High CJD infectivity remains after prion protein is destroyed

"Prion-like" seeding and propagation of oligomeric protein assemblies in neurodegenerative disorders

Intra- or extracellular aggregates of proteins are central pathogenic features in most neurodegenerative disorders. The accumulation of such proteins in diseased brains is believed to be the end-stage of a stepwise aggregation of misfolded monomers to insoluble cross-β fibrils via a series of differently sized soluble oligomers/protofibrils. Several studies have shown how α-synuclein, amyloid-β, tau and other amyloidogenic proteins can act as nucleating particles and thereby share properties with misfolded forms, or strains, of the prion protein. Although the roles of different protein assemblies in the respective aggregation cascades remain unclear, oligomers/protofibrils are considered key pathogenic species. Numerous observations have demonstrated their neurotoxic effects and a growing number of studies have indicated that they also possess seeding properties, enabling their propagation within cellular networks in the nervous system. The seeding behavior of oligomers differs between the proteins and is also affected by various factors, such as size, shape and epitope presentation. Here, we are providing an overview of the current state of knowledge with respect to the "prion-like" behavior of soluble oligomers for several of the amyloidogenic proteins involved in neurodegenerative diseases. In addition to providing new insight into pathogenic mechanisms, research in this field is leading to novel diagnostic and therapeutic opportunities for neurodegenerative diseases.

Sensitive microscopic quantification of surface-bound prion infectivity for the assessment of surgical instrument decontamination procedures

BACKGROUND

Pathogenic prions (PrP^Sc) are amyloid-rich hydrophobic proteins which bind avidly to surgical surfaces and represent some of the most difficult targets during the reprocessing of reusable surgical instruments. In-vitro methods to amplify and detect the presence of otherwise undetectable prion contamination are available, but they do not measure associated infectivity. Most of these methods rely on the use of proteinase K, however this can lead to the loss of a substantial portion of PrP^Sc, potentially producing false negatives.

AIM

To develop a sensitive in-situ method without proteinase treatment for the dynamic quantification of amyloid accumulation in N2a #58 cells following 22L-prion infection from infected tissues and spiked stainless-steel surfaces.

METHODS

We spiked cultures of N2a #58 cells with the 22L prion strain in solution or dried on stainless-steel wires and directly measured the accumulation of prion amyloid aggregates over several passages using highly sensitive fluorescence microscopy.

FINDINGS

We demonstrated a 10-log dynamic range using our method to test residual prion infectivity, that was validated to show variable decontamination efficacy against prions from commercially available cleaning chemistries.

CONCLUSIONS

The new cell-based infectivity method presented here avoids partial or possibly total proteinase K digestion of PrP^Sc in samples for greater sensitivity, in addition to low cost, no ethical concerns, and adaptability to detect different prion strains. This method can be used to test cleaning chemistries' efficacy with greater sensitivity than measuring total residual proteins, which may not correlate with residual prion infectivity.

Prion disease modelled in Drosophila

Prion diseases are fatal neurodegenerative conditions of humans and various vertebrate species that are transmissible between individuals of the same or different species. A novel infectious moiety referred to as a prion is considered responsible for transmission of these conditions. Prion replication is believed to be the cause of the neurotoxicity that arises during prion disease pathogenesis. The prion hypothesis predicts that the transmissible prion agent consists of PrPSc, which is comprised of aggregated misfolded conformers of the normal host protein PrPC. It is important to understand the biology of transmissible prions and to identify genetic modifiers of prion-induced neurotoxicity. This information will underpin the development of therapeutic and control strategies for human and animal prion diseases. The most reliable method to detect prion infectivity is by in vivo transmission in a suitable experimental host, which to date have been mammalian species. Current prion bioassays are slow, cumbersome and relatively insensitive to low titres of prion infectivity, and do not lend themselves to rapid genetic analysis of prion disease. Here, we provide an overview of our novel studies that have led to the establishment of Drosophila melanogaster, a genetically well-defined invertebrate host, as a sensitive, versatile and economically viable animal model for the detection of mammalian prion infectivity and genetic modifiers of prion-induced toxicity.

