Prion protein fragment PrP-(106-126) induces apoptosis via mitochondrial disruption in human neuronal SH-SY5Y cells

Assessment of the therapeutic potential of Hsp70 activator against prion diseases using in vitro and in vivo models

Introduction

Prion diseases are deadly neurodegenerative disorders in both animals and humans, causing the destruction of neural tissue and inducing behavioral manifestations. Heat shock proteins (Hsps), act as molecular chaperones by supporting the appropriate folding of proteins and eliminating the misfolded proteins as well as playing a vital role in cell signaling transduction, cell cycle, and apoptosis control. SW02 is a potent activator of Hsp 70 kDa (Hsp70).

Methods

In the current study, the protective effects of SW02 against prion protein 106-126 (PrP106-126)-induced neurotoxicity in human neuroblastoma cells (SH-SY5Y) were investigated. In addition, the therapeutic effects of SW02 in ME7 scrapie-infected mice were evaluated.

Results

The results showed that SW02 treatment significantly increased Hsp70 mRNA expression levels and Hsp70 ATPase activity (p < 0.01). SW02 also significantly inhibited cytotoxicity and apoptosis induced by PrP106-126 (p < 0.01) and promoted neurite extension. In vivo, intraperitoneal administration of SW02 did not show a statistically significant difference in survival time (p = 0.16); however, the SW02-treated group exhibited a longer survival time of 223.6 ± 6.0 days compared with the untreated control group survival time of 217.6 ± 5.4 days. In addition, SW02 reduced the PrPSc accumulation in ME7 scrapie-infected mice at 5 months post-injection (p < 0.05). A significant difference was not observed in GFAP expression, an astrocyte marker, between the treated and untreated groups.

Conclusion

In conclusion, the potential therapeutic role of the Hsp70 activator SW02 was determined in the present study and may be a novel and effective drug to mitigate the pathologies of prion diseases and other neurodegenerative diseases. Further studies using a combination of two pharmacological activators of Hsp70 are required to maximize the effectiveness of each intervention.

Human prion disease: molecular pathogenesis, and possible therapeutic targets and strategies

INTRODUCTION

Human prion diseases are heterogeneous, and often rapidly progressive, transmissible neurodegenerative disorders associated with misfolded prion protein (PrP) aggregation and self-propagation. Despite their rarity, prion diseases comprise a broad spectrum of phenotypic variants determined at the molecular level by different conformers of misfolded PrP and host genotype variability. Moreover, they uniquely occur in idiopathic, genetically determined, and acquired forms with distinct etiologies.

AREA COVERED

This review provides an up-to-date overview of potential therapeutic targets in prion diseases and the main results obtained in cell and animal models and human trials. The open issues and challenges associated with developing effective therapies and informative clinical trials are also discussed.

EXPERT OPINION

Currently tested therapeutic strategies target the cellular PrP to prevent the formation of misfolded PrP or to favor its elimination. Among them, passive immunization and gene therapy with antisense oligonucleotides against prion protein mRNA are the most promising. However, the disease's rarity, heterogeneity, and rapid progression profoundly frustrate the successful undertaking of well-powered therapeutic trials and patient identification in the asymptomatic or early stage before the development of significant brain damage. Thus, the most promising therapeutic goal to date is preventing or delaying phenoconversion in carriers of pathogenic mutations by lowering prion protein expression.

Impairment of Neuronal Mitochondrial Quality Control in Prion-Induced Neurodegeneration

Mitochondrial dynamics continually maintain cell survival and bioenergetics through mitochondrial quality control processes (fission, fusion, and mitophagy). Aberrant mitochondrial quality control has been implicated in the pathogenic mechanism of various human diseases, including cancer, cardiac dysfunction, and neurological disorders, such as Alzheimer’s disease, Parkinson’s disease, and prion disease. However, the mitochondrial dysfunction-mediated neuropathological mechanisms in prion disease are still uncertain. Here, we used both in vitro and in vivo scrapie-infected models to investigate the involvement of mitochondrial quality control in prion pathogenesis. We found that scrapie infection led to the induction of mitochondrial reactive oxygen species (mtROS) and the loss of mitochondrial membrane potential (ΔΨm), resulting in enhanced phosphorylation of dynamin-related protein 1 (Drp1) at Ser616 and its subsequent translocation to the mitochondria, which was followed by excessive mitophagy. We also confirmed decreased expression levels of mitochondrial oxidative phosphorylation (OXPHOS) complexes and reduced ATP production by scrapie infection. In addition, scrapie-infection-induced aberrant mitochondrial fission and mitophagy led to increased apoptotic signaling, as evidenced by caspase 3 activation and poly (ADP-ribose) polymerase cleavage. These results suggest that scrapie infection induced mitochondrial dysfunction via impaired mitochondrial quality control processes followed by neuronal cell death, which may have an important role in the neuropathogenesis of prion diseases.

Interaction of Prion Peptides with DNA Structures

Prion protein aggregation is known to be modulated by macromolecules including nucleic acids. To clarify the role of nucleic acids in PrP pathology, we investigated the interaction between nucleic acids and the prion peptide (PrP)-a synthetic prion protein model peptide resembling a portion of the human prion protein in structure and function spanning amino acid residues 106-126. We used synthetic DNA lattices and natural DNA duplexes extracted from salmon (sDNA) bound with PrP and studied their interaction using distinct physical measurements. The formation of DNA lattices with PrP was visualized by atomic force microscopy (AFM) to investigate the influence of the PrP. PrP inhibited the growth of the double-crossover (DX) lattices significantly compared to the control peptide (CoP). We also conducted optical measurements such as ultraviolet-visible (UV-Vis), circular dichroism (CD), and Fourier transform infrared (FTIR) spectroscopies to validate the interaction between PrP and DNA immediately (D0) and after a 30-day incubation (D30) period. UV-Vis spectra showed variation in the absorbance intensities, specific for the binding of CoP and PrP to DNA. The CD analysis revealed the presence of various secondary structures, such as α-helices and β-sheets, in PrP- and PrP-bound sDNA complexes. The PrP-sDNA interaction was confirmed using FTIR by the change and shift of the absorption peak intensity and the alteration of PrP secondary structures in the presence of DNA. The cytotoxic effects of the PrP-bound sDNA complexes were assessed by a cytotoxicity assay in human neuroblastoma cells in culture. It confirmed that PrP with sDNA was less cytotoxic than CoP. This study provides new applications for DNA molecules by investigating their effect in complex with aggregated proteins. Our study unequivocally showed the beneficial effect of the interaction between DNA and the pathological prion protein. It therefore provides valuable information to exploit this effect in the development of potential therapeutics. Moreover, our work might serve as a basis for further studies investigating the role of DNA interactions with other amyloidogenic proteins.

