Estimation of Circadian Variations In Cell Cycle Phase Durations In Murine Epidermal Basal Cells

Circadian variations in the proliferative activity of squamous epithelia are well known. However, circadian variations in the duration of the various cell cycle phases (S, G₂ and mitosis) have been disputed. the percent labelled mitoses method, which is traditionally used to obtain duration of cell cycle phases, is poorly suited for identification of circadian variations. Therefore methods combining changes in compartment size (cell cycle phase) and cellular flux through the compartments have been used. Three different methods using such data are presented. These incorporate various simplifying assumptions that cause methodological errors. Limits for use of the different methods are indicated. the use of all three methods gives comparable and pronounced circadian variations in the duration of S and G₂ phase. These results are also compatible with circadian variations in the mitotic duration, but they may also represent artefacts due to sensitivity to model errors.     

Circadian rhythms in mouse epidermal basal cell proliferation. Variations in compartment size, flux and phase duration.

Several kinetic parameters of basal cell proliferation in hairless mouse epidermis were studied, and all parameters clearly showed circadian fluctuations during two successive 24 hr periods. Mitotic indices and the mitotic rate were studied in histological sections; the proportions of cells with S and G2 phase DNA content were measured by flow cytometry of isolated basal cells, and the [3H]TdR labelling indices and grain densities were determined by autoradiography in smears from basal cell suspensions. The influx and efflux of cells from each cell cycle phase were calculated from sinusoidal curves adapted to the cell kinetic findings and the phase durations were determined. A peak of cells in S phase was observed around midnight, and a cohort of partially synchronized cells passed from the S phase to the G2 phase and traversed the G2 phase and mitosis in the early morning. The fluctuations in the influx of cells into the S phase were small compared with the variations in efflux from the S phase and the flux through the subsequent cell cycle phases. The resulting delay in cell cycle traverse through S phase before midnight could well account for the accumulation of cells in S phase and, therefore, also the subsequent partial synchrony of cell cycle traverse through the G2 phase and mitosis. Circadian variations in the duration of the S phase, the G2 phase and mitosis were clearly demonstrated.     

Cell cycle phase expansion in nitrogen-limited cultures of Saccharomyces cerevisiae

The time and coordination of cell cycle events were examined in the budding yeast Saccharomyces cerevisiae. Whole-cell autoradiographic techniques and time-lapse photography were used to measure the duration of the S, G1, and G2 phases, and the cell cycle positions of "start" and bud emergence, in cells whose growth rates were determined by the source of nitrogen. It was observed that the G1, S, and G2 phases underwent a proportional expansion with increasing cell cycle length, with the S phase occupying the middle half of the cell cycle. In each growth condition, start appeared to correspond to the G1 phase/S phase boundary. Bud emergence did not occur until mid S phase. These results show that the rate of transit through all phases of the cell cycle can vary considerably when cell cycle length changes. When cells growing at different rates were arrested in G1, the following synchronous S phase were of the duration expected from the length of S in each asynchronous population. Cells transferred from a poor nitrogen source to a good one after arrest in G1 went through the subsequent S phase at a rate characteristic of the better medium, indicating that cells are not committed in G1 to an S phase of a particular duration.     

Polysomes in various phases of the cell cycle in synchronized plasmacytoma cells

The fraction of membrane-bound and free polysomes during different phases of the cell cycle was determined in suspension cultures of mouse plasmacytoma cells, synchronized by growth in isoleucine-deficient medium. The membrane-bound polysomes reached a maximum value (about 28 % of total polysomes) during the G 1 phase. In the S phase and G 2 phase only 18 to 20 % of the total polysomes were found to be membrane-bound. A high percentage of membrane-bound polysomes in the G 1 phase of the cell cycle agrees with the earlier finding that maximum synthesis of immunoglobulin light chain takes place on polysomes bound to the membrane in the G 1 phase of the cell cycle. The presence of a significant fraction of membrane-bound polysomes in the S and G 2 phases of the cell cycle would suggest that membrane-bound polysomes are also involved in the synthesis of proteins other than immunoglobulins.The ultrastructure of the cells during the various phases of the cell cycle was also studied. During the G 1 phase the surface of the majority of cells was distinguished by the presence of ruffles and slender villus-like cytoplasmic projections. In the S phase the surface contour tended to become smooth and even. These differences in the surface morphology may reflect the change in function which occurs during the transition from the G 1 to the S phase.     

