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| Raag Airan, Matine Azadian, Payton Martinez, and Yun Xiang in the lab. Azadian is holding a version of their ultrasound apparatus designed for humans. Photo Credit: Andrew Brodhead |
Just like your body needs a bath now and then, so too does your brain – but instead of a tub filled with hot water, your brain has cerebrospinal fluid, which flows around inside the brain and helps clear away waste products, misplaced blood cells, and other sometimes-toxic debris.
The trouble is, that natural brain-bathing system can break down as people age or after a brain injury, such as a stroke – and there aren’t any particularly good ways to help the brain out in those situations. Indeed, current ideas to promote cerebrospinal fluid cleaning are either rather invasive or require drugs that may not be safe or effective in people.
Fortunately, a team of Stanford researchers has found a radically simple tool that may help the brain wash itself out without the need for drugs or invasive procedures: ultrasound, the same tool obstetricians regularly use at prenatal checkups.
Writing in Nature Biotechnology, the research team reported that ultrasound treatments successfully moved molecular junk out of the brains of mice and reduced inflammation in the brain. Both effects, the researchers said, could improve brain function and overall health.
Because the approach is so simple, it should be straightforward to translate to humans, said Wu Tsai Neuro affiliate Raag Airan, a senior author on the new study and an assistant professor of radiology at Stanford Medicine. In fact, with help from a Knight Initiative Catalyst Award, Airan’s lab is already working on it.
“With support from the Knight Initiative for Brain Resilience, we’re making a helmet people can wear that delivers ultrasound to the brain, and we plan to initiate clinical testing of this protocol in the next few months,” Airan said. If the new device is as successful as Airan’s experiments in mice, doctors could soon have a simple new way to clean up the brain and treat neurological disease.
A fortuitous mistake
The idea for the new study stems from Airan’s time as a postdoctoral fellow at Johns Hopkins University. At the time, he was thinking about how to deliver drugs deep into the brain, where they could more effectively treat a variety of neurological conditions.
At the time, doing something like that typically meant sticking a needle into the brain – but there had been other, less anxiety-inducing ideas floating around. One such idea involved using ultrasound to deliver drugs across the blood-brain barrier, which controls what gets in and out of cerebrospinal fluid (CSF) and the brain. The idea was to introduce tiny gas bubbles into the blood, then shake them with ultrasound near the blood-brain barrier. Doing so opens up pores that could let drugs or other therapeutic molecules pass from the blood into the brain.
Although Airan would ultimately go in a different direction, he spent time at Hopkins experimenting with ultrasonic blood-brain barrier opening. In one of the last experiments he conducted before returning to Stanford where he had done his doctoral work in bioengineering, Airan planned to send a series of short pulses of ultrasound into the brain over the course of 2 minutes, then track a drug as it crossed into the brain and CSF.
One day, he made a mistake.
“I accidentally left the ultrasound on continuously the whole time, when I was supposed to pulse it,” Airan said. He only discovered the error after he checked his data and noticed something wrong – but fascinating. “In these experiments, we usually see dots of contrast showing up in the brain indicating where the drug crossed the blood-brain barrier, but in this case, the dots were all smeared out.”
That smearing hinted that ultrasound was somehow stirring up the CSF, moving the delivered drug around much more than Airan had expected. This piqued his curiosity.
In recent years, it’s been hypothesized that the brain does a bit of this CSF stirring on its own in order to clear out waste. That led Airan to wonder whether ultrasound might somehow help that process along.
Cleaning up the brain
At Stanford, Airan enlisted his graduate student Matine Azadian to see if ultrasound could help cleanse the CSF. To find out, Airan, Azadian, and their colleagues first injected blood into mouse brains, simulating a hemorrhagic stroke.
Apart from the loss of oxygen to brain cells, hemorrhagic strokes leave rogue blood cells floating around, blocking CSF circulation pathways and inducing inflammation in the brain that can cause further damage and trigger neurodegeneration. The team’s idea was to see if applying ultrasound to the mice’s brains would clean up those blood cells and improve mouse health.
