Researchers believe that they have found how the body protects itself against diseases like Alzheimer's.
An 'Alzheimer's' brain on the left, compared to one which is unaffected
International experts found a brain enzyme that "snips apart" tangles of a protein linked to a decline in mental abilities and brain cell death.
The scientists said that, in the future, drugs could be used to enhance this natural defence mechanism.
The research is published in the journal Neuron.
A hallmark of Alzheimer's and related diseases are bundles of fibre that are found in the brain's nerve cells.
These "tangles", mainly made up of a protein called tau, are thought to be associated with brain cell death.
The team of scientists found that an enzyme called puromycin-sensitive aminopeptidase, or PSA, was snipping apart tau proteins in human brain tissue.
They also discovered, looking at fruit flies, that higher levels of PSA protected against brain cell death - neurodegeneration, while lower levels speeded up the brain's demise.
The researchers concluded that PSA may play a "pivotal" protective role.
They said their findings were an "important step forward" in the bid to understand the mechanisms involved in neurodegenerative diseases.
In the future, they said, drugs could be used to boost levels of PSA, which in essence would enhance the body's natural defence mechanism.
Dr Martin Westwell, deputy director of the Institute for the Future of the Mind, Oxford University, said: "The jury is still out on whether tau actually causes Alzheimer's disease.
"But, should it prove to be the case, then this study and the models they use should prove important in furthering our understanding of Alzheimer's and perhaps helping us to take steps forward towards a therapy."
Rebecca Wood, chief executive, Alzheimer's Research Trust, said: "This news offers hope that researchers could develop new treatments to enhance the protein and slow down or halt the disease.
"Enzymes which control the process by which tau tangles are formed are big targets for scientists trying to develop Alzheimer's treatments.
"This is exciting progress in the search for ways to tackle this devastating disease."