Unstable activator model for size control of the cell cycle

Cell Size Control via an Unstable Accumulating Activator and the Phenomenon of Excess Mitotic Delay

Unstable Accumulating Activator models for cellular size control propose an activator that accumulates in a size-dependent manner and triggers cell cycle progression once it has reached a certain threshold. Having a short half life makes such an activator responsive to changes in cell size and makes specific predictions for how cells respond to perturbation. In particular, it explains the curious phenomenon of excess mitotic delay. Excess mitotic delay, first observed in Tetrahymena in the '50s, is a phenomenon in which a pulse of protein synthesis inhibition causes a delay in mitotic entry that is longer than the pulse and that gets longer the later in the cell cycle the pulse is delivered. The interpretation of this phenomenon championed by Zeuthen and Mitchison in the '60s and '70s is that an unstable activator of mitosis is degraded during the pulse and has to be resynthesized to a threshold level to trigger mitosis; small cells have more time to resynthesize the activator before mitosis and so suffer less excess delay, whereas, large cells have less time thus suffer greater excess delay. Fifty years later, with our detailed understanding of cell cycle biochemistry, we can identify and test candidate Unstable Accumulating Activators. Here I review the field and further develop this concept.

Size-Dependent Expression of the Mitotic Activator Cdc25 Suggests a Mechanism of Size Control in Fission Yeast

Proper cell size is essential for cellular function. Nonetheless, despite more than 100 years of work on the subject, the mechanisms that maintain cell-size homeostasis are largely mysterious [1]. Cells in growing populations maintain cell size within a narrow range by coordinating growth and division. Bacterial and eukaryotic cells both demonstrate homeostatic size control, which maintains population-level variation in cell size within a certain range and returns the population average to that range if it is perturbed [1, 2]. Recent work has proposed two different strategies for size control: budding yeast has been proposed to use an inhibitor-dilution strategy to regulate size at the G1/S transition [3], whereas bacteria appear to use an adder strategy, in which a fixed amount of growth each generation causes cell size to converge on a stable average [4-6]. Here we present evidence that cell size in the fission yeast Schizosaccharomyces pombe is regulated by a third strategy: the size-dependent expression of the mitotic activator Cdc25. cdc25 transcript levels are regulated such that smaller cells express less Cdc25 and larger cells express more Cdc25, creating an increasing concentration of Cdc25 as cells grow and providing a mechanism for cells to trigger cell division when they reach a threshold concentration of Cdc25. Because regulation of mitotic entry by Cdc25 is well conserved, this mechanism may provide a widespread solution to the problem of size control in eukaryotes.

The Ups and Downs of Modeling the Cell Cycle

We discuss the impact of mathematical modeling on our understanding of the cell cycle. Although existing, detailed models confirm that the known interactions in the cell cycle can produce oscillations and predict behaviors such as hysteresis, they contain many parameters and are poorly constrained by data which are almost all qualitative. Questions about the basic architecture of the oscillator may be more amenable to modeling approaches that ignore molecular details. These include asking how the various elaborations of the basic oscillator affect the robustness of the system and how cells monitor their size and use this information to control the cell cycle.

A purified cellular extract accelerates the cell cycle in Physarum polycephalum

Plasmodia of the myxomycete Physarum polycephalum (strain Cl) were collected at different times during the cell cycle and extracts were prepared from homogenates using a buffer optimized for microinjection into plasmodial veins. These extracts were injected into plasmodia during the first 3 h of the cell cycle. The time of the following mitosis was monitored and compared with that of the buffer-injected controls. Extracts of plasmodia homogenized 45 min before late telophase accelerated the onset of mitosis in the injected plasmodium up to 70 min, i.e., an advance of 10-14% compared to the 8- to 10-h cell cycle duration of the controls. The accelerating activity vanished completely after heating, freezing, or protease digestion, thus indicating the peptide nature of the active agent. Purification of the active compound by means of gel filtration revealed a molecular mass of about 2500 Da. The active portion of the extract was further fractionated by HPLC and the activity determined in a single peak.

