The Prion Protein Modulates A-type K+ Currents Mediated by Kv4.2 Complexes through Dipeptidyl Aminopeptidase-like Protein 6

Widely expressed in the adult central nervous system, the cellular prion protein (PrP^C) is implicated in a variety of processes, including neuronal excitability. Dipeptidyl aminopeptidase-like protein 6 (DPP6) was first identified as a PrP^C interactor using in vivo formaldehyde cross-linking of wild type (WT) mouse brain. This finding was confirmed in three cell lines and, because DPP6 directs the functional assembly of K⁺ channels, we assessed the impact of WT and mutant PrP^C upon Kv4.2-based cell surface macromolecular complexes. Whereas a Gerstmann-Sträussler-Scheinker disease version of PrP with eight extra octarepeats was a loss of function both for complex formation and for modulation of Kv4.2 channels, WT PrP^C, in a DPP6-dependent manner, modulated Kv4.2 channel properties, causing an increase in peak amplitude, a rightward shift of the voltage-dependent steady-state inactivation curve, a slower inactivation, and a faster recovery from steady-state inactivation. Thus, the net impact of wt PrP^C was one of enhancement, which plays a critical role in the down-regulation of neuronal membrane excitability and is associated with a decreased susceptibility to seizures. Insofar as previous work has established a requirement for WT PrP^C in the Aβ-dependent modulation of excitability in cholinergic basal forebrain neurons, our findings implicate PrP^C regulation of Kv4.2 channels as a mechanism contributing to the effects of oligomeric Aβ upon neuronal excitability and viability.     

Disease-associated Mutations in the Prion Protein Impair Laminin-induced Process Outgrowth and Survival

Prions, the agents of transmissible spongiform encephalopathies, require the expression of prion protein (PrP^C) to propagate disease. PrP^C is converted into an abnormal insoluble form, PrP^Sc, that gains neurotoxic activity. Conversely, clinical manifestations of prion disease may occur either before or in the absence of PrP^Sc deposits, but the loss of normal PrP^C function contribution for the etiology of these diseases is still debatable. Prion disease-associated mutations in PrP^C represent one of the best models to understand the impact of PrP^C loss-of-function. PrP^C associates with various molecules and, in particular, the interaction of PrP^C with laminin (Ln) modulates neuronal plasticity and memory formation. To assess the functional alterations associated with PrP^C mutations, wild-type and mutated PrP^C proteins were expressed in a neural cell line derived from a PrP^C-null mouse. Treatment with the laminin γ1 chain peptide (Ln γ1), which mimics the Ln binding site for PrP^C, increased intracellular calcium in cells expressing wild-type PrP^C, whereas a significantly lower response was observed in cells expressing mutated PrP^C molecules. The Ln γ1 did not promote process outgrowth or protect against staurosporine-induced cell death in cells expressing mutated PrP^C molecules in contrast to cells expressing wild-type PrP^C. The co-expression of wild-type PrP^C with mutated PrP^C molecules was able to rescue the Ln protective effects, indicating the lack of negative dominance of PrP^C mutated molecules. These results indicate that PrP^C mutations impair process outgrowth and survival mediated by Ln γ1 peptide in neural cells, which may contribute to the pathogenesis of genetic prion diseases.     

Unexpected Tolerance of α-Cleavage of the Prion Protein to Sequence Variations

The cellular form of the prion protein, PrP^C, undergoes extensive proteolysis at the α site (109K↓H110). Expression of non-cleavable PrP^C mutants in transgenic mice correlates with neurotoxicity, suggesting that α-cleavage is important for PrP^C physiology. To gain insights into the mechanisms of α-cleavage, we generated a library of PrP^C mutants with mutations in the region neighbouring the α-cleavage site. The prevalence of C1, the carboxy adduct of α-cleavage, was determined for each mutant. In cell lines of disparate origin, C1 prevalence was unaffected by variations in charge and hydrophobicity of the region neighbouring the α-cleavage site, and by substitutions of the residues in the palindrome that flanks this site. Instead, α-cleavage was size-dependently impaired by deletions within the domain 106–119. Almost no cleavage was observed upon full deletion of this domain. These results suggest that α-cleavage is executed by an α-PrPase whose activity, despite surprisingly limited sequence specificity, is dependent on the size of the central region of PrP^C.     

