Immortality of cell cultures derived from brains of mice and hamsters infected with Creutzfeldt-Jakob disease agent

Proliferative arrest induces neuron differentiation and innate immune responses in control and Creutzfeldt-Jakob Disease agent infected rat septal neurons

Rat post-mitotic septal (SEP) neurons, engineered to conditionally proliferate at 33°C, differentiate when arrested at 37.5°C and can be maintained for weeks without cytotoxic effects. Nine independent cDNA libraries were made to follow arrest-induced neural differentiation and innate immune responses in normal (Nl) uninfected and CJ agent infected SEP cells. Proliferating Nl versus latently infected (CJ-) cells showed few RNA-seq differences. However arrest induced major changes. Normal cells displayed a plethora of anti-proliferative transcripts. Additionally, known neuron differentiation transcripts, e.g., Agtr2, Neuregulin-1, GDF6, SFRP4 and Prnp were upregulated. These Nl neurons also displayed many activated IFN innate immune genes, e.g., OAS1, RTP4, ISG20, GTB4, CD80 and cytokines, complement, and clusterin (CLU) that binds to misfolded proteins. In contrast, arrested highly infectious CJ+ cells (10 logs/gm) downregulated many replication controls. Furthermore, arrested CJ+ cells suppressed neuronal differentiation transcripts, including Prnp which is essential for CJ agent infection. CJ+ cells also enhanced IFN stimulated pathways, and analysis of the 342 CJ+ unique transcripts revealed additional innate immune and anti-viral-linked transcripts, e.g., Il17, ISG15, and RSAD2 (viperin). These data show: 1) innate immune transcripts are produced by normal neurons during differentiation; 2) CJ infection can enhance and expand anti-viral responses; 3) latent CJ infection epigenetically imprints many proliferative pathways to thwart complete arrest. CJ+ brain microglia, white blood cells and intestinal myeloid cells with shared transcripts may be stimulated to educe latent CJD infections that can be clinically silent for >30 years.

Reduced Expression of Prion Protein With Increased Interferon-β Fail to Limit Creutzfeldt-Jakob Disease Agent Replication in Differentiating Neuronal Cells

Immortalized uninfected septal (SEP) neurons proliferate but after physiological mitotic arrest they express differentiated neuronal characteristics including enhanced cell-to-cell membrane contacts and ≥ 8 fold increases in host prion protein (PrP). We compared proliferating uninfected and Creutzfeldt-Jakob Disease (CJD) agent infected cells with their arrested counterparts over 33 days by quantitative mRNA and protein blot analyses. Surprisingly, uninfected arrested cells increased interferon-β (IFN-β) mRNA by 2.5–8 fold; IFN-β mRNA elevations were not previously associated with neuronal differentiation. SEP cells with high CJD infectivity titers produced a much larger 40–68-fold increase in IFN-β mRNA, a classic host anti-viral response that is virucidal for RNA but not DNA viruses. High titers of CJD agent also induced dramatic decreases in host PrP, a protein needed for productive agent replication. Uninfected arrested cells produced large sustained 20–30-fold increases in PrP mRNA and protein, whereas CJD arrested cells showed only transient small 5-fold increases in PrP. A > 10-fold increase in infectivity, but not PrP misfolding, induced host PrP reductions that can limit CJD agent replication. In contrast to neuronal lineage cells, functionally distinct migratory microglia with high titers of CJD agent do not induce an IFN-β mRNA response. Because they have 1/50th of PrP of an average brain cell, microglia would be unable to produce the many new infectious particles needed to induce a large IFN-β response by host cells. Instead, microglia and related cells can be persistent reservoirs of infection and spread. Phase separations of agent-associated molecules in neurons, microglia and other cell types can yield new insights into the molecular structure, persistent, and evasive behavior of CJD-type agents.

