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ABSTRACT: During mitosis, human cells round up, decreasing their adhesion to extracellular substrates. This must be quickly reestablished by poorly understood cytoskeleton remodeling mechanisms that prevent detachment from epithelia, while ensuring the successful completion of cytokinesis. Here we show that the microtubule end-binding (EB) proteins EB1 and EB3 play temporally distinct roles throughout cell division. Whereas EB1 was involved in spindle orientation before anaphase, EB3 was required for stabilization of focal adhesions and coordinated daughter cell spreading during mitotic exit. Additionally, EB3 promoted midbody microtubule stability and, consequently, midbody stabilization necessary for efficient cytokinesis. Importantly, daughter cell adhesion and cytokinesis completion were spatially regulated by distinct states of EB3 phosphorylation on serine 176 by Aurora B. This EB3 phosphorylation was enriched at the midbody and shown to control cortical microtubule growth. These findings uncover differential roles of EB proteins and explain the importance of an Aurora B phosphorylation gradient for the spatiotemporal regulation of microtubule function during mitotic exit and cytokinesis.
The Journal of Cell Biology 05/2013; 201(5):709-724. · 10.26 Impact Factor
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ABSTRACT: Maintenance of genomic stability during eukaryotic cell division relies on the spindle assembly checkpoint (SAC) that prevents mitotic exit until all chromosomes are properly attached to the spindle. Polo is a mitotic kinase proposed to be involved in SAC function, but its role has remained elusive. We demonstrate that Polo and Aurora B functional interdependency comprises a positive feedback loop that promotes Mps1 kinetochore localization and activity. Expression of constitutively active Polo restores normal Mps1 kinetochore levels even after Aurora B inhibition, highlighting a role for Polo in Mps1 recruitment to unattached kinetochores downstream of Aurora B. We also show that Mps1 kinetochore localization is required for BubR1 hyperphosphorylation and formation of the 3F3/2 phosphoepitope. This is essential to allow recruitment of Cdc20 to unattached kinetochores and the assembly of anaphase-promoting complex/cyclosome-inhibitory complexes to levels that ensure long-term SAC activity. We propose a model in which Polo controls Mps1-dependent BubR1 phosphorylation to promote Cdc20 kinetochore recruitment and sustained SAC function.
The EMBO Journal 05/2013; · 9.20 Impact Factor
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ABSTRACT: According to the "immortal" DNA strand hypothesis (Cairns Nature 255:197-200, 1975), stem cells would keep their template strands in order to prevent the accumulation of mutations, which could occur during DNA replication. Despite the growing number of studies that attempt to test this hypothesis, the conclusions remain highly controversial. In the base of this controversy lie the current limitations of available methodology to selectively and faithfully track the fate of template DNA strands throughout and upon cell division. Here, we developed a method that allows the unequivocal tracking of single chromatids containing template DNA strands in Drosophila S2 cells in culture. This method consists in the induction of mitosis with unreplicated genomes (MUGs) in which cells are allowed to enter mitosis without prior DNA replication. This is achieved by RNAi-mediated knockdown of Double parked, a conserved protein required for the initiation of DNA replication and post-replication checkpoint response. The advantages of this system when compared with MUGs generated in mammalian cells is the preservation of chromatid morphology, the ease of loss-of-function studies and the possibility of in vivo applications. Altogether, this approach allows for the readily visualization and tracking of template DNA strands by simply monitoring cells stably expressing GFP-fusions with either Histone H2B or the centromeric Histone variant CID/CENP-A by time-lapse fluorescence microscopy. This might be useful for the dissection of the molecular mechanism behind asymmetric DNA strand segregation.
Chromosome Research 05/2013; 21(3):329-37. · 3.09 Impact Factor
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ABSTRACT: Accurate chromosome segregation during mitosis relies on a dynamic kinetochore (KT)-microtubule (MT) interface that switches from a labile to a stable condition in response to correct MT attachments. This transition is essential to satisfy the spindle-assembly checkpoint (SAC) and couple MT-generated force with chromosome movements, but the underlying regulatory mechanism remains unclear. In this study, we show that during mitosis the MT- and KT-associated protein CLASP2 is progressively and distinctively phosphorylated by Cdk1 and Plk1 kinases, concomitant with the establishment of KT-MT attachments. CLASP2 S1234 was phosphorylated by Cdk1, which primed CLASP2 for association with Plk1. Plk1 recruitment to KTs was enhanced by CLASP2 phosphorylation on S1234. This was specifically required to stabilize KT-MT attachments important for chromosome alignment and to coordinate KT and non-KT MT dynamics necessary to maintain spindle bipolarity. CLASP2 C-terminal phosphorylation by Plk1 was also required for chromosome alignment and timely satisfaction of the SAC. We propose that Cdk1 and Plk1 mediate a fine CLASP2 "phospho-switch" that temporally regulates KT-MT attachment stability.
