The role of Schizosaccharomyces pombe dma1 in spore formation during meiosis

Sporulation: A response to starvation in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe employs two main strategies to adapt to the environment and survive when starved for nutrients. The strategies employ sporulation via sexual differentiation and extension of the chronological lifespan. When a cell is exposed to nutrient starvation in the presence of a cell of the opposite sex, the cells undergo fusion through conjugation and sporulation through meiosis. S. pombe spores are highly resistant to diverse stresses and may survive for a very long time. In this minireview, among the various sexual differentiation processes induced by starvation, we focused on and summarized the findings of the molecular mechanisms of spore formation in fission yeast. Furthermore, comparative measurements of the chronological lifespan of stationary phase cells and G0 cells and the survival period of spore cells revealed that the spore cells survived for a long period, indicating the presence of an effective mechanism for survival. Currently, many molecules involved in sporulation and their functions are being discovered; however, our understanding of these is not complete. Further understanding of spores may not only deepen our comprehension of sexual differentiation but may also provide hints for sustaining life.

Pcp1/pericentrin controls the SPB number in fission yeast meiosis and ploidy homeostasis

During sexual reproduction, the zygote must inherit exactly one centrosome (spindle pole body [SPB] in yeasts) from the gametes, which then duplicates and assembles a bipolar spindle that supports the subsequent cell division. Here, we show that in the fission yeast Schizosaccharomyces pombe, the fusion of SPBs from the gametes is blocked in polyploid zygotes. As a result, the polyploid zygotes cannot proliferate mitotically and frequently form supernumerary SPBs during subsequent meiosis, which leads to multipolar nuclear divisions and the generation of extra spores. The blockage of SPB fusion is caused by persistent SPB localization of Pcp1, which, in normal diploid zygotic meiosis, exhibits a dynamic association with the SPB. Artificially induced constitutive localization of Pcp1 on the SPB is sufficient to cause blockage of SPB fusion and formation of extra spores in diploids. Thus, Pcp1-dependent SPB quantity control is crucial for sexual reproduction and ploidy homeostasis in fission yeast.

Localization of the ubiquitin ligase Dma1 to the fission yeast contractile ring is modulated by phosphorylation

The timing of cytokinesis relative to other mitotic events in the fission yeast Schizosaccharomyces pombe is controlled by the septation initiation network (SIN). During a mitotic checkpoint, the SIN is inhibited by the E3 ubiquitin ligase Dma1 to prevent chromosome mis-segregation. Dma1 dynamically localizes to spindle pole bodies (SPBs) and the contractile ring (CR) during mitosis, though its role at the CR is unknown. Here, we examined whether Dma1 phosphorylation affects its localization or function. We found that preventing Dma1 phosphorylation by substituting the six phosphosites with alanines diminished its CR localization but did not affect its mitotic checkpoint function. These studies reinforce the conclusion that Dma1 localization to the SPB is key to its role in the mitotic checkpoint.

Targeting Polo-like kinase in space and time during C. elegans meiosis

A central player in meiotic chromosome dynamics is the conserved Polo-like kinase (PLK) family. PLKs are dynamically localized to distinct structures during meiotic prophase and phosphorylate a diverse group of substrates to control homolog pairing, synapsis, and meiotic recombination. In a recent study, we uncovered the mechanisms that control the targeting of a meiosis-specific PLK-2 in C. elegans. In early meiotic prophase, PLK-2 localizes to special chromosome regions known as pairing centers and drives homolog pairing and synapsis. PLK-2 then relocates to the synaptonemal complex (SC) after crossover designation and mediates chromosome remodeling required for homolog separation. What controls this intricate targeting of PLK-2 in space and time? We discuss recent findings and remaining questions for the future.

Schizosaccharomyces pombe: A Promising Biotechnology for Modulating Wine Composition

There are numerous yeast species related to wine making, particularly non-Saccharomyces, that deserve special attention due to the great potential they have when it comes to making certain changes in the composition of the wine. Among them, Schizosaccharomyces pombe stands out for its particular metabolism that gives it certain abilities such as regulating the acidity of wine through maloalcoholic fermentation. In addition, this species is characterized by favouring the formation of stable pigments in wine and releasing large quantities of polysaccharides during ageing on lees. Moreover, its urease activity and its competition for malic acid with lactic acid bacteria make it a safety tool by limiting the formation of ethyl carbamate and biogenic amines in wine. However, it also has certain disadvantages such as its low fermentation speed or the development of undesirable flavours and aromas. In this chapter, the main oenological uses of Schizosaccharomyces pombe that have been proposed in recent years will be reviewed and discussed.

