The first unbiased genetic screen for sleep defects in mice has yielded two interesting mutants, Sleepy, which sleeps excessively, and Dreamless, which lacks rapid eye movement (REM) sleep. The findings are the first step towards discovering the biochemistry that controls the switch from wakefulness to sleep, the researchers say.
Although sleep is essential for life and we spend one third of our lives doing it, the function of sleep, and the physiology that regulates it, remain longstanding mysteries in biology. Researchers have hypothesized that there is a substance that builds up when we are awake, and then has to be discharged or recovered while we are sleeping. "But we still don't understand those processes," said HHMI Investigator Joseph Takahashi at the University of Texas Southwestern Medical Center in Dallas.
To address these issues, Takahashi and former HHMI Investigator Masashi Yanagisawa, now Professor in the International Institute for Integrative Sleep Medicine at the University of Tsukuba in Japan, decided to take an unbiased exploratory approach. Instead of beginning with a hypothesis about specific genes that might be involved, the researchers introduced random genetic mutations in more than 8,000 mice and screened them using electroencephalography (EEG) to determine which ones had abnormal sleep as a result of the genetic perturbations.
"The barrier in the past has been that it's a very laborious process. To do a genetic screen, you should be prepared to screen thousands of animals before you find something interesting, and most people are just not willing to measure EEGs in thousands of mice," explained Takahashi. But by optimizing the surgical methods, electrodes, and the software to analyze the EEGs in automated fashion, the researchers were able to conduct the first unbiased genetic screen of this magnitude for sleep defects in mice, which they report in this week's issue of Nature.
The researchers identified two mutations, which they called Sleepy and Dreamless, and subsequently mapped them to locations in the mouse genome. Sleepy mice, which need approximately one third more sleep than normal mice, carry a mutation in the Sik3 kinase gene. Because the Sik3 kinase can phosphorylate many proteins, it is likely to be involved in many signaling pathways, which makes it trickier to characterize.
To investigate why Sleepy mice need more sleep, the researchers examined the circadian clock in Sleepy mice, but they did not find a circadian rhythm disturbance. They tested whether the mice had defects in their arousal system by stimulating them with environmental (e.g., a new cage) and pharmacological (e.g, caffeine and modafinil) stimuli, but found that the mice had normal arousal responses. They concluded that the Sleepy mice had an increased sleep need, but the physiological reasons for that remained unclear. "Sleep need still remains a mystery, but what we hope is that this kinase is maybe the key, the initial key to this big door," said Yanagisawa.
Dreamless mice, which have reduced rapid eye movement (REM) sleep, carry a mutation in a sodium channel. Understanding the effects of the dreamless mutation was more straightforward. The mutation increases the conductivity of a leaky sodium channel that was previously known to regulate neuronal excitability. The neuronal populations that terminate REM sleep have too much excitability, said Yanagisawa, which is why the mice have reduced REM sleep.
The researchers are optimistic that the screen will yield more mutants with sleep defects to investigate. "We really hope that this is opening up some mysteries ... this is just the beginning," said Yanagisawa.