Microtubule polarity determines the lineage of embryonic neural precursor in zebrafish spinal cord

The phenomenal diversity of neuronal types in the central nervous system is achieved in part by the asymmetric division of neural precursors. In zebrafish neural precursors, asymmetric dispatch of Sara endosomes (with its Notch signaling cargo) functions as fate determinant which mediates asymmetric division. Here, we found two distinct pools of neural precursors based on Sara endosome inheritance and spindle-microtubule enrichment. Symmetric or asymmetric levels of spindle-microtubules drive differently Sara endosomes inheritance and predict neural precursor lineage. We uncover that CAMSAP2a/CAMSAP3a and KIF16Ba govern microtubule asymmetry and endosome motility, unveiling the heterogeneity of neural precursors. Using a plethora of physical and cell biological assays, we determined the physical parameters and molecular mechanisms behind microtubule asymmetries and biased endosome motility. Evolutionarily, the values of those parameters explain why all sensory organ precursor cells are asymmetric in flies while, in zebrafish spinal cord, two populations of neural precursors (symmetric vs asymmetric) are possible.

Chromosomal passenger complex condensates generate parallel microtubule bundles in vitro

The mitotic spindle contains many bundles of microtubules (MTs) including midzones and kinetochore fibers, but little is known about how bundled structures are formed. Here, we show that the chromosomal passenger complex (CPC) purified from Escherichia coli undergoes liquid-liquid demixing in vitro. An emergent property of the resultant condensates is to generate parallel MT bundles when incubated with free tubulin and GTP in vitro. We demonstrate that MT bundles emerge from CPC droplets with protruding minus ends that then grow into long and tapered MT structures. During this growth, we found that the CPC in these condensates apparently reorganize to coat and bundle the resulting MT structures. CPC mutants attenuated for liquid-liquid demixing or MT binding prevented the generation of parallel MT bundles in vitro and reduced the number of MTs present at spindle midzones in HeLa cells. Our data demonstrate that an in vitro biochemical activity to produce MT bundles emerges after the concentration of the CPC and provides models for how cells generate parallel-bundled MT structures that are important for the assembly of the mitotic spindle. Moreover, these data suggest that cells contain MT-organizing centers that generate MT bundles that emerge with the opposite polarity from centrosomes.

A Novel Druggable Dual-Specificity tYrosine-Regulated Kinase3/Calmodulin Kinase-like Vesicle-Associated Signaling Module with Therapeutic Implications in Neuroblastoma

High-risk neuroblastoma is a very aggressive pediatric cancer, accounting for ~15% of childhood cancer mortality. Therefore, novel therapeutic strategies for the treatment of neuroblastoma are urgently sought. Here, we focused on the potential implications of the Dual-specificity tYrosine-Regulated Kinase (DYRK) family and downstream signaling pathways. We used bioinformatic analysis of public datasets from neuroblastoma cohorts and cell lines to search correlations between patient survival and expression of DYRK kinases. Additionally, we performed biochemical, molecular, and cellular approaches to validate and characterize our observations, as well as an in vivo orthotopic murine model of neuroblastoma. We identified the DYRK3 kinase as a critical mediator of neuroblastoma cell proliferation and in vivo tumor growth. DYRK3 has recently emerged as a key regulator of several biomolecular condensates and has been linked to the hypoxic response of neuroblastoma cells. Our data suggest a role for DYRK3 as a regulator of the neuroblastoma-specific protein CAMKV, which is also required for neuroblastoma cell proliferation. CAMKV is a very understudied member of the Ca2+/calmodulin-dependent protein kinase family, originally described as a pseudokinase. We show that CAMKV is phosphorylated by DYRK3, and that inhibition of DYRK3 kinase activity induces CAMKV aggregation, probably mediated by its highly disordered C-terminal half. Importantly, we provide evidence that the DYRK3/CAMKV signaling module could play an important role for the function of the mitotic spindle during cell division. Our data strongly support the idea that inhibition of DYRK3 and/or CAMKV in neuroblastoma cells could constitute an innovative and highly specific intervention to fight against this dreadful cancer.

DNA segregation in mitochondria and beyond: insights from the trypanosomal tripartite attachment complex

The tripartite attachment complex (TAC) of the single mitochondrion of trypanosomes allows precise segregation of its single nucleoid mitochondrial genome during cytokinesis. It couples the segregation of the duplicated mitochondrial genome to the segregation of the basal bodies of the flagella. Here, we provide a model of the molecular architecture of the TAC that explains how its eight essential subunits connect the basal body, across the mitochondrial membranes, with the mitochondrial genome. We also discuss how the TAC subunits are imported into the mitochondrion and how they assemble to form a new TAC. Finally, we present a comparative analysis of the trypanosomal TAC with open and closed mitotic spindles, which reveals conserved concepts between these diverse DNA segregation systems.

Knockdown of TACC3 inhibits tumor cell proliferation and increases chemosensitivity in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a highly malignant digestive tract tumor with limited clinical treatments. Transforming acidic coiled-coil-containing protein 3 (TACC3) is a component of the centrosome axis and a member of the TACC family, which affect mitosis and regulate chromosome stability and are involved in tumor development and progression. However, the role of TACC3 in PDAC remains elusive. In this study, by exploiting the TCGA database, we found that high TACC3 expression in PDAC is associated with poor prognosis. shRNA-mediated TACC3 knockdown caused S phase arrest of the cell cycle and inhibited proliferation in PDAC cell lines. Through RNA sequencing and protein co-immunoprecipitation combined with mass spectrometry, KIF11 was identified as a protein that interacts with TACC3. TACC3 stabilizes and regulates KIF11 protein expression levels in PDAC cells through physical interaction. Knockdown of TACC3 or KIF11 resulted in abnormal spindle formation during cell division both in vitro and in vivo. Pharmacological inhibition of TACC3 or KIF11 can suppress tumor cell proliferation and promote apoptosis. Our studies further demonstrated that high expression of TACC3 and KIF11 mediated the resistance of PDAC to gemcitabine, and deficiency of TACC3 or KIF11 increased the sensitivity of PDAC cells to chemotherapy. In conclusion, our study reveals the fundamental role of TACC3 expression in PDAC cell proliferation and chemoresistance, suggesting that TACC3 can be used as a molecular marker to evaluate the prognosis of PDAC.

Cleavage furrow positioning in dividing Dictyostelium cells

Accurate placement of the cleavage furrow is crucial for successful cell division. Recent advancements have revealed that diverse mechanisms have evolved across different branches of the phylogenetic tree. Here, we employed Dictyostelium cells to validate previous models. We observed that during metaphase and early anaphase, mitotic spindles exhibited random rotary movements which ceased when the spindle elongated by approximately 7 μm. At this point, astral microtubules reached the polar cell cortex and fixed the spindle axis, causing cells to elongate by extending polar pseudopods and divide along the spindle axis. Therefore, the position of the furrow is determined when the spindle orientation is fixed. The distal ends of astral microtubules stimulate the extension of pseudopods at the polar cortex. One signal for pseudopod extension may be phosphatidylinositol trisphosphate in the cell membrane, but there appears to be another unknown signal. At the onset of polar pseudopod extension, cortical flow began from both poles toward the equator. We suggest that polar stimulation by astral microtubules determines the furrow position, induces polar pseudopod extension and cortical flow, and accumulates the elements necessary for the construction of the contractile ring.

