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

PrP meets alpha-synuclein: Molecular mechanisms and implications for disease

The discovery of prions has challenged dogmas and has revolutionized our understanding of protein-misfolding diseases. The concept of self-propagation via protein conformational changes, originally discovered for the prion protein (PrP), also applies to other proteins that exhibit similar behavior, such as alpha-synuclein (aSyn), a central player in Parkinson's disease and in other synucleinopathies. aSyn pathology appears to spread from one cell to another during disease progression, and involves the misfolding and aggregation of aSyn. How the transfer of aSyn between cells occurs is still being studied, but one important hypothesis involves receptor-mediated transport. Interestingly, recent studies indicate that the cellular prion protein (PrPC) may play a crucial role in this process. PrPC has been shown to act as a receptor/sensor for protein aggregates in different neurodegenerative disorders, including Alzheimer's disease and amyotrophic lateral sclerosis. Here, we provide a comprehensive overview of the current state of knowledge regarding the interaction between aSyn and PrPC and discuss its role in synucleinopathies. We examine the properties of PrP and aSyn, including their structure, function, and aggregation. Additionally, we discuss the current understanding of PrPC's role as a receptor/sensor for aSyn aggregates and identify remaining unanswered questions in this area of research. Ultimately, we posit that exploring the interaction between aSyn and PrPC may offer potential treatment options for synucleinopathies.

Cleavage site-directed antibodies reveal the prion protein in humans is shed by ADAM10 at Y226 and associates with misfolded protein deposits in neurodegenerative diseases

Proteolytic cell surface release ('shedding') of the prion protein (PrP), a broadly expressed GPI-anchored glycoprotein, by the metalloprotease ADAM10 impacts on neurodegenerative and other diseases in animal and in vitro models. Recent studies employing the latter also suggest shed PrP (sPrP) to be a ligand in intercellular communication and critically involved in PrP-associated physiological tasks. Although expectedly an evolutionary conserved event, and while soluble forms of PrP are present in human tissues and body fluids, for the human body neither proteolytic PrP shedding and its cleavage site nor involvement of ADAM10 or the biological relevance of this process have been demonstrated thus far. In this study, cleavage site prediction and generation (plus detailed characterization) of sPrP-specific antibodies enabled us to identify PrP cleaved at tyrosin 226 as the physiological and apparently strictly ADAM10-dependent shed form in humans. Using cell lines, neural stem cells and brain organoids, we show that shedding of human PrP can be stimulated by PrP-binding ligands without targeting the protease, which may open novel therapeutic perspectives. Site-specific antibodies directed against human sPrP also detect the shed form in brains of cattle, sheep and deer, hence in all most relevant species naturally affected by fatal and transmissible prion diseases. In human and animal prion diseases, but also in patients with Alzheimer`s disease, sPrP relocalizes from a physiological diffuse tissue pattern to intimately associate with extracellular aggregated deposits of misfolded proteins characteristic for the respective pathological condition. Findings and research tools presented here will accelerate novel insight into the roles of PrP shedding (as a process) and sPrP (as a released factor) in neurodegeneration and beyond.

Physiology of Cellular Prion Proteins in Reproduction

Cellular prion protein (PrPC) encoded at Prnp gene is well-known to form a misfolded isoform, termed scrapie PrP (PrPSC) that cause transmissible degenerative diseases in central nervous system. The physiological role of PrPC has been proposed by many studies, showing that PrPC interacts with various intracellular, membrane, and extracellular molecules including mitochondrial inner membrane as a scaffold. PrPC is expressed in most cell types including reproductive organs. Numerous studies using PrPC knockout rodent models found no obvious phenotypic changes, in particular the clear phenotypes in development and reproduction have not demonstrated in these knockout models. However, various roles of PrPC have been evaluated at the cellular levels. In this review, we summarized the known roles of PrPC in various cell types and tissues with a special emphasis on those involved in reproduction.

C9orf72-associated dipeptide protein repeats form A11-positive oligomers in amyotrophic lateral sclerosis and frontotemporal dementia

Hexanucleotide repeat expansion in C9orf72 is one of the most common causes of amyotrophic lateral sclerosis and frontotemporal dementia. The hexanucleotide expansion, formed by GGGGCC (G4C2) repeats, leads to the production of five dipeptide protein repeats (DPRs) via repeat-associated non-AUG translation. Among the five dipeptide repeats, Gly-Arg, Pro-Arg, and Gly-Ala form neuronal inclusions that contain aggregates of the peptides. Several studies have attempted to model DPR-associated toxicity using various repeat lengths, which suggests a unique conformation that is cytotoxic and is independent of the repeat length. However, the structural characteristics of DPR aggregates have yet to be determined. Increasing evidence suggests that soluble species, such as oligomers, are the main cause of toxicity in proteinopathies, such as Alzheimer's and Parkinson's disease. To investigate the ability of DPRs to aggregate and form toxic oligomers, we adopted a reductionist approach using small dipeptide repeats of 3, 6, and 12. This study shows that DPRs, particularly glycine-arginine and proline-arginine, form oligomers that exhibit distinct dye-binding properties and morphologies. Importantly, we also identified toxic DPR oligomers in amyotrophic lateral sclerosis and frontotemporal dementia postmortem brains that are morphologically similar to those generated recombinantly. This study demonstrates that, similar to soluble oligomers formed by various amyloid proteins, DPR oligomers are toxic, independent of their repeat length.

Ultrasensitive electrochemical detection of amyloid-beta oligomers using double amplification strategy by MXene substrate and covalent organic framework-based probe

Most of the existing electrochemical systems failed to achieve satisfactory results in early diagnosis of Alzheimer's disease (AD) owing to a deficiency of effective signal transduction. A new method for the electrochemical detection of AD biomarkers (amyloid-beta oligomers, Aβ1-42 oligomers) was developed based on a double amplification strategy. Titanium carbide (Ti3C2) MXene decorated by gold nanoparticles (Au-MXene) as the electrode substrate not only gave rise to the electrochemical response due to its paradoxical surface area and conspicuous charge mobility, but also provided vast numbers of binding sites for aptamers (Apt) of Aβ1-42 oligomers. Meanwhile, AuNPs were incorporated into covalent organic frameworks (COFs), which were further modified by Apt and electron mediator (toluidine blue, TB). The Apt/TB-Au@COFs composite was utilized as a label because of their improvement of the electron-hole separation efficiency and optimization of the charge-carrier utilization. The proposed electrochemical assay established highly efficient platform for the detection of Aβ1-42 oligomers with a linear range from 0.01 pg mL-1 to 180 pg mL-1 and an ultralow detection limit of 4.27 fg mL-1 (S/N = 3). This biosensing platform had potential applications in molecular diagnostics of AD serum samples.

