Genetic and Molecular Analysis of Aging in Yeast

In the yeast, Saccharomyces cerevisiae, aging is defined as the number of daughter cells that one mother cell can produce. Aging mother cells undergo a number of morphological changes, yet they retain the capacity to give rise to young daughter cells throughout much of their life span. As they become old, however, the daughters they produce have depleted division potential. Full life span is eventually restored in cells generated by the shortlived daughter. These findings suggest a mechanism of aging in which a toxic cytoplasmic factor accumulates in old mother cells, eventually inhibiting their division potential. The life span potential of many strains correlates with stress resistance. By screening for stress resistant mutants in one such strain, mutants were identified in four genes which had significantly longer life spans. Two of these genes have been cloned and characterized. The first gene identified was identical to the SIR4 gene, which participates in transcriptional silencing. The mutation in SIR4, SIR4-42, was recessive for most phenotypes, including stress resistance and transcriptional silencing at telomeres and silent mating type loci. Yet life span extension by SIR4-42 was a dominant phenotype. These and other findings have led to a model whereby the SIR4-42 mutant allele extends life span by causing a re-direction of the Sir complex from telomeres and silent mating type loci to an unidentified Aging locus. This would result in increased silencing of the Aging locus. Expression of a gene or genes at the Aging locus would accelerate aging resulting in shorter life span potential. The second gene isolated from the stress resistance screen, UTH4, extends life span in a similar manner. Life span extension by UTH4 is absolutely dependent on the presence of intact SIR3 and SIR4 genes. Moreover UTH4 interferes with telomere silencing. Much as with SIR4-42, UTH4 likely extends life span by redirecting silencing factors away from telomeres and silent mating type loci, to the Aging locus. Thesis Supervisor: Leonard Guarente Title: Professor of Biology This work is dedicated to my wife, Brenda, whose love, support, encouragement, and suggestions have been immeasurable, and to my parents, James and Betty Kennedy, who inspired me to pursue a career in science and provided the love and support that made that pursuit possible. Acknowledgments First and foremost, I would like to thank Lenny Guarente and Nick Austriaco. Nick has been essential to this work in several ways. We have worked side by side for 5 years and I am saddened that our collaboration must come to an end. Lenny has been an exceptional graduate advisor, providing guidance and criticism whenever needed. I have learned a great deal from his method of approaching scientific problems. I would also like to note that not many advisors would let two graduate students start a project on a topic like yeast aging. I would like to thank my committee Frank Solomon. Bob Horvitz, and Steve Bell, as well as my outside committee member -Gary Ruvkun. I would also like to thank my undergraduate advisor Rick Gaber, who cultivated my interest in yeast genetics. I would especially like to thank Marc Vidal, who I met in Rick's lab. Marc has served as my model for a researcher. I would like to thank several members of the Guarente lab who have been invaluable. Dave McNabb has helped me to understand biochemical approaches and been a great sounding board. Jisi Zhang did all the experiments I could not get to work. Mala Murthy's efforts have also been vital to this thesis. Neal Silverman not only helped with experiments along the way, but also provided me with interesting new interpretations of my work. Finally, I would like to thank Sally Pak, who has helped me immeasurably since joining the lab. All of these people have displayed an attitude toward life that have made even bad science days enjoyable. I would also like to thank other members of the lab Julie Agapite, Yvette Clark, James Claus, Francesca Cole, John Fikes, Maryann Haldi, Myrna Helfenstein, Jun Horiuchi, Rainer Knaus, Dave Lombard, Bob Marciniak, Stan Mah, Greg Marcus, Kevin Mills, Thomas Oehler, Benjamin Pina, Roy Pollock, Dave Sinclair, Tod Smeal, Mike Stanborough, Paul Sugiono, Sheri Treadway, Bernard Turcotte, Alix Van Geel, Jennifer Zamanian, and Li Zhang. I would like to thank Josie Caloruso and Ruth Cumminsky, for their support, especially of the culinary nature. Michael Henry has been a great friend, neighbor and has taught me the value of a good trout stream. Eric Schmidt has been a partner in crime in every sport imaginable. Sumati Murli has done everything for me and more. Mike Brodsky, John and Ellen Crispino, Pete Espenshade, Sloane Henry, Juli Klemm, Margaret McCormick, Don Rowen, and Tracy Smith have also been great friends. Dave McNabb, Stan Mah, and David Sinclair made helpful editorial comments. Finally, I would like to mention the sacrifice of Anya, who missed out on a lot of walks because of this thesis.