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Study Offers Alzheimer's Insight

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The Associated Press

Published: June 23, 2008

WASHINGTON - Researchers have uncovered a new clue to the cause of Alzheimer's disease. The brains of people with the memory-robbing form of dementia are cluttered with a plaque made up of beta-amyloid, a sticky protein.

But there has been a question whether this is a cause of the disease or a side effect. Also involved are tangles of a protein called tau; some scientists suspect this is the cause.

Researchers have caused Alzheimer's symptoms in rats by injecting them with a particular form of beta-amyloid.

Injections with other beta-amyloid forms did not cause illness, which may explain why some people have beta-amyloid plaque in their brains but don't show disease symptoms.

The findings by a team led by Ganesh M. Shankar and Dennis J. Selkoe of Harvard Medical School were reported in Sunday's online edition of the journal Nature Medicine.

Researchers used extracts from the brains of people who donated their bodies to medicine. Forms of soluble beta-amyloid containing different numbers of molecules, as well as insoluble cores of the brain plaque, were injected into the brains of rats. There was no detectable effect from the insoluble plaque or the soluble one-molecule or three-molecule forms, researchers found.

But the two-molecule form of soluble beta-amyloid produced Alzheimer's characteristics in the rats, they reported.

Those rats had impaired memory function, especially for newly learned behaviors.

Studies were done on mice and when their brains were inspected, the density brain cells were reduced by 47 percent. The beta-amyloid seemed to affect synapses, the connections between cells that are essential for communication between them.

The research, for the first time, showed the effect of a particular type of beta-amyloid in the brain, said Marcelle Morrison-Bogorad, neuroscience unit director at the National Institute on Aging that helped fund the research.

"A lot of work needs to be done," she said. "Nature keeps sending us down paths that look straight at the beginning, but there are a lot of curves before we get to the end."

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