Redox metals and oxidative abnormalities in human prion diseases

Ferritin is closely associated with microglia in amyotrophic lateral sclerosis

Iron deposition is a hallmark of amyotrophic lateral sclerosis (ALS) and has been strongly implicated in its pathogenesis. As a byproduct of cellular oxidative stress, iron dysregulation modifies basal levels of the regulatory iron-binding protein ferritin. Examination of thoracic and lumbar spinal cord tissues found increased ferritin immunostaining in white matter axons that corresponded to areas of increased microgliosis in 8 ALS patients versus 8 normal subjects. Gray matter areas containing the motor neurons also demonstrated increased ferritin and microglia in ALS compared to controls but at lower levels than in the white matter. Motor neurons with or without TDP-43 inclusions did not demonstrate either increased ferritin or associated microglial activation. We also observed an association of ferritin with microglia in cerebral cortical tissue samples of ALS cases and in the spinal cord tissues of transgenic mice expressing the SOD1G93A mutation. Elevated ferritin levels were detected in the insoluble fraction from spinal cord tissues of individuals with ALS. These findings suggest that activated microglia and increased ferritin may play significant roles in ALS progression since they are found closely associated in areas of axonal and cortical degeneration.

Electrochemical biosensors for biometal-protein interactions in neurodegenerative diseases

Electrochemical biosensors have been adopted into a wide range of applications in the study of biometal-protein interactions in neurodegenerative diseases. Transition metals such as zinc, copper, and iron that are significant to biological functions have been shown to have strong implications in the progressive neural degeneration in Alzheimer's disease (AD), Parkinson's disease (PD), and prion protein diseases. This review presents a summative examination of the progress made in the design, fabrication, and applications of electrochemical biosensors in recent literature at understanding the metal-protein interactions in neurodegenerative diseases. The focus will be drawn on disease-causing biomarkers such as amyloid-β (Aβ) and tau proteins for AD, α-synuclein (α-syn) for PD, and prion proteins (PrP). Topics such as the use of electrochemical biosensing in monitoring biometal-induced conformational changes, elucidation of complexation motifs, production of reactive oxygen species (ROS) as well as the influence on downstream biomolecular interactions will be discussed. Major results and important concepts presented in these studies will be summarized in the hope to spark inspiration for the next generation of electrochemical sensors.

Metal homeostasis disturbances in neurodegenerative disorders, with special emphasis on Creutzfeldt-Jakob disease - Potential pathogenetic mechanism and therapeutic implications

Creutzfeldt-Jakob disease (CJD) is characterized by a rapidly progressive dementia often accompanied by myoclonus and other signs of brain dysfunction, relying on the conversion of the normal cellular form of the prion protein (PrPC) to a misfolded form (PrPSc). The neuropathological changes include spongiform degeneration, neuronal loss, astrogliosis, and deposition of PrPSc. It is still unclear how these pathological changes correlate with the development of CJD symptoms because few patients survive beyond 2 years after diagnosis. Inasmuch as the symptoms of CJD overlap some of those observed in Alzheimer's, Parkinson's, and Huntington's diseases, there may be some underlying pathologic mechanisms associated with CJD that may contribute to the symptoms of non-prion neurodegenerative diseases as well. Data suggest that imbalance of metals, including copper, zinc, iron, and manganese, induces abnormalities in processing and degradation of prion proteins that are accompanied by self-propagation of PrPSc. These events appear to be responsible for glutamatergic synaptic dysfunctions, neuronal death, and PrPSc aggregation. Given that the prodromal symptoms of CJD such as sleep disturbances and mood disorders are associated with brain stem and limbic system dysfunction, the pathological changes may initially occur in these brain regions, then spread throughout the entire brain. Alterations in cerebrospinal fluid homeostasis, which may be linked to imbalance of these metals, seem to be more important than neuroinflammation in causing the cell death. It is proposed that metal dyshomeostasis could be responsible for the initiation and progression of the pathological changes associated with symptoms of CJD and other neurodegenerative disorders.

