Strain-specific proteolytic processing of the prion protein in prion diseases of ruminants transmitted in ovine transgenic mice

Cell biology of prion strains in vivo and in vitro

The properties of infectious prions and the pathology of the diseases they cause are dependent upon the unique conformation of each prion strain. How the pathology of prion disease correlates with different strains and genetic backgrounds has been investigated via in vivo assays, but how interactions between specific prion strains and cell types contribute to the pathology of prion disease has been dissected more effectively using in vitro cell lines. Observations made through in vivo and in vitro assays have informed each other with regard to not only how genetic variation influences prion properties, but also how infectious prions are taken up by cells, modified by cellular processes and propagated, and the cellular components they rely on for persistent infection. These studies suggest that persistent cellular infection results from a balance between prion propagation and degradation. This balance may be shifted depending upon how different cell lines process infectious prions, potentially altering prion stability, and how fast they can be transported to the lysosome. Thus, in vitro studies have given us a deeper understanding of the interactions between different prions and cell types and how they may influence prion disease phenotypes in vivo.

Novel Type of Chronic Wasting Disease Detected in Moose (Alces alces), Norway

Chronic wasting disease (CWD) persists in cervid populations of North America and in 2016 was detected for the first time in Europe in a wild reindeer in Norway. We report the detection of CWD in 3 moose (Alces alces) in Norway, identified through a large scale surveillance program. The cases occurred in 13-14-year-old female moose, and we detected an abnormal form of prion protein (PrP^Sc) in the brain but not in lymphoid tissues. Immunohistochemistry revealed that the moose shared the same neuropathologic phenotype, characterized by mostly intraneuronal deposition of PrP^Sc. This pattern differed from that observed in reindeer and has not been previously reported in CWD-infected cervids. Moreover, Western blot revealed a PrP^Sc type distinguishable from previous CWD cases and from known ruminant prion diseases in Europe, with the possible exception of sheep CH1641. These findings suggest that these cases in moose represent a novel type of CWD.

Distinct transmissibility features of TSE sources derived from ruminant prion diseases by the oral route in a transgenic mouse model (TgOvPrP4) overexpressing the ovine prion protein

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases associated with a misfolded form of host-encoded prion protein (PrP). Some of them, such as classical bovine spongiform encephalopathy in cattle (BSE), transmissible mink encephalopathy (TME), kuru and variant Creutzfeldt-Jakob disease in humans, are acquired by the oral route exposure to infected tissues. We investigated the possible transmission by the oral route of a panel of strains derived from ruminant prion diseases in a transgenic mouse model (TgOvPrP4) overexpressing the ovine prion protein (A136R154Q171) under the control of the neuron-specific enolase promoter. Sources derived from Nor98, CH1641 or 87V scrapie sources, as well as sources derived from L-type BSE or cattle-passaged TME, failed to transmit by the oral route, whereas those derived from classical BSE and classical scrapie were successfully transmitted. Apart from a possible effect of passage history of the TSE agent in the inocula, this implied the occurrence of subtle molecular changes in the protease-resistant prion protein (PrPres) following oral transmission that can raises concerns about our ability to correctly identify sheep that might be orally infected by the BSE agent in the field. Our results provide proof of principle that transgenic mouse models can be used to examine the transmissibility of TSE agents by the oral route, providing novel insights regarding the pathogenesis of prion diseases.

Prion protein misfolding, strains, and neurotoxicity: an update from studies on Mammalian prions

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative disorders affecting humans and other mammalian species. The central event in TSE pathogenesis is the conformational conversion of the cellular prion protein, PrP^C, into the aggregate, β-sheet rich, amyloidogenic form, PrP^Sc. Increasing evidence indicates that distinct PrP^Sc conformers, forming distinct ordered aggregates, can encipher the phenotypic TSE variants related to prion strains. Prion strains are TSE isolates that, after inoculation into syngenic hosts, cause disease with distinct characteristics, such as incubation period, pattern of PrP^Sc distribution, and regional severity of histopathological changes in the brain. In analogy with other amyloid forming proteins, PrP^Sc toxicity is thought to derive from the existence of various intermediate structures prior to the amyloid fiber formation and/or their specific interaction with membranes. The latter appears particularly relevant for the pathogenesis of TSEs associated with GPI-anchored PrP^Sc, which involves major cellular membrane distortions in neurons. In this review, we update the current knowledge on the molecular mechanisms underlying three fundamental aspects of the basic biology of prions such as the putative mechanism of prion protein conversion to the pathogenic form PrP^Sc and its propagation, the molecular basis of prion strains, and the mechanism of induced neurotoxicity by PrP^Sc aggregates.

