Quantification of surviving cerebellar granule neurones and abnormal prion protein (PrPSc) deposition in sporadic Creutzfeldt-Jakob disease supports a pathogenic role for small PrPSc deposits common to the various molecular subtypes

Cerebellar compartmentation of prion pathogenesis

In prion diseases, the brain lesion profile is influenced by the prion "strain" properties, the invasion route to the brain, and still unknown host cell-specific parameters. To gain insight into those endogenous factors, we analyzed the histopathological alterations induced by distinct prion strains in the mouse cerebellum. We show that 22L and ME7 scrapie prion proteins (PrP^22L , PrP^ME7 ), but not bovine spongiform encephalopathy PrP^6PB1 , accumulate in a reproducible parasagittal banding pattern in the cerebellar cortex of infected mice. Such banding pattern of PrP^22L aggregation did not depend on the neuroinvasion route, but coincided with the parasagittal compartmentation of the cerebellum mostly defined by the expression of zebrins, such as aldolase C and the excitatory amino acid transporter 4, in Purkinje cells. We provide evidence that Purkinje cells display a differential, subtype-specific vulnerability to 22L prions with zebrin-expressing Purkinje cells being more resistant to prion toxicity, while in stripes where PrP^22L accumulated most zebrin-deficient Purkinje cells are lost and spongiosis accentuated. In addition, in PrP^22L stripes, enhanced reactive astrocyte processes associated with microglia activation support interdependent events between the topographic pattern of Purkinje cell death, reactive gliosis and PrP^22L accumulation. Finally, we find that in preclinically-ill mice prion infection promotes at the membrane of astrocytes enveloping Purkinje cell excitatory synapses, upregulation of tumor necrosis factor-α receptor type 1 (TNFR1), a key mediator of the neuroinflammation process. These overall data show that Purkinje cell sensitivity to prion insult is locally restricted by the parasagittal compartmentation of the cerebellum, and that perisynaptic astrocytes may contribute to prion pathogenesis through prion-induced TNFR1 upregulation.

The Immp2l mutation causes age-dependent degeneration of cerebellar granule neurons prevented by antioxidant treatment

Reactive oxygen species are implicated in age‐associated neurodegeneration, although direct in vivo evidence is lacking. We recently showed that mice with a mutation in the Inner Mitochondrial Membrane Peptidase 2‐like (Immp2l) gene had elevated levels of mitochondrial superoxide, impaired fertility and age‐associated phenotypes, including kyphosis and ataxia. Here we show that ataxia and cerebellar hypoplasia occur in old mutant mice (> 16 months). Cerebellar granule neurons (CGNs) are significantly underrepresented; Purkinje cells and cells in the molecular layer are not affected. Treating mutant mice with the mitochondria‐targeted antioxidant SkQ1 from 6 weeks to 21 months protected cerebellar granule neurons. Apoptotic granule neurons were observed in mutant mice but not in age‐matched normal control mice or SkQ1‐treated mice. Old mutant mice showed increased serum protein carbonyl content, cerebellar 4‐hydroxynonenal (HNE), and nitrotyrosine modification compared to old normal control mice. SOD2 expression was increased in Purkinje cells but decreased in granule neurons of old mutant mice. Mitochondrial marker protein VDAC1 also was decreased in CGNs of old mutant mice, suggesting decreased mitochondrial number. SkQ1 treatment decreased HNE and nitrotyrosine modification, and restored SOD2 and VDAC1 expression in CGNs of old mutant mice. Neuronal expression of nitric oxide synthase was increased in cerebella of young mutant mice but decreased in old mutant mice. Our work provides evidence for a causal role of oxidative stress in neurodegeneration of Immp2l mutant mice. The Immp2l mutant mouse model could be valuable in elucidating the role of oxidative stress in age‐associated neurodegeneration.

A closer look at prion strains: characterization and important implications

Prions are infectious proteins that are responsible for transmissible spongiform encephalopathies (TSEs) and consist primarily of scrapie prion protein (PrP^Sc), a pathogenic isoform of the host-encoded cellular prion protein (PrP^C). The absence of nucleic acids as essential components of the infectious prions is the most striking feature associated to these diseases. Additionally, different prion strains have been isolated from animal diseases despite the lack of DNA or RNA molecules. Mounting evidence suggests that prion-strain-specific features segregate with different PrP^Sc conformational and aggregation states.

Strains are of practical relevance in prion diseases as they can drastically differ in many aspects, such as incubation period, PrP^Sc biochemical profile (e.g., electrophoretic mobility and glycoform ratio) and distribution of brain lesions. Importantly, such different features are maintained after inoculation of a prion strain into genetically identical hosts and are relatively stable across serial passages.

