The Ran-GTP gradient spatially regulates XCTK2 in the spindle

Identification of a motif in TPX2 that regulates spindle architecture in Xenopus egg extracts

A bipolar spindle composed of microtubules and many associated proteins functions to segregate chromosomes during cell division in all eukaryotes, yet spindle size and architecture varies dramatically across different species and cell types. Targeting protein for Xklp2 (TPX2) is one candidate factor for modulating spindle microtubule organization through its roles in branching microtubule nucleation, activation of the mitotic kinase Aurora A, and association with the kinesin-5 (Eg5) motor. Here we identify a conserved nuclear localization sequence (NLS) motif, 123 KKLK 126 in X. laevis TPX2, which regulates astral microtubule formation and spindle pole morphology in Xenopus egg extracts. Addition of recombinant TPX2 with this sequence mutated to AALA dramatically increased spontaneous formation of microtubule asters and recruitment of phosphorylated Aurora A, pericentrin, and Eg5 to meiotic spindle poles. We propose that TPX2 is a linchpin spindle assembly factor whose regulation contributes to the recruitment and activation of multiple microtubule polymerizing and organizing proteins, generating distinct spindle architectures.

Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Despite drastic cellular changes during cleavage, a mitotic spindle assembles in each blastomere to accurately segregate duplicated chromosomes. Mechanisms of mitotic spindle assembly have been extensively studied using small somatic cells. However, mechanisms of spindle assembly in large vertebrate embryos remain little understood. Here, we establish functional assay systems in medaka (Oryzias latipes) embryos by combining CRISPR knock-in with auxin-inducible degron technology. Live imaging reveals several unexpected features of microtubule organization and centrosome positioning that achieve rapid, accurate cleavage. Importantly, Ran-GTP assembles a dense microtubule network at the metaphase spindle center that is essential for chromosome segregation in early embryos. This unique spindle structure is remodeled into a typical short, somatic-like spindle after blastula stages, when Ran-GTP becomes dispensable for chromosome segregation. We propose that despite the presence of centrosomes, the chromosome-derived Ran-GTP pathway has essential roles in functional spindle assembly in large, rapidly dividing vertebrate early embryos, similar to acentrosomal spindle assembly in oocytes.

Augmin is a Ran-regulated spindle assembly factor

Mitotic spindles are composed of microtubules (MTs) that must nucleate at the right place and time. Ran regulates this process by directly controlling the release of spindle assembly factors (SAFs) from nucleocytoplasmic shuttle proteins importin-αβ and subsequently forms a biochemical gradient of SAFs localized around chromosomes. The majority of spindle MTs are generated by branching MT nucleation, which has been shown to require an eight-subunit protein complex known as augmin. InXenopus laevis, Ran can control branching through a canonical SAF, TPX2, which is non-essential in Drosophila melanogaster embryos and HeLa cells. Thus, how Ran regulates branching MT nucleation when TPX2 is not required remains unknown. Here, we use in vitro pulldowns and TIRF microscopy to show that augmin is a Ran-regulated SAF. We demonstrate that augmin directly interacts with both importin-α and importin-β through two nuclear localization sequences on the Haus8 subunit, which overlap with the MT binding site. Moreover, we show Ran controls localization of augmin to MTs in both Xenopus egg extract and in vitro. Our results demonstrate that RanGTP directly regulates augmin, which establishes a new way by which Ran controls branching MT nucleation and spindle assembly both in the absence and presence of TPX2.

RNA interference targeting Ras GTPase gene Ran causes larval and adult lethality in Leptinotarsa decemlineata

BACKGROUND

RNA interference (RNAi) is a breakthrough technology in pest control. It is highly efficient to Coleopteran pests such as the Colorado potato beetle Leptinotarsa decemlineata, a serious pest defoliator mainly attacking potatoes worldwide. The first step for effective pest control by RNAi is the development of effective and reliable target genes.

RESULTS

Our results revealed that continuous ingestion of dsLdRan for 3 days successfully silenced the target gene, inhibited larval growth and killed 100% L. decemlineata larvae. When the bioassay began at the second-, third/fourth-instar larval stages, the larval lethality mainly occurred at the fourth larval instar and prepupal stages, respectively. Importantly, consumption of dsLdRan for 3 days by the newly-emerged males and females effectively knocked down the target transcript, reduced fresh weights and caused 100% of lethality within a week. The LdRan females possessed underdeveloped ovaries.

