Mitosis in primary cultures ofDrosophila melanogasterlarval neuroblasts

Frequent Spindle Assembly Errors Require Structural Rearrangement to Complete Meiosis in Zea mays

The success of an organism is contingent upon its ability to faithfully pass on its genetic material. In the meiosis of many species, the process of chromosome segregation requires that bipolar spindles be formed without the aid of dedicated microtubule organizing centers, such as centrosomes. Here, we describe detailed analyses of acentrosomal spindle assembly and disassembly in time-lapse images, from live meiotic cells of Zea mays. Microtubules organized on the nuclear envelope with a perinuclear ring structure until nuclear envelope breakdown, at which point microtubules began bundling into a bipolar form. However, the process and timing of spindle assembly was highly variable, with frequent assembly errors in both meiosis I and II. Approximately 61% of cells formed incorrect spindle morphologies, with the most prevalent being tripolar spindles. The erroneous spindles were actively rearranged to bipolar through a coalescence of poles before proceeding to anaphase. Spindle disassembly occurred as a two-state process with a slow depolymerization, followed by a quick collapse. The results demonstrate that maize meiosis I and II spindle assembly is remarkably fluid in the early assembly stages, but otherwise proceeds through a predictable series of events.

The rates of stem cell division determine the cell cycle lengths of its lineage

Adult stem cells and their transit-amplifying progeny alter their proliferation rates to maintain tissue homeostasis. To test how the division rates of stem cells and transit-amplifying progeny affect tissue growth and differentiation, we developed a computation strategy that estimates the average cell-cycle lengths (lifespans) of germline stem cells and their progeny from fixed-tissue demography in the Drosophila testis. Analysis of the wild-type data using this method indicated that during the germline transit-amplification, the cellular lifespans extend by nearly 1.3-fold after the first division and shrink by about 2-folds after the second division. Cell-autonomous perturbations of the stem cell lifespan accordingly altered the lifespans of successive transit-amplifying stages. Remarkably, almost 2-fold alterations in the lifespans of stem cells and their immediate daughters did not affect the subsequent differentiation. The results indicate that the early germline division rates can adjust the following division rates and the onset of differentiation.

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Prediction of cellular lifespan from the demography of transit-amplifying cells

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Lifespans of spermatogonial cells change anomalously during transit-amplification

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Anomalous lifespan extension during transit-amplification precedes the onset of Bam

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Lifespan changes of early TA stages readjust that of the subsequent stages

HI-NESS: a family of genetically encoded DNA labels based on a bacterial nucleoid-associated protein

The interplay between three-dimensional chromosome organisation and genomic processes such as replication and transcription necessitates in vivo studies of chromosome dynamics. Fluorescent organic dyes are often used for chromosome labelling in vivo. The mode of binding of these dyes to DNA cause its distortion, elongation, and partial unwinding. The structural changes induce DNA damage and interfere with the binding dynamics of chromatin-associated proteins, consequently perturbing gene expression, genome replication, and cell cycle progression. We have developed a minimally-perturbing, genetically encoded fluorescent DNA label consisting of a (photo-switchable) fluorescent protein fused to the DNA-binding domain of H-NS — a bacterial nucleoid-associated protein. We show that this DNA label, abbreviated as HI-NESS (H-NS-based indicator for nucleic acid stainings), is minimally-perturbing to genomic processes and labels chromosomes in eukaryotic cells in culture, and in zebrafish embryos with preferential binding to AT-rich chromatin.

Emergence of Embryo Shape During Cleavage Divisions

Cells are arranged into species-specific patterns during early embryogenesis. Such cell division patterns are important since they often reflect the distribution of localized cortical factors from eggs/fertilized eggs to specific cells as well as the emergence of organismal form. However, it has proven difficult to reveal the mechanisms that underlie the emergence of cell positioning patterns that underlie embryonic shape, likely because a systems-level approach is required that integrates cell biological, genetic, developmental, and mechanical parameters. The choice of organism to address such questions is also important. Because ascidians display the most extreme form of invariant cleavage pattern among the metazoans, we have been analyzing the cell biological mechanisms that underpin three aspects of cell division (unequal cell division (UCD), oriented cell division (OCD), and asynchronous cell cycles) which affect the overall shape of the blastula-stage ascidian embryo composed of 64 cells. In ascidians, UCD creates two small cells at the 16-cell stage that in turn undergo two further successive rounds of UCD. Starting at the 16-cell stage, the cell cycle becomes asynchronous, whereby the vegetal half divides before the animal half, thus creating 24-, 32-, 44-, and then 64-cell stages. Perturbing either UCD or the alternate cell division rhythm perturbs cell position. We propose that dynamic cell shape changes propagate throughout the embryo via cell-cell contacts to create the ascidian-specific invariant cleavage pattern.

