Prions


Prions have been a mistery for scientists from the day they where
discovered. Prions act like viruses but they are not. Their structure and
chemistry are unknown. They are believed to be proteins but that is yet to be
completely proved.

Prion stands for “proteinaceous infectious particles”. Prions are known
to cause many diseases involved with nervous systems like the brain. They are
the ones that cause the well known “ mad cow ” disesase in Britain and “scrapie”
for animals. For humans they are known to cause a rare disease in Papua New
Guinea called Kuru ( or “laughing death”) which striked only the cannibals in
the Highlander tribes. Investigation led to the discovery of prions inside the
of the victims brains that were eaten by the tribesmen that when they died, as a
sign of respect their brains where eaten and the chain went on and on.

The thing that makes prions so special is the fact that they lack the
basic elements for reproduction, deoxyribonucleic acid and ribonucleic acid DNA
and RNA respectively. This is what has given science a great deal of doubt as
this would give the dogma of the beginning of live a radical turn.

Prions have been in research for many years with experiments like the
one done by Stanley B. Pruiser and his team of scientists at the School of
Medicine of the University of California at San Francisco in which a study was
carried out on mice to see if he was able to purify the scrapie agent ,another
prion disease, in mice. But mice as humans took very long to develope the
disease, for example Gerstmann-Straussler-Scheinker disease or fatal familial
insomnia, which appear mostly on humans which have passed the age of forty and
only in very rare cases before, so the experiment was changed to hamsters as
these die faster because developed the disease earlier. One of the methods used
for this purification process was using a centrifuge, that separates the
component of a mixture according to their size and density. After a decade of
experiments using the centrifuge method and other chemical methods, several
discoveries were made : It was found out that the infectious particles were
extremely heterogencous in size and density, the scrapie agent can be found in
many molecular forms and the biological activity of the scrapie agent depends on
a protein (PrP, called later when discovered it was a single molecular specie
protein). This protein was found to be a glycoprotein (PrP): sugars are bounded
to the amino acid, and it is half the size of hemoglobin. PrP is the protein,
but we can also find today PrPc (for cellular) and PrPsc (for scrapie). Prp can
be found in “steak” ( skeletal muscle) and also on the surface of lymphocytes
present in milk, but there is no evidence that the ingestion of this things can
cause disease in humans , but there is the still a risk.

Amyloid hypothesis.

Prions were found to form rods: long fibrils in brain tissues infected
with scrapie and Creutzfeldt-Jacob disease. They believed that the fibrils can
be distiguished from amyloid; that they represent a filamentous animal virus
causing scrapie and that they are elongated form of prion rods.The most impotant
aspect of these rods is the resemblance to amyloid. Amyloid plaques in the
central nervous system form considered accumulations of waste formed as some
sort of a disease process. This plaques are believed to be aggregations of
prions in an almost crystalline state. The production of antibodies to PrP
allowed to demonstrate that amyloid plaques in the brain of scrapie-infected
hamsters contain prion proteins. This amyloid plaques have been found on
Alzheimer’s disease patients, which leads to the question if prions are related
to that disease. Although it has not been proven yet, the hypothesis is quite
reliable.

Can prions infectivity be reduced or eliminated?

There were some experiments done with substances to see if prion
infectivity could be reduced or eliminated. One of the substances used was
protease, which has only effects on proteins. Protease reduced prion infectivity
indeed, but was not totally effective. Thats why PrPc is known to be “protease
sensitive”and PrPsc is “relatively resistant to proteases” (thats one
difference). Also by boiling a prion solution in “sodium dodecyl sulfate” (SDS)
the infectivity was reduced as the protein was denatured. Finally, extremely
high doses of radiation inactivated the scrapie agent but this was not a good
solution.

How do Prions infect?

There is a theory proposed by the scientists of the National Institute
of Allergy and Infectious Diseases ( NIAID ) which states that prions do not
need DNA but that they