Loss of KLP-19 polar ejection force causes misorientation and missegregation of holocentric chromosomes

Enucleation of the C. elegans embryo revealed dynein-dependent spacing between microtubule asters

The intracellular positioning of the centrosome, a major microtubule-organizing center, is important for cellular functions. One of the features of centrosome positioning is the spacing between centrosomes; however, the underlying mechanisms are not fully understood. To characterize the spacing activity in Caenorhabditis elegans embryos, a genetic setup was developed to produce enucleated embryos. The centrosome was duplicated multiple times in the enucleated embryo, which enabled us to characterize the chromosome-independent spacing activity between sister and non-sister centrosome pairs. We found that the timely spacing depended on cytoplasmic dynein, and we propose a stoichiometric model of cortical and cytoplasmic pulling forces for the spacing between centrosomes. We also observed dynein-independent but non-muscle myosin II-dependent movement of centrosomes in the later cell cycle phase. The spacing mechanisms revealed in this study are expected to function between centrosomes in general, regardless of the presence of a chromosome/nucleus between them, including centrosome separation and spindle elongation.

Redundant microtubule crosslinkers prevent meiotic spindle bending to ensure diploid offspring in C. elegans

Oocyte meiotic spindles mediate the expulsion of ¾ of the genome into polar bodies to generate diploid zygotes in nearly all animal species. Failures in this process result in aneuploid or polyploid offspring that are typically inviable. Accurate meiotic chromosome segregation and polar body extrusion require the spindle to elongate while maintaining its structural integrity. Previous studies have implicated three hypothetical activities during this process, including microtubule crosslinking, microtubule sliding and microtubule polymerization. However, how these activities regulate spindle rigidity and elongation as well as the exact proteins involved in the activities remain unclear. We discovered that C. elegans meiotic anaphase spindle integrity is maintained through redundant microtubule crosslinking activities of the Kinesin-5 family motor BMK-1, the microtubule bundling protein SPD-1/PRC1, and the Kinesin-4 family motor, KLP-19. Using time-lapse imaging, we found that single depletion of KLP-19KIF4A, SPD-1PRC1 or BMK-1Eg5 had minimal effects on anaphase B spindle elongation velocity. In contrast, double depletion of SPD-1PRC1 and BMK-1Eg5 or double depletion of KLP-19KIF4A and BMK-1Eg5 resulted in spindles that elongated faster, bent in a myosin-dependent manner, and had a high rate of polar body extrusion errors. Bending spindles frequently extruded both sets of segregating chromosomes into two separate polar bodies. Normal anaphase B velocity was observed after double depletion of KLP-19KIF4A and SPD-1PRC1. These results suggest that KLP-19KIF4A and SPD-1PRC1 act in different pathways, each redundant with a separate BMK-1Eg5 pathway in regulating meiotic spindle elongation. Depletion of ZYG-8, a doublecortin-related microtubule binding protein, led to slower anaphase B spindle elongation. We found that ZYG-8DCLK1 acts by excluding SPD-1PRC1 from the spindle. Thus, three mechanistically distinct microtubule regulation modules, two based on crosslinking, and one based on exclusion of crosslinkers, power the mechanism that drives spindle elongation and structural integrity during anaphase B of C.elegans female meiosis.

Lack of chromokinesin Klp-19 creates a more rigid midzone and affects force transmission during anaphase in C. elegans

Recent studies have highlighted the significance of the spindle midzone - the region positioned between chromosomes - in ensuring proper chromosome segregation. By combining advanced 3D electron tomography and cutting-edge light microscopy we have discovered a previously unknown role of the regulation of microtubule dynamics within the spindle midzone of C. elegans. Using Fluorescence recovery after photobleaching and a combination of second harmonic generation and two-photon fluorescence microscopy, we found that the length of the antiparallel microtubule overlap zone in the spindle midzone is constant throughout anaphase, and independent of cortical pulling forces as well as the presence of the microtubule bundling protein SPD-1. Further investigations of SPD-1 and the chromokinesin KLP-19 in C. elegans suggest that KLP-19 regulates the overlap length and functions independently of SPD-1. Our data shows that KLP-19 plays an active role in regulating the length and turn-over of microtubules within the midzone as well as the size of the antiparallel overlap region throughout mitosis. Depletion of KLP-19 in mitosis leads to an increase in microtubule length in the spindle midzone, which also leads to increased microtubule - microtubule interaction, thus building up a more robust microtubule network. The spindle is globally stiffer and more stable, which has implications for the transmission of forces within the spindle affecting chromosome segregation dynamics. Our data shows that by localizing KLP-19 to the spindle midzone in anaphase microtubule dynamics can be locally controlled allowing the formation of a functional midzone.

Kinetochore component function in C. elegans oocytes revealed by 4D tracking of holocentric chromosomes

During cell division, chromosome congression to the spindle center, their orientation along the spindle long axis and alignment at the metaphase plate depend on interactions between spindle microtubules and kinetochores, and are pre-requisite for chromosome bi-orientation and accurate segregation. How these successive phases are controlled during oocyte meiosis remains elusive. Here we provide 4D live imaging during the first meiotic division in C. elegans oocytes with wild-type or disrupted kinetochore protein function. We show that, unlike in monocentric organisms, holocentric chromosome bi-orientation is not strictly required for accurate chromosome segregation. Instead, we propose a model in which initial kinetochore-localized BHC module (comprised of BUB-1Bub1, HCP-1/2CENP-F and CLS-2CLASP)-dependent pushing acts redundantly with Ndc80 complex-mediated pulling for accurate chromosome segregation in meiosis. In absence of both mechanisms, homologous chromosomes tend to co-segregate in anaphase, especially when initially mis-oriented. Our results highlight how different kinetochore components cooperate to promote accurate holocentric chromosome segregation in oocytes of C. elegans.

