Evidence that actin and myosin are involved in the poleward flux of tubulin in metaphase kinetochore microtubules of crane-fly spermatocytes

Triggering Cation-Induced Contraction of Cytoskeleton Networks via Microfluidics

The dynamic morphology and mechanics of the cytoskeleton is determined by interacting networks of semiflexible actin filaments and rigid microtubules. Active rearrangement of networks of actin and microtubules can not only be driven by motor proteins but by changes to ionic conditions. For example, high concentrations of multivalent ions can induce bundling and crosslinking of both filaments. Yet, how cytoskeleton networks respond in real-time to changing ion concentrations, and how actin-microtubule interactions impact network response to these changing conditions remains unknown. Here, we use microfluidic perfusion chambers and two-color confocal fluorescence microscopy to show that increasing magnesium ions trigger contraction of both actin and actin-microtubule networks. Specifically, we use microfluidics to vary the Mg²⁺ concentration between 2 and 20 mM while simultaneously visualizing the triggered changes to the overall network size. We find that as Mg²⁺ concentration increases both actin and actin-microtubule networks undergo bulk contraction, which we measure as the shrinking width of each network. However, surprisingly, lowering the Mg²⁺concentration back to 2 mM does not stop or reverse the contraction but rather causes both networks to contract further. Further, actin networks begin to contract at lower Mg²⁺ concentrations and shorter times than actin-microtubule networks. In fact, actin-microtubule networks only undergo substantial contraction once the Mg²⁺ concentration begins to lower from 20 mM back to 2 mM. Our intriguing findings shed new light on how varying environmental conditions can dynamically tune the morphology of cytoskeleton networks and trigger active contraction without the use of motor proteins.

Spindle-F-actin interactions in mitotic spindles in an intact vertebrate epithelium

Mitotic spindles are well known to be assembled from and dependent on microtubules. In contrast, whether actin filaments (F-actin) are required for or are even present in mitotic spindles has long been controversial. Here we have developed improved methods for simultaneously preserving F-actin and microtubules in fixed samples and exploited them to demonstrate that F-actin is indeed associated with mitotic spindles in intact Xenopus laevis embryonic epithelia. We also find that there is an “F-actin cycle,” in which the distribution and organization of spindle F-actin changes over the course of the cell cycle. Live imaging using a probe for F-actin reveals that at least two pools of F-actin are associated with mitotic spindles: a relatively stable internal network of cables that moves in concert with and appears to be linked to spindles, and F-actin “fingers” that rapidly extend from the cell cortex toward the spindle and make transient contact with the spindle poles. We conclude that there is a robust endoplasmic F-actin network in normal vertebrate epithelial cells and that this network is also a component of mitotic spindles. More broadly, we conclude that there is far more internal F-actin in epithelial cells than is commonly believed.

Multiple Pools of Nuclear Actin

While nuclear actin was reported ~50 years ago, it's in vivo prevalence and structure remain largely unknown. Here, we use Drosophila oogenesis, that is, follicle development, to characterize nuclear actin. We find that three different reagents-DNase I, anti-actin C4, and anti-actin AC15-recognize distinct pools of nuclear actin. DNase I labels monomeric or G-actin, and, during follicle development, G-actin is present in the nucleus of every cell. Some G-actin is recognized by the C4 antibody. In particular, C4 nuclear actin colocalizes with DNase I to the nucleolus in anterior escort cells, follicle stem cells, some mitotic follicle cells, and a subset of nurse cells during early oogenesis. C4 also labels polymeric nuclear actin in the nucleoplasm of the germline stem cells, early cystoblasts, and oocytes. The AC15 antibody labels a completely distinct pool of nuclear actin from that of DNase I and C4. Specifically, AC15 nuclear actin localizes to the chromatin in the nurse and follicle cells during mid-to-late oogenesis. Within the oocyte, AC15 nuclear actin progresses from localizing to puncta surrounding the DNA, to forming a filamentous cage around the chromosomes. Together these findings reveal that nuclear actin is highly prevalent in vivo, and multiple pools of nuclear actin exist and can be recognized using different reagents. Additionally, our localization studies suggest that nuclear actin may regulate stemness, nucleolar structure and function, transcription, and nuclear structure. Such findings call for further studies to explore the prevalence, diversity, and functions of nuclear actin across tissues and organisms. Anat Rec, 301:2014-2036, 2018. © 2018 Wiley Periodicals, Inc.

Nuclear Actin: From Discovery to Function

While actin was discovered in the nucleus over 50 years ago, research lagged for decades due to strong skepticism. The revitalization of research into nuclear actin occurred after it was found that cellular stresses induce the nuclear localization and alter the structure of actin. These studies provided the first hints that actin has a nuclear function. Subsequently, it was established that the nuclear import and export of actin is highly regulated. While the structures of nuclear actin remain unclear, it can function as monomers, polymers, and even rods. Furthermore, even within a given structure, distinct pools of nuclear actin that can be differentially labeled have been identified. Numerous mechanistic studies have uncovered an array of functions for nuclear actin. It regulates the activity of RNA polymerases, as well as specific transcription factors. Actin also modulates the activity of several chromatin remodeling complexes and histone deacetylases, to ultimately impinge on transcriptional programing and DNA damage repair. Further, nuclear actin mediates chromatin movement and organization. It has roles in meiosis and mitosis, and these functions may be functionally conserved from ancient bacterial actin homologs. The structure and integrity of the nuclear envelope and sub-nuclear compartments are also regulated by nuclear actin. Furthermore, nuclear actin contributes to human diseases like cancer, neurodegeneration, and myopathies. Here, we explore the early discovery of actin in the nucleus and discuss the forms and functions of nuclear actin in both normal and disease contexts. Anat Rec, 301:1999-2013, 2018. © 2018 Wiley Periodicals, Inc.