Evidence that Alzheimer's Disease Is a Disease of Competitive Synaptic Plasticity Gone Awry

 Mounting evidence indicates that a physiological function of amyloid-β (Aβ) is to mediate neural activity-dependent homeostatic and competitive synaptic plasticity in the brain. I have previously summarized the lines of evidence supporting this hypothesis and highlighted the similarities between Aβ and anti-microbial peptides in mediating cell/synapse competition. In cell competition, anti-microbial peptides deploy a multitude of mechanisms to ensure both self-protection and competitor elimination. Here I review recent studies showing that similar mechanisms are at play in Aβ-mediated synapse competition and perturbations in these mechanisms underpin Alzheimer's disease (AD). Specifically, I discuss evidence that Aβ and ApoE, two crucial players in AD, co-operate in the regulation of synapse competition. Glial ApoE promotes self-protection by increasing the production of trophic monomeric Aβ and inhibiting its assembly into toxic oligomers. Conversely, Aβ oligomers, once assembled, promote the elimination of competitor synapses via direct toxic activity and amplification of "eat-me" signals promoting the elimination of weak synapses. I further summarize evidence that neuronal ApoE may be part of a gene regulatory network that normally promotes competitive plasticity, explaining the selective vulnerability of ApoE expressing neurons in AD brains. Lastly, I discuss evidence that sleep may be key to Aβ-orchestrated plasticity, in which sleep is not only induced by Aβ but is also required for Aβ-mediated plasticity, underlining the link between sleep and AD. Together, these results strongly argue that AD is a disease of competitive synaptic plasticity gone awry, a novel perspective that may promote AD research.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

Finding memo: versatile interactions of the VPS10p-Domain receptors in Alzheimer’s disease

The family of VPS10p-Domain (D) receptors comprises five members named SorLA, Sortilin, SorCS1, SorCS2 and SorCS3. While their physiological roles remain incompletely resolved, they have been recognized for their signaling engagements and trafficking abilities, navigating a number of molecules between endosome, Golgi compartments, and the cell surface. Strikingly, recent studies connected all the VPS10p-D receptors to Alzheimer’s disease (AD) development. In addition, they have been also associated with diseases comorbid with AD such as diabetes mellitus and major depressive disorder. This systematic review elaborates on genetic, functional, and mechanistic insights into how dysfunction in VPS10p-D receptors may contribute to AD etiology, AD onset diversity, and AD comorbidities. Starting with their functions in controlling cellular trafficking of amyloid precursor protein and the metabolism of the amyloid beta peptide, we present and exemplify how these receptors, despite being structurally similar, regulate various and distinct cellular events involved in AD. This includes a plethora of signaling crosstalks that impact on neuronal survival, neuronal wiring, neuronal polarity, and synaptic plasticity. Signaling activities of the VPS10p-D receptors are especially linked, but not limited to, the regulation of neuronal fitness and apoptosis via their physical interaction with pro- and mature neurotrophins and their receptors. By compiling the functional versatility of VPS10p-D receptors and their interactions with AD-related pathways, we aim to further propel the AD research towards VPS10p-D receptor family, knowledge that may lead to new diagnostic markers and therapeutic strategies for AD patients.

The Hidden Role of Non-Canonical Amyloid β Isoforms in Alzheimer's Disease

Recent advances have placed the pro-inflammatory activity of amyloid β (Aβ) on microglia cells as the focus of research on Alzheimer's Disease (AD). Researchers are confronted with an astonishing spectrum of over 100 different Aβ variants with variable length and chemical modifications. With the exception of Aβ1-42 and Aβ1-40, the biological significance of most peptides for AD is as yet insufficiently understood. We therefore aim to provide a comprehensive overview of the contributions of these neglected Aβ variants to microglia activation. First, the impact of Aβ receptors, signaling cascades, scavenger mechanisms, and genetic variations on the physiological responses towards various Aβ species is described. Furthermore, we discuss the importance of different types of amyloid precursor protein processing for the generation of these Aβ variants in microglia, astrocytes, oligodendrocytes, and neurons, and highlight how alterations in secondary structures and oligomerization affect Aβ neurotoxicity. In sum, the data indicate that gene polymorphisms in Aβ-driven signaling pathways in combination with the production and activity of different Aβ variants might be crucial factors for the initiation and progression of different forms of AD. A deeper assessment of their interplay with glial cells may pave the way towards novel therapeutic strategies for individualized medicine.

Early Changes in Transcriptomic Profiles in Synaptodendrosomes Reveal Aberrant Synaptic Functions in Alzheimer’s Disease

Alzheimer’s disease (AD) is one of the most prevalent neurodegenerative disorders characterized by the progressive decline of cognitive functions, and is closely associated with the dysfunction of synapses, which comprise the basic structure that mediates the communication between neurons. Although the protein architecture and machinery for protein translation at synapses are extensively studied, the impact that local changes in the mRNA reservoir have on AD progression is largely unknown. Here, we investigated the changes in transcriptomic profiles in the synaptodendrosomes purified from the cortices of AD mice at ages 3 and 6 months, a stage when early signatures of synaptic dysfunction are revealed. The transcriptomic profiles of synaptodendrosomes showed a greater number of localized differentially expressed genes (DEGs) in 6-month-old AD mice compared with mice 3 months of age. Gene Ontology (GO) analysis showed that these DEGs are majorly enriched in mitochondrial biogenesis and metabolic activity. More specifically, we further identified three representative DEGs in mitochondrial and metabolic pathways—Prnp, Cst3, and Cox6c—that regulate the dendritic spine density and morphology in neurons. Taken together, this study provides insights into the transcriptomic changes in synaptodendrosomes during AD progression, which may facilitate the development of intervention strategies targeting local translation to ameliorate the pathological progression of AD.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

PrP C as a Transducer of Physiological and Pathological Signals

After the discovery of prion phenomenon, the physiological role of the cellular prion protein (PrP ^C ) remained elusive. In the past decades, molecular and cellular analysis has shed some light regarding interactions and functions of PrP ^C in health and disease. PrP ^C , which is located mainly at the plasma membrane of neuronal cells attached by a glycosylphosphatidylinositol (GPI) anchor, can act as a receptor or transducer from external signaling. Although the precise role of PrP ^C remains elusive, a variety of functions have been proposed for this protein, namely, neuronal excitability and viability. Although many issues must be solved to clearly define the role of PrP ^C , its connection to the central nervous system (CNS) and to several misfolding-associated diseases makes PrP ^C an interesting pharmacological target. In a physiological context, several reports have proposed that PrP ^C modulates synaptic transmission, interacting with various proteins, namely, ion pumps, channels, and metabotropic receptors. PrP ^C has also been implicated in the pathophysiological cell signaling induced by β-amyloid peptide that leads to synaptic dysfunction in the context of Alzheimer's disease (AD), as a mediator of Aβ-induced cell toxicity. Additionally, it has been implicated in other proteinopathies as well. In this review, we aimed to analyze the role of PrP ^C as a transducer of physiological and pathological signaling.