Molecular basis of essential and morphological variations across 12 balancer strains in C. elegans

Balancers are primarily chromosomal rearrangements that allow lethal or sterile mutations to be stably maintained as heterozygotes. Strains with balanced lethal/sterile mutations are available at the Caenorhabditis Genetics Center. Such strains harbor morphological markers, with associated molecular changes, that are in trans to the balancer. In many cases, only the genetic position (in cM) has been described for balanced mutations or morphological markers. We used short-read whole genome sequencing to uncover the genomic position of those variants (balanced mutations and linked markers) and predicted their effect. We investigated 12 different strains, and characterized at a molecular level 12 variants.

Kinetochore dynein is sufficient to biorient chromosomes and remodel the outer kinetochore

Multiple microtubule-directed activities concentrate on chromosomes during mitosis to ensure their accurate distribution to daughter cells. These activities include couplers and dynamics regulators localized at the kinetochore, the specialized microtubule interface built on centromeric chromatin, as well as motor proteins recruited to kinetochores and to mitotic chromatin. Here, we describe an in vivo reconstruction approach in which the effect of removing the major microtubule-directed activities on mitotic chromosomes is compared to the selective presence of individual activities. This approach revealed that the kinetochore dynein module, comprised of the minus end-directed motor cytoplasmic dynein and its kinetochore-specific adapters, is sufficient to biorient chromosomes and to remodel outer kinetochore composition following microtubule attachment; by contrast, the kinetochore dynein module is unable to support chromosome congression. The chromosome-autonomous action of kinetochore dynein, in the absence of the other major microtubule-directed factors on chromosomes, rotates and orients a substantial proportion of chromosomes such that their sister chromatids attach to opposite spindle poles. In tight coupling with orientation, the kinetochore dynein module drives removal of outermost kinetochore components, including the dynein motor itself and spindle checkpoint activators. The removal is independent of the other major microtubule-directed activities and kinetochore-localized protein phosphatase 1, suggesting that it is intrinsic to the kinetochore dynein module. These observations indicate that the kinetochore dynein module has the ability coordinate chromosome biorientation with attachment state-sensitive remodeling of the outer kinetochore that facilitates cell cycle progression.

Nuclear-enriched protein phosphatase 4 ensures outer kinetochore assembly prior to nuclear dissolution

A landmark event in the transition from interphase to mitosis in metazoans is nuclear envelope breakdown (NEBD). Important mitotic events occur prior to NEBD, including condensation of replicated chromosomes and assembly of kinetochores to rapidly engage spindle microtubules. Here, we show that nuclear-enriched protein phosphatase 4 (PP4) ensures robust assembly of the microtubule-coupling outer kinetochore prior to NEBD. In the absence of PP4, chromosomes exhibit extended monopolar orientation after NEBD and subsequently mis-segregate. A secondary consequence of diminished outer kinetochore assembly is defective sister chromatid resolution. After NEBD, a cytoplasmic activity compensates for PP4 loss, leading to outer kinetochore assembly and recovery of chromosomes from monopolar orientation to significant bi-orientation. The Ndc80-Ska microtubule-binding module of the outer kinetochore is required for this recovery. PP4 associates with the inner kinetochore protein CENP-C; however, disrupting the PP4-CENP-C interaction does not perturb chromosome segregation. These results establish that PP4-dependent outer kinetochore assembly prior to NEBD is critical for timely and proper engagement of chromosomes with spindle microtubules.

The kinesin KIF4 mediates HBV/HDV entry through the regulation of surface NTCP localization and can be targeted by RXR agonists in vitro

Intracellular transport via microtubule-based dynein and kinesin family motors plays a key role in viral reproduction and transmission. We show here that Kinesin Family Member 4 (KIF4) plays an important role in HBV/HDV infection. We intended to explore host factors impacting the HBV life cycle that can be therapeutically addressed using siRNA library transfection and HBV/NLuc (HBV/NL) reporter virus infection in HepG2-hNTCP cells. KIF4 silencing resulted in a 3-fold reduction in luciferase activity following HBV/NL infection. KIF4 knockdown suppressed both HBV and HDV infection. Transient KIF4 depletion reduced surface and raised intracellular NTCP (HBV/HDV entry receptor) levels, according to both cellular fractionation and immunofluorescence analysis (IF). Overexpression of wild-type KIF4 but not ATPase-null KIF4 mutant regained the surface localization of NTCP and significantly restored HBV permissiveness in these cells. IF revealed KIF4 and NTCP colocalization across microtubule filaments, and a co-immunoprecipitation study revealed that KIF4 interacts with NTCP. KIF4 expression is regulated by FOXM1. Interestingly, we discovered that RXR agonists (Bexarotene, and Alitretinoin) down-regulated KIF4 expression via FOXM1-mediated suppression, resulting in a substantial decrease in HBV-Pre-S1 protein attachment to HepG2-hNTCP cell surface and subsequent HBV infection in both HepG2-hNTCP and primary human hepatocyte (PXB) (Bexarotene, IC₅₀ 1.89 ± 0.98 μM) cultures. Overall, our findings show that human KIF4 is a critical regulator of NTCP surface transport and localization, which is required for NTCP to function as a receptor for HBV/HDV entry. Furthermore, small molecules that suppress or alleviate KIF4 expression would be potential antiviral candidates targeting HBV and HDV entry.

