Glucose limitation and pka1 deletion rescue aberrant mitotic spindle formation induced by Mal3 overexpression in Schizosaccharomyces pombe

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

Pps1, phosphatidylserine synthase, regulates the salt stress response in Schizosaccharomyces pombe

Phosphatidylserine (PS) is important for maintaining growth, cytoskeleton, and various functions in yeast; however, its role in stress responses is poorly understood. In Schizosaccharomyces pombe, the PS synthase deletion (pps1∆) mutant shows defects in growth, morphology, cytokinesis, actin cytoskeleton, and cell wall integrity, and these phenotypes are rescued by ethanolamine supplementation. Here, we evaluated the role of Pps1 in the salt stress response in S. pombe. We found that pps1∆ cells are sensitive to salt stresses such as KCl and CaCl2 even in the presence of ethanolamine. Loss of the functional cAMP-dependent protein kinase (git3∆ or pka1∆) or phospholipase B Plb1 (plb1∆) enhanced the salt stress-sensitive phenotype in pps1∆ cells. Green fluorescent protein (GFP)-Pps1 was localized at the plasma membrane and endoplasmic reticulum regardless of the stress conditions. In pka1∆ cells, GFP-Pps1 was accumulated around the nucleus under the KCl stress. Pka1 was localized in the nucleus and the cytoplasm under normal conditions and transferred from the nucleus to the cytoplasm under salt-stress conditions. Pka1 translocated from the nucleus to the cytoplasm during CaCl2 stress in the wild-type cells, while it remained localized in the nucleus in pps1∆ cells. Expression and phosphorylation of Pka1-GFP were not changed in pps1∆ cells. Our results demonstrate that Pps1 plays an important role in the salt stress response in S. pombe.

Tfs1, transcription elongation factor TFIIS, has an impact on chromosome segregation affected by pka1 deletion in Schizosaccharomyces pombe

The cAMP-dependent protein kinase (PKA) pathway in Schizosaccharomyces pombe plays an important role in microtubule organization and chromosome segregation. Typically, loss of functional Pka1 induces sensitivity to the microtubule-destabilizing drug thiabendazole (TBZ) and chromosome mis-segregation. To determine the mechanism via which Pka1 is involved in these events, we explored the relevance of transcription factors by creating a double-deletion strain of pka1 and 102 individual genes encoding transcription factors. We found that rst2∆, tfs1∆, mca1∆, and moc3∆ suppressed the TBZ-sensitive phenotype of the pka1∆ strain, among which tfs1∆ was the strongest suppressor. All single mutants (rst2∆, tfs1∆, mca1∆, and moc3∆) showed a TBZ-tolerant phenotype. Tfs1 has two transcriptional domains (TFIIS and Zn finger domains), both of which contributed to the suppression of the pka1∆-induced TBZ-sensitive phenotype. pka1∆-induced chromosome mis-segregation was rescued by tfs1∆ in the presence of TBZ. tfs1 overexpression induced the TBZ-sensitive phenotype and a high frequency of chromosome mis-segregation, suggesting that the amount of Tfs1 must be strictly controlled. However, Tfs1-expression levels did not differ between the wild-type and pka1∆ strains, and the Tfs1-GFP protein was localized to the nucleus and cytoplasm in both strains, which excludes the direct regulation of expression and localization of Tfs1 by Pka1. Growth inhibition by TBZ in pka1∆ strains was notably rescued by double deletion of rst2 and tfs1 rather than single deletion of rst2 or tfs1, indicating that Rst2 and Tfs1 contribute independently to counteract TBZ toxicity. Our findings highlight Tfs1 as a key transcription factor for proper chromosome segregation.

Microtubule rescue at midzone edges promotes overlap stability and prevents spindle collapse during anaphase B

During anaphase B, molecular motors slide interpolar microtubules to elongate the mitotic spindle, contributing to the separation of chromosomes. However, sliding of antiparallel microtubules reduces their overlap, which may lead to spindle breakage, unless microtubules grow to compensate sliding. How sliding and growth are coordinated is still poorly understood. In this study, we have used the fission yeast S. pombe to measure microtubule dynamics during anaphase B. We report that the coordination of microtubule growth and sliding relies on promoting rescues at the midzone edges. This makes microtubules stable from pole to midzone, while their distal parts including the plus ends alternate between assembly and disassembly. Consequently, the midzone keeps a constant length throughout anaphase, enabling sustained sliding without the need for a precise regulation of microtubule growth speed. Additionally, we found that in S. pombe, which undergoes closed mitosis, microtubule growth speed decreases when the nuclear membrane wraps around the spindle midzone.

Transcriptome sequencing and screening of genes related to glucose availability in Schizosaccharomyces pombe by RNA-seq analysis

While calorie restriction is the most used experimental intervention to increase lifespan in numerous model organisms, increasing evidence suggests that excess glucose leads to decreased lifespan in various organisms. To fully understand the molecular basis of the pro-aging effect of glucose, it is still important to discover genetic interactions, gene expression patterns, and molecular responses depending on glucose availability. Here, we compared the gene expression profiles in Schizosaccharomyces pombe mid-log-phase cells grown in three different Synthetic Dextrose media with 3%, 5%, and 8% glucose, using the RNA sequencing method. Expression patterns of genes that function in carbohydrate metabolism were downregulated as expected, and these genes were downregulated in line with the increase in glucose content. Significant and consistent changes in the expression were observed such as genes that encoding retrotransposable elements, heat shock proteins, glutathione S-transferase, cell agglutination protein, and conserved fungal proteins. We group some genes that function together in the transcription process and mitotic regulation, which have recently been associated with glucose availability. Our results shed light on the relationship between excess glucose, diverse cellular processes, and aging.

Expression of Mug14 is regulated by the transcription factor Rst2 through the cAMP-dependent protein kinase pathway in Schizosaccharomyces pombe

The cAMP-dependent protein kinase (Pka1) regulates many cellular events, including sexual development and glycogenesis, and response to the limitation of glucose, in Schizosaccharomyces pombe. Despite its importance in many cellular events, the targets of the cAMP/PKA pathway have not been fully investigated. Here, we demonstrate that the expression of mug14 is induced by downregulation of the cAMP/PKA pathway and limitation of glucose. This regulation is dependent on the function of Rst2, a transcription factor that regulates transition from mitosis to meiosis. The loss of the C2H2-type zinc finger domain in Rst2, termed Rst2 (C2H2∆), abolished the induction of Mug14 expression. Upon deletion of the stress starvation response element of the S. pombe (STREP: CCCCTC) sequence, which is a potential binding site of Rst2 on mug14, in the pka1∆ strain, its induction was abolished. The expression of Mug14 was significantly reduced and delayed by the limitation of glucose and also by nitrogen starvation in the rst2∆ strain. Mug14 is known to share a common function with Mde1 and Mta3 in the methionine salvage pathway, but the expression of mde1 and mta3 mRNAs was not enhanced by pka1 deletion and limitation of glucose. We conclude that the expression of Mug14 is upregulated by Rst2 under the control of the cAMP/PKA signaling pathway, which senses the limitation of glucose.