Guanylic nucleotide starvation affects Saccharomyces cerevisiae mother-daughter separation and may be a signal for entry into quiescence
© Sagot et al; licensee BioMed Central Ltd. 2005
Received: 18 February 2005
Accepted: 04 May 2005
Published: 04 May 2005
Guanylic nucleotides are both macromolecules constituents and crucial regulators for a variety of cellular processes. Therefore, their intracellular concentration must be strictly controlled. Consistently both yeast and mammalian cells tightly correlate the transcription of genes encoding enzymes critical for guanylic nucleotides biosynthesis with the proliferation state of the cell population.
To gain insight into the molecular relationships connecting intracellular guanylic nucleotide levels and cellular proliferation, we have studied the consequences of guanylic nucleotide limitation on Saccharomyces cerevisiae cell cycle progression. We first utilized mycophenolic acid, an immunosuppressive drug that specifically inhibits inosine monophosphate dehydrogenase, the enzyme catalyzing the first committed step in de novo GMP biosynthesis. To approach this system physiologically, we next developed yeast mutants for which the intracellular guanylic nucleotide pools can be modulated through changes of growth conditions. In both the pharmacological and genetic approaches, we found that guanylic nucleotide limitation generated a mother-daughter separation defect, characterized by cells with two unseparated daughters. We then showed that this separation defect resulted from cell wall perturbations but not from impaired cytokinesis. Importantly, cells with similar separation defects were found in a wild type untreated yeast population entering quiescence upon nutrient limitation.
Our results demonstrate that guanylic nucleotide limitation slows budding yeast cell cycle progression, with a severe pause in telophase. At the cellular level, guanylic nucleotide limitation causes the emergence of cells with two unseparated daughters. By fluorescence and electron microscopy, we demonstrate that this phenotype arises from defects in cell wall partition between mother and daughter cells. Because cells with two unseparated daughters are also observed in a wild type population entering quiescence, our results reinforce the hypothesis that guanylic nucleotide intracellular pools contribute to a signal regulating both cell proliferation and entry into quiescence.
Guanylic nucleotides are critical for multiple crucial cellular processes such as replication, transcription, translation and signalization via small GTPases. Most cell types recycle GMP from guanosine or guanine, which are either taken up from the surrounding environment or synthesized by the intracellular metabolism. GMP can also be synthesized de novo from IMP via two consecutive enzymatic steps, both highly conserved through evolution. Initially, the inosine monophosphate dehydrogenase (IMPDH) catalyzes conversion of IMP into XMP, the first committed step in de novo GMP biosynthesis. Subsequently, GMP synthetase converts the newly produced XMP into GMP.
The expression of IMPDH encoding genes is tightly regulated. In both yeast and mammalian cells, high guanylic nucleotide levels repress the transcription of IMPDH encoding genes [1, 2]. More strikingly, the transcription of IMPDH encoding genes in mammals is linked to cellular proliferation. In non-dividing cells, the expression level of IMPDHs is low, whereas it is highly increased in actively proliferating cells such as cancerous cells [3–6]. Further, IMPDH over-expression bypasses the anti-proliferative effect of p53, indicating that this p53 function requires proper control of IMPDH activity .
Saccharomyces cerevisiae cells have two major sources of guanylic nucleotides: intracellular IMP and extracellular guanine. Therefore GMP synthesis fully relies on IMPDH and GMP synthetase activities in the absence of guanine in the growth medium. Consistently, mutations in these cognate genes lead to guanine auxotrophy [8, 9]. Similar to mammalian cells, S. cerevisiae genes involved in GMP biosynthesis are highly expressed during exponential growth, but are actively repressed through specific regulatory sequences when cells arrest proliferation upon nutrient limitation . Moreover, the intracellular GTP/GDP ratio drastically decreases when yeast cells enter a quiescent state . These experiments lead to the appealing hypothesis that intracellular guanylic nucleotides levels contribute to a signal regulating cell proliferation. However, the molecular pathways linking cell cycle progression to IMPDH activity, and thus to intracellular guanylic nucleotide pools remain unknown.
