A new approach for the separation, characterization and testing of potential prionoid protein aggregates through hollow-fiber flow field-flow fractionation and multi-angle light scattering

Protein misfolding and aggregation are the common mechanisms in a variety of aggregation-dependent diseases. The compromised proteins often assemble into toxic, accumulating amyloid-like structures of various lengths and their toxicity can also be transferred both in vivo and in vitro a prion-like behavior. The characterization of protein interactions, degradation and conformational dynamics in biological systems still represents an analytical challenge in the prion-like protein comprehension. In our work, we investigated the nature of a transferable cytotoxic agent, presumably a misfolded protein, through the coupling of a multi-detector, non-destructive separation platform based on hollow-fiber flow field-flow fractionation with imaging and downstream in vitro tests. After purification with ion exchange chromatography, the transferable cytotoxic agentwas analyzed with Atomic Force Microscopy and statistical analysis, showing that the concentration of protein dimers and low n-oligomer forms was higher in the cytotoxic sample than in the control preparation. To assess whether the presence of these species was the actual toxic and/or self-propagating factor, we employed HF5 fractionation, with UV and Multi-Angle Light Scattering detection, to define proteins molar mass distribution and abundance, and fractionate the sample into size-homogeneous fractions. These fractions were then tested individually in vitro to investigate the direct correlation with cytotoxicity. Only the later-eluted fraction, which contains high-molar mass aggregates, proved to be toxic onto cell cultures. Moreover, it was observed that the selective transfer of toxicity also occurs for one lower-mass fraction, suggesting that two different mechanisms, acute and later induced toxicity, are in place. These results strongly encourage the efficacy of this platform to enable the identification of protein toxicants.

The Pub1 and Upf1 Proteins Act in Concert to Protect Yeast from Toxicity of the [PSI⁺] Prion

The [PSI⁺] nonsense-suppressor determinant of Saccharomyces cerevisiae is based on the formation of heritable amyloids of the Sup35 (eRF3) translation termination factor. [PSI⁺] amyloids have variants differing in amyloid structure and in the strength of the suppressor phenotype. The appearance of [PSI⁺], its propagation and manifestation depend primarily on chaperones. Besides chaperones, the Upf1/2/3, Siw14 and Arg82 proteins restrict [PSI⁺] formation, while Sla2 can prevent [PSI⁺] toxicity. Here, we identify two more non-chaperone proteins involved in [PSI⁺] detoxification. We show that simultaneous lack of the Pub1 and Upf1 proteins is lethal to cells harboring [PSI⁺] variants with a strong, but not with a weak, suppressor phenotype. This lethality is caused by excessive depletion of the Sup45 (eRF1) termination factor due to its sequestration into Sup35 polymers. We also show that Pub1 acts to restrict excessive Sup35 prion polymerization, while Upf1 interferes with Sup45 binding to Sup35 polymers. These data allow consideration of the Pub1 and Upf1 proteins as a novel [PSI⁺] detoxification system.

Fullerene C₆₀ Derivatives Attenuated Microglia-Mediated Prion Peptide Neurotoxicity

Prion disorders are progressive neurodegenerative diseases characterized by extensive neuronal loss, which is linked to the extracellular accumulation of the scrapie isoform (PrPSC) of the normal cellular prion protein (PrPC). As microglial activation is a central event in pathogenesis of prion disease, the strategies that reduce microglial activation may have therapeutic benefits. In this study, the neuroprotective effects of hydroxylated C60(C60-OH) and amino modified-C60(C60-NH2) were evaluated by using PrP(106-126)-stimulated BV-2 cells as a model of activated microglia. Herein, we showed that microglial activation in response to PrP(106-126) was effectively attenuated by pretreatment with C60-OH as compared with C60-NH2. C60-OH significantly inhibited the excessive production of inflammatory mediators, such as prostaglandin E2 (PGE2), nitric oxide (NO), tumor necrosis factors (TNF)-α, interleukin (IL)-1β, and IL-6, and blocked the expression of cyclooxygenase-2 (COX-2) and inducible nitric-oxide synthase (iNOS) in PrP(106-126)-stimulated BV-2 cells. C60-OH exerted anti-inflammatory potential by up-regulating the expression of antioxidant enzymes via activation of nuclear factor erythroid 2-related factor 2 (Nrf2). The inhibitory effect of C60-OH against PrP(106-126)-induced inflammatory response was abolished by siRNA of Nrf2. In addition, conditioned culture media taken from PrP(106-126)-stimulated microglia cause apoptotic neuronal cell death, which was suppressed by pretreatment with C60-OH. Take together, these results suggest that C60-OH protects neuronal cells against PrP(106-126)-mediated neurotoxicity through activation of Nrf2 pathway, and provides evidence that fullerene derivatives may have therapeutic potential in prion diseases.

In vivo prion models and the disconnection between transmissibility and neurotoxicity

The primary causative event in the development of prion diseases is the misfolding of the normal prion protein (PrP^C) into an ensemble of altered conformers (herein collectively denoted as PrP^Sc) that accumulate in the brain. Prominent amongst currently unresolved key aspects underpinning prion disease pathogenesis is whether transmission and toxicity are sub-served by different molecular species of PrP^Sc, which may directly impact on the development of effective targeted treatments. The use of murine models of prion disease has been of fundamental importance for probing the relationship between hypothesised "neurotoxic" and "transmissible" PrP^Sc and the associated kinetic profiles of their production during disease evolution, but unfortunately consensus has not been achieved. Recent in vivo studies have led to formulation of the "two-phase" hypothesis, which postulates that there is first an exponential increase in transmitting PrP^Sc species followed by an abrupt transition to propagation of neurotoxic PrP^Sc species. Such observations however, appear inconsistent with previous in vivo murine studies employing detailed time-course behavioural testing, wherein evidence of neurotoxicity could be detected early in disease progression. This review analyses the contributions of in vivo murine models attempting to provide insights into the relationship between transmitting and neurotoxic PrP^Sc species and explores possible refinements to the "two-phase hypothesis", that better accommodate the available historical and recent evidence.

Prion-Associated Toxicity is Rescued by Elimination of Cotranslational Chaperones

The nascent polypeptide-associated complex (NAC) is a highly conserved but poorly characterized triad of proteins that bind near the ribosome exit tunnel. The NAC is the first cotranslational factor to bind to polypeptides and assist with their proper folding. Surprisingly, we found that deletion of NAC subunits in Saccharomyces cerevisiae rescues toxicity associated with the strong [PSI+] prion. This counterintuitive finding can be explained by changes in chaperone balance and distribution whereby the folding of the prion protein is improved and the prion is rendered nontoxic. In particular, the ribosome-associated Hsp70 Ssb is redistributed away from Sup35 prion aggregates to the nascent chains, leading to an array of aggregation phenotypes that can mimic both overexpression and deletion of Ssb. This toxicity rescue demonstrates that chaperone modification can block key steps of the prion life cycle and has exciting implications for potential treatment of many human protein conformational disorders.

Misfolded proteins can be toxic to cells, causing pathologies such as Alzheimer’s disease, Parkinson’s disease, prion diseases, and ALS. One mechanism by which organisms combat protein misfolding involves molecular chaperones, proteins that help other proteins fold correctly. Here, we describe a novel role for a family of chaperones called the nascent polypeptide-associated complex (NAC). The NAC is a group of proteins that exist in all multicellular organisms, yet we do not fully understand its function. Using yeast as a model system, we have found that deletion of NAC subunits can reduce the toxicity associated with misfolded proteins. This work has implications for human protein misfolding diseases, as modulation of the NAC may present a viable therapeutic avenue by which to slow the progression of neurodegeneration and other protein conformational disorders.

