Nucleic acid binding proteins in highly purified Creutzfeldt-Jakob disease preparations

The prion protein in neuroimmune crosstalk

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

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.

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.

Generation of monoclonal antibody that distinguishes PrPSc from PrPC and neutralizes prion infectivity

To establish PrP(Sc)-specific mAbs, we immunized Prnp(-/-) mice with PrP(Sc) purified from prion-infected mice. Using this approach, we obtained mAb 6H10, which reacted with PrP(Sc) treated with proteinase K, but not with PrP(Sc) pretreated with more than 3 M GdnHCl. In contrast, reactivity of pan-PrP mAbs increased with increasing concentrations of GdnHCl used for pretreatment of PrP(Sc). In histoblot analysis, mAb 6H10 showed a positive reaction on a non-denatured histoblot but reactivity was lower when the histoblot was pretreated by autoclaving. Epitope analysis suggested that the extreme C-terminus of PrP is likely to be part of the epitope for mAb 6H10. MAb 6H10 immunoprecipitated PrP(Sc) from brains of mice, sheep, and cattle infected with prions. Furthermore, pretreatment of purified PrP(Sc) with mAb 6H10 reduced the infectious titer more than 1 log. Taken together, these results suggest that mAb 6H10 recognizes a conformational epitope on PrP(Sc) that is related to prion infectivity.

Quantitative recovery of scrapie agent with minimal protein from highly infectious cultures

There are few reports on the isolation, quantitative recovery, and relative purification of infectious particles that cause scrapie, Creutzfeldt-Jakob disease (CJD) and epidemic bovine spongiform encephalopathy (BSE). Because pure prion protein (PrP) has failed to show significant infectivity, it is critical to find other molecules that are integral agent components. Only complex diseased tissues such as degenerating brain have been fractionated, and agent recoveries have been quite low in concentrated abnormal prion protein (PrP-res) preparations. To simplify the purification of infectious particles, we evaluated a monotypic cell line that continuously produced high levels of the 22L scrapie agent (N2a-22L). A new rapid and accurate GT1 culture assay was used to titrate infectivity in six representative sucrose gradients. We developed a streamlined approximately 3-h procedure that yielded full recovery of starting infectivity in fractions with only a few selected protein bands (representing <1% of starting protein). Infectious particles reproducibly sedimented through >30% sucrose steps, whereas PrP and PrP-res sedimentation varied depending on the conditions used. Both normal and abnormal PrP could be largely separated from infectivity in a single short centrifugation. Because no foreign enzymes were added to achieve reasonably purified infectious particles, these preparations may be used to elicit diagnostic antibodies to foreign agent proteins.

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.

Scrapie infection activates the replication of ecotropic, xenotropic, and polytropic murine leukemia virus (MuLV) in brains and spinal cords of senescence-accelerated mice: implication of MuLV in progression of scrapie pathogenesis

Senescence-accelerated mice (SAMP8) have a short life span, whereas SAMR1 mice are resistant to accelerated senescence. Previously it has been reported that the Akv strain of ecotropic murine leukemia virus (E-MuLV) was detected in brains of SAMP8 mice but not in brains of SAMR1 mice. In order to determine the change of MuLV levels following scrapie infection, we analyzed the E-MuLV titer and the RNA expression levels of E-MuLV, xenotropic MuLV, and polytropic MuLV in brains and spinal cords of scrapie-infected SAM mice. The expression levels of the 3 types of MuLV were increased in scrapie-infected mice compared to control mice; E-MuLV expression was detected in infected SAMR1 mice, but only in the terminal stage of scrapie disease. We also examined incubation periods and the levels of PrPSc in scrapie-infected SAMR1 (sR1) and SAMP8 (sP8) mice. We confirmed that the incubation period was shorter in sP8 (210+/-5 days) compared to sR1 (235+/-10 days) after intraperitoneal injection. The levels of PrPSc in sP8 were significantly greater than sR1 at 210+/-5 days, but levels of PrPSc at the terminal stage of scrapie in both SAM strains were virtually identical. These results show the activation of MuLV expression by scrapie infection and suggest acceleration of the progression of scrapie pathogenesis by MuLV.

