Cryo-immunogold electron microscopy for prions: toward identification of a conversion site

Diminished Neuronal ESCRT-0 Function Exacerbates AMPA Receptor Derangement and Accelerates Prion-Induced Neurodegeneration

Endolysosomal defects in neurons are central to the pathogenesis of prion and other neurodegenerative disorders. In prion disease, prion oligomers traffic through the multivesicular body (MVB) and are routed for degradation in lysosomes or for release in exosomes, yet how prions impact proteostatic pathways is unclear. We found that prion-affected human and mouse brain showed a marked reduction in Hrs and STAM1 (ESCRT-0), which route ubiquitinated membrane proteins from early endosomes into MVBs. To determine how the reduction in ESCRT-0 impacts prion conversion and cellular toxicity in vivo, we prion-challenged conditional knockout mice (male and female) having Hrs deleted from neurons, astrocytes, or microglia. The neuronal, but not astrocytic or microglial, Hrs-depleted mice showed a shortened survival and an acceleration in synaptic derangements, including an accumulation of ubiquitinated proteins, deregulation of phosphorylated AMPA and metabotropic glutamate receptors, and profoundly altered synaptic structure, all of which occurred later in the prion-infected control mice. Finally, we found that neuronal Hrs (nHrs) depletion increased surface levels of the cellular prion protein, PrPC, which may contribute to the rapidly advancing disease through neurotoxic signaling. Taken together, the reduced Hrs in the prion-affected brain hampers ubiquitinated protein clearance at the synapse, exacerbates postsynaptic glutamate receptor deregulation, and accelerates neurodegeneration.SIGNIFICANCE STATEMENT Prion diseases are rapidly progressive neurodegenerative disorders characterized by prion aggregate spread through the central nervous system. Early disease features include ubiquitinated protein accumulation and synapse loss. Here, we investigate how prion aggregates alter ubiquitinated protein clearance pathways (ESCRT) in mouse and human prion-infected brain, discovering a marked reduction in Hrs. Using a prion-infection mouse model with neuronal Hrs (nHrs) depleted, we show that low neuronal Hrs is detrimental and markedly shortens survival time while accelerating synaptic derangements, including ubiquitinated protein accumulation, indicating that Hrs loss exacerbates prion disease progression. Additionally, Hrs depletion increases the surface distribution of prion protein (PrPC), linked to aggregate-induced neurotoxic signaling, suggesting that Hrs loss in prion disease accelerates disease through enhancing PrPC-mediated neurotoxic signaling.

Mutant Prion Protein Endoggresomes are Hubs for Local Axonal Organelle-Cytoskeletal Remodeling

Dystrophic axons comprising misfolded mutant prion protein (PrP) aggregates are a characteristic pathological feature in the prionopathies. These aggregates form inside endolysosomes -called endoggresomes-, within swellings that line up the length of axons of degenerating neurons. The pathways impaired by endoggresomes that result in failed axonal and consequently neuronal health, remain undefined. Here, we dissect the local subcellular impairments that occur within individual mutant PrP endoggresome swelling sites in axons. Quantitative high-resolution light and electron microscopy revealed the selective impairment of the acetylated vs tyrosinated microtubule cytoskeleton, while micro-domain image analysis of live organelle dynamics within swelling sites revealed deficits uniquely to the MT-based active transport system that translocates mitochondria and endosomes toward the synapse. Cytoskeletal and defective transport results in the retention of mitochondria, endosomes, and molecular motors at swelling sites, enhancing mitochondria-Rab7 late endosome contacts that induce mitochondrial fission via the activity of Rab7, and render mitochondria dysfunctional. Our findings point to mutant Pr Pendoggresome swelling sites as selective hubs of cytoskeletal deficits and organelle retention that drive the remodeling of organelles along axons. We propose that the dysfunction imparted locally within these axonal micro-domains spreads throughout the axon over time, leading to axonal dysfunction in prionopathies.

Cellular toxicity of scrapie prions in prion diseases; a biochemical and molecular overview

Transmissible spongiform encephalopathies (TSEs) or prion diseases consist of a broad range of fatal neurological disorders affecting humans and animals. Contrary to Watson and Crick's 'central dogma', prion diseases are caused by a protein, devoid of DNA involvement. Herein, we briefly review various cellular and biological aspects of prions and prion pathogenesis focusing mainly on historical milestones, biosynthesis, degradation, structure-function of cellular and scrapie forms of prions .

Loss of small GTPase Rab7 activation in prion infection negatively affects a feedback loop regulating neuronal cholesterol metabolism

Prion diseases are fatal and infectious neurodegenerative diseases that occur in humans and animals. They are caused by the misfolding of the cellular prion protein PrP^c into the infectious isoform PrP^Sc. PrP^Sc accumulates mostly in endolysosomal vesicles of prion-infected cells, eventually causing neurodegeneration. In response to prion infection, elevated cholesterol levels and a reduction in membrane-attached small GTPase Rab7 have been observed in neuronal cells. Here, we investigated the molecular events causing an impaired Rab7 membrane attachment and the potential mechanistic link with elevated cholesterol levels in prion infection. We demonstrate that prion infection is associated with reduced levels of active Rab7 (Rab7.GTP) in persistently prion-infected neuronal cell lines, primary cerebellar granular neurons, and neurons in the brain of mice with terminal prion disease. In primary cerebellar granular neurons, levels of active Rab7 were increased during the very early stages of the prion infection prior to a significant decrease concomitant with PrP^Sc accumulation. The reduced activation of Rab7 in prion-infected neuronal cell lines is also associated with its reduced ubiquitination status, decreased interaction with its effector RILP, and altered lysosomal positioning. Consequently, the Rab7-mediated trafficking of low-density lipoprotein to lysosomes is delayed. This results in an impaired feedback regulation of cholesterol synthesis leading to an increase in cholesterol levels. Notably, transient overexpression of the constitutively active mutant of Rab7 rescues the delay in the low-density lipoprotein trafficking, hence reducing cholesterol levels and attenuating PrP^Sc propagation, demonstrating a mechanistic link between the loss of Rab7.GTP and elevated cholesterol levels.

Prions induce an early Arc response and a subsequent reduction in mGluR5 in the hippocampus

Synapse dysfunction and loss are central features of neurodegenerative diseases, caused in part by the accumulation of protein oligomers. Amyloid-β, tau, prion, and α-synuclein oligomers bind to the cellular prion protein (PrPC), resulting in the activation of macromolecular complexes and signaling at the post-synapse, yet the early signaling events are unclear. Here we sought to determine the early transcript and protein alterations in the hippocampus during the pre-clinical stages of prion disease. We used a transcriptomic approach focused on the early-stage, prion-infected hippocampus of male wild-type mice, and identify immediate early genes, including the synaptic activity response gene, Arc/Arg3.1, as significantly upregulated. In a longitudinal study of male, prion-infected mice, Arc/Arg-3.1 protein was increased early (40% of the incubation period), and by mid-disease (pre-clinical), phosphorylated AMPA receptors (pGluA1-S845) were increased and metabotropic glutamate receptors (mGluR5 dimers) were markedly reduced in the hippocampus. Notably, sporadic Creutzfeldt-Jakob disease (sCJD) post-mortem cortical samples also showed low levels of mGluR5 dimers. Together, these findings suggest that prions trigger an early Arc response, followed by an increase in phosphorylated GluA1 and a reduction in mGluR5 receptors.

