Interaction between human prion protein and amyloid-beta (Abeta) oligomers: role OF N-terminal residues

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

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

Alzheimer's Aβ interacts with cellular prion protein inducing neuronal membrane damage and synaptotoxicity

A major feature of Alzheimer's disease is the accumulation of β-amyloid (Aβ) peptide in the brain. Recent studies have indicated that Aβ oligomers (Aβo) can interact with the cellular prion protein (PrPc). Therefore, this interaction might be driving some of Aβ toxic effects in the synaptic region. In the present study, we report that Aβo binds to PrPc in the neuronal membrane playing a role on toxic effects induced by Aβ. Phospholipase C-enzymatic cleavage of PrPc from the plasma membrane attenuated the association of Aβo to the neurons. Furthermore, an anti-PrP antibody (6D11) decreased the association of Aβo to hippocampal neurons with a concomitant reduction in Aβo and PrPc co-localization. Interestingly, this antibody blocked the increase in membrane conductance and intracellular calcium induced by Aβo. Thus, the data indicate that PrPc plays a role on the membrane perforations produced by Aβo, the increase in calcium ions and the release of synaptic vesicles that subsequently leads to synaptic failure. Future studies blocking Aβo interaction with PrPc could be important for the discovery of new therapeutic strategies for Alzheimer's disease.

Heme oxygenase-1 protects against Alzheimer's amyloid-β(1-42)-induced toxicity via carbon monoxide production

Heme oxygenase-1 (HO-1), an inducible enzyme up-regulated in Alzheimer's disease, catabolises heme to biliverdin, Fe²⁺ and carbon monoxide (CO). CO can protect neurones from oxidative stress-induced apoptosis by inhibiting Kv2.1 channels, which mediates cellular K⁺ efflux as an early step in the apoptotic cascade. Since apoptosis contributes to the neuronal loss associated with amyloid β peptide (Aβ) toxicity in AD, we investigated the protective effects of HO-1 and CO against Aβ_1-42 toxicity in SH-SY5Y cells, employing cells stably transfected with empty vector or expressing the cellular prion protein, PrP^c, and rat primary hippocampal neurons. Aβ_1-42 (containing protofibrils) caused a concentration-dependent decrease in cell viability, attributable at least in part to induction of apoptosis, with the PrP^c-expressing cells showing greater susceptibility to Aβ_1-42 toxicity. Pharmacological induction or genetic over-expression of HO-1 significantly ameliorated the effects of Aβ_1-42. The CO-donor CORM-2 protected cells against Aβ_1-42 toxicity in a concentration-dependent manner. Electrophysiological studies revealed no differences in the outward current pre- and post-Aβ_1-42 treatment suggesting that K⁺ channel activity is unaffected in these cells. Instead, Aβ toxicity was reduced by the L-type Ca²⁺ channel blocker nifedipine, and by the CaMKKII inhibitor, STO-609. Aβ also activated the downstream kinase, AMP-dependent protein kinase (AMPK). CO prevented this activation of AMPK. Our findings indicate that HO-1 protects against Aβ toxicity via production of CO. Protection does not arise from inhibition of apoptosis-associated K⁺ efflux, but rather by inhibition of AMPK activation, which has been recently implicated in the toxic effects of Aβ. These data provide a novel, beneficial effect of CO which adds to its growing potential as a therapeutic agent.

Single-molecule imaging reveals that small amyloid-β1-42 oligomers interact with the cellular prion protein (PrP(C))

Oligomers of the amyloid-β peptide (Aβ) play a central role in the pathogenesis of Alzheimer’s disease and have been suggested to induce neurotoxicity by binding to a plethora of cell-surface receptors. However, the heterogeneous mixtures of oligomers of varying sizes and conformations formed by Aβ42 have obscured the nature of the oligomeric species that bind to a given receptor. Here, we have used single-molecule imaging to characterize Aβ42 oligomers (oAβ42) and to confirm the controversial interaction of oAβ42 with the cellular prion protein (PrP^C) on live neuronal cells. Our results show that, at nanomolar concentrations, oAβ42 interacts with PrP^C and that the species bound to PrP^C are predominantly small oligomers (dimers and trimers). Single-molecule biophysical studies can thus aid in deciphering the mechanisms that underlie receptor-mediated oAβ-induced neurotoxicity, and ultimately facilitate the discovery of novel inhibitors of these pathways.

Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity

Although it is well established that misfolding of the cellular prion protein (PrP^C) into the β-sheet-rich, aggregated scrapie conformation (PrP^Sc) causes a variety of transmissible spongiform encephalopathies (TSEs), the physiological roles of PrP^C are still incompletely understood. There is accumulating evidence describing the roles of PrP^C in neurodegeneration and neuroinflammation. Recently, we identified a functional regulation of NMDA receptors by PrP^C that involves formation of a physical protein complex between these proteins. Excessive NMDA receptor activity during conditions such as ischemia mediates enhanced Ca²⁺ entry into cells and contributes to excitotoxic neuronal death. In addition, NMDA receptors and/or PrP^C play critical roles in neuroinflammation and glial cell toxicity. Inhibition of NMDA receptor activity protects against PrP^Sc-induced neuronal death. Moreover, in mice lacking PrP^C, infarct size is increased after focal cerebral ischemia, and absence of PrP^C increases susceptibility of neurons to NMDA receptor-dependent death. Recently, PrP^C was found to be a receptor for oligomeric beta-amyloid (Aβ) peptides, suggesting a role for PrP^C in Alzheimer's disease (AD). Our recent findings suggest that Aβ peptides enhance NMDA receptor current by perturbing the normal copper- and PrP^C-dependent regulation of these receptors. Here, we review evidence highlighting a role for PrP^C in preventing NMDA receptor-mediated excitotoxicity and inflammation. There is a need for more detailed molecular characterization of PrP^C-mediated regulation of NMDA receptors, such as determining which NMDA receptor subunits mediate pathogenic effects upon loss of PrP^C-mediated regulation and identifying PrP^C binding site(s) on the receptor. This knowledge will allow development of novel therapeutic interventions for not only TSEs, but also for AD and other neurodegenerative disorders involving dysfunction of PrP^C.

