Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: random orientation of septa in spherical cells

Experimental evolution of multicellularity via cuboidal cell packing in fission yeast

The evolution of multicellularity represents a major transition in life's history, enabling the rise of complex organisms. Multicellular groups can evolve through multiple developmental modes, but a common step is the formation of permanent cell-cell attachments after division. The characteristics of the multicellular morphology that emerges have profound consequences for the subsequent evolution of a nascent multicellular lineage, but little prior work has investigated these dynamics directly. Here, we examine a widespread yet understudied emergent multicellular morphology: cuboidal packing. Extinct and extant multicellular organisms across the tree of life have evolved to form groups in which spherical cells divide but remain attached, forming approximately cubic subunits. To experimentally investigate the evolution of cuboidal cell packing, we used settling selection to favor the evolution of simple multicellularity in unicellular, spherical Schizosaccharomyces pombe yeast. Multicellular clusters with cuboidal organization rapidly evolved, displacing the unicellular ancestor. These clusters displayed key hallmarks of an evolutionary transition in individuality: groups possess an emergent life cycle driven by physical fracture, group size is heritable, and they respond to group-level selection via multicellular adaptation. In 2 out of 5 lineages, group formation was driven by mutations in the ace2 gene, preventing daughter cell separation after division. Remarkably, ace2 mutations also underlie the transition to multicellularity in Saccharomyces cerevisiae and Candida glabrata, lineages that last shared a common ancestor > 300 million years ago. Our results provide insight into the evolution of cuboidal cell packing, an understudied multicellular morphology, and highlight the deeply convergent potential for a transition to multicellular individuality within fungi.

Sandwich freezing device for rapid freezing of viruses, bacteria, yeast, cultured cells and animal and human tissues in electron microscopy

We have been using sandwich freezing of living yeast and bacteria followed by freeze-substitution for observing close-to-native ultrastructure of cells. Recently, sandwich freezing of glutaraldehyde-fixed cultured cells and human tissues have been found to give excellent preservation of ultrastructure of cells and tissues. These studies, however, have been conducted using a handmade sandwich freezing device and have been limited in a few laboratories. To spread the use of this method to other laboratories, we fabricated and commercialized a new sandwich freezing device. The new device is inexpensive, portable and sterilizable. It can be used to rapid-freeze viruses, bacteria, yeast, cultured cells and animal and human tissues to a depth of 0.2 mm if tissues are prefixed with glutaraldehyde. The commercial availability of this device will expand application of rapid freezing to wide range of biological materials.

Regulation and Assembly of Actomyosin Contractile Rings in Cytokinesis and Cell Repair

Cytokinesis and single-cell wound repair both involve contractile assemblies of filamentous actin (F-actin) and myosin II organized into characteristic ring-like arrays. The assembly of these actomyosin contractile rings (CRs) is specified spatially and temporally by small Rho GTPases, which trigger local actin polymerization and myosin II contractility via a variety of downstream effectors. We now have a much clearer view of the Rho GTPase signaling cascade that leads to the formation of CRs, but some factors involved in CR positioning, assembly, and function remain poorly understood. Recent studies show that this regulation is multifactorial and goes beyond the long-established Ca²⁺ -dependent processes. There is substantial evidence that the Ca²⁺ -independent changes in cell shape, tension, and plasma membrane composition that characterize cytokinesis and single-cell wound repair also regulate CR formation. Elucidating the regulation and mechanistic properties of CRs is important to our understanding of basic cell biology and holds potential for therapeutic applications in human disease. In this review, we present a primer on the factors influencing and regulating CR positioning, assembly, and contraction as they occur in a variety of cytokinetic and single-cell wound repair models. Anat Rec, 301:2051-2066, 2018. © 2018 Wiley Periodicals, Inc.

Equatorial Assembly of the Cell-Division Actomyosin Ring in the Absence of Cytokinetic Spatial Cues

The position of the division site dictates the size and fate of daughter cells in many organisms. In animal cells, division-site placement involves overlapping mechanisms, including signaling from the central spindle microtubules, astral microtubules, and spindle poles and through polar contractions [1, 2, 3]. In fission yeast, division-site positioning requires overlapping mechanisms involving the anillin-related protein Mid1 and the tip complex (comprising the Kelch-repeat protein Tea1, the Dyrk-kinase Pom1, and the SH3-domain protein Tea4) [4, 5, 6, 7, 8, 9, 10, 11]. In addition to these factors, cell shape has also been shown to participate in the maintenance of the position of the actomyosin ring [12, 13, 14]. The first principles guiding actomyosin ring placement, however, have not been elucidated in any organism. Because actomyosin ring positioning, ring assembly, and cell morphogenesis are genetically separable in fission yeast, we have used it to derive actomyosin ring placement mechanisms from first principles. We report that, during ring assembly in the absence of cytokinetic cues (anillin-related Mid1 and tip-complex proteins), actin bundles follow the path of least curvature and assemble actomyosin rings in an equatorial position in spherical protoplasts and along the long axis in cylindrical cells and compressed protoplasts. The equatorial position of rings is abolished upon treatment of protoplasts with an actin-severing compound or by slowing down actin polymerization. We propose that the physical properties of actin filaments/bundles play key roles in actomyosin ring assembly and positioning, and that key cytokinetic molecules may modulate the length of actin filaments to promote ring assembly along the short axis.

