Analysis of Creutzfeldt-Jakob disease infectious fractions by gel permeation chromatography and sedimentation field flow fractionation

Absence of association between the incidence of BK virus and sporadic Creutzfeldt-Jakob disease

BACKGROUND

Many studies suggest that the virus-like particles are required for the infection of Creutzfeldt-Jakob disease (CJD).

OBJECTIVE

To determine the relationship between BK polyomavirus (BKV) and sporadic CJD.

MATERIALS AND METHODS

We investigated the prevalence of BKV in urine samples from 94 sporadic CJD patients and 54 other neurological disease (OND) patients using polymerase chain reaction.

RESULTS

BKV DNA was detected in 16 (17%) and 9 (16.7%) urine samples from sporadic CJD and OND patients, respectively. There was no significant difference in the incidence of BKV infection between Korean sporadic CJD and OND patients (p = 0.9558). In order to investigate the genotypes of BKV, we analyzed 22 BKV isolates obtained from Korean patients by DNA sequencing and nucleotide sequence analysis. Three distinct subtypes, namely I, III, and IV, were found in 66.7, 22.2, and 11.1% of 9 BKV isolates from OND patients, whereas subtypes I and IV were detected in 76.9 and 23.1% of 13 BKV isolates from sporadic CJD patients. Interestingly, subtype III was not detected in sporadic CJD patients. Significant differences in the frequency of BKV genotypes were not observed between sporadic CJD and OND patients.

CONCLUSIONS

These results indicate that BKV may not play an important role in the pathogenesis of prion diseases.

Nuclease resistant circular DNAs copurify with infectivity in scrapie and CJD

In transmissible encephalopathies (TSEs), it is commonly believed that the host prion protein transforms itself into an infectious form that encodes the many distinct TSE agent strains without any nucleic acid. Using a Ф29 polymerase and chromatography strategy, highly infectious culture and brain preparations of three different geographic TSE agents all contained novel circular DNAs. Two circular "Sphinx" sequences, of 1.8 and 2.4 kb, copurified with infectious particles in sucrose gradients and, as many protected viruses, resisted nuclease digestion. Each contained a replicase ORF related to microviridae that infect commensal Acinetobacter. Infectious gradient fractions also contained nuclease-resistant 16 kb mitochondrial DNAs and analysis of >4,000 nt demonstrated a 100% identity with their species-specific sequences. This confirmed the fidelity of the newly identified sequences detailed here. Conserved replicase regions within the two Sphinx DNAs were ultimately detected by PCR in cytoplasmic preparations from normal cells and brain but were 2,500-fold less than in parallel-infected samples. No trace of the two Sphinx replicases was found in enzymes, detergents, or other preparative materials using exhaustive PCR cycles. The Sphinx sequences uncovered here could have a role in TSE infections despite their apparently symbiotic, low-level persistence in normal cells and tissues. These, as well as other cryptic circular DNAs, may cause or contribute to neurodegeneration and infection-associated tumor transformation. The current results also raise the intriguing possibility that mammals may incorporate more of the prokaryotic world in their cytoplasm than previously recognized.

Transcytosis of murine-adapted bovine spongiform encephalopathy agents in an in vitro bovine M cell model

Transmissible spongiform encephalopathies (TSE), including bovine spongiform encephalopathy (BSE), are fatal neurodegenerative disorders in humans and animals. BSE appears to have spread to cattle through the consumption of feed contaminated with BSE/scrapie agents. In the case of an oral infection, the agents have to cross the gut-epithelial barrier. We recently established a bovine intestinal epithelial cell line (BIE cells) that can differentiate into the M cell type in vitro after lymphocytic stimulation (K. Miyazawa, T. Hondo, T. Kanaya, S. Tanaka, I. Takakura, W. Itani, M. T. Rose, H. Kitazawa, T. Yamaguchi, and H. Aso, Histochem. Cell Biol. 133:125-134, 2010). In this study, we evaluated the role of M cells in the intestinal invasion of the murine-adapted BSE (mBSE) agent using our in vitro bovine intestinal epithelial model. We demonstrate here that M cell-differentiated BIE cells are able to transport the mBSE agent without inactivation at least 30-fold more efficiently than undifferentiated BIE cells in our in vitro model. As M cells in the follicle-associated epithelium are known to have a high ability to transport a variety of macromolecules, viruses, and bacteria from gut lumen to mucosal immune cells, our results indicate the possibility that bovine M cells are able to deliver agents of TSE, not just the mBSE agent.

