Deciding when to exit

A new imaging approach can distinguish between cells destined to stop proliferating and those committed to re-entering the cell cycle in live animals.

One way to overcome this challenge is to visually monitor proteins that are only active in certain phases of the cell cycle (Sakaue-Sawano et al., 2008;Zielke et al., 2014;Grant et al., 2018). For example, a group of proteins called cyclin-dependent kinases (or CDKs for short), which drive cells into S-phase, are active in G1 but are permanently turned off when cells enter a state of quiescence.
In 2013, a group of researchers used this property of CDKs to distinguish cultured mammalian cells in G1 that were preparing to proliferate from those entering quiescence (Spencer et al., 2013). To do this they engineered a fluorescent reporter protein which sits in the nucleus when CDKs are inactive and moves into the cytoplasm when modified by active CDKs ( Figure 1A). The nucleus and cytoplasm are easily distinguishable via time-lapse microscopy, making it possible to determine when CDKs are active in individual cells during G1. Now, in eLife, David Matus from Stony Brook University and co-workers -including Rebecca Adikes, Abraham Kohrman, and Michael Martinez as joint first authors -report how they used this technique to visualize when individual cells decide to stop dividing in living animals (Adikes et al., 2020). To adapt the CDK reporter to animals, the team (who are based at Stony Brook, the University of Colorado, Stanford University, Imperial College and University of Virginia) turned to two well studied experimental organisms: the nematode worm C. elegans and the zebrafish D. rerio.
Previous work using the CDK reporter in cultured mammalian cells showed that not all G1 cells behave the same after cell division: some never activate CDKs and enter quiescence (CDK low cells), while others begin increasing CDK activity during G1 (CDK inc cells) and ultimately commit to another cell cycle ( Figure 1A; Spencer et al., 2013). Adikes et al. found that this bifurcation in G1 cells could also be detected in the tissues of living C. elegans and D. rerio ( Figure 1B). They found that the CDK low and CDK inc phenotypes of G1 cells could be used to predict whether a cell would enter quiescence or would re-commit to another cycle. Further experiments revealed that if a cell had high levels of a protein called p21, which inhibits the activity of CDKs, its daughter cells were more likely to become quiescent following division. This suggests that the decision to proliferate or exit the cell cycle may depend on how p21 levels are regulated in proliferating cells (Overton et al., 2014;Hsu et al., 2019).
The CDK reporter has a number of applications. It could make it easier to study how quiescence is regulated in tissues that are typically difficult to image for long periods of time. It might reveal early steps in tissue regeneration when cells are ramping up to re-enter the cell cycle. It could also be used to sort and recover populations of CDK low and CDK inc cells for further experiments to identify the pathways regulating entry into quiescence.
The CDK reporter will allow us to tackle many interesting questions in developmental biology. For instance, how might the organization of a tissue influence the decision to stop dividing and enter quiescence? Is it possible to identify cells very early in the differentiation process before genes that demark differentiation turn on? Whatever the application or research question, Adikes et al. demonstrate once again that important new insights into the complexities of biology arise when new tools are developed to visualize living organisms.