Scientists have identified the molecular accomplice that lets Alzheimer’s disease march relentlessly through the brain — and the discovery opens a door that could one day slam it shut.

In a study published June 30 in Cell, researchers at the University of Utah Health found that a protein called Arc — best known as an intercellular messenger that helps neurons communicate — is being weaponized by Alzheimer’s pathology. Toxic Tau protein, the sticky “glue monster” that forms lethal tangles inside neurons, hitches a ride on Arc to travel from sick cells to healthy ones, seeding destruction in new regions of the brain.

“I’m excited by the fact that we’ve identified a new way of potentially stopping the progression of Alzheimer’s disease,” said Jason Shepherd, PhD, professor of neurobiology at University of Utah Health and senior author of the study.

The Hitchhiker’s Guide to Neurodegeneration

Arc normally does something remarkable: it wraps itself in a microscopic bubble called an extracellular vesicle, or EV, and floats from one neuron to the next carrying important information. It’s part of the brain’s postal system. But the Utah team discovered that in Alzheimer’s, toxic Tau seeds — smaller fragments of the massive Tau tangles that kill neurons — stick to Arc and ride those same EVs straight into healthy brain cells.

When the researchers engineered Alzheimer’s-model mice that lacked the Arc protein, the results were striking. Their brain EVs contained barely any Tau, and the disease could no longer spread to new cells. “When we removed Arc, we saw that the transfer of Tau was severely, severely reduced,” said Mitali Tyagi, PhD, first author on the paper. “It was almost gone.”

A Delicate Balance

The finding comes with a twist. Arc appears to play a protective role in the disease’s early stages: by ejecting excess toxic Tau from sick cells via EVs, Arc actually helps those neurons survive longer. Mice without Arc saw their sick neurons die faster because the toxic protein had no escape route.

This means simply blocking Arc outright would be counterproductive. The smarter strategy, the researchers argue, is to intercept the toxic cargo mid-flight — stopping Tau-laden EVs after they leave sick cells but before they can infect healthy neighbors.

“If we could target these particular EVs, that would be a really useful therapy strategy,” Shepherd said. “For someone with early-onset Alzheimer’s or dementia, if we could stop the spread, then we could prevent further damage and cognitive decline.”

From Mice to Humans

The team also found that human brain tissue contains Arc-and-Tau-carrying EVs, suggesting the same mechanism operates in people. However, Shepherd cautions that the work remains preclinical. “Most of the work we’ve been doing is in mice, not in humans. We have some clues that whatever is happening in these mice could also be happening in humans, but we don’t know that yet.”

The discovery, supported by the National Institutes of Health, the Chan-Zuckerberg Initiative, the Alzheimer’s Association, and the Cure Alzheimer Fund, doesn’t promise to repair existing brain damage. But it does suggest an entirely new class of therapies: ones that freeze Alzheimer’s in place rather than trying to undo what’s already lost.

Sources: Cell — Arc mediates intercellular tau transmission via extracellular vesicles, University of Utah Health, ScienceDaily