The expanding universe of prion diseases

Potential Therapeutic Use of Stem Cells for Prion Diseases

Prion diseases are neurodegenerative disorders that are progressive, incurable, and deadly. The prion consists of PrPSc, the misfolded pathogenic isoform of the cellular prion protein (PrPC). PrPC is involved in a variety of physiological functions, including cellular proliferation, adhesion, differentiation, and neural development. Prion protein is expressed on the membrane surface of a variety of stem cells (SCs), where it plays an important role in the pluripotency and self-renewal matrix, as well as in SC differentiation. SCs have been found to multiply the pathogenic form of the prion protein, implying their potential as an in vitro model for prion diseases. Furthermore, due to their capability to self-renew, differentiate, immunomodulate, and regenerate tissue, SCs are prospective cell treatments in many neurodegenerative conditions, including prion diseases. Regenerative medicine has become a new revolution in disease treatment in recent years, particularly with the introduction of SC therapy. Here, we review the data demonstrating prion diseases' biology and molecular mechanism. SC biology, therapeutic potential, and its role in understanding prion disease mechanisms are highlighted. Moreover, we summarize preclinical studies that use SCs in prion diseases.

A single protective polymorphism in the prion protein blocks cross-species prion replication in cultured cells

The bank vole (BV) prion protein (PrP) can function as a universal acceptor of prions. However, the molecular details of BVPrP's promiscuity for replicating a diverse range of prion strains remain obscure. To develop a cultured cell paradigm capable of interrogating the unique properties of BVPrP, we generated monoclonal lines of CAD5 cells lacking endogenous PrP but stably expressing either hamster (Ha), mouse (Mo), or BVPrP (M109 or I109 polymorphic variants) and then challenged them with various strains of mouse or hamster prions. Cells expressing BVPrP were susceptible to both mouse and hamster prions, whereas cells expressing MoPrP or HaPrP could only be infected with species-matched prions. Propagation of mouse and hamster prions in cells expressing BVPrP resulted in strain adaptation in several instances, as evidenced by alterations in conformational stability, glycosylation, susceptibility to anti-prion small molecules, and the inability of BVPrP-adapted mouse prion strains to infect cells expressing MoPrP. Interestingly, cells expressing BVPrP containing the G127V prion gene variant, identified in individuals resistant to kuru, were unable to become infected with prions. Moreover, the G127V polymorphic variant impeded the spontaneous aggregation of recombinant BVPrP. These results demonstrate that BVPrP can facilitate cross-species prion replication in cultured cells and that a single amino acid change can override the prion-permissive nature of BVPrP. This cellular paradigm will be useful for dissecting the molecular features of BVPrP that allow it to function as a universal prion acceptor.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Primary glia cells from bank vole propagate multiple rodent-adapted scrapie prions

Since the beginning prion research has been largely dependent on animal models for deciphering the disease, drug development or prion detection and quantification. Thereby, ethical as well as cost and labour-saving aspects call for alternatives in vitro. Cell models can replace or at least complement animal studies, but their number is still limited and the application usually restricted to certain strains and host species due to often strong transmission barriers. Bank voles promise to be an exception as they or materials prepared from them are uniquely susceptible to prions from various species in vivo, in vitro and in cell-free applications. Here we present a mainly astrocyte-based primary glia cell assay from bank vole, which is infectible with scrapie strains from bank vole, mouse and hamster. Stable propagation of bank vole-adapted RML, murine 22L and RML, and hamster 263K scrapie is detectable from 20 or 30 days post exposure onwards. Thereby, the infected bank vole glia cells show similar or even faster prion propagation than likewise infected glia cells of the corresponding murine or hamster hosts. We propose that our bank vole glia cell assay could be a versatile tool for studying and comparing multiple prion strains with different species backgrounds combined in one cell assay.

The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

New and distinct chronic wasting disease strains associated with cervid polymorphism at codon 116 of the Prnp gene

Chronic wasting disease (CWD) is a prion disease affecting cervids. Polymorphisms in the prion protein gene can result in extended survival of CWD-infected animals. However, the impact of polymorphisms on cellular prion protein (PrP^C) and prion properties is less understood. Previously, we characterized the effects of a polymorphism at codon 116 (A>G) of the white-tailed deer (WTD) prion protein and determined that it destabilizes PrP^C structure. Comparing CWD isolates from WTD expressing homozygous wild-type (116AA) or heterozygous (116AG) PrP, we found that 116AG-prions were conformationally less stable, more sensitive to proteases, with lower seeding activity in cell-free conversion and reduced infectivity. Here, we aimed to understand CWD strain emergence and adaptation. We show that the WTD-116AG isolate contains two different prion strains, distinguished by their host range, biochemical properties, and pathogenesis from WTD-116AA prions (Wisc-1). Serial passages of WTD-116AG prions in tg(CerPrP)1536^+/+ mice overexpressing wild-type deer-PrP^C revealed two populations of mice with short and long incubation periods, respectively, and remarkably prolonged clinical phase upon inoculation with WTD-116AG prions. Inoculation of serially diluted brain homogenates confirmed the presence of two strains in the 116AG isolate with distinct pathology in the brain. Interestingly, deglycosylation revealed proteinase K-resistant fragments with different electrophoretic mobility in both tg(CerPrP)1536^+/+ mice and Syrian golden hamsters infected with WTD-116AG. Infection of tg60 mice expressing deer S96-PrP with 116AG, but not Wisc-1 prions induced clinical disease. On the contrary, bank voles resisted 116AG prions, but not Wisc-1 infection. Our data indicate that two strains co-existed in the WTD-116AG isolate, expanding the variety of CWD prion strains. We argue that the 116AG isolate does not contain Wisc-1 prions, indicating that the presence of 116G-PrP^C diverted 116A-PrP^C from adopting a Wisc-1 structure. This can have important implications for their possible distinct capacities to cross species barriers into both cervids and non-cervids.

Chronic wasting disease belongs to the family of prion diseases. It is considered the most contagious prion disease and the only one that affects free ranging wildlife. The disease range is expanding in North America and Northern Europe. This work describes the emergence and characterization of new chronic wasting disease strains related to a polymorphism in the prion protein gene. It supports the concept that strains are a dynamic mixture of substrains that can influence and interfere with each other. Because transmission barriers are governed by the compatibility of a particular prion strain with the new host’s prion protein, it is critical to understand the emergence and variety of chronic wasting disease strains circulating in wild animals and their ability to infect new host species including humans.

Oral administration of repurposed drug targeting Cyp46A1 increases survival times of prion infected mice

Prion diseases are fatal, infectious, and incurable neurodegenerative disorders caused by misfolding of the cellular prion protein (PrP^C) into the infectious isoform (PrP^Sc). In humans, there are sporadic, genetic and infectious etiologies, with sporadic Creutzfeldt-Jakob disease (sCJD) being the most common form. Currently, no treatment is available for prion diseases. Cellular cholesterol is known to impact prion conversion, which in turn results in an accumulation of cholesterol in prion-infected neurons. The major elimination of brain cholesterol is achieved by the brain specific enzyme, cholesterol 24-hydroxylase (CYP46A1). Cyp46A1 converts cholesterol into 24(S)-hydroxycholesterol, a membrane-permeable molecule that exits the brain. We have demonstrated for the first time that Cyp46A1 levels are reduced in the brains of prion-infected mice at advanced disease stage, in prion-infected neuronal cells and in post-mortem brains of sCJD patients. We have employed the Cyp46A1 activator efavirenz (EFV) for treatment of prion-infected neuronal cells and mice. EFV is an FDA approved anti-HIV medication effectively crossing the blood brain barrier and has been used for decades to chronically treat HIV patients. EFV significantly mitigated PrP^Sc propagation in prion-infected cells while preserving physiological PrP^C and lipid raft integrity. Notably, oral administration of EFV treatment chronically at very low dosage starting weeks to months after intracerebral prion inoculation of mice significantly prolonged the lifespan of animals. In summary, our results suggest that Cyp46A1 as a novel therapeutic target and that its activation through repurposing the anti-retroviral medication EFV might be valuable treatment approach for prion diseases.

