MAP kinase kinase kinase (MAPKKK)-dependent and -independent activation of Sty1 stress MAPK in fission yeast

A strategy for the investigation of toxic mechanisms and protection by efflux pumps using Schizosaccharomyces pombe strains: Application to rotenone

Effects not related with the inhibition of complex I of the mitochondrial electron transport chain are studied in S. pombe, which lacks it. This study aims: First, the use of a strategy with S. pombe strains to investigate the toxicity, mechanisms of action, interactions and detoxication by efflux pumps. Second, to investigate the mechanisms of toxic action of rotenone. In the dose-response assessment, the yeast presented a good correlation with the toxicity in Daphnia magna for 15 chemicals. In the mechanistic study, the mph1Δ strain presented marked specificity to the interaction with microtubules by carbendazim. DNA damage caused by hydroxyurea, an inhibitor of deoxynucleotide synthesis, was identified with marked specificity with the rad3Δ strain. The sty1Δ strain was very sensitive to the oxidative and osmotic stress induced by hydrogen peroxide and potassium chloride, respectively, being more sensitive to oxidative stress than the pap1Δ strain. The protection by exclusion pumps was also evaluated. Rotenone presented low toxicity in S. pombe due to the lack of its main target, and the marked protection by the exclusion transporters Bfr1, Pmd1, Caf5 and Mfs1. Marked cellular stress was detected. Finally, the toxicity of rotenone could be potentiated by the fungicide carbendazim and the antimetabolite hydroxyurea. In conclusion, the use of S. pombe strains is a valid strategy to: a) assess global toxicity; b) investigate the main mechanisms of toxic action, particularly spindle and DNA interferences, and osmotic and oxidative stress not related to complex I inhibition; c) explore the detoxication by efflux pumps; and d) evaluate possible chemical interactions. Therefore, it should be useful for the investigation of adverse outcome pathways.

Applications of Metabolomics for the Elucidation of Abiotic Stress Tolerance in Plants: A Special Focus on Osmotic Stress and Heavy Metal Toxicity

Plants undergo metabolic perturbations under various abiotic stress conditions; due to their sessile nature, the metabolic network of plants requires continuous reconfigurations in response to environmental stimuli to maintain homeostasis and combat stress. The comprehensive analysis of these metabolic features will thus give an overview of plant metabolic responses and strategies applied to mitigate the deleterious effects of stress conditions at a biochemical level. In recent years, the adoption of metabolomics studies has gained significant attention due to the growing technological advances in analytical biochemistry (plant metabolomics). The complexity of the plant biochemical landscape requires sophisticated, advanced analytical methods. As such, technological advancements in the field of metabolomics have been realized, aided much by the development and refinement of separatory techniques, including liquid and gas chromatography (LC and GC), often hyphenated to state-of-the-art detection instruments such as mass spectrometry (MS) or nuclear resonance magnetic (NMR) spectroscopy. Significant advances and developments in these techniques are briefly highlighted in this review. The enormous progress made thus far also comes with the dawn of the Internet of Things (IoT) and technology housed in machine learning (ML)-based computational tools for data acquisition, mining, and analysis in the 4IR era allowing for broader metabolic coverage and biological interpretation of the cellular status of plants under varying environmental conditions. Thus, scientists can paint a holistic and comprehensive roadmap and predictive models for metabolite-guided crop improvement. The current review outlines the application of metabolomics and related technological advances in elucidating plant responses to abiotic stress, mainly focusing on heavy metal toxicity and subsequent osmotic stress tolerance.

A Redox-Sensitive Thiol in Wis1 Modulates the Fission Yeast Mitogen-Activated Protein Kinase Response to H2O2 and Is the Target of a Small Molecule

Oxidation of a highly conserved cysteine (Cys) residue located in the kinase activation loop of mitogen-activated protein kinase kinases (MAPKK) inactivates mammalian MKK6. This residue is conserved in the fission yeast Schizosaccharomyces pombe MAPKK Wis1, which belongs to the H₂O₂-responsive MAPK Sty1 pathway. Here, we show that H₂O₂ reversibly inactivates Wis1 through this residue (C458) in vitro.