Methods to study meiosis in insect spermatocytes

This chapter covers methods that are useful for the in vitro culture and live-cell study of insect spermatocytes in general and of crane-fly spermatocytes in particular. The merits of crane-fly spermatocytes are detailed in the Introduction section. In the following sections, step-by-step instructions are given for optimizing visualization of meiotic events taking place within living spermatocytes by employing microaspiration to flatten cells and then in subsequent operations to manipulate them via microinjection. Emphasis is on the attributes of ionophoretic injection as a way of introducing fluorescently conjugated proteins into the cytoplasm of flattened spermatocytes. In the last section of this chapter, the presentation of pressure injection is an alternative for delivering cell permeable probes into the interstitial space surrounding spermatocytes within in vitro preparations.

Metaphase Spindle Assembly

A microtubule-based bipolar spindle is required for error-free chromosome segregation during cell division. In this review I discuss the molecular mechanisms required for the assembly of this dynamic micrometer-scale structure in animal cells.

Long-term live cell imaging and automated 4D analysis of drosophila neuroblast lineages

The developing Drosophila brain is a well-studied model system for neurogenesis and stem cell biology. In the Drosophila central brain, around 200 neural stem cells called neuroblasts undergo repeated rounds of asymmetric cell division. These divisions typically generate a larger self-renewing neuroblast and a smaller ganglion mother cell that undergoes one terminal division to create two differentiating neurons. Although single mitotic divisions of neuroblasts can easily be imaged in real time, the lack of long term imaging procedures has limited the use of neuroblast live imaging for lineage analysis. Here we describe a method that allows live imaging of cultured Drosophila neuroblasts over multiple cell cycles for up to 24 hours. We describe a 4D image analysis protocol that can be used to extract cell cycle times and growth rates from the resulting movies in an automated manner. We use it to perform lineage analysis in type II neuroblasts where clonal analysis has indicated the presence of a transit-amplifying population that potentiates the number of neurons. Indeed, our experiments verify type II lineages and provide quantitative parameters for all cell types in those lineages. As defects in type II neuroblast lineages can result in brain tumor formation, our lineage analysis method will allow more detailed and quantitative analysis of tumorigenesis and asymmetric cell division in the Drosophila brain.

Sticky/Citron kinase maintains proper RhoA localization at the cleavage site during cytokinesis

In many organisms, the small guanosine triphosphatase RhoA controls assembly and contraction of the actomyosin ring during cytokinesis by activating different effectors. Although the role of some RhoA effectors like formins and Rho kinase is reasonably understood, the functions of another putative effector, Citron kinase (CIT-K), are still debated. In this paper, we show that, contrary to previous models, the Drosophila melanogaster CIT-K orthologue Sticky (Sti) does not require interaction with RhoA to localize to the cleavage site. Instead, RhoA fails to form a compact ring in late cytokinesis after Sti depletion, and this function requires Sti kinase activity. Moreover, we found that the Sti Citron-Nik1 homology domain interacts with RhoA regardless of its status, indicating that Sti is not a canonical RhoA effector. Finally, Sti depletion caused an increase of phosphorylated myosin regulatory light chain at the cleavage site in late cytokinesis. We propose that Sti/CIT-K maintains correct RhoA localization at the cleavage site, which is necessary for proper RhoA activity and contractile ring dynamics.

Drosophila neural progenitor polarity and asymmetric division

In the Drosophila embryonic central nervous system, the neural precursor cells called neuroblasts undergo a number of asymmetric divisions along the apical-basal axis to give rise to different daughter cells of distinct fates. This review summarizes recent progress in understanding the mechanisms of these asymmetric cell divisions. We discuss proteins that are localized at distinct domains of cortex in the neuroblasts and their role in generating asymmetry. We also review uniformly cortical localized factors and actin cytoskeleton-associated motor proteins with regard to their potential role to serve as a link between distinct cortical domains in the neuroblasts. In this review, asymmetric divisions of sensory organ precursor and larval neuroblasts are also briefly discussed.

