Vaccination with an attenuated Creutzfeldt-Jakob disease strain prevents expression of a virulent agent

Nucleation seed size determines amyloid clearance and establishes a barrier to prion appearance in yeast

Amyloid appearance is a rare event that is promoted in the presence of other aggregated proteins. These aggregates were thought to act by templating the formation of an assembly-competent nucleation seed, but we find an unanticipated role for them in enhancing the persistence of amyloid after it arises. Specifically, Saccharoymyces cerevisiae Rnq1 amyloid reduces chaperone-mediated disassembly of Sup35 amyloid, promoting its persistence in yeast. Mathematical modeling and corresponding in vivo experiments link amyloid persistence to the conformationally defined size of the Sup35 nucleation seed and suggest that amyloid is actively cleared by disassembly below this threshold to suppress appearance of the [PSI⁺] prion in vivo. Remarkably, this framework resolves multiple known inconsistencies in the appearance and curing of yeast prions. Thus, our observations establish the size of the nucleation seed as a previously unappreciated characteristic of prion variants that is key to understanding transitions between prion states.

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.

Type I interferon protects neurons from prions in in vivo models

Infectious prions comprising abnormal prion protein, which is produced by structural conversion of normal prion protein, are responsible for transmissible spongiform encephalopathies including Creutzfeldt-Jakob disease in humans. Prions are infectious agents that do not possess a genome and the pathogenic protein was not thought to evoke any immune response. Although we previously reported that interferon regulatory factor 3 (IRF3) was likely to be involved in the pathogenesis of prion diseases, suggesting the protective role of host innate immune responses mediated by IRF3 signalling, this remained to be clarified. Here, we investigated the reciprocal interactions of type I interferon evoked by IRF3 activation and prion infection and found that infecting prions cause the suppression of endogenous interferon expression. Conversely, treatment with recombinant interferons in an ex vivo model was able to inhibit prion infection. In addition, cells and mice deficient in type I interferon receptor (subunit interferon alpha/beta receptor 1), exhibited higher susceptibility to 22L-prion infection. Moreover, in in vivo and ex vivo prion-infected models, treatment with RO8191, a selective type I interferon receptor agonist, inhibited prion invasion and prolonged the survival period of infected mice. Taken together, these data indicated that the interferon signalling interferes with prion propagation and some interferon-stimulated genes might play protective roles in the brain. These findings may allow for the development of new strategies to combat fatal diseases.

Alteration of Prion Strain Emergence by Nonhost Factors

The prion strain, surface type, and matrix containing PrP^Sc can influence PrP^Sc surface adsorption. The cumulative effect of these factors can result in strain- and soil-specific differences in prion bioavailability. Environmental weathering processes can result in decreases in PrP^Sc conversion efficiency and infectivity. Little is known about how incomplete inactivation of surface-bound PrP^Sc affects transmission and prion strain emergence. Here, we show that strain interference occurs with soil-bound prions and that altering the ratios of prion strains by strain-specific inactivation can affect strain emergence. Additionally, we identify a novel mechanism of inhibition of prion conversion by environmental treatment-induced changes at the soil-protein interface altering strain emergence. These novel findings suggest that environmental factors can influence strain emergence of surface-bound prions.

Prions can persist in the environment for extended periods of time after adsorption to surfaces, including soils, feeding troughs, or fences. Prion strain- and soil-specific differences in prion adsorption, infectivity, and response to inactivation may be involved in strain maintenance or emergence of new strains in a population. Extensive proteinase K (PK) digestion of Hyper (HY) and Drowsy (DY) PrP^Sc resulted in a greater reduction in the level of DY PrP^Sc than of HY PrP^Sc. Use of the PK-digested material in protein misfolding cyclic amplification strain interference (PMCAsi) resulted in earlier emergence of HY PrP^Sc than of undigested controls. This result established that strain-specific alteration of the starting ratios of conversion-competent HY and DY PrP^Sc can alter strain emergence. We next investigated whether environmentally relevant factors such as surface binding and weathering could alter strain emergence. Adsorption of HY and DY PrP^Sc to silty clay loam (SCL), both separately and combined, resulted in DY interfering with the emergence of HY in PMCAsi in a manner similar to that seen with unbound controls. Similarly, repeated cycles of wetting and drying of SCL-bound HY and DY PrP^Sc did not alter the emergence of HY PrP^Sc compared to untreated controls. Importantly, these data indicate that prion strain interference can occur when prions are bound to surfaces. Interestingly, we found that drying of adsorbed brain homogenate on SCL could restore its ability to interfere with the emergence of HY, suggesting a novel strain interference mechanism. Overall, these data provide evidence that the emergence of a strain from a mixture can be influenced by nonhost factors.

IMPORTANCE The prion strain, surface type, and matrix containing PrP^Sc can influence PrP^Sc surface adsorption. The cumulative effect of these factors can result in strain- and soil-specific differences in prion bioavailability. Environmental weathering processes can result in decreases in PrP^Sc conversion efficiency and infectivity. Little is known about how incomplete inactivation of surface-bound PrP^Sc affects transmission and prion strain emergence. Here, we show that strain interference occurs with soil-bound prions and that altering the ratios of prion strains by strain-specific inactivation can affect strain emergence. Additionally, we identify a novel mechanism of inhibition of prion conversion by environmental treatment-induced changes at the soil-protein interface altering strain emergence. These novel findings suggest that environmental factors can influence strain emergence of surface-bound prions.

[Protective Role of the Host Innate Immune System in Prion Pathogenesis]

Prion diseases, including human Creutzfeldt-Jakob disease, are infectious, intractable central neurodegenerative diseases, which are also zoonoses that commonly infect not only higher organisms but also a wide variety of animals. Pathogenic prions induce abnormal prion protein (PrP), which is produced by structural conversion of normal PrP, a beta-sheet-rich structure with high aggregation propensity. Thus, it is believed that the host is immunotolerant against prions because there is no difference in the primary structure of normal and abnormal PrP, and prions do not induce a marked immune response. Recently, using mutated Toll-like receptor (TLR) 4-transgenic mice, a bioassay after prion inoculation has intriguingly found that the TLR4-signaling pathway may have a protective role against prion infection. Meanwhile, we reported that a transcription factor, interferon regulatory factor-3 (IRF-3), located downstream of TLR4 signaling, showed resistance to prions. IRF-3-inducing type I interferon (I-IFN) is a critical factor for the host defense against pathogen invasion. These findings indicate that the TLR-signaling pathway of the innate immune system might regulate prion invasion. However, the details have not been fully determined. In this symposium, we will introduce new findings including the relationship between I-IFN and prions.

Independent amplification of co-infected long incubation period low conversion efficiency prion strains

Prion diseases are caused by a misfolded isoform of the prion protein, PrP^Sc. Prion strains are hypothesized to be encoded by strain-specific conformations of PrP^Sc and prions can interfere with each other when a long-incubation period strain (i.e. blocking strain) inhibits the conversion of a short-incubation period strain (i.e. non-blocking). Prion strain interference influences prion strain dynamics and the emergence of a strain from a mixture; however, it is unknown if two long-incubation period strains can interfere with each other. Here, we show that co-infection of animals with combinations of long-incubation period strains failed to identify evidence of strain interference. To exclude the possibility that this inability of strains to interfere in vivo was due to a failure to infect common populations of neurons we used protein misfolding cyclic amplification strain interference (PMCAsi). Consistent with the animal bioassay studies, PMCAsi indicated that both co-infecting strains were amplifying independently, suggesting that the lack of strain interference is not due to a failure to target the same cells but is an inherent property of the strains involved. Importantly PMCA reactions seeded with long incubation-period strains contained relatively higher levels of remaining PrP^C compared to reactions seeded with a short-incubation period strain. Mechanistically, we hypothesize the abundance of PrP^C is not limiting in vivo or in vitro resulting in prion strains with relatively low prion conversion efficiency to amplify independently. Overall, this observation changes the paradigm of the interactions of prion strains and has implications for interspecies transmission and emergence of prion strains from a mixture.