Circadian G2 arrest as related to circadian gating of cell population growth in Euglena

Cell population growth is gated to occur in particular circadian phases, which has been known for over four decades in various organisms including cyanobacteria and human. However, little is known as to which cell cycle phases from G1 to M are primarily regulated by the circadian rhythm or when in a circadian cycle this primary regulation takes place. We report here that in the flagellate alga Euglena gracilis grown photoautotrophically, the circadian rhythm primarily prevented developmentally matured G2 cells from progressing to mitosis, such that cell population growth occurred only during subjective night. In addition, we found that the circadian rhythm also arrests G1-to-S and S-to-G2 transitions at particular circadian phases.     

Quantitative analysis of cell cycle phase durations and PC12 differentiation using fluorescent biosensors

Cell cycle analysis typically relies on fixed time-point measurements of cells in particular phases of the cell cycle. The cell cycle, however, is a dynamic process whose subtle shifts are lost by fixed time-point methods. Live-cell fluorescent biosensors and time-lapse microscopy allows the collection of temporal information about real time cell cycle progression and arrest. Using two genetically-encoded biosensors, we measured the precision of the G1, S, G2, and M cell cycle phase durations in different cell types and identified a bimodal G1 phase duration in a fibroblast cell line that is not present in the other cell types. Using a cell line model for neuronal differentiation, we demonstrated that NGF-induced neurite extension occurs independently of NGF-induced cell cycle G1 phase arrest. Thus, we have begun to use cell cycle fluorescent biosensors to examine the proliferation of cell populations at the resolution of individual cells and neuronal differentiation as a dynamic process of parallel cell cycle arrest and neurite outgrowth.     

A Stochastic Model For Cell Populations With Circadian Rhythms

A mathematical model for cell kinetics, based on a random walk, is developed. the model allows variations with time of the rates of passage of proliferating cells through the four phases of the mitotic cycle. Circadian variations in the mitotic and labelling indices of the Syrian hamster cheek pouch epithelium have previously been observed, and the random walk model has been used to simulate this phenomenon. Assuming that all basal cells are proliferative and that these cells leave the basal layer randomly throughout the mitotic cycle to become differentiated cells, it was found that the experimentally observed circadian rhythms of the mitotic and labelling indices could be reproduced in the model by postulating a circadian rhythm in the rate of passage of cells through the G₁ and S phases only. Moreover, the growth activity of cells in both the G₁ and S phases appears to reach a peak during the dark hours of the light-dark cycle, and to fall off rapidly in the early hours of daylight. the postulate of Møller, Larsen & Faber (1974) that injection of the animals with tritiated thymidine causes a shortening of the G₂ phase duration has been qualitatively confirmed by using the random walk model to simulate the FLM and MI curves after injection with tritiated thymidine.     

Quantitative analysis of cytoskeletal proteins throughout the cell cycle of the MRC-5 fibroblastic cell line

Fluorescent dyes were used to stain actin, vimentin, tubulin and DNA in the same MRC-5 fibroblastic cells. Cytofluorometry and image analysis were then used to quantitatively evaluate the F actin, vimentin and tubulin content throughout the cell cycle. The results showed that different cells can have the same DNA content while their cytoskeletal protein content is variable. The data also showed that cytoskeletal protein content variations exist throughout the cell cycle of the fibroblastic cell line. The F actin content increased during the cell cycle from G1 to G2 phases and decreased in M phase. The amount of tubulin in the G2 was about twice as much as that in the G1 phase, before decreasing in the M phase; there was a threshold of tubulin content for G2 cells entering S phase.     

Studies on the Growth in Culture of Plant Cells: XVII. ANALYSIS OF THE CELL CYCLE OF ASYNCHRONOUSLY DIVIDING ACER PSEUDOPLATANVS L. CELLS IN SUSPENSION CULTURE

Three methods of cell cycle analysis, involving the use of tritiatedthymidine, have been applied to asynchronously dividing suspensioncultures of sycamore. Conditions for an effective chase of unlabelledthymidine were established from a study of the kinetics of entryand incorporation of tritiated thymidine into the cells. Thelevels of thymidine used did not affect the rate of cell divisionor the duration of the phases of the cell cycle. The analyses of the cell cycle based upon pulse labelling, continuouslabelling, and a combination of densitometry and autoradiographywere in good agreement and showed that the phases S (mean 7.0h), G2 (mean 8.5 h) and mitosis (mean 3.0 h), were of relativelyconstant duration, whereas G1 was of variable duration. No relation between nuclear DNA content and mitotic-cycle timeor the duration of S-phase could be inferred from the data presented.     