Therapeutic molecules and endogenous ligands regulate the interaction between brain cellular prion protein (PrPC) and metabotropic glutamate receptor 5 (mGluR5)

Soluble Amyloid-β oligomers (Aβo) can trigger Alzheimer disease (AD) pathophysiology by binding to cell surface cellular prion protein (PrP(C)). PrP(C) interacts physically with metabotropic glutamate receptor 5 (mGluR5), and this interaction controls the transmission of neurotoxic signals to intracellular substrates. Because the interruption of the signal transduction from PrP(C) to mGluR5 has therapeutic potential for AD, we developed assays to explore the effect of endogenous ligands, agonists/antagonists, and antibodies on the interaction between PrP(C) and mGluR5 in cell lines and mouse brain. We show that the PrP(C) segment of amino acids 91-153 mediates the interaction with mGluR5. Agonists of mGluR5 increase the mGluR5-PrP(C) interaction, whereas mGluR5 antagonists suppress protein association. Synthetic Aβo promotes the protein interaction in mouse brain and transfected HEK-293 cell membrane preparations. The interaction of PrP(C) and mGluR5 is enhanced dramatically in the brains of familial AD transgenic model mice. In brain homogenates with Aβo, the interaction of PrP(C) and mGluR5 is reversed by mGluR5-directed antagonists or antibodies directed against the PrP(C) segment of amino acids 91-153. Silent allosteric modulators of mGluR5 do not alter Glu or basal mGluR5 activity, but they disrupt the Aβo-induced interaction of mGluR5 with PrP(C). The assays described here have the potential to identify and develop new compounds that inhibit the interaction of PrP(C) and mGluR5, which plays a pivotal role in the pathogenesis of Alzheimer disease by transmitting the signal from extracellular Aβo into the cytosol.

A systems biology analysis of brain microvascular endothelial cell lipotoxicity

Background

Neurovascular inflammation is associated with a number of neurological diseases including vascular dementia and Alzheimer’s disease, which are increasingly important causes of morbidity and mortality around the world. Lipotoxicity is a metabolic disorder that results from accumulation of lipids, particularly fatty acids, in non-adipose tissue leading to cellular dysfunction, lipid droplet formation, and cell death.

Results

Our studies indicate for the first time that the neurovascular circulation also can manifest lipotoxicity, which could have major effects on cognitive function. The penetration of integrative systems biology approaches is limited in this area of research, which reduces our capacity to gain an objective insight into the signal transduction and regulation dynamics at a systems level. To address this question, we treated human microvascular endothelial cells with triglyceride-rich lipoprotein (TGRL) lipolysis products and then we used genome-wide transcriptional profiling to obtain transcript abundances over four conditions. We then identified regulatory genes and their targets that have been differentially expressed through analysis of the datasets with various statistical methods. We created a functional gene network by exploiting co-expression observations through a guilt-by-association assumption. Concomitantly, we used various network inference algorithms to identify putative regulatory interactions and we integrated all predictions to construct a consensus gene regulatory network that is TGRL lipolysis product specific.

Conclusion

System biology analysis has led to the validation of putative lipid-related targets and the discovery of several genes that may be implicated in lipotoxic-related brain microvascular endothelial cell responses. Here, we report that activating transcription factors 3 (ATF3) is a principal regulator of TGRL lipolysis products-induced gene expression in human brain microvascular endothelial cell.

The 37kDa/67kDa laminin receptor acts as a receptor for Aβ42 internalization

Neuronal loss is a major neuropathological hallmark of Alzheimer's disease (AD). The associations between soluble Aβ oligomers and cellular components cause this neurotoxicity. The 37 kDa/67 kDa laminin receptor (LRP/LR) has recently been implicated in Aβ pathogenesis. In this study the mechanism underlying the pathological role of LRP/LR was elucidated. Försters Resonance Energy Transfer (FRET) revealed that LRP/LR and Aβ form a biologically relevant interaction. The ability of LRP/LR to form stable associations with endogenously shed Aβ was confirmed by pull down assays and Aβ-ELISAs. Antibody blockade of this association significantly lowered Aβ₄₂ induced apoptosis. Furthermore, antibody blockade and shRNA mediated downregulation of LRP/LR significantly hampered Aβ₄₂ internalization. These results suggest that LRP/LR is a receptor for Aβ₄₂ internalization, mediating its endocytosis and contributing to the cytotoxicity of the neuropeptide by facilitating intra-cellular Aβ₄₂ accumulation. These findings recommend anti-LRP/LR specific antibodies and shRNAs as potential therapeutic tools for AD treatment.

Interaction between prion protein and Aβ amyloid fibrils revisited

Recent studies indicate that the pathogenesis of Alzheimer disease may be related to the interaction between prion protein (PrP) and certain oligomeric species of Aβ peptide. However, the mechanism of this interaction remains unclear and controversial. Here we provide direct experimental evidence that, in addition to previously demonstrated binding to Aβ oligomers, PrP also interacts with mature Aβ fibrils. However, contrary to the recent claim that PrP causes fragmentation of Aβ fibrils into oligomeric species, no evidence for such a disassembly could be detected in the present study. In contrast, our data indicate that the addition of PrP to preformed Aβ fibrils results in a lateral association of individual fibrils into larger bundles. These findings have potentially important implications for understanding the mechanism by which PrP might impact Aβ toxicity as well as for the emerging efforts to use PrP-derived compounds as inhibitors of Aβ-induced neurodegeneration.

Amyloid-β nanotubes are associated with prion protein-dependent synaptotoxicity

Growing evidence suggests water-soluble, non-fibrillar forms of amyloid-β protein (Aβ) have important roles in Alzheimer’s disease with toxicities mimicked by synthetic Aβ_1–42. However, no defined toxic structures acting via specific receptors have been identified and roles of proposed receptors, such as prion protein (PrP), remain controversial. Here we quantify binding to PrP of Aβ_1–42 after different durations of aggregation. We show PrP-binding and PrP-dependent inhibition of long-term potentiation (LTP) correlate with the presence of protofibrils. Globular oligomers bind less avidly to PrP and do not inhibit LTP, whereas fibrils inhibit LTP in a PrP-independent manner. That only certain transient Aβ assemblies cause PrP-dependent toxicity explains conflicting reports regarding the involvement of PrP in Aβ-induced impairments. We show that these protofibrils contain a defined nanotubular structure with a previously unidentified triple helical conformation. Blocking the formation of Aβ nanotubes or their interaction with PrP might have a role in treatment of Alzheimer’s disease.

Prion protein has been suggested to bind toxic amyloid-ß oligomers. Nicoll et al. demonstrate that binding to prion protein and prion protein-dependent synaptotoxicity correlate with the presence of a tubular form of amyloid-ß with a defined triple helical structure.

Soluble Aβ oligomers are rapidly sequestered from brain ISF in vivo and bind GM1 ganglioside on cellular membranes

Soluble Aβ oligomers contribute importantly to synaptotoxicity in Alzheimer's disease, but their dynamics in vivo remain unclear. Here, we found that soluble Aβ oligomers were sequestered from brain interstitial fluid onto brain membranes much more rapidly than nontoxic monomers and were recovered in part as bound to GM1 ganglioside on membranes. Aβ oligomers bound strongly to GM1 ganglioside, and blocking the sialic acid residue on GM1 decreased oligomer-mediated LTP impairment in mouse hippocampal slices. In a hAPP transgenic mouse model, substantial levels of GM1-bound Aβ₄₂ were recovered from brain membrane fractions. We also detected GM1-bound Aβ in human CSF, and its levels correlated with Aβ₄₂, suggesting its potential as a biomarker of Aβ-related membrane dysfunction. Together, these findings highlight a mechanism whereby hydrophobic Aβ oligomers become sequestered onto GM1 ganglioside and presumably other lipids on neuronal membranes, where they may induce progressive functional and structural changes.