Transmissibility of Gerstmann-Sträussler-Scheinker syndrome in rodent models: New insights into the molecular underpinnings of prion infectivity

Prion diseases, or transmissible spongiform encephalopathies, have revealed the bewildering phenomenon of transmissibility in neurodegenerative diseases. Hence, the experimental transmissibility of prion-like neurodegenerative diseases via template directed misfolding has become the focus of intense research. Gerstmann-Sträussler-Scheinker disease (GSS) is an inherited prion disease associated with mutations in the prion protein gene. However, with the exception of a few GSS cases with P102L mutation characterized by co-accumulation of protease-resistant PrP core (PrP^res) of ∼21 kDa, attempts to transmit to rodents GSS associated to atypical misfolded prion protein with ∼8 kDa PrP^res have been unsuccessful. As a result, these GSS subtypes have often been considered as non-transmissible proteinopathies rather than true prion diseases. In a recent study we inoculated bank voles with GSS cases associated with P102L, A117V and F198S mutations and found that they transmitted efficiently and produced distinct pathological phenotypes, irrespective of the presence of 21 kDa PrP^res in the inoculum. This study demonstrates that GSS is a genuine prion disease characterized by both transmissibility and strain variation. We discuss the implications of these findings for the understanding of the heterogeneous clinic-pathological phenotypes of GSS and of the molecular underpinnings of prion infectivity.

Fukuoka-1 strain of transmissible spongiform encephalopathy agent infects murine bone marrow-derived cells with features of mesenchymal stem cells

BACKGROUND

The possible risk of iatrogenic transmissible spongiform encephalopathies (TSEs, prion diseases) from transplantation of marrow-derived mesenchymal stem cells (MSCs) is uncertain. While most cell lines resist infection, a few propagate TSE agents.

STUDY DESIGN AND METHODS

We generated MSC-like (MSC-L) cell cultures from bone marrow (BM) of mice inoculated with the human-derived Fukuoka-1 (Fu) strain of TSE agent. Cultured cells were characterized for various markers and cellular prion protein (PrP(C) ) by fluorescence-activated cell sorting and for PrP(C) and its pathologic TSE-associated form (PrP(TSE) ) by Western blotting (WB). Cell cultures were tested for their susceptibility to infection with Fu in vitro. The infectivity of one Fu-infected cell culture was assayed in mice.

RESULTS

BM cells from Fu-infected mice expressed neither PrP(C) nor PrP(TSE) after 3 days in culture as demonstrated by WB. Cells adherent to plastic and maintained under two different culture conditions became spontaneously immortalized and began to express PrP(C) at about the same time. One culture became transformed shortly after exposure to Fu in vitro and remained persistently infected, continuously generating PrP(TSE) through multiple passages; the infectivity of cultured cells was confirmed by intracerebral inoculation of lysates into mice. Both persistently TSE-infected and uninfected cells expressed a number of typical MSC markers.

CONCLUSION

BM-derived MSC-L cells of mice became persistently infected with the Fu agent under certain conditions in culture-conditions that differ substantially from those currently used to develop investigational human stem cell therapies.

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.

Potential involvement of retroviral elements in human dementias

Creutzfeldt-Jakob disease (CJD) is a dementia of humans caused by a class of infectious agents with several biological properties similar to those of conventional viruses. The molecular nature of this group of agents is enigmatic, for neither an agent-specific nucleic acid nor a non-host protein has yet been identified. Recent transmissions of familial CJD dementias to rodents suggest that this class of agent can be integrated into the germline. Furthermore, tissue culture studies indicate that CJD causes transformation of cells in a manner reminiscent of slowly oncogenic retroviruses. Currently characterized retroviral-like elements include many forms that do not have 'typical' retroviral ultrastructural morphology; several forms are also known to be resistant to various types of standard physicochemical inactivation. We suggest that CJD agents are either constituted by retroviral-like nucleic acids or interact with endogenous retroviral sequences to elicit a slowly progressive disease of the central nervous system. Several overlapping properties between infectious CJD and 'non-infectious' dementias, such as Alzheimer's disease, implicate potential common pathogenic mechanisms.

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.

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

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

Hereditary prion protein amyloidoses

Prion protein (PrP) amyloid accumulation is the pathologic hallmark of some inherited prion diseases such as Gerstmann-Sträussler-Scheinker disease (GSS) and PrP cerebral amyloid angiopathy (PrP-CAA). In GSS, parenchymal amyloidosis may coexist with spongiform degeneration or neurofibrillary tangles, whereas in PrP-CAA, vascular amyloid coexists with neurofibrillary tangles. In GSS, N-truncated and C-truncated proteinase K-resistant PrP isoforms are present in the brain.

