Prion proteins THE 'protein only' hypothesis' states that a modified form of normal prion protein triggers infectious neurodegenerative diseases, such as bovine spongiform encephalopathy (BSE), or Creutzfeldt-Jakob disease (CJD) in humans. Prion proteins are thought to exist in two different conformations: the 'benign' PrPc form, and the infectious 'scrapie form', PrPSc. Knowledge of the three-dimensional structure of prpc is essential for understanding the transition to PrPSc.

Schematic representation of the post-translational modifications (a) and secondary structure (b) of murine PrP

Structure of the PrPc 210 aminoacids 33 - 35 kDa 2 N-linked oligosaccharydes GPI anchor (membrane-bound) C-terminal 3 a-helices and 2 antiparalell b-sheets Solution structure of PrPc obtained last year by Wüthrich, Glockshuber, and coworkers at the Swiss Federal Institute of Technology shows three - helices and an antiparallel ß-sheet at the protein's carboxy terminus and a "flexibly disordered" segment at its amino terminus.

PrP gene MANLGCWMLVLFVATWSDLGLCKKRPKPGGWNTGGSRYPGQGSPGGNRYP PQGGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQ GGGTHSQWNKPSKPKTNMKHMAGAAAAGAVVGGLGGYMLGSAMSRPIIHFGS DYEDRYYRENMHRYPNQ VYYRPMDEYSNQNNFVHDCVNITIKQHTVTTTTKGENFTETDVKMMERVVEQ MCITQYERESQAYYQRGS SMVLFSSPPVILLISFLIFLIVG

Ribbon diagram of the structure of the mouse prion protein domain PrP(121-231)

Globular fold and surface properties of PrP(121-231).

The propagation of infectious prion protein occurs via conversion of normal prion protein (PrPc, left) to a disease-causing form (PrPSc, right). In the refolding process, some of the -helical regions (purple coils) in PrPc unfold, forming an extended ß-sheet region (flat blue arrows). Transmission electron micrographs obtained by Lindquist and coworkers of the University of Chicago show fibers formed by yeast Sup35 protein. At right is a single fiber at higher magnification.

Mutations in the PrP gene

Spongiform Encephalopathies This is your Brain . . . . . . . this is your Brain on Beef CJD - human Kuru - human BSE - (cow) Scrapie - (sheep) Vacuolization of neuronal cytoplasm results in the typical sponge-like appearance of brain parenchyma.

Virus hypothesis The genetic information defining the properties of the various scrapie strains is encoded by a nucleic acid. The important role of PrP in disease might be explained in that PrP plays an essential role in the infection and spreading of the virus in the host. Mutations in PrPC would alter susceptibility to the disease. The species barrier for infection could be caused by a reduced affinity of a given virus to the PrPC molecule of a different host species. Conversion of PrPC into PrPSc would be brought about by the interaction of the virus with PrPC. The virus hypothesis readily explains the existence of distinct scrapie strains; however, it is challenged by the claim that in preparations of highly enriched infectivity, the average size of a nucleic acid per infectious unit is not larger than 80 nucleotides

Virino hypothesis Postulates that the infectious particle consists of a small nucleic acid coated by a host protein. Variations in the nucleic acid sequence could account for the existence of distinct strains, as for example, in viroids. The size of the nucleic acid could be very small since it does not have to encode for any proteins. The unusual resistance to treatments destroying or modifying nucleic acids could result from a protective effect of the coat. The most likely candidate for the host protein forming the coat of the virino is PrPSc. Standard preparations of purified infectivity usually contain approximately 105 PrPSc molecules per infectious Infectivity might be associated to a subpopulation of PrPSc molecules, denominated PrP*, which may have escaped detection because it is extremely rare or difficult to distinguish from the rest of PrPSc.

‘Protein only’ hypothesis The information for strain specificity is carried by an infectious protein. To explain the mechanism of propagation of such an unusual infectious agent, it was proposed that the agent consisted of a modified host protein which was able to convert the host protein into a likeness of itself. In order to account for the strain specificities, the modified host protein would have to exist in various different isoforms. The most likely candidate for an infectious protein causing TSEs is PrPSc (or PrP*) because it represents a disease-associated posttranslational modification of a host-encoded protein and because of its alleged association with infectivity.

Two ‘protein only’ models for the molecular mechanism ‘refolding’ model - the rate-determining step is the conformational conversion of PrPC into PrPSc, a process which is catalyzed by the formation of a PrPC - PrPSc heterodimer ‘nucleation-dependent polymerization’ model the rate-determining step is the formation of a nucleus of polymerized PrPSc, which, once formed, promotes further polymerization of PrPSc. Such nuclei would act as polymerization seeds in an infected host thereby rapidly driving host PrP into the aggregated state.

Microscopy of the normal brain (top left) shows dark-staining neurons in the cerebral cortex with no spongy change in surrounding tissue. In classical CJD (top right), brain cortex contains numerous small vacuoles that give a spongelike appearance. In new-variant CJD (bottom left), the cortex shows less severe sponginess but contains "florid" plaques--aggregates of amloid material surrounded by spongy change. Florid plaques in new-variant CJD (bottom right) stain intensely for prion protein (brown). According to the template assistance (or heterodimer) model, PrPc monomer adds to PrPSc monomer to form PrPSc -PrPC heterdimer, which rapidly converts to PrPSc homodimer. The homodimer may then dissociate, releasing new PrPSc monomer. The nucleated polymerization mechanism proposed that a PrPSc oligomer or polymer recruits PrPc, which then converts to PrPSc.

the conversion of PrPC to PrPSc as a consequence of a disease process in which PrPC might act as a receptor for the infectious agent

Scrapie. Naturally occurring scrapie in sheep and goats has a duration of 2-6 months and typically appears between the age of 2.5-4.5. Bovine Spongiform Encephalopathy Clinical signs of bovine spongiform encephalopathy (BSE) typically appear at the age of 4-5 as progressive apprehension, hyperaesthesia, and incoordination of gait with a duration of 1-6 months. Human TSEs: Creutzfeldt-Jakob disease. The age of onset of the disorder typically lies between the age of 50 and 75. CJD occurs worldwide with an incidence of approximately 0.5 cases per million per annum. Iatrogenic cases are extremely rare. Ca. 15% of all CJD cases represent familial CJD with a dominant pattern of inheritance. GSS (Gertsmann-Straussler syndrome) in patients typically between 35-55 years of age as a slowly progressive ataxia of 2-10 years duration. GSS is nearly always described in a familial context; it occurs in approximately 1 per 107 people. Kuru. It was observed predominantly among children and adult females and had a duration of 3-9 months. It is confined to the Fore tribe of Papua New Guinea (PNG), and is caused by cannibalistic rituals, specifically the preparation and eating of human brains Fatal familial insomnia (FFI) Most patients were between the ages of 40 and 60; disease duration was 7-18 months. The occurrence of fatal familial insomnia (FFI) is associated with the same codon 178 aspartic acid to asparagine mutation. Experimental TSE. Neuropathology of experimental TSE is mostly studied in hamsters and mice because of the relatively short incubation times and the convenience of animal keeping. Animals are usually inoculated intracerebrally.