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1.  SUN proteins facilitate the removal of membranes from chromatin during nuclear envelope breakdown 
The Journal of Cell Biology  2014;204(7):1099-1109.
SUN proteins in the inner nuclear membrane function in mitotic membrane remodeling during nuclear envelope breakdown.
SUN proteins reside in the inner nuclear membrane and form complexes with KASH proteins of the outer nuclear membrane that connect the nuclear envelope (NE) to the cytoskeleton. These complexes have well-established functions in nuclear anchorage and migration in interphase, but little is known about their involvement in mitotic processes. Our analysis demonstrates that simultaneous depletion of human SUN1 and SUN2 delayed removal of membranes from chromatin during NE breakdown (NEBD) and impaired the formation of prophase NE invaginations (PNEIs), similar to microtubule depolymerization or down-regulation of the dynein cofactors NudE/EL. In addition, overexpression of dominant-negative SUN and KASH constructs reduced the occurrence of PNEI, indicating a requirement for functional SUN–KASH complexes in NE remodeling. Codepletion of SUN1/2 slowed cell proliferation and resulted in an accumulation of morphologically defective and disoriented mitotic spindles. Quantification of mitotic timing revealed a delay between NEBD and chromatin separation, indicating a role of SUN proteins in bipolar spindle assembly and mitotic progression.
doi:10.1083/jcb.201310116
PMCID: PMC3971743  PMID: 24662567
2.  Kinetic framework of spindle assembly checkpoint signaling 
Nature cell biology  2013;15(11):1370-1377.
The mitotic spindle assembly checkpoint (SAC) delays anaphase onset until all chromosomes have attached to both spindle poles1,2. Here, we investigated SAC signaling kinetics in response to acute detachment of individual chromosomes using laser microsurgery. Most detached chromosomes delayed anaphase until they had realigned to the metaphase plate. A substantial fraction of cells, however, entered anaphase in the presence of unaligned chromosomes. We identify two mechanisms by which cells can bypass the SAC: First, single unattached chromosomes inhibit the anaphase promoting complex/cyclosome (APC/C) less efficiently than a full complement of unattached chromosomes. Second, because of the relatively slow kinetics of reimposing APC/C inhibition during metaphase, cells were unresponsive to chromosome detachment up to several minutes before anaphase onset. Our study defines when cells irreversibly commit to enter anaphase and shows that the SAC signal strength correlates with the number of unattached chromosomes. Detailed knowledge about SAC signaling kinetics is important for understanding the emergence of aneuploidy and the response of cancer cells to chemotherapeutics targeting the mitotic spindle.
doi:10.1038/ncb2842
PMCID: PMC4067996  PMID: 24096243
3.  Sds22 and Repo-Man stabilize chromosome segregation by counteracting Aurora B on anaphase kinetochores 
The Journal of Cell Biology  2012;198(2):173-183.
Repo-Man and Sds22 counteract Aurora B phosphorylation of Dsn1 and thus regulate the kinetochore–microtubule interface during anaphase.
During mitotic spindle assembly, Aurora B kinase is part of an error correction mechanism that detaches microtubules from kinetochores that are under low mechanical tension. During anaphase, however, kinetochore–microtubule attachments must be maintained despite a drop of tension after removal of sister chromatid cohesion. Consistent with this requirement, Aurora B relocates away from chromosomes to the central spindle at the metaphase–anaphase transition. By ribonucleic acid interference screening using a phosphorylation biosensor, we identified two PP1-targeting subunits, Sds22 and Repo-Man, which counteracted Aurora B–dependent phosphorylation of the outer kinetochore component Dsn1 during anaphase. Sds22 or Repo-Man depletion induced transient pauses during poleward chromosome movement and a high incidence of chromosome missegregation. Thus, our study identifies PP1-targeting subunits that regulate the microtubule–kinetochore interface during anaphase for faithful chromosome segregation.
doi:10.1083/jcb.201112112
PMCID: PMC3410419  PMID: 22801782
4.  Cdk1 Inactivation Terminates Mitotic Checkpoint Surveillance and Stabilizes Kinetochore Attachments in Anaphase 
Current Biology  2014;24(6):638-645.