Polo kinase interacts with RacGAP50C and is required to localize the cytokinesis initiation complex

The assembly and constriction of an actomyosin contractile ring in cytokinesis is dependent on the activation of Rho at the equatorial cortex by a complex, here termed the cytokinesis initiation complex, between a microtubule-associated kinesin-like protein (KLP), a member of the RacGAP family, and the RhoGEF Pebble. Recently, the activity of the mammalian Polo kinase ortholog Plk1 has been implicated in the formation of this complex. We show here that Polo kinase interacts directly with the cytokinesis initiation complex by binding RacGAP50C. We find that a new domain of Polo kinase, termed the intermediate domain, interacts directly with RacGAP50C and that Polo kinase is essential for localization of the KLP-RacGAP centralspindlin complex to the cell equator and spindle midzone. In the absence of Polo kinase, RacGAP50C and Pav-KLP fail to localize normally, instead decorating microtubules along their length. Our results indicate that Polo kinase directly binds the conserved cytokinesis initiation complex and is required to trigger centralspindlin localization as a first step in cytokinesis.

Dual mechanism controls asymmetric spindle position in ascidian germ cell precursors

Mitotic spindle orientation with respect to cortical polarity cues generates molecularly distinct daughter cells during asymmetric cell division (ACD). However, during ACD it remains unknown how the orientation of the mitotic spindle is regulated by cortical polarity cues until furrowing begins. In ascidians, the cortical centrosome-attracting body (CAB) generates three successive unequal cleavages and the asymmetric segregation of 40 localized postplasmic/PEM RNAs in germ cell precursors from the 8-64 cell stage. By combining fast 4D confocal fluorescence imaging with gene-silencing and classical blastomere isolation experiments, we show that spindle repositioning mechanisms are active from prometaphase until anaphase, when furrowing is initiated in B5.2 cells. We show that the vegetal-most spindle pole/centrosome is attracted towards the CAB during prometaphase, causing the spindle to position asymmetrically near the cortex. Next, during anaphase, the opposite spindle pole/centrosome is attracted towards the border with neighbouring B5.1 blastomeres, causing the spindle to rotate (10°/minute) and migrate (3 μm/minute). Dynamic 4D fluorescence imaging of filamentous actin and plasma membrane shows that precise orientation of the cleavage furrow is determined by this second phase of rotational spindle displacement. Furthermore, in pairs of isolated B5.2 blastomeres, the second phase of rotational spindle displacement was lost. Finally, knockdown of PEM1, a protein localized in the CAB and required for unequal cleavage in B5.2 cells, completely randomizes spindle orientation. Together these data show that two separate mechanisms active during mitosis are responsible for spindle positioning, leading to precise orientation of the cleavage furrow during ACD in the cells that give rise to the germ lineage in ascidians.

Morgana/chp-1, a ROCK inhibitor involved in centrosome duplication and tumorigenesis

Centrosome abnormalities lead to genomic instability and are a common feature of many cancer cells. Here we show that mutations in morgana/chp-1 result in centrosome amplification and lethality in both Drosophila and mouse, and that the fly centrosome phenotype is fully rescued by the human ortholog of morgana. In mouse cells, morgana forms a complex with Hsp90 and ROCK I and II, and directly binds ROCK II. Morgana downregulation promotes the interaction between ROCK II and nucleophosmin (NPM), leading to an increased ROCK II kinase activity, which results in centrosome amplification. Morgana(+/-) primary cells and mice display an increased susceptibility to neoplastic transformation. In addition, tumor tissue array histochemical analysis revealed that morgana is underexpressed in a large fraction of breast and lung human cancers. Thus, morgana/chp-1 appears to prevent both centrosome amplification and tumorigenesis.

