-rich structure to an insoluble β-sheet-rich structure (PrP(Sc)). The resulting PrP(Sc) then in some way facilitates the progressive transformation of nearby PrP(C) to PrP(Sc). In time this results in the deposition of insoluble PrP(Sc) aggregates in the brain; aggregate deposition is irreversible and is ultimately fatal. Prion diseases are transmissible orally or through transplantation (including blood transfusion). Current diagnostic methods are limited in that they lack the ability to distinguish qualitatively between PrP(C) and PrP(Sc). PrP has been shown to bind divalent cations including copper and zinc, these cations are toxic and thus of limited use in the removal of PrP from solutions destined for administration to subjects. We have immobilised Fe(3+) to an inert Sepharose resin; this resin was capable of quantitatively removing endogenous and recombinant PrP(C) and recombinant β PrP from complex solutions. The low toxicity of Fe(3+) suggests that the resin described in this report may be of practical use in the depletion of PrP from blood products destined for human use.

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.

Increased proportions of C1 truncated prion protein protect against cellular M1000 prion infection

Prion disease pathogenesis is linked to the cell-associated propagation of misfolded protease-resistant conformers (PrP) of the normal cellular prion protein (PrP). Ongoing PrP expression is the only known absolute requirement for successful prion disease transmission and PrP propagation. Further typifying prion disease is selective neuronal dysfunction and loss, although the precise mechanisms underlying this are undefined. We utilized a single prion strain (M1000) and a range of neuronal and nonneuronal, PrP endogenously expressing and transgenically modified overexpressing cell lines, to evaluate whether PrP glycosylation patterns or constitutive N-terminal cleavage events may be determinants of sustained PrP propagation. Our data demonstrates that relative proportions of full-length and C1 truncated PrP are the most important characteristics influencing susceptibility to sustained M1000 prion infection, supporting PrP alpha-cleavage as a protective event, which may contribute to the selective neuronal vulnerability observed in vivo.

Abnormal brain iron homeostasis in human and animal prion disorders

Neurotoxicity in all prion disorders is believed to result from the accumulation of PrP-scrapie (PrP(Sc)), a beta-sheet rich isoform of a normal cell-surface glycoprotein, the prion protein (PrP(C)). Limited reports suggest imbalance of brain iron homeostasis as a significant associated cause of neurotoxicity in prion-infected cell and mouse models. However, systematic studies on the generality of this phenomenon and the underlying mechanism(s) leading to iron dyshomeostasis in diseased brains are lacking. In this report, we demonstrate that prion disease-affected human, hamster, and mouse brains show increased total and redox-active Fe (II) iron, and a paradoxical increase in major iron uptake proteins transferrin (Tf) and transferrin receptor (TfR) at the end stage of disease. Furthermore, examination of scrapie-inoculated hamster brains at different timepoints following infection shows increased levels of Tf with time, suggesting increasing iron deficiency with disease progression. Sporadic Creutzfeldt-Jakob disease (sCJD)-affected human brains show a similar increase in total iron and a direct correlation between PrP and Tf levels, implicating PrP(Sc) as the underlying cause of iron deficiency. Increased binding of Tf to the cerebellar Purkinje cell neurons of sCJD brains further indicates upregulation of TfR and a phenotype of neuronal iron deficiency in diseased brains despite increased iron levels. The likely cause of this phenotype is sequestration of iron in brain ferritin that becomes detergent-insoluble in PrP(Sc)-infected cell lines and sCJD brain homogenates. These results suggest that sequestration of iron in PrP(Sc)-ferritin complexes induces a state of iron bio-insufficiency in prion disease-affected brains, resulting in increased uptake and a state of iron dyshomeostasis. An additional unexpected observation is the resistance of Tf to digestion by proteinase-K, providing a reliable marker for iron levels in postmortem human brains. These data implicate redox-iron in prion disease-associated neurotoxicity, a novel observation with significant implications for prion disease pathogenesis.

Aconitate hydratase of mammals under oxidative stress

Data on the structure, functions, regulation of activity, and expression of cytosolic and mitochondrial aconitate hydratase isoenzymes of mammals are reviewed. The role of aconitate hydratase and structurally similar iron-regulatory protein in maintenance of homeostasis of cell iron is described. Information on modifications of the aconitate hydratase molecule and changes in expression under oxidative stress is generalized. The role of aconitate hydratase in the pathogenesis of some diseases is considered.

