Nuclease-resistant polyadenylated RNAs of significant size are detected by PCR in highly purified Creutzfeldt-Jakob disease preparations

Proteomics applications in prion biology and structure

Prion diseases are a heterogeneous class of fatal neurodegenerative disorders associated with misfolding of host cellular prion protein (PrP(C)) into a pathological isoform, termed PrP(Sc). Prion diseases affect various mammals, including humans, and effective treatments are not available. Prion diseases are distinguished from other protein misfolding disorders - such as Alzheimer's or Parkinson's disease - in that they are infectious. Prion diseases occur sporadically without any known exposure to infected material, and hereditary cases resulting from rare mutations in the prion protein have also been documented. The mechanistic underpinnings of prion and other neurodegenerative disorders remain poorly understood. Various proteomics techniques have been instrumental in early PrP(Sc) detection, biomarker discovery, elucidation of PrP(Sc) structure and mapping of biochemical pathways affected by pathogenesis. Moving forward, proteomics approaches will likely become more integrated into the clinical and research settings for the rapid diagnosis and characterization of prion pathogenesis.

Uptake and degradation of protease-sensitive and -resistant forms of abnormal human prion protein aggregates by human astrocytes

Sporadic Creutzfeldt-Jakob disease is the most common of the human prion diseases, a group of rare, transmissible, and fatal neurologic diseases associated with the accumulation of an abnormal form (PrP(Sc)) of the host prion protein. In sporadic Creutzfeldt-Jakob disease, disease-associated PrP(Sc) is present not only as an aggregated, protease-resistant form but also as an aggregated protease-sensitive form (sPrP(Sc)). Although evidence suggests that sPrP(Sc) may play a role in prion pathogenesis, little is known about how it interacts with cells during prion infection. Here, we show that protease-sensitive abnormal PrP aggregates derived from patients with sporadic Creutzfeldt-Jakob disease are taken up and degraded by immortalized human astrocytes similarly to abnormal PrP aggregates that are resistant to proteases. Our data suggest that relative proteinase K resistance does not significantly influence the astrocyte's ability to degrade PrP(Sc). Furthermore, the cell does not appear to distinguish between sPrP(Sc) and protease-resistant PrP(Sc), suggesting that sPrP(Sc) could contribute to prion infection.

A specific population of abnormal prion protein aggregates is preferentially taken up by cells and disaggregated in a strain-dependent manner

Prion diseases are characterized by the conversion of the soluble protease-sensitive host-encoded prion protein (PrP(C)) into its aggregated, protease-resistant, and infectious isoform (PrP(Sc)). One of the earliest events occurring in cells following exposure to an exogenous source of prions is the cellular uptake of PrP(Sc). It is unclear how the biochemical properties of PrP(Sc) influence its uptake, although aggregate size is thought to be important. Here we show that for two different strains of mouse prions, one that infects cells (22L) and one that does not (87V), a fraction of PrP(Sc) associated with distinct sedimentation properties is preferentially taken up by the cells. However, while the fraction of PrP(Sc) and the kinetics of uptake were similar for both strains, PrP(Sc) derived from the 87V strain was disaggregated more rapidly than that derived from 22L. The increased rate of PrP(Sc) disaggregation did not correlate with either the conformational or aggregate stability of 87V PrP(Sc), both of which were greater than those of 22L PrP(Sc). Our data suggest that the kinetics of disaggregation of PrP(Sc) following cellular uptake is independent of PrP(Sc) stability but may be dependent upon some component of the PrP(Sc) aggregate other than PrP. Rapid disaggregation of 87V PrP(Sc) by the cell may contribute, at least in part, to the inability of 87V to infect cells in vitro.

In situ photodegradation of incorporated polyanion does not alter prion infectivity

Single-stranded polyanions ≥40 bases in length facilitate the formation of hamster scrapie prions in vitro, and polyanions co-localize with PrP^Sc aggregates in vivo[1], [2]. To test the hypothesis that intact polyanionic molecules might serve as a structural backbone essential for maintaining the infectious conformation(s) of PrP^Sc, we produced synthetic prions using a photocleavable, 100-base oligonucleotide (PC-oligo). In serial Protein Misfolding Cyclic Amplification (sPMCA) reactions using purified PrP^C substrate, PC-oligo was incorporated into physical complexes with PrP^Sc molecules that were resistant to benzonase digestion. Exposure of these nuclease-resistant prion complexes to long wave ultraviolet light (315 nm) induced degradation of PC-oligo into 5 base fragments. Light-induced photolysis of incorporated PC-oligo did not alter the infectivity of in vitro-generated prions, as determined by bioassay in hamsters and brain homogenate sPMCA assays. Neuropathological analysis also revealed no significant differences in the neurotropism of prions containing intact versus degraded PC-oligo. These results show that polyanions >5 bases in length are not required for maintaining the infectious properties of in vitro-generated scrapie prions, and indicate that such properties are maintained either by short polyanion remnants, other co-purified cofactors, or by PrP^Sc molecules alone.