Role of α7 Nicotinic Acetylcholine Receptor in Calcium Signaling Induced by Prion Protein Interaction with Stress-inducible Protein 1

The prion protein (PrP^C) is a conserved glycosylphosphatidylinositol-anchored cell surface protein expressed by neurons and other cells. Stress-inducible protein 1 (STI1) binds PrP^C extracellularly, and this activated signaling complex promotes neuronal differentiation and neuroprotection via the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP-dependent protein kinase 1 (PKA) pathways. However, the mechanism by which the PrP^C-STI1 interaction transduces extracellular signals to the intracellular environment is unknown. We found that in hippocampal neurons, STI1-PrP^C engagement induces an increase in intracellular Ca²⁺ levels. This effect was not detected in PrP^C-null neurons or wild-type neurons treated with an STI1 mutant unable to bind PrP^C. Using a best candidate approach to test for potential channels involved in Ca²⁺ influx evoked by STI1-PrP^C, we found that α-bungarotoxin, a specific inhibitor for α7 nicotinic acetylcholine receptor (α7nAChR), was able to block PrP^C-STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when α7nAChR was transfected into HEK 293 cells, it formed a functional complex with PrP^C and allowed reconstitution of signaling by PrP^C-STI1 interaction. These results indicate that STI1 can interact with the PrP^C·α7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.     

Amyloid-β and Proinflammatory Cytokines Utilize a Prion Protein-Dependent Pathway to Activate NADPH Oxidase and Induce Cofilin-Actin Rods in Hippocampal Neurons

Neurites of neurons under acute or chronic stress form bundles of filaments (rods) containing 1∶1 cofilin∶actin, which impair transport and synaptic function. Rods contain disulfide cross-linked cofilin and are induced by treatments resulting in oxidative stress. Rods form rapidly (5–30 min) in >80% of cultured hippocampal or cortical neurons treated with excitotoxic levels of glutamate or energy depleted (hypoxia/ischemia or mitochondrial inhibitors). In contrast, slow rod formation (50% of maximum response in ∼6 h) occurs in a subpopulation (∼20%) of hippocampal neurons upon exposure to soluble human amyloid-β dimer/trimer (Aβd/t) at subnanomolar concentrations. Here we show that proinflammatory cytokines (TNFα, IL-1β, IL-6) also induce rods at the same rate and within the same neuronal population as Aβd/t. Neurons from prion (PrP^C)-null mice form rods in response to glutamate or antimycin A, but not in response to proinflammatory cytokines or Aβd/t. Two pathways inducing rod formation were confirmed by demonstrating that NADPH-oxidase (NOX) activity is required for prion-dependent rod formation, but not for rods induced by glutamate or energy depletion. Surprisingly, overexpression of PrP^C is by itself sufficient to induce rods in over 40% of hippocampal neurons through the NOX-dependent pathway. Persistence of PrP^C-dependent rods requires the continuous activity of NOX. Removing inducers or inhibiting NOX activity in cells containing PrP^C-dependent rods causes rod disappearance with a half-life of about 36 min. Cofilin-actin rods provide a mechanism for synapse loss bridging the amyloid and cytokine hypotheses for Alzheimer disease, and may explain how functionally diverse Aβ-binding membrane proteins induce synaptic dysfunction.     

