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Enduring chromosome segregation errors represent potential threats to genomic stability due to eventual chromosome copy number alterations (aneuploidy) and formation of micronuclei—key intermediates of a rapid mutational process known as chromothripsis that is found in cancer and congenital disorders. The spindle assembly checkpoint (SAC) has been viewed as the sole surveillance mechanism that prevents chromosome segregation errors during mitosis and meiosis. However, different types of chromosome segregation errors stemming from incorrect kinetochore–microtubule attachments satisfy the SAC and are more frequent than previously anticipated. Remarkably, recent works have unveiled that most of these errors are corrected during anaphase and only rarely result in aneuploidy or formation of micronuclei. Here, we discuss recent progress in our understanding of the origin and fate of chromosome segregation errors that satisfy the SAC and shed light on the surveillance, correction, and clearance mechanisms that prevent their transmission, to preserve genomic stability.

DNA damage response (DDR) during interphase involves active signalling and repair to ensure genomic stability. However, how mitotic cells respond to DNA damage remains poorly understood. Supported by correlative live-/fixed-cell microscopy we found that mitotic cells exposed to several cancer chemotherapy compounds acquire and signal DNA damage, regardless of how they interact with DNA. In-depth analysis upon DNA damage during mitosis revealed a spindle assembly checkpoint (SAC)-dependent, but ATM-independent, mitotic delay. This delay was due to the presence of misaligned chromosomes that ultimately satisfy the SAC and missegregate, leading to micronuclei formation. Mechanistically, we show that mitotic DNA damage causes missegregation of polar chromosomes due to the action of arm-ejection forces by chromokinesins. Importantly, with the exception of DNA damage induced by etoposide - a Topoisomerase II inhibitor - this outcome was independent of a general effect on kinetochore microtubule stability. Colony formation assays in pan-cancer cell line models revealed that mitotic DNA damage causes distinct cytotoxic effects, depending on the nature and extent of the damage. Overall, these findings unveil and raise awareness that therapeutic DNA damage regimens may contribute to genomic instability through a surprising link with chromokinesin-mediated missegregation of polar chromosomes in cancer cells.

CLASPs (cytoplasmic linker-associated proteins) are ubiquitous stabilizers of microtubule dynamics, but their molecular targets at the microtubule plus-end are not understood. Using DNA origami–based reconstructions, we show that clusters of human CLASP2 form a load-bearing bond with terminal non-GTP tubulins at the stabilized microtubule tip. This activity relies on the unconventional TOG2 domain of CLASP2, which releases its high-affinity bond with non-GTP dimers upon their conversion into polymerization-competent GTP-tubulins. The ability of CLASP2 to recognize nucleotide-specific tubulin conformation and stabilize the catastrophe-promoting non-GTP tubulins intertwines with the previously underappreciated exchange between GDP and GTP at terminal tubulins. We propose that TOG2-dependent stabilization of sporadically occurring non-GTP tubulins represents a distinct molecular mechanism to suppress catastrophe at the freely assembling microtubule ends and to promote persistent tubulin assembly at the load-bearing tethered ends, such as at the kinetochores in dividing cells.

α/β-Tubulin posttranslational modifications (PTMs) generate microtubule diversity, but whether they account for cancer cell resistance to microtubule-targeting drugs remains unknown. Here, we performed a pilot dissection of the “cancer tubulin code” using the NCI-60 cancer cell panel. We found that acetylated, detyrosinated, and ∆2-α-tubulin that typically accumulate on stable microtubules were uncoupled in many cancer cells. Acetylated α-tubulin did not affect microtubule dynamics, whereas its levels correlated with, but were not required for, taxol-induced cytotoxicity. In contrast, experimental increase of α-tubulin detyrosination, and/or depletion of the detyrosination-sensitive microtubule-depolymerizing enzyme MCAK, enhanced taxol-induced cytotoxicity by promoting cell death in mitosis and the subsequent interphase, without causing a cumulative effect. Interestingly, only increased detyrosinated α-tubulin aggravated taxol-induced spindle multipolarity. Overall, we identified high α-tubulin acetylation as a potential biomarker for cancer cell response to taxol and uncovered a mechanistic link between α-tubulin detyrosination and the suppression of MCAK activity in taxol-induced cytotoxicity, likely by promoting chromosome missegregation, regardless of spindle defects.

