Fruit fly protein involved in circadian response to light

Raquel Harper

When people travel across time zones, jet lag occurs because the body’s internal clock takes a long time to synchronize to a new day and night schedule. The internal clock, or circadian rhythm, which is synchronized by sunlight, controls functions such as sleep, body temperature and metabolism. Researchers from Howard Hughes Medical Institute at the University of Pennsylvania School of Medicine in Philadelphia recently discovered that a protein — which they labeled JET — is required for fruit flies’ circadian response to light. Their discovery may help in the development of new treatments for jet lag.

According to Amita Sehgal, a member of the team, the fruit fly provides an excellent model for studying genetics and molecular biology because its genes can be easily identified, allowing the researchers to figure out which ones are affecting biological processes. Furthermore, she said that most genes are now known to be shared between flies and humans, so gaining knowledge about the flies may help in understanding how human biology works.

Sehgal explained that while the scientists were routinely studying rest-activity rhythms of various fly strains, they noticed a mutant strain that seemed to have a reduced sensitivity to light. The flies were having trouble synchronizing their internal 24-hour clocks to the day-night cycle. The researchers decided to explore this further.

Jet-laglike behaviors of a mutant fruit fly enabled researchers to identify a protein that is required for fruit flies’ circadian response to light. The protein helps TIM, a “clock” protein (red), degrade, which the flies associate with daytime hours. The images show that TIM’s degradation was substantially less in the mutants than in the control flies, suggesting that the protein is required for TIM’s response to light.

They tested the circadian rhythm of the fruit flies by exposing both wild-type flies and the mutant strain to three environments — constant darkness, constant light, and periods of light and dark. They found that almost all of the mutant flies maintained their regular circadian rhythm in constant light, whereas the wild-type control flies did not. However, in darkness, the mutants’ behavior was the same as the control flies, which suggested that the mutants had a defect in their light-input pathway. And when exposed to various periods of light and dark cycles, the mutant flies took longer to adjust their internal clocks than did the control flies.

A normal fruit fly, when exposed to light, has a photoreceptor called cryptochrome that responds to light by binding with a core “clock” protein called timeless (TIM). A second protein, the one the researchers labeled JET, also binds to TIM, which leads to TIM degradation. Because the degradation of TIM always happens in the presence of light, the fruit flies associate the absence of TIM with daytime hours.

Through genetic analysis, the researchers found a defect in the mutant flies’ gene that encodes JET. They used a confocal microscope from Olympus of Melville, N.Y., to examine the changes in TIM levels in both the mutant and wild-type flies. TIM degradation was substantially less in the mutants than in the control flies, suggesting that JET is required for the TIM response to light.

To explore JET’s role further, the researchers genetically replaced the mutated gene sequence that encodes JET with a normal sequence in a sample of fruit flies. When these insects were exposed to light, regular TIM degradation took place and the flies adjusted better to shifting of the light-dark cycle. The researchers concluded that the JET protein must associate with TIM for TIM degradation to occur so that when JET is absent, the fruit flies have an abnormal circadian rhythm.

To further understand circadian response to light, the scientists will continue to look for other proteins that function in the same pathway. Sehgal hopes that the work will contribute to a better understanding of how the biological clock responds to light, which could ultimately lead to treatments for jet lag.

Science, June 23, 2006, pp. 1809-1812.