Posts tagged SIRT1

Researchers have discovered a cause of aging in mammals that may be reversible.

The essence of this finding is a series of molecular events that enable communication inside cells between the nucleus and mitochondria. As communication breaks down, aging accelerates. By administering a molecule naturally produced by the human body, scientists restored the communication network in older mice. Subsequent tissue samples showed key biological hallmarks that were comparable to those of much younger animals.

“The aging process we discovered is like a married couple—when they are young, they communicate well, but over time, living in close quarters for many years, communication breaks down,” said Harvard Medical School Professor of Genetics David Sinclair, senior author on the study. “And just like with a couple, restoring communication solved the problem.”

This study was a joint project between Harvard Medical School, the National Institute on Aging, and the University of New South Wales, Sydney, Australia, where Sinclair also holds a position.

The findings are published Dec. 19 in Cell.

Communication breakdown

Mitochondria are often referred to as the cell’s “powerhouse,” generating chemical energy to carry out essential biological functions. These self-contained organelles, which live inside our cells and house their own small genomes, have long been identified as key biological players in aging. As they become increasingly dysfunctional overtime, many age-related conditions such as Alzheimer’s disease and diabetes gradually set in.

Researchers have generally been skeptical of the idea that aging can be reversed, due mainly to the prevailing theory that age-related ills are the result of mutations in mitochondrial DNA—and mutations cannot be reversed.

Sinclair and his group have been studying the fundamental science of aging—which is broadly defined as the gradual decline in function with time—for many years, primarily focusing on a group of genes called sirtuins. Previous studies from his lab showed that one of these genes, SIRT1, was activated by the compound resveratrol, which is found in grapes, red wine and certain nuts.

Ana Gomes, a postdoctoral scientist in the Sinclair lab, had been studying mice in which this SIRT1 gene had been removed. While they accurately predicted that these mice would show signs of aging, including mitochondrial dysfunction, the researchers were surprised to find that most mitochondrial proteins coming from the cell’s nucleus were at normal levels; only those encoded by the mitochondrial genome were reduced.

“This was at odds with what the literature suggested,” said Gomes.

As Gomes and her colleagues investigated potential causes for this, they discovered an intricate cascade of events that begins with a chemical called NAD and concludes with a key molecule that shuttles information and coordinates activities between the cell’s nuclear genome and the mitochondrial genome. Cells stay healthy as long as coordination between the genomes remains fluid. SIRT1’s role is intermediary, akin to a security guard; it assures that a meddlesome molecule called HIF-1 does not interfere with communication.

For reasons still unclear, as we age, levels of the initial chemical NAD decline. Without sufficient NAD, SIRT1 loses its ability to keep tabs on HIF-1. Levels of HIF-1 escalate and begin wreaking havoc on the otherwise smooth cross-genome communication. Over time, the research team found, this loss of communication reduces the cell’s ability to make energy, and signs of aging and disease become apparent.

“This particular component of the aging process had never before been described,” said Gomes.

While the breakdown of this process causes a rapid decline in mitochondrial function, other signs of aging take longer to occur. Gomes found that by administering an endogenous compound that cells transform into NAD, she could repair the broken network and rapidly restore communication and mitochondrial function. If the compound was given early enough—prior to excessive mutation accumulation—within days, some aspects of the aging process could be reversed.

Cancer connection

Examining muscle from two-year-old mice that had been given the NAD-producing compound for just one week, the researchers looked for indicators of insulin resistance, inflammation and muscle wasting. In all three instances, tissue from the mice resembled that of six-month-old mice. In human years, this would be like a 60-year-old converting to a 20-year-old in these specific areas.

One particularly important aspect of this finding involvesHIF-1. More than just an intrusive molecule that foils communication, HIF-1 normally switches on when the body is deprived of oxygen. Otherwise, it remains silent. Cancer, however, is known to activate and hijack HIF-1. Researchers have been investigating the precise role HIF-1 plays in cancer growth.

“It’s certainly significant to find that a molecule that switches on in many cancers also switches on during aging,” said Gomes. “We’re starting to see now that the physiology of cancer is in certain ways similar to the physiology of aging. Perhaps this can explain why the greatest risk of cancer is age.”

