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The planarian Schmidtea mediterranea is an important model for stem cell research and regeneration, but adequate genome resources for this species have been lacking. Here we report a highly contiguous genome assembly of S. mediterranea , using long-read sequencing and a de novo assembler (MARVEL) enhanced for low-complexity reads. The S. mediterranea genome is highly polymorphic and repetitive, and harbours a novel class of giant retroelements. Furthermore, the genome assembly lacks a number of highly conserved genes, including critical components of the mitotic spindle assembly checkpoint, but planarians maintain checkpoint function. Our genome assembly provides a key model system resource that will be useful for studying regeneration and the evolutionary plasticity of core cell biological mechanisms. An improved genome assembly for Schmidtea mediterranea shows that the genome is highly polymorphic and repetitive, and lacks multiple genes encoding core components of cell biological mechanisms. Genome of a regenerating worm The flatworm Schmidtea mediterranea is an important model for regeneration. Jochen Rink, Eugene Myers and colleagues report an improved genome assembly for the planarian S. mediterranea using long-read sequencing and a new genome assembler called MARVEL. They find that the S. mediterranea genome is highly polymorphic and repetitive, and includes a novel class of giant retroelements. This improved genome assembly provides a useful resource for studying regeneration and the evolution of cell plasticity.

The strength of the DNA damage checkpoint critically influences cell fate, yet the mechanisms behind the fine tuning of checkpoint strength during the DNA damage response (DDR) are poorly understood. Here we show that Rad54B—a SNF2 helicase-like DNA-repair protein—limits the strength of both the G1/S and G2/M checkpoints. We find that Rad54B functions as a scaffold for p53 degradation via its direct interaction with the MDM2–MDMX ubiquitin–ligase complex. During the early phases of the DDR, Rad54B is upregulated, thereby maintaining low checkpoint strength and facilitating cell cycle progression. Once the p53-mediated checkpoint is established, Rad54B is downregulated, and high checkpoint strength is maintained. Constitutive upregulation of Rad54B activity, which is frequently observed in tumours, promotes genomic instability because of checkpoint override. Thus, the scaffolding function of Rad54B dynamically regulates the maintenance of genome integrity by limiting checkpoint strength. Rad54B is a poorly characterized DNA damage repair protein homologous to Rad54, a protein implicated in DNA damage repair through homologous recombination. Here the authors implicate Rad54B as a modulator of the DNA damage response through its interaction with the MDM2–MDMX complex to regulate p53 degradation.

Blocking mitotic progression has been proposed as an attractive therapeutic strategy to impair proliferation of tumour cells. However, how cells survive during prolonged mitotic arrest is not well understood. We show here that survival during mitotic arrest is affected by the special energetic requirements of mitotic cells. Prolonged mitotic arrest results in mitophagy-dependent loss of mitochondria, accompanied by reduced ATP levels and the activation of AMPK. Oxidative respiration is replaced by glycolysis owing to AMPK-dependent phosphorylation of PFKFB3 and increased production of this protein as a consequence of mitotic-specific translational activation of its mRNA. Induction of autophagy or inhibition of AMPK or PFKFB3 results in enhanced cell death in mitosis and improves the anti-tumoral efficiency of microtubule poisons in breast cancer cells. Thus, survival of mitotic-arrested cells is limited by their metabolic requirements, a feature with potential implications in cancer therapies aimed to impair mitosis or metabolism in tumour cells. Malumbres and colleagues reveal that mitotic arrest is accompanied by reduced mitochondrial mass and oxidative respiration resulting in activation of AMPK and induction of glycolysis to promote cell survival.

