For years, researchers have assumed that the signals in the brain that make mammals sleep come from neurons and that astrocytes, glial cells that outnumber neurons five to one in the brain, were following neuronal cues. In a study published today (September 24) in Current Biology, researchers show that calcium levels—a marker of signaling activity between cells—change in astrocytes as mice sleep and wake. Without this cross-talk between astrocytes, mice don’t make up for lost sleep like they normally do, indicating that these cells in the brain have much more influence on sleep than previously thought.
“There was this idea for a long time . . . that astrocytes were just glue, pulling the brain together and are more like passive cells, but it’s not true,” says Sejal Davla, a postdoc at the City University of New York who was not involved in the work. “It was very intriguing to see that the activity patterns that you see in astrocytes are not directly coordinated with the neuronal activity. It looks like the cells do something at their own rhythm in their own manner.”
Astrocytes use waves of calcium to talk to each other because they aren’t electrically excitable like neurons. Around the time that Ashley Ingiosi started a postdoc in the lab of Marcos Frank, a neuroscientist at Washington State University, in 2015, tools that enabled researchers to look at astrocytic calcium were just becoming available, including a fluorescent sensor that glows with an intensity that corresponds to how much calcium is present in cells. The elevation of calcium concentrations in an astrocyte is a form of activation, and this rise in calcium is a step in processes that allow astrocytes to communicate with each other and with other cell types, including neurons. So the brighter the fluorescence, the more active the cells.
Ingiosi, Frank, and colleagues used those tools to explore how astrocytes behave during the sleep-wake cycle in mice. They monitored calcium activity in the frontal cortex of mice held in place for two-photon imaging, which allowed them to get a detailed look at individual astrocytes. And they fitted other animals with a miniscope—a microscope about as big as a Lego that attaches to the head—to monitor astrocyte activity as the animals moved and behaved as usual, including falling asleep and waking up.
With “the miniscope on the head, they truly have freely behaving mice,” says Philip Haydon, a neuroscientist at Tufts University and past collaborator of Frank who did not participate in the study. “They don’t have to put them under anesthesia, so they’re actually seeing natural transitions between these states. It takes us a long way to seeing what the astrocyte is doing.”
The researchers determined that astrocytic calcium was higher during wakefulness and lower during sleep, when calcium signals were also less synchronized among astrocytes. When they sleep deprived mice, they saw an immediate increase in calcium levels when the mouse’s sleep need was highest. They then dropped as the mice slept.
The synchrony of the calcium signals in astrocytes across sleep states and after sleep deprivation was not exactly in line with the synchronization of neuronal electrical activity. If changes in astrocyte synchrony were driven by neurons, the authors write, they might have expected slower oscillations consistent with how neural activity changes in sleep, rather than less coordination among astrocytes.
“In the past, it was thought that astrocytes were just passively responding to neuronal activity,” Ingiosi says, “but now that we see that the synchrony isn’t just mimicking what we see in terms of neuronal synchrony in the brain across states that tells us that astrocytes might actually be playing a more direct role in regulating our sleep-wake behavior than we had previously considered.”
There’s a whole other level of brain organization that we were unaware of that is just as dynamic as neurons.—Marcos Frank, Washington State University
When the team monitored mice whose astrocytes were missing a protein that allows calcium into cells, these mice had lower levels of astrocytic calcium during waking, but they still spent about the same amount of time in various sleep states as wildtype mice did. Sometimes, though, defects in sleep regulation don’t show up unless mice are kept awake, after which they respond to increased sleep pressure by sleeping for longer to compensate for the sleep loss. When the researchers sleep deprived the mice with impaired calcium entry, those animals slept less than sleep-deprived wildtype mice did, indicating that astrocytic calcium plays a role in regulating the compensatory response to sleep deprivation.
“There’s a whole other level of brain organization that we were unaware of that is just as dynamic as neurons. We never could measure it because all we had at the time were the tools that could measure neurons,” says Frank.
In another paper published in Nature Communications July 6, researchers also found a role for astrocytic calcium signaling in sleep. Those authors used two-photon imaging and a different knockout mouse to show that astrocytic calcium is involved in regulating slow wave sleep. Taken together, the two studies indicate that these cells may have more control over sleep than was previously believed.
“It’s really nice when two independent groups come up with the same results. It makes you want to believe them,” says Haydon, adding that both studies point to a range of open questions.
“With this paper and other recent work, the field is poised to tackle some fascinating, and interrelated, questions about the mechanisms by which astrocytes regulate neuronal activity in sleep,” agrees Kira Poskanzer, a neuroscientist at the University of California, San Francisco, who did not participate in the study, in an email to The Scientist. “These mechanistic goals include identifying the intracellular signaling mechanisms in astrocytes that influence sleep, describing which specific features of sleep are astrocyte-mediated, characterizing the circuit mechanisms that astrocytes regulate, and defining how . . . broad an influence astrocytes exert on the neuronal activity that defines sleep.”
A.M. Ingiosi et al., “A role for astroglial calcium in mammalian sleep and sleep regulation,” Current Biology, doi:10.1016/j.cub.2020.08.052, 2020.
Correction (September 24): The article has been updated to clarify how mice with impaired calcium entry in astrocytes respond to sleep deprivation. The Scientist regrets the error.