Getting a grip on prions: oligomers, amyloids, and pathological membrane interactions

Slow Misfolding of a Molten Globule form of a Mutant Prion Protein Variant into a β-rich Dimer

Misfolding of the prion protein is linked to multiple neurodegenerative diseases. A better understanding of the process requires the identification and structural characterization of intermediate conformations via which misfolding proceeds. In this study, three conserved aromatic residues (Tyr168, Phe174, and Tyr217) located in the C-terminal domain of mouse PrP (wt moPrP) were mutated to Ala. The resultant mutant protein, 3A moPrP, is shown to adopt a molten globule (MG)-like native conformation. Hydrogen-deuterium exchange studies coupled with mass spectrometry revealed that for 3A moPrP, the free energy gap between the MG-like native conformation and misfolding-prone partially unfolded forms is reduced. Consequently, 3A moPrP misfolds in native conditions even in the absence of salt, unlike wt moPrP, which requires the addition of salt to misfold. 3A moPrP misfolds to a β-rich dimer in the absence of salt, which can rapidly form an oligomer upon the addition of salt. In the presence of salt, 3A moPrP misfolds to a β-rich oligomer about a thousand-fold faster than wt moPrP. Importantly, the misfolded structure of the dimer is similar to that of the salt-induced oligomer. Misfolding to oligomer seems to be induced at the level of the dimeric unit by monomer-monomer association, and the oligomer grows by accretion of misfolded dimeric units. Additionally, it is shown that the conserved aromatic residues collectively stabilize not only monomeric protein, but also the structural core of the β-rich oligomers. Finally, it is also shown that 3A moPrP misfolds much faster to amyloid-fibrils than does the wt protein.

Misfolded protein oligomers: mechanisms of formation, cytotoxic effects, and pharmacological approaches against protein misfolding diseases

The conversion of native peptides and proteins into amyloid aggregates is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Increasing evidence implicates misfolded protein oligomers produced during the amyloid formation process as the primary cytotoxic agents in many of these devastating conditions. In this review, we analyze the processes by which oligomers are formed, their structures, physicochemical properties, population dynamics, and the mechanisms of their cytotoxicity. We then focus on drug discovery strategies that target the formation of oligomers and their ability to disrupt cell physiology and trigger degenerative processes.

Roles of Apigenin and Nepetin in the Assembly Behavior and Cytotoxicity of Prion Neuropeptide PrP106-126

Development of potential inhibitors to prevent prion protein (PrP) fibrillation is a therapeutic strategy for prion diseases. The prion neuropeptide PrP106-126, a research model of abnormal PrP (PrPSc), presents similar physicochemical and biochemical characters to PrPSc, which is also a target of potential inhibitors against prion deposition. Many flavones have antioxidant, anti-inflammatory, and antibacterial properties, and they are applied in treating prion disorder and other amyloidosis as well. However, the inhibition mechanism of flavones on PrP106-126 fibrillation is still unclear. In the current work, apigenin and nepetin were used to suppress the aggregation of PrP106-126 and to alleviate the peptide-induced cytotoxicity. The results showed that apigenin and nepetin impeded the fibril formation of PrP106-126 and depolymerized the preformed fibrils. They were bound to PrP106-126 predominantly by hydrophobic and hydrogen bonding interactions. In addition, both flavones upregulated cell viability and decreased membrane leakage through reducing peptide oligomerization. The differences in inhibition and cell protection between the two small molecules were presumably attributed to the substitution of hydroxyl and methoxy groups in nepetin, which demonstrated the significant structure-function relationship of flavones with prion neuropeptide and the prospect of flavonoids as drug candidates against prion diseases.

Protein-lipid interactions and protein anchoring modulate the modes of association of the globular domain of the Prion protein and Doppel protein to model membrane patches

The Prion protein is the molecular hallmark of the incurable prion diseases affecting mammals, including humans. The protein-only hypothesis states that the misfolding, accumulation, and deposition of the Prion protein play a critical role in toxicity. The cellular Prion protein (PrPC) anchors to the extracellular leaflet of the plasma membrane and prefers cholesterol- and sphingomyelin-rich membrane domains. Conformational Prion protein conversion into the pathological isoform happens on the cell surface. In vitro and in vivo experiments indicate that Prion protein misfolding, aggregation, and toxicity are sensitive to the lipid composition of plasma membranes and vesicles. A picture of the underlying biophysical driving forces that explain the effect of Prion protein - lipid interactions in physiological conditions is needed to develop a structural model of Prion protein conformational conversion. To this end, we use molecular dynamics simulations that mimic the interactions between the globular domain of PrPC anchored to model membrane patches. In addition, we also simulate the Doppel protein anchored to such membrane patches. The Doppel protein is the closest in the phylogenetic tree to PrPC, localizes in an extracellular milieu similar to that of PrPC, and exhibits a similar topology to PrPC even if the amino acid sequence is only 25% identical. Our simulations show that specific protein-lipid interactions and conformational constraints imposed by GPI anchoring together favor specific binding sites in globular PrPC but not in Doppel. Interestingly, the binding sites we found in PrPC correspond to prion protein loops, which are critical in aggregation and prion disease transmission barrier (β2-α2 loop) and in initial spontaneous misfolding (α2-α3 loop). We also found that the membrane re-arranges locally to accommodate protein residues inserted in the membrane surface as a response to protein binding.

Channel Activities of the Full-Length Prion and Truncated Proteins

Prion diseases are fatal neurodegenerative disorders characterized by the conversion of the cellular prion protein (PrPC) into a misfolded prion form, which is believed to disrupt the cellular membranes. However, the exact mechanisms underlying prion toxicity, including the formation of membrane pores, are not fully understood. The prion protein consists of two domains: a globular domain (GD) and a flexible N-terminus (FT) domain. Although a proximal polybasic amino acid (FT(23-31) sequence of FT is a prerequisite for cellular membrane permeabilization, other functional domain regions may modulate its effects. Through single-channel electrical recordings and cryo-electron microscopy (cryo-EM), we discovered that the FT(23-50) fragment forms pore-shaped oligomers and plays a dominant role in membrane permeabilization within the full-length mouse prion protein (mPrP(23-230)). In contrast, the FT(51-110) domain or the C-terminal domain downregulate the channel activity of FT(23-50) and mPrP(23-230). The addition of prion mimetic antibody, POM1 significantly amplifies mPrP(23-230) membrane permeabilization, whereas POM1_Y104A, a mutant that binds to PrP but cannot elicit toxicity, has a negligible effect on membrane permeabilization. Additionally, the anti-N-terminal antibody POM2 or Cu2+ binds to the FT domain, subsequently enhancing the FT(23-110) channel activity. Importantly, our setup provides a novel approach without an external fused protein to examine the channel activity of truncated PrP in the lipid membranes. We therefore propose that the primary N-terminal residues are essential for membrane permeabilization, while other functional segments of PrP play a vital role in modulating the pathological effects of PrP-mediated neurotoxicity.

