Contagion

Alzheimer’s disease seems to spread like an infection from brain cell to brain cell, two new studies in mice have found. But instead of viruses or bacteria, what is being spread is a distorted protein known as tau.

The surprising finding answers a longstanding question and has immediate implications for developing treatments, researchers said. And they suspect that other degenerative brain diseases like Parkinson’s may spread in a similar way.

Alzheimer’s researchers have long known that dying, tau-filled cells first emerge in a small area of the brain where memories are made and stored. The disease then slowly moves outward to larger areas that involve remembering and reasoning.

But for more than a quarter-century, researchers have been unable to decide between two explanations. One is that the spread may mean that the disease is transmitted from neuron to neuron, perhaps along the paths that nerve cells use to communicate with one another. Or it could simply mean that some brain areas are more resilient than others and resist the disease longer.

The new studies provide an answer. And they indicate it may be possible to bring Alzheimer’s disease to an abrupt halt early on by preventing cell-to-cell transmission, perhaps with an antibody that blocks tau.

The studies, done independently by researchers at Columbia and Harvard, involved genetically engineered mice that could make abnormal human tau proteins, predominantly in the entorhinal cortex, a sliver of tissue behind the ears, toward the middle of the brain, where cells first start dying in Alzheimer’s disease. As expected, tau showed up there. And, as also expected, entorhinal cortex cells in the mice started dying, filled with tangled, spaghettilike strands of tau.

Over the next two years, the cell death and destruction spread outward to other cells along the same network. Since those other cells could not make human tau, the only way they could get the protein was by transmission from nerve cell to nerve cell.

Although the studies were in mice, researchers say they expect that the same phenomenon occurs in humans because the mice had a human tau gene and the progressive wave of cell death matched what they see in people with Alzheimer’s disease.

Two groups of researchers were inspired by the many observations over the years that Alzheimer’s starts in the entorhinal cortex and spreads.

Researchers knew that something set off Alzheimer’s disease. The most likely candidate is a protein known as beta amyloid, which accumulates in the brain of Alzheimer’s patients, forming hard, barnaclelike plaques. But beta amyloid is very different from tau. It is secreted and clumps outside cells. Although researchers have looked, they have never seen evidence that amyloid spreads from cell to cell in a network.

Still, amyloid creates what amounts to a bad neighbourhood in memory regions of the brain. Then tau comes in — some call it “the executioner” — piling up inside cells and killing them. If some cells take longer than others to succumb to the bad neighbourhood, that would explain the spread of the disease in the brain, and there would be no need to blame something odd, like the spread of tau from cell to cell.

The question of which hypothesis was correct — tau spreading cell to cell, or a bad neighbourhood in the brain and cells with different vulnerabilities to it — remained unanswerable. A Dr  said he tried for 25 years to find a good way to address it. One of his ideas was to find a patient or two who had had a stroke or other injury that severed the entorhinal cortex from the rest of the brain. If the patient developed Alzheimer’s in the entorhinal cortex — and it remained contained there — he would have evidence that the disease spread like an infection.

The solution came when researchers were able to develop genetically engineered mice that expressed abnormal human tau, but only in their entorhinal cortexes. Those mice offered the cleanest way to get an answer, said John Hardy, an Alzheimer's researcher at University College London who was not involved in either of the new studies.

There is another advantage, too,  the Dr  said. The mice give him a tool to test ways to block tau’s spread — and that, he added, “is one of the things we’re excited about.”

But if tau spreads from neuron to neuron,  it may be necessary to block both beta amyloid production, which seems to get the disease going, and the spread of tau, which continues it, to bring Alzheimer’s to a halt. He and others are also asking if other degenerative diseases spread through the brain because proteins pass from nerve cell to nerve cell.

Distorted protein

Alzheimer’s researchers have long known that dying, tau-filled cells first emerge in a small area of the brain where memories are made and stored. The disease then slowly moves outward to larger areas that involve remembering and reasoning.

But for more than a quarter-century, researchers have been unable to decide between two explanations. One explanation is that the spread may mean that the disease is transmitted from neuron to neuron, perhaps along the paths that nerve cells use to communicate with one another. Or it could simply mean that some brain areas are more resilient than others and resist the disease longer.

The new studies provide an answer. And they indicate it may be possible to bring Alzheimer's disease to an abrupt halt early on by preventing cell-to-cell transmission, perhaps with an antibody that blocks tau.