Exploring the Function of Cell Shape and Size during Mitosis

Dividing cells almost always adopt a spherical shape. This is true of most eukaryotic cells lacking a rigid cell wall and is observed in tissue culture and single-celled organisms, as well as in cells dividing inside tissues. While the mechanisms underlying this shape change are now well described, the functional importance of the spherical mitotic cell for the success of cell division has been thus far scarcely addressed. Here we discuss how mitotic rounding contributes to spindle assembly and positioning, as well as the potential consequences of abnormal mitotic cell shape and size on chromosome segregation, tissue growth, and cancer

Etude de l'homéostasie de taille chez les cellules animales

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.Le mécanisme d’homéostasie de taille chez les cellules animales est très peu compris actuellement. Cette question est pourtant d’un intérêt majeur car le maintien de l’homéostasie de taille dans une population de cellules prolifératives doit se faire par une coordination entre la croissance et la division. Chez la levure S. pombe, il a ainsi été montré que la taille est une information cruciale pour déclencher l’entrée en mitose (Fantes, 1977). Chez plusieurs bactéries et les cellules filles de la levure S. cerevisiae au contraire, de récentes études ont au contraire montré que l’homéostasie de taille était le résultat d’une addition constante de volume, indépendamment de la taille initiale des cellules (Campos et al., 2014; Soifer et al., 2016; Taheri-Araghi et al., 2015). Ce mécanisme est appelé « adder » et génère une régression des tailles à la moyenne, génération après génération. Ces résultats ont été possibles grâce au développement de techniques permettant la mesure dynamique du volume à l’échelle de la cellule unique et sur plusieurs générations. Une telle mesure est cependant très difficile chez les cellules de mammifère dont le volume fluctue constamment et qui cyclent sur des temps plus longs (environ 20 heures). Pour cette raison, la plupart des approches proposées sont indirectes (Kafri et al., 2013; Sung et al., 2013; Tzur et al., 2009) ou reposent sur une mesure de la masse plutôt que du volume (Mir et al. 2014; Son et al., 2012). Ensemble, ces études ont montré que les cellules de mammifère croissaient de manière exponentielle. Elles ont aussi remis en cause le modèle traditionnel qui proposait que l’homéostasie de taille reposait sur l’adaptation de la durée du cycle et mis en avant un rôle de la régulation de la vitesse de croissance. Cependant, aucun modèle n’a réellement été proposé ou démontré. La nature et l’existence même d’un mécanisme maintenant l’homéostasie de taille des cellules de mammifère est en fait discutée (Lloyd, 2013).Pour caractériser l’homéostasie de taille des cellules de mammifères, nous avons développé une technique permettant pour la première fois la mesure du volume de ces cellules sur des cycles complets (Cadart et al., 2017; Zlotek-Zlotkiewicz et al. 2015). Nous montrons que plusieurs types cellulaires (HT29, MDCK et HeLa) se comportent d’une manière similaire à celle d’un « adder ». Pour tester davantage cette observation, nous induisons artificiellement des divisions asymétriques en confinant les cellules dans des micro-canaux. Nous observons que les asymétries de tailles sont réduites mais pas complètement corrigées au cours du cycle suivant, à la manière d’un « adder ». Pour comprendre comment la croissance et la progression dans le cycle sont coordonnées et génère cet « adder », nous combinons notre méthode de mesure de volume avec un suivi de la progression dans les différentes phases du cycle. Nous montrons que la durée de la phase G1 est inversement corrélée au volume initial des cellules. Cependant, cette corrélation semble contrainte par une durée minimale de G1 mise en évidence lors de l’étude de cellules artificiellement poussées à atteindre de grandes tailles. Néanmoins, même dans cette condition où la modulation de la durée du cycle est perdue, l’observation du « adder » est maintenue. Ceci suggère un rôle complémentaire de la régulation de la vitesse de croissance des cellules. Nous proposons donc une méthode pour estimer théoriquement la contribution relative de l’adaptation de la vitesse de croissance et de la durée du cycle dans le contrôle de la taille. Nous utilisons cette méthode pour proposer un cadre général où comparer le processus homéostatique des bactéries et de nos cellules. En conclusion, notre travail apporte pour la première fois la démonstration que les cellules de mammifères maintiennent l’homéostasie grâce à un mécanisme similaire au « adder ». Ce mécanisme semble impliquer à la fois une modulation de la durée du cycle et du taux de croissance

