Is AD contagious? Beta-amyloid and Prion Proteins

Contributed by: Michael Rafii, M.D., Ph.D - Director of the Memory Disorders Clinic at the University of California, San Diego. ______________________________________
A prion is an infectious agent composed of protein in a misfolded form. The word prion was coined in 1982 by Dr. Stanley B. Prusiner, and is derived from the words protein and infection.

Normal prion proteins are produced naturally in the brain, but can cause disease when they come into contact with an infectious form of the protein that folds into an unusual conformation. These infectious prions convert innocuous prion proteins into the infectious form, which forms clumps and leads to neurodegenerative diseases, such as variant Creutzfeldt-Jakob disease, the human form of mad cow disease. Prions slowly destroy the brain tissue of infected people by causing a cascade of misshapen proteins. They're known to spread via consumption of contaminated food, by getting a transfusion of blood or tissue transplant from someone who is infected.

In 2009, researchers led by Dr. Stephen Strittmatter at Yale, showed that prion proteins produced naturally in the brain interact with the amyloid-ß peptides that are hallmarks of Alzheimer's disease. Blocking this interaction in preparations made from mouse brains halted some neurological defects caused by the accumulation of amyloid-ß peptide.

Now, researchers led by Dr. Caludio Soto at the University of Texas Health Science Center, have shown that, in fact, Alzheimer's disease itself, may be a prion-like disease. His team injected the brain tissue of a confirmed Alzheimer’s patient into mice and compared the results to those from injected tissue of a control without the disease. None of the mice injected with the control showed signs of Alzheimer’s, whereas all of those injected with Alzheimer’s brain extracts developed plaques and other brain alterations typical of the disease.

These findings suggest that in an experimental setting, misfolded beta-amyloid can behave in a similar way as infectious prions. It remains to be proven whether at least a proportion of human AD cases could be due to a transmissible prion-like mechanism. It must be kept in mind, that prions are not contagious via normal human contact.