The Journal of Cell Biology 10/2012; 199(2):285-301. · 10.26 Impact Factor
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ABSTRACT: A microtubule (MT) subpopulation that emanates from Golgi membrane has been recently shown to comprise a significant part of MT network in interphase cells. In this study, we address whether Golgi membrane, which is being extensively remodeled throughout the cell cycle, retains its ability to nucleate MTs at diverse cell cycle stages. Live cell imaging and immunofluorescence microscopy reveals that Golgi-derived MTs form at multiple stages of the cell cycle, including G(1) , G(2) , and distinct phases of mitosis. However, the capacity of Golgi to nucleate MTs in mitosis is strongly down-regulated as compared with interphase, indicating that this property is cell cycle regulated. We demonstrate that Golgi-derived MTs are indispensable for efficient Golgi assembly in telophase, and speculate that these noncentrosomal MTs may hold specific functions at other cell cycle stages. © 2012 Wiley Periodicals, Inc.
Cytoskeleton 10/2012;
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ABSTRACT: The concept of a spindle matrix has long been proposed. Whether such a structure exists, however, and what its molecular and structural composition are have remained controversial. In this study, using a live-imaging approach in Drosophila syncytial embryos, we demonstrate that nuclear proteins reorganize during mitosis to form a highly dynamic, viscous spindle matrix that embeds the microtubule spindle apparatus, stretching from pole to pole. We show that this "internal" matrix is a distinct structure from the microtubule spindle and from a lamin B-containing spindle envelope. By injection of 2000-kDa dextran, we show that the disassembling nuclear envelope does not present a diffusion barrier. Furthermore, when microtubules are depolymerized with colchicine just before metaphase the spindle matrix contracts and coalesces around the chromosomes, suggesting that microtubules act as "struts" stretching the spindle matrix. In addition, we demonstrate that the spindle matrix protein Megator requires its coiled-coil amino-terminal domain for spindle matrix localization, suggesting that specific interactions between spindle matrix molecules are necessary for them to form a complex confined to the spindle region. The demonstration of an embedding spindle matrix lays the groundwork for a more complete understanding of microtubule dynamics and of the viscoelastic properties of the spindle during cell division.
Molecular biology of the cell 08/2012; 23(18):3532-41. · 5.98 Impact Factor
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ABSTRACT: Chromosome positioning at the equator of the mitotic spindle emerges out of a relatively entropic background. At this moment, termed metaphase, all kinetochores have typically captured microtubules leading to satisfaction of the spindle-assembly checkpoint, but the cell does not enter anaphase immediately. The waiting time in metaphase is related to the kinetics of securin and cyclin B1 degradation, which trigger sister-chromatid separation and promote anaphase processivity, respectively. Yet, as judged by metaphase duration, such kinetics vary widely between cell types and organisms, with no evident correlation to ploidy or cell size. During metaphase, many animal and plant spindles are also characterized by a conspicuous "flux" activity characterized by continuous poleward translocation of spindle microtubules, which maintain steady-state length and position. Whether spindle microtubule flux plays a specific role during metaphase remains arguable. Based on known experimental parameters, we have performed a comparative analysis amongst different cell types from different organisms and show that spindle length, metaphase duration and flux velocity combine within each system to obey a quasi-universal rule. As so, knowledge of two of these parameters is enough to estimate the third. This trend indicates that metaphase duration is tuned to allow approximately one kinetochore-to-pole round of microtubule flux. We propose that the time cells spend in metaphase evolved as a quality enhancement step that allows for the uniform stabilization/correction of kinetochore-microtubule attachments, thereby promoting mitotic fidelity.