Transcriptional stability of cultured cells upon prion infection

Prion infections induce severe disruption of the central nervous system with neuronal vacuolation and extensive glial reactions, and invariably lead to death of affected individuals. The molecular underpinnings of these events are not well understood. To better define the molecular consequences of prion infections, we analyzed the transcriptional response to persistent prion infection in a panel of three murine neural cell lines in vitro. Colony spot immunochemistry assays indicated that 65-100% of cells were infected in each line. Only the Nav1 gene was marginally modulated in one cell line, whereas transcripts previously reported to be derailed in prion-infected cells were not confirmed in the present study. We attribute these discrepancies to the experimental stringency of the current study, which was performed under conditions designed to minimize potential genetic drifts. These findings are at striking variance with gene expression studies performed on whole brains upon prion infections in vivo, suggesting that many of the latter changes represent secondary reactions to infection. We conclude that, surprisingly, there are no universal transcriptional changes induced by prion infection of neural cells in vitro.

Vacuolated neuroblastoma cells mimicking FAB L(3) lymphoblasts in bone marrow aspirates

Abundant cytoplasmic vacuolation of neuroblasts has been noted on bone marrow aspirate (BMA) smears of two patients with metastatic neuroblastoma. Occasional tumor cells were dispersed as individual cells as well as in clumps. These cells had basophilic cytoplasm and several nucleoli, reminiscent of L(3) lymphoblast morphology. Flow cytometric analysis of the bone marrow mononuclear cells and neuron-specific enolase staining of the bone marrow biopsy samples further distinguished the cells as neuroblasts. Cytoplasmic vacuolations of neuroblasts may be a feature of metastatic neuroblastoma cells in BMA smears.

Oxidative stress in the brain at early preclinical stages of mouse scrapie

Oxidative stress has been shown to be involved in the pathogenesis of neurodegenerative diseases including prion diseases. Although a growing body of evidence suggests direct involvement of oxidative stress in the pathogenesis of prion diseases, it is still not clear whether oxidative stress is a causative early event in these conditions or a secondary phenomenon commonly found in the progression of neurodegenerative diseases. Using a mouse scrapie model, we assessed oxidative stress in the brain at various stages of the disease progression and observed significantly increased concentration of lipid peroxidation markers, malondialdehyde and 4-hydroxyalkenals, and mRNA level of an oxidative stress response enzyme, heme oxygenase-1, at early preclinical stages of scrapie. The changes preceded dramatic synaptic loss demonstrated by immunohistochemical staining of a synaptic protein, synaptophysin. These findings imply that the brain undergoes oxidative stress even from an early stage of prion invasion into the brain. Given the well-known deleterious effects of reactive-oxygen-species-mediated damage in the brain, it is considered that the oxidative stress at the preclinical stage of prion diseases may predispose the brain to neurodegenerative mechanisms that characterize the diseases.

Interaction of metals with prion protein: Possible role of divalent cations in the pathogenesis of prion diseases

Prion diseases are fatal neurodegenerative disorders that affect both humans and animals. The rapid clinical progression, change in protein conformation, cross-species transmission and massive neuronal degeneration are some key features of this devastating degenerative condition. Although the etiology is unknown, aberrant processing of cellular prion proteins is well established in the pathogenesis of prion diseases. Normal cellular prion protein (PrP(c)) is highly conserved in mammals and expressed predominantly in the brain. Nevertheless, the exact function of the normal prion protein in the CNS has not been fully elucidated. Prion proteins may function as a metal binding protein because divalent cations such as copper, zinc and manganese can bind to octapeptide repeat sequences in the N-terminus of PrP(c). Since the binding of these metals to the octapeptide has been proposed to influence both structural and functional properties of prion proteins, alterations in transition metal levels can alter the course of the disease. Furthermore, cellular antioxidant capacity is significantly compromised due to conversion of the normal prion protein (PrP(c)) to an abnormal scrapie prion (PrP(sc)) protein, suggesting that oxidative stress may play a role in the neurodegenerative process of prion diseases. The combination of imbalances in cellular transition metals and increased oxidative stress could further exacerbate the neurotoxic effect of PrP(sc). This review includes an overview of the structure and function of prion proteins, followed by the role of metals such as copper, manganese and iron in the physiological function of the PrP(c), and the possible role of transition metals in the pathogenesis of the prion disease.

Transition metal complexes of terminally protected peptides containing histidyl residues

Histidine-containing peptide fragments of prion protein are efficient ligands to bind various transition metal ions and they have high selectivity in metal binding. The metal ion affinity follows the order: Pd(II)>>Cu(II)>>Ni(II)Zn(II)>Cd(II) approximately Co(II)>Mn(II). The high selectivity of metal binding is connected to the involvement of both imidazole and amide nitrogen atoms in metal binding for Pd(II), Cu(II) and Ni(II), while only the monodentate N(im)-coordination is possible with the other metal ions. The stoichiometry and binding mode of palladium(II) complexes show great variety depending on the metal ion to ligand ratio, pH and especially the presence of coordinating donor atoms in the side chains of peptide fragments. It is also clear from our data that the peptide fragments containing histidine outside the octarepeat (His96, His111 and His187) are more efficient ligands than the monomer peptide fragments of the octarepeat domain.

