Two S. pombe septation phases differ in ingression rate, septum structure, and response to F-actin loss

Identification and characterization of a novel antifungal compound tubeimoside I targeting cell wall

Due to fungal diseases that threaten immunocompromised patients, along with the limited availability of antifungal agents, there is an urgent need for new antifungal compounds to treat fungal infections. Here, we aimed to identify potential antifungal drugs from natural products using the fission yeast Schizosaccharomyces pombe as a model organism since it shares many features with some pathogenic fungi. Here, we identified tubeimoside I (TBMS1), an extract from Chinese herbal medicine, that showed strong antifungal activity against S. pombe. To gain insight into the underlying mechanism, we performed transcriptomics analyses of S. pombe cells exposed to TBMS1. A significant proportion of the differential expressed genes were involved in cell wall organization or biogenesis. Additionally, TBMS1 treatment of S. pombe cells resulted in pleiotropic phenotypes, including increased sensitivity to β-glucanase, enhanced calcineurin activity, translocation of GFP-Prz1 to the nucleus, as well as enhanced dephosphorylation of Prz1, suggesting that TBMS1 disrupted cell wall integrity of S. pombe cells. Notably, calcofluor staining showed that abnormal deposits of cell wall materials were observed in the septum and cell wall of the TBMS1-treated cells, which were further corroborated by electron microscopy analysis. We also found that oxidative stress might be involved in the antifungal action of TBMS1. Moreover, we confirmed the antifungal activities of TBMS1 against several clinical isolates of pathogenic fungi. Collectively, our findings suggest that TBMS1, a novel antifungal compound, exerts its antifungal activity by targeting cell walls, which may pave the way for the development of a new class of antifungals.

IMPORTANCE

Fungal infections pose a serious threat to public health and have become an emerging crisis worldwide. The development of new antifungal agents is urgently needed. Here, we identified compound tubeimoside I (TBMS1) for the first time showing strong antifungal activity, and explored the underlying mechanisms of its antifungal action by using the model yeast Schizosaccharomyces pombe. Notably, we presented multiple evidence that TBMS1 exerts its antifungal activity through targeting fungal cell walls. Moreover, we verified the antifungal activities of TBMS1 against several pathogenic fungi. Our work indicated that TBMS1 may serve as a novel antifungal candidate, which provides an important foundation for designing and developing new cell wall-targeting agents for combating life-threatening fungal infections.

The fission yeast cytokinetic ring component Fic1 promotes septum formation

In Schizosaccharomyces pombe, septum formation is coordinated with cytokinetic ring constriction but the mechanisms linking these events are unclear. In this study, we explored the role of the cytokinetic ring component Fic1, first identified by its interaction with the F-BAR protein Cdc15, in septum formation. We found that the fic1 phospho-ablating mutant, fic1-2A, is a gain-of-function allele that suppresses myo2-E1, the temperature-sensitive allele of the essential type-II myosin, myo2. This suppression is achieved by the promotion of septum formation and required Fic1's interaction with the F-BAR proteins Cdc15 and Imp2. Additionally, we found that Fic1 interacts with Cyk3 and that this interaction was likewise required for Fic1's role in septum formation. Fic1, Cdc15, Imp2, and Cyk3 are the orthologs of the Saccharomyces cerevisiae ingression progression complex, which stimulates the chitin synthase Chs2 to promote primary septum formation. However, our findings indicate that Fic1 promotes septum formation and cell abscission independently of the S. pombe Chs2 ortholog. Thus, while similar complexes exist in the two yeasts that each promote septation, they appear to have different downstream effectors.

The fission yeast cytokinetic ring component Fic1 promotes septum formation

In Schizosaccharomyces pombe septum formation is coordinated with cytokinetic ring constriction but the mechanisms linking these events are unclear. In this study, we explored the role of the cytokinetic ring component Fic1, first identified by its interaction with the F-BAR protein Cdc15, in septum formation. We found that the fic1 phospho-ablating mutant, fic1-2A , is a gain-of-function allele that suppresses myo2-E1 , the temperature-sensitive allele of the essential type-II myosin, myo2 . This suppression is achieved by the promotion of septum formation and required Fic1's interaction with the F-BAR proteins Cdc15 and Imp2. Additionally, we found that Fic1 interacts with Cyk3 and that this interaction was likewise required for Fic1's role in septum formation. Fic1, Cdc15, Imp2, and Cyk3 are the orthologs of the Saccharomyces cerevisiae ingression progression complex, which stimulates the chitin synthase Chs2 to promote primary septum formation. However, our findings indicate that Fic1 promotes septum formation and cell abscission independently of the S. pombe Chs2 ortholog. Thus, while similar complexes exist in the two yeasts that each promote septation, they appear to have different downstream effectors.

