A mother's sacrifice: what is she keeping for herself?

Abstract

Individual cells of the budding yeast, Saccharomyces cerevisiae, have a limited life span and undergo a form of senescence termed replicative aging. Replicative life span is defined as the number of daughter cells produced by a yeast mother cell before she ceases dividing. Replicative aging is asymmetric: a mother cell ages but the age of her daughter cells is 'reset' to zero. Thus, one or more senescence factors have been proposed to accumulate asymmetrically between mother and daughter yeast cells and lead to mother-specific replicative senescence once a crucial threshold has been reached. Here we evaluate potential candidates for senescence factors and age-associated phenotypes and discuss potential mechanisms underlying the asymmetry of replicative aging in budding yeast.

Figure. Model for the Molecular Basis of Replicative Age-Asymmetry

A single yeast mother cell (top) undergoes replicative aging as she produces successive smaller daughter cells (below the mother cell). One or more senescence factors accumulate in the mother cell (blue Xs) and cause senescence or an age-associated phenotype once a critical threshold is reached. The senescence factor(s) is asymmetrically distributed between the mother cell and the daughter cells for most of the mother cell life span. Likewise, for most of the mother cell’s life span, replicative aging is asymmetric. Daughter cells do not inherit their mother’s age – the replicative age of the daughter cells is “reset” to zero. In the last half of a mother cell’s life span, there may be a complete breakdown of senescence factor asymmetry. Alternatively, senescence factor asymmetry could still exist but so much senescence factor may have accumulated in the mother cell that even the small proportion of the mother cell’s senescence factor acquired by the daughter cell has a deleterious affect on replicative life span. Once daughter cells start to acquire sufficient levels of senescence factor, they are born with progressively decreasing replicative life spans.