Mycophenolic acid (MPA) is a well-characterized non-competitive and reversible inhibitor of IMPDH that severely depletes intracellular GTP pool down to 10% of normal level [12, 13]. MPA reduces or even abrogates proliferation of various cell types [14, 15]. MPA particularly affects lymphocyte division and because it has few secondary effects, its pro-drug form, the mycophenolate mofetil (Cellcept, Roche), is in path to replace cyclosporine A as a commonly used immunosuppressive drug. At the cellular level, MPA inhibits lymphocyte cell cycle progression by arresting cells in G1. Although this effect correlates with the depletion of guanylic nucleotide pools [13, 16], addition of guanosine and 8-aminoguanosine, which can partially replenish guanylic nucleotide pools, does not allow MPA treated cells to re-enter G2/M . Thus, MPA treatment, although apparently blocking cells in G1, could also affect later steps of the cell cycle. In budding yeast, MPA treatment slows cell proliferation and causes various effects on gene expression and thus on the yeast proteome . In a previous study, we have shown that MPA affects yeast cell size, DNA content, budding pattern and causes occasional perturbations of actin and microtubule cytoskeletons . In addition, several mutants affected for various cellular functions are hypersensitive to MPA . However, these data do not point at an obvious molecular process that would account for the effects of MPA on cell proliferation.
Here, to gain insight into the molecular relationships between intracellular guanylic nucleotide levels and cell cycle progression, we studied the effects of MPA treatment on Saccharomyces cerevisiae cell cycle progression. We first demonstrated that, although cells did not arrest in a particular cell cycle stage (confirming that there was no checkpoint for guanylic nucleotides in yeast), a large proportion of the population was slowed in telophase. We further observed that many MPA treated cells presented two unseparated daughter cells. We have shown that this specific morphology was due to a defect in mother-daughter separation and that it was probably a consequence of cell wall perturbations. To validate the results obtained with MPA, we developed yeast mutants in which guanylic nucleotide pools could be modulated by the composition of the growth medium. Using these genetic tools, we confirmed that cell separation was indeed the cell cycle step mostly perturbed by guanylic nucleotides starvation. Finally, the observation that cells entering quiescence also displayed the characteristic "two-daughter cells" morphology strongly suggests that a decrease in intracellular guanylic nucleotide levels may be part of a signal for yeast cells to enter stationary phase.
Mycophenolic acid treatment particularly affects the last step of the yeast cell cycle
A "bibudded" phenotype could result either from a single mother cell with two daughter cells, or from an unrelated G1 cell "sticking" to a normally budding cell. Since MPA treated cells were found to be sensitive to sonication, we used a fluorescence-based approach to distinguish between these two possibilities. Equal amounts of cells over-expressing a green variant of GFP were mixed with cells over-expressing a blue variant of GFP, grown to OD600 nm 0.2 and then treated with MPA.
After 4 hours of incubation, less than 1% (0.3% ± 0.3, N>100 for each of 4 independent experiments) of "bibudded" cells displayed one apparent daughter expressing a different GFP variant than its joined budding cell. Thus, more than 99% of the cells with two apparent daughters were monocolor, which is far more than the 50% expected for a random population of false "bibudded" cells. This experiment demonstrated that the population is indeed composed of a large proportion of cells with two attached daughters. In conclusion, MPA treatment particularly slowed down telophase and caused the appearance of cells with two unseparated daughters.
MPA effects are reversed by extracellular guanine and are not a consequence of translation inhibition
As 100 μg/mL MPA causes a significant decrease of the translation efficiency , we considered the possibility that "bibudded" cells result from impaired translation. To this end, we examined cells treated for 4 hours with the general translational inhibitor cycloheximide at a concentration that abolished translation . In contrast to MPA-treated cells, cycloheximide-treated cells did not exhibit more than one bud (0.8% ± 0.3% of the budding cells had two buds, N>200). Therefore, by decreasing intracellular guanylic nucleotide pools, MPA treatment caused a slowing down of the yeast cell cycle progression, the telophase being mostly affected. Consequently, "bibudded" cells accumulate and this, independently of MPA effects on translation efficiency.
