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Friday, July 25, 2008

Missing link found between circadian clock and metabolism

Two new research studies have discovered a long sought molecular link between our metabolism and components of the internal clock that drives circadian rhythms, keeping us to a roughly 24-hour schedule. The findings appear in the July 25th issue of the journal Cell, a publication of Cell Press.

The missing link is a well–studied mammalian protein called SIRT1, which was previously known to be switched on and off in accordance with cells' metabolic state and is perhaps best known for its potential life-extending properties.

"We all have noticed in an intuitive manner that the body requires more energy at certain times of day," said Paolo Sassone-Corsi of University of California, Irvine. "That's why we have lunch or dinner—there is a cyclicity in feeding behavior and energy requirement. That suggests there must be a link between the clock and metabolism. Now, in SIRT1, we have found a molecular connection between the circadian machinery and metabolism."

" While it remains a matter of speculation, the findings suggest that drugs that inhibit or activate SIRT1 might have an effect on the clock," added Gad Asher of University of Geneva in Switzerland, noting that such treatments might be a help to people suffering from circadian sleep disturbances. That idea could be easily tested by giving mice resveratrol, a SIRT1-boosting ingredient found in red wine, and examining its effects on clock function, he added.

Although still a matter of some debate, SIRT1 and its equivalent proteins in other organisms (known collectively as Sirtuins) have been shown to prolong life span. Studies have also implicated the protein in the life-extending effects of a calorie restricted diet in some, though not all, organisms.

The physiology and behavior of mammals are subject to daily oscillations driven by an endogenous circadian clock, explained Asher's team led by Ueli Schibler. In mammals, the circadian timing system is composed of a central pacemaker in the brain and subsidiary oscillators in most peripheral tissues. While light-dark cycles are the predominant cue for the brain's pacemaker, cyclic feeding behavior has a strong effect on clocks operating in many other tissues.

Sassone-Corsi's team earlier showed that the clock component aptly known as CLOCK affects the way that DNA is packaged into chromatin through chemical modification of the histone spools that wind DNA up into chromatin. Such "epigenetic" modifications allow for reversible changes in gene activity and are increasingly being recognized as a critical factor in many developmental, physiological, and metabolic processes.

CLOCK specifically acts as a so-called histone acetyltransferase (HAT), meaning that it transfers an acetyl group to histones and other proteins as well. If CLOCK is a HAT, that meant there must be a histone deacetylase (HDAC) that would act in the opposite manner, Sassone-Corsi said, removing the acetyl groups that CLOCK adds to drive daily fluctuations in gene activity.

Sirtuins came to mind, he said, because of their dependence on NAD+, a factor that is often considered a readout of metabolic state. SIRT1 also preferentially deacetylates the same histone that they showed CLOCK acetylates. Like CLOCK, Sirtuins are known to modify proteins other than histones as well, he added.

Now, Sassone-Corsi's team shows that the HDAC activity of the SIRT1 enzyme is controlled in a circadian manner, correlating with rhythmic acetylation of histones and the clock component BMAL1 by CLOCK. SIRT1 also associates with CLOCK and is recruited to the CLOCK:BMAL1 chromatin complex at circadian promoters, where they turn on the transcription of other clock genes, they report. Treatments that block SIRT1 activity lead to disturbances in the circadian cycle and in the acetylation of histones and BMAL1. Finally, in mice lacking SIRT1 only in the liver, they found evidence that SIRT1 normally contributes to circadian control in a living animal.

Asher and Schibler's team made a similar discovery: They show that SIRT1 is required for high-magnitude circadian activity of several core clock genes. SIRT1 binds CLOCK-BMAL1 in a circadian manner, they report, and promotes the deacetylation and degradation of the clock component called PER2. " It's been dogma for years that the circadian clock is regulated by transcription feedback loops," Sassone-Corsi said. "Now we have another loop—an enzymatic loop."

The next step is to understand the connection between changes in metabolism and the circadian cycle in more detail, the researchers said.

The findings also open a door on the possibility that epigenetics might influence behavior, Sassone-Corsi added, with potential implications for understanding the obesity epidemic.

" Genetics can't be the answer because the incidence is on the rise," he said. "Something else must be going on and perhaps epigenetic regulation is the key. In broad terms, that's where we're going."

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