Species barriers in prion diseases--brief review

Transmissible spongiform encephalopathies (TSEs or prion diseases) are neurological disorders associated with the aggregation of a pathologic isoform of a host-encoded protein, termed prion protein (PrP). The pathologic isoform of PrP, termed PrP(Sc), is a major constituent of the infectious agent. TSE diseases are characterized by neurodegenerative failure and inevitable morbidity. Bovine spongiform encephalopathy (BSE) has been transmitted from cattle to humans to cause a new variant of Creutzfeldt-Jakob syndrome. The potential for chronic wasting disease to similarly cross the species barrier from cervids to humans is considered unlikely but possible. Thus, understanding how TSE agents overcome resistance to transmission between species is crucial if we are to prevent future epidemics. The species barrier usually can be abrogated to varying degrees in laboratory animals. Studies done with transgenic animals, tissue culture, and cell-free assays established PrP as being necessary for TSE pathogenesis and illustrated that certain amino acid residues are more influential than others for conferring resistance to TSE agent transmission. The essence of what constitutes a TSE agent's species compatibility is thought to be orchestrated by a complex interplay of contributions from its primary amino acid sequence, its glycoform patterns, and its three-dimensional structure.

Acute Formation of Protease-resistant Prion Protein Does Not Always Lead to Persistent Scrapie Infection in Vitro

Transmissible spongiform encephalopathies are accompanied by the accumulation of a pathologic isoform of a host-encoded protein, termed prion protein (PrP). Despite the widespread distribution of the cellular isoform of PrP (protease-sensitive PrP; PrP-sen), the disease-associated isoform (protease-resistant PrP; PrP-res) appears to be primarily restricted to cells of the nervous and lymphoreticular systems. In order to study why scrapie infection appears to be restricted to certain cells, we followed acute and persistent PrP-res formation upon exposure of cells to different scrapie agents. We found that, independent of the cell type and scrapie strain, initial PrP-res formation occurred rapidly in cells. However, sustained generation of PrP-res and persistent infection did not necessarily follow acute PrP-res formation. Persistent PrP-res formation and scrapie infection was restricted to one cell line inoculated with the mouse scrapie strain 22L. In contrast to cells that did not become scrapie-infected, the level of PrP-res in the 22L-infected cells rapidly increased in the absence of a concomitant increase in the number of PrP-res-producing cells. Furthermore, the protein banding pattern of PrP-res in these cells changed over time as the cells became chronically infected. Thus, our results suggest that the events leading to the initial formation of PrP-res may differ from those required for sustained PrP-res formation and infection. This may, at least in part, explain the observation that not all PrP-sen-expressing cells appear to support transmissible spongiform encephalopathy agent replication.

Scrapie-like prion protein is translocated to the nuclei of infected cells independently of proteasome inhibition and interacts with chromatin

Prion diseases are fatal transmissible neurodegenerative disorders characterized by the accumulation of an abnormally folded isoform of the cellular prion protein (PrPC) denoted PrPSc. Recently, wild-type and pathogenic PrP mutants have been shown to be degraded by the endoplasmic reticulum-associated degradation proteasome pathway after translocation into the cytosol. We show here that a protease resistant form of PrP accumulated in the nuclei of prion-infected cells independently of proteasome activity, and that this nuclear translocation required an intact microtubule network. Moreover, our results show for the first time that nuclear PrP interacts with chromatin in vivo, which may have physiopathological consequences in prion diseases

Characterization of 2'-fluoro-RNA aptamers that bind preferentially to disease-associated conformations of prion protein and inhibit conversion

We have isolated artificial ligands or aptamers for infectious prions in order to investigate conformational aspects of prion pathogenesis. The aptamers are 2'-fluoro-modified RNA produced by in vitro selection from a large, randomized library. One of these ligands (aptamer SAF-93) had more than 10-fold higher affinity for PrPSc than for recombinant PrPC and inhibited the accumulation of PrPres in near physiological cell-free conversion assay. To understand the molecular basis of these properties and to distinguish specific from non-specific aptamer-PrP interactions, we studied deletion mutants of bovine PrP in denatured, alpha-helix-rich and beta-sheet-rich forms. We provide evidence that, like scrapie-associated fibrils (SAF), the beta-oligomer of PrP bound to SAF-93 with at least 10-fold higher affinity than did the alpha-form. This differential affinity could be explained by the existence of two binding sites within the PrP molecule. Site 1 lies within residues 23-110 in the unstructured N terminus and is a nonspecific RNA binding site found in all forms of PrP. The region between residue 90 and 110 forms a hinge region that is occluded in the alpha-rich form of PrP but becomes exposed in the denatured form of PrP. Site 2 lies in the region C-terminal of residue 110. This site is beta-sheet conformation-specific and is not recognized by control RNAs. Taken together, these data provide for the first time a specific ligand for a disease conformation-associated site in a region of PrP critical for conformational conversion. This aptamer could provide tools for the further analysis of the processes of PrP misfolding during prion disease and leads for the development of diagnostic and therapeutic approaches to TSEs.