Hijacking PrP(c)-dependent signal transduction: when prions impair Aβ clearance

The cellular prion protein PrP^c is the normal counterpart of the scrapie prion protein PrP ^Sc, the main component of the infectious agent of transmissible spongiform encephalopathies. The recent discovery that PrP ^c can serve as a receptor for the amyloid beta (Aβ) peptide and relay its neurotoxicity is sparking renewed interest on this protein and its involvement in signal transduction processes. Disease-associated PrP ^Sc shares with Aβ the ability to hijack PrP ^c-dependent signaling cascades, and thereby instigate pathogenic events. Among these is an impairment of Aβ clearance, uncovered in prion-infected neuronal cells. These findings add another facet to the intricate interplay between PrP ^c and Aβ. Here, we summarize the connection between PrP-mediated signaling and Aβ clearance and discuss its pathological implications.

High molecular mass assemblies of amyloid-β oligomers bind prion protein in patients with Alzheimer's disease

Alzheimer's disease is the most common form of dementia and the generation of oligomeric species of amyloid-β is causal to the initiation and progression of it. Amyloid-β oligomers bind to the N-terminus of plasma membrane-bound cellular prion protein (PrP(C)) initiating a series of events leading to synaptic degeneration. Composition of bound amyloid-β oligomers, binding regions within PrP(C), binding affinities and modifiers of this interaction have been almost exclusively studied in cell culture or murine models of Alzheimer's disease and our knowledge on PrP(C)-amyloid-β interaction in patients with Alzheimer's disease is limited regarding occurrence, binding regions in PrP(C), and size of bound amyloid-β oligomers. Here we employed a PrP(C)-amyloid-β binding assay and size exclusion chromatography on neuropathologically characterized Alzheimer's disease and non-demented control brains (n = 15, seven female, eight male, average age: 79.2 years for Alzheimer's disease and n = 10, three female, seven male, average age: 66.4 years for controls) to investigate amyloid-β-PrP(C) interaction. PrP(C)-amyloid-β binding always occurred in Alzheimer's disease brains and was never detected in non-demented controls. Neither expression level of PrP(C) nor known genetic modifiers of Alzheimer's disease, such as the PrP(C) codon 129 polymorphism, influenced this interaction. In Alzheimer's disease brains, binding of amyloid-β to PrP(C) occurred via the PrP(C) N-terminus. For synthetic amyloid-β42, small oligomeric species showed prominent binding to PrP(C), whereas in Alzheimer's disease brains larger protein assemblies containing amyloid-β42 bound efficiently to PrP(C). These data confirm Alzheimer's disease specificity of binding of amyloid-β to PrP(C) via its N-terminus in a large cohort of Alzheimer's disease/control brains. Differences in sizes of separated protein fractions between synthetic and brain-derived amyloid-β binding to PrP(C) suggest that larger assemblies of amyloid-β or additional non-amyloid-β components may play a role in binding of amyloid-β42 to PrP(C) in Alzheimer's disease.

Fyn kinase inhibition as a novel therapy for Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder, afflicting more than one-third of people over the age of 85. While many therapies for AD are in late-stage clinical testing, rational drug design based on distinct signaling pathways in this disorder is only now emerging. Here we review the putative signaling pathway of amyloid-beta (Aβ), by which the tyrosine kinase Fyn is activated via cell surface binding of Aβ oligomers to cellular prion protein. Several lines of evidence implicate Fyn in the pathogenesis of AD, and its interaction with both Aβ and Tau renders Fyn a unique therapeutic target that addresses both of the major pathologic hallmarks of AD. We are currently enrolling patients in a phase Ib study of saracatinib (AZD0530), a small molecule inhibitor with high potency for Src and Fyn, for the treatment of AD. The results of this trial and a planned phase IIa multisite study will provide important data regarding the potential for this therapeutic strategy in AD.

A label-free electrical impedimetric biosensor for the specific detection of Alzheimer's amyloid-beta oligomers

Alzheimer's disease (AD) is the most common form of dementia, with over 37 million sufferers worldwide and a global cost of over $600 billion. There is currently no cure for AD and no reliable method of diagnosis other than post-mortem brain examination. The development of a point-of-care test for AD is an urgent requirement in order to provide earlier diagnosis and, thus, useful therapeutic intervention. Here, we present a novel, label-free impedimetric biosensor for the specific detection of amyloid-beta oligomers (AβO), which are the primary neurotoxic species in AD. AβO have been proposed as the best biomarker for AD and levels of AβO in the blood have been found to correlate with cerebrospinal fluid load. The biorecognition element of our biosensor is a fragment of the cellular prion protein (PrP(C), residues 95-110), a highly expressed synaptic protein which mediates the neuronal binding and toxicity of AβO. During the layer-by-layer sensor construction, biotinylated PrP(C) (95-110) was attached via a biotin/NeutrAvidin bridge to polymer-functionalised gold screen-printed electrodes. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry and scanning electron microscopy were used to validate biosensor assembly and functionality. EIS was employed for biosensor interrogation in the presence of Aβ oligomers or monomers. The biosensor was specific for the detection of synthetic AβO and gave a linear response, without significant detection of monomeric Aβ, down to an equivalent AβO concentration of ~0.5 pM. The biosensor was also able to detect natural, cell-derived AβO present in conditioned medium. The eventual commercialisation of this biosensor system could allow for the early diagnosis and disease monitoring of AD.

The prion protein ligand, stress-inducible phosphoprotein 1, regulates amyloid-β oligomer toxicity

In Alzheimer's disease (AD), soluble amyloid-β oligomers (AβOs) trigger neurotoxic signaling, at least partially, via the cellular prion protein (PrP(C)). However, it is unknown whether other ligands of PrP(C) can regulate this potentially toxic interaction. Stress-inducible phosphoprotein 1 (STI1), an Hsp90 cochaperone secreted by astrocytes, binds to PrP(C) in the vicinity of the AβO binding site to protect neurons against toxic stimuli. Here, we investigated a potential role of STI1 in AβO toxicity. We confirmed the specific binding of AβOs and STI1 to the PrP and showed that STI1 efficiently inhibited AβO binding to PrP in vitro (IC50 of ∼70 nm) and also decreased AβO binding to cultured mouse primary hippocampal neurons. Treatment with STI1 prevented AβO-induced synaptic loss and neuronal death in mouse cultured neurons and long-term potentiation inhibition in mouse hippocampal slices. Interestingly, STI1-haploinsufficient neurons were more sensitive to AβO-induced cell death and could be rescued by treatment with recombinant STI1. Noteworthy, both AβO binding to PrP(C) and PrP(C)-dependent AβO toxicity were inhibited by TPR2A, the PrP(C)-interacting domain of STI1. Additionally, PrP(C)-STI1 engagement activated α7 nicotinic acetylcholine receptors, which participated in neuroprotection against AβO-induced toxicity. We found an age-dependent upregulation of cortical STI1 in the APPswe/PS1dE9 mouse model of AD and in the brains of AD-affected individuals, suggesting a compensatory response. Our findings reveal a previously unrecognized role of the PrP(C) ligand STI1 in protecting neurons in AD and suggest a novel pathway that may help to offset AβO-induced toxicity.

