Feedback control of chromosome separation by a midzone Aurora B gradient

Recent insights into the causes and consequences of chromosome mis-segregation

Mitotic cells face the challenging task of ensuring accurate and equal segregation of their duplicated, condensed chromosomes between the nascent daughter cells. Errors in the process result in chromosome missegregation, a significant consequence of which is the emergence of aneuploidy-characterized by an imbalance in chromosome number-and the associated phenomenon of chromosome instability (CIN). Aneuploidy and CIN are common features of cancer, which leverages them to promote genome heterogeneity and plasticity, thereby facilitating rapid tumor evolution. Recent research has provided insights into how mitotic errors shape cancer genomes by inducing both numerical and structural chromosomal changes that drive tumor initiation and progression. In this review, we survey recent findings regarding the mitotic causes and consequences of aneuploidy. We discuss new findings into the types of chromosome segregation errors that lead to aneuploidy and novel pathways that protect genome integrity during mitosis. Finally, we describe new developments in our understanding of the immediate consequences of chromosome mis-segregation on the genome stability of daughter cells.

Phosphorylation of ELYS promotes its interaction with VAPB at decondensing chromosomes during mitosis

ELYS is a nucleoporin that localizes to the nuclear side of the nuclear pore complex (NPC) in interphase cells. In mitosis, it serves as an assembly platform that interacts with chromatin and then with nucleoporin subcomplexes to initiate post-mitotic NPC assembly. Here we identify ELYS as a major binding partner of the membrane protein VAPB during mitosis. In mitosis, ELYS becomes phosphorylated at many sites, including a predicted FFAT (two phenylalanines in an acidic tract) motif, which mediates interaction with the MSP (major sperm protein)-domain of VAPB. Binding assays using recombinant proteins or cell lysates and co-immunoprecipitation experiments show that VAPB binds the FFAT motif of ELYS in a phosphorylation-dependent manner. In anaphase, the two proteins co-localize to the non-core region of the newly forming nuclear envelope. Depletion of VAPB results in prolonged mitosis, slow progression from meta- to anaphase and in chromosome segregation defects. Together, our results suggest a role of VAPB in mitosis upon recruitment to or release from ELYS at the non-core region of the chromatin in a phosphorylation-dependent manner.

Boveri and beyond: Chromothripsis and genomic instability from mitotic errors

Mitotic cell division is tightly monitored by checkpoints that safeguard the genome from instability. Failures in accurate chromosome segregation during mitosis can cause numerical aneuploidy, which was hypothesized by Theodor Boveri over a century ago to promote tumorigenesis. Recent interrogation of pan-cancer genomes has identified unexpected classes of chromosomal abnormalities, including complex rearrangements arising through chromothripsis. This process is driven by mitotic errors that generate abnormal nuclear structures that provoke extensive yet localized shattering of mis-segregated chromosomes. Here, we discuss emerging mechanisms underlying chromothripsis from micronuclei and chromatin bridges, as well as highlight how this mutational cascade converges on the DNA damage response. A fundamental understanding of these catastrophic processes will provide insight into how initial errors in mitosis can precipitate rapid cancer genome evolution.

A tubule-sheet continuum model for the mechanism of nuclear envelope assembly

Nuclear envelope (NE) assembly defects cause chromosome fragmentation, cancer, and aging. However, major questions about the mechanism of NE assembly and its relationship to nuclear pathology are unresolved. In particular, how cells efficiently assemble the NE starting from vastly different, cell type-specific endoplasmic reticulum (ER) morphologies is unclear. Here, we identify a NE assembly mechanism, "membrane infiltration," that defines one end of a continuum with another NE assembly mechanism, "lateral sheet expansion," in human cells. Membrane infiltration involves the recruitment of ER tubules or small sheets to the chromatin surface by mitotic actin filaments. Lateral sheet expansion involves actin-independent envelopment of peripheral chromatin by large ER sheets that then extend over chromatin within the spindle. We propose a "tubule-sheet continuum" model that explains the efficient NE assembly from any starting ER morphology, the cell type-specific patterns of nuclear pore complex (NPC) assembly, and the obligatory NPC assembly defect of micronuclei.

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.

Connections between sister and non-sister telomeres of segregating chromatids maintain euploidy

The complete separation of sister chromatids during anaphase is a fundamental requirement for successful mitosis. Therefore, divisions with either persistent DNA-based connections or lagging chromosome fragments threaten aneuploidy if unresolved. Here, we demonstrate the existence of an anaphase mechanism in normally dividing cells in which pervasive connections between telomeres of segregating chromosomes aid in rescuing lagging chromosome fragments. We observe that in a large proportion of Drosophila melanogaster neuronal stem cell divisions, early anaphase sister and non-sister chromatids remain connected by thin telomeric DNA threads. Normally, these threads are resolved in mid-to-late anaphase via a spatial mechanism. However, we find that the presence of a nearby unrepaired DNA break recruits histones, BubR1 kinase, Polo kinase, Aurora B kinase, and BAF to the telomeric thread of the broken chromosome, stabilizing it. Stabilized connections then aid lagging chromosome rescue. These results suggest a model in which pervasive anaphase telomere-telomere connections that are normally resolved quickly can instead be stabilized to retain wayward chromosome fragments. Thus, the liability of persistent anaphase inter-chromosomal connections in normal divisions may be offset by their ability to maintain euploidy in the face of chromosome damage and genome loss.

The second half of mitosis and its implications in cancer biology

The nucleus undergoes dramatic structural and functional changes during cell division. With the entry into mitosis, in human cells the nuclear envelope breaks down, chromosomes rearrange into rod-like structures which are collected and segregated by the spindle apparatus. While these processes in the first half of mitosis have been intensively studied, much less is known about the second half of mitosis, when a functional nucleus reforms in each of the emerging cells. Here we review our current understanding of mitotic exit and nuclear reformation with spotlights on the links to cancer biology.