Mutations affecting the guanylic nucleotide biosynthesis cause the emergence of cells with two unseparated daughters
In order to diminish the intracellular guanylic nucleotide pools by another route, we constructed an aah1 Δ amd1 Δ double mutant. The AAH1 gene encodes adenine amino-hydrolase, an enzyme converting adenine into hypoxanthine. The AMD1 gene encodes the AMP deaminase enzyme, which converts AMP into IMP (Fig. 1). The de novo synthesis of IMP from PRPP is inhibited by the presence of adenine in the growth medium . Consequently, in the presence of adenine, the aah1 Δ amd1 Δ double mutant has theoretically no path to synthesize GMP, neither from the de novo pathway nor from adenine. Accordingly, without extracellular guanine (or hypoxanthine), the guanylic nucleotide pools of the aah1 Δ amd1 Δ double mutant cannot be replenished and its growth is strongly affected (Fig. 4C). The aah1 Δ amd1 Δ double mutant was grown in SD medium without purine to OD600 nm = 0.2 and then shifted into a medium containing adenine as a sole source of purine. After 4 hours, the percentage of cells with two unseparated buds was counted. As shown in figure 4D, "bibudded" cells only arose when the aah1 Δ amd1 Δ double mutant cells were grown in a medium containing adenine. Thus, modifying the guanylic nucleotide pools through mutations in the purine nucleotide biosynthesis pathway provoked the same effect that MPA treatment on the yeast cell cycle: drastically impaired cell separation without cell cycle arrest, resulting in cells with two unseparated daughters.
Guanylic nucleotide starvation does not affect the formation of cellular structures required for completion of cytokinesis
The decrease of intracellular guanylic nucleotide pools caused the appearance of cells with two unseparated buds. We further characterized these abnormal cells to identify the cellular process(es) critically impaired by this starvation and thus likely responsible for this particular phenotype.
Microscopic observation of synchronized MPA-treated cells revealed that although anaphase on-set was delayed, its duration was almost similar to the anaphase in control cells (Fig. 2B). Therefore, MPA treatment did not critically affect the mitotic exit network (MEN). Further, MPA treatment did not disrupt proper localization of Tem1p, a RAS GTPase governing the MEN, neither the localization of its GTP exchange factor (GEF) Lte1p (Fig. 5B, left and middle panels). Taken together, these results suggest that MPA treatment does not significantly perturb anaphase progression, and consequently does not activate the anaphase checkpoint.
Because MPA severely affected progression through telophase, we examined the formation of cellular structures essential for the completion of cytokinesis. Figure 5A shows that both the first and the second daughter cell can build an actin cytokinetic ring. Further, more than 95% of the MPA treated cells (N>200) maintain proper localization of the septin Cdc12p (Fig. 5B, right panel), suggesting strongly that MPA-induced guanylic nucleotide pools depletion does not affect the septin ring formation and stability. Moreover, in "bibudded" cells, the septin ring can split to allow acto-myosin ring contraction for both the first and the second daughter cell (Fig. 5B, right panel). Therefore, guanylic nucleotide starved cells displayed all the structures required for cytokinesis completion, and the cellular processes leading to the mother-daughter separation defect must occur later in the cell cycle.
Guanylic nucleotide starvation affects cell wall separation
A large proportion of cells entering quiescence exhibit two daughters
Guanylic nucleotides are not only "building blocks" for nucleic acids but are also crucial for the regulation of many cellular processes such as G-proteins based signaling pathways. Therefore, cells must maintain their concentrations at a critical level. However, the molecular relationships between intracellular guanylic nucleotide levels and cell proliferation crucial events remain poorly understood.
Here, we have demonstrated that MPA treatment does not cause a firm cell cycle arrest in yeast. Treated cells continue to proliferate, although at a reduced rate (this study and ). Further, we have shown that conditional mutants unable to synthesize guanylic nucleotides do not arrest in a particular stage of the cell cycle. Therefore, our results establish that there is no guanylic nucleotide checkpoint is S. cerevisiae. By contrast, in mammalian cells, it was previously shown that MPA treatment cause an arrest of cellular proliferation but no guanylic nucleotide specific checkpoint has clearly been identified. Several reports have described that MPA treatment affects mammalian cells ability to commit into division by blocking the transition from G0 to the S phase of the cell cycle [13, 17, 22]. However, if MPA is added when cells have already entered the S phase, the cell cycle arrest occurs in G2/M . Besides, the MPA-induced arrest is not fully reversed by the replenishment of guanylic nucleotide pools [13, 17, 22]. Here, we have shown that even if MPA treatment slows all stages of the yeast cell cycle progression, the most affected step is the mother-daughter cell wall separation, giving rise to "bibudded" cells. This result supports our previous observation that MPA treatment leads to the appearance of many cells with 3N DNA content . Importantly, MPA treatment has the same effects on both non-synchronized and synchronized cells treated with the drug upon release from G1 (Fig. 2) or upon release from G2/M after nocodazole synchronization (I. S., B. D.-F. unpublished results). Therefore, unlike in mammals, in yeast, MPA treatment causes the same effects whatever the cell cycle stage of the cells at the time of drug addition. Further, our analysis of mutants demonstrates that the mother-daughter separation defect results solely from guanylic nucleotide pools depletion and is independent of potential MPA secondary targets.
