Explore the vast range of titles available.

The cytoplasmic DNA sensor cGAS detects DNA in ruptured micronuclei and activates an innate immune response. Autoimmunity under surveillance Innate immune activation has been implicated in autoimmunity and cancer. Here, Andrew Jackson and colleagues provide evidence for an underlying mechanism whereby ruptured micronuclei, which result from endogenous or exogenous chromosomal damage, activate a cell-autonomous inflammatory response via the cytoplasmic DNA sensor cGAS. They conclude that cGAS recognition of micronuclei may be acting as a kind of immune surveillance system in cells. Elsewhere in this issue, Roger Greenberg and colleagues report a link between mitosis and DNA-damage-induced inflammatory signalling involving cGAS in cancer cells. DNA is strictly compartmentalized within the nucleus to prevent autoimmunity 1 ; despite this, cyclic GMP–AMP synthase (cGAS), a cytosolic sensor of double-stranded DNA, is activated in autoinflammatory disorders and by DNA damage 2 , 3 , 4 , 5 , 6 . Precisely how cellular DNA gains access to the cytoplasm remains to be determined. Here, we report that cGAS localizes to micronuclei arising from genome instability in a mouse model of monogenic autoinflammation, after exogenous DNA damage and spontaneously in human cancer cells. Such micronuclei occur after mis-segregation of DNA during cell division and consist of chromatin surrounded by its own nuclear membrane. Breakdown of the micronuclear envelope, a process associated with chromothripsis 7 , leads to rapid accumulation of cGAS, providing a mechanism by which self-DNA becomes exposed to the cytosol. cGAS is activated by chromatin, and consistent with a mitotic origin, micronuclei formation and the proinflammatory response following DNA damage are cell-cycle dependent. By combining live-cell laser microdissection with single cell transcriptomics, we establish that interferon-stimulated gene expression is induced in micronucleated cells. We therefore conclude that micronuclei represent an important source of immunostimulatory DNA. As micronuclei formed from lagging chromosomes also activate this pathway, recognition of micronuclei by cGAS may act as a cell-intrinsic immune surveillance mechanism that detects a range of neoplasia-inducing processes.

Key Points The chromosomal passenger complex (CPC) is a 'master controller' of cell division that is formed by a kinase module (Aurora B kinase) and a localization module (the scaffolding protein inner centromere protein (INCENP), survivin and borealin). Multiple post-translational modifications of CPC components contribute to the appropriate localization and regulation of Aurora B activity. Full activation of Aurora B kinase is a complex multistage process that is mediated by the other CPC components and other cell cycle kinases. In early mitosis, CPC recruitment to the inner centromere is mediated by post-translational modifications of two histones: phosphorylation of histone H3 (by haspin kinase) and of histone H2A (by Bub1 kinase). The baculovirus IAP repeat (BIR) domain of survivin recognizes H3 phosphorylated at Thr3. Further enrichment of the CPC at the inner centromere is mediated by Aurora B-dependent regulatory feedback loops. Roles of the CPC in early mitosis include the regulation of chromosome structure, kinetochore–microtubule attachments and the spindle assembly checkpoint. The CPC relocalizes to central spindle microtubules at the onset of anaphase in a highly regulated process that is mediated by a decrease of cyclin-dependent kinase 1 (Cdk1) activity, interaction with the kinesin mitotic kinesin-like protein 2 (Mklp2) and under the control of several phosphatases and Aurora B kinase itself. Functions of the CPC in late mitosis include the formation and stabilization of the spindle midzone in anaphase and the regulation of the contractile ring formation. The CPC has further roles later on in cytokinesis, in which it regulates furrow ingression and the abscission checkpoint. The chromosomal passenger complex (CPC), which is formed by inner centromere protein (INCENP), borealin, survivin and Aurora B kinase, targets to different locations at different times during mitosis. As it regulates key events at each of these locations, the CPC can be considered as a master regulator of mitosis. Successful cell division requires the precise and timely coordination of chromosomal, cytoskeletal and membrane trafficking events. These processes are regulated by the competing actions of protein kinases and phosphatases. Aurora B is one of the most intensively studied kinases. In conjunction with inner centromere protein (INCENP), borealin (also known as Dasra) and survivin it forms the chromosomal passenger complex (CPC). This complex targets to different locations at differing times during mitosis, where it regulates key mitotic events: correction of chromosome–microtubule attachment errors; activation of the spindle assembly checkpoint; and construction and regulation of the contractile apparatus that drives cytokinesis. Our growing understanding of the CPC has seen it develop from a mere passenger riding on the chromosomes to one of the main controllers of mitosis.

The improper distribution of chromosomes during mitosis can contribute to malignant transformation. Higher eukaryotes have developed strategies for eliminating mitosis-incompetent cells, one of which is mitotic catastrophe. From a functional perspective, mitotic catastrophe can be defined as an oncosuppressive mechanism that precedes (and is distinct from) apoptosis, necrosis or senescence. The improper distribution of chromosomes during mitosis compromises cellular functions and can reduce cellular fitness or contribute to malignant transformation. As a countermeasure, higher eukaryotes have developed strategies for eliminating mitosis-incompetent cells, one of which is mitotic catastrophe. Mitotic catastrophe is driven by a complex and poorly understood signalling cascade but, from a functional perspective, it can be defined as an oncosuppressive mechanism that precedes (and is distinct from) apoptosis, necrosis or senescence. Accordingly, the disruption of mitotic catastrophe precipitates tumorigenesis and cancer progression, and its induction constitutes a therapeutic endpoint.