This is the first example of prion strains that replicate independently in vitro and in vivo. This observation changes the paradigm of the interactions of prion strains and provides further evidence that strains are a dynamic mixture of substrains. Strain interference is influenced by the abundance of PrP^C that is convertible by the strains involved. These observations have implications for interspecies transmission and emergence of prion strains from a mixture.

Vaccination strategies

Currently all prion diseases are without effective treatment and are universally fatal. It is increasingly being recognized that the pathogenesis of many neurodegenerative diseases, such as Alzheimer disease (AD), includes "prion-like" properties. Hence, any effective therapeutic intervention for prion disease could have significant implications for other neurodegenerative diseases. Conversely, therapies that are effective in AD might also be therapeutically beneficial for prion disease. AD-like prion disease has no effective therapy. However, various vaccine and immunomodulatory approaches have shown great success in animal models of AD, with numerous ongoing clinical trials of these potential immunotherapies. More limited evidence suggests that immunotherapies may be effective in prion models and in naturally occurring prion disease. In particular, experimental data suggest that mucosal vaccination against prions can be effective for protection against orally acquired prion infection. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease, and variant Creutzfeldt-Jakob disease, are thought to be acquired peripherally, mainly by oral exposure. Mucosal vaccination would be most applicable to this form of transmission. In this chapter we review various immunologically based strategies which are under development for prion infection.

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

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

Prion Strain Diversity

Prion diseases affect a wide range of mammal species and are caused by a misfolded self-propagating isoform (PrPSc) of the normal prion protein (PrPC). Distinct strains of prions exist and are operationally defined by differences in a heritable phenotype under controlled experimental transmission conditions. Prion strains can differ in incubation period, clinical signs of disease, tissue tropism, and host range. The mechanism by which a protein-only pathogen can encode strain diversity is only beginning to be understood. The prevailing hypothesis is that prion strain diversity is encoded by strain-specific conformations of PrPSc; however, strain-specific cellular cofactors have been identified in vitro that may also contribute to prion strain diversity. Although much progress has been made on understanding the etiological agent of prion disease, the relationship between the strain-specific properties of PrPSc and the resulting phenotype of disease in animals is poorly understood.

Incongruity between Prion Conversion and Incubation Period following Coinfection

When multiple prion strains are inoculated into the same host, they can interfere with each other. Strains with long incubation periods can suppress conversion of strains with short incubation periods; however, nothing is known about the conversion of the long-incubation-period strain during strain interference. To investigate this, we inoculated hamsters in the sciatic nerve with long-incubation-period strain 139H prior to superinfection with the short-incubation-period hyper (HY) strain of transmissible mink encephalopathy (TME). First, we found that 139H is transported along the same neuroanatomical tracks as HY TME, adding to the growing body of evidence indicating that PrP(Sc) favors retrograde transneuronal transport. In contrast to a previous report, we found that 139H interferes with HY TME infection, which is likely due to both strains targeting the same population of neurons following sciatic nerve inoculation. Under conditions where 139H blocked HY TME from causing disease, the strain-specific properties of PrP(Sc) corresponded with the strain that caused disease, consistent with our previous findings. In the groups of animals where incubation periods were not altered, we found that the animals contained a mixture of 139H and HY TME PrP(Sc) This finding expands the definition of strain interference to include conditions where PrP(Sc) formation is altered yet disease outcome is unaltered. Overall, these results contradict the premise that prion strains are static entities and instead suggest that strain mixtures are dynamic regardless of incubation period or clinical outcome of disease.

IMPORTANCE

Prions can exist as a mixture of strains in naturally infected animals, where they are able to interfere with the conversion of each other and to extend incubation periods. Little is known, however, about the dynamics of strain conversion under conditions where incubation periods are not affected. We found that inoculation of the same animal with two strains can result in the alteration of conversion of both strains under conditions where the resulting disease was consistent with infection with only a single strain. These data challenge the idea that prion strains are static and suggests that strain mixtures are more dynamic than previously appreciated. This observation has significant implications for prion adaptation.

Structural conservation of prion strain specificities in recombinant prion protein fibrils in real-time quaking-induced conversion

A major unsolved issue of prion biology is the existence of multiple strains with distinct phenotypes and this strain phenomenon is postulated to be associated with the conformational diversity of the abnormal prion protein (PrP^Sc). Real-time quaking-induced conversion (RT-QUIC) assay that uses Escherichia coli-derived recombinant prion protein (rPrP) for the sensitive detection of PrP^Sc results in the formation of rPrP-fibrils seeded with various strains. We demonstrated that there are differences in the secondary structures, especially in the β-sheets, and conformational stability between 2 rPrP-fibrils seeded with either Chandler or 22L strains in the first round of RT-QUIC. In particular, the differences in conformational properties of these 2 rPrP-fibrils were common to those of the original PrP^Sc. However, the strain specificities of rPrP-fibrils seen in the first round were lost in subsequent rounds. Instead, our findings suggest that nonspecific fibrils became the major species, probable owing to their selective growth advantage in the RT-QUIC. This study shows that at least some strain-specific conformational properties of the original PrP^Sc can be transmitted to rPrP-fibrils in vitro, but further conservation appears to require unknown cofactors or environmental conditions or both.

Post-translational changes to PrP alter transmissible spongiform encephalopathy strain properties

The agents responsible for transmissible spongiform encephalopathies (TSEs), or prion diseases, contain as a major component PrP(Sc), an abnormal conformer of the host glycoprotein PrP(C). TSE agents are distinguished by differences in phenotypic properties in the host, which nevertheless can contain PrP(Sc) with the same amino-acid sequence. If PrP alone carries information defining strain properties, these must be encoded by post-translational events. Here we investigated whether the glycosylation status of host PrP affects TSE strain characteristics. We inoculated wild-type mice with three TSE strains passaged through transgenic mice with PrP devoid of glycans at the first, second or both N-glycosylation sites. We compared the infectious properties of the emerging isolates with TSE strains passaged in wild-type mice by in vivo strain typing and by the standard scrapie cell assay in vitro. Strain-specific characteristics of the 79A TSE strain changed when PrP(Sc) was devoid of one or both glycans. Thus infectious properties of a TSE strain can be altered by post-translational changes to PrP which we propose result in the selection of mutant TSE strains.

Protective role of MyD88-independent innate immune responses against prion infection

Despite recent progress in the understanding of prion diseases, little is known about the host-defense mechanisms against prion. Although it has long been thought that type I interferon (IFN-I) has no protective effect on prion infection, certain key molecules in innate immunity such as toll-like receptor (TLR) 4 seemed to be involved in the host response. For this reason we decided to focus on TLRs and investigate the role of a transcription factor, interferon regulatory factor 3 (IRF3), because the absence of MyD88, a major adaptor signaling molecule of TLRs, has no effect on the survival of prion infected mice. Intriguingly, survival periods of prion inoculated IRF3-knockout mice became significantly shorter than those of wild-type mice. In addition, IRF3 stimulation inhibited PrP^Sc replication in prion persistently-infected cells, and a de novo prion infection assay revealed that IRF3-overexpression could make host cells resistant to prion infection. Our work suggests that IRF3 may play a key role in innate immune responses against invasion of prion pathogens. Activated IRF3 could upregulate several anti-pathogen factors, including IFN-I, and induce sequential responses. Although the mechanism for the anti-prion effects mediated by IRF3 has yet to be clarified, certain interferon responsive genes might be involved in the anti-prion host-defense mechanism.

Could immunomodulation be used to prevent prion diseases?

