We spend about a third of our lives sleeping, but the purpose and mechanisms of sleep remain somewhat of an enigma. Scientists working at the Perelman School of Medicine at the University of Pennsylvania (UPenn) have now identified a single gene in fruit flies that induces sleep. The nemuri gene’s protein, secreted by neurons in the fly’s brain, acts as an antimicrobial peptide (AMP), but also triggers deep and extended sleep after an infection. “While it’s a common notion that sleep and healing are tightly related, our study directly links sleep to the immune system and provides a potential explanation for how sleep increases during sickness,” said senior author Amita Sehgal, PhD, a professor of neuroscience and director of UPenn’s chronobiology program.

Sehgal, together with co-researchers Hirofumi Toda, PhD; Julie A. Williams, PhD;  and Michael Gulledge PhD, reported on their studies in Science, in a paper titled, “A sleep-inducing gene, nemuri, links sleep and immune function in Drosophila.”

Sleep remains “a mystery of biology,” the authors acknowledged, and the mechanisms that underpin our need for sleep represent “a major research challenge in neuroscience.” We know that sleep is broadly regulated by two biological processes. The circadian system oversees the times of day or night when we sleep, while the homeostatic system that determines the need for sleep. Scientists do now have a basic understanding of the mechanisms involved in modulating the body’s circadian clocks, but little is known about the mechanisms that underpin the homeostatic drive for sleep. And while we recognize that the need for sleep increases during illness, again, we don’t know if the processes that lead to increased sleep after periods of sleep-deprivation also underpin the need to sleep when we’re sick.

Prior genetic screens in fruit flies have identified a number of molecules that might play roles in modulating sleep and wakefulness. However, the authors pointed out, most studies have involved loss-of-function screens, which identified factors that, when deleted, reduce sleep, rather than increase sleep. Overexpressing these factors doesn’t increase sleep above that of normal levels, “which suggests that they are insufficient to induce sleep on their own,” they pointed out.

To try and identify genetic factors that promote sleep, Sehgal’s team instead carried out a genome-wide gain-of-function screen in more than 12,000 Drosophila lines. Their results highlighted just one gene, which they called nemuri (nur)—the Japanese word for sleep—which induced sleep in the fruit flies. Drosophila sleep primarily at night, but (and particularly the males) do also have a midday siesta. Initial studies confirmed that overexpression of nur increased both daytime and night-time sleep patterns in the animals. Nur overexpression also led to greater depth of sleep—assessed by how easily the flies could be roused. “After the mechanical stimulus, the few aroused nur-overexpressing flies were sluggish and had reduced locomotor speed relative to control animals,” the authors wrote.

Further tests found that the nur gene product is a secreted protein that, as well as promoting sleep, has antimicrobial activity. AMPs kill microbes directly, but they can also modulate other aspects of the immune response, and prior studies have found that sleep can boost survival of flies after bacterial infection. The UPenn scientists showed that flies overexpressing nur in their neurons survived longer after bacterial infection than did control flies.

“Flies with increased nur also showed higher amounts of daytime sleep than controls after infection and had a lower bacterial load, consistent with the previous finding that sleep enhanced clearance of bacteria,” they wrote. “Thus, induction of sleep may represent another mechanism by which AMPs combat infection.”

To investigate what would happen if the flies lacked nur, they next used CRISPR-Cas9 technology to knock out the nur gene. Experiments suggested that while depleting nur didn’t have any consistent effect on sleep duration, in males (which more commonly take daytime naps than females), the number of bouts of daytime sleep was increased, while bout duration was decreased. Knocking out nur did impact on depth of sleep. While nur overexpression had rendered flies harder to rouse from sleep than wildtype animals, nur depletion was linked with much easier arousal. “… awakened nur mutants moved faster than aroused control animals,” the team continued. “The nur mutants also took longer to fall asleep again once they had been aroused. Thus, nur is required for sleep depth and for reinitiation of sleep after arousal.” Flies lacking nur, in addition, slept less after bacterial infection than wild-type flies.

“Nur fits the criteria for a somnogen, a secreted molecule that transmits homeostatic sleep need,” the scientists commented. “It increases when sleep need is high, and inducing its expression increases sleep.” They pointed out that prior studies have pointed to cytokines such as interleukin-1 and tumor necrosis factor-alpha as potential somnogens. Levels of these molecules can build up with prolonged periods of wakefulness and may promote sleep, the team noted. Studies have also found that in mammals, cytokines can induce production of AMPs, while AMPs can also affect expression of cytokines. It’s possible that the nemuri protein acts to connect immune function with sleep. “…we now identify an AMP that provides a mechanistic link between immune function and sleep, Sehgal et al., concluded. “Nur is relevant for sickness or stress-induced sleep, which is seen in many organisms.”

“The nemuri protein is a genuine driver of keeping sleep on track under conditions of high sleep need like when we’re sick,” added first author Toda, a postdoctoral fellow in Sehgal’s lab. “In the next phase of our work, we plan to investigate the mechanism by which nemuri drives sleep.”

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