Understanding HBV/HDV entry machinery and the mechanism by which NTCP (HBV/HDV entry receptor) surface expression is regulated is crucial to develop antiviral entry inhibitors. We found that NTCP surface transport is mainly controlled by the motor kinesin KIF4. Surprisingly, KIF4 was negatively regulated by RXR receptors through FOXM1-mediated suppression. This study not only mechanistically correlated the role of RXR receptors in regulating HBV/HDV entry but also suggested a novel approach to develop therapeutic rexinoids for preventing HBV and/or HDV infections in important clinical situations, such as in patients undergoing liver transplantation or those who are at a high risk of HBV infection and unresponsive to HBV vaccination.

Dynein-dynactin segregate meiotic chromosomes in C. elegans spermatocytes

The microtubule motor cytoplasmic dynein 1 (dynein) and its essential activator dynactin have conserved roles in spindle assembly and positioning during female meiosis and mitosis, but their contribution to male meiosis remains poorly understood. Here, we characterize the G33S mutation in the C. elegans dynactin subunit DNC-1, which corresponds to G59S in human p150^Glued that causes motor neuron disease. In spermatocytes, dnc-1(G33S) delays spindle assembly and penetrantly inhibits anaphase spindle elongation in meiosis I, which prevents the segregation of homologous chromosomes. By contrast, chromosomes segregate without errors in the early dnc-1(G33S) embryo. Deletion of the DNC-1 N-terminus shows that defective meiosis in dnc-1(G33S) spermatocytes is not due to the inability of DNC-1 to interact with microtubules. Instead, our results suggest that the DNC-1(G33S) protein, which is aggregation prone in vitro, is less stable in spermatocytes than the early embryo, resulting in different phenotypic severity in the two dividing tissues. Thus, the dnc-1(G33S) mutant reveals that dynein-dynactin drive meiotic chromosome segregation in spermatocytes and illustrates that the extent to which protein misfolding leads to loss of function can vary significantly between cell types.

Chromokinesins

During the cell cycle it is critical that the duplicated DNA faithfully segregates to give rise to two genetically identical daughter cells. An even distribution of the genome during mitosis is mediated by mitotic spindle microtubules, assisted by, among others, motor proteins of the kinesin superfamily. Chromokinesins are members of the kinesin superfamily that harbour a specific DNA-binding domain. The best characterized chromokinesins belong to the kinesin-4/Kif4 and kinesin-10/Kif22 families, respectively. Functional analysis of chromokinesins in several model systems revealed their involvement in chromosome arm orientation and oscillations. This is consistent with their originally proposed role in the generation of polar ejection forces that assist chromosome congression to the spindle equator. Kinesin-12/Kif15 members comprise a third family of chromokinesins, but their role remains less understood. Noteworthy, all chromokinesins exhibit chromosome-independent localization on spindle microtubules, and recent works have significantly extended the portfolio of mitotic processes in which chromokinesins play a role, from error correction and DNA compaction, to the regulation of spindle microtubule dynamics.

Self-Organization of Minimal Anaphase Spindle Midzone Bundles

In anaphase spindles, antiparallel microtubules associate to form tight midzone bundles, as required for functional spindle architecture and correct chromosome segregation. Several proteins selectively bind to these overlaps to control cytokinesis. How midzone bundles assemble is poorly understood. Here, using an in vitro reconstitution approach, we demonstrate that minimal midzone bundles can reliably self-organize in solution from dynamic microtubules, the microtubule crosslinker PRC1, and the motor protein KIF4A. The length of the central antiparallel overlaps in these microtubule bundles is similar to that observed in cells and is controlled by the PRC1/KIF4A ratio. Experiments and computer simulations demonstrate that minimal midzone bundle formation results from promoting antiparallel microtubule crosslinking, stopping microtubule plus-end dynamicity, and motor-driven midzone compaction and alignment. The robustness of this process suggests that a similar self-organization mechanism may contribute to the reorganization of the spindle architecture during the metaphase to anaphase transition in cells.

Mitotic Cell Division in Caenorhabditis elegans

Mitotic cell divisions increase cell number while faithfully distributing the replicated genome at each division. The Caenorhabditis elegans embryo is a powerful model for eukaryotic cell division. Nearly all of the genes that regulate cell division in C. elegans are conserved across metazoan species, including humans. The C. elegans pathways tend to be streamlined, facilitating dissection of the more redundant human pathways. Here, we summarize the virtues of C. elegans as a model system and review our current understanding of centriole duplication, the acquisition of pericentriolar material by centrioles to form centrosomes, the assembly of kinetochores and the mitotic spindle, chromosome segregation, and cytokinesis.

Chromosome Segregation Is Biased by Kinetochore Size

Chromosome missegregation during mitosis or meiosis is a hallmark of cancer and the main cause of prenatal death in humans. The gain or loss of specific chromosomes is thought to be random, with cell viability being essentially determined by selection. Several established pathways including centrosome amplification, sister-chromatid cohesion defects, or a compromised spindle assembly checkpoint can lead to chromosome missegregation. However, how specific intrinsic features of the kinetochore—the critical chromosomal interface with spindle microtubules—impact chromosome segregation remains poorly understood. Here we used the unique cytological attributes of female Indian muntjac, the mammal with the lowest known chromosome number (2n = 6), to characterize and track individual chromosomes with distinct kinetochore size throughout mitosis. We show that centromere and kinetochore functional layers scale proportionally with centromere size. Measurement of intra-kinetochore distances, serial-section electron microscopy, and RNAi against key kinetochore proteins confirmed a standard structural and functional organization of the Indian muntjac kinetochores and revealed that microtubule binding capacity scales with kinetochore size. Surprisingly, we found that chromosome segregation in this species is not random. Chromosomes with larger kinetochores bi-oriented more efficiently and showed a 2-fold bias to congress to the equator in a motor-independent manner. Despite robust correction mechanisms during unperturbed mitosis, chromosomes with larger kinetochores were also strongly biased to establish erroneous merotelic attachments and missegregate during anaphase. This bias was impervious to the experimental attenuation of polar ejection forces on chromosome arms by RNAi against the chromokinesin Kif4a. Thus, kinetochore size is an important determinant of chromosome segregation fidelity.

