In this application comprising four scientific projects and four-cores, we propose to continue our studies focused on neurodegeneration caused by human prion diseases, the most common of which is sporadic (s) CJD. Prions seem to be composed solely of PrPSc molecules, which are derived from a precursor PrPc by a poorly understood process. The studies described here are aimed at defining the structure of PrPSc, characterizing the interactions of small molecules with both PrPc and PrPSc and dissecting the molecular events governing the propagation of different human prion strains. In Project 1, we propose to study the interactions of polyoxometalates (POMs) with PrPSc. The POM phosphotungstate anion [PW12O40] (PTA) binds specifically to PrPSc, but not to PrP. POMs are a large class of inorganic metal oxide clusters with rigid polyhedral structures displaying substantial variations in size, shape, and charge density. In Project 2, we propose to carry out fiber diffraction studies of the 55-residue MoPrP(89-143,P101L) peptide that causes inherited prion disease, a 20 mer wt PrP(106-126) peptide known to form amyloid fibrils as well as purified truncated (PrP 27-30) and full-length PrPSc, both of which are infectious in wild-type animals. In Project 3, we propose to study the prions causing sCJD. These studies are possible because our most sensitive Tg mouse line expressing chimeric human/mouse PrP succumbs to disease in ~80 days after inoculation with sCJD prions. Relatively rapid bioassays of human prions in these Tg mice make it practical to measure the titers of prions throughout brain as well as in peripheral organs and body fluids collected from dead sCJD patients. We also propose to develop guinea pig models of sCJD and variant (v) CJD. In Project 4, we propose to discover new ligands that bind to and stabilize human PrPc. We also plan to determine whether such ligands inhibit the conversion of HuPrPc into HuPrPSc. Using a virtual screening approach, large libraries of organic molecules will be docked against the known structure of HuPrPc. High-scoring compounds will be tested for binding biophysically, controlling for non-specific inhibition to which anti-amyloid inhibitors are prone. The ultimate goal of all the proposed studies is to define the molecular events that feature in the formation of human prions in order to develop therapeutics that cure the human prion diseases.