Rtg2 protein links metabolism and genome stability in yeast longevity

Domains of Tra1 important for activator recruitment and transcription coactivator functions of SAGA and NuA4 complexes

The Tra1 protein is a direct transcription activator target that is essential for coactivator function of both the SAGA and NuA4 histone acetyltransferase (HAT) complexes. The ∼400-kDa Saccharomyces cerevisiae Tra1 polypeptide and its human counterpart TRRAP contain 67 or 68 tandem α-helical HEAT and TPR protein repeats that extend from the N terminus to the conserved yet catalytically inactive phosphatidylinositol 3-kinase (PI3K) domain. We generated a series of mutations spanning the length of the protein and assayed for defects in transcription, coactivator recruitment, and histone acetylation at SAGA- and NuA4-dependent genes. Inviable TRA1 mutants all showed defects in SAGA and NuA4 complex stability, suggesting that similar surfaces of Tra1 mediate assembly of these two very different coactivator complexes. Nearly all of the viable Tra1 mutants showed transcription defects that fell into one of three classes: (i) defective recruitment to promoters, (ii) reduced stability of the SAGA and NuA4 HAT modules, or (iii) normal recruitment of Tra1-associated subunits but reduced HAT activity in vivo. Our results show that Tra1 recruitment at Gcn4-dependent and Rap1-dependent promoters requires the same regions of Tra1 and that separate regions of Tra1 contribute to the HAT activity and stability of the SAGA and NuA4 HAT modules.

Peroxisome metabolism and cellular aging

The essential role of peroxisomes in fatty acid oxidation, anaplerotic metabolism, and hydrogen peroxide turnover is well established. Recent findings suggest that these and other related biochemical processes governed by the organelle may also play a critical role in regulating cellular aging. The goal of this review is to summarize and integrate into a model the evidence that peroxisome metabolism actually helps define the replicative and chronological age of a eukaryotic cell. In this model, peroxisomal reactive oxygen species (ROS) are seen as altering organelle biogenesis and function, and eliciting changes in the dynamic communication networks that exist between peroxisomes and other cellular compartments. At low levels, peroxisomal ROS activate an anti-aging program in the cell; at concentrations beyond a specific threshold, a pro-aging course is triggered.

Comparing the yeast retrograde response and NF-κB stress responses: implications for aging

The mitochondrial retrograde response has been extensively described in Saccharomyces cerevisiae, where it has been found to extend life span during times of mitochondrial dysfunction, damage or low nutrient levels. In yeast, the retrograde response genes (RTG) convey these stress responses to the nucleus to change the gene expression adaptively. Similarly, most classes of higher organisms have been shown to have some version of a central stress-mediating transcription factor, NF-κB. There have been several modifications along the phylogenetic tree as NF-κB has taken a larger role in managing cellular stresses. Here, we review similarities and differences in mechanisms and pathways between RTG genes in yeast and NF-κB as seen in more complex organisms. We perform a structural homology search and reveal similarities of Rtg proteins with eukaryotic transcription factors involved in development and metabolism. NF-κB shows more sophisticated functions when compared to RTG genes including participation in immune responses and induction of apoptosis under high levels of ROS-induced mitochondrial and nuclear DNA damage. Involvement of NF-κB in chromosomal stability, coregulation of mitochondrial respiration, and cross talk with the TOR (target of rapamycin) pathway points to a conserved mechanism also found in yeast.

