Scientists have discovered a surprising link between Alzheimer's disease and mad cow disease. It turns out both diseases involve something called a prion protein.
The finding, which appears in the journal Nature, could explain one of the great mysteries in Alzheimer's disease: How components of the plaques that form in patient's brains are able to damage brain cells. It also could point the way to new treatments for the disease.
"It's very exciting," says Lennart Mucke, director of the Gladstone Institute of Neurological Disease and a professor of neurology and neuroscience at the University of California, San Francisco. "The study shines the light on a very unexpected component."
Mucke was not involved in the study, but wrote an article that accompanied it in Nature.
In mad cow disease, and a similar human condition called new variant Creutzfeldt-Jakob disease, prion proteins fold into an abnormal shape that appears to cause degeneration of the brain and spinal cord. Prion diseases can be transmitted by eating the brain or spinal cord of a sick animal.
In Alzheimer's, prion proteins appear to play a different role, says Stephen Strittmatter, one of the new study's authors and the Vincent Coates Professor of Neurology at Yale University School of Medicine.
Strittmatter says there's no evidence the prion proteins fold into an abnormal shape or actually cause Alzheimer's. Instead, they seem to interact with early stage plaques in the brain in a way that allows those plaques to damage brain cells.
Strittmatter's team made the discovery after looking at hundreds of thousands of molecules that occur naturally in the brain. The prion protein turned out to be the best at interacting with a protein called amyloid-beta, which is what forms the plaques in Alzheimer's
"At first they said, 'No that can't be,'" Strittmatter says. "It's too bizarre that these two diseases would share this common protein."
But he says it seemed less strange when they considered that both diseases affect brain cells and cause dementia.
"Once you start thinking about the details," Strittmatter says, "there are so many shared similarities that it actually begins to make a lot of sense."
After showing that the amyloid-beta from plaque could interact with prion protein, the researchers needed to demonstrate that the combination could harm brain cells.
So the Yale team exposed mouse brain tissue to small clusters of amyloid-beta.
In tissue from normal mice, which contains non-infectious prion proteins, the amyloid interfered with brain cells' ability to communicate.
Then, the team took a slice of tissue from a special mouse whose brain contained no prion protein.
"That slice no longer responded," Strittmatter says. "It carried out completely normal functions."
No prion protein, no problem.
It's still unclear exactly how prion proteins allow amyloid beta to affect brain cells. And the study certainly doesn't suggest that prion proteins cause Alzheimer's.
But Mucke says if prion proteins work the same way in people as in mice, the new research could lead to a drug that would prevent Alzheimer's by keeping prion proteins from interacting with amyloid-beta.
Mucke says finding such a drug could happen relatively quickly because scientists already have spent so many years studying mad cow disease.
"We know a great deal about the biochemistry and biology of prion protein," he says, "which should really facilitate the development of drugs."