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

Post-translational modifications in prion diseases

More than 650 reversible and irreversible post-translational modifications (PTMs) of proteins have been listed so far. Canonical PTMs of proteins consist of the covalent addition of functional or chemical groups on target backbone amino-acids or the cleavage of the protein itself, giving rise to modified proteins with specific properties in terms of stability, solubility, cell distribution, activity, or interactions with other biomolecules. PTMs of protein contribute to cell homeostatic processes, enabling basal cell functions, allowing the cell to respond and adapt to variations of its environment, and globally maintaining the constancy of the milieu interieur (the body's inner environment) to sustain human health. Abnormal protein PTMs are, however, associated with several disease states, such as cancers, metabolic disorders, or neurodegenerative diseases. Abnormal PTMs alter the functional properties of the protein or even cause a loss of protein function. One example of dramatic PTMs concerns the cellular prion protein (PrPC), a GPI-anchored signaling molecule at the plasma membrane, whose irreversible post-translational conformational conversion (PTCC) into pathogenic prions (PrPSc) provokes neurodegeneration. PrPC PTCC into PrPSc is an additional type of PTM that affects the tridimensional structure and physiological function of PrPC and generates a protein conformer with neurotoxic properties. PrPC PTCC into PrPSc in neurons is the first step of a deleterious sequence of events at the root of a group of neurodegenerative disorders affecting both humans (Creutzfeldt-Jakob diseases for the most representative diseases) and animals (scrapie in sheep, bovine spongiform encephalopathy in cow, and chronic wasting disease in elk and deer). There are currently no therapies to block PrPC PTCC into PrPSc and stop neurodegeneration in prion diseases. Here, we review known PrPC PTMs that influence PrPC conversion into PrPSc. We summarized how PrPC PTCC into PrPSc impacts the PrPC interactome at the plasma membrane and the downstream intracellular controlled protein effectors, whose abnormal activation or trafficking caused by altered PTMs promotes neurodegeneration. We discussed these effectors as candidate drug targets for prion diseases and possibly other neurodegenerative diseases.

Titanium dioxide and carbon black nanoparticles disrupt neuronal homeostasis via excessive activation of cellular prion protein signaling

Background

Epidemiological emerging evidence shows that human exposure to some nanosized materials present in the environment would contribute to the onset and/or progression of Alzheimer’s disease (AD). The cellular and molecular mechanisms whereby nanoparticles would exert some adverse effects towards neurons and take part in AD pathology are nevertheless unknown.

Results

Here, we provide the prime evidence that titanium dioxide (TiO₂) and carbon black (CB) nanoparticles (NPs) bind the cellular form of the prion protein (PrP^C), a plasma membrane protein well known for its implication in prion diseases and prion-like diseases, such as AD. The interaction between TiO₂- or CB-NPs and PrP^C at the surface of neuronal cells grown in culture corrupts PrP^C signaling function. This triggers PrP^C-dependent activation of NADPH oxidase and subsequent production of reactive oxygen species (ROS) that alters redox equilibrium. Through PrP^C interaction, NPs also promote the activation of 3-phosphoinositide-dependent kinase 1 (PDK1), which in turn provokes the internalization of the neuroprotective TACE α-secretase. This diverts TACE cleavage activity away from (i) TNFα receptors (TNFR), whose accumulation at the plasma membrane augments the vulnerability of NP-exposed neuronal cells to TNFα -associated inflammation, and (ii) the amyloid precursor protein APP, leading to overproduction of neurotoxic amyloid Aβ40/42 peptides. The silencing of PrP^C or the pharmacological inhibition of PDK1 protects neuronal cells from TiO₂- and CB-NPs effects regarding ROS production, TNFα hypersensitivity, and Aβ rise. Finally, we show that dysregulation of the PrP^C-PDK1-TACE pathway likely occurs in the brain of mice injected with TiO₂-NPs by the intra-cerebro-ventricular route as we monitor a rise of TNFR at the cell surface of several groups of neurons located in distinct brain areas.

Conclusion

Our in vitro and in vivo study thus posits for the first time normal cellular prion protein PrP^C as being a neuronal receptor of TiO₂- and CB-NPs and identifies PrP^C-coupled signaling pathways by which those nanoparticles alter redox equilibrium, augment the intrinsic sensitivity of neurons to neuroinflammation, and provoke a rise of Aβ peptides. By identifying signaling cascades dysregulated by TiO₂- and CB-NPs in neurons, our data shed light on how human exposure to some NPs might be related to AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00490-x.

The Cellular Prion Protein-ROCK Connection: Contribution to Neuronal Homeostasis and Neurodegenerative Diseases

Amyloid-based neurodegenerative diseases such as prion, Alzheimer's, and Parkinson's diseases have distinct etiologies and clinical manifestations, but they share common pathological events. These diseases are caused by abnormally folded proteins (pathogenic prions PrP^Sc in prion diseases, β-amyloids/Aβ and Tau in Alzheimer's disease, α-synuclein in Parkinson's disease) that display β-sheet-enriched structures, propagate and accumulate in the nervous central system, and trigger neuronal death. In prion diseases, PrP^Sc-induced corruption of the physiological functions exerted by normal cellular prion proteins (PrP^C) present at the cell surface of neurons is at the root of neuronal death. For a decade, PrP^C emerges as a common cell surface receptor for other amyloids such as Aβ and α-synuclein, which relays, at least in part, their toxicity. In lipid-rafts of the plasma membrane, PrP^C exerts a signaling function and controls a set of effectors involved in neuronal homeostasis, among which are the RhoA-associated coiled-coil containing kinases (ROCKs). Here we review (i) how PrP^C controls ROCKs, (ii) how PrP^C-ROCK coupling contributes to neuronal homeostasis, and (iii) how the deregulation of the PrP^C-ROCK connection in amyloid-based neurodegenerative diseases triggers a loss of neuronal polarity, affects neurotransmitter-associated functions, contributes to the endoplasmic reticulum stress cascade, renders diseased neurons highly sensitive to neuroinflammation, and amplifies the production of neurotoxic amyloids.