A new model for sensitive detection of zoonotic prions by PrP transgenic Drosophila

Prions are transmissible protein pathogens most reliably detected by a bioassay in a suitable host, typically mice. However, the mouse bioassay is slow and cumbersome, and relatively insensitive to low titers of prion infectivity. Prions can be detected biochemically in vitro by the protein misfolding cyclic amplification (PMCA) technique, which amplifies disease-associated prion protein but does not detect bona fide prion infectivity. Here, we demonstrate that Drosophila transgenic for bovine prion protein (PrP) expression can serve as a model system for the detection of bovine prions significantly more efficiently than either the mouse prion bioassay or PMCA. Strikingly, bovine PrP transgenic Drosophila could detect bovine prion infectivity in the region of a 10^-12 dilution of classical bovine spongiform encephalopathy (BSE) inoculum, which is 10⁶-fold more sensitive than that achieved by the bovine PrP mouse bioassay. A similar level of sensitivity was observed in the detection of H-type and L-type atypical BSE and sheep-passaged BSE by bovine PrP transgenic Drosophila. Bioassays of bovine prions in Drosophila were performed within 7 weeks, whereas the mouse prion bioassay required at least a year to assess the same inoculum. In addition, bovine PrP transgenic Drosophila could detect classical BSE at a level 10⁵-fold lower than that achieved by PMCA. These data show that PrP transgenic Drosophila represent a new tractable prion bioassay for the efficient and sensitive detection of mammalian prions, including those of known zoonotic potential.

Stability of BSE infectivity towards heat treatment even after proteolytic removal of prion protein

The unconventional infectious agents of transmissible spongiform encephalopathies (TSEs) are prions. Their infectivity co-appears with PrPSc, aberrant depositions of the host's cellular prion protein (PrPC). Successive heat treatment in the presence of detergent and proteolysis by a keratinase from Bacillus licheniformis PWD-1 was shown before to destroy PrPSc from bovine TSE (BSE) and sheep scrapie diseased brain, however data regarding expected reduction of infectivity were still lacking. Therefore, transgenic Tgbov XV mice which are highly BSE susceptible were used to quantify infectivity before and after the bovine brain treatment procedure. Also four immunochemical analyses were applied to compare the levels of PrPSc. After heating at 115 °C with or without subsequent proteolysis, the original BSE infectivity of 106.2-6.4 ID50 g-1 was reduced to a remaining infectivity of 104.6-5.7 ID50 g-1 while strain characteristics were unaltered, even after precipitation with methanol. Surprisingly, PrPSc depletion was 5-800 times higher than the loss of infectivity. Similar treatment was applied on other prion strains, which were CWD1 in bank voles, 263 K scrapie in hamsters and sheep PG127 scrapie in tg338 ovinized mice. In these strains however, infectivity was already destroyed by heat only. These findings show the unusual heat resistance of BSE and support a role for an additional factor in prion formation as suggested elsewhere when producing prions from PrPC. Leftover material in the remaining PrPSc depleted BSE preparation offers a unique substrate for searching additional elements for prion infectivity and improving our concept about the nature of prions.

Chemical Inactivation of Prions Is Altered by Binding to the Soil Mineral Montmorillonite

Environmental routes of transmission contribute to the spread of the prion diseases chronic wasting disease of deer and elk and scrapie of sheep and goats. Prions can persist in soils and other environmental matrices and remain infectious for years. Prions bind avidly to the common soil mineral montmorillonite, and such binding can dramatically increase oral disease transmission. Decontamination of soil in captive facilities and natural habitats requires inactivation agents that are effective when prions are bound to soil microparticles. Here, we investigate the inactivation of free and montmorillonite-bound prions with sodium hydroxide, acidic pH, Environ LpH, and sodium hypochlorite. Immunoblotting and bioassays confirm that sodium hydroxide and sodium hypochlorite are effective for prion deactivation, although montmorillonite appears to reduce the efficacy of hypochlorite. Acidic conditions slightly reduce prion infectivity, and the acidic phenolic disinfectant Environ LpH produces slight reductions in infectivity and immunoreactivity. The extent to which the association with montmorillonite protects prions from chemical inactivation appears influenced by the effect of chemical agents on the clay structure and surface pH. When clay morphology remains relatively unaltered, as when exposed to hypochlorite, montmorillonite-bound prions appear to be protected from inactivation. In contrast, when the clay structure is substantially transformed, as when exposed to high concentrations of sodium hydroxide, the attachment to montmorillonite does not slow degradation. A reduction in surface pH appears to cause slight disruptions in clay structure, which enhances degradation under these conditions. We expect our findings will aid the development of remediation approaches for successful decontamination of prion-contaminated sites.

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

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

Viral Infections and Obesity

PURPOSE OF REVIEW

Obesity is a multifactorial disease that is now endemic throughout most of the world. Although addressing proximate causes of obesity (excess energy intake and reduced energy expenditure) have been longstanding global health priorities, the problem has continued to worsen at the global level.