Functional interactions of nucleocapsid protein of feline immunodeficiency virus and cellular prion protein with the viral RNA

All lentiviruses and oncoretroviruses examined so far encode a major nucleic-acid binding protein (nucleocapsid or NC* protein), approximately 2500 molecules of which coat the dimeric RNA genome. Studies on HIV-1 and MoMuLV using in vitro model systems and in vivo have shown that NC protein is required to chaperone viral RNA dimerization and packaging during virus assembly, and proviral DNA synthesis by reverse transcriptase (RT) during infection. The human cellular prion protein (PrP), thought to be the major component of the agent causing transmissible spongiform encephalopathies (TSE), was recently found to possess a strong affinity for nucleic acids and to exhibit chaperone properties very similar to HIV-1 NC protein in the HIV-1 context in vitro. Tight binding of PrP to nucleic acids is proposed to participate directly in the prion disease process. To extend our understanding of lentiviruses and of the unexpected nucleic acid chaperone properties of the human prion protein, we set up an in vitro system to investigate replication of the feline immunodeficiency virus (FIV), which is functionally and phylogenetically distant from HIV-1. The results show that in the FIV model system, NC protein chaperones viral RNA dimerization, primer tRNA(Lys,3) annealing to the genomic primer-binding site (PBS) and minus strand DNA synthesis by the homologous FIV RT. FIV NC protein is able to trigger specific viral DNA synthesis by inhibiting self-priming of reverse transcription. The human prion protein was found to mimic the properties of FIV NC with respect to primer tRNA annealing to the viral RNA and chaperoning minus strand DNA synthesis.

The prion protein has DNA strand transfer properties similar to retroviral nucleocapsid protein

The transmissible spongiform encephalopathies are fatal neurodegenerative diseases that are associated with the accumulation of a protease-resistant form of the cellular prion protein (PrP). Although PrP is highly conserved and widely expressed in vertebrates, its function remains a matter of speculation. Indeed PrP null mice develop normally and are healthy. Recent results show that PrP binds to nucleic acids in vitro and is found associated with retroviral particles. Furthermore, in mice the scrapie infectious process appears to be accelerated by MuLV replication. These observations prompted us to further investigate the interaction between PrP and nucleic acids, and compare it with that of the retroviral nucleocapsid protein (NC). As the major nucleic acid-binding protein of the retroviral particle, NC protein is tightly associated with the genomic RNA in the virion nucleocapsid, where it chaperones proviral DNA synthesis by reverse transcriptase. Our results show that the human prion protein (huPrP) functionally resembles NCp7 of HIV-1. Both proteins form large nucleoprotein complexes upon binding to DNA. They accelerate the hybridization of complementary DNA strands and chaperone viral DNA synthesis during the minus and plus DNA strand transfers necessary to generate the long terminal repeats. The DNA-binding and strand transfer properties of huPrP appear to map to the N-terminal fragment comprising residues 23 to 144, whereas the C-terminal domain is inactive. These findings suggest that PrP could be involved in nucleic acid metabolism in vivo.

Bovine spongiform encephalopathy: an overview

Bovine spongiform encephalopathy (BSE), widely known as "mad cow disease," is a chronic, degenerative disease affecting the central nervous system of cattle. Worldwide, there have been more than 180,000 cases since the disease was first diagnosed in 1986 in Great Britain. Bovine spongiform encephalopathy has had a substantial impact on the livestock industry in the United Kingdom. The disease has also been confirmed in native-born cattle in Belgium, Denmark, France, Ireland, Luxembourg, Liechtenstein, The Netherlands, Northern Ireland, Portugal, and Switzerland. However, over 95% of all BSE cases have occurred in the United Kingdom. Bovine spongiform encephalopathy is not known to exist in the United States.

Viral particles are required for infection in neurodegenerative Creutzfeldt-Jakob disease

