The substance, called Lando, is thought to protect the brain against dementia and could be the key to a breakthrough cure.
Lando levels were lower in dementia patients, the research found.
Experiments also found the condition spreads more rapidly in the brains of mice genetically engineered to lack the protein.
Senior study author Dr Douglas Green, from St Jude Children’s Research Hospital, in Memphis, US, said: “We learned about this pathway in the context of brain tumour research. But it has major implications for neuro-inflammatory and neurodegenerative disease.
“We have shown that deficiency in Lando, combined with ageing, can lead to Alzheimer’s disease in a unique mouse model. And there is evidence suggesting this could also be the case in humans.”
The Lando pathway holds a potential clue to developing the first drug that targets the cause of Alzheimer’s.
Previous research by the same team discovered it in microglial cells, the primary immune cells of the brain and central nervous system.