Feasibility studies of radioiodinated pyridyl benzofuran derivatives as potential SPECT imaging agents for prion deposits in the brain

INTRODUCTION

Prion diseases are fatal neurodegenerative disorders caused by the deposition of abnormal prion protein aggregates (PrPSc) in the central nervous system. This study aimed to evaluate the use of iodinated pyridyl benzofuran (IPBF) derivatives as single-photon emission computed tomography (SPECT) probes for the detection of cerebral PrPSc deposits.

METHODS

In vitro binding assays of IPBF derivatives were carried out in the recombinant mouse prion protein (rMoPrP) and brain sections of mouse-adapted bovine spongiform encephalopathy (mBSE)-infected mice. SPECT imaging of 5-(5-[123I]iodobenzofuran-2-yl)-N-methylpyridin-2-amine ([123I]IPBF-NHMe) was performed on mBSE-infected and mock-infected mice.

RESULTS

Fluorescence microscopy results showed that fluorescence signals of IPBF derivatives corresponded to the thioflavin-T positive amyloid deposits of PrPSc in the brain sections of mouse-adapted bovine spongiform encephalopathy (mBSE)-infected mice. Among the IPBF derivatives, 5-(5-iodobenzofuran-2-yl)-N-methylpyridin-2-amine (IPBF-NHMe) exhibited the highest binding affinity to the recombinant mouse prion protein (rMoPrP) aggregates with a Ki of 14.3 nM. SPECT/computed tomography (CT) imaging and ex vivo autoradiography demonstrated that the [123I]IPBF-NHMe distribution in brain tissues of mBSE-infected mice co-localized with PrPSc deposits.

CONCLUSION

[123I]IPBF-NHMe appears to be a prospective SPECT tracer for monitoring prion deposits in living brain tissues.

The utility of bank voles for studying prion disease

The transmission of prions between species is typically an inefficient process due to the species barrier, which represents incompatibility between prion seed and substrate molecules. Bank voles (Myodes glareolus) are an exception to this rule, as they are susceptible to a diverse range of prion strains from many different animal species. In particular, bank voles can be efficiently infected with most types of human prions and have played a critical role in validating variably protease-sensitive prionopathy (VPSPr) and certain forms of Gerstmann-Sträussler-Scheinker (GSS) disease as bona fide prion disorders rather than non-transmissible proteinopathies. The bank vole prion protein (BVPrP) confers a "universal prion acceptor" phenotype when expressed in mice and when used as a substrate for in vitro prion amplification assays, indicating that the unique prion transmission properties of bank voles are mediated by BVPrP. Over-expression of BVPrP in mice can also promote the spontaneous development of prion disease, indicating that BVPrP is intrinsically prone to both spontaneous and template-directed misfolding. Here, we discuss the utility of bank voles and BVPrP for prion research and how they have provided new tools for establishing rapid animal bioassays, modeling spontaneous prion disease, standardizing prion diagnostics, and understanding the molecular basis of the species barrier.

Global analysis of protein degradation in prion infected cells

Prion diseases are rare, neurological disorders caused by the misfolding of the cellular prion protein (PrP^C) into cytotoxic fibrils (PrP^Sc). Intracellular PrP^Sc aggregates primarily accumulate within late endosomes and lysosomes, organelles that participate in the degradation and turnover of a large subset of the proteome. Thus, intracellular accumulation of PrP^Sc aggregates has the potential to globally influence protein degradation kinetics within an infected cell. We analyzed the proteome-wide effect of prion infection on protein degradation rates in N2a neuroblastoma cells by dynamic stable isotopic labeling with amino acids in cell culture (dSILAC) and bottom-up proteomics. The analysis quantified the degradation rates of more than 4,700 proteins in prion infected and uninfected cells. As expected, the degradation rate of the prion protein is significantly decreased upon aggregation in infected cells. In contrast, the degradation kinetics of the remainder of the N2a proteome generally increases upon prion infection. This effect occurs concurrently with increases in the cellular activities of autophagy and some lysosomal hydrolases. The resulting enhancement in proteome flux may play a role in the survival of N2a cells upon prion infection.

The molecular mechanisms of copper metabolism and its roles in human diseases

Copper is an essential element in cells; it can act as either a recipient or a donor of electrons, participating in various reactions. However, an excess of copper ions in cells is detrimental as these copper ions can generate free radicals and increase oxidative stress. In multicellular organisms, copper metabolism involves uptake, distribution, sequestration, and excretion, at both the cellular and systemic levels. Mammalian enterocytes take in bioavailable copper ions from the diet in a Ctr1-dependent manner. After incorporation, cuprous ions are delivered to ATP7A, which pumps Cu⁺ from enterocytes into the blood. Copper ions arrive at the liver through the portal vein and are incorporated into hepatocytes by Ctr1. Then, Cu⁺ can be secreted into the bile or the blood via the Atox1/ATP7B/ceruloplasmin route. In the bloodstream, this micronutrient can reach peripheral tissues and is again incorporated by Ctr1. In peripheral tissue cells, cuprous ions are either sequestrated by molecules such as metallothioneins or targeted to utilization pathways by chaperons such as Atox1, Cox17, and CCS. Copper metabolism must be tightly controlled in order to achieve homeostasis and avoid disorders. A hereditary or acquired copper unbalance, including deficiency, overload, or misdistribution, may cause or aggravate certain diseases such as Menkes disease, Wilson disease, neurodegenerative diseases, anemia, metabolic syndrome, cardiovascular diseases, and cancer. A full understanding of copper metabolism and its roles in diseases underlies the identification of novel effective therapies for such diseases.

Combining autophagy stimulators and cellulose ethers for therapy against prion disease

Prion diseases are fatal transmissible neurodegenerative disorders that affect animals and humans. Prions are proteinaceous infectious particles consisting of a misfolded isoform of the cellular prion protein PrP^C, termed PrP^Sc. PrP^Sc accumulates in infected neurons due to partial resistance to proteolytic digestion. Using compounds that interfere with the production of PrP^Sc or enhance its degradation cure prion infection in vitro, but most drugs failed when used to treat prion-infected rodents. In order to synergize the effect of anti-prion drugs, we combined drugs interfering with the generation of PrP^Sc with compounds inducing PrP^Sc degradation. Here, we tested autophagy stimulators (rapamycin or AR12) and cellulose ether compounds (TC-5RW or 60SH-50) either as single or combination treatment of mice infected with RML prions. Single drug treatments significantly extended the survival compared to the untreated group. As anticipated, also all the combination therapy groups showed extended survival compared to the untreated group, but no combination treatment showed superior effects to 60SH-50 or TC-5RW treatment alone. Unexpectedly, we later found that combining autophagy stimulator and cellulose ether treatment in cultured neuronal cells mitigated the pro-autophagic activity of AR12 and rapamycin, which can in part explain the in vivo results. Overall, we show that it is critical to exclude antagonizing drug effects when attempting combination therapy. In addition, we identified AR-12 as a pro-autophagic drug that significantly extends survival of prion-infected mice, has no adverse side effects on the animals used in this study, and can be useful in future studies.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