Gene regulatory changes in yeast during life extension by nutrient limitation

Genetic analyses aimed at identification of the pathways and downstream effectors of calorie restriction (CR) in the yeast Saccharomyces cerevisiae suggest the importance of central metabolism for the extension of replicative life span by CR. However, the limited gene expression studies to date are not informative, because they have been conducted using cells grown in batch culture which markedly departs from the conditions under which yeasts are grown during life span determinations. In this study, we have examined the gene expression changes that occur during either glucose limitation or elimination of nonessential-amino acids, both of which enhance yeast longevity, culturing cells in a chemostat at equilibrium, which closely mimics conditions they encounter during life span determinations. Expression of 59 genes was examined quantitatively by real-time, reverse transcriptase polymerase chain reaction (qRT-PCR), and the physiological state of the cultures was monitored. Extensive gene expression changes were detected, some of which were common to both CR regimes. The most striking of these was the induction of tricarboxylic acid (TCA) cycle and retrograde response target genes, which appears to be at least partially due to the up-regulation of the HAP4 gene. These gene regulatory events portend an increase in the generation of biosynthetic intermediates necessary for the production of daughter cells, which is the measure of yeast replicative life span.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

The mitochondrial ribosomal protein of the large subunit, Afo1p, determines cellular longevity through mitochondrial back-signaling via TOR1

Yeast mother cell-specific aging constitutes a model of replicative aging as it occurs in stem cell populations of higher eukaryotes. Here, we present a new long-lived yeast deletion mutation,afo1 (for aging factor one), that confers a 60% increase in replicative lifespan. AFO1/MRPL25 codes for a protein that is contained in the large subunit of the mitochondrial ribosome. Double mutant experiments indicate that the longevity-increasing action of the afo1 mutation is independent of mitochondrial translation, yet involves the cytoplasmic Tor1p as well as the growth-controlling transcription factor Sfp1p. In their final cell cycle, the long-lived mutant cells do show the phenotypes of yeast apoptosis indicating that the longevity of the mutant is not caused by an inability to undergo programmed cell death. Furthermore, the afo1 mutation displays high resistance against oxidants. Despite the respiratory deficiency the mutant has paradoxical increase in growth rate compared to generic petite mutants. A comparison of the single and double mutant strains for afo1 and fob1 shows that the longevity phenotype of afo1 is independent of the formation of ERCs (ribosomal DNA minicircles). AFO1/MRPL25 function establishes a new connection between mitochondria, metabolism and aging.

The absence of a mitochondrial genome in rho0 yeast cells extends lifespan independently of retrograde regulation

The absence of mtDNA in rho0 yeast cells affects both respiration and mitochondrial-nuclear communication (e.g., retrograde regulation, intergenomic signaling, or pleiotropic drug resistance). Previously, it has been reported that some rho0 strains have increased replicative lifespans, attributable to the lack of respiration and retrograde regulation. Here, we have been able to confirm that rho0 cells exhibit increased replicative lifespans but have found that this is not associated with the lack of respiration or reduced oxidative stress but instead, is related to the lack of mtDNA per se in rho0 cells. Also, we find no correlation between the strength of retrograde regulation and lifespan. Furthermore, we find that pdr3- or rtg2- mutations are not responsible for lifespan extension in rho0 cells, ruling out a specific role for PDR3-pleiotropic drug resistance or RGT2-retrograde regulation pathways in the extended lifespans of rho0 cells. Surprisingly, Rtg3p, which acts downstream of Rtg2p, is required for lifespan increase in rho0 cells. Together, these findings indicate that the loss of mtDNA per se and not the lack of respiration lead to extended longevity in rho0 cells. They also suggest that Rtg3p, acting independently of retrograde regulation, mediates this effect, possibly via intergenomic signaling.

ISC1-dependent metabolic adaptation reveals an indispensable role for mitochondria in induction of nuclear genes during the diauxic shift in Saccharomyces cerevisiae

