Ablation of the prion protein (PrP) gene in mice prevents scrapie and facilitates production of anti-PrP antibodies

Expression of the cellular prion protein by mast cells in white-tailed deer carotid body, cervical lymph nodes and ganglia

Chronic wasting disease (CWD) is a transmissible and fatal prion disease that affects cervids. While both oral and nasal routes of exposure to prions cause disease, the spatial and temporal details of how prions enter the central nervous system (CNS) are unknown. Carotid bodies (CBs) are structures that are exposed to blood-borne prions and are densely innervated by nerves that are directly connected to brainstem nuclei, known to be early sites of prion neuroinvasion. All CBs examined contained mast cells expressing the prion protein which is consistent with these cells playing a role in neuroinvasion following prionemia.

Protective role of cytosolic prion protein against virus infection in prion-infected cells

Production of the amyloidogenic prion protein, PrPSc, which forms infectious protein aggregates, or prions, is a key pathogenic event in prion diseases. Functional prion-like protein aggregations, such as the mitochondrial adaptor protein MAVS and the inflammasome component protein ASC, have been identified to play a protective role in viral infections in mammalian cells. In this study, to investigate if PrPSc could play a functional role against external stimuli, we infected prion-infected cells with a neurotropic influenza A virus strain, IAV/WSN. We found that prion-infected cells were highly resistant to IAV/WSN infection. In these cells, NF-κB nuclear translocation was disturbed; therefore, mitochondrial superoxide dismutase (mtSOD) expression was suppressed, and mitochondrial reactive oxygen species (mtROS) was increased. The elevated mtROS subsequently activated NLRP3 inflammasomes, leading to the suppression of IAV/WSN-induced necroptosis. We also found that prion-infected cells accumulated a portion of PrP molecules in the cytosol, and that the N-terminal potential nuclear translocation signal of PrP impeded NF-κB nuclear translocation. These results suggest that PrPSc might play a functional role in protection against viral infections by stimulating the NLRP3 inflammasome-dependent antivirus mechanism through the cytosolic PrP-mediated disturbance of NF-κB nuclear translocation, which leads to suppression of mtSOD expression and consequently upregulation of the NLRP3 inflammasome activator mtROS.

IMPORTANCE

Cytosolic PrP has been detected in prion-infected cells and suggested to be involved in the neurotoxicity of prions. Here, we also detected cytosolic PrP in prion-infected cells. We further found that the nuclear translocation of NF-κB was disturbed in prion-infected cells and that the N-terminal potential nuclear translocation signal of PrP expressed in the cytosol disturbed the nuclear translocation of NF-κB. Thus, the N-terminal nuclear translocation signal of cytosolic PrP might play a role in prion neurotoxicity. Prion-like protein aggregates in other protein misfolding disorders, including Alzheimer's disease were reported to play a protective role against various environmental stimuli. We here showed that prion-infected cells were partially resistant to IAV/WSN infection due to the cytosolic PrP-mediated disturbance of the nuclear translocation of NF-κB, which consequently activated NLRP3 inflammasomes after IAV/WSN infection. It is thus possible that prions could also play a protective role in viral infections.

Single-domain antibodies and aptamers drive new opportunities for neurodegenerative disease research

Neurodegenerative diseases (NDs) in mammals, such as Alzheimer's disease (AD), Parkinson's disease (PD), and transmissible spongiform encephalopathies (TSEs), are characterized by the accumulation of misfolded proteins in the central nervous system (CNS). Despite the presence of these pathogenic proteins, the immune response in affected individuals remains notably muted. Traditional immunological strategies, particularly those reliant on monoclonal antibodies (mAbs), face challenges related to tissue penetration, blood-brain barrier (BBB) crossing, and maintaining protein stability. This has led to a burgeoning interest in alternative immunotherapeutic avenues. Notably, single-domain antibodies (or nanobodies) and aptamers have emerged as promising candidates, as their reduced size facilitates high affinity antigen binding and they exhibit superior biophysical stability compared to mAbs. Aptamers, synthetic molecules generated from DNA or RNA ligands, present both rapid production times and cost-effective solutions. Both nanobodies and aptamers exhibit inherent qualities suitable for ND research and therapeutic development. Cross-seeding events must be considered in both traditional and small-molecule-based immunodiagnostic and therapeutic approaches, as well as subsequent neurotoxic impacts and complications beyond protein aggregates. This review delineates the challenges traditional immunological methods pose in ND research and underscores the potential of nanobodies and aptamers in advancing next-generation ND diagnostics and therapeutics.

The importance of prion research

Over the past four decades, prion diseases have received considerable research attention owing to their potential to be transmitted within and across species as well as their consequences for human and animal health. The unprecedented nature of prions has led to the discovery of a paradigm of templated protein misfolding that underlies a diverse range of both disease-related and normal biological processes. Indeed, the "prion-like" misfolding and propagation of protein aggregates is now recognized as a common underlying disease mechanism in human neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and the prion principle has led to the development of novel diagnostic and therapeutic strategies for these illnesses. Despite these advances, research into the fundamental biology of prion diseases has declined, likely due to their rarity and the absence of an acute human health crisis. Given the past translational influence, continued research on the etiology, pathogenesis, and transmission of prion disease should remain a priority. In this review, we highlight several important "unsolved mysteries" in the prion disease research field and how solving them may be crucial for the development of effective therapeutics, preventing future outbreaks of prion disease, and understanding the pathobiology of more common human neurodegenerative disorders.

Expression of the cellular prion protein by mast cells in the human carotid body

Prion diseases are fatal neurologic disorders that can be transmitted by blood transfusion. The route for neuroinvasion following exposure to infected blood is not known. Carotid bodies (CBs) are specialized chemosensitive structures that detect the concentration of blood gasses and provide feedback for the neural control of respiration. Sensory cells of the CB are highly perfused and densely innervated by nerves that are synaptically connected to the brainstem and thoracic spinal cord, known to be areas of early prion deposition following oral infection. Given their direct exposure to blood and neural connections to central nervous system (CNS) areas involved in prion neuroinvasion, we sought to determine if there were cells in the human CB that express the cellular prion protein (PrPC), a characteristic that would support CBs serving as a route for prion neuroinvasion. We collected CBs from cadaver donor bodies and determined that mast cells located in the carotid bodies express PrPC and that these cells are in close proximity to blood vessels, nerves, and nerve terminals that are synaptically connected to the brainstem and spinal cord.