Assembly and Positioning of the Oocyte Meiotic Spindle

Fertilizable eggs develop from diploid precursor cells termed oocytes. Once every menstrual cycle, an oocyte matures into a fertilizable egg in the ovary. To this end, the oocyte eliminates half of its chromosomes into a small cell termed a polar body. The egg is then released into the Fallopian tube, where it can be fertilized. Upon fertilization, the egg completes the second meiotic division, and the mitotic division of the embryo starts. This review highlights recent work that has shed light on the cytoskeletal structures that drive the meiotic divisions of the oocyte in mammals. In particular, we focus on how mammalian oocytes assemble a microtubule spindle in the absence of centrosomes, how they position the spindle in preparation for polar body extrusion, and how the spindle segregates the chromosomes. We primarily focus on mouse oocytes as a model system but also highlight recent insights from human oocytes.

Gamma-actin is involved in regulating centrosome function and mitotic progression in cancer cells

Reorganization of the actin cytoskeleton during mitosis is crucial for regulating cell division. A functional role for γ-actin in mitotic arrest induced by the microtubule-targeted agent, paclitaxel, has recently been demonstrated. We hypothesized that γ-actin plays a role in mitosis. Herein, we investigated the effect of γ-actin in mitosis and demonstrated that γ-actin is important in the distribution of β-actin and formation of actin-rich retraction fibers during mitosis. The reduced ability of paclitaxel to induce mitotic arrest as a result of γ-actin depletion was replicated with a range of mitotic inhibitors, suggesting that γ-actin loss reduces the ability of broad classes of anti-mitotic agents to induce mitotic arrest. In addition, partial depletion of γ-actin enhanced centrosome amplification in cancer cells and caused a significant delay in prometaphase/metaphase. This prolonged prometaphase/metaphase arrest was due to mitotic defects such as uncongressed and missegregated chromosomes, and correlated with an increased presence of mitotic spindle abnormalities in the γ-actin depleted cells. Collectively, these results demonstrate a previously unknown role for γ-actin in regulating centrosome function, chromosome alignment and maintenance of mitotic spindle integrity.

Movement of chromosomes with severed kinetochore microtubules

Experiments dating from 1966 and thereafter showed that anaphase chromosomes continued to move poleward after their kinetochore microtubules were severed by ultraviolet microbeam irradiation. These observations were initially met with scepticism as they contradicted the prevailing view that kinetochore fibre microtubules pulled chromosomes to the pole. However, recent experiments using visible light laser microbeam irradiations have corroborated these earlier experiments as anaphase chromosomes again were shown to move poleward after their kinetochore microtubules were severed. Thus, multiple independent studies using different techniques have shown that chromosomes can indeed move poleward without direct microtubule connections to the pole, with only a kinetochore 'stub' of microtubules. An issue not yet settled is: what propels the disconnected chromosome? There are two not necessarily mutually exclusive proposals in the literature: (1) chromosome movement is propelled by the kinetochore stub interacting with non-kinetochore microtubules and (2) chromosome movement is propelled by a spindle matrix acting on the stub. In this review, we summarise the data indicating that chromosomes can move with severed kinetochore microtubules and we discuss proposed mechanisms for chromosome movement with severed kinetochore microtubules.

Adducin-1 is essential for mitotic spindle assembly through its interaction with myosin-X

The actin-binding protein ADD1 associates with mitotic spindles through Myo10 and is crucial for proper spindle assembly and mitotic progression.

Mitotic spindles are microtubule-based structures, but increasing evidence indicates that filamentous actin (F-actin) and F-actin–based motors are components of these structures. ADD1 (adducin-1) is an actin-binding protein that has been shown to play important roles in the stabilization of the membrane cortical cytoskeleton and cell–cell adhesions. In this study, we show that ADD1 associates with mitotic spindles and is crucial for proper spindle assembly and mitotic progression. Phosphorylation of ADD1 at Ser12 and Ser355 by cyclin-dependent kinase 1 enables ADD1 to bind to myosin-X (Myo10) and therefore to associate with mitotic spindles. ADD1 depletion resulted in distorted, elongated, and multipolar spindles, accompanied by aberrant chromosomal alignment. Remarkably, the mitotic defects caused by ADD1 depletion were rescued by reexpression of ADD1 but not of an ADD1 mutant defective in Myo10 binding. Together, our findings unveil a novel function for ADD1 in mitotic spindle assembly through its interaction with Myo10.

The role of actin and myosin in PtK2 spindle length changes induced by laser microbeam irradiations across the spindle

This study investigates spindle biomechanical properties to better understand how spindles function. In this report, laser microbeam cutting across mitotic spindles resulted in movement of spindle poles toward the spindle equator. The pole on the cut side moved first, the other pole moved later, resulting in a shorter but symmetric spindle. Intervening spindle microtubules bent and buckled during the equatorial movement of the poles. Because of this and because there were no detectable microtubules within the ablation zone, other cytoskeletal elements would seem to be involved in the equatorial movement of the poles. One possibility is actin and myosin since pharmacological poisoning of the actin-myosin system altered the equatorial movements of both irradiated and unirradiated poles. Immunofluorescence microscopy confirmed that actin, myosin and monophosphorylated myosin are associated with spindle fibers and showed that some actin and monophosphorylated myosin remained in the irradiated regions. Overall, our experiments suggest that actin, myosin and microtubules interact to control spindle length. We suggest that actin and myosin, possibly in conjunction with the spindle matrix, cause the irradiated pole to move toward the equator and that cross-talk between the two half spindles causes the unirradiated pole to move toward the equator until a balanced length is obtained.