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Centromere/kinetochore functional layers scale proportionally with centromere size

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Kinetochore microtubule binding capacity scales with kinetochore size

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Chromosome congression and bi-orientation are biased by kinetochore size

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Error formation leading to chromosome missegregation is biased by kinetochore size

Altered chemomechanical coupling causes impaired motility of the kinesin-4 motors KIF27 and KIF7

Despite their shared ability to regulate microtubule polymerization dynamics, kinesin-4 motors display dramatically different motility properties ranging from fast processive motility to no movement. Yue et al. demonstrate that for KIF7 and KIF27, altered chemomechanical coupling results in immotile behavior and slow processive movement, respectively.

Kinesin-4 motors play important roles in cell division, microtubule organization, and signaling. Understanding how motors perform their functions requires an understanding of their mechanochemical and motility properties. We demonstrate that KIF27 can influence microtubule dynamics, suggesting a conserved function in microtubule organization across the kinesin-4 family. However, kinesin-4 motors display dramatically different motility characteristics: KIF4 and KIF21 motors are fast and processive, KIF7 and its Drosophila melanogaster homologue Costal2 (Cos2) are immotile, and KIF27 is slow and processive. Neither KIF7 nor KIF27 can cooperate for fast processive transport when working in teams. The mechanistic basis of immotile KIF7 behavior arises from an inability to release adenosine diphosphate in response to microtubule binding, whereas slow processive KIF27 behavior arises from a slow adenosine triphosphatase rate and a high affinity for both adenosine triphosphate and microtubules. We suggest that evolutionarily selected sequence differences enable immotile KIF7 and Cos2 motors to function not as transporters but as microtubule-based tethers of signaling complexes.

Dephosphorylation of the Ndc80 Tail Stabilizes Kinetochore-Microtubule Attachments via the Ska Complex

During cell division, genome inheritance is orchestrated by microtubule attachments formed at kinetochores of mitotic chromosomes. The primary microtubule coupler at the kinetochore, the Ndc80 complex, is regulated by Aurora kinase phosphorylation of its N-terminal tail. Dephosphorylation is proposed to stabilize kinetochore-microtubule attachments by strengthening electrostatic interactions of the tail with the microtubule lattice. Here, we show that removal of the Ndc80 tail, which compromises in vitro microtubule binding, has no effect on kinetochore-microtubule attachments in the Caenorhabditis elegans embryo. Despite this, preventing Aurora phosphorylation of the tail results in prematurely stable attachments that restrain spindle elongation. This premature stabilization requires the conserved microtubule-binding Ska complex, which enriches at attachment sites prior to anaphase onset to dampen chromosome motion. We propose that Ndc80-tail dephosphorylation promotes stabilization of kinetochore-microtubule attachments via the Ska complex and that this mechanism ensures accurate segregation by constraining chromosome motion following biorientation on the spindle.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

A SUMO-Dependent Protein Network Regulates Chromosome Congression during Oocyte Meiosis

During Caenorhabditis elegans oocyte meiosis, a multi-protein ring complex (RC) localized between homologous chromosomes, promotes chromosome congression through the action of the chromokinesin KLP-19. While some RC components are known, the mechanism of RC assembly has remained obscure. We show that SUMO E3 ligase GEI-17/PIAS is required for KLP-19 recruitment to the RC, and proteomic analysis identified KLP-19 as a SUMO substrate in vivo. In vitro analysis revealed that KLP-19 is efficiently sumoylated in a GEI-17-dependent manner, while GEI-17 undergoes extensive auto-sumoylation. GEI-17 and another RC component, the kinase BUB-1, contain functional SUMO interaction motifs (SIMs), allowing them to recruit SUMO modified proteins, including KLP-19, into the RC. Thus, dynamic SUMO modification and the presence of SIMs in RC components generate a SUMO-SIM network that facilitates assembly of the RC. Our results highlight the importance of SUMO-SIM networks in regulating the assembly of dynamic protein complexes.

RNAi Screen Identifies Novel Regulators of RNP Granules in the Caenorhabditis elegans Germ Line

Complexes of RNA and RNA binding proteins form large-scale supramolecular structures under many cellular contexts. In Caenorhabditis elegans, small germ granules are present in the germ line that share characteristics with liquid droplets that undergo phase transitions. In meiotically-arrested oocytes of middle-aged hermaphrodites, the germ granules appear to aggregate or condense into large assemblies of RNA-binding proteins and maternal mRNAs. Prior characterization of the assembly of large-scale RNP structures via candidate approaches has identified a small number of regulators of phase transitions in the C. elegans germ line; however, the assembly, function, and regulation of these large RNP assemblies remain incompletely understood. To identify genes that promote remodeling and assembly of large RNP granules in meiotically-arrested oocytes, we performed a targeted, functional RNAi screen and identified over 300 genes that regulate the assembly of the RNA-binding protein MEX-3 into large granules. Among the most common GO classes are several categories related to RNA biology, as well as novel categories such as cell cortex, ER, and chromosome segregation. We found that arrested oocytes that fail to localize MEX-3 into cortical granules display reduced oocyte quality, consistent with the idea that the larger RNP assemblies promote oocyte quality when fertilization is delayed. Interestingly, a relatively small number of genes overlap with the regulators of germ granule assembly during normal development, or with the regulators of solid RNP granules in cgh-1 oocytes, suggesting fundamental differences in the regulation of RNP granule phase transitions during meiotic arrest.

