Kinesin-1 and cytoplasmic dynein act sequentially to move the meiotic spindle to the oocyte cortex in Caenorhabditis elegans

Microtubules oppose cortical actomyosin-driven membrane ingression during C. elegans meiosis I polar body extrusion

During C. elegans oocyte meiosis I cytokinesis and polar body extrusion, cortical actomyosin is locally remodeled to assemble a contractile ring that forms within and remains part of a much larger and actively contractile cortical actomyosin network. This network both mediates contractile ring dynamics and generates shallow ingressions throughout the oocyte cortex during polar body extrusion. Based on our analysis of requirements for CLS-2, a member of the CLASP family of proteins that stabilize microtubules, we recently proposed that a balance of actomyosin-mediated tension and microtubule-mediated stiffness limits membrane ingression throughout the oocyte during meiosis I polar body extrusion. Here, using live cell imaging and fluorescent protein fusions, we show that CLS-2 is part of a group of kinetochore proteins, including the scaffold KNL-1 and the kinase BUB-1, that also co-localize during meiosis I to structures called linear elements, which are present within the assembling oocyte spindle and also are distributed throughout the oocyte in proximity to, but appearing to underlie, the actomyosin cortex. We further show that KNL-1 and BUB-1, like CLS-2, promote the proper organization of sub-cortical microtubules and also limit membrane ingression throughout the oocyte. Moreover, nocodazole or taxol treatment to destabilize or stabilize oocyte microtubules leads to, respectively, excess or decreased membrane ingression throughout the oocyte. Furthermore, taxol treatment, and genetic backgrounds that elevate the levels of cortically associated microtubules, both suppress excess membrane ingression in cls-2 mutant oocytes. We propose that linear elements influence the organization of sub-cortical microtubules to generate a stiffness that limits cortical actomyosin-driven membrane ingression throughout the oocyte during meiosis I polar body extrusion. We discuss the possibility that this regulation of sub-cortical microtubule dynamics facilitates actomyosin contractile ring dynamics during C. elegans oocyte meiosis I cell division.

Cortical microtubules oppose actomyosin-driven membrane ingression during C. elegans meiosis I polar body extrusion

During C. elegans oocyte meiosis I, cortical actomyosin is locally remodeled to assemble a contractile ring near the spindle. In contrast to mitosis, when most cortical actomyosin converges into a contractile ring, the small oocyte ring forms within and remains part of a much larger and actively contractile cortical actomyosin network. This network both mediates contractile ring dynamics and generates shallow ingressions throughout the oocyte cortex during polar body extrusion. Based on our analysis of requirements for CLS-2, a member of the CLASP family of proteins that stabilize microtubules, we recently proposed that a balance of actomyosin-mediated tension and microtubule-mediated stiffness are required for contractile ring assembly within the oocyte cortical actomyosin network. Here, using live cell imaging and fluorescent protein fusions, we show that CLS-2 is part of a complex of kinetochore proteins, including the scaffold KNL-1 and the kinase BUB-1, that also co-localize to patches distributed throughout the oocyte cortex during meiosis I. By reducing their function, we further show that KNL-1 and BUB-1, like CLS-2, are required for cortical microtubule stability, to limit membrane ingression throughout the oocyte, and for meiotic contractile ring assembly and polar body extrusion. Moreover, nocodazole or taxol treatment to destabilize or stabilize oocyte microtubules, respectively, leads to excess or decreased membrane ingression throughout the oocyte and defective polar body extrusion. Finally, genetic backgrounds that elevate cortical microtubule levels suppress the excess membrane ingression in cls-2 mutant oocytes. These results support our hypothesis that CLS-2, as part of a sub-complex of kinetochore proteins that also co-localize to patches throughout the oocyte cortex, stabilizes microtubules to stiffen the oocyte cortex and limit membrane ingression throughout the oocyte, thereby facilitating contractile ring dynamics and the successful completion of polar body extrusion during meiosis I.

Tubulin post-translational modifications in meiosis

Haploid gametes are produced from diploid parents through meiosis, a process inherent to all sexually reproducing eukaryotes. Faithful chromosome segregation in meiosis is essential for reproductive success, although it is less clear how the meiotic spindle achieves this compared to the mitotic spindle. It is becoming increasingly clear that tubulin post-translational modifications (PTMs) play critical roles in regulating microtubule functions in many biological processes, and meiosis is no exception. Here, I review recent advances in the understanding of tubulin PTMs in meiotic spindles, especially focusing on their roles in spindle integrity, oocyte aging, and non-Mendelian transmission.

