Hypocretin is a protein suspected to play a role in both insomnia, which is a decreased ability to fall asleep at night, and in narcolepsy, which is a decreased ability to stay awake during the day. People suffering from insomnia may have too much hypocretin in the brain, while people suffering from narcolepsy have too little. Researchers also suspect hypocretin to play a role in depression, ADHD, and other mental disorders.

Although much is already known about the hypocretin system in the brain, little is understood on how hypocretin is regulated inside the cells, according to Birgitte Kornum, PhD, associate professor in the department of neuroscience, the faculty of health and medical sciences, University of Copenhagen who, along with colleagues, has published a study (“The evolutionarily conserved miRNA-137 targets the neuropeptide hypocretin/orexin and modulates the wake to sleep ratio”) in PNAS. The research combines tests on mice, zebrafish, and human cells.

MicroRNA associated with sleep regulation

The team has spent a number of years of researchers studying one of the cellular mechanisms that affect hypocretin levels. In the current research paper, they focused on microRNA-137 (miR-137).

Hypocretin (Hcrt), also known as orexin, neuropeptide signaling stabilizes sleep and wakefulness in all vertebrates. A lack of Hcrt causes the sleep disorder narcolepsy, and increased Hcrt signaling has been speculated to cause insomnia, but while the signaling pathways of Hcrt are relatively well-described, the intracellular mechanisms that regulate its expression remain unclear,” write the investigators.

“Here, we tested the role of microRNAs (miRNAs) in regulating Hcrt expression. We found that miR-137, miR-637, and miR-654-5p target the human HCRT gene. miR-137 is evolutionarily conserved and also targets mouse Hcrt as does miR-665. Inhibition of miR-137 specifically in Hcrt neurons resulted in Hcrt upregulation, longer episodes of wakefulness, and significantly longer wake bouts in the first 4 h of the active phase.

“IL-13 stimulation upregulated endogenous miR-137, while Hcrt mRNA decreased both in vitro and in vivo. Furthermore, knockdown of miR-137 in zebrafish substantially increased wakefulness.

“Finally, we show that in humans, the MIR137 locus is genetically associated with sleep duration. In conclusion, these results show that an evolutionarily conserved miR-137:Hcrt interaction is involved in sleep–wake regulation.”

Digital illustration of areas of activity during REM sleep in human brain highlighted in red and green
Digital illustration of areas of activity during REM sleep in human brain highlighted in red and green. [Dorling Kindersley/Getty Images]

“We discovered that miR-137 helps regulate hypocretin. To experience normal sleep, you need to have the right amount of hypocretin in the brain at the right time, and miR-137 helps with that. Though MiR-137 is also found in other parts of the body, it is especially pronounced in the brain,” said Kornum.

MicroRNA regulates various cellular processes, including hypocretin levels. Therefore, there is considerable research interest in microRNAs, as they could be targeted in order to regulate such processes.

Previously, the scientists knew little about the role played by miR-137 in the brain, but now Kornum’s research team has demonstrated that it is associated with hypocretin regulation and thus with sleep.

“This is the first time a microRNA is associated with sleep regulation,” she explained. “Drawing on the UK Biobank, we discovered some genetic mutations in miR-137 which cause daytime sleepiness. The study demonstrates this connection in both mice and zebrafish, and we are able to prove the connection with hypocretin. Our discovery shows just how complex the machinery of sleep is. Imagine inheriting a variant of miR-137 that puts you at higher risk of feeling sleepy during the day.”

Hypocretin affects sleep stages

Hypocretin, which has caught the attention of pharmaceutical companies, also affects the order of the four sleep stages, which follow a specific order that is vital to the quality of sleep.

“Narcolepsy patients suffering from low levels of hypocretin experience muddled sleep stages. We know this from mice tests demonstrating that hypocretin affects the order of these stages,” said Anja Holm, PhD, from Aalborg University, who is first author of the study and who did the tests together with Kornum.

Existing research suggests that to solve the problem we need to gain more knowledge of hypocretin regulation. Here the Danish researchers point to a different, but equally important piece of the puzzle, i.e., the immune system.

“Most people know that when you are ill you often feel tired. And when you have a fever and the immune system is hard at work, you often suffer from poor sleep. So we know that something happens to the hypocretin level when the body is trying to fight off a virus infection, for example, and we are trying to understand this process,” noted Kornum.

“In the study, we show that one of the immune system’s transmitter substances, IL-13, has a special effect on hypocretin. We can tell that when we add IL-13, it affects miR-137 and thus also the level of hypocretin in the body. We still do not know why, though, but we are currently doing tests that may be able to give us an answer.”