A role for the yeast cell cycle/splicing factor Cdc40 in the G<Subscript>1</Subscript>/S transition

The CDC40 (PRP17) gene of S. cerevisiae encodes a splicing factor required for multiple events in the mitotic and meiotic cell cycles, linking splicing with cell cycle control. cdc40 mutants exhibit a delayed G₁/S transition, progress slowly through S-phase and arrest at a restrictive temperature in the G₂ phase. In addition, they are hypersensitive to genotoxic agents such as methylmethane sulfonate (MMS) and Hydroxyurea (HU). CDC40 has been suggested to control cell cycle through splicing of intron-containing pre-mRNAs that encode proteins important for cell cycle progression. We screened a cDNA overexpression library and isolated cDNAs that specifically suppress the HU/MMS-sensitivity of cdc40 mutants. Most of these cDNAs surprisingly encode chaperones, translation initiation factors and glycolytic enzymes, and none of them is encoded by an intron-containing gene. Interestingly, the cDNAs suppress the G₁/S transition delay of cdc40 cells, which is exacerbated by HU, suggesting that cdc40 mutants are HU/MMS-sensitive due to their S-phase entry defect. A role of Cdc40p in passage through G₁/S (START) is further supported by the enhanced temperature sensitivity and G₁/S transition phenotype of a cdc40 strain lacking the G₁ cyclin, Cln2p. We provide evidence that the mechanism of suppression by the isolated cDNAs does not (at least solely) involve up-regulation of the known positive START regulators CLN2, CLN3, DCR2 and GID8, or of the large and small essential ribonucleotide reductase (RNR) subunits, RNR1 and RNR2. Finally, we discuss possible mechanisms of suppression by the cDNAs that imply cell cycle regulation by apparently unrelated processes, such as splicing, translation initiation and glycolysis.     

Fission yeast enters the stationary phase G0 state from either mitotic G1 or G2

Summary The cell cycle of Schizosaccharomyces pombe in continuous culture is controlled at two steps, one which limits the transition from G₁ to S phase and the other which determines the timing of cell division. We have investigated, by means of flow-cytofluorometry, the cell cycle characteristics of nutritionally starved cells in stationary phase. Cells were shown to become arrested in either G₁ or G₂, in ratios which depended on the composition of the growth medium. G₁ and G₂ stationary phase cells share certain properties. (1) They become relatively resistant to heat shock. (2) They can reenter the cell cycle after subculture into fresh medium. (3) The G₁ and G₂ arrested populations have equal long-term viability in stationary phase. (4) Both populations require the activity of the cdc2 ⁺ gene for reentry into the cell cycle. We suggest that cell cycle arrest in stationary phase is regulated by the activity of the same G₁ and G₂ controls which limit the rate of cell cycle progression in continuous culture. The data demonstrate that in fission yeast the transition from G₁ to S phase does not mark a point of commitment to the completion of the cell cycle.     

Common genes and pathways in the regulation of the mitotic and meiotic cell cycles of Schizosaccharomyces pombe

Summary Cell division cycle mutants defective in G₁, DNA replication or nuclear division were tested for sporulation at semi-restrictive temperatures. In cdc1-7, cdc5-120, cdc17-L16 and cdc18-46 no abnormalities were observed; cdc10-129, cdc20-M10, cdc21-M6B, cdc23-M36 and cdc24-M38 formed four-spored asci but with a low efficiency; cdc22-M45 was completely defective in meiosis, but could conjugate and formed zygotes with a single nucleus. Mutants defective in the mitotic initiation genes cdc2, cdc25 and cdc13 were blocked in meiosis II. None of the wee1-50, adh.nim1 ⁺ and win1 ⁺ alleles had any affect on sporulation, suggesting that their interactions with cdc25 and cdc2 are specific to mitosis. The meiotic function of cdc13 is TBZ-sensitive and probably exerted downstream of cdc2. Single mutants in cut1 or cut2 did not effect sporulation, whereas the double mutant cut1 cut2 formed two-spored asci. The results demonstrate that the cell division cycle and the meiotic developmental pathway share common genes and regulatory cascades.     