Transforming Growth Factor-β1 and -β2 in Gastric Precancer and Cancer and Roles in Tumor-Cell Interactions with Peripheral Blood Mononuclear Cells In Vitro

Transforming growth factor-β1 (TGF-β1) and -β2 are correlated with poorer prognosis in gastric cancer (GC), which act in both tumor and immune cells. However, their expressions in precancer and tumor-cell interactions with peripheral blood mononuclear cells (PBMCs) remain unclear. Protein levels of TGF-β1 and -β2 were analyzed by immunohistochemistry and corresponding mRNA levels were determined by quantitative real-time polymerase chain reaction in 93 surgical and biopsy specimens. Serum TGF-β concentration was detected by enzyme-linked immunosorbent assays. AGS and MKN45 cell lines were directly or indirectly cocultured with PBMCs in vitro. TGF-β and Smad molecules were detected after cocultures and the growths of GC cells and PBMCs were assessed by cell proliferation assay. The results showed positive staining for TGF-β1 was detected in 20% of control samples, 52.3% of precancer, 59.1% of early GC and 66.7% of advanced GC samples, correlated with lesion progression (χ² = 9.487, P = 0.002). All tissues were positive for TGF-β2. TGF-β1 mRNA levels were increased in advanced cancers, while TGF-β2 increased earlier. TGF-β1 mRNA levels were higher in tumor than in peritumor, which positively correlated with Smad2 and Smad7. Serum TGF-β levels were significantly higher in patients with early and advanced cancers compared to controls (TGF-β1∶50.08±4.38 and 45.76±5.00 vs. 27.78±6.11 ng/mL; TGF-β2∶133.61±21.90 and 111.34±15.76 vs. 59.41±15.42 ng/mL, both P<0.05). The levels of TGF-β1 mRNA and cytokine secretion were higher in GC cells after direct coculture compared to indirect culture. TGF-β1 was decreased and TGF-β2 was increased in PBMCs after cocultures. Moreover, TGF-β1 inhibited the viability of PBMCs but not cancer cells. Collectively, neoplastic transformation may be an early event involving the increase of TGF-β1 in the general and local environment. TGF-β1 production is promoted by the direct interaction between GC cells and PBMCs, which might facilitate cancer development.     

Prion-Protein-interacting Amyloid-β Oligomers of High Molecular Weight Are Tightly Correlated with Memory Impairment in Multiple Alzheimer Mouse Models

Alzheimer disease (AD) is characterized by amyloid-β accumulation, with soluble oligomers (Aβo) being the most synaptotoxic. However, the multivalent and unstable nature of Aβo limits molecular characterization and hinders research reproducibility. Here, we characterized multiple Aβo forms throughout the life span of various AD mice and in post-mortem human brain. Aβo exists in several populations, where prion protein (PrP^C)-interacting Aβo is a high molecular weight Aβ assembly present in multiple mice and humans with AD. Levels of PrP^C-interacting Aβo match closely with mouse memory and are equal or superior to other Aβ measures in predicting behavioral impairment. However, Aβo metrics vary considerably between mouse strains. Deleting PrP^C expression in mice with relatively low PrP^C-interacting Aβo (Tg2576) results in partial rescue of cognitive performance as opposed to complete recovery in animals with a high percentage of PrP^C-interacting Aβo (APP/PSEN1). These findings highlight the relative contributions and interplay of Aβo forms in AD.     

A Short Peptide That Mimics the Binding Domain of TGF-β1 Presents Potent Anti-Inflammatory Activity

The transforming growth factor beta 1 (TGF-β1) is a pleiotropic cytokine with multiple roles in development, wound healing, and immune regulation. TGF-β1-mediated immune dysfunction may lead to pathological conditions, such as inflammation. Chronic inflammatory process is characterized by a continuous release of pro-inflammatory cytokines, and the inhibition or the blockage of these cytokines signaling pathways are considered a target treatment. In this context, despite the high numbers of TGF-β-targeted pathways, the inducible regulatory T cells (iTreg) to control inflammation seems to be a promising approach. Our aim was to develop novel peptides through phage display (PhD) technology that could mimic TGF-β1 function with higher potency. Specific mimetic peptides were obtained through a PhD subtraction strategy from whole cell binding using TGF-β1 recombinant as a competitor during elution step. We have selected a peptide that seems to play an important role on cellular differentiation and modulation of TNF-α and IL-10 cytokines. The synthetic pm26TGF-β1 peptide tested in PBMC significantly down-modulated TNF-α and up-regulated IL-10 responses, leading to regulatory T cells (Treg) phenotype differentiation. Furthermore, the synthetic peptide was able to decrease leukocytes rolling in BALB/C mice and neutrophils migration during inflammatory process in C57BL/6 mice. These data suggest that this peptide may be useful for the treatment of inflammatory diseases, especially because it displays potent anti-inflammatory properties and do not exhibit neutrophils’ chemoattraction.     