Prions are unorthodox infectious agents whose composition remains undetermined. Previous experiments have shown that long, negatively charged polymers such as nucleic acid and carbohydrate molecules promote the formation of purified prions in test tube chemical reactions. Various classes of negatively charged polymers have also been found to co-exist within prion complexes in the brains of infected animals. These observations suggest that negatively charged polymers might act as a structural support necessary for prion infectivity. We tested this possibility by chemically synthesizing a negatively charged polymer that can be degraded by exposure to ultraviolet light. Prions containing this light-sensitive polymer remained infectious after light exposure, indicating that negatively charged polymers are not necessary to maintain the structural shapes of infectious prions.

Comparative profiling of highly enriched 22L and Chandler mouse scrapie prion protein preparations

Transmissible spongiform encephalopathies (TSEs) or prion diseases are characterized by the accumulation of an aggregated isoform of the prion protein (PrP). This pathological isoform, termed PrP(Sc), appears to be the primary component of the TSE infectious agent or prion. However, it is not clear to what extent other protein cofactors may be involved in TSE pathogenesis or whether there are PrP(Sc)-associated proteins which help to determine TSE strain-specific disease phenotypes. We enriched PrP(Sc) from the brains of mice infected with either 22L or Chandler TSE strains and examined the protein content of these samples using nanospray LC-MS/MS. These samples were compared with "mock" PrP(Sc) preparations from uninfected brains. PrP was the major component of the infected samples and ferritin was the most abundant impurity. Mock enrichments contained no detectable PrP but did contain a significant amount of ferritin. Of the total proteins identified, 32% were found in both mock and infected samples. The similarities between PrP(Sc) samples from 22L and Chandler TSE strains suggest that the non-PrP(Sc) protein components found in standard enrichment protocols are not strain specific.

Nanobacteria are mineralo fetuin complexes

“Nanobacteria” are nanometer-scale spherical and ovoid particles which have spurred one of the biggest controversies in modern microbiology. Their biological nature has been severely challenged by both geologists and microbiologists, with opinions ranging from considering them crystal structures to new life forms. Although the nature of these autonomously replicating particles is still under debate, their role in several calcification-related diseases has been reported. In order to gain better insights on this calciferous agent, we performed a large-scale project, including the analysis of “nanobacteria” susceptibility to physical and chemical compounds as well as the comprehensive nucleotide, biochemical, proteomic, and antigenic analysis of these particles. Our results definitively ruled out the existence of “nanobacteria” as living organisms and pointed out the paradoxical role of fetuin (an anti-mineralization protein) in the formation of these self-propagating mineral complexes which we propose to call “nanons.” The presence of fetuin within renal calculi was also evidenced, suggesting its role as a hydroxyapatite nucleating factor.

In the last decade, the exact nature of nanobacteria was one of the most controversial of scientific questions. An audacious theory proposed the existence of nanobacteria, initially discovered in Italian hot spring deposits, as a new life form responsible for a wide range of diseases in humans, thus qualifying them as new agents of emerging infectious diseases. The community of microbiologists remained therefore skeptical about the fact that such structures, 100 times smaller than bacteria and highly resistant to heat and other treatments that would normally kill the latter, could be living entities fully capable of self-replication. Other scientists wondered if they might be an unusual form of crystal rather than micro-organisms. The comprehensive characterization of nanobacteria was the focus of our study. Our results definitively ruled out the existence of nanobacteria as living entities and revealed that they correspond to self-propagating mineral-fetuin complexes that we called “nanons.”

The intriguing prion disorders

Prion diseases are among the most intriguing illnesses. Despite their rare incidence, they have captured enormous attention from the scientific community and general public. One of the most hotly debated issues in these diseases is the nature of the infectious material. In recent years increasing evidence has emerged supporting the protein-only hypothesis of prion transmission. In this model PrPSc (the pathological isoform of the prion protein, PrPC) represents the sole component of the infectious particle. However, uncertainties about possible additional factors involved in the conversion of PrPC into PrPSc remain despite extensive attempts to isolate and characterize these elusive components. In this article, we review recent developments concerning the protein-only hypothesis as well as the possible involvement of cellular factors in PrPC to PrPSc conformational change and their influence on the pathogenesis of prion diseases.