Multiple motors cooperate to establish and maintain acentrosomal spindle bipolarity in elegans oocyte meiosis

While centrosomes organize spindle poles during mitosis, oocyte meiosis can occur in their absence. Spindles in human oocytes frequently fail to maintain bipolarity and consequently undergo chromosome segregation errors, making it important to understand the mechanisms that promote acentrosomal spindle stability. To this end, we have optimized the auxin-inducible degron system in Caenorhabditis elegans to remove the factors from pre-formed oocyte spindles within minutes and assess the effects on spindle structure. This approach revealed that dynein is required to maintain the integrity of acentrosomal poles; removal of dynein from bipolar spindles caused pole splaying, and when coupled with a monopolar spindle induced by depletion of the kinesin-12 motor KLP-18, dynein depletion led to a complete dissolution of the monopole. Surprisingly, we went on to discover that following monopole disruption, individual chromosomes were able to reorganize local microtubules and re-establish a miniature bipolar spindle that mediated chromosome segregation. This revealed the existence of redundant microtubule sorting forces that are undetectable when KLP-18 and dynein are active. We found that the kinesin-5 family motor BMK-1 provides this force, uncovering the first evidence that kinesin-5 contributes to C. elegans meiotic spindle organization. Altogether, our studies have revealed how multiple motors are working synchronously to establish and maintain bipolarity in the absence of centrosomes.

Meiosis is a specialized form of cell division that produces the gametes required for sexual reproduction, such as egg and sperm cells. Before the cell splits, it copies its genome so that it has four sets of chromosomes. Genetic information is then shuffled between the chromosomes, and the cell undergoes two rounds of division, resulting in four gametes that are genetically distinct.

Prior to division, the duplicated chromosomes are separated by rope-like protein polymers called microtubules. In most cells, structures called centrosomes organize these fibers into a spindle shape that emanates from two ‘poles’ on opposite ends of the cell: the microtubules then attach to the chromosomes and pull them apart. Despite not having centrosomes, egg cells, or ‘oocytes’, are still able to arrange their microtubules into a similar bipolar shape. However, how oocytes form these ‘acentrosomal’ spindles is poorly understood.

Centrosomes do not organize the spindle alone, and receive help from various motor proteins such as dynein. Previous work showed that dynein is involved in arranging acentrosomal poles, but it was not known if it was required to hold the poles together after they initially formed. To investigate, Cavin-Meza et al. developed a strategy that can rapidly remove dynein from oocytes of the roundworm Caenorhabditis elegans.

The experiment showed that dynein is required both to assemble and stabilize acentrosomal spindles in C. elegans. When dynein and an additional motor protein, KLP-18, were both removed from oocytes simultaneously, the poles blew apart, completely disrupting spindle organization. Surprisingly, Cavin-Meza et al. found that the spindles were able to reform and separate the chromosomes. Further probing revealed, for the first time, that a third motor protein (called BMK-1) also helps to organize the spindle into its bipolar structure.

These findings reveal the important role motor proteins play in stabilizing spindles and separating chromosomes in oocytes. Meiosis is prone to mistakes, and these errors are a major cause of miscarriages and birth defects in humans. Therefore, understanding the underlying mechanisms of how oocyte spindles form and remain stable could shed light on why chromosomes sometimes fail to segregate. This may eventually lead to new strategies for combating infertility.

Cilium axoneme internalization and degradation in chytrid fungi

Loss of the cilium is important for cell cycle progression and certain developmental transitions. Chytrid fungi are a group of basal fungi that have retained centrioles and cilia, and they can disassemble their cilia via axoneme internalization as part of the transition from free-swimming spores to sessile sporangia. While this type of cilium disassembly has been observed in many single-celled eukaryotes, it has not been well characterized because it is not observed in common model organisms. To better characterize cilium disassembly via axoneme internalization, we focused on chytrids Rhizoclosmatium globosum and Spizellomyces punctatus to represent two lineages of chytrids with different motility characteristics. Our results show that each chytrid species can reel in its axoneme into the cell body along its cortex on the order of minutes, while S. punctatus has additional faster ciliary compartment loss and lash-around mechanisms. S. punctatus retraction can also occur away from the cell cortex and is partially actin dependent. Post-internalization, the tubulin of the axoneme is degraded in both chytrids over the course of about 2 hr. Axoneme disassembly and axonemal tubulin degradation are both partially proteasome dependent. Overall, chytrid cilium disassembly is a fast process that separates axoneme internalization and degradation.

Evidence for anaphase pulling forces during C. elegans meiosis

Anaphase chromosome movement is thought to be mediated by pulling forces generated by end-on attachment of microtubules to the outer face of kinetochores. However, it has been suggested that during C. elegans female meiosis, anaphase is mediated by a kinetochore-independent pushing mechanism with microtubules only attached to the inner face of segregating chromosomes. We found that the kinetochore proteins KNL-1 and KNL-3 are required for preanaphase chromosome stretching, suggesting a role in pulling forces. In the absence of KNL-1,3, pairs of homologous chromosomes did not separate and did not move toward a spindle pole. Instead, each homolog pair moved together with the same spindle pole during anaphase B spindle elongation. Two masses of chromatin thus ended up at opposite spindle poles, giving the appearance of successful anaphase.

