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March 25, 2010
Nature’s most recent “Insight” supplement is devoted to a topic near and dear to our hearts, even when spelled with that superfluous UK “e”: Ageing. From the introductory editorial:
Ageing, the accumulation of damage to molecules, cells and tissues over a lifetime, often leads to frailty and malfunction. Old age is the biggest risk factor for many diseases, including cancer and cardiovascular and neurodegenerative diseases. … Ageing research is clearly gaining momentum, as the reviews in this Insight testify, bringing hope that at some time in the future we will be able to keep age-related diseases at bay by suppressing ageing itself.
The five reviews are all by prominent scholars — many of whose work we’ve discussed here — and cover a wide range of subjects within gerontology and biogerontology:
As always, Nature Insight supplements are free-access, so even if you don’t have access to a university subscription, you can still read these articles.
(For a previous aging-related Nature Insight on DNA repair, see here.)
February 3, 2010
A review I should have cited yesterday addresses the promise and challenges of the two most prominent natural-product candidates for longevity-enhancing therapeutics. The author is prominent biogerontologist and all-around bright feller Matt Kaeberlein (see here for earlier posts on his group’s work).
Resveratrol and rapamycin: are they anti-aging drugs?
Studies of the basic biology of aging have advanced to the point where anti-aging interventions, identified from experiments in model organisms, are beginning to be tested in people. Resveratrol and rapamycin, two compounds that target conserved longevity pathways and may mimic some aspects of dietary restriction, represent the first such interventions. Both compounds have been reported to slow aging in yeast and invertebrate species, and rapamycin has also recently been found to increase life span in rodents. In addition, both compounds also show impressive effects in rodent models of age-associated diseases. Clinical trials are underway to assess whether resveratrol is useful as an anti-cancer treatment, and rapamycin is already approved for use in human patients. Compounds such as these, identified from longevity studies in model organisms, hold great promise as therapies to target multiple age-related diseases by modulating the molecular causes of aging.
Note that resveratrol has been taking a bit of a thrashing of late, with recently released studies calling into question its ability to directly activate sirtuins. Briefly, the critics posit that the early data may have been misinterpreted due to artifacts in a fluorescence-based system used to detect protein-drug interactions — but check comment #32 on that post for David Sinclair’s personal response on this issue.
Kaeberlein, M. (2010). Resveratrol and rapamycin: are they anti-aging drugs? BioEssays, 32 (2), 96-99 DOI: 10.1002/bies.200900171
May 7, 2009
Protein degradation is an essential longevity assurance pathway. Maintaining high levels of autophagy can delay age-related decline in liver function. Obstacles to protein degradation tend to shorten the lifespan: blocking autophagy causes hypersensitivity to stress, and inhibiting the ubiquitin/proteasome pathway damages the mitochondria; both of these treatments kill neurons.
Conversely, longevity enhancement tends to enhance disposal of cellular garbage: In a worm model of Alzheimer’s disease, long-lived daf-2 mutants exhibit slower protein aggregation and decreased proteotoxicity, probably as a result of higher rates of protein degradation.
Despite the overall importance of protein degradation in delaying aging, the destruction of individual proteins is not always a good thing. During a screen of worm E3 ubiquitin ligases, Mehta et al. discovered that blocking the degradation of the HIF-1 protein dramatically increases lifespan and blocked the toxicity of pathogenic, aggregation-prone proteins.
Proteasomal Regulation of the Hypoxic Response Modulates Aging in C. elegans
The Caenorhabditis elegans von Hippel-Lindau tumor suppressor homolog VHL-1 is a cullin E3 ubiquitin ligase that negatively regulates the hypoxic response by promoting ubiquitination and degradation of the hypoxic response transcription factor HIF-1. Here, we report that loss of VHL-1 significantly increased life span and enhanced resistance to polyglutamine and amyloid beta toxicity. Deletion of HIF-1 was epistatic to VHL-1, indicating that HIF-1 acts downstream of VHL-1 to modulate aging and proteotoxicity. VHL-1 and HIF-1 control longevity by a mechanism distinct from both dietary restriction and insulin/IGF-1-like signaling. These findings define VHL-1 and the hypoxic response as an alternative longevity and protein homeostasis pathway.
