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

Abstract

Proliferating cells require control mechanisms to counteract stochastic noise and stabilize the cell mass distribution in a population. It is widely believed that size-dependent timing of the G1/S transition is the predominant control process in mammalian cells. However, a model based only on such a checkpoint cannot explain why cell lines with deficient G1/S control are still able to maintain a stable cell mass distribution or how cell mass is maintained throughout the subsequent phases of the cell cycle. To answer such questions, we used a recently developed form of Quantitative Phase Microscopy (ceQPM), which provides much-improved accuracy of individual cell mass measurement, 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. The coefficient of variation in mass, nevertheless declines well after the G1/S transition and throughout the cell cycle in both transformed and non-transformed cells. Furthermore, the cell growth rate responds to cell mass in different ways in different cell types. Growth rate modulation is conveyed by mTORC1-dependent and mTORC1-independent processes, which are independently regulated. Slightly reduced mass accumulation, below exponential growth can effectively reduce cell mass variation in a population. Both size-dependent cell cycle regulation and size-dependent growth rate modulation contribute to cell mass homeostasis. Together they form a compensatory network that strictly controls the coefficient of variation of cell mass in a population. These findings opens new avenues for the discovery of underlying molecular mechanisms. Furthermore, understanding feedback directly on cell growth may provide insights into fundamental principles of cell size control in normal proliferating and terminally differentiated cells, as well as cells in pathological circumstances.

Delineation of proteome changes driven by cell size and growth rate

“…Increasing cell size results in large-scale remodeling of the proteome ( Lanz et al, 2022 ). Larger cells also have a lower protein synthesis rate per unit volume (hereby referred to as growth rate) ( Tzur et al, 2009 ; Cadart et al, 2018 ; Neurohr et al, 2019 ; Liu et al, 2022 ), so that it is unclear if the size-dependent remodeling of the proteome that we have recently reported ( Lanz et al, 2022 ) was due to changing growth rates or due to cell size per se . We therefore sought to determine if and how the cell’s growth rate contributes to proteome remodeling ( Figure 1B ).…”

“…One possibility for how cell size drives changes in protein concentrations would be that this is an indirect effect through the cell’s protein synthesis rate per unit volume (hereby referred to as growth rate), which decreases in larger cells ( Cadart et al, 2018 ; Neurohr et al, 2019 ; Liu et al, 2022 ) ( Figure 1B ). There seems to be a limit to the size range of effective biosynthesis, with larger cells exhibiting a loss of mitochondrial membrane potential ( Miettinen and Björklund, 2016 ) and reduced proliferative capacity ( Demidenko and Blagosklonny, 2008 ; Cheng et al, 2021 ; Lengefeld et al, 2021 ; Wilson et al, 2021 ; Lanz et al, 2022 ).…”

The cell cycle inhibitor RB is diluted in G1 and contributes to controlling cell size in the mouse liver

“…To control their size to be within a target range, proliferating cells can either regulate their growth such that larger cells grow slower, or they can link cell growth to progression through the cell division cycle [15][16][17][18][19] . Studies in cultured cells have now begun to elucidate some of the molecular mechanisms that are responsible.…”

Single-cell monitoring of dry mass and dry mass density reveals exocytosis of cellular dry contents in mitosis

“…A key discovery made here is the significant loss of dry mass in early mitosis. Importantly, a loss of dry mass in mitosis has been reported in adherent mammalian cells studied using QPM ( Liu et al, 2020 ; Liu et al, 2022 ), but the higher temporal resolution of our results enables us to examine this phenotype in more detail. However, our measurement approach decreased the duration of mitosis and cytokinesis, which could potentially influence the dry mass dynamics we observe in mitosis.…”

“…We found that buoyant mass accumulation rate is high from prophase to metaphase-anaphase transition, that is, early mitosis. In contrast, studies using QPM have suggested that dry mass does not accumulate in early mitosis ( Zlotek-Zlotkiewicz et al, 2015 ), or that cells may even experience a decrease in dry mass in early mitosis ( Liu et al, 2020 ; Liu et al, 2022 ). If cells actually lose dry mass in mitosis, this would indicate previously unrecognized compositional changes taking place in mitosis.…”