We are all familiar with the experience of needing to sleep. This need becomes more pressing, the longer we stay awake. Humans need to sleep for around one-third of their lives, and yet we do not know why. What happens to our brains while we are awake that causes us to become tired? What happens while we sleep that ultimately refreshes us and enables us to wake up and start a new day?
We are not alone in this either. Sleep is a universal phenomenon found in all organisms with a nervous system, including all vertebrates, and invertebrates such as flies, worms and jellyfish. Animals sleep even though they cannot forage or remain vigilant while resting, and even though they risk being discovered by predators. Throughout evolution, sleep has remained essential to these organisms but we do not know how it benefits the brain.
A new study conducted by researchers from the Bar-Ilan University in Israel has helped to shed some light on this mystery. The research elucidates a molecular mechanism involved in the sleep patterns of zebrafish. The study was led by Professor Lior Appelbaum, of Bar-Ilan’s Goodman Faculty of Life Sciences and Gonda (Goldschmied) Multidisciplinary Brain Research Center, along with postdoctoral researcher Dr. David Zada.
In the light of research findings from other studies, the Bar-Ilan scientists hypothesized that DNA damage accumulates in neurons (nerve cells) during waking hours. This damage is caused by various factors, including UV light, normal neuronal activity, radiation, oxidative stress and enzymatic errors. Molecular repair systems within each cell correct the damage continuously, but cannot keep up with the damage that accumulates during times of wakefulness. This leads to a homeostatic pressure (tiredness) that builds up in the body.
The researchers tested their hypothesis in a series of experiments on zebrafish. These small, freshwater fish have been bred to be transparent, enabling the penetration of radiation. They have simple brains and sleep at night. The scientists used irradiation, pharmacology and optogenetics, to induce DNA damage in zebrafish and observed how this affected their sleep.
They found that, as DNA damage was increased, the need for rest also increased. The experiment suggested that at some point the accumulation of DNA damage reached a maximum threshold, and increased sleep (homeostatic) pressure to such an extent that the urge to rest was triggered and the fish went to sleep. During the period of sleep, DNA repair took place, which resulted in reduced DNA damage. The fish then awoke.
By gradually reducing the time of darkness to which the fish were exposed each night and monitoring their sleep behavior, the researchers determined that zebrafish require a minimum of six hours of sleep per night in order to repair the DNA damage such that they could wake up. Surprisingly, after less than six hours of sleep, DNA damage was not adequately reduced, and the zebrafish continued to sleep, even though it was broad daylight.
In more detailed research, the scientists investigated the molecular mechanism whereby the brain tells the organism that it needs to sleep in order to allow repair of the damaged DNA. The protein PARP1, plays a role in the DNA damage repair system, and acts like an “antenna”, signaling to the brain that sleep is needed. PARP1 marks DNA damage sites in cells, and recruits all the relevant systems to repair DNA damage. These protein molecules accumulate at DNA break sites during times of wakefulness and they decrease gradually during times of sleep.
Through genetic and pharmacological manipulation, the overexpression of PARP1 revealed not only that increasing PARP1 promoted sleep, but also that it increased sleep-dependent repair. Conversely, knockdown or inhibition of PARP1 blocked the signal for DNA damage repair. As a result, the fish weren’t fully aware that they were tired, didn’t go to sleep, and no DNA damage repair took place.
To test the validity of the findings, the functioning of PARP1 was also tested in mice, in collaboration with Professor Yuval Nir from Tel Aviv University. Just as with zebrafish, the inhibition of PARP1 activity reduced the duration and quality of non-rapid eye movement (NREM) sleep. “PARP1 pathways are capable of signaling the brain that it needs to sleep in order for DNA repair to occur,” says Prof. Appelbaum.
In a previous study, Professor Appelbaum and team determined that sleep increases chromosome dynamics, the movements and attachments of chromosomes that allow DNA repair to take place effectively. Adding the new understanding about the role of PARP1 to the puzzle, the researchers could state that PARP1 increases sleep and chromosome dynamics, which facilitates efficient repair of DNA damage that has been accumulated during waking hours. If the DNA maintenance process in neurons is not efficient enough during waking hours, then an “offline” sleep period with reduced input to the brain allows all residual damage to be repaired.
This new understanding of the “chain of events” that takes place in terms of the repair of DNA during sleep, may explain the link between sleep disturbances, aging and neurodegenerative disorders, such as Parkinson’s and Alzheimer’s. Professor Appelbaum believes that future research will help to apply this mechanism to other animals, ranging from lower invertebrates to, eventually, humans.
The study is published today in the journal Molecular Cell,