DNA synthesis following gross alterations of the nucleocytoplasmic ratio in the ciliate Stentor coeruleus

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

Lipid Biosynthesis and its Coordination with Cell Cycle Progression

The activation of cell cycle regulators at the G1/S boundary has been linked to the cellular protein synthesis rate. It is conceivable that regulatory mechanisms are required to allow cells to coordinate the synthesis of other macromolecules with cell cycle progression. The availability of highly synchronized cells and flow cytometric methods facilitates investigation of the dynamics of lipid synthesis in the entire cell cycle of the heterotrophic dinoflagellate Crypthecodinium cohnii. Flow cytograms of Nile red-stained cells revealed a stepwise increase in the polar lipid content and a continuous increase in neutral lipid content in the dinoflagellate cell cycle. A cell cycle delay at early G1, but not G2/M, was observed upon inhibition of lipid synthesis. However, lipid synthesis continued during cell cycle arrest at the G1/S transition. A cell cycle delay was not observed when inhibitors of cellulose synthesis and fatty acid synthesis were added after the late G1 phase of the cell cycle. This implicates a commitment point that monitors the synthesis of fatty acids at the late G1 phase of the dinoflagellate cell cycle. Reduction of the glucose concentration in the medium down-regulated the G1 cell size with a concomitant forward shift of the commitment point. Inhibition of lipid synthesis up-regulated cellulose synthesis and resulted in an increase in cellulosic contents, while an inhibition of cellulose synthesis had no effects on lipid synthesis. Fatty acid synthesis and cellulose synthesis are apparently coupled to the cell cycle via independent pathways.

The elusive sizer

Size control has been a topic of interest to cell biologists for over a century, but insights into cell size control mechanisms have until recently been relatively sparse. Determining whether cells have a size measurement mechanism and how it might operate has proven difficult. The nucleocytoplasmic ratio is one of the few conserved features of size control but little is know about how it is measured. Models where growth and division can be uncoupled have been underexploited, but have considerable potential for gaining insights into the contribution of the nucleocytoplasmic ratio to cell size regulation.

How cells coordinate growth and division

Size is a fundamental attribute impacting cellular design, fitness, and function. Size homeostasis requires a doubling of cell mass with each division. In yeast, division is delayed until a critical size has been achieved. In metazoans, cell cycles can be actively coupled to growth, but in certain cell types extracellular signals may independently induce growth and division. Despite a long history of study, the fascinating mechanisms that control cell size have resisted molecular genetic insight. Recently, genetic screens in Drosophila and functional genomics approaches in yeast have macheted into the thicket of cell size control.

Growth during the cell cycle

During the cell cycle, major bulk parameters such as volume, dry mass, total protein, and total RNA double and such growth is a fundamental property of the cell cycle. The patterns of growth in volume and total protein or RNA provide an "envelope" that contains and may restrict the gear wheels. The main parameters of cell cycle growth were established in the earlier work when people moved from this field to the reductionist approaches of molecular biology, but very little is known on the patterns of metabolism. Most of the bulk properties of cells show a continuous increase during the cell cycle, although the exact pattern of this increase may vary. Since the earliest days, there have been two popular models, based on an exponential increase and linear increase. In the first, there is no sharp change in the rate of increase through the cycle but a smooth increase by a factor of two. In the second, the rate of increase stays constant through much of the cycle but it doubles sharply at a rate change point (RCP). It is thought that the exponential increase is caused by the steady growth of ribosome numbers and the linear pattern is caused by a doubling of the structural genes during the S period giving an RCP--a "gene dosage" effect. In budding yeast, there are experiments fitting both models but on balance slightly favoring "gene dosage." In fission yeast, there is no good evidence of exponential increase. All the bulk properties, except O2 consumption, appear to follow linear patterns with an RCP during the short S period. In addition, there is in wild-type cells a minor RCP in G2 where the rate increases by 70%. In mammalian cells, there is good but not extensive evidence of exponential increase. In Escherichia coli, exponential increase appears to be the pattern. There are two important points: First, some proteins do not show peaks of periodic synthesis. If they show patterns of exponential increase both they and the total protein pattern will not be cell cycle regulated. However, if the total protein pattern is not exponential, then a majority of the individual proteins will be so regulated. If this majority pattern is linear, then it can be detected from rate measurements on total protein. However, it would be much harder at the level of individual proteins where the methods are at present not sensitive enough to detect a rate change by a factor of two. At a simple level, it is only the exponential increase that is not cell cycle regulated in a synchronous culture. The existence of a "size control" is well known and the control has been studied for a long time, but it has been remarkably resistant to molecular analysis. The attainment of a critical size triggers the periodic events of the cycle such as the S period and mitosis. This control acts as a homeostatic effector that maintains a constant "average" cell size at division through successive cycles in a growing culture. It is a vital link coordinating cell growth with periodic events of the cycle. A size control is present in all the systems and appears to operate near the start of S or of mitosis when the cell has reached a critical size, but the molecular mechanism by which size is measured remains both obscure and a challenge. A simple version might be for the cell to detect a critical concentration of a gene product.