Unbiased high-content screening reveals Aβ- and tau-independent synaptotoxic activities in human brain homogenates from Alzheimer's patients and high-pathology controls

Alzheimer’s disease (AD) is tightly correlated with synapse loss in vulnerable brain regions. It is assumed that specific molecular entities such as Aβ and tau cause synapse loss in AD, yet unbiased screens for synaptotoxic activities have not been performed. Here, we performed size exclusion chromatography on soluble human brain homogenates from AD cases, high pathology non-demented controls, and low pathology age-matched controls using our novel high content primary cultured neuron-based screening assay. Both presynaptic and postsynaptic toxicities were elevated in homogenates from AD cases and high pathology non-demented controls to a similar extent, with more modest synaptotoxic activities in homogenates from low pathology normal controls. Surprisingly, synaptotoxic activities were found in size fractions peaking between the 17–44 kDa size standards that did not match well with Aβ and tau immunoreactive species in these homogenates. The fractions containing previously identified high molecular weight soluble amyloid beta aggregates/”oligomers” were non-toxic in this assay. Furthermore, immunodepletion of Aβ and tau did not reduce synaptotoxic activity. This result contrasts with previous findings involving the same methods applied to 3xTg-AD mouse brain extracts. The nature of the synaptotoxic species has not been identified. Overall, our data indicates one or more potential Aβ and tau independent synaptotoxic activities in human AD brain homogenates. This result aligns well with the key role of synaptic loss in the early cognitive decline and may provide new insight into AD pathophysiology.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part I. a literature review

INTRODUCTION

The cellular prion protein (PrP^C) is well known for its pathogenic roles in prion diseases, several other neurodegenerative diseases (such as Alzheimer's disease), and multiple types of cancer, but the beneficial aspects of PrP^C and its cleavage products received much less attention.

AREAS COVERED

Here the authors will systematically review the literatures on the negative as well as protective aspects of PrP^C and its derivatives (especially PrP N-terminal N1 peptide and shed PrP). The authors will dissect the current findings on N1 and shed PrP, including evidence for their neuroprotective effects, the categories of PrP^C cleavage, and numerous cleavage enzymes involved. The authors will also discuss the protective effects and therapeutic potentials of PrP^C-rich exosomes. The cited articles were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.

EXPERT OPINION

PrP and its N-terminal fragments have strong neuroprotective activities that should be explored for therapeutics and prophylactics development against prion disease, Alzheimer's disease and a few other neurodegenerative diseases. The strategies to develop PrP-based therapeutics and prophylactics for these neurodegenerative diseases will be discussed in a companion article (Part II).

On the role of the cellular prion protein in the uptake and signaling of pathological aggregates in neurodegenerative diseases

Neurodegenerative disorders are associated with intra- or extra-cellular deposition of aggregates of misfolded insoluble proteins. These deposits composed of tau, amyloid-β or α-synuclein spread from cell to cell, in a prion-like manner. Novel evidence suggests that the circulating soluble oligomeric species of these misfolded proteins could play a major role in pathology, while insoluble aggregates would represent their protective less toxic counterparts. Recent convincing data support the proposition that the cellular prion protein, PrP^C, act as a toxicity-inducing receptor for amyloid-β oligomers. As a consequence, several studies focused their investigations to the role played by PrP^C in binding other protein aggregates, such as tau and α-synuclein, for its possible common role in mediating toxic signalling. The biological relevance of PrP^C as key ligand and potential mediator of toxicity for multiple proteinaceous aggregated species, prions or PrP^Sc included, could lead to relevant therapeutic implications. Here we describe the structure of PrP^C and the proposed interplay with its pathological counterpart PrP^Sc and then we recapitulate the most recent findings regarding the role of PrP^C in the interaction with aggregated forms of other neurodegeneration-associated proteins.

Alzheimer's Disease: A Molecular View of β-Amyloid Induced Morbific Events

Amyloid-β (Aβ) is a dynamic peptide of Alzheimer’s disease (AD) which accelerates the disease progression. At the cell membrane and cell compartments, the amyloid precursor protein (APP) undergoes amyloidogenic cleavage by β- and γ-secretases and engenders the Aβ. In addition, externally produced Aβ gets inside the cells by receptors mediated internalization. An elevated amount of Aβ yields spontaneous aggregation which causes organelles impairment. Aβ stimulates the hyperphosphorylation of tau protein via acceleration by several kinases. Aβ travels to the mitochondria and interacts with its functional complexes, which impairs the mitochondrial function leading to the activation of apoptotic signaling cascade. Aβ disrupts the Ca²⁺ and protein homeostasis of the endoplasmic reticulum (ER) and Golgi complex (GC) that promotes the organelle stress and inhibits its stress recovery machinery such as unfolded protein response (UPR) and ER-associated degradation (ERAD). At lysosome, Aβ precedes autophagy dysfunction upon interacting with autophagy molecules. Interestingly, Aβ act as a transcription regulator as well as inhibits telomerase activity. Both Aβ and p-tau interaction with neuronal and glial receptors elevate the inflammatory molecules and persuade inflammation. Here, we have expounded the Aβ mediated events in the cells and its cosmopolitan role on neurodegeneration, and the current clinical status of anti-amyloid therapy.

Condition-dependent structural collapse in the intrinsically disordered N-terminal domain of prion protein

Prion protein is composed of a structure-unsolved N-terminal domain and a globular C-terminal domain. Under limited trypsin digestion, mouse recombinant prion protein can be cleaved into two parts at residue Lys105. Here, we termed these two fragments as the N-domain (sequence 23-105) and the C-domain (sequence 106-230). In this study, the structural properties of the N-domain, the C-domain, and the full-length protein were explored using small-angle X-ray scattering, analytical ultracentrifugation, circular dichroism spectroscopy, and the 8-anilino-1-naphthalenesulfonic acid binding assay. The conformation and size of the prion protein were found to change sensitively under the solvent conditions. The positive residues in the sequence 23-99 of the N-domain were found to be responsible for the enhanced flexibility with the salt concentration reduced below 5 mM. The C-domain containing a hydrophobic patch tends to unfold and aggregate during a salt-induced structural collapse. The N-domain collapsed together with the C-domain at pH 5.2, whereas it collapsed independently at pH 4.2. The positively charged cluster (sequence 100-105) in the N-domain contributed to protecting the exposed hydrophobic surface of the C-domain.