Changes in N-acetylaspartate and myo-inositol detected in the cerebral cortex of hamsters with Creutzfeldt-Jakob disease

The levels of several low-molecular-weight metabolites were measured in 1H nuclear magnetic resonance (NMR) spectra of extracts of Syrian hamster brain infected with Creutzfeldt-Jakob disease (CJD). Metabolite levels were determined in cerebral cortex in CJD-infected and age-matched controls at defined times (40, 65, 85, 105, and 135 days) during the 130- to 135-day incubation period to terminal disease. At 135 days, CJD-infected hamsters showed a significant decrease in N-acetylaspartate of 32% (p < 0.05) and an increase in myo-inositol of 67% (p < 0.001) from age-matched controls. At earlier times (40 to 110 days) levels of N-acetylaspartate and myo-inositol were not significantly different from controls. No significant changes were detected in the cortical levels of glutamate, aspartate, or GABA between 40 and 135 days. The late changes in N-acetylaspartate and myo-inositol in CJD-infected hamsters are similar to those observed in magnetic resonance spectroscopy studies of human CJD. Because they also correspond to the changes found in other dementias, including Alzheimer's disease and HIV dementia, these changes indicate converging pathogenetic pathways involved in many neurodegenerative diseases.

Prion protein amyloidosis

The prion protein (PrP) plays an essential role in the pathogenesis of a group of sporadic, genetically determined and infectious fatal degenerative diseases, referred to as "prion diseases", affecting the central nervous system of humans and other mammals. The cellular PrP is encoded by a single copy gene, highly conserved across mammalian species. In prion diseases, PrP undergoes conformational changes involving a shift from alpha-helix to beta-sheet structure. This conversion is important for PrP amyloidogenesis, which occurs to the highest degree in the genetically determined Gerstmann-Sträussler-Scheinker disease (GSS) and prion protein cerebral amyloid angiopathy (PrP-CAA), while it is less frequently seen in other prion diseases. GSS and PrP-CAA are associated with point mutations of the prion protein gene (PRNP); these conditions show a broad spectrum of clinical presentation, the main signs being ataxia, spastic paraparesis, extrapyramidal signs and dementia. In GSS, parenchymal amyloid may be associated with spongiform changes or neurofibrillary lesions; in PrP-CAA, vascular amyloid is associated with neurofibrillary lesions. A major component of the amyloid fibrils in the two diseases is a 7 kDa peptide, spanning residues 81-150 of PrP.

Gerstmann-Sträussler-Scheinker disease and the Indiana kindred

Gerstmann-Sträussler-Scheinker disease is an autosomal dominant disorder with a wide spectrum of clinical presentations including ataxia, spastic paraparesis, extrapyramidal signs, and dementia. The patients present with symptoms in the third to sixth decade of life and the mean duration of illness is five years. Mutations at codons 102, 105, 117, 145, 198 and 217 of the open reading frame of the prion protein gene have been associated with GSS disease. As a result of the mutations, a substitution at the corresponding residues of the prion protein occurs, or as in the case of the STOP mutation at codon 145, a truncated protein is produced. Neuropathologically, the common denominator is a cerebral prion protein amyloidosis; however, there is significant variability in the pattern of amyloid deposition in regions of the central nervous system among reported families. Amyloidosis coexists with severe spongiform degeneration in patients with the mutation at codon 102, and with neurofibrillary degeneration in the patients with mutation at codons 145, 198 and 217. The development of a transmissible spongiform encephalopathy in animals inoculated with brain tissue from affected subjects with mutation at codon 102 suggests that in some forms of genetically-determined Gerstmann-Sträussler-Scheinker disease, and particularly those characterized by severe spongiosis, amyloidogenesis and production of an infectious "agent" occur concomitantly via mechanisms that are only partially understood.

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.