HMGB1-triggered inflammation inhibition of notoginseng leaf triterpenes against cerebral ischemia and reperfusion injury via MAPK and NF-κB signaling pathways

Ischemic stroke is a clinically common cerebrovascular disease whose main risks include necrosis, apoptosis and cerebral infarction, all caused by cerebral ischemia and reperfusion (I/R) injury. This process has particular significance for the treatment of stroke patients. Notoginseng leaf triterpenes (PNGL), as a valuable medicine, have been discovered to have neuroprotective effects. However, it was not confirmed that whether PNGL may possess neuroprotective effects against cerebral I/R injury. To explore the neuroprotective effects of PNGL and their underlying mechanisms, a middle cerebral artery occlusion/reperfusion (MCAO/R) model was established. In vivo results suggested that in MCAO/R model rats, PNGL pretreatment (73.0, 146, 292 mg/kg) remarkably decreased infarct volume, reduced brain water content, and improved neurological functions; moreover, PNGL (73.0, 146, 292 mg/kg) significantly alleviated blood-brain barrier (BBB) disruption and inhibited neuronal apoptosis and neuronal loss caused by cerebral I/R injury, while PNGL with a different concertation (146, 292 mg/kg) significantly reduced the concentrations of IL-6, TNF-α, IL-1 β, and HMGB1 in serums in a dose-dependent way, which indicated that inflammation inhibition could be involved in the neuroprotective effects of PNGL. The immunofluorescence and western blot analysis showed PNGL decreased HMGB1 expression, suppressed the HMGB1-triggered inflammation, and inhibited microglia activation (IBA1) in hippocampus and cortex, thus dose-dependently downregulating inflammatory cytokines including VCAM-1, MMP-9, MMP-2, and ICAM-1 concentrations in ischemic brains. Interestingly, PNGL administration (146 mg/kg) significantly downregulated the levels of p-P44/42, p-JNK1/2 and p-P38 MAPK, and also inhibited expressions of the total NF-κB and phosphorylated NF-κB in ischemic brains, which was the downstream pathway triggered by HMGB1. All of these results indicated that the protective effects of PNGL against cerebral I/R injury could be associated with inhibiting HMGB1-triggered inflammation, suppressing the activation of MAPKs and NF-κB, and thus improved cerebral I/R-induced neuropathological changes. This study may offer insight into discovering new active compounds for the treatment of ischemic stroke.

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.

Neuroprotective Effects of Radix Scrophulariae on Cerebral Ischemia and Reperfusion Injury via MAPK Pathways

Ischemic stroke is a clinically common cerebrovascular disease whose main risks include necrosis, apoptosis and cerebral infarction, all caused by cerebral ischemia and reperfusion (I/R). Ischemia and reperfusion-induced injury or apoptosis inhibition in human brain tissue may exert an irreplaceable protective effect on ischemic nerves. This process has particular significance for the treatment of stroke patients. However, the development of neuroprotective drugs remains challenging. Radix Scrophulariae, traditionally considered a valuable medicine, has been discovered to have neuroprotective effects. To explore the neuroprotective effects of an aqueous extract of Radix Scrophulariae (RSAE) on cerebral ischemia/reperfusion and their underlying mechanisms, oxygen-glucose deprivation and reperfusion (OGD/R)-induced PC12 cells were used, and a middle cerebral artery occlusion/reperfusion (MCAO/R) mouse model was established. In vitro results showed that 12.5 μg/mL RSAE markedly improved cell viability; inhibited LDH leakage; increased SOD, GSH-Px and CAT enzyme activity; stabilized the mitochondrial membrane potential; and reduced OGD-induced cell injury and apoptosis. Additionally, in vivo results preliminarily suggested that in MCAO/R model mice, RSAE treatments attenuated infarct volume; reduced brain water content and nitric oxide (NO) and malondialdehyde (MDA) concentrations; inhibited I/R-induced neurological deficits; reduced the levels of lactate dehydrogenase (LDH) leakage release; improved antioxidant capacity by upregulating SOD, GSH-Px and CAT enzyme activity; and reduced neuronal apoptosis, necrosis and loss of neurons. Moreover, it was found that RSAE upregulated the expression of Bcl-2 and downregulated the expression of Bax. In addition, the phosphorylation levels of MAPK signal pathways were elucidated via western blot analysis and immunohistochemical evaluation. In summary, this study investigated the neuroprotective effects and potential mechanisms of RSAE on focal cerebral I/R injury in mice. Radix Scrophulariae has been previously identified as a potential neuroprotective natural plant. Hence, our results may offer insight into discovering new active compounds or drugs for the treatment of ischemic stroke. Many new natural active chemicals in this extract may be discovered by chemical separation and identification and may provide new insights into therapeutic targets in stroke patients.

Role of cellular prion protein in interneuronal amyloid transmission

Several studies have indicated that certain misfolded amyloids composed of tau, β-amyloid or α-synuclein can be transferred from cell to cell, suggesting the contribution of mechanisms reminiscent of those by which infective prions spread through the brain. This process of a 'prion-like' spreading between cells is also relevant as a novel putative therapeutic target that could block the spreading of proteinaceous aggregates throughout the brain which may underlie the progressive nature of neurodegenerative diseases. The relevance of β-amyloid oligomers and cellular prion protein (PrP^C) binding has been a focus of interest in Alzheimer's disease (AD). At the molecular level, β-amyloid/PrP^C interaction takes place in two differently charged clusters of PrP^C. In addition to β-amyloid, participation of PrP^C in α-synuclein binding and brain spreading also appears to be relevant in α-synucleopathies. This review summarizes current knowledge about PrP^C as a putative receptor for amyloid proteins and the physiological consequences of these interactions.

Protective role of cellular prion protein against TNFα-mediated inflammation through TACE α-secretase

Although cellular prion protein PrP^C is well known for its implication in Transmissible Spongiform Encephalopathies, its functions remain elusive. Combining in vitro and in vivo approaches, we here show that PrP^C displays the intrinsic capacity to protect neuronal cells from a pro-inflammatory TNFα noxious insult. Mechanistically, PrP^C coupling to the NADPH oxidase-TACE α-secretase signaling pathway promotes TACE-mediated cleavage of transmembrane TNFα receptors (TNFRs) and the release of soluble TNFR, which limits the sensitivity of recipient cells to TNFα. We further show that PrP^C expression is necessary for TACE α-secretase to stay at the plasma membrane in an active state for TNFR shedding. Such PrP^C control of TACE localization depends on PrP^C modulation of β1 integrin signaling and downstream activation of ROCK-I and PDK1 kinases. Loss of PrP^C provokes TACE internalization, which in turn cancels TACE-mediated cleavage of TNFR and renders PrP^C-depleted neuronal cells as well as PrP^C knockout mice highly vulnerable to pro-inflammatory TNFα insult. Our work provides the prime evidence that in an inflammatory context PrP^C adjusts the response of neuronal cells targeted by TNFα through TACE α-secretase. Our data also support the view that abnormal TACE trafficking and activity in prion diseases originate from a-loss-of-PrP^C cytoprotective function.