Effects of increased cell mass and altered gene dosage on the timing of initiation of macronuclear DNA synthesis in Paramecium tetraurelia. Implications for cell cycle regulation

In Paramecium, cell mass and macronuclear DNA content can vary substantially, and both variables affect the timing of initiation of macronuclear DNA synthesis. Cells normally begin macronuclear DNA synthesis at 0.25 in the cell cycle when the mean cell mass is about 119% of the initial value. Gene mutations were used to alter cell size by temporarily blocking cell cycle progression and to change DNA content by altering the segregation pattern of macronuclear material to daughter nuclei at fission. Changes in cell mass or macronuclear DNA content imposed at fission or in the subsequent G1 interval do not affect the timing of initiation of DNA synthesis in that cell cycle, but do affect the timing of initiation of DNA synthesis in the subsequent cell cycle. The progeny of cells with lower than average macronuclear DNA content tend to initiate DNA synthesis earlier than other cells. The G1 interval is proportionally shortened when initial cell mass is greater than normal, and no measurable G1 interval is present when initial cell mass equals or exceeds the normal cell mass present at initiation of DNA synthesis. These results suggest that the timing of initiation of DNA synthesis is established during the preceding cell cycle and that the 'timer' mechanism is not significantly affected by either drastic changes in gene dosage or gene concentration during the G1 interval.

Causal relations among cell cycle processes in Tetrahymena pyriformis. An analysis employing temperature-sensitive mutants

Utilization of temperature-sensitive mutants of Tetrahymena pyriformis affected in cell division or developmental pathway selection has permitted elucidation of causal dependencies interrelating micronuclear and macronuclear replication and division, oral development, and cytokinesis. In those mutants in which cell division is specifically blocked at restrictive temperatures, micronuclear division proceeds with somewhat accelerated periodicity but maintains normal coupling to predivision oral development. Macronuclear division is almost totally suppressed in an early acting mutant (mola) that prevents formation of the fission zone, and is variably affected in other mutants (such as mo3) that allow the fission zone to form but arrest constriction. However, macronuclear DNA synthesis can proceed for about four cycles in the nondividing mutant cells. A second class of mutants (psm) undergoes a switch of developmental pathway such that cells fail to enter division but instead repeatedly carry out an unusual type of oral replacement while growing in nutrient medium at the restrictive temperature. Under these circumstances no nuclei divide, yet macronuclear DNA accumulation continues. These results suggest that (a) macronuclear division is stringently affected by restriction of cell division, (b) micronuclear division and replication can continue in cells that are undergoing the type of oral development that is characteristic of division cycles, and (c) macronuclear DNA synthesis can continue in growing cells regardless of their developmental status. The observed relationships among events are consistent with the further suggestion that the cell cycle in this organism may consist of separate clusters of events. with a varying degree of coupling among clusters. A minimal model of the Tetrahymena cell cycle that takes these phenomena into account is suggested.

Nucleocytoplasmic ratio requirements for the initiation of DNA replication and fission in Tetrahymena

Hydroxyurea (10 mM) arrests the exponential growth of Tetrahymena by blocking DNA replication during S-phase. After removal of the hydroxyurea (HU), they have a long recovery period during which they are active in DNA synthesis. 3H-TdR uptake showed that on completion of the recovery period, the cells divide (recovery division) and enter a cell cycle which lacks G1. The frequency, size and DNA content of the extranuclear chromatin bodies (ECB) formed at this division are all markedly increased (2-4) over the corresponding values obtained from exponential growth phase controls. Microspectrophotometric analysis of macronuclear DNA content (N) coupled with the cytoplasmic dry mass (C) values suggest that specific N to C ratios (N/C) are required for the initiation of DNA replication and fission: during a normal (exponential growth) cell cycle, both N and C double, but asynchronously, so that the N/C of both post-fission-daughter cells and pre-fission cells is identical (standardized to N/C = 1) but late G1 cells have a low N/C. During a 10 hr exposure to HU, the N remains essentially the same whereas the C increases. When the HU is removed, the N increases by 4X and the C continues to increase until just prior to recovery division when it also reaches a value 4X that of the original daughter cells. Thus, the N/C = 1 is re-established. The enlarged ECB formed during recovery division may function to lower the N/C in the daughter cells, which in turn may in some way stimulate immediate DNA replication, thus eliminating G1. The elimination of G1 (and shortening in a few subsequent cell cycles) allows less time for cytoplasmic growth and results in the return of the cells to the generation time and the N and C values observed prior to the HU treatment.

Form and pattern in ciliated protozoa: analysis of a genic mutant with altered cell shape in Tetrahymena pyriformis, Syngen 1

A single cell isolated from the sexual progeny of mutagenized parents gave rise to a clone of cells with an abnormal, conical shape. Breeding analysis revealed that this shape results from the action of a single recessive gene, co (conical). Homozygous mutant cells are shorter and wider than wild type cells, and have their widest point at a more posterior position. Nonetheless, cortical parameters such as number of ciliary rows, number of ciliary units within these rows, and positions of contractile vacuole pores remain essentially unchanged in conical cells, suggesting a considerable degree of mutual independence of pattern and form. Shape changes prior to cell division bring about some convergence in form of dividing conical and wild type cells. However, in conical cells the new oral apparatus and fission line form well posterior to the cell equator, so the opisthes are invariably smaller than proters. Macronuclei nonetheless undergo constriction at the normal central location, and the characteristic inequality in the DNA content of the two macronuclear division products is not increased by the conical condition. Generation times are, on the average, nearly the same in the two wild type daughter cells and in conical proters, while the small conical opisthes have generation times averaging one-third longer. This prolongation explains why population doubling times are always somewhat longer in cultures of conical cells than in parallel cultures of wild type cells. The unusually long generation times of conical opisthes allows for the compensation of initial size differences. Observations on shape changes in conjugating cells of various genotypic combinations suggest that the wild type gene product is not freely exchangeable across the conjugation bridge. The implications of the conical phenotype for problems of cellular patterning and positioning of organelle systems are considered in the discussion.