Spherical spindle shape promotes perpendicular cortical orientation by preventing isometric cortical pulling on both spindle poles during C. elegans female meiosis

Meiotic spindles are positioned perpendicular to the oocyte cortex to facilitate segregation of chromosomes into a large egg and a tiny polar body. In C. elegans, spindles are initially ellipsoid and parallel to the cortex before shortening to a near-spherical shape with flattened poles and then rotating to the perpendicular orientation by dynein-driven cortical pulling. The mechanistic connection between spindle shape and rotation has remained elusive. Here, we have used three different genetic backgrounds to manipulate spindle shape without eliminating dynein-dependent movement or dynein localization. Ellipsoid spindles with flattened or pointed poles became trapped in either a diagonal or a parallel orientation. Mathematical models that recapitulated the shape dependence of rotation indicated that the lower viscous drag experienced by spherical spindles prevented recapture of the cortex by astral microtubules emanating from the pole pivoting away from the cortex. In addition, maximizing contact between pole dynein and cortical dynein stabilizes flattened poles in a perpendicular orientation, and spindle rigidity prevents spindle bending that can lock both poles at the cortex. Spindle shape can thus promote perpendicular orientation by three distinct mechanisms.

Microtubule cross-linking activity of She1 ensures spindle stability for spindle positioning

Dynein orients the spindle by pulling on astral microtubules from the cortex. In Saccharomyces cerevisiae, the microtubule-associated protein She1 specifically inhibits dynein in the mother compartment to promote spindle movements toward the bud. Zhu et al. demonstrate that She1 also stabilizes interpolar microtubules, ensuring spindle integrity during dynein-mediated spindle positioning.

Dynein mediates spindle positioning in budding yeast by pulling on astral microtubules (MTs) from the cell cortex. The MT-associated protein She1 regulates dynein activity along astral MTs and directs spindle movements toward the bud cell. In addition to localizing to astral MTs, She1 also targets to the spindle, but its role on the spindle remains unknown. Using function-separating alleles, live-cell spindle assays, and in vitro biochemical analyses, we show that She1 is required for the maintenance of metaphase spindle stability. She1 binds and cross-links MTs via a C-terminal MT-binding site. She1 can also self-assemble into ring-shaped oligomers. In cells, She1 stabilizes interpolar MTs, preventing spindle deformations during movement, and we show that this activity is regulated by Ipl1/Aurora B phosphorylation during cell cycle progression. Our data reveal how She1 ensures spindle integrity during spindle movement across the bud neck and suggest a potential link between regulation of spindle integrity and dynein pathway activity.

Pnma5 is essential to the progression of meiosis in mouse oocytes through a chain of phosphorylation

PNMA (paraneoplastic antigen MA) family includes Pnma1–6. Although other members have been found to be involved in paraneoplastic neurological disorders, death receptor-dependent apoptosis, and tumorigenesis, Pnma5 was thought to be a female fertility factor, as indicated by one genome-wide study. But until now there have not been any further functional studies about Pnma5 in female meiosis. Our preliminary study indicated that Pnma5 might play important roles in meiosis. To further address this, Pnma5 was knocked down in in-vitro maturated (IVM) mouse oocytes, which are common models for mammalian female meiosis, by specific siRNA, and results showed that the loss of Pnma5 significantly delayed the progression of meiosis I and increased chromosome segregation errors during anaphase I. In in-vitro fertilization (IVF), Pnma5 knockdown caused significantly lower fertilization. To assess how it affects meiosis, Pnma5 knockdown was found to significantly decrease the stability of spindle microtubules and altered F-actin organization within actin cap regions, cause significantly abnormal mitochondria aggregation and lower ATP concentration. Next we have found that phosphorylation at Thr533 re-located Pnma5 strongly to spindles & cortex and was required for the phosphorylation of Akt and Gsk3β, while Src and Erk1/2 phosphorylation was required for the phosphorylation of Pnma5, indicating that phosphorylated Pnma5 is the active form and subsequently activates Akt and Gsk3β. Collectively this study suggests that Pnma5 is important for meiosis and is the pivot of Src→Erk1/2→Pnma5→Akt→Gsk3β pathway.