The initial finding was that knockdowns of the E3 ligase VHL-1 were long-lived. VHL-1 is known to degrade HIF-1, the transcription factor involved in the hypoxic response. To rule out the possibility that another substrate of VHL-1 was important in the longevity enhancement, the authors used fairly straightforward genetic analysis: Mutating EGL-9, another gene required for HIF-1 degradation, also confers the lifespan extension, but neither vhl-1 not egl- mutants could live long in the absence of HIF-1.
The VHL-1/EGL-9/HIF-1 pathway is distinct from other means of lifespan extension: both daf-2 mutants and calorie restricted animals could extend the lifespan of hif-1 mutants, and conversely vhl-1 mutations could further extend the lifespan of daf-2 animals. This distinction may exist only at the level of the more upstream signaling events, however: DAF-16, the longevity assurance transcription factor that is disinhibited by daf-2 mutation, shares many target genes with HIF-1, so it is possible that the longevity enhancements rely on the same stable of stress-resistance and repair genes.
Will boosting HIF-1 levels also influence lifespan in mammals? Probably not, at least not in any simple way: the proteins involved are highly conserved — but VHL-1 is a tumor suppressor in humans, so targeting it with a drug is almost definitely a bad idea. Since suppression of the hypoxic response (especially angiogenesis) is likely to be important to the mechanism of tumor suppression by VHL-1, the same goes for HIF-1. It wouldn’t be incredibly surprising if this particular mechanism of lifespan regulation weren’t conserved between worms and mammals: worms don’t like oxygen as much as we do, so even if the machinery is conserved, the physiological consequences of activating that machinery might not be.
Still, as the authors point out, there might be some value in exploring manipulations of the hypoxic response in post-mitotic tissue – like brain — where the risk of tumorigenesis would presumably be smaller.
Mehta, R., Steinkraus, K., Sutphin, G., Ramos, F., Shamieh, L., Huh, A., Davis, C., Chandler-Brown, D., & Kaeberlein, M. (2009). Proteasomal Regulation of the Hypoxic Response Modulates Aging in C. elegans Science DOI: 10.1126/science.1173507
July 21, 2008
Our understanding of aging in animals owes a great debt to a large body of careful work in a single-celled organism, the brewer’s yeast Saccharomyces cerevisiae. Indeed, as I’ve argued before, yeast is one of the two organisms with the strongest credible claim to have started modern biogerontology. An unusually large crop of yeast aging papers have appeared over the last few months, and I thought it would be appropriate to spend a few paragraphs describing them — in honor of this humble organism that rises our bread, ferments our beer, and has done so much to open our eyes to the fundamental mechanisms of aging.
For those unfamiliar with the yeast field or simply wishing a clearly written and nearly comprehensive summary, Steinkraus et al. provide the historical perspective. The piece thoroughly reviews the development of yeast as a model system in aging, as well as the arguments in favor of a connection between results in yeast and well-established (but sometimes hard-to-test) hypotheses in animals.
Based on the influence that yeast has already had on biogerontology as a whole, it seems fair to claim that it will continue to reveal fundamentals of aging that are conserved across evolution. Now, however, there is quantitative evidence to back up that claim: Smith et al. have used bioinformatic and genomic approaches to study the conservation between known longevity genes in yeast and worm, and they show that yeast mutants in worm longevity genes are significantly more likely to be long-lived than randomly chosen mutants — suggesting that
genes that modulate aging have been conserved not only in sequence, but also in function, over a billion years of evolution.
Given this functional conservation, it is reasonable to use yeast to help answer questions about aging in general, so long as these questions are cell-biological in scope.