Strain-dependent role of copper in prion disease through binding to histidine residues in the N-terminal domain of prion protein

The cellular prion protein, PrPC , is a copper-binding protein abundantly expressed in the brain, particularly by neurons, and its conformational conversion into the amyloidogenic isoform, PrPSc , plays a key pathogenic role in prion diseases. However, the role of copper binding to PrPC in prion diseases remains unclear. Here, we fed mice with a low-copper or regular diet and intracerebrally inoculated them with two different mouse-adapted RML scrapie and BSE prions. Mice with a low-copper diet developed disease significantly but only slightly later than those with a regular diet after inoculation with BSE prions, but not with RML prions, suggesting that copper could play a minor role in BSE prion pathogenesis, but not in RML prion pathogenesis. We then generated two lines of transgenic mice expressing mouse PrP with copper-binding histidine (His) residues in the N-terminal domain replaced with alanine residues, termed TgPrP(5H > A)-7342/Prnp0/0 and TgPrP(5H > A)-7524/Prnp0/0 mice, and similarly inoculated RML and BSE prions into them. Due to 2-fold higher expression of PrP(5H > A) than PrPC in wild-type (WT) mice, TgPrP(5H > A)-7524/Prnp0/0 mice were highly susceptible to these prions, compared to WT mice. However, TgPrP(5H > A)-7342/Prnp0/0 mice, which express PrP(5H > A) 1.2-fold as high as PrPC in WT mice, succumbed to disease slightly, but not significantly, later than WT mice after inoculation with RML prions, but significantly so after inoculation with BSE prions. Subsequent secondary inoculation experiments revealed that amino acid sequence differences between PrP(5H > A) and WT PrPSc created no prion transmission barrier to BSE prions. These results suggest that copper-binding His residues in PrPC are dispensable for RML prion pathogenesis but have a minor effect on BSE prion pathogenesis. Taken together, our current results suggest that copper could have a minor effect on prion pathogenesis in a strain-dependent manner through binding to His residues in the N-terminal domain of PrPC .

Second passage experiments of chronic wasting disease in transgenic mice overexpressing human prion protein

Chronic wasting disease (CWD) is a prion disease of cervids including deer, elk, reindeer, and moose. Human consumption of cervids is common, therefore assessing the risk potential of CWD transmission to humans is critical. In a previous study, we tested CWD transmission via intracerebral inoculation into transgenic mice (tg66 and tgRM) that over-expressed human prion protein. Mice screened by traditional prion detection assays were negative. However, in a group of 88 mice screened by the ultrasensitive RT-QuIC assay, we identified 4 tg66 mice that produced inconsistent positive RT-QuIC reactions. These data could be false positive reactions, residual input inoculum or indicative of subclinical infections suggestive of cross species transmission of CWD to humans. Additional experiments were required to understand the nature of the prion seeding activity in this model. In this manuscript, second passage experiments using brains from mice with weak prion seeding activity showed they were not infectious to additional recipient tg66 mice. Clearance experiments showed that input CWD prion seeding activity was eliminated by 180 days in tg66 mice and PrPKO mice, which are unable to replicate prion protein, indicating that the weak positive levels of seeding activity detected at later time points was not likely residual inoculum. The failure of CWD prions to cause disease in tg66 after two sequential passages suggested that a strong species barrier prevented CWD infection of mice expressing human prion protein.

Chronological Changes in the Expression Pattern of Hippocampal Prion Proteins During Disease Progression in Sporadic Creutzfeldt-Jakob Disease MM1 Subtype

The differential effects of sporadic Creutzfeldt-Jakob disease (sCJD) on the hippocampus and other neocortical areas are poorly understood. We aimed to reveal the histological patterns of cellular prion protein (PrPC) and abnormal prion protein (PrPSc) in hippocampi of sCJD patients and normal controls (NCs). Our study examined 18 postmortem sCJD patients (MM1, 14 cases; MM1 + 2c, 3 cases; MM1 + 2t, 1 case) and 12 NCs. Immunohistochemistry was conducted using 4 primary antibodies, of which 3 targeted the N-terminus of the prion protein (PrP), and 1 (EP1802Y) targeted the C-terminal domain. PrPC expression was abundant in the hippocampus of NCs, and the distribution of PrPC at CA3/4 was reminiscent of synaptic complexes. In sCJD cases with a disease history of <2 years, antibodies against the N-terminus could not detect synapse-like PrP expression at CA4; however, EP1802Y could characterize the synapse-like expression. PrPSc accumulation and spongiform changes became evident after 2 years of illness, when PrPSc deposits were more noticeably detected by N-terminal-specific antibodies. Our findings highlighted the chronology of histopathological alterations in the CA4 region in sCJD patients.

Central Residues in Prion Protein PrPC Are Crucial for Its Conversion into the Pathogenic Isoform

Conformational conversion of the cellular prion protein, PrP^C, into the amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the conversion mechanism remains to be elucidated. Here, we generated Tg(PrPΔ91-106)-8545/Prnp^0/0 mice, which overexpress mouse PrP lacking residues 91-106. We showed that none of the mice became sick after intracerebral inoculation with RML, 22L, and FK-1 prion strains nor accumulated PrP^ScΔ91-106 in their brains except for a small amount of PrP^ScΔ91-106 detected in one 22L-inoculated mouse. However, they developed disease around 85 days after inoculation with bovine spongiform encephalopathy (BSE) prions with PrP^ScΔ91-106 in their brains. These results suggest that residues 91-106 are important for PrP^C conversion into PrP^Sc in infection with RML, 22L, and FK-1 prions but not BSE prions. We then narrowed down the residues 91-106 by transducing various PrP deletional mutants into RML- and 22L-infected cells and identified that PrP mutants lacking residues 97-99 failed to convert into PrP^Sc in these cells. Our in vitro conversion assay also showed that RML, 22L, and FK-1 prions did not convert PrPΔ97-99 into PrP^ScΔ97-99, but BSE prions did. We further found that PrP mutants with proline residues at positions 97 to 99 or charged residues at positions 97 and 99 completely or almost completely lost their converting activity into PrP^Sc in RML- and 22L-infected cells. These results suggest that the structurally flexible and noncharged residues 97-99 could be important for PrP^C conversion into PrP^Sc following infection with RML, 22L, and FK-1 prions but not BSE prions.

Investigating CRISPR/Cas9 gene drive for production of disease-preventing prion gene alleles

Prion diseases are a group of fatal neurodegenerative disorders that includes chronic wasting disease, which affects cervids and is highly transmissible. Given that chronic wasting disease prevalence exceeds 30% in some endemic areas of North America, and that eventual transmission to other mammalian species, potentially including humans, cannot be ruled out, novel control strategies beyond population management via hunting and/or culling must be investigated. Prion diseases depend upon post-translational conversion of the cellular prion protein, encoded by the Prnp gene, into a disease-associated conformation; ablation of cellular prion protein expression, which is generally well-tolerated, eliminates prion disease susceptibility entirely. Inspired by demonstrations of gene drive in caged mosquito species, we aimed to test whether a CRISPR/Cas9-based gene drive mechanism could, in principle, promote the spread of a null Prnp allele among mammalian populations. First, we showed that transient co-expression of Cas9 and Prnp-directed guide RNAs in RK13 cells generates indels within the Prnp open-reading frame, indicating that repair of Cas9-induced double-strand breaks by non-homologous end-joining had taken place. Second, we integrated a ~1.2 kb donor DNA sequence into the Prnp open-reading frame in N2a cells by homology-directed repair following Cas9-induced cleavages and confirmed that integration occurred precisely in most cases. Third, we demonstrated that electroporation of Cas9/guide RNA ribonucleoprotein complexes into fertilised mouse oocytes resulted in pups with a variety of disruptions to the Prnp open reading frame, with a new coisogenic line of Prnp-null mice obtained as part of this work. However, a technical challenge in obtaining expression of Cas9 in the male germline prevented implementation of a complete gene drive mechanism in mice.

