Unraveling prion strains with cell biology and organic chemistry

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Abstract
Prions are the infectious agents causing transmissible spongiform encephalopathies (TSEs), which comprise human Creutzfeldt–Jakob disease (CJD), scrapie of sheep, bovine spongiform encephalopathy (BSE), and several other rare ailments of various species. According to the protein-only hypothesis (1), prions are composed solely of PrPSc, a misfolded form of the cellular protein PrPC. PrPSc typically forms highly ordered fibrillary aggregates, also termed “amyloid.” The term “prion strain” denotes individual prion isolates sharing the same PrP sequence but giving rise to distinct, stable disease traits with different incubation periods and lesion profiles upon serial transmission in congenic hosts. The propagation of different strains in mice congenic with respect to their Prnp allelotypes is difficult to explain by the protein-only hypothesis because the epigenetic strain characteristics of prions appear to dominate over the primary prion protein sequence of the infected host (2, 3). Circumstantial evidence suggests that strain phenotypes are encoded by distinct conformations of PrPSc (Fig. 1). This was first implied by experiments showing that distinct strains of transmissible mink encephalopathy went along with different protease-exposed sites within PrPSc (4). Great strides have been made since then, yet the final proof that conformational variants of PrPSc represent the biological basis of mammalian prion strains is still elusive. Distinct prion strains may bear highly divergent risks of transmission to humans: Sheep scrapie-derived strains may be mostly innocuous, whereas BSE-derived strains appear to induce variant …