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The brain has a waste removal system and scientists are figuring out how it works

A MARTÍNEZ, HOST:

Turns out the brain needs regular cleaning. Otherwise, it gets clogged up with waste products, including some associated with diseases such as Alzheimer's and Parkinson's. NPR's Jon Hamilton reports that scientists are finally figuring out just how the brain cleans itself.

JON HAMILTON, BYLINE: In most of the body, waste disposal is pretty simple. Jonathan Kipnis of Washington University in St. Louis says the system is a lot like what you'd find in a home.

JONATHAN KIPNIS: You have the water pipes and the sewage pipes. Water comes in clean, and then you wash your hands and then the dirty water goes down.

HAMILTON: Into the sink and down the drain. In the body, this process relies on the lymphatic system, a network of tubes connected to the bloodstream. But that system doesn't extend to the brain. So Kipnis says scientists had a question.

KIPNIS: How does the dirt molecule - let's assume there is a waste molecule from the middle of the brain will make all the way out to the borders of the brain into the brain's sink?

HAMILTON: Part of the answer came in 2012 and 2013. A team led by a Danish scientist found evidence that during sleep, cerebrospinal fluid flowed quickly through the brain, flushing out waste. But what was pushing the fluid? 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. And, he says, they realized something.

KIPNIS: By measuring the wave, we are measuring also the flow of interstitial fluid.

HAMILTON: It turned out the wave acts as a signal, transforming neurons into tiny pumps that push the fluid between cells toward the brain surface. The finding was published in the journal Nature. And in a second paper, also in Nature, the team showed how cerebrospinal fluid could transport waste across the barrier that usually separates brain and body. Kipnis says the process, seen in both mice and people, involves a vein that passes through the brain's protective membrane.

KIPNIS: Around the vein, you have a sleeve and sleeve is never fully sealed, right? You have a cuff. And that's where CSF is coming out from into the junkyard of the brain.

HAMILTON: Where it can interact with the body's lymphatic system. Kipnis says this allows immune cells to monitor what's going on inside the brain by detecting the waste products coming out.

KIPNIS: Imagine that your neighbors will go through your trash can on a daily basis. They will know all about you.

HAMILTON: But Kipnis says, if the system isn't working right, it can allow immune cells from the body to get inside the brain, perhaps causing the sort of inflammation seen in diseases like Alzheimer's. Jeffrey Iliff at the University of Washington, says the new studies suggest that keeping the waste clearance system on track requires two different steps - one to flush waste into the cerebrospinal fluid and another to move it into the bloodstream and eventually out of the body.

JEFFREY ILIFF: We've described them separately and frequently labs will study one or the other, but from a biological perspective, they almost certainly are coupled.

HAMILTON: Iliff says some of the findings still need to be confirmed in people.

ILIFF: The anatomical differences between a rodent and a human - they're pretty substantial.

HAMILTON: But he says the results are consistent with research on what causes neurodegenerative disorders. Iliff says at least one disease is clearly more common in people with conditions that affect waste clearance in the brain.

ILIFF: In people that are aging and in people with traumatic brain injury, and with people with cerebrovascular disease - all of these are risk factors for Alzheimer's disease.

HAMILTON: Iliff says impaired waste removal may also be a factor in Parkinson's, headache, and even depression. And finding ways to keep the brain clean might help people with all of those conditions.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.