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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 License.)

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.