A new paradigm for enzymatic control of α-cleavage and β-cleavage of the prion protein

The cellular form of the prion protein (PrP(C)) is found in both full-length and several different cleaved forms in vivo. Although the precise functions of the PrP(C) proteolytic products are not known, cleavage between the unstructured N-terminal domain and the structured C-terminal domain at Lys-109↓His-110 (mouse sequence), termed α-cleavage, has been shown to produce the anti-apoptotic N1 and the scrapie-resistant C1 peptide fragments. β-Cleavage, residing adjacent to the octarepeat domain and N-terminal to the α-cleavage site, is thought to arise from the action of reactive oxygen species produced from redox cycling of coordinated copper. We sought to elucidate the role of key members of the ADAM (a disintegrin and metalloproteinase) enzyme family, as well as Cu(2+) redox cycling, in recombinant mouse PrP (MoPrP) cleavage through LC/MS analysis. Our findings show that although Cu(2+) redox-generated reactive oxygen species do produce fragmentation corresponding to β-cleavage, ADAM8 also cleaves MoPrP in the octarepeat domain in a Cu(2+)- and Zn(2+)-dependent manner. Additional cleavage by ADAM8 was observed at the previously proposed location of α-cleavage, Lys-109↓His-110 (MoPrP sequencing); however, upon addition of Cu(2+), the location of α-cleavage shifted by several amino acids toward the C terminus. ADAM10 and ADAM17 have also been implicated in α-cleavage at Lys-109↓His-110; however, we observed that they instead cleaved MoPrP at a novel location, Ala-119↓Val-120, with additional cleavage by ADAM10 at Gly-227↓Arg-228 near the C terminus. Together, our results show that MoPrP cleavage is far more complex than previously thought and suggest a mechanism by which PrP(C) fragmentation responds to Cu(2+) and Zn(2+).

Exosomes neutralize synaptic-plasticity-disrupting activity of Aβ assemblies in vivo

Background

Exosomes, small extracellular vesicles of endosomal origin, have been suggested to be involved in both the metabolism and aggregation of Alzheimer’s disease (AD)-associated amyloid β-protein (Aβ). Despite their ubiquitous presence and the inclusion of components which can potentially interact with Aβ, the role of exosomes in regulating synaptic dysfunction induced by Aβ has not been explored.

Results

We here provide in vivo evidence that exosomes derived from N2a cells or human cerebrospinal fluid can abrogate the synaptic-plasticity-disrupting activity of both synthetic and AD brain-derived Aβ. Mechanistically, this effect involves sequestration of synaptotoxic Aβ assemblies by exosomal surface proteins such as PrP^C rather than Aβ proteolysis.

Conclusions

These data suggest that exosomes can counteract the inhibitory action of Aβ, which contributes to perpetual capability for synaptic plasticity.

Immunization with either prion protein fragment 95-123 or the fragment-specific antibodies rescue memory loss and neurodegenerative phenotype of neurons in olfactory bulbectomized mice

Epidemiological studies demonstrated association between head injury (HI) and the subsequent development of Alzheimer's disease (AD). Certain hallmarks of AD, e.g. amyloid-β (Aβ) containing deposits, may be found in patients following traumatic BI (TBI). Recent studies uncover the cellular prion protein, PrP(C), as a receptor for soluble polymeric forms of Aβ (sAβ) which are an intermediate of such deposits. We aimed to test the hypothesis that targeting of PrP(C) can prevent Aβ related spatial memory deficits in olfactory bulbectomized (OBX) mice utilized here to resemble some clinical features of AD, such as increased level of Aβ, memory loss and deficit of the CNS cholin- and serotonin-ergic systems. We demonstrated that immunization with the a.a. 95-123 fragment of cellular prion (PrP-I) recovered cortical and hippocampus neurons from OBX induced degeneration, rescued spatial memory loss in Morris water maze test and significantly decrease the Aβ level in brain tissue of these animals. Affinity purified anti-PrP-I antibodies rescued pre-synaptic biomarker synaptophysin eliciting similar effect on memory of OBX mice, and protected hippocampal neurones from Aβ25-35-induced toxicity in vitro. Immunization OBX mice with a.a. 200-213 fragment of cellular prion (PrP-II) did not reach a significance in memory protection albeit having similar to PrP-I immunization impact on Aβ level in brain tissue. The observed positive effect of targeting the PrP-I by either active or passive immunization on memory of OBX mice revealed the involvement of the PrP(C) in AD-like pathology induced by olfactory bulbectomy. This OBX model may be a useful tool for mechanistic and preclinical therapeutic investigations into the association between PrP(C) and AD.

Metabotropic glutamate receptor 5 is a coreceptor for Alzheimer aβ oligomer bound to cellular prion protein

Soluble amyloid-β oligomers (Aβo) trigger Alzheimer's disease (AD) pathophysiology and bind with high affinity to cellular prion protein (PrP(C)). At the postsynaptic density (PSD), extracellular Aβo bound to lipid-anchored PrP(C) activates intracellular Fyn kinase to disrupt synapses. Here, we screened transmembrane PSD proteins heterologously for the ability to couple Aβo-PrP(C) with Fyn. Only coexpression of the metabotropic glutamate receptor, mGluR5, allowed PrP(C)-bound Aβo to activate Fyn. PrP(C) and mGluR5 interact physically, and cytoplasmic Fyn forms a complex with mGluR5. Aβo-PrP(C) generates mGluR5-mediated increases of intracellular calcium in Xenopus oocytes and in neurons, and the latter is also driven by human AD brain extracts. In addition, signaling by Aβo-PrP(C)-mGluR5 complexes mediates eEF2 phosphorylation and dendritic spine loss. For mice expressing familial AD transgenes, mGluR5 antagonism reverses deficits in learning, memory, and synapse density. Thus, Aβo-PrP(C) complexes at the neuronal surface activate mGluR5 to disrupt neuronal function.

Identification of PrP sequences essential for the interaction between the PrP polymers and Aβ peptide in a yeast-based assay

Alzheimer disease is associated with the accumulation of oligomeric amyloid β peptide (Aβ), accompanied by synaptic dysfunction and neuronal death. Polymeric form of prion protein (PrP), PrP(Sc), is implicated in transmissible spongiform encephalopathies (TSEs). Recently, it was shown that the monomeric cellular form of PrP (PrP(C)), located on the neuron surface, binds Aβ oligomers (and possibly other β-rich conformers) via the PrP(23-27) and PrP(90-110) segments, acting as Aβ receptor. On the other hand, PrP(Sc) polymers efficiently bind to Aβ monomers and accelerate their oligomerization. To identify specific PrP sequences that are essential for the interaction between PrP polymers and Aβ peptide, we have co-expressed Aβ and PrP (or its shortened derivatives), fused to different fluorophores, in the yeast cell. Our data show that the 90-110 and 28-89 regions of PrP control the binding of proteinase-resistant PrP polymers to the Aβ peptide, whereas the 23-27 segment of PrP is dispensable for this interaction. This indicates that the set of PrP fragments involved in the interaction with Aβ depends on PrP conformational state.

