Transgenic mouse models for the study of prion diseases

Prion replication in organotypic brain slice cultures is distinct from in vivo inoculation and is species dependent

Cultured brain slices rapidly replicate murine prions, exhibit prion pathology, and are amenable towards drug discovery, but have not been infected with human prions. As deer mice (Peromyscus maniculatus) are susceptible to human prions in vivo, here we investigated deer mouse organotypic brain slice cultures as a potential model of human prion disease. Deer mouse brain slices supported replication of rodent-adapted strains of scrapie and Creutzfeldt-Jakob disease (CJD), but they resisted infection with primary human prion inoculum. To better understand this discrepancy, we quantified prion replication rates, characterized cellular and molecular changes, and estimated inoculum clearance within wildtype CD1 and deer mouse brain slice cultures. Prion replication rates varied by species, strain, and brain region, independently of PrP sequence homology. Scrapie-infected CD1 cerebellar slice cultures exhibited the fastest prion replication rate, closely matching in vivo bioassay kinetics and showing neuronal and synaptic degeneration at similar timepoints. However, deer mouse slice cultures replicated deer mouse-adapted sCJD MM1 prions less efficiently than in vivo inoculation. These findings clarify both the utility and constraints of brain slice cultures in modeling prion disease and imply that the slice culture molecular environment may be suboptimal for human prion replication.

PrP is cleaved from the surface of mast cells by ADAM10 and proteases released during degranulation

While several functions of the endogenous prion protein have been studied, the homeostatic function of prion protein is still debated. Notably, prion protein is highly expressed on mast cells, granular immune cells that regulate inflammation. When activated, mast cells shed prion protein, although the mechanism and consequences of this are not yet understood. First, we tested several mast cell lines and found that, while prion protein was almost always present, the total amount differed greatly. Activation of mast cells induced a cleavage of the N-terminal region of prion protein, and this was reduced by protease inhibitors. Exogenous mast cell proteases caused a similar loss of the prion protein N-terminus. Additionally, mast cells shed prion protein in an ADAM10-dependent fashion, even in the absence of activation. Our results suggest that prion protein is cleaved from resting mast cells by ADAM10 and from activated mast cells by mast cell proteases. Prion protein also appears to affect mast cell function, as Prnp-/- bone marrow-derived mast cells showed lower levels of degranulation and cytokine release, as well as lower levels of both FcεRI and CD117. Finally, we sought to provide clinical relevance by measuring the levels of prion protein in bodily fluids of asthmatic patients, a disease that involves the activation of mast cells. We found an N-terminal fragment of prion protein could be detected in human sputum and serum, and the amount of this prion protein fragment was decreased in the serum of patients with asthma.

PrP meets alpha-synuclein: Molecular mechanisms and implications for disease

The discovery of prions has challenged dogmas and has revolutionized our understanding of protein-misfolding diseases. The concept of self-propagation via protein conformational changes, originally discovered for the prion protein (PrP), also applies to other proteins that exhibit similar behavior, such as alpha-synuclein (aSyn), a central player in Parkinson's disease and in other synucleinopathies. aSyn pathology appears to spread from one cell to another during disease progression, and involves the misfolding and aggregation of aSyn. How the transfer of aSyn between cells occurs is still being studied, but one important hypothesis involves receptor-mediated transport. Interestingly, recent studies indicate that the cellular prion protein (PrPC) may play a crucial role in this process. PrPC has been shown to act as a receptor/sensor for protein aggregates in different neurodegenerative disorders, including Alzheimer's disease and amyotrophic lateral sclerosis. Here, we provide a comprehensive overview of the current state of knowledge regarding the interaction between aSyn and PrPC and discuss its role in synucleinopathies. We examine the properties of PrP and aSyn, including their structure, function, and aggregation. Additionally, we discuss the current understanding of PrPC's role as a receptor/sensor for aSyn aggregates and identify remaining unanswered questions in this area of research. Ultimately, we posit that exploring the interaction between aSyn and PrPC may offer potential treatment options for synucleinopathies.