Cortical tension drug screen links mitotic spindle integrity to Rho pathway

Mechanical force generation plays an essential role in many cellular functions, including mitosis. Actomyosin contractile forces mediate changes in cell shape in mitosis and are implicated in mitotic spindle integrity via cortical tension. An unbiased screen of 150 small molecules that impact actin organization and 32 anti-mitotic drugs identified two molecular targets, Rho kinase (ROCK) and tropomyosin 3.1/2 (Tpm3.1/2), whose inhibition has the greatest impact on mitotic cortical tension. The converse was found for compounds that depolymerize microtubules. Tpm3.1/2 forms a co-polymer with mitotic cortical actin filaments, and its inhibition prevents rescue of multipolar spindles induced by anti-microtubule chemotherapeutics. We examined the role of mitotic cortical tension in this rescue mechanism. Inhibition of ROCK and Tpm3.1/2 and knockdown (KD) of cortical nonmuscle myosin 2A (NM2A), all of which reduce cortical tension, inhibited rescue of multipolar mitotic spindles, further implicating cortical tension in the rescue mechanism. GEF-H1 released from microtubules by depolymerization increased cortical tension through the RhoA pathway, and its KD also inhibited rescue of multipolar mitotic spindles. We conclude that microtubule depolymerization by anti-cancer drugs induces cortical-tension-based rescue to ensure integrity of the mitotic bipolar spindle mediated via the RhoA pathway. Central to this mechanism is the dependence of NM2A on Tpm3.1/2 to produce the functional engagement of actin filaments responsible for cortical tension.

Mild replication stress causes premature centriole disengagement via a sub-critical Plk1 activity under the control of ATR-Chk1

A tight synchrony between the DNA and centrosome cycle is essential for genomic integrity. Centriole disengagement, which licenses centrosomes for duplication, occurs normally during mitotic exit. We recently demonstrated that mild DNA replication stress typically seen in cancer cells causes premature centriole disengagement in untransformed mitotic human cells, leading to transient multipolar spindles that favour chromosome missegregation. How mild replication stress accelerates the centrosome cycle at the molecular level remained, however, unclear. Using ultrastructure expansion microscopy, we show that mild replication stress induces premature centriole disengagement already in G2 via the ATR-Chk1 axis of the DNA damage repair pathway. This results in a sub-critical Plk1 kinase activity that primes the pericentriolar matrix for Separase-dependent disassembly but is insufficient for rapid mitotic entry, causing premature centriole disengagement in G2. We postulate that the differential requirement of Plk1 activity for the DNA and centrosome cycles explains how mild replication stress disrupts the synchrony between both processes and contributes to genomic instability.

Oncogenic Ras deregulates cell-substrate interactions during mitotic rounding and respreading to alter cell division orientation

Oncogenic Ras has been shown to change the way cancer cells divide by increasing the forces generated during mitotic rounding. In this way, RasV12 enables cancer cells to divide across a wider range of mechanical environments than normal cells. Here, we identify a further role for oncogenic Ras-ERK signaling in division by showing that RasV12 expression alters the shape, division orientation, and respreading dynamics of cells as they exit mitosis. Many of these effects appear to result from the impact of RasV12 signaling on actomyosin contractility, because RasV12 induces the severing of retraction fibers that normally guide spindle positioning and provide a memory of the interphase cell shape. In support of this idea, the RasV12 phenotype is reversed by inhibition of actomyosin contractility and can be mimicked by the loss of cell-substrate adhesion during mitosis. Finally, we show that RasV12 activation also perturbs division orientation in cells cultured in 2D epithelial monolayers and 3D spheroids. Thus, the induction of oncogenic Ras-ERK signaling leads to rapid changes in division orientation that, along with the effects of RasV12 on cell growth and cell-cycle progression, are likely to disrupt epithelial tissue organization and contribute to cancer dissemination.

Integrated model of the vertebrate augmin complex

Accurate segregation of chromosomes is required to maintain genome integrity during cell division. This feat is accomplished by the microtubule-based spindle. To build a spindle rapidly and with high fidelity, cells take advantage of branching microtubule nucleation, which rapidly amplifies microtubules during cell division. Branching microtubule nucleation relies on the hetero-octameric augmin complex, but lack of structure information about augmin has hindered understanding how it promotes branching. In this work, we combine cryo-electron microscopy, protein structural prediction, and visualization of fused bulky tags via negative stain electron microscopy to identify the location and orientation of each subunit within the augmin structure. Evolutionary analysis shows that augmin's structure is highly conserved across eukaryotes, and that augmin contains a previously unidentified microtubule binding site. Thus, our findings provide insight into the mechanism of branching microtubule nucleation.

Cigarette smoke condensate induces centrosome clustering in normal lung epithelial cells

BACKGROUND

Unlike normal cells, cancer cells frequently have multiple centrosomes that can cluster to form bipolar mitotic spindles and allow for successful cell division. Inhibiting centrosome clustering, therefore, holds therapeutic promise to promote cancer cell-specific cell death.

METHODS

We used confocal microscopy, real-time PCR, siRNA knockdown, and western blot to analyze centrosome clustering and declustering using normal lung bronchial epithelial and nonsmall-cell lung cancer (NSCLC) cell lines. Also, we used Ingenuity Pathway Analysis software to identify novel pathways associated with centrosome clustering.

RESULTS

In this study, we found that exposure to cigarette smoke condensate induces centrosome amplification and clustering in human lung epithelial cells. We observed a similar increase in centrosome amplification and clustering in unexposed NSCLC cell lines which may suggest a common underlying mechanism for lung carcinogenesis. We identified a cyclin D2-mediated centrosome clustering pathway that involves a sonic hedgehog-forkhead box protein M1 axis which is critical for mitosis. We also observed that cyclin D2 knockdown induced multipolar mitotic spindles that could eventually lead to cell death.

CONCLUSIONS

Here we report a novel role of cyclin D2 in the regulation of centrosome clustering, which could allow the identification of tumors sensitive to cyclin D2 inhibitors. Our data reveal a pathway that can be targeted to inhibit centrosome clustering by interfering with the expression of cyclin D2-associated genes.

TACC3-ch-TOG interaction regulates spindle microtubule assembly by controlling centrosomal recruitment of γ-TuRC

γ-Tubulin ring complex (γ-TuRC), composed of γ-tubulin and multiple γ-tubulin complex proteins (GCPs), serves as the major microtubule nucleating complex in animal cells. However, several γ-TuRC-associated proteins have been shown to control its function. Centrosomal adaptor protein, TACC3, is one such γ-TuRC-interacting factor that is essential for proper mitotic spindle assembly across organisms. ch-TOG is another microtubule assembly promoting protein, which interacts with TACC3 and cooperates in mitotic spindle assembly. However, the mechanism how TACC3-ch-TOG interaction regulates microtubule assembly and the γ-TuRC functions at the centrosomes remain unclear. Here, we show that deletion of the ch-TOG-binding region in TACC3 enhances recruitment of the γ-TuRC proteins to centrosomes and aggravates spindle microtubule assembly in human cells. Loss of TACC3-ch-TOG binding imparts stabilization on TACC3 interaction with the γ-TuRC proteins and it does so by stimulating TACC3 phosphorylation and thereby enhancing phospho-TACC3 recruitment to the centrosomes. We also show that localization of ch-TOG at the centrosomes is substantially reduced and the same on the spindle microtubules is increased in its TACC3-unbound condition. Additional results reveal that ch-TOG depletion stimulates γ-tubulin localization on the spindles without significantly affecting the centrosomal γ-tubulin level. The results indicate that ch-TOG binding to TACC3 controls TACC3 phosphorylation and TACC3-mediated stabilization of the γ-TuRCs at the centrosomes. They also implicate that the spatio-temporal control of TACC3 phosphorylation via ch-TOG-binding ensures mitotic spindle assembly to the optimal level.