Alzheimer's disease: analysis of a mathematical model incorporating the role of prions

We introduce a mathematical model of the in vivo progression of Alzheimer's disease with focus on the role of prions in memory impairment. Our model consists of differential equations that describe the dynamic formation of β-amyloid plaques based on the concentrations of Aβ oligomers, PrP(C) proteins, and the Aβ-x-Aβ-PrP(C)complex, which are hypothesized to be responsible for synaptic toxicity. We prove the well-posedness of the model and provided stability results for its unique equilibrium, when the polymerization rate of Aβ-amyloid is constant and also when it is described by a power law.

Alzheimer's amyloid-β oligomers rescue cellular prion protein induced tau reduction via the Fyn pathway

Amyloid-β (Aβ) and tau are the pathogenic hallmarks in Alzheimer's disease (AD). Aβ oligomers are considered the actual toxic entities, and the toxicity relies on the presence of tau. Recently, Aβ oligomers have been shown to specifically interact with cellular prion protein (PrP(C)) where the role of PrP(C) in AD is still not fully understood. To investigate the downstream mechanism of PrP(C) and Aβ oligomer interaction and their possible relationships to tau, we examined tau expression in human neuroblastoma BE(2)-C cells transfected with murine PrP(C) and studied the effect under Aβ oligomer treatment. By Western blotting, we found that PrP(C) overexpression down-regulated tau protein and Aβ oligomer binding alleviated the tau reduction induced by wild type but not M128V PrP(C), the high AD risk polymorphic allele in human prion gene. PrP(C) lacking the Aβ oligomer binding site was incapable of rescuing the level of tau reduction. Quantitative RT-PCR showed the PrP(C) effect was attributed to tau reduction at the transcription level. Treatment with Fyn pathway inhibitors, Fyn kinase inhibitor PP2 and MEK inhibitor U0126, reversed the PrP(C)-induced tau reduction and Aβ oligomer treatment modulated Fyn kinase activity. The results suggested Fyn pathway regulated Aβ-PrP(C)-tau signaling. Overall, our results demonstrated that PrP(C) down-regulated tau via the Fyn pathway and the effect can be regulated by Aβ oligomers. Our study facilitated the understanding of molecular mechanisms among PrP(C), tau, and Aβ oligomers.

Effects of FlAsH/tetracysteine (TC) Tag on PrP proteolysis and PrPres formation by TC-scanning

Protein-protein interactions associated with proteolytic processing and aggregation are integral to normal and pathological aspects of prion protein (PrP) biology. Characterization of these interactions requires the identification of amino acid residues involved. The FlAsH/tetracysteine (FlAsH/TC) tag is a small fluorescent tag amenable to insertion at internal sites in proteins. In this study, we used serial FlAsH/TC insertions (TC-scanning) as a probe to characterize sites of protein-protein interaction between PrP and other molecules. To explore this application in the context of substrate-protease interactions, we analyzed the effect of FlAsH/TC insertions on proteolysis of cellular prion protein (PrPsen) in in vitro reactions and generation of the C1 metabolic fragment of PrPsen in live neuroblastoma cells. The influence of FlAsH/TC insertion was evaluated by TC-scanning across the cleavage sites of each protease. The results showed that FlAsH/TC inhibited protease cleavage only within limited ranges of the cleavage sites, which varied from about one to six residues in width, depending on the protease, providing an estimate of the PrP residues interacting with each protease. TC-scanning was also used to probe a different type of protein-protein interaction: the conformational conversion of FlAsH-PrPsen to the prion disease-associated isoform, PrPres. PrP constructs with FlAsH/TC insertions at residues 90-96 but not 97-101 were converted to FlAsH-PrPres, identifying a boundary separating loosely versus compactly folded regions of PrPres. Our observations demonstrate that TC-scanning with the FlAsH/TC tag can be a versatile method for probing protein-protein interactions and folding processes.

Characterizing affinity epitopes between prion protein and β-amyloid using an epitope mapping immunoassay

Cellular prion protein, a membrane protein, is expressed in all mammals. Prion protein is also found in human blood as an anchorless protein, and this protein form is one of the many potential sources of misfolded prion protein replication during transmission. Many studies have suggested that β-amyloid1-42 oligomer causes neurotoxicity associated with Alzheimer's disease, which is mediated by the prion protein that acts as a receptor and regulates the hippocampal potentiation. The prevention of the binding of these proteins has been proposed as a possible preventative treatment for Alzheimer's disease; therefore, a greater understanding of the binding hot-spots between the two molecules is necessary. In this study, the epitope mapping immunoassay was employed to characterize binding epitopes within the prion protein and complementary epitopes in β-amyloid. Residues 23-39 and 93-119 in the prion protein were involved in binding to β-amyloid1-40 and 1-42, and monomers of this protein interacted with prion protein residues 93-113 and 123-166. Furthermore, β-amyloid antibodies against the C-terminus detected bound β-amyloid1-42 at residues 23-40, 104-122 and 159-175. β-Amyloid epitopes necessary for the interaction with prion protein were not determined. In conclusion, charged clusters and hydrophobic regions of the prion protein were involved in binding to β-amyloid1-40 and 1-42. The 3D structure appears to be necessary for β-amyloid to interact with prion protein. In the future, these binding sites may be utilized for 3D structure modeling, as well as for the pharmaceutical intervention of Alzheimer's disease.

Overcoming barriers and thresholds - signaling of oligomeric Aβ through the prion protein to Fyn

Evidence has been mounting for an involvement of the prion protein (PrP) in a molecular pathway assumed to play a critical role in the etiology of Alzheimer disease. A currently popular model sees oligomeric amyloid β (oAβ) peptides bind directly to PrP to emanate a signal that causes activation of the cytoplasmic tyrosine kinase Fyn, an essential player in a cascade of events that ultimately leads to NMDA receptor-mediated excitotoxicity and hyper-phosphorylation of tau. The model does not reveal, however, how extracellular binding of oAβ to PrP is communicated across the plasma membrane barrier to affect activation of Fyn. A scenario whereby PrP may adapt a transmembrane topology to affect Fyn activation in the absence of additional partners is currently not supported by evidence. A survey of known candidate PrP interactors leads to a small number of molecules that are known to acquire a transmembrane topology and understood to contribute to Fyn activation. Because multiple signaling pathways converge onto Fyn, a realistic model needs to take into account a reality of Fyn acting as a hub that integrates signals from multiple inhibitory and activating effectors. To clarify the role of PrP in oAβ-dependent excitotoxicity, future studies may need to incorporate experimental designs that can probe the contributions of Fyn modulator pathways and rely on analogous readouts, rather than threshold effects, known to underlie excitotoxic signaling.