Growth of Saccharomyces cerevisiae following glucose depletion (the diauxic shift) depends on a profound metabolic adaptation accompanied by a global reprogramming of gene expression. In this study, we provide evidence for a heretofore unsuspected role for Isc1p in mediating this reprogramming. Initial studies revealed that yeast cells deleted in ISC1, the gene encoding inositol sphingolipid phospholipase C, which resides in mitochondria in the post-diauxic phase, showed defective aerobic respiration in the post-diauxic phase but retained normal intrinsic mitochondrial functions, including intact mitochondrial DNA, normal oxygen consumption, and normal mitochondrial polarization. Microarray analysis revealed that the Deltaisc1 strain failed to up-regulate genes required for nonfermentable carbon source metabolism during the diauxic shift, thus suggesting a mechanism for the defective supply of respiratory substrates into mitochondria in the post-diauxic phase. This defect in regulating nuclear gene induction in response to a defect in a mitochondrial enzyme raised the possibility that mitochondria may initiate diauxic shift-associated regulation of nucleus-encoded genes. This was established by demonstrating that in respiratory-deficient petite cells these genes failed to be up-regulated across the diauxic shift in a manner similar to the Deltaisc1 strain. Isc1p- and mitochondrial function-dependent genes significantly overlapped with Adr1p-, Snf1p-, and Cat8p-dependent genes, suggesting some functional link among these factors. However, the retrograde response was not activated in Deltaisc1, suggesting that the response of Deltaisc1 cannot be simply attributed to mitochondrial dysfunction. These results suggest a novel role for Isc1p in allowing the reprogramming of gene expression during the transition from anaerobic to aerobic metabolism.

Coordination of gene expression between organellar and nuclear genomes

Following the acquisition of chloroplasts and mitochondria by eukaryotic cells during endosymbiotic evolution, most of the genes in these organelles were either lost or transferred to the nucleus. Encoding organelle-destined proteins in the nucleus allows for host control of the organelle. In return, organelles send signals to the nucleus to coordinate nuclear and organellar activities. In photosynthetic eukaryotes, additional interactions exist between mitochondria and chloroplasts. Here we review recent advances in elucidating the intracellular signalling pathways that coordinate gene expression between organelles and the nucleus, with a focus on photosynthetic plants.

Mitochondrial retrograde signaling

Mitochondrial retrograde signaling is a pathway of communication from mitochondria to the nucleus under normal and pathophysiological conditions. The best understood of such pathways is retrograde signaling in the budding yeast Saccharomyces cerevisiae. It involves multiple factors that sense and transmit mitochondrial signals to effect changes in nuclear gene expression; these changes lead to a reconfiguration of metabolism to accommodate cells to defects in mitochondria. Analysis of regulatory factors has provided us with a mechanistic view of regulation of retrograde signaling. Here we review advances in the yeast retrograde signaling pathway and highlight its regulatory factors and regulatory mechanisms, its physiological functions, and its connection to nutrient sensing, TOR signaling, and aging.

Copper supplementation increases yeast life span under conditions requiring respiratory metabolism

To further exploit yeast as a model for cellular aging we have modified the replicative life span assay to force respiration, by replacing glucose with the non-fermentable carbon source glycerol. The growth rates of several different strains varied greatly, with doubling times ranging from 2.7 to 7 h. Life spans of all strains were lower on media containing glycerol than on media containing glucose. However, supplementation of glycerol-containing media with copper resulted in increases in life span of between 17 and 72%; life spans equivalent to or beyond those obtained on glucose media. Addition of copper to glucose medium had no effect on life span. Microarray analysis showed that genes responsible for high affinity import of copper display reduced expression upon addition of copper, while most genes showed no change in expression. No differences in growth rate, oxygen uptake, or the levels of subunit II of the copper-containing cytochrome c oxidase were found between cultures of yeast grown with or without copper supplementation. Copper supplementation greatly extended the life span of sod1 and sod2 strains, suggesting that addition of copper may reduce the generation of superoxide. Forcing yeast to respire places an emphasis on mitochondrial function and may aid in the identification of factors involved in aging in other respiratory-dependent organisms.