Dephosphorylation in nuclear reassembly after mitosis

In most animal cell types, the interphase nucleus is largely disassembled during mitotic entry. The nuclear envelope breaks down and chromosomes are compacted into separated masses. Chromatin organization is also mostly lost and kinetochores assemble on centromeres. Mitotic protein kinases play several roles in inducing these transformations by phosphorylating multiple effector proteins. In many of these events, the mechanistic consequences of phosphorylation have been characterized. In comparison, how the nucleus reassembles at the end of mitosis is less well understood in mechanistic terms. In recent years, much progress has been made in deciphering how dephosphorylation of several effector proteins promotes nuclear envelope reassembly, chromosome decondensation, kinetochore disassembly and interphase chromatin organization. The precise roles of protein phosphatases in this process, in particular of the PP1 and PP2A groups, are emerging. Moreover, how these enzymes are temporally and spatially regulated to ensure that nuclear reassembly progresses in a coordinated manner has been partly uncovered. This review provides a global view of nuclear reassembly with a focus on the roles of dephosphorylation events. It also identifies important open questions and proposes hypotheses.

Too close for comfort? Endomembranes promote missegregation by enclosing lost chromosomes

Correct segregation of chromosomes during mitosis is essential to prevent aneuploidy. In this issue, Ferrandiz et al. (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202203021) show that endomembranes can promote chromosome missegregation by "ensheathing" misaligned chromosomes, preventing their integration into the metaphase plate. Their findings point toward endomembranes as a potential risk factor for aneuploidy.

The oocyte spindle midzone pauses Cdk1 inactivation during fertilization to enable male pronuclear formation and embryo development

Inactivation of cyclin-dependent kinase 1 (Cdk1), controlled by cyclin B1 proteolysis, orders events during mitotic exit. Here, we used a FRET biosensor to study Cdk1 activity while simultaneously monitoring anaphase II and pronuclear (PN) formation in live mouse eggs throughout fertilization. We find that Cdk1 inactivation occurs over two phases separated by a 3-h pause, the first induces anaphase II and the second induces PN formation. Although both phases require the inhibitory Cdk1 kinase Wee1B, only the first involves cyclin B1 proteolysis. Enforcing the 3-h pause is critical for providing the delay required for male PN formation and is mediated by spindle midzone-dependent sequestration of Wee1B between the first and second phases. Thus, unlike continuous Cdk1 inactivation driven by cyclin B1 proteolysis during mitotic exit, MII oocytes engineer a physiologically important pause during fertilization involving two different pathways to inactivate Cdk1, only the first of which requires proteolysis.

The Cell Biology of Heterochromatin

A conserved feature of virtually all higher eukaryotes is that the centromeres are embedded in heterochromatin. Here we provide evidence that this tight association between pericentric heterochromatin and the centromere is essential for proper metaphase exit and progression into telophase. Analysis of chromosome rearrangements that separate pericentric heterochromatin and centromeres indicates that they must remain associated in order to balance Cohesin/DNA catenation-based binding forces and centromere-based pulling forces during the metaphase–anaphase transition. In addition, a centromere embedded in heterochromatin facilitates nuclear envelope assembly around the entire complement of segregating chromosomes. Because the nuclear envelope initially forms on pericentric heterochromatin, nuclear envelope formation proceeds from the pole, thus providing time for incorporation of lagging and trailing chromosome arms into the newly formed nucleus. Additional analysis of noncanonical mitoses provides further insights into the functional significance of the tight association between heterochromatin and centromeres.

Microtubule rescue at midzone edges promotes overlap stability and prevents spindle collapse during anaphase B

During anaphase B, molecular motors slide interpolar microtubules to elongate the mitotic spindle, contributing to the separation of chromosomes. However, sliding of antiparallel microtubules reduces their overlap, which may lead to spindle breakage, unless microtubules grow to compensate sliding. How sliding and growth are coordinated is still poorly understood. In this study, we have used the fission yeast S. pombe to measure microtubule dynamics during anaphase B. We report that the coordination of microtubule growth and sliding relies on promoting rescues at the midzone edges. This makes microtubules stable from pole to midzone, while their distal parts including the plus ends alternate between assembly and disassembly. Consequently, the midzone keeps a constant length throughout anaphase, enabling sustained sliding without the need for a precise regulation of microtubule growth speed. Additionally, we found that in S. pombe, which undergoes closed mitosis, microtubule growth speed decreases when the nuclear membrane wraps around the spindle midzone.

Genome instability from nuclear catastrophe and DNA damage

The nuclear envelope compartmentalizes the eukaryotic genome, provides mechanical resistance, and regulates access to the chromatin. However, recent studies have identified several conditions where the nuclear membrane ruptures during interphase, breaking down this compartmentalization leading to DNA damage, chromothripsis, and kataegis. This review discusses three major circumstances that promote nuclear membrane rupture, nuclear deformation, chromatin bridges, and micronucleation, and how each of these nuclear catastrophes results in DNA damage. In addition, we highlight recent studies that demonstrate a single chromosome missegregation can initiate a cascade of events that lead to accumulating damage and even multiple rounds of chromothripsis.

