Found in (mis)translation: A novel role for protein synthesis fidelity in aging

The idea that translation fidelity might play a role in aging dates back at least as far as 1963, when Leslie Orgel proposed the “error catastrophe” theory of aging: in this model, mistranslation of the translational machinery creates a feedback loop that leads to further translation errors, ultimately causing loss of cell viability. From the Science of Aging Timeline:

Orgel considers two types of proteins: those involved in metabolism, and those involved in information processing. For metabolic proteins, translational error isn’t a long-term problem for the cell, since a malfunctioning protein is simply one of many. Likewise, for translational errors causing loss of function in information processing proteins: the error isn’t heritable, and a small decrease in the efficiency of gene expression is unlikely to pose a serious problem.

However, information processing proteins can be altered in another way: by mutations that decrease the fidelity with which they process or propagate genetic information. Lower-fidelity transcription and translation will result in more mutations. This is the core of Orgel’s idea: “errors which lead to a reduced specificity of an information-handling enzyme lead to an increasing error frequency. Such processes are clearly cumulative and…in the absence of an imposed selection for “accurate” protein-synthesizing units, must lead ultimately to an error catastrophe; that is, the error frequency must reach a value at which one of the processes necessary for the existence of viable cell becomes critically inefficient.”

The logic of the feedback loop is compelling, but the theory suffered for lack of experimental verification. While there is still some controversy over whether error catastrophe has received a full and fair experimental test, the consensus appears to be that while error catastrophe can take place under some systems (e.g., viral replication in the presence of drugs that reduce polymerase fidelity), this phenomenon does not play a role in mammalian aging: the measured values of the relevant parameters (basal translation error rates; the likelihood that a given error will result in further alteration to translation fidelity; protein lifetimes; etc.) appear to be such that the feedback loop doesn’t actually occur.

The error catastrophe theory is still an important waypoint in the evolution of theories of aging, and it has had tremendous influence in other areas within biogerontology. For example, similar logic has been applied to the role of autophagy in aging, where the feedback loop is called the garbage catastrophe.

And even if the feedback-loop logic doesn’t hold up to experimental scrutiny, recent findings have revealed that there may nonetheless be a relationship between protein translation fidelity and aging. Writing in PLoS ONE, Silva et al. report that in yeast, increasing the rate of translation errors might increase the activity of the longevity assurance gene SIR2:

The Yeast PNC1 Longevity Gene Is Up-Regulated by mRNA Mistranslation

Translation fidelity is critical for protein synthesis and to ensure correct cell functioning. Mutations in the protein synthesis machinery or environmental factors that increase synthesis of mistranslated proteins result in cell death and degeneration and are associated with neurodegenerative diseases, cancer and with an increasing number of mitochondrial disorders. Remarkably, mRNA mistranslation plays critical roles in the evolution of the genetic code, can be beneficial under stress conditions in yeast and in Escherichia coli and is an important source of peptides for MHC class I complex in dendritic cells. Despite this, its biology has been overlooked over the years due to technical difficulties in its detection and quantification. In order to shed new light on the biological relevance of mistranslation we have generated codon misreading in Saccharomyces cerevisiae using drugs and tRNA engineering methodologies. Surprisingly, such mistranslation up-regulated the longevity gene PNC1. Similar results were also obtained in cells grown in the presence of amino acid analogues that promote protein misfolding. The overall data showed that PNC1 is a biomarker of mRNA mistranslation and protein misfolding and that PNC1-GFP fusions can be used to monitor these two important biological phenomena in vivo in an easy manner, thus opening new avenues to understand their biological relevance.

PNC1 is a longevity gene because its biochemical activity feeds into the sirtuin pathway: Pnc1p synthesizes nicotinic acid from nicotinamide, which is an inhibitor of Sir2p, one of the canonical longevity factors in S. cerevisiae. Overexpression of PNC1 increases lifespan, presumably by increasing the activity of Sir2p. (The authors show that Sir2p silencing activity is elevated under conditions that cause mistranslation, and that this is inhibited by exogenous nicotinamide. Missing, as far as I can tell, is the same experiment in ∆pnc1 cells, which according to the authors’ model would not induce silencing during mistranslation.)