Hypothesis: functional age and onset of autosomal dominant genetic prion disease

Autosomal dominant diseases typically have an age-related onset. Here, I focus on genetic prion disease (gPrD), caused by various mutations in the PRNP gene. While gPrD typically occurs at or after middle age, there can be considerable variability in the specific age of onset. This variability can occur among patients with the same PRNP mutation; in some cases, these differences occur not only between families but even within the same family. It is not known why gPrD onset is typically delayed for decades when the causative mutation is present from birth. Mouse models of gPrD manifest disease; however, unlike human gPrD, which typically takes decades to manifest, mouse models exhibit disease within months. Therefore, the time to onset of prion disease is proportional to species lifespan; however, it is not known why this is the case. I hypothesize that the initiation of gPrD is strongly influenced by the process of aging; therefore, disease onset is related to proportional functional age (e.g., mice vs. humans). I propose approaches to test this hypothesis and discuss its significance with respect to delaying prion disease through suppression of aging.

Calcineurin Controls Cellular Prion Protein Expression in Mouse Astrocytes

Prion diseases arise from the conformational conversion of the cellular prion protein (PrP^C) into a self-replicating prion isoform (PrP^Sc). Although this process has been studied mostly in neurons, a growing body of evidence suggests that astrocytes express PrP^C and are able to replicate and accumulate PrP^Sc. Currently, prion diseases remain incurable, while downregulation of PrP^C represents the most promising therapy due to the reduction of the substrate for prion conversion. Here we show that the astrocyte-specific genetic ablation or pharmacological inhibition of the calcium-activated phosphatase calcineurin (CaN) reduces PrP^C expression in astrocytes. Immunocytochemical analysis of cultured CaN-KO astrocytes and isolation of synaptosomal compartments from the hippocampi of astrocyte-specific CaN-KO (ACN-KO) mice suggest that PrP^C is downregulated both in vitro and in vivo. The downregulation occurs without affecting the glycosylation of PrP^C and without alteration of its proteasomal or lysosomal degradation. Direct assessment of the protein synthesis rate and shotgun mass spectrometry proteomics analysis suggest that the reduction of PrP^C is related to the impairment of global protein synthesis in CaN-KO astrocytes. When WT-PrP and PrP-D177N, a mouse homologue of a human mutation associated with the inherited prion disease fatal familial insomnia, were expressed in astrocytes, CaN-KO astrocytes showed an aberrant localization of both WT-PrP and PrP-D177N variants with predominant localization to the Golgi apparatus, suggesting that ablation of CaN affects both WT and mutant PrP proteins. These results provide new mechanistic details in relation to the regulation of PrP expression in astrocytes, suggesting the therapeutic potential of astroglial cells.

Homozygous R136S mutation in PRNP gene causes inherited early onset prion disease

BACKGROUND

More than 40 pathogenic heterozygous PRNP mutations causing inherited prion diseases have been identified to date. Recessive inherited prion disease has not been described to date.

METHODS

We describe the clinical and neuropathological data of inherited early-onset prion disease caused by the rare PRNP homozygous mutation R136S. In vitro PrPSc propagation studies were performed using recombinant-adapted protein misfolding cyclic amplification technique. Brain material from two R136S homozygous patients was intracranially inoculated in TgMet129 and TgVal129 transgenic mice to assess the transmissibility of this rare inherited form of prion disease.

RESULTS

The index case presented symptoms of early-onset dementia beginning at the age of 49 and died at the age of 53. Neuropathological evaluation of the proband revealed abundant multicentric PrP plaques and Western blotting revealed a ~ 8 kDa protease-resistant, unglycosylated PrPSc fragment, consistent with a Gerstmann-Sträussler-Scheinker phenotype. Her youngest sibling suffered from progressive cognitive decline, motor impairment, and myoclonus with onset in her late 30s and died at the age of 48. Genetic analysis revealed the presence of the R136S mutation in homozygosis in the two affected subjects linked to homozygous methionine at codon 129. One sibling carrying the heterozygous R136S mutation, linked to homozygous methionine at codon 129, is still asymptomatic at the age of 74. The inoculation of human brain homogenates from our index case and an independent case from a Portuguese family with the same mutation in transgenic mice expressing human PrP and in vitro propagation of PrPSc studies failed to show disease transmissibility.

CONCLUSION

In conclusion, biallelic R136S substitution is a rare variant that produces inherited early-onset human prion disease with a Gerstmann-Sträussler-Scheinker neuropathological and molecular signature. Even if the R136S variant is predicted to be "probably damaging", heterozygous carriers are protected, at least from an early onset providing evidence for a potentially recessive pattern of inheritance in human prion diseases.