RECENT FINDINGS

Numerous microbial agents cause obesity in various experimental models-a phenomena known as infectobesity. Several of the same agents alter metabolic function in human cells and are associated with human obesity or metabolic dysfunction in humans. We address the evidence for a role in the genesis of obesity for viral agents in five broad categories: adenoviridae, herpesviridae, phages, transmissible spongiform encephalopathies (slow virus), and other encephalitides and hepatitides. Despite the importance of this topic area, there are many persistent knowledge gaps that need to be resolved. We discuss factors motivating further research and recommend that future infectobesity investigation should be more comprehensive, leveraged, interventional, and patient-centered.

Protease resistance of infectious prions is suppressed by removal of a single atom in the cellular prion protein

Resistance to proteolytic digestion has long been considered a defining trait of prions in tissues of organisms suffering from transmissible spongiform encephalopathies. Detection of proteinase K-resistant prion protein (PrP^Sc) still represents the diagnostic gold standard for prion diseases in humans, sheep and cattle. However, it has become increasingly apparent that the accumulation of PrP^Sc does not always accompany prion infections: high titers of prion infectivity can be reached also in the absence of protease resistant PrP^Sc. Here, we describe a structural basis for the phenomenon of protease-sensitive prion infectivity. We studied the effect on proteinase K (PK) resistance of the amino acid substitution Y169F, which removes a single oxygen atom from the β2–α2 loop of the cellular prion protein (PrP^C). When infected with RML or the 263K strain of prions, transgenic mice lacking wild-type (wt) PrP^C but expressing MoPrP^169F generated prion infectivity at levels comparable to wt mice. The newly generated MoPrP^169F prions were biologically indistinguishable from those recovered from prion-infected wt mice, and elicited similar pathologies in vivo. Surprisingly, MoPrP^169F prions showed greatly reduced PK resistance and density gradient analyses showed a significant reduction in high-density aggregates. Passage of MoPrP^169F prions into mice expressing wt MoPrP led to full recovery of protease resistance, indicating that no strain shift had taken place. We conclude that a subtle structural variation in the β2–α2 loop of PrP^C affects the sensitivity of PrP^Sc to protease but does not impact prion replication and infectivity. With these findings a specific structural feature of PrP^C can be linked to a physicochemical property of the corresponding PrP^Sc.

Rapid chemical decontamination of infectious CJD and scrapie particles parallels treatments known to disrupt microbes and biofilms

Neurodegenerative human CJD and sheep scrapie are diseases caused by several different transmissible encephalopathy (TSE) agents. These infectious agents provoke innate immune responses in the brain, including late-onset abnormal prion protein (PrP-res) amyloid. Agent particles that lack detectable PrP sequences by deep proteomic analysis are highly infectious. Yet these agents, and their unusual resistance to denaturation, are often evaluated by PrP amyloid disruption. To reexamine the intrinsic resistance of TSE agents to denaturation, a paradigm for less resistant viruses and microbes, we developed a rapid and reproducible high yield agent isolation procedure from cultured cells that minimized PrP amyloid and other cellular proteins. Monotypic neuronal GT1 cells infected with the FU-CJD or 22L scrapie agents do not have complex brain changes that can camouflage infectious particles and prevent their disruption, and there are only 2 reports on infectious titers of any human CJD strain treated with chemical denaturants. Infectious titers of both CJD and scrapie were reduced by >4 logs with Thiourea-urea, a treatment not previously tested. A mere 5 min exposure to 4M GdnHCl at 22°C reduced infectivity by >5 logs. Infectious 22L particles were significantly more sensitive to denaturation than FU-CJD particles. A protocol using sonication with these chemical treatments may effectively decontaminate complicated instruments, such as duodenoscopes that harbor additional virulent microbes and biofilms associated with recent iatrogenic infections.

Infectivity-associated PrP(Sc) and disease duration-associated PrP(Sc) of mouse BSE prions

Disease-related prion protein (PrP^Sc), which is a structural isoform of the host-encoded cellular prion protein, is thought to be a causative agent of transmissible spongiform encephalopathies. However, the specific role of PrP^Sc in prion pathogenesis and its relationship to infectivity remain controversial. A time-course study of prion-affected mice was conducted, which showed that the prion infectivity was not simply proportional to the amount of PrP^Sc in the brain. Centrifugation (20,000 ×g) of the brain homogenate showed that most of the PrP^Sc was precipitated into the pellet, and the supernatant contained only a slight amount of PrP^Sc. Interestingly, mice inoculated with the obtained supernatant showed incubation periods that were approximately 15 d longer than those of mice inoculated with the crude homogenate even though both inocula contained almost the same infectivity. Our results suggest that a small population of fine PrP^Sc may be responsible for prion infectivity and that large, aggregated PrP^Sc may contribute to determining prion disease duration.

