Calyculin A induces contractile ring-like apparatus formation and condensation of chromosomes in unfertilized sea urchin eggs

Rho-associated kinase regulates Langerhans cell morphology and responsiveness to tissue damage

Skin damage requires efficient immune cell responses to restore organ function. Epidermal-resident immune cells known as Langerhans cells use dendritic protrusions to surveil the skin microenvironment, which contains keratinocytes and peripheral axons. The mechanisms governing Langerhans cell dendrite dynamics and responses to tissue damage are poorly understood. Using skin explants from adult zebrafish, we show that Langerhans cells maintain normal surveillance following axonal degeneration and use their dendrites to engulf small axonal debris. By contrast, a ramified-to-rounded shape transition accommodates the engulfment of larger keratinocyte debris. We find that Langerhans cell dendrites are populated with actin and sensitive to a broad-spectrum actin inhibitor. We show that Rho-associated kinase (ROCK) inhibition leads to elongated dendrites, perturbed clearance of large debris, and reduced Langerhans cell migration to epidermal wounds. Our work describes the dynamics of Langerhans cells and involvement of the ROCK pathway in immune cell responses.

Rho-associated kinase regulates Langerhans cell morphology and responsiveness to tissue damage

Skin is often the first physical barrier to encounter invading pathogens and physical damage. Damage to the skin must be resolved quickly and efficiently to maintain organ homeostasis. Epidermal-resident immune cells known as Langerhans cells use dendritic protrusions to dynamically surveil the skin microenvironment, which contains epithelial keratinocytes and somatosensory peripheral axons. The mechanisms governing Langerhans cell dendrite dynamics and responses to tissue damage are not well understood. Using skin explants from adult zebrafish, we show that Langerhans cells maintain normal surveillance activity following axonal degeneration and use their dynamic dendrites to engulf small axonal debris. By contrast, a ramified-to-rounded shape transition accommodates the engulfment of larger keratinocyte debris. We find that Langerhans cell dendrites are richly populated with actin and sensitive to a broad spectrum actin inhibitor. We further show that Rho-associated kinase (ROCK) inhibition leads to elongated dendrites, perturbed clearance of large debris, and reduced Langerhans cell migration to tissue-scale wounds. Altogether, our work describes the unique dynamics of Langerhans cells and involvement of the ROCK pathway in immune cell responses to damage of varying magnitude.

Ventral stress fibers induce plasma membrane deformation in human fibroblasts

Interactions between the actin cytoskeleton and the plasma membrane are important in many eukaryotic cellular processes. During these processes, actin structures deform the cell membrane outward by applying forces parallel to the fiber's major axis (as in migration) or they deform the membrane inward by applying forces perpendicular to the fiber's major axis (as in the contractile ring during cytokinesis). Here we describe a novel actin-membrane interaction in human dermal myofibroblasts. When labeled with a cytosolic fluorophore, the myofibroblasts displayed prominent fluorescent structures on the ventral side of the cell. These structures are present in the cell membrane and colocalize with ventral actin stress fibers, suggesting that the stress fibers bend the membrane to form a "cytosolic pocket" that the fluorophores diffuse into, creating the observed structures. The existence of this pocket was confirmed by transmission electron microscopy. While dissolving the stress fibers, inhibiting fiber protein binding, or inhibiting myosin II binding of actin removed the observed pockets, modulating cellular contractility did not remove them. Taken together, our results illustrate a novel actin-membrane bending topology where the membrane is deformed outward rather than being pinched inward, resembling the topological inverse of the contractile ring found in cytokinesis.

