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

Deciphering the full-length PrPC(23-231) receptor and characterizing the size/subphase-dependent impact of Aβ oligomers on the PrPC(23-231) receptor: insights from molecular dynamics simulations

As one of the cell surface receptors, cellular prion protein (PrPC) can bind Aβ oligomers (AβOs) and attenuate their neurotoxicity. However, there is still considerable controversy regarding the PrPC-AβO interaction, due to the polymorphism and varying size of the AβO species and the void of a full-length PrPC 3D structure. To solve this problem, we first complemented the missing residues in the residue-lacking crystal structure of PrPC and determined a 3D full-length PrPC receptor using "Alphafold2". We subsequently investigated the complexes formed between the PrPC receptors-both the full-length and those missing the N-terminus-and a variety of Aβ42 species, including Aβ42 monomers, Aβ oligomers (AβOs) of varying sizes across two phases, as well as Aβ42 fibrils (AβFs), using molecular dynamics simulations. The simulated results indicate that the full-length PrPC receptor (23-231) employs a cavity, formed by its amino acid residues 44-51 and AA 95-110 regions, for Aβ42 binding. In contrast, the crystal structure of the PrPC receptor, typically lacking the N-terminal sequence (amino acids 23-87), provides a binding cavity composed of amino acids 95-110 and the C-terminal residues 131-161 to bind Aβ42, which is consistent with the diverse experimental outcomes observed (Nature 2009, 457(7233), 1128-1132; J. Am. Chem. Soc. 2022, 144(21), 9264-9270). This underscores the necessity of the novel full-length PrPC (PrPC23-231) 3D model for replicating experimental findings accurately. Additionally, we utilized both the full-length and truncated models of the PrPC receptor to clarify its disruptive effects on the growth of Aβ42 secondary nuclei structures (AβF) and its inhibitory impact on the disordered AβOs across two phases. This work provides molecular-level insights into the PrP-Aβ interaction, facilitating model selection for future experimental studies and identifying molecular targets for designing drugs intended to alleviate the toxicity of Aβ42 oligomers towards the PrPC receptor.

Identifying the copper coordination environment between interacting neurodegenerative proteins: A new approach using pulsed EPR with 14N/15N isotopic labeling

The trafficking and aggregation of neurodegenerative proteins often involve the interaction between intrinsically disordered domains, stabilized by the inclusion of physiological metal ions such as copper or zinc. Characterizing the metal ion coordination environment is critical for assessing the stability and organization of these relevant protein-protein interactions but is challenging given the lack of regular molecular order or global structure. The cellular prion protein (PrPC) binds both monomers and aggregates of Alzheimer's amyloid-beta (Aβ), promoting Aβ internalization and aberrant signaling, respectively. Both proteins bind Cu2+ with high affinity, opening the potential for copper to form an intermolecular bridge. We describe here a novel approach utilizing multiple EPR experiments to investigate the simultaneous Cu2+ coordination of PrPC and Aβ in a 1:1:1 mixture. Uniformly 15N-labeled PrPC is used in conjunction with natural abundance 14N Aβ, the combination of which leads to distinct energy manifolds for paramagnetic Cu2+ and is resolved by the pulsed EPR experiments ESEEM and HYSCORE. We develop acquisition parameters to simultaneously optimize 14N (I = 1) and 15N (I = ½) pulsed EPR signals and we also advance the theory of ESEEM and HYSCORE to quantitatively describe multiple 15N imidazole coordination. This unique approach provides compelling evidence of a copper-stabilized ternary complex, with equatorial Cu2+ coordination formed by one histidine imidazole from Aβ and three from PrP. Moreover, the methodologies developed here provide a framework for assessing the copper environment in other interacting neurodegenerative proteins.

Factors Affecting Pathological Amyloid Protein Transformation: From Post-Translational Modifications to Chaperones

The review discusses the influence of various factors (e.g., post-translational modifications and chaperones) on the pathological transformation of amyloidogenic proteins involved in the onset and development of neurodegenerative diseases (Alzheimer's and Parkinson's diseases) and spongiform encephalopathies of various origin with special focus on the role of α-synuclein, prion protein, and, to a lesser extent, beta-amyloid peptide. The factors investigated by the authors of this review are discussed in more detail, including posttranslational modifications (glycation and S-nitrosylation), cinnamic acid derivatives and dendrimers, and chaperonins (eukaryotic, bacterial, and phage). A special section is devoted to the role of the gastrointestinal microbiota in the pathogenesis of amyloid neurodegenerative diseases, in particular, its involvement in the transformation of infectious prions and possibly other proteins capable of prion-like transmission of amyloidogenic diseases.

Prion Protein Endoproteolysis: Cleavage Sites, Mechanisms and Connections to Prion Disease

Highly abundant in neurons, the cellular prion protein (PrPC) is an obligatory precursor to the disease-associated misfolded isoform denoted PrPSc that accumulates in the rare neurodegenerative disorders referred to either as transmissible spongiform encephalopathies (TSEs) or as prion diseases. The ability of PrPC to serve as a substrate for this template-mediated conversion process depends on several criteria but importantly includes the presence or absence of certain endoproteolytic events performed at the cell surface or in acidic endolysosomal compartments. The major endoproteolytic events affecting PrPC are referred to as α- and β-cleavages, and in this review we outline the sites within PrPC at which the cleavages occur, the mechanisms potentially responsible and their relevance to pathology. Although the association of α-cleavage with neuroprotection is well-supported, we identify open questions regarding the importance of β-cleavage in TSEs and suggest experimental approaches that could provide clarification. We also combine findings from in vitro cleavage assays and mass spectrometry-based studies of prion protein fragments in the brain to present an updated view in which α- and β-cleavages may represent two distinct clusters of proteolytic events that occur at multiple neighbouring sites rather than at single positions. Furthermore, we highlight the candidate proteolytic mechanisms best supported by the literature; currently, despite several proteases identified as capable of processing PrPC in vitro, in cell-based models and in some cases, in vivo, none have been shown conclusively to cleave PrPC in the brain. Addressing this knowledge gap will be crucial for developing therapeutic interventions to drive PrPC endoproteolysis in a neuroprotective direction. Finally, we end this review by briefly addressing other cleavage events, specifically ectodomain shedding, γ-cleavage, the generation of atypical pathological fragments in the familial prion disorder Gerstmann-Sträussler-Scheinker syndrome and the possibility of an additional form of endoproteolysis close to the PrPC N-terminus.