Several models have been proposed for the infectious agents that cause human Creutzfeldt-Jakob disease (CJD) and sheep scrapie. Purified proteins and extracted nucleic acids are not infectious. To further identify the critical molecular components of the CJD agent, 120S infectious material with reduced prion protein (PrP) was treated with guanidine hydrochloride or SDS. Particulate and soluble components were then separated by centrifugation and molecularly characterized. Conditions that optimally solubilized residual PrP and/or nucleic acid-protein complexes were used to produce subfractions that were assayed for infectivity. All controls retained > 90% of the 120S titer (approximately 15% of that in total brain) but lost > 99.5% of their infectivity after heat-SDS treatment (unlike scrapie fractions enriched for PrP). Exposure to 1% SDS at 22 degrees C produced particulate nucleic acid-protein complexes that were almost devoid of host PrP. These sedimenting complexes were as infectious as the controls. In contrast, when such complexes were solubilized with 2.5 M guanidine hydrochloride, the infectious titer was reduced by > 99.5%. Sedimenting PrP aggregates with little nucleic acid and no detectable nucleic acid-binding proteins had negligible infectivity, as did soluble but multimeric forms of PrP. These data strongly implicate a classical viral structure, possibly with no intrinsic PrP, as the CJD infectious agent. CJD-specific protective nucleic acid-binding protein(s) have already been identified in 120S preparations, and preliminary subtraction studies have revealed several CJD-specific nucleic acids. Such viral candidates deserve more attention, as they may be of use in preventing iatrogenic CJD and in solving a fundamental mystery.

Distinct PrP properties suggest the molecular basis of strain variation in transmissible mink encephalopathy

The molecular basis of strain variation in scrapie diseases is unknown. The only identified component of the agent is the posttranslationally modified host prion protein (PrPSc). The biochemical and physical properties of PrP from two strains of transmissible mink encephalopathy (TME), called hyper (HY) and drowsy (DY), were compared to investigate if PrP heterogeneity could account for strain diversity. The degradation rate of PrPTME digested with proteinase K was found to be strain specific and correlated with inactivation of the TME titer. Edman protein sequencing revealed that the major N-terminal end of HY PrPTME commenced at least 10 amino acid residues prior to that of DY PrPTME after digestion with proteinase K. Analysis of the brain distribution of PrPTME exhibited a strain-specific pattern and localization of PrPTME to the perikarya of specific neuron populations. Our findings are consistent with HY and DY PrPTME having distinct protein conformations and/or strain-specific ligand interactions that influence PrPTME properties. We propose that PrPTME conformation could play a role in targeting TME strains to different neuron populations in which strain-specific formation occurs. These data are consistent with the idea that PrPTME protein structure determines the molecular basis of strain variation.

Dementias, neurodegeneration, and viral mechanisms of disease from the perspective of human transmissible encephalopathies

Our transmission experiments with human CJD emphasize the centrality of an exogenous infectious pathogen that can exist in symbiosis with its host for extended periods. Many latent or persistent viruses can cause neurodegenerative disease and may have a role in late onset dementias. There are reasons to believe that CJD infections may share properties with some of these latent viruses in causing dementia, and several retroviral mechanisms may be operative in CJD. In order to clarify viral-like attributes of the CJD agent we have closely followed infectivity and find the following: 1) the CJD agent has a virus-like size and density, and is biochemically separable from most host-encoded prion protein (PrP); 2) Endogenous retroviral IAP RNA sequences of 5,000 bases, as well as several gag-like nucleic acid binding proteins, co-purify with infectivity in preparations treated with high concentrations of anionic detergents and exhaustive nuclease digestion. They signify the purification of true viral cores rather than aggregation artifacts, and diminish claims that there are no protected nucleic acids of > 50 bases in highly purified infectious preparations; 3) In established hamster CJD, temporal studies show the agent has an effective doubling time of approximately 7.5 days in brain, consistent with complex host-viral interactions common to slow viral infections; 4) PrP-res does not correspond to titered levels of infectivity either in a biochemical or an in vivo setting but may function as a viral receptor that can modulate disease expression. Interestingly, functional changes in glial cells occur earlier than PrP-res changes, and indicate an important role for glial cells in evolving infections; 5) Human-rodent transmission studies suggest that CJD, or a CJD-like variant can be a common but latent infection of humans, with relatively infrequent expression of neurological disease. Susceptibility to disease can rest on host attributes and possibly age-related co-factors. Nonetheless, fundamental viral principles are also operative. Agent strain variants, viral burden, and the routes of infection are critical parameters for latency and disease expression. The properties described above have led me to return to the inclusion of CJD (and scrapie) in the panorama of conventional slow viral infections of the brain, as originally proposed by Sigurdsson. Identification of virus-specific molecules are essential for elucidating the role of these agents in the spectrum of human dementias.