Chronic Wasting Disease (CWD) in Cervids and the Consequences of a Mutable Protein Conformation

Chronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose, etc.). It spreads readily from CWD-contaminated environments and among wild cervids. As of 2022, North American CWD has been found in 29 states, four Canadian provinces and South Korea. The Scandinavian form of CWD originated independently. Prions propagate their pathology by inducing a natively expressed prion protein (PrPC) to adopt the prion conformation (PrPSc). PrPC and PrPSc differ solely in their conformation. Like other prion diseases, transmissible CWD prions can arise spontaneously. The CWD prions can respond to selection pressures resulting in the emergence of new strain phenotypes. Annually, 11.5 million Americans hunt and harvest nearly 6 million deer, indicating that CWD is a potential threat to an important American food source. No tested CWD strain has been shown to be zoonotic. However, this may not be true for emerging strains. Should a zoonotic CWD strain emerge, it could adversely impact the hunting economy and game meat consumers.

Transport of Prions in the Peripheral Nervous System: Pathways, Cell Types, and Mechanisms

Prion diseases are transmissible protein misfolding disorders that occur in animals and humans where the endogenous prion protein, PrPC, undergoes a conformational change into self-templating aggregates termed PrPSc. Formation of PrPSc in the central nervous system (CNS) leads to gliosis, spongiosis, and cellular dysfunction that ultimately results in the death of the host. The spread of prions from peripheral inoculation sites to CNS structures occurs through neuroanatomical networks. While it has been established that endogenous PrPC is necessary for prion formation, and that the rate of prion spread is consistent with slow axonal transport, the mechanistic details of PrPSc transport remain elusive. Current research endeavors are primarily focused on the cellular mechanisms of prion transport associated with axons. This includes elucidating specific cell types involved, subcellular machinery, and potential cofactors present during this process.

Loss of the N-acetylgalactosamine side chain of the GPI-anchor impairs bone formation and brain functions and accelerates the prion disease pathology

Glycosylphosphatidylinositol (GPI) is a posttranslational glycolipid modification of proteins that anchors proteins in lipid rafts on the cell surface. Although some GPI-anchored proteins (GPI-APs), including the prion protein PrP^C, have a glycan side chain composed of N-acetylgalactosamine (GalNAc)−galactose−sialic acid on the core structure of GPI glycolipid, in vivo functions of this GPI-GalNAc side chain are largely unresolved. Here, we investigated the physiological and pathological roles of the GPI-GalNAc side chain in vivo by knocking out its initiation enzyme, PGAP4, in mice. We show that Pgap4 mRNA is highly expressed in the brain, particularly in neurons, and mass spectrometry analysis confirmed the loss of the GalNAc side chain in PrP^C GPI in PGAP4-KO mouse brains. Furthermore, PGAP4-KO mice exhibited various phenotypes, including an elevated blood alkaline phosphatase level, impaired bone formation, decreased locomotor activity, and impaired memory, despite normal expression levels and lipid raft association of various GPI-APs. Thus, we conclude that the GPI-GalNAc side chain is required for in vivo functions of GPI-APs in mammals, especially in bone and the brain. Moreover, PGAP4-KO mice were more vulnerable to prion diseases and died earlier after intracerebral inoculation of the pathogenic prion strains than wildtype mice, highlighting the protective roles of the GalNAc side chain against prion diseases.

Virus Infection, Genetic Mutations, and Prion Infection in Prion Protein Conversion

Conformational conversion of the cellular isoform of prion protein, PrP^C, into the abnormally folded, amyloidogenic isoform, PrP^Sc, is an underlying pathogenic mechanism in prion diseases. The diseases manifest as sporadic, hereditary, and acquired disorders. Etiological mechanisms driving the conversion of PrP^C into PrP^Sc are unknown in sporadic prion diseases, while prion infection and specific mutations in the PrP gene are known to cause the conversion of PrP^C into PrP^Sc in acquired and hereditary prion diseases, respectively. We recently reported that a neurotropic strain of influenza A virus (IAV) induced the conversion of PrP^C into PrP^Sc as well as formation of infectious prions in mouse neuroblastoma cells after infection, suggesting the causative role of the neuronal infection of IAV in sporadic prion diseases. Here, we discuss the conversion mechanism of PrP^C into PrP^Sc in different types of prion diseases, by presenting our findings of the IAV infection-induced conversion of PrP^C into PrP^Sc and by reviewing the so far reported transgenic animal models of hereditary prion diseases and the reverse genetic studies, which have revealed the structure-function relationship for PrP^C to convert into PrP^Sc after prion infection.

Ethanolamine Is a New Anti-Prion Compound

Prion diseases are a group of fatal neurodegenerative disorders caused by accumulation of proteinaceous infectious particles, or prions, which mainly consist of the abnormally folded, amyloidogenic prion protein, designated PrP^Sc. PrP^Sc is produced through conformational conversion of the cellular isoform of prion protein, PrP^C, in the brain. To date, no effective therapies for prion diseases have been developed. In this study, we incidentally noticed that mouse neuroblastoma N2a cells persistently infected with 22L scrapie prions, termed N2aC24L1-3 cells, reduced PrP^Sc levels when cultured in advanced Dulbecco's modified eagle medium (DMEM) but not in classic DMEM. PrP^C levels remained unchanged in prion-uninfected parent N2aC24 cells cultured in advanced DMEM. These results suggest that advanced DMEM may contain an anti-prion compound(s). We then successfully identified ethanolamine in advanced DMEM has an anti-prion activity. Ethanolamine reduced PrP^Sc levels in N2aC24L1-3 cells, but not PrP^C levels in N2aC24 cells. Also, oral administration of ethanolamine through drinking water delayed prion disease in mice intracerebrally inoculated with RML scrapie prions. These results suggest that ethanolamine could be a new anti-prion compound.

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.

Prion potentiation after life-long dormancy in mice devoid of PrP

Prions are neurotropic pathogens composed of misfolded assemblies of the host-encoded prion protein PrP^C which replicate by recruitment and conversion of further PrP^C by an autocatalytic seeding polymerization process. While it has long been shown that mouse-adapted prions cannot replicate and are rapidly cleared in transgenic PrP^0/0 mice invalidated for PrP^C, these experiments have not been done with other prions, including from natural resources, and more sensitive methods to detect prion biological activity. Using transgenic mice expressing human PrP to bioassay prion infectivity and RT-QuIC cell-free assay to measure prion seeding activity, we report that prions responsible for the most prevalent form of sporadic Creutzfeldt-Jakob disease in human (MM1-sCJD) can persist indefinitely in the brain of intra-cerebrally inoculated PrP^0/0 mice. While low levels of seeding activity were measured by RT-QuIC in the brain of the challenged PrP^0/0 mice, the bio-indicator humanized mice succumbed at a high attack rate, suggesting relatively high levels of persistent infectivity. Remarkably, these humanized mice succumbed with delayed kinetics as compared to MM1-sCJD prions directly inoculated at low doses, including the limiting one. Yet, the disease that did occur in the humanized mice on primary and subsequent back-passage from PrP^0/0 mice shared the neuropathological and molecular characteristics of MM1-sCJD prions, suggesting no apparent strain evolution during lifelong dormancy in PrP^0/0 brain. Thus, MM1-sCJD prions can persist for the entire life in PrP^0/0 brain with potential disease potentiation on retrotransmission to susceptible hosts. These findings highlight the capacity of prions to persist and rejuvenate in non-replicative environments, interrogate on the type of prion assemblies at work and alert on the risk of indefinite prion persistence with PrP-lowering therapeutic strategies.