Structures of Oligomeric States of Tau Protein, Amyloid-β, α-Synuclein and Prion Protein Implicated in Alzheimer's Disease, Parkinson's Disease and Prionopathies

In this review, we focus on the biophysical and structural aspects of the oligomeric states of physiologically intrinsically disordered proteins and peptides tau, amyloid-β and α-synuclein and partly disordered prion protein and their isolations from animal models and human brains. These protein states may be the most toxic agents in the pathogenesis of Alzheimer's and Parkinson's disease. It was shown that oligomers are important players in the aggregation cascade of these proteins. The structural information about these structural states has been provided by methods such as solution and solid-state NMR, cryo-EM, crosslinking mass spectrometry, AFM, TEM, etc., as well as from hybrid structural biology approaches combining experiments with computational modelling and simulations. The reliable structural models of these protein states may provide valuable information for future drug design and therapies.

Designed Cell-Penetrating Peptide Constructs for Inhibition of Pathogenic Protein Self-Assembly

Peptides possess a number of pharmacologically desirable properties, including greater chemical diversity than other biomolecule classes and the ability to selectively bind to specific targets with high potency, as well as biocompatibility, biodegradability, and ease and low cost of production. Consequently, there has been considerable interest in developing peptide-based therapeutics, including amyloid inhibitors. However, a major hindrance to the successful therapeutic application of peptides is their poor delivery to target tissues, cells or subcellular organelles. To overcome these issues, recent efforts have focused on engineering cell-penetrating peptide (CPP) antagonists of amyloidogenesis, which combine the attractive intrinsic properties of peptides with potent therapeutic effects (i.e., inhibition of amyloid formation and the associated cytotoxicity) and highly efficient delivery (to target tissue, cells, and organelles). This review highlights some promising CPP constructs designed to target amyloid aggregation associated with a diverse range of disorders, including Alzheimer's disease, transmissible spongiform encephalopathies (or prion diseases), Parkinson's disease, and cancer.

Prion protein pathology in Ubiquilin 2 models of ALS

Mutations in UBQLN2 cause ALS and frontotemporal dementia (FTD). The pathological signature in UBQLN2 cases is deposition of highly unusual types of inclusions in the brain and spinal cord that stain positive for UBQLN2. However, what role these inclusions play in pathogenesis remains unclear. Here we show cellular prion protein (PrPC) is found in UBQLN2 inclusions in both mouse and human neuronal induced pluripotent (IPSC) models of UBQLN2 mutations, evidenced by the presence of aggregated forms of PrPC with UBQLN2 inclusions. Turnover studies indicated that the P497H UBQLN2 mutation slows PrPC protein degradation and leads to mislocalization of PrPC in the cytoplasm. Immunoprecipitation studies indicated UBQLN2 and PrPC bind together in a complex. The abnormalities in PrPC caused by UBQLN2 mutations may be relevant in disease pathogenesis.

Cellular prion protein acts as mediator of amyloid beta uptake by caveolin-1 causing cellular dysfunctions in vitro and in vivo

INTRODUCTION

Cellular prion protein (PrPC) was implicated in amyloid beta (Aβ)-induced toxicity in Alzheimer's disease (AD), but the precise molecular mechanisms involved in this process are unclear.

METHODS

Double transgenic mice were generated by crossing Prnp knockout (KO) with 5xFAD mice, and light-sheet microscopy was used for whole brain tissue analyses. PrPC-overexpressing cells were developed for in vitro studies, and microscopy was used to assess co-localization of proteins of interest. Surface-plasmon resonance (SPR) was used to investigate protein-binding characteristics.

RESULTS

In vivo, PrPC levels correlated with reduced lifespan and cognitive and motor function, and its ablation disconnected behavior deficits from Aβ levels. Light-sheet microscopy showed that PrPC influenced Aβ-plaque burden but not the distribution of those plaques. Interestingly, caveolin-1 (Cav-1) KO neurons significantly reduced intracellular Aβ-oligomer (Aβo) uptake when compared to wild-type neurons.

DISCUSSION

The findings shed new light on the relevance of intracellular Aβo, suggesting that PrPC and Cav-1 modulate intracellular Aβ levels and the Aβ-plaque load.

HIGHLIGHTS

PrPC expression adversely affects lifespan and behavior in 5xFAD mice. PrPC increases Aβ1-40 and Aβ1-42 levels and Aβ-plaque load in 5xFAD mice. Cav-1 interacts with both PrPC and Aβ peptides. Knocking out Cav-1 leads to a significant reduction in intracellular Aβ levels.

MAPT mutations associated with familial tauopathies lead to formation of conformationally distinct oligomers that have cross-seeding ability

The microtubule associated protein, tau, is implicated in a multitude of neurodegenerative disorders that are collectively termed as tauopathies. These disorders are characterized by the presence of tau aggregates within the brain of afflicted individuals. Mutations within the MAPT gene that encodes the tau protein form the genetic backdrop for familial forms of tauopathies, such as frontotemporal dementia (FTD), but the molecular consequences of such alterations and their pathological effects are unclear. We sought to investigate the conformational properties of the aggregates of three tau mutants: A152T, P301L, and R406W, all implicated within FTD, and compare them to those of the native form (WT-Tau 2N4R). Our immunochemical analysis reveals that mutants and WT tau oligomers exhibit similar affinity for conformation-specific antibodies but have distinct morphology and secondary structure. Additionally, these oligomers possess different dye-binding properties and varying sensitivity to proteolytic processing. These results point to conformational variety among them. We then tested the ability of the mutant oligomers to cross-seed the aggregation of WT tau monomer. Using similar array of experiments, we found that cross-seeding with mutant aggregates leads to the formation of conformationally unique WT oligomers. The results discussed in this paper provide a novel perspective on the structural properties of oligomeric forms of WT tau 2N4R and its mutant, along with shedding some light on their cross-seeding behavior.