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.

Functions of the cellular prion protein, the end of Moore's law, and Ockham's razor theory

Since its discovery the cellular prion protein (encoded by the Prnp gene) has been associated with a large number of functions. The proposed functions rank from basic cellular processes such as cell cycle and survival to neural functions such as behavior and neuroprotection, following a pattern similar to that of Moore's law for electronics. In addition, particular interest is increasing in the participation of Prnp in neurodegeneration. However, in recent years a redefinition of these functions has begun, since examples of previously attributed functions were increasingly re-associated with other proteins. Most of these functions are linked to so-called "Prnp-flanking genes" that are close to the genomic locus of Prnp and which are present in the genome of some Prnp mouse models. In addition, their role in neuroprotection against convulsive insults has been confirmed in recent studies. Lastly, in recent years a large number of models indicating the participation of different domains of the protein in apoptosis have been uncovered. However, after more than 10 years of molecular dissection our view is that the simplest mechanistic model in PrP(C)-mediated cell death should be considered, as Ockham's razor theory suggested.

Reelin Expression in Creutzfeldt-Jakob Disease and Experimental Models of Transmissible Spongiform Encephalopathies

Reelin is an extracellular glycoprotein involved in key cellular processes in developing and adult nervous system, including regulation of neuronal migration, synapse formation, and plasticity. Most of these roles are mediated by the intracellular phosphorylation of disabled-1 (Dab1), an intracellular adaptor molecule, in turn mediated by binding Reelin to its receptors. Altered expression and glycosylation patterns of Reelin in cerebrospinal and cortical extracts have been reported in Alzheimer's disease. However, putative changes in Reelin are not described in natural prionopathies or experimental models of prion infection or toxicity. With this is mind, in the present study, we determined that Reelin protein and mRNA levels increased in CJD human samples and in mouse models of human prion disease in contrast to murine models of prion infection. However, changes in Reelin expression appeared only at late terminal stages of the disease, which prevent their use as an efficient diagnostic biomarker. In addition, increased Reelin in CJD and in in vitro models does not correlate with Dab1 phosphorylation, indicating failure in its intracellular signaling. Overall, these findings widen our understanding of the putative changes of Reelin in neurodegeneration.

Regulation of Amyloid β Oligomer Binding to Neurons and Neurotoxicity by the Prion Protein-mGluR5 Complex

The prion protein (PrP^C) has been suggested to operate as a scaffold/receptor protein in neurons, participating in both physiological and pathological associated events. PrP^C, laminin, and metabotropic glutamate receptor 5 (mGluR5) form a protein complex on the plasma membrane that can trigger signaling pathways involved in neuronal differentiation. PrP^C and mGluR5 are co-receptors also for β-amyloid oligomers (AβOs) and have been shown to modulate toxicity and neuronal death in Alzheimer's disease. In the present work, we addressed the potential crosstalk between these two signaling pathways, laminin-PrP^C-mGluR5 or AβO-PrP^C-mGluR5, as well as their interplay. Herein, we demonstrated that an existing complex containing PrP^C-mGluR5 has an important role in AβO binding and activity in neurons. A peptide mimicking the binding site of laminin onto PrP^C (Ln-γ1) binds to PrP^C and induces intracellular Ca²⁺ increase in neurons via the complex PrP^C-mGluR5. Ln-γ1 promotes internalization of PrP^C and mGluR5 and transiently decreases AβO biding to neurons; however, the peptide does not impact AβO toxicity. Given that mGluR5 is critical for toxic signaling by AβOs and in prion diseases, we tested whether mGlur5 knock-out mice would be susceptible to prion infection. Our results show mild, but significant, effects on disease progression, without affecting survival of mice after infection. These results suggest that PrP^C-mGluR5 form a functional response unit by which multiple ligands can trigger signaling. We propose that trafficking of PrP^C-mGluR5 may modulate signaling intensity by different PrP^C ligands.

Secretory pathway retention of mutant prion protein induces p38-MAPK activation and lethal disease in mice

Misfolding of proteins in the biosynthetic pathway in neurons may cause disturbed protein homeostasis and neurodegeneration. The prion protein (PrP^C) is a GPI-anchored protein that resides at the plasma membrane and may be misfolded to PrP^Sc leading to prion diseases. We show that a deletion in the C-terminal domain of PrP^C (PrPΔ214–229) leads to partial retention in the secretory pathway causing a fatal neurodegenerative disease in mice that is partially rescued by co-expression of PrP^C. Transgenic (Tg(PrPΔ214–229)) mice show extensive neuronal loss in hippocampus and cerebellum and activation of p38-MAPK. In cell culture under stress conditions, PrPΔ214–229 accumulates in the Golgi apparatus possibly representing transit to the Rapid ER Stress-induced ExporT (RESET) pathway together with p38-MAPK activation. Here we describe a novel pathway linking retention of a GPI-anchored protein in the early secretory pathway to p38-MAPK activation and a neurodegenerative phenotype in transgenic mice.

Neuroprotective Effects Against POCD by Photobiomodulation: Evidence from Assembly/Disassembly of the Cytoskeleton

Postoperative cognitive dysfunction (POCD) is a decline in memory following anaesthesia and surgery in elderly patients. While often reversible, it consumes medical resources, compromises patient well-being, and possibly accelerates progression into Alzheimer's disease. Anesthetics have been implicated in POCD, as has neuroinflammation, as indicated by cytokine inflammatory markers. Photobiomodulation (PBM) is an effective treatment for a number of conditions, including inflammation. PBM also has a direct effect on microtubule disassembly in neurons with the formation of small, reversible varicosities, which cause neural blockade and alleviation of pain symptoms. This mimics endogenously formed varicosities that are neuroprotective against damage, toxins, and the formation of larger, destructive varicosities and focal swellings. It is proposed that PBM may be effective as a preconditioning treatment against POCD; similar to the PBM treatment, protective and abscopal effects that have been demonstrated in experimental models of macular degeneration, neurological, and cardiac conditions.