CONCLUSION

Considering that the larvae, adults and eggs are simultaneously sited on the potato plants, bacterially-expressed dsLdRan is a potential RNAi-based strategy for managing L. decemlineata in the potato field. © 2022 Society of Chemical Industry.

Using FLIM-FRET for Characterizing Spatial Interactions in the Spindle

Proper spindle assembly and the attachment of chromosomes to the spindle are key for the accurate segregation of chromosomes to daughter cells. Errors in these processes can lead to aneuploidy, which is a hallmark of cancer. Understanding the mechanisms that drive spindle assembly will provide fundamental insights into how accurate chromosome segregation is achieved. One challenge in elucidating the complexities of spindle assembly is to visualize protein interactions in space and time. The Xenopus egg extract system has been a valuable tool to probe protein function during spindle assembly in vitro. Tagging proteins with fluorescent proteins and utilizing fluorescence-based approaches, such as Förster resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM), have provided visual clues about the mechanics of spindle assembly and its regulators. However, elucidating how spindle assembly factors are spatially regulated is still challenging. Combining the egg extract system and visual FRET approaches provides a powerful tool to probe the processes involved in spindle assembly. Here we describe how a FLIM-FRET biosensor can be used to study protein-protein interactions in spindles assembled in Xenopus egg extracts. This approach should be readily adaptable to a wide variety of proteins to allow for new insights into the regulation of spindle assembly.

Mud binds the kinesin-14 Ncd in Drosophila

Maintenance of proper mitotic spindle structure is necessary for error-free chromosome segregation and cell division. Spindle assembly is controlled by force-generating kinesin motors that contribute to its geometry and bipolarity, and balancing motor-dependent forces between opposing kinesins is critical to the integrity of this process. Non-claret dysjunctional (Ncd), a Drosophila kinesin-14 member, crosslinks and slides microtubule minus-ends to focus spindle poles and sustain bipolarity. However, mechanisms that regulate Ncd activity during mitosis are underappreciated. Here, we identify Mushroom body defect (Mud), the fly ortholog of human NuMA, as a direct Ncd binding partner. We demonstrate this interaction involves a short coiled-coil domain within Mud (Mud^CC) binding the N-terminal, non-motor microtubule-binding domain of Ncd (Ncd^nMBD). We further show that the C-terminal ATPase motor domain of Ncd (Ncd^CTm) directly interacts with Ncd^nMBD as well. Mud binding competes against this self-association and also increases Ncd^nMBD microtubule binding in vitro. Our results describe an interaction between two spindle-associated proteins and suggest a potentially new mode of minus-end motor protein regulation at mitotic spindle poles.

RanGTP induces an effector gradient of XCTK2 and importin α/β for spindle microtubule cross-linking

High RanGTP around chromatin is important for governing spindle assembly during meiosis and mitosis by releasing the inhibitory effects of importin α/β. Here we examine how the Ran gradient regulates Kinesin-14 function to control spindle organization. We show that Xenopus Kinesin-14, XCTK2, and importin α/β form an effector gradient that is highest at the poles and diminishes toward the chromatin, which is opposite the RanGTP gradient. Importin α and β preferentially inhibit XCTK2 antiparallel microtubule cross-linking and sliding by decreasing the microtubule affinity of the XCTK2 tail domain. This change in microtubule affinity enables RanGTP to target endogenous XCTK2 to the spindle. We propose that these combined actions of the Ran pathway are critical to promote Kinesin-14 parallel microtubule cross-linking to help focus spindle poles for efficient bipolar spindle assembly. Furthermore, our work illustrates that RanGTP regulation in the spindle is not simply a switch, but rather generates effector gradients where importins α and β gradually tune the activities of spindle assembly factors.

Mechanisms of spindle assembly and size control

The spindle is crucial for cell division by allowing the faithful segregation of replicated chromosomes to daughter cells. Proper segregation is ensured only if microtubules (MTs) and hundreds of other associated factors interact to assemble this complex structure with the appropriate architecture and size. In this review, we describe the latest view of spindle organisation as well as the molecular gradients and mechanisms underlying MT nucleation and spindle assembly. We then discuss the overlapping physical and molecular constraints that dictate spindle morphology, concluding with a focus on spindle size regulation.