Oxidation of a highly conserved cysteine (Cys) residue located in the kinase activation loop of mitogen-activated protein kinase kinases (MAPKK) inactivates mammalian MKK6. This residue is conserved in the fission yeast Schizosaccharomyces pombe MAPKK Wis1, which belongs to the H₂O₂-responsive MAPK Sty1 pathway. Here, we show that H₂O₂ reversibly inactivates Wis1 through this residue (C458) in vitro. We found that C458 is oxidized in vivo and that serine replacement of this residue significantly enhances Wis1 activation upon addition of H₂O₂. The allosteric MAPKK inhibitor INR119, which binds in a pocket next to the activation loop and C458, prevented the inhibition of Wis1 by H₂O₂in vitro and significantly increased Wis1 activation by low levels of H₂O₂in vivo. We propose that oxidation of C458 inhibits Wis1 and that INR119 cancels out this inhibitory effect by binding close to this residue. Kinase inhibition through the oxidation of a conserved Cys residue in MKK6 (C196) is thus conserved in the S. pombe MAPKK Wis1.

AMPKα Subunit Ssp2 and Glycogen Synthase Kinases Gsk3/Gsk31 are involved in regulation of sterol regulatory element-binding protein (SREBP) activity in fission yeast

Sterol regulatory element-binding protein (SREBP), a highly conserved family of membrane-bound transcription factors, is an essential regulator for cellular cholesterol and lipid homeostasis in mammalian cells. Sre1, the homolog of SREBP in the fission yeast Schizosaccharomyces pombe (S. pombe), regulates genes involved in the transcriptional responses to low sterol as well as low oxygen. Previous study reported that casein kinase 1 family member Hhp2 phosphorylated the Sre1 N-terminal transcriptional factor domain (Sre1N) and accelerated Sre1N degradation, and other kinases might exist for regulating the Sre1 function. To gain insight into the mechanisms underlying the Sre1 activity and to identify additional kinases involved in regulation of Sre1 function, we developed a luciferase reporter system to monitor the Sre1 activity through its binding site called SRE2 in living yeast cells. Here we showed that both ergosterol biosynthesis inhibitors and hypoxia-mimic CoCl₂ caused a dose-dependent increase in the Sre1 transcription activity, concurrently, these induced transcription activities were almost abolished in Δsre1 cells. Surprisingly, either AMPKα Subunit Ssp2 deletion or Glycogen Synthase Kinases Gsk3/Gsk31 double deletion significantly suppressed ergosterol biosynthesis inhibitors- or CoCl₂-induced Sre1 activity. Notably, the Δssp2Δgsk3Δgsk31 mutant showed further decreased Sre1 activity when compared with their single or double deletion. Consistently, the Δssp2Δgsk3Δgsk31 mutant showed more marked temperature sensitivity than any of their single or double deletion. Moreover, the fluorescence of GFP-Sre1N localized at the nucleus in wild-type cells, but significantly weaker nuclear fluorescence of GFP-Sre1N was observed in Δssp2, Δgsk3Δgsk31, Δssp2Δgsk3, Δssp2Δgsk31 or Δssp2Δgsk3Δgsk31 cells. On the other hand, the immunoblot showed a dramatic decrease in GST-Sre1N levels in the Δgsk3Δgsk31 or the Δssp2Δgsk3Δgsk31 cells but not in the Δssp2 cells. Altogether, our findings suggest that Gsk3/Gsk31 may regulate Sre1N degradation, while Ssp2 may regulate not only the degradation of Sre1N but also its translocation to the nucleus.

Nutrient Limitation Inactivates Mrc1-to-Cds1 Checkpoint Signalling in Schizosaccharomyces pombe

The S. pombe checkpoint kinase, Cds1, protects the integrity of stalled DNA replication forks after its phosphorylation at threonine-11 by Rad3 (ATR). Modified Cds1 associates through its N-terminal forkhead-associated domain (FHA)-domain with Mrc1 (Claspin) at stalled forks. We report here that nutrient starvation results in post-translational changes to Cds1 and the loss of Mrc1. A drop in glucose after a down-shift from 3% to 0.1–0.3%, or when cells enter the stationary phase, triggers a sharp decline in Mrc1 and the accumulation of insoluble Cds1. Before this transition, Cds1 is transiently activated and phosphorylated by Rad3 when glucose levels fall. Because this coincides with the phosphorylation of histone 2AX at S129 by Rad3, an event that occurs towards the end of every unperturbed S phase, we suggest that a glucose limitation promotes the exit from the S phase. Since nitrogen starvation also depletes Mrc1 while Cds1 is post-translationally modified, we suggest that nutrient limitation is the general signal that promotes exit from S phase before it inactivates the Mrc1–Cds1 signalling component. Why Cds1 accumulates in resting cells while its activator Mrc1 declines is, as yet, unclear but suggests a novel function of Cds1 in non-replicating cells.