“There’s clearly much more work to be done here, but if these results stand, then certain aspects of aging may be reversible if caught early,” said Sinclair.

The researchers are now looking at the longer-term outcomes of the NAD-producing compound in mice and how it affects the mouse as a whole. They are also exploring whether the compound can be used to safely treat rare mitochondrial diseases or more common diseases such as Type 1 and Type 2 diabetes. Longer term, Sinclair plans to test if the compound will give mice a healthier, longer life.

(Source: hms.harvard.edu)

Buck Institute study provides insight for new therapeutics that target the interaction between ApoE4 and a Sirtuin protein

The major genetic risk factor for Alzheimer’s disease (AD), present in about two-thirds of people who develop the disease, is ApoE4, the cholesterol-carrying protein that about a quarter of us are born with. But one of the unsolved mysteries of AD is how ApoE4 causes the risk for the incurable, neurodegenerative disease. In research published this week in The Proceedings of the National Academy of Sciences, researchers at the Buck Institute found a link between ApoE4 and SirT1, an “anti-aging protein” that is targeted by resveratrol, present in red wine.

The Buck researchers found that ApoE4 causes a dramatic reduction in SirT1, which is one of seven human Sirtuins. Lead scientists Rammohan Rao, PhD, and Dale Bredesen, MD, founding CEO of the Buck Institute, say the reduction was found both in cultured neural cells and in brain samples from patients with ApoE4 and AD. “The biochemical mechanisms that link ApoE4 to Alzheimer’s disease have been something of a black box. However, recent work from a number of labs, including our own, has begun to open the box,” said Bredesen.

The Buck group also found that the abnormalities associated with ApoE4 and AD, such as the creation of phospho-tau and amyloid-beta, could be prevented by increasing SirT1. They have identified drug candidates that exert the same effect. “This research offers a new type of screen for Alzheimer’s prevention and treatment,” said Rammohan V. Rao, PhD, co-author of the study, and an Associate Research Professor at the Buck. “One of our goals is to identify a safe, non-toxic treatment that could be given to anyone who carries the ApoE4 gene to prevent the development of AD.”

In particular, the researchers discovered that the reduction in SirT1 was associated with a change in the way the amyloid precursor protein (APP) is processed. Rao said that ApoE4 favored the formation of the amyloid-beta peptide that is associated with the sticky plaques that are one of the hallmarks of the disease. He said with ApoE3 (which confers no increased risk of AD), there was a higher ratio of the anti-Alzheimer’s peptide, sAPP alpha, produced, in comparison to the pro-Alzheimer’s amyloid-beta peptide. This finding fits very well with the reduction in SirT1, since overexpressing SirT1 has previously been shown to increase ADAM10, the protease that cleaves APP to produce sAPP alpha and prevent amyloid-beta.

AD affects over 5 million Americans – there are no treatments that are known to cure, or even halt the progression of symptoms that include loss of memory and language. Preventive treatments are particularly needed for the 2.5% of the population that carry two genes for ApoE4, which puts them at an approximate 10-fold higher risk of developing AD, as well as for the 25% of the population with a single copy of the gene. The group hopes that the current work will identify simple, safe therapeutics that can be given to ApoE4 carriers to prevent the development of Alzheimer’s disease.

(Source: buckinstitute.org)

Clock’s rhythm ensures steady energy supply to cells during times of fasting

Each of our cells has an energy furnace, and it is called a mitochondrion. A Northwestern University-led research team now has identified a new mode of timekeeping that involves priming the cell’s furnace to properly use stored fuel when we are not eating.

The interdisciplinary team has identified the “match” and “flint” responsible for lighting this tiny furnace. And the match is only available when the circadian clock says so, underscoring the importance of the biological timing system to metabolism.

“Circadian clocks are with us on Earth because they have everything to do with energy,” said Joe Bass, M.D., who led the research. “If an organism burns its energy efficiently, it has a better chance of survival. Our results tell us how the circadian clock triggers the cell’s energy-burning process. Cells are most capable of using fuel when the clock is working properly.”

Bass is the Charles F. Kettering Professor and chief of the division of endocrinology, metabolism and molecular medicine at Northwestern University Feinberg School of Medicine and an endocrinologist at Northwestern Memorial Hospital.