Meiosis

Sexual reproduction requires the production of haploid gametes (sperm and egg) with only one copy of each chromosome; fertilization then restores the diploid chromosome content in the next generation. This reduction in genetic content is accomplished during a specialized cell division called meiosis, in which two rounds of chromosome segregation follow a single round of DNA replication. In preparation for the first meiotic division, homologous chromosomes pair and synapse, creating a context that promotes formation of crossover recombination events. These crossovers, in conjunction with sister chromatid cohesion, serve to connect the two homologs and facilitate their segregation to opposite poles during the first meiotic division. During the second meiotic division, which is similar to mitosis, sister chromatids separate; the resultant products are haploid cells that become gametes. In Caenorhabditis elegans (and most other eukaryotes) homologous pairing and recombination are required for proper chromosome inheritance during meiosis; accordingly, the events of meiosis are tightly coordinated to ensure the proper execution of these events. In this chapter, we review the seminal events of meiosis: pairing of homologous chromosomes, the changes in chromosome structure that chromosomes undergo during meiosis, the events of meiotic recombination, the differentiation of homologous chromosome pairs into structures optimized for proper chromosome segregation at Meiosis I, and the ultimate segregation of chromosomes during the meiotic divisions. We also review the regulatory processes that ensure the coordinated execution of these meiotic events during prophase I.

The Effects of Ivermectin on Brugia malayi Females In Vitro: A Transcriptomic Approach

BACKGROUND

Lymphatic filariasis and onchocerciasis are disabling and disfiguring neglected tropical diseases of major importance in developing countries. Ivermectin is the drug of choice for mass drug administration programs for the control of onchocerciasis and lymphatic filariasis in areas where the diseases are co-endemic. Although ivermectin paralyzes somatic and pharyngeal muscles in many nematodes, these actions are poorly characterized in adult filariae. We hypothesize that paralysis of pharyngeal pumping by ivermectin in filariae could result in deprivation of essential nutrients, especially iron, inducing a wide range of responses evidenced by altered gene expression, changes in metabolic pathways, and altered developmental states in embryos. Previous studies have shown that ivermectin treatment significantly reduces microfilariae release from females within four days of exposure in vivo, while not markedly affecting adult worms. However, the mechanisms responsible for reduced production of microfilariae are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed transcriptomic profiles from Brugia malayi adult females, an important model for other filariae, using RNAseq technology after exposure in culture to ivermectin at various concentrations (100 nM, 300 nM and 1 μM) and time points (24, 48, 72 h, and 5 days). Our analysis revealed drug-related changes in expression of genes involved in meiosis, as well as oxidative phosphorylation, which were significantly down-regulated as early as 24 h post-exposure. RNA interference phenotypes of the orthologs of these down-regulated genes in C. elegans include "maternal sterile", "embryonic lethal", "larval arrest", "larval lethal" and "sick".

CONCLUSION/SIGNIFICANCE

These changes provide insight into the mechanisms involved in ivermectin-induced reduction in microfilaria output and impaired fertility, embryogenesis, and larval development.

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.

Caenorhabditis elegans Aurora A kinase is required for the formation of spindle microtubules in female meiosis

Female meiotic spindles are organized in the absence of centrosomes. Caenorhabditis elegans Aurora A (AIR-1) is dispensable for the initial assembly of meiotic microtubules within the oocyte nuclei, but its kinase activity is continuously required for the stabilization of meiotic spindle microtubules after germinal vesicle breakdown.

In many animals, female meiotic spindles are assembled in the absence of centrosomes, the major microtubule (MT)-organizing centers. How MTs are formed and organized into meiotic spindles is poorly understood. Here we report that, in Caenorhabditis elegans, Aurora A kinase/AIR-1 is required for the formation of spindle microtubules during female meiosis. When AIR-1 was depleted or its kinase activity was inhibited in C. elegans oocytes, although MTs were formed around chromosomes at germinal vesicle breakdown (GVBD), they were decreased during meiotic prometaphase and failed to form a bipolar spindle, and chromosomes were not separated into two masses. Whereas AIR-1 protein was detected on and around meiotic spindles, its kinase-active form was concentrated on chromosomes at prometaphase and on interchromosomal MTs during late anaphase and telophase. We also found that AIR-1 is involved in the assembly of short, dynamic MTs in the meiotic cytoplasm, and these short MTs were actively incorporated into meiotic spindles. Collectively our results suggest that, after GVBD, the kinase activity of AIR-1 is continuously required for the assembly and/or stabilization of female meiotic spindle MTs.

Dynactin-dependent cortical dynein and spherical spindle shape correlate temporally with meiotic spindle rotation in Caenorhabditis elegans

Cytoplasmic dynein accumulates on the cortex of Caenorhabditis elegans female meiotic spindles just before they rotate in a dynein-dependent manner. These spindles also shorten to a spherical shape that might reduce the drag that opposes cortical pulling by dynein.