α-Synuclein strain propagation is independent of cellular prion protein expression in a transgenic synucleinopathy mouse model

The cellular prion protein, PrPC, has been postulated to function as a receptor for α-synuclein, potentially facilitating cell-to-cell spreading and/or toxicity of α-synuclein aggregates in neurodegenerative disorders such as Parkinson's disease. Previously, we generated the "Salt (S)" and "No Salt (NS)" strains of α-synuclein aggregates that cause distinct pathological phenotypes in M83 transgenic mice overexpressing A53T-mutant human α-synuclein. To test the hypothesis that PrPC facilitates the propagation of α-synuclein aggregates, we produced M83 mice that either express or do not express PrPC. Following intracerebral inoculation with the S or NS strain, the absence of PrPC in M83 mice did not prevent disease development and had minimal influence on α-synuclein strain-specified attributes such as the extent of cerebral α-synuclein deposition, selective targeting of specific brain regions and cell types, the morphology of induced α-synuclein deposits, and the structural fingerprints of protease-resistant α-synuclein aggregates. Likewise, there were no appreciable differences in disease manifestation between PrPC-expressing and PrPC-lacking M83 mice following intraperitoneal inoculation of the S strain. Interestingly, intraperitoneal inoculation with the NS strain resulted in two distinct disease phenotypes, indicative of α-synuclein strain evolution, but this was also independent of PrPC expression. Overall, these results suggest that PrPC plays at most a minor role in the propagation, neuroinvasion, and evolution of α-synuclein strains in mice that express A53T-mutant human α-synuclein. Thus, other putative receptors or cell-to-cell propagation mechanisms may have a larger effect on the spread of α-synuclein aggregates during disease.

Post-translational modifications in prion diseases

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

Aging-Dependent Loss of Connectivity in Alzheimer's Model Mice with Rescue by mGluR5 Modulator

Amyloid accumulation in Alzheimer's disease (AD) is associated with synaptic damage and altered connectivity in brain networks. While measures of amyloid accumulation and biochemical changes in mouse models have utility for translational studies of certain therapeutics, preclinical analysis of altered brain connectivity using clinically relevant fMRI measures has not been well developed for agents intended to improve neural networks. Here, we conduct a longitudinal study in a double knock-in mouse model for AD ( App NL-G-F /hMapt ), monitoring brain connectivity by means of resting-state fMRI. While the 4-month-old AD mice are indistinguishable from wild-type controls (WT), decreased connectivity in the default-mode network is significant for the AD mice relative to WT mice by 6 months of age and is pronounced by 9 months of age. In a second cohort of 20-month-old mice with persistent functional connectivity deficits for AD relative to WT, we assess the impact of two-months of oral treatment with a silent allosteric modulator of mGluR5 (BMS-984923) known to rescue synaptic density. Functional connectivity deficits in the aged AD mice are reversed by the mGluR5-directed treatment. The longitudinal application of fMRI has enabled us to define the preclinical time trajectory of AD-related changes in functional connectivity, and to demonstrate a translatable metric for monitoring disease emergence, progression, and response to synapse-rescuing treatment.

Brain Hydrophobic Peptides Antagonists of Neurotoxic Amyloid β Peptide Monomers/Oligomers-Protein Interactions

Amyloid β (Aβ) oligomers have been linked to Alzheimer's disease (AD) pathogenesis and are the main neurotoxic forms of Aβ. This review focuses on the following: (i) the Aβ(1-42):calmodulin interface as a model for the design of antagonist Aβ peptides and its limitations; (ii) proteolytic degradation as the major source of highly hydrophobic peptides in brain cells; and (iii) brain peptides that have been experimentally demonstrated to bind to Aβ monomers or oligomers, Aβ fibrils, or Aβ plaques. It is highlighted that the hydrophobic amino acid residues of the COOH-terminal segment of Aβ(1-42) play a key role in its interaction with intracellular protein partners linked to its neurotoxicity. The major source of highly hydrophobic endogenous peptides of 8-10 amino acids in neurons is the proteasome activity. Many canonical antigen peptides bound to the major histocompatibility complex class 1 are of this type. These highly hydrophobic peptides bind to Aβ and are likely to be efficient antagonists of the binding of Aβ monomers/oligomers concentrations in the nanomolar range with intracellular proteins. Also, their complexation with Aβ will protect them against endopeptidases, suggesting a putative chaperon-like physiological function for Aβ that has been overlooked until now. Remarkably, the hydrophobic amino acid residues of Aβ responsible for the binding of several neuropeptides partially overlap with those playing a key role in its interaction with intracellular protein partners that mediates its neurotoxicity. Therefore, these latter neuropeptides are also potential candidates to antagonize Aβ peptides binding to target proteins. In conclusion, the analysis performed in this review points out that hydrophobic endogenous brain neuropeptides could be valuable biomarkers to evaluate the risk of the onset of sporadic AD, as well as for the prognosis of AD.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

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.