Drosophila Klp67A binds prophase kinetochores to subsequently regulate congression and spindle length

The kinesin-8 proteins are a family of microtubule-depolymerising motor molecules, which, despite their highly conserved roles in chromosome alignment and spindle dynamics, remain poorly characterised. Here, we report that the Drosophila kinesin-8 protein, Klp67A, exists in two spatially and functionally separable metaphase pools: at kinetochores and along the spindle. Fixed and live-cell analyses of different Klp67A recombinant variants indicate that this kinesin-8 first collects at kinetochores during prophase and, by metaphase, localises to the kinetochore outerplate. Although the catalytic motor activity of Klp67A is required for efficient kinetochore recruitment at all times, microtubules are entirely dispensable for this process. The tail of Klp67A does not play a role in kinetochore accumulation, but is both necessary and sufficient for spindle association. Using functional assays, we reveal that chromosome position and spindle length are determined by the microtubule-depolymerising motor activity of Klp67A exclusively when located at kinetochores, but not along the spindle. These data reveal that, unlike other metazoan kinesin-8 proteins, Klp67A binds the nascent prophase and mature metaphase kinetochore. From this location, Klp67A uses its motor activity to ensure chromosome alignment and proper spindle length.

Production of transgenic medaka fish carrying fluorescent nuclei and chromosomes

As with zebrafish, attention has focused on the teleost medaka Oryzias latipes as an experimental animal representative of non-mammalian vertebrates in various fields of biological science. To enable real-time analyses of the dynamics of nuclei and chromosomes in living medaka cells, we produced a transgenic medaka expressing a fusion protein between histone H2B and green fluorescent protein (GFP) under the control of a cytomegalovirus (CMV) promoter. Since the nuclei and chromosomes of transgenic medaka cells are labeled with GFP, their morphological changes can be instantly monitored throughout the mitotic cell cycle progression under a fluorescent microscope without any fixation and staining of samples. However, GFP-labeling of nuclei and chromosomes is not successful during early embryonic development until zygotic expression begins and during the meiotic cell cycle progression, because the CMV promoter does not work in these stages. In addition, histone H2B-GFP fusion proteins are expressed in an organ-specific manner; strong and ubiquitous expression occurs in cells comprising the gut and fin, whereas the expression is restricted to certain types of cells in the liver and brain. These findings suggest that the CMV-driven expression of the histone H2B-GFP transgene is modified depending on the integration site of the transgene in the genome. Nevertheless, easy and precise monitoring of cytological changes in nuclei and chromosomes in the majority of mitotic cells by using the transgenic medaka will greatly contribute to a better understanding of control mechanisms of nuclear and chromosomal behaviors in vertebrate cells.

Spindle fusion requires dynein-mediated sliding of oppositely oriented microtubules

BACKGROUND

Bipolar spindle assembly is critical for achieving accurate segregation of chromosomes. In the absence of centrosomes, meiotic spindles achieve bipolarity by a combination of chromosome-initiated microtubule nucleation and stabilization and motor-driven organization of microtubules. Once assembled, the spindle structure is maintained on a relatively long time scale despite the high turnover of the microtubules that comprise it. To study the underlying mechanisms responsible for spindle assembly and steady-state maintenance, we used microneedle manipulation of preassembled spindles in Xenopus egg extracts.

RESULTS

When two meiotic spindles were brought close enough together, they interacted, creating an interconnected microtubule structure with supernumerary poles. Without exception, the perturbed system eventually re-established bipolarity, forming a single spindle of normal shape and size. Bipolar spindle fusion was blocked when cytoplasmic dynein function was perturbed, suggesting a critical role for the motor in this process. The fusion of Eg5-inhibited monopoles also required dynein function but only occurred if the initial interpolar separation was less than twice the microtubule radius of a typical monopole.

CONCLUSIONS

Our experiments uniquely illustrate the architectural plasticity of the spindle and reveal a robust ability of the system to attain a bipolar morphology. We hypothesize that a major mechanism driving spindle fusion is dynein-mediated sliding of oppositely oriented microtubules, a novel function for the motor, and posit that this same mechanism might also be involved in normal spindle assembly and homeostasis.

Role of spindle asymmetry in cellular dynamics

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

Cell division requires a direct link between microtubule-bound RacGAP and Anillin in the contractile ring

The mitotic microtubule array plays two primary roles in cell division. It acts as a scaffold for the congression and separation of chromosomes, and it specifies and maintains the contractile-ring position. The current model for initiation of Drosophila and mammalian cytokinesis [1-5] postulates that equatorial localization of a RhoGEF (Pbl/Ect2) by a microtubule-associated motor protein complex creates a band of activated RhoA [6], which subsequently recruits contractile-ring components such as actin, myosin, and Anillin [1-3]. Equatorial microtubules are essential for continued constriction, but how they interact with the contractile apparatus is unknown. Here, we report the first direct molecular link between the microtubule spindle and the actomyosin contractile ring. We find that the spindle-associated component, RacGAP50C, which specifies the site of cleavage [1-5], interacts directly with Anillin, an actin and myosin binding protein found in the contractile ring [7-10]. Both proteins depend on this interaction for their localization. In the absence of Anillin, the spindle-associated RacGAP loses its association with the equatorial cortex, and cytokinesis fails. These results account for the long-observed dependence of cytokinesis on the continual presence of microtubules at the cortex.