The neuroprotective effects of alpha-lipoic acid on an experimental model of Alzheimer's disease in PC12 cells

With the growth of the aging population, the prevalence of Alzheimer's disease (AD) has increased and influenced the work and daily life of AD patients, imposing a heavy burden on society and the patients' families. AD is a progressive disease with a long duration, and the pathogenesis is very complicated. Here, we found that alpha-lipoic acid (LA), an endogenous, naturally synthesized compound, could attenuate amyloid beta fragment (Aβ_25-35 )-induced PC12 cell toxicity. Aβ_25-35 treatment largely decreased the viability of PC12 cells, increased reactive oxygen species (ROS) levels, and increased the percentage of apoptotic cells, which were accompanied by changes in the expression of the apoptosis-related genes. Further, the Wnt pathway was inactivated, and the expression of Wnt pathway-related proteins such as Frizzled2, GSK3β, and phosphorylated GSK3β were dysregulated after Aβ_25-35 treatment. LA efficiently attenuated Aβ_25-35 -induced PC12 cell apoptosis and downregulated the phosphorylation-mediated degradation of β-catenin as well as GSK3β. Our results demonstrate that LA rescues Aβ_25-35 -induced neurocytotoxicity through the Wnt-β-catenin pathway.

Aβ receptors specifically recognize molecular features displayed by fibril ends and neurotoxic oligomers

Several cell-surface receptors for neurotoxic forms of amyloid-β (Aβ) have been described, but their molecular interactions with Aβ assemblies and their relative contributions to mediating Alzheimer's disease pathology have remained uncertain. Here, we used super-resolution microscopy to directly visualize Aβ-receptor interactions at the nanometer scale. We report that one documented Aβ receptor, PrPC, specifically inhibits the polymerization of Aβ fibrils by binding to the rapidly growing end of each fibril, thereby blocking polarized elongation at that end. PrPC binds neurotoxic oligomers and protofibrils in a similar fashion, suggesting that it may recognize a common, end-specific, structural motif on all of these assemblies. Finally, two other Aβ receptors, FcγRIIb and LilrB2, affect Aβ fibril growth in a manner similar to PrPC. Our results suggest that receptors may trap Aβ oligomers and protofibrils on the neuronal surface by binding to a common molecular determinant on these assemblies, thereby initiating a neurotoxic signal.

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

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

Structural details of amyloid β oligomers in complex with human prion protein as revealed by solid-state MAS NMR spectroscopy

Human PrP (huPrP) is a high-affinity receptor for oligomeric amyloid β (Aβ) protein aggregates. Binding of Aβ oligomers to membrane-anchored huPrP has been suggested to trigger neurotoxic cell signaling in Alzheimer’s disease, while an N-terminal soluble fragment of huPrP can sequester Aβ oligomers and reduce their toxicity. Synthetic oligomeric Aβ species are known to be heterogeneous, dynamic, and transient, rendering their structural investigation particularly challenging. Here, using huPrP to preserve Aβ oligomers by coprecipitating them into large heteroassemblies, we investigated the conformations of Aβ(1–42) oligomers and huPrP in the complex by solid-state MAS NMR spectroscopy. The disordered N-terminal region of huPrP becomes immobilized in the complex and therefore visible in dipolar spectra without adopting chemical shifts characteristic of a regular secondary structure. Most of the well-defined C-terminal part of huPrP is part of the rigid complex, and solid-state NMR spectra suggest a loss in regular secondary structure in the two C-terminal α-helices. For Aβ(1–42) oligomers in complex with huPrP, secondary chemical shifts reveal substantial β-strand content. Importantly, not all Aβ(1–42) molecules within the complex have identical conformations. Comparison with the chemical shifts of synthetic Aβ fibrils suggests that the Aβ oligomer preparation represents a heterogeneous mixture of β-strand-rich assemblies, of which some have the potential to evolve and elongate into different fibril polymorphs, reflecting a general propensity of Aβ to adopt variable β-strand-rich conformers. Taken together, our results reveal structural changes in huPrP upon binding to Aβ oligomers that suggest a role of the C terminus of huPrP in cell signaling. Trapping Aβ(1–42) oligomers by binding to huPrP has proved to be a useful tool for studying the structure of these highly heterogeneous β-strand-rich assemblies.

Evidence for aggregation-independent, PrPC-mediated Aβ cellular internalization

Evidence linking amyloid beta (Aβ) cellular uptake and toxicity has burgeoned, and mechanisms underlying this association are subjects of active research. Two major, interconnected questions are whether Aβ uptake is aggregation-dependent and whether it is sequence-specific. We recently reported that the neuronal uptake of Aβ depends significantly on peptide chirality, suggesting that the process is predominantly receptor-mediated. Over the past decade, the cellular prion protein (PrP^C) has emerged as an important mediator of Aβ-induced toxicity and of neuronal Aβ internalization. Here, we report that the soluble, nonfibrillizing Aβ (1-30) peptide recapitulates full-length Aβ stereoselective cellular uptake, allowing us to decouple aggregation from cellular, receptor-mediated internalization. Moreover, we found that Aβ (1-30) uptake is also dependent on PrP^C expression. NMR-based molecular-level characterization identified the docking site on PrP^C that underlies the stereoselective binding of Aβ (1-30). Our findings therefore identify a specific sequence within Aβ that is responsible for the recognition of the peptide by PrP^C, as well as PrP^C-dependent cellular uptake. Further uptake stereodifferentiation in PrP^C-free cells points toward additional receptor-mediated interactions as likely contributors for Aβ cellular internalization. Taken together, our results highlight the potential of targeting cellular surface receptors to inhibit Aβ cellular uptake as an alternative route for future therapeutic development for Alzheimer's disease.

The Dual Role of Glutamatergic Neurotransmission in Alzheimer's Disease: From Pathophysiology to Pharmacotherapy

Alzheimer's disease (AD) is an age-related dementia and neurodegenerative disorder, characterized by Aβ and tau protein deposition impairing learning, memory and suppressing synaptic plasticity of neurons. Increasing evidence suggests that there is a link between the glucose and glutamate alterations with age that down-regulates glucose utilization reducing glutamate levels in AD patients. Deviations in brain energy metabolism reinforce the development of AD by hampering glutamate levels in the brain. Glutamate is a nonessential amino acid and the major excitatory neurotransmitter synthesized from glucose. Alterations in cerebral glucose and glutamate levels precede the deposition of Aβ plaques. In the brain, over 40% of neuronal synapses are glutamatergic and disturbances in glutamatergic function have been implicated in pathophysiology of AD. Nevertheless, targeting the glutamatergic system seems to be a promising strategy to develop novel, improved therapeutics for AD. Here, we review data supporting the involvement of the glutamatergic system in AD pathophysiology as well as the efficacy of glutamatergic agents in this neurodegenerative disorder. We also discuss exciting new prospects for the development of improved therapeutics for this devastating disorder.