Peptide aptamer targeting Aβ-PrP-Fyn axis reduces Alzheimer's disease pathologies in 5XFAD transgenic mouse model

Currently, no effective therapeutics exist for the treatment of incurable neurodegenerative diseases such as Alzheimer's disease (AD). The cellular prion protein (PrP^C) acts as a high-affinity receptor for amyloid beta oligomers (AβO), a main neurotoxic species mediating AD pathology. The interaction of AβO with PrP^C subsequently activates Fyn tyrosine kinase and neuroinflammation. Herein, we used our previously developed peptide aptamer 8 (PA8) binding to PrP^C as a therapeutic to target the AβO-PrP-Fyn axis and prevent its associated pathologies. Our in vitro results indicated that PA8 prevents the binding of AβO with PrP^C and reduces AβO-induced neurotoxicity in mouse neuroblastoma N2a cells and primary hippocampal neurons. Next, we performed in vivo experiments using the transgenic 5XFAD mouse model of AD. The 5XFAD mice were treated with PA8 and its scaffold protein thioredoxin A (Trx) at a 14.4 µg/day dosage for 12 weeks by intraventricular infusion through Alzet^® osmotic pumps. We observed that treatment with PA8 improves learning and memory functions of 5XFAD mice as compared to Trx-treated 5XFAD mice. We found that PA8 treatment significantly reduces AβO levels and Aβ plaques in the brain tissue of 5XFAD mice. Interestingly, PA8 significantly reduces AβO-PrP interaction and its downstream signaling such as phosphorylation of Fyn kinase, reactive gliosis as well as apoptotic neurodegeneration in the 5XFAD mice compared to Trx-treated 5XFAD mice. Collectively, our results demonstrate that treatment with PA8 targeting the AβO-PrP-Fyn axis is a promising and novel approach to prevent and treat AD.

Prion Propagation is Dependent on Key Amino Acids in Charge Cluster 2 within the Prion Protein

To dissect the N-terminal residues within the cellular prion protein (PrPC) that are critical for efficient prion propagation, we generated a library of point, double, or triple alanine replacements within residues 23-111 of PrP, stably expressed them in cells silenced for endogenous mouse PrPC and challenged the reconstituted cells with four common but biologically diverse mouse prion strains. Amino acids (aa) 105-111 of Charge Cluster 2 (CC2), which is disordered in PrPC, were found to be required for propagation of all four prion strains; other residues had no effect or exhibited strain-specific effects. Replacements in CC2, including aa105-111, dominantly inhibited prion propagation in the presence of endogenous wild type PrPC whilst other changes were not inhibitory. Single alanine replacements within aa105-111 identified leucine 108 and valine 111 or the cluster of lysine 105, threonine 106 and asparagine 107 as critical for prion propagation. These residues mediate specific ordering of unstructured CC2 into β-sheets in the infectious prion fibrils from Rocky Mountain Laboratory (RML) and ME7 mouse prion strains.

Cellular prion protein offers neuroprotection in astrocytes submitted to amyloid β oligomer toxicity

The cellular prion protein (PrP^C), in its native conformation, performs numerous cellular and cognitive functions in brain tissue. However, despite the cellular prion research in recent years, there are still questions about its participation in oxidative and neurodegenerative processes. This study aims to elucidate the involvement of PrP^C in the neuroprotection cascade in the presence of oxidative stressors. For that, astrocytes from wild-type mice and knockout to PrP^C were subjected to the induction of oxidative stress with hydrogen peroxide (H₂O₂) and with the toxic oligomer of the amyloid β protein (AβO). We observed that the presence of PrP^C showed resistance in the cell viability of astrocytes. It was also possible to monitor changes in basic levels of metals and associate them with an induced damage condition, indicating the precise role of PrP^C in metal homeostasis, where the absence of PrP^C leads to metallic unbalance, culminating in cellular vulnerability to oxidative stress. Increased caspase 3, p-Tau, p53, and Bcl2 may establish a relationship between a PrP^C and an induced damage condition. Complementarily, it has been shown that PrP^C prevents the internalization of AβO and promotes its degradation under oxidative stress induction, thus preventing protein aggregation in astrocytes. It was also observed that the presence of PrP^C can be related to translocating SOD1 to cell nuclei under oxidative stress, probably controlling DNA damage. The results of this study suggest that PrP^C acts against oxidative stress activating the cellular response and defense by displaying neuroprotection to neurons and ensuring the functionality of astrocytes.

An optimized Western blot assay provides a comprehensive assessment of the physiological endoproteolytic processing of the prion protein

The prion protein (PrP^C) is subjected to several conserved endoproteolytic events producing bioactive fragments that are of increasing interest for their physiological functions and their implication in the pathogenesis of prion diseases and other neurodegenerative diseases. However, systematic and comprehensive investigations on the full spectrum of PrP^C proteoforms have been hampered by the lack of methods able to identify all PrP^C-derived proteoforms. Building on previous knowledge of PrP^C endoproteolytic processing, we thus developed an optimized Western blot assay able to obtain the maximum information about PrP^C constitutive processing and the relative abundance of PrP^C proteoforms in a complex biological sample. This approach led to the concurrent identification of the whole spectrum of known endoproteolytic-derived PrP^C proteoforms in brain homogenates, including C-terminal, N-terminal and, most importantly, shed PrP^C-derived fragments. Endoproteolytic processing of PrP^C was remarkably similar in the brain of widely used wild type and transgenic rodent models, with α-cleavage-derived C1 representing the most abundant proteoform and ADAM10-mediated shedding being an unexpectedly prominent proteolytic event. Interestingly, the relative amount of shed PrP^C was higher in WT mice than in most other models. Our results indicate that constitutive endoproteolytic processing of PrP^C is not affected by PrP^C overexpression or host factors other than PrP^C but can be impacted by PrP^C primary structure. Finally, this method represents a crucial step in gaining insight into pathophysiological roles, biomarker suitability, and therapeutic potential of shed PrP^C and for a comprehensive appraisal of PrP^C proteoforms in therapies, drug screening, or in the progression of neurodegenerative diseases.

