Transparent Zebrafish Study Reveals How Sleep Repairs Damaged Neuronal DNA

Using transparent zebrafish, Israeli researchers were able to confirm neuronal DNA repair as a function of sleep, also identifying a protein that triggers both DNA repair and sleep.

Background

The functions of sleep, though widely researched, have largely remained a mystery. It is known that sleep influences cognition, benefitting learning and memory, but proposed physiological functions have had little strong evidence. Proposed functions of sleep include removing toxic byproducts in the brain caused by wakefulness, replenishing energy and supplies for cells, and remediating neural damage and cellular stress.

Prior research showed that wakefulness and neuronal activity causes DNA double-strand breaks. These lesions are accumulated during wakefulness, contributing to homeostatic sleep drive (the pressure to sleep that builds up as time awake increases). Sleep has been demonstrated to decrease this DNA damage.

Researchers from Israel’s Bar-Ilan University and Tel Aviv University used zebrafish to study neuronal DNA repair as a potential novel function of sleep. The zebrafish is a tiny freshwater fish that has been widely used as a model organism due to their 70% genetic homology (similarity) to humans. Among other parallel physiologies, zebrafish exhibit a diurnal sleep cycle with states closely resembling mammalian slow-wave sleep (SWS) and rapid eye movement (REM) sleep. A mutated type of zebrafish is transparent, enabling researchers to observe the previously unobservable.

By inducing neuronal DNA damage in zebrafish, they were able to determine its relation to sleep as well as causal proteins.

Results

First, the researchers confirmed a significant positive correlation between levels of neuronal DNA damage and total sleep time (R = 0.76). During wakefulness, zebrafish larvae were treated with pentylenetetrazol, which stimulated their neuronal activity. Consequently, the larvae had increased neuronal DNA damage and a 5-fold increase in total sleep time.

Because neuronal DNA damage is not only caused by cell activity, the researchers then exposed the larvae to UV radiation, which damaged their DNA without increasing their neuronal activity. Their subsequent increased sleep further confirmed that DNA damage was the cause.

By testing the rates at which genes involved in the DNA damage response (DDR) were expressed during sleep, they found that the RAD52 and Ku80 proteins were responsible for repairing double-strand breaks during sleep.

Further analysis uncovered that the PARP1 protein, a DNA damage detector that organizes the DDR, was immediately recruited and activated upon neuronal DNA damage.

When the researchers provoked greater expression of PARP1, total sleep time and depth increased—demonstrating that the protein is what connects the DNA damage response to homeostatic sleep drive. PARP1 was also shown to promote sleep regardless of damage.

Discussion

The study confirms in multiple experiments that neuronal DNA repair is a function of sleep regulated by the PARP1 enzyme and carried out by the DNA repair proteins RAD52 and Ku80.

Triggering neuronal DNA damage via cellular excitation and UV light both caused the expected result of increased sleep, with PARP1 as the protein responsible for detecting the damage, provoking a repair response, and causing increased sleep drive.

The study authors noted their intrigue that FDA-approved PARP1 inhibitors used as antitumor agents all caused fatigue as the prominent side effect, suggesting that inhibiting PARP1 masks its sleep-promoting signals. The use of PARP1 inhibitors was also found to cause increased DNA damage. These results, separate from their study, align with their findings regarding the functions of PARP1 as observed in zebrafish.