Impact of ROS on ageing of two fungal model systems: Saccharomyces cerevisiae and Podospora anserina

To provide a foundation for the development of effective interventions to counteract various age-related diseases in humans, ageing processes have been extensively studied in various model organisms and systems. However, the mechanisms underlying ageing are still not unravelled in detail in any system including rather simple organisms. In this article, we review some of the molecular mechanisms that were found to affect ageing in two fungal models, the unicellular ascomycete Saccharomyces cerevisiae and the filamentous ascomycete Podospora anserina. A selection of issues like retrograde response, genomic instability, caloric restriction, mtDNA reorganisation and apoptosis is presented and discussed with special emphasis on the role reactive oxygen species (ROS) play in these diverse molecular pathways.

Mitochondrial metabolism and aging in the filamentous fungus Podospora anserina

The filamentous fungus Podospora anserina has a limited lifespan. In this organism, aging is systematically associated to mitochondrial DNA instability. We recently provided evidence that the respiratory function is a key determinant of its lifespan. Loss of function of the cytochrome pathway leads to the compensatory induction of an alternative oxidase, to a decreased production of reactive oxygen species and to a striking increase in lifespan. These changes are associated to the stabilization of the mitochondrial DNA. Here we review and discuss the links between these different parameters and their implication in the control of lifespan. Since we demonstrated the central role of mitochondrial metabolism in aging, the same relationship has been evidenced in several model systems from yeast to mice, confirming the usefulness of simple organisms as P. anserina for studying lifespan regulation.

Rtg2 protein: at the nexus of yeast longevity and aging

Firm support for the notion that metabolism and particularly mitochondrial metabolism plays a significant role in aging has been gathered in studies on yeast. As in other organisms, mitochondria contribute to aging through their propensity to generate reactive oxygen species. There is more to the involvement of mitochondria in aging than this, however. Mitochondrial dysfunction, which accumulates during aging, triggers the retrograde response, an intracellular signaling pathway that activates genes that compensate for this dysfunction. A key signaling protein in this pathway is the Rtg2 protein. Recent studies have provided evidence that this protein lies at the nexus of the four major processes that are involved in aging in yeast and in other organisms; namely, metabolism, stress resistance, chromatin-dependent gene regulation, and genome stability. The details of this central role of Rtg2 protein explain the delicate balance between longevity and aging, which ultimately must tip towards the latter. Phenomena that resemble the retrograde response appear to exist in human cells, with both common and cell type-specific gene expression changes as the output.

The retrograde response links metabolism with stress responses, chromatin-dependent gene activation, and genome stability in yeast aging

Yeast can be used as a model to understand the impact mitochondria have on aging in higher organisms. Mitochondrial dysfunction increases with replicative age in yeast, and this is associated with the induction of the retrograde response. This intracellular signaling pathway from the mitochondrion to the nucleus results in changes in the expression of metabolic and stress genes, which adapt the yeast cell to the loss of tricarboxylic acid cycle activity by providing alternate anaplerotic sources of biosynthetic precursors. The induction of the retrograde response increases longevity. Paradoxically, it also leads to the production of extrachromosomal ribosomal DNA circles, which cause yeast demise. The deleterious effects of these circles are mitigated by the retrograde response, which increases longevity in part due to this effect and partly due to other activities. Rtg2p is the retrograde signal transducer proximal to the mitochondrion, and it interacts with several proteins in relaying the retrograde signal to the transcription factor Rtg1p-Rtg3p. Rtg2p also suppresses ribosomal DNA circle production. When it is engaged in retrograde signaling, it cannot fulfill the latter role. The SAGA-like SLIK complex is one of the protein complexes in which Rtg2p has been found. This histone acetyltransferase, transcriptional co-activator complex contains Gcn5p, and it potentiates the activation of retrograde responsive genes. SLIK complex integrity, and in particular Gcn5p, are needed for retrograde response extension of life span. Thus, the retrograde response through SLIK links metabolism, stress responses, chromatin-dependent gene regulation, and genome stability in yeast aging. Gene regulatory phenomena akin to the retrograde response also operate in human cells, which display both common and cell-type specific changes in gene expression on loss of mitochondrial function.

