A cell line infectible by prion strains from different species

In vitro amplification of misfolded prion protein using lysate of cultured cells

Protein misfolding cyclic amplification (PMCA) recapitulates the prion protein (PrP) conversion process under cell-free conditions. PMCA was initially established with brain material and then with further simplified constituents such as partially purified and recombinant PrP. However, availability of brain material from some species or brain material from animals with certain mutations or polymorphisms within the PrP gene is often limited. Moreover, preparation of native PrP from mammalian cells and tissues, as well as recombinant PrP from bacterial cells, involves time-consuming purification steps. To establish a convenient and versatile PMCA procedure unrestricted to the availability of substrate sources, we attempted to conduct PMCA with the lysate of cells that express cellular PrP (PrP^C). PrP^Sc was efficiently amplified with lysate of rabbit kidney epithelial RK13 cells stably transfected with the mouse or Syrian hamster PrP gene. Furthermore, PMCA was also successful with lysate of other established cell lines of neuronal or non-neuronal origins. Together with the data showing that the abundance of PrP^C in cell lysate was a critical factor to drive efficient PrP^Sc amplification, our results demonstrate that cell lysate in which PrP^C is present abundantly serves as an excellent substrate source for PMCA.

Sequence-dependent prion protein misfolding and neurotoxicity

Prion diseases are neurodegenerative disorders caused by misfolding of the normal prion protein (PrP) into a pathogenic "scrapie" conformation. To better understand the cellular and molecular mechanisms that govern the conformational changes (conversion) of PrP, we compared the dynamics of PrP from mammals susceptible (hamster and mouse) and resistant (rabbit) to prion diseases in transgenic flies. We recently showed that hamster PrP induces spongiform degeneration and accumulates into highly aggregated, scrapie-like conformers in transgenic flies. We show now that rabbit PrP does not induce spongiform degeneration and does not convert into scrapie-like conformers. Surprisingly, mouse PrP induces weak neurodegeneration and accumulates small amounts of scrapie-like conformers. Thus, the expression of three highly conserved mammalian prion proteins in transgenic flies uncovered prominent differences in their conformational dynamics. How these properties are encoded in the amino acid sequence remains to be elucidated.

Glycosaminoglycan sulphation affects the seeded misfolding of a mutant prion protein

Background

The accumulation of protease resistant conformers of the prion protein (PrP^res) is a key pathological feature of prion diseases. Polyanions, including RNA and glycosaminoglycans have been identified as factors that contribute to the propagation, transmission and pathogenesis of prion disease. Recent studies have suggested that the contribution of these cofactors to prion propagation may be species specific.

Methodology/Principal Finding

In this study a cell-free assay was used to investigate the molecular basis of polyanion stimulated PrP^res formation using brain tissue or cell line derived murine PrP. Enzymatic depletion of endogenous nucleic acids or heparan sulphate (HS) from the PrP^C substrate was found to specifically prevent PrP^res formation seeded by mouse derived PrP^Sc. Modification of the negative charge afforded by the sulphation of glycosaminoglycans increased the ability of a familial PrP mutant to act as a substrate for PrP^res formation, while having no effect on PrP^res formed by wildtype PrP. This difference may be due to the observed differences in the binding of wild type and mutant PrP for glycosaminoglycans.

Conclusions/Significance

Cofactor requirements for PrP^res formation are host species and prion strain specific and affected by disease associated mutations of the prion protein. This may explain both species and strain dependent propagation characteristics and provide insights into the underlying mechanisms of familial prion disease. It further highlights the challenge of designing effective therapeutics against a disease which effects a range of mammalian species, caused by range of aetiologies and prion strains.

Cell-based quantification of chronic wasting disease prions

Cell-based measurement of prion infectivity is currently restricted to experimental strains of mouse-adapted scrapie. Having isolated cell cultures with susceptibility to prions from diseased elk, we describe a modification of the scrapie cell assay allowing evaluation of prions causing chronic wasting disease, a naturally occurring transmissible spongiform encephalopathy. We compare this cervid prion cell assay to bioassays in transgenic mice, the only other existing method for quantification, and show this assay to be a relatively economical and expedient alternative that will likely facilitate studies of this important prion disease.

