Dev Biol Stand 1993;80:31-44
Department of Neurology, University of California, San Francisco 94143.
Studies over the past decade of the infectious prions causing scrapie and other transmissible neurodegenerative diseases support the hypothesis that these pathogens are novel. After convincing evidence was obtained showing that scrapie infectivity depends upon a protein component, the term "prion" was introduced to distinguish these infectious pathogens from others, including viroids and viruses. Enriching fractions from Syrian hamster (SHa) brain for scrapie prion infectivity led to the discovery of the prion protein (PrP). Transgenic (Tg) mice expressing both SHa and mouse (Mo) PrP genes were used to probe the molecular basis of the species barrier and the mechanism of scrapie prion replication. Bioassays of brain extracts from two scrapie-infected Tg lines showed that the prion inoculum dictates which prions are synthesized de novo, even though the cells express both PrP genes. Discovery of mutations in the PrP gene from humans with Gerstmann-Straussler-Scheinker disease (GSS), familial Creutzfeldt-Jakob disease and fatal familial insomnia established that prion diseases are unique among human illnesses--they are both genetic and infectious. Tg mice expressing MoPrP with the GSS point mutation spontaneously develop neurologic dysfunction, spongiform degeneration and astrocytic gliosis. Inoculation of brain extracts prepared from these Tg(GSSMoPrP) mice into Syrian hamsters and Tg mice expressing wild-type PrP transgenes has produced neurodegeneration in recipient animals after prolonged incubation times. If convincing data on serial passage of prions from the inoculated recipients can be obtained, then these results will argue that prions are devoid of foreign nucleic acid in accord with many other lines of evidence. Although it seems likely that transmissible prions are composed only of PrPSc molecules, a hypothetical second component such as a small polynucleotide remains a formal possibility. The conversion of PrPc into PrPSc is a post-translational process that probably occurs in the endocytic pathway. Studies on the structure of PrPSc and PrPC have been unsuccessful in defining a post-translational chemical modification that distinguishes one PrP isoform from the other. These findings suggest that the difference between PrPSc and PrPc may be conformational. The existence of distinct prion isolates or "strains" with different properties poses a conundrum. Distinct isolates produce the accumulation of PrPSc in particular sets of CNS neurons. Whether different conformers of or Asn-linked CHOs attached to PrPSc are produced in specific neurons and are responsible for the properties of distinct prion isolates is unknown. The study of prion diseases seems to be emerging as a new area of investigation at the interface of such disciplines as genetics, cell biology and virology.
Dev Biol Stand 1991;75:55-74
Considerable progress has been made in deciphering the role of an abnormal isoform of the prion protein (PrP) in scrapie of animals and Gerstmann-Straussler syndrome (GSS) of humans. Transgenic (Tg) mice expressing both Syrian hamster (Ha) and mouse (Mo) PrP genes, which encode proteins differing at 16 residues out of 254, were used to probe the mechanism of scrapie prion replication. Four Tg lines expressing HaPrP exhibited distinct incubation times ranging from 48 to 277 days after Ha prion inoculation, which were inversely correlated with the steady-state levels of HaPrP mRNA and HaPrPc. Bioassays of brain extracts from two scrapie-infected Tg lines showed that the prion inoculum dictates which prions are synthesized de novo, even though the cells express both PrP genes. Tg mice inoculated with Ha prions had approximately 10(9) ID50 units of Ha prions per gram of brain while less than 10 units of Mo prions were found. Conversely, Tg mice inoculated with Mo prions had approximately 10(6) ID50 units of Mo prions and less than 10 units of Ha prions. Consistent with the analysis of prion synthesis, Tg mice inoculated with Ha prions exhibited neuropathologic changes characteristic of hamsters with scrapie while Mo prions produced changes similar to those in non-Tg mice with scrapie. Our results argue that species specificity of scrapie prions resides in the primary structure of PrP and formation of infectious prions is initiated by a species-specific interaction between PrPSc in the inoculum and homologous, cellular PrP. Studies on Syrian, Armenian and Chinese hamsters suggest that the domain of the PrP molecule between codons 100 and 120 controls both the length of the incubation time and the deposition of PrP in amyloid plaques. Ataxic GSS in families shows genetic linkage to a mutation in the PrP gene leading to the substitution of Leu for Pro at codon 102. Discovery of a point mutation in the PrP gene from humans with GSS established that GSS is unique among human diseases--it is both genetic and infectious. These results have revised thinking about sporadic Creutzfeldt-Jakob disease (CJD) suggesting it may arise from a somatic mutation. Pulse-chase radiolabeling experiments of scrapie-infected cultures of mouse neuroblastoma cells indicate that protease-resistant PrPSc is synthesized during the chase period with t1/2 approximately 15 h from a protease-sensitive precursor, consistent with the conclusion that PrPc and PrPSc differ due to a post-translational event.