The role of myosin phosphorylation in anaphase chromosome movement

This work deals with the role of myosin phosphorylation in anaphase chromosome movement. Y27632 and ML7 block two different pathways for phosphorylation of the myosin regulatory light chain (MRLC). Both stopped or slowed chromosome movement when added to anaphase crane-fly spermatocytes. To confirm that the effects of the pharmacological agents were on the presumed targets, we studied cells stained with antibodies against mono- or bi-phosphorylated myosin. For all chromosomes whose movements were affected by a drug, the corresponding spindle fibres of the affected chromosomes had reduced levels of 1P- and 2P-myosin. Thus the drugs acted on the presumed target and myosin phosphorylation is involved in anaphase force production. Calyculin A, an inhibitor of MRLC dephosphorylation, reversed and accelerated the altered movements caused by Y27632 and ML-7, suggesting that another phosphorylation pathway is involved in phosphorylation of spindle myosin. Staurosporine, a more general phosphorylation inhibitor, also reduced the levels of MRLC phosphorylation and caused anaphase chromosomes to stop or slow. The effects of staurosporine on chromosome movements were not reversed by Calyculin A, confirming that another phosphorylation pathway is involved in phosphorylation of spindle myosin.

And the dead shall rise: actin and myosin return to the spindle

The spindle directs chromosome partitioning in eukaryotes and, for the last three decades, has been considered primarily a structure based on microtubules, microtubule motors, and other microtubule binding proteins. However, a surprisingly large body of both old and new studies suggests roles for actin filaments (F-actin) and myosins (F-actin-based motor proteins) in spindle assembly and function. Here we review these data and conclude that in several cases the evidence for the participation of F-actin and myosins in spindle function is very strong, and in the situations where it is less strong, there is nevertheless enough evidence to warrant further investigation.

The type II Arabidopsis formin14 interacts with microtubules and microfilaments to regulate cell division

Formins have long been known to regulate microfilaments but have also recently been shown to associate with microtubules. In this study, Arabidopsis thaliana FORMIN14 (AFH14), a type II formin, was found to regulate both microtubule and microfilament arrays. AFH14 expressed in BY-2 cells was shown to decorate preprophase bands, spindles, and phragmoplasts and to induce coalignment of microtubules with microfilaments. These effects perturbed the process of cell division. Localization of AFH14 to microtubule-based structures was confirmed in Arabidopsis suspension cells. Knockdown of AFH14 in mitotic cells altered interactions between microtubules and microfilaments, resulting in the formation of an abnormal mitotic apparatus. In Arabidopsis afh14 T-DNA insertion mutants, microtubule arrays displayed abnormalities during the meiosis-associated process of microspore formation, which corresponded to altered phenotypes during tetrad formation. In vitro biochemical experiments showed that AFH14 bound directly to either microtubules or microfilaments and that the FH2 domain was essential for cytoskeleton binding and bundling. However, in the presence of both microtubules and microfilaments, AFH14 promoted interactions between microtubules and microfilaments. These results demonstrate that AFH14 is a unique plant formin that functions as a linking protein between microtubules and microfilaments and thus plays important roles in the process of plant cell division.

Impact of marine drugs on cytoskeleton-mediated reproductive events

Marine organisms represent an important source of novel bioactive compounds, often showing unique modes of action. Such drugs may be useful tools to study complex processes such as reproduction; which is characterized by many crucial steps that start at gamete maturation and activation and virtually end at the first developmental stages. During these processes cytoskeletal elements such as microfilaments and microtubules play a key-role. In this review we describe: (i) the involvement of such structures in both cellular and in vitro processes; (ii) the toxins that target the cytoskeletal elements and dynamics; (iii) the main steps of reproduction and the marine drugs that interfere with these cytoskeleton-mediated processes. We show that marine drugs, acting on microfilaments and microtubules, exert a wide range of impacts on reproductive events including sperm maturation and motility, oocyte maturation, fertilization, and early embryo development.

The elasticity of motor-microtubule bundles and shape of the mitotic spindle

In the process of cell division, chromosomes are segregated by mitotic spindles -- bipolar microtubule arrays that have a characteristic fusiform shape. Mitotic spindle function is based on motor-generated forces of hundreds of piconewtons. These forces have to deform the spindle, yet the role of microtubule elastic deformations in the spindle remains unclear. Here we solve equations of elasticity theory for spindle microtubules, compare the solutions with shapes of early Drosophila embryo spindles and discuss the biophysical and cell biological implications of this analysis. The model suggests that microtubule bundles in the spindle behave like effective compressed springs with stiffness of the order of tens of piconewtons per micron, that microtubule elasticity limits the motors' power, and that clamping and cross-linking of microtubules are needed to transduce the motors' forces in the spindle. Some data are hard to reconcile with the model predictions, suggesting that cytoskeletal structures laterally reinforce the spindle and/or that rapid microtubule turnover relieves the elastic stresses.

The actin-capping protein CapG localizes to microtubule-dependent organelles during the cell cycle

Extensive cross-talk between the actin and the microtubule cytoskeletons has been reported. Especially in mitosis, processes dependent on actin- and microtubule-based structures alternate and regulate each other in a complex cascade leading to division into two daughter cells. Here, we have studied the subcellular localization of the filamentous actin-capping protein CapG. Fluorescence microscopy of endogenous CapG and EGFP-tagged CapG revealed CapG localization at the mother centriole in interphase, the mitotic spindle in mitosis and the midbody ring in abscission. Surprisingly, nucleoporin Nup62, an interaction partner of CapG, also localized to the midbody ring at the end of abscission and colocalized with CapG. We propose a role for the actin-binding protein CapG as a mediator of cross-talk between the actin cytoskeleton and microtubule-based organelles that regulate cell division.