Elucidating Critical Proteinopathic Mechanisms and Potential Drug Targets in Neurodegeneration

Neurodegeneration entails progressive loss of neuronal structure as well as function leading to cognitive failure, apathy, anxiety, irregular body movements, mood swing and ageing. Proteomic dysregulation is considered the key factor for neurodegeneration. Mechanisms involving deregulated processing of proteins such as amyloid beta (Aβ) oligomerization; tau hyperphosphorylation, prion misfolding; α-synuclein accumulation/lewy body formation, chaperone deregulation, acetylcholine depletion, adenosine 2A (A2A) receptor hyperactivation, secretase deregulation, leucine-rich repeat kinase 2 (LRRK2) mutation and mitochondrial proteinopathies have deeper implications in neurodegenerative disorders. Better understanding of such pathological mechanisms is pivotal for exploring crucial drug targets. Herein, we provide a comprehensive outlook about the diverse proteomic irregularities in Alzheimer's, Parkinson's and Creutzfeldt Jakob disease (CJD). We explicate the role of key neuroproteomic drug targets notably Aβ, tau, alpha synuclein, prions, secretases, acetylcholinesterase (AchE), LRRK2, molecular chaperones, A2A receptors, muscarinic acetylcholine receptors (mAchR), N-methyl-D-aspartate receptor (NMDAR), glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and mitochondrial/oxidative stress-related proteins for combating neurodegeneration and associated cognitive and motor impairment. Cross talk between amyloidopathy, synucleinopathy, tauopathy and several other proteinopathies pinpoints the need to develop safe therapeutics with ability to strike multiple targets in the aetiology of the neurodegenerative disorders. Therapeutics like microtubule stabilisers, chaperones, kinase inhibitors, anti-aggregation agents and antibodies could serve promising regimens for treating neurodegeneration. However, drugs should be target specific, safe and able to penetrate blood-brain barrier.

Prions Strongly Reduce NMDA Receptor S-Nitrosylation Levels at Pre-symptomatic and Terminal Stages of Prion Diseases

Prion diseases are fatal neurodegenerative disorders characterized by the cellular prion protein (PrP^C) conversion into a misfolded and infectious isoform termed prion or PrP^Sc. The neuropathological mechanism underlying prion toxicity is still unclear, and the debate on prion protein gain- or loss-of-function is still open. PrP^C participates to a plethora of physiological mechanisms. For instance, PrP^C and copper cooperatively modulate N-methyl-D-aspartate receptor (NMDAR) activity by mediating S-nitrosylation, an inhibitory post-translational modification, hence protecting neurons from excitotoxicity. Here, NMDAR S-nitrosylation levels were biochemically investigated at pre- and post-symptomatic stages of mice intracerebrally inoculated with RML, 139A, and ME7 prion strains. Neuropathological aspects of prion disease were studied by histological analysis and proteinase K digestion. We report that hippocampal NMDAR S-nitrosylation is greatly reduced in all three prion strain infections in both pre-symptomatic and terminal stages of mouse disease. Indeed, we show that NMDAR S-nitrosylation dysregulation affecting prion-inoculated animals precedes the appearance of clinical signs of disease and visible neuropathological changes, such as PrP^Sc accumulation and deposition. The pre-symptomatic reduction of NMDAR S-nitrosylation in prion-infected mice may be a possible cause of neuronal death in prion pathology, and it might contribute to the pathology progression opening new therapeutic strategies against prion disorders.

Prion protein cleavage fragments regulate adult neural stem cell quiescence through redox modulation of mitochondrial fission and SOD2 expression

Neurogenesis continues in the post-developmental brain throughout life. The ability to stimulate the production of new neurones requires both quiescent and actively proliferating pools of neural stem cells (NSCs). Actively proliferating NSCs ensure that neurogenic demand can be met, whilst the quiescent pool makes certain NSC reserves do not become depleted. The processes preserving the NSC quiescent pool are only just beginning to be defined. Herein, we identify a switch between NSC proliferation and quiescence through changing intracellular redox signalling. We show that N-terminal post-translational cleavage products of the prion protein (PrP) induce a quiescent state, halting NSC cellular growth, migration, and neurite outgrowth. Quiescence is initiated by the PrP cleavage products through reducing intracellular levels of reactive oxygen species. First, inhibition of redox signalling results in increased mitochondrial fission, which rapidly signals quiescence. Thereafter, quiescence is maintained through downstream increases in the expression and activity of superoxide dismutase-2 that reduces mitochondrial superoxide. We further observe that PrP is predominantly cleaved in quiescent NSCs indicating a homeostatic role for this cascade. Our findings provide new insight into the regulation of NSC quiescence, which potentially could influence brain health throughout adult life.