A critical review of the nature of the spongiform encephalopathy agent: protein theory versus virus theory

All spongiform encephalopathies (SEs) result in brain disorders brought about by a slow virus. Since the origin of bovine SE (BSE), the infectious nature of the disease has been firmly established. Tubulofilamentous particles/scrapie termed nemavirus (NVP) and scrapie-associated fibrils (SAF) are ultrastructural markers, whereas protease-resistant protein (PrP(sc)) is a protein marker. The PrP molecules aggregate to form SAF. Each NVP consists of three layers: an outer protein coat, an intermediate ssDNA layer, and inner PrP/SAF. Therefore, ssDNA and PrP/SAF are physically associated with each other. The existence of at least 20 stable strains of SEs implies that a nucleic acid molecule serves as the information molecule. Animals inoculated with PrP(sc) do not develop the clinical disease, however, ssDNA purified from scrapie-hamster brains by alkaline gel electrophoresis mixed with binding proteins before inoculation developed the clinical disease. It appears that an "accessory protein" coded by the ssDNA of the NVP interacts with normal PrP(c) molecules, resulting in their conversion to PrP(sc)/SAF. The pathogenesis process in the infected animal, with increasing incubation periods, reveals that larger amounts of normal PrP molecules are modified to form SAF. This interferes with the normal supply of PrP to cell membranes, which become disrupted and eventually fragment, resulting in the vacuoles typical of those found in the SEs. Critical review of scientific literature has demonstrated that the agent contains a DNA genome.

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.

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

Although slow and persistent viruses often escape host defenses infection may be prevented by live vaccines. To determine whether an attenuated "slow" strain of the Creutzfeldt-Jakob disease agent (SY) could block expression of a virulent "fast" strain (FU), outbred CD-1 mice were inoculated intracerebrally with low infectious doses of SY and challenged 80 days later with higher doses of FU. For comparison, the same SY and FU samples were inoculated in two parallel control groups. All 18 superinfected mice showed incubation times identical to those inoculated with only the SY strain, yielding clinical disease >110 days later than predicted for the FU strain. Neurological signs, such as scratching and an extended clinical phase, were also characteristic for SY but not FU infection. Moreover, the widespread cortical pathology of FU was not detectable in superinfected mice. Western blot analyses further showed no strain-specific differences in prion protein (PrP) band profiles for all experimental groups, although there was approximately 10-fold more protease-resistant PrP (PrP-res) in FU brains during terminal disease. In contrast, infectivity assays revealed an approximately 10,000-fold difference between SY and FU at terminal stages, indicating that PrP-res content does not correlate with infectivity. In summary, an attenuated strain of the Creutzfeldt-Jakob disease agent evokes substantial interference against a virulent agent. Because superinfected mice had little PrP-res just before the onset of clinical disease and retained abundant cellular PrP, cellular PrP was not the factor limiting FU replication. The mechanisms underlying SY interference are not understood but could be based on host recognition of foreign molecular features shared by this class of invasive agents involving antibody production, and possibly involve defective viral particles produced by attenuated variants.

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.

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.

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.

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.

Transgenetic investigations of prion diseases of humans and animals

Prions cause transmissible and genetic neurodegenerative diseases. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc), which is encoded by a chromosomal gene. Although the PrP gene is single copy, transgenic mice with both alleles of the PrP gene ablated develop normally. A post-translational process, as yet unidentified, converts the cellular prion protein (PrPC) into PrPSc. Scrapie incubation times, neuropathology and prion synthesis in transgenic mice are controlled by the PrP gene. Mutations in the PrP gene are genetically linked to development of neurodegeneration. Transgenic mice expressing mutant PrP spontaneously develop neurological dysfunction and spongiform neuropathology. Investigations of prion diseases using transgenesis promise to yield much new information about these once enigmatic disorders.

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

Gel permeation chromatography and sedimentation field flow fractionation (SF3) were used to further analyze highly infectious fractions from Creutzfeldt-Jakob disease (CJD) infected hamster brain. These analyses defined the relative molecular mass and physical size of the Creutzfeldt-Jakob disease (CJD) agent with greater precision than previously possible. Highly purified disaggregated fractions yielded single, homogeneous Gaussian peaks with both methods. The relevant analytical peaks contained protein-nucleic acid complexes with an M(r) of approximately 1.5 x 10(7) daltons and a mean radius of approximately 30 nm. The experimental evidence further solidifies the concept of an infectious agent that resembles a viral core rather than a simple protein.

Relationship of protease-resistant protein, scrapie-associated fibrils and tubulofilamentous particles to the agent of spongiform encephalopathies

Tubulofilamentous particles and scrapie-associated fibrils (SAF) are ultrastructural markers, while protease-resistant protein (PrP) is a molecular biological marker for all spongiform encephalopathies. Review of all published work has suggested that PrP molecules aggregate to form a three-dimensional SAF. Further reports have suggested that a single-stranded DNA wraps round SAF and acquires an outer protein coat to form tubulofilamentous particles. As incubation period increases in the infected animals, larger amounts of PrP molecules are committed to form SAF, interfering with the normal supply of PrP to cell membranes which become disrupted and eventually fragment, resulting in vacuoles typical of those found in spongiform encephalopathies.

Molecular biology and pathology of scrapie and the prion diseases of humans

Scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans are transmissible and genetic neurodegenerative diseases caused by prions. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein which is encoded by a chromosomal gene. An as yet unidentified post-translational process converts the cellular prion protein into an abnormal isoform. Scrapie neuropathology, incubation times, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neurodegeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Studies of prion diseases may advance investigations of other neurodegenerative disorders and of how neurons differentiate, function for decades and grow senescent.

Molecular biology of prion diseases

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.