Oxidative stress response pathways: Fission yeast as archetype

Schizosaccharomyces pombe is a popular model eukaryotic organism to study diverse aspects of mammalian biology, including responses to cellular stress triggered by redox imbalances within its compartments. The review considers the current knowledge on the signaling pathways that govern the transcriptional response of fission yeast cells to elevated levels of hydrogen peroxide. Particular attention is paid to the mechanisms that yeast cells employ to promote cell survival in conditions of intermediate and acute oxidative stress. The role of the Sty1/Spc1/Phh1 mitogen-activated protein kinase in regulating gene expression at multiple levels is discussed in detail.

A Trichoderma atroviride stress-activated MAPK pathway integrates stress and light signals

Cells possess stress-activated protein kinase (SAPK) signalling pathways, which are activated practically in response to any cellular insult, regulating responses for survival and adaptation to harmful environmental changes. To understand the function of SAPK pathways in T. atroviride, mutants lacking the MAPKK Pbs2 and the MAPK Tmk3 were analysed under several cellular stresses, and in their response to light. All mutants were highly sensitive to cellular insults such as osmotic and oxidative stress, cell wall damage, high temperature, cadmium, and UV irradiation. Under oxidative stress, the Tmk3 pathway showed specific roles during development, which in conidia are essential for tolerance to oxidant agents and appear to play a minor role in mycelia. The function of this pathway was more evident in Δpbs2 and Δtmk3 mutant strains when combining oxidative stress or cell wall damage with light. Light stimulates tolerance to osmotic stress through Tmk3 independently of the photoreceptor Blr1. Strikingly, photoconidiation and expression of blue light regulated genes was severally affected in Δtmk3 and Δpbs2 strains, indicating that this pathway regulates light responses. Furthermore, Tmk3 was rapidly phosphorylated upon light exposure. Thus, our data indicate that Tmk3 signalling cooperates with the Blr photoreceptor complex in the activation of gene expression.

Azoles activate Atf1-mediated transcription through MAP kinase pathway for antifungal effects in fission yeast

Azole antifungals directly inhibit enzymes for ergosterol biosynthesis, and this direct action is thought to underlie antifungal actions of these drugs. Recent studies showed that azoles alter expression of genes for various cellular functions. However, transcription factors regulated by azoles and their roles in antifungal actions remain poorly characterized. Using luciferase assay, we found that miconazole increased luciferase activity under the promoter containing the cAMP response element (CRE) motif. This azole-induced activation of CRE reporter was abolished in Atf1-deficient cells, suggesting that azoles induce Atf1 activation. As Atf1 is activated by stress-activated MAP kinase Sty1 upon various stressors, we examined its involvement. Azoles increased phosphorylation of Sty1 for its activation, and Sty1 deletion impaired azole-induced CRE reporter activation. In contrast, deletion of Pyp1, a tyrosine phosphatase which negatively regulates Sty1, increased CRE reporter activation. In addition, cells deficient in Atf1 and stress-activated MAP kinase pathway showed resistance to azoles, whereas cells lacking Pyp1 increased azole susceptibility, suggesting a critical role for azole-induced activation of MAP kinase-Atf1 pathway in antifungal actions of azoles. Collectively, these results suggest that azoles activate stress-activated MAP kinase pathway, thereby facilitating Atf1-mediated transcription for antifungal effects.

Possible involvement of nitric oxide and reactive oxygen species in glucose deprivation-induced activation of transcription factor rst2

Glucose is one of the most important sources of cellular nutrition and glucose deprivation induces various cellular responses. In Schizosaccharomyces pombe, zinc finger protein Rst2 is activated upon glucose deprivation, and regulates gene expression via the STREP (stress response element of Schizosaccharomyces pombe) motif. However, the activation mechanism of Rst2 is not fully understood. We monitored Rst2 transcriptional activity in living cells using a Renilla luciferase reporter system. Hydrogen peroxide (H2O2) enhanced Rst2 transcriptional activity upon glucose deprivation and free radical scavenger inhibited Rst2 transcriptional activity upon glucose deprivation. In addition, deletion of the trx2 (+) gene encoding mitochondrial thioredoxin enhanced Rst2 transcriptional activity. Notably, nitric oxide (NO) generators enhanced Rst2 transcriptional activity upon glucose deprivation as well as under glucose-rich conditions. Furthermore, NO specific scavenger inhibited Rst2 transcriptional activity upon glucose deprivation. Altogether, our data suggest that NO and reactive oxygen species may be involved in the activation of transcription factor Rst2.

