Intraperitoneal Infection of Wild-Type Mice with Synthetically Generated Mammalian Prion

Excitatory neuron-prone prion propagation and excitatory neuronal loss in prion-infected mice

The accumulation of a disease-specific isoform of prion protein (PrPSc) and histopathological lesions, such as neuronal loss, are unevenly distributed in the brains of humans and animals affected with prion diseases. This distribution varies depending on the diseases and/or the combinations of prion strain and experimental animal. The brain region-dependent distribution of PrPSc and neuropathological lesions suggests a neuronal cell-type-dependent prion propagation and vulnerability to prion infection. However, the underlying mechanism is largely unknown. In this study, we provided evidence that the prion 22L strain propagates more efficiently in excitatory neurons than inhibitory neurons and that excitatory neurons in the thalamus are vulnerable to prion infection. PrPSc accumulation was less intense in the striatum, where GABAergic inhibitory neurons predominate, compared to the cerebral cortex and thalamus, where glutamatergic excitatory neurons are predominant, in mice intracerebrally or intraperitoneally inoculated with the 22L strain. PrPSc stains were observed along the needle track after stereotaxic injection into the striatum, whereas they were also observed away from the needle track in the thalamus. Consistent with inefficient prion propagation in the striatum, the 22L prion propagated more efficiently in glutamatergic neurons than GABAergic neurons in primary neuronal cultures. RNAscope in situ hybridization revealed a decrease in Vglut1- and Vglut2-expressing neurons in the ventral posterolateral nuclei of the thalamus in 22L strain-infected mice, whereas no decrease in Vgat-expressing neurons was observed in the adjacent reticular nucleus, mainly composed of Vgat-expressing interneurons. The excitatory neuron-prone prion propagation and excitatory neuronal loss in 22L strain-infected mice shed light on the neuropathological mechanism of prion diseases.

PMCA for ultrasensitive detection of prions and to study disease biology

The emergence of a novel class of infectious agent composed exclusively of a misfolded protein (termed prions) has been a challenge in modern biomedicine. Despite similarities on the behavior of prions with respect to conventional pathogens, the many uncertainties regarding the biology and virulence of prions make them a worrisome paradigm. Since prions do not contain nucleic acids and rely on a very different way of replication and spreading, it was necessary to invent a novel technology to study them. In this article, we provide an overview of such a technology, termed protein misfolding cyclic amplification (PMCA), and summarize its many applications to detect prions and understand prion biology.

Prion protein amino acid sequence influences formation of authentic synthetic PrPSc

Synthetic prions, generated de novo from minimal, non-infectious components, cause bona fide prion disease in animals. Transmission of synthetic prions to hosts expressing syngeneic PrPC results in extended, variable incubation periods and incomplete attack rates. In contrast, murine synthetic prions (MSP) generated via PMCA with minimal cofactors readily infected mice and hamsters and rapidly adapted to both species. To investigate if hamster synthetic prions (HSP) generated under the same conditions as the MSP are also highly infectious, we inoculated hamsters with HSP generated with either hamster wild type or mutant (ΔG54, ΔG54/M139I, M139I/I205M) recombinant PrP. None of the inoculated hamsters developed clinical signs of prion disease, however, brain homogenate from HSPWT- and HSPΔG54-infected hamsters contained PrPSc, indicating subclinical infection. Serial passage in hamsters resulted in clinical disease at second passage accompanied by changes in incubation period and PrPSc conformational stability between second and third passage. These data suggest the HSP, in contrast to the MSP, are not comprised of PrPSc, and instead generate authentic PrPSc via deformed templating. Differences in infectivity between the MSP and HSP suggest that, under similar generation conditions, the amino acid sequence of PrP influences generation of authentic PrPSc.