Restructuring of Holocentric Centromeres During Meiosis in the Plant Rhynchospora pubera

Centromeres are responsible for the correct segregation of chromosomes during mitosis and meiosis. Holocentric chromosomes, characterized by multiple centromere units along each chromatid, have particular adaptations to ensure regular disjunction during meiosis. Here we show by detecting CENH3, CENP-C, tubulin, and centromeric repeats that holocentromeres may be organized differently in mitosis and meiosis of Rhynchospora pubera Contrasting to the mitotic linear holocentromere organization, meiotic centromeres show several clusters of centromere units (cluster-holocentromeres) during meiosis I. They accumulate along the poleward surface of bivalents where spindle fibers perpendicularly attach. During meiosis II, the cluster-holocentromeres are mostly present in the midregion of each chromatid. A linear holocentromere organization is restored after meiosis during pollen mitosis. Thus, a not yet described case of a cluster-holocentromere organization, showing a clear centromere restructuration between mitosis and meiosis, was identified in a holocentric organism.

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.

Franklin FC, Heckmann S. Atypical centromeres in plants-what they can tell us

The centromere, visible as the primary constriction of condensed metaphase chromosomes, is a defined chromosomal locus essential for genome stability. It mediates transient assembly of a multi-protein complex, the kinetochore, which enables interaction with spindle fibers and thus faithful segregation of the genetic information during nuclear divisions. Centromeric DNA varies in extent and sequence composition among organisms, but a common feature of almost all active eukaryotic centromeres is the presence of the centromeric histone H3 variant cenH3 (a.k.a. CENP-A). These typical centromere features apply to most studied species. However, a number of species display "atypical" centromeres, such as holocentromeres (centromere extension along almost the entire chromatid length) or neocentromeres (ectopic centromere activity). In this review, we provide an overview of different atypical centromere types found in plants including holocentromeres, de novo formed centromeres and terminal neocentromeres as well as di-, tri- and metapolycentromeres (more than one centromere per chromosomes). We discuss their specific and common features and compare them to centromere types found in other eukaryotic species. We also highlight new insights into centromere biology gained in plants with atypical centromeres such as distinct mechanisms to define a holocentromere, specific adaptations in species with holocentromeres during meiosis or various scenarios leading to neocentromere formation.

Crosstalk between microtubule attachment complexes ensures accurate chromosome segregation

The microtubule-based mitotic spindle segregates chromosomes during cell division. During chromosome segregation, the centromeric regions of chromosomes build kinetochores that establish end-coupled attachments to spindle microtubules. Here, we used the Caenorhabditis elegans embryo as a model system to examine the crosstalk between two kinetochore protein complexes implicated in temporally distinct stages of attachment formation. The kinetochore dynein module, which mediates initial lateral microtubule capture, inhibited microtubule binding by the Ndc80 complex, which ultimately forms the end-coupled attachments that segregate chromosomes. The kinetochore dynein module directly regulated Ndc80, independently of phosphorylation by Aurora B kinase, and this regulation was required for accurate segregation. Thus, the conversion from initial dynein-mediated, lateral attachments to correctly oriented, Ndc80-mediated end-coupled attachments is actively controlled.

Elevated polar ejection forces stabilize kinetochore-microtubule attachments

Polar ejection forces, which push chromosomes away from spindle poles, modulate kinetochore–microtubule attachment stability.

Chromosome biorientation promotes congression and generates tension that stabilizes kinetochore–microtubule (kt-MT) interactions. Forces produced by molecular motors also contribute to chromosome alignment, but their impact on kt-MT attachment stability is unclear. A critical force that acts on chromosomes is the kinesin-10–dependent polar ejection force (PEF). PEFs are proposed to facilitate congression by pushing chromosomes away from spindle poles, although knowledge of the molecular mechanisms underpinning PEF generation is incomplete. Here, we describe a live-cell PEF assay in which tension was applied to chromosomes by manipulating levels of the chromokinesin NOD (no distributive disjunction; Drosophila melanogaster kinesin-10). NOD stabilized syntelic kt-MT attachments in a dose- and motor-dependent manner by overwhelming the ability of Aurora B to mediate error correction. NOD-coated chromatin stretched away from the pole via lateral and end-on interactions with microtubules, and NOD chimeras with either plus end–directed motility or tip-tracking activity produced PEFs. Thus, kt-MT attachment stability is modulated by PEFs, which can be generated by distinct force-producing interactions between chromosomes and dynamic spindle microtubules.

Human chromokinesins promote chromosome congression and spindle microtubule dynamics during mitosis

Human chromokinesins hKID and KIF4A contribute to proper attachment of chromosomes by controlling the positioning of the chromosome arms and microtubule dynamics, respectively.