Remodeling Alzheimer-amyloidosis models by seeding

Alzheimer’s disease (AD) is among the most prevalent neurodegenerative diseases, with brain pathology defined by extracellular amyloid beta deposits and intracellular tau aggregates. To aid in research efforts to improve understanding of this disease, transgenic murine models have been developed that replicate aspects of AD pathology. Familial AD is associated with mutations in the amyloid precursor protein and in the presenilins (associated with amyloidosis); transgenic amyloid models feature one or more of these mutant genes. Recent advances in seeding methods provide a means to alter the morphology of resultant amyloid deposits and the age that pathology develops. In this review, we discuss the variety of factors that influence the seeding of amyloid beta pathology, including the source of seed, the time interval after seeding, the nature of the transgenic host, and the preparation of the seeding inoculum.

Current Evidence for a Bidirectional Loop Between the Lysosome and Alpha-Synuclein Proteoforms

Cumulative evidence collected in recent decades suggests that lysosomal dysfunction contributes to neurodegenerative diseases, especially if amyloid proteins are involved. Among these, alpha-synuclein (aSyn) that progressively accumulates and aggregates in Lewy bodies is undisputedly a main culprit in Parkinson disease (PD) pathogenesis. Lysosomal dysfunction is evident in brains of PD patients, and mutations in lysosomal enzymes are a major risk factor of PD. At first glance, the role of protein-degrading lysosomes in a disease with pathological protein accumulation seems obvious and should guide the development of straightforward and rational therapeutic targets. However, our review demonstrates that the story is more complicated for aSyn. The protein can possess diverse posttranslational modifications, aggregate formations, and truncations, all of which contribute to a growing known set of proteoforms. These interfere directly or indirectly with lysosome function, reducing their own degradation, and thereby accelerating the protein aggregation and disease process. Conversely, unbalanced lysosomal enzymatic processes can produce truncated aSyn proteoforms that may be more toxic and prone to aggregation. This highlights the possibility of enhancing lysosomal function as a treatment for PD, if it can be confirmed that this approach effectively reduces harmful aSyn proteoforms and does not produce novel, toxic proteoforms.

Prion protein lowering is a disease-modifying therapy across prion disease stages, strains and endpoints

Lowering of prion protein (PrP) expression in the brain is a genetically validated therapeutic hypothesis in prion disease. We recently showed that antisense oligonucleotide (ASO)-mediated PrP suppression extends survival and delays disease onset in intracerebrally prion-infected mice in both prophylactic and delayed dosing paradigms. Here, we examine the efficacy of this therapeutic approach across diverse paradigms, varying the dose and dosing regimen, prion strain, treatment timepoint, and examining symptomatic, survival, and biomarker readouts. We recapitulate our previous findings with additional PrP-targeting ASOs, and demonstrate therapeutic benefit against four additional prion strains. We demonstrate that <25% PrP suppression is sufficient to extend survival and delay symptoms in a prophylactic paradigm. Rise in both neuroinflammation and neuronal injury markers can be reversed by a single dose of PrP-lowering ASO administered after the detection of pathological change. Chronic ASO-mediated suppression of PrP beginning at any time up to early signs of neuropathology confers benefit similar to constitutive heterozygous PrP knockout. Remarkably, even after emergence of frank symptoms including weight loss, a single treatment prolongs survival by months in a subset of animals. These results support ASO-mediated PrP lowering, and PrP-lowering therapeutics in general, as a promising path forward against prion disease.

The Effects of Immune System Modulation on Prion Disease Susceptibility and Pathogenesis

Prion diseases are a unique group of infectious chronic neurodegenerative disorders to which there are no cures. Although prion infections do not stimulate adaptive immune responses in infected individuals, the actions of certain immune cell populations can have a significant impact on disease pathogenesis. After infection, the targeting of peripherally-acquired prions to specific immune cells in the secondary lymphoid organs (SLO), such as the lymph nodes and spleen, is essential for the efficient transmission of disease to the brain. Once the prions reach the brain, interactions with other immune cell populations can provide either host protection or accelerate the neurodegeneration. In this review, we provide a detailed account of how factors such as inflammation, ageing and pathogen co-infection can affect prion disease pathogenesis and susceptibility. For example, we discuss how changes to the abundance, function and activation status of specific immune cell populations can affect the transmission of prion diseases by peripheral routes. We also describe how the effects of systemic inflammation on certain glial cell subsets in the brains of infected individuals can accelerate the neurodegeneration. A detailed understanding of the factors that affect prion disease transmission and pathogenesis is essential for the development of novel intervention strategies.

Strain-Dependent Prion Infection in Mice Expressing Prion Protein with Deletion of Central Residues 91-106

Conformational conversion of the cellular prion protein, PrP^C, into the abnormally folded isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the exact conversion mechanism remains largely unknown. Transgenic mice expressing PrP with a deletion of the central residues 91–106 were generated in the absence of endogenous PrP^C, designated Tg(PrP∆91–106)/Prnp^0/0 mice and intracerebrally inoculated with various prions. Tg(PrP∆91–106)/Prnp^0/0 mice were resistant to RML, 22L and FK-1 prions, neither producing PrP^Sc∆91–106 or prions in the brain nor developing disease after inoculation. However, they remained marginally susceptible to bovine spongiform encephalopathy (BSE) prions, developing disease after elongated incubation times and accumulating PrP^Sc∆91–106 and prions in the brain after inoculation with BSE prions. Recombinant PrP∆91-104 converted into PrP^Sc∆91–104 after incubation with BSE-PrP^Sc-prions but not with RML- and 22L–PrP^Sc-prions, in a protein misfolding cyclic amplification assay. However, digitonin and heparin stimulated the conversion of PrP∆91–104 into PrP^Sc∆91–104 even after incubation with RML- and 22L-PrP^Sc-prions. These results suggest that residues 91–106 or 91–104 of PrP^C are crucially involved in prion pathogenesis in a strain-dependent manner and may play a similar role to digitonin and heparin in the conversion of PrP^C into PrP^Sc.

The Role of Vesicle Trafficking Defects in the Pathogenesis of Prion and Prion-Like Disorders

Prion diseases are fatal and transmissible neurodegenerative diseases in which the cellular form of the prion protein ‘PrP^c’, misfolds into an infectious and aggregation prone isoform termed PrP^Sc, which is the primary component of prions. Many neurodegenerative diseases, like Alzheimer’s disease, Parkinson’s disease, and polyglutamine diseases, such as Huntington’s disease, are considered prion-like disorders because of the common characteristics in the propagation and spreading of misfolded proteins that they share with the prion diseases. Unlike prion diseases, these are non-infectious outside experimental settings. Many vesicular trafficking impairments, which are observed in prion and prion-like disorders, favor the accumulation of the pathogenic amyloid aggregates. In addition, many of the vesicular trafficking impairments that arise in these diseases, turn out to be further aggravating factors. This review offers an insight into the currently known vesicular trafficking defects in these neurodegenerative diseases and their implications on disease progression. These findings suggest that these impaired trafficking pathways may represent similar therapeutic targets in these classes of neurodegenerative disorders.