Summary
Two mechanisms safeguard the bipolar attachment of chromosomes in mitosis. A correction mechanism destabilizes erroneous attachments that do not generate tension across sister kinetochores [1]. In response to unattached kinetochores, the mitotic checkpoint delays anaphase onset by inhibiting the anaphase-promoting complex/cyclosome (APC/CCdc20) [2]. Upon satisfaction of both pathways, the APC/CCdc20 elicits the degradation of securin and cyclin B [3]. This liberates separase triggering sister chromatid disjunction and inactivates cyclin-dependent kinase 1 (Cdk1) causing mitotic exit. How eukaryotic cells avoid the engagement of attachment monitoring mechanisms when sister chromatids split and tension is lost at anaphase is poorly understood [4]. Here we show that Cdk1 inactivation disables mitotic checkpoint surveillance at anaphase onset in human cells. Preventing cyclin B1 proteolysis at the time of sister chromatid disjunction destabilizes kinetochore-microtubule attachments and triggers the engagement of the mitotic checkpoint. As a consequence, mitotic checkpoint proteins accumulate at anaphase kinetochores, the APC/CCdc20 is inhibited, and securin reaccumulates. Conversely, acute pharmacological inhibition of Cdk1 abrogates the engagement and maintenance of the mitotic checkpoint upon microtubule depolymerization. We propose that the simultaneous destruction of securin and cyclin B elicited by the APC/CCdc20 couples chromosome segregation to the dissolution of attachment monitoring mechanisms during mitotic exit.
Graphical Abstract
Highlights
•Cyclin B degradation stabilizes kinetochore attachments in anaphase•Cdk1 inactivation prevents mitotic checkpoint engagement after anaphase onset•Chromatid disjunction by separase causes APC/C inhibition if Cdk1 remains active•APC/C links separase activation to dissolution of attachment monitoring mechanisms
Vázquez-Novelle et al. propose that Cdk1-cyclin B activity sets the temporal window for microtubule-kinetochore attachment surveillance. Failure to degrade cyclin B at anaphase onset destabilizes attachments and engages the mitotic checkpoint. APC/C-induced proteolysis could couple sister chromatid disjunction to checkpoint dissolution when sister chromatids split.
doi:10.1016/j.cub.2014.01.034
PMCID: PMC3969148  PMID: 24583019
5.  Aurora B and Kif2A control microtubule length for assembly of a functional central spindle during anaphase 
The Journal of Cell Biology  2013;202(4):623-636.
A gradient of Aurora B activity determines the distribution of the microtubule depolymerase Kif2A at the central spindle and specifies the subsequent spindle structure necessary for proper cytokinesis.
The central spindle is built during anaphase by coupling antiparallel microtubules (MTs) at a central overlap zone, which provides a signaling scaffold for the regulation of cytokinesis. The mechanisms underlying central spindle morphogenesis are still poorly understood. In this paper, we show that the MT depolymerase Kif2A controls the length and alignment of central spindle MTs through depolymerization at their minus ends. The distribution of Kif2A was limited to the distal ends of the central spindle through Aurora B–dependent phosphorylation and exclusion from the spindle midzone. Overactivation or inhibition of Kif2A affected interchromosomal MT length and disorganized the central spindle, resulting in uncoordinated cell division. Experimental data and model simulations suggest that the steady-state length of the central spindle and its symmetric position between segregating chromosomes are predominantly determined by the Aurora B activity gradient. On the basis of these results, we propose a robust self-organization mechanism for central spindle formation.
doi:10.1083/jcb.201302123
PMCID: PMC3747305  PMID: 23960144
6.  Live-cell imaging RNAi screen identifies PP2A–B55α and importin-β1 as key mitotic exit regulators in human cells 
Nature cell biology  2010;12(9):10.1038/ncb2092.
When vertebrate cells exit mitosis various cellular structures are re-organized to build functional interphase cells1. This depends on Cdk1 (cyclin dependent kinase 1) inactivation and subsequent dephosphorylation of its substrates2–4. Members of the protein phosphatase 1 and 2A (PP1 and PP2A) families can dephosphorylate Cdk1 substrates in biochemical extracts during mitotic exit5,6, but how this relates to postmitotic reassembly of interphase structures in intact cells is not known. Here, we use a live-cell imaging assay and RNAi knockdown to screen a genome-wide library of protein phosphatases for mitotic exit functions in human cells. We identify a trimeric PP2A–B55α complex as a key factor in mitotic spindle breakdown and postmitotic reassembly of the nuclear envelope, Golgi apparatus and decondensed chromatin. Using a chemically induced mitotic exit assay, we find that PP2A–B55α functions downstream of Cdk1 inactivation. PP2A–B55α isolated from mitotic cells had reduced phosphatase activity towards the Cdk1 substrate, histone H1, and was hyper-phosphorylated on all subunits. Mitotic PP2A complexes co-purified with the nuclear transport factor importin-β1, and RNAi depletion of importin-β1 delayed mitotic exit synergistically with PP2A–B55α. This demonstrates that PP2A–B55α and importin-β1 cooperate in the regulation of postmitotic assembly mechanisms in human cells.
doi:10.1038/ncb2092
PMCID: PMC3839080  PMID: 20711181
7.  Automated microscopy for high-content RNAi screening 
The Journal of Cell Biology  2010;188(4):453-461.