Melatonin-mediated β-catenin activation protects neuron cells against prion protein-induced neurotoxicity

Activation of β-catenin in neurons regulates mitochondrial function and protects against protein misfolding disorders, including Alzheimer's disease and Huntington's disease. Melatonin, a natural secretory product of the pineal gland, exerts neuroprotective effects through the activation of β-catenin. In this study, melatonin increased β-catenin protein expression and activation in human neuroblastoma cell lines SH-SY5Y cells. Melatonin also inhibited PrP (106-126)-induced neurotoxicity and the inhibition attenuated by treatment of β-catenin inhibitor ICG-001. Activation of β-catenin blocked PrP (106-126)-mediated downregulation of anti-apoptotic protein survivin and Bcl-2. Reduction of mitochondrial membrane potential, translocation of Bax, and cytochrome c release which induced by PrP (106-126) treatment were inhibited by β-catenin activation, which contributed to prevented PrP (106-126)-induced neuronal cell death. In conclusion, β-catenin activation by melatonin prevented PrP (106-126)-induced neuronal cell death through regulating anti-apoptotic proteins and mitochondrial pathways. These results also suggest the therapeutic value of Wnt/β-catenin signaling in prion-related disorders as influenced by melatonin.

Rutin alleviates prion peptide-induced cell death through inhibiting apoptotic pathway activation in dopaminergic neuronal cells

Prion disorders are progressive neurodegenerative diseases characterized by extensive neuronal loss and accumulation of the abnormal form of the scrapie prion protein (PrP). Rutin is a flavonoid that occurs naturally in plant-derived beverages and foods and is used in traditional and folkloric medicine worldwide. In the present study, we evaluated the protective effects of rutin against PrP fragment (106-126)-induced neuronal cell death. Rutin treatment blocked PrP (106-126)-mediated increases in reactive oxygen species production and nitric oxide release and helped slowing the decrease of neurotrophic factors that results from PrP accumulation. Rutin attenuated PrP (106-126)-associated mitochondrial apoptotic events by inhibiting mitochondrial permeability transition and caspase-3 activity and blocking expression of the apoptotic signals Bax and PARP. Additionally, rutin treatment significantly decreased the expression of the death receptor Fas and its ligand Fas-L. Overall, our results demonstrated that rutin protects against the neurodegenerative effects of prion accumulation by increasing production of neurotropic factors and inhibiting apoptotic pathway activation in neuronal cells. These results suggested that rutin may have clinical benefits for prion diseases and other neurodegenerative disorders.

Prion protein: structural features and related toxicity

Transmissible spongiform encephalopathies, or prion diseases, is a group of infectious neurodegenerative disorders. The conformational conversion from cellular form (PrP(C)) to disease-causing isoform (PrP(Sc)) is considered to be the most important and remarkable event in these diseases, while accumulation of PrP(Sc) is thought to be the main reason for cell death, inflammation and spongiform degeneration observed in infected individuals. Although these rare but unique neurodegenerative disorders have attracted much attention, there are still many questions that remain to be answered. Knowledge of the scrapie agent structures and the toxic species may have significance for understanding the causes of the diseases, and could be helpful for rational design of novel therapeutic and diagnostic methods. In this review, we summarized the available experimental evidence concerning the relationship among the structural features, aggregation status of misfolded PrP and related neurotoxicity in the course of prion diseases development. In particular, most data supports the idea that the smaller oligomeric PrP(Sc) aggregates, rather than the mature amyloid fibers, exhibit the highest toxicity to the host.

Prion protein oligomer and its neurotoxicity

The prion diseases, also known as transmissible spongiform encephalopathies, are fatal neurodegenerative disorders. According to the 'protein only' hypothesis, the key molecular event in the pathogenesis of prion disease is the conformational conversion of the host-derived cellular prion protein (PrP(C)) into a misfolded form (scrapie PrP, PrP(Sc)). Increasing evidence has shown that the most infectious factor is the smaller subfibrillar oligomers formed by prion proteins. Both the prion oligomer and PrP(Sc) are rich in β-sheet structure and resistant to the proteolysis of proteinase K. The prion oligomer is soluble in physiologic environments whereas PrP(Sc) is insoluble. Various prion oligomers are formed in different conditions. Prion oligomers exhibited more neurotoxicity both in vitro and in vivo than the fibrillar forms of PrP(Sc), implying that prion oligomers could be potential drug targets for attacking prion diseases. In this article, we describe recent experimental evidence regarding prion oligomers, with a special focus on prion oligomer formation and its neurotoxicity.

Functional amyloidogenesis and cytotoxicity-insights into biology and pathology

Prions are self-templating protein structures that can be transferred from organism to organism. The [Het-s] prion propagates as a functional amyloid aggregate in the filamentous fungi Podospora anserina, and is involved in mediating heterokaryon incompatibility. Fusion of a P. anserina strain harboring the [Het-s] prion with another strain expressing the soluble Het-S protein results in cell death. The mechanism of Het-s/Het-S-mediated cell death has now been revealed in a paper just published in PLOS Biology. The study shows that Het-s and Het-S C-terminal domain co-amyloidogenesis induces a profound conformational rearrangement in the N-terminal Het-S HeLo domain, resulting in the exposure of a nascent transmembrane helix. Oligomerization of these helices leads to pore formation, leakage of the cytosolic contents, and subsequent cell death. Thus, Het-s amyloid plays a major role in the life cycle of P. anserina by orchestrating a complex conformational change in the Het-S protein, resulting in cytotoxicity by compromising membrane integrity. This ability of Het-s functional amyloid to initiate programmed cytotoxicity by mediating a conformational change in another protein significantly expands the functional repertoire of amyloid. Moreover, the mechanism of Het-S cell killing may be similar to the mechanism by which some pathological amyloid proteins lead to the demise of post-mitotic tissue.

The mechanism of toxicity in HET-S/HET-s prion incompatibility

The HET-s protein from the filamentous fungus Podospora anserina is a prion involved in a cell death reaction termed heterokaryon incompatibility. This reaction is observed at the point of contact between two genetically distinct strains when one harbors a HET-s prion (in the form of amyloid aggregates) and the other expresses a soluble HET-S protein (96% identical to HET-s). How the HET-s prion interaction with HET-S brings about cell death remains unknown; however, it was recently shown that this interaction leads to a relocalization of HET-S from the cytoplasm to the cell periphery and that this change is associated with cell death. Here, we present detailed insights into this mechanism in which a non-toxic HET-s prion converts a soluble HET-S protein into an integral membrane protein that destabilizes membranes. We observed liposomal membrane defects of approximately 10 up to 60 nm in size in transmission electron microscopy images of freeze-fractured proteoliposomes that were formed in mixtures of HET-S and HET-s amyloids. In liposome leakage assays, HET-S has an innate ability to associate with and disrupt lipid membranes and that this activity is greatly enhanced when HET-S is exposed to HET-s amyloids. Solid-state nuclear magnetic resonance (NMR) analyses revealed that HET-s induces the prion-forming domain of HET-S to adopt the β-solenoid fold (previously observed in HET-s) and this change disrupts the globular HeLo domain. These data indicate that upon interaction with a HET-s prion, the HET-S HeLo domain partially unfolds, thereby exposing a previously buried ∼34-residue N-terminal transmembrane segment. The liberation of this segment targets HET-S to the membrane where it further oligomerizes, leading to a loss of membrane integrity. HET-S thus appears to display features that are reminiscent of pore-forming toxins.