The induction of class I HLA by interferon-α is independent of the cell cycle,but the expression is enhanced by a G1/S block

The induction of class I HLA expression by interferon-α (IFN-α) was studied in lymphoid cells arrested or traversing different stages of the cell cycle. Exponential cultures of MOLT-4 cells and the MOLT-4 cell variant YHHH were treated with the cell cycle inhibitors aphidicolin and colcemid to obtain cell populations arrested in G₁/S and G₂/M, respectively, and also cells traversing from S to M and vice versa. Cytofluorimetry with the monoclonal antibody YTH/76.3 (which specifically detects those class I molecules which are most susceptible to IFN-α induction) was used to quantitate the class I HLA response to IFN-α. The results showed that the response to IFN-α is not restricted to a given stage of the cell cycle. These studies also revealed that when the cells were arrested at G₁/S, the absolute level of class I HLA expression was enhanced 2–3-fold, both in the presence or absence of either IFN-α or IFN-γ. Therefore, even when absolute levels changed, the ratio of IFN-induced expression to basal expression remained constant at all cell cycle stages. The level of expression of another surface antigen (the CD1 antigen HTA-1) was not affected by the G₁/S block. The results were confirmed by dot blot hybridization of poly (A)⁺ RNA using cDNA-specific probes. These findings suggest that the effect of IFN-α is continuous throughout the cell cycle but that a G₁-dependent event determines the extent of class I HLA expression, and leads to a synergistic superinduction by IFN in G₁/S-arrested cells.     

Effect of repeated partial hepatectomy on cell division in the regenerating rat liver

Repeated partial hepatectomy (PH), performed 24 h after a 70% PH, had the following effect on the mitotic cell cycle in the regenerating rat liver: it delayed (by about 2 h) the cells in the G₂ period, left the S period almost unchanged, and delayed the cells for 6–8 h in the G₁ period. A mock repeated operation had a similar effect. This indicates that the influence of the repeated PH on the mitotic cell cycle in the regenerating liver is due to operation stress. Additional stimulation of division by repeated PH affects the character of the regeneration process as a whole.Laboratory of Chemical Factors of Regulation of Growth and Cell Division, Institute of Biological and Medical Chemistry, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR V. N. Orekhovich.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 84, No. 10, pp. 488–491, October, 1977.     

Spontaneous mitotic recombination inSchizosaccharomyces pomhe

Summary The UV sensitive mutantrad2-44 ofSchizosaccharomyces pomhe increases mitotic gene conversion and crossover rates about 10-fold but has little or no effect on meiotic recombination. As inrad2⁺, recombination events on different chromosomes are coincidental inrad2-44, indicating that mitotic recombination takes place in a subpopulation of competent cells. However, the coefficient of coincidence is smaller in the mutant, whereas recombination rates among the competent cells are the same as inrad2⁺. This suggests thatrad2-44 increases mitotic recombination by enhancing the fraction of competent cells. The rate limiting factor in spontaneous mitotic recombination inS. pomhe appears to be the size of the subpopulation of recombinationally competent cells.     

Genetic effects of photoactivated psoralens during meiosis in DNA repair mutantpso3-1 ofSaccharomyces cerevisiae

The influence of the DNA repair genePSO3 on photoactivated psoralen-induced meiotic recombination, gene conversion, reverse mutation, and on survival, was assayed in diploid strains ofSaccharomyces cerevisiae homozygous for the wild-type or thepso3-1 mutant allele. Sporulation was normal in thepso3-1 diploid. Wild-type and mutant strains had the same sensitivity to photoactivated monofunctional psoralen (3-CPs+UVA) in meiosis-uncommitted and meiosis-committed stages. The mutant showed higher sensitivity to photoactivated bifunctional psoralen (8-MOP+UVA) during all stages of the meiotic cycle. Mutation induction by 3-CPs+UVA or 8-MOP+UVA in meiosis-committed cells revealed no significant differences between wild-type and thepso3-1 mutant. The status of thePSO3 gene has no influence on the kinetics of induction of gene conversion and crossing-over after 3-CPs+UVA treatment in meiosis-committed cells: gene conversion was blocked while recombination was induced. After treatment with 8-MOP+UVA gene conversion was also blocked in both strains while crossing-over could only be observed in meiosis-committed wild-type cells.     