Cellulose ether treatment in vivo generates chronic wasting disease prions with reduced protease resistance and delayed disease progression

Chronic wasting disease (CWD) is a prion disease of free-ranging and farmed cervids that is highly contagious because of extensive prion shedding and prion persistence in the environment. Previously, cellulose ether compounds (CEs) have been shown to significantly extend the survival of mice inoculated with mouse-adapted prion strains. In this study, we used CEs, TC-5RW, and 60SH-50, in vitro and in vivo to assess their efficacy to interfere with CWD prion propagation. In vitro, CEs inhibited CWD prion amplification in a dose-dependent manner. Transgenic mice over-expressing elk PrP^C (tgElk) were injected subcutaneously with a single dose of either of the CEs, followed by intracerebral inoculation with different CWD isolates from white tailed deer, mule deer, or elk. All treated groups showed a prolonged survival of up to more than 30 % when compared to the control group regardless of the CWD isolate used for infection. The extended survival in the treated groups correlated with reduced proteinase K resistance of prions. Remarkably, passage of brain homogenates from treated or untreated animals in tgElk mice resulted in a prolonged life span of mice inoculated with homogenates from CE-treated mice (of + 17%) even in the absence of further treatment. Besides the delayed disease onset upon passage in TgElk mice, the reduced proteinase K resistance was maintained but less pronounced. Therefore, these compounds can be very useful in limiting the spread of CWD in captive and wild-ranging cervids.

Extracellular vesicles in host-pathogen interactions and immune regulation - exosomes as emerging actors in the immunological theater of pregnancy

This review correlates and summarizes the role of the maternal-fetal interface in the immune tolerance of the fetus and the processes that lead to infection avoidance, emphasizing the participation of exosomes and other extracellular vesicles in both situations. Exosomes are released into the extracellular medium by several cell types and are excellent carriers of biomolecules. Host-derived exosomes and the transport of pathogen-derived molecules by exosomes impact infections in different ways. The interactions of exosomes with the maternal immune system are pivotal to a favorable gestational outcome. In this review, we highlight the potential role of exosomes in the establishment of an adequate milieu that enables embryo implantation and discuss the participation of exosomes released at the maternal-fetal interface during the establishment of an immune-privileged compartment for fetal development. The placenta is a component where important strategies are used to minimize the risk of infection. To present a contrast, we also discuss possible mechanisms used by pathogens to cross the maternal-fetal interface. We review the processes, mechanisms, and potential consequences of dysregulation in all of the abovementioned phenomena. Basic information about exosomes and their roles in viral immune evasion is also presented. The interactions between extracellular vesicles and bacteria, fungi, parasites and proteinaceous infectious agents are addressed. The discovery of the placental microbiota and the implications of this new microbiota are also discussed, and current proposals that explain fetal/placental colonization by both pathogenic and commensal microbes are addressed. The comprehension of such interactions will help us to understand the immune dynamics of human pregnancy and the mechanisms of immune evasion used by different pathogens.

Engineering a murine cell line for the stable propagation of hamster prions

Prions are infectious protein aggregates that cause several fatal neurodegenerative diseases. Prion research has been hindered by a lack of cellular paradigms for studying the replication of prions from different species. Although hamster prions have been widely used to study prion replication in animals and within in vitro amplification systems, they have proved challenging to propagate in cultured cells. Because the murine catecholaminergic cell line CAD5 is susceptible to a diverse range of mouse prion strains, we hypothesized that it might also be capable of propagating nonmouse prions. Here, using CRISPR/Cas9-mediated genome engineering, we demonstrate that CAD5 cells lacking endogenous mouse PrP expression (CAD5-PrP^-/- cells) can be chronically infected with hamster prions following stable expression of hamster PrP. When exposed to the 263K, HY, or 139H hamster prion strains, these cells stably propagated high levels of protease-resistant PrP. Hamster prion replication required absence of mouse PrP, and hamster PrP inhibited the propagation of mouse prions. Cellular homogenates from 263K-infected cells exhibited prion seeding activity in the RT-QuIC assay and were infectious to naïve cells expressing hamster PrP. Interestingly, murine N2a neuroblastoma cells ablated for endogenous PrP expression were susceptible to mouse prions, but not hamster prions upon expression of cognate PrP, suggesting that CAD5 cells either possess cellular factors that enhance or lack factors that restrict the diversity of prion strains that can be propagated. We conclude that transfected CAD5-PrP^-/- cells may be a useful tool for assessing the biology of prion strains and dissecting the mechanism of prion replication.

Prion Disease in Dromedary Camels, Algeria

Prions cause fatal and transmissible neurodegenerative diseases, including Creutzfeldt-Jakob disease in humans, scrapie in small ruminants, and bovine spongiform encephalopathy (BSE). After the BSE epidemic, and the associated human infections, began in 1996 in the United Kingdom, general concerns have been raised about animal prions. We detected a prion disease in dromedary camels (Camelus dromedarius) in Algeria. Symptoms suggesting prion disease occurred in 3.1% of dromedaries brought for slaughter to the Ouargla abattoir in 2015–2016. We confirmed diagnosis by detecting pathognomonic neurodegeneration and disease-specific prion protein (PrP^Sc) in brain tissues from 3 symptomatic animals. Prion detection in lymphoid tissues is suggestive of the infectious nature of the disease. PrP^Sc biochemical characterization showed differences with BSE and scrapie. Our identification of this prion disease in a geographically widespread livestock species requires urgent enforcement of surveillance and assessment of the potential risks to human and animal health.

Autophagy regulates exosomal release of prions in neuronal cells

Prions are protein-based infectious agents that autocatalytically convert the cellular prion protein PrP^C to its pathological isoform PrP^Sc. Subsequent aggregation and accumulation of PrP^Sc in nervous tissues causes several invariably fatal neurodegenerative diseases in humans and animals. Prions can infect recipient cells when packaged into endosome-derived nanoparticles called exosomes, which are present in biological fluids such as blood, urine, and saliva. Autophagy is a basic cellular degradation and recycling machinery that also affects exosomal processing, but whether autophagy controls release of prions in exosomes is unclear. Our work investigated the effect of autophagy modulation on exosomal release of prions and how this interplay affects cellular prion infection. Exosomes isolated from cultured murine central neuronal cells (CAD5) and peripheral neuronal cells (N2a) contained prions as shown by immunoblotting for PrP^Sc, prion-conversion activity, and cell culture infection. We observed that autophagy stimulation with the mTOR inhibitor rapamycin strongly inhibited exosomal prion release. In contrast, inhibition of autophagy by wortmannin or CRISPR/Cas9-mediated knockout of the autophagy protein Atg5 (autophagy-related 5) greatly increased the release of exosomes and exosome-associated prions. We also show that a difference in exosomal prion release between CAD5 and N2a cells is related to differences at the level of basal autophagy. Taken together, our results indicate that autophagy modulation can control lateral transfer of prions by interfering with their exosomal release. We describe a novel role of autophagy in the prion life cycle, an understanding that may provide useful targets for containing prion diseases.

