|Scientists Observe Infectious Prion
Proteins Invade and Move Within Brain Cells
Scientists for the first time have watched agents
of brain-wasting diseases, called transmissible spongiform
encephalopathies (TSE), as they invade a nerve cell
and then travel along wire-like circuits to points of
contact with other cells. These findings will help scientists
better understand TSE diseases and may lead to ways
to prevent or minimize their effects. TSE, or prion,
diseases include scrapie in sheep and goats; chronic
wasting disease in deer and elk; mad cow disease in
cattle; and Creutzfeldt-Jacob disease in humans.
Under the direction of Byron Caughey, Ph.D., at the
Rocky Mountain Laboratories (RML) and Marco Prado, Ph.D.
at the University of Minas Gerais in Belo Horizonte,
Brazil, the team performed the research in laboratory
cultures using a rodent-adapted form of scrapie protein
and cells taken from the central nervous system of mouse
and hamster brains. The proteins were first “branded” with
fluorescent dyes so they could be easily tracked.
The work also revealed that a similar trafficking process
might occur with the key plaque-forming protein in Alzheimer’s
disease, which is not a TSE but a different type of
degenerative brain disease, according to Gerald Baron,
Ph.D., one of the lead RML researchers. RML, located
in Hamilton, MT, is part of the National Institute of
Allergy and Infectious Diseases (NIAID) of the National
Institutes of Health. The new report appears in the
May 25 issue of The Journal of Neuroscience.
“These findings offer intriguing leads toward developing
therapies to stop the spread of TSE and possibly other
degenerative brain diseases,” says NIAID Director Dr.
Anthony Fauci. “Potentially, it may be possible to block
the pathways that prions use to invade cells, their
exit to other cells or their replication within the
Those are precisely some of the next experiments the
RML group is pursuing, along with trying to move the
fluorescent tracking method from laboratory cell cultures
to live mice and hamsters. Along with Drs. Caughey,
Prado and Baron, other key researchers involved in the
project included Kil Sun Lee, Ph.D., RML, and former
RML employee Ana Cristina Magalhães, Ph.D., also from
the Federal University of Minas Gerais. Dr. Baron explains
that throughout his seven years at RML, he and others
have contemplated how to use fluorescent tracking to
learn more about TSEs, but they struggled to develop
an effective method to do so.
“When I started working on TSEs, I thought about them
as being similar to intracellular bacterial pathogens — something
that replicates within an animal or human host cell,” says
Dr. Baron. “I wanted to know how such a pathogen binds
to the host cell, and how it enters, replicates and
spreads to other cells.”
Dr. Baron says researchers have tracked infectious
prion protein moving through other parts of animal bodies
up to the brain, but no one had ever tracked the protein
movement within animal brain cells. One of the most
difficult aspects of the experiment, he says, was finding
a way to fluorescently tag the TSE prion proteins without
altering them — while still allowing researchers to identify
the prions as they penetrated the cells and spread within
the long projections that nerve cells develop to send
signals to other nerve cells.
“This was difficult from a technical aspect because
the scrapie pathogen is largely a corrupted form of
a host cell protein,” Dr. Baron said. “It can be hard
to detect the corrupted prion protein in living infected
cells and distinguish it from its normal counterpart.”
He explains that once researchers learned how to mark
the prion proteins, they added them to a culture of
nerve cells and then began watching and taking photo
images with a confocal microscope. Confocal microscopy
uses laser light to scan many thin sections of a fluorescent
sample, resulting in a clean three-dimensional image.
The painstaking job of analyzing and deciphering about
1,000 different images primarily belonged to Dr. Magalhães — who
filled a file cabinet drawer with CDs containing microscopic
images. The effort resulted in striking photos that,
when put into a video format, show prion protein moving
within cells, then along narrow cellular projections
called neurites and ultimately into close proximity
with adjacent cells.
Other areas the research group plans to explore include:
- Exactly where within nerve cells does scrapie infection
occur, and how does this happen?
- How and why do large masses of infectious prion
protein attach to host cells and become broken into
smaller units so that they can invade the cell interior?
- What types of chemical messages are sent between
neurites from one cell to another that allow infectious
prions to transfer between cells?
- What happens to the infectious prion protein once
it is transferred to another cell?
- How do the many different possible pathways that
lead into cells determine what happens to prion protein;
some pathways could lead to digestion by the cell,
others lead to transfer — and presumably infection — in
“This has been pretty amazing — certainly a new approach
for our field,” Dr. Baron says.
NIAID is a component of the National Institutes of
Health, an agency of the U.S. Department of Health and
Human Services. NIAID supports basic and applied research
to prevent, diagnose and treat infectious diseases such
as HIV/AIDS and other sexually transmitted infections,
influenza, tuberculosis, malaria and illness from potential
agents of bioterrorism. NIAID also supports research
on transplantation and immune-related illnesses, including
autoimmune disorders, asthma and allergies.
News releases, fact sheets and other NIAID-related
materials are available on the NIAID Web site at http://www.niaid.nih.gov.
Reference: A Magalhães et al. Uptake and neuritic transport of scrapie prion protein coincident with infection of neuronal cells. The
Journal of Neuroscience 25 (21):5207-16 (2005). DOI: 10.1523/JNEUROSCI.0653-05.2005.