Mitosis: new roles for myosin-X and actin at the spindle

Roles for actin and myosin in positioning mitotic spindles in the cell are well established. A recent study of myosin-X function in early Xenopus embryo mitosis now reports that this unconventional myosin is required for pole integrity and normal spindle length by localizing to poles and exerting pulling forces on actin filaments within the spindle.

Spindle positioning in mouse oocytes relies on a dynamic meshwork of actin filaments

Female meiosis in higher organisms consists of highly asymmetric divisions, which retain most maternal stores in the oocyte for embryo development. Asymmetric partitioning of the cytoplasm results from the spindle's "off-center" positioning, which, in mouse oocytes, depends mainly on actin filaments [1, 2]. This is a unique situation compared to most systems, in which spindle positioning requires interactions between astral microtubules and cortical actin filaments [3]. Formin 2, a straight-actin-filament nucleator, is required for the first meiotic spindle migration to the cortex and cytokinesis in mouse oocytes [4, 5]. Although the requirement for actin filaments in the control of spindle positioning is well established in this model, no one has been able to detect them in the cytoplasm [6]. Through the expression of an F-actin-specific probe and live confocal microscopy, we show the presence of a cytoplasmic actin meshwork, organized by Formin 2, that controls spindle migration. In late meiosis I, these filaments organize into a spindle-like F-actin structure, which is connected to the cortex. At anaphase, global reorganization of this meshwork allows polar-body extrusion. In addition, using actin-YFP, our FRAP analysis confirms the presence of a highly dynamic cytoplasmic actin meshwork that is tightly regulated in time and space.

Latrunculin A delays anaphase onset in fission yeast by disrupting an Ase1-independent pathway controlling mitotic spindle stability

It has been proposed previously that latrunculin A, an inhibitor of actin polymerization, delays the onset of anaphase by causing spindle misorientation in fission yeast. However, we show that Delta mto1 cells, which are defective in nucleation of cytoplasmic microtubules, have profoundly misoriented spindles but are not delayed in the timing of sister chromatid separation, providing compelling evidence that fission yeast does not possess a spindle orientation checkpoint. Instead, we show that latrunculin A delays anaphase onset by disrupting interpolar microtubule stability. This effect is abolished in a latrunculin A-insensitive actin mutant and exacerbated in cells lacking Ase1, which cross-links antiparallel interpolar microtubules at the spindle midzone both before and after anaphase. These data indicate that both Ase1 and an intact actin cytoskeleton are required for preanaphase spindle stability. Finally, we show that loss of Ase1 activates a checkpoint that requires only the Mad3, Bub1, and Mph1, but not Mad1, Mad2, or Bub3 checkpoint proteins.

What generates flux of tubulin in kinetochore microtubules?

We discuss models for production of tubulin flux in kinetochore microtubules. Current models concentrate solely on microtubules and their associated motors and enzymes. For example, in some models the driving force for flux is enzymes at the poles and the kinetochores; in others the driving force is motor molecules that are associated with a stationary spindle matrix. We present a different viewpoint, that microtubules are propelled poleward by forces arising from the spindle matrix, that the forces on the microtubules "activate" polymerising and depolymerising enzymes at kinetochores and poles, that matrix forces utilise actin, myosin, and microtubule motors, and that the matrix itself may not necessarily be static.

Actin and myosin inhibitors block elongation of kinetochore fibre stubs in metaphase crane-fly spermatocytes

We used an ultraviolet microbeam to cut individual kinetochore spindle fibres in metaphase crane-fly spermatocytes. We then followed the growth of the "kinetochore stubs", the remnants of kinetochore fibres that remain attached to kinetochores. Kinetochore stubs elongate with constant velocity by adding tubulin subunits at the kinetochore, and thus elongation is related to tubulin flux in the kinetochore microtubules. Stub elongation was blocked by cytochalasin D and latrunculin A, actin inhibitors, and by butanedione monoxime, a myosin inhibitor. We conclude that actin and myosin are involved in generating elongation and thus in producing tubulin flux in kinetochore microtubules. We suggest that actin and myosin act in concert with a spindle matrix to propel kinetochore fibres poleward, thereby causing stub elongation and generating anaphase chromosome movement in nonirradiated cells.

Possible roles of actin and myosin during anaphase chromosome movements in locust spermatocytes

We tested whether the mechanisms of chromosome movement during anaphase in locust (Locusta migratoria L.) spermatocytes might be similar to those described for crane-fly spermatocytes. Actin and myosin have been implicated in anaphase chromosome movements in crane-fly spermatocytes, as indicated by the effects of inhibitors and by the localisations of actin and myosin in spindles. In this study, we tested whether locust spermatocyte spindles also utilise actin and myosin, and whether actin is involved in microtubule flux. Living locust spermatocytes were treated with inhibitors of actin (latrunculin B and cytochalasin D), myosin (BDM), or myosin phosphorylation (Y-27632 and ML-7). We added drugs (individually) during anaphase. Actin inhibitors alter anaphase: chromosomes either completely stop moving, slow, or sometimes accelerate. The myosin inhibitor, BDM, also alters anaphase: in most cases, the chromosomes drastically slow or stop. ML-7, an inhibitor of MLCK, causes chromosomes to stop, slow, or sometimes accelerate, similar to actin inhibitors. Y-27632, an inhibitor of Rho-kinase, drastically slows or stops anaphase chromosome movements. The effects of the drugs on anaphase movement are reversible: most of the half-bivalents resumed movement at normal speed after these drugs were washed out. Actin and myosin were present in the spindles in locations consistent with their possible involvement in force production. Microtubule flux along kinetochore fibres is an actin-dependent process, since LatB completely removes or drastically reduces the gap in microtubule acetylation at the kinetochore. These results suggest that actin and myosin are involved in anaphase chromosome movements in locust spermatocytes.