Cellular and genomic approaches for exploring structural chromosomal rearrangements

Human chromosomes are arranged in a linear and conserved sequence order that undergoes further spatial folding within the three-dimensional space of the nucleus. Although structural variations in this organization are an important source of natural genetic diversity, cytogenetic aberrations can also underlie a number of human diseases and disorders. Approaches for studying chromosome structure began half a century ago with karyotyping of Giemsa-banded chromosomes and has now evolved to encompass high-resolution fluorescence microscopy, reporter-based assays, and next-generation DNA sequencing technologies. Here, we provide a general overview of experimental methods at different resolution and sensitivity scales and discuss how they can be complemented to provide synergistic insight into the study of human chromosome structural rearrangements. These approaches range from kilobase-level resolution DNA fluorescence in situ hybridization (FISH)-based imaging approaches of individual cells to genome-wide sequencing strategies that can capture nucleotide-level information from diverse sample types. Technological advances coupled to the combinatorial use of multiple methods have resulted in the discovery of new rearrangement classes along with mechanistic insights into the processes that drive structural alterations in the human genome.

Lasonolide A, a potent and reversible inducer of chromosome condensation

Lasonolide A (LSA) is a natural product with high and selective cytotoxicity against mesenchymal cancer cells, including leukemia, melanomas and glioblastomas. Here, we reveal that LSA induces rapid and reversible premature chromosome condensation (PCC) associated with cell detachment, plasma membrane smoothening and actin reorganization. PCC is induced at all phases of the cell cycle in proliferative cells as well as in circulating human lymphocytes in G 0. It is independent of Cdk1 signaling, associated with cyclin B downregulation and induced in cells at LSA concentrations that are three orders of magnitude lower than those required to block phosphatases 1 and 2A in vitro. At the epigenetic level, LSA-induced PCC is coupled with histone H3 and H1 hyperphosphorylation and deacetylation. Treatment with SAHA reduced LSA-induced PCC, implicating histone deacetylation as one of the PCC effector mechanisms. In addition, PCC is coupled with topoisomerase II (Top2) and Aurora A hyperphosphorylation and activation. Inhibition of Top2 or Aurora A partially blocked LSA-induced PCC. Our findings demonstrate the profound epigenetic alterations induced by LSA and the potential of LSA as a new cytogenetic tool. Based on the unique cellular effects of LSA, further studies are warranted to uncover the cellular target of lasonolide A ("TOL").

Chromosome formation during fertilization in eggs of the teleost Oryzias latipes

Upon fertilization, eggs shift their cell cycle from the meiotic to the mitotic pattern for embryogenesis. The information on chromosome formation has been accumulated by various experiments using inhibitors to affect formation and behavior of chromosomes in the cycle of cell proliferation. Based on experimental results on meiosis and early stages of development of the teleost Oryzias latipes, we discuss the roles of the activities of histone H1 kinase, microtubule-associated protein kinase, DNA polymerase, DNA topoisomerase, and other cytoplasmic factors that play a crucial role in formation and separation of chromosomes.

Calyculin-A induces cleavage in a random plane in unfertilized sea urchin eggs

Calyculin-A (CLA), a protein phosphatase inhibitor, has been known to induce cleavage resembling normal furrowing in unfertilized sea urchin eggs. In CLA-treated eggs, actin filaments and myosin assemble to form a contractile ring-like structure in the egg cortex; however, this occurs in the absence of a mitotic spindle or asters. Here, we investigated the relationship between the plane of CLA-induced cleavage and the intrinsic animal-vegetal polar axis in sea urchin eggs. The animal-vegetal axis was established using black ink to visualize the jelly canal located at the animal pole in the jelly coat surrounding the egg. We measured the acute angle between the jelly canal axis and the cleavage plane for both fertilized eggs and CLA-treated unfertilized eggs. Although the acute angle lay within 10 degrees for most of the fertilized eggs, it varied widely for CLA-treated unfertilized eggs. Measurements of the diameter of blastomeres revealed that cleavage of fertilized eggs took place in the mid-plane of the egg, but that CLA-induced divisions were unequal. These results suggest that neither the orientation nor the location of the CLA-induced cleavage furrow is related to the animal-vegetal polar axis of the egg, even though the furrowing mechanism itself is not dissimilar to that in fertilized eggs.

Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement

Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.

Protein tyrosine and serine-threonine phosphatases in the sea urchin, Strongylocentrotus purpuratus: identification and potential functions

Protein phosphatases, in coordination with protein kinases, play crucial roles in regulation of signaling pathways. To identify protein tyrosine phosphatases (PTPs) and serine-threonine (ser-thr) phosphatases in the Strongylocentrotus purpuratus genome, 179 annotated sequences were studied (122 PTPs, 57 ser-thr phosphatases). Sequence analysis identified 91 phosphatases (33 conventional PTPs, 31 dual specificity phosphatases, 1 Class III Cysteine-based PTP, 1 Asp-based PTP, and 25 ser-thr phosphatases). Using catalytic sites, levels of conservation and constraint in amino acid sequence were examined. Nine of 25 receptor PTPs (RPTPs) corresponded to human, nematode, or fly homologues. Domain structure revealed that sea urchin-specific RPTPs including two, PTPRLec and PTPRscav, may act in immune defense. Embryonic transcription of each phosphatase was recorded from a high-density oligonucleotide tiling microarray experiment. Most RPTPs are expressed at very low levels, whereas nonreceptor PTPs (NRPTPs) are generally expressed at moderate levels. High expression was detected in MAP kinase phosphatases (MKPs) and numerous ser-thr phosphatases. For several expressed NRPTPs, MKPs, and ser-thr phosphatases, morpholino antisense-mediated knockdowns were performed and phenotypes obtained. Finally, to assess roles of annotated phosphatases in endomesoderm formation, a literature review of phosphatase functions in model organisms was superimposed on sea urchin developmental pathways to predict areas of functional activity.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

Actin-based centripetal flow: phosphatase inhibition by calyculin-A alters flow pattern, actin organization, and actomyosin distribution

Previous studies have suggested that the actin-based centripetal flow process in sea urchin coelomocytes is the result of a two-part mechanism, actin polymerization at the cell edge coupled with actomyosin contraction at the cell center. In the present study, we have extended the testing of this two-part model by attempting to stimulate actomyosin contraction via treatment of coelomocytes with the phosphatase inhibitor Calyculin A (CalyA). The effects of this drug were studied using digitally-enhanced video microscopy of living cells combined with immunofluorescent localization and scanning electron microscopy. Under the influence of CalyA, the coelomocyte actin cytoskeleton undergoes a radical reorganization from a dense network to one displaying an array of tangential arcs and radial rivulets in which actin and the Arp2/3 complex concentrate. In addition, the structure and dynamics of the cell center are transformed due to the accumulation of actin and membrane in this region and the constriction of the central actomyosin ring. Physiological evidence of an increase in actomyosin-based contractility following CalyA treatment was demonstrated in experiments in which cells generated tears in their cell centers in response to the drug. Western blotting and immunofluorescent localization with antibodies against the phosphorylated form of the myosin regulatory light chain (MRLC) suggested that the demonstrated constriction of actomyosin distribution was the result of CalyA-induced phosphorylation of MRLC. Overall, the results suggest that there is significant cross talk between the two underlying mechanisms of actin polymerization and actomyosin contraction, and indicate that changes in actomyosin tension may be translated into alterations in the structural organization of the actin cytoskeleton.