In yeast, the fact that a guanylic nucleotides starvation causes a mother-daughter separation defect was unexpected. Indeed, one could have intuitively supposed that this depletion would rather provoke a drastic defect during DNA replication upon S phase or disturb cell cycle steps for which GTPase driven molecular processes are essential. In fact, upon MPA treatment, cells can still build a daughter cell and cell polarity was found even less affected in this study than in our earlier work . Daughter cell appears to grow normally, mitosis proceeds unperturbed, and mother-daughter closure is properly achieved. Thus, guanylic nucleotides starvation does not critically affect the functions of key GTP binding proteins, such as Cdc42p, Tem1p, tubulin and septins. In this study we observed that in the "bibudded" cells, placement of the second daughter cell properly follows the axial budding pattern of haploid cells, suggesting that the bud-site selection machinery is properly located. In contrast, prolonged MPA treatment (48 hours) leads to a random budding pattern . Thus, long-term guanylic nucleotides starvation may have more drastic effects. Most importantly, our experiments show that complete mother-daughter separation is not required for the mother cell to pass through START and to generate a second daughter cell (Fig. 2E). Therefore, our data confirm that no additional checkpoint blocks the cell cycle progression when anaphase is properly achieved.
What molecular targets trigger the mother-daughter separation defect upon guanylic nucleotide starvation? Observation of "bibudded" cells by electron microscopy revealed no obvious defect in the overall septum architecture of abnormally unseparated daughter cell. Nevertheless, MPA treatment increases cells sensitivity to SDS , zymolyase or sonication (I. S., B. D.-F. unpublished results), strongly suggesting cell wall defects. In fact, previous works have illustrated links between guanylic nucleotides metabolism and cell wall integrity, particularly through the synthesis of mannoproteins, essential components of the fungal cell wall. GDP-mannose is the common substrate for mannosyltransferases, enzymes catalyzing the addition of mannose residues on mannoproteins core oligosaccharides. In budding yeast, the GDP-mannose pyrophosphorylase Psa1p is an essential enzyme that synthesizes GDP-mannose, using GTP as a substrate. It was previously shown that MPA treatment affects Psa1p expression  and that Psa1p depletion leads to cell separation failure . Further, Shimma et al have demonstrated that the major defect of a guk1 conditional mutant strain, that is impaired for GDP biosynthesis (Fig. 1), was a decrease in GDP-mannose level (to about 25% of the wild type levels) that leads to mannose outer chain elongation defects . Accordingly, we have observed cell separation defect in guk1 cells (I. S., B. D.-F., unpublished results). In addition, yeast lacking mannosyltransferase encoding genes, such as OCH1, MNN10 or ANP1 display both hypersensitivity to MPA  and a cell separation defect similar to the one observed for guanylic nucleotide starved cells [25–27]. Therefore, one major consequence of guanylic nucleotide starvation could be a significant decrease in the GDP-mannose pool that in turn leads to a mother-daughter separation defect.
The most intriguing aspect of the regulation of intracellular guanylic nucleotide pools is the correlation between the IMPDH activity and cellular proliferation in mammalians models. Interestingly, the transcription of the IMD2 gene is actively shut off via regulatory sequences when yeast cells enter stationary phase upon nutrients limitation . Transcriptome analyses have demonstrated that AAH1, HPT1 and GUA1 are among the most promptly down regulated genes when nutrients become limiting. Thus, it appears that an entire process is devoted to rapidly decrease the intracellular guanylic nucleotide pools when cells enter stationary phase. Here, we have demonstrated that the typical "bibudded" phenotype obtained during guanylic nucleotide starvation also occurs in untreated wild type cells achieving their last divisions upon nutrients limitation. Thus, an identical morphology is observed for both guanylic nucleotide starvation and entry into quiescence.