Soluble Prion Peptide 107-120 Protects Neuroblastoma SH-SY5Y Cells against Oligomers Associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent form of dementia and soluble amyloid β (Aβ) oligomers are thought to play a critical role in AD pathogenesis. Cellular prion protein (PrPC) is a high-affinity receptor for Aβ oligomers and mediates some of their toxic effects. The N-terminal region of PrPC can interact with Aβ, particularly the region encompassing residues 95-110. In this study, we identified a soluble and unstructured prion-derived peptide (PrP107-120) that is external to this region of the sequence and was found to successfully reduce the mitochondrial impairment, intracellular ROS generation and cytosolic Ca2+ uptake induced by oligomeric Aβ42 ADDLs in neuroblastoma SH-SY5Y cells. PrP107-120 was also found to rescue SH-SY5Y cells from Aβ42 ADDL internalization. The peptide did not change the structure and aggregation pathway of Aβ42 ADDLs, did not show co-localization with Aβ42 ADDLs in the cells and showed a partial colocalization with the endogenous cellular PrPC. As a sequence region that is not involved in Aβ binding but in PrP self-recognition, the peptide was suggested to protect against the toxicity of Aβ42 oligomers by interfering with cellular PrPC and/or activating a signaling that protected the cells. These results strongly suggest that PrP107-120 has therapeutic potential for AD.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

The role of the cellular prion protein in the uptake and toxic signaling of pathological neurodegenerative aggregates

Neurodegenerative disorders are invariably associated with intra- or extra-cellular deposition of aggregates composed of misfolded insoluble proteins. These deposits composed of tau, amyloid-β or α-synuclein spread from cell to cell, in a prion-like manner. Emerging evidence suggests that the circulating soluble species of these misfolded proteins (usually referred as oligomers) could play a major role in pathology, while insoluble aggregates would represent their protective less toxic counterparts. Convincing data support the hypothesis that the cellular prion protein, PrP^C, act as a toxicity-transducing receptor for amyloid-β oligomers. As a consequence, several studies extended investigations to the role played by PrP^C in binding aggregates of proteins other than Aβ, such as tau and α-synuclein, for its possible common role in mediating toxic signaling. A better characterization of the biological relevance of PrP^C as key ligand and potential mediator of toxicity for multiple proteinaceous aggregated species, prions or PrP^Sc included, would bring relevant therapeutic implications. Here we will first describe the structure of the prion protein and the hypothesized interplay with its pathological counterpart PrP^Sc and then we will recapitulate the most relevant discoveries regarding the role of PrP^C in the interaction with aggregated forms of other neurodegeneration-associated proteins.

The Effect of (-)-Epigallocatechin-3-Gallate on the Amyloid-β Secondary Structure

Amyloid-β (Aβ) is a macromolecular structure of great interest because its misfolding and aggregation, along with changes in the secondary structure, have been correlated with its toxicity in various neurodegenerative diseases. Small drug-like molecules can modulate the amyloid secondary structure and therefore have raised significant interest in applications to active and passive therapies targeting amyloids. In this study, we investigate the interactions of epigallocatechin-3-gallate (EGCG), found in green tea, with Aβ polypeptides, using a combination of in vitro immuno-infrared sensor measurements, docking, molecular dynamics simulations, and ab initio calculations. We find that the interactions of EGCG are dominated by only a few residues in the fibrils, including hydrophobic π-π interactions with aromatic rings of side chains and hydrophilic interactions with the backbone of Aβ, as confirmed by extended (1-μs-long) molecular dynamics simulations. Immuno-infrared sensor data are consistent with degradation of Aβ fibril induced by EGCG and inhibition of Aβ fibril and oligomer formation, as manifested by the recovery of the amide-I band of monomeric Aβ, which is red-shifted by 26 cm^-1 when compared to the amide-I band of the fibrillar form. The shift is rationalized by computations of the infrared spectra of Aβ42 model structures, suggesting that the conformational change involves interchain hydrogen bonds in the amyloid fibrils that are broken upon binding of EGCG.

Transgenic Overexpression of the Disordered Prion Protein N1 Fragment in Mice Does Not Protect Against Neurodegenerative Diseases Due to Impaired ER Translocation

The structurally disordered N-terminal half of the prion protein (PrP^C) is constitutively released into the extracellular space by an endogenous proteolytic cleavage event. Once liberated, this N1 fragment acts neuroprotective in ischemic conditions and interferes with toxic peptides associated with neurodegenerative diseases, such as amyloid-beta (Aβ) in Alzheimer’s disease. Since analog protective effects of N1 in prion diseases, such as Creutzfeldt-Jakob disease, have not been studied, and given that the protease releasing N1 has not been identified to date, we have generated and characterized transgenic mice overexpressing N1 (TgN1). Upon intracerebral inoculation of TgN1 mice with prions, no protective effects were observed at the levels of survival, clinical course, neuropathological, or molecular assessment. Likewise, primary neurons of these mice did not show protection against Aβ toxicity. Our biochemical and morphological analyses revealed that this lack of protective effects is seemingly due to an impaired ER translocation of the disordered N1 resulting in its cytosolic retention with an uncleaved signal peptide. Thus, TgN1 mice represent the first animal model to prove the inefficient ER translocation of intrinsically disordered domains (IDD). In contrast to earlier studies, our data challenge roles of cytoplasmic N1 as a cell penetrating peptide or as a potent “anti-prion” agent. Lastly, our study highlights both the importance of structured domains in the nascent chain for proteins to be translocated and aspects to be considered when devising novel N1-based therapeutic approaches against neurodegenerative diseases.

A mechanistic hypothesis for the impairment of synaptic plasticity by soluble Aβ oligomers from Alzheimer's brain

It is increasingly accepted that early cognitive impairment in Alzheimer's disease results in considerable part from synaptic dysfunction caused by the accumulation of a range of oligomeric assemblies of amyloid β-protein (Aβ). Most studies have used synthetic Aβ peptides to explore the mechanisms of memory deficits in rodent models, but recent work suggests that Aβ assemblies isolated from human (AD) brain tissue are far more potent and disease-relevant. Although reductionist experiments show Aβ oligomers to impair synaptic plasticity and neuronal viability, the responsible mechanisms are only partly understood. Glutamatergic receptors, GABAergic receptors, nicotinic receptors, insulin receptors, the cellular prion protein, inflammatory mediators, and diverse signaling pathways have all been suggested. Studies using AD brain-derived soluble Aβ oligomers suggest that only certain bioactive forms (principally small, diffusible oligomers) can disrupt synaptic plasticity, including by binding to plasma membranes and changing excitatory-inhibitory balance, perturbing mGluR, PrP, and other neuronal surface proteins, down-regulating glutamate transporters, causing glutamate spillover, and activating extrasynaptic GluN2B-containing NMDA receptors. We synthesize these emerging data into a mechanistic hypothesis for synaptic failure in Alzheimer's disease that can be modified as new knowledge is added and specific therapeutics are developed.