We have identified USP31 as a microtubule and centrosome associated deubiquitylase, depletion of which leads to an increase in individual cell mass and defective proliferation. We have examined its dynamics and impact during mitosis. GFP-USP31 associates with the mitotic and central spindles, its levels are increased 2-3-fold in prometaphase compared to asynchronous cells and it is dynamically phosphorylated in a CDK1 dependent manner. We find that USP31 depleted cells display altered spindle morphology and chromosomal segregation errors, whilst stable expression of a catalytically inactive form of USP31 leads to polyploidy. At prometaphase, levels of multiple CPC components are destabilised, most prominently INCENP. Under anaphase conditions, depletion of USP31 impairs the translocation of both endogenous and ectopically expressed INCENP to the spindle mid-zone, whilst expression of catalytically inactive USP31 results in multiple ectopic cleavage furrows. In summary, our data indicate a multifaceted regulatory role for USP31 during mitosis, with a profound impact on chromosomal passenger complex abundance, dynamics and function.

DNA damage response (DDR) during interphase involves active signalling and repair to ensure genomic stability. However, how mitotic cells respond to DNA damage remains poorly understood. Supported by correlative live-/fixed-cell microscopy analysis we found that mitotic cells exposed to several cancer chemotherapy compounds acquire and signal DNA damage, regardless of how they interact with DNA. In-depth analysis upon long-term DNA damage during mitosis revealed a spindle assembly checkpoint (SAC)-dependent, but DDR-independent, mitotic delay. This delay was due to the presence of misaligned chromosomes that ultimately satisfy the SAC and missegregate, leading to micronuclei formation. Mechanistically, we show that long-term mitotic DNA damage specifically stabilizes kinetochore-microtubule attachments in cancer cells, causing the missegregation of polar chromosomes due to the action of arm-ejection forces by chromokinesins. Overall, these findings unveil that long-term therapeutic DNA damage regimens contribute to genomic instability through a surprising link between the stabilization of kinetochore-microtubule attachments and chromokinesin-mediated missegregation of polar chromosomes in cancer cells.

SummaryChromosome alignment to the spindle equator is a hallmark of mitosis that is thought to promote chromosome segregation fidelity in metazoans. Yet, chromosome alignment is only indirectly supervised by the spindle assembly checkpoint (SAC) as a byproduct of chromosome bi-orientation, and the consequences of defective chromosome alignment remain unclear. Here we investigated how human cells respond to chromosome alignment defects of distinct molecular nature by following the fate of live HeLa cells after RNAi-mediated depletion of 120 proteins previously implicated in chromosome alignment. Surprisingly, in all cases, cells frequently entered anaphase after a delay with chronically misaligned chromosomes. Using depletion of key proteins as prototypes for defective chromosome alignment, we show that chronically misaligned chromosomes often satisfy the SAC and directly missegregate. In-depth analysis of specific molecular perturbations that prevent proper kinetochore-microtubule attachments revealed that chronically misaligned chromosomes that missegregate frequently result in micronuclei. Higher-resolution live-cell imaging indicated that, contrary to most anaphase lagging chromosomes that correct and reintegrate the main nuclei, chronically misaligned chromosomes are a strong predictor of micronuclei formation in a cancer cell model of chromosomal instability, but not in normal near-diploid cells. We provide evidence supporting that intrinsic differences in kinetochore-microtubule attachment stability on misaligned chromosomes account for this distinct outcome. Thus, chronically misaligned chromosomes that satisfy the SAC may represent a previously overlooked mechanism driving chromosomal/genomic instability during cancer cell division, and we unveil genetic conditions predisposing for these events.