Mutations of evolutionarily conserved aromatic residues suggest that misfolding of the mouse prion protein may commence in multiple ways

The misfolding of the mammalian prion protein from its α-helix rich cellular isoform to its β-sheet rich infectious isoform is associated with several neurodegenerative diseases. The determination of the structural mechanism by which misfolding commences, still remains an unsolved problem. In the current study, native-state hydrogen exchange coupled with mass spectrometry has revealed that the N state of the mouse prion protein (moPrP) at pH 4 is in dynamic equilibrium with multiple partially unfolded forms (PUFs) capable of initiating misfolding. Mutation of three evolutionarily conserved aromatic residues, Tyr168, Phe174, and Tyr217 present at the interface of the β2-α2 loop and the C-terminal end of α3 in the structured C-terminal domain of moPrP significantly destabilize the native state (N) of the protein. They also reduce the free energy differences between the N state and two PUFs identified as PUF1 and PUF2**. It is shown that PUF2** in which the β2-α2 loop and the C-terminal end of α3 are disordered, has the same stability as the previously identified PUF2*, but to have a very different structure. Misfolding can commence from both PUF1 and PUF2**, as it can from PUF2*. Hence, misfolding can commence and proceed in multiple ways from structurally distinct precursor conformations. The increased extents to which PUF1 and PUF2** are populated at equilibrium in the case of the mutant variants, greatly accelerate their misfolding. The results suggest that the three aromatic residues may have been evolutionarily selected to impede the misfolding of moPrP.

Copper drives prion protein phase separation and modulates aggregation

Prion diseases are characterized by prion protein (PrP) transmissible aggregation and neurodegeneration, which has been linked to oxidative stress. The physiological function of PrP seems related to sequestering of redox-active Cu2+, and Cu2+ dyshomeostasis is observed in prion disease brain. It is unclear whether Cu2+ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. This study indicates that Cu2+ promotes PrP condensation in live cells at the cell surface and in vitro through copartitioning. Molecularly, Cu2+ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H2O2, triggered liquid-to-solid transition of PrP:Cu2+ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrPC initially protected against Cu2+ cytotoxicity but led to PrPC aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevents copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.

CD11c is not required by microglia to convey neuroprotection after prion infection

Prion diseases are caused by the misfolding of a normal host protein that leads to gliosis, neuroinflammation, neurodegeneration, and death. Microglia have been shown to be critical for neuroprotection during prion infection of the central nervous system (CNS), and their presence extends survival in mice. How microglia impart these benefits to the infected host are unknown. Previous transcriptomics and bioinformatics studies suggested that signaling through the heterodimeric integrin receptor CD11c/CD18, expressed by microglia in the brain, might be important to microglial function during prion disease. Herein, we intracerebrally challenged CD11c-/- mice with prion strain RML and compared them to similarly infected C57BL/6 mice as controls. We initially assessed changes in the brain that are associated with disease such as astrogliosis, microgliosis, prion accumulation, and survival. Targeted qRT-PCR arrays were used to determine alterations in transcription in mice in response to prion infection. We demonstrate that expression of Itgax (CD11c) and Itgb2 (CD18) increases in the CNS in correlation with advancing prion infection. Gliosis, neuropathology, prion deposition, and disease progression in prion infected CD11c deficient mice were comparable to infected C57BL/6 mice. Additionally, both CD11c deficient and C57BL/6 prion-infected mouse cohorts had a similar consortium of inflammatory- and phagocytosis-associated genes that increased as disease progressed to clinical stages. Ingenuity Pathway Analysis of upregulated genes in infected C57BL/6 mice suggested numerous cell-surface transmembrane receptors signal through Spleen Tyrosine Kinase, a potential key regulator of phagocytosis and innate immune activation in the prion infected brain. Ultimately, the deletion of CD11c did not influence prion pathogenesis in mice and CD11c signaling is not involved in the neuroprotection provided by microglia, but our analysis identified a conspicuous phagocytosis pathway in the CNS of infected mice that appeared to be activated during prion pathogenesis.

(Dys)functional insights into nucleic acids and RNA-binding proteins modulation of the prion protein and α-synuclein phase separation

Prion diseases are prototype of infectious diseases transmitted by a protein, the prion protein (PrP), and are still not understandable at the molecular level. Heterogenous species of aggregated PrP can be generated from its monomer. α-synuclein (αSyn), related to Parkinson's disease, has also shown a prion-like pathogenic character, and likewise PrP interacts with nucleic acids (NAs), which in turn modulate their aggregation. Recently, our group and others have characterized that NAs and/or RNA-binding proteins (RBPs) modulate recombinant PrP and/or αSyn condensates formation, and uncontrolled condensation might precede pathological aggregation. Tackling abnormal phase separation of neurodegenerative disease-related proteins has been proposed as a promising therapeutic target. Therefore, understanding the mechanism by which polyanions, like NAs, modulate phase transitions intracellularly, is key to assess their role on toxicity promotion and neuronal death. Herein we discuss data on the nucleic acids binding properties and phase separation ability of PrP and αSyn with a special focus on their modulation by NAs and RBPs. Furthermore, we provide insights into condensation of PrP and/or αSyn in the light of non-trivial subcellular locations such as the nuclear and cytosolic environments.

Transthyretin Stabilizers and Seeding Inhibitors as Therapies for Amyloid Transthyretin Cardiomyopathy

Transthyretin (TTR) amyloid cardiomyopathy (ATTR-CM) is a progressive and increasingly recognized cause of heart failure which is associated with high mortality and morbidity. ATTR-CM is characterized by the misfolding of TTR monomers and their deposition within the myocardium as amyloid fibrils. The standard of care for ATTR-CM consists of TTR-stabilizing ligands, such as tafamidis, which aim at maintaining the native structure of TTR tetramers, thus preventing amyloid aggregation. However, their efficacy in advanced-staged disease and after long-term treatment is still a source of concern, suggesting the existence of other pathogenetic factors. Indeed, pre-formed fibrils present in the tissue can further accelerate amyloid aggregation in a self-propagating process known as “amyloid seeding”. The inhibition of amyloidogenesis through TTR stabilizers combined with anti-seeding peptides may represent a novel strategy with additional benefits over current therapies. Finally, the role of stabilizing ligands needs to be reassessed in view of the promising results derived from trials which have evaluated alternative strategies, such as TTR silencers and immunological amyloid disruptors.