Duration of the mitotic cycle of chinese hamster cells cultured at 30–39°C

The duration of the mitotic cycle (T) and of its stages at temperatures of 30, 33, 36, and 39°C was studied by an autoradiographic method in Chinese hamster cells of subline 237. The duration of the mitotic cycle was shortest at 39°C and increased as the cultivation temperature fell. Within the temperature range 33–39°C the increase in duration of the mitotic cycle and its stages was proportional to the temperature studied. The slope of the curve of duration of the mitotic cycle as a function of temperature increased sharply as the temperature changed from 33 to 30°C. The G₁ period was most sensitive and the G₂ period least to a change in the cultivation temperature.Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 82, No. 11, pp. 1365–1367, November, 1976.     

Formation of gap junctions by stem cells in the developing retina of the clawed frog (Xenopus laevis)

Summary To ascertain whether gap junctions are preferentially formed by proliferative retinal stem cells, an analysis by electron microscopic autoradiography was made on the retina of the Xenopus embryo at stages 26, 29 and 31, after pulse labelling with ³H-thymidine. When the administration of the isotope was carried out for 0.5 or 3 h, retinal cells in S phase or S to M phase in the cell cycle were marked with the isotope, respectively. In these specimens, most gap junctions were found on the isotope-labelled cells, and few on the unlabelled cells. Several cells in mitosis also formed gap junctions. These results would suggest that gap junctions are formed by retinal stem cells in S to M phases of the cell cycle, but not by cells in G₁ phase and post-mitotic cells.     

Cytokine-induced differentiation and proliferation of human T lymphocytesin vitro: Effects of interleukin 2 and interleukin 6

The contents of CD8⁺, CD4⁺CD8⁺, CD3⁺HLA-DR⁺, CD⁺INF-γ⁺ T cells, and natural killers (CD16⁺56⁺) and NK/T cells (CD16⁺56⁺CD3⁺) increase after 7-day culturing in the presence of interleukin-2. The number of apoptotic cells and cells in S-, and G₂+M phases of the cell cycle also increased. Interleukin-6 predominantly induced proliferation of CD3⁺HLA-DR⁺ T cells and G₂+M mitotic cells. Translated fromByulleten' Eksperimental'noi Biologii I Meditsiny, Vol. 129, No. 6, pp. 667–671, June, 2000     

Mouse p87 wee1 kinase is regulated by M-phase specific phosphorylation

We have cloned a mousewee1 kinase cDNA (mwee1). The clone is 2258 bp in length and its open reading frame corresponds to 646 amino acid residues. The molecular weight of this kinase is 87 kDa in SDS-PAGE, which is about 1.7-fold larger than the human p50 ^wee1 kinase reported previously. In a cell cycle, the mousewee1 kinase is phosphorylated at M-phase, and anin vitro study using a mitotic extract revealed that phosphorylation occurs in the N-terminal domain, which is absent from the humanwee1 kinase, resulting in inactivation of the kinase activity. The N-terminal domain or entire molecule is extensively phosphorylated bycdc2-cyclin B kinase. Furthermore, the activity of thewee1 kinase was reduced by phosphorylation with the mitotic extract which containedcdc2-cyclin B kinase     

An abnormal cell division cycle in an AIR carboxylase-deficient mutant of the fission yeast Schizosaccharomyces pombe

Summary Adenine-requiring mutant strains of S. pombe enter the stationary phase after depleting a culture medium of adenine or its analogues. Stationary phase cells of six mutants defective at different stages of the purine nucleotide synthetic pathway were examined for cell volume and DNA content, and then compared in these respects with those of a prototrophic wild-type strain. The cell cycle of the wild-type strain was arrested in the G₂ phase (2C state) in the nitrogen rich medium, as is evident from DNA content per cell (0.0425 pg) and cell volume (47.7 m³). An AIR carboxylase-deficient (ade6) mutant strain was found to have an unusual cell volume (307.4 m³) and DNA content (0.1187 pg). By DAPI fluorescence microscopy, each mutant cell was seen to contain only one enlarged nucleus, which indicates the absence of cell populations containing cells in the 4C state of the S phase following nuclear division. It then follows that in ade6 mutant cells, DNA synthesis occurs in the absence of a completed nuclear division. Thus in S. pombe cells, the completion of nuclear division is not necessarily required for the next cycle initiation of DNA synthesis under certain physiological conditions.Abbreviation AIR aminoimidazole ribonucleotide - DAPI 4,6-diamidino-2-phenylindole - PCA perchloric acid - DABA 3,5-diaminobenzoic acid     