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.

Microtubule-dependent organization of subcortical microfilaments in the earlyDrosophila embryo

Dynamic alterations in the spatial organization of cytoskeletal elements constitute a prominent morphological feature of the early, syncytial stages of embryogenesis in Drosophila. Here, we describe and characterize the dynamic behavior of cytoplasmic, subcortical microfilaments, which form a series of nucleus-associated structures, at different phases of the simultaneous nuclear division cycles characteristic of early Drosophila embryos. Remodeling of the cytoplasmic microfilament arrays takes place in parallel to the established cyclic reorganization of cortical microfilament structures. We provide evidence that the cortical and subcortical microfilament populations organize independently of each other, and in response to distinct instructive cues. Specifically, formation of subcortical microfilament structures appears to rely on, and spatially mirror, the organization of polarized microtubule arrays, while cortical microfilament restructuring constitutes a centrosome-dependent process. Genetic analysis identifies a requirement for SCAR, a key mediator of Arp2/3-based microfilament dynamics, in organization of subcortical microfilament structures.

Cell and Molecular Biology of the Spindle Matrix

The concept of a spindle matrix has long been proposed to account for incompletely understood features of microtubule spindle dynamics and force production during mitosis. In its simplest formulation, the spindle matrix is hypothesized to provide a stationary or elastic molecular matrix that can provide a substrate for motor molecules to interact with during microtubule sliding and which can stabilize the spindle during force production. Although this is an attractive concept with the potential to greatly simplify current models of microtubule spindle behavior, definitive evidence for the molecular nature of a spindle matrix or for its direct role in microtubule spindle function has been lagging. However, as reviewed here multiple studies spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis.

Redundant mechanisms for anaphase chromosome movements: crane-fly spermatocyte spindles normally use actin filaments but also can function without them

Actin inhibitors block or slow anaphase chromosome movements in crane-fly spermatocytes, but stopping of movement is only temporary; we assumed that cells adapt to loss of actin by switching to mechanism(s) involving only microtubules. To test this, we produced actin-filament-free spindles: we added latrunculin B during prometaphase, 9-80 min before anaphase, after which chromosomes generally moved normally during anaphase. We confirmed the absence of actin filaments by staining with fluorescent phalloidin and by showing that cytochalasin D had no effect on chromosome movement. Thus, actin filaments are involved in normal anaphase movements, but in vivo, spindles nonetheless can function normally without them. We tested whether chromosome movements in actin-filament-free spindles arise via microtubules by challenging such spindles with anti-myosin drugs. Y-27632 and BDM (2,3-butanedione monoxime), inhibitors that affect myosin at different regulatory levels, blocked chromosome movement in normal spindles and in actin-filament-free spindles. We tested whether BDM has side effects on microtubule motors. BDM had no effect on ciliary and sperm motility or on ATPase activity of isolated ciliary axonemes, and thus it does not directly block dynein. Nor does it block kinesin, assayed by a microtubule sliding assay. BDM could conceivably indirectly affect these microtubule motors, though it is unlikely that it would have the same side effect on the motors as Y-27632. Since BDM and Y-27632 both affect chromosome movement in the same way, it would seem that both affect spindle myosin; this suggests that spindle myosin interacts with kinetochore microtubules, either directly or via an intermediate component.

Localization of actin filaments on mitotic apparatus in tobacco BY-2 cells

Actin filaments are among the major components of the cytoskeleton, and participate in various cellular dynamic processes. However, conflicting results had been obtained on the localization of actin filaments on the mitotic apparatus and their participation in the process of chromosome segregation. We demonstrated by using rhodamine-phalloidin staining, the localization of actin filaments on the mitotic spindles of tobacco BY-2 cells when the cells were treated with cytochalasin D. At prophase, several clear spots were observed at or near the kinetochores of the chromosomes. At anaphase, the actin filaments that appeared to be pulling chromosomes toward the division poles were demonstrated. However, as there was a slight possibility that these results might have been the artifacts of cytochalasin D treatment or the phalloidin staining, we analyzed the localization of actin filaments at the mitotic apparatus immunologically. We cloned a novel BY-2 alpha-type actin cDNA and prepared a BY-2 actin antibody. The fluorescence of the anti-BY-2 actin antibody was clearly observed at the mitotic apparatus in both non-treated and cytochalasin D-treated BY-2 cells during mitosis. The facts that similar results were obtained in both actin staining with rhodamine-phalloidin and immunostaining with actin antibody strongly indicate the participation of actin in the organization of the spindle body or in the process of chromosome segregation. Furthermore, both filamentous actin and spindle bodies disappeared in the cells treated with propyzamide, which depolymerizes microtubules, supporting the notion that actin filaments are associated with microtubules organizing the spindle body.

[Centromere elasticity]

In addition to the role in the spindle apparatus and associated motors, the chromosome themselves play an important role in facilitating chromosome segregation. Sister chromatids are joined at the centromere through a protein complex called cohesin. Chromatids separation requires the degradation by separase of specific proteins acting as a glue to form the cohesin complex. This evolutionally complex is required for the establishment and maintenance of sister chromatids in a ring like structure. It is therefore a key question whether cohesin is indeed a main component of active centromere. Cohesin is insufficient to resist the splitting force exerted by microtubules until anaphase and must be renforced by cohesion provided by flanking DNA. The ring model suggests that cohesine might possess a considerable mobility when associated with chromatin. Observations demonstrate that the interior region of the centromere behaves as an elastic element. Chromosomes display remarkable elasticity, returning to their initial shape after being extended by up to 10 times. For larger deformations the thick filament is converted in thin filament which can be stretched six times before breaking. This article suggests an additional and novel role for the protein titin on chromosome structure and dynamic. Titine was identified as a chromosomal component and it was hypothesised that titin may provide elasticity to chromosome and resistance to chromosome breakages during mitosis. The elastic properties of purified titin correspond well to the elastic properties of chromosome in living cells. The deformability and bending rigidity are consistent with a model developed for titin elasticity. The association of the presence of cohesine ring and the activity of titin could be necessary for segregation.