Chromosome length and gene density contribute to micronuclear membrane stability

Micronuclei are derived from missegregated chromosomes and frequently lose membrane integrity, leading to DNA damage, innate immune activation, and metastatic signaling. Here, we demonstrate that two characteristics of the trapped chromosome, length and gene density, are key contributors to micronuclei membrane stability and determine the timing of micronucleus rupture. We demonstrate that these results are not due to chromosome-specific differences in spindle position or initial protein recruitment during post-mitotic nuclear envelope assembly. Micronucleus size strongly correlates with lamin B1 levels and nuclear pore density in intact micronuclei, but, unexpectedly, lamin B1 levels do not completely predict nuclear lamina organization or membrane stability. Instead, small gene-dense micronuclei have decreased nuclear lamina gaps compared to large micronuclei, despite very low levels of lamin B1. Our data strongly suggest that nuclear envelope composition defects previously correlated with membrane rupture only partly explain membrane stability in micronuclei. We propose that an unknown factor linked to gene density has a separate function that inhibits the appearance of nuclear lamina gaps and delays membrane rupture until late in the cell cycle.

Synchronization of HeLa Cells to Mitotic Subphases

The cell cycle is a series of events leading to cell replication. When plated at low cell densities in serum-containing medium, cultured cells start to proliferate, moving through the four phases of the cell cycle: G1, S, G2, and M. Mitosis is the most dynamic period of the cell cycle, involving a major reorganization of virtually all cell components. Mitosis is further divided into prophase, prometaphase, metaphase, anaphase, and telophase, which can be easily distinguished from one another by protein markers and/or comparing their chromosome morphology under fluorescence microscope. The progression of the cell cycle through these mitotic subphases is tightly regulated by complicated molecular mechanisms. Synchronization of cells to the mitotic subphases is important for understanding these molecular mechanisms. Here, we describe a protocol to synchronize Hela cells to prometaphase, metaphase, and anaphase/telophase. In this protocol, Hela cells are first synchronized to the early S phase by a double thymidine block. Following the release of the block, the cells are treated with nocodazole, MG132, and blebbistatin to arrest them at prometaphase, metaphase, and anaphase/telophase, respectively. Successful synchronization is assessed using Western blot and fluorescence microscopy.

Cell Cycle Progression and Synchronization: An Overview

The cell cycle is the series of events that take place in a cell that drives it to divide and produce two new daughter cells. Through more than 100 years of efforts by scientists, we now have a much clearer picture of cell cycle progression and its regulation. The typical cell cycle in eukaryotes is composed of the G1, S, G2, and M phases. The M phase is further divided into prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that controls the activity of various Cdk-cyclin complexes. Most cellular events, including DNA duplication, gene transcription, protein translation, and post-translational modification of proteins, occur in a cell-cycle-dependent manner. To understand these cellular events and their underlying molecular mechanisms, it is desirable to have a population of cells that are traversing the cell cycle synchronously. This can be achieved through a process called cell synchronization. Many methods have been developed to synchronize cells to the various phases of the cell cycle. These methods could be classified into two groups: synchronization methods using chemical inhibitors and synchronization methods without using chemical inhibitors. All these methods have their own merits and shortcomings.

Regulation of Cell Cycle Progression by Growth Factor-Induced Cell Signaling

The cell cycle is the series of events that take place in a cell, which drives it to divide and produce two new daughter cells. The typical cell cycle in eukaryotes is composed of the following phases: G1, S, G2, and M phase. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that control the activity of various Cdk–cyclin complexes. While the mechanism underlying the role of growth factor signaling in G1 phase of cell cycle progression has been largely revealed due to early extensive research, little is known regarding the function and mechanism of growth factor signaling in regulating other phases of the cell cycle, including S, G2, and M phase. In this review, we briefly discuss the process of cell cycle progression through various phases, and we focus on the role of signaling pathways activated by growth factors and their receptor (mostly receptor tyrosine kinases) in regulating cell cycle progression through various phases.

Kinetochore life histories reveal an Aurora-B-dependent error correction mechanism in anaphase

Chromosome mis-segregation during mitosis leads to aneuploidy, which is a hallmark of cancer and linked to cancer genome evolution. Errors can manifest as "lagging chromosomes" in anaphase, although their mechanistic origins and likelihood of correction are incompletely understood. Here, we combine lattice light-sheet microscopy, endogenous protein labeling, and computational analysis to define the life history of >104 kinetochores. By defining the "laziness" of kinetochores in anaphase, we reveal that chromosomes are at a considerable risk of mis-segregation. We show that the majority of lazy kinetochores are corrected rapidly in anaphase by Aurora B; if uncorrected, they result in a higher rate of micronuclei formation. Quantitative analyses of the kinetochore life histories reveal a dynamic signature of metaphase kinetochore oscillations that forecasts their anaphase fate. We propose that in diploid human cells chromosome segregation is fundamentally error prone, with an additional layer of anaphase error correction required for stable karyotype propagation.

An anaphase surveillance mechanism prevents micronuclei formation from frequent chromosome segregation errors

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.

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Anaphase lagging chromosomes are frequent but rarely form micronuclei

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A midzone Aurora B activity gradient prevents micronuclei from segregation errors

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Midzone Aurora B assists spindle forces at the kinetochores to correct errors

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Aurora B spatially regulates nuclear envelope reformation on lagging chromosomes

The Aurora B gradient sustains kinetochore stability in anaphase

Kinetochores assemble on chromosomes in mitosis to allow microtubules to attach and bring about accurate chromosome segregation. The kinases Cyclin B-Cdk1 and Aurora B are crucial for the formation of stable kinetochores. However, the activity of these two kinases appears to decline dramatically at centromeres during anaphase onset, precisely when microtubule attachments are required to move chromosomes toward opposite poles of the dividing cell. We find that, although Aurora B leaves centromeres at anaphase, a gradient of Aurora B activity centered on the central spindle is still able to phosphorylate kinetochore substrates such as Dsn1 to modulate kinetochore stability in anaphase and to regulate kinetochore disassembly as cells enter telophase. We provide a model to explain how Aurora B co-operates with Cyclin B-Cdk1 to maintain kinetochore function in anaphase.