Oocyte meiotic spindles orient with one pole juxtaposed to the cortex to facilitate extrusion of chromosomes into polar bodies. In Caenorhabditis elegans, these acentriolar spindles initially orient parallel to the cortex and then rotate to the perpendicular orientation. To understand the mechanism of spindle rotation, we characterized events that correlated temporally with rotation, including shortening of the spindle in the pole-to pole axis, which resulted in a nearly spherical spindle at rotation. By analyzing large spindles of polyploid C. elegans and a related nematode species, we found that spindle rotation initiated at a defined spherical shape rather than at a defined spindle length. In addition, dynein accumulated on the cortex just before rotation, and microtubules grew from the spindle with plus ends outward during rotation. Dynactin depletion prevented accumulation of dynein on the cortex and prevented spindle rotation independently of effects on spindle shape. These results support a cortical pulling model in which spindle shape might facilitate rotation because a sphere can rotate without deforming the adjacent elastic cytoplasm. We also present evidence that activation of spindle rotation is promoted by dephosphorylation of the basic domain of p150 dynactin.

Kinetochore-independent chromosome segregation driven by lateral microtubule bundles

During cell division, chromosomes attach to spindle microtubules at sites called kinetochores, and force generated at the kinetochore-microtubule interface is the main driver of chromosome movement. Surprisingly, kinetochores are not required for chromosome segregation on acentrosomal spindles in Caenorhabditis elegans oocytes, but the mechanism driving chromosomes apart in their absence is not understood. In this study, we show that lateral microtubule–chromosome associations established during prometaphase remain intact during anaphase to facilitate separation, defining a novel form of kinetochore-independent segregation. Chromosome dynamics during congression and segregation are controlled by opposing forces; plus-end directed forces are mediated by a protein complex that forms a ring around the chromosome center and dynein on chromosome arms provides a minus-end force. At anaphase onset, ring removal shifts the balance between these forces, triggering poleward movement along lateral microtubule bundles. This represents an elegant strategy for controlling chromosomal movements during cell division distinct from the canonical kinetochore-driven mechanism.

DOI:http://dx.doi.org/10.7554/eLife.06462.001

An animal's genetic material is packaged into structures called chromosomes. Most animals have two sets of chromosomes: one from each parent. Sperm and egg cells must contain half the number of chromosomes compared to other cells in the body, so that when they fuse, the resulting embryo receives a full complement of chromosomes.

Egg and sperm cells are made via a type of cell division called meiosis. In meiosis, the genetic material of a cell is copied once but then the cell divides twice. Therefore, at the end of the two divisions, the resulting sperm or egg cells contain half the number of chromosomes as the original cell. During cell division, the genetic material is separated by a structure called the spindle apparatus.

The spindle is made of protein filaments called microtubules. At each end of the spindle, there is a cluster of microtubule ends, known as a ‘pole’. The other ends of the microtubules extend out towards the center of the spindle, where they overlap with the microtubules from the opposite pole. The chromosomes line up in the center of the spindle and then the chromosomes are separated, with half moving to one spindle pole, and half to the other. In most forms of cell division, the microtubules attach to the chromosomes via sites called kinetochores. However, it was recently discovered that kinetochores are not required to separate chromosomes to make egg cells in the worm C. elegans, suggesting that these chromosomes associate with the spindle in a different way.

Muscat, Torre-Santiago et al. have now used high-resolution imaging to look at this chromosome separation process in more detail and to figure out how the chromosomes separate when C. elegans forms egg cells. The experiments revealed that the chromosomes move within the spindle along parallel microtubule bundles, much like trains moving along a track. The chromosomes are moved into position at the center of the spindle by a ring-shaped group (or ‘complex’) of proteins that forms around the center of each chromosome. The protein complex comes off the chromosomes as they separate, and a motor protein called dynein walks along the microtubules to pull the separated chromosomes to the poles.

Muscat, Torre-Santiago et al.'s findings thus show that meiosis in C. elegans during the production of egg cells works in a very different way to other types of cell division. In the future, it will be important to understand how dynein and the ring-shaped complex are regulated, as this may shed light on what causes mistakes in the separation of genetic material during meiosis, which can lead to infertility, miscarriages, and birth defects in humans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.06462.002

The asymmetry of female meiosis reduces the frequency of inheritance of unpaired chromosomes

Trisomy, the presence of a third copy of one chromosome, is deleterious and results in inviable or defective progeny if passed through the germ line. Random segregation of an extra chromosome is predicted to result in a high frequency of trisomic offspring from a trisomic parent. Caenorhabditis elegans with trisomy of the X chromosome, however, have far fewer trisomic offspring than expected. We found that the extra X chromosome was preferentially eliminated during anaphase I of female meiosis. We utilized a mutant with a specific defect in pairing of the X chromosome as a model to investigate the apparent bias against univalent inheritance. First, univalents lagged during anaphase I and their movement was biased toward the cortex and future polar body. Second, late-lagging univalents were frequently captured by the ingressing polar body contractile ring. The asymmetry of female meiosis can thus partially correct pre-existing trisomy.

Caenorhabditis elegans oocyte meiotic spindle pole assembly requires microtubule severing and the calponin homology domain protein ASPM-1

Oocyte meiotic spindles are bipolar but assemble without centrosomes. Three Caenorhabditis elegans genes that contribute are that for the calponin homology domain protein, aspm-1; the katanin mei-1; and the kinesin-12 family member klp-18. The results indicate that both microtubule severing and ASPM-1 promote pole assembly, whereas KLP-18 promotes bipolarity.

