Cultured peripheral neuroglial cells are highly permissive to sheep prion infection

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

Anti-prion Drugs Targeting the Protein Folding Activity of the Ribosome Reduce PABPN1 Aggregation

Prion diseases are caused by the propagation of PrPSc, the pathological conformation of the PrPC prion protein. The molecular mechanisms underlying PrPSc propagation are still unsolved and no therapeutic solution is currently available. We thus sought to identify new anti-prion molecules and found that flunarizine inhibited PrPSc propagation in cell culture and significantly prolonged survival of prion-infected mice. Using an in silico therapeutic repositioning approach based on similarities with flunarizine chemical structure, we tested azelastine, duloxetine, ebastine, loperamide and metixene and showed that they all have an anti-prion activity. Like flunarizine, these marketed drugs reduced PrPSc propagation in cell culture and in mouse cerebellum organotypic slice culture, and inhibited the protein folding activity of the ribosome (PFAR). Strikingly, some of these drugs were also able to alleviate phenotypes due to PABPN1 nuclear aggregation in cell and Drosophila models of oculopharyngeal muscular dystrophy (OPMD). These data emphasize the therapeutic potential of anti-PFAR drugs for neurodegenerative and neuromuscular proteinopathies.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

Prion infection, transmission, and cytopathology modeled in a low-biohazard human cell line

Expanding the toolbox of prion research to a low-biohazard, scalable human cell model.

Transmission of prion infectivity to susceptible murine cell lines has simplified prion titration assays and has greatly reduced the need for animal experimentation. However, murine cell models suffer from technical and biological constraints. Human cell lines might be more useful, but they are much more biohazardous and are often poorly infectible. Here, we describe the human clonal cell line hovS, which lacks the human PRNP gene and expresses instead the ovine PRNP VRQ allele. HovS cells were highly susceptible to the PG127 strain of sheep-derived murine prions, reaching up to 90% infected cells in any given culture and were maintained in a continuous infected state for at least 14 passages. Infected hovS cells produced proteinase K–resistant prion protein (PrP^Sc), pelletable PrP aggregates, and bona fide infectious prions capable of infecting further generations of naïve hovS cells and mice expressing the VRQ allelic variant of ovine PrP^C. Infection in hovS led to prominent cytopathic vacuolation akin to the spongiform changes observed in individuals suffering from prion diseases. In addition to expanding the toolbox for prion research to human experimental genetics, the hovS cell line provides a human-derived system that does not require human prions. Hence, the manipulation of scrapie-infected hovS cells may present fewer biosafety hazards than that of genuine human prions.

An astrocyte cell line that differentially propagates murine prions

Prion diseases are fatal infectious neurodegenerative disorders in human and animals caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. Elucidating the molecular and cellular mechanisms underlying prion propagation may help to develop disease interventions. Cell culture systems for prion propagation have greatly advanced molecular insights into prion biology, but translation of in vitro to in vivo findings is often disappointing. A wider range of cell culture systems might help overcome these shortcomings. Here, we describe an immortalized mouse neuronal astrocyte cell line (C8D1A) that can be infected with murine prions. Both PrP^C protein and mRNA levels in astrocytes were comparable with those in neuronal and non-neuronal cell lines permitting persistent prion infection. We challenged astrocytes with three mouse-adapted prion strains (22L, RML, and ME7) and cultured them for six passages. Immunoblotting results revealed that the astrocytes propagated 22L prions well over all six passages, whereas ME7 prions did not replicate, and RML prions replicated only very weakly after five passages. Immunofluorescence analysis indicated similar results for PrP^Sc. Interestingly, when we used prion conversion activity as a readout in real-time quaking-induced conversion assays with RML-infected cell lysates, we observed a strong signal over all six passages, comparable with that for 22L-infected cells. These data indicate that the C8D1A cell line is permissive to prion infection. Moreover, the propagated prions differed in conversion and proteinase K–resistance levels in these astrocytes. We propose that the C8D1A cell line could be used to decipher prion strain biology.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Biological and biochemical characterization of M2B cells: Classical BSE prion is conserved in transgenic mice overexpressing bovine prion protein gene

M2B cells with persistent classical bovine spongiform encephalopathy (C-BSE) have been established previously. In this study, we performed strain characterization of the M2B cell line in bovine PrP^C overexpressing mice (Tg 1896). Mice intracranially inoculated with M2B cells and C-BSE survived for 451 ± 7 and 465 ± 31 d post inoculation, respectively. Although biochemical properties, including deglycosylation and conformational stability, differed between M2B cells and C-BSE, inoculation with M2B cell lysate and C-BSE resulted in comparable phenotypes. Comparable vacuolation scores and PrP^Sc depositions were observed in the brain of Tg 1896 inoculated with both M2B cell lysate and C-BSE. Our results show that biochemical and biological characteristics of M2B cells and C-BSE are classifiable in the same strain.

Chronic wasting disease prion infection of differentiated neurospheres

A possible strategy to develop more diverse cell culture systems permissive to infection with naturally occurring prions is to exploit culture of neurospheres from transgenic mice expressing the normal prion protein (PrP) of the native host species. Accordingly, we developed differentiated neurosphere cultures from the cervid PrP-expressing mice to investigate whether this in vitro system would support replication of non-adapted cervid-origin chronic wasting disease (CWD) prions. Here we report the successful amplification of disease-associated PrP in differentiated neurosphere cultures within 3 weeks after exposure to CWD prions from both white-tailed deer or elk. This neurosphere culture system provides a new in vitro tool that can be used to assess non-adapted CWD prion propagation and transmission.