International audience; MBoC | ARTICLE Centrosome-nuclear axis repositioning drives the assembly of a bipolar spindle scaffold to ensure mitotic fidelity ABSTRACT During the initial stages of cell division, the cytoskeleton is extensively reorganized so that a bipolar mitotic spindle can be correctly assembled. This process occurs through the action of molecular motors, cytoskeletal networks, and the nucleus. How the combined activity of these different components is spatiotemporally regulated to ensure efficient spindle assembly remains unclear. To investigate how cell shape, cytoskeletal organization, and molecular motors cross-talk to regulate initial spindle assembly, we use a combination of micropatterning with high-resolution imaging and 3D cellular reconstruction. We show that during prophase, centrosomes and nucleus reorient so that centrosomes are positioned on the shortest nuclear axis at nuclear envelope (NE) breakdown. We also find that this orientation depends on a combination of centrosome movement controlled by Arp2/3-mediated regulation of microtubule dynamics and Dynein-generated forces on the NE that regulate nuclear reorientation. Finally, we observe this centrosome configuration favors the establishment of an initial bipolar spindle scaffold, facilitating chromosome capture and accurate segregation, without compromising division plane orientation.

The first breath of air at birth marks the beginning of extrauterine life, and breathing problems due to incomplete lung development or acute respiratory distress are common in premature babies and respiratory diseases. However, the underlying molecular mechanisms remain poorly understood. Here we show that the microtubule plus-end-tracking protein CLASP1 is required for neonatal lung function and survival. CLASP1 is expressed in the lungs and associated respiratory structures throughout embryonic development. Clasp1 disruption in mice caused intrauterine growth restriction and neonatal lethality due to acute respiratory failure. Knockout animals showed impaired lung inflation associated with smaller rib cage formation and abnormal diaphragm innervation. Live-cell analysis of microtubule dynamics in cultured hippocampal neurons revealed an increased catastrophe rate, consistent with a role of CLASP1 in neurite outgrowth. Histological and gene expression studies indicated that CLASP1 is required for normal pneumocyte differentiation and fetal lung maturation. Thus, CLASP1-mediated regulation of microtubule dynamics assists multiple systems essential for neonatal lung function and survival.

CLASPs are ubiquitous stabilizers of microtubule dynamics but their molecular targets at the microtubule plus-end are not understood. Using DNA origami-based reconstructions we show that clusters of human CLASP2 form a load-bearing bond with terminal GDP-tubulins at the stabilized microtubule tip. This activity relies on the unconventional TOG2 domain of CLASP2, which releases its high-affinity bond with the GDP-dimers upon their conversion into polymerization-competent GTP-tubulin. By tethering dynamic microtubule ends near immobilized CLASP2, we show that the targets for CLASP2 binding at the polymerizing tip arise stochastically, leading to nanoscale disruptions in microtubule tip integrity. The ability of CLASP2 to recognize nucleotide-specific tubulin conformation and stabilize the catastrophe-promoting GDP-tubulins intertwines with the previously underappreciated exchange between GDP and GTP at terminal tubulins, providing a distinct molecular mechanism to suppress microtubule catastrophe without affecting tubulin incorporation.

Summary Micronuclei are a hallmark of cancer and several other human disorders. Recently, micronuclei were implicated in chromothripsis, a series of massive genomic rearrangements that may drive tumor evolution and progression. Here, we show that Aurora B kinase mediates a surveillance mechanism that integrates error correction during anaphase with spatial control of nuclear envelope reassembly to prevent micronuclei formation. Using high-resolution live-cell imaging of human cancer and non-cancer cells, we uncover that anaphase lagging chromosomes are more frequent than previously anticipated, yet they rarely form micronuclei. Micronuclei formation from anaphase lagging chromosomes is prevented by a midzone-based Aurora B phosphorylation gradient that stabilizes kinetochore-microtubule attachments and assists spindle forces required for anaphase error correction while delaying nuclear envelope reassembly on lagging chromosomes, independently of microtubule density. We propose that a midzone-based Aurora B phosphorylation gradient actively monitors and corrects frequent chromosome segregation errors to prevent micronuclei formation during human cell division., Graphical abstract, Highlights • Anaphase lagging chromosomes are frequent but rarely form micronuclei • A midzone Aurora B activity gradient prevents micronuclei from segregation errors • Midzone Aurora B assists spindle forces at the kinetochores to correct errors • Aurora B spatially regulates nuclear envelope reformation on lagging chromosomes, Orr et al. show that a spindle midzone-based Aurora B phosphorylation gradient mediates a surveillance mechanism that prevents micronuclei formation from frequent chromosome segregation errors by integrating error correction during anaphase with spatial control of nuclear envelope reassembly on lagging chromosomes in human cells.