In many animals, including vertebrates, oocyte meiotic spindles are bipolar but assemble in the absence of centrosomes. Although meiotic spindle positioning in oocytes has been investigated extensively, much less is known about their assembly. In Caenorhabditis elegans, three genes previously shown to contribute to oocyte meiotic spindle assembly are the calponin homology domain protein encoded by aspm-1, the katanin family member mei-1, and the kinesin-12 family member klp-18. We isolated temperature-sensitive alleles of all three and investigated their requirements using live-cell imaging to reveal previously undocumented requirements for aspm-1 and mei-1. Our results indicate that bipolar but abnormal oocyte meiotic spindles assemble in aspm-1(-) embryos, whereas klp-18(-) and mei-1(-) mutants assemble monopolar and apolar spindles, respectively. Furthermore, two MEI-1 functions—ASPM-1 recruitment to the spindle and microtubule severing—both contribute to monopolar spindle assembly in klp-18(-) mutants. We conclude that microtubule severing and ASPM-1 both promote meiotic spindle pole assembly in C. elegans oocytes, whereas the kinesin 12 family member KLP-18 promotes spindle bipolarity.

Molecular pathways regulating mitotic spindle orientation in animal cells

Orientation of the cell division axis is essential for the correct development and maintenance of tissue morphology, both for symmetric cell divisions and for the asymmetric distribution of fate determinants during, for example, stem cell divisions. Oriented cell division depends on the positioning of the mitotic spindle relative to an axis of polarity. Recent studies have illuminated an expanding list of spindle orientation regulators, and a molecular model for how cells couple cortical polarity with spindle positioning has begun to emerge. Here, we review both the well-established spindle orientation pathways and recently identified regulators, focusing on how communication between the cell cortex and the spindle is achieved, to provide a contemporary view of how positioning of the mitotic spindle occurs.

F-actin asymmetry and the endoplasmic reticulum-associated TCC-1 protein contribute to stereotypic spindle movements in the Caenorhabditis elegans embryo

The position of the spindle apparatus determines the plane of cell cleavage and, therefore, the size and position of daughter cells, as well as the decision between symmetric and asymmetric cell division. We show that asymmetry in cortical actin and, remarkably, an endoplasmic reticulum–localized protein contribute to proper spindle positioning in the Caenorhabditis elegans embryo.

The microtubule spindle apparatus dictates the plane of cell cleavage in animal cells. During development, dividing cells control the position of the spindle to determine the size, location, and fate of daughter cells. Spindle positioning depends on pulling forces that act between the cell periphery and astral microtubules. This involves dynein recruitment to the cell cortex by a heterotrimeric G-protein α subunit in complex with a TPR-GoLoco motif protein (GPR-1/2, Pins, LGN) and coiled-coil protein (LIN-5, Mud, NuMA). In this study, we searched for additional factors that contribute to spindle positioning in the one-cell Caenorhabditis elegans embryo. We show that cortical actin is not needed for Gα–GPR–LIN-5 localization and pulling force generation. Instead, actin accumulation in the anterior actually reduces pulling forces, possibly by increasing cortical rigidity. Examining membrane-associated proteins that copurified with GOA-1 Gα, we found that the transmembrane and coiled-coil domain protein 1 (TCC-1) contributes to proper spindle movements. TCC-1 localizes to the endoplasmic reticulum membrane and interacts with UNC-116 kinesin-1 heavy chain in yeast two-hybrid assays. RNA interference of tcc-1 and unc-116 causes similar defects in meiotic spindle positioning, supporting the concept of TCC-1 acting with kinesin-1 in vivo. These results emphasize the contribution of membrane-associated and cortical proteins other than Gα–GPR–LIN-5 in balancing the pulling forces that position the spindle during asymmetric cell division.

Mechanisms of spindle positioning

Accurate positioning of spindles is essential for asymmetric mitotic and meiotic cell divisions that are crucial for animal development and oocyte maturation, respectively. The predominant model for spindle positioning, termed "cortical pulling," involves attachment of the microtubule-based motor cytoplasmic dynein to the cortex, where it exerts a pulling force on microtubules that extend from the spindle poles to the cell cortex, thereby displacing the spindle. Recent studies have addressed important details of the cortical pulling mechanism and have revealed alternative mechanisms that may be used when microtubules do not extend from the spindle to the cortex.

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.

