Calorie restriction

(previous session)

Craig Skinner (Lin Lab, UC Davis): Identification of potential calorie restriction mimics in yeast using a nitric oxide-based screen. Yeast are an important model system in biogerontology, useful not only for genetic studies of longevity control but also for discovery of bioactive compounds. Calorie restriction (CR) in yeast causes increased levels of nitric oxide (NO) — somewhat surprising in that yeast cells lack a homolog of nitric oxide synthase — and elevated NO is sufficient to extend yeast lifespan. These observations led Skinner to screen a yeast deletion library for elevated NO levels, yielding several genes that extend lifespan.

Mark Lucanic (Lithgow Lab, Buck): Endocannabinoid signaling mediates the effect of diet on lifespan in C. elegans. Mutants in the dauer pathway in C. elegans often influence longevity; the daf-2 mutation, which causes constitutive dauer formation at elevated temperatures, extends lifespan by several fold. Lucanic discovered that endocannabinoids are involved in the regulation of the dauer pathway — and therefore, of longevity — either independently of or far downstream of daf-2 and daf-16. Endocannabinoids are upregulated under well-fed conditions, and shorten lifespan.

Delia David (Kenyon Lab, UCSF): Widespread protein aggregation is an inherent part of aging in C. elegans. Protein aggregates are a hallmark of many age-related neurodegenerative diseases, leading to the hypotheses that the cellular mileu changes with age in a manner that causes native, aggregation-prone proteins to form aggregates. David used mass spectrometry to identify a subset of normal worm proteins aggregate as a function of age. As with the proteins associated with neurodegeneration, specific proteins aggregate in specific cell types. Mutations that extend lifespan (such as daf-2) decrease aggregation, and tend to downregulate the expression of genes encoding aggregation-prone proteins. Curiously, regulators of protein homeostasis tend to aggregate themselves, leading to a destructive positive feedback loop in which the very factors that protect the cell from proteotoxicity disappear into aggregates, leading to further aggregation.

Cherry Tang (Zhong Lab, Berkeley): The Clearance of Ubiquitinated Protein Aggregates Via Autophagy. Autophagic protein degradation has been implicated in control of lifespan: autophagy slows cell and tissue aging. Tang has identified a protein that participates in degradation of ubiquitinated proteins and co-localizes with autophagosomes; when the protein is knocked down, protein aggregates become more toxic.

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Subhash Katewa (Kapahi lab, Buck Institute) talked about the metabolic adaptations that occur in flies whose lifespan is being extended by dietary restriction (DR). Katewa is studying translational control in DR using a method called translational profiling, which uses the number of ribosomes bound to each mRNA as an index of translational activity (more ribosomes = more translation). He found that DR increases translation of messages that encode a variety of mitochondrial functions; this observation led to some interesting findings about the differential turnover of triglycerides in DR vs ad libitum flies.

Adam Freund (Campisi lab, Buck Institute) spoke about the sources of age-related inflammation, focusing on the senescence-associated secretory phenotype (SASP). Freund has elucidated mechanisms of SASP control that intermediate between the most upstream events in senescence (DNA damage) and its downstream effects (secretion of inflammatory factors). I have it on good authority that he has a completed manuscript on the subject, hopefully to be publshed soon, so I won’t say more about his story here. (Mr. Freund happens to be my baymate.)

Dario Valenzano (Brunet lab, Stanford University) is studying the genetic architecture of longevity in a short-lived fish Nothobranchius furzeri, the shortest-living vertebrate that can be reared in captivity. As a graduate student, Valenzano developed a system of biomarkers for tracking the progress of aging in skin, brain and other tissues – not only physical markers like the senescence-associated beta-galactosidase but also behavioral markers that change over the lifespan. He is now proceeding to map the longevity-associated genes in N. furzeri and testing the sufficiency of the genes he finds. Early results indicate that short-lived and long-lived fish are dying from different causes, as evidenced by a bimodal distribution of death rate vs. age.

