A few days ago I visited Leonard Guarante, Director of the Glenn Laboratory for the Science of Aging at MIT and pioneer in the investigation of sirtuins and their longevity properties. The lead line of the Laboratory’s web site is “We work on mechanisms of aging so that people may lead healthier lives,” an intention that generates a lot of empathy in me. We talked about some of the current investigations going on in the laboratory and some of Leonard’s views on aging science. I cover high points of our discussion here, in the processes reviewing some of Leonard’s and the laboratory’s past achievements. I also cite selected literature references pertinent to the science involved.
Leonard started investigating aging genes in yeast back in 1991 and his 2003 book Ageless Quest: One Scientist’s Search for Genes That Prolong Youth presents a personal and very readable view of his investigations up to the time of its writing. His studied the roles of sirtuins in yeast complexes and among his first major contributions over the years was identifying that the genetic pathways activated by the sirtuin SIR2 are the same ones activated by calorie restriction, an evolutionary-conserved pathway known to be capable of reliably conveying longevity across a number of species. The 1999 publication co-authored by Guarante The SIR2/3/4 complex and SIR2 alone promote longevity in Saccharomyces cerevisiae by two different mechanisms has been cited by hundreds of subsequent publications and helped to prime the pump leading to what is now a steady river of publications related to sirtuins. Leonard told me that a year or so ago, one new article relating to sirtuins appeared just about every day and now about two such articles are appearing daily. (I would love to cover those here but the task would clearly be impossible.)
Guarante’s 2004 publication The Sir2 family of protein deacetylases tells the story of actions of sirtuins in different organisms. “We summarize the current knowledge of the Sir2 homologs from different organisms, and finally we discuss the role of Sir2 in caloric restriction and aging.” This article is cited by 87 others in Pubmed Central. A short video in which Guarante emphasizes how he believes the benefits of calorie restriction can be realized through activating sirtuins can be found here. For discussions of pathways involved in calorie restriction and its roles in cancers, see my December 2009 blog entries Calorie restriction research roundup – Part I and Calorie restriction research roundup – Part II.
The Glenn lab started to focus on sirtuin proteins about a dozen years ago. Currently, about half of the activity in the lab is focused on the mammalian gene and sirtuin SIRT1, and the rest of the research relates to the other 6 members of the mammalian sirtuin family. SIRT1 is a mammalian homolog gene corresponding to the SIR2 yeast gene. The working hypothesis with respect to longevity has been that the SIRT genes mediate the pathways of longevity related to calorie restriction, and that better understanding of the actions of the sirtuins could lead to practical interventions that postpone aging.
The same interventions, it turns out, are likely to be highly useful for prevention or management of many late-onset diseases and diseases that flare with advancing age such as diabetes, Parkinson’s, Huntington’s and Alzheimer’s. In fact, Guarante points out that delaying aging is the same as delaying onset of the major diseases of aging, and I heartily agree with him.
About sirtuins, “Sir2 is active as an NAD+-dependent deacetylase, which is broadly conserved from bacteria to higher eukaryotes(ref).” NAD+ dependency means essential involvement in metabolism. The discovery of NAD dependency of sirtuins was in Guarante’s lab. Being a deacetylase (or an HDAC) means a capability for transcriptional silencing of gene expression. See the blog entry Histone acetylase and deacetylase inhibitors for an explanation. “In mammalian cells, Sir2 proteins also deacetylate non-histone proteins such as the p53 tumour suppressor protein, alpha-tubulin and forkhead transcription factors to mediate diverse biological processes including metabolism, cell motility and cancer(ref).”