As cells prepare to divide, they must ensure that enough space is available to assemble the mitotic machinery without perturbing tissue homeostasis1. To do so, cells undergo a series of biochemical reactions regulated by cyclin B1-CDK1 that trigger the reorganization of the actomyosin cytoskeleton2 and ensure the coordination of cytoplasmic and nuclear events3.Along with the biochemical events that control mitotic entry, mechanical forces have recently emerged as important players in the regulation of cell cycle events4–6. However, the exact link between mechanical forces and the biochemical pathways that control mitotic progression remains to be established. Here, we identify a mechanical signal on the nucleus that helps set the time for nuclear envelope permeabilization (NEP) and mitotic entry. This signal relies on actomyosin contractility, which leads to nuclear unfolding during the G2-M transition, activating the stretch-sensitive cPLA2 on the nuclear envelope. This contributes to the spatiotemporal translocation of cyclin B1 to the nucleus. Our data demonstrate how nuclear mechanics during the G2-M transition contribute to timely and efficient mitotic spindle assembly and prevents chromosomal instability.

Chromosome alignment to the spindle equator is a hallmark of mitosis thought to promote chromosome segregation fidelity in metazoans. Yet chromosome alignment is only indirectly supervised by the spindle assembly checkpoint (SAC) as a byproduct of chromosome bi-orientation, and the consequences of defective chromosome alignment remain unclear. Here, we investigated how human cells respond to chromosome alignment defects of distinct molecular nature by following the fate of live HeLa cells after RNAi-mediated depletion of 125 proteins previously implicated in chromosome alignment. We confirmed chromosome alignment defects upon depletion of 108/125 proteins. Surprisingly, in all confirmed cases, depleted cells frequently entered anaphase after a delay with misaligned chromosomes. Using depletion of prototype proteins resulting in defective chromosome alignment, we show that misaligned chromosomes often satisfy the SAC and directly missegregate without lagging behind in anaphase. In-depth analysis of specific molecular perturbations that prevent proper kinetochore-microtubule attachments revealed that misaligned chromosomes that missegregate frequently result in micronuclei. Higher-resolution live-cell imaging indicated that, contrary to most anaphase lagging chromosomes that correct and reintegrate the main nuclei, misaligned chromosomes are a strong predictor of micronuclei formation in a cancer cell model of chromosomal instability, but not in non-transformed near-diploid cells. We provide evidence supporting that intrinsic differences in kinetochore-microtubule attachment stability on misaligned chromosomes account for this distinct outcome. Thus, misaligned chromosomes that satisfy the SAC may represent a previously overlooked mechanism driving chromosomal/genomic instability during cancer cell division, and we unveil genetic conditions predisposing for these events.

Chromosome segregation in mammals relies on the maturation of a thick bundle of kinetochore-attached microtubules known as k-fibers. How k-fibers mature from initial kinetochore-microtubule attachments remains a fundamental question. By combining functional perturbations in Indian muntjac, a placental mammal with the lowest known chromosome number (n=3) and distinctively large kinetochores, with fixed- and subsecond live-cell super-resolution CH-STED nanoscopy, we uncovered the mechanism by which Augmin mediates k-fiber maturation. Augmin promoted kinetochore microtubule turnover by sustaining microtubule formation from kinetochores and poleward flux, regardless of pioneer centrosomal microtubules. Tracking of microtubule growth events within individual k-fibers revealed a wide angular dispersion, consistent with Augmin-mediated branched microtubule nucleation. Augmin depletion reduced the frequency of microtubule growth events within k-fibers and hampered normal repair after acute k-fiber injury by laser microsurgery. Our work underscores the contribution of microtubule formation from kinetochores and Augmin-mediated microtubule amplification for k-fiber maturation and spindle assembly in mammals.