Changing the solvent accessibility of the prion protein disulfide bond markedly influences its trafficking and effect on cell function

Prion diseases are fatal transmissible neurodegenerative diseases that result from structural conversion of the prion protein into a disease-associated isoform. The prion protein contains a single disulfide bond. Our analysis of all NMR structures of the prion protein (total of 440 structures over nine species) containing an explicit disulfide bond reveals that the bond exists predominantly in a stable low-energy state, but can also adopt a high-energy configuration. The side chains of two tyrosine residues and one phenylalanine residue control access of solvent to the disulfide bond. Notably, the side chains rotate away from the disulfide bond in the high-energy state, exposing the disulfide bond to solvent. The importance of these aromatic residues for protein function was analysed by mutating them to alanine residues and analysing the properties of the mutant proteins using biophysical and cell biological approaches. Whereas the mutant protein behaved similarly to wild-type prion protein in recombinant systems, the mutants were retained in the endoplasmic reticulum of mammalian cells and degraded by the proteasomal system. The cellular behaviour of the aromatic residue mutants was similar to the cellular behaviour of a disulfide bond mutant prion protein in which the cysteine residues were replaced with alanine, a result which is consistent with an unstable disulfide bond in the aromatic residue mutants. These observations suggest that the conformation of the prion protein disulfide bond may have implications for correct maturation and function of this protein.

Residues surrounding the glycosylphosphatidylinositol anchor attachment site of PrP modulate prion infection: insight from the resistance of rabbits to prion disease

Prion diseases are a group of transmissible, invariably fatal neurodegenerative diseases that affect both humans and animals. According to the protein-only hypothesis, the infectious agent is a prion (proteinaceous infectious particle) that is composed primarily of PrP(Sc), the disease-associated isoform of the cellular prion protein, PrP. PrP(Sc) arises from the conformational change of the normal, glycosylphosphatidylinositol (GPI)-anchored protein, PrP(C). The mechanism by which this process occurs, however, remains enigmatic. Rabbits are one of a small number of mammalian species reported to be resistant to prion infection. Sequence analysis of rabbit PrP revealed that its C-terminal amino acids differ from those of PrP from other mammals and may affect the anchoring of rabbit PrP through its GPI anchor. Using a cell culture model, this study investigated the effect of the rabbit PrP-specific C-terminal amino acids on the addition of the GPI anchor to PrP(C), PrP(C) localization, and PrP(Sc) formation. The incorporation of rabbit-specific C-terminal PrP residues into mouse PrP did not affect the addition of a GPI anchor or the localization of PrP. However, these residues did inhibit PrP(Sc) formation, suggesting that these rabbit-specific residues interfere with a C-terminal PrP(Sc) interaction site.

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.

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.

Increased proportions of C1 truncated prion protein protect against cellular M1000 prion infection

Prion disease pathogenesis is linked to the cell-associated propagation of misfolded protease-resistant conformers (PrP) of the normal cellular prion protein (PrP). Ongoing PrP expression is the only known absolute requirement for successful prion disease transmission and PrP propagation. Further typifying prion disease is selective neuronal dysfunction and loss, although the precise mechanisms underlying this are undefined. We utilized a single prion strain (M1000) and a range of neuronal and nonneuronal, PrP endogenously expressing and transgenically modified overexpressing cell lines, to evaluate whether PrP glycosylation patterns or constitutive N-terminal cleavage events may be determinants of sustained PrP propagation. Our data demonstrates that relative proportions of full-length and C1 truncated PrP are the most important characteristics influencing susceptibility to sustained M1000 prion infection, supporting PrP alpha-cleavage as a protective event, which may contribute to the selective neuronal vulnerability observed in vivo.

Inducible expression of chimpanzee prion protein (PrP) in murine PrP knock-out cells

