I was doing some research on Huntington’s disease when I came across the idea that prions, infectious proteins, may be more common than we previously thought. As we all know, prions are accepted as the disease-causing agent in transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease. These infectious proteins form aggregates, and the breakage of the aggregates represents protein replication.
Amyloid plaques, seen in a variety of neurodegenerative diseases, are also are created by the misfolding and aggregation of proteins. Studies on Alzheimer’s and Parkinson’s disease have given some researchers reason to suspect that the proteins involved in these diseases may exhibit prion-like behavior. Apparently an early study on Alzheimer’s found that the disease was transmissible from the white blood cells of people to hamsters. However, this finding was never reproduced. Another study found that aggregates of AB peptide from brains of Alzheimer’s patients could be passed to mice genetically engineered to produce large amounts of the AB precursor protein. Another study grafted health brain tissue into the brains of Parkinson’s patients, and the tissue acquired protein aggregates associated with the disease pathology. Amyloid A amyloidosis can be excreted in feces, and at least in geese, it can induce amyloidosis if ingested.
The author of the 2005 paper published in Nature voicing this idea makes an important distinction between prions and what he calls “prionoids.” Prions are transmissible through populations and can cause epidemics, but “prionoids” can only affect neighboring molecules or cells. This seems like a very significant distinction to me, and if this is the case, calling the proteins in these neurodegenerative diseases prions seems downright confusing. However, it’s a very interesting idea and once again blurs the distinction of what we consider life.
Aguzzi, Adriano. “Cell biology: Beyond the prion principle.” Nature 18 Jun. 2009; 459:924-925.