Anti-Aging Research: Seeking Certainty on Sirtuins

By Jeffrey Norris For decades, scientists have been kicking around theories as to why we age. But in recent years, researchers have been starting to identify specific molecules that may be involved. Is a fountain of youth at hand? One line of research points to certain enzymes - named sirtuins - that play a role in the metabolism of cells. Experiments show that increasing levels of one primitive sirtuin can increase life span. These experiments have progressed up an evolutionary ladder from yeast, to tiny roundworms, to fruit flies and finally to mice. Researchers have started biotech companies with major programs aimed at investigating how sirtuin activity might be manipulated with small molecules to fight diseases of aging and aging itself. According to sirtuin expert Eric Verdin, MD, associate director of the Gladstone Institute of Virology and Immunology and professor of medicine at UCSF, sirtuins are clearly implicated in biochemical processes thought to affect aging, but there's a lot more to learn.

Eric Verdin

"We are just at the stage where we are identifying the enzymes and their targets," he says. "Integrating all the information in the long run and understanding how these factors modulate life span are going to be a challenge." Meanwhile, new empirical results continue to astound, even if there remains a devil in the molecular details. Primates in their Prime One way to increase life span in various organisms is to limit energy intake - a process referred to as calorie restriction. Mice consistently fed 30 percent to 40 percent fewer calories than normal live about 30 percent to 40 percent longer, Verdin notes. Sirtuin research was off and running after Leonard Guarente, PhD, of MIT discovered that the longevity-promoting effects of calorie restriction in yeast required the activity of a primitive sirtuin called SIR2. Guarente and Cynthia Kenyon, PhD, a UCSF professor who studies the control of life span in roundworms, have since founded Elixir Pharmaceuticals. Elixir aims to develop drugs to target age-related diseases and metabolic disorders. Will calorie restriction work to promote longevity in humans? Some people are trying it on themselves. However, a requirement that participants cut calories by 30 percent for years or decades may not bode well for large-scale clinical trial designs. Rhesus monkeys on calorie-restricted diets are reported to be aging much more gradually than similarly raised monkeys on normal diets. But that study is ongoing, and life span data are not available - most of the monkeys on both diets are still alive. Anyway, calorie restriction sounds more like deprivation than libation. Where's the evidence for a fountain of youth?

A fluorescent version of SIRT3 (green) shows a unique distribution of the enzyme around the nucleus and throughout the cytoplasm of two cells (left panel). Staining the same cells to highlight mitochondria (red) gives the same staining pattern (middle panel). This indicates that SIRT3 is located in the mitochondria. This is further confirmed in the perfect overlap (yellow) of the two merged pictures (right panel).

It turns out that there is a substance in grapes that activates a sirtuin called SIRT1. When mice are treated with high doses of this substance, called resveratrol, they can indulge in high-calorie diets and still live more youthful lives than their similarly indulgent fellow rodents. Resveratrol is relatively abundant in wine - especially pinot noir - but it is not possible to drink enough wine to even come close to matching the resveratrol dosage that was so beneficial to the mice. Nonetheless, Verdin suggests, "Activating sirtuins with small-molecule drugs might recapitulate some of the benefits of calorie restriction." Mitochondria and Aging Verdin's own lab has worked on several of the mammalian sirtuins identified thus far. A recent focus in his lab has been on SIRT3. Verdin discovered that SIRT3 resides in the cell's mitochondria. Like the nucleus, the mitochondria are little organs - organelles - within the cell. In the mitochondria, metabolites from food are converted into the cell's energy currency, a molecule called ATP. Mitochondria are thought to play a critical role in aging. Oxidative damage, a side effect of ATP formation, is viewed as a potential culprit. Disturbances of metabolism, as in diabetes and obesity, are also thought to originate in the mitochondria, Verdin notes. Verdin's interest in sirtuins grew from a more global interest in what now appears to be a major form of chemical signaling in cells. Just in the past decade or so, researchers have discovered that the addition or subtraction of small acetyl molecules from enzymes and other proteins governs their activity and how they send signals to other proteins. Sirtuins are also called protein deacetylases because they target specific proteins and remove their acetyl groups. Acetylation is now known to affect more than a thousand different cellular proteins. According to Verdin, acetylation and deacetylation appear to be as important in regulating cellular activities as the enzymatic addition and subtraction of protein phosphate groups - a canonical mode of signaling that figures largely in the control of cellular functions and in drug research. Verdin's group was the first to identify acetylated proteins in the mitochondria, as well as their regulation by SIRT3. Most of the acetylated proteins they have identified are enzymes involved in metabolic pathways. Verdin is now leading experiments comparing life span and many other parameters in normal mice with a strain of mice engineered to lack SIRT3. "Metabolome" Verdin has also begun working with Duke University mass spectrometry expert Chris Newgard, PhD, to exhaustively measure metabolites in the mice lacking SIRT3. Verdin and Newgard aim to identify the "metabolome" of mice lacking SIRT3, and to compare it with the metabolome of normal mice. This approach is analogous to now widespread efforts to identify within organisms a complete set of genes - a genome - or a complete set of proteins - a proteome. Verdin suspects that SIRT3, like SIRT1, will prove to be an important link between calorie restriction and its effects in mammals. His lab group is currently testing this hypothesis in the mice lacking SIRT3. "We really don't know until we do the experiments," he says. Verdin's findings related to SIRT1, SIRT2 and SIRT3 have been licensed by the University of California to Elixir. Meanwhile, Verdin has recently begun consulting for a second company, Sirtris Pharmaceuticals, which already has a drug in clinical trials that activates SIRT1.

"Reversible lysine acetylation controls the activity of the mitochondrial enzyme acetyl-CoA synthetase 2" Bjoern Schwer, Jakob Bunkenborg, Regis O. Verdin, Jens S. Andersen, and Eric Verdin PNAS 103;27:10224-10229 July 5, 2006 2006 145(4):284-293 Abstract | Full Text | Full Text (PDF)

Related Links: Verdin Lab Gladstone Institute of Virology and Immunology Kenyon Lab Can Kenyon's Roundworms Lead Us to the Fountain of Youth? UCSF Today, July 7, 2006 Elixir Pharmaceuticals Sirtris Pharmaceuticals