The physical relationship between infectivity and prion protein aggregates is strain-dependent

Prions are unconventional infectious agents thought to be primarily composed of PrP(Sc), a multimeric misfolded conformer of the ubiquitously expressed host-encoded prion protein (PrP(C)). They cause fatal neurodegenerative diseases in both animals and humans. The disease phenotype is not uniform within species, and stable, self-propagating variations in PrP(Sc) conformation could encode this 'strain' diversity. However, much remains to be learned about the physical relationship between the infectious agent and PrP(Sc) aggregation state, and how this varies according to the strain. We applied a sedimentation velocity technique to a panel of natural, biologically cloned strains obtained by propagation of classical and atypical sheep scrapie and BSE infectious sources in transgenic mice expressing ovine PrP. Detergent-solubilized, infected brain homogenates were used as starting material. Solubilization conditions were optimized to separate PrP(Sc) aggregates from PrP(C). The distribution of PrP(Sc) and infectivity in the gradient was determined by immunoblotting and mouse bioassay, respectively. As a general feature, a major proteinase K-resistant PrP(Sc) peak was observed in the middle part of the gradient. This population approximately corresponds to multimers of 12-30 PrP molecules, if constituted of PrP only. For two strains, infectivity peaked in a markedly different region of the gradient. This most infectious component sedimented very slowly, suggesting small size oligomers and/or low density PrP(Sc) aggregates. Extending this study to hamster prions passaged in hamster PrP transgenic mice revealed that the highly infectious, slowly sedimenting particles could be a feature of strains able to induce a rapidly lethal disease. Our findings suggest that prion infectious particles are subjected to marked strain-dependent variations, which in turn could influence the strain biological phenotype, in particular the replication dynamics.

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.

A rapid accurate culture assay for infectivity in Transmissible Encephalopathies

The molecular and structural features of infectious agents that cause CJD, scrapie and BSE remain controversial. A major impediment for agent resolution is the very long and expensive animal assays of infectivity. It is crucial to develop a rapid and broadly applicable cell culture assay to titer and compare different TSE agent strains. Because we found GT1 hypothalamic cells, unlike neuroblastoma N2a clones, were highly susceptible to a variety of TSE agents, and could stably produce high agent titers for >1 year, we studied the progressive display of abnormal prion protein (PrP-res) in GT1 cells following exposure to serially diluted 22L scrapie brain homogenates; PrP-res was used as a surrogate, but non-quantitative marker of GT1 infection. Even as early as the first cell split after 22L exposure, GT1 cells produced their own PrP-res bands that were clearly different than brain bands. Plots from passages 3-7 showed a good discrimination of 3 fold differences in titer over a range of >2 logs, with the same endpoint sensitivity (2 x 10(8) LD(50)/gm) as animal assays. Interestingly, the rapid production of de novo PrP-res suggested that GT1 PrP-res might be induced by interaction with an early-intermediate form of a particle that was not fully infectious. The GT1 assay here was also invaluable for rapidly identifying cell cultures with variant titers, even after detergent lysis. Additionally, in-situ PrP amyloid staining provided an independent measure of the minimum infectious dose per cell. Standardized GT1 assays can be used for direct comparison of different agent strains, and will facilitate the rapid isolation of essential agent components.