Isolation of Exosomes and Microvesicles from Cell Culture Systems to Study Prion Transmission

Extracellular vesicles (EVs) are composed of microvesicles and exosomes. Exosomes are small membrane vesicles (40-120 nm sized) of endosomal origin released in the extracellular medium from cells when multivesicular bodies fuse with the plasma membrane, whereas microvesicles (i.e., shedding vesicles, 100 nm to 1 μm sized) bud from the plasma membrane. Exosomes and microvesicles carry functional proteins and nucleic acids (especially mRNAs and microRNAs) that can be transferred to surrounding cells and tissues and can impact multiple dimensions of the cellular life. Most of the cells, if not all, from neuronal to immune cells, release exosomes and microvesicles in the extracellular medium, and all biological fluids including blood (serum/plasma), urine, cerebrospinal fluid, and saliva contain EVs.Prion-infected cultured cells are known to secrete infectivity into their environment. We characterized this cell-free form of prions and showed that infectivity was associated with exosomes. Since exosomes are produced by a variety of cells, including cells that actively accumulate prions, they could be a vehicle for infectivity in body fluids and could participate to the dissemination of prions in the organism. In addition, such infectious exosomes also represent a natural, simple, biological material to get key information on the abnormal PrP forms associated with infectivity.In this chapter, we describe first a method that allows exosomes and microvesicles isolation from prion-infected cell cultures and in a second time the strategies to characterize the prions containing exosomes and their ability to disseminate the prion agent.

Mapping PrPScPropagation in Experimental and Natural Scrapie in Sheep with Different PrP Genotypes

Twenty-one orally inoculated and seven naturally infected sheep with scrapie were examined for PrPScin peripheral tissues and in the central nervous system (CNS), using immunohistochemistry. In the inoculated group, VRQ (valine at codon 136, arginine at codon 154 and glutamine at codon 171)/VRQ sheep generally had a greater accumulation of the pathologic form of prion protein (PrPSc) in peripheral tissues, as compared with VRQ/ARQ (alanine at codon 136, arginine at codon 154, and glutamine at codon 171) animals at corresponding time points after inoculation. PrPScwas not detected in the ileal Peyer's patch, the spleen, the superficial cervical lymph node, and peripheral nervous tissues of several inoculated VRQ/ARQ animals. All inoculated VRQ/VRQ sheep, but only one of eight inoculated VRQ/ARQ animals, were PrPSc-positive in the CNS. Thus, the propagation of PrPScseemed slower and more limited in VRQ/ARQ animals. Tissue and cellular localization of PrPScsuggested that PrPScwas disseminated through three different routes. PrPSc-positive cells in lymph node sinuses and in lymphatics indicated spreading by lymph. The sequential appearance of PrPScin the peripheral nervous system and the CNS, with satellite cells as early targets, suggested the periaxonal transportation of PrPScthrough supportive cells. Focal areas of vascular amyloid-like PrPScin the brain of five sheep, suggested the hematogenous dissemination of PrPSc. There was a poor correlation between the amount of PrPScin the CNS and clinical signs. One subclinically affected sheep showed widespread PrPScaccumulation in the CNS, whereas three sheep had early clinical signs without detectable PrPScin the CNS. A VV136(homozygous for valine at codon 136) sheep inoculated with ARQ/ARR (alanine at codon 136, arginine at codon 154, and arginine at codon 171) tissue succumbed to disease, demonstrating successful heterologous transmission. Less susceptible sheep receiving VRQ/VRQ or ARQ/ARR material were PrPSc-negative by immunohistochemistry, enzyme-linked immunosorbent assay, and western blot.

Efficient inhibition of infectious prions multiplication and release by targeting the exosomal pathway

Exosomes are secreted membrane vesicles of endosomal origin present in biological fluids. Exosomes may serve as shuttles for amyloidogenic proteins, notably infectious prions, and may participate in their spreading in vivo. To explore the significance of the exosome pathway on prion infectivity and release, we investigated the role of the endosomal sorting complex required for transport (ESCRT) machinery and the need for ceramide, both involved in exosome biogenesis. Silencing of HRS-ESCRT-0 subunit drastically impairs the formation of cellular infectious prion due to an altered trafficking of cholesterol. Depletion of Tsg101-ESCRT-I subunit or impairment of the production of ceramide significantly strongly decreases infectious prion release. Together, our data reveal that ESCRT-dependent and -independent pathways can concomitantly regulate the exosomal secretion of infectious prion, showing that both pathways operate for the exosomal trafficking of a particular cargo. These data open up a new avenue to regulate prion release and propagation.

Interaction of prion protein with acetylcholinesterase: potential pathobiological implications in prion diseases

INTRODUCTION

The prion protein (PrP) binds to various molecular partners, but little is known about their potential impact on the pathogenesis of prion diseases

RESULTS

Here, we show that PrP can interact in vitro with acetylcholinesterase (AChE), a key protein of the cholinergic system in neural and non-neural tissues. This heterologous association induced aggregation of monomeric PrP and modified the structural properties of PrP amyloid fibrils. Following its recruitment into PrP fibrils, AChE loses its enzymatic activity and enhances PrP-mediated cytotoxicity. Using several truncated PrP variants and specific tight-binding AChE inhibitors (AChEis), we then demonstrate that the PrP-AChE interaction requires two mutually exclusive sub-sites in PrP N-terminal domain and an aromatic-rich region at the entrance of AChE active center gorge. We show that AChEis that target this site impair PrP-AChE complex formation and also limit the accumulation of pathological prion protein (PrPSc) in prion-infected cell cultures. Furthermore, reduction of AChE levels in prion-infected heterozygous AChE knock-out mice leads to slightly but significantly prolonged incubation time. Finally, we found that AChE levels were altered in prion-infected cells and tissues, suggesting that AChE might be directly associated with abnormal PrP.