Recombinant Mammalian Prions: The “Correctly” Misfolded Prion Protein Conformers

Generating a prion with exogenously produced recombinant prion protein is widely accepted as the ultimate proof of the prion hypothesis. Over the years, a plethora of misfolded recPrP conformers have been generated, but despite their seeding capability, many of them have failed to elicit a fatal neurodegenerative disorder in wild-type animals like a naturally occurring prion. The application of the protein misfolding cyclic amplification technique and the inclusion of non-protein cofactors in the reaction mixture have led to the generation of authentic recombinant prions that fully recapitulate the characteristics of native prions. Together, these studies reveal that recPrP can stably exist in a variety of misfolded conformations and when inoculated into wild-type animals, misfolded recPrP conformers cause a wide range of outcomes, from being completely innocuous to lethal. Since all these recPrP conformers possess seeding capabilities, these results clearly suggest that seeding activity alone is not equivalent to prion activity. Instead, authentic prions are those PrP conformers that are not only heritable (the ability to seed the conversion of normal PrP) but also pathogenic (the ability to cause fatal neurodegeneration). The knowledge gained from the studies of the recombinant prion is important for us to understand the pathogenesis of prion disease and the roles of misfolded proteins in other neurodegenerative disorders.

Beyond Amyloid Fibers: Accumulation, Biological Relevance, and Regulation of Higher-Order Prion Architectures

The formation of amyloid fibers is associated with a diverse range of disease and phenotypic states. These amyloid fibers often assemble into multi-protofibril, high-order architectures in vivo and in vitro. Prion propagation in yeast, an amyloid-based process, represents an attractive model to explore the link between these aggregation states and the biological consequences of amyloid dynamics. Here, we integrate the current state of knowledge, highlight opportunities for further insight, and draw parallels to more complex systems in vitro. Evidence suggests that high-order fibril architectures are present ex vivo from disease relevant environments and under permissive conditions in vivo in yeast, including but not limited to those leading to prion formation or instability. The biological significance of these latter amyloid architectures or how they may be regulated is, however, complicated by inconsistent experimental conditions and analytical methods, although the Hsp70 chaperone Ssa1/2 is likely involved. Transition between assembly states could form a mechanistic basis to explain some confounding observations surrounding prion regulation but is limited by a lack of unified methodology to biophysically compare these assembly states. Future exciting experimental entryways may offer opportunities for further insight.

The protease-sensitive N-terminal polybasic region of prion protein modulates its conversion to the pathogenic prion conformer

Conversion of normal prion protein (PrP^C) to the pathogenic PrP^Sc conformer is central to prion diseases such as Creutzfeldt-Jakob disease and scrapie; however, the detailed mechanism of this conversion remains obscure. To investigate how the N-terminal polybasic region of PrP (NPR) influences the PrP^C-to-PrP^Sc conversion, we analyzed two PrP mutants: ΔN6 (deletion of all six amino acids in NPR) and Met4-1 (replacement of four positively charged amino acids in NPR with methionine). We found that ΔN6 and Met4-1 differentially impacted the binding of recombinant PrP (recPrP) to the negatively charged phospholipid 1-palmitoyl-2-oleoylphosphatidylglycerol, a nonprotein cofactor that facilitates PrP conversion. Both mutant recPrPs were able to form recombinant prion (recPrP^Sc) in vitro, but the convertibility was greatly reduced, with ΔN6 displaying the lowest convertibility. Prion infection assays in mammalian RK13 cells expressing WT or NPR-mutant PrPs confirmed these differences in convertibility, indicating that the NPR affects the conversion of both bacterially expressed recPrP and post-translationally modified PrP in eukaryotic cells. We also found that both WT and mutant recPrP^Sc conformers caused prion disease in WT mice with a 100% attack rate, but the incubation times and neuropathological changes caused by two recPrP^Sc mutants were significantly different from each other and from that of WT recPrP^Sc. Together, our results support that the NPR greatly influences PrP^C-to-PrP^Sc conversion, but it is not essential for the generation of PrP^Sc. Moreover, the significant differences between ΔN6 and Met4-1 suggest that not only charge but also the identity of amino acids in NPR is important to PrP conversion.