CJD and Scrapie Require Agent-Associated Nucleic Acids for Infection

Unlike Alzheimer's and most other neurodegenerative diseases, Transmissible Spongiform Encephalopathies (TSEs) are all caused by actively replicating infectious particles of viral size and density. Different strain-specific TSE agents cause CJD, kuru, scrapie and BSE, and all behave as latent viruses that evade adaptive immune responses and can persist for years in lymphoreticular tissues. A foreign viral structure with a nucleic acid genome best explains these TSE strains and their endemic and epidemic spread in susceptible species. Nevertheless, it is widely believed that host prion protein (PrP), without any genetic material, encodes all these strains. We developed rapid infectivity assays that allowed us to reproducibly isolate infectious particles where >85% of the starting titer separated from the majority of host components, including PrP. Remarkably, digestion of all forms of PrP did not reduce brain particle titers. To ask if TSE agents, as other viruses, require nucleic acids, we exposed high titer FU-CJD and 22L scrapie particles to potent nucleases. Both agent-strains were propagated in GT1 neuronal cells to avoid interference by complex degenerative brain changes that can impede nuclease digestions. After exposure to nucleases that are active in sarkosyl, infectivity of both agents was reproducibly reduced by ≥99%. No gold-stained host proteins or any form of PrP were visibly altered by these nucleases. In contrast, co-purifying protected mitochondrial DNA and circular SPHINX DNAs were destroyed. These findings demonstrate that TSE agents require protected genetic material to infect their hosts, and should reopen investigation of essential agent nucleic acids. J. Cell. Biochem. 117: 1947-1958, 2016. © 2016 Wiley Periodicals, Inc.

Highly infectious CJD particles lack prion protein but contain many viral-linked peptides by LC-MS/MS

It is widely believed that host prion protein (PrP), without nucleic acid, converts itself into an infectious form (PrP‐res) that causes transmissible encephalopathies (TSEs), such as human sporadic CJD (sCJD), endemic sheep scrapie, and epidemic BSE. There are many detailed investigations of PrP, but proteomic studies of other proteins in verified infectious TSE particles have not been pursued, even though brain homogenates without PrP retain their complete infectious titer. To define proteins that may be integral to, process, or protect an agent genome, we developed a streamlined, high‐yield purification of infectious FU‐CJD mouse brain particles with minimal PrP. Proteinase K (PK) abolished all residual particle PrP, but did not reduce infectivity, and viral‐size particles lacking PrP were ∼70S (vs. 90–120S without PK). Furthermore, over 1,500 non‐PrP proteins were still present and positively identified in high titer FU‐CJD particles without detectable PrP by mass spectrometry (LC‐MS/MS); 114 of these peptides were linked to viral motifs in the environmental–viral database, and not evident in parallel uninfected controls. Host components were also identified in both PK and non‐PK treated particles from FU‐CJD mouse brain and human sCJD brain. This abundant cellular data had several surprises, including finding Huntingtin in the sCJD but not normal human brain samples. Similarly, the neural Wiskott–Aldrich sequence and multivesicular and endosome components associated with retromer APP (Alzheimer amyloid) processing were only in sCJD. These cellular findings suggest that new therapies directed at retromer–vesicular trafficking in other neurodegenerative diseases may also counteract late‐onset sCJD PrP amyloid pathology. J. Cell. Biochem. 115: 2012–2021, 2014. © 2014 Wiley Periodicals, Inc.

Proteomic analysis of host brain components that bind to infectious particles in Creutzfeldt-Jakob disease

Transmissible encephalopathies (TSEs), such as Creutzfeldt-Jakob disease (CJD) and scrapie, are caused by infectious agents that provoke strain-specific patterns of disease. Misfolded host prion protein (PrP-res amyloid) is believed to be the causal infectious agent. However, particles that are stripped of PrP retain both high infectivity and viral proteins not detectable in uninfected mouse controls. We here detail host proteins bound with FU-CJD agent infectious brain particles by proteomic analysis. More than 98 proteins were differentially regulated, and 56 FU-CJD exclusive proteins were revealed after PrP, GFAP, C1q, ApoE, and other late pathologic response proteins were removed. Stripped FU-CJD particles revealed HSC70 (144× the uninfected control), cyclophilin B, an FU-CJD exclusive protein required by many viruses, and early endosome-membrane pathways known to facilitate viral processing, replication, and spread. Synaptosomal elements including synapsin-2 (at 33×) and AP180 (a major FU-CJD exclusive protein) paralleled the known ultrastructural location of 25 nm virus-like TSE particles and infectivity in synapses. Proteins without apparent viral or neurodegenerative links (copine-3), and others involved in viral-induced protein misfolding and aggregation, were also identified. Human sCJD brain particles contained 146 exclusive proteins, and heat shock, synaptic, and viral pathways were again prominent, in addition to Alzheimer, Parkinson, and Huntington aggregation proteins. Host proteins that bind TSE infectious particles can prevent host immune recognition and contribute to prolonged cross-species transmissions (the species barrier). Our infectious particle strategy, which reduces background sequences by >99%, emphasizes host targets for new therapeutic initiatives. Such therapies can simultaneously subvert common pathways of neurodegeneration.