Scaling of biosynthesis and metabolism with cell size

Cells adopt a size that is optimal for their function, and pushing them beyond this limit can cause cell aging and death by senescence or reduce proliferative potential. However, by increasing their genome copy number (ploidy), cells can increase their size dramatically and homeostatically maintain physiological properties such as biosynthesis rate. Recent studies investigating the relationship between cell size and rates of biosynthesis and metabolism under normal, polyploid, and pathological conditions are revealing new insights into how cells attain the best function or fitness for their size by tuning processes including transcription, translation, and mitochondrial respiration. A new frontier is to connect single-cell scaling relationships with tissue and whole-organism physiology, which promises to reveal molecular and evolutionary principles underlying the astonishing diversity of size observed across the tree of life

Cell Biology: The Health Hazards of Super-Sizing

Cells typically occupy a narrow range of sizes according to their type. A new study reveals that cells grown to gigantic proportions fail to synthesize sufficient macromolecules, resulting in cytoplasm dilution and a loss of fitness reminiscent of old cells

Dodecaploid Xenopus longipes provides insight into the emergence of size scaling relationships during development

Genome and cell size are strongly correlated across species1,2,3,4,5,6 and influence physiological traits like developmental rate.7,8,9,10,11,12 Although size scaling features such as the nuclear-cytoplasmic (N/C) ratio are precisely maintained in adult tissues,13 it is unclear when during embryonic development size scaling relationships are established. Frogs of the genus Xenopus provide a model to investigate this question, since 29 extant Xenopus species vary in ploidy from 2 to 12 copies (N) of the ancestral frog genome, ranging from 20 to 108 chromosomes.14,15 The most widely studied species, X. laevis (4N = 36) and X. tropicalis (2N = 20), scale at all levels, from body size to cellular and subcellular levels.16 Paradoxically, the rare, critically endangered dodecaploid (12N = 108) Xenopus longipes (X. longipes) is a small frog.15,17 We observed that despite some morphological differences, X. longipes and X. laevis embryogenesis occurred with similar timing, with genome to cell size scaling emerging at the swimming tadpole stage. Across the three species, cell size was determined primarily by egg size, whereas nuclear size correlated with genome size during embryogenesis, resulting in different N/C ratios in blastulae prior to gastrulation. At the subcellular level, nuclear size correlated more strongly with genome size, whereas mitotic spindle size scaled with cell size. Our cross-species study indicates that scaling of cell size to ploidy is not due to abrupt changes in cell division timing, that different size scaling regimes occur during embryogenesis, and that the developmental program of Xenopus is remarkably consistent across a wide range of genome and egg sizes

Polyploidy in Xenopus lowers metabolic rate by decreasing total cell surface area.

Although polyploidization is frequent in development, cancer, and evolution, impacts on animal metabolism are poorly understood. In Xenopus frogs, the number of genome copies (ploidy) varies across species and can be manipulated within a species. Here, we show that triploid tadpoles contain fewer, larger cells than diploids and consume oxygen at a lower rate. Drug treatments revealed that the major processes accounting for tadpole energy expenditure include cell proliferation, biosynthesis, and maintenance of plasma membrane potential. While inhibiting cell proliferation did not abolish the oxygen consumption difference between diploids and triploids, treatments that altered cellular biosynthesis or electrical potential did. Combining these results with a simple mathematical framework, we propose that the decrease in total cell surface area lowered production and activity of plasma membrane components including the Na+/K+ ATPase, reducing energy consumption in triploids. Comparison of Xenopus species that evolved through polyploidization revealed that metabolic differences emerged during development when cell size scaled with genome size. Thus, ploidy affects metabolism by altering the cell surface area to volume ratio in a multicellular organism

The Empirical Fluctuation Pattern of E. coli Division Control

In physics, it is customary to represent the fluctuations of a stochastic system at steady state in terms of linear response to small random perturbations. Previous work has shown that the same framework describes effectively the trade-off between cell-to-cell variability and correction in the control of cell division of single E. coli cells. However, previous analyses were motivated by specific models and limited to a subset of the measured variables. For example, most analyses neglected the role of growth rate variability. Here, we take a comprehensive approach and consider several sets of available data from both microcolonies and microfluidic devices in different growth conditions. We evaluate all the coupling coefficients between the three main measured variables (interdivision times, cell sizes and individual-cell growth rates). The linear-response framework correctly predicts consistency relations between a priori independent experimental measurements, which confirms its validity. Additionally, the couplings between the cell-specific growth rate and the other variables are typically non zero. Finally, we use the framework to detect signatures of mechanisms in experimental data involving growth rate fluctuations, finding that (1) noise-generating coupling between size and growth rate is a consequence of inter-generation growth rate correlations and (2) the correlation patterns agree with a near-adder model where the added size has a dependence on the single-cell growth rate. Our findings define relevant constraints that any theoretical description should reproduce, and will help future studies aiming to falsify some of the competing models of the cell cycle existing today in the literature