TORC1 signaling is governed by two negative regulators in fission yeast

The target of rapamycin (TOR) is a highly conserved protein kinase that regulates cell growth and metabolism. Here we performed a genome-wide screen to identify negative regulators of TOR complex 1 (TORC1) in Schizosaccharomyces pombe by isolating mutants that phenocopy Δtsc2, in which TORC1 signaling is known to be up-regulated. We discovered that Δnpr2 displayed similar phenotypes to Δtsc2 in terms of amino acid uptake defects and mislocalization of the Cat1 permease. However, Δnpr2 and Δtsc2 clearly showed different phenotypes in terms of rapamycin supersensitivity and Isp5 transcription upon various treatments. Furthermore, we showed that Tor2 controls amino acid homeostasis at the transcriptional and post-transcriptional levels. Our data reveal that both Npr2 and Tsc2 negatively regulate TORC1 signaling, and Npr2, but not Tsc2, may be involved in the feedback loop of a nutrient-sensing pathway.

Genome-wide screening for genes associated with valproic acid sensitivity in fission yeast

We have been studying the action mechanisms of valproic acid (VPA) in fission yeast Schizosaccharomyces pombe by developing a genetic screen for mutants that show hypersensitivity to VPA. In the present study, we performed a genome-wide screen of 3004 haploid deletion strains and confirmed 148 deletion strains to be VPA sensitive. Of the 148 strains, 93 strains also showed sensitivity to another aliphatic acids HDAC inhibitor, sodium butyrate (SB), and 55 strains showed sensitivity to VPA but not to SB. Interestingly, we found that both VPA and SB treatment induced a marked increase in the transcription activity of Atf1 in wild-type cells. However, in clr6-1, a mutant allele the clr6⁺ gene encoding class I HDAC, neither VPA- nor SB induced the activation of Atf1 transcription activity. We also found that VPA, but not SB, caused an increase in cytoplasmic Ca²⁺ level. We further found that the cytoplasmic Ca²⁺ increase was caused by Ca²⁺ influx from extracellular medium via Cch1-Yam8 channel complex. Altogether, our present study indicates that VPA and SB play similar but distinct roles in multiple physiological processes in fission yeast.

Stress activated protein kinase pathway modulates homologous recombination in fission yeast

Rad52 is a key player in homologous recombination (HR), a DNA repair pathway that is dedicated to double strand breaks repair and recovery of perturbed replication forks. Here we show that fission yeast Rad52 homologue is phosphorylated when S phase cells are exposed to ROS inducers such as ultraviolet A radiation or hydrogen peroxide, but not to ultraviolet C or camptothecin. Phosphorylation does not depend on kinases Chk1, Rad3, Tel1 or Cdc2, but depends on a functional stress activated protein kinase (SAPK) pathway and can be partially prevented by anti-oxidant treatment. Indeed, cells lacking Sty1, the major fission yeast MAP kinase of the SAPK pathway, do not display Rad52 phosphorylation and have UVA induced Rad52 foci that persist longer if compared to wild type cells. In addition, spontaneous intrachromosomal HR is diminished in cells lacking Sty1 and, more precisely, gene conversion is affected. Moreover, HR induced by site-specific arrest of replication forks is twice less efficient in cells that do not express Sty1. Importantly, impairing HR by deletion of the gene encoding the recombinase Rhp51 leads to Sty1 dependent Rad52 phosphorylation. Thus, SAPK pathway impinges on early step of HR through phosphorylation of Rad52 in cells challenged by oxidative stress or lacking Rhp51 and is required to promote spontaneous gene conversion and recovery from blocked replication forks.

Genome-wide screen reveals novel mechanisms for regulating cobalt uptake and detoxification in fission yeast

Cobalt is an essential micronutrient but is toxic when present in excess. To study cobalt homeostasis we performed a genome-wide screen for deletion strains that show sensitivity or resistance to CoCl(2). Among 54 cobalt-sensitive strains, 18 are supersensitive strains, which are involved in histidine biosynthetic process, ubiquitination, mitochondria function, membrane trafficking, transporter and a variety of other known functions or still unknown functions. Furthermore, we identified 56 cobalt-resistant deletion strains, which are mainly involved in mitochondria function, signal transduction, ubiquitination, and gene expression and chromatin remodeling. Notably, deletion of the zhf1(+) gene, encoding a zinc ion transporter, confers supersensitivity to cobalt and overexpression of the zhf1(+) gene confers marked tolerance to cobalt, indicating that Zhf1 play key roles in cobalt detoxification. Interestingly, all the histidine-auxotrophic mutants displayed cobalt sensitivity and deletion of cationic amino acid transporter Cat1, which was shown to be involved in histidine uptake, suppressed the CoCl(2)-sensitive growth defect of the his2 mutants, suggesting that CoCl(2) may be transported into the cell together with histidine via histidine transporters including Cat1. In addition, we obtained results suggesting that the E2 ubiquitin conjugating enzyme Rhp6 and Sty1 stress MAP kinase pathway are involved in the regulation of cobalt homeostasis. Altogether, our genome-wide study demonstrates for the first time the mechanisms of cobalt homeostasis, particularly its uptake and detoxification in fission yeast.

