Cell cycle and sporulation in Bacillus subtilis

Recent work on cell division and chromosome orientation and partitioning in Bacillus subtilis has provided insights into cell cycle regulation during growth and development. The cell cycle is an integral part of development and entrance into sporulation is modulated by signals that transmit the status of DNA integrity, chromosome replication and segregation. In addition, B. subtilis modifies cell division and DNA segregation to establish cell-type-specific gene expression during sporulation.     

A conservative pathway for coordination of cell wall biosynthesis and cell cycle progression in plants

The mechanism that coordinates cell growth and cell cycle progression remains poorly understood; in particular, whether the cell cycle and cell wall biosynthesis are coordinated remains unclear. Recently, cell wall biosynthesis and cell cycle progression were reported to respond to wounding. Nonetheless, no genes are reported to synchronize the biosynthesis of the cell wall and the cell cycle. Here, we report that wounding induces the expression of genes associated with cell wall biosynthesis and the cell cycle, and that two genes, AtMYB46 in Arabidopsis thaliana and RrMYB18 in Rosa rugosa, are induced by wounding. We found that AtMYB46 and RrMYB18 promote the biosynthesis of the cell wall by upregulating the expression of cell wall-associated genes, and that both of them also upregulate the expression of a battery of genes associated with cell cycle progression. Ultimately, this response leads to the development of curled leaves of reduced size. We also found that the coordination of cell wall biosynthesis and cell cycle progression by AtMYB46 and RrMYB18 is evolutionarily conservative in multiple species. In accordance with wounding promoting cell regeneration by regulating the cell cycle, these findings also provide novel insight into the coordination between cell growth and cell cycle progression and a method for producing miniature plants.     

Cell cycle analysis of fetal germ cells during sex differentiation in mice

Background information. Primordial germ cells in developing male and female gonads are responsive to somatic cell cues that direct their sex‐specific differentiation into functional gametes. The first divergence of the male and female pathways is a change in cell cycle state observed from 12.5 dpc (days post coitum) in mice. At this time XY and XX germ cells cease mitotic division and enter G₁/G₀ arrest and meiosis prophase I respectively. Aberrant cell cycle regulation at this time can lead to disrupted ovarian development, germ cell apoptosis, reduced fertility and/or the formation of germ cell tumours. Results. In order to unravel the mechanisms utilized by germ cells to achieve and maintain the correct cell cycle states, we analysed the expression of a large number of cell cycle genes in purified germ cells across the crucial time of sex differentiation. Our results revealed common signalling for both XX and XY germ cell survival involving calcium signalling. A robust mechanism for apoptosis and checkpoint control was observed in XY germ cells, characterized by p53 and Atm (ataxia telangiectasia mutated) expression. Additionally, a member of the retinoblastoma family and p21 were identified, linking these factors to XY germ cell G₁/G₀ arrest. Lastly, in XX germ cells we observed a down‐regulation of genes involved in both G₁‐ and G₂‐phases of the cell cycle consistent with their entry into meiosis. Conclusion. The present study has provided a detailed analysis of cell cycle gene expression during fetal germ cell development and identified candidate factors warranting further investigation in order to understand cases of aberrant cell cycle control in these specialized cells.     

Number of dictyosomes in a single cell of the green algaMicrasterias

The numbers of dictyosomes in cells ofM. crux-melitensis andM. pinnatifida were counted at various stages in the cell cycle. Dictyosomes synchronously doubled in number by dividing at the premitotic stage and then were separated into two groups by the septum, thus reducing the dictyosomal number to the ordinal number in each new cell. The number remained the same throughout the cell cycle until the next premitotic stage.     

Building a plasmodium: Development in the acellular slime mould Physarum polycephalum

The two vegetative cell types of the acellular slime mould Physarum polycephalum - amoebae and plasmodia - differ greatly in cellular organisation and behaviour as a result of differences in gene expression. The development of uninucleate amoebae into multinucleate, syncytial plasmodia is under the control of the mating-type locus matA, which is a complex, multi-functional locus. A key period during plasmodium development is the extended cell cycle, which occurs in the developing uninucleate cell. During this long cell cycle, many of the changes in cellular organisation that accompany development into the multinucleate stage are initiated including, for example, alterations in microtubule organisation. Genes have been identified that show cell-type specific expression in either amoebae or plasmodia and many of these genes alter their pattern of expression during the extended cell cycle. With the introduction of a DNA transformation system for P. polycephalum, it is now possible to investigate the functions of genes in the vegetative cell types and their roles in the cellular reorganisations accompanying development.     

