Wac: a new Augmin subunit required for chromosome alignment but not for acentrosomal microtubule assembly in female meiosis

The augmin complex architecture reveals structural insights into microtubule branching

In mitosis, the augmin complex binds to spindle microtubules to recruit the γ-tubulin ring complex (γ-TuRC), the principal microtubule nucleator, for the formation of branched microtubules. Our understanding of augmin-mediated microtubule branching is hampered by the lack of structural information on the augmin complex. Here, we elucidate the molecular architecture and conformational plasticity of the augmin complex using an integrative structural biology approach. The elongated structure of the augmin complex is characterised by extensive coiled-coil segments and comprises two structural elements with distinct but complementary functions in γ-TuRC and microtubule binding, linked by a flexible hinge. The augmin complex is recruited to microtubules via a composite microtubule binding site comprising a positively charged unordered extension and two calponin homology domains. Our study provides the structural basis for augmin function in branched microtubule formation, decisively fostering our understanding of spindle formation in mitosis.

The formation of branched microtubule networks in mitotic spindles depends on the augmin complex. Zupa, Würtz et al. elucidate the molecular architecture and conformational plasticity of the augmin complex using integrative structural biology, providing structural insights into microtubule branching.

Reconstitution and mechanistic dissection of the human microtubule branching machinery

Branching microtubule (MT) nucleation is mediated by the augmin complex and γ-tubulin ring complex (γ-TuRC). However, how these two complexes work together to promote this process remains elusive. Here, using purified components from native and recombinant sources, we demonstrate that human augmin and γ-TuRC are sufficient to reconstitute the minimal MT branching machinery, in which NEDD1 bridges between augmin holo complex and GCP3/MZT1 subcomplex of γ-TuRC. The single-molecule experiment suggests that oligomerization of augmin may activate the branching machinery. We provide direct biochemical evidence that CDK1- and PLK1-dependent phosphorylation are crucial for NEDD1 binding to augmin, for their synergistic MT-binding activities, and hence for branching MT nucleation. In addition, we unveil that NEDD1 possesses an unanticipated intrinsic affinity for MTs via its WD40 domain, which also plays a pivotal role in the branching process. In summary, our study provides a comprehensive understanding of the underlying mechanisms of branching MT nucleation in human cells.

The phospho-docking protein 14-3-3 regulates microtubule-associated proteins in oocytes including the chromosomal passenger Borealin

Global regulation of spindle-associated proteins is crucial in oocytes due to the absence of centrosomes and their very large cytoplasmic volume, but little is known about how this is achieved beyond involvement of the Ran-importin pathway. We previously uncovered a novel regulatory mechanism in Drosophila oocytes, in which the phospho-docking protein 14-3-3 suppresses microtubule binding of Kinesin-14/Ncd away from chromosomes. Here we report systematic identification of microtubule-associated proteins regulated by 14-3-3 from Drosophila oocytes. Proteins from ovary extract were co-sedimented with microtubules in the presence or absence of a 14-3-3 inhibitor. Through quantitative mass-spectrometry, we identified proteins or complexes whose ability to bind microtubules is suppressed by 14-3-3, including the chromosomal passenger complex (CPC), the centralspindlin complex and Kinesin-14/Ncd. We showed that 14-3-3 binds to the disordered region of Borealin, and this binding is regulated differentially by two phosphorylations on Borealin. Mutations at these two phospho-sites compromised normal Borealin localisation and centromere bi-orientation in oocytes, showing that phospho-regulation of 14-3-3 binding is important for Borealin localisation and function.

Augmin deficiency in neural stem cells causes p53-dependent apoptosis and aborts brain development

Microtubules that assemble the mitotic spindle are generated by centrosomal nucleation, chromatin-mediated nucleation, and nucleation from the surface of other microtubules mediated by the augmin complex. Impairment of centrosomal nucleation in apical progenitors of the developing mouse brain induces p53-dependent apoptosis and causes non-lethal microcephaly. Whether disruption of non-centrosomal nucleation has similar effects is unclear. Here, we show, using mouse embryos, that conditional knockout of the augmin subunit Haus6 in apical progenitors led to spindle defects and mitotic delay. This triggered massive apoptosis and abortion of brain development. Co-deletion of Trp53 rescued cell death, but surviving progenitors failed to organize a pseudostratified epithelium, and brain development still failed. This could be explained by exacerbated mitotic errors and resulting chromosomal defects including increased DNA damage. Thus, in contrast to centrosomes, augmin is crucial for apical progenitor mitosis, and, even in the absence of p53, for progression of brain development.

The metaphase spindle at steady state - Mechanism and functions of microtubule poleward flux

The mitotic spindle is a bipolar cellular structure, built from tubulin polymers, called microtubules, and interacting proteins. This macromolecular machine orchestrates chromosome segregation, thereby ensuring accurate distribution of genetic material into the two daughter cells during cell division. Powered by GTP hydrolysis upon tubulin polymerization, the microtubule ends exhibit a metastable behavior known as the dynamic instability, during which they stochastically switch between the growth and shrinkage phases. In the context of the mitotic spindle, dynamic instability is furthermore regulated by microtubule-associated proteins and motor proteins, which enables the spindle to undergo profound changes during mitosis. This highly dynamic behavior is essential for chromosome capture and congression in prometaphase, as well as for chromosome alignment to the spindle equator in metaphase and their segregation in anaphase. In this review we focus on the mechanisms underlying microtubule dynamics and sliding and their importance for the maintenance of shape, structure and dynamics of the metaphase spindle. We discuss how these spindle properties are related to the phenomenon of microtubule poleward flux, highlighting its highly cooperative molecular basis and role in keeping the metaphase spindle at a steady state.

A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes

Mammalian oocytes segregate chromosomes with a microtubule spindle that lacks centrosomes, but the mechanisms by which acentrosomal spindles are organized and function are largely unclear. In this study, we identify a conserved subcellular structure in mammalian oocytes that forms by phase separation. This structure, which we term the liquid-like meiotic spindle domain (LISD), permeates the spindle poles and forms dynamic protrusions that extend well beyond the spindle. The LISD selectively concentrates multiple microtubule regulatory factors and allows them to diffuse rapidly within the spindle volume. Disruption of the LISD via different means disperses these factors and leads to severe spindle assembly defects. Our data suggest a model whereby the LISD promotes meiotic spindle assembly by serving as a reservoir that sequesters and mobilizes microtubule regulatory factors in proximity to spindle microtubules.

Direct observation of branching MT nucleation in living animal cells

Branching microtubule nucleation by its molecular mediators has never been directly observed in animal cells. By imaging augmin, γ-TuRC, and microtubules with high spatiotemporal resolution, Verma and Maresca quantitatively define the sequential steps of augmin-mediated branching microtubule nucleation in dividing Drosophila cells.