In transmissible spongiform encephalopathy (TSE) pathogenesis the cellular prion protein (PrP(C)) is converted into its pathogenic PrP(Sc) isoform. Prion protein gene (Prnp) deficient mice (PrP(0/0)) are resistant to PrP(Sc) infection, but following reconstitution of Prnp they regain their susceptibility to infection. Therefore, it is challenging to simulate this natural situation in a cell culture model. We have previously reported the inducible stable expression of a human PrP(C) in murine 3T3 cells. In this study, we used murine PrP(0/0) cells stably expressing exemplarily the chimpanzee Prnp under the control of inducible tetracycline (Tet) system. The Prnp was integrated using a lentiviral vector. Its expression in the engineered PrP(0/0)Chimp1/Tet-Off cell line was analyzed by Western blot (Wb) and fluorescence activated cell sorting (FACS) analyses. PrP(C) was partially purified by using immobilized metal affinity chromatography (IMAC). Compared to all the other cell systems which possess an endogenous PrP(C) expression, here described cell line contains only an overexpressing species specific PrP(C) expression which is tightly regulated and can be turned-off at any time without showing any endogenous host PrP(C) expression. Consequently, a contamination of the isolated PrP(C) is impossible. This cell line potentially offers a new tool for simulation of mice bioassays widely used in TSE infection studies.

Quantitative and qualitative analysis of cellular prion protein (PrP(C)) expression in bovine somatic tissues

The host encoded cellular prion protein (PrP(C)) is an N-linked glycoprotein tethered to the cell membrane by a glycophosphatidylinositol (GPI) anchor. Under certain conditions, PrP(C) can undergo conversion into a conformationally-altered isoform (PrP(Sc)) widely believed to be the pathogenic agent of transmissible spongiform encephalopathies (TSEs). Understanding the tissue-specific expression of PrP(C) is crucial considering that cells expressing high levels of PrP(C) bear a risk for conversion and accumulation of PrP(Sc). In the present study, fifteen bovine somatic tissues were analyzed for PrP(C) expression by quantitative western blot and immunohistochemistry. Quantitative western blot analysis revealed highest expression of PrP(C) in cerebellum, obex and spinal cord. Intermediate levels were detected in thymus, intestine, nerve, heart and spleen, and lower levels in lung, muscle, kidney, lymph node, skin, pancreas and liver. Immunohistochemical analysis detected intense cellular-specific PrP(C) staining in neurons, thymocytes and lymphocytes. PrP(C) was also detected in the enteric wall, pancreatic islets of langerhans, myocardium, pulmonary alveolar sacs, renal glomeruli and dermal epithelial cells. This study demonstrated the quantitatively varied, wide-spread, tissue- and cell-specific expression pattern of PrP(C) in bovine somatic tissues. The importance of this study is to lay the foundation for understanding the tissue-specific expression of PrP(C) and to consider the potential participation of more bovine tissues in the transmission of BSE infection.

Persistent propagation of variant Creutzfeldt-Jakob disease agent in murine spleen stromal cell culture with features of mesenchymal stem cells

The transmission of variant Creutzfeldt-Jakob disease (vCJD) through blood transfusions has created new concerns about the iatrogenic spread of transmissible spongiform encephalopathies (TSEs)/prion diseases through blood and plasma-derived products and has increased the need to develop efficient methods for detection of the agent in biologics. Here, we report the first successful generation of spleen-derived murine stromal cell cultures that persistently propagate two mouse-adapted isolates of human TSE agents, mouse-adapted vCJD, and Fukuoka 1. These new cell cultures can be used efficiently for studies of the pathogenesis of the disease, for development of diagnostics and therapeutics, and as a rapid ex vivo assay for TSE inactivation/removal procedures.

Mouse-adapted sporadic human Creutzfeldt-Jakob disease prions propagate in cell culture

Cell based models used for the study of prion diseases have traditionally employed mouse-adapted strains of sheep scrapie prions. To date, attempts to generate human prion propagation in cell culture have been unsuccessful. Rabbit kidney epithelial cells (RK13) are permissive to infection with prions from a variety of species upon expression of cognate PrP transgenes. We explored RK13 cells expressing human PrP for their utility as a cell line capable of sustaining infection with human prions. RK13 cells processed exogenously expressed human PrP similarly to exogenously expressed mouse PrP but were not permissive to infection when exposed to sporadic Creutzfeldt-Jakob disease prions. Transmission of the same sporadic Creutzfeldt Jakob disease prions to wild-type mice generated a strain of mouse-adapted human prions, which efficiently propagated in RK13 cells expressing mouse PrP, demonstrating these cells are permissive to infection by mouse-adapted human prions. Our observations underscore the likelihood that, in contrast to prions derived from non-human mammals, additional unidentified cofactors or subcellular environment are critical for the generation of human prions.

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.