Efficient interspecies transmission of synthetic prions

Prions are comprised solely of PrP^Sc, the misfolded self-propagating conformation of the cellular protein, PrP^C. Synthetic prions are generated in vitro from minimal components and cause bona fide prion disease in animals. It is unknown, however, if synthetic prions can cross the species barrier following interspecies transmission. To investigate this, we inoculated Syrian hamsters with murine synthetic prions. We found that all the animals inoculated with murine synthetic prions developed prion disease characterized by a striking uniformity of clinical onset and signs of disease. Serial intraspecies transmission resulted in a rapid adaptation to hamsters. During the adaptation process, PrP^Sc electrophoretic migration, glycoform ratios, conformational stability and biological activity as measured by protein misfolding cyclic amplification remained constant. Interestingly, the strain that emerged shares a strikingly similar transmission history, incubation period, clinical course of disease, pathology and biochemical and biological features of PrP^Sc with 139H, a hamster adapted form of the murine strain 139A. Combined, these data suggest that murine synthetic prions are comprised of bona fide PrP^Sc with 139A-like strain properties that efficiently crosses the species barrier and rapidly adapts to hamsters resulting in the emergence of a single strain. The efficiency and specificity of interspecies transmission of murine synthetic prions to hamsters, with relevance to brain derived prions, could be a useful model for identification of structure function relationships between PrP^Sc and PrP^C from different species.

Prions have zoonotic potential as illustrated by the interspecies transmission of bovine spongiform encephalopathy to humans resulting in the emergence of a novel human prion disease. It is unknown if other prion diseases of animals, such as chronic wasting disease, can be transmitted to other species. Models to predict prion zoonotic potential do not exist, in part, due to the lack of understanding of how the structure of PrP^Sc from one species can convert PrP^C from another species. Towards this end, we determined that murine synthetic prions, made from minimal components, can efficiently establish infection in hamsters whose transmission history, clinical features, pathology and biochemical properties of PrP^Sc are consistent with the reisolation of a known prion strain. We conclude that murine synthetic prions can recapitulate interspecies transmission and adaptation allowing for a more detailed mechanistic analysis in a simplified, trackable system.

Oral Ingestion of Synthetically Generated Recombinant Prion Is Sufficient to Cause Prion Disease in Wild-Type Mice

Prion disease is a group of transmissible neurodegenerative disorders affecting humans and animals. The prion hypothesis postulates that PrP^Sc, the pathogenic conformer of host-encoded prion protein (PrP), is the unconventional proteinaceous infectious agent called prion. Supporting this hypothesis, highly infectious prion has been generated in vitro with recombinant PrP plus defined non-protein cofactors and the synthetically generated prion (recPrP^Sc) is capable of causing prion disease in wild-type mice through intracerebral (i.c.) or intraperitoneal (i.p.) inoculation. Given that many of the naturally occurring prion diseases are acquired through oral route, demonstrating the capability of recPrP^Sc to cause prion disease via oral transmission is important, but has never been proven. Here we showed for the first time that oral ingestion of recPrP^Sc is sufficient to cause prion disease in wild-type mice, which was supported by the development of fatal neurodegeneration in exposed mice, biochemical and histopathological analyses of diseased brains, and second round transmission. Our results demonstrate the oral transmissibility of recPrP^Sc and provide the missing evidence to support that the in vitro generated recPrP^Sc recapitulates all the important properties of naturally occurring prions.

Prion protein PrP nucleic acid binding and mobilization implicates retroelements as the replicative component of transmissible spongiform encephalopathy

The existence of more than 30 strains of transmissible spongiform encephalopathy (TSE) and the paucity of infectivity of purified PrP^Sc, as well as considerations of PrP structure, are inconsistent with the protein-only (prion) theory of TSE. Nucleic acid is a strong contender as a second component. We juxtapose two key findings: (i) PrP is a nucleic-acid-binding antimicrobial protein that is similar to retroviral Gag proteins in its ability to trigger reverse transcription. (ii) Retroelement mobilization is widely seen in TSE disease. Given further evidence that PrP also mediates nucleic acid transport into and out of the cell, a strong case is to be made that a second element – retroelement nucleic acid – bound to PrP constitutes the second component necessary to explain the multiple strains of TSE.