Endosomal sorting drives the formation of axonal prion protein endoggresomes

The pathogenic aggregation of misfolded prion protein (PrP) in axons underlies prion disease pathologies. The molecular mechanisms driving axonal misfolded PrP aggregate formation leading to neurotoxicity are unknown. We found that the small endolysosomal guanosine triphosphatase (GTPase) Arl8b recruits kinesin-1 and Vps41 (HOPS) onto endosomes carrying misfolded mutant PrP to promote their axonal entry and homotypic fusion toward aggregation inside enlarged endomembranes that we call endoggresomes. This axonal rapid endosomal sorting and transport-dependent aggregation (ARESTA) mechanism forms pathologic PrP endoggresomes that impair calcium dynamics and reduce neuronal viability. Inhibiting ARESTA diminishes endoggresome formation, rescues calcium influx, and prevents neuronal death. Our results identify ARESTA as a key pathway for the regulation of endoggresome formation and a new actionable antiaggregation target to ameliorate neuronal dysfunction in the prionopathies.

Multisite interactions of prions with membranes and native nanodiscs

Although prions are known as protein-only infectious particles, they exhibit lipid specificities, cofactor dependencies and membrane-dependent activities. Such membrane interactions play key roles in how prions are processed, presented and regulated, and hence have significant functional consequences. The expansive literature related to prion protein interactions with lipids and native nanodiscs is discussed, and provides a unique opportunity to re-evaluate the molecular composition and mechanisms of its infectious and cellular states. A family of crystal and solution structures of prions are analyzed here for the first time using the membrane optimal docking area (MODA) program, revealling the presence of structured binding elements that could mediate specific lipid recognition. A set of motifs centerred around W99, L125, Y169 and Y226 are consistently predicted as being membrane interactive and form an exposed surface which includes α helical, β strand and loop elements involving the prion protein (PrP) structural domain, while the scrapie form is radically different and doubles the size of the membrane interactive site into an extensible surface. These motifs are highly conserved throughout mammalian evolution, suggesting that prions have long been intrinsically attached to membranes at central and N- and C-terminal points, providing several opportunities for stable and specific bilayer interactions as well as multiple complexed orientations. Resistance or susceptibility to prion disease correlates with increased or decreased membrane binding propensity by mutant forms, respectively, indicating a protective role by lipids. The various prion states found in vivo are increasingly resolvable using native nanodiscs formed by styrene maleic acid (SMA) and stilbene maleic acid (STMA) copolymers rather than classical detergents, allowing the endogenous states to be tackled. These copolymers spontaneously fragment intact membranes into water-soluble discs holding a section of native bilayer, and can accommodate prion multimers and mini-fibrils. Such nanodiscs have also proven useful for understanding how β amyloid and α synuclein proteins contribute to Alzheimer's and Parkinson's diseases, providing further biomedical applications. Structural and functional insights of such proteins in styrene maleic acid lipid particles (SMALPs) can be resolved at high resolution by methods including cryo-electron microscopy (cEM), motivating continued progress in polymer design to resolve biological and pathological mechanisms.

Prion protein glycans reduce intracerebral fibril formation and spongiosis in prion disease

Posttranslational modifications (PTMs) are common among proteins that aggregate in neurodegenerative disease, yet how PTMs impact the aggregate conformation and disease progression remains unclear. By engineering knockin mice expressing prion protein (PrP) lacking 2 N-linked glycans (Prnp180Q/196Q), we provide evidence that glycans reduce spongiform degeneration and hinder plaque formation in prion disease. Prnp180Q/196Q mice challenged with 2 subfibrillar, non-plaque-forming prion strains instead developed plaques highly enriched in ADAM10-cleaved PrP and heparan sulfate (HS). Intriguingly, a third strain composed of intact, glycophosphatidylinositol-anchored (GPI-anchored) PrP was relatively unchanged, forming diffuse, HS-deficient deposits in both the Prnp180Q/196Q and WT mice, underscoring the pivotal role of the GPI-anchor in driving the aggregate conformation and disease phenotype. Finally, knockin mice expressing triglycosylated PrP (Prnp187N) challenged with a plaque-forming prion strain showed a phenotype reversal, with a striking disease acceleration and switch from plaques to predominantly diffuse, subfibrillar deposits. Our findings suggest that the dominance of subfibrillar aggregates in prion disease is due to the replication of GPI-anchored prions, with fibrillar plaques forming from poorly glycosylated, GPI-anchorless prions that interact with extracellular HS. These studies provide insight into how PTMs impact PrP interactions with polyanionic cofactors, and highlight PTMs as a major force driving the prion disease phenotype.

Modeling PrPSc Generation Through Deformed Templating

Deformed templating is the process by which self-replicating protein conformations with a given cross-β folding pattern can seed formation of an alternative self-replicating state with different cross-β folding pattern. In particular, uninfectious but propagative PrP amyloid can transform into a bona fide infectious conformer, PrP^Sc through deformed templating. The process can take many rounds of replication (if taking place in vitro) or even several passages of the evolving PrP conformers through successive brains if in vivo, through experimental transmission. In all cases, deformed templating involves a forced conversion in which there is a mismatch between the template and the substrate and/or the templating environment, typically a recombinant PrP amyloid, adept at converting recombinant PrP under denaturing conditions (e.g., presence of chaotropic agents), encountering a glycosylated, GPI-anchored PrP^C substrate under physiological conversion conditions. Deformed templating is characterized by emergence of intermediate conformers that exhibit biochemical characteristics that are intermediate between those of the initial PrP amyloid and the final PrP^Sc conformers. Here, we took advantage of the recent elucidation of the structure of a PrP amyloid by cryo-EM and the availability of a physically plausible atomistic model of PrP^Sc that we have recently proposed. Using modeling and Molecular Dynamics (MD) approaches, we built a complete molecular modelization of deformed templating, including an atomistic model of a glycosylated intermediate conformer and a modified model of PrP^Sc. Among other unanticipated outcomes, our results show that fully glycosylated PrP can be stacked in-register, and how 4-rung β-solenoid (4RβS) PrP architectures can share key structural motifs with parallel-in register intermolecular sheet (PIRIBS) PrP amyloids. Our results shed light on the mechanisms of prion replication.