Synthetic scrapie infectivity: interaction between recombinant PrP and scrapie brain-derived RNA

The key molecular event in human cerebral proteinopathies, which include Alzheimer's, Parkinson's and Huntington's diseases, is the structural conversion of a specific host protein into a β-sheet-rich conformer. With regards to this common mechanism, it appears difficult to explain the outstanding infectious properties attributed to PrP(Sc), the hallmark of another intriguing family of cerebral proteinopathies known as transmissible spongiform encephalopathies (TSE) or prion diseases. The infectious PrP(Sc) or "prion" is thought to be composed solely of a misfolded form of the otherwise harmless cellular prion protein (PrP(c)). To gain insight into this unique situation, we used the 263K scrapie hamster model to search for a putative PrP(Sc)-associated factor that contributes to the infectivity of PrP(Sc) amyloid. In a rigorously controlled set of experiments that included several bioassays, we showed that originally innocuous recombinant prion protein (recPrP) equivalent to PrP(c) is capable of initiating prion disease in hamsters when it is converted to a prion-like conformation (β-sheet-rich) in the presence of RNA purified from scrapie-associated fibril (SAF) preparations. Analysis of the recPrP-RNA infectious mixture reveals the presence of 2 populations of small RNAs of approximately 27 and 55 nucleotides. These unprecedented findings are discussed in light of the distinct relationship that may exist between this RNA material and the 2 biological properties, infectivity and strain features, attributed to prion amyloid.

Influence of breed and genotype on the onset and distribution of infectivity and disease-associated prion protein in sheep following oral infection with the bovine spongiform encephalopathy agent

The onset and distribution of infectivity and disease-specific prion protein (PrP(d)) accumulation was studied in Romney and Suffolk sheep of the ARQ/ARQ, ARQ/ARR and ARR/ARR prion protein gene (Prnp) genotypes (where A stands for alanine, R for arginine and Q for glutamine at codons 136, 154 and 171 of PrP), following experimental oral infection with cattle-derived bovine spongiform encephalopathy (BSE) agent. Groups of sheep were killed at regular intervals and a wide range of tissues taken for mouse bioassay or immunohistochemistry (IHC), or both. Bioassay results for infectivity were mostly coincident with those of PrP(d) detection by IHC both in terms of tissues and time post infection. Neither PrP(d) nor infectivity was detected in any tissues of BSE-dosed ARQ/ARR or ARR/ARR sheep or of undosed controls. Moreover, four ARQ/ARQ Suffolk sheep, which were methionine (M)/threonine heterozygous at codon 112 of the Prnp gene, did not show any biological or immunohistochemical evidence of infection, while those homozygous for methionine (MARQ/MARQ) did. In MARQ/MARQ sheep of both breeds, initial PrP(d) accumulation was identified in lymphoreticular system (LRS) tissues followed by the central nervous system (CNS) and enteric nervous system (ENS) and finally by the autonomic nervous system and peripheral nervous system and other organs. Detection of infectivity closely mimicked this sequence. No PrP(d) was observed in the ENS prior to its accumulation in the CNS, suggesting that ENS involvement occurred simultaneously to that of, or followed centrifugal spread from, the CNS. The distribution of PrP(d) within the ENS further suggested a progressive spread from the ileal plexus to other ENS segments via neuronal connections of the gut wall. Differences between the two breeds were noted in terms of involvement of LRS and ENS tissues, with Romney sheep showing a more delayed and less consistent PrP(d) accumulation than Suffolk sheep in such tissues. Whether this accounted for the slight delay (∼5 months) in the appearance of clinical signs in Romney sheep is debatable since by the last scheduled kill before animals reached clinical end point, both breeds showed widespread accumulation and similar magnitudes of PrP(d) accumulation in the brain.