Development of a new largely scalable in vitro prion propagation method for the production of infectious recombinant prions for high resolution structural studies

The resolution of the three-dimensional structure of infectious prions at the atomic level is pivotal to understand the pathobiology of Transmissible Spongiform Encephalopathies (TSE), but has been long hindered due to certain particularities of these proteinaceous pathogens. Difficulties related to their purification from brain homogenates of disease-affected animals were resolved almost a decade ago by the development of in vitro recombinant prion propagation systems giving rise to highly infectious recombinant prions. However, lack of knowledge about the molecular mechanisms of the misfolding event and the complexity of systems such as the Protein Misfolding Cyclic Amplification (PMCA), have limited generating the large amounts of homogeneous recombinant prion preparations required for high-resolution techniques such as solid state Nuclear Magnetic Resonance (ssNMR) imaging. Herein, we present a novel recombinant prion propagation system based on PMCA that substitutes sonication with shaking thereby allowing the production of unprecedented amounts of multi-labeled, infectious recombinant prions. The use of specific cofactors, such as dextran sulfate, limit the structural heterogeneity of the in vitro propagated prions and makes possible, for the first time, the generation of infectious and likely homogeneous samples in sufficient quantities for studies with high-resolution structural techniques as demonstrated by the preliminary ssNMR spectrum presented here. Overall, we consider that this new method named Protein Misfolding Shaking Amplification (PMSA), opens new avenues to finally elucidate the three-dimensional structure of infectious prions.

Preserving prion strain identity upon replication of prions in vitro using recombinant prion protein

Last decade witnessed an enormous progress in generating authentic infectious prions or PrP^Sc in vitro using recombinant prion protein (rPrP). Previous work established that rPrP that lacks posttranslational modification is able to support replication of highly infectious PrP^Sc with assistance of cofactors of polyanionic nature and/or lipids. Unexpectedly, previous studies also revealed that seeding of rPrP by brain-derived PrP^Sc gave rise to new prion strains with new disease phenotypes documenting loss of a strain identity upon replication in rPrP substrate. Up to now, it remains unclear whether prion strain identity can be preserved upon replication in rPrP. The current study reports that faithful replication of hamster strain SSLOW could be achieved in vitro using rPrP as a substrate. We found that a mixture of phosphatidylethanolamine (PE) and synthetic nucleic acid polyA was sufficient for stable replication of hamster brain-derived SSLOW PrP^Sc in serial Protein Misfolding Cyclic Amplification (sPMCA) that uses hamster rPrP as a substrate. The disease phenotype generated in hamsters upon transmission of recombinant PrP^Sc produced in vitro was strikingly similar to the original SSLOW diseases phenotype with respect to the incubation time to disease, as well as clinical, neuropathological and biochemical features. Infrared microspectroscopy (IR-MSP) indicated that PrP^Sc produced in animals upon transmission of recombinant PrP^Sc is structurally similar if not identical to the original SSLOW PrP^Sc. The current study is the first to demonstrate that rPrP can support replication of brain-derived PrP^Sc while preserving its strain identity. In addition, the current work is the first to document that successful propagation of a hamster strain could be achieved in vitro using hamster rPrP.