Chromokinesins are microtubule plus end–directed motor proteins that bind to chromosome arms. In Xenopus egg cell-free extracts, Xkid and Xklp1 are essential for bipolar spindle formation but the functions of the human homologues, hKID (KIF22) and KIF4A, are poorly understood. By using RNAi-mediated protein knockdown in human cells, we find that only co-depletion delayed progression through mitosis in a Mad2-dependent manner. Depletion of hKID caused abnormal chromosome arm orientation, delayed chromosome congression, and sensitized cells to nocodazole. Knockdown of KIF4A increased the number and length of microtubules, altered kinetochore oscillations, and decreased kinetochore microtubule flux. These changes were associated with failures in establishing a tight metaphase plate and an increase in anaphase lagging chromosomes. Co-depletion of both chromokinesins aggravated chromosome attachment failures, which led to mitotic arrest. Thus, hKID and KIF4A contribute independently to the rapid and correct attachment of chromosomes by controlling the positioning of chromosome arms and the dynamics of microtubules, respectively.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

In sexually reproducing animals, oocytes arrest at diplotene or diakinesis and resume meiosis (meiotic maturation) in response to hormones. Chromosome segregation errors in female meiosis I are the leading cause of human birth defects, and age-related changes in the hormonal environment of the ovary are a suggested cause. Caenorhabditis elegans is emerging as a genetic paradigm for studying hormonal control of meiotic maturation. The meiotic maturation processes in C. elegans and mammals share a number of biological and molecular similarities. Major sperm protein (MSP) and luteinizing hormone (LH), though unrelated in sequence, both trigger meiotic resumption using somatic Gα(s)-adenylate cyclase pathways and soma-germline gap-junctional communication. At a molecular level, the oocyte responses apparently involve the control of conserved protein kinase pathways and post-transcriptional gene regulation in the oocyte. At a cellular level, the responses include cortical cytoskeletal rearrangement, nuclear envelope breakdown, assembly of the acentriolar meiotic spindle, chromosome segregation, and likely changes important for fertilization and the oocyte-to-embryo transition. This chapter focuses on signaling mechanisms required for oocyte growth and meiotic maturation in C. elegans and discusses how these mechanisms coordinate the completion of meiosis and the oocyte-to-embryo transition.

The chromosomal passenger complex is required for meiotic acentrosomal spindle assembly and chromosome biorientation

During meiosis in the females of many species, spindle assembly occurs in the absence of the microtubule-organizing centers called centrosomes. In the absence of centrosomes, the nature of the chromosome-based signal that recruits microtubules to promote spindle assembly as well as how spindle bipolarity is established and the chromosomes orient correctly toward the poles is not known. To address these questions, we focused on the chromosomal passenger complex (CPC). We have found that the CPC localizes in a ring around the meiotic chromosomes that is aligned with the axis of the spindle at all stages. Using new methods that dramatically increase the effectiveness of RNA interference in the germline, we show that the CPC interacts with Drosophila oocyte chromosomes and is required for the assembly of spindle microtubules. Furthermore, chromosome biorientation and the localization of the central spindle kinesin-6 protein Subito, which is required for spindle bipolarity, depend on the CPC components Aurora B and Incenp. Based on these data we propose that the ring of CPC around the chromosomes regulates multiple aspects of meiotic cell division including spindle assembly, the establishment of bipolarity, the recruitment of important spindle organization factors, and the biorientation of homologous chromosomes.

A minimal midzone protein module controls formation and length of antiparallel microtubule overlaps

During cell division, microtubules are arranged in a large bipolar structure, the mitotic spindle, to segregate the duplicated chromosomes. Antiparallel microtubule overlaps in the spindle center are essential for establishing bipolarity and maintaining spindle stability throughout mitosis. In anaphase, this antiparallel microtubule array is tightly bundled forming the midzone, which serves as a hub for the recruitment of proteins essential for late mitotic events. The molecular mechanism of midzone formation and the control of its size are not understood. Using an in vitro reconstitution approach, we show here that PRC1 autonomously bundles antiparallel microtubules and recruits Xklp1, a kinesin-4, selectively to overlapping antiparallel microtubules. The processive motor Xklp1 controls overlap size by overlap length-dependent microtubule growth inhibition. Our results mechanistically explain how the two conserved, essential midzone proteins PRC1 and Xklp1 cooperate to constitute a minimal protein module capable of dynamically organizing the core structure of the central anaphase spindle.

A kinetochore-independent mechanism drives anaphase chromosome separation during acentrosomal meiosis

Although assembly of acentrosomal meiotic spindles has been extensively studied, little is known about the segregation of chromosomes on these spindles. Here, we show in Caenorhabditis elegans oocytes that the kinetochore protein, KNL-1, directs assembly of meiotic kinetochores that orient chromosomes. However, in contrast to mitosis, chromosome separation during meiotic anaphase is kinetochore-independent. Before anaphase, meiotic kinetochores and spindle poles disassemble along with the microtubules on the poleward side of chromosomes. During anaphase, microtubules then form between the separating chromosomes. Functional analysis implicated a set of proteins that localize to a ring-shaped domain between kinetochores during pre-anaphase spindle assembly and anaphase separation. These proteins are localized by the chromosomal passenger complex, which regulates the loss of meiotic chromosome cohesion. Thus, meiotic segregation in C. elegans is a two-stage process, where kinetochores orient chromosomes, but are then dispensable for their separation. We suggest that separation is controlled by a meiosis-specific chromosomal domain to coordinate cohesin removal and chromosome segregation.

Still entangled: assembly of the central spindle by multiple microtubule modulators

The central spindle is a microtubule-based structure that assembles during anaphase of mitosis in animal cells and is essential for multiple steps of cytokinesis. Central spindle assembly occurs by the cooperative action of multiple microtubule motors and modulators. Here, we review the mechanism by which the central spindle is formed, the role of several key proteins in this process and how central spindle assembly is temporally and spatially coordinated with mitosis.