Dissociation of prion protein amyloid seeding from transmission of a spongiform encephalopathy

Misfolding and aggregation of proteins are common pathogenic mechanisms of a group of diseases called proteinopathies. The formation and spread of proteinaceous lesions within and between individuals were first described in prion diseases and proposed as the basis of their infectious nature. Recently, a similar "prion-like" mechanism of transmission has been proposed in other neurodegenerative diseases such as Alzheimer's disease. We investigated if misfolding and aggregation of corrupted prion protein (PrP(TSE)) are always associated with horizontal transmission of disease. Knock-in transgenic mice (101LL) expressing mutant PrP (PrP-101L) that are susceptible to disease but do not develop any spontaneous neurological phenotype were inoculated with (i) brain extracts containing PrP(TSE) from healthy 101LL mice with PrP plaques in the corpus callosum or (ii) brain extracts from mice overexpressing PrP-101L with neurological disease, severe spongiform encephalopathy, and formation of proteinase K-resistant PrP(TSE). In all instances, 101LL mice developed PrP plaques in the area of inoculation and vicinity in the absence of clinical disease or spongiform degeneration of the brain. Importantly, 101LL mice did not transmit disease on serial passage, ruling out the presence of subclinical infection. Thus, in both experimental models the formation of PrP(TSE) is not infectious. These results have implications for the interpretation of tests based on the detection of protein aggregates and suggest that de novo formation of PrP(TSE) in the host does not always result in a transmissible prion disease. In addition, these results question the validity of assuming that all diseases due to protein misfolding can be transmitted between individuals.

Evaluation of a combinatorial approach to prion inactivation using an oxidizing agent, SDS, and proteinase K

BACKGROUND

Prions demonstrate an unusual resistance to methods effective at inactivating conventional microorganisms. This has resulted in a very tangible and difficult infection control challenge to the medical and veterinary communities, as well as animal agriculture and related industries. Currently accepted practices of harsh chemical treatments such as prolonged exposure to sodium hydroxide or sodium hypochlorite, or autoclaving are not suitable in many situations. Less caustic and more readily applicable treatments to contaminated environments are therefore desirable. We recently demonstrated that exposure of the RML scrapie agent to a commercial product containing sodium percarbonate (SPC-P) with or without sodium dodecyl sulfate (SDS) rendered PrP(Sc) sensitive to proteinase K (PK), but did not eliminate infectivity. The current study was designed to evaluate the efficacy of a combinatorial approach to inactivating prions by exposing RML-positive brain homogenate to SPC-P and SDS followed by PK. Treated samples were evaluated for PrP(Sc)-immunoreactivity by western blot, and residual infectivity by mouse bioassay.

RESULTS

Treatment of infected brain homogenate with SPC-P and SDS followed by PK exposure resulted in a 4-5 log10 reduction in infectivity when bioassayed in tga20 mice.

CONCLUSIONS

This study demonstrates that exposure of the RML scrapie agent to SPC-P and SDS followed by PK markedly reduces, but does not eliminate infectivity. The results of this study encourage further investigation into whether consecutive or concomitant exposure to sodium percarbonate, SDS, and a protease may serve as a viable and non-caustic option for prion inactivation.

Re-assessment of PrP(Sc) distribution in sporadic and variant CJD

Human prion diseases are fatal neurodegenerative disorders associated with an accumulation of PrP^Sc in the central nervous system (CNS). Of the human prion diseases, sporadic Creutzfeldt-Jakob disease (sCJD), which has no known origin, is the most common form while variant CJD (vCJD) is an acquired human prion disease reported to differ from other human prion diseases in its neurological, neuropathological, and biochemical phenotype. Peripheral tissue involvement in prion disease, as judged by PrP^Sc accumulation in the tonsil, spleen, and lymph node has been reported in vCJD as well as several animal models of prion diseases. However, this distribution of PrP^Sc has not been consistently reported for sCJD. We reexamined CNS and non-CNS tissue distribution and levels of PrP^Sc in both sCJD and vCJD. Using a sensitive immunoassay, termed SOFIA, we also assessed PrP^Sc levels in human body fluids from sCJD as well as in vCJD-infected humanized transgenic mice (Tg666). Unexpectedly, the levels of PrP^Sc in non-CNS human tissues (spleens, lymph nodes, tonsils) from both sCJD and vCJD did not differ significantly and, as expected, were several logs lower than in the brain. Using protein misfolding cyclic amplification (PMCA) followed by SOFIA, PrP^Sc was detected in cerebrospinal fluid (CSF), but not in urine or blood, in sCJD patients. In addition, using PMCA and SOFIA, we demonstrated that blood from vCJD-infected Tg666 mice showing clinical disease contained prion disease-associated seeding activity although the data was not statistically significant likely due to the limited number of samples examined. These studies provide a comparison of PrP^Sc in sCJD vs. vCJD as well as analysis of body fluids. Further, these studies also provide circumstantial evidence that in human prion diseases, as in the animal prion diseases, a direct comparison and intraspecies correlation cannot be made between the levels of PrP^Sc and infectivity.