Is this simply an example of a general stressor activating a general stress response, whose constitutive activation in turn makes cells more stress-resistant and therefore longer-lived? For example, one could imagine a translation fidelity problem resulting in synthesis of lots of poorly folded proteins, leading to activation of the heat shock response and expression of chaperones (indeed, in the worm, heat shock transcription factor HSF-1 is required for life extension by daf-2 mutations). This doesn’t appear to be that. Instead, loss of protein fidelity causes upregulation of a major longevity assurance pathway, which acts primarily at the level of transcriptional silencing.

A couple of questions:

• What is the relevant molecular correlate of translation infidelity? Unfolded proteins would be the most likely culprit (prediction: whether or not it’s involved in the lifespan extension, there should be some heat shock response under these conditions), but one can imagine more elaborate scenarios: Suppose an inhibitor of PNC1 translation is encoded by an mRNA that is particularly likely to be mistranslated under normal conditions (e.g., because of weird codon usage, secondary structure, or some other quirk) and is now translated so poorly that it loses its inhibitory activity altogether (or acquires a new activity).

• How is the translational upregulation of PNC1 mediated? This is particularly curious given that, by assumption, a cell with a high rate of translation infidelity is having difficulty with translation. Teleologically, there’s no reason not to regulate gene expression at this level — if the gene were upregulated transcriptionally, the mRNA would still have to be translated — but it still strikes me as odd. If this is a bona fide evolved response to translation problems, wouldn’t it be better to pre-synthesize PNC1 and then activate it post-translationally (e.g. by proteolysis)?

• Is SIR2 involved in translation fidelity? Looking at this story as a straightforward stress response, one would expect some action of SIR2 to help mitigate the stress that started the whole process. So I’d be curious to know whether SIR2 mutants have lower translation fidelity, and if so, how it is that SIR2 is involved in improving the accuracy of translation?

Silva, R., Duarte, I., Paredes, J., Lima-Costa, T., Perrot, M., Boucherie, H., Goodfellow, B., Gomes, A., Mateus, D., Moura, G., & Santos, M. (2009). The Yeast PNC1 Longevity Gene Is Up-Regulated by mRNA Mistranslation PLoS ONE, 4 (4) DOI: 10.1371/journal.pone.0005212

 

Is it hot in here, or is it just SIRT1? Sirtuins activate heat shock transcription

Longevity is correlated with stress resistance. This makes abundant sense: Aging is (among other things) the decreasing ability to maintain cellular homeostasis over time. Cellular stress responses, broadly speaking, detect suboptimal conditions and activation of gene expression aimed at correcting the problem — a fairly reasonably definition of maintaining homeostasis. So it seems reasonable that more robust stress responses (or high basal expression of stress response target genes) would be associated with delayed aging and extended lifespan; indeed, this relationship has been used more than once to screen for long-lived mutants.

The heat shock proteins (HSPs), originally characterized (and therefore named) in the context of cellular responses to high-temperature stress, appear to play a critical role in regulation of lifespan, as illustrated by two examples from C. elegans: Expression levels of hsp-16.2, which vary stochastically even within clonal populations, are correlated with lifespan. Consistent with this, heat shock factor (HSF), the transcription factor that governs the heat shock response, is required for the lifespan extension caused by daf-2 mutations. (Mechanistically, HSF may serve this function by activating target genes that drive disaggregation and degradation of malfolded proteins.)