The elegans of spindle assembly

The Caenorhabditis elegans one-cell embryo is a powerful system in which to study microtubule organization because this large cell assembles both meiotic and mitotic spindles within the same cytoplasm over the course of 1 h in a stereotypical manner. The fertilized oocyte assembles two consecutive acentrosomal meiotic spindles that function to reduce the replicated maternal diploid set of chromosomes to a single-copy haploid set. The resulting maternal DNA then unites with the paternal DNA to form a zygotic diploid complement, around which a centrosome-based mitotic spindle forms. The early C. elegans embryo is amenable to live-cell imaging and electron tomography, permitting a detailed structural comparison of the meiotic and mitotic modes of spindle assembly.

Coordinating cohesion, co-orientation, and congression during meiosis: lessons from holocentric chromosomes

Organisms that reproduce sexually must reduce their chromosome number by half during meiosis to generate haploid gametes. To achieve this reduction in ploidy, organisms must devise strategies to couple sister chromatids so that they stay together during the first meiotic division (when homologous chromosomes separate) and then segregate away from one another during the second division. Here we review recent findings that shed light on how Caenorhabditis elegans, an organism with holocentric chromosomes, deals with these challenges of meiosis by differentiating distinct chromosomal subdomains and remodeling chromosome structure during prophase. Furthermore, we discuss how features of chromosome organization established during prophase affect later chromosome behavior during the meiotic divisions. Finally, we illustrate how analysis of holocentric meiosis can inform our thinking about mechanisms that operate on monocentric chromosomes.

Lateral microtubule bundles promote chromosome alignment during acentrosomal oocyte meiosis

Although centrosomes serve to organize microtubules in most cell types, oocyte spindles form and mediate meiotic chromosome segregation in their absence. Here, we used high-resolution imaging of both bipolar and experimentally generated monopolar spindles in Caenorhabditis elegans to reveal a surprising organization of microtubules and chromosomes within acentrosomal structures. We found that homologous chromosome pairs (bivalents) are surrounded by microtubule bundles running along their sides, whereas microtubule density is extremely low at chromosome ends despite a high concentration of kinetochore proteins at those regions. Furthermore, we found that the chromokinesin KLP-19 (kinesin-like protein 19) is targeted to a ring around the centre of each bivalent and provides a polar ejection force that is required for congression. Together, these observations create a new picture of chromosome-microtubule association in acentrosomal spindles and reveal a mechanism by which metaphase alignment can be achieved using this organization. Specifically, we propose that ensheathment by lateral microtubule bundles places spatial constraints on the chromosomes, thereby promoting biorientation, and that localized motors mediate movement along these bundles, thereby promoting alignment.

The 3Ms of central spindle assembly: microtubules, motors and MAPs

During metaphase, sister chromatids are positioned at the midpoint of the microtubule-based mitotic spindle in preparation for their segregation. The onset of anaphase triggers inactivation of the key mitotic kinase cyclin-dependent kinase 1 (CDK1) and the polewards movement of sister chromatids. During anaphase, the mitotic spindle reorganizes in preparation for cytokinesis. Kinesin motor proteins and microtubule-associated proteins bundle the plus ends of interpolar microtubules and generate the central spindle, which regulates cleavage furrow initiation and the completion of cytokinesis. Complementary approaches, including cell biology, genetics and computational modelling, have provided new insights into the mechanism and regulation of central spindle assembly.

Disruption of four kinesin genes in dictyostelium

BACKGROUND

Kinesin and dynein are the two families of microtubule-based motors that drive much of the intracellular movements in eukaryotic cells. Using a gene knockout strategy, we address here the individual function(s) of four of the 13 kinesin proteins in Dictyostelium. The goal of our ongoing project is to establish a minimal motility proteome for this basal eukaryote, enabling us to contrast motor functions here with the often far more elaborate motor families in the metazoans.

RESULTS

We performed individual disruptions of the kinesin genes, kif4, kif8, kif10, and kif11. None of the motors encoded by these genes are essential for development or viability of Dictyostelium. Removal of Kif4 (kinesin-7; CENP-E family) significantly impairs the rate of cell growth and, when combined with a previously characterized dynein inhibition, results in dramatic defects in mitotic spindle assembly. Kif8 (kinesin-4; chromokinesin family) and Kif10 (kinesin-8; Kip3 family) appear to cooperate with dynein to organize the interphase radial microtubule array.

CONCLUSION

The results reported here extend the number of kinesin gene disruptions in Dictyostelium, to now total 10, among the 13 isoforms. None of these motors, individually, are required for short-term viability. In contrast, homologs of at least six of the 10 kinesins are considered essential in humans. Our work underscores the functional redundancy of motor isoforms in basal organisms while highlighting motor specificity in more complex metazoans. Since motor disruption in Dictyostelium can readily be combined with other motility insults and stresses, this organism offers an excellent system to investigate functional interactions among the kinesin motor family.

The C. elegans Myt1 ortholog is required for the proper timing of oocyte maturation

Maturation promoting factor (MPF), a complex of cyclin-dependent kinase 1 and cyclin B, drives oocyte maturation in all animals. Mechanisms to block MPF activation in developing oocytes must exist to prevent precocious cell cycle progression prior to oocyte maturation and fertilization. This study sought to determine the developmental consequences of precociously activating MPF in oocytes prior to fertilization. Whereas depletion of Myt1 in Xenopus oocytes causes nuclear envelope breakdown in vitro, we found that depletion of the Myt1 ortholog WEE-1.3 in C. elegans hermaphrodites causes precocious oocyte maturation in vivo. Although such oocytes are ovulated, they are fertilization incompetent. We have also observed novel phenotypes in these precociously maturing oocytes, such as chromosome coalescence, aberrant meiotic spindle organization, and the expression of a meiosis II post-fertilization marker. Furthermore, co-depletion studies of CDK-1 and WEE-1.3 demonstrate that WEE-1.3 is dispensable in the absence of CDK-1, suggesting that CDK-1 is a major target of WEE-1.3 in C. elegans oocytes.