This review focuses on the characterization of prion strains and on the wide range of important implications that the study of prion strains involves.

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.

Comparison of the pathologic and pathogenic features in six different regions of postmortem brains of three patients with fatal familial insomnia

Fatal familial insomnia (FFI) is an autosomal dominant prion disease clinically characterized by rapidly progressive insomnia, prominent autonomic alterations and behavioral disturbance. The D178N mutation of the prion protein gene (PRNP) on chromosome 20 in conjunction with methionine at codon 129 is a molecular feature. Although the neuropathological characteristics of FFI are well documented, the neuropathologic and pathogenic features of FFI patients remain poorly understood. Six brain regions of postmortem brains from 3 FFI patients were examined using immunohistochemistry, western blot analyses and quantitative real-time PCR. In all 3 brain specimens, reactive astrogliosis was found to be more severe in the thalamus than in the cortex regions. Western blot analyses showed that all three brains expressed PrP, but only 2 were associated with significantly weak proteinase K (PK) resistance. However, the conformational stabilities of PrPSc in the 3 FFI brains were significantly weaker than those presented in a G114V genetic Creutzfeldt-Jakob disease (gCJD) case. Immunohistochemistry and western blot analyses showed comparable amounts of neuron-specific enolase (NSE)-positive stained cells and NSE protein among the different regions in the three brains. In addition, the transcriptional levels of glial fibrillary acidic protein (GFAP) and NSE-specific mRNAs were coincident with the expression of these proteins. In conclusion, in the present study, we described the detailed regional neuropathology of FFI cases.

Disease-associated mutations in the prion protein impair laminin-induced process outgrowth and survival

Prions, the agents of transmissible spongiform encephalopathies, require the expression of prion protein (PrP(C)) to propagate disease. PrP(C) is converted into an abnormal insoluble form, PrP(Sc), that gains neurotoxic activity. Conversely, clinical manifestations of prion disease may occur either before or in the absence of PrP(Sc) deposits, but the loss of normal PrP(C) function contribution for the etiology of these diseases is still debatable. Prion disease-associated mutations in PrP(C) represent one of the best models to understand the impact of PrP(C) loss-of-function. PrP(C) associates with various molecules and, in particular, the interaction of PrP(C) with laminin (Ln) modulates neuronal plasticity and memory formation. To assess the functional alterations associated with PrP(C) mutations, wild-type and mutated PrP(C) proteins were expressed in a neural cell line derived from a PrP(C)-null mouse. Treatment with the laminin γ1 chain peptide (Ln γ1), which mimics the Ln binding site for PrP(C), increased intracellular calcium in cells expressing wild-type PrP(C), whereas a significantly lower response was observed in cells expressing mutated PrP(C) molecules. The Ln γ1 did not promote process outgrowth or protect against staurosporine-induced cell death in cells expressing mutated PrP(C) molecules in contrast to cells expressing wild-type PrP(C). The co-expression of wild-type PrP(C) with mutated PrP(C) molecules was able to rescue the Ln protective effects, indicating the lack of negative dominance of PrP(C) mutated molecules. These results indicate that PrP(C) mutations impair process outgrowth and survival mediated by Ln γ1 peptide in neural cells, which may contribute to the pathogenesis of genetic prion diseases.

Biochemical and strain properties of CJD prions: complexity versus simplicity

Prions, the agents responsible for transmissible spongiform encephalopathies, are infectious proteins consisting primarily of scrapie prion protein (PrP(Sc)), a misfolded, β-sheet enriched and aggregated form of the host-encoded cellular prion protein (PrP(C)). Their propagation is based on an autocatalytic PrP conversion process. Despite the lack of a nucleic acid genome, different prion strains have been isolated from animal diseases. Increasing evidence supports the view that strain-specific properties may be enciphered within conformational variations of PrP(Sc). In humans, sporadic Creutzfeldt-Jakob disease (sCJD) is the most frequent form of prion diseases and has demonstrated a wide phenotypic and molecular spectrum. In contrast, variant Creutzfeldt-Jakob disease (vCJD), which results from oral exposure to the agent of bovine spongiform encephalopathy, is a highly stereotyped disease, that, until now, has only occurred in patients who are methionine homozygous at codon 129 of the PrP gene. Recent research has provided consistent evidence of strain diversity in sCJD and also, unexpectedly enough, in vCJD. Here, we discuss the puzzling biochemical/pathological diversity of human prion disorders and the relationship of that diversity to the biological properties of the agent as demonstrated by strain typing in experimental models.