Development of an Automated Capillary Immunoassay to Detect Prion Glycotypes in Creutzfeldt-Jakob Disease

Creutzfeldt-Jakob disease (CJD) comprises a group of transmissible neurodegenerative diseases with vast phenotypic diversity. Sporadic CJD heterogeneity is predominantly influenced by the genotype at codon 129 of the prion-encoding gene and the molecular weight of PrP^Sc fragments after protease digestion, resulting in a classification of 6 subtypes of CJD (MM1, MM2, MV1, MV2, VV1, and VV2). The majority of cases with CJD can be distinguished using this classification system. However, a number of reported CJD cases are phenotypically unique from others within their same subtype, such as variably protease-sensitive prionopathies, or exist as a mixture of subtypes within the same patient. Western blotting of brain tissue, along with the genotyping of codon 129 of the prion-encoding gene, is considered the "gold standard" for the biochemical characterization of CJD. Western blotting requires a significant amount of prion protein for detection, is labor-intensive, and is also associated with high interassay variability. In addition to these limitations, a growing body of research suggests that unique subtypes of CJD are often undetected or misdiagnosed using standard diagnostic western blotting protocols. Consequently, we successfully optimized and developed a capillary-based western assay using the JESS Simple Western (ProteinSimple) to detect and characterize prion proteins from patients with CJD. We found that this novel assay consistently differentiated CJD type 1 and type 2 cases with a limit of detection 10 to 100× higher than traditional western blotting. Cases with CJD in which type 1 and type 2 coexist within the same brain region can be detected using type 1-specific and type 2-specific antibodies, and we found that there was remarkable specificity for the detection of cases with variably protease-sensitive prionopathy. The assay presented displays outstanding sensitivity, allowing for the preservation of valuable samples and enhancing current detection methods.

The Prion Basis of Progressive Neurodegenerative Disorders

The discovery of proteinaceous infectious agents by Prusiner in 1982 was sensational. All previously known pathogens contained nucleic acids, the code of life, that enabled them to reproduce. In contrast, the proteinaceous agents of disease, called prion proteins (PrP), lacked nucleic acids and propagated by binding to the functional, endogenous form of cellular prion protein (referred to as PrP^C) and altering its conformation to produce the infectious disease-causing misfolded protein (referred to as PrP^Sc). The accumulation and aggregation of these infectious prion proteins within the brain cause destruction of neural tissue and lead to fatal spongiform encephalopathies. In this review, we present the molecular pathology of prion-based diseases. These insights are of particular importance since the principles of prion pathogenesis apply to other neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Collectively, the global prevalence of these diseases is rapidly increasing while effective therapies against them are still lacking. Thus, the need to understand their etiology and pathogenesis is urgent, and it holds profound implications for societal health.

Preparation and Fractionation of Heterogeneous Aβ42 Oligomers with Different Aggregation Properties

Deposition of amyloid-β (Aβ) aggregates in the form of amyloid plaques is a central feature of Alzheimer's disease. The end products of Aβ aggregation are amyloid fibrils. Soluble Aβ aggregates called oligomers are also formed either on or off the pathway of fibril formation. The amyloid fibrils from different clinical subtypes of Alzheimer's disease have been found to adopt different structures, a phenomenon called fibril polymorphism. Meanwhile, different types of Aβ oligomers have also been found. Recently, it has been shown that different types of Aβ42 oligomers may form fibrils of different structures, linking oligomer heterogeneity to fibril polymorphism. In this chapter, we describe methods to prepare heterogeneous Aβ42 oligomers and to quantify the concentration of these oligomers at a low micromolar range using a fluorescamine method. Fractionation of these oligomers by size using ultrafiltration filters allows for the formation of Aβ42 fibrils with different structural properties.

Extracellular vesicles: A new paradigm in understanding, diagnosing and treating neurodegenerative disease

Neurodegenerative disorders (NDs) are becoming one of the leading causes of disability and death across the globe due to lack of timely preventions and treatments. Concurrently, intensive research efforts are being carried out to understand the etiology of these age-dependent disorders. Extracellular vesicles (EVs)-biological nanoparticles released by cells-are gaining tremendous attention in understanding their role in pathogenesis and progression of NDs. EVs have been found to transmit pathogenic proteins of NDs between neurons. Moreover, the ability of EVs to exquisitely surmount natural biological barriers, including blood-brain barrier and in vivo safety has generated interest in exploring them as potential biomarkers and function as natural delivery vehicles of drugs to the central nervous system. However, limited knowledge of EV biogenesis, their heterogeneity and lack of adequate isolation and analysis tools have hampered their therapeutic potential. In this review, we cover the recent advances in understanding the role of EVs in neurodegeneration and address their role as biomarkers and delivery vehicles to the brain.

Microglia have limited influence on early prion pathogenesis, clearance, or replication

Microglia (MG) are critical to host defense during prion infection, but the mechanism(s) of this neuroprotection are poorly understood. To better examine the influence of MG during prion infection, we reduced MG in the brains of C57BL/10 mice using PLX5622 and assessed prion clearance and replication using multiple approaches that included bioassay, immunohistochemistry, and Real-Time Quaking Inducted Conversion (RT-QuIC). We also utilized a strategy of intermittent PLX5622 treatments to reduce MG and allow MG repopulation to test whether new MG could alter prion disease progress. Lastly, we investigated the influence of MG using tga20 mice, a rapid prion model that accumulates fewer pathological features and less PrPres in the infected brain. In C57BL/10 mice we found that MG were excluded from the inoculation site early after infection, but Iba1 positive infiltrating monocytes/macrophage were present. Reducing MG in the brain prior to prion inoculation did not increase susceptibility to prion infection. Short intermittent treatments with PLX5622 in prion infected C57BL/10 mice after 80 dpi were unsuccessful at altering the MG population, gliosis, or survival. Additionally, MG depletion using PLX5622 in tga20 mice had only a minor impact on prion pathogenesis, indicating that the presence of MG might be less important in this fast model with less prion accumulation. In contrast to the benefits of MG against prion disease in late stages of disease, our current experiments suggest MG do not play a role in early prion pathogenesis, clearance, or replication.

Populations of Tau Conformers Drive Prion-like Strain Effects in Alzheimer's Disease and Related Dementias

Recent findings of diverse populations of prion-like conformers of misfolded tau protein expand the prion concept to Alzheimer's disease (AD) and monogenic frontotemporal lobar degeneration (FTLD)-MAPT P301L, and suggest that distinct strains of misfolded proteins drive the phenotypes and progression rates in many neurodegenerative diseases. Notable progress in the previous decades has generated many lines of proof arguing that yeast, fungal, and mammalian prions determine heritable as well as infectious traits. The extraordinary phenotypic diversity of human prion diseases arises from structurally distinct prion strains that target, at different progression speeds, variable brain structures and cells. Although human prion research presents beneficial lessons and methods to study the mechanism of strain diversity of protein-only pathogens, the fundamental molecular mechanism by which tau conformers are formed and replicate in diverse tauopathies is still poorly understood. In this review, we summarize up to date advances in identification of diverse tau conformers through biophysical and cellular experimental paradigms, and the impact of heterogeneity of pathological tau strains on personalized structure- and strain-specific therapeutic approaches in major tauopathies.

