Oxidation sets off fatal structural change of human prion proteins

Prion diseases like the Creutzfeldt-Jakob disease mainly appear spontaneously in humans. They are characterised by the aggregation of a misfolded isoform of the cellular prion protein. Scientists at the Max Planck Institute of Biochemistry and the LMU Munich have now uncovered the cause of the misfolding: an oxidation within the prion molecule.

Prion diseases can be sporadic, inherited and infectious. The vast majority (85 percent) of prion diseases in humans can be attributed to a spontaneous structural conversion of the cellular prion protein: Originally dominated by alpha-helices as structural elements, the prion protein is transformed into its misfolded 'scrapie' isoform which is dominated by accordion-like folded protein sheets, so called beta-sheets. This changes its chemical properties: The molecule becomes less water-soluble and has a strong aggregation tendency.

'Once there is a misfolded and aggregated prion protein present in the tissue, a chain reaction is triggered where one protein after another changes its shape, like dominos knocking over each other,' says Professor Armin Giese (Centre for Neuropathology and Prion Research, LMU Munich). The initial event in this misfolding cascade was so far widely unknown. Now the scientists have identified an oxidation within the prion protein as the cause of the structural conversion:

'Although other mechanisms are also discussed, we are convinced that the oxidation of the amino acid methionine within the prion protein plays a key role,' reports PD Dr Nediljko Budisa, the head of the research group 'Molecular Biotechnology' at the Max Planck Institute. While the hydrophobic methionine usually stabilises alpha-helices effectively, its oxidized form supports the structural conversion into beta-sheets. If the oxidative stress within the cell is sufficient enough to oxidise certain methionine molecules within the prion protein, an irreversible process with serious consequences starts: 'The prion protein gets literally pushed apart because of the oxidation,' says Budisa, 'obviously this is devastating for the folding.'

To prove these findings conclusively the scientists used an elegant trick: The methionine molecules were replaced with isosteric, chemically stable, non-oxidisable analogues, i.e. with the more hydrophobic norleucine (simulates non-oxidised methionine) and the highly hydrophilic methoxinine (simulates oxidised methionine). 'In this way, we created artificial prion proteins, which, like Yin and Yang, reflect two extreme conditions: One prion, that contains non-oxidised methionine molecules only, and one in which all methionine molecules are oxidised,' explains Budisa. The norleucine variant resulted in an alpha-helix rich protein that lacks the in vitro aggregation protein of the parent protein. In contrast, the methoxinine variant resulted in a beta-sheet rich protein with strong aggregation tendency.

These results support a correlation of oxidative stress in cells and the misfolding of proteins. They are highly relevant not only for prion research, but also for other neurodegenerative diseases that are associated with protein misfolding. Research in this field is of general importance, because it can give new insights in neurodegenerative diseases and help with the development of new therapeutic strategies.