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

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

Early detection of prion protein aggregation with a fluorescent pentameric oligothiophene probe using spectral confocal microscopy

Misfolding of the prion protein (PrP) and templating of its pathological conformation onto cognate proteins causes a number of lethal disorders of central nervous system in humans and animals, such as Creutzfeldt-Jacob disease, chronic wasting disease and bovine spongiform encephalopathy. Structural rearrangement of PrP^C into PrP^Sc promotes aggregation of misfolded proteins into β-sheet-rich fibrils, which can be visualized by conformationally sensitive fluorescent probes. Early detection of prion misfolding and deposition might provide useful insights into its pathophysiology. Pentameric formyl thiophene acetic acid (pFTAA) is a novel amyloid probe that was shown to sensitively detect various misfolded proteins, including PrP. Here, we compared sensitivity of pFTAA staining and spectral microscopy with conventional methods of prion detection in mouse brains infected with mouse-adapted 22L prions. pFTAA bound to prion deposits in mouse brain sections exhibited a red-shifted fluorescence emission spectrum, which quantitatively increased with disease progression. Small prion deposits were detected as early as 50 days post-inoculation, well before appearance of clinical signs. Moreover, we detected significant spectral shifts in the greater brain parenchyma as early as 25 days post-inoculation, rivaling the most sensitive conventional method (real-time quaking-induced conversion). These results showcase the potential of pFTAA staining combined with spectral imaging for screening of prion-infected tissue. Not only does this method have comparable sensitivity to established techniques, it is faster and technically simpler. Finally, this readout provides valuable information about the spatial distribution of prion aggregates across tissue in the earliest stages of infection, potentially providing valuable pathophysiological insight into prion transmission.

Isolation of infectious, non-fibrillar and oligomeric prions from a genetic prion disease

Prions are transmissible agents causing lethal neurodegenerative diseases that are composed of aggregates of misfolded cellular prion protein (PrPSc). Despite non-fibrillar oligomers having been proposed as the most infectious prion particles, prions purified from diseased brains usually consist of large and fibrillar PrPSc aggregates, whose protease-resistant core (PrPres) encompasses the whole C-terminus of PrP. In contrast, PrPSc from Gerstmann-Sträussler-Scheinker disease associated with alanine to valine substitution at position 117 (GSS-A117V) is characterized by a small protease-resistant core, which is devoid of the C-terminus. We thus aimed to investigate the role of this unusual PrPSc in terms of infectivity, strain characteristics, and structural features. We found, by titration in bank voles, that the infectivity of GSS-A117V is extremely high (109.3 ID50 U/g) and is resistant to treatment with proteinase K (109.0 ID50 U/g). We then purified the proteinase K-resistant GSS-A117V prions and determined the amount of infectivity and PrPres in the different fractions, alongside the morphological characteristics of purified PrPres aggregates by electron microscopy. Purified pellet fractions from GSS-A117V contained the expected N- and C-terminally cleaved 7 kDa PrPres, although the yield of PrPres was low. We found that this low yield depended on the low density/small size of GSS-A117V PrPres, as it was mainly retained in the last supernatant fraction. All fractions were highly infectious, thus confirming the infectious nature of the 7 kDa PrPres, with infectivity levels that directly correlated with the PrPres amount detected. Finally, electron microscopy analysis of these fractions showed no presence of amyloid fibrils, but only very small and indistinct, non-fibrillar PrPresparticles were detected and confirmed to contain PrP via immunogold labelling. Our study demonstrates that purified aggregates of 7 kDa PrPres, spanning residues ∼90-150, are highly infectious oligomers that encode the biochemical and biological strain features of the original sample. Overall, the autocatalytic behaviour of the prion oligomers reveals their role in the propagation of neurodegeneration in patients with Gerstmann-Sträussler-Scheinker disease and implies that the C-terminus of PrPSc is dispensable for infectivity and strain features for this prion strain, uncovering the central PrP domain as the minimal molecular component able to encode infectious prions. These findings are consistent with the hypothesis that non-fibrillar prion particles are highly efficient propagators of disease and provide new molecular and morphological constraints on the structure of infectious prions.

Direct Observation of Murine Prion Protein Replication in Vitro

Prions are believed to propagate when an assembly of prion protein (PrP) enters a cell and replicates to produce two or more fibrils, leading to an exponential increase in PrP aggregate number with time. However, the molecular basis of this process has not yet been established in detail. Here, we use single-aggregate imaging to study fibril fragmentation and elongation of individual murine PrP aggregates from seeded aggregation in vitro. We found that PrP elongation occurs via a structural conversion from a PK-sensitive to PK-resistant conformer. Fibril fragmentation was found to be length-dependent and resulted in the formation of PK-sensitive fragments. Measurement of the rate constants for these processes also allowed us to predict a simple spreading model for aggregate propagation through the brain, assuming that doubling of the aggregate number is rate-limiting. In contrast, while α-synuclein aggregated by the same mechanism, it showed significantly slower elongation and fragmentation rate constants than PrP, leading to much slower replication rate. Overall, our study shows that fibril elongation with fragmentation are key molecular processes in PrP and α-synuclein aggregate replication, an important concept in prion biology, and also establishes a simple framework to start to determine the main factors that control the rate of prion and prion-like spreading in animals.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Cellular mechanisms responsible for cell-to-cell spreading of prions

Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases.

Effects of pH and aggregation in the human prion conversion into scrapie form: a study using molecular dynamics with excited normal modes

There are two different prion conformations: (1) the cellular natural (PrP^C) and (2) the scrapie (PrP^Sc), an infectious form that tends to aggregate under specific conditions. PrP^C and PrP^Sc are widely different regarding secondary and tertiary structures. PrP^Sc contains more and longer β-strands compared to PrP^C. The lack of solved PrP^Sc structures precludes a proper understanding of the mechanisms related to the transition between cellular and scrapie forms, as well as the aggregation process. In order to investigate the conformational transition between PrP^C and PrP^Sc, we applied MDeNM (molecular dynamics with excited normal modes), an enhanced sampling simulation technique that has been recently developed to probe large structural changes. These simulations yielded new structural rearrangements of the cellular prion that would have been difficult to obtain with standard MD simulations. We observed an increase in β-sheet formation under low pH (≤ 4) and upon oligomerization, whose relevance was discussed on the basis of the energy landscape theory for protein folding. The characterization of intermediate structures corresponding to transition states allowed us to propose a conversion model from the cellular to the scrapie prion, which possibly ignites the fibril formation. This model can assist the design of new drugs to prevent neurological disorders related to the prion aggregation mechanism.

Prion Proteins Without the Glycophosphatidylinositol Anchor: Potential Biomarkers in Neurodegenerative Diseases

Prion protein (PrP) is a biomolecule that is involved in neuronal signaling, myelinization, and the development of neurodegenerative diseases. In the cell, PrP is shed by the ADAM10 protease. This process generates PrP molecules that lack glycophosphatidylinositol anchor, and these molecules incorporate into toxic aggregates and neutralize toxic oligomers. Due to this dual role, these molecules are important biomarkers for neurodegenerative diseases. In this review, we present shed PrP as a potential biomarker, with a focus on PrP226*, which may be the main biomarker for predicting neurodegenerative diseases in humans.