Genes Important for Schizosaccharomyces pombe Meiosis Identified Through a Functional Genomics Screen

Meiosis is a specialized cell division that generates gametes, such as eggs and sperm. Errors in meiosis result in miscarriages and are the leading cause of birth defects; however, the molecular origins of these defects remain unknown. Studies in model organisms are beginning to identify the genes and pathways important for meiosis, but the parts list is still poorly defined. Here we present a comprehensive catalog of genes important for meiosis in the fission yeast, Schizosaccharomyces pombe. Our genome-wide functional screen surveyed all nonessential genes for roles in chromosome segregation and spore formation. Novel genes important at distinct stages of the meiotic chromosome segregation and differentiation program were identified. Preliminary characterization implicated three of these genes in centrosome/spindle pole body, centromere, and cohesion function. Our findings represent a near-complete parts list of genes important for meiosis in fission yeast, providing a valuable resource to advance our molecular understanding of meiosis.

Pombe's thirteen - control of fission yeast cell division by the septation initiation network

The septation initiation network (SIN) regulates aspects of cell growth and division in Schizosaccharomyces pombe and is essential for cytokinesis. Insufficient signalling results in improper assembly of the contractile ring and failure of cytokinesis, generating multinucleated cells, whereas too much SIN signalling uncouples cytokinesis from the rest of the cell cycle. SIN signalling is therefore tightly controlled to coordinate cytokinesis with chromosome segregation. Signalling originates from the cytoplasmic face of the spindle pole body (SPB), and asymmetric localisation of some SIN proteins to one of the two SPBs during mitosis is important for regulation of the SIN. Recent studies have identified in vivo substrates of the SIN, which include components involved in mitotic control, those of the contractile ring and elements of the signalling pathway regulating polarised growth. The SIN is also required for spore formation following meiosis. This has provided insights into how the SIN performs its diverse functions in the cell cycle and shed new light on its regulation.

Meikin is a conserved regulator of meiosis-I-specific kinetochore function

The kinetochore is the crucial apparatus regulating chromosome segregation in mitosis and meiosis. Particularly in meiosis I, unlike in mitosis, sister kinetochores are captured by microtubules emanating from the same spindle pole (mono-orientation) and centromeric cohesion mediated by cohesin is protected in the following anaphase. Although meiotic kinetochore factors have been identified only in budding and fission yeasts, these molecules and their functions are thought to have diverged earlier. Therefore, a conserved mechanism for meiotic kinetochore regulation remains elusive. Here we have identified in mouse a meiosis-specific kinetochore factor that we termed MEIKIN, which functions in meiosis I but not in meiosis II or mitosis. MEIKIN plays a crucial role in both mono-orientation and centromeric cohesion protection, partly by stabilizing the localization of the cohesin protector shugoshin. These functions are mediated mainly by the activity of Polo-like kinase PLK1, which is enriched to kinetochores in a MEIKIN-dependent manner. Our integrative analysis indicates that the long-awaited key regulator of meiotic kinetochore function is Meikin, which is conserved from yeasts to humans.

The meiosis-specific nuclear passenger protein is required for proper assembly of forespore membrane in fission yeast

Sporulation, gametogenesis in yeast, consists of meiotic nuclear division and spore morphogenesis. In the fission yeast Schizosaccharomyces pombe, the four haploid nuclei produced after meiosis II are encapsulated by the forespore membrane (FSM), which is newly synthesized from spindle pole bodies (SPBs) in the cytoplasm of the mother cell as spore precursors. Although the coordination between meiosis and FSM assembly is vital for proper sporulation, the underlying mechanism remains unclear. In the present study, we identified a new meiosis-specific protein Npg1, and found that it was involved in the efficient formation of spores and spore viability. The accumulation and organization of the FSM was compromised in npg1-null cells, leading to the error-prone envelopment of nuclei. Npg1 was first seen as internuclear dots and translocated to the SPBs before the FSM assembled. Genetic analysis revealed that Npg1 worked in conjunction with the FSM proteins Spo3 and Meu14. These results suggest a possible signaling link from the nucleus to the meiotic SPBs in order to associate the onset of FSM assembly with meiosis II, which ensures the successful partitioning of gametic nuclei.

