Aging Yeast Cells Undergo a Sharp Entry into Senescence Unrelated to the Loss of Mitochondrial Membrane Potential

In budding yeast, a mother cell can produce a finite number of daughter cells before it stops dividing and dies. Such entry into senescence is thought to result from a progressive decline in physiological function, including a loss ofmitochondrial membrane potential (Delta Psi). Here, we developed a microfluidic device to monitor the dynamics of cell division and Delta Psi in real time at single- cell resolution. We show that cells do not enter senescence gradually but rather undergo an abrupt transition to a slowly dividing state. Moreover, we demonstrate that the decline in Delta Psi, which is observed only in a fraction of cells, is not responsible for entry into senescence. Rather, the loss of Delta Psi is an age- independent and heritable process that leads to clonal senescence and is therefore incompatible with daughter cell rejuvenation. These results emphasize the importance of quantitative single- cell measurements to decipher the causes of cellular aging.