Uncoupling reproduction from metabolism extends chronological lifespan in yeast

Significance All cells age and do so in relation to how many times a cell divides (replicative aging) and how long a nondividing cell can live (chronological aging). Bakers’ yeast has been used to study both, but because yeast divides when nutrient levels permit, the genetics of its chronological lifespan has only been studied under calorie restriction, mimicked by starvation. Because many terminally differentiated animal cells are long-lived and rarely starve, we developed a model of cell lifespan under calorie-unrestricted conditions. When encapsulated and fed ad libitum, yeast goes into cell cycle arrest, continues to be metabolically active, and remains viable for weeks, offering a new experimental paradigm to study chronological lifespan in the absence of calorie restriction. Studies of replicative and chronological lifespan in Saccharomyces cerevisiae have advanced understanding of longevity in all eukaryotes. Chronological lifespan in this species is defined as the age-dependent viability of nondividing cells. To date this parameter has only been estimated under calorie restriction, mimicked by starvation. Because postmitotic cells in higher eukaryotes often do not starve, we developed a model yeast system to study cells as they age in the absence of calorie restriction. Yeast cells were encapsulated in a matrix consisting of calcium alginate to form ∼3 mm beads that were packed into bioreactors and fed ad libitum. Under these conditions cells ceased to divide, became heat shock and zymolyase resistant, yet retained high fermentative capacity. Over the course of 17 d, immobilized yeast cells maintained >95% viability, whereas the viability of starving, freely suspended (planktonic) cells decreased to <10%. Immobilized cells exhibited a stable pattern of gene expression that differed markedly from growing or starving planktonic cells, highly expressing genes in glycolysis, cell wall remodeling, and stress resistance, but decreasing transcription of genes in the tricarboxylic acid cycle, and genes that regulate the cell cycle, including master cyclins CDC28 and CLN1. Stress resistance transcription factor MSN4 and its upstream effector RIM15 are conspicuously up-regulated in the immobilized state, and an immobilized rim15 knockout strain fails to exhibit the long-lived, growth-arrested phenotype, suggesting that altered regulation of the Rim15-mediated nutrient-sensing pathway plays an important role in extending yeast chronological lifespan under calorie-unrestricted conditions.