Yeast replicative aging leads to permanent cell cycle arrest in G1 effectuated by the start repressor Whi5

Abstract

Yeast replicative aging has been a canonical model for aging research. Since replicative aging eventually leads to permanent cell cycle arrest, a fundamental question is how cells sense the signals from aging and communicate that to the cell cycle control machineries. Using microfluidic devices to track individual mother cells labeled by two different cell cycle markers Whi5-tdTomato and Myo1-EGFP, we measured the length of different cell cycle phases as a function of age and the distribution of cell death in different cell cycle phases. We found that the majority of the cells died in the G1 phase, and their G1 cell cycle length increased drastically in the last few cell divisions. This increase of G1 length correlates with the increase of the nuclear concentration of Whi5, which is a major transcriptional suppressor of the cell cycle start check point. Interestingly, this correlation is apparent only above a threshold concentration of Whi5. We show that in response to external stress, Whi5 concentration increases and cell growth slows down in a Whi5 dependent manner, and that Whi5 deletion significantly extends the lifespan. Together these data suggest the existence of a programmed control to arrest cell cycle in G1 in response to stress signals due to aging, and that Whi5 is a major mediator of this process. Our findings may have important implications in understanding senescence and cancer in mammalian cells, which have a parallel G1/S control system with Rb (a well known tumor suppressor) as the analog of Whi5.\n\nSignificance statementIn this work, we used microfluidic devices to track individual mother cells labeled by two cell cycle markers Whi5-tdTomato and Myo1-EGFP. We found that aging leads to significant lengthening of G1 phase in old cells and the eventual permanent cell cycle arrest in G1, and Whi5 plays an important role in implementing such a program. We show that oxidative stress can lead to the increase of Whi5 expression and the slow-down of cell division. Furthermore, Whi5 deletion significantly extends the lifespan. The result suggest the existence of a programmed control to arrest cell cycle in G1 in response to stress signals due to aging, and that Whi5 is a major mediator of this process.