Properties of the scrapie prion protein: quantitative analysis of protease resistance

The disease-specific isoform of the prion protein (PrPSc) is an essential part of the infectious particle which causes spongiform degeneration in various mammalian species. PrPSc differs from PrP of normal animals (PrPc) by its relative protease resistance. The physical nature of this difference is still unknown. We analyzed the protease resistance of PrPSc quantitatively using an enzyme-linked immunofiltration assay. PrPSc was rendered completely protease-sensitive at alkaline pH or in > 1.5 M guanidinium thiocyanate (GdnSCN). Denaturation in 4 M GdnSCN completely abolished the protease resistance of PrPSc within 15 min, while denaturation in 7.2 M urea showed a slower time course. In the presence of ethanol, PrPSc was protected from denaturation by GdnSCN or alkaline pH. Denaturation curves were used to calculate the free energy (delta GD) as a function of different denaturant concentrations. Linear regression of delta GD values was used to extrapolate the free energy in the absence of denaturants (delta GH2O), yielding similar values (delta GH2O,GdnSCN = -2.3 kcal/mol; delta GH2O,urea = -3.1 kcal/mol). The linear relationship between delta GD and the denaturant concentration is suggestive of a two-state model involving the conformational change of a single protein domain. This is also reflected in the small number of side chains (11.6) additionally exposed to the solvent upon conversion of PrPSc to its protease-sensitive isoform. Our results suggest that only minor rearrangements of the structure of PrP are needed to abolish the protease resistance of PrPSc.

Molecular biology and genetics of prion diseases

Scrapie was thought for many years to be caused by a virus. Enriching fractions from Syrian hamster (SHa) brain for scrapie infectivity led to the discovery of the prion protein (PrP). To date, no scrapie-specific nucleic acid has been found. As well as scrapie, prion diseases include bovine spongiform encephalopathy (BSE) of cattle, as well as Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome (GSS) of humans. Transgenic (Tg) mice expressing both SHa and mouse (Mo) PrP genes were used to probe the molecular basis of the species barrier and the mechanism of scrapie prion replication. The prion inoculum was found to dictate which prions are synthesized de novo, even though the cells express both PrP genes. Discovery of mutations in the PrP genes of humans with GSS and familial CJD established that prion diseases are both genetic and infectious. Tg mice expressing MoPrP with the GSS point mutation spontaneously develop neurologic dysfunction, spongiform degeneration and astrocytic gliosis. Inoculation of brain extracts prepared from these Tg(MoPrP-P101L) mice produced neurodegeneration in many of the recipient animals after prolonged incubation times. These and other results suggest that prions are devoid of foreign nucleic acid and are thus different from viruses and viroids. Studies on the structure of PrPSc and PrPC suggest that the difference is conformational. Whether one or more putative alpha-helices in PrPC are converted into beta-sheets during synthesis of PrPSc is unknown. Distinct prion isolates or 'strains' exhibit different patterns of PrPSc accumulation which are independent of incubation times. Whether variations in PrPSc conformation are responsible for prion diversity remains to be established. Prion studies have given new insights into the etiologies of infectious, sporadic and inherited degenerative diseases.

Endogenous viral complexes with long RNA cosediment with the agent of Creutzfeldt-Jakob disease

A class of viruslike agents that induces Creutzfeldt-Jakob Disease (CJD) and scrapie remains undefined at the molecular level. Several investigators believe this infectious agent is constituted by a single host protein or 'prion', and have emphasized data that would seem to exclude the presence of any viral nucleic acids. However, more rigorous evaluations in scrapie have shown reasonably abundant nucleic acids. Additionally, in highly purified 120S CJD preparations that have been treated with nucleases, RNAs as long as 6,000 bases have been detected. Few nucleic acids have been characterized in either scrapie or CJD, but previous cloning experiments delineated relatively short LTR regions of the endogenous IAP retrovirus in 120S CJD preparations. We therefore used specific primers encompassing the entire IAP genome to test for the presence of long viral RNAs, and here show approximately 5,000 contiguous bases of the IAP RNA genome can be recovered from reasonable amounts of starting brain. The 3' env region of IAP is comparably truncated in CJD and normal preparations, and we find no evidence for IAP transduction of CJD-specific sequences. Because IAP cores can coencapsidate unrelated sequences, and are unusually resistant to physical and chemical treatments, it was relevant to find if cosedimenting cognate proteins of the IAP core, such as gag, could be detected. The predicted approximately 65 kd acidic gag protein, showing appropriate antigenic and nucleic acid binding features, was apparent in both one and 2-D Western blots. This data strongly indicates specific viral complexes cofractionate with the CJD agent. Interestingly, these nuclease resistant IAPs do not appear to be in morphologically recognizable 'R' particles. This cosedimenting viral assembly therefore provides a paradigm for non-particulate CJD complexes in infectious preparations. In developing strategies to identify a CJD specific sequence, cosedimenting IAPs can be used to assess the quality, length and recovery of RNAs extracted from highly resistant viral complexes.