THURSDAY, July 24, 2008 (HealthDay News) -- Researchers report today the identification of a new cog in the machinery of the molecular clock that controls mammalian circadian rhythms.
But the protein, SIRT1, is not merely some new component. It can also sense and act on the cell's metabolic state. And it is related to genes that have been implicated in longevity. Thus, these findings link for the first time the molecular mechanisms of circadian rhythms, metabolism and longevity in a single protein.
"Everyone feels that their normal life is dominated by circadian rhythms, and they might feel also that metabolism is circadian: hormones, temperature, desire to eat, sleep -- all that is metabolism," explained Paolo Sassone-Corsi of the University of California, Irvine, who led one of the two research teams that reported the findings. "The fact that we have found a molecular link between internal clock and metabolism explains why these are so interconnected."
The findings "are actually very exciting, because they link the very interesting sirtuin-1 pathway to the circadian clock for the first time," said Joseph Takahashi, a professor of neurobiology and physiology at Northwestern University, who was not involved in this research. "It is a direct molecular link to the clock mechanism."
The findings were published in a pair of reports in the July 25 issue of Cell.
Sassone-Corsi and Ueli Schibler, of the University of Geneva, Switzerland, led independent research teams that made the discoveries. Each approached the problem from a different angle.
Sassone-Corsi's group was looking specifically for an enzyme that could counterbalance the activity of another integral clock component, a protein called CLOCK.
According to Sassone-Corsi, approximately 10 percent to 15 percent of all cellular genes are expressed or regulated in a circadian manner; that is, their abundance or activity fluctuates over the course of the day.
Key to that fluctuation is the so-called molecular clock. At the heart of the clock mechanism are two proteins, CLOCK and BMAL1. These two proteins interact to form a complex that binds to DNA to activate the expression of several other circadian genes, including Period (PER) and cryptochrome (CRY). It takes a while for PER and CRY proteins to accumulate, but once they do (by late afternoon), they form a complex that blocks CLOCK and BMAL1 activity. PER and CRY expression then stops (because CLOCK/BMAL1 activity turns the genes on), and the PER and CRY proteins slowly degrade, allowing the clock to reset itself. This entire process occurs over about 24 hours, hence, a circadian rhythm.
Interestingly, CLOCK does more than bind DNA. It also functions as an enzyme that catalyzes the transfer of a small molecular mark (an acetyl group) to certain proteins, thereby changing their activities, much like flipping a switch. One of those targets is BMAL1.
Sassone-Corsi reasoned that for every enzyme that can add a molecular mark, there must be another that can remove it. "That's the way biology works; there is yin and yang to any function in the cell," he said.
His search led him to SIRT1, an enzyme that is responsive to metabolism and that requires, among other things, a molecule called nicotinamide adenine dinucleotide (NAD) to function. Because NAD levels fluctuate with the cell's metabolic state, SIRT1 is likewise responsive to metabolism.
"The key finding [for both papers is] that SIRT1 is a deacetylase involved in regulating circadian biology," said Sassone-Corsi. "And because SIRT1 is regulated by NAD, it links metabolism with circadian rhythms."
Schibler's team arrived at the same conclusion from a completely different angle. Recognizing that feeding cycles are critical to synchronizing the molecular clock, Schibler reasoned there must be a clock component capable of sensing metabolic state. As NAD seemed a good candidate for such a sensor, he looked specifically at NAD-binding proteins.
"You need something that senses feeding and metabolism, and there are many [candidates]," Schibler said. "The one we report on in this paper is SIRT1. We believe we have a very good candidate with this SIRT1."
Together, the two reports demonstrate that SIRT1 activity operates in a circadian manner; that SIRT1 binds directly to CLOCK/BMAL1 in a circadian manner; that SIRT1 deacetylates both BMAL1 and PER2, leading to their degradation and/or loss of activity; and (4) that loss of SIRT1 activity dampens circadian rhythms.
Takahashi noted several interesting aspects to this study. First is the fact that SIRT1 is regulated by NAD.
"There are many deacetylases," he said. "So, it certainly didn't have to be SIRT1. The fact that the deacetylase is also regulated by a metabolic indicator is exciting."
Indeed, Schibler said he suspects, but does not yet know for certain, that NAD levels also fluctuate in a circadian manner.
Also surprising, Takahashi said, is the fact that SIRT1 is an integral component of the clock itself, like a cog in the machinery, rather than some downstream player.
"And then," he added, "because the SIRT1 pathway itself is so interesting, because of its role in longevity, that suggests a new direct link between the longevity/metabolism pathway and the circadian clock, a direct molecular link that wasn't known before."
Takahashi noted this study has potential, albeit very long-term, therapeutic implications.
"If you screw up circadian rhythm enough, you can end up with metabolic disorders," he said, citing CLOCK-mutant mice which, in addition to having disrupted circadian cycles, are also obese and predisposed to diabetes.
Said Sassone-Corsi, "SIRT1 or CLOCK might make useful drug targets. Not today or tomorrow, but in the future. I have a feeling there will be a lot of interest in these studies once they come out."
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