Calcium binding promotes prion protein fragment 90-231 conformational change toward a membrane destabilizing and cytotoxic structure

The pathological form of prion protein (PrP(Sc)), as other amyloidogenic proteins, causes a marked increase of membrane permeability. PrP(Sc) extracted from infected Syrian hamster brains induces a considerable change in membrane ionic conductance, although the contribution of this interaction to the molecular mechanism of neurodegeneration process is still controversial. We previously showed that the human PrP fragment 90-231 (hPrP₉₀₋₂₃₁) increases ionic conductance across artificial lipid bilayer, in a calcium-dependent manner, producing an alteration similar to that observed for PrP(Sc). In the present study we demonstrate that hPrP₉₀₋₂₃₁, pre-incubated with 10 mM Ca⁺⁺ and then re-suspended in physiological external solution increases not only membrane conductance but neurotoxicity as well. Furthermore we show the existence of a direct link between these two effects as demonstrated by a highly statistically significant correlation in several experimental conditions. A similar correlation between increased membrane conductance and cell degeneration has been observed assaying hPrP₉₀₋₂₃₁ bearing pathogenic mutations (D202N and E200K). We also report that Ca⁺⁺ binding to hPrP₉₀₋₂₃₁ induces a conformational change based on an alteration of secondary structure characterized by loss of alpha-helix content causing hydrophobic amino acid exposure and proteinase K resistance. These features, either acquired after controlled thermal denaturation or induced by D202N and E200K mutations were previously identified as responsible for hPrP₉₀₋₂₃₁ cytotoxicity. Finally, by in silico structural analysis, we propose that Ca⁺⁺ binding to hPrP₉₀₋₂₃₁ modifies amino acid orientation, in the same way induced by E200K mutation, thus suggesting a pathway for the structural alterations responsible of PrP neurotoxicity.

Cellular prion protein released on exosomes from macrophages binds to Hsp70

Prion diseases are infectious and fatal neurodegenerative disorders. The cellular prion protein (PrP(C)) converting into misfolded isoform of prion protein (PrP(Sc)) is responsible for prion disease infection. Immune system plays an important role in facilitating the spread of prion infections from the periphery to the central nervous system. Macrophages were considered associated with the transportation and replication of PrP(Sc). So, understanding the PrP(C) trafficking in macrophages is important to explore the transport mechanism for PrP(Sc). Here, we isolated exosomes from the culture medium of Ana-1 macrophage cell line and investigated the PrP(C) trafficked by exosomes and the interaction of PrP(C) with Hsp70 in secreted exosomes by western blotting, immunoelectron microscopy, and co-immunoprecipitation. The results showed that the isolated vesicles from the culture medium of macrophages were characterized by exosomes and bore PrP(C). And PrP(C) bound to Hsp70 both in intracellular environment and secreted exosomes. In contrast, PrP(C) had no interaction with marker proteins of exosomes, Tag101 and Flotillin-1. These results suggested that PrP(C) present in extracellular space might be externalized through secreted exosomes from macrophages, and Hsp70 may play roles in the process of PrP(C) released via secreted exosomes.

The interaction of humic substances with the human prion protein fragment 90-231 affects its protease K resistance and cell internalization

In this paper we analyzed the determinants and the structural effects of the interaction of human prion protein fragment 90-231 (HuPrP) with humic substances, (HS) including humic (HA) and fulvic (FA) acids, natural refractory organic polyanions widely diffused in soils and waters. We show that this interaction is mainly driven by non-specific electrostatic attraction involving regions situated within alpha-helix A and beta-sheet S1 of human PrP. FA binding to HuPrP altered its ability to acquire some PrPSc-like characteristics induced by the mild thermal denaturation of the peptide (1 h at 53 degrees C). In particular, in the presence of FA, HuPrP shows a reduced amount of beta-sheet content (as demonstrated by the reduced binding of thioflavin T), an increased sensitivity to protease K and an inhibition of the entering in the fibrillogenic pathway. FA/HuPrP interaction caused the aggregation of the peptide in unstructured macrocomplexes, as demonstrated by the altered electrophoretic migration in semi-denaturing detergent-agarose gel assay. Importantly, in the presence of FA the rate of internalization of HuPrP in human neuroblastoma cells was significantly reduced as compared to that of the beta-structured peptide. Therefore, HS inhibited the acquisition of PrP(Sc)-like structural properties that, in turn, are responsible for HuPrP intracellular accumulation and lead to neuronal death. Important implications of these data are that HuPrP-HS complexes, being unable to be internalized in living cells may represent a molecular mechanism for the reduced transmission of prion transmission from HS-rich soil also in the presence of contamination from infected animals.

Synthetic lipid vesicles recruit native-like aggregates and affect the aggregation process of the prion Ure2p: insights on vesicle permeabilization and charge selectivity

The yeast prion Ure2p polymerizes into native-like fibrils, retaining the overall structure and binding properties of the soluble protein. Recently we have shown that, similar to amyloid oligomers, the native-like Ure2p fibrils and their precursor oligomers are highly toxic to cultured mammalian cells when added to the culture medium, whereas Ure2p amyloid fibrils generated by heating the native-like fibrils are substantially harmless. We show here that, contrary to the nontoxic amyloid fibrils, the toxic, native-like Ure2p assemblies induce a significant calcein release from negatively charged phosphatidylserine vesicles. A minor and less-specific effect was observed with zwitterionic phosphatidylcholine vesicles, suggesting that the toxic aggregates preferentially bind to negatively charged sites on lipid membranes. We also found that cholesterol-enriched phospholipid membranes are protected against permeabilization by native-like Ure2p assemblies. Moreover, vesicle permeabilization appears charge-selective, allowing calcium, but not chloride, influx to be monitored. Finally, we found that the interaction with phosphatidylserine membranes speeds up Ure2p polymerization into oligomers and fibrils structurally and morphologically similar to the native-like Ure2p assemblies arising in free solution, although less cytotoxic. These data suggest that soluble Ure2p oligomers and native-like fibrils, but not amyloid fibrils, interact intimately with negatively charged lipid membranes, where they allow selective cation influx.

Dual modulation of ERK1/2 and p38 MAP kinase activities induced by minocycline reverses the neurotoxic effects of the prion protein fragment 90-231

Several in vitro and in vivo studies addressed the identification of molecular determinants of the neuronal death induced by PrP(Sc) or related peptides. We developed an experimental model to assess PrP(Sc) neurotoxicity using a recombinant polypeptide encompassing amino acids 90-231 of human PrP (hPrP90-231) that corresponds to the protease-resistant core of PrP(Sc) identified in prion-infected brains. By means of mild thermal denaturation, we can convert hPrP90-231 from a PrP(C)-like conformation into a PrP(Sc)-like structure. In virtue of these structural changes, hPrP90-231 powerfully affected the survival of SH-SY5Y cells, inducing caspase 3 and p38-dependent apoptosis, while in the native alpha-helix-rich conformation, hPrP90-231 did not induce cell toxicity. The aim of this study was to identify drugs able to block hPrP90-231 neurotoxic effects, focusing on minocycline, a tetracycline with known neuroprotective activity. hPrP90-231 caused a caspase 3-dependent apoptosis via the blockade of ERK1/2 activation and the subsequent activation of p38 MAP kinase. We propose that hPrP90-231-induced apoptosis is dependent on the inhibition of ERK1/2 responsiveness to neurotrophic factors, removing a tonic inhibition of p38 activity and resulting in caspase 3 activation. Minocycline prevented hPrP90-231-induced toxicity interfering with this mechanism: the pretreatment with this tetracycline restored ERK1/2 activity and reverted p38 and caspase 3 activities. The effects of minocycline were not mediated by the prevention of hPrP90-231 structural changes or cell internalization (differently from Congo Red). In conclusion, minocycline elicits anti-apoptotic effects against the neurotoxic activity of hPrP90-231 and these effects are mediated by opposite modulation of ERK1/2 and p38 MAP kinase activities.