Genetic screening for regulators of Prz1, a transcriptional factor acting downstream of calcineurin in fission yeast

Calcineurin phosphatase plays crucial roles in a wide variety of cell types and organisms. Dephosphorylation of the nuclear factor of activated T-cell (NFAT) family of transcriptional factors by calcineurin is essential for activating immune-responsive genes in mammals. NFAT activity is also regulated by diverse signaling pathways, which affect NFAT kinases and nuclear partner proteins. In fission yeast, calcineurin dephosphorylates and activates Prz1, a C2H2-type zinc finger transcriptional factor. Calcineurin-Prz1 signaling regulates the expression of the Pmc1 Ca(2+) pump. Prz1-overexpressing cells showed extremely slow growth and high transcriptional activity of Prz1 in the absence of stimulation. Here, we isolated seven genes as dosage-dependent suppressors of this slow growth phenotype. These seven genes encode Rad24, Rad25, Pka1, Msn5 (SPAC328.01c), Pac1, Ape2, and Tfs1. All of them decreased the high transcriptional activity caused by Prz1 overexpression. Overexpression of Pka1, Rad24, and Rad25 also repressed the Ca(2+)-induced transcriptional activity in cells with Prz1 expressed at wild-type levels. Knock-out of rad24 or rad25 significantly enhanced the transcriptional activity of Prz1, whereas knock-out or mutation of other genes did not enhance the activity. The 14-3-3 proteins, Rad24 and Rad25, bound Prz1 and the Rad24-binding site located at residues 421-426 of Prz1. In msn5 deletion mutants, GFP-Prz1 localized at nucleus in the absence of Ca(2+) stimulation, suggesting that Msn5 functions as an exportin for Prz1. In summary, our data suggest that Rad24 and Rad25 negatively regulate Prz1 and that Pka1, Msn5, Pac1, Tfs1, and Ape2 also regulate Prz1.

A measurable activation of the bZIP transcription factor Atf1 in a fission yeast strain devoid of stress-activated and cell integrity mitogen-activated protein kinase (MAPK) activities

In Schizosaccharomyces pombe, the stress-activated Sty1 MAPK pathway is essential for cell survival under stress conditions. The Sty1 MAPK regulates Atf1 transcription factor to elicit stress responses in extreme conditions of osmolarity and reactive oxygen species-generating agents such as hydrogen peroxide, heat, low glucose, and heavy metal. Herein, using a newly developed Renilla luciferase reporter assay with enhanced detection sensitivity and accuracy, we show that distinct signaling pathways respond to cadmium and other reactive oxygen species-generating agents for the activation of Atf1. Also, surprisingly, a measurable activation of Atf1 transcription factor was still observed devoid of Sty1 MAPK activity. Further genetic and biological analyses revealed that the residual activation is caused by the activation of the cell wall integrity Pmk1 MAPK pathway and a redox-mediated activation of Atf1.

Regulation of HMG-CoA reductase in mammals and yeast

HMG-CoA reductase (HMGR), a highly conserved, membrane-bound enzyme, catalyzes a rate-limiting step in sterol and isoprenoid biosynthesis and is the primary target of hypocholesterolemic drug therapy. HMGR activity is tightly regulated to ensure maintenance of lipid homeostasis, disruption of which is a major cause of human morbidity and mortality. HMGR regulation takes place at the levels of transcription, translation, post-translational modification and degradation. In this review, we discuss regulation of mammalian, Saccharomyces cerevisiae and Schizosaccharomyces pombe HMGR and highlight recent advances in the field. We find that the general features of HMGR regulation, including a requirement for the HMGR-binding protein Insig, are remarkably conserved between mammals and ascomycetous fungi, including S. cerevisiae and S. pombe. However the specific details by which this regulation occurs differ in surprising ways, revealing the broad evolutionary themes underlying both HMGR regulation and Insig function.