Adolfo Sánchez-Blanco (Kim lab, Stanford University Medical School) described the “molecular odometer” for aging in the worm C. elegans. He began with the observation that lifespan is variable, even among clonally identical individuals kept under identical conditions. With genetics and environment taken out of the picture, what makes some individuals live longer than others? In order to address this question, SB had to develop a molecular marker (e.g., promoter activity of some gene) that measures physiological age (as opposed to chronological age), and then determine whether the expression level of that marker in individual worms is predictive of lifespan. SB has identified several such genes whose expression at middle age strongly predicts remaining lifespan. He is now actively looking for interventions that abolish the correlation between marker expression and longevity: if the marker gene’s activity is serving to overcome the life-shortening effect of some stress, then removing that stress will not necessarily abolish the variability in the marker, but will eliminate the correlation between marker levels and lifespan. (This is a subtle but important logical issue; I would have thought that one should look for interventions that drove the population distribution of marker levels toward the favorable side of the distribution. It was clear from questions that a lot of audience members had trouble with this logic, and I’m still not sure I understand it myself.)

(next session)

We know that exercise is good for us, and increasingly we’re understanding how it works at the molecular and cellular level: Physical activity boosts levels of heat shock proteins, which help cells resist stress; it also improves mitochondrial function in a manner reminiscent of calorie restriction (CR). Our knowledge is sophisticated enough that we can identify and develop small-molecule exercise mimetics and drugs that improve exercise tolerance.

Overall, then, exercise and its molecular/cellular consequences are consistent with longevity assurance pathways and life extension interventions. However, there are complications emerging.

One of the results of exercise is increased activity of anabolic pathways, especially in muscle. Building up tissues require new protein synthesis, and new protein synthesis requires activity of the TOR pathway. TOR is increasingly thought to be a pro-aging or gerontogenic pathway: rapamycin, a drug that inhibits TOR, blocks senescence and extends lifespan in mice (we already knew that TOR inhibition increased longevity in worms and yeast).

Until recently, we’d believed that exercise modulated TOR in the “right” direction for longevity assurance (i.e., down). For instance, AMPK, a target of exercise mimetics, appears to downregulate TOR signaling.

But it would appear that the above result, obtained using exercise mimetics, may not be generally applicable to all exercise — in particular, it does not extend to a specific regimen of exercise designed to stimulate anabolism and muscle growth. In blood flow restriction (BFR) exercise, resistance training is combined with pressure cuffs that significantly decrease blood flow to the exercising muscle; it increases protein synthesis in muscle cells and activates the TOR pathway. Now, Fry et al. have shown that in older men (who don’t increase muscle mass in response to ordinary resistance training), BFR activates TOR.

Superficially, this would seem to represent a contradiction: a lifespan-extending intervention (exercise) activates a lifespan-shortening biochemical signaling pathway (TOR). How might this seeming paradox be resolved?

  • TOR activity in the muscle might be irrelevant to lifespan control. Testing this hypothesis is a special case of a broader question, which is the determination of the key tissues responsible for the lifespan extension by rapamycin. This will probably require tissue-specific conditional knockdowns of either TOR or downstream pathways (e.g., S6K), and will take a while.
  • Not all exercise is lifespan-extending. Perhaps exercise regimens specifically optimized to stimulate anabolism might be gerontogenic, while those that create acute stress and activate hormetic pathways might extend lifespan.

It’s also worth mentioning that BFR exercise may be uniquely bad vis-a-vis longevity control. In worms, one of the targets of TOR is HIF-1, the hypoxia inducible factor. HIF-1 is a gerontogene: knocking it down extends longevity, so its wildtype function must shorten lifespan. I wonder whether the blood flow restriction in BFR exercise might create low-grade hypoxia in the muscle tissue, inducing HIF-1 activity and incurring some gerontogenic effect. It certainly wouldn’t be the first time that an intervention that helped older men increase muscle mass ended up being bad for them in the long run (e.g., hGH).

ResearchBlogging.orgFry, C., Glynn, E., Drummond, M., Timmerman, K., Fujita, S., Abe, T., Dhanani, S., Volpi, E., & Rasmussen, B. (2010). Blood flow restriction exercise stimulates mTORC1 signaling and muscle protein synthesis in older men Journal of Applied Physiology DOI: 10.1152/japplphysiol.01266.2009

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Not only does the mammalian sirtuin SIRT1 mediate the lifespan extension phenotype of caloric restriction (CR), it is also involved in controlling behavior (such as food intake) in response to CR (and possibly during ad libitum feeding as well).

Two recent papers with consistent results address the issue. Both studies employed brain-specific knockouts of SIRT1; Cohen et al. used a brain-specific knockout, whereas Çakir et al. used both pharmacologic inhibition and an siRNA in the hypothalamus. The latter paper implicates the FoxO1 transcription factor and S6 kinase signaling, implying cross-talk with both the IGF-1 and TOR pathways.