Integration site preferences of endogenous retroviruses

Retroviruses have the ability to integrate into the genome of their host, in many cases with little apparent sequence or site specificity. However, relatively few studies have addressed more general features of chromosomal integration. In this study we directly visualized the chromosomal organization of three representative endogenous retroviruses by in situ hybridization. Because there are 50-1000 copies of each of these retroviruses in the genome, it was possible to evaluate repeated integration events. Each retroviral sequence exhibited a unique and markedly different integration pattern. In order to characterize more precisely the chromosomal domains targeted by each retrovirus, later replicating domains were differentially labeled. Additionally, prototypic SINES and LINES (short and long interspersed reiterated sequences), which are inhomogeneously distributed on chromosome arms, were simultaneously detected. Retroviral copies of greater than or equal to 2 kb were found (i) exclusively in a discrete set of later replicating domains, most of which have the staining characteristics of constitutive heterochromatin, (ii) widely represented in disparate types of chromosome domains, or (iii) almost completely confined to CpG Alu-rich regions that are known to be early replicating. Retroviral elements in Alu-rich domains would be expected to be actively transcribed in all cells. Surprisingly, hybridization to blots of brain RNA showed an approximately 25 fold lower level of transcripts from these Alu associated elements than from retroviral sequences restricted to later replicating, heterochromatic domains. Retroviral insertions may subvert more typical transcriptional characteristics of a domain. The present results indicate that there are highly specific integration patterns for each endogenous retrovirus that do not readily relate to their sequence or particle classification. Each host genome may utilize these elements for contrary, and possibly beneficial functions.

Potential retroviral RNAs in Creutzfeldt-Jakob disease

The molecular nature of the related infectious agents that cause Creutzfeldt-Jakob disease (CJD) and scrapie is poorly understood, and an agent-specific nucleic acid genome has not yet been identified. Several biological manifestations of these agents resemble those seen in retrovirus-induced diseases. We therefore attempted to identify an agent-specific retrovirus-like RNA transcript in CJD infectious fractions. A series of synthetic oligonucleotides complementary to known mammalian retroviral primer binding sites were used in a primer extension assay. Substrate nucleic acids isolated from partially purified hamster brain CJD infectious fractions and from parallel normal brain fractions were compared with total starting brain RNA. This sensitive exogenous strong-stop reaction revealed that CJD infectious fractions contained a series of potential retroviral RNAs including apparent transcripts of endogenous hamster IAP genes. Most transcripts selectively recovered in the fractions were substantially protected from micrococcal nuclease digestion, and at least one substrate RNA, consistent with an intracisternal A particle, was packaged in a form that had the same buoyant density as CJD infectivity. Although a completely CJD-specific transcript was not identified, the copurification of potential retroviral transcripts with CJD infectivity suggests that models of disease involving retrovirus-like nucleic acid elements deserve further consideration.

Growth factor production by Creutzfeldt-Jakob disease cell lines

Creutzfeldt-Jakob disease (CJD), a progressive dementia of humans, is caused by an infectious agent that is closely related to the scrapie agent of sheep. Although the molecular nature of these "unconventional" agents is still a matter of speculation and controversy, even less is known concerning the mechanism(s) of their effects on the central nervous system. To gain insight into the cellular effects of these agents, we have examined a series of cell lines derived directly from CJD-infected hamster brain or produced from nontransformed rodent lines by exposure to CJD infectious fractions in vitro. These cell lines appear transformed by a variety of criteria and secrete growth factors into the culture medium. All CJD lines produce a factor that is like alpha-transforming growth factor (alpha-TGF). Conditioned medium from these CJD lines also stimulates the synthesis of glial fibrillary acidic protein in normal astrocytic cells in vitro. This effect is mimicked by purified alpha-TGF and platelet-derived growth factors. Further study of CJD-induced growth factor production may elucidate fundamental properties of these unconventional agents.

Astrocyte gene expression in Creutzfeldt-Jakob disease

Gliosis (hyperplasia and hypertrophy of astrocytes), the fundamental response of the central nervous system to tissue destruction, typically becomes apparent only several weeks after injury. The biochemical hallmark of this response is a marked accumulation of the specific astrocyte intermediate filament glial fibrillary acidic protein (GFAP). To date despite its importance, the mechanisms of GFAP gene regulation have not been studied in any developmental or pathological system to our knowledge, and the molecular signals for GFAP mRNA and protein accumulation are not defined. In Creutzfeldt-Jakob disease, a progressive dementing illness caused by an "unconventional agent," we find steadily increasing elevations of GFAP mRNA throughout the later stages of disease, using two independent GFAP cDNA clones, representing the entire insert or the 3'-noncoding region (pScr-1). The accumulation of GFAP, assessed immunocytochemically, follows GFAP mRNA elevation. A 5-fold stimulation of GFAP gene expression precedes the development of florid histologic lesions in the cerebrum, and in the cerebellum 5- to 6-fold increases occurred with no detectable spongiform changes at any time during disease. Therefore, these GFAP changes cannot be simply a response to neuronal damage. These effects are directly or indirectly caused by high local concentrations of agent and possibly involve a humoral factor.