Matrin3 regulates mitotic spindle dynamics by controlling alternative splicing of CDC14B

Matrin3 is an RNA-binding protein that regulates diverse RNA-related processes, including mRNA splicing. Although Matrin3 has been intensively studied in neurodegenerative diseases, its function in cancer remains unclear. Here, we report Matrin3-mediated regulation of mitotic spindle dynamics in colorectal cancer (CRC) cells. We comprehensively identified RNAs bound and regulated by Matrin3 in CRC cells and focused on CDC14B, one of the top Matrin3 targets. Matrin3 knockdown results in increased inclusion of an exon containing a premature termination codon in the CDC14B transcript and simultaneous down-regulation of the standard CDC14B transcript. Knockdown of CDC14B phenocopies the defects in mitotic spindle dynamics upon Matrin3 knockdown, and the elongated and misoriented mitotic spindle observed upon Matrin3 knockdown are rescued upon overexpression of CDC14B, suggesting that CDC14B is a key downstream effector of Matrin3. Collectively, these data reveal a role for the Matrin3/CDC14B axis in control of mitotic spindle dynamics.

Aurora kinases: Generators of spatial control during mitosis

Cell division events require regulatory systems to ensure that events happen in a distinct order. The classic view of temporal control of the cell cycle posits that cells order events by linking them to changes in Cyclin Dependent Kinase (CDK) activities. However, a new paradigm is emerging from studies of anaphase where chromatids separate at the central metaphase plate and then move to opposite poles of the cell. These studies suggest that distinct events are ordered depending upon the location of each chromosome along its journey from the central metaphase plate to the elongated spindle poles. This system is dependent upon a gradient of Aurora B kinase activity that emerges during anaphase and acts as a spatial beacon to control numerous anaphase/telophase events and cytokinesis. Recent studies also suggest that Aurora A kinase activity specifies proximity of chromosomes or proteins to spindle poles during prometaphase. Together these studies argue that a key role for Aurora kinases is to provide spatial information that controls events depending upon the location of chromosomes or proteins along the mitotic spindle.

Mitotic outcomes and errors in fibrous environments

During mitosis, cells round up and utilize the interphase adhesion sites within the fibrous extracellular matrix (ECM) as guidance cues to orient the mitotic spindles. Here, using suspended ECM-mimicking nanofiber networks, we explore mitotic outcomes and error distribution for various interphase cell shapes. Elongated cells attached to single fibers through two focal adhesion clusters (FACs) at their extremities result in perfect spherical mitotic cell bodies that undergo significant 3-dimensional (3D) displacement while being held by retraction fibers (RFs). Increasing the number of parallel fibers increases FACs and retraction fiber-driven stability, leading to reduced 3D cell body movement, metaphase plate rotations, increased interkinetochore distances, and significantly faster division times. Interestingly, interphase kite shapes on a crosshatch pattern of four fibers undergo mitosis resembling single-fiber outcomes due to rounded bodies being primarily held in position by RFs from two perpendicular suspended fibers. We develop a cortex-astral microtubule analytical model to capture the retraction fiber dependence of the metaphase plate rotations. We observe that reduced orientational stability, on single fibers, results in increased monopolar mitotic defects, while multipolar defects become dominant as the number of adhered fibers increases. We use a stochastic Monte Carlo simulation of centrosome, chromosome, and membrane interactions to explain the relationship between the observed propensity of monopolar and multipolar defects and the geometry of RFs. Overall, we establish that while bipolar mitosis is robust in fibrous environments, the nature of division errors in fibrous microenvironments is governed by interphase cell shapes and adhesion geometries.

SH2D4A promotes centrosome maturation to support spindle microtubule formation and mitotic progression

Mitotic progression requires the precise formation of spindle microtubules based on mature centrosomes. During the G2/M transition, centrosome maturation progresses, and associated microtubules bundle to form mitotic spindle fibers and capture the chromosomes for alignment at the cell equator. Mitotic kinases-induced phosphorylation signaling is necessary for these processes. Here, we identified SH2 domain-containing protein 4A (SH2D4A/PPP1R38) as a new mitotic regulator. SH2D4A knockdown delays mitotic progression. The time-lapse imaging analysis showed that SH2D4A specifically contributes to the alignment of chromosomes. The cold treatment assay and microtubule regrowth assay indicated that SH2D4A promotes microtubule nucleation to support kinetochore-microtubule attachment. This may be due to the centrosome maturation by SH2D4A via centrosomal recruitment of pericentriolar material (PCM) such as cep192, γ-tubulin, and PLK1. SH2D4A was found to be a negative regulator of PP1 phosphatase. Consistently, treatment with a PP1 inhibitor rescues SH2D4A-knockdown-induced phenotypes, including the microtubule nucleation and centrosomal recruitment of active PLK1. These results suggest that SH2D4A is involved in PCM recruitment to centrosomes and centrosome maturation through attenuation of PP1 phosphatases, accelerating the spindle formation and supporting mitotic progression.

CTCF is essential for proper mitotic spindle structure and anaphase segregation

Mitosis is an essential process in which the duplicated genome is segregated equally into two daughter cells. CTCF has been reported to be present in mitosis but its importance for mitotic fidelity remains to be determined. To evaluate the importance of CTCF in mitosis, we tracked mitotic behaviors in wild type and two different CTCF CRISPR-based genetic knockdowns. We find that knockdown of CTCF results in prolonged mitoses and failed anaphase segregation via time lapse imaging of SiR-DNA. CTCF knockdown did not alter cell cycling or the mitotic checkpoint, which was activated upon nocodazole treatment. Immunofluorescence imaging of the mitotic spindle in CTCF knockdowns revealed disorganization via tri/tetrapolar spindles and chromosomes behind the spindle pole. Imaging of interphase nuclei showed that nuclear size increased drastically, consistent with failure to divide the duplicated genome in anaphase. Population measurements of nuclear shape in CTCF knockdowns do not display decreased circularity or increased nuclear blebbing relative to wild type. However, failed mitoses do display abnormal nuclear morphologies relative to successful mitoses, suggesting population images do not capture individual behaviors. Thus, CTCF is important for both proper metaphase organization and anaphase segregation which impacts the size and shape of the interphase nucleus.

Inhibition of acetyl-CoA carboxylase impaired tubulin palmitoylation and induced spindle abnormalities

Tubulin s-palmitoylation involves the thioesterification of a cysteine residue in tubulin with palmitate. The palmitate moiety is produced by the fatty acid synthesis pathway, which is rate-limited by acetyl-CoA carboxylase (ACC). While it is known that ACC is phosphorylated at serine 79 (pSer⁷⁹) by AMPK and accumulates at the spindle pole (SP) during mitosis, a functional role for tubulin palmitoylation during mitosis has not been identified. In this study, we found that modulating pSer⁷⁹-ACC level at the SP using AMPK agonist and inhibitor induced spindle defects. Loss of ACC function induced spindle abnormalities in cell lines and in germ cells of the Drosophila germarium, and palmitic acid (PA) rescued the spindle defects in the cell line treated transiently with the ACC inhibitor, TOFA. Furthermore, inhibition of protein palmitoylating or depalmitoylating enzymes also induced spindle defects. Together, these data suggested that precisely regulated cellular palmitate level and protein palmitoylation may be required for accurate spindle assembly. We then showed that tubulin was largely palmitoylated in interphase cells but less palmitoylated in mitotic cells. TOFA treatment diminished tubulin palmitoylation at doses that disrupt microtubule (MT) instability and cause spindle defects. Moreover, spindle MTs comprised of α-tubulins mutated at the reported palmitoylation site exhibited disrupted dynamic instability. We also found that TOFA enhanced the MT-targeting drug-induced spindle abnormalities and cytotoxicity. Thus, our study reveals that precise regulation of ACC during mitosis impacts tubulin palmitoylation to delicately control MT dynamic instability and spindle assembly, thereby safeguarding nuclear and cell division.