The vascular endothelial cell-expressed prion protein doppel promotes angiogenesis and blood-brain barrier development

The central nervous system (CNS) contains a complex network of blood vessels that promote normal tissue development and physiology. Abnormal control of blood vessel morphogenesis and maturation is linked to the pathogenesis of various neurodevelopmental diseases. The CNS-specific genes that regulate blood vessel morphogenesis in development and disease remain largely unknown. Here, we have characterized functions for the gene encoding prion protein 2 (Prnd) in CNS blood vessel development and physiology. Prnd encodes the glycosylphosphatidylinositol (GPI)-linked protein doppel, which is expressed on the surface of angiogenic vascular endothelial cells, but is absent in quiescent endothelial cells of the adult CNS. During CNS vascular development, doppel interacts with receptor tyrosine kinases and activates cytoplasmic signaling pathways involved in endothelial cell survival, metabolism and migration. Analysis of mice genetically null for Prnd revealed impaired CNS blood vessel morphogenesis and associated endothelial cell sprouting defects. Prnd^-/- mice also displayed defects in endothelial barrier integrity. Collectively, these data reveal novel mechanisms underlying doppel control of angiogenesis in the developing CNS, and may provide new insights about dysfunctional pathways that cause vascular-related CNS disorders.

Transgenic mouse models for the study of prion diseases

Prions are unique agents that challenge the molecular biology dogma by transmitting information on the protein level. They cause neurodegenerative diseases that lack of any cure or treatment called transmissible spongiform encephalopathies. The function of the normal form of the prion protein, the exact mechanism of prion propagation between species as well as at the cellular level and neuron degeneration remains elusive. However, great amount of information known for all these aspects has been achieved thanks to the use of animal models and more precisely to transgenic mouse models. In this chapter, the main contributions of these powerful research tools in the prion field are revised.

N-Terminal Regions of Prion Protein: Functions and Roles in Prion Diseases

The normal cellular isoform of prion protein, designated PrP^C, is constitutively converted to the abnormally folded, amyloidogenic isoform, PrP^Sc, in prion diseases, which include Creutzfeldt-Jakob disease in humans and scrapie and bovine spongiform encephalopathy in animals. PrP^C is a membrane glycoprotein consisting of the non-structural N-terminal domain and the globular C-terminal domain. During conversion of PrP^C to PrP^Sc, its 2/3 C-terminal region undergoes marked structural changes, forming a protease-resistant structure. In contrast, the N-terminal region remains protease-sensitive in PrP^Sc. Reverse genetic studies using reconstituted PrP^C-knockout mice with various mutant PrP molecules have revealed that the N-terminal domain has an important role in the normal function of PrP^C and the conversion of PrP^C to PrP^Sc. The N-terminal domain includes various characteristic regions, such as the positively charged residue-rich polybasic region, the octapeptide repeat (OR) region consisting of five repeats of an octapeptide sequence, and the post-OR region with another positively charged residue-rich polybasic region followed by a stretch of hydrophobic residues. We discuss the normal functions of PrP^C, the conversion of PrP^C to PrP^Sc, and the neurotoxicity of PrP^Sc by focusing on the roles of the N-terminal regions in these topics.

Cofactor molecules: Essential partners for infectious prions

The protein-only hypothesis predicts that infectious mammalian prions are composed solely of PrP^Sc, a misfolded conformer of the normal prion protein, PrP^C. However, to date, all wild type protein-only PrP^Sc preparations lack significant levels of prion infectivity. Using a systemic biochemical approach, our laboratory isolated and identified two different endogenous cofactor molecules, RNA (Deleault et al., 2003 [50]; Deleault et al., 2007 [59]) and phosphatidylethanolamine (Deleault et al., 2012 [61]; Deleault et al., 2012 [18]), which facilitate the formation of prions with high levels of specific infectivity, leading us to propose to the alternative hypothesis that cofactor molecules are required to form wild type infectious prions (Deleault et al., 2007 [59]; Deleault et al., 2012 [18]; Geoghegan et al., 2007 [57]). In addition, we found that purified cofactor molecules restrict the strain properties of chemically defined infectious prions (Deleault et al., 2012 [18]), suggesting a "cofactor selection" model in which natural variation in the distribution of strain-specific cofactor molecules in different parts of the brain may be responsible for strain-dependent patterns of neurotropism (Deleault et al., 2012 [18]; Geoghegan et al., 2007 [57]).

Protective anti-prion antibodies in human immunoglobulin repertoires

Prion immunotherapy may hold great potential, but antibodies against certain PrP epitopes can be neurotoxic. Here, we identified > 6,000 PrP-binding antibodies in a synthetic human Fab phage display library, 49 of which we characterized in detail. Antibodies directed against the flexible tail of PrP conferred neuroprotection against infectious prions. We then mined published repertoires of circulating B cells from healthy humans and found antibodies similar to the protective phage-derived antibodies. When expressed recombinantly, these antibodies exhibited anti-PrP reactivity. Furthermore, we surveyed 48,718 samples from 37,894 hospital patients for the presence of anti-PrP IgGs and found 21 high-titer individuals. The clinical files of these individuals did not reveal any enrichment of specific pathologies, suggesting that anti-PrP autoimmunity is innocuous. The existence of anti-prion antibodies in unbiased human immunological repertoires suggests that they might clear nascent prions early in life. Combined with the reported lack of such antibodies in carriers of disease-associated PRNP mutations, this suggests a link to the low incidence of spontaneous prion diseases in human populations.

Novel regulators of PrPC expression as potential therapeutic targets in prion diseases

INTRODUCTION

Prion diseases are rare and fatal neurodegenerative disorders. The key molecular event in these disorders is the misfolding of the physiological form of the cellular prion protein, PrP^C, leading to the accumulation of a pathological isoform, PrP^Sc, with unique features. Both isoforms share the same primary sequence, lacking detectable differences in posttranslational modification, a major hurdle for their biochemical or biophysical independent characterization. The mechanism underlying the conversion of PrP^C to PrP^Sc is not completely understood, so finding an effective therapy to cure prion disorders is extremely challenging.

AREAS COVERED

This review discusses the strategies for decreasing prion replication and throws a spotlight on the relevance of PrP^C in the prion accumulation process.

EXPERT OPINION

PrP^C is the key substrate for prion pathology; hence, the most promising therapeutic approach appears to be the targeting of PrP^C to block the production of the infectious isoform. The use of RNA interference and antisense oligonucleotide technologies may offer opportunities for treatment because of their success in clinical trials for other neurodegenerative diseases.

α2,3 linkage of sialic acid to a GPI anchor and an unpredicted GPI attachment site in human prion protein

Prion diseases are transmissible, lethal neurodegenerative disorders caused by accumulation of the aggregated scrapie form of the prion protein (PrP^Sc) after conversion of the cellular prion protein (PrP^C). The glycosylphosphatidylinositol (GPI) anchor of PrP^C is involved in prion disease pathogenesis, and especially sialic acid in a GPI side chain reportedly affects PrP^C conversion. Thus, it is important to define the location and structure of the GPI anchor in human PrP^C Moreover, the sialic acid linkage type in the GPI side chain has not been determined for any GPI-anchored protein. Here we report GPI glycan structures of human PrP^C isolated from human brains and from brains of a knock-in mouse model in which the mouse prion protein (Prnp) gene was replaced with the human PRNP gene. LC-electrospray ionization-MS analysis of human PrP^C from both biological sources indicated that Gly²²⁹ is the ω site in PrP^C to which GPI is attached. Gly²²⁹ in human PrP^C does not correspond to Ser²³¹, the previously reported ω site of Syrian hamster PrP^C We found that ∼41% and 28% of GPI anchors in human PrP^Cs from human and knock-in mouse brains, respectively, have N-acetylneuraminic acid in the side chain. Using a sialic acid linkage-specific alkylamidation method to discriminate α2,3 linkage from α2,6 linkage, we found that N-acetylneuraminic acid in PrP^C's GPI side chain is linked to galactose through an α2,3 linkage. In summary, we report the GPI glycan structure of human PrP^C, including the ω-site amino acid for GPI attachment and the sialic acid linkage type.