Differentiating ovine BSE from CH1641 scrapie by serial protein misfolding cyclic amplification

Whilst ovine BSE displays distinct pathological characteristics to ovine CH1641-like scrapie upon passage in rodents, they have very similar molecular phenotypes. As such, the in vitro differentiation of these strains in routine surveillance programmes presents a significant diagnostic challenge. In this study, using serial protein-misfolding cyclic amplification (sPMCA), ovine BSE was readily amplified in vitro in brain substrates from sheep with V₁₃₆R₁₅₄Q₁₇₁/V₁₃₆R₁₅₄Q₁₇₁ or AHQ/AHQ PRNP genotypes. In contrast, the CH1641 strain was refractory to such amplification. This method allowed for complete and unequivocal differentiation of experimental BSE from CH1641 prion strains within an ovine host.

Selection of distinct strain phenotypes in mice infected by ovine natural scrapie isolates similar to CH1641 experimental scrapie

A few cases of transmissible spongiform encephalopathies in sheep have been described in France in which the protease-resistant prion protein (PrP(res)) exhibited some features in Western blot of experimental bovine spongiform encephalopathy in sheep. Their molecular characteristics were indistinguishable from those produced in the CH1641 experimental scrapie isolate. Four of these CH1641-like isolates were inoculated intracerebrally into wild-type C57Bl/6 mice. In striking contrast to previous results in ovine transgenic mice, CH1641 transmission in wild-type mice was efficient. Several components of the strain signature, that is, PrP(res) profile, brain distribution, and morphology of the deposits of the disease-associated prion protein, had some similarities with "classical" scrapie and clearly differed from both bovine spongiform encephalopathy in sheep and CH1641 transmission in ovine transgenic mice. These results on CH1641-like isolates in wild-type mice may be consistent with the presence in these isolates of mixed conformers with different abilities to propagate and mediate specific disease phenotypes in different species.

Cell-surface expression of PrPC and the presence of scrapie prions in the blood of goats

Although host-encoded prion protein (PrP(C)) expression in ovine PBMCs and prion infectivity in scrapie-infected sheep blood have been demonstrated, such studies have not been reported in goats. Therefore, this study characterized cell-surface expression of PrP(C) on PBMC subsets derived from normal goats and sheep, by flow cytometry, and determined prion infectivity in blood from a scrapie-infected goat using a transfusion bioassay in goat kids. Cell-surface PrP(C) expression was detected on all subsets of goat PBMCs. The highest PrP(C) cell-surface expression was found in CD2(+) T lymphocytes in goats. Transmission of infection was detected in all three recipients who received whole blood from a goat with classical scrapie. It was concluded that caprine PBMCs express PrP(C) similarly to sheep but with relative differences among PBMCs subsets, and that blood-borne infectious prions can be detected in scrapie-infected goats. Thus, similar to sheep, goat blood may be a suitable diagnostic target for the detection of scrapie infection.