Kinetochore- and chromosome-driven transition of microtubules into bundles promotes spindle assembly

Mitotic spindle assembly is crucial for chromosome segregation and relies on bundles of microtubules that extend from the poles and overlap in the middle. However, how these structures form remains poorly understood. Here we show that overlap bundles arise through a network-to-bundles transition driven by kinetochores and chromosomes. STED super-resolution microscopy reveals that PRC1-crosslinked microtubules initially form loose arrays, which become rearranged into bundles. Kinetochores promote microtubule bundling by lateral binding via CENP-E/kinesin-7 in an Aurora B-regulated manner. Steric interactions between the bundle-associated chromosomes at the spindle midplane drive bundle separation and spindle widening. In agreement with experiments, theoretical modeling suggests that bundles arise through competing attractive and repulsive mechanisms. Finally, perturbation of overlap bundles leads to inefficient correction of erroneous kinetochore-microtubule attachments. Thus, kinetochores and chromosomes drive coarsening of a uniform microtubule array into overlap bundles, which promote not only spindle formation but also chromosome segregation fidelity.

Chromosome segregation fidelity requires microtubule polyglutamylation by the cancer downregulated enzyme TTLL11

Regulation of microtubule (MT) dynamics is key for mitotic spindle assembly and faithful chromosome segregation. Here we show that polyglutamylation, a still understudied post-translational modification of spindle MTs, is essential to define their dynamics within the range required for error-free chromosome segregation. We identify TTLL11 as an enzyme driving MT polyglutamylation in mitosis and show that reducing TTLL11 levels in human cells or zebrafish embryos compromises chromosome segregation fidelity and impairs early embryonic development. Our data reveal a mechanism to ensure genome stability in normal cells that is compromised in cancer cells that systematically downregulate TTLL11. Our data suggest a direct link between MT dynamics regulation, MT polyglutamylation and two salient features of tumour cells, aneuploidy and chromosome instability (CIN).

Non-transport roles of nuclear import receptors: In need of the right balance

Nuclear import receptors ensure the recognition and transport of proteins across the nuclear envelope into the nucleus. In addition, as diverse processes as mitosis, post-translational modifications at mitotic exit, ciliogenesis, and phase separation, all share a common need for regulation by nuclear import receptors - particularly importin beta-1 and importin beta-2/transportin - independent on nuclear import. In particular, 1) nuclear import receptors regulate the mitotic spindle after nuclear envelope breakdown, 2) they shield cargoes from unscheduled ubiquitination, regulating their timely proteolysis; 3) they regulate ciliary factors, crucial to cell communications and tissue architecture during development; and 4) they prevent phase separation of toxic proteins aggregates in neurons. The balance of nuclear import receptors to cargoes is critical in all these processes, albeit in opposite directions: overexpression of import receptors, as often found in cancer, inhibits cargoes and impairs downstream processes, motivating the therapeutic design of specific inhibitors. On the contrary, elevated expression is beneficial in neuronal contexts, where nuclear import receptors are regarded as potential therapeutic tools in counteracting the formation of aggregates that may cause neurodegeneration. This paradox demonstrates the amplitude of nuclear import receptors-dependent functions in different contexts and adds complexity in considering their therapeutic implications.

Diversity is the spice of life: An overview of how cytokinesis regulation varies with cell type

Cytokinesis is required to physically cleave a cell into two daughters at the end of mitosis. Decades of research have led to a comprehensive understanding of the core cytokinesis machinery and how it is regulated in animal cells, however this knowledge was generated using single cells cultured in vitro, or in early embryos before tissues develop. This raises the question of how cytokinesis is regulated in diverse animal cell types and developmental contexts. Recent studies of distinct cell types in the same organism or in similar cell types from different organisms have revealed striking differences in how cytokinesis is regulated, which includes different threshold requirements for the structural components and the mechanisms that regulate them. In this review, we highlight these differences with an emphasis on pathways that are independent of the mitotic spindle, and operate through signals associated with the cortex, kinetochores, or chromatin.

Augmin prevents merotelic attachments by promoting proper arrangement of bridging and kinetochore fibers

The human mitotic spindle is made of microtubules nucleated at centrosomes, at kinetochores, and from pre-existing microtubules by the augmin complex. However, it is unknown how the augmin-mediated nucleation affects distinct microtubule classes and thereby mitotic fidelity. Here, we use superresolution microscopy to analyze the previously indistinguishable microtubule arrangements within the crowded metaphase plate area and demonstrate that augmin is vital for the formation of uniformly arranged parallel units consisting of sister kinetochore fibers connected by a bridging fiber. This ordered geometry helps both prevent and resolve merotelic attachments. Whereas augmin-nucleated bridging fibers prevent merotelic attachments by creating a nearly parallel and highly bundled microtubule arrangement unfavorable for creating additional attachments, augmin-nucleated k-fibers produce robust force required to resolve errors during anaphase. STED microscopy revealed that bridging fibers were impaired twice as much as k-fibers following augmin depletion. The complete absence of bridging fibers from a significant portion of kinetochore pairs, especially in the inner part of the spindle, resulted in the specific reduction of the interkinetochore distance. Taken together, we propose a model where augmin promotes mitotic fidelity by generating assemblies consisting of bridging and kinetochore fibers that align sister kinetochores to face opposite poles, thereby preventing erroneous attachments.

Chromosome instability in neuroblastoma: A pathway to aggressive disease

For over 100-years, genomic instability has been investigated as a central player in the pathogenesis of human cancer. Conceptually, genomic instability includes an array of alterations from small deletions/insertions to whole chromosome alterations, referred to as chromosome instability. Chromosome instability has a paradoxical impact in cancer. In most instances, the introduction of chromosome instability has a negative impact on cellular fitness whereas in cancer it is usually associated with a worse prognosis. One exception is the case of neuroblastoma, the most common solid tumor outside of the brain in children. Neuroblastoma tumors have two distinct patterns of genome instability: whole-chromosome aneuploidy, which is associated with a better prognosis, or segmental chromosomal alterations, which is a potent negative prognostic factor. Through a computational screen, we found that low levels of the de- ubiquitinating enzyme USP24 have a highly significant negative impact on survival in neuroblastoma. At the molecular level, USP24 loss leads to destabilization of the microtubule assembly factor CRMP2 - producing mitotic errors and leading to chromosome missegregation and whole-chromosome aneuploidy. This apparent paradox may be reconciled through a model in which whole chromosome aneuploidy leads to the subsequent development of segmental chromosome alterations. Here we review the mechanisms behind chromosome instability and the evidence for the progressive development of segmental alterations from existing numerical aneuploidy in support of a multi-step model of neuroblastoma progression.