The Hidden Cell-to-Cell Trail of α-Synuclein Aggregates

The progressive accumulation of insoluble aggregates of the presynaptic protein alpha-synuclein (α-Syn) is a hallmark of neurodegenerative disorders including Parkinson's disease (PD), Multiple System Atrophy, and Dementia with Lewy Bodies, commonly referred to as synucleinopathies. Despite considerable progress on the structural biology of these aggregates, the molecular mechanisms mediating their cell-to-cell transmission, propagation, and neurotoxicity remain only partially understood. Numerous studies have highlighted the stereotypical spatiotemporal spreading of pathological α-Syn aggregates across different tissues and anatomically connected brain regions over time. Experimental evidence from various cellular and animal models indicate that α-Syn transfer occurs in two defined steps: the release of pathogenic α-Syn species from infected cells, and their uptake via passive or active endocytic pathways. Once α-Syn aggregates have been internalized, little is known about what drives their toxicity or how they interact with the endogenous protein to promote its misfolding and subsequent aggregation. Similarly, unknown genetic factors modulate different cellular responses to the aggregation and accumulation of pathogenic α-Syn species. Here we discuss the current understanding of the molecular phenomena associated with the intercellular spreading of pathogenic α-Syn seeds and summarize the evidence supporting the transmission hypothesis. Understanding the molecular mechanisms involved in α-Syn aggregates transmission is essential to develop novel targeted therapeutics against PD and related synucleinopathies.

Development of Alkaline Phosphatase-Fused Mouse Prion Protein and Its Application in Toxic Aβ Oligomer Detection

Amyloid β (Aβ) oligomers play a key role in the progression of Alzheimer's disease (AD). Multiple forms of Aβ assemblies have been identified by in vitro and in vivo analyses; however, it is uncertain which oligomer is highly neurotoxic. Thus, understanding the pathogenesis of AD by detecting toxic Aβ oligomers is crucial. In this study, we report a fusion protein of cellular prion protein (PrPc) and alkaline phosphatase (ALP) from Escherichia coli as a sensing element for toxic Aβ oligomers. Since the N-terminus domain of PrPc (residue 23-111) derived from mice is known to bind to toxic Aβ oligomers in vitro, we genetically fused PrPc_23-111 to ALP. The developed fusion protein, PrP-ALP, retained both the binding ability of PrPc and enzymatic activity of ALP. We showed that PrP-ALP strongly bound to high molecular weight (HMW) oligomers but showed little or no affinity toward monomers. The observation that PrP-ALP neutralized the toxic effect of Aβ oligomers indicated an interaction between PrP-ALP and toxic HMW oligomers. Based on ALP activity, we succeeded in detecting Aβ oligomers. PrP-ALP may serve as a powerful tool for detecting toxic Aβ oligomers that may be related to AD progression.

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.

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

Background

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

Results

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

Conclusion

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

Supplementary Information

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

The prion protein and its ligands: Insights into structure-function relationships

The prion protein is a multifunctional protein that exists in at least two different folding states. It is subject to diverse proteolytic processing steps that lead to prion protein fragments some of which are membrane-bound whereas others are soluble. A multitude of ligands bind to the prion protein and besides proteinaceous binding partners, interaction with metal ions and nucleic acids occurs. Although of great importance, information on structural and functional consequences of prion protein binding to its partners is limited. Here, we will reflect on the structure-function relationship of the prion protein and its binding partners considering the different folding states and prion protein fragments.

Reversal of synapse loss in Alzheimer mouse models by targeting mGluR5 to prevent synaptic tagging by C1Q

Microglia-mediated synaptic loss contributes to the development of cognitive impairments in Alzheimer's disease (AD). However, the basis for this immune-mediated attack on synapses remains to be elucidated. Treatment with the metabotropic glutamate receptor 5 (mGluR5) silent allosteric modulator (SAM), BMS-984923, prevents β-amyloid oligomer-induced aberrant synaptic signaling while preserving physiological glutamate response. Here, we show that oral BMS-984923 effectively occupies brain mGluR5 sites visualized by [¹⁸F]FPEB positron emission tomography (PET) at doses shown to be safe in rodents and nonhuman primates. In aged mouse models of AD (APPswe/PS1ΔE9 overexpressing transgenic and App^NL-G-F/hMapt double knock-in), SAM treatment fully restored synaptic density as measured by [¹⁸F]SynVesT-1 PET for SV2A and by histology, and the therapeutic benefit persisted after drug washout. Phospho-TAU accumulation in double knock-in mice was also reduced by SAM treatment. Single-nuclei transcriptomics demonstrated that SAM treatment in both models normalized expression patterns to a far greater extent in neurons than glia. Last, treatment prevented synaptic localization of the complement component C1Q and synaptic engulfment in AD mice. Thus, selective modulation of mGluR5 reversed neuronal gene expression changes to protect synapses from damage by microglial mediators in rodents.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

AN ENANTIOMERIC FRAGMENT PAIR (EFP) APPROACH FOR THE STUDY OF CELLULAR UPTAKE OF INTRINSICALLY DISORDERED PROTEINS

The study of intrinsically disordered and amyloidogenic proteins poses a major challenge to researchers: the propensity of the system to aggregate and to form amyloid fibrils and deposits . This intrinsic nature limits the way amyloids can be studied and increases the level of complexity of the techniques needed to study the system of interest. Recent reports suggest that cellular recognition and internalization of pre-fibrillary species of amyloidogenic peptides and proteins may initiate some of its toxic actions. Therefore, developing novels tools to facilitate the understanding and determination of the interactions between intrinsically disordered proteins and the cellular membrane is becoming increasingly valuable. Here, we present and propose an approach for the study of the interactions of intrinsically disordered proteins with the cellular surface based on the use of enantiomeric fragment pairs (EFPs). By following a stepwise methodology in which the amyloidogenic peptide or protein is fragmented into specific segments, we show how this approach can be exploited to differentiate between different types of cellular uptake, to determine the degree of receptor-mediated cellular internalization of intrinsically disordered peptides and proteins, and to pinpoint the specific regions within the amino acid sequence responsible for the cellular recognition. Adopting this approach overcomes aggregation-related challenges and offers a particularly well-suited platform for the elucidation of receptor-intermediated recognition, uptake, and toxicity.