Interaction between Rtg2p and Mks1p in the regulation of the RTG pathway of Saccharomyces cerevisiae

Retrograde signaling mediates nuclear gene expression in response to changes in the functional state of mitochondria. In budding yeast, retrograde signaling, also termed the RTG pathway, relies on the heterodimeric, basic helix-loop-helix zipper transcription factors, Rtg1p and Rtg3p, for the activation of target gene expression. Activation of the RTG pathway leads to partial dephosphorylation of Rtg3p and its translocation, together with Rtg1p, from the cytoplasm to the nucleus. These processes depend on a positive regulatory factor, Rtg2p, a novel protein with a ATP binding domain similar to that of the Hsp70/actin/sugar kinase superfamily. Four negative regulatory factors, Lst8p, Mks1p, and two redundant 14-3-3 proteins, Bmh1/2p, function between Rtg2p and Rtg1/3p. Alternative interaction between Mks1p and Rtg2p or Bmh1/2p provides a means for regulation of the RTG pathway. When the RTG pathway is on, Mks1p is inactivated by its association with Rtg2p; and when the RTG pathway is off, Mks1p dissociates from Rtg2p and forms a complex with Bmh1/2p, which is the negative regulatory form of Mks1p. Here we show that Rtg2p and Mks1p can interact in the absence of other factors, and is thereby the minimal binary switch for regulation of the RTG pathway. Gel filtration experiments indicate that both Rtg2p and Mks1p exist in high molecular weight complexes. In response to changes in the activity of the RTG pathway, both Rtg2p and Mks1p shift to different sized high molecular weight complexes. Together, our data suggest that dynamic association between Mks1p and Rtg2p in high molecular weight complexes provides a means to regulate the RTG pathway.

The histone acetyltransferase GCN5 modulates the retrograde response and genome stability determining yeast longevity

Transcriptional silencing decreases at both subtelomeric and silent mating-type loci and increases at the ribosomal DNA locus during the replicative life span of the yeast Saccharomyces cerevisiae . Evidence exists that epigenetic changes in the regulatory state of chromatin may be a causal factor in determining yeast longevity and that histone deacetylases play a role. The significance of histone acetylation has been examined here in more detail. Deletion of the histone acetyltransferase gene GCN5 suppressed the extension of replicative life span afforded by the induction of the retrograde response, which signals mitochondrial dysfunction and leads to changes in nuclear gene expression. It was difficult to ascribe this effect to changes in transcriptional silencing in any of the three known types of heterochromatin. However, a promoter related effect was uncovered by the participation of GCN5 in the induction of the retrograde response. Gcn5p and the retrograde signal transducer Rtg2p are components of the histone acetyltransferase coactivator complex SLIK. Rtg2p blocks the production of extrachromosomal ribosomal DNA circles when it is not engaged in transmission of the retrograde signal. Deletion of GCN5 , which disrupts the integrity of SLIK, suppressed circle accumulation. The results indicate that Gcn5p and SLIK impact the interplay between the retrograde response signal and Rtg2p with consequences for the induction of the response and circle production. Rtg2p and Gcn5p in the SLIK complex link metabolism to stress responses, chromatin-dependent gene regulation, and genome stability in yeast aging.