Direct visualization of microtubule flux during metaphase and anaphase in crane-fly spermatocytes

Microtubule flux in spindles of insect spermatocytes, long-used models for studies on chromosome behavior during meiosis, was revealed after iontophoretic microinjection of rhodamine-conjugated (rh)-tubulin and fluorescent speckle microscopy. In time-lapse movies of crane-fly spermtocytes, fluorescent speckles generated when rh-tubulin incorporated at microtubule plus ends moved poleward through each half-spindle and then were lost from microtubule minus ends at the spindle poles. The average poleward velocity of approximately 0.7 microm/min for speckles within kinetochore microtubules at metaphase increased during anaphase to approximately 0.9 microm/min. Segregating half-bivalents had an average poleward velocity of approximately 0.5 microm/min, about half that of speckles within shortening kinetochore fibers. When injected during anaphase, rhtubulin was incorporated at kinetochores, and kinetochore fiber fluorescence spread poleward as anaphase progressed. The results show that tubulin subunits are added to the plus end of kinetochore microtubules and are removed from their minus ends at the poles, all while attached chromosomes move poleward during anaphase A. The results cannot be explained by a Pac-man model, in which 1) kinetochore-based, minus end-directed motors generate poleward forces for anaphase A and 2) kinetochore microtubules shorten at their plus ends. Rather, in these cells, kinetochore fiber shortening during anaphase A occurs exclusively at the minus ends of kinetochore microtubules.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

Conserved microtubule-actin interactions in cell movement and morphogenesis

Interactions between microtubules and actin are a basic phenomenon that underlies many fundamental processes in which dynamic cellular asymmetries need to be established and maintained. These are processes as diverse as cell motility, neuronal pathfinding, cellular wound healing, cell division and cortical flow. Microtubules and actin exhibit two mechanistic classes of interactions--regulatory and structural. These interactions comprise at least three conserved 'mechanochemical activity modules' that perform similar roles in these diverse cell functions.

Actomyosin transports microtubules and microtubules control actomyosin recruitment during Xenopus oocyte wound healing

BACKGROUND

Interactions between microtubules and actin filaments (F-actin) are critical for cellular motility processes ranging from directed cell locomotion to cytokinesis. However, the cellular bases of these interactions remain poorly understood. We have analyzed the role of microtubules in generation of a contractile array comprised of F-actin and myosin-2 that forms around wounds made in Xenopus oocytes.

RESULTS

After wounding, microtubules are transported to the wound edge in association with F-actin that is itself recruited to wound borders via actomyosin-powered cortical flow. This transport generates sufficient force to buckle and break microtubules at the wound edge. Transport is complemented by local microtubule assembly around wound borders. The region of microtubule breakage and assembly coincides with a zone of actin assembly, and perturbation of the microtubule cytoskeleton disrupts this zone as well as local recruitment of the Arp2/3 complex and myosin-2.

CONCLUSIONS

The results reveal transport of microtubules in association with F-actin that is pulled to wound borders via actomyosin-based contraction. Microtubules, in turn, focus zones of actin assembly and myosin-2 recruitment at the wound border. Thus, wounding triggers the formation of a spatially coordinated feedback loop in which transport and assembly of microtubules maintains actin and myosin-2 in close proximity to the closing contractile array. These results are surprisingly reminiscent of recent findings in locomoting cells, suggesting that similar feedback interactions may be generally employed in a variety of fundamental cell motility processes.

Myosin localization during meiosis I of crane-fly spermatocytes gives indications about its role in division

We showed previously that in crane-fly spermatocytes myosin is required for tubulin flux [Silverman-Gavrila and Forer, 2000a: J Cell Sci 113:597-609], and for normal anaphase chromosome movement and contractile ring contraction [Silverman-Gavrila and Forer, 2001: Cell Motil Cytoskeleton 50:180-197]. Neither the identity nor the distribution of myosin(s) were known. In the present work, we used immunofluorescence and confocal microscopy to study myosin during meiosis-I of crane-fly spermatocytes compared to tubulin, actin, and skeletor, a spindle matrix protein, in order to further understand how myosin might function during cell division. Antibodies to myosin II regulatory light chain and myosin II heavy chain gave similar staining patterns, both dependent on stage: myosin is associated with nuclei, asters, centrosomes, chromosomes, spindle microtubules, midbody microtubules, and contractile rings. Myosin and actin colocalization along kinetochore fibers from prometaphase to anaphase are consistent with suggestions that acto-myosin forces in these stages propel kinetochore fibres poleward and trigger tubulin flux in kinetochore fibres, contributing in this way to poleward chromosome movement. Myosin and actin colocalization at the cell equator in cytokinesis, similar to studies in other cells [e.g., Fujiwara and Pollard, 1978: J Cell Biol 77:182-195], supports a role of actin-myosin interactions in contractile ring function. Myosin and skeletor colocalization in prometaphase spindles is consistent with a role of these proteins in spindle formation. After microtubules or actin were disrupted, myosin remained in spindles and contractile rings, suggesting that the presence of myosin in these structures does not require the continued presence of microtubules or actin. BDM (2,3 butanedione, 2 monoxime) treatment that inhibits chromosome movement and cytokinesis also altered myosin distributions in anaphase spindles and contractile rings, consistent with the physiological effects, suggesting also that myosin needs to be active in order to be properly distributed.