New Drosophila models to uncover the intrinsic and extrinsic factors that mediate the toxicity of the human prion protein

Misfolding of the prion protein (PrP) is responsible for devastating neurological disorders in humans and other mammals. An unresolved problem in the field is unraveling the mechanisms governing PrP conformational dynamics, misfolding, and the cellular mechanism leading to neurodegeneration. The variable susceptibility of mammals to prion diseases is a natural resource that can be exploited to understand the conformational dynamics of PrP. Here we present a new fly model expressing human PrP with new, robust phenotypes in brain neurons and the eye. By using comparable attP2 insertions, we demonstrated the heightened toxicity of human PrP compared to rodent PrP along with a specific interaction with the amyloid-β peptide. By using this new model, we started to uncover the intrinsic (sequence/structure) and extrinsic (interactions) factors regulating PrP toxicity. We described PERK (officially known as EIF2AK3 in humans) and activating transcription factor 4 (ATF4) as key in the cellular mechanism mediating the toxicity of human PrP and uncover a key new protective activity for 4E-BP (officially known as Thor in Drosophila and EIF4EBP2 in humans), an ATF4 transcriptional target. Lastly, mutations in human PrP (N159D, D167S, N174S) showed partial protective activity, revealing its high propensity to misfold into toxic conformations.

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.

Prion Protein: The Molecule of Many Forms and Faces

Cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored protein most abundantly found in the outer membrane of neurons. Due to structural characteristics (a flexible tail and structured core), PrP^C interacts with a wide range of partners. Although PrP^C has been proposed to be involved in many physiological functions, only peripheral nerve myelination homeostasis has been confirmed as a bona fide function thus far. PrP^C misfolding causes prion diseases and PrP^C has been shown to mediate β-rich oligomer-induced neurotoxicity in Alzheimer’s and Parkinson’s disease as well as neuroprotection in ischemia. Upon proteolytic cleavage, PrP^C is transformed into released and attached forms of PrP that can, depending on the contained structural characteristics of PrP^C, display protective or toxic properties. In this review, we will outline prion protein and prion protein fragment properties as well as overview their involvement with interacting partners and signal pathways in myelination, neuroprotection and neurodegenerative diseases.

Recent Advances in the Application Peptide and Peptoid in Diagnosis Biomarkers of Alzheimer's Disease in Blood

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases with irreversible damage of the brain and a continuous pathophysiological process. Early detection and accurate diagnosis are essential for the early intervention of AD. Precise detection of blood biomarkers related to AD could provide a shortcut to identifying early-stage patients before symptoms. In recent years, targeting peptides or peptoids have been chosen as recognition elements in nano-sensors or fluorescence detection to increase the targeting specificity, while peptide-based probes were also developed considering their specific advantages. Peptide-based sensors and probes have been developed according to different strategies, such as natural receptors, high-throughput screening, or artificial design for AD detection. This review will briefly summarize the recent developments and trends of AD diagnosis platforms based on peptide and peptoid as recognition elements and provide insights into the application of peptide and peptoid with different sources and characteristics in the diagnosis of AD biomarkers.

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.

In Situ Assembly of Nanomaterials and Molecules for the Signal Enhancement of Electrochemical Biosensors

The physiochemical properties of nanomaterials have a close relationship with their status in solution. As a result of its better simplicity than that of pre-assembled aggregates, the in situ assembly of nanomaterials has been integrated into the design of electrochemical biosensors for the signal output and amplification. In this review, we highlight the significant progress in the in situ assembly of nanomaterials as the nanolabels for enhancing the performances of electrochemical biosensors. The works are discussed based on the difference in the interactions for the assembly of nanomaterials, including DNA hybridization, metal ion-ligand coordination, metal-thiol and boronate ester interactions, aptamer-target binding, electrostatic attraction, and streptavidin (SA)-biotin conjugate. We further expand the range of the assembly units from nanomaterials to small organic molecules and biomolecules, which endow the signal-amplified strategies with more potential applications.

β-Cleavage of the prion protein in the human eye: Implications for the spread of infectious prions and human ocular disorders

Recently, we reported β-cleavage of the prion protein (PrP^C) in human ocular tissues. Here, we explored whether this is unique to the human eye, and its functional implications. A comparison of the cleavage pattern of PrP^C in human ocular tissues with common nocturnal and diurnal animals revealed mainly β-cleavage in humans, and mostly full-length PrP^C in animal retinas. Soluble FL PrP^C and N-terminal fragment (N2) released from β-cleavage was observed in the aqueous and vitreous humor (AH & VH). Expression of human PrP^C in ARPE-19 cells, a human retinal pigmented epithelial cell line, also showed β-cleaved PrP^C. Surprisingly, β-cleavage was not altered by a variety of insults, including oxidative stress, suggesting a unique role of this cleavage in the human eye. It is likely that β-cleaved C- or N-terminal fragments of PrP^C protect from various insults unique to the human eye. On the contrary, β-cleaved C-terminus of PrP^C is susceptible to conversion to the pathological PrP-scrapie form, and includes the binding sites for β1-integrin and amyloid-β, molecules implicated in several ocular disorders. Considering the species and tissue-specific cleavage of PrP^C, our data suggest re-evaluation of prion infectivity and other ocular disorders of the human eye conducted in mouse models.

Therapeutic Strategies Targeting Amyloid-β Receptors and Transporters in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disease that has been recognized as one of the most intractable medical problems with heavy social and economic costs. Amyloid-β (Aβ) has been identified as a major factor that participates in AD progression through its neurotoxic effects. The major mechanism of Aβ-induced neurotoxicity is by interacting with membrane receptors and subsequent triggering of aberrant cellular signaling. Besides, Aβ transporters also plays an important role by affecting Aβ homeostasis. Thus, these Aβ receptors and transporters are potential targets for the development of AD therapies. Here, we summarize the reported therapeutic strategies targeting Aβ receptors and transporters to provide a molecular basis for future rational design of anti-AD agents.

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.