Cellular copper homeostasis: current concepts on its interplay with glutathione homeostasis and its implication in physiology and human diseases

Copper is a trace element essential for almost all living organisms. But the level of intracellular copper needs to be tightly regulated. Dysregulation of cellular copper homeostasis leading to various diseases demonstrates the importance of this tight regulation. Copper homeostasis is regulated not only within the cell but also within individual intracellular compartments. Inactivation of export machinery results in excess copper being redistributed into various intracellular organelles. Recent evidence suggests the involvement of glutathione in playing an important role in regulating copper entry and intracellular copper homeostasis. Therefore interplay of both homeostases might play an important role within the cell. Similar to copper, glutathione balance is tightly regulated within individual cellular compartments. This review explores the existing literature on the role of glutathione in regulating cellular copper homeostasis. On the one hand, interplay of glutathione and copper homeostasis performs an important role in normal physiological processes, for example neuronal differentiation. On the other hand, perturbation of the interplay might play a key role in the pathogenesis of copper homeostasis disorders.

Interaction of Peptide Aptamers with Prion Protein Central Domain Promotes α-Cleavage of PrPC

Prion diseases are infectious and fatal neurodegenerative diseases affecting humans and animals. Transmission is possible within and between species with zoonotic potential. Currently, no prophylaxis or treatment exists. Prions are composed of the misfolded isoform PrP^Sc of the cellular prion protein PrP^C. Expression of PrP^C is a prerequisite for prion infection, and conformational conversion of PrP^C is induced upon its direct interaction with PrP^Sc. Inhibition of this interaction can abrogate prion propagation, and we have previously established peptide aptamers (PAs) binding to PrP^C as new anti-prion compounds. Here, we mapped the interaction site of PA8 in PrP and modeled the complex in silico to design targeted mutations in PA8 which presumably enhance binding properties. Using these PA8 variants, we could improve PA-mediated inhibition of PrP^Sc replication and de novo infection of neuronal cells. Furthermore, we demonstrate that binding of PA8 and its variants increases PrP^C α-cleavage and interferes with its internalization. This gives rise to high levels of the membrane-anchored PrP-C1 fragment, a transdominant negative inhibitor of prion replication. PA8 and its variants interact with PrP^C at its central and most highly conserved domain, a region which is crucial for prion conversion and facilitates toxic signaling of Aβ oligomers characteristic for Alzheimer's disease. Our strategy allows for the first time to induce α-cleavage, which occurs within this central domain, independent of targeting the responsible protease. Therefore, interaction of PAs with PrP^C and enhancement of α-cleavage represent mechanisms that can be beneficial for the treatment of prion and other neurodegenerative diseases.

Prion Strains and Transmission Barrier Phenomena

Several experimental evidences show that prions are non-conventional pathogens, which physical support consists only in proteins. This finding raised questions regarding the observed prion strain-to-strain variations and the species barrier that happened to be crossed with dramatic consequences on human health and veterinary policies during the last 3 decades. This review presents a focus on a few advances in the field of prion structure and prion strains characterization: from the historical approaches that allowed the concept of prion strains to emerge, to the last results demonstrating that a prion strain may in fact be a combination of a few quasi species with subtle biophysical specificities. Then, we will focus on the current knowledge on the factors that impact species barrier strength and species barrier crossing. Finally, we present probable scenarios on how the interaction of strain properties with host characteristics may account for differential selection of new conformer variants and eventually species barrier crossing.

Specification of Physiologic and Disease States by Distinct Proteins and Protein Conformations

Protein conformational states-from intrinsically disordered ensembles to amyloids that underlie the self-templating, infectious properties of prion-like proteins-have attracted much attention. Here, we highlight the diversity, including differences in biophysical properties, that drive distinct biological functions and pathologies among self-templating proteins. Advances in chemical genomics, gene editing, and model systems now permit deconstruction of the complex interplay between these protein states and the host factors that react to them. These methods reveal that conformational switches modulate normal and abnormal information transfer and that intimate relationships exist between the intrinsic function of proteins and the deleterious consequences of their misfolding.

The celecoxib derivatives AR-12 and AR-14 induce autophagy and clear prion-infected cells from prions

Prion diseases are fatal infectious neurodegenerative disorders that affect both humans and animals. The autocatalytic conversion of the cellular prion protein (PrPC) into the pathologic isoform PrPSc is a key feature in prion pathogenesis. AR-12 is an IND-approved derivative of celecoxib that demonstrated preclinical activity against several microbial diseases. Recently, AR-12 has been shown to facilitate clearance of misfolded proteins. The latter proposes AR-12 to be a potential therapeutic agent for neurodegenerative disorders. In this study, we investigated the role of AR-12 and its derivatives in controlling prion infection. We tested AR-12 in prion infected neuronal and non-neuronal cell lines. Immunoblotting and confocal microscopy results showed that AR-12 and its analogue AR-14 reduced PrPSc levels after only 72 hours of treatment. Furthermore, infected cells were cured of PrPSc after exposure of AR-12 or AR-14 for only two weeks. We partially attribute the influence of the AR compounds on prion propagation to autophagy stimulation, in line with our previous findings that drug-induced stimulation of autophagy has anti-prion effects in vitro and in vivo. Taken together, this study demonstrates that AR-12 and the AR-14 analogue are potential new therapeutic agents for prion diseases and possibly protein misfolding disorders involving prion-like mechanisms.

Strains of Pathological Protein Aggregates in Neurodegenerative Diseases

The presence of protein aggregates in the brain is a hallmark of neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Considerable evidence has revealed that the pathological protein aggregates in many neurodegenerative diseases are able to self-propagate, which may enable pathology to spread from cell-to-cell within the brain. This property is reminiscent of what occurs in prion diseases such as Creutzfeldt-Jakob disease. A widely recognized feature of prion disorders is the existence of distinct strains of prions, which are thought to represent unique protein aggregate structures. A number of recent studies have pointed to the existence of strains of protein aggregates in other, more common neurodegenerative illnesses such as AD, PD, and related disorders. In this review, we outline the pathobiology of prion strains and discuss how the concept of protein aggregate strains may help to explain the heterogeneity inherent to many human neurodegenerative disorders.

Real-time Quaking-induced Conversion Assay for Detection of CWD Prions in Fecal Material

The RT-QuIC technique is a sensitive in vitro cell-free prion amplification assay based mainly on the seeded misfolding and aggregation of recombinant prion protein (PrP) substrate using prion seeds as a template for the conversion. RT-QuIC is a novel high-throughput technique which is analogous to real-time polymerase chain reaction (PCR). Detection of amyloid fibril growth is based on the dye Thioflavin T, which fluoresces upon specific interaction with ᵦ-sheet rich proteins. Thus, amyloid formation can be detected in real time. We attempted to develop a reliable non-invasive screening test to detect chronic wasting disease (CWD) prions in fecal extract. Here, we have specifically adapted the RT-QuIC technique to reveal PrP^Sc seeding activity in feces of CWD infected cervids. Initially, the seeding activity of the fecal extracts we prepared was relatively low in RT-QuIC, possibly due to potential assay inhibitors in the fecal material. To improve seeding activity of feces extracts and remove potential assay inhibitors, we homogenized the fecal samples in a buffer containing detergents and protease inhibitors. We also submitted the samples to different methodologies to concentrate PrP^Sc on the basis of protein precipitation using sodium phosphotungstic acid, and centrifugal force. Finally, the feces extracts were tested by optimized RT-QuIC which included substrate replacement in the protocol to improve the sensitivity of detection. Thus, we established a protocol for sensitive detection of CWD prion seeding activity in feces of pre-clinical and clinical cervids by RT-QuIC, which can be a practical tool for non-invasive CWD diagnosis.