CONCLUSION

Our results indicate that AChE deserves consideration as a new actor in expanding pathologically relevant PrP morphotypes and as a therapeutic target.

hTERT-immortalized ovine microglia propagate natural scrapie isolates

Ex vivo propagation of natural prion isolates (i.e., propagated solely in the natural host) is crucial for the characterization and study of transmissible spongiform encephalopathies (TSEs). Several well-established, prion-permissive cell culture systems are available; however, only a few cell lines are permissive to natural prion isolates and these cells are not pathophysiologically relevant (e.g., renal epithelium and fibroblast-like cells). Therefore, a pathophysiologically relevant cell line derived from a natural TSE host could be used for propagation of natural prion isolates. In this study, ovine brain macrophages (microglia) were immortalized by transfection with the human telomerase reverse transcriptase (hTERT) gene to identify cell lines (hTERT-microglia) permissive to natural scrapie prion isolates. Following transfection, hTERT-microglia were passaged up to 100 times and their lifespan was significantly longer compared to parental cells (Fisher's exact test, P<0.001). Multiple sublines were permissive to cell culture-adapted prions; two sublines were also permissive to natural scrapie isolates (i.e., derived from brain homogenates of sheep infected with scrapie). Prion infectivity and partial protease resistance of the prion protein were maintained in hTERT-microglia. Comparisons between scrapie-permissive and non-permissive hTERT-microglia sublines revealed that overall quantity of the normal cellular prion protein was not associated with prion permissiveness. The use of hTERT-microglia in future TSE studies may be more germane to the characterization of the cellular and subcellular pathophysiology of natural scrapie prion isolates and to investigate host-specific factors involved in prion replication.

A bovine cell line that can be infected by natural sheep scrapie prions

Cell culture systems represent a crucial part in basic prion research; yet, cell lines that are susceptible to prions, especially to field isolated prions that were not adapted to rodents, are very rare. The purpose of this study was to identify and characterize a cell line that was susceptible to ruminant-derived prions and to establish a stable prion infection within it. Based on species and tissue of origin as well as PrP expression rate, we pre-selected a total of 33 cell lines that were then challenged with natural and with mouse propagated BSE or scrapie inocula. Here, we report the successful infection of a non-transgenic bovine cell line, a sub-line of the bovine kidney cell line MDBK, with natural sheep scrapie prions. This cell line retained the scrapie infection for more than 200 passages. Selective cloning resulted in cell populations with increased accumulation of PrP^res, although this treatment was not mandatory for retaining the infection. The infection remained stable, even under suboptimal culture conditions. The resulting infectivity of the cells was confirmed by mouse bioassay (Tgbov mice, Tgshp mice). We believe that PES cells used together with other prion permissive cell lines will prove a valuable tool for ongoing efforts to understand and defeat prions and prion diseases.

Structure-activity relationship study around guanabenz identifies two derivatives retaining antiprion activity but having lost α2-adrenergic receptor agonistic activity

Guanabenz (GA) is an orally active α2-adrenergic agonist that has been used for many years for the treatment of hypertension. We recently described that GA is also active against both yeast and mammalian prions in an α2-adrenergic receptor-independent manner. These data suggest that this side-activity of GA could be explored for the treatment of prion-based diseases and other amyloid-based disorders. In this perspective, the potent antihypertensive activity of GA happens to be an annoying side-effect that could limit its use. In order to get rid of GA agonist activity at α2-adrenergic receptors, we performed a structure-activity relationship study around GA based on changes of the chlorine positions on the benzene moiety and then on the modifications of the guanidine group. Hence, we identified the two derivatives 6 and 7 that still possess a potent antiprion activity but were totally devoid of any agonist activity at α2-adrenergic receptors. Similarly to GA, 6 and 7 were also able to inhibit the protein folding activity of the ribosome (PFAR) which has been suggested to be involved in prion appearance/maintenance. Therefore, these two GA derivatives are worth being considered as drug candidates.

The potential of mesenchymal stem cell in prion research

Scrapie and bovine spongiform encephalopathy are fatal neurodegenerative diseases caused by the accumulation of a misfolded protein (PrP(res)), the pathological form of the cellular prion protein (PrP(C)). For the last decades, prion research has greatly progressed, but many questions need to be solved about prion replication mechanisms, cell toxicity, differences in genetic susceptibility, species barrier or the nature of prion strains. These studies can be developed in murine models of transmissible spongiform encephalopathies, although development of cell models for prion replication and sample titration could reduce economic and timing costs and also serve for basic research and treatment testing. Some murine cell lines can replicate scrapie strains previously adapted in mice and very few show the toxic effects of prion accumulation. Brain cell primary cultures can be more accurate models but are difficult to develop in naturally susceptible species like humans or domestic ruminants. Stem cells can be differentiated into neuron-like cells and be infected by prions. However, the use of embryo stem cells causes ethical problems in humans. Mesenchymal stem cells (MSCs) can be isolated from many adult tissues, including bone marrow, adipose tissue or even peripheral blood. These cells differentiate into neuronal cells, express PrP(C) and can be infected by prions in vitro. In addition, in the last years, these cells are being used to develop therapies for many diseases, including neurodegenerative diseases. We review here the use of cell models in prion research with a special interest in the potential use of MSCs.

Decontamination of prions in a plasma product manufacturing environment

BACKGROUND

The high resistance of prions to inactivating treatments requires the proper management of decontaminating procedures of equipment in contact with materials of human or animal origin destined for medical purposes. Sodium hydroxide (NaOH) is widely used today for this purpose as it inactivates a wide variety of pathogens including prions.

STUDY DESIGN AND METHODS

Several NaOH treatments were tested on prions bound to either stainless steel or chromatographic resins in industrial conditions with multiple prion strains.

