Marcia Barinaga
One of our most indispensable biological machines is our circadian clock, which acts like a multifunction timer to regulate sleep and activity, hormone levels, appetite, and other bodily functions with 24-hour cycles. The clock generally runs a bit fast or slow and must be reset daily by sunlight. Although many components of the clockwork are known, the crucial photoreceptor that passes light's signal to the clock is still at large.
Two suspects, the light-sensitive pigments in the rod and cone cells of the mammalian eye, are eliminated by two papers in this issue. "The really important conclusion from these experiments is that there is another photoreceptor" affecting the clock, says circadian biologist Michael Menaker of the University of Virginia, Charlottesville.
One candidate for that photoreceptor is a protein called cryptochrome. But a report in yesterday's issue of Nature puts an intriguing wrinkle in that story, fingering cryptochrome as a likely part of the clock itself. In mice that lack cryptochrome, the group found, the clock doesn't run at all. "We have never seen [in mice] a mutant like this, where there is instant arrhythmicity," says clock researcher Steve Kay of The Scripps Research Institute in La Jolla, California. That means cryptochrome is essential for clock function, but leaves open the question of whether it is the long-sought circadian photoreceptor in mammals.
Biologists have known since the 1960s that the clock-setting light signal in mammals normally comes via the eyes, because eyeless rodents and humans with few exceptions are unable to reset their clocks to light. One obvious possibility is that the molecules that capture light for vision--the opsins in the rod and cone cells of the retina--also send light signals to the clock.
Evidence against that has mounted as researchers have found that mice lacking either rods or cones have clocks that respond to light. But the chance remained that rods and cones both can do the job, and either can do it alone. The reports on pages 502 and 505 by Russell Foster of the Imperial College of Science, Technology and Medicine in London and his colleagues rule that out.
The researchers introduced genes that destroy retinal rod and cone cells into mice. They found that in those mice, just as in normal mice, light resets the clock and suppresses production of the clock-controlled nocturnal hormone, melatonin. "That says that you don't need rods and cones" for the light response, says Menaker, and means another photoreceptor must do the job.
Cryptochrome, which is found in the eye, became a hot candidate for the photoreceptor last fall, when three teams reported that it seems to help light reset the clock in plants, fruit flies, and mice (Science, 27 November 1998, p. 1628). A group led by Aziz Sancar at the University of North Carolina, Chapel Hill, and Joseph Takahashi at Northwestern University in Chicago mutated cry2, one of two mammalian cryptochrome genes, in mice. The animals' clocks lost some light responsiveness, suggesting that Cry2 is a light sensor, but not the only one. Researchers wondered if Cry1 might be the other, and waited to see if mice missing both cryptochromes could adapt to light.
In Nature this week, Jan Hoeijmakers at Erasmus University in Rotterdam, Netherlands, Akira Yasui of Tohoku University in Sendai, Japan, and their co-workers report the first tests on such mice. But instead of providing an answer about light response, the results delivered a surprise: The mice have no clock. Under conditions of 12 hours of light followed by 12 hours of dark, they act like normal mice, running in their exercise wheels in the dark and sleeping when it is light. But in constant darkness, when the clock would normally maintain the alternating cycles, their behavior loses that pattern; they run on and off around the clock.
Those results suggest the animals' clocks fail in constant darkness. But further tests show they actually have no clock at all. When normal animals are subjected to a new light-dark pattern, they begin to adapt their clocks, a slow process as any jet-lagged traveler knows. But the mutant mice instantly adjust to any light pattern; they run when the lights go out and stop when they come on. That, says clock researcher Jeff Hall of Brandeis University in Waltham, Massachusetts, is the kind of behavior observed in clockless animals. Without a clock to control their behavior, it is driven directly by the light. Sancar and Takahashi, working with Takeshi Todo of Kyoto University in Japan, have also made double cryptochrome knockout mice and have preliminary results similar to those of the Dutch-Japanese team.
Slight abnormalities in cry2 mutant mice that Sancar and Takahashi reported last fall suggested that cry2 might play a central role in the clock, but most researchers were surprised to learn it is essential for the clock to work. That creates a new mystery: What is the cryptochrome doing in the clock? But it does little to solve the old puzzle of whether cryptochrome transmits light signals.
"Perhaps both functions, the clock and the light input, are being taken out" in the double mutants, says Takahashi. Ironically, the lack of a clock in the mutant mice makes it hard to test that hypothesis; one can't measure the effect of light on a nonexistent clock. But Kay notes that even if the clock is disabled, some of its molecular parts remain and should be able to respond to light. He suggests the authors check the behavior of those proteins to see if a light signal is getting through, something both groups plan to do. Wherever the search for the circadian photopigment leads as it moves beyond the rods and cones, one thing is for sure: Cryptochrome has guaranteed itself a place in the story of the circadian clock.