The mcm2-1 mutation of yeast causes DNA damage with a RAD9 requirement for repair

The minichromosome maintenance mutation, mcm2-1, has been found to synthesize damaged DNA at 35°C. Growth at this temperature rendered the mutant strain more sensitive to killing by ultraviolet irradiation. DNA damage could also be detected by pulsed-field gel electrophoresis, where a higher fraction of the DNA loaded was retained in the inserts at the wells. During the exponential phase of growth at this temperature about 50% of the cells had large buds, with the nucleus at or near the neck of the bud in most cases. The incorporation of the rad9 deletion in the mcm2-1-carrying strain caused a reduction in the percentage of large-budded cells and a moderate loss of cell viability. The results are consistent with mcm2-1 causing DNA damage leading to the arrest of cells in the S/G₂ phase of the cell cycle, which was partially dependent on the RAD9 gene product.     

RAN1 + controls the transition from mitotic division to meiosis in fission yeast

Summary We have investigated the genetic and physiological control of meiosis in fission yeast. Nutritionally depleted h ⁺/h ^– diploid cells become irreversibly commited to meiosis immediately prior to the initiation of premeiotic S phase. Premeiotic DNA synthesis requires matP ⁺, matM ⁺, mei2 ⁺ and mei3 ⁺ but not the mitotic cell cycle control gene, cdc2 ⁺, ran1 ⁺ is an essential gene, loss of which provokes sexual conjugation, premeiotic DNA synthesis, pseudo-meiosis and the sporulation of haploid cells. Our experiments suggest that sexual differentiation is achieved physiologically by the inhibition of ran1 ⁺ activity in a two-step process. In the first step, partial inhibition of ran1 ⁺ in starved haploid cells, leads to cell cycle arrest in G₁ followed by sexual conjugation. In the second step, a pathway requiring the matP ⁺, matM ⁺ and mei3 ⁺ genes of the newly-formed zygote, further inhibits ran1 ⁺ and thereby commits the cell to meiosis. mei2 ⁺ is required for meiotic commitment after full inhibition of ran1 ⁺. ran1 ⁺ is normally essential for vegetative cell reproduction but is inessential in cells which have abnormally high levels of cAMP-dependent protein kinase. We propose that the ran1 ⁺ gene encodes a highly controlled protein kinase which shares key substrates with cAMP-dependent protein kinase.     

Cytotoxicity by Tumor Necrosis Factor is Linked with the Cell Cycle But Does Not Require DNA Synthesis

Abstract

The relationship between the kinetics of cell death induced by TNF and the cell cycle in L929.10 target cells was investigated by comparison of growing, asynchronous cells with target cells synchronized at G₁/S using a double thymidine block. The induction phase of lysis, the time following TNF addition but before loss of cell viability, was shortened in asynchronous cells by increasing the level of saturation of the TNF receptor. However, in synchronized target cells, the length of the induction phase showed no dependence on receptor occupancy. Almost all cell death occurred within a 3 hr period 4-7 hr after the addition of TNF regardless of the concentration of TNF. Target cell lysis in Synchronized cells was concomitant with mitosis as verified by flow cytometry and DNA staining with propidium iodide. The narrow window of cytotoxicity was not due to cell cycle-related changes in the expression of the TNF receptor as measured by [¹²⁵I]TNF binding. Treatment with TNF did not accelerate or retard the progression of cells through S and G₂/M nor did target cells accumulate at G₂/M. When the kinetic experiments were repeated in the presence of 2 MM thymidine, TNF-treated cells died with identical dose and kinetic responses as those in which the thymidine block had been removed. Under these conditions, flow cytometric analysis revealed that DNA synthesis remained inhibited. These results suggest that TNF-induced cytotoxicity is linked to cell cycle-associated processes and that TNF is capable of overriding the normal cellular controls that coordinately link the DNA replicative cycle with the mitotic cycle. In the L929.10 target cell, TNF may induce a fatal mitosis-linked event.     

