The transmissible agent causing scrapie must contain more than protein

Cell-associated variants of disease-specific prion protein immunolabelling are found in different sources of sheep transmissible spongiform encephalopathy

Scrapie and bovine spongiform encephalopathy (BSE) are transmissible spongiform encephalopathies (TSEs) or prion diseases affecting domestic and exotic ruminants. In previous immunohistochemical studies, we have shown that different sheep TSE sources may be distinguished by both the proportion of disease-specific prion protein (PrP(d)) accumulation relative to different cell types in the brain (the 'PrP(d) profile') and by different labelling patterns for PrP peptide sequences within phagocytic cells. In the present study, we have further characterized the intracellular accumulation patterns of PrP(d) in the lymphoreticular system (LRS) and in the brain of sheep clinically affected with scrapie or BSE. BSE-infected PrP(ARQ/ARQ) sheep of different breeds were compared with scrapie-infected sheep of different PrP genotypes. Cases of BSE infection could be distinguished from scrapie cases by a marked reduction in labelling of PrP(d) containing the 84-105 amino acid residues in phagocytic cells of the LRS and in neurones and glia of the brain. These results therefore indicate that TSE agent-dependent processing of PrP in specific cell types within the brain and LRS can be used to distinguish between BSE in PrP(ARQ/ARQ) sheep and scrapie in sheep of several PrP genotypes. Three different N-terminal peptide antibody labelling patterns were recognized for different cell types in different tissues of BSE-infected sheep, suggesting that different truncated forms of PrP(d) are formed following infections with this agent strain. These variations in the cleavage sites of BSE PrP(d) may be due to cell-specific variation in endosomal-lysosomal digestion or to cell- and tissue-specific differences in BSE PrP(d) conformation.

TSE agent strains and PrP: reconciling structure and function

The molecular structures of the infectious agents that cause transmissible spongiform encephalopathy (TSE) diseases are still not known despite the current wide acceptance of the prion hypothesis as the basis for their resolution. Here, data supporting and challenging the prion hypothesis in relation to both the biochemical and biological properties of TSE infectious agents are discussed. The need for the independent transmission of TSE agent-specific genetic information is described and the requirements for the molecule to carry this information are proposed. Such a molecule is likely to be a small nucleic acid encoding information to determine the diversity of the pathogenesis of TSE agents.

Scrapie

Scrapie and other transmissible spongiform encephalopathies (TSEs) are characterized by similar pathology, biochemistry and genetics. The PrP protein and its conversion to the disease-related isoform, PrPSC, are crucial for the development of all TSEs. Although scrapie is more often studied in laboratory rodents, it is not a natural disease of these animals, and much can be learned from the normal hosts, sheep. Disease incidence is linked to polymorphisms and mutations of the PrP gene. The complex relationships between PrP genotype and the survival of sheep subjected to scrapie infection are now being investigated in terms of the different structure of the PrP protein molecules produced by each allele. It is these structures and their differing abilities to convert to PrPSC that hold the key to understanding why TSEs occur.

PrP genetics in sheep and the applications for scrapie and BSE

The strong links between PrP genotype and the occurrence of scrapie in sheep strengthen evidence supporting the central importance of the PrP protein in the development of transmissible spongiform encephalopathies, despite the fact that the cattle PrP gene has, so far, failed to show any association between PrP alleles and susceptibility to BSE.

The association between PrP and infectivity in scrapie and BSE infected mouse brain

The structure of the scrapie agent remains unknown. However, scrapie infectivity tends to co-sediment with an infection specific fraction of the glycoprotein PrP (PrPSc) under conditions which solubilise the normal form of this protein (PrPc); accordingly, PrP has been proposed as a candidate component of the agent. To investigate this further we have been examining a new scrapie-related murine model in conjunction with established scrapie models. A bovine spongiform encephalopathy (BSE) derived murine model has short incubation periods, high infectivity titre and low amounts of PrP deposited in the brain. A membrane fraction from scrapie/BSE infected brain is solubilised with Sarkosyl at pH > or = 9.0. Most PrP is also solubilised. In models of the disease with little deposition of the PrP in the brain, this solubilisation step is particularly effective in reducing the amounts of PrP sedimented from brain extracts. Gradient centrifugation of the sedimented fraction shows further separation of infectivity and the residual PrP. It is concluded that at least some PrPSc in the brain need not be associated directly with infectious agents but is deposited in brain solely as a pathological product of infection. However, a residual sedimentable fraction contains PrP which may be a component of the agent.

Murine scrapie-infected neurons in vivo release excess prion protein into the extracellular space

An originally heretical proposition that the transmissible spongiform encephalopathies are caused by a host-coded protein (the prion hypothesis) is now current dogma. Indeed these disorders are commonly called prion diseases but the prion hypothesis provides no readily acceptable explanation for the source of the informational component of the agent necessary to code for the diversity of strains of scrapie. Ultrastructural immunolocalisation of prion protein (PrP) in murine scrapie shows that PrP accumulates in association with the plasmalemma of neurones, diffusing from the neuronal cell surface into the extracellular space around small neurites prior to aggregation and fibril assembly. These events occur without the involvement of other cell types. The area of neuropil infiltrated with extracellular PrP around infected neurons and neurites indicates that the form of PrP initially produced is not immediately amyloidogenic.