cAMP-mediated increase in the critical cell size required for the G1 to S transition in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, cyclic AMP is required for cellular growth. In this study we show that cAMP also specifically inhibits the G1-S transition of the S. cerevisiae cell cycle by increasing the critical cell size required at start, the major yeast cell cycle control step. In fact: (a) addition of cAMP delays the time of entering into the S budded phase of small G1 cells, while it is ineffective on large fast-growing cells. (b) If cell growth is strongly depressed, cAMP permanently inhibits cell cycle commitment of cells arrested at the α-factor-sensitive step. The cell fraction inhibited by cAMP is inversely correlated with the average cell size of treated populations. (c) The critical protein content (P_s) and the critical cell volume (V_B) required for budding in unperturbed exponentially growing yeast populations are largely increased by cAMP. On these bases, we propose a new cAMP role at start.     

The role of the cell cycle in differentiation of the cellular slime mould Dictyostelium discoideum

Summary During development and differentiation of the cellular slime mould Dictyostelium discoideum there appears to be a relationship between the cell cycle and cell fate: amoebae halted in G2 phase during early development differentiate into spores whereas stalk cells are formed from amoebae halted in GI phase. It is proposed that this is because a major effect of the cell cycle is to generate heterogeneity in the cell surface properties of the developing amoebae.     

Cauliflower mosaic virus 35 S promoter directs S phase specific expression in plant cells

Summary An experimental system to study cell cycle specific gene expression in plant cells was developed using protoplasts from tobacco cells synchronized by aphidicolin treatment. Chimeric plasmids consisting either of the chloramphenicol acetyltransferase (CAT) gene downstream of the cauliflower mosaic virus (CaMV) 35 S promoter or the nopaline synthase (nos) promoter were introduced into synchronized protoplasts of four cell cycle stages by electroporation. In the case of the CaMV 35 S promoter cyclic oscillation of CAT activity was observed which paralleled the cell cycle of the recipient cells. The peak of CAT activity was found in the S phase, while no such cyclic change was observed in the case of the nos promoter. This system clearly shows that it is feasible to search for a cell cycle specific promoter. The significance of these observations is discussed in relation to the study of plant cells.     

Regulation of Cell Division in Arabidopsis

The study of cell cycle control in plants is expected to contribute to the understanding of plants' unique developmental features. The principal regulators of the eukaryotic cell cycle, namely, cyclin-dependent kinases (CDKs) and cyclins, are also conserved in plants. This review is concerned with our present knowledge on cell cycle regulation in Arabidopsis thaliana, which is widely accepted as a model plant for the study of a broad range of biological questions. Up to the present, 2 CDKs and 11 cyclins have been identified in Arabidopsis. While the expression of one of these CDKs has been found to be positively correlated with the competence of cells to divide, cyc1A1 expression of the cyclin has been almost exclusively confined to dividing cells. Although much remains to be studied concerning upstream regulators of these genes, the successful introduction of mutant CDKs into plants demonstrates the potential of using such an approach to intentionally modulate the plant cell cycle and development.     

Two Geminin homologs regulate DNA replication in silkworm,Bombyx mori

DNA replication is rigorously controlled in cells to ensure that the genome duplicates exactly once per cell cycle. Geminin is a small nucleoprotein, which prevents DNA rereplication by directly binding to and inhibiting the DNA replication licensing factor, Cdt1. In this study, we have identified 2 Geminin genes, BmGeminin1 and BmGeminn2, in silkworm, Bombyx mori. These genes contain the Geminin conserved coiled-coil domain and are periodically localized in the nucleus during the S-G2 phase but are degraded at anaphase in mitosis. Both BmGeminin1 and BmGeminin2 are able to homodimerize and interact with BmCdt1 in cells. In addition, BmGeminin1 and BmGeminin2 can interact with each other. Overexpression of BmGeminin1 affects cell cycle progression: cell cycle is arrested in S phase, and RNA interference of BmGeminin1 leads to rereplication. In contrast, overexpression or knockdown of BmGeminin2 with RNAi did not significantly affect cell cycle, while more rereplication occurred when BmGeminin1 and BmGeminin2 together were knocked down in cells than when only BmGeminin1 was knocked down. These data suggest that both BmGeminin1 and BmGeminin2 are involved in the regulation of DNA replication. These findings provide insight into the function of Geminin and contribute to our understanding of the regulation mechanism of cell cycle in silkworm.     

