Developmental Regulation of Nucleolus Size during Drosophila Eye Differentiation

When cell cycle withdrawal accompanies terminal differentiation, biosynthesis and cellular growth are likely to change also. In this study, nucleolus size was monitored during cell fate specification in the Drosophila eye imaginal disc using fibrillarin antibody labeling. Nucleolus size is an indicator of ribosome biogenesis and can correlate with cellular growth rate. Nucleolar size was reduced significantly during cell fate specification and differentiation, predominantly as eye disc cells entered a cell cycle arrest that preceded cell fate specification. This reduction in nucleolus size required Dpp and Hh signaling. A transient enlargement of the nucleolus accompanied cell division in the Second Mitotic Wave. Nucleoli continued to diminish in postmitotic cells following fate specification. These results suggest that cellular growth is regulated early in the transition from proliferating progenitor cells to terminal cell fate specification, contemporary with regulation of the cell cycle, and requiring the same extracellular signals.     

Mother-daughter precursor cell fate transformation after Cdc2 down-regulation in the Drosophila bristle lineage

The Drosophila bristle lineage is an excellent system in which to study how cell cycle and fate determination are synchronized in invariant cell lineages. In this model, five different cells arise from a single precursor cell, pI, after four asymmetric cell divisions. Cell diversity is achieved by the asymmetric segregation of cell determinants, such as Numb and Neuralized (Neur), resulting in differential activation of the Notch (N) pathway. We show that down-regulation of Cdc2, by over-expressing Tribbles, Dwee1, and Dmyt1 (three negative regulators of Cdc2) or by using thermo-sensitive Cdc2 mutant flies, delayed pI mitosis, and altered the polarity and the number of subsequent cell divisions. These modifications were associated with a mother-daughter cell fate transformation as the pI cell acquired the identity of the secondary precursor cell, pIIb. This type of change in cell identity only occurred when the N signaling pathway was inactive since ectopic N signaling transformed pI to pIIa-progeny fate. These transformations in cell identity suggest that, although synchronized, cell cycle and fate determination are independent phenomena in the bristle lineage.     

Control of vulval cell division number in the nematode Oscheius/Dolichorhabditis sp. CEW1

Spatial patterning of vulval precursor cell fates is achieved through a different two-stage induction mechanism in the nematode Oscheius/Dolichorhabditis sp. CEW1 compared with Caenorhabditis elegans. We therefore performed a genetic screen for vulva mutants in Oscheius sp. CEW1. Most mutants display phenotypes unknown in C. elegans. Here we present the largest mutant category, which affects division number of the vulva precursors P(4-8).p without changing their fate. Among these mutations, some reduce the number of divisions of P4.p and P8.p specifically. Two mutants omit the second cell cycle of all vulval lineages. A large subset of mutants undergo additional rounds of vulval divisions. We also found precocious and retarded heterochronic mutants. Whereas the C. elegans vulval lineage mutants can be interpreted as overall (homeotic) changes in precursor cell fates with concomitant cell cycle changes, the mutants described in Oscheius sp. CEW1 do not affect overall precursor fate and thereby dissociate the genetic mechanisms controlling vulval cell cycle and fate. Laser ablation experiments in these mutants reveal that the two first vulval divisions in Oscheius sp. CEW1 appear to be redundantly controlled by a gonad-independent mechanism and by a gonadal signal that operates partially independently of vulval fate induction.     

Control of cell cycle timing during C. elegans embryogenesis

As a fundamental process of development, cell proliferation must be coordinated with other processes such as fate differentiation. Through statistical analysis of individual cell cycle lengths of the first 8 out of 10 rounds of embryonic cell division in Caenorhabditis elegans, we identified synchronous and invariantly ordered divisions that are tightly associated with fate differentiation. Our results suggest a three-tier model for fate control of cell cycle pace: the primary control of cell cycle pace is established by lineage and the founder cell fate, then fine-tuned by tissue and organ differentiation within each lineage, then further modified by individualization of cells as they acquire unique morphological and physiological roles in the variant body plan. We then set out to identify the pace-setting mechanisms in different fates. Our results suggest that ubiquitin-mediated degradation of CDC-25.1 is a rate-determining step for the E (gut) and P₃ (muscle and germline) lineages but not others, even though CDC-25.1 and its apparent decay have been detected in all lineages. Our results demonstrate the power of C. elegans embryogenesis as a model to dissect the interaction between differentiation and proliferation, and an effective approach combining genetic and statistical analysis at single-cell resolution.     