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

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

Double-Edge Sword of Sustained ROCK Activation in Prion Diseases through Neuritogenesis Defects and Prion Accumulation

In prion diseases, synapse dysfunction, axon retraction and loss of neuronal polarity precede neuronal death. The mechanisms driving such polarization defects, however, remain unclear. Here, we examined the contribution of RhoA-associated coiled-coil containing kinases (ROCK), key players in neuritogenesis, to prion diseases. We found that overactivation of ROCK signaling occurred in neuronal stem cells infected by pathogenic prions (PrP^Sc) and impaired the sprouting of neurites. In reconstructed networks of mature neurons, PrP^Sc-induced ROCK overactivation provoked synapse disconnection and dendrite/axon degeneration. This overactivation of ROCK also disturbed overall neurotransmitter-associated functions. Importantly, we demonstrated that beyond its impact on neuronal polarity ROCK overactivity favored the production of PrP^Sc through a ROCK-dependent control of 3-phosphoinositide-dependent kinase 1 (PDK1) activity. In non-infectious conditions, ROCK and PDK1 associated within a complex and ROCK phosphorylated PDK1, conferring basal activity to PDK1. In prion-infected neurons, exacerbated ROCK activity increased the pool of PDK1 molecules physically interacting with and phosphorylated by ROCK. ROCK-induced PDK1 overstimulation then canceled the neuroprotective α-cleavage of normal cellular prion protein PrP^C by TACE α-secretase, which physiologically precludes PrP^Sc production. In prion-infected cells, inhibition of ROCK rescued neurite sprouting, preserved neuronal architecture, restored neuronal functions and reduced the amount of PrP^Sc. In mice challenged with prions, inhibition of ROCK also lowered brain PrP^Sc accumulation, reduced motor impairment and extended survival. We conclude that ROCK overactivation exerts a double detrimental effect in prion diseases by altering neuronal polarity and triggering PrP^Sc accumulation. Eventually ROCK emerges as therapeutic target to combat prion diseases.

Transmissible Spongiform Encephalopathies (TSEs), commonly named prion diseases, are caused by deposition in the brain of pathogenic prions PrP^Sc that trigger massive neuronal death. Because of our poor understanding of the mechanisms sustaining prion-induced neurodegeneration, there is to date no effective medicine to combat TSEs. The current study demonstrates that ROCK kinases are overactivated in prion-infected cells and contribute to prion pathogenesis at two levels. First, PrP^Sc-induced ROCK overactivation affects neuronal polarity with synapse disconnection, axon/dendrite degradation, and disturbs neuronal functions. Second, ROCK overactivity amplifies the production of pathogenic prions. The pharmacological inhibition of ROCK protects diseased neurons from PrP^Sc toxicity by preserving neuronal architecture and functions and lowering PrP^Sc level. Inhibition of ROCK in prion-infected mice reduces brain PrP^Sc levels, improves motor activity and extends lifespan. This study opens up new avenues to design ROCK-based therapeutic strategies to fight TSEs.

Review of studies that have used knockout mice to assess normal function of prion protein under immunological or pathophysiological stress

Deletion of cellular isoform of prion protein (PrP(C)) increases neuronal predisposition to damage by modulating apoptosis and the negative consequences of oxidative stress. In vivo studies have demonstrated that PrP(C)-deficient mice are more prone to seizure, depression, and induction of epilepsy and experience extensive cerebral damage following ischemic challenge or viral infection. In addition, adenovirus-mediated overexpression of PrP(C) reduces brain damage in rat models of cerebral ischemia. In experimental autoimmune encephalomyelitis, PrP(C)-deficient mice reportedly have a more aggressive disease onset and less clinical improvement during the chronic phase than wild-type mice mice. In mice given oral dextran sulfate, PrP(C) has a potential protective role against inflammatory bowel disease. PrP(C)-deficient mice demonstrate significantly greater increases in blood glucose concentrations after intraperitoneal injection of glucose than wild-type mice. Further in vivo challenges to PrP gene-deficient models and conditional knockout models with siRNA and in vivo administration of PrP-ligating agents may assist in refining knowledge of the lymphoid function of PrP(C) and predicting the effects of anti-PrP treatment on the immune system. Together, these findings indicate that PrP(C) may have multiple neuroprotective and anti-inflammatory roles, which explains why this protein is so widely expressed.

Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3

Prion protein (PrP) plays crucial roles in regulating antioxidant systems to improve cell defenses against cellular stress. Here, we show that the interactions of PrP with the excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (γ-GT), and multi-drug resistance protein 1 (MRP1) in astrocytes and the interaction between PrP and EAAT3 in neurons regulate the astroglial and neuronal metabolism of the antioxidant glutathione. Ablation of PrP in astrocytes and cerebellar neurons leads to dysregulation of EAAT3-mediated uptake of glutamate and cysteine, which are precursors for the synthesis of glutathione. In PrP-deficient astrocytes, levels of intracellular glutathione are increased, and under oxidative stress, levels of extracellular glutathione are increased, due to (i) increased glutathione release via MRP1 and (ii) reduced activity of the glutathione-degrading enzyme γ-GT. In PrP-deficient cerebellar neurons, cell death is enhanced under oxidative stress and glutamate excitotoxicity, when compared to wild-type cerebellar neurons. These results indicate a functional interplay of PrP with EAAT3, MRP1 and γ-GT in astrocytes and of PrP and EAAT3 in neurons, suggesting that these interactions play an important role in the metabolic cross-talk between astrocytes and neurons and in protection of neurons by astrocytes from oxidative and glutamate-induced cytotoxicity. Interactions of prion protein (PrP) with excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (GGT) and multi-drug resistance protein 1 (MRP1) regulate the astroglial and neuronal metabolism of glutathione (GSH) which protects cells against the cytotoxic oxidative stress. PrP controls the release of GSH from astrocytes via MRP1 and regulates the hydrolysis of extracellular GSH by GGT as well as the neuronal and astroglial glutamate and cysteine uptake via EAAT3.