Goats naturally devoid of PrPC are resistant to scrapie

Prion diseases are progressive and fatal, neurodegenerative disorders described in humans and animals. According to the "protein-only" hypothesis, the normal host-encoded prion protein (PrPC) is converted into a pathological and infectious form (PrPSc) in these diseases. Transgenic knockout models have shown that PrPC is a prerequisite for the development of prion disease. In Norwegian dairy goats, a mutation (Ter) in the prion protein gene (PRNP) effectively blocks PrPC synthesis. We inoculated 12 goats (4 PRNP+/+, 4 PRNP+/Ter, and 4 PRNPTer/Ter) intracerebrally with goat scrapie prions. The mean incubation time until clinical signs of prion disease was 601 days post-inoculation (dpi) in PRNP+/+ goats and 773 dpi in PRNP+/Ter goats. PrPSc and vacuolation were similarly distributed in the central nervous system (CNS) of both groups and observed in all brain regions and segments of the spinal cord. Generally, accumulation of PrPSc was limited in peripheral organs, but all PRNP+/+ goats and 1 of 4 PRNP+/Ter goats were positive in head lymph nodes. The four PRNPTer/Ter goats remained healthy, without clinical signs of prion disease, and were euthanized 1260 dpi. As expected, no accumulation of PrPSc was observed in the CNS or peripheral tissues of this group, as assessed by immunohistochemistry, enzyme immunoassay, and real-time quaking-induced conversion. Our study shows for the first time that animals devoid of PrPC due to a natural mutation do not propagate prions and are resistant to scrapie. Clinical onset of disease is delayed in heterozygous goats expressing about 50% of PrPC levels.

Cellular and Molecular Mechanisms of Prion Disease

Prion diseases are rapidly progressive, incurable neurodegenerative disorders caused by misfolded, aggregated proteins known as prions, which are uniquely infectious. Remarkably, these infectious proteins have been responsible for widespread disease epidemics, including kuru in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids, the latter of which has spread across North America and recently appeared in Norway and Finland. The hallmark histopathological features include widespread spongiform encephalopathy, neuronal loss, gliosis, and deposits of variably sized aggregated prion protein, ranging from small, soluble oligomers to long, thin, unbranched fibrils, depending on the disease. Here, we explore recent advances in prion disease research, from the function of the cellular prion protein to the dysfunction triggering neurotoxicity, as well as mechanisms underlying prion spread between cells. We also highlight key findings that have revealed new therapeutic targets and consider unanswered questions for future research.

Prion neurotoxicity

Although the mechanisms underlying prion propagation and infectivity are now well established, the processes accounting for prion toxicity and pathogenesis have remained mysterious. These processes are of enormous clinical relevance as they hold the key to identification of new molecular targets for therapeutic intervention. In this review, we will discuss two broad areas of investigation relevant to understanding prion neurotoxicity. The first is the use of in vitro experimental systems that model key events in prion pathogenesis. In this context, we will describe a hippocampal neuronal culture system we developed that reproduces the earliest pathological alterations in synaptic morphology and function in response to PrP^Sc . This system has allowed us to define a core synaptotoxic signaling pathway involving the activation of NMDA and AMPA receptors, stimulation of p38 MAPK phosphorylation and collapse of the actin cytoskeleton in dendritic spines. The second area concerns a striking and unexpected phenomenon in which certain structural manipulations of the PrP^C molecule itself, including introduction of N-terminal deletion mutations or binding of antibodies to C-terminal epitopes, unleash powerful toxic effects in cultured cells and transgenic mice. We will describe our studies of this phenomenon, which led to the recognition that it is related to the induction of large, abnormal ionic currents by the structurally altered PrP molecules. Our results suggest a model in which the flexible N-terminal domain of PrP^C serves as a toxic effector which is regulated by intramolecular interactions with the globular C-terminal domain. Taken together, these two areas of study have provided important clues to underlying cellular and molecular mechanisms of prion neurotoxicity. Nevertheless, much remains to be done on this next frontier of prion science.

Cellular prion protein-mediated protection against influenza A virus infection

The cellular prion protein, termed PrPC, is a glycoprotein abundantly expressed in brains and to a lesser extent in non-neuronal tissues including lungs. It was reported that PrPC is expressed by lung epithelial cells in mice, and that it may play a protective role against lethal infection with influenza A viruses (IAVs). This may occur by regulating Cu content and  superoxide dismutase (SOD) activity, eventually reducing oxidative stress in infected lungs. Antioxidative therapeutics have been demonstrated to protect mice from lethal infection with IAVs. Therefore, PrPC might be a new target molecule for development of IAV therapeutics. Here, we introduce the antiviral mechanism of PrPC against IAV infection and discuss perspectives of PrPC-targeting therapeutics against IAV infection.

The prion protein in neuroimmune crosstalk

The cellular prion protein (PrP^C) is a medium-sized glycoprotein, attached to the cell surface by a glycosylphosphatidylinositol anchor. PrP^C is encoded by a single-copy gene, PRNP, which is abundantly expressed in the central nervous system and at lower levels in non-neuronal cells, including those of the immune system. Evidence from experimental knockout of PRNP in rodents, goats, and cattle and the occurrence of a nonsense mutation in goat that prevents synthesis of PrP^C, have shown that the molecule is non-essential for life. Indeed, no easily recognizable phenotypes are associate with a lack of PrP^C, except the potentially advantageous trait that animals without PrP^C cannot develop prion disease. This is because, in prion diseases, PrP^C converts to a pathogenic "scrapie" conformer, PrP^Sc, which aggregates and eventually induces neurodegeneration. In addition, endogenous neuronal PrP^C serves as a toxic receptor to mediate prion-induced neurotoxicity. Thus, PrP^C is an interesting target for treatment of prion diseases. Although loss of PrP^C has no discernable effect, alteration of its normal physiological function can have very harmful consequences. It is therefore important to understand cellular processes involving PrP^C, and research of this topic has advanced considerably in the past decade. Here, we summarize data that indicate the role of PrP^C in modulating immune signaling, with emphasis on neuroimmune crosstalk both under basal conditions and during inflammatory stress.

Prions activate a p38 MAPK synaptotoxic signaling pathway

Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrP^Sc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrP^C by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrP^Sc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrP^Sc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrP^C mediates the synaptotoxic effects of both PrP^Sc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.