Mitotic bookmarking transcription factors (TFs) are thought to mediate rapid and accurate reactivation after mitotic gene silencing. However, the loss of individual bookmarking TFs often leads to the deregulation of only a small proportion of their mitotic targets, raising doubts on the biological significance and importance of their bookmarking function. Here we used targeted proteomics of the mitotic bookmarking TF ESRRB, an orphan nuclear receptor, to discover a large redundancy in mitotic binding among members of the protein super-family of nuclear receptors. Focusing on the nuclear receptor NR5A2, which together with ESRRB is essential in maintaining pluripotency in mouse embryonic stem cells, we demonstrate conjoint bookmarking activity of both factors on promoters and enhancers of a large fraction of active genes, particularly those most efficiently reactivated in G1. Upon fast and simultaneous degradation of both factors during mitotic exit, hundreds of mitotic targets of ESRRB/NR5A2, including key players of the pluripotency network, display attenuated transcriptional reactivation. We propose that redundancy in mitotic bookmarking TFs, especially nuclear receptors, confers robustness to the reestablishment of gene regulatory networks after mitosis. Using targeted proteomics, the authors reveal concurrent mitotic binding of nuclear receptors, a super-family of transcription factors that emerge as recurrent mitotic bookmarking factors, promoting the reactivation of the pluripotency network in embryonic stem cells.

BRCA2-mutant cells are defective in homologous recombination, making them vulnerable to the inactivation of other pathways for the repair of DNA double-strand breaks (DSBs). This concept can be clinically exploited but is currently limited due to insufficient knowledge about how DSBs are repaired in the absence of BRCA2. We show that DNA polymerase θ (POLθ)-mediated end joining (TMEJ) repairs DSBs arising during the S phase in BRCA2-deficient cells only after the onset of the ensuing mitosis. This process is regulated by RAD52, whose loss causes the premature usage of TMEJ and the formation of chromosomal fusions. Purified RAD52 and BRCA2 proteins both block the DNA polymerase function of POLθ, suggesting a mechanism explaining their synthetic lethal relationships. We propose that the delay of TMEJ until mitosis ensures the conversion of originally one-ended DSBs into two-ended DSBs. Mitotic chromatin condensation might further serve to juxtapose correct break ends and limit chromosomal fusions. Löbrich and colleagues report that RAD52 and BRCA2 limit polymerase θ activity until the onset of mitosis to ensure double-strand breaks arising in the S phase are repaired by polymerase θ-mediated end joining in mitosis, not in S phase.

Key Points The spindle checkpoint signalling cascade prevents anaphase onset until all chromosomes are correctly attached, through their kinetochores, to spindle microtubules. Molecular interactions between kinetochore and spindle checkpoint proteins have been defined and characterized. There have been significant advances in understanding the molecular details of phosphoregulation and checkpoint scaffolding. Monopolar spindle protein 1 (MPS1) has emerged as a direct activating kinase of the checkpoint. The checkpoint response strength is variable and corresponds with the number of unattached kinetochores. Inactivation of cyclin-dependent kinase 1 (CDK1) by cyclin B degradation is a basis for checkpoint inactivation during anaphase. Nuclear pore complexes, in addition to kinetochores, signal the checkpoint. The mechanisms underlying spindle checkpoint signalling at the kinetochore, which ensures faithful chromosome segregation during cell division, are being unravelled. They indicate that the checkpoint response is graded rather than switch-like (completely on or off) as traditionally thought, and provide insights for the treatment of cancers in which the checkpoint is bypassed. The spindle checkpoint ensures proper chromosome segregation during cell division. Unravelling checkpoint signalling has been a long-standing challenge owing to the complexity of the structures and forces that regulate chromosome segregation. New reports have now substantially advanced our understanding of checkpoint signalling mechanisms at the kinetochore, the structure that connects microtubules and chromatin. In contrast to the traditional view of a binary checkpoint response — either completely on or off — new findings indicate that the checkpoint response strength is variable. This revised perspective provides insight into how checkpoint bypass can lead to aneuploidy and informs strategies to exploit these errors for cancer treatments.

In early mitosis, the duplicated chromosomes are held together by the ring-shaped cohesin complex 1 . Separation of chromosomes during anaphase is triggered by separase—a large cysteine endopeptidase that cleaves the cohesin subunit SCC1 (also known as RAD21 2 – 4 ). Separase is activated by degradation of its inhibitors, securin 5 and cyclin B 6 , but the molecular mechanisms of separase regulation are not clear. Here we used cryogenic electron microscopy to determine the structures of human separase in complex with either securin or CDK1–cyclin B1–CKS1. In both complexes, separase is inhibited by pseudosubstrate motifs that block substrate binding at the catalytic site and at nearby docking sites. As in Caenorhabditis elegans 7 and yeast 8 , human securin contains its own pseudosubstrate motifs. By contrast, CDK1–cyclin B1 inhibits separase by deploying pseudosubstrate motifs from intrinsically disordered loops in separase itself. One autoinhibitory loop is oriented by CDK1–cyclin B1 to block the catalytic sites of both separase and CDK1 9 , 10 . Another autoinhibitory loop blocks substrate docking in a cleft adjacent to the separase catalytic site. A third separase loop contains a phosphoserine 6 that promotes complex assembly by binding to a conserved phosphate-binding pocket in cyclin B1. Our study reveals the diverse array of mechanisms by which securin and CDK1–cyclin B1 bind and inhibit separase, providing the molecular basis for the robust control of chromosome segregation. Structures of separase in complex with either securin or cyclin B–CDK1 shed light on the regulation of chromosome separation during the cell cycle.