Tribbles, a cell-cycle brake that coordinates proliferation and morphogenesis during Drosophila gastrulation

BACKGROUND: The final shape and size of an organism is determined by both morphogenetic processes and cell proliferation and it is essential that these processes be properly coordinated. In particular, cell division is incompatible with certain types of morphogenetic cell behaviour, such as migration, adhesion and changes in cell shape. Mechanisms must therefore exist to ensure that one does not interfere with the other. RESULTS: We address here the coordination of proliferation and morphogenesis during the development of the mesoderm in Drosophila. We show that it is essential that mitosis be blocked in the mesoderm during early gastrulation, and identify the putative serine/threonine kinase Tribbles as controlling this block. In its absence, the mitotic block is lifted, resulting in severe defects during early gastrulation. Tribbles, a homologue of a group of vertebrate proteins of unknown function, acts in concert with another, as yet unidentified, factor to counteract the activity of the protein phosphatase Cdc25/String. CONCLUSIONS: In a finely tuned balance with Cdc25/String, Tribbles controls the timing of mitosis in the prospective mesoderm, allowing cell-shape changes to be completed. This mechanism for coordinating cell division and cell-shape changes may have helped Drosophila to evolve its mode of rapid early development.     

Tribbles coordinates mitosis and morphogenesis in Drosophila by regulating string/CDC25 proteolysis

Morphogenesis and cell differentiation in multicellular organisms often require accurate control of cell divisions. We show that a novel cell cycle regulator, tribbles, is critical for this control during Drosophila development. During oogenesis, the level of tribbles affects the number of germ cell divisions as well as oocyte determination. The mesoderm anlage enters mitosis prematurely in tribbles mutant embryos, leading to gastrulation defects. We show that Tribbles acts by specifically inducing degradation of the CDC25 mitotic activators String and Twine via the proteosome pathway. By regulating CDC25, Tribbles serves to coordinate entry into mitosis with morphogenesis and cell fate determination.     

Tribbles: a family of kinase-like proteins with potent signalling regulatory function

The recent identification of tribbles as regulators of signal processing systems and physiological processes, including development, together with their potential involvement in diabetes and cancer, has generated considerable interest in these proteins. Tribbles have been reported to regulate activation of a number of intracellular signalling pathways with roles extending from mitosis and cell activation to apoptosis and modulation of gene expression. The current review summarises our current understanding of interactions between tribbles and various other proteins. Since our understanding on the molecular basis of tribbles function is far from complete, we also describe a bioinformatic analysis of various segments of tribbles proteins, which has revealed a number of highly conserved peptide motifs with potentially important functional roles.     

MAPK对高等植物细胞分裂和生长的调节

综述了MAPKs参与植物细胞周期调控的最新进展 ,植物激素与MAPK ,MAPK与植物细胞有丝分裂 ,以及MAPK与植物细胞分裂和生长的调控等三方面 ,阐述MAPK参与调控高等植物细胞分化和有丝分裂的机制。     

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.     

Chalones and the Control of Normal, Regenerative, and Neoplastic Growth of the Skin

SYNOPSIS Chalones,inhibitors of cell dmsion have been isolatedand studied from a number of mammalian tissues, most notably,the epidermis The epidermal rhalone is a glycoprotein It exhibitsconsiderable, but not complete specificity The epidermal chalone decreases mitotic activity by inhibitingcells in the G 2 phase of the cell cycle from entering mitosis,and probably also by inhibiting ceils in the G 1 phase of thecell cycle from entering mitosis To inhibit cells in G 2 fromentering mitosis the chilone requnes adrenalin, and for maximalactivity hydrocortisone It is not known if idrenalin and hydrocortisoneare required for chalone inhibition of cells in G 1 In addition to inhibiting cell division in normal epidermalcells the epidermal chalone can inhibit cell division in regeneratingepidermal cells induced to proliferate by chemical damage Thephase of the cell cycle in which the chalone inhibits legeneratingepidermal cells from entering mitosis is not known Epidermal tumors contain a decreased amount of chalone Mitosisin epidermal tumors is inhibited by treatment with epidermalchalone Tumor cells are inhibitedfrom entering mitosis fromeither the G 1 or G 2 phases of the cell cycle Chalones are said to inhibit mitosis by a negative feedbackmechanism However, experiments which presumably result in adecrease in chalone concentration do not result in an increasein mitotic activity It is suggested that if chalones are physiological controllers of cell division they do not act by a simplenegative feedback mechanism but require the action of a substanceto decrease their concentration     