Prion Protein Signaling in the Nervous System—A Review and Perspective

Prion protein (PrPC) was originally known as the causative agent of transmissible spongiform encephalopathy (TSE) but with recent research, its true function in cells is becoming clearer. It is known to act as a scaffolding protein, binding multiple ligands at the cell membrane and to be involved in signal transduction, passing information from the extracellular matrix (ECM) to the cytoplasm. Its role in the coordination of transmitters at the synapse, glyapse, and gap junction and in short- and long-range neurotrophic signaling gives PrPC a major part in neural transmission and nervous system signaling. It acts to regulate cellular function in multiple targets through its role as a controller of redox status and calcium ion flux. Given the importance of PrPC in cell physiology, this review considers its potential role in disease apart from TSE. The putative functions of PrPC point to involvement in neurodegenerative disease, neuropathic pain, chronic headache, and inflammatory disease including neuroinflammatory disease of the nervous system. Potential targets for the treatment of disease influenced by PrPC are discussed.

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.

The mechanism of membrane disruption by cytotoxic amyloid oligomers formed by prion protein(106-126) is dependent on bilayer composition

The formation of fibrillar aggregates has long been associated with neurodegenerative disorders such as Alzheimer and Parkinson diseases. Although fibrils are still considered important to the pathology of these disorders, it is now widely understood that smaller amyloid oligomers are the toxic entities along the misfolding pathway. One characteristic shared by the majority of amyloid oligomers is the ability to disrupt membranes, a commonality proposed to be responsible for their toxicity, although the mechanisms linking this to cell death are poorly understood. Here, we describe the physical basis for the cytotoxicity of oligomers formed by the prion protein (PrP)-derived amyloid peptide PrP(106-126). We show that oligomers of this peptide kill several mammalian cells lines, as well as mouse cerebellar organotypic cultures, and we also show that they exhibit antimicrobial activity. Physical perturbation of model membranes mimicking bacterial or mammalian cells was investigated using atomic force microscopy, polarized total internal reflection fluorescence microscopy, and NMR spectroscopy. Disruption of anionic membranes proceeds through a carpet or detergent model as proposed for other antimicrobial peptides. By contrast, when added to zwitterionic membranes containing cholesterol-rich ordered domains, PrP(106-126) oligomers induce a loss of domain separation and decreased membrane disorder. Loss of raft-like domains may lead to activation of apoptotic pathways, resulting in cell death. This work sheds new light on the physical mechanisms of amyloid cytotoxicity and is the first to clearly show membrane type-specific modes of action for a cytotoxic peptide.

Neurotoxicity of prion peptides mimicking the central domain of the cellular prion protein

The physiological functions of PrP(C) remain enigmatic, but the central domain, comprising highly conserved regions of the protein may play an important role. Indeed, a large number of studies indicate that synthetic peptides containing residues 106-126 (CR) located in the central domain (CD, 95-133) of PrP(C) are neurotoxic. The central domain comprises two chemically distinct subdomains, the charge cluster (CC, 95-110) and a hydrophobic region (HR, 112-133). The aim of the present study was to establish the individual cytotoxicity of CC, HR and CD. Our results show that only the CD peptide is neurotoxic. Biochemical, Transmission Electron Microscopy and Atomic Force Microscopy experiments demonstrated that the CD peptide is able to activate caspase-3 and disrupt the cell membrane, leading to cell death.

Molecular nexopathies: a new paradigm of neurodegenerative disease

Neural networks provide candidate substrates for the spread of proteinopathies causing neurodegeneration, and emerging data suggest that macroscopic signatures of network disintegration differentiate diseases. However, how do protein abnormalities produce network signatures? The answer may lie with 'molecular nexopathies': specific, coherent conjunctions of pathogenic protein and intrinsic network characteristics that define network signatures of neurodegenerative pathologies. Key features of the paradigm that we propose here include differential intrinsic network vulnerability to propagating protein abnormalities, in part reflecting developmental structural and functional factors; differential vulnerability of neural connection types (e.g., clustered versus distributed connections) to particular pathogenic proteins; and differential impact of molecular effects (e.g., toxic-gain-of-function versus loss-of-function) on gradients of network damage. The paradigm has implications for understanding and predicting neurodegenerative disease biology.

Prion protein and its role in signal transduction

Prion diseases are a class of fatal neurodegenerative disorders that can be sporadic, genetic or iatrogenic. They are characterized by the unique nature of their etiologic agent: prions (PrP(Sc)). A prion is an infectious protein with the ability to convert the host-encoded cellular prion protein (PrP(C)) into new prion molecules by acting as a template. Since Stanley B. Prusiner proposed the "protein-only" hypothesis for the first time, considerable effort has been put into defining the role played by PrP(C) in neurons. However, its physiological function remains unclear. This review summarizes the major findings that support the involvement of PrP(C) in signal transduction.

Modelling neurodegeneration in prion disease - applications for drug development

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

Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance

The subversion of the normal function exerted by the cellular prion protein (PrP^C) in neurons by pathogenic prions is assumed to have a central role in the pathogenesis of transmissible spongiform encephalopathies. Using two murine models of prion infection, the 1C11 neuronal cell line and neurospheres, we document that prion infection is associated with the constitutive activation of signaling targets normally coupled with PrP^C, including the Fyn kinase, the mitogen-associated protein kinases ERK1/2 and the CREB transcription factor. PrP^C-dependent signaling overactivation in infected cells is associated with the recruitment of p38 and JNK stress-associated kinases. Downstream from CREB, prion-infected cells exhibit reduced activity of the matrix metalloprotease (MMP)-9. As MMP-9 catalyzes the degradation of the amyloid A-beta peptide, the decrease in MMP-9 activity in prion-infected cells causes a significant impairment of the clearance of A-beta, leading to its accumulation. By exploiting two 1C11-infected clones accumulating high or moderate levels of prions, we show that the prion-induced changes are correlated with the level of infectivity. Of note, a dose-dependent increase in A-beta levels was also found in the cerebrospinal fluid of mice inoculated with these infected clones. By demonstrating that pathogenic prions trigger increases in A-beta levels through the deviation of PrP^C signaling, our data argue that A-beta may exacerbate prion-induced toxicity.

