Longevity regulation in Saccharomyces cerevisiae: linking metabolism, genome stability, and heterochromatin.

When it was first proposed that the budding yeast Saccharomyces cerevisiae might serve as a model for human aging in 1959, the suggestion was met with considerable skepticism. Although yeast had proved a valuable model for understanding basic cellular processes in humans, it was difficult to accept that such a simple unicellular organism could provide information about human aging, one of the most complex of biological phenomena. While it is true that causes of aging are likely to be multifarious, there is a growing realization that all eukaryotes possess surprisingly conserved longevity pathways that govern the pace of aging. This realization has come, in part, from studies of S. cerevisiae, which has emerged as a highly informative and respected model for the study of life span regulation. Genomic instability has been identified as a major cause of aging, and over a dozen longevity genes have now been identified that suppress it. Here we present the key discoveries in the yeast-aging field, regarding both the replicative and chronological measures of life span in this organism. We discuss the implications of these findings not only for mammalian longevity but also for other key aspects of cell biology, including cell survival, the relationship between chromatin structure and genome stability, and the effect of internal and external environments on cellular defense pathways. We focus on the regulation of replicative life span, since recent findings have shed considerable light on the mechanisms controlling this process. We also present the specific methods used to study aging and longevity regulation in S. cerevisiae.     

Longevity Regulation in Saccharomyces cerevisiae: Linking Metabolism, Genome Stability, and Heterochromatin

When it was first proposed that the budding yeast Saccharomyces cerevisiae might serve as a model for human aging in 1959, the suggestion was met with considerable skepticism. Although yeast had proved a valuable model for understanding basic cellular processes in humans, it was difficult to accept that such a simple unicellular organism could provide information about human aging, one of the most complex of biological phenomena. While it is true that causes of aging are likely to be multifarious, there is a growing realization that all eukaryotes possess surprisingly conserved longevity pathways that govern the pace of aging. This realization has come, in part, from studies of S. cerevisiae, which has emerged as a highly informative and respected model for the study of life span regulation. Genomic instability has been identified as a major cause of aging, and over a dozen longevity genes have now been identified that suppress it. Here we present the key discoveries in the yeast-aging field, regarding both the replicative and chronological measures of life span in this organism. We discuss the implications of these findings not only for mammalian longevity but also for other key aspects of cell biology, including cell survival, the relationship between chromatin structure and genome stability, and the effect of internal and external environments on cellular defense pathways. We focus on the regulation of replicative life span, since recent findings have shed considerable light on the mechanisms controlling this process. We also present the specific methods used to study aging and longevity regulation in S. cerevisiae.     

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.     

酵母Sirtuins在细胞寿命中的作用

酿酒酵母(以下简略为酵母)作为寿命分析模型广泛应用于寿命研究领域。酵母寿命分析方法有两种,分别是复制型酵母寿命分析法和时序型酵母寿命分析法。目前,通过酵母寿命分析模型已识别出包括SIR2在内的多个寿命调节基因。SIR2是目前较好的被确立起来的寿命调节基因,具有NAD依赖型脱乙酰化酶的活性,从原核生物到真核生物都有良好的保守性。Sirtuins (Sir2蛋白家族的总称)在细胞内具有功能上的多样性,其中包括对于压力耐受的调节、基因转录的调节、代谢通路的调节以及寿命调节作用等。Sir2是Sirtuins家族最早发现的成员,其功能是参与异染色质结构域转录的沉默调节,同时还参与复制型酵母寿命的调节。已证明,SIR2的缺失会缩短酵母的寿命,基因表达的增高会延长寿命。Sir2的高等真核生物的同源蛋白也被证实参与衰老相关疾病的调节。本文中,我们将阐述Sir2以及Sir2的酵母同源蛋白Hst1-Hst4的功能,以及由它们调节的酵母寿命。     

Mutations in the RAD27 and SGS1 genes differentially affect the chronological and replicative lifespan of yeast cells growing on glucose and glycerol