Cyclic AMP-dependent and -independent protein phosphokinase activity in subcellular fractions of synchronously growing leydig I-10 cells.

The Leydig I-10 tumor cell line was synchronized by the double thymidine block method using 1.0 mM thymidine. Protein phosphokinase activity of subcellular fractions was determined at various times throughout the cell cycle. Microsomal cAMP-independent kinase activity increased in G2 and decreased during the S and G1 phases. Except for relatively small increases during the G1 and late S phases, microsomal cAMP-dependent kinase activity remained unchanged throughout most of the cycle. In the lysosomal-mitochondrial fraction, cAMP-dependent and cAMP-independent protein kinase activity increased during the S phase. Independent kinase activity peaked again during G1, while the dependent kinase became depressed. Phosphokinase activity increased in the nuclear fraction in late G2 and during mitosis, and was due to increases in both cAMP-independent and cAMP-dependent kinase activity. Cytosol cAMP-dependent kinase activity increased in G2 and during mitosis; cAMP-independent kinase activity showed some increased activity during late G2 and mitosis. These temporal variations in the subcellular kinase activities throughout the cell cycle may act to phosphorylate subcellular protein substrates in a cell cycle-specific fashion.     

Measurement of c-myc protein content and cell cycle kinetics of normal and spontaneously transformed murine mastocytes by bivariate flow cytometry

Progressive in vitro culturing of interleukin-3 (IL-3) dependent normal murine mastocytes (PB-3) resulted in a variant cell line (PB-1) able to grow without exogenous IL-3 and which was tumorogenic in syngenic mice. Bivariate flow cytometry was used to evaluate the c-myc protein and DNA content of PB-3 and PB-1 cells. The c-myc protein was detected by specific monoclonal antibodies. Kinetic characteristics of PB-3 and PB-1 cell lines, namely, the duration of the G1, S and G2 + M cell cycle phases were also evaluated using the bromodeoxyuridine (BrdU) pulse-chase method and BrdU/DNA flow cytometry. Levels of c-myc protein in PB-1 cells were about two-fold higher than those of PB-3 cells in all cell cycle phases. Mean duration of the cell cycle (Tc) was 15.3 h for PB-3 cells and 12.4 h for PB-1 cells. Shortening in Tc for the transformed cells was due to a decrease of nearly 30% in mean duration of the G1 phase (from 8 h to 5.7 h). No significant differences were found in the duration of the S and G2 + M phases. These results indicate that acquired IL-3 independency in vitro and tumorogenicity of PB-1 cells were accompanied by a doubling of c-myc protein level and by a parallel shortening, or bypass, of the regulatory events within the G1 phase of the cell cycle.     

Characteristics of the cell cycle of matrix cells in the mouse embryo during histogenesis of telencephalon

The cell cycle of matrix cells in the telencephalon of the mouse embryo at different stages at day 10, 13, and 17 of gestation was investigated by means of ³H-thymidine autoradiography.The cell cycle time of matrix cells in the day 10 group was found to be 7.0 h, and lengthened linearly with embryonic age. The cell cycle times of day 13 and 17 groups were 15.5 and 26.0 h, respectively.The duration of G1 and S phases also lengthened linearly with embryonic age. The durations of G1 phase were 0.1, 6.8, and 13.8 h, for day 10, 13, and 17 groups, respectively, and those of S phase were 5.1, 6.9, and 10.4 h, for day 10, 13, and 17 groups, respectively. On the other hand, the durations of both G2 and M phases remained unchanged and these were 1.0 and 0.8 h, respectively, throughout the embryonic stages.It was a characteristic of the alteration of the cell cycle of the telencephalon during mouse embryonic life that not only G1 but also S phases lengthened linearly with embryonic age and both G2 and M phases remained constant.     