RESULTS

Data show a strong correlation between inactivation results obtained by immunochemical detection of the prion protein and those obtained with infectivity assays and establish effective inactivation treatments for prions bound to stainless steel or chromatographic resins (ion exchange and affinity), including treatments with lower NaOH concentrations. Furthermore, no obvious strain-specific behavior difference was observed between experimental models.

CONCLUSION

The results generated by these investigations show that industrial NaOH decontamination regimens (in combination with the NaCl elution in the case of the chromatography process) attain substantial prion inactivation and/or removal between batches, thus providing added assurance to the biologic safety of the final plasma-derived medicinal products.

The toll-like receptor agonist imiquimod is active against prions

Using a yeast-based assay, a previously unsuspected antiprion activity was found for imiquimod (IQ), a potent Toll-like receptor 7 (TLR7) agonist already used for clinical applications. The antiprion activity of IQ was first detected against yeast prions [PSI⁺] and [URE3], and then against mammalian prion both ex vivo in a cell-based assay and in vivo in a transgenic mouse model for prion diseases. In order to facilitate structure-activity relationship studies, we conducted a new synthetic pathway which provides a more efficient means of producing new IQ chemical derivatives, the activity of which was tested against both yeast and mammalian prions. The comparable antiprion activity of IQ and its chemical derivatives in the above life forms further emphasizes the conservation of prion controlling mechanisms throughout evolution. Interestingly, this study also demonstrated that the antiprion activity of IQ and IQ-derived compounds is independent from their ability to stimulate TLRs. Furthermore, we found that IQ and its active chemical derivatives inhibit the protein folding activity of the ribosome (PFAR) in vitro.

Targeted knock-down of cellular prion protein expression in myelinating Schwann cells does not alter mouse prion pathogenesis

In naturally acquired transmissible spongiform encephalopathies, the pathogenic agents or prions spread from the sites of initial peripheral uptake or replication to the brain where they cause progressive and fatal neurodegeneration. Routing via the peripheral nervous system is considered to be one of the main pathways to the central nervous system. Replication of prions in Schwann cells is viewed as a potentially important mechanism for efficient prion spread along nerves. Here we used a Cre-loxP mouse transgenetic approach to disrupt host-encoded prion protein (PrP(C)) specifically in myelinating Schwann cells. Despite the use of infection routes targeting highly myelinated nerves, there was no alteration in mouse prion pathogenesis, suggesting that conversion-dependent, centripetal spread of prions does not crucially rely on PrP(C) expressed by myelinating Schwann cells.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Prion propagation and toxicity occur in vitro with two-phase kinetics specific to strain and neuronal type

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are fatal neurodegenerative disorders that occur in humans and animals. The neuropathological hallmarks of TSEs are spongiosis, glial proliferation, and neuronal loss. The only known specific molecular marker of TSEs is the abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(C)), which accumulates in the brain of infected subjects and forms infectious prion particles. Although this transmissible agent lacks a specific nucleic acid component, several prion strains have been isolated. Prion strains are characterized by differences in disease outcome, PrP(Sc) distribution patterns, and brain lesion profiles at the terminal stage of the disease. The molecular factors and cellular mechanisms involved in strain-specific neuronal tropism and toxicity remain largely unknown. Currently, no cellular model exists to facilitate in vitro studies of these processes. A few cultured cell lines that maintain persistent scrapie infections have been developed, but only two of them have shown the cytotoxic effects associated with prion propagation. In this study, we have developed primary neuronal cultures to assess in vitro neuronal tropism and toxicity of different prion strains (scrapie strains 139A, ME7, and 22L). We have tested primary neuronal cultures enriched in cerebellar granular, striatal, or cortical neurons. Our results showed that (i) a strain-specific neuronal tropism operated in vitro; (ii) the cytotoxic effect varied among strains and neuronal cell types; (iii) prion propagation and toxicity occurred in two kinetic phases, a replicative phase followed by a toxic phase; and (iv) neurotoxicity peaked when abnormal PrP accumulation reached a plateau.

A brain aggregate model gives new insights into the pathobiology and treatment of prion diseases

Brain aggregates (BrnAggs) derived from fetal mouse brains contain mature neurons and glial cells. We determined that BrnAggs are consistently infected with Rocky Mountain Laboratory scrapie strain prions and produce increasing levels of the pathogenic form of the prion protein (PrP). Their abundant dendrites undergo degeneration shortly after prion infection. Treatment of prion-infected BrnAggs with drugs, such as a γ-secretase inhibitors and quinacrine (Qa), which stop PrP formation and dendritic degeneration, mirrors the results from rodent studies. Because PrP is trafficked into lysosomes by endocytosis and autophagosomes by phagocytosis in neurons of prion strain-infected BrnAggs, we studied the effects of drugs that modulate subcellular trafficking. Rapamycin (Rap), which activates autophagy, markedly increased light-chain 3-II (LC3-II)-positive autophagosomes and cathepsin D-positive lysosomes in BrnAggs but could not eliminate the intracellular PrP within them. Adding Qa to Rap markedly reduced the number of LC3-II-positive autolysosomes. Rap + Qa created a competition between Rap increasing and Qa decreasing LC3-II. Rapamycin + Qa decreased total PrP by 56% compared with that of Qa alone, which reduced PrP by 37% relative to Rap alone. We conclude that the decrease was dominated by the ability of Qa to decrease the formation of PrP. Therefore, BrnAggs provide an efficient in vitro tool for screening drug therapies and studying the complex biology of prions.