Differentially expressed genes in iron-induced prion protein conversion

The conversion of the cellular prion protein (PrP^C) to the protease-resistant isoform is the key event in chronic neurodegenerative diseases, including transmissible spongiform encephalopathies (TSEs). Increased iron in prion-related disease has been observed due to the prion protein-ferritin complex. Additionally, the accumulation and conversion of recombinant PrP (rPrP) is specifically derived from Fe(III) but not Fe(II). Fe(III)-mediated PK-resistant PrP (PrP^res) conversion occurs within a complex cellular environment rather than via direct contact between rPrP and Fe(III). In this study, differentially expressed genes correlated with prion degeneration by Fe(III) were identified using Affymetrix microarrays. Following Fe(III) treatment, 97 genes were differentially expressed, including 85 upregulated genes and 12 downregulated genes (≥1.5-fold change in expression). However, Fe(II) treatment produced moderate alterations in gene expression without inducing dramatic alterations in gene expression profiles. Moreover, functional grouping of identified genes indicated that the differentially regulated genes were highly associated with cell growth, cell maintenance, and intra- and extracellular transport. These findings showed that Fe(III) may influence the expression of genes involved in PrP folding by redox mechanisms. The identification of genes with altered expression patterns in neural cells may provide insights into PrP conversion mechanisms during the development and progression of prion-related diseases.

Rational approach to an antiprion compound with a multiple mechanism of action

Background: The main pathogenic event of prion disorders has been identified in the deposition of the disease-associated prion protein (PrPSc), which is accompanied by metal dyshomeostasis. Results: The multitarget-directed ligand 1, designed by combining a heteroaromatic prion recognition motif to an 8-hydroxyquinoline metal chelator, has been developed as a potential antiprion disease-modifying agent. Importantly, 1 was found to effectively clear PrPSc from scrapie-infected cells, and, at the same time, inhibit metal-induced prion aggregation and reactive oxygen species generation. 1 was also characterized in terms of pharmacokinetic properties in a preliminary in vitro investigation. Conclusion: Compound 1 has emerged as a suitable lead candidate against prion diseases and as a good starting point for a further optimization process.

Role of Oxidative RNA Damage in Chronic-Degenerative Diseases

Normal cellular metabolism and exposure to ionizing and ultraviolet radiations and exogenous agents produce reactive oxygen species (ROS). Due to their reactivity, they can interact with many critical biomolecules and induce cell damage. The reaction of ROS with free nucleobases, nucleosides, nucleotides, or oligonucleotides can generate numerous distinct modifications in nucleic acids. Oxidative damage to DNA has been widely investigated and is strongly implicated in the development of many chronic-degenerative diseases. In contrast, RNA damage is a poorly examined field in biomedical research. In this review, I discuss the importance of RNA as a target of oxidative damage and the role of oxidative damage to RNA in the pathogenesis of some chronic-degenerative diseases, such as neurological disorders, atherosclerosis, and cancer. Furthermore, I review recent evidence suggesting that RNA may be the target for toxic agents and indicating RNA degradation as a powerful tool to treat any pathology in which there is an aberrant expression of mRNA and/or its gene products.

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.

Antioxidant and Metal Chelation-Based Therapies in the Treatment of Prion Disease

Many neurodegenerative disorders involve the accumulation of multimeric assemblies and amyloid derived from misfolded conformers of constitutively expressed proteins. In addition, the brains of patients and experimental animals afflicted with prion disease display evidence of heightened oxidative stress and damage, as well as disturbances to transition metal homeostasis. Utilising a variety of disease model paradigms, many laboratories have demonstrated that copper can act as a cofactor in the antioxidant activity displayed by the prion protein while manganese has been implicated in the generation and stabilisation of disease-associated conformers. This and other evidence has led several groups to test dietary and chelation therapy-based regimens to manipulate brain metal concentrations in attempts to influence the progression of prion disease in experimental mice. Results have been inconsistent. This review examines published data on transition metal dyshomeostasis, free radical generation and subsequent oxidative damage in the pathogenesis of prion disease. It also comments on the efficacy of trialed therapeutics chosen to combat such deleterious changes.

The iron regulatory capability of the major protein participants in prevalent neurodegenerative disorders

As with most bioavailable transition metals, iron is essential for many metabolic processes required by the cell but when left unregulated is implicated as a potent source of reactive oxygen species. It is uncertain whether the brain’s evident vulnerability to reactive species-induced oxidative stress is caused by a reduced capability in cellular response or an increased metabolic activity. Either way, dys-regulated iron levels appear to be involved in oxidative stress provoked neurodegeneration. As in peripheral iron management, cells within the central nervous system tightly regulate iron homeostasis via responsive expression of select proteins required for iron flux, transport and storage. Recently proteins directly implicated in the most prevalent neurodegenerative diseases, such as amyloid-β precursor protein, tau, α-synuclein, prion protein and huntingtin, have been connected to neuronal iron homeostatic control. This suggests that disrupted expression, processing, or location of these proteins may result in a failure of their cellular iron homeostatic roles and augment the common underlying susceptibility to neuronal oxidative damage that is triggered in neurodegenerative disease.