For instance: NAD+/NADH ratios are thought to be an important metric of the cellular energy balance, and appear to have effects both within the mitochondria and the cytosol. The mitochondrial inner membrane, however, is impermeable to both NAD+ and NADH. How, then, is information about energy balance communicated between the two cellular compartments? Easlon et al. report that two components of the malate-aspartate NADH shuttle (which transports metabolites across the mitochondrial membrane, resulting in equilibration of the cytosolic and mitochondrial NAD+/NADH pools) are involved in controlling longevity. The two proteins, Mdh1 and Aat1, are required for longevity enhancement by calorie restriction (CR), and overexpression of both proteins can increase lifespan independent of caloric conditions (but in a Sir2-dependent manner, about which see more below).
Another outstanding question involves how cellular energy balance is coordinated with the rates of catabolic and anabolic processes, and how this coordination impinges on regulation of longevity. We know that in yeast, the effects of CR are mediated by pathways involving the nutrient sensor TOR and the kinase Sch9. (Brief aside: longevity-enhancing mutations of Sch9 can also suppress genomic instability; new results from Qin et al. show that genomic instability is also associated with lifespan variation in yeast). Sch9 regulates, among other things, ribosome biogenesis; both CR and Sch9 mutation cause ribosome synthesis to decrease — but are the ribosome and longevity phenotypes related? Very likely yes: Steffen et al. report that multiple means of downregulating ribosome synthesis all extend lifespan, implying that reducing production of ribosomes is essential in order to reap the benefits of CR.
As the tools of biology have adapted, so has the yeast field (sometimes leading the charge, as in the case of the earliest microarray-based expression profiling experiments). Murakami et al. have developed a high-throughput method for measuring yeast lifespan. In this first report, the authors primarily demonstrate the use of their method on known mutants, arguing that their results are similar but with lower variance. (Brief aside: they also demonstrate that CR-induced lifespan extension does not require SIR2 or any other yeast sirtuin, adding fuel to the controversy about whether sirtuins play any role in CR in yeast; for more, see here and here.) The increased precision of their technique will allow detection of subtler aging-related phenotypes than were previously detectable, very likely allowing us to add to the list of genes known to regulate lifespan. The high-throughput aspects of the method, of course, open the door to testing small-molecule drugs that could delay aging in yeast — historically a fruitful approach though not without its potential pitfalls.
If you’ve made it this far, feel free to toast S. cerevisiae, perhaps with a beer.
(Before I depart, I just want to mention — since it’s not necessarily clear from the first authors’ names — that four of the papers mentioned above, as well as many of the papers described in earlier Ouroboros posts linked above, are the result of the combined work of the Kaeberlein and Kennedy labs at U-Wash Seattle. Both of them worked together in the Guarente lab back in the day, and they’ve been in the yeast aging field from its very beginning. Clearly, their combined work is continuing to advance the field.)
January 10, 2008
Posted by ouroboros under Calorie restriction
A longstanding controversy about the benefits of calorie restriction (CR) involves a concern about the experimental animals used in CR studies: Most of them have been bred in the lab for generations, potentially resulting in substantial genetic changes, and they may not be particularly good representatives of their wild counterparts.
Mice, for example, have been bred for a century to select for fast and fecund reproduction. Studies by Steven Austad, one of the world’s leading experts on mouse longevity, have suggested that wild-caught mice don’t experience longevity benefits from CR.
The situation may be different in the nematode C. elegans, grand-hermaphrodite of the genetic study of aging and a key model in the analysis of CR’s role in longevity. Sutphin and Kaeberlein report that wild-derived strains of C. elegans as well as field-caught isolates of a related nematode species all benefit to some extent from CR:
Dietary restriction by bacterial deprivation increases life span in wild-derived nematodes.