p75(NTR) activation of NF-kappaB is involved in PrP106-126-induced apoptosis in mouse neuroblastoma cells

Neuronal death is a pathological hallmark of prion diseases. Synthetic prion peptide PrP106-126 can convert PrP(C) into protease-resistant aggregates, which can cause neurotoxicity in vivo and in vitro. Various cell surface proteins can participate in the infection process of prions. p75(NTR) can interact with PrP106-126 and has a neurotoxic effect on neurons. However, for p75(NTR) lacking intrinsic catalytic activity domain in cytoplasm, p75(NTR) -associated signaling molecular and the signaling events in cytoplasm in p75(NTR)-mediated apoptosis responding to PrP106-126 remain still unknown. Thus p75(NTR) -associated NF-kappaB signaling pathway was investigated in this study. Herein PrP106-126-induced apoptosis in mouse neuroblastoma cell line N2a, PrP106-126 significantly up-regulated p75(NTR) expression on mRNA and protein levels. For the first time we found that PrP106-126 induced activation of NF-kappaB by Western blot assay, and blocking the interaction of p75(NTR) with PrP106-126 by p75(NTR) polyclonal antibody sc-6189 or pretreatment with inhibitor NF-kappaB SN50 reduced the activation of NF-kappaB and attenuated the apoptotic effect by PrP106-126. This study offers a possible interpretation that NF-kappaB signaling pathway was activated by the interaction of PrP106-126 with p75(NTR), and NF-kappaB activity showed the pro-apoptotic effect in PrP106-126-induced apoptosis in N2a cells. Involvement of NF-kappaB signaling pathway in p75(NTR)-mediated apoptosis may partially account for the PrP106-126-induced neurotoxicity in N2a cells.

[Occupational exposure to prions due to contact with meat and bone meal (MBM)]

Meat and bone meal (MBM) is free from conventional biological factors if it is not subjected to a secondary pollution, but it may be a potential source of prions. Prions are widely accepted as being the casual agents of a range of serious infectious diseases characterized by degenerative processes in the nervous system. These diseases occur in both humans and animals. The recommended method of producing MBM is now regarded by experts as optimal as it sufficiently eliminates prions, which may be present in the animal material used in MBM production. However, it has been stressed that neither this nor any other method used can guarantee that all prions are completely neutralized. Workers employed in utilizing plants, which produce MBM, plants producing fertilizers and animal fodder, power stations using MBM as an alternative fuel, MBM store-houses, as well as those dealing with MBM distribution and transport form occupational groups at a higher risk. In the opinion of experts, the risk of infection with prions among workers having contact with MBM is very low and it should be regarded as acceptable for this kind of exposure. It is worth noting that guidelines on how to prevent and reduce contacts with MBM has already been elaborated.

Nonpsychoactive cannabidiol prevents prion accumulation and protects neurons against prion toxicity

Prion diseases are transmissible neurodegenerative disorders characterized by the accumulation in the CNS of the protease-resistant prion protein (PrPres), a structurally misfolded isoform of its physiological counterpart PrPsen. Both neuropathogenesis and prion infectivity are related to PrPres formation. Here, we report that the nonpsychoactive cannabis constituent cannabidiol (CBD) inhibited PrPres accumulation in both mouse and sheep scrapie-infected cells, whereas other structurally related cannabinoid analogs were either weak inhibitors or noninhibitory. Moreover, after intraperitoneal infection with murine scrapie, peripheral injection of CBD limited cerebral accumulation of PrPres and significantly increased the survival time of infected mice. Mechanistically, CBD did not appear to inhibit PrPres accumulation via direct interactions with PrP, destabilization of PrPres aggregates, or alteration of the expression level or subcellular localization of PrPsen. However, CBD did inhibit the neurotoxic effects of PrPres and affected PrPres-induced microglial cell migration in a concentration-dependent manner. Our results suggest that CBD may protect neurons against the multiple molecular and cellular factors involved in the different steps of the neurodegenerative process, which takes place during prion infection. When combined with its ability to target the brain and its lack of toxic side effects, CBD may represent a promising new anti-prion drug.

In vitro and in vivo neurotoxicity of prion protein oligomers

The mechanisms underlying prion-linked neurodegeneration remain to be elucidated, despite several recent advances in this field. Herein, we show that soluble, low molecular weight oligomers of the full-length prion protein (PrP), which possess characteristics of PrP to PrPsc conversion intermediates such as partial protease resistance, are neurotoxic in vitro on primary cultures of neurons and in vivo after subcortical stereotaxic injection. Monomeric PrP was not toxic. Insoluble, fibrillar forms of PrP exhibited no toxicity in vitro and were less toxic than their oligomeric counterparts in vivo. The toxicity was independent of PrP expression in the neurons both in vitro and in vivo for the PrP oligomers and in vivo for the PrP fibrils. Rescue experiments with antibodies showed that the exposure of the hydrophobic stretch of PrP at the oligomeric surface was necessary for toxicity. This study identifies toxic PrP species in vivo. It shows that PrP-induced neurodegeneration shares common mechanisms with other brain amyloidoses like Alzheimer disease and opens new avenues for neuroprotective intervention strategies of prion diseases targeting PrP oligomers.

Oxidation reduces the fibrillation but not the neurotoxicity of the prion peptide PrP106-126

There is increasing evidence that soluble oligomers of misfolded protein may play a role in the pathogenesis of protein misfolding diseases including the transmissible spongiform encephalopathies (TSE) where the protein involved is the prion protein, PrP. The effect of oxidation on fibrillation tendency and neurotoxicity of different molecular variants of the prion peptide PrP106-126 was investigated. It was found that methionine oxidation significantly reduced amyloid fibril formation and proteinase K resistance, but it did not reduce (but rather increase slightly) the neurotoxicity of the peptides in vivo (electroretinography after intraocular injections in mice) and in vitro (in primary neuronal cultures). We furthermore found that the bovine variant of PrP106-126, containing only one methionine residue, showed both reduced fibril forming capacity and in vivo and in vitro neurotoxicity. The findings imply (I) that there is not a simple relation between the formation of amyloid fibrils and neurotoxicity of PrP106-126 derived peptides, (II) that putative, soluble, non-amyloid protofibrils, presumed to be present in increased proportions in oxidized PrP106-126, could play a role in the pathogenesis of TSE and III) that the number of methionine residues in the PrP106-126 peptide seems to have a pivotal role in determining the physical and biological properties of PrP106-126.

Insights into prion strains and neurotoxicity

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that are caused by prions and affect humans and many animal species. It is now widely accepted that the infectious agent that causes TSEs is PrP(Sc), an aggregated moiety of the host-derived membrane glycolipoprotein PrP(C). Although PrP(C) is encoded by the host genome, prions themselves encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, after inoculation into distinct hosts, cause disease with consistent characteristics, such as incubation period, distinct patterns of PrP(Sc) distribution and spongiosis and relative severity of the spongiform changes in the brain. The existence of such strains poses a fascinating challenge to prion research.