Structural biology of ex vivo mammalian prions

The structures of prion protein (PrP)-based mammalian prions have long been elusive. However, cryo-EM has begun to reveal the near-atomic resolution structures of fully infectious ex vivo mammalian prion fibrils as well as relatively innocuous synthetic PrP amyloids. Comparisons of these various types of PrP fibrils are now providing initial clues to structural features that correlate with pathogenicity. As first indicated by electron paramagnetic resonance and solid-state NMR studies of synthetic amyloids, all sufficiently resolved PrP fibrils of any sort (n > 10) have parallel in-register intermolecular β-stack architectures. Cryo-EM has shown that infectious brain-derived prion fibrils of the rodent-adapted 263K and RML scrapie strains have much larger ordered cores than the synthetic fibrils. These bona fide prion strains share major structural motifs, but the conformational details and the overall shape of the fibril cross sections differ markedly. Such motif variations, as well as differences in sequence within the ordered polypeptide cores, likely contribute to strain-dependent templating. When present, N-linked glycans and glycophosphatidylinositol (GPI) anchors project outward from the fibril surface. For the mouse RML strain, these posttranslational modifications have little effect on the core structure. In the GPI-anchored prion structures, a linear array of GPI anchors along the twisting fibril axis appears likely to bind membranes in vivo, and as such, may account for pathognomonic membrane distortions seen in prion diseases. In this review, we focus on these infectious prion structures and their implications regarding prion replication mechanisms, strains, transmission barriers, and molecular pathogenesis.

Quercetin binding accelerates prion fibrillation into proteinase sensitive and loosely structured amyloids

Amyloidoses are caused by the deposition of amyloid fibrils ascribed to protein misfolding. In this study, we examined the antiamyloidogenic and antioxidative activities of quercetin, a plant flavonol from the flavonoid group of polyphenols, on mouse prion protein (moPrP) with biophysical approaches. As the results show, quercetin binds to the C-terminal region of moPrP, and quercetin binding does not affect the structure of moPrP. However, quercetin binding accelerates moPrP fibrillation and changes the structure of moPrP fibrils. Unlike typical prion fibrils, quercetin-bound fibrils are sensitive to proteinase K and are loosely structured. Moreover, due to high antioxidant activity of flavonoid, quercetin-bound fibrils lack imbalance of free radicals and, therefore, they are nontoxic towards neuroblastoma cells. The quercetin shows its uniqueness from typical antiamyloidogenic drugs which either suppress the development of amyloid or eliminate formed amyloids. Quercetin binding converts moPrP into protease-sensitive and non-cytotoxic fibrils. This work provides a powerful resolution in the advancement of antiamyloidogenic treatment.

Aptamers Targeting Hallmark Proteins of Neurodegeneration

Neurodegeneration is a progressive deterioration of neural structures leading to cognitive or motor impairment of the affected patient. There is still no effective therapy for any of the most common neurodegenerative diseases (NDs) such as Alzheimer's or Parkinson's disease. Although NDs exhibit distinct clinical characteristics, many are characterized by the accumulation of misfolded proteins or peptide fragments in the brain and/or spinal cord. The presence of similar inclusion bodies in patients with diverse NDs provides a rationale for developing therapies directed at overlapping disease mechanisms. A novel targeting strategy involves the use of aptamers for therapeutic development. Aptamers are short nucleic acid ligands able to recognize molecular targets with high specificity and high affinity. Despite the fact that several academic groups have shown that aptamers have the potential to be used in therapeutic and diagnostic applications, their clinical translation is still limited. In this study, we describe aptamers that have been developed against proteins relevant to NDs, including prion protein and amyloid beta (Aβ), cell surface receptors and other cytoplasmic proteins. This review also describes advances in the application of these aptamers in imaging, protein detection, and protein quantification, and it provides insights about their accelerated clinical use for disease diagnosis and therapy.

Amyloid seeding as a disease mechanism and treatment target in transthyretin cardiac amyloidosis

Transthyretin (TTR) is a tetrameric transport protein mainly synthesized by the liver and choroid plexus. ATTR amyloidosis is characterized by the misfolding of TTR monomers and their accumulation within tissues as amyloid fibres. Current therapeutic options rely on the blockade of TTR production, TTR stabilization to maintain the native structure of TTR, amyloid degradation, or induction of amyloid removal from tissues. “Amyloid seeds” are defined as small fibril fragments that induce amyloid precursors to assume a structure rich in β-sheets, thus promoting fibrillogenesis. Amyloid seeds are important to promote the amplification and spread of amyloid deposits. Further studies are needed to better understand the molecular structure of ATTR seeds (i.e. the characteristics of the most amyloidogenic species), and the conditions that promote the formation and multiplication of seeds in vivo. The pathological cascade may begin months to years before symptom onset, suggesting that seeds in tissues might potentially be used as biomarkers for the early disease stages. Inhibition of amyloid aggregation by anti-seeding peptides may represent a disease mechanism and treatment target in ATTR amyloidosis, with an additional benefit over current therapies.

Fibrinogen Mitigates Prion-Mediated Platelet Activation and Neuronal Cell Toxicity

Prion peptide (PrP) misfolds to infectious scrapie isoform, the β pleat-rich insoluble fibrils responsible for neurodegeneration and fatal conformational diseases in humans. The amino acid sequence 106–126 from prion proteins, PrP(106–126), is highly amyloidogenic and implicated in prion-induced pathologies. Here, we report a novel interaction between PrP(106–126) and the thrombogenic plasma protein fibrinogen that can lead to mitigation of prion-mediated pro-thrombotic responses in human platelets as well as significant decline in neuronal toxicity. Thus, prior exposure to fibrinogen-restrained PrP-induced rise in cytosolic calcium, calpain activation, and shedding of extracellular vesicles in platelets while it, too, averted cytotoxicity of neuronal cells triggered by prion peptide. Interestingly, PrP was found to accelerate fibrin-rich clot formation, which was resistant to plasmin-mediated fibrinolysis, consistent with enhanced thrombus stability provoked by PrP. We propose that PrP-fibrinogen interaction can be clinically exploited further for prevention and management of infectious prion related disorders. Small molecules or peptides mimicking PrP-binding sites on fibrinogen can potentially mitigate PrP-induced cellular toxicity while also preventing the negative impact of PrP on fibrin clot formation and lysis.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

Distinct populations of highly potent TAU seed conformers in rapidly progressing Alzheimer's disease

[Figure: see text].

High-resolution structure and strain comparison of infectious mammalian prions

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼10⁹ lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain). Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs. Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis. This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.

Deletion of Kif5c Does Not Alter Prion Disease Tempo or Spread in Mouse Brain

In prion diseases, the spread of infectious prions (PrPSc) is thought to occur within nerves and across synapses of the central nervous system (CNS). However, the mechanisms by which PrPSc moves within axons and across nerve synapses remain undetermined. Molecular motors, including kinesins and dyneins, transport many types of intracellular cargo. Kinesin-1C (KIF5C) has been shown to transport vesicles carrying the normal prion protein (PrPC) within axons, but whether KIF5C is involved in PrPSc axonal transport is unknown. The current study tested whether stereotactic inoculation in the striatum of KIF5C knock-out mice (Kif5c^−/−) with 0.5 µL volumes of mouse-adapted scrapie strains 22 L or ME7 would result in an altered rate of prion spreading and/or disease timing. Groups of mice injected with each strain were euthanized at either pre-clinical time points or following the development of prion disease. Immunohistochemistry for PrP was performed on brain sections and PrPSc distribution and tempo of spread were compared between mouse strains. In these experiments, no differences in PrPSc spread, distribution or survival times were observed between C57BL/6 and Kif5c^−/− mice.