All prion diseases are currently without effective treatment and are universally fatal. The underlying pathogenesis of prion diseases (prionoses) is related to an autocatalytic conformational conversion of PrP(C) (C for cellular) to a pathological and infectious conformer known as PrP(Sc) (Sc for scrapie) or PrP(Res) (Res for proteinase K resistant). The past experience with variant Creutzfeldt-Jakob disease, which originated from bovine spongiform encephalopathy, as well as the ongoing epidemic of chronic wasting disease has highlighted the necessity for effective prophylactic and/or therapeutic approaches. Human prionoses are most commonly sporadic, and hence therapy is primarily directed to stop progression; however, in animals the majority of prionoses are infectious and, as a result, the emphasis is on prevention of transmission. These infectious prionoses are most commonly acquired via the alimentary tract as a major portal of infectious agent entry, making mucosal immunization a potentially attractive method to produce a local immune response that can partially or completely prevent prion entry across the gut barrier, while at the same time producing a modulated systemic immunity that is unlikely to be associated with toxicity. A critical factor in any immunomodulatory methodology that targets a self-antigen is the need to delicately balance an effective humoral immune response with potential autoimmune inflammatory toxicity. The ongoing epidemic of chronic wasting disease affecting the USA and Korea, with the potential to spread to human populations, highlights the need for such immunomodulatory approaches.

High CJD infectivity remains after prion protein is destroyed

The hypothesis that host prion protein (PrP) converts into an infectious prion form rests on the observation that infectivity progressively decreases in direct proportion to the decrease of PrP with proteinase K (PK) treatment. PrP that resists limited PK digestion (PrP-res, PrP(sc)) has been assumed to be the infectious form, with speculative types of misfolding encoding the many unique transmissible spongiform encephalopathy (TSE) agent strains. Recently, a PK sensitive form of PrP has been proposed as the prion. Thus we re-evaluated total PrP (sensitive and resistant) and used a cell-based assay for titration of infectious particles. A keratinase (NAP) known to effectively digest PrP was compared to PK. Total PrP in FU-CJD infected brain was reduced to ≤0.3% in a 2 h PK digest, yet there was no reduction in titer. Remaining non-PrP proteins were easily visualized with colloidal gold in this highly infectious homogenate. In contrast to PK, NAP digestion left 0.8% residual PrP after 2 h, yet decreased titer by >2.5 log; few residual protein bands remained. FU-CJD infected cells with 10× the infectivity of brain by both animal and cell culture assays were also evaluated. NAP again significantly reduced cell infectivity (>3.5 log). Extreme PK digestions were needed to reduce cell PrP to <0.2%, yet a very high titer of 8 logs survived. Our FU-CJD brain results are in good accord with the only other report on maximal PrP destruction and titer. It is likely that one or more residual non-PrP proteins may protect agent nucleic acids in infectious particles.

Replication and spread of CJD, kuru and scrapie agents in vivo and in cell culture

Transmissible Spongiform Encephalopathy (TSE) agents are defined by their virulence for particular species, their spread in the population, their incubation time to cause disease, and their neuropathological sequelae. Murine adapted human agents, including sporadic CJD (sCJD), New Guinea kuru, and Japanese CJD agents, display particularly distinct incubation times and maximal infectious brain titers. They also induce agent-specific patterns of neurodegeneration. When these TSE agents are transmitted to cultured hypothalamic GT1 cells they maintain their unique identities. Nevertheless, the human kuru (kCJD) and Japanese FU-CJD agents, as well as the sheep 22L and 263K scrapie agents display doubling times that are 8x to 33x faster in cells than in brain, indicating release from complex innate immune responses. These data are most consistent with a foreign viral structure, rather than an infectious form of host prion protein (PrP-res). Profound agent-specific inhibitory effects are also apparent in GT1 cells, and maximal titer plateau in kCJD and FU-CJD differed by 1,000-fold in a cell-based assay. Remarkably, the lower titer kCJD agent rapidly induced de novo PrP-res in GT1 cells, whereas the high titer FU-CJD agent replicated silently for multiple passages. Although PrP-res is often considered to be toxic, PrP-res instead may be part of a primal defense and/or clearance mechanism against TSE environmental agents. Limited spread of particular TSE agents through nanotubes and cell-to-cell contacts probably underlies the long peripheral phase of human CJD.

Transmissible encephalopathy agents: virulence, geography and clockwork

Transmissible spongiform encephalopathies (TSEs) are caused by infectious agents with stable virulence characteristics that are not encoded by the host. Agent-specific features of virulence include variable disease latency and tissue pathology in a given host, as well as the ability to spread to many species. Such cross-species infections contradict predictions based on the prion hypothesis. Recent transmissions of several human agents to normal mice and to monotypic neural cells in culture, underscore the existence of unique agent clades that are prevalent in particular geographic regions. Examples include the epidemic UK bovine agent (BSE) and the New Guinea kuru agent. The virus-like biology of unique TSE agents, including epidemic spread, mutation, and superinfection, can be used to systematically define features of virulence that distinguish common endemic from newly emerging strains.

Tracking protein aggregate interactions

Amyloid fibrils share a structural motif consisting of highly ordered β-sheets aligned perpendicular to the fibril axis ( 1, 2) . At each fibril end, β-sheets provide a template for recruiting and converting monomers ( 3) . Various amyloid fibrils often occur in the same individual, yet whether distinct protein aggregates aid or inhibit the assembly of heterologous proteins is unclear. In prion disease, different amyloid-like prion aggregate structures, or strains, are thought to be the basis of disparate disease phenotypes in the same species expressing identical prion protein sequences ( 4-7) . Here we focus on the interactions reported to occur when two pre-existing amyloids or two distinct prion strains occur together in the central nervous system.

Immunomodulation for prion and prion-related diseases

Prion diseases are a unique category of illness, affecting both animals and humans, where the underlying pathogenesis is related to a conformational change of a normal self protein called cellular prion protein to a pathological and infectious conformer known as scrapie prion protein (PrP(Sc)). Currently, all prion diseases lack effective treatment and are universally fatal. Past experiences with bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease mainly in Europe, as well as the current epidemic of chronic wasting disease in North America, have highlighted the need to develop prophylactic and/or therapeutic approaches. In Alzheimer's disease that, like prion disease, is a conformational neurodegenerative disorder, both passive and active immunization has been shown to be highly effective in model animals at preventing disease and cognitive deficits, with emerging data from human trials suggesting that this approach is able to reduce amyloid-related pathology. However, any immunomodulatory approach aimed at a self-antigen has to finely balance an effective humoral immune response with potential autoimmune toxicity. The prion diseases most commonly acquired by infection typically have the alimentary tract as a portal of infectious agent entry. This makes mucosal immunization a potentially attractive method to produce a local immune response that partially or completely prevents prion entry across the gut barrier, while at the same time producing modulated systemic immunity that is unlikely to be associated with toxicity. Our results using an attenuated Salmonella vaccine strain expressing the prion protein showed that mucosal vaccination can protect against prion infection from a peripheral source, suggesting the feasibility of this approach. It is also possible to develop active and/or passive immunomodulatory approaches that more specifically target PrP(Sc) or target the shared pathological conformer found in numerous conformational disorders. Such approaches could have a significant impact on many of the common age-associated dementias.

Prion strain interactions are highly selective

Various misfolded and aggregated neuronal proteins commonly coexist in neurodegenerative disease, but whether the proteins coaggregate and alter the disease pathogenesis is unclear. Here, we used mixtures of distinct prion strains, which are believed to differ in conformation, to test the hypothesis that two different aggregates interact and change the disease in vivo. We tracked two prion strains in mice histopathologically and biochemically, as well as by spectral analysis of plaque-bound PTAA (polythiophene acetic acid), a conformation-sensitive fluorescent amyloid ligand. We found that prion strains interacted in a highly selective and strain-specific manner, with (1) no interaction, (2) hybrid plaque formation, or (3) blockage of one strain by a second (interference). The hybrid plaques were maintained on additional passage in vivo and each strain seemed to maintain its original conformational properties, suggesting that one strain served only as a scaffold for aggregation of the second strain. These findings not only further our understanding of prion strain interactions but also directly demonstrate interactions that may occur in other protein aggregate mixtures.