The heat shock response has now been connected to another major player in lifespan regulation: SIRT1, the most well-studied member of the sirtuin family. In mammals, HSF is subject to acetylation, which diminishes its ability to bind DNA and activate transcription – but this modification can be removed by the longevity assurance factor SIRT1, which is a protein deacetylase. From Westerheide et al. (see also the Perspectives piece in the same issue of Science):

Stress-Inducible Regulation of Heat Shock Factor 1 by the Deacetylase SIRT1

Heat shock factor 1 (HSF1) is essential for protecting cells from protein-damaging stress associated with misfolded proteins and regulates the insulin-signaling pathway and aging. Here, we show that human HSF1 is inducibly acetylated at a critical residue that negatively regulates DNA binding activity. Activation of the deacetylase and longevity factor SIRT1 prolonged HSF1 binding to the heat shock promoter Hsp70 by maintaining HSF1 in a deacetylated, DNA–binding competent state. Conversely, down-regulation of SIRT1 accelerated the attenuation of the heat shock response (HSR) and release of HSF1 from its cognate promoter elements. These results provide a mechanistic basis for the requirement of HSF1 in the regulation of life span and establish a role for SIRT1 in protein homeostasis and the HSR.

The relationship between a master regulator of aging (SIRT1) and an effector pathway (HSF and its target genes) is another example of an emerging trend in the biogerontological literature: the unification of separate longevity control mechanisms. (By “unification”, I don’t mean that these separate mechanisms are shown to be literally equivalent; I simply mean that our increasing knowledge of the connections between genes and their functions has revealed that many phenomena previously thought to act independently are in fact coordinated by regulatory factors).

These findings may also give mechanistic insight into a curious observation from a couple of years ago: resveratrol, an activator of SIRT1, induces the heat shock response. (When we discussed that study, I lamented that the authors hadn’t determined whether SIRT1 was required for the effect — in light of this paper, it does seem that they missed a pretty big boat.) It now seems reasonable to explain those data as follows: resveratrol activates SIRT1, which deacetylates HSF, which in turn binds DNA more efficiently and increases transcription of heat shock response genes. A strong prediction of this model is that HSF should be necessary for any lifespan extension resulting from resveratrol treatment.

 

SIRT6 follows in SIRT1’s footsteps

Sirtuins are involved in longevity assurance in organisms as evolutionarily diverse as yeast, worms, and mice. All members of the family have homology to histone deacetylases (HDACs), but each protein has unique characteristics as well. Individual family members have distinct tissue expression profiles, subcellular localization, and substrate specificity. Over the past few years, we’ve begun to learn a great deal about the specific targets and interactions of each sirtuin, and how these interaction contribute to their functions in prolonging lifespan.

The SIRT6 protein, one of seven sirtuins encoded by mammalian genomes, came onto biogerontologists’ radar with a report from Katrin Chua’s group that its histone H3K9 deacetylase activity is required to maintain telomeric chromatin in a healthy state. Furthermore, SIRT6 is required for the proper localization of the Werner’s syndrome protein, WRN, to telomeres: in the absence of SIRT6, the WRN-telomere association becomes unstable, recapitulating several of the cellular phenotypes of Werner’s progeria. (SIRT6 isn’t the only sirtuin involved in WRN biology: SIRT1, the most well-studied member of the family, appears to directly deacetylate WRN).

A second association between SIRT6 and aging has been revealed by a new study from the Chua lab: SIRT6 associates with the transcription factor NF-κB and deacetylates histones at NF-κB-bound promoters, causing them to become less active. Genetic suppression studies suggest that SIRT6’s influence on lifespan might be primarily mediated by NF-κB. Kawahara et al.:

SIRT6 Links Histone H3 Lysine 9 Deacetylation to NF-κB-Dependent Gene Expression and Organismal Life Span

Members of the sirtuin (SIRT) family of NAD-dependent deacetylases promote longevity in multiple organisms. Deficiency of mammalian SIRT6 leads to shortened life span and an aging-like phenotype in mice, but the underlying molecular mechanisms are unclear. Here we show that SIRT6 functions at chromatin to attenuate NF-κB signaling. SIRT6 interacts with the NF-κB RELA subunit and deacetylates histone H3 lysine 9 (H3K9) at NF-κB target gene promoters. In SIRT6-deficient cells, hyperacetylation of H3K9 at these target promoters is associated with increased RELA promoter occupancy and enhanced NF-κB-dependent modulation of gene expression, apoptosis, and cellular senescence. Computational genomics analyses revealed increased activity of NF-κB-driven gene expression programs in multiple Sirt6-deficient tissues in vivo. Moreover, haploinsufficiency of RelA rescues the early lethality and degenerative syndrome of Sirt6-deficient mice. We propose that SIRT6 attenuates NF-κB signaling via H3K9 deacetylation at chromatin, and hyperactive NF-κB signaling may contribute to premature and normal aging.