Yeast replicative life span--the mitochondrial connection

Mitochondria have been associated with aging in many experimental systems through the damaging action of reactive oxygen species. There is more, however, to the connection between mitochondria and Saccharomyces cerevisiae longevity and aging. Induction of the retrograde response, a pathway signaling mitochondrial dysfunction, results in the extension of life span and postponement of the manifestations of aging, changing the metabolic and stress resistance status of the cell. A paradox associated with the retrograde response is the simultaneous triggering of extrachromosomal ribosomal DNA circle (ERC) production, because of the deleterious effect these circles have on yeast longevity. The retrograde response gene RTG2 appears to play a pivotal role in ERC production, linking metabolism and genome stability. In addition to mother cell aging, mitochondria are important in establishment of age asymmetry between mother and daughter cells. The results more generally point to the existence of a mechanism to "filter" damaged components from daughter cells, a form of checkpoint control. Mitochondrial integrity is affected by the PHB1 and PHB2 genes, which encode inner mitochondrial membrane chaperones called prohibitins. The Phb1/2 proteins protect the cell from imbalances in the production of mitochondrial proteins. Such imbalances appear to cause a stochastic stratification of the yeast population with the appearance of short-lived cells. Ras2p impacts this process. Maintenance of mitochondrial membrane potential and the provision of Krebs cycle intermediates for biosyntheses appear to be crucial elements in yeast longevity. In sum, it is clear that mitochondria lie at the nexus of yeast longevity and aging.

Yeast longevity and aging—the mitochondrial connection

Studies of the yeast Saccharomyces cerevisiae reveal four processes determining life span: metabolism, stress resistance, chromatin-dependent gene regulation, and genome stability. The retrograde response, which signals mitochondrial dysfunction resulting in changes in nuclear gene expression, extends yeast life span and is induced during normal aging. This response involves extensive metabolic adaptations. The retrograde response links metabolism and genome stability during yeast aging. A reduction in the availability of nutrients also extends yeast life span. This metabolic mechanism operates by pathways distinct from the retrograde response, although it shares with the latter some longevity effectors. Life extension by calorie restriction entails re-modeling of mitochondrial function. The retrograde response appears to compensate for age changes, while calorie restriction may be a preventive mechanism. The maintenance of age asymmetry between the mother and daughter yeast cells also depends on mitochondrial function. Loss of this age asymmetry occurs during normal yeast aging and may be a paradigm for stem cell aging. The importance of mitochondrial integrity in yeast longevity is emphasized by the role of prohibition function in attenuating oxidative damage. Our studies point to the central role of mitochondria in yeast aging. They highlight the importance of the maintenance of mitochondrial membrane potential, which drives the transport of biosynthetic precursors derived from the Krebs cycle. Common threads weave their way through the studies of aging in yeast and in other model organisms. This suggests conserved features of aging across phyla.

Common and cell type-specific responses of human cells to mitochondrial dysfunction

In yeast, mitochondrial dysfunction activates a specific pathway, termed retrograde regulation, which alters the expression of specific nuclear genes and results in increased replicative life span. In mammalian cells, the specific nuclear genes induced in response to loss of mitochondrial function are less well defined. This study characterizes responses in nuclear gene expression to loss of mitochondrial DNA sequences in three different human cell types: T143B, an osteosarcoma-derived cell line; ARPE19, a retinal pigment epithelium cell line; and GMO6225, a fibroblast cell population from an individual with Kearns-Sayre syndrome (KSS). Quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure gene expression of a selection of glycolysis, TCA cycle, mitochondrial, peroxisomal, extracellular matrix, stress response, and regulatory genes. Gene expression changes that were common to all three cell types included up-regulation of GCK (glucokinase), CS (citrate synthase), HOX1 (heme oxygenase 1), CKMT2 (mitochondrial creatine kinase 2), MYC (v-myc myelocytomatosis viral oncogene homolog), and WRN (Werner syndrome helicase), and down-regulation of FBP1 (fructose-1, 6-bisphosphatase 1) and COL4A1 (collagen, type IV, alpha 1). RNA interference experiments show that induction of MYC is important in rho0 cells for the up-regulation of glycolysis. In addition, a variety of cell type-specific gene changes was detected and most likely depended upon the differentiated functions of the individual cell types. These expression changes may help explain the response of different tissues to the loss of mitochondrial function due to aging or disease.