Fluorescence microscopy is one of the most powerful tools to investigate complex cellular processes such as cell division, cell motility, or intracellular trafficking. The availability of RNA interference (RNAi) technology and automated microscopy has opened the possibility to perform cellular imaging in functional genomics and other large-scale applications. Although imaging often dramatically increases the content of a screening assay, it poses new challenges to achieve accurate quantitative annotation and therefore needs to be carefully adjusted to the specific needs of individual screening applications. In this review, we discuss principles of assay design, large-scale RNAi, microscope automation, and computational data analysis. We highlight strategies for imaging-based RNAi screening adapted to different library and assay designs.
doi:10.1083/jcb.200910105
PMCID: PMC2828931  PMID: 20176920
8.  The nuclear F-actin interactome of Xenopus oocytes reveals an actin-bundling kinesin that is essential for meiotic cytokinesis 
The EMBO Journal  2013;32(13):1886-1902.
Nuclei of Xenopus laevis oocytes grow 100 000-fold larger in volume than a typical somatic nucleus and require an unusual intranuclear F-actin scaffold for mechanical stability. We now developed a method for mapping F-actin interactomes and identified a comprehensive set of F-actin binders from the oocyte nuclei. Unexpectedly, the most prominent interactor was a novel kinesin termed NabKin (Nuclear and meiotic actin-bundling Kinesin). NabKin not only binds microtubules but also F-actin structures, such as the intranuclear actin bundles in prophase and the contractile actomyosin ring during cytokinesis. The interaction between NabKin and F-actin is negatively regulated by Importin-β and is responsive to spatial information provided by RanGTP. Disconnecting NabKin from F-actin during meiosis caused cytokinesis failure and egg polyploidy. We also found actin-bundling activity in Nabkin's somatic paralogue KIF14, which was previously shown to be essential for somatic cell division. Our data are consistent with the notion that NabKin/KIF14 directly link microtubules with F-actin and that such link is essential for cytokinesis.
The presence and role of actin filaments in cell nuclei remains incompletely understood. A proteomics approach now reveals a highly distinct set of F-actin-binding proteins in the nucleus, including a novel kinesin family member.
doi:10.1038/emboj.2013.108
PMCID: PMC3981176  PMID: 23727888
cytokinesis; kinesins; meiosis; nuclear actin; phalloidin
9.  CellH5: a format for data exchange in high-content screening 
Bioinformatics  2013;29(12):1580-1582.
Summary: High-throughput microscopy data require a diversity of analytical approaches. However, the construction of workflows that use algorithms from different software packages is difficult owing to a lack of interoperability. To overcome this limitation, we present CellH5, an HDF5 data format for cell-based assays in high-throughput microscopy, which stores high-dimensional image data along with inter-object relations in graphs. CellH5Browser, an interactive gallery image browser, demonstrates the versatility and performance of the file format on live imaging data of dividing human cells. CellH5 provides new opportunities for integrated data analysis by multiple software platforms.
Availability: Source code is freely available at www.github.com/cellh5 under the GPL license and at www.bioconductor.org/packages/release/bioc/html/rhdf5.html under the Artistic-2.0 license. Demo datasets and the CellH5Browser are available at www.cellh5.org. A Fiji importer for cellh5 will be released soon.
Contact: daniel.gerlich@imba.oeaw.ac.at or christoph.sommer@imba.oeaw.ac.at
Supplementary information: Supplementary data are available at Bioinformatics online.
doi:10.1093/bioinformatics/btt175
PMCID: PMC3673213  PMID: 23595665
10.  Phenotypic profiling of the human genome by time-lapse microscopy reveals cell division genes 
Nature  2010;464(7289):721-727.
