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[EN]Fidelity in chromosome duplication and segregation is indispensable for maintaining genomic stability and the perpetuation of life. Challenges to genome integrity jeopardize cell survival and are at the root of di erent types of pathologies, such as cancer. The following three main sources of genomic instability exist: DNA damage, replicative stress, and chromosome segregation defects. In response to these challenges, eukaryotic cells have evolved control mechanisms, also known as checkpoint systems, which sense under-replicated or damaged DNA and activate specialized DNA repair machineries. Cells make use of these checkpoints throughout interphase to shield genome integrity before mitosis. Later on, when the cells enter into mitosis, the spindle assembly checkpoint (SAC) is activated and remains active until the chromosomes are properly attached to the spindle apparatus to ensure an equal segregation among daughter cells. All of these processes are tightly interconnected and under strict regulation in the context of the cell division cycle. The chromosomal instability underlying cancer pathogenesis has recently emerged as a major source for understanding the mitotic processes that helps to safeguard genome integrity. Here, we review the special interconnection between the S-phase and mitosis in the presence of under-replicated DNA regions. Furthermore, we discuss what is known about the DNA damage response activated in mitosis that preserves chromosomal integrity.

The accurate and complete replication of genomic DNA is essential for all life. In eukaryotic cells, the assembly of the multi-enzyme replisomes that perform replication is divided into stages that occur at distinct phases of the cell cycle. Replicative DNA helicases are loaded around origins of DNA replication exclusively during G1 phase. The loaded helicases are then activated during S phase and associate with the replicative DNA polymerases and other accessory proteins. The function of the resulting replisomes is monitored by checkpoint proteins that protect arrested replisomes and inhibit new initiation when replication is inhibited. The replisome also coordinates nucleosome disassembly, assembly, and the establishment of sister chromatid cohesion. Finally, when two replisomes converge they are disassembled. Studies in Saccharomyces cerevisiae have led the way in our understanding of these processes. Here, we review our increasingly molecular understanding of these events and their regulation.

A functional mitotic spindle is essential for accurate chromosome congression and segregation during cell proliferation; however, the underlying mechanisms of its assembly remain unclear. Here we show that NuMA regulates this assembly process via phase separation regulated by Aurora A. NuMA undergoes liquid-liquid phase separation during mitotic entry and KifC1 facilitates NuMA condensates concentrating on spindle poles. Phase separation of NuMA is mediated by its C-terminus, whereas its dynein-dynactin binding motif also facilitates this process. Phase-separated NuMA droplets concentrate tubulins, bind microtubules, and enrich crucial regulators, including Kif2A, at the spindle poles, which then depolymerizes spindle microtubules and promotes poleward spindle microtubule flux for spindle assembly and structural dynamics. In this work, we show that NuMA orchestrates mitotic spindle assembly, structural dynamics and function via liquid-liquid phase separation regulated by Aurora A phosphorylation. Mitotic spindle assembly is required for proper cell division, but many underlying mechanisms remain unclear. Here, the authors show that NuMa undergoes liquid-liquid phase separation, condensing on spindle poles during mitotic entry and enriching critical components to promote spindle assembly.

Metaplastic breast carcinoma (MBC) is a highly aggressive form of triple-negative cancer (TNBC), defined by the presence of metaplastic components of spindle, squamous, or sarcomatoid histology. The protein profiles underpinning the pathological subtypes and metastatic behavior of MBC are unknown. Using multiplex quantitative tandem mass tag-based proteomics we quantify 5798 proteins in MBC, TNBC, and normal breast from 27 patients. Comparing MBC and TNBC protein profiles we show MBC-specific increases related to epithelial-to-mesenchymal transition and extracellular matrix, and reduced metabolic pathways. MBC subtypes exhibit distinct upregulated profiles, including translation and ribosomal events in spindle, inflammation- and apical junction-related proteins in squamous, and extracellular matrix proteins in sarcomatoid subtypes. Comparison of the proteomes of human spindle MBC with mouse spindle ( CCN6 knockout) MBC tumors reveals a shared spindle-specific signature of 17 upregulated proteins involved in translation and 19 downregulated proteins with roles in cell metabolism. These data identify potential subtype specific MBC biomarkers and therapeutic targets. Metaplastic breast carcinoma (MBC) is among the most aggressive subtypes of triple-negative breast cancer (TNBC) but the underlying proteome profiles are unknown. Here, the authors characterize the protein signatures of human MBC tissue samples and their relationship to TNBC and normal breast tissue.