Prion infectivity is encoded exclusively within the structure of proteinase K-resistant fragments of synthetically generated recombinant PrPSc

Transmissible spongiform encephalopathies, also known as prion diseases, are a group of fatal neurodegenerative disorders affecting both humans and animals. The central pathogenic event in prion disease is the misfolding of normal prion protein (PrP^C) into the pathogenic conformer, PrP^Sc, which self-replicates by converting PrP^C to more of itself. The biochemical hallmark of PrP^Sc is its C-terminal resistance to proteinase K (PK) digestion, which has been historically used to define PrP^Sc and is still the most widely used characteristic for prion detection. We used PK-resistance as a biochemical measure for the generation of recombinant prion from bacterially expressed recombinant PrP. However, the existence of both PK- resistant and -sensitive PrP^Sc forms in animal and human prion disease led to the question of whether the in vitro-generated recombinant prion infectivity is due to the PK-resistant or -sensitive recombinant PrP forms. In this study, we compared undigested and PK-digested recombinant prions for their infectivity using both the classical rodent bioassay and the cell-based prion infectivity assay. Similar levels of infectivity were detected in PK-digested and -undigested samples by both assays. A time course study of recombinant prion propagation showed that the increased capability to seed the conversion of endogenous PrP in cultured cells coincided with an increase of the PK-resistant form of recombinant PrP. Moreover, prion infectivity diminished when recombinant prion was subjected to an extremely harsh PK digestion. These results demonstrated that the infectivity of recombinant prion is encoded within the structure of the PK-resistant PrP fragments. This characteristic of recombinant prion, that a simple PK digestion is able to eliminate all PK-sensitive (non-infectious) PrP species, makes possible a more homogenous material that will be ideal for dissecting the molecular basis of prion infectivity.

Elezgarai S, Bravo S, Vázquez-Fernández E, Rosa I, Eraña H, Gil D, Veiga S, Vidal E, Erickson-Beltran ML, Guitián E, Silva CJ, Nonno R, Ma J, Castilla J, R. Requena J. Recombinant PrPSc shares structural features with brain-derived PrPSc: Insights from limited proteolysis

Very solid evidence suggests that the core of full length PrPSc is a 4-rung β-solenoid, and that individual PrPSc subunits stack to form amyloid fibers. We recently used limited proteolysis to map the β-strands and connecting loops that make up the PrPSc solenoid. Using high resolution SDS-PAGE followed by epitope analysis, and mass spectrometry, we identified positions ~116/118, 133-134, 141, 152-153, 162, 169 and 179 (murine numbering) as Proteinase K (PK) cleavage sites in PrPSc. Such sites likely define loops and/or borders of β-strands, helping us to predict the threading of the β-solenoid. We have now extended this approach to recombinant PrPSc (recPrPSc). The term recPrPSc refers to bona fide recombinant prions prepared by PMCA, exhibiting infectivity with attack rates of ~100%. Limited proteolysis of mouse and bank vole recPrPSc species yielded N-terminally truncated PK-resistant fragments similar to those seen in brain-derived PrPSc, albeit with varying relative yields. Along with these fragments, doubly N- and C-terminally truncated fragments, in particular ~89/97-152, were detected in some recPrPSc preparations; similar fragments are characteristic of atypical strains of brain-derived PrPSc. Our results suggest a shared architecture of recPrPSc and brain PrPSc prions. The observed differences, in particular the distinct yields of specific PK-resistant fragments, are likely due to differences in threading which result in the specific biochemical characteristics of recPrPSc. Furthermore, recombinant PrPSc offers exciting opportunities for structural studies unachievable with brain-derived PrPSc.

REST alleviates neurotoxic prion peptide-induced synaptic abnormalities, neurofibrillary degeneration and neuronal death partially via LRP6-mediated Wnt-β-catenin signaling