Key Points Aneuploidy is extraordinarily common in humans, occurring in an estimated 20–40% of all conceptions. It is the most common cause of miscarriages and congenital defects in our species and is a leading impediment to the treatment of infertility. Most aneuploidy results from maternal meiotic nondisjunctional errors. However, there is remarkable variation among chromosomes in the way in which these errors originate, indicating that there are multiple mechanisms by which human aneuploidy occurs. Studies of human fetal oocytes indicate a high level of recombination errors, indicating that some oocytes are predisposed to nondisjoin because of events that occurred before birth. Cell cycle control checkpoints that operate in meiotic prophase and at the metaphase–anaphase transition are less stringent in females than in males. Consequently, abnormal cells that are eliminated in spermatogenesis may escape detection in the female, ultimately leading to aneuploid eggs. Studies from mice suggest that loss of cohesin proteins over the reproductive life of the female contribute to the maternal age effect on human trisomy. Exposure to endocrine disruptors (for example, bisphenol A) disrupts oogenesis at multiple stages and predisposes the oocyte to aneuploidy. Aneuploidy is the leading cause of congenital defects in humans and nearly always results from errors occurring in oocytes. Here, the authors review the evidence pointing towards the mechanistic basis of meiotic defects leading to aneuploidy and discuss the potential role of environmental factors. Trisomic and monosomic (aneuploid) embryos account for at least 10% of human pregnancies and, for women nearing the end of their reproductive lifespan, the incidence may exceed 50%. The errors that lead to aneuploidy almost always occur in the oocyte but, despite intensive investigation, the underlying molecular basis has remained elusive. Recent studies of humans and model organisms have shed new light on the complexity of meiotic defects, providing evidence that the age-related increase in errors in the human female is not attributable to a single factor but to an interplay between unique features of oogenesis and a host of endogenous and exogenous factors.

Inducible degradation is a powerful approach for identifying the function of a specific protein or protein complex. Recently, a plant auxin-inducible degron (AID) system has been shown to degrade AID-tagged target proteins in nonplant cells. Here, we demonstrate that an AID-tagged protein can functionally replace an endogenous protein depleted by RNAi, leading to an inducible null phenotype rapidly after auxin addition. The AID system is shown to be capable of controlling the stability of AID-tagged proteins that are in either nuclear or cytoplasmic compartments and even when incorporated into protein complexes. Induced degradation occurs rapidly after addition of auxin with protein half-life reduced to as little as 9 min and proceeding to completion with first-order kinetics. AID-mediated instability is demonstrated to be rapidly reversible. Induced degradation is shown to initiate and continue in all cell cycle phases, including mitosis, making this system especially useful for identifying the function(s) of proteins of interest during specific points in the mammalian cell cycle.

Drugs that impair microtubule dynamics alter microtubule-kinetochore attachment and invoke the mitotic checkpoint which arrests cells in mitosis. The arrest can last for hours, but it is leaky: cells adapt (i.e., slip out of it) and exit from mitosis. Here, we investigate the mechanism that allows cells to escape, and whether it is possible to prevent it. Based on a model of the mitotic checkpoint which includes the presence of a positive feedback loop, the escape from the arrest is described as a stochastic transition driven by fluctuations of molecular components from a checkpoint ON to a checkpoint OFF state. According to the model, drug removal further facilitates adaptation, a prediction we confirmed in budding yeast. The model suggests two ways to avoid adaptation: inhibition of APC/C and strengthening the mitotic checkpoint. We confirmed experimentally that both alterations decrease the chance of cells slipping out of mitosis, during a prolonged arrest and after washing out the drug. Our results may be relevant for increasing the efficiency of microtubule depolymerizing drugs.

Mitosis is a highly dynamic process, aimed at separating identical copies of genomic material into two daughter cells. A failure of the mitotic process generates cells that carry abnormal chromosome numbers. Such cells are predisposed to become tumorigenic upon continuous cell division and thus need to be removed from the population to avoid cancer formation. Cells that fail in mitotic progression indeed activate cell death or cell cycle arrest pathways; however, these mechanisms are not well understood. Growing evidence suggests that the formation of de novo DNA damage during and after mitotic failure is one of the causal factors that initiate those pathways. Here, we analyze several distinct malfunctions during mitosis and cytokinesis that lead to de novo DNA damage generation.