Chromosome Research 07/2012; 20(5):563-77. · 3.09 Impact Factor
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ABSTRACT: After slightly more than a decade since it was first established, fluorescent speckle microscopy (FSM) has been intensively used to investigate macromolecular dynamics, such as microtubule flux in mitosis and meiosis, microtubule translocation in neurons, microtubule-binding proteins, and focal adhesion proteins, as well as the assembly of actin filaments. This state-of-the-art technique is based on nonuniform distribution of fluorescently labeled subunits diluted in the endogenous, unlabeled ones, resulting in microscopy-detectable speckled patterns. In order to enable sufficient contrast between neighboring diffraction-limited image regions, a low ratio between labeled and endogenous molecules is required, which can be achieved either by microinjection or by expression of limited amounts of fluorescently labeled subunits in cells. Over the years, the initial settings for FSM have been significantly improved by introduction of more sensitive cameras and spinning-disk confocal units, as well as by the development of specialized algorithms for image analysis. In this chapter, we describe our current FSM setup and detail on the necessary experimental approaches for its use in cultured cells, while discussing the present and future challenges of this powerful technique.
Methods in enzymology 01/2012; 504:147-61. · 1.90 Impact Factor
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ABSTRACT: Mitochondria proliferate by growth and partition during every cell-division cycle. Recently, Kashatus et al. (2011) reported that Aurora A kinase regulates the small GTPase RalA to mediate mitochondrial fission. This work illuminates the molecular mechanism behind mitochondrial inheritance in mammals and extends the functional repertoire of a key mitotic regulator.
Developmental cell 09/2011; 21(3):387-8. · 13.36 Impact Factor
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ABSTRACT: The centromere-specific histone H3 variant CENH3 (also known as CENP-A) is considered to be an epigenetic mark for establishment and propagation of centromere identity. Pulse induction of CENH3 (Drosophila CID) in Schneider S2 cells leads to its incorporation into non-centromeric regions and generates CID islands that resist clearing from chromosome arms for multiple cell generations. We demonstrate that CID islands represent functional ectopic kinetochores, which are non-randomly distributed on the chromosome and show a preferential localization near telomeres and pericentric heterochromatin in transcriptionally silent, intergenic chromatin domains. Although overexpression of heterochromatin protein 1 (HP1) or increasing histone acetylation interferes with CID island formation on a global scale, induction of a locally defined region of synthetic heterochromatin by targeting HP1-LacI fusions to stably integrated Lac operator arrays produces a proximal hotspot for CID deposition. These data indicate that the characteristics of regions bordering heterochromatin promote de novo kinetochore assembly and thereby contribute to centromere identity.
Nature Cell Biology 06/2011; 13(7):799-808. · 19.49 Impact Factor
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ABSTRACT: Accurate chromosome segregation during mitosis requires precise coordination of various processes, such as chromosome alignment, maturation of proper kinetochore-microtubule (kMT) attachments, correction of erroneous attachments, and silencing of the spindle assembly checkpoint (SAC). How these fundamental aspects of mitosis are coordinately and temporally regulated is poorly understood. In this study, we show that the temporal regulation of kMT attachments by CLASP1, astrin and Kif2b is central to mitotic progression and chromosome segregation fidelity. In early mitosis, a Kif2b-CLASP1 complex is recruited to kinetochores to promote chromosome movement, kMT turnover, correction of attachment errors, and maintenance of SAC signalling. However, during metaphase, this complex is replaced by an astrin-CLASP1 complex, which promotes kMT stability, chromosome alignment, and silencing of the SAC. We show that these two complexes are differentially recruited to kinetochores and are mutually exclusive. We also show that other kinetochore proteins, such as Kif18a, affect kMT attachments and chromosome movement through these proteins. Thus, CLASP1-astrin-Kif2b complex act as a central switch at kinetochores that defines mitotic progression and promotes fidelity by temporally regulating kMT attachments.
The EMBO Journal 10/2010; 29(20):3531-43. · 9.20 Impact Factor
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ABSTRACT: Although applicability of kymographs is limited to nearly one-dimensional (1D) processes, they have been instrumental in the analysis and interpretation of a wide range of dynamic biological processes. We focus here on some applications of kymography in the study of one among the range of 'nearly-1D' processes -mitosis. Using this biological context, we suggest generalized procedures in kymograph assembly that allow a partial retrieval of spatial information which is typically lost or distorted in conventional kymography. These kymograph variations, namely guided-kymography and chromo-kymography, are helpful in the determination of actual velocities and discrimination of structures when using thick regions of interest (ROIs). The method used to generate chromo-kymographs is generalized to other (non-kymograph) projection techniques, which include time-stack and z-stack projections.