Molecular architecture of the augmin complex

Accurate segregation of chromosomes during mitosis depends on the correct assembly of the mitotic spindle, a bipolar structure composed mainly of microtubules. The augmin complex, or homologous to augmin subunits (HAUS) complex, is an eight-subunit protein complex required for building robust mitotic spindles in metazoa. Augmin increases microtubule density within the spindle by recruiting the γ-tubulin ring complex (γ-TuRC) to pre-existing microtubules and nucleating branching microtubules. Here, we elucidate the molecular architecture of augmin by single particle cryo-electron microscopy (cryo-EM), computational methods, and crosslinking mass spectrometry (CLMS). Augmin's highly flexible structure contains a V-shaped head and a filamentous tail, with the head existing in either extended or contracted conformational states. Our work highlights how cryo-EM, complemented by computational advances and CLMS, can elucidate the structure of a challenging protein complex and provides insights into the function of augmin in mediating microtubule branching nucleation.

Separate To Operate: the Centriole-Free Inner Core of the Centrosome Regulates the Assembly of the Intranuclear Spindle in Toxoplasma gondii

Centrosomes are the main microtubule-organizing center of the cell. They are normally formed by two centrioles, embedded in a cloud of proteins known as pericentriolar material (PCM). The PCM ascribes centrioles with their microtubule nucleation capacity. Toxoplasma gondii, the causative agent of toxoplasmosis, divides by endodyogeny. Successful cell division is critical for pathogenesis. The centrosome, one of the microtubule organizing centers of the cell, plays central roles in orchestrating the temporal and physical coordination of major organelle segregation and daughter cell formation during endodyogeny. The Toxoplasma centrosome is constituted by multiple domains: an outer core, distal from the nucleus; a middle core; and an inner core, proximal to the nucleus. This modular organization has been proposed to underlie T. gondii's cell division plasticity. However, the role of the inner core remains undeciphered. Here, we focus on understanding the function of the inner core by finely studying the localization and role of its only known molecular marker; TgCep250L1. We show that upon conditional degradation of TgCep250L1 parasites are unable to survive. Mutants exhibit severe nuclear segregation defects. In addition, the rest of the centrosome, defined by the position of the centrioles, disconnects from the nucleus. We explore the structural defects underlying these phenotypes by ultrastructure expansion microscopy. We show that TgCep250L1's location changes with respect to other markers, and these changes encompass the formation of the mitotic spindle. Moreover, we show that in the absence of TgCep250L1, the microtubule binding protein TgEB1, fails to localize at the mitotic spindle, while unsegregated nuclei accumulate at the residual body. Overall, our data support a model in which the inner core of the T. gondii centrosome critically participates in cell division by directly impacting the formation or stability of the mitotic spindle. IMPORTANCE Toxoplasma gondii parasites cause toxoplasmosis, arguably the most widespread and prevalent parasitosis of humans and animals. During the clinically relevant stage of its life cycle, the parasites divide by endodyogeny. In this mode of division, the nucleus, containing loosely packed chromatin and a virtually intact nuclear envelope, parcels into two daughter cells generated within a common mother cell cytoplasm. The centrosome is a microtubule-organizing center critical for orchestrating the multiple simultaneously occurring events of endodyogeny. It is organized in two distinct domains: the outer and inner cores. We demonstrate here that the inner core protein TgCEP250L1 is required for replication of T. gondii. Lack of TgCEP250L1 renders parasites able to form daughter cells, while unable to segregate their nuclei. We determine that, in the absence of TgCEP250L1, the mitotic spindle, which is responsible for karyokinesis, does not assemble. Our results support a role for the inner core in nucleation or stabilization of the mitotic spindle in T. gondii.

Length-dependent poleward flux of sister kinetochore fibers promotes chromosome alignment

Chromosome alignment at the spindle equator promotes proper chromosome segregation and depends on pulling forces exerted at kinetochore fiber tips together with polar ejection forces. However, kinetochore fibers are also subjected to forces driving their poleward flux. Here we introduce a flux-driven centering model that relies on flux generated by forces within the overlaps of bridging and kinetochore fibers. This centering mechanism works so that the longer kinetochore fiber fluxes faster than the shorter one, moving the kinetochores toward the center. We develop speckle microscopy in human spindles and confirm the key prediction that kinetochore fiber flux is length dependent. Kinetochores are better centered when overlaps are shorter and the kinetochore fiber flux slower than the bridging fiber flux. We identify Kif18A and Kif4A as overlap and flux regulators and NuMA as a fiber coupler. Thus, length-dependent sliding forces exerted by the bridging fiber onto kinetochore fibers support chromosome alignment.

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A flux-driven centering model explains chromosome alignment at the spindle equator

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Kinetochore fiber poleward flux is driven by overlap microtubules

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Kinetochore centering requires faster flux of the longer sister kinetochore fiber

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Chromosome alignment depends on the overlap length of bridging microtubules

Centrosome function is critical during terminal erythroid differentiation

Red blood cells are produced by terminal erythroid differentiation, which involves the dramatic morphological transformation of erythroblasts into enucleated reticulocytes. Microtubules are important for enucleation, but it is not known if the centrosome, a key microtubule-organizing center, is required as well. Mice lacking the conserved centrosome component, CDK5RAP2, are likely to have defective erythroid differentiation because they develop macrocytic anemia. Here, we show that fetal liver-derived, CDK5RAP2-deficient erythroid progenitors generate fewer and larger reticulocytes, hence recapitulating features of macrocytic anemia. In erythroblasts, but not in embryonic fibroblasts, loss of CDK5RAP2 or pharmacological depletion of centrosomes leads to highly aberrant spindle morphologies. Consistent with such cells exiting mitosis without chromosome segregation, tetraploidy is frequent in late-stage erythroblasts, thereby giving rise to fewer but larger reticulocytes than normal. Our results define a critical role for CDK5RAP2 and centrosomes in spindle formation specifically during blood production. We propose that disruption of centrosome and spindle function could contribute to the emergence of macrocytic anemias, for instance, due to nutritional deficiency or exposure to chemotherapy.

Aggressive uveal melanoma displays a high degree of centrosome amplification, opening the door to therapeutic intervention

Uveal melanoma (UM) is the most common intraocular cancer in adults. Whilst treatment of primary UM (PUM) is often successful, around 50% of patients develop metastatic disease with poor outcomes, linked to chromosome 3 loss (monosomy 3, M3). Advances in understanding UM cell biology may indicate new therapeutic options. We report that UM exhibits centrosome abnormalities, which in other cancers are associated with increased invasiveness and worse prognosis, but also represent a potential Achilles' heel for cancer-specific therapeutics. Analysis of 75 PUM patient samples revealed both higher centrosome numbers and an increase in centrosomes with enlarged pericentriolar matrix (PCM) compared to surrounding normal tissue, both indicative of centrosome amplification. The PCM phenotype was significantly associated with M3 (t-test, p < 0.01). Centrosomes naturally enlarge as cells approach mitosis; however, whilst UM with higher mitotic scores had enlarged PCM regardless of genetic status, the PCM phenotype remained significantly associated with M3 in UM with low mitotic scores (ANOVA, p = 0.021) suggesting that this is independent of proliferation. Phenotypic analysis of patient-derived cultures and established UM lines revealed comparable levels of centrosome amplification in PUM cells to archetypal triple-negative breast cancer cell lines, whilst metastatic UM (MUM) cell lines had even higher levels. Importantly, many UM cells also exhibit centrosome clustering, a common strategy employed by other cancer cells with centrosome amplification to survive cell division. As UM samples with M3 display centrosome abnormalities indicative of amplification, this phenotype may contribute to the development of MUM, suggesting that centrosome de-clustering drugs may provide a novel therapeutic approach.