Role of proteomics in understanding prion infection

Transmissible spongiform encephalopathies or prion diseases are fatal neurodegenerative pathologies characterized by the autocatalytic misfolding and polymerization of a cellular glycoprotein (cellular prion protein [PrP(C)]) that accumulates in the CNS and leads to neurodegeneration. The detailed mechanics of PrP(C) conversion to its pathological isoform (PrP(TSE)) are unclear but one or more exogenous factors are likely involved in the process of PrP misfolding. In the last 20 years, proteomic investigations have identified several endogenous proteins that interact with PrP(C), PrP(TSE) or both, which are possibly involved in the prion pathogenetic process. However, current approaches have not yet produced convincing conclusions on the biological value of such PrP interactors. Future advancements in the comprehension of the molecular pathogenesis of prion diseases, in experimental techniques and in data analysis procedures, together with a boost in more productive international collaborations, are therefore needed to improve the understanding on the role of PrP interactors. Finally, the advancement of 'omics' techniques in prion diseases will contribute to the development of novel diagnostic tests and effective drugs.

Infectious particles, stress, and induced prion amyloids: a unifying perspective

Transmissible encephalopathies (TSEs) are believed by many to arise by spontaneous conversion of host prion protein (PrP) into an infectious amyloid (PrP-res, PrP (Sc) ) without nucleic acid. Many TSE agents reside in the environment, with infection controlled by public health measures. These include the disappearance of kuru with the cessation of ritual cannibalism, the dramatic reduction of epidemic bovine encephalopathy (BSE) by removal of contaminated feed, and the lack of endemic scrapie in geographically isolated Australian sheep with susceptible PrP genotypes. While prion protein modeling has engendered an intense focus on common types of protein misfolding and amyloid formation in diverse organisms and diseases, the biological characteristics of infectious TSE agents, and their recognition by the host as foreign entities, raises several fundamental new directions for fruitful investigation such as: (1) unrecognized microbial agents in the environmental metagenome that may cause latent neurodegenerative disease, (2) the evolutionary social and protective functions of different amyloid proteins in diverse organisms from bacteria to mammals, and (3) amyloid formation as a beneficial innate immune response to stress (infectious and non-infectious). This innate process however, once initiated, can become unstoppable in accelerated neuronal aging.

Cellular aspects of prion replication in vitro

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

Exposure of RML scrapie agent to a sodium percarbonate-based product and sodium dodecyl sulfate renders PrPSc protease sensitive but does not eliminate infectivity

BACKGROUND

Prions, the causative agents of the transmissible spongiform encephalopathies, are notoriously difficult to inactivate. Current decontamination recommendations by the World Health Organization include prolonged exposure to 1 N sodium hydroxide or > 20,000 ppm sodium hypochlorite, or autoclaving. For decontamination of large stainless steel surfaces and equipment as in abattoirs, for example, these methods are harsh or unsuitable. The current study was designed to evaluate the effectiveness of a commercial product containing sodium percarbonate to inactivate prions. Samples of mouse brain infected with a mouse-adapted strain of the scrapie agent (RML) were exposed to a sodium percarbonate-based product (SPC-P). Treated samples were evaluated for abnormal prion protein (PrPSc)-immunoreactivity by western blot analysis, and residual infectivity by mouse bioassay.

RESULTS

Exposure to a 21% solution of SPC-P or a solution containing either 2.1% or 21% SPC-P in combination with sodium dodecyl sulfate (SDS) resulted in increased proteinase K sensitivity of PrPSc. Limited reductions in infectivity were observed depending on treatment condition. A marginal effect on infectivity was observed with SPC-P alone, but an approximate 2-3 log10 reduction was observed with the addition of SDS, though exposure to SDS alone resulted in an approximate 2 log10 reduction.

CONCLUSIONS

This study demonstrates that exposure of a mouse-adapted scrapie strain to SPC-P does not eliminate infectivity, but does render PrPSc protease sensitive.

Infectious titres of sheep scrapie and bovine spongiform encephalopathy agents cannot be accurately predicted from quantitative laboratory test results