ResearchBlogging.orgÇakir, I., Perello, M., Lansari, O., Messier, N., Vaslet, C., & Nillni, E. (2009). Hypothalamic Sirt1 Regulates Food Intake in a Rodent Model System PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008322

Cohen, D., Supinski, A., Bonkowski, M., Donmez, G., & Guarente, L. (2009). Neuronal SIRT1 regulates endocrine and behavioral responses to calorie restriction Genes & Development, 23 (24), 2812-2817 DOI: 10.1101/gad.1839209

To the growing list of small-molecule drugs that have a measurable effect on lifespan or healthspan (e.g., resveratrol and rapamycin) we should add metformin, an anti-diabetic drug that has shown promise as a calorie restriction mimetic. Onken & Driscoll determined some of the genetic requirements for metformin’s anti-aging properties in the worm C. elegans:

Metformin induces a dietary restriction-like state and the oxidative stress response to extend C. elegans Healthspan via AMPK, LKB1, and SKN-1

Metformin, a biguanide drug commonly used to treat type-2 diabetes, has been noted to extend healthspan of nondiabetic mice, but this outcome, and the molecular mechanisms that underlie it, have received relatively little experimental attention. To develop a genetic model for study of biguanide effects on healthspan, we investigated metformin impact on aging Caenorhabditis elegans. We found that metformin increases nematode healthspan, slowing lipofuscin accumulation, extending median lifespan, and prolonging youthful locomotory ability in a dose-dependent manner. Genetic data suggest that metformin acts through a mechanism similar to that operative in eating-impaired dietary restriction (DR) mutants, but independent of the insulin signaling pathway. Energy sensor AMPK and AMPK-activating kinase LKB1, which are activated in mammals by metformin treatment, are essential for health benefits in C. elegans, suggesting that metformin engages a metabolic loop conserved across phyla. We also show that the conserved oxidative stress-responsive transcription factor SKN-1/Nrf2 is essential for metformin healthspan benefits in C. elegans, a mechanistic requirement not previously described in mammals. skn-1, which functions in nematode sensory neurons to promote DR longevity benefits and in intestines for oxidative stress resistance lifespan benefits, must be expressed in both neurons and intestines for metformin-promoted healthspan extension, supporting that metformin improves healthy middle-life aging by activating both DR and antioxidant defense longevity pathways. In addition to defining molecular players operative in metformin healthspan benefits, our data suggest that metformin may be a plausible pharmacological intervention to promote healthy human aging.

Some of those genes might sound familiar…

SKN-1 encodes a transcription factor required for longevity enhancement by calorie restriction (CR) (but not intermittent fasting) in worms and flies. The observation that metformin requires SKN-1 in order to extend median lifespan strongly suggests that the drug is acting via the same metabolic pathway as CR.

AMP kinase, a target of resveratrol and exercise mimetics, has also been implicated in the genetics of CR lifespan extension.

A quick aside about model systems: As it says in the first sentence of the abstract, metformin has already been studied in mice — so, why go “backward” to a simpler, smaller, and more divergent model system? Why not move forward, into humans? The answer has to do with the comparative ease with which certain types of genetic experiments can be performed in different systems; it’s simply easier in worms. Now that the genetics have been worked out, it will be more straightforward to do meaningful experiments in mammalian systems that are closer to our beloved H. sapiens.

Onken, B., & Driscoll, M. (2010). Metformin Induces a Dietary Restriction–Like State and the Oxidative Stress Response to Extend C. elegans Healthspan via AMPK, LKB1, and SKN-1 PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008758

More elsewhere:

The TOR (“target of rapamycin”) protein is a master regulator of cell growth, governing connect nutrient sensing, protein synthesis, and proliferation. It has become increasingly clear that the TOR pathway plays an essential role in longevity determination — specifically, higher TOR activity is associated with more rapid aging and shorter lifespan.

In mammals, TOR interferes with stem cell functions, and TOR activity is downregulated by exercise. It has been proposed that TOR inhibitors might even be used as anti-aging drugs (and in fact we’re going to investigate some recent relevant tests of that idea, sometime next week). The relationship between TOR and lifespan holds true across great evolutionary distances: loss of TOR function (in conjunction with other mutations) can dramatically increase the chronological lifespan of yeast.