Bistability and the species barrier in prion diseases: stepping across the threshold or not

The infectious agent of transmissible spongiform encephalopathies is thought to be a cellular protein, the prion protein, which undergoes, under some circumstances, a dramatic conformational change leading to pathogenesis. The conversion between the normal and pathogenic isoforms corresponds to a autocatalytic mechanism and the metabolism of the prion protein exhibits switches between a normal, stable steady state and a pathogenic one. When the disease can be transmitted between two species, a primary infection from a heterologous donor has to be followed by two passages in the same host species so that the incubation period is stabilized. Sometimes, no pathogenic isoform of the prion protein is detected after the first passage, although corresponding brain extracts remain infectious. The observation that three and only three passages are needed in order to stabilize the strain strongly suggests that, during the course of the primary infection by the heterologous donor, an intermediary conformational species is formed. Within this assumption, a common mechanism involving only conformational changes of the prion protein can give a unifying interpretation of the problem of species barrier, lag characteristics and apparent lack of detection of the pathogenic isoform after the first passage in experiments dealing with interspecies transmission of prion diseases.

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.

Comparison of two gel filtration chromatographic methods for the purification of Lily symptomless virus

Lily symptomless virus (LSV) occurs frequently in many Lilium species worldwide and often causes developmental abnormalities such as a smaller flower and lower bulb yield. In this study, two moderate and efficient gel filtration chromatography (GFC) methods was compared, these two techniques were, respectively, based on Superdex-200 HR and Sephacryl S-1000 SF. The products purified by the two methods were then characterized by measurements with UV-spectrophotometer, reverse transcriptase (RT)-PCR, transmission electron microscope (TEM), polyacrylamide gel electrophoresis (PAGE), Western blotting and matrix assisted laser desorption-ionisation/time-of-flight mass spectrometry (MALDI-TOF-MS). The final yield of purified LSV by the Superdex-200 HR GFC was 9.4 mg from 50 g of fresh infected tissues of Lanzhou lily. However, from the same amount tissues, only 5.6 mg of LSV were obtained by using Sephacryl S-1000 SF GFC. The Superdex-200 HR method was thus shown to be more suitable for the purification of LSV than the Sephacryl S-1000 SF GFC. The Superdex-200 HR method does not require costly equipment for density centrifugation and ultracentrifugation. Furthermore, it can provide an economical and efficient way to obtain purified products for the preparation of antibodies for serological diagnosis or LSV infection and related investigations.

Prions and transmissible spongiform encephalopathy (TSE) chemotherapeutics: A common mechanism for anti-TSE compounds?

No validated treatments exist for transmissible spongiform encephalopathies (TSEs or prion diseases) in humans or livestock. The search for TSE therapeutics is complicated by persistent uncertainties about the nature of mammalian prions and their pathogenic mechanisms. In pursuit of anti-TSE drugs, we and others have focused primarily on blocking conversion of normal prion protein, PrP(C), to the TSE-associated isoform, PrP(Sc). Recently developed high-throughput screens have hastened the identification of new inhibitors with strong in vivo anti-TSE activities such as porphyrins, phthalocyanines, and phosphorthioated oligonucleotides. New routes of administration have enhanced beneficial effects against established brain infections. Several different classes of TSE inhibitors share structural similarities, compete for the same site(s) on PrP(C), and induce the clustering and internalization of PrP(C) from the cell surface. These activities may represent a common mechanism of action for these anti-TSE compounds.

Fractionation of Prion Protein Aggregates by Asymmetrical Flow Field‐Flow Fractionation

Achieving the successful separation and analysis of amyloid and other large protein aggregates can be a difficult proposition. Field-flow fractionation (FFF) is a flow-based separation method like chromatography; however, FFF is capable of high-resolution separations in the absence of a stationary matrix. Thus, FFF is a relatively gentle technique and is well suited to the task of separating large macromolecules and macromolecular complexes. Flow field-flow fractionation (FlFFF), one of the techniques in the FFF family, has been used to successfully fractionate a wide size range of prion protein aggregates, allowing their subsequent characterization by several biophysical and biochemical methods. The ability to easily adjust the strength of the field used during separation means that FlFFF could be applied to particles ranging from 1 nm to nearly 100 mum in size. This flexibility, coupled with the ability to produce fast, high-resolution separations, makes FFF a potentially valuable tool in the field of amyloid research.