NF-κB has been widely implicated in the aging process, especially in the context of inflammatory transcription resulting in “inflammaging.” Indeed, a very recent study has suggested that knocking down NF-κB activity is sufficient to reverse the effects of chronological aging in the skin, at least at the level of gene expression, possibly by blocking inflammatory transcription and allowing the tissue’s natural regenerative capacity to proceed without obstacle.

As with the WRN story, this isn’t the first time a sirtuin has been implicated in regulating the activity of NF-κB — but also as with WRN, the mechanisms of sirtuin action are distinct. Studies of chronic obstructive pulmonary disease have revealed that SIRT1 directly deacetylates NF-κB, reducing its activity. In contrast, SIRT6 appears to associated with NF-κB but then exploit this interaction to “follow” the transcription factor to promoters, where it deacetylates histone H3K9 and facilitates formation of a closed or inactive chromatin state. Kind of a neat team: SIRT1 directly deacetylates proteins of interest, while SIRT6 acts in the same location but operates on chromatin. Working together, the proteins may well have greater than additive impact.

Thus, there is partial redundancy of ultimate function, even though the proteins operate via different mechanisms. This might actually make it easier to intervene favorably in the affected processes, if separate agonists of SIRT1 and SIRT6 end up having a synergistic effect at target promoters (and telomeres).

(There’s also a nice preview/summary piece in the same issue of Cell, by Gioacchino Natoli.)

 

Reviewing the reviews

Here is the next in what will likely be a long series of semi-regular review roundups — links, without extensive further comment, to the reviews I found most intriguing over the past few weeks months (I went on hiatus during the winter holidays). For the previous foray into the secondary literature, see here.

Alzheimer’s:

• Threats to the Mind. Aging, Amyloid, and Hypertension, Iadecola et al.

Apoptosis & cancer:

• The genetics of the p53 pathway, apoptosis and cancer therapy, Vazquez et al.

Calorie restriction:

• Endocrine alterations in response to calorie restriction in humans, Redman & Ravussin

Diabetes:

• Diabetes and the risk of multi-system aging phenotypes: a systematic review and meta-analysis, Lu et al.

Klotho:

• The Klotho gene family, the endocrine fibroblast growth factors, Kurosu and Kuro-o

Sirtuins:

• How does SIRT1 affect metabolism, senescence and cancer?, Brooks & Gu

Stem cells:

• Stem Cells, Their Niches and the Systemic Environment: An Aging Network, Drummond-Barbossa

Telomeres:

• …and progeria: Telomere dysfunction and dyskeratosis congenita, Walne & Dokal

• …and structure: Telomere-associated proteins: cross-talk between telomere maintenance and telomere-lengthening mechanisms, De Boeck et al.

 

A bridge between CR, DNA damage, and transcriptional dysregulation in aging?

In past years, I was fond of comparing biogerontology to the tale of the blind men and the elephant: everyone was approaching the problem from different directions, unable to see the big picture — and reaching conclusions that had more to do with the direction of approach (i.e., initial biases) than the fundamental importance of any given observation.

But this analogy is becoming increasingly less apt, and we may be on the verge of the era of unifying theories in the biology of aging.

What causes aging? The various subfields of biogerontology answer this question in very different ways. To vastly oversimplify: In one corner we have metabolism, including the related stories of sirtuins and calorie restriction; in another corner, we have DNA damage and stochasticity of gene expression. (Already we’re seeing the unifying tendency of recent findings: a few years ago I might have put those four items in four separate corners, but on the basis of recent reports I feel comfortable starting to bin them together. There are those, however, who would argue I’m being premature if not outright inaccurate in so doing.) In both corners, one could make a legitimate claim that the phenomenon in question has serious explanatory power regarding a fundamental mechanism of aging or longevity assurance — but is there a connection between the two?