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Spheroplasts lacking cytokinetic spatial cues assemble equatorial actomyosin rings

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An actin-severing compound abolishes equatorial ring assembly in spheroplasts

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Actin bundles favor the path of least curvature in the absence of cytokinetic cues

HDAC8 functions in spindle assembly during mouse oocyte meiosis

HDAC8 is a class I histone deacetylase that functions in a variety of biological processes through its non-histone substrates. However, its roles during oocyte meiosis remain elusive. Here, we document that HDAC8 localizes at spindle poles and positively participates in the regulation of microtubule organization and spindle assembly in mouse oocytes. Depletion of HDAC8 by siRNA-based gene silencing results in various spindle defects and chromosome misalignment during oocyte meiotic maturation, accompanied by impaired kinetochore-microtubule attachments. Consequently, a higher incidence of aneuploidy is generated in HDAC8-depleted MII eggs. In addition, inhibition of HDAC8 activity with its selective inhibitor PCI-34051 phenocopies the spindle/chromosome defects resulting from HDAC8 depletion by siRNA injection. Finally, we find that HDAC8 is required for the correct localization of ϕ-tubulin to spindle poles. Collectively, these data reveal that HDAC8 plays a significant role in regulating spindle assembly and thus ensuring the euploidy in mouse eggs.

Visualization of Fission Yeast Cells by Transmission Electron Microscopy

This chapter deals with the preparation of fission yeast (Schizosaccharomyces) cells for ultrastructural examination. The structure of the cell must be preserved as close to the in vivo situation as possible. This can be achieved by either chemical or cryofixation; the latter will not be dealt with in this chapter. Aldehydes that cross-link proteins and permanganates that besides cross-linking also stain membranous and cell wall structures are used for chemical fixation. This step is followed by dehydration and embedding of the cells in epoxy or acrylic resin. Sectioning of the embedded material produces slices of the cells that have to be stained with heavy metals to increase contrast differences between different structures or can be used for immunodetection of antigens (polysaccharides or proteins) with specific primary antibodies and gold-conjugated secondary antibodies.

Extracellular cell wall β(1,3)glucan is required to couple septation to actomyosin ring contraction

β(1,3)glucan is critical for contractile ring positioning and for coupling septum synthesis to constriction of the contractile ring and plasma membrane extension during cytokinesis.

Cytokinesis has been extensively studied in different models, but the role of the extracellular cell wall is less understood. Here we studied this process in fission yeast. The essential protein Bgs4 synthesizes the main cell wall β(1,3)glucan. We show that Bgs4-derived β(1,3)glucan is required for correct and stable actomyosin ring positioning in the cell middle, before the start of septum formation and anchorage to the cell wall. Consequently, β(1,3)glucan loss generated ring sliding, oblique positioned rings and septa, misdirected septum synthesis indicative of relaxed rings, and uncoupling between a fast ring and membrane ingression and slow septum synthesis, suggesting that cytokinesis can progress with defective septum pushing and/or ring pulling forces. Moreover, Bgs4-derived β(1,3)glucan is essential for secondary septum formation and correct primary septum completion. Therefore, our results show that extracellular β(1,3)glucan is required for cytokinesis to connect the cell wall with the plasma membrane and for contractile ring function, as proposed for the equivalent extracellular matrix in animal cells.

Regulatory dephosphorylation of CDK at G₂/M in plants: yeast mitotic phosphatase cdc25 induces cytokinin-like effects in transgenic tobacco morphogenesis

BACKGROUND

During the last three decades, the cell cycle and its control by cyclin-dependent kinases (CDKs) have been extensively studied in eukaryotes. This endeavour has produced an overall picture that basic mechanisms seem to be largely conserved among all eukaryotes. The intricate regulation of CDK activities includes, among others, CDK activation by CDC25 phosphatase at G₂/M. In plants, however, studies of this regulation have lagged behind as a plant Cdc25 homologue or other unrelated phosphatase active at G₂/M have not yet been identified.