The amyloid-inhibiting NCAM-PrP peptide targets Aβ peptide aggregation in membrane-mimetic environments

Substantial research efforts have gone into elucidating the role of protein misfolding and self-assembly in the onset and progression of Alzheimer's disease (AD). Aggregation of the Amyloid-β (Aβ) peptide into insoluble fibrils is closely associated with AD. Here, we use biophysical techniques to study a peptide-based approach to target Aβ amyloid aggregation. A peptide construct, NCAM-PrP, consists of a largely hydrophobic signal sequence linked to a positively charged hexapeptide. The NCAM-PrP peptide inhibits Aβ amyloid formation by forming aggregates which are unavailable for further amyloid aggregation. In a membrane-mimetic environment, Aβ and NCAM-PrP form specific heterooligomeric complexes, which are of lower aggregation states compared to Aβ homooligomers. The Aβ:NCAM-PrP interaction appears to take place on different aggregation states depending on the absence or presence of a membrane-mimicking environment. These insights can be useful for the development of potential future therapeutic strategies targeting Aβ at several aggregation states.

The Cellular Prion Protein Increases the Uptake and Toxicity of TDP-43 Fibrils

Cytoplasmic aggregation of the primarily nuclear TAR DNA-binding protein 43 (TDP-43) affects neurons in most amyotrophic lateral sclerosis (ALS) and approximately half of frontotemporal lobar degeneration (FTLD) cases. The cellular prion protein, PrP^C, has been recognized as a common receptor and downstream effector of circulating neurotoxic species of several proteins involved in neurodegeneration. Here, capitalizing on our recently adapted TDP-43 real time quaking induced reaction, we set reproducible protocols to obtain standardized preparations of recombinant TDP-43 fibrils. We then exploited two different cellular systems (human SH-SY5Y and mouse N2a neuroblastoma cells) engineered to express low or high PrP^C levels to investigate the link between PrP^C expression on the cell surface and the internalization of TDP-43 fibrils. Fibril uptake was increased in cells overexpressing either human or mouse prion protein. Increased internalization was associated with detrimental consequences in all PrP-overexpressing cell lines but was milder in cells expressing the human form of the prion protein. As described for other amyloids, treatment with TDP-43 fibrils induced a reduction in the accumulation of the misfolded form of PrP^C, PrP^Sc, in cells chronically infected with prions. Our results expand the list of misfolded proteins whose uptake and detrimental effects are mediated by PrP^C, which encompass almost all pathological amyloids involved in neurodegeneration.

Biological Functions of the Intrinsically Disordered N-Terminal Domain of the Prion Protein: A Possible Role of Liquid-Liquid Phase Separation

The mammalian prion protein (PrPC) is composed of a large intrinsically disordered N-terminal and a structured C-terminal domain, containing three alpha-helical regions and a short, two-stranded beta-sheet. Traditionally, the activity of a protein was linked to the ability of the polypeptide chain to adopt a stable secondary/tertiary structure. This concept has been extended when it became evident that intrinsically disordered domains (IDDs) can participate in a broad range of defined physiological activities and play a major functional role in several protein classes including transcription factors, scaffold proteins, and signaling molecules. This ability of IDDs to engage in a variety of supramolecular complexes may explain the large number of PrPC-interacting proteins described. Here, we summarize diverse physiological and pathophysiological activities that have been described for the unstructured N-terminal domain of PrPC. In particular, we focus on subdomains that have been conserved in evolution.

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.

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 N-terminal domain of the prion protein is required and sufficient for liquid-liquid phase separation: A crucial role of the Aβ-binding domain

Formation of biomolecular condensates through liquid–liquid phase separation (LLPS) has been described for several pathogenic proteins linked to neurodegenerative diseases and is discussed as an early step in the formation of protein aggregates with neurotoxic properties. In prion diseases, neurodegeneration and formation of infectious prions is caused by aberrant folding of the cellular prion protein (PrP^C). PrP^C is characterized by a large intrinsically disordered N-terminal domain and a structured C-terminal globular domain. A significant fraction of mature PrP^C is proteolytically processed in vivo into an entirely unstructured fragment, designated N1, and the corresponding C-terminal fragment C1 harboring the globular domain. Notably, N1 contains a polybasic motif that serves as a binding site for neurotoxic Aβ oligomers. PrP can undergo LLPS; however, nothing is known how phase separation of PrP is triggered on a molecular scale. Here, we show that the intrinsically disordered N1 domain is necessary and sufficient for LLPS of PrP. Similar to full-length PrP, the N1 fragment formed highly dynamic liquid-like droplets. Remarkably, a slightly shorter unstructured fragment, designated N2, which lacks the Aβ-binding domain and is generated under stress conditions, failed to form liquid-like droplets and instead formed amorphous assemblies of irregular structures. Through a mutational analysis, we identified three positively charged lysines in the postoctarepeat region as essential drivers of condensate formation, presumably largely via cation–π interactions. These findings provide insights into the molecular basis of LLPS of the mammalian prion protein and reveal a crucial role of the Aβ-binding domain in this process.

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.

Accumulation of cellular prion protein within β-amyloid oligomer plaques in aged human brains

Alzheimer’s disease (AD) is the main cause of dementia, and β‐amyloid (Aβ) is a central factor in the initiation and progression of the disease. Different forms of Aβ have been identified as monomers, oligomers, and amyloid fibrils. Many proteins have been implicated as putative receptors of respective forms of Aβ. Distinct forms of Aβ oligomers are considered to be neurotoxic species that trigger the pathophysiology of AD. It was reported that cellular prion protein (PrP^C) is one of the most selective and high‐affinity binding partners of Aβ oligomers. The interaction of Aβ oligomers with PrP^C is important to synaptic dysfunction and loss. The binding of Aβ oligomers to PrP^C has mostly been studied with synthetic peptides, cell culture, and murine models of AD by biochemical and biological methods. However, the molecular mechanisms underlying the relationship between Aβ oligomers and PrP^C remain unclear, especially in the human brain. We immunohistochemically investigated the relationship between Aβ oligomers and PrP^C in human brain tissue with and without amyloid pathology. We histologically demonstrate that PrP^C accumulates with aging in human brain tissue even prior to AD mainly within diffuse‐type amyloid plaques, which are composed of more soluble Aβ oligomers without stacked β‐sheet fibril structures. Our results suggest that PrP^C accumulating plaques are associated with more soluble Aβ oligomers, and appear even prior to AD. The investigation of PrP^C accumulating plaques may provide new insights into AD.