Variation in Chst8 gene expression level affects PrPC to PrPSc conversion efficiency in prion-infected Mov cells

The conversion of the endogenous cellular prion protein to an abnormally folded isoform is a hallmark of transmissible spongiform encephalopathies. It occurs when a misfolded prion protein contacts the cellular PrP. Among the molecular partners suggested to be involved in the misfolding process, the glycosaminoglycans seem to be good candidates. The present study was aimed to examine a possible link between PrP conversion efficiency and transcript level of Chst8 gene that encodes the carbohydrate N-acetylgalactosamine 4-O-sulfotransferase 8. Mov cells expressing ovine PrP were transfected with shRNA directed against Chst8 transcripts. Resulting clones were characterized for their Chst8 and Prnp transcript levels, and for their content in sulfated glycosaminoglycans, more particularly sulfated chondroitins. Unexpectedly, the decreased amount of Chst8 transcript induced an increase of the chondroitin sulfate percentage among total GAGs, with an increased amount of 4-O-sulfation of GalNAc residues. Upon to infection by a sheep prion, a slight amount of PrP(Sc) was observed, which rapidly disappeared upon subpassaging. Together, these findings indicate that the Chst8 transcript level affects the glycosaminoglycan environment of the cellular prion protein, and as a consequence its ability for conversion into PrP(Sc).

A simple, versatile and sensitive cell-based assay for prions from various species

Detection and quantification of prion infectivity is a crucial step for various fundamental and applied aspects of prion research. Identification of cell lines highly sensitive to prion infection led to the development of cell-based titration procedures aiming at replacing animal bioassays, usually performed in mice or hamsters. However, most of these cell lines are only permissive to mouse-adapted prions strains and do not allow titration of prions from other species. In this study, we show that epithelial RK13, a cell line permissive to mouse and bank vole prion strains and to natural prion agents from sheep and cervids, enables a robust and sensitive detection of mouse and ovine-derived prions. Importantly, the cell culture work is strongly reduced as the RK13 cell assay procedure designed here does not require subcultivation of the inoculated cultures. We also show that prions effectively bind to culture plastic vessel and are quantitatively detected by the cell assay. The possibility to easily quantify a wider range of prions, including rodent experimental strains but also natural agents from sheep and cervids, should prompt the spread of cell assays for routine prion titration and lead to valuable information in fundamental and applied studies.

Characterization of the role of dendritic cells in prion transfer to primary neurons

TSEs (transmissible spongiform encephalopathies) are neurodegenerative diseases caused by pathogenic isoforms (PrPSc) of the host-encoded PrPc (cellular prion protein). After consumption of contaminated food, PrPSc deposits rapidly accumulate in lymphoid tissues before invasion of the CNS (central nervous system). However, the mechanisms of prion spreading from the periphery to the nervous system are still unclear. In the present study, we investigated the role of DCs (dendritic cells) in the spreading of prion infection to neuronal cells. First, we determined that BMDCs (bone-marrow-derived DCs) rapidly uptake PrPSc after exposure to infected brain homogenate. Next, we observed a progressive catabolism of the internalized prion aggregates. Similar experiments performed with BMDCs isolated from KO (knockout) mice or mice overexpressing PrP (tga20) indicate that both PrPSc uptake and catabolism are independent of PrPc expression in these cells. Finally, using co-cultures of prion-loaded BMDCs and cerebellar neurons, we characterized the transfer of the prion protein and the resulting infection of the neuronal cultures. Interestingly, the transfer of PrPSc was triggered by direct cell-cell contact. As a consequence, BMDCs retained the prion protein when cultured alone, and no transfer to the recipient neurons was observed when a filter separated the two cultures or when neurons were exposed to the BMDC-conditioned medium. Additionally, fixed BMDCs also failed to transfer prion infectivity to neurons, suggesting an active transport of prion aggregates, in accordance with a role of TNTs (tunnelling nanotubes) observed in the co-cultures.

Synthesis of conjugates of 6-aminophenanthridine and guanabenz, two structurally unrelated prion inhibitors, for the determination of their cellular targets by affinity chromatography

The synthesis of affinity matrices for 6-aminophenanthridine (6AP) and 2,6-dichlorobenzylidenaminoguanidine (Guanabenz, GA), two unrelated prion inhibitors, is described. In both cases, the same simple spacer, epsilon-aminocaproylaminopentanol, was introduced by a Mitsunobu reaction and the choice of the anchoring position of the linker was determined by the study of the residual antiprion activity of the corresponding 6AP or GA conjugates. Very recently, these two affinity matrices were used for chromatography assays leading to the identification of ribosome (via the rRNA) as a common target of these two antiprion drugs. Here, we show, using competition experiments with Quinacrine (QC) and Chlorpromazine (CPZ), two other antiprion drugs, that QC, but not CPZ, may also directly target the rRNA.

Prion infection of differentiated neurospheres

Until now only a few cell lines have been proved able to propagate prions and only limited prion strains have been replicated in cell models. Neurosphere lines isolated from the brains of mice at embryonic day 14 grow as aggregates and contain CNS stem cells. Others authors have previously reported that cultured neurospheres expressing cellular prion protein (PrP(C)) can be infected with prions. As potential neural progenitors the neurosphere cultures are supposed to differentiate into neurons and astrocytes which represent the main cell types infected by prions in vivo. Here we study the ability of undifferentiated and differentiated neurospheres to replicate several prion strains. Neurosphere cultures were isolated from 129/ola, FVB, Prnp(0/0) and Tga20 mice, which over-express murine PrP. We were not able to detect PrP(res) accumulation in dividing neurosphere cultures after prion exposure to two different mouse adapted scrapie inocula (RML and 22L). In contrast, with differentiated neurosphere cultures expressing PrP(C) (129/ola, FVB and Tga20) a successful PrP(Res) amplification was observed in very short time experiments when infected with the same inocula, implying that cell differentiation improve prion replication in these cultured cells. The mouse BSE adapted inocula (301C) was not amplified in these neurosphere cultures neither before nor after differentiation, suggesting that these cell cultures showed a differential prion strain susceptibility. These results suggest that differentiated neurosphere cultures can complement prion bioassays in mouse models.