Flow Cytometric Detection of PrPSc in Neurons and Glial Cells from Prion-Infected Mouse Brains

In prion diseases, an abnormal isoform of prion protein (PrP^Sc) accumulates in neurons, astrocytes, and microglia in the brains of animals affected by prions. Detailed analyses of PrP^Sc-positive neurons and glial cells are required to clarify their pathophysiological roles in the disease. Here, we report a novel method for the detection of PrP^Sc in neurons and glial cells from the brains of prion-infected mice by flow cytometry using PrP^Sc-specific staining with monoclonal antibody (MAb) 132. The combination of PrP^Sc staining and immunolabeling of neural cell markers clearly distinguished neurons, astrocytes, and microglia that were positive for PrP^Sc from those that were PrP^Sc negative. The flow cytometric analysis of PrP^Sc revealed the appearance of PrP^Sc-positive neurons, astrocytes, and microglia at 60 days after intracerebral prion inoculation, suggesting the presence of PrP^Sc in the glial cells, as well as in neurons, from an early stage of infection. Moreover, the kinetic analysis of PrP^Sc revealed a continuous increase in the proportion of PrP^Sc-positive cells for all cell types with disease progression. Finally, we applied this method to isolate neurons, astrocytes, and microglia positive for PrP^Sc from a prion-infected mouse brain by florescence-activated cell sorting. The method described here enables comprehensive analyses specific to PrP^Sc-positive neurons, astrocytes, and microglia that will contribute to the understanding of the pathophysiological roles of neurons and glial cells in PrP^Sc-associated pathogenesis.IMPORTANCE Although formation of PrP^Sc in neurons is associated closely with neurodegeneration in prion diseases, the mechanism of neurodegeneration is not understood completely. On the other hand, recent studies proposed the important roles of glial cells in PrP^Sc-associated pathogenesis, such as the intracerebral spread of PrP^Sc and clearance of PrP^Sc from the brain. Despite the great need for detailed analyses of PrP^Sc-positive neurons and glial cells, methods available for cell type-specific analysis of PrP^Sc have been limited thus far to microscopic observations. Here, we have established a novel high-throughput method for flow cytometric detection of PrP^Sc in cells with more accurate quantitative performance. By applying this method, we succeeded in isolating PrP^Sc-positive cells from the prion-infected mouse brains via fluorescence-activated cell sorting. This allows us to perform further detailed analysis specific to PrP^Sc-positive neurons and glial cells for the clarification of pathological changes in neurons and pathophysiological roles of glial cells.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Establishment of a simple cell-based ELISA for the direct detection of abnormal isoform of prion protein from prion-infected cells without cell lysis and proteinase K treatment

Prion-infected cells have been used for analyzing the effect of compounds on the formation of abnormal isoform of prion protein (PrP(Sc)). PrP(Sc) is usually detected using anti-prion protein (PrP) antibodies after the removal of the cellular isoform of prion protein (PrP(C)) by proteinase K (PK) treatment. However, it is expected that the PK-sensitive PrP(Sc) (PrP(Sc)-sen), which possesses higher infectivity and conversion activity than the PK-resistant PrP(Sc) (PrP(Sc)-res), is also digested through PK treatment. To overcome this problem, we established a novel cell-based ELISA in which PrP(Sc) can be directly detected from cells persistently infected with prions using anti-PrP monoclonal antibody (mAb) 132 that recognizes epitope consisting of mouse PrP amino acids 119-127. The novel cell-based ELISA could distinguish prion-infected cells from prion-uninfected cells without cell lysis and PK treatment. MAb 132 could detect both PrP(Sc)-sen and PrP(Sc)-res even if all PrP(Sc) molecules were not detected. The analytical dynamic range for PrP(Sc) detection was approximately 1 log. The coefficient of variation and signal-to-background ratio were 7%-11% and 2.5-3.3, respectively, demonstrating the reproducibility of this assay. The addition of a cytotoxicity assay immediately before PrP(Sc) detection did not affect the following PrP(Sc) detection. Thus, all the procedures including cell culture, cytotoxicity assay, and PrP(Sc) detection were completed in the same plate. The simplicity and non-requirement for cell lysis or PK treatment are advantages for the high throughput screening of anti-prion compounds.

The Structure of the Infectious Prion Protein and Its Propagation

The prion diseases, which include Creutzfeldt-Jakob disease in humans, chronic wasting disease in cervids (i.e., deer, elk, moose, and reindeer), bovine spongiform encephalopathy in cattle, as well as sheep and goat scrapie, are caused by the conversion of the cellular prion protein (PrP^C) into a disease-causing conformer (PrP^Sc). PrP^C is a regular, GPI-anchored protein that is expressed on the cell surface of neurons and many other cell types. The structure of PrP^C is well studied, based on analyses of recombinant PrP, which is thought to mimic the structure of native PrP^C. The mature protein contains an N-terminal, unfolded domain and a C-terminal, globular domain that consists of three α-helices and only a small, two-stranded β-sheet. In contrast, PrP^Sc was found to contain predominantly β-structure and to aggregate into a variety of quaternary structures, such as oligomers, amorphous aggregates, amyloid fibrils, and two-dimensional crystals. The tendency of PrP^Sc to aggregate into these diverse forms is also responsible for our incomplete knowledge about its molecular structure. Nevertheless, the repeating nature of the more regular PrP^Sc aggregates has provided informative insights into the structure of the infectious conformer, albeit at limited resolution. These data established a four-rung β-solenoid architecture as the main element of its structure. Moreover, the four-rung β-solenoid architecture provides a molecular framework for an autocatalytic propagation mechanism, which could explain the conversion of PrP^C into PrP^Sc.

Cell Biology of Prion Protein

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP^C remains elusive but may depend upon its cellular localization. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP^Sc, suggesting that molecular species of PrP distinct from PrP^Sc may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP^Sc but rather ^CtmPrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP^Sc and that associated with ^CtmPrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP^C biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.

The double life of the ribosome: When its protein folding activity supports prion propagation

It is no longer necessary to demonstrate that ribosome is the central machinery of protein synthesis. But it is less known that it is also key player of the protein folding process through another conserved function: the protein folding activity of the ribosome (PFAR). This ribozyme activity, discovered more than 2 decades ago, depends upon the domain V of the large rRNA within the large subunit of the ribosome. Surprisingly, we discovered that anti-prion compounds are also potent PFAR inhibitors, highlighting an unexpected link between PFAR and prion propagation. In this review, we discuss the ancestral origin of PFAR in the light of the ancient RNA world hypothesis. We also consider how this ribosomal activity fits into the landscape of cellular protein chaperones involved in the appearance and propagation of prions and other amyloids in mammals. Finally, we examine how drugs targeting the protein folding activity of the ribosome could be active against mammalian prion and other protein aggregation-based diseases, making PFAR a promising therapeutic target for various human protein misfolding diseases.

The cellular basis of platelet secretion: Emerging structure/function relationships

Platelet activation has long been known to be accompanied by secretion from at least three types of compartments. These include dense granules, the major source of small molecules; α-granules, the major protein storage organelle; and lysosomes, the site of acid hydrolase storage. Despite ~60 years of research, there are still many unanswered questions about the cell biology of platelet secretion: for example, how are these secretory organelles organized to support cargo release and what are the key routes of cargo release, granule to plasma membrane or granule to canalicular system. Moreover, in recent years, increasing evidence points to the platelet being organized for secretion of the contents from other organelles, namely the dense tubular system (endoplasmic reticulum) and the Golgi apparatus. Conceivably, protein secretion is a widespread property of the platelet and its organelles. In this review, we concentrate on the cell biology of the α-granule and its structure/function relationships. We both review the literature and discuss the wide array of 3-dimensional, high-resolution structural approaches that have emerged in the last few years. These have begun to reveal new and unanticipated outcomes and some of these are discussed. We are hopeful that the next several years will bring rapid advances to this field that will resolve past controversies and be clinically relevant.

