Re: What is the difference between proteins that cause Alzheimers and prions?

Lisa, you have asked an excellent question.

Proteins are long linear chains of amino acids that fold up into specific 
shapes.  Occasionally, proteins become folded into the wrong shapes.  
These misfolded proteins are unable to perform their natural functions in 
the body’s biochemical pathways.  Usually this is harmless, and the body 
can do a little housekeeping to remove them, but in some cases, these 
misfolded proteins can actually cause damage.  This is the case with 
Alzheimer’s disease.  There is a protein found within neuronal cells 
called “amyloid beta.”  The natural function of this protein is not well 
understood, but if this protein becomes misfolded, it can precipitate out 
of cellular solution.  Essentially, the misfolded protein begins to form 
aggregates within and around brain cells.  The aggregates build up and 
become the plaques that are found within the brains of Alzheimer’s 
patients.  The damage to the brain cells caused by the plaques leads to 
the dementia that is so characteristic of Alzheimer’s disease.  As people 
get older, the chance that their amyloid beta will become misfolded 
increases;  therefore Alzheimer’s patients are primarily elderly.  There 
are also inherited forms of Alzheimer’s disease in which an individual has 
a mutated form of amyloid beta that is much more prone to misfolding and 
the disease can strike at an early age.  Alzheimer’s disease, however, is 
not contagious – it cannot be transmitted from one person to another.

Creutzfeldt-Jakob disease (CJD) is one of a small group of fatal diseases 
caused by infectious agents called prions.  The term prion comes 
from “proteinaceous infectious particle.”  Prions are normal proteins that 
are misfolded.  Unlike the amyloid beta protein, prions are infectious.  
In other words, if they are physically transported from one organism to 
another, the disease that they cause can be spread.  Bovine spongiform 
encephalopathy (“mad cow disease,” another prion disease) was spread in 
Europe in the 1990’s when meat products from infected animals were fed to 
other animals.  In another example, the prion disease kuru was spread 
among cannibalistic tribes in New Guinea when individuals ate the brains 
of others who were already infected with the disease.

So to answer your question, you are correct in stating that both 
Alzheimer’s disease and CJD are caused by the same fundamental problem – 
misfolded proteins.  But Alzheimer’s disease is not generally considered a 
prion disease because it is not infectious – it cannot be transmitted from 
one person to another.

If you are interested in learning more about prion diseases, I found a 
good web page written by Stanley B. Prusiner who won the Nobel Prize for 
Medicine in 1997 for his pioneering research into prion diseases:
 http://www.nmia.com/~mdibble/prion.html

I also found on this web page an excellent essay on protein folding:
 http://www.faseb.org/opar/protfold/protein.html