Prion diseases: dynamics of the infection and properties of the bistable transition

Prion diseases are thought to result from a pathogenic, conformational change in a cellular protein, the prion protein. The pathogenic isoform seems to convert the normal isoform in an autocatalytic process. In contrast to the conditions used for in vitro studies of enzyme kinetics, the concentration of the catalyst is not much lower than that of the substrate in the course of infection. This feature may endow the system with a time-hierarchy allowing the pathogenic isoform to relax very slowly in the course of infection. This may contribute to the long incubation periods observed in prion diseases. The dynamic process of prion propagation, including turnover of the cellular prion protein, displays bistable properties. Sporadic prion diseases may result from a change in one of the parameters associated with metabolism of the prion protein. The bistable transition observed in sporadic disease is reversible, whereas that observed in cases of exogenous contamination is irreversible. This model is consistent with the occurrence of rare, sporadic forms of prion diseases. It may also explain why only some individuals of a cohort develop a prion disease following transient food contamination.

Scrapie infectivity is independent of amyloid staining properties of the N-terminally truncated prion protein

The prion protein undergoes a profound conformational change when the cellular isoform (PrP(C)) is converted into the disease-causing form (PrP(Sc)). Limited proteolysis of PrP(Sc) produces PrP 27-30, which readily polymerizes into amyloid. To study the relationship between PrP amyloid and infectivity, we employed organic solvents that perturb protein conformation. Hexafluoro-2-propanol (HFIP), which promotes alpha-helix formation, modified the ultrastructure of PrP amyloid and decreased the beta-sheet content as well as prion infectivity. HFIP reversibly decreased the binding of Congo red dye to the PrP amyloid rods while inactivation of prion infectivity was irreversible. In contrast, 1,1,1-trifluoro-2-propanol (TFIP) did not inactivate prion infectivity but like HFIP, TFIP did alter the morphology of the rods and abolished Congo red binding. Solubilization using various solvents and detergents produced monomeric and dimeric PrP that lacked infectivity. Proteinase K resistance of detergent-treated PrP 27-30 showed no correlation with scrapie infectivity. Our results separate prion infectivity from the amyloid properties of PrP 27-30 and underscore the dependence of prion infectivity on PrP(Sc) conformation. These findings also demonstrate that the specific beta-sheet-rich structures required for prion infectivity can be differentiated from those required for amyloid formation.

Microglial activation varies in different models of Creutzfeldt-Jakob disease

Progressive changes in host mRNA expression can illuminate crucial pathogenetic pathways in infectious disease. We examined general and specific approaches to mRNA expression in three rodent models of Creutzfeldt-Jakob disease (CJD). Each of these models displays distinctive neuropathology. Although mRNAs for the chemokine receptor CCR5, the lysosomal protease cathepsin S, and the pleiotropic cytokine transforming growth factor beta1 (TGF-beta1) were progressively upregulated in rodent CJD, the temporal patterns and peak magnitudes of each of these transcripts varied substantially among models. Cathepsin S and TGF-beta1 were elevated more than 15-fold in mice and rats infected with two different CJD strains, but not in CJD-infected hamsters. In rats, an early activation of microglial transcripts preceded obvious deposits of prion protein (PrP) amyloid. However, in each of the three CJD models, the upregulation of CCR5, cathepsin S, and TGF-beta1 was variable with respect to the onset of PrP pathology. These results show glial cell involvement varies as a consequence of the agent strain and species infected. Although neurons are generally assumed to be the primary sites for agent replication and abnormal PrP formation, microglia may be targeted by some agent strains. In such instances, microglia can both process PrP to become amyloid and can enhance neuronal destruction. Because microglia can participate in agent clearance, they may also act as chronic reservoirs of infectivity. Finally, the results here strongly suggest that TGF-beta1 can be an essential signal for amyloid deposition.

Ultrastructural studies on scrapie prion protein crystals obtained from reverse micellar solutions

The structural transition from the cellular prion protein (PrPC) that is rich in alpha-helices to the pathological form (PrPSc) that has a high beta-sheet content seems to be the fundamental event underlying the prion diseases. Determination of the structure of PrPSc and the N-terminally truncated PrP 27-30 has been complicated by their insolubility. Here we report the solubilization of PrP 27-30 through a system of reverse micelles that yields monomeric and dimeric PrP. Although solubilization of PrP 27-30 was not accompanied by any recognizable change in secondary structure as measured by FTIR spectroscopy, it did result in a loss of prion infectivity. The formation of small two- and three-dimensional crystals upon exposure to uranyl salts argues that soluble PrP 27-30 possesses considerable tertiary structure. The crystals of PrP 27-30 grown from reverse micellar solutions suggest a novel crystallization mechanism that might be applicable for other membrane proteins. A variety of different crystal lattices diffracted up to 1.85 nm by electron microscopy. Despite the lack of measurable biological activity, the structure of PrP 27-30 in these crystals may provide insight into the structural transition that occurs during PrPSc formation.