Increased prion protein processing and expression of metabotropic glutamate receptor 1 in a mouse model of Alzheimer's disease

Prion protein (PrP(C) ), a glycosylphosphatidylinositol-anchored protein corrupted in prion diseases, has been shown recently to interact with group I metabotropic glutamate receptors (mGluRs). Moreover, both PrP(C) and mGluRs were proposed to function as putative receptors for β-amyloid in Alzheimer's disease. PrP(C) can be processed in neurons via α or β-cleavage to produce PrP(C) fragments that are neuroprotective or toxic, respectively. We found PrP(C) α-cleavage to be 2-3 times higher in the cortex of APPswe/PS1dE9 mice, a mouse model of Alzheimer's disease. A similar age-dependent increase was observed for PrP(C) β-cleavage. Moreover, we observed considerable age-dependent increase in cortical expression of mGluR1, but not mGluR5. Exposure of cortical neuronal cultures to β-amyloid oligomers upregulated mGluR1 and PrP(C) α-cleavage, while activation of group I mGluRs increased PrP(C) shedding from the membrane, likely due to increased levels of a disintegrin and metalloprotease10, a key disintegrin for PrP(C) shedding. Interestingly, a similar increase in a disintegrin and metalloprotease10 was detected in the cortex of 9-month-old APPswe/PS1dE9 animals. Our experiments reveal novel and complex processing of PrP(C) in connection with mGluR overexpression that seems to be triggered by β-amyloid peptides. Prion protein (PrP(C) ) and metabotropic glutamate receptors (mGluR) are implicated in Alzheimer's disease (AD). We found age-dependent increase in PrP(C) processing, ADAM10 and mGluR1 levels in AD mouse model. These changes could be reproduced in cultured cortical neurons treated with Aβ peptide. Our findings suggest that increased levels of Aβ can trigger compensatory responses that may affect neuronal toxicity.

Alzheimer's disease and prion protein

Alzheimer's disease (AD) is a devastating neurodegenerative disease with progressive loss of memory and cognitive function, pathologically hallmarked by aggregates of the amyloid-beta (Aβ) peptide and hyperphosphorylated tau in the brain. Aggregation of Aβ under the form of amyloid fibrils has long been considered central to the pathogenesis of AD. However, recent evidence has indicated that soluble Aβ oligomers, rather than insoluble fibrils, are the main neurotoxic species in AD. The cellular prion protein (PrP(C)) has newly been identified as a cell surface receptor for Aβ oligomers. PrP(C) is a cell surface glycoprotein that plays a key role in the propagation of prions, proteinaceous infectious agents that replicate by imposing their abnormal conformation to PrP(C) molecules. In AD, PrP(C) acts to transduce the neurotoxic signals arising from Aβ oligomers, leading to synaptic failure and cognitive impairment. Interestingly, accumulating evidence has also shown that aggregated Aβ or tau possesses prion-like activity, a property that would allow them to spread throughout the brain. In this article, we review recent findings regarding the function of PrP(C) and its role in AD, and discuss potential therapeutic implications of PrP(C)-based approaches in the treatment of AD.

A mutant prion protein sensitizes neurons to glutamate-induced excitotoxicity

Growing evidence suggests that a physiological activity of the cellular prion protein (PrP(C)) plays a crucial role in several neurodegenerative disorders, including prion and Alzheimer's diseases. However, how the functional activity of PrP(C) is subverted to deliver neurotoxic signals remains uncertain. Transgenic (Tg) mice expressing PrP with a deletion of residues 105-125 in the central region (referred to as ΔCR PrP) provide important insights into this problem. Tg(ΔCR) mice exhibit neonatal lethality and massive degeneration of cerebellar granule neurons, a phenotype that is dose dependently suppressed by the presence of wild-type PrP. When expressed in cultured cells, ΔCR PrP induces large, ionic currents that can be detected by patch-clamping techniques. Here, we tested the hypothesis that abnormal ion channel activity underlies the neuronal death seen in Tg(ΔCR) mice. We find that ΔCR PrP induces abnormal ionic currents in neurons in culture and in cerebellar slices and that this activity sensitizes the neurons to glutamate-induced, calcium-mediated death. In combination with ultrastructural and biochemical analyses, these results demonstrate a role for glutamate-induced excitotoxicity in PrP-mediated neurodegeneration. A similar mechanism may operate in other neurodegenerative disorders attributable to toxic, β-rich oligomers that bind to PrP(C).

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.

An N-terminal fragment of the prion protein binds to amyloid-β oligomers and inhibits their neurotoxicity in vivo

A hallmark of Alzheimer disease (AD) is the accumulation of the amyloid-β (Aβ) peptide in the brain. Considerable evidence suggests that soluble Aβ oligomers are responsible for the synaptic dysfunction and cognitive deficit observed in AD. However, the mechanism by which these oligomers exert their neurotoxic effect remains unknown. Recently, it was reported that Aβ oligomers bind to the cellular prion protein with high affinity. Here, we show that N1, the main physiological cleavage fragment of the cellular prion protein, is necessary and sufficient for binding early oligomeric intermediates during Aβ polymerization into amyloid fibrils. The ability of N1 to bind Aβ oligomers is influenced by positively charged residues in two sites (positions 23–31 and 95–105) and is dependent on the length of the sequence between them. Importantly, we also show that N1 strongly suppresses Aβ oligomer toxicity in cultured murine hippocampal neurons, in a Caenorhabditis elegans-based assay, and in vivo in a mouse model of Aβ-induced memory dysfunction. These data suggest that N1, or small peptides derived from it, could be potent inhibitors of Aβ oligomer toxicity and represent an entirely new class of therapeutic agents for AD.

The cellular prion protein traps Alzheimer's Aβ in an oligomeric form and disassembles amyloid fibers

There is now strong evidence to show that the presence of the cellular prion protein (PrP(C)) mediates amyloid-β (Aβ) neurotoxicity in Alzheimer's disease (AD). Here, we probe the molecular details of the interaction between PrP(C) and Aβ and discover that substoichiometric amounts of PrP(C), as little as 1/20, relative to Aβ will strongly inhibit amyloid fibril formation. This effect is specific to the unstructured N-terminal domain of PrP(C). Electron microscopy indicates PrP(C) is able to trap Aβ in an oligomeric form. Unlike fibers, this oligomeric Aβ contains antiparallel β sheet and binds to a oligomer specific conformational antibody. Our NMR studies show that a specific region of PrP(C), notably residues 95-113, binds to Aβ oligomers, but only once Aβ misfolds. The ability of PrP(C) to trap and concentrate Aβ in an oligomeric form and disassemble mature fibers suggests a mechanism by which PrP(C) might confer Aβ toxicity in AD, as oligomers are thought to be the toxic form of Aβ. Identification of a specific recognition site on PrP(C) that traps Aβ in an oligomeric form is potentially a therapeutic target for the treatment of Alzheimer's disease.