In the dividing eukaryotic cell, the spindle assembly checkpoint (SAC) ensures that each daughter cell inherits an identical set of chromosomes. The SAC coordinates the correct attachment of sister chromatid kinetochores to the mitotic spindle with activation of the anaphase-promoting complex (APC/C), the E3 ubiquitin ligase responsible for initiating chromosome separation. In response to unattached kinetochores, the SAC generates the mitotic checkpoint complex (MCC), which inhibits the APC/C and delays chromosome segregation. By cryo-electron microscopy, here we determine the near-atomic resolution structure of a human APC/C–MCC complex (APC/C MCC ). Degron-like sequences of the MCC subunit BubR1 block degron recognition sites on Cdc20, the APC/C coactivator subunit responsible for substrate interactions. BubR1 also obstructs binding of the initiating E2 enzyme UbcH10 to repress APC/C ubiquitination activity. Conformational variability of the complex enables UbcH10 association, and structural analysis shows how the Cdc20 subunit intrinsic to the MCC (Cdc20 MCC ) is ubiquitinated, a process that results in APC/C reactivation when the SAC is silenced. A high-resolution structure of a complex between the anaphase-promoting complex (APC/C) and the mitotic checkpoint complex (MCC) reveals how MCC interacts with and represses APC/C by obstructing substrate recognition and suppressing E3 ligase activity. Basis of mitotic regulation The spindle assembly checkpoint (SAC) is a surveillance mechanism that detects incorrect chromatid kinetochore attachments and delays chromosome segregation by generating a 'wait anaphase' signal. It is activated via the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex (APC/C), a multimeric E3 ligase. Here, David Barford and colleagues use cryo-electron microscopy to determine near-atomic resolution structures of the APC/C–MCC complex. The structures reveal how MCC interacts with and represses APC/C by obstructing substrate recognition and suppressing E3 ligase activity.

The transition from mitosis into the first gap phase of the cell cycle in budding yeast is controlled by the Mitotic Exit Network (MEN). The network interprets spatiotemporal cues about the progression of mitosis and ensures that release of Cdc14 phosphatase occurs only after completion of key mitotic events. The MEN has been studied intensively; however, a unified understanding of how localisation and protein activity function together as a system is lacking. In this paper, we present a compartmental, logical model of the MEN that is capable of representing spatial aspects of regulation in parallel to control of enzymatic activity. We show that our model is capable of correctly predicting the phenotype of the majority of mutants we tested, including mutants that cause proteins to mislocalise. We use a continuous time implementation of the model to demonstrate that Cdc14 Early Anaphase Release (FEAR) ensures robust timing of anaphase, and we verify our findings in living cells. Furthermore, we show that our model can represent measured cell–cell variation in Spindle Position Checkpoint (SPoC) mutants. This work suggests a general approach to incorporate spatial effects into logical models. We anticipate that the model itself will be an important resource to experimental researchers, providing a rigorous platform to test hypotheses about regulation of mitotic exit.

The near-complete in vitro reconstitution of the mitotic spindle assembly checkpoint reveals how the assembly of its effector, the mitotic checkpoint complex, is catalysed. Assembly of the mitotic checkpoint complex During mitotic cell division, chromatids attach to the spindle microtubules through protein structures called kinetochores. If even one kinetochore remains unattached, the spindle assembly checkpoint (SAC) is activated to prevent the progress of mitosis. In cells, the SAC response is established within minutes. In vitro , however, the process (activation of the SAC effector protein MAD2 through conformational change to allow the assembly of the mitotic checkpoint complex) takes hours. Andrea Musacchio and colleagues now resolve this discrepancy by reconstituting a near-complete SAC signalling system and analysing it using real-time sensors. Their observations offer a mechanistic explanation for the accelerated and spontaneous conversion of MAD2 in cells. In mitosis, for each daughter cell to inherit an accurate copy of the genome from the mother cell, sister chromatids in the mother cell must attach to microtubules emanating from opposite poles of the mitotic spindle, a process known as bi-orientation. A surveillance mechanism, termed the spindle assembly checkpoint (SAC), monitors the microtubule attachment process and can temporarily halt the separation of sister chromatids and the completion of mitosis until bi-orientation is complete 1 . SAC failure results in abnormal chromosome numbers, termed aneuploidy, in the daughter cells, a hallmark of many tumours. The HORMA-domain-containing protein mitotic arrest deficient 2 (MAD2) is a subunit of the SAC effector mitotic checkpoint complex (MCC). Structural conversion from the open to the closed conformation of MAD2 is required for MAD2 to be incorporated into the MCC 1 . In vitro , MAD2 conversion and MCC assembly take several hours 2 , 3 , 4 , but in cells the SAC response is established in a few minutes 5 , 6 , 7 . Here, to address this discrepancy, we reconstituted a near-complete SAC signalling system with purified components and monitored assembly of the MCC in real time. A marked acceleration in MAD2 conversion and MCC assembly was observed when monopolar spindle 1 (MPS1) kinase phosphorylated the MAD1–MAD2 complex, triggering it to act as the template for MAD2 conversion and therefore contributing to the establishment of a physical platform for MCC assembly. Thus, catalytic activation of the SAC depends on regulated protein–protein interactions that accelerate the spontaneous but rate-limiting conversion of MAD2 required for MCC assembly.