Mitosis-dependent phosphorylation and activation of LIM-kinase 1

LIM-kinases (LIMK1 and LIMK2) regulate actin cytoskeletal reorganization through phosphorylation of cofilin, an actin-depolymerizing factor of actin filaments. Here, we describe a detailed analysis of the cell-cycle-dependent activity of endogenous LIMK1. When HeLa cells were synchronized at prometaphase by nocodazole-treatment, LIMK1 was hyperphosphorylated, and its activity toward cofilin phosphorylation was markedly increased. During cell cycle progression, LIMK1 activity was low in interphase but reached a maximal level during mitosis. Activation of LIMK1 during mitosis was abrogated by roscovitine, a specific inhibitor of cyclin-dependent kinases (CDKs), suggesting that activation of CDKs directly or indirectly participates in LIMK1 activation. These results strongly suggest that LIMK1 may play an important role in the cell cycle progression through regulation of actin cytoskeletal rearrangements.

Effects of anti-myosin drugs on anaphase chromosome movement and cytokinesis in crane-fly primary spermatocytes

To investigate whether myosin is involved in crane-fly primary spermatocyte division, we studied the effects of myosin inhibitors on chromosome movement and on cytokinesis. With respect to chromosome movement, the myosin ATPase inhibitor 2,3-butanedione 2-monoxime (BDM) added during autosomal anaphase reversibly perturbed the movements of all autosomes: autosomes stopped, slowed, or moved backwards during treatment. BDM added before anaphase onset altered chromosome movement less than when BDM was added during anaphase: chromosome movements only rarely were stopped. They often were normal initially and then, if altered at all, were slowed. To confirm that the effects of BDM were due to myosin inhibition, we treated cells with ML-7, a drug that inhibits myosin light chain kinase (MLCK), an enzyme necessary to activate myosin. ML-7 affected anaphase movement only when added in early prometaphase: this treatment prevented chromosome attachment to the spindle. We treated cells with H-7 as a control for possible non-myosin effects of ML-7. H-7, which has a lower affinity than ML-7 for MLCK but a higher affinity than ML-7 for other potential targets, had no effect. These data confirm that the BDM effect is on myosin and indicate that the myosin used for chromosome movement is activated near the start of prometaphase. With respect to cytokinesis, BDM did not block furrow initiation but did block subsequent contraction of the contractile ring. When BDM was added after initiation of the furrow, the contractile ring either stalled or relaxed. ML-7 blocked contractile ring contraction when added at all stages after autosomal anaphase onset, including when added during cytokinesis. H-7 had no effect. These results confirm that the effects of BDM are on myosin and indicate that the myosin used for cytokinesis is activated starting from autosomal anaphase and continuing throughout cytokinesis.

Microtubule flux mediates poleward motion of acentric chromosome fragments during meiosis in insect spermatocytes

We applied a combination of laser microsurgery and quantitative polarization microscopy to study kinetochore-independent forces that act on chromosome arms during meiosis in crane fly spermatocytes. When chromosome arms located within one of the half-spindles during prometa- or metaphase were cut with the laser, the acentric fragments (lacking kinetochores) that were generated moved poleward with velocities similar to those of anaphase chromosomes (approximately 0.5 microm/min). To determine the mechanism underlying this poleward motion of detached arms, we treated spermatocytes with the microtubule-stabilizing drug taxol. Spindles in taxol-treated cells were noticeably short, yet with polarized light, the distribution and densities of microtubules in domains where fragment movement occurred were not different from those in control cells. When acentric fragments were generated in taxol-treated spermatocytes, 22 of 24 fragments failed to exhibit poleward motion, and the two that did move had velocities attenuated by 80% (to approximately 0.1 microm/min). In these cells, taxol did not inhibit the disjunction of chromosomes nor prevent their poleward segregation during anaphase, but the velocity of anaphase was also decreased 80% (approximately 0.1 microm/min) relative to untreated controls. Together, these data reveal that microtubule flux exerts pole-directed forces on chromosome arms during meiosis in crane fly spermatocytes and strongly suggest that the mechanism underlying microtubule flux also is used in the anaphase motion of kinetochores in these cells.

Antagonistic forces generated by myosin II and cytoplasmic dynein regulate microtubule turnover, movement, and organization in interphase cells

Photoactivation of caged fluorescent tubulin was used mark the microtubule (MT) lattice and monitor MT behavior in interphase cells. A broadening of the photoactivated region occurred as MTs moved bidirectionally. MT movement was not inhibited when MT assembly was suppressed with nocodazole or Taxol; MT movement was suppressed by inhibition of myosin light chain kinase with ML7 or by a peptide inhibitor. Conversely, MT movement was increased after inhibition of cytoplasmic dynein with the antibody 70.1. In addition, the half-time for MT turnover was decreased in cells treated with ML7. These results demonstrate that myosin II and cytoplasmic dynein contribute to a balance of forces that regulates MT organization, movement, and turnover in interphase cells.