The Aurora protein kinases are well-established regulators of spindle building and chromosome segregation in mitotic and meiotic cells. In mouse oocytes, there is significant Aurora kinase A (AURKA) compensatory abilities when the other Aurora kinase homologs are deleted. Whether the other homologs, AURKB or AURKC can compensate for loss of AURKA is not known. Using a conditional mouse oocyte knockout model, we demonstrate that this compensation is not reciprocal because female oocyte-specific knockout mice are sterile, and their oocytes fail to complete meiosis I. In determining AURKA-specific functions, we demonstrate that its first meiotic requirement is to activate Polo-like kinase 1 at acentriolar microtubule organizing centers (aMTOCs; meiotic spindle poles). This activation induces fragmentation of the aMTOCs, a step essential for building a bipolar spindle. We also show that AURKA is required for regulating localization of TACC3, another protein required for spindle building. We conclude that AURKA has multiple functions essential to completing MI that are distinct from AURKB and AURKC.

Faithful chromosome segregation depends on the ability of sister kinetochores to attach to spindle microtubules. The outer layer of kinetochores transiently expands in early mitosis to form a fibrous corona, and compacts following microtubule capture. Here we show that the dynein adaptor Spindly and the RZZ (ROD–Zwilch–ZW10) complex drive kinetochore expansion in a dynein-independent manner. C-terminal farnesylation and MPS1 kinase activity cause conformational changes of Spindly that promote oligomerization of RZZ-Spindly complexes into a filamentous meshwork in cells and in vitro. Concurrent with kinetochore expansion, Spindly potentiates kinetochore compaction by recruiting dynein via three conserved short linear motifs. Expanded kinetochores unable to compact engage in extensive, long-lived lateral microtubule interactions that persist to metaphase, and result in merotelic attachments and chromosome segregation errors in anaphase. Thus, dynamic kinetochore size regulation in mitosis is coordinated by a single, Spindly-based mechanism that promotes initial microtubule capture and subsequent correct maturation of attachments. Sacristan et al. show that the dynein adaptor Spindly facilitates oligomerisation of the RZZ complex to expand the kinetochore, after which Spindly-associated dynein compacts the kinetochore to allow for faithful chromosome segregation.

The position and orientation of the mitotic spindle is precisely regulated to ensure the accurate partition of the cytoplasm between daughter cells and the correct localization of the daughters within growing tissue. Using magnetic tweezers to perturb the position of the spindle in intact cells, we discovered a force-generating machinery that maintains the spindle at the cell center during metaphase and anaphase in one- and two-cell Caenorhabditis elegans embryos. The forces increase with the number of microtubules and are larger in smaller cells. The machinery is rigid enough to suppress thermal fluctuations to ensure precise localization of the mitotic spindle, yet compliant enough to allow molecular force generators to fine-tune the position of the mitotic spindle to facilitate asymmetric division.