Polar body cytokinesis

Polar body cytokinesis is the physical separation of a small polar body from a larger oocyte or ovum. This maternal meiotic division shares many similarities with mitotic and spermatogenic cytokinesis, but there are several distinctions, which will be discussed in this review. We synthesize results from many different model species, including those popular for their genetics and several that are more obscure in modern cell biology. The site of polar body division is determined before anaphase, by the eccentric, cortically associated meiotic spindle. Depending on the species, either the actin or microtubule cytoskeleton is required for spindle anchoring. Chromatin is necessary and sufficient to elicit differentiation of the associated cortex, via Ran-based signaling. The midzone of the anaphase spindle serves as a hub for regulatory complexes that elicit Rho activation, and ultimately actomyosin contractile ring assembly and contraction. Polar body cytokinesis uniquely requires another Rho family GTPase, Cdc42, for dynamic reorganization of the polar cortex. This is perhaps due to the considerable asymmetry of this division, wherein the polar body and the oocyte/ovum have distinct fates and very different sizes. Thus, maternal meiotic cytokinesis appears to occur via simultaneous polar relaxation and equatorial contraction, since the polar body is extruded from the spherical oocyte through the nascent contractile ring. As such, polar body cytokinesis is an interesting and important variation on the theme of cell division.

Kinesin-1 prevents capture of the oocyte meiotic spindle by the sperm aster

Centrioles are lost during oogenesis and inherited from the sperm at fertilization. In the zygote, the centrioles recruit pericentriolar proteins from the egg to form a mature centrosome that nucleates a sperm aster. The sperm aster then captures the female pronucleus to join the maternal and paternal genomes. Because fertilization occurs before completion of female meiosis, some mechanism must prevent capture of the meiotic spindle by the sperm aster. Here we show that in wild-type Caenorhabditis elegans zygotes, maternal pericentriolar proteins are not recruited to the sperm centrioles until after completion of meiosis. Depletion of kinesin-1 heavy chain or its binding partner resulted in premature centrosome maturation during meiosis and growth of a sperm aster that could capture the oocyte meiotic spindle. Kinesin prevents recruitment of pericentriolar proteins by coating the sperm DNA and centrioles and thus prevents triploidy by a nonmotor mechanism.

Centrosomes can initiate a polarity axis from any position within one-cell C. elegans embryos

The stereotyped asymmetry of one-cell C. elegans embryos has proven to be an important model for identifying molecular determinants of cell polarity. How polarity is initiated is less well understood. Polarity establishment depends on centrosomes, which use two molecularly distinct pathways to break symmetry. In both, the centrosome's position adjacent to the cell cortex is thought to determine where polarization starts. Defects in centrosome-cortex juxtaposition correlate with defects in polarity establishment in several mutants, suggesting that these processes may be linked, but there is no direct test of this. Here we assess how centrosome position relative to the cortex affects polarity establishment. We find that centrosomes can initiate polarity from any position within the embryo volume, but centrosome-cortex proximity decreases the time required to initiate polarity. Polarization itself brings about close centrosome-cortex proximity. Prior to polarization, cytoplasmic microtubules constrain centrosome movement near the cortex, expanding the controversial role of microtubules during polarity establishment. The ability of centrosomes to induce a single polarity axis from any position within the egg emphasizes the flexible, self-organizing properties of polarization in C. elegans embryos and contrasts the common view of C. elegans development as invariant.

Initial diameter of the polar body contractile ring is minimized by the centralspindlin complex

Polar body formation is an essential step in forming haploid eggs from diploid oocytes. This process involves completion of a highly asymmetric cytokinesis that results in a large egg and two small polar bodies. Unlike mitotic contractile rings, polar body contractile rings assemble over one spindle pole so that the spindle must move through the contractile ring before cytokinesis. During time-lapse imaging of C. elegans meiosis, the contractile ring moved downward along the length of the spindle and completed scission at the midpoint of the spindle, even when spindle length or rate of ring movement was increased. Patches of myosin heavy chain and dynamic furrowing of the plasma membrane over the entire embryo suggested that global cortical contraction forces the meiotic spindle and overlying membrane out through the contractile ring center. Consistent with this model, depletion of myosin phosphatase increased the velocity of ring movement along the length of the spindle. Global dynamic furrowing, which was restricted to anaphase I and II, was dependent on myosin II, the anaphase promoting complex and separase, but did not require cortical contact by the spindle. Large cortical patches of myosin during metaphase I and II indicated that myosin was already in the active form before activation of separase. To identify the signal at the midpoint of the anaphase spindle that induces scission, we depleted two proteins that mark the exact midpoint of the spindle during late anaphase, CYK-4 and ZEN-4. Depletion of either protein resulted in the unexpected phenotype of initial ingression of a polar body ring with twice the diameter of wild type. This phenotype revealed a novel mechanism for minimizing polar body size. Proteins at the spindle midpoint are required for initial ring ingression to occur close to the membrane-proximal spindle pole.

CDK-1 inhibits meiotic spindle shortening and dynein-dependent spindle rotation in C. elegans

Before chromosome expulsion into polar bodies during female meiosis, the APC inhibits CDK-1 to allow dynein-driven spindle rotation.