Calyculin A causes the activation of histone H1 kinase and condensation of chromosomes in unfertilized sea urchin eggs independently of the maturation-promoting factor

Calyculin A is known to inhibit the type-1 and type-2A phosphatases. We previously reported that calyculin A induces contractile ring formation in unfertilized sea urchin eggs, an increase in histone H(1) kinase activity, and chromosome condensation in the calyculin A-treated unfertilized eggs, and the changes induced by calyculin A are not affected by emetine, an inhibitor of protein synthesis. These observations suggest that the mechanism by which histone H(1) kinases are activated by calyculin A is different from that of maturation-promoting factor (MPF), which is activated by a molecular modification of existed cdc2 and newly synthesized cyclin B. We report here that no cyclin B was detected by immunoblotting of unfertilized calyculin A-treated eggs. In addition, no DNA synthesis was induced by calyculin A. As well, butyrolactone I (an inhibitor of cdc2 and cdk2 kinase) had no effect on the increase in histone H(1) kinase activity nor the chromosome condensation, both of which were induced by calyculin A. Thus, we conclude that calyculin A induces histone H(1) phosphorylation in an MPF-independent manner through inhibition of type-1 phosphatase, and that the chromosome condenses as a result of histone H(1) phosphorylation.

Chromatin compaction and tumor cell radiosensitivity at 2 gray

Mammalian cells at mitosis, differentiated lymphocytes, and some radiation-hypersensitive mutants in interphase contain all or a measurable portion of their chromatin in condensed/compacted form and are hypersensitive to ionizing radiation by the mechanism described by single-hit inactivation kinetics (alpha). These observations led to the investigation as to whether compacted chromatin in interphase is the target that determines the widely variable alpha-parameters and surviving fractions of 2 Gy (SF2Gy) measured for human tumor cell lines. Six cell lines whose SF2Gy ranged from 0.29 to 0.73 were used for this study. Their different radiosensitivities were associated mainly with differences in their single-hit inactivation parameters (alpha). Electron microscope images of interphase nuclei were optically scanned, and the pixel densities were digitized for quantitative analyses. A significant correlation between the percentage of nuclear pixels with densities similar to those found in mitotic chromosomes (percent compacted chromatin) and the alpha-inactivation parameters was observed. Digital analyses of electron and/or confocal microscope images of chromatin in interphase tumor cells in biopsy specimens could become a rapid assay for predicting the intrinsic radiosensitivity of tumor clonogens. This research has also identified some inhibitors of protein (histone) phosphatases that promote chromatin compaction and radiosensitize cells to 2-Gy dose fractions.

Calyculin-A, an inhibitor for protein phosphatases, induces cortical contraction in unfertilized sea urchin eggs

When an unfertilized sea urchin egg was exposed to calyculin-A (CL-A), an inhibitor of protein phosphatases, for a short period and then lysed, the cortex contracted to exclude cytoplasm and became a cup-shaped mass. We call the contracted cortex "actin cup" since actin filaments were major structural components. Electron microscopic observation revealed that the cup consisted of inner electron-dense layer, middle microfilamentous layer, and outermost granular region. Microfilaments were heavily accumulated in the inner electron-dense layer. The middle layer also contained numerous microfilaments, which were determined to be actin filaments by myosin S1 decoration, and they were aligned so that their barbed ends directed toward the outermost region. Myosin II, Arp2, Arp3, and spectrin were concentrated in the actin cup. Immuno-electron microscopy revealed that myosin II was localized to the electron-dense layer. We further found that the cortical tension of the egg increased just after application of CL-A and reached maximum within 10 min. Cytochalasin B or butanedione monoxime blocked the contraction, which suggested that both actin filaments and myosin ATPase activity were required for the contraction. Myosin regulatory light chain (MRLC) in the actin cup was shown to be phosphorylated at the activation sites Ser-19 and Thr-18, by immunoblotting with anti-phosphoepitope antibodies. The phosphorylation of MRLC was also confirmed by a (32)P in vivo labeling experiment. The CL-A-induced cortical contraction may be a good model system for studying the mechanism of cytokinesis.