Guarante has very recently co-authored a review Ten years of NAD-dependent SIR2 family deacetylases: implications for metabolic diseases, a March 10, 2010 e-publication ahead of print. “Since the discovery of NAD-dependent deacetylase activity of the silent information regulator-2 (SIR2) family (‘sirtuins’), many exciting connections between protein deacetylation and energy metabolism have been revealed. The importance of sirtuins in the regulation of many fundamental biological responses to various nutritional and environmental stimuli has been firmly established. Sirtuins have also emerged as critical regulators for aging and longevity in model organisms. Their absolute requirement of NAD has revived an enthusiasm in the study of mammalian biosynthesis of NAD. Sirtuin-targeted pharmaceutical and nutriceutical interventions against age-associated diseases are also on the horizon. This review summarizes the recent progress in sirtuin research (particularly in mammalian sirtuin biology) and re-evaluates the connection between sirtuins, metabolism, and age-associated diseases (e.g., type-2 diabetes) to set a basis for the next ten years of sirtuin research. Copyright © 2010 Elsevier Ltd. All rights reserved.”
Earlier papers of relevance authored or co-authored by Guarante include Genetics and the specificity of the aging process (2003), Calorie restriction extends life span by lowering the level of NADH (2004), Mammalian SIRT1 represses forkhead transcription factors (2004), Calorie restriction and SIR2 genes—Towards a mechanism (2005), and Calorie restriction–the SIR2 connection (2005).
The Glenn lab works basically with mice, either mice where a gene such as SIRT1 is knocked out in selected tissues or in mice where an extra copy of the gene has been added to the genome, super-sirtuin mice. In typical past experiments, mice might be challenged, say by feeding them with high-calorie high-fat diets to attempt to induce diabetes in them. Or a mouse might be genetically modified both to increase susceptibility to Alzheimer’s disease and to express increased amounts of SIRT1.
Another area of research being pursued in Guarante’s lab involves the roles of sirtuins in the brain. Guarante thinks sirtuins play important roles in brain aging and repair. In the 2009 publication Neuronal SIRT1 regulates endocrine and behavioral responses to calorie restriction, Guarante and his colleagues reported “Since the somatotropic axis is controlled by the brain, we created mice lacking Sirt1 specifically in the brain and examined the impacts of this manipulation on somatotropic signaling and the CR response. These mutant mice displayed defects in somatotropic signaling when fed ad libitum, and defects in the endocrine and behavioral responses to CR. We conclude that Sirt1 in the brain is a link between somatotropic signaling and CR in mammals.” In a late 2008 publication by some of Guarante’s counterpart colleagues at Harvard they reported “Using embryonic stem cells, we show that mammalian Sir2, SIRT1, represses repetitive DNA and a functionally diverse set of genes across the mouse genome. In response to DNA damage, SIRT1 dissociates from these loci and relocalizes to DNA breaks to promote repair, resulting in transcriptional changes that parallel those in the aging mouse brain. Increased SIRT1 expression promotes survival in a mouse model of genomic instability and suppresses age-dependent transcriptional changes. Thus, DNA damage-induced redistribution of SIRT1 and other chromatin-modifying proteins may be a conserved mechanism of aging in eukaryotes.”
The work of Guarante and his colleagues has led to the identification of resveratrol as an activator of SIRT1 and later to the establishment of Sirtris Pharmaceuticals, a company devoted to the discovery of small-molecule activators of sirtuiins that could address diseases of aging. Reported in the Sitris web site, “A long-term study of middle-aged mice shows resveratrol improves health and mimics some benefits of dietary restriction(ref).”
David Sinclair, a key player in sirtuins research at Harvard and founder of Sirtris, originally came from Australia to work at MIT in Guarante’s lab. Leonard is on the Board of Sirtris which has been acquired by GlaxoSmith Kline. The 2007 publication Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes by Sinclair and other Sirtris-affiliated authors is one of a number of publications relating SIRT1 to type 2 diabetes. “Resveratrol, a polyphenolic SIRT1 activator, mimics the anti-ageing effects of calorie restriction in lower organisms and in mice fed a high-fat diet ameliorates insulin resistance, increases mitochondrial content, and prolongs survival10–14 Here we describe the identification and characterization of small molecule activators of SIRT1 that are structurally unrelated to, and 1,000-fold more potent than, resveratrol. These compounds bind to the SIRT1 enzyme—peptide substrate complex at an allosteric site amino-terminal to the catalytic domain and lower the Michaelis constant for acetylated substrates. In diet-induced obese and genetically obese mice, these compounds improve insulin sensitivity, lower plasma glucose, and increase mitochondrial capacity. In Zucker fa/fa rats, hyperinsulinaemic-euglycaemic clamp studies demonstrate that SIRT1 activators improve whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver. Thus, SIRT1 activation is a promising new therapeutic approach for treating diseases of ageing such as type 2 diabetes(ref).” Sirtris currently has four SIRT1 activator substances in Phase IIa clinical trials(ref). Trials relate to metabolic disease (Type 2 Diabetes), inflammation, cardiovascular disease and oncology.