Coinfecting prion strains compete for a limiting cellular resource

Prion strain interference can influence the emergence of a dominant strain from a mixture; however, the mechanisms underlying prion strain interference are poorly understood. In our model of strain interference, inoculation of the sciatic nerve with the drowsy (DY) strain of the transmissible mink encephalopathy (TME) agent prior to superinfection with the hyper (HY) strain of TME can completely block HY TME from causing disease. We show here that the deposition of PrP(Sc), in the absence of neuronal loss or spongiform change, in the central nervous system corresponds with the ability of DY TME to block HY TME infection. This suggests that DY TME agent-induced damage is not responsible for strain interference but rather prions compete for a cellular resource. We show that protein misfolding cyclic amplification (PMCA) of DY and HY TME maintains the strain-specific properties of PrP(Sc) and replicates infectious agent and that DY TME can interfere, or completely block, the emergence of HY TME. DY PrP(Sc) does not convert all of the available PrP(C) to PrP(Sc) in PMCA, suggesting the mechanism of prion strain interference is due to the sequestering of PrP(C) and/or other cellular components required for prion conversion. The emergence of HY TME in PMCA was controlled by the initial ratio of the TME agents. A higher ratio of DY to HY TME agent is required for complete blockage of HY TME in PMCA compared to several previous in vivo studies, suggesting that HY TME persists in animals coinfected with the two strains. This was confirmed by PMCA detection of HY PrP(Sc) in animals where DY TME had completely blocked HY TME from causing disease.

Agent-specific Shadoo responses in transmissible encephalopathies

Transmissible spongiform encephalopathies (TSE) are neurodegenerative diseases caused by an infectious agent with viral properties. Host prion protein (PrP), a marker of late stage TSE pathology, is linked to a similar protein called Shadoo (Sho). Sho is reduced in mice infected with the RML scrapie agent, but has not been investigated in other TSEs. Although PrP is required for infection by TSE agents, it is not known if Sho is similarly required. Presumably Sho protects cells from toxic effects of misfolded PrP. We compared Sho and PrP changes after infection by very distinct TSE agents including sporadic CJD, Asiatic CJD, New Guinea kuru, vCJD (the UK epidemic bovine agent) and 22L sheep scrapie, all passaged in standard mice. We found that Sho reductions were agent-specific. Variable Sho reductions in standard mice could be partly explained by agent-specific differences in regional neuropathology. However, Sho did not follow PrP misfolding in any quantitative or consistent way. Tga20 mice with high murine PrP levels revealed additional agent-specific differences. Sho was unaffected by Asiatic CJD yet was markedly reduced by the kuru agent in Tga20 mice; in standard mice both agents induced the same Sho reductions. Analyses of neural GT1 cells demonstrated that Sho was not essential for TSE infections. Furthermore, because all infected GT1 cells appeared as healthy as uninfected controls, Sho was not needed to protect infected cells from their "toxic" burden of abundant abnormal PrP and intracellular amyloid.

The kuru infectious agent is a unique geographic isolate distinct from Creutzfeldt-Jakob disease and scrapie agents

Human sporadic Creutzfeldt-Jakob disease (sCJD), endemic sheep scrapie, and epidemic bovine spongiform encephalopathy (BSE) are caused by a related group of infectious agents. The new U.K. BSE agent spread to many species, including humans, and clarifying the origin, specificity, virulence, and diversity of these agents is critical, particularly because infected humans do not develop disease for many years. As with viruses, transmissible spongiform encephalopathy (TSE) agents can adapt to new species and become more virulent yet maintain fundamentally unique and stable identities. To make agent differences manifest, one must keep the host genotype constant. Many TSE agents have revealed their independent identities in normal mice. We transmitted primate kuru, a TSE once epidemic in New Guinea, to mice expressing normal and approximately 8-fold higher levels of murine prion protein (PrP). High levels of murine PrP did not prevent infection but instead shortened incubation time, as would be expected for a viral receptor. Sporadic CJD and BSE agents and representative scrapie agents were clearly different from kuru in incubation time, brain neuropathology, and lymphoreticular involvement. Many TSE agents can infect monotypic cultured GT1 cells, and unlike sporadic CJD isolates, kuru rapidly and stably infected these cells. The geographic independence of the kuru agent provides additional reasons to explore causal environmental pathogens in these infectious neurodegenerative diseases.

Strain-specific viral properties of variant Creutzfeldt-Jakob disease (vCJD) are encoded by the agent and not by host prion protein

Human CJD, endemic sheep scrapie, epidemic bovine spongiform encephalopathy (BSE), and other transmissible spongiform encephalopathies (TSEs), are caused by a group of related but molecularly uncharacterized infectious agents. The UK-BSE agent infected many species, including humans where it causes variant CJD (vCJD). As in most viral infections, different TSE disease phenotypes are determined by both the agent strain and the host species. TSE strains are most reliably classified by incubation time and regional neuropathology in mice expressing wild-type (wt) prion protein (PrP). We compared vCJD to other human and animal derived TSE strains in both mice and neuronal cultures expressing wt murine PrP. Primary and serial passages of the human vCJD agent, as well as the highly selected mutant 263K sheep scrapie agent, revealed profound strain-specific characteristics were encoded by the agent, not by host PrP. Prion theory posits that PrP converts itself into the infectious agent, and thus short incubations require identical PrP sequences in the donor and recipient host. However, wt PrP mice injected with human vCJD brain homogenates showed dramatically shorter primary incubation times than mice expressing only human PrP, a finding not in accord with a PrP species barrier. All mouse passage brains showed the vCJD agent derived from a stable BSE strain. Additionally, both vCJD brain and monotypic neuronal cultures produced a diagnostic 19 kDa PrP fragment previously observed only in BSE and vCJD primate brains. Monotypic cultures can be used to identify the intrinsic, strain-determining molecules of TSE infectious particles.

Heterozygous inhibition in prion infection: the stone fence model

The human PrP gene (PRNP) has two major polymorphic codons: 129 for methionine (M) or valine (V) and 219 for glutamate (E) or lysine (K). The PRNP heterozygotes appear to be protected from sporadic CJD compared to the PRNP homozygotes. The molecular mechanism responsible for these protective effects of PRNP heterozygosity has remained elusive. In this review, we describe the inhibition of PrP conversion observed in a series of transmission studies using PRNP heterozygous animal models. In vCJD infection, the conversion incompetent human PrP 129V molecules showed an inhibitory effect on the conversion of human PrP 129M molecules in the 129M/V heterozygous mice. Furthermore, though the human PrP 219E and PrP 219K were both conversion competent in vCJD infection, these conversion competent PrP molecules showed an inhibitory effect in the 219E/K heterozygous animals. To explain this heterozygous inhibition, we propose a possible mechanism designated as the stone fence model.

Molecular mechanisms of prion pathogenesis

Prion diseases are infectious neurodegenerative diseases occurring in humans and animals with an invariably lethal outcome. One fundamental mechanistic event in prion diseases is the aggregation of aberrantly folded prion protein into large amyloid plaques and fibrous structures associated with neurodegeneration. The cellular prion protein (PrPC) is absolutely required for disease development, and prion knockout mice are not susceptible to prion disease. Prions accumulate not only in the central nervous system but also in lymphoid organs, as shown for new variant and sporadic Creutzfeldt-Jakob patients and for some animals. To date it is largely accepted that prions consist primarily of PrPSc, a misfolded and aggregated beta-sheet-rich isoform of PrPC. However, PrPSc may or may not be completely congruent with the infectious moiety. Here, we discuss the molecular mechanisms leading to neurodegeneration, the role of the immune system in prion pathogenesis, and the existence of prion strains that appear to have different tropisms and biochemical characteristics.

Prion interference with multiple prion isolates

Co-inoculation of prion strains into the same host can result in interference, where replication of one strain hinders the ability of another strain to cause disease. The drowsy (DY) strain of hamster-adapted transmissible mink encephalopathy (TME) extends the incubation period or completely blocks the hyper (HY) strain of TME following intracerebral, intraperitoneal or sciatic nerve routes of inoculation. However, it is not known if the interfering effect of the DY TME agent is exclusive to the HY TME agent by these experimental routes of infection. To address this issue, we show that the DY TME agent can block hamster-adapted chronic wasting disease (HaCWD) and the 263K scrapie agent from causing disease following sciatic nerve inoculation. Additionally, per os inoculation of DY TME agent slightly extends the incubation period of per os superinfected HY TME agent. These studies suggest that prion strain interference can occur by a natural route of infection and may be a more generalized phenomenon of prion strains.