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Central spindle Aurora B targets kinetochore substrates in anaphase

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Phosphorylation of Dsn1 by Aurora B stabilizes kinetochores in anaphase

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Dsn1 phosphorylation modulates chromosome movements in anaphase

Micronuclei in Kif18a mutant mice form stable micronuclear envelopes and do not promote tumorigenesis

Micronuclei, whole or fragmented chromosomes spatially separated from the main nucleus, are associated with genomic instability and have been identified as drivers of tumorigenesis. Paradoxically, Kif18a mutant mice produce micronuclei due to asynchronous segregation of unaligned chromosomes in vivo but do not develop spontaneous tumors. We report here that micronuclei in Kif18a mutant mice form stable nuclear envelopes. Challenging Kif18a mutant mice via deletion of the Trp53 gene led to formation of thymic lymphoma with elevated levels of micronuclei. However, loss of Kif18a had modest or no effect on survival of Trp53 homozygotes and heterozygotes, respectively. Micronuclei in cultured KIF18A KO cells form stable nuclear envelopes characterized by increased recruitment of nuclear envelope components and successful expansion of decondensing chromatin compared with those induced by nocodazole washout or radiation. Lagging chromosomes were also positioned closer to the main chromatin masses in KIF18A KO cells. These data suggest that not all micronuclei actively promote tumorigenesis.

Changing places: Chromosomal Passenger Complex relocation in early anaphase

The Chromosomal Passenger Complex (CPC) regulates a plethora of processes during multiple stages of nuclear and cytoplasmic division. Early during mitosis, the CPC is recruited to centromeres and kinetochores, and ensures that the duplicated chromosomes become properly connected to microtubules from opposite poles of the mitotic spindle. Progression into anaphase is accompanied by a striking relocation of the CPC from centromeres to the antiparallel microtubule overlaps of the anaphase spindle and to the equatorial cortex. This translocation requires direct interactions of the CPC with the kinesin-6 family member MKLP2/KIF20A, and the inactivation of cyclin B-cyclin-dependent kinase-1 (CDK1). Here, we review recent progress in the regulation of this relocation event. Furthermore, we discuss why the CPC must be relocated during early anaphase in light of recent advances in the functions of the CPC post metaphase.

Aurora B-dependent polarization of the cortical actomyosin network during mitotic exit

The isotropic metaphase actin cortex progressively polarizes as the anaphase spindle elongates during mitotic exit. This involves the loss of actomyosin cortex from opposing cell poles and the accumulation of an actomyosin belt at the cell centre. Although these spatially distinct cortical remodelling events are coordinated in time, here we show that they are independent of each other. Thus, actomyosin is lost from opposing poles in anaphase cells that lack an actomyosin ring owing to centralspindlin depletion. In examining potential regulators of this process, we identify a role for Aurora B kinase in actin clearance at cell poles. Upon combining Aurora B inhibition with centralspindlin depletion, cells exiting mitosis fail to change shape and remain completely spherical. Additionally, we demonstrate a requirement for Aurora B in the clearance of cortical actin close to anaphase chromatin in cells exiting mitosis with a bipolar spindle and in monopolar cells forced to divide while flat. Altogether, these data suggest a novel role for Aurora B activity in facilitating DNA-mediated polar relaxation at anaphase, polarization of the actomyosin cortex, and cell division.

Anaphase B: Long-standing models meet new concepts

Mitotic cell divisions ensure stable transmission of genetic information from a mother to daughter cells in a series of generations. To ensure this crucial task is accomplished, the cell forms a bipolar structure called the mitotic spindle that divides sister chromatids to the opposite sides of the dividing mother cell. After successful establishment of stable attachments of microtubules to chromosomes and inspection of connections between them, at the heart of mitosis, the cell starts the process of segregation. This spectacular moment in the life of a cell is termed anaphase, and it involves two distinct processes: depolymerization of microtubules bound to chromosomes, which is also known as anaphase A, and elongation of the spindle or anaphase B. Both processes ensure physical separation of disjointed sister chromatids. In this chapter, we review the mechanisms of anaphase B spindle elongation primarily in mammalian systems, combining different pioneering ideas and concepts with more recent findings that shed new light on the force generation and regulation of biochemical modules operating during spindle elongation. Finally, we present a comprehensive model of spindle elongation that includes structural, biophysical, and molecular aspects of anaphase B.

Mitotic disassembly and reassembly of nuclear pore complexes

Nuclear pore complexes (NPCs) are huge protein assemblies within the nuclear envelope (NE) that serve as selective gates for macromolecular transport between nucleus and cytoplasm. When higher eukaryotic cells prepare for division, they rapidly disintegrate NPCs during NE breakdown such that nuclear and cytoplasmic components mix to enable the formation of a cytoplasmic mitotic spindle. At the end of mitosis, reassembly of NPCs is coordinated with the establishment of the NE around decondensing chromatin. We review recent progress on mitotic NPC disassembly and reassembly, focusing on vertebrate cells. We highlight novel mechanistic insights into how NPCs are rapidly disintegrated into conveniently reusable building blocks, and put divergent models of (post-)mitotic NPC assembly into a spatial and temporal context.

Scaling Laws for Mitotic Chromosomes

During mitosis in higher eukaryotes, each chromosome condenses into a pair of rod-shaped chromatids. This process is co-regulated by the activity of several gene families, and the underlying biophysics remains poorly understood. To better understand the factors regulating chromosome condensation, we compiled a database of mitotic chromosome size and DNA content from the tables and figures of >200 published papers. A comparison across vertebrate species shows that chromosome width, length and volume scale with DNA content to the powers ∼1/4, ∼1/2, and ∼1, respectively. Angiosperms (flowering plants) show a similar length scaling, so this result is not specific to vertebrates. Chromosome shape and size thus satisfy two conditions: (1) DNA content per unit volume is approximately constant and (2) the cross-sectional area increases proportionately with chromosome length. Since viscous drag forces during chromosome movement are expected to scale with length, we hypothesize that the cross-section increase is necessary to limit the occurrence of large chromosome elongations that could slow or stall mitosis. Lastly, we note that individual vertebrate karyotypes typically exhibit a wider range of chromosome lengths as compared with angiosperms.

