Another expert, Dr. Juan C. Troncoso of Johns Hopkins University School of Medicine, said the finding “opens a very good avenue, but it’s not without a lot of technical challenges.”

Drugs that activate sirtuin already exist, including resveratrol, a minor ingredient of red wine and other foods, and small-molecule chemicals designed to mimic resveratrol. Sirtris, the company that developed the drugs, is testing them against diabetes and other diseases. This generation of drugs does not cross the blood-brain barrier so would not work against Alzheimer’s.

But George P. Vlasuk, Sirtris’s chief executive, said the company had developed other sirtuin-activating chemicals that do reach the brain and are in preclinical trials. “We think it has very significant potential in neurodegenerative diseases,” Dr. Vlasuk said.

Sirtuin has been the subject of intense research in the last few years because it seems to protect the body’s various organs against disease by stepping up maintenance programs. The substance came to light through studies of longevity, particularly the discovery that reduced-calorie diets could lengthen the lifespan of mice by 30 percent. Sirtuin appears to convey much of the beneficial effect of such diets, even though drugs that activate sirtuin have not yet been shown to prolong mice’s lifespan in experiments.

Dr. Guarente, a leading sirtuin researcher, said the protein’s protective power against other diseases made him wonder if it might also help against Alzheimer’s. He obtained mice that tend to develop Alzheimer’s-like symptoms because they are genetically engineered to carry two mutated human genes that cause a buildup of plaque in the brain. The mice were crossed with a strain of mice in which the sirtuin-making gene is particularly active. They were also crossed with a strain in which the sirtuin gene was deleted entirely. Dr. Guarente’s team could thus test the effect of having either more or less sirtuin in the brains of Alzheimer’s-prone mice.