Smmary Microtubule attachments to kinetochores cause their deformation — a murky phenomenon known as intra-kinetochore stretching. A new study proposes that intra-kinetochore stretching is independent of microtubule-pulling forces and mediates efficient spindle assembly checkpoint silencing to prevent chromosomal instability.

SummaryMicronuclei are a hallmark of cancer and other human disorders and have recently been implicated in chromothripsis, a series of massive genomic rearrangements that may drive tumor evolution and progression. Here we show that Aurora B kinase mediates a surveillance mechanism that integrates error correction during anaphase with spatial control of nuclear envelope reformation to protect against micronuclei formation during human cell division. Using high-resolution live-cell imaging of human cancer and non-cancer cells we found that anaphase lagging chromosomes are often transient and rarely formed micronuclei. This strong bias against micronuclei formation relied on a midzone-based Aurora B phosphorylation gradient that assisted the mechanical transduction of spindle forces at the kinetochore-microtubule interface required for anaphase error correction, while delaying nuclear envelope reformation on lagging chromosomes, independently of microtubules. Our results uncover a new layer of protection against genomic instability and provide a strategy for the rational design of micronuclei-targeting therapies.

CLASPs are key modulators of microtubule dynamics throughout the cell cycle. During mitosis, CLASPs independently associate with growing microtubule plus-ends and kinetochores and play essential roles in chromosome segregation. In a proteomic survey for human CLASP1-interacting proteins during mitosis, we have previously identified SOGA1 and SOGA2/MTCL1, whose mitotic roles remained uncharacterized. Here we performed an initial functional characterization of human SOGA1 and SOGA2/MTCL1 during mitosis. Using specific polyclonal antibodies raised against SOGA proteins, we confirmed their expression and reciprocal interaction with CLASP1 and CLASP2 during mitosis. In addition, we found that both SOGA1 and SOGA2/MTCL1 are phospho-regulated during mitosis by CDK1. Immunofluorescence analysis revealed that SOGA2/MTCL1 co-localizes with mitotic spindle microtubules and spindle poles throughout mitosis and both SOGA proteins are enriched at the midbody during mitotic exit/cytokinesis. GFP-tagging of SOGA2/MTCL1 further revealed a microtubule-independent localization at kinetochores. Live-cell imaging after siRNA-mediated knockdown of SOGA1 and SOGA2/MTCL1 showed that they are independently required for distinct aspects of chromosome segregation. Thus, SOGA1 and SOGA2/MTCL1 are bona fide CLASP-interacting proteins during mitosis required for faithful chromosome segregation in human cells.

Mitotic spindle microtubules (MTs) undergo continuous poleward flux, whose driving force and function in humans remain unclear. Here, we combined loss‐of‐function screenings with analysis of MT‐dynamics in human cells to investigate the molecular mechanisms underlying MT‐flux. We report that kinesin‐7/CENP‐E at kinetochores (KTs) is the predominant driver of MT‐flux in early prometaphase, while kinesin‐4/KIF4A on chromosome arms facilitates MT‐flux during late prometaphase and metaphase. Both these activities work in coordination with kinesin‐5/EG5 and kinesin‐12/KIF15, and our data suggest that the MT‐flux driving force is transmitted from non‐KT‐MTs to KT‐MTs by the MT couplers HSET and NuMA. Additionally, we found that the MT‐flux rate correlates with spindle length, and this correlation depends on the establishment of stable end‐on KT‐MT attachments. Strikingly, we find that MT‐flux is required to regulate spindle length by counteracting kinesin 13/MCAK‐dependent MT‐depolymerization. Thus, our study unveils the long‐sought mechanism of MT‐flux in human cells as relying on the coordinated action of four kinesins to compensate for MT‐depolymerization and regulate spindle length.