Finally, the GTP/GDP ratio is very sensitive to growth conditions, rapidly decreasing during the diauxic shift and drastically dropping upon nutrients starvation. Further, this ratio may regulate RAS GTPases activity by influencing its guanylic nucleotide loading equilibrium . RAS and TOR pathways are key regulators that coordinate yeast proliferation with nutrients availability. Recent work has suggested that the TOR protein acts as ATP sensor in mammals . Thus it is appealing to speculate that in parallel to the TOR pathway, intracellular guanylic nucleotides levels are part of a signal that regulate cell proliferation via the modulation of RAS GTPases activity.
Using either mycophenolic acid, a molecule that specifically inhibits the first committed step in de novo GMP biosynthesis or mutations in the guanylic nucleotide biosynthesis pathway, we have demonstrated that intracellular guanylic nucleotides limitation causes a mother-daughter cell wall separation defect in budding yeast. This defect leads to the emergence of cells with two unseparated daughters. These "bibudded" cells are also found in a population of cells entering quiescence upon nutrient limitation. These observations further suggest that guanylic nucleotide intracellular pools might contribute to a signal that regulates cell proliferation, particularly upon nutrients limitation and entry into stationary phase.
Strains, media and reagents
Yeast strains used were purchased from Euroscarf (Frankfurt, Germany) and are derivatives of BY4741 or BY4742 . SD and SD casa media were described previously  and were supplemented when required with tryptophan (0.2 mM), uracil (1.8 mM), guanine (0.3 mM), adenine (0.3 mM), hypoxanthine (0.3 mM). Cycloheximide was purchased from Sigma and used at 50 μg/mL. Mycophenolic acid (MPA) was purchased from Amresco, (Ohio, USA) and was used at 100 μg/mL.
pB1594 integrates three tandem copies of GFP at the 3' end of the LTE1 coding region (Lte1p-3xGFP) and is a kind gift of D. Pellman . pB1598 integrates three tandem copies of GFP at the 3' end of the TEM1 coding region (Tem1p-3xGFP) and is a kind gift of D. Pellman. pYB407 (GFP-Cdc12p – LEU2 – CEN) is a kind gift of Y. Barral . pVTYS65T (URA3, 2μ) allows the expression of the GFP variant containing the S65T mutant (fluorescence maxima: Ex: 488 nm, Em: 511 nm) and has been previously described . pVTYBFP2 allows the expression of a GFP variant containing mutations F64L, S65T, Y66H et Y145F which coding sequence has been optimized for expression in yeast (fluorescence maxima: Ex 380 nm, Em 440 nm see ). Details of the mutagenesis and constructions are available upon request.
Cell Biology techniques and Fluorescence Microscopy
Cells were synchronized by addition of alpha-factor at the final concentration of 10 μg/mL directly into the growth medium. After 3 hours of incubation at 30°C with agitation, more than 99% of the cells were unbudded. Cells were washed twice and released into fresh SD medium containing or not MPA. Propidium iodide (Sigma) staining was performed as described in  except that cells were fixed with ethanol 70%. Alexa-568-Phalloidin (Molecular Probes, Eugene, OR) was used to stain filamentous actin as described previously . Zymolyase 20T (ICN Biomedicals, Costa Mesa, CA) was used at 0.2 mg/mL on formaldehyde fixed cells resuspended in PBS. Images were acquired with a Marianas system and analyzed with the Slidebook software (Intelligent Imaging Innovations, Inc. Denver, CO) except for GFP-Cdc12p expressing cells that were imaged with a previously described imaging system .
Cells were grown in SD medium to OD600 nm = 0.2 at 30°C. MPA was added to the final concentration of 100 μg/mL and cells were grown for four more hours. Cells were then fixed with glutaraldehyde and osmic acid, and then stained by the method of Thiéry as previously described .
The authors thank Dr. Y. Barral and Dr. D. Pellman for kindly providing us plasmids and other reagents. We thank C. Saint-Marc for technical assistance in constructing and characterizing the ade8 Δ gua1 Δ and amd1 Δ aah1 Δ double mutants. We specially want to express our gratitude to Dr. D. Pellman for having kindly allowed us to use his microscope and to Dr. Z. Storchova for her expertise and precious help for FACs experiments. We also acknowledge B. Pinson and A. Breton for helpful discussion and comments on the manuscript. We are very grateful to J. Moseley and E. Coic for their help in proofreading the manuscript. This work was supported by Grant # 4749 from The Association pour la Recherche Contre le Cancer.
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