Researchers have spotted a new clue to the cause of Alzheimer's disease in a particular form of protein found in the patient's brain.

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

The brains of people with the memory-robbing form of dementia are cluttered with a plaque made up of beta-amyloid, a sticky protein, the findings said.

However, some people have beta-amyloid plaque in their brains but do not show disease symptoms.

To get to the bottom of the question, researchers injected into rats three forms of beta-amyloid.

They were surprised to find that injections of the two-molecule form of soluble beta-amyloid produced characteristics of Alzheimer's in the rats, while the other two forms of the protein did not cause illness.

Those rats had impaired memory function, especially for newly learned behaviors. When their brains were inspected, the density brain cells was reduced by 47 percent with the beta-amyloid seeming to affect synapses, the connections between cells that are essential for communication between them.

This may explain why some people have beta-amyloid plaque in their brains but do not show disease symptoms.

The answer may lie in the two types of beta-amyloid that do not cause symptoms.

Now, the question is why one has the damaging effect and not others.

"A lot of work needs to be done," said Dr. Marcelle Morrison-Bogorad, director of the division of neuroscience at the National Institute on Aging, which helped fund the research