Histopathological studies of "CH1641-like" scrapie sources versus classical scrapie and BSE transmitted to ovine transgenic mice (TgOvPrP4)

The possibility of the agent causing bovine spongiform encephalopathy (BSE) infecting small ruminants is of serious concern for human health. Among scrapie cases, the CH1641 source in particular appears to have certain biochemical properties similar to the BSE strain. In France, several natural scrapie cases were identified as “CH1641-like” natural scrapie isolates in sheep and goats. The Tg(OvPrP4) mouse line expressing the ovine prion protein is a sensitive model for studying and identifying strains of agents responsible for scrapie and BSE. This model is also very useful when studying specific scrapie source CH1641, known to be not transmissible to wild-type mice despite the similarity of some of its biochemical properties to those of the BSE strain. As it is important to be able to fully distinguish CH1641 from BSE, we herein report the histopathological data from CH1641 scrapie transmission experiments compared to specific cases of “CH1641-like” natural scrapie isolates in sheep, murine scrapie strains and BSE. In addition to the conventional vacuolar lesion profile approach and PrP^d brain mappings, an innovative differential PET-blot analysis was introduced to classify the different strains of agent and revealed the first direct concordance between ways of grouping strains on the basis of PrP^d biochemical characteristics.

Emergence of classical BSE strain properties during serial passages of H-BSE in wild-type mice

Background

Two distinct forms of atypical spongiform encephalopathies (H-BSE and L-BSE) have recently been identified in cattle. Transmission studies in several wild-type or transgenic mouse models showed that these forms were associated with two distinct major strains of infectious agents, which also differed from the unique strain that had been isolated from cases of classical BSE during the food-borne epizootic disease.

Methodology/Principal Findings

H-BSE was monitored during three serial passages in C57BL/6 mice. On second passage, most of the inoculated mice showed molecular features of the abnormal prion protein (PrP^d) and brain lesions similar to those observed at first passage, but clearly distinct from those of classical BSE in this mouse model. These features were similarly maintained during a third passage. However, on second passage, some of the mice exhibited distinctly different molecular and lesion characteristics, reminiscent of classical BSE in C57Bl/6 mice. These similarities were confirmed on third passage from such mice, for which the same survival time was also observed as with classical BSE adapted to C57Bl/6 mice. Lymphotropism was rarely detected in mice with H-BSE features. In contrast, PrP^d was detectable, on third passage, in the spleens of most mice exhibiting classical BSE features, the pattern being indistinguishable from that found in C57Bl/6 mice infected with classical BSE.

Conclusion/Significance

Our data demonstrate the emergence of a prion strain with features similar to classical BSE during serial passages of H-BSE in wild-type mice. Such findings might help to explain the origin of the classical BSE epizootic disease, which could have originated from a putatively sporadic form of BSE.

Prions of ruminants show distinct splenotropisms in an ovine transgenic mouse model

Background

Transmissible agents involved in prion diseases differ in their capacities to target different regions of the central nervous system and lymphoid tissues, which are also host-dependent.

Methodology/Principal Findings

Protease-resistant prion protein (PrP^res) was analysed by Western blot in the spleen of transgenic mice (TgOvPrP4) that express the ovine prion protein under the control of the neuron-specific enolase promoter, after infection by intra-cerebral route with a variety of transmissible spongiform encephalopathies (TSEs) from cattle and small ruminants. Splenic PrP^res was consistently detected in classical BSE and in most natural scrapie sources, the electrophoretic pattern showing similar features to that of cerebral PrP^res. However splenic PrP^res was not detected in L-type BSE and TME-in-cattle, or in the CH1641 experimental scrapie isolate, indicating that some TSE strains showed reduced splenotropism in the ovine transgenic mice. In contrast with CH1641, PrP^res was also consistently detected in the spleen of mice infected with six natural “CH1641-like” scrapie isolates, but then showed clearly different molecular features from those identified in the brains (unglycosylated PrP^res at ∼18 kDa with removal of the 12B2 epitope) of ovine transgenic mice or of sheep. These features included different cleavage of the main PrP^res cleavage product (unglycosylated PrP^res at ∼19 kDa with preservation of the 12B2 epitope) and absence of the additional C-terminally cleaved PrP^res product (unglycosylated form at ∼14 kDa) that was detected in the brain.

Conclusion/Significance

Studies in a transgenic mouse model expressing the sheep prion protein revealed different capacities of ruminant prions to propagate in the spleen. They showed unexpected features in “CH1641-like” ovine scrapie suggesting that such isolates contain mixed conformers with distinct capacities to propagate in the brain or lymphoid tissues of these mice.