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.

Markers of oxidant stress that are clinically relevant in aging and age-related disease

Despite the long held hypothesis that oxidant stress results in accumulated oxidative damage to cellular macromolecules and subsequently to aging and age-related chronic disease, it has been difficult to consistently define and specifically identify markers of oxidant stress that are consistently and directly linked to age and disease status. Inflammation because it is also linked to oxidant stress, aging, and chronic disease also plays an important role in understanding the clinical implications of oxidant stress and relevant markers. Much attention has focused on identifying specific markers of oxidative stress and inflammation that could be measured in easily accessible tissues and fluids (lymphocytes, plasma, serum). The purpose of this review is to discuss markers of oxidant stress used in the field as biomarkers of aging and age-related diseases, highlighting differences observed by race when data is available. We highlight DNA, RNA, protein, and lipid oxidation as measures of oxidative stress, as well as other well-characterized markers of oxidative damage and inflammation and discuss their strengths and limitations. We present the current state of the literature reporting use of these markers in studies of human cohorts in relation to age and age-related disease and also with a special emphasis on differences observed by race when relevant.

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.

RNA modifications by oxidation: a novel disease mechanism?

The past decade has provided exciting insights into a novel class of central (small) RNA molecules intimately involved in gene regulation. Only a small percentage of our DNA is translated into proteins by mRNA, yet 80% or more of the DNA is transcribed into RNA, and this RNA has been found to encompass various classes of novel regulatory RNAs, including, e.g., microRNAs. It is well known that DNA is constantly oxidized and repaired by complex genome maintenance mechanisms. Analogously, RNA also undergoes significant oxidation, and there are now convincing data suggesting that oxidation, and the consequent loss of integrity of RNA, is a mechanism for disease development. Oxidized RNA is found in a large variety of diseases, and interest has been especially devoted to degenerative brain diseases such as Alzheimer disease, in which up to 50-70% of specific mRNA molecules are reported oxidized, whereas other RNA molecules show virtually no oxidation. The iron-storage disease hemochromatosis exhibits the most prominent general increase in RNA oxidation ever observed. Oxidation of RNA primarily leads to strand breaks and to oxidative base modifications. Oxidized mRNA is recognized by the ribosomes, but the oxidation results in ribosomal stalling and dysfunction, followed by decreased levels of functional protein as well as the production of truncated proteins that do not undergo proper folding and may result in protein aggregation within the cell. Ribosomal dysfunction may also signal apoptosis by p53-independent pathways. There are very few reports on interventions that reduce RNA oxidation, one interesting observation being a reduction in RNA oxidation by ingestion of raw olive oil. High urinary excretion of 8-oxo-guanosine, a biomarker for RNA oxidation, is highly predictive of death in newly diagnosed type 2 diabetics; this demonstrates the clinical relevance of RNA oxidation. Taken collectively the available data suggest that RNA oxidation is a contributing factor in several diseases such as diabetes, hemochromatosis, heart failure, and β-cell destruction. The mechanism involves free iron and hydrogen peroxide from mitochondrial dysfunction that together lead to RNA oxidation that in turn gives rise to truncated proteins that may cause aggregation. Thus RNA oxidation may well be an important novel contributing mechanism for several diseases.

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.

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.

Gerstmann-Straüssler-Scheinker PRNP P102L-129V mutation

Neuropathological and biochemical studies in a case of Gerstmann-Straüssler-Scheinker disease bearing the PRNP P102L-129V mutation showed numerous multicentric PrPres in the cerebral cortex, striatum, thalamus and cerebellum, PrPres globular deposits in the anterior and posterior horns of the spinal cord, and multiple granular PrPres deposits in the grey and white matter of the encephalon and spinal cord. Western blots with antiPrPres antibodies revealed several weak bands ranging from 36 to 66 kDa, weak bands of 29 and 24 kDa, a strong band of about 20 kDa, a low band of molecular weight around 15 kDa and a weaker band of about 7 kDa. Spongiform degeneration was absent. Hyper-phosphorylated 3R and 4R tau occurred in dystrophic neurites surrounding PrPres plaques, neuropil threads and, to a lesser degree, in the form of neurofibrillary tangles. Gel electrophoresis of sarkosyl-insoluble fractions and western blotting with anti-phospho-tau antibodies showed a pattern similar to that seen in Alzheimer disease cases run in parallel. Dystrophic neurites in the vicinity of PrPres plaques were enriched in voltage dependent anion channel thus suggesting abnormal accumulation of mitochondria. These changes were associated with increased oxidative damage in neurons and astrocytes, Finally, increased expression of active stress kinases, that have the capacity to phosphorylate tau in vitro, p38 (p-38-P) and SAPK/ JNK (SAPK/JNK-P) was found in cell processes surrounding PrP plaques. Together, these observations provide evidences of mitochondrial abnormalities, and increased oxidative stress damage and oxidative stress responses in GSS bearing the PRNP P102L-129V mutation.