Regulation of the tip-high [Ca2+] gradient in growing hyphae of the fungus Neurospora crassa

Previous work has shown that hyphal elongation in the fungus Neurospora crassa requires a tip-high cytosolic Ca2+ gradient. The source of the Ca2+ appears to be intracellular stores as there is no net transplasma membrane Ca2+ flux at the elongating hyphal tip and modification of ion fluxes across the plasma membrane using voltage clamp is without effect on tip growth. To decode the internal mechanisms which generate and maintain the tip-high Ca2+ gradient we first identified calcium regulators which affect hyphal growth and morphology, then determined how they modify cytosolic [Ca2+] and the actin cytoskeleton using fluorescent dyes and confocal microscopy. Cyclopiazonic acid (a known inhibitor of the endoplasmic reticulum calcium ATPase) inhibits growth and increases cytoplasmic [Ca2+] in the basal region 10-25 microm behind the hyphal tip. 2-APB (2-aminoethoxydiphenyl borate, an inhibitor of IP3-induced Ca2+ release) inhibits hyphal elongation and dissipates the tip-high Ca2 gradient 0-10 microm from the tip. Microinjections of the IP3 receptor agonists adenophostin A and IP3 (but not control microinjections of the biologically inactive L-IP3) transiently inhibited growth and induced subapical branches. IP3 microinjections, but not L-IP3, lowered tip-localized [Ca2+] and increased basal [Ca2+]. Even though their effect on [Ca2+] gradients was different, both cyclopiazonic acid and 2-APB disrupted similarly the normal actin pattern at the hyphal apex. Conversely, disruption of actin with latrunculin B dissipated tip-localized Ca2+. We conclude that the tip-high Ca2+ gradient is generated internally by Ca2+ sequestration into endoplasmic reticulum behind the tip and Ca2+ release via an IP3 receptor from tip-localized vesicles whose location is maintained by the actin cytoskeleton.

Phallacidin stains the kinetochore region in the mitotic spindle of the green algae Oedogonium spp

We found previously that in living cells of Oedogonium cardiacum and O. donnellii, mitosis is blocked by the drug cytochalasin D (CD). We now report on the staining observed in these spindles with fluorescently actin-labeling reagents, particularly Bodipy FL phallacidin. Normal mitotic cells exhibited spots of staining associated with chromosomes; frequently the spots appeared in pairs during prometaphase-metaphase. During later anaphase and telophase, the staining was confined to the region between chromosomes and poles. The texture of the staining appeared to be somewhat dispersed by CD treatment but it was still present, particularly after shorter (< 2 h) exposure. Electron microscopy of CD-treated cells revealed numerous spindle microtubules (MTs); many kinetochores had MTs associated with them, often laterally and some even terminating in the kinetochore as normal, but the usual bundle of kinetochore MTs was never present. As treatment with CD became prolonged, the kinetochores became shrunken and sunk into the chromosomes. These results support the possibility that actin is present in the kinetochore of Oedogonium spp. The previous observations on living cells suggest that it is a functional component of the kinetochore-MT complex involved in the correct attachment of chromosomes to the spindle.

Pac-Man does not resolve the enduring problem of anaphase chromosome movement

The Pac-Man hypothesis suggests that poleward movement of chromosomes during anaphase A is brought about by: disassembly of kinetochore microtubules (MTs) at the kinetochore; generation of the poleward force exclusively at or very close to the kinetochore; and the required energy coming from coupled disassembly of these MTs. This model has become widely accepted and cited as the sole or major mechanism of anaphase A. Rarely acknowledged are several significant phenomena that refute some or all of these postulates. We summarise these anomalies as follows: poleward movement of chromosomes occurring without insertion of any MTs at the kinetochore; "anaphase" shortening of kinetochore fibres in spindles entirely devoid of chromosomes and, presumably, kinetochores; continued movement of chromosomes while their severed kinetochore stub elongated poleward after treatment with UV microbeams; and fluxing of tubulin subunits through kinetochore MTs during anaphase A, indicating that during anaphase, kinetochore MTs disassemble partly or solely at the poles.

Cellular Samurai: katanin and the severing of microtubules

Recent biochemical studies of the AAA ATPase, katanin, provide a foundation for understanding how microtubules might be severed along their length. These in vitro studies are complemented by a series of recent reports of direct in vivo observation of microtubule breakage, which indicate that the in vitro phenomenon of catalysed microtubule severing is likely to be physiological. There is also new evidence that microtubule severing by katanin is important for the production of non-centrosomal microtubules in cells such as neurons and epithelial cells. Although it has been difficult to establish the role of katanin in mitosis, new genetic evidence indicates that a katanin-like protein, MEI-1, plays an essential role in meiosis in C. elegans. Finally, new proteins involved in the severing of axonemal microtubules have been discovered in the deflagellation system of Chlamydomonas.

Analysis of cortical flow models in vivo

Cortical flow, the directed movement of cortical F-actin and cortical organelles, is a basic cellular motility process. Microtubules are thought to somehow direct cortical flow, but whether they do so by stimulating or inhibiting contraction of the cortical actin cytoskeleton is the subject of debate. Treatment of Xenopus oocytes with phorbol 12-myristate 13-acetate (PMA) triggers cortical flow toward the animal pole of the oocyte; this flow is suppressed by microtubules. To determine how this suppression occurs and whether it can control the direction of cortical flow, oocytes were subjected to localized manipulation of either the contractile stimulus (PMA) or microtubules. Localized PMA application resulted in redirection of cortical flow toward the site of application, as judged by movement of cortical pigment granules, cortical F-actin, and cortical myosin-2A. Such redirected flow was accelerated by microtubule depolymerization, showing that the suppression of cortical flow by microtubules is independent of the direction of flow. Direct observation of cortical F-actin by time-lapse confocal analysis in combination with photobleaching showed that cortical flow is driven by contraction of the cortical F-actin network and that microtubules suppress this contraction. The oocyte germinal vesicle serves as a microtubule organizing center in Xenopus oocytes; experimental displacement of the germinal vesicle toward the animal pole resulted in localized flow away from the animal pole. The results show that 1) cortical flow is directed toward areas of localized contraction of the cortical F-actin cytoskeleton; 2) microtubules suppress cortical flow by inhibiting contraction of the cortical F-actin cytoskeleton; and 3) localized, microtubule-dependent suppression of actomyosin-based contraction can control the direction of cortical flow. We discuss these findings in light of current models of cortical flow.