Astrocyte networks in mice connect distant brain regions in a way that nerve cell wiring does not
Like ancient subway tracks buried deep beneath modern lines, a previously unknown transportation system transports materials around the brain. But this system, made up of star-shaped cells called astrocytes, is not a relic, a new study reports. It is operational, connecting places near and far in flexible and mysterious ways.
The results, published on April 22 in Nature, reveal a system by which brain regions could communicatea network that differs from the more familiar systems made up of nerve cells. Additionally, astrocyte networks may also play a role in disorders such as Alzheimer’s disease, head trauma, and stroke – perhaps in varied roles that could both damage and heal.
Discovering these networks will open up a new area of research, says neurobiologist Bess Frost of Brown University in Providence, R.I., who was not involved in the study. “It’s pretty incredible to discover something like this, because it’s so fundamental,” she says. It’s the kind of work that “makes you think, ‘What else don’t we know?’ »
Astrocytes were once thought to be support personnel in the brain. These magnanimous cells keep their neuronal neighbors nourished and clean. But brain specialists are increasingly realizing that these cells play big roles in the exchange of information in and around the brain.
The newly discovered networks suggest how important these roles might be, says Melissa Cooper, a neuroscientist at New York University’s Grossman School of Medicine in New York. But hint is the key word. As of yet, no one knows what astrocyte networks do.
Cooper and his colleagues used a chemical trick to mark some of the traffic flowing in mouse brains through gap junctions, the pores that connect adjacent astrocytes. This tag, a kind of stamp later decorated with a fluorescent molecule, marked a molecular cargo moving among the astrocytes. After the mice’s brains were removed and processed to become clear, the scientists were able to view these patches under a microscope, revealing the network of astrocytes followed by the cargo.
Some researchers thought that astrocytes covered the brain evenly, Cooper says. But the trajectories of the cargo ships told a different story. Astrocytes, so named for their star shape, form large, long-range connections, looking a bit like “galaxies across the brain,” Cooper says. But they also connect in specific and selective ways, forming bonds where nerve cells do not. “This means that astrocytes directly connect these regions of the brain that we didn’t know could communicate with each other before.”
Cooper, a New York resident, likens the astrocyte system to a second subway system that could “move people between parts of the city we didn’t know they could get to before.”
Further experiments showed that these networks could remodel themselves in response to the environment. The mice’s whiskers pick up information and send it to the opposite side of the brain. When the mice had their whiskers cut off on one side, thereby limiting this intake, the astrocytes on the other side of the brain changed their connections and reduced this network.
Human astrocytes function similarly to those of mice. “I would be completely shocked if humans didn’t have the same thing in their brains,” Frost says. But Cooper cautions that there is no obvious way to detect these networks, especially in living people.
As for the tasks of these networks, Frost says they could help allocate resources. Like the underground fungal partners that connect trees, astrocytes may monitor the health of neurons and move energy or nutrients to certain locations when the need arises.
Understanding what these networks do, what passes through them and why will keep scientists busy for a long time. These studies could also lead to a deeper understanding of head injuries and disorders such as Alzheimer’s disease and stroke, all of which have been linked to problems with gap junctions.
“There have been so many questions in neuroscience that we’ve always looked for and haven’t been able to answer,” Cooper says. “And maybe that’s because we haven’t been able to see this big missing piece that’s been there all this time.”