High titers of mucosal and systemic anti-PrP antibodies abrogate oral prion infection in mucosal-vaccinated mice

Significant outbreaks of prion disease linked to oral exposure of the prion agent have occurred in animal and human populations. These disorders are associated with a conformational change of a normal protein, PrP(C) (C for cellular), to a toxic and infectious form, PrP(Sc) (Sc for scrapie). None of the prionoses currently have an effective treatment. Some forms of prion disease are thought to be spread by oral ingestion of PrP(Sc), such as chronic wasting disease and variant Creutzfeldt-Jakob disease. Attempts to obtain an active immunization in wild-type animals have been hampered by auto-tolerance to PrP and potential toxicity. Previously, we demonstrated that it is possible to overcome tolerance and obtain a specific anti-PrP antibody response by oral inoculation of the PrP protein expressed in an attenuated Salmonella vector. This past study showed that 30% of vaccinated animals were free of disease more than 350 days post-challenge. In the current study we have both optimized the vaccination protocol and divided the vaccinated mice into low and high immune responder groups prior to oral challenge with PrP(Sc) scrapie strain 139A. These methodological refinements led to a significantly improved therapeutic response. 100% of mice with a high mucosal anti-PrP titer immunoglobulin (Ig) A and a high systemic IgG titer, prior to challenge, remained without symptoms of PrP infection at 400 days (log-rank test P<0.0001 versus sham controls). The brains from these surviving clinically asymptomatic mice were free of PrP(Sc) infection by Western blot and histological examination. These promising findings suggest that effective mucosal vaccination is a feasible and useful method for overcoming tolerance to PrP and preventing prion infection via an oral route.

Therapeutic approaches for prion and Alzheimer's diseases

Alzheimer's and prion diseases belong to a category of conformational neurodegenerative disorders [Prusiner SB (2001) N Eng J Med344, 1516-1526; Sadowski M & Wisniewski T (2007) Curr Pharm Des 13, 1943-1954; Beekes M (2007) FEBS J 274, 575]. Treatments capable of arresting or at least effectively modifying the course of disease do not yet exist for either one of these diseases. Alzheimer's disease is the major cause of dementia in the elderly and has become an ever greater problem with the aging of Western societies. Unlike Alzheimer's disease, prion diseases are relatively rare. Each year only approximately 300 people in the USA and approximately 100 people in the UK succumb to various forms of prion diseases [Beekes M (2007) FEBS J 274, 575; Sigurdsson EM & Wisniewski T (2005) Exp Rev Vaccines 4, 607-610]. Nevertheless, these disorders have received great scientific and public interest due to the fact that they can be transmissible among humans and in certain conditions from animals to humans. The emergence of variant Creutzfeld-Jakob disease demonstrated the transmissibility of the bovine spongiform encephalopathy to humans [Beekes M (2007) FEBS J 274, 575]. Therefore, the spread of bovine spongiform encephalopathy across Europe and the recently identified cases in North America have put a large human population at risk of prion infection. It is estimated that at least several thousand Britons are asymptomatic carriers of prion infections and may develop variant Creutzfeld-Jakob disease in the future [Hilton DA (2006) J Pathol 208, 134-141]. This delayed emergence of human cases following the near elimination of bovine spongiform encephalopathy in the UK may occur because prion disease have a very prolonged incubation period, ranging from months to decades, which depends on the amount of inoculum, the route of infection and the genetic predisposition of the infected subject [Hilton DA (2006) J Pathol 208, 134-141]. Therefore, there is a great need for effective therapies for both Alzheimer's disease and prion diseases.

Insights into prion strains and neurotoxicity

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that are caused by prions and affect humans and many animal species. It is now widely accepted that the infectious agent that causes TSEs is PrP(Sc), an aggregated moiety of the host-derived membrane glycolipoprotein PrP(C). Although PrP(C) is encoded by the host genome, prions themselves encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, after inoculation into distinct hosts, cause disease with consistent characteristics, such as incubation period, distinct patterns of PrP(Sc) distribution and spongiosis and relative severity of the spongiform changes in the brain. The existence of such strains poses a fascinating challenge to prion research.

A 25 nm virion is the likely cause of transmissible spongiform encephalopathies

The transmissible spongiform encephalopathies (TSEs) such as endemic sheep scrapie, sporadic human Creutzfeldt-Jakob disease (CJD), and epidemic bovine spongiform encephalopathy (BSE) may all be caused by a unique class of "slow" viruses. This concept remains the most parsimonious explanation of the evidence to date, and correctly predicted the spread of the BSE agent to vastly divergent species. With the popularization of the prion (infectious protein) hypothesis, substantial data pointing to a TSE virus have been largely ignored. Yet no form of prion protein (PrP) fulfills Koch's postulates for infection. Pathologic PrP is not proportional to, or necessary for infection, and recombinant and "amplified" prions have failed to produce significant infectivity. Moreover, the "wealth of data" claimed to support the existence of infectious PrP are increasingly contradicted by experimental observations, and cumbersome speculative notions, such as spontaneous PrP mutations and invisible strain-specific forms of "infectious PrP" are proposed to explain the incompatible data. The ability of many "slow" viruses to survive harsh environmental conditions and enzymatic assaults, their stealth invasion through protective host-immune defenses, and their ability to hide in the host and persist for many years, all fit nicely with the characteristics of TSE agents. Highly infectious preparations with negligible PrP contain nucleic acids of 1-5 kb, even after exhaustive nuclease digestion. Sedimentation as well as electron microscopic data also reveal spherical infectious particles of 25-35 nm in diameter. This particle size can accommodate a viral genome of 1-4 kb, sufficient to encode a protective nucleocapsid and/or an enzyme required for its replication. Host PrP acts as a cellular facilitator for infectious particles, and ultimately accrues pathological amyloid features. A most significant advance has been the development of tissue culture models that support the replication of many different strains of agent and can produce high levels of infectivity. These models provide new ways to rapidly identify intrinsic viral and strain-specific molecules so important for diagnosis, prevention, and fundamental understanding.

Cells infected with scrapie and Creutzfeldt-Jakob disease agents produce intracellular 25-nm virus-like particles

We had repeatedly found approximately 25-nm-diameter virus-like particles in highly infectious brain fractions with little prion protein (PrP), and therefore we searched for similar virus-like particles in situ in infected cell lines with high titers. Neuroblastoma cells infected with the 22L strain of scrapie as well as hypothalamic GT cells infected with the FU Creutzfeldt-Jakob disease agent, but not parallel mock controls, displayed dense 25-nm virus-like particles in orthogonal arrays. These particles had no relation to abnormal PrP amyloid in situ, nor were they labeled by PrP antibodies that faithfully recognized rough endoplasmic reticulum membranes and amyloid fibrils, the predicted sites of normal and pathological intracellular PrP. Additionally, phorbol ester stimulated the production of abnormal PrP gel bands by >5-fold in infected N2a + 22L cells, yet this did not increase either the number of virus-like arrays or the infectious titer of these cells. Thus, the 25-nm infection-associated particles could not be prions. Synaptic differentiation and neurodegeneration, as well as retroviruses that populate the rough endoplasmic reticulum of neuroblastoma cells, were not required for particle production. The 25-nm particle arrays in cultured cells strongly resembled those first described in 1968 in synaptic regions of scrapie-infected brain and subsequently identified in many natural and experimental TSEs. The high infectivity of comparable, isolated virus-like particles that show no intrinsic PrP by antibody labeling, combined with their loss of infectivity when nucleic acid-protein complexes are disrupted, make it likely that these 25-nm particles are the causal TSE virions that induce late-stage PrP brain pathology.