In animals, the female meiotic spindle is positioned at the egg cortex in a perpendicular orientation to facilitate the disposal of half of the chromosomes into a polar body. In Caenorhabditis elegans, the metaphase spindle lies parallel to the cortex, dynein is dispersed on the spindle, and the dynein activators ASPM-1 and LIN-5 are concentrated at spindle poles. Anaphase-promoting complex (APC) activation results in dynein accumulation at spindle poles and dynein-dependent rotation of one spindle pole to the cortex, resulting in perpendicular orientation. To test whether the APC initiates spindle rotation through cyclin B–CDK-1 inactivation, separase activation, or degradation of an unknown dynein inhibitor, CDK-1 was inhibited with purvalanol A in metaphase-I–arrested, APC-depleted embryos. CDK-1 inhibition resulted in the accumulation of dynein at spindle poles and dynein-dependent spindle rotation without chromosome separation. These results suggest that CDK-1 blocks rotation by inhibiting dynein association with microtubules and with LIN-5–ASPM-1 at meiotic spindle poles and that the APC promotes spindle rotation by inhibiting CDK-1.

Control of mitotic spindle length

The mitotic spindle accurately segregates genetic instructions by moving chromosomes to spindle poles (anaphase A) and separating the poles (anaphase B) so that, in general, the chromosomes and poles are positioned near the centers of the nascent daughter cell products of each cell division. Because the size of different types of dividing cells, and thus the spacing of their daughter cell centers, can vary significantly, the length of the metaphase or postanaphase B spindle often scales with cell size. However, significant exceptions to this scaling rule occur, revealing the existence of cell size–independent, spindle-associated mechanisms of spindle length control. The control of spindle length reflects the action of mitotic force-generating mechanisms, and its study may illuminate general principles by which cells regulate the size of internal structures. Here we review molecules and mechanisms that control spindle length, how these mechanisms are deployed in different systems, and some quantitative models that describe the control of spindle length.

Nuclear and spindle positioning during oocyte meiosis

Female meiosis is unique in that an asymmetrically positioned meiotic spindle expels chromosomes into tiny, non-developing polar bodies. The extrusion of chromosomes into polar bodies is always mediated by meiotic spindles that are attached to the oocyte cortex by one pole. The asymmetric, cortical positioning of the oocyte meiotic spindle preserves the volume and contents of the oocyte. Recent work in C. elegans and mouse has provided mechanistic details of spindle positioning in oocytes.

How kinesin motor proteins drive mitotic spindle function: Lessons from molecular assays

Kinesins are enzymes that use the energy of ATP to perform mechanical work. There are approximately 14 families of kinesins within the kinesin superfamily. Family classification is derived primarily from alignments of the sequences of the core motor domain. For this reason, the enzymatic behavior and motility of each motor generally reflects its family. At the cellular level, kinesin motors perform a variety of functions during cell division and within the mitotic spindle to ensure that chromosomes are segregated with the highest fidelity possible. The cellular functions of these motors are intimately related to their mechanical and enzymatic properties at the single molecule level. For this reason, motility studies designed to evaluate the activity of purified molecular motors are a requirement in order to understand, mechanistically, how these motors make the mitotic spindle work and what can cause the spindle to fail. This review will focus on a selection of illustrative kinesins, which have been studied at the molecular level in order to inform our understanding of their function in cells. In addition, the review will endeavor to point out some kinesins that have been studied extensively but which still lack sufficient molecular underpinnings to fully predict their contribution to spindle function.

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.

Kinesin-dependent transport results in polarized migration of the nucleus in oocytes and inward movement of yolk granules in meiotic embryos

During female meiosis, meiotic spindles are positioned at the oocyte cortex to allow expulsion of chromosomes into polar bodies. In C. elegans, kinesin-dependent translocation of the entire spindle to the cortex precedes dynein-dependent rotation of one spindle pole toward the cortex. To elucidate the role of kinesin-1 in spindle translocation, we examined the localization of kinesin subunits in meiotic embryos. Surprisingly, kinesin-1 was not associated with the spindle and instead was restricted to the cytoplasm in the middle of the embryo. Yolk granules moved on linear tracks, in a kinesin-dependent manner, away from the cortex, resulting in their concentration in the middle of the embryo where the kinesin was concentrated. These results suggest that cytoplasmic microtubules might be arranged with plus ends extending inward, away from the cortex. This microtubule arrangement would not be consistent with direct transport of the meiotic spindle toward the cortex by kinesin-1. In maturing oocytes, the nucleus underwent kinesin-dependent migration to the future site of spindle attachment at the anterior cortex. Thus the spindle translocation defect observed in kinesin-1 mutants may be a result of failed nuclear migration, which places the spindle too far from the cortex for the spindle translocation mechanism to function.