Prion diseases are a group of fatal neurodegenerative disorders that includes Creutzfeldt-Jakob disease and kuru in humans, and bovine spongiform encephalopathy in cattle. The infectious agent, or prion, that transmits these diseases is a naked protein molecule, the prion protein (PrP), which is an altered form of a normal, cellular protein. Although a great deal is known about how prions propagate themselves and transmit infection, the process by which they actually cause neurons to degenerate has remained mysterious. Here, we have used a specialized neuronal culture system to dissect the cellular and molecular mechanisms by which prions damage synapses, the structures that connect nerve cells and that play a crucial role in learning, memory, and neurological disease. Our results define a stepwise molecular pathway underlying prion synaptic toxicity, which involves activation of glutamate neurotransmitter receptors, influx of calcium ions into the neuron, and stimulation of specific mitogen-activated protein kinases, which attach phosphate groups to proteins to regulate their activity. We demonstrate that specific drugs, as well as a dominant-negative kinase mutant, block these steps and thereby prevent the synaptic degeneration produced by prions. Our results provide new insights into the pathogenesis of prion diseases, they uncover new drug targets for treating these diseases, and they allow us to compare prion diseases to other, more common neurodegenerative disorders like Alzheimer’s disease.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

Epigenetic Control of the Notch and Eph Signaling Pathways by the Prion Protein: Implications for Prion Diseases

Among the ever-growing number of self-replicating proteins involved in neurodegenerative diseases, the prion protein PrP remains the most infamous for its central role in transmissible spongiform encephalopathies (TSEs). In these diseases, pathogenic prions propagate through a seeding mechanism, where normal PrP^C molecules are converted into abnormally folded scrapie isoforms termed PrP^Sc. Since its discovery over 30 years ago, much advance has contributed to define the host-encoded cellular prion protein PrP^C as a critical relay of prion-induced neuronal cell demise. A current consensual view is that the conversion of PrP^C into PrP^Sc in neuronal cells diverts the former from its normal function with subsequent molecular alterations affecting synaptic plasticity. Here, we report that prion infection is associated with reduced expression of key effectors of the Notch pathway in vitro and in vivo, recapitulating changes fostered by the absence of PrP^C. We further show that both prion infection and PrP^C depletion promote drastic alterations in the expression of a defined set of Eph receptors and their ephrin ligands, which represent important players in synaptic function. Our data indicate that defects in the Notch and Eph axes can be mitigated in response to histone deacetylase inhibition in PrP^C-depleted as well as prion-infected cells. We thus conclude that infectious prions cause a loss-of-function phenotype with respect to Notch and Eph signaling and that these alterations are sustained by epigenetic mechanisms.

The intrigue is infectious: Impacts of prion protein during neural development

Normally folded prion protein is abundant in the CNS and remarkably conserved, suggesting that it has important functions, yet these functions have remained elusive. Now the work of Parrie et al. has codified a requirement for prion protein in adult neurogenesis. Their insightful use of prion protein knockout and over-expressing mice, combined with the well-characterized olfactory system site of neurogenesis, demonstrated that prion protein promotes proliferation and survival of adult neurons. The work provides a unique independent confirmation of prion protein playing a role in neuroprotection, especially extending the conclusion beyond models using acute injury. Parrie et al. (2018) further show that prion protein is required for CNS axon guidance. A growing list of phenotypes associated with prion protein loss are coincident with symptoms of neurodegenerative disease and dementia, though it remains contentious whether any such disruption of prion protein function contributes to disease aetiology. Perhaps most intriguingly, identifying the developmental functions for prion protein opens new avenues to understand the evolution of prion protein: what history led to a CNS protein that is conserved and abundant paradoxically being both dispensable for life and the template for devastating disease?

Treatment with a non-toxic, self-replicating anti-prion delays or prevents prion disease in vivo

Transmissible spongiform encephalopathies (TSEs) are fatal neurological disorders caused by prions, which are composed of a misfolded protein (PrP^Sc) that self-propagates in the brain of infected individuals by converting the normal prion protein (PrP^C) into the pathological isoform. Here, we report a novel experimental strategy for preventing prion disease based on producing a self-replicating, but innocuous PrP^Sc-like form, termed anti-prion, which can compete with the replication of pathogenic prions. Our results show that a prophylactic inoculation of prion-infected animals with an anti-prion delays the onset of the disease and in some animals completely prevents the development of clinical symptoms and brain damage. The data indicate that a single injection of the anti-prion eliminated ~99% of the infectivity associated to pathogenic prions. Furthermore, this treatment caused significant changes in the profile of regional PrP^Sc deposition in the brains of animals that were treated, but still succumbed to the disease. Our findings provide new insights for a mechanistic understanding of prion replication and support the concept that prion replication can be separated from toxicity, providing a novel target for therapeutic intervention.

The function of the cellular prion protein in health and disease

The essential role of the cellular prion protein (PrP^C) in prion disorders such as Creutzfeldt-Jakob disease is well documented. Moreover, evidence is accumulating that PrP^C may act as a receptor for protein aggregates and transduce neurotoxic signals in more common neurodegenerative disorders, such as Alzheimer's disease. Although the pathological roles of PrP^C have been thoroughly characterized, a general consensus on its physiological function within the brain has not yet been established. Knockout studies in various organisms, ranging from zebrafish to mice, have implicated PrP^C in a diverse range of nervous system-related activities that include a key role in the maintenance of peripheral nerve myelination as well as a general ability to protect against neurotoxic stimuli. Thus, the function of PrP^C may be multifaceted, with different cell types taking advantage of unique aspects of its biology. Deciphering the cellular function(s) of PrP^C and the consequences of its absence is not simply an academic curiosity, since lowering PrP^C levels in the brain is predicted to be a powerful therapeutic strategy for the treatment of prion disease. In this review, we outline the various approaches that have been employed in an effort to uncover the physiological and pathological functions of PrP^C. While these studies have revealed important clues about the biology of the prion protein, the precise reason for PrP^C's existence remains enigmatic.

Human transmissible spongiform encephalopathies: historic view

The first of several pivotal moments leading to current understanding of human transmissible spongiform encephalopathies (TSEs) occurred in 1959 when veterinary pathologist W.J. Hadlow first recognized several similarities between scrapie-a slow infection of sheep caused by an unusual infectious agent-and kuru, a fatal exotic neurodegenerative disease affecting only people of a single language group in the remote mountainous interior of New Guinea, described two years earlier by D.C. Gajdusek and V. Zigas. Based on the knowledge of scrapie, Gajdusek, C.J. Gibbs, Jr., and M.P. Alpers soon initiated efforts to transmit kuru by inoculating kuru brain tissue into non-human primates, that-although requiring several years-ultimately proved successful. In the same year that Hadlow first proposed that kuru and scrapie might have similar etiology, I. Klatzo noted that kuru's histopathology resembled that of Creutzfeldt-Jakob disease (CJD), another progressive fatal neurodegenerative disease of unknown etiology that A.M. Jakob had first described in 1921. Gajdusek and colleagues went on to demonstrate that not only the more common sporadic form of CJD but also familial CJD and a generally similar familial brain disease (Gerstmann-Sträussler-Scheinker syndrome) were also transmissible, first to non-human primates and later to other animals. (Other investigators later transmitted an even rarer brain disease, fatal familial insomnia, to animals.) Iatrogenic CJD (spread by human pituitary-derived hormones and tissue grafts) was also transmitted to animals. Much later, in 1996, a new variant of CJD was attributed to human infection with the agent of bovine spongiform encephalopathy; vCJD itself caused an iatrogenic TSE spread by blood transfusion (and probably by a human-plasma-derived clotting factor). Starting in the 1930s, the scrapie agent was found to have a unique constellation of physical properties (marked resistance to inactivation by chemicals, heat and radiation), eventually interpreted as suggesting that it might be an unconventional self-replicating pathogen based on protein and containing no nucleic acid. The work of S.B. Prusiner led to the recognition in the early 1980s that a misfolded form of a ubiquitous normal host protein was usually if not always detectable in tissues containing TSE agents, greatly facilitating the diagnosis and TSEs and understanding their pathogenesis. Prusiner proposed that the TSE agent was likely to be composed partly if not entirely of the abnormal protein, for which he coined the term "prion" protein and "prion" for the agent. Expression of the prion protein by animals-while not essential for life-was later found to be obligatory to infect them with TSEs, and a variety of mutations in the protein clearly tracked with TSEs in families, explaining the autosomal dominant pattern of disease and confirming a central role for the protein in pathogenesis. Prusiner's terminology and the prion hypothesis came to be widely though not universally accepted. A popular corollary proposal, that prions arise by spontaneous misfolding of normal prion protein leading to sporadic cases of CJD, BSE, and scrapie, is more problematic and may serve to discourage continued search for environmental sources of exposure to TSE agents.