Contact regulation of cell division in an epithelial-like cell line

The rate of cell division in an epithelial-like cell line, 1S1, was examined by time-lapse cinemicrography. When precautions were taken to insure a sufficient nutrient supply, the number of mitoses per unit time in any given area of a confluent monolayer remained constant. This “contact regulation of cell division” resulted in a steadily decreasing frequency of mitosis per cell as the culture became crowded. With the decrease was associated a gradual change in cell shape, from maximally flattened to maximally compact, due to contact inhibition of the movement of cells across one another. When cells were removed along a line scraped on a dense culture, the cells at the edge of the scrape flattened, migrated into the vacant area, and subsequently increased their frequency of mitosis to that characteristic of non-confluent cells. Inhibition of mitosis caused by a limitation on the nutrient supply was also reversed at a line-scrape. These observations suggest that cell flattening promoted mitosis by causing the cell membrane to expand, thereby facilitating the uptake of nutrients. The cell membrane would thus function in the mechanism of contact regulation as a transducer, for converting the pressure of the surrounding cell population into a restraining force upon the metabolism of cell division.     

Activating the DNA damage checkpoint in a developmental context

BACKGROUND: Studies in unicellular systems have established that DNA damage by irradiation invokes a checkpoint that acts to stall cell division. During metazoan development, the modulation of cell division by checkpoints must occur in the context of gastrulation, differential gene expression and changes in cell cycle regulation. To understand the effects of checkpoint activation in a developmental context, we examined the effect of X-rays on post-blastoderm embryos of Drosophila melanogaster. RESULTS: In Drosophila, DNA damage was previously found to delay anaphase chromosome separation during cleavage cycles that lack a G2 phase. In post-blastoderm cycles that included a G2 phase, we found that irradiation delayed the entry into mitosis. Gastrulation and the developmental program of string (Cdc25) gene expression, which normally regulates the timing of mitosis, occurred normally after irradiation. The radiation-induced delay of mitosis accompanied the exclusion of mitotic cyclins from the nucleus. Furthermore, a mutant form of the mitotic kinase Cdk1 that cannot be inhibited by phosphorylation drove a mitotic cyclin into the nucleus and overcame the delay of mitosis induced by irradiation. CONCLUSIONS: Developmental changes in the cell cycle, for example, the introduction of a G2 phase, dictate the response to checkpoint activation, for example, delaying mitosis instead of or in addition to delaying anaphase. This unprecedented finding suggests that different mechanisms are used at different points during metazoan development to stall cell division in response to checkpoint activation. The delay of mitosis in post-blastoderm embryos is due primarily to inhibitory phosphorylation of Cdk1, whereas nuclear exclusion of a cyclin-Cdk1 complex might play a secondary role. Delaying cell division has little effect on gastrulation and developmentally regulated string gene expression, supporting the view that development generally dictates cell proliferation and not vice versa.     

Cell wall regeneration and cell division in isolated tobacco mesophyll protoplasts

Summary Protoplasts were isolated from palisade tissue of tobacco leaves by treatment with pectinase and cellulase under aseptic conditions, and were cultured in a synthetic liquid medium. Calcofluor, a fluorescent brightener, was found to be an excellent stain for plant cell walls and was used to demonstrate regeneration of cell walls in these protoplasts. The cultured protoplasts regenerated cell walls by the 3rd day of culture, giving rise to spherical cells. The majority of the protoplasts regenerating cell walls underwent mitosis and cell division. The cycle of mitosis and cell division was repeated 2–3 times during 2 weeks of culture. Some of the nutritional conditions affecting division in the cultured protoplasts were studied.     