How does TOR control the rate of aging? In order to answer this question, we must look downstream, to proteins that are controlled by TOR. A recent study from the Kapahi lab (our neighbors at the Buck Institute for Age Research) investigated the role of one such TOR target: HIF-1 (“hypoxia inducible factor”; it is also involved in metabolism). The authors find that loss-of-function mutations in HIF-1 result in longer-lived C. elegans. Chen et al.:

HIF-1 Modulates Dietary Restriction-Mediated Lifespan Extension via IRE-1 in Caenorhabditis elegans

Dietary restriction (DR) extends lifespan in various species and also slows the onset of age-related diseases. Previous studies from flies and yeast have demonstrated that the target of rapamycin (TOR) pathway is essential for longevity phenotypes resulting from DR. TOR is a conserved protein kinase that regulates growth and metabolism in response to nutrients and growth factors. While some of the downstream targets of TOR have been implicated in regulating lifespan, it is still unclear whether additional targets of this pathway also modulate lifespan. It has been shown that the hypoxia inducible factor-1 (HIF-1) is one of the targets of the TOR pathway in mammalian cells. HIF-1 is a transcription factor complex that plays key roles in oxygen homeostasis, tumor formation, glucose metabolism, cell survival, and inflammatory response. Here, we describe a novel role for HIF-1 in modulating lifespan extension by DR in Caenorhabditis elegans. We find that HIF-1 deficiency results in extended lifespan, which overlaps with that by inhibition of the RSKS-1/S6 kinase, a key component of the TOR pathway. Using a modified DR method based on variation of bacterial food concentrations on solid agar plates, we find that HIF-1 modulates longevity in a nutrient-dependent manner. The hif-1 loss-of-function mutant extends lifespan under rich nutrient conditions but fails to show lifespan extension under DR. Conversely, a mutation in egl-9, which increases HIF-1 activity, diminishes the lifespan extension under DR. This deficiency is rescued by tissue-specific expression of egl-9 in specific neurons and muscles. Increased lifespan by hif-1 or DR is dependent on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress. Therefore, our results demonstrate a tissue-specific role for HIF-1 in the lifespan extension by DR involving the IRE-1 ER stress pathway.

The mutants’ life extension was observed when the worms could eat ad libitum but not when they were dietarily restricted (DR), implying that the mechanism of the HIF-1 mutation is similar to that of DR. Conversely, activation of HIF-1 expression (by mutating EGL-9, which ubiquitinates HIF-1) decreases the lifespan extension due to DR. Taken together, the findings imply that downregulation of HIF-1 expression is both necessary and sufficient for DR-mediated longevity enhancement.

One more step down the rabbit hole, then: What are HIF-1 and DR doing? The authors find that lifespan extension requires the IRE1-gene, a principle mediator of the unfolded protein response (UPR). The UPR is activated when the endoplasmic reticulum (ER) is stressed — when protein folding is inefficient, or the secretory machinery is overloaded; the pathway returns the cell to homeostasis by inducing expression of genes that fold, sort, and process proteins in the ER (or degrade the proteins that can’t be saved). Perhaps lifespan extension requires increased ER capacity, or more efficient degradation of misfolded proteins?

On a closing note: Attentive readers will have recalled that not very long ago, we reported on a paper that appears to have reached the opposite conclusion — specifically, that high expression of HIF-1 (induced the same way as here, by mutation in the ubiquitin E3 ligase EGL-9) results in extended lifespan and decreased proteotoxicity. I don’t want to get in the middle of this controversy, except to point out that the systems were different in a number of ways, and that it is a formal possibility that a gene’s activity could be “tuned” such that either an increase or a decrease in expression could increase lifespan (implying that the wildtype expression levels are at a “sweet spot” of lower lifespan but presumably higher fitness, due to some sort of tradeoff between longevity and reproductive success). I am sure that the authors of both studies are working to reconcile the apparent contradiction. We’ll look forward to learning more as the story develops.

ResearchBlogging.orgChen, D., Thomas, E., & Kapahi, P. (2009). HIF-1 Modulates Dietary Restriction-Mediated Lifespan Extension via IRE-1 in Caenorhabditis elegans PLoS Genetics, 5 (5) DOI: 10.1371/journal.pgen.1000486

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.

ResearchBlogging.orgMehta, 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

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