Methods 06/2010; 51(2):214-9. · 4.01 Impact Factor
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Helder Maiato
Cellular and Molecular Life Sciences CMLS 03/2010; 67(13):2141-3. · 6.57 Impact Factor
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ABSTRACT: One of the most extraordinary events in the lifetime of a cell is the coordinated separation of sister chromatids during cell division. This is truly the essence of the entire mitotic process and the reason for the most profound morphological changes in cytoskeleton and nuclear organization that a cell may ever experience. It all occurs within a very short time window known as "anaphase", as if the cell had spent the rest of its existence getting ready for this moment in an ultimate act of survival. And there is a good reason for this: no space for mistakes. Problems in the distribution of chromosomes during cell division have been correlated with aneuploidy, a common feature observed in cancers and several birth defects, and the main cause of spontaneous abortion in humans. In this paper, we critically review the mechanisms of anaphase chromosome motion that resisted the scrutiny of more than 100 years of research, as part of a tribute to the pioneering work of Miguel Mota.
Cellular and Molecular Life Sciences CMLS 03/2010; 67(13):2251-69. · 6.57 Impact Factor
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ABSTRACT: In order to perpetuate their genetic content, eukaryotic cells have developed a microtubule-based machine known as the mitotic spindle. Independently of the system studied, mitotic spindles share at least one common characteristic--the dynamic nature of microtubules. This property allows the constant plasticity needed to assemble a bipolar structure, make proper kinetochore-microtubule attachments, segregate chromosomes, and finally disassemble the spindle and reform an interphase microtubule array. Here, we describe a variety of experimental approaches currently used in our laboratory to study microtubule dynamics during mitosis using Drosophila melanogaster S2 cells as a model. By using quantitative live cell imaging microscopy in combination with an advantageous labeling background, we illustrate how several cooperative pathways are used to build functional mitotic spindles. We illustrate different ways of perturbing spindle microtubule dynamics, including pharmacological inhibition and RNA interference of proteins that directly or indirectly impair microtubule dynamics. Additionally, we demonstrate the advantage of using fluorescent speckle microscopy to investigate an intrinsic property of spindle microtubules known as poleward flux. Finally, we developed a set of laser microsurgery-based experiments that allow, with unique spatiotemporal resolution, the study of specific spindle structures (e.g., centrosomes, microtubules, and kinetochores) and their respective roles during mitosis.
Methods in cell biology 01/2010; 97:243-57. · 2.05 Impact Factor
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Stefano Maffini,
Ana R R Maia,
Amity L Manning,
Zoltan Maliga,
Ana L Pereira,
Magno Junqueira,
Andrej Shevchenko,
Anthony Hyman,
John R Yates,
Niels Galjart,
Duane A Compton, Helder Maiato
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ABSTRACT: Efficient chromosome segregation during mitosis relies on the coordinated activity of molecular motors with proteins that regulate kinetochore attachments to dynamic spindle microtubules [1]. CLASPs are conserved kinetochore- and microtubule-associated proteins encoded by two paralog genes, clasp1 and clasp2, and have been previously implicated in the regulation of kinetochore microtubule dynamics [2-4]. However, it remains unknown how CLASPs work in concert with other proteins to form a functional kinetochore microtubule interface. Here we have identified mitotic interactors of human CLASP1 via a proteomic approach. Among these, the microtubule plus-end-directed motor CENP-E [5] was found to form a complex with CLASP1 that colocalizes to multiple structures of the mitotic apparatus in human cells. We found that CENP-E recruits both CLASP1 and CLASP2 to kinetochores independently of its motor activity or the presence of microtubules. Depletion of CLASPs or CENP-E by RNA interference in human cells causes a significant and comparable reduction of kinetochore microtubule poleward flux and turnover rates and rescues spindle bipolarity in Kif2a-depleted cells. We conclude that CENP-E integrates two critical functions that are important for accurate chromosome movement and spindle architecture: one relying directly on its motor activity, and the other involving the targeting of key microtubule regulators to kinetochores.
