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A group of French researchers has conducted
a study that led to the development of a test using baker's yeast (Saccharomyces
cerevisiae) to isolate compounds that are active against prions. The
study was carried out by Marc Blondel, Stéphane Bach and Laurent
Meijer of the Roscoff Biological Station (CNRS – University of Paris
6,), in collaboration with the teams of Christophe Cullin (University
of Bordeaux 2), Hervé Galons (University of Paris 5) and Dominique
Dormont (CEA, French Atomic Energy Commission). These promising findings
were published in the September issue of the journal Nature Biotechnology.
Prions are infectious proteins that cause neuro-degenerative
spongiform encephalopathy-like diseases in mammals, such as Creutzfeldt-Jakob
in humans and the so-called mad cow disease in bovines and scrapie in
sheep. The primary characteristic of these diseases is that they are caused
by non-conventional infectious agents; unlike classic infectious agents
(for example, bacteria and viruses), they do not contain nucleic acid.
This led 1997 Nobel Prizewinner Stanley Prusiner to put forward the hypothesis
that the infectious agent is made up of a single protein. Certain molecules
that are effective inhibitors of mammalian prions have already been identified
thanks to a system based on mice nerve cells that are chronically infected
by the mammalian protein.
The idea for the new method to use in the yeast-based screening assay
came about following two observations:
1- given the highly infectious nature of mammalian prions, all experiments
must be carried out in high-security, bio-safety level 3 (P3) laboratories.
In addition, the complexity of the mammalian system does not permit a
high-throughput screening approach;
2- baker's yeast contains several proteins that behave like prions and
that, unlike mammalian prions, are entirely harmless, both for the yeast
and for humans. The Roscoff researchers imagined that the mechanisms controlling
the appearance and/or the maintenance of the prions are conserved from
yeast to humans, and put forward the idea that the characteristics of
yeast (easy to use, inexpensive, and harmless) could provide the basis
to develop a high-throughput screening method.
They thus devised a two-step, yeast-based method to screen for anti-prion
drugs: during the first step (primary screening), collections of molecules
(chemical databases) were tested to identify anti-prion activity. When
a compound exhibits an anti-prion effect, it causes a reddening of the
yeast colonies around the disk of filter paper on which it was deposited
(see photograph). The compounds that promote prion clearance then undergo
secondary screening based on another, very different yeast prion. Given
the considerable difference between the two prions, the researchers reasoned
that compounds that are active against both are active against general
mechanisms of the control of yeast prions, and not against mechanisms
that are specific to one or the other of the two prions.
The initial assumption concerning the conservation of prion control mechanisms
from yeast to humans was then proven by testing, in the yeast system,
the identified compounds that were effective in vitro against the mammalian
prions. These molecules also turned out to be active against yeast prions.
Thanks to collaborative work with Dominique Dormont's laboratory, the
researchers moreover observed that the compounds identified in the yeast
test also were also effective against the mammalian prion.
These findings are very important because they currently represent the
first functional indication of the conservation of prion control mechanisms
from yeast to humans. They therefore validate the yeast-based screening
method.
The CNRS has filed a patent application to protect this method as well
as the first molecules isolated using the method.
For more information:
Nature Biotechnology: http://www.nature.com/nbt/
pdf format article: http://www.sb-roscoff.fr/CyCell/Page12.htm
Web site of the cellular cycle team: http://www.sb-roscoff.fr/CyCell/
Researcher
contact:
Marc Blondel, CNRS chargé de recherche
Tel: +33 2 98 29 23 22
E-mail: Blondel@sb-roscoff.fr
Press contact:
Martine Hasler
Tel: +33 1 44 96 46 35
E-mail: martine.hasler@cnrs-dir.fr
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