Kinesin-14 motor protein KIFC1 participates in DNA synthesis and chromatin maintenance

The nuclear localization signal (NLS) in kinesin-14 KIFC1 is associated with nuclear importins and Ran gradient, but detailed mechanism remains unknown. In this study, we found that KIFC1 proteins have specific transport characteristics during cell cycle. In the absence of KIFC1, cell cycle kinetics decrease significantly with a prolonged S phase. After KIFC1 overexpression, the duration of S phase becomes shorten. KIFC1 may transport the recombinant/replicate-related proteins into the nucleus, meanwhile avoiding excessive KIFC1 in the cytoplasm, which results in aberrant microtubule bundling. Interestingly, the deletion of kifc1 in human cells results in a higher ratio of aberrant nuclear membrane, and the degradation of lamin B and lamin A/C. We also found that kifc1 deletion leads to defects in metaphase mitotic spindle assembly, and then results in chromosome structural abnormality. The kifc1^-/- cells finally form micronuclei in daughter cells, and results in aneuploidy and chromosome loss in cell cycle. In this study, we demonstrate that kinesin-14 KIFC1 proteins involve in regulating DNA synthesis in S phase, and chromatin maintenance in mitosis, and maintain cell growth in a nuclear transport-independent way.

Mitotic Motor KIFC1 Is an Organizer of Microtubules in the Axon

KIFC1 (also called HSET or kinesin-14a) is best known as a multifunctional motor protein essential for mitosis. The present studies are the first to explore KIFC1 in terminally postmitotic neurons. Using RNA interference to partially deplete KIFC1 from rat neurons (from animals of either gender) in culture, pharmacologic agents that inhibit KIFC1, and expression of mutant KIFC1 constructs, we demonstrate critical roles for KIFC1 in regulating axonal growth and retraction as well as growth cone morphology. Experimental manipulations of KIFC1 elicit morphological changes in the axon as well as changes in the organization, distribution, and polarity orientation of its microtubules. Together, the results indicate a mechanism by which KIFC1 binds to microtubules in the axon and slides them into alignment in an ATP-dependent fashion and then cross-links them in an ATP-independent fashion to oppose their subsequent sliding by other motors.SIGNIFICANCE STATEMENT Here, we establish that KIFC1, a molecular motor well characterized in mitosis, is robustly expressed in neurons, where it has profound influence on the organization of microtubules in a number of different functional contexts. KIFC1 may help answer long-standing questions in cellular neuroscience such as, mechanistically, how growth cones stall and how axonal microtubules resist forces that would otherwise cause the axon to retract. Knowledge about KIFC1 may help researchers to devise strategies for treating disorders of the nervous system involving axonal retraction given that KIFC1 is expressed in adult neurons as well as developing neurons.

Chromosome alignment maintenance requires the MAP RECQL4, mutated in the Rothmund-Thomson syndrome

RECQL4, which is mutated in the Rothmund–Thomson syndrome characterized by premature aging and cancer susceptibility, is a microtubule-associated protein required for mitotic chromosome alignment.

RecQ-like helicase 4 (RECQL4) is mutated in patients suffering from the Rothmund–Thomson syndrome, a genetic disease characterized by premature aging, skeletal malformations, and high cancer susceptibility. Known roles of RECQL4 in DNA replication and repair provide a possible explanation of chromosome instability observed in patient cells. Here, we demonstrate that RECQL4 is a microtubule-associated protein (MAP) localizing to the mitotic spindle. RECQL4 depletion in M-phase–arrested frog egg extracts does not affect spindle assembly per se, but interferes with maintaining chromosome alignment at the metaphase plate. Low doses of nocodazole depolymerize RECQL4-depleted spindles more easily, suggesting abnormal microtubule–kinetochore interaction. Surprisingly, inter-kinetochore distance of sister chromatids is larger in depleted extracts and patient fibroblasts. Consistent with a role to maintain stable chromosome alignment, RECQL4 down-regulation in HeLa cells causes chromosome misalignment and delays mitotic progression. Importantly, these chromosome alignment defects are independent from RECQL4’s reported roles in DNA replication and damage repair. Our data elucidate a novel function of RECQL4 in mitosis, and defects in mitotic chromosome alignment might be a contributing factor for the Rothmund–Thomson syndrome.