Evolution of a strain of CJD that induces BSE-like plaques

Bovine spongiform encephalopathy (BSE) has become a public health issue because a recently evolved BSE agent has infected people, yielding an unusual form of Creutzfeld-Jakob disease (CJD). A new CJD agent that provokes similar amyloid plaques and cerebellar pathology was serially propagated. First-passage rats showed obvious clinical signs and activated microglia but had negligible PrP-res (the more protease-resistant form of host PrP) or cerebellar lesions. Microglia and astrocytes may participate in strain selection because the agent evolved, stabilized, and reproducibly provoked BSE-like disease in subsequent passages. Early vacuolar change involving activated microglia and astrocytes preceded significant PrP-res accumulation by more than 50 days. These studies reveal several inflammatory host reactions to an exogenous agent.

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.

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.

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

Mice, homozygous for prion protein (PrP) gene ablation (Prn-p0/0), develop normally and remain well > 500 days after inoculation with murine scrapie prions. In contrast, wild-type mice developed scrapie < 165 days after inoculation and most Prn-p0/+ mice, heterozygous for disruption of the PrP gene, exhibited signs of central nervous system dysfunction between 400 and 465 days after inoculation. In situ immunoblots showed widespread deposition of scrapie PrP (PrPSc) in the brains of both wild-type Prn-p+/+ and Prn-p0/+ mice, while neither cellular PrP (PrPC) nor PrPSc was detected in the brains of Prn-p0/0 mice. In contrast to Prn-p+/+ and Prn-p0/+ mice, Prn-p0/0 mice failed to propagate prion infectivity as measured by bioassays. Syrian hamster (SHa) PrP transgenes rendered Prn-p0/0 mice susceptible to prions containing SHaPrPSc. Immunization of Prn-p0/0 mice with purified, infectious mouse or SHa prions dispersed in Freund's adjuvant produced antisera that bound mouse, SHa, and human PrP on Western blots. Presumably, the lack of PrPC expression in Prn-p0/0 mice prevents them from becoming tolerant to the immunogen. The resistance of Prn-p0/0 mice to developing scrapie after inoculation with murine prions supports the hypothesis that PrPSc is essential for both transmission and pathogenesis of the prion diseases.

Protected endogenous retroviral sequences copurify with infectivity in experimental Creutzfeldt-Jakob disease

Scrapie and Creutzfeldt-Jakob disease (CJD) are caused by infectious agents that are defined phenomenologically. No agent-specific molecules or particles have been identified. Biological properties, such as exponential agent replication and strain variation, as well as physical characteristics of infectivity indicate a protected viral structure. A host membrane glycoprotein of 34 kDa ("prion" protein) that aggregates at end stages of disease is clearly important in pathology and susceptibility to infection, but has no demonstrable infectivity in any purified or recombinant form. Thus a characterization of more viral-like molecules is important. In order to identify viral-like nucleic acids we previously developed methods to substantially purify the human CJD agent from experimentally infected hamster brains, and demonstrated selected retroviral-like LTR bands at pg levels that were insufficient for sequencing. To further define these and other viral-like sequences we cloned nucleic acids from highly infectious CJD fractions, and tested the efficacy of our methods using a selected retroviral probe. RNA extracted from an infectious 120 S Gaussian peak, which is reproducibly purified approximately 100,000 fold with respect to starting nucleic acids, and contains approximately 20% of the initial brain infectivity, was used to generate a cDNA library in a sequence independent amplification strategy for low levels of RNA (< 6 ng). Reconstituted strong stop experiments using several retroviral tRNA primers had indicated that Syrian hamster IAP (SHIAP) sequences should be present in both CJD and uninfected control fractions. Because SHIAP particles are extremely resistant to denaturation, their representation in a cDNA library would imply adequate extraction of other protected RNAs of viral origin. At least 900 bases of the Syrian hamster retroviral IAP genome were unambiguously identified in the cDNA library, and in independent PCR walks with selected primers, all of which were based on our cloned sequences. Sequencing confirmed the presence of protected LTR and adjacent retroviral motifs. Because these sequences were also present in control preparations they may represent normal endogenous viral contaminants that cosediment with infectivity in size and density gradients. On the other hand, LTRs can drive the expression of many diverse sequences, and it remains to be seen if CJD specific sequences are either transduced, or copackaged with, protected IAP complexes. The effective extraction and amplification of highly protected SHIAP nucleic acids of significant length sets the stage for identifying additional protected viral elements that may specify the CJD agent.