Mitochondria regulate the supply of energy to cells when we are at rest, with no glucose available from food. In a study of mice, the researchers found that the circadian clock supplies the match to light the furnace and on the match tip is a critical compound called NAD+. It combines with an enzyme in mitochondria called Sirtuin 3, which acts as the flint, to light the furnace. When the clock in an animal isn’t working, the animal can’t metabolize stored energy and the process doesn’t ignite.

This pathway through which the body clock controls activities within the mitochondria shows how energy generation is tied tightly to the light-dark/activity-rest cycle each day. 

The findings, which could be useful in the development of therapies to treat metabolic disorders related to circadian disruption, is published today (Sept. 19) by the journal Science.

The results demonstrate that the circadian clock, a genetic timekeeper that evolved to enable organisms to track the daily transition from light to darkness early in evolution, generates oscillations in mitochondrial energy capacity through rhythmic regulation of NAD+ biosynthesis.

The clock facilitates oxidative rhythms that anticipate an animal’s fasting/feeding cycle that occurs during the transition from light to darkness and wakefulness to sleep each day, and, in so doing, prevents the cell from “starving” during the night.

To understand how mitochondria are affected by circadian clock disorder, the researchers genetically removed the clocks in laboratory mice and compared them to controls. Both groups of mice were studied in a state of fasting; this “stress” test enabled the researchers to pinpoint just how the clock maintains “energy reserves” (akin to stress testing of a bank).

Bass and his research group worked together with Navdeep S. Chandel, a colleague of Bass’ at Feinberg, and John M. Denu, at the University of Wisconsin-Madison. They found the mice lacking clocks had defects in their mitochondria: the mitochondria could not metabolize stored energy and had no reserve to prevent depletion of the main currency, ATP. (Adenosine triphosphate is an energy-bearing molecule found in all living cells.)

Working with Northwestern colleague Milan Mrksich, they went on to show that removal of the clock depletes the necessary ingredient to turn on an enzyme within mitochondria, Sirtuin 3, which activates energy burning during fasting.

The researchers also showed that when the circadian clock was disrupted, resulting in a lack of NAD+, they could provide NAD+ supplements and restore function to the mitochondrion.

The findings expand the understanding of the molecular pathways linking the circadian clock with metabolism and show that the clock provides an essential buffer to stabilize the cell as organisms transition between eating and fasting each day. This knowledge has implications for disease intervention and prevention, including of diabetes, and potentially for states of increased cell demand for metabolism (including inflammation and cancer). 

“We have established the chain of events that couples the clock’s control switch with the machinery of the mitochondria,” said Bass, who also is a member of the department of neurobiology at the Weinberg College of Arts and Sciences. “We now have identified an additional link in the supply chain that provides energy to the cell at different phases of our daily sleep-wake cycle. These findings establish a key role for the NAD+ biosynthetic cycle in this process.”

Major senior authors from Northwestern include Chandel, a professor in medicine-pulmonary and cell and molecular biology at Feinberg, and Mrksich, the Henry Wade Rogers Professor of Biomedical Engineering, Chemistry and Cell and Molecular Biology at Feinberg, Weinberg and the McCormick School of Engineering and Applied Science. Chandel and Mrksich are members of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.

The co-first authors are Clara Bien Peek, a postdoctoral fellow, and Alison H. Affinati, an M.D./Ph.D. candidate, both working in Bass’ lab. They have literally worked around the clock on the research, which builds on the earlier work of co-author Kathryn Moynihan Ramsey. In 2009, she and colleagues reported in Science that the compound NAD, together with the enzyme SIRT1, functions as a molecular “switch” to coordinate the internal clock with metabolic systems.

The current research team combined Northwestern expertise in basic circadian clock research, chemistry and physiology with outside collaborators who were able to verify the Northwestern findings.

Co-author Eric Goetzman, from the University of Pittsburgh School of Medicine, an expert in the rare children’s disease called metabolic myopathy, was able to confirm that the pattern the researchers observed in mice was the same as that seen in these children. Fasting can be life-threatening for children with this disorder because they can’t metabolize stored energy due to defects in their mitochondria.

Analyses by co-author Christopher B. Newgard at Duke University Medical Center identified a signature profile of the metabolic myopathy in mice with altered circadian clock genes.