Comparison of abnormal isoform of prion protein in prion-infected cell lines and primary-cultured neurons by PrPSc-specific immunostaining

We established abnormal isoform of prion protein (PrPSc)-specific double immunostaining using mAb 132, which recognizes aa 119-127 of the PrP molecule, and novel PrPSc-specific mAb 8D5, which recognizes the N-terminal region of the PrP molecule. Using the PrPSc-specific double immunostaining, we analysed PrPSc in immortalized neuronal cell lines and primary cerebral-neuronal cultures infected with prions. The PrPSc-specific double immunostaining showed the existence of PrPSc positive for both mAbs 132 and 8D5, as well as those positive only for either mAb 132 or mAb 8D5. This indicated that double immunostaining detects a greater number of PrPSc species than single immunostaining. Double immunostaining revealed cell-type-dependent differences in PrPSc staining patterns. In the 22 L prion strain-infected Neuro2a (N2a)-3 cells, a subclone of N2a neuroblastoma cell line, or GT1-7, a subclone of the GT1 hypothalamic neuronal cell line, granular PrPSc stains were observed at the perinuclear regions and cytoplasm, whereas unique string-like PrPSc stains were predominantly observed on the surface of the 22 L strain-infected primary cerebral neurons. Only 14 % of PrPSc in the 22 L strain-infected N2a-3 cells were positive for mAb 8D5, indicating that most of the PrPSc in N2a-3 lack the N-terminal portion. In contrast, nearly half PrPSc detected in the 22 L strain-infected primary cerebral neurons were positive for mAb 8D5, suggesting the abundance of full-length PrPSc that possesses the N-terminal portion of PrP. Further analysis of prion-infected primary neurons using PrPSc-specific immunostaining will reveal the neuron-specific mechanism for prion propagation.

Prion infection impairs lysosomal degradation capacity by interfering with rab7 membrane attachment in neuronal cells

Prions are proteinaceous infectious particles which cause fatal neurodegenerative disorders in humans and animals. They consist of a mostly β-sheeted aggregated isoform (PrP^Sc) of the cellular prion protein (PrP^c). Prions replicate autocatalytically in neurons and other cell types by inducing conformational conversion of PrP^c into PrP^Sc. Within neurons, PrP^Sc accumulates at the plasma membrane and in vesicles of the endocytic pathway. To better understand the mechanisms underlying neuronal dysfunction and death it is critical to know the impact of PrP^Sc accumulation on cellular pathways. We have investigated the effects of prion infection on endo-lysosomal transport. Our study demonstrates that prion infection interferes with rab7 membrane association. Consequently, lysosomal maturation and degradation are impaired. Our findings indicate a mechanism induced by prion infection that supports stable prion replication. We suggest modulation of endo-lysosomal vesicle trafficking and enhancement of lysosomal maturation as novel targets for the treatment of prion diseases.

Astrocyte-to-neuron intercellular prion transfer is mediated by cell-cell contact

Prion diseases are caused by misfolding of the cellular protein PrP(C) to an infectious conformer, PrP(Sc). Intercellular PrP(Sc) transfer propagates conversion and allows infectivity to move from the periphery to the brain. However, how prions spread between cells of the central nervous system is unclear. Astrocytes are specialized non-neuronal cells within the brain that have a number of functions indispensable for brain homeostasis. Interestingly, they are one of the earliest sites of prion accumulation in the brain. A fundamental question arising from this observation is whether these cells are involved in intercellular prion transfer and thereby disease propagation. Using co-culture systems between primary infected astrocytes and granule neurons or neuronal cell lines, we provide direct evidence that prion-infected astrocytes can disseminate prion to neurons. Though astrocytes are capable of secreting PrP, this is an inefficient method of transferring prion infectivity. Efficient transfer required co-culturing and direct cell contact. Astrocytes form numerous intercellular connections including tunneling nanotubes, containing PrP(Sc), often colocalized with endolysosomal vesicles, which may constitute the major mechanism of transfer. Because of their role in intercellular transfer of prions astrocytes may influence progression of the disease.

Focus on Extracellular Vesicles: Physiological Role and Signalling Properties of Extracellular Membrane Vesicles

Extracellular vesicles (EVs) are a heterogeneous population of secreted membrane vesicles, with distinct biogenesis routes, biophysical properties and different functions both in physiological conditions and in disease. The release of EVs is a widespread biological process, which is conserved across species. In recent years, numerous studies have demonstrated that several bioactive molecules are trafficked with(in) EVs, such as microRNAs, mRNAs, proteins and lipids. The understanding of their final impact on the biology of specific target cells remains matter of intense debate in the field. Also, EVs have attracted great interest as potential novel cell-free therapeutics. Here we describe the proposed physiological and pathological functions of EVs, with a particular focus on their molecular content. Also, we discuss the advances in the knowledge of the mechanisms regulating the secretion of EV-associated molecules and the specific pathways activated upon interaction with the target cell, highlighting the role of EVs in the context of the immune system and as mediators of the intercellular signalling in the brain.

Efficient inhibition of infectious prions multiplication and release by targeting the exosomal pathway

Exosomes are secreted membrane vesicles of endosomal origin present in biological fluids. Exosomes may serve as shuttles for amyloidogenic proteins, notably infectious prions, and may participate in their spreading in vivo. To explore the significance of the exosome pathway on prion infectivity and release, we investigated the role of the endosomal sorting complex required for transport (ESCRT) machinery and the need for ceramide, both involved in exosome biogenesis. Silencing of HRS-ESCRT-0 subunit drastically impairs the formation of cellular infectious prion due to an altered trafficking of cholesterol. Depletion of Tsg101-ESCRT-I subunit or impairment of the production of ceramide significantly strongly decreases infectious prion release. Together, our data reveal that ESCRT-dependent and -independent pathways can concomitantly regulate the exosomal secretion of infectious prion, showing that both pathways operate for the exosomal trafficking of a particular cargo. These data open up a new avenue to regulate prion release and propagation.

Subcellular distribution of the prion protein in sickness and in health

The cellular prion protein (PrP(C)) is an ubiquitously expressed glycoprotein that is most abundant in the central nervous system. It is thought to play a role in many cellular processes, including neuroprotection, but may also contribute to Alzheimer's disease and some cancers. However, it is best known for its central role in the prion diseases, such as Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE), and scrapie. These protein misfolding diseases can be sporadic, acquired, or genetic and are caused by refolding of endogenous PrP(C) into a beta sheet-rich, pathogenic form, PrP(Sc). Once prions are present in the central nervous system, they increase and spread during a long incubation period that is followed by a relatively short clinical disease phase, ending in death. PrP molecules can be broadly categorized as either 'good' (cellular) PrP(C) or 'bad' (scrapie prion-type) PrP(Sc), but both populations are heterogeneous and different forms of PrP(C) may influence various cellular activities. Both PrP(C) and PrP(Sc) are localized predominantly at the cell surface, with the C-terminus attached to the plasma membrane via a glycosyl-phosphatidylinositol (GPI) anchor and both can exist in cleaved forms. PrP(C) also has cytosolic and transmembrane forms, and PrP(Sc) is known to exist in a variety of conformations and aggregation states. Here, we discuss the roles of different PrP isoforms in sickness and in health, and show the subcellular distributions of several forms of PrP that are particularly relevant for PrP(C) to PrP(Sc) conversion and prion-induced pathology in the hippocampus.