Precise control of microtubule disassembly in living cells

Microtubules tightly regulate various cellular activities. Our understanding of microtubules is largely based on experiments using microtubule‐targeting agents, which, however, are insufficient to dissect the dynamic mechanisms of specific microtubule populations, due to their slow effects on the entire pool of microtubules. To overcome this technological limitation, we have used chemo and optogenetics to disassemble specific microtubule subtypes, including tyrosinated microtubules, primary cilia, mitotic spindles, and intercellular bridges, by rapidly recruiting engineered microtubule‐cleaving enzymes onto target microtubules in a reversible manner. Using this approach, we show that acute microtubule disassembly swiftly halts vesicular trafficking and lysosomal dynamics. It also immediately triggers Golgi and ER reorganization and slows the fusion/fission of mitochondria without affecting mitochondrial membrane potential. In addition, cell rigidity is increased after microtubule disruption owing to increased contractile stress fibers. Microtubule disruption furthermore prevents cell division, but does not cause cell death during interphase. Overall, the reported tools facilitate detailed analysis of how microtubules precisely regulate cellular architecture and functions.

Spindle motility skews division site determination during asymmetric cell division in Physcomitrella

Asymmetric cell division (ACD) underlies the development of multicellular organisms. In animal ACD, the cell division site is determined by active spindle-positioning mechanisms. In contrast, it is considered that the division site in plants is determined prior to mitosis by the microtubule-actin belt known as the preprophase band (PPB) and that the localization of the mitotic spindle is typically static and does not govern the division plane. However, in some plant species, ACD occurs in the absence of PPB. Here, we isolate a hypomorphic mutant of the conserved microtubule-associated protein TPX2 in the moss Physcomitrium patens (Physcomitrella) and observe spindle motility during PPB-independent cell division. This defect compromises the position of the division site and produces inverted daughter cell sizes in the first ACD of gametophore (leafy shoot) development. The phenotype is rescued by restoring endogenous TPX2 function and, unexpectedly, by depolymerizing actin filaments. Thus, we identify an active spindle-positioning mechanism that, reminiscent of acentrosomal ACD in animals, involves microtubules and actin filaments, and sets the division site in plants.

Catastrophic DNA replication in unscheduled tetraploid cells

Unscheduled tetraploidy is a metastable state that rapidly evolves into aneuploidy. Recent findings reported by Gemble et al. demonstrate that freshly formed tetraploid cells fail to accumulate the required amounts of DNA replication factors during the first G₁ phase after whole-genome duplication (WGD), culminating in genetic instability in the subsequent S phase and extensive karyotypic alterations.

Augmin-dependent microtubule self-organization drives kinetochore fiber maturation in mammals

Chromosome segregation in mammals relies on the maturation of a thick bundle of kinetochore-attached microtubules known as k-fiber. How k-fibers mature from initial kinetochore microtubule attachments remains a fundamental question. By combining molecular perturbations and phenotypic analyses in Indian muntjac fibroblasts containing the lowest known diploid chromosome number in mammals (2N = 6) and distinctively large kinetochores, with fixed/live-cell super-resolution coherent-hybrid stimulated emission depletion (CH-STED) nanoscopy and laser microsurgery, we demonstrate a key role for augmin in kinetochore microtubule self-organization and maturation, regardless of pioneer centrosomal microtubules. In doing so, augmin promotes kinetochore and interpolar microtubule turnover and poleward flux. Tracking of microtubule growth events within individual k-fibers reveals a wide angular dispersion, consistent with augmin-mediated branched microtubule nucleation. Augmin depletion reduces the frequency of kinetochore microtubule growth events and hampers efficient repair after acute k-fiber injury by laser microsurgery. Together, these findings underscore the contribution of augmin-mediated microtubule amplification for k-fiber self-organization and maturation in mammals.

Mitotic spindle formation in the absence of Polo kinase

SignificanceMitosis is an essential process in all eukaryotes, but paradoxically, genes required for mitosis vary among species. The essentiality of many mitotic genes was bypassed by activating alternative mechanisms during evolution. However, bypass events have rarely been recapitulated experimentally. Here, using the fission yeast Schizosaccharomyces pombe, the essentiality of a kinase (Plo1) required for bipolar spindle formation was bypassed by other mutations, many of which are associated with glucose metabolism. The Plo1 bypass by the reduction in glucose uptake was dependent on another kinase (casein kinase I), which potentiated spindle microtubule formation. This study illustrates a rare experimental bypass of essentiality for mitotic genes and provides insights into the molecular diversity of mitosis.

Contribution of cytoplasm viscoelastic properties to mitotic spindle positioning

The regulation of mitotic spindle positioning is a key process for tissue architecture, embryo development, and stem cells. To date, most models have assumed that spindles are positioned by forces exerted by polar cytoskeleton networks, like microtubule asters or actomyosin bundles. Here, using in situ magnetic tweezers to apply calibrated forces and torques to mitotic spindles in live dividing sea urchin cells, we found that the viscoelastic properties of the cytoplasm medium in which spindles are embedded can hold spindles in place and move them back if their original position is perturbed. These viscoelastic forces are large and may significantly participate in the force balance that position and orient mitotic spindles in many cell types.

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics, and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division, remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original positions. These forces are independent of cytoskeletal force generators yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization.

Phylogenetic convergence of phase separation and mitotic function in the disordered protein BuGZ

Intrinsically disordered proteins (IDPs) effect biological function despite their sequence-encoded lack of preference for stable three-dimensional structure. Among their many functions, IDPs form membraneless cellular compartments through liquid-liquid phase separation (LLPS), also termed biomolecular condensation. The extent to which LLPS has been evolutionarily selected remains largely unknown, as the complexities of IDP evolution hamper progress. Unlike structured proteins, rapid sequence divergence typical of IDPs confounds inference of their biophysical or biological functions from comparative sequence analyses. Here, we leverage mitosis as a universal eukaryotic feature to interrogate condensate evolutionary history. We observe that evolution has conserved the ability for six homologs of the mitotic IDP BuGZ to undergo LLPS and to serve the same mitotic function, despite low sequence conservation. We also observe that cellular context may tune LLPS. The phylogenetic correlation of LLPS and mitotic function in one protein raises the possibility of an ancient evolutionary interplay between LLPS and biological function, dating back at least 1.6 billion years to the last common ancestor of plants and animals. This article is protected by copyright. All rights reserved.

Complementation of fission yeast kinesin-5/Cut7 with human Eg5 provides a versatile platform for screening of anti-cancer compounds

Kinesin-5 family proteins are essential for bipolar spindle assembly to ensure mitotic fidelity. Here we demonstrate evolutionary functional conservation of kinesin-5 between human and fission yeast. Human Eg5 expressed in the nucleus replaces fission yeast counterpart Cut7. Intriguingly, Eg5 overproduction results in cytotoxicity. This phenotype provides a useful platform for the development of novel kinesin-5 inhibitors as anti-cancer drugs.

TH588 and Low-Dose Nocodazole Impair Chromosome Congression by Suppressing Microtubule Turnover within the Mitotic Spindle

Simple Summary

A promising anti-cancer compound TH588 has been recently identified as a microtubule-targeting agent that inhibits tubulin polymerization in vitro and interferes with microtubule dynamics in interphase cells. Although it was shown to arrest cells in mitosis, its effect on microtubule dynamics in dividing cells remained unknown. By analyzing microtubule dynamics in living cells treated with either TH588 or low-dose nocodazole, we revealed that both of these drugs stabilize microtubules within the mitotic spindle, leading to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes. This causes mitotic arrest, ultimately resulting in cell death or cell division with uncongressed chromosomes. Both of these cell fates could contribute to the selective effect associated with the activity of TH588 in cancer cells.