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.

Prion protein at the crossroads of physiology and disease

The presence of the cellular prion protein (PrP(C)) on the cell surface is critical for the neurotoxicity of prions. Although several biological activities have been attributed to PrP(C), a definitive demonstration of its physiological function remains elusive. In this review, we discuss some of the proposed functions of PrP(C), focusing on recently suggested roles in cell adhesion, regulation of ionic currents at the cell membrane and neuroprotection. We also discuss recent evidence supporting the idea that PrP(C) may function as a receptor for soluble oligomers of the amyloid β peptide and possibly other toxic protein aggregates. These data suggest surprising new connections between the physiological function of PrP(C) and its role in neurodegenerative diseases beyond those caused by prions.

Hypoxia-inducible factor-1 α regulates prion protein expression to protect against neuron cell damage

The human prion protein fragment, PrP (106-126), may contain a majority of the pathological features associated with the infectious scrapie isoform of PrP, known as PrP(Sc). Based on our previous findings that hypoxia protects neuronal cells from PrP (106-126)-induced apoptosis and increases cellular prion protein (PrP(C)) expression, we hypothesized that hypoxia-related genes, including hypoxia-inducible factor-1 alpha (HIF-1α), may regulate PrP(C) expression and that these genes may be involved in prion-related neurodegenerative diseases. Hypoxic conditions are known to elicit cellular responses designed to improve cell survival through adaptive processes. Under normoxic conditions, a deferoxamine-mediated elevation of HIF-1α produced the same effect as hypoxia-inhibited neuron cell death. However, under hypoxic conditions, doxorubicin-suppressed HIF-1α attenuated the inhibitory effect on neuron cell death mediated by PrP (106-126). Knock-down of HIF-1α using lentiviral short hairpin (sh) RNA-induced downregulation of PrP(C) mRNA and protein expression under hypoxic conditions, and sensitized neuron cells to prion peptide-mediated cell death even in hypoxic conditions. In PrP(C) knockout hippocampal neuron cells, hypoxia increased the HIF-1α protein but the cells did not display the inhibitory effect of prion peptide-induced neuron cell death. Adenoviruses expressing the full length Prnp gene (Ad-Prnp) were utilized for overexpression of the Prnp gene in PrP(C) knockout hippocampal neuron cells. Adenoviral transfection of PrP(C) knockout cells with Prnp resulted in the inhibition of prion peptide-mediated cell death in these cells. This is the first report demonstrating that expression of normal PrP(C) is regulated by HIF-1α, and PrP(C) overexpression induced by hypoxia plays a pivotal role in hypoxic inhibition of prion peptide-induced neuron cell death. These results suggest that hypoxia-related genes, including HIF-1α, may be involved in the pathogenesis of prion-related diseases and as such may be a therapeutic target for prion-related neurodegenerative diseases.

Neuritogenesis: the prion protein controls β1 integrin signaling activity

Cytoskeleton modifications are required for neuronal stem cells to acquire neuronal polarization. Little is known, however, about mechanisms that orchestrate cytoskeleton remodeling along neuritogenesis. Here, we show that the silencing of the cellular prion protein (PrP(C)) impairs the initial sprouting of neurites upon induction of differentiation of the 1C11 neuroectodermal cell line, indicating that PrP(C) is necessary to neuritogenesis. Such PrP(C) function relies on its capacity to negatively regulate the clustering, activation, and signaling activity of β1 integrins at the plasma membrane. β1 Integrin aggregation caused by PrP(C) depletion triggers overactivation of the RhoA-Rho kinase-LIMK-cofilin pathway, which, in turn, alters the turnover of focal adhesions, increases the stability of actin microfilaments, and in fine impairs neurite formation. Inhibition of Rho kinases is sufficient to compensate for the lack of PrP(C) and to restore neurite sprouting. We also observe an increased secretion of fibronectin in the surrounding milieu of PrP(C)-depleted 1C11 cells, which likely self-sustains β1 integrin signaling overactivation and contributes to neuritogenesis defect. Our overall data reveal that PrP(C) contributes to the acquisition of neuronal polarization by modulating β1 integrin activity, cell interaction with fibronectin, and cytoskeleton dynamics.

Acute exposure to prion infection induces transient oxidative stress progressing to be cumulatively deleterious with chronic propagation in vitro

Neuronal loss is a pathological feature of prion diseases for which increased reactive oxygen species (ROS) and consequent oxidative stress is one proposed mechanism. The processes underlying ROS production in prion disease and the precise relationship to misfolding of the prion protein remain obscure. Using cell culture models of prion infection we found that cells demonstrate a rapid, prion protein (PrP) dependent, increase in intracellular ROS following exposure to infectious inoculum. ROS production correlated with internalisation and increased intracellular protease resistant PrP (PrP(Res)). The ROS increase was predominantly lysosomal in origin but not sustained, with cells adapting within 48 hours. Overall ROS levels remained normal in the chronically prion infected cell population; however a subpopulation characterised by loss of membrane phosphatidylserine asymmetry exhibited highly peroxidised intracellular aggregates that localised with PrP and intense caspase activation. These apoptotic cells showed increased ROS closely correlating with increased PrP(Res). Our findings demonstrate that a PrP-dependent, transient, increase in intracellular ROS is characteristic of acute cellular prion infection, while chronic phases of prion infection in vitro are associated with a significant subpopulation manifesting apoptosis accompanying heightened oxidative stress and increased PrP(Res) burden. Such observations strengthen the direct links between heightened ROS and ongoing prion propagation with eventual cellular demise.

The extracellular regulated kinase-1 (ERK1) controls regulated alpha-secretase-mediated processing, promoter transactivation, and mRNA levels of the cellular prion protein

The α-secretases A disintegrin and metalloprotease 10 (ADAM10) and ADAM17 trigger constitutive and regulated processing of the cellular prion protein (PrP(c)) yielding N1 fragment. The latter depends on protein kinase C (PKC)-coupled M1/M3 muscarinic receptor activation and subsequent phosphorylation of ADAM17 on its intracytoplasmic threonine 735. Here we show that regulated PrP(c) processing and ADAM17 phosphorylation and activation are controlled by the extracellular-regulated kinase-1/MAP-ERK kinase (ERK1/MEK) cascade. Thus, reductions of ERK1 or MEK activities by dominant-negative analogs, pharmacological inhibition, or genetic ablation all impair N1 secretion, whereas constitutively active proteins increase N1 recovery in the conditioned medium. Interestingly, we also observed an ERK1-mediated enhanced expression of PrP(c). We demonstrate that the ERK1-associated increase in PrP(c) promoter transactivation and mRNA levels involve transcription factor AP-1 as a downstream effector. Altogether, our data identify ERK1 as an important regulator of PrP(c) cellular homeostasis and indicate that this kinase exerts a dual control of PrP(c) levels through transcriptional and post-transcriptional mechanisms.