It is widely accepted that abnormal forms of the prion protein (PrP) are the best surrogate marker for the infectious agent of prion diseases and, in practice, the detection of such disease-associated (PrP(d)) and/or protease-resistant (PrP(res)) forms of PrP is the cornerstone of diagnosis and surveillance of the transmissible spongiform encephalopathies (TSEs). Nevertheless, some studies question the consistent association between infectivity and abnormal PrP detection. To address this discrepancy, 11 brain samples of sheep affected with natural scrapie or experimental bovine spongiform encephalopathy were selected on the basis of the magnitude and predominant types of PrP(d) accumulation, as shown by immunohistochemical (IHC) examination; contra-lateral hemi-brain samples were inoculated at three different dilutions into transgenic mice overexpressing ovine PrP and were also subjected to quantitative analysis by three biochemical tests (BCTs). Six samples gave 'low' infectious titres (10⁶·⁵ to 10⁶·⁷ LD₅₀ g⁻¹) and five gave 'high titres' (10⁸·¹ to ≥ 10⁸·⁷ LD₅₀ g⁻¹) and, with the exception of the Western blot analysis, those two groups tended to correspond with samples with lower PrP(d)/PrP(res) results by IHC/BCTs. However, no statistical association could be confirmed due to high individual sample variability. It is concluded that although detection of abnormal forms of PrP by laboratory methods remains useful to confirm TSE infection, infectivity titres cannot be predicted from quantitative test results, at least for the TSE sources and host PRNP genotypes used in this study. Furthermore, the near inverse correlation between infectious titres and Western blot results (high protease pre-treatment) argues for a dissociation between infectivity and PrP(res).

Small protease sensitive oligomers of PrPSc in distinct human prions determine conversion rate of PrP(C)

The mammalian prions replicate by converting cellular prion protein (PrP^C) into pathogenic conformational isoform (PrP^Sc). Variations in prions, which cause different disease phenotypes, are referred to as strains. The mechanism of high-fidelity replication of prion strains in the absence of nucleic acid remains unsolved. We investigated the impact of different conformational characteristics of PrP^Sc on conversion of PrP^C in vitro using PrP^Sc seeds from the most frequent human prion disease worldwide, the Creutzfeldt-Jakob disease (sCJD). The conversion potency of a broad spectrum of distinct sCJD prions was governed by the level, conformation, and stability of small oligomers of the protease-sensitive (s) PrP^Sc. The smallest most potent prions present in sCJD brains were composed only of∼20 monomers of PrP^Sc. The tight correlation between conversion potency of small oligomers of human sPrP^Sc observed in vitro and duration of the disease suggests that sPrP^Sc conformers are an important determinant of prion strain characteristics that control the progression rate of the disease.

Mammalian prion diseases were originally characterized by accumulation of protease-resistant prion protein (PrP^Sc), often forming large amyloid deposits and fibrils. However, the apparent absence of protease-resistant PrP^Sc or amyloid fibrils in growing number of prion diseases raised several fundamental questions; specifically, whether presumably protease-sensitive forms of PrP^Sc exist as distinct conformers; and whether they comprise the initial steps in prion replication or are related to the alternative misfolding pathway generating noninfectious aggregates. We investigated the conformational characteristics of protease sensitive conformers of PrP^Sc and their role in the pathogenesis of sporadic Creutzfeldt-Jakob disease (sCJD). Using two different in vitro prion protein (PrP^C) conversion techniques in tandem with biophysical methods, we identified small oligomers of protease sensitive PrP^Sc present in sCJD brains as the most potent initiators of PrP^C conversion. Their concentration and conformational stability determine the distinctly different replication potency of PrP^Sc in individual isolates of sCJD and each of these characteristics correlates tightly with duration of the disease. These features argue for a broad range of distinct prion strains causing the sCJD and imply that small oligomers of protease sensitive conformers of pathogenic prion protein are encoding incubation time and progression rate of the disease.

Continuous production of prions after infectious particles are eliminated: implications for Alzheimer's disease

Rat septal cells, induced to enter a terminal differentiation-like state by temperature shift, produce prion protein (PrP) levels 7x higher than their proliferative counterparts. Host PrP accumulates on the plasma membrane, newly elaborated nanotubes, and cell-to-cell junctions, important conduits for viral spread. To find if elevated PrP increased susceptibility to FU-CJD infection, we determined agent titers under both proliferating and arresting conditions. A short 5 day arrest and a prolonged 140 day arrest increased infectivity by 5x and 122x (>2 logs) respectively as compared to proliferating cells. Total PrP rapidly increased 7x and was even more elevated in proliferating cells that escaped chronic arrest conditions. Amyloid generating PrP (PrP-res), the “infectious prion” form, present at ∼100,000 copies per infectious particle, also increased proportionately by 140 days. However, when these highly infectious cells were switched back to proliferative conditions for 60 days, abundant PrP-res continued to be generated even though 4 logs of titer was lost. An identical 4 log loss was found with maximal PrP and PrP-res production in parallel cells under arresting conditions. While host PrP is essential for TSE agent spread and replication, excessive production of all forms of PrP can be inappropriately perpetuated by living cells, even after the initiating infectious agent is eliminated. Host PrP changes can start as a protective innate immune response that ultimately escapes control. A subset of other neurodegenerative and amyloid diseases, including non-transmissible AD, may be initiated by environmental infectious agents that are no longer present.