An essential G1 function for cyclin-like proteins in yeast

Cyclins were discovered in marine invertebrates based on their dramatic cell cycle periodicity. Recently, the products of three genes associated with cell cycle progression in S. cerevisiae were found to share limited homology with cyclins. Mutational elimination of the CLN1, CLN2, and DAF1/WHI1 products leads to cell cycle arrest independent of cell type, while expression of any one of the genes allows cell proliferation. Using strains where CLN1 was expressed conditionally, the essential function of Cln proteins was found to be limited to the G1 phase. Furthermore, the ability of the Cln proteins to carry out this function was found to decay rapidly upon cessation of Cln biosynthesis. The data are consistent with the hypothesis that Cln proteins activate the Cdc28 protein kinase, shown to be essential for the G1 to S phase transition in S. cerevisiae. Because of the apparent functional redundancy of these genes, DAF1/WHI1 has been renamed CLN3.

The timing of initiation of DNA synthesis in Paramecium tetraurelia is established during the preceding cell cycle as cells become committed to cell division

The timing of initiation of DNA synthesis (IDS) in Paramecium is established before cell division at a point located at about 0.75 in the preceding cell cycle. This point occurs about 90 min prior to fission and coincides with the point at which cells become committed to cell division. The location of the point at which the timing of IDS is set was deduced from a series of nutrient-shift experiments. Changes in nutrient level lead to changes in the duration of the subsequent G1 interval when they occur more than 90 min prior to fission. Perturbation of the cell cycle so that the timing of commitment to cell division is altered, results in a parallel shift in the point at which the timing of IDS is established.

The timing of initiation of macronuclear DNA synthesis is set during the preceding cell cycle in Paramecium tetraurelia. Analysis of the effects of abrupt changes in nutrient level

In many eukaryotic organisms, initiation of DNA synthesis is associated with a major control point within the cell cycle and reflects the commitment of the cell to the DNA replication-division portion of the cell cycle. In Paramecium, the timing of DNA synthesis initiation is established prior to fission during the preceding cell cycle. DNA synthesis normally starts at 0.25 in the cell cycle. When dividing cells are subjected to abrupt nutrient shift-up by transfer from a chemostat culture to medium with excess food, or shift-down from a well-fed culture to exhausted medium. DNA synthesis initiation in the post-shift cell cycle occurs at 0.25 of the parental cell cycle and not at either 0.25 in the post-shift cell cycle or at 0.25 in the equilibrium cell cycle produced under the post-shift conditions. The long delay prior to initiation of DNA synthesis following nutritional shift-up is not a consequence of continued slow growth because the rate of protein synthesis increases rapidly to the normal level after shift-up. Analysis of the relation between increase in cell mass and initiation of DNA synthesis following nutritional shifts indicates that increase in cell mass, per se, is neither a necessary nor a sufficient condition for initiation of DNA synthesis, in spite of the strong association between accumulation of cell mass and initiation of DNA synthesis in cells growing under steady-state conditions.

Bud formation by the yeast Saccharomyces cerevisiae is directly dependent on "start"

Cells of the yeast Saccharomyces cerevisiae, which bear a cdc4 gene mutation, arrest early in the cell cycle but continue to produce buds in a periodic fashion. We show here that this periodic bud formation by cells already arrested at the CDC4 step is inhibited if the cell cycle regulatory step "start" is also specifically blocked by mutation or by the presence of the yeast mating pheromone alpha-factor. Thus, the characteristic periodic bud formation by cdc4 mutant cells requires the continued ability to perform start. This finding raises questions concerning the nature of start; these issues are discussed.

Unstable activator models for size control of the cell cycle

The unstable activator model of Wheals & Silverman (1982) is extended to account for the delay of nuclear division in the acellular slime mold, Physarum polycephalum, that is caused by pulse treatments with inhibitors of protein synthesis. The model is solved exactly to predict the delay as a function of the half-life of the activator. The Wheals-Silverman model is found to give results comparable, but not superior, to other unstable activator models of the cell cycle.