Disease-associated prion protein oligomers inhibit the 26S proteasome

The mechanism of cell death in prion disease is unknown but is associated with the production of a misfolded conformer of the prion protein. We report that disease-associated prion protein specifically inhibits the proteolytic beta subunits of the 26S proteasome. Using reporter substrates, fluorogenic peptides, and an activity probe for the beta subunits, this inhibitory effect was demonstrated in pure 26S proteasome and three different cell lines. By challenge with recombinant prion and other amyloidogenic proteins, we demonstrate that only the prion protein in a nonnative beta sheet conformation inhibits the 26S proteasome at stoichiometric concentrations. Preincubation with an antibody specific for aggregation intermediates abrogates this inhibition, consistent with an oligomeric species mediating this effect. We also present evidence for a direct relationship between prion neuropathology and impairment of the ubiquitin-proteasome system (UPS) in prion-infected UPS-reporter mice. Together, these data suggest a mechanism for intracellular neurotoxicity mediated by oligomers of misfolded prion protein.

Changes in neuropeptide expression in mice infected with prions

Prion diseases are neurodegenerative disorders characterized by accumulation of an aberrantly folded isoform (PrP(Sc)) of the normal prion protein (PrP(C)). Using in situ hybridization and immunohistochemistry, we have studied changes in the expression of neuropeptides, acetylcholinesterase and tyrosine hydroxylase in CD1 and FVB wild-type mouse strains, as well as in PrP(C) null mice and in mice overexpressing PrP(C) following intracerebral inoculation with RML or Me7 prions. In the immunohistochemical analysis, neuropeptide Y (NPY), enkephalin and dynorphin-like immunoreactivities increased in mossy fibers of CD1 and FVB mice inoculated with either RML- or Me7 prions, whereas cholecystokinin-like immunoreactivity was decreased. These changes in peptide levels were paralleled by an increase in the transcripts in granule cells for neuropeptide Y, enkephalin, and cholecystokinin. However, the dynorphin transcript was decreased in the granule cells. The changes occurred more rapidly in PrP(C)-overexpressing compared to wild-type mice, and could not be found at all in PrP(C)-knockout mice. These changes in peptide expression, which mostly occur before appearance of symptoms of disease, may reflect attempts to initiate protective and/or regenerative processes.

Neurotoxic prion protein (PrP) fragment 106-126 requires the N-terminal half of the hydrophobic region of PrP in the PrP-deficient neuronal cell line

The cytotoxicity of aged PrP(106-126) was examined using an immortalized prion protein (PrP) gene-deficient neuronal cell line. The N-terminal half of the hydrophobic region (HR) but not the octapeptide repeat (OR) of PrP was required for aged PrP(106-126) neurotoxicity, suggesting that neurotoxic signals of aged PrP(106-126) are mediated by this region.

Study on the toxic mechanism of prion protein peptide 106-126 in neuronal and non neuronal cells

A synthetic peptide corresponding to the 106-126 amyloidogenic region of the cellular human prion protein (PrP(c)) is useful for in vitro study of prion-induced neuronal cell death. The aim of the present work was to examine the implication of the cellular prion protein in the toxicity mechanism induced by PrP 106-126. The effect of PrP 106-126 was investigated both on human neuroblastoma SH-SY5Y cells and on SH-SY5Y overexpressing murine cellular prions (wtPrP). We show by metabolic assay tests and ATP assays that PrP(c) expression does not modulate the toxicity of the prion peptide. Moreover, we investigated the effect of this peptide on an established non neuronal model, rabbit kidney epithelial A74 cells that express a doxycycline-inducible murine PrP(c) gene. We show for the first time that the prion peptide 106-126 does not exert any toxic effect on this cell line in the presence or absence of doxycycline. Our results show that the PrP 106-126-induced cell alteration is independent of PrP(c) expression. Rather, it seems to act via an interaction with lipidic components of the plasma membrane as strengthened by our results showing the differential susceptibility of neuronal and non neuronal cell lines that significantly differ by their membrane fatty acid composition.

Prions and their partners in crime

Prions, the infectious agents of transmissible spongiform encephalopathies (TSEs), have defied full characterization for decades. The dogma has been that prions lack nucleic acids and are composed of a pathological, self-inducing form of the host's prion protein (PrP). Recent progress in propagating TSE infectivity in cell-free systems has effectively ruled out the involvement of foreign nucleic acids. However, host-derived nucleic acids or other non-PrP molecules seem to be crucial. Interactions between TSE-associated PrP and its normal counterpart are also pathologically important, so the physiological functions of normal PrP and how they might be corrupted by TSE infections have been the subject of recent research.

Overstimulation of PrPC signaling pathways by prion peptide 106-126 causes oxidative injury of bioaminergic neuronal cells

Transmissible spongiform encephalopathies, also called prion diseases, are characterized by neuronal loss linked to the accumulation of PrP(Sc), a pathologic variant of the cellular prion protein (PrP(C)). Although the molecular and cellular bases of PrP(Sc)-induced neuropathogenesis are not yet fully understood, increasing evidence supports the view that PrP(Sc) accumulation interferes with PrP(C) normal function(s) in neurons. In the present work, we exploit the properties of PrP-(106-126), a synthetic peptide encompassing residues 106-126 of PrP, to investigate into the mechanisms sustaining prion-associated neuronal damage. This peptide shares many physicochemical properties with PrP(Sc) and is neurotoxic in vitro and in vivo. We examined the impact of PrP-(106-126) exposure on 1C11 neuroepithelial cells, their neuronal progenies, and GT1-7 hypothalamic cells. This peptide triggers reactive oxygen species overflow, mitogen-activated protein kinase (ERK1/2), and SAPK (p38 and JNK1/2) sustained activation, and apoptotic signals in 1C11-derived serotonergic and noradrenergic neuronal cells, while having no effect on 1C11 precursor and GT1-7 cells. The neurotoxic action of PrP-(106-126) relies on cell surface expression of PrP(C), recruitment of a PrP(C)-Caveolin-Fyn signaling platform, and overstimulation of NADPH-oxidase activity. Altogether, these findings provide actual evidence that PrP-(106-126)-induced neuronal injury is caused by an amplification of PrP(C)-associated signaling responses, which notably promotes oxidative stress conditions. Distorsion of PrP(C) signaling in neuronal cells could hence represent a causal event in transmissible spongiform encephalopathy pathogenesis.

Molecular morphology and toxicity of cytoplasmic prion protein aggregates in neuronal and non-neuronal cells

Recent studies have revealed that accumulation of prion protein (PrP) in the cytoplasm results in the production of aggregates that are insoluble in non-ionic detergents and partially resistant to proteinase K. Transgenic mice expressing PrP in the cytoplasm develop severe ataxia with cerebellar degeneration and gliosis, suggesting that cytoplasmic PrP may play a role in the pathogenesis of prion diseases. The mechanism of cytoplasmic PrP neurotoxicity is not known. In this report, we determined the molecular morphology of cytoplasmic PrP aggregates by immunofluorescence and electron microscopy, in neuronal and non-neuronal cells. Transient expression of cytoplasmic PrP produced juxtanuclear aggregates reminiscent of aggresomes in human embryonic kidney 293 cells, human neuroblastoma BE2-M17 cells and mouse neuroblastoma N2a cells. Time course studies revealed that discrete aggregates form first throughout the cytoplasm, and then coalesce to form an aggresome. Aggresomes containing cytoplasmic PrP were 1-5-microm inclusion bodies and were filled with electron-dense particles. Cytoplasmic PrP aggregates induced mitochondrial clustering, reorganization of intermediate filaments, prevented the secretion of wild-type PrP molecules and diverted these molecules to the cytoplasm. Cytoplasmic PrP decreased the viability of neuronal and non-neuronal cells. We conclude that any event leading to accumulation of PrP in the cytoplasm is likely to result in cell death.