Prion interference is due to a reduction in strain-specific PrPSc levels

When two prion strains infect a single host, one strain can interfere with the ability of the other to cause disease but it is not known whether prion replication of the second strain is also diminished. To further investigate strain interference, we infected hamsters in the sciatic nerve with the long-incubation-period transmissible mink encephalopathy (TME) agent DY TME prior to superinfection of hamsters with the short-incubation-period HY TME agent. Increases in the interval between TME agent inoculations resulted in an extension of the incubation period of HY TME or a complete block of the ability of the HY TME agent to cause disease. The sciatic nerve route of inoculation gave the two TME strains access to the same population of neurons, allowing for the potential of prion interference in the lumbar spinal cord. The ability of the DY TME agent to extend the incubation period of HY TME corresponds with detection of DY TME PrP(Sc), the abnormal isoform of the prion protein, in the lumbar spinal cord. The increased incubation period of HY TME or the inability of the HY TME agent to cause disease in the coinfected animals corresponds with a reduction in the abundance of HY TME PrP(Sc) in the lumbar spinal cord. When the two strains were not directed to the same populations of neurons within the lumbar spinal cord, interference between HY TME and DY TME did not occur. This suggests that DY TME agent replication interferes with HY TME agent replication when the two strains infect a common population of neurons.

Reciprocal interference between specific CJD and scrapie agents in neural cell cultures

Infection of mice with an attenuated Creutzfeldt-Jakob disease agent (SY-CJD) interferes with superinfection by a more virulent human-derived CJD agent (FU-CJD) and does not require pathological prion protein (PrPres). Using a rapid coculture system, we found that a neural cell line free of immune system cells similarly supported substantial CJD agent interference without PrPres. In addition, SY-CJD prevented superinfection by sheep-derived Chandler (Ch) and 22L scrapie agents. However, only 22L and not Ch prevented FU-CJD infection, even though both scrapie strains provoked abundant PrPres. This relationship between particular strains of sheep- and human-derived agents is likely to affect their prevalence and epidemic spread.

New molecular markers of early and progressive CJD brain infection

Transmissible spongiform encephalopathies (TSEs), including human Creutzfeldt-Jakob disease (CJD), are caused by a related group of infectious agents that can be transmitted to many mammalian species. Because the infectious component of TSE agents has not been identified, we examined myeloid cell linked inflammatory pathways to find if they were activated early in CJD infection. We here identify a specific set of transcripts in CJD infected mouse brains that define early and later stages of progressive disease. Serum amyloid A3 and L-selectin mRNAs were elevated as early as 20 days after intracerebral inoculation. Transcripts of myeloid cell recruitment factors such as MIP-1alpha, MIP-1beta, and MCP1, as well as IL1alpha and TNFalpha were upregulated > 10 fold between 30 and 40 days, well before prion protein (PrP) abnormalities that begin only after 80 days. At later stages of symptomatic neurodegenerative disease (100-110 days), a selected set of transcripts rose by as much as 100 fold. In contrast, normal brain inoculated controls showed no similar sequential changes. In sum, rapid and simple PCR tests defined progressive stages of CJD brain infection. These markers may also facilitate early diagnosis of CJD in accessible peripheral tissues such as spleen and blood. Because some TSE strains can differentially target particular cell types such as microglia, several of these molecular changes may also distinguish specific agent strains. The many host responses to the CJD agent challenge the assumption that the immune system does not recognize TSE infections because these agents are composed only of the host's own PrP.

Testing the Possibility to Protect Bovine PrPC Transgenic Swiss Mice Against Bovine PrPSc Infection by DNA Vaccination Using Recombinant Plasmid Vectors Harboring and Expressing the Complete or Partial cDNA Sequences of Bovine PrPc

The objective of this study was to investigate the molecular mechanisms of neurobiological processes involved in the degeneration of the central nervous system. The bovine spongiform encephalopathy (BSE) was used as experimental model system for investigation of transmissible spongiform encephalopathy (TSE). The experimental strategy was to evaluate the possibility for protection of bovine PrP(C) transgenic mice against a bovine PrP(Sc) infection by DNA vaccination using the complete or partial cDNA sequences of the bovine prion protein. Three recombinant plasmids pCR3.1-EX-PrP-BSE-C20 (C20), pCR3.1-EX-PrP-BSE-90-235-C4 (C4), and pCR3.1-EX-PrP-BSE-106-131-C14 (C14) were constructed. These mammalian expression vectors harbor complete (C20) or partial (C4 and C14) cDNA sequences of the Bos taurus PrP(C) (BTPrP(C)) encoding for amino acid residues 1-264 (C20), 90-235 (C4), and 106-131 (C14) of the BTPrP(C). Transgenic mice harboring and expressing BTPrP(C) were generated using the donor strain C57/CBA, receptor strain Swiss mouse, and recombinant plasmid MoPrPXho-boPrP. Crossing of positive transgenic mice to bovine PrP and negative to murine PrP with 129/OLA (murine PrP-/-) and C57BL6x129/OLA (murine PrP+/-) mice was carried out to amplify the colony of transgenic mice termed bovine PrP(C) transgenic Swiss mice (BTPrP-TgM). The capabilities of C20, C4, and C14 to express the corresponding cDNA sequence of BTPrP(C) in vitro and in vivo were confirmed prior to DNA vaccination of the BTPrP-TgM using NIH 3T3 cells and BALB/c mice, respectively. In order to prove the capability of the constructed expression vectors to protect BTPrP-TgM in vivo against a BSE infection 80 female BTPrP-TgM were vaccinated intramuscularly and subcutaneously with DNA of the plasmids C20, C4, C14, and parental vector pCR3.1 (100 microg DNA corresponding to about 26-30 pmol DNA/animal and application) in four groups (each consists of 20 animals). DNA vaccination was followed by three additional boosters. The vaccinated animals (15 animals of each group) were challenged twice per oral with homogenates of brain material obtained from BSE cattle containing the infectious PrP(Sc) (100 microl/animal which corresponds to 15 mg of a 15% brain homogenate). The first and second challenge experiments were performed 76-83 and 181 days post DNA vaccination, respectively. A part of the vaccinated animals (3-5 animals of each group) that served as internal negative control were mock infected using the brain homogenate of healthy cattle or Phosphate saline buffer (PBS). A variety of symptoms and clinical pictures were observed during the monitoring of DNA vaccinated animals. However, the observed diseases seem to be similar in all experimental animal groups. After an observation period of 14 months post the second challenge experiment the remaining animals (some animals died or were sacrificed when moribund during the study) were sacrificed after expiration of the experimental schedule. The right hemisphere of the brain and a half of the spleen tissue of the individual animals were used for detection of PrP(Sc) by Western blot analysis. The misfolded bovine PrP(Sc) was not detected in the brain or spleen tissues of those animals that were vaccinated with DNA of C20, which was able to express the complete bovine PrP(C) protein in vitro and in vivo. In contrast, the bovine PrP(Sc) was detected in the brain or spleen tissues of animals that were DNA vaccinated with DNA of the parental vector pCR3.1, with DNA of C4, or with DNA of C14. The results of these studies underline that the constructed expression vector C20 possesses the protective capacity to inhibit the formation of misfolded bovine PrP(Sc) in BTPrP-TgM under the conditions used. A delay of occurrence of TSE-specific symptoms in the majority of the vaccinated animals seems to be due to the prolonged incubation time of BSE infection.

Passive amyloid immunotherapy clears amyloid and transiently activates microglia in a transgenic mouse model of amyloid deposition

The role of microglia in the removal of amyloid deposits after systemically administered anti-Abeta antibodies remains unclear. In the current study, we injected Tg2576 APP transgenic mice weekly with an anti-Abeta antibody for 1, 2, or 3 months such that all mice were 22 months at the end of the study. In mice immunized for 3 months, we found an improvement in alternation performance in the Y maze. Histologically, we were able to detect mouse IgG bound to congophilic amyloid deposits in those mice treated with the anti-Abeta antibody but not in those treated with a control antibody. We found that Fcgamma receptor expression on microglia was increased after 1 month of treatment, whereas CD45 was increased after 2 months of treatment. Associated with these microglial changes was a reduction in both diffuse and compact amyloid deposits after 2 months of treatment. Interestingly, the microglia markers were reduced to control levels after 3 months of treatment, whereas amyloid levels remained reduced. Serum Abeta levels and anti-Abeta antibody levels were elevated to similar levels at all three survival times in mice given anti-Abeta injections rather than control antibody injections. These data show that the antibody is able to enter the brain and bind to the amyloid deposits, likely opsonizing the Abeta and resulting in Fcgamma receptor-mediated phagocytosis. Together with our earlier work, our data argue that all proposed mechanisms of anti-Abeta antibody-mediated amyloid removal can be simultaneously active.