Chromokinesins: multitalented players in mitosis

Molecular motors generate cellular forces and act in a multitude of intracellular transport processes. The chromokinesins are a subgroup of kinesin motors. Chromokinesins act in various steps of mitosis, including chromosome condensation, metaphase alignment, chromosome segregation, cytokinesis and they help maintain genome stability. The emerging multifunctional nature of the chromokinesins provides insights into the coordination of distinct mitotic steps, and their role in maintenance of genome stability makes them attractive potential targets for therapeutic intervention.

Spindle microtubules in flux

Accurate and timely chromosome segregation is a task performed within meiotic and mitotic cells by a specialized force-generating structure--the spindle. This micromachine is constructed from numerous proteins, most notably the filamentous microtubules that form a structural framework for the spindle and also transmit forces through it. Poleward flux is an evolutionarily conserved mechanism used by spindle microtubules both to move chromosomes and to regulate spindle length. Recent studies have identified a microtubule-depolymerizing kinesin as a key force-generating component required for flux. On the basis of these findings, we propose a new model for flux powered by a microtubule-disassembly mechanism positioned at the spindle pole. In addition, we use the flux model to explain the results of spindle manipulation experiments to illustrate the importance of flux for proper chromosome positioning.

Kiss and break up—a safe passage to anaphase in mitosis and meiosis

Eukaryotic chromosomes have many challenges to overcome between DNA replication and sister chromatid segregation. If these challenges are not met, cell death or unregulated cell division (cancer) may result. During prophase, chromosomes condense, the nuclear membrane breaks down and cohesins are removed from chromosome arms. In prometaphase, initial spindle attachments are made by sister kinetochores followed by correction of erroneous attachments, centromere oscillation between spindle poles and congression towards the cell's equator. In metaphase, all chromosomes attain stable bipolar spindle attachments and align at the metaphase plate, ready for the metaphase-anaphase transition when all ties between sister chromatids are broken. This review concentrates on recent developments that have revealed the intricacies of these processes. We now know more about how the mechanisms of cohesin removal differ between prophase and the metaphase-anaphase transition, the processes for detection and correction of improper spindle-kinetochore attachments and the concept that tension between sister kinetochores is the driving factor for satisfying the spindle checkpoint. We are also beginning to gain some understanding of the mechanisms behind the co-segregation of sister chromatids at the first meiotic division.

Essential kinesins: characterization of Caenorhabditis elegans KLP-15

Kinesins form a superfamily of molecular motors involved in cell division and intracellular transport. Twenty kinesins have been found in the Caenorhabditis elegans genome, and four of these belong to the kinesin-14 subfamily, i.e., kinesins with C-terminal motor domains. Three of these kinesin-14s, KLP-15, KLP-16, and KLP-17, form a distinct subgroup in which KLP-15 and KLP-16 are more than 90% identical and appear to be related by a relatively recent gene duplication. They are essential for meiotic spindle organization and chromosome segregation, and are mostly expressed in the germline. With 587 amino acids each, they are among the smallest kinesins known. Using bacterially expressed KLP-15 constructs with different length extensions preceding the motor domain, we have determined in vitro the following characteristic properties: ATPase activity, microtubule binding, oligomeric state, microtubule gliding activity, and direction of movement. The constructs exhibit a monomer-dimer equilibrium that depends on the length of the predicted alpha-helical coiled-coil region preceding the motor domain. The longest construct with the complete coiled-coil domain is a stable dimer, and the shortest construct with only seven amino acids preceding the motor domain is a monomer. In microtubule gliding assays, the monomer is immobile whereas the fully dimeric KLP-15 construct supports gliding at 2.3 microm/min and moves toward microtubule minus ends, like other members of the kinesin-14 subfamily studied to date.

Functional analysis of cytoplasmic dynein heavy chain in Caenorhabditis elegans with fast-acting temperature-sensitive mutations

Cytoplasmic dynein, a minus-end-directed microtubule motor, has been implicated in many cellular and developmental processes. Identification of specific cellular processes that rely directly on dynein would be facilitated by a means to induce specific and rapid inhibition of its function. We have identified conditional variants of a Caenorhabditis elegans dynein heavy chain (DHC-1) that lose function within a minute of a modest temperature upshift. Mutant embryos generated at elevated temperature show defects in centrosome separation, pronuclear migration, rotation of the centrosome/nucleus complex, bipolar spindle assembly, anaphase chromosome segregation, and cytokinesis. Our analyses of mutant embryos generated at permissive temperature and then upshifted quickly just before events of interest indicate that DHC-1 is required specifically for rotation of the centrosome/nucleus complex, for chromosome congression to a well ordered metaphase plate, and for timely initiation of anaphase. Our results do not support the view that DHC-1 is required for anaphase B separation of spindle poles and chromosomes. A P-loop mutation identified in two independent dominant temperature-sensitive alleles of dhc-1, when engineered into the DHC1 gene of Saccharomyces cerevisiae, conferred a dominant temperature-sensitive dynein loss-of-function phenotype. This suggests that temperature-sensitive mutations can be created for time-resolved function analyses of dyneins and perhaps other P-loop proteins in a variety of model systems.