The neurodegeneration in Alzheimer disease and the prion protein

The concept of "prion-like" has been proposed to explain the pathogenic mechanism of the principal neurodegenerative disorders associated with protein misfolding, including Alzheimer disease (AD). Other evidence relates prion protein with AD: the cellular prion protein (PrP(C)) binds β amyloid oligomers, allegedly responsible for the neurodegeneration in AD, mediating their toxic effects. We and others have confirmed the high-affinity binding between β amyloid oligomers and PrP(C), but we were not able to assess the functional consequences of this interaction using behavioral investigations and in vitro tests. This discrepancy rather than being resolved with the classic explanations, differencies in methodological aspects, has been reinforced by new data from different sources. Here we present data obtained with PrP antibody that not interfere with the neurotoxic activity of β amyloid oligomers. Since the potential role of the PrP(C) in the neuronal dysfunction induced by β amyloid oligomers is an important issue, find reasonable explanation of the inconsistent results is needed. Even more important however is the relevance of this interaction in the context of the disease, so as to develop valid therapeutic strategies.

Modeling amyloid-beta as homogeneous dodecamers and in complex with cellular prion protein

Soluble amyloid beta (Aβ) peptide has been linked to the pathology of Alzheimer's disease. A variety of soluble oligomers have been observed to be toxic, ranging from dimers to protofibrils. No tertiary structure has been identified as a single biologically relevant form, though many models are comprised of highly ordered β-sheets. Evidence exists for much less ordered toxic oligomers. The mechanism of toxicity remains highly debated and probably involves multiple pathways. Interaction of Aβ oligomers with the N-terminus of the cellular form of the prion protein (PrP(c)) has recently been proposed. The intrinsically disordered nature of this protein and the highly polymorphic nature of Aβ oligomers make structural resolution of the complex exceptionally challenging. In this study, molecular dynamics simulations are performed for dodecameric assemblies of Aβ comprised of monomers having a single, short antiparallel β-hairpin at the C-terminus. The resulting models, devoid of any intermolecular hydrogen bonds, are shown to correlate well with experimental data and are found to be quite stable within the hydrophobic core, whereas the α-helical N-termini transform to a random coil state. This indicates that highly ordered assemblies are not required for stability and less ordered oligomers are a viable component in the population of soluble oligomers. In addition, a tentative model is proposed for the association of Aβ dimers with a double deletion mutant of the intrinsically disordered N-terminus of PrP(c). This may be useful as a conceptual working model for the binding of higher order oligomers and in the design of further experiments.

Amyloid-β induced signaling by cellular prion protein and Fyn kinase in Alzheimer disease

Alzheimer disease (AD) is the most prevalent cause of dementia. Amyloid-β (Aβ) oligomers are potent synaptotoxins thought to mediate AD-related phenotypes. Cellular prion protein (PrP(C)) has been identified as a high-affinity receptor for Aβ oligomers. Herein, we review the functional consequences of Aβ oligomer binding to PrP(C) on the neuronal surface. We highlight recent evidence that Fyn kinase mediates signal transduction downstream of the PrP(C)-Aβ oligomer complex. These studies suggest that PrP(C) has a central role in AD pathogenesis and may provide a target for therapeutic intervention in AD.

The N-terminal, polybasic region of PrP(C) dictates the efficiency of prion propagation by binding to PrP(Sc)

Prion propagation involves a templating reaction in which the infectious form of the prion protein (PrP(Sc)) binds to the cellular form (PrP(C)), generating additional molecules of PrP(Sc). While several regions of the PrP(C) molecule have been suggested to play a role in PrP(Sc) formation based on in vitro studies, the contribution of these regions in vivo is unclear. Here, we report that mice expressing PrP deleted for a short, polybasic region at the N terminus (residues 23-31) display a dramatically reduced susceptibility to prion infection and accumulate greatly reduced levels of PrP(Sc). These results, in combination with biochemical data, demonstrate that residues 23-31 represent a critical site on PrP(C) that binds to PrP(Sc) and is essential for efficient prion propagation. It may be possible to specifically target this region for treatment of prion diseases as well as other neurodegenerative disorders due to β-sheet-rich oligomers that bind to PrP(C).

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.

Soluble prion protein inhibits amyloid-β (Aβ) fibrillization and toxicity

The pathogenesis of Alzheimer disease appears to be strongly linked to the aggregation of amyloid-β (Aβ) peptide and, especially, formation of soluble Aβ1-42 oligomers. It was recently demonstrated that the cellular prion protein, PrP(C), binds with high affinity to these oligomers, acting as a putative receptor that mediates at least some of their neurotoxic effects. Here we show that the soluble (i.e. glycophosphatidylinositol anchor-free) prion protein and its N-terminal fragment have a strong effect on the aggregation pathway of Aβ1-42, inhibiting its assembly into amyloid fibrils. Furthermore, the prion protein prevents formation of spherical oligomers that normally occur during Aβ fibrillogenesis, acting as a potent inhibitor of Aβ1-42 toxicity as assessed in experiments with neuronal cell culture. These findings may provide a molecular level foundation to explain the reported protective action of the physiologically released N-terminal N1 fragment of PrP(C) against Aβ neurotoxicity. They also suggest a novel approach to pharmacological intervention in Alzheimer disease.

Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons

Amyloid-beta (Aβ) oligomers are thought to trigger Alzheimer’s disease (AD) pathophysiology. Cellular Prion Protein (PrP^C) selectively binds oligomeric Aβ and can mediate AD-related phenotypes. Here, we examined the specificity, distribution and signaling from Aβ/PrP complexes, seeking to explain how they might alter the function of NMDA receptors in neurons. PrP^C is enriched in post-synaptic densities, and Aβ/PrP^C interaction leads to Fyn kinase activation. Soluble Aβ assemblies derived from human AD brain interact with PrP^C to activate Fyn. Aβ engagement of PrP^C/Fyn signaling yields phosphorylation of the NR2B subunit of NMDA-receptors, which is coupled to an initial increase and then loss of surface NMDA-receptors. Aβ-induced LDH release and dendritic spine loss require both PrP^C and Fyn, and human familial AD transgene-induced convulsive seizures do not occur in mice lacking PrP^C. These results delineate an Aβ oligomer signal transduction pathway requiring PrP^C and Fyn to alter synaptic function with relevance to AD.