Role of Lipid Rafts and GM1 in the Segregation and Processing of Prion Protein

The prion protein (PrP^C) is highly expressed within the nervous system. Similar to other GPI-anchored proteins, PrP^C is found in lipid rafts, membrane domains enriched in cholesterol and sphingolipids. PrP^C raft association, together with raft lipid composition, appears essential for the conversion of PrP^C into the scrapie isoform PrP^Sc, and the development of prion disease. Controversial findings were reported on the nature of PrP^C-containing rafts, as well as on the distribution of PrP^C between rafts and non-raft membranes. We investigated PrP^C/ganglioside relationships and their influence on PrP^C localization in a neuronal cellular model, cerebellar granule cells. Our findings argue that in these cells at least two PrP^C conformations coexist: in lipid rafts PrP^C is present in the native folding (α-helical), stabilized by chemico-physical condition, while it is mainly present in other membrane compartments in a PrP^Sc-like conformation. We verified, by means of antibody reactivity and circular dichroism spectroscopy, that changes in lipid raft-ganglioside content alters PrP^C conformation and interaction with lipid bilayers, without modifying PrP^C distribution or cleavage. Our data provide new insights into the cellular mechanism of prion conversion and suggest that GM1-prion protein interaction at the cell surface could play a significant role in the mechanism predisposing to pathology.     

Prion Protein Paralog Doppel Protein Interacts with Alpha-2-Macroglobulin: A Plausible Mechanism for Doppel-Mediated Neurodegeneration

Doppel protein (Dpl) is a paralog of the cellular form of the prion protein (PrP^C), together sharing common structural and biochemical properties. Unlike PrP^C, which is abundantly expressed throughout the central nervous system (CNS), Dpl protein expression is not detectable in the CNS. Interestingly, its ectopic expression in the brain elicits neurodegeneration in transgenic mice. Here, by combining native isoelectric focusing plus non-denaturing polyacrylamide gel electrophoresis and mass spectrometry analysis, we identified two Dpl binding partners: rat alpha-1-inhibitor-3 (α₁I₃) and, by sequence homology, alpha-2-macroglobulin (α₂M), two known plasma metalloproteinase inhibitors. Biochemical investigations excluded the direct interaction of PrP^C with either α₁I₃ or α₂M. Nevertheless, enzyme-linked immunosorbent assays and surface plasmon resonance experiments revealed a high affinity binding occurring between PrP^C and Dpl. In light of these findings, we suggest a mechanism for Dpl-induced neurodegeneration in mice expressing Dpl ectopically in the brain, linked to a withdrawal of natural inhibitors of metalloproteinase such as α₂M. Interestingly, α₂M has been proven to be a susceptibility factor in Alzheimer''s disease, and as our findings imply, it may also play a relevant role in other neurodegenerative disorders, including prion diseases.     

Loss of prion protein control of glucose metabolism promotes neurodegeneration in model of prion diseases