Quite possibly. A major paper from Oberdoerffer et al. has investigated the role of sirtuins (specifically, yeast Sir2 and mammalian SIRT1) in chromatin. Over the course of their elegant study, the authors trace the parallels between sirtuins’ roles in yeast and human genome stability — and in the process, build a bridge between sirtuin activity, DNA damage, and transcriptional dysregulation:

SIRT1 Redistribution on Chromatin Promotes Genomic Stability but Alters Gene Expression during Aging

Genomic instability and alterations in gene expression are hallmarks of eukaryotic aging. The yeast histone deacetylase Sir2 silences transcription and stabilizes repetitive DNA, but during aging or in response to a DNA break, the Sir complex relocalizes to sites of genomic instability, resulting in the desilencing of genes that cause sterility, a characteristic of yeast aging. 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.

In both yeast and mammals, Sir2/SIRT1 relocalizes from its original location to sites of DNA damage. This may be a good thing — the sirtuin appears to be involved in recruiting DNA repair factors to break sites — but it has a negative consequence: The departure of Sir2/SIRT1 from its original perch can result in transcriptional derepression (recall that SIR genes were originally identified in S. cerevisiae as silent information regulators). The extent of such derepression will depend on SIRT1 levels/activity and also the extent of the damage — but it’s fairly obvious that the resulting transcriptional changes will have a stochastic component.

This probably isn’t such a bad thing when cells are young and damage is rare — but as either chronological or replicative age increases, damage becomes more prevalent (and even persistent), and the transcriptional changes due to SIRT1 relocalization could become quite severe.

That’s only part of the story: At damage sites, SIRT1 is promoting DNA repair and genomic stability (though evidence for the latter presented here is rather indirect, inferred from tumor spectra and cancer survival in a mouse model).

At both silenced promoters and DNA breaks, more SIRT1 (or SIRT1 activity; there’s a subtle but important distinction having to do with whether the protein is playing a stoichiometric or catalytic role) appears to be a good thing: More protein means that the “dilution” resulting from SIRT1 relocalization will result in less net loss from sites of transcriptional repression; similarly, higher levels of SIRT1 result in lower levels of genomic instability in response to DNA damage, presumably because there’s more SIRT1 around to do its thing at break sites. To complete their unifying bridge, the authors close the paper with a speculation that calorie restriction’s effect on lifespan might be mediated by increasing SIRT1 levels or activity in both arenas.

According to the authors’ interpretation, SIRT1 is performing two important functions — controlling transcription and ensuring efficient DNA repair — both of which (the authors claim) are essential for longevity assurance. This seems very reasonable, though the formal logic is slightly strained: just because SIRT1 improves genomic stability, prevents transcriptional derepression and extends lifespan doesn’t mean that the (now clearly related) former two functions are in any way related to the latter one. Sorting out causation will be a challenge, requiring some high-impact genetics — perhaps involving an approach that uncouples SIRT1’s silencing function at promoters with its affinity for or activity in damage sites. Probably crazy hard, but not impossible, and logically rather important to the model.

(See also the treatment of this article at ScienceNOW.)

 

SIRT1 and SIRT2 expression analysis in human CR

As we know, a calorie restriction (CR) diet improves the health and lifespan in animal models. In the ongoing search for human biomarkers to assess the effects of CR in humans, Crujeiras et al. analyzed sirtuin gene expression changes and blood antioxidant markers in obese patients who followed a 30% calorie restricted diet for 8 weeks.

The researchers selected two of the seven human sirtuins to study, SIRT1 and SIRT2, because of their known inhibitory role in adipocyte differentiation and lipid accumulation. They performed their analysis in circulating peripheral blood mononuclear cells (PBMCs). Many human gene expression analyses under calorie restriction have been done in adipose tissue or skeletal muscle. If changes could be seen in PBMCs, however, these cells could offer a less invasive method to assess biomarkers of calorie restriction diets in humans.

Sirtuin gene expression in human mononuclear cells is modulated by caloric restriction.