Centrosome-mediated microtubule (MT) nucleation has been well characterized; however, numerous noncentrosomal MT nucleation mechanisms exist. The branching MT nucleation pathway envisages that the γ-tubulin ring complex (γ-TuRC) is recruited to MTs by the augmin complex to initiate nucleation of new MTs. While the pathway is well conserved at a molecular and functional level, branching MT nucleation by core constituents has never been directly observed in animal cells. Here, multicolor TIRF microscopy was applied to visualize and quantitatively define the entire process of branching MT nucleation in dividing Drosophila cells during anaphase. The steps of a stereotypical branching nucleation event entailed augmin binding to a mother MT and recruitment of γ-TuRC after 15 s, followed by nucleation 16 s later of a daughter MT at a 36° branch angle. Daughters typically remained attached throughout their ∼40-s lifetime unless the mother depolymerized past the branch point. Assembly of branched MT arrays, which did not require Drosophila TPX2, enhanced localized RhoA activation during cytokinesis.

Spindle assembly and chromosome dynamics during oocyte meiosis

Organisms that reproduce sexually utilize a specialized form of cell division called meiosis to reduce their chromosome number by half to generate haploid gametes. Meiosis in females is especially error-prone, and this vulnerability has a profound impact on human health: it is estimated that 10-25% of human embryos are chromosomally abnormal, and the vast majority of these defects arise from problems with the female reproductive cells (oocytes). Here, we highlight recent studies that explore how these important cells divide. Although we focus on work in the model organism Caenorhabditis elegans, we also discuss complementary studies in other organisms that together provide new insights into this crucial form of cell division.

A computational model of the early stages of acentriolar meiotic spindle assembly

The mitotic spindle is an ensemble of microtubules responsible for the repartition of the chromosomal content between the two daughter cells during division. In metazoans, spindle assembly is a gradual process involving dynamic microtubules and recruitment of numerous associated proteins and motors. During mitosis, centrosomes organize and nucleate the majority of spindle microtubules. In contrast, oocytes lack canonical centrosomes but are still able to form bipolar spindles, starting from an initial ball that self-organizes in several hours. Interfering with early steps of meiotic spindle assembly can lead to erroneous chromosome segregation. Although not fully elucidated, this process is known to rely on antagonistic activities of plus end- and minus end-directed motors. We developed a model of early meiotic spindle assembly in mouse oocytes, including key factors such as microtubule dynamics and chromosome movement. We explored how the balance between plus end- and minus end-directed motors, as well as the influence of microtubule nucleation, impacts spindle morphology. In a refined model, we added spatial regulation of microtubule stability and minus-end clustering. We could reproduce the features of early stages of spindle assembly from 12 different experimental perturbations and predict eight additional perturbations. With its ability to characterize and predict chromosome individualization, this model can help deepen our understanding of spindle assembly.

Female Meiosis: Synapsis, Recombination, and Segregation in Drosophila melanogaster

A century of genetic studies of the meiotic process in Drosophila melanogaster females has been greatly augmented by both modern molecular biology and major advances in cytology. These approaches, and the findings they have allowed, are the subject of this review. Specifically, these efforts have revealed that meiotic pairing in Drosophila females is not an extension of somatic pairing, but rather occurs by a poorly understood process during premeiotic mitoses. This process of meiotic pairing requires the function of several components of the synaptonemal complex (SC). When fully assembled, the SC also plays a critical role in maintaining homolog synapsis and in facilitating the maturation of double-strand breaks (DSBs) into mature crossover (CO) events. Considerable progress has been made in elucidating not only the structure, function, and assembly of the SC, but also the proteins that facilitate the formation and repair of DSBs into both COs and noncrossovers (NCOs). The events that control the decision to mature a DSB as either a CO or an NCO, as well as determining which of the two CO pathways (class I or class II) might be employed, are also being characterized by genetic and genomic approaches. These advances allow a reconsideration of meiotic phenomena such as interference and the centromere effect, which were previously described only by genetic studies. In delineating the mechanisms by which the oocyte controls the number and position of COs, it becomes possible to understand the role of CO position in ensuring the proper orientation of homologs on the first meiotic spindle. Studies of bivalent orientation have occurred in the context of numerous investigations into the assembly, structure, and function of the first meiotic spindle. Additionally, studies have examined the mechanisms ensuring the segregation of chromosomes that have failed to undergo crossing over.

A novel microtubule nucleation pathway for meiotic spindle assembly in oocytes

The meiotic spindle in oocytes is assembled in the absence of centrosomes, the major microtubule nucleation sites in mitotic and male meiotic cells. A crucial, yet unresolved question in meiosis is how spindle microtubules are generated without centrosomes and only around chromosomes in the exceptionally large volume of oocytes. Here we report a novel oocyte-specific microtubule nucleation pathway that is essential for assembling most spindle microtubules complementarily with the Augmin pathway. This pathway is mediated by the kinesin-6 Subito/MKlp2, which recruits the γ-tubulin complex to the spindle equator to nucleate microtubules in Drosophila oocytes. Away from chromosomes, Subito interaction with the γ-tubulin complex is suppressed by its N-terminal region to prevent ectopic microtubule assembly in oocytes. We further demonstrate in vitro that the Subito complex from ovaries can nucleate microtubules from pure tubulin dimers. Collectively, microtubule nucleation regulated by Subito drives spatially restricted spindle assembly in oocytes.

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

Combining microscopy and biochemistry to study meiotic spindle assembly in Drosophila oocytes

Studies using Drosophila have played pivotal roles in advancing our understanding of molecular mechanisms of mitosis throughout the past decades, due to the short generation time and advanced genetic research of this organism. Drosophila is also an excellent model to study female meiosis in oocytes. Pathways such as the acentrosomal assembly of the meiotic spindle in oocytes are conserved from fly to humans. Collecting and manipulating large Drosophila oocytes for microscopy and biochemistry are both time and cost efficient, offering advantages over mouse or human oocytes. Therefore, Drosophila oocytes serve as an excellent platform for molecular studies of female meiosis using a combination of genetics, microscopy, and biochemistry. Here we describe key methods to observe the formation of the meiotic spindle either in fixed or in live oocytes. Moreover, biochemical methods are described to identify protein-protein interactions in vivo.