The Sgs1 protein from Saccharomyces cerevisiae is a member of the RecQ helicases. Defects in RecQ helicases result in premature aging phenotypes in both yeasts and humans, which appear to be promoted by replicative stress. Yeast rad27 mutants also suffer from premature aging. As the human Rad27p and Sgs1p homologs interact, a similar interaction between the yeast proteins could be important for promoting longevity in S. cerevisiae. We tested the contribution of a potential interaction between Rad27p and Sgs1p to longevity by analyzing lifespan and parameters associated with longevity in rad27 and sgs1 mutants. The carbon source supporting growth also modulated longevity as evaluated by replicative and chronological lifespan measurements. Growth on glycerol promoted chronological lifespan, while maximum replicative lifespan was obtained with glucose-supported growth. In comparison to the individual mutants, the sgs1 rad27 double mutant displayed a shortened replicative lifespan and was also more sensitive to DNA-damaging agents. In addition to promoting replicative lifespan, the activity of Rad27p was critical for achieving full chronological lifespan. The rad27 mutants exhibited increased oxidative stress levels along with an elevated spontaneous mutation rate. Removal of Sgs1p activity additionally increased the oxidative stress and spontaneous mutation rate in rad27 mutants without affecting the chronological lifespan.     

啤酒酵母复制衰老的分子机制

 复制衰老是啤酒酵母衰老形式之一,表现出芽痕累积、细胞体积变大、不对称分裂丧失、不育、核仁脆裂和代谢变化等特征.染色体外rDNA环累积是啤酒酵母复制衰老的重要原因,而组蛋白去乙酰化酶家族成员Sir2蛋白在调节染色体外rDNA环累积、啤酒酵母衰老和寿命方面起到核心作用.作为去乙酰化反应底物的NAD+正性调节Sir2组蛋白去乙酰化酶活性,NAD+代谢产物尼克酰胺对Sir2有负性调节作用,而有尼克酰胺参与的NAD+补救合成途径对于Sir2活性十分重要.目前,已经在人等动物细胞中发现参与这些调节过程的相关蛋白的同源基因,在功能上也表现出一定的相似性.啤酒酵母的衰老机制研究将为人体衰老的认识提供重要线索.     

Calorie restriction extends the chronological lifespan of Saccharomyces cerevisiae independently of the Sirtuins

Calorie restriction (CR) extends the mean and maximum lifespan of a wide variety of organisms ranging from yeast to mammals, although the molecular mechanisms of action remain unclear. For the budding yeast Saccharomyces cerevisiae reducing glucose in the growth medium extends both the replicative and chronological lifespans (CLS). The conserved NAD(+)-dependent histone deacetylase, Sir2p, promotes replicative longevity in S. cerevisiae by suppressing recombination within the ribosomal DNA locus and has been proposed to mediate the effects of CR on aging. In this study, we investigated the functional relationships of the yeast Sirtuins (Sir2p, Hst1p, Hst2p, Hst3p and Hst4p) with CLS and CR. SIR2, HST2, and HST4 were not major regulators of CLS and were not required for the lifespan extension caused by shifting the glucose concentration from 2 to 0.5% (CR). Deleting HST1 or HST3 moderately shortened CLS, but did not prevent CR from extending lifespan. CR therefore works through a Sirtuin-independent mechanism in the chronological aging system. We also show that low temperature or high osmolarity additively extends CLS when combined with CR, suggesting that these stresses and CR act through separate pathways. The CR effect on CLS was not specific to glucose. Restricting other simple sugars such as galactose or fructose also extended lifespan. Importantly, growth on nonfermentable carbon sources that force yeast to exclusively utilize respiration extended lifespan at nonrestricted concentrations and provided no additional benefit when restricted, suggesting that elevated respiration capacity is an important determinant of chronological longevity.     