Summary Statement

The S. pombe cytokinetic ring protein Fic1 promotes septum formation in a manner dependent on interactions with the cytokinetic ring components Cdc15, Imp2, and Cyk3.

Analysis of the Localization of Schizosaccharomyces pombe Glucan Synthases in the Presence of the Antifungal Agent Caspofungin

In recent years, invasive fungal infections have emerged as a common source of infections in immunosuppressed patients. All fungal cells are surrounded by a cell wall that is essential for cell integrity and survival. It prevents cell death and lysis resulting from high internal turgor pressure. Since the cell wall is not present in animal cells, it is an ideal target for selective invasive fungal infection treatments. The antifungal family known as echinocandins, which specifically inhibit the synthesis of the cell wall β(13)glucan, has been established as an alternative treatment for mycoses. To explore the mechanism of action of these antifungals, we analyzed the cell morphology and glucan synthases localization in Schizosaccharomyces pombe cells during the initial times of growth in the presence of the echinocandin drug caspofungin. S. pombe are rod-shaped cells that grow at the poles and divide by a central division septum. The cell wall and septum are formed by different glucans, which are synthesized by four essential glucan synthases: Bgs1, Bgs3, Bgs4, and Ags1. Thus, S. pombe is not only a perfect model for studying the synthesis of the fungal β(1-3)glucan, but also it is ideal for examining the mechanisms of action and resistance of cell wall antifungals. Herein, we examined the cells in a drug susceptibility test in the presence of either lethal or sublethal concentrations of caspofungin, finding that exposure to the drug for long periods at high concentrations (>10 µg/mL) induced cell growth arrest and the formation of rounded, swollen, and dead cells, whereas low concentrations (<10 µg/mL) permitted cell growth with a mild effect on cell morphology. Interestingly, short-term treatments with either high or low concentrations of the drug induced effects contrary to those observed in the susceptibility tests. Thus, low drug concentrations induced a cell death phenotype that was not observed at high drug concentrations, which caused transient fungistatic cell growth arrest. After 3 h, high concentrations of the drug caused the following: (i) a decrease in the GFP-Bgs1 fluorescence level; (ii) altered locations of Bgs3, Bgs4, and Ags1; and (iii) a simultaneous accumulation of cells with calcofluor-stained incomplete septa, which at longer times resulted in septation uncoupling from plasma membrane ingression. The incomplete septa revealed with calcofluor were found to be complete when observed via the membrane-associated GFP-Bgs or Ags1-GFP. Finally, we found that the accumulation of incomplete septa depended on Pmk1, the last kinase of the cell wall integrity pathway.

Echinocandin Drugs Induce Differential Effects in Cytokinesis Progression and Cell Integrity

Fission yeast contains three essential β(1,3)-D-glucan synthases (GSs), Bgs1, Bgs3, and Bgs4, with non-overlapping roles in cell integrity and morphogenesis. Only the bgs4⁺ mutants pbr1-8 and pbr1-6 exhibit resistance to GS inhibitors, even in the presence of the wild-type (WT) sequences of bgs1⁺ and bgs3⁺. Thus, Bgs1 and Bgs3 functions seem to be unaffected by those GS inhibitors. To learn more about echinocandins' mechanism of action and resistance, cytokinesis progression and cell death were examined by time-lapse fluorescence microscopy in WT and pbr1-8 cells at the start of treatment with sublethal and lethal concentrations of anidulafungin, caspofungin, and micafungin. In WT, sublethal concentrations of the three drugs caused abundant cell death that was either suppressed (anidulafungin and micafungin) or greatly reduced (caspofungin) in pbr1-8 cells. Interestingly, the lethal concentrations induced differential phenotypes depending on the echinocandin used. Anidulafungin and caspofungin were mostly fungistatic, heavily impairing cytokinesis progression in both WT and pbr1-8. As with sublethal concentrations, lethal concentrations of micafungin were primarily fungicidal in WT cells, causing cell lysis without impairing cytokinesis. The lytic phenotype was suppressed again in pbr1-8 cells. Our results suggest that micafungin always exerts its fungicidal effect by solely inhibiting Bgs4. In contrast, lethal concentrations of anidulafungin and caspofungin cause an early cytokinesis arrest, probably by the combined inhibition of several GSs.

