The G1 distribution of "G1-less" V79 Chinese hamster cells

Cell Cycle Commitment and the Origins of Cell Cycle Variability

Exit of cells from quiescence following mitogenic stimulation is highly asynchronous, and there is a great deal of heterogeneity in the response. Even in a single, clonal population, some cells re-enter the cell cycle after a sub-optimal mitogenic signal while other, seemingly identical cells, do not, though they remain capable of responding to a higher level of stimulus. This review will consider the origins of this variability and heterogeneity, both in cells re-entering the cycle from quiescence and in the context of commitment decisions in continuously cycling populations. Particular attention will be paid to the role of two interacting molecular networks, namely the RB-E2F and APC/C^CDH1 "switches." These networks have the property of bistability and it seems likely that they are responsible for dynamic behavior previously described kinetically by Transition Probability models of the cell cycle. The relationship between these switches and the so-called Restriction Point of the cell cycle will also be considered.

Extensions of the bifurcating autoregressive model for cell lineage studies

The bifurcating autoregressive model has been used previously to model cell lineage data. A feature of this model is that each line of descendants from an initial cell follows an AR(1) model, and that the environmental effects on sisters are correlated. However, this model concentrates on modelling the correlations between mother and daughter cells and between sister cells, and does not explain the large correlations between more distant relatives observed by some authors. Here the model is extended, firstly by allowing lines of descent to follow an ARMA(p,q) model rather than an AR(1) model, and secondly by allowing correlations between the environmental effects of relatives more distant than sisters. The models are applied to several data sets consisting of independent cell lineage trees.

Extensions of the bifurcating autoregressive model for cell lineage studies

The bifurcating autoregressive model has been used previously to model cell lineage data. A feature of this model is that each line of descendants from an initial cell follows an AR(1) model, and that the environmental effects on sisters are correlated. However, this model concentrates on modelling the correlations between mother and daughter cells and between sister cells, and does not explain the large correlations between more distant relatives observed by some authors. Here the model is extended, firstly by allowing lines of descent to follow an ARMA(p,q) model rather than an AR(1) model, and secondly by allowing correlations between the environmental effects of relatives more distant than sisters. The models are applied to several data sets consisting of independent cell lineage trees.

Concurrent targeted exchange of three bases in mammalian hprt by oligonucleotides

The repair of point mutations in hprt gene by single-stranded oligonucleotides represents a model to test targeted nucleotide exchange. We studied the concurrent nucleotide exchange of two or three nucleotides in the hprt deficient hamster cell line V79-151. The used oligonucleotides resulted in mismatches at two (151, 159) or three (151, 144, and 159) hprt positions. The hprt point mutation at position 151 was repaired in about 2/10(6) cells as shown by hprt sequencing in clones surviving HAT selection. The second nucleotide exchange at hprt position 159 was found in 7% of these HAT selected clones. Using oligonucleotides resulting in three mismatches, 29% of the clones showed nucleotide exchanges at the two hprt positions (151, 144) and about 4% at three positions (151, 144, and 159). These results indicate that single-stranded oligonucleotides can generate two or three nucleotide exchanges in a mammalian chromosomal gene.

Heterogeneity in nuclear transport does not affect the timing of DNA synthesis in quiescent mammalian nuclei induced to replicate in Xenopus egg extracts

Intact G0 nuclei from quiescent mammalian cells initiate DNA synthesis asynchronously in Xenopus egg extracts, despite exposure to the same concentration of replication factors. This indicates that individual nuclei differ in their ability to respond to the inducers of DNA replication. Since the induction of DNA synthesis requires the accumulation of replication factors by active nuclear transport, any variation in the rate of transport among nuclei could contribute to the variability of DNA replication. Using the naturally fluorescent protein allophycocyanin (APC) coupled with the nuclear localization sequence (NLS) of SV40 T antigen, as a marker of nuclear uptake, we show here that individual G0 nuclei differ in their rate of transport over a range of more than 20-fold. Surprisingly, this variation has no direct influence on the timing or extent of DNA synthesis. Similar results were obtained by monitoring the uptake of nucleoplasmin, a nuclear protein present at high levels in egg extracts. These experiments show that the initiation of DNA synthesis is not driven merely by the accumulation of replication factors to some threshold concentration. Instead, some other explanation is needed to account for the timing of initiation.

The continuum model and G1-control of the mammalian cell cycle

The continuum model of the mammalian division cycle proposes that there are no G1-phase specific controls or events. The G1 phase is simply the time when processes begun in the previous cell cycle are completed. In this review, the continuum model is applied the variability of the G1-phase, the existence of G1-less cells, the ubiquitous G1-phase arrest phenomenon, the effect of over-expressed cyclins on G1-phase length, the statistical variation of the cell cycle, the reports of G1-phase syntheses, the proposed variation in retinoblastoma protein phosphorylation in G1-phase, and the myriad findings put forward to support the G1-control model of the mammalian division cycle. The continuum model is a valid description of the mammalian division cycle.