A genomic analysis of chronological longevity factors in budding yeast

Chronological life span (CLS) has been studied as an aging paradigm in yeast. A few conserved aging genes have been identified that modulate both chronological and replicative longevity in yeast as well as longevity in the nematode Caenorhabditis elegans; however, a comprehensive analysis of the relationship between genetic control of chronological longevity and aging in other model systems has yet to be reported. To address this question, we performed a functional genomic analysis of chronological longevity for 550 single-gene deletion strains, which accounts for approximately 12% of the viable homozygous diploid deletion strains in the yeast ORF deletion collection. This study identified 33 previously unknown determinants of CLS. We found no significant enrichment for enhanced CLS among deletions corresponding to yeast orthologs of worm aging genes or among replicatively long-lived deletion strains, although a trend toward overlap was noted. In contrast, a subset of gene deletions identified from a screen for reduced acidification of culture media during growth to stationary phase was enriched for increased CLS. These results suggest that genetic control of CLS under the most commonly utilized assay conditions does not strongly overlap with longevity determinants in C. elegans, with the existing confined to a small number of genetic pathways. These data also further support the model that acidification of the culture medium plays an important role in survival during chronological aging in synthetic medium, and suggest that chronological aging studies using alternate medium conditions may be more informative with regard to aging of multicellular eukaryotes.     

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.     

A genomic analysis of chronological longevity factors in budding yeast

Chronological life span (CLS) has been studied as an aging paradigm in yeast. A few conserved aging genes have been identified that modulate both chronological and replicative longevity in yeast as well as longevity in the nematode Caenorhabditis elegans; however, a comprehensive analysis of the relationship between genetic control of chronological longevity and aging in other model systems has yet to be reported. To address this question, we performed a functional genomic analysis of chronological longevity for 550 single-gene deletion strains, which accounts for approximately 12% of the viable homozygous diploid deletion strains in the yeast ORF deletion collection. This study identified 33 previously unknown determinants of CLS. We found no significant enrichment for enhanced CLS among deletions corresponding to yeast orthologs of worm aging genes or among replicatively long-lived deletion strains, although a trend toward overlap was noted. In contrast, a subset of gene deletions identified from a screen for reduced acidification of culture media during growth to stationary phase was enriched for increased CLS. These results suggest that genetic control of CLS under the most commonly utilized assay conditions does not strongly overlap with longevity determinants in C. elegans, with the existing confined to a small number of genetic pathways. These data also further support the model that acidification of the culture medium plays an important role in survival during chronological aging in synthetic medium, and suggest that chronological aging studies using alternate medium conditions may be more informative with regard to aging of multicellular eukaryotes.Key words: aging, genomic, screen, lifespan, yeast, C. elegans, pH, chronological, replicative     

Implication of Ca2+ in the regulation of replicative life span of budding yeast

In eukaryotic cells, Ca(2+)-triggered signaling pathways are used to regulate a wide variety of cellular processes. Calcineurin, a highly conserved Ca(2+)/calmodulin-dependent protein phosphatase, plays key roles in the regulation of diverse biological processes in organisms ranging from yeast to humans. We isolated a mutant of the SIR3 gene, implicated in the regulation of life span, as a suppressor of the Ca(2+) sensitivity of zds1Δ cells in the budding yeast Saccharomyces cerevisiae. Therefore, we investigated a relationship between Ca(2+) signaling and life span in yeast. Here we show that Ca(2+) affected the replicative life span (RLS) of yeast. Increased external and intracellular Ca(2+) levels caused a reduction in their RLS. Consistently, the increase in calcineurin activity by either the zds1 deletion or the constitutively activated calcineurin reduced RLS. Indeed, the shortened RLS of zds1Δ cells was suppressed by the calcineurin deletion. Further, the calcineurin deletion per se promoted aging without impairing the gene silencing typically observed in short-lived sir mutants, indicating that calcineurin plays an important role in a regulation of RLS even under normal growth condition. Thus, our results indicate that Ca(2+) homeostasis/Ca(2+) signaling are required to regulate longevity in budding yeast.     

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 impact of media composition and petite mutation on the longevity of a polyploid brewing yeast strain

Ageing in Saccharomyces cerevisiae is a finite phenomenon, determined by replicative, rather than chronological lifespan. Yeast physiological condition is known to influence industrial fermentation performance, however, until recently cellular senescence has not been considered as a brewing yeast stress factor. A polyploid lager yeast (BB11) and a brewery isolate, exhibiting petite mutation were analysed for longevity. It was observed that mitochondrial deficiency induced a reduction in lifespan. In addition, replicative capacity was perceived to be dependent on environmental conditions.     