Calcineurin Activation by Prion Protein Induces Neurotoxicity via Mitochondrial Reactive Oxygen Species

Prion diseases are caused by PrPsc accumulation in the brain, which triggers dysfunctional mitochondrial injury and reactive oxygen species (ROS) generation in neurons. Recent studies on prion diseases suggest that endoplasmic reticulum (ER) stress induced by misfolding proteins such as misfolded prion protein results in activation of calcineurin. Calcineurin is a calcium-related protein phosphatase of type 2B that exists in copious quantities in the brain and acts as a critical nodal component in the control of cellular functions. To investigate the relationship between calcineurin and intracellular ROS, we assessed the alteration of CaN and ROS induced by prion peptide (PrP) 106-126. Human prion peptide increased mitochondrial ROS by activating calcineurin, and the inhibition of calcineurin activity protected mitochondrial function and neuronal apoptosis in neuronal cells. These results suggest that calcineurin plays a pivotal role in neuronal apoptosis by mediating mitochondrial injury and ROS in prion diseases.

The pro-apoptotic domain of BIM protein forms toxic amyloid fibrils

BIM is a key apoptotic protein, participating in diverse cellular processes. Interestingly, recent studies have hypothesized that BIM is associated with the extensive neuronal cell death encountered in protein misfolding diseases, such as Alzheimer's disease. Here, we report that the core pro-apoptotic domain of BIM, the BIM-BH3 motif, forms ubiquitous amyloid fibrils. The BIM-BH3 fibrils exhibit cytotoxicity, disrupt mitochondrial functions, and modulate the structures and dynamics of mitochondrial membrane mimics. Interestingly, a slightly longer peptide in which BIM-BH3 was flanked by four additional residues, widely employed as a model of the pro-apoptotic core domain of BIM, did not form fibrils, nor exhibited cell disruptive properties. The experimental data suggest a new mechanistic role for the BIM-BH3 domain, and demonstrate, for the first time, that an apoptotic peptide forms toxic amyloid fibrils.

Prion peptide-mediated calcium level alteration governs neuronal cell damage through AMPK-autophagy flux

BACKGROUND

The distinctive molecular structure of the prion protein, PrPsc, is established only in mammals with infectious prion diseases. Prion protein characterizes either the transmissible pathogen itself or a primary constituent of the disease. Our report suggested that prion protein-mediated neuronal cell death is triggered by the autophagy flux. However, the alteration of intracellular calcium levels, AMPK activity in prion models has not been described. This study is focused on the effect of the changes in intracellular calcium levels on AMPK/autophagy flux pathway and PrP (106-126)-induced neurotoxicity.

METHODS

Western blot and Immunocytochemistry was used to detect AMPK and autophagy-related protein expression. Flow cytometry and a TdT-mediated biotin-16-dUTP nick-end labeling (TUNEL) assay were used to detect the percentage of apoptotic cells. Calcium measurement was employed using fluo-4 by confocal microscope.

RESULTS

We examined the effect of calcium homeostasis alterations induced by human prion peptide on the autophagy flux in neuronal cells. Treatment with human prion peptide increased the intracellular calcium concentration and induced cell death in primary neurons as well as in a neuronal cell line. Using pharmacological inhibitors, we showed that the L-type calcium channel is involved in the cellular entry of calcium ions. Inhibition of calcium uptake prevented autophagic cell death and reduction in AMP-activated protein kinase (AMPK) activity induced by human prion peptide.

CONCLUSION

Our data demonstrated that prion peptide-mediated calcium inflow plays a pivotal role in prion peptide-induced autophagic cell death, and reduction in AMPK activity in neurons. Altogether, our results suggest that calcium influx might play a critical role in neurodegenerative diseases, including prion diseases. Video Abstract.

Revisiting thioflavin T (ThT) fluorescence as a marker of protein fibrillation - The prominent role of electrostatic interactions

Thioflavin T (ThT), a benzothiazole-based fluorophore, is a prominent dye widely employed for monitoring amyloid fibril assembly. Despite the near-universal presumption that ThT binds to β-sheet domains upon fibrillar surface via hydrophobic forces, the contribution of the positive charge of ThT to fibril binding and concomitant fluorescence enhancement have not been thoroughly assessed. Here we demonstrate a considerable interdependence between ThT fluorescence and electrostatic charges of peptide fibrils. Specifically, by analyzing both fibril-forming synthetic peptides and prominent natural fibrillar peptides, we demonstrate pronounced modulations of ThT fluorescence signal that were solely dependent upon electrostatic interactions between ThT and peptide surface. The results further attest to the fact that fibril ζ-potential rather than pH-dependent assembly of the fibrils constitute the primary factor affecting ThT binding and fluorescence. This study provides the first quantitative assessment of electrostatically driven ThT fluorescence upon adsorption to amyloid fibrils.

Imaging Flow Cytometry Illuminates New Dimensions of Amyloid Peptide-Membrane Interactions

Membrane interactions of amyloidogenic proteins constitute central determinants both in protein aggregation as well as in amyloid cytotoxicity. Most reported studies of amyloid peptide-membrane interactions have employed model membrane systems combined with application of spectroscopy methods or microscopy analysis of individual binding events. Here, we applied for the first time, to our knowledge, imaging flow cytometry for investigating interactions of representative amyloidogenic peptides, namely, the 106-126 fragment of prion protein (PrP(106-126)) and the human islet amyloid polypeptide (hIAPP), with giant lipid vesicles. Imaging flow cytometry was also applied to examine the inhibition of PrP(106-126)-membrane interactions by epigallocatechin gallate, a known modulator of amyloid peptide aggregation. We show that imaging flow cytometry provided comprehensive population-based statistical information upon morphology changes of the vesicles induced by PrP(106-126) and hIAPP. Specifically, the experiments reveal that both PrP(106-126) and hIAPP induced dramatic transformations of the vesicles, specifically disruption of the spherical shapes, reduction of vesicle circularity, lobe formation, and modulation of vesicle compactness. Interesting differences, however, were apparent between the impact of the two peptides upon the model membranes. The morphology analysis also showed that epigallocatechin gallate ameliorated vesicle disruption by PrP(106-126). Overall, this study demonstrates that imaging flow cytometry provides powerful means for disclosing population-based morphological membrane transformations induced by amyloidogenic peptides and their inhibition by aggregation modulators.

