Researchers Find 2 Keys to Body’s Internal Clock : Science: A pair of proteins regulate circadian rhythms. They provide major tool in studying mysterious cycle.
Researchers have obtained their first peek into circadian rhythms, the hitherto mysterious internal clock that tells humans and other animals when to fall asleep and wake up and that regulates a host of other biological functions, such as hormone release and body temperature.
Gaining control of this clock, researchers say, could produce a wide variety of benefits, from helping workers stay awake on the night shift to resetting biological clocks to overcome jet lag, as well as lowering blood pressure and improving drug metabolism.
Until now, scientists have been stymied in their search for the intracellular mechanisms that control the 24-hour cycle of the clock.
In three papers in today’s issue of the journal Science, however, a team headed by geneticist Michael W. Young of Rockefeller University reports identifying a pair of proteins whose delicate waltz with each other and with the genes that produce them is the metronomic underpinning of the cycle.
Researchers do not yet know how exposure to light resets the cycle and precisely how the proteins regulate other body processes, but the newly discovered proteins should provide a long-sought entree into those systems, experts said.
“We are really . . . putting a lot of pieces of the jigsaw down,” said biologist Steve Kay of the University of Virginia.
Investigators have long recognized the existence of circadian rhythms, particularly in the sleep-wake cycle and body temperatures. Individuals living in darkened rooms and not exposed to sunlight have been found to have a natural cycle that is just a little bit more than 24 hours, indicating that control of the sleep-wake cycle is internal rather than directed by sunlight. Researchers also have observed that in the late afternoon, body temperature can be as much as two degrees Fahrenheit higher than in the morning.
Scientists have found at least 100 different circadian cycles in the human body, most of them less obvious. Blood clots most rapidly about 8 a.m., which may help explain why the largest number of strokes and heart attacks occur in the morning. Pain tolerance and athletic performance peak in the late afternoon, as does susceptibility to anesthesia used in surgery.
Drugs are metabolized more rapidly at different times of the day, a finding that is leading researchers to look for times when drugs are the most helpful with the fewest side effects. A few investigators even argue that manipulating biorhythms with the crude drugs available today can control some diseases, such as Type 2 diabetes. Almost everyone agrees that the ability to manipulate circadian rhythms in a more sophisticated manner could bring improvements in all those areas.
The researchers, who are part of the National Science Foundation’s Science and Technology Center for Biological Timing, studied the fruit fly, which has a 24-hour circadian cycle similar to that of humans.
They already knew about one gene and the protein it produces, called PER (for period)--previously discovered at Caltech. The concentration of PER in cells of the fly rises and falls over a 24-hour period, but researchers could not explain how that would influence cell cycles.
The key to deciphering the molecular gears and pendulums of the biological clock was the discovery and cloning of a second gene and its associated protein, TIM (for timeless). It is the interaction of PER and TIM that provides the mechanism for the biological clock.
All cells in the fly have the two genes, but those in the brain dominate the body’s activity. Researchers believe the waxing and waning of the proteins control the sleep-wake cycle and others, but that has not yet been proved.
The two genes in the nucleus of each brain cell become active at midday, when the individual is most active. During that time, DNA in the genes is transcribed into messenger RNA (mRNA), which serves as the actual blueprint for protein production.
The mRNA accumulates in cytoplasm outside the cell’s nucleus throughout the afternoon until about dusk, when there is enough for the cell to begin producing protein. Over the course of the evening, while the person is becoming sleepy, PER and TIM bind together to form a stable molecule that is able to enter the nucleus of the cell.
About four hours before dawn, the PER and TIM proteins reach their maximum concentration in the nucleus, which signals the two genes to stop making mRNA. Near dawn, when the person is waking up, the protein complex begins disintegrating naturally, allowing the genes to begin making mRNA again, and the whole process starts over.
The pace of the clock, Young said, appears to stem from the gradual, coordinated accumulation of the mRNA of both genes over a period of hours, as well as from the attraction of the two proteins to each other.
Though the scheme is still theoretical, Young emphasizes, a variety of evidence supports the view that the interaction of those genes and proteins is what controls the circadian cycle. For instance, if either of the two genes is mutated so that no protein is produced, the flies lose their circadian rhythm.
If the proteins are changed to bind less tightly to each other, the circadian cycle is expanded to about 29 hours. If PER is mutated so that it binds more tightly to TIM, the cycle is shortened.
Many questions still remain to be answered, Young concedes. Researchers must find out, for example, how exposure to daylight resets the cycle. They also need to know what else the PER-TIM complex is doing in the nucleus--whether, for example, it is turning on and off other genes that involve hormone release or other functions.
Nonetheless, the discovery of the PER-TIM interaction is a key step, Young said, because it provides researchers a tool with which to study the circadian system.
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The Waltz of the Proteins
After years of searching, researchers finally have their first insight into the molecular mechanism that creates circadian rhythms, the 24-hour cycle that regulates most life processes.
Process Begins: The per and tim genes in the nucleus become active at midday, producing messenger RNA, which serves as the blueprint for protein production.
Leaving the Nucleus: mRNA accumulates in cytoplasm throughout the afternoon until about dusk, when there is enough for the cell to begin producing protein.
Returning in Pairs: When enough PER and TIM accumulate over the course of the evening, they bind together, which enables them to enter the nucleus.
Beaking Down: About four hours before dawn, the PER-TIM complex reaches its maximum concentration in the nucleus, which signals the per and tim genes to stop making mRNA. Near dawn, the whole process starts over.
Source: The journal Science
