Proof Positive for Prions

21 April 2005 (All day)

Prion factory. A schematic representation of the cyclic process by which a molecule of PrPc (left side of circle) interacts with and is converted into PrPSc (right side of circle). PrPSc damages normal brain tissue (left panel) by forming long fibers (right panel).

A new study has taken another step toward proving the "prion hypothesis"—the idea that rogue proteins are responsible for a host of fatal neurological diseases, including Mad Cow Disease and scrapie.

According to the prion hypothesis, the disease-causing protein, PrPSc, makes copies of itself inside the brain, multiplying until it forms fibers that destroy neurons and eventually kill the animal. PrPSc has the same amino acid sequence as a normal brain protein called PrPc, but folds into a different three-dimensional shape. It replicates by corrupting any PrPc it encounters, twisting it into a new PrPSc, which in turn transforms more PrPc. Although recent research has overwhelmingly supported the prion hypothesis [ScienceNOW 15 October, 2003], one vital experiment was missing: No one had induced a prion disease in a healthy animal using pure PrPSc made in a test tube.

In work reported in the 22 April Cell, neurobiologist Claudio Soto of the University of Texas Medical Branch in Galveston and colleagues come close to achieving that goal. To generate high levels of PrPSc, Soto's group performed 20 rounds of “infection” in a test tube. In the first round, the group mixed ground up healthy brain with brain material from a scrapie-infected animal. Just as in an intact animal, the PrPc in the healthy brain material was converted to PrPSc. In subsequent rounds, they used material from the previous round to infect fresh healthy brain material. The procedure yielded billions of molecules of PrPSc from just a few starting molecules. Although some of the original scrapie brain material was carried through the early rounds of the experiment, it eventually became diluted to the point where it disappeared from the sample. Nonetheless, the material was very infectious, efficiently causing scrapie when injected into the brains of hamsters.

The caveat, UCSF neurobiologist Giuseppe Legname notes, is that because the researchers started with scrapie brain, not pure PrPSc, something else that contributes to scrapie may have been amplified along with PrPSc. However, Legname says, this system could be used to detect the low levels of prions present in the blood of infected animals. Such a test is "desperately needed," he says, to reduce the chances that prions will be spread through food or medical blood products.

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