Abstract

Microtubule-targeting agents (MTAs) have been used for decades to treat different hematologic and solid cancers. The mode of action of these drugs mainly relies on their ability to bind tubulin subunits and/or microtubules and interfere with microtubule dynamics. In addition to its MTH1-inhibiting activity, TH588 has been recently identified as an MTA, whose anticancer properties were shown to largely depend on its microtubule-targeting ability. Although TH588 inhibited tubulin polymerization in vitro and reduced microtubule plus-end mobility in interphase cells, its effect on microtubule dynamics within the mitotic spindle of dividing cells remained unknown. Here, we performed an in-depth analysis of the impact of TH588 on spindle-associated microtubules and compared it to the effect of low-dose nocodazole. We show that both treatments reduce microtubule turnover within the mitotic spindle. This microtubule-stabilizing effect leads to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes, which consequently cannot be transported to the cell equator, thereby delaying cell division and leading to cell death or division with uncongressed chromosomes.

Automated tracking of S. pombe spindle elongation dynamics

The mitotic spindle is a microtubule-based machine that pulls the two identical sets of chromosomes to opposite ends of the cell during cell division. The fission yeast Schizosaccharomyces pombe is an important model organism for studying mitosis due to its simple, stereotyped spindle structure and well-established genetic toolset. S. pombe spindle length is a useful metric for mitotic progression, but manually tracking spindle ends in each frame to measure spindle length over time is laborious and can limit experimental throughput. We have developed an ImageJ plugin that can automatically track S. pombe spindle length over time and replace manual or semi-automated tracking of spindle elongation dynamics. Using an algorithm that detects the principal axis of the spindle and then finds its ends, we reliably track the length of the spindle as the cell divides. The plugin integrates with existing ImageJ features, exports its data for further analysis outside of ImageJ and does not require any programming by the user. Thus, the plugin provides an accessible tool for quantification of S. pombe spindle length that will allow automatic analysis of large microscopy data sets and facilitate screening for effects of cell biological perturbations on mitotic progression.

Intracellular functions and motile properties of bi-directional kinesin-5 Cin8 are regulated by neck linker docking

In this study, we analyzed intracellular functions and motile properties of neck-linker (NL) variants of the bi-directional S. cerevisiae kinesin-5 motor, Cin8. We also examined – by modeling – the configuration of H-bonds during NL docking. Decreasing the number of stabilizing H-bonds resulted in partially functional variants, as long as a conserved backbone H-bond at the N-latch position (proposed to stabilize the docked conformation of the NL) remained intact. Elimination of this conserved H-bond resulted in production of a non-functional Cin8 variant. Surprisingly, additional H-bond stabilization of the N-latch position, generated by replacement of the NL of Cin8 by sequences of the plus-end directed kinesin-5 Eg5, also produced a nonfunctional variant. In that variant, a single replacement of N-latch asparagine with glycine, as present in Cin8, eliminated the additional H-bond stabilization and rescued the functional defects. We conclude that exact N-latch stabilization during NL docking is critical for the function of bi-directional kinesin-5 Cin8.

M2 Muscarinic Receptor Activation Impairs Mitotic Progression and Bipolar Mitotic Spindle Formation in Human Glioblastoma Cell Lines

Background: Glioblastoma multiforme (GBM) is characterized by several genetic abnormalities, leading to cell cycle deregulation and abnormal mitosis caused by a defective checkpoint. We previously demonstrated that arecaidine propargyl ester (APE), an orthosteric agonist of M2 muscarinic acetylcholine receptors (mAChRs), arrests the cell cycle of glioblastoma (GB) cells, reducing their survival. The aim of this work was to better characterize the molecular mechanisms responsible for this cell cycle arrest. Methods: The arrest of cell proliferation was evaluated by flow cytometry analysis. Using immunocytochemistry and time-lapse analysis, the percentage of abnormal mitosis and aberrant mitotic spindles were assessed in both cell lines. Western blot analysis was used to evaluate the modulation of Sirtuin2 and acetylated tubulin—factors involved in the control of cell cycle progression. Results: APE treatment caused arrest in the M phase, as indicated by the increase in p-HH3 (ser10)-positive cells. By immunocytochemistry, we found a significant increase in abnormal mitoses and multipolar mitotic spindle formation after APE treatment. Time-lapse analysis confirmed that the APE-treated GB cells were unable to correctly complete the mitosis. The modulated expression of SIRT2 and acetylated tubulin in APE-treated cells provides new insights into the mechanisms of altered mitotic progression in both GB cell lines. Conclusions: Our data show that the M2 agonist increases aberrant mitosis in GB cell lines. These results strengthen the idea of considering M2 acetylcholine receptors a novel promising therapeutic target for the glioblastoma treatment.

Mitotic poleward flux: Finding balance between microtubule dynamics and sliding

Continuous poleward motion of microtubules in metazoan mitotic spindles has been fascinating generations of cell biologists over the last several decades. In human cells, this so-called poleward flux was recently shown to be driven by the coordinated action of four mitotic kinesins. The sliding activities of kinesin-5/EG5 and kinesin-12/KIF15 are sequentially supported by kinesin-7/CENP-E at kinetochores and kinesin-4/KIF4A on chromosome arms, with the individual contributions peaking during prometaphase and metaphase, respectively. Although recent data elucidate the molecular mechanism underlying this cellular phenomenon, the functional roles of microtubule poleward flux during cell division remain largely elusive. Here, we discuss potential contribution of microtubule flux engine to various essential processes at different stages of mitosis.

Kinesin-6 Klp9 orchestrates spindle elongation by regulating microtubule sliding and growth

Mitotic spindle function depends on the precise regulation of microtubule dynamics and microtubule sliding. Throughout mitosis, both processes have to be orchestrated to establish and maintain spindle stability. We show that during anaphase B spindle elongation in Schizosaccharomyces pombe, the sliding motor Klp9 (kinesin-6) also promotes microtubule growth in vivo. In vitro, Klp9 can enhance and dampen microtubule growth, depending on the tubulin concentration. This indicates that the motor is able to promote and block tubulin subunit incorporation into the microtubule lattice in order to set a well-defined microtubule growth velocity. Moreover, Klp9 recruitment to spindle microtubules is dependent on its dephosphorylation mediated by XMAP215/Dis1, a microtubule polymerase, creating a link between the regulation of spindle length and spindle elongation velocity. Collectively, we unravel the mechanism of anaphase B, from Klp9 recruitment to the motors dual-function in regulating microtubule sliding and microtubule growth, allowing an inherent coordination of both processes.