The analysis and interpretation of DNA distributions measured by flow cytometry

A principal use of flow cytometers is for the measurement of fluorescence distributions of cells stained with DNA specific dyes. A large amount of effort has been and is being expended currently in the analysis of these distributions for the fractions of cells in the G1, S, and G2 + M phases. Several methods of analysis have been proposed and are being used; new methods continue to be introduced. Many, if not most, of these methods differ only in the mathematical function used to represent the phases of the cell cycle and represent attempts to fit exactly distributions with known phase fractions or unusual shapes. In this paper we show that these refinements probably are not necessary because of cell staining and sampling variability. This hypothesis was tested by measuring fluorescence distributions for Chinese hamster ovary and KHT mouse sarcoma cells stained with Hoechst-33258, chromomycin A3, propidium iodide, and acriflavine. Our results show that: a) single measurements can result in phase fraction estimates that are in error by as much as 40% for G2 + M phase and 15-20% for G1 and S phases; b) different dyes can yield phase fraction estimates that differ by as much as 40% due to differences in DNA specificity; c) the shapes of fluorescence distributions and their interpretation are very dependent on the dye being used and on its binding mechanism.     

Cell Cycle Markers for Live Cell Analyses

Many cellular processes are regulated by cell cycle dependent changes in protein dynamics and localization. Studying these changes in vivo requires methods to distinguish the different cell cycle stages. Here we demonstrate the use of DNA Ligase I fused to DsRed1 as an in situ marker to identify S phase and the subsequent transition to G2 in live cells. Using this marker, we observed changes in the nuclear distribution of Dnmt1 during cell cycle progression. Based on the different nuclear distribution of DNA Ligase I and Dnmt1 in G2 and G1, we demonstrate that the combination of both proteins allows the direct discrimination of all cell cycle phases using either immunostainings or fusions with fluorescent proteins. These markers are new tools to directly study cell cycle dependent processes in both, fixed and living cells.     

CIRCADIAN RHYTHMS IN MOUSE EPIDERMAL BASAL CELL PROLIFERATION

Several kinetic parameters of basal cell proliferation in hairless mouse epidermis were studied, and all parameters clearly showed circadian fluctuations during two successive 24 hr periods. Mitotic indices and the mitotic rate were studied in histological sections; the proportions of cells with S and G₂ phase DNA content were measured by flow cytometry of isolated basal cells, and the [³H]TdR labelling indices and grain densities were determined by autoradiography in smears from basal cell suspensions. The influx and efflux of cells from each cell cycle phase were calculated from sinusoidal curves adapted to the cell kinetic findings and the phase durations were determined. A peak of cells in S phase was observed around midnight, and a cohort of partially synchronized cells passed from the S phase to the G₂ phase and traversed the G₂ phase and mitosis in the early morning. The fluctuations in the influx of cells into the S phase were small compared with the variations in efflux from the S phase and the flux through the subsequent cell cycle phases. The resulting delay in cell cycle traverse through S phase before midnight could well account for the accumulation of cells in S phase and, therefore, also the subsequent partial synchrony of cell cycle traverse through the G₂ phase and mitosis. Circadian variations in the duration of the S phase, the G₂ phase and mitosis were clearly demonstrated.     

Mode of estrogen action on cell proliferation in CAMA-1 cells: II. Sensitivity of G1 phase population

The mammary cancer cell line CAMA-1 synchronized at the G1/S boundary by thymidine block or at the G1/M boundary by nocodazole was used to evaluate 1) the sensitivity of a specific cell cycle phase or phases to 17 beta-estradiol (E2), 2) the effect of E2 on cell cycle kinetics, and 3) the resultant E2 effect on cell proliferation. In synchronized G1/S cells, E2-induced 3H-thymidine uptake, which indicated a newly formed S population, was observed only when E2 was added during, but not after, thymidine synchronization. Synchronized G2/M cells, enriched by Percoll gradient centrifugation to approximately 90% mitotic cells, responded to E2 added immediately following selection; the total E2-treated population traversed the cycle faster and reached S phase approximately 4 hr earlier than cells not exposed to E2. When E2 was added during the last hour of synchronization (ie, at late G2 or G2/M), or for 1 hr during mitotic cell enrichment, a mixed response occurred: a small portion had an accelerated G1 exit, while the majority of cells behaved the same as controls not incubated with E2. When E2 addition was delayed until 2 hr, 7 hr, or 12 hr following cell selection, to allow many early G1 phase cells to miss E2 exposure, the response to E2 was again mixed. When E2 was added during the 16 hr of nocodazole synchronization, when cells were largely at S or possibly at early G2, it inhibited entry into S phase. The E2-induced increase or decrease of S phase cells in the nocodazole experiments also showed corresponding changes in mitotic index and cell number. These results showed that the early G1 phase and possibly the G2/M phase are sensitive to E2 stimulation, late G1, G1/S, or G2 are refractory; the E2 stimualtion of cell proliferation is due primarily to an increased proportion of G1 cells that traverse the cell cycle and a shortened G1 period, E2 does not facilitate faster cell division; and estrogen-induced cell proliferation or G1/S transition occurs only when very early G1 phase cells are exposed to estrogen. These results are consistent with the constant transition probability hypothesis, that is, E2 alters the probability of cells entering into DNA synthesis without significantly affecting the duration of other cell cycle phases. Results from this study provide new information for further studies aimed at elucidating E2-modulated G1 events related to tumor growth.     

