Prions are infectious misfolded proteins (first purified in 1982 by Stanley Prusiner and his team from the University of California, San Francisco). Once introduced, prions induce normal proteins to misfold as well, propagating the malformed proteins throughout the infected tissue. Creutzfeldt–Jakob disease is a fatal neurological disease caused by prions in humans. Similar diseases in cows and sheep are called bovine spongiform encephalopathy (mad cow disease) and scrapie, respectively. Now Prusiner, along with Jan Stöhr, Joel Watts and their colleagues from the University of California, San Francisco have shown that Alzheimer’ disease (AD) may also be caused by prions.
Autopsies of AD patients show that their brains contain plaques made of amyloid beta peptides. As these plaques spread throughout the brain, they are believed to be responsible for the neurological problems associated with AD. Prusiner and the other researchers hypothesized that AD is initiated by the introduction of an amyloid beta prion that sets off a wave of plaque-creating misfolded amyloid beta peptides.
To test this, the scientists used mice that were genetically bred so that any plaques in their brains would fluoresce. Injecting the mice with either purified amyloid beta protein or with synthetically constructed amyloid beta peptides initiated a chain reaction of plaque formation in the mice’s brains. This strongly suggests that the peptides are acting as prions, that is, that they are infections proteins.