A transmissible Creutzfeldt-Jakob disease-like agent is prevalent in the human population

The etiology of most human dementias is unknown. Creutzfeldt-Jakob disease (CJD), a relatively uncommon human dementia, is caused by a transmissible virus-like agent. Molecular markers that are specific for the agent have not yet been defined. However, the infectious disease can be transmitted to rodents from both brain and infected buffy coat (blood) samples. To determine whether human CJD infections are more widespread than is apparent from the low incidence of neurological disease, we attempted to transmit CJD from buffy coat samples of 30 healthy volunteers who had no family history of dementing illness. Primary transmissions from 26 of 30 individuals produced CJD-like spongiform changes in the brains of recipient hamsters at 200-500 days postinoculation. This positive evidence of viremia was found for individuals in all age groups (20-30, 40-50, and 61-71 years old), whereas 12 negatively scored brain samples failed to produce similar changes in hamsters observed for > 900 days in the same setting. We suggest that a CJD agent endemically infects humans but only infrequently produces an infectious dementia. Disease expression is likely to be influenced by several host factors in combination with viral variants that have altered neurovirulence.

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

The nature of the infectious agent causing human Creutzfeldt-Jakob disease (CJD), a slowly progressive dementia, is controversial. As in scrapie, no agent-specific proteins or nucleic acids have been identified. However, biological features of exponential replication and agent strain variation, as well as physical size and density data, are most consistent with a viral structure--i.e., a nucleic acid-protein complex. It is often assumed that nuclease treatment, which does not reduce infectious titer, leaves no nucleic acids of > 50 bp. However, nucleic acids of 500-6000 bp can be extracted from highly purified infectious complexes with a mass of approximately 1.5 x 10(7) daltons. It was therefore germane to search for nucleic acid binding proteins that might protect an agent genome. We here use Northwestern blotting to show that there are low levels of nonhistone nucleic acid binding proteins in highly purified infectious 120S gradient fractions. Several nucleic acid binding proteins were clearly host encoded, whereas others were apparent only in CJD, but not in parallel preparations from uninfected brain. Small amounts of residual host Gp34 (prion protein) did not bind any 32P-labeled nucleic acid probes. Most of the minor "CJD-specific" proteins had an acidic pI, a characteristic of many viral core proteins. Such proteins deserve further study, as they probably contribute to unique properties of resistance described for these agents. It remains to be seen if any of these proteins are agent encoded.

Field-flow fractionation: analysis of macromolecular, colloidal, and particulate materials

Field-flow fractionation (FFF) is a family of flexible elution techniques capable of simultaneous separation and measurement. Its sample domain extends across a broad macromolecular-colloidal-particulate continuum from about 1 nanometer to more than 100 micrometers and incorporates both simple and complex macromaterials of biological, biomedical, industrial, and environmental relevance. Complex materials are separated into components to simplify measurement. Component properties measurable by FFF include mass, size, density, charge, diffusivity, and thickness of adsorbed layers. When characterization by these properties is inadequate, other measurement tools can be readily coupled to FFF, either off-line or on-line, by virtue of its flow-elution operation. This article describes the principles and major subtechniques of the FFF family along with application of its measurement and separative capabilities.