Mitch a rapidly evolving component of the Ndc80 kinetochore complex required for correct chromosome segregation in Drosophila

We identified an essential kinetochore protein, Mitch, from a genetic screen in D. melanogaster. Mitch localizes to the kinetochore, and its targeting is independent of microtubules (MTs) and several other known kinetochore components. Animals carrying mutations in mitch die as late third-instar larvae; mitotic neuroblasts in larval brains exhibit high levels of aneuploidy. Analysis of fixed D. melanogaster brains and mitch RNAi in cultured cells, as well as video recordings of cultured mitch mutant neuroblasts, reveal that chromosome alignment in mitch mutants is compromised during spindle formation, with many chromosomes displaying persistent mono-orientation. These misalignments lead to aneuploidy during anaphase. Mutations in mitch also disrupt chromosome behavior during both meiotic divisions in spermatocytes: the entire chromosome complement often moves to only one spindle pole. Mutant mitotic cells exhibit contradictory behavior with respect to the spindle assembly checkpoint (SAC). Anaphase onset is delayed in untreated cells, probably because incorrect kinetochore attachment maintains the SAC. However, mutant brain cells and mitch RNAi cells treated with MT poisons prematurely disjoin their chromatids, and exit mitosis. These data suggest that Mitch participates in SAC signaling that responds specifically to disruptions in spindle microtubule dynamics. The mitch gene corresponds to the transcriptional unit CG7242, and encodes a protein that is a possible ortholog of the Spc24 or Spc25 subunit of the Ndc80 kinetochore complex. Despite the crucial role of Mitch in cell division, the mitch gene has evolved very rapidly among species in the genus Drosophila.

Spindle orientation, asymmetric division and tumour suppression in Drosophila stem cells

Recent genetic studies in flies have added further support to an increasing body of evidence that suggests that stem cells might be the cell-of-origin of certain tumours. Malfunction of the mechanisms that control the division of stem cells and the developmental fate of the two resulting daughters could be one of the initial events that steers cells into malignant transformation. These studies suggest a role for controlled spindle orientation in suppressing stem-cell overgrowth. In parallel, the machinery that drives asymmetry in stem cells has been further characterized, identifying new components and uncovering the unique, highly sophisticated behaviour of centrosomes in these cells.

Functionally unequal centrosomes drive spindle orientation in asymmetrically dividing Drosophila neural stem cells

Stem cell asymmetric division requires tight control of spindle orientation. To study this key process, we have recorded Drosophila larval neural stem cells (NBs) engineered to express fluorescent reporters for microtubules, pericentriolar material (PCM), and centrioles. We have found that early in the cell cycle, the two centrosomes become unequal: one organizes an aster that stays near the apical cortex for most of the cell cycle, while the other loses PCM and microtubule-organizing activity, and moves extensively throughout the cell until shortly before mitosis when, located near the basal cortex, it recruits PCM and organizes the second mitotic aster. Upon division, the apical centrosome remains in the stem cell, while the other goes into the differentiating daughter. Apical aster maintenance requires the function of Pins. These results reveal that spindle orientation in Drosophila larval NBs is determined very early in the cell cycle, and is mediated by asymmetric centrosome function.

Flies without a spindle checkpoint

Mad2 has a key role in the spindle-assembly checkpoint (SAC) - the mechanism delaying anaphase onset until all chromosomes correctly attach to the spindle. Here, we show that unlike every other reported case of SAC inactivation in metazoans, mad2-null Drosophila are viable and fertile, and their cells almost always divide correctly despite having no SAC and an accelerated 'clock', which is caused by premature degradation of cyclin B. Mitosis in Drosophila does not need the SAC because correct chromosome attachment is achieved very rapidly, before even the cell lacking Mad2 can initiate anaphase. Experimentally reducing spindle-assembly efficiency renders the cells Mad2-dependent. In fact, the robustness of the SAC may generally mask minor mitotic defects of mutations affecting spindle function. The reported lethality of other Drosophila SAC mutations may be explained by their multifunctionality, and thus the 'checkpoint' phenotypes previously ascribed to these mutations should be considered the consequence of eliminating both the checkpoint and a second mitotic function.