An intervention resembling caloric restriction prolongs life span and retards aging in yeast.

The yeast Saccharomyces cerevisiae has a finite life span that is measured by the number of daughter cells an individual produces. The 20 genes known to determine yeast life span appear to function in more than one pathway, implicating a variety of physiological processes in yeast longevity. Less attention has been focused on environmental effects on yeast aging. We have examined the role that nutritional status plays in determining yeast life span. Reduction of the glucose concentration in the medium led to an increase in life span and to a delay in appearance of an aging phenotype. The increase in life span was the more extensive the lower the glucose levels. Life extension was also elicited by decreasing the amino acids content of the medium. This suggests that it is the decline in calories and not a particular nutrient that is responsible, in striking similarity to the effect on aging of caloric restriction in mammals. The caloric restriction effect did not require the induction of the retrograde response pathway, which signals the functional status of the mitochondrion and determines longevity. Furthermore, deletion of RTG3, a downstream mediator in this pathway, and caloric restriction had an additive effect, resulting in the largest increase (123%) in longevity described thus far in yeast. Thus, retrograde response and caloric restriction operate along distinct pathways in determining yeast longevity. These pathways may be exclusive, at least in part. This provides evidence for multiple mechanisms of metabolic control in yeast aging. Inasmuch as caloric restriction lowers blood glucose levels, this study raises the possibility that reduced glucose alters aging at the cellular level in mammals.     

Rtg2 protein links metabolism and genome stability in yeast longevity

Mitochondrial dysfunction induces a signaling pathway, which culminates in changes in the expression of many nuclear genes. This retrograde response, as it is called, extends yeast replicative life span. It also results in a marked increase in the cellular content of extrachromosomal ribosomal DNA circles (ERCs), which can cause the demise of the cell. We have resolved the conundrum of how these two molecular mechanisms of yeast longevity operate in tandem. About 50% of the life-span extension elicited by the retrograde response involves processes other than those that counteract the deleterious effects of ERCs. Deletion of RTG2, a gene that plays a central role in relaying the retrograde response signal to the nucleus, enhances the generation of ERCs in cells with (grande) or in cells without (petite) fully functional mitochondria, and it curtails the life span of each. In contrast, overexpression of RTG2 diminishes ERC formation in both grandes and petites. The excess Rtg2p did not augment the retrograde response, indicating that it was not engaged in retrograde signaling. FOB1, which is known to be required for ERC formation, and RTG2 were found to be in converging pathways for ERC production. RTG2 did not affect silencing of ribosomal DNA in either grandes or petites, which were similar to each other in the extent of silencing at this locus. Silencing of ribosomal DNA increased with replicative age in either the presence or the absence of Rtg2p, distinguishing silencing and ERC accumulation. Our results indicate that the suppression of ERC production by Rtg2p requires that it not be in the process of transducing the retrograde signal from the mitochondrion. Thus, RTG2 lies at the nexus of cellular metabolism and genome stability, coordinating two pathways that have opposite effects on yeast longevity.     

Replicative and chronological aging in Saccharomyces cerevisiae

Saccharomyces cerevisiae has directly or indirectly contributed to the identification of arguably more mammalian genes that affect aging than any other model organism. Aging in yeast is assayed primarily by measurement of replicative or chronological life span. Here, we review the genes and mechanisms implicated in these two aging model systems and key remaining issues that need to be addressed for their optimization. Because of its well-characterized genome that is remarkably amenable to genetic manipulation and high-throughput screening procedures, S. cerevisiae will continue to serve as a leading model organism for studying pathways relevant to human aging and disease.     

酿酒酵母衰老机制研究进展

酿酒酵母衰老机制的研究对解析高等真核生物衰老的分子机制具有重要意义。酿酒酵母有两种衰老形式：时序衰老（chronologicalaging）和复制衰老（replicative aging）。酿酒酵母衰老研究中通常使用的寿命定义有两种：世代寿命和时序寿命。前者是指单个酿酒酵母细胞在死亡之前的分裂次数;后者是指一定数量的酵母细胞在后二次生长和稳定期的存活时间。本文分别综述了这两种衰老形式的分子机制及两者的相同点和不同点。