SCOPE

Failure to identify a plant mitotic CDK activatory phosphatase led to characterization of the effects of alien cdc25 gene expression in plants. Tobacco, expressing the Schizosaccharomyces pombe mitotic activator gene, Spcdc25, exhibited morphological, developmental and biochemical changes when compared with wild type (WT) and, importantly, increased CDK dephosphorylation at G₂/M. Besides changes in leaf shape, internode length and root development, in day-neutral tobacco there was dramatically earlier onset of flowering with a disturbed acropetal floral capacity gradient typical of WT. In vitro, de novo organ formation revealed substantially earlier and more abundant formation of shoot primordia on Spcdc25 tobacco stem segments grown on shoot-inducing media when compared with WT. Moreover, in contrast to WT, stem segments from transgenic plants formed shoots even without application of exogenous growth regulator. Spcdc25-expressing BY-2 cells exhibited a reduced mitotic cell size due to a shortening of the G₂ phase together with high activity of cyclin-dependent kinase, NtCDKB1, in early S-phase, S/G₂ and early M-phase. Spcdc25-expressing tobacco ('Samsun') cell suspension cultures showed a clustered, more circular, cell phenotype compared with chains of elongated WT cells, and increased content of starch and soluble sugars. Taken together, Spcdc25 expression had cytokinin-like effects on the characteristics studied, although determination of endogenous cytokinin levels revealed a dramatic decrease in Spcdc25 transgenics.

CONCLUSIONS

The data gained using the plants expressing yeast mitotic activator, Spcdc25, clearly argue for the existence and importance of activatory dephosphorylation at G₂/M transition and its interaction with cytokinin signalling in plants. The observed cytokinin-like effects of Spcdc25 expression are consistent with the concept of interaction between cell cycle regulators and phytohormones during plant development. The G₂/M control of the plant cell cycle, however, remains an elusive issue as doubts persist about the mode of activatory dephosphorylation, which in other eukaryotes is provided by Cdc25 phosphatase serving as a final all-or-nothing mitosis regulator.

Positioning cytokinesis

Cytokinesis is the terminal step of the cell cycle during which a mother cell divides into daughter cells. Often, the machinery of cytokinesis is positioned in such a way that daughter cells are born roughly equal in size. However, in many specialized cell types or under certain environmental conditions, the cell division machinery is placed at nonmedial positions to produce daughter cells of different sizes and in many cases of different fates. Here we review the different mechanisms that position the division machinery in prokaryotic and eukaryotic cell types. We also describe cytokinesis-positioning mechanisms that are not adequately explained by studies in model organisms and model cell types.

Tobacco cells transformed with the fission yeast Spcdc25 mitotic inducer display growth and morphological characteristics as well as starch and sugar status evocable by cytokinin application

In plants, the G2/M control of cell cycle remains an elusive issue as doubts persist about activatory dephosphorylation--in other eukaryotes provided by CDC25 phosphatase and serving as a final all-or-nothing mitosis regulator. We report on the effects of tobacco (Nicotiana tabacum L., cv. Samsun) transformation with fission yeast (Schizosaccharomyces pombe) cdc25 (Spcdc25) on cell characteristics. Transformed cell suspension cultures showed higher dry mass accumulation during the exponential phase and clustered more circular cell phenotypes compared to chains of elongated WT cells. Similar cell parameters, as in the transformants, can be induced in WT by cytokinins. Spcdc25 cells, after cytokinin treatment, showed giant cell clusters and growth inhibition. In addition, Spcdc25 expression led to altered carbohydrate status: increased starch and soluble sugars with higher sucrose:hexoses ratio, inducible in WT by cytokinin treatment. Taken together, the Spcdc25 transformation had a cytokinin-like effect on studied characteristics. However, endogenous cytokinin determination revealed markedly lower cytokinin levels in Spcdc25 transformants. This indicates that the cells sense Spcdc25 expression as an increased cytokinin availability, manifested by changed cell morphology, and in consequence decrease endogenous cytokinin levels. Clearly, the results on cell growth and morphology are consistent with the model of G2/M control including cytokinin-regulated activatory dephosphorylation. Nevertheless, no clear link is obvious between Spcdc25 transformation and carbohydrate status and thus the observed cytokinin-like effect on carbohydrate levels poses a problem. Hence, we propose that Spcdc25-induced higher CDK(s) activity at G2/M generates a signal-modifying carbohydrate metabolism to meet high energy and C demands of forthcoming cell division.