The anticonvulsant valproate teratogen restricts the glial cell cycle at a defined point in the mid-G1 phase

Direct cell counting and extent of [3H]thymidine incorporation demonstrated valproate to inhibit C6 glioma proliferation rate in a dose-dependent manner with a 1 mM concentration achieving 50% inhibition. The antiproliferative effect was reversible and could not be attributed to cytotoxicity at the valproate concentrations employed. The site of valproate action within the cell cycle was determined to be in the G1 phase, at a point 6-6.5 h prior to S phase, by estimating the time to increased [3H]thymidine incorporation following release from a 70% proliferative arrest. Synchronised cells obtained by a mitotic selection procedure required 11-12 h to enter S phase and demonstrated the valproate restriction point to be 5 h into the G1 phase of the C6 cell cycle. Exposure of valproate to the part of the G1 period which follows the restriction point was without effect on cell entry into S phase.

Differences in growth factor sensitivity between individual 3T3 cells arise at high frequency: possible relevance to cell senescence

At low serum concentrations (3% or less), individual Swiss 3T3 cells display marked heterogeneity in proliferative capacity. Here we show that this heterogeneity arises at extremely high frequency within a clone, often with sister cells showing considerable differences in capacity for further proliferation. The heterogeneity is unlikely to be due to genetic instability or mutation. Instead, it appears to reflect physiological differences between cells in their requirement for serum growth factors. It is suggested that these differences arise because cells are unable to sustain production, at low growth factor concentrations, of some rare component which is itself required for growth factor action. We believe that the generation of heterogeneity in 3T3 cells has much in common with the phenomenon of senescence in diploid cells.

Cell cycle controls in higher eukaryotic cells: resting state or a prolonged G1 period?

We express the viewpoint that control over cell growth in higher eukaryotes is achieved predominantly by regular transition of cells from proliferation to rest and vice versa as a result of coordinated interrelationship between intracellular growth inhibitors and extracellular growth factors. The resting state is considered as a special physiological state of a cell where the prereplicative reactions necessary for the onset of DNA synthesis are inhibited. Cells pass into a resting state at each successive cell cycle, with regard to the next cycle, once the threshold level of growth inhibitors has been attained. Cellular rest may thus initiate and proceed in parallel with conventional periods of the cell cycle but in a hidden way. Its termination strictly depends on the appropriate concentration of extracellular growth factors. In the absence of growth factors cells, after completing mitosis, pass into an overt state of rest metabolically different from any period of the cell cycle including G1.

Density-dependent regulation of growth in somatic hybrids between normal Chinese hamster fibroblasts and V79-8 (G1-) cells

The purpose of this work was to determine the relationship between the presence of a G1 period in the mitotic cycle and a cell's ability to respond to density-dependent regulation of growth (DDR). Somatic hybrids were obtained between normal fibroblasts from newborn Chinese hamsters, which show a strong response to DDR, and V79-8 Chinese hamster cells, which are insensitive to DDR. Two variant V79-8 sublines were used, one reported to lack a G1 period (G1-) and the other with a G1 period (G1+). Fourteen hybrid clones were isolated in selective medium and analysed for growth properties and cell cycle parameters; their hybrid nature was supported by chromosome counts. All hybrid clones, irrespective of whether a V79-8 G1- or G1+ cell was one of the parents, showed pronounced DDR and had G1 periods of various lengths. Previous experiments had shown the absence of G1 to be dominant in somatic hybrids between V79-8 G1- and G1+ cell lines. Our results may mean that the G1- property provided by V79-8 is unable to overcome the very long G1 of normal fibroblasts, or in cells that can be arrested in G1 in response to DDR, some function prevents the dominant effect of the G1- cell on at least part of the G1 period.

Apparent heterogeneity in the response of quiescent swiss 3T3 cells to serum growth factors: implications for the transition probability model and parallels with "cellular senescence" and "competence"

When subconfluent, Swiss 3T3 cells made quiescent by serum deprivation are stimulated with low concentrations of serum (ca. 1%), only a proportion of them (roughly 50%) enter S phase despite daily replacement with fresh, low-serum medium. The cells that fail to enter S phase are not incapable of doing so, since most of them initiate DNA synthesis after transfer to 10% serum. It would appear that individual cells vary in their growth factor requirements. Using time-lapse cinemicroscopy a few of the cells that respond to low serum were seen to give rise to several generations of progeny, while the majority of cells failed to divide at all, or divided once at most. Despite this, differences between cells in growth factor requirements do not seem to be heritable in the long term, since attempts to enrich for responding cells by prolonged culture in 1% serum have been unsuccessful. Rather, it would appear that the capacity to respond to low serum is an unstable property lost after a few generations in low serum. The loss of responsiveness shows parallels with "cellular senescence" and could conceivably result from decay of the platelet-derived growth factor-induced state of "competence." But regardless of why some cells respond to low serum while others do not, it is clear that the kinetics of entry into S phase after serum stimulation of quiescent 3T3 cells are not strictly first-order, since the labelling index plateaus after roughly 3 days at values substantially below 100%. As such, the kinetics, though not contradicting the transition probability model, cannot be taken to support it as was previously thought.