(Source: northwestern.edu)

Scientists answer hotly debated questions about how calorie restriction delays aging process

Among scientists, the role of proteins called sirtuins in enhancing longevity has been hotly debated, driven by contradictory results from many different scientists. But new research at Washington University School of Medicine in St. Louis may settle the dispute.

Reporting Sept. 3 in Cell Metabolism, Shin-ichiro Imai, MD, PhD, and his colleagues have identified the mechanism by which a specific sirtuin protein called Sirt1 operates in the brain to bring about a significant delay in aging and an increase in longevity. Both have been associated with a low-calorie diet.

The Japanese philosopher and scientist Ekiken Kaibara first described the concept of dietary control as a method to achieve good health and longevity in 1713. He died the following year at the ripe old age of 84—a long life for someone in the 18th century.

Since then, science has proven a link between a low-calorie diet (without malnutrition) and longevity in a variety of animal models. In the new study, Imai and his team have shown how Sirt1 prompts neural activity in specific areas of the hypothalamus of the brain, which triggers dramatic physical changes in skeletal muscle and increases in vigor and longevity.

“In our studies of mice that express Sirt1 in the brain, we found that the skeletal muscular structures of old mice resemble young muscle tissue,” said Imai. “Twenty-month-old mice (the equivalent of 70-year-old humans) look as active as five-month-olds.”

Imai and his team began their quest to define the critical junctures responsible for the connection between dietary restriction and longevity with the knowledge from previous studies that the Sirt1 protein played a role in delaying aging when calories are restricted. But the specific mechanisms by which it carried out its function were unknown.

Imai’s team studied mice that had been genetically modified to overproduce Sirt1 protein. Some of the mice had been engineered to overproduce Sirt1 in body tissues, while others were engineered to produce more of the Sirt1 protein only in the brain.

“We found that only the mice that overexpressed Sirt1 in the brain (called BRASTO) had significant lifespan extension and delay in aging, just like normal mice reared under dietary restriction regimens,” said Imai, an expert in aging research and a professor in the departments of Developmental Biology and Medicine.

The BRASTO mice demonstrated significant life span extension without undergoing dietary restriction. “They were free to eat regular chow whenever they wished,” he said.

In addition to positive skeletal muscle changes in the BRASTO mice, the investigators also observed significant increases in nighttime physical activity, body temperature and oxygen consumption compared with age-matched controls.

Mice are characteristically most active at night. The BRASTO mice also experienced better or deeper sleep, and both males and females had significant increases in longevity.

The median life span of BRASTO mice in the study was extended by 16 percent for females and 9 percent for males. Translated to humans, this could mean an extra 13 or 14 years for women, making their average life span almost 100 years, Shin said. For men, this would add another seven years, increasing their average life span to the mid-80s.

Delay in cancer-dependent death also was observed in the BRASTO mice relative to control mice, the researchers noted.

Imai said that the longevity and health profile associated with the BRASTO mice appears to be the result of a shift in the onset of aging rather than the pace of aging. “What we have observed in BRASTO mice is a delay in the time when age-related decline begins, so while the rate of aging does not change, aging and the risk of cancer has been postponed.”

Having narrowed control of aging to the brain, Imai’s team then traced the control center of aging regulation to two areas of the hypothalamus called the dorsomedial and lateral hypothalamic nuclei. They then were able to identify specific genes within those areas that partner with Sirt1 to kick off the neural signals that elicit the physical and behavioral responses observed.

“We found that overexpression of Sirt1 in the brain leads to an increase in the cellular response of a receptor called orexin type 2 receptor in the two areas of the hypothalamus,” said first author Akiko Satoh, PhD, a postdoctoral staff scientist in Imai’s lab.

“We have demonstrated that the increased response by the receptor initiates signaling from the hypothalamus to skeletal muscles,” said Satoh. She noted that the mechanism by which the signal is specifically directed to skeletal muscle remains to be discovered.

According to Imai, the tight association discovered between Sirt1-prompted brain activation and the regulation of aging and longevity raises the tantalizing possibility of a “control center of aging and longevity” in the brain, which could be manipulated to maintain youthful physiology and extend life span in other mammals as well.

(Source: news.wustl.edu)