Geraniol inhibits prostate cancer growth by targeting cell cycle and apoptosis pathways

The progression of prostate cancer is associated with escape from cell cycle arrest and apoptosis under androgen-depleted conditions. Here, we found that geraniol, a naturally occurring monoterpene, induces cell cycle arrest and apoptosis in cultured cells and tumor grafted mice using PC-3 prostate cancer cells. Geraniol modulated the expression of various cell cycle regulators and Bcl-2 family proteins in PC-3 cells in vitro and in vivo. Furthermore, we showed that the combination of sub-optimal doses of geraniol and docetaxel noticeably suppresses prostate cancer growth in cultured cells and tumor xenograft mice. Therefore, our findings provide insight into unraveling the mechanisms underlying escape from cell cycle arrest and apoptosis and developing therapeutic strategies against prostate cancer.     

Progranulin regulates neurogenesis in the developing vertebrate retina

We evaluated the expression and function of the microglia‐specific growth factor, Progranulin‐a (Pgrn‐a) during developmental neurogenesis in the embryonic retina of zebrafish. At 24 hpf pgrn‐a is expressed throughout the forebrain, but by 48 hpf pgrn‐a is exclusively expressed by microglia and/or microglial precursors within the brain and retina. Knockdown of Pgrn‐a does not alter the onset of neurogenic programs or increase cell death, however, in its absence, neurogenesis is significantly delayed—retinal progenitors fail to exit the cell cycle at the appropriate developmental time and postmitotic cells do not acquire markers of terminal differentiation, and microglial precursors do not colonize the retina. Given the link between Progranulin and cell cycle regulation in peripheral tissues and transformed cells, we analyzed cell cycle kinetics among retinal progenitors following Pgrn‐a knockdown. Depleting Pgrn‐a results in a significant lengthening of the cell cycle. These data suggest that Pgrn‐a plays a dual role during nervous system development by governing the rate at which progenitors progress through the cell cycle and attracting microglial progenitors into the embryonic brain and retina. Collectively, these data show that Pgrn‐a governs neurogenesis by regulating cell cycle kinetics and the transition from proliferation to cell cycle exit and differentiation. © 2017 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 77: 1114–1129, 2017     

Serum requirement and density dependent inhibition of human malignant glioma cells in c lture

Comparative studies on growth control in a human glia/glioma cell system, including the response to wounding of cell layers, show differences of both quantitative and qualitative nature between normal and malignant cells. Glioma cells grow to higher terminal cell density and are less serum dependent. A plateau phase with retention of cells in the G1 phase of the cell cycle is reached even under steady-state conditions. This inhibition of growth was not complete, however, as constant escape of a small fraction of cells into the cell cycle could be recorded. A partially retained density-dependent inhibition of growth of glioma cells was also demonstrated in the complete absence of serum, arguing against growth factor depletion as the sole reason for growth inhibition in crowded cultures of these malignant cells. It is postulated that the reduced density-dependent growth inhibition of glioma cells may be due to two operationally distinct and not necessarily related malfunctions: (1) a decreased serum requirement, accounting for a capacity to grow well beyond confluency; (2) an inability to accomplish perfect physiological intercellular contact relations, which would explain the apparently unavoidable escape of a proportion of the cells into the cell cycle even at very high cell density under these conditions.     

A dual‐color marker system for in vivo visualization of cell cycle progression in Arabidopsis

Visualization of the spatiotemporal pattern of cell division is crucial to understand how multicellular organisms develop and how they modify their growth in response to varying environmental conditions. The mitotic cell cycle consists of four phases: S (DNA replication), M (mitosis and cytokinesis), and the intervening G1 and G2 phases; however, only G2/M‐specific markers are currently available in plants, making it difficult to measure cell cycle duration and to analyze changes in cell cycle progression in living tissues. Here, we developed another cell cycle marker that labels S‐phase cells by manipulating Arabidopsis CDT1a, which functions in DNA replication origin licensing. Truncations of the CDT1a coding sequence revealed that its carboxy‐terminal region is responsible for proteasome‐mediated degradation at late G2 or in early mitosis. We therefore expressed this region as a red fluorescent protein fusion protein under the S‐specific promoter of a histone 3.1‐type gene, HISTONE THREE RELATED2 (HTR2), to generate an S/G2 marker. Combining this marker with the G2/M‐specific CYCB1‐GFP marker enabled us to visualize both S to G2 and G2 to M cell cycle stages, and thus yielded an essential tool for time‐lapse imaging of cell cycle progression. The resultant dual‐color marker system, Cell Cycle Tracking in Plant Cells (Cytrap), also allowed us to identify root cells in the last mitotic cell cycle before they entered the endocycle. Our results demonstrate that Cytrap is a powerful tool for in vivo monitoring of the plant cell cycle, and thus for deepening our understanding of cell cycle regulation in particular cell types during organ development.     

