Replicative senescence in Saccharomyces cerevisiae

Kennedy et al. (1994) studied the replicative aging of the budding yeast S. cerevisiae by microscopically following the mother cells through a number of cell divisions. They noticed that the mother cell underwent a finite number of divisions and that the size of both the mother cell and the daughter cell increased with age. They also noticed that older mother cells underwent a more symmetrical division in which the daughter cell that budded from the mother cell was almost the same size as the mother cell at division. Similarly, the daughter cells of older mother cells had a shorter lifespan than those of younger mother cells. The daughter cells of older mother cells in the last 10% of their lifespan was found to undergo only 7.9 divisions, whereas those daughter cells budded from the mother cells in the first 70% of the lifespan, on an average, divided 26.5 times. Symmetrical division, therefore, does not give any advantage to the daughter cells as they undergo a smaller number of divisions than those derived from younger mother cell.

Aging is associated with the accumulation of damaged proteins, genetic materials or dysfunctional mitochondria. In S. cerevisiae, the levels of protein oxidation increase with the replicative age of mother cells (Aguilaniu et al. 2003). Oxidized proteins are unevenly distributed between the mother and daughter cells during cytokinesis, with mother cells retaining most of the oxidized proteins. However, with increasing age, mother cells lose their ability to retain the oxidized proteins (Aguilaniu et al. 2003). Thus, the amount of oxidized proteins gradually increases in daughter cells as the mother cell undergoes aging.

What happens to the progeny produced from symmetrical daughters? Even though the lifespan of the daughters arising from symmetric divisions is reduced, daughters formed from the daughters of old mother cells are restored back to normal (Kennedy et al. 1994). In other words, progeny from symmetrical daughters recover full life span potential following asymmetric division, indicating that the decrease in the lifespan in daughters of old mother cells is not heritable. Thus the damages accumulated in older mother cells may be diluted in subsequent generations.

Next- Replicative senescence in Schizosaccharomyces pombe

Aguilaniu et al. (2003). Asymmetric inheritance of oxidatively damaged proteins during cytokinesis. Science 299(5613), 1751-3.

Kennedy et al. (1994). Daughter cells of Saccharomyces cerevisiae from old mothers display a reduced life span. J Cell Biol 127(6 Pt 2), 1985-93.