The multifunctional spindle midzone in vertebrate cells at a glance

During anaphase, a microtubule-containing structure called the midzone forms between the segregating chromosomes. The midzone is composed of an antiparallel array of microtubules and numerous microtubule-associated proteins that contribute to midzone formation and function. In many cells, the midzone is an important source of signals that specify the location of contractile ring assembly and constriction. The midzone also contributes to the events of anaphase by generating forces that impact chromosome segregation and spindle elongation; some midzone components contribute to both processes. The results of recent experiments have increased our understanding of the importance of the midzone, a microtubule array that has often been overlooked. This Journal of Cell Science at a Glance article will review, and illustrate on the accompanying poster, the organization, formation and dynamics of the midzone, and discuss open questions for future research.

Microtubule-sliding modules based on kinesins EG5 and PRC1-dependent KIF4A drive human spindle elongation

Proper chromosome segregation into two future daughter cells requires the mitotic spindle to elongate in anaphase. However, although some candidate proteins are implicated in this process, the molecular mechanism that drives spindle elongation in human cells is unknown. Using combined depletion and inactivation assays together with CRISPR technology to explore redundancy between multiple targets, we discovered that the force-generating mechanism of spindle elongation consists of EG5/kinesin-5 together with the PRC1-dependent motor KIF4A/kinesin-4, with contribution from kinesin-6 and kinesin-8. Disruption of EG5 and KIF4A leads to total failure of chromosome segregation due to blocked spindle elongation, despite poleward chromosome motion. Tubulin photoactivation, stimulated emission depletion (STED), and expansion microscopy show that perturbation of both proteins impairs midzone microtubule sliding without affecting microtubule stability. Thus, two mechanistically distinct sliding modules, one based on a self-sustained and the other on a crosslinker-assisted motor, power the mechanism that drives spindle elongation in human cells.

Back to the new beginning: Mitotic exit in space and time

The ultimate goal of cell division is to generate two identical daughter cells that resemble the mother cell from which they derived. Once all the proper attachments to the spindle have occurred, the chromosomes have aligned at the metaphase plate and the spindle assembly checkpoint (a surveillance mechanism that halts cells form progressing in the cell cycle in case of spindle - microtubule attachment errors) has been satisfied, mitotic exit will occur. Mitotic exit has the purpose of completing the separation of the genomic material but also to rebuild the cellular structures necessary for the new cell cycle. This stage of mitosis received little attention until a decade ago, therefore our knowledge is much patchier than the molecular details we now have for the early stages of mitosis. However, it is emerging that mitotic exit is not just the simple reverse of mitotic entry and it is highly regulated in space and time. In this review I will discuss the main advances in the field that provided us with a better understanding on the key role of protein phosphorylation/de-phosphorylation in this transition together with the concept of their spatial regulation. As this field is much younger, I will highlight general consensus, contrasting views together with the outstanding questions awaiting for answers.

The coordination of nuclear envelope assembly and chromosome segregation in metazoans

The nuclear envelope (NE) is composed of two lipid bilayer membranes that enclose the eukaryotic genome. In interphase, the NE is perforated by thousands of nuclear pore complexes (NPCs), which allow transport in and out of the nucleus. During mitosis in metazoans, the NE is broken down and then reassembled in a manner that enables proper chromosome segregation and the formation of a single nucleus in each daughter cell. Defects in coordinating NE reformation and chromosome segregation can cause aberrant nuclear architecture. This includes the formation of micronuclei, which can trigger a catastrophic mutational process commonly observed in cancers called chromothripsis. Here, we discuss the current understanding of the coordination of NE reformation with chromosome segregation during mitotic exit in metazoans. We review differing models in the field and highlight recent work suggesting that normal NE reformation and chromosome segregation are physically linked through the timing of mitotic spindle disassembly.

Small but mighty: the causes and consequences of micronucleus rupture

Micronuclei are small DNA-containing nuclear structures that are spatially isolated from the main nucleus. They are frequently found in pathologies, including cancer. It was recently shown that these nuclear structures are not only biomarkers of disease but also play an active role in tumor biology. Many consequences of micronucleus formation on tumor biology are dependent on the frequent and irreversible rupture of their nuclear envelopes, which results in the exposure of their DNA contents to the cytoplasm. In this review, we discuss models of defective nuclear envelope deposition on missegregated chromosomes that lead to nuclear envelope rupture. Furthermore, we expound upon the various downstream consequences of micronucleus nuclear envelope rupture on cells. These consequences include a massive DNA rearrangement phenomenon called chromothripsis and activation of the cGAS-STING innate immune signaling pathway, which can be a double-edged sword with tumorigenesis and tumor prevention functions. Although micronuclei are small structures, the impact they have on cells and their microenvironment is quite large.

Micronuclei, which contain faulty chromosomes or chromosome fragments and occur outside the main cellular nucleus, are prone to rupturing, which leads to DNA changes that can drive tumor development. A team led by Mijung Kwon from Ewha Womans University in Seoul and Jae-Ho Lee of Ajou University School of Medicine in Suwon, both in South Korea, review how these micronuclei tend to burst, spilling their contents into the cell with devastating consequences. The chromosomes they contain break into tiny fragments and this broken DNA finds its way into the main nucleus, leading to chromosomal rearrangements that can permanently alter genomic function. The rupture of micronuclei also activates a part of the innate immune system that can promote cancer cell invasion and spread. Drugs targeting these processes could aid in the treatment of cancer.