Guarante told me that activation of SIRT1 in the brain might be a useful strategy for dealing with neurodegenerative diseases like Huntington’s, Alzheimer’s and Parkinson’s. Work done in his laboratory related to this point showing initial positive results is still to be published. Also, it appears that sirtuins in the brain can play roles in mental states such as anxiety. One of the main problems in developing a therapy for neurodegenerative diseases, he said, will be finding a SIRT1 activator that can readily cross the blood-brain barrier. The lab is also studying the roles of sirtuins in growth and reproduction. Guarante thinks these roles will turn out to be very important. The question is “how does the body manage completely to reverse aging in the germline and what roles do sirtuins play in the process?” Finally, SIRT1 appears to play a major role in bone health, a field very relevant to aging. This too is being studied in Leonard’s lab. We did not get into the details of the current work in the lab but at some point perhaps I might be able to look a little deeper into one or two of the areas they are investigating.
Writing this particular blog entry has been a particularly frustrating experience for me because I have wanted to develop an overview of the current science relating to sirtuins and so have spent a great deal of time wending my way through the current literature, often finding myself afar from the work done at the MIT Glenn laboratory. This task has proven to be too daunting though I do intend to chew away at it in subsequent blog entries. I will mention one thing that has come to me attention however, and that is the role of SIRT1 as related to PARP1 (Poly(ADP-ribose) polymerase 1) in the presence of DNA damage (ref)(ref)(ref). Apparently SIRT1 is the lawyer for cell survival and PARP1 is the lawyer for cell apoptosis when there is DNA damage. I hope to get to this topic in a subsequent blog entry. I am also fascinated by reports like Interplay among BRCA1, SIRT1, and Survivin during BRCA1-associated tumorigenesis that suggest “These findings suggest that resveratrol treatment serves as an excellent strategy for targeted therapy for BRCA1-associated breast cancer.” My personal hunger for this kind of research outpaces my ability to metabolize it.
A final comment about Leonard and his colleagues who are diligently pursuing scientific truth for its own sake: The press and cable news channels are full of unending economic news and opinion relating to deficit, national debt, trade imbalance, cost of health care, etc. The figures bandied about often involve hundreds of billions of dollars, or even a few trillion dollars. Incredible contortions are engaged in to save a few billion dollars here or there and opinions of economists are desperately sought. Leonard told me he sees the possibility of healthy life extension of up to an average of 10 years based on what is known now, and I think he is right. I have pointed out in a previous blog entry “If we could all extend our healthy lifespans by ten years it would be worth about ten trillion dollars in decreased health care costs and perhaps twice that much more in productivity gains, over 10 years. (Current US health care costs are something like 3 trillion dollars representing over 17% of gross domestic product(ref), and a disproportionally large slice of the cost is for people in the last 10 years of their lives.). Longevity is by far the best area of investment for economic development. With an increase of 10 years in our average healthy lifespan, we could quickly wipe out both the national budget deficit and the national debt.” And we would see a new wave of prosperity. Assuming we start to achieve that 10 years of average life extension and $30 trillion in economic benefit as SIRT1-activating nutritional supplements and pharmaceuticals become available, I think it is important that Leonard and his colleagues are given full acknowledgement for their role in the economic miracle that ensues. The miracle will have gotten started in a MIT laboratory in Cambridge Massachusetts, a place occupied by modest biologists, not economists.