Two Creutzfeldt-Jakob disease agents reproduce prion protein-independent identities in cell cultures

Human Creutzfeldt-Jakob disease (CJD) and similar neurodegenerative diseases such as sheep scrapie are caused by a variety of related infectious agents. They are associated with abnormal host prion protein (PrP), which is assessed by limited proteolysis to yield resistant PrP bands (PrP-res). Although PrP-res has been posited as the infectious agent, purified PrP-res itself is not infectious. To establish the independence of CJD agent characteristics from those of PrP-res, two different mouse-passaged CJD strains were propagated in neuronal cell lines whose PrP-res patterns differ markedly from each other and from those found in infected brain. In mouse brain, the fast CJD strain, FU, elicits many PrP-res deposits, whereas the slow SY strain elicits few. Both strains evoked PrP-res in cultured murine cells, although SY induced PrP-res only transiently. PrP-res patterns in FU- and SY-infected GT1 cells were identical, and were significantly different from those in brain and in N2a cells. Nevertheless, all FU-infected cell lines reproduced their original fast disease in mice, even after extensive subculture, whereas SY-infected cells produced only slow disease. These data indicate PrP-res neither encodes nor alters agent-specific characteristics. PrP-res was also a poor predictor of infectivity because SY cells that had lost PrP-res were approximately 10-fold more infectious than PrP-res-positive cultures. Furthermore, FU titers increased 650-fold, whereas PrP-res remained constant. Passaged FU-infected cells had titers comparable to brain, and >30% of cells displayed abundant cytoplasmic PrP-res aggregates that may trap agent. The continuous substantial replication of CJD in monotypic cells will further the discrimination of agent-specific molecules from pathological host responses to infection.

Early induction of interferon-responsive mRNAs in Creutzfeldt-Jakob disease

Foreign infectious agents typically evoke a host immune response. In scrapie and Creutzfeldt-Jakob disease (CJD), no immune response has been detectable. However, many latent or persistent viruses evade immune recognition but still activate inflammatory pathways. Unique microglial responses in late CJD infection that could be part of a host defense mechanism were previously delineated, although changes secondary to neurodegeneration could not be excluded. Data here show these microglial transcriptional changes are detectable in CJD brain beginning at 30 days after innoculation. In addition, 10 other interferon-sensitive genes were similarly upregulated at very early stages of infection. These responses occurred well before abnormal prion protein (PrP) and clinical signs of CJD were detectable. Further analyses in very pure microglia from CJD brain suggested the CJD agent activated signaling pathways distinct from those induced by amyloidogenic proteins (including abnormal PrP). Although increases in interferon-alpha or -beta transcript levels were not seen in cultures or in whole brain, CJD microglia exhibited a potentiated interferon response when challenged with double-stranded RNA. The induction of interferon-sensitive genes without appreciable interferon synthesis was strikingly similar to that seen in some viral infections. These data suggest the CJD agent is recognized as a foreign virus-like entity. Moreover, the early reactive gene expression profiles described here may be useful in preclinical diagnosis.

Two different scrapie prions isolated in Japanese sheep flocks

Two different scrapie prion strains with different characteristics were obtained from two sheep naturally infected with scrapie in Japan. In mice transmission, one (Tsukuba-1) showed shorter incubation periods (133+/-2 days) than the other (Tsukuba-2) (288+/-5 days). Spongiform changes and accumulation of an abnormal isoform of prion protein (PrP(Sc)) were observed throughout the brain in Tsukuba-1 inoculated mice, while the lesions and the PrP(Sc) accumulation were localized in the brain stem of mice with Tsukuba-2. Western blot analysis showed that there was no strain-specific glycoform of PrP(Sc) within these two strains. A super-infection experiment revealed that neither strain interfered with the other's propagation.

Hypothesis of interference to superinfection between bovine spastic paresis and bovine spongiform encephalopathy; suggestions for experimentation, theoretical and practical interest

Sub-acute transmissible spongiform encephalopathies (TSEs) or prion diseases are diseases of little known etiology. The origin of these diseases would appear to be an abnormal protease-resistant prion protein (PrP(res)) which would be infectious by directly inducing its defective conformation to the normal native protein (PrP(C)). This hypothesis does not account for certain aspects of TSEs, such as interference to superinfection: in laboratory animals, inoculation by means of an attenuated strain with a long incubation period protects against later infection by a very virulent strain with a short incubation period. The hypothesis is put forward that there exists a possibility of interference to superinfection between neurodegenerative diseases of unknown origin, thought to be similar to TSEs, and a later infection by a TSE. The study of this interference between bovine spastic paresis (BSP) and bovine spongiform encephalopathy (BSE) could be used as a model for this hypothesis. BSP is a very rare disease among cattle, of unknown etiology; it is curable, in the very early stages, by using tryptophan and especially lithium, potentiated by copper and manganese. An etiology close to that of TSEs has been suggested on several occasions. If interference could be demonstrated between BSP and BSE, interesting data would be provided concerning the etiology, the pathogenesis and possibly the treatment and prevention of these diseases. Notably, such data could lead to the development of a treatment and a prevention with lithium and amino acids precursors of neuromediators (tryptophan, tyrosine, glutamic acid, etc.), as well as the developing of a vaccine to combat TSEs, especially BSE and scrapie.

Delay in onset of prion disease for the HY strain of transmissible mink encephalopathy as a result of prior peripheral inoculation with the replication-deficient DY strain

We report that the replication-deficient DY strain of transmissible mink encephalopathy (TME) can delay disease caused by the pathogenic HY TME strain. In this study, competition between the HY and DY TME agents was investigated following superinfection of the sciatic nerve and peritoneal cavity. Initially, DY TME infection was examined in the absence of superinfection and it was found that inoculation into the brain and sciatic nerve resulted in prion disease and PrP(Sc) deposition in brain but not lymphoreticular tissues. Conversely, intraperitoneal inoculation of the DY TME agent did not result in clinical symptoms, DY TME agent replication or PrP(Sc) deposition 400-600 days after infection. These findings indicate that the DY TME agent does not replicate in secondary lymphoid organs and is non-pathogenic when neuroinvasion is dependent on prior infection of the lymphoreticular system. However, intraperitoneal inoculation of the DY TME agent at 60 days, but not at 30 days, prior to intraperitoneal inoculation of the HY TME agent resulted in an extension of the HY TME incubation period. Inoculation of the DY TME agent into the sciatic nerve at 60 days prior to intrasciatic nerve inoculation of the HY TME agent did not delay the incubation period of HY TME. The ability of the DY TME agent to delay HY TME infection following extraneural inoculation, but not neural infection, suggests that HY and DY TME agent competition can occur in a common replication site whose cellular location precedes infection of both the lymphoreticular and peripheral nervous systems.

Transmissible encephalopathies: speculations and realities

Virtually all transmissible encephalopathies (TSEs), such as scrapie, CJD, and BSE, are caused by a type of infectious particle that remains enigmatic. The language of prion theory supersedes the reality of what is, and what is not known. This review questions the predictive value, consistency and accuracy of this now dominant assumption. Many people believe the normal cellular prion protein (PrP) self-converts into an infectious amyloid protein or prion. Although the amyloidogenic capacity of proteins is well established, the concept of an infectious protein without nucleic acid was "revolutionary." Diverse experiments have repeatedly shown, however, that this protein alone, in any form, is incapable of reproducing transmissible infection. In contrast, the infectious agent copurifies with many other molecules, including nucleic acids, while it separates from the majority of PrP. The infectious particle has a homogeneous viral size of ~25 nm, and infectivity is markedly reduced by conditions that disrupt viral core components but do not disrupt multimers of PrP amyloid. Additionally, the infectious agent replicates to high levels before any PrP abnormalities can be detected. Hence, we initially proposed that PrP changes are part of the host's pathologic response to high levels of infectious agent, but not the agent itself. Newer data clarifying a role for myeloid cells in the spread of infection, the unique character of two different agent strains propagated in a single animal, and the demonstration of long nucleic acids in a variety of simplified high titer preparations continue to raise serious questions for the prion hypothesis. Moreover, the epidemic spread of TSEs, and the activation of host innate immune mechanisms by infection, further indicate these agents are recognizably foreign, and probably viral.