Exposure of cdc8 homozygous diploids of Saccharomyces cerevisiae to nonpermissive temperature leads to an increase in mitotic conversion frequencies

Summary In a diploid strain homozygous for the cdc8-1 mutation, a block in DNA synthesis caused by restrictive temperature resulted in a significant increase in the frequency of intragenic recombination at the HOM2 locus. Under restrictive conditions, incorporation of radioactivity into DNA was reduced to 2% of the control and alkaline sucrose gradient centrifugation revealed that only short DNA fragments were synthesized. There was no considerable fragmentation of template DNA during incubation of cdc8-1 strains under restrictive conditions.     

Sorting of B lymphoblasts based upon cell diameter provides cell populations enriched in different stages of cell cycle

Here we report analysis and correlation of changes in cell size and cycle state resulting from exposure of murine B lymphocytes to the mitogens lipopolysaccharide (LPS) and dextran sulfate (DxSO₄). Cell cycle changes are assessed by flow cytofluorometric analysis of acridine orange stained cells. Cell diameters are determined by flow cytometric analysis of the pulse-width (time of flight) of the axial light extinction signal. Results indicate that within 12 h of exposure of B cell populations to these mitogens, cells displaying increased diameter and containing increased RNA can be detected. Under these conditions, increased RNA content is considered indicative of G₀ to G₁ transition or entry into cell cycle (Darzynkiewicz et al., 1976). Progressive increases in cell size and transition through G₁, S, G₂, and M occur in parallel during 48 h of culture with mitogens. Sorting of cells based upon size followed by cell cycle analysis reveals a direct correlation between cell size and cycle phase. Specifically, cells 4.5–5.5 μm in diameter are in primarily G₀. Cells 5.5–7.0 μm in diameter are in early G₁. Populations of cells 7.0–10 consist of S, G₂, and M phase cells. The importance of this correlation is discussed in view of needs to more rigorously define B cell populations for investigations of biochemical events of and accessory cell requirements for activation of B lymphocytes.     

Synchronized meiosis and recombination in fission yeast: observations with pat1-114 diploid cells

Summary The mutation pat1-114 has been used to synchronize meiosis in the fission yeast Schizosaccharomyces pombe. We have investigated several aspects of such synchronized meiotic cultures. In both pat1-114 and pat1 ⁺ diploids, meiotic landmark events are initiated at the same time after meiosis induction, but synchrony is much more pronounced in the pat1-114-driven meiosis. Commitment to recombination and to meiosis have been timed at 2 h after meiotic induction. Due to a seven-fold reduction of intragenic recombination frequency in the ade6 region of pat1-114 diploids, physical analysis of recombination has not been possible. We have distinguished three factors that influence intragenic recombination frequencies: temperature, azygotic versus zygotic meiosis, and the nature of the pat1 allele. Differences and similarities in the timing of meiotic landmarks in S. cerevisiae and S. pombe are discussed.     

Cellular effects of leishmanial tubulin inhibitors on L. donovani

To aid our investigation of tubulin as an antileishmanial drug target, the effects of the mammalian antimicrotubule agents ansamitocin P3, taxol, and hemiasterlin on Leishmania donovani promastigotes were described. These drugs affected the assembly of purified leishmanial tubulin and inhibited the growth of L. donovani promastigotes at micromolar concentrations. When promastigotes were treated with these agents, mitotic partitioning of nuclear DNA and cytokinesis were usually inhibited. The spatial orientation of kinetoplasts was often disturbed, suggesting a role for microtubules in the segregation of these organelles during mitosis. Aberrant cell types produced in drug-treated cultures included parasites with one nucleus and two geometrically distinct kinetoplasts, parasites with multiple kinetoplasts, and cytoplasts containing a kinetoplast but no nucleus. A subset of unique cell types, parasites containing two nuclei, a spindle fiber, and two geometrically distinct kinetoplasts, were observed in hemiasterlin-treated cultures. Flow cytometric analysis of L. donovani promastigotes treated with these three drugs indicated a dramatic shift toward the G₂+M phase of the cell cycle, with some cells containing four times the amount of DNA present in G₁. These results were used to evaluate the cellular effects of WR85915, an aromatic thiocyanate with in vitro antileishmanial and anti-tubulin activity, on L. donovani. Treatment of parasites with WR85915 did not produce the unusual cell types described above and did not cause the accumulation of parasites in G₂+M, suggesting that WR85915 acts on target(s) in Leishmania in addition to tubulin. These studies validate tubulin as a suitable antileishmanial drug target and provide criteria to assess the cellular mechanism of action of new candidate antileishmanial agents.