BACKGROUND: Sirtuins may provide novel targets for treating some diseases associated with oxidative stress, such as obesity and its comorbidities. However, there are a few in vivo studies in humans about the potential role of sirtuins as therapeutic targets among obese patients undergoing caloric restriction. Therefore, the aim of this study was to assess if the gene expression of sirtuins is modulated in peripheral blood mononuclear cells (PBMC) by a hypocaloric diet devised to lose weight in humans. MATERIALS AND METHODS: Gene expression of two sirtuins (SIRT1 and SIRT2) in the PBMC of obese subjects (32.3 +/- 5.5 kg m(-2)) before and following an 8-week hypocaloric diet was investigated. NADH-coenzyme Q reductase (NDUFS2) and cytochrome c oxidase assembly protein (COX15) gene expression was selected together with plasma antioxidant power and nitric oxide as markers of antioxidant status. A quantitative real-time polymerase chain reaction approach was performed to assess the nutrigenomics outcome. Moreover, 2-keto[1-(13)C]isocaproate breath test (KICA-BT) parameters were evaluated to study mitochondrial oxidation in vivo. RESULTS: The intervention up-regulated the expression of both sirtuins, being inversely associated with total antioxidant capacity and directly related to nitric oxide, mitochondrial oxidation assessed by the KICA-BT and the expression of the mitochondrial proteins COX15 and NDUFS2. CONCLUSION: SIRT1 and SIRT2 may serve as key regulators for some obesity comorbidities related to antioxidant status, while PBMC could be a model to study the effect of the sirtuin response in obesity therapy.

Overall, patients had a significant reduction in body weight, BMI, fat mass and cholesterol but showed no significant changes in glucose, triglycerides, insulin or insulin resistance (It’s possible that if these patients had maintained a CR diet for a longer period of time, significant changes would have been noted.) Total antioxidant capacity (AOP) and nitric oxide levels were significantly increased, and glutathione peroxidase was significantly decreased, indicating an improvement in the oxidative stress response.

The CR diet significantly upregulated expression of SIRT1 and SIRT2 in PBMCs, consistent with results obtained in the adipose tissue of calorie restricted mice. I would have liked to see a parallel gene expression analysis in the adipose tissue or skeletal muscle of these patients, in addition to PBMCs, to determine whether the gene expression changes observed in PBMCs were mirrored in the other tissues. That data would support the author’s claim that PBMCs are a valid source of cells for gene expression analysis in humans.

Finally, the authors utilized a non-invasive ¹³C-KICA breath test and gene expression analysis of two mitochondrial respiratory chain proteins (NDUFS2 and COX15) to evaluate in vivo mitochondrial oxidation in these patients before and after calorie restriction. Expression of NDUFS2 increased significantly after the 8-week diet, whereas expression of COX15 remained unchanged. No changes were noted in the ¹³C-KICA mitochondrial oxidation rate evolution after the dieting period.

While the researchers did not see significant changes in some of these assays, it is important to remember that short-term CR for weight loss has different physiological consequences than long-term CR for longevity. Hopefully, with additional human studies we will obtain data that will help us better understand the molecular pathways involved in CR diets for longevity in humans.

 

SIRT1 in metabolism, signaling and disease

Racing toward its ultimate goal of being involved in every aspect of biology, the mammalian sirtuin SIRT1 has been the subject of a number of recent papers, each dealing with a different aspect of the protein’s role. (Abstracts are excerpted; ellipses, emphases, and interpolated commentary are mine.)

In energy metabolism and liver cirrhosis: Sirt1 is involved in energy metabolism: The role of chronic ethanol feeding and resveratrol, Oliva et al.:

These results support the concept that ethanol induces the Sirt1/PGC1α pathway of gene regulation and both naringin and resveratrol prevent the activation of this pathway by ethanol. However, resveratrol did not reduce the liver pathology caused by chronic ethanol feeding [In other words, it's probably not a good idea to get your resveratrol by drinking 1000 bottles of red wine a day.]

