Intellectual Merit: All organisms tune their gene expression and phenotypic programs to current conditions. Budding yeast, which has long been cultivated for making bread, wine and beer, is adapted to high concentrations of simple carbohydrates, such as glucose. Growth at lower glucose levels extends the number of times that a yeast mother cell can produced daughter cells (replicative longevity). Associated with lower glucose, there are a variety of known gene expression and epigenetic changes and a known set of enzymes, which are required for the lifespan extension due to glucose restriction. Among these enzymes are Sir2, a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, and enzymes that perform NAD salvage biosynthesis. Earlier investigators in this field hypothesized that lower glucose would alter the levels of intracellular NAD metabolites in a manner that would tune Sir2 activity. However, recent work has indicated that, while increasing intracellular NAD can extend yeast lifespan, glucose restriction does not substantively alter the levels of intracellular NAD metabolites. Rather, the extracellular metabolome of glucose restricted yeast cells is required for lifespan extension and an extracellular fraction has been obtained, which contains a metabolite 300-fold induced by glucose restriction. The goal of this EAGER proposal is to use a combination of mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy to identify the molecule and initiate characterization of its mechanism of action. Given that yeast cells live as communities, the research has the potential to reveal new insights about how a diffusible factor that extends lifespan might act to extend the lifespan of a community and not simply for the benefit of the metabolite-exporting cell.
Broader Impacts: The project will contribute to training of a graduate student and a postdoctoral associate, who will directly carry out the research and also participate in outreach a local high school.
Baker’s yeast is a primitive single-celled fungus that is used to dissect fundamental problems in gene regulation and cellular function. Yeast cells divide by budding—the mother yeast cell has a finite lifetime in which it can produce daughter cells. In yeast as in animals, calorie restriction extends lifespan. How calorie restriction extends lifespan is not understood. The obvious difference between animals and yeast involves multi-cellularity. Animals have different organs, such as brain, heart, skeletal muscle, liver, pancreas and fat, whereas yeast form a colony of identical cells. Because of the lack of specialized cells, it was assumed that a single yeast cell would sense reduced calories and alter its program in a manner that would extend lifespan in the absence of signals from other cells. This type of regulation is termed "cell autonomy." In this project, researchers developed a new method to determine the level of gene expression of a set of proteins that might have been affected by calorie restriction. The results of this analysis suggested that yeast cells might alter their environment when they are calorie restricted and that this behavior might be required to extend lifespan. Surprisingly, yeast mother cells that are moved away from the location of calorie restriction lose the longevity benefit of low glucose, while the media from calorie-restricted yeast cells transmits a lifespan benefit. Transmission of a longevity factor by yeast cells was not anticipated and suggests that yeast cells have a non-cell autonomous program for lifespan extension. Technology developed in this project will aid multiple lines of investigation into quantitative analysis of yeast protein expression. The major scientific conclusion from this project suggests that glucose restriction allows yeast cells to extend the lifespan of an entire colony of cells. Significantly, restricting calories of yeast cells results in lifespan extension of other cells, which could be termed altruistic behavior. This project also contributed to the training of a post-doctoral fellow, who learned how to publish original research in a competitive field.