Interactions of the prion peptide (PrP 106-126) with brain capillary endothelial cells: coordinated cell killing and remodeling of intercellular junctions

We studied here the interactions of PrP 106-126, a peptide corresponding to the prion protein (PrP) amyloidogenic region, with a blood-brain barrier in vitro model consisting of confluent porcine brain endothelial cells (PBEC). PrP 106-126 interacted selectively with PBEC via their luminal side, and caused cumulative cell death, as shown by lactate dehydrogenase release, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, Caspase 3 induction and direct cell counting. In addition, PrP 106-126, but not its corresponding scrambled peptide, produced a 50% reduction of the trans-endothelial electrical resistance, while the PBEC maintained confluency. This process was accompanied by a 23% increase of PBEC average size and the selective disappearance from the cell borders of the junction proteins occludin, claudin-5 and VE-cadherin (but not ZO-1), as evaluated by immunostaining. These results fit with a mechanism by which PrP 106-126 initiates a coordinated cell killing process ultimately causing the remaining cells to undergo a coordinated remodeling of the intercellular junctions and an expansion of their cell territory.

Inhibition of phospholipase A2 increased the removal of the prion derived peptide PrP82-146 from cultured neurons

The prion diseases are characterised by the formation of the disease-associated isoform of the prion protein (PrP(Sc)) and the production of disease-related peptides. The prion derived peptide PrP82-146 bound readily to cortical neurons and was found within detergent resistant membranes that are commonly called lipid rafts. It was not found within lysosomes and the slow degradation of PrP82-146 resulted in a half-life of approximately 5 days. In cortical neurons pre-treated with phospholipase A(2) (PLA(2)) inhibitors (AACOCF(3) or MAFP) less PrP82-146 entered lipid rafts, more PrP82-146 was found within lysosomes and the half-life of PrP82-146 was reduced to 24 h. Similarly, pre-treatment of neurons with platelet-activating factor (PAF) receptor antagonists (Hexa-PAF and ginkgolide B) increased the entry of PrP82-146 into lysosomes and reduced its half-life. Furthermore, the addition of PAF reversed the effects of PLA(2) inhibitors on PrP82-146 trafficking. PAF controlled the amount of cholesterol in cell membranes and the effects of PAF receptor antagonists on the trafficking of PrP82-146 were reversed by the addition of cholesterol. We conclude that activation of PLA(2) and the production of PAF control a cholesterol-sensitive pathway that affects the cellular localisation and hence the fate of PrP82-146 in neurons.

Cellular cholesterol enrichment prevents prion peptide-induced neuron cell damages

The prion diseases are neurodegenerative disorders characterized by the conversion of the PrPc (normal cellular prion) to the PrPsc (misfolded isoform). The accumulation of PrPsc within the central nervous system (CNS) leads to neurocytotoxicity by increasing oxidative stress. In addition, many neurodegenerative disorders including prion, Parkinson's and Alzheimer's diseases may be regulated by cholesterol homeostasis. The effects of cholesterol balance on prion protein-mediated neurotoxicity and ROS (reactive oxygen species) generation were the focus of this study. Cholesterol treatment inhibited PrP (106-126)-induced neuronal cell death and ROS generation in SH-SY5Y neuroblastoma cells. In addition, the PrP (106-126)-mediated increase of p53, p-p38, p-ERK and the decrease of Bcl-2 were blocked by cholesterol treatment. These results indicated that cellular cholesterol enrichment is a key regulator of PrP-106-126-mediated oxidative stress and neurotoxicity. Taken together, the results of this study suggest that modulation of cellular cholesterol appears to prevent the neuronal cell death caused by prion peptides.

Aberrant ERK 1/2 complex activation and localization in scrapie-infected GT1-1 cells

Background

Fatal neurodegenerative disorders such as Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases in humans, scrapie and bovine spongiform encephalopathy in animals, are characterized by the accumulation in the brain of a pathological form of the prion protein (PrP) denominated PrP^Sc. The latter derives from the host cellular form, PrP^C, through a process whereby portions of its α-helical and coil structures are refolded into β-sheet structures.

Results

In this work, the widely known in vitro model of prion replication, hypothalamic GT1-1 cell line, was used to investigate cellular and molecular responses to prion infection. The MAP kinase cascade was dissected to assess the phosphorylation levels of src, MEK 1/2 and ERK 1/2 signaling molecules, both before and after prion infection. Our findings suggest that prion replication leads to a hyper-activation of this pathway. Biochemical analysis was complemented with immunofluorescence studies to map the localization of the ERK complex within the different cellular compartments. We showed how the ERK complex relocates in the cytosol upon prion infection. We correlated these findings with an impairment of cell growth in prion-infected GT1-1 cells as probed by MTT assay. Furthermore, given the persistent urgency in finding compounds able to cure prion infected cells, we tested the effects on the ERK cascade of two molecules known to block prion replication in vitro, quinacrine and Fab D18. We were able to show that while these two compounds possess similar effects in curing prion infection, they affect the MAP kinase cascade differently.

Conclusions

Taken together, our results help shed light on the molecular events involved in neurodegeneration and neuronal loss in prion infection and replication. In particular, the combination of chronic activation and aberrant localization of the ERK complex may lead to a lack of essential neuroprotective and survival factors. Interestingly, these data seem to define some common traits with other neurodegenerative disorders such as, for example, Alzheimer's disease.

Caveolin regulation of neuronal intracellular signaling

Caveolin proteins physically interact with and compartmentalize membrane-localized signaling proteins to facilitate high-fidelity intracellular signaling. Though primarily studied outside the nervous system, recent investigations have revealed that caveolin proteins are key modulators of a variety of neuronal intracellular signaling pathways. Through both protein aggregation and segregation, caveolin proteins can exert positive and negative influences on intracellular signaling. This review will detail recent findings regarding caveolin function in the brain.

