Scientists have produced a prion protein that can trigger the development of a neurological disorder in mice that is similar to "mad cow" disease, according to a new study supported by the National Institute on Aging, a part of the National Institutes of Health.
The findings demonstrate that prions, an unusual class of infectious proteins, can make copies of themselves without the presence of viral DNA or RNA, damage brain tissue, and cause neurological diseases.
The work by Nobel Laureate Stanley B. Prusiner, M.D., and colleagues at the University of California, San Francisco, and Heinrich-Heine Universitat in Germany, appears in the July 30, 2004, issue of Science. For the study, Dr. Prusiner and his colleagues produced prion protein fragments in bacteria, folded them into larger protein structures called amyloid fibrils, and then injected them into the brains of susceptible mice. The mice began exhibiting symptoms of disease in their central nervous systems between 380 and 660 days after they were given the synthetic prion proteins. The amyloid form of the prion protein, which is thought to cause prion disease, was also found in the brains of the diseased mice. The researchers then administered brain extracts from these animals to another group of mice, which subsequently developed similar symptoms 90 to 150 days later. The disorder seems to be distinct from that caused by other known strains of prions, suggesting that the synthetic prion didn't merely activate a pre-existing prion in these mice and that the synthesized prion protein itself is sufficient to make infectious and disease-causing prions.
Prusiner received the 1997 Nobel Prize in physiology or medicine for his discovery of prions. Unlike viruses, bacteria, fungi and parasites, prions contain no DNA or RNA. Instead, they are a type of protein normally found within cells in humans and other organisms. In some cases, the structure of prions can change into a disease-causing form. These abnormal proteins appear to convert other, normal prions to the abnormal shape. Many scientists now believe this conversion process leads to several dementing diseases in humans, including Creutzfeldt-Jakob disease. Similar diseases in animals include bovine spongiform encephalopathy ("mad cow" disease) in cattle and scrapie in sheep.
Abnormal, misfolded proteins contribute to other age-related neurological diseases such as Alzheimer's and Parkinson's diseases, and so these new findings may provide insights into the cause and possible prevention of other brain disorders.