Targeting of Disordered Proteins by Small Molecules in Neurodegenerative Diseases

The formation of protein aggregates and inclusions in the brain and spinal cord is a common neuropathological feature of a number of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and many others. These are commonly referred as neurodegenerative proteinopathies or protein-misfolding diseases. The main characteristic of protein aggregates in these disorders is the fact that they are enriched in amyloid fibrils. Since protein aggregation is considered to play a central role for the onset of neurodegenerative proteinopathies, research is ongoing to develop strategies aimed at preventing or removing protein aggregation in the brain of affected patients. Numerous studies have shown that small molecule-based approaches may be potentially the most promising for halting protein aggregation in neurodegenerative diseases. Indeed, several of these compounds have been found to interact with intrinsically disordered proteins and promote their clearing in experimental models. This notwithstanding, at present small molecule inhibitors still awaits achievements for clinical translation. Hopefully, if we determine whether the formation of insoluble inclusions is effectively neurotoxic and find a valid biomarker to assess their protein aggregation-inhibitory activity in the human central nervous system, the use of small molecule inhibitors will be considered as a cure for neurodegenerative protein-misfolding diseases.

Prion Protein Devoid of the Octapeptide Repeat Region Delays Bovine Spongiform Encephalopathy Pathogenesis in Mice

Conformational conversion of the cellular isoform of prion protein, PrP^C, into the abnormally folded, amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases, including Creutzfeldt-Jakob disease in humans and scrapie and bovine spongiform encephalopathy (BSE) in animals. We previously reported that the octapeptide repeat (OR) region could be dispensable for converting PrP^C into PrP^Sc after infection with RML prions. We demonstrated that mice transgenically expressing mouse PrP with deletion of the OR region on the PrP knockout background, designated Tg(PrPΔOR)/Prnp^0/0 mice, did not show reduced susceptibility to RML scrapie prions, with abundant accumulation of PrP^ScΔOR in their brains. We show here that Tg(PrPΔOR)/Prnp^0/0 mice were highly resistant to BSE prions, developing the disease with markedly elongated incubation times after infection with BSE prions. The conversion of PrPΔOR into PrP^ScΔOR was markedly delayed in their brains. These results suggest that the OR region may have a crucial role in the conversion of PrP^C into PrP^Sc after infection with BSE prions. However, Tg(PrPΔOR)/Prnp^0/0 mice remained susceptible to RML and 22L scrapie prions, developing the disease without elongated incubation times after infection with RML and 22L prions. PrP^ScΔOR accumulated only slightly less in the brains of RML- or 22L-infected Tg(PrPΔOR)/Prnp^0/0 mice than PrP^Sc in control wild-type mice. Taken together, these results indicate that the OR region of PrP^C could play a differential role in the pathogenesis of BSE prions and RML or 22L scrapie prions.IMPORTANCE Structure-function relationship studies of PrP^C conformational conversion into PrP^Sc are worthwhile to understand the mechanism of the conversion of PrP^C into PrP^Sc We show here that, by inoculating Tg(PrPΔOR)/Prnp^0/0 mice with the three different strains of RML, 22L, and BSE prions, the OR region could play a differential role in the conversion of PrP^C into PrP^Sc after infection with RML or 22L scrapie prions and BSE prions. PrPΔOR was efficiently converted into PrP^ScΔOR after infection with RML and 22L prions. However, the conversion of PrPΔOR into PrP^ScΔOR was markedly delayed after infection with BSE prions. Further investigation into the role of the OR region in the conversion of PrP^C into PrP^Sc after infection with BSE prions might be helpful for understanding the pathogenesis of BSE prions.

Recombinant PrP and Its Contribution to Research on Transmissible Spongiform Encephalopathies

The misfolding of the cellular prion protein (PrPC) into the disease-associated isoform (PrPSc) and its accumulation as amyloid fibrils in the central nervous system is one of the central events in transmissible spongiform encephalopathies (TSEs). Due to the proteinaceous nature of the causal agent the molecular mechanisms of misfolding, interspecies transmission, neurotoxicity and strain phenomenon remain mostly ill-defined or unknown. Significant advances were made using in vivo and in cellula models, but the limitations of these, primarily due to their inherent complexity and the small amounts of PrPSc that can be obtained, gave rise to the necessity of new model systems. The production of recombinant PrP using E. coli and subsequent induction of misfolding to the aberrant isoform using different techniques paved the way for the development of cell-free systems that complement the previous models. The generation of the first infectious recombinant prion proteins with identical properties of brain-derived PrPSc increased the value of cell-free systems for research on TSEs. The versatility and ease of implementation of these models have made them invaluable for the study of the molecular mechanisms of prion formation and propagation, and have enabled improvements in diagnosis, high-throughput screening of putative anti-prion compounds and the design of novel therapeutic strategies. Here, we provide an overview of the resultant advances in the prion field due to the development of recombinant PrP and its use in cell-free systems.

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.

Novel amplification mechanism of prions through disrupting sortilin-mediated trafficking

Conformational conversion of the cellular prion protein, PrP^C, into the abnormally folded isoform of prion protein, PrP^Sc, which leads to marked accumulation of PrP^Sc in brains, is a key pathogenic event in prion diseases, a group of fatal neurodegenerative disorders caused by prions. However, the exact mechanism of PrP^Sc accumulation in prion-infected neurons remains unknown. We recently reported a novel cellular mechanism to support PrP^Sc accumulation in prion-infected neurons, in which PrP^Sc itself promotes its accumulation by evading the cellular inhibitory mechanism, which is newly identified in our recent study. We showed that the VPS10P sorting receptor sortilin negatively regulates PrP^Sc accumulation in prion-infected neurons, by interacting with PrP^C and PrP^Sc and trafficking them to lysosomes for degradation. However, PrP^Sc stimulated lysosomal degradation of sortilin, disrupting the sortilin-mediated degradation of PrP^C and PrP^Sc and eventually evoking further accumulation of PrP^Sc in prion-infected neurons. These findings suggest a positive feedback amplification mechanism for PrP^Sc accumulation in prion-infected neurons.

Hepatitis C virus-induced prion protein expression facilitates hepatitis C virus replication

Hepatitis C virus (HCV) infects approximately 180 million people worldwide. Significant progress has been made since the establishment of in vitro HCV infection models in cells. However, the replication of HCV is complex and not completely understood. Here, we found that the expression of host prion protein (PrP) was induced in an HCV replication cell model. We then showed that increased PrP expression facilitated HCV genomic replication. Finally, we demonstrated that the KKRPK motif on the N-terminus of PrP bound nucleic acids and facilitated HCV genomic replication. Our results provided important insights into how viruses may harness cellular protein to achieve propagation.