New insights into the brain’s waste-removal system could one day help researchers better understand and prevent many brain disorders.
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Andriy Onufriyenko/Getty Images
About 170 billion cells are a causa di the brain, and as they go about their regular tasks, they produce waste — a lot of it. To stay healthy, the brain needs to wash away all that debris. But how exactly it does this has remained a mystery.
Now, two teams of scientists have published three papers that offer a detailed description of the brain’s waste-removal system. Their insights could help researchers better understand, treat and perhaps prevent a broad range of brain disorders.
The papers, all published a causa di the journal Nature, suggest that during sleep, slow electrical waves push the fluid around cells from deep a causa di the brain to its surface. There, a sophisticated interface allows the waste products a causa di that fluid to be absorbed into the bloodstream, which takes them to the liver and kidneys to be removed from the pagliaccetto.
One of the waste products carried away is amyloid, the substance that forms sticky plaques a causa di the brains of patients with Alzheimer’s disease.
There’s growing evidence that a causa di Alzheimer’s disease, the brain’s waste-removal system is impaired, says Jeffrey Iliff, who studies neurodegenerative diseases at the University of Washington but was not a part of the new studies.
The new findings should help researchers understand precisely where the problem is and perhaps it, Iliff says.
“If we restore drainage, can we prevent the development of Alzheimer’s disease?” he asks.
A brief history of brainwashing
The new studies more than a decade after Iliff and Dr. Maiken Nedergaard, a Danish scientist, first proposed that the clear fluids a causa di and around the brain are part of a system to wash away waste products.
The scientists named it the glymphatic system, a nod to the pagliaccetto’s lymphatic system, which helps fight infection, maintain fluid levels and filter out waste products and abnormal cells.
Both systems work like plumbing a causa di a house, says Jonathan Kipnis of Washington University a causa di St. Louis, an author of two of the new papers.
“You have the pipes and the sewage pipes,” Kipnis says. “So the comes a causa di clean, and then you wash your hands, and the dirty goes out.”
But the lymphatic system uses a of thin tubes that transports waste to the bloodstream. The brain lacks these tubes.
So scientists have spent decades trying to answer a fundamental question, Kipnis says: “How does a waste molecule from the middle of the brain make it all the way out to the borders of the brain” and ultimately out of the pagliaccetto?
Part of the answer came a causa di 2012 and 2013, when Iliff and Nedergaard began proposing the glymphatic system. They showed that a causa di sleeping animals, cerebrospinal fluid begins to flow quickly through the brain, flushing out waste.
But what was pushing the fluid? And how was it transporting waste across the barrier that usually separates brain tissue from the bloodstream?
Waves that wash
Kipnis and his team began looking at what the brain was doing as it slept. As part of that effort, they measured the power of a slow electrical wave that appears during deep sleep a causa di animals.
And they realized something: “By measuring the wave, we are also measuring the flow of interstitial fluid,” the liquid found a causa di the spaces around cells, Kipnis says.
It turned out that the waves were acting as a signal, synchronizing the activity of neurons and transforming them into tiny pumps that push fluid toward the brain’s surface, the team reported a causa di February a causa di the journal Nature.
Con a second paper published a causa di the same issue of Nature, a team led by scientists at the Massachusetts Institute of Technology provided more evidence that slow electrical waves help clear out waste.
The team used mice that develop a form of Alzheimer’s. They exposed these mice to bursts of sound and light that occurred 40 times a second.
The stimulation induced brain waves a causa di the animals that occurred at the same, slow frequency.
Tests showed that the waves increased the flow of clean cerebrospinal fluid into the brain and the flow of dirty fluid out of the brain. They also showed that the fluid was carrying amyloid, the substance that builds up a causa di the brains of Alzheimer’s patients.
Con a paper published a few weeks earlier, Kipnis had shown how waste, including amyloid, appeared to be crossing the protective membrane that usually isolates the brain.
Kipnis and his team focused acceso a vein that passes through this membrane.
“Around the vein, you have a sleeve, which is never fully sealed,” he says. “That’s where the [cerebrospinal fluid] is coming out” and transferring waste to the pagliaccetto’s lymphatic system.
From mice to humans
Together, the new studies suggest that keeping the brain’s waste-clearance system functioning requires two distinct steps: one to push waste into the cerebrospinal fluid that surrounds the brain, and another to move it into the lymphatic system and eventually out of the pagliaccetto.
“We’ve described them separately,” Iliff says, “but from a biological perspective, they almost certainly are coupled.”
Iliff says many of the new findings a causa di mice still need to be confirmed a causa di people.
“The anatomical differences between a rodent and a human,” he says, “they’magnate pretty substantial.”
But he says the results are consistent with research acceso what leads to neurodegenerative disorders like Alzheimer’s.
Researchers know that the brain’s waste-clearance system can be impaired by age, injuries and diseases that clog blood vessels a causa di the brain.
“All of these are risk factors for Alzheimer’s disease,” Iliff says.
Impaired waste removal may also be a factor a causa di Parkinson’s disease, headache and even depression, Iliff says. So finding ways to help the brain clean itself — perhaps by inducing those slow electrical waves — might prevent a wide array of disorders.