ESCRT-III-mediated membrane fusion drives chromosome fragments through nuclear envelope channels

Mitotic cells must form a single nucleus during telophase or exclude part of their genome as damage-prone micronuclei. While research has detailed how micronuclei arise from cells entering anaphase with lagging chromosomes, cellular mechanisms allowing late-segregating chromosomes to rejoin daughter nuclei remain underexplored. Here, we find that late-segregating acentric chromosome fragments that rejoin daughter nuclei are associated with nuclear membrane but devoid of lamin and nuclear pore complexes in Drosophila melanogaster. We show that acentrics pass through membrane-, lamin-, and nuclear pore-based channels in the nuclear envelope that extend and retract as acentrics enter nuclei. Membrane encompassing the acentrics fuses with the nuclear membrane, facilitating integration of the acentrics into newly formed nuclei. Fusion, mediated by the membrane fusion protein Comt/NSF and ESCRT-III components Shrub/CHMP4B and CHMP2B, facilitates reintegration of acentrics into nuclei. These results suggest a previously unsuspected role for membrane fusion, similar to nuclear repair, in the formation of a single nucleus during mitotic exit and the maintenance of genomic integrity.

Ordered dephosphorylation initiated by the selective proteolysis of cyclin B drives mitotic exit

APC/C-mediated proteolysis of cyclin B and securin promotes anaphase entry, inactivating CDK1 and permitting chromosome segregation, respectively. Reduction of CDK1 activity relieves inhibition of the CDK1-counteracting phosphatases PP1 and PP2A-B55, allowing wide-spread dephosphorylation of substrates. Meanwhile, continued APC/C activity promotes proteolysis of other mitotic regulators. Together, these activities orchestrate a complex series of events during mitotic exit. However, the relative importance of regulated proteolysis and dephosphorylation in dictating the order and timing of these events remains unclear. Using high temporal-resolution proteomics, we compare the relative extent of proteolysis and protein dephosphorylation. This reveals highly-selective rapid proteolysis of cyclin B, securin and geminin at the metaphase-anaphase transition, followed by slow proteolysis of other substrates. Dephosphorylation requires APC/C-dependent destruction of cyclin B and was resolved into PP1-dependent categories with unique sequence motifs. We conclude that dephosphorylation initiated by selective proteolysis of cyclin B drives the bulk of changes observed during mitotic exit.

A fraction of barrier-to-autointegration factor (BAF) associates with centromeres and controls mitosis progression

Barrier-to-Autointegration Factor (BAF) is a conserved nuclear envelope (NE) component that binds chromatin and helps its anchoring to the NE. Cycles of phosphorylation and dephosphorylation control BAF function. Entering mitosis, phosphorylation releases BAF from chromatin and facilitates NE-disassembly. At mitotic exit, PP2A-mediated dephosphorylation restores chromatin binding and nucleates NE-reassembly. Here, we show that in Drosophila a small fraction of BAF (cenBAF) associates with centromeres. We also find that PP4 phosphatase, which is recruited to centromeres by CENP-C, prevents phosphorylation and release of cenBAF during mitosis. cenBAF is necessary for proper centromere assembly and accurate chromosome segregation, being critical for mitosis progression. Disrupting cenBAF localization prevents PP2A inactivation in mitosis compromising global BAF phosphorylation, which in turn leads to its persistent association with chromatin, delays anaphase onset and causes NE defects. These results suggest that, together with PP4 and CENP-C, cenBAF forms a centromere-based mechanism that controls chromosome segregation and mitosis progression.

Mechanisms driving acentric chromosome transmission

The kinetochore-microtubule association is a core, conserved event that drives chromosome transmission during mitosis. Failure to establish this association on even a single chromosome results in aneuploidy leading to cell death or the development of cancer. However, although many chromosomes lacking centromeres, termed acentrics, fail to segregate, studies in a number of systems reveal robust alternative mechanisms that can drive segregation and successful poleward transport of acentrics. In contrast to the canonical mechanism that relies on end-on microtubule attachments to kinetochores, mechanisms of acentric transmission largely fall into three categories: direct attachments to other chromosomes, kinetochore-independent lateral attachments to microtubules, and long-range tether-based attachments. Here, we review these "non-canonical" methods of acentric chromosome transmission. Just as the discovery and exploration of cell cycle checkpoints provided insight into both the origins of cancer and new therapies, identifying mechanisms and structures specifically involved in acentric segregation may have a significant impact on basic and applied cancer research.

Understanding the birth of rupture-prone and irreparable micronuclei

Micronuclei are extra-nuclear bodies mainly derived from ana-telophase lagging chromosomes/chromatins (LCs) that are not incorporated into primary nuclei at mitotic exit. Unlike primary nuclei, most micronuclei are enclosed by nuclear envelope (NE) that is highly susceptible to spontaneous and irreparable rupture. Ruptured micronuclei act as triggers of chromothripsis-like chaotic chromosomal rearrangements and cGAS-mediated innate immunity and inflammation, raising the view that micronuclei play active roles in human aging and tumorigenesis. Thus, understanding the ways in which micronuclear envelope (mNE) goes awry acquires increased importance. Here, we review the data to present a general framework for this question. We firstly describe NE reassembly after mitosis and NE repair during interphase. Simultaneously, we briefly discuss how mNE is organized and how mNE rupture controls the fate of micronuclei and micronucleated cells. As a focus of this review, we highlight current knowledge about why mNE is rupture-prone and irreparable. For this, we survey observations from a series of elegant studies to provide a systematic overview. We conclude that the birth of rupture-prone and irreparable micronuclei may be the cumulative effects of their intracellular geographic origins, biophysical properties, and specific mNE features. We propose that DNA damage and immunogenicity in micronuclei increase stepwise from altered mNE components, mNE rupture, and refractory to repair. Throughout our discussion, we note interesting issues in mNE fragility that have yet to be resolved.

