Cell size homeostasis in animal cells

The way proliferating mammalian cells maintain a constant size through generations is still unknown. This question is however central because size homeostasis is thought to occur through the coordination of growth and cell cycle progression. In the yeast S. pombe for example, the trigger for cell division is the reach of a target size (Fantes, 1977). This mechanism is referred to as ‘sizer’. The homeostatic behavior of bacteria and daughter cells of the yeast S. cerevisiae on the contrary was recently characterized as an ‘adder’ where all cells grow by the same absolute amount of volume at each cell cycle. This leads to a passive regression towards the mean generation after generation (Campos et al., 2014; Soifer et al., 2016; Taheri-Araghi et al., 2015). These findings were made possible by the development of new technologies enabling direct and dynamic measurement of volume over full cell cycle trajectories. Such measurement is extremely challenging in mammalian cells whose shape constantly fluctuate over time and cycle over 20 hours long periods. Studies therefore privileged indirect approaches (Kafri et al., 2013; Sung et al., 2013; Tzur et al., 2009) or indirect measurement of cell mass rather than cell volume (Mir et al. 2014; Son et al., 2012). These studies showed that cells overall grew exponentially and challenged the classical view that cell cycle duration was adapted to size and instead proposed a role for growth rate regulation. To date however, no clear model was reached. In fact, the nature and even the existence of the size homeostasis behavior of mammalian cells is still debated (Lloyd, 2013). In order to characterize the homeostatic process of mammalian cells, we developed a technique that enable measuring, for the first time, single cell volume over full cell cycle trajectories (Cadart et al., 2017; Zlotek-Zlotkiewicz et al. 2015). We found that several cell types, HT29, HeLa and MDCK cells behaved in an adder-like manner. To further test the existence of homeostasis, we artificially induced asymmetrical divisions through confinement in micro-channels. We observed that asymmetries of sizes were reduced within the following cell cycle through an ‘adder’-like behavior. To then understand how growth and cell cycle progression were coordinated in way that generates the ‘adder’, we combined our volume measurement method with cell cycle tracking. We showed that G1 phase duration is negatively correlated with initial size. This adaptation is however limited by a minimum duration of G1, unraveled by the study of artificially-induced very large cells. Nevertheless, the adder behavior is maintained even in the absence of time modulation, thus suggesting a complementary growth regulatory mechanism. Finally, we propose a method to estimate theoretically the relative contribution of growth and timing modulation in the overall size control and use this framework to compare our results with that of bacteria. Overall, our work provides the first evidence that proliferating mammalian cells behave in an adder-like manner and suggests that both growth and cell cycle duration are involved in size control.