Cyclin E and CDK-2 regulate proliferative cell fate and cell cycle progression in the C. elegans germline

The C. elegans germline provides an excellent model for analyzing the regulation of stem cell activity and the decision to differentiate and undergo meiotic development. The distal end of the adult hermaphrodite germline contains the proliferative zone, which includes a population of mitotically cycling cells and cells in meiotic S phase, followed by entry into meiotic prophase. The proliferative fate is specified by somatic distal tip cell (DTC) niche-germline GLP-1 Notch signaling through repression of the redundant GLD-1 and GLD-2 pathways that promote entry into meiosis. Here, we describe characteristics of the proliferative zone, including cell cycle kinetics and population dynamics, as well as the role of specific cell cycle factors in both cell cycle progression and the decision between the proliferative and meiotic cell fate. Mitotic cell cycle progression occurs rapidly, continuously, with little or no time spent in G1, and with cyclin E (CYE-1) levels and activity high throughout the cell cycle. In addition to driving mitotic cell cycle progression, CYE-1 and CDK-2 also play an important role in proliferative fate specification. Genetic analysis indicates that CYE-1/CDK-2 promotes the proliferative fate downstream or in parallel to the GLD-1 and GLD-2 pathways, and is important under conditions of reduced GLP-1 signaling, possibly corresponding to mitotically cycling proliferative zone cells that are displaced from the DTC niche. Furthermore, we find that GLP-1 signaling regulates a third pathway, in addition to the GLD-1 and GLD-2 pathways and also independent of CYE-1/CDK-2, to promote the proliferative fate/inhibit meiotic entry.     

When timing is everything: role of cell cycle regulation in asymmetric division

Asymmetric division is a fundamental mechanism of generating cell diversity during development. One of its hallmarks is asymmetric localization during mitosis of proteins that specify daughter cell fate. Studies in Drosophila show that subcellular localization of many proteins required for asymmetric division of neuronal progenitors correlates with progression through mitosis. Yet, how cell cycle and asymmetric division machineries cooperate remains unclear. Recent data show that (1) key cell cycle regulators are required for asymmetric localization of cell fate determinants and for cell fate determination and (2) molecules that mediate asymmetric division can also act to modulate proliferation potential of progenitor cells.     

Cell cycle-dependent sequencing of cell fate decisions in Caenorhabditis elegans vulva precursor cells

In Caenorhabditis elegans, the fates of the six multipotent vulva precursor cells (VPCs) are specified by extracellular signals. One VPC expresses the primary (1 degrees ) fate in response to a Ras-mediated inductive signal from the gonad. The two VPCs flanking the 1 degrees cell each express secondary (2 degrees ) fates in response to lin-12-mediated lateral signaling. The remaining three VPCs each adopt the non-vulval tertiary (3 degrees ) fate. Here I describe experiments examining how the selection of these vulval fates is affected by cell cycle arrest and cell cycle-restricted lin-12 activity. The results suggest that lin-12 participates in two developmental decisions separable by cell cycle phase: lin-12 must act prior to the end of VPC S phase to influence a 1 degrees versus 2 degrees cell fate choice, but must act after VPC S phase to influence a 3 degrees versus 2 degrees cell fate choice. Coupling developmental decisions to cell cycle transitions may provide a mechanism for prioritizing or ordering choices of cell fates for multipotential cells.     

Chromatin assembly factor 1 regulates the cell cycle but not cell fate during male gametogenesis in Arabidopsis thaliana

The interdependence of cell cycle control, chromatin remodeling and cell fate determination remains unclear in flowering plants. Pollen development provides an interesting model, as it comprises only two cell types produced by two sequential cell divisions. The first division separates the vegetative cell from the generative cell. The generative cell divides and produces the two sperm cells, transported to the female gametes by the pollen tube produced by the vegetative cell. We show in Arabidopsis thaliana that loss of activity of the Chromatin assembly factor 1 (CAF1) pathway causes delay and arrest of the cell cycle during pollen development. Prevention of the second pollen mitosis generates a fraction of CAF1-deficient pollen grains comprising a vegetative cell and a single sperm cell, which both express correctly cell fate markers. The single sperm is functional and fertilizes indiscriminately either female gamete. Our results thus suggest that pollen cell fate is independent from cell cycle regulation.     

Distinct mechanisms triggering glial differentiation in Drosophila thoracic and abdominal neuroblasts 6-4

Neurons and glia are produced in stereotyped patterns after neuroblast cell division during development of the Drosophila central nervous system. The first cell division of thoracic neuroblast 6-4 (NB6-4T) is asymmetric, giving rise to a glial precursor cell and a neuronal precursor cell. In contrast, abdominal NB6-4 (NB6-4A) divides symmetrically to produce two glial cells. To understand the relationship between cell division and glia-neuron cell fate determination, we examined and compared the effects of known cell division mutations on the NB6-4T and NB6-4A lineages. Based on observation of expression of glial fate determination and early glial differentiation genes, the onset of glial differentiation occurred in NB6-4A but not in NB6-4T when both cell cycle progression and cytokinesis were genetically arrested. On the other hand, glial differentiation started in both lineages when cytokinesis was blocked with intact cell cycle progression. These results showed that NB6-4T, but not NB6-4A, requires cell cycle progression for acquisition of glial fate, suggesting that distinct mechanisms trigger glial differentiation in the different lineages.     

