Study Title:

Circadian Clock Tied to Metabolism

Study Abstract

The circadian clock is encoded by a transcription-translation feedback loop that synchronizes behavior and metabolism with the light-dark cycle. Here, we report that both the rate-limiting enzyme in mammalian NAD+ biosynthesis, nicotinamide phosphoribosyltransferase (NAMPT), and levels of NAD+ display circadian oscillations which are regulated by the core clock machinery in mice. Inhibition of NAMPT promotes oscillation of the clock gene Per2 by releasing CLOCK:BMAL1 from suppression by SIRT1. In turn, the circadian transcription factor CLOCK binds to and up-regulates Nampt, thus completing a feedback loop involving NAMPT/NAD+ and SIRT1/CLOCK:BMAL1.

From press release:

UC Irvine researchers have discovered that circadian rhythms – our own body clock – regulate energy levels in cells. The findings have far-reaching implications, from providing greater insights into the bond between the body's day-night patterns and metabolism to creating new ways to treat cancer, diabetes, obesity and a host of related diseases.

In addition, Paolo Sassone-Corsi, Distinguished Professor and Chair of Pharmacology, and his colleagues found that the proteins involved with circadian rhythms and metabolism are intrinsically linked and dependent upon each other. Their study appears online in Science Express on March 12.

"Our circadian rhythms and metabolism are closely partnered to ensure that cells function properly and remain healthy," Sassone-Corsi said. "This discovery opens a new window for us to understand how these two fundamental processes work together, and it can have a great impact on new treatments for diseases caused by cell energy deficiencies."

Circadian rhythms of 24 hours govern fundamental physiological functions in almost all organisms. The circadian clocks are the essential time-tracking systems in our bodies that anticipate environmental changes and adapt to the appropriate time of day. Disruption of these rhythms can profoundly influence human health and has been linked to obesity, diabetes, insomnia, depression, coronary heart diseases and cancer.

Sassone-Corsi already had identified that the enzyme protein CLOCK is an essential molecular gear of the circadian machinery and interacts with a protein, SIRT1, which senses cell energy levels and modulates aging and metabolism.

In this study, he and his colleagues show that CLOCK works in balance with SIRT1 to direct activity in a cell pathway by which metabolic proteins send signals called the NAD+ salvage pathway. In turn, a key protein in that pathway, NAMPT, helps control CLOCK levels, creating a tightly regulated codependency between our circadian clock and metabolism.

"When the balance between these two vital processes is upset, normal cellular function can be disrupted," Sassone-Corsi said. "And this can lead to illness and disease."

The findings suggest that proper sleep and diet may help maintain or rebuild this balance, he said, and also help explain why lack of rest or disruption of normal sleep patterns can increase hunger, leading to obesity-related illnesses and accelerated aging.

The specific interaction between CLOCK and SIRT1 and the NAD+ salvage pathway also presents a starting point for drug development aimed at curbing cell dysfunction and death, thereby helping to solve major medical problems such cancer and diabetes.

From a second press release:

All animals, including humans, have an internal 24-hour clock or circadian rhythm that creates a daily oscillation of body temperature, brain activity, hormone production and metabolism. Studying mice, researchers at Washington University School of Medicine in St. Louis and Northwestern University found how the biological circadian clock mechanism communicates with processes that govern aging and metabolism.

Their findings can potentially explain why the waning of the circadian rhythm with age could contribute to age-related disorders such as insulin resistance and type 2 diabetes.

"Our study establishes a detailed scheme linking metabolism and aging to the circadian rhythm," says one of the lead authors, Shin-ichiro Imai, M.D., Ph.D., who researches aging at Washington University School of Medicine. "This opens the door to new avenues for treating age-related disorders and ways to restore a healthy daily circadian rhythm. It could also yield new interventions to alleviate metabolic disorders such as obesity and diabetes."

Imai, associate professor of medicine and of developmental biology, focuses on the molecular mechanisms of aging and longevity. Earlier, he demonstrated that a gene called SIRT1 was at the center of a network that regulates aging. A form of the gene is found in every organism on earth, and seven forms of the gene exist in humans.

SIRT1 has a broad reach, influencing glucose breakdown and production, cholesterol metabolism, fat burning and insulin sensitivity. Basically, the gene coordinates metabolic reactions throughout the body and manages the body's response to nutrition.

Interestingly, increasing the activity of proteins related to SIRT1 extends the life span of organisms such as yeast, worms and flies. SIRT1 is activated when calories are restricted below normal, which has been shown to extend the life spans of some laboratory animals. "Under nutritional scarcity, SIRT1 may delay aging and extend life span to assure survival until food becomes more readily available," Imai explains.

Imai's collaborator in the current study, Joseph Bass, M.D., Ph.D., assistant professor of medicine and neurobiology at Northwestern University, earlier demonstrated that interfering with the circadian clock of mice led to metabolic complications including obesity and type 2 diabetes.

Now their joint research, led by Kathryn Moynihan Ramsey, Ph.D., at Northwestern and Jun Yoshino, M.D., Ph.D., and Cynthia S. Brace, both at Washington University, has linked the circadian clock to SIRT1 through a key metabolite that serves as the energy currency of the body.

As a result, they have defined a biochemical mechanism by which the body's metabolic and nutritional status can directly drive the oscillation of the body's daily clock as well as influence aging and longevity. This new information points potentially to innovative ways to correct metabolic disorders and improve health as people age.

Studying laboratory mice, the researchers found a daily oscillation of the metabolite NAD (nicotinamide adenine dinucleotide), an important compound that is the body's way of exchanging energy and moving it where it's needed. Previously, scientists believed the amount of NAD in the body's cells stayed fairly constant.

"Seeing this striking abnormality in the NAD levels was like discovering the cause of a disease in a patient after running a blood test," Bass says.

Importantly, the researchers found that this NAD rhythm was linked to the daily rise and fall of the activity of "clock" genes, the genes that serve as the gears that run the body's internal clock. They discovered that the clock genes directly interact with a biochemical process that produces NAD.

NAD is required for SIRT1 to function, suggesting that SIRT1 activity increased and decreased along with NAD oscillation in the mice. Since SIRT1 is known to inhibit the clock genes, the cycle of its activity feeds back into the clock mechanism.

Studying the mice under controlled conditions of light and dark, the researchers established the details of the NAD-SIRT1-clock gene loop and showed that it functions in liver and fat cells. "We showed that this feedback cycle is driven by NAD," Imai says. "Because NAD levels reflect nutrition and energy levels, NAD's link to the circadian and aging mechanisms makes them sensitive to the nutritional status of the organism."

Next, Imai and members of his laboratory will look at whether manipulating components of the NAD biochemical pathway could have therapeutic effects on metabolism through insulin secretion and insulin sensitivity as well as on health in aging individuals.

Takahasi is a cofounder of ReSet Therapeutics Inc., and he and Bass are members of its scientific advisory board. Bass is an advisor and receives support from Amylin Pharmaceuticals. Imai holds a patent related to this research.


Study Information

Kathryn Moynihan Ramsey, Jun Yoshino, Cynthia S. Brace, Dana Abrassart, Yumiko Kobayashi, Biliana Marcheva, Hee-Kyung Hong, Jason L. Chong, Ethan D. Buhr, Choogon Lee, Joseph S. Takahashi, Shin-ichiro Imai, and Joseph Bass
Circadian Clock Feedback Cycle Through NAMPT-Mediated NAD+ Biosynthesis
Science
2009 March
UC Irvine, Washington University School of Medicine in St. Louis, and Northwestern University.

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