Redox control of prion and disease pathogenesis

Imbalance of brain metal homeostasis and associated oxidative stress by redox-active metals like iron and copper is an important trigger of neurotoxicity in several neurodegenerative conditions, including prion disorders. Whereas some reports attribute this to end-stage disease, others provide evidence for specific mechanisms leading to brain metal dyshomeostasis during disease progression. In prion disorders, imbalance of brain-iron homeostasis is observed before end-stage disease and worsens with disease progression, implicating iron-induced oxidative stress in disease pathogenesis. This is an unexpected observation, because the underlying cause of brain pathology in all prion disorders is PrP-scrapie (PrP(Sc)), a beta-sheet-rich conformation of a normal glycoprotein, the prion protein (PrP(C)). Whether brain-iron dyshomeostasis occurs because of gain of toxic function by PrP(Sc) or loss of normal function of PrP(C) remains unclear. In this review, we summarize available evidence suggesting the involvement of oxidative stress in prion-disease pathogenesis. Subsequently, we review the biology of PrP(C) to highlight its possible role in maintaining brain metal homeostasis during health and the contribution of PrP(Sc) in inducing brain metal imbalance with disease progression. Finally, we discuss possible therapeutic avenues directed at restoring brain metal homeostasis and alleviating metal-induced oxidative stress in prion disorders.

Glimepiride reduces the expression of PrPc, prevents PrPSc formation and protects against prion mediated neurotoxicity in cell lines

BACKGROUND

A hallmark of the prion diseases is the conversion of the host-encoded cellular prion protein (PrP(C)) into a disease related, alternatively folded isoform (PrP(Sc)). The accumulation of PrP(Sc) within the brain is associated with synapse loss and ultimately neuronal death. Novel therapeutics are desperately required to treat neurodegenerative diseases including the prion diseases.

PRINCIPAL FINDINGS

Treatment with glimepiride, a sulphonylurea approved for the treatment of diabetes mellitus, induced the release of PrP(C) from the surface of prion-infected neuronal cells. The cell surface is a site where PrP(C) molecules may be converted to PrP(Sc) and glimepiride treatment reduced PrP(Sc) formation in three prion infected neuronal cell lines (ScN2a, SMB and ScGT1 cells). Glimepiride also protected cortical and hippocampal neurones against the toxic effects of the prion-derived peptide PrP82-146. Glimepiride treatment significantly reduce both the amount of PrP82-146 that bound to neurones and PrP82-146 induced activation of cytoplasmic phospholipase A(2) (cPLA(2)) and the production of prostaglandin E(2) that is associated with neuronal injury in prion diseases. Our results are consistent with reports that glimepiride activates an endogenous glycosylphosphatidylinositol (GPI)-phospholipase C which reduced PrP(C) expression at the surface of neuronal cells. The effects of glimepiride were reproduced by treatment of cells with phosphatidylinositol-phospholipase C (PI-PLC) and were reversed by co-incubation with p-chloromercuriphenylsulphonate, an inhibitor of endogenous GPI-PLC.

CONCLUSIONS

Collectively, these results indicate that glimepiride may be a novel treatment to reduce PrP(Sc) formation and neuronal damage in prion diseases.

Lipid rafts and caveolae and their role in compartmentation of redox signaling

Membrane (lipid) rafts and caveolae, a subset of rafts, are cellular domains that concentrate plasma membrane proteins and lipids involved in the regulation of cell function. In addition to providing signaling platforms for G-protein-coupled receptors and certain tyrosine kinase receptors, rafts/caveolae can influence redox signaling. This review discusses molecular characteristics of and methods to study rafts/caveolae, determinants that contribute to the localization of molecules in these entities, an overview of signaling molecules that show such localization, and the contribution of rafts/caveolae to redox signaling. Of particular note is the evidence that endothelial nitric oxide synthase (eNOS), NADPH oxygenase, and heme oxygenase, along with other less well-studied redox systems, localize in rafts and caveolae. The precise basis for this localization and the contribution of raft/caveolae-localized redox components to physiology and disease are important issues for future studies.

Core structure of amyloid fibrils formed by residues 106-126 of the human prion protein

Peptides comprising residues 106-126 of the human prion protein (PrP) exhibit many features of the full-length protein. PrP(106-126) induces apoptosis in neurons, forms fibrillar aggregates, and can mediate the conversion of native cellular PrP (PrP(C)) to the scrapie form (PrP(Sc)). Despite a wide range of biochemical and biophysical studies on this peptide, including investigation of its propensity for aggregation, interactions with cell membranes, and PrP-like toxicity, the structure of amyloid fibrils formed by PrP(106-126) remains poorly defined. In this study we use solid-state nuclear magnetic resonance to define the secondary and quaternary structure of PrP(106-126) fibrils. Our results reveal that PrP(106-126) forms in-register parallel beta sheets, stacked in an antiparallel fashion within the mature fibril. The close intermolecular contacts observed in the fibril core provide a rational for the sequence-dependent behavior of PrP(106-126), and provide a basis for further investigation of its biological properties.

PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway

Cellular prion protein (PrP(c)) is a ubiquitous glycoprotein, whose physiological role is poorly characterized. It has been suggested that PrP(c) participates in neuritogenesis, neuroprotection, copper metabolism, and signal transduction. In this study we detailed the intracellular events induced by PrP(c) antibody-mediated cross-linking in PC12 cells. We found a Fyn-dependent activation of the Ras-Raf pathway, which leads to a rapid and transient phosphorylation of extracellular regulated kinases. In addition, this activation cascade relies on the engagement of integrins, and involves focal adhesion kinase activation. We demonstrated the tyrosine phosphorylation of caveolin-1 as a consequence of PrP(c) stimulation, and showed that phosphocaveolin-1 scaffolds and coordinates protein complexes involved in PrP(c)-dependent signaling. Moreover, we found that caveolin-1 phosphorylation, is a mechanism for recruiting the C-terminal Src kinase and inactivating Fyn, so as to terminate cell signaling. Furthermore our data support a significant role for PrP(c) as a response mediator in neuritogenesis and cell differentiation.