In normal cells prion protein (PrP) is expressed mainly on the cell surface and is anchored by a glycophosphatidylinositol (GPI) linker. In previous experiments we generated mutant anchorless (GPInegative) PrP, and when these molecules were coincubated in vitro with scrapie brain extracts, they were converted to the abnormal scrapie-associated PrPres isoform, characterized by its partial resistance to protease digestion. In more recent experiments we have generated transgenic (Tg) mice expressing only the anchorless form of PrP. After scrapie infection these Tg mice were found to replicate scrapie infectivity, and deposited PrPres primarily as large amyloid plaques particularly in white matter areas of brain. Surprisingly mice with this accumulation of abnormal PrP did not show the typical fatal clinical course of scrapie disease, but instead lived for over 600days with minimal neurological deficits in most individuals. However, these mice were not clinically normal and showed abnormal exploratory behavior and abnormal neurophysiology. Possibly cell surface anchored PrP is required to mediate the severe pathogenic response seen in typical prion diseases, whereas when membrane anchored PrP is absent a different less virulent pathogenic process might predominate. In scrapie-infected anchorless PrP Tg mice amyloid PrPres was also found to accumulate in heart muscle tissue particularly associated with the small blood vessels. Cardiac catheterization indicated that heart function was compromised and had the hallmarks of the early stages of restrictive cardiomyopathy, as seen in human amyloid heart disease. This appears to be a new model for infectious amyloid heart disease and raises the question of whether heart involvement might occur in other human or animal prion diseases. Studies of the roles of different cell types in retinal and brain prion diseases have used transgenic mice expressing PrP in neurons only or in multiple cell types. After intraocular scrapie inoculation both retinal microglial activation and retinal degeneration were increased in mice expressing PrP in multiple cell types compared to mice expressing PrP in neurons only. This difference was not seen in brain tissue where activation and degeneration were extensive in both mouse strains. These results suggest that different microglial activation mechanisms occur after scrapie infection in retina and brain, and that PrP expression by retinal microglia might be an important factor in this process. Genomics studies of mice infected with three different scrapie strains has identified numerous genes which have altered regulation in brain tissue as a results of the scrapie infection and/or pathogenesis process
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