The Role of the Cell Integrity Pathway in Septum Assembly in Yeast

Cytokinesis divides a mother cell into two daughter cells at the end of each cell cycle and proceeds via the assembly and constriction of a contractile actomyosin ring (CAR). Ring constriction promotes division furrow ingression, after sister chromatids are segregated to opposing sides of the cleavage plane. Cytokinesis contributes to genome integrity because the cells that fail to complete cytokinesis often reduplicate their chromosomes. While in animal cells, the last steps of cytokinesis involve extracellular matrix remodelling and mid-body abscission, in yeast, CAR constriction is coupled to the synthesis of a polysaccharide septum. To preserve cell integrity during cytokinesis, fungal cells remodel their cell wall through signalling pathways that connect receptors to downstream effectors, initiating a cascade of biological signals. One of the best-studied signalling pathways is the cell wall integrity pathway (CWI) of the budding yeast Saccharomyces cerevisiae and its counterpart in the fission yeast Schizosaccharomyces pombe, the cell integrity pathway (CIP). Both are signal transduction pathways relying upon a cascade of MAP kinases. However, despite strong similarities in the assembly of the septa in both yeasts, there are significant mechanistic differences, including the relationship of this process with the cell integrity signalling pathways.

Contractile ring constriction and septation in fission yeast are integrated mutually stabilizing processes.. [DOI: 10.1101/2021.06.25.449700] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In common with other cellular machineries, the actomyosin contractile ring that divides cells during cytokinesis does not operate in isolation. Contractile rings in animal cells interact with contiguous actomyosin cortex, while ring constriction in many cell-walled organisms couples tightly to cell wall growth. In fission yeast, a septum grows in the wake of the constricting ring, ensuring cytokinesis leaves two daughter cells fully enclosed by cell wall. Here we mathematical modeled the integrated constriction-septation system in fission yeast, with a kinetic growth model evolving the 3D septum shape coupled to a molecularly explicit simulation of the contractile ring highly constrained by experimental data. Simulations revealed influences in both directions, stabilizing the ring-septum system as a whole. By providing a smooth circular anchoring surface for the ring, the inner septum leading edge stabilized ring organization and tension production; by mechanically regulating septum circularity and in-plane growth, ring tension stabilized septum growth and shape. Genetic or pharmacological perturbation of either subsystem destabilized this delicate balance, precipitating uncontrolled positive feedback with disastrous morphological and functional consequences. Thus, high curvature septum irregularities triggered bridging instabilities, in which contractile ring segments became unanchored. Bridging abolished the local tension-mediated septum shape regulation, exacerbating the irregularity in a mutually destabilizing runaway process. Our model explains a number of previously mysterious experimental observations, including unanchoring of ring segments observed in cells with mutations in the septum-growing β-glucan synthases, and irregular septa in cells with mutations in the contractile ring myosin-II Myo2. Thus, the contractile ring and cell wall growth cellular machineries operate as a single integrated system, whose stability relies on mutual regulation by the two subsystems.

Characterization of the nanomechanical properties of the fission yeast (Schizosaccharomyces pombe) cell surface by atomic force microscopy

Variations in cell wall composition and biomechanical properties can contribute to the cellular plasticity required during complex processes such as polarized growth and elongation in microbial cells. This study utilizes atomic force microscopy (AFM) to map the cell surface topography of fission yeast, Schizosaccharomyces pombe, at the pole regions and to characterize the biophysical properties within these regions under physiological, hydrated conditions. High-resolution images acquired from AFM topographic scanning reveal decreased surface roughness at the cell poles. Force extension curves acquired by nanoindentation probing with AFM cantilever tips under low applied force revealed increased cell wall deformation and decreased cellular stiffness (cellular spring constant) at cell poles (17 ± 4 mN/m) relative to the main body of the cell that is not undergoing growth and expansion (44 ± 10 mN/m). These findings suggest that the increased deformation and decreased stiffness at regions of polarized growth at fission yeast cell poles provide the plasticity necessary for cellular extension. This study provides a direct biophysical characterization of the S. pombe cell surface by AFM, and it provides a foundation for future investigation of how the surface topography and local nanomechanical properties vary during different cellular processes.