Cross-linking mass spectrometry identifies new interfaces of Augmin required to localise the γ-tubulin ring complex to the mitotic spindle

The hetero-octameric protein complex, Augmin, recruits γ-Tubulin ring complex (γ-TuRC) to pre-existing microtubules (MTs) to generate branched MTs during mitosis, facilitating robust spindle assembly. However, despite a recent partial reconstitution of the human Augmin complex in vitro, the molecular basis of this recruitment remains unclear. Here, we used immuno-affinity purification of in vivo Augmin from Drosophila and cross-linking/mass spectrometry to identify distance restraints between residues within the eight Augmin subunits in the absence of any other structural information. The results allowed us to predict potential interfaces between Augmin and γ-TuRC. We tested these predictions biochemically and in the Drosophila embryo, demonstrating that specific regions of the Augmin subunits, Dgt3, Dgt5 and Dgt6 all directly bind the γ-TuRC protein, Dgp71WD, and are required for the accumulation of γ-TuRC, but not Augmin, to the mitotic spindle. This study therefore substantially increases our understanding of the molecular mechanisms underpinning MT-dependent MT nucleation.

Summary: A map of subunit positional restrictions in Drosophila Augmin shows that Dgt3, Dgt5 and Dgt6 are all required to localise gamma-TuRC to mitotic spindles.

Mitotic spindle assembly in animal cells: a fine balancing act

The mitotic spindle has a crucial role in ensuring the accurate segregation of chromosomes into the two daughter cells during cell division, which is paramount for maintaining genome integrity. It is a self-organized and dynamic macromolecular structure that is constructed from microtubules, microtubule-associated proteins and motor proteins. Thirty years of research have led to the identification of centrosome-, chromatin- and microtubule-mediated microtubule nucleation pathways that each contribute to mitotic spindle assembly. Far from being redundant pathways, data are now emerging regarding how they function together to ensure the timely completion of mitosis. We are also beginning to comprehend the multiple mechanisms by which cells regulate spindle scaling. Together, this research has increased our understanding of how cells coordinate hundreds of proteins to assemble the dynamic, precise and robust structure that is the mitotic spindle.

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.

The chromosomal basis of meiotic acentrosomal spindle assembly and function in oocytes

Several aspects of meiosis are impacted by the absence of centrosomes in oocytes. Here, we review four aspects of meiosis I that are significantly affected by the absence of centrosomes in oocyte spindles. One, microtubules tend to assemble around the chromosomes. Two, the organization of these microtubules into a bipolar spindle is directed by the chromosomes. Three, chromosome bi-orientation and attachment to microtubules from the correct pole require modification of the mechanisms used in mitotic cells. Four, chromosome movement to the poles at anaphase cannot rely on polar anchoring of spindle microtubules by centrosomes. Overall, the chromosomes are more active participants during acentrosomal spindle assembly in oocytes, compared to mitotic and male meiotic divisions where centrosomes are present. The chromosomes are endowed with information that can direct the meiotic divisions and dictate their own behavior in oocytes. Processes beyond those known from mitosis appear to be required for their bi-orientation at meiosis I. As mitosis occurs without centrosomes in many systems other than oocytes, including all plants, the concepts discussed here may not be limited to oocytes. The study of meiosis in oocytes has revealed mechanisms that are operating in mitosis and will probably continue to do so.

Kdm5/Lid Regulates Chromosome Architecture in Meiotic Prophase I Independently of Its Histone Demethylase Activity

During prophase of the first meiotic division (prophase I), chromatin dynamically reorganises to recombine and prepare for chromosome segregation. Histone modifying enzymes are major regulators of chromatin structure, but our knowledge of their roles in prophase I is still limited. Here we report on crucial roles of Kdm5/Lid, one of two histone demethylases in Drosophila that remove one of the trimethyl groups at Lys4 of Histone 3 (H3K4me3). In the absence of Kdm5/Lid, the synaptonemal complex was only partially formed and failed to be maintained along chromosome arms, while localisation of its components at centromeres was unaffected. Kdm5/Lid was also required for karyosome formation and homologous centromere pairing in prophase I. Although loss of Kdm5/Lid dramatically increased the level of H3K4me3 in oocytes, catalytically inactive Kdm5/Lid can rescue the above cytological defects. Therefore Kdm5/Lid controls chromatin architecture in meiotic prophase I oocytes independently of its demethylase activity.

Accurate transmission of chromosomes carrying genetic materials from generation to generation is essential for life. Cell divisions that generate gametes, such as eggs and sperm, are critical, as chromosomes inherited from both parents recombine and are accurately sorted into gametes. Errors in these cell divisions often result in infertility, miscarriages or birth defects such as Down syndrome in humans. During these divisions, chromosomes undergo dramatic reorganisation but the molecular mechanisms are not well understood. Chromosome organisation is known to be regulated by various epigenetic marks, which are chemical marks on chromatin crucial for regulating gene expression. We found that an enzyme (Kdm5/Lid) that erases a mark linked to active gene expression regulates multiple aspects of meiotic chromatin organisation in oocytes, including stability of the recombination machinery. Unexpectedly, this function does not require its enzymatic activity. Our findings provide novel insights into how chromosomes are reorganised during reproduction and prompt re-evaluation of the role of this eraser enzyme.

Non-centrosomal nucleation mediated by augmin organizes microtubules in post-mitotic neurons and controls axonal microtubule polarity

Neurons display a highly polarized microtubule network that mediates trafficking throughout the extensive cytoplasm and is crucial for neuronal differentiation and function. In newborn migrating neurons, the microtubule network is organized by the centrosome. During neuron maturation, however, the centrosome gradually loses this activity, and how microtubules are organized in more mature neurons remains poorly understood. Here, we demonstrate that microtubule organization in post-mitotic neurons strongly depends on non-centrosomal nucleation mediated by augmin and by the nucleator γTuRC. Disruption of either complex not only reduces microtubule density but also microtubule bundling. These microtubule defects impair neurite formation, interfere with axon specification and growth, and disrupt axonal trafficking. In axons augmin does not merely mediate nucleation of microtubules but ensures their uniform plus end-out orientation. Thus, the augmin-γTuRC module, initially identified in mitotic cells, may be commonly used to generate and maintain microtubule configurations with specific polarity.

In mature neurons the centrosome no longer functions as the main microtubule organizer and it is unclear how ordered microtubule arrays are assembled. Here, the authors show that in post-mitotic neurons this process depends on non-centrosomal nucleation mediated by the protein complex augmin and the nucleator gamma-TuRC.