Endogenous proteolytic cleavage of disease-associated prion protein to produce C2 fragments is strongly cell- and tissue-dependent

The abnormally folded form of the prion protein (PrP(Sc)) accumulating in nervous and lymphoid tissues of prion-infected individuals can be naturally cleaved to generate a N-terminal-truncated fragment called C2. Information about the identity of the cellular proteases involved in this process and its possible role in prion biology has remained limited and controversial. We investigated PrP(Sc) N-terminal trimming in different cell lines and primary cultured nerve cells, and in the brain and spleen tissue from transgenic mice infected by ovine and mouse prions. We found the following: (i) the full-length to C2 ratio varies considerably depending on the infected cell or tissue. Thus, in primary neurons and brain tissue, PrP(Sc) accumulated predominantly as untrimmed species, whereas efficient trimming occurred in Rov and MovS cells, and in spleen tissue. (ii) Although C2 is generally considered to be the counterpart of the PrP(Sc) proteinase K-resistant core, the N termini of the fragments cleaved in vivo and in vitro can actually differ, as evidenced by a different reactivity toward the Pc248 anti-octarepeat antibody. (iii) In lysosome-impaired cells, the ratio of full-length versus C2 species dramatically increased, yet efficient prion propagation could occur. Moreover, cathepsin but not calpain inhibitors markedly inhibited C2 formation, and in vitro cleavage by cathepsins B and L produced PrP(Sc) fragments lacking the Pc248 epitope, strongly arguing for the primary involvement of acidic hydrolases of the endolysosomal compartment. These findings have implications on the molecular analysis of PrP(Sc) and cell pathogenesis of prion infection.

Dramatic reduction of PrP C level and glycosylation in peripheral nerves following PrP knock-out from Schwann cells does not prevent transmissible spongiform encephalopathy neuroinvasion

Expression of the prion protein (PrP(C)) is a requirement for host susceptibility to the transmissible spongiform encephalopathies (TSEs) and thought to be necessary for the replication and transport of the infectious agent. The mechanism of TSE neuroinvasion is not fully understood, although the routing of infection has been mapped through the peripheral nervous system (PNS) and Schwann cells have been implicated as a potential conduit for transport of the TSE infectious agent. To address whether Schwann cells are a requirement for spread of the TSE agent from the site of infection to the CNS, PrP(C) expression was selectively removed from Schwann cells in vivo. This dramatically reduced total PrP(C) within peripheral nerves by 90%, resulting in the selective loss of glycosylated PrP(C) species. Despite this, 139A and ME7 mouse-passaged scrapie agent strains were efficiently replicated and transported to the CNS following oral and intraperitoneal exposure. Thus, the myelinating glial cells within the PNS do not appear to play a significant role in TSE neuroinvasion.

Infection of cell lines with experimental and natural ovine scrapie agents

Mouse bioassay remains the gold standard for determining proof of infectivity, strain type, and infectious titer estimation in prion disease research. The development of an approach using ex vivo cell-based assays remains an attractive alternative, both in order to reduce the use of mice and to hasten results. The main limitation of a cell-based approach is the scarcity of cell lines permissive to infection with natural transmissible spongiform encephalopathy strains. This study combines two advances in this area, namely, the standard scrapie cell assay (SSCA) and the Rov9 and MovS6 cell lines, which both express the ovine PrP VRQ allele, to assess to what extent natural and experimental ovine scrapie can be detected ex vivo. Despite the Rov9 and MovS6 cell lines being of different biological origin, they were both permissive and resistant to infection with the same isolates of natural sheep scrapie as detected by SSCA. Rov9 subclones that are 20 times more sensitive than Rov9 to SSBP/1-like scrapie infection were isolated, but all the subclones maintained their resistance to isolates that failed to transmit to the parental line. The most sensitive subclone of the Rov9 cell line was used to estimate the infectious titer of a scrapie brain pool (RBP1) and proved to be more sensitive than the mouse bioassay using wild-type mice. Increasing the sensitivity of the Rov9 cell line to SSBP/1 infection did not correlate with broadening susceptibility, as the specificity of permissiveness and resistance to other scrapie isolates was maintained.

Protein folding activity of ribosomal RNA is a selective target of two unrelated antiprion drugs

Background

6-Aminophenanthridine (6AP) and Guanabenz (GA, a drug currently in use for the treatment of hypertension) were isolated as antiprion drugs using a yeast-based assay. These structurally unrelated molecules are also active against mammalian prion in several cell-based assays and in vivo in a mouse model for prion-based diseases.

Methodology/Principal Findings

Here we report the identification of cellular targets of these drugs. Using affinity chromatography matrices for both drugs, we demonstrate an RNA-dependent interaction of 6AP and GA with the ribosome. These specific interactions have no effect on the peptidyl transferase activity of the ribosome or on global translation. In contrast, 6AP and GA specifically inhibit the ribosomal RNA-mediated protein folding activity of the ribosome.

Conclusion/Significance

6AP and GA are therefore the first compounds to selectively inhibit the protein folding activity of the ribosome. They thus constitute precious tools to study the yet largely unexplored biological role of this protein folding activity.

Antihypertensive drug guanabenz is active in vivo against both yeast and mammalian prions

Background

Prion-based diseases are incurable transmissible neurodegenerative disorders affecting animals and humans.