Corruption of cellular prion protein (PrP^C) function(s) at the plasma membrane of neurons is at the root of prion diseases, such as Creutzfeldt-Jakob disease and its variant in humans, and Bovine Spongiform Encephalopathies, better known as mad cow disease, in cattle. The roles exerted by PrP^C, however, remain poorly elucidated. With the perspective to grasp the molecular pathways of neurodegeneration occurring in prion diseases, and to identify therapeutic targets, achieving a better understanding of PrP^C roles is a priority. Based on global approaches that compare the proteome and metabolome of the PrP^C expressing 1C11 neuronal stem cell line to those of PrP^null-1C11 cells stably repressed for PrP^C expression, we here unravel that PrP^C contributes to the regulation of the energetic metabolism by orienting cells towards mitochondrial oxidative degradation of glucose. Through its coupling to cAMP/protein kinase A signaling, PrP^C tones down the expression of the pyruvate dehydrogenase kinase 4 (PDK4). Such an event favors the transfer of pyruvate into mitochondria and its conversion into acetyl-CoA by the pyruvate dehydrogenase complex and, thereby, limits fatty acids β-oxidation and subsequent onset of oxidative stress conditions. The corruption of PrP^C metabolic role by pathogenic prions PrP^Sc causes in the mouse hippocampus an imbalance between glucose oxidative degradation and fatty acids β-oxidation in a PDK4-dependent manner. The inhibition of PDK4 extends the survival of prion-infected mice, supporting that PrP^Sc-induced deregulation of PDK4 activity and subsequent metabolic derangements contribute to prion diseases. Our study posits PDK4 as a potential therapeutic target to fight against prion diseases.     

Unaltered prion protein expression in Alzheimer disease patients

The suggested role of cellular prion protein (PrP^C) in mediating the toxic effects of oligomeric amyloid β peptide (Aβ) in Alzheimer disease (AD) is controversial. To address the hypothesis that variable PrP^C expression is involved in AD pathogenesis, we analyzed PrP^C expression in the frontal and temporal cortices and hippocampus of individuals with no cognitive impairment (NCI), amnestic mild cognitive impairment (aMCI), mild AD (mAD) and AD. We found that PrP^C expression in all brain regions was not significantly altered among the various patient groups. In addition, PrP^C levels in all groups did not correlate with expression of methionine (M) or valine (V) at codon 129 of the PrP gene, a polymorphism that has been linked in some studies to increased risk for AD, and which occurs in close proximity to the proposed binding region for the oligomeric Aβ peptide. Our results indicate that, if PrP^C is involved in mediating the toxic effects of the oligomeric Aβ peptide, these effects occur independently of steady state levels of PrP or the codon 129 polymorphism.Key words: prion protein, PrP codon 129 polymorphism, Alzheimer disease, oligomeric Aβ, Alzheimer precursor protein     

The P’s and Q’s of cellular PrP-Aβ interactions

Prion disease research has opened up the “black-box” of neurodegeneration, defining a key role for protein misfolding wherein a predominantly alpha-helical precursor protein, PrP^C, is converted to a disease-associated, β-sheet enriched isoform called PrP^Sc. In Alzheimer disease (AD) the Aβ peptide derived from the β-amyloid precuror protein APP folds in β-sheet amyloid. Early thoughts along the lines of overlap may have been on target,¹ but were eclipsed by a simultaneous (but now anachronistic) controversy over the role of PrP^Sc in prion diseases.^2,3 Nonetheless, as prion diseases such as Creutzfeldt-Jakob Disease (CJD) are themselves rare and can include an overt infectious mode of transmission, and as familial prion diseases and familial AD involve different genes, an observer might reasonably have concluded that prion research could occasionally catalyze ideas in AD, but could never provide concrete overlaps at the mechanistic level. Surprisingly, albeit a decade or three down the road, several prion/AD commonalities can be found within the contemporary literature. One important prion/AD overlap concerns seeded spread of Aβ aggregates by intracerebral inoculation much like prions,⁴ and, with a neuron-to-neuron ‘spreading’ also reported for pathologic forms of other misfolded proteins, Tau^5,6 and α-synuclein in the case of Parkinson Disease.^7,8 The concept of seeded spread has been discussed extensively elsewhere, sometimes under the rubric of “prionoids”⁹, and lies outside the scope of this particular review where we will focus upon PrP^C. From this point the story can now be subdivided into four strands of investigation: (1) pathologic effects of Aβ can be mediated by binding to PrP^C,¹⁰ (2) the positioning of endoproteolytic processing events of APP by pathologic (β-cleavage + γ-cleavage) and non-pathologic (α-cleavage + γ-cleavage) secretase pathways is paralleled by seemingly analogous α- and β-like cleavage of PrP^C (Fig. 1) (3) similar lipid raft environments for PrP^C and APP processing machinery,^11-13 and perhaps in consequence, overlaps in repertoire of the PrP^C and APP protein interactors (“interactomes”),^14,15 and (4) rare kindreds with mixed AD and prion pathologies.¹⁶ Here we discuss confounds, consensus and conflict associated with parameters that apply to these experimental settings.     