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.

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.

Oxidative damage to RNA in neurodegenerative diseases

Since 1999, oxidative damage to RNA molecules has been described in several neurological diseases including Alzheimer's disease, Parkinson's disease, Down syndrome, dementia with Lewy bodies, prion disease, subacute sclerosing panencephalitis, and xeroderma pigmentosum. An early involvement of RNA oxidation of vulnerable neuronal population in the neurodegenerative diseases has been demonstrated, which is strongly supported by a recent observation of increased RNA oxidation in brains of subjects with mild cognitive impairment. Until recently, little is known about consequences and cellular handling of the RNA damage. However, increasing body of evidence suggests detrimental effects of the RNA damage in protein synthesis and the existence of several coping mechanisms including direct repair and avoiding the incorporation of the damaged ribonucleotides into translational machinery. Further investigations toward understanding of the consequences and cellular handling mechanisms of the oxidative RNA damage may provide significant insights into the pathogenesis and therapeutic strategies of the neurodegenerative diseases.

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.

Oxidative damage to RNA: mechanisms, consequences, and diseases

Overproduction of free radicals can damage cellular components resulting in progressive physiological dysfunction, which has been implicated in many human diseases. Oxidative damage to RNA received little attention until the past decade. Recent studies indicate that RNA, such as messenger RNA and ribosomal RNA, is very vulnerable to oxidative damage. RNA oxidation is not a consequence of dying cells but an early event involved in pathogenesis. Oxidative modification to RNA results in disturbance of the translational process and impairment of protein synthesis, which can cause cell deterioration or even cell death. In this review, we discuss the mechanisms of oxidative damage to RNA and the possible biological consequences of damaged RNA. Furthermore, we review recent evidence suggesting that oxidative damage to RNA may contribute to progression of many human diseases.

Involvement of Dab1 in APP processing and beta-amyloid deposition in sporadic Creutzfeldt-Jakob patients

Alzheimer's disease and prion pathologies (e.g., Creutzfeldt-Jakob disease (CJD)) display profound neural lesions associated with aberrant protein processing and extracellular amyloid deposits. Dab1 has been implicated in the regulation of amyloid precursor protein (APP), but a direct link between human prion diseases and Dab1/APP interactions has not been published. Here we examined this putative relationship in 17 cases of sporadic CJD (sCJD) post-mortem. Biochemical analyses of brain tissue revealed two groups, which also correlated with PrP(sc) types 1 and 2. One group with PrP(sc) type 1 showed increased Dab1 phosphorylation and lower betaCTF production with an absence of Abeta deposition. The second sCJD group, which carried PrP(sc) type 2, showed lower levels of Dab1 phosphorylation and betaCTF production, and Abeta deposition. Thus, the present observations suggest a correlation between Dab1 phosphorylation, Abeta deposition and PrP(sc) type in sCJD.

RNA oxidation in Alzheimer disease and related neurodegenerative disorders

RNA oxidation and its biological effects are less well studied compared to DNA oxidation. However, RNA may be more susceptible to oxidative insults than DNA, for RNA is largely single-stranded and its bases are not protected by hydrogen bonding and less protected by specific proteins. Also, cellular RNA locates in the vicinity of mitochondria, the primary source of reactive oxygen species. Oxidative modification can occur not only in protein-coding RNAs, but also in non-coding RNAs that have been recently revealed to contribute towards the complexity of the mammalian brain. Damage to coding and non-coding RNAs will cause errors in proteins and disturbances in the regulation of gene expression. While less lethal than mutations in the genome and not inheritable, such sublethal damage to cells might be associated with underlying mechanisms of degeneration, especially age-associated neurodegeneration that is commonly found in the elderly population. Indeed, oxidative RNA damage has been described recently in most of the common neurodegenerative disorders including Alzheimer disease, Parkinson disease, dementia with Lewy bodies and amyotrophic lateral sclerosis. Of particular interest, the accumulating evidence obtained from studies on either human samples or experimental models coincidentally suggests that oxidative RNA damage is a feature in vulnerable neurons at early-stage of these neurodegenerative disorders, indicating that RNA oxidation actively contributes to the onset or the development of the disorders. Further investigations aimed at understanding of the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative disorders and lead to better therapeutic strategies.