A de novo 10p11.23-p12.1 deletion recapitulates the phenotype observed in WAC mutations and strengthens the role of WAC in intellectual disability and behavior disorders

Chromosomal microarray analysis has become a powerful diagnostic tool in the investigation of patients with intellectual disability leading to the discovery of dosage sensitive genes implicated in the manifestation of various genomic disorders. Interstitial deletions of the short arm of chromosome 10 represent rare genetic abnormalities, especially those encompassing the chromosomal region 10p11-p12. To date, only 10 postnatal cases with microdeletion of this region have been described, and all patients shared a common phenotype, including intellectual disability, abnormal behavior, distinct dysmorphic features, visual impairment, and cardiac malformations. WAC was suggested to be the main candidate gene for intellectual disability associated with 10 p11-p12 deletion syndrome. Here, we describe a new case of de novo 10p11.23-p12.1 microdeletion in a patient with intellectual disability, abnormal behavior, and distinct dysmorphic features. Our observation allows us to redefine the smallest region of overlap among patients reported so far, with a size of 80 Kb and which contains only the WAC gene. These findings strengthen the hypothesis that haploinsufficency of WAC gene might be likely responsible for intellectual disability and behavior disorders. Our data also led us to propose a clinical pathway for patients with this recognizable genetic syndrome depending on the facial dysmorphisms. © 2016 Wiley Periodicals, Inc.

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.

Intra-spindle Microtubule Assembly Regulates Clustering of Microtubule-Organizing Centers during Early Mouse Development

Errors during cell division in oocytes and early embryos are linked to birth defects in mammals. Bipolar spindle assembly in early mouse embryos is unique in that three or more acentriolar microtubule-organizing centers (MTOCs) are initially formed and are then clustered into two spindle poles. Using a knockout mouse and live imaging of spindles in embryos, we demonstrate that MTOC clustering during the blastocyst stage requires augmin, a critical complex for MT-dependent MT nucleation within the spindle. Functional analyses in cultured cells with artificially increased numbers of centrosomes indicate that the lack of intra-spindle MT nucleation, but not loss of augmin per se or overall reduction of spindle MTs, is the cause of clustering failure. These data suggest that onset of mitosis with three or more MTOCs is turned into a typical bipolar division through augmin-dependent intra-spindle MT assembly.

Oocyte Meiotic Spindle Assembly and Function

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

The microtubule catastrophe promoter Sentin delays stable kinetochore-microtubule attachment in oocytes

The microtubule catastrophe-promoting complex Sentin-EB1 delays stable kinetochore–microtubule attachment and facilitates bipolar attachment of homologous chromosomes in Drosophila oocytes.

The critical step in meiosis is to attach homologous chromosomes to the opposite poles. In mouse oocytes, stable microtubule end-on attachments to kinetochores are not established until hours after spindle assembly, and phosphorylation of kinetochore proteins by Aurora B/C is responsible for the delay. Here we demonstrated that microtubule ends are actively prevented from stable attachment to kinetochores until well after spindle formation in Drosophila melanogaster oocytes. We identified the microtubule catastrophe-promoting complex Sentin-EB1 as a major factor responsible for this delay. Without this activity, microtubule ends precociously form robust attachments to kinetochores in oocytes, leading to a high proportion of homologous kinetochores stably attached to the same pole. Therefore, regulation of microtubule ends provides an alternative novel mechanism to delay stable kinetochore–microtubule attachment in oocytes.

The Drosophila histone variant H2A.V works in concert with HP1 to promote kinetochore-driven microtubule formation

Unlike other organisms that have evolved distinct H2A variants for different functions, Drosophila melanogaster has just one variant which is capable of filling many roles. This protein, H2A.V, combines the features of the conserved variants H2A.Z and H2A.X in transcriptional control/heterochromatin assembly and DNA damage response, respectively. Here we show that mutations in the gene encoding H2A.V affect chromatin compaction and perturb chromosome segregation in Drosophila mitotic cells. A microtubule (MT) regrowth assay after cold exposure revealed that loss of H2A.V impairs the formation of kinetochore-driven (k) fibers, which can account for defects in chromosome segregation. All defects are rescued by a transgene encoding H2A.V that lacks the H2A.X function in the DNA damage response, suggesting that the H2A.Z (but not H2A.X) functionality of H2A.V is required for chromosome segregation. We also found that loss of H2A.V weakens HP1 localization, specifically at the pericentric heterochromatin of metaphase chromosomes. Interestingly, loss of HP1 yielded not only telomeric fusions but also mitotic defects similar to those seen in H2A.V null mutants, suggesting a role for HP1 in chromosome segregation. We also show that H2A.V precipitates HP1 from larval brain extracts indicating that both proteins are part of the same complex. Moreover, we found that the overexpression of HP1 rescues chromosome missegregation and defects in the kinetochore-driven k-fiber regrowth of H2A.V mutants indicating that both phenotypes are influenced by unbalanced levels of HP1. Collectively, our results suggest that H2A.V and HP1 work in concert to ensure kinetochore-driven MT growth.

Centrosomin represses dendrite branching by orienting microtubule nucleation

Neuronal dendrite branching is fundamental for building nervous systems. Branch formation is genetically encoded by transcriptional programs to create dendrite arbor morphological diversity for complex neuronal functions. In Drosophila sensory neurons, the transcription factor Abrupt represses branching via an unknown effector pathway. Targeted screening for branching-control effectors identified Centrosomin, the primary centrosome-associated protein for mitotic spindle maturation. Centrosomin repressed dendrite branch formation and was used by Abrupt to simplify arbor branching. Live imaging revealed that Centrosomin localized to the Golgi cis face and that it recruited microtubule nucleation to Golgi outposts for net retrograde microtubule polymerization away from nascent dendrite branches. Removal of Centrosomin enabled the engagement of wee Augmin activity to promote anterograde microtubule growth into the nascent branches, leading to increased branching. The findings reveal that polarized targeting of Centrosomin to Golgi outposts during elaboration of the dendrite arbor creates a local system for guiding microtubule polymerization.

WAC loss-of-function mutations cause a recognisable syndrome characterised by dysmorphic features, developmental delay and hypotonia and recapitulate 10p11.23 microdeletion syndrome

BACKGROUND

Rare de novo mutations have been implicated as a significant cause of idiopathic intellectual disability. Large deletions encompassing 10p11.23 have been implicated in developmental delay, behavioural abnormalities and dysmorphic features, but the genotype-phenotype correlation was not delineated. Mutations in WAC have been recently reported in large screening cohorts of patients with intellectual disability or autism, but no full phenotypic characterisation was described.

METHODS

Clinical and molecular characterisation of six patients with loss-of-function WAC mutations identified by whole exome sequencing was performed. Clinical data were obtained by retrospective chart review, parental interviews, direct patient interaction and formal neuropsychological evaluation.

RESULTS

Five heterozygous de novo WAC mutations were identified in six patients. Three of the mutations were nonsense, and two were frameshift; all are predicted to cause loss of function either through nonsense-mediated mRNA decay or protein truncation. Clinical findings included developmental delay (6/6), hypotonia (6/6), behavioural problems (5/6), eye abnormalities (5/6), constipation (5/6), feeding difficulties (4/6), seizures (2/6) and sleep problems (2/6). All patients exhibited common dysmorphic features, including broad/prominent forehead, synophrys and/or bushy eyebrows, depressed nasal bridge and bulbous nasal tip. Posteriorly rotated ears, hirsutism, deep-set eyes, thin upper lip, inverted nipples, hearing loss and branchial cleft anomalies were also noted.

