Sleep is critical for survival, and many diseases are linked with long-term poor sleep quality. Researchers at the University of California, San Francisco (UCSF), have now identified a gene mutation in members of a family who need much less sleep than average. Reporting on their studies in rodents engineered to carry the same mutation, the team suggests the findings could ultimately lead to drug targets for treating sleep-related disorders.
“It’s remarkable that we know so little about sleep, given that the average person spends a third of their lives doing it,” commented Louis Ptáček, PhD, a neurologist at UCSF and one of two senior authors of their paper, which is published in Neuron. “This research is an exciting new frontier that allows us to dissect the complexity of circuits in the brain and the different types of neurons that contribute to sleep and wakefulness.” The team’s paper is titled, “A Rare Mutation of β1-Adrenergic Receptor Affects Sleep/Wake Behaviors.”
It’s long been thought that sleep behavior is regulated by interactions between the body’s circadian clock and sleep regulatory pathways. There is known to be some genetic component to sleep patterns in humans, the authors stated. “In humans, variations of genetically inherited sleep features in the population have been recognized for a long time. How and when different species sleep varies. Humans tend to sleep for one consolidated period, whereas mice will sleep throughout the 24-hour day, with a tendency to sleep more during the light than during the dark.”
The scientists had previously identified gene variations that influence the timing of sleep in humans, and confirmed that mouse models with the same genetic variants also tended to mirror the same sleep behaviors. “Timing of sleep is heavily influenced by the circadian clock, which has been intensely studied, and we now have a large and growing body of knowledge on how the clock is regulated at the molecular level. On the other hand, our understanding of sleep homeostasis regulation for human lags behind,” they wrote.
As the team pointed out, identifying genes that are in involved in modulating human sleep duration will provide a unique way to increase knowledge of genes and pathways that are critical for human sleep homeostasis and regulation. In their reported study the researchers searched for mutations in people naturally sleep for just 4–6 hours per night (natural short sleep; NSS), which is less than the average, but will still feel well rested. The researchers’ prior research had already identified a gene that, when mutated, caused carriers “to sleep 6 h nightly for their entire lives without apparent negative effects,” they wrote. “Another mutation in DEC2 was later reported in a single individual who is a short sleeper and resistant to sleep deprivation to sleep for less time than normal every night.”
For their research reported in Neuron, the team was particularly interested in NSS that ran in families. “While NSS has long been recognized in sporadic cases from the general population, familial NSS was only first reported in 2009, thus enabling the use of human genetics to identify novel sleep genes,” they explained.
Their search identified a family exhibiting familial natural short sleep (FNSS), in which individuals who carry a very rare variant of the β1 adrenergic receptor (ADRB1), which is a G-protein-coupled receptor (GPCR). Individuals in the family who carried this ADRB1 gene variant sleep for only about six hours a night—about two hours less than average—without any effect on their daytime functioning.
The gene variant was identified using genetic linkage studies and whole exome sequencing. “In the human population, this is a rare mutation, with an incidence of 4.028/100,000 according to the Exome Aggregation Consortium database,” the investigators stated.
Having identified ADRB1, the team first assessed its effects in laboratory grown cells. “We wanted to determine if these mutations caused any functional alterations compared with the wild type,” said co-senior author Hing-Ui Fu, PhD. “We found that this gene codes for ß1-adrenergic receptor, and that the mutant version of the protein is much less stable, altering the receptor’s function. This suggested it was likely to have functional consequences in the brain.”
The researchers next studied the effects of the variant in mouse models, and found that animals carrying the ADRB1 variant slept on average 55 minutes less per 24 hours than did control mice. Further analyses showed that the ADRB1 gene variant was expressed at high levels in the dorsal pons (DP), part of the brainstem that is involved in subconscious activities such as respiration, eye movement, and sleep. Their studies using fluorescence signaling techniques in live mice showed that the normal ADRB1 neurons in this region of the brain were more active during wakefulness, and also during REM (rapid eye movement) sleep, but were not activated during non-REM sleep. “We found that this receptor is highly expressed in the dorsal pons and that these ADRB1+ neurons are active during rapid eye movement (REM) sleep and wakefulness,” the researchers reported.
Neurons with the mutant gene were more active than their normal counterparts, which may contribute to the shortened sleep time. “Neuronal activity measured by calcium imaging in this region demonstrated that ADRB1+ neurons in DP are wake and REM sleep active,” the authors stated. “ADRB1+ neurons in DP change their population activity across sleep-wake states and offer a mechanistic framework for their participation in the regulation of sleep and wakefulness.”
“Another way we confirmed the role of the protein was using optogenetics,” Fu explained. “When we used light to activate the ADRB1 neurons, the mice immediately woke up from sleep.” Ptáček acknowledges that among the limitations of the study is the fact that mice do naturally exhibit different sleep patterns than humans. “But it’s challenging to study sleep in humans, too, because sleep is a behavior as well as a function of biology,” he stated. “We drink coffee and stay up late and do other things that go against our natural biological tendencies.”
The researchers say their findings support the role of the ADRB1 mutation as a “causative” mutation in FNSS, and of DP βARs in the regulation of sleep/wake behavior. “This, in turn, provides an opportunity to further explore the mechanisms and potential drug targets of β1-AR for the treatment of sleep-related disorders. Much more work is needed to dissect the complex circuitry underlying sleep/wake regulation.”
The investigators plan to study the function of the ADRB1 protein in other parts of the brain. They also are looking at other families for additional genes that are likely to be important. “Sleep is complicated,” Ptáček noted. “We don’t think there’s one gene or one region of the brain that’s telling our bodies to sleep or wake. This is only one of many parts.”
Fu suggested that the work may eventually help scientists develop new types of drugs to control sleep and wakefulness. “Sleep is one of the most important things we do,” she stated. “Not getting enough sleep is linked to an increase in the incidence of many conditions, including cancer, autoimmune disorders, cardiovascular disease, and Alzheimer’s.”