Thymosin beta 4 attenuates PrP(106-126)-induced human brain endothelial cells dysfunction

The blood-brain barrier (BBB) is a highly selective permeability barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). The BBB is formed by cerebral endothelial cells connected by tight junctions. Prion diseases are neurodegenerative pathologies characterized by the accumulation of altered forms of the prion protein (PrP), named PrP^Sc. Thymosin beta 4 (Tβ₄) is an actin-sequestering peptide known to bind monomeric actin and inhibit its polymerization, and it is known to have a neuroprotective effect. However, the effect of Tβ₄ on prion disease has not yet been investigated. Therefore, in this study, we investigated the effect of Tβ₄ on prion-induced BBB dysfunction in hCMEC/D3 human cerebral endothelial cells. We found that Tβ₄ increased the expression of tight junction protein, but reduced the ratio of F-actin to G-actin. Moreover, we showed that Tβ₄ significantly improved PrP (106-126)-induced vascular permeability dysfunction in hCMEC/D3 cells. Through human BBB in vitro model, we found that PrP (106-126) could disrupt tight junctions and cytoskeleton arrangement. These results suggest that Tβ₄ may play a critical role in barrier stabilization. Furthermore, Tβ₄ may prevent neurodegenerative diseases caused by prion-induced BBB dysfunction.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual's brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual’s brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

REST alleviates neurotoxic prion peptide-induced synaptic abnormalities, neurofibrillary degeneration and neuronal death partially via LRP6-mediated Wnt-β-catenin signaling

Prion diseases are a group of infectious neurodegenerative diseases characterized by multiple neuropathological hallmarks including synaptic damage, spongiform degeneration and neuronal death. The factors and mechanisms that maintain cellular morphological integrity and protect against neurodegeneration in prion diseases are still unclear. Here we report that after stimulation with the neurotoxic PrP106-126 fragment in primary cortical neurons, REST translocates from the cytoplasm to the nucleus and protects neurons from harmful effects of PrP106-126. Overexpression of REST reduces pathological damage and abnormal biochemical alterations of neurons induced by PrP106-126 and maintains neuronal viability by stabilizing the level of pro-survival protein FOXO1 and inhibiting the permeability of the mitochondrial outer membrane, release of cytochrome c from mitochondria to cytoplasm and the activation of Capase3. Conversely, knockdown of REST exacerbates morphological damage and inhibits the expression of FOXO1. Additionally, by overexpression or knockdown of LRP6, we further show that LRP6-mediated Wnt-β-catenin signaling partly regulates the expression of REST. Collectively, we demonstrate for the first time novel neuroprotective function of REST in prion diseases and hypothesise that the LRP6-Wnt-β-catenin/REST signaling plays critical and collaborative roles in neuroprotection. This signaling of neuronal survival regulation could be explored as a viable therapeutic target for prion diseases and associated neurodegenerative diseases.

The Pathogenic A116V Mutation Enhances Ion-Selective Channel Formation by Prion Protein in Membranes

Prion diseases are a group of fatal neurodegenerative disorders that afflict mammals. Misfolded and aggregated forms of the prion protein (PrP(Sc)) have been associated with many prion diseases. A transmembrane form of PrP favored by the pathogenic mutation A116V is associated with Gerstmann-Sträussler-Scheinker syndrome, but no accumulation of PrP(Sc) is detected. However, the role of the transmembrane form of PrP in pathological processes leading to neuronal death remains unclear. This study reports that the full-length mouse PrP (moPrP) significantly increases the permeability of living cells to K(+), and forms K(+)- and Ca(2+)-selective channels in lipid membranes. Importantly, the pathogenic mutation A116V greatly increases the channel-forming capability of moPrP. The channels thus formed are impermeable to sodium and chloride ions, and are blocked by blockers of voltage-gated ion channels. Hydrogen-deuterium exchange studies coupled with mass spectrometry (HDX-MS) show that upon interaction with lipid, the central hydrophobic region (109-132) of the protein is protected against exchange, making it a good candidate for inserting into the membrane and lining the channel. HDX-MS also shows a dramatic increase in the protein-lipid stoichiometry for A116V moPrP, providing a rationale for its increased channel-forming capability. The results suggest that ion channel formation may be a possible mechanism of PrP-mediated neurodegeneration by the transmembrane forms of PrP.

Membrane Disruption Mechanism of a Prion Peptide (106-126) Investigated by Atomic Force Microscopy, Raman and Electron Paramagnetic Resonance Spectroscopy

A fragment of the human prion protein spanning residues 106-126 (PrP106-126) recapitulates many essential properties of the disease-causing protein such as amyloidogenicity and cytotoxicity. PrP106-126 has an amphipathic characteristic that resembles many antimicrobial peptides (AMPs). Therefore, the toxic effect of PrP106-126 could arise from a direct association of monomeric peptides with the membrane matrix. Several experimental approaches are employed to scrutinize the impacts of monomeric PrP106-126 on model lipid membranes. Porous defects in planar bilayers are observed by using solution atomic force microscopy. Adding cholesterol does not impede defect formation. A force spectroscopy experiment shows that PrP106-126 reduces Young's modulus of planar lipid bilayers. We use Raman microspectroscopy to study the effect of PrP106-126 on lipid atomic vibrational dynamics. For phosphatidylcholine lipids, PrP106-126 disorders the intrachain conformation, while the interchain interaction is not altered; for phosphatidylethanolamine lipids, PrP106-126 increases the interchain interaction, while the intrachain conformational order remains similar. We explain the observed differences by considering different modes of peptide insertion. Finally, electron paramagnetic resonance spectroscopy shows that PrP106-126 progressively decreases the orientational order of lipid acyl chains in magnetically aligned bicelles. Together, our experimental data support the proposition that monomeric PrP106-126 can disrupt lipid membranes by using similar mechanisms found in AMPs.