Caprine PRNP polymorphisms N146S and Q222K are associated with proteolytic cleavage of PrPC

Expression of the cellular prion protein (PrP^C) is crucial for the development of prion diseases. Amino acid changes in PrP^C or a reduced amount of PrP^C may modulate disease resistance. The relative abundance of C1, a natural α-cleavage fragment of PrP^C, was previously found to be associated with a resistant PRNP genotype in sheep. Goats are another small ruminant where classical scrapie susceptibility is under strong genetic control. In this study, we assessed PrP^C in goats for the existence of similar associations between PrP^C fragments and genotype. Brain tissue homogenates from scrapie-free goats with wild type PRNP or polymorphisms (I142M, H143R, N146S, or Q222K) were deglycosylated prior to immunoblot for assessment of the relative abundance of the C1 fragment of PrP^C. The presence of K₂₂₂ or S₁₄₆ alleles demonstrated significantly different relative levels of C1 compared to that observed in wild type goats, which suggests that the genotype association with C1 is neither unique to sheep nor exclusive to the ovine Q171R dimorphism.

Structurally distinct external solvent-exposed domains drive replication of major human prions

There is a limited understanding of structural attributes that encode the iatrogenic transmissibility and various phenotypes of prions causing the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD). Here we report the detailed structural differences between major sCJD MM1, MM2, and VV2 prions determined with two complementary synchrotron hydroxyl radical footprinting techniques—mass spectrometry (MS) and conformation dependent immunoassay (CDI) with a panel of Europium-labeled antibodies. Both approaches clearly demonstrate that the phenotypically distant prions differ in a major way with regard to their structural organization, and synchrotron-generated hydroxyl radicals progressively inhibit their seeding potency in a strain and structure-specific manner. Moreover, the seeding rate of sCJD prions is primarily determined by strain-specific structural organization of solvent-exposed external domains of human prion particles that control the seeding activity. Structural characteristics of human prion strains suggest that subtle changes in the organization of surface domains play a critical role as a determinant of human prion infectivity, propagation rate, and targeting of specific brain structures.

Sporadic human prion diseases are conceivably the most heterogenous neurodegenerative disorders and a growing body of research indicates that they are caused by distinct strains of prions. By parallel monitoring their replication potency and progressive hydroxyl radical modification of amino acid side chains during synchrotron irradiation, we identified major differences in the structural organization that correlate with distinct inactivation susceptibility of a given human prion strain. Furthermore, our data demonstrated, for the first time, that seeding activity of different strains of infectious brain-derived human prions is primarily function of distinct solvent-exposed structural domains, and implicate them in the initial binding of cellular isoform of prion protein (PrPC) as a critical step in human prion replication and infectivity.

Probing amyloid fibril secondary structures by infrared nanospectroscopy: experimental and theoretical considerations

Amyloid fibrils are composed of aggregated peptides or proteins in a fibrillary structure with a higher β-sheet content than their native structure. Attenuated total reflection Fourier transform infrared spectroscopy only provides bulk analysis of a sample therefore it is impossible to discriminate between different aggregated structures. To overcome this limitation, near-field techniques like AFM-IR have emerged in the last twenty years to allow infrared nanospectroscopy. This technique obtains IR spectra with a spatial resolution of ten nanometres, the size of isolated fibrils. Here, we present essential practical considerations to avoid misinterpretations and artefacts during these analyses. Effects of polarization of the incident IR laser, illumination configuration and coating of the AFM probes are discussed, including the advantages and drawbacks of their use. This approach will improve interpretation of AFM-IR spectra especially for the determination of secondary structures of species not accessible using classical ATR-FTIR.

Human cerebral organoids as a therapeutic drug screening model for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob Disease (CJD) is a fatal, currently incurable, neurodegenerative disease. The search for candidate treatments would be greatly facilitated by the availability of human cell-based models of prion disease. Recently, an induced pluripotent stem cell derived human cerebral organoid model was shown to take up and propagate human CJD prions. This model offers new opportunities to screen drug candidates for the treatment of human prion diseases in an entirely human genetic background. Here we provide the first evidence that human cerebral organoids can be a viable model for CJD drug screening by using an established anti-prion compound, pentosan polysulfate (PPS). PPS delayed prion propagation in a prophylactic-like treatment paradigm and also alleviated propagation when applied following establishment of infection in a therapeutic-like treatment paradigm. This study demonstrates the utility of cerebral organoids as the first human 3D cell culture system for screening therapeutic drug candidates for human prion diseases.

Prion-induced photoreceptor degeneration begins with misfolded prion protein accumulation in cones at two distinct sites: cilia and ribbon synapses

Accumulation of misfolded host proteins is central to neuropathogenesis of numerous human brain diseases including prion and prion-like diseases. Neurons of retina are also affected by these diseases. Previously, our group and others found that prion-induced retinal damage to photoreceptor cells in mice and humans resembled pathology of human retinitis pigmentosa caused by mutations in retinal proteins. Here, using confocal, epifluorescent and electron microscopy we followed deposition of disease-associated prion protein (PrPSc) and its association with damage to critical retinal structures following intracerebral prion inoculation. The earliest time and place of retinal PrPSc deposition was 67 days post-inoculation (dpi) on the inner segment (IS) of cone photoreceptors. At 104 and 118 dpi, PrPSc was associated with the base of cilia and swollen cone inner segments, suggesting ciliopathy as a pathogenic mechanism. By 118 dpi, PrPSc was deposited in both rods and cones which showed rootlet damage in the IS, and photoreceptor cell death was indicated by thinning of the outer nuclear layer. In the outer plexiform layer (OPL) in uninfected mice, normal host PrP (PrPC) was mainly associated with cone bipolar cell processes, but in infected mice, at 118 dpi, PrPSc was detected on cone and rod bipolar cell dendrites extending into ribbon synapses. Loss of ribbon synapses in cone pedicles and rod spherules in the OPL was observed to precede destruction of most rods and cones over the next 2–3 weeks. However, bipolar cells and horizontal cells were less damaged, indicating high selectivity among neurons for injury by prions. PrPSc deposition in cone and rod inner segments and on the bipolar cell processes participating in ribbon synapses appear to be critical early events leading to damage and death of photoreceptors after prion infection. These mechanisms may also occur in human retinitis pigmentosa and prion-like diseases, such as AD.

Quercetin Disaggregates Prion Fibrils and Decreases Fibril-Induced Cytotoxicity and Oxidative Stress

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases caused by misfolding and aggregation of prion protein (PrP). Previous studies have demonstrated that quercetin can disaggregate some amyloid fibrils, such as amyloid β peptide (Aβ) and α-synuclein. However, the disaggregating ability is unclear in PrP fibrils. In this study, we examined the amyloid fibril-disaggregating activity of quercetin on mouse prion protein (moPrP) and characterized quercetin-bound moPrP fibrils by imaging, proteinase resistance, hemolysis assay, cell viability, and cellular oxidative stress measurements. The results showed that quercetin treatment can disaggregate moPrP fibrils and lead to the formation of the proteinase-sensitive amorphous aggregates. Furthermore, quercetin-bound fibrils can reduce the membrane disruption of erythrocytes. Consequently, quercetin-bound fibrils cause less oxidative stress, and are less cytotoxic to neuroblastoma cells. The role of quercetin is distinct from the typical function of antiamyloidogenic drugs that inhibit the formation of amyloid fibrils. This study provides a solution for the development of antiamyloidogenic therapy.