Evidence of a Prion-Like Transmission of p53 Amyloid in Saccharomyces cerevisiae

Loss of p53 function is largely responsible for the occurrence of cancer in humans. Aggregation of mutant p53 has been found in multiple cancer cell types, suggesting a role of aggregation in loss of p53 function and cancer development. The p53 protein has recently been hypothesized to possess a prion-like conformation, although experimental evidence is lacking. Here, we report that human p53 can be inactivated upon exposure to preformed fibrils containing an aggregation-prone sequence-specific peptide, PILTIITL, derived from p53, and the inactive state was found to be stable for many generations. Importantly, we provide evidence of a prion-like transmission of these p53 aggregates. This study has significant implications for understanding cancer progression due to p53 malfunctioning without any loss-of-function mutation or occurrence of transcriptional inactivation. Our data might unlock new possibilities for understanding the disease and will lead to rational design of p53 aggregation inhibitors for the development of drugs against cancer.

Destabilizing polymorphism in cervid prion protein hydrophobic core determines prion conformation and conversion efficiency

Prion diseases are infectious neurodegenerative disorders of humans and animals caused by misfolded forms of the cellular prion protein PrP^C. Prions cause disease by converting PrP^C into aggregation-prone PrP^Sc. Chronic wasting disease (CWD) is the most contagious prion disease with substantial lateral transmission, affecting free-ranging and farmed cervids. Although the PrP primary structure is highly conserved among cervids, the disease phenotype can be modulated by species-specific polymorphisms in the prion protein gene. How the resulting amino-acid substitutions impact PrP^C and PrP^Sc structure and propagation is poorly understood. We investigated the effects of the cervid 116A>G substitution, located in the most conserved PrP domain, on PrP^C structure and conversion and on 116AG-prion conformation and infectivity. Molecular dynamics simulations revealed structural de-stabilization of 116G-PrP, which enhanced its in vitro conversion efficiency when used as recombinant PrP substrate in real-time quaking-induced conversion (RT-QuIC). We demonstrate that 116AG-prions are conformationally less stable, show lower activity as a seed in RT-QuIC and exhibit reduced infectivity in vitro and in vivo. Infectivity of 116AG-prions was significantly enhanced upon secondary passage in mice, yet conformational features were retained. These findings indicate that structurally de-stabilized PrP^C is readily convertible by cervid prions of different genetic background and results in a prion conformation adaptable to cervid wild-type PrP. Conformation is an important criterion when assessing transmission barrier, and conformational variants can target a different host range. Therefore, a thorough analysis of CWD isolates and re-assessment of species-barriers is important in order to fully exclude a zoonotic potential of CWD.

Chronic wasting disease (CWD) is a prion disease which affects wild and captive cervids. Prion diseases are infectious neurodegenerative disorders, and the causative agent consists of abnormally folded prion protein termed PrP^Sc. Prions replicate without genetic information, and their three-dimensional structure is thought to encode heritable information necessary to propagate using the cellular prion protein PrP^C as a substrate for conversion. In this study, we use in vitro and in vivo techniques to analyze the effect of a polymorphism at codon 116 (A>G) of the white-tailed deer prion protein on CWD prion conformation, propagation and pathogenesis. We observed differences in conformation, infectivity and seeding activity in vitro between CWD prions isolated from white-tailed deer encoding wild-type (116AA) PrP^C or 116AG-PrP^C. In mouse bioassays conformational differences are retained, however, 116AG CWD prions resulted in significantly shortened incubation times upon passages. Molecular dynamics simulations suggest that the structure of 116G-PrP^C is more flexible, which is supported by an improved convertibility in an in vitro conversion assay. Altogether these data indicate the importance of a variation in the most conserved PrP domain, and highlight the relationship between PrP^C structural flexibility, prion conformation and conversion, and pathogenesis of prion disease in vivo.

Amyloid Prions in Fungi

Prions are infectious protein polymers that have been found to cause fatal diseases in mammals. Prions have also been identified in fungi (yeast and filamentous fungi), where they behave as cytoplasmic non-Mendelian genetic elements. Fungal prions correspond in most cases to fibrillary β-sheet-rich protein aggregates termed amyloids. Fungal prion models and, in particular, yeast prions were instrumental in the description of fundamental aspects of prion structure and propagation. These models established the "protein-only" nature of prions, the physical basis of strain variation, and the role of a variety of chaperones in prion propagation and amyloid aggregate handling. Yeast and fungal prions do not necessarily correspond to harmful entities but can have adaptive roles in these organisms.

Early and Non-Invasive Detection of Chronic Wasting Disease Prions in Elk Feces by Real-Time Quaking Induced Conversion

Chronic wasting disease (CWD) is a fatal prion disease of wild and captive cervids in North America. Prions are infectious agents composed of a misfolded version of a host-encoded protein, termed PrP^Sc. Infected cervids excrete and secrete prions, contributing to lateral transmission. Geographical distribution is expanding and case numbers in wild cervids are increasing. Recently, the first European cases of CWD have been reported in a wild reindeer and two moose from Norway. Therefore, methods to detect the infection early in the incubation time using easily available samples are desirable to facilitate effective disease management. We have adapted the real-time quaking induced conversion (RT-QuIC) assay, a sensitive in vitro prion amplification method, for pre-clinical detection of prion seeding activity in elk feces. Testing fecal samples from orally inoculated elk taken at various time points post infection revealed early shedding and detectable prion seeding activity throughout the disease course. Early shedding was also found in two elk encoding a PrP genotype associated with reduced susceptibility for CWD. In summary, we suggest that detection of CWD prions in feces by RT-QuIC may become a useful tool to support CWD surveillance in wild and captive cervids. The finding of early shedding independent of the elk’s prion protein genotype raises the question whether prolonged survival is beneficial, considering accumulation of environmental prions and its contribution to CWD transmission upon extended duration of shedding.

CWD prions remain infectious after passage through the digestive system of coyotes (Canis latrans)

Chronic wasting disease (CWD) is a geographically expanding prion disease of wild and captive cervids in North America. Disease can be transmitted directly, animal to animal, or indirectly via the environment. CWD contamination can occur residually in the environment via soil, water, and forage following deposition of bodily fluids such as urine, saliva, and feces, or by the decomposition of carcasses. Recent work has indicated that plants may even take up prions into the stems and leaves. When a carcass or gut pile is present in the environment, a large number of avian and mammalian species visit and consume the carrion. Additionally, predators like coyotes, likely select for disease-compromised cervids. Natural cross-species CWD transmission has not been documented, however, passage of infectious prion material has been observed in the feces of crows. In this study we evaluated the ability of CWD-infected brain material to pass through the gastrointestinal tract of coyotes (Canis latrans) following oral ingestion, and be infectious in a cervidized transgenic mouse model. Results from this study indicate that coyotes can pass infectious prions via their feces for at least 3 days post ingestion, demonstrating that mammalian scavengers could contribute to the translocation and contamination of CWD in the environment.

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.

Genetics of Prion Disease in Cattle

Bovine spongiform encephalopathy (BSE) is a prion disease that is invariably fatal in cattle and has been implicated as a significant human health risk. As a transmissible disease of livestock, it has impacted food safety, production practices, global trade, and profitability. Genetic polymorphisms that alter the prion protein in humans and sheep are associated with transmissible spongiform encephalopathy susceptibility or resistance. In contrast, there is no strong evidence that nonsynonymous mutations in the bovine prion gene (PRNP) are associated with classical BSE (C-BSE) disease susceptibility, though two bovine PRNP insertion/deletion polymorphisms, in the putative region, are associated with susceptibility to C-BSE. However, these associations do not explain the full extent of BSE susceptibility, and loci outside of PRNP appear to be associated with disease incidence in some cattle populations. This article provides a review of the current state of genetic knowledge regarding prion diseases in cattle.