Changes in the activities of protein phosphatase type 1 and type 2A in sea urchin embryos during early development

In the eggs and embryos of sea urchins, the activity of protein phosphatase type 2A (PP2A) increased during the developmental period between fertilization and the morula stage, decreased after the prehatching blastula stage and increased again after hatching. The PP2A activity changed keeping pace with alteration to the activities of cAMP-dependent protein kinase (A kinase), Ca2+/calmodulin-dependent protein kinase (CaM kinase) and casein kinase. Probably, PP2A contributes to the quick turning off of cellular signals because of protein phosphorylation. The activity of protein phosphatase type 1 (PP1) was not detectable up to the morula stage and appreciably increased thereafter. In the isolated nucleus fraction, specific activities of PP1 and PP2A were higher than in whole embryos at all stages in early development. Exponential increase in the number of nuclei because of egg cleavage probably makes PP1 activity detectable in whole embryos after the morula stage. In isolated nuclei, the activities of PP1 and PP2A appreciably decreased after hatching, whereas the activities of A kinase, Ca2+/phospholipid-dependent protein kinase (C kinase) and CaM kinase, as well as casein kinase, became higher. In nuclei, cellular signals caused by protein phosphorylation after hatching do not seem to be turned off by these protein kinases so quickly as before hatching. The PP1 and PP2A in nuclei also seem to contribute to the elimination of signal noise.

Effect of calyculin A on the surface structure of unfertilized sea urchin eggs

Calyculin A, a potent inhibitor of type 1 and type 2A protein phosphatases, induces contractile ring formation when applied to unfertilized sea urchin eggs [Tosuji et al., 1992: Proc. Natl. Acad. Sci. USA 89:10613-10617]. We report here the elongation of microvilli in the unfertilized eggs exposed to calyculin A. The elongated microvilli and associated sperm-egg binding sites (egg receptor for sperm) then became concentrated into a constriction site corresponding to the cleavage furrow. The egg receptor for sperm was also in close connection to the microfilaments. Okadaic acid is another known inhibitor of protein phosphatase type-1 and type-2A. Its effect, however, is about a hundredfold feebler for type-1 phosphatase than type-2A. Even after treatment with okadaic acid, no change was observed, suggesting that these morphological changes were induced by calyculin A solely though its inhibitory effect on the type-1 protein phosphatase.

Molecular cloning of the gene for p85 that regulates the initiation of cytokinesis in Tetrahymena

Tetrahymena p85 is localized to the presumptive division plane before division furrow formation; its molecular weight in SDS-polyacrylamide gel electrophoresis differs in wild-type and temperature-sensitive cell-division-arrest mutant cdaA1 cells. At the restrictive temperature, p85 localization and division furrow formation are not observed in cdaA1 cells. In this study, we purified p85 and cloned a wild-type p85 cDNA. The deduced amino acid sequence of p85 was composed mainly of two kinds of repeat sequences. One of these contained regions homologous to a calmodulin-binding site and a part of actin, and the other contained a region homologous to a part of a cdc2 kinase homologue. Moreover, we cloned a cDNA encoding the cdaA1 p85. There was no difference in the predicted amino acid sequences of wild-type and cdaA1 p85, suggesting that the difference in molecular weight between p85 in wild-type and mutant cells is caused by a disorder of posttranslational-modification mechanisms of p85 in the cdaA1 cell.

Microinjection of Ca2+ store-enriched microsome fractions to dividing newt eggs induces extra-cleavage furrows via inositol 1,4,5-trisphosphate-induced Ca2+ release

The cleavage signal transferred to the future cleavage cortex during anaphase has been proposed as "cleavage stimulus," but no signal has proved to induce cleavage furrows. The local Ca2+ transient along the cleavage furrow has been reported, but the Ca2+ source has remained unknown. To address these questions, we studied functions of Ca2+ stores in dividing newt eggs and found that microinjection of the Ca2+ store-enriched microsome fraction to the dividing newt egg induced a local extra-cleavage furrow at the injection site in 64-67% of the injected newt eggs while coinjection with inositol 1,4, 5-trisphosphate receptor (IP(3)R) antagonists heparin or anti-type 1-IP(3)R antibody clearly suppressed this induction (5 and 11% in induction rates, respectively). Injection of cerebellar microsomes from the type 1-IP(3)R-deficient mice induced extracleavage furrows albeit at a low rate (19%). Our observations strongly suggest that Ca2+ stores with IP(3)R induce and position a cleavage furrow via IP(3)-induced Ca2+ release (IICR) as Ca(2+)-releasing machinery and putative cleavage stimulus itself.