Fission yeast Pak1 phosphorylates anillin-like Mid1 for spatial control of cytokinesis

Protein kinases direct polarized growth by regulating the cytoskeleton in time and space and could play similar roles in cell division. We found that the Cdc42-activated polarity kinase Pak1 colocalizes with the assembling contractile actomyosin ring (CAR) and remains at the division site during septation. Mutations in pak1 led to defects in CAR assembly and genetic interactions with cytokinesis mutants. Through a phosphoproteomic screen, we identified novel Pak1 substrates that function in polarized growth and cytokinesis. For cytokinesis, we found that Pak1 regulates the localization of its substrates Mid1 and Cdc15 to the CAR. Mechanistically, Pak1 phosphorylates the Mid1 N-terminus to promote its association with cortical nodes that act as CAR precursors. Defects in Pak1-Mid1 signaling lead to misplaced and defective division planes, but these phenotypes can be rescued by synthetic tethering of Mid1 to cortical nodes. Our work defines a new signaling mechanism driven by a cell polarity kinase that promotes CAR assembly in the correct time and place.

Comparative Analysis of the Roles of Non-muscle Myosin-IIs in Cytokinesis in Budding Yeast, Fission Yeast, and Mammalian Cells

The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.

Inhibition of cell membrane ingression at the division site by cell walls in fission yeast

Eukaryotic cells assemble actomyosin rings during cytokinesis to function as force-generating machines to drive membrane invagination and to counteract the intracellular pressure and the cell surface tension. How the extracellular matrix affects actomyosin ring contraction has not been fully explored. While studying the Schizosaccharomyces pombe 1,3-β-glucan-synthase mutant cps1-191, which is defective in division septum synthesis and arrests with a stable actomyosin ring, we found that weakening of the extracellular glycan matrix caused the generated spheroplasts to divide under the nonpermissive condition. This nonmedial slow division was dependent on a functional actomyosin ring and vesicular trafficking, but independent of normal septum synthesis. Interestingly, the high intracellular turgor pressure appears to play a minimal role in inhibiting ring contraction in the absence of cell wall remodeling in cps1-191 mutants, as decreasing the turgor pressure alone did not enable spheroplast division. We propose that during cytokinesis, the extracellular glycan matrix restricts actomyosin ring contraction and membrane ingression, and remodeling of the extracellular components through division septum synthesis relieves the inhibition and facilitates actomyosin ring contraction.

Roles of Mso1 and the SM protein Sec1 in efficient vesicle fusion during fission yeast cytokinesis

Membrane trafficking during cytokinesis is essential for the delivery of membrane lipids and cargoes to the division site. However, the molecular mechanisms are still incompletely understood. In this study, we demonstrate the importance of uncharacterized fission yeast proteins Mso1 and Sec1 in membrane trafficking during cytokinesis. Fission yeast Mso1 shares homology with budding yeast Mso1 and human Mint1, proteins that interact with Sec1/Munc18 family proteins during vesicle fusion. Sec1/Munc18 proteins and their interactors are important regulators of SNARE complex formation during vesicle fusion. The roles of these proteins in vesicle trafficking during cytokinesis have been barely studied. Here, we show that fission yeast Mso1 is also a Sec1-binding protein and Mso1 and Sec1 localize to the division site interdependently during cytokinesis. The loss of Sec1 localization in mso1Δ cells results in a decrease in vesicle fusion and cytokinesis defects such as slow ring constriction, defective ring disassembly, and delayed plasma membrane closure. We also find that Mso1 and Sec1 may have functions independent of the exocyst tethering complex on the plasma membrane at the division site. Together, Mso1 and Sec1 play essential roles in regulating vesicle fusion and cargo delivery at the division site during cytokinesis.