Recent research has shed new light on the sleep/wake cycle. In two papers featured on the cover of the July 10 issue of Cell (J.L. Price et al., Cell, 94:83-95, 1998; B. Kloss et al., Cell, 94:97-107, 1998), scientists from Rockefeller University reported the discovery of a gene in Drosophila, dubbed double-time (dbt). The dbt gene is believed to regulate the molecular cycles underlying circadian rhythms--patterns of activity that, in humans, regulate body temperature, mental alertness, pain sensitivity, hormone production, and the sleep/wake cycle. Better understanding such patterns may help researchers elucidate the mechanics of sleeping disorders, make drug delivery more effective and efficient, and improve the acclimation of persons working night shifts.
In 1995, the senior author of both papers, Michael W. Young , director of the NSF Science and Technology Center for Biological Timing at Rockefeller, and colleagues showed that circadian rhythms in Drosophila require the interaction of the two proteins PERIOD (PER) and TIMELESS (TIM) (N. Gekakis et al., Science, 270:811-5, 1995). Follow-up studies suggested that similar mechanisms were at work in mice and in humans.
DNAGOLIATH DNA: Geneticists did not slay a giant for this DNA trophy hanging in the newest life sciences building at the University of California, Davis. Instead they turned to the skilled hands of Roger Berry (below), a Clarksburg, Calif., artist. He pieced together 400 feet of stainless steel and 603 wafers of glass to produce "Portrait of a DNA Sequence," an accurate representation of a fragment of a kinesin-related protein discovered by UC-Davis molecular and cellular biologist Jonathan M. Scholey. Inspiration for the 48-foot-tall, 18-inch-diameter sculpture initially came to Mark G.McNamee, dean of the division of biological sciences. In 1996, he had visited the unfinished building and the massive four-story spiral staircase reminded him of the DNA double helix.
The period and timeless genes start making the RNA molecules essential to create PER and TIM proteins at around noon; but only after sunset does the accumulated RNA prompt the cell to stockpile, and then pair, these proteins. He and other researchers surmised two years ago that the delay was due to the failure of PER to accumulate despite the presence of the necessary RNA--but they weren't sure why this occurred. They now believe that the DOUBLETIME (DBT) protein regulates the buildup of PER during the day by degrading it, probably via phosphorylation. The nighttime partnership between PER and TIM, on the other hand, most likely prevents such a phosphorylation event and thus allows the cycle to progress.
"We haven't shown the direct phosphorylation of PER by DOUBLETIME," reports Jeffrey Price , an assistant professor of biology at West Virginia University and a co-author of both Cell papers. "What we've shown so far is an association." Price is quick to point out, however, that the research team did produce dbt mutants that showed an increased accumulation of PER and had altered PER phosphorylation and sleep/wake cycles. "That's very strong evidence that phosphorylation regulates at least the stability of PER," he maintains.
The Cell papers come in the wake of a pair of related Science papers published in June (N. Gekakis et al., Science, 280:1564-8, 1998; and T.K. Darlington et al., Science, 280:1599-1603, 1998). According to Steven Kay , senior author of the second Science paper, and an associate professor of cell biology at the Scripps Research Institute in La Jolla, Calif., he and his colleagues demonstrated how two other players in the cycle, the CLOCK and BMAL1 proteins, interact with PER and TIM to yield periodicity in both fly and mammalian models. The four proteins are involved in a negative feedback loop: CLOCK and BMAL1 drive PER and TIM expression; but when enough PER and TIM accumulate, they inhibit CLOCK and BMAL1.
"We sort of closed the circadian loop by showing that PER and TIM specifically inhibited these two other proteins CLOCK and BMAL1," notes Kay. The main point of the Science papers, he says, is that the "batteries of the clock [CLOCK and BMAL1] are the same in flies and mammals."
"Mike Young's paper then talks to a different part of the clock," he continues. "That is, how do you stretch this clock out to be a 24-hour one? How do you make sure that PER is removed from the cell at the right time? That's what this protein kinase [DBT] is doing."
According to Kay, the most intriguing thing about Young's findings is that some of the dbt deletions proved lethal--a finding that might indicate that the dbt alleles have a role in the biology of the organism other than that of regulating circadian rhythms.
While Young contends that researchers have discovered all the major players in the cycle, he acknowledges that other features have yet to be clarified. For example, light, when shown on the system, causes the rapid destruction of TIM by an unknown mechanism. Kay hopes to understand how light "resets the clock" by finding the actual photoreceptor responsible for TIM elimination.
"We don't have a DOUBLETIME-like activity that's been identified that comes over and grabs timeless when the lights are on," says Young. "That's an important piece of the puzzle that needs to be found."