Parkin Overexpression Ameliorates PrP106-126-Induced Neurotoxicity via Enhanced Autophagy in N2a Cells

Transmissible spongiform encephalopathies (TSEs) are caused by the accumulation of the abnormal prion protein scrapie (PrPSc). Prion protein aggregation, misfolding, and cytotoxicity in the brain are the major causes of neuronal dysfunction and ultimate neurodegeneration in all TSEs. Parkin, an E3 ubiquitin ligase, has been studied extensively in all major protein misfolding aggregating diseases, especially Parkinson's disease and Alzheimer's disease, but the role of parkin in TSEs remains unknown. Here we investigated the role of parkin in a prion disease cell model in which neuroblastoma2a (N2a) cells were treated with prion peptide PrP106-126. We observed a gradual decrease in the soluble parkin level upon treatment with PrP106-126 in a time-dependent manner. Furthermore, endogenous parkin colocalized with FITC-tagged prion fragment106-126. Overexpression of parkin in N2a cells via transfection repressed apoptosis by enhancing autophagy. Parkin-overexpressing cells also showed reductions in apoptotic BAX translocation to the mitochondria and cytochrome c release to the cytosol, which ultimately inhibited activation of proapoptotic caspases. Taken together, our findings reveal a parkin-mediated cytoprotective mechanism against PrP106-126 toxicity, which is a novel potential therapeutic target for treating prion diseases.

Cellular prion protein is present in mitochondria of healthy mice

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycophosphatidylinositol (GPI) anchor. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. The precise function of PrP^C remains elusive but may depend upon its cellular localization. Here we show that PrP^C is present in brain mitochondria from 6–12 week old wild-type and transgenic mice in the absence of disease. Mitochondrial PrP^C was fully processed with mature N-linked glycans and did not require the GPI anchor for localization. Protease treatment of purified mitochondria suggested that mitochondrial PrP^C exists as a transmembrane isoform with the C-terminus facing the mitochondrial matrix and the N-terminus facing the intermembrane space. Taken together, our data suggest that PrP^C can be found in mitochondria in the absence of disease, old age, mutation, or overexpression and that PrP^C may affect mitochondrial function.

Chaperone-like Activity of Calnuc Prevents Amyloid Aggregation

Calnuc is a ubiquitously expressed protein of the EF-hand Ca²⁺-binding superfamily. Previous studies have implicated it in Ca²⁺-sensitive physiological processes, whereas details of its function and involvement in human diseases are lacking. Drawing upon the sequence homology of calnuc with calreticulin, we propose it functions as a molecular chaperone-like protein. In cells under thermal, chemical [urea and guanidinium chloride (GdmCl)], and acidic stress, calnuc exhibits properties similar to those of established chaperone-like proteins (GRP78, spectrin, and α-crystallin), effectively demonstrated by its ability to suppress aggregation of malate dehydrogenase (MDH), alcohol dehydrogenase, and catalase. Calnuc aids in refolding of MDH with retention of 80% of its enzymatic activity. In HEK293 cells subjected to heat shock, calnuc chaperones luciferase, protecting its activity. Our in vitro and cell culture results establish the ability of calnuc to inhibit fibrillation of insulin and lysozyme and validate its neuroprotective role in cells treated with amyloid fibrils. Calnuc also rescues cells from fibrillar toxicity (caused by misfolded or aggregated proteins), providing a plausible explanation for the previous observation of its low level of expression in brains affected by Alzheimer's disease. We propose that calnuc is possibly involved in controlling protein unfolding diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), prion disease, and type II diabetes.

EGCG-mediated autophagy flux has a neuroprotection effect via a class III histone deacetylase in primary neuron cells

Prion diseases caused by aggregated misfolded prion protein (PrP) are transmissible neurodegenerative disorders that occur in both humans and animals. Epigallocatechin-3-gallate (EGCG) has preventive effects on prion disease; however, the mechanisms related to preventing prion diseases are unclear. We investigated whether EGCG, the main polyphenol in green tea, prevents neuron cell damage induced by the human prion protein. We also studied the neuroprotective mechanisms and proper signals mediated by EGCG. The results showed that EGCG protects the neuronal cells against human prion protein-induced damage through inhibiting Bax and cytochrome c translocation and autophagic pathways by increasing LC3-II and reducing and blocking p62 by using ATG5 small interfering (si) RNA and autophagy inhibitors. We further demonstrated that the neuroprotective effects of EGCG were exhibited by a class III histone deacetylase; sirt1 activation and the neuroprotective effects attenuated by sirt1 inactivation using sirt1 siRNA and sirtinol. We demonstrated that EGCG activated the autophagic pathways by inducing sirt1, and had protective effects against human prion protein-induced neuronal cell toxicity. These results suggest that EGCG may be a therapeutic agent for treatment of neurodegenerative disorders including prion diseases.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Influence of oxodiperoxovanadate complexes on prion neuropeptide fibril formation

Different oxodiperoxovanadate complexes inhibit the fibril formation of prion neuropeptides by different action modes.

Neuroprotective effect of cellular prion protein (PrPC) is related with activation of alpha7 nicotinic acetylcholine receptor (α7nAchR)-mediated autophagy flux

Activation of the alpha7 nicotinic acetylcholine receptor (α7nAchR) is regulated by prion protein (PrPC) expression and has a neuroprotective effect by modulating autophagic flux. In this study, we hypothesized that PrPC may regulate α7nAchR activation and that may prevent prion-related neurodegenerative diseases by regulating autophagic flux. PrP(106-126) treatment decreased α7nAchR expression and activation of autophagic flux. In addition, the α7nAchR activator PNU-282987 enhanced autophagic flux and protected neuron cells against PrP(106-126)-induced apoptosis. However, activation of autophagy and the protective effects of PNU-282987 were inhibited in PrPC knockout hippocampal neuron cells. In addition, PrPC knockout hippocampal neuron cells showed decreased α7nAchR expression levels. Adenoviral overexpression of PrPC in PrPC knockout hippocampal neuron cells resulted in activation of autophagic flux and inhibition of prion peptide-mediated cell death via α7nAchR activation. This is the first report demonstrating that activation of α7nAchR-mediated autophagic flux is regulated by PrPC, and that activation of α7nAchR regulated by PrPC expression may play a pivotal role in protection of neuron cells against prion peptide-induced neuron cell death by autophagy. These results suggest that α7nAchR-mediated autophagic flux may be involved in the pathogenesis of prion-related diseases and may be a therapeutic target for prion-related neurodegenerative diseases.