An endoplasmic-reticulum-specific apoptotic pathway is involved in prion and amyloid-beta peptides neurotoxicity

Prion (PrP) and amyloid-beta (Abeta) peptides are involved in the neuronal loss that occurs in Prion disorders (PrD) and Alzheimer's disease (AD), respectively, partially due to Ca(2+) dysregulation. Besides, the endoplasmic reticulum (ER) stress has an active role in the neurotoxic mechanisms that lead to these pathologies. Here, we analyzed whether the ER-mediated apoptotic pathway is involved in the toxic effect of synthetic PrP and Abeta peptides. In PrP106-126- and Abeta1-40-treated cortical neurons, the release of Ca(2+) through ER ryanodine (RyR) and inositol 1,4,5-trisphosphate (IP(3)R) receptors induces ER stress and leads to increased cytosolic Ca(2+) and reactive oxygen species (ROS) levels and subsequently to apoptotic death involving mitochondrial cytochrome c release and caspases activation. These results demonstrate that the early PrP- and Abeta-induced perturbation of ER Ca(2+) homeostasis is a death message that leads to neuronal loss, suggesting that the regulation of ER Ca(2+) levels may be a potential therapeutical target for PrD and AD.

Altered expression of the prion gene in rat astrocyte and neuron cultures treated with prion peptide 106-126

Neuronal degeneration and astrogliosis are hallmarks of prion disease. Synthetic prion protein (PrP) peptide 106-126 (PrP106-126) can induce death of neurons and proliferation of astrocytes in vitro and this neurotoxic effect depends on the expression of cellular PrP (PrPC) and is hence believed to be PrP(C) -mediated. To further elucidate the involvement of PrPC in PrP106-126-induced neurotoxicity, we determined the expression of PrP mRNA in primary culture of rat cortical neuron cells, cerebellar granule cells, and astrocytes following treatment with 50 microM of PrP106-126 scrambled PrP106-126 by quantitative real-time RT-PCR. As shown by MTT test, PrP106-126 induced significant death of neuron cells and marked proliferation of astrocytes after 10 days of treatment. Under the same treatment regimens, the level of PrP gene expression was significantly down-regulated in cortical neuron cell cultures and cerebellar granule cell cultures and was up-regulated in astrocyte cultures. The altered PrP gene expression occurred as early as 3 days after the treatment. After 10 days of treatment, while the cultured cortical neurons underwent further apoptosis, their expression of PrP gene started to recover gradually. These findings indicate that PrP 106-126 regulates transcription of the PrP gene and this activity is associated with its neurotoxicity in primary rat neuronal cultures.

Amyloid Fibrils of Mammalian Prion Protein Are Highly Toxic to Cultured Cells and Primary Neurons

A growing body of evidence indicates that small, soluble oligomeric species generated from a variety of proteins and peptides rather than mature amyloid fibrils are inherently highly cytotoxic. Here, we show for the first time that mature amyloid fibrils produced from full-length recombinant mammalian prion protein (rPrP) were highly toxic to cultured cells and primary hippocampal and cerebella neurons. Fibrils induced apoptotic cell death in a time- and dose-dependent manner. The toxic effect of fibrils was comparable with that exhibited by soluble small beta-oligomers generated from the same protein. Fibrils prepared from insulin were not toxic, suggesting that the toxic effect was not solely due to the highly polymeric nature of the fibrillar form. The cell death caused by rPrP fibrils or beta-oligomers was substantially reduced when expression of endogenous PrP(C) was down-regulated by small interfering RNAs. In opposition to the beta-oligomer and amyloid fibrils of rPrP, the monomeric alpha-helical form of rPrP stimulated neurite out-growth and survival of neurons. These studies illustrated that both soluble beta-oligomer and amyloid fibrils of the prion protein are intrinsically toxic and confirmed that endogenously expressed PrP(C) is required for mediating the toxicity of abnormally folded external PrP aggregates.

New Insights into Prion Structure and Toxicity

Prion diseases in humans and animals are due to conformational conversion of PrP(C), a cellular glycoprotein of unknown function, into PrP(Sc), an isoform that appears to be infectious in the absence of nucleic acids. Proteins that behave as prions are also found in yeast and filamentous fungi. Although there is now strong experimental support for the hypothesis that prions are infectious proteins, two subjects have remained poorly understood: the structure of prions, and the mechanisms by which they kill neurons. In this review, we will highlight recent studies that shed new light on these important issues.

The yeast prion Ure2p native-like assemblies are toxic to mammalian cells regardless of their aggregation state

The yeast prion Ure2p assembles in vitro into oligomers and fibrils retaining the alpha-helix content and binding properties of the soluble protein. Here we show that the different forms of Ure2p native-like assemblies (dimers, oligomers, and fibrils) are similarly toxic to murine H-END cells when added to the culture medium. Interestingly, the amyloid fibrils obtained by heat treatment of the toxic native-like fibrils appear harmless. Moreover, the Ure2p C-terminal domain, lacking the N-terminal segment necessary for aggregation but containing the glutathione binding site, is not cytotoxic. This finding strongly supports the idea that Ure2p toxicity depends on the structural properties of the flexible N-terminal prion domain and can therefore be considered as an inherent feature of the protein, unrelated to its aggregation state but rather associated with a basic toxic fold shared by all of the Ure2p native-like assemblies. Indeed, the latter are able to interact with the cell surface, leading to alteration of calcium homeostasis, membrane permeabilization, and oxidative stress, whereas the heat-treated amyloid fibrils do not. Our results support the idea of a general mechanism of toxicity of any protein/peptide aggregate endowed with structural features, making it able to interact with cell membranes and to destabilize them. This evidence extends the widely accepted view that the toxicity by protein aggregates is restricted to amyloid prefibrillar aggregates and provides new insights into the mechanism by which native-like oligomers compromise cell viability.

The interaction between cytoplasmic prion protein and the hydrophobic lipid core of membrane correlates with neurotoxicity

Prion protein (PrP), normally a cell surface protein, has been detected in the cytosol of a subset of neurons. The appearance of PrP in the cytosol could result from either retro-translocation of misfolded PrP from the endoplasmic reticulum (ER) or impaired import of PrP into the ER. Transgenic mice expressing cytoplasmic PrP (cyPrP) developed neurodegeneration in cerebellar granular neurons, although no detectable pathology was observed in other brain regions. In order to understand why granular neurons in the cerebellum were most susceptible to cyPrP-induced degeneration, we investigated the subcellular localization of cyPrP. Interestingly, we found that cyPrP is membrane-bound. In transfected cells, it binds to the ER and plasma/endocytic vesicular membranes. In transgenic mice, it is associated with synaptic and microsomal membranes. Furthermore, the cerebellar neurodegeneration in transgenic mice correlates with the interaction between cyPrP and the hydrophobic lipid core of the membrane but not with either the aggregation status or the dosage of cyPrP. These results suggest that lipid membrane perturbation could be a cellular mechanism for cyPrP-induced neurotoxicity and explain the seemingly conflicting results concerning cyPrP.