Cell cycle variations in chromatin structure detected by DNase I

We have recently developed a reproducible method for the use of DNase I as a sensitive probe of chromatin structure (Prentice, D A & Gurley, L R, Biochim biophys acta 740 (1983) 134) [12] and have used this probe to investigate chromatin structure during the interphase of the cell cycle. Chinese hamster cells (line CHO) were synchronized by: (1) mitotic detachment, to obtain M-phase cells; (2) isoleucine deprivation, to obtain G1-phase cells; and (3) sequential use of isoleucine deprivation followed by release into the presence of hydroxyurea, to obtain cells blocked at the start of S phase. The cells were released from the various blocking schemes and nuclei were isolated and digested with DNase I at various times. The digestion kinetics were monitored to detect possible changes in chromatin condensation through the cell cycle. The chromatin was much more accessible to DNase I in G1 phase than in S or G2 phase, with only small variations in structure detected in late G1 and very early S phase. From early S phase up to mitosis, the chromatin became increasingly condensed and inaccessible to DNase I action. These results support the concept of a chromatin condensation cycle during interphase as well as during mitosis.     

Duration of the cell cycle phases in mutants for the tumor suppressor Merlin in Drosophila melanogaster

The cell cycle duration was estimated in Drosophila melanogaster mutants for the tumor suppressor Merlin with the use of different approaches. Experiments on induction of mosaic clones in tissues of the larval wing imaginal disc showed that the cell cycle in mutant discs is shorter than that in control. Flow fluorescence cytometry revealed no differences between mutant and normal animals in the relative duration of the cell cycle phases, which suggests proportional shortening of the cell cycle phases. The study with pulse-labeled mitoses confirmed these results and showed that the length of the cell cycle is 7 h (S phase duration 3 h) in control individuals and 5 h (S phase duration 2 h) in Merlin gene mutants.     

Mathematical Models for Cell Migration with Real-Time Cell Cycle Dynamics

The fluorescent ubiquitination-based cell cycle indicator, also known as FUCCI, allows the visualization of the G1 and S/G2/M cell cycle phases of individual cells. FUCCI consists of two fluorescent probes, so that cells in the G1 phase fluoresce red and cells in the S/G2/M phase fluoresce green. FUCCI reveals real-time information about cell cycle dynamics of individual cells, and can be used to explore how the cell cycle relates to the location of individual cells, local cell density, and different cellular microenvironments. In particular, FUCCI is used in experimental studies examining cell migration, such as malignant invasion and wound healing. Here we present, to our knowledge, new mathematical models that can describe cell migration and cell cycle dynamics as indicated by FUCCI. The fundamental model describes the two cell cycle phases, G1 and S/G2/M, which FUCCI directly labels. The extended model includes a third phase, early S, which FUCCI indirectly labels. We present experimental data from scratch assays using FUCCI-transduced melanoma cells, and show that the predictions of spatial and temporal patterns of cell density in the experiments can be described by the fundamental model. We obtain numerical solutions of both the fundamental and extended models, which can take the form of traveling waves. These solutions are mathematically interesting because they are a combination of moving wavefronts and moving pulses. We derive and confirm a simple analytical expression for the minimum wave speed, as well as exploring how the wave speed depends on the spatial decay rate of the initial condition.