Dissecting the first and the second meiotic divisions using a marker-less drug-hypersensitive fission yeast

Faithful chromosome segregation during meiosis is indispensable to prevent birth defects and infertility. Canonical genetic manipulations have not been very useful for studying meiosis II, since mutations of genes involved in cell cycle regulation or chromosome segregation may affect meiosis I, making interpretations of any defects observed in meiosis II complicated. Here we present a powerful strategy to dissect meiosis I and meiosis II, using chemical inhibitors in genetically tractable model organism fission yeast (Schizosaccharomyces pombe). As various chemical probes are not active in fission yeast, mainly due to an effective multidrug resistance (MDR) response, we have recently developed a drug-hypersensitive MDR-sup strain by suppression of the key genes responsible for MDR response. We further developed the MDR-supML (marker-less) strain by deleting 7 MDR genes without commonly used antibiotic markers. The new strain makes fluorescent tagging and gene deletion much simpler, which enables effective protein visualization in varied genetic backgrounds. Using the MDR-supML strain with chemical inhibitors and live cell fluorescence microscopy, we established cell cycle arrest at meiosis I and meiosis II and examined Aurora-dependent spindle assembly checkpoint (SAC) regulation during meiosis. We found that Aurora B/Ark1 kinase activity is required for recruitment of Bub1, an essential SAC kinase, to unattached kinetochore in prometaphase I and prometaphase II as in mitosis. Thus, Aurora's role in SAC activation is likely conserved in mitosis, meiosis I, and meiosis II. Together, our MDR-supML strain will be useful to dissect complex molecular mechanisms in mitosis and 2 successive meiotic divisions.

Dma1-dependent degradation of Septation Initiation Network proteins during meiosis in Schizosaccharomyces pombe

The Schizosaccharomyces pombe septation initiation network (SIN) is required for cytokinesis during vegetative growth and spore formation during meiosis. Regulation of the SIN during mitosis has been studied extensively, but less is known about its meiotic regulation. Here, we show that several aspects of the SIN regulation differ between mitosis and meiosis. First, the presence of GTP-bound spg1p is not the main determinant of the timing of cdc7p and sid1p association with the SPB during meiosis. Second, the localisation dependencies of SIN proteins differ from those in mitotic cells, suggesting a modified functional organisation of the SIN during meiosis. Third, there is stage-specific degradation of SIN components in meiosis; byr4p is degraded after meiosis I, while the degradation of cdc7p, cdc11p and sid4p occurs after the second meiotic division and depends upon the ubiquitin ligase dma1p. Finally, dma1p-dependent degradation is not restricted to the SIN, for we show that dma1p is needed for the degradation of mcp6p/hrs1p in meiosis I. Together, these data suggest that stage-specific targetted proteolysis will play an important role in regulating meiotic progression.

Microtubules and Alp7-Alp14 (TACC-TOG) reposition chromosomes before meiotic segregation

Tethering kinetochores at spindle poles facilitates their efficient capture and segregation by microtubules at mitotic onset in yeast. During meiotic prophase of fission yeast, however, kinetochores are detached from the poles, which facilitates meiotic recombination but may cause a risk of chromosome mis-segregation during meiosis. How cells circumvent this dilemma remains unclear. Here we show that an extensive microtubule array assembles from the poles at meiosis I onset and retrieves scattered kinetochores towards the poles to prevent chromosome drift. Moreover, the microtubule-associated protein complex Alp7-Alp14 (the fission yeast orthologues of mammalian TACC-TOG) is phosphorylated by Polo kinase, which promotes its meiosis-specific association to the outer kinetochore complex Nuf2-Ndc80 of scattered kinetochores, thereby assisting in capturing remote kinetochores. Although TOG was recently characterized as a microtubule polymerase, Dis1 (the other TOG orthologue in fission yeast), together with the Dam1 complex, plays a role in microtubule shortening to pull kinetochores polewards. Thus, microtubules and their binding proteins uniquely reconstitute chromosome configuration during meiosis.

Polar opposites: Fine-tuning cytokinesis through SIN asymmetry

Mitotic exit and cell division must be spatially and temporally integrated to facilitate equal division of genetic material between daughter cells. In the fission yeast, Schizosaccharomyces pombe, a spindle pole body (SPB) localized signaling cascade termed the septation initiation network (SIN) couples mitotic exit with cytokinesis. The SIN is controlled at many levels to ensure that cytokinesis is executed once per cell cycle and only after cells segregate their DNA. An interesting facet of the SIN is that its activity is asymmetric on the two SPBs during anaphase; however, how and why the SIN is asymmetric has remained elusive. Many key factors controlling SIN asymmetry have now been identified, shedding light on the significance of SIN asymmetry in regulating cytokinesis. In this review, we highlight recent advances in our understanding of SIN regulation, with an emphasis on how SIN asymmetry is achieved and how this aspect of SIN regulation fine-tunes cytokinesis.