Increased susceptibility to oxidative stress in scrapie-infected neuroblastoma cells is associated with intracellular iron status

The molecular mechanism of neurodegeneration in prion diseases remains largely uncertain, but one of the features of infected cells is higher sensitivity to induced oxidative stress. In this study, we have investigated the role of iron in hydrogen peroxide (H(2)O(2))-induced toxicity in scrapie-infected mouse neuroblastoma N2a (ScN 2 a) cells. ScN 2 a cells were significantly more susceptible to H(2)O(2) toxicity than N2a cells as revealed by cell viability (MTT) assay. After 2h exposure, significant decrease in cell viability in ScN 2 a cells was observed at low concentrations of extracellular H(2)O(2) (5-10 microM), whereas N2a cells were not affected. The increased H(2)O(2) toxicity in ScN 2 a cells may be related to intracellular iron status since ferrous iron (Fe(2+)) chelator 2,2'-bipyridyl (BIP) prevented H(2)O(2)-induced decrease in cell viability. Further, the level of calcein-sensitive labile iron pool (LIP) was significantly increased in ScN 2 a cells after H(2)O(2) treatment. Finally, the production of reactive oxygen species (ROS) was inhibited by 30% by iron chelators desferrioxamine (DFO) and BIP in ScN 2 a cells, whereas no significant effect of iron chelators on basal ROS production was observed in N2a cells. This study indicates that cellular resistance to oxidative stress in ScN 2 a cells is associated with intracellular status of reactive iron.

Redox metals and oxidative abnormalities in human prion diseases

Prion diseases are characterized by the accumulation of diffuse and aggregated plaques of protease-resistant prion protein (PrP) in the brains of affected individuals and animals. Whereas prion diseases in animals appear to be almost exclusively transmitted by infection, human prion diseases most often occur sporadically and, to a lesser extent, by inheritance or infection. In the sporadic cases (sporadic Creutzfeld-Jakob disease, sCJD), PrP-containing plaques are infrequent, whereas in transmitted (variant CJD) and inherited (Gerstmann-Straussler-Scheinker Syndrome) cases, plaques are a usual feature. In the current study, representative cases from each of the classes of human prion disease were analyzed for the presence of markers of oxidative damage that have been found in other neurodegenerative diseases. Interestingly, we found that the pattern of deposition of PrP, amyloid-beta, and redox active metals was distinct for the various prion diseases. Whereas 8-hydroxyguanosine has been shown to be increased in sCJD, and inducible NOS is increased in scrapie-infected mice, well-studied markers of oxidative damage that accumulate in the lesions of other neurodegenerative diseases (such as Alzheimer's disease, progressive supranuclear palsy, and Parkinson's disease), such as heme oxygenase-1 and lipid peroxidation, were not found around PrP deposits or in vulnerable neurons. These findings suggest an important distinction in prion-related oxidative stress, indicating that different neurodegenerative pathways are involved in different prion diseases.

Increased iron-induced oxidative stress and toxicity in scrapie-infected neuroblastoma cells

The mechanisms behind the pathology of prion diseases are still unknown, but accumulating evidence suggests oxidative impairment along with metal imbalances in scrapie-infected brains. In this study, we have investigated iron-induced oxidative stress in scrapie-infected mouse neuroblastoma N2a (ScN2a) cells. Uninfected N2a and ScN2a cells were treated with ferric ammonium citrate (FAC) for 1-16 h, and the levels of labile iron pool (LIP), the formation of reactive oxygen species (ROS), cell viability and ferritin protein levels were measured. The increase in LIP in N2a cells was transient with a quick recovery to normal levels within 4h accompanied by a moderate increase of formation of ROS after 3h followed by the decrease to the basal level. In ScN2a cells, the increase in LIP was lower, but the process of recovery was prolonged and accompanied by high ROS formation and decreased cell viability. Ferritin protein levels were significantly lower in ScN2a cells than in wild-type cells in all iron treatments. These results suggest that ScN2a cells are more sensitive to iron treatment as compared to wild-type cells with respect to ROS formation and cell viability, and that ferritin deficiency in infected cells may contribute to iron-induced oxidative stress in scrapie-infected cells.