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

PrP is a central player in toxicity mediated by soluble aggregates of neurodegeneration-causing proteins

Neurodegenerative diseases are an enormous public health problem, affecting tens of millions of people worldwide. Nearly all of these diseases are characterized by oligomerization and fibrillization of neuronal proteins, and there is great interest in therapeutic targeting of these aggregates. Here, we show that soluble aggregates of α-synuclein and tau bind to plate-immobilized PrP in vitro and on mouse cortical neurons, and that this binding requires at least one of the same N-terminal sites at which soluble Aβ aggregates bind. Moreover, soluble aggregates of tau, α-synuclein and Aβ cause both functional (impairment of LTP) and structural (neuritic dystrophy) compromise and these deficits are absent when PrP is ablated, knocked-down, or when neurons are pre-treated with anti-PrP blocking antibodies. Using an all-human experimental paradigm involving: (1) isogenic iPSC-derived neurons expressing or lacking PRNP, and (2) aqueous extracts from brains of individuals who died with Alzheimer's disease, dementia with Lewy bodies, and Pick's disease, we demonstrate that Aβ, α-synuclein and tau are toxic to neurons in a manner that requires PrPC. These results indicate that PrP is likely to play an important role in a variety of late-life neurodegenerative diseases and that therapeutic targeting of PrP, rather than individual disease proteins, may have more benefit for conditions which involve the aggregation of more than one protein.

Sleep deprivation regulates availability of PrPC and Aβ peptides which can impair interaction between PrPC and laminin and neuronal plasticity

PrP^C is a glycoprotein capable to interact with several molecules and mediates diverse signaling pathways. Among numerous ligands, laminin (LN) is known to promote neurite outgrowth and memory consolidation, while amyloid‐beta oligomers (Aβo) trigger synaptic dysfunction. In both pathways, mGluR1 is recruited as co‐receptor. The involvement of PrP^C/mGluR1 in these opposite functions suggests that this complex is a key element in the regulation of synaptic activity. Considering that sleep‐wake cycle is important for synaptic homeostasis, we aimed to investigate how sleep deprivation affects the expression of PrP^C and its ligands, laminin, Aβo, and mGluR1, a multicomplex that can interfere with neuronal plasticity. To address this question, hippocampi of control (CT) and sleep deprived (SD) C57BL/6 mice were collected at two time points of circadian period (13 hr and 21 hr). We observed that sleep deprivation reduced PrP^C and mGluR1 levels with higher effect in active state (21 hr). Sleep deprivation also caused accumulation of Aβ peptides in rest period (13 hr), while laminin levels were not affected. In vitro binding assay showed that Aβo can compete with LN for PrP^C binding. The influence of Aβo was also observed in neuritogenesis. LN alone promoted longer neurite outgrowth than non‐treated cells in both Prnp^+/+ and Prnp^0/0 genotypes. Aβo alone did not show any effects, but when added together with LN, it attenuated the effects of LN only in Prnp^+/+ cells. Altogether, our findings indicate that sleep deprivation regulates the availability of PrP^C and Aβ peptides, and based on our in vitro assays, these alterations induced by sleep deprivation can negatively affect LN–PrP^C interaction, which is known to play roles in neuronal plasticity.

Membrane Binding Strongly Affecting the Dopamine Reactivity Induced by Copper Prion and Copper/Amyloid-β (Aβ) Peptides. A Ternary Copper/Aβ/Prion Peptide Complex Stabilized and Solubilized in Sodium Dodecyl Sulfate Micelles

The combination between dyshomeostatic levels of catecholamine neurotransmitters and redox-active metals such as copper and iron exacerbates the oxidative stress condition that typically affects neurodegenerative diseases. We report a comparative study of the oxidative reactivity of copper complexes with amyloid-β (Aβ₄₀) and the prion peptide fragment 76-114 (PrP_76-114), containing the high-affinity binding site, toward dopamine and 4-methylcatechol, in aqueous buffer and in sodium dodecyl sulfate micelles, as a model membrane environment. The competitive oxidative and covalent modifications undergone by the peptides were also evaluated. The high binding affinity of Cu/peptide to micelles and lipid membranes leads to a strong reduction (Aβ₄₀) and quenching (PrP_76-114) of the oxidative efficiency of the binary complexes and to a stabilization and redox silencing of the ternary complex Cu^II/Aβ₄₀/PrP_76-114, which is highly reactive in solution. The results improve our understanding of the pathological and protective effects associated with these complexes, depending on the physiological environment.

Amyloid Beta Oligomers Target to Extracellular and Intracellular Neuronal Synaptic Proteins in Alzheimer's Disease

Introduction: β-Amyloid protein (Aβ) putatively plays a seminal role in synaptic loss in Alzheimer's disease (AD). While there is no consensus regarding the synaptic-relevant species of Aβ, it is known that Aβ oligomers (AβOs) are noticeably increased in the early stages of AD, localizing at or within the synapse. In cell and animal models, AβOs have been shown to attach to synapses and instigate synapse dysfunction and deterioration. To establish the pathological mechanism of synaptic loss in AD, it will be important to identify the synaptic targets to which AβOs attach. Methods: An unbiased approach using far western ligand blots has identified three synaptic proteins to which AβOs specifically attach. These proteins (p100, p140, and p260) were subsequently enriched by detergent extraction, ultracentrifugation, and CHT-HPLC column separation, and sequenced by LC-MS/MS. P100, p140, and p260 were identified. These levels of AβOs targets in human AD and aging frontal cortexes were analyzed by quantitative proteomics and western-blot. The polyclonal antibody to AβOs was developed and used to block the toxicity of AβOs. The data were analyzed with one-way analysis of variance. Results: AβOs binding proteins p100, p140, and p260 were identified as Na/K-ATPase, synGap, and Shank3, respectively. α3-Na/K-ATPase, synGap, and Shank3 proteins showed loss in the postsynaptic density (PSD) of human AD frontal cortex. In short term experiments, oligomers of Aβ inhibited Na/K-ATPase at the synapse. Na/K-ATPase activity was restored by an antibody specific for soluble forms of Aβ. α3-Na/K-ATPase protein and synaptic β-amyloid peptides were pulled down from human AD synapses by co-immunoprecipitation. Results suggest synaptic dysfunction in early stages of AD may stem from inhibition of Na/K-ATPase activity by Aβ oligomers, while later stages could hypothetically result from disrupted synapse structure involving the PSD proteins synGap and Shank3. Conclusion: We identified three AβO binding proteins as α3-Na/K-ATPase, synGap, and Shank3. Soluble Aβ oligomers appear capable of attacking neurons via specific extracellular as well as intracellular synaptic proteins. Impact on these proteins hypothetically could lead to synaptic dysfunction and loss, and could serve as novel therapeutic targets for AD treatment by antibodies or other agents.

