Secrets for Staying Young

As organisms age, they accumulate cellular damage. Stem cells are needed to replenish tissues, but the damage they sustain over time compromises their ability to maintain healthy tissue function. A number of recent papers further our understanding of how stem cells are vulnerable to or protected from damage. Stem cells may possess intrinsic protective mechanisms that keep them healthy. For instance, one possibility is that upon dividing, they favor one of their progeny by bestowing it with the newest and healthiest cellular components. Now a recent study by David Sabatini and colleagues presents evidence in support of this idea (Katajisto et al., 2015). By using various labeling methods to distinguish between old and young organelles in human mammary stem-like cells, they find that unlike other organelles examined, mitochondria are divvied up unequally, with young mitochondria going preferentially to the daughter cell that retains stem-cell-like properties. Live imaging shows that even before division, old and new mitochondria are separated, with old mitochondria sticking close to the nucleus, but young ones venturing further out. Inhibiting mitochondrial fission prevents this partitioning, somitochondria have to stay nimble for this process to occur. But how exactly do how young and old mitochondria get asymmetrically segregated to promote stemness?