Half of the mice with simulated strokes got three 10-minute ultrasound treatments, while the others got a “sham” treatment, where researchers applied the ultrasound device but didn’t turn it on. On a number of measures, mice that got the ultrasound treatment fared better than others.
First, ultrasound did actually clean up cerebrospinal fluid. Examining tissue samples taken from both sets of mice, the researchers found less than half as much blood still floating around from the simulated stroke – not as clean as healthy control mice, who had no blood in their CSF, but a substantial improvement nonetheless.
Mice that got the ultrasound treatment also showed fewer signs of damaging brain inflammation, and they had less difficulty with a standard mobility test involving navigating out of tight corners when startled. Finally, treated mice also lived longer: Without treatment, only half the mice survived past two weeks. But 83 percent of those with ultrasound treatment lived on.
Simple tool, complicated process
It was clear that ultrasound helped remove blood cells from CSF, but how did it work? Airan at first assumed that ultrasound was stirring the fluid directly, which could conceivably facilitate the cleaning process. But there was a problem with that hypothesis. After the team turned off the ultrasound, they could see that CSF kept circulating – as if they’d kept stirring – for as much as an hour and a half. Whatever was going on, it was more complicated than Airan and colleagues originally thought.
Azadian, Airan’s graduate student, had a thought. He’d been researching mechanically sensitive processes in cells – for instance, channels in and out of cells that open up in response to vibrations. He wondered if such vibration-sensitive channels might play a role in the team’s ultrasound experiments. Rather than stirring CSF, he thought, perhaps ultrasound was stimulating a cellular process that complemented or mediated the mixing mechanisms.
To test that idea, the researchers gave mice a molecule derived from spider venom that blocks cells’ vibration sensors. Doing so quickly eliminated ultrasound’s ability to clean CSF, indicating Azadian’s hypothesis might be right.
The researchers also uncovered initial evidence tying mechanically sensitive processes to the immune system and inflammation via two pathways. The first involves immune cells called microglia, which are key to overall brain maintenance. Airan and colleagues found that ultrasound activates vibration-sensitive channels in microglia, shifting them from a state that encourages inflammation to one where they perform a different function: breaking down misplaced cells and other detritus so that they’re easier to remove.
Second, the team learned that ultrasound may activate similar channels in astrocytes, cells that form part of the connective tissue of the brain and that also play a role in the brain’s immune responses. Activating those channels in turn activates processes that allow fluids to flow more easily through the brain and across the blood-brain barrier, which would reduce resistance to the CSF-cleaning process.
Airan and his team plan to investigate those possibilities in the future, but in the meantime they’ve found a drug-free, non-invasive approach to cleaning CSF, which could help patients living with neurodegeneration or recovering from brain injuries.
“Ultrasound is giving us access to the biology the brain uses to clean CSF and enhancing that in a way that’s more naturalistic” than the alternatives, Airan said. “We’re on our way. We plan to have the system ready for human tests soon, and I call dibs.”
Funding: The research was funded by a Wu Tsai Neurosciences Institute Seed Grant, the National Institutes of Health BRAIN Initiative (RF1MH114252, UG3NS114438), NIH HEAL Initiative (UG3NS115637), NIH National Institute Neurological Disease and Stroke (R01NS126761), a Focused Ultrasound Foundation High Risk Award, an anonymous donor to the Stanford School of Medicine Radiology Department, a Ford Foundation Predoctoral Fellowship, an NSF-Graduate Research Fellowship, a Hydrocephalus Association Innovator Award, The Shurl and Kay Curci Foundation, and a Complete Genomics Spatial Biology Grant.
Airan has received consulting fees from Cordance Medical and Lumos Labs and grant funding from AbbVie Inc. All other authors declare no conflicts of interest.
Published in journal: Nature Biotechnology
Authors: Matine M. Azadian, Sepideh Kiani Shabestari, Arjun Rajan, Payton J. Martinez, Nicholas Macedo, Eric Markarian, Yun Xiang, Brenda J. Yu, Paul M. George, Ryann M. Fame, and Raag D. Airan
Source/Credit: Stanford University
Reference Number: beng111125_02
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