The intricate mechanisms of neurodegeneration in prion diseases

Prion diseases are a group of infectious neurodegenerative diseases with an entirely novel mechanism of transmission, involving a protein-only infectious agent that propagates the disease by transmitting protein conformational changes. The disease results from extensive and progressive brain degeneration. The molecular mechanisms involved in neurodegeneration are not entirely known but involve multiple processes operating simultaneously and synergistically in the brain, including spongiform degeneration, synaptic alterations, brain inflammation, neuronal death and the accumulation of protein aggregates. Here, we review the pathways implicated in prion-induced brain damage and put the pieces together into a possible model of neurodegeneration in prion disorders. A more comprehensive understanding of the molecular basis of brain degeneration is essential to develop a much needed therapy for these devastating diseases.

Cytosolically expressed PrP GPI-signal peptide interacts with mitochondria

We previously reported that PrP GPI-anchor signal peptide (GPI-SP) is specifically degraded by the proteasome. Additionally, we showed that the point mutation P238S, responsible for a genetic form of prion diseases, while not affecting the GPI-anchoring process, results in the accumulation of PrP GPI-SP, suggesting the possibility that PrP GPI-anchor signal peptide could play a role in neurodegenerative prion diseases. We now show that PrP GPI-SP, when expressed as a cytosolic peptide, is able to localize to the mitochondria and to induce mitochondrial fragmentation and vacuolarization, followed by loss in mitochondrial membrane potential, ultimately resulting in apoptosis. Our results identify the GPI-SP of PrP as a novel candidate responsible for the impairment in mitochondrial function involved in the synaptic pathology observed in prion diseases, establishing a link between PrP GPI-SP accumulation and neuronal death.

In vitro neurotoxicity evaluation of piperazine designer drugs in differentiated human neuroblastoma SH-SY5Y cells

Abuse of synthetic drugs is widespread worldwide. Studies indicate that piperazine designer drugs act as substrates at dopaminergic and serotonergic receptors and/or transporters in the brain. This work aimed to investigate the cytotoxicity of N-benzylpiperazine, 1-(3-trifluoromethylphenyl)piperazine, 1-(4-methoxyphenyl)piperazine and 1-(3,4-methylenedioxybenzyl)piperazine in the differentiated human neuroblastoma SH-SY5Y cell line. Cytotoxicity was evaluated after 24 h incubations through the MTT reduction and neutral red uptake assays. Oxidative stress (reactive oxygen and nitrogen species production and glutathione content) and energetic (ATP content) parameters, as well as intracellular Ca(2+), mitochondrial membrane potential, DNA damage (comet assay) and cell death mode were also evaluated. Complete cytotoxicity curves were obtained after 24 h incubations with each drug. A significant decrease in intracellular total glutathione content was noted for all the tested drugs. All drugs caused a significant increase of intracellular free Ca(2+) levels, accompanied by mitochondrial hyperpolarization. However, ATP levels remained unchanged. The investigation of cell death mode revealed a predominance of early apoptotic cells. No genotoxicity was found in the comet assay. Among the tested drugs, 1-(3-trifluoromethylphenyl)piperazine was the most cytotoxic. Overall, piperazine designer drugs are potentially neurotoxic, supporting concerns on risks associated with the abuse of these drugs.

The Unexposed Secrets of Prion Protein Oligomers

According to the "protein-only" hypothesis, the misfolding and conversion of host-derived cellular prion protein (PrP(C)) into pathogenically misfolded PrP are believed to be the key procedure in the pathogenesis of prion diseases. Intermediate, soluble oligomeric prion protein (PrP) aggregates were considered a critical process for prion diseases. Several independent studies on PrP oligomers gained insights into oligomers' formation, biophysical and biochemical characteristics, structure conversion, and neurotoxicity. PrP oligomers are rich in β-sheet structure and slightly resistant to proteinase K digestion. PrP oligomers exhibited more neurotoxicity and induced neuronal apoptosis in vivo and/or in vitro. In this review, we summarized recent studies regarding PrP oligomers and the relationship between misfolded PrP aggregates and neuronal death in the course of prion diseases.

Methionine oxidation of amyloid peptides by peroxovanadium complexes: inhibition of fibril formation through a distinct mechanism

Peroxovanadium complexes inhibit the fibril formation of neurodegenerative amyloid peptides by oxidizing methionine residues.

Baicalein prevents human prion protein-induced neuronal cell death by regulating JNK activation

Prion diseases are neurodegenerative disorders characterized by the accumulation of an abnormal isoform of the protease-insensitive isoform (PrPSc) of prion protein. Human prion protein fragment 106‑126 [PrP (106‑126)] contains most of the pathological characteristics associated with PrPSc. Although a number of compounds have been identified to inhibit PrP accumulation or dissolve fibrils and aggregates in vitro, there is currenlty no treatment available for these progressive neurodegenerative diseases. Baicalein, the dried root of Scutellaria baicalensis (S. baicalensis) Georgi (known as Huang-qin in traditional Chinese medicine) has been reported to exert neuroprotective effects on neurodegenerative diseases. In the present study, we investigated the effects of baicalein on the development of prion diseases using SH-SY5Y and SK-N-SH cells in vitro. We found that baicalein protected the cells against PrP‑induced neuronal cell death by inhibiting the production of reactive oxygen species (ROS) and mitochondrial dysfunction using ROS detection assay and MTP assay. We demonstrated that baicalein treatment regulated the phosphorylation of c-Jun N-terminal kinase (JNK) by using western blot analysis and Annexin V assay. Our data suggest that baicalein has potential for use as a therapeutic drug for the treatment of various neurodegenerative diseases, including prion diseases.

Dysfunction of the PI3K-Akt-GSK-3 pathway is a common feature in cell culture and in vivo models of prion disease

AIMS

Transmissible spongiform encephalopathies, also called prion diseases, are characterized by the cerebral accumulation of misfolded prion protein (PrP(SC) ) and subsequent neurodegeneration. However, despite considerable research effort, the molecular mechanisms underlying prion-induced neurodegeneration are poorly understood. Here, we explore the hypothesis that prions induce dysfunction of the PI3K/Akt/GSK-3 signalling pathway.

METHODS

We employed two parallel approaches. Using cell cultures derived from mouse primary neurones and from a human neuronal cell line, we identified common elements that were modified by the neurotoxic fragment of PrP(106-126) . These studies were then complemented by comparative analyses in a mouse model of prion infection.