A roadmap for investigating the role of the prion protein in depression associated with neurodegenerative disease

The physiological properties of the native, endogenous prion protein (PrP^C) is a matter of concern, due to its pleiotropic functions and links to neurodegenerative disorders and cancer. In line with our hypothesis that the basic function of PrP^C is to serve as a cell surface scaffold for the assembly of signaling modules, multiple interactions have been identified of PrP^C with signaling molecules, including neurotransmitter receptors. We recently reported evidence that PrP^C may modulate monoaminergic neurotransmission, as well as depressive-like behavior in mice. Here, we discuss how those results, together with a number of other studies, including our previous demonstration that both inflammatory and behavioral stress modulate PrP^C content in neutrophils, suggest a distributed role of PrP^C in clinical depression and inflammation associated with neurodegenerative diseases. An overarching understanding of the multiple interventions of PrP^C upon physiological events may both shed light on the pathogenesis of, as well as help the identification of novel therapeutic targets for clinical depression, Prion and Alzheimer's Diseases.     

��-Cleavage of cellular prion protein

The cellular prion protein (PrP^C) is subjected to various processing under physiological and pathological conditions, of which the α-cleavage within the central hydrophobic domain not only disrupts a region critical for both PrP toxicity and PrP^C to PrP^Sc conversion but also produces the N1 fragment that is neuroprotective and the C1 fragment that enhances the pro-apoptotic effect of staurosporine in one report and inhibits prion in another. The proteases responsible for the α-cleavage of PrP^C are controversial. The effect of ADAM10, ADAM17, and ADAM9 on N1 secretion clearly indicates their involvement in the α-cleavage of PrP^C, but there has been no report of direct PrP^C α-cleavage activity with any of the three ADAMs in a purified protein form. We demonstrated that, in muscle cells, ADAM8 is the primary protease for the α-cleavage of PrP^C, but another unidentified protease(s) must also play a minor role. We also found that PrP^C regulates ADAM8 expression, suggesting that a close examination on the relationships between PrP^C and its processing enzymes may reveal novel roles and underlying mechanisms for PrP^C in non-prion diseases such as asthma and cancer.     

Prion Protein Is Decreased in Alzheimer's Brain and Inversely Correlates with BACE1 Activity,Amyloid-β Levels and Braak Stage

The cellular prion protein (PrP^C) has been implicated in the development of Alzheimer''s disease (AD). PrP^C decreases amyloid-β (Aβ) production, which is involved in AD pathogenesis, by inhibiting β-secretase (BACE1) activity. Contactin 5 (CNTN5) has also been implicated in the development of AD by a genome-wide association study. Here we measured PrP^C and CNTN5 in frontal cortex samples from 24 sporadic AD and 24 age-matched control brains and correlated the expression of these proteins with markers of AD. PrP^C was decreased in sporadic AD compared to controls (by 49%, p = 0.014) but there was no difference in CNTN5 between sporadic AD and controls (p = 0.217). PrP^C significantly inversely correlated with BACE1 activity (r_s = −0.358, p = 0.006), Aβ load (r_s = −0.456, p = 0.001), soluble Aβ (r_s = −0.283, p = 0.026) and insoluble Aβ (r_s = −0.353, p = 0.007) and PrP^C also significantly inversely correlated with the stage of disease, as indicated by Braak tangle stage (r_s = −0.377, p = 0.007). CNTN5 did not correlate with Aβ load (r_s = 0.040, p = 0.393), soluble Aβ (r_s = 0.113, p = 0.223) or insoluble Aβ (r_s = 0.169, p = 0.125). PrP^C was also measured in frontal cortex samples from 9 Down''s syndrome (DS) and 8 age-matched control brains. In contrast to sporadic AD, there was no difference in PrP^C in the DS brains compared to controls (p = 0.625). These data are consistent with a role for PrP^C in regulating Aβ production and indicate that brain PrP^C level may be important in influencing the onset and progression of sporadic AD.     