CONCLUSIONS

Our case series show that loss-of-function mutations in WAC cause a recognisable genetic syndrome characterised by a neurocognitive phenotype and facial dysmorphism. Our data highly suggest that WAC haploinsufficiency is responsible for most of the phenotypic features associated with deletions encompassing 10p11.23.

Dissecting the function and assembly of acentriolar microtubule organizing centers in Drosophila cells in vivo

Acentriolar microtubule organizing centers (aMTOCs) are formed during meiosis and mitosis in several cell types, but their function and assembly mechanism is unclear. Importantly, aMTOCs can be overactive in cancer cells, enhancing multipolar spindle formation, merotelic kinetochore attachment and aneuploidy. Here we show that aMTOCs can form in acentriolar Drosophila somatic cells in vivo via an assembly pathway that depends on Asl, Cnn and, to a lesser extent, Spd-2--the same proteins that appear to drive mitotic centrosome assembly in flies. This finding enabled us to ablate aMTOC formation in acentriolar cells, and so perform a detailed genetic analysis of the contribution of aMTOCs to acentriolar mitotic spindle formation. Here we show that although aMTOCs can nucleate microtubules, they do not detectably increase the efficiency of acentriolar spindle assembly in somatic fly cells. We find that they are required, however, for robust microtubule array assembly in cells without centrioles that also lack microtubule nucleation from around the chromatin. Importantly, aMTOCs are also essential for dynein-dependent acentriolar spindle pole focusing and for robust cell proliferation in the absence of centrioles and HSET/Ncd (a kinesin essential for acentriolar spindle pole focusing in many systems). We propose an updated model for acentriolar spindle pole coalescence by the molecular motors Ncd/HSET and dynein in conjunction with aMTOCs.

The NuRD nucleosome remodelling complex and NHK-1 kinase are required for chromosome condensation in oocytes

Chromosome condensation during cell division is one of the most dramatic events in the cell cycle. Condensin and topoisomerase II are the most studied factors in chromosome condensation. However, their inactivation leads to only mild defects and little is known about the roles of other factors. Here, we took advantage of Drosophilaoocytes to elucidate the roles of potential condensation factors by performing RNA interference (RNAi). Consistent with previous studies, depletion of condensin I subunits or topoisomerase II in oocytes only mildly affected chromosome condensation. In contrast, we found severe undercondensation of chromosomes after depletion of the Mi-2-containing NuRD nucleosome remodelling complex or the protein kinase NHK-1 (also known as Ballchen in Drosophila). The further phenotypic analysis suggests that Mi-2 and NHK-1 are involved in different pathways of chromosome condensation. We show that the main role of NHK-1 in chromosome condensation is to phosphorylate Barrier-to-autointegration factor (BAF) and suppress its activity in linking chromosomes to nuclear envelope proteins. We further show that NHK-1 is important for chromosome condensation during mitosis as well as in oocytes.

Meiosis: an overview of key differences from mitosis

Meiosis is the specialized cell division that generates gametes. In contrast to mitosis, molecular mechanisms and regulation of meiosis are much less understood. Meiosis shares mechanisms and regulation with mitosis in many aspects, but also has critical differences from mitosis. This review highlights these differences between meiosis and mitosis. Recent studies using various model systems revealed differences in a surprisingly wide range of aspects, including cell-cycle regulation, recombination, postrecombination events, spindle assembly, chromosome-spindle interaction, and chromosome segregation. Although a great degree of diversity can be found among organisms, meiosis-specific processes, and regulation are generally conserved.

Differing requirements for Augmin in male meiotic and mitotic spindle formation in Drosophila

Animal cells divide using a microtubule-based, bipolar spindle. Both somatic, mitotic cells and sperm-producing male meiotic spermatocytes use centrosome-dependent and acentrosomal spindle-forming mechanisms. Here, we characterize the largely undefined, centrosome-independent spindle formation pathway used during male meiosis. Our live and fixed cell analyses of Drosophila spermatocytes reveal that acentrosomal microtubules are nucleated at kinetochores and in the vicinity of chromatin and that together these assemble into functional spindles. Mutational studies indicate that γ-tubulin and its extra-centrosomal targeting complex, Augmin, are vital for this process. In addition, Augmin facilitates efficient spindle assembly in the presence of centrosomes. In contrast to the pronounced recruitment of Augmin on spindles in other cell types, the complex is absent from those of spermatocytes but does accumulate on kinetochores. Polo kinase facilitates this kinetochore recruitment while inhibiting Augmin's spindle association, and this in turn dictates γ-tubulin distribution and spindle density. Polo's negative regulation of Augmin in male meiosis contrasts with its requirement in loading Augmin along mitotic spindles in somatic Drosophila cells. Together our data identify a novel mechanism of acentrosomal spindle formation in spermatocytes and reveal its divergence from that used in mitotic cells.

Identification of the augmin complex in the filamentous fungus Aspergillus nidulans

Augmin is a protein complex that binds to spindle microtubules (MTs), recruits the potent MT nucleator, γ-tubulin, and thereby promotes the centrosome-independent MT generation within mitotic and meiotic spindles. Augmin is essential for acentrosomal spindle assembly, which is commonly observed during mitosis in plants and meiosis in female animals. In many animal somatic cells that possess centrosomes, the centrosome- and augmin-dependent mechanisms work cooperatively for efficient spindle assembly and cytokinesis. Yeasts have lost the augmin genes during evolution. It is hypothesized that their robust MT nucleation from the spindle pole body (SPB), the centrosome-equivalent structure in fungi, compensates for the lack of augmin. Intriguingly, however, a gene homologous to an augmin subunit (Aug6/AUGF) has been found in the genome of filamentous fungi, which has the SPB as a robust MT nucleation centre. Here, we aimed to clarify if the augmin complex is present in filamentous fungi and to identify its role in mitosis. By analysing the Aug6-like gene in the filamentous fungus Aspergillus nidulans, we found that it forms a large complex with several other proteins that share weak but significant homology to known augmin subunits. In A. nidulans, augmin was enriched at the SPB and also associated with spindle MTs during mitosis. However, the augmin gene disruptants did not exhibit growth defects under normal, checkpoint-deficient, or MT-destabilised conditions. Moreover, we obtained no evidence that A. nidulans augmin plays a role in γ-tubulin recruitment or in mitotic cell division. Our study uncovered the conservation of the augmin complex in the fungal species, and further suggests that augmin has several functions, besides mitotic spindle MT nucleation, that are yet to be identified.

γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning

γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examine a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, we propose a role for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.

An anti-inflammatory NOD-like receptor is required for microglia development

Microglia are phagocytic cells that form the basis of the brain's immune system. They derive from primitive macrophages that migrate into the brain during embryogenesis, but the genetic control of microglial development remains elusive. Starting with a genetic screen in zebrafish, we show that the noncanonical NOD-like receptor (NLR) nlrc3-like is essential for microglial formation. Although most NLRs trigger inflammatory signaling, nlrc3-like acts cell autonomously in microglia precursor cells to suppress unwarranted inflammation in the absence of overt immune challenge. In nlrc3-like mutants, primitive macrophages initiate a systemic inflammatory response with increased proinflammatory cytokines and actively aggregate instead of migrating into the brain to form microglia. NLRC3-like requires both its pyrin and NACHT domains, and it can bind the inflammasome component apoptosis-associated speck-like protein. Our studies suggest that NLRC3-like may regulate the inflammasome and other inflammatory pathways. Together, these results demonstrate that NLRC3-like prevents inappropriate macrophage activation, thereby allowing normal microglial development.

Meiosis-specific stable binding of augmin to acentrosomal spindle poles promotes biased microtubule assembly in oocytes

In the oocytes of many animals including humans, the meiotic spindle assembles without centrosomes. It is still unclear how multiple pathways contribute to spindle microtubule assembly, and whether they are regulated differently in mitosis and meiosis. Augmin is a γ-tubulin recruiting complex which “amplifies” spindle microtubules by generating new microtubules along existing ones in mitosis. Here we show that in Drosophila melanogaster oocytes Augmin is dispensable for chromatin-driven assembly of bulk spindle microtubules, but is required for full microtubule assembly near the poles. The level of Augmin accumulated at spindle poles is well correlated with the degree of chromosome congression. Fluorescence recovery after photobleaching shows that Augmin stably associates with the polar regions of the spindle in oocytes, unlike in mitotic cells where it transiently and uniformly associates with the metaphase spindle. This stable association is enhanced by γ-tubulin and the kinesin-14 Ncd. Therefore, we suggest that meiosis-specific regulation of Augmin compensates for the lack of centrosomes in oocytes by actively biasing sites of microtubule generation within the spindle.

Although centrosomes are the main sites of microtubule assembly in mitotic cells, the meiotic spindle assembles without centrosomes in the oocytes of many animals including humans. It has also been shown that bipolar spindles can be assembled in mitotic cells even when functional centrosomes are artificially eliminated. It is still unclear how spindle microtubules assemble without centrosomes, and whether microtubule assembly is regulated differently in mitosis and meiosis. Here we investigated the role and regulation of the conserved protein complex Augmin, which recruits γ-tubulin to microtubules to generate new microtubules, in Drosophila oocytes. We found that meiosis-specific regulation of Augmin substitutes for a lack of centrosomal activity in oocytes by biasing microtubule assembly towards poles. As Augmin is conserved widely in higher eukaryotes, our finding has an important implication in our understanding of chromosome segregation and mis-segregation in human oocytes.

Subdiffraction imaging of centrosomes reveals higher-order organizational features of pericentriolar material

The centrosome is the main microtubule organization centre of animal cells. It is composed of a centriole pair surrounded by pericentriolar material (PCM). Traditionally described as amorphous, the architecture of the PCM is not known, although its intricate mode of assembly alludes to the presence of a functional, hierarchical structure. Here we used subdiffraction imaging to reveal organizational features of the PCM. Interphase PCM components adopt a concentric toroidal distribution of discrete diameter around centrioles. Positional mapping of multiple non-overlapping epitopes revealed that pericentrin (PCNT) is an elongated molecule extending away from the centriole. We find that PCM components occupy separable spatial domains within mitotic PCM that are maintained in the absence of microtubule nucleation complexes and further implicate PCNT and CDK5RAP2 in the organization and assembly of PCM. Globally, this work highlights the role of higher-order PCM organization in the regulation of centrosome assembly and function.

The conserved kinase SRPK regulates karyosome formation and spindle microtubule assembly in Drosophila oocytes

In Drosophila oocytes, after the completion of recombination, meiotic chromosomes form a compact cluster called the karyosome within the nucleus, and later assemble spindle microtubules without centrosomes. Although these oocyte-specific phenomena are also observed in humans, their molecular basis is not well understood. Here, we report essential roles for the conserved kinase SRPK in both karyosome formation and spindle microtubule assembly in oocytes. We have identified a female-sterile srpk mutant through a cytological screen for karyosome defects. Unlike most karyosome mutants, the karyosome defect is independent of the meiotic recombination checkpoint. Heterochromatin clustering found within the wild-type karyosome is disrupted in the mutant. Strikingly, a loss of SRPK severely prevents microtubule assembly for acentrosomal spindles in mature oocytes. Subsequently, bi-orientation and segregation of meiotic chromosomes are also defective. Therefore, this study demonstrates new roles of this conserved kinase in two independent meiotic steps specific to oocytes.

New insights into subcomplex assembly and modifications of centrosomal proteins

This review provides a brief overview of the recent work on centrosome proteomics, protein complex identification and functional characterization with an emphasis on the literature of the last three years. Proteomics, genetic screens and comparative genomics studies in different model organisms have almost exhaustively identified the molecular components of the centrosome. However, much knowledge is still missing on the protein-protein interactions, protein modifications and molecular changes the centrosome undergoes throughout the cell cycle and development. The dynamic nature of this large multi-protein complex is reflected in the variety of annotated subcellular locations and biological processes of its proposed components. Some centrosomal proteins and complexes have been studied intensively in different organisms and provided detailed insight into centrosome functions. For example, the molecular, structural and functional characterization of the γ-Tubulin ring complex (γ-TuRC) and the the discovery of the Augmin/HAUS complex has advanced our understanding of microtubule (MT) capture, nucleation and organization. Surprising findings revealed new functions and localizations of proteins that were previously regarded as bona fide centriolar or centrosome components, e.g. at the kinetochore or in the nuclear pore complex regulating MT plus end capture or mRNA processing. Many centrosome components undergo posttranslational modifications such as phosphorylation, SUMOylation and ubiquitylation that are critical in modulating centrosome function and biology. A wealth of information has recently become available driven by new developments in technologies such as mass spectrometry, light and electron microscopy providing more detailed molecular and structural definition of the centrosome and particular roles of proteins throughout the cell cycle and development.