RESULTS

The presence of a polymerized fragment of the prion protein (PrP(106-126) ) or of a prion strain altered PI3K-mediated signalling, as evidenced by Akt inhibition and GSK-3 activation. PI3K activation by the addition of insulin or the expression of a constitutively active Akt mutant restored normal levels of Akt and GSK-3 activity. These changes were correlated with a reduction in caspase activity and an increase in neuronal survival. Moreover, we found that activation of caspase 3, Erk and GSK-3 are common features of PrP(106-126) -mediated neurotoxicity in cellular systems and prion infection in the mouse cerebellum, while activation of caspase 12 and JNK was observed in cellular models.

CONCLUSIONS

Our findings in cell culture and in vivo models of prion disease demonstrate marked alterations to the PI3K/Akt/GSK-3 pathway and suggest that two additional pathways contribute to PrP-induced neurotoxicity as responsible of JNK and caspase 12 activation.

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.

Gingerol prevents prion protein-mediated neuronal toxicity by regulating HIF prolyl hydroxylase 2 and prion protein

Prion diseases are a family of progressive neurodegenerative disorders, which are fatal in the majority of cases and affect both humans and domestic animals. Prion protein (PrP) (106–126) retains the neurotoxic properties of the entire pathological PrPsc and it is generally used as a reasonable model to study the mechanisms responsible for prion diseases. In our previous studies, we demonstrated that hypoxia-inducible factor (HIF)-1α is involved in the gingerol-mediated protection of neuronal cells. HIF mediates cellular adaptations to low oxygen. Prolyl hydroxylase domain-containing protein 2 (PHD2) is an oxygen sensor that hydroxylates the HIF-α-subunit, promoting its proteasomal degradation under normoxic conditions. Thus, in the present study we wished to determine whether gingerol inhibits the catalytic activity of PHD2 and prevents HIF-1α protein proteasomal degradation, thereby preventing the occurrence of PrP (106–126)-induced neuronal apoptosis. We used the pharmacological inhibition of PHD2 by dimethyloxalylglycine (DMOG) or deferoxamine (DFO) and the genetic inhibition of HIF-1α by HIF-1α small interfering RNA (siRNA) to block the effects of gingerol against PrP (106–126)-induced neurotoxicity. Our results demonstrated that gingerol prevented PrP (106–126)-induced neuronal apoptosis by upregulating HIF-1α and inhibiting the catalytic activity of PHD2 under normoxic conditions. Moreover, the protective effects of gingerol against PrP (106–126)-induced neuronal apoptosis were associated with the upregulation of the expression of cellular prion protein (PrPc). In conclusion, our results indicate that gingerol has therapeutic potential for use in the treatment or prevention of prion diseases, and its inhibitory effects on the catalytic activity of PHD2 may be of clinical benefit.

Activation of pro-survival CaMK4β/CREB and pro-death MST1 signaling at early and late times during a mouse model of prion disease

Background

The signaling pathways most critical to prion disease pathogenesis are as yet incompletely characterized. We have developed a kinomics approach to identify signaling pathways that are dysregulated during prion pathogenesis. The approach is sensitive and specific enough to detect signaling pathways dysregulated in a simple in vitro model of prion pathogenesis. Here, we used this approach to identify signaling pathways dysregulated during prion pathogenesis in vivo.

Methods

Mice intraperitoneally infected with scrapie (strain RML) were euthanized at 70, 90, 110, 130 days post-infection (dpi) or at terminal stages of disease (155–190 dpi). The levels of 139 protein kinases in brainstem-cerebellum homogenates were analyzed by multiplex Western blots, followed by hierarchical clustering and analyses of activation states.

Results

Hierarchical and functional clustering identified CaMK4β and MST1 signaling pathways as potentially dysregulated. Targeted analyses revealed that CaMK4β and its downstream substrate CREB, which promotes neuronal survival, were activated at 70 and 90 dpi in cortical, subcortical and brainstem-cerebellum homogenates from scrapie-infected mice. The activation levels of CaMK4β/CREB signaling returned to those in mock-infected mice at 110 dpi, whereas MST1, which promotes neuronal death, became activated at 130 dpi.

Conclusion

Pro-survival CaMK4β/CREB signaling is activated in mouse scrapie at earlier times and later inhibited, whereas pro-death MST1 signaling is activated at these later times.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-422X-11-160) contains supplementary material, which is available to authorized users.

Sulforaphane-induced autophagy flux prevents prion protein-mediated neurotoxicity through AMPK pathway

Prion diseases are neurodegenerative and infectious disorders that involve accumulation of misfolded scrapie prion protein, and which are characterized by spongiform degeneration. Autophagy, a major homeostatic process responsible for the degradation of cytoplasmic components, has garnered attention as the potential target for neurodegenerative diseases such as prion disease. We focused on protective effects of sulforaphane found in cruciferous vegetables on prion-mediated neurotoxicity and the mechanism of sulforaphane related to autophagy. In human neuroblastoma cells, sulforaphane protected prion protein (PrP) (106-126)-mediated neurotoxicity and increased autophagy flux marker microtubule-associated protein 1 light chain 3-II protein levels, following a decrease of p62 protein level. Pharmacological and genetical inhibition of autophagy by 3MA, wortmannin and knockdown of autophagy-related 5 (ATG5) led to block the effect of sulforaphane against PrP (106-126)-induced neurotoxicity. Furthermore we demonstrated that both sulforaphane-induced autophagy and protective effect of sulforaphane against PrP (106-126)-induced neurotoxicity are dependent on the AMP-activated protein kinase (AMPK) signaling. The present results indicated that sulforaphane of cruciferous vegetables enhanced autophagy flux led to the protection effects against prion-mediated neurotoxicity, which was regulated by AMPK signaling pathways in human neuron cells. Our data also suggest that sulforaphane has a potential value as a therapeutic tool in neurodegenerative disease including prion diseases.

Overexpression of BAT3 alleviates prion protein fragment PrP106-126-induced neuronal apoptosis

BACKGROUNDS AND AIMS

Prion diseases are a group of infectious neurodegenerative diseases characterized by neuronal death and degeneration. Human leukocyte antigen-B-associated transcript 3 (BAT3) is an important apoptosis regulator. We therefore investigated the interactions between BAT3 and prion protein and the potential role of BAT3 in PrP106-126-induced apoptosis.

METHODS

BAT3 and prion protein were overexpressed in Hela, Neuro2A, or primary neuronal cells by transfection with BAT3-HA or PRNP-EGFP expression plasmids and their relationship studied by immunofluorescence and Western blotting. The effect of BAT3 on PrP106-126-induced cytotoxicity and apoptosis was detected by the CCK-8 assay and terminal-deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) assay. The expression of cytochrome c and Bcl-2 was examined by Western blotting.