Prion Protein-mediated Toxicity of Amyloid-β Oligomers Requires Lipid Rafts and the Transmembrane LRP1

Soluble oligomers of the amyloid-β (Aβ) peptide cause neurotoxicity, synaptic dysfunction, and memory impairments that underlie Alzheimer disease (AD). The cellular prion protein (PrP^C) was recently identified as a high affinity neuronal receptor for Aβ oligomers. We report that fibrillar Aβ oligomers recognized by the OC antibody, which have been shown to correlate with the onset and severity of AD, bind preferentially to cells and neurons expressing PrP^C. The binding of Aβ oligomers to cell surface PrP^C, as well as their downstream activation of Fyn kinase, was dependent on the integrity of cholesterol-rich lipid rafts. In SH-SY5Y cells, fluorescence microscopy and co-localization with subcellular markers revealed that the Aβ oligomers co-internalized with PrP^C, accumulated in endosomes, and subsequently trafficked to lysosomes. The cell surface binding, internalization, and downstream toxicity of Aβ oligomers was dependent on the transmembrane low density lipoprotein receptor-related protein-1 (LRP1). The binding of Aβ oligomers to cell surface PrP^C impaired its ability to inhibit the activity of the β-secretase BACE1, which cleaves the amyloid precursor protein to produce Aβ. The green tea polyphenol (−)-epigallocatechin gallate and the red wine extract resveratrol both remodeled the fibrillar conformation of Aβ oligomers. The resulting nonfibrillar oligomers displayed significantly reduced binding to PrP^C-expressing cells and were no longer cytotoxic. These data indicate that soluble, fibrillar Aβ oligomers bind to PrP^C in a conformation-dependent manner and require the integrity of lipid rafts and the transmembrane LRP1 for their cytotoxicity, thus revealing potential targets to alleviate the neurotoxic properties of Aβ oligomers in AD.     

The Heat Shock Response Is Modulated by and Interferes with Toxic Effects of Scrapie Prion Protein and Amyloid β

The heat shock response (HSR) is an evolutionarily conserved pathway designed to maintain proteostasis and to ameliorate toxic effects of aberrant protein folding. We have studied the modulation of the HSR by the scrapie prion protein (PrP^Sc) and amyloid β peptide (Aβ) and investigated whether an activated HSR or the ectopic expression of individual chaperones can interfere with PrP^Sc- or Aβ-induced toxicity. First, we observed different effects on the HSR under acute or chronic exposure of cells to PrP^Sc or Aβ. In chronically exposed cells the threshold to mount a stress response was significantly increased, evidenced by a decreased expression of Hsp72 after stress, whereas an acute exposure lowered the threshold for stress-induced expression of Hsp72. Next, we employed models of PrP^Sc- and Aβ-induced toxicity to demonstrate that the induction of the HSR ameliorates the toxic effects of both PrP^Sc and Aβ. Similarly, the ectopic expression of cytosolic Hsp72 or the extracellular chaperone clusterin protected against PrP^Sc- or Aβ-induced toxicity. However, toxic signaling induced by a pathogenic PrP mutant located at the plasma membrane was prevented by an activated HSR or Hsp72 but not by clusterin, indicating a distinct mode of action of this extracellular chaperone. Our study supports the notion that different pathological protein conformers mediate toxic effects via similar cellular pathways and emphasizes the possibility to exploit the heat shock response therapeutically.