Cellular Prion Protein as a Receptor of Toxic Amyloid-β42 Oligomers Is Important for Alzheimer's Disease

The pathological features of Alzheimer's disease (AD) include senile plaques induced by amyloid-β (Aβ) protein deposits, neurofibrillary tangles formed by aggregates of hyperphosphorylated tau proteins and neuronal cell loss in specific position within the brain. Recent observations have suggested the possibility of an association between AD and cellular prion protein (PrP ^C ) levels. PrP ^C is a high affinity receptor for oligomeric Aβ and is important for Aβ-induced neurotoxicity and thus plays a critical role in AD pathogenesis. The determination of the relationship between PrP ^C and AD and the characterization of PrP ^C binding to Aβ will facilitate the development of novel therapies for AD.

Alzheimer Disease Pathogenesis: Insights From Molecular and Cellular Biology Studies of Oligomeric Aβ and Tau Species

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary human and financial burdens. Studies of pathogenesis are essential for defining targets for discovering disease-modifying treatments. Past studies of AD neuropathology provided valuable, albeit limited, insights. Nevertheless, building on these findings, recent studies have provided an increasingly rich harvest of genetic, molecular and cellular data that are creating unprecedented opportunities to both understand and treat AD. Among the most significant are those documenting the presence within the AD brain of toxic oligomeric species of Aβ and tau. Existing data support the view that such species can propagate and spread within neural circuits. To place these findings in context we first review the genetics and neuropathology of AD, including AD in Down syndrome (AD-DS). We detail studies that support the existence of toxic oligomeric species while noting the significant unanswered questions concerning their precise structures, the means by which they spread and undergo amplification and how they induce neuronal dysfunction and degeneration. We conclude by offering a speculative synthesis for how oligomers of Aβ and tau initiate and drive pathogenesis. While 100 years after Alzheimer's first report there is much still to learn about pathogenesis and the discovery of disease-modifying treatments, the application of new concepts and sophisticated new tools are poised to deliver important advances for combatting AD.

The Amyloid-β Oligomer Hypothesis: Beginning of the Third Decade

The amyloid-β oligomer (AβO) hypothesis was introduced in 1998. It proposed that the brain damage leading to Alzheimer’s disease (AD) was instigated by soluble, ligand-like AβOs. This hypothesis was based on the discovery that fibril-free synthetic preparations of AβOs were potent CNS neurotoxins that rapidly inhibited long-term potentiation and, with time, caused selective nerve cell death (Lambert et al., 1998). The mechanism was attributed to disrupted signaling involving the tyrosine-protein kinase Fyn, mediated by an unknown toxin receptor. Over 4,000 articles concerning AβOs have been published since then, including more than 400 reviews. AβOs have been shown to accumulate in an AD-dependent manner in human and animal model brain tissue and, experimentally, to impair learning and memory and instigate major facets of AD neuropathology, including tau pathology, synapse deterioration and loss, inflammation, and oxidative damage. As reviewed by Hayden and Teplow in 2013, the AβO hypothesis “has all but supplanted the amyloid cascade.” Despite the emerging understanding of the role played by AβOs in AD pathogenesis, AβOs have not yet received the clinical attention given to amyloid plaques, which have been at the core of major attempts at therapeutics and diagnostics but are no longer regarded as the most pathogenic form of Aβ. However, if the momentum of AβO research continues, particularly efforts to elucidate key aspects of structure, a clear path to a successful disease modifying therapy can be envisioned. Ensuring that lessons learned from recent, late-stage clinical failures are applied appropriately throughout therapeutic development will further enable the likelihood of a successful therapy in the near-term.

Binding between Prion Protein and Aβ Oligomers Contributes to the Pathogenesis of Alzheimer's Disease

A plethora of evidence suggests that protein misfolding and aggregation are underlying mechanisms of various neurodegenerative diseases, such as prion diseases and Alzheimer's disease (AD). Like prion diseases, AD has been considered as an infectious disease in the past decades as it shows strain specificity and transmission potential. Although it remains elusive how protein aggregation leads to AD, it is becoming clear that cellular prion protein (PrP^C) plays an important role in AD pathogenesis. Here, we briefly reviewed AD pathogenesis and focused on recent progresses how PrP^C contributed to AD development. In addition, we proposed a potential mechanism to explain why infectious agents, such as viruses, conduce AD pathogenesis. Microbe infections cause Aβ deposition and upregulation of PrP^C, which lead to high affinity binding between Aβ oligomers and PrP^C. The interaction between PrP^C and Aβ oligomers in turn activates the Fyn signaling cascade, resulting in neuron death in the central nervous system (CNS). Thus, silencing PrP^C expression may turn out be an effective treatment for PrP^C dependent AD.

A novel crosslinking protocol stabilizes amyloid β oligomers capable of inducing Alzheimer's-associated pathologies

Amyloid β oligomers (AβOs) accumulate early in Alzheimer's disease (AD) and experimentally cause memory dysfunction and the major pathologies associated with AD, for example, tau abnormalities, synapse loss, oxidative damage, and cognitive dysfunction. In order to develop the most effective AβO-targeting diagnostics and therapeutics, the AβO structures contributing to AD-associated toxicity must be elucidated. Here, we investigate the structural properties and pathogenic relevance of AβOs stabilized by the bifunctional crosslinker 1,5-difluoro-2,4-dinitrobenzene (DFDNB). We find that DFDNB stabilizes synthetic Aβ in a soluble oligomeric conformation. With DFDNB, solutions of Aβ that would otherwise convert to large aggregates instead yield solutions of stable AβOs, predominantly in the 50-300 kDa range, that are maintained for at least 12 days at 37°C. Structures were determined by biochemical and native top-down mass spectrometry analyses. Assayed in neuronal cultures and i.c.v.-injected mice, the DFDNB-stabilized AβOs were found to induce tau hyperphosphorylation, inhibit choline acetyltransferase, and provoke neuroinflammation. Most interestingly, DFDNB crosslinking was found to stabilize an AβO conformation particularly potent in inducing memory dysfunction in mice. Taken together, these data support the utility of DFDNB crosslinking as a tool for stabilizing pathogenic AβOs in structure-function studies.