Accumulation of WGA receptors in the cleavage furrow during cytokinesis of sea urchin eggs

Sea urchin eggs stained with fluorescein-conjugated wheat germ agglutinin (F-WGA) before or after fixation showed a marked accumulation of fluorescence at the cleavage furrow in the first and the second cell divisions. WGA receptors (WGA-binding membrane glycoproteins) were redistributed to the equatorial region through several steps in compressed eggs. Accumulated WGA receptors showed a distribution similar to that of contractile-ring microfilaments throughout most of the steps. Therefore, the former is probably associated with the latter directly or indirectly. Labeling with F-WGA provides a simple method to detect contractile-ring microfilaments in living eggs. Treatment of eggs with colcemid shortly before cytokinesis dispersed the ring-like accumulation of WGA receptors together with contractile-ring microfilaments. This result suggests that microtubule structures, probably asters, are involved in the redistribution of WGA receptors. Cytochalasin B prevented furrowing when it was applied shortly before cytokinesis. While contractile-ring microfilaments showed a spotty distribution in the expected furrow region, WGA receptors were normally redistributed. Furthermore, a higher concentration of the drug allowed the appearance of accumulated WGA receptors in compressed eggs although the development into a ring-like configuration was inhibited. These observations suggest the possibility that the redistribution of WGA receptors is involved in the formation of contractile ring.

Effect of protein phosphatase inhibitors on cleavage furrow formation in newt eggs: inhibition of normal furrow formation and concomitant induction of furrow-like dents

The effects of three protein phosphatase inhibitors, okadaic acid, calyculin A and tautomycin, on the formation of cleavage furrows and the induction of furrow-like dents in the egg of the newt, Cynops pyrrhogaster, were examined. Solutions of the individual compound were injected into the animal hemisphere of one of the two presumptive blastomere regions of the embryo during the first cleavage. Injection of a solution containing any of the chemicals often disturbed the formation of a normal furrow in the injected blastomere at second cleavage. Injection with okadaic acid or calyculin A often induced furrow-like dents on the surface of the injected blastomere at the same time as second cleavage in control embryos, while that with tautomycin usually did not induce them. In an injected blastomere, formation of dents started in the animal half and moved towards the vegetal half as the furrow in its counterpart blastomere extended from the animal half towards the vegetal. Dents gradually became slightly deeper and formed cytoplasmic projections that later degenerated, leaving a surface scar. Cytological observations on blastomeres injected with calyculin A revealed that nuclear division occurred normally.

Formation of a phragmosome-like structure in centrifuged protonemal cells of Adiantum capillus-veneris L

A phragmosome (PS) is a transvacuolar aggregation of cytoplasm that develops in the plane of future cytokinesis and is found specifically in highly vacuolated cells. Although protonemal cells of Adiantum capillus-veneris L. usually do not form a PS, a PS-like structure developed at the site of a preprophase band (PPB) of microtubules (MTs) when the nucleus and endoplasm were displaced from the division site by centrifugation, leaving a PPB in the cortical cytoplasm. The PS-like structure contained endoplasmic MTs, Factin, oil droplets and mitochondria. The structure did not develop when the cells were centrifuged before the formation of a PPB. Application of amiprophos-methyl (APM) before development of the PPB strongly inhibited the formation of the PS-like structure after centrifugation. The PS-like structure was dispersed after cytokinesis which occurred in the region of the displaced nucleus. Treatment with APM after the formation of the PS-like structure arrested the cell cycle at the M phase and inhibited the degradation of this structure. These results suggest that development of a PS-like structure is associated both with the formation of a PPB and with the stage of the cell cycle.

