Beyond G1/S regulation: how cell size homeostasis is tightly controlled throughout the cell cycle?

Proliferating cells develop mechanisms to counteract stochastic noise in order to stabilize the cell mass distribution in a population. It is widely believed that cell size is controlled by size-dependent timing of the G1/S transition. However, a model based only on such a size checkpoint cannot explain why cell lines with deficient G1/S control are able to maintain a very stable cell mass distribution and how cell mass is maintained throughout the S and G2 phases of the cell cycle. To answer such questions, we applied a recently developed form of computationally enhanced Quantitative Phase Microscopy (ceQPM), which provides much-improved accuracy of cell mass measurement for individual cells. ceQPM allows us to investigate the factors contributing to cell mass homeostasis. We find that cell mass homeostasis is robustly maintained, despite disruptions of the normal G1/S transition or cell growth rate. Cell mass control is exerted throughout the cell cycle, where the coefficient of variation in cell mass for the population declines well before the G1/S transition and throughout the cell cycle in both transformed and non-transformed cells. Furthermore, the detailed response of cell growth rate to cell mass differs in different cell types. This size-dependent growth rate modulation occurs through both mTORC1-dependent and mTORC1-independent processes, which are independently regulated. The reduction of mass accumulation rate, slightly below that of exponential growth, is used to effectively reduce cell mass variation in the population. Both size-dependent cell cycle regulation and size-dependent growth rate modulation contribute to cell mass homeostasis by strictly controlling the coefficient of variation of cell mass in the population. These findings expose new principles in cell size control for normal and pathological proliferating cells, as well as terminally differentiated cells. Further, they suggest new avenues for the discovery of the underlying molecular mechanisms.