A d-enantiomeric peptide interferes with heteroassociation of amyloid-β oligomers and prion protein

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects millions of people worldwide. One AD hallmark is the aggregation of β-amyloid (Aβ) into soluble oligomers and insoluble fibrils. Several studies have reported that oligomers rather than fibrils are the most toxic species in AD progression. Aβ oligomers bind with high affinity to membrane-associated prion protein (PrP), leading to toxic signaling across the cell membrane, which makes the Aβ-PrP interaction an attractive therapeutic target. Here, probing this interaction in more detail, we found that both full-length, soluble human (hu) PrP(23-230) and huPrP(23-144), lacking the globular C-terminal domain, bind to Aβ oligomers to form large complexes above the megadalton size range. Following purification by sucrose density-gradient ultracentrifugation, the Aβ and huPrP contents in these heteroassemblies were quantified by reversed-phase HPLC. The Aβ:PrP molar ratio in these assemblies exhibited some limited variation depending on the molar ratio of the initial mixture. Specifically, a molar ratio of about four Aβ to one huPrP in the presence of an excess of huPrP(23-230) or huPrP(23-144) suggested that four Aβ units are required to form one huPrP-binding site. Of note, an Aβ-binding all-d-enantiomeric peptide, RD2D3, competed with huPrP for Aβ oligomers and interfered with Aβ-PrP heteroassembly in a concentration-dependent manner. Our results highlight the importance of multivalent epitopes on Aβ oligomers for Aβ-PrP interactions and have yielded an all-d-peptide-based, therapeutically promising agent that competes with PrP for these interactions.

Prion protein stabilizes amyloid-β (Aβ) oligomers and enhances Aβ neurotoxicity in a Drosophila model of Alzheimer's disease

The cellular prion protein (PrP^C) can act as a cell-surface receptor for β-amyloid (Aβ) peptide; however, a role for PrP^C in the pathogenesis of Alzheimer's disease (AD) is contested. Here, we expressed a range of Aβ isoforms and PrP^C in the Drosophila brain. We found that co-expression of Aβ and PrP^C significantly reduces the lifespan, disrupts circadian rhythms, and increases Aβ deposition in the fly brain. In contrast, under the same conditions, expression of Aβ or PrP^C individually did not lead to these phenotypic changes. In vitro studies revealed that substoichiometric amounts of PrP^C trap Aβ as oligomeric assemblies and fragment-preformed Aβ fibers. The ability of membrane-anchored PrP^C to trap Aβ as cytotoxic oligomers at the membrane surface and fragment inert Aβ fibers suggests a mechanism by which PrP^C exacerbates Aβ deposition and pathogenic phenotypes in the fly, supporting a role for PrP^C in AD. This study provides a second animal model linking PrP^C expression with Aβ toxicity and supports a role for PrP^C in AD pathogenesis. Blocking the interaction of Aβ and PrP^C represents a potential therapeutic strategy.

Alterations in the brain interactome of the intrinsically disordered N-terminal domain of the cellular prion protein (PrPC) in Alzheimer's disease

The cellular prion protein (PrPC) is implicated in neuroprotective signaling and neurotoxic pathways in both prion diseases and Alzheimer's disease (AD). Specifically, the intrinsically disordered N-terminal domain (N-PrP) has been shown to interact with neurotoxic ligands, such as Aβ and Scrapie prion protein (PrPSc), and to be crucial for the neuroprotective activity of PrPC. To gain further insight into cellular pathways tied to PrP, we analyzed the brain interactome of N-PrP. As a novel approach employing recombinantly expressed PrP and intein-mediated protein ligation, we used N-PrP covalently coupled to beads as a bait for affinity purification. N-PrP beads were incubated with human AD or control brain lysates. N-PrP binding partners were then identified by electrospray ionization tandem mass spectrometry (nano ESI-MS/MS). In addition to newly identified proteins we found many previously described PrP interactors, indicating a crucial role of the intrinsically disordered part of PrP in mediating protein interactions. Moreover, some interactors were found only in either non-AD or AD brain, suggesting aberrant PrPC interactions in the pathogenesis of AD.

Prion Protein Interactome: Identifying Novel Targets in Slowly and Rapidly Progressive Forms of Alzheimer's Disease

Rapidly progressive Alzheimer's disease (rpAD) is a variant of AD distinguished by a rapid decline in cognition and short disease duration from onset to death. While attempts to identify rpAD based on biomarker profile classifications have been initiated, the mechanisms which contribute to the rapid decline and prion mimicking heterogeneity in clinical signs are still largely unknown. In this study, we characterized prion protein (PrP) expression, localization, and interactome in rpAD, slow progressive AD, and in non-dementia controls. PrP along with its interacting proteins were affinity purified with magnetic Dynabeads Protein-G, and were identified using Q-TOF-ESI/MS analysis. Our data demonstrated a significant 1.2-fold decrease in di-glycosylated PrP isoforms specifically in rpAD patients. Fifteen proteins appeared to interact with PrP and only two proteins3/4histone H2B-type1-B and zinc alpha-2 protein3/4were specifically bound with PrP isoform isolated from rpAD cases. Our data suggest distinct PrP involvement in association with the altered PrP interacting protein in rpAD, though the pathophysiological significance of these interactions remains to be established.

Structural and mechanistic aspects influencing the ADAM10-mediated shedding of the prion protein

Background

Proteolytic processing of the prion protein (PrPC) by endogenous proteases generates bioactive membrane-bound and soluble fragments which may help to explain the pleiotropic roles of this protein in the nervous system and in brain diseases. Shedding of almost full-length PrPC into the extracellular space by the metalloprotease ADAM10 is of peculiar relevance since soluble PrP stimulates axonal outgrowth and is protective in neurodegenerative conditions such as Alzheimer’s and prion disease. However, molecular determinates and mechanisms regulating the shedding of PrP are entirely unknown.

Methods

We produced an antibody recognizing the neo-epitope of shed PrP generated by ADAM10 in biological samples and used it to study structural and mechanistic aspects affecting the shedding. For this, we investigated genetically modified cellular and murine models by biochemical and morphological approaches.

Results

We show that the novel antibody specifically detects shed PrP in cell culture supernatants and murine brain. We demonstrate that ADAM10 is the exclusive sheddase of PrPC in the nervous system and reveal that the glycosylation state and type of membrane-anchorage of PrPC severely affect its shedding. Furthermore, we provide evidence that PrP shedding can be modulated by pharmacological inhibition and stimulation and present data suggesting that shedding is a relevant part of a compensatory network ensuring PrPC homeostasis of the cell.

Conclusions

With the new antibody, our study introduces a new tool to reliably investigate PrP-shedding. In addition, this study provides novel and important insight into the regulation of this cleavage event, which is likely to be relevant for diagnostic and therapeutic approaches even beyond neurodegeneration.

Role of cellular prion protein in interneuronal amyloid transmission

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