Dgp71WD is required for the assembly of the acentrosomal Meiosis I spindle, and is not a general targeting factor for the γ-TuRC

Dgp71WD/Nedd1 proteins are essential for mitotic spindle formation. In human cells, Nedd1 targets γ-tubulin to both centrosomes and spindles, but in other organisms the function of Dgp71WD/Nedd1 is less clear. In Drosophila cells, Dgp71WD plays a major part in targeting γ-tubulin to spindles, but not centrosomes, while in Xenopus egg extracts, Nedd1 acts as a more general microtubule (MT) organiser that can function independently of γ-tubulin. The interpretation of these studies, however, is complicated by the fact that some residual Dgp71WD/Nedd1 is likely present in the cells/extracts analysed. Here we generate a Dgp71WD null mutant lacking all but the last 12 nucleotides of coding sequence. The complete loss of Dgp71WD has no quantifiable effect on γ-tubulin or Centrosomin recruitment to the centrosome in larval brain cells. The recruitment of γ-tubulin to spindle MTs, however, is severely impaired, and spindle MT density is reduced in a manner that is indistinguishable from cells lacking Augmin or γ-TuRC function. In contrast, the absence of Dgp71WD leads to defects in the assembly of the acentrosomal female Meiosis I spindle that are more severe than those seen in Augmin or γ-TuRC mutants, indicating that Dgp71WD has additional functions that are independent of these complexes in oocytes. Moreover, the localisation of bicoid RNA during oogenesis, which requires γ-TuRC function, is unperturbed in Dgp71WD(120) mutants. Thus, Dgp71WD is not simply a general cofactor required for γ-TuRC and/or Augmin targeting, and it appears to have a crucial role independent of these complexes in the acentrosomal Meiosis I spindle.

Characterization of the Arabidopsis augmin complex uncovers its critical function in the assembly of the acentrosomal spindle and phragmoplast microtubule arrays

Plant cells assemble the bipolar spindle and phragmoplast microtubule (MT) arrays in the absence of the centrosome structure. Our recent findings in Arabidopsis thaliana indicated that AUGMIN subunit3 (AUG3), a homolog of animal dim γ-tubulin 3, plays a critical role in γ-tubulin-dependent MT nucleation and amplification during mitosis. Here, we report the isolation of the entire plant augmin complex that contains eight subunits. Among them, AUG1 to AUG6 share low sequence similarity with their animal counterparts, but AUG7 and AUG8 share homology only with proteins of plant origin. Genetic analyses indicate that the AUG1, AUG2, AUG4, and AUG5 genes are essential, as stable mutations in these genes could only be transmitted to heterozygous plants. The sterile aug7-1 homozygous mutant in which AUG7 expression is significantly reduced exhibited pleiotropic phenotypes of seriously retarded vegetative and reproductive growth. The aug7-1 mutation caused delocalization of γ-tubulin in the mitotic spindle and phragmoplast. Consequently, spindles were abnormally elongated, and their poles failed to converge, as MTs were splayed to discrete positions rendering deformed arrays. In addition, the mutant phragmoplasts often had disorganized MT bundles with uneven edges. We conclude that assembly of MT arrays during plant mitosis depends on the augmin complex, which includes two plant-specific subunits.

An inducible RNA interference system in Physcomitrella patens reveals a dominant role of augmin in phragmoplast microtubule generation

Mitosis is a fundamental process of eukaryotic cell proliferation. However, the molecular mechanisms underlying mitosis remain poorly understood in plants partly because of the lack of an appropriate model cell system in which loss-of-function analyses can be easily combined with high-resolution microscopy. Here, we developed an inducible RNA interference (RNAi) system and three-dimensional time-lapse confocal microscopy in the moss Physcomitrella patens that allowed in-depth phenotype characterization of the moss genes essential for cell division. We applied this technique to two microtubule regulators, augmin and γ-tubulin complexes, whose mitotic roles remain obscure in plant cells. Live imaging of caulonemal cells showed that they proceed through mitosis with continual generation and self-organization of acentrosomal microtubules. We demonstrated that augmin plays an important role in γ-tubulin localization and microtubule generation from prometaphase to cytokinesis. Most evidently, microtubule formation in phragmoplasts was severely compromised after RNAi knockdown of an augmin subunit, leading to incomplete expansion of phragmoplasts and cytokinesis failure. Knockdown of the γ-tubulin complex affected microtubule formation throughout mitosis. We conclude that postanaphase microtubule generation is predominantly stimulated by the augmin/γ-tubulin machinery in moss and further propose that this RNAi system serves as a powerful tool to dissect the molecular mechanisms underlying mitosis in land plants.

Microtubule nucleation by γ-tubulin complexes

Microtubule nucleation is regulated by the γ-tubulin ring complex (γTuRC) and related γ-tubulin complexes, providing spatial and temporal control over the initiation of microtubule growth. Recent structural work has shed light on the mechanism of γTuRC-based microtubule nucleation, confirming the long-standing hypothesis that the γTuRC functions as a microtubule template. The first crystallographic analysis of a non-γ-tubulin γTuRC component (γ-tubulin complex protein 4 (GCP4)) has resulted in a new appreciation of the relationships among all γTuRC proteins, leading to a refined model of their organization and function. The structures have also suggested an unexpected mechanism for regulating γTuRC activity via conformational modulation of the complex component GCP3. New experiments on γTuRC localization extend these insights, suggesting a direct link between its attachment at specific cellular sites and its activation.

Kebab: kinetochore and EB1 associated basic protein that dynamically changes its localisation during Drosophila mitosis

Microtubule plus ends are dynamic ends that interact with other cellular structures. Microtubule plus end tracking proteins are considered to play important roles in the regulation of microtubule plus ends. Recent studies revealed that EB1 is the central regulator for microtubule plus end tracking proteins by recruiting them to microtubule plus ends through direct interaction. Here we report the identification of a novel Drosophila protein, which we call Kebab (kinetochore and EB1 associated basic protein), through in vitro expression screening for EB1-interacting proteins. Kebab fused to GFP shows a novel pattern of dynamic localisation in mitosis. It localises to kinetochores weakly in metaphase and accumulates progressively during anaphase. In telophase, it associates with microtubules in central-spindle and centrosomal regions. The localisation to kinetochores depends on microtubules. The protein has a domain most similar to the atypical CH domain of Ndc80, and a coiled-coil domain. The interaction with EB1 is mediated by two SxIP motifs but is not required for the localisation. Depletion of Kebab in cultured cells by RNA interference did not show obvious defects in mitotic progression or microtubule organisation. Generation of mutants lacking the kebab gene indicated that Kebab is dispensable for viability and fertility.