Methodology/Principal Findings

Here we report the discovery of the in vivo antiprion activity of Guanabenz (GA), an agonist of α2-adrenergic receptors routinely used in human medicine as an antihypertensive drug. We isolated GA in a screen for drugs active in vivo against two different yeast prions using a previously described yeast-based two steps assay. GA was then shown to promote ovine PrP^Sc clearance in a cell-based assay. These effects are very specific as evidenced by the lack of activity of some GA analogues that we generated. GA antiprion activity does not involve its agonist activity on α2-adrenergic receptors as other chemically close anti-hypertensive agents possessing related mechanism of action were found inactive against prions. Finally, GA showed activity in a transgenic mouse-based in vivo assay for ovine prion propagation, prolonging slightly but significantly the survival of treated animals.

Conclusion/Significance

GA thus adds to the short list of compounds active in vivo in animal models for the treatment of prion-based diseases. Because it has been administrated for many years to treat hypertension on a daily basis, without major side-effects, our results suggest that it could be evaluated in human as a potential treatment for prion-based diseases.

Evidence for prion protein expression in enteroglial cells of the myenteric plexus of mouse intestine

Transmissible spongiform encephalopathies (TSEs) are slowly progressive and fatal neurodegenerative diseases affecting man and animals. They are caused by pathological isoforms (PrP(Sc)) of the host-encoded cellular prion protein (PrP(C)). There are two crucial factors for the initiation of infection, namely host cells PrP(C) expression and sufficient sequence homology between the PrP(Sc) to which the animal is exposed and its own PrP(C). In acquired TSEs, the gastrointestinal tract (GIT) is the main prion entry site. Hence, it is of paramount importance to an understanding of the early pathogenesis of prion infections, to characterize the GIT cell types constitutively expressing PrP(C). Twenty-three mice were utilized, including wild-type (WT), Prnp knock-out (KO), and PrP(C)-overexpressing (tga20/tga20) animals, of 20-30 g in weight and of either sex. In all three groups of mice, PrP(C)-immunoreactivity (IR), along with glial fibrillary acidic protein (GFAP)-IR and synaptophysin (Syn)-IR were investigated by means of indirect immunofluorescence in wholemount preparations from several gut regions, from duodenum to rectum. In WT mice, PrP(C)-IR and GFAP-IR co-localization was observed in enteric glial cells (EGCs) from all intestinal segments. PrP(C)-overexpressing mice showed a stronger PrP(C)-IR in EGCs, whereas the same cells exhibited no PrP(C)-IR in Prnp-KO mice. Our findings clearly indicate that EGCs of the mouse intestine constitutively express PrP(C); thus they could be a potential target for infectious prions.

Prnp knockdown in transgenic mice using RNA interference

RNA interference has become a widely used approach to perform gene knockdown experiments in cell cultures and more recently transgenic animals. A designed miRNA targeting the prion protein mRNA was built and expressed using the human PRNP promoter. Its efficiency was confirmed in transfected cells and it was used to generate several transgenic mouse lines. Although expressed at low levels, it was found to downregulate the endogenous mouse Prnp gene expression to an extent that appears to be directly related with the transgene expression level and that could reach up to 80% inhibition. This result highlights the potential and limitations of the RNA interference approach when applied to disease resistance.

Production of biological active murine IFN-gamma by recombinant Lactococcus lactis

IFN-gamma is a cytokine produced primarily by both T lymphocytes and natural killer cells and it is considered to be an attractive therapeutic molecule. In the present study, a DNA sequence encoding the mature murine IFN-gamma (muIFN-gamma) protein was cloned and expressed in the food-grade lactic acid bacterium Lactococcus lactis. The activity of recombinant muIFN-gamma produced by genetically engineered L. lactis was confirmed in an antiviral assay using MoV cells infected with Vesicular Stomatitis Virus. The data provide the first demonstration that a Gram-positive bacterium, L. lactis, is able to produce functional muIFN-gamma. This recombinant strain could lead to the development of a new, well-tolerated vector to deliver active muIFN-gamma at the mucosal level.

Susceptibility of cell substrates to PrPSc infection and safety control measures related to biological and biotherapeutical products

Concerns over the potential for infectious prion proteins to contaminate human biologics and biotherapeutics have been raised from time to time. Transmission of the pathogenic form of prion protein (PrP(Sc)) through veterinary vaccines has been observed, yet no human case through the use of vaccine products has been reported. However, iatrogenic transmissions of PrP(Sc) in humans through blood components, tissues and growth hormone have been reported. These findings underscore the importance of reliable detection or diagnostic methods to prevent the transmission of prion diseases, given that the number of asymptomatic infected individuals remains unknown, the perceived incubation time for human prion diseases could be decades, and no cure of the diseases has been found yet. A variety of biochemical and molecular methods can selectively concentrate PrP(Sc) to facilitate its detection in tissues and cells. Furthermore, some methods routinely used in the manufacturing process of biological products have been found to be effective in reducing PrP(Sc) from the products. Questions remain unanswered as to the validation criteria of these methods, the minimal infectious dose of the PrP(Sc) required to cause infection and the susceptibility of cells used in gene therapy or the manufacturing process of biological products to PrP(Sc) infections. Here, we discuss some of these challenging issues.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Cell models of prion infection

Due to recent renewal of interest and concerns in prion diseases, a number of cell systems permissive to prion multiplication have been generated in the last years. These include established cell lines, neuronal stem cells and primary neuronal cultures. While most of these models are permissive to experimental, mouse-adapted strains of prions, the propagation of natural field isolates from sheep scrapie and chronic wasting disease has been recently achieved. These models have improved our knowledge on the molecular and cellular events controlling the conversion of the PrP(C) protein into abnormal isoforms and on the cell-to-cell spreading of prions. Infected cultured cells will also facilitate investigations on the molecular basis of strain identity and on the mechanisms that lead to neurodegeneration. The ongoing development of new cell models with improved characteristics will certainly be useful for a number of unanswered critical issues in the prion field.