Transgenic fatal familial insomnia mice indicate prion infectivity-independent mechanisms of pathogenesis and phenotypic expression of disease

Fatal familial insomnia (FFI) and a genetic form of Creutzfeldt-Jakob disease (CJD¹⁷⁸) are clinically different prion disorders linked to the D178N prion protein (PrP) mutation. The disease phenotype is determined by the 129 M/V polymorphism on the mutant allele, which is thought to influence D178N PrP misfolding, leading to the formation of distinctive prion strains with specific neurotoxic properties. However, the mechanism by which misfolded variants of mutant PrP cause different diseases is not known. We generated transgenic (Tg) mice expressing the mouse PrP homolog of the FFI mutation. These mice synthesize a misfolded form of mutant PrP in their brains and develop a neurological illness with severe sleep disruption, highly reminiscent of FFI and different from that of analogously generated Tg(CJD) mice modeling CJD¹⁷⁸. No prion infectivity was detectable in Tg(FFI) and Tg(CJD) brains by bioassay or protein misfolding cyclic amplification, indicating that mutant PrP has disease-encoding properties that do not depend on its ability to propagate its misfolded conformation. Tg(FFI) and Tg(CJD) neurons have different patterns of intracellular PrP accumulation associated with distinct morphological abnormalities of the endoplasmic reticulum and Golgi, suggesting that mutation-specific alterations of secretory transport may contribute to the disease phenotype.

Genetic prion diseases are degenerative brain disorders caused by mutations in the gene encoding the prion protein (PrP). Different PrP mutations cause different diseases, including Creutzfeldt-Jakob disease (CJD) and fatal familial insomnia (FFI). The reason for this variability is not known, but assembly of the mutant PrPs into distinct aggregates that spread in the brain by promoting PrP aggregation may contribute to the disease phenotype. We previously generated transgenic mice modeling genetic CJD, clinically identified by dementia and motor abnormalities. We have now generated transgenic mice carrying the PrP mutation associated with FFI, and found that they develop severe sleep abnormalities and other key features of the human disorder. Thus, transgenic mice recapitulate the phenotypic differences seen in humans. The mutant PrPs in FFI and CJD mice are aggregated but unable to promote PrP aggregation. They accumulate in different intracellular compartments and cause distinct morphological abnormalities of transport organelles. These results indicate that mutant PrP has disease-encoding properties that are independent of its ability to self-propagate, and suggest that the phenotypic heterogeneity may be due to different effects of aggregated PrP on intracellular transport. Our study provides new insights into the mechanisms of selective neuronal dysfunction due to protein aggregation.

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.

Yeast studies reveal moonlighting functions of the ancient actin cytoskeleton

Classic functions of the actin cytoskeleton include control of cell size and shape and the internal organization of cells. These functions are manifest in cellular processes of fundamental importance throughout biology such as the generation of cell polarity, cell migration, cell adhesion, and cell division. However, studies in the unicellular model eukaryote Saccharomyces cerevisiae (Baker's yeast) are giving insights into other functions in which the actin cytoskeleton plays a critical role. These include endocytosis, control of protein translation, and determination of protein 3-dimensional shape (especially conversion of normal cellular proteins into prions). Here, we present a concise overview of these new "moonlighting" roles for the actin cytoskeleton and how some of these roles might lie at the heart of important molecular switches. This is an exciting time for researchers interested in the actin cytoskeleton. We show here how studies of actin are leading us into many new and exciting realms at the interface of genetics, biochemistry, and cell biology. While many of the pioneering studies have been conducted using yeast, the conservation of the actin cytoskeleton and its component proteins throughout eukaryotes suggests that these new roles for the actin cytoskeleton may not be restricted to yeast cells but rather may reflect new roles for the actin cytoskeleton of all eukaryotes.

Translation of the prion protein mRNA is robust in astrocytes but does not amplify during reactive astrocytosis in the mouse brain

Prion diseases induce neurodegeneration in specific brain areas for undetermined reasons. A thorough understanding of the localization of the disease-causing molecule, the prion protein (PrP), could inform on this issue but previous studies have generated conflicting conclusions. One of the more intriguing disagreements is whether PrP is synthesized by astrocytes. We developed a knock-in reporter mouse line in which the coding sequence of the PrP expressing gene (Prnp), was replaced with that for green fluorescent protein (GFP). Native GFP fluorescence intensity varied between and within brain regions. GFP was present in astrocytes but did not increase during reactive gliosis induced by scrapie prion infection. Therefore, reactive gliosis associated with prion diseases does not cause an acceleration of local PrP production. In addition to aiding in Prnp gene activity studies, this reporter mouse line will likely prove useful for analysis of chimeric animals produced by stem cell and tissue transplantation experiments.

Evidence that bank vole PrP is a universal acceptor for prions

Bank voles are uniquely susceptible to a wide range of prion strains isolated from many different species. To determine if this enhanced susceptibility to interspecies prion transmission is encoded within the sequence of the bank vole prion protein (BVPrP), we inoculated Tg(M109) and Tg(I109) mice, which express BVPrP containing either methionine or isoleucine at polymorphic codon 109, with 16 prion isolates from 8 different species: humans, cattle, elk, sheep, guinea pigs, hamsters, mice, and meadow voles. Efficient disease transmission was observed in both Tg(M109) and Tg(I109) mice. For instance, inoculation of the most common human prion strain, sporadic Creutzfeldt-Jakob disease (sCJD) subtype MM1, into Tg(M109) mice gave incubation periods of ∼200 days that were shortened slightly on second passage. Chronic wasting disease prions exhibited an incubation time of ∼250 days, which shortened to ∼150 days upon second passage in Tg(M109) mice. Unexpectedly, bovine spongiform encephalopathy and variant CJD prions caused rapid neurological dysfunction in Tg(M109) mice upon second passage, with incubation periods of 64 and 40 days, respectively. Despite the rapid incubation periods, other strain-specified properties of many prion isolates—including the size of proteinase K–resistant PrP^Sc, the pattern of cerebral PrP^Sc deposition, and the conformational stability—were remarkably conserved upon serial passage in Tg(M109) mice. Our results demonstrate that expression of BVPrP is sufficient to engender enhanced susceptibility to a diverse range of prion isolates, suggesting that BVPrP may be a universal acceptor for prions.

Prions are infectious proteins that cause devastating neurodegenerative diseases in both humans and animals. Unlike other rodents, bank voles are highly susceptible to prions from many different species, suggesting that bank voles do not impose a “species barrier,” which normally restricts the transmission of prions from one species to another. We were curious as to whether the unprecedented promiscuity of bank voles for prions is due to the specific prion protein sequence expressed, or to some other factor inherent to bank vole physiology. To answer this question, we inoculated transgenic mice that express bank vole prion protein [Tg(BVPrP) mice] with a diverse set of prions deriving from eight different species. Like bank voles, Tg(BVPrP) mice were highly susceptible to prions from all species tested, demonstrating that the BVPrP sequence mediates the enhanced susceptibility of bank voles to prions. Because the amino acid sequences of mouse and BVPrP differ at only eight positions, our results demonstrate that alterations to a small subset of residues within PrP can have a profound effect on the susceptibility of an organism to prions from another species.