Recent advances in understanding oogenesis: interactions with the cytoskeleton, microtubule organization, and meiotic spindle assembly in oocytes

Maternal control of development begins with production of the oocyte during oogenesis. All of the factors necessary to complete oocyte maturation, meiosis, fertilization, and early development are produced in the transcriptionally active early oocyte. Active transcription of the maternal genome is a mechanism to ensure that the oocyte and development of the early embryo begin with all of the factors needed for successful embryonic development. To achieve the maximum maternal store, only one functional cell is produced from the meiotic divisions that produce the oocyte. The oocyte receives the bulk of the maternal cytoplasm and thus is significantly larger than its sister cells, the tiny polar bodies, which receive a copy of the maternal genome but essentially none of the maternal cytoplasm. This asymmetric division is accomplished by an enormous cell that is depleted of centrosomes in early oogenesis; thus, meiotic divisions in oocytes are distinct from those of mitotic cells. Therefore, these cells must partition the chromosomes faithfully to ensure euploidy by using mechanisms that do not rely on a conventional centrosome-based mitotic spindle. Several mechanisms that contribute to assembly and maintenance of the meiotic spindle in oocytes have been identified; however, none is fully understood. In recent years, there have been many exciting and significant advances in oogenesis, contributed by studies using a myriad of systems. Regrettably, I cannot adequately cover all of the important advances here and so I apologize to those whose beautiful work has not been included. This review focuses on a few of the most recent studies, conducted by several groups, using invertebrate and vertebrate systems, that have provided mechanistic insight into how microtubule assembly and meiotic spindle morphogenesis are controlled in the absence of centrosomes.

Minus end-directed kinesin-14 KIFC1 regulates the positioning and architecture of the Golgi apparatus

The Golgi apparatus is the central organelle along the eukaryotic secretory and endocytic pathway. In non-polarized mammalian cells, the Golgi complex is usually located proximal to the nucleus at the cell center and is closely associated with the microtubule organizing center. Microtubule networks are essential in the organization and central localization of the Golgi apparatus, but the molecular basis underlying these processes are poorly understood. Here we reveal that minus end-directed kinesin-14 KIFC1 proteins are required for the structural integrity and positioning of the Golgi complex in non-polarized mammalian cells. Remarkably, we found that the motor domain of kinesin-14 KIFC1 regulates the recognition and binding of the Golgi and KIFC1 also statically binds to the microtubules via its tail domain. These findings reveal a new stationary binding model that kinesin-14 KIFC1 proteins function as crosslinkers between the Golgi apparatus and the microtubules and contribute to the central positioning and structural maintenance of the Golgi apparatus.

Two-step interphase microtubule disassembly aids spindle morphogenesis

BACKGROUND

Entry into mitosis triggers profound changes in cell shape and cytoskeletal organisation. Here, by studying microtubule remodelling in human flat mitotic cells, we identify a two-step process of interphase microtubule disassembly.

RESULTS

First, a microtubule-stabilising protein, Ensconsin/MAP7, is inactivated in prophase as a consequence of its phosphorylation downstream of Cdk1/cyclin B. This leads to a reduction in interphase microtubule stability that may help to fuel the growth of centrosomally nucleated microtubules. The peripheral interphase microtubules that remain are then rapidly lost as the concentration of tubulin heterodimers falls following dissolution of the nuclear compartment boundary. Finally, we show that a failure to destabilise microtubules in prophase leads to the formation of microtubule clumps, which interfere with spindle assembly.

CONCLUSIONS

This analysis highlights the importance of the step-wise remodelling of the microtubule cytoskeleton and the significance of permeabilisation of the nuclear envelope in coordinating the changes in cellular organisation and biochemistry that accompany mitotic entry.

Catch and release: 14-3-3 controls Ncd in meiotic spindles

Dasso discusses work from Beaven et al. on the regulation of Ncd in the meiotic spindle by 14-3-3 proteins.