Chromosomal instability (CIN) is defined as the perpetual missegregation of whole chromosomes during mitosis and represents a hallmark of human cancer. However, the mechanisms influencing CIN and its consequences on tumour growth are largely unknown. We identified an increase in microtubule plus-end assembly rates as a mechanism influencing CIN in colorectal cancer cells. This phenotype is induced by overexpression of the oncogene AURKA or by loss of the tumour suppressor gene CHK2 , a genetic constitution found in 73% of human colorectal cancers. Increased microtubule assembly rates are associated with transient abnormalities in mitotic spindle geometry promoting the generation of lagging chromosomes and influencing CIN. Reconstitution of proper microtubule assembly rates by chemical or genetic means suppresses CIN and thereby, unexpectedly, accelerates tumour growth in vitro and in vivo . Thus, we identify a fundamental mechanism influencing CIN in cancer cells and reveal its adverse consequence on tumour growth. Chromosomal instability (CIN) is a common feature of colorectal cancer cells and several mechanisms have been suggested for CIN generation. Bastians and colleagues find that increased microtubule plus-end stability can be triggered by AURKA overexpression and is associated with abnormal spindles and lagging chromosomes in colorectal cancer cells.

In the Caenorhabditis elegans zygote, mutations in zyg-8 DCLK1 , the sole Doublecortin-family member, disrupt mitotic spindle positioning, as seen by immunofluorescence. Doublecortin proteins bind microtubules and are thought to stabilise or rigidify them. In the zygote, ZYG-8 only modestly affects microtubule growth and nucleation. We thus investigated whether these moderate dynamic perturbations alone could explain the spindle mispositioning observed in zyg-8 mutants. Using three complementary genetic perturbations—RNAi-mediated depletion of ZYG-8, its overexpression, and the thermosensitive zyg-8(or484ts) mutant (that disrupts microtubule binding)—we observed altered spindle pole oscillations and changes in microtubule cortical-contact behaviour, indicative of impaired cortical forces. Importantly, these phenotypes could not be fully explained by previously reported alterations in microtubule dynamics, suggesting an additional mechanism. Our findings indicate that ZYG-8 increases microtubule rigidity: ZYG-8 depletion or mutation led to more frequent microtubule bending and higher curvature and tortuosity. Simulations confirmed that reduced rigidity prolongs cortical contact lifetimes, an effect we experimentally observed in zyg-8(RNAi) embryos. Using custom biophysical assays, we showed that microtubule softening in zyg-8(RNAi) embryos and zyg-8 mutants reduced the efficiency of centring forces, leading to exaggerated spindle-pole oscillations. In mutants, the largest oscillations caused spindle poles to move closer to the cell periphery, preventing re-centring and resulting in spindle mispositioning and misorientation during late anaphase. Importantly, reducing cortical pulling forces rescued orientation defects, highlighting the importance of balanced pulling-pushing forces for proper spindle positioning. We propose that sufficient microtubule rigidity is essential for generating effective cortical pushing forces, potentially in synergy with other microtubule properties, which contribute to centring mechanisms that ensure accurate spindle orientation in late mitosis. Given that DCLK1 is frequently deregulated in human cancers and that accurate spindle positioning is essential for maintaining cell proliferation-differentiation balance, these findings may have implications for understanding how disruptions in microtubule mechanics contribute to carcinogenesis.

Eukaryotic chromosome segregation relies on the assembly of a bipolar machinery based on microtubules (MTs), named the mitotic spindle. Formation of the mitotic spindle follows a force balance mechanism that ensures the proper capture and separation of sister chromatids. Many proteins have been involved in the establishment of this force balance, although kinesin 5 is well recognized as the major outward pushing force generator, since its inactivation results in monopolar, non-functional spindles. In order to find additional players in the force balance mechanism, we have performed a suppressor screen using a conditional allele of the fission yeast kinesin 5 ortholog Cut7. This screen identified that the lack of the PP6 phosphatase partially suppresses cut7 phenotypes, at least by defective translation of MT regulators, such as the minus end-directed kinesin Klp2, the MT stabilizer Alp7 and the MT bundler Ase1, impacting on the force balance mechanism. Additionally, our data show that the Elongator complex, a target activated by PP6 for efficient tRNA modification, also contributes to the force balance, albeit to a lesser extent. Importantly, this complex has recently been implicated in direct MT polymerization in metazoans, a role not shared by its fission yeast counterpart.