Prion protein expression is required for susceptibility to scrapie infection, but the cell types required for this effect are not known. To study the cell type-specific effects of prion protein expression in vivo mice expressing prion protein in unique cell types, such as neurons only or astrocytes only, were studied. Intraocular infection induced scrapie brain disease in mice expressing PrP in neurons only, or astrocytes only, however, no retinal damage was observed. In contrast, intraocular infection of mice expressing PrP in multiple cell types lead to severe retinal degeneration. Therefore, PrP expression in additional cell types besides neurons and astrocytes is required for scrapie-induced retinal damage. Using accurate in vivo tests, mice devoid of PrP expression were found to have deficits in hippocampal memory as well as hippocampal electrophysiology. These deficits could be reversed by the presence of a transgene which induced PrP expression in neurons only, indicating that presence of PrP on neurons is required for normal function in these tests. Normally prion protein is expressed as a cell surface protein anchored to the cell membrane by a GPI linker molecule. To study the role of PrP membrane anchoring on scrapie infection, transgenic mice which expressed only an anchorless PrP were generated. After infection of these transgenic mice, scrapie agent was found to replicate and scrapie-associated protease-resistant prion protein was deposited in brain as amyloid plaques. However, these mice survived over 500 days post-infection and failed to show typical signs of scrapie. These results indicate that by itself amyloid protease-resistant PrP may not be able to cause the rapid brain damage usually seen in prion diseases. Most likely membrane-anchored PrP may also be required to receive the maximal neurotoxic signals.
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