A role for a novel centrosome cycle in asymmetric cell division

Tissue stem cells play a key role in tissue maintenance. Drosophila melanogaster central brain neuroblasts are excellent models for stem cell asymmetric division. Earlier work showed that their mitotic spindle orientation is established before spindle formation. We investigated the mechanism by which this occurs, revealing a novel centrosome cycle. In interphase, the two centrioles separate, but only one is active, retaining pericentriolar material and forming a “dominant centrosome.” This centrosome acts as a microtubule organizing center (MTOC) and remains stationary, forming one pole of the future spindle. The second centriole is inactive and moves to the opposite side of the cell before being activated as a centrosome/MTOC. This is accompanied by asymmetric localization of Polo kinase, a key centrosome regulator. Disruption of centrosomes disrupts the high fidelity of asymmetric division. We propose a two-step mechanism to ensure faithful spindle positioning: the novel centrosome cycle produces a single interphase MTOC, coarsely aligning the spindle, and spindle–cortex interactions refine this alignment.

A primary cell culture of Drosophila postembryonic larval neuroblasts to study cell cycle and asymmetric division

Drosophila melanogaster is a key model system that has greatly contributed to the advance of developmental biology through its extensive and sophisticated genetics. Nevertheless, only a few in vitro approaches are available in Drosophila to complement genetic studies in order to better elucidate developmental mechanisms at the cellular and molecular level. Here we present a dissociated cell culture system generated from the optic lobes of Drosophila larval brain. This culture system makes it feasible to study the proliferative properties of Drosophila postembryonic Nbs by allowing BrdU pulse and chase assays, as well as detailed immunocytochemical analysis with molecular markers. These immunofluorescence experiments allowed us to conclude that localization of asymmetric cell division markers such as Inscuteable, Miranda, Prospero and Numb is cell autonomous. By time-lapse video recording we have observed interesting cellular features of postembryonic neurogenesis such us the polarized genesis of the neuroblast progeny, the extremely short ganglion mother cell (GMC) cell cycle, and the last division of a neuroblast lineage. The combination of this cell culture system and genetic tools of Drosophila will provide a powerful experimental model for the analysis of cell cycle and asymmetric cell division of neural progenitor cells.

Live imaging of Drosophila brain neuroblasts reveals a role for Lis1/dynactin in spindle assembly and mitotic checkpoint control

Lis1 is required for nuclear migration in fungi, cell cycle progression in mammals, and the formation of a folded cerebral cortex in humans. Lis1 binds dynactin and the dynein motor complex, but the role of Lis1 in many dynein/dynactin-dependent processes is not clearly understood. Here we generate and/or characterize mutants for Drosophila Lis1 and a dynactin subunit, Glued, to investigate the role of Lis1/dynactin in mitotic checkpoint function. In addition, we develop an improved time-lapse video microscopy technique that allows live imaging of GFP-Lis1, GFP-Rod checkpoint protein, green fluorescent protein (GFP)-labeled chromosomes, or GFP-labeled mitotic spindle dynamics in neuroblasts within whole larval brain explants. Our mutant analyses show that Lis1/dynactin have at least two independent functions during mitosis: first promoting centrosome separation and bipolar spindle assembly during prophase/prometaphase, and subsequently generating interkinetochore tension and transporting checkpoint proteins off kinetochores during metaphase, thus promoting timely anaphase onset. Furthermore, we show that Lis1/dynactin/dynein physically associate and colocalize on centrosomes, spindle MTs, and kinetochores, and that regulation of Lis1/dynactin kinetochore localization in Drosophila differs from both Caenorhabditis elegans and mammals. We conclude that Lis1/dynactin act together to regulate multiple, independent functions in mitotic cells, including spindle formation and cell cycle checkpoint release.