Despite our rapidly growing knowledge about the human genome, we do not know all of the genes required for some of the most basic functions of life. To start to fill this gap we developed a high-throughput phenotypic screening platform combining potent gene silencing by RNA interference, time-lapse microscopy and computational image processing. We carried out a genome-wide phenotypic profiling of each of the ~21,000 human protein-coding genes by two-day live imaging of fluorescently labelled chromosomes. Phenotypes were scored quantitatively by computational image processing, which allowed us to identify hundreds of human genes involved in diverse biological functions including cell division, migration and survival. As part of the Mitocheck consortium, this study provides an in-depth analysis of cell division phenotypes and makes the entire high-content data set available as a resource to the community.
doi:10.1038/nature08869
PMCID: PMC3108885  PMID: 20360735
11.  Tubulin polyglutamylation stimulates spastin-mediated microtubule severing 
The Journal of Cell Biology  2010;189(6):945-954.
Microtubules with long polyglutamylated C-terminal tails are more prone to severing by spastin, establishing the importance of tubulin posttranslational modifications.
Posttranslational glutamylation of tubulin is present on selected subsets of microtubules in cells. Although the modification is expected to contribute to the spatial and temporal organization of the cytoskeleton, hardly anything is known about its functional relevance. Here we demonstrate that glutamylation, and in particular the generation of long glutamate side chains, promotes the severing of microtubules. In human cells, the generation of long side chains induces spastin-dependent microtubule disassembly and, consistently, only microtubules modified by long glutamate side chains are efficiently severed by spastin in vitro. Our study reveals a novel control mechanism for microtubule mass and stability, which is of fundamental importance to cellular physiology and might have implications for diseases related to microtubule severing.
doi:10.1083/jcb.201001024
PMCID: PMC2886356  PMID: 20530212
12.  The Cul3–KLHL21 E3 ubiquitin ligase targets Aurora B to midzone microtubules in anaphase and is required for cytokinesis 
The Journal of Cell Biology  2009;187(6):791-800.
Selective ubiquitination of Aurora B by different Cul3 adaptors targets it at the correct time to the correct place during mitosis.
Cul3 (Cullin3)-based E3 ubiquitin ligases recently emerged as critical regulators of mitosis. In this study, we identify two mammalian BTB (Bric-a-brac–Tramtrack–Broad complex)-Kelch proteins, KLHL21 and KLHL22, that interact with Cul3 and are required for efficient chromosome alignment. Interestingly, KLHL21 but not KLHL22 is necessary for cytokinesis and regulates translocation of the chromosomal passenger complex (CPC) from chromosomes to the spindle midzone in anaphase, similar to the previously described BTB-Kelch proteins KLHL9 and KLHL13. KLHL21 directly binds to Aurora B and mediates ubiquitination of Aurora B in vitro. In contrast to KLHL9 and KLHL13, KLHL21 localizes to midzone microtubules in anaphase and recruits Aurora B and Cul3 to this region. Together, our results suggest that different Cul3 adaptors nonredundantly regulate Aurora B during mitosis, possibly by ubiquitinating different pools of Aurora B at distinct subcellular localizations.
doi:10.1083/jcb.200906117
PMCID: PMC2806313  PMID: 19995937
13.  Mitotic control of kinetochore-associated dynein and spindle orientation by human Spindly 
The Journal of Cell Biology  2009;185(5):859-874.
Mitotic spindle formation and chromosome segregation depend critically on kinetochore–microtubule (KT–MT) interactions. A new protein, termed Spindly in Drosophila and SPDL-1 in C. elegans, was recently shown to regulate KT localization of dynein, but depletion phenotypes revealed striking differences, suggesting evolutionarily diverse roles of mitotic dynein. By characterizing the function of Spindly in human cells, we identify specific functions for KT dynein. We show that localization of human Spindly (hSpindly) to KTs is controlled by the Rod/Zw10/Zwilch (RZZ) complex and Aurora B. hSpindly depletion results in reduced inter-KT tension, unstable KT fibers, an extensive prometaphase delay, and severe chromosome misalignment. Moreover, depletion of hSpindly induces a striking spindle rotation, which can be rescued by co-depletion of dynein. However, in contrast to Drosophila, hSpindly depletion does not abolish the removal of MAD2 and ZW10 from KTs. Collectively, our data reveal hSpindly-mediated dynein functions and highlight a critical role of KT dynein in spindle orientation.
doi:10.1083/jcb.200812167
PMCID: PMC2711594  PMID: 19468067

Results 1-13 (13)