Alzeihmer's Disease and Prions

Scientific American cited a study in the journal Nature which found that prions can team up with amyloid beta, the protein that forms the big plaques in Alzheimer's, to make the condition much worse. Another article in Science news claims "Prions Complicit in Alzheimer's Disease" and calls the findings "sensational" although the research is still in its early stages.

What are prions?

Proteinaceous infectious particles- or PRIONS- are infectious self-reproducing protein structures2. Prions cause a number of degenerative brain diseases: scrapie (a fatal disease in sheep and goats), mad cow disease, Creutzfeldt-Jacob disease, fatal familial insomnia and others1. MedicineNet.com defines a prion as a disease-causing agent that is neither bacterial nor fungal nor viral and contains no genetic material1. A prion is a protein that is harmless in its normally occurring form, but when it folds into an aberrant shape it turns into a rouge agent. It then causes other normal prions to become rouge prions.

How do they replicate?

Prions replicate within the cell by converting other normal cell proteins into other prions3. This method of replication is unique to prions since other agents-like viruses- that cause disease contain genetic material which is necessary for the to spread themselves through the victim5. Since prions do not have genetic material this method is irrelevant. Instead, the prion corrupts a perfectly normal protein, PrP, which usually sits on the external surface of brain cells5.
In general, healthy prion protein that does not cause disease is made of alpha helices. The infectious prion protein is mainly made of beta-pleated sheets4. The infectious prion has a changed structural form dominated by conversion of protein helix structure into flat sheets, even though the amino acid sequence is identical in both strains6. The mechanism for the conversion of the α-helices by infectious ß-pleated sheet prion is not exactly known. Experiments have shown that normal protein interacts with the beta sheet prion form and then changes its structure4.


References:

1. Definition of Prion. 1/22/2004. MedicineNet.com 02/01/2009. http://www.medterms.com/script/main/art.asp?articlekey=5047
2. Prion. Bio-Medicine. http://www.bio-medicine.org/biology-definition/Prion/
3. Pall Medical. http://www.pall.com/pdf/prions-QandA.pdf
4. SFN. How does a prion reproduce? http://www.scienceforums.net/forum/showthread.php?t=18577
5. The prion: simply mad. BBC News. 5/19/1998. http://news.bbc.co.uk/1/hi/health/background_briefings/bse/82972.stm
6. Another hazard: prions and mad cow disease. http://peer.tamu.edu/curriculum_modules/cell_Biology/module_5/hazards3.htm