After drinking a mysterious potion, Rip Van Winkle slept for 20 years and finally awoke an old man. Perhaps he would have avoided the potion had it been labeled “oxidative stress,” which refers to byproducts of mitochondrial activity that have been associated with lifespan, aging, and degenerative disease. Oxidative stress, scientists now report, influences sleep-control neurons via ion transport channels that are named, appropriately enough, Sandman and Shaker.
According to scientists based at the University of Oxford, the new findings about oxidative stress brings us closer to understanding how sleep is regulated, to developing new drugs to treat sleep disorders, and to confirming the suspected link between chronic lack of sleep and shorter lifespans.
“It’s no accident that oxygen tanks carry explosion hazard labels: uncontrolled combustion is dangerous,” said Gero Miesenböck, PhD, director, Centre for Neural Circuits and Behaviour and Waynflete professor of physiology, who led the Oxford team. “Animals, including humans, face a similar risk when they use the oxygen they breathe to convert food into energy: imperfectly contained combustion leads to “oxidative stress” in the cell.
“This is believed to be a cause of aging and a culprit for the degenerative diseases that blight our later years. Our new research shows that oxidative stress also activates the neurons that control whether we go to sleep.”
The new research appeared March 21 in the journal Nature, in an article titled, “A potassium channel β-subunit couples mitochondrial electron transport to sleep.” The article describes how the Oxford team studied the regulation of sleep in fruit flies—the animal that also provided the first insight into the circadian clock nearly 50 years ago. Each fly has a special set of sleep-control neurons, brain cells that are also found in other animals and believed to exist in people.
“In the fruit fly Drosophila,” the article explained, “about two dozen sleep-inducing neurons with projections to the dorsal fan-shaped body (dFB) adjust their electrical output to sleep need, via the antagonistic regulation of two potassium conductances: the leak channel Sandman imposes silence during waking, whereas increased A-type currents through Shaker support tonic firing during sleep.”
In previous research, Miesenböck and colleagues discovered that dFB neurons act like an on-off switch: if the neurons are electrically active, the fly is asleep; when they are silent, the fly is awake.
“We show that oxidative byproducts of mitochondrial electron transport regulate the activity of dFB neurons through a nicotinamide adenine dinucleotide phosphate (NADPH) cofactor bound to the oxidoreductase domain of Shaker’s KVβ subunit, Hyperkinetic,” the new study indicates. “Sleep loss elevates mitochondrial reactive oxygen species in dFB neurons, which register this rise by converting Hyperkinetic to the NADP+-bound form. The oxidation of the cofactor slows the inactivation of the A-type current and boosts the frequency of action potentials, thereby promoting sleep.”
The Shaker and Sandman channels generate and control the electrical impulses through which brain cells communicate. A main difference between sleep and waking is how much electrical current flows through Shaker and Sandman. During sleep, most of the current goes through Shaker.
“This turned the big, intractable question ‘Why do we sleep?’ into a concrete, solvable problem,” said Seoho Song, PhD, one of the current study’s co-authors. “What causes the electrical current to flow through Shaker?”
The team found the answer in a component of the Shaker channel itself.
“Suspended underneath the electrically conducting portion of Shaker is another part, like the gondola under a hot air balloon,” noted lead author Anissa Kempf, PhD, a postdoc at the University of Oxford. “A passenger in the gondola, the small molecule NADPH, flips back and forth between two chemical states—this regulates the Shaker current. The state of NADPH, in turn, reflects the degree of oxidative stress the cell has experienced. Sleeplessness causes oxidative stress, and this drives the chemical conversion.”
In a striking demonstration of this mechanism, a flash of light that flipped the chemical state of NADPH put flies to sleep.
According to Miesenböck, drugs that change the chemistry of Shaker-bound NADPH in the same way could be a powerful new type of sleeping pill: “Sleep disturbances are very common, and sleeping pills are among the most commonly prescribed drugs. But existing medications carry risks of confusion, forgetfulness, and addiction. Targeting the mechanism we have discovered could avoid some of these side effects.”