Morphology transition genes in the dimorphic fission yeast Schizosaccharomyces japonicus

The ability of the saprophytic fungus Schizosaccharomyces japonicus to alternate between unicellular yeast form and multicellular true mycelium makes this organism an attractive non-pathogenic model for the investigation of di- and polymorphism critical for pathogenicity in many pathogenic fungi. In a previous work we described three mutations that made the cells unable to form hyphae. Here we report on the isolation of additional mycelium-minus mutants and show that the mutations represent seven genes. Apart from the inhibition of the yeast-to-mycelium transition, the mutations also cause drastic changes in the yeast phase. Cells of myc3-34 and myc4-35 grow isotropically with apolar distribution of actin and randomised localisation of division planes. The mutants myc1-4, myc1-10, myc1-36, myc5-39 and myc6-43 form sep-like, branching microhyphae containing bipolarly growing cells. All mutants are defective in the polarisation of vacuole distribution, and myc3-34, myc4-35 and myc7-56 are also defective in vacuole fusion. The diversity of the mutant phenotypes indicates that several cellular processes must take place simultaneously for the transition from the yeast phase into the mycelial phase.

Fission yeast cytoskeletons and cell polarity factors: connecting at the cortex

Cell polarity is a fundamental property of cells from unicellular to multicellular organisms. Most of the time, it is essential so that the cells can achieve their function. The fission yeast Schizosaccharomyces pombe is a powerful genetic model organism for studying the molecular mechanisms of the cell polarity process. Indeed, S. pombe cells are rod-shaped and cell growth is restricted at the poles. The accurate localization of the cell growth machinery at the cell cortex, which involves the actin cytoskeleton, depends on cell polarity pathways that are temporally and spatially regulated. The importance of interphase microtubules and cell polarity factors acting at the cortex of cell ends in this process has been shown. Here, we review recent advances in knowledge of molecular pathways leading to the establishment of a cellular axis in fission yeast. We also describe the role of cortical proteins and mitotic cytoskeletal rearrangements that control the symmetry of cell division.

The novel fission yeast protein Pal1p interacts with Hip1-related Sla2p/End4p and is involved in cellular morphogenesis

The establishment and maintenance of characteristic cellular morphologies is a fundamental property of all cells. Here we describe Schizosaccharomyces pombe Pal1p, a protein important for maintenance of cylindrical cellular morphology. Pal1p is a novel membrane-associated protein that localizes to the growing tips of interphase cells and to the division site in cells undergoing cytokinesis in an F-actin- and microtubule-independent manner. Cells deleted for pal1 display morphological defects, characterized by the occurrence of spherical and pear-shaped cells with an abnormal cell wall. Pal1p physically interacts and displays overlapping localization with the Huntingtin-interacting-protein (Hip1)-related protein Sla2p/End4p, which is also required for establishment of cylindrical cellular morphology. Sla2p is important for efficient localization of Pal1p to the sites of polarized growth and appears to function upstream of Pal1p. Interestingly, spherical pal1Delta mutants polarize to establish a pearlike morphology before mitosis in a manner dependent on the kelch-repeat protein Tea1p and the cell cycle inhibitory kinase Wee1p. Thus, overlapping mechanisms involving Pal1p, Tea1p, and Sla2p contribute to the establishment of cylindrical cellular morphology, which is important for proper spatial regulation of cytokinesis.

Ace2p controls the expression of genes required for cell separation in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells divide by medial fission through contraction of an actomyosin ring and deposition of a multilayered division septum that must be cleaved to release the two daughter cells. Here we describe the identification of seven genes (adg1(+), adg2(+), adg3(+), cfh4(+), agn1(+), eng1(+), and mid2(+)) whose expression is induced by the transcription factor Ace2p. The expression of all of these genes varied during the cell cycle, maximum transcription being observed during septation. At least three of these proteins (Eng1p, Agn1p, and Cfh4p) localize to a ring-like structure that surrounds the septum region during cell separation. Deletion of the previously uncharacterized genes was not lethal to the cells, but produced defects or delays in cell separation to different extents. Electron microscopic observation of mutant cells indicated that the most severe defect is found in eng1Delta agn1Delta cells, lacking the Eng1p endo-beta-1,3-glucanase and the Agn1p endo-alpha-glucanase. The phenotype of this mutant closely resembled that of ace2Delta mutants, forming branched chains of cells. This suggests that these two proteins are the main activities required for cell separation to be completed.

Multifunctional cytokinesis genes in Schizosaccharomyces pombe

The proper division of cells is essential for the production of viable daughter cells. In plants and fungi, the dividing cell produces a cross-wall or septum that bisects the cytoplasm. For separation of the daughter cells, the septum has to be cleaved. To study the regulation of this process, we isolated mutants defective in septum cleavage. The mutants showed highly pleiotropic phenotypes and defined 17 novel genes. The deduced amino acid sequences of the products of the cloned genes exhibited homologies to various transcription regulators of other organisms. The homologies and the pleiotropic effects of the mutations on sexual development, stress response, mitotic stability, septum initiation and septum placement indicated that these genes affect cell separation indirectly, through multifunctional regulatory modules.