Virus-like interference in the latency and prevention of Creutzfeldt-Jakob disease

We previously showed that intracerebral (ic) inoculation of the attenuated SY strain of Creutzfeld-Jakob disease in mice could delay clinical signs and widespread neuropathology evoked by subsequent ic challenge with the more virulent FU strain. Using lower doses of SY and FU ic, we here demonstrate that mice can be protected well into old age without demonstrable neuropathology or pathologic prion protein (PrP-res). In contrast, parallel FU only controls became terminally diseased 1 year earlier. To determine whether factors elaborated in response to SY might be part of this effect, we evaluated brain and serum samples from additional parallel mice at 90 days after SY infection and just before FU challenge. The infectivity of FU preparations was significantly reduced by mixing with these fresh SY brain homogenates but not by mixing with SY serum samples, suggesting that brain cells were elaborating labile inhibitory factors that were part of the protective response. SY infectivity was too low to be detected in these brain homogenates. Although suppression could be overcome by higher FU doses ic, strong protection against maximal doses of FU was observed by using i.v. inoculations. Because myeloid microglia are infectious and also elaborate many factors in response to the foreign Creutzfeld-Jakob disease agent, it is likely that innate immunity underlies the profound protection shown here. In principle, it should be possible to artificially stimulate relevant myeloid pathways to better prevent and/or delay the clinical and pathological sequelae of these infections.

Therapeutic approaches to prion diseases

Prion diseases are unique in that they comprise sporadic, genetic, and iatrogenically or environmentally acquired forms. When disease is acquired by peripheral route, neuroinvasion occurs via at least two different neural pathways (vague and splanchnic nerves) and is usually preceded by prion propagation in secondary lymphoid organs. Conversely, in the other etiologic forms, PrPSc formation occurs within, and is apparently limited to, the CNS. Longitudinal studies on experimental scrapie indicate that substantial neuropathologic changes (i.e., glial activation and nerve cell degeneration) already are present before the onset of symptoms and are topographically related to PrPSc deposits. Accordingly, any effective intervention should start during the preclinical stage of disease, and be aimed at preventing neuroinvasion or PrPSc propagation in the CNS. Unfortunately, no tests are available currently to detect presymptomatic individuals, except for carriers of pathogenic mutations of the PRNP gene. Inhibition of PrPSc formation can be achieved through (1) abrogation of PrPC synthesis or prevention of its transport to the cell surface; (2) stabilization of the PrPC structure to make its conformational change unfavorable; (3) sequestration of PrPSc; (4) reversion of PrPSc to a protease-sensitive form; or (5) interference with the interaction between PrPC, PrPSc, and other macromolecules that feature in the conversion process. The compounds that have some effectiveness in in vitro, cell culture, or animal models of prion disease seem to operate through one of these mechanisms (see Table 1); however, even the most effective drugs only work when administered at the time of infection or very short thereafter, and these conditions are incurable at present. The heterogeneity and complexity of the etiopathogenesis of prion diseases suggest that various strategies and a combination of several compounds with different modes of actions are likely necessary for prevention and treatment. Major efforts should be focused on the development of preclinical diagnostic tests in conjunction with immunization strategies for diseases acquired by peripheral route and identification of more effective compounds for the other etiological forms.

Microglia from Creutzfeldt-Jakob disease-infected brains are infectious and show specific mRNA activation profiles

Neurons are often assumed to be the principal sites for replication of the infectious agents causing Creutzfeldt-Jakob disease (CJD), scrapie, and bovine spongiform encephalopathy because they express high levels of normal and pathological prion protein (PrP). However, isolated brain cell types have not been evaluated for either infection or gene expression. Microglia purified from CJD-infected mice showed infectivity comparable to that of starting brain homogenate but expressed approximately 50-fold less PrP. CJD-infected microglia also displayed morphological changes indicative of cellular activation. To determine the molecular pathways of activation, we evaluated pertinent transcripts, including those linked to inflammation. Semiquantitative reverse transcription-PCR showed a >4-fold increase in cathepsin S, an enzyme important in antigen presentation, the cytokine interleukin-1beta, and the chemokine B-lymphocyte chemoattractant. The profile of microglial changes induced by the CJD agent differed substantially from activation induced by bacterial lipopolysaccharide or by beta-amyloid, a structure comparable to pathological PrP. These microglial studies emphasize migratory hematopoietic cells in the dispersion, and possibly replication, of the CJD agent. The low PrP levels in these highly infectious and activated cells further support the concept that pathological PrP is the result of infection rather than the infectious agent itself. Because microglia develop a specific pattern of responses to the CJD agent, microglial markers may be exploited in the diagnosis of these spongiform encephalopathies.

The use of antioxidants in transmissible spongiform encephalopathies: a case report

BACKGROUND

Transmissible spongiform encephalopathies (TSE), which include Creutzfeldt-Jakob disease and new variant Creutzfeldt-Jakob disease, are diseases characterized by progressive deterioration in the central nervous system with neuronal degeneration, vacuolatization of the neuropil, and gliosis. Little is known about the pathogenic mechanisms of infection, and controversy exits around the inciting infective agent. It has been shown that an important factor in pathogenesis is the immune system.

CASE

The reported case points to beneficial effects when antioxidant therapies are used in transmissible spongiform encephalopathies. The case revealed an early reversal in cognitive decline and subsequent improvements in myoclonus, apnea and rigidity. Although death was the ultimate outcome, the patient succumbed to the illness over 22 months after the onset of symptoms when the early rapid decline predicted demise within a few months.

CONCLUSION

It is possible that strategies blocking the effect of proinflammatory cytokines and the resulting oxidative damage may stem the progressive damage to the neuropil that occurs in spongiform encephalopathies. Further investigation into the use of antioxidants and other types of agents quelling inflammation needs to be undertaken. If antioxidants could be combined with treatments for the inciting infective agent, a new direction could be taken in the outcome of transmissible spongiform encephalopathies including CJD and vCJD.

The transmissible spongiform encephalopathies: pathogenic mechanisms and strategies for therapeutic intervention

Primary neurodegenerative diseases tend to be intractable and largely affect the elderly. There is rarely the opportunity to identify individuals at risk and the appearance of clinical symptoms usually signifies the occurrence of irreversible neurological damage. This situation describes sporadic Creutzfeldt-Jakob disease which occurs world-wide, affecting one person per million per annum. The epidemic of bovine spongiform encephalopathy in the UK in the 1980s and the subsequent causal appearance of variant Creutzfeldt-Jakob disease in young UK residents in the 1990s has refocused attention on this whole group of diseases, known as the transmissible spongiform encephalopathies or prion diseases. The potentially lengthy incubation period of variant Creutzfeldt-Jakob disease, including perhaps an obligate peripheral phase, prior to neuroinvasion, marks variant Creutzfeldt-Jakob disease out as different from sporadic Creutzfeldt-Jakob disease. The formal possibility of detecting individuals infected with the bovine spongiform encephalopathy agent during this asymptomatic peripheral phase provides a strong incentive for the development of therapies for transmissible spongiform encephalopathies. This review focuses on recent advances in the understanding of the pathogenesis of these diseases, with particular reference to in vitro and animal model systems. Such systems have proved invaluable in the identification of potential therapeutic strategies that either specifically target the prion protein or more generally target peripheral pathogenesis. Furthermore, recent experiments in animal models suggest that even after neuroinvasion there may be pharmacological avenues to explore that might retard or even halt the degenerative process.