Prion diseases are a group of infectious neurodegenerative diseases characterized by multiple neuropathological hallmarks including synaptic damage, spongiform degeneration and neuronal death. The factors and mechanisms that maintain cellular morphological integrity and protect against neurodegeneration in prion diseases are still unclear. Here we report that after stimulation with the neurotoxic PrP106-126 fragment in primary cortical neurons, REST translocates from the cytoplasm to the nucleus and protects neurons from harmful effects of PrP106-126. Overexpression of REST reduces pathological damage and abnormal biochemical alterations of neurons induced by PrP106-126 and maintains neuronal viability by stabilizing the level of pro-survival protein FOXO1 and inhibiting the permeability of the mitochondrial outer membrane, release of cytochrome c from mitochondria to cytoplasm and the activation of Capase3. Conversely, knockdown of REST exacerbates morphological damage and inhibits the expression of FOXO1. Additionally, by overexpression or knockdown of LRP6, we further show that LRP6-mediated Wnt-β-catenin signaling partly regulates the expression of REST. Collectively, we demonstrate for the first time novel neuroprotective function of REST in prion diseases and hypothesise that the LRP6-Wnt-β-catenin/REST signaling plays critical and collaborative roles in neuroprotection. This signaling of neuronal survival regulation could be explored as a viable therapeutic target for prion diseases and associated neurodegenerative diseases.

Self-propagating, protease-resistant, recombinant prion protein conformers with or without in vivo pathogenicity

Prions, characterized by self-propagating protease-resistant prion protein (PrP) conformations, are agents causing prion disease. Recent studies generated several such self-propagating protease-resistant recombinant PrP (rPrP-res) conformers. While some cause prion disease, others fail to induce any pathology. Here we showed that although distinctly different, the pathogenic and non-pathogenic rPrP-res conformers were similarly recognized by a group of conformational antibodies against prions and shared a similar guanidine hydrochloride denaturation profile, suggesting a similar overall architecture. Interestingly, two independently generated non-pathogenic rPrP-res were almost identical, indicating that the particular rPrP-res resulted from cofactor-guided PrP misfolding, rather than stochastic PrP aggregation. Consistent with the notion that cofactors influence rPrP-res conformation, the propagation of all rPrP-res formed with phosphatidylglycerol/RNA was cofactor-dependent, which is different from rPrP-res generated with a single cofactor, phosphatidylethanolamine. Unexpectedly, despite the dramatic difference in disease-causing capability, RT-QuIC assays detected large increases in seeding activity in both pathogenic and non-pathogenic rPrP-res inoculated mice, indicating that the non-pathogenic rPrP-res is not completely inert in vivo. Together, our study supported a role of cofactors in guiding PrP misfolding, indicated that relatively small structural features determine rPrP-res’ pathogenicity, and revealed that the in vivo seeding ability of rPrP-res does not necessarily result in pathogenicity.

Many neurodegenerative disorders, including Alzheimer’s disease, Parkinson’s disease and Prion disease, are caused by misfolded proteins that can self-propagate in vivo and in vitro. Misfolded self-replicating recombinant prion protein (PrP) conformers have been generated in vitro with defined cofactors, some of which are highly infectious and cause bona fide prion diseases, while others completely fail to induce any pathology. Here we compare these misfolded recombinant PrP conformers and show that the non-pathogenic misfolded recombinant PrP is not completely inert in vivo. We also found that the pathogenic and non-pathogenic recombinant PrP conformers share a similar overall architecture. Importantly, our study clearly shows that in vivo seeded spread of misfolded conformation does not necessarily lead to pathogenic change or cause disease. These findings not only are important for understanding the molecular basis for prion infectivity, but also may have important implications for the “prion-like” spread of misfolded proteins in other neurodegenerative diseases.

Lithium alleviates neurotoxic prion peptide-induced synaptic damage and neuronal death partially by the upregulation of nuclear target REST and the restoration of Wnt signaling

