Investigating the Mad Cow Prion
By: Carol A. Rouzer, VICB Communications
Published: March 14, 2010
Structural studies of prion proteins provide clues to neurodegenerative diseases.
Beef eaters throughout the western world took notice when cattle in the United Kingdom
started becoming ill with a strange neurological disease. Since its appearance in the mid
1980s, over 180,000 cases of mad cow disease or bovine spongiform encephalopathy (BSE)
have been diagnosed in the UK. Concern turned to fear as a similar progressive and fatal
neuro-degenerative disease began to appear in humans, especially when that disease was attributed to eating meat from the affected animals.
BSE and its human equivalent, Creutzfeldt-Jacob Disease (CJD), are caused not by an
infecting virus, bacterium, or parasite, but by a simple protein called a prion. The infectious
prion protein, PrPSc, is an abnormal version of a membrane protein, PrPC, that is widely
distributed in cells of the neurological and immune systems. Most investigators believe that the
only difference between PrPSc and PrPC is the way that the amino acid chain that makes up the
protein is folded. The aberrant shape of PrPSc causes it to aggregate into insoluble fibers called
amyloid, deposits of which can be seen in the
brains of cattle and humans suffering from BSE or
CJD. If a healthy cow or person eats meat
contaminated with neural tissue that contains
PrPSc, and the abnormal protein enters cells in the
brain or spinal column, it induces the normal PrPC
that is present in the cells to change shape and
form amyloid aggregates. Ultimately, this process
damages the cells, leading to cell death. The
infected brain tissue takes on the look of a sponge
due to holes left in spaces once occupied by
healthy neurons (Figure 1).
Figure 1. Micrographs of brain tissue from a cow with
BSE showing amyloid deposits (arrows, A) and
spongiform degeneration (B).
(Images courtesy of Wikimedia Commons under the
GNU Free Documentation License.)
Fortunately, changes in the way cattle are fed has stopped the
spread of BSE, and CJD remains a rare disease in humans. However, the much more common
neurodegenerative diseases, Alzheimer’s disease and Parkinson’s disease, are also
characterized by abnormal accumulations of proteins as amyloid fibers. This has led Vanderbilt
Institute of Chemical Biology investigator Gerald Stubbs and his collaborator Stanley Prusiner at
the Institute for Neurodegenerative Diseases at the University of California San Francisco to
believe that discovering the mechanism by which prion proteins form amyloid fibers will provide keys to understanding the mechanism of brain damage in all of these conditions.
Because the key to a prion’s toxicity is its shape, understanding the three-dimensional
structure of PrPSc is critical. Most proteins that form amyloid-type fibers take on a structure
characterized by layers of amino acid chains running perpendicular to the axis of the fiber
(called a cross-β structure, Figure 2).
Figure 2. Cross-β amyloid structure
consists of layers of amino acid chains
lying perpendicular to the fiber axis.
(Image courtesy of Wikimedia Commons
under the GNU Free Documentation
Investigators have generally believed that PrPSc amyloid
fibers contain this same structure. However, PrPSc fibers are highly unstable and insoluble, so it
has been difficult to confirm this hypothesis. Now the Stubbs laboratory and their UCSF
collaborators have achieved this goal [Wille et al. (2009) Proc. Natl. Acad. Sci. U.S.A., 106, 16990]. The UCSF investigators isolated “natural” amyloid
fibers from the brains of infected hamsters and mice, and
“recombinant” PrP expressed in genetically engineered
bacteria. In each case, stable amyloid preparations allowed
the investigators to obtain structural data from a process
called X-ray fiber diffraction, which was performed at the
Stanford Synchrotron Radiation Laboratory, the Advanced
Photon Source at Argonne National Laboratory, and at the
ALS synchrotron, Berkeley. Data analysis by the Stubbs lab
confirmed the presence of cross-β structure in both natural
and recombinant amyloid samples. However, closer
examination revealed striking differences between the
structures of the amyloid samples isolated from the brains of
infected animals and those prepared from bacteria.
Specifically, samples from animal sources displayed evidence
of a helical structure, while those from bacteria appeared to be formed from multiple stacks of
protein sheets. Since the recombinant samples showed lower levels of infectivity than those
from animal sources, these structural differences provide clues to the mechanisms by which
PrPSc proteins cause neurotoxicity. The Stubbs group is now working to obtain more detailed
structural data on these valuable amyloid preparations, and they are looking forward to
extending their success to the study of amyloid fibers from other neurodegenerative diseases.