During Drosophila melanogaster oogenesis, spindle assembly occurs without centrosomes and relies on signals from chromosomes. Beaven et al. (2017. J. Cell. Biol.https://doi.org/10.1083/jcb.201704120) show that 14-3-3 proteins bind and inhibit a key microtubule motor, Ncd, during oogenesis, but Aurora B releases Ncd inhibition near chromosomes, allowing Ncd to work in the right time and place.

Molecular mechanisms of kinesin-14 motors in spindle assembly and chromosome segregation

During eukaryote cell division, molecular motors are crucial regulators of microtubule organization, spindle assembly, chromosome segregation and intracellular transport. The kinesin-14 motors are evolutionarily conserved minus-end-directed kinesin motors that occur in diverse organisms from simple yeasts to higher eukaryotes. Members of the kinesin-14 motor family can bind to, crosslink or slide microtubules and, thus, regulate microtubule organization and spindle assembly. In this Commentary, we present the common subthemes that have emerged from studies of the molecular kinetics and mechanics of kinesin-14 motors, particularly with regard to their non-processive movement, their ability to crosslink microtubules and interact with the minus- and plus-ends of microtubules, and with microtubule-organizing center proteins. In particular, counteracting forces between minus-end-directed kinesin-14 and plus-end-directed kinesin-5 motors have recently been implicated in the regulation of microtubule nucleation. We also discuss recent progress in our current understanding of the multiple and fundamental functions that kinesin-14 motors family members have in important aspects of cell division, including the spindle pole, spindle organization and chromosome segregation.

KIFC1: a promising chemotherapy target for cancer treatment?

The kinesin motor KIFC1 has been suggested as a potential chemotherapy target due to its critical role in clustering of the multiple centrosomes found in cancer cells. In this regard, KIFC1 seems to be non-essential in normal somatic cells which usually possess only two centrosomes. Moreover, KIFC1 is also found to initiatively drive tumor malignancy and metastasis by stabilizing a certain degree of genetic instability, delaying cell cycle and protecting cancer cell surviving signals. However, that KIFC1 also plays roles in other specific cell types complicates the question of whether it is a promising chemotherapy target for cancer treatment. For example, KIFC1 is found functionally significant in vesicular and organelle trafficking, spermiogenesis, oocyte development, embryo gestation and double-strand DNA transportation. In this review we summarize a recent collection of information so as to provide a generalized picture of ideas and mechanisms against and in favor of KIFC1 as a chemotherapy target. And we also drew the conclusion that KIFC1 is a promising chemotherapy target for some types of cancers, because the side-effects of inhibiting KIFC1 mentioned in this review are theoretically easy to avoid, while KIFC1 is functionally indispensable during mitosis and malignancy of multi-centrosome cancer cells. Further investigations of how KIFC1 is regulated throughout the mitosis in cancer cells are needed for the understanding of the pathways where KIFC1 is involved and for further exploitation of indirect KIFC1 inhibitors.

Thirty years of search and capture: The complex simplicity of mitotic spindle assembly

Cell division is enacted by a microtubule-based, self-assembling macromolecular machine known as the mitotic spindle. In 1986, Kirschner and Mitchison proposed that by undergoing dynamic cycles of growth and disassembly, microtubules search for chromosomes. Capture of microtubules by the kinetochores progressively connects chromosomes to the bipolar spindle. 30 years later, “search and capture” remains the cornerstone of spindle assembly. However, a variety of facilitating mechanisms such as regulation of microtubule dynamics by diffusible gradients, spatially selective motor activities, and adaptive changes in chromosome architecture have been discovered. We discuss how these mechanisms ensure that the spindle assembles rapidly and with a minimal number of errors.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

Acentrosomal Microtubule Assembly in Mitosis: The Where, When, and How

In mitosis the cell assembles the bipolar spindle, a microtubule (MT)-based apparatus that segregates the duplicated chromosomes into two daughter cells. Most animal cells enter mitosis with duplicated centrosomes that provide an active source of dynamic MTs. However, it is now established that spindle assembly relies on the nucleation of acentrosomal MTs occurring around the chromosomes after nuclear envelope breakdown, and on pre-existing microtubules. Where chromosome-dependent MT nucleation occurs, when MT amplification takes place and how the two pathways function are still key questions that generate some controversies. We reconcile the data and present an integrated model accounting for acentrosomal microtubule assembly in the dividing cell.