Binding of Amyloid β(1-42)-Calmodulin Complexes to Plasma Membrane Lipid Rafts in Cerebellar Granule Neurons Alters Resting Cytosolic Calcium Homeostasis

Lipid rafts are a primary target in studies of amyloid β (Aβ) cytotoxicity in neurons. Exogenous Aβ peptides bind to lipid rafts, which in turn play a key role in Aβ uptake, leading to the formation of neurotoxic intracellular Aβ aggregates. On the other hand, dysregulation of intracellular calcium homeostasis in neurons has been observed in Alzheimer's disease (AD). In a previous work, we showed that Aβ(1-42), the prevalent Aβ peptide found in the amyloid plaques of AD patients, binds with high affinity to purified calmodulin (CaM), with a dissociation constant ≈1 nM. In this work, to experimentally assess the Aβ(1-42) binding capacity to intracellular CaM, we used primary cultures of mature cerebellar granule neurons (CGN) as a neuronal model. Our results showed a large complexation of submicromolar concentrations of Aβ(1-42) dimers by CaM in CGN, up to 120 ± 13 picomoles of Aβ(1-42) /2.5 × 106 cells. Using fluorescence microscopy imaging, we showed an extensive co-localization of CaM and Aβ(1-42) in lipid rafts in CGN stained with up to 100 picomoles of Aβ(1-42)-HiLyteTM-Fluor555 monomers. Intracellular Aβ(1-42) concentration in this range was achieved by 2 h incubation of CGN with 2 μM Aβ(1-42), and this treatment lowered the resting cytosolic calcium of mature CGN in partially depolarizing 25 mM potassium medium. We conclude that the primary cause of the resting cytosolic calcium decrease is the inhibition of L-type calcium channels of CGN by Aβ(1-42) dimers, whose activity is inhibited by CaM:Aβ(1-42) complexes bound to lipid rafts.

Epilepsy and prion diseases: A narrative review

Epileptic seizures have been described as one feature of prion diseases, but are an unusual clinical presentation. The aim of this narrative Review was to summarize current knowledge of epileptic seizures in the various forms of prion diseases, from a clinical perspective. Examination of the published literature identified no systematic studies; the evidence base is largely anecdotal, consisting mainly of case studies and small case series. Hence, uncertainty prevails as to seizure frequency, semiology, treatment, and pathogenesis in prion diseases. Seizures probably occur in around 10% of sporadic cases but less frequently in iatrogenic and familial forms, with the possible exception of the E200K mutation. The literature suggests a predominance of focal motor and nonconvulsive status epilepticus. Electroencephalographic accompaniments include periodic lateralized or generalized periodic epileptiform discharges (PLEDs, GPEDs), sometimes predating the more typical periodic sharp wave complexes. There are no convincing accounts of successful antiepileptic drug therapy. The underlying mechanisms of epileptogenesis in prion diseases may include loss of cellular prion protein function (PrP^c) and aggregation of abnormally folded prion protein (PrP^Sc). The need for systematic studies and clinical trials to expand the evidence base surrounding epilepsy and prion diseases is evident.

The Dynamism of Intrinsically Disordered Proteins: Binding-Induced Folding, Amyloid Formation, and Phase Separation

Intrinsically disordered proteins (IDPs) or natively unfolded proteins do not undergo autonomous folding into a well-defined 3-D structure and challenge the conventional structure-function paradigm. They are involved in a multitude of critical physiological functions by adopting various structural states via order-to-disorder transitions or by maintaining their disordered characteristics in functional complexes. In recent times, there has been a burgeoning interest in the investigation of intriguing behavior of IDPs using highly multidisciplinary and complementary approaches due to the pivotal role of this unique class of protein chameleons in physiology and disease. Over the past decade or so, our laboratory has been actively investigating the unique physicochemical properties of this class of highly dynamic, flexible, rapidly interconverting proteins. We have utilized a diverse array of existing and emerging tools involving steady-state and time-resolved fluorescence, Raman spectroscopy, circular dichroism, light scattering, fluorescence microscopy, and atomic force microscopy coupled with site-directed mutagenesis and other biochemical and biophysical tools to study a variety of interesting and important aspects of IDPs. In this Feature Article, I describe our work on the conformational characteristics, solvation dynamics, binding-induced folding, amyloid formation, and liquid-liquid phase separation of a number of amyloidogenic IDPs. A series of these studies described here captures the role of conformational plasticity and dynamics in directing binding, folding, assembly, aggregation, and phase transitions implicated in physiology and pathology.

Rodent Models of Amyloid-Beta Feature of Alzheimer's Disease: Development and Potential Treatment Implications

Alzheimer's disease (AD) is the most common neurodegenerative disorder worldwide and causes severe financial and social burdens. Despite much research on the pathogenesis of AD, the neuropathological mechanisms remain obscure and current treatments have proven ineffective. In the past decades, transgenic rodent models have been used to try to unravel this disease, which is crucial for early diagnosis and the assessment of disease-modifying compounds. In this review, we focus on transgenic rodent models used to study amyloid-beta pathology in AD. We also discuss their possible use as promising tools for AD research. There is still no effective treatment for AD and the development of potent therapeutics are urgently needed. Many molecular pathways are susceptible to AD, ranging from neuroinflammation, immune response, and neuroplasticity to neurotrophic factors. Studying these pathways may shed light on AD pathophysiology as well as provide potential targets for the development of more effective treatments. This review discusses the advantages and limitations of these models and their potential therapeutic implications for AD.