Prion strain- and species-dependent effects of antiprion molecules in primary neuronal cultures

Transmissible spongiform encephalopathies (TSE) arise as a consequence of infection of the central nervous system by prions and are incurable. To date, most antiprion compounds identified by in vitro screening failed to exhibit therapeutic activity in animals, thus calling for new assays that could more accurately predict their in vivo potency. Primary nerve cell cultures are routinely used to assess neurotoxicity of chemical compounds. Here, we report that prion strains from different species can propagate in primary neuronal cultures derived from transgenic mouse lines overexpressing ovine, murine, hamster, or human prion protein. Using this newly developed cell system, the activity of three generic compounds known to cure prion-infected cell lines was evaluated. We show that the antiprion activity observed in neuronal cultures is species or strain dependent and recapitulates to some extent the activity reported in vivo in rodent models. Therefore, infected primary neuronal cultures may be a relevant system in which to investigate the efficacy and mode of action of antiprion drugs, including toward human transmissible spongiform encephalopathy agents.

Efficient dissemination of prions through preferential transmission to nearby cells

Despite circumstantial evidence that prions can be found extracellularly or at the surface of infected cells, little is known about how these infectious agents spread from cell to cell. In order to gain better insight into this critical issue, this study used two different cell lines (neuroglial MovS and epithelial Rov cells) that have previously been shown to be permissive for ovine prion multiplication. Co-culture of infected cells and uninfected target cells at a ratio of 1 : 9 resulted in total infection of MovS cells within 10 days but not of Rov cell cultures, suggesting that the efficiency of prion dissemination may vary greatly depending on the type of permissive cell. Analysis of the spatial distribution of the newly infected cells revealed that, although long-range spread could also occur, cells proximal to the infected donor cells consistently accumulated more abnormal PrP, consistent with preferential infection of nearby cells. This experimental approach, focused on dissemination among living cells, could help in the analysis of mechanisms involved in the cell-to-cell spread of prion infections.

An overview of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders of humans and animals associated with an accumulation of abnormal isoforms of prion protein (PrP) in nerve cells. The pathogenesis of TSEs involves conformational conversions of normal cellular PrP (PrPc) to abnormal isoforms of PrP (PrPSc). While the protein-only hypothesis has been widely accepted as a causal mechanism of prion diseases, evidence from more recent research suggests a possible involvement of other cellular component(s) or as yet undefined infectious agent(s) in PrP pathogenesis. Although the underlying mechanisms of PrP strain variation and the determinants of interspecies transmissibility have not been fully elucidated, biochemical and molecular findings indicate that bovine spongiform encephalopathy in cattle and new-variant Creutzfeldt–Jakob disease in humans are caused by indistinguishable etiological agent(s). Cumulative evidence suggests that there may be risks of humans acquiring TSEs via a variety of exposures to infected material. The development of highly precise ligands is warranted to detect and differentiate strains, allelic variants and infectious isoforms of these PrPs. This article describes the general features of TSEs and PrP, the current understanding of their pathogenesis, recent advances in prion disease diagnostics, and PrP inactivation.

Prion infection of muscle cells in vitro

The prion agent has been detected in skeletal muscle of humans and animals with prion diseases. Here we report scrapie infection of murine C2C12 myoblasts and myotubes in vitro following coculture with a scrapie-infected murine neuroblastoma (N2A) cell line but not following incubation with a scrapie-infected nonneuronal cell line or a scrapie brain homogenate. Terminal differentiation of scrapie-infected C2C12 myoblasts into myotubes resulted in an increase in the expression of the disease-specific prion protein, PrP(Sc). The amount of scrapie infectivity or PrP(Sc) in C2C12 myotubes was comparable to the levels found in scrapie-infected N2A cells, indicating that a high level of infection was established in muscle cells. Subclones of scrapie-infected C2C12 cells produced high levels of PrP(Sc) in myotubes, and the C-terminal C2 polypeptide fragment of PrP(Sc) was found based on deglycosylation and PrP(Sc)-specific immunoprecipitation of cell lysates. This is the first report of a stable prion infection in muscle cells in vitro and of a long-term prion infection in a nondividing, differentiated peripheral cell type in culture. These in vitro studies also suggest that in vivo prion infection of skeletal muscle requires contact with prion-infected neurons or, possibly, nerve terminals.

Nitric oxide synthase immunoreactivity and NADPH-d histochemistry in the enteric nervous system of Sarda breed sheep with different PrP genotypes in whole-mount and cryostat preparations

Until now, significant differences in the neurochemical pattern of enteric neurons have been demonstrated in all species studied; however, some strong similarities also occur across species, such as the occurrence of nitric oxide synthase immunoreactivity (NOS-IR) in inhibitory motor neurons to muscle. In consideration of the insufficient data regarding the enteric nervous system (ENS) of sheep, we investigated the myenteric plexus and submucosal plexus of the ovine ileum. Since the pivotal role of the ENS in the early pathogenesis of sheep scrapie, the "prototype" of prion diseases, has been suggested, we have focused our observations also on the host's PrP genotype. We have studied the morphology and distribution of NOS-IR neurons and their relationships with the enteric glia in whole-mount preparations and in cryostat sections. NOS-IR neurons, always encircled by glial processes, were located in both plexuses. Many NOS-IR fibers were seen in the circular muscle layer, in the submucosa, and in the mucosa. In the submucosa they were close to the lymphoid tissue. No differences in the distribution and percentage of NOS-IR fibers and neurons were observed among sheep carrying different PrP genotype, thus making unlikely their contribution in the determinism of susceptibility/resistance to scrapie infection.