Early detection of chronic wasting disease prions in urine of pre-symptomatic deer by real-time quaking-induced conversion assay

Chronic wasting disease (CWD) is a prion disease of captive and free-ranging deer (Odocoileus spp), elk (Cervus elaphus nelsonii) and moose (Alces alces shirasi). Unlike in most other prion diseases, in CWD prions are shed in urine and feces, which most likely contributes to the horizontal transmission within and between cervid species. To date, CWD ante-mortem diagnosis is only possible by immunohistochemical detection of protease resistant prion protein (PrP^Sc) in tonsil or recto-anal mucosa-associated lymphoid tissue (RAMALT) biopsies, which requires anesthesia of animals. We report on detection of CWD prions in urine collected from pre-symptomatic deer and in fecal extracts by using real time quaking-induced conversion (RT-QuIC). This assay can be useful for non-invasive pre-symptomatic diagnosis and surveillance of CWD.

Synaptic dysfunction in prion diseases: a trafficking problem?

Synaptic dysfunction is an important cause of neurological symptoms in prion diseases, a class of clinically heterogeneous neurodegenerative disorders caused by misfolding of the cellular prion protein (PrP^C). Experimental data suggest that accumulation of misfolded PrP^C in the endoplasmic reticulum (ER) may be crucial in synaptic failure, possibly because of the activation of the translational repression pathway of the unfolded protein response. Here, we report that this pathway is not operative in mouse models of genetic prion disease, consistent with our previous observation that ER stress is not involved. Building on our recent finding that ER retention of mutant PrP^C impairs the secretory trafficking of calcium channels essential for synaptic function, we propose a model of pathogenicity in which intracellular retention of misfolded PrP^C results in loss of function or gain of toxicity of PrP^C-interacting proteins. This neurotoxic modality may also explain the phenotypic heterogeneity of prion diseases.

A low-molecular-weight ferroxidase is increased in the CSF of sCJD cases: CSF ferroxidase and transferrin as diagnostic biomarkers for sCJD

AIMS

Most biomarkers used for the premortem diagnosis of sporadic Creutzfeldt-Jakob disease (CJD) are surrogate in nature, and provide suboptimal sensitivity and specificity.

RESULTS

We report that CJD-associated brain iron dyshomeostasis is reflected in the cerebrospinal fluid (CSF), providing disease-specific diagnostic biomarkers. Analysis of 290 premortem CSF samples from confirmed cases of CJD, Alzheimer's disease, and other dementias (DMs), and 52 non-DM (ND) controls revealed a significant difference in ferroxidase (Frx) activity and transferrin (Tf) levels in sporadic Creutzfeldt-Jakob disease (sCJD) relative to other DM and ND controls. A combination of CSF Frx and Tf discriminated sCJD from other DMs with a sensitivity of 86.8%, specificity of 92.5%, accuracy of 88.9%, and area-under-the receiver-operating-characteristic (ROC) curve of 0.94. This combination provided a similar diagnostic accuracy in discriminating CJD from rapidly progressing cases who died within 6 months of sample collection. Surprisingly, ceruloplasmin and amyloid precursor protein, the major brain Frxs, displayed minimal activity in the CSF. Most of the Frx activity was concentrated in the <3-kDa fraction in normal and diseased CSF, and resisted heat and proteinase-K treatment.

INNOVATION

(i) A combination of CSF Frx and Tf provides disease-specific premortem diagnostic biomarkers for sCJD. (ii) A novel, nonenzymatic, nonprotein Frx predominates in human CSF that is distinct from the currently known CSF Frxs.

CONCLUSION

The underlying cause of iron imbalance is distinct in sCJD relative to other DMs associated with the brain iron imbalance. Thus, change in the CSF levels of iron-management proteins can provide disease-specific biomarkers and insight into the cause of iron imbalance in neurodegenerative conditions.

Ethics in prion disease

This paper is intended to discuss some of the scientific and ethical issues that are created by increased research efforts towards earlier diagnosis, as well as to treatment of, human prion diseases (and related dementias), including the resulting consequences for individuals, their families, and society. Most patients with prion disease currently are diagnosed when they are about 2/3 of the way through their disease course (Geschwind et al., 2010a; Paterson et al., 2012b), when the disease has progressed so far that even treatments that stop the disease process would probably have little benefit. Although there are currently no treatments available for prion diseases, we and others have realized that we must diagnose patients earlier and with greater accuracy so that future treatments have hope of success. As approximately 15% of prion diseases have a autosomal dominant genetic etiology, this further adds to the complexity of ethical issues, particularly regarding when to conduct genetic testing, release of genetic results, and when or if to implement experimental therapies. Human prion diseases are both infectious and transmissible; great care is required to balance the needs of the family and individual with both public health needs and strained hospital budgets. It is essential to proactively examine and address the ethical issues involved, as well as to define and in turn provide best standards of care.

Profoundly different prion diseases in knock-in mice carrying single PrP codon substitutions associated with human diseases

In man, mutations in different regions of the prion protein (PrP) are associated with infectious neurodegenerative diseases that have remarkably different clinical signs and neuropathological lesions. To explore the roots of this phenomenon, we created a knock-in mouse model carrying the mutation associated with one of these diseases [Creutzfeldt-Jakob disease (CJD)] that was exactly analogous to a previous knock-in model of a different prion disease [fatal familial insomnia (FFI)]. Together with the WT parent, this created an allelic series of three lines, each expressing the same protein with a single amino acid difference, and with all native regulatory elements intact. The previously described FFI mice develop neuronal loss and intense reactive gliosis in the thalamus, as seen in humans with FFI. In contrast, CJD mice had the hallmark features of CJD, spongiosis and proteinase K-resistant PrP aggregates, initially developing in the hippocampus and cerebellum but absent from the thalamus. A molecular transmission barrier protected the mice from any infectious prion agents that might have been present in our mouse facility and allowed us to conclude that the diseases occurred spontaneously. Importantly, both models created agents that caused a transmissible neurodegenerative disease in WT mice. We conclude that single codon differences in a single gene in an otherwise normal genome can cause remarkably different neurodegenerative diseases and are sufficient to create distinct protein-based infectious elements.

In vitro prion protein conversion suggests risk of bighorn sheep (Ovis canadensis) to transmissible spongiform encephalopathies

Background

Transmissible spongiform encephalopathies (TSEs) affect both domestic sheep (scrapie) and captive and free-ranging cervids (chronic wasting disease; CWD). The geographical range of bighorn sheep (Ovis canadensis; BHS) overlaps with states or provinces that have contained scrapie-positive sheep or goats and areas with present epizootics of CWD in cervids. No TSEs have been documented in BHS, but the susceptibility of this species to TSEs remains unknown.

Results

We acquired a library of BHS tissues and found no evidence of preexisting TSEs in these animals. The prion protein gene (Prnp) in all BHS in our library was identical to scrapie-susceptible domestic sheep (A¹³⁶R¹⁵⁴Q¹⁷¹ genotype). Using an in vitro prion protein conversion assay, which has been previously used to assess TSE species barriers and, in our study appears to recollect known species barriers in mice, we assessed the potential transmissibility of TSEs to BHS. As expected based upon Prnp genotype, we observed BHS prion protein conversion by classical scrapie agent and evidence for a species barrier between transmissible mink encephalopathy (TME) and BHS. Interestingly, our data suggest that the species barrier of BHS to white-tailed deer or wapiti CWD agents is likely low. We also used protein misfolding cyclic amplification to confirm that CWD, but not TME, can template prion protein misfolding in A¹³⁶R¹⁵⁴Q¹⁷¹ genotype sheep.