Molecular basis of MKLP2-dependent Aurora B transport from chromatin to the anaphase central spindle

The Aurora B chromosomal passenger complex (CPC) is a conserved regulator of mitosis. Its functions require localization first to the chromosome arms and then centromeres in mitosis and subsequently the central spindle in anaphase. Here, we analyze the requirements for core CPC subunits, survivin and INCENP, and the mitotic kinesin-like protein 2 (MKLP2) in targeting to these distinct localizations. Centromere recruitment of the CPC requires interaction of survivin with histone H3 phosphorylated at threonine 3, and we provide a complete structure of this assembly. Furthermore, we show that the INCENP RRKKRR-motif is required for both centromeric localization of the CPC in metaphase and MKLP2-dependent transport in anaphase. MKLP2 and DNA bind competitively to this motif, and INCENP T59 phosphorylation acts as a switch preventing MKLP2 binding in metaphase. In anaphase, CPC binding promotes the microtubule-dependent ATPase activity of MKLP2. These results explain how centromere targeting of the CPC in mitosis is coupled to its movement to the central spindle in anaphase.

Phosphoproteome and drug-response effects mediated by the three protein phosphatase 2A inhibitor proteins CIP2A, SET, and PME-1

Protein phosphatase 2A (PP2A) critically regulates cell signaling and is a human tumor suppressor. PP2A complexes are modulated by proteins such as cancerous inhibitor of protein phosphatase 2A (CIP2A), protein phosphatase methylesterase 1 (PME-1), and SET nuclear proto-oncogene (SET) that often are deregulated in cancers. However, how they impact cellular phosphorylation and how redundant they are in cellular regulation is poorly understood. Here, we conducted a systematic phosphoproteomics screen for phosphotargets modulated by siRNA-mediated depletion of CIP2A, PME-1, and SET (to reactivate PP2A) or the scaffolding A-subunit of PP2A (PPP2R1A) (to inhibit PP2A) in HeLa cells. We identified PP2A-modulated targets in diverse cellular pathways, including kinase signaling, cytoskeleton, RNA splicing, DNA repair, and nuclear lamina. The results indicate nonredundancy among CIP2A, PME-1, and SET in phosphotarget regulation. Notably, PP2A inhibition or reactivation affected largely distinct phosphopeptides, introducing a concept of nonoverlapping phosphatase inhibition- and activation-responsive sites (PIRS and PARS, respectively). This phenomenon is explained by the PPP2R1A inhibition impacting primarily dephosphorylated threonines, whereas PP2A reactivation results in dephosphorylation of clustered and acidophilic sites. Using comprehensive drug-sensitivity screening in PP2A-modulated cells to evaluate the functional impact of PP2A across diverse cellular pathways targeted by these drugs, we found that consistent with global phosphoproteome effects, PP2A modulations broadly affect responses to more than 200 drugs inhibiting a broad spectrum of cancer-relevant targets. These findings advance our understanding of the phosphoproteins, pharmacological responses, and cellular processes regulated by PP2A modulation and may enable the development of combination therapies.

Intracellular morphogens: Specifying patterns at the subcellular scale

The notion that graded distributions of signals underlie the spatial organization of biological systems has long been a central pillar in the fields of cell and developmental biology. During morphogenesis, morphogens spread across tissues to guide development of the embryo. Similarly, a variety of dynamic gradients and pattern-forming networks have been discovered that shape subcellular organization. Here we discuss the principles of intracellular pattern formation by these intracellular morphogens and relate them to conceptually similar processes operating at the tissue scale. We will specifically review mechanisms for generating cellular asymmetry and consider how intracellular patterning networks are controlled and adapt to cellular geometry. Finally, we assess the general concept of intracellular gradients as a mechanism for positional control in light of current data, highlighting how the simple readout of fixed concentration thresholds fails to fully capture the complexity of spatial patterning processes occurring inside cells.

Micronuclei-based model system reveals functional consequences of chromothripsis in human cells

Cancer cells often harbor chromosomes in abnormal numbers and with aberrant structure. The consequences of these chromosomal aberrations are difficult to study in cancer, and therefore several model systems have been developed in recent years. We show that human cells with extra chromosome engineered via microcell-mediated chromosome transfer often gain massive chromosomal rearrangements. The rearrangements arose by chromosome shattering and rejoining as well as by replication-dependent mechanisms. We show that the isolated micronuclei lack functional lamin B1 and become prone to envelope rupture, which leads to DNA damage and aberrant replication. The presence of functional lamin B1 partly correlates with micronuclei size, suggesting that the proper assembly of nuclear envelope might be sensitive to membrane curvature. The chromosomal rearrangements in trisomic cells provide growth advantage compared to cells without rearrangements. Our model system enables to study mechanisms of massive chromosomal rearrangements of any chromosome and their consequences in human cells.