Effects of propranolol on mitosis-inhibiting activity of G2-chalone and adrenaline in cell culture of Ehrlich's ascites tumor]

Chalone-containing preparation from ascite Ehrlich's tumour blocks these cell transition from G2-phase to mitosis and its motion in mitosis in vitro (G2-block, M-block). Adrenaline blocks these cell transition from G2-phase to mitosis. Propranolol raises G2-block of chalone or adrenaline. Consequently this way of chalones action on cell division includes beta-adrenergic receptors influence of preparation on cell motion in mitosis doesn't change with addition of propranolol. Consequently this way of chalone system action on mitosis doesn't include beta-adrenergic receptors.     

Fission yeast cells undergo nuclear division in the absence of spindle microtubules

Mitosis in eukaryotic cells employs spindle microtubules to drive accurate chromosome segregation at cell division. Cells lacking spindle microtubules arrest in mitosis due to a spindle checkpoint that delays mitotic progression until all chromosomes have achieved stable bipolar attachment to spindle microtubules. In fission yeast, mitosis occurs within an intact nuclear membrane with the mitotic spindle elongating between the spindle pole bodies. We show here that in fission yeast interference with mitotic spindle formation delays mitosis only briefly and cells proceed to an unusual nuclear division process we term nuclear fission, during which cells perform some chromosome segregation and efficiently enter S-phase of the next cell cycle. Nuclear fission is blocked if spindle pole body maturation or sister chromatid separation cannot take place or if actin polymerization is inhibited. We suggest that this process exhibits vestiges of a primitive nuclear division process independent of spindle microtubules, possibly reflecting an evolutionary intermediate state between bacterial and Archeal chromosome segregation where the nucleoid divides without a spindle and a microtubule spindle-based eukaryotic mitosis.     

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.     

Direct Evidence for Cytodifferentiation to Tracheary Elements without Intervening Mitosis in a Culture of Single Cells Isolated from the Mesophyll of Zinnia elegans

A serial observation of the process of tracheary element differentiation from single cells isolated from the mesophyll of Zinnia elegans L. cv. Canary bird provided the first direct evidence for the cytodifferentiation without intervening mitosis. Percentage of the tracheary elements formed without cell division was about 60% of total tracheary elements formed on the 4th day of culture. The number of tracheary elements formed without intervening mitosis was not reduced in the presence of colchicine at the concentrations blocking cell division. These facts clearly indicate that cell division is not a prerequisite for tracheary element differentiation in this system.     

Remote changes in the course of mitosis in synchronized cultures of Chinese hamster cells following inhibition of RNA and protein synthesis

The effect of inhibition of the synthesis of some types of RNA and proteins was determined in the synchronized culture of Chinese hamster cells at various stages of the interphase on the course of remote mitosis. Analysis of MI and some forms of pathological cell division showed that the action of different doses of AMD and pyromycin during the first part of the interphase provoked an identical effect--C-mitosis at the immediate and remote waves of cell division. Suppression of the synthesis of whole cell RNA and proteins during the second half of the interphase was accompanied by a metaphase cell delay with scattering chromosomes, this indicating derangement of the synthesis of the division spindle component. It is suggested that proteins (tubulins) and RNA participate in the organization of the division spindle during remote mitosis as a "reserve pool".     

Asymmetric cell division and neoplastic growth

Asymmetric division was formed as an evolutionary conserved mechanism of self-maintenance of cellular populations and creation of a variety of cell types during ontogenesis. Asymmetric division enables a special mechanism of determinant segregation, which further defines development of daughter cells. As a result two unequal cells are developed. Recent research demonstrates the interplay of disturbed asymmetric division of stem cells and tumorigenesis. Genes implicated in cell’s polarization and normal progression of asymmetric mitosis were identified in Drosophila. Other genes regulating asymmetric mitosis were described as tumor suppressors, and their mutations were shown to initiate neoplastic growth. Comparative study of gene expression suggests that the disturbance of asymmetric division might be one of the reasons for neoplasm progression in vertebrates.     