Conclusions

Our results indicate the in vitro conversion assay used in our study does mimic the species barrier of mice to the TSE agents that we tested. Based on Prnp genotype and results from conversion assays, BHS are likely to be susceptible to infection by classical scrapie. Despite mismatches in amino acids thought to modulate prion protein conversion, our data indicate that A¹³⁶R¹⁵⁴Q¹⁷¹ genotype sheep prion protein is misfolded by CWD agent, suggesting that these animals could be susceptible to CWD. Further investigation of TSE transmissibility to BHS, including animal studies, is warranted. The lack of reported TSEs in BHS may be attributable to other host factors or a lack of TSE surveillance in this species.

Could avian scavengers translocate infectious prions to disease-free areas initiating new foci of chronic wasting disease?

Mechanisms for the spread of transmissible spongiform encephalopathy diseases, including chronic wasting disease (CWD) in North American cervids, are incompletely understood, but primary routes include horizontal and environmental transmission. Birds have been identified as potential vectors for a number of diseases, where they ingest or are exposed to infected material and later shed the disease agent in new areas after flying substantial distances. We recently identified American crows (Corvus brachyrhynchos) as having the potential to translocate infectious prions in their feces. Our results suggest that this common, migratory North American scavenger is capable of translocating infectious prions to disease-free areas, potentially seeding CWD infection where no other initial source of pathogen establishment is forthcoming. Here we speculate on the role avian scavengers, like American crows, might play in the spatial dissemination of CWD. We also consider the role mammalian scavengers may play in dispersing prions.

Critical significance of the region between Helix 1 and 2 for efficient dominant-negative inhibition by conversion-incompetent prion protein

Prion diseases are fatal infectious neurodegenerative disorders in man and animals associated with the accumulation of the pathogenic isoform PrP^Sc of the host-encoded prion protein (PrP^c). A profound conformational change of PrP^c underlies formation of PrP^Sc and prion propagation involves conversion of PrP^c substrate by direct interaction with PrP^Sc template. Identifying the interfaces and modalities of inter-molecular interactions of PrPs will highly advance our understanding of prion propagation in particular and of prion-like mechanisms in general. To identify the region critical for inter-molecular interactions of PrP, we exploited here dominant-negative inhibition (DNI) effects of conversion-incompetent, internally-deleted PrP (ΔPrP) on co-expressed conversion-competent PrP. We created a series of ΔPrPs with different lengths of deletions in the region between first and second α-helix (H1∼H2) which was recently postulated to be of importance in prion species barrier and PrP fibril formation. As previously reported, ΔPrPs uniformly exhibited aberrant properties including detergent insolubility, limited protease digestion resistance, high-mannose type N-linked glycans, and intracellular localization. Although formerly controversial, we demonstrate here that ΔPrPs have a GPI anchor attached. Surprisingly, despite very similar biochemical and cell-biological properties, DNI efficiencies of ΔPrPs varied significantly, dependant on location and inversely correlated with the size of deletion. This data demonstrates that H1∼H2 and the region C-terminal to it are critically important for efficient DNI. It also suggests that this region is involved in PrP-PrP interaction and conversion of PrP^C into PrP^Sc. To reconcile the paradox of how an intracellular PrP can exert DNI, we demonstrate that ΔPrPs are subject to both proteasomal and lysosomal/autophagic degradation pathways. Using autophagy pathways ΔPrPs obtain access to the locale of prion conversion and PrP^Sc recycling and can exert DNI there. This shows that the intracellular trafficking of PrPs is more complex than previously anticipated.

Prion diseases are deadly infectious diseases of the brain characterized by accumulation of a pathologic protein (PrP^Sc) which is derived from the normal prion protein (PrP^c). Prions replicate by direct contact in a template-directed refolding process which involves conversion of PrP^C into PrP^Sc. Identifying the modalities of this interaction can advance our molecular understanding of prion diseases. Like substrates and competitive inhibitors of enzymes, a conversion-incompetent PrP can inhibit conversion of normal PrP^C, a phenomenon known as dominant-negative inhibition (DNI). Interestingly, some conversion-incompetent PrPs efficiently cause DNI but others do not, presumably depending on affinity for PrP^Sc and integrity of interaction interface. We utilized DNI to characterize the PrP-PrP interaction interface in cultured cells. We created a series of PrPs with internal deletions in the region between helix 1 and 2 and evaluated their DNI. We found an inverse correlation between deletion size and DNI which suggests that this region plays an important role in PrP-PrP interaction. We also found that such PrPs are subject to various cellular degradation pathways and that a fraction of them reaches the intracellular locale of prion conversion. Further investigation of such prion proteins might help elucidating the cellular mechanisms of the PrP^C-PrP^Sc interaction.

Structural plasticity of the cellular prion protein and implications in health and disease

Two lines of transgenic mice expressing mouse/elk and mouse/horse prion protein (PrP) hybrids, which both form a well-structured β2-α2 loop in the NMR structures at 20 °C termed rigid-loop cellular prion proteins (RL-PrP(C)), presented with accumulation of the aggregated scrapie form of PrP in brain tissue, and the mouse/elk hybrid has also been shown to develop a spontaneous transmissible spongiform encephalopathy. Independently, there is in vitro evidence for correlations between the amino acid sequence in the β2-α2 loop and the propensity for conformational transitions to disease-related forms of PrP. To further contribute to the structural basis for these observations, this paper presents a detailed characterization of RL-PrP(C) conformations in solution. A dynamic local conformational polymorphism involving the β2-α2 loop was found to be evolutionarily preserved among all mammalian species, including those species for which the WT PrP forms an RL-PrP(C). The interconversion between two ensembles of PrP(C) conformers that contain, respectively, a 310-helix turn or a type I β-turn structure of the β2-α2 loop, exposes two different surface epitopes, which are analyzed for their possible roles in the still evasive function of PrP(C) in healthy organisms and/or at the onset of a transmissible spongiform encephalopathy.

Does the central dogma still stand?

Prions are agents of analog, protein conformation-based inheritance that can confer beneficial phenotypes to cells, especially under stress. Combined with genetic variation, prion-mediated inheritance can be channeled into prion-independent genomic inheritance. Latest screening shows that prions are common, at least in fungi. Thus, there is non-negligible flow of information from proteins to the genome in modern cells, in a direct violation of the Central Dogma of molecular biology. The prion-mediated heredity that violates the Central Dogma appears to be a specific, most radical manifestation of the widespread assimilation of protein (epigenetic) variation into genetic variation. The epigenetic variation precedes and facilitates genetic adaptation through a general 'look-ahead effect' of phenotypic mutations. This direction of the information flow is likely to be one of the important routes of environment-genome interaction and could substantially contribute to the evolution of complex adaptive traits.

A novel expression system for production of soluble prion proteins in E. coli

Expression of eukaryotic proteins in Escherichia coli is challenging, especially when they contain disulfide bonds. Since the discovery of the prion protein (PrP) and its role in transmissible spongiform encephalopathies, the need to obtain large quantities of the recombinant protein for research purposes has been essential. Currently, production of recombinant PrP is achieved by refolding protocols. Here, we show that the co-expression of two different PrP with the human Quiescin Sulfhydryl OXidase (QSOX), a human chaperone with thiol/disulfide oxidase activity, in the cytoplasm of E. coli produces soluble recombinant PrP. The structural integrity of the soluble PrP has been confirmed by nuclear magnetic resonance spectroscopy, demonstrating that properly folded PrP can be easily expressed in bacteria. Furthermore, the soluble recombinant PrP produced with this method can be used for functional and structural studies.