The Putative RNA-Binding Protein Dri1 Promotes the Loading of Kinesin-14/Klp2 to the Mitotic Spindle and Is Sequestered into Heat-Induced Protein Aggregates in Fission Yeast

Cells form a bipolar spindle during mitosis to ensure accurate chromosome segregation. Proper spindle architecture is established by a set of kinesin motors and microtubule-associated proteins. In most eukaryotes, kinesin-5 motors are essential for this process, and genetic or chemical inhibition of their activity leads to the emergence of monopolar spindles and cell death. However, these deficiencies can be rescued by simultaneous inactivation of kinesin-14 motors, as they counteract kinesin-5. We conducted detailed genetic analyses in fission yeast to understand the mechanisms driving spindle assembly in the absence of kinesin-5. Here, we show that deletion of the dri1 gene, which encodes a putative RNA-binding protein, can rescue temperature sensitivity caused by cut7-22, a fission yeast kinesin-5 mutant. Interestingly, kinesin-14/Klp2 levels on the spindles in the cut7 mutants were significantly reduced by the dri1 deletion, although the total levels of Klp2 and the stability of spindle microtubules remained unaffected. Moreover, RNA-binding motifs of Dri1 are essential for its cytoplasmic localization and function. We have also found that a portion of Dri1 is spatially and functionally sequestered by chaperone-based protein aggregates upon mild heat stress and limits cell division at high temperatures. We propose that Dri1 might be involved in post-transcriptional regulation through its RNA-binding ability to promote the loading of Klp2 on the spindle microtubules.

Mdm2-Mediated Downmodulation of GRK2 Restricts Centrosome Separation for Proper Chromosome Congression

The timing of centrosome separation and the distance moved apart influence the formation of the bipolar spindle, affecting chromosome stability. Epidermal growth factor receptor (EGFR) signaling induces early centrosome separation through downstream G protein-coupled receptor kinase GRK2, which phosphorylates the Hippo pathway component MST2 (Mammalian STE20-like protein kinase 2), in turn allowing NIMA kinase Nek2A activation for centrosomal linker disassembly. However, the mechanisms that counterbalance centrosome disjunction and separation remain poorly understood. We unveil that timely degradation of GRK2 by the E3 ligase Mdm2 limits centrosome separation in the G2. Both knockout expression and catalytic inhibition of Mdm2 result in GRK2 accumulation and enhanced centrosome separation before mitosis onset. Phosphorylation of GRK2 on residue S670 enables a complex pattern of non-K48-linked polyubiquitin chains assembled by Mdm2, which correlate with kinase protein degradation. Remarkably, GRK2-S670A protein fails to phosphorylate MST2 despite overcoming Mdm2-dependent degradation, which results in defective centrosome separation, shorter spindles, and abnormal chromosome congression. Conversely, extra levels of wild-type kinase in the G2 cause increased inter-centrosome distances with longer spindles, also converging in congression issues. Our findings show that the signals enabling activity of the GRK2/MST2/Nek2A axis for separation also switches on Mdm2 degradation of GRK2 to ensure accurate centrosome dynamics and proper mitotic spindle functionality.

A non-canonical function for Centromere-associated protein-E controls centrosome integrity and orientation of cell division

Centromere-associated protein-E (CENP-E) is a kinesin motor localizing at kinetochores. Although its mitotic functions have been well studied, it has been challenging to investigate direct consequences of CENP-E removal using conventional methods because CENP-E depletion resulted in mitotic arrest. In this study, we harnessed an auxin-inducible degron system to achieve acute degradation of CENP-E. We revealed a kinetochore-independent role for CENP-E that removes pericentriolar material 1 (PCM1) from centrosomes in late S/early G2 phase. After acute loss of CENP-E, centrosomal Polo-like kinase 1 (Plk1) localization is abrogated through accumulation of PCM1, resulting in aberrant phosphorylation and destabilization of centrosomes, which triggers shortened astral microtubules and oblique cell divisions. Furthermore, we also observed centrosome and cell division defects in cells from a microcephaly patient with mutations in CENPE. Orientation of cell division is deregulated in some microcephalic patients, and our unanticipated findings provide additional insights into how microcephaly can result from centrosomal defects.

The myosin regulatory light chain Myl5 localizes to mitotic spindle poles and is required for proper cell division

Myosins are ATP-dependent actin-based molecular motors critical for diverse cellular processes like intracellular trafficking, cell motility, and cell invasion. During cell division, myosin MYO10 is important for proper mitotic spindle assembly, the anchoring of the spindle to the cortex, and positioning of the spindle to the cell mid-plane. However, myosins are regulated by myosin regulatory light chains (RLCs), and whether RLCs are important for cell division has remained unexplored. Here, we have determined that the previously uncharacterized myosin RLC Myl5 associates with the mitotic spindle and is required for cell division. We show that Myl5 localizes to the leading edge and filopodia during interphase and to mitotic spindle poles and spindle microtubules during early mitosis. Importantly, depletion of Myl5 led to defects in mitotic spindle assembly, chromosome congression, and chromosome segregation and to a slower transition through mitosis. Furthermore, Myl5 bound to MYO10 in vitro and co-localized with MYO10 at the spindle poles. These results suggest that Myl5 is important for cell division and that it may be performing its function through MYO10.

Synthesis and Anticancer Activity of Mitotic-Specific 3,4-Dihydropyridine-2(1H)-thiones

Most anticancer drugs target mitosis as the most crucial and fragile period of rapidly dividing cancer cells. However the limitations of classical chemotherapeutics drive the search for new more effective and selective compounds. For this purpose structural modifications of the previously characterized pyridine aalog (S1) were incorporated aiming to obtain an antimitotic inhibitor of satisfactory and specific anticancer activity. Structure-activity relationship analysis of the compounds against a panel of cancer cell lines allowed to select a compound with a thiophene ring at C5 of a 3,4-dihydropyridine-2(1H)-thione (S22) with promising antiproliferative activity (IC50 equal 1.71 ± 0.58 µM) and selectivity (SI = 21.09) against melanoma A375 cells. Moreover, all three of the most active compounds from the antiproliferative study, namely S1, S19 and S22 showed better selectivity against A375 cells than reference drug, suggesting their possible lower toxicity and wider therapeutic index. As further study revealed, selected compounds inhibited tubulin polymerization via colchicine binding site in dose dependent manner, leading to aberrant mitotic spindle formation, cell cycle arrest and apoptosis. Summarizing, the current study showed that among obtained mitotic-specific inhibitors analogue with thiophene ring showed the highest antiproliferative activity and selectivity against cancer cells.

Spindle Architectural Features Must Be Considered Along With Cell Size to Explain the Timing of Mitotic Checkpoint Silencing

Mitosis proceeds through a defined series of events that is largely conserved, but the amount of time needed for their completion can vary in different cells and organisms. In many systems, mitotic duration depends on the time required to satisfy and silence the spindle assembly checkpoint (SAC), also known as the mitotic checkpoint. Because SAC silencing involves trafficking SAC molecules among kinetochores, spindle, and cytoplasm, the size and geometry of the spindle relative to cell volume are expected to affect mitotic duration by influencing the timing of SAC silencing. However, the relationship between SAC silencing, cell size, and spindle dimensions is unclear. To investigate this issue, we used four DLD-1 tetraploid (4N) clones characterized by small or large nuclear and cell size. We found that the small 4N clones had longer mitotic durations than the parental DLD-1 cells and that this delay was due to differences in their metaphase duration. Leveraging a previous mathematical model for spatiotemporal regulation of SAC silencing, we show that the difference in metaphase duration, i.e., SAC silencing time, can be explained by the distinct spindle microtubule densities and sizes of the cell, spindle, and spindle poles in the 4N clones. Lastly, we demonstrate that manipulating spindle geometry can alter mitotic and metaphase duration, consistent with a model prediction. Our results suggest that spindle size does not always scale with cell size in mammalian cells and cell size is not sufficient to explain the differences in metaphase duration. Only when a number of spindle architectural features are considered along with cell size can the kinetics of SAC silencing, and hence mitotic duration, in the different clones be explained.