Isolation and characterization of pos mutants defective in correct positioning of septum in Schizosaccharomyces pombe

We have isolated mutants of fission yeast defective in correct positioning of septum. In visual screening, we obtained 16 clones showing unequal septation at restrictive temperature, which were classified into three complementation groups. At restrictive temperature, all the mutants underwent nuclear division normally. In cytokinesis, however, a contractile ring was formed at the site independent of the mitotic spindle. These results suggest that positional information for cytokinesis are not accurately transmitted to the cell equator. Furthermore, all the mutants frequently displayed incorrect orientation and/or distortion of septum, which suggests that the septum positioning is closely related to correct orientation and organization of septum.

Chromosome condensation induced by fostriecin does not require p34cdc2 kinase activity and histone H1 hyperphosphorylation, but is associated with enhanced histone H2A and H3 phosphorylation

Chromosome condensation at mitosis correlates with the activation of p34cdc2 kinase, the hyperphosphorylation of histone H1 and the phosphorylation of histone H3. Chromosome condensation can also be induced by treating interphase cells with the protein phosphatase 1 and 2A inhibitors okadaic acid and fostriecin. Mouse mammary tumour FT210 cells grow normally at 32 degrees C, but at 39 degrees C they lose p34cdc2 kinase activity and arrest in G2 because of a temperature-sensitive lesion in the cdc2 gene. The treatment of these G2-arrested FT210 cells with fostriecin or okadaic acid resulted in full chromosome condensation in the absence of p34cdc2 kinase activity or histone H1 hyperphosphorylation. However, phosphorylation of histones H2A and H3 was strongly stimulated, partly through inhibition of histone H2A and H3 phosphatases, and cyclins A and B were degraded. The cells were unable to complete mitosis and divide. In the presence of the protein kinase inhibitor starosporine, the addition of fostriecin did not induce histone phosphorylation and chromosome condensation. The results show that chromosome condensation can take place without either the histone H1 hyperphosphorylation or the p34cdc2 kinase activity normally associated with mitosis, although it requires a staurosporine-sensitive protein kinase activity. The results further suggest that protein phosphatases 1 and 2A may be important in regulating chromosome condensation by restricting the level of histone phosphorylation during interphase, thereby preventing premature chromosome condensation.

Cleavage furrow: timing of emergence of contractile ring actin filaments and establishment of the contractile ring by filament bundling in sea urchin eggs

Cleavage furrow formation at the first cell division of sea urchin and sand dollar eggs was investigated in detail by fluorescence staining of actin filaments with rhodamine-phalloidin of either whole eggs or isolated egg cortices. Cortical actin filaments were clustered at anaphase and then the clusters became fibrillar at the end of anaphase. The timing when the contractile ring actin filaments appear was precisely determined in the course of mitosis: accumulation of the contractile ring actin filaments at the equatorial cell cortex is first noticed at the beginning of telophase (shortly before furrow formation), when the chromosomal vesicles are fusing with each other. The accumulated actin filaments were not well organized at the early stage but were organized into parallel bundles as the furrowing progressed. The bundles were finally fused into a tightly packed filament belt. Wheat germ agglutinin (WGA)-binding sites were distributed on the surface of the egg in a manner similar to the actin filaments after anaphase. The WGA-binding sites became accumulated in the contractile ring together with the contractile ring actin filaments, indicating an intimate relationship between these sites and actin filament-anchoring sites on the plasma membrane. Myosin also appeared in the contractile ring together with the actin filaments. The ‘cleavage stimulus’, a signal hypothesized by Rappaport (reviewed by R. Rappaport (1986) Int. Rev. Cytol. 105, 245–281) was suggested to induce aggregation or bundling of the actin filaments in the cortical layer.