Areas of the brain that keep you awake during the day are damaged in the early stages of the memory-robbing disease, which is why people with Alzheimer's may nap excessively long before they start to struggle with forgetting things, the study authors said.
Not only that, the scientists also found that damage to brain regions involved in daytime wakefulness was caused by a protein called tau. This provides more evidence that tau may play a larger role in Alzheimer's than the more extensively studied amyloid protein, the researchers noted.
"Our work shows definitive evidence that the brain areas promoting wakefulness degenerate due to the accumulation of tau - not amyloid protein - from the very earliest stages of the disease," said study senior author Dr. Lea Grinberg. She is an associate professor of neurology and pathology at the Memory and Aging Center and a member of the Global Brain Health Institute at the University of California, San Francisco (UCSF).
Previous research has suggested that excessive napping is due to poor sleep caused by Alzheimer's-related disruptions in brain regions that promote sleep, or that sleep problems themselves contribute to Alzheimer's disease progression.
In this study, the researchers analyzed the brains of 13 deceased Alzheimer's patients and seven people without the disease. The investigators concluded that Alzheimer's disease attacks brain regions responsible for wakefulness during the day, and that these regions are among the first damaged by the disease.
The findings suggest that excessive daytime napping could serve as an early harbinger of Alzheimer's.
In the Alzheimer's-affected brains, significant tau buildup was found in all three wakefulness-promoting centers examined by the researchers, and those regions had lost as many as 75% of their neurons.
The findings were published Aug. 12 in the journal Alzheimer's and Dementia.
According to study lead author Jun Oh, a Grinberg lab research associate, "It's remarkable because it's not just a single brain nucleus that's degenerating, but the whole wakefulness-promoting network.
Crucially, this means that the brain has no way to compensate because all of these functionally related cell types are being destroyed at the same time," Oh explained.
"It seems that the wakefulness-promoting network is particularly vulnerable in Alzheimer's disease," Oh said in a UCSF news release. "Understanding why this is the case is something we need to follow up in future research."
This and other findings suggest that tau buildup plays a greater role in Alzheimer's than the more widely studied amyloid protein. Research into amyloid has so far failed to result in effective Alzheimer's treatments, according to the UCSF team.
Grinberg said that the "research adds to a growing body of work showing that tau burden is likely a direct driver of mental decline." - Robert Preidt.
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