Lymphocyte Toxicity of Prion Fragments

Prion protein fragments that are extracted from the brains of patients with Gerstmann-Straussler-Scheinker disease are known to have stimulating action on circulating leukocytes. In particular, the amyloidogenic hydrophobic prion peptide HuPrP (113-127) AGAAAAGAVVGGLGG has been reported to be associated with significant cellular toxicity. In this paper we show that the self assembled form of HuPrP (113-127) and its valine rich domains viz. GAVVGGLG [HuPrP (119-126)] and VVGGLGG [HuPrP (121-127)] are toxic to peripheral lymphocytes. To explore the cytotoxic mechanism of these fragments, we studied 3-(4,5-dimethylthiazol-2yl)-2-5-diphenyltetrazolium bromide (MTT) reduction, reactive oxygen species (ROS) generation, calcium influx and raft sequestration of' peptide treated lymphocytes. Langmuir monolayer studies on these peptides showed a maximum lipid perturbing property of HuPrP (121-127) as compared to the other two fragments. MTT reduction assays on lymphocytes treated with peptides indicated that the prion peptide fibrils are relatively more toxic than freshly solubilized peptide preparations. Lymphocytes treated with HuPrP (121-127), HuPrP (113-127) and HuPrP (119-126) fibrils underwent 60%, 30% and 40% cell death, respectively. Abeta(1-42), HuPrP (119-126) and HuPrP (121-127) fibrils caused 4 fold increases in intracellular ROS as compared with control cells. However, HuPrP (113-127) fibrils lacked such a significant ROS generating activity, indicating that a subtle difference in sequence leads to a difference in the toxic mechanism in the cell. HuPrP (119-126) and HuPrP (121-127) fibrils also produced maximum raft sequestration and calcium influx. Taken together, these data suggest that the assemblage of prion fragments has significant toxic activity on peripheral lymphocytes, a finding with implications for controlling reactive lymphocytes in prion infected subjects.

BSE risk assessments in the UK: a risk tradeoff?

Risk assessments for bovine spongiform encephalopathy (BSE) should be based on the group risk and not the median individual risk. The group risk is calculated from the arithmetic mean risk, which in the case of dorsal root ganglia, is a factor of 50-fold higher than the median. For environmental routes, the arithmetic mean exposure is sufficient for risk assessment, while for food-borne routes failure to accommodate the variation in exposures to individuals across the UK population could overestimate the group risk considerably. Ignoring prion destruction by cooking could overestimate the food-borne risks still further. The recent estimate for the arithmetic mean cow-to-man species barrier of 4000 does not take into accounts either of these factors and thus may be too high. Until evidence for a threshold dose is demonstrated, public health scientists should avoid assessing safety on the basis of a 'minimum infective dose'. The incubation period observed in cattle-feeding studies, when completed, would continue to increase with decreasing dose below the ID50if there is a threshold or co-operative effect. The question is raised of whether fears over BSE in drinking water contributed to the spread of foot-and-mouth disease across the UK in 2001; a risk tradeoff.

High incidence of scrapie induced by repeated injections of subinfectious prion doses

To clarify the mechanisms leading to the development of Creutzfeldt-Jakob disease in some recipients of pituitary-derived human growth hormone (hGH), we investigated the effects of repeated injections of low prion doses in mice. The injections were performed, as in hGH-treated children, by a peripheral route at short intervals and for an extended period. Twelve groups of 24 mice were intraperitoneally inoculated one, two, or five times per week for 200 days with 2 x 10(-5) to 2 x 10(-8) dilutions of brain homogenate containing the mouse-adapted C506M3 scrapie strain. Sixteen control mice were injected once a week for 200 days with a 2 x 10(-4) dilution of normal brain homogenate. Of mice injected in a single challenge with a scrapie inoculum of a 2 x 10(-4), 2 x 10(-5), or 2 x 10(-6) dilution, 2/10, 1/10, and 0/10 animals developed scrapie, respectively. Control mice remained healthy. One hundred thirty-five of 135 mice injected with repeated prion doses of a 2 x 10(-5) or 2 x 10(-6) dilution succumbed to scrapie. Of mice injected with repeated scrapie doses of a 2 x 10(-7) or 2 x 10(-8) dilution, 52/59 and 38/67 animals died of scrapie, respectively. A high incidence of scrapie was observed in mice receiving repeated doses at low infectivity, whereas there was no disease in mice that were injected once with the same doses. Repeated injections of low prion doses thus constitute a risk for development of prion disease even if the same total dose inoculated in a single challenge does not induce the disease.

Cytosolic prion protein (PrP) is not toxic in N2a cells and primary neurons expressing pathogenic PrP mutations

Inherited prion diseases are linked to mutations in the prion protein (PrP) gene, which favor conversion of PrP into a conformationally altered, pathogenic isoform. The cellular mechanism by which this process causes neurological dysfunction is unknown. It has been proposed that neuronal death can be triggered by accumulation of PrP in the cytosol because of impairment of proteasomal degradation of misfolded PrP molecules retrotranslocated from the endoplasmic reticulum (Ma, J., Wollmann, R., and Lindquist, S. (2002) Science 298, 1781-1785). To test whether this neurotoxic mechanism is operative in inherited prion diseases, we evaluated the effect of proteasome inhibitors on the viability of transfected N2a cells and primary neurons expressing mouse PrP homologues of the D178N and nine octapeptide mutations. We found that the inhibitors caused accumulation of an unglycosylated, aggregated form of PrP exclusively in transfected N2a expressing PrP from the cytomegalovirus promoter. This form contained an uncleaved signal peptide, indicating that it represented polypeptide chains that had failed to translocate into the ER lumen during synthesis, rather than retrogradely translocated PrP. Quantification of N2a viability in the presence of proteasome inhibitors demonstrated that accumulation of this form was not toxic. No evidence of cytosolic PrP was found in cerebellar granule neurons from transgenic mice expressing wild-type or mutant PrPs from the endogenous promoter, nor were these neurons more susceptible to proteasome inhibitor toxicity than neurons from PrP knock-out mice. Our analysis fails to confirm the previous observation that mislocation of PrP in the cytosol is neurotoxic, and argues against the hypothesis that perturbation of PrP metabolism through the proteasomal pathway plays a pathogenic role in prion diseases.

The neurotoxicity of prion protein (PrP) peptide 106–126 is independent of the expression level of PrP and is not mediated by abnormal PrP species

A synthetic peptide homologous to region 106-126 of the prion protein (PrP) is toxic to cells expressing PrP, but not to PrP knockout neurons, arguing for a specific role of PrP in mediating the peptide's activity. Whether this is related to a gain of toxicity or a loss of function of PrP is not clear. We explored the possibility that PrP106-126 triggered formation of PrP(Sc) or other neurotoxic PrP species. We found that PrP106-126 did not induce detergent-insoluble and protease-resistant PrP, nor did it alter its membrane topology or cellular distribution. We also found that neurons expressing endogenous or higher level of either wild-type PrP or a nine-octapeptide insertional mutant were equally susceptible to PrP106-126, and that sub-physiological PrP expression was sufficient to restore vulnerability to the peptide. These results indicate that PrP106-126 interferes with a PrP function that requires only low protein levels, and is not impaired by a pathogenic insertion in the octapeptide region.