Prion diseases are a group of infectious neurodegenerative diseases characterized by multiple neuropathological hallmarks, including accumulation of PrP^Sc, synaptic damage, and neuronal death. We previously reported that the repressor element 1-silencing transcription factor (REST), a novel neuroprotective marker in neurodegeneration, protects neurons against neurotoxic peptide (PrP106-126)-induced neurotoxicity, but fails to maintain survival following prolonged exposure to PrP106-126. Because Wnt signaling partially induces REST and is activated by lithium, we investigated the effects of lithium on REST in prion diseases. Lithium restores nuclear expression of REST, which is essential for regulating survival proteins. Lithium also mimics neuroprotective functions when REST is blocked, and these beneficial effects are additive with REST overexpression under physiological conditions. Reciprocally, under PrP106-126-stimulated pathological conditions, REST plays a critical role in the neuroprotective mechanisms of lithium treatment. Although lithium recovers Wnt signaling by inhibiting glycogen synthase kinase-3β and stabilizing β-catenin, restores survival associated proteins after exposure to PrP106-126 in primary cortical neurons. Knockdown of REST significantly suppresses the neuroprotective function of lithium. Conversely, overexpression of REST partially recovers its actions. Notably, lithium directly alleviates PrP106-126-induced synaptic damage and neuronal cell death by preventing changes in presynaptic and postsynaptic marker proteins and promoting survival pathways also partially via the expression of REST. Our results suggest that REST acts as a novel and important nuclear target for lithium. We hypothesize that PrP106-126-stimulated neurotoxicity induces Wnt signaling dysfunction and lithium mimics this signaling cascade, suggesting that lithium should be considered as a potential therapeutic agent against prion diseases.

Downregulation of the Repressor Element 1-Silencing Transcription Factor (REST) Is Associated with Akt-mTOR and Wnt-β-Catenin Signaling in Prion Diseases Models

Prion diseases are a group of infectious diseases characterized by multiple neuropathological changes, yet the mechanisms that preserve function and protect against prion-associated neurodegeneration are still unclear. We previously reported that the repressor element 1-silencing transcription factor (REST) alleviates neurotoxic prion peptide (PrP106-126)-induced toxicity in primary neurons. Here we confirmed the findings of the in vitro model in 263K infected hamsters, an in vivo model of prion diseases and further showed the relationships between REST and related signaling pathways. REST was depleted from the nucleus in prion infected brains and taken up by autophagosomes in the cytoplasm, co-localizing with LC3-II. Importantly, downregulation of the Akt–mTOR and at least partially inactivation of LRP6-Wnt-β-catenin signaling pathways correlated with the decreased levels of REST in vivo in the brain of 263K-infected hamsters and in vitro in PrP106-126-treated primary neurons. Overexpression of REST in primary cortical neurons alleviated PrP106-126 peptide-induced neuronal oxidative stress, mitochondrial damage and partly inhibition of the LRP6-Wnt-β-catenin and Akt–mTOR signaling. Based on our findings, a model of REST-mediated neuroprotection in prion infected animals is proposed, with Akt–mTOR and Wnt-β-catenin signaling as the key pathways. REST-mediated neuronal survival signaling could be explored as a viable therapeutic target for prion diseases and related neurodegenerative diseases.

The role of the unusual threonine string in the conversion of prion protein

The conversion of normal prion protein (PrP) into pathogenic PrP conformers is central to prion disease, but the mechanism remains unclear. The α-helix 2 of PrP contains a string of four threonines, which is unusual due to the high propensity of threonine to form β-sheets. This structural feature was proposed as the basis for initiating PrP conversion, but experimental results have been conflicting. We studied the role of the threonine string on PrP conversion by analyzing mouse Prnp^a and Prnp^b polymorphism that contains a polymorphic residue at the beginning of the threonine string, and PrP mutants in which threonine 191 was replaced by valine, alanine, or proline. The PMCA (protein misfolding cyclic amplification) assay was able to recapitulate the in vivo transmission barrier between PrP^a and PrP^b. Relative to PMCA, the amyloid fibril growth assay is less restrictive, but it did reflect certain properties of in vivo prion transmission. Our results suggest a plausible theory explaining the apparently contradictory results in the role of the threonine string in PrP conversion and provide novel insights into the complicated relationship among PrP stability, seeded conformational change, and prion structure, which is critical for understanding the molecular basis of prion infectivity.