Prion aggregates transfer through tunneling nanotubes in endocytic vesicles

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases caused by the misfolding of the cellular prion protein to an infectious form PrP(Sc). The intercellular transfer of PrP(Sc) is a question of immediate interest as the cell-to-cell movement of the infectious particle causes the inexorable propagation of disease. We have previously identified tunneling nanotubes (TNTs) as one mechanism by which PrP(Sc) can move between cells. Here we investigate further the details of this mechanism and show that PrP(Sc) travels within TNTs in endolysosomal vesicles. Additionally we show that prion infection of CAD cells increases both the number of TNTs and intercellular transfer of membranous vesicles, thereby possibly playing an active role in its own intercellular transfer via TNTs.

Cholesterol balance in prion diseases and Alzheimer's disease

Prion diseases are transmissible and fatal neurodegenerative disorders of humans and animals. They are characterized by the accumulation of PrP^Sc, an aberrantly folded isoform of the cellular prion protein PrP^C, in the brains of affected individuals. PrP^C is a cell surface glycoprotein attached to the outer leaflet of the plasma membrane by a glycosyl-phosphatidyl-inositol (GPI) anchor. Specifically, it is associated with lipid rafts, membrane microdomains enriched in cholesterol and sphinoglipids. It has been established that inhibition of endogenous cholesterol synthesis disturbs lipid raft association of PrP^C and prevents PrP^Sc accumulation in neuronal cells. Additionally, prion conversion is reduced upon interference with cellular cholesterol uptake, endosomal export, or complexation at the plasma membrane. Altogether, these results demonstrate on the one hand the importance of cholesterol for prion propagation. On the other hand, growing evidence suggests that prion infection modulates neuronal cholesterol metabolism. Similar results were reported in Alzheimer’s disease (AD): whereas amyloid β peptide formation is influenced by cellular cholesterol, levels of cholesterol in the brains of affected individuals increase during the clinical course of the disease. In this review, we summarize commonalities of alterations in cholesterol homeostasis and discuss consequences for neuronal function and therapy of prion diseases and AD.

Structural and dynamic properties of the human prion protein

Prion diseases involve the conformational conversion of the cellular prion protein (PrP(C)) to its misfolded pathogenic form (PrP(Sc)). To better understand the structural mechanism of this conversion, we performed extensive all-atom, explicit-solvent molecular-dynamics simulations for three structures of the wild-type human PrP (huPrP) at different pH values and temperatures. Residue 129 is polymorphic, being either Met or Val. Two of the three structures have Met in position 129 and the other has Val. Lowering the pH or raising the temperature induced large conformational changes of the C-terminal globular domain and increased exposure of its hydrophobic core. In some simulations, HA and its preceding S1-HA loop underwent large displacements. The C-terminus of HB was unstable and sometimes partially unfolded. Two hydrophobic residues, Phe-198 and Met-134, frequently became exposed to solvent. These conformational changes became more dramatic at lower pH or higher temperature. Furthermore, Tyr-169 and the S2-HB loop, or the X-loop, were different in the starting structures but converged to common conformations in the simulations for the Met-129, but not the Val-129, protein. α-Strands and β-strands formed in the initially unstructured N-terminus. α-Strand propensity in the N-terminus was different between the Met-129 and Val129 proteins, but β-strand propensity was similar. This study reveals detailed structural and dynamic properties of huPrP, providing insight into the mechanism of the conversion of PrP(C) to PrP(Sc).

Targeting prion propagation using peptide constructs with signal sequence motifs

Synthetic peptides with sequences derived from the cellular prion protein (PrP(C)) unprocessed N-terminus are able to counteract the propagation of proteinase K resistant prions (PrP(Res), indicating the presence of the prion isoform of the prion protein) in cell cultures (Löfgren et al., 2008). The anti-prion peptides have characteristics like cell penetrating peptides (CPPs) and consist of the prion protein hydrophobic signal sequence followed by a polycationic motif (residues KKRPKP), in mouse PrP(C) corresponding to residues 1-28. Here we analyze the sequence elements required for the anti-prion effect of KKRPKP-conjugates. Neuronal GT1-1 cells were infected with either prion strain RML or 22L. Variable peptide constructs originating from the mPrP1-28 sequence were analyzed for anti-prion effects, measured as disappearance of proteinase K resistant prions (PrP(Res)) in the infected cell cultures. We find that even a 5 amino acid N-terminal shortening of the signal peptide abolishes the anti-prion effect. We show that the signal peptide from PrP(C) can be replaced with the signal peptide from the Neural cell adhesion molecule-1; NCAM11-19, with a retained capacity to reduce PrP(Res) levels. The anti-prion effect is lost if the polycationic N-terminal PrP(C)-motif is conjugated to any conventional CPP, such as TAT48-60, transportan-10 or penetratin. We propose a mechanism by which a signal peptide from a secretory or cell surface protein acts to promote the transport of a prion-binding polycationic PrP(C)-motif to a subcellular location where prion conversion occurs (most likely the Endosome Recycling Compartment), thereby targeting prion propagation.

Polymorphisms at amino acid residues 141 and 154 influence conformational variation in ovine PrP

Polymorphisms in ovine PrP at amino acid residues 141 and 154 are associated with susceptibility to ovine prion disease: Leu141Arg154 with classical scrapie and Phe141Arg154 and Leu141His154 with atypical scrapie. Classical scrapie is naturally transmissible between sheep, whereas this may not be the case with atypical scrapie. Critical amino acid residues will determine the range or stability of structural changes within the ovine prion protein or its functional interaction with potential cofactors, during conversion of PrPC to PrPSc in these different forms of scrapie disease. Here we computationally identified that regions of ovine PrP, including those near amino acid residues 141 and 154, displayed more conservation than expected based on local structural environment. Molecular dynamics simulations showed these conserved regions of ovine PrP displayed genotypic differences in conformational repertoire and amino acid side-chain interactions. Significantly, Leu141Arg154 PrP adopted an extended beta sheet arrangement in the N-terminal palindromic region more frequently than the Phe141Arg154 and Leu141His154 variants. We supported these computational observations experimentally using circular dichroism spectroscopy and immunobiochemical studies on ovine recombinant PrP. Collectively, our observations show amino acid residues 141 and 154 influence secondary structure and conformational change in ovine PrP that may correlate with different forms of scrapie.