Genes determining yeast replicative life span in a long-lived genetic background

Here we describe the replicative life spans of more than 50 congenic Saccharomyces cerevisiae strains, each carrying a mutation previously implicated in yeast aging. This analysis provides a direct comparison, in a single, long-lived strain background, of a majority of reported yeast aging genes. Of the eleven deletion mutations previously reported to increase yeast life span, we find that deletion of FOB1, deletion of SCH9, and deletion of GPA2, GPR1, or HXK2 (three genetic models of calorie restriction) significantly enhanced longevity. In addition, over-expression of SIR2 or growth on low glucose increased life span. These results define a limited number of genes likely to regulate replicative life span in a strain-independent manner, and create a basis for future epistasis analysis to determine genetic pathways of aging.

Higher respiratory activity decreases mitochondrial reactive oxygen release and increases life span in Saccharomyces cerevisiae

Increased replicative longevity in Saccharomyces cerevisiae because of calorie restriction has been linked to enhanced mitochondrial respiratory activity. Here we have further investigated how mitochondrial respiration affects yeast life span. We found that calorie restriction by growth in low glucose increased respiration but decreased mitochondrial reactive oxygen species production relative to oxygen consumption. Calorie restriction also enhanced chronological life span. The beneficial effects of calorie restriction on mitochondrial respiration, reactive oxygen species release, and replicative and chronological life span could be mimicked by uncoupling agents such as dinitrophenol. Conversely, chronological life span decreased in cells treated with antimycin (which strongly increases mitochondrial reactive oxygen species generation) or in yeast mutants null for mitochondrial superoxide dismutase (which removes superoxide radicals) and for RTG2 (which participates in retrograde feedback signaling between mitochondria and the nucleus). These results suggest that yeast aging is linked to changes in mitochondrial metabolism and oxidative stress and that mild mitochondrial uncoupling can increase both chronological and replicative life span.

Sir2-independent life span extension by calorie restriction in yeast

Calorie restriction slows aging and increases life span in many organisms. In yeast, a mechanistic explanation has been proposed whereby calorie restriction slows aging by activating Sir2. Here we report the identification of a Sir2-independent pathway responsible for a majority of the longevity benefit associated with calorie restriction. Deletion of FOB1 and overexpression of SIR2 have been previously found to increase life span by reducing the levels of toxic rDNA circles in aged mother cells. We find that combining calorie restriction with either of these genetic interventions dramatically enhances longevity, resulting in the longest-lived yeast strain reported thus far. Further, calorie restriction results in a greater life span extension in cells lacking both Sir2 and Fob1 than in cells where Sir2 is present. These findings indicate that Sir2 and calorie restriction act in parallel pathways to promote longevity in yeast and, perhaps, higher eukaryotes.

This study indicates that calorie restriction and Sir2 promote longevity in yeast through distinct pathways. This undermines the accepted view, and has implications for aging in higher organisms

Small molecules that regulate lifespan: evidence for xenohormesis

Barring genetic manipulation, the diet known as calorie restriction (CR) is currently the only way to slow down ageing in mammals. The fact that CR works on most species, even microorganisms, implies a conserved underlying mechanism. Recent findings in the yeast Saccharomyces cerevisiae indicate that CR extends lifespan because it is a mild biological stressor that activates Sir2, a key component of yeast longevity and the founding member of the sirtuin family of deacetylases. The sirtuin family appears to have first arisen in primordial eukaryotes, possibly to help them cope with adverse conditions. Today they are found in plants, yeast, and animals and may underlie the remarkable health benefits of CR. Interestingly, a class of polyphenolic molecules produced by plants in response to stress can activate the sirtuins from yeast and metazoans. At least in the case of yeast, these molecules greatly extend lifespan by mimicking CR. One explanation for this surprising observation is the 'xenohormesis hypothesis', the idea that organisms have evolved to respond to stress signalling molecules produced by other species in their environment. In this way, organisms can prepare in advance for a deteriorating environment and/or loss of food supply.