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MELISSA BLOCK, Host:

This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

MICHELE NORRIS, Host:

And I'm Michele Norris. Researchers have discovered a surprising link between Alzheimer's disease and mad cow disease. It turns out both involve a strange protein called a prion. NPR's Jon Hamilton reports.

JON HAMILTON: Alzheimer's and mad cow both destroy the brain but in very different ways. Mad cow and a similar disease in people are caused by prions - not the normal prion protein that all brains have, but a deformed infectious prion that creates holes in the brain. Alzheimer occurs as sticky plaques build up around brain cells, but it's been unclear how these plaques interrupt the workings of the brain. They don't seem to affect brain cells directly. Researchers at Yale University thought the answer might be that plaques team up with some other molecule, but they were caught off guard by what they found.

STEPHEN STRITTMATTER: At first, we said, no that can't be it. It's, it's too bizarre that these two diseases would share this common protein.

HAMILTON: Stephen Strittmatter and his team had screened hundreds of thousands of molecules that occur naturally in the brain. Only one was a good match - the prion protein, or as some people say it prion.

STRITTMATTER: Now that was a real surprise, not what we were expecting at all. And it's really quite striking because there's this whole - separate from Alzheimer's disease - an entire set of diseases called prion diseases that are based on the cellular prion protein.

HAMILTON: But Strittmatter says maybe it isn't so surprising that there's a connection between mad cow and Alzheimer's.

STRITTMATTER: Once you start thinking about the details, there's so many shared similarities, that it's - actually begins to make a lot of sense.

HAMILTON: For example, both diseases kill brain cells and both cause dementia. The next question was what did prions and plaque working together do to brain tissue? To find out, the Yale team exposed mouse brain tissue to the main substance in Alzheimer's plaques, amyloid peptide. In tissue from normal mice, which contains non-infectious prion protein, the Alzheimer's peptide interfered with the brain cells ability to communicate. Then the team took a slice of tissue from a special mouse, whose brain contains no prion protein.

STRITTMATTER: That slice no longer responded to the presence of this Alzheimer's peptide. Instead, it carried out completely normal functions whether the A beta-peptide from Alzheimer's was there or not.

HAMILTON: In other words, no prions, no problem. It's still unclear exactly how prions allow the plaque proteins to affect brain cells and the study certainly doesn't suggest that prions themselves cause Alzheimer's. But Lennart Mucke, a brain researcher at the Gladstone Institutes at the University of California San Francisco, says although the finding is only in mice, it could be very exciting.

LENNART MUCKE: One would want to know whether these disease causing (unintelligible) between amyloid proteins and the prion proteins happen in the human brain.

HAMILTON: And if it does, could a drug prevent Alzheimer's by keeping prions from interacting with amyloid? Mucke says these questions will be easier to answer because scientists have already spent so many years studying mad cow disease.

MUCKE: We know a great deal about the biochemistry and biology of prion protein, which should really facilitate the development of drugs and specifically at the prion protein.

HAMILTON: The new research appears in the current issue of the journal Nature.

Jon Hamilton, NPR News.

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