RESULTS

BAT3 interacted with prion protein and enhanced PrP expression. After PrP106-126 peptide treated, BAT3 was transported from the nucleus to cytoplasm, increased cell viability, and protected neurons from PrP106-126-induced apoptosis through stabilizing the level of Bcl-2 protein and inhibiting the release of cytochrome c to cytoplasm.

CONCLUSIONS

Our present data showed a novel molecular mechanism of PrP106-126-induced apoptotic process regulation through the overexpression of BAT3, which may be important for the basic regulatory mechanism of neuron survival in prion diseases and associated neurodegenerative diseases in vivo.

Lactoferrin from bovine colostrum regulates prolyl hydroxylase 2 activity and prevents prion protein-mediated neuronal cell damage via cellular prion protein

Prion disorders are associated with the conversion of normal cellular prion protein (PrPc) to the abnormal scrapie isoform of prion protein (PrPsc). Recent studies have shown that expression of normal PrPc is regulated by hypoxia-inducible factor-1 alpha (HIF-1α), and that lactoferrin increases full-length PrPc on the cell surface. Lactoferrin is an 80-kDa iron-binding glycoprotein with various biological activities, including iron-chelating ability. HIF-1α and the associated ubiquitin-proteasome pathway are regulated by HIF prolyl-hydroxylases 2 (PHD2). We hypothesized that lactoferrin regulates PHD2 expression and enzymatic activity, and the PHD2 regulation promotes HIF-1α stability and prevention of neuronal cell death mediated by prion protein (PrP) residues (106-126). Lactoferrin prevented PrP (106-126)-induced neurotoxicity by the induction of PrPc expression via promoting HIF-1α stability in neuronal cells. Our results demonstrated that lactoferrin prevented PrP (106-126)-induced neurotoxicity via the up-regulation of HIF-1α stability determined by PHD2 expression and enzymatic activity. These findings suggest that possible therapies such as PHD2 inhibition, or promotion of lactoferrin secretion, may have clinical benefits in neurodegenerative diseases, including prion disease.

Prion fragment peptides are digested with membrane type matrix metalloproteinases and acquire enzyme resistance through Cu²⁺-binding

Prions are the cause of neurodegenerative disease in humans and other mammals. The structural conversion of the prion protein (PrP) from a normal cellular protein (PrPC) to a protease-resistant isoform (PrPSc) is thought to relate to Cu2+ binding to histidine residues. In this study, we focused on the membrane-type matrix metalloproteinases (MT-MMPs) such as MT1-MMP and MT3-MMP, which are expressed in the brain as PrPC-degrading proteases. We synthesized 21 prion fragment peptides. Each purified peptide was individually incubated with recombinant MT1-MMP or MT3-MMP in the presence or absence of Cu2+ and the cleavage sites determined by LC-ESI-MS analysis. Recombinant MMP-7 and human serum (HS) were also tested as control. hPrP61-90, from the octapeptide-repeat region, was cleaved by HS but not by the MMPs tested here. On the other hand, hPrP92-168 from the central region was cleaved by MT1-MMP and MT3-MMP at various sites. These cleavages were inhibited by treatment with Cu2+. The C-terminal peptides had higher resistance than the central region. The data obtained from this study suggest that MT-MMPs expressed in the brain might possess PrPC-degrading activity.

FTY720 protects neuronal cells from damage induced by human prion protein by inactivating the JNK pathway

Prion diseases affect the central nervous system (CNS) in humans and animals, and are associated with the conversion of the cellular prion protein (PrPC) to the misfolded isoform (PrPSc). FTY720, an immune modulator and synthetic analogue of sphingosine-1-phosphate (S1P), activates S1P receptors and has been shown to be effective in experimental models of transplantation and autoimmunity, including multiple sclerosis. Whereas the immune modulatory functions of FTY720 have been extensively investigated, the other functions of FTY720 are not yet well understood. In this study, we investigated the effects of FTY720 phosphate (FTY720-p) on prion protein-mediated neuronal cell death, as well as its effects on intracellular apoptotic pathways. Treatment with FTY720-p protected neuronal cells from synthetic human prion protein peptide [PrP (106‑126)]-mediated damage and prevented mitochondrial dysfunction by inhibiting the activation of c-jun N-terminal kinase. Moreover, FTY720-p prevented the PrP (106‑126)-induced reduction in mitochondrial potential, the translocation of Bax to the mitochondria and the release of cytochrome c. To the best of our knowledge, this study is the first to demonstrate the effects of FTY720 on prion protein-mediated neurotoxicity and to suggest that FTY720 has therapeutic potential in prion diseases.

Differential roles for caspase-mediated and calpain-mediated cell death in 1- and 3-week-old rat cortical cultures

Necrosis and apoptosis are well established as two primary cell death pathways. Mixed neuroglial cultures are commonly used to study cell death mechanisms in neural cells. However, the ages of these cultures vary across studies and little attention has been paid to how cell death processes may change as the cultures mature. To clarify whether neuroglial culture age affects cell death mechanisms, we treated 1- and 3-week-old neuroglial cultures with either the excitotoxic stimulus, N-methyl-D-aspartate (NMDA), or with the oxidative stressor, hydrogen peroxide (H2O2). Although NMDA is known to be toxic only in cultures that are at least 2 weeks old, H2O2 is toxic in cultures of all ages. Here, we confirm that, in 1-week-old neuroglial cultures, NMDA does not induce toxicity, whereas H2O2 induces both calpain-mediated and caspase-mediated neuronal death. In 3-week-old cultures, both NMDA and H2O2 trigger calpain-mediated, but not caspase-mediated, neuronal death. Further, we observed a decrease in caspase-3 levels and an increase in calpain levels in untreated neuroglial cultures as they aged. The findings presented here show that neuronal cell death mechanisms vary with culture age and highlight the necessity of considering culture age when interpreting neural cell culture data.