Using a histone yellow fluorescent protein fusion for tagging and tracking endothelial cells in ES cells and mice

We report the first endothelial lineage-specific transgenic mouse allowing live imaging at subcellular resolution. We generated an H2B-EYFP fusion protein which can be used for fluorescent labeling of nucleosomes and used it to specifically label endothelial cells in mice and in differentiating embryonic stem (ES) cells. A fusion cDNA encoding a human histone H2B tagged at its C-terminus with enhanced yellow fluorescent protein (EYFP) was expressed under the control of an Flk1 promoter and intronic enhancer. The Flk1::H2B-EYFP transgenic mice are viable and high levels of chromatin-localized reporter expression are maintained in endothelial cells of developing embryos and in adult animals upon breeding. The onset of fluorescence in differentiating ES cells and in embryos corresponds with the beginning of endothelial cell specification. These transgenic lines permit real-time imaging in normal and pathological vasculogenesis and angiogenesis to track individual cells and mitotic events at a level of detail that is unprecedented in the mouse.

Recruitment of Mad2 to the kinetochore requires the Rod/Zw10 complex

Compromising the activity of the spindle checkpoint permits mitotic exit in the presence of unattached kinetochores and, consequently, greatly increases the rate of aneuploidy in the daughter cells. The metazoan checkpoint mechanism is more complex than in yeast in that it requires additional proteins and activities besides the classical Mads and Bubs. Among these are Rod, Zw10, and Zwilch, components of a 700 Kdal complex (Rod/Zw10) that is required for recruitment of dynein/dynactin to kinetochores but whose role in the checkpoint is poorly understood. The dynamics of Rod and Mad2, examined in different organisms, show intriguing similarities as well as apparent differences. Here we simultaneously follow GFP-Mad2 and RFP-Rod and find they are in fact closely associated throughout early mitosis. They accumulate simultaneously on kinetochores and are shed together along microtubule fibers after attachment. Their behavior and position within attached kinetochores is distinct from that of BubR1; Mad2 and Rod colocalize to the outermost kinetochore region (the corona), whereas BubR1 is slightly more interior. Moreover, Mad2, but not BubR1, Bub1, Bub3, or Mps1, requires Rod/Zw10 for its accumulation on unattached kinetochores. Rod/Zw10 thus contributes to checkpoint activation by promoting Mad2 recruitment and to checkpoint inactivation by recruiting dynein/dynactin that subsequently removes Mad2 from attached kinetochores.

Asymmetric cell division in C. elegans: cortical polarity and spindle positioning

The one-cell Caenorhabditis elegans embryo divides asymmetrically into a larger and smaller blastomere, each with a different fate. How does such asymmetry arise? The sperm-supplied centrosome establishes an axis of polarity in the embryo that is transduced into the establishment of anterior and posterior cortical domains. These cortical domains define the polarity of the embryo, acting upstream of the PAR proteins. The PAR proteins, in turn, determine the subsequent segregation of fate determinants and the plane of cell division. We address how cortical asymmetry could be established, relying on data from C. elegans and other polarized cells, as well as from applicable models. We discuss how cortical polarity influences spindle position to accomplish an asymmetric division, presenting the current models of spindle orientation and anaphase spindle displacement. We focus on asymmetric cell division as a function of the actin and microtubule cytoskeletons, emphasizing the cell biology of polarity.

Citron kinase is an essential effector of the Pbl-activated Rho signalling pathway in Drosophila melanogaster

Pebble (Pbl)-activated RhoA signalling is essential for cytokinesis in Drosophila melanogaster. Here we report that the Drosophila citron gene encodes an essential effector kinase of Pbl-RhoA signalling in vivo. Drosophila citron is expressed in proliferating tissues but is downregulated in differentiating tissues. We find that Citron can bind RhoA and that localisation of Citron to the contractile ring is dependent on the cytokinesis-specific Pbl-RhoA signalling. Phenotypic analysis of mutants showed that citron is required for cytokinesis in every tissue examined, with mutant cells exhibiting multinucleate and hyperploid phenotypes. Strong genetic interactions were observed between citron and pbl alleles and constructs. Vertebrate studies implicate at least two Rho effector kinases, Citron and Rok, in cytokinesis. By contrast, we failed to find evidence for a role for the Drosophila ortholog of Rok in cell division. We conclude that Citron plays an essential, non-redundant role in the Rho signalling pathway during Drosophila cytokinesis.

