Alzheimer’s disease builds up a harmful protein called tau. Too much tau hurts brain cells and eventually kills them. As tau moves into new parts of the brain, memory gets worse and thinking becomes harder.
New research with mice shows another protein, Arc, may be helping tau travel. Arc normally helps brain cells talk to each other. The study found that Arc also carries toxic tau from sick cells to healthy ones.
Why Arc Matters
Researchers compared normal mice with mice that do not make Arc. They saw that without Arc, tau could not jump between cells as easily.
Arc works by packing itself into tiny bubbles called extracellular vesicles (EVs). These bubbles float from one neuron to another, delivering important messages.
Unfortunately, tau can hitch a ride on these bubbles. When tau sticks to Arc inside an EV, it can move into a healthy neuron and start causing damage there.
What Happens When Tau Gets Inside a Healthy Cell
Every neuron has tau, but in Alzheimer’s the protein sticks together and forms clumps. The clumps block the cell’s internal transport system and eventually kill the cell.
Scientists call the small pieces that break off “tau seeds.” These seeds can travel to another neuron. When they meet normal tau, they turn it into more sticky clumps, spreading the disease.
In the mouse brains, the researchers found EVs that contained both Arc and sticky tau. Those EVs entered healthy neurons and made new tau clumps appear.
What Changes When Arc Is Missing
When the mice lacked Arc, the EVs carried very little tau. The disease could not spread well to nearby cells. The scientists said the transfer of tau was "almost gone."
Both Good and Bad Sides of Arc
Arc is not all bad. Early in the disease, it helps sick neurons get rid of extra tau, letting them survive a little longer.
Without Arc, tau stays trapped inside the sick cell and builds up to dangerous levels, causing the cell to die faster.
So, stopping Arc completely might hurt the brain. A better idea could be to block the EVs that carry tau from entering healthy neurons.
A New Hope for Treatments
The team also found EVs with Arc and tau in human brain samples, suggesting the same process may happen in people.
Future medicines might aim to catch these EVs after they leave a diseased cell but before they reach a healthy one. Such a therapy would not fix damage that is already done, but it could slow or stop the spread of Alzheimer’s.
Scientists say a lot more work is needed before any drug reaches patients, but the discovery opens a new path for research.