Dietary restriction is known to promote longevity in a variety of eukaryotic organisms. Most studies of dietary restriction have been performed on animals bred for many generations under conditions that differ substantially from their natural environment, raising the possibility that some apparent beneficial effects of dietary restriction are due to adaptation to laboratory conditions. To address this question in an invertebrate model, we determined the effect of dietary restriction by bacterial deprivation on life span in five different wild-derived Caenorhabditis elegans strains and two strains of the related species Caenorhabditis remanei. Longevity was enhanced in each of the wild-derived C. elegans strains, in most cases to a degree similar to that observed in N2, the standard laboratory strain. Both strains of C. remanei were substantially longer lived any of the C. elegans isolates, produced larger brood sizes, and retained the ability to produce offspring for a longer period of time. Dietary restriction failed to increase mean life span in one C. remanei isolate, but significantly increased the maximum life span of both C. remanei strains. Thus, we find no evidence that adaptation to laboratory conditions has significantly altered the aging process in C. elegans under either standard or food-restricted conditions.
This paper demonstrates fairly conclusively that N2, the lab strain of C. elegans, isn’t unusual in its response to CR, and is therefore a good model for the study of the phenomenon in “true wild-type” worms. The question that remains, however, is whether C. elegans are a good model for a phenomenon that actually occurs in true wild-type mammals, like Steve Austad’s mice…or us.
July 1, 2007
The family of proteins called sirtuins (named after the founding member, the yeast gene Silent Information Repressor-2) are intimately connected with the history of modern biogerontology. Originally identified as key players in the determination of yeast replicative lifespan, these proteins were subsequently shown to play essential roles in life-extension pathways in worms. More recent findings suggest that sirtuins are also important in regulation of mammalian aging — though the story there is more complex, with seven SIR2 homologs in the human and mouse genomes, each with its own tissue specificity and subcellular localizations.
Small-molecule sirtuin activators such as resveratrol have been shown to promote longevity in specific animal models (see our earlier articles, Resveratrol, lifespan and an unhealthy diet and Resveratrol: Breakfast of champions), raising the hope that such compounds could be developed as a means of therapeutically intervening in the natural aging process or as a prophylactic against age-related diseases (see Toward sirtuin activators in the clinic).
Where questions are asked about diseases of aging, the discussion will eventually turn toward the great scourge of age-related neurodegenerative disease. Given that sirtuins have already demonstrated potential to positively impact the aging process in a wide range of animals, it seems logical to ask whether they might also have therapeutic or prophylactic potential against neurodegeneration.
The answer may end up being murky, and rely on the specific details of the specific illness and sirtuin family member in question. Two recent papers have studied the effect of sirtuin expression (and pharmaceutical modulation of sirtuin activity) on neurodegenerative disease — and come up with two diametrically opposing answers.
The first study yielded result that one might expect, given the well-documented pro-longevity effects of sirtuins discussed above. Kim et al. showed that both overexpression of SIRT1 and administration of the activator resveratrol had a salutary effect in models of two kinds of neurodegeneration, Alzheimer’s disease (AD) and ALS:
SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer’s disease and amyotrophic lateral sclerosis
A progressive loss of neurons with age underlies a variety of debilitating neurological disorders, including Alzheimer’s disease (AD) and amyotrophic lateral sclerosis (ALS), yet few effective treatments are currently available. The SIR2 gene promotes longevity in a variety of organisms and may underlie the health benefits of caloric restriction, a diet that delays aging and neurodegeneration in mammals. Here, we report that a human homologue of SIR2, SIRT1, is upregulated in mouse models for AD, ALS and in primary neurons challenged with neurotoxic insults. In cell-based models for AD/tauopathies and ALS, SIRT1 and resveratrol, a SIRT1-activating molecule, both promote neuronal survival. In the inducible p25 transgenic mouse, a model of AD and tauopathies, resveratrol reduced neurodegeneration in the hippocampus, prevented learning impairment, and decreased the acetylation of the known SIRT1 substrates PGC-1alpha and p53. Furthermore, injection of SIRT1 lentivirus in the hippocampus of p25 transgenic mice conferred significant protection against neurodegeneration. Thus, SIRT1 constitutes a unique molecular link between aging and human neurodegenerative disorders and provides a promising avenue for therapeutic intervention.