A CtrA homolog affects swarming motility and encystment in <Emphasis Type="Italic">Rhodospirillum centenum</Emphasis>

The α-proteobacterium, Rhodospirillum centenum, has a complex life cycle that allows adaptation to different environments. Transitions between vegetative swim cell and swarmer cell types depend on whether the organism is growing in liquid surroundings or on a solid substrate. Moreover, starvation can induce vegetative cells to differentiate into quiescent cysts. This paper describes the results of our investigation into the role of a putative DNA-binding response regulator that is homologous to CtrA, the cell cycle regulator from Caulobacter crescentus. Deletion of ctrA from the R. centenum genome resulted in a viable strain with impaired swarming motility coupled with an increased tendency to form cysts. Conversely, overexpression of wild type CtrA or a phosphomimetic allele, CtrAD51E, suppressed cyst cell formation, whereas overexpression of a CtrAD51A allele failed to suppress encystment but did prevent swarming motility. Thus, we propose that CtrA participates within a two-component signal transduction pathway that promotes swarming motility while contributing to the suppression of cyst cell formation.     

Fusion of cell ghosts with sexually opposite type cells in Dictyostelium discoideum

In the sexual cycle of Dictyostelium discoideum, haploid cells of two opposite mating types, strains HM1 and NC4, acquire fusion-competence under certain conditions, such as suspension culture in the dark, and fuse specifically to form giant zygote cells. Each giant cell engulfs the surrounding cells, gradually increases in size, and finally develops into a macrocyst that is a sexual structure in D. discoideum. Fusion-competent HM1 cells suspended in a solution were frozen and thawed to make cell ghosts. When cell ghosts were introduced into fusion-competent and -incompetent intact NC4 cells, the cell ghosts killed them in a short time, but the fusion-competent cells were killed in preference to the fusion-incompetent cells. This killing occurred through the fusion of the cell ghosts directly to intact cell membranes. Since the fusion was specific, the fusion between ghosts and cells appears to be essentially the same as that between intact cells during the sexual cycle in molecular mechanisms.     

Cell wall formation in yeast

Summary Incorporation of tritiated glucose into cell walls of growingSaccharomyces cerevisiac andSchizosaccharomyces pombe was studied using electron microscopic autoradiography. The pattern and the extent of labelling ofS. cerevisiae cell walls depended on the cell stage in the cell cycle. Quantitative evaluation of autoradiographs showed that the highest rate of wall synthesis took place during bud growth. The incorporation of new material into the wall of growing bud showed an increasing rate with the magnitude of the bud. The incorporation into the mother cell wall was almost negligible during bud growth. The rate of wall synthesis in double cells decreased during cell division. This period and that before new bud initiation was found to be the time of substantially reduced rate of wall replication inS. cerevisiae. A significant random incorporation was observed into the walls of post-division adult cells, both parental and daughter. The cell walls ofS. pombe were labelled almost exclusively at growing tips. The incorporation of tritiated carbohydrates into non-extensile regions ofS. pombe cell walls was found to be only about 5% of the total wall labelling.     

Cell cycle re-entry sensitizes podocytes to injury induced death

Podocytes are terminally differentiated renal cells, lacking the ability to regenerate by proliferation. However, during renal injury, podocytes re-enter into the cell cycle but fail to divide. Earlier studies suggested that re-entry into cell cycle results in loss of podocytes, but a direct evidence for this is lacking. Therefore, we established an in vitro model to test the consequences of re-entry into the cell cycle on podocyte survival. A mouse immortalized podocyte cell line was differentiated to non-permissive podocytes and stimulated with e.g. growth factors. Stimulated cells were analyzed for mRNA-expression or stained for cell cycle analysis using flow cytometry and immunocytofluorescence microscopy. After stimulation to re-entry into cell cycle, podocytes were stressed with puromycin aminonucleoside (PAN) and analyzed for survival. During permissive stage more than 40% of immortalized podocytes were in the S-phase. In contrast, S-phase in non-permissive differentiated podocytes was reduced to 5%. Treatment with b-FGF dose dependently induced re-entry into cell cycle increasing the number of podocytes in the S-phase to 10.7% at an optimal bFGF dosage of 10 ng/ml. Forty eight hours after stimulation with bFGF the number of bi-nucleated podocytes significantly increased. A secondary injury stimulus significantly reduced podocyte survival preferentially in bi-nucleated podocytes In conclusion, stimulation of podocytes using bFGF was able to induce re-entry of podocytes into the cell cycle and to sensitize the cells for cell death by secondary injuries. Therefore, this model is appropriate for testing new podocyte protective substances that can be used for therapy.