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Key Points The G1–S transcriptional programme is robustly activated by positive feedback mechanisms, creating an 'all-or-none' switch that leads to cell cycle commitment. Inactivation of G1–S transcription in both yeast and humans involves negative feedback loops. The wave of G1–S transcripts consists of subgroups based on their function, timing and mechanism of regulation. G1–S transcription is mechanistically linked to the DNA replication checkpoint by shared transcription factors in both yeast and humans in order to promote genomic stability during replication stress. Systems level properties associated with G1 control, such as the commitment point to cell division, the temporal pattern of G1–S transcription and its response to genotoxic stress, are likely to be conserved across eukaryotes despite frequent lack of protein sequence homology within the regulatory network. Recent work revealed new insights into the temporal regulation of G1–S cell cycle transcription, during proliferation and in response to activation of the DNA replication checkpoint. This has established the importance of G1–S transcription for both cell cycle progression and the maintenance of genome stability. The accurate transition from G1 phase of the cell cycle to S phase is crucial for the control of eukaryotic cell proliferation, and its misregulation promotes oncogenesis. During G1 phase, growth-dependent cyclin-dependent kinase (CDK) activity promotes DNA replication and initiates G1-to-S phase transition. CDK activation initiates a positive feedback loop that further increases CDK activity, and this commits the cell to division by inducing genome-wide transcriptional changes. G1–S transcripts encode proteins that regulate downstream cell cycle events. Recent work is beginning to reveal the complex molecular mechanisms that control the temporal order of transcriptional activation and inactivation, determine distinct functional subgroups of genes and link cell cycle-dependent transcription to DNA replication stress in yeast and mammals.

Key Points Activation of initiation of DNA replication occurs at only a subset of replication origins that were previously assembled in the G1 phase of the cell cycle. This is achieved through a highly regulated sequential two-step process: origin licensing in the G1 phase and origin activation during the S phase. DNA replication origins from Saccharomyces cerevisiae have a sequence consensus, whereas metazoan origins are more plastic and are determined by both sequence preferences, such as G-rich elements, and epigenetic features. Chromosomal environment and transcriptional status also influence the activation of origins. Origins selected in adjacent replication units are synchronously activated and form replication domains, which are activated at specific times during the S phase. Replication timing domains correlate with topologically associated domains (TADs). Replication timing is regulated by specific proteins and chromatin marks. The activation of DNA replication origins is developmentally controlled. Specific checkpoints regulate the initiation of DNA replication in response to replication stress. During the G1–S phase transition of the cell cycle, a variable subset of previously 'licensed' origins of replication is activated to initiate DNA synthesis. Insight is being gained into the mechanisms underlying which origins are activated and when; these mechanisms are associated with nuclear organization, cell differentiation and replication stress. DNA replication begins with the assembly of pre-replication complexes (pre-RCs) at thousands of DNA replication origins during the G1 phase of the cell cycle. At the G1–S-phase transition, pre-RCs are converted into pre-initiation complexes, in which the replicative helicase is activated, leading to DNA unwinding and initiation of DNA synthesis. However, only a subset of origins are activated during any S phase. Recent insights into the mechanisms underlying this choice reveal how flexibility in origin usage and temporal activation are linked to chromosome structure and organization, cell growth and differentiation, and replication stress.

Progression through the stages of lymphocyte development requires coordination of the cell cycle. Such coordination ensures genomic integrity while cells somatically rearrange their antigen receptor genes [in a process called variable-diversity-joining (VDJ) recombination] and, upon successful rearrangement, expands the pools of progenitor lymphocytes. Here we show that in developing B lymphocytes, the RNA-binding proteins (RBPs) ZFP36L1 and ZFP36L2 are critical for maintaining quiescence before precursor B cell receptor (pre-BCR) expression and for reestablishing quiescence after pre-BCR–induced expansion. These RBPs suppress an evolutionarily conserved posttranscriptional regulon consisting of messenger RNAs whose protein products cooperatively promote transition into the S phase of the cell cycle. This mechanism promotes VDJ recombination and effective selection of cells expressing immunoglobulin-μ at the pre-BCR checkpoint.

Circadian disruption has multiple pathological consequences, but the underlying mechanisms are largely unknown. To address such mechanisms, we subjected transformed cultured cells to chronic circadian desynchrony (CCD), mimicking a chronic jet-lag scheme, and assayed a range of cellular functions. The results indicated a specific circadian clock-dependent increase in cell proliferation. Transcriptome analysis revealed up-regulation of G1/S phase transition genes (myelocytomatosis oncogene cellular homolog [Myc], cyclin D1/3, chromatin licensing and DNA replication factor 1 [Cdt1]), concomitant with increased phosphorylation of the retinoblastoma (RB) protein by cyclin-dependent kinase (CDK) 4/6 and increased G1-S progression. Phospho-RB (Ser807/811) was found to oscillate in a circadian fashion and exhibit phase-shifted rhythms in circadian desynchronized cells. Consistent with circadian regulation, a CDK4/6 inhibitor approved for cancer treatment reduced growth of cultured cells and mouse tumors in a time-of-day-specific manner. Our study identifies a mechanism that underlies effects of circadian disruption on tumor growth and underscores the use of treatment timed to endogenous circadian rhythms.