In contrast, Outeiro et al. found that inhibition of another sirtuin, SIRT2, was neuroprotective in a model of Parkinson’s disease:
Sirtuin 2 Inhibitors Rescue alpha-Synuclein-Mediated Toxicity in Models of Parkinson’s Disease
The sirtuins are members of the histone deacetylase family of proteins that participate in a variety of cellular functions and play a role in aging. Here, we identified a potent inhibitor of sirtuin 2 (SIRT2), and found that inhibition of SIRT2 rescued alpha-synuclein toxicity and modified inclusion morphology in a cellular model of Parkinson’s disease. Genetic inhibition of SIRT2 via siRNA similarly rescued alpha-synuclein toxicity. Furthermore, the inhibitors protected against dopaminergic cell death both in vitro and in a Drosophila model of Parkinson’s disease. The results suggest a link between neurodegeneration and aging.
Why the dramatic difference in results? The answer could lie either in differences between the diseases studied or in the functions of the sirtuin family members that were targeted.
AD and Parkinson’s are both characterized by protein misfolding and amyloid aggregation of specific proteins (Aß and alpha-synuclein, respectively), but there are distinct differences at the cellular level: Aß plaques tend to be extensively deposited outside the cell, whereas alpha-synuclein inclusion bodies are almost entirely intracellular. Although both diseases result in neuronal cell death, they affect cells in different parts of the brain, and consequently have very different clinical presentations and symptomatology. My expertise in the specifics of neurodegenerative disease is rather limited, so I’ll close this thought by merely pointing out the formal possibility that despite some superficial similarities in cellular etiology, idiosyncrasies of AD or Parkinson’s might be sufficient to explain the seemingly contradictory findings.
It is also possible that SIRT1 (expressed/activated in the AD paper) and SIRT2 (inhibited in the Parkinson’s paper) have very different biochemical functions, and that this difference explains their opposing influences on neurodegeneration. We already know from the work of Matt Kaeberlein and colleagues that resveratrol potently stimulates SIRT1 but not SIRT2, suggesting that despite their homology and conserved deacetylase activities, these proteins differ substantially in molecular detail. We also know that the preferred substrates of the two proteins differ: SIRT1 acts primarily on histones, and thereby influences chromatin state and transcription; in contrast, SIRT2 targets tubulin and appears to play a role in the control of differentiation and mitosis.
It is therefore tempting to speculate that SIRT1 activity either triggers expression of neuroprotective genes or represses genes actively involved in cell death; stimulation of this protein, either by ectopic overexpression or the administration of an activator, would delay the progress of Alzheimer’s pathology. In contrast, SIRT2 lacks the gene-regulatory activity of SIRT1, and thus inhibiting it should have no preventive effect on cell death.
It remains a mystery how tubulin acetylation might influence the life-or-death outcome of protein aggregation in neurons. Certainly, tubulin is an essential component of the complex neuronal cytoskeleton and the transport machinery that delivers critical materials back and forth along the axons and dendrites. Hence, it’s not too much of a stretch to imagine that alterations in tubulin acetylation could dramatically impact a cell already under stress due to protein aggregation toxicity.
Taken together, these two studies underscore the importance of understanding the detailed molecular mechanism of drug action. While resveratrol appears to be selective for SIRT1 vs SIRT2, it is not necessary for all chemical modulators of sirtuin activity to observe the same preference. A hypothetical broad-spectrum sirtuin activator might end up doing more harm than good — regardless of its pharmacokinetics, bioavailability, or for that matter patentability/profitability — if it delayed Alzheimer’s (via SIRT1) only to speed the progress of Parkinson’s (via SIRT2). As we develop more compounds to target sirtuins, then, it will be critical not only to monitor efficacy in limited contexts, but also to carefully enumerate off-target effects on proteins within the same family.
May 30, 2007
Two excellent recent reviews by Matt Kaeberlein and colleagues address issues in the budding yeast model of aging.
