UTH1 and the genetic control of aging in the yeast, Saccharomyces

Individual yeast cells have a restricted division capacity called their lifespan after which they stop cycling and lyse. Furthermore, the mortality kinetics of a population of yeast cells can be described by the Gompertz equation, the classical definition for aging populations, which states that the mortality rate, the probability of dying in the next time interval or generation, increases exponentially with age. Aging cells increase in size, experience a lengthening of their cell cycle times, and accumulate lipid containing granules which are expelled into the media when the cells lyse. Intriguingly, in several wildtype backgrounds, the length of a strain's lifespan correlated with its ability to remain viable during starvation. To test the hypothesis that the same genes may regulate both the starvation response and the lifespan in yeast, a screen was performed to isolate stress-resistant mutants which were also long-lived. The screen identified three recessive complementation groups and a dominant mutation with both increased viability after starvation and extended lifespan. These youth (UTH) mutants have a variety of phenotypes which suggest that the length of the lifespan of a strain is a reflection of its fitness and its ability to cope with stress. Cloning of UTH1 was done by complementation of the paraquat sensitivity of the mutant. Paraquat is a drug which is thought to generate superoxide radicals in vivo. UTH1 is a novel gene located on chromosome XI. Disruption of the putative open reading frame extends longevity suggesting that UTHI is a yeast aging gene. Epistasis analysis demonstrated that UTH1 is genetically upstream of the SIR genes which regulate transcriptional silencing in yeast. Furthermore, the analysis of uthl mutations in strains where telomeres or the HM loci are marked with a reporter sensitive to silencing levels suggested that UTH1 encodes an "anti-silencer". We propose that loss of UTH1 would lead to enhanced silencing throughout the genome and a subsequent increase in lifespan from the silencing of an aging gene(s). UTH1 is a founding member of the SUN (SIM1, UTH1, NCA3) family of Saccharomyces genes which share a 215 amino acid domain at their C-termini. Two additional SUN genes have recently been identified including SUN4, another gene from Saccharomyces, and bglA, a putative beta-glucosidase from Candida wickerhamii. Null mutations in the other Saccharomyces SUN genes decreased the mean lifespan but not the maximum lifespan of the strain and it is still not clear if these genes directly affect the yeast aging process. Characterization of the UTH genes provided the first links between transcriptional silencing and aging in the yeast cell and suggested that still unidentified aging genes (called AGE) were subject to gene silencing. Two other regions of the yeast genome, the telomeres and the HM loci, are known to be silenced. To examine the relationship between these known silenced regions and the hypothetical AGE locus, we determined the effects of changing telomere length on the lifespan of the yeast cell. We discovered an inverse correlation between telomere length and lifespan. Mutations which lengthen telomeres including rapls and Arifl lead to a shortening of lifespan. The significant shortening of lifespan in the Arifl strain was partially suppressed by alleles of SIR4 which are thought to mislocalize the silencing complex to AGE. In contrast, overexpression of truncated TLC], the gene encoding the yeast telomerase RNA, leads to a shortening of telomeres and a delay in senescence. Strikingly, this extension in lifespan requires SIR3 which encodes an essential component of the silencing machinery. We propose that short telomeres increase the pool of available silencing complexes which enhances silencing at AGE delaying senescence. Our findings suggest that the telomere shortening seen in senescing mammalian cells may not cause senescence but may actually be a cellular response to an age-dependent loss of genomic silencing: telomeres shorten so that cells can continue proliferating. In summary, the identification of single gene mutations which modulate longevity has opened up the field of aging to the power of yeast genetics. It remains to be seen, however, if any of these regulatory pathways is functionally conserved in models of replicative senescence in higher cells. What is clear is that yeast aging has become more than just a bud scar phenomenon. It is an area of research which is still young. Thesis Supervisor: Leonard P. Guarente Title: Professor of Biology