Current biology: CB 10/2009; 19(18):1566-72. · 10.99 Impact Factor
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ABSTRACT: The synchronous movement of chromosomes during anaphase ensures their correct inheritance in every cell division. This reflects the uniformity of spindle forces acting on chromosomes and their simultaneous entry into anaphase. Although anaphase onset is controlled by the spindle assembly checkpoint, it remains unknown how spindle forces are uniformly distributed among different chromosomes. In this paper, we show that tension uniformity at metaphase kinetochores and subsequent anaphase synchrony in Drosophila S2 cells are promoted by spindle microtubule flux. These results can be explained by a mechanical model of the spindle where microtubule poleward translocation events associated with flux reflect relaxation of the kinetochore-microtubule interface, which accounts for the redistribution and convergence of kinetochore tensions in a timescale comparable to typical metaphase duration. As predicted by the model, experimental acceleration of mitosis precludes tension equalization and anaphase synchrony. We propose that flux-dependent equalization of kinetochore tensions ensures a timely and uniform maturation of kinetochore-microtubule interfaces necessary for error-free and coordinated segregation of chromosomes in anaphase.
The Journal of Cell Biology 08/2009; 186(1):11-26. · 10.26 Impact Factor
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ABSTRACT: The chromodomain protein, Chromator, has been shown to have multiple functions that include regulation of chromatin structure as well as coordination of muscle remodeling during metamorphosis depending on the developmental context. In this study we show that mitotic neuroblasts from brain squash preparations from larvae heteroallelic for the two Chromator loss-of-function alleles Chro(71) and Chro(612) have severe microtubule spindle and chromosome segregation defects that were associated with a reduction in brain size. The microtubule spindles formed were incomplete, unfocused, and/or without clear spindle poles and at anaphase chromosomes were lagging and scattered. Time-lapse analysis of mitosis in S2 cells depleted of Chromator by RNAi treatment suggested that the lagging and scattered chromosome phenotypes were caused by incomplete alignment of chromosomes at the metaphase plate, possibly due to a defective spindle-assembly checkpoint, as well as of frayed and unstable microtubule spindles during anaphase. Expression of full-length Chromator transgenes under endogenous promoter control restored both microtubule spindle morphology as well as brain size strongly indicating that the observed mutant defects were directly attributable to lack of Chromator function.
Developmental Biology 08/2009; 334(1):253-63. · 4.07 Impact Factor
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ABSTRACT: Establishment and maintenance of the mitotic spindle requires the balanced activity of microtubule-associated proteins and motors. In this study we have addressed how the microtubule plus-end tracking protein mast/orbit/CLASP and cytoplasmic dynein regulate this process in Drosophila melanogaster embryos and S2 cells. We show that mast accumulates at kinetochores early in mitosis, which is followed by a poleward streaming upon microtubule attachment. This leads to a reduction of mast levels at kinetochores during metaphase and anaphase that depends largely on the microtubule minus end-directed motor cytoplasmic dynein. Surprisingly, we also found that co-depletion of dynein rescues spindle bipolarity in mast-depleted cells, while restoring normal microtubule poleward flux. Our results suggest that mast and dynein have antagonistic roles in the local regulation of microtubule plus-end dynamics at kinetochores, which are important for the maintenance of spindle bipolarity and normal spindle length.
Journal of Cell Science 07/2009; 122(Pt 14):2543-53. · 6.11 Impact Factor
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ABSTRACT: Cytoskeleton microtubules undergo a reversible metamorphosis as cells enter and exit mitosis to build a transient mitotic spindle required for chromosome segregation. Centrosomes play a dominant but dispensable role in microtubule (MT) organization throughout the animal cell cycle, supporting the existence of concurrent mechanisms that remain unclear. Here we investigated MT organization at the entry and exit from mitosis, after perturbation of centriole function in Drosophila S2 cells. We found that several MTs originate from acentriolar microtubule-organizing centers (aMTOCs) that contain gamma-tubulin and require Centrosomin (Cnn) for normal architecture and function. During spindle assembly, aMTOCs associated with peripheral MTs are recruited to acentriolar spindle poles by an Ncd/dynein-dependent clustering mechanism to form rudimentary aster-like structures. At anaphase onset, down-regulation of CDK1 triggers massive formation of cytoplasmic MTs de novo, many of which nucleated directly from aMTOCs. CDK1 down-regulation at anaphase coordinates the activity of Msps/XMAP215 and the kinesin-13 KLP10A to favor net MT growth and stability from aMTOCs. Finally, we show that microtubule nucleation from aMTOCs also occurs in cells containing centrosomes. Our data reveal a new form of cell cycle-regulated MTOCs that contribute for MT cytoskeleton remodeling during mitotic spindle assembly/disassembly in animal somatic cells, independently of centrioles.
Molecular biology of the cell 05/2009; 20(11):2796-808. · 5.98 Impact Factor