Challenges and Advances in Antemortem Diagnosis of Human Transmissible Spongiform Encephalopathies

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, arise from the structural conversion of the monomeric, cellular prion protein (PrPC) into its multimeric scrapie form (PrPSc). These pathologies comprise a group of intractable, rapidly evolving neurodegenerative diseases. Currently, a definitive diagnosis of TSE relies on the detection of PrPSc and/or the identification of pathognomonic histological features in brain tissue samples, which are usually obtained postmortem or, in rare cases, by brain biopsy (antemortem). Over the past two decades, several paraclinical tests for antemortem diagnosis have been developed to preclude the need for brain samples. Some of these alternative methods have been validated and can provide a probable diagnosis when combined with clinical evaluation. Paraclinical tests include in vitro cell-free conversion techniques, such as the real-time quaking-induced conversion (RT-QuIC), as well as immunoassays, electroencephalography (EEG), and brain bioimaging methods, such as magnetic resonance imaging (MRI), whose importance has increased over the years. PrPSc is the main biomarker in TSEs, and the RT-QuIC assay stands out for its ability to detect PrPSc in cerebrospinal fluid (CSF), olfactory mucosa, and dermatome skin samples with high sensitivity and specificity. Other biochemical biomarkers are the proteins 14-3-3, tau, neuron-specific enolase (NSE), astroglial protein S100B, α-synuclein, and neurofilament light chain protein (NFL), but they are not specific for TSEs. This paper reviews the techniques employed for definite diagnosis, as well as the clinical and paraclinical methods for possible and probable diagnosis, both those in use currently and those no longer employed. We also discuss current criteria, challenges, and perspectives for TSE diagnosis. An early and accurate diagnosis may allow earlier implementation of strategies to delay or stop disease progression.

Conformation-dependent membrane permeabilization by neurotoxic PrP oligomers: The role of the H2H3 oligomerization domain

The relationship between prion propagation and the generation of neurotoxic species and clinical onset remains unclear. Several converging lines of evidence suggest that interactions with lipids promote various precursors to form aggregation-prone states that are involved in amyloid fibrils. Here, we compared the cytotoxicities of different soluble isolated oligomeric constructs from murine full-length PrP and from the restricted helical H2H3 domain with their effects on lipid vesicles. The helical H2H3 domain is suggested to be the minimal region of PrP involved in the oligomerization process. The discrete PrP oligomers of both the full-length sequence and the H2H3 domain have de novo β-sheeted structure when interacting with the membrane. They were shown to permeabilize synthetic negatively charged vesicles in a dose-dependent manner. Restricting the polymerization domain of the full-length PrP to the H2H3 helices strongly diminished the ability of the corresponding oligomers to associate with the lipid vesicles. Furthermore, the membrane impairment mechanism occurs differently for the full-length PrP oligomers and the H2H3 helices, as shown by dye-release and black lipid membrane experiments. The membrane damage caused by the full-length PrP oligomers is correlated to their neuronal toxicity at submicromolar concentrations, as shown by cell culture assays. Although oligomers of synthetic H2H3 could compromise in vitro cell homeostasis, they followed a membrane-disruptive pattern that was different from the full-length oligomers, as revealed by the role of PrP^C in cell viability assays.

Paired Helical Filament-Forming Region of Tau (297-391) Influences Endogenous Tau Protein and Accumulates in Acidic Compartments in Human Neuronal Cells

Assembly of tau protein into paired helical filaments and straight filaments is a key feature of Alzheimer's disease. Aggregation of tau has been implicated in neurodegeneration, cellular toxicity and the propagation, which accompanies disease progression. We have reported previously that a region of tau (297–391), referred to as dGAE, assembles spontaneously in physiological conditions to form paired helical filament-like fibres in vitro in the absence of additives such as heparin. This provides a valuable tool with which to explore the effects of tau in cell culture. Here we have studied the cellular uptake of soluble oligomeric and fibrillar forms of dGAE and examined the downstream consequences of tau internalisation into differentiated SH-SY5Y neuroblastoma cells using fluorescence and electron microscopy alongside structural and biochemical analyses. The assembled dGAE shows more acute cytotoxicity than the soluble, non-aggregated form. Conversely, the soluble form is much more readily internalised and, once within the cell, is able to associate with endogenous tau resulting in increased phosphorylation and aggregation of endogenous tau, which accumulates in lysosomal/endosomal compartments. It appears that soluble oligomeric forms are able to propagate tau pathology without being acutely toxic. The model system we have developed now permits the molecular mechanisms of propagation of tau pathology to be studied in vitro in a more physiological manner with a view to development of novel therapeutic approaches.

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Tau297–391 aggregates to form filaments toxic to cells in the absence of additives.

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Internalisation of soluble tau297–391 into cells leads to increased insoluble Ptau.

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Soluble tau297–391 accumulates with endogenous tau in endo-lysosomal compartments.

α-Synuclein aggregation and transmission in Parkinson's disease: a link to mitochondria and lysosome

The presence of intraneuronal Lewy bodies (LBs) and Lewy neurites (LNs) in the substantia nigra (SN) composed of aggregated α-synuclein (α-syn) has been recognized as a hallmark of pathological changes in Parkinson's disease (PD). Numerous studies have shown that aggregated α-syn is necessary for neurotoxicity. Meanwhile, the mitochondrial and lysosomal dysfunctions are associated with α-syn pathogenicity The hypothesis that α-syn transmission in the human brain contributes to the instigation and progression of PD has provided insights into PD pathology. This review will provide a brief overview of increasing researches that shed light on the relationship of α-syn aggregation with mitochondrial and lysosomal dysfunctions, and highlight recent understanding of α-syn transmission in PD pathology.