Quiescence is a fundamental feature of plant life, which enables plasticity, renewal and fidelity of the somatic cell line. Cellular quiescence is defined by arrest in a particular phase of the cell cycle, typically G1 or G2; however, the regulation of quiescence and proliferation can also be considered across wider scales in space and time. As such, quiescence is a defining feature of plant development and phenology, from meristematic stem cell progenitors to terminally differentiated cells, as well as dormant or suppressed seeds and buds. While the physiology of each of these states differs considerably, each is referred to as 'cell cycle arrest' or 'G1 arrest'. Here the physiology and molecular regulation of (1) meristematic quiescence, (2) dormancy and (3) terminal differentiation (cell cycle exit) are considered in order to determine whether and how the molecular decisions guiding these nuclear states are distinct. A brief overview of the canonical cell cycle regulators is provided, and the genetic and genomic, as well as physiological, evidence is considered regarding two primary questions: (1) Are the canonical cell cycle regulators superior or subordinate in the regulation of quiescence? (2) Are these three modes of quiescence governed by distinct molecular controls? Meristematic quiescence, dormancy and terminal differentiation are each predominantly characterized by G1 arrest but regulated distinctly, at a level largely superior to the canonical cell cycle. Meristematic quiescence is intrinsically linked to non-cell-autonomous regulation of meristem cell identity, and particularly through the influence of ubiquitin-dependent proteolysis, in partnership with reactive oxygen species, abscisic acid and auxin. The regulation of terminal differentiation shares analogous features with meristematic quiescence, albeit with specific activators and a greater role for cytokinin signalling. Dormancy meanwhile appears to be regulated at the level of chromatin accessibility, by Polycomb group-type histone modifications of particular dormancy genes.

The far-red fluorescent protein mMaroon1 and a reporter based on stem-loop binding protein enables the generation of Fucci4, a 4-color cell cycle reporter system that can be used to distinguish all phases of the cell cycle. Also online, a paper by Laviv et al . uses mMaroon1 as a FRET acceptor for the newly developed CyRFP1. A robust method for simultaneous visualization of all four cell cycle phases in living cells is highly desirable. We developed an intensiometric reporter of the transition from S to G2 phase and engineered a far-red fluorescent protein, mMaroon1, to visualize chromatin condensation in mitosis. We combined these new reporters with the previously described Fucci system to create Fucci4, a set of four orthogonal fluorescent indicators that together resolve all cell cycle phases.

A mathematical method, known as ergodic rate analysis, has been developed and used to study the rates of molecular events from single time measurements of large populations of fixed cells; this new method is able to overcome some of the previous limitations with regards to studying cell-size control. Cell size through the cell cycle How cell size is maintained is a fundamental question in cell biology. In this study, Marc Kirschner and colleagues describe a new analytical method, called ergodic rate analysis (ERA), which measures the dynamics of cellular processes based on single-cell measurements in fixed steady-state populations. They use the method to monitor how proliferating cells constrain variation in cell size, and to calculate the rate of cell growth in relation to their position in the cell cycle. The results suggest that just before S phase, there is a sharp transition in the dependence of growth rate on cell size that acts to limit variation. Biologists have long been concerned about what constrains variation in cell size, but progress in this field has been slow and stymied by experimental limitations 1 . Here we describe a new method, ergodic rate analysis (ERA), that uses single-cell measurements of fixed steady-state populations to accurately infer the rates of molecular events, including rates of cell growth. ERA exploits the fact that the number of cells in a particular state is related to the average transit time through that state 2 . With this method, it is possible to calculate full time trajectories of any feature that can be labelled in fixed cells, for example levels of phosphoproteins or total cellular mass. Using ERA we find evidence for a size-discriminatory process at the G1/S transition that acts to decrease cell-to-cell size variation.

The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.

Cell growth and division must be coordinated to maintain a stable cell size, but how this coordination is implemented in multicellular tissues remains unclear. In unicellular eukaryotes, autonomous cell size control mechanisms couple cell growth and division with little extracellular input. However, in multicellular tissues we do not know if autonomous cell size control mechanisms operate the same way or whether cell growth and cell cycle progression are separately controlled by cell-extrinsic signals. Here, we address this question by tracking single epidermal stem cells growing in the mouse ear. We find that a cell-autonomous size control mechanism, dependent on the RB pathway, sets the timing of S phase entry based on the cell’s current size. Cell-extrinsic variations in the cellular microenvironment affect cell growth rates but not this autonomous coupling. Our work reassesses long-standing models of cell cycle regulation in complex animal tissues and identifies cell-autonomous size control as a critical mechanism regulating cell division. In vivo imaging reveals that vertebrate cells use RB pathway-dependent, cell-autonomous size control to trigger G1/S at a fixed size. Extrinsic cues alter growth rates but not the size threshold for S phase entry.

Hutchinson–Gilford progeria syndrome (HGPS) is a severe human premature aging disorder caused by a lamin A mutant named progerin. Death occurs at a mean age of 13 y from cardiovascular problems. Previous studies revealed loss of vascular smooth muscle cells (SMCs) in the media of large arteries in a patient with HGPS and two mouse models, suggesting a causal connection between the SMC loss and cardiovascular malfunction. However, the mechanisms of how progerin leads to massive SMC loss are unknown. In this study, using SMCs differentiated from HGPS induced pluripotent stem cells, we show that HGPS SMCs exhibit a profound proliferative defect, which is primarily caused by caspase-independent cell death. Importantly, progerin accumulation stimulates a powerful suppression of PARP1 and consequently triggers an activation of the error-prone nonhomologous end joining response. As a result, most HGPS SMCs exhibit prolonged mitosis and die of mitotic catastrophe. This study demonstrates a critical role of PARP1 in mediating SMC loss in patients with HGPS and elucidates a molecular pathway underlying the progressive SMC loss in progeria.