The first covers the history of yeast as a model system of aging (and Prof. Kaeberlein would know, inasmuch as he’s been involved in the field for a decade, and has broke some of its earliest stories), and provides a thorough background on a matter that might seem to require some explanation: specifically, why we believe that a single-celled fungus can teach us about the genetic control of aging in the metazoans. From the article in PLoS Genetics (with co-author Brian Kennedy, himself an old hand in yeast lifespan):
In the last decade, research into the molecular determinants of aging has progressed rapidly and much of this progress can be attributed to studies in invertebrate eukaryotic model organisms. Of these, single-celled yeast is the least complicated and most amenable to genetic and molecular manipulations. Supporting the use of this organism for aging research, increasing evidence has accumulated that a subset of pathways influencing longevity in yeast are conserved in other eukaryotes, including mammals. Here we briefly outline aging in yeast and describe recent findings that continue to keep this “simple” eukaryote at the forefront of aging research.
That review is required reading before you jump into this next one, in which Matt takes on the question of whether Sir2 (the founding member of the sirtuins, which certainly do regulate lifespan in yeast, and possibly in other organisms) is involved in the life extension resulting from calorie restriction (CR) in yeast. From Kaeberlein & Powers:
Activation of Sir2-family proteins in response to calorie restriction (CR) has been proposed as an evolutionarily conserved mechanism for life span extension. This idea has been called into question with the discovery that Sir2-family proteins are not required for life span extension from CR in yeast. We present here a historical perspective and critical evaluation of the model that CR acts through Sir2 in yeast, and interpret prior reports in light of more recent discoveries. Several specific cases where the Sir2 model of CR is inconsistent with experimental data are noted. These shortcomings must be considered along with evidence supporting a role for Sir2 in CR in order to fully evaluate the validity of this model.
The article is valuable not only as a treatment of a controversy (on which Kaeberlein definitely falls on one side), but also as an exemplar of the types of genetic reasoning (and associated challenges) involved in lifespan studies of many kinds in all types of organisms.
January 8, 2007
We already know that in many species, decreasing caloric intake can result in dramatic increases in lifespan. Nonetheless, one would not be tempted to extrapolate the result to the absurd extreme of removing food altogether…unless, apparently, one happens to be a C. elegans, in which it appears that total withdrawal of all food extends adult lifespan, and to a greater extent than calorie restriction (CR). Kaeberlein et al. make the startling report:
A partial reduction in food intake has been found to increase lifespan in many different organisms. We report here a new dietary restriction regimen in the nematode Caenorhabditis elegans, based on the standard agar plate lifespan assay, in which adult worms are maintained in the absence of a bacterial food source. These findings represent the first report in any organism of lifespan extension in response to prolonged starvation. Removal of bacterial food increases lifespan to a greater extent than partial reduction of food through a mechanism that is distinct from insulin/IGF-like signaling and the Sir2-family deacetylase, SIR-2.1. Removal of bacterial food also increases lifespan when initiated in postreproductive adults, suggesting that dietary restriction started during middle age can result in a substantial longevity benefit that is independent of reproduction.
The independence from daf-2/IGF-1 and sirtuin pathways suggests that this lifespan extension is distinct from conventional calorie restriction and is mediated by a genetically novel mechanism that remains to be elucidated — but then again, if you ask the Kaeberlein group, at least some examples of CR life extension are independent of sirtuins anyway, and if that’s true in the worm, then the sirtuin-independence of the full-starvation life extension doesn’t address the point either way.
This result is more than a little stunning. It’s difficult to imagine that it will generalize to larger metazoans: I am pretty sure that complete withdrawal of food from a mouse will dramatically shorten the animal’s lifespan, not extend it. But assuming (1) that this result reproduces in the worm, and (2) that it is indeed mechanistically distinct from conventional CR, one could certainly speculate that the salubrious effects of regular fasting might be mediated by this novel “full withdrawal” pathway rather than by the periodic acute engagement of the CR response.