PrPSc Oligomerization Appears Dynamic, Quickly Engendering Inherent M1000 Acute Synaptotoxicity

Prion diseases are neurodegenerative disorders pathogenically linked to cellular prion protein (PrPC) misfolding into abnormal conformers (PrPSc), with PrPSc underpinning both transmission and synaptotoxicity. Although the biophysical features of PrPSc required to induce acute synaptic dysfunction remain incompletely defined, we recently reported that acutely synaptotoxic PrPSc appeared to be oligomeric. We herein provide further insights into the kinetic and requisite biophysical characteristics of acutely synaptotoxic ex vivo PrPSc derived from the brains of mice dying from M1000 prion disease. Pooled fractions of M1000 PrPSc located within the molecular weight range approximating monomeric PrP (mM1000) generated through size exclusion chromatography were found to harbor acute synaptotoxicity equivalent to preformed oligomeric fractions (oM1000). Subsequent investigation showed mM1000 corresponded to PrPSc rapidly concatenating in physiological buffer to exist as predominantly, closely associated, small oligomers. The oligomerization of PrP in mM1000 could be substantially mitigated by treatment with the antiaggregation compound epigallocatechin gallate, thereby maintaining the PrPSc as primarily nonoligomeric with completely abrogated acute synaptotoxicity; moreover, despite epigallocatechin gallate treatment, pooled oM1000 remained oligomeric and acutely synaptotoxic. A similar tendency to rapid formation of oligomers was observed for PrPC when monomeric fractions derived from size exclusion chromatography of normal brain homogenates (mNBH) were pooled, but neither mNBH nor preformed higher-order NBH complexes (oNBH) were acutely synaptotoxic. Oligomers formed from mNBH could be reduced to mainly monomers (<100 kDa) after enzymatic digestion of nucleic acids, whereas higher-order PrP assemblies derived from pooled mM1000, oM1000, and oNBH resisted such treatment. Collectively, these findings support that oligomerization of PrPSc into small multimeric assemblies appears to be a critical biophysical feature for engendering inherent acute synaptotoxicity, with preformed oligomers found in oM1000 appearing to be stable, tightly self-associated ensembles that coexist in dynamic equilibrium with mM1000, with the latter appearing capable of rapid aggregation, albeit initially forming smaller, weakly self-associated, acutely synaptotoxic oligomers.

Diverse, evolving conformer populations drive distinct phenotypes in frontotemporal lobar degeneration caused by the same MAPT-P301L mutation

Tau protein accumulation is a common denominator of major dementias, but this process is inhomogeneous, even when triggered by the same germline mutation. We considered stochastic misfolding of human tau conformers followed by templated conversion of native monomers as an underlying mechanism and derived sensitive conformational assays to test this concept. Assessments of brains from aged TgTauP301L transgenic mice revealed a prodromal state and three distinct signatures for misfolded tau. Frontotemporal lobar degeneration (FTLD)-MAPT-P301L patients with different clinical phenotypes also displayed three signatures, two resembling those found in TgTauP301L mice. As physicochemical and cell bioassays confirmed diverse tau strains in the mouse and human brain series, we conclude that evolution of diverse tau conformers is intrinsic to the pathogenesis of this uni-allelic form of tauopathy. In turn, effective therapeutic interventions in FTLD will need to address evolving repertoires of misfolded tau species rather than singular, static molecular targets.

RNA-seq and network analysis reveal unique glial gene expression signatures during prion infection

Background

Prion diseases and prion-like disorders, including Alzheimer’s disease and Parkinson’s disease, are characterized by gliosis and accumulation of misfolded aggregated host proteins. Ablating microglia in prion-infected brain by treatment with the colony-stimulating factor-1 receptor (CSF-1R) inhibitor, PLX5622, increased accumulation of misfolded prion protein and decreased survival time.

Methods

To better understand the role of glia during neurodegeneration, we used RNA-seq technology, network analysis, and hierarchical cluster analysis to compare gene expression in brains of prion-infected versus mock-inoculated mice. Comparisons were also made between PLX5622-treated prion-infected mice and untreated prion-infected mice to assess mechanisms involved in disease acceleration in the absence of microglia.

Results

RNA-seq and network analysis suggested that microglia responded to prion infection through activation of integrin CD11c/18 and did not adopt the expression signature associated with other neurodegenerative disease models. Instead, microglia acquired an alternative molecular signature late in the disease process. Furthermore, astrocytes expressed a signature pattern of genes which appeared to be specific for prion diseases. Comparisons were also made with prion-infected mice treated with PLX5622 to assess the impact of microglia ablation on astrocyte gene expression during prion infection. In the presence of microglia, a unique mix of transcripts associated with A1- and A2-reactive astrocytes was increased in brains of prion-infected mice. After ablation of microglia, this reactive astrocyte expression pattern was enhanced. Thus, after prion infection, microglia appeared to decrease the overall A1/A2-astrocyte responses which might contribute to increased survival after infection.

Conclusions

RNA-seq analysis indicated dysregulation of over 300 biological processes within the CNS during prion disease. Distinctive microglia- and astrocyte-associated expression signatures were identified during prion infection. Furthermore, astrogliosis and the unique astrocyte-associated expression signature were independent of microglial influences. Astrogliosis and the unique astrocyte-associated gene expression pattern were increased when microglia were ablated. Our findings emphasize the potential existence of alternative pathways for activating the A1/A2 paradigm in astrocytes during neurodegenerative disease.

Immunotherapy against Prion Disease

The term "prion disease" encompasses a group of neurodegenerative diseases affecting both humans and animals. Currently, there is no effective therapy and all forms of prion disease are invariably fatal. Because of (a) the outbreak of bovine spongiform encephalopathy in cattle and variant Creutzfeldt-Jakob disease in humans; (b) the heated debate about the prion hypothesis; and (c) the availability of a natural prion disease in rodents, the understanding of the pathogenic process in prion disease is much more advanced compared to that of other neurodegenerative disorders, which inspired many attempts to develop therapeutic strategies against these fatal diseases. In this review, we focus on immunotherapy against prion disease. We explain our rationale for immunotherapy as a plausible therapeutic choice, review previous trials using either active or passive immunization, and discuss potential strategies for overcoming the hurdles in developing a successful immunotherapy. We propose that immunotherapy is a plausible and practical therapeutic strategy and advocate more studies in this area to develop effective measures to control and treat these devastating disorders.

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.

Atomic insights into the effects of pathological mutants through the disruption of hydrophobic core in the prion protein

Destabilization of prion protein induces a conformational change from normal prion protein (PrPC) to abnormal prion protein (PrPSC). Hydrophobic interaction is the main driving force for protein folding, and critically affects the stability and solvability. To examine the importance of the hydrophobic core in the PrP, we chose six amino acids (V176, V180, T183, V210, I215, and Y218) that make up the hydrophobic core at the middle of the H2-H3 bundle. A few pathological mutants of these amino acids have been reported, such as V176G, V180I, T183A, V210I, I215V, and Y218N. We focused on how these pathologic mutations affect the hydrophobic core and thermostability of PrP. For this, we ran a temperature-based replica-exchange molecular dynamics (T-REMD) simulation, with a cumulative simulation time of 28 μs, for extensive ensemble sampling. From the T-REMD ensemble, we calculated the protein folding free energy difference between wild-type and mutant PrP using the thermodynamic integration (TI) method. Our results showed that pathological mutants V176G, T183A, I215V, and Y218N decrease the PrP stability. At the atomic level, we examined the change in pair-wise hydrophobic interactions from valine-valine to valine-isoleucine (and vice versa), which is induced by mutation V180I, V210I (I215V) at the 180th-210th (176th-215th) pair. Finally, we investigated the importance of the π-stacking between Y218 and F175.