Gene and protein patterns of potential prion-related markers in the central nervous system of clinical and preclinical infected sheep

The molecular pathogenic mechanisms of prion diseases are far from clear. Genomic analyses have revealed genetic biomarkers potentially involved in prion neuropathology in naturally scrapie-infected sheep, a good animal model of infectious prionopathies. However, these biomarkers must be validated in independent studies at different stages of the disease. The gene and protein expression profiles and protein distribution of six potential genetic biomarkers (i.e., CAPN6, COL1A2, COL3A1, GALA1, MT2A and MTNR1B) are presented here for both the early and terminal stages of scrapie in five different brain regions. Gene transcription changes were confirmed in the medulla oblongata, and the expression profiles were generally similar in other central nervous system regions. The changes were more substantial in clinical animals compared to preclinical animals. The expression of the CAPN6 protein increased in the spinal cord and cerebellum of the clinical and preclinical brains. The distribution of the GALA1 was identified in glial cells from the cerebellum of scrapie-infected animals, GALA1 protein expression was increased in clinical animals in the majority of regions, and the increase of MT2A was in agreement with previous reports. The downregulation of MTNR1B was especially marked in the Purkinje cells. Finally, although collagen genes were downregulated the protein immunostaining did not reveal significant changes between the scrapie-infected and control animals. In conclusion, this study of gene transcription and protein expression and distribution confirm CAPN6, GALA1, MTNR1B and MT2A as potential targets for further prion disease research.

Melatonin-induced autophagy protects against human prion protein-mediated neurotoxicity

  Melatonin has neuroprotective effects in the models of neurodegenerative disease including Alzheimer's and Parkinson's disease. Several studies have shown that melatonin prevents neurodegeneration by regulation of mitochondrial function. However, the protective action of melatonin has not been reported in prion disease. We investigated the influence of melatonin on prion-mediated neurotoxicity. Melatonin rescued neuronal cells from PrP(106-126)-induced neurotoxicity by prevention of mitochondrial dysfunction. Moreover, the protective effect of melatonin against mitochondrial dysfunction was related with autophagy activation. Melatonin-treated cells were dose-dependently increased in LC3-II, an autophagy marker. Melatonin-induced autophagy prevented a PrP(106-126)-induced reduction in mitochondrial potential and translocation of Bax to the mitochondria and cytochrome c release. On the other hand, downregulation of autophagy protein 5 with Atg5 siRNA or the autophagy blocker 3-methyladenine prevented the melatonin-mediated neuroprotective effects. This is the first report demonstrating that treatment with melatonin appears to protect against prion-mediated neurotoxicity and that the neuroprotection is induced by melatonin-mediated autophagy signals. The results of this study suggest that regulation of melatonin is a therapeutic strategy for prion peptide-induced apoptosis.

Induction of ligand-specific PrP (C) signaling in human neuronal cells

Cellular prion protein (PrP (C) ) has attracted considerable attention for its role in transmissible spongiform encephalopathies (TSEs). In spite of being a point of intense research effort critical questions still remain regarding the physiological function of PrP (C) and how these functions may change with the conversion of the protein into the infectious and pathological conformation (PrP (Sc) ). While emerging evidence suggests PrP (C/Sc) are involved in signal transduction there is little consensus on the signaling pathways associated with the normal and diseased states. The purported involvement of PrP (C) in signal transduction, and the association of TSEs with neural pathology, makes kinome analysis of human neurons an interesting and appropriate model to characterize patterns of signal transduction following activation of PrP (C) by two commonly employed experimental ligands; antibody-induced dimerization by 6H4 and the amino acids 106-126 PrP peptide fragment (PrP 106-126). Analysis of the induced kinome responses reveals distinct patterns of signaling activity following each treatment. Specifically, stimulation of human neurons with the 6H4 antibody results in alterations in mitogen activated protein kinase (MAPK) signaling pathways while the 106-126 peptide activates growth factor related signaling pathways including vascular endothelial growth factor (VEGF) signaling and the phosphoinositide-3 kinase (PI3K) pathway. These pathways were validated through independent functional assays. Collectively these results indicate that stimulation of PrP (C) with distinct ligands, even within the same cell type, results in unique patterns of signaling. While this investigation highlights the apparent functional versatility of PrP (C) as a signaling molecule and may offer insight into cellular mechanisms of TSE pathology it also emphasizes the potential dangers associated with attributing activation of specific intracellular events to particular receptors through artificial models of receptor activation.

Role of prion protein aggregation in neurotoxicity

In several neurodegenerative diseases, such as Parkinson, Alzheimer’s, Huntington, and prion diseases, the deposition of aggregated misfolded proteins is believed to be responsible for the neurotoxicity that characterizes these diseases. Prion protein (PrP), the protein responsible of prion diseases, has been deeply studied for the peculiar feature of its misfolded oligomers that are able to propagate within affected brains, inducing the conversion of the natively folded PrP into the pathological conformation. In this review, we summarize the available experimental evidence concerning the relationship between aggregation status of misfolded PrP and neuronal death in the course of prion diseases. In particular, we describe the main findings resulting from the use of different synthetic (mainly PrP106-126) and recombinant PrP-derived peptides, as far as mechanisms of aggregation and amyloid formation, and how these different spatial conformations can affect neuronal death. In particular, most data support the involvement of non-fibrillar oligomers rather than actual amyloid fibers as the determinant of neuronal death.

Recombinant human prion protein fragment 90-231, a useful model to study prion neurotoxicity

Transmissible spongiform encephalopathies (TSE), or prion diseases, are a group of fatal neurodegenerative disorders of animals and humans. Human diseases include Creutzfeldt-Jakob (CJD) and Gerstmann-Straussler-Scheinker (GSSD) diseases, fatal familial insomnia, and Kuru. Human and animal TSEs share a common histopathology with a pathognomonic triad: spongiform vacuolation of the grey matter, neuronal death, glial proliferation, and, more inconstantly, amyloid deposition. According to the "protein only" hypothesis, TSEs are caused by a unique post-translational conversion of normal, host-encoded, protease-sensitive prion protein (PrP(sen) or PrP(C)) to an abnormal disease-associated isoform (PrP(res) or PrP(Sc)). To investigate the molecular mechanism of neurotoxicity induced by PrP(Sc) we developed a protocol to obtain millimolar amounts of soluble recombinant polypeptide encompassing the amino acid sequence 90-231 of human PrP (hPrP90-231). This protein corresponds to the protease-resistant prion protein fragment that originates after amino-terminal truncation. Importantly, hPrP90-231 has a flexible backbone that, similar to PrP(C), can undergo to structural rearrangement. This peptide, structurally resembling PrP(C), can be converted in a PrP(Sc)-like conformation, and thus represents a valuable model to study prion neurotoxicity. In this article we summarized our experimental evidence on the molecular and structural mechanisms responsible of hPrP90-231 neurotoxicity on neuroectodermal cell line SHSY5Y and the effects of some PrP pathogen mutations identified in familial TSE.