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

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

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.

RNA oxidation: a contributing factor or an epiphenomenon in the process of neurodegeneration

In the past decade, RNA oxidation has caught the attention of many researchers, working to uncover its role in the pathogenesis of neurodegenerative diseases. It has been well documented that RNA oxidation is involved in a wide variety of neurological diseases and is an early event in the process of neurodegeneration. The analysis of oxidized RNA species revealed that at least messenger RNA (mRNA) and ribosomal RNA (rRNA) are damaged in several neurodegenerative diseases, including Alzheimer's disease and amyotrophic lateral sclerosis (ALS). The magnitude of the RNA oxidation, at least in mRNA, is significantly high at the early stage of the disease. Oxidative damage to mRNA is not random but selective and many oxidized mRNAs are related to the pathogenesis of the disease. Several studies have suggested that oxidative modification of RNA affects the translational process and consequently produces less protein and/or defective protein. Furthermore, several proteins have been identified to be involved in handling of damaged RNA. Although a growing body of studies suggests that oxidative damage to RNA may be associated with neuron deterioration, further investigation and solid evidence are needed. In addition, further uncovering of the consequences and cellular handling of the oxidatively damaged RNA should be important focuses in this area and may provide significant insights into the pathogenesis of neurodegenerative diseases.

Increased oxidation, glycoxidation, and lipoxidation of brain proteins in prion disease

The basic molecular underpinnings of the pathological changes that unfold in prion disease remain elusive. A key role of increased oxidative stress has been hypothesized. Given the transient nature of most intermediate molecules implicated, increased oxidative stress is better assessed by quantitating the damage it causes to macromolecules. We used mass spectrometry-based methods to measure specific products of protein oxidation, glycoxidation, and lipoxidation in brains from patients suffering from Creutzfeldt-Jakob disease and Syrian hamsters affected by scrapie. In both cases, increased amounts of glutamic and aminoadipic semialdehydes, products of metal-catalyzed oxidation, malondialdehydelysine (a product of lipoxidation), N-epsilon-carboxyethyllysine (a product of glycoxidation), and N-epsilon-carboxymethyllysine (generated by lipoxidation and glycoxidation) were measured. PrP(Sc), the infectious isoform of the prion protein that accumulates in prion disease, was itself shown to be a target of increased oxidative modification. These changes were accompanied by alterations in fatty acid composition and increased phosphorylation of ERK(1/2) and p38, protein kinases known to respond to increased flows of ROS. These data support an important role of oxidative damage in the pathology of prion disease.

Modeling by assembly and molecular dynamics simulations of the low Cu2+ occupancy form of the mammalian prion protein octarepeat region: gaining insight into Cu2+-mediated beta-cleavage

The prion protein has garnered considerable interest because of its involvement in prion disease as well as its unresolved cellular function. The octarepeat region in the flexible N-domain is capable of binding copper through multiple coordination modes. Under conditions of low pH and low Cu(2+) concentration, the four octarepeats (ORs) cooperatively coordinate a single copper ion. Based on the average structure of the PHGG and GWGQ portions of a copper-free OR(2) model from molecular dynamics simulations, the starting structures of the OR(4) complex could be constructed by assembling the repeating structure of PHGG and GWGQ fragments. The resulting model contains a preformed site suitable for Cu(2+) coordination. Molecular dynamics simulations of Cu(2+) bound to the assembled OR(4) model (Cu:OR(4)) reveal a close association of specific Trp and Gly residues with the Cu(2+) center. This low Cu(2+)-occupancy form of prion protein is redox-active and can readily initiate cleavage of the OR region, mediated by reactive oxygen species generated by Cu(+). The OR region is known to be required for beta-cleavage, as are the Trp residues within the OR region. The beta-cleaved form of the prion protein accumulates in amyloid fibrils. Hence, the close approach of Trp and Gly residues to the Cu(2+) coordination site in the low Cu(2+)-occupancy form of the OR region may signal an important interaction for the initiation of prion disease.

Messenger RNA oxidation occurs early in disease pathogenesis and promotes motor neuron degeneration in ALS

Background

Accumulating evidence indicates that RNA oxidation is involved in a wide variety of neurological diseases and may be associated with neuronal deterioration during the process of neurodegeneration. However, previous studies were done in postmortem tissues or cultured neurons. Here, we used transgenic mice to demonstrate the role of RNA oxidation in the process of neurodegeneration.

Methodology/Principal Findings

We demonstrated that messenger RNA (mRNA) oxidation is a common feature in amyotrophic lateral sclerosis (ALS) patients as well as in many different transgenic mice expressing familial ALS-linked mutant copper-zinc superoxide dismutase (SOD1). In mutant SOD1 mice, increased mRNA oxidation primarily occurs in the motor neurons and oligodendrocytes of the spinal cord at an early, pre-symptomatic stage. Identification of oxidized mRNA species revealed that some species are more vulnerable to oxidative damage, and importantly, many oxidized mRNA species have been implicated in the pathogenesis of ALS. Oxidative modification of mRNA causes reduced protein expression. Reduced mRNA oxidation by vitamin E restores protein expression and partially protects motor neurons.

Conclusion/Significance

These findings suggest that mRNA oxidation is an early event associated with motor neuron deterioration in ALS, and may be also a common early event preceding neuron degeneration in other neurological diseases.

A manganese-superoxide dismutase/catalase mimetic extends survival in a mouse model of human prion disease

Animal models, and human postmortem studies, of prion disease have demonstrated the presence of heightened oxidative stress in the brain, with additional findings supporting the likelihood that the normal isoform of prion protein directly contributes to neuronal antioxidant defences. Although such data are consistent with the postulate that oxidative stress plays a salient pathogenic role in prion disease, it remains possible that oxidative damage represents a secondary or relatively less important phenomenon in neurons already rendered dysfunctional from other primary insults. To provide further insights into the relative pathogenic importance of oxidative stress, we employed a potent manganese-superoxide dismutase/catalase mimetic, EUK-189, as a therapeutic in our mouse model of human prion disease. A significant but relatively modest prolongation of survival in EUK-189-treated mice was observed, which correlated with reductions in oxidative, especially nitrative, damage to proteins when compared to untreated disease controls. Lesion profiling also revealed reductions in spongiform change in specific brain regions of terminally sick EUK-189-treated mice. Our results are consistent with heightened oxidative stress playing a pathogenic role in prion disease but underscore the need for more biologically potent and, most likely, broader spectrum antioxidant treatments if more successful amelioration is to be achieved.

Sublethal RNA oxidation as a mechanism for neurodegenerative disease

Although cellular RNA is subjected to the same oxidative insults as DNA and other cellular macromolecules, oxidative damage to RNA has not been a major focus in investigations of the biological consequences of free radical damage. In fact, because it is largely single-stranded and its bases lack the protection of hydrogen bonding and binding by specific proteins, RNA may be more susceptible to oxidative insults than is DNA. Oxidative damage to protein-coding RNA or non-coding RNA will, in turn, potentially cause errors in proteins and/or dysregulation of gene expression. While less lethal than mutations in the genome, such sublethal insults to cells might be associated with underlying mechanisms of several chronic diseases, including neurodegenerative disease. Recently, oxidative RNA damage has been described in several neurodegenerative diseases including Alzheimer disease, Parkinson disease, dementia with Lewy bodies, and prion diseases. Of particular interest, oxidative RNA damage can be demonstrated in vulnerable neurons early in disease, suggesting that RNA oxidation may actively contribute to the onset of the disease. An increasing body of evidence suggests that, mechanistically speaking, the detrimental effects of oxidative RNA damage to protein synthesis are attenuated, at least in part, by the existence of protective mechanisms that prevent the incorporation of the damaged ribonucleotides into the translational machinery. Further investigations aimed at understanding the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative and other degenerative diseases and lead to better therapeutic strategies.

Cellular prion protein protects against reactive-oxygen-species-induced DNA damage

Although the cellular form of the prion protein (PrPC) is critical for the development of prion disease through its conformational conversion into the infectious form (PrPSc), the physiological role of PrPC is less clear. Using alkaline single-cell gel electrophoresis (the Comet assay), we show that expression of PrPC protects human neuroblastoma SH-SY5Y cells against DNA damage under basal conditions and following exposure to reactive oxygen species, either hydroxyl radicals following exposure to Cu2+ or Fe2+ or singlet oxygen following exposure to the photosensitizer methylene blue and white light. Cells expressing either PrPDeltaoct which lacks the octapeptide repeats or the prion-disease-associated mutants A116V or PG14 had increased levels of DNA damage compared to cells expressing PrPC. In PrPSc-infected mouse ScN2a cells there was a significant increase in DNA damage over noninfected N2a cells (median tail DNA 2.87 and 7.33%, respectively). Together, these data indicate that PrPC has a critical role to play in protecting cells against reactive-oxygen-species-mediated DNA damage; a function which requires the octapeptide repeats in the protein, is lost in disease-associated mutants of the protein or upon conversion to PrPSc, and thus provide further support for the neuroprotective role for PrPC.

Modulation of proteinase K-resistant prion protein in cells and infectious brain homogenate by redox iron: implications for prion replication and disease pathogenesis

The principal infectious and pathogenic agent in all prion disorders is a beta-sheet-rich isoform of the cellular prion protein (PrP(C)) termed PrP-scrapie (PrP(Sc)). Once initiated, PrP(Sc) is self-replicating and toxic to neuronal cells, but the underlying mechanisms remain unclear. In this report, we demonstrate that PrP(C) binds iron and transforms to a PrP(Sc)-like form (*PrP(Sc)) when human neuroblastoma cells are exposed to an inorganic source of redox iron. The *PrP(Sc) thus generated is itself redox active, and it induces the transformation of additional PrP(C), simulating *PrP(Sc) propagation in the absence of brain-derived PrP(Sc). Moreover, limited depletion of iron from prion disease-affected human and mouse brain homogenates and scrapie-infected mouse neuroblastoma cells results in 4- to 10-fold reduction in proteinase K (PK)-resistant PrP(Sc), implicating redox iron in the generation, propagation, and stability of PK-resistant PrP(Sc). Furthermore, we demonstrate increased redox-active ferrous iron levels in prion disease-affected brains, suggesting that accumulation of PrP(Sc) is modulated by the combined effect of imbalance in brain iron homeostasis and the redox-active nature of PrP(Sc). These data provide information on the mechanism of replication and toxicity by PrP(Sc), and they evoke predictable and therapeutically amenable ways of modulating PrP(Sc) load.

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.

Prion protein reduces both oxidative and non-oxidative copper toxicity

The prion protein is a membrane tethered glycoprotein that binds copper. Conversion to an abnormal isoform is associated with neurodegenerative diseases known as prion diseases. Expression of the prion protein has been suggested to prevent cell death caused by oxidative stress. Using cell based models we investigated the potential of the prion protein to protect against copper toxicity. Although prion protein expression effectively protected neurones from copper toxicity, this protection was not necessarily associated with reduction in oxidative damage. We also showed that glycine and the prion protein could both protect neuronal cells from oxidative stress. Only the prion protein could protect these cells from the toxicity of copper. In contrast glycine increased copper toxicity without any apparent oxidative stress or lipid peroxidation. Mutational analysis showed that protection by the prion protein was dependent upon the copper binding octameric repeat region. Our findings demonstrate that copper toxicity can be independent of measured oxidative stress and that prion protein expression primarily protects against copper toxicity independently of the mechanism of cell death.

Oxidation, glycoxidation, lipoxidation, nitration, and responses to oxidative stress in the cerebral cortex in Creutzfeldt-Jakob disease

Gel electrophoresis and Western blotting of frontal cortex homogenates have been carried out in sporadic Creutzfeldt-Jakob disease (CJD) cases and age-matched controls to gain understanding of the expression of glycation-end products (AGEs). N-Carboxymethyl-lysine (CML) and N-carboxyethyl-lysine (CEL) were used as markers of glycoxidation; 4-hydroxynonenal (4-HNE) and malondialdehyde-lysine (MDAL) as markers of lipoxidation; and nitrotyrosine (N-tyr) and neuronal, endothelial and inducible nitric oxide synthase (nNOS, eNos and iNos) as markers of protein nitration and as sources of NO production, respectively. Age receptor (RAGE) and Cu/Zn superoxide dismutase (SOD1) and Mn superoxide dismutase (SOD2) expression levels were also examined. The results showed a significant increase in the expression levels of AGE (p<0.05), CEL (p<0.001), RAGE (p<0.05), HNE-modified proteins (p<0.01), nNOS, iNOS and eNOS (p<0.01 and p<0.05, respectively), N-tyr (p<0.05), and SOD1 (p<0.05) and SOD2 (p<0.05). No relationship was observed between PrP genotype, PrP type, PrP burden, and expression levels of oxidative stress markers. The present findings demonstrate oxidative, glycoxidative, lipoxidative and nitrative protein damage, accompanied by increased oxidative responses, in the cerebral cortex in sporadic CJD. These results provide support for the concept that oxidative stress may have important implications in the pathogenesis of prion diseases.