神经发育中的细胞周期时程

哺乳动物的神经发育经历一系列神经前体细胞的形态结构和机能改变,其细胞周期时程也呈现动态变化,从神经发生早期至后期,神经前体细胞的细胞周期时程逐渐延长,并与细胞发育命运转归有关,其调节因素包括周期蛋白复合体、Notch信号通路、原神经基因靶向蛋白、微管与分子马达蛋白等。细胞周期长度假说认为,细胞周期的长度影响到命运决定子的积累,因而决定细胞的命运。文章综述了相关的研究进展。     

Mathematical modeling of cell cycle regulation in response to DNA damage: exploring mechanisms of cell-fate determination

After DNA damage, cells activate p53, a tumor suppressor gene, and select a cell fate (e.g., DNA repair, cell cycle arrest, or apoptosis). Recently, a p53 oscillatory behavior was observed following DNA damage. However, the relationship between this p53 oscillation and cell-fate selection is unclear. Here, we present a novel model of the DNA damage signaling pathway that includes p53 and whole cell cycle regulation and explore the relationship between p53 oscillation and cell fate selection. The simulation run without DNA damage qualitatively realized experimentally observed data from several cell cycle regulators, indicating that our model was biologically appropriate. Moreover, the comprehensive sensitivity analysis for the proposed model was implemented by changing the values of all kinetic parameters, which revealed that the cell cycle regulation system based on the proposed model has robustness on a fluctuation of reaction rate in each process. Simulations run with four different intensities of DNA damage, i.e. Low-damage, Medium-damage, High-damage, and Excess-damage, realized cell cycle arrest in all cases. Low-damage, Medium-damage, High-damage, and Excess-damage corresponded to the DNA damage caused by 100, 200, 400, and 800 J/m² doses of UV-irradiation, respectively, based on expression of p21, which plays a crucial role in cell cycle arrest. In simulations run with High-damage and Excess-damage, the length of the cell cycle arrest was shortened despite the severe DNA damage, and p53 began to oscillate. Cells initiated apoptosis and were killed at 400 and 800 J/m² doses of UV-irradiation, corresponding to High-damage and Excess-damage, respectively. Therefore, our model indicated that the oscillatory mode of p53 profoundly affects cell fate selection.     

Transitions between epithelial and mesenchymal states during cell fate conversions

Cell fate conversion is considered as the changing of one type of cells to another type including somatic cell reprogramming (de-differentiation), differentiation, and trans-differentiation. Epithelial and mesenchymal cells are two major types of cells and the transitions between these two cell states as epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) have been observed during multiple cell fate conversions including embryonic development, tumor progression and somatic cell reprogramming. In addition, MET and sequential EMT-MET during the generation of induced pluripotent stem cells (iPSC) from fibroblasts have been reported recently. Such observation is consistent with multiple rounds of sequential EMT-MET during embryonic development which could be considered as a reversed process of reprogramming at least partially. Therefore in current review, we briefly discussed the potential roles played by EMT, MET, or even sequential EMT-MET during different kinds of cell fate conversions. We also provided some preliminary hypotheses on the mechanisms that connect cell state transitions and cell fate conversions based on results collected from cell cycle, epigenetic regulation, and stemness acquisition.     

Msx2 alters the timing of retinal ganglion cells fate commitment and differentiation

Timing of cell fate commitment determines distinct retinal cell types, which is believed to be controlled by a tightly coordinated regulatory program of proliferation, cell cycle exit and differentiation. Although homeobox protein Msx2 could induce apoptosis of optic vesicle, it is unclear whether Msx2 regulates differentiation and cell fate commitment of retinal progenitor cells (RPCs) to retinal ganglion cells (RGCs). In this study, we show that overexpression of Msx2 transiently suppressed the expression of Cyclin D1 and blocked cell proliferation. Meanwhile, overexpression of Msx2 delayed the expression of RGC-specific differentiation markers (Math5 and Brn3b), which showed that Msx2 could affect the timing of RGCs fate commitment and differentiation by delaying the timing of cell cycle exit of retinal progenitors. These results indicate Msx2 possesses dual regulatory functions in controlling cell cycle progression of retinal RPCs and timing of RGCs differentiation.     

Metabolic remodeling: a pyruvate transport affair

Metabolic remodeling is a major determinant for many cell fate decisions, and a switch from respiration to aerobic glycolysis is generally considered as a hallmark of cancer cell transformation. Pyruvate is a key metabolite at the major junction of carbohydrate metabolism between cytosolic glycolysis and the mitochondrial Krebs cycle. In this issue of The EMBO Journal, Bender et al show that yeast cells regulate pyruvate uptake into mitochondria, and thus its metabolic fate, by expressing alternative pyruvate carrier complexes with different activities.