The "tubulin code" combines different a/ß-tubulin isotypes with several post-translational modifications (PTMs) to generate microtubule diversity in cells. During cell division, specific microtubule populations in the mitotic spindle are differentially modified, but only recently, the functional significance of the tubulin code, with particular emphasis on the role specified by tubulin PTMs, started to be elucidated. This is the case of a-tubulin detyrosination, which was shown to guide chromosomes during congression to the metaphase plate and allow the discrimination of mitotic errors, whose correction is required to prevent chromosomal instability-a hallmark of human cancers implicated in tumor evolution and metastasis. Although alterations in the expression of certain tubulin isotypes and associated PTMs have been reported in human cancers, it remains unclear whether and how the tubulin code has any functional implications for cancer cell properties. Here, we review the role of the tubulin code in chromosome segregation during mitosis and how it impacts cancer cell properties. In this context, we discuss the existence of an emerging "cancer tubulin code" and the respective implications for diagnostic, prognostic and therapeutic purposes. Danilo Lopes is a student of Programa de Pós-Graduação Ciência para o Desenvolvimento (PGCD) from Instituto Gulbenkian de Ciência (IGC), Oeiras, Portugal and recipient of a studentship (SFRH/BD/135077/2017) from Fundação para a Ciência e a Tecnologia of Portugal. Work in the laboratory of H.M. is funded by the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (grant agreement No 681443).

Mitotic spindle microtubules (MTs) undergo continuous poleward flux, whose driving force and function in humans remain unclear. Here, we combined loss-of-function screenings with analysis of MT dynamics in human cells to investigate the molecular mechanisms underlying MT-flux. We report that kinesin-7/CENP-E at kinetochores (KTs) is the predominant driver of MT-flux in early prometaphase, while kinesin-4/KIF4A on chromosome arms facilitates MT-flux during late prometaphase and metaphase. We show that both of these activities work in coordination with MT-crosslinking motors kinesin-5/EG5 and kinesin-12/KIF15. Our data further indicate that MT-flux driving force is transmitted from non-KT MTs to KT-MTs via MT-coupling by HSET and NuMA. Moreover, we found that MT-flux rate correlates with spindle size and this correlation depends on the establishment of stable end-on KT-MT attachments. Strikingly, we revealed that flux is required to counteract the kinesin 13/MCAK-dependent MT-depolymerization to regulate spindle length. Thus, our study demonstrates that MT-flux in human cells is driven by the coordinated action of four kinesins, and is required to regulate mitotic spindle size in response to MCAK-mediated MT-depolymerizing activity at KTs.

The assembly of a microtubule (MT)-based bipolar spindle is a distinct hallmark of mitosis. Mitotic spindle assembly is tightly controlled in order to ensure proper chromosome attachment and segregation during mitosis. This regulation is achieved by the coordinated action of many motor and nonmotor MT-associated proteins (MAPs). Owing to the distinctive capacity of MAPs to associate with the MT minus and/or plus ends, as well as with the MT lattice, they play an essential role in the spatial and temporal regulation of MT behaviour during mitotic spindle assembly. Key Concepts The mitotic spindle is a dynamic structure composed of microtubules, motor and nonmotor proteins. Proper chromosome alignment and segregation during mitosis requires bipolar spindle assembly. Nonmotor microtubule-associated proteins are involved in spindle pole focusing, microtubule dynamics, kinetochore-microtubule attachment stability and cytokinesis. Nonmotor microtubule-associated proteins associate with microtubule plus and minus ends (+TIPs and −TIPs, respectively) as well as with the microtubule lattice. Nonmotor microtubule-associated proteins are tightly regulated during the cell cycle. Keywords: mitosis; mitotic spindle; microtubule; microtubule dynamics; microtubule-associated proteins; +TIPs; −TIPs