Live imaging of prions reveals nascent PrPSc in cell-surface, raft-associated amyloid strings and webs

Mammalian prions refold host glycosylphosphatidylinositol-anchored PrP(C) into β-sheet-rich PrP(Sc). PrP(Sc) is rapidly truncated into a C-terminal PrP27-30 core that is stable for days in endolysosomes. The nature of cell-associated prions, their attachment to membranes and rafts, and their subcellular locations are poorly understood; live prion visualization has not previously been achieved. A key obstacle has been the inaccessibility of PrP27-30 epitopes. We overcame this hurdle by focusing on nascent full-length PrP(Sc) rather than on its truncated PrP27-30 product. We show that N-terminal PrP(Sc) epitopes are exposed in their physiological context and visualize, for the first time, PrP(Sc) in living cells. PrP(Sc) resides for hours in unexpected cell-surface, slow moving strings and webs, sheltered from endocytosis. Prion strings observed by light and scanning electron microscopy were thin, micrometer-long structures. They were firmly cell associated, resisted phosphatidylinositol-specific phospholipase C, aligned with raft markers, fluoresced with thioflavin, and were rapidly abolished by anti-prion glycans. Prion strings and webs are the first demonstration of membrane-anchored PrP(Sc) amyloids.

Glycosylphosphatidylinositol anchoring directs the assembly of Sup35NM protein into non-fibrillar, membrane-bound aggregates

In prion-infected hosts, PrPSc usually accumulates as non-fibrillar, membrane-bound aggregates. Glycosylphosphatidylinositol (GPI) anchor-directed membrane association appears to be an important factor controlling the biophysical properties of PrPSc aggregates. To determine whether GPI anchoring can similarly modulate the assembly of other amyloid-forming proteins, neuronal cell lines were generated that expressed a GPI-anchored form of a model amyloidogenic protein, the NM domain of the yeast prion protein Sup35 (Sup35(GPI)). We recently reported that GPI anchoring facilitated the induction of Sup35(GPI) prions in this system. Here, we report the ultrastructural characterization of self-propagating Sup35(GPI) aggregates of either spontaneous or induced origin. Like membrane-bound PrPSc, Sup35(GPI) aggregates resisted release from cells treated with phosphatidylinositol-specific phospholipase C. Sup35(GPI) aggregates of spontaneous origin were detergent-insoluble, protease-resistant, and self-propagating, in a manner similar to that reported for recombinant Sup35NM amyloid fibrils and induced Sup35(GPI) aggregates. However, GPI-anchored Sup35 aggregates were not stained with amyloid-binding dyes, such as Thioflavin T. This was consistent with ultrastructural analyses, which showed that the aggregates corresponded to dense cell surface accumulations of membrane vesicle-like structures and were not fibrillar. Together, these results showed that GPI anchoring directs the assembly of Sup35NM into non-fibrillar, membrane-bound aggregates that resemble PrPSc, raising the possibility that GPI anchor-dependent modulation of protein aggregation might occur with other amyloidogenic proteins. This may contribute to differences in pathogenesis and pathology between prion diseases, which uniquely involve aggregation of a GPI-anchored protein, versus other protein misfolding diseases.

Molecular dynamics simulations capture the misfolding of the bovine prion protein at acidic pH

Bovine spongiform encephalopathy (BSE), or mad cow disease, is a fatal neurodegenerative disease that is transmissible to humans and that is currently incurable. BSE is caused by the prion protein (PrP), which adopts two conformers; PrPC is the native innocuous form, which is α-helix rich; and PrPSc is the β-sheet rich misfolded form, which is infectious and forms neurotoxic species. Acidic pH induces the conversion of PrPC to PrPSc. We have performed molecular dynamics simulations of bovine PrP at various pH regimes. An acidic pH environment induced conformational changes that were not observed in neutral pH simulations. Putative misfolded structures, with nonnative β-strands formed in the flexible N-terminal domain, were found in acidic pH simulations. Two distinct pathways were observed for the formation of nonnative β-strands: at low pH, hydrophobic contacts with M129 nucleated the nonnative β-strand; at mid-pH, polar contacts involving Q168 and D178 facilitated the formation of a hairpin at the flexible N-terminus. These mid- and low pH simulations capture the process of nonnative β-strand formation, thereby improving our understanding of how PrPC misfolds into the β-sheet rich PrPSc and how pH factors into the process.

Characterization of intracellular dynamics of inoculated PrP-res and newly generated PrP(Sc) during early stage prion infection in Neuro2a cells

To clarify the cellular mechanisms for the establishment of prion infection, we analyzed the intracellular dynamics of inoculated and newly generated abnormal isoform of prion protein (PrP(Sc)) in Neuro2a cells. Within 24h after inoculation, the newly generated PrP(Sc) was evident at the plasma membrane, in early endosomes, and in late endosomes, but this PrP(Sc) was barely evident in lysosomes; in contrast, the majority of the inoculated PrP(Sc) was evident in late endosomes and lysosomes. However, during the subsequent 48 h, the newly generated PrP(Sc) increased remarkably in early endosomes and recycling endosomes. Overexpression of wild-type and mutant Rab proteins showed that membrane trafficking along not only the endocytic-recycling pathway but also the endo-lysosomal pathway is involved in de novo PrP(Sc) generation. These results suggest that the trafficking of exogenously introduced PrP(Sc) from the endo-lysosomal pathway to the endocytic-recycling pathway is important for the establishment of prion infection.

Prion protein misfolding, strains, and neurotoxicity: an update from studies on Mammalian prions

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative disorders affecting humans and other mammalian species. The central event in TSE pathogenesis is the conformational conversion of the cellular prion protein, PrP^C, into the aggregate, β-sheet rich, amyloidogenic form, PrP^Sc. Increasing evidence indicates that distinct PrP^Sc conformers, forming distinct ordered aggregates, can encipher the phenotypic TSE variants related to prion strains. Prion strains are TSE isolates that, after inoculation into syngenic hosts, cause disease with distinct characteristics, such as incubation period, pattern of PrP^Sc distribution, and regional severity of histopathological changes in the brain. In analogy with other amyloid forming proteins, PrP^Sc toxicity is thought to derive from the existence of various intermediate structures prior to the amyloid fiber formation and/or their specific interaction with membranes. The latter appears particularly relevant for the pathogenesis of TSEs associated with GPI-anchored PrP^Sc, which involves major cellular membrane distortions in neurons. In this review, we update the current knowledge on the molecular mechanisms underlying three fundamental aspects of the basic biology of prions such as the putative mechanism of prion protein conversion to the pathogenic form PrP^Sc and its propagation, the molecular basis of prion strains, and the mechanism of induced neurotoxicity by PrP^Sc aggregates.