Spindle pole body components are reorganized during fission yeast meiosis

We show that spindle pole body (SPB) remodeling during meiosis in fission yeast is essential for meiosis. Many SPB components disappear during meiotic prophase and return to the SPBs at meiosis I onset. We found novel functions for Polo kinase/Plo1 and centrin/Cdc31 in the meiotic reorganization of SPB components.

During meiosis, the centrosome/spindle pole body (SPB) must be regulated in a manner distinct from that of mitosis to achieve a specialized cell division that will produce gametes. In this paper, we demonstrate that several SPB components are localized to SPBs in a meiosis-specific manner in the fission yeast Schizosaccharomyces pombe. SPB components, such as Cut12, Pcp1, and Spo15, which stay on the SPB during the mitotic cell cycle, disassociate from the SPB during meiotic prophase and then return to the SPB immediately before the onset of meiosis I. Interestingly, the polo kinase Plo1, which normally localizes to the SPB during mitosis, is excluded from them in meiotic prophase, when meiosis-specific, horse-tail nuclear movement occurs. We found that exclusion of Plo1 during this period was essential to properly remodel SPBs, because artificial targeting of Plo1 to SPBs resulted in an overduplication of SPBs. We also found that the centrin Cdc31 was required for meiotic SPB remodeling. Thus Plo1 and a centrin play central roles in the meiotic SPB remodeling, which is essential for generating the proper number of meiotic SPBs and, thereby provide unique characteristics to meiotic divisions.

Roles of the DYRK kinase Pom2 in cytokinesis, mitochondrial morphology, and sporulation in fission yeast

Pom2 is predicted to be a dual-specificity tyrosine-phosphorylation regulated kinase (DYRK) related to Pom1 in Schizosaccharomyces pombe. DYRKs share a kinase domain capable of catalyzing autophosphorylation on tyrosine and exogenous phosphorylation on serine/threonine residues. Here we show that Pom2 is functionally different from the well-characterized Pom1, although they share 55% identity in the kinase domain and the Pom2 kinase domain functionally complements that of Pom1. Pom2 localizes to mitochondria throughout the cell cycle and to the contractile ring during late stages of cytokinesis. Overexpression but not deletion of pom2 results in severe defects in cytokinesis, indicating that Pom2 might share an overlapping function with other proteins in regulating cytokinesis. Gain and loss of function analyses reveal that Pom2 is required for maintaining mitochondrial morphology independently of microtubules. Intriguingly, most meiotic pom2Δ cells form aberrant asci with meiotic and/or forespore membrane formation defects. Taken together, Pom2 is a novel DYRK kinase involved in regulating cytokinesis, mitochondrial morphology, meiosis, and sporulation in fission yeast.

A novel role of Dma1 in regulating forespore membrane assembly and sporulation in fission yeast

By characterizing the fission yeast Dma1's function during meiosis, we revealed that Dma1 is required for spore formation, while it is dispensable for fidelity of nuclear divisions. We also found that Dma1 is functionally related to SIN pathway and meiosis-specific kinase Slk1 during sporulation.

In fission yeast Schizosaccharomyces pombe, a diploid mother cell differentiates into an ascus containing four haploid ascospores following meiotic nuclear divisions, through a process called sporulation. Several meiosis-specific proteins of fission yeast have been identified to play essential roles in meiotic progression and sporulation. We report here an unexpected function of mitotic spindle checkpoint protein Dma1 in proper spore formation. Consistent with its function in sporulation, expression of dma1⁺ is up-regulated during meiosis I and II. We showed that Dma1 localizes to the SPB during meiosis and the maintenance of this localization at meiosis II depends on septation initiation network (SIN) scaffold proteins Sid4 and Cdc11. Cells lacking Dma1 display defects associated with sporulation but not nuclear division, leading frequently to formation of asci with fewer spores. Our genetic analyses support the notion that Dma1 functions in parallel with the meiosis-specific Sid2-related protein kinase Slk1/Mug27 and the SIN signaling during sporulation, possibly through regulating proper forespore membrane assembly. Our studies therefore revealed a novel function of Dma1 in regulating sporulation in fission yeast.