Detyrosination is a frequent posttranslational modification of long-lived microtubules that inhibits microtubule depolymerase activity in vitro. Ferreira et al. combine manipulation of tubulin tyrosine ligase and carboxypeptidase (Vasohibins-SVBP) activities with state-of-the-art microscopy in human cells to show that α-tubulin detyrosination allows centromeric MCAK to discriminate between correct and incorrect kinetochore–microtubule attachments and ensure mitotic fidelity., Incorrect kinetochore–microtubule attachments during mitosis can lead to chromosomal instability, a hallmark of human cancers. Mitotic error correction relies on the kinesin-13 MCAK, a microtubule depolymerase whose activity in vitro is suppressed by α-tubulin detyrosination—a posttranslational modification enriched on long-lived microtubules. However, whether and how MCAK activity required for mitotic error correction is regulated by α-tubulin detyrosination remains unknown. Here we found that detyrosinated α-tubulin accumulates on correct, more stable, kinetochore–microtubule attachments. Experimental manipulation of tubulin tyrosine ligase (TTL) or carboxypeptidase (Vasohibins-SVBP) activities to constitutively increase α-tubulin detyrosination near kinetochores compromised efficient error correction, without affecting overall kinetochore microtubule stability. Rescue experiments indicate that MCAK centromeric activity was required and sufficient to correct the mitotic errors caused by excessive α-tubulin detyrosination independently of its global impact on microtubule dynamics. Thus, microtubules are not just passive elements during mitotic error correction, and the extent of α-tubulin detyrosination allows centromeric MCAK to discriminate correct vs. incorrect kinetochore–microtubule attachments, thereby promoting mitotic fidelity., Graphical Abstract

The study of microtubule dynamics is of utmost importance for the understanding of the mechanisms underlying mitotic fidelity. During mitosis, the microtubular cytoskeleton reorganizes to assemble a mitotic spindle necessary for chromosome segregation. Several methods, such as controlled exposure to cold, high pressure, high calcium concentration, or microtubule depolymerizing drugs, have been widely used to evaluate the dynamic properties of specific spindle microtubule populations. However, while these methods offer a qualitative approach that is sufficient to discern differences among specific spindle microtubule populations, they fall short in providing a robust quantitative picture that is sensitive enough to highlight minor differences, for example when comparing spindle microtubule dynamics in different genetic backgrounds. In this chapter we describe a detailed methodology to measure spindle microtubule dynamics using photoactivation of fluorescently tagged tubulin in living cells. This methodology allows the quantitative discrimination of the turnover of specific microtubule populations (e.g., kinetochore vs. non-kinetochore microtubules), as well as determination of microtubule poleward flux rates. These two conspicuous features of metazoan spindles must be tightly regulated to allow, on the one hand, efficient error correction, and on the other hand the satisfaction of the spindle assembly checkpoint that controls mitotic fidelity.

During cell division in eukaryotes a microtubule-based network undergoes drastic changes and remodeling to assemble a mitotic spindle competent to segregate chromosomes. Several model systems have been widely used to dissect the molecular and structural mechanisms behind mitotic spindle assembly and function. These include budding and fission yeasts, which are ideal for genetic and molecular approaches, but show limitations in high-resolution live-cell imaging, while being evolutionarily distant from humans. On the other hand, systems that were historically used for their exceptional properties for live-cell imaging of mitosis (e.g., newt lung cells and Haemanthus endosperm cells) lack the necessary genomic tools for molecular studies. In a CRISPR-Cas9 era, human cultured cells have conquered the privilege to be positioned among the most powerful genetically manipulatable systems, but their high chromosome number remains a significant bottleneck for the molecular dissection of mitosis in mammals. We believe that we can significantly broaden this scenario by establishing a unique placental mammal model system that combines the powerful genetic tools and low chromosome number of fission yeast and Drosophila melanogaster, with the exceptional cytological features of a rat kangaroo cell. This system is based on hTERT-immortalized fibroblasts from a female Indian muntjac, a placental mammal with the lowest known chromosome number (n = 3). Here we describe a series of methodologies established in our laboratory for the study of mitosis in Indian muntjac. These include standard techniques such as immunofluorescence, western blotting, and FISH, but also several state-of-the-art methodologies, including live-cell imaging, cell confinement, RNAi, super-resolution STED microscopy, and laser microsurgery.