Investigating the Role of the Prion Protein Polybasic Domain in PrPSc Formation and Neuroprotection

Prion diseases are characterized by the conformational conversion of the cellular prion protein (PrPC) into the misfolded PrP Sc isoform. However, both the normal, physiological function of PrP C and the mechanism of its conversion into PrPSc remain undefined. Several lines of evidence suggest that the expression of PrP can be neuroprotective, including its ability to abrogate the neurotoxic activity of PrP deletion mutants. In my thesis work, I characterized the role of the polybasic domain (residues 23-31) in the neuroprotective effect of WT PrP and examined the role of these residues in the formation of the infectious PrPSc protein.;Previously generated transgenic mice expressing PrP deleted for residues 32-134 displayed a spontaneous neurodegenerative phenotype rescued by co-expression of WT PrP. In order to define the domains of PrP involved in this neuroprotective activity, I have analyzed the ability of several N-terminal truncation mutants, all deleted for PrP residues 23-31, to prevent the neurotoxicity induced by expression of Delta32-134 PrP. My data show that deletion of these residues greatly diminishes the ability of PrP to rescue from Delta32-134, pinpointing the N-terminal, polybasic domain as a critical determinant of PrPC neuroprotective activity.;In the second part of my thesis work, I examined the role of residues 23-31 in conversion to PrPSc. Scrapie infection of transgenic mice expressing Delta23-31 PrP resulted in a reduced accumulation of PrP Sc and a lengthened incubation time compared to control mice expressing full-length PrP. Further investigation demonstrated that Delta23-31 PrP is a poor substrate for conversion, displaying a reduced ability to form Delta23-31 PrPSc both in vivo and in vitro. Co-immunoprecipitation assays indicated that the polybasic domain is an essential part of the PrPC-PrPSc binding site. These experiments provide the first direct in vivo evidence that residues 23-31 play an important role in the conversion of PrPC to PrPSc.;The critical involvement of PrP's polybasic domain in both conversion and neuroprotection suggests that the identification of molecules interacting with this region may help to elucidate the normal and disease-associated functions of the protein. My work also implies that this nine amino acid stretch represents a therapeutic target for the treatment of infectious prion diseases.