Chronological and replicative lifespan in yeast: do they meet in the middle?

Budding yeast is a preeminent model organism in studies of cellular aging pathways that are conserved in eukaryotes, including humans. There are two primary ways to query the lifespan of this organism. If one asks how many times a cell can divide, the answer will be its replicative life span (RLS). If, on the other hand, one asks how long a cell can stay alive without dividing, the answer will be its chronological life span (CLS).

Budding yeast is a facultative aerobe with exceptional genetic tractability. Hence, many environmental and genetic factors are known to affect replicative and chronological lifespan. Since the context of the RLS and CLS assays is different, with dividing vs. non-dividing cells, it is not immediately obvious whether these factors should be overlapping. The results to date are ambiguous. Some well-studied interventions like dietary restriction as well as reduced TOR and protein kinase A signaling, extend both replicative and chronological lifespan. However, in a quantitative comparison of gene deletions that extend lifespan in both assays, no significant overlap was observed.

Further complicating the issue is the differences in methodology regarding chronological aging. When performed in synthetic-defined complete (SDC) media, it was recently shown that acidification of the medium during the growth phase accelerates mortality. Lifespan is extended by buffering the culture medium to pH 6.0 or performing the experiment in rich YEPD medium, which is more refractory to acidification. Interestingly, a recent report indicates that media acidification may be a limiting component to long-term survival of non-proliferative mammalian cells as well. To what extent acidification accelerates normal CLS aging mechanisms and whether it relates at all to replicative aging remain unknown.

Instead of focusing on the factors that are shared, or not, between CLS and RLS pathways, a different way to probe the relationship between CLS and RLS is to examine how one aging process affects the other. More than a decade ago, it was reported that the longer cells age chronologically, the fewer times they can divide when nutrients are restored.

A new study by Murakami et al. found that chronologically aged cells had a reduced replicative lifespan, confirming the earlier report. In addition to replicating the initial study, in which the CLS portion of the assay was performed in YEPD, this study compared three CLS conditions: YEPD, SDC and buffered SDC, finding that replicative lifespan is dramatically shortened in the SDC conditions associated with acidification. These findings indicate that conditions associated with acidification and rapid chronological aging impact the replicative lifespan of the cells, suggesting that the consequences of acidification are related to those of slower aging in YPD and possibly replicative aging as well.

Murakami et al. went further. The CLS to RLS transition is essentially a transition from a non-dividing state, to a dividing one. Hence, querying parameters associated with cell cycle progression ought to be pertinent for the CLS to RLS transition. Indeed, Murakami et al. found that cells with the greatest replicative potential after quiescence were smaller and arrested properly in the G1 phase of the cell cycle, before DNA replication. These results further support the significance of G1 control mechanisms in aging., Why would cells that are chronologically aged have a reduced replicative lifespan? All cells in a quiescent population would be exposed to damage, either due to acidification or other causes. However, the authors note that once the population reenters a proliferative state, this damage may stay with the mother cells. This would “free up” the daughters, maximizing the fitness of the population as a whole. This model is appealing and far-reaching. Cycles of quiescent and proliferative states are the norm not only for single-celled organisms in the wild, but also for cells in animal tissues.

Many questions remain. For example, what is the mechanistic basis for the interventions (e.g., buffering acidification) that extend CLS, which then also extend RLS? Which aspect of G1 control is causally related to the CLS→RLS transition? Finally, while Murakami et al. examined how CLS influences RLS, the reverse relationship is also worth examining. Do replicatively older mothers have a reduced CLS and, if so, why? While standard CLS methods cannot address this issue, a plate-based CLS assay is theoretically amenable. Whatever the answers to the above questions might be, yeast aging with its chronological and replicative flavors will continue to drive progress in the field (Fig. 1).

Figure 1. Schematic of the relationship between the non-dividing state of chronological aging (shown in yellow), and the dividing state during which cells age replicatively (shown in blue). Conditions such as acid stress, which accelerate chronological aging, also shorten replicative lifespan.