Calorie restriction is one of the most widely studied interventions known to extend lifespan and/or healthspan in species as diverse as yeast and humans. Yeast has proven to be a fantastic model organism for the discovery of genes that influence human aging and age-related disease, due to the ease of study and the conservation of nutrient sensing and signaling pathways that underlie many aging processes. Initial studies in yeast pointed to Sir2, a histone deacetylase, as being responsible for mediating the life-extending properties of calorie restriction, while later studies called this research into question Confusion seems to relate to minor differences in strain background and media composition as well as more significant problems with the way calorie restriction is defined in yeast. Under a new yeast calorie restriction paradigm I developed, experiments point to significant Sir2-dependent and Sir2-independent pathways regulating chronological lifespan (the time a non-dividing cell remains viable in a stationary phase culture) in response to changing glucose concentrations. The major goal of this proposal is to investigate and define these pathways. My central hypotheses are that the Sirtuin-dependent and Sirtuin-independent pathways mediating yeast longevity will continue to be easier to elucidate under this experimental paradigm, that genes involved in traditional caloric restriction-mediated longevity operate across a much broader range of caloric conditions, and that Sir2 is likely affecting chronological lifespan through one or more of several known Sirtuin-linked longevity pathways in response to changes in sugar concentration.
In Aim 1, both candidate and unbiased genetic approaches will be used to identify genes involved in the Sir2-independent glucose-regulated longevity pathway. Genes implicated previously in calorie restriction-mediated chronological longevity will be assayed for chronological lifespan in both high and low glucose conditions, and a pooled culture of the yeast knockout collection will be assayed for chronological longevity with short- and long-lived mutants identified by sequencing counts of unique DNA barcodes associated with each mutant. Mutants that fail to exhibit lifespan extension under calorie restriction conditions will be investigated further.
In Aim 2, candidate and unbiased approaches will be used to identify genes involved in the Sir2-dependent pathway. Mutants that are defective in known or suspected Sir2- interacting pathways will be assayed for chronological lifespan in both caloric conditions, looking for epistatic interactions with sir2 . Additionally, deacetylation targets of Sir2 will be identifid using a process that biotinylates lysine residues specifically deacetylated by Sir2, allowing their precipitation and identification by tandem mass spectroscopy. Peptides subject to differential deacetylation under high and low calorie conditions will be investigated further for a role in chronological lifespan regulation. This work will identify new genes relevant to yeast chronological aging and calorie restriction-mediated longevity, and finally resolve the role of Sir in altering chronological lifespan in response to changing caloric conditions of growth media.
S. cerevisiae is a genetically tractable model organism with a full host of molecular biological tools available for use and a long history of study. As a result f the high level of functional conservation between yeast and humans, especially of the nutrient sensing and signaling pathways central to aging and age-related disease, more genes relevant to human aging have been identified in yeast than any other system. This project, in addition to identifying and confirming new genes related to the aging of post-mitotic human cell populations, will hopefully settle one of the biggest controversies in research of calorie restriction by defining the role of Sir2 in calorie restriction-mediated longevity. Specific Aims: Calorie restriction is one of the most widely studied interventions known to extend lifespan and healthspan in species as diverse as yeast and rhesus monkeys1. Although data on lifespan extension in humans is inconclusive, possibly due to a shortage of carefully controlled long-term studies, many positive impacts of calorie restriction on overall health have been documented1-2. Nutrient sensing and signaling pathways such as TOR, PKA, and Insulin/IGF-1 have all been implicated in mediating this effect in various species1. Despite a wealth of evidence linking these pathways to lifespan and calorie restriction, much remains unknown about the mechanisms of nutrient sensing and downstream effectors involved. Even more controversial is the possible role Sirtuins play in mediating lifespan changes in response to calorie restriction. Sirtuins are a class of NAD-dependent protein deacetylases, the founding member being Sir2 from S. cerevisiae. Initial studies pointed to Sir2 being responsible for mediating the life-extending properties of calorie restriction in yeast, while later studies have called this research into question, especially as it relates to the measure of chronological lifespan in yeast. Based on the work of others and initial studies I have performed, this confusion seems to relate to minor differences in strain background and media composition as well as much more significant problems with the way calorie restriction is defined in yeast. While wild S. cerevisiae isolates ar generally found on substrates with a high caloric content (10-25% sugar), such as angiosperm fruits and tree saps, the unrestricted laboratory diet used in calorie restriction experiments is significantly lower (2% sugar), with the restricted diet being even lower still (d0.5% sugar). Usin 12% glucose as my unrestricted diet and 2% glucose as the calorie restriction diet, my initial experiments seem to point to significant Sir2- dependent and Sir2-independent pathways that regulate chronological lifespan in response to changing glucose concentrations, phenomena that aren't evident under traditional calorie restriction conditions. The major goal of this proposal is to investigate and define these Sir2-dependent and Sir2-independent pathways regulating lifespan under the new yeast calorie restriction paradigm I have developed. My central hypothesis is that much of the confusion regarding the role of yeast Sir2 in calorie restriction-mediated longevity relates to the arbitrary and illogical conditions traditionally usedin experiments. I hypothesize that the true roles of Sirtuins and other pathways in mediating calorie restriction-induced longevity in yeast will be easier to elucidate under these new conditions, and that Sir2 is likely affecting lifespan through one or more of several known Sirtuin-linked longevity pathways in response to changes in sugar concentration.
|McCleary, David F; Steakley, David Lee; Rine, Jasper (2016) Sir protein-independent repair of dicentric chromosomes in Saccharomyces cerevisiae. Mol Biol Cell 27:2879-83|