Getting out of mitosis: spatial and temporal control of mitotic exit and cytokinesis by PP1 and PP2A

Here, we will review the evidence showing that mitotic exit is initiated by regulated proteolysis and then driven by the PPP family of phosphoserine/threonine phosphatases. Rapid APC/CCDC20 and ubiquitin-dependent proteolysis of cyclin B and securin initiates sister chromatid separation, the first step of mitotic exit. Because proteolysis of Aurora and Polo family kinases dependent on APC/CCDH1 is relatively slow, this creates a new regulatory state, anaphase, different to G2 and M-phase. We will discuss how the CDK1-counteracting phosphatases PP1 and PP2A-B55, together with Aurora and Polo kinases, contribute to the temporal regulation and order of events in the different stages of mitotic exit from anaphase to cytokinesis. For PP2A-B55, these timing properties are created by the ENSA-dependent inhibitory pathway and differential recognition of phosphoserine and phosphothreonine. Finally, we will discuss how Aurora B and PP2A-B56 are needed for the spatial regulation of anaphase spindle formation and how APC/C-dependent destruction of PLK1 acts as a timer for abscission, the final event of cytokinesis.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Switching of INCENP paralogs controls transitions in mitotic chromosomal passenger complex functions

A single inner centromere protein (INCENP) found throughout eukaryotes modulates Aurora B kinase activity and chromosomal passenger complex (CPC) localization, which is essential for timely mitotic progression. It has been proposed that INCENP might act as a rheostat to regulate Aurora B activity through mitosis, with successively higher activity threshold levels for chromosome alignment, the spindle checkpoint, anaphase spindle transfer and finally spindle elongation and cytokinesis. It remains mechanistically unclear how this would be achieved. Here, we reveal that the urochordate, Oikopleura dioica, possesses two INCENP paralogs, which display distinct localizations and subfunctionalization in order to complete M-phase. INCENPa was localized on chromosome arms and centromeres by prometaphase, and modulated Aurora B activity to mediate H3S10/S28 phosphorylation, chromosome condensation, spindle assembly and transfer of the CPC to the central spindle. Polo-like kinase (Plk1) recruitment to CDK1 phosphorylated INCENPa was crucial for INCENPa-Aurora B enrichment on centromeres. The second paralog, INCENPb was enriched on centromeres from prometaphase, and relocated to the central spindle at anaphase onset. In the absence of INCENPa, meiotic spindles failed to form, and homologous chromosomes did not segregate. INCENPb was not required for early to mid M-phase events but became essential for the activity and localization of Aurora B on the central spindle and midbody during cytokinesis in order to allow abscission to occur. Together, our results demonstrate that INCENP paralog switching on centromeres modulates Aurora B kinase localization, thus chronologically regulating CPC functions during fast embryonic divisions in the urochordate O. dioica. Abbreviations: CCAN: constitutive centromere-associated network; CENPs: centromere proteins; cmRNA: capped messenger RNA; CPC: chromosomal passenger complex; INCENP: inner centromere protein; Plk1: polo-like kinase 1; PP1: protein phosphatase 1; PP2A: protein phosphatase 2A; SAC: spindle assembly checkpoint; SAH: single α-helix domain.

Spatiotemporal control of mitotic exit during anaphase by an aurora B-Cdk1 crosstalk

According to the prevailing 'clock' model, chromosome decondensation and nuclear envelope reformation when cells exit mitosis are byproducts of Cdk1 inactivation at the metaphase-anaphase transition, controlled by the spindle assembly checkpoint. However, mitotic exit was recently shown to be a function of chromosome separation during anaphase, assisted by a midzone Aurora B phosphorylation gradient - the 'ruler' model. Here we found that Cdk1 remains active during anaphase due to ongoing APC/CCdc20- and APC/CCdh1-mediated degradation of B-type Cyclins in Drosophila and human cells. Failure to degrade B-type Cyclins during anaphase prevented mitotic exit in a Cdk1-dependent manner. Cyclin B1-Cdk1 localized at the spindle midzone in an Aurora B-dependent manner, with incompletely separated chromosomes showing the highest Cdk1 activity. Slowing down anaphase chromosome motion delayed Cyclin B1 degradation and mitotic exit in an Aurora B-dependent manner. Thus, a crosstalk between molecular 'rulers' and 'clocks' licenses mitotic exit only after proper chromosome separation.

Classical and Emerging Regulatory Mechanisms of Cytokinesis in Animal Cells

The primary goal of cytokinesis is to produce two daughter cells, each having a full set of chromosomes. To achieve this, cells assemble a dynamic structure between segregated sister chromatids called the contractile ring, which is made up of filamentous actin, myosin-II, and other regulatory proteins. Constriction of the actomyosin ring generates a cleavage furrow that divides the cytoplasm to produce two daughter cells. Decades of research have identified key regulators and underlying molecular mechanisms; however, many fundamental questions remain unanswered and are still being actively investigated. This review summarizes the key findings, computational modeling, and recent advances in understanding of the molecular mechanisms that control the formation of the cleavage furrow and cytokinesis.

The Role of Phosphatases in Nuclear Envelope Disassembly and Reassembly and Their Relevance to Pathologies

The role of kinases in the regulation of cell cycle transitions is very well established, however, over the past decade, studies have identified the ever-growing importance of phosphatases in these processes. It is well-known that an intact or otherwise non-deformed nuclear envelope (NE) is essential for maintaining healthy cells and any deviation from this can result in pathological conditions. This review aims at assessing the current understanding of how phosphatases contribute to the remodelling of the nuclear envelope during its disassembling and reformation after cell division and how errors in this process may lead to the development of diseases.

Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole during metaphase I. We have found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) regulate sister centromere fusion in Drosophila oocytes. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion. Maintenance of sister centromere fusion by SPC105R depends on Separase, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, maintenance of sister centromere fusion by PP1-87B does not depend on either Separase or WAPL. Instead, PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. We demonstrate that this regulation is through antagonizing Polo kinase and BubR1, two proteins known to promote stability of kinetochore-microtubule (KT-MT) attachments, suggesting that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, C(3)G, the transverse element of the synaptonemal complex (SC), is also required for centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions, that might involve regulating microtubule dynamics. Together, we propose two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments.