Oxidative impairment in scrapie-infected mice is associated with brain metals perturbations and altered antioxidant activities

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

PrP (58-93) peptide from unstructured N-terminal domain of human prion protein forms amyloid-like fibrillar structures in the presence of Zn2+ ions

Many transition metal ions modulate the aggregation of different amyloid peptides. Substoichiometric zinc concentrations can inhibit aggregation, while an excess of zinc can accelerate the formation of cytotoxic fibrils. In this study, we report the fibrillization of the octarepeat domain to amyloid-like structures. Interestingly, this self-assembling process occurred only in the presence of Zn(ii) ions. The formed peptide aggregates are able to bind amyloid specific dyes thioflavin T and Congo red. Atomic force microscopy and transmission electron microscopy revealed the formation of long, fibrillar structures. X-ray diffraction and Fourier transform infrared spectroscopy studies of the formed assemblies confirmed the presence of cross-β structure. Two-component analysis of synchrotron radiation SAXS data provided the evidence for a direct decrease in monomeric peptide species content and an increase in the fraction of aggregates as a function of Zn(ii) concentration. These results could shed light on Zn(ii) as a toxic agent and on the metal ion induced protein misfolding in prion diseases.

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.

Substitutions of PrP N-terminal histidine residues modulate scrapie disease pathogenesis and incubation time in transgenic mice

Prion diseases have been linked to impaired copper homeostasis and copper induced-oxidative damage to the brain. Divalent metal ions, such as Cu2+ and Zn2+, bind to cellular prion protein (PrPC) at octapeptide repeat (OR) and non-OR sites within the N-terminal half of the protein but information on the impact of such binding on conversion to the misfolded isoform often derives from studies using either OR and non-OR peptides or bacterially-expressed recombinant PrP. Here we created new transgenic mouse lines expressing PrP with disrupted copper binding sites within all four histidine-containing OR's (sites 1-4, H60G, H68G, H76G, H84G, "TetraH>G" allele) or at site 5 (composed of residues His-95 and His-110; "H95G" allele) and monitored the formation of misfolded PrP in vivo. Novel transgenic mice expressing PrP(TetraH>G) at levels comparable to wild-type (wt) controls were susceptible to mouse-adapted scrapie strain RML but showed significantly prolonged incubation times. In contrast, amino acid replacement at residue 95 accelerated disease progression in corresponding PrP(H95G) mice. Neuropathological lesions in terminally ill transgenic mice were similar to scrapie-infected wt controls, but less severe. The pattern of PrPSc deposition, however, was not synaptic as seen in wt animals, but instead dense globular plaque-like accumulations of PrPSc in TgPrP(TetraH>G) mice and diffuse PrPSc deposition in (TgPrP(H95G) mice), were observed throughout all brain sections. We conclude that OR and site 5 histidine substitutions have divergent phenotypic impacts and that cis interactions between the OR region and the site 5 region modulate pathogenic outcomes by affecting the PrP globular domain.

Metal Dyshomeostasis and Their Pathological Role in Prion and Prion-Like Diseases: The Basis for a Nutritional Approach

Metal ions are key elements in organisms' life acting like cofactors of many enzymes but they can also be potentially dangerous for the cell participating in redox reactions that lead to the formation of reactive oxygen species (ROS). Any factor inducing or limiting a metal dyshomeostasis, ROS production and cell injury may contribute to the onset of neurodegenerative diseases or play a neuroprotective action. Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of fatal neurodegenerative disorders affecting the central nervous system (CNS) of human and other mammalian species. The causative agent of TSEs is believed to be the scrapie prion protein PrP^Sc, the β sheet-rich pathogenic isoform produced by the conformational conversion of the α-helix-rich physiological isoform PrP^C. The peculiarity of PrP^Sc is its ability to self-propagate in exponential fashion in cells and its tendency to precipitate in insoluble and protease-resistance amyloid aggregates leading to neuronal cell death. The expression “prion-like diseases” refers to a group of neurodegenerative diseases that share some neuropathological features with prion diseases such as the involvement of proteins (α-synuclein, amyloid β, and tau) able to precipitate producing amyloid deposits following conformational change. High social impact diseases such as Alzheimer's and Parkinson's belong to prion-like diseases. Accumulating evidence suggests that the exposure to environmental metals is a risk factor for the development of prion and prion-like diseases and that metal ions can directly bind to prion and prion-like proteins affecting the amount of amyloid aggregates. The diet, source of metal ions but also of natural antioxidant and chelating agents such as polyphenols, is an aspect to take into account in addressing the issue of neurodegeneration. Epidemiological data suggest that the Mediterranean diet, based on the abundant consumption of fresh vegetables and on low intake of meat, could play a preventive or delaying role in prion and prion-like neurodegenerative diseases. In this review, metal role in the onset of prion and prion-like diseases is dealt with from a nutritional, cellular, and molecular point of view.

Production and removal of superoxide anion radical by artificial metalloenzymes and redox-active metals

Generation of reactive oxygen species is useful for various medical, engineering and agricultural purposes. These include clinical modulation of immunological mechanism, enhanced degradation of organic compounds released to the environments, removal of microorganisms for the hygienic purpose, and agricultural pest control; both directly acting against pathogenic microorganisms and indirectly via stimulation of plant defense mechanism represented by systemic acquired resistance and hypersensitive response. By aiming to develop a novel classes of artificial redox-active biocatalysts involved in production and/or removal of superoxide anion radicals, recent attempts for understanding and modification of natural catalytic proteins and functional DNA sequences of mammalian and plant origins are covered in this review article.

Prion Protein Does Not Confer Resistance to Hippocampus-Derived Zpl Cells against the Toxic Effects of Cu2+, Mn2+, Zn2+ and Co2+ Not Supporting a General Protective Role for PrP in Transition Metal Induced Toxicity

The interactions of transition metals with the prion protein (PrP) are well-documented and characterized, however, there is no consensus on their role in either the physiology of PrP or PrP-related neurodegenerative disorders. PrP has been reported to protect cells from the toxic stimuli of metals. By employing a cell viability assay, we examined the effects of various concentrations of Cu2+, Zn2+, Mn2+, and Co2+ on Zpl (Prnp-/-) and ZW (Prnp+/+) hippocampus-derived mouse neuronal cells. Prnp-/- Zpl cells were more sensitive to all four metals than PrP-expressing Zw cells. However, when we introduced PrP or only the empty vector into Zpl cells, we could not discern any protective effect associated with the presence of PrP. This observation was further corroborated when assessing the toxic effect of metals by propidium-iodide staining and fluorescence activated cell sorting analysis. Thus, our results on this mouse cell culture model do not seem to support a strong protective role for PrP against transition metal toxicity and also emphasize the necessity of extreme care when comparing cells derived from PrP knock-out and wild type mice.

Prion infection in cells is abolished by a mutated manganese transporter but shows no relation to zinc

The cellular prion protein has been identified as a metalloprotein that binds copper. There have been some suggestions that prion protein also influences zinc and manganese homeostasis. In this study we used a series of cell lines to study the levels of zinc and manganese under different conditions. We overexpressed either the prion protein or known transporters for zinc and manganese to determine relations between the prion protein and both manganese and zinc homeostasis. Our observations supported neither a link between the prion protein and zinc metabolism nor any effect of altered zinc levels on prion protein expression or cellular infection with prions. In contrast we found that a gain of function mutant of a manganese transporter caused reduction of manganese levels in prion infected cells, loss of observable PrP(Sc) in cells and resistance to prion infection. These studies strengthen the link between manganese and prion disease.

Antioxidant peroxiredoxin 6 protein rescues toxicity due to oxidative stress and cellular hypoxia in vitro, and attenuates prion-related pathology in vivo

Protein misfolding, mitochondrial dysfunction and oxidative stress are common pathomechanisms that underlie neurodegenerative diseases. In prion disease, central to these processes is the post-translational transformation of cellular prion protein (PrP(c)) to the aberrant conformationally altered isoform; PrP(Sc). This can trigger oxidative reactions and impair mitochondrial function by increasing levels of peroxynitrite, causing damage through formation of hydroxyl radicals or via nitration of tyrosine residues on proteins. The 6 member Peroxiredoxin (Prdx) family of redox proteins are thought to be critical protectors against oxidative stress via reduction of H2O2, hydroperoxides and peroxynitrite. In our in vitro studies cellular metabolism of SK-N-SH human neuroblastoma cells was significantly decreased in the presence of H2O2 (oxidative stressor) or CoCl2 (cellular hypoxia), but was rescued by treatment with exogenous Prdx6, suggesting that its protective action is in part mediated through a direct action. We also show that CoCl2-induced apoptosis was significantly decreased by treatment with exogenous Prdx6. We proposed a redox regulator role for Prdx6 in regulating and maintaining cellular homeostasis via its ability to control ROS levels that could otherwise accelerate the emergence of prion-related neuropathology. To confirm this, we established prion disease in mice with and without astrocyte-specific antioxidant protein Prdx6, and demonstrated that expression of Prdx6 protein in Prdx6 Tg ME7-animals reduced severity of the behavioural deficit, decreased neuropathology and increased survival time compared to Prdx6 KO ME7-animals. We conclude that antioxidant Prdx6 attenuates prion-related neuropathology, and propose that augmentation of endogenous Prdx6 protein represents an attractive adjunct therapeutic approach for neurodegenerative diseases.

Hematological shift in goat kids naturally devoid of prion protein

The physiological role of the cellular prion protein (PrP^C) is incompletely understood. The expression of PrP^C in hematopoietic stem cells and immune cells suggests a role in the development of these cells, and in PrP^C knockout animals altered immune cell proliferation and phagocytic function have been observed. Recently, a spontaneous nonsense mutation at codon 32 in the PRNP gene in goats of the Norwegian Dairy breed was discovered, rendering homozygous animals devoid of PrP^C. Here we report hematological and immunological analyses of homozygous goat kids lacking PrP^C (PRNP^Ter/Ter) compared to heterozygous (PRNP^+/Ter) and normal (PRNP^+/+) kids. Levels of cell surface PrP^C and PRNP mRNA in peripheral blood mononuclear cells (PBMCs) correlated well and were very low in PRNP^Ter/Ter, intermediate in PRNP^+/Ter and high in PRNP^+/+ kids. The PRNP^Ter/Ter animals had a shift in blood cell composition with an elevated number of red blood cells (RBCs) and a tendency toward a smaller mean RBC volume (P = 0.08) and an increased number of neutrophils (P = 0.068), all values within the reference ranges. Morphological investigations of blood smears and bone marrow imprints did not reveal irregularities. Studies of relative composition of PBMCs, phagocytic ability of monocytes and T-cell proliferation revealed no significant differences between the genotypes. Our data suggest that PrP^C has a role in bone marrow physiology and warrant further studies of PrP^C in erythroid and immune cell progenitors as well as differentiated effector cells also under stressful conditions. Altogether, this genetically unmanipulated PrP^C-free animal model represents a unique opportunity to unveil the enigmatic physiology and function of PrP^C.

The secret life of extracellular vesicles in metal homeostasis and neurodegeneration

Biologically active metals such as copper, zinc and iron are fundamental for sustaining life in different organisms with the regulation of cellular metal homeostasis tightly controlled through proteins that coordinate metal uptake, efflux and detoxification. Many of the proteins involved in either uptake or efflux of metals are localised and function on the plasma membrane, traffic between intracellular compartments depending upon the cellular metal environment and can undergo recycling via the endosomal pathway. The biogenesis of exosomes also occurs within the endosomal system, with several major neurodegenerative disease proteins shown to be released in association with these vesicles, including the amyloid-β (Aβ) peptide in Alzheimer's disease and the infectious prion protein involved in Prion diseases. Aβ peptide and the prion protein also bind biologically active metals and are postulated to play important roles in metal homeostasis. In this review, we will discuss the role of extracellular vesicles in Alzheimer's and Prion diseases and explore their potential contribution to metal homeostasis.

Effects of heme-PrP complex on cell-free conversion and peroxidase-linked immunodetection of prions in blood-based assays

Prion protein (PrP) binding to natural and synthetic porphyrins has been previously demonstrated but the effects of endogenous heme interactions with PrP remain uncertain. This study investigated implications of this interaction in blood-based peroxidase-linked prion immunodetection and seeded conversion of cellular prion (PrP(C)) into disease associated form (PrP(Sc)). Heme binding to recombinant PrP(C) enhanced intrinsic peroxidase activity (POD) by 2.5-fold and POD inherent to denatured blood accounted for over 84% of luminol-based substrate oxidation in a prion immunodetection assay. An immuno-capture assay showed that 75-98% of blood POD was attributable to binding of PrP(C) with endogenous heme. Additionally, 10 μM heme inhibited (P<0.05) the seeded conversion of PrP(C) to PrP(Sc) through the protein misfolding cycling amplification assay. We conclude that the observed effects can interfere with cell-free conversion and peroxidase-linked immunodetection of prions in blood-based assays. These results indicate that heme-PrP interactions could modulate intrinsic POD and protect PrP(C) from conversion into PrP(Sc).

Anxiolytic and antidepressant profile of the methanolic extract of Piper nigrum fruits in beta-amyloid (1-42) rat model of Alzheimer's disease

Background

Piper nigrum L. (Piperaceae) is employed in traditional medicine of many countries as analgesic, antiinflammatory, anticonvulsant, antioxidant, antidepressant and cognitive-enhancing agent. This study was undertaken in order to evaluate the possible anxiolytic, antidepressant and antioxidant properties of the methanolic extract of Piper nigrum fruits in beta-amyloid (1–42) rat model of Alzheimer’s disease.

Methods

The anxiolytic- and antidepressant-like effects of the methanolic extract were studied by means of in vivo (elevated plus-maze and forced swimming tests) approaches. Also, the antioxidant activity in the amygdala was assessed using superoxide dismutase, glutathione peroxidase and catalase specific activities, the total content of the reduced glutathione, protein carbonyl and malondialdehyde levels. Statistical analyses were performed using one-way analysis of variance (ANOVA). Significant differences were determined by Tukey’s post hoc test. F values for which p < 0.05 were regarded as statistically significant. Pearson’s correlation coefficient and regression analysis were used in order to evaluate the connection between behavioral measures, the antioxidant defence and lipid peroxidation.

Results

The beta-amyloid (1–42)-treated rats exhibited the following: decrease of the exploratory activity, the percentage of the time spent and the number of entries in the open arm within elevated plus-maze test and decrease of swimming time and increase of immobility time within forced swimming test. Administration of the methanolic extract significantly exhibited anxiolytic- and antidepressant-like effects and also antioxidant potential.

Conclusions

Taken together, our results suggest that the methanolic extract ameliorates beta-amyloid (1–42)-induced anxiety and depression by attenuation of the oxidative stress in the rat amygdala.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

Proteomics analysis of amyloid and nonamyloid prion disease phenotypes reveals both common and divergent mechanisms of neuropathogenesis

Prion diseases are a heterogeneous group of neurodegenerative disorders affecting various mammals including humans. Prion diseases are characterized by a misfolding of the host-encoded prion protein (PrP^C) into a pathological isoform termed PrP^Sc. In wild-type mice, PrP^C is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor and PrP^Sc typically accumulates in diffuse nonamyloid deposits with gray matter spongiosis. By contrast, when mice lacking the GPI anchor are infected with the same prion inoculum, PrP^Sc accumulates in dense perivascular amyloid plaques with little or no gray matter spongiosis. In order to evaluate whether different host biochemical pathways were implicated in these two phenotypically distinct prion disease models, we utilized a proteomics approach. In both models, infected mice displayed evidence of a neuroinflammatory response and complement activation. Proteins involved in cell death and calcium homeostasis were also identified in both phenotypes. However, mitochondrial pathways of apoptosis were implicated only in the nonamyloid form, whereas metal binding and synaptic vesicle transport were more disrupted in the amyloid phenotype. Thus, following infection with a single prion strain, PrP^C anchoring to the plasma membrane correlated not only with the type of PrP^Sc deposition but also with unique biochemical pathways associated with pathogenesis.

The cellular prion protein counteracts cardiac oxidative stress

AIMS

The cellular prion protein, PrP(C), whose aberrant isoforms are related to prion diseases of humans and animals, has a still obscure physiological function. Having observed an increased expression of PrP(C) in two in vivo paradigms of heart remodelling, we focused on isolated mouse hearts to ascertain the capacity of PrP(C) to antagonize oxidative damage induced by ischaemic and non-ischaemic protocols.

METHODS AND RESULTS

Hearts isolated from mice expressing PrP(C) in variable amounts were subjected to different and complementary oxidative perfusion protocols. Accumulation of reactive oxygen species, oxidation of myofibrillar proteins, and cell death were evaluated. We found that overexpressed PrP(C) reduced oxidative stress and cell death caused by post-ischaemic reperfusion. Conversely, deletion of PrP(C) increased oxidative stress during both ischaemic preconditioning and perfusion (15 min) with H2O2. Supporting its relation with intracellular systems involved in oxidative stress, PrP(C) was found to influence the activity of catalase and, for the first time, the expression of p66(Shc), a protein implicated in oxidative stress-mediated cell death.

CONCLUSIONS

Our data demonstrate that PrP(C) contributes to the cardiac mechanisms antagonizing oxidative insults.

Isoprostanes and neuroprostanes as biomarkers of oxidative stress in neurodegenerative diseases

Accumulating data shows that oxidative stress plays a crucial role in neurodegenerative disorders. The literature data indicate that in vivo or postmortem cerebrospinal fluid and brain tissue levels of F2-isoprostanes (F2-IsoPs) especially F4-neuroprotanes (F4-NPs) are significantly increased in some neurodegenerative diseases: multiple sclerosis, Alzheimer's disease, Huntington's disease, and Creutzfeldt-Jakob disease. Central nervous system is the most metabolically active organ of the body characterized by high requirement for oxygen and relatively low antioxidative activity, what makes neurons and glia highly susceptible to destruction by reactive oxygen/nitrogen species and neurodegeneration. The discovery of F2-IsoPs and F4-NPs as markers of lipid peroxidation caused by the free radicals has opened up new areas of investigation regarding the role of oxidative stress in the pathogenesis of human neurodegenerative diseases. This review focuses on the relationship between F2-IsoPs and F4-NPs as biomarkers of oxidative stress and neurodegenerative diseases. We summarize the knowledge of these novel biomarkers of oxidative stress and the advantages of monitoring their formation to better define the involvement of oxidative stress in neurological diseases.

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.

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.

The physiological role of reversible methionine oxidation

Sulfur-containing amino acids such as cysteine and methionine are particularly vulnerable to oxidation. Oxidation of cysteine and methionine in their free amino acid form renders them unavailable for metabolic processes while their oxidation in the protein-bound state is a common post-translational modification in all organisms and usually alters the function of the protein. In the majority of cases, oxidation causes inactivation of proteins. Yet, an increasing number of examples have been described where reversible cysteine oxidation is part of a sophisticated mechanism to control protein function based on the redox state of the protein. While for methionine the dogma is still that its oxidation inhibits protein function, reversible methionine oxidation is now being recognized as a powerful means of triggering protein activity. This mode of regulation involves oxidation of methionine to methionine sulfoxide leading to activated protein function, and inactivation is accomplished by reduction of methionine sulfoxide back to methionine catalyzed by methionine sulfoxide reductases. Given the similarity to thiol-based redox-regulation of protein function, methionine oxidation is now established as a novel mode of redox-regulation of protein function. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Neuronal zinc regulation and the prion protein

Zinc, the most abundant trace metal in the brain, has numerous functions in health and disease. It is released into the synaptic cleft alongside glutamate and this connection between zinc and glutamatergic neurotransmission allows the ion to modulate overall excitability of the brain and influence synaptic plasticity. To maintain healthy synapses, extracellular zinc levels need to be tightly regulated. We recently reported that the cellular prion protein (PrP (C) ) can directly influence neuronal zinc concentrations by promoting zinc uptake via AMPA receptors. The octapeptide repeat region of PrP (C) is involved in zinc sensing or scavenging and the AMPA receptor provides the channel for transport of the metal across the membrane, facilitated by a direct interaction between the N-terminal polybasic region of PrP (C) and AMPA receptors. PrP (C) has been evolutionarily linked to the Zrt/Irt-like protein (ZIP) metal ion transport family with the C-terminus of PrP (C) sharing sequence similarities with the N-terminal extracellular domains of ZIP 5, 6 and 10. By incorporating the properties of ZIP transporters (both zinc sensing and zinc transport) into two existing neuronal proteins, (PrP (C) as zinc sensor, AMPA receptor as zinc transporter), neuronal cells are enhancing their biological efficiency and functionality.

Conformational conversion of prion protein in prion diseases

Prion diseases are a group of infectious fatal neurodegenerative diseases. The conformational conversion of a cellular prion protein (PrP(C)) into an abnormal misfolded isoform (PrP(Sc)) is the key event in prion diseases pathology. Under normal conditions, the high-energy barrier separates PrP(C) from PrP(Sc) isoform. However, pathogenic mutations, modifications as well as some cofactors, such as glycosaminoglycans, nucleic acids, and lipids, could modulate the conformational conversion process. Understanding the mechanism of conformational conversion of prion protein is essential for the biomedical research and the treatment of prion diseases. Particularly, the characterization of cofactors interacting with prion protein might provide new diagnostic and therapeutic strategies.

Cytosolic caspases mediate mislocalised SOD2 depletion in an in vitro model of chronic prion infection

Oxidative stress as a contributor to neuronal death during prion infection is supported by the fact that various oxidative damage markers accumulate in the brain during the course of this disease. The normal cellular substrate of the causative agent, the prion protein, is also linked with protective functions against oxidative stress. Our previous work has found that, in chronic prion infection, an apoptotic subpopulation of cells exhibit oxidative stress and the accumulation of oxidised lipid and protein aggregates with caspase recruitment. Given the likely failure of antioxidant defence mechanisms within apoptotic prion-infected cells, we aimed to investigate the role of the crucial antioxidant pathway components, superoxide dismutases (SOD) 1 and 2, in an in vitro model of chronic prion infection. Increased total SOD activity, attributable to SOD1, was found in the overall population coincident with a decrease in SOD2 protein levels. When apoptotic cells were separated from the total population, the induction of SOD activity in the infected apoptotic cells was lost, with activity reduced back to levels seen in mock-infected control cells. In addition, mitochondrial superoxide production was increased and mitochondrial numbers decreased in the infected apoptotic subpopulation. Furthermore, a pan-caspase probe colocalised with SOD2 outside of mitochondria within cytosolic aggregates in infected cells and inhibition of caspase activity was able to restore cellular levels of SOD2 in the whole unseparated infected population to those of mock-infected control cells. Our results suggest that prion propagation exacerbates an apoptotic pathway whereby mitochondrial dysfunction follows mislocalisation of SOD2 to cytosolic caspases, permitting its degradation. Eventually, cellular capacity to maintain oxidative homeostasis is overwhelmed, thus resulting in cell death.

Modelling neurodegeneration in prion disease - applications for drug development

Prion diseases are a group of neurodegenerative diseases that affect mammals, including humans and ruminants such as sheep. They are believed to be caused by the conversion of the prion protein (PrP), a host expressed protein, into a toxic form (PrP(sc)). PrP(sc) accumulates in the brain, resulting in neuronal loss and the typical spongiform appearance of the brain. So far, there are no effective therapies available for prion diseases. This review discusses possible therapies for prion diseases and the models available for advancing research into the disease.

Gene regulation as a potential avenue for the treatment of neurodegenerative disorders

BACKGROUND

As more people live to an older age, the frequency of diseases associated with longer life begins to increase. Neurodegenerative disorders are the worst of these in that there is now no treatment that offers any real improvement. For this reason, any new avenue of research that could lead to a treatment needs to be rigorously pursued. In many cases, neurodegenerative diseases are associated with the expression of a protein with an altered conformation or that generates a breakdown product associated with the cause. Clearly, the prevention of this process is a key therapeutic target.

OBJECTIVE

In this review, the potential for regulating gene expression to prevent or reverse neurodegenerative disease is explored.

CONCLUSIONS

Whereas much research has been directed at the proteins associated with neurodegeneration, understanding what controls their expression presents a new way this issue could be studied.

Low copper and high manganese levels in prion protein plaques

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

Prion protein facilitates uptake of zinc into neuronal cells

Zinc is released into the synaptic cleft upon exocytotic stimuli, although the mechanism for its reuptake into neurons is unresolved. Here we show that the cellular prion protein enhances the uptake of zinc into neuronal cells. This prion-protein-mediated zinc influx requires the octapeptide repeats and amino-terminal polybasic region in the prion protein, but not its endocytosis. Selective antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors block the prion protein-mediated zinc uptake, and the prion protein co-immunoprecipitates with both GluA1 and GluA2 AMPA receptor subunits. Zinc-sensitive intracellular tyrosine phosphatase activity is decreased in cells expressing prion protein and increased in the brains of prion-protein-null mice, providing evidence of a physiological consequence of this process. Prion protein-mediated zinc uptake is ablated in cells expressing familial associated mutants of the protein and in prion-infected cells. These data suggest that alterations in the cellular prion protein-mediated zinc uptake may contribute to neurodegeneration in prion and other neurodegenerative diseases.

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.

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.

Prions and manganese: A maddening beast

The prion protein is well known because of its association with prion diseases. These diseases, which include variant CJD, are unusual because they are neurodegenerative diseases that can be transferred between individuals experimentally. The prion protein is also widely known as a copper binding protein. The binding of copper to the prion protein is possibly necessary for its normal cellular function. The prion protein has also been suggested to bind other metals, and among these, manganese. Despite over ten years of research on manganese and prion disease, this interaction has often been dismissed or at best seen as a poor cousin to the involvement of copper. However, recent data has shown that manganese could stabilise prions in the environment and that chelation therapy specifically aimed at manganese can extend the life of animals with prion disease. This article reviews the evidence for a link between prions and manganese.

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.

Consequences of dietary manganese and copper imbalance on neuronal apoptosis in a murine model of scrapie

AIMS

Copper and manganese levels are altered in mice both lacking PrPc and prion-infected brains. The aim of this study was to analyse the effects of manganese and copper imbalance on neuronal apoptosis in a scrapie-infected Tga20 mouse model.

METHODS

Immunoreactivities for the apoptotic proteins Bax and active caspase-3 were evaluated in nine regions of the brain of scrapie-infected and control Tga20 mice treated with one of several diets: depleted cooper (-Cu), loaded manganese (+Mn), depleted copper/loaded manganese (-Cu+Mn) and regular diet. Immunohistochemical determination of NeuN was used to detect possible neuronal loss.

RESULTS

Intracellular Bax detection was significantly decreased in animals fed with modified diets, particularly in those treated with copper-depleted diets. A decrease in active caspase-3 was primarily observed in animals fed with enhanced manganese diets. Our results show that the -Cu, -Cu+Mn and +Mn diets protected against apoptosis in scrapie-infected mice. However, NeuN immunolabelling quantification revealed that no diet was sufficient to arrest neuronal death.

CONCLUSIONS

With regard to apoptosis induction, the response of Tga20 mice to prion infection was similar to that reported for other mice models. Our results demonstrate the neuroprotective effects of -Cu, -Cu+Mn and +Mn diets in a murine model of scrapie. However, neuronal death induced by infection with prions seems to be independent of apoptosis marker signalling. Moreover, copper-modified diets were neuroprotective against the possible toxicity of the prion transgene in Tga20 control and infected mice even though manganese supplementation could not counteract this toxicity.

Metal ion physiopathology in neurodegenerative disorders

Metal dyshomeostasis in the brain (BMD) has often been proposed as a possible cause for several neurodegenerative disorders (NDs). Nevertheless, the precise nature of the biochemical mechanisms of metal involvement in NDs is still largely unknown. Mounting evidence suggests that normal aging itself is characterized by, among other features, a significant degree of metal ion dysmetabolism in the brain. This is probably the result of a progressive deterioration of the metal regulatory systems and, at least in some cases, of life-long metal exposure and brain accumulation. Although alterations of metal metabolism do occur to some extent in normal aging, they appear to be highly enhanced under various neuropathological conditions, causing increased oxidative stress and favoring abnormal metal-protein interactions. Intriguingly, despite the fact that most common NDs have a distinct etiological basis, they share striking similarities as they are all characterized by a documented brain metal impairment. This review will primarily focus on the alterations of metal homeostasis that are observed in normal aging and in Alzheimer's disease. We also present a brief survey on BMD in other NDs (Amyotrophic Lateral Sclerosis, Parkinson's, and Prion Protein disease) in order to highlight what represents the most reliable evidence supporting a crucial involvement of metals in neurodegeneration. The opportunities for metal-targeted pharmacological strategies in the major NDs are briefly outlined as well.

Manganese-induced changes of the biochemical characteristics of the recombinant wild-type and mutant PrPs

Manganese may play some roles in the pathogenesis of prion diseases. In this study, recombinant human wild-type (WT) PrP and PrP mutants with deleted or inserted octarepeats were exposed to manganese, and their biochemical and biophysical characteristics were evaluated by proteinase K (PK) digestion, sedimentation experiments, transmission electron microscopy and circular dichroism. It demonstrated that incubation of manganese remarkably increased PK-resistances, protein aggregations and beta-sheet contents of the PrPs. Moreover, the PrP mutants of inserted or deleted octarepeats were much vulnerable to the influence of manganese, which showed obviously more aggregation and higher beta-sheet content than that of WT-PrP. It highlights that the effect of manganese on the PrP seems to lie on the incorrectness of the octarepeats numbers. The association of the octarepeats number of PrP with manganese may further provide insight into the unresolved biological function of PrP in the neurons.

Recent advances in prion chemotherapeutics

The transmissible spongiform encephalopathies are rapidly progressive and invariably fatal neurodegenerative diseases for which there are no proven efficacious treatments. Many approaches have been undertaken to find ways to prevent, halt, or reverse these prion diseases, with limited success to date. However, as both our understanding of pathogenesis and our ability to detect early disease increases, so do our potential therapeutic targets and our chances of finding effective drugs. There is increasing pressure to find effective decontaminants for blood supplies, as variant Creutzfeldt Jakob Disease (vCJD) has been shown to be transmissible by blood, and to find non-toxic preventative therapies, with ongoing cases of Bovine Spongiform Encephalopathy (BSE) and the spread of Chronic Wasting Disease (CWD). Within the realm of chemotherapeutic approaches, much research has focussed on blocking the conversion of the normal form of prion protein (PrP(c)) to its abnormal counterpart (PrP(res)). Structurally, these chemotherapeutic agents are often polyanionic or polycyclic and may directly bind PrP(c) or PrP(res), or act by redistributing, sequestering, or down-regulating PrP(c), thus preventing its conversion. There are also some polycationic compounds which proport to enhance the clearance of PrP(res). Other targets include accessory molecules such as the laminin receptor precursor which influences conversion, or cell signalling molecules which may be required for pathogenesis. Of recent interest are the possible neuroprotective effects of some drugs. Importantly, there is evidence that combining compounds may provide synergistic responses. This review provides an update on current testing methods, therapeutic targets, and promising candidates for chemical-based therapy.

Retinal Function and Morphology Are Altered in Cattle Infected with the Prion Disease Transmissible Mink Encephalopathy

Transmissible spongiform encephalopathies (TSEs) are a group of diseases that result in progressive and invariably fatal neurologic disease in both animals and humans. TSEs are characterized by the accumulation of an abnormal protease-resistant form of the prion protein in the central nervous system. Transmission of infectious TSEs is believed to occur via ingestion of prion protein–contaminated material. This material is also involved in the transmission of bovine spongiform encephalopathy (“mad cow disease”) to humans, which resulted in the variant form of Creutzfeldt-Jakob disease. Abnormal prion protein has been reported in the retina of TSE-affected cattle, but despite these observations, the specific effect of abnormal prion protein on retinal morphology and function has not been assessed. The objective of this study was to identify and characterize potential functional and morphologic abnormalities in the retinas of cattle infected with a bovine-adapted isolate of transmissible mink encephalopathy. We used electroretinography and immunohistochemistry to examine retinas from 10 noninoculated and 5 transmissible mink encephalopathy–inoculated adult Holstein steers. Here we show altered retinal function, as evidenced by prolonged implicit time of the electroretinogram b-wave, in transmissible mink encephalopathy–infected cattle before the onset of clinical illness. We also demonstrate disruption of rod bipolar cell synaptic terminals, indicated by decreased immunoreactivity for the alpha isoform of protein kinase C and vesicular glutamate transporter 1, and activation of Müller glia, as evidenced by increased glial fibrillary acidic protein and glutamine synthetase expression, in the retinas of these cattle at the time of euthanasia due to clinical deterioration. This is the first study to identify both functional and morphologic alterations in the retinas of TSE-infected cattle. Our results support future efforts to focus on the retina for the development of new strategies for the diagnosis of TSEs.

Copper(II) coordination outside the tandem repeat region of an unstructured domain of chicken prion protein

Combined potentiometric, calorimetric and spectroscopic methods were used to investigate the Cu(2+) binding ability and coordination behaviour of some peptide fragments related to the neurotoxic region of chicken Prion Protein. The systems studied were the following protein fragments: chPrP(106-114), chPrP(119-126), chPrP(108-127), chPrP(105-127) and chPrP(105-133).The complex formation always starts around pH 4 with the coordination of an imidazole nitrogen, followed by the deprotonation and binding of amide nitrogens from the peptidic backbone. At neutral pH, the {N(im), 3N(-)} binding mode is the preferred one. The amide nitrogens participating in the binding to the Cu(2+) ion derive from residues from the N-terminus side, with the formation of a six-membered chelate ring with the imidazolic side chain.Comparison of thermodynamic data for the two histydyl binding domains (around His-110 and His-124), clearly indicates that the closest to the hexarepeat domain (His-110) has the highest ability to bind Cu(2+) ions, although both of them have the same coordination mode. Conversely, in the case of the human neurotoxic peptide region, between the two binding sites, located at His-96 and His-111, the farthest from the tandem repeat region is the strongest one. Finally, thermodynamic data show that chicken peptide is a distinctly better ligand for coordination of copper ions with respect to the human fragment.

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.

The propagation of hamster-adapted scrapie PrPSc can be enhanced by reduced pyridine nucleotide in vitro

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders caused by an infectious agent termed a prion, which can convert normal cellular prion protein (PrP(C)) into a pathologically misfolded isoform (PrP(Sc)). Taking advantage of protein misfolding cyclic amplification (PMCA), a series of experiments was conducted to investigate the possible influences of pyridine nucleotides on the propagation activities of hamster-adapted scrapie agents 263K and 139A in vitro using normal hamster brain homogenates and recombinant hamster PrP as the substrates. The results showed that PrP(Sc) from both scrapie agent 263K- and 139A-infected brains propagated more efficiently in PMCA with the addition of reduced NADPH, showing an obvious dose-dependent enhancement. Reduced NADH also prompted PrP(Sc) propagation, whereas NADP, NAD and vitamin C failed. Moreover, following incubation with NADPH, recombinant hamster PrP could be efficiently converted into the proteinase K-resistant form when exposed to the trace of PrP(Sc) from infected hamsters. Our data provide evidence that the reduced pyridine nucleotide plays an important role in the propagation of prion and this process seems to target PrP(C) molecules.

Free tyrosine and tyrosine-rich peptide-dependent superoxide generation catalyzed by a copper-binding, threonine-rich neurotoxic peptide derived from prion protein

Previously, generation of superoxide anion (O(2)(*-)) catalyzed by Cu-binding peptides derived from human prion protein (model sequence for helical Cu-binding motif VNITKQHTVTTTT was most active) in the presence of catecholamines and related aromatic monoamines such as phenylethylamine and tyramine, has been reported [Kawano, T., Int J Biol Sci 2007; 3: 57-63]. The peptide sequence (corresponding to helix 2) tested here is known as threonine-rich neurotoxic peptide. In the present article, the redox behaviors of aromatic monoamines, 20 amino acids and prion-derived tyrosine-rich peptide sequences were compared as putative targets of the oxidative reactions mediated with the threonine-rich prion-peptide. For detection of O(2)(*-), an O(2)(*-)-specific chemiluminescence probe, Cypridina luciferin analog was used. We found that an aromatic amino acid, tyrosine (structurally similar to tyramine) behaves as one of the best substrates for the O(2)(*-) generating reaction (conversion from hydrogen peroxide) catalyzed by Cu-bound prion helical peptide. Data suggested that phenolic moiety is required to be an active substrate while the presence of neither carboxyl group nor amino group was necessarily required. In addition to the action of free tyrosine, effect of two tyrosine-rich peptide sequences YYR and DYEDRYYRENMHR found in human prion corresponding to the tyrosine-rich region was tested as putative substrates for the threonine-rich neurotoxic peptide. YYR motif (found twice in the Y-rich region) showed 2- to 3-fold higher activity compared to free tyrosine. Comparison of Y-rich sequence consisted of 13 amino acids and its Y-to-F substitution mutant sequence revealed that the tyrosine-residues on Y-rich peptide derived from prion may contribute to the higher production of O(2)(*-). These data suggest that the tyrosine residues on prion molecules could be additional targets of the prion-mediated reactions through intra- or inter-molecular interactions. Lastly, possible mechanism of O(2)(*-) generation and the impacts of such self-redox events on the conformational changes in prion are discussed.

The ubiquitin-proteasome system in spongiform degenerative disorders

Spongiform degeneration is characterized by vacuolation in nervous tissue accompanied by neuronal death and gliosis. Although spongiform degeneration is a hallmark of prion diseases, this pathology is also present in the brains of patients suffering from Alzheimer’s disease, diffuse Lewy body disease, human immunodeficiency virus (HIV) infection, and Canavan’s spongiform leukodystrophy. The shared outcome of spongiform degeneration in these diverse diseases suggests that common cellular mechanisms must underlie the processes of spongiform change and neurodegeneration in the central nervous system. Immunohistochemical analysis of brain tissues reveals increased ubiquitin immunoreactivity in and around areas of spongiform change, suggesting the involvement of ubiquitin–proteasome system dysfunction in the pathogenesis of spongiform neurodegeneration. The link between aberrant ubiquitination and spongiform neurodegeneration has been strengthened by the discovery that a null mutation in the E3 ubiquitin–protein ligase mahogunin ring finger-1 (Mgrn1) causes an autosomal recessively inherited form of spongiform neurodegeneration in animals. Recent studies have begun to suggest that abnormal ubiquitination may alter intracellular signaling and cell functions via proteasome-dependent and proteasome-independent mechanisms, leading to spongiform degeneration and neuronal cell death. Further elucidation of the pathogenic pathways involved in spongiform neurodegeneration should facilitate the development of novel rational therapies for treating prion diseases, HIV infection, and other spongiform degenerative disorders.