Kinetochore-driven formation of kinetochore fibers contributes to spindle assembly during animal mitosis

It is now clear that a centrosome-independent pathway for mitotic spindle assembly exists even in cells that normally possess centrosomes. The question remains, however, whether this pathway only activates when centrosome activity is compromised, or whether it contributes to spindle morphogenesis during a normal mitosis. Here, we show that many of the kinetochore fibers (K-fibers) in centrosomal Drosophila S2 cells are formed by the kinetochores. Initially, kinetochore-formed K-fibers are not oriented toward a spindle pole but, as they grow, their minus ends are captured by astral microtubules (MTs) and transported poleward through a dynein-dependent mechanism. This poleward transport results in chromosome bi-orientation and congression. Furthermore, when individual K-fibers are severed by laser microsurgery, they regrow from the kinetochore outward via MT plus-end polymerization at the kinetochore. Thus, even in the presence of centrosomes, the formation of some K-fibers is initiated by the kinetochores. However, centrosomes facilitate the proper orientation of K-fibers toward spindle poles by integrating them into a common spindle.

Spindle mechanics and dynamics during mitosis in Drosophila

Drosophila melanogaster is an excellent model for studying mitosis. Syncytial embryos are amenable to time-lapse imaging of hundreds of synchronously dividing spindles, allowing the quantitation of spindle and chromosome dynamics with unprecedented fidelity. Other Drosophila cell types, including neuroblasts, cultured cells, spermatocytes and oocytes, contain spindles that differ in their design, providing cells amenable to different types of experiments and allowing identification of common core mechanisms. The function of mitotic proteins can be studied using mutants, inhibitor microinjection and RNA interference (RNAi) to identify the full inventory of mitotic proteins encoded by the genome. Here, we review recent advances in understanding how ensembles of mitotic proteins coordinate spindle assembly and chromosome motion in this system.

Flattening Drosophila cells for high-resolution light microscopic studies of mitosis in vitro

Here we briefly review techniques used to flatten cells that otherwise round in culture, so that their division can be more clearly analyzed in vitro by high resolution light microscopy. We then describe an agar overlay procedure for use with isolated Drosophila neuroblasts, which promotes their long-term viability while also allowing for correlative studies of the same cell in the living and fixed state. This same procedure can also be used to obtain high temporal and spatial resolution images of mitosis and cytokinesis in cultured Drosophila Schneider S2 cells, which are a popular model for RNAi studies.

Technicolour transgenics: imaging tools for functional genomics in the mouse

Over the past decade, a battery of powerful tools that encompass forward and reverse genetic approaches have been developed to dissect the molecular and cellular processes that regulate development and disease. The advent of genetically-encoded fluorescent proteins that are expressed in wild type and mutant mice, together with advances in imaging technology, make it possible to study these biological processes in many dimensions. Importantly, these technologies allow direct visual access to complex events as they happen in their native environment, which provides greater insights into mammalian biology than ever before.

Dissecting mitosis by RNAi in Drosophila tissue culture cells

Here we describe a detailed methodology to study the function of genes whose products function during mitosis by dsRNA-mediated interference (RNAi) in cultured cells of Drosophila melanogaster. This procedure is particularly useful for the analysis of genes for which genetic mutations are not available or for the dissection of complicated phenotypes derived from the analysis of such mutants. With the advent of whole genome sequencing it is expected that RNAi-based screenings will be one method of choice for the identification and study of novel genes involved in particular cellular processes. In this paper we focused particularly on the procedures for the proper phenotypic analysis of cells after RNAi-mediated depletion of proteins required for mitosis, the process by which the genetic information is segregated equally between daughter cells. We use RNAi of the microtubule-associated protein MAST/Orbit as an example for the usefulness of the technique.

A RhoGEF and Rho family GTPase-activating protein complex links the contractile ring to cortical microtubules at the onset of cytokinesis

The mechanism that positions the cytokinetic contractile ring is unknown, but derives from the spindle midzone. We show that an interaction between the Rho GTP exchange factor, Pebble, and the Rho family GTPase-activating protein, RacGAP50C, connects the contractile ring to cortical microtubules at the site of furrowing in D. melanogaster cells. Pebble regulates actomyosin organization, while RacGAP50C and its binding partner, the Pavarotti kinesin-like protein, regulate microtubule bundling. All three factors are required for cytokinesis. As furrowing begins, these proteins colocalize to a cortical equatorial ring. We propose that RacGAP50C-Pavarotti complexes travel on cortical microtubules to the cell equator, where they associate with the Pebble RhoGEF to position contractile ring formation and coordinate F-actin and microtubule remodeling during cytokinesis.