Alzheimer's disease Aβ assemblies mediating rapid disruption of synaptic plasticity and memory

Alzheimer’s disease (AD) is characterized by episodic memory impairment that often precedes clinical diagnosis by many years. Probing the mechanisms of such impairment may provide much needed means of diagnosis and therapeutic intervention at an early, pre-dementia, stage. Prior to the onset of significant neurodegeneration, the structural and functional integrity of synapses in mnemonic circuitry is severely compromised in the presence of amyloidosis. This review examines recent evidence evaluating the role of amyloid-ß protein (Aβ) in causing rapid disruption of synaptic plasticity and memory impairment. We evaluate the relative importance of different sizes and conformations of Aβ, including monomer, oligomer, protofibril and fibril. We pay particular attention to recent controversies over the relevance to the pathophysiology of AD of different water soluble Aβ aggregates and the importance of cellular prion protein in mediating their effects. Current data are consistent with the view that both low-n oligomers and larger soluble assemblies present in AD brain, some of them via a direct interaction with cellular prion protein, cause synaptic memory failure. At the two extremes of aggregation, monomers and fibrils appear to act in vivo both as sources and sinks of certain metastable conformations of soluble aggregates that powerfully disrupt synaptic plasticity. The same principle appears to apply to other synaptotoxic amyloidogenic proteins including tau, α-synuclein and prion protein.

Prion protein expression alters APP cleavage without interaction with BACE-1

The prion protein (PrP) and the beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE-1) are both copper binding proteins, but are associated with two separate neurodegenerative diseases. The role of BACE-1 in the formation of beta-amyloid has made it a major target in attempts to reduce the formation of beta-amyloid in Alzheimer's diseases. However, the suggestion that PrP, normally associated with prion diseases, binds to BACE-1 and reduces its activity has led to the suggestion that the study of this interaction could be of considerable importance to Alzheimer's disease. We therefore undertook to investigate the possible interaction of these two proteins physically and at the level of transcription, translation and APP cleavage. Our findings suggest that mature PrP and BACE-1 do not physically interact, but that altered PrP expression results in altered BACE-1 protein expression and promoter activity. Additionally, overexpression of PrP results in increased cleavage of APP in contrast to previous datas suggesting a reduction. Our findings suggest that any relation between PrP and BACE-1 is indirect. Altered expression of PrP causes changes in the expression of many other proteins which may be as a result of altered copper metabolism.

The association between prion proteins and Aβ₁₋₄₂ oligomers in cytotoxicity and apoptosis

Misfolding of prion protein (PrP to PrPSc) can cause neurodegenerative prion diseases. As a glycosylphosphatidylinositol (GPI)-anchored membrane protein, the normal form of PrP (PrPC) can function as a receptor for ligands in the extracellular space. PrPC was suggested to be involved in memory, synaptic neuronal communication, and anti-oxidation as a neuroprotective agent. The recently identified interaction between PrPC and Aβ(1-42) oligomers suggested another role for PrP as a receptor for Aβ(1-42) oligomers, thereby influencing cytotoxicity and apoptosis. Here, the association between PrPC and Aβ(1-42) oligomers was investigated by visualizing protein localization in neuronal cells by immunocytochemistry. Aβ(1-42) oligomer-induced cytotoxicity was tested in respective expressions of PrPC by using mouse neuroblastoma-2a (N2a) cells, the prion protein overexpressed cells (L2-2B1), and a Prnp-null mouse hippocampal cell line (HpL 3-4). Moreover, apoptotic proteins such as caspase-8 were used to assess the effect of PrPC on Aβ(1-42) oligomer-mediated apoptosis. In L2-2B1 and HpL 3-4 cells, the difference in the cytotoxicity of Aβ(1-42) oligomers could be clearly distinguished. In addition, Aβ(1-42) oligomers induced mitochondria dysfunction, reactive oxygen species (ROS) generation, and calcium influx PrPC-dependently. Apoptosis, related to mitochondria dysfunction, was further investigated to determine the cytotoxic pathway; the results suggest that PrPC could be involved in both the intrinsic and extrinsic apoptotic pathways. Finally, cells with abundant PrPC expression seemed to be more susceptible to Aβ(1-42) oligomer toxicity, suggesting the importance of the level of PrPC expression in the induction of apoptosis.

Interaction between pathogenic proteins in neurodegenerative disorders

The misfolding and progressive aggregation of specific proteins in selective regions of the nervous system is a seminal occurrence in many neurodegenerative disorders, and the interaction between pathological/toxic proteins to cause neurodegeneration is a hot topic of current neuroscience research. Despite clinical, genetic and experimental differences, increasing evidence indicates considerable overlap between synucleinopathies, tauopathies and other protein-misfolding diseases. Inclusions, often characteristic hallmarks of these disorders, suggest interactions of pathological proteins enganging common downstream pathways. Novel findings that have shifted our understanding in the role of pathologic proteins in the pathogenesis of Alzheimer, Parkinson, Huntington and prion diseases, have confirmed correlations/overlaps between these and other neurodegenerative disorders. Emerging evidence, in addition to synergistic effects of tau protein, amyloid-β, α-synuclein and other pathologic proteins, suggests that prion-like induction and spreading, involving secreted proteins, are major pathogenic mechanisms in various neurodegenerative diseases, depending on genetic backgrounds and environmental factors. The elucidation of the basic molecular mechanisms underlying the interaction and spreading of pathogenic proteins, suggesting a dualism or triad of neurodegeneration in protein-misfolding disorders, is a major challenge for modern neuroscience, to provide a deeper insight into their pathogenesis as a basis of effective diagnosis and treatment.

Proteolytic processing of the prion protein in health and disease

A variety of physiological functions, not only restricted to the nervous system, are discussed for the cellular prion protein (PrP(C)). A prominent, non-physiological property of PrPC is the conversion into its pathogenic isoform (PrP(Sc)) during fatal, transmissible, and neurodegenerative prion diseases. The prion protein is subject to posttranslational proteolytic processing and these cleavage events have been shown i) to regulate its physiological functions, ii) to produce biologically active fragments, and iii) to potentially influence the course of prion disease. Here, we give an overview on the proteolytic processing under physiological and pathological conditions and critically review what is currently known about the three main cleavage events of the prion protein, namely α-cleavage, β-cleavage, and ectodomain shedding. The biological relevance of resulting fragments as well as controversies regarding candidate proteases, with special emphasis on members of the A-disintegrin-and-metalloproteinase (ADAM) family, will be discussed. In addition, we make suggestions aimed at facilitating clarity and progress in this important research field. The better understanding of this issue will not only answer basic questions in prion biology but will likely impact research on other neurodegenerative diseases as well.

Allosteric function and dysfunction of the prion protein

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases associated with progressive oligo- and multimerization of the prion protein (PrP(C)), its conformational conversion, aggregation and precipitation. We recently proposed that PrP(C) serves as a cell surface scaffold protein for a variety of signaling modules, the effects of which translate into wide-range functional consequences. Here we review evidence for allosteric functions of PrP(C), which constitute a common property of scaffold proteins. The available data suggest that allosteric effects among PrP(C) and its partners are involved in the assembly of multi-component signaling modules at the cell surface, impose upon both physiological and pathological conformational responses of PrP(C), and that allosteric dysfunction of PrP(C) has the potential to entail progressive signal corruption. These properties may be germane both to physiological roles of PrP(C), as well as to the pathogenesis of the TSEs and other degenerative/non-communicable diseases.