In diseases of protein aggregation: The role of calorie restriction and SIRT1 in prion-mediated neurodegeneration, Chen et al. [a collaboration between the Lindquist and Guarente labs]:

We tested the role of SIRT1 in mediating the effects of CR in a mouse model of prion disease. … We report that the onset of prion disease is delayed by CR and in the SIRT1 KO mice fed ad libitum. CR exerts no further effect on the SIRT1 KO strain, suggesting the effects of CR and SIRT1 deletion are mechanistically coupled. In conjunction, SIRT1 is downregulated in certain brain regions of CR mice. … Surprisingly, CR greatly shortens the duration of clinical symptoms of prion disease and ultimately shortens lifespan of prion-inoculated mice in a manner that is independent of SIRT1. [i.e., CR isn't actually therapeutically beneficial since the mice die young.]

In inflammation, inflammaging, and HIV/AIDS: SIRT1 longevity factor suppresses NFκB -driven immune responses: regulation of aging via NFκB acetylation?, Salminen et al. (review):

HIV-1 Tat protein binds to SIRT1 protein, a well-known longevity factor, and inhibits the SIRT1-mediated deacetylation of the p65 component of the NFκB complex. As a consequence, the transactivation efficiency of the NFκB factor was greatly potentiated, leading to the activation of immune system and later to the decline of adaptive immunity. … Longevity factors, such as SIRT1 and its activators, might regulate the efficiency of the NFκB signaling, the major outcome of which is inflamm-aging via proinflammatory responses.

In Notch regulation of stem cell aging: Sirt1, Notch and stem cell “age asymmetry”, Mantel et al. (review):

The protein-deacetylase, SIRT1, has received much attention because of its roles in oxygen metabolism, cellular stress response, aging, and has been investigated in various species and cell types including embryonic stem cells. However, there is a dearth of information on SIRT1 in adult stem cells, which have a pivotal role in adult aging processes. Here, we discuss the potential relationships between SIRT1 and the surface receptor protein, Notch, with stem cell self-renewal, asymmetric cell division, signaling, and stem cell aging.

 

CSHL 2008, Session II: Genome stability, damage & repair

In this first entry on Session II, I’ll focus on the non-telomere-related talks from this session:

Jan Vijg gave a review of his published data about stochasticity and transcriptional noise in response to aging and damage. He also described some newer analysis suggesting that transcriptional noise is present in young animals as well, but that the variation for a given gene depends on the function of that gene — in other words, different functional categories of genes generally exhibited different variation profiles.

Christian Beauséjour argued that mice do not clear DNA-damaged cells — in particular, that neither the adaptive nor the innate immune system is involved in eliminating cells exhibiting signs of persistent DNA damage. This is a foray into controversial territory: Recently published data from other labs suggests that DNA damage causes cells to express ligands for immune cells, resulting in attack and clearance by e.g. NK cells. Beauséjour could reproduce these findings in marrow stem cells (the cell type used in the earlier studies) in vitro, but observed no evidence of clearance of damaged MSCs in vivo.

Philipp Oberdoerffer (from David Sinclair’s lab) asked: What drives chromatin reorganization and how does it relate to mammalian aging? Based on findings in yeast, he looked at mammalian SIRT1 relocalization in response to genotoxic stress, and found that SIRT1 is transiently associated with DNA double-strand break sites.

Session index:

• I. Genetics of simple organisms.
• IIa. Genome stability, damage and repair
• IIb. Telomeres
• VI. Senescence, apoptosis and stress
• VII. Stem cells
• X. Environmental interventions

 

Exercise as a calorie restriction mimetic?

According to a recent study by Lanza et al., endurance exercise increases mitochondrial protein levels, metabolic enzyme activity, and expression of SIRT3 (a sirtuin thought to be involved in longevity assurance). At the organismal level, insulin sensitivity goes up (this is good: insulin resistance leads to type II diabetes) and gluconeogenesis goes down.

So far, so good, but hardly surprising: file under “exercise is good for you, item #68232″. The interesting bit is that the mitochondrial and other changes are very similar to the physiological consequences of calorie restriction (CR), an intervention that is known to extend lifespan in model organisms and to delay age-related disease in humans. The authors argue that exercise may promote longevity through the same pathways as CR.