Plasma membrane invaginations containing clusters of full-length PrPSc are an early form of prion-associated neuropathology in vivo

During prion disease, cellular prion protein (PrP(C)) is refolded into a pathogenic isoform (PrP(Sc)) that accumulates in the central nervous system and causes neurodegeneration and death. We used immunofluorescence, quantitative cryo-immunogold EM, and tomography to detect nascent, full-length PrP(Sc) in the hippocampus of prion-infected mice from early preclinical disease stages onward. Comparison of uninfected and infected brains showed that sites containing full-length PrP(Sc) could be recognized in the neuropil by bright spots and streaks of immunofluorescence on semi-thin (200-nm) sections, and by clusters of cryo-immunogold EM labeling. PrP(Sc) was found mainly on neuronal plasma membranes, most strikingly on membrane invaginations and sites of cell-to-cell contact, and was evident by 65 days postinoculation, or 54% of the incubation period to terminal disease. Both axons and dendrites in the neuropil were affected. We hypothesize that closely apposed plasma membranes provide a favorable environment for prion conversion and intercellular prion transfer. Only a small proportion of clustered PrP immunogold labeling was found at synapses, indicating that synapses are not targeted specifically in prion disease.

Disturbed vesicular trafficking of membrane proteins in prion disease

The pathogenic mechanism of prion diseases remains unknown. We recently reported that prion infection disturbs post-Golgi trafficking of certain types of membrane proteins to the cell surface, resulting in reduced surface expression of membrane proteins and abrogating the signal from the proteins. The surface expression of the membrane proteins was reduced in the brains of mice inoculated with prions, well before abnormal symptoms became evident. Prions or pathogenic prion proteins were mainly detected in endosomal compartments, being particularly abundant in recycling endosomes. Some newly synthesized membrane proteins are delivered to the surface from the Golgi apparatus through recycling endosomes, and some endocytosed membrane proteins are delivered back to the surface through recycling endosomes. These results suggest that prions might cause neuronal dysfunctions and cell loss by disturbing post-Golgi trafficking of membrane proteins via accumulation in recycling endosomes. Interestingly, it was recently shown that delivery of a calcium channel protein to the cell surface was impaired and its function was abrogated in a mouse model of hereditary prion disease. Taken together, these results suggest that impaired delivery of membrane proteins to the cell surface is a common pathogenic event in acquired and hereditary prion diseases.

Absence of CD14 delays progression of prion diseases accompanied by increased microglial activation

Prion diseases are fatal neurodegenerative disorders characterized by accumulation of PrP(Sc), vacuolation of neurons and neuropil, astrocytosis, and microglial activation. Upregulation of gene expressions of innate immunity-related factors, including complement factors and CD14, is observed in the brains of mice infected with prions even in the early stage of infections. When CD14 knockout (CD14(-/-)) mice were infected intracerebrally with the Chandler and Obihiro prion strains, the mice survived longer than wild-type (WT) mice, suggesting that CD14 influences the progression of the prion disease. Immunofluorescence staining that can distinguish normal prion protein from the disease-specific form of prion protein (PrP(Sc)) revealed that deposition of PrP(Sc) was delayed in CD14(-/-) mice compared with WT mice by the middle stage of the infection. Immunohistochemical staining with Iba1, a marker for activated microglia, showed an increased microglial activation in prion-infected CD14(-/-) mice compared to WT mice. Interestingly, accompanied by the increased microglial activation, anti-inflammatory cytokines interleukin-10 (IL-10) and transforming growth factor β (TGF-β) appeared to be expressed earlier in prion-infected CD14(-/-) mice. In contrast, IL-1β expression appeared to be reduced in the CD14(-/-) mice in the early stage of infection. Double immunofluorescence staining demonstrated that CD11b- and Iba1-positive microglia mainly produced the anti-inflammatory cytokines, suggesting anti-inflammatory status of microglia in the CD14(-/-) mice in the early stage of infection. These results imply that CD14 plays a role in the disease progression by suppressing anti-inflammatory responses in the brain in the early stage of infection.

Probing early misfolding events in prion protein mutants by NMR spectroscopy

The post-translational conversion of the ubiquitously expressed cellular form of the prion protein, PrPC, into its misfolded and pathogenic isoform, known as prion or PrPSc, plays a key role in prion diseases. These maladies are denoted transmissible spongiform encephalopathies (TSEs) and affect both humans and animals. A prerequisite for understanding TSEs is unraveling the molecular mechanism leading to the conversion process whereby most α-helical motifs are replaced by β-sheet secondary structures. Importantly, most point mutations linked to inherited prion diseases are clustered in the C-terminal domain region of PrPC and cause spontaneous conversion to PrPSc. Structural studies with PrP variants promise new clues regarding the proposed conversion mechanism and may help identify "hot spots" in PrPC involved in the pathogenic conversion. These investigations may also shed light on the early structural rearrangements occurring in some PrPC epitopes thought to be involved in modulating prion susceptibility. Here we present a detailed overview of our solution-state NMR studies on human prion protein carrying different pathological point mutations and the implications that such findings may have for the future of prion research.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Exosomes: vesicular carriers for intercellular communication in neurodegenerative disorders

The intercellular transfer of misfolded proteins has received increasing attention in various neurodegenerative diseases characterized by the aggregation of specific proteins, as observed in Alzheimer's, Parkinson's and Huntington's disease. One hypothesis holds that intercellular dissemination of these aggregates within the central nervous system results in the seeded assembly of the cognate soluble protein in target cells, similar to that proposed for transmissible prion diseases. The molecular mechanisms underlying the intercellular transfer of these proteinaceous aggregates are poorly understood. Various transfer modes of misfolded proteins including continuous cell-cell contacts such as nanotubes, unconventional secretion or microvesicle/exosome-associated dissemination have been suggested. Cells can release proteins, lipids and nucleic acids by vesicular exocytosis pathways destined for horizontal transfer. Encapsulation into microvesicular/exosomal vehicles not only protects these molecules from degradation and dilution in the extracellular space but also facilitates delivery over large distances, e.g. within the blood flow or interstitial fluid. Specific surface ligands might allow the highly efficient and targeted uptake of these vesicles by recipient cells. In this review, we focus on the cell biology and function of neuronal microvesicles/exosomes and discuss the evidence for pathogenic intercellular protein transfer mediated by vesicular carriers.

Disruption of the X-loop turn of the prion protein linked to scrapie resistance

The prion diseases are a class of neurodegenerative diseases caused by the misfolding and aggregation of the prion protein (PrP(C)) into toxic and infectious oligomers (PrP(Sc)). These oligomers are critical to understanding and combating these diseases. Differences in the sequence of PrP affect disease susceptibility, likely by shifting the tolerance of the protein for adaptation to PrP(Sc) conformations and/or the recognition event between PrP(Sc) and PrP(C) prior to conversion of the PrP(C). We selected two sets of PrP(Sc)-resistant mutant sequences for solvated atomistic molecular dynamics simulation to investigate the structural basis of resistance. The first group involved mutation in the X-loop (residues 164-171) resulting from selective breeding of sheep. The second group included eight mutants in mice identified by random mutagenesis targeting helix C followed by screening in cell cultures. Multiple simulations were performed of 14 different mutant and control constructs under different pH conditions for a total of 3.6 μs of simulation time. The X-loop formed a stable turn at neutral pH in wild-type PrP from both species. PrP(Sc)-resistant mutations disrupted this turn even though only one of the mutants is in the X-loop. The X-loop is compact and buried in our previously described spiral models of PrP(Sc)-like oligomers. On the basis of the findings presented here and in the context of the spiral oligomer model, we propose that expansion of the X-loop disrupts protofibril packing, providing a structural basis for resistance.