Encephalopathy induced by Alzheimer brain inoculation in a non-human primate

Alzheimer’s disease is characterized by cognitive alterations, cerebral atrophy and neuropathological lesions including neuronal loss, accumulation of misfolded and aggregated β-amyloid peptides (Aβ) and tau proteins. Iatrogenic induction of Aβ is suspected in patients exposed to pituitary-derived hormones, dural grafts, or surgical instruments, presumably contaminated with Aβ. Induction of Aβ and tau lesions has been demonstrated in transgenic mice after contamination with Alzheimer’s disease brain homogenates, with very limited functional consequences. Unlike rodents, primates naturally express Aβ or tau under normal conditions and attempts to transmit Alzheimer pathology to primates have been made for decades. However, none of earlier studies performed any detailed functional assessments. For the first time we demonstrate long term memory and learning impairments in a non-human primate (Microcebus murinus) following intracerebral injections with Alzheimer human brain extracts. Animals inoculated with Alzheimer brain homogenates displayed progressive cognitive impairments (clinical tests assessing cognitive and motor functions), modifications of neuronal activity (detected by electroencephalography), widespread and progressive cerebral atrophy (in vivo MRI assessing cerebral volume loss using automated voxel-based analysis), neuronal loss in the hippocampus and entorhinal cortex (post mortem stereology). They displayed parenchymal and vascular Aβ depositions and tau lesions for some of them, in regions close to the inoculation sites. Although these lesions were sparse, they were never detected in control animals. Tau-positive animals had the lowest performances in a memory task and displayed the greatest neuronal loss. Our study is timely and important as it is the first one to highlight neuronal and clinical dysfunction following inoculation of Alzheimer’s disease brain homogenates in a primate. Clinical signs in a chronic disease such as Alzheimer take a long time to be detectable. Documentation of clinical deterioration and/or dysfunction following intracerebral inoculations with Alzheimer human brain extracts could lead to important new insights about Alzheimer initiation processes.

Direct interaction of DNMT inhibitors to PrPC suppresses pathogenic process of prion

The conversion of the normal cellular prion protein (PrPC) to the misfolded pathogenic scrapie prion protein (PrPSc) is the biochemical hallmark of prion replication. So far, various chemical compounds that inhibit this conformational conversion have been identified. Here, we report the novel anti-prion activity of SGI-1027 and its meta/meta analogue (M/M), previously known only as potent inhibitors of DNA methyltransferases (DNMTs). These compounds effectively decreased the level of PrPSc in cultured cells with permanent prion infection, without affecting PrPC at the transcriptional or translational levels. Furthermore, SGI-1027 prevented effective prion infection of the cells. In a PrP aggregation assay, both SGI-1027 and M/M blocked the formation of misfolded PrP aggregates, implying that binding of these compounds hinders the PrP conversion process. A series of binding and docking analyses demonstrated that both SGI-1027 and M/M directly interacted with the C-terminal globular domain of PrPC, but only SGI-1027 bound to a specific region of PrPC with high affinity, which correlates with its potent anti-prion efficacy. Therefore, we report SGI-1027 and related compounds as a novel class of potential anti-prion agents that preferentially function through direct interaction with PrPC.

Altered distribution, aggregation, and protease resistance of cellular prion protein following intracranial inoculation

Prion protein (PrP^C) is a protease-sensitive and soluble cell surface glycoprotein expressed in almost all mammalian cell types. PrP^Sc, a protease-resistant and insoluble form of PrP^C, is the causative agent of prion diseases, fatal and transmissible neurogenerative diseases of mammals. Prion infection is initiated via either ingestion or inoculation of PrP^Sc or when host PrP^C stochastically refolds into PrP^Sc. In either instance, the early events that occur during prion infection remain poorly understood. We have used transgenic mice expressing mouse PrP^C tagged with a unique antibody epitope to monitor the response of host PrP^C to prion inoculation. Following intracranial inoculation of either prion-infected or uninfected brain homogenate, we show that host PrP^C can accumulate both intra-axonally and within the myelin membrane of axons suggesting that it may play a role in axonal loss following brain injury. Moreover, in response to the inoculation host PrP^C exhibits an increased insolubility and protease resistance similar to that of PrP^Sc, even in the absence of infectious prions. Thus, our results raise the possibility that damage to the brain may be one trigger by which PrP^C stochastically refolds into pathogenic PrP^Sc leading to productive prion infection.

Understanding Prion Strains: Evidence from Studies of the Disease Forms Affecting Humans

Prion diseases are a unique group of rare neurodegenerative disorders characterized by tissue deposition of heterogeneous aggregates of abnormally folded protease-resistant prion protein (PrPSc), a broad spectrum of disease phenotypes and a variable efficiency of disease propagation in vivo. The dominant clinicopathological phenotypes of human prion disease include Creutzfeldt⁻Jakob disease, fatal insomnia, variably protease-sensitive prionopathy, and Gerstmann⁻Sträussler⁻Scheinker disease. Prion disease propagation into susceptible hosts led to the isolation and characterization of prion strains, initially operatively defined as "isolates" causing diseases with distinctive characteristics, such as the incubation period, the pattern of PrPSc distribution, and the regional severity of neuropathological changes after injection into syngeneic hosts. More recently, the structural basis of prion strains has been linked to amyloid polymorphs (i.e., variant amyloid protein conformations) and the concept extended to all protein amyloids showing polymorphic structures and some evidence of in vivo or in vitro propagation by seeding. Despite the significant advances, however, the link between amyloid structure and disease is not understood in many instances. Here we reviewed the most significant contributions of human prion disease studies to current knowledge of the molecular basis of phenotypic variability and the prion strain phenomenon and underlined the unsolved issues from the human disease perspective.

Molecular Mechanisms of TDP-43 Misfolding and Pathology in Amyotrophic Lateral Sclerosis

TAR DNA binding protein 43 (TDP-43) is a versatile RNA/DNA binding protein involved in RNA-related metabolism. Hyper-phosphorylated and ubiquitinated TDP-43 deposits act as inclusion bodies in the brain and spinal cord of patients with the motor neuron diseases: amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). While the majority of ALS cases (90-95%) are sporadic (sALS), among familial ALS cases 5-10% involve the inheritance of mutations in the TARDBP gene and the remaining (90-95%) are due to mutations in other genes such as: C9ORF72, SOD1, FUS, and NEK1 etc. Strikingly however, the majority of sporadic ALS patients (up to 97%) also contain the TDP-43 protein deposited in the neuronal inclusions, which suggests of its pivotal role in the ALS pathology. Thus, unraveling the molecular mechanisms of the TDP-43 pathology seems central to the ALS therapeutics, hence, we comprehensively review the current understanding of the TDP-43's pathology in ALS. We discuss the roles of TDP-43's mutations, its cytoplasmic mis-localization and aberrant post-translational modifications in ALS. Also, we evaluate TDP-43's amyloid-like in vitro aggregation, its physiological vs. pathological oligomerization in vivo, liquid-liquid phase separation (LLPS), and potential prion-like propagation propensity of the TDP-43 inclusions. Finally, we describe the various evolving TDP-43-induced toxicity mechanisms, such as the impairment of endocytosis and mitotoxicity etc. and also discuss the emerging strategies toward TDP-43 disaggregation and ALS therapeutics.