Replicative aging in yeast: the means to the end

Abstract

Progress in aging research is now rapid, and surprisingly, studies in a single-celled eukaryote are a driving force. The genetic modulators of replicative life span in yeast are being identified, the molecular events that accompany aging are being discovered, and the extent to which longevity pathways are conserved between yeast and multicellular eukaryotes is being tested. In this review, we provide a brief retrospective view on the development of yeast as a model for aging and then turn to recent discoveries that have pushed aging research into novel directions and also linked aging in yeast to well-developed hypotheses in mammals. Although the question of what causes aging still cannot be answered definitively, that day may be rapidly approaching.

Figure 1

Replicative and chronological life span assays in yeast. (Left) In a replicative life span (RLS) assay, one mother cell is allowed to divide, and the smaller daughters cells are removed by microdissection. The number of daughters produced is tallied as the RLS. (Right) In a chronological life span (CLS) assay, cells are maintained in an undividing state. At given time (t) intervals, a subset of the population is placed onto media to allow cell division to resume. The life span is determined as the time point at which those cells are unable to reenter the cell cycle.

Figure 2

The asymmetry of division is lost in old mother cells and sir2Δ cells. The three aging factors diagrammed here are extrachromosomal ribosomal DNA circles (ERCs), oxidatively damaged proteins, and protein aggregates. (a) In young wild-type cells, levels of all three factors are low, and the factors are preferentially sequestered in the mother cell. Note the size differential between the mother cell and the budding daughter cell. Both mother and daughter will go on to further budding. (b) In old wild-type cells, the three factors accumulate to high levels. The budding daughter cell is similar in size to the mother cell and inherits all three of the aging factors. The old mother cell can no longer divide at the end of its replicative life span (RLS). The daughter cell, although the product of asymmetrical division, will proceed to normal asymmetric division. (c) sir2Δ cells accumulate abnormally high levels of ERCs and protein aggregates. Division is partially symmetric in that, although the daughter cell is much smaller than the mother, these aging factors are not preferentially sequestered away from the daughter. The precocious accumulation of aging factors limits the RLS of the mother, although the daughter can go on to divide.

Figure 3

Old mother cells represent a fraction of an experimental population. The schematic shows the distribution of ages of cells in a dividing population. The number of divisions each mother cell has undergone is indicated in the subtext. Virgin daughters (purple) represent half of the population at each division. Among the remaining population are the younger mothers (blue) and the original mother (green).

Figure 4

Mitochondria, reactive oxygen species (ROS) production, oxidative damage, and aging. Yeast cells produce energy via different pathways, depending on the carbon source. Pathways that promote aging are diagrammed in red; pathways that do not promote aging are diagrammed in blue. When in the presence of glucose, fermentation results in low levels of ROS production. However, in the presence of glycerol, respiration results in higher levels of ROS production. Elevated levels of ROS may lead to increased oxidative damage, which in turn may promote aging. The effects of two different mitochondrial uncouplers are also diagrammed: Carbonyl cyanide 3-chlorophenylhydrazone (CCCP) leads to increased ROS production and a shortened life span, whereas dinitrophenol (DNP) treatment results in decreased levels of ROS and enhanced life span.

Figure 5

Model for the mechanism of dietary restriction (DR). In this proposed model, DR inhibits the three protein kinases Tor, protein kinase A (PKA), and Sch9. This inhibition results in increased levels of respiration and activation of the stress response proteins Msn2/4. Inhibition of Msn2/4 results in loss of activity of Pnc1, a nicotinamide deaminase. Both events lead to the activation of Sir2, which then affects aging by inhibiting ERC formation and by an unknown pathway. Inhibition of the three kinases may also result in altered translation levels as well as higher levels of autophagy. The cumulative effect of all events is the extension of life span. The asterisks indicate that the relevance of these branches of the pathway is in question among different research groups. The pound sign indicates where a role for autophagy in regulating yeast replicative life span is possible but not formally demonstrated. ERCs, extrachromosomal ribosomal DNA circles.