50 ways to build a spindle: the complexity of microtubule generation during mitosis

The accurate segregation of duplicated chromosomes, essential for the development and viability of a eukaryotic organism, requires the formation of a robust microtubule (MT)-based spindle apparatus. Entry into mitosis or meiosis precipitates a cascade of signalling events which result in the activation of pathways responsible for a dramatic reorganisation of the MT cytoskeleton: through changes in the properties of MT-associated proteins, local concentrations of free tubulin dimer and through enhanced MT nucleation. The latter is generally thought to be driven by localisation and activation of γ-tubulin-containing complexes (γ-TuSC and γ-TuRC) at specific subcellular locations. For example, upon entering mitosis, animal cells concentrate γ-tubulin at centrosomes to tenfold the normal level during interphase, resulting in an aster-driven search and capture of chromosomes and bipolar mitotic spindle formation. Thus, in these cells, centrosomes have traditionally been perceived as the primary microtubule organising centre during spindle formation. However, studies in meiotic cells, plants and cell-free extracts have revealed the existence of complementary mechanisms of spindle formation, mitotic chromatin, kinetochores and nucleation from existing MTs or the cytoplasm can all contribute to a bipolar spindle apparatus. Here, we outline the individual known mechanisms responsible for spindle formation and formulate ideas regarding the relationship between them in assembling a functional spindle apparatus.

Regulation of microtubule-based microtubule nucleation by mammalian polo-like kinase 1

Bipolar spindle formation is pivotal for accurate segregation of mitotic chromosomes during cell division. A growing body of evidence suggests that, in addition to centrosome- and chromatin-based microtubule (MT) nucleation, MT-based MT nucleation plays an important role for proper bipolar spindle formation in various eukaryotic organisms. Although a recently discovered Augmin complex appears to play a central role in this event, how Augmin is regulated remains unknown. Here we provide evidence that a mammalian polo-like kinase 1 (Plk1) localizes to mitotic spindles and promotes MT-based MT nucleation by directly regulating Augmin. Mechanistically, we demonstrated that Cdc2-dependent phosphorylation on a γ-tubulin ring complex (γ-TuRC) recruitment protein, Nedd1/GCP-WD, at the previously uncharacterized S460 residue induces the Nedd1-Plk1 interaction. This step appeared to be critical to allow Plk1 to phosphorylate the Hice1 subunit of the Augmin complex to promote the Augmin-MT interaction and MT-based MT nucleation from within the spindle. Loss of either the Nedd1 S460 function or the Plk1-dependent Hice1 phosphorylation impaired both the Augmin-MT interaction and γ-tubulin recruitment to the spindles, thus resulting in improper bipolar spindle formation that ultimately leads to mitotic arrest and apoptotic cell death. Thus, via the formation of the Nedd1-Plk1 complex and subsequent Augmin phosphorylation, Plk1 regulates spindle MT-based MT nucleation to accomplish normal bipolar spindle formation and mitotic progression.

Augmin plays a critical role in organizing the spindle and phragmoplast microtubule arrays in Arabidopsis

In higher plant cells, microtubules (MTs) are nucleated and organized in a centrosome-independent manner. It is unclear whether augmin-dependent mechanisms underlie spindle MT organization in plant cells as they do in animal cells. When AUGMIN subunit3 (AUG3), which encodes a homolog of animal dim γ-tubulin 3/human augmin-like complex, subunit 3, was disrupted in Arabidopsis thaliana, gametogenesis frequently failed due to defects in cell division. Compared with the control microspores, which formed bipolar spindles at the cell periphery, the mutant cells often formed peripheral half spindles that only attached to condensed chromosomes or formed elongated spindles with unfocused interior poles. In addition, defective cells exhibited disorganized phragmoplast MT arrays, which caused aborted cytokinesis. The resulting pollen grains were either shrunken or contained two nuclei in an undivided cytoplasm. AUG3 was localized along MTs in the spindle and phragmoplast, and its signal was pronounced in anaphase spindle poles. An AUG3-green fluorescent protein fusion exhibited a dynamic distribution pattern, similar to that of the γ-tubulin complex protein2. When AUG3 was enriched from seedlings by affinity chromatography, AUG1 was detected by immunoblotting, suggesting an augmin-like complex was present in vivo. We conclude that augmin plays a critical role in MT organization during plant cell division.

Aurora-A phosphorylates Augmin complex component Hice1 protein at an N-terminal serine/threonine cluster to modulate its microtubule binding activity during spindle assembly

Proper assembly of mitotic spindles requires Hice1, a spindle-associated protein. Hice1 possesses direct microtubule binding activity at its N-terminal region and contributes to intraspindle microtubule nucleation as a subunit of the Augmin complex. However, whether microtubule binding activity of Hice1 is modulated by mitotic regulators remains unexplored. Here, we found that Aurora-A kinase, a major mitotic kinase, specifically binds to and phosphorylates Hice1. We identified four serine/threonine clusters on Hice1 that can be phosphorylated by Aurora-A in vitro. Of the four clusters, the Ser/Thr-17-21 cluster was the most critical for bipolar spindle assembly, whereas other phospho-deficient point mutants had a minimal effect on spindle assembly. Immunostaining with a phospho-Ser-19/20 phospho-specific antibody revealed that phosphorylated Hice1 primarily localizes to spindle poles during prophase to metaphase but gradually diminishes after anaphase. Consistently, the phospho-mimic 17-21E mutant reduced microtubule binding activity in vitro and diminished localization to spindles in vivo. Furthermore, expression of the 17-21E mutant led to decreased association of Fam29a, an Augmin component, with spindles. On the other hand, expression of the phospho-deficient 17-21A mutant permitted intraspindle nucleation but delayed the separation of early mitotic spindle poles and the timely mitotic progression. Taken together, these results suggest that Aurora-A modulates the microtubule binding activity of Hice1 in a spatiotemporal manner for proper bipolar spindle assembly.

Genomic and clinical characteristics of six patients with partially overlapping interstitial deletions at 10p12p11

With the clinical implementation of genomic microarrays, the detection of cryptic unbalanced rearrangements in patients with syndromic developmental delay has improved considerably. Here we report the molecular karyotyping and phenotypic description of six new unrelated patients with partially overlapping microdeletions at 10p12.31p11.21 ranging from 1.0 to 10.6 Mb. The smallest region of overlap is 306 kb, which includes WAC gene, known to be associated with microtubule function and to have a role in cell division. Another patient has previously been described with a 10 Mb deletion, partially overlapping with our six patients. All seven patients have developmental delay and a majority of the patients have abnormal behaviour and dysmorphic features, including bulbous nasal tip, deep set eyes, synophrys/thick eyebrows and full cheeks, whereas other features varied. All patients also displayed various visual impairments and six out of seven patients had cardiac malformations. Taken together with the previously reported patient, our study suggests that the detected deletions may represent a new contiguous gene syndrome caused by dosage-sensitive genes that predispose to developmental delay.