Role of glycosylphosphatidylinositols in the activation of phospholipase A2 and the neurotoxicity of prions

Prion-induced neuronal injury in vivo is associated with prostaglandin E(2) production, a process that can be reproduced in tissue-culture models of prion disease. In the present study, neuronal phospholipase A(2) was activated by glycosylphosphatidylinositols (GPIs) isolated from the cellular prion protein (PrP(c)) or from disease-associated isoforms (PrP(Sc)), resulting in prostaglandin E(2) production, but not by GPIs isolated from Thy-1. The ability of GPIs to activate neuronal phospholipase A(2) was lost following the removal of acyl chains or cleavage of the phosphatidylinositol-glycan linkage, and was inhibited by a mAb that recognized phosphatidylinositol. In competition assays, pretreatment of neurons with partial GPIs, inositol monophosphate or sialic acid reduced the production of prostaglandin E(2) in response to a synthetic miniprion (sPrP106), a synthetic correlate of a PrP(Sc) species found in Gerstmann-Straussler-Scheinker disease (HuPrP82-146), prion preparations or high concentrations of PrP-GPIs. In addition, neurons treated with inositol monophosphate or sialic acid were resistant to the otherwise toxic effects of sPrP106, HuPrP82-146 or prion preparations. This protective effect was selective, as inositol monophosphate- or sialic acid-treated neurons remained susceptible to the toxicity of arachidonic acid or platelet-activating factor. Addition of PrP-GPIs to cortical neuronal cultures increased caspase-3 activity, a marker of apoptosis that is elevated in prion diseases. In contrast, treatment of such cultures with inositol monophosphate or sialic acid greatly reduced sPrP106-induced caspase-3 activity and, in co-cultures, reduced the killing of sPrP106-treated neurons by microglia. These results implicate phospholipase A(2) activation by PrP-GPIs as an early event in prion-induced neurodegeneration.

Oligodendrocytes are susceptible to apoptotic cell death induced by prion protein-derived peptides

Neurodegenerative prion diseases, characterized by a progressive dementia, are associated with the accumulation of abnormal forms of the prion (PrPc) protein, potentially due to an aberrant regulation of PrPc biogenesis and/or topology. One of these forms, termed ctmPrP, displays a transmembrane conformation and might trigger neuronal cell death in Gerstmann-Straüssler-Scheinker (GSS) syndrome and other prion-associated diseases in humans. Although the primary target cells involved in the progression of prion diseases remain unidentified, it was recently suggested that modifications of the oligodendroglial cells occur early in prion diseases. In the present study, we demonstrate that a putative transmembrane domain of the human PrPc, i.e., amino acids 118-135, induces oligodendrocyte (OLG) death in vitro in a time- and dose-dependent manner. The process leading to OLG death and induced by the PrP[118-135] peptide was characterized by DNA fragmentation, cytoskeletal disruption, and caspase activation. Protection against the PrP[118-135] peptide-induced OLG apoptosis by several antioxidant molecules, such as probucol, propylgallate, and promethazine, suggests that oxidative injuries contribute to the PrP[118-135] cytotoxicity to OLGs. These results suggest a potential pathophysiological role of the ctmPrP- and/or PrP fragment-mediated OLG cytotoxicity in spongiform encephalopathies.

Accumulation of prion protein in muscle fibers of experimental chloroquine myopathy: in vivo model for deposition of prion protein in non-neuronal tissues

Prion protein (PrP) is known to accumulate in some non-neuronal tissues under conditions unrelated to prion diseases. The biochemical and biological nature of such accumulated PrP molecules, however, has not been fully evaluated. In this study, we established experimental myopathy in hamsters by long-term administration of chloroquine, and we examined the nature of the PrP molecules that accumulated. PrP accumulation was immunohistochemically demonstrated in autophagic vacuoles in degenerated muscle fibers, and this was accompanied by the accumulation of other molecules related to the neuropathogenesis of prion diseases such as clathrin, cathepsin B, heparan sulfate, and apolipoprotein J. Accumulated PrP molecules were partially insoluble in detergent solution and were slightly less sensitive to proteinase K digestion than normal cellular PrP. Muscle homogenates containing these PrP molecules did not cause disease in inoculated hamsters. The findings indicate that the PrP molecules that accumulated in muscle fibers have distinct biochemical and biological properties. Therefore, experimental chloroquine myopathy is a novel and useful model to investigate the mechanism of deposition of PrP in non-neuronal tissues and might provide new insights in the pathogenesis of prion diseases.

Humanin rescues cortical neurons from prion-peptide-induced apoptosis

We recently demonstrated that a soluble oligomeric prion peptide, the putative 118-135 transmembrane domain of prion protein (PrP), exhibited membrane fusogenic properties and induced apoptotic cell death both in vitro and in vivo. A recently discovered rescue factor humanin (HN) was shown to protect neuronal cells from various insults involved in human neurodegenerative diseases. We thus addressed the question of whether HN might modulate the apoptosis induced by the soluble PrP(118-135) fragment. We found that the incubation of rat cortical neurons with 10 microM HN prevented soluble PrP(118-135) fragment-induced cell death concomitantly with inhibition of apoptotic events. An HN variant, termed HNG, exhibited a 500-fold increase in the protective activity in cortical neurons, whereas the HNA variant displayed no protective effect. The effects of HN and HNG peptides did not require a preincubation with the PrP(118-135) fragment, strongly suggesting that these peptides rescue cells independently of a direct interaction with the prion peptide. By contrast, and in agreement with a previous study, HN had no effect on the fibrillar PrP(106-126) peptide-induced cell death. This protective effect for neurons from PrP(118-135)-induced cell death strongly suggests that PrP(118-135) and PrP(106-126) peptides may trigger different pathways leading to neuronal apoptosis.

Copper(II) complexes of peptide fragments of the prion protein. Conformation changes induced by copper(II) and the binding motif in C-terminal protein region

In this paper, we report the characterization of copper(II) complexes with two prion (PrP) protein peptide fragment analogues (VNITKQHTVTTTT), one with the N-terminus acetylated and the C-terminus amidated (PrP Ac180-193NH2) and the other with both the C- and N-termini free (PrP 180-193). Such peptide sequence almost entirely encompasses the PrPC's helix 2 in the C-terminal region. The stoichiometry, the binding modes and the conformational features of the copper(II) complexes with the above mentioned two peptides were investigated by electrospray ionization-mass spectrometry (ESI-MS), UV-visible (UV-Vis) spectrometry and electron paramagnetic resonance (EPR) spectrometry as well as by circular dichroism (CD) measurements. The binding site location of copper(II) in the structured region of the protein can be here suggested on the basis of our findings that show the involvement of His 187 residue. The similarity of the EPR parameters suggests that the anchoring imidazole residue drives the copper(II) coordination environment towards a common binding motif in different regions of the prion protein.

The role of platelet activating factor in prion and amyloid-β neurotoxicity

In the prion diseases, neurodegeneration is preceded by the accumulation of the disease-associated isoform of the prion protein (PrP). In the present study, neurones treated with three different phospholipase A2 inhibitors were resistant to the toxic effects of PrP peptides or a synthetic miniprion (sPrP106). Phospholipase A2 inhibitors also protected neurones against a toxic peptide found in Alzheimer's disease (amyloid-beta1-42). Further studies showed that neurones pre-treated with platelet activating factor (PAF) antagonists were equally resistant to PrP peptides or amyloid-beta1-42. Moreover, both phospholipase A2 inhibitors and PAF antagonists reduced the activation of caspase-3, a marker of apoptosis, and the production of prostaglandin E2 that is closely associated with neuronal death in prion or Alzheimer's diseases.