Prof Lior Appelbaum explains why DNA repair seen in the neurons of sleeping zebrafish may hold the key to understanding why we sleep.

Sleep. Everyone knows it’s vital for our very survival. But why? What is happening in our bodies while we sleep? Understanding this fundamental question is the focus of research groups like the Appelbaum Lab at Bar-Ilan University in Israel. In 2019, Appelbaum, along with Dr David Zada and colleagues, published the results of a study in zebrafish, which proposed that the restorative function of sleep is DNA repair. More recently, the same lab published another paper showing that accumulation of DNA damage drives sleep pressure, and that the PARP1 protein promotes sleep and its repair activities.

Longevity.Technology: Sleep is inherently linked to our longevity and healthspan, but simply knowing that it’s good for us doesn’t mean we understand it. We spoke to the head of the Appelbaum Lab, Professor Lior Appelbaum, to learn more about his work in this intriguing field of research.

The phenomenon of sleep fascinates Appelbaum because it is conserved across almost every animal on our planet and yet, he says, it makes no sense from an evolutionary perspective.

“Sleep is conserved across evolution, even jellyfish, worms, and flies sleep – but why? All animals with a nervous system sleep, but it’s against evolution, against survival. If you sleep, it makes you vulnerable to predators – it doesn’t make sense at all.”

A radical view on sleep

Appelbaum says that most people agree that the purpose of sleep is for the benefit of the brain/nervous system, and that sleep is typically thought of as a “brain state” that involves networks of neurons operating together. His lab decided to look at things slightly differently.

“Many studies show that sleep is important for learning and memory – if you don’t sleep for several hours, your performance will be reduced, and you will be less successful in tests,” says Appelbaum. “But we took a more radical view. We thought sleep might be important for individual neurons and consequently also to the entire networks, because even animals with simple neural nets, like jellyfish and worms, sleep.

“This made sense to us, because it’s hard to believe that jellyfish need to consolidate memory, so why do they sleep? This was the starting point for our experiment.”

The next step was to find a suitable animal model to observe. Studying individual neurons in live mammals is challenging due to the complexity of accessing the brain, which is protected by the cranium. So the researchers eventually opted for the zebrafish.

“The zebrafish is a unique animal model because it a transparent vertebrate which makes it easier to study,” says Appelbaum. “In addition, the brain is simpler, but with the same structure and functions as humans have.”

Sleep promotes DNA repair in neurons

The scientists developed several tools to allow them to look inside a single cell inside an intact animal. This allowed them to observe what was happening inside a single neuron while a fish was asleep (or awake), which led to a ground-breaking observation.

“We found that DNA damage was accumulated in neurons during wakefulness, and this damage was reduced during sleep,” says Appelbaum. “DNA damage is a natural process that happens all the time in our cells, and there are many repair systems that fix this damage. But for some reason, in neurons, this ongoing repair is not efficient enough during wakefulness, so damage continues to accumulate and is only reduced during sleep, when the repair activity appears to be more efficient.”

The lab also saw that sleep deprivation increased the DNA damage levels in neurons, and this was only repaired when normal sleep patterns were resumed. Interestingly, the phenomenon was only observed in neurons and not in any other cells. So why neurons?

“Neurons are non-dividing cells that you carry with you until you die, they can’t be replaced, so we need to take care of them,” says Appelbaum. “So maybe this is why we have developed this strange behaviour called sleep – to allow our neurons to perform nuclear or cellular maintenance. Sleep is the offline mode when you can access this repair process, and it appears you cannot do it while the brain is awake.”

Following its initial discovery, the Appelbaum Lab’s work has continued to delve further into the mechanisms behind sleep, leading to the most recent paper about the connection to the PARP1 gene.

“It appears that PARP1 is a detector – it’s accumulated in the neuron while the DNA damage is accumulated and it signals to the brain it’s time to sleep,” says Appelbaum. “So whenever you overexpress PARP1 the fish go to sleep and when you knock out PARP1, the fish stay awake because nothing tells them that it’s time to go to sleep.”

Sleep and neurodegenerative disease

So how might all this zebrafish-specific data relate to human longevity?

“Well, according to our hypothesis, if you are chronically sleep deprived you will accumulate DNA damage and you will start to see neurodegenerative effects,” says Appelbaum. “It’s been known for years that Alzheimer’s, Parkinson’s and ALS all are linked to sleep disturbances, and there are those that suggest sleep disturbances increase the risk for neurodegenerative disease.

“Our mechanism suggests why this might happen: it may be that, if you don’t sleep well, you accumulate DNA damage to your neurons and, if it goes on long enough, eventually those cells start to die and then you increase the risk for neurodegenerative disease.”

Appelbaum also points to a recent study of doctors working nightshifts that linked sleep deprivation to DNA damage in humans.

“It definitely shows there is correlation between chronic sleep deprivation and DNA damage accumulation, so it’s a risk for longevity,” he says. “Of course, they could not do in humans what we did in zebrafish. We manipulate DNA damage and see its effect on sleep, or the opposite. But that’s in a model animal, and it’s easier.”

How much sleep to repair DNA?

For years, people have been interested to know how much sleep they should be getting, and there are many different views on this. While Appelbaum can’t comment directly on what humans might need, he has conducted this exact experiment on zebrafish.

“The fish are very sensitive to light – they only sleep when it’s dark, so it is easy to control when they sleep and wake,” he says, explaining that the researchers monitored DNA damage to neurons depending on how many hours the fish spent asleep.

“We saw that if the fish slept for only two to four hours, the DNA damage state remained high. But after six hours, the DNA damage reduced back to normal levels. So, in zebrafish at least, six hours is the optimal amount of sleep time, from a DNA repair perspective. If they slept eight hours or 10 hours, the effect was the same as for six hours.”

Next steps for sleep research

There are still plenty questions for Appelbaum’s lab to answer when it comes to understanding sleep.

“In zebrafish, we still need to answer a big question around whether the DNA repair is global or circuit specific,” he says. “I believe it’s circuit specific. How does the brain know that it’s time to repair DNA in a specific neuron? Not every cell will put you to sleep, so there must be a threshold where many cells somehow signal the brain that it’s time to go to sleep.”

There is also the question of neurons that remain active while we sleep – those that control our dreams, for example.

“Maybe they rest during wakeful hours – we don’t know yet,” says Appelbaum, who also intends to extend this work into other animal models, including humans.

“We aim to extend the studies in zebrafish to other animal models. We will start to collaborate with hospitals to try to do some applications in humans, but also into simpler animal models like invertebrates to see if what we found in the fish is also found there. Because our hypothesis is that this is an evolutionarily conserved phenomenon.

“No matter what species we are, we need to take care of our neurons. Whether you’re a worm or a human, you must sleep.”