Timing of micronuclear mitosis and its relation to commitment to division inParamecium tetraurelia

This study examines the timing of micronuclear mitosis during the vegetative cell cycle and shows that mitosis begins early in the division process and coincides approximately with the earliest stages of oral morphogenesis (about 0.6 in the cell cycle in synchronous cell samples). The cc1 mutation blocks cell cycle progression prior to the point of commitment to division. Although the cc1 mutation blocks macronuclear DNA synthesis under restrictive conditions, it does not block micronuclear DNA synthesis. However, absence of functional cc1 gene product leads to blockage of micronuclear mitosis prior to completion of anaphase. This point coincides with commitment to division and is also the point at which oral morphogenesis is blocked in cc1 cells. The tim-ings of the transition points for micronuclear mitosis and oral morphogenesis in cc1 cells are closely associated in both synchronous cell samples and in asynchronous cultures. © 1992 Wiley-Liss, Inc.     

Teilungsverhalten der Chromatophoren in bezug auf die Mitose während des Lebenszyklus vonDiatoma hiemale var.mesodon

The vegetative life cycle ofDiatoma hiemale var.mesodon (Ehr.)Grun. living in a spring has been studied under natural conditions. In the beginning the cells have a constant number of 8 chromatophores which are divided into 16 during cell growth. Chloroplast division is finished before nuclear division starts. The young daughter cells have again 8 chromatophores. In the course of cell division a plastic remodelling of the chromatophores and a simplifying of their shape occurs. Besides single cells also populations have been studied to follow the temporal progress of chromatophore division, mitosis and cell growth. The results are evaluated by indices and demonstrated by a diagram. The maximum of chromatophore divisions preceds the maximum of mitoses by several hours, while the cell growth is in correlation with the chromatophore division. Minima of the other parameters were found before mitosis is starting and after it is finished. Our results are discussed with regard to the semiautonomy of the plastids. From the morphological point of view this concept is supported by the mode of division and by the anticipation of the chromatophore division. The number of chromatophores at the beginning (8) and at the end (16) of the life cycle is constant. The life cycle is classified into stages of cell growth, chromatophore division, stagnation, mitosis and differentiation of the daughter cells.     

Cyclin-like accumulation and loss of the putative kinetochore motor CENP-E results from coupling continuous synthesis with specific degradation at the end of mitosis

CENP-E is a kinesin-like protein that binds to kinetochores through the early stages of mitosis, but after initiation of anaphase, it relocalizes to the overlapping microtubules in the midzone, ultimately concentration in the developing midbody. By immunoblotting of cells separated at various positions in the cell cycle using centrifugal elutriation, we show that CENP-E levels increase progressively across the cycle peaking at approximately 22,000 molecules/cell early in mitosis, followed by an abrupt (> 10 fold) loss at the end of mitosis. Pulse-labeling with [35S]methionine reveals that beyond a twofold increase in synthesis between G1 and G2, interphase accumulation results primarily from stabilization of CENP-E during S and G2. Despite localizing in the midbody during normal cell division, CENP-E loss at the end of mitosis is independent of cytokinesis, since complete blockage of division with cytochalasin has no affect on CENP-E loss at the M/G1 transition. Thus, like mitotic cyclins, CENP-E accumulation peaks before cell division, and it is specifically degraded at the end of mitosis. However, CENP-E degradation kinetically follows proteolysis of cyclin B in anaphase. Combined with cyclin A destruction before the end of metaphase, degradation of as yet unidentified components at the metaphase/anaphase transition, and cyclin B degradation at or after the anaphase transition, CENP-E destruction defines a fourth point in a mitotic cascade of timed proteolysis.     

Move,Stop, Learn: Illustrating Mitosis through Stop-Motion Animation

Learning about microscopic things, such as cells, can often be mundane to students because they are not able to see or manipulate what they are learning about. Students often recall learning about cell division through memorization—thus they find it tedious and dull. Few opportunities exist that allow students to explore and manipulate cells or the process of cellular division. This activity attempts to combat the monotony that is often associated with learning mitosis by engaging students in creating a stop-motion animation video using iPads. Our students found the activity fun, enjoyed it more than learning mitosis through traditional methods, and believed making the video helped them understand cell division.