This fits in nicely with recent observations connecting exercise and CR: for example, resveratrol, thought to be a CR mimetic, improves exercise tolerance in mice, consistent with the idea that exercise and CR have something in common.

The next obvious question: Do exercise mimetics also promote longevity, and if so, do they do so by the same mechanism as CR?

 

Just breathe: In yeast, aerobic metabolism required for longevity enhancement by calorie restriction

Calorie restriction (CR) extends lifespan in most organisms studied, including some of our more distant relatives — e.g. the baker’s brewer’s yeast Saccharomyces cerevisiae. The genetics underlying CR-mediated life extension are currently being worked out (for details, see our earlier piece, Biogerontology rising); despite lingering controversy, the story is starting to converge. Specifically, it’s becoming clear that TOR, Sch9 kinase and regulation of ribosome synthesis play an important role — and, in contrast to earlier models, it’s seeming less and less likely that sirtuins are involved.

A new twist in the plot comes from a comparative study of two budding yeasts, S. cerevisiae and its close relative Kluyveromyces lactis. Brewer’s yeast prefers to ferment (grow anaerobically) in glucose-rich environments (like an extract of malted barley), but when carbon is limiting, it starts to grow aerobically. According to Oliveira et al. this increase in respiratory capacity is essential to the lifespan extension mediated by CR in yeast:

Increased aerobic metabolism is essential for the beneficial effects of caloric restriction on yeast life span

Calorie restriction is a dietary regimen capable of extending life span in a variety of multicellular organisms. A yeast model of calorie restriction has been developed in which limiting the concentration of glucose in the growth media of Saccharomyces cerevisiae leads to enhanced replicative and chronological longevity. Since S. cerevisiae are Crabtree-positive cells that present repression of aerobic catabolism when grown in high glucose concentrations, we investigated if this phenomenon participates in life span regulation in yeast. S. cerevisiae only exhibited an increase in chronological life span when incubated in limited concentrations of glucose. Limitation of galactose, raffinose or glycerol plus ethanol as substrates did not enhance life span. Furthermore, in Kluyveromyces lactis, a Crabtree-negative yeast, glucose limitation did not promote an enhancement of respiratory capacity nor a decrease in reactive oxygen species formation, as is characteristic of conditions of caloric restriction in S. cerevisiae. In addition, K. lactis did not present an increase in longevity when incubated in lower glucose concentrations. Altogether, our results indicate that release from repression of aerobic catabolism is essential for the beneficial effects of glucose limitation in the yeast calorie restriction model. Potential parallels between these changes in yeast and hormonal regulation of respiratory rates in animals are discussed.

For those of you whose yeast metabolic biochemistry is a little bit rusty: The alternate carbon sources (carbohydrates other than glucose) are ones that S. cerevisiae must metabolize aerobically (to a greater or lesser extent: they can grow anaerobically, though poorly, on non-glucose sugars, but not at all on glycerol, which absolutely requires respiration).

To summarize: Only in S. cerevisiae and only in the context of growth on glucose metabolism (which happens anaerobically at high concentrations but aerobically at low concentrations) does CR results in lifespan extension. When limitation of a carbon source does not result in a net increase in respiration — in S. cerevisiae growing on alternate sugars, or in K. lactis, which prefers to grow aerobically even under glucose-rich conditions — CR does not extend longevity.

The title is too strongly worded for my taste. The data are ultimately correlative, and I would liketo see more genetic manipulation that tests the hypothesis: for example, using S. cerevisiae mutants that don’t undergo the shift to aerobic metabolism in response to limiting glucose, or “high respiratory” strains that respire constitutively or at least undergo the metabolic shift earlier in the glucose-limitation curve. (My K. lactis genetics is non-existent, so I don’t know whether the converse mutants — i.e., reluctant respirers or “ready fermenters” — exist in that species, but if they do, it would be nice to see whether they exhibit CR-mediated life extension.)

But given the huge contributions that yeast has made to biogerontology in general, and to CR in particular, it will be interesting to see whether CR in metazoans is also accompanied by an increase in aerobic metabolism. If so, is it required for the benefits of CR, and more importantly, what are the molecular mechanisms underlying the metabolic shift?